34404 Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50 and 58 

[EPA–HQ–OAR–2006–0922; FRL–8926–3] 

RIN 2060–AO19 

Primary National Ambient Air Quality 

Standard for Nitrogen Dioxide 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Proposed rule. 

SUMMARY: Based on its review of the air 

quality criteria for oxides of nitrogen 

and the primary national ambient air 

quality standard (NAAQS) for oxides of 

nitrogen as measured by nitrogen 

dioxide (NO2), EPA proposes to make 

revisions to the primary NO2 NAAQS in 

order to provide requisite protection of 

public health. Specifically, EPA 

proposes to supplement the current 

annual standard by establishing a new 

short-term NO2 standard based on the 3- 

year average of the 99th percentile (or 

4th highest) of 1-hour daily maximum 

concentrations. EPA proposes to set the 

level of this new standard within the 

range of 80 to 100 ppb and solicits 

comment on standard levels as low as 

65 ppb and as high as 150 ppb. EPA also 

proposes to establish requirements for 

an NO2 monitoring network that will 

include monitors within 50 meters of 

major roadways. In addition, EPA is 

soliciting comment on an alternative 

approach to setting the standard and 

revising the monitoring network. 

Consistent with the terms of a consent 

decree, the Administrator will sign a 

notice of final rulemaking by January 

22, 2010. 

DATES: Comments must be received on 

or before September 14, 2009. Under the 

Paperwork Reduction Act, comments on 

the information collection provisions 

must be received by OMB on or before 

August 14, 2009. 

Public Hearings: EPA intends to hold 

public hearings on this proposed rule in 

August 2009 in Los Angeles, California 

and Arlington, VA. These will be 

announced in a separate Federal 

Register notice that provides details, 

including specific times and addresses, 

for these hearings. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–HQ– 

OAR–2006–0922 by one of the following 

methods: 

• http://www.regulations.gov: Follow 

the on-line instructions for submitting 

comments. 

• E-mail: a-and-r-Docket@epa.gov. 

• Fax: 202–566–9744 

• Mail: Docket No. EPA–HQ–OAR– 

2006–0922, Environmental Protection 

Agency, Mail code 6102T, 1200 

Pennsylvania Ave., NW., Washington, 

DC 20460. Please include a total of two 

copies. 

• Hand Delivery: Docket No. EPA– 

HQ–OAR–2006–0922, Environmental 

Protection Agency, EPA West, Room 

3334, 1301 Constitution Ave., NW., 

Washington, DC. Such deliveries are 

only accepted during the Docket’s 

normal hours of operation, and special 

arrangements should be made for 

deliveries of boxed information. 

Instructions: Direct your comments to 

Docket ID No. EPA–HQ–OAR–2006– 

0922. EPA’s policy is that all comments 

received will be included in the public 

docket without change and may be 

made available online at http:// 

www.regulations.gov, including any 

personal information provided, unless 

the comment includes information 

claimed to be Confidential Business 

Information (CBI) or other information 

whose disclosure is restricted by statute. 

Do not submit information that you 

consider to be CBI or otherwise 

protected through http:// 

www.regulations.gov or e-mail. The 

http://www.regulations.gov Web site is 

an ‘‘anonymous access’’ system, which 

means EPA will not know your identity 

or contact information unless you 

provide it in the body of your comment. 

If you send an e-mail comment directly 

to EPA without going through http:// 

www.regulations.gov your e-mail 

address will be automatically captured 

and included as part of the comment 

that is placed in the public docket and 

made available on the Internet. If you 

submit an electronic comment, EPA 

recommends that you include your 

name and other contact information in 

the body of your comment and with any 

disk or CD–ROM you submit. If EPA 

cannot read your comment due to 

technical difficulties and cannot contact 

you for clarification, EPA may not be 

able to consider your comment. 

Electronic files should avoid the use of 

special characters, any form of 

encryption, and be free of any defects or 

viruses. For additional information 

about EPA’s public docket visit the EPA 

Docket Center homepage at http:// 

www.epa.gov/epahome/dockets.htm. 

Docket: All documents in the docket 

are listed in the http:// 

www.regulations.gov index. Although 

listed in the index, some information is 

not publicly available, e.g., CBI or other 

information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy. Publicly available docket 

materials are available either 

electronically in http:// 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., 

NW., Washington, DC. The Public 

Reading Room is open from 8:30 a.m. to 

4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone 

number for the Public Reading Room is 

(202) 566–1744 and the telephone 

number for the Air and Radiation 

Docket and Information Center is (202) 

566–1742. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Scott Jenkins, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail Code C504–06, Research Triangle 

Park, NC 27711; telephone: 919–541– 

1167; fax: 919–541–0237; e-mail: 

jenkins.scott@epa.gov. 

SUPPLEMENTARY INFORMATION: 

General Information 

What Should I Consider as I Prepare My 

Comments for EPA? 

1. Submitting CBI. Do not submit this 

information to EPA through http:// 

www.regulations.gov or e-mail. Clearly 

mark the part or all of the information 

that you claim to be CBI. For CBI 

information in a disk or CD ROM that 

you mail to EPA, mark the outside of the 

disk or CD ROM as CBI and then 

identify electronically within the disk or 

CD ROM the specific information that is 

claimed as CBI. In addition to one 

complete version of the comment that 

includes information claimed as CBI, a 

copy of the comment that does not 

contain the information claimed as CBI 

must be submitted for inclusion in the 

public docket. Information so marked 

will not be disclosed except in 

accordance with procedures set forth in 

40 CFR part 2. 

2. Tips for Preparing Your Comments. 

When submitting comments, remember 

to: 

• Identify the rulemaking by docket 

number and other identifying 

information (subject heading, Federal 

Register date and page number). 

• Follow directions—the agency may 

ask you to respond to specific questions 

or organize comments by referencing a 

Code of Federal Regulations (CFR) part 

or section number. 

• Explain why you agree or disagree, 

suggest alternatives, and substitute 

language for your requested changes. 

• Describe any assumptions and 

provide any technical information and/ 

or data that you used. 

• Provide specific examples to 

illustrate your concerns, and suggest 

alternatives. 


srobinson on DSKHWCL6B1PROD with PROPOSALS2
Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules 34405 

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). 

• Explain your views as clearly as 

possible, avoiding the use of profanity 

or personal threats. 

• Make sure to submit your 

comments by the comment period 

deadline identified. 

Availability of Related Information 

A number of the documents that are 

relevant to this rulemaking are available 

through EPA’s Office of Air Quality 

Planning and Standards (OAQPS) 

Technology Transfer Network (TTN) 

Web site at http://www.epa.gov/ttn/ 

naaqs/standards/nox/s_nox_index.html. 

These documents include the Integrated 

Review Plan and the Health Assessment 

Plan, available at http://www.epa.gov/ 

ttn/naaqs/standards/nox/ 

s_nox_cr_pd.html, the Integrated 

Science Assessment (ISA), available at 

http://cfpub.epa.gov/ncea/cfm/ 

recordisplay.cfm?deid=194645, and the 

Risk and Exposure Assessment (REA), 

available at http://www.epa.gov/ttn/ 

naaqs/standards/nox/ 

s_nox_cr_rea.html. These and other 

related documents are also available for 

inspection and copying in the EPA 

docket identified above. 

Table of Contents 

The following topics are discussed in 

this preamble: 

I. Background 

A. Legislative Requirements 

B. Related NO2 Control Programs 

C. Review of the Air Quality Criteria and 

Standards for Oxides of Nitrogen 

II. Rationale for Proposed Decisions on the 

Primary Standard 

A. Characterization of NO2 Air Quality 

1. Current Patterns of NO2 Air Quality 

2. NO2 Air Quality and Gradients Around 

Roadways 

B. Health Effects Information 

1. Adverse Respiratory Effects and ShortTerm Exposure to NO2

a. Emergency Department Visits and 

Hospital Admissions 

b. Respiratory Symptoms 

c. Impaired Host Defense 

d. Airway Response 

e. Airway Inflammation 

f. Lung Function 

g. Conclusions From the ISA 

2. Other Effects With Short-Term Exposure 

to NO2

a. Mortality 

b. Cardiovascular Effects 

3. Health Effects With Long-Term Exposure 

to NO2

a. Respiratory Morbidity 

b. Mortality 

c. Carcinogenic, Cardiovascular, and 

Reproductive/Developmental Effects 

4. NO2-Related Impacts on Public Health 

a. Pre-Existing Disease 

b. Age 

c. Genetics 

d. Gender 

e. Proximity to Roadways 

f. Socioeconomic Status 

g. Size of the At-Risk Population 

C. Human Exposure and Health Risk 

Characterization 

1. Evidence Base for the Risk 

Characterization 

2. Overview of Approaches 

3. Key Limitations and Uncertainties 

D. Considerations in Review of the 

Standard 

1. Background on the Current Standard 

2. Approach for Reviewing the Need to 

Retain or Revise the Current Standard 

E. Adequacy of the Current Standard 

1. Evidence-Based Considerations 

2. Exposure- and Risk-Based 

Considerations 

3. Summary of Considerations From the 

REA 

4. CASAC Views 

5. Administrator’s Conclusions Regarding 

Adequacy of the Current Standard 

F. Conclusions on the Elements of a New 

Short-Term Standard and an Annual 

Standard 

1. Indicator 

2. Averaging Time 

a. Short-Term Averaging Time 

b. Long-Term Averaging Time 

c. CASAC Views 

d. Administrator’s Conclusions on 

Averaging Time 

3. Form 

4. Level 

a. Evidence-Based Considerations 

b. Exposure- and Risk-Based 

Considerations 

c. Summary of Consideration From the 

REA 

d. CASAC Views 

e. Administrator’s Conclusions on Level for 

a 1-Hour Standard 

f. Alternative Approach to Setting the 1- 

Hour Standard Level 

g. Level of the Annual Standard 

G. Summary of Proposed Decisions on the 

Primary Standard 

III. Proposed Amendments to Ambient 

Monitoring and Reporting Requirements 

A. Monitoring Methods 

B. Network Design 

1. Background 

2. Proposed Changes 

a. Monitoring in Areas of Expected 

Maximum Concentrations Near Major 

Roads 

b. Area-Wide Monitoring at Neighborhood 

and Larger Spatial Scales 

3. Solicitation for Comment on an 

Alternative Network Design 

C. Data Reporting 

IV. Proposed Appendix S—Interpretation of 

the Primary NAAQS for Oxides of 

Nitrogen and Proposed Revisions to the 

Exceptional Events Rule 

A. Background 

B. Interpretation of the Primary NAAQS for 

Oxides of Nitrogen 

1. Annual Primary Standard 

2. 1-Hour Primary Standard Based on the 

Annual 4th Highest Daily Value Form 

3. 1-Hour Primary Standard Based on the 

Annual 99th Percentile Value Form 

C. Exceptional Events Information 

Submission Schedule 

V. Clean Air Act Implementation 

Requirements 

A. Designations 

B. Classifications 

C. Attainment Dates 

1. Attaining the NAAQS 

2. Consequences of Failing to Attain by the 

Statutory Attainment Date 

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

Requirements 

E. Attainment Planning Requirements 

1. Nonattainment Area SIPs 

2. New Source Review and Prevention of 

Significant Deterioration Requirements 

3. General Conformity 

4. Transportation Conformity 

VI. Communication of Public Health 

Information 

VII. Statutory and Executive Order Reviews 

References 

I. Background 

A. Legislative Requirements 

Two sections of the Clean Air Act 

(Act or CAA) govern the establishment 

and revision of the NAAQS. Section 108 

of the Act directs the Administrator to 

identify and list air pollutants that meet 

certain criteria, including that the air 

pollutant ‘‘in his judgment, cause[s] or 

contribute[s] to air pollution which may 

reasonably be anticipated to endanger 

public health and welfare’’ and ‘‘the 

presence of which in the ambient air 

results from numerous or diverse mobile 

or stationary sources.’’ 42 U.S.C. 21 

7408(a)(1)(A) & (B). For those air 

pollutants listed, section 108 requires 

the Administrator to issue air quality 

criteria that ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in ambient air 

* * *’’ 42 U.S.C. 7408(2). 

Section 109(a) of the Act directs the 

Administrator to promulgate ‘‘primary’’ 

and ‘‘secondary’’ NAAQS for pollutants 

for which air quality criteria have been 

issued. 42 U.S.C. 7409(1). Section 

109(b)(1) defines a primary standard as 

one ‘‘the attainment and maintenance of 

which in the judgment of the 

Administrator, based on [the air quality] 

criteria and allowing an adequate 

margin of safety, are requisite to protect 

the public health.’’ 1 42 U.S.C. 

7409(b)(1). A secondary standard, in 

turn, must ‘‘specify a level of air quality 

the attainment and maintenance of 

which, in the judgment of the 

Administrator, based on [the air quality] 

criteria, is requisite to protect the public 


srobinson on DSKHWCL6B1PROD with PROPOSALS2
34406 Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules 

2 EPA is currently conducting a separate review 

of the secondary NO2 NAAQS jointly with a review 

of the secondary SO2 NAAQS. 

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. 

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 Petroleum 

Institute v. Costle, 665 F.2d 1176, 1186 

(D.C. Cir. 1981), cert. denied, 455 U.S. 

1034 (1982). Both kinds of uncertainties 

are components of the risk associated 

with pollution at levels below those at 

which human health effects can be said 

to occur with reasonable scientific 

certainty. Thus, in selecting primary 

standards that include an adequate 

margin of safety, the Administrator is 

seeking not only to prevent pollution 

levels that have been demonstrated to be 

harmful but also to prevent lower 

pollutant levels that may pose an 

unacceptable risk of harm, even if the 

risk is not precisely identified as to 

nature or degree. 

In addressing the requirement for a 

margin of safety, EPA considers such 

factors as the nature and severity of the 

health effects involved, the size of the 

at-risk population(s), and the kind and 

degree of the uncertainties that must be 

addressed. The selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment. Lead 

Industries Association v. EPA, supra, 

647 F.2d at 1161–62. 

In setting standards that are 

‘‘requisite’’ to protect public health and 

welfare, as provided in section 109(b), 

EPA’s task is to establish standards that 

are neither more nor less stringent than 

necessary for these purposes. In so 

doing, EPA may not consider the costs 

of implementing the standards. 

Whitman v. American Trucking 

Associations, 531 U.S. 457, 471, 475–76 

(2001). 

Section 109(d)(1) of the Act requires 

the Administrator to periodically 

undertake a thorough review of the air 

quality criteria published under section 

108 and the NAAQS and to revise the 

criteria and standards as may be 

appropriate. 42 U.S.C. 7409(d)(1). The 

Act also requires the Administrator to 

appoint an independent scientific 

review committee composed of seven 

members, including at least one member 

of the National Academy of Sciences, 

one physician, and one person 

representing State air pollution control 

agencies, to review the air quality 

criteria and NAAQS and to 

‘‘recommend to the Administrator any 

new standards and revisions of existing 

criteria and standards as may be 

appropriate under section 108 and 

subsection (b) of this section.’’ 42 U.S.C. 

7409(d)(2). This independent review 

function is performed by the Clean Air 

Scientific Advisory Committee (CASAC) 

of EPA’s Science Advisory Board. 

B. Related NO2 Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once EPA 

has established them. Under section 110 

of the Act, 42 U.S.C. 7410, and related 

provisions, States are to submit, for EPA 

approval, State implementation plans 

(SIPs) that provide for the attainment 

and maintenance of such standards 

through control programs directed to 

sources of the pollutants involved. The 

States, in conjunction with EPA, also 

administer the prevention of significant 

deterioration program that covers these 

pollutants. See 42 U.S.C. 7470–7479. In 

addition, Federal programs provide for 

nationwide reductions in emissions of 

these and other air pollutants under 

Title II of the Act, 42 U.S.C. 7521— 

7574, which involves controls for 

automobile, truck, bus, motorcycle, 

nonroad engine and equipment, and 

aircraft emissions; the new source 

performance standards under section 

111 of the Act, 42 U.S.C. 7411; and the 

national emission standards for 

hazardous air pollutants under section 

112 of the Act, 42 U.S.C. 7412. 

Currently there are no areas in the 

United States that are designated as 

nonattainment of the NO2 NAAQS. If 

the NO2 NAAQS is revised as a result 

of this review, however, some areas 

could be classified as non-attainment. 

Certain States would then be required to 

develop SIPs that identify and 

implement specific air pollution control 

measures to reduce ambient NO2

concentrations to attain and maintain 

the revised NO2 NAAQS, most likely by 

requiring air pollution controls on 

sources that emit oxides of nitrogen 

(NOX 3). 

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 lowerNOX vehicles and engines will become 

an increasingly large fraction of in-use 

mobile sources, effecting large NOX

emission reductions. 

C. Review of the Air Quality Criteria and 

Standards for Oxides of Nitrogen 

On April 30, 1971, EPA promulgated 

identical primary and secondary 

NAAQS for NO2 under section 109 of 

the Act. The standards were set at 0.053 

parts per million (ppm) (53 ppb), annual 

average (36 FR 8186). EPA completed 

reviews of the air quality criteria and 

NO2 standards in 1985 and 1996 with 

decisions to retain the standard (50 FR 

25532, June 19, 1985; 61 FR 52852, 

October 8, 1996). 

EPA initiated the current review of 

the air quality criteria for oxides of 

nitrogen and the NO2 primary NAAQS 

on December 9, 2005 (70 FR 73236) with 

a general call for information. EPA’s 

draft Integrated Review Plan for the 

Primary National Ambient Air Quality 

Standard for Nitrogen Dioxide (EPA, 

2007a) was made available in February 

2007 for public comment and was 

discussed by the CASAC via a publicly 

accessible teleconference on May 11, 

2007. As noted in that plan, NOX

includes multiple gaseous (e.g., NO2, 

NO) and particulate (e.g., nitrate) 

species. Because the health effects 


srobinson on DSKHWCL6B1PROD with PROPOSALS2
Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules 34407 

4The ‘‘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. 

associated with particulate species of 

NOX have been considered within the 

context of the health effects of ambient 

particles in the Agency’s review of the 

NAAQS for particulate matter (PM), the 

current review of the primary NO2

NAAQS is focused on the gaseous 

species of NOX and does not consider 

health effects directly associated with 

particulate species. 

The first draft of the Integrated 

Science Assessment for Oxides of 

Nitrogen-Health Criteria (ISA) and the 

Nitrogen Dioxide Health Assessment 

Plan: Scope and Methods for Exposure 

and Risk Assessment (EPA, 2007b) were 

reviewed by CASAC at a public meeting 

held on October 24–25, 2007. Based on 

comments received from CASAC and 

the public, EPA developed the second 

draft of the ISA and the first draft of the 

Risk and Exposure Assessment to 

Support the Review of the NO2 Primary 

National Ambient Air Quality Standard 

(Risk and Exposure Assessment (REA)). 

These documents were reviewed by 

CASAC at a public meeting held on May 

1–2, 2008. Based on comments received 

from CASAC and the public at this 

meeting, EPA released the final ISA in 

July of 2008 (EPA, 2008a). In addition, 

comments received were considered in 

developing the second draft of the REA, 

which was released for public review 

and comment in two parts. The first part 

of this document, containing chapters 

1–7, 9 and appendices A and C as well 

as part of appendix B, was released in 

August, 2008. The second part of this 

document, containing chapter 8 

(describing the Atlanta exposure 

assessment) and a completed appendix 

B, was released in October of 2008. This 

document was the subject of CASAC 

reviews at public meetings on 

September 9 and 10, 2008 (for the first 

part) and on October 22, 2008 (for the 

second part). In preparing the final REA 

(EPA, 2008b), EPA considered 

comments received from the CASAC 

and the public at those meetings. 

In the course of reviewing the second 

draft REA, CASAC expressed the view 

that the document would be incomplete 

without the addition of a policy 

assessment chapter presenting an 

integration of evidence-based 

considerations and risk and exposure 

assessment results. CASAC stated that 

such a chapter would be ‘‘critical for 

considering options for the NAAQS for 

NO2’’ (Samet, 2008a). In addition, 

within the period of CASAC’s review of 

the second draft REA, EPA’s Deputy 

Administrator indicated in a letter to the 

chair of CASAC, addressing earlier 

CASAC comments on the NAAQS 

review process (Henderson, 2008), that 

the risk and exposure assessment will 

include ‘‘a broader discussion of the 

science and how uncertainties may 

effect decisions on the standard’’ and 

‘‘all analyses and approaches for 

considering the level of the standard 

under review, including risk assessment 

and weight of evidence methodologies’’ 

(Peacock, 2008, p.3; September 8, 2008). 

Accordingly, the final REA included a 

new policy assessment chapter. This 

policy assessment chapter considered 

the scientific evidence in the ISA and 

the exposure and risk characterization 

results presented in other chapters of 

the REA as they relate to the adequacy 

of the current NO2 primary NAAQS and 

potential alternative primary NO2

standards. In considering the current 

and potential alternative standards, the 

final REA document focused on the 

information that is most pertinent to 

evaluating the basic elements of 

national ambient air quality standards: 

indicator, averaging time, form 4, and 

level. These elements, which together 

serve to define each standard, must be 

considered collectively in evaluating the 

health protection afforded. CASAC 

discussed the final version of the REA, 

with an emphasis on the policy 

assessment chapter, during a public 

teleconference held on December 5, 

2008. Following that teleconference, 

CASAC offered comments and advice 

on the NO2 primary NAAQS in a letter 

to the Administrator (Samet, 2008b). 

The schedule for completion of this 

review is governed by a judicial order 

resolving a lawsuit filed in September 

2005, concerning the timing of the 

current review. The order that now 

governs this review, entered by the 

court in August 2007 and amended in 

December 2008, provides that the 

Administrator will sign, for publication, 

notices of proposed and final 

rulemaking concerning the review of the 

primary NO2 NAAQS no later than June 

26, 2009 and January 22, 2010, 

respectively. 

This action presents the 

Administrator’s proposed decisions on 

the current primary NO2 standard. 

Throughout this preamble a number of 

conclusions, findings, and 

determinations proposed by the 

Administrator are noted. While they 

identify the reasoning that supports this 

proposal, they are not intended to be 

final or conclusive in nature. The EPA 

invites general, specific, and/or 

technical comments on all issues 

involved with this proposal, including 

all such proposed judgments, 

conclusions, findings, and 

determinations. Further, EPA invites 

specific comments from CASAC on the 

proposed approach of establishing a 

new 1-hour NO2 standard in 

conjunction with a revised monitoring 

network that includes a substantial 

number of monitors placed near major 

roads. In addition to requesting 

comment on the overall approach, EPA 

invites specific comment on the level, or 

range of levels, appropriate for such a 

standard, as well as on the rationale that 

would support that level or range of 

levels. 

II. Rationale for Proposed Decisions on 

the Primary Standard 

This section presents the rationale for 

the Administrator’s proposed decision 

to revise the existing NO2 primary 

standard by supplementing the current 

annual standard with a 1-hour standard 

and to specify the standards to the 

nearest parts per billion (ppb). As 

discussed more fully below, this 

rationale takes into account: (1) 

Judgments and conclusions presented in 

the ISA and the REA; (2) CASAC advice 

and recommendations, as reflected in 

discussions of drafts of the ISA and REA 

at public meetings, in separate written 

comments, and in CASAC’s letter to the 

Administrator (Samet, 2008b); and (3) 

public comments received at CASAC 

meetings during the development of the 

ISA and the REA. 

In developing this rationale, EPA has 

drawn upon an integrative synthesis of 

the entire body of evidence on human 

health effects associated with the 

presence of NO2 in the air. As discussed 

below, this body of evidence addresses 

a broad range of health endpoints 

associated with exposure to NO2. In 

considering this entire body of 

evidence, EPA focuses in particular on 

those health endpoints for which the 

ISA finds associations with NO2 to be 

causal or likely causal (see section II.B 

below). This rationale also draws upon 

the results of quantitative exposure and 

risk assessments. 

As discussed below, a substantial 

amount of new research has been 

conducted since the last review of the 

NO2 NAAQS, with important new 

information coming from epidemiologic 

studies in particular. The newly 

available research studies evaluated in 

the ISA have undergone intensive 

scrutiny through multiple layers of peer 

review and opportunities for public 

review and comment. While important 

uncertainties remain in the qualitative 

and quantitative characterizations of 

health effects attributable to exposure to 

ambient NO2, the review of this 


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5 It should be noted that the ISA Section 2.4.1 

references a different number of active monitors in 

the NO2 network. The discrepancy between the ISA 

numbers and the number presented here is due to 

differing metrics used in pulling data from AQS. 

The ISA only references SLAMS, NAMS, and 

PAMS sites with defined monitoring objectives, 

while the Watkins and Thompson, 2008 value 

represents all NO2 sites reporting data at any point 

during the year. These differences in numbers of 

active monitors per year also explain why the 

Watkins and Thompson 2008 document 

characterized the NO2 network size as relatively 

stable since the early 1980s. 

information has been extensive and 

deliberate. 

The remainder of this section 

discusses the rationale for the 

Administrator’s proposed decisions on 

the primary standard. Section II.A 

presents a discussion of NO2 air quality, 

including discussion of the NO2

concentration gradients that can exist 

around roadways, and the current NO2

monitoring network. Section II.B 

includes an overview of the scientific 

evidence related to health effects 

associated with NO2 exposure. This 

overview includes discussion of the 

health endpoints and at-risk 

populations considered in the ISA. 

Section II.C discusses the approaches 

taken by EPA to assess exposures and 

health risks associated with NO2, 

including a discussion of key 

uncertainties associated with the 

analyses. Section II.D presents the 

approach that is being used in the 

current review of the NO2 NAAQS with 

regard to consideration of the scientific 

evidence and exposure-/risk-based 

results related to the adequacy of the 

current standard and potential 

alternative standards. Sections II.E and 

II.F discuss the scientific evidence and 

the exposure-/risk-based results 

specifically as they relate to the current 

and potential alternative standards, 

including discussion of the 

Administrator’s proposed decisions on 

the standard. Section II.G summarizes 

the Administrator’s proposed decisions 

with regard to the NO2 primary NAAQS. 

A. Characterization of NO2 Air Quality 

1. Current patterns of NO2 Air Quality 

The size of the State and local NO2

monitoring network has remained 

relatively stable since the early 1980s, 

and currently has approximately 400 

monitors reporting data to EPA’s Air 

Quality System (AQS) database. 5 At 

present, there are no minimum 

monitoring requirements for NO2 in 40 

CFR part 58 Appendix D, other than a 

requirement for EPA Regional 

Administrator approval before removing 

any existing monitors, and that any 

ongoing NO2 monitoring must have at 

least one monitor sited to measure the 

maximum concentration of NO2 in that 

area (though, as discussed below 

monitors in the current network do not 

measure peak concentrations associated 

with on-road mobile sources that can 

occur near major roadways because the 

network was not designed for this 

purpose). EPA removed the specific 

minimum monitoring requirements for 

NO2 of two monitoring sites per area 

with a population of 1,000,000 or more 

in the 2006 monitoring rule revisions 

(71 FR 61236), based on the fact that 

there were no NO2 nonattainment areas 

at that time, coupled with trends 

evidence showing an increasing gap 

between national average NO2

concentrations and the current annual 

standard. Additionally, the minimum 

requirements were removed to provide 

State, local, and Tribal air monitoring 

agencies flexibility in meeting higher 

priority monitoring needs for pollutants 

such as ozone and PM2.5, or 

implementing the new multi-pollutant 

sites (NCore network) required by the 

2006 rule revisions, by allowing them to 

discontinue lower priority monitoring. 

There are requirements in 40 CFR part 

58 Appendix D for NO2 monitoring as 

part of the Photochemical Assessment 

Monitoring Stations (PAMS) network. 

However, of the approximately 400 NO2

monitors currently in operation, only 

about 10 percent may be due to the 

PAMS requirements. 

An analysis of the approximately 400 

monitors comprising the current NO2

monitoring network (Watkins and 

Thompson, 2008) indicates that the 

current NO2 network has largely 

remained unchanged in terms of size 

and target monitor objective categories 

since it was introduced in the May 10, 

1979 monitoring rule (44 FR 27571). 

The review of the current network 

found that the assessment of 

concentrations for general population 

exposure and maximum concentrations 

at neighborhood and larger scales were 

the top objectives. A review of the 

distribution of listed spatial scales of 

representation shows that only 

approximately 3 monitors are described 

as microscale, representing an area on 

the order of several meters to 100 

meters, and approximately 23 monitors 

are described as middle scale, which 

represents an area on the order of 100 

to 500 meters. This low percentage of 

smaller spatially representative scale 

sites within the network of 

approximately 400 monitoring sites 

indicates that the majority of monitors 

have, in fact, been sited to assess areawide exposures on the neighborhood, 

urban, and regional scales, as would be 

expected for a network sited to support 

the current annual NO2 standard and 

PAMS objectives. The current network 

does not include monitors placed near 

major roadways and, therefore, monitors 

in the current network do not 

necessarily measure the maximum 

concentrations that can occur on a 

localized scale near these roadways (as 

discussed in the next section). It should 

be noted that the network not only 

accommodates NAAQS related 

monitoring, but also serves other 

monitoring objectives such as support 

for photochemistry analysis, ozone 

modeling and forecasting, and 

particulate matter precursor tracking. 

2. NO2 Air Quality and Gradients 

Around Roadways 

On-road and non-road mobile sources 

account for approximately 60% of NOX

emissions (ISA, table 2.2–1) and trafficrelated exposures can dominate 

personal exposures to NO2 (ISA section 

2.5.4). While driving, personal exposure 

concentrations in the cabin of a vehicle 

could be substantially higher than 

ambient concentrations measured 

nearby (ISA, section 2.5.4). For example, 

mean in-vehicle NO2 concentrations 

have been reported to be 2 to 3 times 

higher than non-traffic ambient 

concentrations (ISA, sections 2.5.4 and 

4.3.6). In addition, estimates presented 

in the REA suggest that on/near 

roadway NO2 concentrations could be 

approximately 40% (REA, compare 

Tables 7–11 and 7–13) or 80% (REA, 

section 7.3.2) higher on average than 

concentrations away from roadways and 

that roadway-associated environments 

could be responsible for the large 

majority of 1-hour peak NO2 exposures 

(REA, Figures 8–17 and 8–18). Because 

monitors in the current network are not 

sited to measure peak roadwayassociated NO2 concentrations, 

individuals who spend time on and/or 

near major roadways could experience 

NO2 concentrations that are 

considerably higher than indicated by 

monitors in the current area-wide NO2

monitoring network. 

Research suggests that the 

concentrations of on-road mobile source 

pollutants such as NOX, carbon 

monoxide (CO), directly emitted air 

toxics, and certain size distributions of 

particulate matter (PM), such as 

ultrafine PM, typically display peak 

concentrations on or immediately 

adjacent to roads (ISA, section 2.5). This 

situation typically produces a gradient 

in pollutant concentrations, with 

concentrations decreasing with 

increasing distance from the road, and 

concentrations generally decreasing 

back to near area-wide ambient levels, 

or typical upwind urban background 


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levels, within several hundred meters 

downwind. While this general concept 

is applicable to almost all roads, the 

actual characteristics of the gradient and 

the distance that the mobile source 

pollutant signature from an individual 

road can be differentiated from 

background or upwind concentrations 

are heavily dependent on factors 

including traffic volumes, local 

topography, roadside features, 

meteorology, and photochemical 

reactivity conditions (Baldauf, et al., 

2009; Beckerman et al., 2008; Clements 

et al., 2008; Hagler et al., 2009; Janssen 

et al., 2001; Rodes and Holland, 1980; 

Roorda-Knape et al., 1998; Singer et al., 

2004; Zhou and Levy, 2007). 

Because NO2 in the ambient air is due 

largely to the atmospheric oxidation of 

NO emitted from combustion sources 

(ISA, section 2.2.1), elevated NO2

concentrations can extend farther away 

from roadways than the primary 

pollutants also emitted by on-road 

mobile sources. More specifically, 

review of the technical literature 

suggests that NO2 concentrations may 

return to area-wide or typical urban 

background concentrations within 

distances up to 500 meters of roads, 

though the actual distance will vary 

with topography, roadside features, 

meteorology, and photochemical 

reactivity conditions (Baldauf et al., 

2009; Beckerman et al., 2008; Clements 

et al., 2008; Gilbert et al. 2003; Rodes 

and Holland, 1980; Singer et al., 2004; 

Zhou and Levy, 2007). Efforts to 

quantify the extent and slope of the 

concentration gradient that may exist 

from peak near-road concentrations to 

the typical urban background 

concentrations must consider the 

variability that exists across locations 

and for a given location over time. As 

a result, we have identified a range of 

concentration gradients in the technical 

literature which indicate that, on 

average, peak NO2 concentrations on or 

immediately adjacent to roads may 

typically be between 30 and 100 percent 

greater than concentrations monitored 

in the same area but farther away from 

the road (ISA, Section 2.5.4; Beckerman 

et al., 2008; Gilbert et al., 2003; Rodes 

and Holland, 1980; Roorda-Knape et al., 

1998; Singer et al., 2004). This range of 

concentration gradients has 

implications for revising the NO2

primary standard and for the NO2

monitoring network (see sections II.F.4 

and III). 

B. Health Effects Information 

In the last review of the NO2 NAAQS, 

the 1993 NOX Air Quality Criteria 

Document (1993 AQCD) (EPA, 1993) 

concluded that there were two key 

health effects of greatest concern at 

ambient or near-ambient concentrations 

of NO2 (ISA, section 5.3.1). The first was 

increased airway responsiveness in 

asthmatic individuals after short-term 

exposures. The second was increased 

respiratory illness among children 

associated with longer-term exposures 

to NO2. Evidence also was found for 

increased risk of emphysema, but this 

appeared to be of major concern only 

with exposures to NO2 at levels much 

higher than then current ambient levels 

(ISA, section 5.3.1). Controlled human 

exposure and animal toxicological 

studies provided qualitative evidence 

for airway hyperresponsiveness and 

lung function changes while 

epidemiologic studies provided 

evidence for increased respiratory 

symptoms with increased indoor NO2

exposures. Animal toxicological 

findings of lung host defense system 

changes with NO2 exposure provided a 

biologically-plausible basis for the 

epidemiologic results. Subpopulations 

considered potentially more susceptible 

to the effects of NO2 exposure included 

persons with preexisting respiratory 

disease, children, and the elderly. The 

epidemiologic evidence for respiratory 

health effects was limited, and no 

studies had considered endpoints such 

as hospital admissions, emergency 

department visits, or mortality (ISA, 

section 5.3.1). 

As discussed below, evidence 

published since the last review 

generally has confirmed and extended 

the conclusions articulated in the 1993 

AQCD (ISA, section 5.3.2). The 

epidemiologic evidence has grown 

substantially with the addition of field 

and panel studies, intervention studies, 

time-series studies of endpoints such as 

hospital admissions, and a substantial 

number of studies evaluating mortality 

risk associated with short-term NO2

exposures. While not as marked as the 

growth in the epidemiologic literature, a 

number of recent toxicological and 

controlled human exposure studies also 

provide insights into relationships 

between NO2 exposure and health 

effects. The body of evidence that has 

become available since the last review 

focuses the current review on NO2- 

related respiratory effects at lower 

ambient and exposure concentrations. 

The ISA, along with its associated 

annexes, provides a comprehensive 

review and assessment of the scientific 

evidence related to the health effects 

associated with NO2 exposures. For 

these health effects, the ISA 

characterized judgments about causality 

with a hierarchy that contains five 

levels (ISA, section 1.3): sufficient to 

infer a causal relationship, sufficient to 

infer a likely causal relationship (i.e., 

more likely than not), suggestive but not 

sufficient to infer a causal relationship, 

inadequate to infer the presence or 

absence of a causal relationship, and 

suggestive of no causal relationship. 

Judgments about causality were 

informed by a series of aspects that are 

based on those set forth by Sir Austin 

Bradford Hill in 1965 (ISA, Table 1.3– 

1). These aspects include strength of the 

observed association, availability of 

experimental evidence, consistency of 

the observed association, biological 

plausibility, coherence of the evidence, 

temporal relationship of the observed 

association, and the presence of an 

exposure-response relationship. A 

summary of each of the five levels of the 

hierarchy is provided in Table 1.3–2 of 

the ISA. 

Judgments made in the ISA about the 

extent to which relationships between 

various health endpoints and exposure 

to NO2 are likely causal have been 

informed by several factors. As 

discussed in the ISA in section 1.3, 

these factors include the nature of the 

evidence (i.e., controlled human 

exposure, epidemiological, and/or 

toxicological studies) and the weight of 

evidence. The weight of evidence takes 

into account such considerations as 

biological plausibility, coherence of the 

evidence, strength of associations, and 

consistency of the evidence. Controlled 

human exposure studies provide 

directly applicable information for 

determining causality because these 

studies are not limited by differences in 

dosimetry and species sensitivity, 

which would need to be addressed in 

extrapolating animal toxicology data to 

human health effects, and because they 

provide data relating health effects 

specifically to NO2 exposures, in the 

absence of the co-occurring pollutants 

present in ambient air. Epidemiologic 

studies provide evidence of associations 

between NO2 concentrations and more 

serious health endpoints (e.g., hospital 

admissions and emergency department 

visits) that cannot be assessed in 

controlled human exposure studies. For 

these studies the degree of uncertainty 

introduced by confounding variables 

(e.g., other pollutants) affects the level 

of confidence that the health effects 

being investigated are attributable to 

NO2 exposures alone and/or in 

combination with co-occurring 

pollutants. 

In using a weight of evidence 

approach to inform judgments about the 

degree of confidence that various health 

effects are likely to be caused by 

exposure to NO2, confidence increases 

with the number of studies consistently 

reporting a particular health endpoint, 


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with increasing support for the 

biological plausibility of the health 

effects, and with the strength and 

coherence of the evidence. Conclusions 

regarding biological plausibility, 

consistency, and coherence of evidence 

of NO2-related health effects are drawn 

from the integration of epidemiologic 

studies with controlled human exposure 

studies and with mechanistic 

information from animal toxicological 

studies. As discussed below, the weight 

of evidence is strongest for respiratory 

morbidity endpoints (e.g., respiratory 

symptoms, hospital admissions, and 

emergency department visits) associated 

with short-term (e.g., 1 to 24 hours) NO2

exposures. 

For epidemiologic studies, strength of 

association refers to the magnitude of 

the association and its statistical 

strength, which includes assessment of 

both effect estimate size and precision. 

In general, when associations yield large 

relative risk estimates, it is less likely 

that the association could be completely 

accounted for by a potential confounder 

or some other bias. Consistency refers to 

the persistent finding of an association 

between exposure and outcome in 

multiple studies of adequate power in 

different persons, places, circumstances 

and times. Based on the information 

presented in the ISA and summarized 

below in sections II.B.1–II.B.3, this 

section discusses judgments concerning 

the extent to which relationships 

between various health endpoints and 

ambient NO2 exposures have been 

judged in the ISA to be likely causal. 

As noted above, this section is 

devoted to discussion of health effects 

associated with NO2 exposure, as 

assessed in the ISA. Section II.B.1 below 

discusses respiratory morbidity 

associated with short-term exposure to 

NO2. The specific endpoints considered 

in this section are respiratory-related 

emergency department visits and 

hospital admissions, respiratory 

symptoms, lung host defense and 

immunity, airway responsiveness, 

airway inflammation, and lung function. 

Section II.B.2 discusses mortality and 

cardiovascular effects associated with 

short-term exposures. Section II.B.3 

discusses effects that have been 

associated with long-term NO2

exposures including respiratory 

morbidity, mortality, cancer, 

cardiovascular effects, and 

reproductive/developmental effects. 

Section II.B.4 discusses the potential 

NO2-related impacts on public health. 

1. Adverse Respiratory Effects and 

Short-Term Exposure to NO2

The ISA concluded that, taken 

together, recent studies provide 

scientific evidence that is sufficient to 

infer a likely causal relationship 

between short-term NO2 exposure and 

adverse effects on the respiratory system 

(ISA, section 5.3.2.1). This 

determination was based on 

consideration of the broad array of 

relevant scientific evidence, as well as 

the uncertainties associated with that 

evidence. Specifically, this 

determination is supported by the large 

body of recent epidemiologic evidence 

as well as findings from human and 

animal experimental studies. 

In considering the uncertainties 

associated with the epidemiologic 

evidence, the ISA (section 5.4) noted 

that it is difficult to determine ‘‘the 

extent to which NO2 is independently 

associated with respiratory effects or if 

NO2 is a marker for the effects of 

another traffic-related pollutant or mix 

of pollutants.’’ On-road vehicle exhaust 

emissions are a nearly ubiquitous source 

of combustion pollutant mixtures that 

include NOX and can be an important 

contributor to NO2 levels in near-road 

locations. Although this complicates 

efforts to quantify specific NO2-related 

health effects, a number of 

epidemiologic studies have evaluated 

associations with NO2 in models that 

also include co-occurring pollutants 

such as PM, O3, CO, and/or SO2. The 

evidence summarized in the ISA 

indicates that NO2 associations 

generally remain robust in these multipollutant models and supports a direct 

effect of short-term NO2 exposure on 

respiratory morbidity (see ISA Figures 

3.1–7, 3.1–10, 3.1–11 and Figures 1 

through 3 below). The plausibility and 

coherence of these effects are also 

supported by epidemiologic studies of 

indoor NO2 as well as experimental (i.e., 

toxicologic and controlled human 

exposure) studies that have evaluated 

host defense and immune system 

changes, airway inflammation, and 

airway responsiveness (see subsequent 

sections of this proposal and the ISA, 

section 5.3.2.1). The ISA (section 5.4) 

concluded that the robustness of 

epidemiologic findings to adjustment 

for co-pollutants, coupled with data 

from animal and human experimental 

studies, support a determination that 

the relationship between NO2 and 

respiratory morbidity is likely causal, 

while still recognizing the relationship 

between NO2 and other traffic related 

pollutants. 

BILLING CODE 6560–50–P 


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6Effect estimates in the ISA were standardized to 

a 30 ppb increase in NO2 concentrations and to a 

20 ppb increase for studies that evaluated 24-hour 

average concentrations. 

BILLING CODE 6560–50–C 

The epidemiologic and experimental 

studies encompass a number of 

endpoints, including emergency 

department visits and hospitalizations, 

respiratory symptoms, airway 

hyperresponsiveness, airway 

inflammation, and lung function. Effect 

estimates from epidemiologic studies 

conducted in the United States and 

Canada generally indicate a 2–20%6

increase in risks for emergency 

department visits and hospital 

admissions and higher risks for 

respiratory symptoms (ISA, section 5.4). 

The findings relevant to these 

endpoints, which provide the rationale 

to support the judgment of a likely 

causal relationship, are described in 

more detail below. 

a. Emergency Department Visits and 

Hospital Admissions 

Epidemiologic evidence exists for 

positive associations of short-term 

ambient NO2 concentrations below the 

current NAAQS with increased numbers 

of emergency department visits and 

hospital admissions for respiratory 

causes, especially asthma (ISA, section 

5.3.2.1). Total respiratory causes for 

emergency department visits and 

hospitalizations typically include 

asthma, bronchitis and emphysema 

(collectively referred to as COPD), 

pneumonia, upper and lower respiratory 

infections, and other minor categories. 

Temporal associations between 

respiratory emergency department visits 

or hospital admissions and ambient 

levels of NO2 have been the subject of 

over 50 peer-reviewed research 

publications since the review of the NO2

NAAQS that was completed in 1996. 

These studies have examined morbidity 

in different age groups and have often 

utilized multi-pollutant models to 

evaluate potential confounding effects 

of co-pollutants. Associations are 

particularly consistent among children 

(< 14 years) and older adults (> 65 years) 

when all respiratory outcomes are 

analyzed together (ISA, Figures 3.1–8 

and 3.1–9) and among children and 

subjects of all ages for asthma 

admissions (ISA, Figures 3.1–12 and 

3.1–13). When examined with copollutant models, associations of NO2

with respiratory emergency department 

visits and hospital admissions were 

generally robust and independent of the 

effects of co-pollutants (i.e., magnitude 

of effect estimates remained relatively 

unchanged) (ISA, Figures 3.1–10 and 

3.1–11). The plausibility and coherence 

of these effects are supported by 

experimental (i.e., toxicologic and 

controlled human exposure) studies that 

evaluate host defense and immune 

system changes, airway inflammation, 

and airway responsiveness (see 

subsequent sections of this document 

and ISA, section 5.3.2.1). 

Of the respiratory emergency 

department visit and hospital admission 

studies reviewed in the ISA, 6 key 

studies were conducted in the United 

States (ISA, Table 5.4–1). Of these 6 

studies, 4 evaluated associations with 

NO2 using multi-pollutant models (Peel 

et al., 2005 and updated in Tolbert et 

al., 2007 in Atlanta; New York 

Department of Health (NYDOH), 2006 

and Ito et al., 2007 in New York City), 

while 2 studies evaluated only single 

pollutant models (Linn et al., 2000 in 

Los Angeles; Jaffe et al., 2003 in 

Cleveland/Cincinnati, OH). In the study 

by Peel and colleagues, investigators 

evaluated respiratory emergency 

department visits among all ages in 

Atlanta, GA during the period from 

1993 to 2000. Using single pollutant 

models, a 2.4% (95% CI: 0.9%, 4.1%) 

increase in respiratory emergency 

department visits was associated with a 

30-ppb increase in 1-hour maximum 

NO2 concentrations. For asthma visits, a 

4.1% (95% CI: 0.8%, 7.6%) increase 

was estimated in individuals 2 to 18 

years of age. Tolbert and colleagues 

reanalyzed these data with 4 additional 

years of information and found 

essentially similar results in single 

pollutant models (2.0% increase, 95% 

CI: 0.5%, 3.3%). This same study found 

that the associations were positive, but 

not statistically significant, in multipollutant models that included PM10 or 

O3 (Figure 2 in published manuscript). 

In the study conducted by the NYDOH, 

investigators evaluated asthma 


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emergency department visits in Bronx 

and Manhattan, New York over the 

period of January 1999 to November 

2000. In Bronx, a 6% (95% CI: 1%, 

10%) increase in visits was estimated 

per 20 ppb increase in 24-hour average 

concentrations of NO2 and a 7% (95% 

CI: 2%, 12%) increase in visits was 

estimated per 30 ppb increase in daily 

1-hour maximum concentrations. These 

effects were not statistically significant 

in 2-pollutant models that included 

PM2.5 or SO2 (Tables 4a and 9 in 

manuscript). In Manhattan, the authors 

found non-significant decreases (3% for 

24-hour and a 2% for daily 1-hour 

maximum) in asthma-related emergency 

department visits associated with 

increasing NO2. In the study by Ito and 

colleagues (2007), investigators 

evaluated respiratory emergency 

department visits for asthma in New 

York City during the years 1999 to 2002. 

A 12% (95% CI: 7%, 15%) increase in 

risk was estimated per 20 ppb increase 

in 24-hour ambient NO2. Risk estimates 

were robust and remained statistically 

significant in multi-pollutant models 

that included PM2.5, O3, CO, and SO2

(figure 8 in manuscript). With regard to 

the studies that evaluated only single 

pollutant models, Linn et al. (2000) 

detected a statistically significant 

increase in respiratory hospital 

admissions and Jaffe et al. (2003) 

detected a positive, but not statistically 

significant, increase in respiratory 

emergency department visits associated 

with 24-hour NO2 concentrations. 

b. Respiratory Symptoms 

Evidence for associations between 

NO2 and respiratory symptoms is 

derived primarily from the 

epidemiologic literature, although the 

experimental evidence for airway 

inflammation and immune system 

effects (described in the ISA, section 

3.1) does provide support for the 

plausibility and coherence for the 

epidemiologic results (ISA, section 

5.3.2.1). Consistent evidence has been 

observed for an association of 

respiratory effects with indoor and 

personal NO2 exposures in children 

(ISA, sections 3.1.5.1 and 5.3.2.1) and 

with ambient levels of NO2, as measured 

by area-wide monitors (ISA, sections 

3.1.4.2 and 5.3.2.1, see Figure 3.1–6). In 

the results of multi-pollutant models, 

NO2 associations in multicity studies 

are generally robust to adjustment for 

co-pollutants including O3, CO, and 

PM10 (ISA, sections 3.1.4.3, 5.3.2.1 and 

Figure 3.1–7). Specific studies of 

respiratory symptoms are discussed in 

more detail below. 

Epidemiologic studies using 

community ambient monitors have 

found associations between ambient 

NO2 concentrations and respiratory 

symptoms (ISA, sections 3.1.4.2 and 

5.3.2.1, Figure 3.1–6) in cities where the 

entire range of 24-hour average NO2

concentrations were well below the 

level of the current NAAQS (0.053 ppm 

annual average). Several studies have 

been published since the last review 

including single-city studies (e.g., Ostro 

et al., 2001; Delfino et al., 2002) and 

multicity studies in urban areas 

covering the continental United States 

and southern Ontario (Schwartz et al., 

1994; Mortimer et al., 2002; Schildcrout 

et al., 2006). 

Schwartz et al. (1994) studied 1,844 

schoolchildren, followed for 1 year, as 

part of the Six Cities Study that 

included the cities of Watertown, MA, 

St. Louis, MO, Kingston-Harriman, TN, 

Steubenville, OH, Topeka, KS, and 

Portage, WI. Respiratory symptoms were 

recorded daily. The authors reported a 

significant association between 4-day 

mean NO2 levels and incidence of cough 

among all children in single-pollutant 

models, with an odds ratio (OR) of 1.61 

(95% CI: 1.08, 2.43) standardized to a 

20-ppb increase in NO2. The incidence 

of cough increased up to approximately 

mean NO2 levels (13 ppb) (p = 0.01), 

after which no further increase was 

observed. The significant association 

between cough and 4-day mean NO2

level remained unchanged in models 

that included O3 but lost statistical 

significance in two-pollutant models 

that included PM10 (OR = 1.37 [95% CI: 

0.88, 2.13]) or SO2 (OR = 1.42 [95% CI: 

0.90, 2.28]). 

Mortimer et al. (2002) studied the risk 

of asthma symptoms among 864 

asthmatic children in New York City, 

NY, Washington, DC, Cleveland, OH, 

Detroit, MI, St Louis, MO, and Chicago, 

IL. Subjects were followed daily for four 

2-week periods over the course of nine 

months with morning and evening 

asthma symptoms and peak flow 

recorded. The greatest effect was 

observed for morning symptoms using a 

6-day moving average, with a reported 

OR of 1.48 (95% CI: 1.02, 2.16) per 20 

ppb increase in NO2. Although the 

magnitudes of effect estimates were 

generally robust in multi-pollutant 

models that included O3 (OR for 20-ppb 

increase in NO2 = 1.40 [95% CI: 0.93, 

2.09]), O3 and SO2 (OR for NO2 = 1.31 

[95% CI: 0.87, 2.09]), or O3, SO2, and 

PM10 (OR for NO2 = 1.45 [95% CI: 0.63, 

3.34]), they were not statistically 

significant. 

Schildcrout et al. (2006) investigated 

the association between ambient NO2

and respiratory symptoms and rescue 

inhaler use as part of the Childhood 

Asthma Management Program (CAMP) 

study. The study reported on 990 

asthmatic children living within 50 

miles of an NO2 monitor in Boston, MA, 

Baltimore, MD, Toronto, ON, St. Louis, 

MO, Denver, CO, Albuquerque, NM, or 

San Diego, CA. Symptoms and use of 

rescue medication were recorded daily, 

resulting in each subject having an 

average of approximately two months of 

data. The authors reported the strongest 

association between NO2 and increased 

risk of cough for a 2-day lag, with an OR 

of 1.09 (95% CI: 1.03, 1.15) for each 20- 

ppb increase in NO2 occurring 2 days 

before measurement. Multi-pollutant 

models that included CO, PM10, or SO2

produced similar results (ISA, Figure 

3.1–5, panel A). Additionally, increased 

NO2 exposure was associated with 

increased use of rescue medication, 

with the strongest association for a 2- 

day lag. In the single-pollutant model, 

the relative risk (RR) for increased 

inhaler usage was 1.05 (95% CI: 1.01, 

1.09). 

Evidence supporting increased 

respiratory symptoms following NO2

exposures is found in studies focused 

on indoor sources of NO2 (ISA, section 

3.1.4.1). These studies are not 

confounded by the same mix of copollutants 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 concentrationresponse relationship, strengthening the 

argument that NO2 is causally related to 

respiratory morbidity. A number of 

other indoor studies conducted in 

homes with gas appliances have also 

detected significant associations 

between indoor NO2 and respiratory 

symptoms (ISA, section 3.1.4.1). 

c. Impaired Host Defense 

Impaired host-defense systems and 

increased risk of susceptibility to both 

viral and bacterial infections after NO2

exposures have been observed in 


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7The updated meta-analysis added a study that 

evaluated non-specific airway responsiveness 

following exposure to 260 ppb NO2 and removed 

a study that evaluated allergen-induced airway 

responsiveness following exposure to 100 ppb NO2. 

epidemiologic, controlled human 

exposure, and animal toxicological 

studies (ISA, section 3.1.1 and 5.3.2.1). 

A recent epidemiologic study (Chauhan 

et al., 2003) provides evidence that 

increased personal exposure to NO2

worsened virus-associated symptoms 

and decreased lung function in children 

with asthma. The limited evidence from 

controlled human exposure studies 

indicates that NO2 may increase 

susceptibility to lung injury by 

subsequent viral challenge at exposures 

of as low as 600 ppb for 3 hours in 

healthy adults (Frampton et al., 2002). 

Toxicological studies have shown that 

lung host defenses, including 

mucociliary clearance and immune cell 

function, are sensitive to NO2 exposure, 

with effects observed at concentrations 

of less than 1000 ppb (ISA, section 

3.1.7). When taken together, 

epidemiologic and experimental studies 

linking NO2 exposure with viral 

illnesses provide coherent and 

consistent evidence that NO2 exposure 

can result in lung host defense or 

immune system effects (ISA, sections 

3.1.7 and 5.3.2.1). This group of 

outcomes also provides some 

plausibility for other respiratory system 

effects. For example, effects on ciliary 

action (clearance) or immune cell 

function (i.e. macrophage phagocytosis) 

could be the basis for the effects 

observed in epidemiologic studies, 

including increased respiratory illness 

or respiratory symptoms (ISA, section 

5.3.2.1). Proposed mechanisms by 

which NO2, in conjunction with viral 

infections, may exacerbate airway 

symptoms are summarized in the ISA 

(Table 3.1–1). 

d. Airway Response 

In acute exacerbations of asthma, 

bronchial smooth muscle contraction 

occurs quickly to narrow the airway in 

response to exposure to various stimuli 

including allergens or irritants. 

Bronchoconstriction is the dominant 

physiological event leading to clinical 

symptoms and interference with airflow 

(National Heart, Lung, and Blood 

Institute, 2007). Inhaled pollutants such 

as NO2 may enhance the inherent 

responsiveness of the airway to a 

challenge by allergens and nonspecific 

agents (ISA, section 3.1.3). In the 

laboratory, airway responses can be 

measured by assessing changes in 

pulmonary function (e.g., decline in 

FEV1) or changes in the inflammatory 

response (e.g., using markers in 

bronchoalveolar lavage (BAL) fluid or 

induced sputum) (ISA, section 3.1.3). 

The ISA (section 5.3.2.1) drew two 

broad conclusions regarding airway 

responsiveness in asthmatics following 

NO2 exposure. First, the ISA concluded 

that NO2 exposure may enhance the 

sensitivity to allergen-induced 

decrements in lung function and 

increase the allergen-induced airway 

inflammatory response at exposures as 

low as 260 ppb NO2 for 30 minutes 

(ISA, section 5.3.2.1 and Figure 3.1–2). 

Second, exposure to NO2 has been 

found to enhance the inherent 

responsiveness of the airway to 

subsequent nonspecific challenges in 

controlled human exposure studies 

(section 3.1.3.2). In general, small but 

significant increases in nonspecific 

airway responsiveness were observed in 

the range of 200 to 300 ppb NO2 for 30- 

minute exposures and at 100 ppb NO2

for 60-minute exposures in asthmatics. 

These conclusions are consistent with 

results from animal toxicological studies 

which have detected 1) increased 

immune-mediated pulmonary 

inflammation in rats exposed to house 

dust mite allergen following exposure to 

5000 ppb NO2 for 3-h and 2) increased 

responsiveness to non-specific 

challenges following sub-chronic (6–12 

weeks) exposure to 1000 to 4000 ppb 

NO2 (ISA, section 5.3.2.1). 

Enhanced airway responsiveness 

could have important clinical 

implications for asthmatics since 

transient increases in airway 

responsiveness following NO2 exposure 

have the potential to increase symptoms 

and worsen asthma control (ISA, section 

5.4). In addition, the ISA cited the 

controlled human exposure literature on 

the NO2 airway response as being 

supportive of the epidemiologic 

evidence on respiratory morbidity (ISA, 

section 5.4). Because studies on airway 

responsiveness have been used to 

identify potential health effect 

benchmark values and to inform the 

identification of potential alternative 

standards for evaluation (see REA, 

sections 4.5 and 5), more detail is 

provided below on the specific studies 

that form the basis for the conclusions 

in the ISA regarding this endpoint. 

Folinsbee (1992) conducted a metaanalysis 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 metaanalysis, which focused only on data for 

nonspecific responsiveness, are 

presented in the ISA (Table 3.1–3).7

When exposed at rest, 66% of 

asthmatics experienced an increase in 

airway responsiveness following 

exposure to 100 ppb NO2, 67% of 

asthmatics experienced an increase in 

airway responsiveness following 

exposure to NO2 concentrations 

between 100 and 150 ppb (inclusively), 

75% of subjects experienced an increase 

in airway responsiveness following 

exposure to NO2 concentrations 

between 200 and 300 ppb (inclusively), 

and 73% of subjects experienced an 

increase in airway responsiveness 

following exposure to NO2

concentrations above 300 ppb. Effects of 

NO2 exposure on the direction of airway 

responsiveness were statistically 

significant at all of these levels. Because 

this meta-analysis evaluated only the 

direction of the change in airway 

responsiveness, it is not possible to 

discern the magnitude of the change 

from these data. However, the results do 

suggest that short-term (i.e., 30-min to 3- 

h) exposures to NO2 at near-ambient 

levels (<300 ppb) can alter airway 

responsiveness in people with mild 

asthma (ISA, section 3.1.3.2). 

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 allergeninduced 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 


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increase in the BAL recovery of 

polymorphonuclear (PMN) cells and 

eosinophil cationic protein (ECP) as 

well as a reduction in total BAL fluid 

volume and cell viability. ECP is 

released by degranulating eosinophils, 

is toxic to respiratory epithelial cells, 

and is thought to play a role in the 

pathogenesis of airway injury in asthma. 

Subsequently, Barck et al. (2005) 

exposed 18 mild asthmatics to air or 260 

ppb NO2 for 15 minutes on day 1, 

followed by two 15 minute exposures 

separated by 1 hour on day 2, with 

allergen challenge after exposures on 

both days 1 and 2. Sputum was induced 

before exposure on day 1 and after 

exposures (morning of day 3). Compared 

to air plus allergen, NO2 plus allergen 

resulted in increased levels of ECP in 

both sputum and blood and increased 

myeloperoxidase levels in blood. 

All exposures in these studies (Barck 

et al., 2002, 2005; Strand et al., 1997, 

1998) used subjects at rest. They used 

an adequate number of subjects, 

included air control exposures, 

randomized exposure order, and 

separated exposures by at least 2 weeks. 

Together, they indicate the possibility 

for effects on allergen responsiveness in 

some asthmatics following brief 

exposures to 260 ppb NO2. Other recent 

studies have failed to find effects using 

similar, but not identical, approaches 

(ISA, section 3.1.3.1). The differing 

findings may relate in part to differences 

in timing of the allergen challenge, the 

use of multiple versus single-dose 

allergen challenge, the use of BAL 

versus sputum induction, exercise 

versus rest during exposure, and 

differences in subject susceptibility 

(ISA, section 3.1.3.1). 

e. Airway Inflammation 

Effects of NO2 on airway 

inflammation have been observed in 

controlled human exposure and animal 

toxicological studies at higher than 

ambient levels (400–5000 ppb). 

Controlled human exposure studies 

provide evidence for increased airway 

inflammation at NO2 concentrations of 

<2000 ppb. The onset of inflammatory 

responses in healthy subjects appears to 

be between 100 and 200 ppm-minutes, 

i.e., 1000 ppb for 2 to 3 hours (ISA, 

Figure 3.1–1). Increases in biological 

markers of inflammation were not 

observed consistently in healthy 

animals at levels of less than 5000 ppb; 

however, increased susceptibility (as 

indicated by biochemical markers of 

inflammation) to NO2 concentrations of 

as low as 400 ppb was observed when 

lung vitamin C was reduced (by diet) to 

levels that were <50% of normal. The 

few available epidemiologic studies 

were suggestive of an association 

between ambient NO2 concentrations 

and inflammatory response in the 

airway in children, though the 

associations were inconsistent in the 

adult populations examined (ISA, 

section 3.1.2 and 5.3.2.1). These data 

provide some evidence for biological 

plausibility and one potential 

mechanism for other respiratory effects, 

such as exacerbation of asthma 

symptoms and increased emergency 

department visits for asthma (ISA, 

section 5.3.2.1). 

f. Lung Function 

Recent epidemiologic studies that 

examined the association between 

ambient NO2 concentrations and lung 

function in children and adults have 

produced inconsistent results (ISA, 

sections 3.1.5.1 and 5.3.2.1). Controlled 

human exposure studies generally did 

not find direct effects of NO2 on lung 

function in healthy adults at levels as 

high as 4000 ppb (ISA, section 5.3.2.1). 

For asthmatics, the direct effects of NO2

on lung function also have been 

inconsistent at exposure concentrations 

of less than 1000 ppb NO2. 

g. Conclusions From the ISA 

As noted previously, the ISA 

concluded that the findings of 

epidemiologic, controlled human 

exposure, and animal toxicological 

studies provide evidence that is 

sufficient to infer a likely causal 

relationship for respiratory effects 

following short-term NO2 exposure 

(ISA, sections 3.1.7 and 5.3.2.1). The 

ISA (section 5.4) concluded that the 

strongest evidence for an association 

between NO2 exposure and adverse 

human health effects comes from 

epidemiologic studies of respiratory 

symptoms, emergency department 

visits, and hospital admissions. These 

studies include panel and field studies, 

studies that control for the effects of cooccurring pollutants, and studies 

conducted in areas where the whole 

distribution of ambient 24-hour average 

NO2 concentrations was below the 

current NAAQS level of 53 ppb (annual 

average). With regard to this evidence, 

the ISA concluded that NO2

epidemiologic studies provide ‘‘little 

evidence of any effect threshold’’ (ISA, 

section 5.3.2.9, p. 5–15). In studies that 

have evaluated concentration-response 

relationships, they appear linear within 

the observed range of data (ISA, section 

5.3.2.9). 

Overall, the epidemiologic evidence 

for respiratory effects has been 

characterized in the ISA as consistent, 

in that associations are reported in 

studies conducted in numerous 

locations with a variety of 

methodological approaches. 

Considering this large body of 

epidemiologic studies alone, the 

findings have also been characterized as 

coherent in that the studies report 

associations with respiratory health 

outcomes that are logically linked 

together. In addition, a number of these 

associations are statistically significant, 

particularly the more precise effect 

estimates (ISA, section 5.3.2.1). These 

epidemiologic studies are supported by 

evidence from toxicological and 

controlled human exposure studies, 

particularly those that evaluated airway 

hyperresponsiveness in asthmatic 

individuals (ISA, section 5.4). The ISA 

concluded that together, the 

epidemiologic and experimental data 

sets form a plausible, consistent, and 

coherent description of a relationship 

between NO2 exposures and an array of 

adverse respiratory health effects that 

range from the onset of respiratory 

symptoms to hospital admissions. 

2. Other Effects With Short-Term 

Exposure to NO2

a. Mortality 

The ISA concluded that the 

epidemiologic evidence is suggestive, 

but not sufficient, to infer a causal 

relationship between short-term 

exposure to NO2 and all-cause and 

cardiopulmonary-related mortality (ISA, 

section 5.3.2.3). Results from several 

large U.S. and European multicity 

studies and a meta-analysis study 

indicate positive associations between 

ambient NO2 concentrations and the 

risk of all-cause (nonaccidental) 

mortality, with effect estimates ranging 

from 0.5 to 3.6% excess risk in mortality 

per standardized increment (20 ppb for 

24-hour averaging time, 30 ppb for 1- 

hour averaging time) (ISA, section 3.3.1, 

Figure 3.3–2, section 5.3.2.3). In general, 

the NO2 effect estimates were robust to 

adjustment for co-pollutants. Both 

cardiovascular and respiratory mortality 

have been associated with increased 

NO2 concentrations in epidemiologic 

studies (ISA, Figure 3.3–3); however, 

similar associations were observed for 

other pollutants, including PM and SO2. 

The range of risk estimates for excess 

mortality is generally smaller than that 

for other pollutants such as PM. In 

addition, while NO2 exposure, alone or 

in conjunction with other pollutants, 

may contribute to increased mortality, 

evaluation of the specificity of this 

effect is difficult. Clinical studies 

showing hematologic effects and animal 

toxicological studies showing 

biochemical, lung host defense, 

permeability, and inflammation changes 


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with short-term exposures to NO2

provide limited evidence of plausible 

pathways by which risks of mortality 

may be increased, but no coherent 

picture is evident at this time (ISA, 

section 5.3.2.3). 

b. Cardiovascular Effects 

The ISA concluded that the available 

evidence on cardiovascular health 

effects following short-term exposure to 

NO2 is inadequate to infer the presence 

or absence of a causal relationship at 

this time (ISA, section 5.3.2.2). Evidence 

from epidemiologic studies of heart rate 

variability, repolarization changes, and 

cardiac rhythm disorders among heart 

patients with ischemic cardiac disease 

are inconsistent (ISA, section 5.3.2.2). In 

most studies, associations with PM were 

found to be similar or stronger than 

associations with NO2. Generally 

positive associations between ambient 

NO2 concentrations and hospital 

admissions or emergency department 

visits for cardiovascular disease have 

been reported in single-pollutant 

models (ISA, section 5.3.2.2); however, 

most of these effect estimate values were 

diminished in multi-pollutant models 

that also contained CO and PM indices 

(ISA, section 5.3.2.2). Mechanistic 

evidence of a role for NO2 in the 

development of cardiovascular diseases 

from studies of biomarkers of 

inflammation, cell adhesion, 

coagulation, and thrombosis is lacking 

(ISA, section 5.3.2.2). Furthermore, the 

effects of NO2 on various hematological 

parameters in animals are inconsistent 

and, thus, provide little biological 

plausibility for effects of NO2 on the 

cardiovascular system (ISA, section 

5.3.2.2). 

3. Health Effects With Long-Term 

Exposure to NO2

a. Respiratory Morbidity 

The ISA concluded that overall, the 

epidemiologic and experimental 

evidence is suggestive, but not 

sufficient, to infer a causal relationship 

between long-term NO2 exposure and 

respiratory morbidity (ISA, section 

5.3.2.4). The available database 

evaluating the relationship between 

respiratory illness in children and longterm exposures to NO2 has increased 

since the 1996 review of the NO2

NAAQS. A number of epidemiologic 

studies have examined the effects of 

long-term exposure to NO2 and reported 

positive associations with decrements in 

lung function and partially irreversible 

decrements in lung function growth 

(ISA, section 3.4.1, Figures 3.4–1 and 

3.4–2). Specifically, results from the 

California-based Children’s Health 

Study, which evaluated NO2 exposures 

in children over an 8-year period, 

demonstrated deficits in lung function 

growth (Gauderman et al., 2004). This 

effect has also been observed in Mexico 

City, Mexico (Rojas-Martinez et al., 

2007a,b) and in Oslo, Norway (Oftedal 

et al., 2008), with decrements ranging 

from 1 to 17.5 ml per 20-ppb increase 

in annual NO2 concentration. Similar 

associations have been found for PM, 

O3, and proximity to traffic (<500 m), 

though these studies did not report the 

results of co-pollutant models. The high 

correlation among traffic-related 

pollutants makes it difficult to 

accurately estimate independent effects 

in these long-term exposure studies 

(ISA, section 5.3.2.4). With regard to 

asthma incidence and long-term NO2, 

two major cohort studies, the Children’s 

Health Study (Gauderman et al., 2005) 

and a birth cohort study in the 

Netherlands (Brauer et al., 2007), 

observed significant associations. 

However, several other studies failed to 

find consistent associations between 

long-term NO2 exposure and asthma 

outcomes (ISA, section 5.3.2.4). 

Similarly, epidemiologic studies 

conducted in the United States and 

Europe reported inconsistent results 

regarding an association between longterm exposure to NO2 and respiratory 

symptoms (ISA, sections 3.4.3 and 

5.3.2.4). While some positive 

associations were noted, a large number 

of symptom outcomes were examined 

and the results across specific outcomes 

were inconsistent (ISA, section 5.3.2.4). 

Animal toxicological studies may 

provide biological plausibility for the 

chronic effects of NO2 that have been 

observed in epidemiologic studies (ISA, 

sections 3.4.5 and 5.3.2.4). The main 

biochemical targets of NO2 exposure 

appear to be antioxidants, membrane 

polyunsaturated fatty acids, and thiol 

groups. NO2 effects include changes in 

oxidant/antioxidant homeostasis and 

chemical alterations of lipids and 

proteins. Lipid peroxidation has been 

observed at NO2 exposures as low as 40 

ppb for 9 months and at exposures of 

1200 ppb for 1 week, suggesting lower 

effect thresholds with longer durations 

of exposure. Other studies showed 

decreases in formation of key 

arachidonic acid metabolites in alveolar 

macrophages following NO2 exposures 

of 500 ppb. NO2 has been shown to 

increase collagen synthesis rates at 

concentrations as low as 500 ppb. This 

could indicate increased total lung 

collagen, which is associated with 

pulmonary fibrosis, or increased 

collagen turnover, which is associated 

with remodeling of lung connective 

tissue. Morphological effects following 

chronic NO2 exposures have been 

identified in animal studies that link to 

these increases in collagen synthesis 

and may provide plausibility for the 

deficits in lung function growth 

described in epidemiologic studies of 

long-term exposure to NO2 (ISA, section 

3.4.5). 

b. Mortality 

The ISA concluded that the 

epidemiologic evidence is inadequate to 

infer the presence or absence of a causal 

relationship between long-term 

exposure to NO2 and mortality (ISA, 

section 5.3.2.6). In the United States and 

European cohort studies examining the 

relationship between long-term 

exposure to NO2 and mortality, results 

have been inconsistent (ISA, section 

5.3.2.6). Further, when associations 

were suggested, they were not specific 

to NO2 but also implicated PM and 

other traffic indicators. The relatively 

high correlations reported between NO2

and PM indices make it difficult to 

interpret these observed associations at 

this time (ISA, section 5.3.2.6). 

c. Carcinogenic, Cardiovascular, and 

Reproductive/Developmental Effects 

The ISA concluded that the available 

epidemiologic and toxicological 

evidence is inadequate to infer the 

presence or absence of a causal 

relationship for carcinogenic, 

cardiovascular, and reproductive and 

developmental effects related to longterm NO2 exposure (ISA, section 

5.3.2.5). Epidemiologic studies 

conducted in Europe have shown an 

association between long-term NO2

exposure and increased incidence of 

cancer (ISA, section 5.3.2.5). However, 

the animal toxicological studies have 

provided no clear evidence that NO2

acts as a carcinogen (ISA, section 

5.3.2.5). The very limited epidemiologic 

and toxicological evidence do not 

suggest that long-term exposure to NO2

has cardiovascular effects (ISA, section 

5.3.2.5). The epidemiologic evidence is 

not consistent for associations between 

NO2 exposure and fetal growth 

retardation; however, some evidence is 

accumulating for effects on preterm 

delivery (ISA, section 5.3.2.5). Scant 

animal evidence supports a weak 

association between NO2 exposure and 

adverse birth outcomes and provides 

little mechanistic information or 

biological plausibility for the 

epidemiologic findings. 

4. NO2-Related Impacts on Public 

Health 

Specific groups within the general 

population are likely at increased risk 


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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 preexisting disease/condition (e.g., obesity, 

diabetes, respiratory disease, asthma, 

chronic obstructive pulmonary disease 

(COPD), cardiovascular disease, airway 

hyperresponsiveness, respiratory 

infection, adverse birth outcome) (ISA, 

sections 4.3.1, 4.3.5, and 5.3.2.8). In 

addition, certain groups may experience 

relatively high exposure to NO2, thus 

forming a potentially vulnerable 

population (ISA, section 4.3.6). Factors 

that may influence exposures and/or 

susceptibility to air pollution include 

socioeconomic status (SES), education 

level, air conditioning use, proximity to 

roadways, geographic location, level of 

physical activity, and work environment 

(e.g., indoor versus outdoor) (ISA, 

section 4.3.5). The ISA discussed factors 

that can confer susceptibility and/or 

vulnerability to air pollution with most 

of the discussion devoted to factors for 

which NO2-specific evidence exists 

(ISA, section 4.3). These factors are 

discussed below. 

a. Pre-Existing Disease 

A number of health conditions have 

been found to put individuals at greater 

risk for adverse events following 

exposure to air pollution. In general, 

these include asthma, COPD, respiratory 

infection, cardiac conduction disorders, 

congestive heart failure (CHF), diabetes, 

past myocardial infarction (MI), obesity, 

coronary artery disease, low birth 

weight/prematurity, and hypertension 

(ISA, sections 4.3.1, 4.3.5, and 5.3.2.9). 

In addition to these conditions, 

epidemiologic evidence indicates that 

individuals with bronchial or airway 

hyperresponsiveness, as determined by 

methacholine provocation, may be at 

increased risk for experiencing 

respiratory symptoms (ISA, section 

4.3.1). In considering NO2 specifically, 

the ISA evaluated studies on asthmatics, 

individuals with cardiopulmonary 

disease, and diabetics (ISA, sections 

4.3.1.1 and 4.3.1.2). These groups are 

discussed in more detail below. 

Epidemiologic and controlled human 

exposure studies, supported by animal 

toxicology studies, have provided 

evidence for associations between NO2

exposure and respiratory effects in 

asthmatics (ISA, section 4.3.1.1). The 

ISA found evidence from epidemiologic 

studies for an association between 

ambient NO2 and children’s hospital 

admissions, emergency department 

visits, and calls to doctors for asthma. 

Long-term NO2 exposure was associated 

with aggravation of asthma effects that 

include symptoms, medication use, and 

lung function. Time-series studies 

demonstrated a relationship in children 

between hospital admissions or 

emergency department visits for asthma 

and ambient NO2 levels, even after 

adjusting for co-pollutants such as PM 

and CO (ISA, section 4.3.1.1). Important 

evidence was available from 

epidemiologic studies of indoor NO2

exposures. Recent studies have shown 

associations with asthma attacks and 

severity of virus-induced asthma (ISA, 

section 4.3.1.1). In addition, in 

controlled human exposure studies, 

airway hyperresponsiveness in 

asthmatics occurred following exposure 

to ambient or near-ambient NO2

concentrations (ISA, sections 5.3.2.1– 

5.3.2.6). Compared to asthma, less 

evidence is available to support 

cardiovascular disease as a mediator of 

susceptibility to NO2. However, recent 

epidemiologic studies report that 

individuals with preexisting conditions 

(e.g., including diabetes, CHF, prior MI) 

may be at increased risk for adverse 

cardiac health events associated with 

ambient NO2 concentrations (ISA, 

section 4.3.1.2). The small number of 

controlled human exposure and animal 

toxicological studies that have evaluated 

cardiovascular endpoints provide only 

limited supporting evidence for 

susceptibility to NO2 in persons with 

cardiovascular disease (ISA, section 

4.3.1.2). 

b. Age 

The ISA identified infants, children 

(i.e., <18 years of age), and older adults 

(i.e., >65 years of age) as groups that are 

potentially more susceptible than the 

general population to the health effects 

associated with ambient NO2

concentrations (ISA, section 4.3.2). The 

ISA found evidence that associations of 

NO2 with respiratory emergency 

department visits and hospitalizations 

were stronger among children and older 

adults, though not all studies had 

comparable findings on this issue (ISA, 

section 4.3.2). In addition, long-term 

exposure studies suggest effects in 

children that include impaired lung 

function growth, increased respiratory 

symptoms and infections, and onset of 

asthma (ISA, section 3.4 and 4.3.2). In 

some studies, associations between NO2

and hospitalizations or emergency 

department visits for CVD have been 

observed in elderly populations. Among 

studies that observed positive 

associations between NO2 and mortality, 

a comparison indicated that, in general, 

the elderly population was more 

susceptible than the non-elderly 

population to NO2 effects (ISA, section 

4.3.2). 

c. Genetics 

As noted in the ISA (section 4.3.4), 

genetic factors related to health 

outcomes and ambient pollutant 

exposures merit consideration. Several 

criteria should be satisfied in selecting 

and establishing useful links between 

polymorphisms in candidate genes and 

adverse respiratory effects. First, the 

candidate gene must be significantly 

involved in the pathogenesis of the 

adverse effect of interest. Second, 

polymorphisms in the gene must 

produce a functional change in either 

the protein product or in the level of 

expression of the protein. Third, in 

epidemiologic studies, the issue of 

confounding by other environmental 

exposures must be carefully considered 

(ISA, section 4.3.4). Investigation of 

genetic susceptibility to NO2 effects has 

focused on the glutathione S-tranferase 

(GST) gene. Several GST genes have 

common, functionally-important alleles 

that affect host defense in the lung (ISA, 

section 4.3.4). GST genes are inducible 

by electrophilic species (e.g., reactive 

oxygen species) and individuals with 

genotypes that result in enzymes with 

reduced or absent peroxidase activity 

are likely to have reduced defenses 

against oxidative insult. This could 

potentially result in increased 

susceptibility to inhaled oxidants and 

radicals. However, data on genetic 

susceptibility to NO2 are only beginning 

to emerge and, while it remains 

plausible that there are genetic factors 

that can influence health responses to 

NO2, the few available studies do not 

provide specific support for genetic 

susceptibility to NO2 exposure (ISA, 

section 4.3.4). 

d. Gender 

As reported in the ISA, a limited 

number of NO2 studies have stratified 

results by gender. The results of these 

studies were mixed, and the ISA did not 


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8The most current American Housing Survey 

(http://www.census.gov/hhes/www/housing/ahs/ 

ahs.html) is from 2007 and lists a higher fraction 

of housing units within the 300 foot boundary than 

do prior surveys. According to Table IA–6 from that 

report (http://www.census.gov/hhes/www/housing/ 

ahs/ahs07/tab1a–6.pdf), out of 128,303,000 total 

housing units in the United States, 20,016,000 were 

reported by the surveyed occupant or landlord as 

being within 300 feet of a 4-or-more lane highway, 

railroad, or airport. That constitutes 15.613% of the 

total housing units in the U.S. Assuming equal 

distributions, with a current population of 

306,330,199, that means that there would be 47.8 

million people meeting the 300 foot criteria. 

draw conclusions regarding the 

potential for gender to confer 

susceptibility to the effects of NO2 (ISA, 

section 4.3.3). 

e. Proximity to Roadways 

Certain groups may experience 

relatively high exposure to NO2, thus 

forming a potentially vulnerable 

population. The ISA included 

discussion of populations reported to 

experience increased NO2 exposures on 

or near roadways (ISA, section 4.3.6). 

Large gradients in NOX concentrations 

near roadways may lead to increased 

exposures for individuals residing, 

working, traveling, or attending school 

in the vicinity of roadways. Many 

studies find that indoor, personal, and 

outdoor NO2 levels are strongly 

associated with proximity to traffic or to 

traffic density (ISA, section 4.3.6). 

That adverse respiratory effects can be 

associated with proximity to roadways 

has been demonstrated in a number of 

studies. For example, Gauderman and 

colleagues (2007) reported reduced lung 

function growth in children who lived 

within 500 m of a freeway compared to 

children who lived at least 1500 m from 

a freeway. In a separate study, 

Gauderman and colleagues (2005) 

reported that the incidence of 

physician-diagnosed asthma increased 

with both increasing NO2

concentrations outside the child’s 

residence and decreasing distance 

between the child’s residence and a 

major freeway. 

In addition to those who live near 

major roadways, individuals who spend 

time commuting on major roadways can 

also be exposed to relatively higher 

concentrations of NO2 than the ones 

reported at monitors away from the 

roads. Due to high air exchange rates, 

NO2 concentrations inside a vehicle can 

rapidly approach ambient 

concentrations on the roadway during 

commuting (ISA, section 4.3.6). Mean 

in-vehicle NO2 concentrations are often 

between 2 and 3 times higher than 

ambient levels measured at monitors 

located away from the road (ISA, section 

4.3.6). Due to the potential for high peak 

exposures while driving, total personal 

exposure could be underestimated if 

exposures while commuting are not 

considered. Therefore, individuals with 

occupations that require them to be in 

traffic or close to traffic (e.g., bus and 

taxi drivers, highway patrol officers, toll 

collectors) and individuals with long 

commutes could be exposed to 

relatively high levels of NO2 compared 

to the ambient levels measured at fixedsite monitors located away from the 

roadway. 

f. Socioeconomic Status 

The ISA discussed evidence that SES 

modifies the effects of air pollution 

(section 4.3.6). Many recent studies 

examined modification by SES 

indicators on the association between 

mortality and PM or other indices such 

as traffic density, distance to roadway, 

or a general air pollution index (ISA, 

section 4.3.6). SES modification of NO2

associations has been examined in fewer 

studies. However, in a study conducted 

in Seoul, South Korea, community-level 

SES indicators modified the association 

of air pollution with emergency 

department visits for asthma. Of the five 

criteria air pollutants evaluated, NO2

showed the strongest association in 

lower SES districts compared to high 

SES districts (Kim et al., 2007). In 

addition, Clougherty et al. (2007) 

evaluated exposure to violence (a 

potential surrogate for SES) as a 

modifier of the effect of traffic-related 

air pollutants, including NO2, on 

childhood asthma. The authors reported 

an elevated risk of asthma with an 

increase in NO2 exposure solely among 

children with above-median exposure to 

violence in their neighborhoods (ISA, 

section 4.3.6). Although these recent 

studies have evaluated the impact of 

SES on vulnerability to NO2, they are 

too few in number to draw definitive 

conclusions (ISA, section 5.3.2.8). 

g. Size of the At-Risk Population 

The population potentially affected by 

NO2 is large. A considerable fraction of 

the population resides, works, or 

attends school near major roadways, 

and these individuals are likely to have 

increased exposure to NO2 (ISA, section 

4.4). Based on data from the 2003 

American Housing Survey, 

approximately 36 million individuals 

live within 300 feet (∼90 meters) of a 

four-lane highway, railroad, or airport 

(ISA, section 4.4).8 Furthermore, in 

California, 2.3% of schools with a total 

enrollment of more than 150,000 

students were located within 

approximately 500 feet of high-traffic 

roads, with a higher proportion of nonwhite and economically disadvantaged 

students attending those schools (ISA, 

section 4.4). Of this population, 

asthmatics and members of other 

susceptible groups discussed above will 

have even greater risks of experiencing 

health effects related to NO2 exposure. 

In the United States, approximately 

10% of adults and 13% of children have 

been diagnosed with asthma, and 6% of 

adults have been diagnosed with COPD 

(ISA, section 4.4). The prevalence and 

severity of asthma is higher among 

certain ethnic or racial groups such as 

Puerto Ricans, American Indians, 

Alaskan Natives, and African Americans 

(ISA, section 4.4). A higher prevalence 

of asthma among persons of lower SES 

and an excess burden of asthma 

hospitalizations and mortality in 

minority and inner-city communities 

have been observed (ISA, section 4.4). In 

addition, based on U.S. census data 

from 2000, about 72.3 million (26%) of 

the U.S. population are under 18 years 

of age, 18.3 million (7.4%) are under 5 

years of age, and 35 million (12%) are 

65 years of age or older. Therefore, large 

portions of the U.S. population are in 

age groups that are likely at-risk for 

health effects associated with exposure 

to ambient NO2. The size of the 

potentially at-risk population suggests 

that exposure to ambient NO2 could 

have a significant impact on public 

health in the United States. 

C. Human Exposure and Health Risk 

Characterization 

To put judgments about NO2- 

associated health effects into a broader 

public health context, EPA has drawn 

upon the results of the quantitative 

exposure and risk assessments. 

Judgments reflecting the nature of the 

evidence and the overall weight of the 

evidence are taken into consideration in 

these quantitative exposure and risk 

assessments, discussed below. These 

assessments provide estimates of the 

likelihood that asthmatic individuals 

would experience exposures of potential 

concern and estimates of the incidence 

of NO2-associated respiratory emergency 

department visits under varying air 

quality scenarios (e.g., just meeting the 

current or alternative standards), as well 

as characterizations of the kind and 

degree of uncertainties inherent in such 

estimates. 

This section describes the approach 

taken in the REA to characterize NO2- 

related exposures and health risks. 

Goals of the REA included estimating 

short-term exposures and potential 

human health risks associated with (1) 

recent levels of ambient NO2; (2) NO2

levels adjusted to simulate just meeting 

the current standard; and (3) NO2 levels 

adjusted to simulate just meeting 


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potential alternative standards. This 

section discusses the scientific evidence 

from the ISA that was used as the basis 

for the risk characterization (II.C.1), the 

approaches used in characterizing 

exposures and risks (II.C.2), and 

important uncertainties associated with 

these analyses (II.C.3). The results of the 

exposure and risk analyses, as they 

relate to the current and potential 

alternative standards, are discussed in 

subsequent sections of this proposal 

(sections II.E and II.F, respectively). 

1. Evidence Base for the Risk 

Characterization 

For purposes of the quantitative 

characterization of NO2 health risks, the 

REA determined that it was appropriate 

to focus on endpoints for which the ISA 

concluded that the available evidence is 

sufficient to infer either a causal or a 

likely causal relationship. This was 

generally consistent with judgments 

made in other recent NAAQS reviews 

(e.g., see EPA, 2005). 

As noted above in section II.A, the 

only health effect category for which the 

evidence was judged in the ISA to be 

sufficient to infer either a causal or a 

likely causal relationship is respiratory 

morbidity following short-term NO2

exposure. Therefore, for purposes of 

characterizing health risks associated 

with NO2, the REA focused on 

respiratory morbidity endpoints that 

have been associated with short-term 

NO2 exposures. Other health effects 

(e.g., those associated with long-term 

exposures) are considered as part of the 

evidence-based evaluation of potential 

alternative standards (see section II.F.2). 

In evaluating the appropriateness of 

specific endpoints for use in the NO2

risk characterization, the REA 

considered both epidemiologic and 

controlled human exposure studies. 

When evaluating epidemiologic 

studies as to their appropriateness for 

use as the basis for a quantitative risk 

assessment, the REA considered several 

factors. First, the REA concluded that 

studies conducted in the United States 

are preferable to those conducted 

outside the United States given the 

potential for effect estimates to be 

impacted by factors such as the ambient 

pollutant mix, the placement of 

monitors, activity patterns of the 

population, and characteristics of the 

healthcare system. Second, the REA 

concluded that studies of ambient NO2

are preferable to those of indoor NO2, 

which focus on individuals exposed to 

NO2 from indoor sources. These indoor 

sources can result in exposure patterns, 

NO2 levels, and co-pollutants that are 

different from those typically associated 

with ambient NO2. Therefore, although 

indoor studies made important 

contributions to the evidence base for 

causality judgments in the ISA, the 

preferred approach for conducting a 

quantitative risk assessment based on 

the epidemiologic literature to inform 

decisions regarding an ambient NO2

standard is to consider studies of 

ambient NO2. Third, the REA concluded 

that it was appropriate to focus on 

studies of emergency department visits 

and hospital admissions given the clear 

public health significance of these 

endpoints and the availability of 

baseline incidence data. Finally, the 

REA concluded that it was appropriate 

to focus on studies that evaluated NO2

health effect associations using both 

single- and multi-pollutant models. 

Taking these factors into consideration, 

the epidemiology-based risk assessment 

in the REA focused on the study 

conducted in Atlanta, Georgia by 

Tolbert et al. (2007). This assessment is 

described in more detail in the REA 

(chapter 9). 

In identifying health endpoints from 

controlled human exposure studies on 

which to focus the characterization of 

NO2 health risks, the REA concluded 

that it was appropriate to focus on 

endpoints that occur at or near ambient 

levels of NO2 and endpoints that may be 

important from a public health 

perspective. Controlled human exposure 

studies have addressed the 

consequences of short-term (e.g., 30- 

minutes to several hours) NO2

exposures for a number of health 

endpoints including airway 

responsiveness, host defense and 

immunity, inflammation, and lung 

function (ISA, section 3.1). With regard 

to the NO2 levels at which different 

effects have been documented, the ISA 

concluded: (1) In asthmatics NO2 may 

increase the allergen-induced airway 

inflammatory response at exposures as 

low as 260 ppb for 30 min (ISA, Figure 

3.1–2), and NO2 exposures between 200 

and 300 ppb for 30 minutes or 100 ppb 

for 60-minutes can result in small, but 

significant, increases in nonspecific 

airway responsiveness (ISA, section 

5.3.2.1); (2) limited evidence indicates 

that NO2 may increase susceptibility to 

injury by subsequent viral challenge 

following exposures of 600–1500 ppb 

for 3 hours; (3) evidence exists for 

increased airway inflammation at NO2

concentrations less than 2000 ppb; and 

(4) the direct effects of NO2 on lung 

function in asthmatics have been 

inconsistent at exposure concentrations 

below 1000 ppb (ISA, section 5.3.2.1). 

Therefore, of the health effects caused 

by NO2 in controlled human exposure 

studies, the only effect identified by the 

ISA to occur at or near ambient levels 

is increased airway responsiveness in 

asthmatics. 

The REA concluded that airway 

responsiveness in the asthmatic 

population is an appropriate focus for 

the risk characterization for several 

reasons. First, the ISA concluded that 

‘‘persons with preexisting pulmonary 

conditions are likely at greater risk from 

ambient NO2 exposures than the general 

public, with the most extensive 

evidence available for asthmatics as a 

potentially susceptible group’’ (ISA, 

section 5.3.2.8). Second, when 

discussing the clinical significance of 

NO2-related airway 

hyperresponsiveness in asthmatics, the 

ISA concluded that ‘‘transient increases 

in airway responsiveness following NO2

exposure have the potential to increase 

symptoms and worsen asthma control’’ 

(ISA, sections 3.1.3 and 5.4). That this 

effect could have public health 

implications is suggested by the large 

size of the asthmatic population in the 

United States (ISA, Table 4.4–1). Third, 

NO2 effects on airway responsiveness in 

asthmatics are part of the body of 

experimental evidence that provides 

plausibility and coherence for the 

effects observed on hospital admissions 

and emergency department visits in 

epidemiologic studies (ISA, section 

5.3.2.1). As a result of these 

considerations, of the endpoints from 

controlled human exposure studies, the 

REA focused on airway responsiveness 

in asthmatics for purposes of 

quantifying risks associated with 

ambient NO2 (see below). 

Because many of the studies of airway 

responsiveness evaluated only a single 

level of NO2 and because of 

methodological differences between the 

studies, the data are not sufficient to 

derive an exposure-response 

relationship in the range of interest. 

Therefore, the REA concluded that the 

most appropriate approach to 

characterizing risks based on the 

controlled human exposure evidence for 

airway responsiveness was to compare 

estimated NO2 air quality and exposure 

levels with potential health effect 

benchmark levels. In this review, the 

term ‘‘exposures of potential concern’’ is 

defined as personal exposures to 1-hour 

ambient NO2 concentrations at and 

above specific benchmark levels. 

Benchmark levels represent NO2

exposure concentrations reported to 

increase airway responsiveness in most 

asthmatics, as discussed above in 

section II.B.1.d. Although the analysis of 

exposures of potential concern was 

conducted using discrete benchmark 

levels (i.e., 100, 150, 200, 250, 300 ppb), 

EPA recognizes that there is no sharp 


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9Estimated emissions from Hartsfield 

International Airport in Atlanta, a non-road mobile 

source, were also included in this analysis. 

breakpoint within the continuum 

ranging from at and above 300 ppb 

down to 100 ppb. In considering the 

concept of exposures of potential 

concern, it is important to balance 

concerns about the potential for health 

effects and their severity with the 

increasing uncertainty associated with 

our understanding of the likelihood of 

such effects at lower NO2 levels. Within 

the context of this continuum, estimates 

of exposures of potential concern at 

discrete benchmark levels provide some 

perspective on the potential public 

health impacts of NO2-related health 

effects that have been demonstrated in 

controlled human exposure studies but 

cannot be evaluated in quantitative risk 

assessments (i.e., increased airway 

responsiveness). They also help in 

understanding the extent to which such 

impacts could change by just meeting 

the current and potential alternative 

standards. 

The NO2-related increase in airway 

responsiveness is plausibly linked to the 

NO2-associated morbidity reported in 

epidemiologic studies (e.g., increased 

respiratory symptoms, emergency 

department visits and hospital 

admissions). However, estimates of the 

number of asthmatics likely to 

experience exposures of potential 

concern cannot be translated directly 

into quantitative estimates of the 

number of people likely to experience 

specific health effects, since sufficient 

information to draw such comparisons 

is not available. Due to individual 

variability in responsiveness, only a 

subset of asthmatics exposed at and 

above a specific benchmark level can be 

expected to experience health effects. 

The amount of weight to place on the 

estimates of exposures of potential 

concern at any of these benchmark 

levels depends in part on the weight of 

the scientific evidence concerning 

health effects associated with NO2

exposures at and above that benchmark 

level. It also depends on judgments 

about the importance from a public 

health perspective of the health effects 

that are known or can reasonably be 

inferred to occur as a result of exposures 

at and above the benchmark level. Such 

public health policy judgments are 

embodied in the NAAQS standard 

setting criteria (i.e., standards that, in 

the judgment of the Administrator, are 

requisite to protect public health with 

an adequate margin of safety). 

2. Overview of Approaches 

As noted above, the purpose of the 

assessments described in the REA was 

to characterize air quality, exposures, 

and health risks associated with recent 

ambient levels of NO2, with NO2 levels 

that could be associated with just 

meeting the current NO2 NAAQS, and 

with NO2 levels that could be associated 

with just meeting potential alternative 

standards. To characterize health risks, 

we employed three approaches in the 

REA. In the first approach, for each air 

quality scenario, NO2 concentrations at 

fixed-site monitors and simulated 

concentrations on/near roadways were 

compared to potential health effect 

benchmark values derived from the 

controlled human exposure literature. In 

the second approach, modeled estimates 

of actual exposures in asthmatics were 

compared to potential health effect 

benchmarks. In the third approach, 

concentration-response relationships 

from an epidemiologic study were used 

in conjunction with baseline incidence 

data and recent or simulated ambient 

concentrations to estimate health 

impacts. An overview of the approaches 

to characterizing health risks is 

provided below and each approach has 

been described in more detail in the 

REA (chapters 6 through 9). 

In the first approach, we compared 

ambient NO2 concentrations with 

potential health effect benchmark levels 

for NO2. The ambient NO2

concentrations used in these analyses 

were based on those measured at 

monitors in the current NO2 monitoring 

network. These monitored 

concentrations were compared to 

benchmark levels directly and were also 

used, in conjunction with literaturederived characterizations of the NO2

concentration gradient around 

roadways, as the basis for estimating 

NO2 concentrations on/near roadways. 

Scenario-driven air quality analyses 

were performed using ambient NO2

concentrations for the years 1995 

though 2006. With this approach, NO2

air quality serves as a surrogate for 

exposure. All U.S. monitoring sites 

where NO2 data have been collected, 

and that met completeness criteria 

(REA, chapter 7), were represented by 

this analysis. As such, the results 

generated were considered a broad 

characterization of national air quality 

and human exposures that might be 

associated with these concentrations. 

An advantage of this approach is its 

relative simplicity; however, there is 

uncertainty associated with the 

assumption that NO2 air quality can 

serve as an adequate surrogate for total 

exposure to ambient NO2. Actual 

exposures might be influenced by 

factors not considered by this approach, 

including small scale spatial variability 

in ambient NO2 concentrations (which 

might not be captured by the network of 

fixed-site ambient monitors) and 

spatial/temporal variability in human 

activity patterns. 

In the second approach, we used an 

inhalation exposure model to generate 

more realistic estimates of personal 

exposures in asthmatics (REA, chapter 8 

for more detail on this assessment). This 

analysis estimated temporally and 

spatially variable ambient NO2

concentrations and simulated human 

contact with these pollutant 

concentrations. The approach was 

designed to incorporate exposures that 

are not necessarily captured by the 

existing ambient monitoring data, 

including those that occur on or near 

roadways. AERMOD, an EPA dispersion 

model, was used to estimate 1-hour 

ambient NO2 concentrations using 

emissions estimates from stationary and 

on-road mobile sources.9 The Air 

Pollutants Exposure (APEX) model, an 

EPA human exposure model, was then 

used to estimate population exposures 

using the hourly census block level NO2

concentrations estimated by AERMOD. 

A probabilistic approach was used to 

model individual exposures considering 

the time people spend in different 

microenvironments and the variable 

NO2 concentrations that occur within 

these microenvironments across time, 

space, and microenvironment type. 

Estimates of personal exposure were 

compared to potential NO2 health 

benchmark levels. This approach to 

assessing exposures was more resource 

intensive than using ambient levels as a 

surrogate for exposure; therefore, the 

final REA included the analysis of only 

one specific location in the U.S. (Atlanta 

MSA). Although the geographic scope of 

this analysis was restricted, the 

approach provided estimates of NO2

exposures in asthmatics in Atlanta, 

particularly those exposures associated 

with important emission sources of 

NOX, and the analysis served to 

complement the broad air quality 

characterization. 

For the characterization of risks in 

both the air quality analysis and the 

exposure modeling analysis described 

above, the REA used a range of shortterm 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 


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NO2 air quality was used as a surrogate 

for exposure, the output of the analysis 

was an estimate of the number of times 

per year specific locations experience 1- 

hour levels of NO2 that exceed a 

particular benchmark. When personal 

exposures were simulated, the output of 

the analysis was an estimate of the 

number of asthmatics at risk for 

experiencing daily maximum 1-hour 

levels of NO2 of ambient origin that 

exceed a particular benchmark. An 

advantage of using the benchmark 

approach to characterize health risks is 

that the effects observed in controlled 

human exposure studies clearly result 

from NO2 exposure. A disadvantage of 

this approach is that the magnitude of 

the NO2 effect on airway responsiveness 

can vary considerably from individual 

to individual and not all asthmatics 

would be expected to respond to the 

same levels of NO2 exposure. Therefore, 

the public health impacts of NO2- 

induced airway hyperresponsiveness 

are difficult to quantify. 

In the third approach, we estimated 

respiratory emergency department visits 

as a function of ambient levels of NO2

measured at a fixed-site monitor 

representing ambient air quality for an 

urban area. In this approach, 

concentration-response functions from 

an epidemiologic study (Tolbert et al., 

2007) were used, in combination with 

baseline incidence data for respiratory 

emergency department visits in the 

Atlanta area and ambient NO2

monitoring data, to estimate the impact 

on emergency department visits of 

ambient levels of NO2. Compared to the 

risk characterization based on the air 

quality and exposure analyses described 

above, this approach to characterizing 

health risks has several advantages. For 

example, the public health significance 

of respiratory emergency department 

visits is less ambiguous, in terms of its 

impact on individuals, than is an 

increase of unknown magnitude in the 

airway response. In addition, the 

concentration-response relationship 

reflects real-world levels of NO2 and copollutants 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 cooccurring pollutants. 

3. Key Limitations and Uncertainties 

A number of key uncertainties should 

be considered when interpreting the 

results of these analyses. While the air 

quality, exposure, and quantitative risk 

analyses are each associated with 

unique uncertainties, they also share 

some uncertainties in common. 

Important uncertainties shared by these 

analyses, as well as uncertainties 

specifically associated with the air 

quality, exposure, and risk analyses, are 

discussed below. 

In order to simulate just meeting the 

current annual standard and many of 

the alternative 1-hour standards 

analyzed, an adjustment (either upward 

or downward) of recent ambient NO2

concentrations was required. As noted 

in the REA, an upward adjustment does 

not reflect a judgment that levels of NO2

are likely to increase across the country 

or in any specific location under the 

current standard or any of the potential 

alternative standards. However, it does 

acknowledge that, under the current 

standard and some of the alternative 

standards evaluated, an increase in NO2

concentrations would be permitted. The 

benefit of these air quality adjustments 

is that they can inform consideration of 

the current and alternative standards by 

providing estimates of health risks that 

could be associated with ambient air 

quality levels that just meet these 

standards. In adjusting air quality to 

simulate just meeting these standards, 

the analyses in the REA assumed that 

the overall shape of the distribution of 

NO2 concentrations in an area would 

not change. While the REA concluded 

that this is a reasonable assumption in 

the absence of evidence supporting a 

different distribution, and while 

available analyses support this approach 

(Rizzo, 2008), the REA recognized this 

as an important uncertainty. It may be 

an especially important uncertainty for 

those scenarios where considerable 

adjustment is required to simulate just 

meeting one or more of the standards 

(REA, section 8.12). 

In addition, simulation of just meeting 

different alternative standards was 

achieved by adjusting NO2

concentrations at monitors in the 

current area-wide network. Therefore, 

resulting estimates of the potential 

public health implications of different 

decisions are most directly relevant to a 

standard focused specifically on the 

area-wide NO2 concentrations that are 

the primary target of the current 

monitoring network. However, as 

discussed below (sections II.F.4.e and 

III), with this notice the Administrator 

is proposing to establish a standard 

focused specifically on the peak 

concentrations to which individuals can 

be exposed from on-road mobile source 

emissions on or near major roadways 

and to support such a standard with a 

monitoring network that includes 

monitors placed near major roadways. 

This proposed shift in the monitoring 

network introduces uncertainty in the 

extent to which the exposure and risk 

analyses presented in the REA can 

directly inform decisions on the 

proposed standard. 

In addition to the general 

uncertainties discussed above, some 

uncertainties are specific to the air 

quality analyses. In order to estimate 

ambient NO2 concentrations on or near 

roadways in the air quality analyses, the 

REA used empirically-derived 

relationships between ambient 

concentrations measured at fixed-site 

monitors in the current NO2 monitoring 

network and on/near-road 

concentrations. The data used to 

develop the relationships were likely 

collected under different conditions 

(e.g., different meteorological conditions 

which can affect important parameters 

in this relationship, such as the 

production of NO2 from NO). The REA 

noted that the extent to which these 

conditions are representative of the 

times and places included in our 

analyses is unknown. Therefore, there is 

uncertainty in the degree to which the 

relationships used to estimate on/nearroad NO2 concentrations reflect the 

actual relationship in the locations and 

over the time periods of interest. 

Potential health benchmark levels 

used in the air quality analyses were 

based largely on a meta-analysis (ISA, 

Table 3.1–3) of controlled human 

exposure studies of airway 

hyperresponsiveness. One important 

source of uncertainty with regard to this 

approach is that controlled human 

exposure studies have typically 

involved volunteers with mild asthma. 

Data are lacking for more severely 

affected asthmatics, who may be more 

susceptible (ISA, section 3.1.3.2). As a 

result, the potential health effect 

benchmarks could underestimate risks 

in populations with greater 

susceptibility. While approaches to 

classifying asthma severity differ, some 

estimates indicate that over half of 

asthmatics could be classified as 

moderate or severe (Fuhlbrigge et al., 

2002; Stout et al., 2006). A second 

important source of uncertainty with 

regard to this approach is that the metaanalysis 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. 


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Therefore, although the ISA did 

conclude that increased airway 

responsiveness associated with NO2

exposure could increase symptoms and 

worsen asthma control (ISA, section 

5.4), the full public health implications 

of benchmark exceedances are 

uncertain. 

The Atlanta exposure assessment was 

also associated with a number of key 

uncertainties that should be considered 

when interpreting the results with 

regard to decisions on the standard. 

Some of these uncertainties, including 

those associated with benchmark levels, 

were shared with the air quality 

analyses. Additional uncertainties 

associated specifically with the Atlanta 

exposure assessment are discussed 

briefly below. 

When compared to ambient 

measurement data, predicted upper 

percentile NO2 concentrations may be 

10–50% higher. Because these predicted 

concentrations are used as inputs for the 

exposure modeling, this suggests the 

possibility that the exposure assessment 

is over-predicting upper percentile NO2

exposures. Other approaches used to 

evaluate exposure results (i.e., 

comparison to personal exposure 

monitoring results and comparison of 

exposure-to-ambient concentration 

ratios with those identified in the ISA) 

have suggested that exposure estimates 

are reasonable. However, the possibility 

cannot be ruled out that benchmark 

exceedances are over-predicted in the 

Atlanta exposure analysis. 

The exposure assessment was limited 

to Atlanta and the extent to which these 

results are representative of other 

locations in the U.S. is uncertain. The 

REA (section 8.11) concluded that the 

Atlanta exposure estimates are likely 

representative of other moderate to large 

urban areas. However, the REA also 

recognized that, given the greater 

proximity of the population to mobile 

sources in large urban areas such as Los 

Angeles, New York, and Chicago (see 

REA, Tables 8–14 and 8–15), the 

estimates of benchmark exceedances in 

Atlanta may be smaller than in these 

larger cities. 

A number of key uncertainties should 

also be considered when interpreting 

the results of the Atlanta risk 

assessment with regard to decisions on 

the standard. Some of these, including 

the appropriateness of generalizing 

results from Atlanta, are shared with the 

Atlanta exposure assessment. 

Additional uncertainties associated 

specifically with the Atlanta risk 

assessment are discussed briefly below. 

There is uncertainty about whether 

the association between NO2 and 

emergency department visits actually 

reflects a causal relationship across the 

range of daily and hourly concentration 

levels in the epidemiologic studies. The 

ISA (section 5.4, p. 5–15) noted that 

when interpreting the NO2

epidemiologic results, ‘‘It is difficult to 

determine * * * the extent to which 

NO2 is independently associated with 

respiratory effects or if NO2 is a marker 

for the effects of another traffic-related 

pollutant or mix of pollutants (see 

section 5.2.2 for more details on 

exposure issues). A factor contributing 

to uncertainty in estimating the NO2- 

related effect from epidemiologic 

studies is that NO2 is a component of a 

complex air pollution mixture from 

traffic related sources that include CO 

and various forms of PM.’’ This 

uncertainty should be considered when 

interpreting the quantitative NO2 risk 

estimates based on the Atlanta 

epidemiologic study. However, in 

discussing these uncertainties, the ISA 

(section 5.4, p. 5–16) concluded that, 

‘‘Although this complicates the efforts 

to disentangle specific NO2-related 

health effects, the evidence summarized 

in this assessment indicates that NO2

associations generally remain robust in 

multi-pollutant models and supports a 

direct effect of short-term NO2 exposure 

on respiratory morbidity at ambient 

concentrations below the current 

NAAQS. The robustness of 

epidemiologic findings to adjustment 

for co-pollutants, coupled with data 

from animal and human experimental 

studies, support a determination that 

the relationship between NO2 and 

respiratory morbidity is likely causal, 

while still recognizing the relationship 

between NO2 and other traffic-related 

pollutants.’’ 

A related uncertainty is that 

associated with the estimated NO2

coefficient in the concentrationresponse 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, singlepollutant models may produce 

overestimates of the NO2 effects if some 

of those effects are really due in whole 

or part to one or more of the other 

pollutants. On the other hand, effect 

estimates based on multi-pollutant 

models can be uncertain, and can result 

in statistically non-significant estimates 

where a true relationship exists, if the 

co-pollutants included in the model are 

highly correlated with NO2. As a result 

of these considerations, we report risk 

estimates based on both the single- and 

multi-pollutant models from Tolbert et 

al. (2007). 

D. Considerations in Review of the 

Standard 

This section presents the integrative 

synthesis of the evidence and 

information contained in the ISA and 

the REA with regard to the current and 

potential alternative standards. EPA 

notes that the final decision on retaining 

or revising the current primary NO2

standard is a public health policy 

judgment to be made by the 

Administrator. This judgment will be 

informed by a recognition that the 

available health effects evidence reflects 

a continuum consisting of ambient 

levels of NO2 at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. The Administrator’s final 

decision will draw upon scientific 

information and analyses related to 

health effects, population exposures, 

and risks; judgments about the 

appropriate response to the range of 

uncertainties that are inherent in the 

scientific evidence and analyses; and 

comments received in response to this 

proposal. 

1. Background on the Current Standard 

The current standard, which is an 

annual average of 0.053 ppm (53 ppb), 

was retained by the Administrator in the 

most recent review in 1996 (61 FR 

52854 (October 8, 1996)). The decision 

in that review to retain the annual 

standard was based on consideration of 

available scientific evidence for health 

effects associated with NO2 and on air 

quality information. With regard to 

these considerations, the Administrator 

noted that ‘‘a 0.053 ppm annual 

standard would keep annual NO2

concentrations considerably below the 

long-term levels for which serious 

chronic effects have been observed in 

animals’’ and that ‘‘[r]etaining the 

existing standard would also provide 

protection against short-term peak NO2

concentrations at the levels associated 

with mild changes in pulmonary 

function and airway responsiveness 

observed in controlled human studies’’ 

(60 FR 52874, 52880 (Oct. 11, 1995)). As 

a result, the Administrator concluded 

that ‘‘the existing annual primary 

standard appears to be both adequate 

and necessary to protect human health 

against both long- and short-term NO2

exposures’’ and that ‘‘retaining the 

existing annual standard is consistent 


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with the scientific data assessed in the 

Criteria Document (EPA, 1993) and the 

Staff Paper (EPA, 1995) and with the 

advice and recommendations of 

CASAC’’ (61 FR 52852 at 52854). 

As noted previously, the 1993 AQCD 

concluded that there were two key 

health effects of greatest concern at 

ambient or near-ambient levels of NO2: 

increased airway responsiveness in 

asthmatic individuals after short-term 

exposures and increased occurrence of 

respiratory illness in children with 

longer-term exposures. Evidence also 

was found for increased risk of 

emphysema, but this was of major 

concern only with exposures to levels of 

NO2 much higher than then-current 

ambient concentrations. The evidence 

regarding airway responsiveness was 

drawn largely from controlled human 

exposure studies. The evidence for 

respiratory illness was drawn from 

epidemiologic studies that reported 

associations between respiratory 

symptoms and indoor exposures to NO2

in people living in homes with gas 

stoves. The biological plausibility of the 

epidemiologic results was supported by 

toxicological studies that detected 

changes in lung host defenses following 

NO2 exposure. Subpopulations 

considered potentially more susceptible 

to the effects of NO2 included 

individuals with preexisting respiratory 

disease, children, and the elderly. 

In that review, health risks were 

characterized by comparing ambient 

monitoring data, which were used as a 

surrogate for exposure, with potential 

health benchmark levels identified from 

controlled human exposure studies. At 

the time of the review, a meta-analysis 

of controlled human exposure studies 

indicated the possibility for adverse 

health effects due to short-term (e.g., 1- 

hour) exposures between 200 ppb and 

300 ppb NO2. Therefore, the focus of the 

assessment was on the potential for 

short-term (i.e., 1-hour) exposures to 

NO2 levels above potential health 

benchmarks in this range. The 

assessment used monitoring data from 

the years 1988–1992 and screened for 

sites with one or more hourly 

exceedances of potential short-term 

health effect benchmarks. Predictive 

models were then constructed to relate 

the frequency of hourly concentrations 

above short-term health effect 

benchmarks to a range of annual average 

concentrations, including the current 

standard. Based on the results of this 

analysis, both CASAC (Wolff, 1995) and 

the Administrator (60 FR 52874) 

concluded that the minimal occurrence 

of short-term peak concentrations at or 

above a potential health effect 

benchmark of 200 ppb (1-hour average) 

indicated that the existing annual 

standard would provide adequate health 

protection against short-term exposures. 

This conclusion, combined with the 

conclusion that the current annual 

standard would maintain annual 

average levels well-below those 

associated with serious effects in animal 

toxicological studies, formed a large part 

of the basis for the decision in the 1996 

review to retain the existing annual 

standard. 

2. Approach for Reviewing the Need To 

Retain or Revise the Current Standard 

The decision in the present review on 

whether the current annual standard is 

requisite to protect public health with 

an adequate margin of safety will be 

informed by a number of scientific 

studies and analyses that were not 

available in the 1996 review. 

Specifically, as discussed above (section 

II), a large number of epidemiologic 

studies have been published since the 

1996 review. Many of these studies 

evaluate associations between NO2 and 

adverse respiratory endpoints (e.g., 

respiratory symptoms, emergency 

department visits, hospital admissions) 

in locations where annual average NO2

concentrations are well-below the level 

allowed by the current standard (53 

ppb). In addition, the meta-analysis of 

controlled human exposure studies has 

been updated for this review to include 

information on additional exposure 

concentrations. Finally, the REA 

described estimates of NO2-associated 

health risks that could be present in 

locations that just meet the current 

annual standard. These types of risk 

estimates were not available in the last 

review. The approach for considering 

this scientific evidence and exposure/ 

risk information is discussed below. 

To evaluate whether the current 

primary NO2 standard is adequate or 

whether consideration of revisions is 

appropriate, EPA is using an approach 

in this review that has been described 

in chapter 10 of the REA. The approach 

outlined in the REA builds upon the 

approaches used in reviews of other 

criteria pollutants, including the most 

recent reviews of the Pb, O3, and PM 

NAAQS (EPA, 2007d; EPA, 2007e; EPA, 

2005), and reflects the body of evidence 

and information that is currently 

available. As in other recent reviews, 

EPA’s considerations will include the 

implications of placing more or less 

weight or emphasis on different aspects 

of the scientific evidence and the 

exposure/risk-based information, 

recognizing that the weight to be given 

to various elements of the evidence and 

exposure/risk information is part of the 

public health policy judgments that the 

Administrator will make in reaching 

decisions on the standard. 

A series of general questions frames 

this approach to considering the 

scientific evidence and exposure-/riskbased information. First, EPA’s 

consideration of the scientific evidence 

and exposure/risk information with 

regard to the adequacy of the current 

standard is framed by the following 

questions: 

• To what extent does evidence that has 

become available since the last review 

reinforce or call into question evidence for 

NO2-associated effects that were identified in 

the last review? 

• To what extent has evidence for different 

health effects and/or sensitive populations 

become available since the last review? 

• To what extent have uncertainties 

identified in the last review been reduced 

and/or have new uncertainties emerged? 

• To what extent does evidence and 

exposure-/risk-based information that has 

become available since the last review 

reinforce or call into question any of the 

basic elements of the current standard? 

To the extent that the available 

evidence and exposure-/risk-based 

information suggests it may be 

appropriate to consider revision of the 

current standard, EPA considers that 

evidence and information with regard to 

its support for consideration of a 

standard that is either more or less 

protective than the current standard. 

This evaluation is framed by the 

following questions: 

• Is there evidence that associations, 

especially causal or likely causal 

associations, extend to ambient NO2

concentrations as low as, or lower than, the 

concentrations that have previously been 

associated with health effects? If so, what are 

the important uncertainties associated with 

that evidence? 

• Are exposures above benchmark levels 

and/or health risks estimated to occur in 

areas that meet the current standard? If so, 

are the estimated exposures and health risks 

important from a public health perspective? 

What are the important uncertainties 

associated with the estimated risks? 

To the extent that there is support for 

consideration of a revised standard, EPA 

then considers the specific elements of 

the standard (indicator, averaging time, 

form, and level) within the context of 

the currently available information. In 

so doing, the Agency addresses the 

following questions: 

• Does the evidence provide support for 

considering a different indicator for gaseous 

NOX? 

• Does the evidence provide support for 

considering different averaging times? 

• What ranges of levels and forms of 

alternative standards are supported by the 

evidence, and what are the associated 

uncertainties and limitations? 


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• To what extent do specific averaging 

times, levels, and forms of alternative 

standards reduce the estimated exposures 

above benchmark levels and risks attributable 

to NO2, and what are the uncertainties 

associated with the estimated exposure and 

risk reductions? 

The questions outlined above have 

been addressed in the REA. The 

following sections present 

considerations regarding the adequacy 

of the current standard and potential 

alternative standards, as discussed in 

chapter 10 of the REA, in terms of 

indicator, averaging time, form, and 

level. 

E. Adequacy of the Current Standard 

In considering the adequacy of the 

current standard, the policy assessment 

chapter of the REA considered the 

scientific evidence assessed in the ISA 

and the quantitative exposure- and riskbased information presented in the REA. 

A summary of this evidence and 

information as well as CASAC 

recommendations and the 

Administrator’s conclusions regarding 

the adequacy of the current standard are 

presented below. 

1. Evidence-Based Considerations 

As discussed in chapter 10 of the 

REA, evidence published since the last 

review generally has confirmed and 

extended the conclusions articulated in 

the 1993 AQCD (ISA, section 5.3.2). The 

epidemiologic evidence has grown 

substantially with the addition of field 

and panel studies, intervention studies, 

time-series studies of effects such as 

emergency department visits and 

hospital admissions, and a substantial 

number of studies evaluating mortality 

risk associated with short-term NO2

exposures. As noted above, no 

epidemiologic studies were available in 

1993 that assessed relationships 

between NO2 and outcomes such as 

hospital admissions, emergency 

department visits, or mortality. In 

contrast, dozens of epidemiologic 

studies on such outcomes, conducted at 

recent and current ambient NO2

concentrations, are now included in this 

evaluation (ISA, chapter 3). While not as 

marked as the growth in the 

epidemiologic literature, a number of 

recent toxicological and human clinical 

studies also provide insights into 

relationships between NO2 exposure 

and health effects. 

As an initial consideration with 

regard to the adequacy of the current 

standard, the REA noted that the 

evidence relating long-term (weeks to 

years) NO2 exposures at current ambient 

concentrations to adverse health effects 

was judged in the ISA to be either 

‘‘suggestive but not sufficient to infer a 

causal relationship’’ (respiratory 

morbidity) or ‘‘inadequate to infer the 

presence or absence of a causal 

relationship’’ (mortality, cancer, 

cardiovascular effects, reproductive/ 

developmental effects) (ISA, sections 

5.3.2.4–5.3.2.6). In contrast, the 

evidence relating short-term (minutes to 

hours) NO2 exposures to respiratory 

morbidity was judged to be ‘‘sufficient 

to infer a likely causal relationship’’ 

(ISA, section 5.3.2.1). This judgment 

was supported primarily by a large body 

of recent epidemiologic evidence that 

evaluated associations of short-term 

NO2 concentrations with respiratory 

symptoms, emergency department 

visits, and hospital admissions. These 

conclusions from the ISA suggest that, 

at a minimum, consideration of the 

adequacy of the current annual standard 

should take into account the extent to 

which that standard provides protection 

against respiratory effects associated 

with short-term NO2 exposures. As 

noted in the REA, such an emphasis on 

health endpoints for which evidence 

has been judged to be sufficient to infer 

a likely causal relationship would be 

consistent with other recent NAAQS 

reviews (e.g., EPA, 2005; EPA, 2007d; 

EPA, 2007e). 

In considering the NO2 epidemiologic 

studies as they relate to the adequacy of 

the current standard, the REA noted that 

annual average NO2 concentrations were 

below the level of the current annual 

NO2 NAAQS in many of the locations 

where positive, and often statistically 

significant, associations with respiratory 

morbidity endpoints have been reported 

(ISA, section 5.4). As discussed 

previously, the ISA characterized that 

evidence for respiratory effects as 

consistent and coherent. The evidence 

is consistent in that associations are 

reported in studies conducted in 

numerous locations and with a variety 

of methodological approaches (ISA, 

section 5.3.2.1). It is coherent in the 

sense that the studies report 

associations with respiratory health 

outcomes that are logically linked 

together (ISA, section 5.3.2.1). The ISA 

noted that when the epidemiologic 

literature is considered as a whole, there 

are generally positive associations 

between NO2 and respiratory symptoms, 

hospital admissions, and emergency 

department visits. A number of these 

associations are statistically significant, 

particularly the more precise effect 

estimates (ISA, section 5.3.2.1). 

As discussed previously, the 

interpretation of these NO2

epidemiologic studies is complicated by 

the fact that on-road vehicle exhaust 

emissions are a nearly ubiquitous source 

of combustion pollutant mixtures that 

include NO2. In order to provide some 

perspective on the uncertainty related to 

the presence of co-pollutants, the ISA 

evaluated epidemiologic studies that 

employed multi-pollutant models, 

epidemiologic studies of indoor and 

personal NO2 exposure, and 

experimental studies. Specifically, the 

ISA noted that a number of NO2

epidemiologic studies have attempted to 

disentangle the effects of NO2 from 

those of co-occurring pollutants by 

employing multi-pollutant models. 

When evaluated as a whole, NO2 effect 

estimates in these models generally 

remained robust when co-pollutants 

were included. Therefore, despite 

uncertainties associated with separating 

the effects of NO2 from those of cooccurring 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 multipollutant 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 copollutants present in the ambient air or 

by the contribution of NO2 to the 

formation of secondary particles or O3

(ISA, section 3.1.4.1). The ISA noted 

that the findings of indoor NO2 studies 

are consistent with those of studies 

using ambient concentrations from 

central site monitors and concluded that 

indoor studies provide evidence of 

coherence for respiratory effects (ISA, 

section 3.1.4.1). With regard to 

experimental studies, the REA noted 

that they have the advantage of 

providing information on health effects 

that are specifically associated with 

exposure to NO2 in the absence of copollutants. 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 


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considered the epidemiologic and 

experimental evidence as well as the 

uncertainties associated with that 

evidence. When this evidence and its 

associated uncertainties are taken 

together, the ISA concluded that the 

results of epidemiologic and 

experimental studies form a plausible 

and coherent data set that supports a 

relationship between NO2 exposures 

and respiratory endpoints, including 

respiratory symptoms and emergency 

department visits, at ambient 

concentrations that are present in areas 

that meet the current NO2 NAAQS. 

Thus, taking into consideration the 

evidence discussed above, particularly 

the epidemiologic studies reporting 

NO2-associated health effects in 

locations that meet the current standard, 

the REA concluded that the scientific 

evidence calls into question the 

adequacy of the current standard to 

protect public health. 

2. Exposure- and Risk-Based 

Considerations 

In addition to the evidence-based 

considerations described above, the 

REA considered the extent to which 

exposure- and risk-based information 

can inform decisions regarding the 

adequacy of the current annual NO2

standard, taking into account key 

uncertainties associated with the 

estimated exposures and risks. As noted 

above, NO2-associated health risks were 

characterized with three approaches. In 

the first, NO2 air quality from locations 

across the country was used as a 

surrogate for exposure. In the second, 

exposures were estimated for all 

asthmatics and for asthmatic children 

considering time spent in different 

microenvironments in one urban area, 

Atlanta, GA. For both of these analyses, 

health risks were characterized by 

comparing estimates of air quality or 

exposure to potential health benchmark 

levels. Benchmark levels spanned the 

range of NO2 concentrations that have 

been reported to increase airway 

responsiveness in asthmatics (i.e., 100– 

300 ppb). In the third approach to 

characterizing NO2-related health risks, 

occurrences of NO2-related respiratory 

emergency department visits were 

estimated for Atlanta. This quantitative 

risk assessment was based on NO2

concentration-response relationships 

identified in an epidemiologic study of 

air pollution-related emergency 

department visits in Atlanta. The results 

of each of these analyses are discussed 

in this section, specifically as they relate 

to the current standard. 

When considering the Atlanta risk 

assessment results as they relate to the 

adequacy of the current standard, the 

REA noted that central estimates of 

incidence of NO2-related respiratory 

emergency department visits in Atlanta 

ranged from approximately 8 to 9% of 

total respiratory-related emergency 

department visits per year (or 9,800– 

10,900 NO2-related incidences) based on 

single pollutant models when air quality 

is adjusted upward to simulate a 

situation where Atlanta just meets the 

current standard. Central estimates of 

incidence of NO2-related respiratory 

emergency department visits ranged 

from 2.9–7.7% of total respiratoryrelated emergency department visits per 

year (or 3,600–9,400 NO2-related 

incidences) based on two-pollutant 

models. Inclusion of O3 and/or PM10 in 

multi-pollutant models resulted in the 

inclusion of an estimate of zero NO2- 

related respiratory emergency 

department visits within the 95% 

confidence intervals. 

When considering the Atlanta 

exposure results as they relate to the 

adequacy of the current standard, the 

REA noted the number of days per year 

asthmatics could experience exposure to 

NO2 concentrations greater than or 

equal to potential health benchmark 

levels, given air quality that is adjusted 

upward to simulate just meeting the 

current standard. If NO2 concentrations 

were such that the Atlanta area just 

meets the current standard, nearly all 

asthmatics in Atlanta (>97%) would be 

estimated to experience six or more 

days per year with 1-hour NO2 exposure 

concentrations greater than or equal to 

our highest benchmark level (300 ppb) 

(REA, Figure 8–22). Six days per year 

was the largest number of days 

specifically considered in the REA, but 

these results suggest that some 

asthmatics could experience 1-hour NO2

exposure concentrations greater than or 

equal to 300 ppb on more than six days 

per year. In addition, more frequent 

exceedances would be expected for the 

lower benchmark levels. 

When considering the air qualitybased 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 fixedsite monitors in locations across the 

U.S. could exceed benchmark levels. 

Most locations were estimated to 

experience at least 50 days per year with 

1-hour ambient NO2 concentrations at 

fixed-site monitors in the current 

network greater than or equal to 100 ppb 

(Figures 7–2 and 7–3 in the REA) under 

this hypothetical scenario. Far fewer 

ambient exceedances were predicted for 

the higher benchmark levels. For 

example, only 5 areas were estimated to 

experience any days with 1-hour 

ambient NO2 concentrations at fixed-site 

monitors greater than or equal to 300 

ppb, and none of those locations were 

estimated to experience more than 2 

such days per year, on average (REA, 

Appendix A). 

However, on-road NO2 concentrations 

were estimated in this analysis to be an 

average of 80% higher than 

concentrations at fixed-site monitors 

(though this relationship will vary 

across locations and with time). In the 

majority of locations evaluated, roadway 

exceedances of the 100 ppb benchmark 

level could occur on most days of the 

year when air quality is adjusted 

upward to simulate just meeting the 

current standard (Figure 7–6 in the 

REA). Even for higher benchmark levels, 

most locations were estimated to have 

exceedances on roadways. All locations 

evaluated except one (Boston) were 

estimated to experience on-road NO2

concentrations greater than or equal to 

300 ppb (REA, Appendix A). Four of 

these locations were estimated to 

experience an average of greater than 20 

days per year with on-road NO2

concentrations greater than or equal to 

300 ppb (REA, Appendix A). 

3. Summary of Considerations From the 

REA 

As noted above, the policy assessment 

chapter of the REA considered the 

scientific evidence with regard to the 

current standard. This included 

consideration of causality judgments 

made in the ISA regarding the level of 

support for effects associated with shortterm and long-term exposures, the 

epidemiologic evidence described in the 

ISA including associated uncertainties, 

the conclusions in the ISA regarding the 

robustness of this evidence, and the 

support provided for epidemiologic 

findings by experimental studies. The 

REA concluded that, given these 

considerations, particularly the 

evidence for NO2-associated effects in 

locations that meet the current standard, 

the adequacy of the current standard to 

protect the public health is clearly 

called into question. This evidence 

provides support for consideration of an 

NO2 standard that would provide 

increased health protection for at-risk 

groups, including asthmatics and 

individuals who spend time on or near 

major roadways, against health effects 

associated with short-term exposures 

ranging from increased asthma 

symptoms to respiratory-related 

emergency department visits and 


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hospital admissions, in addition to 

potential effects associated with longterm exposures. 

In examining the exposure- and riskbased 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 riskbased results reinforce the scientific 

evidence in supporting the conclusion 

that consideration should be given to 

revising the current standard so as to 

provide increased public health 

protection, especially for at-risk groups, 

from NO2-related adverse health effects 

associated with short-term, and 

potential long-term, exposures. 

4. CASAC Views 

With regard to the adequacy of the 

current standard, CASAC conclusions 

were consistent with the views 

expressed in the policy assessment 

chapter of the REA. CASAC agreed that 

the primary concern in this review is to 

protect against health effects that have 

been associated with short-term NO2

exposures. CASAC also agreed that the 

current annual standard is not sufficient 

to protect public health against the 

types of exposures that could lead to 

these health effects. Given these 

considerations, and as noted in their 

letter to the EPA Administrator, 

‘‘CASAC concurs with EPA’s judgment 

that the current NAAQS does not 

protect the public’s health and that it 

should be revised’’ (Samet, 2008b). 

CASAC’s views on how the standard 

should be revised are provided below 

within the context of discussions on the 

elements (i.e., indicator, averaging time, 

form, level) of a new short-term 

standard. 

5. Administrator’s Conclusions 

Regarding Adequacy of the Current 

Standard 

In considering the adequacy of the 

current NO2 NAAQS, the Administrator 

has considered the conclusions of the 

ISA, the conclusions of the policy 

assessment chapter of the REA, and the 

views expressed by CASAC. In 

particular, the ISA concluded that the 

results of epidemiologic and 

experimental studies form a plausible 

and coherent data set that supports a 

likely causal relationship between shortterm 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 shortterm exposures. In considering 

approaches to revising the current 

standard, the Administrator concludes 

that it is appropriate to consider setting 

a new short-term standard to 

supplement the current annual 

standard. The Administrator notes that 

such a short-term standard could 

provide increased public health 

protection, especially for members of atrisk groups, from effects described in 

both epidemiologic and controlled 

human exposure studies to be 

associated with short-term exposures to 

NO2. 

F. Conclusions on the Elements of a 

New Short-Term Standard and an 

Annual Standard 

In considering alternative NO2

primary NAAQS, the Administrator 

notes the need to protect at-risk 

individuals from short-term exposures 

to NO2 air quality that could cause the 

types of respiratory morbidity effects 

reported in epidemiologic studies and 

the need to protect at-risk individuals 

from short-term exposure to NO2

concentrations reported in controlled 

human exposure studies to increase 

airway responsiveness in asthmatics. 

Considerations with regard to potential 

alternative standards and the specific 

options being proposed are discussed in 

the following sections in terms of 

indicator, averaging time, form, and 

level (sections II.F.1–II.F.4). 

1. Indicator 

In past reviews, EPA has focused on 

NO2 as the most appropriate indicator 

for ambient NOX. In making a decision 

in the current review on the most 

appropriate indicator, the Administrator 

has considered the conclusions of the 

ISA and REA as well as the view 

expressed by CASAC. The REA noted 

that, while the presence of NOX species 

other than NO2 has been recognized, no 

alternative to NO2 has been advanced as 

being a more appropriate surrogate. 

Controlled human exposure studies and 

animal toxicology studies provide 

specific evidence for health effects 

following exposure to NO2. 

Epidemiologic studies also typically 

report levels of NO2 though the degree 

to which monitored NO2 reflects actual 

NO2 levels, as opposed to NO2 plus 

other gaseous NOX, can vary (REA, 

section 2.2.3). In addition, because 

emissions that lead to the formation of 

NO2 generally also lead to the formation 

of other NOX oxidation products, 

measures leading to reductions in 

population exposures to NO2 can 

generally be expected to lead to 

reductions in population exposures to 

other gaseous NOX. Therefore, an NO2

standard can also be expected to 

provide some degree of protection 

against potential health effects that may 

be independently associated with other 

gaseous NOX even though such effects 

are not discernable from currently 

available studies indexed by NO2 alone. 

Given these key points, the REA 

concluded that the evidence supports 

retaining NO2 as the indicator. 

Consistent with this conclusion, the 

CASAC Panel recommended in its letter 

to the EPA Administrator that it 

‘‘concurs with retention of NO2 as the 

indicator’’ (Samet, 2008b). In light of the 

above considerations, the Administrator 

proposes to retain NO2 as the indicator 

in the current review. 

2. Averaging Time 

The current annual averaging time for 

the NO2 NAAQS was originally set in 

1971, based on epidemiologic studies 

that supported a link between adverse 

respiratory effects and long-term 

exposure to low levels of NO2. As noted 

above, that annual standard was 

retained in subsequent reviews in part 

because an air quality assessment 

conducted by EPA concluded that areas 

that meet the annual standard would be 

unlikely to experience short-term 

ambient peaks above concentrations that 

had been reported in a meta-analysis of 

controlled human exposure studies to 

increase airway responsiveness in 

asthmatics. In the current review, 

additional scientific evidence is 

available to inform a decision on 

averaging time. This includes the 

availability of a number of 

epidemiologic studies that have 

evaluated endpoints including 

respiratory symptoms, emergency 


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10As 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. 

department visits, and hospital 

admissions as well as an updated metaanalysis of controlled human exposure 

studies of airway responsiveness in 

asthmatics. 

In order to inform conclusions with 

regard to averaging time in this review, 

the REA considered judgments on the 

evidence from the ISA, results from 

experimental and epidemiologic 

studies, and an analysis of correlations 

between short- and long-term ambient 

NO2 concentrations. These 

considerations are described in more 

detail below. 

a. Short-Term Averaging Time 

As described previously, the evidence 

relating short-term (minutes to hours) 

NO2 exposures to respiratory morbidity 

was judged in the ISA to be ‘‘sufficient 

to infer a likely causal relationship’’ 

(ISA, section 5.3.2.1) while the evidence 

relating long-term (weeks to years) NO2

exposures to adverse health effects was 

judged to be either ‘‘suggestive but not 

sufficient to infer a causal relationship’’ 

(respiratory morbidity) or ‘‘inadequate 

to infer the presence or absence of a 

causal relationship’’ (mortality, cancer, 

cardiovascular effects, reproductive/ 

developmental effects) (ISA, sections 

5.3.2.4–5.3.2.6). Thus, the REA 

concluded that these judgments most 

directly support an averaging time that 

focuses protection on short-term 

exposures to NO2. 

As in past reviews of the NO2

NAAQS, it is instructive to evaluate the 

potential for a standard based on annual 

average NO2 concentrations, as is the 

current standard, to provide protection 

against short-term NO2 exposures. To 

this end, Table 10–1 in the REA 

reported the ratios of short-term to 

annual average NO2 concentrations. 

Ratios of 1-hour daily maximum 

concentrations (98th and 99th 

percentile) 10 to annual average 

concentrations across 14 locations 

ranged from 2.5 to 8.7 while ratios of 24- 

hour average concentrations to annual 

average concentrations ranged from 1.6 

to 3.8 (see Thompson, 2008 for more 

details). The REA concluded that the 

variability in these ratios across 

locations, particularly those for 1-hour 

concentrations, suggested that a 

standard based on annual average NO2

concentrations would not likely be an 

effective or efficient approach to focus 

protection on short-term NO2 exposures. 

For example, in an area with a relatively 

high ratio (e.g., 8), the current annual 

standard (53 ppb) would be expected to 

allow 1-hour daily maximum NO2

concentrations of about 400 ppb. In 

contrast, in an area with a relatively low 

ratio (e.g., 3), the current standard 

would be expected to allow 1-hour daily 

maximum NO2 concentrations of about 

150 ppb. Thus, for purposes of 

protecting against the range of 1-hour 

NO2 exposures, the REA noted that a 

standard based on annual average 

concentrations would likely require 

more control than necessary in some 

areas and less control than necessary in 

others, depending on the standard level 

selected. 

In considering the level of support 

available for specific short-term 

averaging times, the policy assessment 

chapter of the REA noted evidence from 

both experimental and epidemiologic 

studies. Controlled human exposure 

studies and animal toxicological studies 

provide evidence that NO2 exposures 

from less than 1-hour up to 3-hours can 

result in respiratory effects such as 

increased airway responsiveness and 

inflammation (ISA, section 5.3.2.7). 

Specifically, the ISA concluded that 

NO2 exposures of 100 ppb for 1-hour (or 

200 ppb to 300 ppb for 30-min) can 

result in small but significant increases 

in nonspecific airway responsiveness 

(ISA, section 5.3.2.1). In contrast, the 

epidemiologic literature provides 

support for short-term averaging times 

ranging from approximately 1-hour up 

to 24-hours (ISA, section 5.3.2.7). A 

number of epidemiologic studies have 

detected positive associations between 

respiratory morbidity and 1-hour (daily 

maximum) and/or 24-hour NO2

concentrations. A few epidemiologic 

studies have considered both 1-hour 

and 24-hour averaging times, allowing 

comparisons to be made. The ISA 

reported that such comparisons in 

studies that evaluate asthma emergency 

department visits failed to reveal 

differences between effect estimates 

based on a 1-hour averaging time and 

those based on a 24-hour averaging time 

(ISA, section 5.3.2.7). Therefore, the ISA 

concluded that it is not possible, from 

the available epidemiologic evidence, to 

discern whether effects observed are 

attributable to average daily (or multiday) concentrations (24-hour average) or 

high, peak exposures (1-hour maximum) 

(ISA, section 5.3.2.7). 

As noted in the policy assessment 

chapter of the REA, given the above 

conclusions, the experimental evidence 

provides support for an averaging time 

of shorter duration than 24 hours (e.g., 

1-h) while the epidemiologic evidence 

provides support for both 1-hour and 

24-hour averaging times. At a minimum, 

this suggests that a primary concern 

with regard to averaging time is the 

level of protection provided against 1- 

hour daily maximum NO2

concentrations. However, it is also 

important to consider the ability of a 1- 

hour (daily maximum) averaging time to 

protect against 24-hour average NO2

concentrations. To this end, Table 10– 

2 in the REA presented correlations 

between 1-hour daily maximum NO2

concentrations and 24-hour average NO2

concentrations (98th and 99th 

percentile) across 14 locations (see 

Thompson, 2008 for more detail). 

Typical ratios ranged from 1.5 to 2.0, 

though one ratio (Las Vegas) was 3.1. 

These ratios were far less variable than 

those discussed above for annual 

average concentrations, suggesting that a 

standard based on 1-hour daily 

maximum NO2 concentrations could 

also be effective at protecting against 24- 

hour NO2 concentrations. The REA 

concluded that the scientific evidence, 

combined with the air quality 

correlations described above, support 

the appropriateness of a standard based 

on 1-hour daily maximum NO2

concentrations to protect against health 

effects associated with short-term 

exposures. 

b. Long-Term Averaging Time 

While the REA concluded that the 

combination of the scientific evidence 

from the ISA and air quality analyses 

most directly support an averaging time 

that focuses protection on short-term 

exposures to NO2, some evidence does 

support the need to also consider health 

effects potentially associated with longterm 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 longterm 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 


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link to these increases in collagen 

synthesis and may provide plausibility 

for the deficits in lung function growth 

described in epidemiologic studies of 

long-term exposure to NO2 (ISA, section 

3.4.5). 

Therefore, though the evidence 

provides strong support for the need to 

protect against health effects associated 

with short-term NO2 exposures, it may 

also be appropriate to consider the 

extent to which the NO2 standard could 

protect against potential effects 

associated with long-term exposures. To 

address this issue, the REA estimated 

annual average NO2 concentrations 

assuming different 1-hour standards 

were just met. For the locations 

evaluated, a 1-hour area-wide standard 

with a level at or below 100 ppb was 

estimated to be associated with annual 

average NO2 concentrations below the 

level of the current annual standard (53 

ppb) (REA, section 10.4.2). Therefore, it 

is possible that a 1-hour standard could 

also provide protection against potential 

effect associated with long-term 

exposures, depending on the level of the 

standard. 

c. CASAC Views 

CASAC agreed with the conclusions 

of the policy assessment chapter of the 

REA that a primary consideration of the 

NO2 NAAQS should be the protection 

provided against health effects 

associated with short-term exposures. In 

their letter to the EPA Administrator, 

CASAC stated that they concur ‘‘with 

having a short-term NAAQS primary 

standard for oxides of nitrogen and 

using the one-hour maximum NO2

value.’’ In addition, the letter noted that 

‘‘CASAC also recommends retaining the 

current standard based on the annual 

average.’’ CASAC based this 

recommendation on the ‘‘limited 

evidence related to potential long-term 

effects of NO2 exposure and the lack of 

strong evidence of no effect.’’ In 

addition, CASAC concluded that ‘‘the 

findings of the REA do not provide 

assurance that a short-term standard 

based on the one-hour maximum will 

necessarily protect the population from 

long-term exposures at levels potentially 

leading to adverse health effects’’ 

(Samet, 2008b). 

d. Administrator’s Conclusions on 

Averaging Time 

In considering the most appropriate 

averaging time(s) for the NO2 primary 

NAAQS, the Administrator notes the 

conclusions and judgments made in the 

ISA about available scientific evidence, 

conclusions from the REA, and CASAC 

recommendations discussed above. 

Based on these considerations, the 

Administrator proposes to set a new 

standard based on 1-hour daily 

maximum NO2 concentrations. In 

addition, the Administrator notes that 

CASAC recommended retaining the 

current annual standard to account for 

the fact that some evidence suggests that 

long-term NO2 exposures could cause 

adverse effects on respiratory health. 

Taking into account these 

considerations, in addition to proposing 

a new 1-hour NO2 primary NAAQS to 

provide increased protection against 

effects associated with short-term 

exposures, the Administrator also 

proposes to retain an annual standard. 

3. Form 

When evaluating alternative forms in 

conjunction with specific levels, the 

REA considered the adequacy of the 

public health protection provided by the 

combination of level and form to be the 

foremost consideration. In addition, the 

REA recognized that it is desirable to 

have a form that is reasonably stable and 

insulated from the impacts of extreme 

meteorological events. As noted in the 

review of the O3 NAAQS (EPA, 2007e), 

forms that call for averaging of 

concentrations over three years better 

reflect pollutant-associated health risks 

than forms based on expected 

exceedances. This is because such 

‘‘concentration-based’’ forms give 

proportionally greater weight to periods 

of time when pollutant concentrations 

are well above the level of the standard 

than to times when the concentrations 

are just above the standard, while an 

expected exceedance form would give 

the same weight to periods of time with 

concentrations that just exceed the 

standard as to times when 

concentrations greatly exceed the 

standard. Averaging concentrations over 

three years also provides greater 

regulatory stability than a form based on 

allowing only a single expected 

exceedance in a year. Therefore, 

consistent with recent reviews of the O3

and PM NAAQS, the REA focused on 

concentration-based forms averaged 

over 3 years. 

In considering specific concentrationbased forms, the REA focused on 98th 

and 99th percentile concentrations 

averaged over 3 years. With regard to 

these alternative forms, the REA noted 

that a 99th percentile form for a 1-hour 

daily maximum standard would 

correspond approximately to the 4th 

highest daily maximum concentration 

in a year (which is the form of the 

current O3 NAAQS) while a 98th 

percentile form (which is the form of the 

current short-term PM2.5 NAAQS) 

would correspond approximately to the 

7th or 8th highest daily maximum 

concentration in a year (Table 10–4 in 

the REA; see Thompson, 2008 for 

methods). The REA concluded that 

either of these forms could provide an 

appropriate balance between limiting 

peak NO2 concentrations and providing 

sufficient regulatory stability. This is 

consistent with judgments made in the 

2006 review of the PM NAAQS (EPA, 

2005). 

When considering the extent to which 

exposure and risk analyses inform 

judgments on the form of the standard, 

the REA noted that a 99th percentile 

form could be appreciably more 

protective than a 98th percentile form 

(for the same standard level) in some 

locations, as shown by the results of air 

quality analyses. For example, a 99th 

percentile standard of 200 ppb was 

estimated to decrease the number of 

benchmark exceedances, relative to a 

98th percentile form, by approximately 

50–70% in Boston, Philadelphia, and 

Washington, DC (Table 10–5 in the 

REA). However, a 99th percentile form 

was estimated to decrease the number of 

benchmark exceedances by only 

approximately 10% in St. Louis, Detroit, 

and Las Vegas (Table 10–5 in the REA). 

For most locations analyzed, the 

difference was estimated to be between 

approximately 10 and 50% (Table 10–5 

in the REA). With regard to the Atlanta 

exposure assessment, a 99th percentile 

form was estimated to decrease the 

number of days with 6 or more 

benchmark exceedances (for 300 ppb), 

relative to a 98th percentile form, by 5– 

35% depending on the standard level 

selected (REA Appendix B, table B–48). 

With regard to the Atlanta risk 

assessment, a 99th percentile form was 

estimated to be associated with 

approximately 6% to 8% fewer NO2- 

related emergency department visits 

than a 98th percentile form, across the 

levels of the potential 1-hour standards 

examined. 

When considering these results as 

they relate to the form of the standard, 

the REA noted that a decision on form 

must be made in conjunction with 

selection of a particular standard level. 

The primary emphasis in such a 

decision will be on the degree of public 

health protection provided by the 

combination of form and level. 

CASAC agreed with the importance of 

considering the public health protection 

provided by the combination of form 

and level. In its letter to the EPA 

Administrator with regard to the final 

REA, the CASAC panel stated that it 

‘‘advises that EPA choose a health 

protective percentile appropriate for the 

level chosen for the one-hour standard.’’ 

CASAC went on to recommend that a 

98th percentile form would be 


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appropriate for a standard level at the 

lower boundary of the range evaluated 

(50 ppb, see below) but that a higher 

percentile should be considered for 

higher levels (Samet, 2008b). 

When considering alternative forms, 

the Administrator notes the views 

expressed in the REA and the 

recommendations from CASAC, as 

described above. In particular, she notes 

that a 99th percentile (or 4th highest) 

form could be appreciably more 

protective in some locations than a 98th 

(or 7th or 8th highest) form. Given these 

considerations, and in light of the 

specific range proposed for level below, 

the Administrator proposes to adopt 

either a 99th percentile or a 4th highest 

form, averaged over 3 years. In addition, 

the Administrator notes that a 98th 

percentile form could be appropriate, 

particularly for standard levels at the 

low end of the range considered in the 

REA. Therefore, she also solicits 

comment on both 98th percentile and 

7th or 8th highest forms. 

4. Level 

In assessing the level of the standard 

to propose, the Administrator has 

considered the broad range of scientific 

evidence assessed in the ISA, including 

the epidemiologic studies and 

controlled human exposure studies, as 

well as the results of exposure/risk 

analyses presented in the REA. In light 

of this body of evidence and analyses, 

she has determined that it is necessary 

to provide increased public health 

protection for at-risk individuals against 

an array of adverse respiratory health 

effects related to short-term (i.e., 30 

minutes to 24 hours) exposures to 

ambient NO2. Such health effects have 

been associated with exposure to the 

distribution of short-term ambient NO2

concentrations across an area. This 

distribution includes both the higher 

short-term (i.e., peak) exposure 

concentrations that can occur on or near 

major roadways and the lower shortterm exposure concentrations that can 

occur in areas not near major roadways. 

In considering the most appropriate 

approach to providing this protection, 

the Administrator is mindful of the 

extent to which the available evidence 

and analyses can inform a decision on 

standard level. Specifically, the range of 

proposed standard levels discussed 

below (section II.F.4.e) is informed by 

controlled human exposure and 

epidemiologic studies. 

As discussed above (section II.B.1.d), 

controlled human exposure studies have 

reported associations between various 

levels of NO2 exposures and increased 

airway responsiveness in asthmatics. 

These studies can inform an evaluation 

of the risks associated with exposure to 

specific NO2 concentrations, regardless 

of where those exposures occur in an 

area. Controlled human exposure 

studies most directly inform 

consideration of the risks associated 

with peak short-term NO2 exposure 

concentrations, such as those that can 

occur on or near major roadways. This 

is the case because NO2 concentrations 

around major roadways could include 

concentrations within the range 

evaluated in the studies. Controlled 

human exposure studies have not been 

conducted at the lower concentrations 

of NO2 typically expected in areas not 

near major roadways. 

In addition, epidemiologic studies 

(section II.B.1.a and b) have reported 

associations between ambient NO2

concentrations, measured at area-wide 

monitors in the current network, and 

increased respiratory symptoms, 

emergency department visits, and 

hospital admissions. Area-wide 

monitors in the urban areas in which 

these epidemiologic studies were 

conducted do not measure the full range 

of ambient NO2 concentrations that can 

occur anywhere in the area, because 

they are not sited in locations with more 

localized peak concentrations. Thus, 

they do not measure the full range of 

ambient NO2 concentrations that are 

likely responsible for the exposures 

linked to the NO2-associated health 

effects reported in the studies. Rather, 

the area-wide NO2 concentrations 

measured by these monitors are used as 

surrogates for the entire distribution of 

ambient NO2 concentrations across the 

area, a distribution that includes NO2

concentrations that are both higher and 

lower than the area-wide concentrations 

reported for the study locations. 

Specifically, this distribution of 

concentrations includes the higher 

short-term peak NO2 concentrations that 

occur on or near major roadways and 

the lower short-term concentrations that 

occur away from roadways. Thus, the 

epidemiologic studies can inform an 

evaluation of the risks associated with 

the full range of exposures likely to 

occur across an area. 

The available evidence and analyses 

support the importance of roadwayassociated NO2 exposures for public 

health. Specifically, the exposure 

assessment presented in the REA 

estimated that roadway-associated 

exposures account for the great majority 

of exposures to peak NO2 concentrations 

(REA, Figures 8–17 and 8–18). In 

addition, the ISA (section 2.5.4) noted 

that in-vehicle NO2 exposures could be 

2–3 times higher than indicated by 

ambient monitors in the current area 

wide-oriented network. Millions of 

people in the U.S. live, work, and/or 

attend school near important sources of 

NO2 such as major roadways (ISA, 

section 4.4) and ambient NO2

concentrations in these locations are 

strongly associated with distance from 

major roads (i.e., the closer to a major 

road, the higher the NO2 concentration) 

(ISA, section 2.5.4). Therefore, these 

populations, which likely include a 

disproportionate number of individuals 

in groups with higher prevalence of 

asthma and higher hospitalization rates 

for asthma (e.g. ethnic or racial 

minorities and individuals of low 

socioeconomic status ) (ISA, section 

4.4), are likely exposed to NO2

concentrations higher than those that 

occur away from major roadways. 

Given the above considerations, the 

Administrator proposes to set a level for 

the 1-hour NO2 primary NAAQS that 

reflects the maximum allowable NO2

concentration anywhere in an area. This 

concentration is likely to occur on or 

near a major roadway. As discussed 

above (section II.A.2), monitoring 

studies suggest that NO2 concentrations 

near roadways can be approximately 30 

to 100% higher than concentrations in 

the same area but not near the road. 

This NO2 concentration gradient around 

roadways is one factor considered by 

the Administrator in determining the 

appropriate standard level to propose. 

EPA proposes to set the level of the 

standard such that, when available 

information regarding the concentration 

gradient around roadways is considered, 

appropriate public health protection 

would be provided by limiting the 

higher short-term peak exposure 

concentrations expected to occur on and 

near major roadways, as well as the 

lower short-term exposure 

concentrations expected to occur away 

from those roadways. 

The Administrator notes that this 

approach to setting the standard would 

provide a relatively high degree of 

confidence regarding the level of 

protection provided by the standard 

against peak exposures, such as those 

that can occur on or near major 

roadways. This is a particularly 

important consideration given the 

available information and the air quality 

and exposure analyses, discussed above 

in section II.F.4.b, which indicated that 

roadway-associated exposures account 

for the majority of exposures to peak 

NO2 concentrations. The Administrator 

concludes that the proposed approach 

would directly address the great 

majority of peak exposures and 

associated health effects. In addition, 

the range of standard levels proposed 

below (section II.F.4.e) would provide a 

reasonable degree of confidence that the 


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accompanying area-wide NO2

concentrations would be maintained 

well below concentrations that have 

occurred in locations where 

epidemiologic studies have reported 

associations between ambient NO2

concentrations and health endpoints 

such as increased respiratory symptoms, 

emergency department visits, and 

hospital admissions. Therefore, the 

Administrator proposes to set a standard 

level reflecting the maximum allowable 

NO2 concentration anywhere in an area 

that, in combination with the proposed 

decisions on indicator, averaging time, 

and form, will protect public health 

with an adequate margin of safety 

against the array of NO2-associated 

health effects. 

The remainder of this section 

describes the considerations relevant to 

the Administrator’s proposed decisions 

on standard levels for a new 1-hour 

standard and the annual standard. 

Specifically, with regard to a 1-hour 

standard evidence-based considerations 

drawn from the ISA and discussed in 

the policy-assessment chapter of the 

REA are discussed in section II.F.4.a. 

Exposure- and risk-based considerations 

for a 1-hour standard drawn from the 

analyses in the REA and discussed in 

the policy assessment chapter are 

discussed in section II.F.4.b. A summary 

of the considerations relating to a 1-hour 

standard from the policy assessment 

chapter of the REA is presented in 

section II.F.4.c and CASAC views 

expressed in the context of their 

comments on the final REA are 

presented in section II.F.4.d. The 

Administrator’s proposed approach to 

setting a 1-hour standard and her 

conclusions regarding the level of such 

a standard are presented in section 

II.F.4.e. An alternative approach to 

setting a 1-hour standard is discussed in 

section II.E.4.f. Comment is solicited on 

both approaches. Finally, the 

Administrator’s proposed conclusions 

on the level of the annual standard are 

presented in section II.E.4.g. 

a. Evidence-Based Considerations 

Evidence-based considerations take 

into account the full body of scientific 

evidence assessed in the ISA. When 

considering the extent to which this 

scientific evidence can inform a 

decision on the level of a 1-hour 

standard, the policy assessment chapter 

of the REA notes that NO2

concentrations represent different 

measures of exposure when drawn from 

experimental versus epidemiologic 

studies. Concentrations of NO2 tested in 

experimental studies, such as controlled 

human exposure studies, represent 

exposure concentrations in the 

breathing zone of the individual test 

subjects. In cases where controlled 

human exposure studies report effects, 

those effects are caused directly by 

exposure to a specified concentration of 

NO2. In contrast, concentrations of NO2

drawn from epidemiologic studies are 

often based on ambient monitoring data. 

In the case of key U.S. studies that have 

been specifically considered within the 

context of assessing the appropriate 

level for the standard, these monitors 

measure area-wide NO2 concentrations 

that occur away from major roadways. 

NO2 concentrations recorded at these 

ambient monitors are used as surrogates 

for the distribution of NO2 exposures 

across the study area and over the time 

period of the study. As noted above, 

these monitors do not measure the full 

range of ambient NO2 concentrations 

that can occur in an area and, thus, they 

do not measure the full range of ambient 

NO2 concentrations that are likely 

responsible for the NO2-associated 

health effects reported in the studies. 

Instead they capture one part of the 

distribution (the area-wide 

concentration) and this is used as a 

surrogate for the entire distribution, 

which includes peak roadwayassociated concentrations. As noted in 

the REA, the interpretation of NO2

concentrations from different types of 

studies is an important consideration for 

decisions on standard level. These 

implications are discussed in more 

detail below in section II.F.4.e. 

In considering the epidemiologic 

evidence, the REA noted the ISA 

conclusion that epidemiologic studies 

provide the strongest support for the 

link between short-term NO2 exposure 

and respiratory morbidity. In addition, 

epidemiologic studies provide evidence 

for the most serious NO2-associated 

respiratory effects, including 

respiratory-related hospital admissions 

and emergency department visits. As 

noted above, these effects have been 

reported to be associated with area-wide 

NO2 concentrations in key U.S. 

epidemiologic studies. Because areawide NO2 concentrations are used as 

surrogates for the distribution of NO2

exposures across the study area and 

over the time period of the study (see 

above), the health effects reported in 

these epidemiologic studies are 

reasonably inferred to be associated 

with exposure to ambient NO2

concentrations that are both higher and 

lower than the area-wide concentrations 

reported for the study locations. As 

noted above, this distribution of 

exposure concentrations includes both 

the higher short-term peak NO2

concentrations that occur on or near 

major roadways and the lower shortterm concentrations that occur away 

from roadways. 

When evaluating the epidemiologic 

literature for its potential to inform the 

selection of an appropriate range of 

standard levels, the REA noted the ISA 

conclusion that NO2 epidemiologic 

studies provide ‘‘little evidence of any 

effect threshold’’ (ISA, section 5.3.2.9, p. 

5–15). In studies that have evaluated 

concentration-response relationships, 

those relationships appear linear within 

the observed range of data (ISA, section 

5.3.2.9). Given this lack of an apparent 

threshold below which effects do not 

occur, an important consideration with 

regard to providing an adequate margin 

of safety is the extent to which it is 

appropriate for the range of proposed 

standard levels to extend below NO2

concentrations that have been 

associated with health effects in these 

studies. For purposes of using the 

epidemiologic evidence to identify a 

range of standard levels for evaluation 

in the absence of an apparent threshold, 

the REA considered the range of NO2

concentrations that have been 

monitored in locations, and during time 

periods, of key U.S. epidemiologic 

studies (ISA, Table 5.4–1). 

Figures 4 and 5 below (REA, Figures 

5–1 and 5–2) show standardized effect 

estimates from single pollutant models 

and the 99th and 98th percentiles of the 

1-hour daily maximum NO2

concentrations recorded at area-wide 

monitors in the locations, and during 

the time periods, of key U.S. studies. 

The peak NO2 concentrations to which 

individuals were exposed on and/or 

near major roadways in these locations 

during the study periods would be 

expected to be substantially higher than 

the concentrations recorded at these 

area-wide monitors. The lowest areawide 1-hour daily maximum 

concentrations, 53 (99th percentile) and 

50 (98th percentile) ppb, were 

monitored in the location of the study 

by Delfino et al. (2002). This single 

study reported mixed results for 

respiratory symptoms with most 

reported NO2 effect estimates being 

positive, and with some but not all 

positive effect estimates being 

statistically significant. A cluster of 5 

studies (Ito et al., 2007; Jaffe et al., 2003; 

NYDOH, 2006; Peel et al., 2005; Tolbert 

et al., 2007) were conducted in locations 

with area-wide 1-hour daily maximum 

NO2 concentrations ranging from 93 to 

112 ppb (99th percentile) and from 85 

to 94 ppb (98th percentile). In these 

studies, single pollutant models yielded 

generally positive and often statistically 

significant NO2 effect estimates for 

respiratory-related emergency 


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11 In this study, multi-pollutant models were 

evaluated only for the warm months. Single 

pollutant effect estimates for NO2 were statistically 

significant for the warm months, but not for the 

cold months. 

12As discussed above in section II.B.1, the 

conclusion from the ISA that NO2 effect estimates 

generally remain robust in multi-pollutant models 

is based on evaluation of the broader body of 

epidemiologic evidence which includes, but is not 

limited to, these U.S. studies (e.g., see Figures 1– 

3 above and ISA, Figures 3.1–7, 3.1–10, and 3.1– 

11). Effect estimates from these U.S. studies were 

not included in the multi-pollutant figures in the 

ISA because the studies generally reported multipollutant model results only qualitatively. They 

generally did not report the quantitative 

information that would have been necessary to 

include the results in the ISA figures. 

13Effect estimates presented in Figures 4 and 5 

are from single pollutant models. 

14Authors of relevant U.S. and Canadian studies 

were contacted and, for each study, air quality 

statistics were requested from the monitor that 

recorded the highest NO2 concentrations. In cases 

where authors provided 1-hour daily maximum air 

quality statistics, this information is presented in 

Figures 4 and 5 (studies by Tolbert, Peel, NYDOH, 

Delfino). In four cases (studies by Ito, Jaffe, Linn, 

Ostro), we were not able to identify 1-hour NO2

statistics from the information provided by the 

authors. In these cases, we evaluated monitored 

NO2 concentrations reported to EPA’s Air Quality 

System (AQS) for the location and time of the 

study. Figures 4 and 5 present the highest 98th/99th 

percentile 1-hour daily maximum NO2

concentrations that correspond to each study 

location and time period. Prior to identifying 

potential alternative standards, we did not receive 

air quality information from any of the Canadian 

authors contacted and we were unable to 

reconstruct the air quality data sets for the Canadian 

studies. Therefore, for purposes of identifying levels 

of potential alternative standards, our analysis was 

based on these key U.S. studies. Note that the NO2

concentrations reported in the study by Jaffe are 

labeled as 24-hour concentrations, but the author 

indicated in a personal communication (Jaffe, 2008) 

that they actually represent 1-hour daily maximum 

concentrations. 

department visits and hospital 

admissions in a variety of locations 

across the U.S. Of these 5 studies, 4 

studies (Ito, 2007; NYDOH, 2006; Peel et 

al., 2005; Tolbert et al., 2007) also 

reported NO2 effect estimates using 

multi-pollutant models, as discussed 

above (section II.B.1.a). In the study by 

Ito (2007), risk estimates were robust 

and remained statistically significant in 

multi-pollutant models that included 

PM2.5, O3, CO, and SO2.11 In the study 

by Peel et al. (2005), the authors 

reported that ‘‘The estimates for NO2

were generally not attenuated in 

multipollutant models, while the 

estimates for the other pollutants [PM10, 

ozone, NO2, and CO] suggested weaker 

or no associations in the multipollutant 

models.’’ The quantitative results for 

these multi-pollutant models were not 

presented in this study. In the 

remaining 2 studies (NYDOH, 2006; 

Tolbert et al., 2007), NO2 effect 

estimates that were positive in single 

pollutant models remained positive but 

not statistically significant in multipollutant models.12 Two additional 

studies which evaluated only single 

pollutant models (Linn et al., 2000; 

Ostro et al., 2001) reported positive and 

statistically significant NO2 effect 

estimates in locations with appreciably 

higher area-wide 1-hour daily maximum 

NO2 concentrations (i.e., around 200 

ppb). 


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15When the asthmatic results were grouped 

together for all exposures, both at rest and during 

exercise, the percent of asthmatics with increased 

airway responsiveness decreased at the higher 

exposure concentrations. This result could be 

attributed to the lack of an effect in the asthmatics 

exposed during exercise. 

When evaluating the controlled 

human exposure literature for its 

potential to inform the selection of a 

range of appropriate standard levels for 

evaluation, the REA noted that available 

studies have addressed the 

consequences of short-term (e.g., 30- 

minutes to several hours) NO2

exposures for a number of health 

endpoints including increased airway 

responsiveness, reduced host defense 

and immunity, inflammation, and 

decreased lung function (ISA, section 

3.1). In identifying health endpoints on 

which to focus for purposes of 

informing decisions about potential 

alternative standard levels, the REA 

concluded that it was appropriate to 

focus on those endpoints that occur at 

or near ambient levels of NO2 and 

endpoints that are of potential public 

health significance. As described above 

in more detail (section II.C.1), the only 

endpoint to meet both of these criteria 

is increased airway responsiveness in 

asthmatics. The ISA concluded that NO2

exposures between 200 and 300 ppb for 

30 minutes and 100 ppb for 60-minutes 

can result in small but significant 

increases in nonspecific airway 

responsiveness (ISA, section 5.3.2.1) 

and that ‘‘transient increases in airway 

responsiveness following NO2 exposure 

have the potential to increase symptoms 

and worsen asthma control’’ (ISA, 

sections 3.1.3 and 5.4). This effect could 

have important public health 

implications due to the large size of the 

asthmatic population in the United 

States (ISA, Table 4.4–1). In addition, 

NO2 effects on airway responsiveness in 

asthmatics are part of the body of 

experimental evidence that provides 

plausibility and coherence for the 

observed NO2-related increase in 

hospital admissions and emergency 

department visits in epidemiologic 

studies (ISA, section 5.3.2.1). For all of 

these reasons, the REA considered the 

extent to which results reported for the 

NO2-associated increase in airway 

responsiveness in asthmatics could 

inform decisions on alternative standard 

levels. 

With regard to controlled human 

exposure studies of airway 

responsiveness, the ISA and the REA 

discussed an update to a meta-analysis 

that was originally published by 

Folinsbee in 1992 and considered in the 

1993 NOX AQCD. The original analysis 

by Folinsbee (1992) included individual 

level data from 19 studies involving 

asthmatic volunteers. Folinsbee 

reported that 65% of resting asthmatics 

(57 of 88) exposed to NO2

concentrations between 100 and 140 

ppb experienced an increase in airway 

responsiveness. In addition, 76% (25 of 

33) of resting asthmatics experienced 

increased airway responsiveness 

following exposure to NO2

concentrations between 200 and 300 

ppb. These results in resting asthmatics 

were statistically significant. Smaller, 

and statistically non-significant, 

percentages of exercising asthmatics 

experienced increased airway 

responsiveness following exposure to 

NO2 concentrations (ISA, section 

3.1.3.2). The reason for this difference is 

not known as the factors that predispose 

some asthmatics to NO2 responsiveness 

are not understood (ISA, section 

3.1.3.2).15


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16The updated meta-analysis added a study that 

evaluated non-specific airway responsiveness 

following exposure to 260 ppb NO2 and removed a study that evaluated allergen-induced airway 

responsiveness following exposure to 100 ppb NO2. 

The update of this meta-analysis 

presented in the ISA (Table 3.1–3) 

included one additional study of nonspecific responsiveness and removed an 

allergen responsiveness study that was 

included in the original 16 (see ISA, 

section 3.1.3.2 for more discussion). 

While the updated analysis does not 

include new results at lower 

concentrations (100–250 ppb), we 

interpreted the results with a greater 

focus on 100 ppb due, in part, to the 

greater body of evidence available, 

including new epidemiologic evidence. 

Therefore, the updated analysis also 

reported results specifically for an NO2

exposure concentration of 100 ppb. As 

with the original analysis by Folinsbee 

(1992), the updated meta-analysis 

reported that a larger percentage of 

resting asthmatics, as opposed to 

exercising asthmatics, experienced an 

NO2-related increase in airway 

responsiveness. The updated analysis 

reported that, when exposed at rest, 

66% (33 of 50) of asthmatics 

experienced an increase in airway 

responsiveness following exposure to 

100 ppb NO2, 67% (47 of 70) of 

asthmatics experienced an increase in 

airway responsiveness following 

exposure to NO2 concentrations from 

100 to 150 ppb, 75% (38 of 51) of 

asthmatics experienced an increase in 

airway responsiveness following 

exposure to NO2 concentrations from 

200 to 300 ppb, and 73% (24 of 33) of 

asthmatics experienced an increase in 

airway responsiveness following 

exposure to NO2 concentrations above 

300 ppb. The fraction of resting 

asthmatics experiencing an increase in 

airway responsiveness was statistically 

significant at each of these NO2

concentrations. 

Based on this evidence, we have 

identified exposure to NO2 at a level of 

100 ppb to be the lowest level at which 

effects have been observed in controlled 

human exposure studies, noting that it 

is also the lowest level tested in the 

studies used in the meta-analysis. There 

is no evidence from this meta-analysis, 

however, of a threshold below which 

NO2-related effects do not occur. 

b. Exposure- and Risk-Based 

Considerations 

Chapters 7–9 of the REA estimated 

exposures and health risks associated 

with recent air quality and with air 

quality, as measured at monitors in the 

current area-wide network, which had 

been adjusted to simulate just meeting 

the current and potential alternative 

standards. The specific standard levels 

evaluated, for an area-wide standard 

based on the 3-year average of the 98th 

and 99th percentile 1-hour daily 

maximum NO2 concentrations, were 50, 

100, 150, and 200 ppb. 

The results of the air quality, 

exposure, and risk analyses are 

presented below in Table 1. With regard 

to the air quality results, Table 1 

presents the number of days per year 

that NO2 concentrations on/near roads 

were estimated to equal or exceed the 

lowest and the highest health 

benchmarks evaluated (100 and 300 

ppb). Compared to just meeting the 

current annual standard, exceedances 

estimated to be associated with just 

meeting 99th percentile 1-hour daily 

maximum area-wide standard levels of 

either 50 or 100 ppb were substantially 

lower. In contrast, exceedances 

estimated to be associated with 1-hour 

area-wide standards of 150 or 200 ppb 

were either similar to, or slightly higher 

than, those estimated for just meeting 

the current standard. Table 1 also 

presents the results of the Atlanta 

exposure and risk assessments. As is the 

case for the air quality analyses, NO2

exposures and risks estimated to be 

associated with just meeting 1-hour 

area-wide standard levels of either 50 or 

100 ppb were substantially lower than 

those associated with just meeting the 

current annual standard. Exposures and 

risks estimated to be associated with 1- 

hour area-wide standard levels of 150 or 

200 ppb were somewhat lower than, or 

similar to, those estimated for just 

meeting the current annual standard. 

TABLE 1—SUMMARY OF RESULTS OF THE EXPOSURE AND RISK ANALYSES PRESENTED IN THE REA 

Air quality 

Mean estimated number of days 

per year with 1-hour NO2

concentrations on/near roads 

greater than or equal to benchmark levels (in location with largest number of estimate 

exceedances) 

Mean percent of Atlanta 

asthmatics estimated to 

experience 6 or more days per 

year with 1-hour NO2 exposure 

concentrations greater than or 

equal to benchmark levels (based 

on the year 2002) 

Mean percent of total respiratory 

ED visits in Atlanta estimated to 

be related to NO2

(based on the year 2007) 

100 ppb 

benchmark 

300 ppb 

benchmark 

100 ppb 

benchmark 

(percent) 

300 ppb 

benchmark 

(percent) 

Single 

pollutant 

estimate 

Multipollutant 

estimates* 

Current annual standard .............. 338 38 100 97 8.1 1.7–6.9 

Potential Alternative Standards Evaluated in the REA 

99th 1-hour: 200 ppb ................... 350 56 100 89 7.1 1.5–6.1 

99th 1-hour: 150 ppb ................... 337 13 100 57 5.4 1.1–4.6 

99th 1-hour: 100 ppb ................... 229 4 100 11 3.6 0.7–3.1 

99th 1-hour: 50 ppb ..................... 13 1 57 0 1.8 0.4–1.6 

* Ranges represent the range of risk estimates that result from including different co-pollutants in the model. 

c. Summary of Considerations From the 

REA 

The policy assessment chapter of the 

REA considered the scientific evidence 

and the exposure/risk information as 

they relate to considering alternative 1- 

hour NO2 standards that could be 

judged to be requisite to protect public 

health with an adequate margin of 

safety. The conclusions of the REA were 

based, in large part, on scientific 

evidence (i.e., key U.S. epidemiologic 

studies) and exposure/risk analyses that 

were based on the use of the available 

NO2 air quality data from area-wide 

monitors, as discussed above in sections 

II.B and II.C. The implications of these 

conclusions for a standard level that 

reflects the maximum allowable 

concentration anywhere in an area (a 


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17As noted above, the health effects reported in 

epidemiologic studies are reasonably inferred to be 

associated with exposure to ambient NO2

concentrations that are both higher than and lower 

than the area-wide concentrations reported for the 

study location. 

18Earlier CASAC letters focused on their review 

of the air quality, exposure, and risk analyses as 

presented in other chapters of the draft REA. 

concentration likely to occur near major 

roads) are discussed below in section 

II.F.4.e. 

When considering an appropriate 

upper end of the range of 1-hour daily 

maximum standard levels supported by 

the scientific evidence, the REA noted 

the following: 

• Positive and statistically significant 

associations were observed in several 

key U.S. epidemiologic studies in 

locations with area-wide 98th and 99th 

percentile 1-hour daily maximum NO2

concentrations ranging from 85 to 112 

ppb 17 (Peel et al., 2005; NYDOH, 2006; 

Ito et al., 2007; Tolbert et al., 2007) (see 

Figure 4 above). 

• The meta-analysis of airway 

responsiveness presented in the ISA 

reported increased airway 

responsiveness in most asthmatics (66% 

or 33 out of 50) following short-term 

exposures to 100 ppb NO2, which was 

the lowest concentration for which such 

data were available. Although some 

uncertainties associated with this 

evidence, as described above, provide 

support for considering standard levels 

below 100 ppb (i.e., studies have 

typically involved volunteers with mild 

asthma and data are lacking from more 

severely affected asthmatics, who may 

be more susceptible (ISA, p. 3–16)), 

other uncertainties (i.e., the 

undetermined magnitude and clinical 

significance of the NO2-associated 

increase in airway responsiveness) 

provide support for considering higher 

standard levels. 

Given these considerations, the REA 

concluded that the scientific evidence 

provides support for a standard level up 

to 100 ppb. The REA also noted that, to 

the extent more emphasis is placed on 

the uncertainties associated with 

ascribing effects to NO2 in the cluster of 

epidemiologic studies and on the 

magnitude and clinical significance of 

the NO2-associated increase in airway 

responsiveness following exposure to 

NO2, standard levels higher than 100 

ppb could be considered. However, the 

strongest support was concluded to be 

for standard levels at or below 100 ppb. 

When considering an appropriate 

lower end of a range of levels supported 

by the scientific evidence, the REA 

noted the following: 

• The epidemiologic study by Delfino 

et al., (2002) evaluated associations 

between short-term ambient NO2

concentrations and respiratory 

symptoms in a location (Alpine, CA) 

where area-wide NO2 concentrations 

were well below levels in other key U.S. 

epidemiologic studies. As noted above, 

this single study provides mixed 

evidence for NO2-associated effects in a 

location with 99th and 98th percentile 

1-hour daily maximum area-wide NO2

concentrations of 53 and 50 ppb, 

respectively. 

• The meta-analysis of controlled 

human exposure studies reported 

increased airway responsiveness in 

asthmatics at the lowest NO2

concentration for which data were 

available (i.e., 100 ppb). In identifying 

the specific lower level for the standard 

that could be reasonably supported by 

this controlled human exposure 

evidence, there are several reasons why 

it is appropriate to consider levels 

below 100 ppb. First, the meta-analysis 

did not provide information on the 

potential for an NO2-induced increase in 

airway responsiveness at concentrations 

below 100 ppb, leaving open the 

possibility for effects following 

exposures to lower concentrations. 

Second, the studies included in the 

meta-analysis did not evaluate severe 

asthmatics and most of the subjects 

included in these studies were mild 

asthmatics. Asthmatics characterized as 

having more severe asthma may be more 

susceptible than mild asthmatics to the 

effects of NO2 exposure (ISA, section 

3.1.3.2). 

Thus, the REA concluded that it was 

appropriate to base the lower end of the 

range of standard levels on NO2

concentrations in the location of the 

epidemiologic study by Delfino and on 

providing increased protection relative 

to the lowest level at which increased 

airway responsiveness in asthmatics 

was reported in controlled human 

exposure studies. Given the mixed 

results reported in the Delfino study, the 

REA concluded that it was appropriate 

to consider standard levels 

approximately equal to, rather than 

below, those measured in the location of 

the study. Given these considerations, 

the REA concluded that the lower end 

of the range of levels that is reasonably 

supported by the scientific evidence is 

50 ppb for a 1-hour standard that would 

protect public health with an adequate 

margin of safety. 

In addition to these evidence-based 

considerations, the REA compared the 

health risks estimated to be associated 

with just meeting the current standard 

to those estimated to be associated with 

different 1-hour standards. As noted 

above (section II.C), the REA 

characterized NO2-associated health 

risks by estimating the potential 

occurrence of ambient NO2

concentrations greater than or equal to 

concentrations reported to increase 

airway responsiveness, exposures of 

asthmatics to NO2 concentrations 

reported to increase airway 

responsiveness, and the incidence of 

NO2-associated emergency department 

visits. Given the REA conclusion that 

the available evidence and information 

clearly call into question the adequacy 

of the current standard, the adequacy of 

alternative 1-hour standards would also 

be called into question if those 

standards were estimated to be 

associated with similar or higher risks. 

In considering the three analyses that 

characterized NO2-associated health 

risks, the REA noted that just meeting 1- 

hour area-wide standard levels of 150 

and 200 ppb was estimated to be 

associated with risks ranging from 

somewhat lower to slightly higher than 

those estimated for the just meeting the 

current standard. In contrast, just 

meeting 1-hour standard levels of 50 or 

100 ppb, in conjunction with the 

current area-wide monitoring network, 

was estimated to result in appreciably 

lower health risks than the current 

standard. Given this, the REA 

concluded that the exposure/risk 

information reinforces the scientific 

evidence in supporting a standard level 

from 50 to 100 ppb. 

d. CASAC Views 

CASAC expressed their views in a 

letter to the EPA Administrator (Samet, 

2008b) within the context of their 

review of the final REA, a review which 

focused primarily on the policy 

assessment chapter.18 In drawing 

conclusions regarding the level of a 

short-term standard, CASAC considered 

the scientific evidence evaluated in the 

ISA, the exposure and risk results 

presented in the REA, and the evidenceand 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 


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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 their review of the 

final REA. As noted above, the 

conclusions from the policy assessment 

chapter of the final REA were based, in 

large part, on scientific evidence and 

exposure/risk information based on NO2

air quality data from the current areawide NO2 monitoring network. 

Therefore, it is not clear the degree to 

which CASAC recommendations might 

differ for a standard level that reflects 

the maximum allowable NO2

concentration anywhere in an area, 

including near major roads. As noted in 

section I.C above, we are specifically 

soliciting CASAC comment on the use 

of this approach and on the proposed 

range of levels for a standard set using 

this approach. 

In drawing conclusions regarding the 

level of an annual standard, CASAC 

noted the scientific evidence assessed in 

the ISA. Specifically, CASAC concluded 

that while there is evidence supporting 

the link between long-term NO2

exposure and adverse health effects, this 

evidence does not provide a strong 

quantitative basis for changing the level 

of the current annual standard. 

Therefore, with regard to the annual 

standard, CASAC recommended 

‘‘retaining the current level, as evidence 

has not been cited that would lead to 

either an increase or decrease’’ (Samet, 

2008b). 

e. Administrator’s Conclusions on Level 

for a 1-Hour Standard 

In considering the appropriate level 

for an NO2 standard based on the 3-year 

average of the 99th percentile (or 4th 

highest) 1-hour daily maximum NO2

concentration, the Administrator has 

considered the broad body of scientific 

evidence and exposure/risk information. 

She draws from that evidence and 

information the need to protect at-risk 

individuals against the distribution of 

short-term ambient NO2 exposure 

concentrations across an area and the 

array of health effects that have been 

linked to such NO2 exposures. 

Specifically, the Administrator has 

considered the extent to which a variety 

of levels, which would reflect the 

maximum allowable 1-hour NO2

concentration anywhere in an area, 

would be expected to protect at-risk 

individuals against increased airway 

responsiveness, respiratory symptoms, 

and respiratory-related emergency 

department visits and hospital 

admissions. The Administrator notes 

that these health endpoints are logically 

linked together in that the evidence for 

increased airway responsiveness in 

asthmatics is part of the body of 

experimental evidence that the ISA 

recognized as supporting the 

plausibility of associations between 

ambient NO2 and the respiratory 

morbidity endpoints (i.e., respiratory 

symptoms, emergency department 

visits, and hospital admissions) reported 

in epidemiologic studies. 

As noted above, NO2 exposure 

patterns associated with respiratory 

morbidity in epidemiologic studies are 

reasonably expected to include shortterm peak exposures on and/or near 

major roadways of a magnitude that has 

been reported to increase airway 

responsiveness in asthmatics. Therefore, 

to inform the identification of an 

appropriate range of standard levels to 

propose, the Administrator has 

considered the scientific evidence, the 

exposure/risk results, and information 

on the NO2 concentration gradient 

around roadways. 

In making judgments regarding the 

weight to place on the scientific 

evidence and exposure/risk information, 

the Administrator has considered the 

results of epidemiologic studies, 

controlled human exposure studies, and 

exposure/risk analyses as well as the 

uncertainties associated with this 

evidence and these analyses. 

Specifically, she notes the following: 

• The ISA concluded that 

epidemiologic studies provide the 

strongest support for the relationship 

between short-term exposure to NO2

and respiratory morbidity. Despite the 

possibility that associations between 

health effects and NO2 in epidemiologic 

studies may be confounded by the 

presence of co-occurring pollutants, 

particularly other traffic-related 

pollutants, the ISA concluded that NO2

effect estimates remain robust in multipollutant models and that the evidence 

supports a direct effect of NO2

exposures on respiratory morbidity, 

independent of associations with other 

traffic-related pollutants. Given this 

conclusion, along with conclusions 

from the ISA regarding the consistency 

and the coherence of results across the 

relatively large number of NO2

epidemiologic studies (both indoor and 

outdoor) and the supporting evidence 

from experimental studies, the 

Administrator has judged it appropriate 

to place substantial weight on 

epidemiologic studies in identifying an 

appropriate range of levels to propose. 

• Controlled human exposure studies 

report that short-term exposures to NO2

can increase airway responsiveness in 

asthmatics. With regard to this 

evidence, the Administrator also has 

considered the uncertainties associated 

with the magnitude and the clinical 

relevance of the NO2-associated increase 

in airway responsiveness, noting that 

this effect may or may not be clinically 

significant for any given asthmatic. 

However, given the potential public 

health importance of this effect, due to 

the large size of the asthmatic 

population in the U.S. and the 

possibility that the NO2-associated 

increase in airway responsiveness could 

worsen asthma symptoms and decrease 

control of asthma, the Administrator 

judges that it is also appropriate to place 

weight on this evidence when 

identifying an appropriate range of 

levels to propose. 

• The results of the risk and exposure 

analyses presented in the REA provide 

information on the potential public 

health implications of setting the 

standard at different levels. The 

Administrator acknowledges the 

uncertainties associated with these 

analyses which, as discussed in the 

REA, could result in either over- or 

underestimates of NO2-associated health 

risks. However, she also notes that those 

uncertainties should be similar across 

different air quality simulations within 

the air quality, exposure, and risk 

analyses. Therefore, the Administrator 

judges that these analyses are 

potentially useful for considering the 

relative levels of public health 

protection that could be provided by 

specific standard levels. 

After considering the scientific 

evidence and the exposure/risk 

information (see sections II.B, II.C, and 

II.F.4.a through II.F.4.c), as well as the 

available information on the NO2

concentration gradient around roadways 

(section II.A.2), as they relate to a 

standard level reflecting the maximum 

allowable NO2 concentration in an area, 

the Administrator concludes that the 

strongest support is for a standard level 

at or somewhat below 100 ppb. The 

Administrator’s rationale in reaching 

this conclusion is provided below. 

First, the Administrator notes that a 

standard level of 100 ppb or lower 

under the proposed approach would be 

expected to limit short-term peak NO2

exposures to concentrations that have 

been reported to increase airway 

responsiveness in asthmatics. With 

regard to this, the Administrator 

specifically notes the following: 

• The meta-analysis of controlled 

human exposure data in the ISA 

reported increased airway 


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responsiveness in asthmatics at rest 

following exposure at and above 100 

ppb NO2, the lowest NO2 concentration 

for which airway responsiveness data 

are available in humans. 

• This meta-analysis does not provide 

any evidence of a threshold below 

which effects do not occur. The studies 

included in the meta-analysis evaluated 

primarily mild asthmatics while more 

severely affected individuals could 

respond to lower concentrations. Given 

this, it is possible that exposure to NO2

concentrations below 100 ppb could 

increase airway responsiveness in some 

asthmatics. 

• However, the magnitude of the 

NO2-induced increase in airway 

responsiveness, and its clinical 

implications, cannot be quantified from 

the meta-analysis. As noted previously, 

the NO2-induced increase in airway 

responsiveness may or may not be 

clinically significant. Further, there was 

a lack of an effect in asthmatics exposed 

during exercise. 

Given the above considerations, the 

Administrator concludes that the 

controlled human exposure studies of 

airway responsiveness provide support 

for limiting exposure to NO2

concentrations at or somewhat below 

100 ppb. While she acknowledges that 

exposure to lower concentrations could 

increase airway responsiveness in some 

asthmatics, the Administrator concludes 

that, given the uncertainties regarding 

the magnitude and the clinical 

significance of the NO2-induced 

increase in airway responsiveness, the 

greatest support is for limiting 

exposures to 100 ppb. 

Second, the Administrator notes that 

a standard level at or somewhat below 

100 ppb under the proposed approach 

would be expected to maintain peak 

area-wide NO2 concentrations 

considerably below peak area-wide 

concentrations measured in locations 

where multiple key U.S. epidemiologic 

studies have reported associations with 

emergency department visits and 

hospital admissions. With regard to this, 

the Administrator specifically notes that 

5 key U.S. studies provide evidence for 

effects in locations where 99th 

percentile 1-hour daily maximum NO2

concentrations measured at area-wide 

monitors ranged from 93 to 112 ppb. 

The Administrator notes that the study 

by Delfino provides mixed evidence for 

effects in a location with a 99th 

percentile 1-hour daily maximum NO2

concentration, as measured by an areawide monitor, of 53 ppb. In that study, 

most of the reported NO2 effect 

estimates were positive, but not 

statistically significant. Focusing on 

these studies, the Administrator 

concludes that they provide support for 

limiting area-wide NO2 concentrations 

to below 90 ppb (99th percentile) in 

order to provide protection against the 

reported effects. She also concludes that 

limiting area-wide concentrations to 

considerably below 90 ppb would be 

appropriate in order to provide an 

adequate margin of safety. Given the 

mixed results of the Delfino study, the 

Administrator concludes that it may not 

be necessary to maintain area-wide NO2

concentrations at or below 50 ppb to 

provide protection against the effects 

reported in epidemiologic studies. 

Given that NO2 concentrations near 

roads may be 30 to 100% higher than 

concentrations away from roads (see 

section II.A.2), the Administrator notes 

that a standard level at or somewhat 

below 100 ppb under the proposed 

approach could limit area-wide NO2

concentrations to well below 90 ppb 

(99th percentile). With regard to this, 

she specifically notes the following: 

• If NO2 concentrations near roads are 

30% higher than concentrations away 

from roads, a standard level of 100 ppb 

could limit area-wide concentrations to 

approximately 75 ppb. 

• If NO2 concentrations near roads are 

65% higher than concentrations away 

from roads (the mid-range of the 30% to 

100% gradients), a standard level of 100 

ppb could limit area-wide NO2

concentrations to approximately 60 ppb. 

• If NO2 concentrations near roads are 

100% higher than concentrations away 

from roads, a standard level of 100 ppb 

could limit area-wide concentrations to 

approximately 50 ppb. 

Therefore, a standard level at or 

somewhat below 100 ppb under the 

proposed approach would be expected 

to maintain area-wide NO2

concentrations well below 90 ppb across 

locations despite the expected variation 

in the NO2 concentration gradient that 

can exist around roadways in different 

locations and over time. Such a 

standard level recognizes the substantial 

weight that the Administrator judges is 

appropriate to place on the cluster of 

key U.S. epidemiologic studies that 

reported positive, and often statistically 

significant, associations between NO2

and emergency department visits and 

hospital admissions. This judgment 

takes into account the determinations in 

the ISA, based on a much broader body 

of evidence, that there is a likely causal 

association between exposure to NO2

and these kinds of morbidity effects, 

and that there is no evidence of a 

threshold below which such effects 

would not occur. 

As noted above, based on the 

Administrator’s consideration of the 

controlled human exposure and 

epidemiologic evidence, she concludes 

that the strongest support is for a 

standard level reflecting the maximum 

allowable NO2 concentration in an area 

at or somewhat below 100 ppb. In 

addition to these evidence-based 

considerations, the Administrator notes 

that a standard level of 100 ppb under 

the proposed approach would be 

consistent with the results of the 

exposure and risk analyses presented in 

the REA. As described in sections 

II.F.4.b and II.F.4.c above, the results of 

these analyses supported limiting areawide NO2 concentrations to between 50 

and 100 ppb, which would be expected 

with a standard level at or below 100 

ppb under the proposed approach. 

Given all of these considerations, the 

Administrator concludes that a standard 

level at or somewhat below 100 ppb 

under the proposed approach would be 

requisite to protect public health with 

an adequate margin of safety against the 

array of NO2-associated health effects. 

To the extent it is determined 

appropriate to emphasize the possibility 

that NO2-induced airway 

responsiveness in asthmatics could 

occur following exposures below 100 

ppb and/or the clinical significance of 

such increase in airway responsiveness, 

the Administrator notes that the 

evidence would support setting the 

standard level below 100 ppb. The 

Administrator also notes that a standard 

level below 100 ppb would be 

consistent with placing greater 

emphasis on the mixed results reported 

in the epidemiologic study by Delfino et 

al. (2002). Specifically, she notes that a 

standard level of 80 ppb would be 

expected to limit area-wide NO2

concentrations to approximately 50 ppb 

(80 is 65% higher than 50) and that a 

standard level of 80 ppb would be 

expected to provide protection against 

exposure concentrations below those 

that have been reported to increase 

airway responsiveness in asthmatics. 

For the reasons stated above, the 

Administrator proposes to set the level 

of a new 1-hour standard between 80 

ppb and 100 ppb. In so doing, the 

Administrator proposes to place 

emphasis on reported findings from 

both epidemiologic studies and from 

controlled human exposure studies. In 

order to protect against NO2-associated 

emergency department visits and 

hospital admissions reported in 

multiple key U.S. epidemiologic 

studies, and against reported NO2- 

induced increases in airway 

responsiveness, the Administrator 

proposes to set the standard level no 

higher than 100 ppb. In addition, in 

light of the fact that the Administrator 

is considering, and soliciting comment 


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on, the appropriate weight to place on 

the potential risk of NO2-associated 

effects in locations with relatively low 

area-wide NO2 concentrations and on 

the significance of potential NO2- 

induced increases in airway 

responsiveness in some asthmatics 

following exposures to concentrations 

below 100 ppb, the Administrator is 

proposing to set a standard level within 

a range that includes 100 ppb but is no 

lower than 80 ppb. 

The Administrator solicits comment 

on the appropriateness of this proposed 

range of standard levels as well as on 

the approach she has used to identify 

the range. Specifically, the 

Administrator solicits comment on the 

following: 

• The weight she has placed on the 

epidemiologic evidence, the controlled 

human exposure evidence, the 

exposure/risk information, and the 

uncertainties associated with each of 

these. 

• Her use of available information on 

the NO2 concentration gradient around 

roadways (i.e., that concentrations near 

roadways can be 30 to 100% higher than 

concentrations in the same area but not 

near the road) to inform an appropriate 

range of standard levels. 

• The most appropriate part of the 

proposed range in which to set the 

standard level given the available 

scientific evidence, exposure/risk 

information, NO2 air quality 

information, and the uncertainties 

associated with each. 

With regard to the proposed range of 

standard levels, the Administrator notes 

that the proposed range is consistent 

with the recommendation by CASAC to 

set a standard level no higher than 100 

ppb. However, much of the evidence 

and exposure/risk information that 

informed CASAC’s advice was based on 

NO2 concentrations measured at areawide monitors in the current monitoring 

network. CASAC did not explicitly 

address whether or how the standard 

level should differ if it reflects the 

maximum allowable NO2 concentration 

in a location (including near major 

roads) rather than the maximum 

allowable area-wide concentration. 

The Administrator also solicits 

comment on setting a standard level 

above 100 ppb and up to 150 ppb. In so 

doing, the Administrator recognizes that 

there are uncertainties with the 

scientific evidence, such as that 

associated with the magnitude and 

clinical significance of the NO2-induced 

increase in airway responsiveness in 

asthmatics and with attributing effects 

reported in epidemiologic studies 

specifically to NO2 given the presence of 

co-occurring pollutants. The 

Administrator invites comment on the 

extent to which it is appropriate to 

emphasize these uncertainties in 

considering the standard level and on 

whether it would be appropriate to set 

a standard level as high as 150 ppb. 

The Administrator notes that, in order 

to consider the potential implications of 

a standard level as high as 150 ppb, it 

is important to put such a standard in 

the context of potential ambient 

concentrations. A standard level of 150 

ppb under the proposed approach could 

be associated with 1-hour area-wide 

NO2 concentrations of approximately 90 

ppb (150 is approximately 65% higher 

than 90), and potentially with 

concentrations ranging from 75 to 115 

ppb (150 is approximately 100% higher 

than 75 and 30% higher than 115) 

depending on location. 

The Administrator notes that a 

standard level as high as 150 ppb would 

place more emphasis on uncertainties 

associated with the scientific evidence. 

Specifically, a standard level of 150 ppb 

would emphasize the uncertainty 

associated with the magnitude and the 

clinical significance of the NO2-induced 

increase in airway responsiveness in 

asthmatics and would be based on an 

assumption that NO2-associated health 

effects reported in epidemiologic 

studies are due in large part to exposure 

to co-occurring pollutants, rather than 

exposure to NO2. As noted above, the 

Administrator seeks comment on the 

extent to which it would be appropriate 

to emphasize these uncertainties in 

considering the standard level and the 

extent to which the scientific evidence 

would support levels up to 150 ppb. 

In addition, the Administrator notes 

that a standard level lower than 80 ppb 

could be appropriate to the extent that 

near-road concentrations are determined 

to be closer to 30% higher than areawide concentrations or to the extent that 

additional emphasis is placed on the 

possibility that exposure to NO2

concentrations below 100 ppb could 

increase airway responsiveness in some 

asthmatics. Accordingly, the 

Administrator also solicits comment on 

standard levels as low as 65 ppb (30% 

higher than an area-wide concentration 

of 50 ppb). 

f. Alternative Approach to Setting the 

1-Hour Standard Level 

As discussed above, the 

Administrator is proposing a standard 

level reflecting the maximum allowable 

NO2 concentration anywhere in an area. 

However, for the reasons discussed 

below, EPA also solicits comment on an 

alternative approach to setting a 1-hour 

NO2 standard. Under this alternative 

approach, the standard level would 

reflect the maximum allowable NO2

concentration measured at an area-wide 

monitoring site. Such a site would not 

be located in close proximity to major 

roads and, for a given area, would not 

be the location of the maximum NO2

concentration anywhere in that area. In 

conjunction with soliciting comment on 

this alternative approach, EPA solicits 

comment on setting the level of such a 

standard within the range of 50 to 75 

ppb. In addition, as with the proposed 

standard, EPA solicits comment on NO2

as the indicator, a 1-hour (daily 

maximum) averaging time, and the 3- 

year average of the 99th percentile (or 

4th highest) or 98th percentile (or the 

7th or 8th highest) as the form. 

With regard to the range of levels from 

50 to 75 ppb, which would reflect 

maximum allowable area-wide NO2

concentrations under this approach, the 

Administrator notes the following. First, 

a standard level within in this range 

would be expected to maintain areawide NO2 concentrations below peak 

1-hour area-wide concentrations 

measured in locations where key U.S. 

epidemiologic studies have reported 

associations with respiratory-related 

emergency department visits and 

hospital admissions. Second, she notes 

that standard levels from the lower end 

of this range would be expected to limit 

roadway-associated exposures to NO2

concentrations that have been reported 

in controlled human exposure studies to 

increase airway responsiveness in 

asthmatics. A standard level of 50 ppb 

under this approach could limit nearroad concentrations to between 65 and 

100 ppb, given that near-road NO2

concentrations can range from 30% to 

100% higher than area-wide 

concentrations. Assuming the mid-point 

of the range of gradients (i.e., that nearroad concentrations are 65% higher 

than area-wide concentrations), a 

standard level of 50 ppb under this 

approach could limit near-road 

concentrations to approximately 80 ppb 

and a standard level of 60 ppb could 

limit near-road concentrations to 

approximately 100 ppb. Third, to the 

extent that relatively more emphasis is 

placed on the uncertainties regarding 

the magnitude and clinical significance 

of the NO2-induced increase in airway 

responsiveness, the Administrator notes 

that a standard level from the upper end 

of the range could be determined to be 

appropriate. Finally, this approach 

would provide more confidence than 

the proposed approach regarding the 

degree to which a specific standard 

level would limit area-wide NO2

concentrations but less confidence 

regarding the degree to which a specific 


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standard level would limit the peak NO2

concentrations likely to occur near 

major roadways. 

The Administrator recognizes that her 

proposed approach results from a 

comprehensive evaluation of alternative 

approaches to determining the level of 

the NO2 primary NAAQS, but that these 

approaches have not previously been 

presented to CASAC, or other 

stakeholders, for their evaluation and 

public discussion. More specifically, the 

Administrator notes that much of the 

information included in the policy 

assessment chapter of the REA, which 

formed the foundation for CASAC’s 

recommendations regarding standard 

level, was based on evaluation of data 

drawn from the current area wideoriented monitoring network. Further, 

the Administrator notes that CASAC did 

not explicitly discuss in their 

recommendations whether and how the 

standard level should differ if that level 

reflects the maximum allowable NO2

concentration anywhere in an area 

rather than the maximum allowable NO2

concentration measured at an area-wide 

monitoring site. Given this, the 

Administrator recognizes the possibility 

that comments received on this 

proposal, particularly those received 

from CASAC, could provide important 

new information for consideration. 

g. Level of the Annual Standard 

With regard to the annual standard, 

the Administrator notes that the ISA 

concluded that the scientific evidence is 

suggestive but not sufficient to infer a 

causal relationship between long-term 

NO2 exposure and respiratory 

morbidity. While some studies have 

reported associations between long-term 

NO2 exposure and respiratory endpoints 

such as decrements in lung function 

growth (Gauderman et al., 2004; RojasMartinez et al., 2007a and b; Oftedal et 

al., 2008), the ISA notes that the high 

correlation among traffic-related 

pollutants makes it difficult to 

accurately estimate independent effects 

in these long-term studies. CASAC 

recommended retaining an annual 

standard in order to provide protection 

against potential health effects 

associated with long-term exposures. 

They based this recommendation on 

‘‘the limited evidence related to 

potential long-term effects of NO2

exposure and the lack of strong 

evidence of no effect’’ (Samet, 2008b). 

With regard to the level of an annual 

standard, CASAC recommended 

retaining the current level as the 

evidence considered did not provide a 

basis for either increasing or decreasing 

it. Given these considerations, and 

recognizing that a new 1-hour standard 

level as proposed would also provide 

some degree of protection from longterm exposures, the Administrator 

proposes to take a cautious approach 

and retain the current annual standard. 

The Administrator solicits comment on 

this approach. 

G. Summary of Proposed Decisions on 

the Primary Standard 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the ISA and 

REA as well as the advice and 

recommendations of CASAC, the 

Administrator proposes that the current 

annual standard is not requisite to 

protect public health with an adequate 

margin of safety. The Administrator 

proposes to establish a new short-term 

standard that will afford increased 

protection for asthmatics and other atrisk populations against an array of 

adverse respiratory health effects related 

to short-term NO2 exposure. These 

effects include increased asthma 

symptoms, worsened control of asthma, 

an increase in respiratory illnesses and 

symptoms, and related serious 

indicators of respiratory morbidity 

including emergency department visits 

and hospital admissions for respiratory 

causes. 

Specifically, the Administrator 

proposes to set a new short-term 

primary NO2 standard, with a 1-hour 

(daily maximum) averaging time, a form 

defined as the 3-year average of the 99th 

percentile or the 4th highest daily 

maximum concentration. The level for 

the new standard is proposed to be 

within the range of 80 to 100 ppb, 

reflecting maximum allowable 

concentrations anywhere in an area. In 

conjunction with this proposed 

standard, the Administrator also solicits 

comment on levels as low as 65 ppb and 

as high as 150 ppb, and on alternative 

forms including the 3-year average of 

the 98th percentile or the 7th or 8th 

highest daily maximum concentration. 

In addition, the Administrator also 

solicits comment on an alternative 

approach to setting a new 1-hour 

standard. Under this alternative, the 

NO2 NAAQS would reflect the 

maximum allowable area-wide NO2

concentration, which would be 

measured away from major roads. With 

regard to this approach, the 

Administrator solicits comment on a 

level within the range from 50 to 75 ppb 

and on the same alternative forms as 

noted above. 

In addition to setting a new 1-hour 

standard, the Administrator proposes to 

retain the current annual standard. The 

current annual standard together with a 

new 1-hour standard would provide 

protection against health effects 

potentially associated with long-term 

exposures to NO2. The Administrator 

solicits comment on this approach. 

III. Proposed Amendments to Ambient 

Monitoring and Reporting 

Requirements 

The EPA is proposing changes to the 

ambient air monitoring, reporting, and 

network design requirements for the 

NO2 NAAQS. This section discusses the 

changes we are proposing which are 

intended to support the proposed 1- 

hour NAAQS and proposed retention of 

the current annual NAAQS in Section II. 

Ambient NO2 monitoring data are used 

to determine whether an area is in 

violation of the NO2 NAAQS. Ambient 

NO2 monitoring data are collected by 

state, local, and Tribal monitoring 

agencies (‘‘monitoring agencies’’) in 

accordance with the monitoring 

requirements contained in 40 CFR parts 

50, 53, and 58. 

A. Monitoring Methods 

To be used in a determination of 

compliance with the NO2 NAAQS, NO2

data must be collected using a Federal 

Reference Method (FRM) or a Federal 

Equivalent Method (FEM) analyzer. The 

current monitoring method in use by 

most State and local monitoring 

agencies is the gas-phase 

chemiluminescence FRM (40 CFR Part 

50, Appendix F), which was 

implemented into the NO2 monitoring 

network in the early 1980s. The current 

list of all approved FRMs and FEMs 

capable of providing ambient NO2 data 

for use in attainment designations may 

be found on the EPA Web site (http:// 

www.epa.gov/ttn/amtic/files/ambient/ 

criteria/reference-equivalent-methodslist.pdf). It must be noted, however, that 

due to the proposal of a new 1-hour 

NAAQS, wet chemical based FEMs 

would not be appropriate for use in 

determining compliance of the proposed 

1-hour NAAQS, since such methods are 

incapable of providing hourly averaged 

data. Therefore, we propose that any 

NO2 FRM or FEM used for making 

primary NAAQS decisions must be 

capable of providing hourly averaged 

concentration data. We propose to only 

allow FRM or FEMs capable of 

providing hourly averaged 

concentration data to be used to 

produce data for comparison to the 

NAAQS, and solicit comment on this 

proposed requirement. 

The sum of nitric oxide (NO) and NO2

is commonly called NOX. Nitrogen 

oxides, technically the total reactive 

nitrogen oxide family, known as NOY, is 

defined as the sum of NO, NO2, and the 

higher nitrogen oxides collectively 


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termed NOZ. Important components of 

ambient NOZ include nitrous acid 

(HNO2), nitric acid (HNO3), and the 

peroxyacetyl nitrates (PANs). However, 

NO2 is the indicator for the nitrogen 

oxides NAAQS. In the ambient 

monitoring network, very nearly all 

measurements of NO2 are collected by 

the chemiluminescence FRM. However, 

this technique directly measures only 

NO by the principle of gas-phase 

chemiluminescence induced by the 

reaction of NO with O3 at low pressure. 

NO2 concentrations are determined 

indirectly by the analyzer in two steps: 

(1) By first measuring the ambient NO 

concentration, and (2) determining total 

NOX, including NO2, by measuring a 

second NO concentration after reducing 

the NO2 in the sample air stream to NO 

(most often through the use of a 

molybdenum oxide (MoOX) substrate 

heated to between 300 °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 analyzers are subject to varying 

interferences produced by the presence 

in the air sample of the NOZ species 

listed above and others occurring in 

trace amounts in ambient air. This 

interference is often termed a ‘‘positive 

artifact’’ in the reported NO2

concentration since the presence of NOZ

results in an over-estimate in the 

reported measurement of the actual 

ambient NO2 concentration. This 

interference by NOZ compounds has 

long been known and evaluated 

(Fehsenfeld et al., 1987; Nunnermacker 

et al., 1998; Parrish and Fehsenfeld, 

2000; McClenny et al., 2002; U.S. 

Environmental Protection Agency, 1993, 

2006a). The sensitivity of the 

chemiluminescence FRM to potential 

interference by individual NOZ

compounds is variable and depends in 

part on characteristics of individual 

monitors, such as the design of the 

instrument inlet, the temperature and 

composition of the reducing substrate, 

and the interactions of atmospheric 

species with the reducing substrate. 

Furthermore, the concentrations of NOZ

compounds in ambient air are variable 

with time and distance from the sources 

of NO and NO2, chiefly the point source 

and both on-road and non-road mobile 

source combustion of fossil fuels. Nearer 

to these sources, the potential 

interference is lower than it is farther 

away because more of the measured 

nitrogen oxides are present as the 

emitted NO and quickly formed NO2, 

rather than NOZ. This is because 

oxidation to the NOZ compounds from 

NO and NO2 requires time and the 

presence of other atmospheric 

compounds like the hydroxyl radical. 

Overall, as noted in the ISA, it 

appears that interference by NOZ on 

chemiluminescence FRMs is not more 

than 10 percent of the reported NO2

concentration during most or all of the 

day during winter (cold temperatures), 

but larger interference ranging up to 70 

percent can be found during summer 

(warm temperatures) in the afternoon at 

sites away and downwind from strong 

emission sources. In general, the NOZ

interference in the reported NO2

concentrations collected downwind of 

source areas and NO2 concentrations 

collected in relatively remote areas 

away from concentrated point, area, or 

mobile sources is larger than the NOZ

interference in NO2 measurements taken 

in urban cores or other areas with fresh 

NOX emissions. 

The chemiluminescence FRM is well 

established, comprising a large majority 

of the current operating network, and 

has served as the principal monitoring 

method in the NO2 network for more 

than thirty years. Many of the 

epidemiologic studies referenced in the 

REA as the health basis for the proposed 

primary NO2 NAAQS utilized ambient 

NO2 data obtained from 

chemiluminescence FRMs, and 

subsequently, the uncertainties that may 

occur from the potential positive 

influence of NOZ species on NO2 values 

provided by the ambient FRM 

monitoring network are already 

reflected in those studies. Therefore, for 

purposes of comparing NO2 monitoring 

data to the NO2 NAAQS, the EPA 

believes that the chemiluminescence 

FRMs are appropriate for continued use 

under the current standard and under 

any of the options being considered for 

a new 1-hour averaged primary NO2

NAAQS. 

EPA is aware of the more recent 

development of an alternative method 

in determining NO2 concentrations by 

chemiluminescence, specifically 

through the use of a photolytic 

converter, which uses specific 

wavelengths of ultraviolet light to 

reduce NO2 to NO in lieu of the FRM’s 

MoOX substrate converter. The 

advantage of the photolyticchemiluminescence 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 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 openpath and remote sensing methods, 

which use spectrometers to detect 

pollutant concentrations by light 

absorption over an optical path length, 

are suitable for continued use in the 

ambient monitoring network as they can 

provide NO2 measurements with 

reduced influences of NOZ species on 

the reported NO2 concentrations, 

relative to the chemiluminescence FRM. 

However, these methods do not provide 

point specific concentrations like those 

provided by chemiluminescence FRMs 

that are typically expected and seen in 

the monitoring network, and may be one 

of the reasons these methods are not 

more widely used. 

In recognition of the existence of 

alternative methods that may be useful 

in the measurement of NO2 for NAAQS 

compliance purposes, as well as other 

objectives, EPA solicits comment on the 

advantages and disadvantages of 

advancing technology, such as the 

photolytic-chemiluminescence method, 

or the use of existing open-path or 

remote sensing FRM and FEM 

technology, as alternative methods to 

supplement the approved 

chemiluminescence FRMs already 

deployed across the U.S. at NO2

monitoring sites. 

B. Network Design 

1. Background 

The basic objectives of an ambient 

monitoring network, as noted in 40 CFR 

Part 58 Appendix D, include (1) 

providing air pollution data to the 

general public in a timely manner, (2) 

supporting compliance with ambient air 

quality standards and emissions strategy 

development, and (3) providing support 

for air pollution research. Section II.A.1 

notes that there are currently no 

minimum monitoring requirements for 

NO2 in 40 CFR part 58 Appendix D, 


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19 It should be noted that the ISA Section 2.4.1 

references a different number of active monitors in 

the NO2 network. The difference stems from how 

‘currently operating monitors’ were defined when 

extracting data from AQS. The ISA only references 

SLAMS, NAMS, and PAMS sites with defined 

montoring objectives, while the Watkins and 

Thompson, 2008 value represents all NO2 sites 

reporting data at any point during the year. 

20The ISA references studies of heavy-duty diesel 

vehicles retrofitted with a CDPF in describing the 

range of NO2 to NOX ratios from diesel vehicles. 

These studies are based on vehicles equipped with 

CDPFs prior to 2009. However, as of January 1, 

2009, EPA’s National Clean Diesel Campaign 

requires that emission control devices included on 

its Verified Technologies List raise the fraction of 

NO2 in exhaust NOX from an engine no more than 

20% above the baseline engine NO2 to NOX ratio. 

Retrofit technologies sold after January 1, 2009 that 

do not meet the NO2 emission limit may not be 

installed or sold as EPA verified technologies. 

21For purposes of the discussion, near-road NO2

monitors are defined to be no greater than 50 meters 

from the nearest traffic lane of target road segments. 

The details of appropriately placing NO2 monitors 

near roads are explained in Section III.2.a of this 

document. 

other than the requirement for EPA 

Regional Administrator approval before 

removing any existing monitors, and 

that any ongoing NO2 monitoring must 

have at least one monitor sited to 

measure the maximum concentration of 

NO2 in that area. As discussed in 

Section II.A.2, an analysis of the 

approximately 400 19 monitors 

comprising the current NO2 monitoring 

network (Watkins and Thompson, 2008) 

indicates that the most frequently stated 

monitor objectives for sites in the 

current NO2 network are for the 

assessment of concentrations for general 

population exposure and maximum 

(highest) concentrations typically at the 

neighborhood and urban scales. Spatial 

scales are defined in 40 CFR Part 58 

Appendix D, Section 1.2, where the 

scales of representativeness of most 

interest for the monitoring site types 

include: 

1. Microscale—Defines the 

concentration in air volumes associated 

with area dimensions ranging from 

several meters up to about 100 meters. 

2. Middle scale—Defines the 

concentration typical of areas up to 

several city blocks in size, with 

dimensions ranging from about 100 

meters to 0.5 kilometers. 

3. Neighborhood scale—Defines 

concentrations within some extended 

area of the city that has relatively 

uniform land use with dimensions in 

the 0.5 to 4.0 kilometers range. 

4. Urban scale—Defines 

concentrations within an area of citylike dimensions, on the order of 4 to 50 

kilometers. Within a city, the geographic 

placement of sources may result in there 

being no single site that can be said to 

represent air quality on an urban scale. 

The neighborhood and urban scales 

have the potential to overlap in 

applications that concern secondarily 

formed or homogeneously distributed 

air pollutants. 

5. Regional scale—Defines usually a 

rural area of reasonably homogeneous 

geography without large sources, and 

extends from tens to hundreds of 

kilometers. 

The ISA and REA indicate that one of 

the largest factors affecting ambient 

exposures to NO2 above health 

benchmark concentrations are mobile 

source emissions, particularly at 

locations near major roads. Information 

in the ISA and the REA shows that 

concentrations of mobile source 

pollutants, including NO2, typically 

display peak concentrations on or 

immediately adjacent to roads, 

producing a gradient in pollutant 

concentrations where concentrations 

decrease with increasing distance from 

roads (Section II.A.2 above, ISA sections 

2.5.4 and 4.3.6 and Table 2.2–1; REA 

section 7.3.2 and Figures 8–17 and 8– 

18). In the ambient environment, NO2 is 

largely a secondary pollutant resulting 

from the reaction of NO with available 

ozone (O3), the concentrations of which 

depend on photochemical reactions of 

ambient hydrocarbons and prior (precursory) NOX emissions. The ISA notes 

that the direct emission of NO2 from 

mobile sources is estimated to be only 

a few percent of the total NOX emissions 

for light-duty gasoline vehicles, and 

anywhere from less than 10 percent up 

to 70 percent of the total NOX emission 

from heavy-duty diesel vehicles, 

depending on the engine, the use of 

emission control technologies such as 

catalyzed diesel particulate filters 

(CDPFs), and mode of vehicle 

operation.20 However, since the rate of 

conversion of mobile source NO to NO2

as described above is a generally rapid 

process, (i.e., on the order of a minute 

(ISA Section 2.2.2)), NO2 behaves like a 

primary pollutant in the near-road 

environment, exhibiting peak 

concentrations on or closely adjacent to 

roads. However, due to the secondary 

formation characteristic of NO2, its rate 

of decay with increasing distance from 

a road can be slower than that of the 

other pollutants directly emitted from 

mobile sources including carbon 

monoxide (CO), ultrafine particulates, 

air toxics, and black carbon. Literature 

values indicate that the distance 

required for NO2 concentrations to 

return to near area-wide or background 

concentrations away from major 

roadways can range up to 500 meters. 

The actual distance is variable, and 

highly dependent on topography, 

roadside features, meteorology, and the 

related photochemical reactivity 

conditions (Baldauf et al., 2008; 

Beckerman et al., 2007; Clements et al., 

2008; Gilbert et al. 2003; Hagler et al., 

2009; Rodes and Holland, 1980; Singer 

et al., 2003; Zhou and Levy, 2007). 

Nonetheless, any efforts to measure 

peak ambient NO2 concentrations from 

on-road mobile sources, or other mobile 

source pollutant of interest noted above, 

would be best served by monitoring as 

near as practicable to roadways of 

interest. 

2. Proposed Changes 

In conjunction with the proposed 1- 

hour NAAQS and the proposed 

retention of the current annual NAAQS, 

we propose a number of changes to the 

NO2 monitoring network. As described 

above in Section II.F.4, we are 

proposing a 1-hour NO2 NAAQS that 

reflects the maximum allowable NO2

concentration in an area. However, the 

current network is not oriented to 

address peak concentrations, such as the 

on-road and near-road environment, but 

many sites may be situated to assess 

high concentrations at the neighborhood 

or larger spatial scales. The EPA is 

proposing a two-tier network design to 

monitor ambient concentrations of NO2

and assess compliance with the NO2

NAAQS. The two tiers would provide 

data for comparison with both the 1- 

hour and annual standards, and would 

be comprised of (1) monitoring in areas 

of expected maximum 1-hour 

concentrations and (2) monitoring to 

characterize areas with the highest 

expected NO2 concentrations at the 

neighborhood and larger spatial scales, 

or ‘‘area-wide’’ scales. Because the 

maximum hourly NO2 concentrations in 

many areas are expected to be due to onroad mobile emissions, the EPA believes 

that the first tier of the monitoring 

network should include a component 

requiring monitoring near major roads, 

where higher NO2 concentrations have 

been identified and there are no 

significant monitoring efforts to address 

roadway exposures. The EPA recognizes 

that requiring a component of the 

ambient NO2 monitoring network to 

characterize the peak NO2

concentrations derived from on-road 

mobile sources, using monitors placed 

near major roadways (‘‘near-road 

monitors’’), will introduce new 

requirements for monitoring sites that, 

for a majority of the state and local 

monitoring networks, currently do not 

exist.21 However, the monitoring of 

maximum hourly concentrations of 

NO2, particularly in the near-road 

environment, is an essential component 


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22We 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. 

of an ambient monitoring network 

designed to determine compliance with 

the proposed 1-hour NAAQS. In 

addition, the EPA recognizes that the 

establishment of near-road monitoring 

sites will produce certain other 

advantages, by providing a new data 

source for public health studies that will 

support future NAAQS reviews, 

allowing for the tracking of mobile 

source emission reductions progress, 

providing monitoring infrastructure that 

may be of use for mixtures of pollutants 

in a multi-pollutant paradigm, and 

supporting scientific studies of other 

mobile source pollutants like CO, 

ultrafine particulate matter, black 

carbon, and air toxics. 

The second tier of the proposed 

network design, the area-wide 

monitoring component, is intended to 

characterize the highest concentrations 

of NO2 typical or representative of 

neighborhood and larger spatial scales, 

to address the wider area impact of NO2

sources on urban populations. Further, 

a requirement for the continuation of 

area-wide monitoring of NO2 serves to 

maintain continuity in collecting areawide data that have served to inform 

long-term pollutant concentration 

trends analysis and health and scientific 

research for more than thirty years. 

We propose that state and, when 

appropriate, local air monitoring 

agencies provide a plan for deploying 

monitors in accordance with the 

following proposed network design by 

July 1, 2011. We also propose that the 

NO2 network being proposed be 

physically established no later than 

January 1, 2013. Considering the 

proposed timeline and criteria 

presented in the network design, we 

solicit comment on whether state and 

local monitoring agencies should be 

required to deploy monitors sooner than 

January 1, 2013. 

a. Monitoring in Areas of Expected 

Maximum Concentrations Near Major 

Roads 

We are proposing to require 

monitoring in locations of expected 

maximum concentrations near major 

roads in larger urban areas, with 

minimum monitoring requirements 

triggered for metropolitan areas based 

on Core Based Statistical Area (CBSA) 

population thresholds and the traffic 

related metric annual average daily 

traffic (AADT). The U.S. Department of 

Transportation (U.S. DOT) Federal 

Highway Administration’s Status of the 

Nation’s Highways, Bridges, and 

Transit: 2006 Conditions and 

Performance document (http:// 

www.fhwa.dot.gov/policy/2006cpr/ 

es02h.htm) states that ‘‘while urban 

mileage constitutes only 24.9 percent of 

total (US) mileage, these roads carried 

64.1 percent of the 3 trillion vehicles 

miles (VMT) travelled in the United 

States in 2004.’’ The document also 

states that ‘‘urban interstate highways 

made up only 0.4 percent of total (US) 

mileage but carried 15.5 percent of total 

VMT.’’ These statements indicate how 

much more traffic volume exists on 

roads in urban areas versus the more 

rural areas that have significant amounts 

mileage of the total public road 

inventory. Because the combination of 

increased mobile source emissions and 

increased urban population densities 

can lead to increased exposures and 

associated risks, urban areas are the 

appropriate areas to concentrate 

required near-road monitoring efforts. 

Therefore, we propose that one nearroad NO2 monitor be required in CBSAs 

with a population greater than or equal 

to 350,000 persons. This population 

threshold is proposed to provide the 

near-road monitoring component of the 

network an appropriate spatial extent 

across the country, given the limited 

availability of routine measurements in 

these environments. Based on 2007 

Census Bureau statistics, this will result 

in approximately 142 sites in as many 

CBSAs.22

We also propose that a second nearroad monitor be required in CBSAs with 

a population greater than or equal to 

2,500,000 persons, or in any CBSAs 

with one or more road segments with an 

AADT count greater than or equal to 

250,000. Based on 2007 Census Bureau 

statistics and data from the 2007 

Highway Performance Monitoring 

System (HPMS) maintained by the U.S. 

DOT Federal Highway Administration 

(FHWA), this particular element of the 

minimum monitoring requirements will 

add approximately 23 sites to the 

approximate 142 near-road sites in 

CBSAs that already will have one nearroad monitor required due to the 

350,000 population threshold. Of the 23 

additional sites, two sites are due to the 

250,000 AADT threshold and are 

attributed to the Las Vegas, Nevada and 

Sacramento, California CBSAs. The 

2,500,000 population threshold is 

proposed as a second threshold to allow 

for further characterization of larger 

urban areas that are more likely to have 

a greater number of major roads across 

a potentially larger geographic area, and 

a corresponding increase in potential for 

exposure. Of the approximate 1.66 

million public road segments tracked in 

the HPMS, road segments of 250,000 

AADT or greater make up the top 0.03 

percent of the most traveled public road 

segments. The FHWA has also used this 

threshold on its Web site to give an 

indication of the most travelled urban 

highways in the country (http:// 

www.fhwa.dot.gov/policyinformation/ 

tables/02.cfm). We proposed to use 

HPMS-reported AADT as the traffic 

volume metric because AADT appears 

to be the most widely used traffic 

volume metric in the scientific 

literature, is widely available, and offers 

the most objective and consistent metric 

available to indicate traffic volumes 

across the country. These AADT data 

are typically available from local 

Metropolitan Planning Organizations 

(MPOs), state departments of 

transportation, and from the FHWA’s 

HPMS. The FHWA also provides 

national guidance on the appropriate 

measurement and estimation of AADT 

for different road types in their HPMS 

Field Manual (http://www.fhwa.dot.gov/ 

ohim/hpmsmanl/hpms.cfm). We are 

therefore proposing the 250,000 AADT 

threshold for requiring a near-road 

monitor because that threshold 

represents the highest traffic volume 

road segments in the country, which 

may correspond to the greatest potential 

for high exposures directly connected to 

motor vehicle emissions. 

In summary, the combination of the 

above proposed minimum monitoring 

requirement thresholds for the near-road 

monitors as part of the ambient NO2

monitoring network are anticipated to 

require approximately 165 near-road 

sites in 142 CBSAs. We solicit comment 

on the proposed CBSA population 

threshold values (i.e., 350,000 and 

2,500,000) and on the use of population 

thresholds both lower and higher than 

those proposed, the use of the traffic 

volume metric AADT, and the 250,000 

AADT threshold in establishing the 

minimum number of required near-road 

sites for urban areas. 

In choosing these population and 

traffic related thresholds for the 

minimum monitoring requirements, it 

should be noted that, based on 2007 

Census Bureau statistics, the U.S. Virgin 

Islands and seven states (Delaware, 

Montana, North Dakota, South Dakota, 

Vermont, West Virginia, and Wyoming) 

currently would not have required nearroad monitoring sites under this current 

proposal. Considering the relative lack 

of near-road monitoring data 

nationwide, the new level and averaging 

time of the NAAQS being proposed, and 

the desire to establish a spatially 

representative and protective network, 

we solicit comment on the inclusion or 


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exclusion of an additional or alternative 

monitoring requirement such that each 

state and territory would have at least 

one near-road monitoring site. 

The EPA recognizes that in certain 

cases, there can be an area or areas of 

expected maximum hourly 

concentration in a CBSA due to a major 

stationary source or to the combination 

of multiple sources that could include 

point, area, and non-road source 

emissions in addition to on-road mobile 

source emissions. Such locations might 

be identified through data analysis, such 

as the evaluation of existing ambient 

data and/or emissions data, or through 

air quality modeling. An example of 

such a location might be away from 

roads and downwind of a stationary 

source or sources in situations where 

the required near-road monitors do not 

represent a location or locations of 

expected maximum hourly NO2

concentrations in a CBSA. In these 

situations, where such locations are 

known, we propose that the Regional 

Administrator will have discretion to 

require monitoring above the minimum 

requirements as necessary to address 

situations where the required near-road 

monitors do not represent a location or 

locations where the expected maximum 

hourly NO2 concentrations exist in a 

CBSA. The EPA also proposes to allow 

Regional Administrators the ability to 

require additional near-road monitoring 

sites to address situations where 

minimum monitoring requirements are 

not sufficient to meet monitoring 

objectives, such as a situation where 

there is a variety of exposure potential 

in an area due to variety in the amount 

or types of fleet mix, congestion 

patterns, terrain, or geographic areas 

within a CBSA. An example of requiring 

an additional near-road monitor might 

be a case where a particular community 

or neighborhood is significantly or 

uniquely affected by road emissions, but 

the site or area is not monitored even 

though the responsible State or local 

monitoring agency is fulfilling the 

minimum monitoring requirements. 

In all cases, the Regional 

Administrator and the responsible State 

or local air monitoring agency should 

work together to design and/or maintain 

the most appropriate NO2 network to 

service the variety of data needs for an 

area. We solicit comment on the 

proposal to allow Regional 

Administrators the discretion to require 

monitoring above the minimum 

requirements for any CBSA where 

required near-road monitors do not 

represent a location or locations where 

the expected maximum hourly NO2

concentrations exist in a CBSA. We also 

solicit comment on the proposal to 

allow Regional Administrators to 

require additional near-road NO2

monitoring stations above the minimum 

required in situations where the 

minimum monitoring requirements are 

not sufficient to meet monitoring 

objectives as noted above. 

The new near-road monitoring sites 

that are to be part of the NO2 ambient 

monitoring network will require specific 

site selection criteria to focus 

monitoring efforts on one or a few major 

roads in a given CBSA. The EPA 

anticipates that these near-road 

monitoring sites will likely be best 

characterized as microscale, mobile 

source oriented sites. We propose that 

monitoring agencies be required to 

select their near-road monitoring site 

location(s) to characterize the largest 

traffic volume segment(s) in the CBSA, 

determined by ranking all road 

segments by AADT, and identifying a 

location or locations adjacent to those 

top ranked AADT segments where 

motor vehicle emission-derived NO2

concentrations are expected to be at a 

maximum. Where a state or local air 

monitoring agency identifies multiple 

acceptable candidate sites where 

maximum hourly NO2 concentrations 

are expected to occur, the monitoring 

agency should consider taking into 

account the potential for population 

exposure in the criteria utilized to select 

the final site location. 

We propose that near-road NO2

monitoring stations must be sited so that 

the NO2 monitor probe is no greater 

than 50 meters away, horizontally, from 

the outside nearest edge of the traffic 

lanes of the target road segment, and 

shall have no obstructions in the fetch 

between the monitor probe and roadway 

traffic such as noise barriers or 

vegetation higher than the monitor 

probe height. Baldauf et al. (2009) 

indicate that the NO2 probe would 

ideally be situated between 10 and 20 

meters from the nearest traffic lane. We 

are not proposing that the near-road 

NO2 monitor be on the predominantly 

downwind side of the target roadway, 

however, we solicit comment on 

whether this requirement is necessary to 

ensure near-road NO2 sites capture 

maximum expected hourly 

concentrations. 

We propose that the monitor probe be 

located within 2 to 7 meters above the 

ground, as is required for microscale 

PM2.5 sites. EPA recognizes that these 

near-road monitoring sites will be 

adjacent to a variety of road types, 

where some target roads will be on an 

even plane with the monitoring station, 

while others may be cut roads, (i.e., 

below the plane of the monitoring 

station), or fill and open elevated roads, 

(i.e., where the road plane is above the 

monitoring station). In any given case, it 

is most appropriate to place the NO2

monitor probe as close to the plane of 

the target road segment as possible, 

while staying between 2 to 7 meters 

above the ground. In addition, we 

propose that monitor probe placement 

on noise barriers or buildings, where the 

inlet probe height is no less than 2 

meters and no more than 7 meters above 

the target road, will be acceptable, so 

long as the inlet probe is at least 1 meter 

vertically or horizontally away (in the 

direction of the target road) from any 

supporting wall or structure, and the 

subsequent residence time of the 

pollutant in the sample line between the 

inlet probe and the analyzer does not 

exceed 20 seconds. Although a wallmounted 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. 

As noted above, we are proposing a 

siting criterion for NO2 monitor probe 

placement to be no greater than 50 

meters away from the outside nearest 

edge of the traffic lanes of the target 

road segment. Based on a review of the 

scientific literature, as discussed in 

Section II.A and the background portion 

of this section, locations on or 

immediately adjacent to roads typically 

exhibit the peak concentrations for 

mobile source pollutants, therefore 

monitor probe placement at increasing 

distances from a road will 

correspondingly decrease the potential 

for sampling maximum concentrations 

of NO2. In addition, monitor probe 

placement within 50 meters of a target 

road allows for increased probability of 

reading elevated concentrations from 

the mobile source emissions even when 

wind conditions cause the near-road 

monitoring site to be upwind of the 

target road. Research literature indicates 

that in certain cases, mobile source 

derived pollutant concentrations, 

including NO2, can be detected upwind 

of roads, above background levels, due 

to a phenomenon called upwind 

meandering. Kalthoff et al. (2007) 

indicates that mobile source derived 

pollutants can meander upwind on the 

order of tens of meters, mainly due to 

vehicle induced turbulence, while 

Beckerman et al. (2008) note that nearroad pollutant concentrations on the 

predominantly upwind side of their 

study sites dropped off to near 

background levels within the first 50 

meters, but were above background in 

this short and variable upwind range, 

which could be due to, at least in part, 


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vehicle induced turbulence. This 

upwind meandering characteristic of 

pollutants in the near-road environment 

provides an additional basis for locating 

near-road sites within 50 meters of 

target road segments because of the 

increased opportunity to monitor 

mobile source derived NO2

concentrations that, although not peak 

concentrations, are still elevated above 

background levels, in meteorological 

conditions where the site is upwind of 

the target road. 

We solicit comment on the proposed 

near-road NO2 monitor siting criteria 

presented here, particularly: (1) The 

requirement for monitoring agencies to 

select near-road NO2 monitor sites by 

ranking all road segments in a given 

CBSA by AADT, (2) selecting a site 

adjacent to a top ranked AADT road 

segment where motor vehicle emissionderived NO2 concentrations are 

expected to be at a maximum, (3) the 

consideration of population exposure as 

a selection criterion in situations where 

a state or local air monitoring agency 

identifies multiple acceptable candidate 

sites where maximum hourly NO2

concentrations are expected to occur, (4) 

the requirement for near-road NO2

monitor probes to be no greater than 50 

meters in the horizontal from the 

outside nearest edge of the traffic lanes 

of the target road segment, and (5) the 

requirement for monitor probes to be 

between 2 to 7 meters above the ground, 

and when located on a wall or 

supporting structure, that the inlet 

probe be at least 1 meter vertically or 

horizontally away from any supporting 

wall or structure. 

We also solicit comment on an 

alternative approach that would allow 

state and local agencies greater 

discretion in selecting monitoring 

locations to fulfill minimum monitoring 

requirements for measurements of 

expected maximum NO2 concentrations 

in each CBSA. In this alternative 

approach, an NO2 monitor would still 

be required in locations of expected 

maximum NO2 concentrations in CBSAs 

with a population greater than or equal 

to 350,000 persons. An additional 

monitor would be required in CBSAs 

with a population greater than or equal 

to 2,500,000, or in any CBSAs with one 

or more road segments with an AADT 

count greater than or equal to 250,000. 

Under this approach, states would not 

be specifically required to place 

monitors near roads, but would have 

flexibility to place monitors at locations 

of expected maximum concentrations. 

However, if a location or locations of 

expected maximum concentration were 

near roads in a CBSA, we would expect 

the NO2 monitor to be placed near those 

roads. Further, we solicit comment on 

alternative ways of considering 

population exposure, in concert with 

the identification of locations of 

maximum expected NO2 concentrations, 

in determining where to place near-road 

NO2 monitors. In suggesting an 

appropriate role for population 

exposure, we invite comment on how 

the suggested role would take into 

account the fact that NAAQS are 

designed to protect all of the public, 

including at-risk or sensitive subpopulations, which can include smaller 

sub-populations that may be exposed to 

higher concentrations. We also invite 

comment on how any suggested role 

would compare with EPA’s historic 

practice of placing monitors at locations 

of maximum concentration at the 

appropriate spatial scale, reflecting 

consideration of the averaging time of 

the NAAQS. 

In situations where open-path 

monitors are used at near-road NO2

sites, we have not identified an 

appropriate path length for this 

microscale monitoring site. For the 

purpose of this proposal, we propose a 

path length range of 50 to 300 meters as 

an appropriate path length range for 

open-path near-road NO2 monitors. The 

high end of this proposed range 

coincides with path lengths identified 

for other pollutants at the micro and 

middle-scales. We solicit comment on 

the appropriate path length for a nearroad NO2 open-path monitor. 

During the near-road monitor site 

selection process, monitoring agencies 

may utilize forms of quantitative 

analysis, such as emissions and/or air 

quality modeling, data analysis, or 

saturation studies, to better evaluate 

which of their top ranked AADT road 

segments may exhibit the potential for 

creating the highest NO2 concentrations 

that might be monitored in the CBSA. 

As an example, such an analysis might 

indicate that of the top ranked AADT 

road segments in a given area, those 

segments that are part of or adjacent to 

interchanges and toll plazas, that have 

higher ratios of heavy duty diesel traffic 

to light duty traffic, have a high fraction 

of rapidly accelerating or grade-climbing 

vehicles, or that are located in or near 

particular terrain or land features, may 

exhibit higher potential maximum NO2

concentrations. In addition, top ranked 

AADT road segment analysis may allow 

the monitoring agencies to select a nearroad monitoring site located in a more 

densely populated area or a location 

representing more vulnerable 

populations from a pool of otherwise 

similarly categorized site candidates. In 

CBSAs required to have two near-road 

monitoring sites, we propose that the 

second site be selected based on AADT 

ranking and expected maximum 

concentration, but differentiated from 

the first site by factors such as: Fleet 

mix, congestion patterns, terrain, or 

geographic area within the CBSA, or at 

minimum, selecting a site along a 

different road with a different route, 

interstate, or freeway designation. This 

differentiation is to avoid having the 

two sites characterize the same traffic 

when there are potentially other road 

segments with different traffic 

characteristics available that meet siting 

criteria for the second near-road 

monitor. We solicit comment on the 

factors and methods to be used to 

differentiate a second required nearroad NO2 monitoring site from the first 

such site in a given CBSA. 

In further support of characterizing 

the peak NO2 concentrations occurring 

in the near-road environment, the EPA 

proposes to require three-dimensional 

anemometry, providing wind vector 

data in the horizontal and vertical 

planes, along with temperature and 

relative humidity measurements, at all 

required near-road monitoring sites. Due 

to the near-road NO2 site being a 

somewhat specialized microscale site, 

we propose that the meteorological 

measurement hardware would be 

required to be situated at the same 

height as the NO2 monitor probe, as 

opposed to a standardized height, to aid 

in characterizing what NO2 analyzers 

are measuring from the target road 

segments. The requirement of threedimensional anemometry is to allow for 

the determination of the standard 

deviation of vertical wind velocities 

(sw). Venkatram et al. (2007) notes that 

sw 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 nearroad site is relatively upwind or 

downwind of the target road, provide a 

method to potentially identify the 

magnitude of vehicle induced 

turbulence, permit calculation of sw in 

the near-road environment to provide a 

better understanding of the mixing of 

mobile source pollutants at the 

monitoring site and how site 

characteristics influence mixing, and, 

with the inclusion of temperature and 

relative humidity, provide basic 

meteorological data. We solicit 

comment on the proposed requirement 

for three-dimensional anemometry, the 

placement of the meteorological 

equipment at the same height of the NO2

monitor probe height, and the 

requirement for meteorological 


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measurements in general at all required 

near-road monitoring sites. 

b. Area-Wide Monitoring at 

Neighborhood and Larger Spatial Scales 

As the second tier of the NO2 ambient 

monitoring network, we are proposing a 

minimum number of monitors to 

characterize that area with highest 

expected NO2 concentrations at the 

neighborhood and larger (area-wide) 

spatial scales. We are proposing to 

require one area-wide monitoring site in 

each CBSA with a population greater 

than or equal to 1,000,000, to be sited 

to represent an area of maximum 

concentration at the neighborhood or 

larger spatial scales. This minimum 

monitoring requirement is expected to 

trigger 52 monitoring sites in as many 

CBSAs. Many of these monitors are 

likely already in place as part of the 

approximately 400 NO2 monitoring sites 

that are currently operating across the 

country. Further, the EPA proposes to 

allow any current photochemical 

assessment monitoring station (PAMS) 

sites that are situated to address the 

highest NO2 concentrations in an urban 

area and sited at neighborhood or urban 

scales to satisfy this proposed area-wide 

monitoring requirement. While in many 

cases it may be found that these areawide monitors may show lower 

concentrations than the maximum 

concentration near-road NO2 monitors, 

data from these larger spatially 

representative sites would provide 

information on area-wide exposures 

from an individual or a group of point, 

area, on-road and/or non-road mobile 

sources. These area-wide monitoring 

data may also, when coupled with the 

near-road monitoring data, assist in the 

determination of spatial variation of 

NO2 concentrations across a given area, 

and assist in providing insight to the 

gradients that exist between local nearroad or stationary source derived 

concentration maxima and the areawide concentration levels. 

The EPA recognizes that the 

minimum number of area-wide 

monitors required in this proposal may 

be less than the total number of NO2

monitoring sites needed to satisfy the 

multiple monitoring objectives that 

neighborhood and larger scale sites can 

serve. These additional monitoring 

objectives include ambient 

photochemical pollutant assessment, 

aiding in ozone forecasting, aiding in 

PM precursor analysis and PM 

forecasting, and characterization of 

point and area sources that may be 

impacting certain communities. We 

propose that EPA Regional 

Administrators have the discretion to 

require additional area-wide NO2

monitoring sites above the minimum 

monitoring requirements where the 

minimum monitoring requirements for 

area-wide monitors are not sufficient to 

meet monitoring objectives. For 

example, the Regional Administrator 

may require additional NO2 monitors in 

certain communities, both inside and 

outside of CBSAs, which are affected by 

an individual or group of sources but 

are not required to have an NO2 monitor 

as part of the minimum monitoring 

requirements. The Regional 

Administrator and the responsible State 

or local air monitoring agency should 

work together to design and/or maintain 

the most appropriate NO2 network to 

service the variety of data needs for an 

area. 

We solicit comment on the proposed 

minimum monitoring requirement of 

approximately 52 monitors to 

characterize areas with highest expected 

NO2 concentrations at the area-wide 

(neighborhood and larger) spatial scales 

in CBSAs with populations of 1,000,000 

or more persons. We also solicit 

comment on the proposal that the 

Regional Administrator can require 

additional monitoring sites on a case-bycase basis, to address situations where 

the minimum monitoring requirements 

for area-wide monitoring sites are not 

sufficient for an area. 

3. Solicitation for Comment on an 

Alternative Network Design 

In conjunction with the solicitation of 

comment on an alternative NAAQS that 

is discussed in Section II.F.4, the 

complementary network design would 

not reflect peak NO2 concentrations 

anywhere in an area. Instead, the 

alternative network design would rely 

on monitors sited at the neighborhood 

and larger spatially representative 

scales, which is identical to the second 

component of the two-tiered network 

design being proposed except for having 

different population thresholds for 

minimum required monitoring. The 

currently operating NO2 network would 

likely satisfy a portion of this alternative 

network design, however the entire 

network would need to be assessed 

before state or local agencies could 

make such determinations. State and 

local agencies would have to determine 

what each currently operating site is 

actually assessing to identify if any 

given site represents the highest 

concentrations for a given CBSA at the 

neighborhood and larger spatial scales. 

We solicit comment on an alternative 

network design where near-road 

monitors are not specifically included 

in the minimum monitoring 

requirements, and only monitors sited at 

the neighborhood and larger spatial 

scales are required. In this alternative 

network design, minimum monitoring 

requirements would apply to CBSAs 

based on population thresholds, where 

one monitor would be required in 

CBSAs with populations of 350,000 or 

more persons and a second monitor 

would be required for CBSAs with 

populations of 1,000,000 or more 

persons. Based on 2007 U.S. Census 

Bureau statistics, we estimate that these 

population thresholds would require 

approximately 194 monitoring sites in 

142 CBSAs. The first monitor required 

in any CBSA would be expected to be 

sited at the neighborhood or larger scale 

to characterize that area with highest 

expected NO2 concentrations. Any 

second monitor required in a CBSA 

would be expected to characterize a 

separate area within the same CBSA, 

also with expected high NO2

concentrations. All such monitor site 

locations are anticipated to be in areas 

of higher population densities of CBSAs 

and in, or adjacent to, urban cores. The 

alternative network design would allow 

the Regional Administrators to use their 

discretion to require monitoring above 

the minimum requirements to address 

community impacts from the variety of 

NO2 emission sources. EPA expects that 

this network design will result in little 

or no progress being made in the 

development of long-term near-road 

monitoring capabilities due to the lack 

of specific network design requirements. 

EPA seeks comment on this alternative 

network design. 

In addition to soliciting comment 

generally on this alternative area-wide 

monitoring approach, the Administrator 

specifically requests comment on the 

appropriate definition of area-wide NO2

concentrations and how best to use data 

representing these concentrations to 

determine compliance with a 1-hour 

standard reflecting the alternative 

approach of selecting a level for 

maximum area-wide concentrations on 

which EPA is soliciting comment. 

Comparing NO2 concentrations 

measured near major roadways to a 

level meant to reflect the maximum 

allowable NO2 concentrations at 

neighborhood and larger spatially 

representative scales would have the 

effect of increasing the stringency of the 

standard beyond that intended. With 

regard to this specific request for 

comment, the Administrator notes that 

the definition of area-wide 

concentrations could include a 

provision requiring that they be 

monitored at a distance greater than or 

equal to some prescribed distance from 

the nearest roadway. The Administrator 

notes that, while it is clear that peak 


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roadway-associated NO2 concentrations 

occur on or very near major roads, the 

point at which these concentrations 

return to area-wide concentrations 

comparable to the area-wide standard is 

less certain and may vary considerable 

by location. As discussed above (section 

II.A.2), the scientific literature suggests 

that concentrations can return to typical 

urban background concentrations 

within distances of up to 500 meters 

from roads, though the actual distance 

will vary with topography, roadside 

features, meteorology, and 

photochemical reactivity conditions. 

The REA notes that studies suggest the 

return to background concentrations can 

occur from within distances of up to 200 

to 500 m from the roads. Therefore, the 

Administrator requests comment on the 

degree to which these distances (up to 

200 m, and up to 500m) serve to further 

define the distance from major roads 

that would represent concentrations 

comparable to the alternative standard. 

Further, since roadways of various sizes 

and traffic volumes can affect nearby 

NO2 concentrations and roadways are 

ubiquitous in urban areas, the 

Administrator notes that defining 

representative area-wide concentrations 

could require more than a uniform 

assumption of a single specific distance 

from a class of roadway. The 

Administrator notes that the approach 

to defining representative area-wide 

distances could include consideration of 

location-specific roadway traffic volume 

and location-specific roadway 

characteristics such as topography, 

presence of sound walls, vehicle mix, 

and traffic patterns, to adequately 

address the variability. Given these 

considerations, the Administrator 

solicits comment on how to define the 

minimum distance to the nearest major 

roadway such that measured 

concentrations at this distance (or 

farther) would represent area-wide NO2

concentrations for comparison to the 

alternative standard. 

C. Data Reporting 

NO2 chemiluminescence FRMs are 

continuous gas analyzers, producing 

updated data values on the order of 

every 20 seconds. Data values are 

typically aggregated into minute 

averages and then compiled into hourly 

averages for reporting purposes. State 

and local monitoring agencies are 

required to report hourly NO, NO2, and 

NOX data to AQS within 90 days of the 

end of each calendar quarter. Some 

agencies also voluntarily report their 

pre-validated data on an hourly basis to 

EPA’s real time AIRNow data system, 

where the data may be used by air 

quality forecasters to assist in ozone 

forecasting. The EPA believes these data 

reporting procedures are appropriate to 

support the current NO2 NAAQS and 

any options being considered for a 

revised primary NO2 NAAQS. 

As a part of the larger data quality 

performance requirements of the 

ambient monitoring program, we are 

proposing to develop data quality 

objectives (DQOs) for the proposed NO2

network. The DQOs are meant to 

identify measurement uncertainty for a 

given pollutant method. We propose a 

goal for acceptable measurement 

uncertainty for NO2 methods to be 

defined for precision as an upper 90 

percent confidence limit for the 

coefficient of variation (CV) of 15 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent. We solicit comment 

on the proposed goals for acceptable 

measurement uncertainty. 

IV. Proposed Appendix S— 

Interpretation of the Primary NAAQS 

for Oxides of Nitrogen and Proposed 

Revisions to the Exceptional Events 

Rule 

The EPA is proposing to add 

Appendix S, Interpretation of the 

Primary National Ambient Air Quality 

Standards for Oxides of Nitrogen, to 40 

CFR part 50 in order to provide data 

handling procedures for the proposed 

NO2 1-hour primary standard and for 

the existing NO2 annual primary 

standard. The proposed Appendix S 

would detail the computations 

necessary for determining when the 

proposed 1-hour and existing annual 

primary NO2 NAAQS are met. The 

proposed Appendix S also would 

address data reporting, data 

completeness considerations, and 

rounding conventions. 

Two versions of the proposed 

Appendix S are printed at the end of 

this notice. The first applies to an 

annual primary standard and a 1-hour 

primary standard based on the annual 

4th high value form, while the second 

applies to an annual primary standard 

and a 1-hour primary standard based on 

the 99th percentile daily value form. 

The discussion here addresses the first 

of these versions, followed by a brief 

description of the differences found in 

the second version. 

Both versions of the proposed 

Appendix S are based on a nearroadway approach to the setting the 

level of the 1-hour standard and to 

siting monitors. As such, these versions 

place no geographical restrictions on 

which monitoring sites’ concentration 

data can and will be compared to the 

standard when making nonattainment 

determinations and other findings 

related to attainment or violation of the 

standard. If the final rule adopts the 

area-wide approach on which section 

II.F.4.e of this notice invites comment, 

provisions would be added to section 2 

of Appendix S to specify geographical 

criteria for determining which 

monitoring sites’ data can and will be 

compared to the standard consistent 

with the area-wide approach as 

described in that section. 

The EPA is proposing to amend and 

move the provisions of 40 CFR 50.11 

related to data completeness for the 

existing annual primary standard to the 

new Appendix S, and to add provisions 

for the proposed 1-hour primary 

standard. Substantively, the proposed 

data handling procedures for the annual 

primary standard in Appendix S are the 

same as the existing provisions in 40 

CFR 50.11 for that standard, except for 

a proposed addition of a cross-reference 

to the Exceptional Events Rule, a 

proposed addition of Administrator 

discretion to consider otherwise 

incomplete data complete, and a 

proposed provision addressing the 

possibility of there being multiple NO2

monitors at one site. The proposed 

procedures for the 1-hour primary 

standard are entirely new. 

The EPA is also proposing NO2- 

specific changes to the deadlines, in 40 

CFR 50.14, by which States must flag 

ambient air data that they believe have 

been affected by exceptional events and 

submit initial descriptions of those 

events, and the deadlines by which 

States must submit detailed 

justifications to support the exclusion of 

that data from EPA determinations of 

attainment or nonattainment with the 

NAAQS. The deadlines now contained 

in 40 CFR 50.14 are generic, and are not 

always appropriate for NO2 given the 

anticipated schedule for the 

designations of areas under the 

proposed NO2 NAAQS. 

A. Background 

The purpose of a data interpretation 

appendix in general is to provide the 

practical details on how to make a 

comparison between multi-day and 

possibly multi-monitor ambient air 

concentration data and the level of the 

NAAQS, so that determinations of 

compliance and violation are as 

objective as possible. Data interpretation 

guidelines also provide criteria for 

determining whether there are sufficient 

data to make a NAAQS level 

comparison at all. 

The regulatory language for the 

current NO2 NAAQS, originally adopted 

in 1977, contains data interpretation 

instructions only for the issue of data 

completeness. This situation contrasts 


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with the situations for ozone, PM2.5, 

PM10, and most recently Pb for which 

there are detailed data interpretation 

appendices in 40 CFR part 50 

addressing more issues that can arise in 

comparing monitoring data to the 

NAAQS. EPA has used its experience 

drafting and applying these other data 

interpretation appendices to develop the 

proposed text for Appendix S. 

An exceptional event is defined in 40 

CFR 50.1 as an event that affects air 

quality, is not reasonably controllable or 

preventable, is an event caused by 

human activity that is unlikely to recur 

at a particular location or a natural 

event, and is determined by the 

Administrator in accordance with 40 

CFR 50.14 to be an exceptional event. 

Air quality data that is determined to 

have been affected by an exceptional 

event under the procedural steps and 

substantive criteria specified in section 

50.14 may be excluded from 

consideration when EPA makes a 

determination that an area is meeting or 

violating the associated NAAQS. The 

key procedural deadlines in section 

50.14 are that a State must notify EPA 

that data have been affected by an event, 

i.e., ‘‘flag’’ the data in the Air Quality 

Systems (AQS) database, and provide an 

initial description of the event by July 

1 of the year after the data are collected, 

and that the State must submit the full 

justification for exclusion within 3 years 

after the quarter in which the data were 

collected. However, if a regulatory 

decision based on the data, for example 

a designation action, is anticipated, the 

schedule is foreshortened and all 

information must be submitted to EPA 

no later than a year before the decision 

is to be made. This generic schedule 

presents problems when a NAAQS has 

been recently revised, as discussed 

below. 

The REA did not address data 

interpretation details. However, the 

approach to data interpretation used in 

the REA, for example to report the 

number of cities which would violate 

possible 1-hour primary NAAQS, was 

generally consistent with the proposed 

data interpretation procedures. 

B. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen 

The purpose of a data interpretation 

rule for the NO2 NAAQS is to give effect 

to the form, level, averaging time, and 

indicator specified in the proposed 

regulatory text at 40 CFR 50.11, 

anticipating and resolving in advance 

various future situations that could 

occur. The proposed Appendix S 

provides common definitions and 

requirements that apply to both the 

annual and the 1-hour primary 

standards for NO2. The common 

requirements concern how ambient data 

are to be reported, what ambient data 

are to be considered (including the issue 

of which of multiple monitors’ data sets 

will be used when more than one 

monitor has operated at a site), and the 

applicability of the Exceptional Events 

Rule to the primary NO2 NAAQS. 

The proposed Appendix S also 

addresses several issues in ways which 

are specific to the individual primary 

NO2 standards, as described below. 

1. Annual Primary Standard 

The proposed data interpretation 

provisions for the annual standard are 

consistent with the current instructions 

included along with the statement of the 

level and form of the standard in 40 CFR 

53.11. These are the following: (1) At 

least 75% of the hours in the year must 

have reported concentration data. (2) 

The available hourly data are 

arithmetically averaged, and then 

rounded (not truncated) to whole parts 

per billion. (3) The design value is this 

rounded annual average concentration. 

(4) The design value is compared with 

the level of the annual primary standard 

(expressed in parts per billion). 

It would be possible to introduce 

additional steps for the annual primary 

standard which in principle could make 

the design value a more reliable 

indicator of actual annual average 

concentration in cases where some 

monitoring data have been lost. For 

example, averaging within a calendar 

quarter first and then averaging across 

quarters could help compensate for 

uneven data capture across the year. For 

some aspects of the data interpretation 

procedures for some other pollutants, 

the current data interpretation 

appendices do contain such additional 

steps. The proposed provisions for the 

proposed 1-hour NO2 standard 

(described immediately below) also 

incorporate some such features. 

However, we believe that such 

complexity is not needed to 

appropriately implement the annual 

primary standard, especially since no 

area presently comes close to violating 

the standard. EPA invites comment on 

whether the annual primary standard 

design value should be a weighted 

annual mean (e.g. averaging within 

calendar quarters before averaging 

across quarters), rather than the mean of 

all available hourly values. 

2. 1-Hour Primary Standard Based on 

the Annual 4th High Value Form 

With regard to data completeness for 

the proposed 1-hour primary standard, 

the proposed Appendix follows past 

EPA practice for other NAAQS 

pollutants by requiring that in general at 

least 75% of the monitoring data that 

should have resulted from following the 

planned monitoring schedule in a 

period must be available for the key air 

quality statistic from that period to be 

considered valid. For the proposed 1- 

hour primary NO2 NAAQS, the key air 

quality statistics are the daily maximum 

1-hour concentrations in three 

successive years. It is important that 

sampling within a day encompass the 

period when concentrations are likely to 

be highest and that all seasons of the 

year are well represented. Hence, the 

75% requirement is proposed to be 

applied at the daily and quarterly levels. 

EPA invites comment on the proposed 

completeness requirements. 

Recognizing that there may be years 

with incomplete data, the proposed text 

provides that a design value derived 

from incomplete data will nevertheless 

be considered valid in either of two 

situations. 

First, if the design value calculated 

from at least four days of monitoring 

observations in each of these years 

exceeds the level of the 1-hour primary 

standard, it would be valid. This 

situation could arise if monitoring was 

intermittent but high NO2 levels were 

measured on enough hours and days for 

the mean of the three annual 4th values 

to exceed the standard. In this situation, 

more complete monitoring could not 

possibly have indicated that the 

standard was actually met. 

Second, we are proposing a diagnostic 

data substitution test which is intended 

to identify those cases with incomplete 

data in which it nevertheless is very 

likely, if not virtually certain, that the 

daily 1-hour design value would have 

been observed to be below the level of 

the NAAQS if monitoring data had been 

minimally complete. 

The diagnostic test would be applied 

only if there is at least 50% data capture 

in each quarter of each year and if the 

3-year mean of the observed annual 4th 

highest maximum hourly values in the 

incomplete data is below the NAAQS 

level. The test would substitute a high 

hypothetical concentration for as much 

of the missing data as needed to meet 

the 100% requirement in each quarter. 

The value that is substituted for the 

missing values is the highest daily 

maximum 1-hour observed in the same 

quarter, looking across all three years 

under evaluation. If the resulting 3-year 

design value is below the NAAQS, it is 

highly likely that the design value 

calculated from complete data would 

also have been below the NAAQS, so 

the original design value indicating 

compliance would be considered valid. 


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It should be noted that one outcome 

of applying the proposed substitution 

test is that a year with incomplete data 

may nevertheless be determined to not 

have a valid design value and thus to be 

unusable in making 1-hour primary 

NAAQS compliance determinations for 

that 3-year period. EPA invites comment 

on incorporating into the final rule the 

proposed substitution test. 

Also, we are proposing that the 

Administrator have general discretion to 

use incomplete data based on casespecific factors, either at the request of 

a state or at her own initiative. Similar 

provisions exist already for some other 

NAAQS. 

3. 1-Hour Primary Standard Based on 

the Annual 99th Percentile Daily Value 

Form 

The second version of the proposed 

Appendix S appearing at the end of this 

notice contains proposed interpretation 

procedures for a 1-hour primary 

standard based on the 99th percentile 

daily value form. The 4th high daily 

value form and the 99th percentile daily 

value form would yield the same design 

value in a situation in which every hour 

and day of the year has reported 

monitoring data, since the 99th 

percentile of 365 daily values is the 4th 

highest value. However, the two forms 

diverge if data completeness is 82% or 

less, because in that case the 99th 

percentile value is the 3rd highest (or 

higher) value, to compensate for the lack 

of monitoring data on days when 

concentrations could also have been 

high. 

Logically, provisions to address 

possible data incompleteness under the 

99th percentile daily value form should 

be somewhat different from those for the 

4th highest form. With a 4th highest 

form, incompleteness should not 

invalidate a design value that exceeds 

the standard, for reasons explained 

above. With the 99th percentile form, 

however, a design value exceeding the 

standard stemming from incomplete 

data should not automatically be 

considered valid, because 

concentrations on the unmonitored days 

could have been relatively low, such 

that the actual 99th percentile value for 

the year could have been lower, and the 

design value could have been below the 

standard. The second proposed version 

of Appendix S accordingly has 

somewhat different provisions for 

dealing with data incompleteness. One 

difference is the addition of another 

diagnostic test based on data 

substitution, which in some cases can 

validate a design value based on 

incomplete data that exceeds the 

standard. 

The second version of the proposed 

Appendix S provides a table for 

determining which day’s maximum 1- 

hour concentration will be used as the 

99th percentile concentration for the 

year. The proposed table is similar to 

one used now for the 24-hour PM2.5 

NAAQS, which is based on a 98th 

percentile form, but adjusted to reflect 

a 99th percentile form for the 1-hour 

primary NO2 standard. The proposed 

Appendix S also provides instructions 

for rounding (not truncating) the average 

of three annual 99th percentile hourly 

concentrations before comparison to the 

level of the primary NAAQS. 

C. Exceptional Events Information 

Submission Schedule 

The Exceptional Events Rule at 40 

CFR 50.14 contains generic deadlines 

for a state to submit to EPA specified 

information about exceptional events 

and associated air pollutant 

concentration data. A state must 

initially notify EPA that data has been 

affected by an event by July 1 of the year 

after the data are collected; this is done 

by flagging the data in AQS and 

providing an initial event description. 

The state must also, after notice and 

opportunity for public comment, submit 

a demonstration to justify any claim 

within 3 years after the quarter in which 

the data were collected. However, if a 

regulatory decision based on the data 

(for example, a designation action) is 

anticipated, the schedule to flag data in 

AQS and submit complete 

documentation to EPA for review is 

foreshortened, and all information must 

be submitted to EPA no later than one 

year before the decision is to be made. 

These generic deadlines are suitable 

for the period after initial designations 

have been made under a NAAQS, when 

the decision that may depend on data 

exclusion is a redesignation from 

attainment to nonattainment or from 

nonattainment to attainment. However, 

these deadlines present problems with 

respect to initial designations under a 

newly revised NAAQS. One problem is 

that some of the deadlines, especially 

the deadlines for flagging some relevant 

data, may have already passed by the 

time the revised NAAQS is 

promulgated. Until the level and form of 

the NAAQS have been promulgated a 

state does not know whether the criteria 

for excluding data (which are tied to the 

level and form of the NAAQS) were met 

on a given day. The only way a state 

could guard against this possibility is to 

flag all data that could possibly be 

eligible for exclusion under a future 

NAAQS. This could result in flagging 

far more data than will eventually be 

eligible for exclusion. EPA believes this 

is an inefficient use of state and EPA 

resources, and is potentially confusing 

and misleading to the public and 

regulated entities. Another problem is 

that it may not be feasible for 

information on some exceptional events 

that may affect final designations to be 

collected and submitted to EPA at least 

one year in advance of the final 

designation decision. This could have 

the unintended consequence of EPA 

designating an area nonattainment as a 

result of uncontrollable natural or other 

qualified exceptional events. 

When Section 50.14 was revised in 

March 2007, EPA was mindful that 

designations were needed under the 

recently revised PM2.5 NAAQS, so 

exceptions to the generic deadline were 

included for PM2.5. The EPA was also 

mindful that similar issues would arise 

for subsequent new or revised NAAQS. 

The Exceptional Events Rule at section 

50.14(c)(2)(v) indicates ‘‘when EPA sets 

a NAAQS for a new pollutant, or revises 

the NAAQS for an existing pollutant, it 

may revise or set a new schedule for 

flagging data for initial designation of 

areas for those NAAQS.’’ 

For the specific case of NO2, EPA 

anticipates that initial designations 

under the revised NAAQS may be made 

by January 22, 2012 based on air quality 

data from the years 2008–2010. (See 

Section VI below for more detailed 

discussion of the designation schedule 

and what data EPA intends to use.) If 

final designations are made by January 

22, 2012, all events to be considered 

during the designations process must be 

flagged and fully documented by states 

one year prior to designations, by 

January 22, 2011. This date also 

coincides with the Clean Air Act 

deadline for Governors to submit to EPA 

their recommendations for designating 

all areas of their states. 

EPA is proposing revisions to 40 CFR 

50.14 to change submission dates for 

information supporting claimed 

exceptional events affecting NO2 data. 

The proposed rule text at the end of this 

notice shows the changes that would 

apply if a revised NO2 NAAQS is 

promulgated by January 22, 2010, and 

designations are made two years after 

promulgation of a NO2 NAAQS revision. 

For air quality data collected in 2008, 

we propose to extend the generic July 1, 

2009 deadline for flagging data (and 

providing a brief initial description of 

the event) to July 1, 2010. EPA believes 

this extension provides adequate time 

for states to review the impact of 

exceptional events from 2008 on the 

revised standard and notify EPA by 

flagging the relevant data in AQS. EPA 

is not proposing to change the generic 

deadline of January 22, 2011 for 


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23Since EPA is proposing to retain the annual 

standard without revision, the discussion in this 

section relates to implementation of the proposed 

1-hour standard, rather than the annual standard. 

submitting documentation to justify an 

NO2-related exceptional event from 

2008. We believe the generic deadline 

provides adequate time for states to 

develop and submit proper 

documentation. 

For data collected in 2009, EPA does 

not believe it is necessary to change the 

generic deadline of July 1, 2010 for 

flagging data and providing initial event 

descriptions. Similarly, EPA does not 

believe it is necessary to change the 

generic deadline of January 22, 2011 for 

states to submit documentation to 

justify an NO2-related exceptional event 

from 2009. 

For data collected in 2010, EPA 

believes the designations deadline of 

January 22, 2011 for flagging data and 

providing initial event descriptions does 

not provide states with adequate time to 

review and identify potential 

exceptional events that occur in 

calendar year 2010, especially events 

that might occur late in the year. 

Therefore, EPA is proposing that states 

may flag and provide initial event 

descriptions for 2010 data no later than 

April 1, 2011. This affords states more 

than 2 additional months than would be 

provided under the generic schedule to 

review and identify exceptional events 

affecting 2010 NO2 data. Similarly, EPA 

believes the designations schedule that 

would require states to submit detailed 

documentation to justify 2010 events 

claims by January 22, 2011 is not 

reasonable, because it would potentially 

preclude states from completing the 

required public review of the 

documentation prior to submitting to 

EPA. Therefore, EPA is proposing to 

extend this deadline to July 1, 2011. 

This would afford states more than 5 

additional months than provided by the 

generic schedule to complete the 

required public review and submit full 

supporting documentation, yet would 

still allow EPA adequate time to review 

the documentation and develop its final 

plans for designations by January 22, 

2012. 

Table 2 below summarizes the 

proposed two year designation 

deadlines discussed in this section. If 

the promulgation date for a revised NO2

NAAQS will occur on a different date 

than January 22, 2010, EPA will revise 

the final NO2 exceptional event flagging 

and documentation submission 

deadlines accordingly, consistent with 

this proposal, to provide states with 

reasonably adequate opportunity to 

review, identify, and document 

exceptional events that may affect an 

area designation under a revised 

NAAQS. EPA invites comment on these 

proposed changes in the exceptional 

event flagging and documentation 

submission deadlines. 

TABLE 2—SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS pollutant/standard/(level)/ 

promulgation date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description 

deadline 

Detailed documentation submission 

deadline 

PM2.5/24-Hr Standard (35 μg/m3) Promulgated October 17, 2006.2004–2006 ....... October 1, 2007 a ................................... April 15, 2008.a

Ozone/8–Hr .............................................. 2005–2007 ....... June 18, 2009 b ...................................... June 18, 2009.b

Standard (0.075 ppm) Promulgated 

March 12, 2008.

2008 ................. June 18, 2009b ....................................... June 18, 2009.b

2009 ................. 60 Days after the end of the calendar 

quarter in which the event occurred 

or February 5, 2010, whichever date 

occurs first b.

60 Days after the end of the calendar 

quarter in which the event occurred 

or February 5, 2010, whichever date 

occurs first.b

NO2/1–Hour Standard (80–100 PPB, final 

level TBD).

2008 ................. July 1, 2010 b ......................................... January 22, 2011. 

2009 ................. July 1, 2010 ........................................... January 22, 2011. 

2010 ................. April 1, 2011 b ......................................... July 1, 2011.b

aThese dates are unchanged from those published in the original rulemaking, and are shown in this table for informational purposes. b Indicates change from general schedule in 40 CFR 50.14. 

NOTE: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. 

V. Clean Air Act Implementation 

Requirements 

This section of the preamble discusses 

the Clean Air Act (CAA) requirements 

that states and emissions sources must 

address when implementing new or 

revised NO2 NAAQS based on the 

structure outlined in the CAA and 

existing rules.23 EPA may provide 

additional guidance in the future, as 

necessary, to assist states and emissions 

sources to comply with the CAA 

requirements for implementing new or 

revised NO2 NAAQS. 

The CAA assigns important roles to 

EPA, states, and, in specified 

circumstances, Tribal governments to 

achieve the NAAQS. States have the 

primary responsibility for developing 

and implementing State Implementation 

Plans (SIPs) that contain state measures 

necessary to achieve the air quality 

standards in each area. EPA provides 

assistance to states by providing 

technical tools, assistance, and 

guidance, including information on the 

potential control measures that may 

assist in helping areas attain the 

standards. 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once they 

have been established by EPA. Under 

section 110 of the CAA, 42 U.S.C. 7410, 

and related provisions, states are 

required to submit, for EPA approval, 

SIPs that provide for the attainment and 

maintenance of such standards through 

control programs directed at sources of 

NO2 emissions. If a state fails to adopt 

and implement the required SIPs by the 

time periods provided in the CAA, the 

EPA has responsibility under the CAA 

to adopt a Federal Implementation Plan 

(FIP) to assure that areas attain the 

NAAQS in an expeditious manner. 

The states, in conjunction with EPA, 

also administer the prevention of 

significant deterioration (PSD) program 

for NO2. See sections 160–169 of the 

CAA. In addition, Federal programs 

provide for nationwide reductions in 

emissions of NO2 and other air 

pollutants under Title II of the Act, 42 


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U.S.C. 7521–7574, which involves 

controls for automobiles, trucks, buses, 

motorcycles, nonroad engines, and 

aircraft emissions; the new source 

performance standards (NSPS) for 

stationary sources under section 111 of 

the CAA, 42 U.S.C. 7411; and the 

national emission standards for 

hazardous air pollutants for stationary 

sources under section 112 of the CAA, 

42 U.S.C. 7412. 

CAA Section 301(d) authorizes EPA to 

treat eligible Indian Tribes in the same 

manner as states (TAS) under the CAA 

and requires EPA to promulgate 

regulations specifying the provisions of 

the statute for which such treatment is 

appropriate. EPA has promulgated these 

regulations—known as the Tribal 

Authority Rule or TAR—at 40 CFR Part 

49. See 63 FR 7254 (February 12, 1998). 

The TAR establishes the process for 

Indian Tribes to seek TAS eligibility and 

sets forth the CAA functions for which 

TAS will be available. Under the TAR, 

eligible Tribes may seek approval for all 

CAA and regulatory purposes other than 

a small number of functions enumerated 

at section 49.4. Implementation plans 

under section 110 are included within 

the scope of CAA functions for which 

eligible Tribes may obtain approval. 

Section 110(o) also specifically 

describes Tribal roles in submitting 

implementation plans. Eligible Indian 

Tribes may thus submit implementation 

plans covering their reservations and 

other areas under their jurisdiction. 

Under the CAA and TAR, Tribes are 

not, however, required to apply for TAS 

or implement any CAA program. In 

promulgating the TAR EPA explicitly 

determined that it was not appropriate 

to treat Tribes similarly to states for 

purposes of, among other things, 

specific plan submittal and 

implementation deadlines for NAAQSrelated requirements. 40 CFR 49.4(a). In 

addition, where Tribes do seek approval 

of CAA programs, including section 110 

implementation plans, the TAR 

provides flexibility and allows them to 

submit partial program elements, so 

long as such elements are reasonably 

severable—i.e., ‘‘not integrally related to 

program elements that are not included 

in the plan submittal, and are consistent 

with applicable statutory and regulatory 

requirements’’. 40 CFR 49.7. 

To date, very few Tribes have sought 

TAS for purposes of section 110 

implementation plans. However, some 

Tribes may be interested in pursuing 

such plans to implement today’s 

proposed standard. As noted above, 

such Tribes may seek approval of 

partial, reasonably severable plan 

elements, or they may seek to 

implement all relevant components of 

an air quality program for purposes of 

meeting the requirements of the Act. In 

several sections of this preamble, EPA 

describes the various roles and 

requirements states will address in 

implementing today’s proposed 

standard. Such references to states are 

generally intended to include eligible 

Indian Tribes to the extent consistent 

with the flexibility provided to Tribes 

under the TAR. Where Tribes do not 

seek TAS for section 110 

implementation plans, EPA will 

promulgate Federal implementation 

plans as ‘‘necessary or appropriate to 

protect air quality.’’ 40 CFR 49.11(a) 

EPA also notes that some Tribes 

operate air quality monitoring networks 

in their areas. For such monitors to be 

used to measure attainment with this 

primary NAAQS for NO2, the criteria 

and procedures identified in this rule 

would apply. 

A. Designations 

After EPA establishes or revises a 

NAAQS, the CAA requires EPA and the 

states to begin taking steps to ensure 

that the new or revised NAAQS are met. 

The first step is to identify areas of the 

country that do not meet the new or 

revised NAAQS. The CAA defines 

EPA’s authority to designate areas that 

do not meet a new or revised NAAQS. 

Section 107(d)(1) provides that, ‘‘By 

such date as the Administrator may 

reasonably require, but not later than 1 

year after promulgation of a new or 

revised NAAQS for any pollutant under 

section 109, the Governor of each state 

shall * * * submit to the Administrator 

a list of all areas (or portions thereof) in 

the state’’ that designates those areas as 

nonattainment, attainment, or 

unclassifiable. Section 107(d)(1)(B)(i) 

further provides, ‘‘Upon promulgation 

or revision of a NAAQS, the 

Administrator shall promulgate the 

designations of all areas (or portions 

thereof) * * * as expeditiously as 

practicable, but in no case later than 2 

years from the date of promulgation. 

Such period may be extended for up to 

one year in the event the Administrator 

has insufficient information to 

promulgate the designations. ‘‘The term 

‘‘promulgation’’ has been interpreted by 

the courts to be signature and 

dissemination of a rule. By no later than 

120 days prior to promulgating 

designations, EPA is required to notify 

states of any intended modifications to 

their boundaries as EPA may deem 

necessary. States then have an 

opportunity to comment on EPA’s 

tentative decision. Whether or not a 

state provides a recommendation, EPA 

must promulgate the designation that it 

deems appropriate. 

Thus, following promulgation of the 

revised NO2 NAAQS in January 2010, 

EPA must promulgate initial 

designations by January 2012 (2 years 

after promulgation of the revised 

NAAQS), or, by January 2013 in the 

event that the Administrator has 

insufficient information to promulgate 

initial designations within 2 years. In 

the case of the NO2 NAAQS, in today’s 

action EPA is proposing new NO2

monitor siting rules that focus on 

roadways. EPA anticipates that it will 

require up to 3 years to get a new 

monitoring network in place, plus an 

additional 3 years of monitoring 

thereafter in order to determine 

compliance with the revised standard. 

This means that a full set of air quality 

data from the new network will not be 

available until approximately 2016. 

Since data from the new network will 

not be available prior to the CAA 

designation deadlines even if EPA takes 

an additional year, EPA intends to 

complete initial designations in 2012 

using air quality data from the current 

NO2 monitoring network in place, using 

NO2 monitoring data from the years 

2008–2010. 

Accordingly, Governors will be 

required to submit their initial 

designation recommendations to EPA 

no later than January 2011. If the 

Administrator intends to modify any 

state area recommendation, EPA will 

notify the Governor no later than 120 

days prior to initial designations in 

January 2012. States that believe the 

Administrator’s modification is 

inappropriate will have an opportunity 

to demonstrate why they believe their 

recommendation is more appropriate 

before designations are promulgated in 

January 2012. As explained below in 

more detail, we intend to designate 

areas under the current NO2 monitoring 

network as ‘‘unclassifiable’’ or 

‘‘nonattainment’’ based on the data set 

for 2008–2010. 

We intend to designate areas that do 

not show violations of the revised NO2

NAAQS as ‘‘unclassifiable’’ since the 

existing area-wide monitoring network 

does not fully satisfy the near roadwayoriented NO2 monitoring requirements 

proposed in this notice. Because there 

are no monitors in the current NO2

network that meet the proposed 

definition of ‘‘near-roadway,’’ 

monitoring data that does not indicate a 

violation of the NAAQS would not 

provide a sufficient basis for concluding 

that an area is meeting the revised NO2

NAAQS. Rather, an area-wide monitor 

may record concentrations that are 

below the revised NO2 NAAQS because 

it is not sited where concentrations in 

the area are highest. Thus, we do not 


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believe the current monitoring network 

provides information that supports 

designating an area as ‘‘attainment’’ 

with today’s proposed standards. 

The EPA anticipates that areas 

designated as ‘‘unclassifiable’’ in 

January 2012 will remain so until a new 

NO2 monitoring network is deployed 

and 3 years of monitoring data have 

been collected. Once the NO2 monitors 

are placed in locations meeting the 

proposed near-roadway siting 

requirements and monitoring data 

become available, the Agency could 

subsequently redesignate areas as 

‘‘nonattainment’’ or ‘‘attainment’’ under 

section 107(d)(3). 

In January 2012 we intend to 

designate as ‘‘nonattainment’’ areas that 

show violations of the revised standard 

under the current monitoring network. 

As discussed above, the current 

monitoring network may not record NO2

concentrations near roadways where 

NO2 concentrations are highest. We thus 

anticipate that any area showing 

violations of the revised NO2 standard 

based on the current monitoring 

network will continue to show 

violations when monitors are placed in 

near-roadway locations. 

In summary, as required by section 

107(d)(1)(A)(i) of the CAA, in January 

2012 the EPA must designate as 

‘‘nonattainment’’ any areas with 

monitors within the existing network 

that report violations of the revised NO2

NAAQS. All other areas not indicating 

a violation of the revised NO2 NAAQS 

will be designated as ‘‘unclassifiable.’’ 

While the CAA provides the Agency an 

additional third year from promulgation 

of a NAAQS to complete designations in 

the event that there is insufficient 

information to make NAAQS 

compliance determinations, we 

anticipate that delaying designations for 

this additional year would not result in 

significant additional data that would 

allow EPA to designate areas that would 

otherwise be designated 

‘‘unclassifiable.’’ Once a near-roadway 

network has been deployed and 3 years 

of air quality data has been collected, 

we anticipate redesignating 

unclassifiable areas as ‘‘attainment’’ or 

‘‘nonattainment’’ where additional data 

from the new network provides a basis 

for such a designation. 

EPA is also taking comment on the 

area-wide approach discussed in section 

II.F.4.e above. If this approach is 

finalized, we anticipate designating 

areas as either ‘‘attainment,’’ 

‘‘nonattainment’’ or ‘‘unclassifiable’’ in 

2012, based on air quality data for years 

2008–2010. Unlike the near-roadway 

approach, we would expect to have 

sufficient data to designate some areas 

showing no violations of the revised 

NAAQS as ‘‘attainment’’ rather than 

‘‘unclassifiable.’’ As required by CAA 

section 107(d), we would expect to 

designate areas with violating monitors 

and nearby areas, including those with 

major roadways that contribute to such 

violations, as ‘‘nonattainment.’’ Any 

areas which EPA cannot classify on the 

basis of available information as 

meeting or not meeting the revised 

NAAQS would be designated as 

‘‘unclassifiable.’’ 

B. Classifications 

Section 172(a)(1)(A) of the CAA 

authorizes EPA to classify areas 

designated as nonattainment for the 

purpose of applying an attainment date 

pursuant to section 172(a)(2), or for 

other reasons. In determining the 

appropriate classification, EPA may 

consider such factors as the severity of 

the nonattainment problem and the 

availability and feasibility of pollution 

control measures (see section 

172(a)(1)(A) of the CAA). The EPA may 

classify NO2 nonattainment areas, but is 

not required to do so. The primary 

reason to establish classifications is to 

set different deadlines for each class of 

nonattainment area to complete the 

planning process and to provide for 

different attainment dates based upon 

the severity of the nonattainment 

problem for the affected area. However, 

the CAA separately establishes specific 

planning and attainment deadlines in 

sections 191 and 192: 18 months for the 

submittal of an attainment plan and as 

expeditiously as possible but no later 

than 5 years for areas to attain standard. 

EPA believes that classifications are 

unnecessary in light of these relatively 

short deadlines. Therefore, EPA is not 

proposing to establish classifications for 

a revised NO2 NAAQS. 

C. Attainment Dates 

The maximum deadline date by 

which an area is required to attain the 

NO2 NAAQS is determined from the 

effective date of the nonattainment 

designation for the affected area. For 

areas designated nonattainment for the 

revised NO2 NAAQS, SIPs must provide 

for attainment of the NAAQS as 

expeditiously as practicable, but no later 

than 5 years from the date of the 

nonattainment designation for the area 

(see section 192(a) of the CAA). The 

EPA will determine whether an area has 

demonstrated attainment of the NO2

NAAQS by evaluating air quality 

monitoring data consistent with the 

form of the NO2 NAAQS if revised, 

which will be codified at 40 CFR part 

50, Appendix F. 

1. Attaining the NAAQS 

In order for an area to be redesignated 

as attainment, the state must comply 

with the five requirements as provided 

under section 107(d)(3)(E) of the CAA. 

This section requires that: 

—EPA must have determined that the 

area has met the NO2 NAAQS; 

—EPA has fully approved the state’s 

implementation plan; 

—the improvement in air quality in the 

affected area is due to permanent and 

enforceable reductions in emissions; 

—EPA has fully approved a 

maintenance plan for the area; and 

—The state(s) containing the area have 

met all applicable requirements under 

section 110 and part D. 

2. Consequences of Failing To Attain by 

the Statutory Attainment Date 

Any NO2 nonattainment area that fails 

to attain by its statutory attainment date 

would be subject to the requirements of 

sections 179(c) and (d) of the CAA. EPA 

is required to make a finding of failure 

to attain no later than 6 months after the 

specified attainment date and publish a 

notice in the Federal Register. The state 

would be required to submit an 

implementation plan revision, no later 

than one year following the effective 

date of the Federal Register notice 

making the determination of the area’s 

failure to attain, which demonstrates 

that the standard will be attained as 

expeditiously as practicable, but no later 

than 5 years from the effective date of 

EPA’s finding that the area failed to 

attain. In addition, section 179(d)(2) 

provides that the SIP revision must 

include any specific additional 

measures as may be reasonably 

prescribed by EPA, including ‘‘all 

measures that can be feasibly 

implemented in the area in light of 

technological achievability, costs, and 

any nonair quality and other air qualityrelated health and environmental 

impacts.’’ 

D. Section 110(a)(2) NAAQS 

Infrastructure Requirements 

Section 110(a)(2) of the CAA requires 

all states to develop and maintain a 

solid air quality management 

infrastructure, including enforceable 

emission limitations, an ambient 

monitoring program, an enforcement 

program, air quality modeling, and 

adequate personnel, resources, and legal 

authority. Section 110(a)(2)(D) also 

requires state plans to prohibit 

emissions from within the state which 

contribute significantly to 

nonattainment or maintenance areas in 

any other State, or which interfere with 

programs under part C to prevent 


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24Two 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. 

significant deterioration of air quality or 

to achieve reasonable progress toward 

the national visibility goal for Federal 

class I areas (national parks and 

wilderness areas). 

Under section 110(a)(1) and (2) of the 

CAA, all states are required to submit 

SIPs to EPA which demonstrate that 

basic program elements have been 

addressed within 3 years of the 

promulgation of any new or revised 

NAAQS. Subsections (A) through (M) of 

section 110(a)(2) listed below, set forth 

the elements that a State’s program must 

contain in the SIP.24 The list of section 

110(a)(2) NAAQS implementation 

requirements are the following: 

• Ambient air quality monitoring/ 

data system: Section 110(a)(2)(B) 

requires SIPs to provide for setting up 

and operating ambient air quality 

monitors, collecting and analyzing data 

and making these data available to EPA 

upon request. 

• Program for enforcement of control 

measures: Section 110(a)(2)(C) requires 

SIPs to include a program providing for 

enforcement of measures and regulation 

and permitting of new/modified 

sources. 

• Interstate transport: Section 

110(a)(2)(D) requires SIPs to include 

provisions prohibiting any source or 

other type of emissions activity in the 

state from contributing significantly to 

nonattainment in another state or from 

interfering with measures required to 

prevent significant deterioration of air 

quality or to protect visibility. 

• Adequate resources: Section 

110(a)(2)(E) requires states to provide 

assurances of adequate funding, 

personnel and legal authority for 

implementation of their SIPs. 

• Stationary source monitoring 

system: Section 110(a)(2)(F) requires 

states to establish a system to monitor 

emissions from stationary sources and 

to submit periodic emissions reports to 

EPA. 

• Emergency power: Section 

110(a)(2)(G) requires states to include 

contingency plans, and adequate 

authority to implement them, for 

emergency episodes in their SIPs. 

• Provisions for SIP revision due to 

NAAQS changes or findings of 

inadequacies: Section 110(a)(2)(H) 

requires states to provide for revisions 

of their SIPs in response to changes in 

the NAAQS, availability of improved 

methods for attaining the NAAQS, or in 

response to an EPA finding that the SIP 

is inadequate. 

• Consultation with local and Federal 

government officials: Section 110(a)(2)(J) 

requires states to meet applicable local 

and Federal government consultation 

requirements when developing SIP and 

reviewing preconstruction permits. 

• Public notification of NAAQS 

exceedances: Section 110(a)(2)(J) 

requires states to adopt measures to 

notify the public of instances or areas in 

which a NAAQS is exceeded. 

• PSD and visibility protection: 

Section 110(a)(2)(J) also requires states 

to adopt emissions limitations, and such 

other measures, as may be necessary to 

prevent significant deterioration of air 

quality in attainment areas and protect 

visibility in Federal Class I areas in 

accordance with the requirements of 

CAA Title I, part C. 

• Air quality modeling/data: Section 

110(a)(2)(K) requires that SIPs provide 

for performing air quality modeling for 

predicting effects on air quality of 

emissions of any NAAQS pollutant and 

submission of data to EPA upon request. 

• Permitting fees: Section 110(a)(2)(L) 

requires the SIP to include requirements 

for each major stationary source to pay 

permitting fees to cover the cost of 

reviewing, approving, implementing 

and enforcing a permit. 

• Consultation/participation by 

affected local government: Section 

110(a)(2)(M) requires states to provide 

for consultation and participation by 

local political subdivisions affected by 

the SIP. 

E. Attainment Planning Requirements 

1. Nonattainment Area SIPs 

Any state containing an area 

designated as nonattainment with 

respect to the NO2 NAAQS must 

develop for submission a SIP meeting 

the requirements of part D, Title I, of the 

CAA, providing for attainment by the 

applicable statutory attainment date (see 

sections 191(a) and 192(a) of the CAA). 

As indicated in section 191(a) all 

components of the NO2 part D SIP must 

be submitted within 18 months of the 

effective date of an area’s designation as 

nonattainment. 

Section 172 of the CAA includes 

general requirements for all designated 

nonattainment areas. Section 172(c)(1) 

requires that each nonattainment area 

plan ‘‘provide for the implementation of 

all reasonably available control 

measures (RACM) as expeditiously as 

practicable (including such reductions 

in emissions from existing sources in 

the area as may be obtained through the 

adoption, at a minimum, of Reasonably 

Available Control Technology (RACT)), 

and shall provide for attainment of the 

national primary ambient air quality 

standards.’’ States are required to 

implement RACM and RACT in order to 

attain ‘‘as expeditiously as practicable’’. 

Section 172(c) requires states with 

nonattainment areas to submit a SIP for 

these areas which contain an attainment 

demonstration which shows that the 

affected area will attain the standard by 

the applicable statutory attainment date. 

The State must also show that the area 

will attain the standards as 

expeditiously as practicable, and it must 

include an analysis of whether 

implementation of reasonably available 

measures will advance the attainment 

date for the area. 

Part D SIPs must also provide for 

reasonable further progress (RFP) (see 

section 172(c)(2) of the CAA). The CAA 

defines RFP as ‘‘such annual 

incremental reductions in emissions of 

the relevant air pollution as are required 

by part D, or may reasonably be required 

by the Administrator for the purpose of 

ensuring attainment of the applicable 

NAAQS by the applicable attainment 

date.’’ (See section 171 of the CAA) 

Historically, for some pollutants, RFP 

has been met by showing annual 

incremental emission reductions 

sufficient to maintain generally linear 

progress toward attainment by the 

applicable attainment date. 

All NO2 nonattainment area SIPs must 

include contingency measures which 

must be implemented in the event that 

an area fails to meet RFP or fails to 

attain the standards by its attainment 

date. (See section 172(c)(9)) These 

contingency measures must be fully 

adopted rules or control measures that 

take effect without further action by the 

state or the Administrator. The EPA 

interprets this requirement to mean that 

the contingency measures must be 

implemented with only minimal further 

action by the state or the affected 

sources with no additional rulemaking 

actions such as public hearings or 

legislative review. 

Emission inventories are also critical 

for the efforts of State, local, and Federal 

agencies to attain and maintain the 

NAAQS that EPA has established for 

criteria pollutants including NO2. 

Section 191(a) in conjunction with 

section 172(c) requires that areas 

designated as nonattainment for NO2

submit an emission inventory to EPA no 

later than 18 months after designation as 

nonattainment. In the case of NO2, 

sections 191(a) and 172(c) also require 

that states submit periodic emission 


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25The 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. 

26 In addition, the PSD program applies to noncriteria pollutants subject to regulation under the 

Act, except those pollutants regulated under section 

112 and pollutants subject to regulation only under 

section 211(o). 

27Criteria pollutants are those pollutants for 

which EPA has established a NAAQS under section 

109 of the CAA. 

inventories for nonattainment areas. The 

periodic inventory must include 

emissions of NO2 for point, nonpoint, 

mobile (on-road and non-road), and area 

sources. 

2. New Source Review and Prevention 

of Significant Deterioration 

Requirements 

The Prevention of Significant 

Deterioration (PSD) and nonattainment 

New Source Review (NSR) programs 

contained in parts C and D of Title I of 

the CAA govern preconstruction review 

of any new or modified major stationary 

sources of air pollutants regulated under 

the CAA as well as any precursors to the 

formation of that pollutant when 

identified for regulation by the 

Administrator.25 The EPA rules 

addressing these programs can be found 

at 40 CFR 51.165, 51.166, 52.21, 52.24, 

and part 51, appendix S. States which 

have areas designated as nonattainment 

for the NO2 NAAQS must submit, as a 

part of the SIP due 18 months after an 

area is designated as nonattainment, 

provisions requiring permits for the 

construction and operation of new or 

modified stationary sources anywhere 

in the nonattainment area. SIPs that 

address the PSD requirements related to 

attainment areas are due no later than 3 

years after the promulgation of a revised 

NAAQS for NO2. 

The NSR program is composed of 

three different permit programs: 

• Prevention of Significant 

Deterioration (PSD). 

• Nonattainment NSR (NA NSR). 

• Minor NSR. 

The PSD program applies when a 

major source, that is located in an area 

that is designated as attainment or 

unclassifiable for any criteria pollutant, 

is constructed, or undergoes a major 

modification.26 The nonattainment NSR 

program applies on a pollutant-specific 

basis when a major source constructs or 

modifies in an area that is designated as 

nonattainment for that pollutant. The 

minor NSR program addresses both 

major and minor sources that undergo 

construction or modification activities 

that do not qualify as major, and it 

applies, as necessary to ensure 

attainment, regardless of the designation 

of the area in which a source is located. 

The PSD requirements include but are 

not limited to the following: 

• Installation of Best Available 

Control Technology (BACT); 

• Air quality monitoring and 

modeling analyses to ensure that a 

project’s emissions will not cause or 

contribute to a violation of any NAAQS 

or maximum allowable pollutant 

increase (PSD increment); 

• Notification of Federal Land 

Manager of nearby Class I areas; and 

• Public comment on permit. 

Nonattainment NSR requirements 

include but are not limited to: 

• Installation of Lowest Achievable 

Emissions Rate (LAER) control 

technology; 

• Offsetting new emissions with 

creditable emissions reductions; 

• A certification that all major 

sources owned and operated in the state 

by the same owner are in compliance 

with all applicable requirements under 

the CAA; 

• An alternative siting analysis 

demonstrating that the benefits of a 

proposed source significantly outweigh 

the environmental and social costs 

imposed as a result of its location, 

construction, or modification; and 

• Public comment on the permit. 

Minor NSR programs must meet the 

statutory requirements in section 

110(a)(2)(C) of the CAA which requires 

‘‘* * * regulation of the modification 

and construction of any stationary 

source * * * as necessary to ensure that 

the [NAAQS] are achieved.’’ Areas 

which are newly designated as 

nonattainment for the NO2 NAAQS as a 

result of any changes made to the 

NAAQS will be required to adopt a 

nonattainment NSR program to address 

major sources of NO2 where the program 

does not currently exist for the NO2

NAAQS and may need to amend their 

minor source program as well. Prior to 

adoption of the SIP revision addressing 

major source nonattainment NSR for 

NO2 nonattainment areas, the 

requirements of 40 CFR part 51, 

appendix S will apply. 

3. General Conformity 

Section 176(c) of the CAA, as 

amended (42 U.S.C. 7401 et seq.), 

requires that all Federal actions conform 

to an applicable implementation plan 

developed pursuant to section 110 and 

part D of the CAA. The EPA rules, 

developed under the authority of 

section 176(c) of the CAA, prescribe the 

criteria and procedures for 

demonstrating and assuring conformity 

of Federal actions to a SIP. Each Federal 

agency must determine that any actions 

covered by the general conformity rule 

conform to the applicable SIP before the 

action is taken. The criteria and 

procedures for conformity apply only in 

nonattainment areas and those areas 

redesignated attainment since 1990 

(‘‘maintenance areas’’) with respect to 

the criteria pollutants under the CAA: 27

Carbon monoxide (CO), lead (Pb), 

nitrogen dioxide (NO2), ozone (O3), 

particulate matter (PM2.5 and PM10), and 

sulfur dioxide (SO2). The general 

conformity rules apply one year 

following the effective date of 

designations for any new or revised 

NAAQS. 

The general conformity determination 

examines the impacts of direct and 

indirect emissions related to Federal 

actions. The general conformity rule 

provides several options to satisfy air 

quality criteria, such as modeling or 

offsets, and requires the Federal action 

to also meet any applicable SIP 

requirements and emissions milestones. 

The general conformity rule also 

requires that notices of draft and final 

general conformity determinations be 

provided directly to air quality 

regulatory agencies and to the public by 

publication in a local newspaper. 

4. Transportation Conformity 

Transportation conformity is required 

under CAA section 176(c) (42 U.S.C. 

7506(c)) to ensure that transportation 

plans, transportation improvement 

programs (TIPs) and Federally 

supported highway and transit projects 

will not cause new air quality 

violations, worsen existing violations, or 

delay timely attainment of the relevant 

NAAQS or interim reductions and 

milestones. Transportation conformity 

applies to areas that are designated 

nonattainment and maintenance for 

transportation-related criteria 

pollutants: carbon monoxide (CO), 

ozone (O3), nitrogen dioxide (NO2), and 

particulate matter (PM2.5 and PM10). 

Transportation conformity for a revised 

NO2 NAAQS does not apply until one 

year after the effective date of a 

nonattainment designation. (See CAA 

section 176(c)(6) and 40 CFR 93.102(d)). 

EPA’s Transportation Conformity 

Rule (40 CFR Part 51, Subpart T, and 

Part 93, Subpart A establishes the 

criteria and procedures for determining 

whether transportation activities 

conform to the SIP. The EPA is not 

proposing changes to the Transportation 

Conformity rule in this proposed 

rulemaking. However, in the future, 

EPA will review the need to conduct a 


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rulemaking to establish any new or 

revised transportation conformity tests 

that would apply under a revision to the 

NO2 NAAQS for transportation plans, 

TIPs, and applicable highway and 

transit projects. 

VI. Communication of Public Health 

Information 

Information on the public health 

implications of ambient concentrations 

of criteria pollutants is currently made 

available primarily through EPA’s Air 

Quality Index (AQI) program. The 

current Air Quality Index has been in 

use since its inception in 1999 (64 FR 

42530). It provides accurate, timely, and 

easily understandable information about 

daily levels of pollution (40 CFR 58.50). 

The AQI establishes a nationally 

uniform system of indexing pollution 

levels for NO2, carbon monoxide, ozone, 

particulate matter and sulfur dioxide. 

The AQI converts pollutant 

concentrations in a community’s air to 

a number on a scale from 0 to 500. 

Reported AQI values enable the public 

to know whether air pollution levels in 

a particular location are characterized as 

good (0–50), moderate (51–100), 

unhealthy for sensitive groups (101– 

150), unhealthy (151–200), very 

unhealthy (201–300), or hazardous 

(300–500). The AQI index value of 100 

typically corresponds to the level of the 

short-term NAAQS for each pollutant. 

An AQI value greater than 100 means 

that a pollutant is in one of the 

unhealthy categories (i.e., unhealthy for 

sensitive groups, unhealthy, very 

unhealthy, or hazardous) on a given 

day; an AQI value at or below 100 

means that a pollutant concentration is 

in one of the satisfactory categories (i.e., 

moderate or good). Decisions about the 

pollutant concentrations at which to set 

the various AQI breakpoints, that 

delineate the various AQI categories, 

draw directly from the underlying 

health information that supports the 

NAAQS review. 

The Agency recognizes the 

importance of revising the AQI in a 

timely manner to be consistent with any 

revisions to the NAAQS. Therefore EPA 

proposes to finalize conforming changes 

to the AQI, in connection with the 

Agency’s final decision on the NO2

NAAQS if revisions to the primary 

standard are promulgated. Currently, no 

AQI breakpoints are identified below an 

AQI value of 200 since there is no shortterm NO2 NAAQS. Therefore, if a shortterm NO2 NAAQS is promulgated, 

conforming changes would include 

setting the 100 level of the AQI at the 

same level as the revised primary NO2

NAAQS and also setting the other AQI 

breakpoints at the lower end of the AQI 

scale (i.e., AQI values of 50 and 150). 

EPA does not propose to change 

breakpoints at the higher end of the AQI 

scale (from 200 to 500), which would 

apply to state contingency plans or the 

Significant Harm Level (40 CFR 51.16), 

because the information from this 

review does not inform decisions about 

breakpoints at those higher levels. 

With regard to an AQI value of 50, the 

breakpoint between the good and 

moderate categories, historically this 

value is set at the level of the annual 

NAAQS, if there is one, or one-half the 

level of the short-term NAAQS in the 

absence of an annual NAAQS (63 FR 

67823, Dec. 12, 1998). Taking into 

consideration this practice, EPA is 

proposing to set the AQI value of 50 to 

be between 0.040 and 0.053 ppm NO2, 

1-hour average. EPA anticipates that 

figures towards the lower end of this 

range would be appropriate if the 

standard is set towards the lower end of 

the proposed range for the standard (e.g. 

80 ppb), while figures towards the 

higher end of the range would be more 

appropriate for standards set at the 

higher end of the range for the standard 

(e.g., 100 ppb). EPA solicits comments 

on this range for an AQI of 50, and the 

appropriate basis for selecting an AQI of 

50 both within this range and, in light 

of EPA’s solicitation of comment on 

standard levels below 80 ppb and above 

100 ppb, above or below this range. 

With regard to an AQI value of 150, 

the breakpoint between the unhealthy 

for sensitive groups and unhealthy 

categories, historically values between 

the short-term standard and an AQI 

value of 500 are set at levels that are 

approximately equidistant between the 

AQI values of 100 and 500 unless there 

is health evidence that suggests a 

specific level would be appropriate (63 

FR 67829, Dec. 12, 1998). For an AQI 

value of 150, the range of 0.360 to 0.370 

ppm NO2, 1-hour average, represents the 

midpoint between the proposed range 

for the short-term standard and the level 

of an AQI value of 200 (0.64 ppm NO2, 

1-hour average). Therefore, EPA is 

proposing to set the AQI value of 150 to 

be between 0.360 and 0.370 ppm NO2, 

1-hour average. 

VII. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under section 3(f)(1) of Executive 

Order 12866 (58 FR 51735, October 4, 

1993), this action is an ‘‘economically 

significant regulatory action’’ because it 

is likely to have an annual effect on the 

economy of $100 million or more. 

Accordingly, EPA submitted this action 

to the Office of Management and Budget 

(OMB) for review under EO 12866 and 

any changes made in response to OMB 

recommendations have been 

documented in the docket for this 

action. In addition, EPA prepared a 

Regulatory Impact Analysis (RIA) of the 

potential costs and benefits associated 

with this action. However, the CAA and 

judicial decisions make clear that the 

economic and technical feasibility of 

attaining ambient standards are not to 

be considered in setting or revising 

NAAQS, although such factors may be 

considered in the development of State 

plans to implement the standards. 

Accordingly, although an RIA has been 

prepared, the results of the RIA have not 

been considered in developing this 

proposed rule. 

B. Paperwork Reduction Act 

The information collection 

requirements in this proposed rule have 

been submitted for approval to the 

Office of Management and Budget 

(OMB) under the Paperwork Reduction 

Act, 44 U.S.C. 3501 et seq. The 

Information Collection Request (ICR) 

document prepared by EPA for these 

proposed revisions to part 58 has been 

assigned EPA ICR number 2358.01. 

The information collected under 40 

CFR part 53 (e.g., test results, 

monitoring records, instruction manual, 

and other associated information) is 

needed to determine whether a 

candidate method intended for use in 

determining attainment of the National 

Ambient Air Quality Standards 

(NAAQS) in 40 CFR part 50 will meet 

the design, performance, and/or 

comparability requirements for 

designation as a Federal reference 

method (FRM) or Federal equivalent 

method (FEM). We do not expect the 

number of FRM or FEM determinations 

to increase over the number that is 

currently used to estimate burden 

associated with NO2 FRM/FEM 

determinations provided in the current 

ICR for 40 CFR part 53 (EPA ICR 

numbers 2358.01). As such, no change 

in the burden estimate for 40 CFR part 

53 has been made as part of this 

rulemaking. 

The information collected and 

reported under 40 CFR part 58 is needed 

to determine compliance with the 

NAAQS, to characterize air quality and 

associated health impacts, to develop 

emissions control strategies, and to 

measure progress for the air pollution 

program. The proposed amendments 

would revise the technical requirements 

for NO2 monitoring sites, require the 

siting and operation of additional NO2

ambient air monitors, and the reporting 

of the collected ambient NO2 monitoring 


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

The annual average reporting burden for 

the collection under 40 CFR part 58 

(averaged over the first 3 years of this 

ICR) is $3,616,487. Burden is defined at 

5 CFR 1320.3(b). State, local, and Tribal 

entities are eligible for State assistance 

grants provided by the Federal 

government under the CAA which can 

be used for monitors and related 

activities. 

An agency may not conduct or 

sponsor, and a person is not required to 

respond to, a collection of information 

unless it displays a currently valid OMB 

control number. The OMB control 

numbers for EPA’s regulations in 40 

CFR are listed in 40 CFR part 9. 

To comment on the Agency’s need for 

this information, the accuracy of the 

provided burden estimates, and any 

suggested methods for minimizing 

respondent burden, EPA has established 

a public docket for this rule, which 

includes this ICR, under Docket ID 

number EPA–HQ–OAR–2006–0922. 

Submit any comments related to the ICR 

to EPA and OMB. See ADDRESSES

section at the beginning of this notice 

for where to submit comments to EPA. 

Send comments to OMB at the Office of 

Information and Regulatory Affairs, 

Office of Management and Budget, 725 

17th Street, NW., Washington, DC 

20503, Attention: Desk Office for EPA. 

Since OMB is required to make a 

decision concerning the ICR between 30 

and 60 days after July 15, 2009, a 

comment to OMB is best assured of 

having its full effect if OMB receives it 

by August 14, 2009. The final rule will 

respond to any OMB or public 

comments on the information collection 

requirements contained in this proposal. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of this rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of this proposed rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This proposed rule will not impose any 

requirements on small entities. Rather, 

this rule establishes national standards 

for allowable concentrations of NO2 in 

ambient air as required by section 109 

of the CAA. American Trucking Assn’s 

v. EPA, 175 F. 3d 1027, 1044–45 (D.C. 

cir. 1999) (NAAQS do not have 

significant impacts upon small entities 

because NAAQS themselves impose no 

regulations upon small entities). 

Similarly, the proposed amendments to 

40 CFR part 58 address the requirements 

for States to collect information and 

report compliance with the NAAQS and 

will not impose any requirements on 

small entities. We continue to be 

interested in the potential impacts of the 

proposed rule on small entities and 

welcome comments on issues related to 

such impacts. 

D. Unfunded Mandates Reform Act 

Title II of the Unfunded Mandates 

Reform Act of 1995 (UMRA), Public 

Law 104–4, establishes requirements for 

Federal agencies to assess the effects of 

their regulatory actions on State, local, 

and Tribal governments and the private 

sector. Unless otherwise prohibited by 

law, under section 202 of the UMRA, 

EPA generally must prepare a written 

statement, including a cost-benefit 

analysis, for proposed and final rules 

with ‘‘Federal mandates’’ that may 

result in expenditures to State, local, 

and Tribal governments, in the 

aggregate, or to the private sector, of 

$100 million or more in any one year. 

Before promulgating an EPA rule for 

which a written statement is required 

under section 202, section 205 of the 

UMRA generally requires EPA to 

identify and consider a reasonable 

number of regulatory alternatives and to 

adopt the least costly, most costeffective or least burdensome alternative 

that achieves the objectives of the rule. 

The provisions of section 205 do not 

apply when they are inconsistent with 

applicable law. Moreover, section 205 

allows EPA to adopt an alternative other 

than the least costly, most cost-effective 

or least burdensome alternative if the 

Administrator publishes with the final 

rule an explanation why that alternative 

was not adopted. Before EPA establishes 

any regulatory requirements that may 

significantly or uniquely affect small 

governments, including Tribal 

governments, it must have developed 

under section 203 of the UMRA a small 

government agency plan. The plan must 

provide for notifying potentially 

affected small governments, enabling 

officials of affected small governments 

to have meaningful and timely input in 

the development of EPA regulatory 

proposals with significant Federal 

intergovernmental mandates, and 

informing, educating, and advising 

small governments on compliance with 

the regulatory requirements. 

This action is not subject to the 

requirements of sections 202 and 205 of 

the UMRA. EPA has determined that 

this proposed rule does not contain a 

Federal mandate that may result in 

expenditures of $100 million or more 

for State, local, and Tribal governments, 

in the aggregate, or the private sector in 

any one year. The revisions to the NO2

NAAQS impose no enforceable duty on 

any State, local or Tribal governments or 

the private sector. The expected costs 

associated with the monitoring 

requirements are described in EPA’s ICR 

document, but those costs are not 

expected to exceed $100 million in the 

aggregate for any year. Furthermore, as 

indicated previously, in setting a 

NAAQS, EPA cannot consider the 

economic or technological feasibility of 

attaining ambient air quality standards. 

Because the Clean Air Act prohibits 

EPA from considering the types of 

estimates and assessments described in 

section 202 when setting the NAAQS, 

the UMRA does not require EPA to 

prepare a written statement under 

section 202 for the revisions to the NO2

NAAQS. 

With regard to implementation 

guidance, the CAA imposes the 

obligation for States to submit SIPs to 

implement the NO2 NAAQS. In this 

proposed rule, EPA is merely providing 

an interpretation of those requirements. 

However, even if this rule did establish 

an independent obligation for States to 

submit SIPs, it is questionable whether 

an obligation to submit a SIP revision 

would constitute a Federal mandate in 

any case. The obligation for a State to 

submit a SIP that arises out of section 

110 and section 191 of the CAA is not 

legally enforceable by a court of law, 

and at most is a condition for continued 

receipt of highway funds. Therefore, it 

is possible to view an action requiring 

such a submittal as not creating any 

enforceable duty within the meaning of 

2 U.S.C. 658 for purposes of the UMRA. 

Even if it did, the duty could be viewed 

as falling within the exception for a 

condition of Federal assistance under 2 

U.S.C. 658. 


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EPA has determined that this 

proposed rule contains no regulatory 

requirements that might significantly or 

uniquely affect small governments 

because it imposes no enforceable duty 

on any small governments. Therefore, 

this rule is not subject to the 

requirements of section 203 of the 

UMRA. 

E. Executive Order 13132: Federalism 

Executive Order 13132, entitled 

‘‘Federalism’’ (64 FR 43255, August 10, 

1999), requires EPA to develop an 

accountable process to ensure 

‘‘meaningful and timely input by State 

and local officials in the development of 

regulatory policies that have federalism 

implications.’’ ‘‘Policies that have 

federalism implications’’ is defined in 

the Executive Order to include 

regulations that have ‘‘substantial direct 

effects on the States, on the relationship 

between the national government and 

the States, or on the distribution of 

power and responsibilities among the 

various levels of government.’’ 

This proposed rule does not have 

federalism implications. It will not have 

substantial direct effects on the States, 

on the relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. The rule does 

not alter the relationship between the 

Federal government and the States 

regarding the establishment and 

implementation of air quality 

improvement programs as codified in 

the CAA. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, CAA section 116 preserves the 

rights of States to establish more 

stringent requirements if deemed 

necessary by a State. Furthermore, this 

rule does not impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. Finally, 

as noted in section E (above) on UMRA, 

this rule does not impose significant 

costs on State, local, or Tribal 

governments or the private sector. Thus, 

Executive Order 13132 does not apply 

to this rule. 

However, EPA recognizes that States 

will have a substantial interest in this 

rule and any corresponding revisions to 

associated air quality surveillance 

requirements, 40 CFR part 58. 

Therefore, in the spirit of Executive 

Order 13132, and consistent with EPA 

policy to promote communications 

between EPA and State and local 

governments, EPA specifically solicits 

comment on this proposed rule from 

State and local officials. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

Executive Order 13175, entitled 

‘‘Consultation and Coordination with 

Indian Tribal Governments’’ (65 FR 

67249, November 9, 2000), requires EPA 

to develop an accountable process to 

ensure ‘‘meaningful and timely input by 

Tribal officials in the development of 

regulatory policies that have Tribal 

implications.’’ This proposed rule does 

not have Tribal implications, as 

specified in Executive Order 13175. It 

does not have a substantial direct effect 

on one or more Indian Tribes, on the 

relationship between the Federal 

government and Indian Tribes, or on the 

distribution of power and 

responsibilities between the Federal 

government and Tribes. The rule does 

not alter the relationship between the 

Federal government and Tribes as 

established in the CAA and the TAR. 

Under section 109 of the CAA, EPA is 

mandated to establish NAAQS; 

however, this rule does not infringe 

existing Tribal authorities to regulate air 

quality under their own programs or 

under programs submitted to EPA for 

approval. Furthermore, this rule does 

not affect the flexibility afforded to 

Tribes in seeking to implement CAA 

programs consistent with the TAR, nor 

does it impose any new obligation on 

Tribes to adopt or implement any 

NAAQS. Finally, as noted in section E 

(above) on UMRA, this rule does not 

impose significant costs on Tribal 

governments. Thus, Executive Order 

13175 does not apply to this rule. 

However, EPA recognizes that Tribes 

may be interested in this rule and any 

corresponding revisions to associated 

air quality surveillance requirements. 

Therefore, in the spirit of Executive 

Order 13175, and consistent with EPA 

policy to promote communications 

between EPA and Tribes, EPA 

specifically solicits additional comment 

on this proposed rule from Tribal 

officials. 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

This action is subject to Executive 

Order (62 FR 19885, April 23, 1997) 

because it is an economically significant 

regulatory action as defined by 

Executive Order 12866, and we believe 

that the environmental health risk 

addressed by this action has a 

disproportionate effect on children. The 

proposed rule will establish uniform 

national ambient air quality standards 

for NO2; these standards are designed to 

protect public health with an adequate 

margin of safety, as required by CAA 

section 109. The protection offered by 

these standards may be especially 

important for asthmatics, including 

asthmatic children, because respiratory 

effects in asthmatics are among the most 

sensitive health endpoints for NO2

exposure. Because asthmatic children 

are considered a sensitive population, 

we have evaluated the potential health 

effects of exposure to NO2 pollution 

among asthmatic children. These effects 

and the size of the population affected 

are discussed in chapters 3 and 4 of the 

ISA; chapters 3, 4, and 8 of the REA, 

and sections II.A through II.E of this 

preamble. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This rule is not a ‘‘significant energy 

action’’ as defined in Executive Order 

13211, ‘‘Actions Concerning Regulations 

That Significantly Affect Energy Supply, 

Distribution, or Use’’ (66 FR 28355 (May 

22, 2001)) because it is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this rule is to establish 

revised NAAQS for NO2. The rule does 

not prescribe specific control strategies 

by which these ambient standards will 

be met. Such strategies will be 

developed by States on a case-by-case 

basis, and EPA cannot predict whether 

the control options selected by States 

will include regulations on energy 

suppliers, distributors, or users. Thus, 

EPA concludes that this rule is not 

likely to have any adverse energy 

effects. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (NTTAA), Public Law 104– 

113, section 12(d) (15 U.S.C. 272 note) 

directs EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. The NTTAA directs EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

This proposed rulemaking involves 

technical standards with regard to 

ambient monitoring of NO2. The use of 

this voluntary consensus standard 

would be impractical because the 


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analysis method does not provide for 

the method detection limits necessary to 

adequately characterize ambient NO2

concentrations for the purpose of 

determining compliance with the 

proposed revisions to the NO2 NAAQS. 

EPA welcomes comments on this 

aspect of the proposed rule, and 

specifically invites the public to identify 

potentially applicable voluntary 

consensus standards and to explain why 

such standards should be used in the 

regulation. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629; 

Feb. 16, 1994) establishes Federal 

executive policy on environmental 

justice. Its main provision directs 

Federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this 

proposed rule will not have 

disproportionately high and adverse 

human health or environmental effects 

on minority or low-income populations 

because it increases the level of 

environmental protection for all affected 

populations without having any 

disproportionately high and adverse 

human health effects on any population, 

including any minority or low-income 

population. The proposed rule will 

establish uniform national standards for 

NO2 in ambient air. EPA solicits 

comment on environmental justice 

issues related to the proposed revision 

of the NO2 NAAQS. 

References 

Baldauf, R, Watkins N, Heist D, Bailey C, 

Rowley P, Shores R. (2009). Near-road air 

quality monitoring: Factors affecting 

network design and interpreation of data. 

Air Qual. Atmos. Health. 2:1–9. 

Barck, C, Sandstro ̈m T, Lundahl J, Hallde ́n 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, Hallde ́n G, Bylin G. 

(2005). Brief exposures to NO2 augment 

the allergic inflammation in asthmatics. 

Environ Res. 97:58–66. 

Beckerman, B, Jerrett M, Brook JR, Verma DK, 

Arain MA, Finkelstein MM. (2008). 

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List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: June 26, 2009. 

Lisa P. Jackson, 

Administrator. 

For the reasons stated in the 

preamble, title 40, chapter I of the Code 

of Federal Regulations is proposed to be 

amended as follows: 

PART 50—NATIONAL PRIMARY 

AMBIENT AIR QUALITY STANDARDS 

1. The authority citation for part 50 

continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

Subpart A—General Provisions 

2. Section 50.11 is revised to read as 

follows: 

§ 50.11 National primary and secondary 

ambient air quality standards for oxides of 

nitrogen (nitrogen dioxide). 

(a) The level of the national primary 

annual ambient air quality standard for 

oxides of nitrogen is 53 parts per billion 

(ppb, which is 1 part in 1,000,000,000), 

annual average concentration, measured 

in the ambient air as nitrogen dioxide. 

(b) The level of the national primary 

1-hour ambient air quality standard for 

oxides of nitrogen is (80–100) ppb, 1- 

hour average concentration, measured 

in the ambient air as nitrogen dioxide. 

(c) The level of the national secondary 

ambient air quality standard for nitrogen 

dioxide is 0.053 parts per million (100 

micrograms per cubic meter), annual 

arithmetic mean concentration. 

(d) The levels of the standards shall 

be measured by: 

(1) A reference method based on 

appendix F to this part; or 

(2) By a Federal equivalent method 

(FEM) designated in accordance with 

part 53 of this chapter. 

(e) The annual primary standard is 

met when the annual average 

concentration in a calendar year is less 

than or equal to 53 ppb, as determined 

in accordance with Appendix S of this 

part for the annual standard. 

(f) The 1-hour primary standard is met 

when the three-year average of the 

annual (99th percentile)(fourth highest) 

of the daily maximum 1-hour average 

concentration is less than or equal to 

(80–100) ppb, as determined in 

accordance with Appendix S of this part 

for the 1-hour standard. 

(g) The secondary standard is attained 

when the annual arithmetic mean 

concentration in a calendar year is less 

than or equal to 0.053 ppm, rounded to 

three decimal places (fractional parts 

equal to or greater than 0.0005 ppm 

must be rounded up). To demonstrate 

attainment, an annual mean must be 

based upon hourly data that are at least 

75 percent complete or upon data 

derived from manual methods that are 

at least 75 percent complete for the 

scheduled sampling days in each 

calendar quarter. 

3. Section 50.14 is amended by 

revising paragraph (c)(2)(vi) to read as 

follows: 

§ 50.14 Treatment of air quality monitoring 

data influenced by exceptional events. 

* * * * * 

(c) * * * 

(2) * * * 

(vi) When EPA sets a NAAQS for a 

new pollutant or revises the NAAQS for 

an existing pollutant, it may revise or 

set a new schedule for flagging 

exceptional event data, providing initial 

data descriptions and providing detailed 

data documentation in AQS for the 

initial designations of areas for those 

NAAQS: Table 1 provides the schedule 

for submission of flags with initial 

descriptions in AQS and detailed 

documentation and the schedule shall 

apply for those data which will or may 

influence the initial designation of areas 

for those NAAQS. EPA anticipates 

revising Table 1 as necessary to 

accommodate revised data submission 

schedules for new or revised NAAQS. 

TABLE 1—TO PARAGRAPH (C)(2)(VI): SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION

FOR DATA TO BE USED IN DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS pollutant/standard/(level)/ 

promulgation date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description 

deadline 

Detailed documentation submission 

deadline 

PM2.5/24-Hr Standard (35μg/m3) Promulgated October 17, 2006.2004–2006 October 1, 2007 a .................................. April 15, 2008. a

Ozone/8-Hr Standard (0.075 ppm) Promulgated March 12, 2008.2005–2007 June 18, 2009 b ..................................... June 18, 2009.b

2008 June 18, 2009 b ..................................... June 18, 2009.b

2009 60 Days after the end of the calendar 

quarter in which the event occurred 

or February 5, 2010, whichever date 

occurs first. b

60 Days after the end of the calendar 

quarter in which the event occurred 

or February 5, 2010, whichever date 

occurs first.b

NO2/1-Hour Standard (80–100 ppb, final 

level TBD).

2008 July 1, 2010 b ........................................ January 22, 2011. 

2009 July 1, 2010 ........................................... January 22, 2011. 

2010 April 1, 2011 b ........................................ July 1, 2011.b

aThese dates are unchanged from those published in the original rulemaking, and are shown in this table for informational purposes. b Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. 


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* * * * * 

4. Appendix S is added to read as 

follows: 

Option 1 for Appendix S to Part 50: 

Appendix S to Part 50—Interpretation 

of the Primary National Ambient Air 

Quality Standards for Oxides of 

Nitrogen (Nitrogen Dioxide) (1-Hour 

Primary Standard Based on the 4th 

Highest Daily Maximum Value Form) 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the primary 

national ambient air quality standards for 

oxides of nitrogen as measured by nitrogen 

dioxide (‘‘NO2 NAAQS’’) specified in § 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 from 

midnight to midnight (local standard time) 

that are used in NAAQS computations. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

specified in section 5 of this appendix. The 

design values for the primary NAAQS are: 

(1) The annual mean value for a monitoring 

site for one year (referred to as the ‘‘annual 

primary standard design value’’). 

(2) The 3-year average of annual 4th 

highest daily maximum 1-hour values for a 

monitoring site (referred to as the ‘‘1-hour 

primary standard design value’’). 

Annual 4th highest daily maximum 

1-hour value refers to the 4th highest daily 

1-hour maximum value at a site in a 

particular year. 

Quarter refers to a calendar quarter. 

Year refers to a calendar year. 

2. Requirements for Data Used for 

Comparisons With the NO2 NAAQS and 

Data Reporting Considerations 

(a) All valid FRM/FEM NO2 hourly data 

required to be submitted to EPA’s Air Quality 

System (AQS), or otherwise available to EPA, 

meeting the requirements of part 58 of this 

chapter including appendices A, C, and E 

shall be used in design value calculations. 

Multi-hour average concentration values 

collected by wet chemistry methods shall not 

be used. 

(b) When two or more NO2 monitors are 

operated at a site, the state may in advance 

designate one of them as the primary 

monitor. If the state has not made this 

designation in advance, the Administrator 

will make the designation, either in advance 

or retrospectively. Design values will be 

developed using only the data from the 

primary monitor, if this results in a valid 

design value. If data from the primary 

monitor do not allow the development of a 

valid design value, data solely from the other 

monitor(s) will be used in turn to develop a 

valid design value, if this results in a valid 

design value. If there are three or more 

monitors, the order for such comparison of 

the other monitors will be determined by the 

Administrator. The Administrator may 

combine data from different monitors in 

different years for the purpose of developing 

a valid 1-hour primary standard design value, 

if a valid design value cannot be developed 

solely with the data from a single monitor. 

However, data from two or more monitors in 

the same year at the same site will not be 

combined in an attempt to meet data 

completeness requirements, except if one 

monitor has physically replaced another 

instrument permanently, in which case the 

two instruments will be considered to be the 

same monitor, or if the state has switched the 

designation of the primary monitor from one 

instrument to another during the year. 

(c) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

3. Comparisons With the NO2 NAAQS 

3.1 The Annual Primary NO2 NAAQS 

(a) The annual primary NO2 NAAQS is met 

at a site when the valid annual primary 

standard design value is less than or equal to 

53 parts per billion (ppb). 

(b) An annual primary standard design 

value is valid when at least 75 percent of the 

hours in the year are reported. 

(c) An annual primary standard design 

value based on data that do not meet the 

completeness criteria stated in 3.1(b) may 

also be considered valid with the approval of, 

or at the initiative of, the Administrator, who 

may consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(d) The procedures for calculating the 

annual primary standard design values are 

given in section 5.1 of this appendix. 

3.2 The 1-Hour Primary NO2 NAAQS 

(a) The 1-hour primary NO2 NAAQS is met 

at a site when the valid 1-hour primary 

standard design value is less than or equal to 

[80–100] parts per billion (ppb). 

(b) An NO2 1-hour primary standard design 

value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all 4 quarters are complete. A quarter 

is complete when at least 75 percent of the 

sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3.2(b) of this 

appendix and thus would normally not be 

useable for the calculation of a valid 3-year 

1-hour primary standard design value, the 

3-year 1-hour primary standard design value 

shall nevertheless be considered valid if 

either of the following conditions is true. 

(i) If there are at least four days in each of 

the 3 years that have at least one reported 

hourly value, and the resulting 

3-year 1-hour primary standard design value 

exceeds the 1-hour primary NAAQS. In this 

situation, more complete data capture could 

not possibly have resulted in a design value 

below the 1-hour primary NAAQS. 

(ii)(A) A 1-hour primary standard design 

value that is below the level of the NAAQS 

can be validated if the substitution test in 

section 3.2(c)(ii)(B) results in a ‘‘test design 

value’’ that is below the level of the NAAQS. 

The test substitutes actual ‘‘high’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the calendar quarter) for 

unknown values that were not successfully 

measured. Note that the test is merely 

diagnostic in nature, intended to confirm that 

there is a very high likelihood that the 

original design value (the one with less than 

75 percent data capture of hours by day and 

of days by quarter) reflects the true underNAAQS-level status for that 3-year period; 

the result of this data substitution test (the 

‘‘test design value,’’ as defined in section 

3.2(c)(ii)(B)), is not considered the actual 

design value. For this test, substitution is 

permitted only if there are at least 200 days 

across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day have reported 

concentrations. However, maximum 1-hour 

values from days with less than 75 percent 

of the hours reported shall also be considered 

in identifying the high value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily maximum 

1-hour value for that quarter, looking across 

those three months of all three years under 

consideration. All daily maximum 1-hour 

values from all days in the quarter period 

shall be considered when identifying this 

highest value, including days with less than 

75 percent data capture. If after substituting 

the highest reported daily maximum 1-hour 

value for a quarter for as much of the missing 

daily data in the matching deficient 

quarter(s) as is needed to make them 100 

percent complete, the procedure in section 

5.2 yields a recalculated 3-year 1-hour 

standard ‘‘test design value’’ below the level 

of the standard, then the 1-hour primary 


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standard design value is deemed to have 

passed the diagnostic test and is valid, and 

the level of the standard is deemed to have 

been met in that 3-year period. As noted in 

section 3.2(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(d) A 1-hour primary standard design value 

based on data that do not meet the 

completeness criteria stated in section 3.2(b) 

and also do not satisfy section 3.2(c), may 

also be considered valid with the approval of, 

or at the initiative of, the Administrator, who 

may consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 1- 

hour primary standard design values are 

given in section 5.2 of this appendix. 

4. Rounding Conventions 

4.1 Rounding Conventions for the Annual 

Primary NO2 NAAQS 

(a) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) The annual primary standard design 

value is calculated pursuant to section 5.1 

and then rounded to the nearest whole 

number or 1 ppb (decimals 0.5 and greater 

are rounded up to the nearest whole number, 

and any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

4.2 Rounding Conventions for the 1-Hour 

Primary NO2 NAAQS 

(a) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily maximum 1-hour values, 

including the annual 4th highest of those 

daily values, are not rounded. 

(c) The 1-hour primary standard design 

value is calculated pursuant to section 5.2 

and then rounded to the nearest whole 

number or 1 ppb (decimals 0.5 and greater 

are rounded up to the nearest whole number, 

and any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

5. Calculation Procedures for the Primary 

NO2 NAAQS 

5.1 Calculation Procedures for the Annual 

Primary NO2 NAAQS 

(a) When the data for a site and year meet 

the data completeness requirements in 

section 3.1(b) of this appendix, or if the 

Administrator exercises the discretionary 

authority in section 3.1(c), the annual mean 

is simply the arithmetic average of all of the 

reported 1-hour values. 

(b) The annual primary standard design 

value for a site is the valid annual mean 

rounded according to the conventions in 

section 4.1. 

5.2 Calculation Procedures for the 1-Hour 

Primary NO2 NAAQS 

(a) When the data for a particular site and 

year meet the data completeness 

requirements in section 3.2(b), or if one of the 

conditions of section 3.2(c) is met, or if the 

Administrator exercises the discretionary 

authority in section 3.2(d), calculation of the 

4th highest daily 1-hour maximum is 

accomplished as follows. 

(i) For each year, select from each day the 

highest hourly value. All daily maximum 1- 

hour values from all days in the quarter 

period shall be considered at this step, 

including days with less than 75 percent data 

capture. 

(ii) For each year, order these daily values 

and take the 4th highest. 

(iii) The 1-hour primary standard design 

value for a site is mean of the three annual 

4th highest values, rounded according to the 

conventions in section 4.2. 

Option 2 for Appendix S to Part 50: 

Appendix S to Part 50—Interpretation 

of the Primary National Ambient Air 

Quality Standards for Oxides of 

Nitrogen (Nitrogen Dioxide) (1-Hour 

Primary Standard Based on the 99th 

Percentile Form) 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the primary 

national ambient air quality standards for 

oxides of nitrogen as measured by nitrogen 

dioxide (‘‘NO2 NAAQS’’) specified in § 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 from 

midnight to midnight (local standard time) 

that are used in NAAQS computations. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

specified in section 5 of this appendix. The 

design values for the primary NAAQS are: 

(1) The annual mean value for a monitoring 

site for one year (referred to as the ‘‘annual 

primary standard design value’’). 

(2) The 3-year average of annual 99th 

percentile daily maximum 1-hour values for 

a monitoring site (referred to as the ‘‘1-hour 

primary standard design value’’). 

99th percentile daily maximum 1-hour 

value is the value below which nominally 99 

percent of all daily maximum 1-hour 

concentration values fall, using the ranking 

and selection method specified in section 5.2 

of this appendix. 

Quarter refers to a calendar quarter. 

Year refers to a calendar year. 

2. Requirements for Data Used for 

Comparisons With the NO2 NAAQS and 

Data Reporting Considerations 

(a) All valid FRM/FEM NO2 hourly data 

required to be submitted to EPA’s Air Quality 

System (AQS), or otherwise available to EPA, 

meeting the requirements of part 58 of this 

chapter including appendices A, C, and E 

shall be used in design value calculations. 

Multi-hour average concentration values 

collected by wet chemistry methods shall not 

be used. 

(b) When two or more NO2 monitors are 

operated at a site, the state may in advance 

designate one of them as the primary 

monitor. If the state has not made this 

designation, the Administrator will make the 

designation, either in advance or 

retrospectively. Design values will be 

developed using only the data from the 

primary monitor, if this results in a valid 

design value. If data from the primary 

monitor do not allow the development of a 

valid design value, data solely from the other 

monitor(s) will be used in turn to develop a 

valid design value, if this results in a valid 

design value. If there are three or more 

monitors, the order for such comparison of 

the other monitors will be determined by the 

Administrator. The Administrator may 

combine data from different monitors in 

different years for the purpose of developing 

a valid 1-hour primary standard design value, 

if a valid design value cannot be developed 

solely with the data from a single monitor. 

However, data from two or more monitors in 

the same year at the same site will not be 

combined in an attempt to meet data 

completeness requirements, except if one 

monitor has physically replaced another 

instrument permanently, in which case the 

two instruments will be considered to be the 

same monitor, or if the state has switched the 

designation of the primary monitor from one 

instrument to another during the year. 

(c) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

3. Comparisons With the NO2 NAAQS 

3.1 The Annual Primary NO2 NAAQS 

(a) The annual primary NO2 NAAQS is met 

at a site when the valid annual primary 

standard design value is less than or equal to 

53 parts per billion (ppb). 

(b) An annual primary standard design 

value is valid when at least 75 percent of the 

hours in the year are reported. 

(c) An annual primary standard design 

value based on data that do not meet the 

completeness criteria stated in section 3.1(b) 

may also be considered valid with the 

approval of, or at the initiative of, the 


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Administrator, who may consider factors 

such as monitoring site closures/moves, 

monitoring diligence, the consistency and 

levels of the valid concentration 

measurements that are available, and nearby 

concentrations in determining whether to use 

such data. 

(d) The procedures for calculating the 

annual primary standard design values are 

given in section 5.1 of this appendix. 

3.2 The 1-Hour Primary NO2 NAAQS 

(a) The 1-hour primary NO2 NAAQS is met 

at a site when the valid 1-hour primary 

standard design value is less than or equal to 

[80–100] parts per billion (ppb). 

(b) An NO2 1-hour primary standard design 

value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all 4 quarters are complete. A quarter 

is complete when at least 75 percent of the 

sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3.2(b) of this 

appendix and thus would normally not be 

useable for the calculation of a valid 3-year 

1-hour primary standard design value, the 

3-year 1-hour primary standard design value 

shall nevertheless be considered valid if one 

of the following conditions is true. 

(i) At least 75 percent of the days in each 

quarter of each of three consecutive years 

have at least one reported hourly value, and 

the design value calculated according to the 

procedures specified in section 5.2 is above 

the level of the primary 1-hour standard. 

(ii)(A) A 1-hour primary standard design 

value that is below the level of the NAAQS 

can be validated if the substitution test in 

section 3.2(c)(ii)(B) results in a ‘‘test design 

value’’ that is below the level of the NAAQS. 

The test substitutes actual ‘‘high’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the same calendar quarter) 

for unknown values that were not 

successfully measured. Note that the test is 

merely diagnostic in nature, intended to 

confirm that there is a very high likelihood 

that the original design value (the one with 

less than 75 percent data capture of hours by 

day and of days by quarter) reflects the true 

under-NAAQS-level status for that 3-year 

period; the result of this data substitution test 

(the ‘‘test design value’’, as defined in section 

3.2(c)(ii)(B)) is not considered the actual 

design value. For this test, substitution is 

permitted only if there are at least 200 days 

across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day have reported 

concentrations. However, maximum 1-hour 

values from days with less than 75 percent 

of the hours reported shall also be considered 

in identifying the high value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily maximum 

1-hour value for that quarter, looking across 

those three months of all three years under 

consideration. All daily maximum 1-hour 

values from all days in the quarter period 

shall be considered when identifying this 

highest value, including days with less than 

75 percent data capture. If after substituting 

the highest reported daily maximum 1-hour 

value for a quarter for as much of the missing 

daily data in the matching deficient 

quarter(s) as is needed to make them 100 

percent complete, the procedure in section 

5.2 yields a recalculated 3-year 1-hour 

standard ‘‘test design value’’ below the level 

of the standard, then the 1-hour primary 

standard design value is deemed to have 

passed the diagnostic test and is valid, and 

the level of the standard is deemed to have 

been met in that 3-year period. As noted in 

section 3.2(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(iii)(A) A 1-hour primary standard design 

value that is above the level of the NAAQS 

can be validated if the substitution test in 

section 3.2(c)(iii)(B) results in a ‘‘test design 

value’’ that is above the level of the NAAQS. 

The test substitutes actual ‘‘low’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the same three months of the 

calendar) for unknown values that were not 

successfully measured. Note that the test is 

merely diagnostic in nature, intended to 

confirm that there is a very high likelihood 

that the original design value (the one with 

less than 75 percent data capture of hours by 

day and of days by quarter) reflects the true 

above-NAAQS-level status for that 3-year 

period; the result of this data substitution test 

(the ‘‘test design value,’’ as defined in section 

3.2(c)(iii)(B)) is not considered the actual 

design value. For this test, substitution is 

permitted only if there are a minimum 

number of available daily data points from 

which to identify the low quarter-specific 

daily maximum 1-hour values, specifically if 

there are at least 200 days across the three 

matching quarters of the three years under 

consideration (which is about 75 percent of 

all possible daily values in those three 

quarters) for which 75 percent of the hours 

in the day have reported concentrations. 

Only days with at least 75 percent of the 

hours reported shall be considered in 

identifying the low value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture. Identify for each quarter (e.g., 

January–March) the lowest reported daily 

maximum 1-hour value for that quarter, 

looking across those three months of all three 

years under consideration. All daily 

maximum 1-hour values from all days with 

at least 75 percent capture in the quarter 

period shall be considered when identifying 

this lowest value. If after substituting the 

lowest reported daily maximum 1-hour value 

for a quarter for as much of the missing daily 

data in the matching deficient quarter(s) as is 

needed to make them 75 percent complete, 

the procedure in section 5.2 yields a 

recalculated 3-year 1-hour standard ‘‘test 

design value’’ above the level of the standard, 

then the 1-hour primary standard design 

value is deemed to have passed the 

diagnostic test and is valid, and the level of 

the standard is deemed to have been 

exceeded in that 3-year period. As noted in 

section 3.2(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(d) A 1-hour primary standard design value 

based on data that do not meet the 

completeness criteria stated in 3.2(b) and also 

do not satisfy section 3.2(c), may also be 

considered valid with the approval of, or at 

the initiative of, the Administrator, who may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 1- 

hour primary standard design values are 

given in section 5.2 of this appendix. 

4. Rounding Conventions 

4.1 Rounding Conventions for the Annual 

Primary NO2 NAAQS 

(a) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) The annual primary standard design 

value is calculated pursuant to section 5.1 

and then rounded to the nearest whole 

number or 1 ppb (decimals 0.5 and greater 

are rounded up to the nearest whole number, 

and any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

4.2 Rounding Conventions for the 1-Hour 

Primary NO2 NAAQS 

(a) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily maximum 1-hour values and 

therefore the annual 4th highest of those 

daily values are not rounded. 

(c) The 1-hour primary standard design 

value is calculated pursuant to section 5.2 

and then rounded to the nearest whole 

number or 1 ppb (decimals 0.5 and greater 

are rounded up to the nearest whole number, 

and any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

5. Calculation Procedures for the Primary 

NO2 NAAQS 

5.1 Procedures for the Annual Primary NO2

NAAQS 

(a) When the data for a site and year meet 

the data completeness requirements in 

section 3.1(b) of this appendix, or if the 

Administrator exercises the discretionary 

authority in section 3.1(c), the annual mean 

is simply the arithmetic average of all of the 

reported 1-hour values. 


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(b) The annual primary standard design 

value for a site is the valid annual mean 

rounded according to the conventions in 

section 4.1. 

5.2 Calculation Procedures for the 1-Hour 

Primary NO2 NAAQS 

(a) Procedure for identifying annual 99th 

percentile values. When the data for a 

particular site and year meet the data 

completeness requirements in section 3.2(b), 

or if one of the conditions of section 3.2(c) 

is met, or if the Administrator exercises the 

discretionary authority in section 3.2(d), 

identification of annual 99th percentile 

values will be based on the number of days 

with at least 75 percent of the hourly values 

reported. 

(i) For the year, from only the days with 

at least 75 percent of the hourly values 

reported, select from each day the highest 

hourly value. 

(ii) Sort all the valid daily values from a 

particular site and year by descending value. 

(For example: (x[1], x[2], x[3], * * *, x[n]). 

In this case, x[1] is the largest number and 

x[n] is the smallest value.) The 99th 

percentile is determined from this sorted 

series of daily values which is ordered from 

the highest to the lowest number. Using the 

left column of Table 1, determine the 

appropriate range (i.e., row) for the annual 

number of days with valid data for year y 

(cny). The corresponding ‘‘n’’ value in the 

right column identifies the rank of the annual 

99th percentile value in the descending 

sorted list of daily site values for year y. 

Thus, P0.99, y= the nth largest value. 

TABLE 1—TO SECTION 5.2(A)(II) 

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 for a site is mean of the three annual 

4th highest values, rounded according to the 

conventions in section 4.2. 

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

5. The authority citation for part 58 

continues to read as follows: 

Authority: 42 U.S.C. 7403, 7410, 7601(a), 

7611, and 7619. 

Subpart A [AMENDED] 

6. Section 58.1 is amended by adding 

definitions for ‘‘AADP’’ and ‘‘Near-road 

NO2 Monitor’’ in alphabetical order to 

read as follows: 

§ 58.1 Definitions. 

* * * * * 

AADT means the annual average daily 

traffic. 

* * * * * 

Near-road NO2 Monitor means any 

NO2 monitor meeting the specifications 

in 4.3.2 of Appendix D and paragraphs 

2, 4(b), 6.1, and 6.4 of Appendix E of 

this part. 

* * * * * 

Subpart B [AMENDED] 

7. Section 58.10, is amended by 

adding paragraphs (a)(5) and (b)(12) to 

read as follows: 

§ 58.10 Annual monitoring network plan 

and periodic network assessment. 

(a) * * * 

(5) A plan for establishing NO2

monitoring sites in accordance with the 

requirements of appendix D to this part 

shall be submitted to the Administrator 

by July 1, 2011. The plan shall provide 

for all required stations to be 

operational by January 1, 2013. 

* * * * * 

(b) * * * 

(12) The identification of required 

NO2 monitors as either near-road or 

area-wide sites in accordance with 

Appendix D, Section 4.3 of this part. 

* * * * * 

8. Section 58.13 is amended by 

adding paragraph (c) to read as follows: 

§ 58.13 Monitoring network completion. 

* * * * * 

(c) The network of NO2 monitors must 

be physically established no later than 

January 1, 2013, and at that time, must 

be operating under all of the 

requirements of this part, including the 

requirements of appendices A, C, D, E, 

and G to this part. 

9. Section 58.16 is amended by 

revising paragraph (a) to read as follows: 

§ 58.16 Data submittal and archiving 

requirements. 

(a) The State, or where appropriate, 

local agency, shall report to the 

Administrator, via AQS all ambient air 

quality data and associated quality 

assurance data for SO2; CO; O3; NO2; 

NO; NOY; NOX; Pb-TSP mass 

concentration; Pb-PM10 mass 

concentration; PM10 mass concentration; 

PM2.5 mass concentration; for filterbased PM2.5FRM/FEM the field blank 

mass, sampler-generated average daily 

temperature, and sampler-generated 

average daily pressure; chemically 

speciated PM2.5 mass concentration 

data; PM10–2.5 mass concentration; 

chemically speciated PM10–2.5 mass 

concentration data; meteorological data 

from NCore, PAMS, and near-road NO2

monitoring sites; average daily 

temperature and average daily pressure 

for Pb sites if not already reported from 

sampler generated records; and 

metadata records and information 

specified by the AQS Data Coding 

Manual (http://www.epa.gov/ttn/airs/ 

airsaqs/manuals/manuals.htm). The 

State, or where appropriate, local 

agency, may report site specific 

meteorological measurements generated 

by onsite equipment (meteorological 

instruments, or sampler generated) or 

measurements from the nearest airport 

reporting ambient pressure and 

temperature. Such air quality data and 

information must be submitted directly 

to the AQS via electronic transmission 

on the specified quarterly schedule 

described in paragraph (b) of this 

section. 

* * * * * 

10. Appendix A to Part 58 is amended 

as by adding section 2.3.1.5 to read as 

follows: 

Appendix A to Part 58—Quality 

Assurance Requirements for SLAMS, 

SPMs and PSD Air Monitoring 

* * * * * 

2.3.1.5 Measurement Uncertainty for 

NO2. The goal for acceptable measurement 

uncertainty is defined for precision as an 

upper 90 percent confidence limit for the 

coefficient of variation (CV) of 15 percent and 

for bias as an upper 95 percent confidence 

limit for the absolute bias of 15 percent. 

* * * * * 

11. Appendix C to Part 58 is amended 

as by adding section 2.1.1 to read as 

follows: 

Appendix C to Part 58—Ambient Air 

Quality Monitoring Methodology 

* * * * * 

2.1.1 Any NO2 FRM or FEM used for 

making primary NAAQS decisions must be 

capable of providing hourly averaged 

concentration data. 

* * * * * 

12. Appendix D to Part 58 is amended 

by revising section 4.3 to read as 

follows: 

Appendix D to Part 58—Network 

Design Criteria for Ambient Air Quality 

Monitoring 

* * * * * 

4.3 Nitrogen Dioxide (NO2) Design Criteria 

4.3.1 General Requirements. (a) State and, 

where appropriate, local agencies must 

operate a minimum number of required NO2

monitoring sites as described below. 

4.3.2 Requirement for Near-road NO2

Monitors. (a) Within the NO2 network, there 

must be one microscale near-road NO2

monitoring station in each CBSA with a 

population of 350,000 or more persons to 

monitor a location of expected maximum 

hourly concentrations sited near a major road 

with high AADT counts as specified in 


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paragraph 4.3.2(a)(1) of this appendix. An 

additional near-road NO2 monitoring station 

is required for any CBSA with a population 

of 2,500,000 persons or more, or in any CBSA 

with a population of 350,000 or more persons 

that has one or more roadway segments with 

250,000 or greater AADT counts to monitor 

a second location of expected maximum 

hourly concentrations. CBSA populations 

shall be based on the latest available census 

figures. 

(1) The near-road NO2 monitoring stations 

shall be selected by ranking all road segments 

within a CBSA by AADT and then 

identifying a location or locations adjacent to 

those highest ranked road segments where 

maximum hourly NO2 concentrations are 

expected to be highest and siting criteria can 

be met in accordance with appendix E of this 

part. Where a state or local air monitoring 

agency identifies multiple acceptable 

candidate sites where maximum hourly NO2

concentrations are expected to occur, the 

monitoring agency should consider taking 

into account the potential for population 

exposure in the criteria utilized to select the 

final site location. Where one CBSA is 

required to have two near-road NO2

monitoring stations, the sites shall be 

differentiated from each other by one or more 

of the following factors: fleet mix; congestion 

patterns; terrain; geographic area within the 

CBSA; or different route, interstate, or 

freeway designation. 

(b) Measurements at required near-road 

NO2 monitor sites must include at a 

minimum: NO, NO2, NOX, wind vector data 

in the horizontal and vertical planes, ambient 

temperature, and ambient relative humidity. 

4.3.3 Requirement for Area-wide NO2

Monitoring. (a) Within the NO2 network, 

there must be one monitoring station in each 

CBSA with a population of 1,000,000 or more 

persons to monitor a location of expected 

highest NO2 concentrations representing the 

neighborhood or larger spatial scales. PAMS 

sites collecting NO2 data that are situated in 

an area of expected high NO2 concentrations 

at the neighborhood or larger spatial scale 

may be used to satisfy this minimum 

monitoring requirement when the NO2

monitor is operated year round. Emission 

inventories and meteorological analysis 

should be used to identify the appropriate 

locations within a CBSA for locating required 

area-wide NO2 monitoring stations. CBSA 

populations shall be based on the latest 

available census figures. 

4.3.4 Regional Administrator Required 

Monitoring. (a) The Regional Administrator 

may require additional NO2 monitoring 

stations above the minimum requirements to 

monitor in locations away from roads, or 

sites that do not meet near-road NO2 monitor 

siting criteria noted in appendix E of this 

part, where required near-road monitors do 

not represent a location or locations where 

the expected maximum hourly NO2

concentrations exist in a CBSA. The Regional 

Administrator may also require additional 

near-road NO2 monitoring stations above the 

minimum required in situations where the 

minimum monitoring requirements are not 

sufficient to meet monitoring objectives, and 

may consider additional locations of 

expected high NO2 concentrations and the 

variety of exposure potential due to increased 

variety in amount or types of fleet mix, 

congestion patterns, terrain, or geographic 

areas within a CBSA. The Regional 

Administrator and the responsible State or 

local air monitoring agency should work 

together to design and/or maintain the most 

appropriate NO2 network to service the 

variety of data needs for an area. 

(b) The Regional Administrator may 

require additional NO2 monitoring stations 

for area-wide NO2 monitors at the 

neighborhood and larger spatial scales above 

the minimum monitoring requirements 

where the minimum monitoring 

requirements are not sufficient to meet 

monitoring objectives for an area, such as 

supporting photochemical pollutant 

assessment, air quality forecasting, PM 

precursor analysis, and characterizing 

impacts of NO2 sources on certain 

communities. The Regional Administrator 

and the responsible State or local air 

monitoring agency should work together to 

design and/or maintain the most appropriate 

NO2 network to service the variety of data 

needs for an area. 

4.3.5 NO2 Monitoring Spatial Scales. (a) 

The most important spatial scale for nearroad NO2 monitoring stations to effectively 

characterize the maximum expected hourly 

NO2 concentration due to mobile source 

emissions on major roadways is the 

microscale. The most important spatial scales 

for other monitoring stations characterizing 

maximum expected hourly NO2

concentrations are the microscale and middle 

scale. The most important spatial scale for 

area-wide monitoring of high NO2

concentrations is the neighborhood scale. 

(1) Microscale—This scale would typify 

areas in close proximity to major roadways 

or point and area sources. Emissions from 

roadways result in high ground level NO2

concentrations at the microscale, where 

concentration gradients generally exhibit a 

marked decrease with increasing downwind 

distance from major roads. As noted in 

appendix E of this part, near-road NO2

monitoring stations are required to be within 

50 meters of target road segments in order to 

measure expected peak concentrations. 

Emissions from stationary point and area 

sources, and non-road sources may, under 

certain plume conditions, result in high 

ground level concentrations at the 

microscale. The microscale typically 

represents an area impacted by the plume 

with dimensions extending up to 

approximately 100 meters. 

(2) Middle scale—This scale generally 

represents air quality levels in areas up to 

several city blocks in size with dimensions 

on the order of approximately 100 meters to 

500 meters. The middle scale may include 

locations of expected maximum hourly 

concentrations due to proximity to major 

NO2 point, area, and/or non-road sources. 

(3) Neighborhood scale—The 

neighborhood scale would characterize air 

quality conditions throughout some 

relatively uniform land use areas with 

dimensions in the 0.5 to 4.0 kilometer range. 

Emissions from stationary point and area 

sources may, under certain plume 

conditions, result in high NO2 concentrations 

at the neighborhood scale. Where a 

neighborhood site is located away from 

immediate NO2 sources, the site may be 

useful in representing typical air quality 

values for a larger residential area, and 

therefore suitable for population exposure 

and trends analyses. 

(4) Urban scale—Measurements in this 

scale would be used to estimate 

concentrations over large portions of an 

urban area with dimensions from 4 to 50 

kilometers. Such measurements would be 

useful for assessing trends in area-wide air 

quality, and hence, the effectiveness of largescale air pollution control strategies. Urban 

scale sites may also support other monitoring 

objectives of the NO2 monitoring network 

identified in paragraph 4.3.4 above. 

4.3.6 NOy Monitoring. (a) NO/NOy

measurements are included within the NCore 

multipollutant site requirements and the 

PAMS program. These NO/NOy

measurements will produce conservative 

estimates for NO2 that can be used to ensure 

tracking continued compliance with the NO2

NAAQS. NO/NOy monitors are used at these 

sites because it is important to collect data 

on total reactive nitrogen species for 

understanding O3 photochemistry. 

* * * * * 

13. Section Appendix E to part 58 is 

amended as follows: 

a. By revising section 2. 

b. By adding paragraph (d) to section 

4. 

c. By revising section 6.1. 

d. By adding section 6.4. 

e. By revising section 11 including 

Table E–4. 

Appendix E to Part 58—Probe and 

Monitoring Path Siting Criteria for 

Ambient Air Quality Monitoring 

* * * * * 

2. Horizontal and Vertical Placement 

The probe or at least 80 percent of the 

monitoring path must be located between 2 

and 15 meters above ground level for all 

ozone and sulfur dioxide monitoring sites, 

and for neighborhood or larger spatial scale 

Pb, PM10, PM10–2.5, PM2.5, NO2 and carbon 

monoxide sites. Middle scale PM10–2.5 sites 

are required to have sampler inlets between 

2 and 7 meters above ground level. 

Microscale Pb, PM10, PM10–2.5 and PM2.5 sites 

are required to have sampler inlets between 

2 and 7 meters above ground level. 

Microscale near-road NO2 monitoring sites 

are required to have sampler inlets between 

2 and 7 meters above ground level. The inlet 

probes for microscale carbon monoxide 

monitors that are being used to measure 

concentrations near roadways must be 3±1⁄2

meters above ground level. The probe or at 

least 90 percent of the monitoring path must 

be at least 1 meter vertically or horizontally 

away from any supporting structure, walls, 

parapets, penthouses, etc., and away from 

dusty or dirty areas. If the probe or a 

significant portion of the monitoring path is 

located near the side of a building or wall, 

then it should be located on the windward 

side of the building relative to the prevailing 

wind direction during the season of highest 


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Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules 34465 

concentration potential for the pollutant 

being measured. 

* * * * * 

4. Spacing From Obstructions 

* * * * * 

(d) For near-road NO2 monitoring stations, 

the monitor probe shall have an unobstructed 

air flow, where no obstacles exist at or above 

the height of the monitor probe, between the 

monitor probe and the outside nearest edge 

of the traffic lanes of the target road segment. 

* * * * * 

6. * * * 

6.1 Spacing for Ozone Probes and 

Monitoring Paths. In siting an O3 analyzer, it 

is important to minimize destructive 

interferences form sources of NO, since NO 

readily reacts with O3. Table E–1 of this 

appendix provides the required minimum 

separation distances between a roadway and 

a probe or, where applicable, at least 90 

percent of a monitoring path for various 

ranges of daily roadway traffic. A sampling 

site having a point analyzer probe located 

closer to a roadway than allowed by the 

Table E–1 requirements should be classified 

as microscale or middle scale, rather than 

neighborhood or urban scale, since the 

measurements from such a site would more 

closely represent the middle scale. If an open 

path analyzer is used at a site, the monitoring 

path(s) must not cross over a roadway with 

an average daily traffic count of 10,000 

vehicles per day or more. For those situations 

where a monitoring path crosses a roadway 

with fewer than 10,000 vehicles per day, 

monitoring agencies must consider the entire 

segment of the monitoring path in the area 

of potential atmospheric interference from 

automobile emissions. Therefore, this 

calculation must include the length of the 

monitoring path over the roadway plus any 

segments of the monitoring path that lie in 

the area between the roadway and minimum 

separation distance, as determined from 

Table E–1 of this appendix. The sum of these 

distances must not be greater than 10 percent 

of the total monitoring path length. 

* * * * * 

6.4 Spacing for Nitrogen Dioxide (NO2) 

Probes and Monitoring Paths (a) In siting 

near-road NO2 monitors as required in 

paragraph 4.3.2 of appendix D of this part, 

the monitor probe shall be as near as 

practicable to the outside nearest edge of the 

traffic lanes of the target road segment; but 

shall not be located at a distance greater than 

50 meters, in the horizontal, from the outside 

nearest edge of the traffic lanes of the target 

road segment. 

(b) In siting NO2 monitors for 

neighborhood and larger scale monitoring, it 

is important to minimize near-road 

influences. Table E–1 of this appendix 

provides the required minimum separation 

distances between a roadway and a probe or, 

where applicable, at least 90 percent of a 

monitoring path for various ranges of daily 

roadway traffic. A sampling site having a 

point analyzer probe located closer to a 

roadway than allowed by the Table E–1 

requirements should be classified as 

microscale or middle scale rather than 

neighborhood or urban scale. If an open path 

analyzer is used at a site, the monitoring 

path(s) must not cross over a roadway with 

an average daily traffic count of 10,000 

vehicles per day or more. For those situations 

where a monitoring path crosses a roadway 

with fewer than 10,000 vehicles per day, 

monitoring agencies must consider the entire 

segment of the monitoring path in the area 

of potential atmospheric interference form 

automobile emissions. Therefore, this 

calculation must include the length of the 

monitoring path over the roadway plus any 

segments of the monitoring path that lie in 

the area between the roadway and minimum 

separation distance, as determined from 

Table E–1 of this appendix. The sum of these 

distances must not be greater than 10 percent 

of the total monitoring path length. 

* * * * * 

11. Summary 

Table E–4 of this appendix presents a 

summary of the general requirements for 

probe and monitoring path siting criteria 

with respect to distances and heights. It is 

apparent from Table E–4 that different 

elevation distances above the ground are 

shown for the various pollutants. The 

discussion in this appendix for each of the 

pollutants describes reasons for elevating the 

monitor, probe, or monitoring path. The 

differences in the specified range of heights 

are based on the vertical concentration 

gradients. For CO and near-road NO2

monitors, the gradients in the vertical 

direction are very large for the microscale, so 

a small range of heights are used. The upper 

limit of 15 meters is specified for the 

consistency between pollutants and to allow 

the use of a single manifold or monitoring 

path for monitoring more than one pollutant. 

TABLE E–4 OF APPENDIX E TO PART 58—SUMMARY OF PROBE AND MONITORING PATH SITING CRITERIA

Pollutant Scale (maximum monitoring path 

length, meters) 

Height from 

ground to 

probe, inlet or 

80% of monitoring path 1

Horizontal and 

vertical distance 

from supporting 

structures 2 to 

probe, inlet or 

90% of monitoring path 1

(meters) 

Distance from 

trees to probe, 

inlet or 90% of 

monitoring 

path 1 (meters) 

Distance from roadways to 

probe, inlet or monitoring path 1

(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).31⁄2: 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 (Near-road [50–300]) ........ 2–7 (micro) ....... > 1 .................... > 10 .................. ≤ 50 meters for near-road 

microscale. 

Middle (300m) ............................ 2–15 (all other 

scales).

Neighborhood, Urban, and Regional (1 km).See Table E–1 of this appendix 

for all other scales. 

Ozone precursors 

(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 E–1 of 

this appendix for all other 

scales. 

N/A—Not applicable. 

1 Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and 

regional scale NO2 monitoring, and all applicable scales for monitoring SO2,O3, and O3 precursors. 2When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof.

3Should 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. 


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34466 Federal Register / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules 

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. 6The 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. 7For microscale CO monitoring sites, the probe must be > 10 meters from a street intersection and preferably at a midblock location. 8 Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 200 liters/min or at least 1 

meter apart for samplers having flow rates less than 200 liters/min to preclude airflow interference. 

* * * * * 

14. Appendix G to Part 58 is amended 

by revising section 9 and table 2 to read 

as follows: 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

* * * * * 

9. How Does the AQI Relate to Air Pollution 

Levels? 

For each pollutant, the AQI transforms 

ambient concentrations to a scale from 0 to 

500. The AQI is keyed as appropriate to the 

national ambient air quality standards 

(NAAQS) for each pollutant. In most cases, 

the index value of 100 is associated with the 

numerical level of the short-term (i.e., 

averaging time of 24-hours or less) standard 

for each pollutant. The index value of 50 is 

associated with one of the following: The 

numerical level of the annual standard for a 

pollutant, if there is one; one-half the level 

of the short-term standard for the pollutant; 

or the level at which it is appropriate to begin 

to provide guidance on cautionary language. 

Higher categories of the index are based on 

increasingly serious health effects that affect 

increasing proportions of the population. An 

index value is calculated each day for each 

pollutant (as described in section 12 of this 

appendix), unless that pollutant is 

specifically excluded (see section 8 of this 

appendix). The pollutant with the highest 

index value for the day is the ‘‘critical’’ 

pollutant, and must be included in the daily 

AQI report. As a result, the AQI for any given 

day is equal to the index value of the critical 

pollutant for that day. For the purposes of 

reporting the AQI, the indexes for PM10 and 

PM2.5 are to be considered separately. 

* * * * * 

TABLE 2—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQI’s 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour1

PM2.5

(μg/m3) 

PM10

(μg/m3) CO (ppm) SO2

(ppm) 

NO2 (ppm) 

1-hour AQI Category 

0.000–0.059 ............................ .................. 0.0–15.4 0–54 0.0–4.4 0.000–0.034 0–(0.040– 

0.053) 

0–50 Good. 

0.060–0.075 ............................ .................. 15.5–40.4 55–154 4.5–9.4 0.035–0.144 (0.041– 

0.054)– 

(0.080–0.100) 

51–100 Moderate. 

0.076–0.095 ............................ 0.125–0.164 40.5–65.4 155–254 9.5–12.4 0.145–0.224 (0.081– 

0.101)– 

(0.360–0.370) 

101–150 Unhealthy for Sensitive 

Groups. 

0.096–0.115 ............................ 0.165–0.204 3 65.5–150.4 255–354 12.5–15.4 0.225–0.304 (0.361– 

0.371)–0.64 

151–200 Unhealthy. 

0.116–0.374 ............................ 0.205–0.404 3 150.5– 

250.4 

355–424 15.5–30.4 0.305–0.604 0.65–1.24 201–300 Very Unhealthy. 

(2)............... ............................. 0.405–0.504 3 250.5– 

350.4 

425–504 30.5–40.4 0.605–0.804 1.25–1.64 301–400 Hazardous. 

(2)............... ............................. 0.505–0.604 3 350.5– 

500.4 

505–604 40.5–50.4 0.805–1.004 1.65–2.04 401–500 Hazardous. 

1Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour ozone 

values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the 

maximum of the two values reported. 2 8-hour O3 values do not define higher AQI values (≥301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations. 3 If a different SHL for PM2.5 is promulgated, these numbers will change accordingly. 

[FR Doc. E9–15944 Filed 7–14–09; 8:45 am] 

BILLING CODE 6560–50–P 


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