[Federal Register Volume 73, Number 98 (Tuesday, May 20, 2008)]
[Proposed Rules]
[Pages 29184-29291]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E8-10808]
[[Page 29183]]
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Part II
Environmental Protection Agency
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40 CFR Parts 50, 51, 53 et al.
National Ambient Air Quality Standards for Lead; Proposed Rule
Federal Register / Vol. 73, No. 98 / Tuesday, May 20, 2008 / Proposed
Rules
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 51, 53 and 58
[EPA-HQ-OAR-2006-0735; FRL-8563-9]
RIN 2060-AN83
National Ambient Air Quality Standards for Lead
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
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SUMMARY: Based on its review of the air quality criteria and national
ambient air quality standards (NAAQS) for lead (Pb), EPA proposes to
make revisions to the primary and secondary NAAQS for Pb to provide
requisite protection of public health and welfare, respectively. EPA
proposes to revise various elements of the primary standard to provide
increased protection for children and other at-risk populations against
an array of adverse health effects, most notably including neurological
effects, particularly neurocognitive and neurobehavioral effects, in
children. With regard to the level and indicator of the standard, EPA
proposes to revise the level to within the range of 0.10 to 0.30 [mu]g/
m\3\ in conjunction with retaining the current indicator of Pb in total
suspended particles (Pb-TSP) but with allowance for the use of Pb-
PM10 data, and solicits comment on alternative levels up to
0.50 [mu]g/m\3\ and down below 0.10 [mu]g/m\3\. With regard to the
averaging time and form of the standard, EPA proposes two options: To
retain the current averaging time of a calendar quarter and the current
not-to-be-exceeded form, revised to apply across a 3-year span; and to
revise the averaging time to a calendar month and the form to the
second-highest monthly average across a 3-year span. EPA also solicits
comment on revising the indicator to Pb-PM10 and on the same
broad range of levels on which EPA is soliciting comment for the Pb-TSP
indicator (up to 0.50 [mu]g/m\3\). EPA also invites comment on when, if
ever, it would be appropriate to set a NAAQS for Pb at a level of zero.
EPA proposes to make the secondary standard identical in all respects
to the proposed primary standard.
EPA is also proposing corresponding changes to data handling
procedures, including the treatment of exceptional events, and to
ambient air monitoring and reporting requirements for Pb including
those related to sampling and analysis methods, network design,
sampling schedule, and data reporting. Finally, EPA is providing
guidance on its proposed approach for implementing the proposed revised
primary and secondary standards for Pb.
Consistent with the terms of a court order, by September 15, 2008
the Administrator will sign a notice of final rulemaking for
publication in the Federal Register.
DATES: Comments must be received by July 21, 2008. Under the Paperwork
Reduction Act, comments on the information collection provisions must
be received by OMB on or before June 19, 2008.
Public Hearings: EPA intends to hold public hearings on this
proposed rule in June 2008 in St. Louis, Missouri and Baltimore,
Maryland. These will be announced in a separate Federal Register notice
that provides details, including specific times and addresses, for
these hearings.
ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2006-0735 by one of the following methods:
http://www.regulations.gov: Follow the online instructions
for submitting comments.
E-mail: [email protected].
Fax: 202-566-9744.
Mail: Docket No. EPA-HQ-OAR-2006-0735, Environmental
Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave., NW.,
Washington, DC 20460. Please include a total of two copies.
Hand Delivery: Docket No. EPA-HQ-OAR-2006-0735,
Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution
Ave., NW., Washington, DC. Such deliveries are only accepted during the
Docket's normal hours of operation, and special arrangements should be
made for deliveries of boxed information.
Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2006-0735. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at http://www.regulations.gov, including any personal
information provided, unless the comment includes information claimed
to be Confidential Business Information (CBI) or other information
whose disclosure is restricted by statute. Do not submit information
that you consider to be CBI or otherwise protected through http://www.regulations.gov or e-mail. The http://www.regulations.gov Web site
is an ``anonymous access'' system, which means EPA will not know your
identity or contact information unless you provide it in the body of
your comment. If you send an e-mail comment directly to EPA without
going through http://www.regulations.gov, your e-mail address will be
automatically captured and included as part of the comment that is
placed in the public docket and made available on the Internet. If you
submit an electronic comment, EPA recommends that you include your name
and other contact information in the body of your comment and with any
disk or CD-ROM you submit. If EPA cannot read your comment due to
technical difficulties and cannot contact you for clarification, EPA
may not be able to consider your comment. Electronic files should avoid
the use of special characters, any form of encryption, and be free of
any defects or viruses. For additional information about EPA's public
docket, visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm.
Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although listed in the index, some
information is not publicly available, e.g., CBI or other information
whose disclosure is restricted by statute. Certain other material, such
as copyrighted material, will be publicly available only in hard copy.
Publicly available docket materials are available either electronically
in http://www.regulations.gov or in hard copy at the Air and Radiation
Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301
Constitution Ave., NW., Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is (202)
566-1744 and the telephone number for the Air and Radiation Docket and
Information Center is (202) 566-1742.
FOR FURTHER INFORMATION CONTACT: For further information in general or
specifically with regard to sections I through III or VII, contact Dr.
Deirdre Murphy, Health and Environmental Impacts Division, Office of
Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Mail code C504-06, Research Triangle Park, NC 27711; telephone:
919-541-0729; fax: 919-541-0237; e-mail: [email protected]. With
regard to Section IV, contact Mr. Mark Schmidt, Air Quality Analysis
Division, Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Mail code C304-04, Research Triangle
Park, NC 27711; telephone: 919-541-2416; fax: 919-541-1903; e-mail:
[email protected]. With regard to Section V, contact Mr. Kevin
Cavender,
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Air Quality Analysis Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail code C304-06,
Research Triangle Park, NC 27711; telephone: 919-541-2364; fax: 919-
541-1903; e-mail: [email protected]. With regard to Section VI,
contact Mr. Larry Wallace, Ph.D., Air Quality Policy Division, Office
of Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Mail code C539-01, Research Triangle Park, NC 27711; telephone:
919-541-0906; fax: 919-541-0824; e-mail: [email protected].
SUPPLEMENTARY INFORMATION:
General Information
What Should I Consider as I Prepare My Comments for EPA?
1. Submitting CBI. Do not submit this information to EPA through
http://www.regulations.gov or e-mail. Clearly mark the part or all of
the information that you claim to be CBI. For CBI information in a disk
or CD-ROM that you mail to EPA, mark the outside of the disk or CD-ROM
as CBI and then identify electronically within the disk or CD-ROM the
specific information that is claimed as CBI. In addition to one
complete version of the comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.
2. Tips for Preparing Your Comments. When submitting comments,
remember to:
Identify the rulemaking by docket number and other
identifying information (subject heading, Federal Register date and
page number).
Follow directions--the agency may ask you to respond to
specific questions or organize comments by referencing a Code of
Federal Regulations (CFR) part or section number.
Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
Describe any assumptions and provide any technical
information and/or data that you used.
If you estimate potential costs or burdens, explain how
you arrived at your estimate in sufficient detail to allow for it to be
reproduced.
Provide specific examples to illustrate your concerns, and
suggest alternatives.
Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
Make sure to submit your comments by the comment period
deadline identified.
Availability of Related Information
A number of documents relevant to this rulemaking, including the
advance notice of proposed rulemaking (72 FR 71488), the Air Quality
Criteria for Lead (Criteria Document) (USEPA, 2006a), the Staff Paper,
related risk assessment reports, and other related technical documents
are available on EPA's Office of Air Quality Planning and Standards
(OAQPS) Technology Transfer Network (TTN) Web site at http://www.epa.gov/ttn/naaqs/standards/pb/s_pb_index.html. These and other
related documents are also available for inspection and copying in the
EPA docket identified above.
Table of Contents
The following topics are discussed in this preamble:
I. Background
A. Legislative Requirements
B. History of Lead NAAQS Reviews
C. Current Related Lead Control Programs
D. Current Lead NAAQS Review
II. Rationale for Proposed Decision on the Primary Standard
A. Multimedia, Multipathway Considerations and Background
1. Atmospheric Emissions and Distribution of Lead
2. Air-Related Human Exposure Pathways
3. Nonair-Related and Air-Related Background Human Exposure
Pathways
4. Contributions to Children's Lead Exposures
B. Health Effects Information
1. Blood Lead
a. Internal Disposition of Lead
b. Use of Blood Lead as Dose Metric
c. Air-to-Blood Relationships
2. Nature of Effects
a. Broad Array of Effects
b. Neurological Effects in Children
3. Lead-Related Impacts on Public Health
a. At-Risk Subpopulations
b. Potential Public Health Impacts
4. Key Observations
C. Human Exposure and Health Risk Assessments
1. Overview of Risk Assessment From Last Review
2. Design Aspects of Exposure and Risk Assessments
a. CASAC Advice
b. Health Endpoint, Risk Metric and Concentration-response
Functions
c. Case Study Approach
d. Air Quality Scenarios
e. Categorization of Policy-Relevant Exposure Pathways
f. Analytical Steps
g. Generating Multiple Sets of Risk Results
h. Key Limitations and Uncertainties
3. Summary of Estimates and Key Observations
a. Blood Pb Estimates
b. IQ Loss Estimates
D. Conclusions on Adequacy of the Current Primary Standard
1. Background
a. The Current Standard
b. Policy Options Considered in the Last Review
2. Considerations in the Current Review
a. Evidence-Based Considerations
b. Exposure- and Risk-Based Considerations
3. CASAC Advice and Recommendations
4. Administrator's Proposed Conclusions Concerning Adequacy
E. Conclusions on the Elements of the Standard
1. Indicator
2. Averaging Time and Form
3. Level for a Pb NAAQS With Pb-TSP Indicator
a. Evidence-Based Considerations
b. Exposure- and Risk-Based Considerations
c. CASAC Advice and Recommendations
d. Administrator's Proposed Conclusion Concerning Level
4. Level for a Pb NAAQS With Pb-PM10 Indicator
a. Considerations With Regard to Particles Not Captured by
PM10
b. CASAC Advice
c. Approaches for Levels for a PM10-Based Standard
F. Proposed Decision on the Primary Standard
III. Rationale for Proposed Decision on the Secondary Standard
A. Welfare Effects Information
B. Screening Level Ecological Risk Assessment
1. Design Aspects of the Assessment and Associated Uncertainties
2. Summary of Results
C. The Secondary Standard
1. Background on the Current Standard
2. Approach for Current Review
3. Conclusions on Adequacy of the Current Standard
a. Evidence-Based Considerations
b. Risk-Based Considerations
c. CASAC Advice and Recommendations
d. Administrator's Proposed Conclusions on Adequacy of Current
Standard
4. Conclusions and Proposed Decision on the Elements of the
Secondary Standard
IV. Proposed Appendix R on Interpretation of the NAAQS for Lead and
Proposed Revisions to the Exceptional Events Rule
A. Background
B. Interpretation of the NAAQS for Lead
1. Interpretation of a Standard Based on Pb-TSP
2. Interpretation of Alternative Elements
C. Exceptional Events Information Submission Schedule
V. Proposed Amendments to Ambient Monitoring Requirements
A. Sampling and Analysis Methods
1. Background
2. Proposed Changes
a. Pb-TSP Sampling Method
b. Pb-PM10 Sampling Method
c. Analysis Method
d. FEM Criteria
e. Quality Assurance
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B. Network Design
1. Background
2. Proposed Changes
C. Sampling Schedule
1. Background
2. Proposed Changes
D. Monitoring for the Secondary NAAQS
1. Background
2. Proposed Changes
E. Other Monitoring Regulation Changes
1. Reporting of Average Pressure and Temperature
2. Special Purpose Monitoring Exemption
VI. Implementation Considerations
A. Designations for the Lead NAAQS
1. Potential Schedule for Designations of A Revised Lead NAAQS
B. Lead Nonattainment Area Boundaries
1. County-Based Boundaries
2. MSA-Based Boundaries
C. Classifications
D. Section 110(a)(2) Lead NAAQS Infrastructure Requirements
E. Attainment Dates
F. Attainment Planning Requirements
1. Schedule for Attaining a Revised Pb NAAQS
2. RACM for Lead Nonattainment Areas
3. Demonstration of Attainment for Lead Nonattainment Areas
4. Reasonable Further Progress (RFP)
5. Contingency Measures
6. Nonattainment New Source Review (NSR) and Prevention of
Significant Deterioration (PSD) Requirements
7. Emissions Inventories
8. Modeling
G. General Conformity
H. Transition From the Current NAAQS to a Revised NAAQS for Lead
VII. Statutory and Executive Order Reviews
References
I. Background
A. Legislative Requirements
Two sections of the Clean Air Act (Act) govern the establishment
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list each air pollutant that ``in his
judgment, cause or contribute to air pollution which may reasonably be
anticipated to endanger public health and welfare'' and whose
``presence * * * in the ambient air results from numerous or diverse
mobile or stationary sources'' and to issue air quality criteria for
those that are listed. Air quality criteria are to ``accurately reflect
the latest scientific knowledge useful in indicating the kind and
extent of all identifiable effects on public health or welfare which
may be expected from the presence of [a] pollutant in ambient air * *
*''. Section 109 (42 U.S.C. 7409) directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants
listed under section 108. Section 109(b)(1) defines a primary standard
as one ``the attainment and maintenance of which in the judgment of the
Administrator, based on [air quality] criteria and allowing an adequate
margin of safety, are requisite to protect the public health.'' \1\ A
secondary standard, as defined in Section 109(b)(2), must ``specify a
level of air quality the attainment and maintenance of which, in the
judgment of the Administrator, based on criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of [the] pollutant in the ambient
air.'' \2\
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level * * * which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
\2\ Welfare effects as defined in section 302(h) (42 U.S.C.
7602(h)) include, but are not limited to, ``effects on soils, water,
crops, vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
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The requirement that primary standards include an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. Lead Industries Association v. EPA, 647 F.2d 1130, 1154
(D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum
Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert.
denied, 455 U.S. 1034 (1982). Both kinds of uncertainties are
components of the risk associated with pollution at levels below those
at which human health effects can be said to occur with reasonable
scientific certainty. Thus, in selecting primary standards that include
an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that may pose an unacceptable
risk of harm, even if the risk is not precisely identified as to nature
or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentration
levels, see Lead Industries Association v. EPA, 647 F.2d at 1156 n. 51,
but rather at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
The selection of any particular approach to providing an adequate
margin of safety is a policy choice left specifically to the
Administrator's judgment. Lead Industries Association v. EPA, 647 F.2d
at 1161-62. In addressing the requirement for an adequate margin of
safety, EPA considers such factors as the nature and severity of the
health effects involved, the size of the population(s) at risk, and the
kind and degree of the uncertainties that must be addressed.
In setting standards that are ``requisite'' to protect public
health and welfare, as provided in section 109(b), EPA's task is to
establish standards that are neither more nor less stringent than
necessary for these purposes. Whitman v. American Trucking
Associations, 531 U.S. 457, 473. Further the Supreme Court ruled that
``[t]he text of Sec. 109(b), interpreted in its statutory and
historical context and with appreciation for its importance to the CAA
as a whole, unambiguously bars cost considerations from the NAAQS-
setting process * * *'' Id. at 472.\3\ Section 109(d)(1) of the Act
requires that ``[n]ot later than December 31, 1980, and at 5-year
intervals thereafter, the Administrator shall complete a thorough
review of the criteria published under section 108 and the national
ambient air quality standards promulgated under this section and shall
make such revisions in such criteria and standards and promulgate such
new standards as may be appropriate in accordance with section 108 and
subsection (b) of this section.'' Section 109(d)(2)(A) requires that
``The Administrator shall appoint an independent scientific review
committee composed of seven members including at least one member of
the National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) requires that, ``[n]ot later than January 1, 1980, and at
five-year intervals thereafter, the committee referred to in
subparagraph (A) shall complete a review of the criteria published
under section 108 and the national primary and secondary ambient air
quality standards promulgated under this section and shall recommend to
the Administrator any new national ambient air quality standards and
revisions of existing criteria and standards as may be appropriate
under section 108 and subsection (b) of this
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section.'' Since the early 1980's, this independent review function has
been performed by the Clean Air Scientific Advisory Committee (CASAC)
of EPA's Science Advisory Board.
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\3\ In considering whether the CAA allowed for economic
considerations to play a role in the promulgation of the NAAQS, the
Supreme Court rejected arguments that because many more factors than
air pollution might affect public health, EPA should consider
compliance costs that produce health losses in setting the NAAQS.
531 U.S. at 466. Thus, EPA may not take into account possible public
health impacts from the economic cost of implementation. Id.
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B. History of Lead NAAQS Reviews
On October 5, 1978 EPA promulgated primary and secondary NAAQS for
Pb under section 109 of the Act (43 FR 46246). Both primary and
secondary standards were set at a level of 1.5 micrograms per cubic
meter ([mu]g/m\3\), measured as Pb in total suspended particulate
matter (Pb-TSP), not to be exceeded by the maximum arithmetic mean
concentration averaged over a calendar quarter. This standard was based
on the 1977 Air Quality Criteria for Lead (USEPA, 1977).
A review of the Pb standards was initiated in the mid-1980s. The
scientific assessment for that review is described in the 1986 Air
Quality Criteria for Lead (USEPA, 1986a), the associated Addendum
(USEPA, 1986b) and the 1990 Supplement (USEPA, 1990a). As part of the
review, the Agency designed and performed human exposure and health
risk analyses (USEPA, 1989), the results of which were presented in a
1990 Staff Paper (USEPA, 1990b). Based on the scientific assessment and
the human exposure and health risk analyses, the 1990 Staff Paper
presented options for the Pb NAAQS level in the range of 0.5 to 1.5
[mu]g/m3, and suggested the second highest monthly average
in three years for the form and averaging time of the standard (USEPA,
1990b). After consideration of the documents developed during the
review and the significantly changed circumstances since Pb was listed
in 1976, the Agency did not propose any revisions to the 1978 Pb NAAQS.
In a parallel effort, the Agency developed the broad, multi-program,
multimedia, integrated U.S. Strategy for Reducing Lead Exposure (USEPA,
1991). As part of implementing this strategy, the Agency focused
efforts primarily on regulatory and remedial clean-up actions aimed at
reducing Pb exposures from a variety of nonair sources judged to pose
more extensive public health risks to U.S. populations, as well as on
actions to reduce Pb emissions to air, such as bringing more areas into
compliance with the existing Pb NAAQS (USEPA, 1991).
C. Current Related Lead Control Programs
States are primarily responsible for ensuring attainment and
maintenance of national ambient air quality standards once EPA has
established them. Under section 110 of the Act (42 U.S.C. 7410) and
related provisions, States are to submit, for EPA approval, State
implementation plans (SIPs) that provide for the attainment and
maintenance of such standards through control programs directed to
sources of the pollutants involved. The States, in conjunction with
EPA, also administer the prevention of significant deterioration
program (42 U.S.C. 7470-7479) for these pollutants. In addition,
Federal programs provide for nationwide reductions in emissions of
these and other air pollutants through the Federal Motor Vehicle
Control Program under Title II of the Act (42 U.S.C. 7521-7574), which
involves controls for automobile, truck, bus, motorcycle, nonroad
engine, and aircraft emissions; the new source performance standards
under section 111 of the Act (42 U.S.C. 7411); and the national
emission standards for hazardous air pollutants under section 112 of
the Act (42 U.S.C. 7412).
As Pb is a multimedia pollutant, a broad range of Federal programs
beyond those that focus on air pollution control provide for nationwide
reductions in environmental releases and human exposures. In addition,
the Centers for Disease Control and Prevention (CDC) programs provide
for the tracking of children's blood Pb levels nationally and provide
guidance on levels at which medical and environmental case management
activities should be implemented (CDC, 2005a; ACCLPP, 2007).\4\ In
1991, the Secretary of the Health and Human Services (HHS)
characterized Pb poisoning as the ``number one environmental threat to
the health of children in the United States'' (Alliance to End
Childhood Lead Poisoning, 1991). In 1997, President Clinton created, by
Executive Order 13045, the President's Task Force on Environmental
Health Risks and Safety Risks to Children in response to increased
awareness that children face disproportionate risks from environmental
health and safety hazards (62 FR 19885).\5\ By Executive Orders issued
in October 2001 and April 2003, President Bush extended the work for
the Task Force for an additional three and a half years beyond its
original charter (66 FR 52013 and 68 FR 19931). The Task Force set a
Federal goal of eliminating childhood Pb poisoning by the year 2010 and
reducing Pb poisoning in children was the Task Force's top priority.
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\4\ As described in Section III below the CDC stated in 2005
that no ``safe'' threshold for blood Pb levels in young children has
been identified (CDC, 2005a).
\5\ Co-chaired by the Secretary of the HHS and the Administrator
of the EPA, the Task Force consisted of representatives from 16
Federal departments and agencies.
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Federal abatement programs provide for the reduction in human
exposures and environmental releases from in-place materials containing
Pb (e.g., Pb-based paint, urban soil and dust, and contaminated waste
sites). Federal regulations on disposal of Pb-based paint waste help
facilitate the removal of Pb-based paint from residences.\6\ Further,
in 1991, EPA lowered the maximum levels of Pb permitted in public water
systems from 50 parts per billion (ppb) to 15 ppb (56 FR 26460).
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\6\ See ``Criteria for Classification of Solid Waste Disposal
Facilities and Practices and Criteria for Municipal Solid Waste
Landfills: Disposal of Residential Lead-Based Paint Waste; Final
Rule'' EPA-HQ-RCRA-2001-0017.
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Federal programs to reduce exposure to Pb in paint, dust, and soil
are specified under the comprehensive federal regulatory framework
developed under the Residential Lead-Based Paint Hazard Reduction Act
(Title X). Under Title X and Title IV of the Toxic Substances Control
Act, EPA has established regulations and associated programs in the
following five categories: (1) Training and certification requirements
for persons engaged in lead-based paint activities; accreditation of
training providers; authorization of State and Tribal lead-based paint
programs; and work practice standards for the safe, reliable, and
effective identification and elimination of lead-based paint hazards;
(2) ensuring that, for most housing constructed before 1978, lead-based
paint information flows from sellers to purchasers, from landlords to
tenants, and from renovators to owners and occupants; (3) establishing
standards for identifying dangerous levels of Pb in paint, dust and
soil; (4) providing grant funding to establish and maintain State and
Tribal lead-based paint programs, and to address childhood lead
poisoning in the highest-risk communities; and (5) providing
information on Pb hazards to the public, including steps that people
can take to protect themselves and their families from lead-based paint
hazards.
Under Title IV of TSCA, EPA established standards identifying
hazardous levels of lead in residential paint, dust, and soil in 2001.
This regulation supports the implementation of other regulations which
deal with worker training and certification, Pb hazard disclosure in
real estate transactions, Pb hazard evaluation and control in
Federally-owned housing prior to sale and housing receiving Federal
assistance, and U.S. Department of Housing and Urban Development grants
to local jurisdictions to perform
[[Page 29188]]
Pb hazard control. The TSCA Title IV term ``lead-based paint hazard''
implemented through this regulation identifies lead-based paint and all
residential lead-containing dust and soil regardless of the source of
Pb, which, due to their condition and location, would result in adverse
human health effects. One of the underlying principles of Title X is to
move the focus of public and private decision makers away from the mere
presence of lead-based paint, to the presence of lead-based paint
hazards, for which more substantive action should be undertaken to
control exposures, especially to young children. In addition the
success of the program will rely on the voluntary participation of
states and tribes as well as counties and cities to implement the
programs and on property owners to follow the standards and EPA's
recommendations. If EPA were to set unreasonable standards (e.g.,
standards that would recommend removal of all Pb from paint, dust, and
soil), States and Tribes may choose to opt out of the Title X Pb
program and property owners may choose to ignore EPA's advice believing
it lacks credibility and practical value. Consequently, EPA needed to
develop standards that would not waste resources by chasing risks of
negligible importance and that would be accepted by States, Tribes,
local governments and property owners. In addition, a separate
regulation establishes, among other things, under authority of TSCA
section 402, residential Pb dust cleanup levels and amendments to dust
and soil sampling requirements (66 FR 1206).
On March 31, 2008, the Agency issued a new rule (Lead: Renovation,
Repair and Painting [RRP] Program) to protect children from lead-based
paint hazards. This rule applies to renovators and maintenance
professionals who perform renovation, repair, or painting in housing,
child-care facilities, and schools built prior to 1978. It requires
that contractors and maintenance professionals be certified; that their
employees be trained; and that they follow protective work practice
standards. These standards prohibit certain dangerous practices, such
as open flame burning or torching of lead-based paint. The required
work practices also include posting warning signs, restricting
occupants from work areas, containing work areas to prevent dust and
debris from spreading, conducting a thorough cleanup, and verifying
that cleanup was effective. The rule will be fully effective by April
2010. States and tribes may become authorized to implement this rule,
and the rule contains procedures for the authorization of states,
territories, and tribes to administer and enforce these standards and
regulations in lieu of a federal program. In announcing this rule, EPA
noted that almost 38 million homes in the United States contain some
lead-based paint, and that this rule's requirements were key components
of a comprehensive effort to eliminate childhood Pb poisoning. To
foster adoption of the rule's measures, EPA also intends to conduct an
extensive education and outreach campaign to promote awareness of these
new requirements.
Programs associated with the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA or Superfund) and Resource
Conservation Recovery Act (RCRA) also implement abatement programs,
reducing exposures to Pb and other pollutants. For example, EPA
determines and implements protective levels for Pb in soil at Superfund
sites and RCRA corrective action facilities. Federal programs,
including those implementing RCRA, provide for management of hazardous
substances in hazardous and municipal solid waste.\7\ For example,
Federal regulations concerning batteries in municipal solid waste
facilitate the collection and recycling or proper disposal of batteries
containing Pb.\8\ Similarly, Federal programs provide for the reduction
in environmental releases of hazardous substances such as Pb in the
management of wastewater (http://www.epa.gov/owm/).
---------------------------------------------------------------------------
\7\ See, e.g., ``Hazardous Waste Management System;
Identification and Listing of Hazardous Waste: Inorganic Chemical
Manufacturing Wastes; Land Disposal Restrictions for Newly
Identified Wastes and CERCLA Hazardous Substance Designation and
Reportable Quantities; Final Rule'', http://www.epa.gov/epaoswer/hazwaste/state/revision/frs/fr195.pdf and http://www.epa.gov/epaoswer/hazwaste/ldr/basic.htm.
\8\ See, e.g., ``Implementation of the Mercury-Containing and
Rechargeable Battery Management Act'' http://www.epa.gov/epaoswer/hazwaste/recycle/battery.pdf and ``Municipal Solid Waste Generation,
Recycling, and Disposal in the United States: Facts and Figures for
2005'' http://www.epa.gov/epaoswer/osw/conserve/resources/msw-2005.pdf.
---------------------------------------------------------------------------
A variety of federal nonregulatory programs also provide for
reduced environmental release of Pb containing materials through more
general encouragement of pollution prevention, promotion of reuse and
recycling, reduction of priority and toxic chemicals in products and
waste, and conservation of energy and materials. These include the
Resource Conservation Challenge (http://www.epa.gov/epaoswer/osw/conserve/index.htm), the National Waste Minimization Program (http://www.epa.gov/epaoswer/hazwaste/minimize/leadtire.htm), ``Plug in to
eCycling'' (a partnership between EPA and consumer electronics
manufacturers and retailers; http://www.epa.gov/epaoswer/hazwaste/recycle/electron/crt.htm#crts), and activities to reduce the practice
of backyard trash burning (http://www.epa.gov/msw/backyard/pubs.htm).
Efforts such as those programs described above have been successful
in that blood Pb levels in all segments of the population have dropped
significantly from levels observed around 1990. In particular, blood Pb
levels for the general population of children 1 to 5 years of age have
dropped to a median level of 1.6 [mu]g/dL and a level of 3.9 [mu]g/dL
for the 90th percentile child in the 2003-2004 National Health and
Nutrition Examination Survey (NHANES) as compared to median and 90th
percentile levels in 1988-1991 of 3.5 [mu]g/dL and 9.4 [mu]g/dL,
respectively (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm). These levels (median and 90th percentile) for the
general population of young children \9\ are at the low end of the
historic range of blood Pb levels for general population of children
aged 1-5 years. However, as discussed in Section II.B.1.b, levels have
been found to vary among children of different socioeconomic status and
other demographic characteristics (CD, p. 4-21) and racial/ethnic and
income disparities in blood Pb levels in children persist. The decline
in blood Pb levels in the United States has resulted from coordinated,
intensive efforts at the national, state, and local levels. The Agency
has continued to grapple with soil and dust Pb levels from the
historical use of Pb in paint and gasoline and other sources.
---------------------------------------------------------------------------
\9\ The 95th percentile value for the 2003-2004 NHANES is 5.1
[mu]g/dL (Axelrad, 2008).
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EPA's research program, with other Federal agencies, defines,
encourages and conducts research needed to locate and assess serious
risks and to develop methods and tools to characterize and help reduce
risks. For example, EPA's Integrated Exposure Uptake Biokinetic Model
for Lead in Children (IEUBK model) for Pb in children and the Adult
Lead Methodology are widely used and accepted as tools that provide
guidance in evaluating site specific data. More recently, in
recognition of the need for a single model that predicts Pb
concentrations in tissues for children and adults, EPA is developing
the All Ages Lead Model (AALM) to provide researchers and risk
assessors with a
[[Page 29189]]
pharmacokinetic model capable of estimating blood, tissue, and bone
concentrations of Pb based on estimates of exposure over the lifetime
of the individual. EPA research activities on substances including Pb
focus on better characterizing aspects of health and environmental
effects, exposure, and control or management of environmental releases
(see http://www.epa.gov/ord/researchaccomplishments/index.html).
D. Current Lead NAAQS Review
EPA initiated the current review of the air quality criteria for Pb
on November 9, 2004, with a general call for information (69 FR 64926).
A project work plan (USEPA, 2005a) for the preparation of the Criteria
Document was released in January 2005 for CASAC and public review. EPA
held a series of workshops in August 2005, inviting recognized
scientific experts to discuss initial draft materials that dealt with
various lead-related issues being addressed in the Pb air quality
criteria document. The first draft of the Criteria Document (USEPA,
2005b) was released for CASAC and public review in December 2005 and
discussed at a CASAC meeting held on February 28-March 1, 2006.
A second draft Criteria Document (USEPA, 2006b) was released for
CASAC and public review in May 2006, and discussed at the CASAC meeting
on June 28, 2006. A subsequent draft of Chapter 7--Integrative
Synthesis (Chapter 8 in the final Criteria Document), released on July
31, 2006, was discussed at an August 15, 2006, CASAC teleconference.
The final Criteria Document was released on September 30, 2006 (USEPA,
2006a; cited throughout this preamble as CD). While the Criteria
Document focuses on new scientific information available since the last
review, it integrates that information with scientific criteria from
previous reviews.
In February 2006, EPA released the Plan for Review of the National
Ambient Air Quality Standards for Lead (USEPA, 2006c) that described
Agency plans and a timeline for reviewing the air quality criteria,
developing human exposure and risk assessments and an ecological risk
assessment, preparing a policy assessment, and developing the proposed
and final rulemakings.
In May 2006, EPA released for CASAC and public review a draft
Analysis Plan for Human Health and Ecological Risk Assessment for the
Review of the Lead National Ambient Air Quality Standards (USEPA,
2006d), which was discussed at a June 29, 2006, CASAC meeting
(Henderson, 2006). The May 2006 assessment plan discussed two
assessment phases: A pilot phase and a full-scale phase. The pilot
phase of both the human health and ecological risk assessments was
presented in the draft Lead Human Exposure and Health Risk Assessments
and Ecological Risk Assessment for Selected Areas (ICF, 2006;
henceforth referred to as the first draft Risk Assessment Report) which
was released for CASAC and public review in December 2006. The first
draft Staff Paper, also released in December 2006, discussed the pilot
assessments and the most policy-relevant science from the Criteria
Document. These documents were reviewed by CASAC and the public at a
public meeting on February 6-7, 2007 (Henderson, 2007a).
Subsequent to that meeting, EPA conducted full-scale human exposure
and health risk assessments, although no further work was done on the
ecological assessment due to resource limitations. A second draft Risk
Assessment Report (USEPA, 2007a), containing the full-scale human
exposure and health risk assessments, was released in July 2007 for
review by CASAC at a meeting held on August 28-29, 2007. Taking into
consideration CASAC comments (Henderson, 2007b) and public comments on
that document, we conducted additional human exposure and health risk
assessments. A final Risk Assessment Report (USEPA, 2007b) and final
Staff Paper (USEPA, 2007c) were released on November 1, 2007.
The final Staff Paper presents OAQPS staff's evaluation of the
public health and welfare policy implications of the key studies and
scientific information contained in the Criteria Document and presents
and interprets results from the quantitative risk/exposure analyses
conducted for this review. Further, the Staff Paper presents OAQPS
staff recommendations on a range of policy options for the
Administrator to consider concerning whether, and if so how, to revise
the primary and secondary Pb NAAQS. Such an evaluation of policy
implications is intended to help ``bridge the gap'' between the
scientific assessment contained in the Criteria Document and the
judgments required of the EPA Administrator in determining whether it
is appropriate to retain or revise the NAAQS for Pb. In evaluating the
adequacy of the current standard and a range of alternatives, the Staff
Paper considered the available scientific evidence and quantitative
risk-based analyses, together with related limitations and
uncertainties, and focused on the information that is most pertinent to
evaluating the basic elements of national ambient air quality
standards: indicator,\10\ averaging time, form,\11\ and level. These
elements, which together serve to define each standard, must be
considered collectively in evaluating the public health and welfare
protection afforded by the Pb standards. The information, conclusions,
and OAQPS staff recommendations presented in the Staff Paper were
informed by comments and advice received from CASAC in its reviews of
the earlier draft Staff Paper and drafts of related risk/exposure
assessment reports, as well as comments on these earlier draft
documents submitted by public commenters.
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\10\ The ``indicator'' of a standard defines the chemical
species or mixture that is to be measured in determining whether an
area attains the standard.
\11\ The ``form'' of a standard defines the air quality
statistic that is to be compared to the level of the standard in
determining whether an area attains the standard.
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Subsequent to completion of the Staff Paper, EPA issued an advance
notice of proposed rulemaking (ANPR) that was signed by the
Administrator on December 5, 2007 (72 FR 71488-71544). The ANPR is one
of the key features of the new NAAQS review process that EPA has
instituted over the past two years to help to improve the efficiency of
the process the Agency uses in reviewing the NAAQS while ensuring that
the Agency's decisions are informed by the best available science and
broad participation among experts in the scientific community and the
public. The ANPR provided the public an opportunity to comment on a
wide range of policy options that could be considered by the
Administrator. The substantial number of comments we received on the Pb
NAAQS ANPR helped inform the narrower range of options we are proposing
and taking comment on today. The new process (described at http://www.epa.gov/ttn/naaqs/.) is being incorporated into the various ongoing
NAAQS reviews being conducted by the Agency, including the current
review of the Pb NAAQS.
A public meeting of the CASAC was held on December 12-13, 2007 to
provide advice and recommendations to the Administrator based on its
review of the ANPR and the previously released final Staff Paper and
Risk Assessment Report. Information about this meeting was published in
the Federal Register on November 20, 2007 (72 FR 65335-65336),
transcripts of the meeting are in the Docket for this review and
CASAC's letter to the Administrator (Henderson, 2008) is also available
on the EPA Web site (http://www.epa.gov/sab).
[[Page 29190]]
A public comment period for the ANPR extended from December 17,
2007 through January 16, 2008 and comments received are in the Docket
for this review. Comments were received from nearly 9000 private
citizens (roughly 200 of them were not part of one of several mass
comment campaign), 13 state and local agencies, one federal agency,
three regional or national associations of government agencies or
officials, 15 nongovernmental environmental or public health
organizations (including one submission on behalf of a coalition of 23
organizations) and five industries or industry organizations. Although
the Agency has not developed formal responses to comments received on
the ANPR, these comments have been considered in the development of
this notice and are generally described in subsequent sections on
proposed conclusions with regard to the adequacy of the standards and
with regard to the Administrator's proposed decisions on revisions to
the standards.
The schedule for completion of this review is governed by a
judicial order in Missouri Coalition for the Environment, v. EPA (No.
4:04CV00660 ERW, Sept. 14, 2005). The order governing this review,
entered by the court on September 14, 2005 and amended on April 29,
2008, specifies that EPA sign, for publication, notices of proposed and
final rulemaking concerning its review of the Pb NAAQS no later than
May 1, 2008 and September 15, 2008, respectively. In light of the
compressed schedule ordered by the court for issuing the final rule,
EPA may be able to respond only to those comments submitted during the
public comment period on this proposal. EPA has considered all of the
comments submitted to date in preparing this proposal, but if
commenters believe that comments submitted on the ANPR are fully
applicable to the proposal and wish to ensure that those comments are
addressed by EPA as part of the final rulemaking, the earlier comments
should be resubmitted during the comment period on this proposal.
This action presents the Administrator's proposed decisions on the
review of the current primary and secondary Pb standards. Throughout
this preamble a number of judgments, conclusions, findings, and
determinations proposed by the Administrator are noted. While they
identify the reasoning that supports this proposal, they are not
intended to be final or conclusive in nature. The EPA invites general,
specific, and/or technical comments on all issues involved with this
proposal, including all such proposed judgments, conclusions, findings,
and determinations.
II. Rationale for Proposed Decision on the Primary Standard
This section presents the rationale for the Administrator's
proposed decision that the current primary standard is not requisite to
protect public health with an adequate margin of safety, and that the
existing Pb primary standard should be revised. With regard to the
primary standard for Pb, EPA is proposing options for the revision of
the various elements of the standard to provide increased protection
for children and other at-risk populations against an array of adverse
health effects, most notably including neurological effects in
children, particularly neurocognitive and neurobehavioral effects. With
regard to the level and indicator of the standard, EPA proposes to
revise the level of the standard to a level within the range of 0.10 to
0.30 [mu]g/m\3\ in conjunction with retaining the current indicator of
Pb in total suspended particles (Pb-TSP) but with allowance for the use
of Pb-PM10 data. With regard to the form and averaging time of the
standard, EPA proposes the following options: (1) To retain the current
averaging time of a calendar quarter and the current not-to-be-exceeded
form, revised so as to apply across a 3-year span, and (2) to revise
the averaging time to a calendar month and the form to be the second-
highest monthly average across a 3-year span. EPA also solicits comment
on revising the indicator to Pb-PM10.
As discussed more fully below, this proposal is based on a thorough
review, in the Criteria Document, of the latest scientific information
on human health effects associated with the presence of Pb in the
ambient air. This proposal also takes into account: (1) Staff
assessments of the most policy-relevant information in the Criteria
Document and staff analyses of air quality, human exposure, and health
risks presented in the Staff Paper, upon which staff recommendations
for revisions to the primary Pb standard are based; (2) CASAC advice
and recommendations, as reflected in discussions of the ANPR and drafts
of the Criteria Document and Staff Paper at public meetings, in
separate written comments, and in CASAC's letters to the Administrator;
and (3) public comments received during the development of these
documents, either in connection with CASAC meetings or separately.
In developing this proposal, EPA has drawn upon an integrative
synthesis of the entire body of evidence, published through late 2006,
on human health effects associated with Pb exposure. Some 6000 newly
available studies were considered in this review. As discussed below in
section II.B, this body of evidence addresses a broad range of health
endpoints associated with exposure to Pb (EPA, 2006a, chapter 8), and
includes hundreds of epidemiologic studies conducted in the U.S.,
Canada, and many countries around the world since the time of the last
review (EPA, 2006a, chapter 6). This proposal also draws upon the
results of the quantitative exposure and risk assessments, discussed
below in section II.C. Evidence- and exposure/risk-based considerations
that form the basis for the Administrator's proposed decisions on the
adequacy of the current standard and on the elements of the proposed
alternative standards are discussed below in section II.D.2 and II.D.3,
respectively.
A. Multimedia, Multipathway Considerations and Background
1. Atmospheric Emissions and Distribution of Lead
Lead is emitted into the air from many sources encompassing a wide
variety of source types (Staff Paper, Section 2.2). Further, once
deposited out of the air, Pb can subsequently be resuspended into the
air (CD, pp. 2-62 to 2-66). There are over 100 categories of sources of
Pb emissions included in the EPA's 2002 National Emissions Inventory
(NEI),\12 \ the top five of which include: Mobile sources (leaded
aviation gas) \13\; industrial, commercial, institutional and process
boilers; utility boilers; iron and steel foundries; and primary Pb
smelting (Staff Paper Section 2.2). Further, there are some 13,000
industrial, commercial or institutional point sources in the 2002 NEI,
each with one or more processes that emit Pb to the atmosphere. In
addition to these 13,000 sources, there are approximately 3,000
airports at which leaded gasoline is used (Staff Paper, p. 2-8). Among
these sources, more than one thousand are estimated to emit at least a
tenth of a ton of Pb per year (Staff Paper, Section 2.2.3). Because of
its persistence, Pb emissions contribute to media
[[Page 29191]]
concentrations for some time into the future.
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\12\ As noted in the Staff Paper, quantitative estimates of
emissions associated with resuspension of soil-bound Pb particles
and contaminated road dust are not included in the 2002 NEI.
\13\ The emissions estimates identified as mobile sources in the
current NEI are currently limited to combustion of leaded aviation
gas in piston-engine aircraft. Lead emissions estimates for other
mobile source emissions of Pb (e.g., brake wear, tire wear, loss of
Pb wheel weights and others) are not included in the current NEI.
---------------------------------------------------------------------------
Lead emitted to the air is predominantly in particulate form, with
the particles occurring in many sizes. Once emitted, Pb particles can
be transported long or short distances depending on their size, which
influences the amount of time spent in aerosol phase. In general,
larger particles tend to deposit more quickly, within shorter distances
from emissions points, while smaller particles will remain in aerosol
phase and travel longer distances before depositing. Additionally, once
deposited, Pb particles can be resuspended back into the air and
undergo a second dispersal. Thus, the atmospheric transport processes
of Pb contribute to its broad dispersal, with larger particles
generally occurring as a greater contribution to total airborne Pb at
locations closer to the point of emission than at more distant
locations where the relative contribution from smaller particles is
greater (CD, Section 2.3.1 and p. 3-3).
Airborne concentrations of Pb in total suspended particulate matter
(Pb-TSP) in the United States have fallen substantially since the
current Pb NAAQS was set in 1978.\14\ Despite this decline, there have
still been a small number of areas, associated with large stationary
sources of Pb, that have not met the NAAQS over the past few years. The
average maximum quarterly mean concentration for the time period 2003-
2005 among source-oriented monitoring sites in the U.S. is 0.48 [mu]g/
m3, while the corresponding average for non-source-oriented
sites is 0.03 [mu]g/m3.\15\ The average and median among all
monitoring-site-specific maximum quarterly mean concentrations for this
time period are 0.17 [mu]g/m3 and 0.03 [mu]g/m3,
respectively. Coincident with the historical trend in reduction in Pb
levels, however, there has also been a substantial reduction in number
of Pb-TSP monitoring sites. As described below in section II.B.3.b,
many of the highest Pb emitting sources in the 2002 NEI do not have
nearby Pb-TSP monitors, which may lead to underestimates of the extent
of occurrences of relatively higher Pb concentrations (as recognized in
the Staff Paper, Section 2.3.2 and, with regard to more recent
analysis, in section II.B.3.b below).
---------------------------------------------------------------------------
\14\ Air Pb concentrations nationally are estimated to have
declined more than 90% since the early 1980s, in locations not known
to be directly influenced by stationary sources (Staff Paper, pp. 2-
22 to 2-23).
\15\ The data set included data for 189 monitor sites meeting
the data analysis screening criteria. Details with regard to the
data set and analyses supporting the values provided here are
presented in Section 2.3.2 of the Staff Paper.
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2. Air-Related Human Exposure Pathways
As when the standard was set in 1978, we recognize that exposure to
air Pb can occur directly by inhalation, or indirectly by ingestion of
Pb-contaminated food, water or nonfood materials including dust and
soil (43 FR 46247). This occurs as Pb emitted into the ambient air is
distributed to other environmental media and can contribute to human
exposures via indoor and outdoor dusts, outdoor soil, food and drinking
water, as well as inhalation of air (CD, pp. 3-1 to 3-2). Accordingly,
people are exposed to Pb emitted into ambient air by both inhalation
and ingestion pathways. In general, air-related pathways include those
pathways where Pb passes through ambient air on its path from a source
to human exposure. EPA considers risks to public health from exposure
to Pb that was emitted into the air as relevant to our consideration of
the primary standard. Therefore , we consider these air-related
pathways to be policy-relevant in this review. Air-related Pb exposure
pathways include: Inhalation of airborne Pb (that may include Pb
emitted into the air and deposited and then resuspended); and ingestion
of Pb that, once airborne, has made its way into indoor dust, outdoor
dust or soil, dietary items (e.g., crops and livestock), and drinking
water (e.g., CD, Figure 3-1).
Ambient air Pb contributes to Pb in indoor dust through transport
of Pb suspended in ambient air that is then deposited indoors and
through transport of Pb that has deposited outdoors from ambient air
and is transported indoors in ways other than through ambient air (CD,
Section 3.2.3; Adgate et al., 1998). For example, infiltration of
ambient air into buildings brings airborne Pb indoors where deposition
of particles contributes to Pb in dust on indoor surfaces (CD, p. 3-28;
Caravanos et al., 2006a). Indoor dust may be ingested (e.g., via hand-
to-mouth activity by children; CD, p. 8-12) or may be resuspended
through household activities and inhaled (CD, p. 8-12). Ambient air Pb
can also deposit onto outdoor surfaces (including surface soil) with
which humans may come into contact (CD, Section 2.3.2; Farfel et al.,
2003; Caravanos et al., 2006a, b). Human contact with this deposited Pb
may result in incidental ingestion from this exposure pathway and may
also result in some of this Pb being carried indoors (e.g., on clothes
and shoes) adding to indoor dust Pb (CD, p. 3-28; von Lindern et al.,
2003a, b). Additionally, Pb from ambient air that deposits on outdoor
surfaces may also be resuspended and carried indoors in the air where
it can be inhaled. Thus, indoor dust receives air-related Pb directly
from ambient air coming indoors and also more indirectly, after
deposition from ambient air onto outdoor surfaces.
As mentioned above, humans may contact Pb in dust on outdoor
surfaces, including surface soil and other materials, that has
deposited from ambient air (CD, Section 3.2; Caravanos et al., 2006a;
Mielke et al., 1991; Roels et al., 1980). Human exposure to this
deposited Pb can occur through incidental ingestion, and, when the
deposited Pb is resuspended, by inhalation. Atmospheric deposition of
Pb also contributes to Pb in vegetation, both as a result of contact
with above ground portions of the plant and through contributions to
soil and transport of Pb into roots (CD, pp. 7-9 and AXZ7-39; USEPA,
1986a, Sections 6.5.3 and 7.2.2.2.1). Livestock may subsequently be
exposed to Pb in vegetation (e.g., grasses and silage) and in surface
soils via incidental ingestion of soil while grazing (USEPA 1986a,
Section 7.2.2.2.2). Atmospheric deposition is estimated to comprise a
significant proportion of Pb in food (CD, p. 3-48; Flegel et al., 1990;
Juberg et al., 1997; Dudka and Miller, 1999). Atmospheric deposition
outdoors also contributes to Pb in surface waters, although given the
widespread use of settling or filtration in drinking water treatment,
air-related Pb is generally a small component of Pb in treated drinking
water (CD, Section 2.3.2 and p. 3-33).
Air-related exposure pathways are affected by changes to air
quality, including changes in concentrations of Pb in air and/or
changes in atmospheric deposition of Pb. Further, because of its
persistence in the environment, Pb deposited from the air may
contribute to human and ecological exposures for years into the future
(CD, pp. 3-18 to 3-19, pp. 3-23 to 2-24). Thus, because of the roles in
human exposure pathways of both air concentration and air deposition,
and of the persistence of Pb, once deposited, some pathways respond
more quickly to changes in air quality than others. Pathways most
directly involving Pb in ambient air and exchanges of ambient air with
indoor air respond more quickly while pathways involving exposure to Pb
deposited from ambient air into the environment generally respond more
slowly (CD, pp. 3-18 to 3-19).
[[Page 29192]]
Exposure pathways tied most directly to ambient air, and that
consequently have the potential to respond relatively more quickly to
changes in air Pb, include inhalation of ambient air, and ingestion of
Pb in indoor dust directly contaminated with Pb from ambient air.\16\
Lead from ambient air contaminates indoor dust directly when outdoor
air comes inside (through open doors or windows, for example) and Pb in
that air deposits to indoor surfaces (Caravanos et al., 2006a; CD, p.
8-22). This includes Pb that was previously deposited outdoors and is
then resuspended and transported indoors. Lead in dust on outdoor
surfaces also responds to air deposition (Caravanos et al., 2006).
Pathways in which the air quality impact is reflected over a somewhat
longer time frame generally are associated with outdoor atmospheric
deposition, and include ingestion pathways such as the following: (1)
Ingestion of Pb in outdoor soil; (2) ingestion of Pb in indoor dust
indirectly contaminated with Pb from the outdoor air (e.g, ``tracking
in'' of Pb deposited to outdoor surface soil, as compared to ambient
air transport of resuspended outdoor soil); (3) ingestion of Pb in diet
that is attributable to deposited air Pb, and; (4) ingestion of Pb in
drinking water that is attributable to deposited air Pb (e.g., Pb
entering water bodies used for drinking supply).
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\16\ We note that in the risk assessment, we only assessed
alternate standard impacts on the subset of air-related pathways
that respond relatively quickly to changes in air Pb.
---------------------------------------------------------------------------
3. Nonair-Related and Air-Related Background Human Exposure Pathways
As when the standard was set in 1978, there continue to be multiple
sources of exposure, both air-related and others (nonair-related).
Human exposure pathways that are not air-related are those in which Pb
does not pass through ambient air. These pathways as well as air-
related human exposure pathways that involve natural sources of Pb to
air are considered policy-relevant background in this review. In the
context of NAAQS for other criteria pollutants which are not multimedia
in nature, such as ozone, the term policy-relevant background is used
to distinguish anthropogenic air emissions from naturally occurring
non-anthropogenic emissions to separate pollution levels that can be
controlled by U.S. regulations from levels that are generally
uncontrollable by the United States (USEPA, 2007d). In the case of Pb,
however, due to the multimedia, multipathway nature of human exposures
to Pb, policy-relevant background is defined more broadly to include
not only the ``quite low'' levels of naturally occurring Pb emissions
into the air from non-anthropogenic sources such as volcanoes, sea
salt, and windborne soil particles from areas free of anthropogenic
activity (see below), but also Pb from nonair sources. These are
collectively referred to as ``policy-relevant background.''
The pathways of human exposure to Pb that are not air-related
include ingestion of Pb from indoor Pb paint \17\, Pb in diet as a
result of inadvertent additions during food processing, and Pb in
drinking water attributable to Pb in distribution systems (CD, Chapter
3). Other less prevalent, potential pathways of Pb exposure that are
not air-related include ingestion of some calcium supplements or of
food contaminated during storage in some Pb glazed glassware, and hand-
to-mouth contact with some imported vinyl miniblinds or with some hair
dyes containing Pb acetate, as well as some cosmetics and folk remedies
(CD, pp. 3-50 to 3-51).
---------------------------------------------------------------------------
\17\ Weathering of outdoor Pb paint may also contribute to soil
Pb levels adjacent to the house.
---------------------------------------------------------------------------
Some amount of Pb in the air derives from background sources, such
as volcanoes, sea salt, and windborne soil particles from areas free of
anthropogenic activity (CD, Section 2.2.1). The impact of these sources
on current air concentrations is expected to be quite low (relative to
current concentrations) and has been estimated to fall within the range
from 0.00002 [mu]g/m3 and 0.00007 [mu]g/m3 based
on mass balance calculations for global emissions (CD, Section 3.1 and
USEPA 1986, Section 7.2.1.1.3). The midpoint in this range, 0.00005
[mu]g/m3, has been used in the past to represent the
contribution of naturally occurring air Pb to total human exposure
(USEPA 1986, Section 7.2.1.1.3). The data available to derive such an
estimate are limited and such a value might be expected to vary
geographically with the natural distribution of Pb. Comparing this to
reported air Pb measurements is complicated by limitations of the
common analytical methods and by inconsistent reporting practices. This
value is one half the lowest reported nonzero value in AQS. Little
information is available regarding anthropogenic sources of airborne Pb
located outside of North America, which would also be considered
policy-relevant background. In considering contributions from policy-
relevant background to human exposures and associated health effects,
however, any credible estimate of policy-relevant background in air is
likely insignificant in comparison to the contributions from exposures
to nonair media.
4. Contributions to Children's Lead Exposures
As when the standard was set in 1978, EPA recognizes that there
remain today contributions to blood Pb levels from nonair sources. The
relative contribution of Pb in different exposure media to human
exposure varies, particularly for different age groups. For example,
some studies have found that dietary intake of Pb may be a predominant
source of Pb exposure among adults, greater than consumption of water
and beverages or inhalation (CD, p. 3-43).\18\ For young children,
however, ingestion of indoor dust can be a significant Pb exposure
pathway, such that dust ingested via hand-to-mouth activity can be a
more important source of Pb exposure than inhalation, although indoor
dust can also be resuspended through household activities and pose an
inhalation risk as well (CD, p. 3-27 to 3-28; Melnyk et al. 2000).\19\
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\18\ ``Some recent exposure studies have evaluated the relative
importance of diet to other routes of Pb exposure. In reports from
the NHEXAS, Pb concentrations measured in households throughout the
Midwest were significantly higher in solid food compared to
beverages and tap water (Clayton et al., 1999; Thomas et al., 1999).
However, beverages appeared to be the dominant dietary pathway for
Pb according to the statistical analysis (Clayton et al., 1999),
possibly indicating greater bodily absorption of Pb from liquid
sources (Thomas et al., 1999). Dietary intakes of Pb were greater
than those calculated for intake from home tap water or inhalation
on a [mu]g/day basis (Thomas et al., 1999). The NHEXAS study in
Arizona showed that, for adults, ingestion was a more important Pb
exposure route than inhalation (O'Rourke et al., 1999).'' (CD, p. 3-
43)
\19\ For example, the Criteria Document states the following:
``Given the large amount of time people spend indoors, exposure to
Pb in dusts and indoor air can be significant. For children, dust
ingested via hand-to-mouth activity is often a more important source
of Pb exposure than inhalation. Dust can be resuspended through
household activities, thereby posing an inhalation risk as well.
House dust Pb can derive both from Pb-based paint and from other
sources outside the home. The latter include Pb-contaminated
airborne particles from currently operating industrial facilities or
resuspended soil particles contaminated by deposition of airborne Pb
from past emissions.'' (CD, p. E-6)
---------------------------------------------------------------------------
Estimating contributions from nonair sources is complicated by the
existence of multiple and varied air-related pathways (as described in
section II.A.2 above), as well as the persistent nature of Pb. For
example, Pb that is a soil or dust contaminant today may have been
airborne yesterday or many years ago. The studies currently available
and reviewed in the Criteria Document that evaluate the multiple
pathways of Pb exposure, when considering exposure contributions from
outdoor dust/soil, do
[[Page 29193]]
not usually distinguish between outdoor soil/dust Pb resulting from
historical emissions and outdoor soil/dust Pb resulting from recent
emissions. Further, while indoor dust Pb has been identified as being a
predominant contributor to children's blood Pb, available studies do
not generally distinguish the different pathways (air-related and
other) contributing to indoor dust Pb. The exposure assessment for
children performed for this review has employed available data and
methods to develop estimates intended to inform a characterization of
these pathways (as described in section II.C below).
Relative contributions to a child's total Pb exposure from air-
related exposure pathways (such as those identified in the sections
above) compared to other (nonair-related) Pb exposures depends on many
factors including ambient air concentrations and air deposition in the
area where the child resides (as well as in the area from which the
child's food derives), access to other sources of Pb exposure such as
Pb paint, tap water affected by plumbing containing Pb and access to
Pb-tainted products. Studies indicate that in the absence of paint-
related exposures, Pb from other sources such as stationary sources of
Pb emissions may dominate a child's Pb exposures (CD, section 3.2). In
other cases, such as children living in older housing with peeling
paint or where renovations have occurred, the dominant source may be
lead paint used in the house in the past (CD, pp. 3-50 and 3-51).
Depending on Pb levels in a home's tap water, drinking water can
sometimes be a significant source (CD, section 3.3). And in still other
cases, there may be more of a mixture of contributions from multiple
sources, with no one source dominating (CD, Chapter 3).
As recognized in sections B.1.1 and II.B.3.a, blood Pb levels are
the commonly used index of exposure for Pb and they reflect external
sources of exposure, behavioral characteristics and physiological
factors. Lead derived from differing sources or taken into the body as
a result of differing exposure pathways (e.g., air- as compared to
nonair-related), is not easily distinguished. As mentioned above,
complications to consideration of estimates of air-related or
conversely, nonair, blood Pb levels are the roles of air Pb in human
exposure pathways and the persistence of Pb in the environment. As
described in section II.A.2, air-related pathways (those in which Pb
passes through the air on its path from source to human exposure) are
varied, including inhalation and ingestion, indoor dust, outdoor dust/
soil and diet, Pb suspended in and deposited from air, and encompassing
a range of time frames from more immediate to less so. Estimates of
blood Pb levels associated with air-related exposure pathways or only
with nonair exposure pathways will vary depending on how completely the
air-related pathways are characterized.
Consistent with reductions in air Pb concentrations (as described
in section II.A.1 above) which contribute to blood Pb, nonair
contributions have also been reduced. For example, the use of Pb paint
in new houses has declined substantially over the 20th century, such
that according to the National Survey of Lead and Allergens in Housing
(USHUD, 2002) an estimated 24% of U.S. housing constructed between 1960
and 1978; 69% of the housing constructed between 1940 and 1959; and 87%
of the pre-1940 housing contains lead-based paint. Additionally, Pb
contributions to diet have been reported to have declined significantly
since 1978, perhaps as much as 70% or more between then and 1990 (WHO,
1995) and the 2006 Criteria Document identifies a drop in dietary Pb
intake by 2 to 5 year olds of 96% between the early 1980s and mid 1990s
(CD, Section 3.4 and p. 8-14).\20\ These reductions are generally
attributed to reductions in gasoline-related airborne Pb as well as the
reduction in use of Pb solder in canning food products (CD, Section
3.4).\21\ There have also been reductions in tap water Pb levels (CD,
section 3.3 and pp. 8-13 to 8-14). Contamination from the distribution/
plumbing system appears to remain the predominant source of Pb in the
drinking water (CD, section 3.3 and pp. 8-013 to 8-14).
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\20\ Additionally, the 1977 Criteria Document included a dietary
Pb intake estimate for the general population of 100 to 350 [mu]g
Pb/day, with estimates near and just below 100 [mu]g/day for young
children (USEPA 1977, pp. 1-2 and 12-32) and the 2006 Criteria
Document cites recent studies (for the mid-1990s) indicating a
dietary intake ranging from 2 to 10 [mu]g Pb/day for children (CD,
Section 3.4 and p. 8-14).
\21\ Sources of Pb in food were identified in the 1986 Criteria
Document as including air-related sources, metals used in processing
raw foodstuffs, solder used in packaging and water used in cooking
(1986a, section 3.1.2).
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The availability of estimates of blood Pb levels resulting only
from air-related sources and exposures or only from those unrelated to
air is limited and, given the discussion above, would be expected to
vary for different populations. In addition to potential differences in
air-related and nonair-related blood Pb levels among populations with
different exposure circumstances (e.g., relatively more or lesser
exposure to air-related Pb), the absolute levels may also vary among
different age groups. As described in section II.B.1.b, average total
blood Pb levels in the U.S. differ among age groups, with levels being
highest in children aged one to five years old. We also note that
behavioral characteristics that influence Pb exposures vary among age
groups. For example as noted above, the predominant Pb exposure
pathways may differ between adults and children. The extent of any
quantitative impact of these differences on estimates of nonair blood
Pb levels is unknown.\22\
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\22\ As noted earlier in this section, for children, dust
ingestion by hand-to-mouth activity can be an important source of Pb
exposure, while for adults, dietary Pb can be predominant.
---------------------------------------------------------------------------
In their advice to the Agency on levels for the standard, the CASAC
Pb Panel explored several approaches to deriving a level, one of which
required an estimate of the nonair component of blood Pb for the
average child. They recommended consideration of 1.0 to 1.4 [mu]g/dL or
lower for such an estimate for the average nonair blood Pb level for
young children (Henderson, 2007a, p. D-1). This range was developed
with consideration of simulations of the integrated exposure and uptake
biokinetic (IEUBK) model for lead for which the exposure concentration
inputs included zero air concentration and concentrations for soil and
dust of 50 ppm and 35 ppm, respectively (Henderson, 2007a, p. F-
60).\23\ \24\ \25\
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\23\ The soil and dust levels are described as ``typical
geochemical non-air input levels for dust and soil'' (Henderson,
2007a, p. F-60). The values used for these levels in this simulation
fall within the range of 1 to 200 ppm described in the Criteria
Document for soil not influenced by sources (CD, p. 3-18).
\24\ The other IEUBK inputs (e.g., exposure and biokinetic
factors) were those used in the IEUBK modeling for the risk
assessment in this review (Henderson, 2007a, p. F-60).
\25\ Individual CASAC member comments describing the IEUBK
simulations stated that the modeling produced a nonair blood Pb
level of ``1.4 [mu]g/dL as a geometric mean'' (Henderson, 2007a, p.
F-61).
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As is evident from the prior discussion, the many different
exposure pathways contributing to children's blood Pb levels, and other
factors, complicate our consideration of the available data with regard
to characterization of levels particular to specific pathways, air-
related or otherwise.
B. Health Effects Information
The following summary focuses on health endpoints associated with
the range of exposures considered to be most relevant to current
exposure levels and makes note of several key aspects of the health
evidence for Pb. First (as
[[Page 29194]]
described in Section II.A, above), because exposure to atmospheric Pb
particles occurs not only via direct inhalation of airborne particles,
but also via ingestion of deposited ambient Pb, the exposure considered
is multimedia and multipathway in nature, occurring via both the
inhalation and ingestion routes. Second, the exposure index or dose
metric most commonly used and associated with health effects
information is an internal biomarker (i.e., blood Pb). Additionally,
the exposure duration of interest (i.e., that influencing internal dose
pertinent to health effects of interest) may span months to potentially
years, as does the time scale of the environmental processes
influencing Pb deposition and fate. Lastly, the nature of the evidence
for the health effects of greatest interest for this review,
neurological effects, particularly neurocognitive and neurobehavioral
effects, in young children, are epidemiological data substantiated by
toxicological data that provide biological plausibility and insights on
mechanisms of action (CD, sections 5.3, 6.2 and 8.4.2).
In recognition of the multi-pathway aspects of Pb, and the use of
an internal exposure metric in health risk assessment, the next section
describes the internal disposition or distribution of Pb, and the use
of blood Pb as an internal exposure or dose metric. This is followed by
a discussion of the nature of Pb-induced health effects that emphasizes
those with the strongest evidence. Potential impacts of Pb exposures on
public health, including recognition of potentially susceptible or
vulnerable subpopulations, are then discussed. Finally, key
observations about Pb-related health effects are summarized.
1. Blood Lead
The health effects of Pb are remote from the portals of entry to
the body (i.e., the respiratory system and gastrointestinal tract).
Consequently, the internal disposition and distribution of Pb in the
blood is an integral aspect of the relationship between exposure and
effect. Additionally, the focus on blood Pb as the dose metric in
consideration of the Pb health effects evidence, while reducing our
uncertainty with regard to causality, leads to an additional
consideration with regard to contribution of air-related sources and
exposure pathways to blood Pb.
a. Internal Disposition of Lead
This section briefly summarizes the current state of knowledge of
Pb disposition pertaining to both inhalation and ingestion routes of
exposure as described in the Criteria Document.
Inhaled Pb particles deposit in the different regions of the
respiratory tract as a function of particle size (CD, pp. 4-3 to 4-4).
Lead associated with smaller particles, which are predominantly
deposited in the pulmonary region, may, depending on solubility, be
absorbed into the general circulation or transported to the
gastrointestinal tract (CD, pp. 4-3). Lead associated with larger
particles, which are predominantly deposited in the head and conducting
airways (e.g., nasal pharyngeal and tracheobronchial regions of
respiratory tract), may be transported into the esophagus and
swallowed, thus making its way to the gastrointestinal tract (CD, pp.
4-3 to 4-4), where it may be absorbed into the blood stream. Thus, Pb
can reach the gastrointestinal tract either directly through the
ingestion route or indirectly following inhalation.
Once in the blood stream, where approximately 99% of the Pb
associates with red blood cells, the Pb is quickly distributed
throughout the body (e.g., within days) with the bone serving as a
large, long-term storage compartment, and soft tissues (e.g., kidney,
liver, brain, etc.) serving as smaller compartments, in which Pb may be
more mobile (CD, sections 4.3.1.4 and 8.3.1.). Additionally, the
epidemiologic evidence indicates that Pb freely crosses the placenta
resulting in continued fetal exposure throughout pregnancy, and that
exposure increases during the later half of pregnancy (CD, section
6.6.2).
During childhood development, bone represents approximately 70% of
a child's body burden of Pb, and this accumulation continues through
adulthood, when more than 90% of the total Pb body burden is stored in
the bone (CD, section 4.2.2). Accordingly, levels of Pb in bone are
indicative of a person's long-term, cumulative exposure to Pb. In
contrast, blood Pb levels are usually indicative of recent exposures.
Depending on exposure dynamics, however, blood Pb may--through its
interaction with bone--be indicative of past exposure or of cumulative
body burden (CD, section 4.3.1.5).
Throughout life, Pb in the body is exchanged between blood and
bone, and between blood and soft tissues (CD, section 4.3.2), with
variation in these exchanges reflecting ``duration and intensity of the
exposure, age and various physiological variables'' (CD, p. 4-1). Past
exposures that contribute Pb to the bone, consequently, may influence
current levels of Pb in blood. Where past exposures were elevated in
comparison to recent exposures, this influence may complicate
interpretations with regard to recent exposure (CD, sections 4.3.1.4 to
4.3.1.6). That is, higher blood Pb concentrations may be indicative of
higher cumulative exposures or of a recent elevation in exposure (CD,
pp. 4-34 and 4-133).
In several studies investigating the relationship between Pb
exposure and blood Pb in children (e.g., Lanphear and Roghmann 1997;
Lanphear et al., 1998), blood Pb levels have been shown to reflect Pb
exposures, with particular influence associated with exposures to Pb in
surface dust. Further, as stated in the Criteria Document ``these and
other studies of populations near active sources of air emissions
(e.g., smelters, etc.) substantiate the effect of airborne Pb and
resuspended soil Pb on interior dust and blood Pb'' (CD, p. 8-22).
b. Use of Blood Lead as Dose Metric
Blood Pb levels are extensively used as an index or biomarker of
exposure by national and international health agencies, as well as in
epidemiological (CD, sections 4.3.1.3 and 8.3.2) and toxicological
studies of Pb health effects and dose-response relationships (CD,
Chapter 5). The prevalence of the use of blood Pb as an exposure index
or biomarker is related to both the ease of blood sample collection
(CD, p. 4-19; Section 4.3.1) and by findings of association with a
variety of health effects (CD, Section 8.3.2). For example, the U.S.
Centers for Disease Control and Prevention (CDC), and its predecessor
agencies, have for many years used blood Pb level as a metric for
identifying children at risk of adverse health effects and for
specifying particular public health recommendations (CDC, 1991; CDC,
2005a). In 1978, when the current Pb NAAQS was established, the CDC
recognized a blood Pb level of 30 [mu]g/dL as a level warranting
individual intervention (CDC, 1991). In 1985, the CDC recognized a
level of 25 [mu]g/dL for individual child intervention, and in 1991,
they recognized a level of 15 [mu]g/dL for individual intervention and
a level of 10 [mu]g/dL for implementing community-wide prevention
activities (CDC, 1991; CDCa, 2005). In 2005, with consideration of a
review of the evidence by their advisory committee, CDC revised their
statement on Preventing Lead Poisoning in Young Children, specifically
recognizing the evidence of adverse health effects in children with
blood Pb levels below 10 [mu]g/dL \26\ and the data demonstrating that
[[Page 29195]]
no ``safe'' threshold for blood Pb had been identified, and emphasizing
the importance of preventative measures (CDC, 2005a, ACCLPP, 2007).\27\
---------------------------------------------------------------------------
\26\ As described by the Advisory Committee on Childhood Lead
Poisoning Prevention, ``In 1991, CDC defined the blood lead level
(BLL) that should prompt public health actions as 10 [mu]g/dL.
Concurrently, CDC also recognized that a BLL of 10 [mu]g/dL did not
define a threshold for the harmful effects of lead. Research
conducted since 1991 has strengthened the evidence that children's
physical and mental development can be affected at BLLS <10 [mu]g/
dL'' (ACCLPP, 2007).
\27\ With the 2005 statement, CDC did not lower the 1991 level
of concern and identified a variety of reasons, reflecting both
scientific and practical considerations, for not doing so, including
a lack of effective clinical or public health interventions to
reliably and consistently reduce blood Pb levels that are already
below 10 [mu]g/dL, the lack of a demonstrated threshold for adverse
effects, and concerns for deflecting resources from children with
higher blood Pb levels (CDC, 2005a). CDC's Advisory Committee on
Childhood Lead Poisoning Prevention recently provided
recommendations regarding interpreting and managing blood Pb levels
below 10 [mu]g/dL in children and reducing childhood exposures to Pb
(ACCLPP, 2007).
---------------------------------------------------------------------------
Since 1976, the CDC has been monitoring blood Pb levels nationally
through the National Health and Nutrition Examination Survey (NHANES).
This survey monitors blood Pb levels in multiple age groups in the U.S.
This information indicates variation in mean blood Pb levels across the
various age groups monitored. For example, mean values in 2001-2002 for
ages 1-5, 6-11, 12-19 and greater than or equal to 20 years of age, are
1.70, 1.25, 0.94, and 1.56, respectively (CD, p. 4-22).
The NHANES information has documented the dramatic decline in mean
blood Pb levels in the U.S. population that has occurred since the
1970s and that coincides with regulations regarding leaded fuels,
leaded paint, and Pb-containing plumbing materials that have reduced Pb
exposure among the general population (CD, Sections 4.3.1.3 and 8.3.3;
Schwemberger et al., 2005). The Criteria Document summarizes related
information as follows (CD, p. E-6).
In the United States, decreases in mobile sources of Pb,
resulting from the phasedown of Pb additives created a 98% decline
in emissions from 1970 to 2003. NHANES data show a consequent
parallel decline in blood-Pb levels in children aged 1 to 5 years
from a geometric mean of ~15 [mu]g/dL in 1976-1980 to ~1-2 [mu]g/dL
in the 2000-2004 period.
While levels in the U.S. general population, including geometric mean
levels in children aged 1-5, have declined significantly, levels have
been found to vary among children of different socioeconomic status
(SES) and other demographic characteristics (CD, p. 4-21). For example,
while the 2001-2004 median blood level for children aged 1-5 of all
races and ethnic groups is 1.6 [mu]g/dL, the median for the subset
living below the poverty level is 2.3 [mu]g/dL and 90th percentile
values for these two groups are 4.0 [mu]g/dL and 5.4 [mu]g/dL,
respectively. Similarly, the 2001-2004 median blood level for black,
non-Hispanic children aged 1-5 is 2.5 [mu]g/dL, while the median level
for the subset of that group living below the poverty level is 2.9
[mu]g/dL and the median level for the subset living in more well-off
households (i.e., with income more than 200% of the poverty level) is
1.9 [mu]g/dL. Associated 90th percentile values for 2001-2004 are 6.4
[mu]g/dL (for black, non-Hispanic children aged 1-5), 7.7 [mu]g/dL (for
the subset of that group living below the poverty level) and 4.1 [mu]g/
dL (for the subset living in a household with income more than 200% of
the poverty level).\28\ The recently released RRP rule (discussed above
in section I.C) is expected to contribute to further reductions in BLL
for children living in houses with Pb paint.
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\28\ This information is available at: http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm (click on
``Download a universal spreadsheet file of the Body Burdens data
tables'').
Bone measurements, as a result of the generally slower Pb turnover
in bone, are recognized as providing a better measure of cumulative Pb
exposure (CD, Section 8.3.2). The bone pool of Pb in children, however,
is thought to be much more labile than that in adults due to the more
rapid turnover of bone mineral as a result of growth (CD, p. 4-27). As
a result, changes in blood Pb concentration in children more closely
parallel changes in total body burden (CD, pp. 4-20 and 4-27). This is
in contrast to adults, whose bone has accumulated decades of Pb
exposures (with past exposures often greater than current ones), and
for whom the bone may be a significant source long after exposure has
ended (CD, Section 4.3.2.5).
c. Air-to-Blood Relationships
As described in Section II.A, Pb in ambient air contributes to Pb
in blood by multiple pathways, with the pertinent exposure routes
including both inhalation and ingestion (CD, Sections 3.1.3.2, 4.2 and
4.4; Hilts, 2003). The quantitative relationship between ambient air Pb
and blood Pb, which is often termed a slope or ratio, describes the
increase in blood Pb (in [mu]g/dL) per unit of air Pb (in [mu]g/m
\3\).\29\
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\29\ Ratios are presented in the form of 1:x, with the 1
representing air Pb (in [mu]g/m\3\) and x representing blood Pb (in
[mu]g/dL). Description of ratios as higher or lower refers to the
values for x (i.e., the change in blood Pb per unit of air Pb).
Slopes are presented as simply the value of x.
---------------------------------------------------------------------------
The evidence on this quantitative relationship is now, as in the
past, limited by the circumstances in which the data are collected.
These estimates are generally developed from studies of populations in
various Pb exposure circumstances. The 1986 Criteria Document discussed
the studies available at that time that addressed the relationship
between air Pb and blood Pb,\30\ recognizing that there is significant
variability in air-to-blood ratios for different populations exposed to
Pb through different air-related exposure pathways and at different
exposure levels.
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\30\ We note that the 2006 Criteria Document did not include a
discussion of more recent studies on air-to-blood ratios.
---------------------------------------------------------------------------
In discussing the available evidence, the 1986 Criteria Document
observed that estimates of air-to-blood ratios that included air-
related ingestion pathways in addition to the inhalation pathway are
``necessarily higher'' (in terms of blood Pb response) than those
estimates based on inhalation alone (USEPA 1986a, p. 11-106). Thus, the
extent to which studies account for the full set of air-related
exposure pathways affects the magnitude of the resultant air-to-blood
estimates, such that fewer pathways included as ``air-related'' yield
lower ratios. The 1986 Criteria Document also observed that ratios
derived from studies focused only on inhalation pathways (e.g., chamber
studies, occupational studies) have generally been on the order of 1:2
or lower, while ratios derived from studies including more air-related
pathways were generally higher (USEPA, 1986a, p. 11-106). Further, the
current evidence appears to indicate higher ratios for children as
compared to those for adults (USEPA, 1986a), perhaps due to behavioral
differences between the age groups.
Reflecting these considerations, the 1986 Criteria Document
identified a range of air-to-blood ratios for children that reflected
both inhalation and ingestion-related air Pb contributions as generally
ranging from 1:3 to 1:5 based on the information available at that time
(USEPA 1986a, p. 11-106). Table 11-36 (p. 11-100) in the 1986 Criteria
Document (drawn from Table 1 in Brunekreef, 1984) presents air-to-blood
ratios from a number of studies in children (i.e., those with
identified air monitoring methods and reliable blood Pb data). For
example, air-to-blood ratios from the subset of those studies that used
quality control protocols and presented adjusted slopes \31\ include
[[Page 29196]]
adjusted ratios of 3.6 (Zielhuis et al., 1979), 5.2 (Billick et al.,
1979, 1980), 2.9 (Billick et al., 1983), and 8.5 (Brunekreef et al,
1983).
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\31\ Brunekreef et al. (1984) discusses potential confounders to
the relationship between air Pb and blood Pb, recognizing that
ideally all possible confounders should be taken into account in
deriving an adjusted air-to-blood relationship from a community
study. The studies cited here adjusted for parental education
(Zielhuis et al., 1979), age and race (Billick et al., 1979, 1980)
and additionally measuring height of air Pb (Billick et al., 1983);
Brunekreef et al. (1984) used multiple regression to control for
several confounders. The authors conclude that ``presentation of
both unadjusted and (stepwise) adjusted relationships is advisable,
to allow insight in the range of possible values for the
relationship'' (p. 83). Unadjusted ratios were presented for two of
these studies, including ratios of 4.0 (Zielhuis et al., 1979) and
18.5 (Brunekreef et al., 1983). Note, that the Brunekreef et al.,
1983 study is subject to a number of sources of uncertainty that
could result in air-to-blood Pb ratios that are biased high,
including the potential for underestimating ambient air Pb levels
due to the use of low volume British Smoke air monitors and the
potential for ongoing (higher historical) ambient air Pb levels to
have influenced blood Pb levels (see Section V.B.2 of the 1989 Pb
Staff Report for the Pb NAAQS review, EPA, 1989). In addition, the
1989 Staff Report notes that the higher air-to-blood ratios obtained
from this study could reflect the relatively lower blood Pb levels
seen across the study population (compared with blood Pb levels
reported in other studies from that period).
---------------------------------------------------------------------------
Additionally, the 1986 Criteria Document noted that ratios derived
from studies involving higher blood and air Pb levels are generally
smaller than ratios from studies involving lower blood and air Pb
levels (USEPA, 1986a. p. 11-99). In consideration of this factor, we
note that the range of 1:3 to 1:5 in air-to-blood ratios for children
noted in the 1986 Criteria Document generally reflected study
populations with blood Pb levels in the range of approximately 10-30
[mu]g/dL (USEPA 1986a, pp. 11-100; Brunekreef, 1984), much higher than
those common in today's population. This observation suggests that air-
to-blood ratios relevant for today's population of children would
likely extend higher than the 1:3 to 1:5 range identified in the 1986
Criteria Document.
More recently, a study of changes in children's blood Pb levels
associated with reduced Pb emissions and associated air concentrations
near a Pb smelter in Canada (for children through six years of age)
reports a ratio of 1:6 and additional analysis of the data by EPA for
the initial time period of the study resulted in a ratio of 1:7 (CD,
pp. 3-23 to 3-24; Hilts, 2003).\32\ Ambient air and blood Pb levels
associated with the Hilts (2003) study range from 1.1 to 0.03 [mu]g/
m\3\, and associated population mean blood Pb levels range from 11.5 to
4.7 [mu]g/dL, which are lower than levels associated with the older
studies cited in the 1986 Criteria Document (USEPA, 1986).
---------------------------------------------------------------------------
\32\ This study considered changes in ambient air Pb levels and
associated blood Pb levels over a five-year period which included
closure of an older Pb smelter and subsequent opening of a newer
facility in 1997 and a temporary (3 month) shutdown of all smelting
activity in the summer of 2001. The author observed that the air-to-
blood ratio for children in the area over the full period was
approximately 1:6. The author noted limitations in the dataset
associated with exposures in the second time period, after the
temporary shutdown of the facility in 2001, including sampling of a
different age group at that time and a shorter time period (3
months) at these lower ambient air Pb levels prior to collection of
blood Pb levels. Consequently, EPA calculated an alternate air-to-
blood Pb ratio based on consideration for ambient air Pb and blood
Pb reductions in the first time period (after opening of the new
facility in 1997).
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Sources of uncertainty related to air-to-blood ratios obtained from
Hilts (2003) study have been identified. One such area of uncertainty
relates to the pattern of changes in indoor Pb dustfall (presented in
Table 3 in the article) which suggests a potentially significant
decrease in Pb impacts to indoor dust prior to closure of an older Pb
smelter and start-up of a newer facility in 1997. Some have suggested
that this earlier reduction in indoor dustfall suggests that a
significant portion of the reduction in Pb exposure (and therefore, the
blood Pb reduction reflected in air-to-blood ratios) may have resulted
from efforts to increase public awareness of the Pb contamination issue
(e.g., through increased cleaning to reduce indoor dust levels) rather
than reductions in ambient air Pb and associated indoor dust Pb
contamination. In addition, notable fluctuations in blood Pb levels
observed prior to 1997 (as seen in Figure 2 of the article) have raised
questions as to whether factors other than ambient air Pb reduction
could be influencing decreases in blood Pb.\33\
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\33\ In the publication, the author acknowledges that remedial
programs (e.g., community and home-based dust control and education)
may have been responsible for some of the blood Pb reduction seen
during the study period (1997 to 2001). However, the author points
out that these programs were in place in 1992 and he suggests that
it is unlikely that they contributed to the sudden drop in blood Pb
levels occurring after 1997. In addition, the author describes a
number of aspects of the analysis, which could have implications for
air-to-blood ratios including a tendency over time for children with
lower blood Pb levels to not return for testing, and inclusion of
children aged 6 to 36 months in Pb screening in 2001 (in contrast to
the wider age range up to 60 months as was done in previous years).
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In addition to the study by Hilts (2003), we are aware of two other
studies published since the 1986 Criteria Document that report air-to-
blood ratios for children (Tripathi et al., 2001 and Hayes et al.,
1994). These studies were not cited in the 2006 Criteria Document, but
were referenced in public comments received by EPA during this
review.\34\ The study by Tripathi et al. (2001) reports an air-to-blood
ratio of approximately 1:3.6 for an analysis of children aged six
through ten in India. The ambient air and blood Pb levels in this study
(geometric mean blood Pb levels generally ranged from 10 to 15 [mu]g/
dL) are similar to levels reported in older studies reviewed in the
1986 Criteria Document and are much higher than current conditions in
the U.S. The study by Hayes (1994) compared patterns of ambient air Pb
reductions and blood Pb reductions for large numbers of children in
Chicago between 1971 and 1988, a period when significant reductions
occurred in both measures. The study reports an air-to-blood ratio of
1:5.6 associated with ambient air Pb levels near 1 [mu]g/m\3\ and a
ratio of 1:16 for ambient air Pb levels in the range of 0.25 [mu]g/
m\3\, indicating a pattern of higher ratios with lower ambient air Pb
and blood Pb levels consistent with conclusions in the 1986 Criteria
Document.\35\
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\34\ EPA is not basing its proposed decisions on these two
studies, but notes that these estimates are consistent with other
studies that were included in the 1986 and 2006 Criteria Documents
and accordingly considered by CASAC and the public.
\35\ As with all studies, we note that there are strengths and
limitations for these two studies which may affect the specific
magnitudes of the reported ratios, but that the studies' findings
and trends are generally consistent with the conclusions from the
1986 Criteria Document.
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In their advice to the Agency, CASAC identified air-to-blood ratios
of 1:5, as used by the World Health Organization (2000), and 1:10, as
supported by an empirical analysis of changes in air Pb and changes in
blood Pb between 1976 and the time when the phase-out of Pb from
gasoline was completed (Henderson, 2007a).\36\
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\36\ The CASAC Panel stated ``The Schwartz and Pitcher analysis
showed that in 1978, the midpoint of the National Health and
Nutrition Examination Survey (NHANES) II, gasoline Pb was
responsible for 9.1 [mu]g/dL of blood Pb in children. Their estimate
is based on their coefficient of 2.14 [mu]g/dL per 100 metric tons
(MT) per day of gasoline use, and usage of 426 MT/day in 1976.
Between 1976 and when the phase-out of Pb from gasoline was
completed, air Pb concentrations in U.S. cities fell a little less
than 1 [mu]g/m\3\ (24). These two facts imply a ratio of 9-10 [mu]g/
dL per [mu]g/m\3\ reduction in air Pb, taking all pathways into
account.'' (Henderson, 2007a, pp. D-2 to D-3).
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Beyond considering the evidence presented in the published
literature and that reviewed in Pb Criteria Documents, we have also
considered air-to-blood ratios derived from the exposure assessment for
this review (discussed below in section II.C). In that assessment,
current modeling tools and information on children's activity patterns,
behavior and physiology (e.g., CD, Section 4.4) were used to estimate
blood Pb levels associated with
[[Page 29197]]
multimedia and multipathway Pb exposure. The results from the various
case studies included in this assessment, with consideration of the
context in which they were derived (e.g., the extent to which the range
of air-related pathways were simulated), are also informative to our
understanding of air-to-blood ratios.
For the general urban case study, air-to-blood ratios ranged from
1:2 to 1:9 across the alternative standard levels assessed, which
ranged from the current standard of 1.5 [mu]g/m\3\ down to a level of
0.02 [mu]g/m\3\. This pattern of model-derived ratios generally
supports the range of ratios obtained from the literature and also
supports the observation that lower ambient air Pb levels are
associated with higher air-to-blood ratios. There are a number of
sources of uncertainty associated with these model-derived ratios. The
hybrid indoor dust Pb model, which is used in estimating indoor dust Pb
levels for the urban case studies, uses a HUD dataset reflecting
housing constructed before 1980 in establishing the relationship
between dust loading and concentration, which is a key component in the
hybrid dust model (see Section Attachment G-1 of the Risk Assessment,
Volume II). Given this application of the HUD dataset, there is the
potential that the non-linear relationship between indoor dust Pb
loading and concentration (which is reflected in the structure of the
hybrid dust model) could be driven more by the presence of indoor Pb
paint than contributions from outdoor ambient air Pb. We also note that
only recent air pathways were adjusted in modeling the impact of
ambient air Pb reductions on blood Pb levels in the urban case studies,
which could have implications for the air-to-blood ratios.
For the primary Pb smelter (subarea) case study, air-to-blood
ratios ranged from 1:10 to 1:19 across the same range of alternative
standard levels, from 1.5 down to 0.02 [mu]g/m\3\.\37\ Because these
ratios are based on regression modeling developed using empirical data,
there is the potential for these ratios to capture more fully the
impact of ambient air on indoor dust Pb (and ultimately blood Pb),
including longer timeframe impacts resulting from changes in outdoor
deposition. Therefore, given that these ratios are higher than ratios
developed for the general urban case study using the hybrid indoor dust
Pb model (which only considers reductions in recent air), the ratios
estimated for the primary Pb smelter (subarea) support the evidence-
based observation discussed above that consideration of more of the
exposure pathways relating ambient air Pb to blood Pb, may result in
higher air-to-blood Pb ratios. In considering this case study, some
have suggested, however, that the regression modeling fails to
accurately reflect the temporal relationship between reductions in
ambient air Pb and indoor dust Pb, which could result in an over-
estimate of the degree of dust Pb reduction associated with a specified
degree of ambient air Pb reduction, which in turn could produce air-to-
blood Pb ratios that are biased high.
---------------------------------------------------------------------------
\37\ As noted below in section II.C.3.a, air-to-blood ratios for
the primary Pb smelter (full study area) range from 1:3 to 1:7
across the same range of alternative standard levels (from 1.5 down
to 0.02 [mu]g/m\3\).
---------------------------------------------------------------------------
In summary, in EPA's view, the current evidence in conjunction with
the results and observations drawn from the exposure assessment,
including related uncertainties, supports consideration of a range of
air-to-blood ratios for children ranging from 1:3 to 1:7, reflecting
multiple air-related pathways beyond simply inhalation and the lower
air and blood Pb levels pertinent to this review. In light of the
uncertainties that remain in the available information on air-to-blood
ratios, EPA requests comment on this range and on the appropriate
weight to place on specific ratios within this range.
2. Nature of Effects
a. Broad Array of Effects
Lead has been demonstrated to exert ``a broad array of deleterious
effects on multiple organ systems via widely diverse mechanisms of
action'' (CD, p. 8-24 and Section 8.4.1). This array of health effects
includes effects on heme biosynthesis and related functions;
neurological development and function; reproduction and physical
development; kidney function; cardiovascular function; and immune
function. The weight of evidence varies across this array of effects
and is comprehensively described in the Criteria Document. There is
also some evidence of Pb carcinogenicity, primarily from animal
studies, together with limited human evidence of suggestive
associations (CD, Sections 5.6.2, 6.7, and 8.4.10).\38\
---------------------------------------------------------------------------
\38\ Lead has been classified as a probable human carcinogen by
the International Agency for Research on Cancer, based mainly on
sufficient animal evidence, and as reasonably anticipated to be a
human carcinogen by the U.S. National Toxicology Program (CD,
Section 6.7.2). U.S. EPA considers Pb a probable carcinogen (http://www.epa.gov/iris/subst/0277.htm; CD, p. 6-195).
---------------------------------------------------------------------------
This review is focused on those effects most pertinent to ambient
exposures, which given the reductions in ambient Pb levels over the
past 30 years, are generally those associated with individual blood Pb
levels in children and adults in the range of 10 [mu]g/dL and lower.
Tables 8-5 and 8-6 in the Criteria Document highlight the key such
effects observed in children and adults, respectively (CD, pp. 8-60 to
8-62). The effects include neurological, hematological and immune
effects for children, and hematological, cardiovascular and renal
effects for adults. As evident from the discussions in Chapters 5, 6
and 8 of the Criteria Document, ``neurotoxic effects in children and
cardiovascular effects in adults are among those best substantiated as
occurring at blood Pb concentrations as low as 5 to 10 [mu]g/dL (or
possibly lower); and these categories are currently clearly of greatest
public health concern'' (CD, p. 8-60).\39\ The toxicological and
epidemiological information available since the time of the last review
``includes assessment of new evidence substantiating risks of
deleterious effects on certain health endpoints being induced by
distinctly lower than previously demonstrated Pb exposures indexed by
blood Pb levels extending well below 10 [mu]g/dL in children and/or
adults'' (CD, p. 8-25). Some health effects associated with individual
blood Pb levels extend below 5 [mu]g/dL, and some studies have observed
these effects at the lowest blood levels considered.
---------------------------------------------------------------------------
\39\ With regard to blood Pb levels in individual children
associated with particular neurological effects, the Criteria
Document states ``Collectively, the prospective cohort and cross-
sectional studies offer evidence that exposure to Pb affects the
intellectual attainment of preschool and school age children at
blood Pb levels <10 [mu]g/dL (most clearly in the 5 to 10 [mu]g/dL
range, but, less definitively, possibly lower).'' (p. 6-269)
---------------------------------------------------------------------------
With regard to population mean levels, the Criteria Document points
to studies reporting ``Pb effects on the intellectual attainment of
preschool and school age children at population mean concurrent blood-
Pb levels ranging down to as low as 2 to 8 [mu]g/dL'' (CD, p. E-9).
We note that many studies over the past decade have, in
investigating effects at lower blood Pb levels, utilized the CDC
advisory level for individual children (10 [mu]g/dL) as a benchmark for
assessment, and this is reflected in the numerous references in the
Criteria Document to 10 [mu]g/dL. Individual study conclusions stated
with regard to effects observed below 10 [mu]g/dL are usually referring
to individual blood Pb levels. In fact, many such study groups have
been restricted to individual blood Pb levels below 10 [mu]g/dL or
below levels lower than 10 [mu]g/dL. We note that the
[[Page 29198]]
mean blood Pb level for these groups will necessarily be lower than the
blood Pb level they are restricted below.
Threshold levels, in terms of blood Pb levels in individual
children, for neurological effects cannot be discerned from the
currently available studies (CD, pp. 8-60 to 8-63). The Criteria
Document states ``There is no level of Pb exposure that can yet be
identified, with confidence, as clearly not being associated with some
risk of deleterious health effects'' (CD, p. 8-63). As discussed in the
Criteria Document, ``a threshold for Pb neurotoxic effects may exist at
levels distinctly lower than the lowest exposures examined in these
epidemiologic studies'' (CD, p. 8-67).\40\
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\40\ In consideration of the evidence from experimental animal
studies with regard to the issue of threshold for neurotoxic
effects, the CD notes that there is little evidence that allows for
clear delineation of a threshold, and that ``blood-Pb levels
associated with neurobehavioral effects appear to be reasonably
parallel between humans and animals at reasonably comparable blood-
Pb concentrations; and such effects appear likely to occur in humans
ranging down at least to 5-10 [mu]g/dL, or possibly lower (although
the possibility of a threshold for such neurotoxic effects cannot be
ruled out at lower blood-Pb concentrations)'' (CD, p. 8-38).
---------------------------------------------------------------------------
In summary, the Agency has identified neurological, hematological
and immune effects in children and neurological, hematological,
cardiovascular and renal effects in adults as the effects observed at
blood Pb levels near or below 10 [mu]g/dL and further considers
neurological effects in children and cardiovascular effects in adults
to be categories of effects that ``are currently clearly of greatest
public health concern'' (CD, pp. 8-60 to 8-62). Neurological effects in
children are discussed further below.
b. Neurological Effects in Children
Among the wide variety of health endpoints associated with Pb
exposures, there is general consensus that the developing nervous
system in young children is among, if not, the most sensitive. As
described in the Criteria Document, neurotoxic effects in children and
cardiovascular effects in adults are categories of effects that are
``currently clearly of greatest public health concern'' (CD, p. 8-
60).\41\ While also recognizing the occurrence of adult cardiovascular
effects at somewhat similarly low blood Pb levels \42\, neurological
effects in children are considered to be the sentinel effects in this
review and are the focus of the quantitative risk assessment conducted
for this review (discussed below in section III.C).
---------------------------------------------------------------------------
\41\ The Criteria Document states ``neurotoxic effects in
children and cardiovascular effects in adults are among those best
substantiated as occurring at blood-Pb concentrations as low as 5 to
10 [mu]g/dL (or possibly lower); and these categories of effects are
currently clearly of greatest public health concern (CD, p. 8-60).''
\42\ For example, the Criteria Document describes associations
of blood Pb in adults with blood pressure in studies with population
mean blood Pb levels ranging from approximately 2 to 6 [mu]g/dL (CD,
section 6.5.2 and Table 6-2).
---------------------------------------------------------------------------
The nervous system has long been recognized as a target of Pb
toxicity, with the developing nervous system affected at lower
exposures than the mature system (CD, Sections 5.3, 6.2.1, 6.2.2, and
8.4). While blood Pb levels in U.S. children ages one to five years
have decreased notably since the late 1970s, newer studies have
investigated and reported associations of effects on the
neurodevelopment of children with these more recent blood Pb levels
(CD, Chapter 6). Functional manifestations of Pb neurotoxicity during
childhood include sensory, motor, cognitive and behavioral impacts.
Numerous epidemiological studies have reported neurocognitive,
neurobehavioral, sensory, and motor function effects in children with
blood Pb levels below 10 [mu]g/dL (CD, Sections 6.2 and 8.4). \43\ As
discussed in the Criteria Document, ``extensive experimental laboratory
animal evidence has been generated that (a) substantiates well the
plausibility of the epidemiologic findings observed in human children
and adults and (b) expands our understanding of likely mechanisms
underlying the neurotoxic effects'' (CD, p. 8-25; Section 5.3).
---------------------------------------------------------------------------
\43\ Further, neurological effects in general include behavioral
effects, such as delinquent behavior (CD, sections 6.2.6 and
8.4.2.2), sensory effects, such as those related to hearing and
vision (CD, sections 6.2.7 and 8.4.2.3), and deficits in neuromotor
function (CD, p. 8-36).
---------------------------------------------------------------------------
The evidence for neurotoxic effects in children is a robust
combination of epidemiological and toxicological evidence (CD, Sections
5.3, 6.2 and 8.5). The epidemiological evidence is supported by animal
studies that substantiate the biological plausibility of the
associations, and contributes to our understanding of mechanisms of
action for the effects (CD, Section 8.4.2).
Cognitive effects associated with Pb exposures that have been
observed in epidemiological studies have included decrements in
intelligence test results, such as the widely used IQ score, and in
academic achievement as assessed by various standardized tests as well
as by class ranking and graduation rates (CD, Section 6.2.16 and pp 8-
29 to 8-30). As noted in the Criteria Document with regard to the
latter, ``Associations between Pb exposure and academic achievement
observed in the above-noted studies were significant even after
adjusting for IQ, suggesting that Pb-sensitive neuropsychological
processing and learning factors not reflected by global intelligence
indices might contribute to reduced performance on academic tasks''
(CD, pp 8-29 to 8-30).
Other cognitive effects observed in studies of children have
included effects on attention, executive functions, language, memory,
learning and visuospatial processing (CD, Sections 5.3.5, 6.2.5 and
8.4.2.1), with attention and executive function effects associated with
Pb exposures indexed by blood Pb levels below 10 [mu]g/dL (CD, Section
6.2.5 and pp. 8-30 to 8-31). The evidence for the role of Pb in this
suite of effects includes experimental animal findings (discussed in
CD, Section 8.4.2.1; p. 8-31), which provide strong biological
plausibility of Pb effects on learning ability, memory and attention
(CD, Section 5.3.5), as well as associated mechanistic findings. With
regard to persistence of effects the Criteria Document states the
following (CD, p. 8-67):
Persistence or apparent ``irreversibility'' of effects can
result from two different scenarios: (1) Organic damage has occurred
without adequate repair or compensatory offsets, or (2) exposure
somehow persists. As Pb exposure can also derive from endogenous
sources (e.g., bone), a performance deficit that remains detectable
after external exposure has ended, rather than indicating
irreversibility, could reflect ongoing toxicity due to Pb remaining
at the critical target organ or Pb deposited at the organ post-
exposure as the result of redistribution of Pb among body pools. The
persistence of effect appears to depend on the duration of exposure
as well as other factors that may affect an individual's ability to
recover from an insult. The likelihood of reversibility also seems
to be related, at least for the adverse effects observed in certain
organ systems, to both the age-at-exposure and the age-at-
assessment.
The evidence with regard to persistence of Pb-induced deficits observed
in animal and epidemiological studies is described in discussion of
those studies in the Criteria Document (CD, Sections 5.3.5, 6.2.11, and
8.5.2). It is additionally important to note that there may be long-
term consequences of such deficits over a lifetime. Poor academic
skills and achievement can have ``enduring and important effects on
objective parameters of success in real life,'' as well as increased
risk of antisocial and delinquent behavior (CD, Section 6.2.16).
As discussed in the Criteria Document, while there is no direct
animal test parallel to human IQ tests, ``in animals a wide variety of
tests that assess attention, learning, and memory suggest that Pb
exposure {of animals{time} results in a global deficit in functioning,
[[Page 29199]]
just as it is indicated by decrements in IQ scores in children'' (CD,
p. 8-27). The animal and epidemiological evidence for this endpoint are
consistent and complementary (CD, p. 8-44). As stated in the Criteria
Document (p. 8-44):
Findings from numerous experimental studies of rats and of
nonhuman primates, as discussed in Chapter 5, parallel the observed
human neurocognitive deficits and the processes responsible for
them. Learning and other higher order cognitive processes show the
greatest similarities in Pb-induced deficits between humans and
experimental animals. Deficits in cognition are due to the combined
and overlapping effects of Pb-induced perseveration, inability to
inhibit responding, inability to adapt to changing behavioral
requirements, aversion to delays, and distractibility. Higher level
neurocognitive functions are affected in both animals and humans at
very low exposure levels (<10 [mu]g/dL), more so than simple
cognitive functions.
Epidemiologic studies of Pb and child development have demonstrated
inverse associations between blood Pb concentrations and children's IQ
and other cognitive-related outcomes at successively lower Pb exposure
levels over the past 30 years (CD, p. 6-64). This is supported by
multiple studies performed over the past 15 years (as discussed in the
CD, Section 6.2.13). For example, the overall weight of the available
evidence, described in the Criteria Document, provides clear
substantiation of neurocognitive decrements being associated in
children with mean blood Pb levels in the range of 5 to 10 [mu]g/dL,
and some analyses indicate Pb effects on intellectual attainment of
children for which population mean blood Pb levels in the analysis
ranged from 2 to 8 [mu]g/dL (CD, Sections 6.2, 8.4.2 and 8.4.2.6).\44\
That is, while blood Pb levels in U.S. children have decreased notably
since the late 1970s, newer studies have investigated and reported
associations of effects on the neurodevelopment of children with blood
Pb levels similar to the more recent blood Pb levels (CD, Chapter 6).
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\44\ ``The overall weight of the available evidence provides
clear substantiation of neurocognitive decrements being associated
in young children with blood-Pb concentrations in the range of 5-10
[mu]g/dL, and possibly somewhat lower. Some newly available analyses
appear to show Pb effects on the intellectual attainment of
preschool and school age children at population mean concurrent
blood-Pb levels ranging down to as low as 2 to 8 [mu]g/dL.'' (CD, p.
E-9)
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The evidence described in the Criteria Document with regard to the
effect on children's cognitive function of blood Pb levels at the lower
concentration range includes the international pooled analysis by
Lanphear and others (2005), studies of individual cohorts such as the
Rochester, Boston, and Mexico City cohorts (Canfield et al., 2003a;
Canfield et al., 2003b; Bellinger and Needleman, 2003; Tellez-Rojo et
al., 2006), the study of African-American inner-city children from
Detroit (Chiodo et al., 2004), the cross-sectional study of young
children in three German cities (Walkowiak et al., 1998) and the cross-
sectional analysis of a nationally representative sample from the
NHANES III \45\ (Lanphear et al., 2000). These studies included
differing adjustments for different important potential confounders
(e.g., parental IQ or HOME score) or surrogates of these measures
(e.g., parental education and SES factors) through multivariate
analyses.46 47 Each of these studies has individual
strengths and limitations, however, a pattern of positive findings is
demonstrated across the studies. In these studies, statistically
significant associations of neurocognitive decrement \48\ with blood Pb
were found in the full study cohorts, as well as in some subgroups
restricted to children with lower blood Pb levels for which mean blood
Pb levels extended below 5 [mu]g/dL. More specifically, a statistically
significant association was reported for full-scale IQ with blood Pb at
age five in a subset analysis (n=71) of the Rochester cohort for which
the population mean blood Pb level was 3.32 [mu]g/dL, as well as in the
full study group (mean=5.8 [mu]g/dL, n=171) (Canfield et al., 2003a;
Canfield, 2008). Full-scale IQ was also significantly associated with
blood Pb at age seven and a half in a subset analysis (n=200) in the
Detroit inner-city study for which the population mean blood Pb level
was 4.1 [mu]g/dL, as well as the other subgroup with higher blood Pb
levels (mean=4.6 [mu]g/dL, n=224) and in the full study group (mean=5.4
[mu]g/dL, n=246); additionally, performance IQ was significantly
associated with blood Pb in those analyses as well as in the subset
analysis (n=120) for which the population mean blood Pb level was 3
[mu]g/dL (although full-scale IQ was not significantly associated with
blood Pb in this lowest blood Pb subgroup) (Chiodo et al., 2004,
Chiodo, 2008). Vocabulary, one of ten subtests of the full-scale IQ,
was significantly associated with blood
[[Page 29200]]
Pb at age six in the German three-city study (n=384) in which the mean
blood Pb level was 4.2 [mu]g/dL (Walkowiak et al., 1998). In a Mexico
City cohort of infants age two, the mental development index (MDI) and
psychomotor development index (PDI) were significantly associated with
blood Pb in the full study group (mean=4.28 [mu]g/dL, n=294); further,
the MDI (but not the PDI) was significantly associated with blood Pb in
the subset analysis (n=193) for which the population mean blood Pb
level was 2.9 [mu]g/dL, and PDI (but not the MDI) was significantly
associated with blood Pb in the subset analysis (n=101) for which the
population mean blood Pb was 6.9 [mu]g/dL (Tellez-Rojo et al., 2006;
Tellez-Rojo, 2008). Scores on academic achievement tests for reading
and math were significantly associated with blood Pb at age six through
sixteen in a subgroup analysis (n=4043) of the NHANES III data for
which the population mean blood Pb level was 1.7 [mu]g/dL, as discussed
below (Lanphear et al. 2000; Auinger, 2008).
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\45\ The NHANES III survey was conducted in 1988-1994.
\46\ Some studies also employed exclusion criteria which limited
variation in socioeconomic status across the study population.
Further, with regard to adjustment for potential confounders in the
large pooled international analysis (Lanphear et al. 2005),
discussed below, the authors adjusted for HOME score, birth weight,
maternal IQ and maternal education. Canfield et al. (2003) adjusted
for maternal IQ, maternal education, HOME score, birth weight, race,
tobacco use during pregnancy, household income, gender, and iron
status. Bellinger and Needleman (2003) adjusted for maternal IQ,
HOME score, SES, child stress, maternal age, race, gender, birth
order, marital status. Chiodo et al. (2004) adjusted for primary
care-giver education and vocabulary, HOME score, family environment
scale, SES, gender, number of children under 18, birth order.
Tellez-Rojo et al. (2006) adjusted for maternal IQ, birth weight and
gender; the authors also state that other potentially confounding
variables that were not found to be significant at p<.10 were not
adjusted for. Walkoviak et al. (1998) adjusted for parental
education, breastfeeding, nationality and gender. In Lanphear et al.
(2000), the authors adjusted for race/ethnicity and poverty index
ratio, as surrogates for HOME score/SES status, and adjusted for the
parental education level as a surrogate for maternal IQ; they also
adjusted for gender, serum ferritin level and serum cotinine level.
\47\ The Criteria Document notes that a ``major challenge to
observational studies examining the impact of Pb on parameters of
child development has been the assessment and control for
confounding factors'' (CD, p. 6-73). However, the Criteria Document
further recognizes that ``[m]ost of the important confounding
factors in Pb studies have been identified, and efforts have been
made to control them in studies conducted since the 1990
Supplement'' (CD, p. 6-75). On this subject, the Criteria Document
further concludes the following: ``Invocation of the poorly measured
confounder as an explanation for positive findings is not
substantiated in the database as a whole when evaluating the impact
of Pb on the health of U.S. children (Needleman, 1995). Of course,
it is often the case that following adjustment for factors such as
social class, parental neurocognitive function, and child rearing
environment using covariates such as parental education, income, and
occupation, parental IQ, and HOME scores, the Pb coefficients are
substantially reduced in size and statistical significance (Dietrich
et al., 1991). This has sometimes led investigators to be quite
cautious in interpreting their study results as being positive
(Wasserman et al., 1997). This is a reasonable way of appraising any
single study, and such extreme caution would certainly be warranted
if forced to rely on a single study to confirm the Pb effects
hypothesis. Fortunately, there exists a large database of high
quality studies on which to base inferences regarding the
relationship between Pb exposure and neurodevelopment. In addition,
Pb has been extensively studied in animal models at doses that
closely approximate the human situation. Experimental animal studies
are not compromised by the possibility of confounding by such
factors as social class and correlated environmental factors. The
enormous experimental animal literature that proves that Pb at low
levels causes neurobehavioral deficits and provides insights into
mechanisms must be considered when drawing causal inferences
(Bellinger, 2004; Davis et al., 1990; U.S. Environmental Protection
Agency, 1986a, 1990).'' (CD, p. 6-75)
\48\ The tests for cognitive function in these studies include
age-appropriate Wechsler intelligence tests (Lanphear et al., 2005),
the Stanford-Binet intelligence test (Canfield et al., 2003a), and
the Bayley Scales of Infant Development (Tellez-Rojo et al., 2006).
In some cases, individual subtests of the Wechsler intelligence
tests (Lanphear et al., 2000; Walkowiak et al., 1998), and
individual subtests of the Wide Range Achievement Test (Lanphear et
al., 2000) were used. The Wechsler and Stanford-Binet tests are
widely used to assess neurocognitive function in children and
adults, however, these tests are not appropriate for children under
age three. For such children, studies generally use the age-
appropriate Bayley Scales of Infant Development as a measure of
cognitive development. See footnote 63 for further information.
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The study by Lanphear et al. (2000) is a large cross-sectional
study using NHANES III dataset, with 4853 subjects in the full study
and more than 4000 in the subgroup analyses, that reports statistically
significant \49\ associations of concurrent blood Pb levels \50\ with
neurocognitive decrements in the full study population and in subgroup
analyses down to and including the subgroup with individual blood Pb
levels below 5 [mu]g/dL (CD, pp. 6-31 to 6-32; Lanphear et al., 2000).
Specifically the study by Lanphear et al. (2000) reported a
statistically significant association between math (p<0.001), reading
(p<0.001), block design (p=0.009), and digit span (p=0.04) scores and
blood Pb levels in the analysis that included all study subjects.
Additionally, the study reports statistically significant associations
for block design and digit span scores down to and including the
subgroup with individual blood Pb levels below 7.5 [mu]g/dL and 10
[mu]g/dL, respectively.\51\ Further, statistically significant
associations were observed for reading and math scores down to and
including the subgroup with individual blood Pb levels below 5 [mu]g/
dL, which included 4043 of the 4853 children.\52\ A similar pattern in
the magnitude of the effect estimates was observed across all the
subgroup analyses and for all four tests, including the subgroup with
individual blood Pb levels less than 2.5 [mu]g/dL, although not all the
effect estimates were statistically significant (Lanphear et al.,
2000).\53\ In particular, the lack of statistical significance in the
subset of individuals with blood Pb levels less than 2.5 [mu]g/dL may
be attributable to the smaller sample size (2467 children) and reduced
variability of blood Pb levels.\54\ Blood Pb levels in the full study
population ranged from below detection to above 10 [mu]g/dL, with a
population geometric mean of 1.9 [mu]g/dL, and the subgroups were
composed of children with blood Pb levels less than 10 [mu]g/dL
(geometric mean of 1.8 [mu]g/dL), less than 7.5 [mu]g/dL (geometric
mean of 1.8 [mu]g/dL), less than 5 [mu]g/dL (geometric mean of 1.7
[mu]g/dL), and less than 2.5 [mu]g/dL (geometric mean of 1.2 [mu]g/dL),
respectively (Lanphear et al., 2000; Auinger, 2008).\55\
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\49\ The statistical significance refers to the effect estimate
of the linear relationship across the range of data, as presented in
Table 4 of Lanphear et al. (2000).
\50\ A limitation noted for this study is with regard to the use
of concurrent blood Pb levels in children of this age. The authors
state that ``it is not clear whether the cognitive and academic
deficits observed in the present analysis are due to lead exposure
that occurred during early childhood or due to concurrent
exposure'', however, they further note that ``concurrent blood lead
concentration was the best predictor of adverse neurobehavioral
effects of lead exposure in all but one of the published prospective
studies''. The average blood Pb level for 1-5 year olds was
approximately 15 [mu]g/dL in the 1976-1980 NHANES. When in that age
range, some of the children included in the NHANES III dataset may
have had blood Pb levels comparable to those of the earlier NHANES.
The general issue regarding blood Pb metrics is further discussed in
subsequent text.
\51\ The associations with block design score were not
statistically significant for subgroups limited to blood Pb of <5
and <2.5 [mu]g/dL. The associations with digit span score were not
statistically significant for the blood Pb subgroups of <7.5 and
lower.
\52\ The associations with math and reading scores were not
statistically significant for the subgroup limited to blood Pb <2.5
[mu]/dL.
\53\ For example, for reading scores, effect estimates were -
0.99, -1.44, -1.53, -1.66, and -1.71 points per [mu]g/dL for all
children, the subgroup with blood Pb <10 [mu]g/dL, the subgroup with
blood Pb <7.5, the subgroup with blood Pb <5 and the subgroup with
blood Pb<2.5, respectively (Lanphear et al., 2000, Table 4).
\54\ The authors state ``Indeed, while the average effects of
lead exposure on reading scores were not significant for blood lead
concentrations less that 2.5 [mu]g/dL, the size of the effect and
the borderline significance level ([beta] = -1.71, p=0.07) suggests
that the smaller sample size and the imprecision of the relationship
of blood Pb concentration with performance on the reading subtest--
as indicated by the large standard error--may be the reason we did
not find a statistically significant association for children in
that range.''
\55\ We note that the datasets for each subgroup include
children for the lower blood Pb subgroups (in Table 4 of Lanphear et
al., 2000). For example, the dataset of children with blood Pb
levels <2.5 is a component of the dataset of children with blood Pb
levels <5 (Lanphear et al., 2000).
---------------------------------------------------------------------------
The epidemiological studies that have investigated blood Pb effects
on IQ (as discussed in the CD, Section 6.2.3) have considered a variety
of specific blood Pb metrics, including: (1) Blood concentration
``concurrent'' with the response assessment (e.g., at the time of IQ
testing), (2) average blood concentration over the ``lifetime'' of the
child at the time of response assessment (e.g., average of measurements
taken over child's first 6 or 7 years), (3) peak blood concentration
during a particular age range, and (4) early childhood blood
concentration (e.g., the mean of measurements between 6 and 24 months
age). With regard to the latter two, the Criteria Document (e.g., CD,
chapters 3 and 6) has noted that age has been observed to strongly
predict the period of peak exposure (around 18-27 months when there is
maximum hand-to-mouth activity). The CD further notes, this maximum
exposure period coincides with a period of time in which major events
are occurring in central nervous system (CNS) development (CD, p. 6-
60). Accordingly, the belief that the first few years of life are a
critical window of vulnerability is evident particularly in the earlier
literature (CD, p. 6-60). However, more recent analyses have found even
stronger associations between blood Pb at school age and IQ at school
age (i.e., concurrent blood Pb), indicating the important role that is
continued to be played by Pb exposures later in life. In fact,
concurrent and lifetime averaged measurements were stronger predictors
of adverse neurobehavioral effects (better than the peak or 24 month
metrics) in all but one of the prospective cohort studies (CD, pp. 6-61
to 6-62). While all four specific blood Pb metrics were correlated with
IQ in the international pooled analysis by Lanphear and others (2005),
the concurrent blood Pb level exhibited the strongest relationship with
intellectual deficits (CD, p. 6-29).
The Criteria Document presentation on toxicological evidence also
recognizes neurological effects elicited by exposures subsequent to
earliest childhood (CD, sections 5.3.5 and 5.3.7). For example,
research with monkeys has indicated that while exposure only during
infancy may elicit a response, exposures (with similar blood Pb levels)
that only occurred post-infancy also elicit responses. Further, in the
monkey research, exposures limited to post-infancy resulted in a
greater response than exposures limited to infancy (Rice and Gilbert,
1990; Rice, 1992).
A study by Chen and others (2005) involving 622 children has
attempted to directly address the question regarding periods of
enhanced susceptibility to Pb effects (CD, pp. 6-62 to 6-64).\56\ The
authors found that the concurrent blood
[[Page 29201]]
Pb association with IQ was always stronger than that for 24-month blood
Pb. As children aged, the relationship with concurrent blood Pb grew
stronger while that with 24-month blood Pb grew weaker. Further, in
models including both prior blood Pb (at 24-months age) and concurrent
blood Pb (at 7-years age), concurrent blood Pb was always more
predictive of IQ. In fact, concurrent blood Pb explained most of Pb-
related variation in IQ such that prior blood Pb (at 24-months age) was
rendered nonsignificant and nearly null.\57\ The effect estimate for
concurrent blood Pb was robust and remained significant, little changed
from its value without adjustment for 24-month blood Pb level. The
Criteria Document concluded the following regarding the results of this
study (CD, pp. 6-63 to 6-64).
---------------------------------------------------------------------------
\56\ In the children in this study, the mean blood Pb
concentration was 26.2 [mu]g/dL at age 2, 12.0 [mu]g/dL at age 5 and
8.0 [mu]g/dL at age 7 (Chen et al. 2005).
\57\ We note that blood Pb levels at any point in time are
influenced by current as well as past exposures, e.g., through
exchanges between blood and bone (as summarized in section II.B.1
above and discussed in more detail in the Criteria Document).
These results support the idea that Pb exposure continues to be
toxic to children as they reach school age, and do not lend support
to the interpretation that all the damage is done by the time the
child reaches 2 to 3 years of age. These findings also imply that
cross-sectional associations seen in children, such as the study
recently conducted by Lanphear et al. (2000) using data from NHANES
III, should not be dismissed. Chen et al. (2005) concluded that if
concurrent blood Pb remains important until school age for optimum
cognitive development, and if 6- and 7-year-olds are as or more
sensitive to Pb effects than 2-year-olds, then the difficulties in
preventing Pb exposure are magnified but the potential benefits of
---------------------------------------------------------------------------
prevention are greater.
In addition to findings of association with neurocognitive
decrement (including IQ) at study group mean blood Pb levels well below
10 [mu]g/dL, the evidence indicates that the slope for Pb effects on IQ
is steeper at lower blood Pb levels (CD, section 6.2.13). As stated in
the CD, ``the most compelling evidence for effects at blood Pb levels
<10 [mu]g/dL, as well as a nonlinear relationship between blood Pb
levels and IQ, comes from the international pooled analysis of seven
prospective cohort studies (n=1,333) by Lanphear et al. (2005)'' (CD,
pp. 6-67 and 8-37 and section 6.2.3.1.11).\58\ Using the full pooled
dataset with concurrent blood Pb level as the exposure metric and IQ as
the response from the pooled dataset of seven international studies,
Lanphear and others (2005) employed mathematical models of various
forms, including linear, cubic spline, log-linear, and piece-wise
linear, in their investigation of the blood Pb concentration-response
relationship (CD, p. 6-29; Lanphear et al., 2005). They observed that
the shape of the concentration-response relationship is nonlinear and
the log-linear model provides a better fit over the full range of blood
Pb measurements \59\ than a linear one (CD, p. 6-29 and pp. 6-67 to 6-
70; Lanphear et al., 2005). In addition, they found that no individual
study among the seven was responsible for the estimated nonlinear
relationship between Pb and deficits in IQ (CD p. 6-30). Others have
also analyzed the same dataset and similarly concluded that, across the
range of the dataset's blood Pb levels, a log-linear relationship was a
significantly better fit than the linear relationship (p=0.009) with
little evidence of residual confounding from included model variables
(CD, Section 6.2.13; Rothenberg and Rothenberg, 2005).
---------------------------------------------------------------------------
\58\ We note that a public comment submitted on March 19, 2008
on behalf of the Association of Battery Recyclers described concerns
the commenter had with the conclusion by Lanphear et al. (2005) of a
nonlinear relationship of blood Pb with IQ, citing a publication by
Surkan et al. (2007), a study published since the completion of the
Criteria Document, and the Tellez-Rojo et al. (2006) finding,
discussed in the Criteria Document, of two different slopes for
their study subgroups of young children with blood Pb levels below 5
[mu]g/d (n=193, for which the slope of -1.7 was statistically
significant, p=0.01) and those with blood Pb levels between 5 and 10
[mu]g/dL (n=101, for which the slope of -0.94 was not statistically
significant, p=0.12). The commenter also cites another publication
published since the completion of the Criteria Document, Jusko et
al. (2007) related to this issue. EPA notes that it is not basing
its proposed decisions on studies that are not included in the
Criteria Document.
\59\ The geometric mean of the concurrent blood Pb levels
modeled was 9.7 [mu]g/dL; the 5th and 95th percentile values were
2.5 and 33.2 [mu]g/dL, respectively (Lanphear et al., 2005).
---------------------------------------------------------------------------
The impact of the nonlinear slope is illustrated by the log-linear
model-based estimates of IQ decrements for similar changes in blood Pb
level at different absolute values of blood Pb level (Lanphear et al.,
2005). These estimates of IQ decrement are 3.9 (with 95% confidence
interval, CI, of 2.4-5.3), 1.9 (95% CI, 1.2-2.6) and 1.1 IQ points per
[mu]g/dL blood Pb (95% CI, 0.7-1.5), for increases in concurrent blood
Pb from 2.4 to 10 [mu]g/dL, 10 to 20 [mu]g/dL, and 20 to 30 [mu]g/dL,
respectively (Lanphear et al., 2005). For an increase in concurrent
blood Pb levels from <1 to 10 [mu]g/dL, the log-linear model estimates
a decline of 6.2 points in full scale IQ which is comparable to the 7.4
point decrement in IQ for an increase in lifetime mean blood Pb levels
up to 10 [mu]g/dL observed in the Rochester study (CD, pp. 6-30 to 6-
31).
A nonlinear blood Pb concentration-response relationship is also
suggested by several other analyses that have observed that each [mu]g/
dL increase in blood Pb may have a greater effect on IQ at lower blood
Pb levels (e.g., below 10 [mu]g/dL) than at higher levels (CD, pp. 8-63
to 8-64; Figure 8-7). As noted in the Criteria Document, while this may
at first seem at odds with certain fundamental toxicological concepts,
a number of examples of non- or supralinear dose-response relationships
exist in toxicology (CD, pp. 6-76 and 8-38 to 8-39). With regard to the
effects of Pb on neurodevelopmental outcome such as IQ, the CD suggests
that initial neurodevelopmental effects at lower Pb levels may be
disrupting very different biological mechanisms (e.g., early
developmental processes in the central nervous system) than more severe
effects of high exposures that result in symptomatic Pb poisoning and
frank mental retardation (CD, p. 6-76).
The Criteria Document describes this issue with regard to Pb as
follows (CD, p. 8-39).
In the case of Pb, this nonlinear dose-effect relationship
occurs in the pattern of glutamate release (Section 5.3.2), in the
capacity for long term potentiation (LTP; Section 5.3.3), and in
conditioned operant responses (Section 5.3.5). The 1986 Lead AQCD
also reported U-shaped dose-effect relationships for maze
performance, discrimination learning, auditory evoked potential, and
locomotor activity. Davis and Svendsgaard (1990) reviewed U-shaped
dose-response curves and their implications for Pb risk assessment.
An important implication is the uncertainty created in
identification of thresholds and ``no-observed-effect-levels''
(NOELS). As a nonlinear relationship is observed between IQ and low
blood Pb levels in humans, as well as in new toxicologic studies
wherein neurotransmitter release and LTP show this same
relationship, it is plausible that these nonlinear cognitive
outcomes may be due, in part, to nonlinear mechanisms underlying
these observed Pb neurotoxic effects.
More specifically, various findings within the toxicological
evidence presented in the Criteria Document provides biologic
plausibility for a steeper IQ loss at low blood levels, with a
potential explanation being that the predominant mechanism at very low
blood-Pb levels is rapidly saturated and that a different, less-
rapidly-saturated process, becomes predominant at blood-Pb levels
greater than 10 [mu]g/dL.\60\
---------------------------------------------------------------------------
\60\ The toxicological evidence presented in the Criteria
Document of biphasic dose-effect relationships includes: Suppression
of stimulated hippocampal glutamate release at low exposure levels
and induction of glutamate exocytosis at higher exposure levels (CD,
Section 5.3.2); downregulation of NMDA receptors at low blood Pb
levels and upregulation at higher levels (CD, section 5.3.2); Pb
causes elevated induction threshold and diminished magnitude of
long-term potentiation at low exposures, but not at higher exposures
(CD, section 5.3.3); and low-level Pb exposures increase fixed-
interval response rates and high-level Pb exposures decrease fixed
interval response rates in learning deficit testing in rats (CD,
section 5.3.5). Additional in vitro evidence includes Pb stimulation
of PKC activity at picomolar concentrations and inhibition of PKC
activity at nano- and micro-molar concentrations (CD, section
5.3.2).
---------------------------------------------------------------------------
[[Page 29202]]
In addition to the observed associations between neurocognitive
decrement (including IQ) and blood Pb at study group mean levels well
below 10 [mu]g/dL (described above), the current evidence includes
multiple studies that have examined the quantitative relationship
between IQ and blood Pb level in analyses of children with individual
blood Pb concentrations below 10 [mu]g/dL. In comparing across the
individual epidemiological studies and the international pooled
analysis, the Criteria Document observed that at higher blood Pb levels
(e.g., above 10 [mu]g/dL), the slopes (for change in IQ with blood Pb)
derived for log-linear and linear models are almost identical, and for
studies with lower blood Pb levels, the slopes appear to be steeper
than those observed in studies involving higher blood Pb levels (CD, p.
8-78, Figure 8-7). In making these observations, the Criteria Document
focused on the curves from the models from the 10th percentile to the
90th percentile saying that the ``curves are restricted to that range
because log-linear curves become very steep at the lower end of the
blood Pb levels, and this may be an artifact of the model chosen.''
The quantitative relationship between IQ and blood Pb level has
been examined in the Criteria Document using studies where all or the
majority of study subjects had blood Pb levels below 10 [mu]g/dL and
also where an analysis was performed on a subset of children whose
blood Pb levels have never exceeded 10 [mu]g/dL (CD, Table 6-1). The
datasets for three of these studies included concurrent blood Pb levels
above 10 [mu]g/dL; the C-R relationship reported for one of the three
was linear while it was log-linear for the other two. For the one of
these three studies with the linear C-R relationship, the highest blood
Pb level was just below 12 [mu]g/dL (Kordas et al., 2006). Of the two
studies with log-linear functions, one reported 69% of the children
with blood Pb levels below 10 [mu]g/dL and a population mean blood Pb
level of 7.44 [mu]g/dL (Al-Saleh et al., 2001), and the second reported
a population median blood Pb level of 9.7 [mu]g/dL and a 95th
percentile of 33.2 [mu]g/dL (Lanphear et al., 2005). In order to
compare slopes across all of these studies (linear and log-linear), EPA
estimated, for each, the average slope of change in IQ with change in
blood Pb between the 10th percentile \61\ blood Pb level and 10 [mu]g/
dL (CD, Table 6-1). The resultant group of reported and estimated
average linear slopes for IQ change with blood Pb levels up to 10
[mu]g/dL range from -0.4 to -1.8 IQ points per [mu]g/dL blood Pb (CD,
Tables 6-1 and 8-7), with a median of -0.9 IQ points per [mu]g/dL blood
Pb (CD, pp. 8-80).\62\
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\61\ In the Criteria Document analysis, the 10th percentile was
chosen as a common point of comparison for the loglinear (and
linear) models at a point prior to the lowest end of the blood Pb
levels.
\62\ Among this group of slopes (CD, Table 6-1) is that from the
analysis of the IQ-blood Pb (concurrent) relationship for children
whose peak blood Pb levels are below 10 [mu]g/dL in the
international pooled dataset studied by Lanphear and others (2005);
these authors reported this slope along with the companion slope for
blood Pb levels for the remaining children with peak blood Pb level
equal to or above 10 [mu]g/dL (Lanphear et al., 2005). In the
economic analysis for EPA's recent Lead Renovation, Repair and
Painting (RRP) Program rule (described above in section I.C),
changes in IQ loss as a function of changes in lifetime average
blood Pb level were estimated using the corresponding piecewise
model for lifetime average blood Pb derived from the pooled dataset
(USEPA, 2008; USEPA, 2007e). Selection of this model for the RRP
economic analysis reflects consideration of the distribution of
blood Pb levels in that analysis, those for children living in
houses with Pb-based paint. With consideration of these blood Pb
levels, the economic analysis document states that ``[s]electing a
model with a node, or changing one segment to the other, at a
lifetime average blood Pb concentration of 10 [mu]g/dL rather than
at 7.5 [mu]g/dL, is a small protection against applying an
incorrectly rapid change (steep slope with increasingly smaller
effect as concentrations lower) to the calculation''. We note that
the slope for the less-than-10-[mu]g/dL portion of the model used in
the RRP analysis (-0.88) is similar to the median for the slopes
included in the Criteria Document analysis of quantitative
relationships for distributions of blood Pb levels extending from
just below 10 [mu]g/dL and lower.
---------------------------------------------------------------------------
Among this group of quantitative IQ-blood Pb relationships examined
in the Criteria Document (CD, Tables 6-1 and 8-7), the steepest slopes
for change in IQ with change in blood Pb level are those derived for
the subsets of children in the Rochester and Boston cohorts for which
peak blood Pb levels were <10 [mu]g/dL; these slopes, in terms of IQ
points per [mu]g/dL blood Pb, are -1.8 (for concurrent blood Pb
influence on IQ) and -1.6 (for 24-month blood Pb influence on IQ),
respectively. The mean blood Pb levels for children in these subsets of
the Rochester and Boston cohorts are 3.32 and 3.8 [mu]g/dL,
respectively, which are the lowest population mean levels among the
datasets included in the table (Canfield, 2008; Bellinger, 2008). Other
studies with analyses involving similarly low blood Pb levels (e.g.,
mean levels below 4 [mu]g/dL) also had slopes steeper than -1.5 points
per [mu]g/dL blood Pb. These include the slope of -1.71 points per
[mu]g/dL blood Pb \63\ for the subset of 24-month-old children in the
Mexico City cohort with blood Pb levels less than 5 [mu]g/dL (n=193),
for which the mean concurrent blood Pb level was 2.9 [mu]g/dL (Tellez-
Rojo et al. 2006, 2008) \64\ and also the slope of -2.94 points per
[mu]g/dL blood Pb for the subset of 6-10-year-old children whose peak
blood Pb levels never exceeded 7.5 [mu]g/dL (n=112), and for which the
mean concurrent blood Pb level was 3.24 [mu]g/dL (Lanphear et al. 2005;
Hornung 2008). Thus, from these subset analyses, the slopes range from
-1.71 to -2.94 IQ points per [mu]g/dL of concurrent blood Pb. We also
note that the nonlinear C-R function in which greatest confidence is
placed in estimating IQ loss in the quantitative risk assessment
(described below in section II.C) has a slope that falls
[[Page 29203]]
intermediate between these two for blood Pb levels up to approximately
3.7 [mu]g/dL (USEPA, 2007b).
---------------------------------------------------------------------------
\63\ This slope reflects effects on cognitive development in
this cohort of 24-month-old children based on the age-appropriate
test described earlier, and is similar in magnitude to slopes for
the cohorts of older children described here. The strengths and
limitations of this age-appropriate text, the Mental Development
Index (MDI) of the Bayley Scales of Infant Development (BSID), were
discussed in a letter to the editor by Black and Baqui (2005). The
authors state that ``the MDI is a well-standardized,
psychometrically strong measure of infant mental development.'' The
MDI represents a complex integration of empirically-derived
cognitive skills, for example, sensory/perceptual acuities,
discriminations, and response; acquisition of object constancy;
memory learning and problem solving; vocalization and beginning of
verbal communication; and basis of abstract thinking. Black and
Baqui state that although the MDI is one of the most well-
standardized, widely used assessment of infant mental development,
evidence indicates low predictive validity of the MDI for infants
younger than 24 months to subsequent measures of intelligence. They
explain that the lack of continuity may be partially explained by
``the multidimensional and rapidly changing aspects of infant mental
development and by variations in performance during infancy,
variations in tasks used to measure intellectual functioning
throughout childhood, and variations in environmental challenges and
opportunities that may influence development.'' Martin and Volkmar
(2007) also noted that correlations between BSID performance and
subsequent IQ assessments were variable, but they also reported high
test-retest reliability and validity, as indicated by the
correlation coefficients of 0.83 to 0.91, as well as high interrater
reliability, correlation coefficient of 0.96, for the MDI.
Therefore, the BSID has been found to be a reliable indicator of
current development and cognitive functioning of the infant. Martin
and Volkmar (2007) further note that ``for the most part,
performance on the BSID does not consistently predict later
cognitive measures, particularly when socioeconomic status and level
of functioning are controlled''.
\64\ In this study, the slope for blood Pb levels between 5 and
10 [mu]g/dL (population mean blood Pb of 6.9 [mu]g/dL; n=101) was -
0.94 points per [mu]g/dL blood Pb but was not statistically
significant, with a P value of 0.12. The difference in the slope
between the <5 [mu]g/dL and the 5-10 [mu]g/dL groups was not
statistically significant (Tellez-Rojo et al., 2006; Tellez-Rojo,
2008).
---------------------------------------------------------------------------
The C-R functions discussed above are presented in two sets in
Table 1 below.
Table 1. Summary of Quantitative Relationships of IQ and Blood Pb for Two Sets of Studies Discussed Above
--------------------------------------------------------------------------------------------------------------------------------------------------------
Form of model Average
Range BLL ([mu]g/ Geometric mean BLL from which linear slope
Study/Analysis Study cohort Analysis dataset N dL) 5th-95th ([mu]g/dL) average slope \A\ (points
percentile] derived per [mu]g/dL)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Set of studies from which steeper slopes are drawn
--------------------------------------------------------------------------------------------------------------------------------------------------------
Tellez-Rojo <5 subgroup based Mexico City, age Children--BLL<5 193 0.8-4.9........... 2.9............... Linear.......... -1.71
on Lanphear et al. 2005,\B\ 24 mo. [mu]g/dL.
Log-linear with low-exposure
linearization (LLL) \B\.
Dataset from which the log-linear function is derived is the pooled International LLL\C\.......... -2.29 at 2
dataset of 1333 children, age 6-10 yr, having median blood Pb of 9.7 [mu]g/dL and 5th- [mu]g/dL\C\
95th percentile of 2.5-33.2 [mu]g/dL.Slope presented here is the slope at a blood Pb
level of 2 [mu]g/dL.\C\
Lanphear et al. 2005,\B\ <7.5 Pooled Children--peak 103 [1.3-6.0]......... 3.24.............. Linear.......... -2.94
peak subgroup. International, BLL <7.5 [mu]g/
age 6-10 yr. dL.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Set of studies with shallower slopes (Criteria Document, Table 6-1) \D\
--------------------------------------------------------------------------------------------------------------------------------------------------------
Canfield et al. 2003 \B\, <10 Rochester, age 5 Children--peak 71 Unspecified....... 3.32.............. Linear.......... -1.79
peak subgroup. yr. BLL <10 [mu]g/dL.
Bellinger and Needleman Boston\A\ \E\.... Children--peak 48 1-9.3\E\.......... 3.8\E\............ Linear.......... -1.56
2003\B\. BLL <10 [mu]g/dL.
Tellez-Rojo et al. 2006....... Mexico City, age Full dataset..... 294 0.8-<10........... 4.28.............. Linear.......... -1.04
24 mo.
Tellez-Rojo et al. 2006 full-- Mexico City, age Full dataset..... 294 0.8-<10........... 4.28.............. Log-linear...... -0.94
loglinear. 24 mo.
Lanphear et al. 2005,\B\ <10 Pooled Children--peak 244 [1.4-8.0]......... 4.30.............. Linear.......... -0.80
peak\F\ subgroup. International, BLL <10 [mu]g/dL.
age 6-10 yr.
Al-Saleh et al. 2001 full-- Saudi Arabia, age Full dataset..... 533 2.3-27.36\G\...... 7.44.............. Log-linear...... -0.76
loglinear. 6-12 yr.
Kordas et al. 2006, <12 Torreon, Mexico, Children--BLL<12 377 2.3-<12........... 7.9............... Linear.......... -0.40
subgroup. age 7 yr. [mu]g/dL.
Lanphear et al. 2005\B\ full-- Pooled Full dataset..... 1333 [2.5-33.2]........ 9.7 (median)...... Log-linear...... -0.41
loglinear. International,
age 6-10 yr.
Median value.... -0.9 \D\
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Average slope for change in IQ from 10th percentile to 10 [mu]g/dL Slope estimates here are for relationship between IQ and concurrent blood Pb
levels (BLL), except for Bellinger & Needleman which used 24 month BLLs with 10 year old IQ.
\B\ The Lanphear et al. 2005 pooled International study includes blood Pb data from the Rochester and Boston cohorts, although for different ages (6 and
5 years, respectively) than the ages analyzed in Canfield et al. 2003 and Bellinger and Needleman 2003.
\C\ The LLL function (described in section II.C.2.b) was developed from Lanphear et al. 2005 loglinear model with a linearization of the slope at BLL
below 1 [mu]g/dL. The slope shown is that at 2 [mu]g/dL. In estimating IQ loss with this function in the risk assessment (section II.C) and in the
evidence-based considerations in section II.E.3, the nonlinear form of the model was used, with varying slope for all BLL above 1 [mu]g/dL.
\D\ These studies and quantitative relationships are discussed in the Criteria Document (CD, sections 6.2, 6.2.1.3 and 8.6.2).
\E\ The BLL for Bellinger and Needleman (2003) are for age 24 months.
\F\ As referenced above and in section II.C.2.b, the form of this function derived for lifetime average blood Pb was used in the economic analysis for
the RRP rule. The slope for that function was -0.88 IQ points per [mu]g/dL lifetime averaged blood Pb.
\G\ 69% of children in Al-Saleh et al. (2001) study had BLL<10 [mu]g/dL.
[[Page 29204]]
3. Lead-Related Impacts on Public Health
In addition to the advances in our knowledge and understanding of
Pb health effects at lower exposures (e.g., using blood Pb as the
index), there has been some change with regard to the U.S. population
Pb burden since the time of the last Pb NAAQS review. For example, the
geometric mean blood Pb level for U.S. children aged 1-5, as estimated
by the U.S. Centers for Disease Control, declined from 2.7 [mu]g/dL
(95% CI: 2.5-3.0) in the 1991-1994 survey period to 1.7 [mu]g/dL (95%
CI: 1.55-1.87) in the 2001-2002 survey period (CD, Section 4.3.1.3) and
1.8 [mu]g/dL in the 2003-2004 survey period (Axelrad, 2008).\65\ Blood
Pb levels have also declined in the U.S. adult population over this
time period (CD, Section 4.3.1.3).\66\ As noted in the Criteria
Document, ``blood-Pb levels have been declining at differential rates
for various general subpopulations, as a function of income, race, and
certain other demographic indicators such as age of housing'' (CD, pp.
8-21). For example, the geometric mean blood Pb level for children
(aged one to five) living in poverty in the 2003-2004 survey period is
2.4 [mu]g/dL. For black, non-Hispanic children, the geometric mean is
2.7 [mu]g/dL, and for the subset of this group that is living in
poverty, the geometric mean is 3.1 [mu]g/dL. Further, the 95th
percentile blood Pb level in the 2003-2004 NHANES for children aged 1-5
of all races and ethnic groups is 5.1 [mu]g/dL, while the corresponding
level for the subset of children living below the poverty level is 6.6
[mu]g/dL. The 95th percentile level for black, non-Hispanic children is
8.9 [mu]g/dL, and for the subset of that group living below the poverty
level, it is 10.5 [mu]g/dL (Axelrad, 2008).\67\
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\65\ These levels are in contrast to the geometric mean blood Pb
level of 14.9 [mu]g/dL reported for U.S. children (aged 6 months to
5 years) in 1976-1980 (CD, Section 4.3.1.3).
\66\ For example, NHANES data for older adults (60 years of age
and older) indicate a decline in overall population geometric mean
blood Pb level from 3.4 [mu]g/dL in 1991-1994 to 2.2 [mu]g/dL in
1999-2002; the trend for adults between 20 and 60 years of age is
similar to that for children 1 to 5 years of age (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5420a5.htm).
\67\ Although the 90th percentile statistic for these subgroups
is not currently available for the 2003-04 survey period, the 2001-
2004 90th percentile blood Pb level for children aged 1-5 of all
races and ethnic groups is 4.0 [mu]g/dL, while the corresponding
level for the subset of children living below the poverty level is
5.4 [mu]g/dL, and that level for black, non-Hispanic children living
below the poverty level is 7.7 [mu]g/dL (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm--then click on
``Download a universal spreadsheet file of the Body Burdens data
tables'').
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a. At-Risk Subpopulations
Potentially at-risk subpopulations include those with increased
susceptibility (i.e., physiological factors contributing to a greater
response for the same exposure) and those with increased exposure
(including that resulting from behavior leading to increased contact
with contaminated media) (USEPA 1986a, pp. 1-154). A behavioral factor
of great impact on Pb exposure is the incidence of hand-to-mouth
activity that is prevalent in very young children (CD, Section 4.4.3).
Physiological factors include both conditions contributing to a
subgroup's increased risk of effects at a given blood Pb level, and
those that contribute to blood Pb levels higher than those otherwise
associated with a given Pb exposure (CD, Section 8.5.3). These factors
include nutritional status (e.g., iron deficiency, calcium intake), as
well as genetic and other factors (CD, chapter 4 and sections 3.4,
5.3.7 and 8.5.3).
We also considered evidence pertaining to vulnerability to
pollution-related effects which additionally encompasses situations of
elevated exposure, such as residing in older housing with Pb-containing
paint or near sources of ambient Pb, as well as socioeconomic factors,
such as reduced access to health care or low socioeconomic status (SES)
(USEPA, 2003, 2005c) that can contribute to increased risk of adverse
health effects from Pb. With regard to elevated exposures in particular
socioeconomic and minority subpopulations, we observe notably higher
blood Pb levels in children in poverty and in black, non-Hispanic
children compared to those for more economically well-off children and
white children, in general (as recognized in section II.B.1.b above).
Three particular physiological factors contributing to increased
risk of Pb effects at a given blood Pb level are recognized in the
Criteria Document (e.g., CD, Section 8.5.3): age, health status, and
genetic composition. With regard to age, the susceptibility of young
children to the neurodevelopmental effects of Pb is well recognized
(e.g., CD, Sections 5.3, 6.2, 8.4, 8.5, 8.6.2), although the specific
ages of vulnerability have not been established (CD, pp. 6-60 to 6-64).
Early childhood may also be a time of increased susceptibility for Pb
immunotoxicity (CD, Sections 5.9.10, 6.8.3 and 8.4.6). Further early
life exposures have been associated with increased risk of
cardiovascular effects in humans later in life (CD, pp. 8-74). Early
life exposures have also been associated with increased risk, in
animals, of neurodegenerative effects later in life (CD, pp. 8-74).\68\
Health status is another physiological factor in that subpopulations
with pre-existing health conditions may be more susceptible (as
compared to the general population) for particular Pb-associated
effects, with this being most clear for renal and cardiovascular
outcomes. For example, African Americans as a group have a higher
frequency of hypertension than the general population or other ethnic
groups (NCHS, 2005), and as a result may face a greater risk of adverse
health impact from Pb-associated cardiovascular effects. A third
physiological factor relates to genetic polymorphisms. That is,
subpopulations defined by particular genetic polymorphisms (e.g.,
presence of the [delta]-aminolevulinic acid dehydratase-2 [ALAD-2]
allele) have also been recognized as sensitive to Pb toxicity, which
may be due to increased susceptibility to the same internal dose and/or
to increased internal dose associated with the same exposure (CD, pp.
8-71, Sections 6.3.5, 6.4.7.3 and 6.3.6).
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\68\ Specifically, among young adults who lived as children in
an area heavily polluted by a smelter and whose current Pb exposure
was low, higher bone Pb levels were associated with higher systolic
and diastolic blood pressure (CD, pp. 8-74). In adult rats, greater
early exposures to Pb are associated with increased levels of
amyloid protein precursor, a marker of risk for neurodegenerative
disease (CD, pp. 8-74).
---------------------------------------------------------------------------
Childhood is well recognized as a time of increased susceptibility,
and as summarized in section II.B.2.b above and described in more
detail in the Criteria Document, a large body of epidemiological
evidence describes neurological effects on children at low blood Pb
levels. The toxicological evidence further helps inform an
understanding of specific periods of development with increased
vulnerability to specific types of neurological effect (CD, Section
5.3). Additionally, the toxicological evidence of a differing
sensitivity of the immune system to Pb across and within different
periods of life stages indicates the potential importance of exposures
of duration as short as weeks to months. For example, the animal
studies suggest that, for immune effects, the gestation period is the
most sensitive life stage followed by early neonatal stage, and that
within these life stages, critical windows of vulnerability are likely
to exist (CD, Section 5.9 and p. 5-245).
In summary, there are a variety of ways in which Pb exposed
populations might be characterized and stratified for consideration of
public health impacts. Age or lifestage was used to distinguish
[[Page 29205]]
potential groups on which to focus the quantitative risk assessment
because of its influence on exposure and susceptibility. Young children
were selected as the priority population for the risk assessment in
consideration of the health effects evidence regarding endpoints of
greatest public health concern. The Criteria Document recognizes,
however, other population subgroups as described above may also be at
risk of Pb-related health effects of public health concern.
b. Potential Public Health Impacts
As discussed in the Criteria Document, there are potential public
health implications of low-level Pb exposure, indexed by blood Pb
levels, associated with several health endpoints identified in the
Criteria Document (CD, Section 8.6).\69\ These include potential
impacts on population IQ, which is the focus of the quantitative risk
assessment conducted for this review, as well as heart disease and
chronic kidney disease, which are not included in the quantitative risk
assessment (CD, Sections 8.6, 8.6.2, 8.6.3 and 8.6.4). It is noted that
there is greater uncertainty associated with effects at the lower
levels of blood Pb, and that there are differing weights of evidence
across the effects observed.\70\ With regard to potential implications
of Pb effects on IQ, the Criteria Document recognizes the ``critical''
distinction between population and individual risk, noting that a
``point estimate indicating a modest mean change on a health index at
the individual level can have substantial implications at the
population level'' (CD, p. 8-77).\71\ A downward shift in the mean IQ
value is associated with both substantial decreases in percentages
achieving very high scores and substantial increases in the percentage
of individuals achieving very low scores (CD, p. 8-81).\72\ For an
individual functioning in the low IQ range due to the influence of
developmental risk factors other than Pb, a Pb-associated IQ decline of
several points might be sufficient to drop that individual into the
range associated with increased risk of educational, vocational, and
social handicap (CD, p. 8-77).
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\69\ The differing evidence and associated strength of the
evidence for these different effects is described in detail in the
Criteria Document.
\70\ As is described in Section II.C.2.a, CASAC, in their
comments on the analysis plan for the risk assessment described in
this notice, placed higher priority on modeling the child IQ metric
than the adult endpoints (e.g., cardiovascular effects).
\71\ Similarly, ``although an increase of a few mmHg in blood
pressure might not be of concern for an individual's well-being, the
same increase in the population mean might be associated with
substantial increases in the percentages of individuals with values
that are sufficiently extreme that they exceed the criteria used to
diagnose hypertension'' (CD, p. 8-77).
\72\ For example, for a population mean IQ of 100 (and standard
deviation of 15), 2.3% of the population would score above 130, but
a shift of the population to a mean of 95 results in only 0.99% of
the population scoring above 130 (CD, pp. 8-81 to 8-82).
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The magnitude of a public health impact is dependent upon the size
of population affected and type or severity of the effect. As
summarized above, there are several population groups that may be
susceptible or vulnerable to effects associated with exposure to Pb,
including young children, particularly those in families of low SES
(CD, p. E-15), as well as individuals with hypertension, diabetes, and
chronic renal insufficiency (CD, p. 8-72). Although comprehensive
estimates of the size of these groups residing in proximity to sources
of ambient Pb have not been developed, total estimates of these
population subpopulations within the U.S. are substantial (as noted in
Table 3-3 of the Staff Paper).\73\
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\73\ For example, approximately 4.8 million children live in
poverty, while the estimates of numbers of adults with hypertension,
diabetes or chronic kidney disease are on the order of 20 to 50
million (see Table 3-3 of Staff Paper).
---------------------------------------------------------------------------
With regard to estimates of the size of potentially vulnerable
subpopulations living in areas of increased exposure related to ambient
Pb, the information is still more limited. The limited information
available on air and surface soil concentrations of Pb indicates
elevated concentrations near stationary sources as compared with areas
remote from such sources (CD, Sections 3.2.2 and 3.8). Air quality
analyses (presented in Chapter 2 of the Staff Paper) indicate
dramatically higher Pb concentrations at monitors near sources as
compared with those more remote. As described in Section 2.3.2.1 of the
Staff Paper, however, since the 1980s the number of Pb monitors has
been significantly reduced by states (with EPA guidance that monitors
well below the current NAAQS could be shut down) and a lack of monitors
near some large sources may lead to underestimates of the extent of
occurrences of relatively higher Pb concentrations. The significant
limitations of our monitoring and emissions information constrain our
efforts to characterize the size of at-risk populations in areas
influenced by sources of ambient Pb. For example, the limited size and
spatial coverage of the current Pb monitoring network constrains our
ability to characterize current levels of airborne Pb in the U.S.
Further, as noted above in section II.A.1, the Staff Paper review of
the available information on emissions and locations of sources (as
described in section 2.3.2.1 of the Staff Paper) indicates that the
network is inconsistent in its coverage of the largest sources
identified in the 2002 National Emissions Inventory (NEI). The most
recent analysis of monitors near sources greater than 1 ton per year
(tpy) indicates that less than 15% of stationary sources with emissions
greater than or equal to 1 tpy have a monitor within one mile.
Additionally, there are various uncertainties and limitations
associated with source information in the NEI (as described in section
2.2.5 of the Staff Paper; USEPA, 2007c).
In recognition of the significant limitations associated with the
currently available information on Pb emissions and airborne
concentrations in the U.S. and the associated exposure of potentially
at-risk populations, Chapter 2 of the Staff Paper summarizes the
information in several different ways. For example, analyses of the
current monitoring network indicated the numbers of monitoring sites
that would exceed alternate standard levels, taking into consideration
different statistical forms. These analyses are also summarized with
regard to population size in counties home to those monitoring sites
(as presented in Appendix 5.A of the Staff Paper). Information for the
monitors and from the NEI indicates a range of source sizes in
proximity to monitors at which various levels of Pb are reported.
Together this information suggests that there is variety in the
magnitude of Pb emissions from sources that could influence air Pb
concentrations. Identifying specific emissions levels of sources
expected to result in air Pb concentrations of interest, however, would
be informed by a comprehensive analysis using detailed source
characterization information, which was not feasible within the time
and data constraints of this review. Instead, we have developed a
summary of the emissions and demographic information for Pb sources
that includes estimates of the numbers of people residing in counties
in which the aggregate Pb emissions from NEI sources is greater than or
equal to 0.1 tpy or in counties in which the aggregate Pb emissions is
greater than or equal to 0.1 tpy per 1000 square miles (as presented in
Tables 3-4 and 3-5, respectively, in the Staff Paper).
Additionally, the potential for resuspension of recently and
historically deposited Pb near roadways to contribute to increased
risks of Pb exposure to populations residing nearby is suggested in the
Criteria Document (e.g., CD, pp. 2-62 and 3-32).
[[Page 29206]]
4. Key Observations
The following key observations are based on the available health
effects evidence and the evaluation and interpretation of that evidence
in the Criteria Document.
Lead exposures occur both by inhalation and by ingestion
(CD, Chapter 3). As stated in the Criteria Document, ``given the large
amount of time people spend indoors, exposure to Pb in dusts and indoor
air can be significant'' (CD, p. 3-27).
Children, in general and especially those of low SES, are
at increased risk for Pb exposure and Pb-induced adverse health
effects. This is due to several factors, including enhanced exposure to
Pb via ingestion of soil Pb and/or dust Pb due to normal childhood
hand-to-mouth activity (CD, p. E-15, Chapter 3 and Section 6.2.1).
Once inhaled or ingested, Pb is distributed by the blood,
with long-term storage accumulation in the bone. Bone Pb levels provide
a strong measure of cumulative exposure which has been associated with
many of the effects summarized below, although difficulty of sample
collection has precluded widespread use in epidemiological studies to
date (CD, Chapter 4).
Blood levels of Pb are well accepted as an index of
exposure (or exposure metric) for which associations with the key
effects (see below) have been observed. In general, associations with
blood Pb are most robust for those effects for which past exposure
history poses less of a complicating factor, i.e., for effects during
childhood (CD, Section 4.3).
Both epidemiological and toxicologic studies have shown
that environmentally relevant levels of Pb affect many different organ
systems (CD, p. E-8). With regard to the most important such effects
observed in children and adults, the Criteria Document states (CD, p.
8-60) that ``neurotoxic effects in children and cardiovascular effects
in adults are among those best substantiated as occurring at blood-Pb
concentrations as low as 5 to 10 [mu]g/dL (or possibly lower); and
these categories of effects are currently clearly of greatest public
health concern. Other newly demonstrated immune and renal system
effects among general population groups are also emerging as low-level
Pb-exposure effects of potential public health concern.''
Many associations of health effects with Pb exposure have
been found at levels of blood Pb that are currently relevant for the
U.S. population, with individual children having blood Pb levels of 5-
10 [mu]g/dL and lower, being at risk for neurological effects (as
described in the subsequent bullet). Supportive evidence from
toxicological studies provides biological plausibility for the observed
effects. (CD, Chapters 5, 6 and 8)
Pb exposure is associated with a variety of neurological
effects in children, notably intellectual attainment and school
performance. Both qualitative and quantitative evidence, with further
support from animal research, indicates a robust and consistent effect
of Pb exposure on neurocognitive ability at mean concurrent blood Pb
levels in the range of 5 to 10 [mu]g/dL. Specific epidemiological
analyses have further indicated association with neurocognitive effects
in analyses restricted to children with individual blood Pb levels
below 5-10 [mu]g/dL, and for which group mean levels are lower.
Further, ``[s]ome newly available analyses appear to show Pb effects on
the intellectual attainment of preschool and school age children at
population mean concurrent blood-Pb levels ranging down to as low as 2
to 8 [mu]g/dL'' (CD, p. E-9; Sections 5.3, 6.2, 8.4.2 and 6.10).
Deficits in cognitive skills may have long-term
consequences over a lifetime. Poor academic skills and achievement can
have enduring and important effects on objective parameters of success
in life as well as increased risk of antisocial and delinquent
behavior. (CD, Sections 6.1 and 8.4.2)
The current epidemiological evidence indicates a steeper
slope of the blood Pb concentration-response relationship at lower
blood Pb levels, particularly those below 10 [mu]g/dL (CD, Sections
6.2.13 and 8.6).
At mean blood Pb levels, in children, on the order of 10
[mu]g/dL, and somewhat lower, associations have been found with effects
to the immune system, including altered macrophage activation,
increased IgE levels and associated increased risk for autoimmunity and
asthma (CD, Sections 5.9, 6.8, and 8.4.6).
In adults, with regard to cardiovascular outcomes, the
Criteria Document included the following summary (CD, p. E-10).
Epidemiological studies have consistently demonstrated
associations between Pb exposure and enhanced risk of deleterious
cardiovascular outcomes, including increased blood pressure and
incidence of hypertension.\74\ A meta-analysis of numerous studies
estimates that a doubling of blood-Pb level (e.g., from 5 to 10
[mu]g/dL) is associated with ~1.0 mm Hg increase in systolic blood
pressure and ~0.6 mm Hg increase in diastolic pressure. Studies have
also found that cumulative past Pb exposure ( e.g., bone Pb) may be
as important, if not more, than present Pb exposure in assessing
cardiovascular effects. The evidence for an association of Pb with
cardiovascular morbidity and mortality is limited but supportive.
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\74\ The Criteria Document states that ``While several studies
have demonstrated a positive correlation between blood pressure and
blood Pb concentration, others have failed to show such association
when controlling for confounding factors such as tobacco smoking,
exercise, body weight, alcohol consumption, and socioeconomic
status. Thus, the studies that have employed blood Pb level as an
index of exposure have shown a relatively weak association with
blood pressure. In contrast, the majority of the more recent studies
employing bone Pb level have found a strong association between
long-term Pb exposure and arterial pressure (Chapter 6). Since the
residence time of Pb in the blood is relatively short but very long
in the bone, the latter observations have provided rather compelling
evidence for a positive relationship between Pb exposure and a
subsequent rise in arterial pressure'' (CD, pp. 5-102 to 5-103).
Further, in consideration of the meta-analysis also described here,
the Criteria Document stated that ``The meta-analysis provides
strong evidence for an association between increased blood Pb and
increased blood pressure over a wide range of populations'' (CD, p.
6-130) and ``the meta-analyses results suggest that studies not
detecting an effect may be due to small sample sizes or other
factors affecting precision of estimation of the exposure effect
relationship'' (CD, p. 6-133).
Studies of nationally representative U.S. samples observed associations
between blood Pb levels and increased systolic blood pressure at
population mean blood Pb levels less than 5 [mu]g/dL, particularly
among African Americans (CD, Section 6.5.2). With regard to gender
---------------------------------------------------------------------------
differences, the Criteria Document states the following (CD, p. 6-154).
Although females often show lower Pb coefficients than males,
and Blacks higher Pb coefficients than Whites, where these
differences have been formally tested, they are usually not
statistically significant. The tendencies may well arise in the
differential Pb exposure in these strata, lower in women than in
men, higher in Blacks than in Whites. The same sex and race
differential is found with blood pressure.
Animal evidence provides confirmation of Pb effects on cardiovascular
functions (CD, Sections 5.5, 6.5, 8.4.3 and 8.6.3).
Renal effects, evidenced by reduced renal filtration, have
also been associated with Pb exposures indexed by bone Pb levels and
also with mean blood Pb levels in the range of 5 to 10 [mu]g/dL in the
general adult population, with the potential adverse impact of such
effects being enhanced for susceptible subpopulations including those
with diabetes, hypertension, and chronic renal insufficiency (CD,
Sections 6.4, 8.4.5, and 8.6.4). The full significance of this effect
is unclear,
[[Page 29207]]
given that other evidence of more marked signs of renal dysfunction
have not been detected at blood Pb levels below 30-40 [mu]g/dL in large
studies of occupationally exposed Pb workers (CD, pp. 6-270 and 8-
50).\75\
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\75\ In the general population, both cumulative and circulating
Pb has been found to be associated with longitudinal decline in
renal functions. In the large NHANES III study, alterations in
urinary creatinine excretion rate (one indicator of possible renal
dysfunction) were observed in hypertensives at a mean blood Pb of
only 4.2 [mu]g/dL. These results provide suggestive evidence that
the kidney may well be a target organ for effects from Pb in adults
at current U.S. environmental exposure levels. The magnitude of the
effect of Pb on renal function ranged from 0.2 to -1.8 mL/min change
in creatinine clearance per 1.0 [mu]g/dL increase in blood Pb in
general population studies. However, the full significance of this
effect is unclear, given that other evidence of more marked signs of
renal dysfunction have not been detected at blood Pb levels below
30-40 [mu]g/dL among thousands of occupationally exposed Pb workers
that have been studied (CD, p. 6-270).
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Other Pb associated effects in adults occurring at or just
above 10 [mu]g/dL include hematological (e.g., impact on heme synthesis
pathway) and neurological effects, with animal evidence providing
support of Pb effects on these systems and evidence regarding mechanism
of action (CD, Sections 5.2, 5.3, 6.3 and 6.9.2).
C. Human Exposure and Health Risk Assessments
This section presents a brief summary of the human exposure and
health risk assessments conducted by EPA for this review. The complete
full-scale assessment, which includes specific analyses conducted to
address CASAC comments and advice on an earlier draft assessment, is
presented in the final Risk Assessment Report (USEPA, 2007b).
The focus of this Pb NAAQS risk assessment is on characterizing
risk resulting from exposure to policy-relevant Pb (i.e., exposure to
Pb that has passed through ambient air on its path from source to human
exposure--as described in section II.A.2). The design and
implementation of this assessment needed to address significant
limitations and complexity that go far beyond the situation for similar
assessments typically performed for other criteria pollutants. Not only
was the risk assessment constrained by the timeframe allowed for this
review in the context of breadth of information to address, it was also
constrained by significant limitations in data and modeling tools for
the assessment, as discussed further in section II.C.2.h below.
Furthermore, the multimedia and persistent nature of Pb, and the role
of multiple exposure pathways (discussed in section II.A), add
significant complexity to the assessment as compared to other
assessments that focus only on the inhalation pathway. The impact of
this on our estimates for air-related exposure pathways is discussed in
section II.C.2.e.
The remainder of this overview of the human health risk assessment
is organized as follows. An overview of the human health risk
assessment completed in the last review of the Pb NAAQS in 1990 (USEPA,
1990a) is presented first. Next, design aspects of the current risk
assessment are presented, including: (a) CASAC advice regarding the
design of the risk assessment, (b) description of health endpoints and
associated risk metrics modeled, including the concentration-response
functions used, (c) overview of the case study approach employed, (d)
description of air quality scenarios modeled, (e) explanation of air-
related versus background classification of risk results in the context
of this analysis, (f) overview of analytical (modeling) steps completed
for the risk assessment and (g) description of the multiple sets of
risk results generated for the analysis. Then, key sources of
uncertainty associated with the analysis are presented. And finally, a
summary of exposure and risk estimates and key observations is
presented.
1. Overview of Risk Assessment From Last Review
The risk assessment conducted in support of the last review used a
case study approach to compare air quality scenarios in terms of their
impact on the percentage of modeled populations that exceeded specific
blood Pb levels chosen with consideration of the health effects
evidence at that time (USEPA, 1990b; USEPA, 1989). The case studies in
that analysis, however, focused exclusively on Pb smelters including
two secondary and one primary smelter and did not consider exposures in
a more general urban context. The analysis focused on children (birth
through 7 years of age) and middle-aged men. The assessment evaluated
impacts of alternate NAAQS on numbers of children and men with blood Pb
levels above levels of concern based on health effects evidence at that
time. The primary difference between the risk assessment approach used
in the current analysis and the assessment completed in 1990 involves
the risk metric employed. Rather than estimating the percentage of
study populations with exposures above blood Pb levels of interest as
was done in the last review (i.e., 10, 12 and 15 [mu]g/dL), the current
analysis estimates changes in health risk, specifically IQ loss,
associated with Pb exposure for child populations at each of the case
study locations with that estimated IQ loss further differentiated
between air-related and background Pb exposure categories.
2. Design Aspects of Exposure and Risk Assessments
This section provides an overview of key elements of the assessment
design, inputs, and methods, and includes identification of key
uncertainties and limitations.
a. CASAC Advice
The CASAC conducted a consultation on the draft analysis plan for
the risk assessment (USEPA, 2006c) in June, 2006 (Henderson, 2006).
Some key comments provided by CASAC members on the plan included: (1)
Placing a higher priority on modeling the child IQ metric than the
adult endpoints (e.g., cardiovascular effects), (2) recognizing the
importance of indoor dust loading by Pb contained in outdoor air as a
factor in Pb-related exposure and risk for sources considered in this
analysis, and (3) concurring with use of the IEUBK biokinetic blood Pb
model. Taking these comments into account, a pilot phase assessment was
conducted to test the risk assessment methodology being developed for
the subsequent full-scale assessment. The pilot phase assessment is
described in the first draft Staff Paper and accompanying technical
report (ICF 2006), which was discussed by the CASAC Pb panel on
February 6-7 (Henderson, 2007a).
Results from the pilot assessment, together with comments received
from CASAC and the public, informed the design of the full-scale
analysis. The full-scale analysis included a substitution of a more
generalized urban case study for the location-specific near-roadway
case study evaluated in the pilot. In addition, a number of changes
were made in the exposure and risk assessment approaches, including the
development of a new indoor dust Pb model focused specifically on urban
residential locations and specification of additional IQ loss
concentration-response (C-R) functions to provide greater coverage for
potential impacts at lower exposure levels.
The draft full-scale assessment was presented in the July 2007
draft risk assessment report (USEPA, 2007a) that was released for
public comment and provided to CASAC for review. In their review of the
July draft risk assessment report, the CASAC Pb Panel made several
recommendations for additional exposure and health risk analyses
(Henderson, 2007b). These included a recommendation that the general
urban
[[Page 29208]]
case study be augmented by the inclusion of risk analyses in specific
urban areas of the U.S. In this regard, they specifically stated the
following (Henderson, 2007b, p. 3)
* * * the CASAC strongly believes that it is important that EPA
staff make estimates of exposure that will have national
implications for, and relevance to, urban areas; and that,
significantly, the case studies of both primary lead (Pb) smelter
sites as well as secondary smelter sites, while relevant to a few
atypical locations, do not meet the needs of supporting a Lead
NAAQS. The Agency should also undertake case studies of several
urban areas with varying lead exposure concentrations, based on the
prototypic urban risk assessment that OAQPS produced in the 2nd
Draft Lead Human Exposure and Health Risk Assessments. In order to
estimate the magnitude of risk, the Agency should estimate exposures
and convert these exposures to estimates of blood levels and IQ loss
for children living in specific urban areas.
Hence, EPA included additional case studies in the risk assessment
focused on characterizing risk for residential populations in three
specific urban locations. Further, CASAC recommended using a
concentration-response function with a change in slope near 7.5 [mu]g/
dL. Accordingly, EPA included such an additional concentration-response
function in the risk assessment. Results from the initial full-scale
analyses, along with comments from CASAC, such as those described here,
and the public resulted in a final version of the full-scale
assessments which is briefly summarized here and presented in greater
detail in the Risk Assessment Report and associated appendices (USEPA,
2007b).
In their review of the final risk assessment, CASAC expressed
strong support, stating as follows (Henderson, 2008a, p. 4):
The Final Risk Assessment report captures the breadth of issues
related to assessing the potential public health risk associated
with lead exposures; it competently documents the universe of
knowledge and interpretations of the literature on lead toxicity,
exposures, blood lead modeling and approaches for conducting risk
assessments for lead.
b. Health Endpoint, Risk Metric and Concentration-Response Functions
The health endpoint on which the quantitative health risk
assessment focuses is developmental neurotoxicity in children, with IQ
decrement (or loss) as the risk metric. Among the wide variety of
health endpoints associated with Pb exposures, there is general
consensus that the developing nervous system in young children is the
most sensitive and that neurobehavioral effects (specifically
neurocognitive deficits), including IQ decrements, appear to occur at
lower blood levels than previously believed (i.e., at levels <10 [mu]g/
dL). The selection of children's IQ for the quantitative risk
assessment reflects consideration of the evidence presented in the
Criteria Document as well as advice received from CASAC (Henderson,
2006, 2007a).
Given the evidence described in detail in the Criteria Document
(Chapters 6 and 8), and in consideration of CASAC recommendations
(Henderson, 2006, 2007a, 2007b), the risk assessment for this review
relies on the functions presented by Lanphear and others (2005) that
relate absolute IQ as a function of concurrent blood Pb or of the log
of concurrent blood Pb, and lifetime average blood Pb, respectively. As
discussed in the Criteria Document (CD, p. 8-63 to 8-64), the slope of
the concentration-response relationship described by these functions is
greater at the lower blood Pb levels (e.g., less than 10 [mu]g/dL). As
discussed in the Criteria Document and summarized in section II.B.2,
threshold blood Pb levels for these effects cannot be discerned from
the currently available epidemiological studies, and the evidence in
the animal Pb neurotoxicity literature does not define a threshold for
any of the toxic mechanisms of Pb (CD, Sections 5.3.7 and 6.2).
In applying relationships observed with the international pooled
analysis by Lanphear and others (2005) to the risk assessment, which
includes blood Pb levels below the range represented by the pooled
analysis, several alternative blood Pb concentration-response models
were considered in recognition of a reduced confidence in our ability
to characterize the quantitative blood Pb concentration-response
relationship at the lowest blood Pb levels represented in the recent
epidemiological studies. The functions considered and employed in the
initial risk analyses for this review include the following.
Log-linear function with low-exposure linearization, for
both concurrent and lifetime average blood metrics, applies the
nonlinear relationship down to the blood Pb concentration representing
the lower bound of blood Pb levels for that blood metric in the pooled
analysis and applies the slope of the tangent at that point to blood Pb
concentrations estimated in the risk assessment to fall below that
level.
Log-linear function with cutpoint, for both concurrent and
lifetime average blood metrics, also applies the nonlinear relationship
at blood Pb concentrations above the lower bound of blood Pb
concentrations in the pooled analysis dataset for that blood metric,
but then applies zero risk to all lower blood Pb concentrations
estimated in the risk assessment (this cutpoint is 1 [mu]g/dL for the
concurrent blood Pb).
In the additional risk analyses performed subsequent to the August
2007 CASAC public meeting, the two functions listed above and the
following two functions were employed (details on the forms of these
functions as applied in this risk assessment are described in Section
5.3.1 of the Risk Assessment Report).
Population stratified dual linear function for concurrent
blood Pb, derived from the pooled dataset stratified at peak blood Pb
of 10 [mu]g/dL \76\ and
---------------------------------------------------------------------------
\76\ As mentioned above (section II.B.2.b), this function
(derived for lifetime average blood Pb), was used in the economic
analysis for the RRP rule. This model was selected for the RRP
economic analysis with consideration of advice from CASAC and of the
distribution of blood Pb levels being considered in that analysis,
which focused on children living in houses with lead-based paint
(USEPA, 2008). With consideration of these blood Pb levels, the
economic analysis document states that ``[s]electing a model with a
node, or changing one segment to the other, at a lifetime average
blood Pb concentration of 10 [mu]g/dL rather than at 7.5 [mu]g/dL,
is a small protection against applying an incorrectly rapid change
(steep slope with increasingly smaller effect as concentrations
lower) to the calculation'' (USEPA, 2008).
---------------------------------------------------------------------------
Population stratified dual linear function for concurrent
blood Pb, derived from the pooled dataset stratified at 7.5 [mu]g/dL
peak blood Pb.
In interpreting risk estimates derived using the various functions,
consideration should be given to the uncertainties with regard to the
precision of the coefficients used for each analysis. The coefficients
for the log-linear model from Lanphear et al. (2005) had undergone a
careful development process, including sensitivity analyses, using all
available data from 1,333 children. The shape of the exposure-response
relationship was first assessed through tests of linearity, then by
evaluating the restricted cubic spline model. After determining that
the log-linear model provided a good fit to the data, covariates to
adjust for potential confounding were included in the log-linear model
with careful consideration of the stability of the parameter estimates.
After the multiple regression models were developed, regression
diagnostics were employed to ascertain whether the Pb coefficients were
affected by collinearity or influential observations. To further
investigate the stability of the model, a random-effects model (with
sites
[[Page 29209]]
random) was applied to evaluate the results and also the effect of
omitting one of the seven cohorts on the Pb coefficient. In the various
sensitivity analyses performed, the coefficient from the log-linear
model was found to be robust and stable. The log-linear model, however,
is not biologically plausible at the very lowest blood Pb
concentrations as they approach zero; therefore, in the first two
functions the log-linear model is applied down to a cutpoint (of 1
[mu]g/dL for the concurrent blood Pb metric), selected based on the low
end of the blood Pb levels in the pooled dataset, followed by a
linearization or an assumption of zero risk at levels below that point.
In contrast, the coefficients from the two analyses using the
population stratified dual linear function with stratification at 7.5
[mu]g/dL and 10 [mu]g/dL,\77\ peak blood Pb, have not undergone as
careful development. These analyses were primarily done to compare the
lead-associated decrement at lower blood Pb concentrations and higher
blood Pb concentrations. For these analyses, the study population was
stratified at the specified peak blood Pb level and separate linear
models were fitted to the concurrent blood Pb data for the children in
the two study population subgroups.\78\ While these analyses are quite
suitable for the purpose of investigating whether the slope at lower
concentration levels is greater compared to higher concentration
levels, use of such coefficients as the primary C-R function in a risk
analysis such as this may be inappropriate. Further, only 103 children
had maximal blood Pb levels less than 7.5 [mu]g/dL and 244 children had
maximal blood Pb levels less than 10 [mu]g/dL. While these children may
better represent current blood Pb levels, not fitting a single model
using all available data may lead to bias. Slob et al. (2005) noted
that the usual argument for not considering data from the high dose
range is that different biological mechanisms may play a role at higher
doses compared to lower doses. However, this does not mean a single
curve across the entire exposure range cannot describe the
relationship. The fitted curve merely assumes that the underlying dose-
response follows a smooth curve over the whole dose range. If
biological mechanisms change when going from lower to higher doses,
this change will result in a gradually changing slope of the dose-
response. The major strength of the Lanphear et al. (2005) study was
the large sample size and the pooled analysis of data from seven
different cohorts. In the case of the study population subgroup with
peak blood Pb below 7.5 [mu]g/dL, less than 10% of the available data
is used in the analysis (103 of the 1333 subjects in the pooled
dataset), with more than half of the data coming from one cohort
(Rochester) and the six other cohorts contributing zero to 13 children
to the analysis. Such an analysis consequently does not make full use
of the strength of the pooled study by Lanphear and others (2005).
---------------------------------------------------------------------------
\77\ See previous footnote.
\78\ Neither fit of the model nor other sensitivity analyses
were conducted (or reported) for these coefficients.
---------------------------------------------------------------------------
In consideration of the preceding discussion and the range of blood
Pb levels assessed in this analysis,\79\ greater confidence is placed
in the log-linear model form compared to the dual-linear stratified
models for purposes of the risk assessment described in this notice.
Further, in considering risk estimates derived from the four core
functions (log-linear function with low-exposure linearization, log-
linear function with cutpoint, dual linear function, stratified at 7.5
[mu]g/dL peak blood Pb, and dual linear function, stratified at 10
[mu]g/dL peak blood Pb), greatest confidence is assigned to risk
estimates derived using the log-linear function with low-exposure
linearization since this function (a) is a nonlinear function that
describes greater response per unit blood Pb at lower blood Pb levels
consistent with multiple studies identified in the discussion above,
(b) is based on fitting a function to the entire pooled dataset (and
hence uses all of the data in describing response across the range of
exposures), (c) is supported by sensitivity analyses showing the model
coefficients to be robust, and (d) provides an approach for predicting
IQ loss at the lowest exposures simulated in the assessment (consistent
with the lack of evidence for a threshold). Note, however, that risk
estimates generated using the other three concentration-response
functions are also presented to provide perspective on the impact of
uncertainty in this key modeling step. We additionally note that the
CASAC Pb Panel recommended that C-R function derived from the pooled
dataset stratified at 7.5 [mu]g/dL, peak blood Pb, be given weight in
this analysis (Henderson, 2008).
---------------------------------------------------------------------------
\79\ The median concurrent values in all case studies and air
quality scenarios are below 5 [mu]g/dL and those for air quality
scenarios within the range of standard levels proposed in this
notice are below 3 [mu]g/dL (as shown in Table 1).
---------------------------------------------------------------------------
c. Case Study Approach
For the risk assessment described in this notice, a case study
approach was employed as described in Sections 2.2 (and subsections)
and 5.1.3 of the Risk Assessment Report (USEPA, 2007b). In summarizing
the assessment in this proposal, we have focused on five \80\ case
studies that generally represent two types of population exposures: (1)
More highly air-pathway exposed children (as described below) residing
in small neighborhoods or localized residential areas with air
concentrations somewhat near the standard level being evaluated, and
(2) urban populations with a broader range of air-related exposures.
These five case studies are:
---------------------------------------------------------------------------
\80\ A sixth case study (the secondary Pb smelter case study) is
also described in the Risk Assessment Report. However, as discussed
in Section 4.3.1 of that document (USEPA, 2007b), significant
limitations in the approaches employed for this case study have
contributed to large uncertainties in the corresponding estimates.
---------------------------------------------------------------------------
A general urban case study: This case study is not based
on a specific geographic location and reflects several simplifying
assumptions used in representing exposure including uniform ambient air
Pb levels associated with the standard of interest across the
hypothetical study area and a uniform study population. This case study
characterizes risk for a localized part of an urban area at different
standard levels, but based on national average estimates of the
relationships between the different standard form assessed and ambient
air exposure concentrations. Thus, while this provides characterization
of risk to children that are relatively more highly air pathway exposed
(as compared to the location-specific case studies), this case study is
not considered to represent a high-end scenario with regard to the
characterization of ambient air Pb levels and associated risk.\81\
---------------------------------------------------------------------------
\81\ In representing the different forms of each standard level
assessed (maximum monthly or maximum quarterly) as annual air
concentrations for input to the blood Pb model for this case study,
however, we relied on averages of these relationships for large
urban areas nationally. As the averages are higher than the medians,
localized areas near more than half the urban monitoring locations
would have higher exposures and associated risks than those reported
for this case study. Further, we note that exposure concentrations
would be twice those used here if the 25th percentile values for
these relationships had been used in place of the averages. For this
reason, this case study should not be interpreted as representing a
high-end scenario with regard to the characterization of ambient air
Pb levels and associated risk.
---------------------------------------------------------------------------
A primary Pb smelter case study: \82\ This case study
estimates risk for children living in an area currently not in
attainment with the current NAAQS that is impacted by Pb emissions from
a primary Pb smelter. Results described
[[Page 29210]]
here are those for the area within 1.5 km of the facility (the
``subarea'') where airborne Pb concentrations are closest to the
current standard. As such, this case study characterizes risk for a
specific more highly exposed population and also provides insights on
risk to child populations living in areas near large sources of Pb
emissions.\83\
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\82\ See Section II.C.2.a for a summary of CASAC's comment with
regard to the primary and secondary Pb smelter case studies.
\83\ Result for the full study area, which extends 10 km out
from the facility, are presented in the Risk Assessment Report
(USEPA, 2007a), but are not presented here. Exposures in the full
study area were dominated by modeled children farther from the
facility where, as discussed in the ANPR (section III.B.2.h), there
is likely underestimation of ambient air-related Pb exposure due to
increasing influence of other sources relative to that of the
facility, which were not included in the dispersion modeling
performed to estimate air concentrations for this case study.
---------------------------------------------------------------------------
Three location-specific urban case studies: These urban
case studies focus on specific urban areas (Cleveland, Chicago and Los
Angeles) to provide representations of the distribution of ambient air-
related risk in specific densely populated urban locations. These case
studies represent areas with specific population distributions and that
experience a broader range of air-related exposures due both to
potential spatial gradients in ambient air Pb levels and population
density. A large majority of the population in these case studies
resides in areas with much lower air concentrations than those in the
very small subareas of these case studies with the highest
concentrations. Ambient air Pb concentrations are characterized using
source-oriented and other Pb-TSP monitors in these cities, while
location-specific U.S. Census demographic data are used to characterize
the spatial distribution of residential child populations in these
study areas.
These different case studies generally represent two types of
population exposures. The general urban and primary Pb smelter subarea
provide estimates of risk for more highly air-pathway exposed children
residing in small neighborhoods or localized residential areas with air
concentrations somewhat near the standard level being evaluated. By
contrast, the three location-specific urban case studies included in
the analysis provide risk estimates for an urban population with a
broader range of air-related exposures. In fact, for the location-
specific urban case studies, the majority of the modeled populations
experience ambient air Pb levels significantly lower than the standard
level being evaluated, with only a small population experiencing
ambient air Pb levels at or near the standard.\84\
---------------------------------------------------------------------------
\84\ Based on the nature of the population exposures represented
by the two categories of case study, the first category (the general
urban and primary Pb smelter case studies) relates more closely to
the second evidence-based framework (see Sections II.D.2.a and
II.E.3.a) with regard to estimates of air-related IQ loss. As
mentioned above these case studies, as compared to the other
category of case studies, include populations that are relatively
more highly air pathway exposed to air Pb concentrations somewhat
near the standard level evaluated.
---------------------------------------------------------------------------
In considering risk results generated for the location-specific
urban case studies, we note that, given the wide range of monitored Pb
levels in urban areas, combined with the relatively limited monitoring
network characterizing ambient levels in the urban setting, it is not
possible to determine where these case studies fall within the
distribution of ambient air-related risk in U.S. cities.
d. Air Quality Scenarios
Air quality scenarios assessed include (a) a current conditions
scenario for the location-specific urban case studies and the general
urban case study, (b) a current NAAQS scenario for the location-
specific urban case studies, the general urban case study and the
primary Pb smelter case study, and (c) a range of alternative NAAQS
scenarios for all case studies. The alternative NAAQS scenarios include
levels of 0.5, 0.2, 0.05, and 0.02 [mu]g/m\3\, with a monthly averaging
time, as well as a level of 0.2 [mu]g/m\3\ scenario using a quarterly
averaging time.\85\
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\85\ For further discussion of the air quality scenarios and
averaging times included in the risk assessment, see section 2.3.1
of the Risk Assessment Report (USEPA, 2007b).
---------------------------------------------------------------------------
The current NAAQS scenario for the urban case studies assumes
ambient air Pb concentrations higher than those currently occurring in
nearly all urban areas nationally.\86\ While it is extremely unlikely
that Pb concentrations in urban areas would rise to meet the current
NAAQS and there are limitations and uncertainties associated with the
roll-up procedure used for the location-specific urban case studies (as
described in Section III.B.2.h below), this scenario was included for
those case studies to provide perspective on potential risks associated
with raising levels to the point that the highest level across the
study area just meets the current NAAQS. When evaluating these results
it is important to keep these limitations and uncertainties in mind.
---------------------------------------------------------------------------
\86\ This scenario was simulated for the location-specific urban
case studies using a proportional roll-up procedure. For the general
urban case study, the maximum quarterly average ambient air
concentration was set equal to the current NAAQS.
---------------------------------------------------------------------------
Current conditions for the three location-specific urban case
studies in terms of maximum quarterly average air Pb concentrations are
0.09, 0.14 and 0.36 [mu]g/m\3\ for the study areas in Los Angeles,
Chicago and Cleveland, respectively. In terms of maximum monthly
average the values are 0.17 [mu]g/m\3\, 0.31 [mu]g/m\3\ and 0.56 [mu]g/
m\3\ for the study areas in Los Angeles, Chicago and Cleveland,
respectively.
Details of the assessment scenarios, including a description of the
derivation of Pb concentrations for air and other media are presented
in Sections 2.3 (and subsections) and Section 5.1.1 of the Risk
Assessment Report (USEPA, 2007b).
e. Categorization of Policy-Relevant Exposure Pathways
As discussed in Section IIA, this review focuses on air-related
exposure pathways (i.e., those pathways where Pb passes through ambient
air on its path from source to human exposure). These include both
inhalation of ambient air Pb (including both Pb emitted directly into
ambient air as well as resuspended Pb); and ingestion of Pb that, once
airborne, has made its way into indoor dust, outdoor dust or soil,
dietary items (e.g., crops and livestock), and drinking water. Because
of the nonlinear response of blood Pb to exposure (simulated in the
IEUBK blood Pb model) and also the nonlinearity reflected in the C-R
functions for estimation of IQ loss, this assessment first estimates
total blood Pb and risk (air- and nonair-related), and then separates
out those estimates of blood Pb and associated risk associated with the
pathways of interest in this review.
To separate out risk for the pathways of interest in this review,
we split the estimates of total (all-pathway) blood Pb and IQ loss into
background and two air-related categories (referred to as ``recent
air'' and ``past air''). However, significant limitations in our
modeling tools and data resulted in an inability to parse specific risk
estimates into specific pathways, such that we have approximated
estimates for the air-related and background categories.
Those Pb exposure pathways identified in section II.A.2 as being
tied most directly to ambient air, which consequently have the
potential to respond relatively more quickly to changes in air Pb
(inhalation and ingestion of indoor dust loaded directly from ambient
air Pb) were placed into the ``recent air'' category. The other air-
related Pb exposure pathways, associated with atmospheric deposition,
were placed into the ``past air'' category. These include ingestion of
Pb in
[[Page 29211]]
outdoor dust/soil and ingestion of the portion of Pb in indoor dust
that after deposition from ambient air outdoors is carried indoors with
humans (as described in section II.A.2 above).\87\
---------------------------------------------------------------------------
\87\ As discussed below, due to technical limitations related to
indoor dust Pb modeling, dust from Pb paint may be included to some
extent in the ``past air'' category of exposure pathways.
---------------------------------------------------------------------------
Thus, total blood Pb and IQ loss estimates were apportioned into
the following pathways or pathway combinations:
Inhalation of ambient air Pb (i.e., ``recent air'' Pb):
This is derived using the blood Pb estimate resulting from Pb exposure
limited to the inhalation pathway (and includes inhalation of Pb in
ambient air from all sources contributing to the ambient air
concentration estimate, including potentially resuspension).
Ingestion of ``recent air'' indoor dust Pb: This is
derived using the blood Pb estimate resulting from Pb exposure limited
to ingestion of the Pb in indoor dust that is predicted in this
assessment from infiltration of ambient air indoors and subsequent
deposition.\88\
---------------------------------------------------------------------------
\88\ Recent air indoor dust Pb was estimated using the
mechanistic component of the hybrid blood Pb model (see Section
3.1.4 of the Risk Assessment Report). For the primary Pb smelter
case study, estimates for this pathway are not separated from
estimates for the pathway described in the subsequent bullet due to
uncertainty regarding this categorization with the model used for
this case study (Section 3.1.4.2 of the Risk Assessment Report).
---------------------------------------------------------------------------
Ingestion of ``other'' indoor dust Pb (considered part of
``past air'' exposure): This is derived using the blood Pb estimate
resulting from Pb exposure limited to ingestion of the Pb in indoor
dust that is not predicted from infiltration of ambient air indoors and
subsequent deposition.\89\ This is interpreted to represent indoor
paint, outdoor soil/dust, and additional sources of Pb to indoor dust
including historical air (as discussed in the Risk Assessment Report,
Section 2.4.3). As the intercept in regression dust models will be
inclusive of error associated with the model coefficients, this
category also includes some representation of dust Pb associated with
current ambient air concentrations (described in previous bullet). For
the primary Pb smelter case study, estimates for this pathway are not
separated from estimates for the pathway described above due to
uncertainty regarding this categorization with the model used for this
case study (Risk Assessment Report, Section 3.1.4.2). This pathway is
included in the ``past air'' category.
---------------------------------------------------------------------------
\89\ ``Other'' indoor dust Pb is estimated using the intercept
in the dust models plus that predicted by the outdoor soil
concentration coefficient (for models that include soil Pb as a
predictor of indoor dust Pb) (Section 3.1.4 of the Risk Assessment
Report).
---------------------------------------------------------------------------
Ingestion of outdoor soil/dust Pb: This is derived using
the blood Pb estimate resulting from Pb exposure limited to ingestion
of outdoor soil/dust Pb. This pathway is included in the ``past air''
category (and could include contamination from historic Pb emissions
from automobiles and Pb paint).
Ingestion of drinking water Pb: This is derived using the
blood Pb estimate resulting from Pb exposure limited to ingestion of
drinking water Pb. This pathway is included in the policy-relevant
background category.
Ingestion of dietary Pb: This is derived using the blood
Pb estimate resulting from Pb exposure limited to ingestion of dietary
Pb. This pathway is included in the policy-relevant background
category.
As noted above, significant limitations in our modeling tools and
data resulted in an inability to parse risk estimates for specific
pathways, such that we approximated estimates for the air-related and
background categories. Of note in this regard is the apportionment of
background (nonair) pathways. For example, while conceptually indoor Pb
paint contributions to indoor dust Pb would be considered background
and included in the ``background'' category for this assessment, due to
technical limitations related to indoor dust Pb modeling, ultimately,
dust from Pb paint was included as part of ``other'' indoor dust Pb
(i.e., as part of past air exposure). The inclusion of indoor lead Pb
as a component of ``other'' indoor air (and consequently as a component
of the ``past air'' category) represents a source of potential high
bias in our prediction of exposure and risk associated with the ``past
air'' category because conceptually, exposure to indoor paint Pb is
considered part of background exposure. Further, Pb in ambient air does
contribute to the exposure pathways included in the ``background''
category (drinking water and diet), and is likely a substantial
contribution to diet (CD, p. 3-48). But we could not separate the air
contribution from the nonair contributions, and the total contribution
from both the drinking water and diet pathways are categorized as
``background'' in this assessment. As a result, our ``background'' risk
estimate includes some air-related risk.
Further, we note that in simulating reductions in exposure
associated with reducing ambient air Pb levels through alternative
NAAQS (and increases in exposure if the current NAAQS was reached in
certain case studies) only the exposure pathways categorized as
``recent air'' (inhalation and ingestion of that portion of indoor dust
associated with outdoor ambient air) were varied with changes in air
concentration. The assessment did not simulate decreases in ``past
air'' exposure pathways (e.g., reductions in outdoor soil Pb levels
following reduction in ambient air Pb levels and a subsequent decrease
in exposure through incidental soil ingestion and the contribution of
outdoor soil to indoor dust). These exposures were held constant across
all air quality scenarios. In comparing total risk estimates between
alternate NAAQS scenarios, this aspect of the analysis will tend to
underestimate the reductions in risk associated with alternative NAAQS.
However, this does not mean that overall risk has been underestimated.
The net effect of all sources of uncertainty or bias in the analysis,
which may also tend to under- or overestimate risk, could not be
quantified. Interpretation of risk estimates is discussed more fully in
section II.C.3.b.
In summary, because of limitations in the assessment design, data
and modeling tools, our risk estimates for the ``past air'' category
include both risks that are truly air-related and potentially, some
background risk. Because we could not sharply separate Pb linked to
ambient air from Pb that is background, some of the three categories of
risk are underestimated and others overestimated. On balance, we
believe this limitation leads to a slight overestimate of the risks in
the ``past air'' category. At the same time, as discussed above, the
``recent air'' category does not fully represent the risk associated
with all air-related pathways. Thus, we consider the risk attributable
to air-related exposure pathways to be bounded on the low end by the
risk estimated for the ``recent air'' category and on the upper end by
the risk estimated for the ``recent air'' plus ``past air'' categories.
f. Analytical Steps
The risk assessment includes four analytical steps, briefly
described below and presented in detail in Sections 2.4.4, 3.1, 3.2,
4.1, and 5.1 of the Risk Assessment Report (USEPA, 2007b).
Characterization of Pb in ambient air: The
characterization of outdoor ambient air Pb levels uses different
approaches depending on the case study (as explained in more detail
below): (a) source-oriented and non-source oriented monitors are
assumed to represent different exposure zones in the city-specific case
studies, (b) a single exposure level is assumed for the entire
[[Page 29212]]
population in the general urban case study, and (c) ambient levels are
estimated using air dispersion modeling based on Pb emissions from a
particular facility in the primary Pb smelter case study.
Characterization of outdoor soil/dust and indoor dust Pb
concentrations: Outdoor soil Pb levels are estimated using empirical
data and fate and transport modeling. Indoor dust Pb levels are
predicted using a combination of (a) regression-based models that
relate indoor dust to ambient air Pb and outdoor soil Pb, and (b)
mechanistic models.\90\
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\90\ Indoor dust Pb modeling for the urban case studies is based
on a hybrid mechanistic-empirical model which considers the direct
impact of Pb in ambient air on indoor dust Pb (i.e., which models
the infiltration of ambient air indoors and subsequent deposition of
Pb to indoor surfaces). This modeling does not consider other
ambient air-related contributions to indoor dust, such as ``tracking
in'' of outdoor soil Pb. By contrast, indoor dust Pb modeling for
the primary Pb smelter case study subarea uses a site-specific
regression model which relates average dust Pb values (based on a
recent multi-year dataset) to annual average air Pb concentrations
(based on air dispersion modeling). In this way, modeling for the
primary Pb smelter subarea may reflect some contributions to indoor
dust Pb that relate to longer term impacts of ambient air (e.g.,
``tracking in'' of outdoor soil), as well as contributions from
infiltration of ambient air. Additional detail on the methods used
in characterizing Pb concentrations in outdoor soil and indoor dust
are presented in Sections 3.1.3 and 3.1.4 of the Risk Assessment,
respectively. Data, methods and assumptions here used in
characterizing Pb concentrations in these exposure media may differ
from those in other analyses that serve different purposes.
---------------------------------------------------------------------------
Characterization of blood Pb levels: Blood Pb levels for
each exposure zone are derived from central-tendency blood Pb
concentrations estimated using the Integrated Exposure and Uptake
Biokinetic (IEUBK) model, and concurrent or lifetime average blood Pb
is estimated from these outputs as described in Section 3.2.1.1 of the
Risk Assessment Report (USEPA, 2007b). For the point source and
location-specific urban case studies, a probabilistic exposure model is
used to generate population distributions of blood Pb concentrations
based on: (a) The central tendency blood Pb levels for each exposure
zone, (b) demographic data for the distribution of children (less than
7 years of age) across exposure zones in a study area, and (c) a
geometric standard deviation (GSD) intended to characterize
interindividual variability in blood Pb (e.g., reflecting differences
in behavior and biokinetics related to Pb). For the general urban case
study, as demographic data for a specific location are not considered,
the GSD is applied directly to the central tendency blood Pb level to
estimate a population distribution of blood Pb levels. Additional
detail on the methods used to model population blood Pb levels is
presented in Sections 3.2.2 and 5.2.2.3 of the Risk Assessment Report
(USEPA, 2007b).
Risk characterization (estimating IQ loss): Concurrent or
lifetime average blood Pb estimates for each simulated child in each
case study population are converted into total Pb-related IQ loss
estimates using the concentration-response functions described above in
section II.C.2.b.\91\
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\91\ The four C-R functions applied in the risk assessment,
which are based on analyses presented in Lanphear et al. (2005)
include a log-linear function with low-exposure linearization, a
log-linear function with a cutpoint, and two dual linear functions
(based on population stratification at peak blood Pb levels of 7.5
and 10 [mu]g/dL) (see section II.C.2.b).
---------------------------------------------------------------------------
We have also used the results of exposure modeling to estimate air-
to-blood ratios for two of the case studies (the general urban and
primary Pb smelter case studies). Specifically, we compared the change
in ambient air Pb between adjacent NAAQS levels with the associated
reduction in concurrent blood Pb levels (for the median population
percentile) to derive air-to-blood ratios. As they relate air
concentrations \92\ input to the first analytical step to blood Pb
estimates output from the third analytical step, they may be viewed as
a collapsed alternate to the three steps for the exposure pathways
directly linked to air concentrations in this assessment. The values
for these ratios are affected by design aspects of the risk assessment,
most notably those identified here:
---------------------------------------------------------------------------
\92\ Because the IEUBK blood Pb model runs with an annual time
step, the air concentrations input to the ``recent air'' pathways
modeling steps were in terms of annual average air concentration.
---------------------------------------------------------------------------
Because they are derived from differences in blood Pb
estimates between air quality scenarios and the only pathways varied
with air quality scenarios are ambient air and indoor dust (as
described in section II.C.2.e above), the exposure pathways reflected
in the ratios are generally the ``recent air'' pathways (described in
section II.C.2.e above), which include inhalation of ambient air and
ingestion of indoor dust loaded by infiltration of ambient air. Ratios
for the primary Pb smelter case study subarea may additionally reflect
some contributions to indoor dust from other ambient air-related
pathways (e.g., ``tracking in'' of soil containing ambient air Pb), yet
still not all air-related pathways. Thus, the air-to-blood ratios
derived for both case studies (described in section II.C.3.a) are lower
than they would be if they reflected all air-related pathways.
The blood Pb estimates used in this calculation are for
the ``concurrent'' metric (i.e., concentrations during the 7th year of
life). Accordingly, the resultant air-to-blood ratios are lower than
they would be if based on blood Pb estimates for the 2nd year of life
(e.g., peak) or estimates averaged over the exposure period.
Key limitations and uncertainties associated with the application
of these specific analytical steps are summarized in Section III.B.2.k
below.
g. Generating Multiple Sets of Risk Results
In the initial analyses for the full-scale assessment (USEPA,
2007a), EPA implemented multiple modeling approaches for each case
study scenario in an effort to characterize the potential impact on
exposure and risk estimates of uncertainty associated with the
limitations in the tools, data and methods available for this risk
assessment and with key analytical steps in the modeling approach.
These multiple modeling approaches are described in Section 2.4.6.2 of
the final Risk Assessment Report (USEPA, 2007b). In consideration of
comments provided by CASAC (Henderson, 2007b) on these analyses
regarding which modeling approach they felt had greater scientific
support, a pared down set of modeling combinations was identified as
the core approach for the subsequent analyses. The core modeling
approach includes the following key elements:
Ambient air Pb estimates (based on monitors or modeling
and proportional rollbacks, as described below),
Background exposure from food and water (as described
above),
The hybrid indoor dust model specifically developed for
urban residential applications (which predicts Pb in indoor dust as a
function of ambient air Pb and nonair contribution),
The IEUBK blood Pb model (which predicts blood Pb in young
children exposed to Pb from multiple exposure pathways),
The concurrent blood Pb metric,
A GSD for concurrent blood Pb of 2.1 to characterize
interindividual variability in blood Pb levels for a given ambient
level for the urban case studies,\93\ and
---------------------------------------------------------------------------
\93\ In the economic analysis for the RRP rule, a GSD of 1.6 was
used in its probabilistic simulations, reflecting the fact that the
simulated exposures focus on a subset of Pb exposure pathways
(exposure to dust and airborne Pb resulting from renovation
activity) and a CASAC recommendation to use the IEUBK-recommended
GSD with the Leggett model, where no GSD is provided. In addition,
the accompanying sensitivity analysis used a GSD of 2.1 to consider
the impact on IQ change estiamtes of using a larger GSD, which would
reflect greater heterogeneity in the study population with regard to
Pb exposure and blood Pb response.
---------------------------------------------------------------------------
[[Page 29213]]
Four different functions relating concurrent blood Pb to
IQ loss (described in section II.C.2.b), including two log-linear
models (one with a cutpoint and one with low-exposure linearization)
and two dual-linear models with stratification, one stratified at 7.5
[mu]g/dL peak blood Pb and the other at 10 [mu]g/dL peak blood Pb.
For each case study, the core modeling approach employs a single
set of modeling elements to estimate exposure and the four different
concentration-response functions referenced above to derive four sets
of risk results from the single set of exposure estimates. The spread
of estimates resulting from application of all four functions captures
much of the uncertainty associated model choice in this analytical
step. Among these four functions, EPA has greater confidence in
estimates derived using the log-linear with low-exposure linearization
concentration-response function as discussed above.
In addition to employing multiple concentration-response functions,
the assessment includes various sensitivity analyses to characterize
the potential impact of uncertainty in other key analysis steps on
exposure and risk estimates. The sensitivity analyses and uncertainty
characterization completed for the risk analysis are described in
Sections 3.5, 4.3, 5.2.5 and 5.3.3 of the Risk Assessment Report
(USEPA, 2007b).
h. Key Limitations and Uncertainties
As recognized above, EPA has made simplifying assumptions in
several areas of this assessment due to the limited data, models, and
time available. These assumptions and related limitations and
uncertainties are described in the Risk Assessment Report (USEPA,
2007b). Key assumptions, limitations and uncertainties are briefly
identified below, with emphasis on those sources of uncertainty
considered most critical in interpreting risk results. In the
presentation below, limitations (and associated uncertainty) are
listed, beginning with those regarding design of the assessment or case
studies, followed by those regarding estimation of Pb concentrations in
ambient air indoor dust, outdoor soil/dust, and blood, and lastly
regarding estimation of Pb-related IQ loss.
Temporal aspects: Exposure modeling uses a 7 year exposure
period for each simulated child, during which time, media
concentrations remain fixed (at levels associated with the ambient air
Pb level being modeled) and the child remains at the same residence,
while exposure factors and physiological parameters are adjusted to
match the age of the child. These aspects are a simplification of
population exposures that contributes some uncertainty to our exposure
and risk estimates.
General urban case study: As described in section
II.C.2.c, this case study is not based on a specific location and is
instead intended to represent a smaller neighborhood experiencing
ambient air Pb levels at or near the standard of interest.
Consequently, it assumes (a) a single exposure zone within which all
media concentrations of Pb are assumed to be spatially uniform and (b)
a uniformly distributed population of unspecified size. While these
assumptions are reasonable in the context of evaluating risk for a
smaller subpopulation located close to a monitor reporting values at or
near the standard of interest, there is significant uncertainty
associated with extrapolating these risks to a specific urban location,
particularly if that urban location is relatively large, given that
larger urban areas are expected to have increasingly varied patterns of
ambient air Pb levels and population density. The risk estimates for
this general urban case study, while generally representative of an
urban residential population exposed to the specified ambient air Pb
levels, cannot be readily related to a specific large urban population.
Location-specific urban case studies: The Pb-TSP
monitoring network is currently quite limited and consequently, the
number of monitors available to represent air concentrations in these
case studies is limited, ranged from six for Cleveland to 11 for
Chicago. Accordingly, our estimates of the magnitude of and spatial
variation of air Pb concentrations are subject to uncertainty
associated with the limited monitoring data and method used in
extrapolating from those data to characterize an ambient air Pb level
surface for these modeled urban areas. Details on the approach used to
derive ambient air Pb surfaces for the urban case studies based on
monitoring data are presented in Section 5.1.3 of the Risk Assessment
Report (USEPA, 2007b). As recognized in Section, III.B.2.a, the
analyses for these case studies were developed in response to CASAC
recommendations on the July 2007 draft Risk Assessment (Henderson,
2007b). Subsequently, the CASAC has reviewed the approach used in
conducting the final draft of the full-scale risk assessment, including
the inclusion of the location-specific urban case studies and expressed
broad support for the technical approach used (Henderson, 2008).
Current NAAQS air quality scenarios: For the location-
specific urban case studies, proportional roll-up procedures were used
to adjust ambient air Pb concentrations up to just meet the current
NAAQS (a detailed discussion is provided in Sections 2.3.1 and 5.2.2.1
of the Risk Assessment Report, USEPA, 2007b). This procedure was used
to provide insights into the degree of risk which could be associated
with ambient air Pb levels at or near the current standard in urban
areas. EPA recognizes that it is extremely unlikely that Pb
concentrations would rise to just meet the current NAAQS in urban areas
nationwide and that there is substantial uncertainty with our
simulation of such conditions. For the primary Pb smelter case study,
where current conditions exceed the current NAAQS, attainment of the
current NAAQS was simulated using air quality modeling, emissions and
source parameters used in developing the 2007 proposed revision to the
State Implementation Plan for the area (described in Section 3.1.1.2 of
the Risk Assessment Report (USEPA, 2007b)).
Alternative NAAQS air quality scenarios: In all case
studies, proportional roll-down procedures were used to adjust ambient
air Pb concentrations downward to attain alternative NAAQS (described
in Sections 2.3.1 and 5.2.2.1 of the Risk Assessment Report, USEPA,
2007b). There is significant uncertainty in simulating conditions
associated with the implementation of emissions reduction actions to
meet a lower standard.
Estimates of outdoor soil/dust Pb concentrations: Outdoor
soil Pb concentration for both the urban case studies and the primary
Pb smelter case study are based on empirical data (as described in
Section 3.1.3 of the Risk Assessment). To the extent that these data
are from areas containing older structures, the impact of Pb paint
weathered from older structures on soil Pb levels will be reflected in
these empirical estimates. In the case of the urban case studies, a
mean value from a sample of houses built between 1940 and 1998 was used
to represent soil Pb levels (as described in Section 3.1.3.1 of the
Risk Assessment). In the case of the primary Pb smelter case study
subarea, site-specific data are used. As there has been remediation of
soil in this subarea, the measurements do not reflect historical air
quality. Additionally,
[[Page 29214]]
studies since remediation have reported increasing soil Pb levels
indicating that soil concentrations are still responding to current air
quality, and consequently underestimate eventual steady state
conditions for the current air quality. In all case studies, the same
outdoor soil/dust Pb concentrations (based on these datasets) are used
for all air quality scenarios (i.e., the potential longer-term impact
of reductions in ambient air Pb on outdoor soil/dust Pb levels and
associated impacts on indoor dust Pb have not be simulated). In areas
where air concentrations have been greater in the past, however,
implementation of a reduced NAAQS might be expected to yield reduced
soil Pb levels over the long term. As described in Section 2.3.3 of the
Risk Assessment Report (USEPA, 2007b), however, there is potentially
significant uncertainty associated with this conclusion, particularly
with regard to implications for areas in which a Pb source may locate
where one of comparable size had not been previously. Additionally, it
is possible that control measures implemented to meet alternative NAAQS
may result in changes to soil Pb concentrations; these are not
reflected in the assessment.
Estimates of indoor dust Pb concentrations for the urban
case studies (application of the hybrid model): The hybrid mechanistic-
empirical model for estimating indoor dust Pb for the urban case
studies (as described in Section 3.1.4.1 of the Risk Assessment Report,
USEPA, 2007b) utilizes a mechanistic model to simulate the exchange of
outdoor ambient air Pb indoors and subsequent deposition (and buildup)
of Pb on indoor surfaces, which relies on a number of empirical
measurements for parameterization (e.g., infiltration rates, deposition
velocities, cleaning frequencies and efficiencies). There is
considerable uncertainty associated with these parameter estimates. In
addition, there is uncertainty associated with the partitioning of
total indoor dust Pb estimates between the infiltration-related
(``recent air'') component and other contributions (``other'' as
described in section II.C.2.e).
Estimates of indoor dust Pb concentrations for the primary
Pb smelter case study (application of the site-specific regression
model): There is uncertainty associated with the site-specific
regression model applied in the remediation zone (as described in
Section 3.1.4.2 of the Risk Assessment Report), and relatively greater
uncertainty associated with its application to air quality scenarios
that simulate notably lower air Pb levels (as is typically the case
when applying regression-based models beyond the bounds of the datasets
used in their derivation). The log-log form of the regression model
prevents the ready identification of an intercept term handicapping us
in partitioning estimates of air-related indoor dust (and consequently
exposure and risk estimates) between ``recent air'' and ``other''
components. In addition, limitations in the model-derived air estimates
used in deriving the regression model prevented effective consideration
for the role of ambient air Pb related to resuspension in influencing
indoor dust Pb levels. A public commenter suggested that indoor dust Pb
levels using this model may be overestimated due to factors associated
with the model's derivation. Factors identified by the commenter,
however, may contribute to a potential for either over- or
underestimation, and as noted by the commenter, additional research
might reduce this uncertainty.
Characterizing interindividual variability using a GSD:
There is uncertainty associated with the GSD specified for each case
study (as described in Sections 3.2.3 and 5.2.2.3 of the Risk
Assessment Report). Two factors are described here as contributors to
that uncertainty. Interindividual variability in blood Pb levels for
any study population (as described by the GSD) will reflect, to a
certain extent, spatial variation in media concentrations, including
outdoor ambient air Pb levels and indoor dust Pb levels, as well as
differences in physiological response to Pb exposure. For each case
study, there is significant uncertainty in the specification of spatial
variability in ambient air Pb levels and associated indoor dust Pb
levels, as noted above. In addition, there are a limited number of
datasets for different types of residential child populations from
which a GSD can be derived (e.g., NHANES datasets \94\ for more
heterogeneous populations and individual study datasets for likely more
homogeneous populations near specific industrial Pb sources). This
uncertainty associated with the GSDs introduces significant uncertainty
in exposure and risk estimates for the 95th population percentile.
---------------------------------------------------------------------------
\94\ The GSD for the urban case studies, in the risk assessment
described in this notice, was derived using NHANES data for the
years 1999-2000.
---------------------------------------------------------------------------
Exposure pathway apportionment for higher percentile blood
Pb level and IQ loss estimates: Apportionment of blood Pb levels for
higher population percentiles is assumed to be the same as that
estimated using the central tendency estimate of blood Pb in an
exposure zone. This introduces significant uncertainty into projections
of pathway apportionment for higher population percentiles of blood Pb
and IQ loss. In reality, pathway apportionment may differ in higher
exposure percentiles. For example, paint and/or drinking water
exposures may increase in importance, with air-related contributions
decreasing as an overall percentage of blood Pb levels and associated
risk. Because of this uncertainty related to pathway apportionment, as
mentioned earlier, greater confidence is placed in estimates of total
Pb exposure and risk in evaluating the impact of the current NAAQS and
alternative NAAQS relative to current conditions.
Relating blood Pb levels to IQ loss: Specification of the
quantitative relationship between blood Pb level and IQ loss is subject
to significant uncertainty at lower blood Pb levels (e.g., below 5
[mu]g/dL concurrent blood Pb). As discussed earlier, there are
limitations in the datasets and concentration-response analyses
available for characterizing the concentration-response relationship at
these lower blood Pb levels. For example, the pooled international
dataset analyzed by Lanphear and others (2005) includes relatively few
children with blood Pb levels below 5 [mu]g/dL and no children with
levels below 1 [mu]g/dL. In recognition of the uncertainty in
specifying a quantitative concentration-response relationship at such
levels, our core modeling approach involves the application of four
different functions to generate a range of risk estimates (as described
in Section 4.2.6 and Section 5.3.1 of the Risk Assessment Report,
USEPA, 2007b). The difference in absolute IQ loss estimates for the
four concentration-response functions for a given case study/air
quality scenario combination is typically close to a factor of 3.
Estimates of differences in IQ loss between air quality scenarios (in
terms of percent), however, are more similar across the four functions,
although the function producing higher overall risk estimates (the dual
linear function, stratified at 7.5 [mu]g/dL, peak blood Pb) also
produces larger absolute reductions in IQ loss compared with the other
three functions.
3. Summary of Estimates and Key Observations
This section presents blood Pb and IQ loss estimates generated in
the exposure and risk assessments. Blood Pb estimates (and air-to-blood
Pb ratios) are presented first, followed by IQ loss estimates.
[[Page 29215]]
a. Blood Pb Estimates
This section presents a summary of blood Pb modeling results for
concurrent blood Pb drawn from the more detailed presentation in the
Staff Paper and the Risk Assessment Report (USEPA, 2007a, 2007b,
2007c).
Blood Pb level estimates for the current conditions air quality
scenarios for these case studies differ somewhat from the national
values associated with recent NHANES information. For example, median
blood Pb levels for the current conditions scenario for the urban case
studies are somewhat larger than the national median from the NHANES
data for 2003-2004. Specifically, values for the three location-
specific urban case studies range from 1.7 to 1.8 [mu]g/dL with the
general urban case study having a value of 1.9 [mu]g/dL (current-
conditions mean) (presented in Risk Assessment Report, Volume I, Table
5-5), while the median value from NHANES (2003-2004) is 1.6 [mu]g/dL
(http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm).
Additionally, NHANES values for the 90th percentile (for 2003-2004)
were identified and these values can be compared against 90th
percentile estimates generated for the urban case studies (see Risk
Assessment Report, Appendix O, Section O.3.2 for the location-specific
urban case study and Appendix N, Section N.2.1.2 for the general urban
case study). The 90th percentile blood Pb levels for the current
conditions scenario, for the three location-specific urban case studies
range from 4.5 to 4.6 [mu]g/dL, while the estimate for the general
urban case study is 5.0 [mu]g/dL. These 90th percentile values for the
case study populations are larger than the 90th percentile value of 3.9
[mu]g/dL reported by NHANES for all children in 2003-2004. It is noted
that ambient air levels reflected in the urban case studies are likely
to differ from those underlying the NHANES data.\95\
---------------------------------------------------------------------------
\95\ The maximum quarterly mean Pb concentrations in the
location-specific case studies ranged from 0.09-0.36 [mu]g/m\3\,
which are higher levels than the maximum quarterly mean values in
most monitoring sites in the U.S. The median of the maximum
quarterly mean values across all sites in the 2003-05 national
dataset is 0.03 [mu]g/m\3\ (USEPA, 2007a, appendix A).
---------------------------------------------------------------------------
Table 2 presents total blood Pb estimates for alternative
standards, focusing on the median in the assessed population, and
associated estimates for the air-related percentage of total blood Pb
(i.e., bounded on the low end by the ``recent air'' contributions and
on the high end by the ``recent'' plus ``past air'' contribution to
total Pb exposure).
Generally, 95th percentile blood Pb estimates across air quality
scenarios for all case studies (not shown here) are 2-3 times higher
than the median estimates in Table 2. For example, 95th percentile
estimates of total blood Pb for the current NAAQS scenario are 10.6
[mu]g/dL for the general urban case study, 12.3 [mu]g/dL for the
primary Pb smelter subarea, and 7.4 to 10.2 [mu]g/dL for the three
location-specific urban case studies (Staff Paper, Table 4-2). While
the estimates indicate similar fractions of total blood Pb that is air-
related between the 95th percentile and median, there is greater
uncertainty in pathway apportionment among air-related and other
sources for higher percentiles, including the 95th percentile.
Table 2.--Summary of Median Blood Pb Estimates for Concurrent Blood Pb
[Total]
--------------------------------------------------------------------------------------------------------------------------------------------------------
Total blood Pb ([mu]g/dL) (air-related percentage) \A\
--------------------------------------------------------------------------------------------------------------------
NAAQS Level simulated ([mu]g/m\3\ Location-specific urban case studies
max monthly, except as noted below) General urban case Primary Pb smelter --------------------------------------------------------------------
study (subarea) case studyB Cleveland (0.56 [mu]g/ Chicago (0.31 [mu]g/ Los Angeles (0.17
C m\3\) m\3\) [mu]g/m\3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
1.5 max quarterly \D\.............. 3.1 (61 to 84%)....... 4.6 (up to 87%)....... 2.1 \D\ (57 to 86%).. 3.0 \E\ (63 to 83%).. 2.6E (50 to 81%).
0.50............................... 2.2 (41 to 73%)....... 3.2 (up to 81%)....... 1.8 (39 to 72%)...... (\F\)................ (\F\)
0.20............................... 1.9 (26 to 74%)....... 2.3 (up to 78%)....... 1.7 (6 to 65%)....... 1.8 (17 to 67%)...... 1.7 (\G\) (18 to
71%).
0.05............................... 1.7 (12 to 65%)....... 1.7 (up to 65%)....... 1.6 (1 to 63%)....... 1.6 (6 to 69%)....... 1.6 (13 to 69%).
0.02............................... 1.6 (6 to 69%)........ 1.6 (up to 69%)....... 1.6 (1 to 63%)....... 1.6 (1 to 63%)....... 1.6 (6 to 63%).
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ --Blood Pb estimates are rounded to one decimal place. Air-related percentage is bracketed by ``recent air'' (lower bound of presented range) and
``recent'' plus ``past air'' (upper bound of presented range). The term ``past air'' includes contributions from the outdoor soil/dust contribution to
indoor dust, historical air contribution to indoor dust, and outdoor soil/dust pathways; ``recent air'' refers to contributions from inhalation of
ambient air Pb or ingestion of indoor dust Pb predicted to be associated with outdoor ambient air Pb levels, with outdoor ambient air also potentially
including resuspended, previously deposited Pb (see Section II.C.2.e).
\B\ --In the case of the primary Pb smelter subarea, only recent plus past air estimates are available.
\C\ --Median blood Pb levels for the primary smelter (full study area) are estimated at 1.5 [mu]g/dL (for the 1.5 [mu]g/m\3\ max quarterly level) and
1.4 [mu]g/dL for the remaining NAAQS levels simulated. The air-related percentages for these standard levels range from 36% to 79%.
\D\ --This corresponds to roughly 0.7-1.0 [mu]g/m\3\ maximum monthly mean, across the urban case studies.
\E\ --A ``roll-up'' was performed so that the highest monitor in the study area is increased to just meet this level.
\F\ --A ``roll-up'' to this level was not performed.
\G\ --A ``roll-up'' to this level was not performed; these estimates are based on current conditions in this area.
As described in section II.C.2.f, the risk assessment also
developed estimates for air-to-blood ratios, which are described in
section 5.2.5.2 of the Risk Assessment Report (USEPA, 2007b). These
ratios reflect a subset of air-related pathways related to inhalation
and ingestion of indoor dust; inclusion of the remaining pathways would
be expected to yield higher ratios. Additionally, these ratios are
based on blood Pb estimates for the 7th year of exposure (concurrent
blood Pb) which are lower than blood Pb estimates at younger ages (and
than the lifetime-averaged blood Pb metric). Ratios based on other
blood Pb estimates (e.g., lifetime-averaged or peak blood Pb) would be
higher.
For the general urban case study, estimates of air-to-
blood ratios, presented in section 5.2.5.2 of the Risk Assessment
Report (USEPA, 2007b) ranged from 1:2 to 1:9, with the majority of the
estimates ranging from 1:4 to 1:6.\96\ As noted in Section II.C.2.f,
[[Page 29216]]
because the risk assessment only reflects the impact of reductions on
recent air-related pathways in predicting changes in indoor dust Pb for
urban case studies, these ratios are lower than they would be if they
had also reflected potential reductions in other air-related pathways
(e.g., changes in outdoor surface soil/dust Pb levels and diet with
changes in ambient air Pb levels). We also note that the median blood
Pb levels associated with exposure pathways that were not varied in
this assessment (and consequently are not reflected in these ratios)
generally range from 1.3 to 1.5 [mu]g/dL for this case study.
---------------------------------------------------------------------------
\96\ The ratios increase as the level of the alternate standard
decreases. This reflects nonlinearity in the Pb response, which is
greater on a per-unit basis for lower ambient air Pb levels.
---------------------------------------------------------------------------
For the primary Pb smelter subarea, estimates of air-to-
blood ratios, presented in section 5.2.5.2 of the Risk Assessment
Report (USEPA, 2007b) ranged from 1:10 and higher.97 98 One
reason for these estimates being higher than those for the urban case
study is that the dust Pb model used may reflect somewhat ambient air-
related pathways other than that of ambient air infiltrating a home (as
described in Section II.C.2.f above).\99\
---------------------------------------------------------------------------
\97\ As with such estimates for the urban case study, ratios are
higher at lower ambient air Pb levels, reflecting the nonlinearity
of the dust Pb response with air concentration.
\98\ For the primary Pb smelter (full study area), for which
limitations are noted above in section II.C.2.c, the air-to-blood
ratio estimates, presented in section 5.2.5.2 of the Risk Assessment
Report (USEPA, 2007b), ranged from 1:3 to 1:7. As in the other case
studies, ratios are higher at lower ambient air Pb levels. It is
noted that the underlying changes in both ambient air Pb and blood
Pb across standard levels are extremely small, introducing
uncertainty into ratios derived using these data.
\99\ Also, as noted above (Section II.C.2.h), there is increased
uncertainty with application of this regression-based model in air
quality scenarios of notably lower air Pb levels than the data set
used in its derivation.
---------------------------------------------------------------------------
b. IQ Loss Estimates
The risk assessment estimated IQ loss associated with both total Pb
exposure and air-related Pb exposure. This section focuses on findings
in relation to air-related Pb exposure, since this is the category of
risk results considered most relevant to the review in considering
whether the current NAAQS and potential alternative NAAQS provide
protection of public health with an adequate margin of safety
(additional categories of risk results, including IQ loss estimates
based on total Pb exposure and population incidence results, are
presented at the end of the section).\100\
---------------------------------------------------------------------------
\100\ The detailed results are provided in the Risk Assessment
Report (USEPA, 2007b).
---------------------------------------------------------------------------
In considering air-related risk results, we note that IQ loss
associated with air-related exposure for each NAAQS scenario is bounded
by recent-air on the low-end and recent plus past air on the high-end
(as described in section II.C.2.e above). In considering differences in
these risk estimates (or in the total risk estimates presented in the
final Risk Assessment Report) for alternative NAAQS, we note that these
comparisons underestimate the true impacts of the alternate NAAQS and
accordingly, the benefit to public health that would result from lower
NAAQS levels. This is due to our inability to simulate in this
assessment reductions in several outdoor air deposition-related
pathways (e.g., diet, ingestion of outdoor surface soil). The magnitude
of this underestimation is unknown.
As with the discussion of blood Pb results, the IQ loss estimates
are summarized here according to air quality scenario and case study
category (Table 3). In presenting these results, we have focused this
presentation on estimates for the median in each case study population
of children because of the greater confidence associated with estimates
for the median as compared to those for 95th percentile.\101\
Generally, 95th percentile IQ loss estimates for all case studies are
80 to 100% higher than the median results in Table 3. The fraction of
total IQ loss that is air-related for the 95th percentile is generally
similar to that for the median (for a particular combination of case
study and air quality scenario).
---------------------------------------------------------------------------
\101\ A complete presentation of risk estimates is available in
the final Risk Assessment Report, including a presentation of
estimates for the 95th percentile in Table 5-10 of that report.
---------------------------------------------------------------------------
The risk estimates presented in boldface in Table 3 are those
derived using the log-linear with low-exposure linearization
concentration-response function, while the range of estimates
associated with all four concentration-response functions is presented
in parentheses. These functions are discussed above in section
II.C.2.b.
[[Page 29217]]
Table 3.--Summary of Risk Attributable to Air-Related Pb Exposure
----------------------------------------------------------------------------------------------------------------
Median air-related IQ loss \A\
-------------------------------------------------------------------------------
NAAQS level simulated ([mu]g/m Location-specific urban case studies
\3\ max monthly, except as noted Primary Pb -----------------------------------------------
below) General urban smelter Cleveland Los Angeles
case study (subarea) case (0.56 [mu]g/m Chicago (0.31 (0.17 [mu]g/m
study \B, C\ \3\) [mu]g/m \3\) \3\)
----------------------------------------------------------------------------------------------------------------
1.5 max quarterly \D\........... 3.5-4.8 < 6 2.8-3.9 \E\ 3.4-4.7 \E\ 2.7-4.2 \E\
(1.5-7.7) <(3.2-9.4) (0.6-4.6) (1.4-7.4) (1.1-6.2)
0.5............................. 1.9-3.6 < 4.5 0.6-2.9 \F\ \F\
(0.7-4.8) <(2.1-7.7) (0.2-3.9)
0.2............................. 1.2-3.2 < 3.7 0.6-2.8 0.6-2.9 0.7-2.9 \G\
(0.4-4.0) <(1.2-5.1) (0.1-3.2) (0.3-3.6) (0.2-3.5)
0.05............................ 0.5-2.8 < 2.8 0.1-2.6 0.2-2.6 0.3-2.7
(0.2-3.3) <(0.9-3.4) (<0.1-3.1) (0.1-3.2) (0.1-3.2)
0.02............................ 0.3-2.6 < 2.9 <0.1-2.6 0.1-2.6 0.1-2.6
(0.1-3.1) <(0.9-3.3) (<0.1-3.0) (<0.1-3.1) (<0.1-3.1)
----------------------------------------------------------------------------------------------------------------
\A\--Air-related risk is bracketed by ``recent air'' (lower bound of presented range) and ``recent'' plus ``past
air'' (upper bound of presented range). While differences between standard levels are better distinguished by
differences in the ``recent'' plus ``past air'' estimates (upper bounds shown here), these differences are
inherently underestimates. The term ``past air'' includes contributions from the outdoor soil/dust
contribution to indoor dust, historical air contribution to indoor dust, and outdoor soil/dust pathways;
``recent air'' refers to contributions from inhalation of ambient air Pb or ingestion of indoor dust Pb
predicted to be associated with outdoor ambient air Pb levels, with outdoor ambient air also potentially
including resuspended, previously deposited Pb (see Section II.C.2.e). Boldface values are estimates generated
using the log-linear with low-exposure linearization function. Values in parentheses reflect the range of
estimates associated with all four concentration-response functions.
\B\--In the case of the primary Pb smelter case study, only recent plus past air estimates are available.
\C\--Median air-related IQ loss estimates for the primary Pb smelter (full study area) range from <1.7 to <2.9
points, with no consistent pattern across simulated NAAQS levels. This lack of a pattern reflects inclusion of
a large fraction of the study population with relatively low ambient air impacts such that there is lower
variation (at the population median) across standard levels (see Section 4.2 of the Risk Assessment, Volume
1).
\D\--This corresponds to roughly 0.7--1.0 [mu]g/m3 maximum monthly mean, across the urban case studies
\E\--A ``roll-up'' was performed so that the highest monitor in the study area is increased to just meet this
level.
\F\--A ``roll-up'' to this level was not performed.
\G\--A ``roll-up'' to this level was not performed; these estimates are based on current conditions in this
area.
Key observations regarding the median estimates of air-related risk
for the current NAAQS and alternative standards presented in Table 3
include:
For the scenario for the current NAAQS (1.5 [mu]g/m\3\,
maximum quarterly average), air-related risk exceeds 2 points IQ loss
at the median and the upper bound of air-related risk is near or above
4 points IQ loss in all five case studies.\102\
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\102\ As noted in Table 3 and section II.C.2.d above, and
discussed further, with regard to associated limitations and
uncertainties, in section II.C.2.h above, a proportional roll-up
procedure was used to estimate air Pb concentrations in this
scenario for the location-specific case studies.
---------------------------------------------------------------------------
Alternate standards provide substantial reduction in
estimates of air-related risk across the full set of alternative NAAQS
considered in this analysis (i.e., 0.5 to 0.02 [mu]g/m\3\ max monthly).
This is particularly the case for the lower bounds of the air-related
estimates presented in Table 3, which reflect the estimates for
``recent air''-related pathways, which are the pathways that were
varied with changes in air concentrations (as described above in
section II.C.2.e). There is less risk reduction associated with the
upper bounds of these estimates as the upper bound values are inclusive
of the exposure pathways categorized as ``past air'' which were not
varied with changes in air concentrations (as described in section
II.C.2.3). The upper bound estimates for the lowest level assessed
(0.02 [mu]g/m\3\) are 2.6-2.9 points IQ loss.
In the general urban case study, the lower bound of air-
related risk falls below 2 points IQ loss for an alternative NAAQS of
0.5 [mu]g/m\3\ max monthly, and below 1 point IQ loss somewhere between
an alternative NAAQS of 0.2 and 0.05 [mu]g/m\3\ max monthly.
The upper-bound of air-related risk for the primary Pb
smelter subarea is generally higher than that for the general urban
case study, likely due to the difference in indoor dust models used for
the two case studies. The indoor dust Pb model used for the primary Pb
smelter considered more completely, the impact of outdoor ambient air
Pb on indoor dust (compared to the hybrid indoor dust Pb model used in
the urban case studies). Specifically, the regression model used for
the primary Pb smelter included consideration for longer-term
relationships between outdoor ambient air and indoor dust (e.g.,
changes in outdoor soil and subsequent tracking in of soil Pb).
As noted above (section II.C.2.c), the three location-
specific urban case studies provide risk estimates for populations with
a broader range of air-related exposures. Accordingly, because of the
population distribution in these three case studies, the air-related
risk is smaller for them than for the other case studies, particularly
at the population median. Further, the majority of the population in
each case study resides in areas with ambient air Pb levels well below
each standard level assessed, particularly for levels above 0.05 [mu]g/
m\3\ max monthly. Consequently, risk estimates indicate little response
to alternative standard levels above 0.05 [mu]g/m\3\ max monthly.
In addition to the air-related risk results described above, we
present two additional categories of risk results, including (a)
estimates of median IQ loss based on total Pb exposure for each case
study (Table 4) and (b) IQ loss incidence estimates for each of the
location-specific case studies (Tables 4 and 5).\103\ Each of these
categories of risk results are described in creater detail below:
---------------------------------------------------------------------------
\103\ As recognized in section II.C.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
---------------------------------------------------------------------------
Estimates of IQ loss for all air quality scenarios (based
on total Pb exposure): Table 4 presents median IQ loss estimates for
total Pb exposure for each of the air quality scenarios simulated for
each case study (as noted earlier in this section, there is greater
uncertainty associated with higher-end risk percentiles and therefore,
they are
[[Page 29218]]
not presented in tabular format here--see Table 5-10 of Risk Assessment
Volume 1 for 95th percentile total IQ loss estimates). As with the
incremental risk results presented in Table 3 above, in order to
reflect the variation in estimates derived from the four different
concentration-response functions included in the analysis, three
categories of estimates are presented in Table 4 including (a) IQ loss
estimates generated using the low concentration-response function (the
model that generated the lowest IQ loss estimates), (b) estimates
generated using the log-linear with low-exposure linearization (LLL)
model, and (c) IQ loss estimates generated using the high
concentration-response function (the model that generated the highest
IQ loss estimates). It is important to emphasize, that, as noted in
Section II.C.2.e, because of limitations in modeling methods, we were
only able to simulate reduction in recent air-related exposures in
considering alternate standard levels and could not simulate reduction
in past air-related exposures. This likely results in an underestimate
of the total degree of reduction in exposure and risk associated with
each standard level. Therefore, in comparing total risk estimates
between alternate NAAQS scenarios (i.e., considering incremental risk
reductions), this aspect of the analysis will tend to underestimate the
reductions in risk associated with alternative NAAQS.
IQ loss incidence estimates for the three location-
specific urban case studies: Estimates of the number of children for
each location-specific urban case study projected to have total Pb-
related IQ loss greater than one point are summarized in Table 5, and
similar estimates for IQ loss greater than 7 points are summarized in
Table 6. Also presented are the changes in incidence of the current
NAAQS and alternative NAAQS scenarios compared to current conditions,
with emphasis placed on estimates generated using the LLL
concentration-response function. Estimates are presented for each of
the four concentration-response functions used in the risk analysis.
This metric illustrates the overall number of children within a given
urban case study location projected to experience various levels of IQ
loss due to Pb exposure and how that distribution of incidence changes
with alternate standard levels. These incidence estimates were only
generated for the location-specific urban case studies, since these
have larger enumerated study populations (additional detail on the
derivation of these incidence estimates is presented in Section 5.3.1.2
of the Risk Assessment Report). The complete set of incidence results
is presented in Risk Assessment Report Appendix O, Section O.3.4.
Total IQ loss results presented in Table 4 for the primary Pb
smelter case study (full study area) illustrate the reason why these
results were not presented earlier in summarizing air-related IQ loss
estimates for the primary Pb smelter case study in Table 3 (and
instead, results for the subarea were presented). As mentioned earlier
in Section II.C.2.c, the full study area for the primary Pb smelter
case study incorporates a large number of simulated children with
relatively low air-related impacts, which results in little
differentiation between alternate standard levels in terms of total IQ
loss (as well as air-related IQ loss). This can be seen by considering
the results in Table 4 for the primary Pb smelter (full study area).
Those results suggest that total IQ loss varies little across alternate
standard levels for the full study area simulation, with the only
noticeable difference in total IQ loss resulting from analysis of the
current standard (when compared to alternate levels). By contrast,
there are notable differences in total IQ loss between alternative
standard levels for the sub-area of the primary Pb smelter case study.
Table 4.--Summary of Risk Estimates for Medians of Total-Exposure Risk Distributions
----------------------------------------------------------------------------------------------------------------
Points IQ loss (total Pb exposure) \a\
-----------------------------------------------
Case study and air quality scenario Low C-R High C-R
function LLL \b\ function
estimate estimate
----------------------------------------------------------------------------------------------------------------
Location-specific (Chicago)
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)................... 2.4 5.6 8.8
Current conditions (0.14 [mu]g/m\3\ max quarterly; 0.31 [mu]g/ 1.4 4.2 5.2
m\3\ max monthly)..............................................
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly)................. 1.4 4.2 5.2
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.8
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.7
----------------------------------------------------------------------------------------------------------------
Location-specific (Cleveland)
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)................... 1.7 4.7 6.3
Current conditions (0.36 [mu]g/m\3\ max quarterly; 0.56 [mu]g/ 1.4 4.2 5.2
m\3\ max monthly)..............................................
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly)................. 1.4 4.2 5.2
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)............... 1.4 4.1 5.0
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly)................. 1.3 4.1 4.9
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.7
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................ 1.2 3.9 4.6
----------------------------------------------------------------------------------------------------------------
Location-specific (Los Angeles)
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)................... 2.1 5.3 7.7
Current conditions (0.09 [mu]g/m\3\ max quarterly; 0.17 [mu]g/ 1.4 4.2 5.1
m\3\ max monthly)..............................................
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.8
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.7
----------------------------------------------------------------------------------------------------------------
General Urban
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)................... 2.5 5.8 9.2
[[Page 29219]]
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly)................. 1.7 4.8 6.4
Current conditions--high-end (0.87 [mu]g/m\3\ max quarterly).... 1.7 4.7 6.3
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)............... 1.6 4.6 5.9
Current conditions--mean (0.14 [mu]g/m\3\ max quarterly)........ 1.5 4.5 5.6
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly)................. 1.5 4.4 5.6
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................ 1.3 4.1 5.0
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.8
----------------------------------------------------------------------------------------------------------------
Primary Pb smelter--full study area
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)................... 1.2 3.8 4.4
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly)................. 1.0 3.7 4.2
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)............... 0.9 3.6 4.2
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly)................. 0.9 3.6 4.1
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................ 0.9 3.6 4.0
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................ 0.9 3.6 4.1
----------------------------------------------------------------------------------------------------------------
Primary Pb smelter--1.5km subarea
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)................... 3.7 6.8 11.2
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly)................. 2.6 5.8 9.4
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)............... 2.0 5.2 7.4
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly)................. 1.9 5.0 6.9
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................ 1.4 4.2 5.1
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................ 1.3 4.0 4.8
----------------------------------------------------------------------------------------------------------------
\a\ --These columns present the estimates of total IQ loss resulting from total Pb exposure (policy-relevant
plus background). Estimates below 1.0 are rounded to one decimal place, all values below 0.05 are presented as
<0.1 and values between 0.05 and 0.1 as 0.1. All values above 1.0 are rounded to the nearest whole number.
\b\ --Log-linear with low-exposure linearization concentration-response function.
Table 5.--Incidence of Children With >1 Point Pb-Related IQ Loss
--------------------------------------------------------------------------------------------------------------------------------------------------------
Dual linear--stratified Log-linear with Dual linear--stratified Log-linear with cutpoint
at 7.5 mg/dl peak blood linearization at 10 m/dL peak blood -------------------------
Pb -------------------------- Pb
-------------------------- -------------------------- Delta
Air quality scenario (for location-specific Delta Delta Delta (change in
urban case studies) (change Incidence (change in (change in Incidence incidence
Incidence inincidence of >1 point incidence Incidence incidence of >1 point compared to
of >1 point compared to IQ loss compared to of >1 point compared to IQ loss current
IQ loss current current IQ loss current conditions)
conditions) conditions) conditions)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Chicago (total modeled child population:
396,511):
Chicago Current Conditions.................. 391,602 ........... 389,754 ........... 271,031 ........... 236,257
Current NAAQS (1.5 mg/m\3\ Maximum 395,797 4,195 395,528 5,773 347,415 76,384 314,053 77,795
Quarterly).................................
Alternative NAAQS (0.2 mg/m\3\ Maximum 391,158 -444 389,461 -293 271,444 412 235,559 -698
Monthly)...................................
Alternative NAAQS (0.05 mg/m\3\ Maximum 389,572 -2,030 387,407 -2,347 253,775 -17,256 224,394 -11,864
Monthly)...................................
Alternative NAAQS (0.02 mg/m\3\ Maximum 389,176 -2,427 386,630 -3,125 249,865 -21,166 219,294 -16,963
Monthly)...................................
Cleveland (total modeled child population:
13,990):
Cleveland Current Conditions................ 13,809 ........... 13,745 ........... 9,526 ........... 8,515
Current NAAQS (1.5 mg/m\3\ Maximum 13,893 84 13,857 112 10,664 1,137 9,769 1,254
Quarterly).................................
Alternative NAAQS (0.2 mg/m\3\ Maximum 13,770 -38 13,703 -42 9,221 -305 8,160 -354
Quarterly).................................
Alternative NAAQS (0.5 mg/m\3\ Maximum 13,789 -20 13,720 -25 9,497 -29 8,464 -51
Monthly)...................................
Alternative NAAQS (0.2 mg/m\3\ Maximum 13,759 -50 13,694 -51 9,083 -443 8,010 -505
Monthly)...................................
Alternative NAAQS (0.05 mg/m\3\ Maximum 13,729 -80 13,642 -103 8,785 -741 7,720 -795
Monthly)...................................
Alternative NAAQS (0.02 mg/m\3\ Maximum 13,720 -88 13,628 -117 8,736 -790 7,668 -846
Monthly)...................................
Los Angeles (total modeled child population:
372,252):
Los Angeles Current Conditions.............. 282,216 ........... 280,711 ........... 191,675 ........... 170,474 ...........
Current NAAQS (1.5 mg/m\3\ Maximum, 285,272 3,056 284,945 4,234 240,988 49,313 226,608 56,134
Quarterly).................................
[[Page 29220]]
Alternative NAAQS (0.05 mg/m\3\ Maximum 281,112 -1,104 279,658 -1,053 183,395 -8,280 161,914 -8,560
Monthly)...................................
Alternative NAAQS (0.02 mg/m\3\ Maximum 280,740 -1,476 279,057 -1,654 180,745 -10,929 158,234 -12,240
Monthly)...................................
--------------------------------------------------------------------------------------------------------------------------------------------------------
Table 6.--Incidence of Children With >7 Points Pb-Related IQ Loss
--------------------------------------------------------------------------------------------------------------------------------------------------------
Dual linear--stratified Log-linear with Dual linear--stratified Log-linear with cutpoint
at 7.5 ug/dL peak blood linearization at 10 ug/dL peak blood -------------------------
Pb -------------------------- Pb
-------------------------- -------------------------- Delta
Air quality scenario (location-specific urban Delta Delta Delta Incidence (change in
case studies) Incidence (change in Incidence (change in Incidence (change in of > 7 incidence
of > 7 incidence of > 7 incidence of > 7 incidence points IQ compared to
points IQ compared to points IQ compared to points IQ compared to loss current
loss current loss current loss current conditions)
conditions) conditions) conditions)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Chicago (total modeled child population:
396,511):
Chicago Current Conditions.................. 136,709 ........... 33,664 ........... 63 ........... 1,015 ...........
Current NAAQS (1.5 [mu]g/m\3\ Maximum 244,401 107,692 100,159 66,495 555 492 5,226 4,211
Quarterly).................................
Alternative NAAQS (0.2 [mu]g/\3\ Maximum 136,067 -642 32,546 -1,118 48 -16 1,007 -8
Monthly)...................................
Alternative NAAQS (0.05 [mu]g/\3\ Maximum 120,706 -16,003 27,367 -6,297 16 -48 864 -151
Monthly)...................................
Alternative NAAQS (0.02 [mu]g/\3\ Maximum 117,819 -18,890 26,027 -7,637 8 -56 690 -325
Monthly)...................................
Cleveland (total modeled child population:
13,990):
Cleveland Current Conditions................ 4,834 ........... 1,212 ........... 3 ........... 46 ...........
Current NAAQS (1.5 [mu]g/m\3\ Maximum 6,139 1,305 1,858 647 4 2 105 59
Quarterly).................................
Alternative NAAQS (0.2 [mu]g/m\3\ Maximum 4,525 -309 1,073 -139 1 -2 40 -6
Quarterly).................................
Alternative NAAQS (0.5 [mu]g/m\3\ Maximum 4,806 -28 1,180 -31 1 -2 43 -3
Monthly)...................................
Alternative NAAQS (0.2 [mu]g/m\3\ Maximum 4,424 -410 1,026 -186 1 -2 43 -3
Monthly)...................................
Alternative NAAQS (0.05 [mu]g/m\3\ Maximum 4,106 -728 886 -326 0 -3 24 -22
Monthly)...................................
Alternative NAAQS (0.02 [mu]g/m\3\ Maximum 4,051 -783 866 -345 0 -3 27 -18
Monthly)...................................
Los Angeles (total modeled child population:
372,252):
Los Angeles Current Conditions.............. 94,684 ........... 22,665 ........... 23 ........... 732 ...........
Current NAAQS (1.5 [mu]g/m\3\ Maximum, 158,171 63,487 57,834 35,168 183 160 3,771 3,038
Quarterly).................................
Alternative NAAQS (0.05 [mu]g/m\3\ Maximum, 87,303 -7,382 19,781 -2,884 11 -11 624 -109
Monthly)...................................
Alternative NAAQS (0.02 [mu]g/m\3\ Maximum, 83,909 -10,775 17,939 -4,726 17 -6 498 -235
Monthly)...................................
--------------------------------------------------------------------------------------------------------------------------------------------------------
D. Conclusions on Adequacy of the Current Primary Standard
The initial issue to be addressed in the current review of the
primary Pb standard is whether, in view of the advances in scientific
knowledge and additional information, the existing standard should be
retained or revised. In evaluating whether it is appropriate to retain
or revise the current standard, the Administrator builds on the general
approach used in the initial setting of the standard, as well as that
used in the last review, and reflects the broader body of evidence and
information now available.
The approach used is based on an integration of information on
health effects associated with exposure to ambient Pb; expert judgment
on the adversity of such effects on individuals; and policy judgments
as to when the standard is requisite to protect public health with an
adequate margin of safety, which are informed by air quality and
related analyses, quantitative exposure and risk assessments when
possible, and qualitative assessment of impacts that could not be
quantified.
The Administrator has taken into account both evidence-based \104\
and quantitative exposure- and risk-based considerations in developing
conclusions on the adequacy of the current primary Pb standard.
Evidence-based considerations include the assessment of evidence for a
variety of
[[Page 29221]]
Pb-related health endpoints from epidemiological, and animal
toxicological studies. Consideration of quantitative exposure- and
risk-based information draws from the results of the exposure and risk
assessments described above. More specifically, estimates of the
magnitude of Pb-related exposures and risks associated with air quality
levels associated with just meeting the current primary Pb NAAQS have
been considered.\105\
---------------------------------------------------------------------------
\104\ The term ``evidence-based'' as used here refers to the
drawing of information directly from published studies, with
specific attention to those reviewed and described in the Criteria
Document, and is distinct from considerations that draw from the
results of the quantitative exposure and risk assessement.
\105\ As described in seciton II.C.2.d above, levels in the
location-specific urban case studies were increased from current
conditions such that the portion of each case study with highest
concentrations would just meet the current NAAQS.
---------------------------------------------------------------------------
In this review, a series of general questions frames the approach
to reaching a decision on the adequacy of the current standard, such as
the following: (1) To what extent does newly available information
reinforce or call into question evidence of associations of Pb
exposures with effects identified when the standard was set?; (2) to
what extent has evidence of new effects or at-risk populations become
available since the time the standard was set?; (3) to what extent have
important uncertainties identified when the standard was set been
reduced and have new uncertainties emerged?; and (4) to what extent
does newly available information reinforce or call into question any of
the basic elements of the current standard?
The question of whether the available evidence supports
consideration of a standard that is more protective than the current
standard includes consideration of: (1) Whether there is evidence that
associations with blood Pb in epidemiological studies extend to ambient
Pb concentration levels that are as low as or lower than had previously
been observed, and the important uncertainties associated with that
evidence; (2) the extent to which exposures of potential concern and
health risks are estimated to occur in areas upon meeting the current
standard and the important uncertainties associated with the estimated
exposures and risks; and (3) the extent to which the Pb-related health
effects indicated by the evidence and the exposure and risk assessments
are considered important from a public health perspective, taking into
account the nature and severity of the health effects, the size of the
at-risk populations, and the kind and degree of the uncertainties
associated with these considerations.
This approach is consistent with the requirements of the NAAQS
provisions of the Act and with how EPA and the courts have historically
interpreted the Act. These provisions require the Administrator to
establish primary standards that, in the Administrator's judgment, are
requisite to protect public health with an adequate margin of safety.
In so doing, the Administrator seeks to establish standards that are
neither more nor less stringent than necessary for this purpose. The
Act does not require that primary standards be set at a zero-risk level
but rather at a level that avoids unacceptable risks to public health,
including the health of sensitive groups.
The following discussion starts with background information on the
current standard (section II.D.1), including both the basis for
derivation of the current standard and considerations and conclusions
from the 1990 Staff Paper (USEPA, 1990b). This is followed by a
discussion of the Agency's approach in this review for evaluating the
adequacy of the current standard, in section II.D.2, including both
evidence-based and exposure/risk-based considerations (sections
II.D.2.a and b, respectively). CASAC advice and recommendations
concerning adequacy of the current standard are summarized in section
II.D.3. Lastly, the Administrator's proposed conclusions with regard to
the adequacy of the current standard are presented in section II.D.4.
1. Background
a. The Current Standard
The current primary standard is set at a level of 1.5 [mu]g/m\3\,
measured as Pb-TSP, not to be exceeded by the maximum arithmetic mean
concentration averaged over a calendar quarter. The standard was set in
1978 to provide protection to the public, especially children as the
particularly sensitive population subgroup, against Pb-induced adverse
health effects (43 FR 46246). In setting the standard, EPA relied on
conclusions regarding sources of exposure, air-related exposure
pathways, variability and susceptibility of young children, the most
sensitive health endpoints, blood Pb level thresholds for various
health effects and the stability and distributional characteristics of
Pb (both in the human body and in the environment) (43 FR 46247). The
specific basis for selecting each of the elements of the standard is
described below.
i. Level
EPA's objective in selecting the level of the current standard was
``to estimate the concentration of Pb in the air to which all groups
within the general population can be exposed for protracted periods
without an unacceptable risk to health'' (43 FR 46252). As stated in
the notice of final rulemaking, ``This estimate was based on EPA's
judgment in four key areas:
(1) Determining the `sensitive population' as that group within the
general population which has the lowest threshold for adverse effects
or greatest potential for exposure. EPA concludes that young children,
aged 1 to 5, are the sensitive population.
(2) Determining the safe level of total lead exposure for the
sensitive population, indicated by the concentration of lead in the
blood. EPA concludes that the maximum safe level of blood lead for an
individual child is 30 [mu]g Pb/dl and that population blood lead,
measured as the geometric mean, must be 15 [mu]g Pb/dl in order to
place 99.5 percent of children in the United States below 30 [mu]g Pb/
dl.
(3) Attributing the contribution to blood lead from nonair
pollution sources. EPA concludes that 12 [mu]g Pb/dl of population
blood lead for children should be attributed to nonair exposure.
(4) Determining the air lead level which is consistent with
maintaining the mean population blood lead level at 15 [mu]g Pb/dl [the
maximum safe mean level]. Taking into account exposure from other
sources (12 [mu]g Pb/dl), EPA has designed the standard to limit air
contribution after achieving the standard to 3 [mu]g Pb/dl. On the
basis of an estimated relationship of air lead to blood lead of 1 to 2,
EPA concludes that the ambient air standard should be 1.5 [mu]g Pb/
m\3\.'' (43 FR 46252)
EPA's judgments in these key areas, as well as margin of safety
considerations, are discussed below.
The assessment of the science that was presented in the 1977
Criteria Document (USEPA, 1977), indicated young children, aged 1 to 5,
as the population group at particular risk from Pb exposure. Children
were recognized to have a greater physiological sensitivity than adults
to the effects of Pb and a greater exposure. In identifying young
children as the sensitive population, EPA also recognized the
occurrence of subgroups with enhanced risk due to genetic factors,
dietary deficiencies or residence in urban areas. Yet information was
not available to estimate a threshold for adverse effects for these
subgroups separate from that of all young children. Additionally, EPA
recognized both a concern regarding potential risk to pregnant women
and fetuses, and a lack of information to establish that these
subgroups are more at risk than young children. Accordingly, young
children, aged 1 to 5, were identified as the group which has the
lowest threshold for adverse
[[Page 29222]]
effects of greatest potential for exposure (i.e., the sensitive
population) (43 FR 46252).
In identifying the maximum safe exposure, EPA relied upon the
measurement of Pb in blood (43 FR 46252-46253). The physiological
effect of Pb that had been identified as occurring at the lowest blood
Pb level was inhibition of an enzyme integral to the pathway by which
heme (the oxygen carrying protein of human blood) is synthesized, i.e.,
delta-aminolevulinic acid dehydratase ([delta]-ALAD). The 1977 Criteria
Document reported a threshold for inhibition of this enzyme in children
at 10 [mu]g Pb/dL. The 1977 Criteria Document also reported a threshold
of 15-20 [mu]g/dL for elevation of erythrocyte protoporphyrin (EP),
which is an indication of some disruption of the heme synthesis
pathway. EPA concluded that this effect on the heme synthesis pathway
(indicated by EP) was potentially adverse. EPA further described a
range of blood levels associated with a progression in detrimental
impact on the heme synthesis pathway. At the low end of the range (15-
20 [mu]g/dL), the initial detection of EP associated with blood Pb was
not concluded to be associated with a significant risk to health. The
upper end of the range (40 [mu]g/dL), the threshold associated with
clear evidence of heme synthesis impairment and other effects
contributing to clinical symptoms of anemia, was regarded by EPA as
clearly adverse to health. EPA also noted that for some children with
blood Pb levels just above those for these effects (e.g., 50 [mu]g/dL),
there was risk for additional adverse effects (e.g., nervous system
deficits). Additionally, in the Agency's statement of factors on which
the conclusion as to the maximum safe blood Pb level for an individual
child was based, EPA stated that the maximum safe blood level should be
``no higher than the blood Pb range characterized as undue exposure by
the Center for Disease Control of the Public Health Service, as
endorsed by the American Academy of Pediatrics, because of elevation of
erythrocyte protoporphyrin (above 30 [mu]g Pb/dL)''.\106\
---------------------------------------------------------------------------
\106\ The CDC subsequently revised their advisory level for
children's blood Pb to 25 [mu]g/dL in 1985, and to 10 [mu]g/dL in
1991. In 2005, with consideration of a review of the evidence by
their advisory committee, CDC revised their statement on Preventing
Lead Poisoning in Young Children, specifically recognizing the
evidence of adverse health effects in children with blood Pb levels
below 10 [mu]g/dL and the data demonstrating that no ``safe''
threshold for blood Pb in children had been identified, and
emphasizing the importance of preventative measures (CDC, 2005a).
Recently, CDC's Advisory Committee on Childhood Lead Poisoning
Prevention noted the 2005 CDC statements and reported on a review of
the clinical interpretation and management of blood Pb levels below
10 [mu]g/dL (ACCLPP, 2007). More details on this level are provided
in Section II.B.1.
---------------------------------------------------------------------------
Having identified the maximum safe blood level in individual
children, EPA next made a public health policy judgment regarding the
target mean blood level for the U.S. population of young children (43
FR 46252-46253). With this judgment, EPA identified a target of 99.5
percent of this population to be brought below the maximum safe blood
Pb level. This judgment was based on consideration of the size of the
sensitive subpopulation, and the recognition that there are special
high-risk groups of children within the general population. The
population statistics available at the time (the 1970 U.S. Census)
indicated a total of 20 million children younger than 5 years of age,
with 15 million residing in urban areas and 5 million in center cities
where Pb exposure was thought likely to be ``high''. Concern about
these high-risk groups influenced EPA's determination of 99.5 percent,
deterring EPA from selecting a population percentage lower than 99.5
(43 FR 46253). EPA then used standard statistical techniques to
calculate the population mean blood Pb level that would place 99.5
percent of the population below the maximum safe level. Based on the
then available data, EPA concluded that blood Pb levels in the
population of U.S. children were normally distributed with a GSD of
1.3. Based on standard statistical techniques, EPA determined that a
thus described population in which 99.5 percent of the population has
blood Pb levels below 30 [mu]g/dL would have a geometric mean blood
level of 15 [mu]g/dL. EPA described 15 [mu]g/dL as ``the maximum safe
blood lead level (geometric mean) for a population of young children''
(43 FR 46247).
When setting the current NAAQS, EPA recognized that the air
standard needed to take into account the contribution to blood Pb
levels from Pb sources unrelated to air pollution. Consequently, the
calculation of the current NAAQS included the subtraction of Pb
contributed to blood Pb from nonair sources, from the estimate of a
safe mean population blood Pb level. Without this subtraction, EPA
recognized that the combined exposure to Pb from air and nonair sources
would result in a blood Pb concentration exceeding the safe level (43
FR 46253). In developing an estimate of this nonair contribution, EPA
recognized the lack of detailed or widespread information about the
relative contribution of various sources to children's blood Pb levels,
such that an estimate could only be made by inference from other
empirical or theoretical studies, often involving adults. Additionally,
EPA recognized the expectation that the contribution to blood Pb levels
from nonair sources would vary widely, was probably not in constant
proportion to air Pb contribution, and in some cases may alone exceed
the target mean population blood Pb level (43 FR 46253-46254). The
amount of blood Pb attributed to nonair sources was selected based
primarily on findings in studies of blood Pb levels in areas where air
Pb levels were low relative to other locations in U.S. The air Pb
levels in these areas ranged from 0.1 to 0.7 [mu]g/m\3\. The average of
the reported blood Pb levels for children of various ages in these
areas was on the order of 12 [mu]g/dL. Thus, 12 [mu]g/dL was identified
as the nonair contribution, and subtracted from the population mean
target level of 15 [mu]g/dL to yield a value of 3 [mu]g/dL as the limit
on the air contribution to blood Pb.
In determining the air Pb level consistent with an air contribution
of 3 [mu]g Pb/dL, EPA reviewed studies assessed in the 1977 Criteria
Document that reported changes in blood Pb with different air Pb
levels. These studies included a study of children exposed to Pb from a
primary Pb smelter, controlled exposures of adult men to Pb in fine
particulate matter, and a personal exposure study involving several
male cohorts exposed to Pb in a large urban area in the early 1970s (43
FR 46254).\107\ Using all three studies, EPA calculated an average
slope or ratio over the entire range of data. That value was 1.95
(rounded to 2 [mu]g/dL blood Pb concentration to 1 [mu]g/m\3\ air Pb
concentration), and is recognized to fall within the range of values
reported in the 1977 Criteria Document. On the basis of this 2-to-1
relationship, EPA concluded that the ambient air standard should be 1.5
[mu]g Pb/m\3\ (43 FR 46254).
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\107\ Mean blood Pb levels in the adult study groups ranged from
10 [mu]g/dL to approximately 30 [mu]g/dL and in the child groups
they ranged from approximately 20 [mu]g/dL up to 65 [mu]g/dL (USEPA,
1986a, section 11.4.1).
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In consideration of the appropriate margin of safety during the
development of the current NAAQS, EPA identified the following factors:
(1) The 1977 Criteria Document reported multiple biological effects of
Pb in practically all cell types, tissues and organ systems, of which
the significance for health had not yet been fully studied; (2) no
beneficial effects of Pb at then current environmental levels were
recognized;
[[Page 29223]]
(3) data were incomplete as to the extent to which children are
indirectly exposed to air Pb that has moved to other environmental
media, such as water, soil and dirt, and food; (4) Pb is chemically
persistent and with continued uncontrolled emissions would continue to
accumulate in human tissue and the environment; and (5) the possibility
that exposure associated with blood Pb levels previously considered
safe might influence neurological development and learning abilities of
the young child (43 FR 46255). Recognizing that estimating an
appropriate margin of safety for the air Pb standard was complicated by
the multiple sources and media involved in Pb exposure, EPA chose to
use margin of safety considerations principally in establishing a
maximum safe blood Pb level for individual children (30 [mu]g Pb/dL)
and in determining the percentage of children to be placed below this
maximum level (about 99.5 percent). Additionally, in establishing other
factors used in calculating the standard, EPA used margin of safety
considerations in the sense of making careful judgment based on
available data, but these judgments were not considered to be at the
precautionary extreme of the range of data available at the time (43 FR
46251).
EPA further recognized that, because of the variability between
individuals in a population experiencing a given level of Pb exposure,
it was considered impossible to provide the same margin of safety for
all members in the sensitive population or to define the margin of
safety in the standard as a simple percentage. EPA believed that the
factors it used in designing the standards provided an adequate margin
of safety for a large proportion of the sensitive population. The
Agency did not believe that the margin was excessively large or on the
other hand that the air standard could protect everyone from elevated
blood Pb levels (43 FR 46251).
ii. Averaging Time, Form, and Indicator
The averaging time for the current standard is a calendar quarter.
In the decision for this aspect of the standard, the Agency also
considered a monthly averaging period, but concluded that ``a
requirement for the averaging of air quality data over calendar quarter
will improve the validity of air quality data gathered without a
significant reduction in the protectiveness of the standards.'' As
described in the notice for this decision (43 FR 46250), this
conclusion was based on several points, including the following:
An analysis of ambient measurements available at the time
indicated that the distribution of air Pb levels was such that there
was little possibility that there could be sustained periods greatly
above the average value in situations where the quarterly standard was
achieved.
A recognition that the monitoring network may not actually
represent the exposure situation for young children, such that it
seemed likely that elevated air Pb levels when occurring would be close
to Pb air pollution sources where young children would typically not
encounter them for the full 24-hour period reported by the monitor.
Medical evidence available at the time indicated that
blood Pb levels re-equilibrate slowly to changes in air exposure, a
finding that would serve to dampen the impact of short-term period of
exposure to elevated air Pb.
Direct exposure to air is only one of several routes of
total exposure, thus lessening the impact of a change in air Pb on
blood Pb levels.
The statistical form of the current standard is a not-to-be-
exceeded or maximum value. EPA set the standard as a ceiling value with
the conclusion that this air level would be safe for indefinite
exposure for young children (43 FR 46250).
The indicator is total airborne Pb collected by a high volume
sampler (43 FR 46258). EPA's selection of Pb-TSP as the indicator for
the standard was based on explicit recognition both of the significance
of ingestion as an exposure pathway for Pb that had deposited from the
air and of the potential for Pb deposited from the air to become re-
suspended in respirable size particles in the air and available for
human inhalation exposure. As stated in the final rule, ``a significant
component of exposure can be ingestion of materials contaminated by
deposition of lead from the air,'' and that, ``in addition to the
indirect route of ingestion and absorption from the gastrointestinal
tract, non-respirable Pb in the environment may, at some point become
respirable through weathering or mechanical action'' (43 FR 46251).
b. Policy Options Considered in the Last Review
During the 1980s, EPA initiated a review of the air quality
criteria and NAAQS for Pb. CASAC and the public were fully involved in
this review, which led to the publication of a criteria document with
associated addendum and a supplement (USEPA, 1986a, 1986b, 1990a), an
exposure analysis methods document (USEPA, 1989), and a staff paper
(USEPA, 1990b).
Total emissions to air were estimated to have dropped by 94 percent
between 1978 and 1987, with the vast majority of it attributed to the
reduction of Pb in gasoline. Accordingly, the focus of the last review
was on areas near stationary sources of Pb emissions. Although such
sources were not considered to have made a significant contribution (as
compared to Pb in gasoline) to the overall Pb pollution across large-
urban or regional areas, Pb emissions from such sources were considered
to have the potential for a significant impact on a local scale. Air Pb
concentrations, and especially soil and dust Pb concentrations, had
been associated with elevated levels of Pb absorption in children and
adults in numerous Pb point source community studies. Exceedances of
the current NAAQS were found at that time only in the vicinity of
nonferrous smelters or other point sources of Pb.
In summarizing and interpreting the health evidence presented in
the 1986 Criteria Document and associated documents, the 1990 Staff
Paper described the collective impact on children of the effects at
blood Pb levels above 15 [mu]g/dL as representing a clear pattern of
adverse effects worthy of avoiding. This is in contrast to EPA's
identification of 30 [mu]g/dL as a safe blood Pb level for individual
children when the NAAQS was set in 1978. The Staff Paper further stated
that at blood Pb levels of 10-15 [mu]g/dL, there was a convergence of
evidence of Pb-induced interference with a diverse set of physiological
functions and processes, particularly evident in several independent
studies showing impaired neurobehavioral function and development.
Further, the available data did not indicate a clear threshold in this
blood Pb range. Rather, it suggested a continuum of health risks down
to the lowest levels measured.\108\
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\108\ In 1991, the CDC reduced their advisory level for
children's blood Pb from 25 [mu]g/dL to 10 [mu]g/dL.
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For the purposes of comparing the relative protectiveness of
alternative Pb NAAQS, the staff conducted analyses to estimate the
percentages of children with blood Pb levels above 10 [mu]g/dL and
above 15 [mu]g/dL for several air quality scenarios developed for a
small set of stationary source exposure case studies. The results of
the analyses of child populations living near two Pb smelters indicated
that substantial reductions in Pb exposure could be achieved through
just meeting the current Pb NAAQS. According to the best estimate
analyses, over 99.5% of children living in areas significantly affected
by the smelters would have blood Pb levels below 15
[[Page 29224]]
[mu]g/dL if the current standard was achieved. Progressive changes in
this number were estimated for the alternative monthly Pb NAAQS levels
evaluated in those analyses, which ranged from 1.5 [mu]g/m\3\ to 0.5
[mu]g/m\3\.
In light of the health effects evidence available at the time, the
1990 Staff Paper presented air quality, exposure, and risk analyses,
and other policy considerations, as well as the following staff
conclusions with regard to the primary Pb NAAQS (USEPA, 1990b, pp. xii
to xiv):
(1) ``The range of standards * * * should be from 0.5 to 1.5 [mu]g/
m\3\.''
(2) ``A monthly averaging period would better capture short-term
increases in lead exposure and would more fully protect children's
health than the current quarterly average.''
(3) ``The most appropriate form of the standard appears to be the
second highest monthly averages {sic{time} in a 3-year span. This form
would be nearly as stringent as a form that does not permit any
exceedances and allows for discounting of one `bad' month in 3 years
which may be caused, for example, by unusual meteorology.''
(4) ``With a revision to a monthly averaging time more frequent
sampling is needed, except in areas, like roadways remote from lead
point sources, where the standard is not expected to be violated. In
those situations, the current 1-in-6 day sampling schedule would
sufficiently reflect air quality and trends.''
(5) ``Because exposure to atmospheric lead particles occurs not
only via direct inhalation, but via ingestion of deposited particles as
well, especially among young children, the hi-volume sampler provides a
reasonable indicator for determining compliance with a monthly standard
and should be retained as the instrument to monitor compliance with the
lead NAAQS until more refined instruments can be developed.''
Based on its review of a draft Staff Paper, which contained the
above recommendations, the CASAC strongly recommended to the
Administrator that EPA should actively pursue a public health goal of
minimizing the Pb content of blood to the extent possible, and that the
Pb NAAQS is an important component of a multimedia strategy for
achieving that goal (CASAC, 1990, p. 4). In noting the range of levels
recommended by staff, CASAC recommended consideration of a revised
standard that incorporates a ``wide margin of safety, because of the
risk posed by Pb exposures, particularly to the very young whose
developing nervous system may be compromised by even low level
exposures'' (id., p. 3). More specifically, CASAC judged that a
standard within the range of 1.0 to 1.5 [mu]g/m\3\ would have
``relatively little, if any, margin of safety;'' that greater
consideration should be given to a standard set below 1.0 [mu]g/m\3\;
and, to provide perspective in setting the standard, it would be
appropriate to consider the distribution of blood Pb levels associated
with meeting a monthly standard of 0.25 [mu]g/m\3\, a level below the
range considered by staff (id.).
After consideration of the documents developed during the review,
EPA chose not to propose revision of the NAAQS for Pb. During the same
time period, the Agency published and embarked on the implementation of
a broad, multi-program, multi-media, integrated national strategy to
reduce Pb exposures (USEPA, 1991). As discussed above in section I.C.,
as part of implementing this integrated Pb strategy, the Agency focused
efforts primarily on regulatory and remedial clean-up actions aimed at
reducing Pb exposures from a variety of nonair sources judged to pose
more extensive public health risks to U.S. populations, as well as on
actions to reduce Pb emissions to air, particularly near stationary
sources.\109\
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\109\ A description of the various programs implemented since
1990 to reduce Pb exposures, including the recent RRP rule, is
provided in section I.C.
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2. Considerations in the Current Review
a. Evidence-Based Considerations
In considering the broad array of health effects evidence assessed
in the Criteria Document with respect to the adequacy of the current
standard, the discussion here, like that in the Staff Paper and ANPR,
focuses on those health endpoints associated with the Pb exposure and
blood levels most pertinent to ambient exposures. In so doing, EPA
gives particular weight to evidence available today that differs from
that available at the time the standard was set with regard to its
support of the current standard.
First, with regard to the sensitive population, the susceptibility
of young children to the effects of Pb is well recognized, in addition
to more recent recognition of effects of chronic or cumulative Pb
exposure with advancing age (CD, Sections 5.3.7 and pp. 8-73 to 8-75).
The prenatal period and early childhood are periods of increased
susceptibility to Pb exposures, with evidence of adverse effects on the
developing nervous system that generally appear to persist into later
childhood and adolescence (CD, Section 6.2).\110\ Thus, while the
sensitivity of the elderly and other particular subgroups is
recognized, as at the time the standard was set, young children
continue to be recognized as a key sensitive population for Pb
exposures.
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\110\ For example, the following statement is made in the
Criteria Document ``Negative Pb impacts on neurocognitive ability
and other neurobehavioral outcomes are robust in most recent studies
even after adjustment for numerous potentially confounding factors
(including quality of care giving, parental intelligence, and
socioeconomic status). These effects generally appear to persist
into adolescence and young adulthood.'' (CD, p.E-9)
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With regard to the exposure levels at which adverse health effects
occur, the current evidence demonstrates the occurrence of adverse
health effects at appreciably lower blood Pb levels than those
demonstrated by the evidence at the time the standard was set, at which
time the Agency identified 30 [mu]g/dL as the maximum safe blood Pb
level for individual children and 15 [mu]g/dL as the maximum safe
geometric mean blood Pb level for a population of children (as
described in section II.D.1.a above). This change in the evidence since
the time the standard was set is reflected in changes made by the CDC
in their advisory level for Pb in children's blood, and changes they
have made in their characterization of that level (as described in
section II.B.1.b). Although CDC recognized a level of 30 [mu]g/dL blood
Pb as warranting individual intervention in 1978 when the Pb NAAQS was
set, in 2005 they recognized the evidence of adverse health effects in
children with blood Pb levels below 10 [mu]g/dL and the data
demonstrating that no ``safe'' threshold for blood Pb had been
identified (CDC, 1991; CDC, 2005).
As summarized in section II.B above, the Criteria Document
describes current evidence regarding the occurrence of a variety of
health effects, including neurological effects in children associated
with blood Pb levels extending well below 10 [mu]g/dL (CD, Sections
6.2, 8.4 and 8.5).\111\ As stated
[[Page 29225]]
in the Criteria Document, ``The overall weight of the available
evidence provides clear substantiation of neurocognitive decrements
being associated in young children with blood-Pb concentrations in the
range of 5-10 [mu]g/dL, and possibly somewhat lower. Some newly
available analyses appear to show Pb effects on the intellectual
attainment of preschool and school age children at population mean
concurrent blood-Pb levels ranging down to as low as 2 to 8 [mu]g/dL''
(CD, p. E-9). With regard to the evidence of neurological effects at
these low levels, EPA notes, in particular (and discusses more
completely in section II.B.2.b above), the international pooled
analysis by Lanphear and others (2005), studies of individual cohorts
such as the Rochester, Boston, and Mexico City cohorts (Canfield et
al., 2003a; Canfield et al., 2003b; Bellinger and Needleman, 2003;
Tellez-Rojo et al., 2006), the study of African-American inner-city
children from Detroit (Chiodo et al., 2004), the cross-sectional study
of young children in three German cities (Walkowiak et al., 1998) and
the cross-sectional analysis of a nationally representative sample from
the NHANES III (collected from 1988-1994) (Lanphear et al., 2000). In
the study by Lanphear et al (2000), the mean blood Pb for the full
study group was 1.9 [mu]g/dL and the mean blood Pb level in the lowest
blood Pb subgroup with which a statistically significant association
with neurocognitive effects was found (individual blood Pb values <5
[mu]g/dL) was 1.7 [mu]g/dL (CD, pp. 6-31 to 6-32; Lanphear et al.,
2000; Auinger, 2008).\112\ These studies and associated limitations are
discussed above in section II.B.2.b.
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\111\ For context, it is noted that the 2001-2004 median blood
level for children aged 1-5 of all races and ethnic groups is 1.6
[mu]g/dL, the median for the subset living below the poverty level
is 2.3 [mu]g/dL and 90th percentile values for these two groups are
4.0 [mu]g/dL and 5.4 [mu]g/dL, respectively. Similarly, the 2001-
2004 median blood level for black, non-hispanic children aged 1-5 is
2.5 [mu]g/dL, while the median level for the subset of that group
living below the poverty level is 2.9 [mu]g/dL and the median level
for the subset living in a household with income more than 200% of
the poverty level is 1.9 [mu]g/dL. Associated 90th percentile values
for 2001-2004 are 6.4 [mu]g/dL (for black, non-hispanic children
aged 1-5), 7.7 [mu]g/dL (for the subset of that group living below
the poverty level) and 4.1 [mu]g/dL (for the subset living in a
household with income more than 200% of the poverty level). (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm--then
click on ``Download a universal spreadsheet file of the Body Burdens
data tables'').
\112\ These findings include significant associations in some of
the study sample subsets of children, namely those with blood Pb
levels less than 10 [mu]g/dL, less than 7.5 [mu]g/dL, and less than
5 [mu]g/dL. The mean blood Pb level in the third subset was 1.7
[mu]g/dL (Auinger, 2008). A positive, but not statistically
significant association, was observed in the less than 2.5 [mu]g/dL
subset (mean blood Pb of 1.2 [mu]g/dL [Auinger, 2008]), although the
effect estimate for this subset was largest among all the subsets
(Lanphear et al., 2000). The lack of statistical significance for
this subset may be due to the smaller sample size of this subset
which would lead to lower statistical power.
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As stated in the Criteria Document with regard to the
neurocognitive effects in children, the ``weight of overall evidence
strongly substantiates likely occurrence of type of effect in
association with blood-Pb concentrations in range of 5-10 [mu]g/dL, or
possibly lower, as implied by (???) [in associated Table 8-5 of
Criteria Document]. Although no evident threshold has yet been clearly
established for those effects, the existence of such effects at still
lower blood-Pb levels cannot be ruled out based on available data.''
(CD, p. 8-61). The Criteria Document further notes that any such
threshold may exist ``at levels distinctly lower than the lowest
exposures examined in these epidemiological studies'' (CD, p. 8-67).
i. Evidence-Based Framework Considered in the Staff Paper
In considering the adequacy of the current standard, the Staff
Paper considered the evidence in the context of the framework used to
determine the standard in 1978, as adapted to reflect the current
evidence. In so doing, the Staff Paper recognized that the health
effects evidence with regard to characterization of a threshold for
adverse effects has changed since the standard was set in 1978, as have
the Agency's views on the characterization of a safe blood Pb level. As
described in section II.D.1.a, parameters for this framework include
estimates for average nonair blood Pb level, and air-to-blood ratio, as
well as a maximum safe individual and/or geometric mean blood Pb level.
For this last parameter, the Staff Paper for the purposes of this
evaluation considered the lowest population mean blood Pb levels with
which some neurocognitive effects have been associated in the evidence.
As when the standard was set in 1978, there remain today
contributions to blood Pb levels from nonair sources. In 1978, the
Agency estimated the average blood Pb level for young children
associated with nonair sources to be 12 [mu]g/dL (as described in
section II.D.1.a). However, consistent with reductions since that time
in air Pb concentrations \113\ which contribute to blood Pb, nonair
contributions have also been reduced (as described in section II.A.4
above). The Staff Paper noted that the current evidence is limited with
regard to estimates of the aggregate reduction since 1978 of all nonair
sources to blood Pb and with regard to an estimate of current nonair
blood Pb levels (discussed in sections II.A.4). In recognition of
temporal reductions in nonair sources discussed in section II.A.4 and
in the context of estimates pertinent to an application of the 1978
framework, the CASAC Pb Panel recommended consideration of 1.0-1.4
[mu]g/dL or lower as an estimate of the nonair component of blood Pb
pertinent to average blood Pb levels (as more fully described in
section II.A.4 above; Henderson, 2007b).
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\113\ Air Pb concentrations nationally are estimated to have
declined more than 90% since the early 1980s, in locations not known
to be directly influenced by stationary sources (Staff Paper, pp. 2-
22 to 2-23).
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As in 1978, the evidence demonstrates that Pb in ambient air
contributes to Pb in blood, with the pertinent exposure routes
including both inhalation and ingestion (CD, Sections 3.1, 3.2, 4.2 and
4.4). In 1978, the evidence indicated a quantitative relationship
between ambient air Pb and blood Pb in terms of an air-to-blood ratio
that ranged from 1:1 to 1:2 (USEPA, 1977). In setting the standard, the
Agency relied on a ratio of 1:2, i.e., 2 [mu]g/dL blood Pb per 1 [mu]g/
m\3\ air Pb (as described in section II.D.1.a above). The Staff Paper
observed that ``[W]hile there is uncertainty and variability in the
absolute value of an air-to-blood relationship, the current evidence
indicates a notably greater ratio * * * e.g., on the order of 1:3 to
1:10'' (USEPA, 2007c).
Based on the information described above, the Staff Paper concluded
that young children remain the sensitive population of primary focus in
this review, ``there is now no recognized safe level of Pb in
children's blood and studies appear to show adverse effects at
population mean concurrent blood Pb levels as low as approximately 2
[mu]g/dL (CD, pp. 6-31 to 6-32; Lanphear et al., 2000)'' (USEPA,
2007c). The Staff Paper further stated that ``while the nonair
contribution to blood Pb has declined, perhaps to a range of 1.0-1.4
[mu]g/dL, the air-to-blood ratio appears to be higher at today's lower
blood Pb levels than the estimates at the time the standard was set,
with current estimates on the order of 1:3 to 1:5 and perhaps up to
1:10'' (USEPA, 2007c). Adapting the framework employed in setting the
standard in 1978, the Staff Paper concluded that ``the more recently
available evidence suggests a level for the standard that is lower by
an order of magnitude or more'' (USEPA, 2007c).
ii. Air-Related IQ Loss Evidence-Based Framework
Since completion of the Staff Paper and ANPR, the Agency has
further considered the evidence with regard to adequacy of the current
standard using an approach other than the adapted 1978 framework
considered in the Staff Paper. This alternative evidence-based
framework, referred to as the air-related IQ loss framework, shifts
focus from identifying an appropriate target population mean blood lead
level and instead focuses on the magnitude of effects of air-related Pb
on neurocognitive functions. This framework builds on a recommendation
by the CASAC Pb Panel to consider the evidence in a more quantitative
manner,
[[Page 29226]]
and is discussed in more detail below in section II.E.3.a, concerning
the level of the standard.
In this air-related IQ loss framework, we have drawn from the
entire body of evidence as a basis for concluding that there are causal
associations between air-related Pb exposures and population IQ
loss.\114\ We have also drawn more quantitatively from the evidence by
using evidence-based C-R functions to quantify the association between
air Pb concentrations and air-related population mean IQ loss. Thus,
this framework more fully considers the evidence with regard to the
concentration-response relationship for the effect of Pb on IQ, and it
also draws from estimates for air-to-blood ratios.
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\114\ For example, as stated in the Criteria Document,
``Fortunately, there exists a large database of high quality studies
on which to base inferences regarding the relationship between Pb
exposure and neurodevelopment. In addition, Pb has been extensively
studied in animal models at doses that closely approximate the human
situation. Experimental animal studies are not compromised by the
possibility of confounding by such factors as social class and
correlated environmental factors. The enormous experimental animal
literature that proves that Pb at low levels causes neurobehavioral
deficits and provides insights into mechanisms must be considered
when drawing causal inferences (Bellinger, 2004; Davis et al., 1990;
U.S. Environmental Protection Agency, 1986a, 1990).'' (CD, p. 6-75)
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While we note the evidence of steeper slope for the C-R
relationship for blood Pb concentration and IQ loss at lower blood Pb
levels (described in sections II.B.2.b and II.E.3.a), for purposes of
consideration of the adequacy of the current standard we are concerned
with the C-R relationship for blood Pb levels that would be associated
with exposure to air-related Pb at the level of the current standard.
For this purpose, we have focused on a median linear estimate of the
slope of the C-R function for blood Pb levels up to, but no higher
than, 10 [mu]g/dL (described in section II.B.2.b above). The median
slope estimate is -0.9 IQ points per [mu]g/dL blood Pb \115\ (CD, p. 8-
80).
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\115\ As noted above (in section II.B.2.b), this slope is
similar to the slope for the below 10 [mu]g/dL piece of the
piecewise model used in the RRP rule economic analysis.
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Applying estimates of air-to-blood ratios ranging from 1:3 to 1:5,
drawing from the discussion of air-to-blood ratios in section II.B.1.c
above, a population of children exposed at the current level of the
standard might be expected to result in an average air-related blood Pb
level above 4 [mu]g/dL.\116\ Multiplying these blood Pb levels by the
slope estimate, identified above, for blood Pb levels extending up to
10 [mu]g/dL (-0.9 IQ points per [mu]g/dL), would imply an average air-
related IQ loss for such a group of children on the order of 4 or more
IQ points.
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\116\ This is based on the calculation in which 1.5 [mu]g/m\3\
is multiplied by a ratio of 3 [mu]g blood Pb per 1 [mu]g/m\3\ air Pb
to yield an air-related blood Pb estimate of 4.5 [mu]g/dL; using a
1:5 ratio yields an estimate of 7.5 [mu]g/dL. As with the 1978
framework considered in the Staff Paper, the context for use of the
air-to-blood ratio here is a population being exposed at the level
of the standard.
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b. Exposure- and Risk-Based Considerations
As discussed above in section II.C, we have estimated exposures and
health risks associated with air quality that just meets the current
standard to help inform judgments about whether or not the current
standard provides adequate protection of public health, taking into
account key uncertainties associated with the estimated exposures and
risks (summarized above in section II.C and more fully in the Risk
Assessment Report).
As discussed above, children are the sensitive population of
primary focus in this review. The exposure and risk assessment
estimates Pb exposure for children (less than 7 years of age), and
associated risk of neurocognitive effects in terms of IQ loss. In
addition to the risks (IQ loss) that were quantitatively estimated, EPA
recognizes that there may be long-term adverse consequences of such
deficits over a lifetime, and there are other, unquantified adverse
neurocognitive effects that may occur at similarly low exposures which
might additionally contribute to reduced academic performance, which
may have adverse consequences over a lifetime (CD, pp. 8-29 to 8-
30).\117\ Other impacts at low levels of childhood exposure that were
not quantified in the risk assessment include: other neurological
effects (sensory, motor, cognitive and behavioral), immune system
effects (including some related to allergic responses and asthma), and
early effects related to anemia. Additionally, as noted in section
II.B.2, other health effects evidence demonstrates associations between
Pb exposure and adverse health effects in adults (e.g., cardiovascular
and renal effects).\118\
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\117\ For example, the Criteria Document notes particular
findings with regard to academic achievement as ``suggesting that
Pb-sensitive neuropsychological processing and learning factors not
reflected by global intelligence indices might contribute to reduced
performance on academic tasks'' (CD, pp. 8-29 to 8-30).
\118\ The weight of the evidence differs for the different
endpoints.
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As noted in the Criteria Document, a modest change in the
population mean of a health index, that is quantified for each
individual, can have substantial implications at the population level
(CD, p. 8-77, Sections 8.6.1 and 8.6.2; Bellinger, 2004; Needleman et
al., 1982; Weiss, 1988; Weiss, 1990)). For example, for an individual
functioning in the low range of IQ due to the influence of risk factors
other than Pb, a Pb-associated IQ loss of a few points might be
sufficient to drop that individual into the range associated with
increased risk of educational, vocational, and social handicap (CD, p.
8-77), while such a decline might create less significant impacts for
the individual near the mean of the population. Further, given a
uniform manifestation of Pb-related decrements across the range of IQ
scores in a population, a downward shift in the mean IQ value is
associated not only with a substantial increase in the percentage of
individuals achieving very low scores, but also with substantial
decreases in percentages achieving very high scores (CD, p. 8-81). The
CASAC Pb Panel has advised on this point that ``a population loss of 1-
2 IQ points is highly significant from a public health perspective''
(Henderson, 2007a, p. 6).
In considering exposure and risk estimates with regard to adequacy
of the current standard, EPA has focused on IQ loss for air-related
exposure pathways. As described in section II.C.2.e above, limitations
in our data and modeling tools have resulted in an inability to develop
specific estimates such that we have approximated estimates for the
air-related pathways, bounded on the low end by exposure/risk estimated
for the ``recent air'' category and on the upper end by the exposure/
risk estimated for the ``recent air'' plus ``past air'' categories.
Thus, the following discussion presents air-related IQ loss estimates
in terms of upper and lower bounds. In addition, as noted above
(section II.C.3.b), this discussion focuses predominantly on risk
estimates derived using the log-linear with low-exposure linearization
(LLL) C-R function, with the range associated with the other three
functions used in the assessment also being noted. Further, air-related
risk estimates are presented for the median and for an upper percentile
(i.e., the 95th percentile of the population assessed).
EPA and CASAC recognize uncertainties in the risk estimates in the
tails of the distribution and consequently the 95th percentile is
reported as the estimate of the high end of the risk distribution
(Henderson, 2007b, p. 3). In so doing, however, EPA notes that it is
important to consider that there are individuals in the population
expected to have higher risk, particularly in light of the risk
management objectives for the current standard which was set in 1978 to
[[Page 29227]]
protect the 99.5th percentile. Further, we note an increased
uncertainty in our estimates of air-related risk for the upper
percentiles, such as the 95th percentile, due to limitations in the
data and tools available to us to estimate pathway contributions to
blood Pb and associated risk for individuals at the upper ends of the
distribution.
In order to consider exposure and risk associated with the current
standard, EPA developed estimates for a case study based on air quality
projected to just meet the standard in a location of the country where
air concentrations currently do not meet the current standard (the
primary Pb smelter case study). Estimates of median air-related IQ loss
associated with just meeting the current NAAQS in the primary Pb
smelter case study subarea had a lower bound estimate of <3.2 points IQ
loss (``recent air'' category of Pb exposures) and an upper bound
estimate of <9.4 points IQ loss (``recent air'' plus ``past air''
category) for the range of C-R functions (Table 3). This estimate
(recent air plus past air) for the subarea based on the LLL C-R
function is 6.0 points IQ loss for the median and 8.0 points IQ loss
for the 95th percentile, with which we note a greater uncertainty than
for the median estimate (as discussed above).\119\ Modeling limitations
have affected our ability to derive lower bound estimates for this case
study (as described above in section II.C.2.c).
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\119\ We note that while we have termed risk estimates derived
for the sum of ``recent air'' plus ``past air'' exposure pathways as
``upper bound'' estimates of air-related risk, the primary Pb
smelter subarea is an area where soil has been remediated and thus
does not reflect any historical deposition. Further, soil Pb
concentrations in this area are not stable and may be increasing,
seeming to indicate ongoing response to current atmospheric
depositon in the area. Thus, for this case study, the ``recent air''
plus ``past air'' estimates are less of an ``upper bound'' for air-
related risk than in other case studies where historical Pb
deposition may have some representation in the ``past air'' soil
ingestion pathway.
---------------------------------------------------------------------------
Additionally, we developed estimates of blood Pb and associated IQ
loss associated with the current standard for the urban case studies.
We note that we consider it extremely unlikely that air concentrations
in urban areas across the U.S. that are currently well below the
current standard would increase to just meet the standard. However, we
recognize the potential, although not the likelihood, for air Pb
concentrations in some limited areas currently well below the standard
to increase to just meet the standard by way of, for example, expansion
of existing sources (e.g., facilities operating as secondary smelters
may exercise previously used capabilities as primary smelters) or by
the congregation of multiple Pb sources in adjacent locations. We have
simulated this scenario (increased Pb concentrations to just meet the
current standard) in a general urban case study and three location-
specific urban case studies. For the location-specific urban case
studies, we note substantial uncertainty in simulating how the profile
of Pb concentrations might change in the hypothetical case where
concentrations increase to just meet the current standard.
Turning first to the exposure/risk estimates for the current NAAQS
scenario simulated for the general urban case study, which is a
simplified representation of a location within an urban area (described
in section II.C.2.h above), median estimates of air-related IQ loss
range from 1.5 to 7.7 points (across all four C-R functions), with an
estimate based on the LLL function bounded at the low end by 3.4 points
and at the high end by 4.8 points (Table 3). At the 95th percentile for
total IQ loss (LLL estimate), IQ loss associated with air-related Pb is
estimated to fall somewhere between 5.5 and 7.6 points (Staff Paper,
Table 4-6).
In considering the estimates for the three location-specific urban
case studies, we first note the extent to which exposures associated
with increased air Pb concentrations that simulate just meeting the
current standard are estimated to increase blood Pb levels in young
children. The magnitude of this for the median total blood Pb ranges
from 0.3 [mu]g/dL (an increase of 20 percent) in the case of the
Cleveland study area (where the highest monitor is estimated to be
approximately one fourth of the current NAAQS), up to approximately 1
[mu]g/dL (an increase of 50 to 70%) for the Chicago and Los Angeles
study areas, where the highest monitor is estimated to be at or below
one tenth of the current NAAQS (Table 1). Median estimates of air-
related risk for these case studies range from 0.6 points IQ loss
(recent air estimate using low-end C-R function) to 7.4 points IQ loss
(recent plus past air estimate using the high-end C-R function). The
corresponding estimates based on the LLL C-R function range from 2.7
points (lowest location-specific recent air estimate) to 4.7 points IQ
loss (highest location-specific recent plus past air estimate). The
comparable estimates of air-related risk for children at the 95th
percentile in these three case studies range from 2.6 to 7.6 points IQ
loss for the LLL C-R function (Staff paper, Table 4-6), although we
note increased uncertainty in the magnitude of these 95th percentile
air-related estimates.
Another way in which the risk assessment results might be
considered is by comparing current NAAQS scenario estimates to current
conditions, although in so doing, it is important to recognize that, as
stated below and described in section II.C., this will underestimate
air-related impacts associated with the current NAAQS. In making such a
comparison of estimates for the three location-specific urban case
studies, the estimated difference in total Pb-related IQ loss for the
median child is about 0.5 to 1.4 points using the LLL C-R function and
a similar magnitude of difference is estimated for the 95th percentile.
The corresponding comparison for the general urban case study indicates
the current NAAQS scenario median total Pb-related IQ loss is 1.1 to
1.3 points higher than the two current conditions scenarios. As
described in section II.C, such comparisons are underestimates of air-
related impacts brought about as a result of increased air Pb
concentrations, and consequently they are inherently underestimates of
the true impact of an increased NAAQS level on public health.
In considering the exposure/risk information with regard to
adequacy of the current standard, the Staff Paper first considered the
estimates described above, particularly those associated with air-
related risk.\120\ The Staff Paper described these estimates for the
current NAAQS as being indicative of levels of IQ loss associated with
air-related risk that may ``reasonably be judged to be highly
significant from a public health perspective'' (USEPA, 2007c).
---------------------------------------------------------------------------
\120\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard, nor for the
general urban case study.
---------------------------------------------------------------------------
The Staff Paper also describes a different risk metric that
estimated differences in the numbers of children with different amounts
of Pb-related IQ loss between air quality scenarios for current
conditions and for the current NAAQS in the three location-specific
urban case studies. For example, estimates of the additional number of
children with IQ loss greater than one point (based on the LLL C-R
function) in these three study areas, for the current NAAQS scenario as
compared to current conditions, range from 100 to 6,000 across the
three locations (as shown above in Table 5). The corresponding
estimates for the additional number of children with IQ
[[Page 29228]]
loss greater than seven points, for the current NAAQS as compared to
current conditions, range from 600 to 66,000 (as shown above in Table
6). These latter values for the change in incidence of children with
greater than seven points Pb-related IQ loss represent 5 to 17 percent
of the children (aged less than 7 years of age) in these study areas.
This increase corresponds to approximately a doubling in the number of
children with this magnitude of Pb-related IQ loss in the study area
most affected. The Staff Paper concluded that these estimates indicate
the potential for significant numbers of children to be negatively
affected if air Pb concentrations increased to levels just meeting the
current standard.
Beyond the findings related to quantified IQ loss, the Staff Paper
recognized the potential for other, unquantified adverse effects that
may occur at similarly low exposures. In summary, the Staff Paper
concluded that taken together, ``the quantified IQ effects associated
with the current NAAQS and other, nonquantified effects are important
from a public health perspective, indicating a need for consideration
of revision of the standard to provide an appreciable increase in
public health protection'' (USEPA, 2007c).
3. CASAC Advice and Recommendations and Public Comment
CASAC's recommendations in this review builds upon the CASAC
recommendations during the 1990 review, which also advised on
consideration of more health protective NAAQS. In CASAC's review of the
1990 Staff Paper, as discussed in Section II.D.1.b, they generally
recommended consideration of levels below 1.0 [mu]g/m\3\, specifically
recommended analyses of a standard set at 0.25 [mu]g/m\3\, and also
recommended a revision to a monthly averaging time (CASAC, 1990).
In its letter to the Administrator subsequent to consideration of
the ANPR, the final Staff Paper and the final Risk Assessment Report,
the CASAC Pb Panel unanimously and fully supported ``Agency staff's
scientific analyses in recommending the need to substantially lower the
level of the primary (public-health based) Lead NAAQS, to an upper
bound of no higher than 0.2 [mu]g/m\3\ with a monthly averaging time''
(Henderson, 2008, p. 1). This recommendation is consistent with their
recommendations conveyed in two earlier letters in the course of this
review (Henderson, 2007a, 2007b). Further, in their advice to the
Agency over the course of this review, CASAC has provided rationale for
their conclusions that has included their statement that the current Pb
NAAQS ``are totally inadequate for assuring the necessary decreases of
lead exposures in sensitive U.S. populations below those current health
hazard markers identified by a wealth of new epidemiological,
experimental and mechanistic studies'', and stated that ``Consequently,
it is the CASAC Lead Review Panel's considered judgment that the NAAQS
for Lead must be decreased to fully-protect both the health of children
and adult populations'' (Henderson, 2007a, p. 5). CASAC drew support
for their recommendation from the current evidence, described in the
Criteria Document, of health effects occurring at dramatically lower
blood Pb levels than those indicated by the evidence available when the
standard was set and of a recognition of effects that extend beyond
children to adults.
The Agency has also received comments from the public on drafts of
the Staff Paper and related technical support document, as well as on
the ANPR.\121\ Public comments received to date that have addressed
adequacy of the current standard overwhelmingly concluded that the
current standard is inadequate and should be substantially revised, in
many cases suggesting specific reductions to a level at or below 0.2
[mu]g/m\3\. Two comments were received from specific industries
expressing the view that the current standard might need little or no
adjustment. One comment received early in the review stated that
current conditions justified revocation of the standard.
---------------------------------------------------------------------------
\121\ All written comments submitted to the Agency are available
in the docket for this rulemaking, are transcripts of the public
meetings held in conjunction with CASAC's review of the Staff Paper,
the Risk Assessment Report, the Criteria Document and the ANPR.
---------------------------------------------------------------------------
4. Administrator's Proposed Conclusions Concerning Adequacy
Based on the large body of evidence concerning the public health
impacts of Pb, including significant new evidence concerning effects at
blood Pb concentrations substantially below those identified when the
current standard was set, the Administrator proposes that the current
standard does not protect public health with an adequate margin of
safety and should be revised to provide additional public health
protection.
In considering the adequacy of the current standard, the
Administrator has carefully considered the conclusions contained in the
Criteria Document, the information, exposure/risk assessments,
conclusions, and recommendations presented in the Staff Paper, the
advice and recommendations from CASAC, and public comments received on
the ANPR and other documents to date.
The Administrator notes that the body of available evidence,
summarized above in section III.B and discussed in the Criteria
Document, is substantially expanded from that available when the
current standard was set three decades ago. The Criteria Document
presents evidence of the occurrence of health effects at appreciably
lower blood Pb levels than those demonstrated by the evidence at the
time the standard was set. Subsequent to the setting of the standard,
the Pb NAAQS criteria review during the 1980s and the current review
have provided (a) expanded and strengthened evidence of still lower Pb
exposure levels associated with slowed physical and neurobehavioral
development, lower IQ, impaired learning, and other indicators of
adverse neurological impacts; and (b) other effects of Pb on
cardiovascular function, immune system components, calcium and vitamin
D metabolism and other health endpoints (discussed fully in the
Criteria Document).
The Administrator notes particularly the robust evidence of
neurotoxic effects of Pb exposure in children, both with regard to
epidemiological and toxicological studies. While blood Pb levels in
U.S. children have decreased notably since the late 1970s, newer
studies have investigated and reported associations of effects on the
neurodevelopment of children with these more recent blood Pb levels.
The toxicological evidence includes extensive experimental laboratory
animal evidence that substantiates well the plausibility of the
epidemiologic findings observed in human children and expands our
understanding of likely mechanisms underlying the neurotoxic effects.
Further, the Administrator notes the current evidence that suggests a
steeper dose-response relationship at these lower blood Pb levels than
at higher blood Pb levels, indicating the potential for greater
incremental impact associated with exposure at these lower levels.
In addition to the evidence of health effects occurring at
significantly lower blood Pb levels, the Administrator recognizes that
the current health effects evidence together with findings from the
exposure and risk assessments (summarized above in section III.B), like
the information available at the time the standard was set, supports
our finding that air-related Pb exposure pathways contribute to blood
Pb levels in young children, by inhalation and ingestion. Furthermore,
the Administrator takes
[[Page 29229]]
note of the information that suggests that the air-to-blood ratio
(i.e., the quantitative relationship between air concentrations and
blood concentrations) is now likely larger, when air inhalation and
ingestion are considered, than that estimated when the standard was
set.
Based on evidence discussed above, the Administrator first
considered the evidence in the context of an adaptation of the 1978
framework, as presented in the Staff Paper, recognizing that the health
effects evidence with regard to characterization of a threshold for
adverse effects has changed dramatically since the standard was set in
1978. As discussed above, however, the 1978 framework was premised on
an evidentiary basis that clearly identified an adverse health effect
and a health-based policy judgment that identified a level that would
be safe for an individual child with respect to this adverse health
effect. The adaptation to the 1978 framework applies this framework to
a situation where there is no longer an evidentiary basis to determine
a safe level for individual children. In addition, this approach does
not address explicitly what magnitude of effect should be considered
adverse. Given these two limitations, the Administrator has focused
primarily instead on the air-related IQ loss evidence-based framework
described above in considering the adequacy of the current standard.
In considering the application the air-related IQ loss framework to
the current evidence as discussed above in section II.D.2.a, the
Administrator notes that this framework suggests an average air-related
IQ loss for a population of children exposed at the level of the
current standard on the order of 4 or more IQ points. The Administrator
judges that an air-related IQ loss of this magnitude is large from a
public health perspective and that this evidence-based framework
supports a conclusion that the current standard does not protect public
health with an adequate margin of safety. Further, the Administrator
believes that the current evidence indicates the need for a standard
level that is substantially lower than the current level to provide
increased public health protection, especially for at-risk groups,
including most notably children, against an array of effects, most
importantly including effects on the developing nervous system.
The Administrator has also considered the results of the exposure
and risk assessments conducted for this review, which provides some
further perspective on the potential magnitude of air-related IQ loss.
However, taking into consideration the uncertainties and limitations in
the assessments, notably including questions as to whether the
assessment scenarios that roll up current air quality to simulate just
meeting the current standard are realistic in wide areas across the
U.S., the Administrator has not placed primary reliance on the exposure
and risk assessments. Nonetheless, the Administrator observes that in
areas projected to just meet the current standard, the quantitative
estimates of IQ loss associated with air-related Pb, as summarized
above in section II.D.2.b, indicate risk of a magnitude that in his
judgment is significant from a public health perspective. Further,
although the current monitoring data indicate few areas with airborne
Pb near or just exceeding the current standard, the Administrator
recognizes significant limitations with the current monitoring network
and thus the potential that the prevalence of such levels of Pb
concentrations may be underestimated by currently available data.
The Administrator believes that the air-related blood Pb and IQ
loss estimates discussed in the Staff Paper and Risk Assessment Report,
summarized above, as well as the estimates of air-related IQ loss
suggested by this evidence-based framework, are important from a public
health perspective and are indicative of potential risks to susceptible
and vulnerable groups. In reaching this proposed judgment, the
Administrator considered the following factors: (1) The estimates of
blood Pb and IQ loss for children from air-related Pb exposures
associated with the current standard, (2) the estimates of numbers of
children with different amounts of increased Pb-related IQ loss
associated with the current standard, (3) the variability within and
among areas in both the exposure and risk estimates, (4) the
uncertainties in these estimates, and (5) the recognition that there is
a broader array of Pb-related adverse health outcomes for which risk
estimates could not be quantified and that the scope of the assessment
was limited to a sample of case studies and to some but not all at-risk
populations, leading to an incomplete estimation of public health
impacts associated with Pb exposures across the country.\122\ In
addition to the evidence-based and risk-based conclusions described
above, the Administrator also notes that it was the unanimous
conclusion of the CASAC Panel that EPA needed to ``substantially
lower'' the level of the primary Pb NAAQS to fully protect the health
of children and adult populations (Henderson, 2007a, 2007b, 2008).
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\122\ While recognizing that there are significant uncertainties
associated with the risk estimates from the case studies, EPA places
an appropriate weight on the risk assessment results for purposes of
evaluating the adequacy of the current standard, given the strength
of the evidence of the existence of effects at blood Pb levels
associated with exposures at the level of the current standard, the
magnitude of the IQ losses that are estimated, and the consistency
of these IQ losses with the estimates of IQ loss derived from the
alternative evidence-based framework. The weight to place on the
risk assessment results for purposes of evaluating alterative levels
of the standard is discussed later in the discussion on the level of
the standard.
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Based on all of these considerations, the Administrator proposes
that the current Pb standard is not requisite to protect public health
with an adequate margin of safety because it does not provide
sufficient protection, and that the standard should be revised to
provide increased public health protection, especially for members of
at-risk groups.
E. Conclusions on the Elements of the Standard
The four elements of the standard--indicator, averaging time, form,
and level--serve to define the standard and must be considered
collectively in evaluating the health and welfare protection afforded
by the standard. In considering revisions to the current primary Pb
standard, as discussed in the following sections, EPA considers each of
the four elements of the standard as to how they might be revised to
provide a primary standard for Pb that is requisite to protect public
health with an adequate margin of safety. Considerations and proposed
conclusions on indicator are discussed in section II.E.1, and on
averaging time and form in section II.E.2. Considerations and proposed
conclusions on a level for a Pb NAAQS with a Pb-TSP indicator are
discussed in section II.E.3, and considerations on a level for a Pb
NAAQS with a Pb-PM10 indicator are discussed in section
II.E.4.
1. Indicator
The indicator for the current standard is Pb-TSP (as described in
section II.D.1.a above).\123\ When the standard was set in 1978, the
Agency proposed Pb-TSP as the indicator, but considered identifying Pb
in particulate matter less than or equal to 10 [mu]m in diameter (Pb-
PM10) as the indicator. EPA had received comments expressing
concern
[[Page 29230]]
that because only a fraction of airborne particulate matter is
respirable, an air standard based on total air Pb would be
unnecessarily stringent. The Agency responded that while it agreed that
some Pb particles are too small or too large to be deposited in the
respiratory system, a significant component of exposures can be
ingestion of materials contaminated by deposition of Pb from the air.
In addition to the route of ingestion and absorption from the
gastrointestinal tract, nonrespirable Pb in the environment may, at
some point, become respirable through weathering or mechanical action.
EPA concluded that total airborne Pb, both respirable and nonrespirable
fractions, should be addressed by the air standard (43 FR 46251). The
federal reference method (FRM) for Pb-TSP specifies the use of the
high-volume FRM sampler for TSP.
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\123\ The current standard specifies the measurement of airborne
Pb with a high-volume TSP federal reference method (FRM) sampler
with atomic absorption spectrometry of a nitric acid extract from
the filter for Pb, or with an approved equivalent method.
---------------------------------------------------------------------------
In the 1990 Staff Paper, this issue was reconsidered in light of
information regarding limitations of the high-volume sampler used for
the Pb-TSP measurements, and the continued use of Pb-TSP as the
indicator was recommended in the Staff Paper (USEPA, 1990):
Given that exposure to lead occurs not only via direct
inhalation, but via ingestion of deposited particles as well,
especially among young children, the hi-vol provides a more complete
measure of the total impact of ambient air lead. * * * Despite its
shortcomings, the staff believes the high-volume sampler will
provide a reasonable indicator for determination of compliance * * *
In the current review, the Staff Paper evaluated the evidence with
regard to the indicator for a revised primary standard. This evaluation
included consideration of the basis for using Pb-TSP as the current
indicator, information regarding the sampling methodology for the
current indicator, and CASAC advice with regard to indicator (described
below). Based on this evaluation, the Staff Paper recommended retaining
Pb-TSP as the indicator for the primary standard. The Staff Paper also
recommended activities intended to encourage collection and development
of datasets that will improve our understanding of national and site-
specific relationships between Pb-PM10 (collected by low-
volume sampler) and Pb-TSP to support a more informed consideration of
indicator during the next review. The Staff Paper suggested that such
activities might include describing a federal equivalence method (FEM)
in terms of PM10 and allowing its use for a TSP-based
standard in certain situations, such as where sufficient data are
available to adequately demonstrate a relationship between Pb-TSP and
Pb-PM10 or, in combination with more limited Pb-TSP
monitoring, in areas where Pb-TSP data indicate Pb levels well below
the NAAQS level.
The ANPR further identified issues and options associated with
consideration of the potential use of Pb-PM10 data for
judging attainment or nonattainment with a Pb-TSP NAAQS. These issues
included the impact of controlling Pb-PM10 for sources
predominantly emitting Pb in particles larger than those captured by
PM10 monitors \124\ (i.e., ultra-coarse), \125\ and the
options included potential application of Pb-PM10 FRM/FEMs
at sites with established relationships between Pb-TSP and Pb-
PM10, and use of Pb-PM10 data, with adjustment,
as a surrogate for Pb-TSP data. The ANPR broadly solicited comment in
these areas.
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\124\ For simplicity, the discussion here and below speaks as if
PM10 samplers have a sharp size cut-off. In reality, they
have a size selection behavior in which 50% of particles 10 microns
in size are captured, with a progressively higher capture rate for
smaller particles and a progressively lower capture rate for larger
particles. The ideal capture efficiency curve for PM10
samplers specifies that particles above 15 microns not be captured
at all, although real samplers may capture a very small percentage
of particles above 15 microns. TSP samplers have 50% capture points
in the range of 25 to 50 microns, which is broad enough to include
virtually all particles capable of being transported any significant
distance from their source except under extreme wind events. As
explained below, the capture efficiency of a high-volume TSP sampler
for any given size particle is affected by wind speed and wind
direction.
\125\ In this notice, we use ``ultra-coarse'' to refer to
particles collected by a TSP sampler but not by a PM10
sampler (we note that CASAC has variously also referred to these
particles as ``very coarse'' or ``larger coarse-mode'' particles),
``fine'' to refer to particles collected by a PM2.5
sampler, and ``coarse'' to refer to particles collected by a
PM10 sampler but not by a PM2.5 sampler,
recognizing that there will be some overlap in the particle sizes in
the three types of collected material.
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In the current review, both the CASAC Pb Panel and members of the
CASAC Ambient Air Monitoring and Methods (AAMM) Subcommittee have
recommended that EPA consider a change in the indicator to
PM10, utilizing low-volume PM10 sampling
(Henderson, 2007a, 2007b, 2008; Russell, 2008). \126\ In their January
2008 letter, the CASAC Lead Panel unanimously recommended that EPA
revise the Pb NAAQS indicator to rely on low-volume PM10
sampling (Henderson, 2008). They indicated support for their
recommendation in a range of areas. First, they noted poor precision in
high-volume TSP sampling, wide variation in the upper particle size-cut
as a function of wind speed and direction, and greater difficulties in
capturing the spatial non-homogeneity of ultra-coarse particles with a
national monitoring network. They stated that the low-volume
PM10 collection method is a much more accurate and precise
collection method, and would provide a more representative
characterization on a large spatial scale of monitored particles which
remain airborne longer, thus providing a characterization that is more
broadly representative of ambient exposures over large spatial scales.
They also noted the automated sequential sampling capability of low-
volume PM10 monitors which would be particularly useful if
the averaging time is revised (i.e., to a monthly averaging time, as
recommended by CASAC), which, in CASAC's view would necessitate an
increased monitoring frequency. Further, they noted the potential for
utilization of the more widespread PM10 sampling network
(Henderson, 2007a, 2007b, 2008).\127\ In their advice, CASAC also
stated that they ``recognize the importance of coarse dust
contributions to total Pb ingestion and acknowledge that TSP sampling
is likely to capture additional very coarse particles which are
excluded by PM10 samplers'' (Henderson 2007b). They
suggested that an adjustment of the NAAQS level would accommodate the
loss of these ultra-coarse Pb particles, and that development of such a
quantitative adjustment might appropriately be based on concurrent Pb-
PM10 and Pb-TSP sampling data \128\ (Henderson, 2007a,
2007b, 2008).
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\126\ ``Low-volume PM10 sampling'' refers to sampling
using any of a number of monitor models that draw 16.67 liters/
minute (1 m3/hour) of air through the filter, in contrast
to ``high-volume'' sampling of either TSP or PM10 in
which the monitor draws 1500 liters/minute (90 m3/hour).
All commercial TSP FRM samplers at this time are high-volume
samplers; both high-volume and low-volume PM10 FRM
samplers are available. Low-volume sampling is the more recently
introduced method. Low-volume and high-volume samplers differ in
many other ways also, including filter size, accuracy of the flow
control, and degree of computerization.
\127\ EPA notes that costs, including those of operating a
monitoring network, may not be considered in establishing or
revising the NAAQS.
\128\ In their advice, CASAC recognized the potential for site-
to-site variability in the relationship between Pb-TSP and Pb-
PM10 (Henderson, 2007a, 2007b). They also stated in their
September 2007 letter, ``The Panel urges that PM10
monitors, with appropriate adjustments, be used to supplement the
data. * * * A single quantitative adjustment factor could be
developed from a short period of collocated sampling at multiple
sites; or a PM10 Pb/TSP Pb 'equivalency ratio' could be
determined on a regional or site-specific basis.''
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The Agency received comments on the discussion of the indicator in
the ANPR from several state and local agencies and national/regional
air pollution control organizations, as well as a national
environmental organization. These public comments
[[Page 29231]]
were somewhat mixed. Most of these commenters recommended maintaining
Pb-TSP as the indicator to ensure that Pb emitted in larger particles
is not overlooked by the Pb NAAQS. Some of those comments and others
suggested keeping TSP as the indicator but revising the FRM to a low-
volume TSP method \129\ and considering tighter sampling height
criteria to reduce variability.\130\ Others, in considering a potential
PM10-based indicator or the use of PM10 data as a
surrogate for Pb-TSP, noted the need for characterization of the
relationship between Pb-PM10 and Pb-TSP, which varies with
proximity to some sources. One state agency and a national organization
of regulatory air agencies expressed clear support for revising the
indicator to Pb-PM10, predominantly citing advantages
associated with improved technology and efficiency in data collection.
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\129\ The Pb-TSP FRM specification, 40 CFR 50 appendix G,
currently explicitly requires the use of the high-volume TSP FRM
sampler which is required by appendix B for the mass of TSP.
Therefore it would require amendments to 40 CFR 50 appendix B and/or
G (or a new dedicated appendix) to establish a low-volume TSP
sampler as the only FRM, or as an alternative FRM, for TSP and/or
Pb-TSP measurement. A number of researchers have utilized both self-
built and commercially available low-volume TSP samplers in ambient
air studies. Typically, these samplers are identical to low-volume
PM10 FRM samplers with the exception that their inlets
and other size separation devices (or lack thereof) are aimed at
collecting TSP. EPA is not aware of any rigorous evaluation of the
performance of these available, non-designated low-volume TSP
samplers or their equivalence to the TSP FRM. No one has applied to
date for designation of a low-volume TSP sampler as a FEM, either
for TSP measurement per se or for purposes of Pb-TSP measurement.
\130\ Currently, probe heights for Pb-TSP and PM10
sampling are allowed to be between 2 and 15 meters above ground
level for neighborhood-scale monitoring sites (those intended to
represent concentrations over a relatively large area around the
site) and between 2 and 7 meters for microscale sites. Near very
low-height sources of TSP, including fugitive dust sources at ground
level, concentrations of TSP, especially the concentrations of
particles larger than 10 microns, can vary substantially across this
height range with higher concentrations closer to the ground; near-
ground concentrations can also vary more in time than concentrations
higher up.
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In considering these issues concerning the appropriate indicator,
EPA takes note of previous Agency conclusions that the health evidence
indicates that Pb in all particle size fractions, not just respirable
Pb, contributes to Pb in blood and to associated health effects.
Further, the evidence and exposure/risk estimates in the current review
indicate that ingestion pathways dominate air-related exposure. Lead is
unlike other criteria pollutants, where inhalation of the airborne
pollutant is the key contributor to exposure. For Pb it is the quantity
of Pb in ambient particles with the potential to deposit indoors or
outdoors, thereby leading to a role in ingestion pathways, that is the
key contributor to air-related exposure. As recognized by the Agency in
setting the standard, and as noted by CASAC in their advice during this
review, these particles include ultra-coarse particles. Thus, choosing
the appropriate indicator requires consideration of the impact of the
indicator on protection from both the inhalation and ingestion pathways
of exposure and Pb in all particle sizes, including ultra-coarse
particles.
As discussed in section V.A., the Agency recognizes the body of
evidence indicating that the high-volume Pb-TSP sampling methodology
contributes to imprecision in resultant Pb measurements due to
variability in the efficiency of capture of particles of different
sizes and thus, in the mass of Pb measured. For example, the measured
values from a high-volume TSP sampler may differ substantially,
depending on wind speed and direction, for the same actual ambient
concentration of Pb-TSP.\131\ Variability is most substantial in
samples with a large portion of Pb particles greater than 10 microns,
such as those samples collected near sources with emissions of ultra-
coarse particles. The result is a clear risk of error from
underestimating the ambient level of total Pb in the air, especially in
areas near sources of ultra-coarse particles, by underestimating the
amount of the ultra-coarse particles. There is also the potential for
overestimation of individual sampling period measurements associated
with high wind events.\132\
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\131\ As noted in section V, the collection efficiency (over the
24-hour collection period) of particles larger than approximately 10
microns in a high-volume TSP FRM sampler varies with wind speed due
to aerodynamic effects, with a lower collection efficiency under
high winds. The collection efficiency also varies with wind
direction due to the non-cylindrical shape of the TSP sampler inlet.
These characteristics tend in the direction of reporting less than
the true TSP concentration over the 24-hour collection period.
\132\ We note that it is possible for high winds to blow Pb
particles onto a high-volume TSP sampler's filter after the end of
its 24-hour collection period before the filter is retrieved,
causing the reported concentration for the 24-hour period to be
higher than the actual 24-hour concentration.
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The low-volume PM10 sampling methodology does not
exhibit such variability \133\ due both to increased precision of the
monitor and decreased spatial variation of Pb-PM10
concentrations. As a result, greater precision is associated with
sample measurements for Pb collected using the PM10 sampling
methodology. The result is a lower risk of error in measuring the
ambient Pb in the PM10 size class than there is risk of
error in measuring the ambient Pb in the TSP size class using Pb TSP
samplers. On the other hand, PM10 samplers do not include
the Pb in particles greater than PM10 that also contributes
to the health risks posed by air-related Pb, especially in areas
influenced by sources of ultra-coarse particles. There are also
concerns over whether control strategies put in place to meet a NAAQS
with a Pb-PM10 indicator will be effective in controlling
ultra-coarse Pb-containing particles. In evaluating these two
indicators, the differences in the nature and degree of these sources
of error between Pb-TSP and Pb-PM10 need to be considered
and weighed, to determine the appropriate way to protect the public
from exposure to air-related Pb.
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\133\ Low-volume PM10 samplers are equipped with an
omni-directional (cylindrical) inlet, which reduces the effect of
wind direction, and a sharp particle separator which excludes most
of the particles greater than 10-15 microns in diameter whose
collection efficiency is most sensitive to wind speed. Also, in low-
volume samplers, the filter is protected from post-sampling
contamination.
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As noted above, EPA is concerned about the total mass of all Pb
particles emitted into the air and subsequently inhaled or ingested.
Measurements of Pb-TSP address a greater fraction of the particles of
concern from a public health perspective than measurements of Pb-
PM10, but limitations with regard to the sampler mean that
these data are less precise. EPA recognizes substantial variability in
the high-volume Pb-TSP method, meaning there is a risk of not
consistently identifying sites that fail to achieve the standard, both
across sites and across time periods for the same site.
Alternatively, using low-volume Pb-PM10 as the indicator
would allow the use of a technology that has better precision in
measuring PM10. In addition, since Pb-PM10
concentrations have less spatial variability, such monitoring data may
be representative of Pb-PM10 air quality conditions over a
larger geographic area (and larger populations) than would Pb-TSP
measurements. The larger scale of representation for Pb-PM10
would mean that reported measurements of this indicator, and hence
designation outcomes, would be less sensitive to exact monitor siting
than with Pb-TSP as the indicator.\134\ However, there would be a
different source of error, in that larger Pb particles not captured by
PM10 samplers would not be measured.
[[Page 29232]]
The fraction of Pb collected with a TSP sampler that would not be
collected by a PM10 sampler varies depending on proximity to
sources of ultra-coarse Pb particles and the size mix of the particles
they emit (as well as the sampling variability inherent in the method
discussed above). This means that this error is of most concern in
locations in closer proximity to such sources, which may also be
locations with some of the higher ambient air levels. As discussed
below, such variability would be a consideration in determining the
appropriate level for a standard based on a Pb-PM10
indicator.
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\134\ The larger scale would also make comparisons between two
or more monitoring sites more indicative of the true comparison
between the areas surrounding the monitoring sites, with regard to
the Pb captured by Pb-PM10 monitors, which could be
informative in studies of Pb uptake and health effects in
populations.
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Accordingly, we believe it is reasonable to consider continued use
of a Pb-TSP indicator, focusing on the fact that it specifically
includes the ultra-coarse Pb particles in the air that are of concern
and need to be addressed in protecting public health from air-related
exposures. In considering the option of retaining Pb-TSP as the
indicator, EPA recognizes that high-volume FRM TSP samplers would
continue to be used at many monitoring sites operated by State and
local agencies. In addition, it is possible that one or more low-volume
TSP monitors would be approved as FEM, under the provisions of 40 CFR
53, Ambient Air Monitoring Reference and Equivalent Methods. EPA
believes, along with some commenters as noted above, that low-volume
Pb-TSP sampling would have important advantages over high-volume Pb-TSP
sampling.\135\ To facilitate the ability of monitor vendors and
monitoring agencies to gain FEM status for low-volume Pb-TSP monitors,
EPA is proposing certain revisions to the side-by-side equivalence
testing requirements in 40 CFR 53 regarding the ambient Pb
concentrations required during testing so that testing is more
practical for a monitor vendor to conduct, as described in more detail
in section V below. We note that 40 CFR 53.7, Testing of Methods at the
Initiative of the Administrator, allows EPA itself to conduct the
required equivalence testing for a method and then determine whether
the requirements for equivalence are met. It would also be possible for
EPA to promulgate amendments to 40 CFR 50 establishing one or more
particular designs of a low-volume sampler as a Pb-TSP FRM, or to
establish performance specifications that would facilitate the approval
of low-volume samplers as FRM on a performance basis rather than a
design basis; this could be done as a replacement for the high-volume
TSP and Pb-TSP FRM or as an alternative TSP and/or Pb-TSP FRM. Either
path to FRM status would avoid the need for the side-by-side testing,
prescribed by 40 CFR 53, of low-volume samplers to demonstrate
equivalence to the high-volume FRM sampler, although some amount and
type of new testing in the field or in a wind tunnel may be appropriate
before such changes should be made. EPA invites comments on the low-
volume TSP sampler concept.
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\135\ Low-volume Pb-TSP samplers could be assembled by making
low-cost parts substitution to either low-volume PM10 or
low-volume PM2.5 samplers; some models would have the
same sequential sampling ability as CASAC has noted for low-volume
Pb-PM10 samplers; sensitivity to wind direction would be
eliminated; and their flow control and data processing and reporting
abilities would be substantially better than high-volume Pb-TSP
samplers. Low-volume Pb-TSP sampling data would have the same
geographic variability as high-volume Pb-TSP sampling data, however.
The size-specific capture efficiency curves of currently available
commercial low-volume sampling systems are not well characterized,
nor their sensitivity to wind speed. EPA therefore recognizes some
uncertainty about their equivalence to high-volume samplers in terms
of the capture of ultra-coarse particles.
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Within the option of continued use of a Pb-TSP indicator, EPA
recognizes that some State, local, or tribal monitoring agencies, or
other organizations, for the sake of the advantages noted above, may
wish to deploy low-volume Pb-PM10 samplers rather than Pb-
TSP samplers. In anticipation of this, we have also considered an
approach within the option of retaining Pb-TSP as the indicator that
would allow the use of Pb-PM10 data (when and if low-volume
Pb-PM10 samplers have been approved by EPA as either FRM or
FEM), with adjustment(s), for monitoring for compliance with the Pb-TSP
NAAQS. This approach would have five components: (1) The establishment
of a FRM specification for low-volume Pb-PM10 monitoring
including both a PM10 sampler specification and a reference
chemical analysis method for determination of Pb in the collected
particulate matter; (2) the establishment of a path to FEM designation
for Pb-PM10 monitoring methods that differ from the FRM in
either the sampler or the analytical method; (3) flexibility for
monitoring agencies to deploy low-volume Pb-PM10 monitors
anywhere that Pb monitoring is required by the revised Pb monitoring
requirements to help implement the revised NAAQS; (4) specific steps
for applying an adjustment to low-volume Pb-PM10 data for
purposes of making comparisons to the level of the NAAQS specified in
terms of Pb-TSP, and (5) a provision in the data interpretation
guidelines that, whenever and wherever Pb-TSP data from a monitoring
site is available and sufficient for determining whether or not the Pb-
TSP standard has been exceeded, any collocated Pb-PM10 data
from that site for the associated time period will not be considered.
The first three and the last components are discussed in depth in
sections IV and V below. Because the issue of adjustment to low-volume
Pb-PM10 data is linked closely to considerations of the
advantages of one indicator option versus another, it is discussed
here.
In considering how to identify the appropriate adjustment(s) to be
made to Pb-PM10 data for purposes of making comparisons to
the level of the NAAQS specified in terms of Pb-TSP, we recognize the
importance to protecting public health of taking into account the
ultra-coarse particles that are not included in Pb-PM10
measurement. As discussed below, one approach to doing so would be to
adjust or scale Pb-PM10 data upwards before comparison to a
Pb-TSP NAAQS level where the data are collected in an area that can be
expected to have ultra-coarse particles present.
Pb-PM10/Pb-TSP relationships vary from site to site and
time to time. These Pb-PM10/Pb-TSP relationships have a
systematic variation with distance from emissions sources emitting
particles larger than would be captured by Pb-PM10 samplers,
such that generally there are larger differences between Pb-
PM10 and Pb-TSP near sources. This is due to the faster
deposition of the ultra-coarse particles (as described in section
II.A.1). The exact size mix of particles at the point(s) of emissions
release and the height of the release point(s) also affect the
relationship. Accordingly, EPA is proposing to require the one-time
development and the continued use of site-specific adjustments for Pb-
PM10 data, for those sites for which a State prefers to
conduct Pb-PM10 monitoring rather than Pb-TSP monitoring.
Site-specific studies to establish the relationships between Pb-TSP and
Pb-PM10, conducted using side-by-side paired samplers, would
allow Pb-PM10 monitoring using locally determined factors
based on local study data to determine compliance with a NAAQS based on
Pb-TSP.
In addition, EPA invites comment on also providing in the final
rule default scaling factor(s) for use of Pb-PM10 data in conjunction
with a Pb-TSP indicator, as an alternative for States which wish to
conduct Pb-PM10 monitoring rather than Pb-TSP monitoring near Pb
sources but prefer not to conduct a site-specific scaling factor study.
EPA has identified and analyzed available collocated Pb-PM10 and Pb-TSP
data from 23 monitoring sites in seven States. (Schmidt and Cavender,
2008). This analysis considered both source-
[[Page 29233]]
oriented and nonsource-oriented sites. In this analysis, EPA identified
only three of the 23 monitoring sites with collocated data as being
source-oriented. One of these sites was near an operating Pb smelter at
the time of the collocated monitoring; Pb emissions from smelters
typically contain both ultra-coarse particles from materials handling
and resuspension of contaminated dust, and fine and coarse particles
from the high temperature smelting operation itself. However, since
this study was conducted, EPA has promulgated a Maximum Achievable
Control Technology (MACT) standard for primary lead smelting that
controls process and fugitive dust emissions. (64 FR 30194, June 4,
1999). The other two source-oriented sites include one located near a
battery manufacturer, and one located near an automobile plant. The
data for the smelter site was collected in 1988 and indicate an average
Pb-TSP concentration of about 2.5 [mu]g/m\3\. The data for the battery
manufacturer site were collected in the mid-1990s and indicate an
average Pb-TSP concentration of about 0.09 [mu]g/m\3\; data for the
third site, located near an automotive plant, collected within the past
5 years, indicate an average Pb-TSP concentration at that site of about
0.03 [mu]g/m\3\. As discussed in Schmidt and Cavender (2008), ratios
between Pb-TSP and Pb-PM10 concentrations varied somewhat
within the data for each site, but the ratios between the Pb-TSP and
Pb-PM10 concentration averages were 2.0 for the smelter site
(based on 20 data pairs), 1.6 at the site near the battery manufacturer
(based on 107 data pairs), and 1.1 at the site located near an
automotive plant (based on 167 data pairs).
Collectively, these three monitoring sites suggest that site-
specific scaling factors for source-oriented monitoring sites may vary
between 1.1 and 2.0; the range may also be greater. EPA notes that in
selecting a default factor for source-oriented monitoring sites, if
that approach is taken in the final rule, it may be appropriate to
consider default adjustment factors from within the mid to upper part
of this range rather than the lower end to avoid the possibility of
underestimating the appropriate scaling factor for a large proportion
of the source-oriented sites for which States might choose the default
factor rather than conduct a local study. On this basis, EPA invites
comment on the possibility of providing a default factor(s) for source-
oriented sites and on the selection of a value(s) from within this
range for all source-oriented monitoring sites, as an option to the
proposed requirement for development a site-specific factor through
analysis of paired monitoring data. EPA invites comment on the
selection of a single or multiple default factors for source-oriented
sites from within this range. While the selection of the scaling factor
in concept could depend on a characterization of the particle size mix
emitted by the Pb source, we note that reliable information on the mix
of coarse and ultra-coarse particles may often be unavailable. For
example, EPA could select a default factor that is at or near the upper
end of the range, 2.0, to avoid the risk of underprotection in
situations in which there is as high or nearly as high a proportion of
ultra-coarse Pb as at the smelter site. Alternatively, EPA could
discount the smelter data set on the basis that the 1988 data set does
not reasonably represent any likely current or future smelter
situation. Similarly, EPA could rely on the data taken near the
automotive plant since it is the most recent and largest dataset. EPA
also invites comment on other sets of paired data from near Pb sources
of which we may be unaware, and comment on other approaches of
selecting a default factor for the final rule based on paired data,
including approaches that might use more than one default factor for
source-oriented monitoring sites with the selection of the factor for a
given monitoring site depending on the characteristics of the nearby
sources, the ambient concentration of Pb-PM10, or other
factors.
EPA also invites comment on whether and what default scaling
factor(s) should be established for monitoring sites which, as far as
is known, are not influenced by nearby emission sources. We have
reviewed paired data from the 20 monitoring sites that appear to fit
this description (Schmidt and Cavender, 2008). Average Pb-TSP
concentrations at nearly all these sites were near to or below the
lowest concentration on which comments are invited as to the NAAQS
level. Judging from ratios at these 20 sites, it appears that site-
specific factors generally range from 1.0 to 1.4 (with the factors for
three sites ranging from 1.8 to 1.9), and the ratios may be influenced
by measurement variability in both samplers as well as by actual air
concentrations. Given the relatively low ambient concentrations that we
believe currently prevail at nonsource-oriented sites, the value of a
default scaling factor selected within the range of 1.0 to 1.4 would
have little effect on the NAAQS compliance determination at such sites.
EPA invites comment on the approach of requiring use of a default
factor(s) for adjusting Pb-PM10 data at nonsource-oriented
sites and on the selection of a value(s) from within the range of 1.0
to 1.4 and also solicits comment on selection of a default scaling
factor from within the broader range of 1.0 to 1.9. We note that
allowing the use of a default scaling factor of 1.0 for nonsource-
oriented sites would in effect allow a State the option of comparing
Pb-PM10 data directly to the level of the Pb-TSP standard at
nonsource-oriented monitoring sites, without conducting a site-specific
study. Below, and in section II.E.4, EPA discusses the possibility of
revising the indicator to Pb-PM10, which would result in
such unadjusted comparisons of Pb-PM10 data to the standard
at all monitoring sites.
EPA recognizes that the available data from collocated monitoring
of Pb-TSP and Pb-PM10, described above, have limitations
which make their interpretation and use in selecting default scaling
factors subject to considerable uncertainty. All of the Pb-
PM10 measurements at these sites were made with high-volume
PM10 samplers, which are more variable than the low-volume
samplers for which scaling factors would actually be applied after the
final rule; this greater variability no doubt has added to the
variation in ratios discussed above. Only three source-oriented sites
have collocated data; with such a small sample of sites both the range
of ratios and the distribution of ratios among all current and future
source-oriented sites remains uncertain. There were many more
nonsource-oriented sites which tended to show notably lower ratios,
implying lower scaling factors, but all had relatively low
concentrations; these ratios may or may not be representative of
monitoring sites near well controlled Pb sources. In many cases, the
period of collocated testing was only a few months; ratios observed in
such a short period may not be representative of ratios that occur at
other times of the year that may be more critical to attainment status.
Also, EPA has not yet had the benefit of CASAC review of the detailed
compilation of these data, as (Schmidt and Cavender, 2008) was prepared
subsequent to the most recent consultation with CASAC's AAMM
Subcommittee. Because of these uncertainties, EPA is proposing to
require States that wish to use Pb-PM10 data for a Pb-TSP
standard to develop site-specific scaling factors based on their own
collocated monitoring using paired Pb-TSP and low-volume Pb-
PM10 samplers over at least a one-year period, as described
in section IV. EPA intends
[[Page 29234]]
to encourage States to consider conducting local studies, even if the
final rule allows the use of default factors. Also, EPA invites comment
on whether to provide for the use of default scaling factors, and the
values of those factors.
As a possible second option, taking into consideration the advice
of the CASAC Pb Panel and members of the CASAC AAMM Subcommittee, EPA
has also considered potential revision of the indicator to Pb-
PM10. In so doing, we recognize several potential important
benefits of such a revision, as well as the need to reflect such a
revision in the selection of level of the standard.\136\ We recognize
that the low volume PM10 sampler provides better precision
and size selection characteristics which would make the associated data
more comparable across sites.
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\136\ EPA recognizes and has specifically considered that such a
decision would affect the selection of the level of the standard,
recognizing that it is the combination of indicator and level (with
averaging and time and form) that determine the degree of protection
afforded by the standard. Section II.E.4 further considers the
impact of adoption of a Pb-PM10 indicator on the
selection of a level for the standard.
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In considering a potential revision of the indicator to Pb-
PM10, we recognize that an important issue is whether
regulating concentrations of Pb-PM10 will lead to
appropriate controls on all particle size Pb emissions from sources.
For example, it would be of concern if a NAAQS based on a Pb-
PM10 indicator resulted in different emissions control
decisions at sources with a large percentage of Pb in the size range
not substantially captured by PM10 sampling (e.g., fugitive
dust emissions from Pb smelters) than the emission control decisions
that would be made if the NAAQS was based on Pb-TSP. In that case, a
PM10-based NAAQS might not yield emissions changes by some
Pb sources which under a Pb-TSP indicator would have contributed to
NAAQS exceedances and subsequent emissions changes. Alternatively,
while collocated Pb-TSP and Pb-PM10 data are lacking for a
broad range of source types, there are likely many sources (e.g., high
temperature combustion processes) for which virtually all of the
emitted particles represented in a Pb-TSP measurement would be captured
by a Pb-PM10 measurement. Further, there are likely other
source types with a range of particle sizes extending beyond Pb-
PM10, for which controls adopted to meet a Pb-
PM10 requirement would also achieve a proportional reduction
in ultra-coarse particles. In these situations, one might not expect
any difference in emissions control decisions whether the NAAQS is Pb-
PM10-based or Pb-TSP-based.
If the indicator were to be revised to Pb-PM10, low-
volume Pb-PM10 samplers would become the required approach
to Pb monitoring at required monitoring sites and would be a logical
choice wherever else NAAQS-oriented Pb monitoring is undertaken.
Nonetheless EPA notes that retaining Pb-TSP monitors at some relatively
small subset of the Pb-PM10 monitoring sites would be
beneficial for purposes of scientific understanding of both ambient
conditions and the performance of the two types of measurement systems.
For reasons discussed here, and taking into account information and
assessments presented in the Criteria Document, Staff Paper, and ANPR,
the advice and recommendations of CASAC and of members of the CASAC
AAMM Subcommittee, and public comments to date, the Administrator
proposes to retain the current indicator of Pb-TSP, measured by the
current FRM, a current FEM, or an FEM approved under the proposed
revisions to 40 CFR part 53, but with expansion of the measurements
accepted for determining attainment or nonattainment of the Pb NAAQS to
provide an allowance for use of Pb-PM10 data, measured by
the new low-volume Pb-PM10 FRM specified in the proposed
appendix Q to 40 CFR part 50 or by a FEM approved under the proposed
revisions to 40 CFR part 53, with site-specific scaling factors as
described above and more specifically below in section IV. The
Administrator invites comment on also providing States the option of
using default scaling factors instead of conducting the testing that
would be needed to develop the site-specific scaling factors. In
consideration of all of the issues discussed above, the Administrator
also invites comment on a second option, a revision of the current
indicator to Pb-PM10. (Considerations related to the level
of a standard based on a PM10 indicator are discussed below
in section II.E.4.) The Administrator solicits comment on all of the
issues discussed above, and specifically with regard to the potential
for a Pb-PM10 indicator to influence implementation of
controls in ways that would lead to less control associated with larger
particles than might be achieved with a Pb-TSP-based NAAQS, taking into
account the variability noted above for TSP sampling.
2. Averaging Time and Form
The statistical form of the current standard is a not-to-be-
exceeded or maximum value, averaged over a calendar quarter. This might
also be described as requiring that no average air Pb concentration
representing a time period of duration as long as calendar quarter (or
longer) may exceed the level of the standard. As noted in section
II.D.1.a, EPA set the standard in 1978 as a ceiling value with the
conclusion that this air level would be safe for indefinite exposure
for young children (43 FR 46250).
The basis for selection of the current standard's averaging time of
calendar quarter reflects consideration of the evidence available when
the Pb NAAQS were promulgated in 1978. At that time, the Agency had
concluded that the level of the standard, 1.5 [mu]g/m\3\, would be a
``safe ceiling for indefinite exposure of young children'' (43 FR
46250), and that the slightly greater possibility of elevated air Pb
levels for shorter periods within the quarterly averaging period as
contrasted to the monthly averaging period proposed in 1977 (43 FR
63076), was not significant for health. These conclusions were based in
part on the Agency's interpretation of the health effects evidence as
indicating that 30 [mu]g/dL was the maximum safe level of blood Pb for
an individual child.
With regard to averaging time, after consideration of the evidence
available at that time, the 1990 Staff Paper concluded that ``[a]
monthly averaging period would better capture short-term increases in
lead exposure and would more fully protect children's health than the
current quarterly average'' (USEPA, 1990b). The 1990 Staff Paper
further concluded that ``[t]he most appropriate form of the standard
appears to be the second highest monthly average in a 3-year span. This
form would be nearly as stringent as a form that does not permit any
exceedances and allows for discounting of one `bad' month in 3 years
which may be caused, for example, by unusual meteorology.'' In their
review of the 1990 Staff Paper, the CASAC Pb Panel concurred with the
staff recommendation to express the lead NAAQS as a monthly standard
not to be exceeded more than once in three years.
As summarized in section II.B above and discussed in detail in the
Criteria Document, the currently available health effects evidence
\137\ indicates a wider variety of neurological effects, as well as
immune system and hematological effects, associated with substantially
lower blood Pb levels in children than were recognized when the
standard was set in 1978. Further, the health effects evidence with
regard to characterization of a threshold for
[[Page 29235]]
adverse effects has changed since the standard was set in 1978, as have
the Agency's views on the characterization of a safe blood Pb
level.\138\ In consideration of averaging time for the Pb NAAQS, we
note the following aspects of the current health effects evidence.
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\137\ The differing evidence and associated strength of the
evidence for these different effects is described in detail in the
Criteria Document.
\138\ For example, EPA recognizes today that ``there is no level
of Pb exposure that can yet be identified, with confidence, as
clearly not being associated with some risk of deleterious health
effects'' (CD, p. 8-63).
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Children are exposed to ambient Pb via inhalation and
ingestion, with Pb that is taken into the body absorbed through the
lungs and through the gastrointestinal tract. Studies on Pb uptake,
elimination, and distribution show that Pb is absorbed into peripheral
tissues in adults within a few days (USEPA 1986a; USEPA 1990b, p. IV-
2). Absorption of Pb from the gastrointestinal tract appears to be
greater and faster in children as compared to adults (CD, Section
4.2.1). Once absorbed, it is quickly distributed from plasma to red
blood cells and throughout the body.
Lead accumulates in the body and is only slowly removed,
with bone Pb serving as a blood Pb source for years after exposure and
as a source of fetal Pb exposure during pregnancy (CD, Sections 4.3.1.4
and 4.3.1.5).
Blood Pb levels, including levels of the toxicologically
active fraction, respond quickly to increased Pb exposure, such that an
abrupt increase in Pb uptake rapidly changes blood Pb levels. The
associated time to reach a new quasi-steady state with the total body
burden after such an occurrence is projected to be approximately 75 to
100 days (CD, p. 4-27).
The elimination half-life, which describes the time for
blood Pb levels to stabilize after a reduction in exposure, for the
dominant phase for blood Pb responses to changes in exposure is on the
order of 20 to 30 days for adults (CD, p. 4-25). Blood elimination
half-lives are influenced by contributions from bone. Given the tighter
coupling in children of bone stores with blood levels, children's blood
Pb is expected to respond more quickly than adults (CD, pp. 4-20 and 4-
27).
Data from NHANES II and an analysis of the temporal
relationship between gasoline consumption data and blood lead data
generally support the inference of a prompt response of children's
blood Pb levels to changes in exposure. Children's blood Pb levels and
the number of children with elevated blood Pb levels appear to respond
to monthly variations in Pb emissions from Pb in gasoline (EPA, 1986a,
p. 11-39; Rabinowitz and Needleman, 1983; Schwartz and Pitcher, 1989;
USEPA, 1990b).
The evidence with regard to sensitive neurological effects
is limited in what it indicates regarding the specific duration of
exposure associated with effect, although it indicates both the
sensitivity of the first 3 years of life and a sustained sensitivity
throughout the lifespan as the human central nervous system continues
to mature and be vulnerable to neurotoxicants (CD, Section 8.4.2.7).
The animal evidence supports our understanding of periods of
development with increased vulnerability to specific types of effect
(CD, Section 5.3), and indicates a potential importance of exposures on
the order of months.
Evidence of a differing sensitivity of the immune system
to Pb across and within different periods of life stages indicates a
potential importance of exposures as short as weeks to months duration.
For example, the animal evidence suggests that the gestation period is
the most sensitive life stage followed by early neonatal stage, and
within these life stages, critical windows of vulnerability are likely
to exist (CD, Section 5.9 and p. 5-245).
Evidence described in the Criteria Document and the risk assessment
indicate that ingestion of dust can be a predominant exposure pathway
for young children to air-related Pb, and that there is a strong
association between indoor dust Pb levels and children's blood Pb
levels. As stated in the Criteria Document, ``given the large amount of
time people spend indoors, exposure to Pb in dusts and indoor air can
be significant'' (CD, p. 3-27). The Criteria Document further describes
studies that evaluated the influence of dust Pb exposure on children's
blood Pb: ``Using a structural equation model, Lanphear and Roghmann
(1997) also found the exposure pathway most influential on blood Pb was
interior dust Pb loading, directly or through its influence on hand Pb.
Both soil and paint Pb influenced interior dust Pb; with the influence
of paint Pb greater than that of soil Pb. Interior dust Pb loading also
showed the strongest influence on blood Pb in a pooled multivariate
regression analysis (Lanphear et al., 1998).'' (CD, p. 4-134). Further,
a recent study of dustfall near an open window in New York City
indicates the potential for a relatively rapid response of indoor dust
Pb loading to ambient airborne Pb, on the order of weeks (CD, p. 3-28;
Caravanos et al., 2006a).
We note that the health effects evidence identifies varying length
durations in exposure that may be relevant and important. In light of
uncertainties in aspects such as response times of children's exposure
to airborne Pb, we recognize, as in the past, that this evidence
provides a basis for consideration of both calendar quarter and
calendar month as averaging times.
In considering averaging time and form, EPA has combined the
current quarterly averaging time with the current not-to-be exceeded
(maximum) form and has also combined a monthly averaging time with a
second maximum form, so as to provide an appropriate degree of year-to-
year stability that a maximum monthly form would not afford. We also
note that, as discussed below, the second maximum monthly form provides
a roughly comparable degree of protection on a broad national scale.
In this consideration of averaging time and form, EPA has taken
into account analyses using air quality data for 2003-2005 that are
presented in the Staff Paper (chapter 2). These analyses consider both
a period of three calendar years and a period of one calendar year
(with the form of the current standard being the maximum quarterly
mean). These analyses indicate that, with regard to either single-year
or 3-year statistics for the 2003-2005 dataset, a second maximum
monthly mean yields very similar, although just slightly greater,
numbers of sites exceeding various alternative levels as a maximum
quarterly mean, with both yielding fewer exceedances than a maximum
monthly mean.\139\ That is, these two averaging time and form
combinations resulted in roughly the same number of areas that would
not attain a standard at any given level on a broad national scale,
suggesting roughly comparable public health protection. However, the
relative protection provided by these two forms may differ from area to
area. For example, some of the areas meeting a maximum quarterly mean
standard over the 2003-2005 period at a given level did not meet a
second maximum monthly mean standard at the same level because there
were at least two months with high monthly concentrations which were
averaged with a lower concentration month in the same quarter. On the
other hand,
[[Page 29236]]
theoretically it is possible for an area to meet a given standard level
with a second maximum monthly mean averaging time and form and not meet
it for a maximum quarterly mean (e.g., the second highest monthly
average may be below the standard level while the quarterly average may
exceed it). Moreover, control programs to reduce quarterly mean
concentrations may not have the same protective effect as control
programs aimed at reducing concentrations in every individual month.
Given the limited scope of the current monitoring network which lacks
monitors near many significant Pb sources and uncertainty about Pb
source emissions and possible controls, it is difficult to more
quantitatively compare the protectiveness of the quarterly mean versus
the second maximum monthly mean approaches.
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\139\ For example, 49 sites (of 189) exceed a standard level of
0.10 [mu]g/m\3\ based on a form of maximum quarterly mean while 54
sites exceed based on a form of second maximum monthly mean.
Further, 25 sites exceed a standard level of 0.30 [mu]g/m\3\ based
on a form of maximum quarterly mean while 29 sites exceed based on a
form of second maximum monthly mean (Staff Paper, Table 2-6).
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In their advice to the Agency in this review, CASAC has recommended
that consideration be given to changing from a calendar quarter to a
monthly averaging time (Henderson, 2007a, 2007b, 2008). In making that
recommendation, CASAC emphasizes support from studies that suggest that
blood Pb concentrations respond at shorter time scales than would be
captured completely by quarterly values, as indicated by their
description of their recommendation for adoption of a monthly averaging
time as ``more protective of human health in light of the response of
blood lead concentrations that occur at sub-quarterly time scales''
(Henderson, 2007a). With regard to form of the standard, CASAC has
stated that one could ``consider having the lead standards based on the
second highest monthly average, a form that appears to correlate well
with using the maximum quarterly value'', while also indicating that
``the most protective form would be the highest monthly average in a
year'' (Henderson, 2007a).
Among the public comments the Agency received on the discussion of
averaging time and form in the ANPR, the majority concurred with the
CASAC recommendation for a revision of the averaging time to a calendar
month.
The 1990 Staff Paper and the Staff Paper for this review both
recommended that the Administrator consider specifying, in the form of
the NAAQS, that compliance with the NAAQS will be evaluated over a 3-
year period. The Administrator has considered this recommendation and
is proposing to adopt it. In the 3-year approach, a monitor would be
considered to be in violation of the NAAQS as of a certain date if in
any of the three previous calendar years with sufficiently complete
data (as explained in detail in section IV below), the value of the
selected form of the indicator (e.g., second maximum monthly average or
maximum quarterly average) exceeded the level of the NAAQS. A monitor,
initially or after once having violated the NAAQS, would not be
considered to have attained the NAAQS until three years have passed
without the form and level of the standard being violated. Many types
of Pb sources have variable emissions from day-to-day and year-to-year
due to market conditions for their products and/or weather variations
that can affect the generation of fugitive dust from contaminated
roadways and grounds. In addition, variations in wind patterns from
year to year can cause a near-source Pb monitor to be exposed to high
concentrations on more days in one year than in another, even if source
emissions are constant, especially if it operates on only some days.
Thus, it is possible for a monitor to indicate a violation of a
hypothetical form and level in one period but not in another, even if
no permanent controls have been applied at nearby source(s). Analysis
of historical Pb air concentration data has confirmed that this pattern
of fluctuating monitoring results can happen at the levels and forms
being proposed. It would potentially reduce the public health
protection afforded by the standard if areas fluctuated in and out of
formal nonattainment status so frequently that states do not have
opportunity and incentive to identify sources in need of more emission
control and to require those controls to be put in place. The 3-year
approach would help ensure that areas initially found to be violating
the NAAQS have effectively controlled the contributing lead emissions
before being redesignated to attainment/maintenance.
In considering averaging time and form for the standard, the
Administrator has considered the information summarized above
(described in more detail in Criteria Document and Staff Paper), as
well as the advice from CASAC and public comments. The Administrator
recognizes that there is support in the evidence for a monthly
averaging time consistent with the following observations: (1) The
health evidence indicates that very short exposures can lead to
increases in blood Pb levels, (2) the time period of response of indoor
dust Pb to airborne Pb can be on the order of weeks, and (3) the health
evidence indicates that adverse effects may occur with exposures during
relatively short windows of susceptibility, such as prenatally and in
developing infants.\140\ The Administrator also recognizes limitations
and uncertainties in the evidence including the limited available
evidence specific to the consideration of the particular duration of
sustained airborne Pb levels having the potential to contribute to the
adverse health effects identified as most relevant to this review, as
well as variability in the response time of indoor dust Pb loading to
ambient airborne Pb.
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\140\ The health evidence with regard to the susceptibility of
the developing fetus and infants is well documented in the evidence
as described in the 1986 Criteria Document, the 1990 Supplement
(e.g., chapter III) and the 2006 Criteria Document. For example,
``[n]eurobehavioral Neurobehavioral effects of Pb-exposure early in
development (during fetal, neonatal, and later postnatal periods) in
young infants and children (7 years old) have been observed
with remarkable consistency across numerous studies involving
varying study designs, different developmental assessment protocols,
and diverse populations.'' (CD, p. E-9)
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Based on these considerations and the air quality analyses
summarized above, the Administrator concludes that this information
provides support for an averaging time no longer than a calendar
quarter. Further, the Administrator recognizes that if substantial
weight is given to the evidence of even shorter times for response of
dust Pb, blood Pb, and associated effects to airborne Pb, a monthly
averaging time may be appropriate. Accordingly, the Administrator is
proposing two options with regard to the form and averaging time for
the standard, and with both he proposes making the time period
evaluated in considering attainment be 3 years. One option is to retain
the current not-to-be-exceeded form with an averaging time of a
calendar quarter, such that the form would be maximum quarterly average
across a 3-year span. The second option is to revise the averaging time
to a calendar month and the form to be the second highest monthly
average across a 3-year span. Based on the considerations discussed
above, EPA requests comment on whether a level for a NAAQS with a
monthly averaging time and a second-highest monthly average form should
be based on an adjustment to a higher level than the level for a NAAQS
with a quarterly averaging time and a not-to-be-exceeded form, and, if
so, on the magnitude of the adjustment that would be appropriate.
3. Level for a Pb NAAQS With a Pb-TSP Indicator
With regard to level of the standard, for a standard using a Pb-TSP
indicator, we first discuss evidence-based and exposure/risk-based
considerations, including considerations and
[[Page 29237]]
conclusions of the Staff Paper, in sections II.E.3.a and II.E.3.b
below. This is followed by a summary of CASAC advice and
recommendations and public comments (section II.E.3.c) and the
Administrator's proposed conclusions (section II.E.3.d). In addition,
we discuss considerations and solicit comment with regard to a level of
a standard using a Pb-PM10 indicator in section II.E.4
below.
a. Evidence-Based Considerations
As a general matter, EPA recognizes that in the case of Pb there
are several aspects to the body of epidemiological evidence that add
complexity to the selection of an appropriate level for the primary
standard. As summarized above and discussed in greater depth in the
Criteria Document (CD, Sections 4.3 and 6.1.3), the epidemiological
evidence that associates Pb exposures with health effects generally
focuses on blood Pb for the dose metric.\141\ In addition, exposure to
Pb comes from various media, only some of which are air-related. This
presents a more complex situation than does evidence of associations
between occurrences of health effects and ambient air concentrations of
an air pollutant, such as is the case for particulate matter and ozone.
Further, for the health effects receiving greatest emphasis in this
review (neurological effects, particularly neurocognitive and
neurobehavioral effects, in children), no threshold levels can be
discerned from the evidence. As was recognized at the time of the last
review, estimating a threshold for toxic effects of Pb on the central
nervous system entails a number of difficulties (CD, pp. 6-10 to 6-11).
The task is made still more complex by support in the evidence for a
nonlinear rather than linear relationship of blood Pb with
neurocognitive decrement, with greater risk of decrement-associated
changes in blood Pb at the lower levels of blood Pb in the exposed
population (Section 3.3.7; CD, Section 6.2.13). In this context EPA
notes that the health effects evidence most useful in determining the
appropriate level of the NAAQS is this large body of epidemiological
studies. Unlike the recent review of the NAAQS for ozone, there are no
clinical studies useful for informing a determination of the
appropriate level for a standard.\142\ The discussion below therefore
focuses on the epidemiological studies, recognizing and taking into
consideration the complexity and resulting uncertainty in using this
body of evidence to determine the appropriate level for the NAAQS.
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\141\ Among the studies of Pb health effects, in which blood Pb
level is generally used as an index of exposure, the sources of
exposure vary and are inclusive of air-related sources of Pb such as
smelters (e.g., CD, chapter 6).
\142\ See, e.g., 72 FR 37878-9 (July 11, 2007) (Ozone NAAQS
Notice of Proposed Rulemaking).
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In considering the evidence with regard to selection of the level
of the standard, the Agency has considered the same evidence-based
frameworks discussed above in section II.D.2.a on the adequacy of the
current standard. That is, the Staff Paper considered how to apply an
adapted 1978 framework to the much expanded body of evidence that is
now available, and the Agency has further considered this evidence in
the context of the air-related IQ loss evidence-based framework that
builds on a recommendation by the CASAC Pb Panel. These evidence-based
approaches are discussed below in considering the appropriate standard
levels to propose.
As noted in section II.D.2.a above, this review focuses on young
children as a key sensitive population for Pb exposures. In this
sensitive population, the current evidence demonstrates the occurrence
of health effects, including neurological effects, associated with
blood Pb levels extending well below 10 [mu]g/dL (CD, sections 6.2, 8.4
and 8.5). As further described in section II.D.2.a above, some studies
indicate Pb effects on intellectual attainment of children for which
population mean blood Pb levels in the analysis ranged from
approximately 2 to 8 [mu]g/dL (CD, Sections 6.2, 8.4.2 and 8.4.2.6).
Further, as noted above, the current evidence does not indicate a
threshold for the more sensitive health endpoints such as neurological
effects in children (CD, pp. 5-71 to 5-74 and Section 6.2.13).\143\
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\143\ This differs from the Agency's recognition in the 1978
rulemaking of a threshold of 40 [mu]g/dL blood Pb for an individual
child for effects of Pb considered clearly adverse to health at that
time, i.e., impairment of heme synthesis and other effects which
result in anemia.
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As when the standard was set in 1978, there remain today
contributions to blood Pb levels from nonair sources. As discussed
above (section II.D.2), current evidence is limited with regard to
estimates of the aggregate reduction since 1978 of all nonair sources
to blood Pb and with regard to an estimate of current nonair blood Pb
levels (discussed more fully in sections II.A.4) In recognition of
temporal reductions in nonair sources discussed in section II.A.4 and
in the context of estimates pertinent to an application of the 1978
framework, the CASAC Pb Panel recommended consideration of 1.0 to 1.4
[mu]g/dL or lower as an estimate of the nonair component of blood Pb
pertinent to average blood Pb levels in children (as described in
section II.A.4 above; Henderson, 2007a). The Staff Paper considered
this range of 1.0 to 1.4 [mu]g/dL for the nonair component of blood Pb
in its application of the adapted 1978 evidence-based framework.
As discussed in section II.B.1.c, the current evidence in
conjunction with the results and observations drawn from the exposure
assessment support a focus on air-to-blood ratios for children in the
range of 1:3 to 1:7, based on consideration of both inhalation and
ingestion exposure pathways and on the lower air and blood Pb levels
pertinent to this review. In considerations here, we have described the
value of 1:5 as falling somewhat central within the range supported by
the evidence.
i. Evidence-Based Framework Considered in the Staff Paper
Recommendations in the Staff Paper on standard levels were based
upon an approach that built upon and adapted the general approach used
by EPA in setting the standard in 1978. In adapting this approach to
the currently available information, the Staff Paper recognized the
more extensive and stronger body of evidence now available on a broader
range of health effects associated with exposure to Pb. For example,
EPA recognizes that today ``there is no level of Pb exposure that can
yet be identified, with confidence, as clearly not being associated
with some risk of deleterious health effects'' (CD, p. 8-63). This is
in contrast to the situation in 1978 when the Agency judged that the
maximum safe individual and geometric mean blood Pb levels for a
population of young children were 30 [mu]g/dL and 15 [mu]g/dL,
respectively.\144\
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\144\ More specifically, when the standard was set in 1978, the
Agency stated that the population mean, measured as the geometric
mean, must be 15 [mu]g/dL in order to ensure that 99.5 percent of
children in the United States would have a blood Pb level below 30
[mu]g/dL, which was identified as the maximum safe blood Pb level
for individual children based on the information available at that
time (43 FR 46247-46252).
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In the Staff Paper application of an adapted 1978 framework, the
focus shifted away from identifying a safe blood Pb level for an
individual child (and then determining an ambient air level that would
keep a very high percentage of children at or below that safe level),
because information was no longer available to identify such a level.
Rather, the Staff Paper approach focused on identifying an appropriate
population mean blood Pb level, and then identifying an ambient air
level that would keep the mean blood Pb levels of children exposed at
that air level below the target population mean blood Pb level. Based
on the review of
[[Page 29238]]
the evidence, the Staff Paper approach substituted a level of 2 [mu]g/
dL for the target population geometric mean blood Pb of 15 [mu]g/dL
used in 1978. In the absence of a demonstrated safe level, at either an
individual or a population level, the Staff Paper used 2 [mu]g/dL as
representative of the lowest population mean level for which there is
evidence of a statistically significant association between blood lead
levels and health effects (e.g., CD, p. E-9; Lanphear et al., 2000).
This approach does not evaluate the magnitude or degree of health
effects occurring across the population at that mean blood lead level.
In this adaptation of the 1978 approach the focus is solely on the
existence of a relationship between blood lead levels and
neurocognitive effects. The approach takes as the public health goal
the identification of an ambient air lead level that can be expected to
keep the mean blood lead level of an exposed population of children at
or below the lowest level at which a statistically significant
association has been demonstrated between blood lead level and
neurocognitive effects.\145\
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\145\ There are some similarities between this approach and the
approach employed in determining the levels for the daily and annual
PM standards in the latest PM review, where EPA determined an
ambient PM level based on the ambient levels in the epidemiology
studies that found statistically significant associations between
changes in ambient PM levels and changes in occurrences of health
effects. See 71 FR 61144 (October 17, 2006). However, there are
several important differences in this adaptation to the 1978
approach for lead. For example, the health effects evaluated in the
PM epidemiological studies were clearly adverse health effects,
ranging from hospital admissions to premature mortality. In
addition, the studies looked directly at the association between
ambient level and occurrences of health effects. Here the
epidemiology studies look at the association between blood lead
level and neurocognitive effect, and there is an additional step to
link the blood lead level to air-related lead. In addition, at a
population level there is a less clear delineation of when the
neurocognitive effect is adverse to public health. This is discussed
below in this section with respect to the impact on public health of
a shift in the mean IQ of a population of children.
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Starting with a target population geometric mean blood lead level
of 2 [mu]g/dL for the population of exposed children, then subtracting
1 to 1.4 [mu]g/dL for the nonair component of blood Pb, yields 0.6 to 1
[mu]g/dL as a target for the geometric mean air contribution to blood
Pb. The adapted 1978 approach divides the air-related target by 5, an
air-to-blood ratio somewhat central within the range of air Pb to blood
Pb ratios generally supported by the currently available evidence. This
resulted in a potential standard level of 0.1 to 0.2 [mu]g/m\3\.
The Staff Paper conclusions on level for the primary Pb standard
built on the staff's conclusion that the overall body of evidence
clearly calls into question the adequacy of the current standard with
regard to health protection afforded to at-risk populations. Based on
consideration of the health effects evidence, as described above, the
Staff Paper concluded that it is reasonable to consider a range for the
level of the standard, for which the upper part is represented by 0.1
to 0.2 [mu]g/m\3\.
ii. Air-related IQ Loss Evidence-Based Framework
As mentioned above, in analyses subsequent to the Staff Paper and
ANPR, the Agency has primarily considered the evidence in the context
of an alternative evidence-based framework, referred to as the air-
related IQ loss framework. This framework focuses on the contribution
of air-related Pb to neurocognitive effects, with a public health goal
of identifying the appropriate ambient air level of Pb to protect
exposed children from health effects that are considered adverse, and
are associated with their exposure to air-related Pb. This framework
does not focus on overall blood lead levels or on nonair contribution
to blood lead levels. While this avoids some of the limitations noted
above with the adapted 1978 approach, EPA recognizes that looking at
air-related Pb in isolation from other sources of Pb could be
considered a limitation for this framework. The different limitations
of each of these frameworks derive from the limitations in the
underlying body of evidence available for this review.
In this air-related IQ loss evidence-based framework, we have drawn
from the entire body of evidence as a basis for concluding that there
are causal associations between air-related Pb exposures and population
IQ loss. We have drawn more quantitatively from the evidence by
combining air-to-blood ratios with evidence-based C-R functions from
the epidemiological studies to quantify the association between air Pb
concentrations and air-related population mean IQ loss in exposed
children. This air-related IQ loss framework focuses on selecting a
standard that would prevent air-related IQ loss (and related effects)
of a magnitude judged by the Administrator to be of concern in
populations of children exposed to the level of the standard, taking
into consideration such factors as the uncertainties inherent in such
estimates. In addition to this judgment by the Administrator, this
framework is also based on specifying an air-to-blood ratio (also used
in the adapted 1978 framework) and a C-R function(s) for population
mean IQ response associated with blood Pb level.
In considering the evidence with regard to C-R functions, and in
recognition of the finding in the evidence of a steeper slope at lower
blood Pb levels (i.e., the nonlinear relationship), we have identified
two sets of C-R functions (discussed more fully above in section
II.B.2.b). The first set focuses on C-R functions reflecting population
mean concurrent blood Pb levels of approximately 3 [mu]g/dL.\146\ The
second set (CD, pp. 8-78 to 8-80) considers functions descriptive of
the C-R relationship from a larger set of studies that include
population mean blood Pb levels ranging from a mean of 3.3 up to a
median of 9.7 [mu]g/dL (see Table 1).\147\
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\146\ As noted above in section II.B.2.b, the log-linear C-R
function with low-exposure linearization (LLL) used in the
quantitative risk assessment, based on log-linear model in Lanphear
et al 2005), has a slope that falls intermediate within this first
set of functions at low blood Pb levels. The log-linear model by
Lanphear et al (2005) is derived from the pooled International
dataset for which the median blood Pb is 9.7 [mu]g/dL.
\147\ For context, it is noted that the 2001-2004 median blood
level for children aged 1-5 of all races and ethnic groups is 1.6
[mu]g/dL, the median for the subset living below the poverty level
is 2.3 [mu]g/dL and 90th percentile values for these two groups are
4.0 [mu]g/dL and 5.4 [mu]g/dL, respectively. Similarly, the 2001-
2004 median blood level for black, non-hispanic children aged 1-5 is
2.5 [mu]g/dL, while the median level for the subset of that group
living below the poverty level is 2.9 [mu]g/dL and the median level
for the subset living in a household with income more than 200% of
the poverty level is 1.9 [mu]g/dL. Associated 90th percentile values
for 2001-2004 are 6.4 [mu]g/dL (for black, non-hispanic children
aged 1-5), 7.7 [mu]g/dL (for the subset of that group living below
the poverty level) and 4.1 [mu]g/dL (for the subset living in a
household with income more than 200% of the poverty level). (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm--then
click on ``Download a universal spreadsheet file of the Body Burdens
data tables'').
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As discussed above in section II.B.2.b, the C-R functions from
analyses involving the lower mean blood Pb levels, that are closer to
current mean blood Pb levels in U.S. children, provide slopes of IQ
loss with increasing blood Pb that range from -1.71 to -2.94 IQ points
per [mu]g/dL blood Pb. These include C-R function from Lanphear et al.
(2005) recommended for consideration by CASAC, in light of the current
blood Pb levels of U.S. children (Henderson, 2008),\148\ and also the
C-R function
[[Page 29239]]
given greatest weight in the risk assessment (discussed above in
section II.C.2.b), the loglinear function with low-exposure
linearization (the LLL function). The function yielding the lowest
slope in this range is from the analysis by Tellez-Rojo and others
(2006) of very young children with blood Pb levels below 5 [mu]g/dL,
with a group mean blood Pb level of 2.9 [mu]g/dL. The function yielding
the highest slope in this range is from the analysis by Lanphear and
others (2005) of children whose blood Pb levels never exceeded 7.5
[mu]g/dL, with a group mean blood Pb level of 3.24 [mu]g/dL. The LLL
function falls within the range of the other two functions at lower
blood Pb levels, with an average slope of -2.29 IQ points per [mu]g/dL
across blood Pb levels extending below 2 [mu]g/dL.
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\148\ In their September 2007 letter, the CASAC Pb Panel
``recommends using the two-piece linear function for relating IQ
alterations to current blood lead levels with a slope change or
``hinge'' point closer to 7.5 [mu]g/dL than 10.82 [mu]g/dL as used
by EPA staff in the second draft exposure/risk assessments document.
The higher value used by staff underestimates risk at lower blood Pb
levels, where most of the population will be located. Epidemiologic
data indicate that the slope of the line below 7.5 [mu]g/dL is
approximately minus three (-3) IQ decrements per 1 [mu]g/dL blood
lead and the vast majority of children in the U.S. have maximal
baseline Pb blood levels below 7.5 [mu]g/dL (Lanphear et al., EHP
2005; MMWR 2005). On a population level, the mean increase in blood
lead concentration from airborne lead would generally be up to, but
not exceeding, a blood lead concentration of 7.5 [mu]g/dL. This
approach should also account for sensitive subpopulations of
children.'' In in their January 2008 letter, the Panel also points
to several other studies ``confirming that the relationship of lead
exposure is non-linear and per-sists at blood lead levels
considerably lower than 5 [mu]g/dL (Lanphear, 2000; Wasserman, 2003;
Kordas, 2006; Tellez-Rojo, 2006). In particular, Tellez-Rojo and co-
workers reported that the slope of the association between 24-month
blood lead and the 24-month Mental Development Index (MDI) for 294
children who had peak blood lead levels below 5 [mu]g/dL was
negative (-1.7 points for each 1 [mu]g/dL increase in blood lead
concentration, p=0.01). Collectively, these studies indicate that
there is sufficient evidence to support the use of the dose-response
relationship from the pooled analysis at blood lead levels < 5
[mu]g/dL (Lanphear, 2005), as described in the Final Lead Staff
Paper and previously recommended by CASAC.''
---------------------------------------------------------------------------
The second set of C-R functions discussed in section II.B.2.b is
drawn from a larger group of studies, although these studies include
groups of children with higher blood Pb levels (CD, pp. 8-78 to 8-80)
such that the population mean levels for these studies include
population mean blood Pb levels ranging from a mean of 3.3 up to a
median of 9.7 [mu]g/dL (see Table 1). This second set of C-R functions
is represented by a median of -0.9 IQ points per [mu]g/dL blood Pb (CD,
p. 8-80).\149\
---------------------------------------------------------------------------
\149\ As noted above (in section II.B.2.b), this slope is
similar to the slope for the below 10 [mu]g/dL piece of the
piecewise model used in the RRP rule economics analysis.
---------------------------------------------------------------------------
In applying the air-related IQ loss evidence-based framework, as
with the adapted 1978 framework, we recognize uncertainty in our
estimates for the two input parameters (air-to-blood ratio and C-R
function slope). Accordingly, in associating various standard levels
with the estimated magnitudes of air-related mean IQ loss that would
likely be prevented by keeping exposed populations below such standard
levels, we have considered combinations of parameter estimates that are
potentially supportable within this framework. With regard to the C-R
functions we have drawn estimates from both sets of functions. For the
first set of C-R functions, we have relied on the upper and lower-end
values to provide a range at lower blood Pb levels, and have focused on
the LLL function for blood Pb levels above approximately 2.5 to 3.0
[mu]g/dL, as shown in Table 7.\150\ From the second set of C-R
functions, we have relied on the median estimate across the range of
blood Pb levels considered. For air-to-blood ratios, we have focused on
the estimate of 1:5 as above, and also provided IQ loss estimates using
higher and lower estimates of air-to-blood ratio (i.e., 1:3 and 1:7)
within the range supported by the evidence. These estimates are
presented in Table 7 below.
---------------------------------------------------------------------------
\150\ We derived estimates of air-related IQ loss using the LLL
(nonlinear) function giving equal weight to all contributions of Pb
to total blood Pb as illustrated by the following example. For a
level of 0.30 [mu]g/m\3\, and an air-to-blood ratio of 1:5, the
resultant estimate of air-related blood Pb is 1.5 [mu]g/dL. Using
estimates for nonair blood Pb levels of 1 and 1.4 [mu]g/dL, the
estimates of total blood Pb are 2.5 and 2.9 [mu]g/dL. The
corresponding total Pb-related IQ loss estimates based on the LLL
function are 5.2 and 5.6 points IQ loss. These estimates are then
multiplied by the fraction of total Pb that is air-related (i.e.,
1.5/2.5 and 1.5/2.9) to derive the estimated range of air-related IQ
loss (2.9-3.1 points).
Table 7.--Estimates of Air-Related Population Mean IQ Loss for Children Exposed at the Level of the Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
Air-related population mean IQ loss (points) for children exposed at level of the standard
---------------------------------------------------------------------------------------------------------------------------
Air-to-blood ratio of Air-to-blood ratio of Air-to-blood ratio of Air-to-blood ratio of Air-to-blood ratio of
Potential level for standard 1:3 1:4 1:5 1:6 1:7
([mu]g/m\3\) ---------------------------------------------------------------------------------------------------------------------------
1st group 2nd group 1st group 2nd group 1st group 2nd group 1st group 2nd group 1st group 2nd group
of C-R of C-R of C-R of C-R of C-R of C-R of C-R of C-R of C-R of C-R
functions functions functions functions functions functions functions functions functions functions
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.50........................ * 2.9-3.1 1.4 * 3.5-3.8 1.8 * 4.1-4.3 2.3 * 4.6-4.8 2.7 * 5.0-5.3 3.2
0.40........................ * 2.4-2.6 1.1 * 3.0-3.2 1.4 * 3.5-3.8 1.8 * 4.0-4.2 2.2 * 4.4-4.6 2.5
0.30........................ 1.5-2.6 0.8 * 2.4-2.6 1.1 * 2.9-3.1 1.4 * 3.3-3.5 1.6 * 3.6-3.9 1.9
0.20........................ 1.0-1.8 0.5 1.4-2.4 0.7 1.7-2.9 0.9 * 2.4-2.6 1.1 * 2.7-3.0 1.3
0.10........................ 0.5-0.9 0.3 0.7-1.2 0.4 0.9-1.5 0.5 1.0-1.8 0.5 1.2-2.1 0.6
0.05........................ 0.3-0.4 0.14 0.3-0.6 0.18 0.4-0.7 0.2 0.5-0.9 0.27 0.6-1.0 0.3
0.02........................ 0.1-0.2 0.05 0.1-0.2 0.07 0.2-0.3 0.09 0.2-0.4 0.1 0.2-0.4 0.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
* These estimates were derived using only the nonlinear C-R function from the risk assessment which, given its nonlinearity, EPA considers to better
assess risk across the range that includes extending into these higher standard levels (and the associated higher blood Pb levels). That is, the upper
and lower values presented in the asterisked cells are both derived using the LLL function, as described in the text and associated footnote above,
rather than using the two linear functions of -1.71 from Tellez-Rojo, 2005 (<5 [mu]g/dL subgroup) and -2.94 from Lanphear, 2005 (<7.5 [mu]g/dL peak
blood Pb subgroup) as is the case in the cells without asterisks.
Using the air-to-blood ratio of 1:5 with the range of slopes from
the first set of C-R functions indicates an air-related mean IQ loss
estimate of 0.9 to 1.5 points for a population of children exposed at
the standard level of 0.10 [mu]g/m\3\. Similarly, the air-related mean
IQ loss estimate for a standard level of 0.20 [mu]g/m\3\ is 1.7 to 2.9
points. Using the air-to-blood ratio of 1:5 and the slope from the
second set of C-R functions (from blood Pb levels extending up to 10
[mu]g/dL) in the calculation indicates an air-related mean IQ loss of
0.5 points for a population of children exposed at the standard level
of 0.10 [mu]g/m\3\; the corresponding air-related mean IQ loss estimate
for a standard level of 0.20 [mu]g/m\3\ is 0.9 points. Using the 1:5
air-to-blood ratio with first set of C-R functions indicates an air-
related mean IQ loss estimate of approximately 3 points for a
population of children exposed at the standard level of 0.30 [mu]g/
m\3\. Using the slope from the second set of C-R functions indicates an
air-related mean IQ loss estimate of 1.4 points for a population of
children exposed at the standard level of 0.30 [mu]g/m\3\.
[[Page 29240]]
As mentioned above, we recognize uncertainty in the air-to-blood
values, and have accordingly also considered estimates of air-to-blood
ratio that are lower and higher than the 1:5 value used above.
Accordingly, we note that using a lower air-to-blood ratio, such as 1:3
(low end of range from evidence) generally results in lower air-related
IQ loss estimates with either set of C-R functions (approximately 40%
lower than those using a ratio of 1:5). Similarly, use of a higher air-
to-blood ratio, such as 1:7, yields higher air-related mean IQ loss
estimates with either set of C-R functions (approximately 40% higher
than those using a ratio of 1:5).
In applying this framework, we have also considered higher standard
levels, above 0.30 [mu]g/m\3\ up to the highest alternative level
included in the risk assessment (e.g., up to 0.50 [mu]g/m\3\). Using
the 1:5 air-to-blood ratio with the first set of C-R functions, the
air-related mean IQ loss estimate for a standard level of 0.50 [mu]g/
m\3\ is approximately 4 points. Using the slope from the second set of
C-R functions indicates an air-related mean IQ loss estimate of 2.3
points for a population of children exposed at the standard level of
0.50 [mu]g/m\3\. Using the 1:3 air-to-blood ratio with the first set of
C-R functions indicates an air-related mean IQ loss estimate of
approximately 3 points for a population of children exposed at the
standard level of 0.50 [mu]g/m\3\. Using the 1:3 air-to-blood ratio and
the slope for the second set of C-R functions indicates an air-related
mean IQ loss estimate of 1.4 points for a population of children
exposed at the standard level of 0.50 [mu]g/m\3\.
Further, we have also considered lower standard levels, down to the
lowest alternative levels included in the risk assessment (e.g., 0.05
to 0.02 [mu]g/m\3\). For example, across both sets of C-R functions and
the range of air-to-blood ratios considered above (1:3 to 1:7), a
standard level of 0.05 [mu]g/m\3\ indicates an air-related mean IQ loss
of approximately 0.1 to 1 point. The estimates for either set of C-R
functions are approximately 50% lower at the standard level of 0.02
[mu]g/m\3\.
b. Exposure- and Risk-Based Considerations
To inform judgments about a range of levels for the standard that
could provide an appropriate degree of public health protection, in
addition to considering the health effects evidence, EPA also
considered the quantitative estimates of exposure and health risks
attributable to air-related Pb upon meeting specific alternative levels
of alternative Pb standards and the uncertainties in the estimated
exposures and risks, as discussed above in Section III.B. As discussed
above, the risk assessment conducted by EPA is based on exposures that
have been estimated for children of less than 7 years of age in several
case studies. The assessment estimated the risk of adverse
neurocognitive effects in terms of IQ loss associated with total and
air-related Pb exposures, including incidence of different magnitudes
of IQ loss in the three location-specific case studies. In so doing,
EPA is mindful of the important uncertainties and limitations that are
associated with the exposure and risk assessments. For example, with
regard to the risk assessment important uncertainties include those
related to estimation of blood Pb C-R functions, particularly for blood
Pb concentrations at and below the lower end of those represented in
the epidemiological studies characterized in the Criteria Document.
EPA also recognizes important limitations in the design of, and
data and methods employed in, the exposure and risk analyses. For
example, the available monitoring data for Pb relied upon for
estimating current conditions for the urban case studies are quite
limited, in that monitors are not located near many of the larger known
Pb sources, which results in potential underestimation of current
conditions, and there is uncertainty about the proximity of existing
monitors to other Pb sources potentially influencing exposures, such as
old urban roadways and areas where housing with Pb paint has been
demolished or has undergone extensive exterior renovation. All of these
limitations raise uncertainty as to whether these data adequately
capture the magnitude of ambient Pb concentrations to which the target
population is currently exposed. Additionally, EPA recognizes that
there is not sufficient information available to evaluate all relevant
sensitive groups (e.g., adults with chronic kidney disease) or all Pb-
related health effects (e.g., neurological effects other than IQ loss,
immune system effects, adult cardiovascular or renal effects), and the
scope of our analyses was generally limited to estimating exposures and
risks in case studies intended to illustrate a variety of Pb exposure
situations across the U.S., with three of them focused on specific
areas in three cities. As noted above, however, coordinated intensive
efforts over the last 20 years have yielded a substantial decline in
blood Pb levels in the United States. Recent NHANES data (2003-2004)
yield blood lead level estimates for children age 1 to 5 years of 1.6
[mu]g/dL (median) and 3.9 [mu]g/dL (90th percentile). These median and
90th percentile national-level data are lower than modeled values
generated for the three location-specific urban case studies current
conditions scenarios (described in section II.C.3.a above). As noted in
section II.C.3.a, however, the urban case studies and the NHANES study
are likely to differ with regard to factors related to Pb exposure,
including ambient air levels (e.g., the national median ambient air Pb
concentrations are generally lower than those in the location-specific
case studies).
As described in section II.C.2.e, we also recognize limitations in
our ability to characterize the contribution of air-related Pb to total
Pb exposure and Pb-related health risk. As a result, we have
approximated estimates for the air-related pathways, bounded on the low
end by exposure/risk estimated for the ``recent air'' category and on
the upper end by the exposure/risk estimated for the ``recent air''
plus ``past air'' categories.\151\
---------------------------------------------------------------------------
\151\ As noted in section II.C.2.e above, the recent air
category does not include a variety of air-related categories
(including some associated with air deposition to outdoor surfaces
and diet) and both the recent air and past categories may include
some Pb in soil or dust from the historical use of Pb in paint.
---------------------------------------------------------------------------
We generally focus in this discussion on risk estimates derived
using the LLL (log-linear with low exposure linearization) C-R
function. Further, in considering the risk estimates in light of IQ
loss estimates (described in section II.E.3.a) of the air-related IQ
loss evidence-based framework, we focus here on risk estimates for the
general urban and primary Pb smelter subarea case studies as these
cases studies generally represent population exposures for more highly
air-pathway exposed children residing in small neighborhoods or
lozalized residential areas with air concentrations nearer the standard
level being evaluated than do the location-specific case studies in
which populations have a broader range of air-related exposures
including many well below the standard level being evaluated.
In considering the results of the risk assessment for the
alternative standard levels assessed, we note that the risk estimates
are roughly consistent with and generally supportive of the evidence-
based mean air-related IQ loss estimates described above (section
II.E.3.a). For example, at a standard level of 0.20 [mu]g/m\3\, the
evidence-based approach indicates estimates of mean air-related IQ loss
ranging from less than
[[Page 29241]]
1 to approximately 3 points IQ loss, while the median air-related risk
estimates for this level in the general urban case study are
represented by a lower bound near 1 point IQ loss and an upper bound
near 3 points IQ loss. The corresponding upper bound air-related IQ
loss estimate for the primary Pb smelter case study subarea is 3.7
points. Alternatively, at a standard level of 0.50 [mu]g/m\3\, the
evidence-based approach indicates estimates of mean air-related IQ loss
ranging from approximately 1.5 points to greater than 4 points, while
the median air-related risk estimates for this level for the general
urban case study are represented by a lower bound near 2 points IQ loss
and an upper bound just below 4 points IQ loss (section II.C.3.b). The
corresponding upper bound air-related IQ loss estimate for the primary
Pb smelter case study subarea is 4.5 points. Also, while the risk
assessment did not specifically assess the standard levels of 0.10 and
0.30 [mu]g/m\3\, we note that estimates for these levels based on
interpolation from the estimates described above are also roughly
consistent with and generally supportive of the evidence-based mean
air-related IQ loss estimates described in section II.E.3.a above
(Murphy and Pekar, 2008).
As mentioned above (section II.E.3.a), the Staff Paper conclusions
on level for the primary Pb standard built on staff 's conclusion that
the overall body of evidence clearly calls into question the adequacy
of the current standard with regard to health protection afforded to
at-risk populations. Drawing from both consideration of the evidence
and consideration of the quantitative risk and exposure information
(described in section II.E.3.b), staff concluded that the available
information provides strong support for consideration of a range of
standard levels that are appreciably below the level of the current
standard in order to provide increased public health protection for
these populations, with support for this conclusion. With regard to the
risk estimates, the Staff Paper recognized that, to the extent one
places weight on risk estimates for the lower standard levels, those
estimates may suggest consideration of a range of levels that extend
down to the lowest levels assessed in the risk assessment, 0.02 to 0.05
[mu]g/m\3\. In summary, the Staff Paper concluded that ``a level for
the standard set in the upper part of [the staff] recommended range
(0.1-0.2 [mu]g/m\3\, particularly with a monthly averaging time) is
well supported by the evidence and also supported by estimates of risk
associated with policy-relevant Pb that overlap with the range of IQ
loss that may reasonably be judged to be highly significant from a
public health perspective, and is judged to be so by CASAC'' (USEPA,
2007c). Further, the Staff Paper concluded that ``a standard set in the
lower part of the range would be more precautionary and would place
weight on the more highly uncertain range of estimates from the risk
assessment'' (USEPA, 2007c).
c. CASAC Advice and Recommendations and Public Comments
Beyond the evidence- and risk/exposure-based information discussed
above, EPA's consideration of the level for the TSP-based standard also
takes into account the advice and recommendations of CASAC, based on
their review of the Criteria Document, the Staff Paper and the related
technical support document, and the ANPR, as well as comments from the
public on drafts of the Staff Paper and related technical support
document and the ANPR.
In their advice to the Agency during this review CASAC has
recognized the importance of both the health effects evidence and the
exposure and risk information in selecting the level for the TSP-based
standard (Henderson, 2007a, 2007b, 2008). In two separate letters sent
prior to publication of the ANPR, CASAC stated that it is the unanimous
judgment of the CASAC Lead Panel that the primary NAAQS should be
``substantially lowered'' to ``a level of about 0.2 [mu]g/m\3\ or
less,'' reflecting their view of the health effects evidence
(Henderson, 2007a,b). In their most recent letter, reflecting their
review of the ANPR and Staff Paper, the Panel reiterated their earlier
judgment, stating that ``[t]he Committee unanimously and fully supports
Agency staff's scientific analyses in recommending the need to
substantially lower the level of the primary (public-health based) Lead
NAAQS, to an upper bound of no higher than 0.2 [mu]g/m\3\ with a
monthly averaging time.''
The CASAC Pb Panel also provided advice regarding how the Agency
should consider IQ loss estimates derived from the risk assessment in
selecting a level for the standard (Henderson, 2007a). The Panel stated
that they consider a population loss of 1-2 IQ points to be ``highly
significant from a public health perspective''.
Among the many public comments the Agency has received in this
review regarding the level of the standard, the overwhelming majority
recommended appreciable reductions in the level, e.g., setting it at
0.2 [mu]g/m\3\ or less, while only a few recommended that the Agency
make no or only a modest adjustment. Among the comments recommending
appreciable reduction, many noted the importance of considering
exposures and risks to vulnerable and susceptible populations. Some
recognized that blood Pb levels are disproportionately elevated among
minority and low-income children, and recommended more explicit
consideration of issues of environmental justice. And some comments
also noted the need for the standard to provide an adequate margin of
safety, indicating that such a need might provide support for
consideration of much lower levels. The American Academy of Pediatrics
recommended that EPA set the level at 0.2 or lower, and also
recommended that EPA consider the approach developed by the State of
California Environmental Protection Agency (Cal-EPA) for the purposes
of school site assessment, which has at its goal prevention of a rise
in blood Pb level that Cal-EPA has predicted to be associated with an
incremental increase estimated to decrease IQ by 1 point.
d. Administrator's Proposed Conclusion Concerning Level
For the reasons discussed below, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and the public
comments to date, the Administrator proposes to revise the existing
primary Pb standard. Specifically, the Administrator proposes to revise
the level of the primary Pb standard, defined in terms of the current
Pb-TSP indicator, to within the range of 0.10 to 0.30 [mu]g/m\3\,
conditional on judgments as to the appropriate values of key parameters
to use in the context of the air-related IQ loss evidence-based
framework discussed below.
Further, in recognition of alternative views of the science, the
exposure and risk assessments, the uncertainties inherent in the
science and these assessments, and the appropriate public health policy
responses based on the currently available information, the
Administrator also solicits comments on whether to proceed instead with
alternative levels of a primary Pb-TSP standard within ranges from
above 0.30 [mu]g/m\3\ up to 0.50 [mu]g/m\3\ and below 0.10 [mu]g/m\3\.
Based on the comments received and the accompanying rationales, the
Administrator may adopt other standards within the range of the
alternative levels identified above in lieu of the standards he is
proposing today. In addition, as discussed below, the Administrator
also solicits comments on when, if ever, it would be
[[Page 29242]]
appropriate to set a NAAQS for Pb at a level of zero.
The Administrator's consideration of alternative levels of the
primary Pb-TSP standard builds on his proposed conclusion, discussed
above in section II.D.4, that the overall body of evidence indicates
that the current Pb standard is not requisite to protect public health
with an adequate margin of safety and that the standard should be
revised to provide increased public health protection, especially for
members of at-risk groups, notably including children, against an array
of adverse health effects. These effects range from IQ loss, a health
outcome that could be quantified in the risk assessment, to health
outcomes that could not be directly estimated, including decrements in
other neurocognitive functions, other neurological effects and immune
system effects, as well as cardiovascular and renal effects in adults.
In reaching a proposed decision about the level of the Pb primary
standard, the Administrator has considered: the evidence-based
considerations from the Criteria Document and the Staff Paper and those
based on the air-related IQ loss evidence-based framework discussed
above; the results of the exposure and risk assessments discussed above
and in the Staff Paper, giving weight to the exposure and risk
assessments as judged appropriate; CASAC advice and recommendations, as
reflected in discussions of the Criteria Document, Staff Paper, and
ANPR at public meetings, in separate written comments, and in CASAC's
letters to the Administrator; EPA staff recommendations; and public
comments received during the development of these documents, either in
connection with CASAC meetings or separately. In considering what
standard is requisite to protect public health with an adequate margin
of safety, the Administrator is mindful that this choice requires
judgment based on an interpretation of the evidence and other
information that neither overstates nor understates the strength and
limitations of the evidence and information nor the appropriate
inferences to be drawn.
In reaching a proposed decision on a range of levels for a revised
standard, as in reaching a proposed decision on the adequacy of the
current standard, the Administrator primarily considered the evidence
in the context of the air-related IQ loss evidence-based framework
described above in section II.E.3.a.ii. As a general matter, in
considering this evidence-based framework, the Administrator recognizes
that in the case of Pb there are several aspects to the body of
epidemiological evidence that add complexity to the selection of an
appropriate level for the primary standard. As discussed above, these
complexities include evidence based on blood Pb as the dose metric,
exposure pathways that are both air-related and nonair-related, and the
absence of any discernible threshold levels in the health effects
evidence. Further, the Administrator recognizes that there are a number
of important uncertainties and limitations inherent in the available
health effects evidence and related information, including
uncertainties in the evidence of associations between total blood Pb
and neurocognitive effects in children, especially at the lowest blood
Pb levels evaluated in such studies, as well as uncertainties in key
parameters used in this evidence-based framework, including C-R
functions and air-to-blood ratios. In addition, the Administrator
recognizes that there are currently no commonly accepted guidelines or
criteria within the public health community that would provide a clear
basis for reaching a judgment as to the appropriate degree of public
health protection that should be afforded to neurocognitive effects in
sensitive populations, such as IQ loss in children.
The air-related IQ loss evidence-based framework considered by the
Administrator focuses on quantitative relationships between air-related
Pb and neurocognitive effects (e.g., IQ loss) in children, building on
recommendations from CASAC to consider the body of evidence in a more
quantitative manner. More specifically, this framework is premised on a
public health goal of selecting a standard level that would prevent
air-related IQ loss (and related effects) of a magnitude judged by the
Administrator to be of concern in populations of children exposed to
the level of the standard, taking into consideration uncertainties
inherent in such estimates. In addition to this public health policy
judgment regarding IQ loss, two other parameters are relevant to this
framework--a C-R function for population IQ response associated with
blood Pb level and an air-to-blood ratio. Based on the discussion of
these parameters in section II.E.3.a above, the Administrator concludes
that, in considering alternative standard levels below the level of the
current standard, it is appropriate to take into account the same two
sets of C-R functions, recognizing uncertainties in the related
evidence, as was done in considering the adequacy of the current
standard (as discussed above in section II.D). He notes that the first
set of C-R functions reflects the evidence indicative of steeper slopes
in relationships between blood Pb and IQ in children, and that the
second set of C-R functions reflects relationships with shallower
slopes between blood Pb and IQ in children. In addition, the
Administrator concludes that it is appropriate to consider various air-
to-blood ratios, again recognizing the uncertainties in the relevant
evidence. He notes that an air-to-blood ratio of 1:5 is within the
reasonable range of values that EPA considers to be generally supported
by the available evidence, which includes ratios of 1:3 up to 1:7.
With regard to making a public health policy judgment as to the
appropriate level of protection against air-related IQ loss and related
effects, the Administrator first notes that ideally air-related (as
well as other) exposures to environmental Pb would be reduced to the
point that no IQ impact in children would occur. The Administrator
recognizes, however, that in the case of setting a NAAQS, he is
required to make a judgment as to what degree of protection is
requisite to protect public health with an adequate margin of safety.
The NAAQS must be sufficient but not more stringent than necessary to
achieve that result, and does not require a zero-risk standard.
Considering the advice of CASAC and public comments on this issue,
notably including the comments of the American Academy of Pediatrics,
the Administrator proposes to conclude that an air-related population
mean IQ loss within the range of 1 to 2 points could be significant
from a public health perspective, and that a standard level should be
selected to provide protection from air-related population mean IQ loss
in excess of this range.
The Administrator considered the application of this air-related IQ
loss framework with this target degree of protection in mind, drawing
from the information presented in Table 7 above in section II.E.3.a.ii
that addresses a broad range of standard levels. In so doing, the
Administrator first focused on the estimates associated with the first
set of C-R functions in conjunction with the range of air-to-blood
ratios considered by EPA in this framework. Specifically, using an air-
to-blood ratio of 1:5, the Administrator notes that a standard level of
0.10 [mu]g/m\3\ would limit the estimated degree of impact on
population mean IQ loss from air-related Pb to no more than 1.5 points,
the mid-point of the proposed range of protection. Using the full range
of air-to-blood ratios considered in this framework (1:3 to 1:7), he
notes that a standard set at this level (0.10 [mu]g/m\3\) would limit
the estimated degree of air-
[[Page 29243]]
related impact on population mean IQ loss to a range from less than 1
point to around 2 points. Again based on the first set of C-R
functions, the Administrator notes that a standard level of 0.20 [mu]g/
m\3\ would also limit the estimated degree of air-related impact on
population mean IQ loss to within the proposed range of protection
based on using an air-to-blood ratio of 1:3.
In considering the use of the second set of C-R functions in
conjunction with the range of air-to-blood ratios considered in this
framework (1:3 to 1:7), the Administrator notes for example that a
standard set within the range of 0.10 to 0.30 [mu]g/m\3\ would limit
the estimated degree of air-related impact on population mean IQ loss
to a range from less than one-half point to just under 2 points. More
specifically, based on using an air-to-blood ratio of 1:5 (the
approximately central estimate) in conjunction with the second set of
C-R functions, the Administrator notes that a standard level of 0.30
[mu]g/m\3\ would limit the estimated degree of impact on population
mean IQ loss from air-related Pb to just under 1.5 points, the mid-
point of the proposed range of protection.
Taking these considerations into account, and based on the full
range of information presented in Table 7 above on estimates of air-
related IQ loss in children over a broad range of alternative standard
levels, the Administrator concludes that it is appropriate to propose a
range of standard levels, and that a range of levels from 0.10 to 0.30
[mu]g/m\3\ is consistent with his target for protection from air-
related IQ loss in children. In recognition of the uncertainties in
these key parameters, the Administrator believes that the selection of
a standard level from within this range is conditional on judgments as
to the most appropriate parameter values to use in the context of this
evidence-based framework. For example, he notes that placing more
weight on the use of a C-R function with a relatively steeper slope
would tend to support a standard level in the lower part of the
proposed range, while placing more weight on a C-R function with a
shallower slope would tend to support a level in the upper part of the
proposed range. Similarly, placing more weight on a higher air-to-blood
ratio would tend to support a standard level in the lower part of the
proposed range, whereas placing more weight on a lower ratio would tend
to support a level in the upper part of the range. In soliciting
comment on a standard level within this proposed range, the
Administrator specifically solicits comment on the appropriate values
to use for these key parameters in the context of this evidence-based
framework, reflecting that his proposal to revise the level of the
primary Pb standard, defined in terms of the current Pb-TSP indicator,
to within the range of 0.10 to 0.30 [mu]g/m\3\ is conditional on
judgments as to the appropriate values of key parameters to use in this
context.
The Administrator has also considered the results of the exposure
and risk assessments conducted for this review to provide some further
perspective on the potential magnitude of air-related IQ loss. The
Administrator finds that these quantitative assessments provide a
useful perspective on the risk from air-related Pb. However, in light
of the important uncertainties and limitations associated with these
assessments, as discussed above in sections II.C and II.E.3.b, for
purposes of evaluating potential new standards, the Administrator
places less weight on the risk estimates than on the evidence-based
assessments. Nonetheless, the Administrator finds that the risk
estimates are roughly consistent with and generally supportive of the
evidence-based air-related IQ loss estimates described above, as
discussed above in section II.E.3.b. This lends support to the proposed
range based on this evidence-based framework.
In the Administrator's view, the above considerations, taken
together, provide no evidence- or risk-based bright line that indicates
a single appropriate level. Instead, there is a collection of
scientific evidence and judgments and other information, including
information about the uncertainties inherent in many relevant factors,
which needs to be considered together in making this public health
policy judgment and in selecting a standard level from a range of
reasonable values. Based on consideration of the entire body of
evidence and information available at this time, as well as the
recommendations of CASAC and public comments, the Administrator is
proposing that a standard level within the range of 0.10 to 0.30 [mu]g/
m\3\ would be requisite to protect public health, including the health
of sensitive groups, with an adequate margin of safety. He also
recognizes that selection of a level from within this range is
conditional on judgments as to what C-R function and what air-to-blood
ratio are most appropriate to use within the context of the air-related
IQ loss framework. The Administrator notes that this proposed range
encompasses the specific level of 0.20 [mu]g/m\3\, the upper end of the
range recommended by CASAC and by many public commenters. The
Administrator provisionally concludes that a standard level selected
from within this range would reduce the risk of a variety of health
effects associated with exposure to Pb, including effects indicated in
the epidemiological studies at low blood Pb levels, particularly
including neurological effects in children, and cardiovascular and
renal effects in adults.
Because there is no bright line clearly directing the choice of
level within this reasonable range, the choice of what is appropriate,
considering the strengths and limitations of the evidence, and the
appropriate inferences to be drawn from the evidence and the exposure
and risk assessments, is a public health policy judgment. To further
inform this judgment, the Administrator solicits comment on the air-
related IQ loss evidence-based framework considered by the Agency and
on appropriate parameter values to be considered in the application of
this framework. More specifically, we solicit comment on the
appropriate C-R function and air-to-blood ratio to be used in the
context of the air-related IQ loss framework. The Administrator also
solicits comment on the degree of impact of air-related Pb on IQ loss
and other related neurocognitive effects in children considered to be
significant from a public health perspective, and on the use of this
framework as a basis for selecting a standard level.
For the reasons discussed above, the Administrator proposes to
revise the level of the primary Pb standard, defined in terms of the
current Pb-TSP indicator, to within the range of 0.10 to 0.30 [mu]g/
m\3\, conditional on judgments as to the appropriate C-R functions and
air-to-blood ratio to use in the context of the air-related IQ loss
framework.
The Administrator notes that this framework indicates that for
standard levels above 0.30 [mu]g/m\3\ up to 0.50 [mu]g/m\3\, the
estimated degree of impact on population mean IQ loss from air-related
Pb would range from approximately 2 points to 5 points or more with the
use of the first set of C-R functions and the full range of air-to-
blood ratios considered, and would extend from somewhere within the
proposed range of 1 to 2 points IQ loss to above that range when using
the second set of C-R functions and the full range of air-to-blood
ratios considered. The Administrator proposes to conclude in light of
his consideration of the evidence in the framework discussed above that
the magnitude of air-related Pb effects at the higher blood Pb levels
that would be allowed by standards above 0.30 up to 0.50 [mu]g/m\3\
would be greater than what is requisite to protect
[[Page 29244]]
public health with an adequate margin of safety.
In addition, the Administrator notes that for standard levels below
0.10 [mu]g/m\3\, the estimated degree of impact on population mean IQ
loss from air-related Pb would generally be somewhat to well below the
proposed range of 1 to 2 points air-related population mean IQ loss
regardless of which set of C-R functions or which air-to-blood ratio
within the range of ratios considered are used. The Administrator
proposes to conclude that the degree of public health protection that
standards below 0.10 [mu]g/m\3\ would likely afford would be greater
than what is requisite to protect public health with an adequate margin
of safety.
Having reached this proposed decision based on the interpretation
of the evidence, the evidence-based frameworks, the exposure/risk
assessment, and the public health policy judgments described above, the
Administrator recognizes that other interpretations, frameworks,
assessments, and judgments are possible. There are also potential
alternative views as to the range of values for relevant parameters
(e.g., C-R function, air-to-blood ratio) in the evidence-based
framework that might be considered supportable and the relative weight
that might appropriately be placed on any specific value for these
parameters within such ranges. In addition, the Administrator
recognizes that there may be other views as to the appropriate degree
of public health protection that should be afforded in terms of air-
related population mean IQ loss in children that would provide support
for alternative standard levels different from the proposed range.
Further, there may be other views as to the appropriate weight and
interpretation to give to the exposure/risk assessment conducted for
this review. Consistent with the goal of soliciting comment on a wide
array of issues, the Administrator solicits comment on these and other
issues.
In particular, the Administrator solicits comment on alternative
levels of a primary Pb-TSP standard of above 0.30 [mu]g/m\3\ up to 0.50
[mu]g/m\3\. In considering the air-related IQ loss framework and the
case when the second set of C-R functions is used in conjunction with
the lowest air-to-blood ratio considered in this framework (i.e., 1:3),
a standard level as high as 0.50 [mu]g/m\3\ would still limit the
estimated degree of impact on population mean IQ loss from air-related
Pb to no more than 1.5 points, the mid-point of the proposed range of
protection. Comment is solicited on levels within this range and the
associated rationale for selecting such a level in terms of the
appropriate weight to place on relevant parameter values that may
extend to values outside the ranges of values considered by EPA, or in
terms of alternative evidence- or risk-based frameworks that might
support standard levels within this range.
In addition, the Administrator solicits comment on alternative
levels below 0.10 [mu]g/m\3\. In considering the evidence-based
framework discussed above, a standard level within this range would
likely provide a degree of protection in terms of air-related
population mean IQ loss that is greater than the proposed range based
on the use of any of the relevant parameter values within the ranges
considered by EPA. Comment is solicited on levels within this range and
the associated rationale for selecting such a level in terms of the
appropriate weight to place on relevant parameter values that may
extend to values outside of the ranges considered by EPA, or
alternative public health policy judgments as to the degree of
protection that is warranted, or the appropriate weight to place on the
results of the risk assessment.
More broadly, as discussed above, the Administrator recognizes that
Pb can be considered a non-threshold pollutant.\152\ In recognizing
that no threshold has been identified below which we are scientifically
confident that there is no risk of harm, EPA's views are consistent
with the views of the CDC, the Federal agency that tracks children's
blood Pb levels nationally and provides guidance on levels at which
medical and environmental case management activities should be
implemented (CDC, 2005a; ACCLPP, 2007). In 2005, CDC revised its
statement on Preventing Lead Poisoning in Young Children, specifically
recognizing the evidence of adverse health effects in children and the
data demonstrating that no ``safe'' threshold for blood Pb had been
identified (CDC, 2005a). EPA's views are also consistent with other
organizations, including, for example, the American Academy of
Pediatrics that recognized in commenting on the ANPR that ``[t]here is
no known ``safe'' level of blood lead in children'' (AAP, 2008). In
addition, the California Environmental Protection Agency, in a recent
risk assessment report, recognizes that ``no safe level has been
definitively established'' for effects of Pb in children (CalEPA, 2007,
p. 1). Given the current state of scientific evidence, which does not
resolve the question of whether or not there is a threshold, we
recognize that there is no level below which we can say with scientific
confidence that there is no risk of harm from exposure to ambient air
related lead.
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\152\ Similarly, in the most recent reviews of the NAAQS for
ozone and PM, EPA recognized that the available epidemiological
evidence neither supports nor refutes the existence of thresholds at
the population level, while noting uncertainties and limitations in
studies that make discerning thresholds in populations difficult
(e.g., 73 FR 16444, March 27, 2008; 71 FR 61158, October 17, 2006).
---------------------------------------------------------------------------
The Administrator also recognizes, as discussed in section I.A
above, that the CAA does not require that NAAQS be established at a
zero-risk level, but rather at a level that reduces risk sufficiently
so as to protect public health with an adequate margin of safety. In
setting primary standards that are ``requisite'' to provide the this
degree of public health protection, the Supreme Court has affirmed that
EPA's task is to establish standards that are neither more nor less
stringent than necessary for this purpose. The question then becomes
how the Agency should reconcile these scientific and legal
understandings in reviewing the Pb NAAQS.
As discussed above, EPA is proposing a range of levels for the
primary Pb NAAQS, with the range extending down to 0.10 [mu]g/m\3\.
This range reflects the Administrator's proposed conclusion that lower
levels would be more than necessary to protect public health with an
adequate margin of safety. This proposed conclusion is based in large
part on EPA's evaluation of the evidence, recognizing important
uncertainties in the scientific evidence and related assessments, and
reflects the proposed public heath policy judgment of the Administrator
on these issues. As discussed above, these uncertainties stem in part
from the complexities of determining the health impact of air-related
Pb given the multi-media exposure pathways for exposure to lead and the
persistence of Pb in the environment. The major areas of uncertainty
include the appropriate air-to-blood ratio; the apportionment of Pb
between air-related and nonair Pb; the increasing uncertainty at lower
blood Pb levels as to the existence, nature, and degree of health
effects; and the uncertainty over the public health significance of
smaller and smaller impacts on IQ or other similar neurocognitive
metrics from exposure to air-related Pb. In recognition of such
uncertainties, EPA is also soliciting comment on a lower range of
standard levels below 0.10 [mu]g/m\3\.
In so doing, EPA fully recognizes that a standard set at the lowest
proposed level of 0.10 [mu]g/m\3\, or any non-zero level, would not be
a risk-free standard.
[[Page 29245]]
As in numerous prior NAAQS reviews, we recognize that the CAA does not
require that EPA set a risk-free standard. Instead, EPA is to recognize
and take risk into account, and set a standard that is requisite to
protect public health with an adequate margin of safety based on the
currently available information. This calls for a public health policy
judgment informed by many factors, most notably the nature and severity
of the health effects at issue, the size of the population(s) at risk,
and the kind and degree of uncertainties involved. After considering
all of these factors in this review, the Administrator's proposed
judgment is that a standard set below 0.10 [mu]g/m\3\ would not satisfy
this statutory directive.
The Administrator recognizes that the current state of the
scientific evidence clearly indicates that health effects from Pb occur
at much lower blood Pb levels than we understood in the past, and that
the appropriate level for ambient air Pb is much lower than we thought
in the past. Further the Administrator expects that, as time goes on,
future scientific studies will continue to enhance our understanding of
Pb, and anticipates that such studies might lead to a situation where
there is very little, if any, remaining uncertainty about human health
impacts from even extremely low levels of Pb in the ambient air. As
noted above, this has the potential to raise fundamental questions as
to how the Agency can continue to reconcile such evidence with the
statutory provision calling for the NAAQS to be set at a level that is
requisite to protect public health with an adequate margin of safety.
Faced with scientific evidence that could reasonably be interpreted as
demonstrating that any ambient Pb level above zero contributes to
adverse health effects in at-risk populations, some might conclude that
the only standard requisite to protect public health with an adequate
margin of safety would be a standard set at zero. While EPA's proposed
conclusions on the current scientific evidence and an appropriate
standard based on that evidence and on its interpretation of the
statute clearly differ from such a view, EPA nonetheless believes that
inviting comment in this review on the views described above and the
issues raised by such circumstances is appropriate.
More specifically, EPA invites comment on when, if ever, it would
be appropriate to set a NAAQS for Pb at a level of zero. Comments on
this question might address issues such as: The level of scientific
certainty that would be needed to support such a decision; the level of
harm, e.g., severity of health effect and size of affected population,
that would be needed to support such a decision; and whether there are
normative or quantitative criteria that could be applied in deciding
whether, and if so, when it would be appropriate to set a standard at
zero. EPA invites comment on how to reconcile the above issues in this
and subsequent NAAQS reviews.
4. Level for a Pb NAAQS with a Pb-PM10 Indicator
EPA is requesting comment on the option of revising the indicator
for the Pb NAAQS from Pb-TSP to Pb-PM10, based on low-volume
sample collection as discussed above in section II.E.1 and below in
section V.A. In this section, we discuss considerations important to
selection of a level for such a Pb-PM10-based standard
(section II.E.4.a) and CASAC's advice and public comments on this issue
(section II.E.4.b). Approaches for adjusting the level of a Pb NAAQS
with Pb-TSP indicator for a Pb-PM10-based standard, and a
range of levels for a Pb-PM10-based standard, under
consideration and on which EPA is soliciting comment are presented in
II.E.4.c.
a. Considerations With Regard to Particles Not Captured by
PM10
In the course of deciding to propose the Pb-TSP indicator approach
as described in section II.E.1 above, EPA has noted the important role
of both respirable and non-respirable Pb particles in air-related Pb
exposure of concern and the lesser capture of these particles by
PM10 samplers compared to TSP samplers. We recognize that
the health evidence indicates that Pb in all particle size fractions,
not just respirable Pb, contributes to Pb in blood and to associated
health effects. Further, the quantity of Pb in ambient particles with
the potential to deposit (indoors and outdoors, leading to a role in
ingestion pathways) is a key contributor to air-related exposure, and
these particles include ultra-coarse mode particles that are not
captured by PM10 samplers (as discussed in section II.E.1
above). In recognition of these considerations, both of the indicator
options discussed in this notice recognize the need to consider use of
an adjustment related to the use of PM10 measurements,
either when considering the optional use of Pb-PM10 data for
comparison with a Pb-TSP-based NAAQS, or when considering a level for a
NAAQS based on a Pb-PM10 indicator.
Section II.E.1 above contains extensive discussion of the
relationship between Pb-PM10 and Pb-TSP, including the fact
that Pb-PM10/Pb-TSP relationships vary from site to site and
from time to time, but have a systematic variation with distance from
emissions sources emitting particles larger than would be captured by
Pb-PM10 samplers, such that generally there are larger
differences between Pb-PM10 and Pb-TSP near sources. Section
II.E.1 goes on to identify and solicit comment on two ranges from which
scaling factors could be chosen that would be applied to the Pb-
PM10 measurements to derive surrogate Pb-TSP concentrations
for use in making comparisons to a Pb-TSP-based NAAQS. In recognition
of the influence of proximity to sources on the relationship between
Pb-TSP and Pb-PM10 measurements for source types with a high
fraction of ultra-coarse particles containing Pb, different scaling
factors are identified for source-oriented monitoring sites and
nonsource-oriented monitoring sites (as described in section II.E.1).
These ranges have been developed based on analyses of the available
collocated Pb-TSP and Pb-PM10 data (Schmidt and Cavender,
2008) and recognition of variability and uncertainty inherent in this
data set.
The data supporting the range for source-oriented scaling factors,
as discussed in Schmidt and Cavender (2008), indicate the potential, in
areas influenced by some types of sources (e.g., Pb smelters), for
PM10 samplers to capture as little as approximately 50% of
the Pb that is measured with Pb-TSP monitors. The data from 20 sites
not known to be near Pb sources show a range of ratios between Pb-TSP
and Pb-PM10 that vary from day to day and between sites.
When rounded to one decimal place, these ratios of the multi-day mean
concentration of Pb-TSP to the same statistic for Pb-PM10 at
each site ranged from 1.0 to 1.9.\153\ Eighty-five percent of the sites
had ratios between 1.0 and 1.4, and slightly over one-half the sites
had ratios between 1.0 and 1.2. This is consistent with the conceptual
model that concentrations of ultra-coarse particles of Pb are quite low
at sites not near the primary sources of such particles, such that Pb-
PM10 monitors at such sites would tend to collect the large
majority, but generally not all, of total airborne Pb.
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\153\ On individual days, the ratio between the two measures was
sometimes below 1.0 or well over 2.0, which may be the result of
sampler errors and data rounding particularly when concentrations of
one or both measures were low. Accordingly, EPA considers the ratio
of the multi-day mean concentration of Pb-TSP to the same statistic
for Pb-PM10 at each site to be a better indicator of
typical monitor behavior.
---------------------------------------------------------------------------
In considering the need for and magnitude of a potential adjustment
to derive a standard level for a Pb-PM10-
[[Page 29246]]
based NAAQS, we note the inherent variability in the TSP sampling
methodology which will contribute variability to relationships derived
between Pb-PM10 and Pb-TSP data. We also note the influence
on such relationships of proximity to sources of Pb particles that
would not be captured by PM10 samplers. This latter
influence is evident in the difference between the two ranges of
scaling factors proposed in section II.E.1 above.
We are also aware of the limitations of the dataset available on
which to base these decisions, including those related to the quantity
of collocated measurements and particularly the very limited number of
source-influenced monitors for which such measurements are available,
and the correspondingly limited number of types of sources represented.
Moreover, the available collocated measurements suggesting the above-
referenced 50% figure in a source-influenced location are from
conditions in which ambient concentrations were above the current
standard level and well above the proposed range of levels. If the
contributing emissions sources had been controlled so that local
concentrations were within or near the range proposed for the revised
standard, it is unclear whether the relationship between Pb-
PM10 and Pb-TSP data would have been different or not. The
Pb-TSP concentrations at sites in the dataset analyzed that were not
known to be source-influenced were well below the proposed range of
standard levels, leaving uncertainty about typical proportions of
ultra-coarse particles in nonsource areas with Pb-TSP concentrations
near the proposed range of levels.
If EPA adopts a PM10 indicator, the approach of using
two adjustment factors representing source-oriented and nonsource-
oriented sites, or the approach of site-specific adjustment factors,
would not be used in setting a standard level.\154\ Rather, the
complexity of the site-to-site variability in the Pb-TSP/Pb-
PM10 relationship would have to be reflected in a decision
about whether and how to adjust the level of the standard to account
for the fact that a Pb-PM10 indicator would be less
inclusive of Pb particles than would a Pb-TSP indicator.
---------------------------------------------------------------------------
\154\ As discussed below in sections IV and VI, however, EPA is
soliciting comment on the potential use of Pb-TSP data for initial
designations for Pb-PM10 standard and whether the
associated use of scaling factors would be appropriate.
---------------------------------------------------------------------------
b. CASAC Advice
As noted above, CASAC has described the use of an adjustment of the
NAAQS level to accommodate the loss of the ultra-coarse Pb particles
that are important contributions to Pb exposure but that are excluded
by PM10 samplers (section II.E.1). For example, in
discussion of the recommendation for the Agency to revise the Pb NAAQS
indicator to Pb-PM10 (using low-volume samplers) in their
February 2007 letter, the CASAC Pb Panel stated that ``Presumably a
downward scaling of the level of the Lead NAAQS could accommodate the
loss of very large coarse-mode lead particles * * * '' (Henderson,
2007a). With regard to the magnitude of such scaling, CASAC has
recognized the usefulness of some ``short period of concurrent
PM10 and TSP lead sampling'' to ``help develop site-specific
scaling factors at sites with highest concentrations'' (Henderson,
2007a) and also indicated an expectation that, in general, Pb-
PM10 will represent a large fraction of, and be highly
correlated with TSP Pb (Henderson, 2007b). In their most recent letter,
the Panel stated generally that ``it would be well within EPA's range
of discretionary options to accept a slight loss of ultra-coarse lead
at some monitoring sites by selecting an appropriately conservative
level for the revised Pb NAAQS'' (Henderson, 2008). In summary, while
the CASAC recognized the appropriateness of making an adjustment to the
level for a Pb-PM10-based NAAQS, they did not provide a
quantitative value, but did note interest in sites with highest
concentrations. Further, CASAC expressed the view that the overall
health-related benefits from moving to a PM10-based standard
could outweigh a small loss in protection from exposure to ultra-coarse
particles in some areas.
The Agency received few public comments with regard to a standard
level for a revised indicator of Pb-PM10. Of these, some
generally agreed with CASAC that an adjustment to the level was
appropriate, recognizing the difference in the two sampling methods.
Some were concerned that the current data may not support the
derivation of a single scaling or adjustment factor that would provide
requisite protection for some communities near some large point source
emitters of dust.
c. Approaches for Levels for a PM10-Based Standard
For the reasons identified in the preceding section and in section
II.E.1 above, EPA's consideration of a Pb-PM10 indicator is
accompanied by consideration of an adjustment of the proposed level for
the standard, in recognition of the importance for public health of
those ultra-coarse dust contributions not captured by PM10
samplers.
In considering the appropriate level for a standard for which the
indicator is Pb-PM10, EPA recognizes the importance of all
particle size fractions and the dominant role of the ingestion pathway
in contributing to human exposures to air-related Pb. We also recognize
that the proportion of Pb captured by TSP monitors that is not captured
by PM10 monitors will vary, not only in reflection of the
inherent greater variability of the TSP sampler (as compared to the
PM10 sampler), but also based on proximity to sources
emitting ultra-coarse Pb particles. An appreciably lower proportion of
the Pb captured by TSP monitors will be captured by PM10
monitors in areas near such sources (e.g., Pb smelters).
However, we are also aware of the limitations with regard to the
available Pb monitoring data on which to base a decision with regard to
an adjustment that appropriately recognizes these considerations. EPA
notes that at lower levels, there is increased uncertainty as to the
appropriate scaling factor to use, particularly in light of the very
limited data we have on which to base an analysis. Additionally, we
take note of advice from CASAC and public comments with regard to
considerations for a level to accompany a Pb-PM10 indicator.
Based on these and other considerations summarized above (II.E.1
and II.E.4.a), including the data indicating the proportion of Pb-TSP
that may not be captured by PM10 samplers in some source-
oriented locations, EPA requests comment on whether a level for a NAAQS
with a Pb-PM10 indicator should be based on an adjustment to
a lower level than the level for a NAAQS with a Pb-TSP indicator, and,
if so, on the magnitude of the adjustment that would be appropriate.
Taking into consideration uncertainties in the appropriate adjustment
for a Pb-PM10 based level (due to the very limited
collocated dataset with which to evaluate relationships between Pb-TSP
and Pb-PM10), and the appropriate policy responses based on
the currently available information, EPA specifically solicits comment
on the appropriate level for a Pb-PM10-based primary
standard within the full range of levels on which comment is being
solicited for a Pb-TSP standard, i.e., levels up to 0.50 [mu]g/m\3\.
Based on the comments received and the accompanying rationales, EPA may
adopt standards within this broad range of alternative levels.
[[Page 29247]]
F. Proposed Decision on the Primary Standard
For the reasons discussed above, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and the public
comments to date, the Administrator is proposing options for the
revision of the various elements of the standard to provide increased
protection for children and other at-risk populations against an array
of adverse health effects, most notably including neurological effects,
including neurocognitive and neurobehavioral effects, in children.
Specifically, with regard to the indicator and level of the standard,
the Administrator proposes to revise the level of the standard to a
level within the range of 0.10 to 0.30 [mu]g/m\3\ in conjunction with
retaining the current indicator of Pb-TSP but with allowance for the
use of Pb-PM10 data. The Administrator also solicits comment
on alternative levels up to 0.50 [mu]g/m\3\ and down below 0.10 [mu]g/
m\3\. With regard to the form and averaging time of the standard, the
Administrator proposes two options: (1) To retain the current averaging
time of a calendar quarter and the current not-to-be-exceeded form, to
apply across a 3-year span, and (2) to revise the averaging time to a
calendar month and the form to be the second-highest monthly average
across a 3-year span.
Corresponding revisions to data handling conventions and the
schedule for States to request exclusion of ambient Pb concentration
data affected by exceptional events are specified in proposed revisions
to Appendix R, as discussed in section IV below. Corresponding
revisions to aspects of the ambient air monitoring and reporting
requirements for Pb are discussed in section V below, including
sampling and analysis methods (e.g., a new Federal reference method for
monitoring Pb in PM10, quality assurance requirements),
network design, sampling schedule, data reporting, and other
miscellaneous requirements.
In recognition of alternative views of the science and the exposure
and risk assessments, the uncertainties inherent in this information,
and the appropriate policy responses based on the currently available
information, the Administrator also solicits comments on other options.
More specifically, the Administrator solicits comment on revising the
indicator to Pb-PM10 and on the same broad range of levels
on which EPA is soliciting comment for the proposed Pb-TSP indicator,
i.e., up to 0.50 [mu]g/m\3\. In addition, the Administrator invites
comment on when, if ever, it would be appropriate to set a NAAQS for Pb
at a level of zero. Based on the comments received and the accompanying
rationales, the Administrator may adopt other standards within the
range of the alternative levels identified above in lieu of the
standards he is proposing today.
III. Rationale for Proposed Decision on the Secondary Standard
This section presents the rationale for the Administrator's
proposed decision to revise the existing secondary NAAQS. In
considering the currently available evidence on Pb-related welfare
effects, the Staff Paper notes that there is much information linking
Pb to potentially adverse effects on organisms and ecosystems. However,
given the evaluation of this information in the Criteria Document and
Staff Paper which highlighted the substantial limitations in the
evidence, especially the lack of evidence linking various effects to
specific levels of ambient Pb, the Administrator concludes that the
available evidence supports revising the secondary standard but does
not provide a sufficient basis for establishing a distinct secondary
standard for Pb.
A. Welfare Effects Information
Welfare effects addressed by the secondary NAAQS include, but are
not limited to, effects on soils, water, crops, vegetation, manmade
materials, animals, wildlife, weather, visibility and climate, damage
to and deterioration of property, and hazards to transportation, as
well as effects on economic values and on personal comfort and well-
being. A qualitative assessment of welfare effects evidence related to
ambient Pb is summarized in this section, drawing from Chapter 6 of the
Staff Paper. The presentation here first recognizes several key aspects
of the welfare evidence for Pb. Lead is persistent in the environment
and accumulates in soils, aquatic systems (including sediments), and
some biological tissues of plants, animals, and other organisms,
thereby providing long-term, multipathway exposures to organisms and
ecosystems.
Additionally, EPA recognizes that there have been a number of uses
of Pb, especially as an ingredient in automobile fuel but also in other
products such as paint, lead-acid batteries, and some pesticides, which
have significantly contributed to widespread increases in Pb
concentrations in the environment, a portion of which remains today
(e.g., CD, Chapters 2 and 3).
Ecosystems near smelters, mines, and other industrial sources of Pb
have demonstrated a wide variety of adverse effects including decreases
in species diversity, loss of vegetation, changes to community
composition, decreased growth of vegetation, and increased number of
invasive species. These sources may have multiple pathways for
discharging Pb to ecosystems, and apportioning effects between air-
related pathways and other pathways (e.g. discharges to water) in such
cases is difficult. Likewise, apportioning these effects between Pb and
other stressors is complicated because these point sources also emit a
wide variety of other heavy metals and sulfur dioxide which may cause
toxic effects. There are no field studies which have investigated
effects of Pb additions alone but some studies near large point sources
of Pb have found significantly reduced species composition and altered
community structures. While these effects are significant, they are
spatially limited: the majority of contamination occurs within 20 to 50
km of the emission source (CD, AX7.1.4.2).
By far, the majority of air-related Pb found in terrestrial
ecosystems was deposited in the past during the use of Pb additives in
gasoline. This gasoline-derived Pb was emitted predominantly in small
size particles which were widely dispersed and transported across large
distances. Many sites receiving Pb predominantly through such long-
range transport have accumulated large amounts of Pb in soils (CD, p.
AX7-98). There is little evidence that terrestrial sites exposed as a
result of this long range transport of Pb have experienced significant
effects on ecosystem structure or function (CD, AX7.1.4.2, p. AX7-98).
Strong complexation of Pb by soil organic matter may explain why few
ecological effects have been observed (CD, p. AX7-98). Studies have
shown decreasing levels of Pb in vegetation which seems to correlate
with decreases in atmospheric deposition of Pb resulting from the
removal of Pb additives to gasoline (CD, AX 7.1.4.2).
Terrestrial ecosystems remain primarily sinks for Pb but amounts
retained in various soil layers vary based on forest type, climate, and
litter cycling (CD, section 7.1). Once in the soil, the migration and
distribution of Pb is controlled by a multitude of factors including
pH, precipitation, litter composition, and other factors which govern
the rate at which Pb is bound to organic materials in the soil (CD,
section 2.3.5).
Like most metals the solubility of Pb is increased at lower pH.
However, the
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reduction of pH may in turn decrease the solubility of dissolved
organic material (DOM). Given the close association between Pb mobility
and complexation with DOM, a reduced pH does not necessarily lead to
increased movement of Pb through terrestrial systems and into surface
waters. In areas with moderately acidic soil (i.e., pH of 4.5 to 5.5)
and abundant DOM, there is no appreciable increase in the movement of
Pb into surface waters compared to those areas with neutral soils
(i.e., pH of approximately 7.0). This appears to support the theory
that the movement of Pb in soils is limited by the solubilization and
transport of DOM. In sandy soils without abundant DOM, moderate
acidification appears likely to increase outputs of Pb to surface
waters (CD, AX 7.1.4.1).
Lead exists in the environment in various forms which vary widely
in their ability to cause adverse effects on ecosystems and organisms.
Current levels of Pb in soil also vary widely depending on the source
of Pb but in all ecosystems Pb concentrations exceed natural background
levels. The deposition of gasoline-derived Pb into forest soils has
produced a legacy of slow moving Pb that remains bound to organic
materials despite the removal of Pb from most fuels and the resulting
dramatic reductions in overall deposition rates. For areas influenced
by point sources of air Pb, concentrations of Pb in soil may exceed by
many orders of magnitude the concentrations which are considered
harmful to laboratory organisms. Adverse effects associated with Pb
include neurological, physiological, and behavioral effects which may
influence ecosystem structure and functioning. Ecological soil
screening levels (Eco-SSLs) have been developed for Superfund site
characterizations to indicate concentrations of Pb in soils below which
no adverse effects are expected to plants, soil invertebrates, birds,
and mammals. Values like these may be used to identify areas in which
there is the potential for adverse effects to any or all of these
receptors based on current concentrations of Pb in soils.
Atmospheric Pb enters aquatic ecosystems primarily through the
erosion and runoff of soils containing Pb and deposition (wet and dry).
While overall deposition rates of atmospheric Pb have decreased
dramatically since the removal of Pb additives from gasoline, Pb
continues to accumulate and may be re-exposed in sediments and water
bodies throughout the United States (CD, section 2.3.6).
Several physical and chemical factors govern the fate and
bioavailability of Pb in aquatic systems. A significant portion of Pb
remains bound to suspended particulate matter in the water column and
eventually settles into the substrate. Species, pH, salinity,
temperature, turbulence, and other factors govern the bioavailability
of Pb in surface waters (CD, section 7.2.2).
Lead exists in the aquatic environment in various forms and under
various chemical and physical parameters which determine the ability of
Pb to cause adverse effects either from dissolved Pb in the water
column or Pb in sediment. Current levels of Pb in water and sediment
also vary widely depending on the source of Pb. Conditions exist in
which adverse effects to organisms and thereby ecosystems may be
anticipated given experimental results. It is unlikely that dissolved
Pb in surface water constitutes a threat to ecosystems that are not
directly influenced by point sources. For Pb in sediment, the evidence
is less clear. It is likely that some areas with long term historical
deposition of Pb to sediment from a variety of sources as well as areas
influenced by point sources have the potential for adverse effects to
aquatic communities. The long residence time of Pb in sediment and its
ability to be resuspended by turbulence make Pb likely to be a factor
for the foreseeable future. Criteria have been developed to indicate
concentrations of Pb in water and sediment below which no adverse
effects are expected to aquatic organisms. These values may be used to
identify areas in which there is the potential for adverse effects to
receptors based on current concentrations of Pb in water and sediment.
B. Screening Level Ecological Risk Assessment
This section presents a brief summary of the screening-level
ecological risk assessment conducted by EPA for this review. The
assessment is described in detail in Lead Human Exposure and Health
Risk Assessments and Ecological Risk Assessment for Selected Areas,
Pilot Phase (ICF, 2006). Funding constraints have precluded performance
of a full-scale ecological risk assessment. The discussion here is
focused on the screening level assessment performed in the pilot phase
(ICF, 2006) and takes into consideration CASAC recommendations with
regard to interpretation of this assessment (Henderson, 2007a, b). The
following summary focuses on key features of the approach used in the
assessment and presents only a brief summary of the results of the
assessment. A complete presentation of results is provided in the pilot
phase Risk Assessment Report (ICF, 2006) and summarized in Chapter 6 of
the Staff Paper.
1. Design Aspects of Assessment and Associated Uncertainties
The screening level risk assessment involved several location-
specific case studies and a national-scale surface water and sediment
screen. The case studies included areas surrounding a primary Pb
smelter and a secondary Pb smelter, as well as a location near a
nonurban roadway. An additional case study for an ecologically
vulnerable location was identified and described (ICF, 2006), but
resource constraints have precluded risk analysis for this location.
The case study analyses were designed to estimate the potential for
ecological risks associated with exposures to Pb emitted into ambient
air. Soil, surface water, and/or sediment concentrations were estimated
from available monitoring data or modeling analysis, and then compared
to ecological screening benchmarks to assess the potential for
ecological impacts from Pb that was emitted into the air. Results of
these comparisons are not definitive estimates of risk, but rather
serve to identify those locations at which there is the greatest
likelihood for adverse effect. Similarly, the national-scale screening
assessment evaluated surface water and sediment monitoring locations
across the United States for the potential for ecological impacts
associated with atmospheric deposition of Pb. The reader is referred to
the pilot phase Risk Assessment Report (ICF, 2006) for details on the
use of this information and models in the screening assessment.
The measures of exposure for these analyses are total Pb
concentrations in soil, dissolved Pb concentrations in fresh surface
waters (water column), and total Pb concentrations in freshwater
sediments. The hazard quotient (HQ) approach was then used to compare
Pb media concentrations with ecological screening values. The exposure
concentrations were estimated for the three case studies and the
national-scale screening analyses as described below:
For the primary Pb smelter case study, measured
concentrations of total Pb in soil, dissolved Pb in surface waters, and
total Pb in sediment were used to develop point estimates for sampling
clusters thought to be associated with atmospheric Pb deposition,
rather than Pb associated with nonair sources, such as runoff from
waste storage piles.
For the secondary Pb smelter case study, concentrations of
Pb in soil were
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estimated using fate and transport modeling based on EPA's MPE
methodology (USEPA, 1998) and data available from similar locations.
For the near roadway nonurban case study, measured soil
concentration data collected from two interstate sampling locations,
one with fairly high-density development (Corpus Christi, Texas) and
another with medium-density development (Atlee, Virginia), were used to
develop estimates of Pb in soils for each location.
For the national-scale surface water and sediment
screening analyses, measurements of dissolved Pb concentrations in
surface water and total Pb in sediment for locations across the United
States were compiled from available databases (USGS, 2004). Air
emissions, surface water discharge, and land use data for the areas
surrounding these locations were assessed to identify locations where
atmospheric Pb deposition may be expected to contribute to potential
ecological impacts. The exposure assessment focused on these locations.
The ecological screening values used in this assessment were
developed from the Eco-SSLs methodology, EPA's recommended ambient
water quality criteria, and sediment screening values developed by
MacDonald and others (2000, 2003). Soil screening values were derived
for this assessment using the Eco-SSL methodology with the toxicity
reference values for Pb (USEPA, 2005d, 2005e) and consideration of the
inputs on diet composition, food intake rates, incidental soil
ingestion, and contaminant uptake by prey (details are presented in
section 7.1.3.1 and Appendix L, of ICF, 2006). Hardness-specific
surface water screening values were calculated for each site based on
EPA's recommended ambient water quality criteria for Pb (USEPA, 1984).
For sediment screening values, the assessment relied on sediment
``threshold effect concentrations'' and ``probable effect
concentrations'' developed by MacDonald et al (2000). The methodology
for these sediment criteria is described more fully in section 7.1.3.3
and Appendix M of the pilot phase Risk Assessment Report (ICF, 2006).
The HQ is calculated as the ratio of the media concentration to the
ecotoxicity screening value, and represented by the following equation:
HQ = (estimated Pb media concentration)/(ecotoxicity screening value)
For each case study, HQ values were calculated for each location
where either modeled or measured media concentrations were available.
Separate soil HQ values were calculated for each ecological receptor
group for which an ecotoxicity screening value has been developed
(i.e., birds, mammals, soil invertebrates, and plants). HQ values less
than 1.0 suggest that Pb concentrations in a specific medium are
unlikely to pose significant risks to ecological receptors. HQ values
greater than 1.0 indicate that the expected exposure exceeds the
ecotoxicity screening value and that there is a potential for adverse
effects.
There are several uncertainties that apply across case studies
noted below:
The ecological risk screen is limited to specific case
study locations and other locations for which dissolved Pb data were
available and evaluated in the national-scale surface water and
sediment screens. In identifying sites for inclusion in the assessment,
efforts were made to ensure that the Pb exposures assessed were
attributable to airborne Pb and not dominated by nonair sources.
However, there is uncertainty as to whether other sources might have
actually contributed to the Pb exposure estimates.
A limitation to using the selected ecotoxicity screening
values is that they might not be sufficient to identify risks to some
threatened or endangered species or unusually sensitive aquatic
ecosystems (e.g., CD, p. AX7-110).
The methods and database from which the surface water
screening values (i.e., the AWQC for Pb) were derived is somewhat
dated. New data and approaches (e.g., use of pH as indicator of
bioavailability) may now be available to estimated the aquatic toxicity
of Pb (CD, sections AX7.2.1.2 and AX7.2.1.3).
No adjustments were made for sediment-specific
characteristics that might affect the bioavailability of Pb in
sediments in the derivation of the sediment quality criteria used for
this ecological risk screen (CD, sections 7.2.1 and AX7.2.1.4; Appendix
M, ICF, 2006). Similarly, characteristics of soils for the case study
locations were not evaluated for measures of bioavailability.
Although the screening value for birds used in this
analysis is based on reasonable estimates for diet composition and
assimilation efficiency parameters, it was based on a conservative
estimate of the relative bioavailability of Pb in soil and natural
diets compared with water soluble Pb added to an experimental pellet
diet (Appendix L, ICF, 2006).
2. Summary of Results
The following is a brief summary of key observations related to the
results of the screening-level ecological risk assessment. A more
complete discussion of the results is provided in Chapter 6 of the
Staff Paper and the complete presentation of the assessment and results
is presented in the pilot phase Risk Assessment Report (ICF, 2006).
The national-scale screen of surface water data initially
identified some 42 sample locations of which 15 were then identified as
unrelated to mining sites and having water column levels of dissolved
Pb that were greater than hardness adjusted chronic criteria for the
protection of aquatic life (with one location having a HQ of 15),
indicating a potential for adverse effect if concentrations were
persistent over chronic periods. Acute criteria were not exceeded at
any of these locations. The extent to which air emissions of Pb have
contributed to these surface water Pb concentrations is unclear.
In the national-scale screen of sediment data associated
with the 15 surface water sites described above, threshold effect
concentration-based HQs at nine of these sites exceeded 1.0.
Additionally, HQs based on probable effect concentrations exceeded 1.0
at five of the sites, indicating probable adverse effects to sediment
dwelling organisms. Thus, sediment Pb concentrations at some sites are
high enough that there is a likelihood that they would cause adverse
effects to sediment dwelling organisms. However, the contribution of
air emissions to these concentrations is unknown.
In the primary Pb smelter case study, for which
measurements were used to estimate nonair media concentrations, all
three of the soil sampling clusters (including the ``reference areas'')
had HQs that exceeded 1.0 for birds. Samples from one cluster also had
HQs greater than 1.0 for plants and mammals. The surface water sampling
clusters all had measurements below the detection limit of 3.0 [mu]g/L.
However, three sediment sample clusters had HQs greater than 1.0. In
summary, the concentrations of Pb in soil and sediments exceed
screening values for these media indicating potential for adverse
effects to terrestrial organisms (plants, birds and mammals) and to
sediment dwelling organisms. While the contribution to these Pb
concentrations from air as compared to nonair sources is not
quantified, air emissions from this facility are substantial (Appendix
D, USEPA 2007b; ICF 2006). Further, the contribution of air Pb under
the current NAAQS to these concentrations as compared to that prior to
the current NAAQS is unknown.
[[Page 29250]]
In the secondary Pb smelter case study, the soil
concentrations, developed from soil data for similar locations,
resulted in avian HQs greater than 1.0 for all distance intervals
evaluated. The soil concentrations within 1 km of the facility, scaled
using a combination of measurements and modeling (as described in the
Staff Paper, Chapter 6) also showed HQs greater than 1.0 for plants,
birds, and mammals. These estimates indicate a potential for adverse
effect to those receptor groups. We note that the contribution of
nonair sources to these concentrations is unknown. Further, the
contribution of air Pb under the current NAAQS to these concentrations
as compared to that prior to the current NAAQS is also unknown.
In the nonurban, near roadway case study, HQs for birds
and mammals were greater than 1.0 at all but one of the distances from
the road. Plant HQs were greater than 1.0 at the closest distance. In
summary, HQs above one were estimated for plants, birds and mammals,
indicating potential for adverse effect to these receptor groups. We
note that the contribution of air Pb under the current NAAQS to these
concentrations as compared to that prior to the current NAAQS is
unknown.
C. The Secondary Standard
The NAAQS provisions of the Act require the Administrator to
establish secondary standards that, in the judgment of the
Administrator, are requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
the pollutant in the ambient air. In so doing, the Administrator seeks
to establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that secondary
standards be set to eliminate all risk of adverse welfare effects, but
rather at a level requisite to protect public welfare from those
effects that are judged by the Administrator to be adverse.
The following discussion starts with background information on the
current standard (section III.C.1). The general approach for this
current review is summarized in section III.C.2. Considerations and
conclusions with regard to the adequacy of the current standard are
discussed in section III.C.3, with evidence and exposure-risk-based
considerations in sections III.C.3.a and b, respectively, followed by a
summary of CASAC advice and recommendations (section III.C.3.c) and the
Administrator's proposed conclusions (section III.C.3.d).
Considerations, conclusions and the Administrator's proposed decision
with regard to elements of the secondary standard are discussed in
section III.C.4.
1. Background on the Current Standard
The current standard was set in 1978 to be identical to the primary
standard (1.5 [mu]g Pb/m\3\, as a maximum arithmetic mean averaged over
a calendar quarter), the basis for which is summarized in Section
II.C.1. At the time the standard was set, the Agency concluded that the
primary air quality standard would adequately protect against known and
anticipated adverse effects on public welfare, as the Agency stated
that it did not have evidence that a more restrictive secondary
standard was justified. In the rationale for this conclusion, the
Agency stated that the available evidence cited in the 1977 Criteria
Document indicated that ``animals do not appear to be more susceptible
to adverse effects from lead than man, nor do adverse effects in
animals occur at lower levels of exposure than comparable effects in
humans'' (43 FR 46256). The Agency recognized that Pb may be deposited
on the leaves of plants and present a hazard to grazing animals. With
regard to plants, the Agency stated that Pb is absorbed but not
accumulated to any great extent by plants from soil, and that although
some plants may be susceptible to Pb, it is generally in a form that is
largely nonavailable to them. Further the Agency stated that there was
no evidence indicating that ambient levels of Pb result in significant
damage to manmade materials and Pb effects on visibility and climate
are minimal.
The secondary standard was subsequently considered during the 1980s
in development of the 1986 Criteria Document (USEPA, 1986a) and the
1990 Staff Paper (USEPA, 1990). In summarizing OAQPS staff conclusions
and recommendations at that time, the 1990 Staff Paper stated that a
qualitative assessment of available field studies and animal
toxicological data suggested that ``domestic animals and wildlife are
as susceptible to the effects of lead as laboratory animals used to
investigate human lead toxicity risks.'' Further, the 1990 Staff Paper
highlighted concerns over potential ecosystem effects of Pb due to its
persistence, but concluded that pending development of a stronger
database that more accurately quantifies ecological effects of
different Pb concentrations, consideration should be given to retaining
a secondary standard at or below the level of the then-current
secondary standard of 1.5 [mu]g/m\3\.
2. Approach for Current Review
In evaluating whether it is appropriate to retain the current
secondary Pb standard, or whether revision is appropriate, the
Administrator has considered the evidence and risk analyses presented
in the Criteria Document, the Staff Paper, the ANPR and the associated
technical support documents, [together with the associated
uncertainties] and CASAC advice and public comment on these documents.
The Staff Paper and ANPR recognize that the available welfare effects
evidence generally reflects laboratory-based evidence of toxicological
effects on specific organisms exposed to concentrations of Pb at which
scientists generally agree that adverse effects are likely to occur. It
is widely recognized, however, that environmental exposures are likely
to be at lower concentrations and/or accompanied by significant
confounding factors (e.g., other metals, acidification), which
increases our uncertainty about the likelihood and magnitude of the
organism and ecosystem response.
3. Conclusions on Adequacy of the Current Standard
a. Evidence-Based Considerations
In considering the welfare effects evidence with respect to the
adequacy of the current standard, the Administrator considers not only
the array of evidence newly assessed in the Criteria Document but also
that assessed in the 1986 Criteria Document and summarized in the 1990
Staff Paper. As discussed extensively in the latter two documents,
there was a significantly improved characterization of environmental
effects of Pb in the ten years after the Pb NAAQS was set. And in the
subsequent nearly 20 years, many additional studies on Pb effects in
the environment are now available (2006 Criteria Document). Some of the
more relevant aspects of the evidence available since the standard was
set include the following:
A more quantitative determination of the mobility,
distribution, uptake, speciation, and fluxes of atmospherically
delivered Pb in terrestrial ecosystems shows that the binding of Pb to
organic materials in the soil slows its mobility through soil and may
prevent uptake by plants (CD, Sections 7.1.2, 7.1.5, AX7.1.4.1,
AX7.1.4.2, AX7.1.4.3 and AX7.1.2 ). Therefore, while atmospheric
deposition of Pb has decreased, Pb may be more persistent in some
ecosystems than others and may remain in the active zone of the soil,
where exposure
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may occur, for decades (CD, Sections 7.1.2, AX7.1.2 and AX7.1.4.3).
Plant toxicity may occur at lower levels than previously
identified as determined by data considered in development of Eco-SSLs
(CD, pp. 7-11 to 7-12, AX7-16 and Section AX7.1.3.2), although the
range of reported soil Pb effect levels is large (tens to thousands of
mg/kg soil).
Avian and mammalian toxicity may occur at lower levels
than those previously identified, although the range of Pb effect
levels is large (<1 to >1,000 mg Pb/kg bw-day) (CD, p. 7-12, Section
AX7.1.3.3).
There is an expanded understanding of the fate and effects
of Pb in aquatic ecosystems and of the distribution and concentrations
of Pb in surface waters throughout the United States (CD, Section
AX7.2.2).
New methods for assessing the toxicity of metals to water
column and sediment-dwelling organisms and data collection efforts (CD,
Sections 7.2.1, 7.2.2, AX7.2.2, and AX7.2.2.2) have improved our
understanding of Pb aquatic toxicity and findings include an indication
that in some estuarine systems Pb deposited during historic usage of
leaded gasoline may remain in surface sediments for decades. (CD, p. 7-
23).
A larger dataset of aquatic species assessed with regard
to Pb toxicity, and findings of lower effect levels for previously
untested species (CD, p. AX7-176 and Section AX7.2.4.3).
Currently available studies have also shown effects on
community structure, function and primary productivity, although some
confounders (such as co-occurring pollutants) have not been well
addressed (CD, Section AX7.1.4.2).
Evidence in ecological research generally indicates the
value of a critical loads approach; however, current information on Pb
critical loads is lacking for many processes and interactions involving
Pb in the environment and work is ongoing (CD, Section 7.3).
Given the full body of current evidence, despite wide variations in
Pb concentrations in soils throughout the country, Pb concentrations
are likely in excess of concentrations expected from geologic or other
non-anthropogenic forces. In particular, the deposition of gasoline-
derived Pb into forest soils has produced a legacy of slow moving Pb
that remains bound to organic materials despite the removal of Pb from
most fuels and the resulting dramatic reductions in overall deposition
rates (CD, Section AX7.1.4.3). For areas influenced by point sources of
air Pb that meet the current standard, concentrations of Pb in soil may
exceed by many orders of magnitude the concentrations which are
considered harmful to laboratory organisms (CD, Section 3.2 and
AX7.1.2.3).
There are several difficulties in quantifying the role of current
ambient Pb in the environment: some Pb deposited before the standard
was enacted is still present in soils and sediments; historic Pb from
gasoline continues to move slowly through systems as does current Pb
derived from both air and nonair sources. Additionally, the evidence of
adversity in natural systems is very sparse due in no small part to the
difficulty in determining the effects of confounding factors such as
multiple metals or factors influencing bioavailability in field
studies. However, the evidence summarized above and in Section 4.2 of
the Staff Paper and described in detail in the Criteria Document
informs our understanding of Pb in the environment today and evidence
of environmental Pb exposures of potential concern.
Conditions exist in which Pb-associated adverse effects to aquatic
organisms and thereby ecosystems may be anticipated given experimental
results. While the evidence does not indicate that dissolved Pb in
surface water constitutes a threat to those ecosystems that are not
directly influenced by point sources, the evidence regarding Pb in
sediment is less clear (CD, Sections AX7.2.2.2.2 and AX7.2.4). It is
likely that some areas with long term historical deposition of Pb to
sediment from a variety of sources as well as areas influenced by point
sources have the potential for adverse effects to aquatic communities.
The Staff Paper concluded based on looking to laboratory studies and
current media concentrations in a wide range of areas, it seems likely
that adverse effects are occurring, particularly near point sources,
under the current standard. The long residence time of Pb in sediment
and its ability to be resuspended by turbulence make Pb contamination
likely to be a factor for the foreseeable future. Based on this
information, the Staff Paper concluded that the evidence suggests that
the environmental levels of Pb occurring under the current standard,
set nearly thirty years ago, may pose risk of adverse environmental
effect.
b. Risk-Based Considerations
In addition to the evidence-based considerations described in the
previous section, the screening level ecological risk assessment is
informative, taking into account key limitations and uncertainties
associated with the analyses.
The screening level risk assessment involved a comparison of
estimates of environmental media concentrations of Pb to ecological
screening levels to assess the potential for ecological impacts from Pb
that was emitted into the air. Results of these comparisons are not
considered to be definite predictors of risk, but rather serve to
identify those locations at which there is greatest likelihood for
adverse effect. Similarly, the national-scale screening assessment
evaluated the potential for ecological impacts associated with the
atmospheric deposition of Pb released into ambient air at surface water
and sediment monitoring locations across the United States.
The ecological screening levels employed in the screening level
risk assessment for different media are drawn from different sources.
Consequently there are somewhat different limitations and uncertainties
associated with each. In general, their use here recognizes their
strength in identifying media concentrations with the potential for
adverse effect and their relative nonspecificity regarding the
magnitude of risk of adverse effect.
As discussed in the previous section, as a result of its
persistence, Pb emitted in the past remains today in aquatic and
terrestrial ecosystems of the United States. Consideration of the
environmental risks associated with the current standard is complicated
by the environmental burden associated with air Pb concentrations that
exceeded the current standard, predominantly in the past.
Concentrations of Pb in soil and sediments associated with the
primary Pb smelter case study exceeded screening values for those
media, indicating potential for adverse effect in terrestrial organisms
(plants, birds, and mammals) and in sediment dwelling organisms. While
the contribution to these Pb concentrations from air as compared to
nonair sources has not been quantified, air emissions from this
facility are substantial (Appendix D, USEPA 2007b; ICF 2006).
Additionally, estimates of Pb concentration in soils associated with
the nonurban near roadway case study and the secondary Pb smelter case
study were also associated with HQs above 1 for plants, birds and
mammals, indicating potential for adverse effect to those receptor
groups. The industrial facility in the secondary Pb smelter case study
is much younger than the primary Pb smelter and apparently became
active less than ten years prior to the establishment of the current
standard.
[[Page 29252]]
The national-scale screens, which are not focused on particular
point source locations, indicate the ubiquitous nature of Pb in aquatic
systems of the United States today. Further, the magnitude of Pb
concentrations in several aquatic systems exceeded screening values. In
the case of the national-scale screen of surface water data, 15
locations were identified with water column levels of dissolved Pb that
were greater than hardness-adjusted chronic criteria for the protection
of aquatic life (with one location having a HQ as high as 15),
indicating a potential for adverse effect if concentrations were
persistent over chronic periods. Further, sediment Pb concentrations at
some sites in the national-scale screen were high enough that the
likelihood that they would cause adverse effects to sediment dwelling
organisms may be considered ``probable''.
A complicating factor in interpreting the findings for the
national-scale screening assessments is the lack of clear apportionment
of Pb contributions from air as compared to nonair sources, such as
industrial and municipal discharges. While the contribution of air
emissions to the elevated concentrations has not been quantified,
documentation of historical trends in the sediments of many water
bodies has illustrated the sizeable contribution that airborne Pb can
have on aquatic systems (e.g., Staff Paper, section 2.8.1). This
documentation also indicates the greatly reduced contribution in many
systems as compared to decades ago (presumably reflecting the banning
of Pb-additives from gasoline used by cars and trucks). However, the
timeframe for removal of Pb from surface sediments into deeper sediment
varies across systems, such that Pb remains available to biological
organisms in some systems for much longer than in others (Staff Paper,
section 2.8; CD, pp. AX7-141 to AX7-145).
The case study locations included in the screening assessment, with
the exception of the primary Pb smelter site, are currently meeting the
current Pb standard, yet Pb occurs in some locations at concentrations,
particularly in soil, and aquatic sediment above the screening levels,
indicative of a potential for harm to some terrestrial and sediment
dwelling organisms. While the role of airborne Pb in determining these
Pb concentrations is unclear, the historical evidence indicates that
airborne Pb can create such concentrations in sediments and soil.
Further, environmental concentrations may be related to emissions prior
to establishment of the current standard and such concentrations appear
to indicate a potential for harm to ecological receptors today.
c. CASAC Advice and Recommendations
In the CASAC letter transmitting advice and recommendations
pertaining to the review of the ANPR and final Staff Paper and Pb
Exposure and Risk Assessments, the CASAC Pb panel provided
recommendations regarding the need for a Pb NAAQS, and the adequacy of
the current Pb NAAQS, as well as comments on the documents. With regard
to the revision of the primary and secondary NAAQS, this CASAC letter
(Henderson, 2008) said:
The Committee unanimously and fully supports Agency staff's
scientific analyses in recommending the need to substantially lower
the level of the primary (public-health based) Lead NAAQS, to an
upper bound of no higher than 0.2 [mu]g/m\3\ with a monthly
averaging time. The CASAC is also unanimous in its recommendation
that the secondary (public-welfare based) standard for lead needs to
be substantially lowered to a level at least as low as the
recommended primary NAAQS for Lead.
In earlier comments on the December 2006 draft documents, the CASAC
Pb Panel concluded they presented ``compelling scientific evidence that
current atmospheric Pb concentrations and deposition--combined with a
large reservoir of historically deposited Pb in soils, sediments and
surface waters--continue to cause adverse environmental effects in
aquatic and/or terrestrial ecosystems, especially in the vicinity of
large emissions sources.'' The Panel went on to state that ``These
effects persist in some cases at locations where current airborne lead
concentrations are below the level of the current primary and secondary
lead standards'' and ``Thus, from an environmental perspective, there
are convincing reasons to both retain lead as a regulated criteria air
pollutant and to lower the level of the current secondary standard''
(Henderson, 2007a).
In making this recommendation, the CASAC Pb Panel also cites the
persistence of Pb in the environment, the possibility of some of the
large amount of historically deposited Pb becoming resuspended by
natural events, and the expectation that humans are not uniquely
sensitive among the many animal and plant species in the environment.
CASAC provided further advice and recommendations on the Agency's
consideration of the secondary standard in this review in their letter
of September 2007 (Henderson, 2007b). In that letter they recognized
the role of the secondary standard in influencing the long-term
environmental burden of Pb and a need for environmental monitoring to
assess the success of the standard in this role.
Similarly, in CASAC's advice on the ANPR and final Staff Paper they
concluded:
[I]t is critical that the secondary Lead NAAQS be set at a
sufficiently-stringent level so as to ensure that there is no
reversal of the current downward trend in lead concentrations in the
environment. Therefore, at a minimum, the level of the secondary
Lead NAAQS should be at least as low as the level of the recommended
primary lead standard. Moreover, the Agency needs to give greater
priority to the monitoring of environmental lead in the ambient air.
However, CASAC also recognized that EPA ``lacks the relevant data
to provide a clear, quantitative basis for setting a secondary Pb NAAQS
that differs from the primary in indicator, averaging time, level or
form'' (Henderson, 2007a).
d. Administrator's Proposed Conclusions on Adequacy of Current Standard
In considering the adequacy of the current standard in providing
requisite protection from Pb-related adverse effects on public welfare,
the Administrator has considered the body of available evidence
(briefly summarized above in Section III.A). Depending on the
interpretation, the available data and evidence, primarily qualitative,
suggests the potential for adverse environmental impacts under the
current standard. Given the limited data on Pb effects in ecosystems,
it is necessary to look at evidence of Pb effects on organisms and
extrapolate to ecosystem effects. Therefore, taking into account the
available evidence and current media concentrations in a wide range of
areas, the Administrator concludes that there is potential for adverse
effects occurring under the current standard, however there are
insufficient data to provide a quantitative basis for setting a
secondary standard different than the primary. While the role of
current airborne emissions is difficult to apportion, it is conclusive
that deposition of Pb from air sources is occurring and that this
ambient Pb is likely to be persistent in the environment. Historically
deposited Pb has persisted, although location-specific dynamics of Pb
in soil result in differences in the timeframe during which Pb is
retained in surface soils or sediments where it may be available to
ecological receptors (USEPA, 2007b, section 2.3.3).
[[Page 29253]]
There is only very limited information available pertinent to
assessing whether groups of organisms which influence ecosystem
function are subject to similar effects as those in humans. The
screening-level risk information, while limited and accompanied by
various uncertainties, also suggests occurrences of environmental Pb
concentrations existing under the current standard that could have
adverse environmental effects. Environmental Pb levels today are
associated with atmospheric Pb concentrations and deposition that have
combined with a large reservoir of historically deposited Pb in
environmental media.
In considering this evidence, as well as the views of CASAC,
summarized above, the Staff Paper and associated support documents, and
views of public commenters on the adequacy of the current standard, the
Administrator proposes to conclude that the current secondary standard
for Pb is not requisite to protect public welfare from known or
anticipated adverse effects.
4. Conclusions and Proposed Decision on the Elements of the Secondary
Standard
The secondary standard is defined in terms of four basic elements:
indicator, averaging time, level and form, which serve to define the
standard and must be considered collectively in evaluating the welfare
protection afforded by the standards.
With regard to the pollutant indicator for use in a secondary NAAQS
that provides protection for public welfare from exposure to Pb, EPA
notes that Pb is a persistent pollutant to which ecological receptors
are exposed via multiple pathways. While the evidence indicates that
the environmental mobility and ecological toxicity of Pb are affected
by various characteristics of its chemical form, and the media in which
it occurs, information is insufficient to identify an indicator other
than total Pb that would provide protection against adverse
environmental effect in all ecosystems nationally. Thus, the same
concerns regarding the relative advantages of TSP and PM10
as the basis for the indicator apply here as for the primary standard.
Lead is a cumulative pollutant with environmental effects that can
last many decades. In considering the appropriate averaging time for a
secondary standard for such a pollutant the concept of critical loads
may be useful (CD, section 7.3). However, information is currently
insufficient for such use in this review.
There is a general lack of data that would indicate the appropriate
level of Pb in environmental media that may be associated with adverse
effects. The EPA notes the influence of airborne Pb on Pb in aquatic
systems and of changes in airborne Pb on aquatic systems, as
demonstrated by historical patterns in sediment cores from lakes and Pb
measurements (section 2.8.1; CD, section AX7.2.2; Yohn et al., 2004;
Boyle et al., 2005), as well as the comments of the CASAC Pb panel that
a significant change to current air concentrations (e.g., via a
significant change to the standard) is likely to have significant
beneficial effects on the magnitude of Pb exposures in the environment
and Pb toxicity impacts on natural and managed terrestrial and aquatic
ecosystems in various regions of the U.S., the Great Lakes and also
U.S. territorial waters of the Atlantic Ocean (Henderson, 2007a,
Appendix E). EPA concurs with CASAC's conclusion that the Agency lacks
the relevant data to provide a clear, quantitative basis for setting a
secondary Pb NAAQS that differs from the primary in indicator,
averaging time, level or form. The Administrator concurs with CASAC's
conclusion that the Agency lacks the relevant data to provide a clear,
quantitative basis for setting a secondary Pb NAAQS that differs from
the primary in indicator, averaging time, level, or form.
Based on these considerations, and taking into account the
observations, analyses, and recommendations discussed above, the
Administrator proposes to revise the current secondary Pb standard by
making it identical in all respects to the proposed primary Pb standard
(described in section II.D.4 above).
IV. Proposed Appendix R--Interpretation of the NAAQS for Lead and
Proposed Revisions to the Exceptional Events Rule
The EPA is proposing to add Appendix R, Interpretation of the
National Ambient Air Quality Standards for Pb, to 40 CFR part 50 in
order to provide data handling procedures for the proposed Pb standard.
The proposed Appendix R would detail the computations necessary for
determining when the proposed Pb NAAQS is met. The proposed appendix
also would address data reporting; sampling frequency and data
completeness considerations; the use of scaled Pb-PM10 data
as a surrogate for Pb-TSP data (or vice versa), including associated
scaling instructions; and rounding conventions. Although the
Administrator is proposing one indicator and inviting comment on
another, and proposing several possible combinations of different
averaging times, forms, and levels, for simplicity the proposed data
handling appendix text only directly addresses one combination: a Pb-
TSP indicator with an option for using scaled Pb-PM10 data
for NAAQS comparisons, an averaging time of monthly, a second maximum
(over three years) form, and a level of 0.20 [mu]g/m\3\. The proposed
appendix text indicates in brackets, as examples, the change that would
be needed if the level of the standard is set at 0.10 or 0.30 [mu]g/
m\3\ rather than at 0.20 [mu]g/m\3\. A decision to adopt Pb-
PM10 as the indicator, to adopt a different indicator,
averaging time, and/or form, or not to make use of surrogate data would
require other differences in the text of the appendix; the proposed
differences in the appendix text to accommodate such difference are
described below, after the explanation of the proposed version of the
appendix.
The EPA is also proposing Pb-specific changes to the deadlines, in
40 CFR 50.14, by which States must flag ambient air data that they
believe has been affected by exceptional events and submit initial
descriptions of those events, and the deadlines by which States must
submit detailed justifications to support the exclusion of that data
from EPA determinations of attainment or nonattainment with the NAAQS.
The deadlines now contained in 40 CFR 50.14 are generic, and are not
always appropriate for Pb given the anticipated schedule for the
designations of areas under the proposed Pb NAAQS.
A. Background
The purpose of a data interpretation guideline in general is to
provide the practical details on how to make a comparison between
multi-day, possibly multi-monitor, and (in the unique instance of this
proposed Pb NAAQS) possibly multi-parameter (i.e., Pb-TSP and/or Pb-
PM10) ambient air concentration data to the level of the
NAAQS, so that determinations of compliance and violation are as
objective as possible. Data interpretation guidelines also provide
criteria for determining whether there are sufficient data to make a
NAAQS level comparison at all. When data are insufficient, for example
because of failure to collect valid ambient data on enough days in
enough months (because of operator error or events beyond the control
of the operator), then no determination of current compliance or
violation is possible.
The regulatory language for the current Pb NAAQS, originally
adopted in 1977, contains no data interpretation instructions. Because
of that, the EPA
[[Page 29254]]
has issued various guidance documents and memoranda relevant to the
topic. This situation contrasts with the situations for ozone,
PM2.5, and PM10 for which there are detailed data
interpretation appendices in 40 CFR part 50. EPA has used its
experience drafting and applying these other data interpretation
appendices to develop the proposed text for appendix R.
An exceptional event is an event that affects air quality, is not
reasonably controllable or preventable, is an event caused by human
activity that is unlikely to recur at a particular location or a
natural event, and is determined by the Administrator in accordance
with 40 CFR 50.14 to be an exceptional event. Air quality data affected
by an exceptional event in certain specified ways may be excluded from
consideration when EPA makes a determination that an area is meeting or
violating the associated NAAQS, subject to EPA review and concurrence.
Section 50.14 contains both substantive criteria that an event and the
associated air concentration data must meet in order to be excluded,
and process steps and deadlines for a State to submit specified
information to EPA. The key deadlines are that a State must initially
notify EPA that data have been affected by an event and provide an
initial description of the event by July 1 of the year after the data
are collected, and that the State must submit the full justification
for exclusion within 3 years after the quarter in which the data were
collected. However, if a regulatory decision based on the data, for
example a designation action, is anticipated, the schedule is
foreshortened and all information must be submitted to EPA no later
than a year before the decision is to be made. This schedule presents
problems when a NAAQS has been recently revised, as discussed below.
The Staff Paper did not address data interpretation details, and
although the ANPR discussed data handling to a limited extent, there
has been only limited comment by CASAC or the public to date (other
than comments on the related issues of form and indicator for the
standard, including scaling factor issues). Similarly, no comments were
received on exceptional event issues.
B. Interpretation of the NAAQS for Lead
1. Interpretation of a Standard Based on Pb-TSP
The purpose of a data interpretation rule for the Pb NAAQS is to
give effect to the form, level, averaging time, and indicator specified
in the proposed regulatory text at 40 CFR 50.16, anticipating and
resolving in advance various future situations that could occur. The
proposed Appendix R, like the existing NAAQS interpretation appendices
for ozone, PM2.5, and PM10, addresses the
possible situation of there being less than 100% complete data
available, which is an issue in common across NAAQS pollutants. It also
addresses several issues which are specific to the proposed Pb NAAQS,
as described below.
With regard to data completeness, the proposed Appendix follows
past EPA practice for other NAAQS pollutants by requiring that in
general at least 75% of the monitoring data that should have resulted
from following the planned monitoring schedule in a period must be
available for the key air quality statistic from that period to be
considered valid. For the combination of NAAQS parameters addressed in
the proposed text, the key air quality statistic is the mean
concentration in an individual month, and so the 75% requirement is
applied for that time period. With the proposed required sampling
schedule of one day in three under a monthly mean form for the standard
(section V), typically there will be 10 required sampling days so a
monthly mean would be considered valid if there were data available for
at least 8 of those days.\155\ EPA invites comment on this proposed 75%
requirement, recognizing that for the current NAAQS based on a
quarterly mean concentration form with a required one-day-in-six
schedule, the current EPA policy is effectively that there be at least
11 days of data in a quarterly mean.
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\155\ Fewer than 10 days could be required, and fewer needed for
the monthly average to be valid, for February at all sites and in
all months for sites approved for only one-day-in-six sampling
because they have a history of recording concentrations well below
the level of the NAAQS. See Section V for more detail on required
sampling schedules.
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The proposed rule text for Pb data interpretation, like the
corresponding existing rule for PM2.5, has two provisions
that help a monitoring agency guard against a month ending up with data
completeness below 75%. First, there is a provision to allow data from
secondary, collocated samplers to substitute for data from a primary
monitor on a day when the primary monitor for some reason fails to
deliver valid data. There is also a provision which would allow a
monitoring agency to make up a sampling day on which no valid data were
collected, and to count the make-up sampling data in the assessment of
data completeness. To help insure that sampling days are well
distributed across the month and that a make-up day will generally fall
within the same source emissions and meteorological regime as the
missed sampling day, a number of specific restrictions are proposed on
the number of make-up days per month and on how soon after the missed
scheduled sampling day they must occur. These restrictions are stated
in the proposed rule text, and are adapted from current practice for
PM2.5 with adaptations to fit the monthly form of the
proposed Pb standard.
A monthly mean Pb concentration for Pb-TSP would be calculated from
all available daily mean concentrations within that calendar month,
including successfully completed sampling days, allowed make-up
sampling days, and any other sampling days actually completed
successfully by the primary monitor or by secondary monitors if there
is no data from a primary monitor. These other sampling days would not
be used in calculating data completeness, however; this follows the
example of the current requirements for PM2.5 data
interpretation.
Recognizing that even allowing for make-up samples, there may be
months with fewer than 75% complete data, the proposed text provides
for two diagnostic tests which are intended to identify those cases
with completeness less than 75% in which it nevertheless is very
likely, if not virtually certain, that the monthly mean concentration
would have been observed to be either above or below the level of the
NAAQS if monitoring data had been complete. One test, to be applied if
the mean of the incomplete data is above the NAAQS level, substitutes
low hypothetical concentrations for as much of the missing data as
needed to meet the 75% requirement; if the resulting mean is still
above the NAAQS level, then the NAAQS level is considered to have been
exceeded for the month. The hypothetical low values would be set equal
to the lowest concentration observed in the same month over the 3-year
period being evaluated, in effect giving the benefit of the doubt as to
the actual concentrations on the days with missing data. If the monthly
mean nevertheless is above the NAAQS, it is virtually certain that the
mean of complete data would also have been above the NAAQS. The other
test, to be applied if the mean of the incomplete data is below the
NAAQS level, works similarly except that at most 50% of the scheduled
data can be missing and all missing data is substituted with the
highest value observed in the same month over the 3-year period, with
the same rationale. If the monthly mean nevertheless is below the
NAAQS, it is virtually certain that the mean of complete data would
also have been
[[Page 29255]]
below the NAAQS. Data substitution tests similar to these are currently
used for ozone and PM2.5. It should be noted that one
outcome of applying the substitution tests proposed for Pb is that a
month with incomplete data may still be determined to not have a valid
monthly mean and to be unusable in making NAAQS exceedance
determinations for that monthly time period. In turn, this may make it
impossible to make a determination of compliance or violation for the
3-year period, depending on the completeness and levels of the
concentration data from the other months.
EPA invites comment on also incorporating into the final rule two
other possible tests that could allow a NAAQS exceedance determination
to be made on the basis of monthly data that is not at least 75%
complete. EPA may incorporate a version of either or both of these
additional tests into the final rule. The first additional test would
allow use of the monthly mean based on data that is between 50% and 75%
complete if that monthly mean were below some percentage (for example,
50%) the NAAQS, on the rationale that if the available daily values
(typically there would be 5 values in a month with 50% complete data)
have a mean below some sufficiently low limit, day-to-day variability
at the site must be small and the actual concentrations on the days
with missing data are very unlikely to have been high enough to make
the true monthly mean exceed the NAAQS level.
The second additional test would be more statistically rigorous,
yet will allow compliance determinations to be made on some smaller
data sets by considering uncertainty bounds. The test would use the
available data to create a two-sided statistical confidence interval
around the calculated monthly mean concentration. A reduced minimum
completeness percentage such as 50% would still be applied to ensure
that there are enough sampling days that they could not all be from
within a very short period of time. As expected, the uncertainty range
about the monthly mean would increase as the number of samples
decreases, and as there is more variability in the data that were
collected (more high concentrations days mixed with low concentration
days). If the prescribed two-sided confidence interval is entirely
above the level of the NAAQS, then the NAAQS would be deemed to have
been exceeded in that month. Note that the calculated monthly mean in
this situation would also have been above the NAAQS level. If the
confidence interval is entirely below the level of the NAAQS, then the
NAAQS would be deemed to have not been exceeded in that month. EPA
invites comment on the statistical assumptions that should be
considered to create a confidence interval from the available data, for
example the assumed distribution of the underlying ambient data and how
the confidence intervals should be constructed. For example, the
confidence interval could be constructed based on an assumption of a
log-normal distribution for daily concentrations combined with the
concept of a ``finite population correction factor,'' where means based
on data with between 50 and 75% completeness would have an associated
uncertainty range.\156\ Any data that is at least 75% complete could be
considered ``complete'' and would have no confidence interval. This
approach would make the general completeness test and this statistical
test yield the same result for a month with at least 75% completeness.
EPA notes that such a statistical confidence interval approach is not
presently used in data interpretation for any other NAAQS, but no other
NAAQS involves the combination of an averaging period as short as a
month with a sampling schedule as infrequent as one day in three.
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\156\ See, for example, the explanation of the finite population
correction factor approach at grants.nih.gov/grants/funding/modular/eval/Sample_MGAP.doc. Another useful reference is ``Sampling:
Design and Analysis'', Lohr, Sharon L., Brooks/Cole Publishing Co.,
Pacific Grove, CA, 1999.
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Section V.C. contains provisions which interact with the proposed
data completeness requirements described above. EPA invites comment on
whether the proposed data completeness provisions taken together
provide a good balance between avoiding situations in which no
determination of attainment or nonattainment can be made until more
data are collected during another calendar year, and avoiding erroneous
determinations caused by reliance on small sample sizes affected by
data variability. EPA also plans to explore this question prior to the
final rule, by analyzing hypothetical cases reflecting the variability
seen in historical monitoring data, and may make adjustments to the
proposed provisions for the final rule.\157\
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\157\ This exploration will be somewhat similar to the work EPA
did on data quality objectives for the PM2.5 monitoring
network, but likely will be more simplistic in light of the more
limited available data. See ``Data Quality Objectives (DQOs) for
PM2.5,'' July 25, 2001, http://www.epa.gov/ttn/amtic/files/ambient/pm25/qa/2001Dqo.pdf.
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The proposed rule text would require that only a minimum of two
valid monthly means be available over the 3-year period in order to
determine that a site has violated the NAAQS, since if the NAAQS has
been observed to be exceeded twice the concentrations in the other
months would be irrelevant to a finding of NAAQS violation. Valid
monthly means would be required for all 36 possible months in the 3-
year period in order to make a finding that the NAAQS has been met. An
exception would be allowed if there are 35 valid monthly means and none
of them exceed the NAAQS, because in that case it is irrelevant whether
the one month with incomplete data experienced an exceedance or not.
The proposed text of Appendix R has provisions to implement the
proposal that Pb-PM10 data adjusted by the application of
site-specific scaling factors be treated as surrogate Pb-TSP data.
These provisions are somewhat complex, to be able to address various
possible situations without ambiguity. These situations arise from the
possibility that both Pb-TSP and Pb-PM10 monitoring might
take place at a single site, with differences from day to day within
the 3-year period as to which samplers were operating and yielded valid
data for the day. The proposed approach is to consider all Pb-TSP and
Pb-PM10 data that have been collected and submitted by the
monitoring agency, i.e., once Pb-PM10 data have been
collected and submitted the monitoring agency could not choose to have
them ignored.\158\ However, where and when both types of data exist,
the Pb-TSP data would be given first consideration. Specifically the
proposed approach is to treat as separate questions whether the Pb-TSP
monitor and the Pb-PM10 monitor have produced a valid
monthly mean concentration, taking into account the provisions for
make-up samples and data substitution from secondary monitors, but not
mixing Pb-TSP and Pb-PM10 data within the month. If valid
monthly means for both Pb-TSP and Pb-PM10 have been
achieved, i.e., the main or a supplemental data completeness test has
been passed, the Pb-TSP data takes precedence and the Pb-
PM10 data for
[[Page 29256]]
that month are ignored. However, across the 3-year period, monthly
means for Pb-TSP and scaled Pb-PM10 can be considered
together in determining whether more than one monthly mean Pb
concentration has exceeded the level of the NAAQS. This allows for the
possibility that a monitoring agency may have switched from one type of
monitoring to the other during the 3 years, or that it has been more
successful in getting complete Pb-TSP data in some months than in
others.
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\158\ Section 3(a) of the proposed Appendix R has a more
detailed statement of what ambient data will be considered when
determining compliance with the NAAQS than is given in other data
interpretation appendices to 40 CFR part 50. EPA invites comment on
this codification of current practice. One new feature is a
provision for the use of data collected before the promulgation of
the proposed changes and additions to the FRM/FEM criteria, to make
it clear that these changes and additions are in effect retroactive.
FRM/FEM revisions and new FRM/FEM designations have not always been
treated as retroactive but in the case of the revised Pb NAAQS EPA
wishes to maximize the available data for making designations.
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The proposed Appendix R addresses the procedures and criteria for
development and use of site-specific scaling factors for Pb-
PM10 data. The scaling factor is the number that would
multiply Pb-PM10 data to get a surrogate for Pb-TSP data.
The proposal would require States to develop a site-specific scaling
factor for each monitoring site at which the State wishes to use Pb-
PM10 data as a surrogate for Pb-TSP data, either to allow it
to only operate a Pb-PM10 monitor or to make a Pb-
PM10 monitor eligible as a back-up source of Pb data for
greater data completeness. The site-specific scaling factor would have
to be based on at least a year of measurements of both types at the
site in question. EPA invites comment on the detailed criteria for
developing such local scaling factors, given in section 2(b) of the
proposed Appendix.
The existing FRM for Pb-TSP, Appendix G of 40 CFR part 50, contains
procedures for calculating Pb concentration data in micrograms per
cubic meter at standard conditions of temperature and pressure (STP).
The proposed FRM for low-volume Pb-PM10, Appendix Q of 40
CFR part 50, requires reporting of concentration data at local
conditions of temperature and pressure, for reasons explained in
section V. For consistency going forward, we are proposing in the
proposed appendix R that for monitoring conducted on or after January
1, 2009, Pb-TSP data should be reported at local conditions of
temperature and pressure also. The first deadline for such reporting
will be about June 30, 2009 (to be exact, 90 days from March 31, 2009)
so monitoring agencies will have ample lead time to change their
reporting procedures. However, EPA believes it would be an unnecessary
burden to require monitoring agencies to re-submit pre-January 1, 2009
Pb-TSP data corrected to local conditions, given that the adjustment
would in most cases be small. The proposed Appendix R would provide
that pre-2009 Pb-TSP data reported in STP is to be compared directly to
the level of the standard with no adjustment for the difference in
reporting forms, but gives the monitoring agency the option of re-
submitting the data corrected to local conditions. EPA invites comment
on this approach.
Both FRM rules require reporting of daily Pb concentrations with
three decimal places. When monthly means are calculated, they are to be
rounded to two decimal places for purposes of comparing to the level of
the NAAQS, which is expressed to two decimal places.
2. Interpretation of Alternative Elements
This section addresses changes that would be made to the proposed
Appendix R as printed at the end of this notice, if the Administrator
decides to adopt certain features which are being proposed today in the
alternative to those described above, or on which comment is invited.
If a quarterly maximum mean form is adopted for the final standard,
we propose that the basic period for assessing completeness would still
be the month. An equation would be added for calculating a quarterly
mean from three monthly means. The two supplemental diagnostic
completeness tests would be changed so that the outcome depends on
whether the quarterly mean with substituted data included for one or
more incomplete months meets or exceeds the standard, rather than the
monthly mean. The design value would be defined as the maximum
quarterly mean concentration in the 3-year period. To be determined to
violate the standard, at least one valid quarterly mean in the 3-year
period would be required. To be determined to meet the standard, 12
valid quarterly means in the 3-year period would be required. EPA
invites comment on the alternative of applying completeness tests only
for whole calendar quarters rather than individual months, an approach
that might allow attainment determinations to be made in some cases in
which the by-month approach just described would prevent a
determination.
As discussed in section II.E.1, EPA is inviting comment on the
possibility of the final rule containing default scaling factors for
adjusting Pb-PM10 data for use as a surrogate for Pb-TSP
data. This would give States the option of using a default scaling
factor rather than conducting the site-specific paired monitor testing
required in the proposed text of Appendix R. If EPA adopts this
approach in the final rule, Appendix R would be modified to provide the
default scaling factor values and explain their application. The
appropriate default scaling factor would be used in calculation
formulas exactly as the proposed Appendix R text requires the use of a
site-specific scaling factor; other provisions would be unaffected.
Because TSP samplers collect a broader range of particle sizes than
PM10 samplers, the scaling factor logically can not be less
than 1.0. EPA is inviting comment on the selection of default scaling
factors from within two ranges. The first range is 1.1 to 2.0 and would
apply to Pb-PM10 data collected at source-oriented
monitoring sites. The other range is 1.0 to 1.4 \159\ and would apply
to Pb-PM10 data collected at monitoring sites that are not
source-oriented. These ranges are based on historical data from sites
where the two types of monitors were operated on the same days, as
explained in section II.E.1. Because there would be different default
scaling factors for the two monitoring site types, a modification of
the proposed Appendix R text would require for each monitoring agency
to determine and designate, subject to EPA review, whether each Pb-
PM10 site is in fact source-oriented and to document that
determination in the Annual Monitoring Plan required by 40 CFR 58.10
(see section V for more information on the requirement for this plan
and for designating sites as source-oriented or not).
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\159\ EPA is also soliciting comment on a broader range of 1.0
to 1.9 for nonsource-oriented sites as discussed in section II.E.1.
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As explained in section II.E, EPA is inviting comment on the
possibility of revising the Pb indicator to be Pb-PM10. If a
Pb-PM10 indicator is adopted in the final rule, references
to the two types of data would be reversed from the way they appear in
the proposed text of Appendix R, so that Pb-PM10 data when
available would have primacy over scaled Pb-TSP data. If Pb-
PM10 is adopted as the indicator for the final standard,
many areas may not have sufficient Pb-PM10 data to allow a
determination of compliance or violation with the Pb standard within
the two or three years allowed under the Clean Air Act for initial
designations. EPA is inviting comment on an approach that would allow
the use of Pb-TSP data, with adjustment(s), for comparing ambient
concentrations of Pb to a Pb-PM10 NAAQS for the sole purpose
of making initial designations. The scaling issues, relevant data, and
possible approaches are similar to those described in section II.E.1.
We invite comment on adding language to Appendix R restricting the use
of scaled Pb-TSP data to determinations made for purposes of
designations within three years of promulgation of the revised
standard. (See section VI for discussion
[[Page 29257]]
of the schedule for designations.) This generally would mean that
scaling factors would be used only on 2007-2009 and possibly on earlier
Pb-TSP data, because Pb-PM10 monitoring is proposed to be
required to begin by January 1, 2010. Because scaling factors would
need to be available for designations decisions which must be made
within three years of promulgation of the NAAQS, there would be limited
time for a State to do collocated testing to develop local scaling
factors and then have them reviewed and approved by EPA. Requiring
development of site-specific scaling factors might effectively prevent
use of scaled Pb-TSP data in many States, resulting in more areas
having to be designated unclassifiable initially. Therefore, we invite
comment on removing the passages requiring the development of site-
specific scaling factors from Appendix R and providing default scaling
factors instead. Scaling factors would be 1.0 or less. EPA invites
comment on the selection of appropriate default scaling factors for
this situation.
C. Exceptional Events Information Submission Schedule
As explained above, 40 CFR 50.14 contains generic deadlines for a
State to submit to EPA specified information about exceptional events
and associated air concentration data. A State must initially notify
EPA that data has been affected by an event by July 1 of the year after
the data are collected; this is done by flagging the data in AQS. The
State must also provide an initial description of the event by July 1.
Also, the State must submit the full justification for exclusion within
3 years after the quarter in which the data were collected; however, if
a regulatory decision based on the data (for example, a designation
action) is anticipated, the schedule for the full justification is
foreshortened and all information must be submitted to EPA no later
than a year before the decision is to be made.
These generic deadlines are suitable for the period after initial
designations have been made under a NAAQS, when the decision that may
depend on data exclusion is a redesignation from attainment to
nonattainment or from nonattainment to attainment. However, these
deadlines present problems with respect to initial designations under a
revised NAAQS. One problem is that some of the deadlines, especially
the deadlines for flagging data, can have already passed for some
relevant data by the time the revised NAAQS is promulgated. However,
until the level and form of the NAAQS have been promulgated a State
does not know whether the criteria for excluding data (which are tied
to the level and form of the NAAQS) were met on a given day, so the
only way a State can be sure to have flagged all data of concern and
possible eligibility for exclusion by the deadline is to flag far more
data than will eventually be eligible for exclusion. Another problem is
that some of the data that may be used for final designations may not
be collected and submitted to EPA until later than one year before the
final designation decision, making it impossible to flag that data one
year before the decision. When Section 50.14 was revised to add these
deadlines in March 2007, EPA was mindful that designations were needed
under the recently revised PM2.5 NAAQS, and so exceptions to
the generic deadline were included for PM2.5 only.
The EPA was also mindful that similar issues would arise for
subsequent new or revised NAAQS. The Exceptional Events Rule at section
51.14(c)(2)(v) indicates ``when EPA sets a NAAQS for a new pollutant,
or revises the NAAQS for an existing pollutant, it may revise or set a
new schedule for flagging data for initial designation of areas for
those NAAQS.'' For the specific case of Pb, EPA anticipates that
designations under the revised NAAQS may be made in September 2011
based on 2008-2010 data (or possibly in September 2010 based on 2007-
2009 data if sufficient data is available), and thus will depend in
part on air quality data collected as late as December 2010 (or
December 2009). (See Section VI below for more detailed discussion of
the designation schedule and what data EPA intends to use.) There is no
way for a State to flag and submit documentation regarding events that
happen in October, November, and December 2010 (or 2009) by one year
before designation decisions that are made in September 2011 (or 2010).
The proposed revisions to 40 CFR 50.14 involve only changes in
submission dates for information regarding claimed exceptional events
affecting Pb data. In the proposed rule text at the end of this notice,
only the changes that would apply if designations are made three years
after promulgation are shown; where a deadline would be different if
designations were made at the two-year point, the difference in
deadline is noted in the description immediately below. We propose to
extend the generic deadline for flagging data (and providing a brief
initial description of the event) of July 1 of the year following the
data collection, to July 1, 2009 for data collected in 2006-2007. The
extension includes 2006 and 2007 data because Governors' designation
recommendations will consider 2006-2008 data, and possibly EPA will
consider 2006-2008 or 2007-2009 data if complete data for 2008-2010 are
not available at the time of final designations. EPA does not intend to
use data prior to 2006 in making Pb designation decisions. The generic
event flagging deadline in the Exceptional Events Rule would continue
to apply to data from 2008, and would thus be July 1, 2009. This would
allow a State time following the September 2008 promulgation of the
revised Pb NAAQS to consider what data it wishes to flag and to submit
those flags. The Governor of a State would be required to submit
designation recommendations to EPA in September 2009, and would
therefore know what 2008 data have been flagged when formulating those
recommendations.
For data collected in 2010 (or 2009), we propose to move up the
generic deadline of July 1 for data flagging to May 1, 2011 (or May 1,
2010) (which is also the applicable deadline for certifying data in AQS
as being complete and accurate to the best knowledge of the responsible
monitoring agency head). This would give a State less time, but EPA
believes still sufficient time, to decide what 2010 (or 2009) data to
flag, and would allow EPA to have access to the flags in time for EPA
to develop its own proposed and final plans for designations.
Finally, EPA proposes to make the deadline for submission of
detailed justifications for exclusion of data collected in 2006 through
2008 be September 15, 2010 for the three year designation schedule, or
September 15, 2009 under the two year designation schedule. EPA
generally does not anticipate data from 2006 and 2007 being used in
final Pb designations. Under the three year designation schedule, for
data collected in 2010, EPA proposes to make the deadline for
submission of justifications be May 1, 2011. This is less than a year
before the designation decisions would be made, but we believe it is a
good compromise between giving a State a reasonable period to prepare
the justifications and EPA a reasonable period to consider the
information submitted by the State. Similarly, under the two year
designation schedule, for data collected in 2009, EPA proposes to make
the deadline for submission of justifications be May 1, 2010. Table 8
summarizes the proposed three year designation deadlines discussed in
this section, and Table 9 summarizes the two year designation
deadlines.
[[Page 29258]]
Table 8.--Proposed Schedule for Exceptional Event Flagging and
Documentation Submission if Designations Promulgated in Three Years
------------------------------------------------------------------------
Detailed
Air quality data collected for Event flagging documentation
calendar year deadline submission
deadline
------------------------------------------------------------------------
2006............................ July 1, 2009*..... September 15,
2010*.
2007............................ July 1, 2009*..... September 15,
2010.
2008............................ July 1, 2009...... September 15,
2010*.
2009............................ July 1, 2010...... September 15,
2010*.
2010............................ May 1, 2011*...... May 1, 2011*.
------------------------------------------------------------------------
* Indicates proposed change from generic schedule in 40 CFR 50.14.
Table 9.--Proposed Schedule for Exceptional Event Flagging and
Documentation Submission If Designations Promulgated in Two Years
------------------------------------------------------------------------
Detailed
Air quality data collected for Event flagging documentation
calendar year deadline submission
deadline
------------------------------------------------------------------------
2006............................ July 1, 2009*..... September 15,
2009.
2007............................ July 1, 2009*..... September 15,
2009*.
2008............................ July 1, 2009...... September 15,
2009*.
2009............................ May 1, 2010*...... May 1, 2010*.
------------------------------------------------------------------------
* Indicates proposed change from generic schedule in 40 CFR 50.14.
EPA invites comment on these proposed changes in the exceptional
event flagging and documentation submission deadlines.
V. Proposed Amendments to Ambient Monitoring and Reporting Requirements
As part of our proposal to revise and implement the Pb NAAQS, we
are proposing several changes to the ambient air monitoring and
reporting requirements for Pb. Ambient Pb monitoring data are used to
determine whether an area is in violation of the Pb NAAQS. Ambient data
are collected and reported by State, local, and Tribal monitoring
agencies (``monitoring agencies'') according to the monitoring
requirements contained in 40 CFR parts 50, 53, and 58. This section
explains aspects of the existing Pb monitoring and reporting
requirements as background and discusses the changes we are proposing
to support the changes being proposed in the Pb NAAQS and other options
for the NAAQS on which EPA is inviting comments, discussed above in
section II.E. These aspects include the sampling and analysis methods
(including quality assurance requirements), network design, sampling
schedule, data reporting, and other miscellaneous requirements.
A. Sampling and Analysis Methods
We are proposing changes to the sampling and analysis methods for
the Pb monitoring network. Specifically, we are proposing a new Federal
Reference Method (FRM) for Pb in PM10 (Pb-PM10)
and revised Federal Equivalent Method (FEM) criteria. We are
maintaining the current FRM for Pb in TSP (Pb-TSP) and lowering the Pb
concentration range required during Pb-TSP and Pb-PM10
candidate FEM comparability testing. The following sections provide
background, rationale, and details for the proposed changes to the
sampling and analysis methods.
1. Background
Lead monitoring data must be collected and analyzed using FRM or
FEM methods in order to be comparable to the NAAQS. The current FRM for
Pb sampling and analysis is based on the use of a high-volume TSP FRM
sampler to collect the particulate matter sample and the use of atomic
absorption (AA) spectrometry for the analysis of Pb in a nitric acid
extract of the filter sample (40 CFR part 50, Appendix G). There are 21
FEMs currently approved for Pb-TSP \160\. All 21 FEMs are based on the
use of high-volume TSP samplers and a variety of approved equivalent
analysis methods.\161\
---------------------------------------------------------------------------
\160\ For a list of currently approved FRM/FEMs for Pb-TSP refer
to: http://www.epa.gov/ttn/amtic/criteria.html.
\161\ The 21 distinct approved FEMs represent less than 21
fundamentally different analysis methods, as some differ in only in
minor aspects.
---------------------------------------------------------------------------
Concerns have been raised over the use of the high-volume TSP
samplers to collect samples for subsequent Pb analysis. It is known
that the high-volume TSP sampler's particulate matter capture
efficiency varies as a function of wind speed and wind direction due to
the non-symmetrical inlet design and the lack of an integral particle
separator. Early evaluations of the high-volume TSP sampler
demonstrated that the sampler's 50% collection efficiency cutpoint can
vary between 25 and 50 [mu]m depending on wind speed and direction
(Wedding et al., 1977, McFarland and Rodes, 1979). More recently, a
study was conducted during the last Pb NAAQS review to evaluate the
effect of wind speed and direction on sampler efficiency (Purdue,
1988). This study showed that despite the effect of wind speed and wind
direction on the sampler's collection efficiency for larger particles,
for particle distributions typical of those near industrial sources the
overall Pb collection efficiency of the high-volume TSP sampler ranged
from 80% to 90% over a wide range of wind speeds and directions.
CASAC commented in the context of their review of the Staff Paper
that TSP samplers have poor precision, that the upper particle cut size
of TSP samplers varies widely as a function of wind speed and
direction, and that the spatial non-homogeneity of very coarse
particles cannot be efficiently captured by a national monitoring
network (Henderson, 2007a, Henderson, 2008). For these reasons, CASAC
recommended considering a revision to the Pb reference method to allow
sample collection using low-volume PM10 samplers.\162\
---------------------------------------------------------------------------
\162\ PM10 can be measured with either a ``low-
volume'' or a ``high-volume'' sampler. CASAC specifically
recommended the low-volume sampler, for reasons explained here and
in section II.E.1.
---------------------------------------------------------------------------
As part of preparing the ANPR for this rulemaking, we performed and
reported in the ANPR the results of an analysis of the precision and
bias of the high-volume TSP sampler based on Pb-TSP
[[Page 29259]]
data reported to AQS for collocated samplers and the results of in-
field sampler flow audits and laboratory audits for lead (Camalier and
Rice, 2007). The average precision of the high-volume Pb-TSP sampler
was approximately 12% with a standard deviation of 19% and average
sampling bias (based on flow audits) was -0.7% with a standard
deviation of 4.2%. The average bias for the lab analyses of Pb-spiked
audit strips was -1.1% with a standard deviation of 5.5%. Total bias,
which includes bias from both sampling and laboratory analysis, was
estimated at -1.7% with a standard deviation of 3.4%. These findings
are specific for the times and sites of the sampling, including the
nature and total quantity of TSP and Pb-TSP that prevailed during the
sampling, and may not be indicative of the TSP FRM performance in other
places. Also, we did not investigate to determine whether the physical
arrangement of the collocated samplers was such as to provide a good
test of sensitivity to wind speed and wind direction.\163\ However, we
note that at face value these bias and precision results are not
greatly different than has historically been considered acceptable for
other criteria pollutants.
---------------------------------------------------------------------------
\163\ If the collocated TSP samplers were always oriented in the
same direction, they would be exposed to the same wind speed and
wind direction, and the appearance of good precision between them
would not necessarily be indicative of the sensitivity of Pb-TSP
measurements to wind speed and wind direction.
---------------------------------------------------------------------------
The CASAC and some public comments on the ANPR again stressed
concerns with the use of the high-volume TSP sampler and a strong
interest in moving to a low-volume Pb-PM10 sampler. The
CASAC reiterated the disadvantages of retaining TSP and of utilizing it
as the ``gold standard'' against which new and better technologies are
compared (Henderson 2008). On March 25, 2008, the AAMM Subcommittee of
CASAC and EPA staff conducted a consultation by conference call, at
which the subcommittee members confirmed and elaborated on the views
CASAC expressed in their comments on the ANPR. Public comments were
also generally supportive of moving away from the current high-volume
PM sampling technology and moving toward modern, sequential, low-volume
PM10 monitors, especially if sampling frequencies are
increased. On the other hand, several monitoring agencies cautioned
against moving to Pb-PM10 as the indicator because samplers
for Pb-PM10 would miss much of the Pb in the atmosphere
especially near Pb sources.
CASAC recommended that Pb-PM10 be measured with low-
cost, multi-element analysis methods with improved detection limits
(e.g., x-ray fluorescence, XRF) for measuring concentrations typical of
today's ambient air. One public commenter suggested that the MDL be
significantly reduced to enable measurement of average Pb levels of
0.08 [mu]g/m\3\ or below.
The current post-sampling FRM analysis method for Pb-TSP is atomic
absorption (AA) spectrometry. A typical or nominal lower detectable
limit (LDL) for Pb, for high-volume sample collection followed by AA
analysis, stated in the FRM regulation in Appendix G to Part 50 for
informational purposes only, is 0.07 [mu]g/m\3\. This value was
calculated by doubling the between-laboratory standard deviation
obtained for the lowest measurable lead concentration (Long 1979). This
value can be considered a conservative (i.e., upper bound) estimate of
the sensitivity for the AA method currently used by air monitoring
laboratories, as evidence by the fact that data obtained from AQS
includes reported locally determined MDL values for the AA FRM that are
well below 0.07 [mu]g/m\3\ (typically 0.01 (g/m\3\ or below).
One estimate of the method detection limit (MDL) for AA analysis of
a low-volume sample of either Pb-PM10 or Pb-TSP, taking into
account the nominal LDL of 0.07 [mu]g/m\3\ (or 140 [mu]g/L), and the
smaller sample volume, extraction volume, and filter size for low-
volume sampling, is about 0.12 [mu]g/m\3\ (see Table 10). Assuming an
LDL of 0.01 (g/m\3\ for TSP sampling, the MDL for low-volume sampling
would be about 0.02 (g/m\3\. Other Pb-TSP FEM analysis methods
currently used with the high-volume sampling method, such as XRF,
inductively coupled plasma mass spectrometry (ICP/MS) and graphite
furnace atomic absorption (GFAA) are more sensitive than AA analysis,
and are clearly sensitive enough to support low-volume sampling and a
reduced NAAQS level.
2. Proposed Changes
As discussed in Section II.E.3 of this preamble, after considering
the CASAC and public comments on monitoring issues, we are proposing to
retain Pb-TSP, as measured by the FRM method specified in 40 CFR part
50, appendix G (which cross references appendix B, the specification of
the TSP FRM) as the indicator for the Pb standard, and to invite
comment on a second option which would instead make Pb-PM10
measured by a low-volume monitor the indicator. We further propose that
monitoring agencies should be given the option to use adjusted or
scaled low-volume Pb-PM10 monitoring data as a surrogate for
Pb-TSP data. Details on how this option would work are discussed in the
data handling section of this preamble (section IV). Also, in section
IV.B we are inviting comment on whether, if low-volume Pb-
PM10 is selected as the indicator, Pb-TSP data with an
adjustment should be useable as a surrogate for Pb-PM10 data
for the specific purpose of initial designations under the revised
standard. In this section, we discuss the Pb-TSP and Pb-PM10
sampling and analysis issues themselves and propose approaches for
these issues, as these issues are relevant to the use of data from each
method directly or as surrogates for the other.
a. TSP Sampling Method
If the final standard is based on Pb-TSP we believe it is
appropriate to continue to allow, although perhaps not to encourage,
the use of the current high-volume FRM for measuring Pb-TSP. The
selection of Pb-TSP as the NAAQS indicator would depend on a conclusion
that the precision, bias, and MDL (discussed above) of the TSP sampler
is adequate for continued use in the Pb monitoring network, including a
conclusion that although the TSP sampler's size selection performance
is affected by wind speed and wind direction, we do not believe that
this effect is so significant as to prevent the continued use of this
sampler in the Pb network. EPA proposes to make several minor
clarifying changes in Appendix G to correct long-standing errors in
reference citations. We are not proposing any other substantive changes
to Appendix G.
However, we also believe that low-volume Pb-TSP samplers might be
superior to high-volume TSP samplers. Presently, a low-volume TSP
sampler cannot obtain FRM status, because the FRM is specified in
design terms that preclude designation of a low-volume sampler as a
FRM. A low-volume Pb-TSP monitoring system (including an analytical
method for Pb) can in principle be designated as a FEM Pb-TSP monitor,
if side-by-side testing is performed as prescribed by 40 CFR 53.33. We
are proposing amendments to this CFR section, described below, to make
such testing more practical and to clarify that both high-volume and
low-volume TSP methods may use this route to FEM status. Note that the
terms of the revised FEM procedures can also be used to obtain FEM
status for Pb-PM10 samplers.
[[Page 29260]]
b. PM10 Sampling Method
If the final standard is based on Pb-PM10, or if the
final rule for a standard based on Pb-TSP includes an option to monitor
Pb-PM10 instead of Pb-TSP, we will need to promulgate both
an FRM for measuring Pb-PM10 and an appropriate set of FEM
criteria. Accordingly, we are proposing new FRM and FEM criteria for
measuring Pb-PM10. The proposed FRM for Pb-PM10
can be broken down into two parts: (1) the sampling method (i.e., the
procedures and apparatus used for collecting PM10 on a
filter) and (2) the analysis method (i.e., the procedures and apparatus
used to analyze the collected particulate matter for Pb content).
Currently, the FRM specification for PM10 monitoring,
Appendix J to 40 CFR Part 50, is based on a performance test and does
not specify whether a sampler is high-volume or low-volume. Early
commercialized samplers were high-volume, but more recently a number of
low-volume PM10 samplers have received FRM approvals. To be
certain that Pb-PM10 monitoring is conducted with low-volume
samplers without specifying the use of particular sampler brands or
models, it is necessary to establish a new FRM specification for low-
volume PM10 samplers. There is a recently promulgated FRM
for particulate matter with aerodynamic diameter between 2.5 and 10
microns (PM10-2.5) (Appendix O to 40 CFR part 50) that is
based on a pair of low-volume samplers for PM2.5 and
PM10 to provide a PM10-2.5 concentration by
difference. We are proposing to create a FRM for Pb-PM10
sampling by cross-referencing to the specification for the
PM10 sampler in this paired FRM (referred to as the
PM10C sampler, where the ``C'' refers to the use of this
PM10 sampler as part of a pair for measuring coarse PM). We
are proposing to use the low-volume PM10C sampler for the
FRM for Pb-PM10 rather than the existing PM10 FRM
specified by appendix J, for several reasons. Appendix J to part 50 has
resulted in the designation of both high-volume and low-volume
PM10 samplers as FRM for PM10. We believe high-
volume PM10 sampling should not be used to measure Pb-
PM10 under a revised Pb standard. A low-volume
PM10C FRM sampler must meet more demanding performance
criteria than is required for PM10 samplers in general in
Appendix J. We note the current availability of samplers that meet
these more demanding performance criteria (already in use for
PM2.5 and PM10-2.5 sampling) that are equipped
with sequential sampling capabilities (i.e., the ability to schedule
multiple samples between operator visits, which is desirable if the
proposed sampling frequency requirements are increased to support a
monthly averaging form of Pb NAAQS). The geometry of commercial high-
volume PM10 samplers makes sequential sampling with a single
sampler impossible. The low-volume sampler also precisely maintains a
constant sample flow rate corrected to actual conditions by actively
sensing changes in temperature and pressure and regulating sampling
flow rate. Use of a low-volume sampler for the Pb-PM10 FRM
would also provide network efficiencies and operational consistencies
with the samplers that are in widespread use for the PM2.5
FRM network, and that are seeing growing use in the PM10 and
PM10-2.5 networks. Finally, the use of a low-volume sampler
is consistent with the comments and recommendations from CASAC and
members of CASAC's AAMM (Henderson 2007a, Henderson 2008, Russell
2008).
Low-volume Pb-PM10 samplers and the data systems that
they connect to can be configured to report concentrations corrected to
standard conditions of temperature and pressure or based on local
conditions of temperature and pressure. We are proposing that the FRM
for samplers used to collect Pb data specify reporting of
concentrations based on local conditions, for a few reasons. The actual
concentration of Pb in the atmosphere is a better indicator of the
potential for deposition than the concentration based on standard
pressure and temperature. In addition, there are practical advantages
to moving to local conditions since the FRM for both PM2.5
and PM10-2.5 are also based on local conditions.
c. Analysis Method
There are several potential analysis methods for a Pb-
PM10 FRM. Atomic absorption (AA) is the analysis method for
the current Pb-TSP FRM. In addition, there are several other analysis
methods (e.g., XRF, ICP/MS) approved as FEMs for the measurement of Pb-
TSP. Table 10 summarizes the estimated MDLs for the analysis methods
considered in developing the proposed FRM for Pb-PM10. The
estimated MDLs are based on published instrument detection limits and
LDLs, which typically take into account only instrument signal-to-noise
ratios and laboratory-related variability but not variability related
to sample collection and handling. It is important to note that the
MDLs in Table 10 are estimates and these values will vary as a function
of the specific instrument used, detector age, instrument signal-to-
noise level, etc., and therefore, MDLs must be determined for the
specific instrument used.
Table 10.--Summary of Candidate Analysis Method Detection Limits for a Pb-PM10 FRM or FEM With Low-Volume Sample
Collection
----------------------------------------------------------------------------------------------------------------
Estimated MDL
Analysis method Estimated DLs \a\ \b\ ([mu]g/
m\3\)
----------------------------------------------------------------------------------------------------------------
Atomic Absorption (AA).......................... 0.07 [mu]g/m\3\ \c\......................... 0.12 \f\
0.01 [mu]g/m\3\ \d\......................... 0.02 \f\
X-Ray Fluorescence (XRF)........................ 1.5 ng/cm\2\ \e\............................ 0.001 \g\
Graphite Furnace Atomic Absorption (GFAA)....... 0.05 [mu]g/L \h\............................ 0.00004 \f\
Inductively Coupled Plasma/Mass Spectrometry 0.08 [mu]g/L \e\............................ 0.00006 \f\
(ICP/MS).
----------------------------------------------------------------------------------------------------------------
\a\ Detection limits (DLs) found in available literature as provided in footnotes below.
\b\ Estimated MDLs determined using estimated DL, extraction volume, and sample volume as noted in footnotes
provided.
\c\ The lower detectable limit (LDL) for Pb-TSP taken from Appendix G to Part 50 based on 2400m\3\ sample
volume, 0.10L extraction volume, and 12 strips per filter.
\d\ Based on MDLs reported in AQS.
\e\ DL expressed as nanogram per square centimeter of filter surface is taken from the Compendium of Methods for
the Determination of Inorganic Compounds in Ambient Air (USEPA, 1999).
\f\ Based on 46.2-mm filter extraction volume of 0.020 L and sample volume of 24 m\3\ of air.
\g\ Based on 46.2-mm filter area of 11.86 cm\2\ and sample volume of 24 m\3\ of air.
\h\ Taken from the Perkin Elmer Guide to Atomic Spectroscopy Techniques and Applications (Perkin Elmer, 2000).
[[Page 29261]]
One disadvantage of the low-volume sampler is that the total mass
of the PM10 sample collected is significantly lower than
that of the high-volume sampler due to the lower volume of air sampled
(24 m\3\ per 24 hours for the low-volume sampler versus. over 1500 m\3\
per 24 hours for a high-volume sampler). The lower mass of sample
collected results in higher MDLs for any given analysis method when
coupled with the low-volume sampler. As can be seen in Table 10, even
assuming the smaller LDL reported to AQS for recent sampling, the
estimated MDL for atomic absorption (the current FRM analysis method
for Pb-TSP) when coupled with low-volume sampling is the highest (least
sensitive) of all potential methods for use as an FRM/FEM method for
Pb-PM10.
AA, GFAA, and ICP/MS are destructive methods and require solvent
extractions that possibly involve the use of strong acids to adequately
extract Pb from the collected PM for analysis. The specific extraction
solutions and methods are selected and optimized in order to meet the
required extraction efficiency for a measurement program. Both methods
are destructive, meaning that the sample collected on the filter is
destroyed during analysis. These methods also have higher analysis
costs relative to XRF.
While XRF, GFAA, and ICP/MS all have more than adequate MDLs to
support a reduced NAAQS level, we believe that the XRF analysis method
has several advantages which make it a desirable analysis method to
specify as the FRM. XRF does not require sample preparation or
extraction with acids prior to analysis. It is a non-destructive
method; therefore, the sample is not destroyed during analysis and can
be archived for future analysis or re-analysis if needed. XRF analysis
is a cost-effective approach that could be used at the option of the
monitoring agency to simultaneously analyze for many additional metals
(e.g., arsenic, antimony, and iron) which may be useful in source
apportionment. XRF is also the method used for the urban
PM2.5 speciation monitoring networks and for the mostly
rural visibility monitoring program in Class I visibility areas, and is
being considered for the PM10-2.5 coarse speciation
monitoring network that will be implemented by monitoring agencies as
part of the NCore multi-pollutant network. The XRF analysis method
should have acceptable precision, bias, and MDL for use as the FRM for
Pb-PM10 when coupled with the low-volume PM10
sampler. Finally, CASAC recommended the use of XRF as a low-cost and
sensitive analysis method for the FRM (Henderson 2007a, Henderson
2008). For these reasons, we are proposing to base the analysis method
for the proposed Pb-PM10 FRM on XRF.
d. FEM Criteria
The FEM criteria provide for approval of candidate methods that
employ an alternative analysis method for Pb, an alternative sampler,
or both.
The proposed Pb-PM10 FRM is based on the low-volume
PM10c sampler and XRF analysis. Under the proposed revisions
to 40 CFR 53.33, Pb-PM10 data from any candidate FEM using
an alternative sampler would be compared to side-by-side data from the
low-volume PM10c FRM sampler. An FEM candidate using only an
alternative analysis method would be evaluated by collecting paired
filters from paired low-volume PM10c FRM samplers, and
analyzing one filter of each pair with XRF and the other filter with
the candidate method.
As mentioned above, there are other analysis methods commonly used
which are also expected to meet the precision, bias, and MDLs necessary
to be used in the Pb surveillance monitoring network (e.g., GFAA and
ICP/MS). These analysis methods would be compared to the proposed XRF
method and would be approvable as FEMs through the performance testing
requirements outlined in regulation Sec. 53.33 of 40 CFR part 53,
subpart C. Several of these requirements need revisions for consistency
with a potentially lowered Pb NAAQS and for the potential addition of a
Pb-PM10 FRM. The following paragraphs describe the aspects
of the FEM criteria that we are proposing to revise.
The current FEM requirements state that the ambient Pb
concentration range at which the FEM comparability testing must be
conducted to be valid is 0.5 to 4.0 [mu]g/m\3\. Currently there are few
locations in the United States where FEM testing can be conducted with
assurance that the ambient concentrations during the time of the
testing would exceed 0.5 [mu]g/m\3\. In addition, the Agency is
proposing to lower the Pb NAAQS level to between 0.10 and 0.30 [mu]g/
m\3\. As such, we are proposing to revise the Pb concentration
requirements for candidate FEM testing to a range of 30% of the NAAQS
to 250% of the NAAQS in [mu]g/m\3\. For example, if the level of the Pb
NAAQS is finalized at 0.20 [mu]g/m\3\, the ambient concentrations that
would be required for FEM testing would have to range between 0.06
[mu]g/m\3\ to 0.50 [mu]g/m\3\. The requirements were changed from
actual concentration values to percentages of the NAAQS to allow the
FEM text to remain appropriate if subsequent changes to NAAQS levels
occur in the future.
The current FEM requirements state that the maximum precision and
accuracy for candidate analytical methods must be 15% and 5%
respectively. No changes are proposed for these requirements. Based on
the results for the current high-volume Pb-TSP precision and bias
(Camalier and Rice, 2007), these requirement seem reasonable for the
proposed FEM requirements. The current FEM does not have a requirement
for a maximum MDL. In order to ensure that candidate analytical methods
have adequate sensitivity or MDLs, we are proposing to add a
requirement that as part of the testing of a candidate FEM, the
applicant must demonstrate that the MDL of the method is less than 1%
of the level of Pb NAAQS. We believe this MDL requirement will ensure
that FEM methods will have enough sensitivity to detect Pb
concentrations much less than the proposed NAAQS level, but will not
unnecessarily restrict methods which could be used to provide data
sufficient for the purpose of determining compliance with the NAAQS.
Subsequent users of a previously approved FEM would not be required to
demonstrate the MDL of the method as implemented in their laboratories,
but EPA plans to encourage them to do so periodically as a good quality
assurance practice.
The existing FEM requirements require that audit samples (the known
concentration or reference samples provided on request by EPA used to
verify the accuracy with which a laboratory conducts the FRM analytical
procedure before it may begin comparing the FRM to the candidate FEM)
be analyzed at levels that are equal to 100, 300, and 750 [mu]g per
spiked filter strip (equivalent to 0.5, 1.5, and 3.75 [mu]g/m\3\ of
sampled air). We are proposing to revise the levels of the audit
concentrations to percentages (30%, 100% and 250%) of the Pb NAAQS to
provide for reduced audit concentrations for a lowered NAAQS. These
percentages are roughly equivalent to the percentages of the current
NAAQS level (1.5 [mu]g/m\3\) used to set the spiked filter strip audit
concentrations provided above in the original FEM regulation.
The existing FEM requirements are based on the high-volume TSP
sampler, and as such, refer to \3/4\ x 8-inch glass fiber strips. In
order to also accommodate the use of low-volume sample filters, we are
proposing to add references to 46.2-mm sample filters
[[Page 29262]]
where appropriate. Pairs of these filters will be collected by a pair
of FRM samplers, so that there is no need to cut the 46.2 mm filters
into two parts before analysis.
e. Quality Assurance
Modifications are needed to the quality assurance (QA) requirements
for Pb in 40 CFR part 58, Appendix A paragraph 3.3.4 in order to
accommodate Pb-PM10 monitoring. Paragraph 3.3.4 specifies
requirements for annual flow rate audits for TSP samplers used in Pb
monitoring and Pb strip audits for laboratories performing analysis of
TSP filters for Pb. Other QA requirements specified in paragraph 3.3.1
for all TSP samplers are also applicable to Pb-TSP samplers. As part of
the overall Pb NAAQS review, it is appropriate to revise these
requirements to consolidate all the QA requirements for Pb monitoring
in paragraph 3.3.4, to add provisions specific for Pb-PM10
measurements and to eliminate cross references to the general TSP
provisions. The following paragraphs detail the QA requirements we are
proposing to change.
The collocation requirement for all TSP samplers (paragraph 3.3.1)
applies to TSP samplers used for Pb-TSP monitoring. These requirements
are the same for PM10 (paragraph 3.3.1); as such, no changes
are needed to accommodate low-volume Pb-PM10. However, to
clarify that this requirement also applies to Pb monitoring we are
proposing to add a reference to this requirement in paragraph 3.3.4.
The sampler flow rate verifications requirement (paragraph 3.3.2)
for low-volume PM10 and for TSP are at different intervals.
While this appears appropriate and no change is needed, to clarify that
this requirement also applies to Pb monitoring we are proposing to add
a reference to this requirement in paragraph 3.3.4.
Paragraph 3.3.4.1 has an error in the text that suggests an annual
flow rate audit for Pb, but then includes reference in the text to
semi-annual audits. The correct flow rate audit frequency is semi-
annual. We are proposing to correct this error. Also, we are proposing
to change the references to the Pb FRM to include the proposed Pb-
PM10 FRM.
Paragraph 3.3.4.2 discusses the audit procedures for the lead
analysis method. This section assumes the use of a high-volume TSP
sampler, and we are proposing edits to account for the proposed Pb-
PM10 FRM. In addition, the audit concentration ranges will
not be appropriate if the NAAQS is lowered. We are proposing to lower
the audit ranges for Pb-TSP from the current range of 0.5-1.5 [mu]g/
m\3\ to a range from 30-100% of the proposed Pb NAAQS level for the low
concentration audit and from 3.0-5.0 [mu]g/m\3\ to 200-300% of the
proposed NAAQS for the higher concentration audit standard. The
requirements would also be changed from specific concentration value-
based ranges to ranges based on the percentages of the NAAQS to allow
these QA requirements to remain appropriate if changes to NAAQS levels
occur during future reviews.
Unlike the PM2.5 and PM10-2.5 Performance
Evaluation Program (PEP), the existing QA program requirements for Pb
monitoring do not include a requirement for the collection of data
appropriate for making an independent estimate of the overall sampling
and analysis bias. We are proposing to require one PEP-like audit at
one site within each primary quality assurance organization (PQAO) once
per year. We are also proposing that, for each quarter, one filter of a
collocated sample filter pair from one site within each PQAO be sent to
an independent laboratory for analysis. The independent measurement on
one filter from each pair would be compared to the monitoring agency's
regular laboratory's measurement on the other filter of the pair, to
allow estimation of any bias in the regular laboratory's measurements.
EPA believes that the combination of the PEP data and the independent
collocation data will be enough to provide a reasonable assessment of
overall bias and data comparability on a PQAO basis over the
designation period. As currently is the case for PEP auditing of
PM2.5 and PM10-2.5 monitoring sites, it would be
the responsibility of each State to ensure that Pb PEP testing and
collocation testing as described here is performed as required. EPA
plans to consult with monitoring agencies after completion of this
rulemaking as to whether a centrally run program managed by EPA and
funded with State and Tribal Assistance Grant funds would be a more
efficient and preferred alternative than individual State-managed
programs.
B. Network Design
As a result of this Pb NAAQS review and the proposed tightening of
the standards, EPA recognizes that the current network design
requirements are inadequate to assess compliance and determine the
extent of all the areas that may violate the revised NAAQS. As such, we
are proposing new network design requirements for the Pb NAAQS
surveillance network. The following sections provide background,
rationale, and details for the proposed changes to the Pb network
design requirements.
1. Background
The once large Pb surveillance network of FRM samplers for Pb-TSP
has decreased substantially over the last few decades. In 1980 there
were over 900 Pb surveillance sites. This number has been reduced to
approximately 200 sites today. These reductions were made because of
substantially reduced ambient Pb concentrations causing monitoring
agencies to shift priorities to other criteria pollutants including
PM2.5 and ozone which were believed to pose a greater health
risk. As a result of these reductions, many states currently have no
ambient air Pb monitors resulting in large portions of the country with
no data on current ambient Pb air concentrations. In addition, many of
the largest Pb emitting sources in the country do not have nearby
ambient Pb air monitors.
There is also a smaller network, the National Air Toxics Trends
Stations network, of 27 monitoring sites measuring Pb-PM10.
Some of these use a high-volume PM10 sampler to collect the
particulate matter and some use a low-volume PM10 sampler.
Most are in urban areas.
The current network design requirements for Pb monitoring are given
in 40 CFR part 58 appendix D section 4.5. The current minimum network
design requirements are for two Federal Reference Method (FRM) or
Federal Equivalent Method (FEM) sites in any area where Pb
concentrations exceed or have exceeded the NAAQS in the most recent two
years. These current minimum monitoring requirements cannot be relied
upon to cause monitoring agencies to fill the existing gaps in the
current network, and if they are not revised it will be difficult to
develop the necessary network to properly evaluate ambient air
concentrations during the designation process, especially if the NAAQS
is finalized at a significantly lower level than the current standard.
For these reasons, EPA indicated in the Advanced Notice of Proposed
Rulemaking (72 FR 71488) that the existing Pb NAAQS surveillance
network may not be adequate for a lowered Pb NAAQS, and that if the
NAAQS is substantially lowered as proposed additional monitoring sites
would be needed to provide estimates of ambient Pb air concentrations
near Pb emission sources and for characterizing ambient air
concentrations in large urban areas. Comments received from CASAC and
other public commenters
[[Page 29263]]
on the ANPR stated that the Pb surveillance network should be expanded
in order to provide better coverage of Pb emission sources and to
better understand population exposures to Pb from ambient air. After
considering these comments and evaluating the existing network, EPA is
proposing changes to the network as described below.
2. Proposed Changes
We are proposing to modify the existing network design requirements
for the Pb surveillance monitoring network to achieve better
understanding of ambient Pb air concentrations near Pb emission sources
and to provide better information on population exposure to Pb in large
urban areas. The following paragraphs provide the rationale and details
for the proposed changes.
The primary objective of the Pb monitoring network is to provide
data on the ambient Pb air concentrations in areas where there is the
potential for a violation of the NAAQS. Ambient Pb concentrations have
dropped dramatically in most urban areas due to the elimination of Pb
in gasoline. However, based on our analysis of the ambient Pb data,
relatively large sources of Pb continue to have the potential to cause
ambient air concentrations in excess of the proposed NAAQS (EPA,
2007c). Furthermore, it appears, based on the limited network still
operating, that violations of the proposed range for the revised NAAQS
levels are likely to exist only near such sources of Pb emissions, with
lower levels of Pb away from such sources. Accordingly, we are
proposing to require monitoring near Pb emission sources such as Pb
smelters, metallurgical operations, battery manufacturing, and other
source categories that emit Pb. By implementing the NAAQS through a
source-oriented monitoring network, Pb concentrations will be kept
below the NAAQS level for those living near these sources and for those
living farther away.
The 2002 National Emissions Inventory (NEI) lists over 13,000
sources of Pb, with emission rates from as low as 1 pound to nearly 60
tons per year (according to the NEI 90% of lead sources emit less than
0.1 tpy). It is not practical to conduct monitoring at every Pb
emission source, nor is it likely that very small Pb emission sources
will cause ambient concentrations to exceed the proposed NAAQS.
Therefore, it is appropriate to limit the source oriented monitoring
requirement to emission sources that may have the potential to result
in ambient air concentrations in excess of the proposed NAAQS.
We are proposing that monitoring be presumptively required at
sources that have Pb emissions (as identified in the latest NEI or by
other scientifically justifiable methods and data) that exceed a Pb
``emissions threshold.'' This monitoring requirement would apply not
only to existing industrial sources of lead, but also to fugitive
sources of lead (e.g., mine tailing piles, closed industrial
facilities) and airports where leaded aviation gas is used. In this
context, the emissions threshold is the Pb emission rate for a source
that may reasonably be expected to result in ambient air concentrations
in excess of the proposed Pb NAAQS. We conducted an analysis to
estimate the appropriate emission threshold (Cavender 2008b) which is
available in the docket for this rulemaking. In this analysis, four
different methods were used for calculating an appropriate threshold
emissions rate based on the candidate NAAQS level. The arithmetic mean
of the four methods suggests a maximum emission impact of 0.5 [mu]g/
m\3\ per 1,000 kg Pb emitted per year. Using the results from this
analysis, we propose that the emission threshold be set in the range of
200 kg-600 kg per year total Pb emissions (including point, area, and
fugitive emissions and including Pb in all sizes of PM). We are
proposing a range for the emission threshold since we are proposing a
range for the level of the standard. If the final NAAQS is set at 0.10
[mu]g/m\3\, we would set the emission threshold at 200 kg per year.
Conversely, if the final NAAQS is set at 0.30 [mu]g/m\3\, we would set
the emission threshold at 600 kg per year. We solicit comments on the
various methods for calculating emission rate thresholds, as well as
using the arithmetic mean of these results in choosing the appropriate
threshold for designing the monitoring network.
We recognize that a number of factors influence the actual impact a
source of Pb has on ambient Pb concentrations (e.g., local meteorology,
emission release characteristics, and terrain). As such, we are also
proposing to allow monitoring agencies to petition the EPA Regional
Administrator to waive this requirement for a source that emits less
than 1 ton per year where it can be shown (by demonstrating actual
emissions are less than the threshold, through modeling, historical
monitoring data, or other means) that a source will not cause ambient
air concentrations to exceed 50% of the NAAQS during a three year
period. We are proposing that for facilities identified as emitting
more than 1 tpy in the NEI, a waiver is possible only by demonstrating
that actual emissions are less than the emissions threshold. By
requiring every source actually emitting more than 1 tpy to be
monitored, we will avoid the possibility that faulty or uncertain
modeling demonstrations or past monitoring programs would be the basis
for not monitoring sources that are the most likely to cause NAAQS
violations.
We seek comments on the appropriateness of requiring monitoring
near Pb emissions sources and the proposed emission rate threshold. We
also seek comments on the appropriateness of allowing monitoring
agencies to seek waivers from this requirement and the upper emission
threshold level at which waivers should no longer be allowed.
The required source-oriented monitors shall be located at sites of
maximum impact and will be classified primarily as microscale monitors
representative of small hot-spot areas adjacent or nearly adjacent to
facility fence-lines. EPA takes comment on this monitoring requirement
and whether monitors should only be placed in areas which are
population-oriented. In some cases, source-oriented monitors may be
representative of somewhat bigger areas due to the orientation of
sources with respect to areas with locations appropriate for ambient
monitoring. In these cases, the source-oriented monitors may be
classified as middle-scale, but should still represent the locations
where maximum Pb concentrations around a facility are expected to
occur, consistent with applicable siting regulations and the outputs of
quantitative tools (e.g., dispersion modeling) used to determine
maximum impacts.
We are proposing to require a small network of nonsource-oriented
monitors in urban areas in addition to the source oriented monitors
discussed above, in order to gather information on the general
population exposure to Pb in ambient air. While it is expected that
these nonsource-oriented monitors will show lower concentrations than
source oriented monitors, data from these nonsource-oriented monitors
will be helpful in understanding the risk posed by Pb to the general
population. Data from these monitors will also be useful in determining
impacts on Pb concentrations from re-entrained roadway dust,
construction and demolition projects, other nonpoint area sources; and
in determining the spatial variation in Pb concentrations between areas
that are and are not source impacted. Such data on spatial variations
within an urban area could assist with the determination of non-
attainment boundaries.
[[Page 29264]]
We are proposing to require one nonsource-oriented monitor in each
Core Base Statistical Area (CBSA, as defined by the Office of
Management and Budget)\164\ with a population of 1,000,000 people or
more as determined in the most recent census estimates. Based on the
most current census estimates, 50 CBSAs would be required to have
nonsource-oriented population monitors. We request comments on the
appropriateness of requiring nonsource-oriented monitors and the
proposed population threshold of 1,000,000 people for this requirement.
---------------------------------------------------------------------------
\164\ For the complete definition of CBSA refer to: http://www.census.gov/population/www/estimates/aboutmetro.html.
---------------------------------------------------------------------------
Lead concentrations near roadways are not well understood at this
time. The Pb critieria document discussed data for the South Coast Air
Quality Management District where a modeling effort suggested that Pb
deposited during the years when leaded gasoline was used could be a
significant portion of their ambient Pb inventory. However, this work
was conducted in an area of the country where quarterly average Pb-TSP
concentrations are considerably less than 0.1 [mu]g/m\3\. We analyzed
ambient air Pb concentrations near a number of large roadways (Cavender
2008). Based on this analysis it appears unlikely that roadways will
result in ambient Pb air concentrations in excess of the lowest Pb
NAAQS level being proposed in this action. In addition, members of the
CASAC AAMM Subcommittee agreed that a separate monitoring requirement
for roadways was unnecessary based on the results of this analysis. As
such, the proposed regulatory text does not include a requirement for
Pb monitoring near roadways. We do, however, propose to allow
monitoring near roadways to satisfy the requirements of the nonsource-
oriented monitoring requirement discussed above. For example, a
monitoring agency could place a monitor in a CBSA with a population
greater than one million and locate that monitor nearly adjacent to a
major roadway in a populated area. That monitor would satisfy the
nonsource-oriented requirements while also gathering data on possible
roadway exposure. We request comments on the need for monitoring near
roadways and the appropriateness of allowing near roadway monitoring to
be used to satisfy the requirement for nonsource-oriented monitoring.
Monitoring agencies would need to install new Pb monitoring sites
as a result of these proposed revisions to the Pb monitoring
requirements. We are estimating that the size of the required Pb
network will range from between approximately 160 and 500 sites,
depending on the level of the final standard. If the size of the final
network is on the order of 500 sites, we are proposing to allow
monitoring agencies to stagger the installation of newly required sites
over two years, with at least half the newly required Pb monitoring
sites being installed and operating by January 1, 2010 (16 months after
the court-ordered deadline for promulgation of the final Pb NAAQS
revision) and the remaining newly required monitoring sites installed
and operating by January 1, 2011. As proposed, monitors near the
highest Pb emitting sources would need to be installed in the first
year, with monitors near the lower Pb emitting sources and nonsource-
oriented monitors being installed in the second year. The annual
network plan due on July 1, 2009 would need to include the plan and
schedule for installation and operation of the newly required Pb
monitoring sites necessary to comply with these proposed requirements.
We are also proposing to allow monitoring agencies one year following
the release of updates to the NEI or an update to the census to add new
monitors if these updates would trigger new monitoring requirements.
Monitoring agencies would be required to identify and propose new Pb
monitoring sites as part of their annual network plan required under 40
CFR 58.10. We invite comments on the need for a staggered network
deployment.
The type of monitor that must be used at these required monitoring
sites will depend on whether for a final revised NAAQS based on Pb-TSP
scaled monitoring data for Pb-PM10 may be used as a
surrogate. If cross-use of data is permitted, then either type of
monitor could be used at a required monitoring site. EPA intends to
encourage a relatively small number of sites to operate both types of
monitors. The proposed appendix R (see section IV) explains how data
would be selected for purposes of NAAQS compliance determinations if
both types of monitors operate in the same month or quarter. One
approach on which EPA is seeking comment would be to change the Pb
indicator to Pb-PM10 and allow the use of Pb-TSP data only
for the purpose of initial designations. If this approach is adopted, a
Pb-TSP monitor could not be used in lieu of a Pb-PM10
sampler at a required monitoring site after the area containing the
monitoring site had received its initial designation (see section VI
for an explanation of the anticipated designation schedule).
If the final Pb standard is based on Pb-TSP, the July 1, 2009
monitoring plan would be required to designate which Pb-PM10
monitoring sites, if any, are source-oriented, so that this designation
can be available for public comment and can be reviewed by the EPA
Regional Administrator. This site designation information is needed to
determine scaling factors for the Pb concentration data from these Pb-
PM10 monitoring sites (see section IV). Sites that are
counted towards meeting the required number of source-oriented
monitoring sites should of course be designated as source-oriented. It
may be appropriate to designate other sites as source-oriented also.
Because sources may come and go, or be newly discovered, the revised 40
CFR 58.10 requires the monitoring agency to consider whether revisions
in site designations are needed as part of the preparation of each
year's monitoring plan.
C. Sampling Schedule
We are proposing to increase the sampling frequency if the final Pb
NAAQS is based on a monthly averaging form. Specifically, we are
proposing to increase the sampling frequency to require one 24-hour
sample taken every 3 days (referred to as ``1 in 3 day sampling'') if
the final Pb NAAQS is based on a monthly average. The remainder of this
section provides background, rationale, and details for the proposed
changes to the Pb sampling frequency.
1. Background
The current required sampling frequency requirement for Pb is one
24-hour sample every six days (40 CFR 58.12(b)). For the current form
of the NAAQS that is based on a quarterly average, the 1-in-6 day
sampling schedule yields 15 samples per quarter on average with 100%
completeness, or 12 samples with 75% completeness. A change to a
monthly averaging period would result in between 4 and 6 samples per
month at the current sampling frequency with 100% completeness, or
between 3 and 5 samples with 75% completeness.
In the ANPR, we indicated that if we changed the averaging time to
a monthly average, we would need to consider increasing the required
sampling frequency from 1-in-6 days since 3 to 5 samples would likely
not result in a reasonably confident estimate of the actual air quality
for the period. We suggested several alternatives which included
increasing the sampling frequency to 1-in-3 day, or increasing
[[Page 29265]]
the sampling frequency to 1-in-1 day sampling (i.e., every day
sampling). In addition, we suggested an option that relates sampling
frequency to recent ambient Pb-TSP concentrations, such that an
increased sampling frequency is required as the recent ambient Pb-TSP
concentration approaches the NAAQS level. In addition, we sought
comments on several practices that would help to reduce the burden
associated with more frequent sampling including:
Increasing sampling time duration (e.g., changing from a
24-hour sampling time duration to a 48-hour or 72-hour sampling time
duration),
Allowing for compositing of samples (i.e., extracting and
analyzing several sequential samples together), and
Allowing for multiple samplers at one site.
In CASAC's comments on the ANPR, they recommended increasing the
sampling frequency to 1-in-3 day sampling, or higher. They discouraged
increasing the sample duration and the allowance for compositing of
samples, as these practices would reduce the ability to use the samples
in source apportionment techniques that may be useful in identifying
what sources contributed to the ambient air Pb concentrations.
2. Proposed Changes
We propose increasing the sampling frequency to 1-in-3 day sampling
if we change the form of the revised NAAQS to a monthly average in the
final rule. A 1-in-3 day sampling frequency would yield 9 or 10 samples
per month on average at 100% completeness. At 75% completeness, a 1-in-
3 day sampling frequency would yield 7 or 8 samples per month at a
minimum.
We recognize that at concentrations considerably below the level of
the NAAQS there is less potential to misclassify an area due to the
error resulting from less than complete sampling. We believe it is
appropriate to allow for less frequent sampling in areas with low
ambient air Pb concentrations relative to the level of the NAAQS. As
such, we are proposing to allow monitoring agencies to request a
reduction in the sampling frequency to 1-in-6 day sampling if the most
recent 3-year design value is less than 70% of the NAAQS.
We request comment on the proposed change to 1-in-3 day sampling
and the proposed option to reduce sampling to 1-in-6 day sampling in
areas with low ambient Pb concentrations. We also seek comments on the
need to increase sampling frequency further to 1-in-1 day sampling in
areas with ambient air Pb concentrations near the level of the final
NAAQS.
We are currently assessing how different sampling schedules could
affect the confidence in the estimate of a mean monthly Pb
concentration as part of developing Data Quality Objectives (DQOs) for
Pb monitoring. This assessment will include evaluating temporal
variability at current Pb monitoring sites (both Pb-TSP and Pb-
PM10) in order to provide uncertainty estimates associated
with various sampling frequency scenarios. We will evaluate 1-in-1 day,
1-in-3 day, and 1-in-6 day sampling frequencies, at varying degrees of
completion between 50% and 100%, and for each we plan to estimate the
margin of error about a mean monthly estimate, focusing on sites
assumed to be close to the proposed NAAQS. Based upon this assessment,
expected to be complete in June of 2008, we will be able to better
understand the uncertainties around a monthly estimate. We will use
this better understanding and information provided in public comment to
choose the final sampling frequency requirements.
D. Monitoring for the Secondary NAAQS
We are not proposing additional monitoring requirements for the
secondary NAAQS because the proposed monitoring requirements for the
primary NAAQS will be sufficient to demonstrate compliance with the
secondary NAAQS. The remainder of this section provides background and
rationale on our decision to not propose additional monitoring
requirements for the secondary NAAQS.
1. Background
CASAC has recommended additional monitoring to gather information
to better inform consideration of the secondary NAAQS in the next and
future reviews. Specifically, CASAC stated that ``the EPA needs to
initiate new measurement activities in rural areas--which quantify and
track changes in lead concentrations in the ambient air, soils,
deposition, surface waters, sediments and biota, along with other
information as may be needed to calculate and apply a critical loads
approach for assessing environmental lead exposures and risks in the
next review cycle'' (Henderson, 2007b).
We currently monitor ambient Pb in PM2.5 (Pb-
PM2.5) as part of the Interagency Monitoring of Protected
Visual Environments (IMPROVE) network. There are 110 formally
designated IMPROVE sites located in or near national parks and other
Class I visibility areas, virtually all of these being rural.
Approximately 80 additional sites at various urban and rural locations,
requested and funded by various parties, are also informally treated as
part of the IMPROVE network. While we believe it is not appropriate to
rely on Pb-PM2.5 monitoring to demonstrate compliance with a
Pb-TSP NAAQS, we believe the Pb-PM2.5 measurements provided
by the IMPROVE network can be used as a useful indicator to temporal
and spatial patterns in ambient Pb concentrations and resulting Pb
deposition in rural areas that are not directly impacted by a nearby Pb
emission source. In the ANPR, we suggested it might be desirable to
augment the IMPROVE network with a small ``sentinel'' network of
collocated Pb-TSP monitors for a period of time in order to develop a
better understanding of how Pb-PM2.5 and Pb-TSP relate in
these rural areas. Alternatively, since it is likely that at rural
locations nearly all ambient Pb is in the less than 10 [mu]m size
range, we suggested it might be possible to analyze the IMPROVE
PM10 mass samples (which are already being collected) for Pb
for a period of time to develop a better understanding of how Pb-
PM2.5 and Pb-PM10 relate in these rural areas.
The National Water-Quality Assessment (NAWQA), conducted by the
United States Geological Survey, contains data on Pb concentrations in
surface water, bed sediment, and animal tissue for more than 50 river
basins and aquifers throughout the country (CD, AX7.2.2.2). NAWQA data
are collected during long-term, cyclical investigations wherein study
units undergo intensive sampling for 3 to 4 years, followed by low-
intensity monitoring and assessment of trends every 10 years.
Similarly, the USGS is collaborating with Canadian and Mexican
government agencies on a multi-national project called ``Geochemical
Landscapes'' that has as its long-term goal a soil geochemical survey
of North America (http://minerals.cr.usgs.gov/projects/geochemical_landscapes/index.html). The Geochemical Landscapes project has the
potential to fill the need for periodic Pb soil sampling. We note the
value of the NAWQA and Geochemical Landscapes data in the assessment of
trends in Pb concentrations in both soil and aquatic systems, and
support the continued collection of this data by the USGS.
2. Proposed Changes
As discussed in Section III of this preamble, we are proposing to
set the secondary NAAQS equal to the primary NAAQS. Based on our
analysis of the
[[Page 29266]]
existing ambient Pb monitoring data (EPA 2007c), we do not expect there
to be ambient air concentrations in excess of the proposed secondary
NAAQS in rural areas that are not associated with a Pb emission source.
As noted earlier in this section, we are proposing Pb surveillance
monitoring requirements for Pb sources to demonstrate compliance with
the primary NAAQS that will also be sufficient to determine compliance
with the secondary NAAQS.
The Pb-PM2.5 data collected as part of the IMPROVE
program provides useful information on Pb concentrations in rural areas
that can be used to track trends in ambient air Pb concentrations in
rural areas and important ecosystems. These data are available through
the VIEWS Web portal (http://vista.cira.colostate.edu/views/) and are
also reported to AQS. While collection of a limited amount of
collocated Pb-TSP or Pb-PM10 would be useful in
understanding the relationship between Pb-PM2.5 and Pb-TSP
(or Pb-PM10) in rural areas, we do not believe it is
appropriate to establish a regulatory requirement for the collection of
these data. Rather, we believe it is more appropriate to work with the
monitoring agencies responsible for IMPROVE monitoring to encourage the
collection of a limited amount of collocated Pb data from
PM10 or TSP samplers. We seek comments on our decision to
not require additional monitoring requirements for the proposed
secondary Pb NAAQS.
E. Other Monitoring Regulation Changes
We are proposing to make two other minor changes to various aspects
of the Pb monitoring regulations to make them consistent with the
proposed NAAQS. The remainder of this section discusses the proposed
changes.
1. Reporting of Average Pressure and Temperature
The high-volume FRM for Pb-TSP monitoring is based on standard
pressure and temperature (25 degrees C, and 760 mmHg). We are not
proposing to change this. As discussed in section II.E of this
preamble, we are proposing to adopt a new FRM for low-volume Pb-
PM10 monitoring with concentration reporting based on local
temperature and pressure. We are proposing to specify reporting based
on local temperature and pressure because the actual concentration of
Pb in the atmosphere is a better indicator of the potential for
deposition than the concentration based on standard pressure and
temperature. In addition, there are practical advantages to moving to
local conditions since both PM2.5 and PM10-2.5
are also based on local conditions. We are proposing to revise 40 CFR
58.16(a) to add a requirement that the monitoring agency report the
average pressure and temperature during the time of sampling for both
Pb-TSP monitoring and Pb-PM10 monitoring, consistent with
the requirements for such reporting contained in the PM2.5
and PM10-2.5 FRMs. For low-volume Pb-PM10
monitors, this requirement is easily met because the monitors
incorporate temperature and pressure sensors and the monitor software
makes reporting these parameters automatic. High-volume TSP samplers do
not incorporate these sensors, so more effort may be needed to report
the data. We note that sampler-generated average daily temperature and
pressure are already required to be reported to AQS from filter-based
PM2.5 FRM/FEM samplers, and that the current submission of
these data would fulfill the temperature and pressure reporting
requirements for any Pb-TSP sampling at the same site. Relevant
measurements could also be obtained from nearby National Weather System
(NWS) monitoring sites, nearby low-volume PM2.5 or
PM10 samplers, and other nearby meteorological measurements
that undergo routine quality control checks and quality assurance;
relying on one of these sources would mean that a separate data
submission action would be needed to associate the data with the Pb-TSP
monitoring site. The reporting of average pressure and temperature data
would support the ability to investigate data quality and other data
analysis questions that may be arise with regard to the Pb-TSP or Pb-
PM10 monitors.
We seek comment on the requirement to report the average
temperature and pressure recorded during Pb measurements and the
usefulness of such data in supporting data analysis purposes.
2. Special Purpose Monitoring Exemption
According to 40 CFR 58.20(e) ``If an SPM using an FRM, FEM, or ARM
is discontinued within 24 months of start-up, the Administrator will
not designate an area as nonattainment for the CO, SO2,
NO2, Pb, or 24-hour PM10 NAAQS solely on the
basis of data from the SPM. Such data are eligible for use in
determinations of whether a nonattainment area has attained one of
these NAAQS.'' When this provision was added in the October 2006
revisions to the ambient monitoring regulations, we stated that the
basis for finalizing a prohibition on the use of SPM data to designate
an area as nonattainment for Pb (as well as CO, SO2,
NO2, and PM10) was EPA's discretion to not make a
finding of nonattainment even though a SPM indicated a violation of the
relevant NAAQS (see 71 FR 61252). We stated that even though the NAAQS
for these pollutants have forms that allow a nonattainment finding
based on less than 24 months of data, EPA does not have a mandatory
duty to make nonattainment redesignations until such time as the NAAQS
are revised. Since EPA is proposing to revise the Pb NAAQS, and the
form of the proposed NAAQS would allow a nonattainment finding to be
based on only 1 or 2 years of data, and such a NAAQS revision must be
followed by a mandatory round of designations, we are proposing to
revise 40 CFR Section 58.20(e) by removing the specific reference to Pb
in the rule language.
VI. Implementation Considerations
This section of the proposal discusses the specific CAA
requirements that must be addressed when implementing any new or
revised Pb NAAQS based on the structure outlined in the CAA, existing
rules, existing guidance, and in some cases proposed revised guidance.
We intend the preamble to the final rule revising the Pb NAAQS to
provide EPA's final implementation guidance.
The CAA assigns important roles to EPA, states, and Tribal
governments in implementing NAAQS. States have the primary
responsibility for developing and implementing State Implementation
Plans (SIPs) that contain state measures necessary to achieve the air
quality standards in each area. EPA provides assistance to states and
Tribes by providing technical tools, assistance, and guidance,
including information on the potential control measures.
A SIP is the compilation of regulations and control programs that a
state uses to carry out its responsibilities under the CAA, including
the attainment, maintenance, and enforcement of the NAAQS. States use
the SIP development process to identify the emissions sources that
contribute to the nonattainment problem in a particular area, and to
select the emissions reduction measures most appropriate for the
particular area in question. Under the CAA, SIPs must ensure that areas
reach attainment as expeditiously as practicable.
Currently only two areas in the United States are designated as
nonattainment and eleven areas are designated as maintenance areas for
the current Pb NAAQS. If the Pb NAAQS is lowered to the range proposed,
it is likely (based on a review of the current air quality monitoring
data) that
[[Page 29267]]
additional areas would be designated as nonattainment. States
determined to have lead nonattainment areas would be required to submit
SIPs that identify and implement specific air pollution control
measures to reduce the ambient concentrations of lead to meet the
NAAQS.
The EPA's analysis of the available Pb monitoring data suggests
that a large majority of recent exceedances of Pb levels in the range
of 0.10 to [mu]g/m\3\ have occurred in locations with active or retired
industrial sources of Pb. Accordingly, if this pattern also prevails
for concentrations observed from new monitoring sites, many states may
be able to attain the revised NAAQS by implementing air pollution
control measures on lead emitting industrial sources only. These
controls could include measures such as fabric filter particulate
matter control measures and industrial fugitive dust control measures
applied in plant buildings and on plant grounds. However, it may become
necessary in some areas to also implement controls on non-industrial
sources. Based on these considerations, EPA believes that some of the
regulations and guidance being used to implement the current Pb NAAQS
is still appropriate to implement any of the options being proposed in
this rulemaking for a new or revised Pb NAAQS.
The regulations and guidance for implementing the current NAAQS for
Pb are mainly provided in the following documents: (1) ``State
Implementation Plans; General Preamble for the Implementation of Title
I of the Clean Air Act Amendments of 1990'', 57 FR 13549, April 16,
1992, (2) ``State Implementation Plans for Lead Nonattainment Areas;
Addendum to the General Preamble for the Implementation of Title I of
the Clean Air Act Amendments of 1990'', 58 FR 67748, December 22, 1993,
and (3) regulations at 40 CFR 51.117. The aforementioned documents
address requirements such as designating areas, setting nonattainment
area boundaries, promulgating area classifications, nonattainment area
SIP requirements such as Reasonably Available Control Measures (RACM),
Reasonably Available Control Technology (RACT), New Source Review
(NSR), Prevention of Significant Deterioration (PSD), and emissions
inventory requirements. We have summarized the most relevant
information from these documents below for your convenience. The EPA
believes that there is sufficient guidance and regulations to fully
implement the proposed revised Pb NAAQS, although EPA may review and
revise or update as necessary, policies, guidance, and regulations for
implementing the Pb NAAQS in the future. The EPA solicits comment on
whether additional guidance is necessary for implementation of the
revised Pb NAAQS.
A. Designations for the Lead NAAQS
After EPA establishes or revises a NAAQS, the CAA requires EPA and
the states to begin taking steps to ensure that the new or revised
NAAQS are met. The first step is to identify areas of the country that
do not meet the new or revised NAAQS. The CAA defines EPA's authority
to designate areas that do not meet a new or revised NAAQS. Section
107(d)(1) provides that ``By such date as the Administrator may
reasonably require, but not later than 1 year after promulgation of a
new or revised NAAQS for any pollutant under section 109, the Governor
of each state shall * * * submit to the Administrator a list of all
areas (or portions thereof) in the state'' that designates those areas
as nonattainment, attainment, or unclassifiable. Section
107(d)(1)(B)(i) further provides, ``Upon promulgation or revision of a
NAAQS, the Administrator shall promulgate the designations of all areas
(or portions thereof) * * * as expeditiously as practicable, but in no
case later than 2 years from the date of promulgation. Such period may
be extended for up to one year in the event the Administrator has
insufficient information to promulgate the designations.'' The term
``promulgation'' has been interpreted by the courts to be signature and
dissemination of a rule.\165\ By no later than 120 days prior to
promulgating final designations, EPA is required to notify states or
Tribes of any intended modifications to their boundaries as EPA may
deem necessary. States and Tribes then have an opportunity to comment
on EPA's tentative decision. Whether or not a state or a Tribe provides
a recommendation, EPA must promulgate the designation that it deems
appropriate.
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\165\ American Petroleum Institute v. Costle, 609 F.2d 20 (D.C.
Cir. 1979).
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Accordingly, Governors of states and Tribal leaders will be
required to submit their initial designation recommendations to EPA no
later than September 2009. The initial designation of areas for any new
or revised NAAQS for lead must occur no later than September 2010,
although that date may be extended by up to one year under the CAA (or
no later than September 2011) if EPA has insufficient information to
promulgate the designations. As discussed below, EPA is anticipating a
designations schedule that provides the full 3 years allowed under the
CAA, and is taking comment on issues related to the anticipated
designation schedule.
1. Potential Schedule for Initial Designations of a Revised Lead NAAQS
As stated previously, section 107(d)(1)(B)(i) requires EPA to
promulgate initial designations for all areas of the country for any
new or revised NAAQS, as expeditiously as practicable, but in no case
later than 3 years from the date of promulgation of the new or revised
NAAQS. Two key considerations in establishing a schedule for
designating areas are: (1) The advantages of promulgating all
designations at the same time; and (2) the availability of a monitoring
network and sufficient monitoring data to identify areas that may be
violating the NAAQS.
EPA continues to believe, consistent with its past practice, that
there are important advantages to promulgating designations for all
areas at the same time. This practice provides helpful uniformity for
the deadlines for SIP submissions and attainment. Moreover, since a key
question for the designation process is delineating the boundaries of
nonattainment areas, establishing appropriate nonattainment boundaries
in a two-stage process is likely to generate significant issues. Thus,
EPA intends to promulgate designations for all areas at the same time.
As discussed in section V.B, the existing Pb monitoring network is
not adequate to evaluate attainment of the proposed revised Pb NAAQS at
locations consistent with EPA's proposed new network siting criteria
and data collection requirements. These new requirements would result
in a more strategically targeted network that would begin to be in
operation by January 1, 2010. Thus, taking the additional year provided
under section 107(d)(1)(B)(1) of the CAA (which would allow up to 3
years to promulgate designations following the promulgate of a new
NAAQS) would allow the first year of data from this network to be
available. The EPA believes that, due to the updated network design
requirements, this additional data would be of significant benefit for
designating areas for a new NAAQS. If EPA completes the initial
designations within 2 years of new NAAQS promulgation, it is likely
that large areas of the country will be designated ``unclassifiable''
because the monitoring network will not be sufficient to make clear
decisions. Even if EPA takes an extra year for final initial
designation
[[Page 29268]]
decisions we recognize that some areas may still have to be designated
as unclassifiable or attainment/unclassifiable because of the lack of a
sufficient record of FRM (FEM) monitoring data.\166\ If sufficient
monitoring data become available for ``unclassifiable'' areas
subsequent to the time EPA finalizes initial designations, EPA may use
the discretion provided to the Administrator under the CAA pursuant to
section 107(d)(3) to revise the initial designations for these areas.
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\166\ As discussed in Section IV of this notice, EPA is
soliciting comment on the use of Pb-TSP monitoring data, with or
without a scaling factor, as a surrogate for Pb-PM10 data
where Pb-PM10 data are not available, particularly for
initial designations.
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Under the initial designation schedule described above, states (and
Tribes) would be required to submit designation recommendations to EPA
no later than September 2009 (i.e., one year following promulgation of
a new NAAQS). States will be able to consider ambient data collected
with FRM (FEM) samplers through the end of 2008 and part way through
2009 when formulating their recommendations. As stated previously, by
no later than 120 days prior to promulgating designations, EPA is
required to notify states or Tribes of any intended modifications to
their recommended boundaries as EPA may deem necessary. This would
occur no later than in May 2011. If EPA promulgates designations in
September 2011, EPA will have access to Pb air quality data from 2010
which state monitoring officials have certified is complete and
accurate, since the deadline for such certification is May 1, 2011.
Under this schedule, EPA would consider data from calendar years 2008-
2010 in formulating its proposed revisions, if any, to the designations
recommended by states and Tribes. States and Tribes will then have an
opportunity to comment on EPA's proposed modifications
As described above, EPA is currently anticipating that there will
be insufficient information to promulgate designations in 2010. The EPA
is soliciting comment on whether we have the authority to determine in
the final rule that three years are necessary to promulgate
designations based on the availability of appropriate information. EPA
is also soliciting comment on whether designations should be made
within the 2 year period provided under section 107(d)(1)(B)(i)
utilizing all data available by that time.
2. Ambient Data For Designations
The proposed alternative forms of the NAAQS, maximum quarterly
average concentration over three years and second maximum monthly
concentration over three years, would both allow a nonattainment
determination based on less than three years of data, if the monitoring
data in a more limited time period includes a quarterly average above
the level of the NAAQS or if it includes two monthly averages above the
level of the NAAQS. In such a case, EPA intends to designate the
affected area nonattainment even though less than three years of data
are available. EPA would designate an area attainment only if three
calendar years of data indicate the absence of a violation. As stated
above, EPA anticipates that some areas will have to be designated as
unclassifiable. If sufficient monitoring data become available for
``unclassifiable'' areas subsequent to the time EPA finalizes initial
designations, EPA may use the discretion provided to the Administrator
under the CAA pursuant to section 107(d)(3) to revise the initial
designations for these areas.
B. Lead Nonattainment Area Boundaries
As stated previously, the process for initially designating areas
following the promulgation of a new NAAQS is prescribed in section
107(d)(1) of the CAA. This section of the CAA provides each state
Governor an opportunity to recommend initial designations of
attainment, nonattainment, or unclassifiable for each area in the
state. Section 107(d)(1) of the CAA also directs the state to provide
the appropriate boundaries to EPA for each area of the state, and
provides that EPA may make modifications to the boundaries submitted by
the state as it deems necessary. A lead nonattainment area must consist
of that area that does not meet (or contributes to ambient air quality
in a nearby area that does not meet) the Pb NAAQS. Thus, a key factor
in setting boundaries for nonattainment areas is determining the
geographic extent of nearby source areas contributing to the
nonattainment problem. For each monitor or group of monitors that
exceed a standard, nonattainment boundaries must be set that include a
sufficiently large enough area to include both the area judged to be
violating the standard as well as the source areas that are determined
to be contributing to these violations.
Historically, Pb NAAQS violations have been the result of lead
emissions from large stationary sources and mobile sources that burn
lead-based fuels. In some locations, a limited number of area sources
have also contributed to violations. Since lead has been successfully
phased out of motor vehicle gasoline, mobile sources are no longer a
significant source of violations of the current Pb NAAQS. At the
current standard level, EPA expects stationary sources to be the
primary contributor to violations of the NAAQS. At the lower standard
levels contemplated in this proposal, it is possible that fugitive dust
emissions from area sources containing deposited lead will also
contribute to violations of a revised Pb NAAQS. The location and
dispersion characteristics of these sources of ambient lead
concentrations are important factors in determining nonattainment area
boundaries. The EPA is proposing that the county boundary be the
presumptive boundary for lead nonattainment areas. However, we are also
taking comment on whether urban-based Metropolitan Statistical Area
(MSA) boundaries should be the presumptive boundaries for lead
nonattainment areas.
The EPA is proposing to presumptively define the boundary for
designating a nonattainment area as the perimeter of the county
associated with the air quality monitor(s) which records a violation of
the standard. This presumption is the existing EPA recommendation for
defining the nonattainment boundaries for the current Pb NAAQS, and is
described in the 1992 General Preamble (57 FR 13549). The EPA is also
taking comment on an option to presumptively define the nonattainment
boundary using the OMB-defined Metropolitan Statistical Area (MSA)
associated with the violating monitor(s). This presumption is used, by
CAA requirement, for the ozone and CO NAAQS nonattainment boundaries,
and was recommended by EPA as the appropriate presumption for the 1997
PM2.5 NAAQS nonattainment boundaries. Under either option,
the state and/or EPA may conduct additional area-specific analyses that
could lead EPA to depart from the presumptive boundary. Factors
relevant to such an analysis are described below.
1. County-Based Boundaries
The option being proposed by EPA is that lead nonattainment
boundaries would be presumptively defined by the perimeter of the
county in which the ambient lead monitor(s) recording a violation of
the NAAQS is located, unless area-specific information indicates that
some other boundary is more appropriate. In addition, if the relevant
air quality monitor measuring a violation(s) is located near another
county, then EPA would presume that the contributing county should also
be designated as nonattainment for the Pb NAAQS. In some instances, a
boundary other than the county perimeter, that
[[Page 29269]]
addresses areas impacted by specific sources of lead, may also be
appropriate.
For the new proposed Pb NAAQS, EPA is recommending that
nonattainment area boundaries that deviate from presumptive county
boundaries should be supported by an assessment of several factors,
which are discussed below. The factors for determining nonattainment
area boundaries for the Pb NAAQS under this recommendation closely
resemble the factors identified in recent EPA guidance for the 1997 8-
hour ozone NAAQS, the 1997 PM2.5 NAAQS, and the 2006
PM2.5 NAAQS nonattainment area boundaries. EPA intends to
apply these factors in evaluating boundary modifications. For this
particular option, EPA would consider the following factors in
assessing whether to exclude portions of a county and whether to
include additional nearby areas outside the county as part of the
designated nonattainment area:
Emissions in areas potentially included versus excluded
from the nonattainment area,
Air quality in potentially included versus excluded areas,
Population density and degree of urbanization including
commercial development in included versus excluded areas,
Expected growth (including extent, pattern and rate of
growth),
Meteorology (weather/transport patterns),
Geography/topography (mountain ranges or other air basin
boundaries),
Jurisdictional boundaries (e.g., counties, air districts,
Reservations, etc.),
Level of control of emission sources.
Analyses of these factors may suggest nonattainment boundaries that
are either larger or smaller than the county. A demonstration
supporting the designation of boundaries that are less than the full
county must show both that violation(s) are not occurring in the
excluded portions of the county and that the excluded portions are not
source areas that contribute to the observed violations.
Recommendations to designate a nonattainment area larger than the
county should also be based on an analysis of these factors. EPA will
consider these factors in evaluating state and tribal recommendations
and assessing whether any modifications are appropriate.
Under previous Pb implementation guidance, EPA advised that
Governors could choose to recommend lead nonattainment boundaries by
using any one, or a combination of the following techniques, the
results of which EPA would consider when making a decision as to
whether and how to modify the Governors' recommendations: (1)
Qualitative analysis, (2) spatial interpolation of air quality
monitoring data, or (3) air quality simulation by dispersion modeling.
These techniques are more fully described in ``Procedures for
Estimating Probability of Nonattainment of a PM10 NAAQS
Using Total Suspended Particulate or PM10 Data,'' December
1986 (see 57 FR 13549).
EPA solicits comments on the use of these factors and modeling
techniques, and other approaches, for adjusting county boundaries in
designating nonattainment areas.
2. MSA-Based Boundaries
The EPA is also taking comment on the alternative that lead
nonattainment boundaries should be presumptively defined by the
perimeter of a metropolitan area as defined by OMB's Metropolitan
Statistical Areas (MSAs), or appropriate divisions thereof, within
which a violating monitor(s) is located. The Metropolitan Statistical
Area, as delineated by the Office of Management and Budget (OMB),
provides a presumptive definition of the populated area associated with
a core urban area. Accordingly, EPA is taking comment on the
alternative option that the Metropolitan Statistical Area would provide
the presumptive definition of the source area that contributes to a
lead nonattainment problem. This presumption would take the view that,
in the absence of evidence to the contrary, violations of the Pb NAAQS
in urban-oriented areas may be presumed attributable, at least in part,
to contributions from large sources of lead emissions distributed
throughout the Metropolitan Area. The last revision to the OMB listing
of MSAs was published November 20, 2007. As in the EPA's preferred
proposed option, EPA would consider state, local, and tribal
recommendations of nonattainment area boundaries based on the same set
of factors listed in the previous subsection.
As stated previously, EPA is proposing that the county boundaries
be used as the presumptive boundaries for any new or revised Pb NAAQS,
but is also requesting comments the MSA boundaries being used as the
presumptive boundaries for any new or revised Pb NAAQS.
C. Classifications
Section 172(a)(1)(A) of the CAA authorizes EPA to classify areas
designated as nonattainment for the purposes of applying an attainment
date pursuant to section 172(a)(2), or for other reasons. In
determining the appropriate classification, EPA may consider such
factors as the severity of the nonattainment problem and the
availability and feasibility of pollution control measures (see section
172(a)(1)(A) of the CAA). The EPA may classify lead nonattainment
areas, but is not required to do so.
While section 172(a)(1)(A) provides a mechanism to classify
nonattainment areas, section 172(a)(2)(D) provides that the attainment
date extensions described in section 172(a)(2)(A) do not apply to
nonatainment areas having specific attainment dates that are addressed
under other provisions of the part D of the CAA. Section 192(a), of
part D, specifically provides an attainment date for areas designated
as nonattainment for the Pb NAAQS. Therefore, EPA has legal authority
to classify lead nonattainment areas, but the 5 year attainment date
under section 192(a) cannot be extended pursuant to section
172(a)(2)(D).
Based on this limitation, EPA is proposing not to establish
classifications within the 5 year interval for attaining any new or
revised NAAQS. This approach is consistent with EPA's previous
classification decision in the 1992 General Preamble (See 57 FR 13549,
April 16, 1992).
D. Section 110(a)(2) Lead NAAQS Infrastructure Requirements
Under section 110(a)(1) and (2) of the CAA, all states are required
to submit plans to provide for the implementation, maintenance, and
enforcement of any new or revised NAAQS. Section 110(a)(1) and (2)
require states to address basic program elements, including
requirements for emissions inventories, monitoring, and modeling, among
other things. States are required to submit SIPs to EPA demonstrating
these basic program elements within 3 years of the promulgation of any
new or revised NAAQS. Subsections (A) through (M), of section
110(a)(2), set forth the elements that a state's program must contain
in their SIP. The list below identifies the required program elements
contained in section 110(a)(2).\167\ The list of section 110(a)(2)
[[Page 29270]]
NAAQS implementation requirements are the following:
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\167\ Two elements identified in section 110(a)(2) are not
listed below because, as EPA interprets the CAA, SIPs incorporating
any necessary local nonattainment area controls would not be due
within 3 years, but rather are due at the time the nonattainment
area planning requirements are due. The elements are: (1) Emission
limits and other control measures, section 110(a)(2)(A), and (2)
Provisions for meeting part D, section 110(a)(2)(I), which requires
areas designated as nonattainment to meet the applicable
nonattainment planning requirements of part D, title I of the CAA.
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Ambient air quality monitoring/data system: Section
110(a)(2)(B) requires SIPs to provide for setting up and operating
ambient air quality monitors, collecting and analyzing data and making
these data available to EPA upon request.
Program for enforcement of control measures: Section
110(a)(2)(C) requires SIPs to include a program providing for
enforcement of measures and regulation of new/modified (permitted)
sources.
Interstate transport: Section 110(a)(2)(D) requires SIPs
to include provisions prohibiting any source or other type of emissions
activity in one State from contributing significantly to nonattainment
in another State or from interfering with measures required to prevent
significant deterioration of air quality or to protect visibility.
Adequate resources: Section 110(a)(2)(E) requires States
to provide adequate funding, personnel and legal authority for
implementation of their SIPs.
Stationary source monitoring system: Section 110(a)(2)(F)
requires States to establish a system to monitor emissions from
stationary sources and to submit periodic emissions reports to EPA.
Emergency power: Section 110(a)(2)(G) requires States to
provide for authority to implement the emergency episode provisions in
their SIPs.
Provisions for SIP revision due to NAAQS changes or
findings of inadequacies: Section 110(a)(2)(H) requires States to
revise their SIPs in response to changes in the NAAQS, availability of
improved methods for attaining the NAAQS, or in response to an EPA
finding that the SIP is inadequate.
Section 121 consultation with local and Federal government
officials: Section 110(a)(2)(J) requires States to meet applicable
local and Federal government consultation requirements of section 121.
Section 127 public notification of NAAQS exceedances:
Section 110(a)(2)(J) requires States to meet applicable requirements of
section 127 relating to public notification of violating NAAQS.
PSD and visibility protection: Section 110(a)(2)(J) also
requires States to meet applicable requirements of title I part C
related to prevention of significant deterioration and visibility
protection.
Air quality modeling/data: Section 110(a)(2)(K) requires
that SIPs provide for performing air quality modeling for predicting
effects on air quality of emissions from any NAAQS pollutant and
submission of data to EPA upon request.
Permitting fees: Section 110(a)(2)(L) requires the SIP to
include requirements for each major stationary source to pay permitting
fees to cover the cost of reviewing, approving, implementing and
enforcing a permit.
Consultation/participation by affected local government:
Section 110(a)(2)(M) requires States to provide for consultation and
participation by local political subdivisions affected by the SIP.
E. Attainment Dates
Generally, the date by which an area is required to attain the Pb
NAAQS is determined by the effective date of the nonattainment
designation for the area. For areas designated nonattainment for any
new or revised primary Pb NAAQS, SIPs must provide for attainment of
the NAAQS as expeditiously as practicable, but no later than 5 years
from the date of the nonattainment designation for the area (see
section 192(a) of the CAA). So, for example, if final designations are
effective in Fall 2011, then nonattainment areas must plan to attain
the NAAQS by no later than Fall 2016. For an area with an attainment
date of September 2016, EPA would determine whether it had attained the
Pb NAAQS by evaluating air quality monitoring data from the 1, 2, or 3
previous calendar years (i.e., 2013, 2014, and 2015) as available.
F. Attainment Planning Requirements
Any state containing an area designated as nonattainment with
respect to the Pb NAAQS must develop for submission, a SIP meeting the
requirements of part D, Title I, of the CAA, providing for attainment
(see sections 191(a) and 192(a) of the CAA). As indicated in section
191(a) all components of the lead part D SIP must be submitted within
18 months of an areas nonattainment designation. So, for example, if
final designations are effective in Fall 2011, the part D SIPs must be
submitted by Spring 2013. Additional specific plan requirements for
lead nonattainment areas are outlined in 40 CFR 51.117.
The general part D nonattainment plan requirements are set forth in
section 172 of the CAA. Section 172(c) specifies that SIPs submitted to
meet the part D requirements must, among other things, include
Reasonably Available Control Measures (RACM) (which includes Reasonably
Available Control Technology (RACT)), provide for Reasonable Further
Progress (RFP), include an emissions inventory, require permits for the
construction and operation of major new or modified stationary sources
(see also section 173), contain contingency measures, and meet the
applicable provisions of section 110(a)(2) of the CAA related to the
general implementation of a new or revised NAAQS. It is important to
note that lead nonattainment SIPs must meet all of the requirements
related to part D of the CAA, including those specified in section
172(c), even if EPA does not provide separate specific guidance for
each provision (e.g., applicable provisions of section 110(a)(2)).
1. RACM for Lead Nonattainment Areas
Lead nonattainment area SIPs must contain RACM (including RACT)
that addresses sources of ambient lead concentrations. In general, as
stated previously, EPA believes that lead nonattainment area issues are
usually attributed to emissions from stationary sources, but some
emissions may also be attributed to smaller area sources. As a general
rule, the stationary sources in lead nonattainment areas tend to emit a
relatively large amount of particulate matter containing lead. In EPA's
2002 National Emissions Inventory (NEI), there were 29 stationary
sources in the country with lead emissions over 5 tons per year, and
239 sources over 1 ton of lead emissions per year.
At primary lead smelters, for example, the process of reducing
concentrated ore to lead involves a series of steps, some of which are
completed outside of buildings, or inside of buildings which are not
totally enclosed. Over a period of time, emissions from these sources
have been deposited in neighboring communities (e.g., on roadways,
parking lots, yards, and off-plant property). This historically
deposited lead, when disturbed, may be re-entrained into the ambient
air and re-entrained fugitive lead bearing dust may contribute to
violations of the Pb NAAQS in the affected area. There are also
potential sources of lead that are under federal control. As a part of
the Regulatory Impact Analysis for this rule, the EPA is reviewing the
impact of these and other sources of lead emissions to assess their
impact on any new or revised Pb NAAQS. States must also meet the
requirements outlined in 40 CFR 51.117(a) related to control strategy
demonstrations.
The first step in addressing RACM for lead is identifying potential
control measures for sources of lead in the nonattainment area. A
suggested starting point for specifying RACM in lead nonattainment area
SIPs is outlined in appendix 1 of the guidance entitled
[[Page 29271]]
``State Implementation Plans for Lead Nonattainment Areas; Addendum to
the General Preamble for the Implementation of Title I of the Clean Air
Act Amendments of 1990, 58 FR 67752, December 22, 1993. If a state
receives substantive public comments that demonstrate through
appropriate documentation, that additional control measures may be
reasonably available in a particular circumstance for an area, those
measures should be added to the list of available measures for
consideration in that particular area.
While EPA does not presume that these control measures are
reasonably available in all areas, a reasoned justification for
rejection of any available control measure should be prepared. If it
can be shown that measures, considered both individually and as a
group, are unreasonable because emissions from the affected sources are
insignificant, those measures may be excluded from further
consideration as they would not be representative of RACM for an area.
The resulting control measures should then be evaluated for
reasonableness, considering their technological feasibility and the
cost of control in the area for which the SIP applies. In the case of
public sector sources and control measures, this evaluation should
consider the impact and reasonableness of the measures on the
municipal, or other governmental entity that must assume the
responsibility for their implementation. It is important to note that a
state should consider the feasibility of implementing measures in part
when full implementation would be infeasible. A reasoned justification
for partial or full rejection of any available control measure,
including those considered or presented during the state's public
hearing process, should be prepared. The justification should contain
an explanation, with appropriate documentation, as to why each rejected
control measure is deemed infeasible or otherwise unreasonable for
implementation.
Economic feasibility considers the cost of reducing emissions and
the difference between the cost of the emissions reduction approach at
the particular source in question and the costs of emissions reduction
approaches that have been implemented at other similar sources. Absent
other indications, EPA presumes that it is reasonable for similar
sources to bear similar costs of emissions reduction. Economic
feasibility for RACT purposes is largely determined by evidence that
other sources in a source category have in fact applied the control
technology or process change in question. EPA also encourages the
development of innovative measures not previously employed which may
also be technically and economically feasible.
The capital costs, annualized costs, and cost effectiveness of an
emissions reduction technology should be considered in determining
whether a potential control measure is reasonable for an area or state.
One available reference for calculating costs is the EPA Air Pollution
Control Cost Manual,\168\ which describes the procedures EPA uses for
determining these costs for stationary sources. The above costs should
be determined for all technologically feasible emission reduction
options. States may give substantial weight to cost effectiveness in
evaluating the economic feasibility of an emission reduction
technology. The cost effectiveness of a technology is its annualized
cost ($/year) divided by the emissions reduced (i.e., tons/year) which
yields a cost per amount of emission reduction ($/ton). Cost
effectiveness provides a value for each emission reduction option that
is comparable with other options and other facilities. With respect to
a given pollutant, a measure is likely to be reasonable if it has a
cost per ton similar to other measures previously employed for that
pollutant. In addition, a measure is likely to be reasonable from a
cost effectiveness standpoint if it has a cost per ton similar to that
of other measures needed to achieve expeditious attainment in the area
within the CAA's time frames.
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\168\ EPA Air Pollution Control Cost Manual--Sixth Edition (EPA
452/B-02-001), EPA Office of Air Quality Planning and Standards,
Research Triangle Park, NC, Jan 2002.
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The fact that a measure has been adopted or is in the process of
being adopted by other states is an indicator (though not a definitive
one) that the measure may be technically and economically feasible for
another state. We anticipate that states may decide upon RACT and RACM
controls that differ from state to state, based on the state's
determination of the most effective strategies given the relevant
mixture of sources and potential controls in the relevant nonattainment
areas, and differences in difficulty of attaining expeditiously.
Nevertheless, states should consider and address RACT and RACM measures
developed for other areas or other states as part of a well reasoned
RACT and RACM analysis. The EPA's own evaluation of SIPs for compliance
with the RACT and RACM requirements will include comparison of measures
considered or adopted by other states.
In considering what level of control is reasonable, EPA is not
proposing a specific dollar per ton cost threshold for RACT. Areas with
more serious air quality problems typically will need to obtain greater
levels of emissions reductions from local sources than areas with less
serious problems, and it would be expected that their residents could
realize greater public health benefits from attaining the standard. For
these reasons, we believe that it will be reasonable and appropriate
for areas with more serious air quality problems and higher design
values to impose emission reduction requirements with generally higher
costs per ton of reduced emissions than the cost of emissions
reductions in areas with lower design values. In addition, where
essential reductions are more difficult to achieve (e.g., because many
sources are already controlled), the cost per ton of control may
necessarily be higher.
The EPA believes that in determining appropriate emission control
levels, the state should consider the collective public health benefits
that can be realized in the area due to projected improvements in air
quality. Because EPA believes that RACT requirements will be met where
the state demonstrates timely attainment, and areas with more severe
air quality problems typically will need to adopt more stringent
controls, RACT level controls in such areas will require controls at
higher cost effectiveness levels ($/ton) than areas with less severe
air quality problems.
In identifying the range of costs per ton that are reasonable,
information on benefits per ton of emission reduction can be useful as
one factor to consider. The Pb NAAQS RIA will provide information on
the estimated benefits per ton of reducing Pb emissions from various
emissions sources. It should be noted that such benefits estimates are
subject to significant uncertainty, and that benefits per ton vary in
different areas. Nonetheless this information could be used in a way
that recognizes these uncertainties. If a per ton cost of implementing
a measure is significantly less than the anticipated benefits per ton,
this would be an indicator that the cost per ton is reasonable. If a
source contends that a source-specific RACT level should be established
because it cannot afford the technology that appears to be RACT for
other sources in its source category, the source should support its
claim by providing detailed and verified information regarding the
impact of imposing RACT on:
Fixed and variable production costs ($/unit),
[[Page 29272]]
Product supply and demand elasticity,
Product prices (cost absorption vs. cost pass-through),
Expected costs incurred by competitors,
Company profits, and
Employment costs.
The technical guidance entitled ``Fugitive Dust Background Document
and Technical Information Document for Best Available Control
Measures'' (EPA-450/2-92-004, September 1992) provides an example for
states on how to analyze control costs for a given area.
Once the process of determining RACM for an area is completed, the
individual measures should then be converted into a legally enforceable
vehicle (e.g., a regulation or permit program) (see section 172(c)(6)
and section 110(a)(2)(A) of the CAA). The regulations or other measures
submitted should meet EPA's criteria regarding the enforceability of
SIPs and SIP revisions. These criteria were stated in a September 23,
1987 memorandum (with attachments) from J. Craig Potter, Assistant
Administrator for Air and Radiation; Thomas L. Adams, Jr. Assistant
Administrator for Enforcement and Compliance Monitoring; and S. Blake,
General Counsel, Office of the General Counsel; entitled ``Review of
State Implementation Plans and Revisions of Enforceability and Legal
Sufficiency.'' As stated in this memorandum, SIPs and SIP revisions
that fail to satisfy the enforceability criteria should not be
forwarded for approval. If they are submitted, they will be disapproved
if, in EPA's judgment, they fail to satisfy applicable statutory and
regulatory requirements.
The EPA's historic definition of RACT is the lowest emissions
limitation that a particular source is capable of meeting by the
application of control technology that is reasonably available
considering technological and economic feasibility.\169\ RACT applies
to the ``existing sources'' of lead including stack emissions,
industrial process fugitive emissions, and industrial fugitive dust
emissions (e.g., on-site haul roads, unpaved staging areas at the
facility, etc) (see section 172(c)(1)). EPA's most recent guidance for
implementing the current Pb NAAQS recommends that stationary sources
which actually emit a total of 5 tons per year of lead or lead
compounds, measured as elemental lead, be the minimum starting point
for RACT analysis (see 58 FR 67750, December 22, 1993). Further, EPA
recommends that available control technology be applied to those
existing sources in the nonattainment area that are reasonable to
control in light of the attainment needs of the area and the
feasibility of such controls. Thus a state's control technology
analysis may need to include sources which actually emit less than 5
tons per year of lead or lead compounds in the area, or other sources
in the area that are reasonable to control, in light of the attainment
needs and feasibility of control for the area.
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\169\ See for example, 44 FR 53762 (September 17, 1979) and
footnote 3 of that notice. Note that EPA's emissions trading policy
statement has clarified that the RACT requirement may be satisfied
by achieving ``RACT equivalent'' emission reductions in the
aggregate from the full set of existing stationary sources in the
area. See also EPA's economic incentive proposal which reflects the
Agency's policy guidance with respect to emissions trading 58 FR
11110, February 23, 1993.
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Given the proposal for promulgating a new or revised Pb NAAQS
significantly lower than the current standard, EPA is seeking comment
on an appropriate threshold for the minimum starting point for future
Pb RACT analyses for stationary lead sources in nonattainment areas. In
the monitoring section of today's proposal, EPA is taking comment on
minimum network monitoring requirements based on emissions source sizes
ranging from 200 kg/yr to 600 kg/yr. One possible approach for RACT is
to recommend that RACT analyses for Pb sources be consistent with the
monitoring requirements, such that all stationary sources above from
200 kg/yr to 600 kg/yr should undergo a RACT review. EPA is also taking
comment on source monitoring for stationary sources that emit Pb
emissions in amounts that have potential to cause ambient levels at
least one-half the selected NAAQS level. This suggests another
potential recommended starting point for RACT analysis. EPA is seeking
comment on these ideas as well as any information commenters can
provide that would help inform EPA recommendations on an appropriate
emissions threshold for initiating RACT analyses.
2. Demonstration of Attainment for Lead Nonattainment Areas
The SIPs for lead nonattainment areas should provide for the
implementation of control measures for point and area stationary
sources of lead emissions which demonstrate attainment of the Pb NAAQS
as expeditiously as practicable, but no later than the applicable
statutory attainment date for the area (See also 40 CFR 51.117(a) for
additional control strategy requirements). Therefore, if a state adopts
less than all available measures in an area but demonstrates,
adequately, that reasonable further progress (RFP), and attainment of
the Pb NAAQS are assured, and application of all such available
measures would not result in attainment any faster, then a plan which
requires implementation of less than all technologically and
economically available measures may be approved (see 44 FR 20375 (April
4, 1979) and 56 FR 5460 (February 11, 1991)). The EPA believes that it
would be unreasonable to require that a plan which demonstrates
attainment include all technologically and economically available
control measures even though such measures would not expedite
attainment. Thus, for some sources in areas which demonstrate
attainment, it is possible that some available control measures may not
be ``reasonably'' available because their implementation would not
expedite attainment.
3. Reasonable Further Progress (RFP)
Part D SIPs must provide for RFP (see section 172(c)(2) of the
CAA). Section 171 of the CAA defines RFP as ``such annual incremental
reductions in emissions of the relevant air pollution as are required
by part D, or may reasonably be required by the Administrator for the
purpose of ensuring attainment of the applicable NAAQS by the
applicable attainment date.'' Historically, for some pollutants, RFP
has been met by showing annual incremental emission reductions
generally sufficient to maintain linear progress toward attainment by
the applicable attainment date. Requiring linear emission reduction
progress to maintain RFP may be appropriate where:
Pollutants are emitted by numerous and diverse sources;
The relationship between any individual source and the
overall air quality is not explicitly quantified;
There is a chemical transformation involved; and
The emission control system utilized (e.g., at major point
sources) will result in swift and significant emission reductions.
The EPA believes that it may not be reasonable to require linear
reductions in emissions in SIPs for lead nonattainment areas because
the air quality problem is not usually due to a vast inventory of
sources. However, this is not to suggest that generally it would be
unreasonable for EPA to require annual incremental reductions in
emissions in lead nonattainment areas. RFP for lead nonattainment areas
should be met, at least in part, by ``adherence to an ambitious
compliance schedule'' which is expected to periodically yield
significant emission reductions, and as appropriate, linear
[[Page 29273]]
progress.\170\ The EPA recommends that SIPs for lead nonattainment
areas provide a detailed schedule for compliance of RACM (including
RACT) in the areas and accurately indicate the corresponding annual
emission reductions to be achieved. In reviewing the SIP, EPA believes
that it is appropriate to expect early implementation of less
technology-intensive control measures (e.g., controlling fugitive dust
emissions at the stationary source, as well as required controls on
area sources) while phasing in the more technology-intensive control
measures, such as those involving the installation of new hardware.
Finally, it should be noted that failure to implement the SIP
provisions required to meet annual incremental reductions in emissions
(i.e., RFP) in a particular area could result in the application of
sanctions as described in sections 110(m) and 179(b) of the CAA
(pursuant to a finding under section 179(a)(4)), and the implementation
of contingency measures required by section 172(c)(9) of the CAA.
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\170\ As previously stated most of the lead nonattainment
problems are caused by point sources. For this reason EPA believes
that the RFP for Pb should parallel the RFP policy for SO2 (see
General Preamble, 57 FR 13545, April 16, 1992).
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4. Contingency Measures
Section 172(c)(9) of the CAA defines contingency measures as
measures in a SIP that are to be implemented if an area fails to
achieve and maintain RFP, or fails to attain the NAAQS by the
applicable attainment date. Contingency measures must be designed to
become effective without further action by the state or the
Administrator, upon determination by EPA that the area has failed to
achieve or maintain reasonable further progress, or attain the Pb NAAQS
by the applicable statutory attainment date. Contingency measures
should consist of available control measures that are not already
included in the primary control strategy for the affected area.
Contingency measures are important for lead nonattainment areas,
which is generally due to emissions from stationary sources, for
several reasons. First, process and fugitive emissions from these
stationary sources, and the possible re-entrainment of historically
deposited emissions, have historically been difficult to quantify.
Therefore, the analytical tools for determining the relationship
between reductions in emissions, and resulting air quality
improvements, can be subject to some uncertainties. Second, emission
estimates and attainment analysis can be influenced by overly-
optimistic assumptions about fugitive emission control efficiency.
Examples of contingency measures for controlling area fugitive
emissions may include stabilizing additional storage piles, etc.
Examples of contingency measures for processed-related fugitive
emissions include increasing the enclosure of buildings, increasing air
flow in hoods, increasing operation and maintenance procedures, etc.
Examples for contingency measures for stack sources include reducing
hours of operation, changing the feed material to lower lead content,
and reducing the occurrence of malfunctions by increasing operation and
maintenance procedures, etc.
Section 172(c)(9) provides that contingency measures should be
included in the SIP for a lead nonattainment area and shall ``take
effect without further action by the state or the Administrator.'' The
EPA interprets this requirement to mean that no further rulemaking
actions by the state, or EPA, would be needed to implement the
contingency measures (see generally 57 FR 12512 and 13543-13544). The
EPA recognizes that certain actions, such as the notification of
sources, modification of permits, etc., may be needed before a measure
could be implemented. However, states must show that their contingency
measures can be implemented with minimal further action on their part
and with no additional rulemaking actions such as public hearings or
legislative review. After EPA determines that a lead nonattainment area
has failed to maintain RFP or timely attain the Pb NAAQS, EPA generally
expects all actions needed to affect full implementation of the
measures to occur within 60 days after EPA notifies the state of such
failure. The state should ensure that the measures are fully
implemented as expeditiously as practicable after the requirement takes
effect.
5. Nonattainment New Source Review (NSR) and Prevention of Significant
Deterioration (PSD) Requirements
The PSD and nonattainment NSR programs contained in parts C and D
of title I of the CAA govern preconstruction review and permitting
programs for any new or modified major stationary sources of air
pollutants regulated under the CAA as well as any precursors to the
formation of that pollutant when identified for regulation by the
Administrator. EPA rules addressing these regulations can be found at
40 CFR 51.165, 51.166, 52.21, 52.24, and part 51, appendix S.
Areas designated as nonattainment for the Pb NAAQS must submit SIPs
that address the requirements of nonattainment area NSR. Specifically,
section 172(c)(5) of the CAA requires that States which have areas
designated as nonattainment for the Pb NAAQS must submit, as a part of
the nonattainment area SIP, provisions requiring permits for the
construction and operation of new or modified stationary sources
anywhere in the nonattainment area, in accordance with the permit
requirements pursuant to section 173 of the CAA.
Stationary sources that emit lead are currently subject to
regulation under existing requirements for the preconstruction review
and approval of new and modified stationary sources. The existing
requirements, referred to collectively as the New Source Review (NSR)
program, require any major and minor stationary sources of any air
pollutant for which there is a NAAQS to undergo review and approval
prior to the commencement of construction.\171\ The NSR program is
composed of three different permit programs:
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\171\ The terms ``major'' and ``minor'' define the size of a
stationary source, for applicability purposes, in terms of an annual
emissions rate (tons per year, tpy) for a pollutant. Generally, a
minor source is any source that is not ``major.'' ``Major'' is
defined by the applicable regulations--PSD or nonattainment NSR.
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The NSR program is composed of three different permit programs:
Prevention of Significant Deterioration (PSD);
Nonattainment NSR (NA NSR); and,
Minor NSR.
The PSD program and nonattainment NSR programs, contained in parts
C and D, respectively, of Title I of the CAA, are often referred to as
the major NSR program because these programs regulate only major
sources.
The PSD program applies when a major source, that is located in an
area that is designated as attainment or unclassifiable for any
criteria pollutant, is constructed, or undergoes a major
modification.\172\ The NA NSR program applies when a major source that
is located in an area that is designated as nonattainment for any
criteria pollutant is constructed or undergoes a major modification.
The minor NSR program addresses both major and minor sources that
underground construction or modification activities that do not qualify
as major, and it applies regardless of the designation of the area in
which a source is located.
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\172\ In addition, the PSD program applies to most non-criteria
regulated pollutants.
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The national regulations that apply to each of these programs are
located in the CFR as shown below:
[[Page 29274]]
------------------------------------------------------------------------
Applications
------------------------------------------------------------------------
PSD....................................... 40 CFR 52.21, 40 CFR 51.166,
40 CFR 51.165(b).
NA NSR.................................... 40 CFR 52.24, 40 CFR 51.165,
40 CFR part 51, Appendix S.
Minor NSR................................. 40 CFR 51.160-164.
------------------------------------------------------------------------
The PSD requirements include but are not limited to the following:
Installation of Best Available Control Technology (BACT);
Air quality monitoring and modeling analyses to ensure
that a project's emissions will not cause or contribute to a violation
of any NAAQS or maximum allowable pollutant increase (PSD increment);
Notification of Federal Land Manager of nearby Class I
areas; and
Public comment on permit.
Nonattainment NSR requirements include but are not limited to:
Installation of Lowest Achievable Emissions Rate (LAER)
control technology;
Offsetting new emissions with creditable emissions
reductions;
A certification that all major sources owned and operated
in the state by the same owner are in compliance with all applicable
requirements under the CAA;
An alternative citing analysis demonstrating that the
benefits of proposed source significantly outweigh the environmental
and social costs imposed as a result of its location, construction, or
modification; and
Public comment on the permit.
Minor NSR programs must meet the statutory requirements in section
110(a)(2)(C) of the CAA which requires ``* * * regulation of the
modification and construction of any stationary source * * * as
necessary to assure that the [NAAQS] are achieved.''
Areas which are newly designated as nonattainment for the Pb NAAQS
as a result of any changes made to the NAAQS will be required to adopt
the NA NSR program to address major sources of lead where the program
does not currently exist for the Pb NAAQS. Prior to adoption of the SIP
revision addressing NSR for lead nonattainment areas, the requirements
of 40 CFR part 51, appendix S will apply.
6. Emissions Inventories
States must develop and periodically update a comprehensive,
accurate, current inventory of actual emissions affecting ambient lead
concentrations. The emissions inventory is used by states and EPA to
determine the nature and extent of the specific control strategy
necessary to help bring an area into attainment of the NAAQS. Emissions
inventories should be based on measured emissions or documented
emissions factors. Generally, the more comprehensive and accurate the
inventory, the more effective the evaluation of possible control
measures can be for the affected area (see section 172(c)(3) of the
CAA).
Pursuant to its authority under section 110 of Title I of the CAA,
EPA has long required states to submit emission inventories containing
information regarding the emissions of criteria pollutants as well as
their precursors. The EPA codified these requirements in 40 CFR part
51, subpart Q in 1979 and amended them in 1987. The 1990 Clean Air Act
Amendments (CAAA) revised many of the provisions of the CAA related to
attainment of the NAAQS. These revisions established new emission
inventory requirements applicable to certain areas that were designated
as nonattainment for certain pollutants. In the case of lead, the
emission inventory provisions are in the general provisions pursuant to
section 173(c)(3) of the CAA.
In June 2002, EPA promulgated the Consolidated Emissions Reporting
Rule (CERR) (67 FR 39602, June 10, 2002). The CERR consolidates the
various emissions reporting requirements that already exist into one
place in the CFR, and establishes new requirements for the state wide
reporting of area source and mobile source emissions. States should
follow the requirements under the CERR as well as any new or revised
guidance related to emissions inventories for criteria pollutants. The
CERR establishes two types of required emissions inventories: (1)
Annual inventories, and (2) 3-year cycle inventories. The annual
inventory requirement is limited to reporting statewide emissions data
from the larger point sources. For the 3-year cycle inventory, states
will need to report data from all of their point sources plus all of
the area and mobile sources on a statewide basis.
By merging emissions information from relevant point sources, area
sources and mobile sources into a comprehensive emission inventory, the
CERR allows state, local and tribal agencies to do the following:
Set a baseline for SIP development.
Measure their progress in reducing emissions.
Answer the public's request for information.
The EPA uses the data submitted by the states to develop the
National Emission Inventory (NEI). The NEI is used by EPA to show
national emission trends, as modeling input for analysis of potential
regulations, and other purposes.
Most importantly, states need these inventories to help in the
development of control strategies and demonstrations to attain the Pb
NAAQS. While the CERR sets forth requirements for data elements, EPA
guidance complements these requirements and indicates how the data
should be prepared for SIP submissions. Our regulations at 40 CFR
51.117(e) require states to include in the inventory all point sources
that emit 5 or more tons of lead emissions per year. EPA is also
considering whether revision to the recommended threshold for RACT
analysis is appropriate in light of the proposed revision to the Pb
NAAQS. In this proposed rulemaking we are taking comment on whether the
recommended threshold for RACT analysis should be less than the current
5 tons/yr (see section VI.F.1). If EPA lowers the recommended threshold
for RACT at the time of the final rulemaking, we propose also to
revise, to be consistent, the emissions threshold for including sources
in the inventory pursuant to 40 CFR 51.117. We solicit comment on the
appropriate threshold for Pb point source inventory reporting
requirements.
The SIP inventory must be approved by EPA as a SIP element and is
subject to public hearing requirements, whereas the CERR is not.
Because of the regulatory significance of the SIP inventory, EPA will
need more documentation on how the SIP inventory was developed by the
State as opposed to the documentation required for the CERR inventory.
In addition, the geographic area encompassed by some aspects of the SIP
submission inventory will be different from the statewide area covered
by the CERR emissions inventory.
The EPA has proposed the Air Emissions Reporting Rule (AERR) at 71
FR 69 (Jan. 3, 2006). When finalized, the AERR would update the CERR
reporting requirements by consolidating and harmonizing new emissions
reporting requirements with pre-existing sets of reporting requirements
under the Clean Air Interstate Rule (CAIR) and the NOX SIP
Call. At this time, EPA expects to finalize the AERR rulemaking in the
Fall of calendar year 2008. The AERR is expected to be a means by which
the Agency will implement additional data reporting requirements for
the Pb NAAQS SIP emission inventories.
7. Modeling
The lead SIP regulations found at 40 CFR 51.117 require states to
employ atmospheric dispersion modeling for the demonstration of
attainment for areas in
[[Page 29275]]
the vicinity of point sources listed in 40 CFR 51.117(a)(1). To
complete the necessary dispersion modeling, meteorological, and other
data are necessary. Dispersion modeling should follow the procedures
outlined in EPA's latest guidance document entitled ``Guideline on Air
Quality Models''. This guideline indicates the types and historical
records for data necessary for modeling demonstrations (e.g., on-site
meteorological stations are used, 12 months of data are required in
order to demonstrate attainment for the affected area).
G. General Conformity
Section 176(c) of the CAA, as amended (42 U.S.C. 7401 et seq.),
requires that all Federal actions conform to an applicable
implementation plan developed pursuant to section 110 and part D of the
CAA. Section 176(c) of the CAA requires EPA to promulgate criteria and
procedures for demonstrating and assuring conformity of Federal actions
to a SIP. For the purpose of summarizing the general conformity rule,
it can be viewed as containing three major parts: applicability,
procedure, and analysis. These are briefly described below.
The general conformity rule covers direct and indirect emissions of
criteria pollutants or their precursors that are caused by a Federal
action, are reasonably foreseeable, and can practicably be controlled
by the Federal agency through its continuing program responsibility.
The general conformity rule generally applies to Federal actions
except: (1) Actions covered by the transportation conformity rule; (2)
Actions with respect to associated emissions below specified de minimis
levels; and (3) Certain other actions that are exempt or presumed to
conform.
The general conformity rule also establishes procedural
requirements. Federal agencies must make their conformity
determinations available for public review. Notice of draft and final
general conformity determinations must be provided directly to air
quality regulatory agencies and to the public by publication in a local
newspaper.
The general conformity determination examines the impacts of direct
and indirect emissions related to Federal actions. The general
conformity rule provides several options to satisfy air quality
criteria and requires the Federal action to also meet any applicable
SIP requirements and emissions milestones. Each Federal agency must
determine that any actions covered by the general conformity rule
conform to the applicable SIP before the action is taken. The criteria
and procedures for conformity apply only in nonattainment and
maintenance areas with respect to the criteria pollutants under the
CAA: \173\ carbon monoxide (CO), lead (Pb), nitrogen dioxide
(NO2), ozone (O3), particulate matter
(PM-2.5 and PM10), and sulfur dioxide
(SO2). The general conformity rule establishes procedural
requirements for Federal agencies for actions related to all NAAQS
pollutants, both nonattainment and maintenance areas and will apply one
year following the promulgation of designations for any new or revised
Pb NAAQS.\174\
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\173\ Criteria pollutants are those pollutants for which EPA has
established a NAAQS under section 109 of the CAA.
\174\ Transportation conformity is required under CAA section
176(c) (42 U.S.C. 7506(c)) to ensure that federally supported
highway and transit project activities are consistent with
(``conform to'') the purpose of the SIP. Transportation conformity
applies to areas that are designated nonattainment, and those areas
redesignated to attainment after 1990 (``maintenance areas'' with
plans developed under CAA section 175A) for transportation-related
criteria pollutants. In light of the elimination of Pb additives
from gasoline transportation conformity does not apply to the Pb
NAAQS.
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H. Transition From the Current NAAQS to a Revised NAAQS for Lead
EPA is proposing to revise the level of the Pb NAAQS significantly,
as well as changing the indicator and averaging time. The EPA believes
that Congress's intent, as evidenced by section 110(l), 193, and
section 172(e) of the CAA, was to ensure that continuous progress, in
terms of public health protection, takes place in transitioning from a
current NAAQS for a pollutant to a new or revised NAAQS. Therefore, in
this section, EPA is proposing that the existing NAAQS will be revoked
one year following the promulgation of designations for any new NAAQS,
except that the existing NAAQS will not be revoked for any current
nonattainment area until the affected area submits, and EPA approves,
an attainment demonstration which addresses the attainment of the new
Pb NAAQS.
The CAA contains a number of provisions that indicate Congress's
intent to not allow states to alter or remove provisions from
implementation plans if the plan revision would jeopardize the air
quality protection being provided by the plan. For example, section
110(l) provides that EPA may not approve a SIP revision if it
interferes with any applicable requirement concerning attainment and
RFP, or any other applicable requirement under the CAA. In addition
section 193 of the CAA prohibits the modification of a control, or a
control requirement, in effect or required to be adopted as of November
15, 1990 (i.e., following the promulgation of the Clean Air Act
Amendments (CAAA) of 1990), unless such a modification would ensure
equivalent or greater emissions reductions. One other provision of the
CAA provides additional insight into Congress's intent related to the
need to continue progress towards meeting air quality standards during
periods of transition from one standard to another. Section 172(e) of
the CAA, related to future modifications of a standard, applies when
EPA promulgates a new or revised NAAQS and makes it less stringent than
the previous NAAQS. This provision of the CAA specifies that in such
circumstances, States may not relax control obligations that apply in
nonattainment area SIPs, or avoid adopting those controls that have not
yet been adopted as required.
Because it is EPA's belief that Congress did not intend to permit
states to remove control measures when EPA revises a standard until the
new or revised standard is implemented, we believe that controls that
are required under the current Pb NAAQS, or that are currently in place
under the current Pb NAAQS, should remain in place until designations
are promulgated and, for current nonattainment areas, attainment SIPs
are approved for any new or revised standard. As a result, EPA is
proposing that the current Pb NAAQS should stay in place for one year
following the effective date of designations for any new or revised
NAAQS before being revoked, except in current nonattainment areas,
where the existing NAAQS will not be revoked until the affected area
submits, and EPA approves, an attainment demonstration for the revised
Pb NAAQS. Pursuant to CAA section 110(l), any proposed SIP revision
being considered by EPA after the effective date of the revised Pb
NAAQs would be evaluated for its potential to interfere with attainment
or maintenance of the new standard. Unlike the transition from the 1-
hour ozone standard to the 8-hour ozone standard, EPA believes that any
area attaining the revised Pb NAAQS would also attain the existing Pb
NAAQS, and thus reviewing proposed SIP revisions for interference with
the new standard will be sufficient to prevent backsliding.
Consequently, in light of the nature of the proposed revision of the Pb
NAAQS, the lack of classifications (and mandatory controls associated
with such classifications pursuant to the CAA), and the small number of
nonattainment areas, EPA believes that retaining the current standard
for a limited period of time until attainment SIPs are approved for the
new standard
[[Page 29276]]
in current nonattainment areas, or one year after designations in other
areas, will adequately serve the anti-backsliding goals of the CAA. The
EPA requests comment on this proposed approach for transitioning to the
proposed revised Pb NAAQS.
VII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
Under section 3(f)(1) of Executive Order 12866 (58 FR 51735,
October 4, 1993), this action is an ``economically significant
regulatory action'' because it is likely to have an annual effect on
the economy of $100 million or more. Accordingly, EPA submitted this
action to the Office of Management and Budget (OMB) for review under EO
12866 and any changes made in response to OMB recommendations have been
documented in the docket for this action (EPA-HQ-OAR-2006-0735). In
addition, EPA prepared a Regulatory Impact Analysis (RIA) of the
potential costs and benefits associated with this action. A copy of the
analysis is available in the RIA docket (EPA-HQ-OAR-2008-0253) and the
analysis is briefly summarized here. The RIA estimates the costs and
monetized human health and welfare benefits of attaining four
alternative Pb NAAQS nationwide. Specifically, the RIA examines the
alternatives of 0.30 [mu]g/m\3\, 0.20 [mu]g/m\3\, 0.10 [mu]g/m\3\ and
0.05 [mu]g/m\3\. The RIA contains illustrative analyses that consider a
limited number of emissions control scenarios that States and Regional
Planning Organizations might implement to achieve these alternative Pb
NAAQS. However, the CAA and judicial decisions make clear that the
economic and technical feasibility of attaining ambient standards are
not to be considered in setting or revising NAAQS, although such
factors may be considered in the development of State plans to
implement the standards. Accordingly, although an RIA has been
prepared, the results of the RIA have not been considered in issuing
this proposed rule.
B. Paperwork Reduction Act
The information collection requirements in this proposed rule have
been submitted for approval to the Office of Management and Budget
(OMB) under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The
Information Collection Request (ICR) document prepared by EPA for these
proposed revisions to part 58 has been assigned EPA ICR numbers
0940.21.
The information collected under 40 CFR part 53 (e.g., test results,
monitoring records, instruction manual, and other associated
information) is needed to determine whether a candidate method intended
for use in determining attainment of the National Ambient Air Quality
Standards (NAAQS) in 40 CFR part 50 will meet the design, performance,
and/or comparability requirements for designation as a Federal
reference method (FRM) or Federal equivalent method (FEM). While this
proposed rule amends the requirements for Pb FRM and FEM
determinations, they merely provide additional flexibility in meeting
the FRM/FEM determination requirements. Furthermore, we do not expect
the number of FRM or FEM determinations to increase over the number
that is currently used to estimate burden associated with Pb FRM/FEM
determinations provided in the current ICR for 40 CFR part 53 (EPA ICR
numbers 0559.12). As such, no change in the burden estimate for 40 CFR
part 53 has been made as part of this rulemaking.
The information collected and reported under 40 CFR part 58 is
needed to determine compliance with the NAAQS, to characterize air
quality and associated health and ecosystem impacts, to develop
emissions control strategies, and to measure progress for the air
pollution program. The proposed amendments would revise the technical
requirements for Pb monitoring sites, require the siting and operation
of additional Pb ambient air monitors, and the reporting of the
collected ambient Pb monitoring data to EPA's Air Quality System (AQS).
Because this rulemaking includes a range of proposals for the level and
averaging time, it is not possible accurately predict the size of the
final network, and its associated burden. Rather we have estimated the
upper range of burden possible based on the regulatory options being
proposed which would result in a higher reporting burden (i.e., a final
level for the standard of 0.1 [mu]g/m\3\ with a 2nd maximum monthly
averaging form). Based on these assumptions, the annual average
reporting burden for the collection under 40 CFR part 58 (averaged over
the first 3 years of this ICR) for 150 respondents is estimated to
increase by a total of 90,434 labor hours per year with an increase of
$6,599,653 per year. Burden is defined at 5 CFR 1320.3(b). State,
local, and tribal entities are eligible for State assistance grants
provided by the Federal government under the CAA which can be used for
monitors and related activities.
An agency may not conduct or sponsor, and a person is not required
to respond to, a collection of information unless it displays a
currently valid OMB control number. The OMB control numbers for EPA's
regulations in 40 CFR are listed in 40 CFR part 9.
To comment on the Agency's need for this information, the accuracy
of the provided burden estimates, and any suggested methods for
minimizing respondent burden, EPA has established a public docket for
this rule, which includes this ICR, under Docket ID number EPA-HQ-OAR-
2006-0735. Submit any comments related to the ICR to EPA and OMB. See
ADDRESSES section at the beginning of this notice for where to submit
comments to EPA. Send comments to OMB at the Office of Information and
Regulatory Affairs, Office of Management and Budget, 725 17th Street,
NW., Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is
required to make a decision concerning the ICR between 30 and 60 days
after May 20, 2008, a comment to OMB is best assured of having its full
effect if OMB receives it by June 19, 2008. The final rule will respond
to any OMB or public comments on the information collection
requirements contained in this proposal.
C. Regulatory Flexibility Act
The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
For purposes of assessing the impacts of this rule on small
entities, small entity is defined as: (1) A small business that is a
small industrial entity as defined by the Small Business
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not
dominant in its field.
After considering the economic impacts of this proposed rule on
small entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This
proposed rule will not impose any
[[Page 29277]]
requirements on small entities. Rather, this rule establishes national
standards for allowable concentrations of Pb in ambient air as required
by section 109 of the CAA. American Trucking Ass'ns v. EPA, 175 F. 3d
1027, 1044-45 (D.C. cir. 1999) (NAAQS do not have significant impacts
upon small entities because NAAQS themselves impose no regulations upon
small entities). Similarly, the proposed amendments to 40 CFR part 58
address the requirements for States to collect information and report
compliance with the NAAQS and will not impose any requirements on small
entities. We continue to be interested in the potential impacts of the
proposed rule on small entities and welcome comments on issues related
to such impacts.
D. Unfunded Mandates Reform Act
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Unless otherwise prohibited by law,
under section 202 of the UMRA, EPA generally must prepare a written
statement, including a cost-benefit analysis, for proposed and final
rules with ``Federal mandates'' that may result in expenditures to
State, local, and tribal governments, in the aggregate, or to the
private sector, of $100 million or more in any one year. Before
promulgating an EPA rule for which a written statement is required
under section 202, section 205 of the UMRA generally requires EPA to
identify and consider a reasonable number of regulatory alternatives
and to adopt the least costly, most cost-effective or least burdensome
alternative that achieves the objectives of the rule. The provisions of
section 205 do not apply when they are inconsistent with applicable
law. Moreover, section 205 allows EPA to adopt an alternative other
than the least costly, most cost-effective or least burdensome
alternative if the Administrator publishes with the final rule an
explanation why that alternative was not adopted. Before EPA
establishes any regulatory requirements that may significantly or
uniquely affect small governments, including tribal governments, it
must have developed under section 203 of the UMRA a small government
agency plan. The plan must provide for notifying potentially affected
small governments, enabling officials of affected small governments to
have meaningful and timely input in the development of EPA regulatory
proposals with significant Federal intergovernmental mandates, and
informing, educating, and advising small governments on compliance with
the regulatory requirements.
This action is not subject to the requirements of sections 202 and
205 of the UMRA. EPA has determined that this proposed rule does not
contain a Federal mandate that may result in expenditures of $100
million or more for State, local, and tribal governments, in the
aggregate, or the private sector in any one year. The revisions to the
Pb NAAQS impose no enforceable duty on any State, local or Tribal
governments or the private sector. The expected costs associated with
the increased monitoring requirements are described in EPA's ICR
document, but those costs are not expected to exceed $100 million in
the aggregate for any year. Furthermore, as indicated previously, in
setting a NAAQS EPA cannot consider the economic or technological
feasibility of attaining ambient air quality standards. Because the
Clean Air Act prohibits EPA from considering the types of estimates and
assessments described in section 202 when setting the NAAQS, the UMRA
does not require EPA to prepare a written statement under section 202
for the revisions to the Pb NAAQS.
With regard to implementation guidance, the CAA imposes the
obligation for States to submit SIPs to implement the Pb NAAQS. In this
proposed rule, EPA is merely providing an interpretation of those
requirements. However, even if this rule did establish an independent
obligation for States to submit SIPs, it is questionable whether an
obligation to submit a SIP revision would constitute a Federal mandate
in any case. The obligation for a State to submit a SIP that arises out
of section 110 and section 191 of the CAA is not legally enforceable by
a court of law, and at most is a condition for continued receipt of
highway funds. Therefore, it is possible to view an action requiring
such a submittal as not creating any enforceable duty within the
meaning of 2 U.S.C. 658 for purposes of the UMRA. Even if it did, the
duty could be viewed as falling within the exception for a condition of
Federal assistance under 2 U.S.C. 658.
EPA has determined that this proposed rule contains no regulatory
requirements that might significantly or uniquely affect small
governments because it imposes no enforceable duty on any small
governments. Therefore, this rule is not subject to the requirements of
section 203 of the UMRA.
E. Executive Order 13132: Federalism
Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August
10, 1999), requires EPA to develop an accountable process to ensure
``meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications.''
``Policies that have federalism implications'' is defined in the
Executive Order to include regulations that have ``substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.''
This proposed rule does not have federalism implications. It will
not have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. The rule does not alter the
relationship between the Federal government and the States regarding
the establishment and implementation of air quality improvement
programs as codified in the CAA. Under section 109 of the CAA, EPA is
mandated to establish NAAQS; however, CAA section 116 preserves the
rights of States to establish more stringent requirements if deemed
necessary by a State. Furthermore, this rule does not impact CAA
section 107 which establishes that the States have primary
responsibility for implementation of the NAAQS. Finally, as noted in
section E (above) on UMRA, this rule does not impose significant costs
on State, local, or tribal governments or the private sector. Thus,
Executive Order 13132 does not apply to this rule.
However, EPA recognizes that States will have a substantial
interest in this rule and any corresponding revisions to associated air
quality surveillance requirements, 40 CFR part 58. Therefore, in the
spirit of Executive Order 13132, and consistent with EPA policy to
promote communications between EPA and State and local governments, EPA
specifically solicits comment on this proposed rule from State and
local officials.
F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to
[[Page 29278]]
ensure ``meaningful and timely input by tribal officials in the
development of regulatory policies that have tribal implications.''
This proposed rule does not have tribal implications, as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian Tribes, since Tribes are not obligated to adopt or
implement any NAAQS. Thus, Executive Order 13175 does not apply to this
rule. However, EPA specifically solicits additional comment on this
proposed rule from tribal officials.
G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks
This action is subject to Executive Order (62 FR 19885, April 23,
1997) because it is an economically significant regulatory action as
defined by Executive Order 12866, and we believe that the environmental
health risk addressed by this action has a disproportionate effect on
children. The proposed rule will establish uniform national ambient air
quality standards for Pb; these standards are designed to protect
public health with an adequate margin of safety, as required by CAA
section 109. However, the protection offered by these standards may be
especially important for children because neurological effects in
children are among if not the most sensitive health endpoints for Pb
exposure. Because children are considered a sensitive population, we
have carefully evaluated the environmental health effects of exposure
to Pb pollution among children. These effects and the size of the
population affected are summarized in chapters 6 and 8 of the Criteria
Document and sections 3.3 and 3.4 of the Staff Paper, and the results
of our evaluation of the effects of Pb pollution on children are
discussed in sections II.B and II.C of this preamble.
H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution or Use
This rule is not a ``significant energy action'' as defined in
Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 28355
(May 22, 2001)) because it is not likely to have a significant adverse
effect on the supply, distribution, or use of energy. The purpose of
this rule is to establish revised NAAQS for Pb. The rule does not
prescribe specific control strategies by which these ambient standards
will be met. Such strategies will be developed by States on a case-by-
case basis, and EPA cannot predict whether the control options selected
by States will include regulations on energy suppliers, distributors,
or users. Thus, EPA concludes that this rule is not likely to have any
adverse energy effects.
I. National Technology Transfer and Advancement Act
Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (NTTAA), Public Law 104-113, section 12(d) (15 U.S.C. 272
note) directs EPA to use voluntary consensus standards in its
regulatory activities unless to do so would be inconsistent with
applicable law or otherwise impractical. Voluntary consensus standards
are technical standards (e.g., materials specifications, test methods,
sampling procedures, and business practices) that are developed or
adopted by voluntary consensus standards bodies. The NTTAA directs EPA
to provide Congress, through OMB, explanations when the Agency decides
not to use available and applicable voluntary consensus standards.
This proposed rulemaking involves technical standards. EPA proposes
to use low-volume PM10 samplers coupled with XRF analysis as
the FRM for Pb-PM10 measurement. While EPA identified the
ISO standard ``Determination of the particulate lead content of
aerosols collected on filters'' (ISO 9855: 1993) as being potentially
applicable, we do not propose to use it in this rule. The use of this
voluntary consensus standard would be impractical because the analysis
method does not provide for the method detection limits necessary to
adequately characterize ambient Pb concentrations for the purpose of
determining compliance with the proposed revisions to the Pb NAAQS.
EPA welcomes comments on this aspect of the proposed rule, and
specifically invites the public to identify potentially applicable
voluntary consensus standards and to explain why such standards should
be used in the regulation.
J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes
federal executive policy on environmental justice. Its main provision
directs federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
EPA has determined that this proposed rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or low-
income population. The proposed rule will establish uniform national
standards for Pb in ambient air.
EPA is continuing to assess the impact of Pb air pollution on
minority and low-income populations, and plans to prepare a technical
memo as part of its assessment to be placed in the docket by the date
of publication of this proposed rule in the Federal Register. EPA
solicits comment on environmental justice issues related to the
proposed revision of the Pb NAAQS.
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List of Subjects
40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
40 CFR Part 51
Environmental protection, Administrative practice and procedure,
Air pollution control, Carbon monoxide, Intergovernmental relations,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Reporting and
recordkeeping requirements.
40 CFR Part 53
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.
40 CFR Part 58
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.
Dated: May 1, 2008.
Stephen L. Johnson,
Administrator.
For the reasons stated in the preamble, title 40, chapter I of the
Code of Federal Regulations is proposed to be amended as follows:
PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY
STANDARDS
1. The authority citation for part 50 continues to read as follows:
Authority: 42 U.S.C. 7401 et seq.
2. Section 50.3 is revised to read as follows:
[[Page 29282]]
Sec. 50.3 Reference conditions.
All measurements of air quality that are expressed as mass per unit
volume (e.g., micrograms per cubic meter) other than for particulate
matter (PM2.5) standards contained in Sec. Sec. 50.7 and 50.13 and
lead standards contained in Sec. 50.16 shall be corrected to a
reference temperature of 25 (deg) C and a reference pressure of 760
millimeters of mercury (1,013.2 millibars). Measurements of
PM2.5 for purposes of comparison to the standards contained
in Sec. Sec. 50.7 and 50.13 and of lead for purposes of comparison to
the standards contained in Sec. 50.16 shall be reported based on
actual ambient air volume measured at the actual ambient temperature
and pressure at the monitoring site during the measurement period.
3. Section 50.12 is amended by designating the existing text as
paragraph (a) and adding paragraph (b) to read as follows:
Sec. 50.12 National primary and secondary ambient air quality
standards for lead.
* * * * *
(b) The standards set forth in this section will remain applicable
to all areas notwithstanding the promulgation of lead national ambient
air quality standards (NAAQS) in Sec. 50.16. The lead NAAQS set forth
in this section will no longer apply to an area one year after the
effective date of the designation of that area, pursuant to section 107
of the Clean Air Act, for the lead NAAQS set forth in Sec. 50.16;
except that for areas designated nonattainment for the lead NAAQS set
forth in this section as of the effective date of Sec. 50.16, the lead
NAAQS set forth in this section will apply until that area submits,
pursuant to section 191 of the Clean Air Act, and EPA approves, an
implementation plan providing for attainment of the lead NAAQS set
forth in Sec. 50.16.
4. Section 50.14 is amended by:
(a) Revising paragraph (a)(2);
(b) Revising paragraph (c)(2)(iii);
(c) Redesignating paragraph (c)(2)(v) as paragraph (c)(2)(vi) and
adding a new paragraph (c)(2)(v); and
(d) Redesignating existing paragraphs (c)(3)(iii) and (c)(3)(iv) as
paragraphs (c)(3)(iv) and (c)(3)(v), respectively, and adding paragraph
(c)(3)(iii).
The additions and revisions read as follows:
Sec. 50.14 Treatment of air quality monitoring data influenced by
exceptional events.
* * * * *
(a) * * *
* * * * *
(2) Demonstration to justify data exclusion may include any
reliable and accurate data, but must demonstrate a clear causal
relationship between the measured exceedance or violation of such
standard and the event in accordance with paragraph (c)(3)(iv) of this
section.
(c) * * *
(2) * * *
(iii) Flags placed on data as being due to an exceptional event
together with an initial description of the event shall be submitted to
EPA not later than July 1st of the calendar year following the year in
which the flagged measurement occurred, except as allowed under
paragraph (c)(2)(iv) or (c)(2)(v) of this section.
* * * * *
(v) For lead (Pb) data collected during calendar years 2006-2008,
that the State identifies as resulting from an exceptional event, the
State must notify EPA of the flag and submit an initial description of
the event no later than July 1, 2009. For Pb data collected during
calendar year 2009, that the State identifies as resulting from an
exceptional event, the State must notify EPA of the flag and submit an
initial description of the event no later than July 1, 2010. For Pb
data collected during calendar year 2010, that the State identifies as
resulting from an exceptional event, the State must notify EPA of the
flag and submit an initial description of the event no later than May
1, 2011.
* * * * *
(3) * * *
(iii) A State that flags Pb data collected during calendar years
2006-2009, pursuant to paragraph (c)(2)(v) of this section shall, after
notice and opportunity for public comment, submit to EPA a
demonstration to justify exclusion of the data not later than September
15, 2010. A State that flags Pb data collected during calendar year
2010 shall, after notice and opportunity for public comment, submit to
EPA a demonstration to justify the exclusion of the data not later than
May 1, 2011. A state must submit the public comments it received along
with its demonstration to EPA.
* * * * *
5. Section 50.16 is added to read as follows:
Sec. 50.16 National primary and secondary ambient air quality
standards for lead.
(a) The national primary and secondary ambient air quality
standards for lead (Pb) and its compounds is [0.10-0.30] micrograms per
cubic meter ([mu]/m\3\), [arithmetic mean concentration averaged over a
calendar quarter or second highest arithmetic mean concentration
averaged over a calendar month] measured in the ambient air as Pb
either by:
(1) A reference method based on (Appendix G or Appendix Q of this
part) and designated in accordance with part 53 of this chapter; or
(2) An equivalent method designated in accordance with part 53 of
this chapter.
(b) The national primary and secondary ambient air quality
standards for Pb are met when the [quarterly or second highest monthly]
arithmetic mean concentration, as determined in accordance with
Appendix R of this part, is less than or equal to [0.10-0.30]
micrograms per cubic meter.
6. Appendix G is amended as follows:
a. In section 10.2 the definition of the term ``VSTP''
in the equation is revised; and
b. In section 14 reference 10 is added and reference 15 is revised.
Appendix G to Part 50--Reference Method for the Determination of Lead
in Suspended Particulate Matter Collected From Ambient Air
* * * * *
10.2 * * *
VSTP= Air volume from section 10.1.
* * * * *
14. * * *
10. Intersociety Committee (1972). Methods of Air Sampling and
Analysis. 1015 Eighteenth Street, NW., Washington, DC: American
Public Health Association. 365-372.
* * *
15. Sharon J. Long, et. al., ``Lead Analysis of Ambient Air
Particulates: Interlaboratory Evaluation of EPA Lead Reference
Method,'' APCA Journal, 29, 28-31 (1979).
* * * * *
7. Appendix Q is added to read as follows:
Appendix Q to Part 50--Reference Method for the Determination of Lead
in Particulate Matter as PM10 Collected From Ambient Air
This Federal Reference Method (FRM) draws heavily from the
specific analytical protocols used by the U.S. EPA.
1. Applicability and Principle
1.1 This method provides for the measurement of the lead (Pb)
concentration in particulate matter that is 10 micrometers or less
(PM10) in ambient air. PM10 is collected on a
46.2 mm diameter polytetrafluoroethylene (PTFE) filter for 24 hours
using active sampling at local conditions with a low-volume air
sampler. The low-volume sampler has an average flow rate of 16.7
liters per minute (Lpm) and total sampled volume of 24 cubic meters
(m\3\) of air. The analysis of Pb in PM10 is performed on
each individual 24-hour sample. For the purpose of this method,
PM10 is defined as particulate matter having an
aerodynamic
[[Page 29283]]
diameter in the nominal range of 10 micrometers (10 [mu]m) or less.
1.2 For this reference method, PM10 shall be
collected with the PM10c federal reference method (FRM)
sampler as described in Appendix O to Part 50 using the same sample
period, measurement procedures, and requirements specified in
Appendix L of Part 50. The PM10c sampler is also being
used for measurement PM10-2.5 mass by difference and as
such, the PM10c sampler must also meet all of the
performance requirements specified for PM2.5 in Appendix
L. The concentration of Pb in the atmosphere is determined in the
total volume of air sampled and expressed in micrograms per cubic
meter ([mu]g/m\3\) at local temperature and pressure conditions.
1.3 The FRM will serve as the basis for approving Federal
Equivalent Methods (FEMs) as specified in 40 CFR part 53 (Reference
and Equivalent Methods).
1.4 An electrically powered air sampler for PM10c
draws ambient air at a constant volumetric flow rate into a
specially shaped inlet and through an inertial particle size
separator, where the suspended particulate matter in the
PM10 size range is separated for collection on a PTFE
filter over the specified sampling period. The lead content of the
PM10c sample is analyzed by energy-dispersive X-ray
fluorescence spectrometry (EDXRF). Energy-dispersive X-ray
fluorescence spectrometry provides a means for identification of an
element by measurement of its characteristic X-ray emission energy.
The method allows for quantification of the element by measuring the
emitted characteristic line intensity and then relating this
intensity to the elemental concentration. The number or intensity of
X-rays produced at a given energy provides a measure of the amount
of the element present by comparisons with calibration standards.
The X-rays are detected and the spectral signals are acquired and
processed with a personal computer. EDXRF is commonly used as a non-
destructive method for quantifying trace elements in PM. An EPA
method for the EDXRF analysis of ambient particulate matter is
described in reference 1 of section 8. A detailed explanation of
quantitative X-ray spectrometry is described in references 2 and 3.
1.5 Quality assurance (QA) procedures for the collection of
monitoring data are contained in Part 58, Appendix A.
2. PM10c Lead Measurement Range and Method Detection Limit. The
values given below in section 2.1 and 2.2 are typical of the method
capabilities. Absolute values will vary for individual situations
depending on the instrument, detector age, and operating conditions
used. Data are typically reported in ng/m\3\ for ambient air
samples; however, for this reference method, data will be reported
in [mu]g/m\3\ at local temperature and pressure conditions.
2.1 EDXRF Measurement Range. The typical ambient air measurement
range is 0.001 to 30 [mu]g Pb/m\3\, assuming an upper range
calibration standard of about 60 [mu]g Pb per square centimeter
(cm\2\), a filter deposit area of 11.86 cm\2\, and an air volume of
24-m\3\. The top range of the EDXRF instrument is much greater than
what is stated here. The top measurement range of quantification is
defined by the level of the high concentration calibration standard
used and can be increased to expand the measurement range as needed.
2.2 Method Detection Limit (MDL). A typical one-sigma estimate
of the method detection limit (MDL) is about 1.5 ng Pb/cm\2\ or
0.001 [mu]g Pb/m\3\, assuming a filter size of 46.2-mm (filter
deposit area of 11.86 cm\2\) and a sample air volume of 24-m\3\. The
MDL is an estimate of the lowest amount of lead that can be detected
by the analytical instrument. The one-sigma detection limit for Pb
is calculated as the average overall uncertainty or propagated error
for Pb, determined from measurements on a series of blank filters.
The sources of random error which are considered are calibration
uncertainty; system stability; peak and background counting
statistics; uncertainty in attenuation corrections; uncertainty in
peak overlap corrections; and uncertainty in flow rate, but the
dominating source is by far peak and background counting statistics.
Laboratories are to estimate the MDLs using 40 CFR Part 136,
Appendix B, ``Definition and Procedure for the Determination of the
Method Detection Limit.'' (Reference 4).
3. Factors Affecting Bias and Precision of Lead Determination by
EDXRF
3.1 Filter Deposit. Too much deposit material can be problematic
because XRF analysis and data processing programs for aerosol
samples are designed specifically for a thin film or thin layer of
material to be analyzed. The X-ray spectra are subject to distortion
if unusually heavy deposits are analyzed. This is the result of
internal absorption of both primary and secondary X-rays within the
sample. The optimum filter loading is about 150 [mu]g/cm\2\ or 1.6
mg/filter for a 46.2-mm filter. Too little deposit material can also
be problematic due to low counting statistics and signal noise. The
particle mass deposit should minimally be 15 [mu]g/cm\2\. A properly
collected sample will have a uniform deposit over the entire
collection area. Sample heterogeneity can lead to very large
systematic errors. Samples with physical deformities (including a
visually non-uniform deposit area) should not be quantitatively
analyzed.
3.2 Spectral Interferences and Spectral Overlap. Spectral
interference occurs when the entirety of the analyte spectral lines
of two species are nearly 100% overlapped. There are only a few
cases where this may occur and they are instrument specific: Si/Rb,
Si/Ta, S/Mo, S/Tl, Al/Br, Al/Tm. These interferences are determined
during instrument calibration and automatically corrected for by the
XRF instrument software. Interferences need to be addressed when
multi-elemental analysis is performed. The presence of arsenic (As)
is a problematic interference for EDXRF systems which use the Pb
L[alpha] line exclusively to quantify the Pb
concentration. This is because the Pb L[alpha] line and
the As K[alpha] lines severely overlap. However, if the
instrument software is able to use multiple Pb lines, including the
L[beta] and/or the L[gamma] lines for
quantification, then the uncertainty in the Pb determination in the
presence of As can be significantly reduced. There can be instances
when lines partially overlap the Pb spectral lines, but with the
energy resolution of most detectors, these overlaps are typically
de-convoluted using standard spectral de-convolution software
provided by the instrument vendor. An EDXRF protocol for Pb must
define which Pb lines are used for quantification and where spectral
overlaps occur. Some of the overlaps may be very small and some
severe. A de-convolution protocol must be used to separate all the
lines which overlap with Pb.
3.3 Particle Size Effects and Attenuation Correction Factors. X-
ray attenuation is dependent on the X-ray energy, mass sample
loading, composition, and particle size. In some cases, the
excitation and fluorescent X-rays are attenuated as they pass
through the sample. In order to relate the measured intensity of the
X-rays to the thin-film calibration standards used, the magnitude of
any attenuation present must be corrected for. The effect is
especially significant and more complex for PM10
measurements, especially for the lighter elements that may also be
measured. An average attenuation and uncertainty for each coarse
particle element is based on a broad range of mineral compositions
and is a one-time calculation that gives an attenuation factor for
use in all subsequent particle analyses. See references 6, 7, and 8
of section 8 for more discussion on addressing this issue.
Essentially no attenuation corrections are necessary for Pb in
PM10: both the incoming excitation X-rays used for
analyzing lead and the fluoresced Pb X-rays are sufficiently
energetic that for particles in this size range and for normal
filter loadings, the Pb x-ray yield is not significantly impacted by
attenuation. However, this issue must be addressed when doing multi-
element analyses.
4. Precision
4.1 Measurement system precision is assessed according to the
procedures set forth in Appendix A to part 58. Measurement method
precision is assessed from collocated sampling and analysis. The
goal for acceptable measurement uncertainty, as precision, is
defined as an upper 90 percent confidence limit for the coefficient
of variation (CV) of 15 percent.
5. Bias
5.1 Measurement system bias for monitoring data is assessed
according to the procedures set forth in Appendix A of part 58. The
bias is assessed through an audit using spiked filters. The goal for
measurement bias is defined as an upper 95 percent confidence limit
for the absolute bias of 10 percent.
6. Measurement of PTFE Filters by EDXRF
6.1 Sampling
6.1.1 Low-Volume PM10c Sampler. The low-volume PM10c
sampler shall be used for sample collection and operated in
accordance with the performance specifications described in Part 50,
Appendix L.
6.1.2 PTFE Filters and Filter Acceptance Testing. The PTFE
filters used for PM10c sample collection shall meet the
specifications provided in Part 50, Appendix L. The following
requirements are similar to those currently specified for the
acceptance of PM2.5 filters that are tested for trace
[[Page 29284]]
elements by EDXRF. For large batches of filters (greater than 500
filters) randomly select 50 filters from a given batch. For small
batches (less than 500 filters) a lesser number of filters may be
taken. Analyze each filter separately and calculate the average lead
concentration in ng/cm\2\. Ninety percent, or 45 of the 50 filters,
must have an average lead concentration that is less than 4.8 ng Pb/
cm\2\.
6.2 Analysis. The four main categories of random and systematic
error encountered in X-ray fluorescence analysis include errors from
sample collection, the X-ray source, the counting process, and
inter-element effects. These errors are addressed through the
calibration process and mathematical corrections in the instrument
software.
6.2.1 EDXRF Analysis Instrument. An energy-dispersive XRF system
is used. Energy-dispersive XRF systems are available from a number
of commercial vendors including Thermo (www.thermo.com) and
PANalytical (www.panalytical.com). Note the mention of commercial
products does not imply endorsement by the U.S. Environmental
Protection Agency. The analysis is performed at room temperature in
either vacuum or in a helium atmosphere. The specific details of the
corrections and calibration algorithms are typically included in
commercial analytical instrument software routines for automated
spectral acquisition and processing and vary by manufacturer. It is
important for the analyst to understand the correction procedures
and algorithms of the particular system used, to ensure that the
necessary corrections are applied.
6.2.2 Thin film standards. Thin film standards are used for
calibration because they most closely resemble the layer of
particles on a filter. Thin films standards are typically deposited
on Nuclepore substrates. The preparation of thin film standards is
discussed in reference 6, and 9. Thin film standards are
commercially available from MicroMatter Inc. (Arlington, WA).\1\
6.2.3 Filter Preparation. Filters used for sample collection are
46.2-mm PTFE filters with a pore size of 2 microns and filter
deposit area 11.86 cm\2\. Filters are typically archived in cold
storage prior to analysis. Filters that are scheduled for XRF
analysis are removed from storage and allowed to reach room
temperature. All filter samples received for analysis are checked
for any holes, tears, or a non-uniform deposit which would prevent
quantitative analysis. A properly collected sample will have a
uniform deposit over the entire collection area. Samples with
physical deformities are not quantitatively analyzable. The filters
are carefully removed with tweezers from the Petri dish and securely
placed into the instrument-specific sampler holder for analysis.
Care must be taken to protect filters to avoid contamination prior
to analysis. Filters must be kept covered when not being analyzed.
No other preparation of the samples is required.
6.2.4 Calibration. In general, calibration determines each
element's sensitivity, i.e., its response in X-ray counts/sec to
each [mu]g/cm\2\ of a standard and an interference coefficient for
each element that causes interference with another one (See section
3.2 above). The sensitivity can be determined by a linear plot of
count rate versus concentration ([mu]g/cm\2\) in which the slope is
the instrument's sensitivity for that element. A more precise way,
which requires fewer standards, is to fit sensitivity versus atomic
number. Calibration is a complex task in the operation of an XRF
system. Two major functions accomplished by calibration are the
production of reference spectra which are used for fitting and the
determination of the elemental sensitivities. Included in the
reference spectra (referred to as ``shapes'') are background-
subtracted peak shapes of the elements to be analyzed, as well as
peak shapes for interfering element energies and spectral
backgrounds. Pure element thin film standards are used for the
element peak shapes and clean filter blanks from the same lot as
unknowns are used for the background. The analysis of PM filter
deposits is based on the assumption that the thickness of the
deposit is small with respect to the characteristic lead X-ray
transmission thickness. Therefore, the concentration of lead in a
sample is determined by first calibrating the spectrometer with thin
film standards to determine sensitivity factors and then analyzing
the unknown samples under identical excitation conditions as used to
determine the calibration factors. Calibration is performed only
when significant repairs occur or when a change in fluorescers, X-
ray tubes, or detector is made. Calibration establishes the
elemental sensitivity factors and the magnitude of interference or
overlap coefficients. See reference 7 for more detailed discussion
of calibration and analysis of shapes standards for background
correction, coarse particle absorption corrections, and spectral
overlap.
6.2.4.1 Spectral Peak Fitting. The EPA uses a library of pure
element peak shapes (shape standards) to extract the elemental
background-free peak areas from an unknown spectrum. It is also
possible to fit spectra using peak stripping or analytically defined
functions such as modified Gaussian functions. The EPA shape
standards are generated from pure, mono-elemental thin film
standards. The shape standards are acquired for sufficiently long
times to provide a large number of counts in the peaks of interest.
It is not necessary for the concentration of the standard to be
known. A slight contaminant in the region of interest in a shape
standard can have a significant and serious effect on the ability of
the least squares fitting algorithm to fit the shapes to the unknown
spectrum. It is these elemental shapes, that are fitted to the peaks
in an unknown sample during spectral processing by the analyzer. In
addition to this library of elemental shapes, there is also a
background shape spectrum for the filter type used as discussed
below in section 6.2.4.2 of this section.
6.2.4.2 Background Measurement and Correction. A background
spectrum generated by the filter itself must be subtracted from the
X-ray spectrum prior to extracting peak areas. The background shape
standards which are used for background fitting are created at the
time of calibration. About 20-30 clean blank filters are kept in a
sealed container and are used exclusively for background measurement
and correction. The spectra acquired on individual blank filters are
added together to produce a single spectrum for each of the
secondary targets or fluorescers used in the analysis of lead.
Individual blank filter spectra which show contamination are
excluded from the summed spectra. The summed spectra are fitted to
the appropriate background during spectral processing. Background
correction is automatically included during spectral processing of
each sample.
7. Calculation.
7.1 The PM10 lead concentration in the atmosphere
([mu]g/m\3\) is calculated using the following equation:
[GRAPHIC] [TIFF OMITTED] TP20MY08.006
Where,
MPb is the mass per unit volume for lead in [mu]g/m\3\;
CPb is the mass per unit area for lead in [mu]g/cm\2\ as provided by
the XRF instrument software;
A is the filter deposit area in cm\2\;
VLC is the total volume of air sampled by the PM10c
sampler in actual volume units measured at local conditions of
temperature and pressure, as provided by the sampler in m\3\.
8. References
1. Inorganic Compendium Method IO-3.3; Determination of Metals
in Ambient Particulate Matter Using X-Ray Fluorescence (XRF)
Spectroscopy; U.S. Environmental Protection Agency, Cincinnati, OH
45268. EPA/625/R-96/010a. June 1999.
2. Jenkins, R., Gould, R.W., and Gedcke, D. Quantitative X-ray
Spectrometry: Second Edition. Marcel Dekker, Inc., New York, NY.
1995.
3. Jenkins, R. X-Ray Fluorescence Spectrometry: Second Edition
in Chemical Analysis, a Series of Monographs on Analytical Chemistry
and Its Applications, Volume 152. Editor J.D.Winefordner; John Wiley
& Sons, Inc. New York, NY. 1999.
4. Code of Federal Regulations (CFR) 40 part 136, Appendix B;
Definition and Procedure for the Determination of the Method
Detection Limit--Revision 1.11
5. Dzubay, T.G. X-ray Fluorescence Analysis of Environmental
Samples, Ann Arbor Science Publishers Inc., 1977.
6. Drane, E.A, Rickel, D.G., and Courtney, W.J., ``Computer Code
for Analysis X-Ray Fluorescence Spectra of Airborne Particulate
Matter,'' in Advances in X-Ray Analysis, J.R. Rhodes, Ed., Plenum
Publishing Corporation, New York, NY, p. 23 (1980).
7. Analysis of Energy-Dispersive X-ray Spectra of ambient
Aerosols with Shapes Optimization, Guidance Document; TR-WDE-06-02;
prepared under contract EP-D-05-065 for the U.S. Environmental
Protection Agency, National Exposure Research Laboratory. March
2006.
8. Billiet, J., Dams, R., and Hoste, J. (1980) Multielement Thin
Film Standards for XRF Analysis, X-Ray Spectrometry, 9(4): 206-211.
8. Appendix R is added to read as follows:
[[Page 29285]]
Appendix R to Part 50--Interpretation of the National Ambient Air
Quality Standards for Lead
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining when the primary and
secondary national ambient air quality standards (NAAQS) for lead
(Pb) specified in Sec. 50.16 are met. The NAAQS indicator for Pb is
defined as: lead and its compounds, measured as elemental lead in
total suspended particulate (Pb-TSP), sampled and analyzed by a
Federal reference method (FRM) based on appendix G to this part or
by a Federal equivalent method (FEM) designated in accordance with
part 53 of this chapter. Although Pb-TSP is the lead NAAQS
indicator, surrogate Pb-TSP concentrations shall also be used for
NAAQS comparisons; specifically, valid surrogate Pb-TSP data are
concentration data for lead and its compounds, measured as elemental
lead, in particles with an aerodynamic size of 10 microns or less
(Pb-PM10), sampled and analyzed by an FRM based on
appendix Q to this part or by an FEM designated in accordance with
part 53 of this chapter, the resulting concentrations then
multiplied by an appropriate site-specific scaling factor to
represent Pb-TSP. Data handling and computation procedures to be
used in making comparisons between reported and/or surrogate Pb-TSP
concentrations and the level of the Pb NAAQS, including Pb-
PM10 to Pb-TSP scaling instructions, are specified in the
following sections.
(b) Whether to exclude, retain, or make adjustments to the data
affected by exceptional events, including natural events, is
determined by the requirements and process deadlines specified in
Sec. Sec. 50.1, 50.14, and 51.930 of this chapter.
(c) The terms used in this appendix are defined as follows:
Annual monitoring plan refers to the plan required by section
58.10 of this chapter.
Creditable samples are samples that are given credit for data
completeness. They include valid samples collected on required
sampling days and valid ``make-up'' samples taken for missed or
invalidated samples on required sampling days.
Daily values for Pb refers to the 24-hour mean concentrations of
Pb (Pb-TSP or Pb-PM10) measured from midnight to midnight
(local standard time) that are used in NAAQS computations.
Design value is the site-level metric (i.e., statistic) that is
compared to the NAAQS level to determine compliance; the design
value for the Pb NAAQS is the second highest monthly mean Pb-TSP or
surrogate Pb-TSP concentration for the most recent valid 3-year
calendar period.
Extra samples are non-creditable samples. They are daily values
that do not occur on scheduled sampling days and that can not be
used as make-ups for missed or invalidated scheduled samples. Extra
samples are used in mean calculations. For purposes of determining
whether a sample must be treated as a make-up sample or an extra
sample, Pb-TSP and Pb-PM10 data collected before January
1, 2009 will be treated with an assumed scheduled sampling frequency
of every sixth day.
Make-up samples are samples taken to supplant missed or
invalidated required scheduled samples. Make-ups can be made by
either the primary or collocated (same size cut) instruments. Make-
up samples are either taken before the next required sampling day or
exactly one week after the missed (or voided) sampling day. Make-up
samples can not span years; that is, if a scheduled sample for
December is missed (or voided), it can not be made up in January.
Make-up samples, however, may span months, for example a missed
sample on January 31 may be made up on February 1, 2, or 6. Section
3(e) explains how such month-spanning make-up samples are to be
treated for purposes of data completeness and monthly means. Only
two make-up samples are permitted each calendar month; these are
counted according to the month in which the miss and not the makeup
occurred Also, to be considered a valid make-up, the sampling must
be conducted with equipment and procedures that meet the
requirements for scheduled sampling. For purposes of determining
whether a sample must be treated as a make-up sample or an extra
sample, Pb-TSP and Pb-PM10 data collected before January
1, 2009 will be treated with an assumed scheduled sampling frequency
of every sixth day.
Monthly mean refers to an arithmetic mean, as defined in section
4.3 of this appendix. Monthly means are one of two specific types,
``monthly parameter means'' or ``monthly site means''. Monthly means
are computed at each monitoring site separately for Pb-TSP and Pb-
PM10 (i.e., by site-parameter-year-month); these
parameter-specific means are referred to as monthly parameter means.
Monthly parameter means are validated according to the criteria
stated in section 4 of this appendix. A ``monthly site mean'' (i.e.,
one for a site-year-month level) will be the valid monthly Pb-TSP
mean if available, or the valid Pb-PM10 (scaled) monthly
mean when it is available and a valid Pb-TSP monthly mean is not. If
neither a valid Pb-TSP nor a valid Pb-PM10 monthly
(parameter) mean exists for a particular site-year-month then there
will be no corresponding valid monthly site mean.
Parameter refers either to Pb-TSP or to Pb-PM10.
Scheduled sampling day means a day on which sampling is
scheduled based on the required sampling frequency for the
monitoring site, as provided in section 58.12 of this chapter.
Year refers to a calendar year.
2. Monitoring Considerations for Use of Scaled Pb-PM10 Data as
Surrogate Pb-TSP Data
(a) Monitoring agencies are permitted to monitor for Pb-
PM10 at a required Pb monitoring site rather than
monitoring for Pb-TSP, but only after the monitoring agency
develops, and the Regional Administrator approves, a site-specific
scaling factor to be used to adjust Pb-PM10 data before
comparison to the standard. The development of such a factor must
meet the criteria stated below (in sections 2(b)(i) through
2(b)(iv)), and the factor and associated analysis must be documented
in the monitoring agency's Annual Monitoring Network Plan. The site-
specific scaling factor meeting all of these requirements shall take
effect on January 1 following Regional Administrator approval of the
Plan. The data criteria for establishing a site-specific alternative
Pb-PM10 to Pb-TSP scaling factor are:
(i) A scaling factor shall be based on a minimum of 12
consecutive months of collocated Pb-TSP and Pb-PM10 FRM/
FEM monitoring which produces at least 6 pairs of valid collocated
measurements for each of at least 10 months of each period of 12
months.
(ii) Calculated Pearson correlation coefficients for the paired
data shall equal or exceed 0.60 for each individual month of the
evaluation period (for months containing at least 6 pairs), and a
calculated overall (using all 10 or more months with at least 6
pairs of valid collocated measurements) Pearson correlation
coefficient shall equal or exceed 0.80.
(iii) The site-specific scaling factor shall be equal to the
mean of the ratios of monthly mean Pb-TSP concentration to monthly
mean Pb-PM10 concentration, using all 10 or more months
with at least 6 pairs of valid collocated measurements and only
using the days with valid collocated measurements. The scaling
factor shall be rounded to two decimal places.
(iv) Each monthly ratio of Pb-TSP to Pb-PM10 shall be
within twenty percent of the 10-month (or more) mean ratio. Ratios
shall be computed from unrounded means but monthly ratios shall be
rounded to two decimal places before making the comparison.
3. Requirements for Data Used for Comparisons With the Pb NAAQS and
Data Reporting Considerations
(a) All valid FRM/FEM Pb-TSP data and all valid FRM/FEM Pb-
PM10 data submitted to EPA's Air Quality System (AQS), or
otherwise available to EPA, meeting the requirements of part 58 of
this chapter including appendices A, C, and E shall be used in
design value calculations. Pb-TSP and Pb-PM10 data
representing sample collection periods prior to January 1, 2009
(i.e., ``pre-rule'' data) will also be considered valid for NAAQS
comparisons and related attainment/nonattainment determinations if
the sampling and analysis methods that were utilized to collect that
data were consistent with previous or newly designated FRMs or FEMs
and with either the provisions of part 58 of this chapter including
appendices A, C, and E that were in effect at the time of original
sampling or that are in effect at the time of the attainment/
nonattainment determination, and if such data are submitted to AQS
prior to September 1, 2009.
(b) Pb-TSP and Pb-PM10 measurement data shall be
reported to AQS in units of micrograms per cubic meter ([mu]g/m\3\)
at local conditions (local temperature and pressure, LC) to three
decimal places, with additional digits to the right being truncated.
Pb-PM10 data shall be reported without application of a
scaling factor. Pre-rule Pb-TSP and Pb-PM10 concentration
data that were reported in standard conditions (standard temperature
[[Page 29286]]
and standard pressure, STP) will not require a conversion to local
conditions but rather, after truncating to three decimal places and
processing as stated in this appendix, shall compared ``as is'' to
the NAAQS (i.e., the LC to STP conversion factor will be assumed to
be one). However, if the monitoring agency has retroactively
resubmitted Pb-TSP or Pb-PM10 pre-rule data converted
from STP to LC based on suitable meteorological data, only the LC
data will be used.
(c) At each monitoring location (site), Pb-TSP and Pb-
PM10 data are to be processed separately when selecting
daily data by day (as specified in 3(d) below) and when aggregating
daily data by month (per 4(2)(a) below), however, when deriving the
design value for the three-year period, monthly means for the two
data types may be combined; see section 4(e) below.
(d) Daily values for sites will be selected for a site on a size
cut (Pb-TSP or Pb-PM10, i.e., ``parameter'') basis; Pb-
TSP concentrations and Pb-PM10 concentrations shall not
be commingled in these determinations. Site level, parameter-
specific daily values will be selected as follows:
(i) The starting dataset for a site-parameter shall consist of
the measured daily concentrations recorded from the designated
primary FRM/FEM monitor for that parameter. The primary monitor for
each parameter shall be designated in the appropriate State or local
agency annual Monitoring Network Plan. If no primary monitor is
designated, the Administrator will select which monitor to treat as
primary. All daily values produced by the primary sampler are
considered part of the site-parameter composite record (i.e., that
site-parameter's set of daily values); this includes all creditable
samples and all extra samples.
(ii) Data for the primary monitor for each parameter shall be
augmented as much as possible with data from collocated (same
parameter) FRM/FEM monitors. If a valid 24-hour measurement is not
produced from the primary monitor for a particular day (scheduled or
otherwise), but a valid sample is generated by a collocated (same
parameter) FRM/FEM instrument, then that collocated value shall be
considered part of the site-parameter data record (i.e., that site-
parameter's monthly set of daily values). If more than one valid
collocated FRM/FEM value is available, the mean of those valid
collocated values shall be used as the daily value.
(e) All daily values in the composite site-parameter record are
used in monthly mean calculations. However, not all daily values are
given credit towards data completeness requirements. Only
``creditable'' samples are given credit for data completeness.
Creditable samples include valid samples on scheduled sampling days
and valid make-up samples. All other types of daily values are
referred to as ``extra'' samples. Make-up samples taken in the
(first week of the) month after the one in which the miss/void
occurred will be credited for data capture in the month of the miss/
void but will be included in the month actually taken when computing
monthly means.
4. Comparisons With the Pb NAAQS
(a) The Pb NAAQS is met at a monitoring site when the identified
design value is valid and less than or equal to 0.20 [0.10, 0.30]
micrograms per cubic meter ([mu]g/m\3\). A Pb design value of 0.20
[0.10, 0.30] [mu]g/m\3\ or less is valid if it encompasses 3
consecutive calendar years of valid monthly means (i.e., 36 valid
monthly means). See 4(c) below for the definition of a valid monthly
mean and 6(c) below for the definition of the design value. A Pb
design value of 0.20 [0.10, 0.30] [mu]g/m\3\ or less will also be
considered valid if it encompasses 35 valid monthly means (out of 36
possible over 3 consecutive calendar years) and the highest of the
35 is equal to or less than 0.20 [0.10, 0.30] [mu]g/m\3\.
(b) The Pb NAAQS is violated at a monitoring site when the
identified design value is valid and is greater than 0.20 [0.10,
0.30] micrograms per cubic meter ([mu]g/m\3\). A Pb design value
greater than 0.20 [0.10, 0.30] [mu]g/m\3\ is valid if it encompasses
at least two valid monthly means. A site does not have to have valid
monitoring data for three full calendar years in order to have a
valid violating design value. For example, a site could start
monitoring in November of a given calendar year and violate the
NAAQS for any three-year period that includes that given calendar
year, if the November and December means are valid and greater than
0.20 [0.10, 0.30] [mu]g/m\3\.
(c) (i) A monthly mean is considered valid (i.e., meets data
completeness requirements) if for one or both of the Pb parameters
measured at the site, the data capture rate is greater than or equal
to 75 percent. Monthly data capture rates (expressed as a
percentage) are specifically calculated as the number of creditable
samples for the month (including any make-up samples taken the
subsequent month for missed samples in the (previous) month in
question) divided by the number of scheduled samples for the month,
the result then multiplied by 100 and rounded to the nearest
integer. As noted above, Pb-TSP and Pb-PM10 daily values
are processed separately when calculating monthly means and data
capture rates; a Pb-TSP value cannot be used as a make-up for a
missing Pb-PM10 value or vice versa. For purposes of
assessing data capture, Pb-TSP and Pb-PM10 data collected
before January 1, 2009 will be treated with an assumed scheduled
sampling frequency of every sixth day.
(ii) A monthly parameter mean that does not have at least 75
percent data capture and thus cannot be considered valid under
4(c)(1) shall still be considered valid (and complete) if it passes
either of the two following ``data substitution'' tests, one such
test for validating an above NAAQS-level mean (using actual ``low''
reported values from the site), and the second test for validating a
below-NAAQS level mean (using actual ``high'' values reported for
the site). Note that both tests are merely diagnostic in nature,
intending to confirm that there is a very high likelihood if not
certainty that that original mean (the one with less than 75% data
capture) reflects the true over/under NAAQS-level status for that
month; the result of these data substitution tests (i.e., the test
means, as described below) is never considered the actual monthly
parameter mean and shall not be used to determine the design value.
For both types of data substitution, substitution is permitted only
if there are a sufficient number of available data points from which
to identify the high or low 3-year month-specific values,
specifically if there are at least 10 data points total from at
least two of the three possible year-months. Data substitution may
only use data of the same parameter type. For Pb-PM10
data, the ``test'' monthly mean after data substitution shall be
scaled using Equation 2 of section 6(b) before being compared to the
level of the standard.
(A) The ``above NAAQS level'' test is as follows: If by
substituting the lowest reported daily value for that month over the
3-year design value period in question (year non-specific; e.g., for
January) for missing scheduled data in the deficient months
(substituting only enough to meet the 75 percent data capture
minimum), the computation yields a recalculated test monthly
parameter mean concentration above the level of the standard, then
the month is deemed to have passed the diagnostic test and the level
of the standard is deemed to have been exceeded in that month. As
noted above, in such a case, the monthly parameter mean of the data
actually reported, not the recalculated (``test'') result including
the low values, shall be used to determine the design value.
(B) The ``below NAAQS level'' test is as follows: A monthly
parameter mean that does not have at least 75 percent data capture
but does have at least 50 percent data capture shall still be
considered valid (and complete) if, by substituting the highest
reported daily value for that month over the 3-year design value
period in question, for all missing scheduled data in the deficient
months (i.e., bringing the data capture rate up to 100%), the
computation yields a recalculated monthly parameter mean
concentration equal or less than the level of the standard, then the
month is deemed to have passed the diagnostic test and the level of
the standard is deemed not to have been exceeded in that month. As
noted above, in such a case, the monthly parameter mean of the data
actually reported, not the recalculated (``test'') result including
the high values, shall be used to determine the design value.
(d) Months that do not meet the completeness criteria stated in
4(c)(i) or 4(c)(ii) above, and design values that do not meet the
completeness criteria stated in 4(a) or 4(b) above, may also be
considered valid (and complete) with the approval of, or at the
initiative of, the Administrator, who may consider factors such as
monitoring site closures/moves, monitoring diligence, the
consistency and levels of the valid concentration measurements that
are available, and nearby concentrations in determining whether to
use such data.
(e) The site-level design value for a three calendar year period
is identified from the available valid monthly parameter means. In a
situation where there are valid monthly means for both parameters
(Pb-TSP and Pb-PM10), the mean originating from the
reported Pb-TSP data will be the one deemed the site-level monthly
mean and used in design value identifications. A monitoring site
will have only one site-level monthly
[[Page 29287]]
mean per month; however, the set of site-level monthly means
considered for design value identification (i.e., two to 36 site-
level monthly means) can be a combination of Pb-TSP and scaled Pb-
PM10 data.
(f) The procedures for calculating monthly means, scaling Pb-
PM10 monthly means to a surrogate Pb-TSP basis, and
identifying Pb design values are given in section 6 of this
appendix.
5. Rounding Conventions
(a) Monthly means shall be rounded to the nearest hundredth
[mu]g/m\3\ (0.xx). Decimals 0.xx5 and greater are rounded up, and
any decimal lower than 0.xx5 is rounded down; e.g., a monthly mean
of 0.104925 rounds to 0.10, and a monthly mean of .10500 rounds to
0.11.
(b) Because a Pb design value is simply a (second highest)
monthly mean and because the NAAQS level is stated to two decimal
places, no additional rounding beyond what is specified for monthly
means is required before a design value is compared to the NAAQS.
6. Procedures and Equations for the Pb NAAQS.
(a) A monthly mean value for Pb-TSP (or Pb-PM10) is
determined by averaging the daily values of a calendar month using
equation 1 of this appendix:
[GRAPHIC] [TIFF OMITTED] TP20MY08.007
Where:
Xm,y,s = the mean for quarter q of the year y for site s;
and
nm = the number of daily values in the month; and
Xi,m,y,s = the ith value in month m for year y
for site s.
(b) Monthly means for reported Pb-PM10 data are
scaled to a surrogate Pb-TSP basis using Equation 2 of this
appendix.
[GRAPHIC] [TIFF OMITTED] TP20MY08.008
Where:
Zm,y,s = the surrogate Pb-TSP mean for month m of the
year y for site s; and
Xm,y,s = the Pb-PM10 mean for month m of the
year y for site s; and
Fm,y,s = the scaling factor for year y and for site s
determined through collocated testing in accordance with section
2.0(b).
(c) The site-level identified Pb design value is the second
highest valid site-level monthly mean over the most recent 3-year
period. Section 4 above explains when the identified design value is
itself considered valid for purposes of determining that the NAAQS
is met or violated at a site.
PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS
9. The authority citation for part 53 continues to read as follows:
Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec.
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat.
1713, unless otherwise noted.
Subpart C--[Amended]
10. Section 53.33 is revised to read as follows:
Sec. 53.33 Test Procedure for Methods for Lead (Pb).
(a) General. The reference method for collection of Pb in TSP
includes two parts, the reference method for high-volume sampling of
TSP as specified in 40 CFR part 50, appendix B and the analysis method
for Pb in TSP as specified in 40 CFR part 50, appendix G.
Correspondingly, the reference method for Pb in PM10
includes the reference method for low-volume sampling of
PM10 in 40 CFR part 50, appendix O and the analysis method
of Pb in PM10 as specified in 40 CFR part 50, appendix Q.
This section explains the procedures for demonstrating the equivalence
of either a candidate method for Pb in TSP to the high-volume reference
methods, or a candidate method for Pb in PM10 to the low-
volume reference methods.
(1) Pb in TSP--A candidate method for Pb in TSP specifies reporting
of Pb concentrations in terms of standard temperature and pressure.
Comparisons of candidate methods to the reference method in 40 CFR part
50, appendix G must be made in a consistent manner with regard to
temperature and pressure.
(2) Pb in PM10--A candidate method for Pb in
PM10 must specify reporting of Pb concentrations in terms of
local conditions of temperature and pressure, which will be compared to
similarly reported concentrations from the reference method in 40 CFR
part 50, appendix Q.
(b) Comparability. Comparability is shown for Pb methods when the
differences between:
(1) Measurements made by a candidate method, and
(2) Measurements made by the reference method on simultaneously
collected Pb samples (or the same sample, if applicable), are less than
or equal to the values specified in table C-3 of this subpart.
(c) Test measurements. Test measurements may be made at any number
of test sites. Augmentation of pollutant concentrations is not
permitted, hence an appropriate test site or sites must be selected to
provide Pb concentrations in the specified range.
(d) Collocated samplers. The ambient air intake points of all the
candidate and reference method collocated samplers shall be positioned
at the same height above the ground level, and between 2 meters (1
meter for samplers with flow rates less than 200 liters per minute (L/
min)) and 4 meters apart. The samplers shall be oriented in a manner
that will minimize spatial and wind directional effects on sample
collection.
(e) Sample collection. Collect simultaneous 24-hour samples
(filters) of Pb at the test site or sites with both the reference and
candidate methods until at least 10 filter pairs have been obtained. A
candidate method for Pb in TSP which employs a sampler and sample
collection procedure that are identical to the sampler and sample
collection procedure specified in the reference method in 40 CFR part
50, appendix B, but uses a different analytical procedure than
specified in 40 CFR part 50, appendix G, may be tested by analyzing
pairs of filter strips taken from a single TSP reference sampler
operated according to the procedures specified by that reference
method. A candidate method for Pb in PM10 which employs a
sampler and sample collection procedure that are identical to the
sampler and sample collection procedure specified in the reference
method in 40 CFR part 50, appendix O, but uses a different analytical
procedure than specified in 40 CFR part 50, appendix Q, requires the
use of two PM10 reference samplers because a single 46.2-mm
filter from a reference sampler may not be divided prior to analysis.
(f) Audit samples. Three audit samples must be obtained from the
address given in Sec. 53.4(a). For Pb in TSP collected by the high-
volume sampling method, the audit samples are \3/4\ x 8-inch glass
fiber strips containing known amounts of Pb in micrograms per strip
([mu]g/strip) equivalent to the following nominal percentages of the
National Ambient Air Quality Standard (NAAQS): 30%, 100%, and 250%. For
Pb in PM10 collected by the low-volume sampling method, the
audit samples are 46.2-mm polytetrafluorethylene (PTFE) filters
containing known amounts of Pb in micrograms per filter ([mu]g/filter)
equivalent to the same percentages of the NAAQS: 30%, 100%, and 250%.
The true amount of Pb (Tqi), in total [mu]g/strip (for TSP) or total
[mu]g/filter (for PM10), will be provided with each audit
sample.
(g) Filter analysis.
(1) For both the reference method samples and the audit samples,
analyze each filter or filter extract three times in accordance with
the reference method analytical procedure. This applies to both the Pb
in TSP and Pb in PM10 methods. The analysis of replicates
[[Page 29288]]
should not be performed sequentially, i.e., a single sample should not
be analyzed three times in sequence. Calculate the indicated Pb
concentrations for the reference method samples in micrograms per cubic
meter ([mu]g/m\3\) for each analysis of each filter. Calculate the
indicated total Pb amount for the audit samples in [mu]g/strip for each
analysis of each strip or [mu]g/filter for each analysis of each audit
filter. Label these test results as R1A, R1B,
R1C, R2A, R2B, * * *, Q1A,
Q1B, Q1C, * * *, where R denotes results from the
reference method samples; Q denotes results from the audit samples; 1,
2, 3 indicate the filter number, and A, B, C indicate the first,
second, and third analysis of each filter, respectively.
(2) For the candidate method samples, analyze each sample filter or
filter extract three times and calculate, in accordance with the
candidate method, the indicated Pb concentration in [mu]g/m\3\ for each
analysis of each filter. The analysis of replicates should not be
performed sequentially. Label these test results as C1A,
C1B, C2C, * * *, where C denotes results from the
candidate method. For candidate methods which provide a direct
measurement of Pb concentrations without a separable procedure,
C1A = C1B = C1C, C2A =
C2B = C2C, etc.
(h) Average Pb concentration. For the reference method, calculate
the average Pb concentration for each filter by averaging the
concentrations calculated from the three analyses as described in
paragraph (g)(1) of this section using equation 1 of this section:
[GRAPHIC] [TIFF OMITTED] TP20MY08.009
Where, i is the filter number.
(i) Accuracy.
(1)(i) For the audit samples, calculate the average Pb
concentration for each strip or filter by averaging the concentrations
calculated from the three analyses as described in (g)(1) using
equation 2 of this section:
[GRAPHIC] [TIFF OMITTED] TP20MY08.010
Where, i is audit sample number.
(ii) Calculate the percent difference (Dq) between the
indicated Pb concentration for each audit sample and the true Pb
concentration (Tq) using equation 3 of this section:
[GRAPHIC] [TIFF OMITTED] TP20MY08.011
(2) If any difference value (Dqi) exceeds 5
percent, the accuracy of the reference method analytical procedure is
out-of-control. Corrective action must be taken to determine the source
of the error(s) (e.g., calibration standard discrepancies, extraction
problems, etc.) and the reference method and audit sample
determinations must be repeated according to paragraph (g) of this
section, or the entire test procedure (starting with paragraph (e) of
this section) must be repeated.
(j) Acceptable filter pairs. Disregard all filter pairs for which
the Pb concentration, as determined in paragraph (h) of this section by
the average of the three reference method determinations, falls outside
the range of 30% to 250% of the Pb NAAQS level in [mu]g/m\3\ for Pb in
both TSP and PM10. All remaining filter pairs must be
subjected to the tests for precision and comparability in paragraphs
(k) and (l) of this section. At least five filter pairs must be within
the specified concentration range for the tests to be valid.
(k) Test for precision.
(1) Calculate the precision (P) of the analysis (in percent) for
each filter and for each method, as the maximum minus the minimum
divided by the average of the three concentration values, using
equation 4 or equation 5 of this section:
[GRAPHIC] [TIFF OMITTED] TP20MY08.012
or
[GRAPHIC] [TIFF OMITTED] TP20MY08.013
where, i indicates the filter number.
(2) If any reference method precision value (PRi)
exceeds 15 percent, the precision of the reference method analytical
procedure is out-of-control. Corrective action must be taken to
determine the source(s) of imprecision, and the reference method
determinations must be repeated according to paragraph (g) of this
section, or the entire test procedure (starting with paragraph (e) of
this section) must be repeated.
(3) If any candidate method precision value (PCi)
exceeds 15 percent, the candidate method fails the precision test.
(4) The candidate method passes this test if all precision values
(i.e., all PRi's and all PCi's) are less than 15
percent.
(l) Test for comparability. (1) For each filter or analytical
sample pair, calculate all nine possible percent differences (D)
between the reference and candidate methods, using all nine possible
combinations of the three determinations (A, B, and C) for each method
using equation 6 of this section:
[GRAPHIC] [TIFF OMITTED] TP20MY08.014
where, i is the filter number, and n numbers from 1 to 9 for the
nine possible difference combinations for the three determinations
for each method (j = A, B, C, candidate; k = A, B, C, reference).
(2) If none of the percent differences (D) exceeds 20
percent, the candidate method passes the test for comparability.
(3) If one or more of the percent differences (D) exceed 20 percent, the candidate method fails the test for
comparability.
(4) The candidate method must pass both the precision test
(paragraph (k) of this section) and the comparability test (paragraph
(l) of this section) to qualify for designation as an equivalent
method.
(m) Method Detection Limit (MDL). Calculate the estimated MDL using
the guidance provided in 40 CFR Part 136, Appendix B. It is essential
that all sample processing steps of the analytical method be included
in the determination of the method detection limit. Take a minimum of
seven aliquots of the sample to be used to calculate the method
detection limit and process each through the entire analytical method.
Make all computations according to the defined method with the final
results in [mu]g/m\3\. The MDL must be equal to, or less than 1% of the
level of the Pb NAAQS.
10a. Revise Table C-3 to Subpart C of Part 53 to read as follows:
Table C-3 to Subpart C of Part 53.--Test Specifications for Pb in TSP
and Pb in PM10 Methods
------------------------------------------------------------------------
------------------------------------------------------------------------
Concentration range equivalent to 30% to 250%.
percentage of NAAQS in [mu]g/m\3\.
Minimum number of 24-hr measurements...... 5.
Maximum precision, PR or PC............... <=15%.
Maximum analytical accuracy, Dq........... 5%
Maximum difference (D), percent of 20%.
reference method.
[[Page 29289]]
Estimated Method Detection Limit (MDL), 1% of NAAQS level.
[mu]g/m\3\.
------------------------------------------------------------------------
PART 58--AMBIENT AIR QUALITY SURVEILLANCE
11. The authority citation for part 58 continues to read as
follows:
Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.
Subpart B--[Amended]
12. Section 58.10, is amended by adding paragraphs (a)(4) and
(b)(9) to read as follows:
Sec. 58.10 Annual monitoring network plan and periodic network
assessment.
(a) * * *
(4) A plan for establishing Pb monitoring sites in accordance with
the requirements of appendix D to this part shall be submitted to the
EPA Regional Administrator by July 1, 2009. The plan shall provide for
at least one half of the required Pb monitoring sites to be operational
by January 1, 2010, and for all required Pb monitoring sites to be
operational by January 1, 2011. Source oriented Pb monitoring sites for
the highest emitting half of Pb sources shall be installed by January
1, 2010.
(b) * * *
(9) The designation of any Pb monitors as either source-oriented or
non-source oriented according to appendix D to this part.
* * * * *
13. Section 58.12 is amended by revising paragraph (b) to read as
follows:
Sec. 58.12 Operating schedules.
* * * * *
(b) For Pb manual methods, at least one 24-hour sample must be
collected every 3 days except during periods or seasons exempted by the
Regional Administrator. The Regional Administrator can allow a
reduction in the sampling schedule to one 24-hour sample every 6 days
if the Pb design value over the previous 3 years is less than 70% of
the Pb NAAQS.
14. Section 58.13 is amended by revising paragraph (b) to read as
follows:
Sec. 58.13 Monitoring network completion.
* * * * *
(b) Not withstanding specific dates included in this part,
beginning January 1, 2008, when existing networks are not in
conformance with the minimum number of required monitors specified in
this part, additional required monitors must be identified in the next
applicable annual monitoring network plan, with monitoring operation
beginning by January 1 of the following year. To allow sufficient time
to prepare and comment on Annual Monitoring Network Plans, only
monitoring requirements effective 120 days prior to the required
submission date of the plan (i.e., 120 days prior to July 1 of each
year) shall be included in that year's annual monitoring network plan.
15. Section 58.16 is amended by revising paragraph (a) to read as
follows:
Sec. 58.16 Data submittal and archiving requirements.
(a) The State, or where appropriate, local agency, shall report to
the Administrator, via AQS all ambient air quality data and associated
quality assurance data for SO2; CO; O3;
NO2; NO; NOY; NOX; Pb-TSP mass
concentration; Pb-PM10 mass concentration; PM10
mass concentration; PM2.5 mass concentration; for filter-
based PM2.5 FRM/FEM the field blank mass, sampler-generated
average daily temperature, and sampler-generated average daily
pressure; chemically speciated PM2.5 mass concentration
data; PM10-2.5 mass concentration; chemically speciated
PM10-2.5 mass concentration data; meteorological data from
NCore and PAMS sites; average daily temperature and average daily
pressure for Pb sites if not already reported from sampler generated
records; and metadata records and information specified by the AQS Data
Coding Manual (http://www.epa.gov/ttn/airs/airsaqs/manuals/manuals.htm). Such air quality data and information must be submitted
directly to the AQS via electronic transmission on the specified
quarterly schedule described in paragraph (b) of this section.
* * * * *
Subpart C--[Amended]
16. Section 58.20 is amended by revising paragraph (e) to read as
follows:
Sec. 58.20 Special purpose monitors (SPM).
* * * * *
(e) If an SPM using an FRM, FEM, or ARM is discontinued within 24
months of start-up, the Administrator will not designate an area as
nonattainment for the CO, SO2, NO2, or 24-hour
PM10 NAAQS solely on the basis of data from the SPM. Such
data are eligible for use in determinations of whether a nonattainment
area has attained one of these NAAQS.
* * * * *
17. Appendix A to part 58 is amended by revising paragraph 3.3.4
and Table A-2.
Appendix A to Part 58--Quality Assurance Requirements for SLAMS, SPMs
and PSD Air Monitoring
* * * * *
3.3.4 Pb Methods.
3.3.4.1 Flow Rates. For the Pb Reference Methods (40 CFR part
50, appendix G and appendix Q) and associated FEMs, the flow rates
of the Pb samplers shall be verified and audited using the same
procedures described in sections 3.3.2 and 3.3.3 of this appendix.
3.3.4.2 Pb Analysis Audits. Each calendar quarter or sampling
quarter (PSD), audit the Pb Reference Method analytical procedure
using filters containing a known quantity of Pb. These audit filters
are prepared by depositing a Pb solution on unexposed filters and
allowing them to dry thoroughly. The audit samples must be prepared
using batches of reagents different from those used to calibrate the
Pb analytical equipment being audited. Prepare audit samples in the
following concentration ranges:
------------------------------------------------------------------------
Equivalent ambient Pb
Range concentration, [mu]g/m\3\
\1\
------------------------------------------------------------------------
1......................................... 30-100% of Pb NAAQS.
2......................................... 200-300% of Pb NAAQS.
------------------------------------------------------------------------
\1\ Equivalent ambient Pb concentration in [mu]g/m\3\ is based on
sampling at 1.7 m\3\/min for 24 hours on a 20.3 cm x 25.4 cm (8 inch x
10 inch) glass fiber filter.
(a) Audit samples must be extracted using the same extraction
procedure used for exposed filters.
(b) Analyze three audit samples in each of the two ranges each
quarter samples are analyzed. The audit sample analyses shall be
distributed as much as possible over the entire calendar quarter.
(c) Report the audit concentrations (in [mu]g Pb/filter or
strip) and the corresponding measured concentrations (in [mu]g Pb/
filter or strip) using AQS unit code 077. The relative percent
differences between the concentrations are used to calculate
analytical accuracy as described in section 4.4.2 of this appendix.
(d) The audits of an equivalent Pb method are conducted and
assessed in the same manner as for the reference method. The flow
auditing device and Pb analysis audit samples must be compatible
with the specific requirements of the equivalent method.
3.3.4.3 Collocated Sampling. The collocated sampling
requirements for Pb-TSP and Pb-PM10 shall be determined
using the same procedures described in sections 3.3.1 of this
appendix.
3.3.4.4 Pb Performance Evaluation Program (PEP) Procedures. One
performance evaluation audit, as described in section 3.2.7 of this
appendix must be performed at one Pb site in each primary quality
assurance organization each year. The calculations for evaluating
bias between the primary monitor(s) and the performance evaluation
monitors for Pb are the same as those for PM10-2.5 which
are described in section 4.1.3 of this appendix. In addition, for
each
[[Page 29290]]
quarter, one half of a collocated sample pair (from the designated
collocated sampler) from one site within each PQAO must sent to an
independent laboratory for analysis.
* * * * *
Table A-2 of Appendix A to Part 58.--Minimum Data Assessment Requirements for SLAMS Sites
----------------------------------------------------------------------------------------------------------------
Parameters
Method Assessment method Coverage Minimum frequency reported
----------------------------------------------------------------------------------------------------------------
Automated Methods
----------------------------------------------------------------------------------------------------------------
1-Point QC for SO2, NO2, O3, CO. Response check at Each analyzer..... Once per 2 weeks.. Audit
concentration concentration\1\
0.01-0.1 ppm SO2, and measured
NO2, O3, and 1-10 concentration.\2\
ppm CO.
Annual performance evaluation See section 3.2.2 Each analyzer..... Once per year..... Audit
for SO2, NO2, O3, CO. of this appendix. concentration\1\
and measured
concentration\2\
for each level.
Flow rate verification PM10, Check of sampler Each sampler...... Once every month.. Audit flow rate
PM2.5, PM10 2.5. flow rate. and measured flow
rate indicated by
the sampler.
Semi-annual flow rate audit Check of sampler Each sampler...... Once every 6...... Audit flow rate
PM10, PM2.5, PM10 2.5. flow rate using and measured flow
independent rate indicated by
standard. the sampler.
Collocated sampling PM2.5, Collocated 15%............... Every 12 days..... Primary sampler
PM10 2.5. samplers. concentration and
duplicate sampler
concentration
Performance evaluation program Collocated 1. 5 valid audits Over all 4 Primary sampler
PM2.5, PM10 2.5. samplers. for primary QA quarters. concentration and
orgs, with <= 5 performance
sites 2. 8 valid evaluation
audits for sampler
primary QA orgs, concentration.
with > 5 sites 3.
All samplers in 6
years.
----------------------------------------------------------------------------------------------------------------
Manual Methods
----------------------------------------------------------------------------------------------------------------
Collocated sampling PM10, TSP, Collocated 15%............... Every 12 days PSD-- Primary sampler
PM10 2.5, PM2.5, Pb-TSP, Pb- samplers. every 6 days. concentration and
PM10. duplicate sampler
concentration.
Flow rate verification PM10 (low Check of sampler Each sampler...... Once every month.. Audit flow rate
Vol), PM10 2.5, PM2.5, Pb-PM10. flow rate. and measured flow
rate indicated by
the sampler.
Flow rate verification PM10 Check of sampler Each sampler...... Once every quarter Audit flow rate
(High-Vol), TSP, Pb-TSP. flow rate. and measured flow
rate indicated by
the sampler.
Semi-annual flow rate audit Check of sampler Each sampler, all Once every 6 Audit flow rate
PM10, TSP, PM10 2.5, PM2.5, Pb- flow rate using locations. months. and measured flow
TSP, Pb-PM10. independent rate indicated by
standard. the sampler.
Pb audit strips Pb-TSP, Pb-PM10. Check of Analytical........ Each quarter...... Actual
analytical system concentration.
with Pb audit
strips.
Performance evaluation program Collocated 1. 5 valid audits Over all 4 Primary sampler
PM2.5, PM10 2.5. samplers. for primary QA quarters. concentration and
orgs, with <= 5 performance
sites 2. 8 valid evaluation
audits for sampler
primary QA orgs, concentration.
with [gteqt] 5
sites 3. All
samplers in 6
years.
Performance evaluation program Collocated 1 valid audit for Over all 4 Primary sampler
Pb-TSP, Pb-PM10. samplers. primary QA orgs. quarters. concentration and
performance
evaluation
sampler
concentration.
----------------------------------------------------------------------------------------------------------------
\1\ Effective concentration for open path analyzers.
\2\ Corrected concentration, if applicable, for open path analyzers.
* * * * *
18. Appendix D to part 58 is amended as by revising paragraph 4.5
to read as follows:
Appendix D to Part 58--Network Design Criteria for Ambient Air Quality
Monitoring
* * * * *
4.5 Lead (Pb) Design Criteria. (a) State and, where appropriate,
local agencies are required to conduct Pb monitoring near lead
sources which emit more than [200 to 600] kilograms per year. At a
minimum, there must be one source-oriented SLAMS site located
(taking into account logistics and other limitations) to measure the
maximum Pb concentration in ambient air resulting from the lead
source.
(b) The Regional Administrator may waive the requirement in
paragraph 4.5(a) for monitoring near Pb sources emitting less than
1000 kilograms if the State or, where appropriate, local agency can
demonstrate (via historical monitoring data, modeling, or other
means) that the Pb source will not contribute to a maximum Pb
concentration in ambient air in excess of 50% of the NAAQS.
(c) State and, where appropriate, local agencies are required to
conduct Pb
[[Page 29291]]
monitoring in each CBSA with a population greater than 1,000,000
people as determined based on the latest available census figures.
At a minimum, there must be one nonsource-oriented SLAMS site
located to estimate typical Pb concentrations in the urban area.
Consideration should be given to locating these monitors in
neighborhoods near heavily trafficked roadways.
(d) The most important spatial scales for source-oriented sites
to effectively characterize the emissions from point sources are
microscale and middle scale. The most important spatial scale for
nonsource-oriented sites to characterize typical lead concentrations
in urban areas is the neighborhood scale.
(1) Microscale--This scale would typify areas in close proximity
to lead point sources. Emissions from point sources such as primary
and secondary lead smelters, and primary copper smelters may under
fumigation conditions likewise result in high ground level
concentrations at the microscale. In the latter case, the microscale
would represent an area impacted by the plume with dimensions
extending up to approximately 100 meters. Data collected at
microscale sites provide information for evaluating and developing
``hot-spot'' control measures.
(2) Middle scale--This scale generally represents Pb air quality
levels in areas up to several city blocks in size with dimensions on
the order of approximately 100 meters to 500 meters. The middle
scale may for example, include schools and playgrounds in center
city areas which are close to major Pb point sources. Pb monitors in
such areas are desirable because of the higher sensitivity of
children to exposures of elevated Pb concentrations (reference 3 of
this appendix). Emissions from point sources frequently impact on
areas at which single sites may be located to measure concentrations
representing middle spatial scales.
(3) Neighborhood scale--The neighborhood scale would
characterize air quality conditions throughout some relatively
uniform land use areas with dimensions in the 0.5 to 4.0 kilometer
range. Sites of this scale would provide monitoring data in areas
representing conditions where children live and play. Monitoring in
such areas is important since this segment of the population is more
susceptible to the effects of Pb. Where a neighborhood site is
located away from immediate Pb sources, the site may be very useful
in representing typical air quality values for a larger residential
area, and therefore suitable for population exposure and trends
analyses.
(e) Pb monitoring required in paragraphs 4.5(a) and 4.5(c) can
be conducted with either Pb-TSP or Pb-PM10.
(f) Technical guidance is found in references 4 and 5 of this
appendix. These documents provide additional guidance on locating
sites to meet specific urban area monitoring objectives and should
be used in locating new sites or evaluating the adequacy of existing
sites.
* * * * *
[FR Doc. E8-10808 Filed 5-19-08; 8:45 am]
BILLING CODE 6560-50-P