1

Cover Sheet for Transmitting Documents under E.O. 12866 

Information about the Regulatory Action Related to This Document 

Title of the Regulatory Action: Primary National Ambient Air Quality Standard for Nitrogen 

Dioxide 

Regulatory Identifier Number (RIN): 2060-AO19 

Stage of Action: Proposed rule 

Information about This Document 

Document Creation Date: May 22, 2009 

Document Type: 

(X) Draft preamble and regulatory text (i.e., this is the regulatory action) 

( ) Draft Technical Support Document 

( ) Draft Regulatory Impact Analyses 

( ) Draft Information Collection Request 

( ) Other: _________________________________________________ 

Document Version: 

(X) Version sent to OMB for E.O. 12866 review 

( ) Version substantively changed as a result of E.O. 12866 review 
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6560-50-P 

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, 58

 [EPA-HQ-OAR-2006-0922; FRL- ] 

Primary National Ambient Air Quality Standard for Nitrogen Dioxide

AGENCY: Environmental Protection Agency (EPA). 

ACTION: Proposed Rule 

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 125 ppb. EPA also proposes to 

establish requirements for an NO2 monitoring network that will include monitors near major 

roadways. 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 [insert date 60 days after date of 

publication in the Federal Register]. Under the Paperwork Reduction Act, comments on the 

information collection provisions must be received by OMB on or before {insert date thirty 
3

days after date of publication in the Federal Register.}

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: 

• www.regulations.gov: Follow the on-line instructions for submitting comments. 

• Email: 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 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 www.regulations.gov or e-mail. The 

www.regulations.gov website is an “anonymous access” system, which means EPA will not 
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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 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 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 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; 
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telephone: 919-541-1167; fax: 919-541-0237; email: 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 

www.regulations.gov or email. 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 NO2

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 
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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 considerations from the REA 

 d. CASAC views 

 e. Administrator’s conclusions on level 

G. Summary of proposed decisions on the primary standard 

III. Proposed Amendments to Ambient Monitoring and Reporting Requirements 
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 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 NAAQS for NO2

 1. Annual primary standard 

 2. 1-hour primary standard based on the 99th percentile value form 

 3. 1-hour standard based on the annual 4th highest daily value form 

 C. Exceptional Events Information Submission Schedule 

V. Clean Air Act Implementation Requirements for NO2 NAAQS 

A. Designations for the NO2 NAAQS 

B. Classifications 

C. Attainment Dates 

1. Attaining the NO2 NAAQS 

2. Consequences of failing to attain by the Statutory Attainment Date 

D. Section 110(a)(2) NO2 NAAQS Infrastructure Requirements 

E. Attainment Planning Requirements for NO2

 1. NO2 Nonattainment Area SIPs 

2. New Source Review and Prevention of Significant Deterioration 

Requirements 

 3. General Conformity 

 4. Transportation Conformity 

 5. Transition from the Existing NO2 NAAQS to a Revised NO2 NAAQS 

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 
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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 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). 

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 

 

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. 
10

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 
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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 
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pollution controls on sources that emit oxides of nitrogen (NOX

3

). 

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 NO2 

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). 

 

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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 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 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 
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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 
15

quality standards: indicator, averaging time, form4, 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). 

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 

 

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. 
16

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 
17

characterizations of health effects attributable to exposure to ambient NO2, the review of this 

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 NO2 monitoring network 

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 

 

5

 It should be noted that the ISA Section 2.4.1references 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 
18

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. 

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 

 

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. 
19

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 exposure 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 
20

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 areawide 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 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 
21

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 (sees 

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, 
22

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 
23

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 
24

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 cooccurring 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, 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 
25

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
26

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. 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. 

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. 

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 

 

6

 Effect estimates in the ISA were standardized to a 30 ppb increase in NO2 for studies that evaluated 1-hour daily 

maximum NO2 concentrations and to a 20 ppb increase for studies that evaluated 24-hour average concentrations. 
27

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 
28

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 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 asthmarelated 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. 
29

(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, 
30

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, 
31

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 singlepollutant 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). 
32

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 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. 
33

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 subchronic (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 
34

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 (results combined only 

from nonspecific responsiveness studies) are presented in the ISA (Table 3.1-3). 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. 
35

However, the results do suggest that short-term (i.e., 30-min to 3-h) exposures to NO2 at nearambient levels (<300 ppb) can alter airway responsiveness in people with mild asthma (ISA, 

section 3.1.3.2). 

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 allergeninduced inflammatory response. Compared with air followed by allergen, NO2 followed by 

allergen caused an 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 
36

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 
37

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 
38

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, 
39

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 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, 
40

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 
41

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 
42

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 
43

Specific groups within the general population are likely at increased risk 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 
44

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. 
45

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 
46

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 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, 
47

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 
48

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 abovemedian 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)7. Furthermore, in 

 

7

 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 1A-6 from that report (http://www.census.gov/hhes/www/housing/ahs/ahs07/tab1a-6.pdf), out of 128,203,000 

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 
49

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 atrisk for health effects associated with exposure to ambient NO2. The size of the potentially atrisk population suggests that exposure to ambient NO2 could have a significant impact on public 

health in the United States. 

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 

 

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. 
50

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 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 
51

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 multipollutant models. Taking these factors into consideration, the epidemiology-based risk 
52

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 allergeninduced 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
53

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 
54

that there is no sharp 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 
55

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. 
56

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 

recommended dispersion model, was used to estimate 1-hour ambient NO2 concentrations using 

emissions estimates from stationary and on-road mobile sources.8

 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 

 

8

 Estimated emissions from Hartsfield International Airport in Atlanta, a non-road mobile source, were also included 

in this analysis. 
57

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. 

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 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 
58

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 concentrationresponse 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 
59

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 
60

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 
61

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. 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 overpredicting upper percentile NO2 exposures. Other approaches used to evaluate exposure results 

(i.e., comparison to personal exposure monitoring results and comparison of exposure-toambient 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 
62

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 
63

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 copollutants 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 
64

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 with the scientific data assessed in the Criteria Document (EPA, 
65

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 
66

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 
67

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? 
68

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? 

• 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? 
69

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. 
70

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 statisticallysignificant, 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 
71

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 
72

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 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 
73

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 concentrationresponse 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 respiratoryrelated 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 
74

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 
75

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 longterm 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
76

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 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 atrisk groups, from NO2-related adverse health effects associated with short-term, and potential 

long-term, exposures. 

4. CASAC views 
77

 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 
78

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
79

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 department visits, and hospital admissions as well as an updated meta-analysis of 

controlled human exposure studies of airway responsiveness in asthmatics. 
80

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 percentile9

) 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 

 

9

 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. 
81

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. 

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 
82

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-
83

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 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 longterm 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 areawide 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. 
84

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. 
85

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 
86

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, D.C. (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. 
87

 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 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 

Monitoring studies show that a gradient in NO2 concentrations exists around roadways with 

maximum concentrations occurring on or near major roadways and decreasing relatively quickly 

with distance from the road. These monitoring studies suggest that NO2 concentrations within 

50 m of a major roadway can be approximately 30 to 100% higher than concentrations at areawide monitors located away from major roadways. Similarly, the REA used data comparison 

and modeling approaches to estimate that NO2 concentrations on or near roads could be 
88

approximately 40% or 80% higher than concentrations away from roads, on average across 

multiple locations (see section II.A.2 above). In addition, the gradient of NO2 concentrations 

around roadways would not be expected to change substantially even as the absolute 

concentrations of NO2 continue to decrease due to ongoing implementation of mobile source 

standards (see section I.B). That is, as NO2 concentrations around roadways change, 

approximately proportional changes would also be expected farther away from those roadways. 

In identifying an appropriate range of standard levels to propose for a 1-hour standard, 

the Administrator notes the need to protect at-risk individuals against the distribution of exposure 

concentrations across an area, including both the higher peak exposure concentrations that can 

occur on or near major roadways and the lower exposure concentrations that can occur generally 

across an area away from roadways, which can also reflect exposures related to stationary source 

emissions. In considering how to address the health risks associated with exposures to these 

different concentrations, our evaluation has been informed by information drawn from the 

different types of scientific studies. 

Specifically, 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 they occur in an area. They most directly inform 

consideration of the risks associated with exposure to the higher concentrations of NO2 expected 

to occur on or near roadways, which could include concentrations within the range evaluated in 

the human exposure studies. These human exposure studies are less informative in evaluating 

the risks associated with exposures to the lower concentrations of NO2 expected away from 

roadways, as these concentrations are typically lower than those evaluated in the studies. 
89

In addition, epidemiologic studies have reported associations between the patterns of NO2

air quality across an area, as indexed by the lower concentrations measured at area-wide 

monitors in the current network, and respiratory symptoms, emergency department visits, and 

hospital admissions. The health effects reported in the epidemiologic studies are reasonably 

inferred to be associated with exposure to both the higher short-term peak NO2 concentrations 

that could occur on or near major roadways and the lower short-term concentrations that could 

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 Administrator has considered two approaches to setting the level of a 1-hour NO2

primary NAAQS in order to provide appropriate protection from the adverse health effects 

associated with short-term exposures to ambient NO2. In one approach (hereafter referred to as 

the near-road approach), the level of the NO2 primary NAAQS would reflect the maximum 

allowable NO2 concentration anywhere in the area, a concentration that would typically be 

expected to occur on or near major roadways.10 The level of a standard under the near-road 

approach would be set such that, when available information regarding the concentration 

gradient around roadways is considered, appropriate protection would be provided against the 

higher exposure concentrations expected to occur on and near major roadways, as well as the 

lower exposure concentrations expected to occur away from those roadways (i.e., area-wide 

concentrations as defined below). In the other approach (hereafter referred to as the area-wide 

approach), the level of the NO2 primary NAAQS would reflect a maximum allowable area-wide 

NO2 concentration. For convenience, the term “area-wide concentration” will be used to refer to 

ambient concentrations that occur away from major roadways, such as those concentrations 

 

10 For ease of discussion,, the phrase “maximum allowable concentration near major roadways” will be used to refer 

to the maximum allowable concentration anywhere in an area, given that we would expect maximum concentrations 

across an area to typically occur near major roadways. 
90

measured by monitors sited for the purpose of monitoring area-wide concentrations as in the 

current monitoring network (see sections II.A.1, II.F.4.e and III.B.3). Given the NO2

concentration gradients that exist around roadways, a range of higher near-roadway 

concentrations would likely be associated with a given area-wide concentration. The level of a 

standard under the area-wide approach would be set such that, when available information 

regarding the concentration gradient around roadways is considered, appropriate protection 

would be provided against exposures to area-wide NO2 concentrations as well as the higher NO2

concentrations expected to occur on and near major roadways. 

In considering these two approaches, the Administrator notes that, in order to achieve a 

given degree of public health protection, the appropriate standard level under the area-wide 

approach would be lower than the appropriate standard level under the near-road approach.11 

The magnitude of the difference between the levels would depend on the NO2 concentration 

gradient around roadways. This issue is discussed in more detail below (section II.E.4.e). 

The remainder of this section describes the considerations relevant to the Administrator’s 

proposed decision on standard level. Specifically, evidence-based considerations drawn from the 

ISA and discussed in the policy-assessment chapter of the REA are discussed in section II.E.4.a. 

Exposure- and risk-based considerations drawn from the analyses in the REA and discussed in 

the policy assessment chapter are discussed in section II.E.4.b. A summary of the considerations 

from the policy assessment chapter of the REA is presented in section II.E.4.c and CASAC 

views expressed in the context of their comments on the final REA are presented in section 

II.E.4.d. Finally, the Administrator’s proposed conclusions regarding standard level are 

presented in section II.E.4.e. 

 

11 Thus, the remainder of this section will distinguish between appropriate standard levels reflecting either a 

maximum allowable area-wide concentration or a maximum allowable concentration anywhere in a given area (e.g., 

likely to occur near major roads). 
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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 standard level, the policy assessment chapter of the REA notes that NO2

concentrations represent different measures of exposure when drawn from experimental versus 

epidemiologic studies. NO2 concentrations tested in experimental studies, such as controlled 

human exposure studies, represent exposure levels 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 the ambient monitors used in 

these studies are used as surrogates for the distribution of NO2 exposures across the study area 

and over the time period of the study. These ambient monitors do not directly measure the full 

range of concentrations in an area, instead they capture one part of the distribution (the area-wide 

concentration) and this is used as a surrogate for the entire distribution. Thus, the area-wide 

monitored NO2 concentrations associated with the studies would not be expected to directly 

identify the full range of personal exposure levels that are responsible for the NO2-associated 

health effects reported in the studies. 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.E.4.e. 

In considering the epidemiologic evidence, the REA noted the ISA conclusion that 
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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. studies, and are reasonably 

inferred to be associated with exposure to both the higher short-term peak NO2 concentrations 

that could occur on or near roadways and the lower short-term concentrations that could occur 

away from roadways. The reported effects are logically linked to the types of peak exposures 

that can occur on or near major roadways and that have been reported in controlled human 

exposure studies to increase airway responsiveness in asthmatics. 

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, they 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 1 and 2 below (REA, Figures 5-1 and 5-2) show standardized effect estimates 
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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 roadways in these locations during the 

study periods could be substantially higher than the concentrations recorded at these area-wide 

monitors (e.g., up to 2-fold higher). 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). These studies reported generally 

positive and often statistically-significant NO2 effect estimates for respiratory-related emergency 

department visits and hospital admissions in a variety of locations across the U.S. Two 

additional studies (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). 
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Figure 1. NO2 effect estimates12 (95% CI) for emergency department visits/hospital 

admissions and 1-hour daily maximum NO2 concentrations (98th and 99th percentile values 

in boxes13) 

 

12Effect estimates presented in Figures 1 and 2 are from single pollutant models. 

13 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 1 and 2 (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 1 and 2 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. 
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Figure 2. NO2 effect estimates for respiratory symptoms and associated 1-hour daily 

maximum NO2 levels (98th and 99th percentile values in boxes) 
96

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 airway responsiveness, host defense and 

immunity, inflammation, and 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 focused on 

increased airway responsiveness in asthmatics when considering the controlled human exposure 

literature in terms of its ability to inform decisions on alternative standard levels. 

With regard to controlled human exposure studies of airway responsiveness, the ISA and 

the REA discussed a meta-analysis of individual level data from 19 studies involving asthmatic 
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volunteers (ISA, Table 3.1-3; REA, section 3.1.3.2 and Table 4-5). This meta-analysis is an 

update of an analysis considered in the 1993 NOX AQCD and published by Folinsbee (1992). 

The studies included in the original meta-analysis by Folinsbee used NO2 exposure levels 

between 100 and 1000 ppb. Most of the studies included in the original meta-analysis used 

nonspecific bronchoconstricting agents such as methacholine, carbachol, histamine, or cold air. 

Compared to asthmatics exposed to NO2 while exercising, a larger percentage of asthmatics 

exposed during rest experienced an NO2-related increase in airway responsiveness. The reason 

for this is not known as the factors that predispose some asthmatics to NO2 responsiveness are 

not understood (ISA, section 3.1.3.2). Among subjects exposed at rest, 76% experienced 

increased airway responsiveness following exposure to NO2 levels between 200 and 300 ppb. 

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 (see ISA, section 3.1.3.2 for more discussion). It also reported 

results for an additional NO2 concentration, 100 ppb. This updated analysis reported that, 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 from 100 to 150 ppb, 75% of asthmatics experienced 

an increase in airway responsiveness following exposure to NO2 concentrations from 200 to 300 

ppb, and 73% of asthmatics experienced an increase in airway responsiveness following 

exposure to NO2 concentrations above 300 ppb. The fraction of asthmatics experiencing an 

increase in airway responsiveness was statistically significant at each of these NO2

concentrations. 

Based on this evidence, we have identified exposure to NO2 at a level of 100 ppb to be 
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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, that 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. Similar to 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 
99

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. 
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Table 1. Summary of results of the exposure and risk analyses presented in the REA
101

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 the level of a 1-hour NO2 standard. The REA did not 

explicitly address whether the range of standard levels supported by the evidence and 

information would differ depending on whether the standard is set in conjunction with the areawide approach (maximum allowable area-wide level) or in conjunction with a near-road 

approach (maximum allowable peak level). However, the analyses presented in the REA were 

premised on the use of area-wide monitors. That is, in simulating just meeting different 

standards, NO2 concentrations were adjusted at monitors in the current area-wide network. 

Therefore, the estimates of potential public health risks associated with each standard evaluated 

in the REA are applicable to the area-wide approach. Further, while the REA did not explicitly 

address whether its conclusions were more appropriately applied to an area-wide approach 

versus a near-road approach, they are more directly applicable to the former than the latter. The 

implications of the REA conclusions regarding standard level, as they relate to each of the 

approaches considered by the Administrator (i.e., area-wide and near-road), are discussed below 

in section II.B.4.e. 

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) provides evidence for associations 

between short-term ambient NO2 concentrations and respiratory morbidity in a 

location (Alpine, CA) where area-wide NO2 concentrations were well below levels in 

other key U.S. epidemiologic studies. This study reported mostly positive 

associations, some of which were statistically significant, between 1-hour and 8-hour 
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(only 8-hour associations were statistically significant) levels of NO2 and asthma 

symptoms in a location where the 99th and 98th percentile 1-hour daily maximum NO2

concentrations recorded at an area-wide monitor during the time period of the study 

were 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 that could be 

reasonably supported by this controlled human exposure evidence, there were several 

reasons why it was appropriate to consider levels below 100 ppb. First, the metaanalysis 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 area-wide standard levels equal to, rather than below, those measured in the location of 
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the study. Given these considerations, the REA concluded that an appropriate lower end of the 

range of area-wide standard levels supported by the scientific evidence is 50 ppb. 

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/99th percentile 1-hour daily 

maximum NO2 concentrations ranging from 85 to 112 ppb (Peel et al., 2005; NYDOH, 

2006; Ito et al., 2007; Tolbert et al., 2007) (see Figure 1 above). To the extent that 

uncertainties associated with ascribing effects reported in the Delfino study to NO2 are 

emphasized, this cluster of studies provides stronger support for 1-hour standard levels 

close to 100 ppb. 

• The meta-analysis of airway responsiveness presented in the ISA reported increased 

airway responsiveness in most asthmatics 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, others (i.e., the magnitude and clinicalsignificance 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 an area-wide 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-
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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. 

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 area-wide standards. As noted above (section II.C), the REA 

characterized NO2-associated health risks by estimating air quality exceedances of NO2

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 any 

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 an area-wide 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 
105

assessment chapter. 14 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 

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. 

CASAC comments were offered within the context of the final REA, which analyzed the 

potential public health implications of different area-wide standards and did not discuss potential 

differences between area-wide and near-road standards. Thus, it is not clear the degree to which 

CASAC recommendations might differ for a standard level that would be determined with 

respect to maximum allowable near-road NO2 concentrations. As noted in section I.C above, we 

are specifically soliciting CASAC input on the appropriate level, or range of levels, with respect 

to a standard set using the near-road approach, reflecting a maximum allowable NO2

concentration near major roads. 

 

14 Earlier CASAC letters focused on their review of the air quality, exposure, and risk analyses as presented in other 

chapters of the draft REA. 
106

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 

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 

notes the need to protect at-risk individuals from an array of health effects that have been 

associated with NO2 exposures. Specifically, the Administrator has considered the extent to 

which a variety of levels under two separate approaches for setting a 1-hour NO2 standard would 

be expected to protect at-risk individuals against exposures to NO2 concentrations reported in 

controlled human exposure studies to increase airway responsiveness in asthmatics as well as 

against the patterns of NO2 air quality, as indicated by concentrations measured at monitors in 

the current area wide-oriented network, reported in epidemiologic studies to be associated with 

respiratory symptoms, 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 is part of the body of experimental evidence that the ISA recognized as 

supporting the plausibility of associations reported in epidemiologic studies and given that NO2

exposure patterns associated with respiratory morbidity in epidemiologic studies plausibly 

include short-term peak exposures on and/or near major roadways that could increase airway 

responsiveness in asthmatics. As discussed previously, the Administrator has considered two 
107

approaches to setting a 1-hour primary NO2 standard in order to provide appropriate protection 

of public health against this array of effects. 

In one approach (i.e., the area-wide approach), the NO2 primary NAAQS level would 

reflect maximum allowable area-wide NO2 concentrations. The area-wide level under this 

approach would be chosen to protect against health effects associated with the overall pattern of 

NO2 air quality across an area, as indicated by concentrations at area-wide monitors. In addition, 

the level of a standard under this approach would be set such that, when available information 

regarding the concentration gradient around roadways is considered, appropriate protection 

would also be provided against the higher NO2 exposures on and near major roadways. For 

purposes of determining compliance with the NAAQS, it would be appropriate to compare the 

level of such an area-wide standard to area-wide NO2 concentrations (see section III.B.2.c). 

Comparing a standard level set under this approach to NO2 concentrations that occur near major 

roadways, where considerably higher concentrations would be expected, would have the effect of 

increasing the stringency of the area-wide standard relative to what was determined to be 

requisite in setting such a standard. 

In the other approach (i.e. the near road approach), the NO2 primary NAAQS level would 

reflect maximum allowable NO2 concentrations anywhere in an area, which we would expect to 

occur near major roadways. Under this approach, the standard level would be chosen to protect 

against health effects associated with the higher NO2 exposures expected on and near major 

roadways. In addition, the level of a standard under this approach would be set such that, when 

available information regarding the concentration gradient around roadways is considered, 

appropriate protection would also be provided against exposure to the overall distribution of NO2

air quality, including the lower concentrations expected to occur away from major roadways. 
108

In considering these two approaches, the Administrator notes that area-wide 

concentrations would be expected to be lower in a given area than near-road concentrations in 

that same area because of the NO2 concentration gradient around roadways. Therefore, in order 

to achieve a given degree of public health protection, the appropriate standard level would differ 

depending on the approach chosen. Specifically, in order to provide a comparable level of public 

health protection, the level of a standard reflecting maximum allowable area-wide NO2

concentrations would be lower than the level of a standard reflecting maximum allowable NO2

concentrations near major roadways.15 

As discussed above, the magnitude of the difference between the appropriate levels under 

these two approaches depends on the nature and magnitude of the NO2 concentration gradient 

around roadways. In considering this issue, the Administrator notes that (1) monitoring studies 

suggest that NO2 concentrations within 50 m of a major roadway can be approximately 30 to 

100% higher than concentrations away from major roadways and (2) estimates from the REA, 

which are based on monitor comparisons and model simulations, suggest that NO2

concentrations on or near roads could be 40% or 80% higher than concentrations away from 

roads, on average across multiple locations. In considering this information, the Administrator 

notes that, in order to provide a comparable degree of public health protection, a standard level 

reflecting maximum allowable NO2 concentrations near major roadways could appropriately be 

from 30 to 100% higher than a level reflecting maximum allowable area-wide NO2

concentrations. Further, the Administrator notes that an assumption that NO2 concentrations on 

or near roadways are approximately 65% higher than concentrations away from roadways would 

 

15 As noted above, for ease of discussion, the phrases maximum allowable peak concentration or maximum 

allowable concentration near roadways will be used to refer to the maximum allowable concentration anywhere in 

an area, which we would expect to occur near major roadways. 
109

reflect a reasonable mid-point based on the available information. The impact of the gradient on 

the proposed range of standard levels is discussed more specifically below. 

The remainder of this section describes the following. First, the Administrator’s 

conclusions are described with regard to the emphasis placed on the different scientific evidence 

and exposure/risk information. Second, the Administrator’s conclusions are described with 

regard to the appropriate range of levels to consider in conjunction with the area-wide approach. 

Third, the Administrator’s conclusions are described with regard to the appropriate range of 

levels to consider in conjunction with the near-road approach. Fourth, the Administrator’s 

conclusions with regard to the strengths and limitations of each approach are described followed 

by the Administrator’s proposed approach to setting a 1-hour primary NO2 standard and her 

range of proposed levels. Finally, the Administrator’s conclusions with regard to the level of the 

annual standard are described. 

To inform the appropriate range of levels to consider under each approach, the 

Administrator has considered the scientific evidence and exposure/risk results. In considering 

the weight to place on different evidence and 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 information. 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 
110

robust in multi-pollutant models and that the evidence supports a direct effect of NO2

exposures on respiratory morbidity, independent of associations with other trafficrelated 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 determined 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 significance 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 concludes that it is appropriate to also 

place some 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 
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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 concludes that 

these analyses are potentially useful for considering the relative levels of public 

health protection that could be provided by specific standard levels. 

To inform the identification of an appropriate range of levels that would reflect maximum 

allowable area-wide NO2 concentrations, the Administrator notes that it is appropriate to 

consider directly the NO2 concentrations measured by monitors in the current area-wide network 

in locations of key U.S. epidemiologic studies, without adjustment to account for the roadway 

gradient. Similarly, it is appropriate to consider directly the health risks estimated in the REA to 

be associated with just meeting different 1-hour standards, as NO2 air quality was simulated in 

these analyses by adjusting NO2 concentrations at monitors in the current area-wide network. In 

contrast, because area-wide concentrations would be related to but lower than the maximum NO2

concentrations likely to occur in a given area (i.e., on or near major roadways), the Administrator 

notes that it is appropriate to consider NO2 concentrations reported to increase airway 

responsiveness in asthmatics within the context of the NO2 concentration gradient that exists 

around roadways. The Administrator’s consideration of this evidence and information is 

discussed in more detail below. 

In considering the epidemiologic evidence as it relates to the level of a standard reflecting 

maximum allowable area-wide NO2 concentrations, the Administrator first notes the NO2

concentrations that have been measured at area-wide monitors in locations where key U.S. 

epidemiologic studies have reported associations with respiratory morbidity. Specifically, the 
112

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. In addition, she notes that 5 key U.S. studies provide stronger 

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. Given that there is 

not an observable threshold concentration below which effects do not occur, the Administrator 

also notes the need to consider how far below these concentrations it is appropriate to set an 

area-wide standard level, taking into consideration the need to provide an adequate margin of 

safety. 

Focusing on the cluster of 5 epidemiologic studies, the Administrator notes that they 

provide support for a standard level below 90 ppb to provide protection against the reported 

effects. Given the mixed results of the Delfino study, the Administrator concludes that it is not 

necessary to set the level below about 50 ppb to provide protection against the effects reported in 

epidemiologic studies. Therefore, for an NO2 primary standard level reflecting maximum 

allowable area-wide concentrations, the Administrator concludes that the epidemiologic evidence 

provides support for standard levels from 50 ppb to below about 90 ppb, though she notes that 

standard levels considerably below 90 ppb could be appropriate in order to provide an adequate 

margin of safety against these effects. 

In considering the controlled human exposure evidence as it relates to the level of a 

standard reflecting maximum allowable area-wide NO2 concentrations, the Administrator notes 

that 100 ppb is the lowest NO2 concentration for which airway responsiveness data are available 

in humans. In considering the implications of this evidence, she also notes the following: 
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• The meta-analysis of controlled human exposure data in the ISA reported increased 

airway responsiveness in asthmatics following exposure to 100 ppb NO2. 

• This meta-analysis does not provide information regarding effects at lower levels, nor 

does it provide any evidence of an effects threshold below which effects would not occur. 

Further, the studies included in the meta-analysis evaluated primarily mild asthmatics 

while more severely affected individuals could respond to lower concentrations. 

Therefore, it is possible that exposure to NO2 concentrations below 100 ppb could 

increase airway responsiveness in some asthmatics. 

• The magnitude of the NO2-induced increase in airway responsiveness, and its clinical 

implications for a given asthmatic, cannot be quantified from the meta-analysis. As 

noted previously, the NO2-induced increase in airway responsiveness may or may not be 

clinically significant for a given asthmatic. 

• The controlled human exposure studies of airway responsiveness evaluated effects 

following actual exposure to NO2. However, if the level of the standard is set with 

respect to maximum allowable area-wide concentrations, individuals are likely to 

experience higher exposure concentrations on or near major roadways. If NO2

concentrations on or near major roads are 65% higher than those away from roads, a 

concentration of 60 ppb measured at an area-wide monitor could correspond 

approximately to 100 ppb on or near the road. 

Given these considerations, for an NO2 primary standard level reflecting maximum 

allowable area-wide concentrations, the Administrator concludes that the controlled human 

exposure evidence provides the greatest support for an area-wide standard level of about 60 ppb. 
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She notes that an area-wide standard level of 60 ppb would generally be expected to limit 

exposures on or near roads to approximately 100 ppb NO2. However, given the variability in the 

NO2 concentration gradient around roadways across locations and time, the Administrator also 

notes that the controlled human exposure evidence could provide support for area-wide standard 

levels as high as 75 ppb, reflecting an assumption that concentrations on/near roadways are 30% 

higher than concentrations away from roadways (100 ppb is approximately 30% higher than 75 

ppb) and as low as 50 ppb, reflecting an assumption that concentrations on/near roadways are 

100% higher than concentrations away from roadways (100 ppb is 100% higher than 50 ppb). 

In considering the implications for standard level of estimates of NO2-associated 

exposures and risks, the Administrator notes the conclusions of the REA. Specifically, she notes 

that of the levels examined, an area-wide NO2 primary NAAQS would need to have a level at or 

below approximately 100 ppb in order to achieve substantial risk reductions compared to the 

current annual standard. Given that the Administrator has concluded that the current standard is 

not requisite to protect public health with an adequate margin of safety (see above), she notes 

that the exposure/risk results provide support for a standard level reflecting maximum allowable 

area-wide concentrations at or below 100 ppb. 

Given all of the above considerations, the Administrator concludes that, for an area-wide 

standard, the greatest support is for a standard level of approximately 60 ppb. With regard to this 

level, the Administrator notes that 60 ppb is well-below peak 1-hour NO2 concentrations in 

multiple U.S. locations where epidemiologic studies have reported positive, and often 

statistically significant, associations between NO2 and emergency department visits and hospital 

admissions, recognizing the substantial weight that is appropriate to place on these studies. The 

Administrator also notes that while it is above the 50 ppb level reported in the Delfino study, the 
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limited inferences to be drawn from the results of that one study, as discussed above, indicate 

that it could be appropriate to set a level significantly below 90 ppb but above 50 ppb. In 

addition, the Administrator notes that an area-wide concentration of 60 ppb could correspond to 

NO2 concentrations on or near a major roadway of approximately 100 ppb (i.e., based on our 

evaluation that it is reasonable to estimate concentrations on or near roadways to be 65% higher 

than concentrations away from those roads), providing protection against roadway-associated 

exposures to NO2 concentrations that have been reported to increase airway responsiveness in 

asthmatics. The Administrator also notes that, compared to the current annual standard, a 1-hour 

area-wide standard with a level of 60 ppb would be estimated to substantially reduce allowable 

risks. Given these considerations, the Administrator concludes that the evidence and other 

information provides the most support for an area-wide 1-hour standard with a level of 60 ppb as 

requisite to protect public health against the array of NO2-associated health effects with an 

adequate margin of safety. 

However, the Administrator also notes that, to the extent a final decision on standard 

level emphasizes the results reported in the study by Delfino and colleagues, the possibility that 

NO2-induced airway responsiveness in asthmatics could occur following exposures below 100 

ppb, or the possibility that NO2 concentrations on/near roadways could be closer to 100% higher 

than area-wide concentrations, the evidence would support setting the level below 60 ppb. 

Specifically, she notes that a standard level of 50 ppb would provide some protection against 

potential respiratory effects in locations with relatively low area-wide NO2 concentrations and 

that a level of 50 ppb would be expected to provide protection against exposure concentrations 

below those that have been reported to increase airway responsiveness in asthmatics (i.e., an 

area-wide concentration of 50 ppb could correspond to a concentration on/near roads of 
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approximately 80 ppb based on our evaluation that it is reasonable to estimate concentrations on 

or near roadways to be 65% higher than concentrations away from those roads). She also notes 

that, if on-/near-roadway concentrations are 100% higher than area-wide concentrations, an areawide standard level of 50 ppb could limit roadway-associated exposures to 100 ppb. 

In addition, the Administrator notes that, to the extent a final decision on standard level 

emphasizes the possibility that NO2 concentrations on/near roadways are closer to 30% higher 

than concentrations measured at area-wide monitors or the uncertainty associated with the 

magnitude and the clinical significance of the NO2-induced increase in airway responsiveness, 

the evidence could support setting the level above 60 ppb. Specifically, if NO2 concentrations 

on/near roads are 30% higher than those measured at area-wide monitors in the current network, 

then an area-wide standard level of 75 ppb would limit roadway-associated exposures to 

concentrations of 100 ppb. However, if the difference in concentrations is greater (e.g., closer to 

65%), the Administrator notes that an area-wide NO2 concentration of 75 ppb could correspond 

to NO2 concentrations greater than 100 ppb on or near roadways (i.e., 125 ppb if concentrations 

on or near roadways are 65% higher than concentrations away from those roads). In addition, 

she notes that a level of 75 ppb would still maintain area-wide NO2 concentrations considerably 

lower than those in the cluster of key U.S. studies that provide evidence for NO2-associated 

emergency department visits and hospital admissions. 

To inform the identification of a range of standard levels reflecting maximum allowable 

NO2 concentrations near major roadways that would provide public health protection equivalent 

to that provided by an area-wide standard level of approximately 60 ppb, the Administrator has 

considered the NO2 concentration gradient around roadways. Specifically, she notes that an 

area-wide NO2 concentration of 60 ppb could correspond to an NO2 concentration on or near a 
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roadway in the same area of approximately 100 ppb, based on our evaluation that it is reasonable 

to estimate concentrations on or near roadways to be 65% higher than concentrations away from 

those roads. Therefore, a near-road standard level of 100 ppb could provide a level of public 

health protection that is equivalent to that provided by an area-wide standard level of 60 ppb. In 

addition, if NO2 concentrations on/near major roads are 30% higher than those measured at areawide monitors in the current network, then a near-roadway standard level of 100 ppb could 

provide a level of public health protection that is equivalent to that provided by an area-wide 

standard level of 75 ppb. However, if the difference in concentrations is greater (e.g., closer to 

100%), the Administrator notes that a near-roadway standard level of 100 ppb could provide a 

level of public health protection equivalent to that provided by an area-wide standard level of 50 

ppb. The Administrator also notes that a near-road standard with a level of 100 ppb could limit 

roadway-associated exposures to NO2 that have been reported to increase airway responsiveness 

in asthmatics. Therefore, with regard to a standard level reflecting maximum allowable NO2

concentrations near major roadways, the Administrator concludes that the greatest support is for 

a level of 100 ppb. 

However, the Administrator notes that levels both below and above 100 ppb may also be 

appropriate to the extent that a different emphasis is placed on the various evidence and 

information and their associated uncertainties. Based on our evaluation that it is reasonable to 

estimate concentrations on or near roadways to be 65% higher than concentrations away from 

those roads, the Administrator concludes that the available evidence and information could 

support a near-road standard level of 80 ppb (approximately 65% higher than 50 ppb) or a level 

as high as 125 ppb (approximately 65% higher than 75). 
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In considering the area-wide and near-road approaches described above, the 

Administrator is mindful that there are strengths and limitations associated with each approach. 

In considering those strengths and limitations, the Administrator notes the following: 

• The near-road approach would provide more certainty than the area-wide approach 

regarding the level of protection provided against peak exposures that can occur on or 

near major roadways. In contrast, the area-wide approach would provide more certainty 

than the near-road approach with regard to the level of protection provided against NO2

exposures indicated by area-wide concentrations. 

• The available scientific evidence and exposure information support the importance of 

roadway-associated exposures for public health. The ISA noted that in-vehicle NO2

exposures could be 2-3 times higher than indicated by ambient monitors in the current 

network. In addition, millions of people in the U.S. live, work, and/or attend school near 

major roadways, which are important sources of NO2. The exposure assessment 

presented in the REA estimated that roadway-associated exposures account for the great 

majority of exposures to peak NO2 concentrations. 

After considering the strengths and limitations of both of the approaches described above, 

the Administrator concludes that the near-roadway approach, in combination with the standard 

levels and ranges of levels discussed below, is the most appropriate approach to revising the NO2

primary NAAQS in order to protect public health with an adequate margin of safety. This 

conclusion emphasizes the degree of certainty regarding the level of protection provided by the 

standard against peak exposures, such as those that can occur on or near major roadways. 

Available information and analyses clearly indicate that roadway-associated exposures account 
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for the great majority of exposures to peak NO2 concentrations, and the near-road approach 

would therefore directly address the great majority of peak exposures, which are expected to be 

the primary contributor to the health effects discussed above. In contrast, given the geographic 

variability in the NO2 concentration gradient around roadways, the level of protection against 

roadway-associated exposures would be more uncertain for any given location under the areawide approach. In addition, the range of near-roadway standard levels described below would 

provide a reasonable degree of certainty that the accompanying area-wide levels could be 

approximately 60 ppb, with significantly more certainty that the area-wide levels would be 

within a range of 50–75 ppb. Therefore, the Administrator proposes to set such a near-roadway 

standard. 

For the reasons stated above, the Administrator proposes to set the level of a new 1-hour 

standard to between 80 ppb and 100 ppb, with 100 ppb appearing to have the greatest support. 

With regard to the standard level, 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 observed NO2-induced increases in 

airway responsiveness, the Administrator proposes to set the standard level no higher than 100 

ppb. In addition, if more weight is put on the risk of potential NO2-associated effects that could 

occur in locations with relatively low area-wide NO2 concentrations, and the possibility that NO2

could increase airway responsiveness in some asthmatics following exposures to concentrations 

below 100 ppb, the Administrator also proposes to set a standard level within a range that 

includes 100 ppb but is no lower than 80 ppb. 
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With regard to this range, the Administrator notes that the levels proposed are consistent 

with the recommendation by CASAC to set a standard level no higher than 100 ppb. However, 

CASAC did not explicitly address whether or how the standard level should differ with a nearroad approach for the standard, as described in this proposal. Since a near-road standard level 

greater than 100 ppb could provide public health protection equivalent to that provided by an 

area-wide standard level below 100 ppb, such as 75 ppb, the Administrator also solicits comment 

on near-road standard levels as high as 125 ppb. The Administrator notes that a near-road 

standard level as high as 125 ppb could be appropriate to the extent that near-road concentrations 

are determined to be closer to 100% higher than area-wide concentrations, or to the extent more 

emphasis is placed on uncertainties associated with the magnitude and clinical significance of the 

NO2-induced increase in airway responsiveness in asthmatics, particularly at the lower 

concentrations evaluated in the controlled human exposure studies. In addition, the 

Administrator notes that a near-road 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. Accordingly, the Administrator also solicits comment on standard levels as low 

as 65 ppb (30% higher than an area-wide concentration of 50 ppb). 

For the reasons discussed above, the Administrator is proposing a level reflecting the 

maximum allowable near-road concentrations, and is not proposing to set the level based on 

maximum allowable area-wide concentrations. 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 
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of the REA, which formed the foundation for CASAC’s recommendations regarding standard 

level, was based on evaluation of data drawn from the current 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 near-road 

NO2 concentration rather than the maximum allowable area-wide NO2 concentration. 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. 

Therefore, EPA also solicits comment on the option of revising the NO2 primary NAAQS such 

that the level would reflect the maximum allowable area-wide NO2 concentration, and on setting 

the level of such a standard within the range of 50 to 75 ppb. 

In addition to soliciting comment generally on this area-wide 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 an areawide standard. As described above, comparing NO2 concentrations measured near major 

roadways to a level meant to reflect the maximum allowable area-wide NO2 concentrations 

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 areawide 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 roadway-associated NO2 concentrations occur within about 

50 m of the 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 
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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 urban 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 above (50m to 200m, and 50m to 500m) serve to further define the distance from 

roadway that would represent concentrations comparable to the area-wide 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 area-wide standard. 

With regard to the annual standard, the Administrator notes that CASAC recommended 

retaining an annual standard in order to provide protection against potential health effects 

associated with long-term exposures. With regard to the level of an annual standard, CASAC 

recommended retaining the current level as the evidence considered did not suggest that either an 

increase or decrease would be appropriate. Given these considerations, and recognizing that a 
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new 1-hour standard level as proposed would also provide some degree of protection from longterm exposures, the Administrator also proposes to retain the current annual standard level. 

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 a1-hour (daily maximum) averaging time and a form defined as the 3-year average of the 

99th percentile or the 4th highest daily maximum concentration The proposed range of levels, 

reflecting maximum allowable concentrations in an area, such as those that occur near roadways, 

is 80 to 100 ppb. In conjunction with this proposed near-road standard, the Administrator also 

solicits comment on levels as low as 65 ppb and as high as 125 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 solicits comment on using the area-wide approach 

to set a new 1-hour standard, and on a range of levels, reflecting maximum allowable area-wide 

concentrations, from 50 to 75 ppb. 
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With regard to the current annual standard, the Administrator proposes to retain the 

current standard to supplement the new short-term standard, such that the combination would 

provide requisite protection for public health with an adequate margin of safety against health 

effects potentially associated with long-term exposures to NO2. 

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, discussed above 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 website (http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalentmethods-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. 
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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 solicit comment on only 

allowing FRM of FEMs capable of providing hourly averaged concentration data to used to 

produce data for comparison to the NAAQS. 

The sum of nitric oxide (NO) and NO2 is commonly referred to as NOX. Nitrogen 

oxides, technically the total reactive nitrogen oxide family, referred to as NOY, is defined as the 

sum of NO, NO2, and the higher nitrogen oxides collectively termed NOZ. Important 

components of ambient NOZ include nitrous acid (HNO2), nitric acid (HNO3), and the 

peroxyacetyl nitrates (PANs). More specifically, NO2 is the indicator of nitrogen oxides for 

purposes of the NAAQS, as noted in section II.F.1, as the specific health effects associated with 

exposure to NO2 make it the component of greatest interest for the nitrogen oxides. 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 ozone (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°C and 400°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 
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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 tested (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 as high as 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 
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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 and comprises a large majority of the 

current operating network. 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 photolyticchemiluminescence 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 
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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 photolyticchemiluminescence 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, 
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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, 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 40016 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: 

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. The 

neighborhood and urban scales listed below have the potential to overlap in applications that 

concern secondarily formed or homogeneously distributed air pollutants. 

 

16 It should be noted that the ISA Section 2.4.1 references a different number of active monitors in the NO2 network. 

The difference between the ISA number 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. 
130

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. 

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 the road (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 primary 

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.17 However, since the reaction rates converting mobile source NO to 

 

17 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 
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NO2 as described above are indicative of a fast process, on the order of a minute (ISA section 

2.2.2), NO2 behaves like a primary pollutant in the near-road environment. It exhibits peak 

concentrations on or closely adjacent to roads, although resulting from the secondary formation 

characteristic of NO2, its decay rate 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, some 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, though this distance is 

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 pollutants of interest noted above, would be best served 

by monitoring as near as practicable to roadways of interest. 

2. Proposed Changes 

In conjunction with the proposed 1-hour NAAQS, based on the near-road approach that 

reflects maximum allowable NO2 concentrations in an area such as those near major roadways, 

and the proposed retention of the current annual NAAQS, we propose a number of changes to 

the NO2 monitoring network. Since the current network is not oriented to address peak 

concentrations in the on-road and near-road environment, the EPA is proposing a two-tier NO2

network design to monitor ambient concentrations of NO2 and assess compliance with the NO2

NAAQS. The two tiers would provide data to be compared to both the 1-hour and annual 

 

20% above the baseline engine NO2 to NOX ratio. Retrofit technologies sold after January 1, 2009 that do not meet 

the NO2 emission limit may not be installed or sold as EPA verified technologies. 
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standards, and would be comprised of 1) monitoring near major roads 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 NO2 concentrations in an area are expected to be due to on-road mobile 

source and dominating stationary source 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 “near-road monitors,” will introduce new requirements for monitoring sites that, 

for a majority of the state and local monitoring networks, currently does not exist. However, the 

monitoring of maximum concentrations of NO2, including the near-road environment, is a 

critical network design component if the ambient monitoring network is to appropriately function 

as a mechanism to support the proposed 1-hour averaged NAAQS and provide determinations on 

violations of the NAAQS. In addition, the EPA recognizes that the establishment of near-road 

monitoring sites may have certain other advantages, particularly by supporting future NAAQS 

reviews, allowing for the tracking of mobile source emission reductions progress, providing 

monitoring infrastructure that may be of use for other criteria pollutants in the multi-pollutant 

paradigm, providing a new data source for public health studies, and supporting scientific studies 

of other criteria pollutants like CO and ultrafine particulate matter and non-criteria pollutants 

such as some air toxics and black carbon. 

The second tier of the proposed network design, the area-wide monitoring component, is 

intended to characterize the highest concentrations of NO2 that typically representing 
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neighborhood and larger spatial scales which can address the wider area impact of NO2 sources 

on populations. Further, a requirement for the continuation of area-wide monitoring of NO2

serves to maintain continuity in collecting area-wide data that has served to inform long-term 

pollutant concentration trends analysis and health and scientific research in the past. 

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 could 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 US 

Department of Transportation (US 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 
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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 because it will give the nearroad monitoring component of the network a greater spatial extent across the country than using 

a higher population threshold, which may be important when we consider that this near-road 

monitoring approach is a new requirement for monitoring NO2, and there are very few data sets 

indicating what levels might be seen in any given area. 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. Based on 2007 Census 

Bureau statistics, this will result in approximately 142 sites in as many CBSAs. 

We also propose that a second near-road monitor be required in CBSAs with a population 

greater than or equal to 2,500,000 million, 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 

US 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 likely to have an increased number of large roads across a potentially 
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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 website 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 metric in the scientific literature, is widely available, and offers 

the most objective and consistent metric available to indicate traffic volumes across the country. 

AADT data is typically available from local Metropolitan Planning Organizations (MPOs), state 

departments of transportation, and from the FHWA’s HPMS. 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. 

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, non-road and on-road mobile 

sources emissions. Such locations might be identified through data analysis, such as the 

evaluation of existing ambient data and/or emissions data. In these situations, we propose that the 

Regional Administrator will have discretion in requiring monitoring to address individual cases, 
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in cooperation with the State and local monitoring agencies, and state and local agencies, of 

course, can choose to monitor additional locations beyond the proposed requirements. 

In summary, the combination of the above proposed minimum monitoring requirement 

thresholds for the near-road monitors as part of the maximum concentration monitoring 

component 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), 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 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. 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 exclusion of an additional or alternative monitoring requirement so that each 

state and territory would have at least one near-road monitoring site. The EPA also recognizes 

that while there are states that would have no required near-road sites with the current proposal, 

there are other states with larger numbers of required near-road sites relative to all the states that 

would have required near-road monitoring sites. Of the 42 states, the District of Columbia, and 

Puerto Rico, which are anticipated to have near-road sites under this proposal, we estimate that 

six states have seven or more CBSAs with required near road monitors: California has 16 

CBSAs, Florida has 12 CBSAs, Texas has 10 CBSAs, Ohio has 8 CBSAs, Pennsylvania has 8 

CBSAs, and North Carolina has 7 CBSAs. The EPA solicits comment on a cap of the number of 
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CBSAs subject to near-road monitoring requirements per state, particularly for a cap of 6 CBSAs 

per state. The use of such a cap would be to allow for a balance between appropriate protection 

through the use of population and traffic-based triggered minimum monitoring requirements and 

the avoidance of overburdening or redundant monitoring requirements. Further, we solicit 

comment on the scenario where a CBSA cap is implemented, if the selection of the 6 CBSAs in 

states with 7 or more CBSAs meeting the population or AADT thresholds proposed to trigger 

near-road monitoring sites should be based on population, the amount of traffic volume, or the 

amount of population estimated to live near major roads in those eligible CBSAs. 

The EPA also solicits comment on the proposal to allow Regional Administrators the 

ability require additional near road monitoring sites to address situations where minimum 

monitoring requirements are not sufficient. An example of requiring an additional near road 

monitor might be a case where a particular community or neighborhood is significantly 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. 

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. 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. The EPA anticipates that these near-road 

monitoring sites will likely be best characterized as microscale, mobile source oriented sites. 

Specifically, near-road monitoring stations must be sited so that the NO2 monitor probe is no 
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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, below the plane of the 

monitoring station, or fill and open elevated roads, 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 above the target road is no closer than 2 meters and does not exceed 

7 meters, 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 nearroad 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, of being 

adjacent to a top ranked AADT road segment. 
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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 gather 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: selecting a road with a different fleet mix, different congestion 

patterns, different terrain, different geographic area within the CBSA, or at minimum, selecting a 

site along a different road, in an effort to avoid having the two sites characterize the same traffic 

when there are potentially other available road segments with different traffic characteristics that 

meet siting criteria for the second near-road monitor. 

The EPA proposes the 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 
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concentrations for mobile source pollutants, therefore monitoring 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, 

which could be due to, at least in part, 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 are not peak concentrations, 

are still elevated above background levels, in meteorological conditions where the site is upwind 

of the target road. 

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. 
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In further support of characterizing the peak NO2 concentrations occurring in the nearroad 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 (σw) and is of particular 

interest if meteorological data are to be collected at near-road monitoring sites, as Venkatram et 

al. (2007) notes that σw 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 identify the magnitude of vehicle induced turbulence, calculating σw in the near road 

environment and providing 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, would 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 of meteorological measurements in general at all required near-road monitoring 

sites. 

b. Area-wide Monitoring at Neighborhood and Larger Spatial Scales 
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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, which shall 

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 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. This 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 total number of NO2 monitoring sites needed to fully 

support the multiple monitoring objectives that neighborhood and larger scale sites can serve in 

addition to determining compliance with the NAAQS may be more than the minimum monitors 

required. These monitoring objective include ambient photochemical pollutant assessment, 
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aiding in ozone forecasting, aiding in PM precursor analysis and PM forecasting, and to 

characterize point and area sources that may be impacting certain communities. We propose that 

EPA Regional Administrators have the ability to require additional area-wide NO2 monitoring 

sites above the minimum monitoring requirements where the minimum monitoring requirements 

for area-wide monitors or the current network 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 impacted by an individual or group 

of sources that does not have an NO2 monitor as part of the minimum monitoring requirements. 

It is expected that a Regional Administrator and the responsible State or local air monitoring 

agency will need to work together to design and/or maintain the most appropriate area-wide NO2

network component to service the variety of data needs in a given 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 based on a case by case basis, to address situations where the 

minimum monitoring requirements for area-wide monitoring sites are not sufficient. 

3. Solicitation for Comment on an Alternative Network Design 

In conjunction with the option of setting a new short-term NO2 primary NAAQS such 

that the level would reflect the maximum allowable area-wide, rather than near-road, NO2

concentrations that EPA is soliciting comment on but not proposing, we solicit comment on an 

alternative network design where near road monitors are not specifically included in the 

minimum monitoring requirements. This alternative is similar in concept to the second tier of 
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the two-tiered network design being proposed, relying on the continued use of primarily 

neighborhood and larger spatially scaled monitors to determine attainment of the NAAQS. We 

expect that 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 US Census Bureau statistics, we 

estimate that these population thresholds would trigger requiring 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. Such monitor 

site locations are anticipated to be in areas of higher population densities of CBSAs and in, or 

adjacent in the downwind direction of, urban cores. We would also allow for middle- or microscale locations that represent many such locations throughout a metropolitan area; in these cases, 

the smaller spatial scales may also represent air quality that exists in multiple other and similar 

such locations in a given CBSA. In areas where a significant fraction of the population is located 

near similar primary emission sources, such middle- or micro-scale monitors may be considered 

representative of area-wide exposures. The EPA expects such an alternative network design to 

retain the characteristics of the current proposal where the Regional Administrator can require 

monitoring above the minimum requirements, in particular to address community impacts from 

the variety of NO2 emission sources. If the alternative network design is followed, EPA expects 

that little or no progress would be made in the development of long-term near-road monitoring 

capabilities due to the lack of specific network design requirements. Nevertheless, EPA seeks 
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comment on this alternative area-wide approach to network design.

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. 

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 (“NO2 NAAQS”), 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 99th percentile 

value form, while the second applies to an annual primary standard and a 1-hour primary 
146

standard based on the annual 4th highest 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 not proposing any change in the provisions of 40 CFR 50.11 related to data 

completeness for the existing annual secondary standard. Substantively, the proposed data 

handling procedures for the annual primary standard are the same as these existing provisions for 

the existing annual secondary 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 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 
147

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 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 
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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 counties which would 

violate possible 1-hour primary NAAQS, was generally consistent with the proposed data 

interpretation procedures. 

B. Interpretation of the Primary NAAQS for Nitrogen Dioxide 

 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 
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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 is arithmetically averaged, and then 

rounded (not truncated) to whole parts per billion. (3) The design value is the 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. 

2. 1-hour primary standard based on the 99th percentile 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 
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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 

for two diagnostic data substitution tests which are intended to identify those cases with 

incomplete data in which it nevertheless is very likely, if not virtually certain, that the daily 1-

hour maximum concentration would have been observed to be either above or below the level of 

the NAAQS if monitoring data had been minimally complete. Data substitution tests are 

currently used for ozone, PM2.5 and Pb. 

 The first test would be applied (regardless of data capture in each quarter) if the annual 

99th percentile daily maximum 1-hour of the incomplete data is above the NAAQS level. The 

test would substitute a low hypothetical concentration for as much of the missing data as needed 

to meet the normal 75% requirement in each quarter. The value that is substituted for the missing 

values is the lowest daily maximum 1-hour observed in the same quarter, looking across all three 

years under evaluation. If the resulting three-year design value is still above the NAAQS, it is 

virtually certain that the annual 99th percentile daily maximum 1-hour of complete data would 

also have been above the NAAQS, so the original design value indicating a violation would be 

considered valid. 

 The second test is similar but works in the opposite direction. It would be applied only 

if there is at least 50% data capture in each quarter and if the annual 99th percentile daily 

maximum 1-hour of 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% 
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requirement. 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 three-year design value is below the NAAQS, it is virtually certain that the annual 99th

percentile daily maximum 1-hour of complete data would also have been below the NAAQS, so 

the original design value indicating compliance would be considered valid. 

 It should be noted that one outcome of applying the substitution tests proposed is that a 

year with incomplete data may nevertheless be determined to not have a valid annual 99th

percentile daily maximum 1-hour and thus to be unusable in making 1-hour primary NAAQS 

compliance determinations for any three-year period that encompasses that year. EPA invites 

comment on incorporating into the final rule the proposed substitution tests. 

 Two other proposed provisions also address data incompleteness. If a day has valid 

data for fewer than 75% of its hours but at least one hourly value is above the level of the 1-hour 

standard or if treating the day (and similarly incomplete days) as if it were complete results in a 

three-year design value above the standard, the day would be treated as if it met the 75% 

requirement. 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. 

 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 the 99th percentile form of the proposed 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 
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to the level of the primary NAAQS. 

3. 1-hour standard based on the annual 4th highest 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 annual 

4th highest daily value form, rather than the 99th percentile form. These two forms would yield 

the same design value in a situation in which every hour and day of the year has reported 

monitoring data. However, the provisions to logically address possible data incompleteness 

under the annual 4th highest daily value form are necessarily somewhat different. The second 

version of Appendix S is partially based on the existing Appendix P for the 8-hour ozone 

NAAQS adopted in 2008, which employs the annual 4th highest daily value form, with certain 

differences to maintain substantive consistency with the first version of Appendix S on aspects 

not tied to the form of the standard. 

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. 
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 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 
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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 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 
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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 indentify 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 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. 
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Table 2. Proposed Schedule for NO2 Exceptional Event Flagging and Documentation 

Submission If Designations Promulgated in Two Years from January 22, 2010 

Air Quality Data Collected for 

Calendar Year 

Event Flagging Deadline Detailed Documentation 

Submission Deadline 

2008 July 1, 2010* January 22, 2011 

2009 July 1, 2010 January 22, 2011 

2010 April 1, 2011* July 1, 2011* 

* Indicates proposed change from generic schedule in 40 CFR 50.14. 

V. Clean Air Act Implementation Requirements for NO2 NAAQS

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. 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. 

The CAA assigns important roles to EPA, states, and 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 and Tribes 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. 
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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 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. 

A. Designations for the NO2 NAAQS 

After EPA establishes or revises a NAAQS, the CAA requires EPA and the states to 

begin taking steps to ensure that the new or revised NAAQS are met. The first step is to identify 

areas of the country that do not meet the new or revised NAAQS. The CAA defines EPA's 

authority to designate areas that do not meet a new or revised NAAQS. Section 107(d)(1) 

provides that, “By such date as the Administrator may reasonably require, but not later than 1 

year after promulgation of a new or revised NAAQS for any pollutant under section 109, the 

Governor of each state shall * * * submit to the Administrator a list of all areas (or portions 

thereof) in the state” that designates those areas as nonattainment, attainment, or unclassifiable. 

Section 107(d)(1)(B)(i) further provides, “Upon promulgation or revision of a NAAQS, the 

Administrator shall promulgate the designations of all areas (or portions thereof) * * * as 

expeditiously as practicable, but in no case later than 2 years from the date of promulgation. 

Such period may be extended for up to one year in the event the Administrator has insufficient 

information to promulgate the designations. “The term “promulgation” has been interpreted by 

the courts to be signature and dissemination of a rule. By no later than 120 days prior to 
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promulgating designations, EPA is required to notify states or Tribes of any intended 

modifications to their boundaries as EPA may deem necessary. States and Tribes then have an 

opportunity to comment on EPA's tentative decision. Whether or not a state or a Tribe provides a 

recommendation, EPA must promulgate the designation that it deems appropriate. 

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 and Tribal leaders will be required to submit their initial 

designation recommendations to EPA no later than January 2011. If the Administrator intends to 

modify any state or Tribal area recommendation, EPA will notify the Governor or Tribal leader 

no later than 120 days prior to initial designations in January 2012. States or Tribes 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 
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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 necessarily mean 

that an area is meeting the revised NO2 NAAQS. Rather, an area-wide monitor may not record 

concentrations that violate the revised NO2 NAAQS because it is not sited where concentrations 

in the area are highest. Thus, we do not believe the current monitoring network provides 

information that supports designating an area as “attainment” with today’s proposed standards. 

Furthermore, as discussed in other parts of this action, the revised NO2 NAAQS that EPA is 

proposing is based, in part, on NO2 concentrations that cause health effects in controlled human 

exposure studies. These studies provide information on the relationship between exposure to 

specific concentrations of NO2 and specific health effects. In order to protect and to notify the 

public of the health risks where NO2 levels have the greatest potential to be harmful to human 

health, a near-roadway network would need to be deployed and data collected. 

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). 
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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 

believe that there is little value in taking an additional year to collect more data from the existing 

NO2 network when we expect to initially designate most areas in the country as “unclassifiable.” 

Once a near-roadway network has been deployed and 3 years of air quality data has been 

collected, we anticipate redesignating areas to “attainment” or “nonattainment.” 

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. Accordingly, the primary reason to establish 
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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, a maximum 18 months for the 

submittal of an attainment plan and a maximum of 5 years for areas to attain standard. These 

relatively short deadlines provide a limited opportunity to establish even nearer deadlines 

through classifications. 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 NO2 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 
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reductions in emissions; 

- EPA has fully approved a maintenance plan for the area; 

- 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) NO2 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 
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areas in any other state, or which interfere with programs under part C to prevent 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.18 The list of 

section 110(a)(2) NAAQS implementation requirements are the following: 

• 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.

 

18 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. 
164

• 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, 
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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 for NO2 

1. NO2 Nonattainment Area SIP Requirements 

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”. However, under the Tribal Authority Rule (TAR), requirements for RACT and 

RACM may be considered to be severable elements of implementation plan requirements for 

Tribes. 

 Section 172(c) requires states with nonattainment areas to submit a SIP for these areas 
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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 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 
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states submit periodic emission 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.19 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. 

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 

 

19 The terms “major” and “minor” define the size of a stationary source, for applicability purposes, in terms of an 

annual emissions rate (tons per year, tpy) for a pollutant. Generally, a minor source is any source that is not 

“major.” “Major” is defined by the applicable regulations—PSD or nonattainment NSR. 
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designated as attainment or unclassifiable for any criteria pollutant, is constructed, or undergoes 

a major modification.20 The nonattainment NSR program applies when a major source of a 

criteria pollutant that is located in an area that is designated as nonattainment for that pollutant is 

constructed or undergoes a major modification. 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 regardless of the designation of the area in which a source is located. 

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; 

 

20 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). 
169

• 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 assure 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. Prior to adoption of the SIP revision 

addressing 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 
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CAA:21 carbon monoxide (CO), 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 NO2 NAAQS. 

 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 

 

21 Criteria pollutants are those pollutants for which EPA has established a NAAQS under section 109 of the CAA. 
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to conduct a 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. 

5. Transition from the Existing NO2 NAAQS to a Revised NO2 NAAQS 

As stated in section I of this proposed rulemaking, EPA is proposing to retain the current 

annual mean NO2 NAAQS (0.053 ppm, or 100 μg/m3) and is also proposing to promulgate a 

short-term 1-hour NO2 NAAQS. The EPA is taking comment on both of the aforementioned 

actions in this rulemaking. After considering public comments, if EPA decides to promulgate 

only a short-term NO2 NAAQS and revoke the current annual mean standard, EPA would need 

to promulgate anti-backsliding provisions. The CAA establishes anti-backsliding requirements 

where EPA relaxes a NAAQS. Here, however, replacing the annual standard with a short term 

standard would be a tightening of the standard and 172(e) would not directly apply. 

Because section 172(e) does not directly apply for a transition to a more stringent 

NAAQS, EPA has previously looked to other provisions of the CAA to determine how to 

address anti-backsliding. EPA has recognized that there are a number of CAA provisions that 

indicate Congress’s intent to not allow states to alter or remove provisions from implementation 

plans if the plan revision would jeopardize the air quality protection being provided by the 

existing plan when EPA tightens a NAAQS. For example, section 110(l) provides that EPA may 

not approve a SIP revision if it interferes with any applicable requirement concerning attainment 

and RFP, or any other applicable requirement under the CAA. In addition, section 193 of the 

CAA prohibits the modification of a control, or a control requirement, in effect or required to be 

adopted as of November 15, 1990 (i.e., prior to the promulgation of the Clean Air Act 

Amendments (CAAA) of 1990), unless such a modification would ensure equivalent or greater 
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emissions reductions. Further, section 172(e) of the CAA specifies that if EPA revises a NAAQS 

to make it less stringent than a previous NAAQS, states may not relax control obligations that 

apply in nonattainment area SIPs, or avoid adopting those controls that have not yet been 

adopted as required. Looking to these provisions, EPA determined for the transition from the 1-

hour ozone NAAQS to the 1997 8-hour ozone NAAQS, that it was consistent with 

Congressional intent to require areas to continue to meet control obligations for the revoked 1-

hour NAAQS until the area attained the revised 8-hour NAAQS. This interpretation was upheld 

in South Coast Air Quality Management Dist. v. EPA, 472 F.3d 882 (D.C. Cir. 2006). 

If EPA revises the NO2 NAAQS by revoking the current standard and replacing it with a shortterm standard, EPA is proposing to apply the principles in section 172(e) in order to ensure that 

the health protection provided under the existing NO2 NAAQS continues to be achieved and 

maintained as states transition to implementation of the revised NAAQS. EPA is proposing that 

the current NO2 NAAQS would remain in effect for one year following effective date of the 

initial designations for the revised NO2 NAAQS before the current NAAQS is revoked. This 

timeframe would ensure that there is no gap in coverage of PSD requirements for areas attaining 

the current annual NO2 standard. It would also ensure that there is no gap in conformity 

requirements for the Los Angeles South Coast Air Basin area, which is the only maintenance 

area in the country. Pursuant to CAA sections 110(l) and 193, any proposed SIP revision being 

considered by EPA after the effective date of the revised NO2 NAAQS would be evaluated for 

its potential to interfere with attainment or maintenance of the new standard. EPA believes any 

area attaining the revised NO2 NAAQS would also attain the existing NO2 NAAQS, and thus 

reviewing proposed SIP provisions for interference with the new standard will be sufficient to 

prevent backsliding. 
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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 
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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 range (i.e., AQI values of 50 and 150). EPA does not 

propose to change breakpoints at the higher end of the range (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 onehalf the level of the short-term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 

12, 1998). If a short-term NO2 NAAQS is promulgated from the upper end of the proposed 

range, then it is likely that an AQI value of 50 would be set at the level of the annual NAAQS 

(53 ppb NO2, 1-hour average). If a short-term NO2 NAAQS is promulgated from the lower end 

of the proposed range, then it is likely that an AQI value of 50 would be set at one-half the level 

of the short-term NO2 standard. EPA is soliciting comment on this approach for selecting an 

AQI value of 50; specifically at what point the Agency should switch from using the level of the 

annual NAAQS to using one-half the level of the short-term NAAQS, or some other level, for an 

AQI value of 50. 

 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 level of 304 ppb NO2, 1-hour average, 

is approximately equidistant between the proposed range for the short-term standard and the 
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level of an AQI value of 200 (640 ppb NO2, 1-hour average). This level also served as a 

benchmark level in the REA, as discussed above (section II.B.1). The level was selected based 

on a meta-analysis of individual level data from 19 NO2 controlled human exposure studies 

measuring airway responsiveness in asthmatics. The meta-analysis indicates that at the level of 

300 ppb NO2, most people with asthma, who compromise a sensitive group for NO2, are likely to 

experience increased airway responsiveness. This health evidence supports using the level of 

300 ppb NO2, 1-hour average, as the upper bound of the unhealthy for sensitive groups category. 

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, 
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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 data to EPA’s Air Quality System (AQS). Because this 

rulemaking includes a range of proposals for the level and form, it is not possible accurately 

predict the size of the final network, and its associated burden. Rather we have estimated the 

upper range of burden possible based on the regulatory options being proposed which would 

result in a higher reporting burden (i.e., a final level for the standard of 50 ppb with a 99th

percentile form). Based on these assumptions, the annual average reporting burden for the 

collection under 40 CFR part 58 (averaged over the first 3 years of this ICR) 7,068,755. Burden 
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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 EPAHQ-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 [Insert date of publication in the Federal Register], a comment to OMB is best assured 

of having its full effect if OMB receives it by [Insert date 30 days after publication in the 

Federal Register.]. 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 
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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 Ass’ns v. EPA, 175 F. 3d 1027, 1044-45 (D.C. cir. 

1999) (NAAQS do not have significant impacts upon small entities because NAAQS themselves 

impose no regulations upon small entities). Similarly, the proposed amendments to 40 CFR part 

58 address the requirements for States to collect information and report compliance with the 

NAAQS and will not impose any requirements on small entities. We continue to be interested in 

the potential impacts of the proposed rule on small entities and welcome comments on issues 

related to such impacts. 

D. Unfunded Mandates Reform Act 

Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law 104-4, 

establishes requirements for Federal agencies to assess the effects of their regulatory actions on 

State, local, and tribal governments and the private sector. Unless otherwise prohibited by law, 
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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 costeffective 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 
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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. 

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.
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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 
182

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, since Tribes are not obligated to adopt or implement any NAAQS. 

Thus, Executive Order 13175 does not apply to this rule. However, EPA specifically solicits 

additional comment on this proposed rule from tribal officials. 

G. Executive Order 13045: Protection of Children from Environmental Health & Safety 

Risks 

This action is subject to Executive Order (62 FR 19885, April 23, 1997) because it is an 

economically significant regulatory action as defined by Executive Order 12866, and we believe 

that the environmental health risk addressed by this action has a disproportionate effect on 

children. The proposed rule will establish uniform national ambient air quality standards for 

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 
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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. 
184

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 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 lowincome 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. 
185

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Barck, C, Sandström T, Lundahl J, Halldén G, Svartengren M, Strand V, Rak S, Bylin G. 

(2002). Ambient level of NO2 augments the inflammatory response to inhaled allergen in 

asthmatics. Respir Med. 96:907-917. 

Barck, C, Lundahl J, Halldén G, Bylin G. (2005). Brief exposures to NO2 augment the allergic 

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List of Subjects 

40 CFR Part 50 

 Environmental protection, Air pollution control, Carbon monoxide, Lead, Nitrogen 

dioxide, Ozone, Particulate matter, Sulfur oxides. 

40 CFR Part 58 

 Environmental protection, Administrative practice and procedure, Air pollution control, 

Intergovernmental relations, Reporting and recordkeeping requirements 
193

Dated: 

Lisa 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: 
194

Part 50-NATIONAL PRIMARY AMBIENT AIR QUALITY STANDARDS 

Section 50.11 is amended by redesignating paragraphs (b) and (c) as paragraphs (c) and (d), 

respectively, and amending the other provisions as follows-- 

§ 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 for the annual 
195

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 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. 

(h) The annual primary standard set forth in this section and the implementation plan obligations 

set forth in sections 110, 191, and 192 of the Clean Air Act associated with that standard, 

notwithstanding the promulgation of the 1-hour primary standard, will remain applicable to all 

areas for one year after the effective date of the initial designation of the areas for the 1-hour 

standard, pursuant to section 107(d)(1) of the Clean Air Act. 

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. 
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(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 § 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 §§ 

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 between 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 
197

determine compliance, calculated as specified in section 5 of this appendix: 

 (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 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. 

(b) When two or more NO2 monitors are operated at a site during a year, 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. If 
198

data from the primary monitor for a year are complete under the requirements of section 3 and 

can be used (in the case of the 1-hour primary standard, along with data from two other years) to 

develop a valid design value for either or both of the primary standards, or if the Administrator 

chooses to consider the design value resulting from the primary monitor’s data to be complete 

under section 3.1(c) or 3.2(d), data for the same year from other monitor(s) at the site will not be 

used in the development of the design value(s) for the site. If data from the primary monitor are 

not complete and are not considered by the Administrator as complete, data solely from the other 

monitor(s) will be used in turn to develop valid design values if such data are sufficiently 

complete. If there are three or more monitors, the order for such comparison of the other 

monitors will be determined by the Administrator. If at its turn a monitor is found to have 

sufficient completeness for the year to allow calculation of a valid design value or is considered 

complete, data from other monitors will not be considered. Hourly and daily 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 when one monitor has physically replaced another monitor 

permanently in which case the two monitors will be considered to be the same monitor. 

(c) Hourly NO2 measurement data shall be reported to AQS in units of parts per billion (ppb) to 

one place after the decimal, with additional digits to the right being truncated. 

3. Comparisons with the NO2 NAAQS. 

3.1 The Annual Primary NO2 NAAQS. 
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(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 mean is valid when at least 75 percent of the hours in the year are reported. 

(c) Years that do not meet the completeness criteria stated in 3.1(c) and the associated annual 

primary standard design values may also be considered complete and 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 3 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 day is complete if 75 percent of the hours are reported; or if 

at least one reported hourly concentration exceeds the level of the 1-hour NAAQS; or if there is 
200

at least one reported hourly concentration and the three-year design value calculated by ignoring 

the 75 percent daily completeness requirement, for all days at once, exceeds the 1-hour NAAQS. 

 (c)(i) 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 three-year 1-hour primary standard design value, the three-year 1-hour primary standard 

design value shall nevertheless be considered valid if either of the following two ‘‘data 

substitution’’ tests is passed. The first test is for validating an above-NAAQS-level (i.e., 

violating) 1-hour primary standard design value, by substituting actual ‘‘low’’ reported values 

from the same site at about the same time of the year (i.e., in the same quarter) looking across 

three years. The second test is for validating a below-NAAQS-level 1-hour primary standard 

design value, by substituting actual ‘‘high’’ values reported for the same site at about the same 

time of the year (i.e., in the same quarter) looking across three years. Note that both tests are 

merely diagnostic in nature, intended to confirm that there is a very high likelihood if not 

certainty that the original design value (the one with less than 75 percent data capture) reflects 

the true over/under-NAAQS-level status for that 3-year period; the result of one of these data 

substitution tests (i.e., a ‘‘test design value”, as defined in section 3(c)(ii) or 3(c)(iii)) is not 

considered the actual design value. For both types of data substitution, substitution is permitted 

only if there are sufficient available data points from which to identify the high or low quarterspecific 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. 
201

(ii) The ‘‘above NAAQS level’’ test is as follows: Data substitution will be performed in all 

quarter periods that have less than 75 percent data capture. If by substituting the lowest reported 

daily maximum 1-hour value for that quarter (year non-specific; e.g., for January-March over the 

three years under consideration) ) for as much of the missing daily data in the deficient quarters 

as is needed to make them 75 percent complete, the computation 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 the level of the standard is 

deemed to have been exceeded in that 3-year period. As noted in section 3(c)(i), in such a case, 

the 3-year design value based on the data actually reported, not the ‘‘test design value’’ including 

the low values, shall be used as the design value. 

(iii) The ‘‘below NAAQS level’’ 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 the quarter can not utilize this substitution 

test. If by substituting the highest reported daily maximum 1-hour value for that quarter (year 

non-specific; e.g., for January-March over the three years under consideration) for as much of 

the missing daily data in the deficient quarters as is needed to make them 100 percent complete, 

the computation 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 the level of the standard is deemed to have been met in that 3-year period. As 

noted in section 3(c)(i), in such a case, the 3-year design value based on the data actually 

reported, not the ‘‘test design value’’ including the low values, shall be used as the design value. 
202

(d) Years that do not meet the completeness criteria stated in 3.2(b) and the associated 1-hour 

primary standard design values may also be considered complete and 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. 

(f) 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 values reported as parts per billion with one decimal digit are not rounded. 

(b) The annual primary standard design value is the annual mean calculated from hourly values 

shall be 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 values reported as parts per billion with one decimal digit are not rounded. 

(b) Daily maximum 1-hour values and therefore the annual 99th percentiles of those daily values 

are not rounded. 
203

(c) The 1-hour primary standard design value calculated from three annual 99th percentile values 

shall be 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 of 

this appendix, 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. 

(c) The annual primary standard design value is rounded according to the conventions in section 

4.1 of this appendix before a comparison with the standard is made. 

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 or 

when they meet one of the diagnostic tests in section 3.2 of this appendix, calculation of the 99th

percentile is accomplished by the steps provided in this subsection. Table 1 of this appendix shall 

be used to identify annual 99th percentile values. 

(1) Procedure for identifying annual 99th percentile values. Identification of annual 98th 

percentile values will be based on the number of days with valid data. 
204

Procedure: 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 

Annual number of days with valid data 

for year “y” (cny) 

P0.99, y is the nth maximum value of the 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 is then calculated by averaging the annual 99th

percentiles of the daily maximum 1-hour values for the most recent 3-year period. 

(c) The 1-hour primary standard design value (3-year average of the 99th percentile daily 

maximum 1-hour values) is rounded according to the conventions in section 4.2 of this appendix 

before a comparison with the standard is made. 
205

PART 58--AMBIENT AIR QUALITY SURVEILLANCE 

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] 

Section 58.1, is amended by adding the following definitions in alphabetical order to read as 

follows: 

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] 

Section 58.10, is amended by adding paragraph subsection (a)(5) and adding paragraph (b)(12) 

to read as follows: 

§ 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. 
206

* * * * * 

(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. 

* * * * * 

Section 58.13 is amended by adding paragraph (c) to read as follows: 

§ 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, operating under all of the requirements of this part, including the 

requirements of appendices A, C, D, E, and G to this part. 

* * * * * 

Section 58.16 is amended by revising paragraph (a) to read as follows: 

§ 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; 
207

NO; NOY; NOX; Pb-TSP mass concentration; Pb-PM10 mass concentration; PM10 mass 

concentration; PM2.5mass 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 nearroad 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. 

* * * * * 

Appendix C to Part 58 is amended as by adding paragraph 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. 

* * * * * 
208

Appendix D to Part 58 is amended as by replacing paragraph 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 Requirement s. (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 located near a 

major road with high AADT counts as specified in paragraph 4.3.2(a)(1) of this appendix. A 

second 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 to appendix E of this part. In situations where one 

CBSA is required to have two near-road NO2 monitoring stations, the sites shall be differentiated 
209

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 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. 
210

(b) The Regional Administrator may require additional NO2 monitoring stations for areawide 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. 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. In these cases, the microscale would represent an area impacted 

by the plume with dimensions extending up to approximately 100 meters. 
211

(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. 
212

* * * * * 

Appendix E to Part 58 is amended by revising paragraph 2, 4(b), 6.1, adding paragraph 6.4 and 

revising Table E-4 to read as follows: 

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 concentration potential for the pollutant being measured. 
213

* * * * * 

4. Spacing from Obstructions 

(a) * * * 

(b) * * * 

(c) * * * 

(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. Spacing from Roadways 

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 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. 
214

For those situations where a monitoring path crosses a roadway with fewer than 10,000 vehicles 

per day, one 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 the 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 nearroad 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 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 
215

roadway with fewer than 10,000 vehicles per day, one 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 form the 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. 
216

Table E-4 of Appendix E to Part 58. Summary of Probe and Monitoring Path 

Siting Criteria 

Pollutant Scale 

(maximum 

monitoring 

path length, 

meters) 

Height 

from 

ground to 

probe, 

inlet or 

80% of 

monitoring 

path1

Horizontal 

and 

vertical 

distance 

from 

supporting 

structures2

to probe, 

inlet or 

90% of 

monitoring 

path1

(meters) 

Distance 

from trees 

to probe, 

inlet or 

90% of 

monitoring 

path1

(meters) 

Distance from 

roadways to 

probe, inlet 

or monitoring 

path1

(meters) 

SO2 3,4,5,6 Middle 

(300 m) 

Neighborhood 

Urban, and 

Regional 

(1 km) 

2-15 > 1 > 10 N/A 

CO 4,5,7 Micro, middle 

(300 m), 

Neighborhood 

(1 km) 

3∀1⁄2: 2-15 > 1 > 10 2-10; see 

Table E-2 of 

this appendix 

for middle 

and 

neighborhood 

scales. 

O3 3,4,5

 

Middle (300 

m) 

Neighborhood, 

Urban, and 

Regional (1 

km) 

2-15 > 1 > 10 See Table E-1 

of this 

appendix for 

all scales. 

NO2 3,4,5 Micro (Nearroad [50-

300}) 

Middle(300m) 

Neighborhood, 

Urban, and 

Regional (1 

km) 

2-7 

(micro); 

2-15(all 

other 

scales) 

> 1 > 10 < 50 meters 

for near-road 

microscale; 

See Table E-1 

of this 

appendix for 

all other 

scales 

Ozone 

precursor

s (for 

PAMS)3,4,5

Neighborhood 

and Urban (1 

km) 

2-15 > 1 > 10 See Table E-4 

of this 

appendix for 

all scales. 

PM,Pb 

3,4,5,6,8

Micro: 

Middle, 

Neighborhood, 

Urban and 

Regional 

2-7 

(micro); 

2-7 

(middle 

PM10-2.5); 

2-15 (all 

other 

scales) 

> 2 (all 

scales, 

horizontal 

distance 

only) 

> 10 (all 

scales) 

2-10 (micro); 

see Figure E1 of this 

appendix for 

all other 

scales. 
217

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. 

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. 

Appendix G to Part 58 is amended as by revising paragraph 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 standard (i.e., averaging time of 24-hours or less) for each pollutant. The index value 

of 50 is associated with the numerical level of the annual standard for a pollutant, if there is one, 
218

at one-half the level of the short-term standard for the pollutant, or at 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 and increasing proportions of the population that 

are likely to be affected. The index is related to other air pollution concentrations through linear 

interpolation based on these levels. The AQI is equal to the highest of the numbers 

corresponding to each pollutant. For the purposes of reporting the AQI, the sub-indexes for PM10 

and PM2.5 are to be considered separately. The pollutant responsible for the highest index value 

(the reported AQI) is called the “critical” pollutant. 

* * * * * 

TABLE 2.—BREAKPOINTS FOR THE AQI 

These breakpoints 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour1

PM2.5

(μg/m3

) 

PM10

(μg/m3

) 

CO (ppm) SO2 (ppm) NO2 (ppm) A

0.000-0.059 .................. 0.0-15.4 0-54 0.0-4.4 0.000-0.034 0 – (0.040 – 

0.050)

0-

0.060-0.075 .................. 15.5-40.4 55-154 4.5-9.4 0.035-0.144 (0.040 – 0.050) – 

(0.080 – 0.100) 

51-

0.076-0.095 0.125-0.164 40.5-65.4 155-254 9.5-12.4 0.145-0.224 (0.080 – 0.10) – 

(0.120 – 0.150) 

101

0.096-0.115 0.165-0.204 365.5-150.4 255-354 12.5-15.4 0.225-0.304 (0.120 – 0.150) – 

(0.160 – 0.200) 

151

0.116-0.374 0.205-0.404 3150.5-250.4 355-424 15.5-30.4 0.305-0.604 (0.160 – 0.200) – 

(0.240 – 0.300) 

201

(

2

)............... 0.405-0.504 3250.5-350.4 425-504 30.5-40.4 0.605-0.804 (0.240 – 0.300) – 

(0.320 – 0.400) 

301

(

2

)............... 0.505-0.604 3350.5-500.4 505-604 40.5-50.4 0.805-1.004 (0.320 – 0.400) – 

(0.400 – 0.500) 

401

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. 
219

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.