[Federal Register Volume 74, Number 134 (Wednesday, July 15, 2009)]
[Proposed Rules]
[Pages 34403-34466]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E9-15944]



[[Page 34403]]

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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: 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 
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: 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 
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.
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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

[[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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    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.
---------------------------------------------------------------------------

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:
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

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:
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

     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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

    \27\ Criteria pollutants are those pollutants for which EPA has 
established a NAAQS under section 109 of the CAA.
---------------------------------------------------------------------------

    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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contaminants and emergency department visits for asthma in the Bronx 
and Manhattan. Prepared for: The U.S. Department of Health and Human 
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Nunnermacker, LJ, Imre D, Daum PH, Kleinman L, Lee YN, Lee JH, 
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(1998). Characterization of the Nashville urban plume on July 3 and 
July 18, 1995. J. Geophys. Res. [Atmos.] 103:28,129-28,148.
Oftedal, B, Brunekreef B, Nystad W, Madsen C, Walker SE, Nafstad P. 
(2008). Residential outdoor air pollution and lung function in 
schoolchildren. Epidemiology 19:129-137.
Ostro, B, Lipsett M, Mann J, Braxton-Owens H, White M. (2001). Air 
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Rizzo (2008). Investigation of how distributions of hourly nitrogen 
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Nitrogen Dioxide NAAQS Review Docket (EPA-HQ-OAR-2006-
0922). Available at http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_rea.html.
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[[Page 34459]]

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Health. 7:89.

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
--------------------------------------------------------------------------------------------------------------------------------------------------------
\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]
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