[Federal Register Volume 74, Number 134 (Wednesday, July 15, 2009)]
[Proposed Rules]
[Pages 34404-34466]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E9-15944]
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Part II
Environmental Protection Agency
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40 CFR Parts 50 and 58
Primary National Ambient Air Quality Standard for Nitrogen Dioxide;
Proposed Rule
Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 /
Proposed Rules
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50 and 58
[EPA-HQ-OAR-2006-0922; FRL-8926-3]
RIN 2060-AO19
Primary National Ambient Air Quality Standard for Nitrogen
Dioxide
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
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SUMMARY: Based on its review of the air quality criteria for oxides of
nitrogen and the primary national ambient air quality standard (NAAQS)
for oxides of nitrogen as measured by nitrogen dioxide
(NO2), EPA proposes to make revisions to the primary
NO2 NAAQS in order to provide requisite protection of public
health. Specifically, EPA proposes to supplement the current annual
standard by establishing a new short-term NO2 standard based
on the 3-year average of the 99th percentile (or 4th highest) of 1-hour
daily maximum concentrations. EPA proposes to set the level of this new
standard within the range of 80 to 100 ppb and solicits comment on
standard levels as low as 65 ppb and as high as 150 ppb. EPA also
proposes to establish requirements for an NO2 monitoring
network that will include monitors within 50 meters of major roadways.
In addition, EPA is soliciting comment on an alternative approach to
setting the standard and revising the monitoring network. Consistent
with the terms of a consent decree, the Administrator will sign a
notice of final rulemaking by January 22, 2010.
DATES: Comments must be received on or before September 14, 2009. Under
the Paperwork Reduction Act, comments on the information collection
provisions must be received by OMB on or before August 14, 2009.
Public Hearings: EPA intends to hold public hearings on this
proposed rule in August 2009 in Los Angeles, California and Arlington,
VA. 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-2006-0922 by one of the following methods:
http://www.regulations.gov: Follow the on-line
instructions for submitting comments.
E-mail: [email protected].
Fax: 202-566-9744
Mail: Docket No. EPA-HQ-OAR-2006-0922, 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-2006-0922,
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-
2006-0922. 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: Dr. Scott Jenkins, 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: 919-541-1167; fax: 919-
541-0237; 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.
Provide specific examples to illustrate your concerns, and
suggest alternatives.
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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 the documents that are relevant to this rulemaking are
available through EPA's Office of Air Quality Planning and Standards
(OAQPS) Technology Transfer Network (TTN) Web site at http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_index.html. These documents
include the Integrated Review Plan and the Health Assessment Plan,
available at http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_pd.html, the Integrated Science Assessment (ISA), available at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=194645, and the Risk and
Exposure Assessment (REA), available at http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_rea.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. Related NO2 Control Programs
C. Review of the Air Quality Criteria and Standards for Oxides
of Nitrogen
II. Rationale for Proposed Decisions on the Primary Standard
A. Characterization of NO2 Air Quality
1. Current Patterns of NO2 Air Quality
2. NO2 Air Quality and Gradients Around Roadways
B. Health Effects Information
1. Adverse Respiratory Effects and Short-Term Exposure to
NO2
a. Emergency Department Visits and Hospital Admissions
b. Respiratory Symptoms
c. Impaired Host Defense
d. Airway Response
e. Airway Inflammation
f. Lung Function
g. Conclusions From the ISA
2. Other Effects With Short-Term Exposure to NO2
a. Mortality
b. Cardiovascular Effects
3. Health Effects With Long-Term Exposure to NO2
a. Respiratory Morbidity
b. Mortality
c. Carcinogenic, Cardiovascular, and Reproductive/Developmental
Effects
4. NO2-Related Impacts on Public Health
a. Pre-Existing Disease
b. Age
c. Genetics
d. Gender
e. Proximity to Roadways
f. Socioeconomic Status
g. Size of the At-Risk Population
C. Human Exposure and Health Risk Characterization
1. Evidence Base for the Risk Characterization
2. Overview of Approaches
3. Key Limitations and Uncertainties
D. Considerations in Review of the Standard
1. Background on the Current Standard
2. Approach for Reviewing the Need to Retain or Revise the
Current Standard
E. Adequacy of the Current Standard
1. Evidence-Based Considerations
2. Exposure- and Risk-Based Considerations
3. Summary of Considerations From the REA
4. CASAC Views
5. Administrator's Conclusions Regarding Adequacy of the Current
Standard
F. Conclusions on the Elements of a New Short-Term Standard and
an Annual Standard
1. Indicator
2. Averaging Time
a. Short-Term Averaging Time
b. Long-Term Averaging Time
c. CASAC Views
d. Administrator's Conclusions on Averaging Time
3. Form
4. Level
a. Evidence-Based Considerations
b. Exposure- and Risk-Based Considerations
c. Summary of Consideration From the REA
d. CASAC Views
e. Administrator's Conclusions on Level for a 1-Hour Standard
f. Alternative Approach to Setting the 1-Hour Standard Level
g. Level of the Annual Standard
G. Summary of Proposed Decisions on the Primary Standard
III. Proposed Amendments to Ambient Monitoring and Reporting
Requirements
A. Monitoring Methods
B. Network Design
1. Background
2. Proposed Changes
a. Monitoring in Areas of Expected Maximum Concentrations Near
Major Roads
b. Area-Wide Monitoring at Neighborhood and Larger Spatial
Scales
3. Solicitation for Comment on an Alternative Network Design
C. Data Reporting
IV. Proposed Appendix S--Interpretation of the Primary NAAQS for
Oxides of Nitrogen and Proposed Revisions to the Exceptional Events
Rule
A. Background
B. Interpretation of the Primary NAAQS for Oxides of Nitrogen
1. Annual Primary Standard
2. 1-Hour Primary Standard Based on the Annual 4th Highest Daily
Value Form
3. 1-Hour Primary Standard Based on the Annual 99th Percentile
Value Form
C. Exceptional Events Information Submission Schedule
V. Clean Air Act Implementation Requirements
A. Designations
B. Classifications
C. Attainment Dates
1. Attaining the NAAQS
2. Consequences of Failing to Attain by the Statutory Attainment
Date
D. Section 110(a)(2) NAAQS Infrastructure Requirements
E. Attainment Planning Requirements
1. Nonattainment Area SIPs
2. New Source Review and Prevention of Significant Deterioration
Requirements
3. General Conformity
4. Transportation Conformity
VI. Communication of Public Health Information
VII. Statutory and Executive Order Reviews
References
I. Background
A. Legislative Requirements
Two sections of the Clean Air Act (Act or CAA) govern the
establishment and revision of the NAAQS. Section 108 of the Act directs
the Administrator to identify and list air pollutants that meet certain
criteria, including that the air pollutant ``in his judgment, cause[s]
or contribute[s] to air pollution which may reasonably be anticipated
to endanger public health and welfare'' and ``the presence of which in
the ambient air results from numerous or diverse mobile or stationary
sources.'' 42 U.S.C. 21 7408(a)(1)(A) & (B). For those air pollutants
listed, section 108 requires the Administrator to issue air quality
criteria that ``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 * * *'' 42 U.S.C. 7408(2).
Section 109(a) of the Act directs the Administrator to promulgate
``primary'' and ``secondary'' NAAQS for pollutants for which air
quality criteria have been issued. 42 U.S.C. 7409(1). Section 109(b)(1)
defines a primary standard as one ``the attainment and maintenance of
which in the judgment of the Administrator, based on [the air quality]
criteria and allowing an adequate margin of safety, are requisite to
protect the public health.'' \1\ 42 U.S.C. 7409(b)(1). A secondary
standard, in turn, must ``specify a level of air quality the attainment
and maintenance of which, in the judgment of the Administrator, based
on [the air quality] criteria, is requisite to protect the public
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welfare from any known or anticipated adverse effects associated with
the presence of such pollutant in the ambient air.'' \2\ 42 U.S.C.
7409(b)(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 \ EPA is currently conducting a separate review of the
secondary NO2 NAAQS jointly with a review of the
secondary SO2 NAAQS.
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The requirement that primary standards include an adequate margin
of safety is intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It is 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.
In addressing the requirement for a margin of safety, EPA considers
such factors as the nature and severity of the health effects involved,
the size of the at-risk population(s), and the kind and degree of the
uncertainties that must be addressed. 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, supra, 647 F.2d at 1161-62.
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. In so doing, EPA may not consider the
costs of implementing the standards. Whitman v. American Trucking
Associations, 531 U.S. 457, 471, 475-76 (2001).
Section 109(d)(1) of the Act requires the Administrator to
periodically undertake a thorough review of the air quality criteria
published under section 108 and the NAAQS and to revise the criteria
and standards as may be appropriate. 42 U.S.C. 7409(d)(1). The Act also
requires the Administrator to 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, to review the air
quality criteria and NAAQS and to ``recommend to the Administrator any
new standards and revisions of existing criteria and standards as may
be appropriate under section 108 and subsection (b) of this section.''
42 U.S.C. 7409(d)(2). This independent review function is performed by
the Clean Air Scientific Advisory Committee (CASAC) of EPA's Science
Advisory Board.
B. Related NO2 Control Programs
States are primarily responsible for ensuring attainment and
maintenance of 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 that covers these pollutants. See 42 U.S.C. 7470-7479. In
addition, Federal programs provide for nationwide reductions in
emissions of these and other air pollutants under Title II of the Act,
42 U.S.C. 7521--7574, which involves controls for automobile, truck,
bus, motorcycle, nonroad engine and equipment, 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.
Currently there are no areas in the United States that are
designated as nonattainment of the NO2 NAAQS. If the
NO2 NAAQS is revised as a result of this review, however,
some areas could be classified as non-attainment. Certain States would
then be required to develop SIPs that identify and implement specific
air pollution control measures to reduce ambient NO2
concentrations to attain and maintain the revised NO2 NAAQS,
most likely by requiring air pollution controls on sources that emit
oxides of nitrogen (NOX \3\).
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\3\ In this document, the terms ``oxides of nitrogen'' and
``nitrogen oxides'' (NOX) refer to all forms of oxidized
nitrogen (N) compounds, including NO, NO2, and all other
oxidized N-containing compounds formed from NO and NO2.
This follows usage in the Clean Air Act Section 108(c): ``Such
criteria [for oxides of nitrogen] shall include a discussion of
nitric and nitrous acids, nitrites, nitrates, nitrosamines, and
other carcinogenic and potentially carcinogenic derivatives of
oxides of nitrogen.'' By contrast, within the air pollution research
and control communities, the terms ``oxides of nitrogen'' and
``nitrogen oxides'' are restricted to refer only to the sum of NO
and NO2, and this sum is commonly abbreviated as
NOX. The category label used by this community for the
sum of all forms of oxidized nitrogen compounds including those
listed in Section 108(c) is NOY.
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While NOX is emitted from a wide variety of source
types, the top three categories of sources of NOX emissions
are on-road mobile sources, electricity generating units, and non-road
mobile sources. EPA anticipates that NOX emissions will
decrease substantially over about the next 20 years as a result of the
ongoing implementation of mobile source emissions standards. In
particular, Tier 2 NOX emission standards for light-duty
vehicle emissions began phasing into the fleet beginning with model
year 2004, in combination with low-sulfur gasoline fuel standards. For
heavy-duty engines, new NOX standards are phasing in between
the 2007 and 2010 model years, following the introduction of ultra-low
sulfur diesel fuel. Lower NOX standards for nonroad diesel
engines, locomotives, and certain marine engines are becoming effective
throughout the next decade. In future decades, these lower-
NOX vehicles and engines will become an increasingly large
fraction of in-use mobile sources, effecting large NOX
emission reductions.
C. Review of the Air Quality Criteria and Standards for Oxides of
Nitrogen
On April 30, 1971, EPA promulgated identical primary and secondary
NAAQS for NO2 under section 109 of the Act. The standards
were set at 0.053 parts per million (ppm) (53 ppb), annual average (36
FR 8186). EPA completed reviews of the air quality criteria and
NO2 standards in 1985 and 1996 with decisions to retain the
standard (50 FR 25532, June 19, 1985; 61 FR 52852, October 8, 1996).
EPA initiated the current review of the air quality criteria for
oxides of nitrogen and the NO2 primary NAAQS on December 9,
2005 (70 FR 73236) with a general call for information. EPA's draft
Integrated Review Plan for the Primary National Ambient Air Quality
Standard for Nitrogen Dioxide (EPA, 2007a) was made available in
February 2007 for public comment and was discussed by the CASAC via a
publicly accessible teleconference on May 11, 2007. As noted in that
plan, NOX includes multiple gaseous (e.g., NO2,
NO) and particulate (e.g., nitrate) species. Because the health effects
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associated with particulate species of NOX have been
considered within the context of the health effects of ambient
particles in the Agency's review of the NAAQS for particulate matter
(PM), the current review of the primary NO2 NAAQS is focused
on the gaseous species of NOX and does not consider health
effects directly associated with particulate species.
The first draft of the Integrated Science Assessment for Oxides of
Nitrogen-Health Criteria (ISA) and the Nitrogen Dioxide Health
Assessment Plan: Scope and Methods for Exposure and Risk Assessment
(EPA, 2007b) were reviewed by CASAC at a public meeting held on October
24-25, 2007. Based on comments received from CASAC and the public, EPA
developed the second draft of the ISA and the first draft of the Risk
and Exposure Assessment to Support the Review of the NO2
Primary National Ambient Air Quality Standard (Risk and Exposure
Assessment (REA)). These documents were reviewed by CASAC at a public
meeting held on May 1-2, 2008. Based on comments received from CASAC
and the public at this meeting, EPA released the final ISA in July of
2008 (EPA, 2008a). In addition, comments received were considered in
developing the second draft of the REA, which was released for public
review and comment in two parts. The first part of this document,
containing chapters 1-7, 9 and appendices A and C as well as part of
appendix B, was released in August, 2008. The second part of this
document, containing chapter 8 (describing the Atlanta exposure
assessment) and a completed appendix B, was released in October of
2008. This document was the subject of CASAC reviews at public meetings
on September 9 and 10, 2008 (for the first part) and on October 22,
2008 (for the second part). In preparing the final REA (EPA, 2008b),
EPA considered comments received from the CASAC and the public at those
meetings.
In the course of reviewing the second draft REA, CASAC expressed
the view that the document would be incomplete without the addition of
a policy assessment chapter presenting an integration of evidence-based
considerations and risk and exposure assessment results. CASAC stated
that such a chapter would be ``critical for considering options for the
NAAQS for NO2'' (Samet, 2008a). In addition, within the
period of CASAC's review of the second draft REA, EPA's Deputy
Administrator indicated in a letter to the chair of CASAC, addressing
earlier CASAC comments on the NAAQS review process (Henderson, 2008),
that the risk and exposure assessment will include ``a broader
discussion of the science and how uncertainties may effect decisions on
the standard'' and ``all analyses and approaches for considering the
level of the standard under review, including risk assessment and
weight of evidence methodologies'' (Peacock, 2008, p.3; September 8,
2008).
Accordingly, the final REA included a new policy assessment
chapter. This policy assessment chapter considered the scientific
evidence in the ISA and the exposure and risk characterization results
presented in other chapters of the REA as they relate to the adequacy
of the current NO2 primary NAAQS and potential alternative
primary NO2 standards. In considering the current and
potential alternative standards, the final REA document focused on the
information that is most pertinent to evaluating the basic elements of
national ambient air quality standards: indicator, averaging time, form
\4\, and level. These elements, which together serve to define each
standard, must be considered collectively in evaluating the health
protection afforded. CASAC discussed the final version of the REA, with
an emphasis on the policy assessment chapter, during a public
teleconference held on December 5, 2008. Following that teleconference,
CASAC offered comments and advice on the NO2 primary NAAQS
in a letter to the Administrator (Samet, 2008b).
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\4\ 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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The schedule for completion of this review is governed by a
judicial order resolving a lawsuit filed in September 2005, concerning
the timing of the current review. The order that now governs this
review, entered by the court in August 2007 and amended in December
2008, provides that the Administrator will sign, for publication,
notices of proposed and final rulemaking concerning the review of the
primary NO2 NAAQS no later than June 26, 2009 and January
22, 2010, respectively.
This action presents the Administrator's proposed decisions on the
current primary NO2 standard. Throughout this preamble a
number of 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. Further, EPA
invites specific comments from CASAC on the proposed approach of
establishing a new 1-hour NO2 standard in conjunction with a
revised monitoring network that includes a substantial number of
monitors placed near major roads. In addition to requesting comment on
the overall approach, EPA invites specific comment on the level, or
range of levels, appropriate for such a standard, as well as on the
rationale that would support that level or range of levels.
II. Rationale for Proposed Decisions on the Primary Standard
This section presents the rationale for the Administrator's
proposed decision to revise the existing NO2 primary
standard by supplementing the current annual standard with a 1-hour
standard and to specify the standards to the nearest parts per billion
(ppb). As discussed more fully below, this rationale takes into
account: (1) Judgments and conclusions presented in the ISA and the
REA; (2) CASAC advice and recommendations, as reflected in discussions
of drafts of the ISA and REA at public meetings, in separate written
comments, and in CASAC's letter to the Administrator (Samet, 2008b);
and (3) public comments received at CASAC meetings during the
development of the ISA and the REA.
In developing this rationale, EPA has drawn upon an integrative
synthesis of the entire body of evidence on human health effects
associated with the presence of NO2 in the air. As discussed
below, this body of evidence addresses a broad range of health
endpoints associated with exposure to NO2. In considering
this entire body of evidence, EPA focuses in particular on those health
endpoints for which the ISA finds associations with NO2 to
be causal or likely causal (see section II.B below). This rationale
also draws upon the results of quantitative exposure and risk
assessments.
As discussed below, a substantial amount of new research has been
conducted since the last review of the NO2 NAAQS, with
important new information coming from epidemiologic studies in
particular. The newly available research studies evaluated in the ISA
have undergone intensive scrutiny through multiple layers of peer
review and opportunities for public review and comment. While important
uncertainties remain in the qualitative and quantitative
characterizations of health effects attributable to exposure to ambient
NO2, the review of this
[[Page 34408]]
information has been extensive and deliberate.
The remainder of this section discusses the rationale for the
Administrator's proposed decisions on the primary standard. Section
II.A presents a discussion of NO2 air quality, including
discussion of the NO2 concentration gradients that can exist
around roadways, and the current NO2 monitoring network.
Section II.B includes an overview of the scientific evidence related to
health effects associated with NO2 exposure. This overview
includes discussion of the health endpoints and at-risk populations
considered in the ISA. Section II.C discusses the approaches taken by
EPA to assess exposures and health risks associated with
NO2, including a discussion of key uncertainties associated
with the analyses. Section II.D presents the approach that is being
used in the current review of the NO2 NAAQS with regard to
consideration of the scientific evidence and exposure-/risk-based
results related to the adequacy of the current standard and potential
alternative standards. Sections II.E and II.F discuss the scientific
evidence and the exposure-/risk-based results specifically as they
relate to the current and potential alternative standards, including
discussion of the Administrator's proposed decisions on the standard.
Section II.G summarizes the Administrator's proposed decisions with
regard to the NO2 primary NAAQS.
A. Characterization of NO2 Air Quality
1. Current patterns of NO2 Air Quality
The size of the State and local NO2 monitoring network
has remained relatively stable since the early 1980s, and currently has
approximately 400 monitors reporting data to EPA's Air Quality System
(AQS) database. \5\ At present, there are no minimum monitoring
requirements for NO2 in 40 CFR part 58 Appendix D, other
than a requirement for EPA Regional Administrator approval before
removing any existing monitors, and that any ongoing NO2
monitoring must have at least one monitor sited to measure the maximum
concentration of NO2 in that area (though, as discussed
below monitors in the current network do not measure peak
concentrations associated with on-road mobile sources that can occur
near major roadways because the network was not designed for this
purpose). EPA removed the specific minimum monitoring requirements for
NO2 of two monitoring sites per area with a population of
1,000,000 or more in the 2006 monitoring rule revisions (71 FR 61236),
based on the fact that there were no NO2 nonattainment areas
at that time, coupled with trends evidence showing an increasing gap
between national average NO2 concentrations and the current
annual standard. Additionally, the minimum requirements were removed to
provide State, local, and Tribal air monitoring agencies flexibility in
meeting higher priority monitoring needs for pollutants such as ozone
and PM2.5, or implementing the new multi-pollutant sites
(NCore network) required by the 2006 rule revisions, by allowing them
to discontinue lower priority monitoring. There are requirements in 40
CFR part 58 Appendix D for NO2 monitoring as part of the
Photochemical Assessment Monitoring Stations (PAMS) network. However,
of the approximately 400 NO2 monitors currently in
operation, only about 10 percent may be due to the PAMS requirements.
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\5\ It should be noted that the ISA Section 2.4.1 references a
different number of active monitors in the NO2 network.
The discrepancy between the ISA numbers and the number presented
here is due to differing metrics used in pulling data from AQS. The
ISA only references SLAMS, NAMS, and PAMS sites with defined
monitoring objectives, while the Watkins and Thompson, 2008 value
represents all NO2 sites reporting data at any point
during the year. These differences in numbers of active monitors per
year also explain why the Watkins and Thompson 2008 document
characterized the NO2 network size as relatively stable
since the early 1980s.
---------------------------------------------------------------------------
An analysis of the approximately 400 monitors comprising the
current NO2 monitoring network (Watkins and Thompson, 2008)
indicates that the current NO2 network has largely remained
unchanged in terms of size and target monitor objective categories
since it was introduced in the May 10, 1979 monitoring rule (44 FR
27571). The review of the current network found that the assessment of
concentrations for general population exposure and maximum
concentrations at neighborhood and larger scales were the top
objectives. A review of the distribution of listed spatial scales of
representation shows that only approximately 3 monitors are described
as microscale, representing an area on the order of several meters to
100 meters, and approximately 23 monitors are described as middle
scale, which represents an area on the order of 100 to 500 meters. This
low percentage of smaller spatially representative scale sites within
the network of approximately 400 monitoring sites indicates that the
majority of monitors have, in fact, been sited to assess area-wide
exposures on the neighborhood, urban, and regional scales, as would be
expected for a network sited to support the current annual
NO2 standard and PAMS objectives. The current network does
not include monitors placed near major roadways and, therefore,
monitors in the current network do not necessarily measure the maximum
concentrations that can occur on a localized scale near these roadways
(as discussed in the next section). It should be noted that the network
not only accommodates NAAQS related monitoring, but also serves other
monitoring objectives such as support for photochemistry analysis,
ozone modeling and forecasting, and particulate matter precursor
tracking.
2. NO2 Air Quality and Gradients Around Roadways
On-road and non-road mobile sources account for approximately 60%
of NOX emissions (ISA, table 2.2-1) and traffic-related
exposures can dominate personal exposures to NO2 (ISA
section 2.5.4). While driving, personal exposure concentrations in the
cabin of a vehicle could be substantially higher than ambient
concentrations measured nearby (ISA, section 2.5.4). For example, mean
in-vehicle NO2 concentrations have been reported to be 2 to
3 times higher than non-traffic ambient concentrations (ISA, sections
2.5.4 and 4.3.6). In addition, estimates presented in the REA suggest
that on/near roadway NO2 concentrations could be
approximately 40% (REA, compare Tables 7-11 and 7-13) or 80% (REA,
section 7.3.2) higher on average than concentrations away from roadways
and that roadway-associated environments could be responsible for the
large majority of 1-hour peak NO2 exposures (REA, Figures 8-
17 and 8-18). Because monitors in the current network are not sited to
measure peak roadway-associated NO2 concentrations,
individuals who spend time on and/or near major roadways could
experience NO2 concentrations that are considerably higher
than indicated by monitors in the current area-wide NO2
monitoring network.
Research suggests that the concentrations of on-road mobile source
pollutants such as NOX, carbon monoxide (CO), directly
emitted air toxics, and certain size distributions of particulate
matter (PM), such as ultrafine PM, typically display peak
concentrations on or immediately adjacent to roads (ISA, section 2.5).
This situation typically produces a gradient in pollutant
concentrations, with concentrations decreasing with increasing distance
from the road, and concentrations generally decreasing back to near
area-wide ambient levels, or typical upwind urban background
[[Page 34409]]
levels, within several hundred meters downwind. While this general
concept is applicable to almost all roads, the actual characteristics
of the gradient and the distance that the mobile source pollutant
signature from an individual road can be differentiated from background
or upwind concentrations are heavily dependent on factors including
traffic volumes, local topography, roadside features, meteorology, and
photochemical reactivity conditions (Baldauf, et al., 2009; Beckerman
et al., 2008; Clements et al., 2008; Hagler et al., 2009; Janssen et
al., 2001; Rodes and Holland, 1980; Roorda-Knape et al., 1998; Singer
et al., 2004; Zhou and Levy, 2007).
Because NO2 in the ambient air is due largely to the
atmospheric oxidation of NO emitted from combustion sources (ISA,
section 2.2.1), elevated NO2 concentrations can extend
farther away from roadways than the primary pollutants also emitted by
on-road mobile sources. More specifically, review of the technical
literature suggests that NO2 concentrations may return to
area-wide or typical urban background concentrations within distances
up to 500 meters of roads, though the actual distance will vary with
topography, roadside features, meteorology, and photochemical
reactivity conditions (Baldauf et al., 2009; Beckerman et al., 2008;
Clements et al., 2008; Gilbert et al. 2003; Rodes and Holland, 1980;
Singer et al., 2004; Zhou and Levy, 2007). Efforts to quantify the
extent and slope of the concentration gradient that may exist from peak
near-road concentrations to the typical urban background concentrations
must consider the variability that exists across locations and for a
given location over time. As a result, we have identified a range of
concentration gradients in the technical literature which indicate
that, on average, peak NO2 concentrations on or immediately
adjacent to roads may typically be between 30 and 100 percent greater
than concentrations monitored in the same area but farther away from
the road (ISA, Section 2.5.4; Beckerman et al., 2008; Gilbert et al.,
2003; Rodes and Holland, 1980; Roorda-Knape et al., 1998; Singer et
al., 2004). This range of concentration gradients has implications for
revising the NO2 primary standard and for the NO2
monitoring network (see sections II.F.4 and III).
B. Health Effects Information
In the last review of the NO2 NAAQS, the 1993
NOX Air Quality Criteria Document (1993 AQCD) (EPA, 1993)
concluded that there were two key health effects of greatest concern at
ambient or near-ambient concentrations of NO2 (ISA, section
5.3.1). The first was increased airway responsiveness in asthmatic
individuals after short-term exposures. The second was increased
respiratory illness among children associated with longer-term
exposures to NO2. Evidence also was found for increased risk
of emphysema, but this appeared to be of major concern only with
exposures to NO2 at levels much higher than then current
ambient levels (ISA, section 5.3.1). Controlled human exposure and
animal toxicological studies provided qualitative evidence for airway
hyperresponsiveness and lung function changes while epidemiologic
studies provided evidence for increased respiratory symptoms with
increased indoor NO2 exposures. Animal toxicological
findings of lung host defense system changes with NO2
exposure provided a biologically-plausible basis for the epidemiologic
results. Subpopulations considered potentially more susceptible to the
effects of NO2 exposure included persons with preexisting
respiratory disease, children, and the elderly. The epidemiologic
evidence for respiratory health effects was limited, and no studies had
considered endpoints such as hospital admissions, emergency department
visits, or mortality (ISA, section 5.3.1).
As discussed below, evidence published since the last review
generally has confirmed and extended the conclusions articulated in the
1993 AQCD (ISA, section 5.3.2). The epidemiologic evidence has grown
substantially with the addition of field and panel studies,
intervention studies, time-series studies of endpoints such as hospital
admissions, and a substantial number of studies evaluating mortality
risk associated with short-term NO2 exposures. While not as
marked as the growth in the epidemiologic literature, a number of
recent toxicological and controlled human exposure studies also provide
insights into relationships between NO2 exposure and health
effects. The body of evidence that has become available since the last
review focuses the current review on NO2-related respiratory
effects at lower ambient and exposure concentrations.
The ISA, along with its associated annexes, provides a
comprehensive review and assessment of the scientific evidence related
to the health effects associated with NO2 exposures. For
these health effects, the ISA characterized judgments about causality
with a hierarchy that contains five levels (ISA, section 1.3):
sufficient to infer a causal relationship, sufficient to infer a likely
causal relationship (i.e., more likely than not), suggestive but not
sufficient to infer a causal relationship, inadequate to infer the
presence or absence of a causal relationship, and suggestive of no
causal relationship. Judgments about causality were informed by a
series of aspects that are based on those set forth by Sir Austin
Bradford Hill in 1965 (ISA, Table 1.3-1). These aspects include
strength of the observed association, availability of experimental
evidence, consistency of the observed association, biological
plausibility, coherence of the evidence, temporal relationship of the
observed association, and the presence of an exposure-response
relationship. A summary of each of the five levels of the hierarchy is
provided in Table 1.3-2 of the ISA.
Judgments made in the ISA about the extent to which relationships
between various health endpoints and exposure to NO2 are
likely causal have been informed by several factors. As discussed in
the ISA in section 1.3, these factors include the nature of the
evidence (i.e., controlled human exposure, epidemiological, and/or
toxicological studies) and the weight of evidence. The weight of
evidence takes into account such considerations as biological
plausibility, coherence of the evidence, strength of associations, and
consistency of the evidence. Controlled human exposure studies provide
directly applicable information for determining causality because these
studies are not limited by differences in dosimetry and species
sensitivity, which would need to be addressed in extrapolating animal
toxicology data to human health effects, and because they provide data
relating health effects specifically to NO2 exposures, in
the absence of the co-occurring pollutants present in ambient air.
Epidemiologic studies provide evidence of associations between
NO2 concentrations and more serious health endpoints (e.g.,
hospital admissions and emergency department visits) that cannot be
assessed in controlled human exposure studies. For these studies the
degree of uncertainty introduced by confounding variables (e.g., other
pollutants) affects the level of confidence that the health effects
being investigated are attributable to NO2 exposures alone
and/or in combination with co-occurring pollutants.
In using a weight of evidence approach to inform judgments about
the degree of confidence that various health effects are likely to be
caused by exposure to NO2, confidence increases with the
number of studies consistently reporting a particular health endpoint,
[[Page 34410]]
with increasing support for the biological plausibility of the health
effects, and with the strength and coherence of the evidence.
Conclusions regarding biological plausibility, consistency, and
coherence of evidence of NO2-related health effects are
drawn from the integration of epidemiologic studies with controlled
human exposure studies and with mechanistic information from animal
toxicological studies. As discussed below, the weight of evidence is
strongest for respiratory morbidity endpoints (e.g., respiratory
symptoms, hospital admissions, and emergency department visits)
associated with short-term (e.g., 1 to 24 hours) NO2
exposures.
For epidemiologic studies, strength of association refers to the
magnitude of the association and its statistical strength, which
includes assessment of both effect estimate size and precision. In
general, when associations yield large relative risk estimates, it is
less likely that the association could be completely accounted for by a
potential confounder or some other bias. Consistency refers to the
persistent finding of an association between exposure and outcome in
multiple studies of adequate power in different persons, places,
circumstances and times. Based on the information presented in the ISA
and summarized below in sections II.B.1-II.B.3, this section discusses
judgments concerning the extent to which relationships between various
health endpoints and ambient NO2 exposures have been judged
in the ISA to be likely causal.
As noted above, this section is devoted to discussion of health
effects associated with NO2 exposure, as assessed in the
ISA. Section II.B.1 below discusses respiratory morbidity associated
with short-term exposure to NO2. The specific endpoints
considered in this section are respiratory-related emergency department
visits and hospital admissions, respiratory symptoms, lung host defense
and immunity, airway responsiveness, airway inflammation, and lung
function. Section II.B.2 discusses mortality and cardiovascular effects
associated with short-term exposures. Section II.B.3 discusses effects
that have been associated with long-term NO2 exposures
including respiratory morbidity, mortality, cancer, cardiovascular
effects, and reproductive/developmental effects. Section II.B.4
discusses the potential NO2-related impacts on public
health.
1. Adverse Respiratory Effects and Short-Term Exposure to
NO2
The ISA concluded that, taken together, recent studies provide
scientific evidence that is sufficient to infer a likely causal
relationship between short-term NO2 exposure and adverse
effects on the respiratory system (ISA, section 5.3.2.1). This
determination was based on consideration of the broad array of relevant
scientific evidence, as well as the uncertainties associated with that
evidence. Specifically, this determination is supported by the large
body of recent epidemiologic evidence as well as findings from human
and animal experimental studies.
In considering the uncertainties associated with the epidemiologic
evidence, the ISA (section 5.4) noted that it is difficult to determine
``the extent to which NO2 is independently associated with
respiratory effects or if NO2 is a marker for the effects of
another traffic-related pollutant or mix of pollutants.'' On-road
vehicle exhaust emissions are a nearly ubiquitous source of combustion
pollutant mixtures that include NOX and can be an important
contributor to NO2 levels in near-road locations. Although
this complicates efforts to quantify specific NO2-related
health effects, a number of epidemiologic studies have evaluated
associations with NO2 in models that also include co-
occurring pollutants such as PM, O3, CO, and/or
SO2. The evidence summarized in the ISA indicates that
NO2 associations generally remain robust in these multi-
pollutant models and supports a direct effect of short-term
NO2 exposure on respiratory morbidity (see ISA Figures 3.1-
7, 3.1-10, 3.1-11 and Figures 1 through 3 below). The plausibility and
coherence of these effects are also supported by epidemiologic studies
of indoor NO2 as well as experimental (i.e., toxicologic and
controlled human exposure) studies that have evaluated host defense and
immune system changes, airway inflammation, and airway responsiveness
(see subsequent sections of this proposal and the ISA, section
5.3.2.1). The ISA (section 5.4) concluded that the robustness of
epidemiologic findings to adjustment for co-pollutants, coupled with
data from animal and human experimental studies, support a
determination that the relationship between NO2 and
respiratory morbidity is likely causal, while still recognizing the
relationship between NO2 and other traffic related
pollutants.
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BILLING CODE 6560-50-C
The epidemiologic and experimental studies encompass a number of
endpoints, including emergency department visits and hospitalizations,
respiratory symptoms, airway hyperresponsiveness, airway inflammation,
and lung function. Effect estimates from epidemiologic studies
conducted in the United States and Canada generally indicate a 2-20%
\6\ increase in risks for emergency department visits and hospital
admissions and higher risks for respiratory symptoms (ISA, section
5.4). The findings relevant to these endpoints, which provide the
rationale to support the judgment of a likely causal relationship, are
described in more detail below.
---------------------------------------------------------------------------
\6\ Effect estimates in the ISA were standardized to a 30 ppb
increase in NO2 concentrations and to a 20 ppb increase
for studies that evaluated 24-hour average concentrations.
---------------------------------------------------------------------------
a. Emergency Department Visits and Hospital Admissions
Epidemiologic evidence exists for positive associations of short-
term ambient NO2 concentrations below the current NAAQS with
increased numbers of emergency department visits and hospital
admissions for respiratory causes, especially asthma (ISA, section
5.3.2.1). Total respiratory causes for emergency department visits and
hospitalizations typically include asthma, bronchitis and emphysema
(collectively referred to as COPD), pneumonia, upper and lower
respiratory infections, and other minor categories. Temporal
associations between respiratory emergency department visits or
hospital admissions and ambient levels of NO2 have been the
subject of over 50 peer-reviewed research publications since the review
of the NO2 NAAQS that was completed in 1996. These studies
have examined morbidity in different age groups and have often utilized
multi-pollutant models to evaluate potential confounding effects of co-
pollutants. Associations are particularly consistent among children (<
14 years) and older adults (> 65 years) when all respiratory outcomes
are analyzed together (ISA, Figures 3.1-8 and 3.1-9) and among children
and subjects of all ages for asthma admissions (ISA, Figures 3.1-12 and
3.1-13). When examined with co-pollutant models, associations of
NO2 with respiratory emergency department visits and
hospital admissions were generally robust and independent of the
effects of co-pollutants (i.e., magnitude of effect estimates remained
relatively unchanged) (ISA, Figures 3.1-10 and 3.1-11). The
plausibility and coherence of these effects are supported by
experimental (i.e., toxicologic and controlled human exposure) studies
that evaluate host defense and immune system changes, airway
inflammation, and airway responsiveness (see subsequent sections of
this document and ISA, section 5.3.2.1).
Of the respiratory emergency department visit and hospital
admission studies reviewed in the ISA, 6 key studies were conducted in
the United States (ISA, Table 5.4-1). Of these 6 studies, 4 evaluated
associations with NO2 using multi-pollutant models (Peel et
al., 2005 and updated in Tolbert et al., 2007 in Atlanta; New York
Department of Health (NYDOH), 2006 and Ito et al., 2007 in New York
City), while 2 studies evaluated only single pollutant models (Linn et
al., 2000 in Los Angeles; Jaffe et al., 2003 in Cleveland/Cincinnati,
OH). In the study by Peel and colleagues, investigators evaluated
respiratory emergency department visits among all ages in Atlanta, GA
during the period from 1993 to 2000. Using single pollutant models, a
2.4% (95% CI: 0.9%, 4.1%) increase in respiratory emergency department
visits was associated with a 30-ppb increase in 1-hour maximum
NO2 concentrations. For asthma visits, a 4.1% (95% CI: 0.8%,
7.6%) increase was estimated in individuals 2 to 18 years of age.
Tolbert and colleagues reanalyzed these data with 4 additional years of
information and found essentially similar results in single pollutant
models (2.0% increase, 95% CI: 0.5%, 3.3%). This same study found that
the associations were positive, but not statistically significant, in
multi-pollutant models that included PM10 or O3
(Figure 2 in published manuscript). In the study conducted by the
NYDOH, investigators evaluated asthma
[[Page 34414]]
emergency department visits in Bronx and Manhattan, New York over the
period of January 1999 to November 2000. In Bronx, a 6% (95% CI: 1%,
10%) increase in visits was estimated per 20 ppb increase in 24-hour
average concentrations of NO2 and a 7% (95% CI: 2%, 12%)
increase in visits was estimated per 30 ppb increase in daily 1-hour
maximum concentrations. These effects were not statistically
significant in 2-pollutant models that included PM2.5 or
SO2 (Tables 4a and 9 in manuscript). In Manhattan, the
authors found non-significant decreases (3% for 24-hour and a 2% for
daily 1-hour maximum) in asthma-related emergency department visits
associated with increasing NO2. In the study by Ito and
colleagues (2007), investigators evaluated respiratory emergency
department visits for asthma in New York City during the years 1999 to
2002. A 12% (95% CI: 7%, 15%) increase in risk was estimated per 20 ppb
increase in 24-hour ambient NO2. Risk estimates were robust
and remained statistically significant in multi-pollutant models that
included PM2.5, O3, CO, and SO2
(figure 8 in manuscript). With regard to the studies that evaluated
only single pollutant models, Linn et al. (2000) detected a
statistically significant increase in respiratory hospital admissions
and Jaffe et al. (2003) detected a positive, but not statistically
significant, increase in respiratory emergency department visits
associated with 24-hour NO2 concentrations.
b. Respiratory Symptoms
Evidence for associations between NO2 and respiratory
symptoms is derived primarily from the epidemiologic literature,
although the experimental evidence for airway inflammation and immune
system effects (described in the ISA, section 3.1) does provide support
for the plausibility and coherence for the epidemiologic results (ISA,
section 5.3.2.1). Consistent evidence has been observed for an
association of respiratory effects with indoor and personal
NO2 exposures in children (ISA, sections 3.1.5.1 and
5.3.2.1) and with ambient levels of NO2, as measured by
area-wide monitors (ISA, sections 3.1.4.2 and 5.3.2.1, see Figure 3.1-
6). In the results of multi-pollutant models, NO2
associations in multicity studies are generally robust to adjustment
for co-pollutants including O3, CO, and PM10
(ISA, sections 3.1.4.3, 5.3.2.1 and Figure 3.1-7). Specific studies of
respiratory symptoms are discussed in more detail below.
Epidemiologic studies using community ambient monitors have found
associations between ambient NO2 concentrations and
respiratory symptoms (ISA, sections 3.1.4.2 and 5.3.2.1, Figure 3.1-6)
in cities where the entire range of 24-hour average NO2
concentrations were well below the level of the current NAAQS (0.053
ppm annual average). Several studies have been published since the last
review including single-city studies (e.g., Ostro et al., 2001; Delfino
et al., 2002) and multicity studies in urban areas covering the
continental United States and southern Ontario (Schwartz et al., 1994;
Mortimer et al., 2002; Schildcrout et al., 2006).
Schwartz et al. (1994) studied 1,844 schoolchildren, followed for 1
year, as part of the Six Cities Study that included the cities of
Watertown, MA, St. Louis, MO, Kingston-Harriman, TN, Steubenville, OH,
Topeka, KS, and Portage, WI. Respiratory symptoms were recorded daily.
The authors reported a significant association between 4-day mean
NO2 levels and incidence of cough among all children in
single-pollutant models, with an odds ratio (OR) of 1.61 (95% CI: 1.08,
2.43) standardized to a 20-ppb increase in NO2. The
incidence of cough increased up to approximately mean NO2
levels (13 ppb) (p = 0.01), after which no further increase was
observed. The significant association between cough and 4-day mean
NO2 level remained unchanged in models that included
O3 but lost statistical significance in two-pollutant models
that included PM10 (OR = 1.37 [95% CI: 0.88, 2.13]) or
SO2 (OR = 1.42 [95% CI: 0.90, 2.28]).
Mortimer et al. (2002) studied the risk of asthma symptoms among
864 asthmatic children in New York City, NY, Washington, DC, Cleveland,
OH, Detroit, MI, St Louis, MO, and Chicago, IL. Subjects were followed
daily for four 2-week periods over the course of nine months with
morning and evening asthma symptoms and peak flow recorded. The
greatest effect was observed for morning symptoms using a 6-day moving
average, with a reported OR of 1.48 (95% CI: 1.02, 2.16) per 20 ppb
increase in NO2. Although the magnitudes of effect estimates
were generally robust in multi-pollutant models that included
O3 (OR for 20-ppb increase in NO2 = 1.40 [95% CI:
0.93, 2.09]), O3 and SO2 (OR for NO2 =
1.31 [95% CI: 0.87, 2.09]), or O3, SO2, and
PM10 (OR for NO2 = 1.45 [95% CI: 0.63, 3.34]),
they were not statistically significant.
Schildcrout et al. (2006) investigated the association between
ambient NO2 and respiratory symptoms and rescue inhaler use
as part of the Childhood Asthma Management Program (CAMP) study. The
study reported on 990 asthmatic children living within 50 miles of an
NO2 monitor in Boston, MA, Baltimore, MD, Toronto, ON, St.
Louis, MO, Denver, CO, Albuquerque, NM, or San Diego, CA. Symptoms and
use of rescue medication were recorded daily, resulting in each subject
having an average of approximately two months of data. The authors
reported the strongest association between NO2 and increased
risk of cough for a 2-day lag, with an OR of 1.09 (95% CI: 1.03, 1.15)
for each 20-ppb increase in NO2 occurring 2 days before
measurement. Multi-pollutant models that included CO, PM10,
or SO2 produced similar results (ISA, Figure 3.1-5, panel
A). Additionally, increased NO2 exposure was associated with
increased use of rescue medication, with the strongest association for
a 2-day lag. In the single-pollutant model, the relative risk (RR) for
increased inhaler usage was 1.05 (95% CI: 1.01, 1.09).
Evidence supporting increased respiratory symptoms following
NO2 exposures is found in studies focused on indoor sources
of NO2 (ISA, section 3.1.4.1). These studies are not
confounded by the same mix of co-pollutants present in the ambient air
or by the contribution of NO2 to the formation of secondary
particles or O3 (ISA, section 3.1.4.1). Specifically, in a
randomized intervention study in Australia (Pilotto et al., 2004),
asthmatic students attending schools that switched out unvented gas
heaters, a major source of indoor NO2, experienced a
decrease in both levels of NO2 and in respiratory symptoms
(e.g., difficulty breathing, chest tightness, and asthma attacks)
compared to students in schools that did not switch out unvented gas
heaters (ISA, section 3.1.4.1). An earlier indoor study by Pilotto and
colleagues (1997) also found that students in classrooms with higher
levels of NO2 due primarily to indoor sources had higher
rates of respiratory symptoms (e.g., sore throat, cold) and absenteeism
than students in classrooms with lower levels of NO2. This
study detected a significant concentration-response relationship,
strengthening the argument that NO2 is causally related to
respiratory morbidity. A number of other indoor studies conducted in
homes with gas appliances have also detected significant associations
between indoor NO2 and respiratory symptoms (ISA, section
3.1.4.1).
c. Impaired Host Defense
Impaired host-defense systems and increased risk of susceptibility
to both viral and bacterial infections after NO2 exposures
have been observed in
[[Page 34415]]
epidemiologic, controlled human exposure, and animal toxicological
studies (ISA, section 3.1.1 and 5.3.2.1). A recent epidemiologic study
(Chauhan et al., 2003) provides evidence that increased personal
exposure to NO2 worsened virus-associated symptoms and
decreased lung function in children with asthma. The limited evidence
from controlled human exposure studies indicates that NO2
may increase susceptibility to lung injury by subsequent viral
challenge at exposures of as low as 600 ppb for 3 hours in healthy
adults (Frampton et al., 2002). Toxicological studies have shown that
lung host defenses, including mucociliary clearance and immune cell
function, are sensitive to NO2 exposure, with effects
observed at concentrations of less than 1000 ppb (ISA, section 3.1.7).
When taken together, epidemiologic and experimental studies linking
NO2 exposure with viral illnesses provide coherent and
consistent evidence that NO2 exposure can result in lung
host defense or immune system effects (ISA, sections 3.1.7 and
5.3.2.1). This group of outcomes also provides some plausibility for
other respiratory system effects. For example, effects on ciliary
action (clearance) or immune cell function (i.e. macrophage
phagocytosis) could be the basis for the effects observed in
epidemiologic studies, including increased respiratory illness or
respiratory symptoms (ISA, section 5.3.2.1). Proposed mechanisms by
which NO2, in conjunction with viral infections, may
exacerbate airway symptoms are summarized in the ISA (Table 3.1-1).
d. Airway Response
In acute exacerbations of asthma, bronchial smooth muscle
contraction occurs quickly to narrow the airway in response to exposure
to various stimuli including allergens or irritants.
Bronchoconstriction is the dominant physiological event leading to
clinical symptoms and interference with airflow (National Heart, Lung,
and Blood Institute, 2007). Inhaled pollutants such as NO2
may enhance the inherent responsiveness of the airway to a challenge by
allergens and nonspecific agents (ISA, section 3.1.3). In the
laboratory, airway responses can be measured by assessing changes in
pulmonary function (e.g., decline in FEV1) or changes in the
inflammatory response (e.g., using markers in bronchoalveolar lavage
(BAL) fluid or induced sputum) (ISA, section 3.1.3).
The ISA (section 5.3.2.1) drew two broad conclusions regarding
airway responsiveness in asthmatics following NO2 exposure.
First, the ISA concluded that NO2 exposure may enhance the
sensitivity to allergen-induced decrements in lung function and
increase the allergen-induced airway inflammatory response at exposures
as low as 260 ppb NO2 for 30 minutes (ISA, section 5.3.2.1
and Figure 3.1-2). Second, exposure to NO2 has been found to
enhance the inherent responsiveness of the airway to subsequent
nonspecific challenges in controlled human exposure studies (section
3.1.3.2). In general, small but significant increases in nonspecific
airway responsiveness were observed in the range of 200 to 300 ppb
NO2 for 30-minute exposures and at 100 ppb NO2
for 60-minute exposures in asthmatics. These conclusions are consistent
with results from animal toxicological studies which have detected 1)
increased immune-mediated pulmonary inflammation in rats exposed to
house dust mite allergen following exposure to 5000 ppb NO2
for 3-h and 2) increased responsiveness to non-specific challenges
following sub-chronic (6-12 weeks) exposure to 1000 to 4000 ppb
NO2 (ISA, section 5.3.2.1).
Enhanced airway responsiveness could have important clinical
implications for asthmatics since transient increases in airway
responsiveness following NO2 exposure have the potential to
increase symptoms and worsen asthma control (ISA, section 5.4). In
addition, the ISA cited the controlled human exposure literature on the
NO2 airway response as being supportive of the epidemiologic
evidence on respiratory morbidity (ISA, section 5.4). Because studies
on airway responsiveness have been used to identify potential health
effect benchmark values and to inform the identification of potential
alternative standards for evaluation (see REA, sections 4.5 and 5),
more detail is provided below on the specific studies that form the
basis for the conclusions in the ISA regarding this endpoint.
Folinsbee (1992) conducted a meta-analysis using individual level
data from 19 NO2 controlled human exposure studies measuring
airway responsiveness in asthmatics (ISA, section 3.1.3.2). These
studies included NO2 exposure levels between 100 and 1000
ppb and most of them used nonspecific bronchoconstricting agents such
as methacholine, carbachol, histamine, or cold air. The largest effects
were observed for asthmatics at rest. Among asthmatics exposed at rest,
76% experienced increased airway responsiveness following exposure to
NO2 levels between 200 and 300 ppb. Results from an update
of this meta-analysis, which focused only on data for nonspecific
responsiveness, are presented in the ISA (Table 3.1-3).\7\ When exposed
at rest, 66% of asthmatics experienced an increase in airway
responsiveness following exposure to 100 ppb NO2, 67% of
asthmatics experienced an increase in airway responsiveness following
exposure to NO2 concentrations between 100 and 150 ppb
(inclusively), 75% of subjects experienced an increase in airway
responsiveness following exposure to NO2 concentrations
between 200 and 300 ppb (inclusively), and 73% of subjects experienced
an increase in airway responsiveness following exposure to
NO2 concentrations above 300 ppb. Effects of NO2
exposure on the direction of airway responsiveness were statistically
significant at all of these levels. Because this meta-analysis
evaluated only the direction of the change in airway responsiveness, it
is not possible to discern the magnitude of the change from these data.
However, the results do suggest that short-term (i.e., 30-min to 3-h)
exposures to NO2 at near-ambient levels (<300 ppb) can alter
airway responsiveness in people with mild asthma (ISA, section
3.1.3.2).
---------------------------------------------------------------------------
\7\ The updated meta-analysis added a study that evaluated non-
specific airway responsiveness following exposure to 260 ppb NO2 and
removed a study that evaluated allergen-induced airway
responsiveness following exposure to 100 ppb NO2.
---------------------------------------------------------------------------
Several studies published since the 1996 review evaluate the
potential for low-level exposures to NO2 to enhance the
response to specific allergen challenge in mild asthmatics (ISA,
section 3.1.3.1). These studies suggest that NO2 may enhance
the sensitivity to allergen-induced decrements in lung function and
increase the allergen-induced airway inflammatory response. Strand et
al. (1997) demonstrated that single 30-minute exposures to 260 ppb
NO2 increased the late phase response to allergen challenge
4 hours after exposure, as measured by changes in lung function. In a
separate study (Strand et al., 1998), 4 daily repeated exposures to 260
ppb NO2 for 30 minutes increased both the early and late-
phase responses to allergen, as measured by changes in lung function.
Barck et al. (2002) used the same exposure and challenge protocol in
the earlier Strand study (260 ppb for 30 min, with allergen challenge 4
hours after exposure), and performed BAL 19 hours after the allergen
challenge to determine NO2 effects on the allergen-induced
inflammatory response. Compared with air followed by allergen,
NO2 followed by allergen caused an
[[Page 34416]]
increase in the BAL recovery of polymorphonuclear (PMN) cells and
eosinophil cationic protein (ECP) as well as a reduction in total BAL
fluid volume and cell viability. ECP is released by degranulating
eosinophils, is toxic to respiratory epithelial cells, and is thought
to play a role in the pathogenesis of airway injury in asthma.
Subsequently, Barck et al. (2005) exposed 18 mild asthmatics to air or
260 ppb NO2 for 15 minutes on day 1, followed by two 15
minute exposures separated by 1 hour on day 2, with allergen challenge
after exposures on both days 1 and 2. Sputum was induced before
exposure on day 1 and after exposures (morning of day 3). Compared to
air plus allergen, NO2 plus allergen resulted in increased
levels of ECP in both sputum and blood and increased myeloperoxidase
levels in blood.
All exposures in these studies (Barck et al., 2002, 2005; Strand et
al., 1997, 1998) used subjects at rest. They used an adequate number of
subjects, included air control exposures, randomized exposure order,
and separated exposures by at least 2 weeks. Together, they indicate
the possibility for effects on allergen responsiveness in some
asthmatics following brief exposures to 260 ppb NO2. Other
recent studies have failed to find effects using similar, but not
identical, approaches (ISA, section 3.1.3.1). The differing findings
may relate in part to differences in timing of the allergen challenge,
the use of multiple versus single-dose allergen challenge, the use of
BAL versus sputum induction, exercise versus rest during exposure, and
differences in subject susceptibility (ISA, section 3.1.3.1).
e. Airway Inflammation
Effects of NO2 on airway inflammation have been observed
in controlled human exposure and animal toxicological studies at higher
than ambient levels (400-5000 ppb). Controlled human exposure studies
provide evidence for increased airway inflammation at NO2
concentrations of <2000 ppb. The onset of inflammatory responses in
healthy subjects appears to be between 100 and 200 ppm-minutes, i.e.,
1000 ppb for 2 to 3 hours (ISA, Figure 3.1-1). Increases in biological
markers of inflammation were not observed consistently in healthy
animals at levels of less than 5000 ppb; however, increased
susceptibility (as indicated by biochemical markers of inflammation) to
NO2 concentrations of as low as 400 ppb was observed when
lung vitamin C was reduced (by diet) to levels that were <50% of
normal. The few available epidemiologic studies were suggestive of an
association between ambient NO2 concentrations and
inflammatory response in the airway in children, though the
associations were inconsistent in the adult populations examined (ISA,
section 3.1.2 and 5.3.2.1). These data provide some evidence for
biological plausibility and one potential mechanism for other
respiratory effects, such as exacerbation of asthma symptoms and
increased emergency department visits for asthma (ISA, section
5.3.2.1).
f. Lung Function
Recent epidemiologic studies that examined the association between
ambient NO2 concentrations and lung function in children and
adults have produced inconsistent results (ISA, sections 3.1.5.1 and
5.3.2.1). Controlled human exposure studies generally did not find
direct effects of NO2 on lung function in healthy adults at
levels as high as 4000 ppb (ISA, section 5.3.2.1). For asthmatics, the
direct effects of NO2 on lung function also have been
inconsistent at exposure concentrations of less than 1000 ppb
NO2.
g. Conclusions From the ISA
As noted previously, the ISA concluded that the findings of
epidemiologic, controlled human exposure, and animal toxicological
studies provide evidence that is sufficient to infer a likely causal
relationship for respiratory effects following short-term
NO2 exposure (ISA, sections 3.1.7 and 5.3.2.1). The ISA
(section 5.4) concluded that the strongest evidence for an association
between NO2 exposure and adverse human health effects comes
from epidemiologic studies of respiratory symptoms, emergency
department visits, and hospital admissions. These studies include panel
and field studies, studies that control for the effects of co-occurring
pollutants, and studies conducted in areas where the whole distribution
of ambient 24-hour average NO2 concentrations was below the
current NAAQS level of 53 ppb (annual average). With regard to this
evidence, the ISA concluded that NO2 epidemiologic studies
provide ``little evidence of any effect threshold'' (ISA, section
5.3.2.9, p. 5-15). In studies that have evaluated concentration-
response relationships, they appear linear within the observed range of
data (ISA, section 5.3.2.9).
Overall, the epidemiologic evidence for respiratory effects has
been characterized in the ISA as consistent, in that associations are
reported in studies conducted in numerous locations with a variety of
methodological approaches. Considering this large body of epidemiologic
studies alone, the findings have also been characterized as coherent in
that the studies report associations with respiratory health outcomes
that are logically linked together. In addition, a number of these
associations are statistically significant, particularly the more
precise effect estimates (ISA, section 5.3.2.1). These epidemiologic
studies are supported by evidence from toxicological and controlled
human exposure studies, particularly those that evaluated airway
hyperresponsiveness in asthmatic individuals (ISA, section 5.4). The
ISA concluded that together, the epidemiologic and experimental data
sets form a plausible, consistent, and coherent description of a
relationship between NO2 exposures and an array of adverse
respiratory health effects that range from the onset of respiratory
symptoms to hospital admissions.
2. Other Effects With Short-Term Exposure to NO2
a. Mortality
The ISA concluded that the epidemiologic evidence is suggestive,
but not sufficient, to infer a causal relationship between short-term
exposure to NO2 and all-cause and cardiopulmonary-related
mortality (ISA, section 5.3.2.3). Results from several large U.S. and
European multicity studies and a meta-analysis study indicate positive
associations between ambient NO2 concentrations and the risk
of all-cause (nonaccidental) mortality, with effect estimates ranging
from 0.5 to 3.6% excess risk in mortality per standardized increment
(20 ppb for 24-hour averaging time, 30 ppb for 1-hour averaging time)
(ISA, section 3.3.1, Figure 3.3-2, section 5.3.2.3). In general, the
NO2 effect estimates were robust to adjustment for co-
pollutants. Both cardiovascular and respiratory mortality have been
associated with increased NO2 concentrations in
epidemiologic studies (ISA, Figure 3.3-3); however, similar
associations were observed for other pollutants, including PM and
SO2. The range of risk estimates for excess mortality is
generally smaller than that for other pollutants such as PM. In
addition, while NO2 exposure, alone or in conjunction with
other pollutants, may contribute to increased mortality, evaluation of
the specificity of this effect is difficult. Clinical studies showing
hematologic effects and animal toxicological studies showing
biochemical, lung host defense, permeability, and inflammation changes
[[Page 34417]]
with short-term exposures to NO2 provide limited evidence of
plausible pathways by which risks of mortality may be increased, but no
coherent picture is evident at this time (ISA, section 5.3.2.3).
b. Cardiovascular Effects
The ISA concluded that the available evidence on cardiovascular
health effects following short-term exposure to NO2 is
inadequate to infer the presence or absence of a causal relationship at
this time (ISA, section 5.3.2.2). Evidence from epidemiologic studies
of heart rate variability, repolarization changes, and cardiac rhythm
disorders among heart patients with ischemic cardiac disease are
inconsistent (ISA, section 5.3.2.2). In most studies, associations with
PM were found to be similar or stronger than associations with
NO2. Generally positive associations between ambient
NO2 concentrations and hospital admissions or emergency
department visits for cardiovascular disease have been reported in
single-pollutant models (ISA, section 5.3.2.2); however, most of these
effect estimate values were diminished in multi-pollutant models that
also contained CO and PM indices (ISA, section 5.3.2.2). Mechanistic
evidence of a role for NO2 in the development of
cardiovascular diseases from studies of biomarkers of inflammation,
cell adhesion, coagulation, and thrombosis is lacking (ISA, section
5.3.2.2). Furthermore, the effects of NO2 on various
hematological parameters in animals are inconsistent and, thus, provide
little biological plausibility for effects of NO2 on the
cardiovascular system (ISA, section 5.3.2.2).
3. Health Effects With Long-Term Exposure to NO2
a. Respiratory Morbidity
The ISA concluded that overall, the epidemiologic and experimental
evidence is suggestive, but not sufficient, to infer a causal
relationship between long-term NO2 exposure and respiratory
morbidity (ISA, section 5.3.2.4). The available database evaluating the
relationship between respiratory illness in children and long-term
exposures to NO2 has increased since the 1996 review of the
NO2 NAAQS. A number of epidemiologic studies have examined
the effects of long-term exposure to NO2 and reported
positive associations with decrements in lung function and partially
irreversible decrements in lung function growth (ISA, section 3.4.1,
Figures 3.4-1 and 3.4-2). Specifically, results from the California-
based Children's Health Study, which evaluated NO2 exposures
in children over an 8-year period, demonstrated deficits in lung
function growth (Gauderman et al., 2004). This effect has also been
observed in Mexico City, Mexico (Rojas-Martinez et al., 2007a,b) and in
Oslo, Norway (Oftedal et al., 2008), with decrements ranging from 1 to
17.5 ml per 20-ppb increase in annual NO2 concentration.
Similar associations have been found for PM, O3, and
proximity to traffic (<500 m), though these studies did not report the
results of co-pollutant models. The high correlation among traffic-
related pollutants makes it difficult to accurately estimate
independent effects in these long-term exposure studies (ISA, section
5.3.2.4). With regard to asthma incidence and long-term NO2,
two major cohort studies, the Children's Health Study (Gauderman et
al., 2005) and a birth cohort study in the Netherlands (Brauer et al.,
2007), observed significant associations. However, several other
studies failed to find consistent associations between long-term
NO2 exposure and asthma outcomes (ISA, section 5.3.2.4).
Similarly, epidemiologic studies conducted in the United States and
Europe reported inconsistent results regarding an association between
long-term exposure to NO2 and respiratory symptoms (ISA,
sections 3.4.3 and 5.3.2.4). While some positive associations were
noted, a large number of symptom outcomes were examined and the results
across specific outcomes were inconsistent (ISA, section 5.3.2.4).
Animal toxicological studies may provide biological plausibility
for the chronic effects of NO2 that have been observed in
epidemiologic studies (ISA, sections 3.4.5 and 5.3.2.4). The main
biochemical targets of NO2 exposure appear to be
antioxidants, membrane polyunsaturated fatty acids, and thiol groups.
NO2 effects include changes in oxidant/antioxidant
homeostasis and chemical alterations of lipids and proteins. Lipid
peroxidation has been observed at NO2 exposures as low as 40
ppb for 9 months and at exposures of 1200 ppb for 1 week, suggesting
lower effect thresholds with longer durations of exposure. Other
studies showed decreases in formation of key arachidonic acid
metabolites in alveolar macrophages following NO2 exposures
of 500 ppb. NO2 has been shown to increase collagen
synthesis rates at concentrations as low as 500 ppb. This could
indicate increased total lung collagen, which is associated with
pulmonary fibrosis, or increased collagen turnover, which is associated
with remodeling of lung connective tissue. Morphological effects
following chronic NO2 exposures have been identified in
animal studies that link to these increases in collagen synthesis and
may provide plausibility for the deficits in lung function growth
described in epidemiologic studies of long-term exposure to
NO2 (ISA, section 3.4.5).
b. Mortality
The ISA concluded that the epidemiologic evidence is inadequate to
infer the presence or absence of a causal relationship between long-
term exposure to NO2 and mortality (ISA, section 5.3.2.6).
In the United States and European cohort studies examining the
relationship between long-term exposure to NO2 and
mortality, results have been inconsistent (ISA, section 5.3.2.6).
Further, when associations were suggested, they were not specific to
NO2 but also implicated PM and other traffic indicators. The
relatively high correlations reported between NO2 and PM
indices make it difficult to interpret these observed associations at
this time (ISA, section 5.3.2.6).
c. Carcinogenic, Cardiovascular, and Reproductive/Developmental Effects
The ISA concluded that the available epidemiologic and
toxicological evidence is inadequate to infer the presence or absence
of a causal relationship for carcinogenic, cardiovascular, and
reproductive and developmental effects related to long-term
NO2 exposure (ISA, section 5.3.2.5). Epidemiologic studies
conducted in Europe have shown an association between long-term
NO2 exposure and increased incidence of cancer (ISA, section
5.3.2.5). However, the animal toxicological studies have provided no
clear evidence that NO2 acts as a carcinogen (ISA, section
5.3.2.5). The very limited epidemiologic and toxicological evidence do
not suggest that long-term exposure to NO2 has
cardiovascular effects (ISA, section 5.3.2.5). The epidemiologic
evidence is not consistent for associations between NO2
exposure and fetal growth retardation; however, some evidence is
accumulating for effects on preterm delivery (ISA, section 5.3.2.5).
Scant animal evidence supports a weak association between
NO2 exposure and adverse birth outcomes and provides little
mechanistic information or biological plausibility for the
epidemiologic findings.
4. NO2-Related Impacts on Public Health
Specific groups within the general population are likely at
increased risk
[[Page 34418]]
for suffering adverse effects from NO2 exposure. This could
occur because they are affected by lower levels of NO2 than
the general population (susceptibility), because they experience a
larger health impact than the general population to a given level of
exposure (susceptibility), and/or because they are exposed to higher
levels of NO2 than the general population (vulnerability).
The term susceptibility generally encompasses innate (e.g., genetic or
developmental) and/or acquired (e.g., age or disease) factors that make
individuals more likely to experience effects with exposure to
pollutants. The severity of health effects experienced by a susceptible
subgroup may be much greater than that experienced by the population at
large. Factors that may influence susceptibility to the effects of air
pollution include age (e.g., infants, children, elderly); gender; race/
ethnicity; genetic factors; and pre-existing disease/condition (e.g.,
obesity, diabetes, respiratory disease, asthma, chronic obstructive
pulmonary disease (COPD), cardiovascular disease, airway
hyperresponsiveness, respiratory infection, adverse birth outcome)
(ISA, sections 4.3.1, 4.3.5, and 5.3.2.8). In addition, certain groups
may experience relatively high exposure to NO2, thus forming
a potentially vulnerable population (ISA, section 4.3.6). Factors that
may influence exposures and/or susceptibility to air pollution include
socioeconomic status (SES), education level, air conditioning use,
proximity to roadways, geographic location, level of physical activity,
and work environment (e.g., indoor versus outdoor) (ISA, section
4.3.5). The ISA discussed factors that can confer susceptibility and/or
vulnerability to air pollution with most of the discussion devoted to
factors for which NO2-specific evidence exists (ISA, section
4.3). These factors are discussed below.
a. Pre-Existing Disease
A number of health conditions have been found to put individuals at
greater risk for adverse events following exposure to air pollution. In
general, these include asthma, COPD, respiratory infection, cardiac
conduction disorders, congestive heart failure (CHF), diabetes, past
myocardial infarction (MI), obesity, coronary artery disease, low birth
weight/prematurity, and hypertension (ISA, sections 4.3.1, 4.3.5, and
5.3.2.9). In addition to these conditions, epidemiologic evidence
indicates that individuals with bronchial or airway
hyperresponsiveness, as determined by methacholine provocation, may be
at increased risk for experiencing respiratory symptoms (ISA, section
4.3.1). In considering NO2 specifically, the ISA evaluated
studies on asthmatics, individuals with cardiopulmonary disease, and
diabetics (ISA, sections 4.3.1.1 and 4.3.1.2). These groups are
discussed in more detail below.
Epidemiologic and controlled human exposure studies, supported by
animal toxicology studies, have provided evidence for associations
between NO2 exposure and respiratory effects in asthmatics
(ISA, section 4.3.1.1). The ISA found evidence from epidemiologic
studies for an association between ambient NO2 and
children's hospital admissions, emergency department visits, and calls
to doctors for asthma. Long-term NO2 exposure was associated
with aggravation of asthma effects that include symptoms, medication
use, and lung function. Time-series studies demonstrated a relationship
in children between hospital admissions or emergency department visits
for asthma and ambient NO2 levels, even after adjusting for
co-pollutants such as PM and CO (ISA, section 4.3.1.1). Important
evidence was available from epidemiologic studies of indoor
NO2 exposures. Recent studies have shown associations with
asthma attacks and severity of virus-induced asthma (ISA, section
4.3.1.1). In addition, in controlled human exposure studies, airway
hyperresponsiveness in asthmatics occurred following exposure to
ambient or near-ambient NO2 concentrations (ISA, sections
5.3.2.1-5.3.2.6). Compared to asthma, less evidence is available to
support cardiovascular disease as a mediator of susceptibility to
NO2. However, recent epidemiologic studies report that
individuals with preexisting conditions (e.g., including diabetes, CHF,
prior MI) may be at increased risk for adverse cardiac health events
associated with ambient NO2 concentrations (ISA, section
4.3.1.2). The small number of controlled human exposure and animal
toxicological studies that have evaluated cardiovascular endpoints
provide only limited supporting evidence for susceptibility to
NO2 in persons with cardiovascular disease (ISA, section
4.3.1.2).
b. Age
The ISA identified infants, children (i.e., <18 years of age), and
older adults (i.e., >65 years of age) as groups that are potentially
more susceptible than the general population to the health effects
associated with ambient NO2 concentrations (ISA, section
4.3.2). The ISA found evidence that associations of NO2 with
respiratory emergency department visits and hospitalizations were
stronger among children and older adults, though not all studies had
comparable findings on this issue (ISA, section 4.3.2). In addition,
long-term exposure studies suggest effects in children that include
impaired lung function growth, increased respiratory symptoms and
infections, and onset of asthma (ISA, section 3.4 and 4.3.2). In some
studies, associations between NO2 and hospitalizations or
emergency department visits for CVD have been observed in elderly
populations. Among studies that observed positive associations between
NO2 and mortality, a comparison indicated that, in general,
the elderly population was more susceptible than the non-elderly
population to NO2 effects (ISA, section 4.3.2).
c. Genetics
As noted in the ISA (section 4.3.4), genetic factors related to
health outcomes and ambient pollutant exposures merit consideration.
Several criteria should be satisfied in selecting and establishing
useful links between polymorphisms in candidate genes and adverse
respiratory effects. First, the candidate gene must be significantly
involved in the pathogenesis of the adverse effect of interest. Second,
polymorphisms in the gene must produce a functional change in either
the protein product or in the level of expression of the protein.
Third, in epidemiologic studies, the issue of confounding by other
environmental exposures must be carefully considered (ISA, section
4.3.4). Investigation of genetic susceptibility to NO2
effects has focused on the glutathione S-tranferase (GST) gene. Several
GST genes have common, functionally-important alleles that affect host
defense in the lung (ISA, section 4.3.4). GST genes are inducible by
electrophilic species (e.g., reactive oxygen species) and individuals
with genotypes that result in enzymes with reduced or absent peroxidase
activity are likely to have reduced defenses against oxidative insult.
This could potentially result in increased susceptibility to inhaled
oxidants and radicals. However, data on genetic susceptibility to
NO2 are only beginning to emerge and, while it remains
plausible that there are genetic factors that can influence health
responses to NO2, the few available studies do not provide
specific support for genetic susceptibility to NO2 exposure
(ISA, section 4.3.4).
d. Gender
As reported in the ISA, a limited number of NO2 studies
have stratified results by gender. The results of these studies were
mixed, and the ISA did not
[[Page 34419]]
draw conclusions regarding the potential for gender to confer
susceptibility to the effects of NO2 (ISA, section 4.3.3).
e. Proximity to Roadways
Certain groups may experience relatively high exposure to
NO2, thus forming a potentially vulnerable population. The
ISA included discussion of populations reported to experience increased
NO2 exposures on or near roadways (ISA, section 4.3.6).
Large gradients in NOX concentrations near roadways may lead
to increased exposures for individuals residing, working, traveling, or
attending school in the vicinity of roadways. Many studies find that
indoor, personal, and outdoor NO2 levels are strongly
associated with proximity to traffic or to traffic density (ISA,
section 4.3.6).
That adverse respiratory effects can be associated with proximity
to roadways has been demonstrated in a number of studies. For example,
Gauderman and colleagues (2007) reported reduced lung function growth
in children who lived within 500 m of a freeway compared to children
who lived at least 1500 m from a freeway. In a separate study,
Gauderman and colleagues (2005) reported that the incidence of
physician-diagnosed asthma increased with both increasing
NO2 concentrations outside the child's residence and
decreasing distance between the child's residence and a major freeway.
In addition to those who live near major roadways, individuals who
spend time commuting on major roadways can also be exposed to
relatively higher concentrations of NO2 than the ones
reported at monitors away from the roads. Due to high air exchange
rates, NO2 concentrations inside a vehicle can rapidly
approach ambient concentrations on the roadway during commuting (ISA,
section 4.3.6). Mean in-vehicle NO2 concentrations are often
between 2 and 3 times higher than ambient levels measured at monitors
located away from the road (ISA, section 4.3.6). Due to the potential
for high peak exposures while driving, total personal exposure could be
underestimated if exposures while commuting are not considered.
Therefore, individuals with occupations that require them to be in
traffic or close to traffic (e.g., bus and taxi drivers, highway patrol
officers, toll collectors) and individuals with long commutes could be
exposed to relatively high levels of NO2 compared to the
ambient levels measured at fixed-site monitors located away from the
roadway.
f. Socioeconomic Status
The ISA discussed evidence that SES modifies the effects of air
pollution (section 4.3.6). Many recent studies examined modification by
SES indicators on the association between mortality and PM or other
indices such as traffic density, distance to roadway, or a general air
pollution index (ISA, section 4.3.6). SES modification of
NO2 associations has been examined in fewer studies.
However, in a study conducted in Seoul, South Korea, community-level
SES indicators modified the association of air pollution with emergency
department visits for asthma. Of the five criteria air pollutants
evaluated, NO2 showed the strongest association in lower SES
districts compared to high SES districts (Kim et al., 2007). In
addition, Clougherty et al. (2007) evaluated exposure to violence (a
potential surrogate for SES) as a modifier of the effect of traffic-
related air pollutants, including NO2, on childhood asthma.
The authors reported an elevated risk of asthma with an increase in
NO2 exposure solely among children with above-median
exposure to violence in their neighborhoods (ISA, section 4.3.6).
Although these recent studies have evaluated the impact of SES on
vulnerability to NO2, they are too few in number to draw
definitive conclusions (ISA, section 5.3.2.8).
g. Size of the At-Risk Population
The population potentially affected by NO2 is large. A
considerable fraction of the population resides, works, or attends
school near major roadways, and these individuals are likely to have
increased exposure to NO2 (ISA, section 4.4). Based on data
from the 2003 American Housing Survey, approximately 36 million
individuals live within 300 feet (~90 meters) of a four-lane highway,
railroad, or airport (ISA, section 4.4).\8\ Furthermore, in California,
2.3% of schools with a total enrollment of more than 150,000 students
were located within approximately 500 feet of high-traffic roads, with
a higher proportion of non-white and economically disadvantaged
students attending those schools (ISA, section 4.4). Of this
population, asthmatics and members of other susceptible groups
discussed above will have even greater risks of experiencing health
effects related to NO2 exposure. In the United States,
approximately 10% of adults and 13% of children have been diagnosed
with asthma, and 6% of adults have been diagnosed with COPD (ISA,
section 4.4). The prevalence and severity of asthma is higher among
certain ethnic or racial groups such as Puerto Ricans, American
Indians, Alaskan Natives, and African Americans (ISA, section 4.4). A
higher prevalence of asthma among persons of lower SES and an excess
burden of asthma hospitalizations and mortality in minority and inner-
city communities have been observed (ISA, section 4.4). In addition,
based on U.S. census data from 2000, about 72.3 million (26%) of the
U.S. population are under 18 years of age, 18.3 million (7.4%) are
under 5 years of age, and 35 million (12%) are 65 years of age or
older. Therefore, large portions of the U.S. population are in age
groups that are likely at-risk for health effects associated with
exposure to ambient NO2. The size of the potentially at-risk
population suggests that exposure to ambient NO2 could have
a significant impact on public health in the United States.
---------------------------------------------------------------------------
\8\ The most current American Housing Survey (http://www.census.gov/hhes/www/housing/ahs/ahs.html) is from 2007 and lists
a higher fraction of housing units within the 300 foot boundary than
do prior surveys. According to Table IA-6 from that report (http://www.census.gov/hhes/www/housing/ahs/ahs07/tab1a-6.pdf), out of
128,303,000 total housing units in the United States, 20,016,000
were reported by the surveyed occupant or landlord as being within
300 feet of a 4-or-more lane highway, railroad, or airport. That
constitutes 15.613% of the total housing units in the U.S. Assuming
equal distributions, with a current population of 306,330,199, that
means that there would be 47.8 million people meeting the 300 foot
criteria.
---------------------------------------------------------------------------
C. Human Exposure and Health Risk Characterization
To put judgments about NO2-associated health effects
into a broader public health context, EPA has drawn upon the results of
the quantitative exposure and risk assessments. Judgments reflecting
the nature of the evidence and the overall weight of the evidence are
taken into consideration in these quantitative exposure and risk
assessments, discussed below. These assessments provide estimates of
the likelihood that asthmatic individuals would experience exposures of
potential concern and estimates of the incidence of NO2-
associated respiratory emergency department visits under varying air
quality scenarios (e.g., just meeting the current or alternative
standards), as well as characterizations of the kind and degree of
uncertainties inherent in such estimates.
This section describes the approach taken in the REA to
characterize NO2-related exposures and health risks. Goals
of the REA included estimating short-term exposures and potential human
health risks associated with (1) recent levels of ambient
NO2; (2) NO2 levels adjusted to simulate just
meeting the current standard; and (3) NO2 levels adjusted to
simulate just meeting
[[Page 34420]]
potential alternative standards. This section discusses the scientific
evidence from the ISA that was used as the basis for the risk
characterization (II.C.1), the approaches used in characterizing
exposures and risks (II.C.2), and important uncertainties associated
with these analyses (II.C.3). The results of the exposure and risk
analyses, as they relate to the current and potential alternative
standards, are discussed in subsequent sections of this proposal
(sections II.E and II.F, respectively).
1. Evidence Base for the Risk Characterization
For purposes of the quantitative characterization of NO2
health risks, the REA determined that it was appropriate to focus on
endpoints for which the ISA concluded that the available evidence is
sufficient to infer either a causal or a likely causal relationship.
This was generally consistent with judgments made in other recent NAAQS
reviews (e.g., see EPA, 2005).
As noted above in section II.A, the only health effect category for
which the evidence was judged in the ISA to be sufficient to infer
either a causal or a likely causal relationship is respiratory
morbidity following short-term NO2 exposure. Therefore, for
purposes of characterizing health risks associated with NO2,
the REA focused on respiratory morbidity endpoints that have been
associated with short-term NO2 exposures. Other health
effects (e.g., those associated with long-term exposures) are
considered as part of the evidence-based evaluation of potential
alternative standards (see section II.F.2). In evaluating the
appropriateness of specific endpoints for use in the NO2
risk characterization, the REA considered both epidemiologic and
controlled human exposure studies.
When evaluating epidemiologic studies as to their appropriateness
for use as the basis for a quantitative risk assessment, the REA
considered several factors. First, the REA concluded that studies
conducted in the United States are preferable to those conducted
outside the United States given the potential for effect estimates to
be impacted by factors such as the ambient pollutant mix, the placement
of monitors, activity patterns of the population, and characteristics
of the healthcare system. Second, the REA concluded that studies of
ambient NO2 are preferable to those of indoor
NO2, which focus on individuals exposed to NO2
from indoor sources. These indoor sources can result in exposure
patterns, NO2 levels, and co-pollutants that are different
from those typically associated with ambient NO2. Therefore,
although indoor studies made important contributions to the evidence
base for causality judgments in the ISA, the preferred approach for
conducting a quantitative risk assessment based on the epidemiologic
literature to inform decisions regarding an ambient NO2
standard is to consider studies of ambient NO2. Third, the
REA concluded that it was appropriate to focus on studies of emergency
department visits and hospital admissions given the clear public health
significance of these endpoints and the availability of baseline
incidence data. Finally, the REA concluded that it was appropriate to
focus on studies that evaluated NO2 health effect
associations using both single- and multi-pollutant models. Taking
these factors into consideration, the epidemiology-based risk
assessment in the REA focused on the study conducted in Atlanta,
Georgia by Tolbert et al. (2007). This assessment is described in more
detail in the REA (chapter 9).
In identifying health endpoints from controlled human exposure
studies on which to focus the characterization of NO2 health
risks, the REA concluded that it was appropriate to focus on endpoints
that occur at or near ambient levels of NO2 and endpoints
that may be important from a public health perspective. Controlled
human exposure studies have addressed the consequences of short-term
(e.g., 30-minutes to several hours) NO2 exposures for a
number of health endpoints including airway responsiveness, host
defense and immunity, inflammation, and lung function (ISA, section
3.1). With regard to the NO2 levels at which different
effects have been documented, the ISA concluded: (1) In asthmatics
NO2 may increase the allergen-induced airway inflammatory
response at exposures as low as 260 ppb for 30 min (ISA, Figure 3.1-2),
and NO2 exposures between 200 and 300 ppb for 30 minutes or
100 ppb for 60-minutes can result in small, but significant, increases
in nonspecific airway responsiveness (ISA, section 5.3.2.1); (2)
limited evidence indicates that NO2 may increase
susceptibility to injury by subsequent viral challenge following
exposures of 600-1500 ppb for 3 hours; (3) evidence exists for
increased airway inflammation at NO2 concentrations less
than 2000 ppb; and (4) the direct effects of NO2 on lung
function in asthmatics have been inconsistent at exposure
concentrations below 1000 ppb (ISA, section 5.3.2.1). Therefore, of the
health effects caused by NO2 in controlled human exposure
studies, the only effect identified by the ISA to occur at or near
ambient levels is increased airway responsiveness in asthmatics.
The REA concluded that airway responsiveness in the asthmatic
population is an appropriate focus for the risk characterization for
several reasons. First, the ISA concluded that ``persons with
preexisting pulmonary conditions are likely at greater risk from
ambient NO2 exposures than the general public, with the most
extensive evidence available for asthmatics as a potentially
susceptible group'' (ISA, section 5.3.2.8). Second, when discussing the
clinical significance of NO2-related airway
hyperresponsiveness in asthmatics, the ISA concluded that ``transient
increases in airway responsiveness following NO2 exposure
have the potential to increase symptoms and worsen asthma control''
(ISA, sections 3.1.3 and 5.4). That this effect could have public
health implications is suggested by the large size of the asthmatic
population in the United States (ISA, Table 4.4-1). Third,
NO2 effects on airway responsiveness in asthmatics are part
of the body of experimental evidence that provides plausibility and
coherence for the effects observed on hospital admissions and emergency
department visits in epidemiologic studies (ISA, section 5.3.2.1). As a
result of these considerations, of the endpoints from controlled human
exposure studies, the REA focused on airway responsiveness in
asthmatics for purposes of quantifying risks associated with ambient
NO2 (see below).
Because many of the studies of airway responsiveness evaluated only
a single level of NO2 and because of methodological
differences between the studies, the data are not sufficient to derive
an exposure-response relationship in the range of interest. Therefore,
the REA concluded that the most appropriate approach to characterizing
risks based on the controlled human exposure evidence for airway
responsiveness was to compare estimated NO2 air quality and
exposure levels with potential health effect benchmark levels. In this
review, the term ``exposures of potential concern'' is defined as
personal exposures to 1-hour ambient NO2 concentrations at
and above specific benchmark levels. Benchmark levels represent
NO2 exposure concentrations reported to increase airway
responsiveness in most asthmatics, as discussed above in section
II.B.1.d. Although the analysis of exposures of potential concern was
conducted using discrete benchmark levels (i.e., 100, 150, 200, 250,
300 ppb), EPA recognizes that there is no sharp
[[Page 34421]]
breakpoint within the continuum ranging from at and above 300 ppb down
to 100 ppb. In considering the concept of exposures of potential
concern, it is important to balance concerns about the potential for
health effects and their severity with the increasing uncertainty
associated with our understanding of the likelihood of such effects at
lower NO2 levels. Within the context of this continuum,
estimates of exposures of potential concern at discrete benchmark
levels provide some perspective on the potential public health impacts
of NO2-related health effects that have been demonstrated in
controlled human exposure studies but cannot be evaluated in
quantitative risk assessments (i.e., increased airway responsiveness).
They also help in understanding the extent to which such impacts could
change by just meeting the current and potential alternative standards.
The NO2-related increase in airway responsiveness is
plausibly linked to the NO2-associated morbidity reported in
epidemiologic studies (e.g., increased respiratory symptoms, emergency
department visits and hospital admissions). However, estimates of the
number of asthmatics likely to experience exposures of potential
concern cannot be translated directly into quantitative estimates of
the number of people likely to experience specific health effects,
since sufficient information to draw such comparisons is not available.
Due to individual variability in responsiveness, only a subset of
asthmatics exposed at and above a specific benchmark level can be
expected to experience health effects. The amount of weight to place on
the estimates of exposures of potential concern at any of these
benchmark levels depends in part on the weight of the scientific
evidence concerning health effects associated with NO2
exposures at and above that benchmark level. It also depends on
judgments about the importance from a public health perspective of the
health effects that are known or can reasonably be inferred to occur as
a result of exposures at and above the benchmark level. Such public
health policy judgments are embodied in the NAAQS standard setting
criteria (i.e., standards that, in the judgment of the Administrator,
are requisite to protect public health with an adequate margin of
safety).
2. Overview of Approaches
As noted above, the purpose of the assessments described in the REA
was to characterize air quality, exposures, and health risks associated
with recent ambient levels of NO2, with NO2
levels that could be associated with just meeting the current
NO2 NAAQS, and with NO2 levels that could be
associated with just meeting potential alternative standards. To
characterize health risks, we employed three approaches in the REA. In
the first approach, for each air quality scenario, NO2
concentrations at fixed-site monitors and simulated concentrations on/
near roadways were compared to potential health effect benchmark values
derived from the controlled human exposure literature. In the second
approach, modeled estimates of actual exposures in asthmatics were
compared to potential health effect benchmarks. In the third approach,
concentration-response relationships from an epidemiologic study were
used in conjunction with baseline incidence data and recent or
simulated ambient concentrations to estimate health impacts. An
overview of the approaches to characterizing health risks is provided
below and each approach has been described in more detail in the REA
(chapters 6 through 9).
In the first approach, we compared ambient NO2
concentrations with potential health effect benchmark levels for
NO2. The ambient NO2 concentrations used in these
analyses were based on those measured at monitors in the current
NO2 monitoring network. These monitored concentrations were
compared to benchmark levels directly and were also used, in
conjunction with literature-derived characterizations of the
NO2 concentration gradient around roadways, as the basis for
estimating NO2 concentrations on/near roadways. Scenario-
driven air quality analyses were performed using ambient NO2
concentrations for the years 1995 though 2006. With this approach,
NO2 air quality serves as a surrogate for exposure. All U.S.
monitoring sites where NO2 data have been collected, and
that met completeness criteria (REA, chapter 7), were represented by
this analysis. As such, the results generated were considered a broad
characterization of national air quality and human exposures that might
be associated with these concentrations. An advantage of this approach
is its relative simplicity; however, there is uncertainty associated
with the assumption that NO2 air quality can serve as an
adequate surrogate for total exposure to ambient NO2. Actual
exposures might be influenced by factors not considered by this
approach, including small scale spatial variability in ambient
NO2 concentrations (which might not be captured by the
network of fixed-site ambient monitors) and spatial/temporal
variability in human activity patterns.
In the second approach, we used an inhalation exposure model to
generate more realistic estimates of personal exposures in asthmatics
(REA, chapter 8 for more detail on this assessment). This analysis
estimated temporally and spatially variable ambient NO2
concentrations and simulated human contact with these pollutant
concentrations. The approach was designed to incorporate exposures that
are not necessarily captured by the existing ambient monitoring data,
including those that occur on or near roadways. AERMOD, an EPA
dispersion model, was used to estimate 1-hour ambient NO2
concentrations using emissions estimates from stationary and on-road
mobile sources.\9\ The Air Pollutants Exposure (APEX) model, an EPA
human exposure model, was then used to estimate population exposures
using the hourly census block level NO2 concentrations
estimated by AERMOD. A probabilistic approach was used to model
individual exposures considering the time people spend in different
microenvironments and the variable NO2 concentrations that
occur within these microenvironments across time, space, and
microenvironment type. Estimates of personal exposure were compared to
potential NO2 health benchmark levels. This approach to
assessing exposures was more resource intensive than using ambient
levels as a surrogate for exposure; therefore, the final REA included
the analysis of only one specific location in the U.S. (Atlanta MSA).
Although the geographic scope of this analysis was restricted, the
approach provided estimates of NO2 exposures in asthmatics
in Atlanta, particularly those exposures associated with important
emission sources of NOX, and the analysis served to
complement the broad air quality characterization.
---------------------------------------------------------------------------
\9\ Estimated emissions from Hartsfield International Airport in
Atlanta, a non-road mobile source, were also included in this
analysis.
---------------------------------------------------------------------------
For the characterization of risks in both the air quality analysis
and the exposure modeling analysis described above, the REA used a
range of short-term potential health effect benchmarks. As noted above,
the levels of potential benchmarks are based on NO2 exposure
levels that have been associated with increased airway responsiveness
in asthmatics in controlled human exposure studies (ISA, section
5.3.2.1). Benchmark values of 100, 150, 200, 250, and 300 ppb were
compared to both NO2 air quality levels and to estimates of
NO2 exposure in asthmatics. When
[[Page 34422]]
NO2 air quality was used as a surrogate for exposure, the
output of the analysis was an estimate of the number of times per year
specific locations experience 1-hour levels of NO2 that
exceed a particular benchmark. When personal exposures were simulated,
the output of the analysis was an estimate of the number of asthmatics
at risk for experiencing daily maximum 1-hour levels of NO2
of ambient origin that exceed a particular benchmark. An advantage of
using the benchmark approach to characterize health risks is that the
effects observed in controlled human exposure studies clearly result
from NO2 exposure. A disadvantage of this approach is that
the magnitude of the NO2 effect on airway responsiveness can
vary considerably from individual to individual and not all asthmatics
would be expected to respond to the same levels of NO2
exposure. Therefore, the public health impacts of NO2-
induced airway hyperresponsiveness are difficult to quantify.
In the third approach, we estimated respiratory emergency
department visits as a function of ambient levels of NO2
measured at a fixed-site monitor representing ambient air quality for
an urban area. In this approach, concentration-response functions from
an epidemiologic study (Tolbert et al., 2007) were used, in combination
with baseline incidence data for respiratory emergency department
visits in the Atlanta area and ambient NO2 monitoring data,
to estimate the impact on emergency department visits of ambient levels
of NO2. Compared to the risk characterization based on the
air quality and exposure analyses described above, this approach to
characterizing health risks has several advantages. For example, the
public health significance of respiratory emergency department visits
is less ambiguous, in terms of its impact on individuals, than is an
increase of unknown magnitude in the airway response. In addition, the
concentration-response relationship reflects real-world levels of
NO2 and co-pollutants present in ambient air. However, as
noted previously, a disadvantage of this approach is the ambiguity and
complexity associated with quantifying the contribution of
NO2 to emergency department visits relative to the
contributions of co-occurring pollutants.
3. Key Limitations and Uncertainties
A number of key uncertainties should be considered when
interpreting the results of these analyses. While the air quality,
exposure, and quantitative risk analyses are each associated with
unique uncertainties, they also share some uncertainties in common.
Important uncertainties shared by these analyses, as well as
uncertainties specifically associated with the air quality, exposure,
and risk analyses, are discussed below.
In order to simulate just meeting the current annual standard and
many of the alternative 1-hour standards analyzed, an adjustment
(either upward or downward) of recent ambient NO2
concentrations was required. As noted in the REA, an upward adjustment
does not reflect a judgment that levels of NO2 are likely to
increase across the country or in any specific location under the
current standard or any of the potential alternative standards.
However, it does acknowledge that, under the current standard and some
of the alternative standards evaluated, an increase in NO2
concentrations would be permitted. The benefit of these air quality
adjustments is that they can inform consideration of the current and
alternative standards by providing estimates of health risks that could
be associated with ambient air quality levels that just meet these
standards. In adjusting air quality to simulate just meeting these
standards, the analyses in the REA assumed that the overall shape of
the distribution of NO2 concentrations in an area would not
change. While the REA concluded that this is a reasonable assumption in
the absence of evidence supporting a different distribution, and while
available analyses support this approach (Rizzo, 2008), the REA
recognized this as an important uncertainty. It may be an especially
important uncertainty for those scenarios where considerable adjustment
is required to simulate just meeting one or more of the standards (REA,
section 8.12).
In addition, simulation of just meeting different alternative
standards was achieved by adjusting NO2 concentrations at
monitors in the current area-wide network. Therefore, resulting
estimates of the potential public health implications of different
decisions are most directly relevant to a standard focused specifically
on the area-wide NO2 concentrations that are the primary
target of the current monitoring network. However, as discussed below
(sections II.F.4.e and III), with this notice the Administrator is
proposing to establish a standard focused specifically on the peak
concentrations to which individuals can be exposed from on-road mobile
source emissions on or near major roadways and to support such a
standard with a monitoring network that includes monitors placed near
major roadways. This proposed shift in the monitoring network
introduces uncertainty in the extent to which the exposure and risk
analyses presented in the REA can directly inform decisions on the
proposed standard.
In addition to the general uncertainties discussed above, some
uncertainties are specific to the air quality analyses. In order to
estimate ambient NO2 concentrations on or near roadways in
the air quality analyses, the REA used empirically-derived
relationships between ambient concentrations measured at fixed-site
monitors in the current NO2 monitoring network and on/near-
road concentrations. The data used to develop the relationships were
likely collected under different conditions (e.g., different
meteorological conditions which can affect important parameters in this
relationship, such as the production of NO2 from NO). The
REA noted that the extent to which these conditions are representative
of the times and places included in our analyses is unknown. Therefore,
there is uncertainty in the degree to which the relationships used to
estimate on/near-road NO2 concentrations reflect the actual
relationship in the locations and over the time periods of interest.
Potential health benchmark levels used in the air quality analyses
were based largely on a meta-analysis (ISA, Table 3.1-3) of controlled
human exposure studies of airway hyperresponsiveness. One important
source of uncertainty with regard to this approach is that controlled
human exposure studies have typically involved volunteers with mild
asthma. Data are lacking for more severely affected asthmatics, who may
be more susceptible (ISA, section 3.1.3.2). As a result, the potential
health effect benchmarks could underestimate risks in populations with
greater susceptibility. While approaches to classifying asthma severity
differ, some estimates indicate that over half of asthmatics could be
classified as moderate or severe (Fuhlbrigge et al., 2002; Stout et
al., 2006). A second important source of uncertainty with regard to
this approach is that the meta-analysis showed increased airway
responsiveness in asthmatics at the lowest NO2 level for
which data were available (i.e. 100 ppb). Controlled human exposure
studies have not evaluated the possibility of NO2 effects on
airway responsiveness in asthmatics at exposure concentrations below
100 ppb. A third important source of uncertainty associated with this
approach is that the meta-analysis provided information on the
direction of the NO2-induced airway response, but not on the
magnitude of the response.
[[Page 34423]]
Therefore, although the ISA did conclude that increased airway
responsiveness associated with NO2 exposure could increase
symptoms and worsen asthma control (ISA, section 5.4), the full public
health implications of benchmark exceedances are uncertain.
The Atlanta exposure assessment was also associated with a number
of key uncertainties that should be considered when interpreting the
results with regard to decisions on the standard. Some of these
uncertainties, including those associated with benchmark levels, were
shared with the air quality analyses. Additional uncertainties
associated specifically with the Atlanta exposure assessment are
discussed briefly below.
When compared to ambient measurement data, predicted upper
percentile NO2 concentrations may be 10-50% higher. Because
these predicted concentrations are used as inputs for the exposure
modeling, this suggests the possibility that the exposure assessment is
over-predicting upper percentile NO2 exposures. Other
approaches used to evaluate exposure results (i.e., comparison to
personal exposure monitoring results and comparison of exposure-to-
ambient concentration ratios with those identified in the ISA) have
suggested that exposure estimates are reasonable. However, the
possibility cannot be ruled out that benchmark exceedances are over-
predicted in the Atlanta exposure analysis.
The exposure assessment was limited to Atlanta and the extent to
which these results are representative of other locations in the U.S.
is uncertain. The REA (section 8.11) concluded that the Atlanta
exposure estimates are likely representative of other moderate to large
urban areas. However, the REA also recognized that, given the greater
proximity of the population to mobile sources in large urban areas such
as Los Angeles, New York, and Chicago (see REA, Tables 8-14 and 8-15),
the estimates of benchmark exceedances in Atlanta may be smaller than
in these larger cities.
A number of key uncertainties should also be considered when
interpreting the results of the Atlanta risk assessment with regard to
decisions on the standard. Some of these, including the appropriateness
of generalizing results from Atlanta, are shared with the Atlanta
exposure assessment. Additional uncertainties associated specifically
with the Atlanta risk assessment are discussed briefly below.
There is uncertainty about whether the association between
NO2 and emergency department visits actually reflects a
causal relationship across the range of daily and hourly concentration
levels in the epidemiologic studies. The ISA (section 5.4, p. 5-15)
noted that when interpreting the NO2 epidemiologic results,
``It is difficult to determine * * * the extent to which NO2
is independently associated with respiratory effects or if
NO2 is a marker for the effects of another traffic-related
pollutant or mix of pollutants (see section 5.2.2 for more details on
exposure issues). A factor contributing to uncertainty in estimating
the NO2-related effect from epidemiologic studies is that
NO2 is a component of a complex air pollution mixture from
traffic related sources that include CO and various forms of PM.'' This
uncertainty should be considered when interpreting the quantitative
NO2 risk estimates based on the Atlanta epidemiologic study.
However, in discussing these uncertainties, the ISA (section 5.4, p. 5-
16) concluded that, ``Although this complicates the efforts to
disentangle specific NO2-related health effects, the
evidence summarized in this assessment indicates that NO2
associations generally remain robust in multi-pollutant models and
supports a direct effect of short-term NO2 exposure on
respiratory morbidity at ambient concentrations below the current
NAAQS. The robustness of epidemiologic findings to adjustment for co-
pollutants, coupled with data from animal and human experimental
studies, support a determination that the relationship between
NO2 and respiratory morbidity is likely causal, while still
recognizing the relationship between NO2 and other traffic-
related pollutants.''
A related uncertainty is that associated with the estimated
NO2 coefficient in the concentration-response function. This
coefficient has been characterized by confidence intervals reflecting
sample size. However, these confidence intervals do not reflect all of
the uncertainties related to the concentration-response functions, such
as whether or not the model used in the epidemiologic study is the
correct model form. Concerning the possible role of co-pollutants in
the Tolbert et al. (2007) study, single-pollutant models may produce
overestimates of the NO2 effects if some of those effects
are really due in whole or part to one or more of the other pollutants.
On the other hand, effect estimates based on multi-pollutant models can
be uncertain, and can result in statistically non-significant estimates
where a true relationship exists, if the co-pollutants included in the
model are highly correlated with NO2. As a result of these
considerations, we report risk estimates based on both the single- and
multi-pollutant models from Tolbert et al. (2007).
D. Considerations in Review of the Standard
This section presents the integrative synthesis of the evidence and
information contained in the ISA and the REA with regard to the current
and potential alternative standards. EPA notes that the final decision
on retaining or revising the current primary NO2 standard is
a public health policy judgment to be made by the Administrator. This
judgment will be informed by a recognition that the available health
effects evidence reflects a continuum consisting of ambient levels of
NO2 at which scientists generally agree that health effects
are likely to occur, through lower levels at which the likelihood and
magnitude of the response become increasingly uncertain. The
Administrator's final decision will draw upon scientific information
and analyses related to health effects, population exposures, and
risks; judgments about the appropriate response to the range of
uncertainties that are inherent in the scientific evidence and
analyses; and comments received in response to this proposal.
1. Background on the Current Standard
The current standard, which is an annual average of 0.053 ppm (53
ppb), was retained by the Administrator in the most recent review in
1996 (61 FR 52854 (October 8, 1996)). The decision in that review to
retain the annual standard was based on consideration of available
scientific evidence for health effects associated with NO2
and on air quality information. With regard to these considerations,
the Administrator noted that ``a 0.053 ppm annual standard would keep
annual NO2 concentrations considerably below the long-term
levels for which serious chronic effects have been observed in
animals'' and that ``[r]etaining the existing standard would also
provide protection against short-term peak NO2
concentrations at the levels associated with mild changes in pulmonary
function and airway responsiveness observed in controlled human
studies'' (60 FR 52874, 52880 (Oct. 11, 1995)). As a result, the
Administrator concluded that ``the existing annual primary standard
appears to be both adequate and necessary to protect human health
against both long- and short-term NO2 exposures'' and that
``retaining the existing annual standard is consistent
[[Page 34424]]
with the scientific data assessed in the Criteria Document (EPA, 1993)
and the Staff Paper (EPA, 1995) and with the advice and recommendations
of CASAC'' (61 FR 52852 at 52854).
As noted previously, the 1993 AQCD concluded that there were two
key health effects of greatest concern at ambient or near-ambient
levels of NO2: increased airway responsiveness in asthmatic
individuals after short-term exposures and increased occurrence of
respiratory illness in children with longer-term exposures. Evidence
also was found for increased risk of emphysema, but this was of major
concern only with exposures to levels of NO2 much higher
than then-current ambient concentrations. The evidence regarding airway
responsiveness was drawn largely from controlled human exposure
studies. The evidence for respiratory illness was drawn from
epidemiologic studies that reported associations between respiratory
symptoms and indoor exposures to NO2 in people living in
homes with gas stoves. The biological plausibility of the epidemiologic
results was supported by toxicological studies that detected changes in
lung host defenses following NO2 exposure. Subpopulations
considered potentially more susceptible to the effects of
NO2 included individuals with preexisting respiratory
disease, children, and the elderly.
In that review, health risks were characterized by comparing
ambient monitoring data, which were used as a surrogate for exposure,
with potential health benchmark levels identified from controlled human
exposure studies. At the time of the review, a meta-analysis of
controlled human exposure studies indicated the possibility for adverse
health effects due to short-term (e.g., 1-hour) exposures between 200
ppb and 300 ppb NO2. Therefore, the focus of the assessment
was on the potential for short-term (i.e., 1-hour) exposures to
NO2 levels above potential health benchmarks in this range.
The assessment used monitoring data from the years 1988-1992 and
screened for sites with one or more hourly exceedances of potential
short-term health effect benchmarks. Predictive models were then
constructed to relate the frequency of hourly concentrations above
short-term health effect benchmarks to a range of annual average
concentrations, including the current standard. Based on the results of
this analysis, both CASAC (Wolff, 1995) and the Administrator (60 FR
52874) concluded that the minimal occurrence of short-term peak
concentrations at or above a potential health effect benchmark of 200
ppb (1-hour average) indicated that the existing annual standard would
provide adequate health protection against short-term exposures. This
conclusion, combined with the conclusion that the current annual
standard would maintain annual average levels well-below those
associated with serious effects in animal toxicological studies, formed
a large part of the basis for the decision in the 1996 review to retain
the existing annual standard.
2. Approach for Reviewing the Need To Retain or Revise the Current
Standard
The decision in the present review on whether the current annual
standard is requisite to protect public health with an adequate margin
of safety will be informed by a number of scientific studies and
analyses that were not available in the 1996 review. Specifically, as
discussed above (section II), a large number of epidemiologic studies
have been published since the 1996 review. Many of these studies
evaluate associations between NO2 and adverse respiratory
endpoints (e.g., respiratory symptoms, emergency department visits,
hospital admissions) in locations where annual average NO2
concentrations are well-below the level allowed by the current standard
(53 ppb). In addition, the meta-analysis of controlled human exposure
studies has been updated for this review to include information on
additional exposure concentrations. Finally, the REA described
estimates of NO2-associated health risks that could be
present in locations that just meet the current annual standard. These
types of risk estimates were not available in the last review. The
approach for considering this scientific evidence and exposure/risk
information is discussed below.
To evaluate whether the current primary NO2 standard is
adequate or whether consideration of revisions is appropriate, EPA is
using an approach in this review that has been described in chapter 10
of the REA. The approach outlined in the REA builds upon the approaches
used in reviews of other criteria pollutants, including the most recent
reviews of the Pb, O3, and PM NAAQS (EPA, 2007d; EPA, 2007e;
EPA, 2005), and reflects the body of evidence and information that is
currently available. As in other recent reviews, EPA's considerations
will include the implications of placing more or less weight or
emphasis on different aspects of the scientific evidence and the
exposure/risk-based information, recognizing that the weight to be
given to various elements of the evidence and exposure/risk information
is part of the public health policy judgments that the Administrator
will make in reaching decisions on the standard.
A series of general questions frames this approach to considering
the scientific evidence and exposure-/risk-based information. First,
EPA's consideration of the scientific evidence and exposure/risk
information with regard to the adequacy of the current standard is
framed by the following questions:
To what extent does evidence that has become available
since the last review reinforce or call into question evidence for
NO2-associated effects that were identified in the last
review?
To what extent has evidence for different health
effects and/or sensitive populations become available since the last
review?
To what extent have uncertainties identified in the
last review been reduced and/or have new uncertainties emerged?
To what extent does evidence and exposure-/risk-based
information that has become available since the last review
reinforce or call into question any of the basic elements of the
current standard?
To the extent that the available evidence and exposure-/risk-based
information suggests it may be appropriate to consider revision of the
current standard, EPA considers that evidence and information with
regard to its support for consideration of a standard that is either
more or less protective than the current standard. This evaluation is
framed by the following questions:
Is there evidence that associations, especially causal
or likely causal associations, extend to ambient NO2
concentrations as low as, or lower than, the concentrations that
have previously been associated with health effects? If so, what are
the important uncertainties associated with that evidence?
Are exposures above benchmark levels and/or health
risks estimated to occur in areas that meet the current standard? If
so, are the estimated exposures and health risks important from a
public health perspective? What are the important uncertainties
associated with the estimated risks?
To the extent that there is support for consideration of a revised
standard, EPA then considers the specific elements of the standard
(indicator, averaging time, form, and level) within the context of the
currently available information. In so doing, the Agency addresses the
following questions:
Does the evidence provide support for considering a
different indicator for gaseous NOX?
Does the evidence provide support for considering
different averaging times?
What ranges of levels and forms of alternative
standards are supported by the evidence, and what are the associated
uncertainties and limitations?
[[Page 34425]]
To what extent do specific averaging times, levels, and
forms of alternative standards reduce the estimated exposures above
benchmark levels and risks attributable to NO2, and what
are the uncertainties associated with the estimated exposure and
risk reductions?
The questions outlined above have been addressed in the REA. The
following sections present considerations regarding the adequacy of the
current standard and potential alternative standards, as discussed in
chapter 10 of the REA, in terms of indicator, averaging time, form, and
level.
E. Adequacy of the Current Standard
In considering the adequacy of the current standard, the policy
assessment chapter of the REA considered the scientific evidence
assessed in the ISA and the quantitative exposure- and risk-based
information presented in the REA. A summary of this evidence and
information as well as CASAC recommendations and the Administrator's
conclusions regarding the adequacy of the current standard are
presented below.
1. Evidence-Based Considerations
As discussed in chapter 10 of the REA, evidence published since the
last review generally has confirmed and extended the conclusions
articulated in the 1993 AQCD (ISA, section 5.3.2). The epidemiologic
evidence has grown substantially with the addition of field and panel
studies, intervention studies, time-series studies of effects such as
emergency department visits and hospital admissions, and a substantial
number of studies evaluating mortality risk associated with short-term
NO2 exposures. As noted above, no epidemiologic studies were
available in 1993 that assessed relationships between NO2
and outcomes such as hospital admissions, emergency department visits,
or mortality. In contrast, dozens of epidemiologic studies on such
outcomes, conducted at recent and current ambient NO2
concentrations, are now included in this evaluation (ISA, chapter 3).
While not as marked as the growth in the epidemiologic literature, a
number of recent toxicological and human clinical studies also provide
insights into relationships between NO2 exposure and health
effects.
As an initial consideration with regard to the adequacy of the
current standard, the REA noted that the evidence relating long-term
(weeks to years) NO2 exposures at current ambient
concentrations to adverse health effects was judged in the ISA to be
either ``suggestive but not sufficient to infer a causal relationship''
(respiratory morbidity) or ``inadequate to infer the presence or
absence of a causal relationship'' (mortality, cancer, cardiovascular
effects, reproductive/developmental effects) (ISA, sections 5.3.2.4-
5.3.2.6). In contrast, the evidence relating short-term (minutes to
hours) NO2 exposures to respiratory morbidity was judged to
be ``sufficient to infer a likely causal relationship'' (ISA, section
5.3.2.1). This judgment was supported primarily by a large body of
recent epidemiologic evidence that evaluated associations of short-term
NO2 concentrations with respiratory symptoms, emergency
department visits, and hospital admissions. These conclusions from the
ISA suggest that, at a minimum, consideration of the adequacy of the
current annual standard should take into account the extent to which
that standard provides protection against respiratory effects
associated with short-term NO2 exposures. As noted in the
REA, such an emphasis on health endpoints for which evidence has been
judged to be sufficient to infer a likely causal relationship would be
consistent with other recent NAAQS reviews (e.g., EPA, 2005; EPA,
2007d; EPA, 2007e).
In considering the NO2 epidemiologic studies as they
relate to the adequacy of the current standard, the REA noted that
annual average NO2 concentrations were below the level of
the current annual NO2 NAAQS in many of the locations where
positive, and often statistically significant, associations with
respiratory morbidity endpoints have been reported (ISA, section 5.4).
As discussed previously, the ISA characterized that evidence for
respiratory effects as consistent and coherent. The evidence is
consistent in that associations are reported in studies conducted in
numerous locations and with a variety of methodological approaches
(ISA, section 5.3.2.1). It is coherent in the sense that the studies
report associations with respiratory health outcomes that are logically
linked together (ISA, section 5.3.2.1). The ISA noted that when the
epidemiologic literature is considered as a whole, there are generally
positive associations between NO2 and respiratory symptoms,
hospital admissions, and emergency department visits. A number of these
associations are statistically significant, particularly the more
precise effect estimates (ISA, section 5.3.2.1).
As discussed previously, the interpretation of these NO2
epidemiologic studies is complicated by the fact that on-road vehicle
exhaust emissions are a nearly ubiquitous source of combustion
pollutant mixtures that include NO2. In order to provide
some perspective on the uncertainty related to the presence of co-
pollutants, the ISA evaluated epidemiologic studies that employed
multi-pollutant models, epidemiologic studies of indoor and personal
NO2 exposure, and experimental studies. Specifically, the
ISA noted that a number of NO2 epidemiologic studies have
attempted to disentangle the effects of NO2 from those of
co-occurring pollutants by employing multi-pollutant models. When
evaluated as a whole, NO2 effect estimates in these models
generally remained robust when co-pollutants were included. Therefore,
despite uncertainties associated with separating the effects of
NO2 from those of co-occurring pollutants, the ISA (section
5.4, p. 5-16) concluded that ``the evidence summarized in this
assessment indicates that NO2 associations generally remain
robust in multi-pollutant models and supports a direct effect of short-
term NO2 exposure on respiratory morbidity at ambient
concentrations below the current NAAQS.'' With regard to indoor
studies, the ISA noted that these studies can test hypotheses related
to NO2 specifically (ISA, section 3.1.4.1). Although
confounding by indoor combustion sources is a concern, indoor studies
are not confounded by the same mix of co-pollutants present in the
ambient air or by the contribution of NO2 to the formation
of secondary particles or O3 (ISA, section 3.1.4.1). The ISA
noted that the findings of indoor NO2 studies are consistent
with those of studies using ambient concentrations from central site
monitors and concluded that indoor studies provide evidence of
coherence for respiratory effects (ISA, section 3.1.4.1). With regard
to experimental studies, the REA noted that they have the advantage of
providing information on health effects that are specifically
associated with exposure to NO2 in the absence of co-
pollutants. The ISA concluded that the NO2 epidemiologic
literature is supported by (1) evidence from controlled human exposure
studies of airway hyperresponsiveness in asthmatics, (2) controlled
human exposure and animal toxicological studies of impaired host-
defense systems and increased risk of susceptibility to viral and
bacterial infection, and (3) controlled human exposure and animal
toxicological studies of airway inflammation (ISA, section 5.3.2.1 and
5.4).
In drawing broad conclusions regarding the evidence, the ISA
[[Page 34426]]
considered the epidemiologic and experimental evidence as well as the
uncertainties associated with that evidence. When this evidence and its
associated uncertainties are taken together, the ISA concluded that the
results of epidemiologic and experimental studies form a plausible and
coherent data set that supports a relationship between NO2
exposures and respiratory endpoints, including respiratory symptoms and
emergency department visits, at ambient concentrations that are present
in areas that meet the current NO2 NAAQS. Thus, taking into
consideration the evidence discussed above, particularly the
epidemiologic studies reporting NO2-associated health
effects in locations that meet the current standard, the REA concluded
that the scientific evidence calls into question the adequacy of the
current standard to protect public health.
2. Exposure- and Risk-Based Considerations
In addition to the evidence-based considerations described above,
the REA considered the extent to which exposure- and risk-based
information can inform decisions regarding the adequacy of the current
annual NO2 standard, taking into account key uncertainties
associated with the estimated exposures and risks. As noted above,
NO2-associated health risks were characterized with three
approaches. In the first, NO2 air quality from locations
across the country was used as a surrogate for exposure. In the second,
exposures were estimated for all asthmatics and for asthmatic children
considering time spent in different microenvironments in one urban
area, Atlanta, GA. For both of these analyses, health risks were
characterized by comparing estimates of air quality or exposure to
potential health benchmark levels. Benchmark levels spanned the range
of NO2 concentrations that have been reported to increase
airway responsiveness in asthmatics (i.e., 100-300 ppb). In the third
approach to characterizing NO2-related health risks,
occurrences of NO2-related respiratory emergency department
visits were estimated for Atlanta. This quantitative risk assessment
was based on NO2 concentration-response relationships
identified in an epidemiologic study of air pollution-related emergency
department visits in Atlanta. The results of each of these analyses are
discussed in this section, specifically as they relate to the current
standard.
When considering the Atlanta risk assessment results as they relate
to the adequacy of the current standard, the REA noted that central
estimates of incidence of NO2-related respiratory emergency
department visits in Atlanta ranged from approximately 8 to 9% of total
respiratory-related emergency department visits per year (or 9,800-
10,900 NO2-related incidences) based on single pollutant
models when air quality is adjusted upward to simulate a situation
where Atlanta just meets the current standard. Central estimates of
incidence of NO2-related respiratory emergency department
visits ranged from 2.9-7.7% of total respiratory-related emergency
department visits per year (or 3,600-9,400 NO2-related
incidences) based on two-pollutant models. Inclusion of O3 and/or PM10
in multi-pollutant models resulted in the inclusion of an estimate of
zero NO2-related respiratory emergency department visits
within the 95% confidence intervals.
When considering the Atlanta exposure results as they relate to the
adequacy of the current standard, the REA noted the number of days per
year asthmatics could experience exposure to NO2
concentrations greater than or equal to potential health benchmark
levels, given air quality that is adjusted upward to simulate just
meeting the current standard. If NO2 concentrations were
such that the Atlanta area just meets the current standard, nearly all
asthmatics in Atlanta (>97%) would be estimated to experience six or
more days per year with 1-hour NO2 exposure concentrations
greater than or equal to our highest benchmark level (300 ppb) (REA,
Figure 8-22). Six days per year was the largest number of days
specifically considered in the REA, but these results suggest that some
asthmatics could experience 1-hour NO2 exposure
concentrations greater than or equal to 300 ppb on more than six days
per year. In addition, more frequent exceedances would be expected for
the lower benchmark levels.
When considering the air quality-based results as they relate to
the adequacy of the current standard, the REA noted the number of
benchmark exceedances estimated to occur in different locations given
air quality that just meets that standard. In situations where annual
NO2 concentrations were adjusted upward to simulate just
meeting the current standard, 1-hour NO2 concentrations
measured at fixed-site monitors in locations across the U.S. could
exceed benchmark levels. Most locations were estimated to experience at
least 50 days per year with 1-hour ambient NO2
concentrations at fixed-site monitors in the current network greater
than or equal to 100 ppb (Figures 7-2 and 7-3 in the REA) under this
hypothetical scenario. Far fewer ambient exceedances were predicted for
the higher benchmark levels. For example, only 5 areas were estimated
to experience any days with 1-hour ambient NO2
concentrations at fixed-site monitors greater than or equal to 300 ppb,
and none of those locations were estimated to experience more than 2
such days per year, on average (REA, Appendix A).
However, on-road NO2 concentrations were estimated in
this analysis to be an average of 80% higher than concentrations at
fixed-site monitors (though this relationship will vary across
locations and with time). In the majority of locations evaluated,
roadway exceedances of the 100 ppb benchmark level could occur on most
days of the year when air quality is adjusted upward to simulate just
meeting the current standard (Figure 7-6 in the REA). Even for higher
benchmark levels, most locations were estimated to have exceedances on
roadways. All locations evaluated except one (Boston) were estimated to
experience on-road NO2 concentrations greater than or equal
to 300 ppb (REA, Appendix A). Four of these locations were estimated to
experience an average of greater than 20 days per year with on-road
NO2 concentrations greater than or equal to 300 ppb (REA,
Appendix A).
3. Summary of Considerations From the REA
As noted above, the policy assessment chapter of the REA considered
the scientific evidence with regard to the current standard. This
included consideration of causality judgments made in the ISA regarding
the level of support for effects associated with short-term and long-
term exposures, the epidemiologic evidence described in the ISA
including associated uncertainties, the conclusions in the ISA
regarding the robustness of this evidence, and the support provided for
epidemiologic findings by experimental studies. The REA concluded that,
given these considerations, particularly the evidence for
NO2-associated effects in locations that meet the current
standard, the adequacy of the current standard to protect the public
health is clearly called into question. This evidence provides support
for consideration of an NO2 standard that would provide
increased health protection for at-risk groups, including asthmatics
and individuals who spend time on or near major roadways, against
health effects associated with short-term exposures ranging from
increased asthma symptoms to respiratory-related emergency department
visits and
[[Page 34427]]
hospital admissions, in addition to potential effects associated with
long-term exposures.
In examining the exposure- and risk-based information with regard
to the adequacy of the current annual NO2 standard to
protect the public health, the REA noted that estimated risks
associated with air quality adjusted upward to simulate just meeting
the current standard can reasonably be concluded to be important from a
public health perspective. In particular, a large percentage (8-9%) of
respiratory-related ED visits in Atlanta could be associated with
short-term NO2 exposures, most asthmatics in Atlanta could
be exposed on multiple days per year to NO2 concentrations
at or above the highest benchmark evaluated, and most locations
evaluated could experience on-/near-road NO2 concentrations
above benchmark levels on more than half of the days in a given year.
Therefore, the REA noted that exposure- and risk-based results
reinforce the scientific evidence in supporting the conclusion that
consideration should be given to revising the current standard so as to
provide increased public health protection, especially for at-risk
groups, from NO2-related adverse health effects associated
with short-term, and potential long-term, exposures.
4. CASAC Views
With regard to the adequacy of the current standard, CASAC
conclusions were consistent with the views expressed in the policy
assessment chapter of the REA. CASAC agreed that the primary concern in
this review is to protect against health effects that have been
associated with short-term NO2 exposures. CASAC also agreed
that the current annual standard is not sufficient to protect public
health against the types of exposures that could lead to these health
effects. Given these considerations, and as noted in their letter to
the EPA Administrator, ``CASAC concurs with EPA's judgment that the
current NAAQS does not protect the public's health and that it should
be revised'' (Samet, 2008b). CASAC's views on how the standard should
be revised are provided below within the context of discussions on the
elements (i.e., indicator, averaging time, form, level) of a new short-
term standard.
5. Administrator's Conclusions Regarding Adequacy of the Current
Standard
In considering the adequacy of the current NO2 NAAQS,
the Administrator has considered the conclusions of the ISA, the
conclusions of the policy assessment chapter of the REA, and the views
expressed by CASAC. In particular, the ISA concluded that the results
of epidemiologic and experimental studies form a plausible and coherent
data set that supports a likely causal relationship between short-term
NO2 exposures and adverse respiratory effects at ambient
NO2 concentrations that are present in locations meeting the
current NO2 NAAQS. With regard to the exposure and risk
results, the REA concludes that central risk estimates suggest that the
current standard could allow important adverse public health impacts.
Based on her consideration of these conclusions, as well as
consideration of CASAC's conclusion that the current NO2
NAAQS does not protect the public's health, the Administrator concludes
that the current NO2 standard does not provide the requisite
degree of protection for public health against adverse effects
associated with short-term exposures. In considering approaches to
revising the current standard, the Administrator concludes that it is
appropriate to consider setting a new short-term standard to supplement
the current annual standard. The Administrator notes that such a short-
term standard could provide increased public health protection,
especially for members of at-risk groups, from effects described in
both epidemiologic and controlled human exposure studies to be
associated with short-term exposures to NO2.
F. Conclusions on the Elements of a New Short-Term Standard and an
Annual Standard
In considering alternative NO2 primary NAAQS, the
Administrator notes the need to protect at-risk individuals from short-
term exposures to NO2 air quality that could cause the types
of respiratory morbidity effects reported in epidemiologic studies and
the need to protect at-risk individuals from short-term exposure to
NO2 concentrations reported in controlled human exposure
studies to increase airway responsiveness in asthmatics. Considerations
with regard to potential alternative standards and the specific options
being proposed are discussed in the following sections in terms of
indicator, averaging time, form, and level (sections II.F.1-II.F.4).
1. Indicator
In past reviews, EPA has focused on NO2 as the most
appropriate indicator for ambient NOX. In making a decision
in the current review on the most appropriate indicator, the
Administrator has considered the conclusions of the ISA and REA as well
as the view expressed by CASAC. The REA noted that, while the presence
of NOX species other than NO2 has been
recognized, no alternative to NO2 has been advanced as being
a more appropriate surrogate. Controlled human exposure studies and
animal toxicology studies provide specific evidence for health effects
following exposure to NO2. Epidemiologic studies also
typically report levels of NO2 though the degree to which
monitored NO2 reflects actual NO2 levels, as
opposed to NO2 plus other gaseous NOX, can vary
(REA, section 2.2.3). In addition, because emissions that lead to the
formation of NO2 generally also lead to the formation of
other NOX oxidation products, measures leading to reductions
in population exposures to NO2 can generally be expected to
lead to reductions in population exposures to other gaseous
NOX. Therefore, an NO2 standard can also be
expected to provide some degree of protection against potential health
effects that may be independently associated with other gaseous
NOX even though such effects are not discernable from
currently available studies indexed by NO2 alone. Given
these key points, the REA concluded that the evidence supports
retaining NO2 as the indicator. Consistent with this
conclusion, the CASAC Panel recommended in its letter to the EPA
Administrator that it ``concurs with retention of NO2 as the
indicator'' (Samet, 2008b). In light of the above considerations, the
Administrator proposes to retain NO2 as the indicator in the
current review.
2. Averaging Time
The current annual averaging time for the NO2 NAAQS was
originally set in 1971, based on epidemiologic studies that supported a
link between adverse respiratory effects and long-term exposure to low
levels of NO2. As noted above, that annual standard was
retained in subsequent reviews in part because an air quality
assessment conducted by EPA concluded that areas that meet the annual
standard would be unlikely to experience short-term ambient peaks above
concentrations that had been reported in a meta-analysis of controlled
human exposure studies to increase airway responsiveness in asthmatics.
In the current review, additional scientific evidence is available to
inform a decision on averaging time. This includes the availability of
a number of epidemiologic studies that have evaluated endpoints
including respiratory symptoms, emergency
[[Page 34428]]
department visits, and hospital admissions as well as an updated meta-
analysis of controlled human exposure studies of airway responsiveness
in asthmatics.
In order to inform conclusions with regard to averaging time in
this review, the REA considered judgments on the evidence from the ISA,
results from experimental and epidemiologic studies, and an analysis of
correlations between short- and long-term ambient NO2
concentrations. These considerations are described in more detail
below.
a. Short-Term Averaging Time
As described previously, the evidence relating short-term (minutes
to hours) NO2 exposures to respiratory morbidity was judged
in the ISA to be ``sufficient to infer a likely causal relationship''
(ISA, section 5.3.2.1) while the evidence relating long-term (weeks to
years) NO2 exposures to adverse health effects was judged to
be either ``suggestive but not sufficient to infer a causal
relationship'' (respiratory morbidity) or ``inadequate to infer the
presence or absence of a causal relationship'' (mortality, cancer,
cardiovascular effects, reproductive/developmental effects) (ISA,
sections 5.3.2.4-5.3.2.6). Thus, the REA concluded that these judgments
most directly support an averaging time that focuses protection on
short-term exposures to NO2.
As in past reviews of the NO2 NAAQS, it is instructive
to evaluate the potential for a standard based on annual average
NO2 concentrations, as is the current standard, to provide
protection against short-term NO2 exposures. To this end,
Table 10-1 in the REA reported the ratios of short-term to annual
average NO2 concentrations. Ratios of 1-hour daily maximum
concentrations (98th and 99th percentile) \10\ to annual average
concentrations across 14 locations ranged from 2.5 to 8.7 while ratios
of 24-hour average concentrations to annual average concentrations
ranged from 1.6 to 3.8 (see Thompson, 2008 for more details). The REA
concluded that the variability in these ratios across locations,
particularly those for 1-hour concentrations, suggested that a standard
based on annual average NO2 concentrations would not likely
be an effective or efficient approach to focus protection on short-term
NO2 exposures. For example, in an area with a relatively
high ratio (e.g., 8), the current annual standard (53 ppb) would be
expected to allow 1-hour daily maximum NO2 concentrations of
about 400 ppb. In contrast, in an area with a relatively low ratio
(e.g., 3), the current standard would be expected to allow 1-hour daily
maximum NO2 concentrations of about 150 ppb. Thus, for
purposes of protecting against the range of 1-hour NO2
exposures, the REA noted that a standard based on annual average
concentrations would likely require more control than necessary in some
areas and less control than necessary in others, depending on the
standard level selected.
---------------------------------------------------------------------------
\10\ As discussed below, 98th and 99th percentile forms were
evaluated in the REA. A 99th percentile form corresponds
approximately to the 4th highest 1-hour concentration in a year
while a 98th percentile form corresponds approximately to the 7th or
8th highest 1-hour concentration in a year. A 4th highest
concentration form has been used previously in the O3
NAAQS while a 98th percentile form has been used previously in the
PM2.5 NAAQS.
---------------------------------------------------------------------------
In considering the level of support available for specific short-
term averaging times, the policy assessment chapter of the REA noted
evidence from both experimental and epidemiologic studies. Controlled
human exposure studies and animal toxicological studies provide
evidence that NO2 exposures from less than 1-hour up to 3-
hours can result in respiratory effects such as increased airway
responsiveness and inflammation (ISA, section 5.3.2.7). Specifically,
the ISA concluded that NO2 exposures of 100 ppb for 1-hour
(or 200 ppb to 300 ppb for 30-min) can result in small but significant
increases in nonspecific airway responsiveness (ISA, section 5.3.2.1).
In contrast, the epidemiologic literature provides support for short-
term averaging times ranging from approximately 1-hour up to 24-hours
(ISA, section 5.3.2.7). A number of epidemiologic studies have detected
positive associations between respiratory morbidity and 1-hour (daily
maximum) and/or 24-hour NO2 concentrations. A few
epidemiologic studies have considered both 1-hour and 24-hour averaging
times, allowing comparisons to be made. The ISA reported that such
comparisons in studies that evaluate asthma emergency department visits
failed to reveal differences between effect estimates based on a 1-hour
averaging time and those based on a 24-hour averaging time (ISA,
section 5.3.2.7). Therefore, the ISA concluded that it is not possible,
from the available epidemiologic evidence, to discern whether effects
observed are attributable to average daily (or multi-day)
concentrations (24-hour average) or high, peak exposures (1-hour
maximum) (ISA, section 5.3.2.7).
As noted in the policy assessment chapter of the REA, given the
above conclusions, the experimental evidence provides support for an
averaging time of shorter duration than 24 hours (e.g., 1-h) while the
epidemiologic evidence provides support for both 1-hour and 24-hour
averaging times. At a minimum, this suggests that a primary concern
with regard to averaging time is the level of protection provided
against 1-hour daily maximum NO2 concentrations. However, it
is also important to consider the ability of a 1-hour (daily maximum)
averaging time to protect against 24-hour average NO2
concentrations. To this end, Table 10-2 in the REA presented
correlations between 1-hour daily maximum NO2 concentrations
and 24-hour average NO2 concentrations (98th and 99th
percentile) across 14 locations (see Thompson, 2008 for more detail).
Typical ratios ranged from 1.5 to 2.0, though one ratio (Las Vegas) was
3.1. These ratios were far less variable than those discussed above for
annual average concentrations, suggesting that a standard based on 1-
hour daily maximum NO2 concentrations could also be
effective at protecting against 24-hour NO2 concentrations.
The REA concluded that the scientific evidence, combined with the air
quality correlations described above, support the appropriateness of a
standard based on 1-hour daily maximum NO2 concentrations to
protect against health effects associated with short-term exposures.
b. Long-Term Averaging Time
While the REA concluded that the combination of the scientific
evidence from the ISA and air quality analyses most directly support an
averaging time that focuses protection on short-term exposures to
NO2, some evidence does support the need to also consider
health effects potentially associated with long-term exposures. As
noted above, the ISA judged the evidence relating long-term (weeks to
years) NO2 exposures to respiratory morbidity to be
``suggestive but not sufficient to infer a causal relationship.'' The
available database supporting the relationship between respiratory
illness in children and long-term exposures to NO2 has
increased since the 1996 review of the NO2 NAAQS. Results
from several studies, including the California-based Children's Health
Study, have reported deficits in lung function growth (Gauderman et
al., 2004) in association with long-term exposure to NO2. In
addition, some studies have reported associations between asthma
incidence and long-term NO2. The plausibility of these
associations is supported by some animal toxicological studies.
Specifically, morphological effects following chronic NO2
exposures have been identified in animal studies that
[[Page 34429]]
link to these increases in collagen synthesis and may provide
plausibility for the deficits in lung function growth described in
epidemiologic studies of long-term exposure to NO2 (ISA,
section 3.4.5).
Therefore, though the evidence provides strong support for the need
to protect against health effects associated with short-term
NO2 exposures, it may also be appropriate to consider the
extent to which the NO2 standard could protect against
potential effects associated with long-term exposures. To address this
issue, the REA estimated annual average NO2 concentrations
assuming different 1-hour standards were just met. For the locations
evaluated, a 1-hour area-wide standard with a level at or below 100 ppb
was estimated to be associated with annual average NO2
concentrations below the level of the current annual standard (53 ppb)
(REA, section 10.4.2). Therefore, it is possible that a 1-hour standard
could also provide protection against potential effect associated with
long-term exposures, depending on the level of the standard.
c. CASAC Views
CASAC agreed with the conclusions of the policy assessment chapter
of the REA that a primary consideration of the NO2 NAAQS
should be the protection provided against health effects associated
with short-term exposures. In their letter to the EPA Administrator,
CASAC stated that they concur ``with having a short-term NAAQS primary
standard for oxides of nitrogen and using the one-hour maximum
NO2 value.'' In addition, the letter noted that ``CASAC also
recommends retaining the current standard based on the annual
average.'' CASAC based this recommendation on the ``limited evidence
related to potential long-term effects of NO2 exposure and
the lack of strong evidence of no effect.'' In addition, CASAC
concluded that ``the findings of the REA do not provide assurance that
a short-term standard based on the one-hour maximum will necessarily
protect the population from long-term exposures at levels potentially
leading to adverse health effects'' (Samet, 2008b).
d. Administrator's Conclusions on Averaging Time
In considering the most appropriate averaging time(s) for the
NO2 primary NAAQS, the Administrator notes the conclusions
and judgments made in the ISA about available scientific evidence,
conclusions from the REA, and CASAC recommendations discussed above.
Based on these considerations, the Administrator proposes to set a new
standard based on 1-hour daily maximum NO2 concentrations.
In addition, the Administrator notes that CASAC recommended retaining
the current annual standard to account for the fact that some evidence
suggests that long-term NO2 exposures could cause adverse
effects on respiratory health. Taking into account these
considerations, in addition to proposing a new 1-hour NO2
primary NAAQS to provide increased protection against effects
associated with short-term exposures, the Administrator also proposes
to retain an annual standard.
3. Form
When evaluating alternative forms in conjunction with specific
levels, the REA considered the adequacy of the public health protection
provided by the combination of level and form to be the foremost
consideration. In addition, the REA recognized that it is desirable to
have a form that is reasonably stable and insulated from the impacts of
extreme meteorological events. As noted in the review of the
O3 NAAQS (EPA, 2007e), forms that call for averaging of
concentrations over three years better reflect pollutant-associated
health risks than forms based on expected exceedances. This is because
such ``concentration-based'' forms give proportionally greater weight
to periods of time when pollutant concentrations are well above the
level of the standard than to times when the concentrations are just
above the standard, while an expected exceedance form would give the
same weight to periods of time with concentrations that just exceed the
standard as to times when concentrations greatly exceed the standard.
Averaging concentrations over three years also provides greater
regulatory stability than a form based on allowing only a single
expected exceedance in a year. Therefore, consistent with recent
reviews of the O3 and PM NAAQS, the REA focused on
concentration-based forms averaged over 3 years.
In considering specific concentration-based forms, the REA focused
on 98th and 99th percentile concentrations averaged over 3 years. With
regard to these alternative forms, the REA noted that a 99th percentile
form for a 1-hour daily maximum standard would correspond approximately
to the 4th highest daily maximum concentration in a year (which is the
form of the current O3 NAAQS) while a 98th percentile form
(which is the form of the current short-term PM2.5 NAAQS)
would correspond approximately to the 7th or 8th highest daily maximum
concentration in a year (Table 10-4 in the REA; see Thompson, 2008 for
methods). The REA concluded that either of these forms could provide an
appropriate balance between limiting peak NO2 concentrations
and providing sufficient regulatory stability. This is consistent with
judgments made in the 2006 review of the PM NAAQS (EPA, 2005).
When considering the extent to which exposure and risk analyses
inform judgments on the form of the standard, the REA noted that a 99th
percentile form could be appreciably more protective than a 98th
percentile form (for the same standard level) in some locations, as
shown by the results of air quality analyses. For example, a 99th
percentile standard of 200 ppb was estimated to decrease the number of
benchmark exceedances, relative to a 98th percentile form, by
approximately 50-70% in Boston, Philadelphia, and Washington, DC (Table
10-5 in the REA). However, a 99th percentile form was estimated to
decrease the number of benchmark exceedances by only approximately 10%
in St. Louis, Detroit, and Las Vegas (Table 10-5 in the REA). For most
locations analyzed, the difference was estimated to be between
approximately 10 and 50% (Table 10-5 in the REA). With regard to the
Atlanta exposure assessment, a 99th percentile form was estimated to
decrease the number of days with 6 or more benchmark exceedances (for
300 ppb), relative to a 98th percentile form, by 5-35% depending on the
standard level selected (REA Appendix B, table B-48). With regard to
the Atlanta risk assessment, a 99th percentile form was estimated to be
associated with approximately 6% to 8% fewer NO2-related
emergency department visits than a 98th percentile form, across the
levels of the potential 1-hour standards examined.
When considering these results as they relate to the form of the
standard, the REA noted that a decision on form must be made in
conjunction with selection of a particular standard level. The primary
emphasis in such a decision will be on the degree of public health
protection provided by the combination of form and level.
CASAC agreed with the importance of considering the public health
protection provided by the combination of form and level. In its letter
to the EPA Administrator with regard to the final REA, the CASAC panel
stated that it ``advises that EPA choose a health protective percentile
appropriate for the level chosen for the one-hour standard.'' CASAC
went on to recommend that a 98th percentile form would be
[[Page 34430]]
appropriate for a standard level at the lower boundary of the range
evaluated (50 ppb, see below) but that a higher percentile should be
considered for higher levels (Samet, 2008b).
When considering alternative forms, the Administrator notes the
views expressed in the REA and the recommendations from CASAC, as
described above. In particular, she notes that a 99th percentile (or
4th highest) form could be appreciably more protective in some
locations than a 98th (or 7th or 8th highest) form. Given these
considerations, and in light of the specific range proposed for level
below, the Administrator proposes to adopt either a 99th percentile or
a 4th highest form, averaged over 3 years. In addition, the
Administrator notes that a 98th percentile form could be appropriate,
particularly for standard levels at the low end of the range considered
in the REA. Therefore, she also solicits comment on both 98th
percentile and 7th or 8th highest forms.
4. Level
In assessing the level of the standard to propose, the
Administrator has considered the broad range of scientific evidence
assessed in the ISA, including the epidemiologic studies and controlled
human exposure studies, as well as the results of exposure/risk
analyses presented in the REA. In light of this body of evidence and
analyses, she has determined that it is necessary to provide increased
public health protection for at-risk individuals against an array of
adverse respiratory health effects related to short-term (i.e., 30
minutes to 24 hours) exposures to ambient NO2. Such health
effects have been associated with exposure to the distribution of
short-term ambient NO2 concentrations across an area. This
distribution includes both the higher short-term (i.e., peak) exposure
concentrations that can occur on or near major roadways and the lower
short-term exposure concentrations that can occur in areas not near
major roadways. In considering the most appropriate approach to
providing this protection, the Administrator is mindful of the extent
to which the available evidence and analyses can inform a decision on
standard level. Specifically, the range of proposed standard levels
discussed below (section II.F.4.e) is informed by controlled human
exposure and epidemiologic studies.
As discussed above (section II.B.1.d), controlled human exposure
studies have reported associations between various levels of
NO2 exposures and increased airway responsiveness in
asthmatics. These studies can inform an evaluation of the risks
associated with exposure to specific NO2 concentrations,
regardless of where those exposures occur in an area. Controlled human
exposure studies most directly inform consideration of the risks
associated with peak short-term NO2 exposure concentrations,
such as those that can occur on or near major roadways. This is the
case because NO2 concentrations around major roadways could
include concentrations within the range evaluated in the studies.
Controlled human exposure studies have not been conducted at the lower
concentrations of NO2 typically expected in areas not near
major roadways.
In addition, epidemiologic studies (section II.B.1.a and b) have
reported associations between ambient NO2 concentrations,
measured at area-wide monitors in the current network, and increased
respiratory symptoms, emergency department visits, and hospital
admissions. Area-wide monitors in the urban areas in which these
epidemiologic studies were conducted do not measure the full range of
ambient NO2 concentrations that can occur anywhere in the
area, because they are not sited in locations with more localized peak
concentrations. Thus, they do not measure the full range of ambient
NO2 concentrations that are likely responsible for the
exposures linked to the NO2-associated health effects
reported in the studies. Rather, the area-wide NO2
concentrations measured by these monitors are used as surrogates for
the entire distribution of ambient NO2 concentrations across
the area, a distribution that includes NO2 concentrations
that are both higher and lower than the area-wide concentrations
reported for the study locations. Specifically, this distribution of
concentrations includes the higher short-term peak NO2
concentrations that occur on or near major roadways and the lower
short-term concentrations that occur away from roadways. Thus, the
epidemiologic studies can inform an evaluation of the risks associated
with the full range of exposures likely to occur across an area.
The available evidence and analyses support the importance of
roadway-associated NO2 exposures for public health.
Specifically, the exposure assessment presented in the REA estimated
that roadway-associated exposures account for the great majority of
exposures to peak NO2 concentrations (REA, Figures 8-17 and
8-18). In addition, the ISA (section 2.5.4) noted that in-vehicle
NO2 exposures could be 2-3 times higher than indicated by
ambient monitors in the current area wide-oriented network. Millions of
people in the U.S. live, work, and/or attend school near important
sources of NO2 such as major roadways (ISA, section 4.4) and
ambient NO2 concentrations in these locations are strongly
associated with distance from major roads (i.e., the closer to a major
road, the higher the NO2 concentration) (ISA, section
2.5.4). Therefore, these populations, which likely include a
disproportionate number of individuals in groups with higher prevalence
of asthma and higher hospitalization rates for asthma (e.g. ethnic or
racial minorities and individuals of low socioeconomic status ) (ISA,
section 4.4), are likely exposed to NO2 concentrations
higher than those that occur away from major roadways.
Given the above considerations, the Administrator proposes to set a
level for the 1-hour NO2 primary NAAQS that reflects the
maximum allowable NO2 concentration anywhere in an area.
This concentration is likely to occur on or near a major roadway. As
discussed above (section II.A.2), monitoring studies suggest that
NO2 concentrations near roadways can be approximately 30 to
100% higher than concentrations in the same area but not near the road.
This NO2 concentration gradient around roadways is one
factor considered by the Administrator in determining the appropriate
standard level to propose. EPA proposes to set the level of the
standard such that, when available information regarding the
concentration gradient around roadways is considered, appropriate
public health protection would be provided by limiting the higher
short-term peak exposure concentrations expected to occur on and near
major roadways, as well as the lower short-term exposure concentrations
expected to occur away from those roadways.
The Administrator notes that this approach to setting the standard
would provide a relatively high degree of confidence regarding the
level of protection provided by the standard against peak exposures,
such as those that can occur on or near major roadways. This is a
particularly important consideration given the available information
and the air quality and exposure analyses, discussed above in section
II.F.4.b, which indicated that roadway-associated exposures account for
the majority of exposures to peak NO2 concentrations. The
Administrator concludes that the proposed approach would directly
address the great majority of peak exposures and associated health
effects. In addition, the range of standard levels proposed below
(section II.F.4.e) would provide a reasonable degree of confidence that
the
[[Page 34431]]
accompanying area-wide NO2 concentrations would be
maintained well below concentrations that have occurred in locations
where epidemiologic studies have reported associations between ambient
NO2 concentrations and health endpoints such as increased
respiratory symptoms, emergency department visits, and hospital
admissions. Therefore, the Administrator proposes to set a standard
level reflecting the maximum allowable NO2 concentration
anywhere in an area that, in combination with the proposed decisions on
indicator, averaging time, and form, will protect public health with an
adequate margin of safety against the array of NO2-
associated health effects.
The remainder of this section describes the considerations relevant
to the Administrator's proposed decisions on standard levels for a new
1-hour standard and the annual standard. Specifically, with regard to a
1-hour standard evidence-based considerations drawn from the ISA and
discussed in the policy-assessment chapter of the REA are discussed in
section II.F.4.a. Exposure- and risk-based considerations for a 1-hour
standard drawn from the analyses in the REA and discussed in the policy
assessment chapter are discussed in section II.F.4.b. A summary of the
considerations relating to a 1-hour standard from the policy assessment
chapter of the REA is presented in section II.F.4.c and CASAC views
expressed in the context of their comments on the final REA are
presented in section II.F.4.d. The Administrator's proposed approach to
setting a 1-hour standard and her conclusions regarding the level of
such a standard are presented in section II.F.4.e. An alternative
approach to setting a 1-hour standard is discussed in section II.E.4.f.
Comment is solicited on both approaches. Finally, the Administrator's
proposed conclusions on the level of the annual standard are presented
in section II.E.4.g.
a. Evidence-Based Considerations
Evidence-based considerations take into account the full body of
scientific evidence assessed in the ISA. When considering the extent to
which this scientific evidence can inform a decision on the level of a
1-hour standard, the policy assessment chapter of the REA notes that
NO2 concentrations represent different measures of exposure
when drawn from experimental versus epidemiologic studies.
Concentrations of NO2 tested in experimental studies, such
as controlled human exposure studies, represent exposure concentrations
in the breathing zone of the individual test subjects. In cases where
controlled human exposure studies report effects, those effects are
caused directly by exposure to a specified concentration of
NO2. In contrast, concentrations of NO2 drawn
from epidemiologic studies are often based on ambient monitoring data.
In the case of key U.S. studies that have been specifically considered
within the context of assessing the appropriate level for the standard,
these monitors measure area-wide NO2 concentrations that
occur away from major roadways. NO2 concentrations recorded
at these ambient monitors are used as surrogates for the distribution
of NO2 exposures across the study area and over the time
period of the study. As noted above, these monitors do not measure the
full range of ambient NO2 concentrations that can occur in
an area and, thus, they do not measure the full range of ambient
NO2 concentrations that are likely responsible for the
NO2-associated health effects reported in the studies.
Instead they capture one part of the distribution (the area-wide
concentration) and this is used as a surrogate for the entire
distribution, which includes peak roadway-associated concentrations. As
noted in the REA, the interpretation of NO2 concentrations
from different types of studies is an important consideration for
decisions on standard level. These implications are discussed in more
detail below in section II.F.4.e.
In considering the epidemiologic evidence, the REA noted the ISA
conclusion that epidemiologic studies provide the strongest support for
the link between short-term NO2 exposure and respiratory
morbidity. In addition, epidemiologic studies provide evidence for the
most serious NO2-associated respiratory effects, including
respiratory-related hospital admissions and emergency department
visits. As noted above, these effects have been reported to be
associated with area-wide NO2 concentrations in key U.S.
epidemiologic studies. Because area-wide NO2 concentrations
are used as surrogates for the distribution of NO2 exposures
across the study area and over the time period of the study (see
above), the health effects reported in these epidemiologic studies are
reasonably inferred to be associated with exposure to ambient
NO2 concentrations that are both higher and lower than the
area-wide concentrations reported for the study locations. As noted
above, this distribution of exposure concentrations includes both the
higher short-term peak NO2 concentrations that occur on or
near major roadways and the lower short-term concentrations that occur
away from roadways.
When evaluating the epidemiologic literature for its potential to
inform the selection of an appropriate range of standard levels, the
REA noted the ISA conclusion that NO2 epidemiologic studies
provide ``little evidence of any effect threshold'' (ISA, section
5.3.2.9, p. 5-15). In studies that have evaluated concentration-
response relationships, those relationships appear linear within the
observed range of data (ISA, section 5.3.2.9). Given this lack of an
apparent threshold below which effects do not occur, an important
consideration with regard to providing an adequate margin of safety is
the extent to which it is appropriate for the range of proposed
standard levels to extend below NO2 concentrations that have
been associated with health effects in these studies. For purposes of
using the epidemiologic evidence to identify a range of standard levels
for evaluation in the absence of an apparent threshold, the REA
considered the range of NO2 concentrations that have been
monitored in locations, and during time periods, of key U.S.
epidemiologic studies (ISA, Table 5.4-1).
Figures 4 and 5 below (REA, Figures 5-1 and 5-2) show standardized
effect estimates from single pollutant models and the 99th and 98th
percentiles of the 1-hour daily maximum NO2 concentrations
recorded at area-wide monitors in the locations, and during the time
periods, of key U.S. studies. The peak NO2 concentrations to
which individuals were exposed on and/or near major roadways in these
locations during the study periods would be expected to be
substantially higher than the concentrations recorded at these area-
wide monitors. The lowest area-wide 1-hour daily maximum
concentrations, 53 (99th percentile) and 50 (98th percentile) ppb, were
monitored in the location of the study by Delfino et al. (2002). This
single study reported mixed results for respiratory symptoms with most
reported NO2 effect estimates being positive, and with some
but not all positive effect estimates being statistically significant.
A cluster of 5 studies (Ito et al., 2007; Jaffe et al., 2003; NYDOH,
2006; Peel et al., 2005; Tolbert et al., 2007) were conducted in
locations with area-wide 1-hour daily maximum NO2
concentrations ranging from 93 to 112 ppb (99th percentile) and from 85
to 94 ppb (98th percentile). In these studies, single pollutant models
yielded generally positive and often statistically significant
NO2 effect estimates for respiratory-related emergency
[[Page 34432]]
department visits and hospital admissions in a variety of locations
across the U.S. Of these 5 studies, 4 studies (Ito, 2007; NYDOH, 2006;
Peel et al., 2005; Tolbert et al., 2007) also reported NO2
effect estimates using multi-pollutant models, as discussed above
(section II.B.1.a). In the study by Ito (2007), risk estimates were
robust and remained statistically significant in multi-pollutant models
that included PM2.5, O3, CO, and
SO2.\11\ In the study by Peel et al. (2005), the authors
reported that ``The estimates for NO2 were generally not
attenuated in multipollutant models, while the estimates for the other
pollutants [PM10, ozone, NO2, and CO] suggested
weaker or no associations in the multipollutant models.'' The
quantitative results for these multi-pollutant models were not
presented in this study. In the remaining 2 studies (NYDOH, 2006;
Tolbert et al., 2007), NO2 effect estimates that were
positive in single pollutant models remained positive but not
statistically significant in multi-pollutant models.\12\ Two additional
studies which evaluated only single pollutant models (Linn et al.,
2000; Ostro et al., 2001) reported positive and statistically
significant NO2 effect estimates in locations with
appreciably higher area-wide 1-hour daily maximum NO2
concentrations (i.e., around 200 ppb).
---------------------------------------------------------------------------
\11\ In this study, multi-pollutant models were evaluated only
for the warm months. Single pollutant effect estimates for
NO2 were statistically significant for the warm months,
but not for the cold months.
\12\ As discussed above in section II.B.1, the conclusion from
the ISA that NO2 effect estimates generally remain robust
in multi-pollutant models is based on evaluation of the broader body
of epidemiologic evidence which includes, but is not limited to,
these U.S. studies (e.g., see Figures 1-3 above and ISA, Figures
3.1-7, 3.1-10, and 3.1-11). Effect estimates from these U.S. studies
were not included in the multi-pollutant figures in the ISA because
the studies generally reported multi-pollutant model results only
qualitatively. They generally did not report the quantitative
information that would have been necessary to include the results in
the ISA figures.
---------------------------------------------------------------------------
---------------------------------------------------------------------------
\13\ Effect estimates presented in Figures 4 and 5 are from
single pollutant models.
---------------------------------------------------------------------------
---------------------------------------------------------------------------
\14\ Authors of relevant U.S. and Canadian studies were
contacted and, for each study, air quality statistics were requested
from the monitor that recorded the highest NO2
concentrations. In cases where authors provided 1-hour daily maximum
air quality statistics, this information is presented in Figures 4
and 5 (studies by Tolbert, Peel, NYDOH, Delfino). In four cases
(studies by Ito, Jaffe, Linn, Ostro), we were not able to identify
1-hour NO2 statistics from the information provided by
the authors. In these cases, we evaluated monitored NO2
concentrations reported to EPA's Air Quality System (AQS) for the
location and time of the study. Figures 4 and 5 present the highest
98th/99th percentile 1-hour daily maximum NO2
concentrations that correspond to each study location and time
period. Prior to identifying potential alternative standards, we did
not receive air quality information from any of the Canadian authors
contacted and we were unable to reconstruct the air quality data
sets for the Canadian studies. Therefore, for purposes of
identifying levels of potential alternative standards, our analysis
was based on these key U.S. studies. Note that the NO2
concentrations reported in the study by Jaffe are labeled as 24-hour
concentrations, but the author indicated in a personal communication
(Jaffe, 2008) that they actually represent 1-hour daily maximum
concentrations.
[GRAPHIC] [TIFF OMITTED] TP15JY09.003
[[Page 34433]]
[GRAPHIC] [TIFF OMITTED] TP15JY09.004
When evaluating the controlled human exposure literature for its
potential to inform the selection of a range of appropriate standard
levels for evaluation, the REA noted that available studies have
addressed the consequences of short-term (e.g., 30-minutes to several
hours) NO2 exposures for a number of health endpoints
including increased airway responsiveness, reduced host defense and
immunity, inflammation, and decreased lung function (ISA, section 3.1).
In identifying health endpoints on which to focus for purposes of
informing decisions about potential alternative standard levels, the
REA concluded that it was appropriate to focus on those endpoints that
occur at or near ambient levels of NO2 and endpoints that
are of potential public health significance. As described above in more
detail (section II.C.1), the only endpoint to meet both of these
criteria is increased airway responsiveness in asthmatics. The ISA
concluded that NO2 exposures between 200 and 300 ppb for 30
minutes and 100 ppb for 60-minutes can result in small but significant
increases in nonspecific airway responsiveness (ISA, section 5.3.2.1)
and that ``transient increases in airway responsiveness following
NO2 exposure have the potential to increase symptoms and
worsen asthma control'' (ISA, sections 3.1.3 and 5.4). This effect
could have important public health implications due to the large size
of the asthmatic population in the United States (ISA, Table 4.4-1). In
addition, NO2 effects on airway responsiveness in asthmatics
are part of the body of experimental evidence that provides
plausibility and coherence for the observed NO2-related
increase in hospital admissions and emergency department visits in
epidemiologic studies (ISA, section 5.3.2.1). For all of these reasons,
the REA considered the extent to which results reported for the
NO2-associated increase in airway responsiveness in
asthmatics could inform decisions on alternative standard levels.
With regard to controlled human exposure studies of airway
responsiveness, the ISA and the REA discussed an update to a meta-
analysis that was originally published by Folinsbee in 1992 and
considered in the 1993 NOX AQCD. The original analysis by
Folinsbee (1992) included individual level data from 19 studies
involving asthmatic volunteers. Folinsbee reported that 65% of resting
asthmatics (57 of 88) exposed to NO2 concentrations between
100 and 140 ppb experienced an increase in airway responsiveness. In
addition, 76% (25 of 33) of resting asthmatics experienced increased
airway responsiveness following exposure to NO2
concentrations between 200 and 300 ppb. These results in resting
asthmatics were statistically significant. Smaller, and statistically
non-significant, percentages of exercising asthmatics experienced
increased airway responsiveness following exposure to NO2
concentrations (ISA, section 3.1.3.2). The reason for this difference
is not known as the factors that predispose some asthmatics to
NO2 responsiveness are not understood (ISA, section
3.1.3.2).\15\
---------------------------------------------------------------------------
\15\ When the asthmatic results were grouped together for all
exposures, both at rest and during exercise, the percent of
asthmatics with increased airway responsiveness decreased at the
higher exposure concentrations. This result could be attributed to
the lack of an effect in the asthmatics exposed during exercise.
---------------------------------------------------------------------------
[[Page 34434]]
The update of this meta-analysis presented in the ISA (Table 3.1-3)
included one additional study of non-specific responsiveness and
removed an allergen responsiveness study that was included in the
original \16\ (see ISA, section 3.1.3.2 for more discussion). While the
updated analysis does not include new results at lower concentrations
(100-250 ppb), we interpreted the results with a greater focus on 100
ppb due, in part, to the greater body of evidence available, including
new epidemiologic evidence. Therefore, the updated analysis also
reported results specifically for an NO2 exposure
concentration of 100 ppb. As with the original analysis by Folinsbee
(1992), the updated meta-analysis reported that a larger percentage of
resting asthmatics, as opposed to exercising asthmatics, experienced an
NO2-related increase in airway responsiveness. The updated
analysis reported that, when exposed at rest, 66% (33 of 50) of
asthmatics experienced an increase in airway responsiveness following
exposure to 100 ppb NO2, 67% (47 of 70) of asthmatics
experienced an increase in airway responsiveness following exposure to
NO2 concentrations from 100 to 150 ppb, 75% (38 of 51) of
asthmatics experienced an increase in airway responsiveness following
exposure to NO2 concentrations from 200 to 300 ppb, and 73%
(24 of 33) of asthmatics experienced an increase in airway
responsiveness following exposure to NO2 concentrations
above 300 ppb. The fraction of resting asthmatics experiencing an
increase in airway responsiveness was statistically significant at each
of these NO2 concentrations.
---------------------------------------------------------------------------
\16\ The updated meta-analysis added a study that evaluated non-
specific airway responsiveness following exposure to 260 ppb
NO2 and removed a study that evaluated allergen-induced
airway responsiveness following exposure to 100 ppb NO2.
---------------------------------------------------------------------------
Based on this evidence, we have identified exposure to
NO2 at a level of 100 ppb to be the lowest level at which
effects have been observed in controlled human exposure studies, noting
that it is also the lowest level tested in the studies used in the
meta-analysis. There is no evidence from this meta-analysis, however,
of a threshold below which NO2-related effects do not occur.
b. Exposure- and Risk-Based Considerations
Chapters 7-9 of the REA estimated exposures and health risks
associated with recent air quality and with air quality, as measured at
monitors in the current area-wide network, which had been adjusted to
simulate just meeting the current and potential alternative standards.
The specific standard levels evaluated, for an area-wide standard based
on the 3-year average of the 98th and 99th percentile 1-hour daily
maximum NO2 concentrations, were 50, 100, 150, and 200 ppb.
The results of the air quality, exposure, and risk analyses are
presented below in Table 1. With regard to the air quality results,
Table 1 presents the number of days per year that NO2
concentrations on/near roads were estimated to equal or exceed the
lowest and the highest health benchmarks evaluated (100 and 300 ppb).
Compared to just meeting the current annual standard, exceedances
estimated to be associated with just meeting 99th percentile 1-hour
daily maximum area-wide standard levels of either 50 or 100 ppb were
substantially lower. In contrast, exceedances estimated to be
associated with 1-hour area-wide standards of 150 or 200 ppb were
either similar to, or slightly higher than, those estimated for just
meeting the current standard. Table 1 also presents the results of the
Atlanta exposure and risk assessments. As is the case for the air
quality analyses, NO2 exposures and risks estimated to be
associated with just meeting 1-hour area-wide standard levels of either
50 or 100 ppb were substantially lower than those associated with just
meeting the current annual standard. Exposures and risks estimated to
be associated with 1-hour area-wide standard levels of 150 or 200 ppb
were somewhat lower than, or similar to, those estimated for just
meeting the current annual standard.
Table 1--Summary of Results of the Exposure and Risk Analyses Presented in the REA
--------------------------------------------------------------------------------------------------------------------------------------------------------
Mean estimated number of days Mean percent of Atlanta Mean percent of total
per year with 1-hour NO2 asthmatics estimated to respiratory ED visits in Atlanta
concentrations on/near roads experience 6 or more days per estimated to be related to NO2
greater than or equal to year with 1-hour NO2 exposure (based on the year 2007)
benchmark levels (in location concentrations greater than or ---------------------------------
Air quality with largest number of estimate equal to benchmark levels (based
exceedances) on the year 2002)
-------------------------------------------------------------------- Single Multi-
100 ppb 300 ppb pollutant pollutant
100 ppb 300 ppb benchmark benchmark estimate estimates*
benchmark benchmark (percent) (percent)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Current annual standard........................... 338 38 100 97 8.1 1.7-6.9
--------------------------------------------------------------------------------------------------------------------------------------------------------
Potential Alternative Standards Evaluated in the REA
--------------------------------------------------------------------------------------------------------------------------------------------------------
99th 1-hour: 200 ppb.............................. 350 56 100 89 7.1 1.5-6.1
99th 1-hour: 150 ppb.............................. 337 13 100 57 5.4 1.1-4.6
99th 1-hour: 100 ppb.............................. 229 4 100 11 3.6 0.7-3.1
99th 1-hour: 50 ppb............................... 13 1 57 0 1.8 0.4-1.6
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Ranges represent the range of risk estimates that result from including different co-pollutants in the model.
c. Summary of Considerations From the REA
The policy assessment chapter of the REA considered the scientific
evidence and the exposure/risk information as they relate to
considering alternative 1-hour NO2 standards that could be
judged to be requisite to protect public health with an adequate margin
of safety. The conclusions of the REA were based, in large part, on
scientific evidence (i.e., key U.S. epidemiologic studies) and
exposure/risk analyses that were based on the use of the available
NO2 air quality data from area-wide monitors, as discussed
above in sections II.B and II.C. The implications of these conclusions
for a standard level that reflects the maximum allowable concentration
anywhere in an area (a
[[Page 34435]]
concentration likely to occur near major roads) are discussed below in
section II.F.4.e.
When considering an appropriate upper end of the range of 1-hour
daily maximum standard levels supported by the scientific evidence, the
REA noted the following:
Positive and statistically significant associations were
observed in several key U.S. epidemiologic studies in locations with
area-wide 98th and 99th percentile 1-hour daily maximum NO2
concentrations ranging from 85 to 112 ppb \17\ (Peel et al., 2005;
NYDOH, 2006; Ito et al., 2007; Tolbert et al., 2007) (see Figure 4
above).
---------------------------------------------------------------------------
\17\ As noted above, the health effects reported in
epidemiologic studies are reasonably inferred to be associated with
exposure to ambient NO2 concentrations that are both
higher than and lower than the area-wide concentrations reported for
the study location.
---------------------------------------------------------------------------
The meta-analysis of airway responsiveness presented in
the ISA reported increased airway responsiveness in most asthmatics
(66% or 33 out of 50) following short-term exposures to 100 ppb
NO2, which was the lowest concentration for which such data
were available. Although some uncertainties associated with this
evidence, as described above, provide support for considering standard
levels below 100 ppb (i.e., studies have typically involved volunteers
with mild asthma and data are lacking from more severely affected
asthmatics, who may be more susceptible (ISA, p. 3-16)), other
uncertainties (i.e., the undetermined magnitude and clinical
significance of the NO2-associated increase in airway
responsiveness) provide support for considering higher standard levels.
Given these considerations, the REA concluded that the scientific
evidence provides support for a standard level up to 100 ppb. The REA
also noted that, to the extent more emphasis is placed on the
uncertainties associated with ascribing effects to NO2 in
the cluster of epidemiologic studies and on the magnitude and clinical
significance of the NO2-associated increase in airway
responsiveness following exposure to NO2, standard levels
higher than 100 ppb could be considered. However, the strongest support
was concluded to be for standard levels at or below 100 ppb.
When considering an appropriate lower end of a range of levels
supported by the scientific evidence, the REA noted the following:
The epidemiologic study by Delfino et al., (2002)
evaluated associations between short-term ambient NO2
concentrations and respiratory symptoms in a location (Alpine, CA)
where area-wide NO2 concentrations were well below levels in
other key U.S. epidemiologic studies. As noted above, this single study
provides mixed evidence for NO2-associated effects in a
location with 99th and 98th percentile 1-hour daily maximum area-wide
NO2 concentrations of 53 and 50 ppb, respectively.
The meta-analysis of controlled human exposure studies
reported increased airway responsiveness in asthmatics at the lowest
NO2 concentration for which data were available (i.e., 100
ppb). In identifying the specific lower level for the standard that
could be reasonably supported by this controlled human exposure
evidence, there are several reasons why it is appropriate to consider
levels below 100 ppb. First, the meta-analysis did not provide
information on the potential for an NO2-induced increase in
airway responsiveness at concentrations below 100 ppb, leaving open the
possibility for effects following exposures to lower concentrations.
Second, the studies included in the meta-analysis did not evaluate
severe asthmatics and most of the subjects included in these studies
were mild asthmatics. Asthmatics characterized as having more severe
asthma may be more susceptible than mild asthmatics to the effects of
NO2 exposure (ISA, section 3.1.3.2).
Thus, the REA concluded that it was appropriate to base the lower
end of the range of standard levels on NO2 concentrations in
the location of the epidemiologic study by Delfino and on providing
increased protection relative to the lowest level at which increased
airway responsiveness in asthmatics was reported in controlled human
exposure studies. Given the mixed results reported in the Delfino
study, the REA concluded that it was appropriate to consider standard
levels approximately equal to, rather than below, those measured in the
location of the study. Given these considerations, the REA concluded
that the lower end of the range of levels that is reasonably supported
by the scientific evidence is 50 ppb for a 1-hour standard that would
protect public health with an adequate margin of safety.
In addition to these evidence-based considerations, the REA
compared the health risks estimated to be associated with just meeting
the current standard to those estimated to be associated with different
1-hour standards. As noted above (section II.C), the REA characterized
NO2-associated health risks by estimating the potential
occurrence of ambient NO2 concentrations greater than or
equal to concentrations reported to increase airway responsiveness,
exposures of asthmatics to NO2 concentrations reported to
increase airway responsiveness, and the incidence of NO2-
associated emergency department visits. Given the REA conclusion that
the available evidence and information clearly call into question the
adequacy of the current standard, the adequacy of alternative 1-hour
standards would also be called into question if those standards were
estimated to be associated with similar or higher risks. In considering
the three analyses that characterized NO2-associated health
risks, the REA noted that just meeting 1-hour area-wide standard levels
of 150 and 200 ppb was estimated to be associated with risks ranging
from somewhat lower to slightly higher than those estimated for the
just meeting the current standard. In contrast, just meeting 1-hour
standard levels of 50 or 100 ppb, in conjunction with the current area-
wide monitoring network, was estimated to result in appreciably lower
health risks than the current standard. Given this, the REA concluded
that the exposure/risk information reinforces the scientific evidence
in supporting a standard level from 50 to 100 ppb.
d. CASAC Views
CASAC expressed their views in a letter to the EPA Administrator
(Samet, 2008b) within the context of their review of the final REA, a
review which focused primarily on the policy assessment chapter.\18\ In
drawing conclusions regarding the level of a short-term standard, CASAC
considered the scientific evidence evaluated in the ISA, the exposure
and risk results presented in the REA, and the evidence- and risk-based
considerations presented in the policy assessment chapter of the REA.
CASAC concurred with the conclusion from the policy assessment chapter
that the strongest support is for standard levels between 50 and 100
ppb. Their letter noted that, ``CASAC firmly recommends that the upper
end of the range not exceed 100 ppb.'' In considering the impact of
margin of safety on standard level, CASAC noted that ``the intent of
the Clean Air Act is to protect public health with an adequate margin
of safety and consequently uncertainty should be considered as a reason
to move towards the lower end of the range of levels and not to the
upper.'' In addition, with regard to the NO2 concentration
gradient
[[Page 34436]]
around roadways, CASAC noted that ``the highest exposures likely occur
when individuals are near roadways.'' As a result they recommended that
the Agency consider the implications of this exposure issue when
interpreting the evidence and when considering the siting of regulatory
monitors.
---------------------------------------------------------------------------
\18\ Earlier CASAC letters focused on their review of the air
quality, exposure, and risk analyses as presented in other chapters
of the draft REA.
---------------------------------------------------------------------------
CASAC comments were offered within the context of their review of
the final REA. As noted above, the conclusions from the policy
assessment chapter of the final REA were based, in large part, on
scientific evidence and exposure/risk information based on
NO2 air quality data from the current area-wide
NO2 monitoring network. Therefore, it is not clear the
degree to which CASAC recommendations might differ for a standard level
that reflects the maximum allowable NO2 concentration
anywhere in an area, including near major roads. As noted in section
I.C above, we are specifically soliciting CASAC comment on the use of
this approach and on the proposed range of levels for a standard set
using this approach.
In drawing conclusions regarding the level of an annual standard,
CASAC noted the scientific evidence assessed in the ISA. Specifically,
CASAC concluded that while there is evidence supporting the link
between long-term NO2 exposure and adverse health effects,
this evidence does not provide a strong quantitative basis for changing
the level of the current annual standard. Therefore, with regard to the
annual standard, CASAC recommended ``retaining the current level, as
evidence has not been cited that would lead to either an increase or
decrease'' (Samet, 2008b).
e. Administrator's Conclusions on Level for a 1-Hour Standard
In considering the appropriate level for an NO2 standard
based on the 3-year average of the 99th percentile (or 4th highest) 1-
hour daily maximum NO2 concentration, the Administrator has
considered the broad body of scientific evidence and exposure/risk
information. She draws from that evidence and information the need to
protect at-risk individuals against the distribution of short-term
ambient NO2 exposure concentrations across an area and the
array of health effects that have been linked to such NO2
exposures.
Specifically, the Administrator has considered the extent to which
a variety of levels, which would reflect the maximum allowable 1-hour
NO2 concentration anywhere in an area, would be expected to
protect at-risk individuals against increased airway responsiveness,
respiratory symptoms, and respiratory-related emergency department
visits and hospital admissions. The Administrator notes that these
health endpoints are logically linked together in that the evidence for
increased airway responsiveness in asthmatics is part of the body of
experimental evidence that the ISA recognized as supporting the
plausibility of associations between ambient NO2 and the
respiratory morbidity endpoints (i.e., respiratory symptoms, emergency
department visits, and hospital admissions) reported in epidemiologic
studies.
As noted above, NO2 exposure patterns associated with
respiratory morbidity in epidemiologic studies are reasonably expected
to include short-term peak exposures on and/or near major roadways of a
magnitude that has been reported to increase airway responsiveness in
asthmatics. Therefore, to inform the identification of an appropriate
range of standard levels to propose, the Administrator has considered
the scientific evidence, the exposure/risk results, and information on
the NO2 concentration gradient around roadways.
In making judgments regarding the weight to place on the scientific
evidence and exposure/risk information, the Administrator has
considered the results of epidemiologic studies, controlled human
exposure studies, and exposure/risk analyses as well as the
uncertainties associated with this evidence and these analyses.
Specifically, she notes the following:
The ISA concluded that epidemiologic studies provide the
strongest support for the relationship between short-term exposure to
NO2 and respiratory morbidity. Despite the possibility that
associations between health effects and NO2 in epidemiologic
studies may be confounded by the presence of co-occurring pollutants,
particularly other traffic-related pollutants, the ISA concluded that
NO2 effect estimates remain robust in multi-pollutant models
and that the evidence supports a direct effect of NO2
exposures on respiratory morbidity, independent of associations with
other traffic-related pollutants. Given this conclusion, along with
conclusions from the ISA regarding the consistency and the coherence of
results across the relatively large number of NO2
epidemiologic studies (both indoor and outdoor) and the supporting
evidence from experimental studies, the Administrator has judged it
appropriate to place substantial weight on epidemiologic studies in
identifying an appropriate range of levels to propose.
Controlled human exposure studies report that short-term
exposures to NO2 can increase airway responsiveness in
asthmatics. With regard to this evidence, the Administrator also has
considered the uncertainties associated with the magnitude and the
clinical relevance of the NO2-associated increase in airway
responsiveness, noting that this effect may or may not be clinically
significant for any given asthmatic. However, given the potential
public health importance of this effect, due to the large size of the
asthmatic population in the U.S. and the possibility that the
NO2-associated increase in airway responsiveness could
worsen asthma symptoms and decrease control of asthma, the
Administrator judges that it is also appropriate to place weight on
this evidence when identifying an appropriate range of levels to
propose.
The results of the risk and exposure analyses presented in
the REA provide information on the potential public health implications
of setting the standard at different levels. The Administrator
acknowledges the uncertainties associated with these analyses which, as
discussed in the REA, could result in either over- or underestimates of
NO2-associated health risks. However, she also notes that
those uncertainties should be similar across different air quality
simulations within the air quality, exposure, and risk analyses.
Therefore, the Administrator judges that these analyses are potentially
useful for considering the relative levels of public health protection
that could be provided by specific standard levels.
After considering the scientific evidence and the exposure/risk
information (see sections II.B, II.C, and II.F.4.a through II.F.4.c),
as well as the available information on the NO2
concentration gradient around roadways (section II.A.2), as they relate
to a standard level reflecting the maximum allowable NO2
concentration in an area, the Administrator concludes that the
strongest support is for a standard level at or somewhat below 100 ppb.
The Administrator's rationale in reaching this conclusion is provided
below.
First, the Administrator notes that a standard level of 100 ppb or
lower under the proposed approach would be expected to limit short-term
peak NO2 exposures to concentrations that have been reported
to increase airway responsiveness in asthmatics. With regard to this,
the Administrator specifically notes the following:
The meta-analysis of controlled human exposure data in the
ISA reported increased airway
[[Page 34437]]
responsiveness in asthmatics at rest following exposure at and above
100 ppb NO2, the lowest NO2 concentration for
which airway responsiveness data are available in humans.
This meta-analysis does not provide any evidence of a
threshold below which effects do not occur. The studies included in the
meta-analysis evaluated primarily mild asthmatics while more severely
affected individuals could respond to lower concentrations. Given this,
it is possible that exposure to NO2 concentrations below 100
ppb could increase airway responsiveness in some asthmatics.
However, the magnitude of the NO2-induced
increase in airway responsiveness, and its clinical implications,
cannot be quantified from the meta-analysis. As noted previously, the
NO2-induced increase in airway responsiveness may or may not
be clinically significant. Further, there was a lack of an effect in
asthmatics exposed during exercise.
Given the above considerations, the Administrator concludes that
the controlled human exposure studies of airway responsiveness provide
support for limiting exposure to NO2 concentrations at or
somewhat below 100 ppb. While she acknowledges that exposure to lower
concentrations could increase airway responsiveness in some asthmatics,
the Administrator concludes that, given the uncertainties regarding the
magnitude and the clinical significance of the NO2-induced
increase in airway responsiveness, the greatest support is for limiting
exposures to 100 ppb.
Second, the Administrator notes that a standard level at or
somewhat below 100 ppb under the proposed approach would be expected to
maintain peak area-wide NO2 concentrations considerably
below peak area-wide concentrations measured in locations where
multiple key U.S. epidemiologic studies have reported associations with
emergency department visits and hospital admissions. With regard to
this, the Administrator specifically notes that 5 key U.S. studies
provide evidence for effects in locations where 99th percentile 1-hour
daily maximum NO2 concentrations measured at area-wide
monitors ranged from 93 to 112 ppb. The Administrator notes that the
study by Delfino provides mixed evidence for effects in a location with
a 99th percentile 1-hour daily maximum NO2 concentration, as
measured by an area-wide monitor, of 53 ppb. In that study, most of the
reported NO2 effect estimates were positive, but not
statistically significant. Focusing on these studies, the Administrator
concludes that they provide support for limiting area-wide
NO2 concentrations to below 90 ppb (99th percentile) in
order to provide protection against the reported effects. She also
concludes that limiting area-wide concentrations to considerably below
90 ppb would be appropriate in order to provide an adequate margin of
safety. Given the mixed results of the Delfino study, the Administrator
concludes that it may not be necessary to maintain area-wide
NO2 concentrations at or below 50 ppb to provide protection
against the effects reported in epidemiologic studies.
Given that NO2 concentrations near roads may be 30 to
100% higher than concentrations away from roads (see section II.A.2),
the Administrator notes that a standard level at or somewhat below 100
ppb under the proposed approach could limit area-wide NO2
concentrations to well below 90 ppb (99th percentile). With regard to
this, she specifically notes the following:
If NO2 concentrations near roads are 30% higher
than concentrations away from roads, a standard level of 100 ppb could
limit area-wide concentrations to approximately 75 ppb.
If NO2 concentrations near roads are 65% higher
than concentrations away from roads (the mid-range of the 30% to 100%
gradients), a standard level of 100 ppb could limit area-wide
NO2 concentrations to approximately 60 ppb.
If NO2 concentrations near roads are 100%
higher than concentrations away from roads, a standard level of 100 ppb
could limit area-wide concentrations to approximately 50 ppb.
Therefore, a standard level at or somewhat below 100 ppb under the
proposed approach would be expected to maintain area-wide
NO2 concentrations well below 90 ppb across locations
despite the expected variation in the NO2 concentration
gradient that can exist around roadways in different locations and over
time. Such a standard level recognizes the substantial weight that the
Administrator judges is appropriate to place on the cluster of key U.S.
epidemiologic studies that reported positive, and often statistically
significant, associations between NO2 and emergency
department visits and hospital admissions. This judgment takes into
account the determinations in the ISA, based on a much broader body of
evidence, that there is a likely causal association between exposure to
NO2 and these kinds of morbidity effects, and that there is
no evidence of a threshold below which such effects would not occur.
As noted above, based on the Administrator's consideration of the
controlled human exposure and epidemiologic evidence, she concludes
that the strongest support is for a standard level reflecting the
maximum allowable NO2 concentration in an area at or
somewhat below 100 ppb. In addition to these evidence-based
considerations, the Administrator notes that a standard level of 100
ppb under the proposed approach would be consistent with the results of
the exposure and risk analyses presented in the REA. As described in
sections II.F.4.b and II.F.4.c above, the results of these analyses
supported limiting area-wide NO2 concentrations to between
50 and 100 ppb, which would be expected with a standard level at or
below 100 ppb under the proposed approach. Given all of these
considerations, the Administrator concludes that a standard level at or
somewhat below 100 ppb under the proposed approach would be requisite
to protect public health with an adequate margin of safety against the
array of NO2-associated health effects.
To the extent it is determined appropriate to emphasize the
possibility that NO2-induced airway responsiveness in
asthmatics could occur following exposures below 100 ppb and/or the
clinical significance of such increase in airway responsiveness, the
Administrator notes that the evidence would support setting the
standard level below 100 ppb. The Administrator also notes that a
standard level below 100 ppb would be consistent with placing greater
emphasis on the mixed results reported in the epidemiologic study by
Delfino et al. (2002). Specifically, she notes that a standard level of
80 ppb would be expected to limit area-wide NO2
concentrations to approximately 50 ppb (80 is 65% higher than 50) and
that a standard level of 80 ppb would be expected to provide protection
against exposure concentrations below those that have been reported to
increase airway responsiveness in asthmatics.
For the reasons stated above, the Administrator proposes to set the
level of a new 1-hour standard between 80 ppb and 100 ppb. In so doing,
the Administrator proposes to place emphasis on reported findings from
both epidemiologic studies and from controlled human exposure studies.
In order to protect against NO2-associated emergency
department visits and hospital admissions reported in multiple key U.S.
epidemiologic studies, and against reported NO2-induced
increases in airway responsiveness, the Administrator proposes to set
the standard level no higher than 100 ppb. In addition, in light of the
fact that the Administrator is considering, and soliciting comment
[[Page 34438]]
on, the appropriate weight to place on the potential risk of
NO2-associated effects in locations with relatively low
area-wide NO2 concentrations and on the significance of
potential NO2-induced increases in airway responsiveness in
some asthmatics following exposures to concentrations below 100 ppb,
the Administrator is proposing to set a standard level within a range
that includes 100 ppb but is no lower than 80 ppb.
The Administrator solicits comment on the appropriateness of this
proposed range of standard levels as well as on the approach she has
used to identify the range. Specifically, the Administrator solicits
comment on the following:
The weight she has placed on the epidemiologic evidence,
the controlled human exposure evidence, the exposure/risk information,
and the uncertainties associated with each of these.
Her use of available information on the NO2
concentration gradient around roadways (i.e., that concentrations near
roadways can be 30 to 100% higher than concentrations in the same area
but not near the road) to inform an appropriate range of standard
levels.
The most appropriate part of the proposed range in which
to set the standard level given the available scientific evidence,
exposure/risk information, NO2 air quality information, and
the uncertainties associated with each.
With regard to the proposed range of standard levels, the
Administrator notes that the proposed range is consistent with the
recommendation by CASAC to set a standard level no higher than 100 ppb.
However, much of the evidence and exposure/risk information that
informed CASAC's advice was based on NO2 concentrations
measured at area-wide monitors in the current monitoring network. CASAC
did not explicitly address whether or how the standard level should
differ if it reflects the maximum allowable NO2
concentration in a location (including near major roads) rather than
the maximum allowable area-wide concentration.
The Administrator also solicits comment on setting a standard level
above 100 ppb and up to 150 ppb. In so doing, the Administrator
recognizes that there are uncertainties with the scientific evidence,
such as that associated with the magnitude and clinical significance of
the NO2-induced increase in airway responsiveness in
asthmatics and with attributing effects reported in epidemiologic
studies specifically to NO2 given the presence of co-
occurring pollutants. The Administrator invites comment on the extent
to which it is appropriate to emphasize these uncertainties in
considering the standard level and on whether it would be appropriate
to set a standard level as high as 150 ppb.
The Administrator notes that, in order to consider the potential
implications of a standard level as high as 150 ppb, it is important to
put such a standard in the context of potential ambient concentrations.
A standard level of 150 ppb under the proposed approach could be
associated with 1-hour area-wide NO2 concentrations of
approximately 90 ppb (150 is approximately 65% higher than 90), and
potentially with concentrations ranging from 75 to 115 ppb (150 is
approximately 100% higher than 75 and 30% higher than 115) depending on
location.
The Administrator notes that a standard level as high as 150 ppb
would place more emphasis on uncertainties associated with the
scientific evidence. Specifically, a standard level of 150 ppb would
emphasize the uncertainty associated with the magnitude and the
clinical significance of the NO2-induced increase in airway
responsiveness in asthmatics and would be based on an assumption that
NO2-associated health effects reported in epidemiologic
studies are due in large part to exposure to co-occurring pollutants,
rather than exposure to NO2. As noted above, the
Administrator seeks comment on the extent to which it would be
appropriate to emphasize these uncertainties in considering the
standard level and the extent to which the scientific evidence would
support levels up to 150 ppb.
In addition, the Administrator notes that a standard level lower
than 80 ppb could be appropriate to the extent that near-road
concentrations are determined to be closer to 30% higher than area-wide
concentrations or to the extent that additional emphasis is placed on
the possibility that exposure to NO2 concentrations below
100 ppb could increase airway responsiveness in some asthmatics.
Accordingly, the Administrator also solicits comment on standard levels
as low as 65 ppb (30% higher than an area-wide concentration of 50
ppb).
f. Alternative Approach to Setting the 1-Hour Standard Level
As discussed above, the Administrator is proposing a standard level
reflecting the maximum allowable NO2 concentration anywhere
in an area. However, for the reasons discussed below, EPA also solicits
comment on an alternative approach to setting a 1-hour NO2
standard. Under this alternative approach, the standard level would
reflect the maximum allowable NO2 concentration measured at
an area-wide monitoring site. Such a site would not be located in close
proximity to major roads and, for a given area, would not be the
location of the maximum NO2 concentration anywhere in that
area. In conjunction with soliciting comment on this alternative
approach, EPA solicits comment on setting the level of such a standard
within the range of 50 to 75 ppb. In addition, as with the proposed
standard, EPA solicits comment on NO2 as the indicator, a 1-
hour (daily maximum) averaging time, and the 3-year average of the 99th
percentile (or 4th highest) or 98th percentile (or the 7th or 8th
highest) as the form.
With regard to the range of levels from 50 to 75 ppb, which would
reflect maximum allowable area-wide NO2 concentrations under
this approach, the Administrator notes the following. First, a standard
level within in this range would be expected to maintain area-wide
NO2 concentrations below peak 1-hour area-wide
concentrations measured in locations where key U.S. epidemiologic
studies have reported associations with respiratory-related emergency
department visits and hospital admissions. Second, she notes that
standard levels from the lower end of this range would be expected to
limit roadway-associated exposures to NO2 concentrations
that have been reported in controlled human exposure studies to
increase airway responsiveness in asthmatics. A standard level of 50
ppb under this approach could limit near-road concentrations to between
65 and 100 ppb, given that near-road NO2 concentrations can
range from 30% to 100% higher than area-wide concentrations. Assuming
the mid-point of the range of gradients (i.e., that near-road
concentrations are 65% higher than area-wide concentrations), a
standard level of 50 ppb under this approach could limit near-road
concentrations to approximately 80 ppb and a standard level of 60 ppb
could limit near-road concentrations to approximately 100 ppb. Third,
to the extent that relatively more emphasis is placed on the
uncertainties regarding the magnitude and clinical significance of the
NO2-induced increase in airway responsiveness, the
Administrator notes that a standard level from the upper end of the
range could be determined to be appropriate. Finally, this approach
would provide more confidence than the proposed approach regarding the
degree to which a specific standard level would limit area-wide
NO2 concentrations but less confidence regarding the degree
to which a specific
[[Page 34439]]
standard level would limit the peak NO2 concentrations
likely to occur near major roadways.
The Administrator recognizes that her proposed approach results
from a comprehensive evaluation of alternative approaches to
determining the level of the NO2 primary NAAQS, but that
these approaches have not previously been presented to CASAC, or other
stakeholders, for their evaluation and public discussion. More
specifically, the Administrator notes that much of the information
included in the policy assessment chapter of the REA, which formed the
foundation for CASAC's recommendations regarding standard level, was
based on evaluation of data drawn from the current area wide-oriented
monitoring network. Further, the Administrator notes that CASAC did not
explicitly discuss in their recommendations whether and how the
standard level should differ if that level reflects the maximum
allowable NO2 concentration anywhere in an area rather than
the maximum allowable NO2 concentration measured at an area-
wide monitoring site. Given this, the Administrator recognizes the
possibility that comments received on this proposal, particularly those
received from CASAC, could provide important new information for
consideration.
g. Level of the Annual Standard
With regard to the annual standard, the Administrator notes that
the ISA concluded that the scientific evidence is suggestive but not
sufficient to infer a causal relationship between long-term
NO2 exposure and respiratory morbidity. While some studies
have reported associations between long-term NO2 exposure
and respiratory endpoints such as decrements in lung function growth
(Gauderman et al., 2004; Rojas-Martinez et al., 2007a and b; Oftedal et
al., 2008), the ISA notes that the high correlation among traffic-
related pollutants makes it difficult to accurately estimate
independent effects in these long-term studies. CASAC recommended
retaining an annual standard in order to provide protection against
potential health effects associated with long-term exposures. They
based this recommendation on ``the limited evidence related to
potential long-term effects of NO2 exposure and the lack of
strong evidence of no effect'' (Samet, 2008b). With regard to the level
of an annual standard, CASAC recommended retaining the current level as
the evidence considered did not provide a basis for either increasing
or decreasing it. Given these considerations, and recognizing that a
new 1-hour standard level as proposed would also provide some degree of
protection from long-term exposures, the Administrator proposes to take
a cautious approach and retain the current annual standard. The
Administrator solicits comment on this approach.
G. Summary of Proposed Decisions on the Primary Standard
For the reasons discussed above, and taking into account
information and assessments presented in the ISA and REA as well as the
advice and recommendations of CASAC, the Administrator proposes that
the current annual standard is not requisite to protect public health
with an adequate margin of safety. The Administrator proposes to
establish a new short-term standard that will afford increased
protection for asthmatics and other at-risk populations against an
array of adverse respiratory health effects related to short-term
NO2 exposure. These effects include increased asthma
symptoms, worsened control of asthma, an increase in respiratory
illnesses and symptoms, and related serious indicators of respiratory
morbidity including emergency department visits and hospital admissions
for respiratory causes.
Specifically, the Administrator proposes to set a new short-term
primary NO2 standard, with a 1-hour (daily maximum)
averaging time, a form defined as the 3-year average of the 99th
percentile or the 4th highest daily maximum concentration. The level
for the new standard is proposed to be within the range of 80 to 100
ppb, reflecting maximum allowable concentrations anywhere in an area.
In conjunction with this proposed standard, the Administrator also
solicits comment on levels as low as 65 ppb and as high as 150 ppb, and
on alternative forms including the 3-year average of the 98th
percentile or the 7th or 8th highest daily maximum concentration.
In addition, the Administrator also solicits comment on an
alternative approach to setting a new 1-hour standard. Under this
alternative, the NO2 NAAQS would reflect the maximum
allowable area-wide NO2 concentration, which would be
measured away from major roads. With regard to this approach, the
Administrator solicits comment on a level within the range from 50 to
75 ppb and on the same alternative forms as noted above.
In addition to setting a new 1-hour standard, the Administrator
proposes to retain the current annual standard. The current annual
standard together with a new 1-hour standard would provide protection
against health effects potentially associated with long-term exposures
to NO2. The Administrator solicits comment on this approach.
III. Proposed Amendments to Ambient Monitoring and Reporting
Requirements
The EPA is proposing changes to the ambient air monitoring,
reporting, and network design requirements for the NO2
NAAQS. This section discusses the changes we are proposing which are
intended to support the proposed 1-hour NAAQS and proposed retention of
the current annual NAAQS in Section II. Ambient NO2
monitoring data are used to determine whether an area is in violation
of the NO2 NAAQS. Ambient NO2 monitoring data are
collected by state, local, and Tribal monitoring agencies (``monitoring
agencies'') in accordance with the monitoring requirements contained in
40 CFR parts 50, 53, and 58.
A. Monitoring Methods
To be used in a determination of compliance with the NO2
NAAQS, NO2 data must be collected using a Federal Reference
Method (FRM) or a Federal Equivalent Method (FEM) analyzer. The current
monitoring method in use by most State and local monitoring agencies is
the gas-phase chemiluminescence FRM (40 CFR Part 50, Appendix F), which
was implemented into the NO2 monitoring network in the early
1980s. The current list of all approved FRMs and FEMs capable of
providing ambient NO2 data for use in attainment
designations may be found on the EPA Web site (http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf). It
must be noted, however, that due to the proposal of a new 1-hour NAAQS,
wet chemical based FEMs would not be appropriate for use in determining
compliance of the proposed 1-hour NAAQS, since such methods are
incapable of providing hourly averaged data. Therefore, we propose that
any NO2 FRM or FEM used for making primary NAAQS decisions
must be capable of providing hourly averaged concentration data. We
propose to only allow FRM or FEMs capable of providing hourly averaged
concentration data to be used to produce data for comparison to the
NAAQS, and solicit comment on this proposed requirement.
The sum of nitric oxide (NO) and NO2 is commonly called
NOX. Nitrogen oxides, technically the total reactive
nitrogen oxide family, known as NOY, is defined as the sum
of NO, NO2, and the higher nitrogen oxides collectively
[[Page 34440]]
termed NOZ. Important components of ambient NOZ
include nitrous acid (HNO2), nitric acid (HNO3),
and the peroxyacetyl nitrates (PANs). However, NO2 is the
indicator for the nitrogen oxides NAAQS. In the ambient monitoring
network, very nearly all measurements of NO2 are collected
by the chemiluminescence FRM. However, this technique directly measures
only NO by the principle of gas-phase chemiluminescence induced by the
reaction of NO with O3 at low pressure. NO2
concentrations are determined indirectly by the analyzer in two steps:
(1) By first measuring the ambient NO concentration, and (2)
determining total NOX, including NO2, by
measuring a second NO concentration after reducing the NO2
in the sample air stream to NO (most often through the use of a
molybdenum oxide (MoOX) substrate heated to between 300
[deg]C and 400 [deg]C in the sample flow path). The difference between
the second concentration (NO plus the NO2 reduced to NO) and
the first concentration (ambient NO only) is reported as the
NO2 concentration.
One issue of note with the chemiluminescence FRM is that the
reduction of NO2 to NO on the MoOX converter
substrate is not specific to NO2; hence, chemiluminescence
method analyzers are subject to varying interferences produced by the
presence in the air sample of the NOZ species listed above
and others occurring in trace amounts in ambient air. This interference
is often termed a ``positive artifact'' in the reported NO2
concentration since the presence of NOZ results in an over-
estimate in the reported measurement of the actual ambient
NO2 concentration. This interference by NOZ
compounds has long been known and evaluated (Fehsenfeld et al., 1987;
Nunnermacker et al., 1998; Parrish and Fehsenfeld, 2000; McClenny et
al., 2002; U.S. Environmental Protection Agency, 1993, 2006a). The
sensitivity of the chemiluminescence FRM to potential interference by
individual NOZ compounds is variable and depends in part on
characteristics of individual monitors, such as the design of the
instrument inlet, the temperature and composition of the reducing
substrate, and the interactions of atmospheric species with the
reducing substrate. Furthermore, the concentrations of NOZ
compounds in ambient air are variable with time and distance from the
sources of NO and NO2, chiefly the point source and both on-
road and non-road mobile source combustion of fossil fuels. Nearer to
these sources, the potential interference is lower than it is farther
away because more of the measured nitrogen oxides are present as the
emitted NO and quickly formed NO2, rather than
NOZ. This is because oxidation to the NOZ
compounds from NO and NO2 requires time and the presence of
other atmospheric compounds like the hydroxyl radical.
Overall, as noted in the ISA, it appears that interference by
NOZ on chemiluminescence FRMs is not more than 10 percent of
the reported NO2 concentration during most or all of the day
during winter (cold temperatures), but larger interference ranging up
to 70 percent can be found during summer (warm temperatures) in the
afternoon at sites away and downwind from strong emission sources. In
general, the NOZ interference in the reported NO2
concentrations collected downwind of source areas and NO2
concentrations collected in relatively remote areas away from
concentrated point, area, or mobile sources is larger than the
NOZ interference in NO2 measurements taken in
urban cores or other areas with fresh NOX emissions.
The chemiluminescence FRM is well established, comprising a large
majority of the current operating network, and has served as the
principal monitoring method in the NO2 network for more than
thirty years. Many of the epidemiologic studies referenced in the REA
as the health basis for the proposed primary NO2 NAAQS
utilized ambient NO2 data obtained from chemiluminescence
FRMs, and subsequently, the uncertainties that may occur from the
potential positive influence of NOZ species on
NO2 values provided by the ambient FRM monitoring network
are already reflected in those studies. Therefore, for purposes of
comparing NO2 monitoring data to the NO2 NAAQS,
the EPA believes that the chemiluminescence FRMs are appropriate for
continued use under the current standard and under any of the options
being considered for a new 1-hour averaged primary NO2
NAAQS.
EPA is aware of the more recent development of an alternative
method in determining NO2 concentrations by
chemiluminescence, specifically through the use of a photolytic
converter, which uses specific wavelengths of ultraviolet light to
reduce NO2 to NO in lieu of the FRM's MoOX
substrate converter. The advantage of the photolytic-chemiluminescence
method is that the photolytic converter is more specific to
NO2, as compared to a MoOX substrate converter,
and does not reduce many NOZ species to NO (Ryerson et al.,
2000), reducing the potential influence of NOZ
concentrations on the reported NO2 concentration. The
photolytic-chemiluminescence method is currently deployed within
certain research networks, but the EPA has not approved this method as
an FRM or an FEM. If this technique is to be advanced to an FRM or FEM,
the method may require additional research and development to ensure
the stability of the photolytic converter rates in a variety of ambient
conditions and monitor set-ups that might be experienced in the field
and a consistent method of mathematically correcting for the known
converter efficiencies.
EPA also recognizes that, although not widely used by state and
local monitoring agencies, the existing FRM and FEM path-integrated
optical remote sensing techniques, also known as open-path and remote
sensing methods, which use spectrometers to detect pollutant
concentrations by light absorption over an optical path length, are
suitable for continued use in the ambient monitoring network as they
can provide NO2 measurements with reduced influences of
NOZ species on the reported NO2 concentrations,
relative to the chemiluminescence FRM. However, these methods do not
provide point specific concentrations like those provided by
chemiluminescence FRMs that are typically expected and seen in the
monitoring network, and may be one of the reasons these methods are not
more widely used.
In recognition of the existence of alternative methods that may be
useful in the measurement of NO2 for NAAQS compliance
purposes, as well as other objectives, EPA solicits comment on the
advantages and disadvantages of advancing technology, such as the
photolytic-chemiluminescence method, or the use of existing open-path
or remote sensing FRM and FEM technology, as alternative methods to
supplement the approved chemiluminescence FRMs already deployed across
the U.S. at NO2 monitoring sites.
B. Network Design
1. Background
The basic objectives of an ambient monitoring network, as noted in
40 CFR Part 58 Appendix D, include (1) providing air pollution data to
the general public in a timely manner, (2) supporting compliance with
ambient air quality standards and emissions strategy development, and
(3) providing support for air pollution research. Section II.A.1 notes
that there are currently no minimum monitoring requirements for
NO2 in 40 CFR part 58 Appendix D,
[[Page 34441]]
other than the requirement for EPA Regional Administrator approval
before removing any existing monitors, and that any ongoing
NO2 monitoring must have at least one monitor sited to
measure the maximum concentration of NO2 in that area. As
discussed in Section II.A.2, an analysis of the approximately 400 \19\
monitors comprising the current NO2 monitoring network
(Watkins and Thompson, 2008) indicates that the most frequently stated
monitor objectives for sites in the current NO2 network are
for the assessment of concentrations for general population exposure
and maximum (highest) concentrations typically at the neighborhood and
urban scales. Spatial scales are defined in 40 CFR Part 58 Appendix D,
Section 1.2, where the scales of representativeness of most interest
for the monitoring site types include:
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\19\ It should be noted that the ISA Section 2.4.1 references a
different number of active monitors in the NO2 network.
The difference stems from how `currently operating monitors' were
defined when extracting data from AQS. The ISA only references
SLAMS, NAMS, and PAMS sites with defined montoring objectives, while
the Watkins and Thompson, 2008 value represents all NO2
sites reporting data at any point during the year.
---------------------------------------------------------------------------
1. Microscale--Defines the concentration in air volumes associated
with area dimensions ranging from several meters up to about 100
meters.
2. Middle scale--Defines the concentration typical of areas up to
several city blocks in size, with dimensions ranging from about 100
meters to 0.5 kilometers.
3. Neighborhood scale--Defines concentrations within some extended
area of the city that has relatively uniform land use with dimensions
in the 0.5 to 4.0 kilometers range.
4. Urban scale--Defines concentrations within an area of city-like
dimensions, on the order of 4 to 50 kilometers. Within a city, the
geographic placement of sources may result in there being no single
site that can be said to represent air quality on an urban scale. The
neighborhood and urban scales have the potential to overlap in
applications that concern secondarily formed or homogeneously
distributed air pollutants.
5. Regional scale--Defines usually a rural area of reasonably
homogeneous geography without large sources, and extends from tens to
hundreds of kilometers.
The ISA and REA indicate that one of the largest factors affecting
ambient exposures to NO2 above health benchmark
concentrations are mobile source emissions, particularly at locations
near major roads. Information in the ISA and the REA shows that
concentrations of mobile source pollutants, including NO2,
typically display peak concentrations on or immediately adjacent to
roads, producing a gradient in pollutant concentrations where
concentrations decrease with increasing distance from roads (Section
II.A.2 above, ISA sections 2.5.4 and 4.3.6 and Table 2.2-1; REA section
7.3.2 and Figures 8-17 and 8-18). In the ambient environment,
NO2 is largely a secondary pollutant resulting from the
reaction of NO with available ozone (O3), the concentrations
of which depend on photochemical reactions of ambient hydrocarbons and
prior (pre-cursory) NOX emissions. The ISA notes that the
direct emission of NO2 from mobile sources is estimated to
be only a few percent of the total NOX emissions for light-
duty gasoline vehicles, and anywhere from less than 10 percent up to 70
percent of the total NOX emission from heavy-duty diesel
vehicles, depending on the engine, the use of emission control
technologies such as catalyzed diesel particulate filters (CDPFs), and
mode of vehicle operation.\20\ However, since the rate of conversion of
mobile source NO to NO2 as described above is a generally
rapid process, (i.e., on the order of a minute (ISA Section 2.2.2)),
NO2 behaves like a primary pollutant in the near-road
environment, exhibiting peak concentrations on or closely adjacent to
roads. However, due to the secondary formation characteristic of
NO2, its rate of decay with increasing distance from a road
can be slower than that of the other pollutants directly emitted from
mobile sources including carbon monoxide (CO), ultrafine particulates,
air toxics, and black carbon. Literature values indicate that the
distance required for NO2 concentrations to return to near
area-wide or background concentrations away from major roadways can
range up to 500 meters. The actual distance is variable, and highly
dependent on topography, roadside features, meteorology, and the
related photochemical reactivity conditions (Baldauf et al., 2008;
Beckerman et al., 2007; Clements et al., 2008; Gilbert et al. 2003;
Hagler et al., 2009; Rodes and Holland, 1980; Singer et al., 2003; Zhou
and Levy, 2007). Nonetheless, any efforts to measure peak ambient
NO2 concentrations from on-road mobile sources, or other
mobile source pollutant of interest noted above, would be best served
by monitoring as near as practicable to roadways of interest.
---------------------------------------------------------------------------
\20\ The ISA references studies of heavy-duty diesel vehicles
retrofitted with a CDPF in describing the range of NO2 to
NOX ratios from diesel vehicles. These studies are based
on vehicles equipped with CDPFs prior to 2009. However, as of
January 1, 2009, EPA's National Clean Diesel Campaign requires that
emission control devices included on its Verified Technologies List
raise the fraction of NO2 in exhaust NOX from
an engine no more than 20% above the baseline engine NO2
to NOX ratio. Retrofit technologies sold after January 1,
2009 that do not meet the NO2 emission limit may not be
installed or sold as EPA verified technologies.
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2. Proposed Changes
In conjunction with the proposed 1-hour NAAQS and the proposed
retention of the current annual NAAQS, we propose a number of changes
to the NO2 monitoring network. As described above in Section
II.F.4, we are proposing a 1-hour NO2 NAAQS that reflects
the maximum allowable NO2 concentration in an area. However,
the current network is not oriented to address peak concentrations,
such as the on-road and near-road environment, but many sites may be
situated to assess high concentrations at the neighborhood or larger
spatial scales. The EPA is proposing a two-tier network design to
monitor ambient concentrations of NO2 and assess compliance
with the NO2 NAAQS. The two tiers would provide data for
comparison with both the 1-hour and annual standards, and would be
comprised of (1) monitoring in areas of expected maximum 1-hour
concentrations and (2) monitoring to characterize areas with the
highest expected NO2 concentrations at the neighborhood and
larger spatial scales, or ``area-wide'' scales. Because the maximum
hourly NO2 concentrations in many areas are expected to be
due to on-road mobile emissions, the EPA believes that the first tier
of the monitoring network should include a component requiring
monitoring near major roads, where higher NO2 concentrations
have been identified and there are no significant monitoring efforts to
address roadway exposures. The EPA recognizes that requiring a
component of the ambient NO2 monitoring network to
characterize the peak NO2 concentrations derived from on-
road mobile sources, using monitors placed near major roadways (``near-
road monitors''), will introduce new requirements for monitoring sites
that, for a majority of the state and local monitoring networks,
currently do not exist.\21\ However, the monitoring of maximum hourly
concentrations of NO2, particularly in the near-road
environment, is an essential component
[[Page 34442]]
of an ambient monitoring network designed to determine compliance with
the proposed 1-hour NAAQS. In addition, the EPA recognizes that the
establishment of near-road monitoring sites will produce certain other
advantages, by providing a new data source for public health studies
that will support future NAAQS reviews, allowing for the tracking of
mobile source emission reductions progress, providing monitoring
infrastructure that may be of use for mixtures of pollutants in a
multi-pollutant paradigm, and supporting scientific studies of other
mobile source pollutants like CO, ultrafine particulate matter, black
carbon, and air toxics.
---------------------------------------------------------------------------
\21\For purposes of the discussion, near-road NO2
monitors are defined to be no greater than 50 meters from the
nearest traffic lane of target road segments. The details of
appropriately placing NO2 monitors near roads are
explained in Section III.2.a of this document.
---------------------------------------------------------------------------
The second tier of the proposed network design, the area-wide
monitoring component, is intended to characterize the highest
concentrations of NO2 typical or representative of
neighborhood and larger spatial scales, to address the wider area
impact of NO2 sources on urban populations. Further, a
requirement for the continuation of area-wide monitoring of
NO2 serves to maintain continuity in collecting area-wide
data that have served to inform long-term pollutant concentration
trends analysis and health and scientific research for more than thirty
years.
We propose that state and, when appropriate, local air monitoring
agencies provide a plan for deploying monitors in accordance with the
following proposed network design by July 1, 2011. We also propose that
the NO2 network being proposed be physically established no
later than January 1, 2013. Considering the proposed timeline and
criteria presented in the network design, we solicit comment on whether
state and local monitoring agencies should be required to deploy
monitors sooner than January 1, 2013.
a. Monitoring in Areas of Expected Maximum Concentrations Near Major
Roads
We are proposing to require monitoring in locations of expected
maximum concentrations near major roads in larger urban areas, with
minimum monitoring requirements triggered for metropolitan areas based
on Core Based Statistical Area (CBSA) population thresholds and the
traffic related metric annual average daily traffic (AADT). The U.S.
Department of Transportation (U.S. DOT) Federal Highway
Administration's Status of the Nation's Highways, Bridges, and Transit:
2006 Conditions and Performance document (http://www.fhwa.dot.gov/policy/2006cpr/es02h.htm) states that ``while urban mileage constitutes
only 24.9 percent of total (US) mileage, these roads carried 64.1
percent of the 3 trillion vehicles miles (VMT) travelled in the United
States in 2004.'' The document also states that ``urban interstate
highways made up only 0.4 percent of total (US) mileage but carried
15.5 percent of total VMT.'' These statements indicate how much more
traffic volume exists on roads in urban areas versus the more rural
areas that have significant amounts mileage of the total public road
inventory. Because the combination of increased mobile source emissions
and increased urban population densities can lead to increased
exposures and associated risks, urban areas are the appropriate areas
to concentrate required near-road monitoring efforts. Therefore, we
propose that one near-road NO2 monitor be required in CBSAs
with a population greater than or equal to 350,000 persons. This
population threshold is proposed to provide the near-road monitoring
component of the network an appropriate spatial extent across the
country, given the limited availability of routine measurements in
these environments. Based on 2007 Census Bureau statistics, this will
result in approximately 142 sites in as many CBSAs.\22\
---------------------------------------------------------------------------
\22\ We also note that this population threshold corresponds to
the minimum population level in which Air Quality Index (AQI) levels
are required to be reported, as noted in 40 CFR Part 58 Subpart F.
---------------------------------------------------------------------------
We also propose that a second near-road monitor be required in
CBSAs with a population greater than or equal to 2,500,000 persons, or
in any CBSAs with one or more road segments with an AADT count greater
than or equal to 250,000. Based on 2007 Census Bureau statistics and
data from the 2007 Highway Performance Monitoring System (HPMS)
maintained by the U.S. DOT Federal Highway Administration (FHWA), this
particular element of the minimum monitoring requirements will add
approximately 23 sites to the approximate 142 near-road sites in CBSAs
that already will have one near-road monitor required due to the
350,000 population threshold. Of the 23 additional sites, two sites are
due to the 250,000 AADT threshold and are attributed to the Las Vegas,
Nevada and Sacramento, California CBSAs. The 2,500,000 population
threshold is proposed as a second threshold to allow for further
characterization of larger urban areas that are more likely to have a
greater number of major roads across a potentially larger geographic
area, and a corresponding increase in potential for exposure. Of the
approximate 1.66 million public road segments tracked in the HPMS, road
segments of 250,000 AADT or greater make up the top 0.03 percent of the
most traveled public road segments. The FHWA has also used this
threshold on its Web site to give an indication of the most travelled
urban highways in the country (http://www.fhwa.dot.gov/policyinformation/tables/02.cfm). We proposed to use HPMS-reported AADT
as the traffic volume metric because AADT appears to be the most widely
used traffic volume metric in the scientific literature, is widely
available, and offers the most objective and consistent metric
available to indicate traffic volumes across the country. These AADT
data are typically available from local Metropolitan Planning
Organizations (MPOs), state departments of transportation, and from the
FHWA's HPMS. The FHWA also provides national guidance on the
appropriate measurement and estimation of AADT for different road types
in their HPMS Field Manual (http://www.fhwa.dot.gov/ohim/hpmsmanl/hpms.cfm). We are therefore proposing the 250,000 AADT threshold for
requiring a near-road monitor because that threshold represents the
highest traffic volume road segments in the country, which may
correspond to the greatest potential for high exposures directly
connected to motor vehicle emissions.
In summary, the combination of the above proposed minimum
monitoring requirement thresholds for the near-road monitors as part of
the ambient NO2 monitoring network are anticipated to
require approximately 165 near-road sites in 142 CBSAs. We solicit
comment on the proposed CBSA population threshold values (i.e., 350,000
and 2,500,000) and on the use of population thresholds both lower and
higher than those proposed, the use of the traffic volume metric AADT,
and the 250,000 AADT threshold in establishing the minimum number of
required near-road sites for urban areas.
In choosing these population and traffic related thresholds for the
minimum monitoring requirements, it should be noted that, based on 2007
Census Bureau statistics, the U.S. Virgin Islands and seven states
(Delaware, Montana, North Dakota, South Dakota, Vermont, West Virginia,
and Wyoming) currently would not have required near-road monitoring
sites under this current proposal. Considering the relative lack of
near-road monitoring data nationwide, the new level and averaging time
of the NAAQS being proposed, and the desire to establish a spatially
representative and protective network, we solicit comment on the
inclusion or
[[Page 34443]]
exclusion of an additional or alternative monitoring requirement such
that each state and territory would have at least one near-road
monitoring site.
The EPA recognizes that in certain cases, there can be an area or
areas of expected maximum hourly concentration in a CBSA due to a major
stationary source or to the combination of multiple sources that could
include point, area, and non-road source emissions in addition to on-
road mobile source emissions. Such locations might be identified
through data analysis, such as the evaluation of existing ambient data
and/or emissions data, or through air quality modeling. An example of
such a location might be away from roads and downwind of a stationary
source or sources in situations where the required near-road monitors
do not represent a location or locations of expected maximum hourly
NO2 concentrations in a CBSA. In these situations, where
such locations are known, we propose that the Regional Administrator
will have discretion to require monitoring above the minimum
requirements as necessary to address situations where the required
near-road monitors do not represent a location or locations where the
expected maximum hourly NO2 concentrations exist in a CBSA.
The EPA also proposes to allow Regional Administrators the ability to
require additional near-road monitoring sites to address situations
where minimum monitoring requirements are not sufficient to meet
monitoring objectives, such as a situation where there is a variety of
exposure potential in an area due to variety in the amount or types of
fleet mix, congestion patterns, terrain, or geographic areas within a
CBSA. An example of requiring an additional near-road monitor might be
a case where a particular community or neighborhood is significantly or
uniquely affected by road emissions, but the site or area is not
monitored even though the responsible State or local monitoring agency
is fulfilling the minimum monitoring requirements.
In all cases, the Regional Administrator and the responsible State
or local air monitoring agency should work together to design and/or
maintain the most appropriate NO2 network to service the
variety of data needs for an area. We solicit comment on the proposal
to allow Regional Administrators the discretion to require monitoring
above the minimum requirements for any CBSA where required near-road
monitors do not represent a location or locations where the expected
maximum hourly NO2 concentrations exist in a CBSA. We also
solicit comment on the proposal to allow Regional Administrators to
require additional near-road NO2 monitoring stations above
the minimum required in situations where the minimum monitoring
requirements are not sufficient to meet monitoring objectives as noted
above.
The new near-road monitoring sites that are to be part of the
NO2 ambient monitoring network will require specific site
selection criteria to focus monitoring efforts on one or a few major
roads in a given CBSA. The EPA anticipates that these near-road
monitoring sites will likely be best characterized as microscale,
mobile source oriented sites. We propose that monitoring agencies be
required to select their near-road monitoring site location(s) to
characterize the largest traffic volume segment(s) in the CBSA,
determined by ranking all road segments by AADT, and identifying a
location or locations adjacent to those top ranked AADT segments where
motor vehicle emission-derived NO2 concentrations are
expected to be at a maximum. Where a state or local air monitoring
agency identifies multiple acceptable candidate sites where maximum
hourly NO2 concentrations are expected to occur, the
monitoring agency should consider taking into account the potential for
population exposure in the criteria utilized to select the final site
location.
We propose that near-road NO2 monitoring stations must
be sited so that the NO2 monitor probe is no greater than 50
meters away, horizontally, from the outside nearest edge of the traffic
lanes of the target road segment, and shall have no obstructions in the
fetch between the monitor probe and roadway traffic such as noise
barriers or vegetation higher than the monitor probe height. Baldauf et
al. (2009) indicate that the NO2 probe would ideally be
situated between 10 and 20 meters from the nearest traffic lane. We are
not proposing that the near-road NO2 monitor be on the
predominantly downwind side of the target roadway, however, we solicit
comment on whether this requirement is necessary to ensure near-road
NO2 sites capture maximum expected hourly concentrations.
We propose that the monitor probe be located within 2 to 7 meters
above the ground, as is required for microscale PM2.5 sites.
EPA recognizes that these near-road monitoring sites will be adjacent
to a variety of road types, where some target roads will be on an even
plane with the monitoring station, while others may be cut roads,
(i.e., below the plane of the monitoring station), or fill and open
elevated roads, (i.e., where the road plane is above the monitoring
station). In any given case, it is most appropriate to place the
NO2 monitor probe as close to the plane of the target road
segment as possible, while staying between 2 to 7 meters above the
ground. In addition, we propose that monitor probe placement on noise
barriers or buildings, where the inlet probe height is no less than 2
meters and no more than 7 meters above the target road, will be
acceptable, so long as the inlet probe is at least 1 meter vertically
or horizontally away (in the direction of the target road) from any
supporting wall or structure, and the subsequent residence time of the
pollutant in the sample line between the inlet probe and the analyzer
does not exceed 20 seconds. Although a wall-mounted or noise barrier-
mounted near-road monitor set-up is not ideal, it may allow for
existing sites to be utilized as near-road monitoring stations if they
also meet the site selection criterion described below.
As noted above, we are proposing a siting criterion for
NO2 monitor probe placement to be no greater than 50 meters
away from the outside nearest edge of the traffic lanes of the target
road segment. Based on a review of the scientific literature, as
discussed in Section II.A and the background portion of this section,
locations on or immediately adjacent to roads typically exhibit the
peak concentrations for mobile source pollutants, therefore monitor
probe placement at increasing distances from a road will
correspondingly decrease the potential for sampling maximum
concentrations of NO2. In addition, monitor probe placement
within 50 meters of a target road allows for increased probability of
reading elevated concentrations from the mobile source emissions even
when wind conditions cause the near-road monitoring site to be upwind
of the target road. Research literature indicates that in certain
cases, mobile source derived pollutant concentrations, including
NO2, can be detected upwind of roads, above background
levels, due to a phenomenon called upwind meandering. Kalthoff et al.
(2007) indicates that mobile source derived pollutants can meander
upwind on the order of tens of meters, mainly due to vehicle induced
turbulence, while Beckerman et al. (2008) note that near-road pollutant
concentrations on the predominantly upwind side of their study sites
dropped off to near background levels within the first 50 meters, but
were above background in this short and variable upwind range, which
could be due to, at least in part,
[[Page 34444]]
vehicle induced turbulence. This upwind meandering characteristic of
pollutants in the near-road environment provides an additional basis
for locating near-road sites within 50 meters of target road segments
because of the increased opportunity to monitor mobile source derived
NO2 concentrations that, although not peak concentrations,
are still elevated above background levels, in meteorological
conditions where the site is upwind of the target road.
We solicit comment on the proposed near-road NO2 monitor
siting criteria presented here, particularly: (1) The requirement for
monitoring agencies to select near-road NO2 monitor sites by
ranking all road segments in a given CBSA by AADT, (2) selecting a site
adjacent to a top ranked AADT road segment where motor vehicle
emission-derived NO2 concentrations are expected to be at a
maximum, (3) the consideration of population exposure as a selection
criterion in situations where a state or local air monitoring agency
identifies multiple acceptable candidate sites where maximum hourly
NO2 concentrations are expected to occur, (4) the
requirement for near-road NO2 monitor probes to be no
greater than 50 meters in the horizontal from the outside nearest edge
of the traffic lanes of the target road segment, and (5) the
requirement for monitor probes to be between 2 to 7 meters above the
ground, and when located on a wall or supporting structure, that the
inlet probe be at least 1 meter vertically or horizontally away from
any supporting wall or structure.
We also solicit comment on an alternative approach that would allow
state and local agencies greater discretion in selecting monitoring
locations to fulfill minimum monitoring requirements for measurements
of expected maximum NO2 concentrations in each CBSA. In this
alternative approach, an NO2 monitor would still be required
in locations of expected maximum NO2 concentrations in CBSAs
with a population greater than or equal to 350,000 persons. An
additional monitor would be required in CBSAs with a population greater
than or equal to 2,500,000, or in any CBSAs with one or more road
segments with an AADT count greater than or equal to 250,000. Under
this approach, states would not be specifically required to place
monitors near roads, but would have flexibility to place monitors at
locations of expected maximum concentrations. However, if a location or
locations of expected maximum concentration were near roads in a CBSA,
we would expect the NO2 monitor to be placed near those
roads. Further, we solicit comment on alternative ways of considering
population exposure, in concert with the identification of locations of
maximum expected NO2 concentrations, in determining where to
place near-road NO2 monitors. In suggesting an appropriate
role for population exposure, we invite comment on how the suggested
role would take into account the fact that NAAQS are designed to
protect all of the public, including at-risk or sensitive sub-
populations, which can include smaller sub-populations that may be
exposed to higher concentrations. We also invite comment on how any
suggested role would compare with EPA's historic practice of placing
monitors at locations of maximum concentration at the appropriate
spatial scale, reflecting consideration of the averaging time of the
NAAQS.
In situations where open-path monitors are used at near-road
NO2 sites, we have not identified an appropriate path length
for this microscale monitoring site. For the purpose of this proposal,
we propose a path length range of 50 to 300 meters as an appropriate
path length range for open-path near-road NO2 monitors. The
high end of this proposed range coincides with path lengths identified
for other pollutants at the micro and middle-scales. We solicit comment
on the appropriate path length for a near-road NO2 open-path
monitor.
During the near-road monitor site selection process, monitoring
agencies may utilize forms of quantitative analysis, such as emissions
and/or air quality modeling, data analysis, or saturation studies, to
better evaluate which of their top ranked AADT road segments may
exhibit the potential for creating the highest NO2
concentrations that might be monitored in the CBSA. As an example, such
an analysis might indicate that of the top ranked AADT road segments in
a given area, those segments that are part of or adjacent to
interchanges and toll plazas, that have higher ratios of heavy duty
diesel traffic to light duty traffic, have a high fraction of rapidly
accelerating or grade-climbing vehicles, or that are located in or near
particular terrain or land features, may exhibit higher potential
maximum NO2 concentrations. In addition, top ranked AADT
road segment analysis may allow the monitoring agencies to select a
near-road monitoring site located in a more densely populated area or a
location representing more vulnerable populations from a pool of
otherwise similarly categorized site candidates. In CBSAs required to
have two near-road monitoring sites, we propose that the second site be
selected based on AADT ranking and expected maximum concentration, but
differentiated from the first site by factors such as: Fleet mix,
congestion patterns, terrain, or geographic area within the CBSA, or at
minimum, selecting a site along a different road with a different
route, interstate, or freeway designation. This differentiation is to
avoid having the two sites characterize the same traffic when there are
potentially other road segments with different traffic characteristics
available that meet siting criteria for the second near-road monitor.
We solicit comment on the factors and methods to be used to
differentiate a second required near-road NO2 monitoring
site from the first such site in a given CBSA.
In further support of characterizing the peak NO2
concentrations occurring in the near-road environment, the EPA proposes
to require three-dimensional anemometry, providing wind vector data in
the horizontal and vertical planes, along with temperature and relative
humidity measurements, at all required near-road monitoring sites. Due
to the near-road NO2 site being a somewhat specialized
microscale site, we propose that the meteorological measurement
hardware would be required to be situated at the same height as the
NO2 monitor probe, as opposed to a standardized height, to
aid in characterizing what NO2 analyzers are measuring from
the target road segments. The requirement of three-dimensional
anemometry is to allow for the determination of the standard deviation
of vertical wind velocities ([sigma]w). Venkatram et al.
(2007) notes that [sigma]w is a key meteorological factor in
governing the dispersion of on road pollutant emissions. Therefore, the
measurement of three dimensional wind would serve to inform when the
near-road site is relatively upwind or downwind of the target road,
provide a method to potentially identify the magnitude of vehicle
induced turbulence, permit calculation of [sigma]w in the
near-road environment to provide a better understanding of the mixing
of mobile source pollutants at the monitoring site and how site
characteristics influence mixing, and, with the inclusion of
temperature and relative humidity, provide basic meteorological data.
We solicit comment on the proposed requirement for three-dimensional
anemometry, the placement of the meteorological equipment at the same
height of the NO2 monitor probe height, and the requirement
for meteorological
[[Page 34445]]
measurements in general at all required near-road monitoring sites.
b. Area-Wide Monitoring at Neighborhood and Larger Spatial Scales
As the second tier of the NO2 ambient monitoring
network, we are proposing a minimum number of monitors to characterize
that area with highest expected NO2 concentrations at the
neighborhood and larger (area-wide) spatial scales. We are proposing to
require one area-wide monitoring site in each CBSA with a population
greater than or equal to 1,000,000, to be sited to represent an area of
maximum concentration at the neighborhood or larger spatial scales.
This minimum monitoring requirement is expected to trigger 52
monitoring sites in as many CBSAs. Many of these monitors are likely
already in place as part of the approximately 400 NO2
monitoring sites that are currently operating across the country.
Further, the EPA proposes to allow any current photochemical assessment
monitoring station (PAMS) sites that are situated to address the
highest NO2 concentrations in an urban area and sited at
neighborhood or urban scales to satisfy this proposed area-wide
monitoring requirement. While in many cases it may be found that these
area-wide monitors may show lower concentrations than the maximum
concentration near-road NO2 monitors, data from these larger
spatially representative sites would provide information on area-wide
exposures from an individual or a group of point, area, on-road and/or
non-road mobile sources. These area-wide monitoring data may also, when
coupled with the near-road monitoring data, assist in the determination
of spatial variation of NO2 concentrations across a given
area, and assist in providing insight to the gradients that exist
between local near-road or stationary source derived concentration
maxima and the area-wide concentration levels.
The EPA recognizes that the minimum number of area-wide monitors
required in this proposal may be less than the total number of
NO2 monitoring sites needed to satisfy the multiple
monitoring objectives that neighborhood and larger scale sites can
serve. These additional monitoring objectives include ambient
photochemical pollutant assessment, aiding in ozone forecasting, aiding
in PM precursor analysis and PM forecasting, and characterization of
point and area sources that may be impacting certain communities. We
propose that EPA Regional Administrators have the discretion to require
additional area-wide NO2 monitoring sites above the minimum
monitoring requirements where the minimum monitoring requirements for
area-wide monitors are not sufficient to meet monitoring objectives.
For example, the Regional Administrator may require additional
NO2 monitors in certain communities, both inside and outside
of CBSAs, which are affected by an individual or group of sources but
are not required to have an NO2 monitor as part of the
minimum monitoring requirements. The Regional Administrator and the
responsible State or local air monitoring agency should work together
to design and/or maintain the most appropriate NO2 network
to service the variety of data needs for an area.
We solicit comment on the proposed minimum monitoring requirement
of approximately 52 monitors to characterize areas with highest
expected NO2 concentrations at the area-wide (neighborhood
and larger) spatial scales in CBSAs with populations of 1,000,000 or
more persons. We also solicit comment on the proposal that the Regional
Administrator can require additional monitoring sites on a case-by-case
basis, to address situations where the minimum monitoring requirements
for area-wide monitoring sites are not sufficient for an area.
3. Solicitation for Comment on an Alternative Network Design
In conjunction with the solicitation of comment on an alternative
NAAQS that is discussed in Section II.F.4, the complementary network
design would not reflect peak NO2 concentrations anywhere in
an area. Instead, the alternative network design would rely on monitors
sited at the neighborhood and larger spatially representative scales,
which is identical to the second component of the two-tiered network
design being proposed except for having different population thresholds
for minimum required monitoring. The currently operating NO2
network would likely satisfy a portion of this alternative network
design, however the entire network would need to be assessed before
state or local agencies could make such determinations. State and local
agencies would have to determine what each currently operating site is
actually assessing to identify if any given site represents the highest
concentrations for a given CBSA at the neighborhood and larger spatial
scales. We solicit comment on an alternative network design where near-
road monitors are not specifically included in the minimum monitoring
requirements, and only monitors sited at the neighborhood and larger
spatial scales are required. In this alternative network design,
minimum monitoring requirements would apply to CBSAs based on
population thresholds, where one monitor would be required in CBSAs
with populations of 350,000 or more persons and a second monitor would
be required for CBSAs with populations of 1,000,000 or more persons.
Based on 2007 U.S. Census Bureau statistics, we estimate that these
population thresholds would require approximately 194 monitoring sites
in 142 CBSAs. The first monitor required in any CBSA would be expected
to be sited at the neighborhood or larger scale to characterize that
area with highest expected NO2 concentrations. Any second
monitor required in a CBSA would be expected to characterize a separate
area within the same CBSA, also with expected high NO2
concentrations. All such monitor site locations are anticipated to be
in areas of higher population densities of CBSAs and in, or adjacent
to, urban cores. The alternative network design would allow the
Regional Administrators to use their discretion to require monitoring
above the minimum requirements to address community impacts from the
variety of NO2 emission sources. EPA expects that this
network design will result in little or no progress being made in the
development of long-term near-road monitoring capabilities due to the
lack of specific network design requirements. EPA seeks comment on this
alternative network design.
In addition to soliciting comment generally on this alternative
area-wide monitoring approach, the Administrator specifically requests
comment on the appropriate definition of area-wide NO2
concentrations and how best to use data representing these
concentrations to determine compliance with a 1-hour standard
reflecting the alternative approach of selecting a level for maximum
area-wide concentrations on which EPA is soliciting comment. Comparing
NO2 concentrations measured near major roadways to a level
meant to reflect the maximum allowable NO2 concentrations at
neighborhood and larger spatially representative scales would have the
effect of increasing the stringency of the standard beyond that
intended. With regard to this specific request for comment, the
Administrator notes that the definition of area-wide concentrations
could include a provision requiring that they be monitored at a
distance greater than or equal to some prescribed distance from the
nearest roadway. The Administrator notes that, while it is clear that
peak
[[Page 34446]]
roadway-associated NO2 concentrations occur on or very near
major roads, the point at which these concentrations return to area-
wide concentrations comparable to the area-wide standard is less
certain and may vary considerable by location. As discussed above
(section II.A.2), the scientific literature suggests that
concentrations can return to typical urban background concentrations
within distances of up to 500 meters from roads, though the actual
distance will vary with topography, roadside features, meteorology, and
photochemical reactivity conditions. The REA notes that studies suggest
the return to background concentrations can occur from within distances
of up to 200 to 500 m from the roads. Therefore, the Administrator
requests comment on the degree to which these distances (up to 200 m,
and up to 500m) serve to further define the distance from major roads
that would represent concentrations comparable to the alternative
standard. Further, since roadways of various sizes and traffic volumes
can affect nearby NO2 concentrations and roadways are
ubiquitous in urban areas, the Administrator notes that defining
representative area-wide concentrations could require more than a
uniform assumption of a single specific distance from a class of
roadway. The Administrator notes that the approach to defining
representative area-wide distances could include consideration of
location-specific roadway traffic volume and location-specific roadway
characteristics such as topography, presence of sound walls, vehicle
mix, and traffic patterns, to adequately address the variability. Given
these considerations, the Administrator solicits comment on how to
define the minimum distance to the nearest major roadway such that
measured concentrations at this distance (or farther) would represent
area-wide NO2 concentrations for comparison to the
alternative standard.
C. Data Reporting
NO2 chemiluminescence FRMs are continuous gas analyzers,
producing updated data values on the order of every 20 seconds. Data
values are typically aggregated into minute averages and then compiled
into hourly averages for reporting purposes. State and local monitoring
agencies are required to report hourly NO, NO2, and
NOX data to AQS within 90 days of the end of each calendar
quarter. Some agencies also voluntarily report their pre-validated data
on an hourly basis to EPA's real time AIRNow data system, where the
data may be used by air quality forecasters to assist in ozone
forecasting. The EPA believes these data reporting procedures are
appropriate to support the current NO2 NAAQS and any options
being considered for a revised primary NO2 NAAQS.
As a part of the larger data quality performance requirements of
the ambient monitoring program, we are proposing to develop data
quality objectives (DQOs) for the proposed NO2 network. The
DQOs are meant to identify measurement uncertainty for a given
pollutant method. We propose a goal for acceptable measurement
uncertainty for NO2 methods to be defined for precision as
an upper 90 percent confidence limit for the coefficient of variation
(CV) of 15 percent and for bias as an upper 95 percent confidence limit
for the absolute bias of 15 percent. We solicit comment on the proposed
goals for acceptable measurement uncertainty.
IV. Proposed Appendix S--Interpretation of the Primary NAAQS for Oxides
of Nitrogen and Proposed Revisions to the Exceptional Events Rule
The EPA is proposing to add Appendix S, Interpretation of the
Primary National Ambient Air Quality Standards for Oxides of Nitrogen,
to 40 CFR part 50 in order to provide data handling procedures for the
proposed NO2 1-hour primary standard and for the existing
NO2 annual primary standard. The proposed Appendix S would
detail the computations necessary for determining when the proposed 1-
hour and existing annual primary NO2 NAAQS are met. The
proposed Appendix S also would address data reporting, data
completeness considerations, and rounding conventions.
Two versions of the proposed Appendix S are printed at the end of
this notice. The first applies to an annual primary standard and a 1-
hour primary standard based on the annual 4th high value form, while
the second applies to an annual primary standard and a 1-hour primary
standard based on the 99th percentile daily value form. The discussion
here addresses the first of these versions, followed by a brief
description of the differences found in the second version.
Both versions of the proposed Appendix S are based on a near-
roadway approach to the setting the level of the 1-hour standard and to
siting monitors. As such, these versions place no geographical
restrictions on which monitoring sites' concentration data can and will
be compared to the standard when making nonattainment determinations
and other findings related to attainment or violation of the standard.
If the final rule adopts the area-wide approach on which section
II.F.4.e of this notice invites comment, provisions would be added to
section 2 of Appendix S to specify geographical criteria for
determining which monitoring sites' data can and will be compared to
the standard consistent with the area-wide approach as described in
that section.
The EPA is proposing to amend and move the provisions of 40 CFR
50.11 related to data completeness for the existing annual primary
standard to the new Appendix S, and to add provisions for the proposed
1-hour primary standard. Substantively, the proposed data handling
procedures for the annual primary standard in Appendix S are the same
as the existing provisions in 40 CFR 50.11 for that standard, except
for a proposed addition of a cross-reference to the Exceptional Events
Rule, a proposed addition of Administrator discretion to consider
otherwise incomplete data complete, and a proposed provision addressing
the possibility of there being multiple NO2 monitors at one
site. The proposed procedures for the 1-hour primary standard are
entirely new.
The EPA is also proposing NO2-specific changes to the
deadlines, in 40 CFR 50.14, by which States must flag ambient air data
that they believe have 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 NO2 given the anticipated
schedule for the designations of areas under the proposed
NO2 NAAQS.
A. Background
The purpose of a data interpretation appendix in general is to
provide the practical details on how to make a comparison between
multi-day and possibly multi-monitor ambient air concentration data and
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.
The regulatory language for the current NO2 NAAQS,
originally adopted in 1977, contains data interpretation instructions
only for the issue of data completeness. This situation contrasts
[[Page 34447]]
with the situations for ozone, PM2.5, PM10, and
most recently Pb for which there are detailed data interpretation
appendices in 40 CFR part 50 addressing more issues that can arise in
comparing monitoring data to the NAAQS. EPA has used its experience
drafting and applying these other data interpretation appendices to
develop the proposed text for Appendix S.
An exceptional event is defined in 40 CFR 50.1 as 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 that is determined to have been affected by an
exceptional event under the procedural steps and substantive criteria
specified in section 50.14 may be excluded from consideration when EPA
makes a determination that an area is meeting or violating the
associated NAAQS. The key procedural deadlines in section 50.14 are
that a State must notify EPA that data have been affected by an event,
i.e., ``flag'' the data in the Air Quality Systems (AQS) database, 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 generic schedule
presents problems when a NAAQS has been recently revised, as discussed
below.
The REA did not address data interpretation details. However, the
approach to data interpretation used in the REA, for example to report
the number of cities which would violate possible 1-hour primary NAAQS,
was generally consistent with the proposed data interpretation
procedures.
B. Interpretation of the Primary NAAQS for Oxides of Nitrogen
The purpose of a data interpretation rule for the NO2
NAAQS is to give effect to the form, level, averaging time, and
indicator specified in the proposed regulatory text at 40 CFR 50.11,
anticipating and resolving in advance various future situations that
could occur. The proposed Appendix S provides common definitions and
requirements that apply to both the annual and the 1-hour primary
standards for NO2. The common requirements concern how
ambient data are to be reported, what ambient data are to be considered
(including the issue of which of multiple monitors' data sets will be
used when more than one monitor has operated at a site), and the
applicability of the Exceptional Events Rule to the primary
NO2 NAAQS.
The proposed Appendix S also addresses several issues in ways which
are specific to the individual primary NO2 standards, as
described below.
1. Annual Primary Standard
The proposed data interpretation provisions for the annual standard
are consistent with the current instructions included along with the
statement of the level and form of the standard in 40 CFR 53.11. These
are the following: (1) At least 75% of the hours in the year must have
reported concentration data. (2) The available hourly data are
arithmetically averaged, and then rounded (not truncated) to whole
parts per billion. (3) The design value is this rounded annual average
concentration. (4) The design value is compared with the level of the
annual primary standard (expressed in parts per billion).
It would be possible to introduce additional steps for the annual
primary standard which in principle could make the design value a more
reliable indicator of actual annual average concentration in cases
where some monitoring data have been lost. For example, averaging
within a calendar quarter first and then averaging across quarters
could help compensate for uneven data capture across the year. For some
aspects of the data interpretation procedures for some other
pollutants, the current data interpretation appendices do contain such
additional steps. The proposed provisions for the proposed 1-hour
NO2 standard (described immediately below) also incorporate
some such features. However, we believe that such complexity is not
needed to appropriately implement the annual primary standard,
especially since no area presently comes close to violating the
standard. EPA invites comment on whether the annual primary standard
design value should be a weighted annual mean (e.g. averaging within
calendar quarters before averaging across quarters), rather than the
mean of all available hourly values.
2. 1-Hour Primary Standard Based on the Annual 4th High Value Form
With regard to data completeness for the proposed 1-hour primary
standard, 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
proposed 1-hour primary NO2 NAAQS, the key air quality
statistics are the daily maximum 1-hour concentrations in three
successive years. It is important that sampling within a day encompass
the period when concentrations are likely to be highest and that all
seasons of the year are well represented. Hence, the 75% requirement is
proposed to be applied at the daily and quarterly levels. EPA invites
comment on the proposed completeness requirements.
Recognizing that there may be years with incomplete data, the
proposed text provides that a design value derived from incomplete data
will nevertheless be considered valid in either of two situations.
First, if the design value calculated from at least four days of
monitoring observations in each of these years exceeds the level of the
1-hour primary standard, it would be valid. This situation could arise
if monitoring was intermittent but high NO2 levels were
measured on enough hours and days for the mean of the three annual 4th
values to exceed the standard. In this situation, more complete
monitoring could not possibly have indicated that the standard was
actually met.
Second, we are proposing a diagnostic data substitution test which
is intended to identify those cases with incomplete data in which it
nevertheless is very likely, if not virtually certain, that the daily
1-hour design value would have been observed to be below the level of
the NAAQS if monitoring data had been minimally complete.
The diagnostic test would be applied only if there is at least 50%
data capture in each quarter of each year and if the 3-year mean of the
observed annual 4th highest maximum hourly values in the incomplete
data is below the NAAQS level. The test would substitute a high
hypothetical concentration for as much of the missing data as needed to
meet the 100% requirement in each quarter. The value that is
substituted for the missing values is the highest daily maximum 1-hour
observed in the same quarter, looking across all three years under
evaluation. If the resulting 3-year design value is below the NAAQS, it
is highly likely that the design value calculated from complete data
would also have been below the NAAQS, so the original design value
indicating compliance would be considered valid.
[[Page 34448]]
It should be noted that one outcome of applying the proposed
substitution test is that a year with incomplete data may nevertheless
be determined to not have a valid design value and thus to be unusable
in making 1-hour primary NAAQS compliance determinations for that 3-
year period. EPA invites comment on incorporating into the final rule
the proposed substitution test.
Also, we are proposing that the Administrator have general
discretion to use incomplete data based on case-specific factors,
either at the request of a state or at her own initiative. Similar
provisions exist already for some other NAAQS.
3. 1-Hour Primary Standard Based on the Annual 99th Percentile Daily
Value Form
The second version of the proposed Appendix S appearing at the end
of this notice contains proposed interpretation procedures for a 1-hour
primary standard based on the 99th percentile daily value form. The 4th
high daily value form and the 99th percentile daily value form would
yield the same design value in a situation in which every hour and day
of the year has reported monitoring data, since the 99th percentile of
365 daily values is the 4th highest value. However, the two forms
diverge if data completeness is 82% or less, because in that case the
99th percentile value is the 3rd highest (or higher) value, to
compensate for the lack of monitoring data on days when concentrations
could also have been high.
Logically, provisions to address possible data incompleteness under
the 99th percentile daily value form should be somewhat different from
those for the 4th highest form. With a 4th highest form, incompleteness
should not invalidate a design value that exceeds the standard, for
reasons explained above. With the 99th percentile form, however, a
design value exceeding the standard stemming from incomplete data
should not automatically be considered valid, because concentrations on
the unmonitored days could have been relatively low, such that the
actual 99th percentile value for the year could have been lower, and
the design value could have been below the standard. The second
proposed version of Appendix S accordingly has somewhat different
provisions for dealing with data incompleteness. One difference is the
addition of another diagnostic test based on data substitution, which
in some cases can validate a design value based on incomplete data that
exceeds the standard.
The second version of the proposed Appendix S provides a table for
determining which day's maximum 1-hour concentration will be used as
the 99th percentile concentration for the year. The proposed table is
similar to one used now for the 24-hour PM2.5 NAAQS, which is based on
a 98th percentile form, but adjusted to reflect a 99th percentile form
for the 1-hour primary NO2 standard. The proposed Appendix S
also provides instructions for rounding (not truncating) the average of
three annual 99th percentile hourly concentrations before comparison to
the level of the primary NAAQS.
C. Exceptional Events Information Submission Schedule
The Exceptional Events Rule at 40 CFR 50.14 contains generic
deadlines for a state to submit to EPA specified information about
exceptional events and associated air pollutant 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 and providing an initial event description.
The state must also, after notice and opportunity for public comment,
submit a demonstration to justify any claim 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 to flag data in AQS and submit complete
documentation to EPA for review is foreshortened, and all information
must be submitted to EPA no later than one 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
newly revised NAAQS. One problem is that some of the deadlines,
especially the deadlines for flagging some relevant data, may have
already passed by the time the revised NAAQS is promulgated. 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. The only way a state
could guard against this possibility is to flag all data that could
possibly be eligible for exclusion under a future NAAQS. This could
result in flagging far more data than will eventually be eligible for
exclusion. EPA believes this is an inefficient use of state and EPA
resources, and is potentially confusing and misleading to the public
and regulated entities. Another problem is that it may not be feasible
for information on some exceptional events that may affect final
designations to be collected and submitted to EPA at least one year in
advance of the final designation decision. This could have the
unintended consequence of EPA designating an area nonattainment as a
result of uncontrollable natural or other qualified exceptional events.
When Section 50.14 was revised in March 2007, EPA was mindful that
designations were needed under the recently revised PM2.5
NAAQS, so exceptions to the generic deadline were included for
PM2.5. The EPA was also mindful that similar issues would
arise for subsequent new or revised NAAQS. The Exceptional Events Rule
at section 50.14(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 NO2, EPA anticipates that
initial designations under the revised NAAQS may be made by January 22,
2012 based on air quality data from the years 2008-2010. (See Section
VI below for more detailed discussion of the designation schedule and
what data EPA intends to use.) If final designations are made by
January 22, 2012, all events to be considered during the designations
process must be flagged and fully documented by states one year prior
to designations, by January 22, 2011. This date also coincides with the
Clean Air Act deadline for Governors to submit to EPA their
recommendations for designating all areas of their states.
EPA is proposing revisions to 40 CFR 50.14 to change submission
dates for information supporting claimed exceptional events affecting
NO2 data. The proposed rule text at the end of this notice
shows the changes that would apply if a revised NO2 NAAQS is
promulgated by January 22, 2010, and designations are made two years
after promulgation of a NO2 NAAQS revision. For air quality
data collected in 2008, we propose to extend the generic July 1, 2009
deadline for flagging data (and providing a brief initial description
of the event) to July 1, 2010. EPA believes this extension provides
adequate time for states to review the impact of exceptional events
from 2008 on the revised standard and notify EPA by flagging the
relevant data in AQS. EPA is not proposing to change the generic
deadline of January 22, 2011 for
[[Page 34449]]
submitting documentation to justify an NO2-related
exceptional event from 2008. We believe the generic deadline provides
adequate time for states to develop and submit proper documentation.
For data collected in 2009, EPA does not believe it is necessary to
change the generic deadline of July 1, 2010 for flagging data and
providing initial event descriptions. Similarly, EPA does not believe
it is necessary to change the generic deadline of January 22, 2011 for
states to submit documentation to justify an NO2-related
exceptional event from 2009.
For data collected in 2010, EPA believes the designations deadline
of January 22, 2011 for flagging data and providing initial event
descriptions does not provide states with adequate time to review and
identify potential exceptional events that occur in calendar year 2010,
especially events that might occur late in the year. Therefore, EPA is
proposing that states may flag and provide initial event descriptions
for 2010 data no later than April 1, 2011. This affords states more
than 2 additional months than would be provided under the generic
schedule to review and identify exceptional events affecting 2010
NO2 data. Similarly, EPA believes the designations schedule
that would require states to submit detailed documentation to justify
2010 events claims by January 22, 2011 is not reasonable, because it
would potentially preclude states from completing the required public
review of the documentation prior to submitting to EPA. Therefore, EPA
is proposing to extend this deadline to July 1, 2011. This would afford
states more than 5 additional months than provided by the generic
schedule to complete the required public review and submit full
supporting documentation, yet would still allow EPA adequate time to
review the documentation and develop its final plans for designations
by January 22, 2012.
Table 2 below summarizes the proposed two year designation
deadlines discussed in this section. If the promulgation date for a
revised NO2 NAAQS will occur on a different date than
January 22, 2010, EPA will revise the final NO2 exceptional
event flagging and documentation submission deadlines accordingly,
consistent with this proposal, to provide states with reasonably
adequate opportunity to review, identify, and document exceptional
events that may affect an area designation under a revised NAAQS. EPA
invites comment on these proposed changes in the exceptional event
flagging and documentation submission deadlines.
Table 2--Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
Designations Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
Air quality data
NAAQS pollutant/standard/(level)/ collected for calendar Event flagging & initial Detailed documentation
promulgation date year description deadline submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/ 2004-2006............... October 1, 2007 \a\...... April 15, 2008.\a\
m3) Promulgated October 17, 2006.
Ozone/8-Hr....................... 2005-2007............... June 18, 2009 \b\........ June 18, 2009.\b\
Standard (0.075 ppm) Promulgated 2008.................... June 18, 2009\b\......... June 18, 2009.\b\
March 12, 2008.
2009.................... 60 Days after the end of 60 Days after the end of
the calendar quarter in the calendar quarter in
which the event occurred which the event
or February 5, 2010, occurred or February 5,
whichever date occurs 2010, whichever date
first \b\. occurs first.\b\
NO2/1-Hour Standard (80-100 PPB, 2008.................... July 1, 2010 \b\......... January 22, 2011.
final level TBD).
2009.................... July 1, 2010............. January 22, 2011.
2010.................... April 1, 2011 \b\........ July 1, 2011.\b\
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, and are shown in this table for
informational purposes.
\b\ Indicates change from general schedule in 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
notably, for data used by EPA for redesignations to attainment.
V. Clean Air Act Implementation Requirements
This section of the preamble discusses the Clean Air Act (CAA)
requirements that states and emissions sources must address when
implementing new or revised NO2 NAAQS based on the structure
outlined in the CAA and existing rules.\23\ EPA may provide additional
guidance in the future, as necessary, to assist states and emissions
sources to comply with the CAA requirements for implementing new or
revised NO2 NAAQS.
---------------------------------------------------------------------------
\23\ Since EPA is proposing to retain the annual standard
without revision, the discussion in this section relates to
implementation of the proposed 1-hour standard, rather than the
annual standard.
---------------------------------------------------------------------------
The CAA assigns important roles to EPA, states, and, in specified
circumstances, Tribal governments to achieve the 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 by providing technical tools, assistance, and guidance,
including information on the potential control measures that may assist
in helping areas attain the standards.
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once they have been
established by EPA. Under section 110 of the CAA, 42 U.S.C. 7410, and
related provisions, states are required to submit, for EPA approval,
SIPs that provide for the attainment and maintenance of such standards
through control programs directed at sources of NO2
emissions. If a state fails to adopt and implement the required SIPs by
the time periods provided in the CAA, the EPA has responsibility under
the CAA to adopt a Federal Implementation Plan (FIP) to assure that
areas attain the NAAQS in an expeditious manner.
The states, in conjunction with EPA, also administer the prevention
of significant deterioration (PSD) program for NO2. See
sections 160-169 of the CAA. In addition, Federal programs provide for
nationwide reductions in emissions of NO2 and other air
pollutants under Title II of the Act, 42
[[Page 34450]]
U.S.C. 7521-7574, which involves controls for automobiles, trucks,
buses, motorcycles, nonroad engines, and aircraft emissions; the new
source performance standards (NSPS) for stationary sources under
section 111 of the CAA, 42 U.S.C. 7411; and the national emission
standards for hazardous air pollutants for stationary sources under
section 112 of the CAA, 42 U.S.C. 7412.
CAA Section 301(d) authorizes EPA to treat eligible Indian Tribes
in the same manner as states (TAS) under the CAA and requires EPA to
promulgate regulations specifying the provisions of the statute for
which such treatment is appropriate. EPA has promulgated these
regulations--known as the Tribal Authority Rule or TAR--at 40 CFR Part
49. See 63 FR 7254 (February 12, 1998). The TAR establishes the process
for Indian Tribes to seek TAS eligibility and sets forth the CAA
functions for which TAS will be available. Under the TAR, eligible
Tribes may seek approval for all CAA and regulatory purposes other than
a small number of functions enumerated at section 49.4. Implementation
plans under section 110 are included within the scope of CAA functions
for which eligible Tribes may obtain approval. Section 110(o) also
specifically describes Tribal roles in submitting implementation plans.
Eligible Indian Tribes may thus submit implementation plans covering
their reservations and other areas under their jurisdiction.
Under the CAA and TAR, Tribes are not, however, required to apply
for TAS or implement any CAA program. In promulgating the TAR EPA
explicitly determined that it was not appropriate to treat Tribes
similarly to states for purposes of, among other things, specific plan
submittal and implementation deadlines for NAAQS-related requirements.
40 CFR 49.4(a). In addition, where Tribes do seek approval of CAA
programs, including section 110 implementation plans, the TAR provides
flexibility and allows them to submit partial program elements, so long
as such elements are reasonably severable--i.e., ``not integrally
related to program elements that are not included in the plan
submittal, and are consistent with applicable statutory and regulatory
requirements''. 40 CFR 49.7.
To date, very few Tribes have sought TAS for purposes of section
110 implementation plans. However, some Tribes may be interested in
pursuing such plans to implement today's proposed standard. As noted
above, such Tribes may seek approval of partial, reasonably severable
plan elements, or they may seek to implement all relevant components of
an air quality program for purposes of meeting the requirements of the
Act. In several sections of this preamble, EPA describes the various
roles and requirements states will address in implementing today's
proposed standard. Such references to states are generally intended to
include eligible Indian Tribes to the extent consistent with the
flexibility provided to Tribes under the TAR. Where Tribes do not seek
TAS for section 110 implementation plans, EPA will promulgate Federal
implementation plans as ``necessary or appropriate to protect air
quality.'' 40 CFR 49.11(a)
EPA also notes that some Tribes operate air quality monitoring
networks in their areas. For such monitors to be used to measure
attainment with this primary NAAQS for NO2, the criteria and
procedures identified in this rule would apply.
A. Designations
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. By no later than 120 days prior to
promulgating designations, EPA is required to notify states of any
intended modifications to their boundaries as EPA may deem necessary.
States then have an opportunity to comment on EPA's tentative decision.
Whether or not a state provides a recommendation, EPA must promulgate
the designation that it deems appropriate.
Thus, following promulgation of the revised NO2 NAAQS in
January 2010, EPA must promulgate initial designations by January 2012
(2 years after promulgation of the revised NAAQS), or, by January 2013
in the event that the Administrator has insufficient information to
promulgate initial designations within 2 years. In the case of the
NO2 NAAQS, in today's action EPA is proposing new
NO2 monitor siting rules that focus on roadways. EPA
anticipates that it will require up to 3 years to get a new monitoring
network in place, plus an additional 3 years of monitoring thereafter
in order to determine compliance with the revised standard. This means
that a full set of air quality data from the new network will not be
available until approximately 2016. Since data from the new network
will not be available prior to the CAA designation deadlines even if
EPA takes an additional year, EPA intends to complete initial
designations in 2012 using air quality data from the current
NO2 monitoring network in place, using NO2
monitoring data from the years 2008-2010.
Accordingly, Governors will be required to submit their initial
designation recommendations to EPA no later than January 2011. If the
Administrator intends to modify any state area recommendation, EPA will
notify the Governor no later than 120 days prior to initial
designations in January 2012. States that believe the Administrator's
modification is inappropriate will have an opportunity to demonstrate
why they believe their recommendation is more appropriate before
designations are promulgated in January 2012. As explained below in
more detail, we intend to designate areas under the current
NO2 monitoring network as ``unclassifiable'' or
``nonattainment'' based on the data set for 2008-2010.
We intend to designate areas that do not show violations of the
revised NO2 NAAQS as ``unclassifiable'' since the existing
area-wide monitoring network does not fully satisfy the near roadway-
oriented NO2 monitoring requirements proposed in this
notice. Because there are no monitors in the current NO2
network that meet the proposed definition of ``near-roadway,''
monitoring data that does not indicate a violation of the NAAQS would
not provide a sufficient basis for concluding that an area is meeting
the revised NO2 NAAQS. Rather, an area-wide monitor may
record concentrations that are below the revised NO2 NAAQS
because it is not sited where concentrations in the area are highest.
Thus, we do not
[[Page 34451]]
believe the current monitoring network provides information that
supports designating an area as ``attainment'' with today's proposed
standards.
The EPA anticipates that areas designated as ``unclassifiable'' in
January 2012 will remain so until a new NO2 monitoring
network is deployed and 3 years of monitoring data have been collected.
Once the NO2 monitors are placed in locations meeting the
proposed near-roadway siting requirements and monitoring data become
available, the Agency could subsequently redesignate areas as
``nonattainment'' or ``attainment'' under section 107(d)(3).
In January 2012 we intend to designate as ``nonattainment'' areas
that show violations of the revised standard under the current
monitoring network. As discussed above, the current monitoring network
may not record NO2 concentrations near roadways where
NO2 concentrations are highest. We thus anticipate that any
area showing violations of the revised NO2 standard based on
the current monitoring network will continue to show violations when
monitors are placed in near-roadway locations.
In summary, as required by section 107(d)(1)(A)(i) of the CAA, in
January 2012 the EPA must designate as ``nonattainment'' any areas with
monitors within the existing network that report violations of the
revised NO2 NAAQS. All other areas not indicating a
violation of the revised NO2 NAAQS will be designated as
``unclassifiable.'' While the CAA provides the Agency an additional
third year from promulgation of a NAAQS to complete designations in the
event that there is insufficient information to make NAAQS compliance
determinations, we anticipate that delaying designations for this
additional year would not result in significant additional data that
would allow EPA to designate areas that would otherwise be designated
``unclassifiable.'' Once a near-roadway network has been deployed and 3
years of air quality data has been collected, we anticipate
redesignating unclassifiable areas as ``attainment'' or
``nonattainment'' where additional data from the new network provides a
basis for such a designation.
EPA is also taking comment on the area-wide approach discussed in
section II.F.4.e above. If this approach is finalized, we anticipate
designating areas as either ``attainment,'' ``nonattainment'' or
``unclassifiable'' in 2012, based on air quality data for years 2008-
2010. Unlike the near-roadway approach, we would expect to have
sufficient data to designate some areas showing no violations of the
revised NAAQS as ``attainment'' rather than ``unclassifiable.'' As
required by CAA section 107(d), we would expect to designate areas with
violating monitors and nearby areas, including those with major
roadways that contribute to such violations, as ``nonattainment.'' Any
areas which EPA cannot classify on the basis of available information
as meeting or not meeting the revised NAAQS would be designated as
``unclassifiable.''
B. Classifications
Section 172(a)(1)(A) of the CAA authorizes EPA to classify areas
designated as nonattainment for the purpose 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 NO2
nonattainment areas, but is not required to do so. The primary reason
to establish classifications is to set different deadlines for each
class of nonattainment area to complete the planning process and to
provide for different attainment dates based upon the severity of the
nonattainment problem for the affected area. However, the CAA
separately establishes specific planning and attainment deadlines in
sections 191 and 192: 18 months for the submittal of an attainment plan
and as expeditiously as possible but no later than 5 years for areas to
attain standard. EPA believes that classifications are unnecessary in
light of these relatively short deadlines. Therefore, EPA is not
proposing to establish classifications for a revised NO2
NAAQS.
C. Attainment Dates
The maximum deadline date by which an area is required to attain
the NO2 NAAQS is determined from the effective date of the
nonattainment designation for the affected area. For areas designated
nonattainment for the revised NO2 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). The EPA will determine
whether an area has demonstrated attainment of the NO2 NAAQS
by evaluating air quality monitoring data consistent with the form of
the NO2 NAAQS if revised, which will be codified at 40 CFR
part 50, Appendix F.
1. Attaining the NAAQS
In order for an area to be redesignated as attainment, the state
must comply with the five requirements as provided under section
107(d)(3)(E) of the CAA. This section requires that:
--EPA must have determined that the area has met the NO2
NAAQS;
--EPA has fully approved the state's implementation plan;
--the improvement in air quality in the affected area is due to
permanent and enforceable reductions in emissions;
--EPA has fully approved a maintenance plan for the area; and
--The state(s) containing the area have met all applicable requirements
under section 110 and part D.
2. Consequences of Failing To Attain by the Statutory Attainment Date
Any NO2 nonattainment area that fails to attain by its
statutory attainment date would be subject to the requirements of
sections 179(c) and (d) of the CAA. EPA is required to make a finding
of failure to attain no later than 6 months after the specified
attainment date and publish a notice in the Federal Register. The state
would be required to submit an implementation plan revision, no later
than one year following the effective date of the Federal Register
notice making the determination of the area's failure to attain, which
demonstrates that the standard will be attained as expeditiously as
practicable, but no later than 5 years from the effective date of EPA's
finding that the area failed to attain. In addition, section 179(d)(2)
provides that the SIP revision must include any specific additional
measures as may be reasonably prescribed by EPA, including ``all
measures that can be feasibly implemented in the area in light of
technological achievability, costs, and any nonair quality and other
air quality-related health and environmental impacts.''
D. Section 110(a)(2) NAAQS Infrastructure Requirements
Section 110(a)(2) of the CAA requires all states to develop and
maintain a solid air quality management infrastructure, including
enforceable emission limitations, an ambient monitoring program, an
enforcement program, air quality modeling, and adequate personnel,
resources, and legal authority. Section 110(a)(2)(D) also requires
state plans to prohibit emissions from within the state which
contribute significantly to nonattainment or maintenance areas in any
other State, or which interfere with programs under part C to prevent
[[Page 34452]]
significant deterioration of air quality or to achieve reasonable
progress toward the national visibility goal for Federal class I areas
(national parks and wilderness areas).
Under section 110(a)(1) and (2) of the CAA, all states are required
to submit SIPs to EPA which demonstrate that basic program elements
have been addressed within 3 years of the promulgation of any new or
revised NAAQS. Subsections (A) through (M) of section 110(a)(2) listed
below, set forth the elements that a State's program must contain in
the SIP.\24\ The list of section 110(a)(2) NAAQS implementation
requirements are the following:
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\24\ 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. These 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 and permitting of new/modified
sources.
Interstate transport: Section 110(a)(2)(D) requires SIPs
to include provisions prohibiting any source or other type of emissions
activity in the 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 assurances of 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
include contingency plans, and adequate authority to implement them,
for emergency episodes in their SIPs.
Provisions for SIP revision due to NAAQS changes or
findings of inadequacies: Section 110(a)(2)(H) requires states to
provide for revisions of 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.
Consultation with local and Federal government officials:
Section 110(a)(2)(J) requires states to meet applicable local and
Federal government consultation requirements when developing SIP and
reviewing preconstruction permits.
Public notification of NAAQS exceedances: Section
110(a)(2)(J) requires states to adopt measures to notify the public of
instances or areas in which a NAAQS is exceeded.
PSD and visibility protection: Section 110(a)(2)(J) also
requires states to adopt emissions limitations, and such other
measures, as may be necessary to prevent significant deterioration of
air quality in attainment areas and protect visibility in Federal Class
I areas in accordance with the requirements of CAA Title I, part C.
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 of 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 Planning Requirements
1. Nonattainment Area SIPs
Any state containing an area designated as nonattainment with
respect to the NO2 NAAQS must develop for submission a SIP
meeting the requirements of part D, Title I, of the CAA, providing for
attainment by the applicable statutory attainment date (see sections
191(a) and 192(a) of the CAA). As indicated in section 191(a) all
components of the NO2 part D SIP must be submitted within 18
months of the effective date of an area's designation as nonattainment.
Section 172 of the CAA includes general requirements for all
designated nonattainment areas. Section 172(c)(1) requires that each
nonattainment area plan ``provide for the implementation of all
reasonably available control measures (RACM) as expeditiously as
practicable (including such reductions in emissions from existing
sources in the area as may be obtained through the adoption, at a
minimum, of Reasonably Available Control Technology (RACT)), and shall
provide for attainment of the national primary ambient air quality
standards.'' States are required to implement RACM and RACT in order to
attain ``as expeditiously as practicable''.
Section 172(c) requires states with nonattainment areas to submit a
SIP for these areas which contain an attainment demonstration which
shows that the affected area will attain the standard by the applicable
statutory attainment date. The State must also show that the area will
attain the standards as expeditiously as practicable, and it must
include an analysis of whether implementation of reasonably available
measures will advance the attainment date for the area.
Part D SIPs must also provide for reasonable further progress (RFP)
(see section 172(c)(2) of the CAA). 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.'' (See section 171
of the CAA) Historically, for some pollutants, RFP has been met by
showing annual incremental emission reductions sufficient to maintain
generally linear progress toward attainment by the applicable
attainment date.
All NO2 nonattainment area SIPs must include contingency
measures which must be implemented in the event that an area fails to
meet RFP or fails to attain the standards by its attainment date. (See
section 172(c)(9)) These contingency measures must be fully adopted
rules or control measures that take effect without further action by
the state or the Administrator. The EPA interprets this requirement to
mean that the contingency measures must be implemented with only
minimal further action by the state or the affected sources with no
additional rulemaking actions such as public hearings or legislative
review.
Emission inventories are also critical for the efforts of State,
local, and Federal agencies to attain and maintain the NAAQS that EPA
has established for criteria pollutants including NO2.
Section 191(a) in conjunction with section 172(c) requires that areas
designated as nonattainment for NO2 submit an emission
inventory to EPA no later than 18 months after designation as
nonattainment. In the case of NO2, sections 191(a) and
172(c) also require that states submit periodic emission
[[Page 34453]]
inventories for nonattainment areas. The periodic inventory must
include emissions of NO2 for point, nonpoint, mobile (on-
road and non-road), and area sources.
2. New Source Review and Prevention of Significant Deterioration
Requirements
The Prevention of Significant Deterioration (PSD) and nonattainment
New Source Review (NSR) programs contained in parts C and D of Title I
of the CAA govern preconstruction review of 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.\25\ The EPA rules addressing these
programs can be found at 40 CFR 51.165, 51.166, 52.21, 52.24, and part
51, appendix S. States which have areas designated as nonattainment for
the NO2 NAAQS must submit, as a part of the SIP due 18
months after an area is designated as nonattainment, provisions
requiring permits for the construction and operation of new or modified
stationary sources anywhere in the nonattainment area. SIPs that
address the PSD requirements related to attainment areas are due no
later than 3 years after the promulgation of a revised NAAQS for
NO2.
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\25\ 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).
Minor NSR.
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.\26\ The nonattainment NSR program applies on a pollutant-
specific basis when a major source constructs or modifies in an area
that is designated as nonattainment for that pollutant. The minor NSR
program addresses both major and minor sources that undergo
construction or modification activities that do not qualify as major,
and it applies, as necessary to ensure attainment, regardless of the
designation of the area in which a source is located.
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\26\ In addition, the PSD program applies to non-criteria
pollutants subject to regulation under the Act, except those
pollutants regulated under section 112 and pollutants subject to
regulation only under section 211(o).
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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 siting analysis demonstrating that the
benefits of a 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 ensure that the [NAAQS] are achieved.'' Areas which are
newly designated as nonattainment for the NO2 NAAQS as a
result of any changes made to the NAAQS will be required to adopt a
nonattainment NSR program to address major sources of NO2
where the program does not currently exist for the NO2 NAAQS
and may need to amend their minor source program as well. Prior to
adoption of the SIP revision addressing major source nonattainment NSR
for NO2 nonattainment areas, the requirements of 40 CFR part
51, appendix S will apply.
3. 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. The EPA rules, developed under the authority of section 176(c) of
the CAA, prescribe the criteria and procedures for demonstrating and
assuring conformity of Federal actions to a SIP. 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 areas and
those areas redesignated attainment since 1990 (``maintenance areas'')
with respect to the criteria pollutants under the CAA: \27\ Carbon
monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone
(O3), particulate matter (PM2.5 and
PM10), and sulfur dioxide (SO2). The general conformity
rules apply one year following the effective date of designations for
any new or revised NAAQS.
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\27\ Criteria pollutants are those pollutants for which EPA has
established a NAAQS under section 109 of the CAA.
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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, such as modeling or offsets, and requires the Federal action
to also meet any applicable SIP requirements and emissions milestones.
The general conformity rule also requires that notices of draft and
final general conformity determinations be provided directly to air
quality regulatory agencies and to the public by publication in a local
newspaper.
4. Transportation Conformity
Transportation conformity is required under CAA section 176(c) (42
U.S.C. 7506(c)) to ensure that transportation plans, transportation
improvement programs (TIPs) and Federally supported highway and transit
projects will not cause new air quality violations, worsen existing
violations, or delay timely attainment of the relevant NAAQS or interim
reductions and milestones. Transportation conformity applies to areas
that are designated nonattainment and maintenance for transportation-
related criteria pollutants: carbon monoxide (CO), ozone (O3), nitrogen
dioxide (NO2), and particulate matter (PM2.5 and
PM10). Transportation conformity for a revised
NO2 NAAQS does not apply until one year after the effective
date of a nonattainment designation. (See CAA section 176(c)(6) and 40
CFR 93.102(d)).
EPA's Transportation Conformity Rule (40 CFR Part 51, Subpart T,
and Part 93, Subpart A establishes the criteria and procedures for
determining whether transportation activities conform to the SIP. The
EPA is not proposing changes to the Transportation Conformity rule in
this proposed rulemaking. However, in the future, EPA will review the
need to conduct a
[[Page 34454]]
rulemaking to establish any new or revised transportation conformity
tests that would apply under a revision to the NO2 NAAQS for
transportation plans, TIPs, and applicable highway and transit
projects.
VI. Communication of Public Health Information
Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily through EPA's Air Quality Index (AQI) program. The current
Air Quality Index has been in use since its inception in 1999 (64 FR
42530). It provides accurate, timely, and easily understandable
information about daily levels of pollution (40 CFR 58.50). The AQI
establishes a nationally uniform system of indexing pollution levels
for NO2, carbon monoxide, ozone, particulate matter and
sulfur dioxide. The AQI converts pollutant concentrations in a
community's air to a number on a scale from 0 to 500. Reported AQI
values enable the public to know whether air pollution levels in a
particular location are characterized as good (0-50), moderate (51-
100), unhealthy for sensitive groups (101-150), unhealthy (151-200),
very unhealthy (201-300), or hazardous (300-500). The AQI index value
of 100 typically corresponds to the level of the short-term NAAQS for
each pollutant. An AQI value greater than 100 means that a pollutant is
in one of the unhealthy categories (i.e., unhealthy for sensitive
groups, unhealthy, very unhealthy, or hazardous) on a given day; an AQI
value at or below 100 means that a pollutant concentration is in one of
the satisfactory categories (i.e., moderate or good). Decisions about
the pollutant concentrations at which to set the various AQI
breakpoints, that delineate the various AQI categories, draw directly
from the underlying health information that supports the NAAQS review.
The Agency recognizes the importance of revising the AQI in a
timely manner to be consistent with any revisions to the NAAQS.
Therefore EPA proposes to finalize conforming changes to the AQI, in
connection with the Agency's final decision on the NO2 NAAQS
if revisions to the primary standard are promulgated. Currently, no AQI
breakpoints are identified below an AQI value of 200 since there is no
short-term NO2 NAAQS. Therefore, if a short-term
NO2 NAAQS is promulgated, conforming changes would include
setting the 100 level of the AQI at the same level as the revised
primary NO2 NAAQS and also setting the other AQI breakpoints
at the lower end of the AQI scale (i.e., AQI values of 50 and 150). EPA
does not propose to change breakpoints at the higher end of the AQI
scale (from 200 to 500), which would apply to state contingency plans
or the Significant Harm Level (40 CFR 51.16), because the information
from this review does not inform decisions about breakpoints at those
higher levels.
With regard to an AQI value of 50, the breakpoint between the good
and moderate categories, historically this value is set at the level of
the annual NAAQS, if there is one, or one-half the level of the short-
term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 12,
1998). Taking into consideration this practice, EPA is proposing to set
the AQI value of 50 to be between 0.040 and 0.053 ppm NO2,
1-hour average. EPA anticipates that figures towards the lower end of
this range would be appropriate if the standard is set towards the
lower end of the proposed range for the standard (e.g. 80 ppb), while
figures towards the higher end of the range would be more appropriate
for standards set at the higher end of the range for the standard
(e.g., 100 ppb). EPA solicits comments on this range for an AQI of 50,
and the appropriate basis for selecting an AQI of 50 both within this
range and, in light of EPA's solicitation of comment on standard levels
below 80 ppb and above 100 ppb, above or below this range.
With regard to an AQI value of 150, the breakpoint between the
unhealthy for sensitive groups and unhealthy categories, historically
values between the short-term standard and an AQI value of 500 are set
at levels that are approximately equidistant between the AQI values of
100 and 500 unless there is health evidence that suggests a specific
level would be appropriate (63 FR 67829, Dec. 12, 1998). For an AQI
value of 150, the range of 0.360 to 0.370 ppm NO2, 1-hour
average, represents the midpoint between the proposed range for the
short-term standard and the level of an AQI value of 200 (0.64 ppm
NO2, 1-hour average). Therefore, EPA is proposing to set the
AQI value of 150 to be between 0.360 and 0.370 ppm NO2, 1-
hour average.
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. In addition, EPA prepared a
Regulatory Impact Analysis (RIA) of the potential costs and benefits
associated with this action. 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
developing 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 number 2358.01.
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). 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
NO2 FRM/FEM determinations provided in the current ICR for
40 CFR part 53 (EPA ICR numbers 2358.01). 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 impacts, to develop emissions control
strategies, and to measure progress for the air pollution program. The
proposed amendments would revise the technical requirements for
NO2 monitoring sites, require the siting and operation of
additional NO2 ambient air monitors, and the reporting of
the collected ambient NO2 monitoring
[[Page 34455]]
data to EPA's Air Quality System (AQS). The annual average reporting
burden for the collection under 40 CFR part 58 (averaged over the first
3 years of this ICR) is $3,616,487. 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-
2006-0922. 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 July 15, 2009, a comment to OMB is best assured of having its
full effect if OMB receives it by August 14, 2009. 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 requirements on small entities.
Rather, this rule establishes national standards for allowable
concentrations of NO2 in ambient air as required by section
109 of the CAA. American Trucking Assn's 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
NO2 NAAQS impose no enforceable duty on any State, local or
Tribal governments or the private sector. The expected costs associated
with the 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 NO2 NAAQS.
With regard to implementation guidance, the CAA imposes the
obligation for States to submit SIPs to implement the NO2
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.
[[Page 34456]]
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 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,
on the relationship between the Federal government and Indian Tribes,
or on the distribution of power and responsibilities between the
Federal government and Tribes. The rule does not alter the relationship
between the Federal government and Tribes as established in the CAA and
the TAR. Under section 109 of the CAA, EPA is mandated to establish
NAAQS; however, this rule does not infringe existing Tribal authorities
to regulate air quality under their own programs or under programs
submitted to EPA for approval. Furthermore, this rule does not affect
the flexibility afforded to Tribes in seeking to implement CAA programs
consistent with the TAR, nor does it impose any new obligation on
Tribes to adopt or implement any NAAQS. Finally, as noted in section E
(above) on UMRA, this rule does not impose significant costs on Tribal
governments. Thus, Executive Order 13175 does not apply to this rule.
However, EPA recognizes that Tribes may be interested in this rule and
any corresponding revisions to associated air quality surveillance
requirements. Therefore, in the spirit of Executive Order 13175, and
consistent with EPA policy to promote communications between EPA and
Tribes, 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 NO2; these standards are designed to
protect public health with an adequate margin of safety, as required by
CAA section 109. The protection offered by these standards may be
especially important for asthmatics, including asthmatic children,
because respiratory effects in asthmatics are among the most sensitive
health endpoints for NO2 exposure. Because asthmatic
children are considered a sensitive population, we have evaluated the
potential health effects of exposure to NO2 pollution among
asthmatic children. These effects and the size of the population
affected are discussed in chapters 3 and 4 of the ISA; chapters 3, 4,
and 8 of the REA, and sections II.A through II.E 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 NO2. 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 with regard
to ambient monitoring of NO2. The use of this voluntary
consensus standard would be impractical because the
[[Page 34457]]
analysis method does not provide for the method detection limits
necessary to adequately characterize ambient NO2
concentrations for the purpose of determining compliance with the
proposed revisions to the NO2 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 effects on
any population, including any minority or low-income population. The
proposed rule will establish uniform national standards for
NO2 in ambient air. EPA solicits comment on environmental
justice issues related to the proposed revision of the NO2
NAAQS.
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0922). Available at http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_rea.html.
Rodes, CE and Holland DM. (1981). Variations of NO, NO2,
and O3 concentrations downwind of a Los Angeles Freeway.
Atmos. Environ. 15:243-250.
Roorda-Knape, MC, Janssen NAH, De Hartog JJ, Van Vliet PHN, Harssema
H, Brunekreef B. (1998). Air pollution from traffic in city
districts near major motorways. Atmos. Environ. 32:1921-1930.
Rojas-Martinez, R, Perez-Padilla R, Olaiz-Fernandez G, Mendoza-
Alvarado L, Moreno-Macias H, Fortoul T, McDonnell W, Loomis D,
Romieu I. (2007a) Lung function growth in children with long-term
exposure to air pollutants in Mexico City. Am. J. Respir. Crit. Care
Med. 176:377-384.
Rojas-Martinez, R, Perez-Padilla R, Olaiz-Fernandez G, Mendoza-
Alvarado L, Moreno-Macias H, Fortoul T, McDonnell W, Loomis D,
Romieu I. (2007b) Lung function growth in children with long-term
exposure to air pollutants in Mexico City. Online data supplement.
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October, 2007].
Ryerson, TB, Williams EJ, Fehsenfeld FC. (2000). An efficient
photolysis system for fast-response NO2 measurements. J.
Geophys. Res. [Atmos.] 105:26,447-26,461.
Samet, J. (2008a). Letter to EPA Administrator Stephen Johnson:
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NO2 Primary National Ambient Air Quality Standard: Second
Draft.'' EPA-CASAC-08-021, September 24.
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``Clean Air Scientific Advisory Committee's (CASAC) Review Comments
on EPA's Risk and Exposure Assessment to Support the Review of the
NO2 Primary National Ambient Air Quality Standard.'' EPA-
CASAC-09-003, December 16.
Schildcrout, JS, Sheppard L, Lumley T, Slaughter JC, Koenig JQ,
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Multipollutant modeling issues in a study of ambient air quality and
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[[Page 34459]]
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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 58
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.
Dated: June 26, 2009.
Lisa P. Jackson,
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 AMBIENT AIR QUALITY STANDARDS
1. The authority citation for part 50 continues to read as follows:
Authority: 42 U.S.C. 7401, et seq.
Subpart A--General Provisions
2. Section 50.11 is revised to read as follows:
Sec. 50.11 National primary and secondary ambient air quality
standards for oxides of nitrogen (nitrogen dioxide).
(a) The level of the national primary annual ambient air quality
standard for oxides of nitrogen is 53 parts per billion (ppb, which is
1 part in 1,000,000,000), annual average concentration, measured in the
ambient air as nitrogen dioxide.
(b) The level of the national primary 1-hour ambient air quality
standard for oxides of nitrogen is (80-100) ppb, 1-hour average
concentration, measured in the ambient air as nitrogen dioxide.
(c) The level of the national secondary ambient air quality
standard for nitrogen dioxide is 0.053 parts per million (100
micrograms per cubic meter), annual arithmetic mean concentration.
(d) The levels of the standards shall be measured by:
(1) A reference method based on appendix F to this part; or
(2) By a Federal equivalent method (FEM) designated in accordance
with part 53 of this chapter.
(e) The annual primary standard is met when the annual average
concentration in a calendar year is less than or equal to 53 ppb, as
determined in accordance with Appendix S of this part for the annual
standard.
(f) The 1-hour primary standard is met when the three-year average
of the annual (99th percentile)(fourth highest) of the daily maximum 1-
hour average concentration is less than or equal to (80-100) ppb, as
determined in accordance with Appendix S of this part for the 1-hour
standard.
(g) The secondary standard is attained when the annual arithmetic
mean concentration in a calendar year is less than or equal to 0.053
ppm, rounded to three decimal places (fractional parts equal to or
greater than 0.0005 ppm must be rounded up). To demonstrate attainment,
an annual mean must be based upon hourly data that are at least 75
percent complete or upon data derived from manual methods that are at
least 75 percent complete for the scheduled sampling days in each
calendar quarter.
3. Section 50.14 is amended by revising paragraph (c)(2)(vi) to
read as follows:
Sec. 50.14 Treatment of air quality monitoring data influenced by
exceptional events.
* * * * *
(c) * * *
(2) * * *
(vi) 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 exceptional event data, providing initial data descriptions
and providing detailed data documentation in AQS for the initial
designations of areas for those NAAQS: Table 1 provides the schedule
for submission of flags with initial descriptions in AQS and detailed
documentation and the schedule shall apply for those data which will or
may influence the initial designation of areas for those NAAQS. EPA
anticipates revising Table 1 as necessary to accommodate revised data
submission schedules for new or revised NAAQS.
Table 1--to Paragraph (c)(2)(vi): Schedule for Exceptional Event Flagging and Documentation Submission for Data
To Be Used in Designations Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
Air quality data
NAAQS pollutant/standard/(level)/ collected for Event flagging & initial Detailed documentation
promulgation date calendar year description deadline submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35[mu]g/m\3\) 2004-2006 October 1, 2007 \a\........ April 15, 2008. \a\
Promulgated October 17, 2006.
Ozone/8-Hr Standard (0.075 ppm) 2005-2007 June 18, 2009 \b\.......... June 18, 2009.\b\
Promulgated March 12, 2008.
2008 June 18, 2009 \b\.......... June 18, 2009.\b\
2009 60 Days after the end of 60 Days after the end of
the calendar quarter in the calendar quarter in
which the event occurred which the event occurred
or February 5, 2010, or February 5, 2010,
whichever date occurs whichever date occurs
first. \b\ first.\b\
NO2/1-Hour Standard (80-100 ppb, 2008 July 1, 2010 \b\........... January 22, 2011.
final level TBD).
2009 July 1, 2010............... January 22, 2011.
2010 April 1, 2011 \b\.......... July 1, 2011.\b\
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, and are shown in this table for
informational purposes.
\b\ Indicates change from general schedule in 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
notably, for data used by EPA for redesignations to attainment.
[[Page 34460]]
* * * * *
4. Appendix S is added to read as follows:
Option 1 for Appendix S to Part 50:
Appendix S to Part 50--Interpretation of the Primary National Ambient
Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide) (1-Hour
Primary Standard Based on the 4th Highest Daily Maximum Value Form)
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining when the primary national
ambient air quality standards for oxides of nitrogen as measured by
nitrogen dioxide (``NO2 NAAQS'') specified in Sec. 50.11
are met. Nitrogen dioxide (NO2) is measured in the
ambient air by a Federal reference method (FRM) based on appendix F
to this part or by a Federal equivalent method (FEM) designated in
accordance with part 53 of this chapter. Data handling and
computation procedures to be used in making comparisons between
reported NO2 concentrations and the levels of the
NO2 NAAQS 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 mean refers to the annual average of all of the 1-hour
concentration values as defined in section 5.1 of this appendix.
Daily maximum 1-hour values for NO2 refers to the
maximum 1-hour NO2 concentration values measured from
midnight to midnight (local standard time) that are used in NAAQS
computations.
Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
specified in section 5 of this appendix. The design values for the
primary NAAQS are:
(1) The annual mean value for a monitoring site for one year
(referred to as the ``annual primary standard design value'').
(2) The 3-year average of annual 4th highest daily maximum 1-
hour values for a monitoring site (referred to as the ``1-hour
primary standard design value'').
Annual 4th highest daily maximum 1-hour value refers to the 4th
highest daily 1-hour maximum value at a site in a particular year.
Quarter refers to a calendar quarter.
Year refers to a calendar year.
2. Requirements for Data Used for Comparisons With the NO2
NAAQS and Data Reporting Considerations
(a) All valid FRM/FEM NO2 hourly data required to be
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. Multi-hour average concentration values collected by
wet chemistry methods shall not be used.
(b) When two or more NO2 monitors are operated at a
site, the state may in advance designate one of them as the primary
monitor. If the state has not made this designation in advance, the
Administrator will make the designation, either in advance or
retrospectively. Design values will be developed using only the data
from the primary monitor, if this results in a valid design value.
If data from the primary monitor do not allow the development of a
valid design value, data solely from the other monitor(s) will be
used in turn to develop a valid design value, if this results in a
valid design value. If there are three or more monitors, the order
for such comparison of the other monitors will be determined by the
Administrator. The Administrator may combine data from different
monitors in different years for the purpose of developing a valid 1-
hour primary standard design value, if a valid design value cannot
be developed solely with the data from a single monitor. However,
data from two or more monitors in the same year at the same site
will not be combined in an attempt to meet data completeness
requirements, except if one monitor has physically replaced another
instrument permanently, in which case the two instruments will be
considered to be the same monitor, or if the state has switched the
designation of the primary monitor from one instrument to another
during the year.
(c) Hourly NO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
3. Comparisons With the NO2 NAAQS
3.1 The Annual Primary NO2 NAAQS
(a) The annual primary NO2 NAAQS is met at a site
when the valid annual primary standard design value is less than or
equal to 53 parts per billion (ppb).
(b) An annual primary standard design value is valid when at
least 75 percent of the hours in the year are reported.
(c) An annual primary standard design value based on data that
do not meet the completeness criteria stated in 3.1(b) may also be
considered valid 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.
(d) The procedures for calculating the annual primary standard
design values are given in section 5.1 of this appendix.
3.2 The 1-Hour Primary NO2 NAAQS
(a) The 1-hour primary NO2 NAAQS is met at a site
when the valid 1-hour primary standard design value is less than or
equal to [80-100] parts per billion (ppb).
(b) An NO2 1-hour primary standard design value is
valid if it encompasses three consecutive calendar years of complete
data. A year meets data completeness requirements when all 4
quarters are complete. A quarter is complete when at least 75
percent of the sampling days for each quarter have complete data. A
sampling day has complete data if 75 percent of the hourly
concentration values are reported.
(c) In the case of one, two, or three years that do not meet the
completeness requirements of section 3.2(b) of this appendix and
thus would normally not be useable for the calculation of a valid 3-
year 1-hour primary standard design value, the 3-year 1-hour primary
standard design value shall nevertheless be considered valid if
either of the following conditions is true.
(i) If there are at least four days in each of the 3 years that
have at least one reported hourly value, and the resulting 3-year 1-
hour primary standard design value exceeds the 1-hour primary NAAQS.
In this situation, more complete data capture could not possibly
have resulted in a design value below the 1-hour primary NAAQS.
(ii)(A) A 1-hour primary standard design value that is below the
level of the NAAQS can be validated if the substitution test in
section 3.2(c)(ii)(B) results in a ``test design value'' that is
below the level of the NAAQS. The test substitutes actual ``high''
reported daily maximum 1-hour values from the same site at about the
same time of the year (specifically, in the calendar quarter) for
unknown values that were not successfully measured. Note that the
test is merely diagnostic in nature, intended to confirm that there
is a very high likelihood that the original design value (the one
with less than 75 percent data capture of hours by day and of days
by quarter) reflects the true under-NAAQS-level status for that 3-
year period; the result of this data substitution test (the ``test
design value,'' as defined in section 3.2(c)(ii)(B)), is not
considered the actual design value. For this test, substitution is
permitted only if there are at least 200 days across the three
matching quarters of the three years under consideration (which is
about 75 percent of all possible daily values in those three
quarters) for which 75 percent of the hours in the day have reported
concentrations. However, maximum 1-hour values from days with less
than 75 percent of the hours reported shall also be considered in
identifying the high value to be used for substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture but at least 50 percent data capture; if any quarter
has less than 50 percent data capture then this substitution test
cannot be used. Identify for each quarter (e.g., January-March) the
highest reported daily maximum 1-hour value for that quarter,
looking across those three months of all three years under
consideration. All daily maximum 1-hour values from all days in the
quarter period shall be considered when identifying this highest
value, including days with less than 75 percent data capture. If
after substituting the highest reported daily maximum 1-hour value
for a quarter for as much of the missing daily data in the matching
deficient quarter(s) as is needed to make them 100 percent complete,
the procedure in section 5.2 yields a recalculated 3-year 1-hour
standard ``test design value'' below the level of the standard, then
the 1-hour primary
[[Page 34461]]
standard design value is deemed to have passed the diagnostic test
and is valid, and the level of the standard is deemed to have been
met in that 3-year period. As noted in section 3.2(c)(i), in such a
case, the 3-year design value based on the data actually reported,
not the ``test design value'', shall be used as the valid design
value.
(d) A 1-hour primary standard design value based on data that do
not meet the completeness criteria stated in section 3.2(b) and also
do not satisfy section 3.2(c), may also be considered valid 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 procedures for calculating the 1-hour primary standard
design values are given in section 5.2 of this appendix.
4. Rounding Conventions
4.1 Rounding Conventions for the Annual Primary NO2
NAAQS
(a) Hourly NO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
(b) The annual primary standard design value is calculated
pursuant to section 5.1 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).
4.2 Rounding Conventions for the 1-Hour Primary NO2
NAAQS
(a) Hourly NO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
(b) Daily maximum 1-hour values, including the annual 4th
highest of those daily values, are not rounded.
(c) The 1-hour primary standard design value is calculated
pursuant to section 5.2 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).
5. Calculation Procedures for the Primary NO2 NAAQS
5.1 Calculation Procedures for the Annual Primary NO2
NAAQS
(a) When the data for a site and year meet the data completeness
requirements in section 3.1(b) of this appendix, or if the
Administrator exercises the discretionary authority in section
3.1(c), the annual mean is simply the arithmetic average of all of
the reported 1-hour values.
(b) The annual primary standard design value for a site is the
valid annual mean rounded according to the conventions in section
4.1.
5.2 Calculation Procedures for the 1-Hour Primary NO2
NAAQS
(a) When the data for a particular site and year meet the data
completeness requirements in section 3.2(b), or if one of the
conditions of section 3.2(c) is met, or if the Administrator
exercises the discretionary authority in section 3.2(d), calculation
of the 4th highest daily 1-hour maximum is accomplished as follows.
(i) For each year, select from each day the highest hourly
value. All daily maximum 1-hour values from all days in the quarter
period shall be considered at this step, including days with less
than 75 percent data capture.
(ii) For each year, order these daily values and take the 4th
highest.
(iii) The 1-hour primary standard design value for a site is
mean of the three annual 4th highest values, rounded according to
the conventions in section 4.2.
Option 2 for Appendix S to Part 50:
Appendix S to Part 50--Interpretation of the Primary National Ambient
Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide) (1-Hour
Primary Standard Based on the 99th Percentile Form)
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining when the primary national
ambient air quality standards for oxides of nitrogen as measured by
nitrogen dioxide (``NO2 NAAQS'') specified in Sec. 50.11
are met. Nitrogen dioxide (NO2) is measured in the
ambient air by a Federal reference method (FRM) based on appendix F
to this part or by a Federal equivalent method (FEM) designated in
accordance with part 53 of this chapter. Data handling and
computation procedures to be used in making comparisons between
reported NO2 concentrations and the levels of the
NO2 NAAQS 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 mean refers to the annual average of all of the 1-hour
concentration values as defined in section 5.1 of this appendix.
Daily maximum 1-hour values for NO2 refers to the
maximum 1-hour NO2 concentration values measured from
midnight to midnight (local standard time) that are used in NAAQS
computations.
Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
specified in section 5 of this appendix. The design values for the
primary NAAQS are:
(1) The annual mean value for a monitoring site for one year
(referred to as the ``annual primary standard design value'').
(2) The 3-year average of annual 99th percentile daily maximum
1-hour values for a monitoring site (referred to as the ``1-hour
primary standard design value'').
99th percentile daily maximum 1-hour value is the value below
which nominally 99 percent of all daily maximum 1-hour concentration
values fall, using the ranking and selection method specified in
section 5.2 of this appendix.
Quarter refers to a calendar quarter.
Year refers to a calendar year.
2. Requirements for Data Used for Comparisons With the NO2
NAAQS and Data Reporting Considerations
(a) All valid FRM/FEM NO2 hourly data required to be
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. Multi-hour average concentration values collected by
wet chemistry methods shall not be used.
(b) When two or more NO2 monitors are operated at a
site, the state may in advance designate one of them as the primary
monitor. If the state has not made this designation, the
Administrator will make the designation, either in advance or
retrospectively. Design values will be developed using only the data
from the primary monitor, if this results in a valid design value.
If data from the primary monitor do not allow the development of a
valid design value, data solely from the other monitor(s) will be
used in turn to develop a valid design value, if this results in a
valid design value. If there are three or more monitors, the order
for such comparison of the other monitors will be determined by the
Administrator. The Administrator may combine data from different
monitors in different years for the purpose of developing a valid 1-
hour primary standard design value, if a valid design value cannot
be developed solely with the data from a single monitor. However,
data from two or more monitors in the same year at the same site
will not be combined in an attempt to meet data completeness
requirements, except if one monitor has physically replaced another
instrument permanently, in which case the two instruments will be
considered to be the same monitor, or if the state has switched the
designation of the primary monitor from one instrument to another
during the year.
(c) Hourly NO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
3. Comparisons With the NO2 NAAQS
3.1 The Annual Primary NO2 NAAQS
(a) The annual primary NO2 NAAQS is met at a site
when the valid annual primary standard design value is less than or
equal to 53 parts per billion (ppb).
(b) An annual primary standard design value is valid when at
least 75 percent of the hours in the year are reported.
(c) An annual primary standard design value based on data that
do not meet the completeness criteria stated in section 3.1(b) may
also be considered valid with the approval of, or at the initiative
of, the
[[Page 34462]]
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.
(d) The procedures for calculating the annual primary standard
design values are given in section 5.1 of this appendix.
3.2 The 1-Hour Primary NO2 NAAQS
(a) The 1-hour primary NO2 NAAQS is met at a site
when the valid 1-hour primary standard design value is less than or
equal to [80-100] parts per billion (ppb).
(b) An NO2 1-hour primary standard design value is
valid if it encompasses three consecutive calendar years of complete
data. A year meets data completeness requirements when all 4
quarters are complete. A quarter is complete when at least 75
percent of the sampling days for each quarter have complete data. A
sampling day has complete data if 75 percent of the hourly
concentration values are reported.
(c) In the case of one, two, or three years that do not meet the
completeness requirements of section 3.2(b) of this appendix and
thus would normally not be useable for the calculation of a valid 3-
year 1-hour primary standard design value, the 3-year 1-hour primary
standard design value shall nevertheless be considered valid if one
of the following conditions is true.
(i) At least 75 percent of the days in each quarter of each of
three consecutive years have at least one reported hourly value, and
the design value calculated according to the procedures specified in
section 5.2 is above the level of the primary 1-hour standard.
(ii)(A) A 1-hour primary standard design value that is below the
level of the NAAQS can be validated if the substitution test in
section 3.2(c)(ii)(B) results in a ``test design value'' that is
below the level of the NAAQS. The test substitutes actual ``high''
reported daily maximum 1-hour values from the same site at about the
same time of the year (specifically, in the same calendar quarter)
for unknown values that were not successfully measured. Note that
the test is merely diagnostic in nature, intended to confirm that
there is a very high likelihood that the original design value (the
one with less than 75 percent data capture of hours by day and of
days by quarter) reflects the true under-NAAQS-level status for that
3-year period; the result of this data substitution test (the ``test
design value'', as defined in section 3.2(c)(ii)(B)) is not
considered the actual design value. For this test, substitution is
permitted only if there are at least 200 days across the three
matching quarters of the three years under consideration (which is
about 75 percent of all possible daily values in those three
quarters) for which 75 percent of the hours in the day have reported
concentrations. However, maximum 1-hour values from days with less
than 75 percent of the hours reported shall also be considered in
identifying the high value to be used for substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture but at least 50 percent data capture; if any quarter
has less than 50 percent data capture then this substitution test
cannot be used. Identify for each quarter (e.g., January-March) the
highest reported daily maximum 1-hour value for that quarter,
looking across those three months of all three years under
consideration. All daily maximum 1-hour values from all days in the
quarter period shall be considered when identifying this highest
value, including days with less than 75 percent data capture. If
after substituting the highest reported daily maximum 1-hour value
for a quarter for as much of the missing daily data in the matching
deficient quarter(s) as is needed to make them 100 percent complete,
the procedure in section 5.2 yields a recalculated 3-year 1-hour
standard ``test design value'' below the level of the standard, then
the 1-hour primary standard design value is deemed to have passed
the diagnostic test and is valid, and the level of the standard is
deemed to have been met in that 3-year period. As noted in section
3.2(c)(i), in such a case, the 3-year design value based on the data
actually reported, not the ``test design value'', shall be used as
the valid design value.
(iii)(A) A 1-hour primary standard design value that is above
the level of the NAAQS can be validated if the substitution test in
section 3.2(c)(iii)(B) results in a ``test design value'' that is
above the level of the NAAQS. The test substitutes actual ``low''
reported daily maximum 1-hour values from the same site at about the
same time of the year (specifically, in the same three months of the
calendar) for unknown values that were not successfully measured.
Note that the test is merely diagnostic in nature, intended to
confirm that there is a very high likelihood that the original
design value (the one with less than 75 percent data capture of
hours by day and of days by quarter) reflects the true above-NAAQS-
level status for that 3-year period; the result of this data
substitution test (the ``test design value,'' as defined in section
3.2(c)(iii)(B)) is not considered the actual design value. For this
test, substitution is permitted only if there are a minimum number
of available daily data points from which to identify the low
quarter-specific daily maximum 1-hour values, specifically if there
are at least 200 days across the three matching quarters of the
three years under consideration (which is about 75 percent of all
possible daily values in those three quarters) for which 75 percent
of the hours in the day have reported concentrations. Only days with
at least 75 percent of the hours reported shall be considered in
identifying the low value to be used for substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture. Identify for each quarter (e.g., January-March) the
lowest reported daily maximum 1-hour value for that quarter, looking
across those three months of all three years under consideration.
All daily maximum 1-hour values from all days with at least 75
percent capture in the quarter period shall be considered when
identifying this lowest value. If after substituting the lowest
reported daily maximum 1-hour value for a quarter for as much of the
missing daily data in the matching deficient quarter(s) as is needed
to make them 75 percent complete, the procedure in section 5.2
yields a recalculated 3-year 1-hour standard ``test design value''
above the level of the standard, then the 1-hour primary standard
design value is deemed to have passed the diagnostic test and is
valid, and the level of the standard is deemed to have been exceeded
in that 3-year period. As noted in section 3.2(c)(i), in such a
case, the 3-year design value based on the data actually reported,
not the ``test design value'', shall be used as the valid design
value.
(d) A 1-hour primary standard design value based on data that do
not meet the completeness criteria stated in 3.2(b) and also do not
satisfy section 3.2(c), may also be considered valid 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 procedures for calculating the 1-hour primary standard
design values are given in section 5.2 of this appendix.
4. Rounding Conventions
4.1 Rounding Conventions for the Annual Primary NO2
NAAQS
(a) Hourly NO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
(b) The annual primary standard design value is calculated
pursuant to section 5.1 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).
4.2 Rounding Conventions for the 1-Hour Primary NO2
NAAQS
(a) Hourly NO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
(b) Daily maximum 1-hour values and therefore the annual 4th
highest of those daily values are not rounded.
(c) The 1-hour primary standard design value is calculated
pursuant to section 5.2 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).
5. Calculation Procedures for the Primary NO2 NAAQS
5.1 Procedures for the Annual Primary NO2 NAAQS
(a) When the data for a site and year meet the data completeness
requirements in section 3.1(b) of this appendix, or if the
Administrator exercises the discretionary authority in section
3.1(c), the annual mean is simply the arithmetic average of all of
the reported 1-hour values.
[[Page 34463]]
(b) The annual primary standard design value for a site is the
valid annual mean rounded according to the conventions in section
4.1.
5.2 Calculation Procedures for the 1-Hour Primary NO2
NAAQS
(a) Procedure for identifying annual 99th percentile values.
When the data for a particular site and year meet the data
completeness requirements in section 3.2(b), or if one of the
conditions of section 3.2(c) is met, or if the Administrator
exercises the discretionary authority in section 3.2(d),
identification of annual 99th percentile values will be based on the
number of days with at least 75 percent of the hourly values
reported.
(i) For the year, from only the days with at least 75 percent of
the hourly values reported, select from each day the highest hourly
value.
(ii) Sort all the valid daily values from a particular site and
year by descending value. (For example: (x[1], x[2], x[3], * * *,
x[n]). In this case, x[1] is the largest number and x[n] is the
smallest value.) The 99th percentile is determined from this sorted
series of daily values which is ordered from the highest to the
lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days with
valid data for year y (cny). The corresponding ``n''
value in the right column identifies the rank of the annual 99th
percentile value in the descending sorted list of daily site values
for year y. Thus, P0.99, y= the nth largest value.
Table 1--to Section 5.2(a)(ii)
------------------------------------------------------------------------
P0.99, y is the
nth maximum
Annual number of days with valid data for year ``y'' value of the
(cny) year, where n is
the listed
number
------------------------------------------------------------------------
1-100................................................. 1
101-200............................................... 2
201-300............................................... 3
301-366............................................... 4
------------------------------------------------------------------------
(b) The 1-hour primary standard design value for a site is mean
of the three annual 4th highest values, rounded according to the
conventions in section 4.2.
PART 58--AMBIENT AIR QUALITY SURVEILLANCE
5. The authority citation for part 58 continues to read as follows:
Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.
Subpart A [AMENDED]
6. Section 58.1 is amended by adding definitions for ``AADP'' and
``Near-road NO2 Monitor'' in alphabetical order to read as
follows:
Sec. 58.1 Definitions.
* * * * *
AADT means the annual average daily traffic.
* * * * *
Near-road NO2 Monitor means any NO2 monitor
meeting the specifications in 4.3.2 of Appendix D and paragraphs 2,
4(b), 6.1, and 6.4 of Appendix E of this part.
* * * * *
Subpart B [AMENDED]
7. Section 58.10, is amended by adding paragraphs (a)(5) and
(b)(12) to read as follows:
Sec. 58.10 Annual monitoring network plan and periodic network
assessment.
(a) * * *
(5) A plan for establishing NO2 monitoring sites in
accordance with the requirements of appendix D to this part shall be
submitted to the Administrator by July 1, 2011. The plan shall provide
for all required stations to be operational by January 1, 2013.
* * * * *
(b) * * *
(12) The identification of required NO2 monitors as
either near-road or area-wide sites in accordance with Appendix D,
Section 4.3 of this part.
* * * * *
8. Section 58.13 is amended by adding paragraph (c) to read as
follows:
Sec. 58.13 Monitoring network completion.
* * * * *
(c) The network of NO2 monitors must be physically
established no later than January 1, 2013, and at that time, must be
operating under all of the requirements of this part, including the
requirements of appendices A, C, D, E, and G to this part.
9. 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 PM2.5FRM/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, PAMS, and near-road NO2 monitoring 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). The State, or where
appropriate, local agency, may report site specific meteorological
measurements generated by onsite equipment (meteorological instruments,
or sampler generated) or measurements from the nearest airport
reporting ambient pressure and temperature. 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.
* * * * *
10. Appendix A to Part 58 is amended as by adding section 2.3.1.5
to read as follows:
Appendix A to Part 58--Quality Assurance Requirements for SLAMS, SPMs
and PSD Air Monitoring
* * * * *
2.3.1.5 Measurement Uncertainty for NO2. The goal for
acceptable measurement uncertainty is defined for precision as an
upper 90 percent confidence limit for the coefficient of variation
(CV) of 15 percent and for bias as an upper 95 percent confidence
limit for the absolute bias of 15 percent.
* * * * *
11. Appendix C to Part 58 is amended as by adding section 2.1.1 to
read as follows:
Appendix C to Part 58--Ambient Air Quality Monitoring Methodology
* * * * *
2.1.1 Any NO2 FRM or FEM used for making primary
NAAQS decisions must be capable of providing hourly averaged
concentration data.
* * * * *
12. Appendix D to Part 58 is amended by revising section 4.3 to
read as follows:
Appendix D to Part 58--Network Design Criteria for Ambient Air Quality
Monitoring
* * * * *
4.3 Nitrogen Dioxide (NO2) Design Criteria
4.3.1 General Requirements. (a) State and, where appropriate,
local agencies must operate a minimum number of required
NO2 monitoring sites as described below.
4.3.2 Requirement for Near-road NO2 Monitors. (a)
Within the NO2 network, there must be one microscale
near-road NO2 monitoring station in each CBSA with a
population of 350,000 or more persons to monitor a location of
expected maximum hourly concentrations sited near a major road with
high AADT counts as specified in
[[Page 34464]]
paragraph 4.3.2(a)(1) of this appendix. An additional near-road
NO2 monitoring station is required for any CBSA with a
population of 2,500,000 persons or more, or in any CBSA with a
population of 350,000 or more persons that has one or more roadway
segments with 250,000 or greater AADT counts to monitor a second
location of expected maximum hourly concentrations. CBSA populations
shall be based on the latest available census figures.
(1) The near-road NO2 monitoring stations shall be
selected by ranking all road segments within a CBSA by AADT and then
identifying a location or locations adjacent to those highest ranked
road segments where maximum hourly NO2 concentrations are
expected to be highest and siting criteria can be met in accordance
with appendix E of this part. Where a state or local air monitoring
agency identifies multiple acceptable candidate sites where maximum
hourly NO2 concentrations are expected to occur, the
monitoring agency should consider taking into account the potential
for population exposure in the criteria utilized to select the final
site location. Where one CBSA is required to have two near-road
NO2 monitoring stations, the sites shall be
differentiated from each other by one or more of the following
factors: fleet mix; congestion patterns; terrain; geographic area
within the CBSA; or different route, interstate, or freeway
designation.
(b) Measurements at required near-road NO2 monitor
sites must include at a minimum: NO, NO2, NOX,
wind vector data in the horizontal and vertical planes, ambient
temperature, and ambient relative humidity.
4.3.3 Requirement for Area-wide NO2 Monitoring. (a)
Within the NO2 network, there must be one monitoring
station in each CBSA with a population of 1,000,000 or more persons
to monitor a location of expected highest NO2
concentrations representing the neighborhood or larger spatial
scales. PAMS sites collecting NO2 data that are situated
in an area of expected high NO2 concentrations at the
neighborhood or larger spatial scale may be used to satisfy this
minimum monitoring requirement when the NO2 monitor is
operated year round. Emission inventories and meteorological
analysis should be used to identify the appropriate locations within
a CBSA for locating required area-wide NO2 monitoring
stations. CBSA populations shall be based on the latest available
census figures.
4.3.4 Regional Administrator Required Monitoring. (a) The
Regional Administrator may require additional NO2
monitoring stations above the minimum requirements to monitor in
locations away from roads, or sites that do not meet near-road
NO2 monitor siting criteria noted in appendix E of this
part, where required near-road monitors do not represent a location
or locations where the expected maximum hourly NO2
concentrations exist in a CBSA. The Regional Administrator may also
require additional near-road NO2 monitoring stations
above the minimum required in situations where the minimum
monitoring requirements are not sufficient to meet monitoring
objectives, and may consider additional locations of expected high
NO2 concentrations and the variety of exposure potential
due to increased variety in amount or types of fleet mix, congestion
patterns, terrain, or geographic areas within a CBSA. The Regional
Administrator and the responsible State or local air monitoring
agency should work together to design and/or maintain the most
appropriate NO2 network to service the variety of data
needs for an area.
(b) The Regional Administrator may require additional
NO2 monitoring stations for area-wide NO2
monitors at the neighborhood and larger spatial scales above the
minimum monitoring requirements where the minimum monitoring
requirements are not sufficient to meet monitoring objectives for an
area, such as supporting photochemical pollutant assessment, air
quality forecasting, PM precursor analysis, and characterizing
impacts of NO2 sources on certain communities. The
Regional Administrator and the responsible State or local air
monitoring agency should work together to design and/or maintain the
most appropriate NO2 network to service the variety of
data needs for an area.
4.3.5 NO2 Monitoring Spatial Scales. (a) The most
important spatial scale for near-road NO2 monitoring
stations to effectively characterize the maximum expected hourly
NO2 concentration due to mobile source emissions on major
roadways is the microscale. The most important spatial scales for
other monitoring stations characterizing maximum expected hourly
NO2 concentrations are the microscale and middle scale.
The most important spatial scale for area-wide monitoring of high
NO2 concentrations is the neighborhood scale.
(1) Microscale--This scale would typify areas in close proximity
to major roadways or point and area sources. Emissions from roadways
result in high ground level NO2 concentrations at the
microscale, where concentration gradients generally exhibit a marked
decrease with increasing downwind distance from major roads. As
noted in appendix E of this part, near-road NO2
monitoring stations are required to be within 50 meters of target
road segments in order to measure expected peak concentrations.
Emissions from stationary point and area sources, and non-road
sources may, under certain plume conditions, result in high ground
level concentrations at the microscale. The microscale typically
represents an area impacted by the plume with dimensions extending
up to approximately 100 meters.
(2) Middle scale--This scale generally represents 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 include locations of expected maximum hourly
concentrations due to proximity to major NO2 point, area,
and/or non-road sources.
(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. Emissions from stationary point and area sources may, under
certain plume conditions, result in high NO2
concentrations at the neighborhood scale. Where a neighborhood site
is located away from immediate NO2 sources, the site may
be useful in representing typical air quality values for a larger
residential area, and therefore suitable for population exposure and
trends analyses.
(4) Urban scale--Measurements in this scale would be used to
estimate concentrations over large portions of an urban area with
dimensions from 4 to 50 kilometers. Such measurements would be
useful for assessing trends in area-wide air quality, and hence, the
effectiveness of large-scale air pollution control strategies. Urban
scale sites may also support other monitoring objectives of the
NO2 monitoring network identified in paragraph 4.3.4
above.
4.3.6 NOy Monitoring. (a) NO/NOy
measurements are included within the NCore multipollutant site
requirements and the PAMS program. These NO/NOy
measurements will produce conservative estimates for NO2
that can be used to ensure tracking continued compliance with the
NO2 NAAQS. NO/NOy monitors are used at these
sites because it is important to collect data on total reactive
nitrogen species for understanding O3 photochemistry.
* * * * *
13. Section Appendix E to part 58 is amended as follows:
a. By revising section 2.
b. By adding paragraph (d) to section 4.
c. By revising section 6.1.
d. By adding section 6.4.
e. By revising section 11 including Table E-4.
Appendix E to Part 58--Probe and Monitoring Path Siting Criteria for
Ambient Air Quality Monitoring
* * * * *
2. Horizontal and Vertical Placement
The probe or at least 80 percent of the monitoring path must be
located between 2 and 15 meters above ground level for all ozone and
sulfur dioxide monitoring sites, and for neighborhood or larger
spatial scale Pb, PM10, PM10-2.5,
PM2.5, NO2 and carbon monoxide sites. Middle
scale PM10-2.5 sites are required to have sampler inlets
between 2 and 7 meters above ground level. Microscale Pb,
PM10, PM10-2.5 and PM2.5 sites are
required to have sampler inlets between 2 and 7 meters above ground
level. Microscale near-road NO2 monitoring sites are
required to have sampler inlets between 2 and 7 meters above ground
level. The inlet probes for microscale carbon monoxide monitors that
are being used to measure concentrations near roadways must be
3\1/2\ meters above ground level. The probe or at least
90 percent of the monitoring path must be at least 1 meter
vertically or horizontally away from any supporting structure,
walls, parapets, penthouses, etc., and away from dusty or dirty
areas. If the probe or a significant portion of the monitoring path
is located near the side of a building or wall, then it should be
located on the windward side of the building relative to the
prevailing wind direction during the season of highest
[[Page 34465]]
concentration potential for the pollutant being measured.
* * * * *
4. Spacing From Obstructions
* * * * *
(d) For near-road NO2 monitoring stations, the
monitor probe shall have an unobstructed air flow, where no
obstacles exist at or above the height of the monitor probe, between
the monitor probe and the outside nearest edge of the traffic lanes
of the target road segment.
* * * * *
6. * * *
6.1 Spacing for Ozone Probes and Monitoring Paths. In siting an
O3 analyzer, it is important to minimize destructive
interferences form sources of NO, since NO readily reacts with
O3. Table E-1 of this appendix provides the required
minimum separation distances between a roadway and a probe or, where
applicable, at least 90 percent of a monitoring path for various
ranges of daily roadway traffic. A sampling site having a point
analyzer probe located closer to a roadway than allowed by the Table
E-1 requirements should be classified as microscale or middle scale,
rather than neighborhood or urban scale, since the measurements from
such a site would more closely represent the middle scale. If an
open path analyzer is used at a site, the monitoring path(s) must
not cross over a roadway with an average daily traffic count of
10,000 vehicles per day or more. For those situations where a
monitoring path crosses a roadway with fewer than 10,000 vehicles
per day, monitoring agencies must consider the entire segment of the
monitoring path in the area of potential atmospheric interference
from automobile emissions. Therefore, this calculation must include
the length of the monitoring path over the roadway plus any segments
of the monitoring path that lie in the area between the roadway and
minimum separation distance, as determined from Table E-1 of this
appendix. The sum of these distances must not be greater than 10
percent of the total monitoring path length.
* * * * *
6.4 Spacing for Nitrogen Dioxide (NO2) Probes and
Monitoring Paths (a) In siting near-road NO2 monitors as
required in paragraph 4.3.2 of appendix D of this part, the monitor
probe shall be as near as practicable to the outside nearest edge of
the traffic lanes of the target road segment; but shall not be
located at a distance greater than 50 meters, in the horizontal,
from the outside nearest edge of the traffic lanes of the target
road segment.
(b) In siting NO2 monitors for neighborhood and
larger scale monitoring, it is important to minimize near-road
influences. Table E-1 of this appendix provides the required minimum
separation distances between a roadway and a probe or, where
applicable, at least 90 percent of a monitoring path for various
ranges of daily roadway traffic. A sampling site having a point
analyzer probe located closer to a roadway than allowed by the Table
E-1 requirements should be classified as microscale or middle scale
rather than neighborhood or urban scale. If an open path analyzer is
used at a site, the monitoring path(s) must not cross over a roadway
with an average daily traffic count of 10,000 vehicles per day or
more. For those situations where a monitoring path crosses a roadway
with fewer than 10,000 vehicles per day, monitoring agencies must
consider the entire segment of the monitoring path in the area of
potential atmospheric interference form automobile emissions.
Therefore, this calculation must include the length of the
monitoring path over the roadway plus any segments of the monitoring
path that lie in the area between the roadway and minimum separation
distance, as determined from Table E-1 of this appendix. The sum of
these distances must not be greater than 10 percent of the total
monitoring path length.
* * * * *
11. Summary
Table E-4 of this appendix presents a summary of the general
requirements for probe and monitoring path siting criteria with
respect to distances and heights. It is apparent from Table E-4 that
different elevation distances above the ground are shown for the
various pollutants. The discussion in this appendix for each of the
pollutants describes reasons for elevating the monitor, probe, or
monitoring path. The differences in the specified range of heights
are based on the vertical concentration gradients. For CO and near-
road NO2 monitors, the gradients in the vertical
direction are very large for the microscale, so a small range of
heights are used. The upper limit of 15 meters is specified for the
consistency between pollutants and to allow the use of a single
manifold or monitoring path for monitoring more than one pollutant.
Table E-4 of Appendix E to Part 58--Summary of Probe and Monitoring Path Siting Criteria
--------------------------------------------------------------------------------------------------------------------------------------------------------
Horizontal and
vertical distance
Height from ground from supporting Distance from trees Distance from roadways
Pollutant Scale (maximum monitoring to probe, inlet or structures \2\ to to probe, inlet or to probe, inlet or
path length, meters) 80% of monitoring probe, inlet or 90% 90% of monitoring monitoring path \1\
path \1\ of monitoring path path \1\ (meters) (meters)
\1\ (meters)
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2 3, 4, 5, 6................... Middle (300 m) 2-15................ > 1................. > 10................ N/A.
Neighborhood Urban, and
Regional (1 km).
CO 4, 5, 7....................... Micro, middle (300 m) 3\1/2\: 2-15........ > 1................. > 10................ 2-10; see Table E-2 of
Neighborhood (1 km). this appendix for
middle and neighborhood
scales.
O3 3, 4, 5....................... Middle (300 m) 2-15................ > 1................. > 10................ See Table E-1 of this
Neighborhood, Urban, and appendix for all
Regional (1 km). scales.
NO2 \3,4,5\...................... Micro (Near-road [50- 2-7 (micro)......... > 1................. > 10................ <= 50 meters for near-
300]). road microscale.
Middle (300m)............ 2-15 (all other
scales).
Neighborhood, Urban, and See Table E-1 of this
Regional (1 km). appendix for all other
scales.
Ozone precursors (for PAMS) 3, 4, Neighborhood and Urban (1 2-15................ > 1................. > 10................ See Table E-4 of this
5. km). appendix for all
scales.
PM,Pb 3, 4, 5, 6, 8.............. Micro: Middle, 2-7 (micro); 2-7 > 2 (all scales, > 10 (all scales)... 2-10 (micro); see Figure
Neighborhood, Urban and (middle PM10 2.5); horizontal distance E-1 of this appendix
Regional. 2-15 (all other only). for all other scales.
scales).
--------------------------------------------------------------------------------------------------------------------------------------------------------
N/A--Not applicable.
\1\ Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and regional
scale NO2 monitoring, and all applicable scales for monitoring SO2,O3, and O3 precursors.
\2\ When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof.
\3\ Should be > 20 meters from the dripline of tree(s) and must be 10 meters from the dripline when the tree(s) act as an obstruction.
[[Page 34466]]
\4\ Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes
above the sampler, probe, or monitoring path. Sites not meeting this criterion may be classified as middle scale (see text).
\5\ Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a wall.
\6\ The probe, sampler, or monitoring path should be away from minor sources, such as furnace or incineration flues. The separation distance is
dependent on the height of the minor source's emission point (such as a flue), the type of fuel or waste burned, and the quality of the fuel (sulfur,
ash, or lead content). This criterion is designed to avoid undue influences from minor sources.
\7\ For microscale CO monitoring sites, the probe must be > 10 meters from a street intersection and preferably at a midblock location.
\8\ Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 200 liters/min or at least 1 meter
apart for samplers having flow rates less than 200 liters/min to preclude airflow interference.
* * * * *
14. Appendix G to Part 58 is amended by revising section 9 and
table 2 to read as follows:
Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily
Reporting
* * * * *
9. How Does the AQI Relate to Air Pollution Levels?
For each pollutant, the AQI transforms ambient concentrations to
a scale from 0 to 500. The AQI is keyed as appropriate to the
national ambient air quality standards (NAAQS) for each pollutant.
In most cases, the index value of 100 is associated with the
numerical level of the short-term (i.e., averaging time of 24-hours
or less) standard for each pollutant. The index value of 50 is
associated with one of the following: The numerical level of the
annual standard for a pollutant, if there is one; one-half the level
of the short-term standard for the pollutant; or the level at which
it is appropriate to begin to provide guidance on cautionary
language. Higher categories of the index are based on increasingly
serious health effects that affect increasing proportions of the
population. An index value is calculated each day for each pollutant
(as described in section 12 of this appendix), unless that pollutant
is specifically excluded (see section 8 of this appendix). The
pollutant with the highest index value for the day is the
``critical'' pollutant, and must be included in the daily AQI
report. As a result, the AQI for any given day is equal to the index
value of the critical pollutant for that day. For the purposes of
reporting the AQI, the indexes for PM10 and
PM2.5 are to be considered separately.
* * * * *
Table 2--Breakpoints for the AQI
--------------------------------------------------------------------------------------------------------------------------------------------------------
These breakpoints Equal these AQI's
--------------------------------------------------------------------------------------------------------------------------------------------------------
PM2.5 PM10
O3 (ppm) 8-hour O3 (ppm) 1- ([mu]g/ ([mu]g/ CO (ppm) SO2 (ppm) NO2 (ppm) 1- AQI Category
hour\1\ m\3\) m\3\) hour
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.000-0.059......................... ........... 0.0-15.4 0-54 0.0-4.4 0.000-0.034 0-(0.040-0.053 0-50 Good.
....... )
0.060-0.075......................... ........... 15.5-40.4 55-154 4.5-9.4 0.035-0.144 (0.041-0.054)- 51-100 Moderate.
....... (0.080-0.100)
0.076-0.095......................... 0.125-0.164 40.5-65.4 155-254 9.5-12.4 0.145-0.224 (0.081-0.101)- 101-150 Unhealthy for Sensitive
(0.360-0.370) Groups.
0.096-0.115......................... 0.165-0.204 3 65.5- 255-354 12.5-15.4 0.225-0.304 (0.361-0.371)- 151-200 Unhealthy.
150.4 0.64
0.116-0.374......................... 0.205-0.404 3 150.5- 355-424 15.5-30.4 0.305-0.604 0.65-1.24 201-300 Very Unhealthy.
250.4
(\2\)............................... 0.405-0.504 3 250.5- 425-504 30.5-40.4 0.605-0.804 1.25-1.64 301-400 Hazardous.
350.4
(\2\)............................... 0.505-0.604 3 350.5- 505-604 40.5-50.4 0.805-1.004 1.65-2.04 401-500 Hazardous.
500.4
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\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour
ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be
calculated, and the maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>=301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations.
\3\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
[FR Doc. E9-15944 Filed 7-14-09; 8:45 am]
BILLING CODE 6560-50-P