Primary National Ambient Air Quality Standard for Nitrogen Dioxide, 34404-34466 [E9-15944]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 74, Number 134 (Wednesday, July 15, 2009)]
[Proposed Rules]
[Pages 34404-34466]
From the Federal Register Online via the Government Printing 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:
     https://www.regulations.gov: Follow the on-line 
instructions for submitting comments.
     E-mail: a-and-r-Docket@epa.gov.
     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 
https://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 https://www.regulations.gov or e-mail. The https://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 https://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 https://www.epa.gov/epahome/dockets.htm.
    Docket: All documents in the docket are listed in the https://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 https://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: jenkins.scott@epa.gov.

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 
https://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 https://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 https://www.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_pd.html, the Integrated Science Assessment (ISA), available at https://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=194645, and the Risk and 
Exposure Assessment (REA), available at https://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.
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    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

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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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    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.
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    \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.
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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 indoo