Tuesday, 

February 9, 2010 

Part III 

Environmental 

Protection Agency 

40 CFR Parts 50 and 58 

Primary National Ambient Air Quality 

Standards for Nitrogen Dioxide; Final 

Rule 


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ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50 and 58 

[EPA–HQ–OAR–2006–0922; FRL 9107–9] 

RIN 2060–AO19 

Primary National Ambient Air Quality 

Standards for Nitrogen Dioxide 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final 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 is making revisions 

to the primary NO2 NAAQS in order to 

provide requisite protection of public 

health. Specifically, EPA is establishing 

a new 1-hour standard at a level of 100 

ppb, based on the 3-year average of the 

98th percentile of the yearly distribution 

of 1-hour daily maximum 

concentrations, to supplement the 

existing annual standard. EPA is also 

establishing requirements for an NO2

monitoring network that will include 

monitors at locations where maximum 

NO2 concentrations are expected to 

occur, including within 50 meters of 

major roadways, as well as monitors 

sited to measure the area-wide NO2

concentrations that occur more broadly 

across communities. 

DATES: This final rule is effective on 

April 12, 2010. 

ADDRESSES: EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2006–0922. All 

documents in the docket are listed on 

the http://www.regulations.gov Web 

site. Although listed in the index, some 

information is not publicly available, 

e.g., confidential business information 

or other information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy form. Publicly available docket 

materials are available either 

electronically through 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: 

Table of Contents 

The following topics are discussed in this 

preamble: 

I. Background 

A. Summary of Revisions to the NO2

Primary NAAQS 

B. Legislative Requirements 

C. Related NO2 Control Programs 

D. Review of the Air Quality Criteria and 

Standards for Oxides of Nitrogen 

E. Summary of Proposed Revisions to the 

NO2 Primary NAAQS 

F. Organization and Approach to Final NO2

Primary NAAQS Decisions 

II. Rationale for Final Decisions on the NO2

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

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 

C. Human Exposure and Health Risk 

Characterization 

D. Approach for Reviewing the Need To 

Retain or Revise the Current Standard 

E. Adequacy of the Current Standard 

1. Rationale for Proposed Decision 

2. Comments on the Adequacy of the 

Current Standard 

a. Comments on EPA’s Interpretation of the 

Epidemiologic Evidence 

b. Comments on EPA’s Interpretation of the 

Controlled Human Exposure Evidence 

c. Comments on EPA’s Characterization of 

NO2-Associated Exposures and Health 

Risks 

3. Conclusions on the Adequacy of the 

Current Standard 

F. Elements of a New Short-Term Standard 

1. Indicator 

a. Rationale for Proposed Decision 

b. Comments on Indicator 

c. Conclusions Regarding Indicator 

2. Averaging Time 

a. Rationale for Proposed Decision 

b. Comments on Averaging Time 

c. Conclusions on Averaging Time 

3. Form 

a. Rationale for Proposed Decision 

b. CASAC and Public Comments on Form 

c. Conclusions on Form 

4. Level 

a. Rationale for Proposed Decisions on 

Approach and Level 

b. Rationale for Alternative Decisions on 

Approach and Level 

c. Comments on Approach and Level 

i. CASAC Comments on the Approach to 

Setting the Standard 

ii. Public Comments on the Approach To 

Setting the Standard 

iii. CASAC Comments on Standard Level 

iv. Public Comments on Standard Level 

d. Conclusions on Approach and Standard 

Level 

G. Annual Standard 

H. Summary of Final Decisions on the 

Primary NO2 Standard 

III. Amendments to Ambient Monitoring and 

Reporting Requirements 

A. Monitoring Methods 

1. Chemiluminescence FRM and 

Alternative Methods 

2. Allowable FRM and FEMs for 

Comparison to the NAAQS 

a. Proposed Changes to FRM and FEMs 

That May Be Compared to the NAAQS 

b. Comments 

c. Decisions on Allowable FRM and FEMs 

for Comparison to the NAAQS 

B. Network Design 

1. Two-Tiered Network Design 

a. Proposed Two-Tier Network Design 

b. Comments 

c. Conclusions Regarding the Two-Tier 

Network Design 

2. First Tier (Near-road Monitoring 

Component) of the NO2 Network Design 

a. Proposed First Tier (Near-road 

Monitoring Component) of the Network 

Design 

b. Comments 

c. Conclusions Regarding the First Tier 

(Near-road Monitoring Component) of 

the Network Design 

3. Second Tier (Area-wide Monitoring 

Component) of the Network Design 

a. Proposed Second Tier (Area-wide 

Monitoring Component) of the Network 

Design 

b. Comments 

c. Conclusions on the Second Tier (Areawide Monitoring Component) of the 

Network Design 

4. Regional Administrator Authority 

a. Proposed Regional Administrator 

Authority 

b. Comments 

c. Conclusions on Regional Administrator 

Authority 

5. Monitoring Network Implementation 

a. Proposed Monitoring Network 

Implementation Approach 

b. Comments 

c. Conclusions on Monitoring Network 

Implementation 

6. Near-Road Site Selection 

a. Proposed Near-Road Site Selection 

Criterion 

b. Comments 

c. Conclusions on Near-Road Site Selection 

7. Near-Road Siting Criteria 

a. Proposed Near-Road Siting Criteria 

b. Comments 

c. Conclusions on Near-Road Siting Criteria 

8. Area-wide Monitor Site Selection and 

Siting Criteria 


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1EPA notes that as the promulgation of a NAAQS 

is identified in section 307(d)(1) of the Clean Air 

Act, all of the provisions of this rulemaking are 

subject to the requirements of section 307(d) of the 

Clean Air Act. 

2The 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). 

3EPA is currently conducting a separate review 

of the secondary NO2 NAAQS jointly with a review 

of the secondary SO2 NAAQS. 

a. Proposed Area-wide Monitor Site 

Selection and Siting Criteria 

b. Comments 

c. Conclusions on Area-Wide Monitor Site 

Selection and Siting Criteria 

9. Meteorological Measurements 

a. Proposed Meteorological Measurements 

b. Comments 

c. Conclusions on Meteorological 

Measurements 

C. Data Reporting 

1. Proposed Data Quality Objectives and 

Measurement Uncertainty 

2. Comments 

3. Conclusions on Data Quality Objectives 

and Measurement Uncertainty 

IV. Appendix S—Interpretation of the 

Primary NAAQS for Oxides of Nitrogen 

and Revisions to the Exceptional Events 

Rule 

A. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen for the Annual 

Primary Standard 

1. Proposed Interpretation of the Annual 

Standard 

2. Comments on Interpretation of the 

Annual Standard 

3. Conclusions on Interpretation of the 

Annual Standard 

B. Interpretation of the Primary NAAQS for 

Oxides of Nitrogen 1-Hour Primary 

Standard 

1. Proposed Interpretation of the 1-Hour 

Standard 

2. Comments on Interpretation of the 

1-Hour Standard 

3. Conclusions on Interpretation of the 

1-Hour Standard 

C. Exceptional Events Information 

Submission Schedule 

V. Designation of Areas 

A. Proposed Process 

B. Public Comments 

C. Final Designations Process 

VI. Clean Air Act Implementation 

Requirements 

A. Classifications 

1. Proposal 

2. Public comments 

3. Final 

B. Attainment Dates 

1. Attaining the NAAQS 

a. Proposal 

b. Final 

2. Consequences of Failing to Attain by the 

Statutory Attainment Date 

a. Proposal 

b. Final 

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

Requirements 

1. Proposal 

2. Final 

D. Attainment Planning Requirements 

1. Nonattainment Area SIPs 

a. Proposal 

b. Public Comments 

c. Final 

2. New Source Review and Prevention of 

Significant Deterioration Requirements 

a. Proposal 

b. Public Comments 

c. Final 

3. General Conformity 

a. Proposal 

4. Transportation Conformity 

a. Proposal 

b. Public Comments 

c. Final 

VII. Communication of Public Health 

Information 

VIII. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Congressional Review Act 

References 

I. Background 

A. Summary of Revisions to the NO2

Primary NAAQS 

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 is making revisions 

to the primary NO2 NAAQS in order to 

provide requisite protection of public 

health as appropriate under section 109 

of the Clean Air Act (Act or CAA). 

Specifically, EPA is supplementing the 

existing annual standard for NO2 of 53 

parts per billion (ppb) by establishing a 

new short-term standard based on the 3- 

year average of the 98th percentile of the 

yearly distribution of 1-hour daily 

maximum concentrations. EPA is setting 

the level of this new standard at 100 

ppb. EPA is making changes in data 

handling conventions for NO2 by adding 

provisions for this new 1-hour primary 

standard. EPA is also establishing 

requirements for an NO2 monitoring 

network. These new provisions require 

monitors at locations where maximum 

NO2 concentrations are expected to 

occur, including within 50 meters of 

major roadways, as well as monitors 

sited to measure the area-wide NO2

concentrations that occur more broadly 

across communities. EPA is making 

conforming changes to the air quality 

index (AQI). 

B. Legislative Requirements 

Two sections of the 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 [her] 

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).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.’’ 2 42 U.S.C. 

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

turn, must ‘‘specify a level of air quality 

the attainment and maintenance of 

which, in the judgment of the 

Administrator, based on [the air quality] 

criteria, is requisite to protect the public 

welfare from any known or anticipated 

adverse effects associated with the 

presence of such pollutant in the 

ambient air.’’ 3 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 

(DC Cir 1980), cert. denied, 449 U.S. 


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4 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. 

1042 (1980); American Petroleum 

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

(DC 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. 

C. 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. With 

the revisions to the NO2 NAAQS that 

result from this review, however, some 

areas could be classified as nonattainment. Certain States will 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).4

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 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. 

D. 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 

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 is not intended to 

address 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 


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5The ‘‘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. 

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, 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 

policy assessment chapter of the final 

REA focused on the information that is 

most pertinent to evaluating the basic 

elements of national ambient air quality 

standards: Indicator, averaging time, 

form,5 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. In accordance with this 

schedule, the Administrator signed a 

notice of proposed rulemaking on June 

26, 2009 (FR 74 34404). This action 

presents the Administrator’s final 

decisions on the primary NO2 standard. 

E. Summary of Proposed Revisions to 

the NO2 Primary NAAQS 

For the reasons discussed in the 

preamble of the proposal for the NO2

primary NAAQS (74 FR 34404), EPA 

proposed to make revisions to the 

primary NO2 NAAQS and to make 

related revisions for NO2 data handling 

conventions in order to provide 

requisite protection of public health. 

EPA also proposed to make 

corresponding changes to the AQI for 

NO2. Specifically, EPA proposed to 

supplement the current annual standard 

by establishing a new short-term NO2

standard that would reflect the 

maximum allowable NO2 concentration 

anywhere in an area. EPA proposed that 

this new short-term standard would be 

based on the 3-year average of the 99th 

percentile (or 4th highest) of the yearly 

distribution of 1-hour daily maximum 

NO2 concentrations and solicited 

comment on using the 3-year average of 

the 98th percentile (or 7th or 8th 

highest) of the yearly distribution of 1- 

hour daily maximum NO2

concentrations. EPA proposed to set the 

level of this new 1-hour standard within 

the range of 80 to 100 ppb and solicited 

comment on standard levels as low as 

65 ppb and as high as 150 ppb. EPA 

proposed to specify the level of the 

standard to the nearest ppb. EPA also 

proposed to establish requirements for 

an NO2 monitoring network at locations 

where maximum NO2 concentrations 

are expected to occur, including 

monitors within 50 meters of major 

roadways, as well as area-wide monitors 

sited to measure the NO2 concentrations 

that can occur more broadly across 

communities. EPA also solicited 

comment on the alternative approach of 

setting a 1-hour standard that would 

reflect the allowable area-wide NO2

concentration. 

F. Organization and Approach to Final 

NO2 Primary NAAQS Decisions 

This action presents the 

Administrator’s final decisions 

regarding the need to revise the current 

NO2 primary NAAQS. Revisions to the 

primary NAAQS for NO2, and the 

rationale supporting those revisions, are 

described below in section II. 

Requirements for the NO2 ambient 

monitoring network are described in 

section III. Related requirements for data 

completeness, data handling, data 

reporting, rounding conventions, and 

exceptional events are described in 

section IV. Implementation of the 

revised NO2 primary NAAQS is 

discussed in sections V and VI. 

Communication of public health 

information through the AQI is 

discussed in section VII and a 

discussion of statutory and executive 

order reviews is provided in section 

VIII. 

Today’s final decisions are based on 

a thorough review in the ISA of 

scientific information on known and 

potential human health effects 

associated with exposure to NO2 in the 

air. These final decisions also take into 

account: (1) Assessments in the REA of 

the most policy-relevant information in 

the ISA as well as quantitative exposure 

and risk analyses based on that 

information; (2) CASAC Panel advice 

and recommendations, as reflected in its 

letters to the Administrator and its 

public discussions of the ISA, the REA, 

and the notice of proposed rulemaking; 

(3) public comments received during the 

development of ISA and REA; and (4) 

public comments received on the 

proposed rulemaking. 

Some commenters have referred to 

and discussed individual scientific 

analyses on the health effects of NO2

that were not included in the ISA (EPA, 

2008a) (‘‘new studies’’). In considering 


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6 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 Watkins and Thompson (2008) considered all 

NO2 sites reporting data at any point during the 

year. Based on this approach, Watkins and 

Thompson (2008) also noted that the size of the 

NO2 monitoring network has remained relatively 

stable since the early 1980s. 

and responding to comments for which 

such ‘‘new studies’’ were cited in 

support, EPA has provisionally 

considered the cited studies in the 

context of the findings of the ISA. 

As in prior NAAQS reviews, EPA is 

basing its decision in this review on 

studies and related information 

included in the ISA and staff’s policy 

assessment, which have undergone 

CASAC and public review. In this NO2

NAAQS review, staff’s policy 

assessment was presented in the form of 

a policy assessment chapter of the REA 

(EPA, 2008b). The studies assessed in 

the ISA and REA, and the integration of 

the scientific evidence presented in 

them, have undergone extensive critical 

review by EPA, CASAC, and the public. 

The rigor of that review makes these 

studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 

NAAQS decisions can have profound 

impacts on public health and welfare, 

and NAAQS decisions should be based 

on studies that have been rigorously 

assessed in an integrative manner not 

only by EPA but also by the statutorily 

mandated independent advisory 

committee, as well as the public review 

that accompanies this process. EPA’s 

provisional consideration of ‘‘new 

studies’’ did not and could not provide 

that kind of in-depth critical review. 

This decision is consistent with EPA’s 

practice in prior NAAQS reviews and its 

interpretation of the requirements of the 

CAA. Since the 1970 amendments, the 

EPA has taken the view that NAAQS 

decisions are to be based on scientific 

studies and related information that 

have been assessed as a part of the 

pertinent air quality criteria, and has 

consistently followed this approach. 

This longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. See 71 FR 61144, 61148 

(October 17, 2006) (final decision on 

review of PM NAAQS) for a detailed 

discussion of this issue and EPA’s past 

practice. 

As discussed in EPA’s 1993 decision 

not to revise the NAAQS for ozone (O3), 

‘‘new studies’’ may sometimes be of such 

significance that it is appropriate to 

delay a decision on revision of a 

NAAQS and to supplement the 

pertinent air quality criteria so the 

studies can be taken into account (58 FR 

at 13013–13014, March 9, 1993). In the 

present case, EPA’s provisional 

consideration of ‘‘new studies’’ 

concludes that, taken in context, the 

‘‘new’’ information and findings do not 

materially change any of the broad 

scientific conclusions regarding the 

health effects of NO2 made in the air 

quality criteria. For this reason, 

reopening the air quality criteria review 

would not be warranted even if there 

were time to do so under the court order 

governing the schedule for this 

rulemaking. 

Accordingly, EPA is basing the final 

decisions in this review on the studies 

and related information included in the 

NO2 air quality criteria that have 

undergone CASAC and public review. 

EPA will consider the ‘‘new studies’’ for 

purposes of decision-making in the next 

periodic review of the NO2 NAAQS, 

which will provide the opportunity to 

fully assess these studies through a 

more rigorous review process involving 

EPA, CASAC, and the public. Further 

discussion of these ‘‘new studies’’ can be 

found below, in section II.E, and in the 

Response to Comments document. 

II. Rationale for Final Decisions on the 

NO2 Primary Standard 

This section presents the rationale for 

the Administrator’s decision to revise 

the existing NO2 primary standard by 

supplementing the current annual 

standard with a new 1-hour standard. 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 

summarized below in section II.B, 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. This rationale also draws 

upon the results of quantitative 

exposure and risk assessments, 

summarized below in section II.C. 

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 

information has been extensive and 

deliberate. 

The remainder of this section 

provides background information that 

informed the Administrator’s decisions 

on the primary standard and discusses 

the rationale for those decisions. Section 

II.A presents a discussion of NO2 air 

quality. 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 

results. Section II.D summarizes the 

approach that was 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–II.G 

discuss the Administrator’s decisions 

regarding the adequacy of the current 

standard, elements of a new 1-hour 

standard, and retention of the current 

annual standard, respectively, taking 

into consideration public comments on 

the proposed decisions. Section II.H 

summarizes the Administrator’s 

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.6 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 


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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 O3 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, O3

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, 

estimates presented in the REA suggest 

that on/near roadway NO2

concentrations could be approximately 

80% (REA, section 7.3.2) higher on 

average across locations than 

concentrations away from roadways and 

that roadway-associated environments 

could be responsible for the 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 to 

near area-wide ambient levels, or typical 

upwind urban background levels, 

within a few hundred meters 

downwind. While such a concentration 

gradient is present on almost all roads, 

the characteristics of the gradient, 

including the distance from the road 

that a mobile source pollutant signature 

can be differentiated from background 

concentrations, are heavily dependent 

on factors such as 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, 1981; 

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, 1981; 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, 1981; 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 (discussed in 

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 


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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 summarized below and discussed 

more fully in section II.B of the proposal 

notice, 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. This body of evidence focuses 

the current review on NO2-related 

respiratory effects at lower ambient and 

exposure concentrations than 

considered in the previous review. 

1. Adverse Respiratory Effects and 

Short-Term Exposure to NO2

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, and 

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. 

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 widespread source of 

combustion pollutant mixtures that 

include NOX and are an important 

contributor to NO2 levels in near-road 

locations. Although the presence of 

other pollutants from vehicle exhaust 

emissions complicates efforts to 

quantify specific NO2-related health 

effects, a number of epidemiologic 

studies have evaluated associations with 

NO2 in models that also include cooccurring pollutants such as PM, O3, 

CO, and/or SO2. The evidence 

summarized in the ISA indicates that 

NO2 associations generally remain 

robust in these multi-pollutant models 

and supports a direct effect of shortterm NO2 exposure on respiratory 

morbidity (see ISA Figures 3.1–7, 3.1– 

10, 3.1–11). The plausibility and 

coherence of these effects are also 

supported by epidemiologic studies of 

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

toxicological and controlled human 

exposure) studies that have evaluated 

host defense and immune system 

changes, airway inflammation, and 

airway responsiveness (see subsequent 

sections of this proposal and the ISA, 

section 5.3.2.1). The ISA (section 5.4) 

concluded that the robustness of 

epidemiologic findings to adjustment 

for co-pollutants, coupled with data 

from animal and human experimental 

studies, support a determination that 

the relationship between NO2 and 

respiratory morbidity is likely causal, 

while still recognizing the relationship 

between NO2 and other traffic related 

pollutants. 

The epidemiologic and experimental 

studies encompass a number of 

respiratory-related health endpoints, 

including emergency department visits 

and hospitalizations, respiratory 

symptoms, airway hyperresponsiveness, 

airway inflammation, and lung function. 

The findings relevant to these 

endpoints, which provide the rationale 

to support the judgment of a likely 

causal relationship, are described in 

more detail in section II.B.1 of the 

proposal. 

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 United States 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 ISA concluded that 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, 


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may contribute to increased mortality, 

evaluation of the specificity of this 

effect is difficult. Clinical studies 

showing hematologic effects and animal 

toxicological studies showing 

biochemical, lung host defense, 

permeability, and inflammation changes 

with short-term exposures to NO2

provide limited evidence of plausible 

pathways by which risks of mortality 

may be increased, but no coherent 

picture is evident at this time (ISA, 

section 5.3.2.3). 

b. Cardiovascular Effects 

The ISA concluded that the available 

evidence on cardiovascular health 

effects following short-term exposure to 

NO2 is inadequate to infer the presence 

or absence of a causal relationship at 

this time (ISA, section 5.3.2.2). Evidence 

from epidemiologic studies of heart rate 

variability, repolarization changes, and 

cardiac rhythm disorders among heart 

patients with ischemic cardiac disease 

are inconsistent (ISA, section 5.3.2.2). In 

most studies, associations with PM were 

found to be similar or stronger than 

associations with NO2. Generally 

positive associations between ambient 

NO2 concentrations and hospital 

admissions or emergency department 

visits for cardiovascular disease have 

been reported in single-pollutant 

models (ISA, section 5.3.2.2); however, 

most of these effect estimate values were 

diminished in multi-pollutant models 

that also contained CO and PM indices 

(ISA, section 5.3.2.2). Mechanistic 

evidence of a role for NO2 in the 

development of cardiovascular diseases 

from studies of biomarkers of 

inflammation, 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 (see section II.B.3 of the 

proposal for a more detailed 

discussion). 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). While 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 high correlation 

among traffic-related pollutants in 

epidemiologic studies makes it difficult 

to accurately estimate independent 

effects (ISA, section 5.3.2.4). 

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 

for suffering adverse effects from NO2

exposure. This could occur because they 

are affected by lower levels of NO2 than 

the general population or 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 susceptibility and 

vulnerability 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 include 

pre-existing disease (e.g., asthma), age 

(i.e., infants, children, older adults), 

genetic factors, gender, socioeconomic 

status, and proximity to roadways (see 

section II.B.4 in proposal for more 

detailed discussion of these factors). 

As discussed in more detail in the 

proposal (section II.B.4), 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 


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7The most current American Housing Survey 

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

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

of housing units within the 300 foot boundary than 

do prior surveys. According to Table 1A–6 from 

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

housing/ahs/ahs07/tab1a-6.pdf), out of 128,203,000 

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

were reported by the surveyed occupant or landlord 

as being within 300 feet of a 4-or-more lane 

highway, railroad, or airport. That constitutes 

15.613% of the total housing units 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. 

8The study by Tolbert et al. (2007) reported 

positive associations between 1-hour ambient NO2

concentrations and respiratory-related emergency 

department visits. The meta-analysis was included 

in the ISA and reported that short-term exposures 

to NO2 concentrations at or above 100 ppb 

increased airway responsiveness in most 

asthmatics. 

from the 2003 American Housing 

Survey, approximately 36 million 

individuals live within 300 feet (∼90 

meters) of a four-lane highway, railroad, 

or airport (ISA, section 4.4).7

Furthermore, in California, 2.3% of 

schools, with a total enrollment of more 

than 150,000 students were located 

within approximately 500 feet of hightraffic roads, with a higher proportion of 

non-white and economically 

disadvantaged students attending those 

schools (ISA, section 4.4). Of this 

population, asthmatics and members of 

other susceptible groups discussed 

above will have even greater risks of 

experiencing health effects related to 

NO2 exposure. In the United States, 

approximately 10% of adults and 13% 

of children (approximately 22.2 million 

people in 2005) 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 United States census 

data from 2000, about 72.3 million 

(26%) of the United States 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 

United States 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. As discussed more fully in 

section II.C of the proposal, this section 

summarizes the approach taken in the 

REA to characterize NO2-related 

exposures and health risks. Goals of the 

REA included estimating short-term 

exposures and potential human health 

risks associated with (1) recent levels of 

ambient NO2; (2) NO2 levels adjusted to 

simulate just meeting the current 

standard; and (3) NO2 levels adjusted to 

simulate just meeting potential 

alternative standards. 

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. 

In evaluating the appropriateness of 

specific endpoints for use in the NO2

risk characterization, the REA 

considered both epidemiologic and 

controlled human exposure studies. As 

described in more detail in the proposal 

(section II.C.1), the characterization of 

NO2-associated health risks was based 

on an epidemiology study conducted in 

Atlanta, Georgia by Tolbert et al. (2007) 

and a meta-analysis of controlled 

human exposure studies of NO2 and 

airway responsiveness in asthmatics 

(ISA, Table 3.1–3).8

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, 

the REA employed three approaches. In 

the first approach, for each air quality 

scenario, NO2 concentrations at fixedsite 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 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 in the proposal (section II.C.2) 

and each approach, along with its 

limitations and uncertainties (see 

proposal, section II.C.3) has been 

described in more detail in the REA 

(chapters 6 through 9). 

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. In interpreting 

these results within the context of the 

current revisions to the NO2 primary 

NAAQS (see below), we note that 

simulation of different standard levels 

was based on adjusting NO2

concentrations at available area-wide 

monitors. Therefore, the standard levels 

referred to above reflect the allowable 

area-wide NO2 concentrations, not the 

maximum allowable concentrations. As 

a consequence, the maximum 

concentrations in an area that just meets 

one of these standard levels would be 

expected to be higher than the standard 

level. For example, given that near-road 


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NO2 concentrations can be 30% to 

100% higher than area-wide 

concentrations (see section II.E.2), an 

area-wide concentration of 50 ppb could 

correspond to near-road concentrations 

from 65 to 100 ppb. 

Key results of the air quality, 

exposure, and risk analyses were 

presented in the policy assessment 

chapter of the REA and summarized in 

the proposal (Table 1 in proposal). In 

considering these results, the policy 

assessment chapter of the REA 

concluded that the risks estimated to be 

associated with just meeting the current 

annual standard can be judged 

important from a public health 

perspective. The results for specific 1- 

hour standard levels estimate that 

limiting the 98th/99th percentile of the 

distribution of 1-hour daily maximum 

NO2 concentrations measured at areawide monitors to 50 or 100 ppb could 

substantially reduce exposures to 

ambient NO2 and associated health risks 

(compared to just meeting the current 

standard). In contrast, limiting these 

area-wide NO2 concentrations to 150 or 

200 ppb is estimated to result in similar, 

or in some cases higher, NO2-associated 

exposures and health risks than just 

meeting the current standard. The 

pattern of results was similar for 

standards just meeting either the 98th or 

the 99th percentile 1-hour daily 

maximum area-wide standards (REA, 

Chapters 7, 8, and 9). 

D. Approach for Reviewing the Need To 

Retain or Revise the Current Standard 

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 has been 

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 

decisions 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 from CASAC and 

the public. 

To evaluate whether the current 

primary NO2 standard is requisite or 

whether consideration of revisions is 

appropriate, EPA has used an approach 

in this review that was described in the 

policy assessment chapter of the REA. 

This approach builds upon those used 

in reviews of other criteria pollutants, 

including the most recent reviews of the 

Pb, O3, and PM NAAQS (EPA, 2007c; 

EPA, 2007d; EPA, 2005), and reflects the 

body of evidence and information that 

is currently available. As in other recent 

reviews, EPA’s considerations included 

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 framed 

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 has been 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 has been 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 has addressed the 

following questions: 

• Does the evidence provide support for 

considering a different indicator for gaseous 

NOX? 

• Does the evidence provide support for 

considering different averaging times? 

• What ranges of levels and forms of 

alternative standards are supported by the 

evidence, and what are the associated 

uncertainties and limitations? 

• To what extent do specific averaging 

times, levels, and forms of alternative 

standards reduce the estimated exposures 

above benchmark levels and risks attributable 

to NO2, and what are the uncertainties 

associated with the estimated exposure and 

risk reductions? 

The questions outlined above have been 

addressed in the REA, the proposal, and 

in this final rulemaking. The following 

sections present the rationale for 

proposed decisions, discussion of 

public comments, and the 

Administrator’s conclusions on the 

adequacy of the current standard and 

potential alternative standards in terms 

of indicator, averaging time, form, and 

level. 

E. Adequacy of the Current Standard 

This section discusses considerations 

related to the decision as to whether the 

current NO2 primary NAAQS is 

requisite to protect public health with 

an adequate margin of safety. 

Specifically, section II.E.1 provides an 

overview of the rationale supporting the 

Administrator’s conclusion in the 

proposal that the current standard alone 

does not provide adequate public health 

protection; section II.E.2 discusses 

comments received on the adequacy of 

the current standard; and section II.E.3 

discusses the Administrator’s final 

decision on whether the current NO2

primary NAAQS is requisite to protect 

public health with an adequate margin 

of safety. 

1. Rationale for Proposed Decision 

In reaching a conclusion regarding the 

adequacy of the current NO2 NAAQS in 

the proposal (section II.E.5), the 

Administrator considered the scientific 

evidence assessed in the ISA and the 

conclusions of the ISA, the exposure 

and risk information presented in the 

REA and the conclusions of the policy 

assessment chapter of the REA, and the 

views expressed by CASAC. These 

considerations are discussed in detail in 

the proposal (II.E.) and are summarized 

in this section. In the proposal, the 


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Administrator noted the following in 

considering the adequacy of the current 

standard: 

• 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 respiratory-related 

hospital admissions and emergency 

department visits, at ambient 

concentrations that are present in areas 

that meet the current NO2 NAAQS (ISA, 

section 5.4). 

• The policy assessment chapter of 

the REA concluded that risks estimated 

to be associated with air quality 

adjusted upward to simulate just 

meeting the current standard can 

reasonably be judged important from a 

public health perspective (REA, section 

10.3.3). 

• The policy assessment chapter of 

the REA concluded that exposure- and 

risk-based results reinforce the scientific 

evidence in supporting the conclusion 

that consideration should be given to 

revising the current NO2 NAAQS 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 (REA, 

section 10.3.3). 

• CASAC agreed that the current 

annual standard alone is not sufficient 

to protect public health against the 

types of exposures that could lead to 

these health effects. Specifically, in 

their letter to the Administrator on the 

final REA, they stated that ‘‘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). 

Based on these considerations 

(discussed in more detail in the 

proposal, section II.E), the 

Administrator concluded in the 

proposal that the current NO2 primary 

NAAQS is not requisite to protect 

public health with an adequate margin 

of safety against adverse respiratory 

effects associated with short-term 

exposures. In considering approaches to 

revising the current standard, the 

Administrator concluded that it is 

appropriate to consider setting a new 

short-term standard in addition to 

retaining the current annual standard. 

The Administrator noted that such a 

short-term standard could provide 

increased public health protection, 

especially for members of at-risk groups, 

from effects described in both 

epidemiologic and controlled human 

exposure studies to be associated with 

short-term exposures to NO2. 

2. Comments on the Adequacy of the 

Current Standard 

This section discusses comments 

received from CASAC and public 

commenters on the proposal that either 

supported or opposed the 

Administrator’s proposed decision to 

revise the current NO2 primary NAAQS. 

Comments on the adequacy of the 

current standard that focused on the 

scientific and/or the exposure/risk basis 

for the Administrator’s proposed 

conclusions are discussed in sections 

II.E.2.a-II.E.2.c. Comments on the 

epidemiologic evidence are considered 

in section II.E.2.a. Comments on the 

controlled human exposure evidence 

are considered in section II.E.2.b. 

Comments on human exposure and 

health risk assessments are considered 

in section II.E.2.c. To the extent these 

comments on the evidence and 

information are also used to justify 

commenters’ conclusions on decisions 

related to indicator, averaging time, 

level, or form, they are noted in the 

appropriate sections below (II.F.1– 

II.F.4). 

In their comments on the proposal 

(Samet, 2009), CASAC reiterated their 

support for the need to revise the 

current annual NO2 NAAQS in order to 

increase public health protection. As 

noted above, in its letter to the 

Administrator on the final REA (Samet, 

2008b) CASAC stated that it ‘‘concurs 

with EPA’s judgment that the current 

NAAQS does not protect the public’s 

health and that it should be revised.’’ In 

supporting adoption of a more stringent 

NAAQS for NO2, CASAC considered the 

assessment of the scientific evidence 

presented in the ISA, the results of 

assessments presented in the REA, and 

the conclusions of the policy assessment 

chapter of the REA. As such, CASAC’s 

rationale for revising the current 

standard was consistent with the 

Administrator’s rationale as discussed 

in the proposal. 

Many public commenters agreed with 

CASAC that, based on the available 

information, the current NO2 standard is 

not requisite to protect public health 

with an adequate margin of safety and 

that revisions to the standard are 

appropriate. Among those calling for 

revisions to the standard were 

environmental groups (e.g., Clean Air 

Council (CAC), Earth Justice (EJ), 

Environmental Defense Fund (EDF), 

Natural Resources Defense Council 

(NRDC), Group Against Smog and 

Pollution (GASP)); medical/public 

health organizations (e.g., American 

Lung Association (ALA), American 

Medical Association (AMA), American 

Thoracic Society (ATS), National 

Association for the Medical Direction of 

Respiratory Care (NAMDRC), National 

Association of Cardiovascular and 

Pulmonary Rehabilitation (NACPR), 

American College of Chest Physicians 

(ACCP)); a large number of State 

agencies and organizations (e.g., 

National Association of Clean Air 

Agencies (NACAA), Northeast States for 

Coordinated Air Use Management 

(NESCAUM), and State or local agencies 

in CA, IA, IL, MI, MO, NC, NM, NY, TX, 

VA, WI); Tribes (e.g., National Tribal Air 

Association (NTAA), Fond du Lac Band 

of Lake Superior Chippewa (Fond du 

Lac)), and a number of individual 

commenters. These commenters 

concluded that the current NO2

standard needs to be revised and that a 

more stringent standard is needed to 

protect the health of sensitive 

population groups. In supporting the 

need to adopt a more stringent NAAQS 

for NO2, these commenters often 

referenced the conclusions of CASAC 

and relied on the evidence and 

information presented in the proposal. 

As such, similar to CASAC, the 

rationale offered by these commenters 

was consistent with that presented in 

the proposal to support the 

Administrator’s proposed decision to 

revise the current NO2 NAAQS. 

Some industry commenters (e.g., 

Alliance of Automobile Manufacturers 

(AAM), American Petroleum Institute 

(API), Interstate Natural Gas Association 

of America (INGAA), Utility Air 

Regulatory Group (UARG)) and one 

State commenter (IN Department of 

Environmental Management) expressed 

support for retaining the current annual 

standard alone. In supporting this view, 

these commenters generally concluded 

that the current standard is requisite to 

protect public health with an adequate 

margin of safety and that the available 

evidence is not sufficient to support 

revision of the standard. For example, 

UARG stated that ‘‘EPA has failed to 

demonstrate that the present NO2

NAAQS is no longer at the level 

requisite to protect public health with 

an adequate margin of safety.’’ In 

addition, INGAA stated that 

‘‘* * * EPA should be compelled to 

retain the current standard and defer a 

decision on a new short-term standard 

until the science is more clearly 

defined.’’ 

In support of their views, these 

commenters provided specific 

comments on the epidemiologic and 

controlled human exposure evidence as 

discussed below. In responding to these 

specific comments, we note that the 

Administrator relied in the proposal on 

the evidence, information and 

judgments contained in the ISA and the 


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REA (including the policy assessment 

chapter) as well as on the advice of 

CASAC. In considering the evidence, 

information, and judgments of the ISA 

and the REA, the Agency notes that 

these documents have been reviewed 

extensively by CASAC and have been 

discussed by CASAC at multiple public 

meetings (see section I.D). In their letter 

to the Administrator regarding the 

second draft ISA (Henderson, 2008), 

CASAC noted the following: 

Panel members concur with the primary 

conclusions reached in the ISA with regard 

to health risks that are associated with NO2

exposure. In particular, the Panel agrees with 

the conclusion that the current scientific 

evidence is ‘‘sufficient to infer a likely causal 

relationship between short-term NO2

exposure and adverse effects on the 

respiratory system.’’ The strongest evidence 

in support of this conclusion comes from 

epidemiology studies that show generally 

positive associations between NO2 and 

respiratory symptoms, hospitalizations or 

emergency department visits, as summarized 

in Figure 5.3.1.’’ 

Similarly, in their letter to the 

Administrator on the final REA (Samet, 

2008b), CASAC noted the following: 

Overall, CASAC found this version of the 

REA satisfactory in its approach to moving 

from the scientific foundation developed in 

the Integrated Science Assessment (ISA) to 

setting out evidence-based options for the 

NAAQS. The REA provides the needed 

bridge from the evidence presented in the 

ISA to a characterization of the exposures 

and the associated risks with different 

profiles of exposure. It draws on toxicological 

and epidemiological evidence and addresses 

risk to an identified susceptible population, 

people with asthmatic conditions. EPA has 

also systematically described uncertainties 

associated with the risk assessments. We 

commend EPA for developing a succinct and 

thoughtfully developed synthesis in chapter 

10. This summary chapter represents a longneeded and transparent model for linking a 

substantial body of scientific evidence to the 

four elements of the NAAQS. 

Therefore, in discussing comments on 

the interpretation of the scientific 

evidence and exposure/risk information, 

we note that CASAC has endorsed the 

approaches and conclusions of the ISA 

and the REA. These approaches and 

conclusions are discussed below in 

more detail, within the context of 

specific public comments. 

a. Comments on EPA’s Interpretation of 

the Epidemiologic Evidence 

Several industry groups (e.g., API, 

National Mining Association (NMA), 

American Chemistry Council (ACC), 

AAM, Annapolis Center for ScienceBased Public Policy (ACSBPP), Engine 

Manufacturers Association (EMA), 

ExxonMobil (Exxon), National 

Association of Manufacturers (NAM)) 

commented that, given the presence of 

numerous co-pollutants in the air, 

epidemiologic studies do not support 

the contention that NO2 itself is causing 

health effects. 

While EPA has recognized that 

multiple factors can contribute to the 

etiology of respiratory disease and that 

more than one air pollutant could 

independently impact respiratory 

health, we continue to judge, as 

discussed in the ISA, that the available 

evidence supports the conclusion that 

there is an independent effect of NO2 on 

respiratory morbidity. In reaching this 

judgment, we recognize that a major 

methodological issue affecting NO2

epidemiologic studies concerns the 

evaluation of the extent to which other 

air pollutants may confound or modify 

NO2-related effect estimates. The use of 

multipollutant regression models is the 

most common approach for controlling 

potential confounding by co-pollutants 

in epidemiologic studies. The issues 

related to confounding and the evidence 

of potential confounding by copollutants has been thoroughly 

reviewed in the ISA (see Figures 3.1–10 

and 3.1–11) and in previous 

assessments (e.g., the criteria document 

for PM) (EPA, 2004). NO2 risk estimates 

for respiratory morbidity endpoints, in 

general, were not sensitive to the 

inclusion of co-pollutants, including 

particulate and gaseous pollutants. As 

observed in Figures 3.1–10 and 3.1–11 

in the ISA, relative risks for hospital 

admissions or emergency department 

visits are generally unchanged, nor is 

their interpretation modified, upon 

inclusion of PM or gaseous copollutants in the models. Similarly, 

associations between short-term NO2

exposure and asthma symptoms are 

generally robust to adjustment for copollutants in multipollutant models, as 

shown in Figures 3.1–5 and 3.1–7 of the 

ISA. These results, in conjunction with 

the results of a randomized intervention 

study evaluating respiratory effects of 

indoor exposure to NO2 (ISA, section 

3.1.4.1), led to the conclusion that the 

effect of NO2 on respiratory health 

outcomes is robust and independent of 

the effects of other ambient copollutants. 

In addition, experimental studies 

conducted in animals and humans 

provide support for the plausibility of 

the associations reported in 

epidemiologic studies. These controlled 

human exposure and animal 

toxicological studies have reported 

effects of NO2 on immune system 

function, lung host defense, airway 

inflammation, and airway 

responsiveness (ISA, section 5.4). These 

experimental study results support an 

independent contribution of NO2 to the 

respiratory health effects reported in 

epidemiologic studies (ISA Section 5.4). 

In considering the entire body of 

evidence, including epidemiologic and 

experimental studies, the ISA (section 

5.4, p. 5–16) concluded the following: 

Although this [presence of co-pollutants] 

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. 

Comments on specific epidemiologic 

studies are discussed below. 

The National Association of 

Manufacturers (NAM) commented that 

the final REA relied on an 

epidemiologic study (Delfino et al. 

2002) not critically reviewed in the final 

ISA. Contrary to NAM’s contention, the 

study by Delfino et al. (2002) was 

critically reviewed by EPA staff and 

pertinent information was extracted 

from the study. The respiratory health 

effects of NO2 on asthma reported in 

this study are included in Figure 5.3–1, 

Table 5.4–1, and Annex Table AX6.3–2 

of the ISA. While NAM comments on 

the narrative discussion of this study in 

the final ISA, their contention that EPA 

scientists did not critically analyze the 

study while preparing the final ISA is 

incorrect. The inclusion of the study in 

the figures and tables in this ISA, as 

well as inclusion in the 2004 PM AQCD, 

indicate critical analysis of the study 

that was implemented throughout the 

review process. The narrative 

discussion in the ISA focused on 

multicity studies (specifically those by 

Schwartz et al. 1994, Mortimer et al. 

2002 and Schildcrout et al. 2006), 

which provide substantial 

epidemiologic evidence for the 

respiratory health effects of NO2 on 

asthma among children. 

Additional comments from NAM 

contend that EPA’s interpretation of 

three individual epidemiologic studies 

(e.g. Krewski et al. 2000; Schildcrout et 

al. 2006; Mortimer et al. 2002) is 

inconsistent across different NAAQS 

reviews. The NAM comments on all 

three studies are discussed below. 

NAM stated the following regarding 

the study by Krewski et al: 

In the Final ISA, EPA cites the Krewski, et 

al. (2000) study as evidence of a significant 


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association between NO2 exposure and 

mortality. Although EPA acknowledges that 

exposure to NO2 was ‘‘highly correlated’’ with 

other pollutants, including PM2.5 and SO2, 

EPA does not consider the analysis of the 

respective contributions of single pollutants 

in the same study that EPA included in its 

prior Staff Paper for Particulate Matter. In 

that document, EPA stated: ‘‘In singlepollutant models, none of the gaseous copollutants was significantly associated with 

mortality except SO2.’’ If EPA has not altered 

its scientific views concerning this study as 

expressed in the PM Staff Paper, it is entirely 

inappropriate for EPA to suggest that the 

Krewski, et al. (2000) study provides any 

evidence of an association between NO2

exposure and mortality. 

In these comments, NAM fails to 

recognize that the report from Krewski 

et al. (2000) contains a reanalysis of two 

cohort studies, the Harvard Six Cities 

and the American Cancer Society (ACS) 

studies. The characterization in the NOX

ISA of the study by Krewski et al. 

(2000), referenced by NAM in their 

comments, refers to the reanalysis of the 

Harvard Six Cities Study. As stated in 

the NOX ISA (p. 3–74): 

Krewski et al. (2000) conducted a 

sensitivity analysis of the Harvard Six Cities 

study and examined associations between 

gaseous pollutants (i.e., O3, NO2, SO2, CO) 

and mortality. NO2 showed risk estimates 

similar to those for PM2.5 per ‘‘low to high’’ 

range increment with total (1.15 [95% CI: 

1.04, 1.27] per 10-ppb increase), 

cardiopulmonary (1.17 [95% CI: 1.02, 1.34]), 

and lung cancer (1.09 [95% CI: 0.76, 1.57]) 

deaths; however, in this dataset NO2 was 

highly correlated with PM2.5 (r = 0.78), SO4

2¥ (r = 0.78), and SO2 (r = 0.84). 

In contrast, the characterization in the 

PM Staff Paper (EPA, 2005) of the study 

by Krewski et al. (2000), referenced by 

NAM in their comments, refers to the 

results of the ACS study. Therefore, 

NAM appears to have confused the 

conclusions on the results of the 

reanalysis of the Harvard Six Cities 

Study in the NOX ISA with the 

conclusions on the results of the 

reanalysis of the ACS study in the PM 

Staff Paper. 

Further, in considering the reanalysis 

of the ACS study by Krewski et al. 

(2000), the NOX ISA observed that ‘‘NO2

showed no associations with mortality 

outcomes’’ (ISA, p. 3–74). This 

statement is consistent with the 

interpretation of that reanalysis as 

discussed in the PM Staff Paper. Thus, 

there is no inconsistency in the 

interpretation of the results of the study 

by Krewski et al. (2000) in the PM Staff 

Paper (EPA, 2005) and the NOX ISA 

(EPA, 2008a). 

NAM also commented that EPA has 

relied on a study by Schildcrout et al. 

(2006) in the NOX ISA but declined to 

rely on the same study for the previous 

review of the O3 NAAQS. NAM made 

the following comment regarding the 

study by Schildcrout et al: 

Another example of how EPA has reached 

different scientific conclusions in the Final 

ISA than in prior NAAQS documents is 

provided by the Schildcrout, et al. (2006) 

study. In the Final ISA, EPA includes an 

extensive discussion of this study of 

asthmatic children and the relationship 

purportedly found in this study between NO2

and various respiratory symptoms. In 

contrast, as part of the NAAQS review for 

ozone, EPA expressly declined to rely on this 

same study because of specific limitations in 

the study design. Among the limitations EPA 

cites were the fact that the Schildcrout, et al. 

(2006) study included ‘‘children in which the 

severity of their asthma was not clearly 

identified,’’ and the use of a study population 

that was ‘‘not comparable to other large 

multi-city studies.’’ EPA must explain why it 

chose to discount the value of the 

Schildcrout, et al. (2006) study when 

evaluating the effects of ozone, but has relied 

on it extensively in the Final ISA for NO2. 

The study by Schildcrout et al. (2006) 

appeared in the peer-review literature 

too late to be considered in the 2006 O3

AQCD; however, this study was 

included in the O3 Provisional 

Assessment. The purpose of the 

Provisional Assessment was to 

determine if new literature materially 

changed any of the broad scientific 

conclusions regarding the health effects 

of O3 exposure as stated in the 2006 O3

AQCD. EPA concluded that, taken in 

context, the ‘‘new’’ information and 

findings did not materially change any 

of the broad scientific conclusions 

regarding the health effects of O3

exposure made in the O3 AQCD. 

Therefore, NAM’s contention that EPA 

‘‘declined’’ to rely on the Schildcrout 

study for the O3 review because of 

limitations in study design is not 

correct. 

The observations NAM draws from 

the O3 Provisional Assessment regarding 

severity of asthma and the study 

population do not indicate limitations 

that resulted in EPA ‘‘discounting’’ the 

study results. Rather, these observations 

were intended to put the study in 

perspective for purposes of interpreting 

the results within the context of the 

larger body of O3 health effects 

evidence. These observations were 

drawn from comments submitted by Dr. 

Schildcrout regarding the interpretation 

of the results of his study in the 

decision to revise the ozone standards 

(see docket ID EPA–HQ–OAR–2005– 

0172–6991). The results of this study are 

being fully considered in the ongoing 

review of the ozone NAAQS. 

Finally, NAM contends that EPA 

reached differing scientific conclusions 

on the use of self-reported peak 

expiratory flow (PEF) depending on 

regulatory context, particularly in the 

large multi-city trial by Mortimer et al. 

(2002). We disagree with this 

contention. EPA consistently examines 

clinical measurements of lung function, 

which include PEF, forced expiratory 

flow in 1 second (FEV1), forced vital 

capacity (FVC), maximal midexpiratory 

flow (MMEF), maximal expiratory flow 

at 50% (MEF50), maximal expiratory 

flow at 25% (MEF25), and forced 

expiratory flow at 25 to 75% of FVC 

(FEF25–75). Evidence for all of these 

clinical measurements is considered 

before drawing a conclusion related to 

the association of lung function with a 

criteria pollutant. In different reviews, 

there may be more evidence from one of 

these clinical measurements than 

another. In the previous review of the 

O3 NAAQS, EPA identified statistically 

significant associations between 

increased ozone levels and morning 

PEF, which remained significant even 

when concentrations exceeding 0.08 

ppm were excluded from the analysis 

(Mortimer et al. 2002). EPA considered 

this evidence, along with evidence of 

other clinical measurements of changes 

in lung function, in drawing 

conclusions on the relationship between 

ozone and lung function. Using a 

similar approach to weigh the evidence 

pertinent to lung function, including 

studies that produced no statistically 

significant results for PEF, the NOX ISA 

(section 3.1.5.3) states: 

In summary, epidemiologic studies using 

data from supervised lung function 

measurements (spirometry or peak flow 

meters) report small decrements in lung 

function (Hoek and Brunekreef, 1994; Linn et 

al., 1996; Moshammer et al., 2006; Peacock 

et al., 2003; Schindler et al., 2001). No 

significant associations were reported in any 

studies using unsupervised, selfadministered peak flow [PEF] measurements 

with portable devices. 

The evaluation of the evidence in the 

NOX ISA is consistent with the way the 

evidence from multiple clinical 

measures of lung function was used in 

the review of the O3 NAAQS. 

b. Comments on EPA’s Interpretation of 

the Controlled Human Exposure 

Evidence 

A number of industry groups (e.g., 

AAM, ACC, API, Dow Chemical 

Company (Dow), EMA, NAM, UARG) 

disagreed with EPA’s reliance on a 

meta-analysis of controlled human 

exposure studies of airway 

responsiveness in asthmatics. Based on 

this meta-analysis (ISA, Table 3.1–3 for 

results), the ISA concluded that ‘‘small 

but significant increases in nonspecific 

airway hyperresponsiveness were 


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9EPA considers the Goodman study to be a ‘‘new 

study’’ on which, as discussed above in section 1.B, 

it would not be appropriate to base a standard in 

the absence of thorough CASAC and public review 

of the study and its methodology. However, as 

discussed below, EPA has considered the study in 

the context of responding to public comments on 

the proposal and has concluded it does not provide 

a basis to materially change any of the broad 

scientific conclusions regarding the health effects of 

NO2 made in the air quality criteria. 

observed * * * at 0.1 ppm NO2 for 60- 

min exposures in asthmatics’’ (ISA, p. 5– 

11). Industry groups raised a number of 

objections to this analysis and the way 

in which it has been used in the current 

review. 

Several of these industry groups 

concluded that, in relying on this 

analysis, EPA has inappropriately relied 

on a new unpublished meta-analysis 

that has not been peer-reviewed, was 

not reviewed by CASAC, and was not 

conducted in a transparent manner. For 

example, as part of a Request for 

Correction submitted under EPA’s 

Information Quality Guidelines, NAM 

stated that ‘‘EPA’s substantial reliance 

on an unpublished assessment 

described as a ‘‘meta-analysis’’ of the 

relation between NO2 exposure and 

changes in airway responsiveness 

violates EPA Guidelines requiring 

‘‘transparency about data and methods.’’ 

EPA disagrees with this 

characterization of the updated metaanalysis included in the final ISA. As 

described in the ISA (p. 3–16), this 

meta-analysis is based on an earlier 

analysis by Folinsbee (1992) that has 

been subject to peer-review, that was 

published in a scientific journal 

(Toxicol Ind Health. 8:1–11, 1992), and 

that was reviewed by CASAC as part of 

the previous review of the NO2 NAAQS 

(EPA, 1993, Table 15–10). The updates 

to this earlier analysis did not include 

substantive changes to the approach. As 

discussed in the final ISA (p. 3–16), the 

changes made to the analysis were to 

remove the results of one allergen study 

and add results from a non-specific 

responsiveness study, which focused 

the meta-analysis on non-specific 

airway responsiveness, and to discuss 

results for an additional exposure 

concentration (i.e., 100 ppb). The 

information needed to reproduce this 

meta-analysis is provided in the ISA 

(Tables 3.1–2 and 3.1–3, including 

footnotes). 

While the ISA meta-analysis reports 

findings on airway responsiveness in 

asthmatics following exposure to 100 

ppb NO2, a concentration not 

specifically discussed in the findings of 

the original report by Folinsbee (1992), 

this does not constitute a substantive 

change to that original analysis. For 

exposures at rest, four of the studies 

included in the analysis by Folinsbee 

evaluated the effects of exposure to 100 

ppb NO2. In that original meta-analysis, 

these studies were grouped with another 

study that evaluated exposures to 140 

ppb NO2. When analyzed together, 

exposures to NO2 concentrations of 100 

ppb and 140 ppb (grouped together in 

the manuscript and described as less 

than 0.2 ppm) increased airway 

responsiveness in 65% of resting 

asthmatics (p < 0.01). Therefore, 

reporting results at 100 ppb NO2 in the 

ISA meta-analysis reflects a change in 

the way the data are presented and does 

not reflect a substantive change to the 

study. This change in presentation 

allows specific consideration of the 

potential for exposures to 100 ppb NO2

to increase airway responsiveness, 

rather than grouping results at 100 ppb 

with results at other exposure 

concentrations. 

In addition, the updated metaanalysis was considered by CASAC 

during their review of the REA (REA, 

Table 4–5 reports the results of the 

updated meta-analysis), which based 

part of the assessment of NO2-associated 

health risks on the results of the metaanalysis. In their letter to the 

Administrator on the final REA (Samet, 

2008b), CASAC stated that ‘‘[t]he 

evidence reviewed in the REA indicates 

that adverse health effects have been 

documented in clinical studies of 

persons with asthma at 100 ppb’’ and 

that ‘‘CASAC firmly recommends that 

the upper end of the range [of standard 

levels] not exceed 100 ppb, given the 

findings of the REA.’’ In addition, in 

their comments on the proposal, CASAC 

reiterated this advice in their statement 

that ‘‘the level of the one-hour NO2

standard should be within the range of 

80–100 ppb and not above 100 ppb.’’ 

These statements indicate that CASAC 

did specifically consider the results of 

the updated meta-analysis and that they 

used those results to inform their 

recommendations on the range of 

standard levels supported by the 

scientific evidence. 

In summary, we note the following: 

• The original meta-analysis was 

published in a peer-reviewed journal 

and was reviewed by CASAC in the 

previous review of the NO2 NAAQS. 

• The updated meta-analysis does not 

include substantive changes to the 

methodology of this original analysis. 

• The changes that were made are 

clearly described in the ISA. 

• CASAC specifically reviewed and 

considered the ISA meta-analysis in 

making recommendations regarding the 

range of standard levels supported by 

the science. 

Many of these same industry groups 

also referred in their comments to a 

recent meta-analysis of controlled 

human exposure studies evaluating the 

airway response in asthmatics following 

NO2 exposure (Goodman et al., 2009). 

These groups generally recommended 

that EPA rely on this meta-analysis and 

on the authors’ conclusions with regard 

to NO2 and airway responsiveness. 

Specific comments based on the 

manuscript by Goodman et al., as well 

as EPA’s responses, are discussed below 

in more detail.9

Industry commenters generally 

claimed that the meta-analysis by 

Goodman et al. supports the conclusion 

that no adverse effects occur following 

exposures up to 600 ppb NO2. However, 

Table 4 of the Goodman study reports 

that 64% (95% Confidence Interval: 

58%, 71%) of resting asthmatics 

exposed to NO2 experienced an increase 

in airway responsiveness. Furthermore, 

Figure 2a of this manuscript reports that 

for exposures < 0.2 ppm, the fraction 

affected is 0.61 (95% CI: 0.52, 0.70) 

while for exposures of 0.2 ppm to < 0.3 

ppm, the fraction affected is 0.66 (95% 

CI: 0.59, 0.74). These findings are 

consistent with those reported in the 

meta-analysis by Folinsbee and in the 

updated meta-analysis that was 

included in the final ISA. 

Also based on the meta-analysis by 

Goodman et al. (2009), several industry 

commenters concluded that NO2- 

induced airway hyperresponsiveness is 

not adverse and, therefore, should not 

be considered in setting standards. The 

basis for this comment appears to be the 

conclusions reached by Goodman et al. 

that there is no dose-response 

relationship for NO2 and that the 

magnitude of any NO2 effect on airway 

responsiveness is too small to be 

considered adverse. 

Due to differences in study protocols 

in the NO2-airway response literature 

(ISA, section 3.1.3), EPA disagrees with 

the approach taken in the Goodman 

study to use existing data to attempt to 

evaluate the presence of a dose-response 

relationship and to determine the 

magnitude of the NO2 response. 

Examples of differences in the study 

protocols include the NO2 exposure 

method (i.e., mouthpiece versus 

chamber), subject activity level (i.e., rest 

versus exercise) during NO2 exposure, 

choice of airway challenge agent, and 

physiological endpoint used to quantify 

airway responses. Goodman et al. (2009) 

also recognized heterogeneity among 

studies as a limitation in their analyses. 

As a result of these differences, EPA 

judged it appropriate in the ISA metaanalysis to assess only the fraction of 

asthmatics experiencing increased or 

decreased airway responsiveness 


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10Once EPA determines whether to retain or 

revise the current standard, the actual air quality 

levels in various areas of the country are clearly 

relevant under the NAAQS implementation 

provisions for the Act, such as the provision for 

designation of areas based on whether or not they 

attain the required NAAQS. 

following NO2 exposure. We have 

acknowledged in the REA, the proposal, 

and in this final rulemaking that there 

is uncertainty with regard to the 

magnitude and the clinical-significance 

of NO2-induced increases in airway 

responsiveness (see sections II.C.3 and 

II.F.4.a in the proposed rulemaking as 

well as II.F.3 in this final rulemaking). 

The REA stated the following (p. 302): 

[O]ne of the important uncertainties 

associated with these [NO2-induced airway 

hyperresponsiveness] results is that, because 

the 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. 

This limitation makes it particularly difficult 

to quantify the public health implications of 

these results. 

While we acknowledge this 

uncertainty, EPA disagrees with the 

conclusion that the NO2-induced 

increase in airway responsiveness in 

asthmatics exposed to NO2

concentrations up to 600 ppb is not 

adverse and should not be considered in 

setting standards. Specifically, we note 

that the ISA concluded that ‘‘[t]ransient 

increases in airway responsiveness 

following NO2 exposure have the 

potential to increase symptoms and 

worsen asthma control’’ (ISA, section 

5.4). The uncertainty over the adversity 

of the response reported in controlled 

human exposure studies does not mean 

that the NO2-induced increase in airway 

responsiveness is not adverse. Rather, it 

means that there is a risk of adversity, 

especially for asthmatics with more than 

mild asthma, but that this risk cannot be 

fully characterized based on existing 

studies. The studies of NO2 and airway 

responsiveness included in the metaanalysis have generally evaluated mild 

asthmatics, rather than more severely 

affected asthmatics who could be more 

susceptible to the NO2-induced increase 

in airway responsiveness (ISA, section 

3.1.3.2). Given that this is the case, and 

given the large percentage of asthmatics 

that experienced an NO2-induced 

increase in airway responsiveness in the 

studies and the large size of the 

asthmatic population in the United 

States, the REA concluded that it is 

appropriate to consider NO2-induced 

airway hyperresponsiveness in 

characterizing NO2-associated health 

risks (REA, section 10.3.2). As noted 

above, CASAC endorsed this conclusion 

in their letters to the Administrator on 

the final REA and on the proposal 

(Samet, 2008b; Samet, 2009). 

c. Comments on EPA’s Characterization 

of NO2-Associated Exposures and 

Health Risks 

Several commenters discussed the 

analyses of NO2-associated exposures 

and health risks presented in the REA. 

As in past reviews (EPA 2005, 2007c, 

2007d), EPA has estimated allowable 

risks associated with the current 

standard and potential alternative 

standards to inform judgments on the 

public health risks that could exist 

under different standard options. Some 

industry commenters (e.g., API, NMA) 

concluded that the Administrator 

should consider modeled exposures and 

risks associated with actual NO2 air 

quality rather than with NO2

concentrations adjusted to simulate just 

meeting the current annual standard or 

potential alternative 1-hour standards. 

These commenters pointed out that 

such simulations require large 

adjustments to air quality and are highly 

uncertain and that NAAQS are intended 

to address actual, rather than highly 

improbable, risks to health. 

We disagree with these commenters 

that exposure- and risk-related 

considerations in the NAAQS review 

should rely only on unadjusted air 

quality. In considering whether the 

current standard is requisite to protect 

public health with an adequate margin 

of safety, air quality adjustments allow 

estimates of NO2-related exposures and 

health risks that could exist in areas that 

just meet that standard. That is, these 

adjustments allow consideration of 

exposures and risks that would be 

permissible under the current standard. 

Therefore, such adjustments are clearly 

useful to inform a decision on the issue 

before EPA (i.e., the adequacy of the 

level of public health protection 

associated with allowable NO2 air 

quality under the standard). Similarly, 

air quality adjustments to simulate 

different potential alternative standards 

provide information on exposures and 

risks that would be permissible under 

these alternatives.10 As noted above, in 

their letter to the Administrator on the 

final REA (Samet, 2008b), CASAC 

concluded that ‘‘The REA provides the 

needed bridge from the evidence 

presented in the ISA to a 

characterization of the exposures and 

the associated risks with different 

profiles of exposure.’’ 

We agree that there are uncertainties 

inherent in air quality adjustments. 

These uncertainties are discussed 

thoroughly in the REA (sections 7.4, 

8.12, 9.6, and 10.3.2.1) and in the 

proposed rule (section II.C.3). For 

example, the policy assessment chapter 

of the REA (section 10.3.2.1) noted the 

following regarding adjustment of NO2

concentrations: 

In order to simulate just meeting the 

current annual standard and many of the 

alternative 1-h standards analyzed, an 

upward adjustment of recent ambient NO2

concentrations was required. We note that 

this adjustment does not reflect a judgment 

that levels of NO2 are likely to increase under 

the current standard or any of the potential 

alternative standards under consideration. 

Rather, these adjustments reflect the fact that 

the current standard, as well as some of the 

alternatives under consideration, could allow 

for such increases in ambient NO2

concentrations. In adjusting air quality to 

simulate just meeting these standards, we 

have assumed that the overall shape of the 

distribution of NO2 concentrations would not 

change. While we believe this is a reasonable 

assumption in the absence of evidence 

supporting a different distribution and we 

note that available analyses support this 

approach (Rizzo, 2008), we recognize this as 

an important uncertainty. It may be an 

especially important uncertainty for those 

scenarios where considerable upward 

adjustment is required to simulate just 

meeting one or more of the standards. 

These air quality adjustments are not 

meant to imply an expectation that NO2

concentrations will increase broadly 

across the United States or in any given 

area (REA, section 10.3.2.1). Rather, as 

noted above, they are meant to estimate 

NO2-related exposures and health risks 

that would be permitted under the 

current and potential alternative 

standards. Such estimates can inform 

decisions on whether the current 

standard, or particular potential 

alternative standards, provide the 

requisite protection of public health. 

3. Conclusions Regarding the Adequacy 

of the Current Standard 

In considering the adequacy of the 

current standard, the Administrator has 

considered the scientific evidence 

assessed in the ISA, the exposure and 

risk results presented in the REA, the 

conclusions of the policy assessment 

chapter of the REA, and comments from 

CASAC and the public. These 

considerations are described below. 

In considering the scientific evidence 

as it relates to the adequacy of the 

current standard, the Administrator 

notes that the epidemiologic evidence 

has grown substantially since the last 

review with the addition of field and 

panel studies, intervention studies, and 

time-series studies of effects such as 

emergency department visits and 

hospital admissions associated with 


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short-term NO2 exposures. No 

epidemiologic studies were available in 

1993 assessing relationships between 

NO2 and outcomes such as hospital 

admissions or emergency department 

visits. 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). 

As an initial consideration with 

regard to the adequacy of the current 

standard, the Administrator notes 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 conclusion was 

supported primarily by a large body of 

recent epidemiologic studies that 

evaluated associations of short-term 

NO2 concentrations with respiratory 

symptoms, emergency department 

visits, and hospital admissions. Given 

these conclusions from the ISA, the 

Administrator judges 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. 

In considering the NO2 epidemiologic 

studies as they relate to the adequacy of 

the current standard, the Administrator 

notes 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 in the proposal (II.E.1) 

and above, the Administrator 

acknowledges that 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. 

She notes that, 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 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 cooccurring pollutants by employing 

multi-pollutant models. When evaluated 

as a whole, NO2 effect estimates in these 

models generally remained robust when 

co-pollutants were included. Therefore, 

despite uncertainties associated with 

separating the effects of NO2 from those 

of co-occurring pollutants, the ISA 

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

‘‘the evidence summarized in this 

assessment indicates that NO2

associations generally remain robust in 

multi-pollutant models and supports a 

direct effect of short-term NO2 exposure 

on respiratory morbidity at ambient 

concentrations below the current 

NAAQS.’’ With regard to indoor studies, 

the ISA noted that these studies can test 

hypotheses related to NO2 specifically 

(ISA, section 3.1.4.1). Although 

confounding by indoor combustion 

sources is a concern, indoor studies are 

not confounded by the same mix of 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). Given the above 

consideration of the evidence, 

particularly the epidemiologic studies 

reporting NO2-associated health effects 

in locations that meet the current 

standard, the Administrator agrees with 

the conclusion in the policy assessment 

chapter of the REA that the scientific 

evidence calls into question the 

adequacy of the current standard to 

protect public health. 

In addition to the evidence-based 

considerations described above, the 

Administrator has considered the extent 

to which exposure- and risk-based 

information can inform decisions 

regarding the adequacy of the current 

annual NO2 standard. While she 

acknowledges the uncertainties 

associated with adjusting air quality in 

these analyses, she judges that such 

analyses are appropriate for 

consideration in this review of the NO2

primary NAAQS. In reaching this 

conclusion she notes the considerations 

discussed above, particularly the 

endorsement by CASAC of the REA and 

its characterization of NO2-associated 

exposures and health risks. 

In considering the exposure- and riskbased information with regard to the 

adequacy of the current annual NO2

standard to protect the public health, 

the Administrator notes the conclusion 

in the policy assessment chapter of the 

REA that risks estimated to be 

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 300 ppb, and 

most locations evaluated could 

experience on-/near-road NO2

concentrations above 100 ppb on more 

than half of the days in a given year. 

Therefore, after considering the results 

of the exposure and risk analyses 

presented in the REA the Administrator 

agrees with the conclusion of the policy 

assessment chapter of the REA that 

exposure- and risk-based results 

reinforce the scientific evidence in 


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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. 

In reaching a conclusion on the 

adequacy of the current standard, the 

Administrator has also considered 

advice received from CASAC. In their 

comments on the final 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. 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). 

Based on the considerations discussed 

above, the Administrator concludes that 

the current NO2 primary NAAQS alone 

is not requisite to protect public health 

with an adequate margin of safety. 

Accordingly, she concludes that the 

NO2 primary standard should be revised 

in order to provide increased public 

health protection against respiratory 

effects associated with short-term 

exposures, particularly for susceptible 

populations such as asthmatics, 

children, and older adults. In 

considering approaches to revising the 

current standard, the Administrator 

concludes that it is appropriate to 

consider setting a new short-term 

standard (see below). 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. Elements of a New Short-Term 

Standard 

In considering a revised 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 shortterm exposure to NO2 concentrations 

reported in controlled human exposure 

studies to increase airway 

responsiveness in asthmatics. The 

Administrator’s considerations with 

regard to her decisions are discussed in 

the following sections in terms of 

indicator (II.F.1), averaging time (II.F.2), 

level (II.F.3), and form (II.F.4). 

1. Indicator 

a. Rationale for Proposed Decision 

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 

considered the conclusions of the ISA 

and the policy assessment chapter of the 

REA as well as the view expressed by 

CASAC. The policy assessment chapter 

of 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 assessed in the ISA 

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 policy 

assessment chapter of the REA 

concluded that the evidence supports 

retaining NO2 as the indicator. 

Consistent with this conclusion, the 

CASAC Panel stated 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 

proposed to retain NO2 as the indicator 

in the current review. 

b. Comments on Indicator 

A relatively small number of 

comments directly addressed the issue 

of the indicator for the standard 

(CASAC, Dow, API, AAM, and the 

Missouri Department of Natural 

Resources Air Pollution Control 

Program (MODNR)). All of these 

commenters endorsed the proposal to 

continue to use NO2 as the indicator for 

ambient NOX. 

c. Conclusions on Indicator 

Based on the available information 

discussed above, and consistent with 

the views of CASAC and other 

commenters, the Administrator 

concludes that it is appropriate to 

continue to use NO2 as the indicator for 

a standard that is intended to address 

effects associated with exposure to NO2, 

alone or in combination with other 

gaseous NOX. In so doing, the 

Administrator recognizes that measures 

leading to reductions in population 

exposures to NO2 will also reduce 

exposures to other nitrogen oxides. 

2. Averaging Time 

This section discusses considerations 

related to the averaging time of the NO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

regarding averaging time (II.F.2.a; see 

section II.F.2 of the proposal for more 

detail), discusses comments related to 

averaging time (II.F.2.b), and presents 

the Administrator’s final conclusions 

regarding averaging time (II.F.2.c). 

a. Rationale for Proposed Decision 

In considering the most appropriate 

averaging time for the NO2 primary 

NAAQS, the Administrator noted in the 

proposal the conclusions and judgments 

made in the ISA about available 

scientific evidence, air quality 

correlations discussed in the REA, 

conclusions of the policy assessment 

chapter of the REA, and CASAC 

recommendations (section II.F.2 in the 

proposal). Specifically, she noted the 

following: 

• Experimental studies in humans 

and animals have reported respiratory 

effects following NO2 exposures lasting 

from less than 1-hour up to several 

hours. Epidemiologic studies have 

reported associations between 

respiratory effects and both 1 hour and 

24-hour NO2 concentrations. Therefore, 

the experimental evidence provides 

support for an averaging time of shorter 

duration than 24 hours (e.g., 1 hour) 

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 NO2 concentrations. 

• Air quality correlations presented 

in the policy assessment chapter of the 

REA illustrated the relatively high 

degree of variability in the ratios of 

annual average to short-term NO2

concentrations (REA, Table 10–2). This 


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11As 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. 

variability suggests that a standard 

based on annual average NO2

concentrations would not likely be an 

effective or efficient approach to focus 

protection on short-term exposures. 

• These air quality correlations (REA, 

Table 10–1) suggested that a standard 

based on 1-hour daily maximum NO2

concentrations could also be effective at 

protecting against 24-hour NO2

concentrations. 

• The policy assessment chapter of 

the REA concluded that the scientific 

evidence, combined with the air quality 

correlations, support the 

appropriateness of a standard based on 

1-hour daily maximum NO2

concentrations to protect against health 

effects associated with short-term 

exposures. 

• CASAC concurred ‘‘with having a 

short-term NAAQS primary standard for 

oxides of nitrogen and using the onehour maximum NO2 value’’ (Samet, 

2008b). 

Based on these considerations, the 

Administrator proposed to set a new 

standard based on 1-hour daily 

maximum NO2 concentrations. 

b. Comments on averaging time 

As discussed above, CASAC endorsed 

the establishment of a new standard 

with a 1-hour averaging time. CASAC 

stated the following in their comments 

on the proposal (Samet, 2009): 

In reviewing the REA, CASAC supported a 

short-term standard for NO2 and in reviewing 

the proposal, CASAC supports the proposed 

one-hour averaging time in EPA’s proposed 

rule. 

The supporting rationale offered by 

CASAC in support of a new 1-hour 

standard was generally the same as that 

put forward in the final REA and the 

proposal. Specifically, that rationale 

considered the available scientific 

evidence, which supports a link 

between 1-hour NO2 concentrations and 

adverse respiratory effects, and air 

quality information presented in the 

REA, which suggests that a 1-hour 

standard can protect against effects 

linked to short-term NO2 exposures 

while an annual standard would not be 

an effective or efficient approach to 

protecting against these effects. 

A large number of public commenters 

also endorsed the establishment of a 

new standard with a 1-hour averaging 

time. These included a number of State 

agencies and organizations (e.g., 

NACAA, NESCAUM and agencies in 

CA, IL, NM, TX, VA); environmental, 

medical, and public health 

organizations (e.g., ACCP, ALA, AMA, 

ATS, CAC, EDF, EJ, GASP, NACPR, 

NAMDRC, NRDC); and most individual 

commenters. The supporting rationales 

offered by these commenters often 

acknowledged the recommendations of 

CASAC and the Administrator’s 

rationale as discussed in the proposal. 

Though many industry commenters 

recommended not revising the current 

annual standard (as discussed above in 

section II.E.2), several of these groups 

did conclude that if a short-term 

standard were to be set, a 1-hour 

averaging time would be appropriate 

(e.g., Colorado Petroleum Association 

(CPA), Dow, NAM, Petroleum 

Association of Wyoming (PAW), Utah 

Petroleum Association (UPA)). As 

discussed above, industry commenters 

who disagreed with setting a new 1- 

hour standard generally based this 

conclusion on their interpretation of the 

scientific evidence and their conclusion 

that this evidence does not support the 

need to revise the current annual 

standard. These comments, and EPA’s 

responses, are discussed in more detail 

above (section II.E) and in the Response 

to Comments document. 

c. Conclusions on Averaging Time 

In considering the most appropriate 

averaging time for the NO2 primary 

NAAQS, the Administrator notes the 

available scientific evidence as assessed 

in the ISA, the air quality analyses 

presented in the REA, the conclusions 

of the policy assessment chapter of the 

REA, CASAC recommendations, and 

public comments received. These 

considerations are described below. 

When considering averaging time, the 

Administrator notes that 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 Administrator 

concludes 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, the Administrator notes that 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, 

the Administrator notes that Table 10– 

1 in the REA reported the ratios of shortterm to annual average NO2

concentrations. Ratios of 1-hour daily 

maximum concentrations (98th and 

99th percentile 11) 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 policy assessment chapter 

of 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 Administrator notes 

that the policy assessment chapter of the 

REA considered 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 


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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. The 

Administrator concludes that, at a 

minimum, this suggests that a primary 

concern with regard to averaging time is 

the level of protection provided against 

1-hour NO2 concentrations. However, 

she also notes that it is important to 

consider the ability of a 1-hour 

averaging time to protect against 24- 

hour average NO2 concentrations. To 

this end, the Administrator notes that 

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. 

Based on these considerations, the 

Administrator concludes that a standard 

with a 1-hour averaging time can 

effectively limit short-term (i.e., 1- to 24- 

hours) exposures that have been linked 

to adverse respiratory effects. This 

conclusion is based on the observations 

summarized above and in more detail in 

the proposal, particularly that: (1) The 

1-hour averaging time has been directly 

associated with respiratory effects in 

both epidemiologic and experimental 

studies and that (2) results from air 

quality analyses suggest that a 1-hour 

standard could also effectively control 

24-hour NO2 concentrations. In 

addition, the Administrator notes the 

support provided for a 1-hour averaging 

time in comments from CASAC, States, 

environmental groups, and medical/ 

public health groups. The Administrator 

notes that arguments offered by some 

industry groups against setting a 1-hour 

NO2 standard generally focus on 

commenters’ conclusions regarding 

uncertainties in the scientific evidence. 

As discussed in more detail above 

(section II.E.2), the Administrator 

disagrees with the conclusions of these 

commenters regarding the appropriate 

interpretation of the scientific evidence 

and associated uncertainties. Given 

these considerations, the Administrator 

judges that it is appropriate to set a new 

NO2 standard with a 1-hour averaging 

time. 

3. Form 

This section discusses considerations 

related to the form of the 1-hour NO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

regarding form (II.F.4.a; see section 

II.F.3 of the proposal for more detail), 

discusses comments related to form 

(II.F.4.b), and presents the 

Administrator’s final conclusions 

regarding form (II.F.4.c). 

a. Rationale For Proposed Decision 

When considering alternative forms in 

the proposal, the Administrator noted 

the conclusions in the policy 

assessment chapter of the REA. 

Specifically, she noted the conclusion 

that the adequacy of the public health 

protection provided by the combination 

of standard level and form should be the 

foremost consideration. With regard to 

this, she noted that concentration-based 

forms can better reflect pollutantassociated health risks than forms based 

on expected exceedances. This is the 

case because concentration-based forms 

give proportionally greater weight to 

years when pollutant concentrations are 

well above the level of the standard than 

to years when the concentrations are 

just above the standard, while an 

expected exceedance form would give 

the same weight to years with 

concentrations that just exceed the 

standard as to years when 

concentrations greatly exceed the 

standard. The Administrator also 

recognized the conclusion in the policy 

assessment chapter of the REA that it is 

desirable from a public health 

perspective to have a form that is 

reasonably stable and insulated from the 

impacts of extreme meteorological 

events. With regard to this, she noted 

that a form that calls for averaging 

concentrations over three years would 

provide greater regulatory stability than 

a form based on a single year of 

concentrations. Therefore, consistent 

with recent reviews of the O3 and PM 

NAAQS, the proposal focused on 

concentration-based forms averaged 

over 3 years, as evaluated in the REA. 

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

and 99th percentile concentrations 

averaged over 3 years. This focus on the 

upper percentiles of the distribution is 

appropriate given the reliance, in part, 

on NO2 health evidence from 

experimental studies, which provide 

information on specific exposure 

concentrations that are linked to 

specific health effects. 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 (REA, Table 10– 

4; see Thompson, 2008 for methods). 

Consideration in the REA of an 

appropriate form for a 1-hour standard 

was based on analyses of standard levels 

that reflected the allowable area-wide 

NO2 concentration, not the maximum 

allowable concentration. Therefore, in 

their review of the final REA, CASAC 

did not have the opportunity to 

comment on the appropriateness of 

specific forms in conjunction with a 

standard level that reflects the 

maximum allowable NO2 concentration 

anywhere in an area. Given this, when 

considering alternative forms for the 

1-hour standard in the proposal, the 

Administrator judged that it was 

appropriate to consider both forms 

evaluated in the REA (i.e., 98th and 99th 

percentiles). Therefore, she proposed to 

adopt either a 99th percentile or a 4th 

highest form, averaged over 3 years, and 

she solicited comment on both 98th 

percentile and 7th or 8th highest forms. 

b. CASAC and Public Comments on 

Form 

In their letter to the Administrator, 

CASAC discussed the issue of form 

within the context of the proposed 


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12 In addition, the air quality analyses presented 

in the REA estimated that on-road NO2

concentrations are about 80% higher on average 

than concentrations away from the road (REA, 

section 7.3.2) and that NO2 monitors within 20 m 

Continued 

approach of setting a 1-hour standard 

level that reflects the maximum 

allowable NO2 concentration anywhere 

in an area. CASAC recommended that, 

for such a standard, EPA adopt a form 

based on the 3-year average of the 98th 

percentile of the distribution of 1-hour 

daily maximum NO2 concentrations. 

Specifically, they stated the following in 

their comments on the proposal (Samet, 

2009): 

The 98th percentile is preferred by CASAC 

for the form, given the likely instability of 

measurements at the upper range and the 

absence of data from the proposed two-tier 

approach. 

As indicated in their letter, CASAC 

concluded that the potential instability 

in higher percentile NO2 concentrations 

near major roads argues for a 98th, 

rather than a 99th, percentile form. 

Several State organizations and agencies 

(e.g., NESCAUM and agencies in IN, NC, 

SD, VA) and industry groups (e.g., 

AAM, ACC, API, AirQuality Research 

and Logistics (AQRL), CPA, Dow, 

ExxonMobil, IPAMS, PAW, UPA) also 

recommended a 98th percentile form in 

order to provide regulatory stability. In 

contrast, a small number of State and 

local agencies (e.g., in MO and TX), 

several environmental organizations 

(e.g., EDF, EJ, GASP, NRDC), and 

medical/public health organizations 

(e.g., ALA, ATS) recommended either a 

99th percentile form or a more stringent 

form (e.g., no exceedance) to further 

limit the occurrence of NO2

concentrations that exceed the standard 

level in locations that attain the 

standard. 

c. Conclusions On Form 

The Administrator recognizes that 

there is not a clear health basis for 

selecting one specific form over another. 

She also recognizes that the analyses of 

different forms in the REA are most 

directly relevant to a standard that 

reflects NO2 concentrations permitted to 

occur broadly across a community, 

rather than the maximum concentration 

that can occur anywhere in the area. In 

contrast, as discussed below (section 

II.F.4.c), the Administrator has judged it 

appropriate to set a new 1-hour standard 

that reflects the maximum allowable 

NO2 concentration anywhere in an area. 

In light of this, the Administrator places 

particular emphasis on the comments 

received on form from CASAC relating 

to a 1-hour standard level that reflects 

the maximum allowable NO2

concentration anywhere in an area. In 

particular, the Administrator notes that 

CASAC recommended a 98th percentile 

form averaged over 3 years for such a 

standard, given the potential for 

instability in the higher percentile 

concentrations around major roadways. 

In considering this recommendation, 

the Administrator recognizes that the 

public health protection provided by the 

1-hour NO2 standard is based on the 

approach used to set the standard and 

the level of the standard (see below), in 

conjunction with the form of the 

standard. Given that the Administrator 

is setting a standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area, rather than a 

standard that reflects the allowable areawide NO2 concentration, she agrees 

with CASAC that an appropriate 

consideration with regard to form is the 

extent to which specific statistics could 

be unstable at locations where 

maximum NO2 concentrations are 

expected, such as near major roads. 

When considering alternative forms for 

the standard, the Administrator notes 

that an unstable form could result in 

areas shifting in and out of attainment, 

potentially disrupting ongoing air 

quality planning without achieving 

public health goals. Given the limited 

available information on the variability 

in peak NO2 concentrations near 

important sources of NO2 such as major 

roadways, and given the 

recommendation from CASAC that the 

potential for instability in the 99th 

percentile concentration is cause for 

supporting a 98th percentile form, the 

Administrator judges it appropriate to 

set the form based on the 3-year average 

of the 98th percentile of the annual 

distribution of 1-hour daily maximum 

NO2 concentrations. 

4. Level 

As discussed below and in more 

detail in the proposal (section II.F.4), 

the Administrator has considered two 

different approaches to setting the 

1-hour NO2 primary NAAQS. In the 

proposal, each of these approaches was 

linked with a different range of standard 

levels. Specifically, the Administrator 

proposed to set a 

1-hour standard reflecting the maximum 

allowable NO2 concentration anywhere 

in an area and to set the level of such 

a standard from 80 to 100 ppb. The 

Administrator also solicited comment 

on the alternative approach of setting a 

standard that reflects the allowable areawide NO2 concentration and setting the 

standard level from 50 to 75 ppb. This 

section summarizes the rationale for the 

Administrator’s proposed approach and 

range of standard levels (II.F.3.a), 

describes the alternative approach and 

range of standard levels (II.F.3.b), 

discusses comments related to each 

approach and range of standard levels 

(II.F.3.c), and presents the 

Administrator’s final conclusions 

regarding the approach and level 

(II.F.3.d). 

a. Rationale For Proposed Decisions on 

Approach and Level 

In assessing the most appropriate 

approach to setting the 1-hour standard 

and the most appropriate range of 

standard levels to propose, the 

Administrator considered the broad 

body of scientific evidence assessed in 

the ISA, including epidemiologic and 

controlled human exposure studies, as 

well as the results of exposure/risk 

analyses presented in the REA. In light 

of the body of available evidence and 

analyses, as described above, the 

Administrator concluded in the 

proposal that it is necessary to provide 

increased public health protection for 

at-risk individuals against an array of 

adverse respiratory health effects linked 

with short-term (i.e., 30 minutes to 24 

hours) exposures to NO2. Such health 

effects have been associated with 

exposure to the distribution of shortterm ambient NO2 concentrations across 

an area, including higher short-term 

(i.e., peak) exposure concentrations, 

such as those that can occur on or near 

major roadways and near other sources 

of NO2, as well as the lower short-term 

exposure concentrations that can occur 

in areas not near major roadways or 

other sources of NO2. The 

Administrator’s proposed decisions on 

approach and level, as discussed in 

detail in the proposal (section II.F.4), are 

outlined below. 

In considering a standard-setting 

approach, the Administrator was 

mindful in the proposal that the 

available evidence and analyses from 

the ISA and REA support the public 

health importance of roadwayassociated NO2 exposures. The exposure 

assessment described in the REA 

estimated that roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations 

(REA, Figures 8–17, 8–18). The ISA 

concluded (section 4.3.6) that NO2

concentrations in heavy traffic or on 

freeways ‘‘can be twice the residential 

outdoor or residential/arterial road 

level.’’ In considering the potential 

variability in the NO2 concentration 

gradient, the proposal noted that 

available monitoring studies suggest 

that NO2 concentrations could be 30 to 

100% higher than those in the same area 

but away from the road.12


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of roads measure NO2 concentrations that are, on 

average across locations, 40% higher than 

concentrations measured by monitors at least 100 

m from the road (REA, compare Tables 7–11 and 

7–13). 

13The 98th percentile concentrations in these 

study locations ranged from 85 to 94 ppb. 

14For a standard of 100 ppb, area-wide 

concentrations would be expected to range from 

approximately 50 ppb (assuming near-road 

concentrations are 100% higher than area-wide 

concentrations) to 75 ppb (assuming near-road 

concentrations are 30% higher than area-wide 

concentrations). 

The Administrator also considered 

that millions of people in the United 

States live, work, and/or attend school 

near important sources of NO2 such as 

major roadways (ISA, section 4.4), and 

that ambient NO2 concentrations in 

these locations vary depending on the 

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 that are 

higher than those occurring away from 

major roadways. 

Given the above considerations, the 

Administrator proposed an approach to 

setting the 1-hour NO2 primary NAAQS 

whereby the standard would reflect the 

maximum allowable NO2 concentration 

anywhere in an area. In many locations, 

this concentration is likely to occur on 

or near a major roadway. EPA proposed 

to set the level of the standard such that, 

when available information regarding 

the concentration gradient around roads 

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 

concluded that this approach to setting 

the 1-hour NO2 NAAQS would be 

expected to protect public health against 

exposure to the distribution of shortterm NO2 concentrations across an area 

and would provide a relatively high 

degree of confidence regarding the 

protection provided against peak 

exposures to higher NO2 concentrations, 

such as those that can occur around 

major roadways. The remainder of this 

section discusses the proposed range of 

standard levels. 

In considering the appropriate range 

of levels to propose for a standard that 

reflects the maximum allowable NO2

concentration anywhere in an area, the 

Administrator considered the broad 

body of scientific evidence and 

exposure/risk information as well as 

available information on the 

relationship between NO2

concentrations near roads and those 

away from roads. Specifically, she 

considered the extent to which a variety 

of levels would be expected to protect 

at-risk individuals against increased 

airway responsiveness, respiratory 

symptoms, and respiratory-related 

emergency department visits and 

hospital admissions. 

After considering the scientific 

evidence and the exposure/risk 

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

II.F.4.a.1 through II.F.4.a.3 in the 

proposal), as well as the available 

information on the NO2 concentration 

gradient around roadways (section 

II.A.2 above and in the proposal), the 

Administrator concluded that the 

strongest support is for a standard level 

at or somewhat below 100 ppb. The 

Administrator’s rationale in reaching 

this proposed conclusion is provided 

below. 

The Administrator noted that a 

standard level at or somewhat below 

100 ppb in conjunction with the 

proposed approach would be expected 

to limit short-term NO2 exposures to 

concentrations that have been reported 

to increase airway responsiveness in 

asthmatics (i.e., at or above 100 ppb). 

While she acknowledged that exposure 

to NO2 concentrations below 100 ppb 

could potentially increase airway 

responsiveness in some asthmatics, the 

Administrator also noted uncertainties 

regarding the magnitude and the clinical 

significance of the NO2-induced 

increase in airway responsiveness, as 

discussed in the policy assessment 

chapter of the REA (section 10.3.2.1, 

discussed in section II.F.4.e in the 

proposal). Given these uncertainties, the 

Administrator concluded in the 

proposal that controlled human 

exposure studies provide support for 

limiting exposures at or somewhat 

below 100 ppb NO2. 

The Administrator also noted that a 

standard level at or somewhat below 

100 ppb in conjunction with the 

proposed approach would be expected 

to maintain peak area-wide NO2

concentrations considerably below 

those measured in locations where key 

U.S. epidemiologic studies have 

reported associations with more serious 

respiratory effects, as indicated by 

increased emergency department visits 

and hospital admissions. Specifically, 

the Administrator noted that 5 key U.S. 

studies provide evidence for such 

associations in locations where the 99th 

percentile of the distribution of 1-hour 

daily maximum NO2 concentrations 

measured at area-wide monitors ranged 

from 93 to 112 ppb (Ito et al., 2007; Jaffe 

et al., 2003; Peel et al., 2005; Tolbert et 

al., 2007; and a study by the New York 

State Department of Health, 2006).13

The Administrator concluded that these 

studies provide support for a 1-hour 

standard that limits the 99th percentile 

of the distribution of 1-hour daily 

maximum area-wide NO2

concentrations to below 90 ppb 

(corresponds to a 98th percentile 

concentration of 85 ppb), and that 

limiting area-wide concentrations to 

considerably below 90 ppb would be 

appropriate in order to provide an 

adequate margin of safety. The 

Administrator noted that, based on 

available information about the NO2

concentration gradient around roads, a 

standard level at or somewhat below 

100 ppb set in conjunction with the 

proposed approach would be expected 

to accomplish this. Specifically, she 

noted that given available information 

regarding NO2 concentration gradients 

around roads (see section II.A.2), a 

standard level at or below 100 ppb (with 

either a 99th or 98th percentile form) 

would be expected to limit peak areawide NO2 concentrations to 

approximately 75 ppb or below.14

Therefore, the Administrator concluded 

that a standard level at or somewhat 

below 100 ppb under the proposed 

approach would be expected to 

maintain peak 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. 

The Administrator also noted that a 

study by Delfino provides mixed 

evidence for effects in a location with 

area-wide 98th and 99th percentile 

1-hour daily maximum NO2

concentrations of 50 and 53 ppb, 

respectively. In that study, NO2 effect 

estimates were positive, but some 

reported 95% confidence limits for the 

odds ratio (OR) that included values less 

than 1.00. Given the mixed results of the 

Delfino study, the Administrator 

concluded 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. 

In addition to these evidence-based 

considerations, the Administrator noted 

that a standard level at or somewhat 

below 100 ppb under the proposed 

approach would be consistent with the 


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15This conclusion assumes that near-road NO2

concentrations are 65% higher than area-wide 

concentrations, reflecting the mid-point in the range 

of 30 to 100%. Based on available information 

suggesting that near-road concentrations can be 30 

to 100% higher than area-wide concentrations, a 

standard level of 80 ppb could limit area-wide 

concentrations to between 40 and 60 ppb. 

16CASAC members were also part of the CASAC 

Panel for the NO2 NAAQS review (i.e., the Oxides 

of Nitrogen Primary National Ambient Air Quality 

Standards Panel). Therefore, references to the 

CASAC Panel include both CASAC members and 

Panel members. 

results of the exposure and risk analyses 

presented in the REA. As discussed in 

section II.C of the proposal, the results 

of these analyses provide support for 

setting a standard that limits 1-hour 

area-wide NO2 concentrations to 

between 50 and 100 ppb. As described 

above, a standard level of 100 ppb that 

reflects the maximum allowable NO2

concentration would be expected to 

maintain area-wide NO2 concentrations 

at or below approximately 75 ppb. 

Given all of these considerations, the 

Administrator concluded in the 

proposal that a standard level at or 

somewhat below 100 ppb (with a 99th 

percentile form), in conjunction with 

the proposed approach, would be 

requisite to protect public health with 

an adequate margin of safety against the 

array of NO2-associated health effects. 

In addition to the considerations 

discussed above, which support setting 

a standard level at or somewhat below 

100 ppb, the Administrator also 

considered the extent to which available 

evidence could support standard levels 

below 100 ppb. The Administrator 

concluded that the evidence could 

support setting the standard level below 

100 ppb to the extent the following were 

emphasized: 

• The possibility that an NO2-induced 

increase in airway responsiveness could 

occur in asthmatics following exposures 

to concentrations below 100 ppb and/or 

the possibility that such an increase 

could be clinically significant. 

• The mixed results reported in the 

study by Delfino et al. (2002) of an 

association between respiratory 

symptoms and the relatively low 

ambient NO2 concentrations measured 

in the study area. 

Specifically, she noted that a standard 

level of 80 ppb (99th percentile form), 

in conjunction with the proposed 

approach, could limit area-wide NO2

concentrations to 50 ppb 15 and would 

be expected to limit exposure 

concentrations to below those that have 

been reported to increase airway 

responsiveness in asthmatics. For the 

reasons stated above, the Administrator 

proposed to set the level of a new 

1-hour standard between 80 ppb and 

100 ppb. 

b. Rationale for the Alternative 

Approach and Range of Levels 

As described above, the Administrator 

proposed to set a 1-hour NO2 NAAQS 

reflecting the maximum allowable NO2

concentration anywhere in an area and 

to set the level of such a standard from 

80 to 100 ppb. However, prior to the 

proposal, the approach of setting a 

1-hour NO2 NAAQS that reflects the 

maximum allowable NO2 concentration 

anywhere in an area had not been 

discussed by EPA in the REA or 

considered by CASAC. Rather, the 

potential alternative standards 

discussed in the REA, and reviewed by 

CASAC, reflected allowable area-wide 

NO2 concentrations (i.e., concentrations 

that occur broadly across communities). 

Given this, the Administrator noted in 

the proposal that comments received on 

the approach to setting the 1-hour 

standard (i.e., from CASAC and from 

members of the public) could provide 

important new information for 

consideration. Therefore, the 

Administrator also solicited comment 

on the alternative approach of setting a 

1-hour NO2 primary NAAQS that would 

reflect the allowable area-wide NO2

concentration, analogous to the 

standards evaluated in the REA, and 

with a level set within the range of 50 

to 75 ppb. In discussing this alternative 

approach with a standard level from 50 

to 75 ppb, the Administrator noted the 

following in the proposal: 

• Such a standard would be expected 

to maintain area-wide NO2

concentrations below peak 1-hour areawide concentrations measured in 

locations where key U.S. epidemiologic 

studies have reported associations with 

respiratory-related emergency 

department visits and hospital 

admissions. 

• Standard levels from the lower end 

of the 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. Specifically, a standard 

level of 50 ppb under this approach 

could limit near-road concentrations to 

between approximately 65 and 100 ppb, 

depending on the relationship between 

near-road NO2 concentrations and areawide concentrations. 

• This alternative approach would 

provide relatively more confidence 

regarding the degree to which a specific 

standard level would limit area-wide 

NO2 concentrations and less confidence 

regarding the degree to which a specific 

standard level would limit the peak NO2

concentrations likely to occur near 

major roadways. 

c. Comments on Approach and Level 

In the proposal, each approach to 

setting the 1-hour standard, and each 

range of standard levels, was linked to 

different requirements for the design of 

the NO2 monitoring network. 

Specifically, in conjunction with the 

proposed approach (i.e., standard 

reflects the maximum allowable NO2

concentration anywhere in an area and 

the level is set within the range of 80 to 

100 ppb), the Administrator proposed to 

establish a 2-tiered monitoring network 

that would include monitors sited to 

measure the maximum NO2

concentrations anywhere in an area, 

including near major roadways, and 

monitors sited to measure maximum 

area-wide NO2 concentrations. In 

conjunction with the alternative 

approach (i.e., standard reflects the 

allowable area-wide NO2 concentration 

and the level is set within the range of 

50 to 75 ppb), the Administrator 

solicited comment on a monitoring 

network that would only include areawide NO2 monitors. Because of these 

linkages in the proposal, most 

commenters combined their comments 

on the approach to setting a 1-hour 

standard and on the standard level with 

their comments on the monitoring 

requirements. In this section, we discuss 

comments from CASAC and public 

commenters on the approach to setting 

a 1-hour standard and on the standard 

level. Comments on the monitoring 

network are also discussed in this 

section to the extent they indicate a 

preference for either the proposed or 

alternative approach to setting the 1- 

hour standard. More specific comments 

on monitor placement and network 

design are discussed below in section 

III.B.2 and in the Response to Comments 

document. EPA responses to technical 

comments on the scientific evidence 

and the exposure/response information 

are discussed above in section II.E.2 and 

in the Response to Comments 

document. The Administrator’s 

response to commenters’ views on the 

approach to setting the 1-hour standard 

and on the standard level is embodied 

in the discussed in section II.F.4.d. 

i. CASAC Comments on the Approach 

to Setting the Standard 

A majority of CASAC and CASAC 

Panel members 16 favored the proposed 

approach of setting a 1-hour standard 

that reflects the maximum allowable 


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NO2 concentration anywhere in an area 

and linking such a standard with a 2- 

tiered monitoring network that would 

include both near-road and area-wide 

monitors, though CASAC did not reach 

consensus on this approach. 

Specifically, in their letter to the 

Administrator (Samet, 2009), CASAC 

stated the following: 

There was a split view on the two 

approaches among both CASAC and CASAC 

panel members with a majority of each 

favoring the Agency’s proposed two-tiered 

monitoring network because they thought 

this approach would be more effective in 

limiting near-roadway exposures that may 

reach levels in the range at which some 

individuals with asthma may be adversely 

affected. Other members acknowledged the 

need for research and development of nearroad monitoring data for criteria pollutants in 

general but favored retention of EPA’s 

current area-wide monitoring for NO2

regulatory purposes, due to the lack of 

epidemiological data based on near-roadway 

exposure measurements and issues related to 

implementing a near-road monitoring system 

for NO2. 

Thus, the recommendation of the 

majority of CASAC Panel members was 

based on their conclusion that the 

proposed approach would be more 

effective than the alternative at limiting 

near-roadway exposures to NO2

concentrations that could adversely 

affect asthmatics. In addition, these 

CASAC Panel members noted important 

uncertainties with the alternative 

approach. Specifically, they stated the 

following (Samet, 2009): 

Panel members also supported the 

proposed two-tiered approach because basing 

regulations on area-wide monitoring alone 

was problematic. Such an approach would 

require EPA to embed uncertainties and 

assumptions about the relationship between 

area-wide and road-side monitoring into the 

area-wide standard. 

A minority of CASAC Panel members 

expressed support for the alternative 

approach of setting a 1-hour standard 

that reflects the allowable area-wide 

NO2 concentration. These CASAC Panel 

members concluded that there would be 

important uncertainties associated with 

the proposed approach. Specifically, 

they noted that the key U.S. NO2

epidemiologic studies relied upon areawide NO2 concentrations. In their view, 

the use of area-wide concentrations in 

these studies introduces uncertainty 

into the selection of a standard level for 

a standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area and that is linked with a 

requirement to place monitors near 

major roads. As a result of this 

uncertainty, CASAC Panel members 

who favored the alternative approach 

noted that ‘‘it would be better to set the 

standard on the same area-wide 

monitoring basis as employed in the 

epidemiologic studies upon which it 

[the standard] now relies’’ (Samet, 2009). 

These CASAC Panel members also 

strongly supported obtaining monitoring 

data near major roads, while recognizing 

uncertainties associated with 

identifying appropriate monitoring sites 

near roads (see section III.B.2 and the 

Response to Comments document for 

more discussion of CASAC’s monitoring 

comments). 

ii. Public Comments on the Approach to 

Setting the Standard 

Consistent with the views expressed 

by the majority of CASAC members, a 

number of commenters concluded that 

the most appropriate approach would be 

to set a 1-hour standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area and to couple that 

standard with a requirement that 

monitors be placed in locations where 

maximum concentrations are expected, 

including near major roads. This view 

was expressed by some State and local 

agencies (e.g., in CA, IA, NY, TX, WA, 

WI), by a number of environmental 

organizations (e.g., CAC, EDF, EJ, GASP, 

NRDC), by the ALA, and individual 

commenters. Several additional medical 

and public health organizations (ACCP, 

AMA, ATS, NADRC, NACPR) did not 

explicitly express a recommendation 

regarding the approach though these 

organizations did recommend that, in 

setting a 1-hour standard, particular 

attention should be paid to NOX

concentrations around major roadways. 

In support of their recommendation to 

adopt the proposed approach and to 

focus monitoring around major roads, 

these commenters generally concluded 

that a primary consideration should be 

the extent to which the NO2 NAAQS 

protects at-risk populations that live 

and/or attend school near important 

sources of NO2 such as major roads. As 

such, these comments supported the 

rationale in the proposal for setting a 1- 

hour standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area. 

A number of State commenters 

expressed the view that area-wide 

monitors should be used for attainment/ 

non-attainment determinations (e.g., 

NACAA, NESCAUM and agencies in IL, 

IN, MI, MS, NC, NM, SC). One State 

commenter (NESCAUM) agreed with 

EPA concerns about near-road 

exposures but concluded that it is 

premature to establish a large near-road 

monitoring network at this time due to 

uncertainty regarding the relationship 

between near-road and area-wide NO2

concentrations and the variability in 

that relationship. NESCAUM 

recommended that EPA work with 

States to establish a targeted monitoring 

program in select urban areas to gather 

data that would inform future 

modifications to the monitoring 

network, but that ‘‘[t]he existing areawide monitoring network should be 

used to identify initial nonattainment 

areas.’’ Other State commenters also 

concluded that the most appropriate 

approach would be to base nonattainment determinations only on areawide monitors. Based on their 

monitoring comments, many of these 

commenters appeared to support setting 

a 1-hour standard that reflects the 

allowable area-wide NO2 concentration. 

State concerns with the proposed 

approach often included uncertainties 

associated with identifying and 

accessing appropriate monitor sites near 

major roads, as well as concerns related 

to implementation and cost to States (as 

discussed further in the Response to 

Comments document, the Administrator 

may not consider cost of 

implementation in decisions on a 

NAAQS). 

One commenter (AAM) concluded 

that the focus of the proposed approach 

on NO2 concentrations around major 

roadways is not justified because the 

REA and the proposal overstate the 

extent to which NO2 concentrations 

near roads are higher than NO2

concentrations farther away from the 

road. This conclusion is based on an 

analysis of 42 existing NO2 monitors in 

6 locations. Comparing NO2

concentrations measured by these 

monitors, some of which are closer to 

roads and others of which are farther 

from roads, AAM concluded that 

‘‘roadside monitors are not measuring 

high NO2 concentrations.’’ 

We agree that there is uncertainty 

associated with estimates of roadwayassociated NO2 concentrations (see REA, 

sections 7.4.6 and 8.4.8.3 for detailed 

discussion of these uncertainties) and in 

identifying locations where maximum 

concentrations are expected to occur. 

However, we note that the 

Administrator’s conclusions regarding 

the relationship between NO2

concentrations near roads and those 

away from roads rely on multiple lines 

of scientific evidence and information. 

Specifically, the Administrator relied in 

the proposal on the following in 

drawing conclusions regarding the 

distribution of NO2 concentrations 

across areas: 

• Monitoring studies discussed in the 

ISA and REA that were designed to 

characterize the NO2 concentration 

gradient around roads, which indicated 

that NO2 concentrations near roads can 


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17To measure maximum concentrations, the 

Administrator proposed monitoring provisions that 

would require monitors within 50 meters of major 

roads and to allow the Regional Administrator to 

require additional monitors in situations where 

maximum concentrations would be expected to 

occur in locations other than near major roads (e.g., 

due to the influence of multiple smaller roads and/ 

or stationary sources). 

be approximately 30 to 100% higher 

than concentrations away from the road 

in the same area. 

• Air quality and exposure analyses 

presented in the REA which estimate 

that, on average across locations, NO2

concentrations on roads could be 80% 

higher than those away from roads and 

that roadway-associated exposures 

account for the majority of exposures to 

NO2 concentrations at or above 100 ppb. 

In contrast, the existing NO2

monitoring network, which was the 

basis for the analysis submitted by 

AAM, was not designed to characterize 

the spatial gradients in NO2

concentrations surrounding roadways. 

Rather, concentrations of NO2 measured 

by existing monitors are likely to reflect 

contributions from a combination of 

mobile and stationary sources, with one 

or the other dominating depending on 

the proximity of these sources to the 

monitors. Therefore, we conclude that 

the analysis submitted by AAM, which 

does not consider other relevant lines of 

evidence and information, does not 

appropriately characterize the 

relationship between NO2

concentrations near roads and those 

away from roads. (See the Response to 

Comments document for a more 

detailed discussion of AAM comments.) 

In addition, we note that, although the 

Administrator concluded in the 

proposal that maximum NO2

concentrations in many areas are likely 

to occur around major roads, she also 

recognized that maximum 

concentrations can occur elsewhere in 

an area. For this reason, she proposed to 

set a 1-hour NO2 standard that reflects 

the maximum allowable NO2

concentration anywhere in an area, 

regardless of where that maximum 

concentration occurs.17 Therefore, the 

proposed approach to setting the 

standard would be expected to limit the 

maximum NO2 concentrations 

anywhere in an area even if in some 

areas, as is contended by AAM, those 

maximum NO2 concentrations do not 

occur near roads. 

iii. CASAC Comments on Standard 

Level 

In commenting on the proposal, 

CASAC discussed both the proposed 

range of standard levels (i.e., 80–100 

ppb) and the alternative range of 

standard levels (i.e., 50–75 ppb). 

CASAC did express the consensus 

conclusion that if the Agency finalizes 

a 1-hour standard in accordance with 

the proposed approach (i.e., standard 

level reflects the maximum allowable 

NO2 concentration anywhere in an 

area), then it is appropriate to consider 

the proposed range of standard levels 

from 80 to 100 ppb. Specifically, the 

CASAC letter to the Administrator on 

the proposal (Samet, 2009) stated the 

following with regard to the proposed 

approach: 

[T]he level of the one-hour NO2 standard 

should be within the range of 80–100 ppb 

and not above 100 ppb. In its letter of 

December 2, 2008, CASAC strongly voiced a 

consensus view that the upper end of the 

range should not exceed 100 ppb, based on 

evidence of risk at that concentration. The 

lower limit of 80 ppb was viewed as 

reasonable by CASAC; selection of a value 

lower than 80 ppb would represent a policy 

judgment based on uncertainty and the 

degree of public health protection sought, 

given the limited health-based evidence at 

concentrations below 100 ppb. 

CASAC also recommended that this 

level be employed with a 98th 

percentile form, in order to promote the 

stability of the standard (see above for 

discussion of form). 

iv. Public Comments on Standard Level 

A number of State and local agencies 

and organizations expressed support for 

setting the level of the 1-hour NO2

standard within the proposed range of 

80 to 100 ppb. While some State and 

local agencies (e.g., in CA, IA, MI, NY, 

TX) made this recommendation in 

conjunction with a recommendation to 

focus monitoring near major roads and 

other important sources of NO2, a 

number of State commenters (e.g., 

NACAA, NESCAUM and agencies in IL, 

NC, NM, TX, VA) recommended a 

standard level from 80 to 100 ppb in 

conjunction with a recommendation 

that only area-wide monitors be 

deployed for purposes of determining 

attainment with the standard. Based on 

these monitoring comments, these State 

commenters appear to favor an 

approach where a standard level from 

80 to 100 ppb would reflect the 

allowable area-wide NO2 concentration. 

As discussed above (and in more detail 

in section III.B.2 and the Response to 

Comments document), State 

commenters often based these 

recommendations on uncertainties 

associated with designing an 

appropriate national near-road 

monitoring network. 

A number of environmental 

organizations (e.g., CAC, EDF, EJ, GASP, 

NRDC) and medical/public health 

organizations (e.g., ACCP, ALA, AMA, 

ATS, NACPR, NAMDRC) supported 

setting a standard level below 80 ppb for 

a standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area. Several of these groups 

recommended a standard level of 50 

ppb. This recommendation was 

typically based on the commenters’ 

interpretation of the epidemiologic and 

controlled human exposure evidence, as 

described below. 

Some of these commenters noted that 

the 98th percentile area-wide NO2

concentration was below 80 ppb in the 

location of a single key U.S. 

epidemiologic study (i.e., 50 ppb in 

study by Delfino). Given this, 

commenters concluded that the 

standard level should be set at 50 ppb. 

Their comments on the monitoring 

network generally favored a requirement 

to place monitors near major roads and, 

therefore, these commenters appeared to 

favor a standard level as low as 50 ppb 

and to recommend that such a standard 

level reflect the maximum allowable 

NO2 concentration anywhere in an area. 

In their comments, the ALA, EDF, EJ, 

and NRDC stated the following: 

Considering the Delfino study alone on 

EPA’s terms, that is, focusing on the 98th 

percentile of the 1-hour daily maximum 

concentrations, EPA reports a concentration 

of 50 ppb where asthma symptoms were 

observed. Based primarily on this study, EPA 

concluded in the REA that it was appropriate 

to set the lower end of the range at 50 ppb, 

which corresponded to the lowest-observed 

effects level of airway hyperresponsiveness 

in asthmatics. To provide the strongest 

public health protection, we therefore urge 

the level of the standard be set at 50 ppb. 

In some cases, the same commenters 

also appeared to recommend setting a 

standard level below 50 ppb because 

mean area-wide NO2 concentrations 

reported in locations of key U.S. 

epidemiologic studies are below this 

concentration. Specifically, with regard 

to the key U.S. epidemiologic studies, 

these commenters (e.g., ALA, EDF, EJ, 

NRDC) stated the following: 

These studies clearly identify adverse 

health effects such as emergency room visits 

and hospital admissions for respiratory 

causes at concentrations currently occurring 

in the United States. Mean concentrations for 

all but two of these studies are about or 

below 50 ppb, suggesting that the standard 

must be set below this level to allow for a 

margin of safety. 

The Administrator’s consideration of 

the Delfino study as it relates to a 

decision on standard level is discussed 

below (section II.F.4.d). Regarding the 

recommendation to set the level below 

50 ppb based on mean area-wide NO2

concentrations in epidemiologic study 


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18As discussed above, the Administrator has 

selected the 98th percentile as the form for the new 

1-hour NO2 standard. 

locations, we note that the 

Administrator proposed to set a 

standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area and to set the form of that 

standard at the upper end of the 

distribution of 1-hour daily maximum 

NO2 concentrations.18 As described in 

the proposal, such a standard, with a 

level from the proposed range of 80 to 

100 ppb, would be expected to maintain 

peak area-wide NO2 concentrations 

below the peak area-wide 

concentrations measured in locations 

where key U.S. epidemiologic studies 

have reported associations with 

respiratory-related emergency 

department visits and hospital 

admissions. Because reducing NOX

emissions to meet a 98th percentile NO2

standard should lower the distribution 

of NO2 concentrations, including the 

mean, a standard that limits the 98th 

percentile of the distribution of 1-hour 

daily maximum concentrations would 

also be expected to limit mean 

concentrations. Therefore, although we 

acknowledge that the relationship 

between peak and mean NO2

concentrations will likely vary across 

locations and over time, if peak areawide NO2 concentrations are 

maintained below those in key 

epidemiologic study locations, mean 

area-wide NO2 concentrations would 

also be expected to be maintained below 

the mean area-wide concentrations in 

those locations (see ISA, figure 2.4–13 

for information on the relationship 

between peak and mean NO2

concentrations). 

As discussed above (section, II.E.2), a 

number of industry groups did not 

support setting a new 1-hour NO2

standard. However, several of these 

groups (e.g., AAM, Dow, NAM, NPRA) 

also concluded that, if EPA does choose 

to set a new 1-hour standard, the level 

of that standard should be above 100 

ppb. As a basis for this 

recommendation, these groups 

emphasized uncertainties in the 

scientific evidence. Specifically, as 

discussed in more detail above (section 

II.E.2), these commenters typically 

concluded that available epidemiologic 

studies do not support the conclusion 

that NO2 causes reported health effects. 

This was based on their assertion that 

the presence of co-pollutants in the 

ambient air precludes the identification 

of a specific NO2 contribution to 

reported effects. As a result, these 

commenters recommended that a 1-hour 

standard should be based on the 

controlled human exposure evidence 

and that, in considering that evidence, 

EPA should rely on the meta-analysis of 

NO2 airway responsiveness studies 

conducted by Goodman et al., (2009) 

rather than the meta-analysis included 

in the final ISA. As described above, 

they concluded that in relying on the 

ISA meta-analysis, EPA has 

inappropriately relied on a new 

unpublished meta-analysis that has not 

been peer-reviewed, was not reviewed 

by CASAC, and was not conducted in a 

transparent manner. EPA recognizes the 

uncertainties in the scientific evidence 

that are discussed by these industry 

commenters; however, we strongly 

disagree with their conclusions 

regarding the implications of these 

uncertainties for decisions on the NO2

NAAQS. These comments, and EPA’s 

responses, are discussed in detail above 

(section II.E.2) and in the Response to 

Comments document and are 

summarized briefly below. 

As noted in section II.E.2, we agree 

that the presence of co-pollutants in the 

ambient air complicates the 

interpretation of epidemiologic studies; 

however, our conclusions regarding 

causality are based on consideration of 

the broad body of epidemiologic studies 

(including those employing multipollutant models) as well as animal 

toxicological and controlled human 

exposure studies. The ISA concluded 

that this body of evidence ‘‘supports a 

direct effect of short-term NO2 exposure 

on respiratory morbidity at ambient 

concentrations below the current 

NAAQS level’’ (ISA, p. 5–16). In 

addition, the ISA (p. 5–15) concluded 

the following: 

[T]he strongest evidence for an association 

between NO2 exposure and adverse human 

health effects comes from epidemiologic 

studies of respiratory symptoms and ED 

visits and hospital admissions. These new 

findings were based on numerous studies, 

including panel and field studies, 

multipollutant studies that control for the 

effects of other pollutants, and studies 

conducted in areas where the whole 

distribution of ambient 24-h avg NO2

concentrations was below the current 

NAAQS level of 0.053 ppm (53 ppb) (annual 

average). 

Given that epidemiologic studies 

provide the strongest support for an 

association between NO2 and 

respiratory morbidity, and that a 

number of these studies controlled for 

the presence of other pollutants with 

multi-pollutant models (in which NO2

effect estimates remained robust), we 

disagree that NO2 epidemiologic studies 

should not be used to inform a decision 

on the level of the 1-hour NO2 standard. 

In addition, we agree that uncertainty 

exists regarding the extent to which the 

NO2-induced increase in airway 

responsiveness is adverse (REA, section 

10.3.2.1); however, as discussed in 

detail above (section II.E.2), we disagree 

with the conclusion by many industry 

commenters that this effect is not 

adverse in asthmatics following 

exposures from 100 to 600 ppb NO2. 

Specifically, we do not agree that the 

approach taken in the study by 

Goodman et al. (2009), which was used 

by many industry commenters to 

support their conclusions, was 

appropriate. The authors of the 

Goodman study used data from existing 

NO2 studies to characterize the doseresponse relationship of NO2 and airway 

responsiveness and to calculate the 

magnitude of the NO2 effect. Given the 

protocol differences in existing studies 

of NO2 and airway responsiveness, we 

do not agree that it is appropriate to 

base such an analysis on these studies. 

The Administrator’s consideration of 

these uncertainties, within the context 

of setting a standard level, is discussed 

in the next section. 

d. Conclusions on Approach and 

Standard Level 

Having carefully considered the 

public comments on the appropriate 

approach and level for a 1-hour NO2

standard, as discussed above, the 

Administrator believes the fundamental 

conclusions reached in the ISA and REA 

remain valid. In considering the 

approach, the Administrator continues 

to place primary emphasis on the 

conclusions of the ISA and the analyses 

of the REA, both of which focus 

attention on the importance of roadways 

in contributing to peak NO2 exposures, 

given that roadway-associated 

exposures can dominate personal 

exposures to NO2. In considering the 

level at which the 1-hour primary NO2

standard should be set, the 

Administrator continues to place 

primary emphasis on the body of 

scientific evidence assessed in the ISA, 

as summarized above in section II.B, 

while viewing the results of exposure 

and risk analyses, discussed above in 

section II.C, as providing information in 

support of her decision. 

With regard to her decision on the 

approach to setting the 1-hour standard, 

the Administrator continues to judge it 

appropriate to provide increased public 

health protection for at-risk individuals 

against an array of adverse respiratory 

health effects linked with short-term 

exposures to NO2, where such health 

effects have been associated with 

exposure to the distribution of shortterm ambient NO2 concentrations across 


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19The 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. 

According to Table 1A–6 from that report (http:// 

www.census.gov/hhes/www/housing/ahs/ahs07/ 

tab1a-6.pdf), out of 128.2 million total housing 

units in the United States, about 20 million 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 15.6% of the 

total housing units in the U.S. Assuming equal 

distributions, with a current population of 306.3 

million, that means that there would be 47.8 

million people meeting the 300 foot criteria. 

an area. In protecting public health 

against exposure to the distribution of 

short-term NO2 concentrations across an 

area, the Administrator is placing 

emphasis on providing a relatively high 

degree of confidence regarding the 

protection provided against exposures 

to peak concentrations of NO2, such as 

those that can occur around major 

roadways. Available evidence and 

information suggest that roadways 

account for the majority of exposures to 

peak NO2 concentrations and, therefore, 

are important contributors to NO2- 

associated public health risks. In 

reaching this conclusion, the 

Administrator notes the following: 

• Mobile sources account for the 

majority of NOX emissions (ISA, Table 

2.2–1). 

• The ISA stated that NO2

concentrations in heavy traffic or on 

freeways ‘‘can be twice the residential 

outdoor or residential/arterial road 

level,’’ that ‘‘exposure in traffic can 

dominate personal exposure to NO2,’’ 

and that ‘‘NO2 levels are strongly 

associated with distance from major 

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

higher the NO2 concentration)’’ (ISA, 

sections 2.5.4, 4.3.6). 

• The exposure assessment presented 

in the REA estimated that roadwayassociated exposures account for the 

majority of exposures to peak NO2

concentrations (REA, Figures 8–17, 8– 

18). 

• Monitoring studies suggest that NO2

concentrations near roads can be 

considerably higher than those in the 

same area but away from roads (e.g., by 

30–100%, see section II.A.2). 

• In their comments on the approach 

to setting the 1-hour NO2 standard, the 

majority of CASAC Panel members 

emphasized the importance of setting a 

standard that limits roadway-associated 

exposures to NO2 concentrations that 

could adversely affect asthmatics. These 

CASAC Panel members favored the 

proposed approach, including its focus 

on roads. 

In addition, the Administrator notes 

that a considerable fraction of the 

population resides, works, or attends 

school near major roadways or other 

sources of NO2 and that these 

populations 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).19

Furthermore, in California, 2.3% of 

schools with a total enrollment of more 

than 150,000 students were located 

within approximately 500 feet of hightraffic roads (ISA, section 4.4). Of this 

population, which likely includes a 

disproportionate number of individuals 

in groups with a 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), 

asthmatics and members of other 

susceptible groups (e.g., children, 

elderly) will have the greatest 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). 

In considering the approach to setting 

the 1-hour standard, the Administrator 

also notes that concerns with the 

proposed approach expressed by the 

minority of CASAC Panel members 

included concern with the uncertainty 

in the relationship between near-road 

and area-wide NO2 concentrations, 

given that U.S. epidemiologic studies 

have been based on concentrations 

measured at area-wide monitors. 

However, as discussed by the majority 

of CASAC Panel members, a similar 

uncertainty would be involved in 

setting a standard with the alternative 

approach (Samet, 2009). The 

Administrator agrees with the majority 

of CASAC Panel members and 

concludes that uncertainty in the 

relationship between near-road and 

area-wide NO2 concentrations should be 

considered regardless of the approach 

selected to set the standard. She 

recognizes that this uncertainty can and 

should be taken into consideration 

when considering the level of the 

standard. 

In drawing conclusions on the 

approach, the Administrator has 

considered the extent to which each 

approach, in conjunction with the 

ranges of standard levels discussed in 

the proposal, would be expected to limit 

the distribution of NO2 concentrations 

across an area and, therefore, would be 

expected to protect against risks 

associated with NO2 exposures. 

Specifically, she has considered the 

extent to which a standard set with each 

approach would be expected to limit 

maximum NO2 concentrations and areawide NO2 concentrations. 

With regard to expected maximum 

concentrations, the Administrator notes 

the following: 

• A standard reflecting the maximum 

allowable NO2 concentration anywhere 

in an area would provide a relatively 

high degree of confidence regarding the 

level of protection provided against 

peak exposures, such as those that can 

occur on or near major roadways. A 

standard level from anywhere within 

the proposed range (i.e., 80 to 100 ppb) 

would be expected to limit exposures to 

NO2 concentrations reported to increase 

airway responsiveness in asthmatics. 

• A standard reflecting the allowable 

area-wide NO2 concentration would not 

provide a high degree of confidence 

regarding the extent to which maximum 

NO2 concentrations would be limited. 

Maximum NO2 concentrations would be 

expected to be controlled to varying 

degrees across locations and over time 

depending on the NO2 concentration 

gradient around roads. Given the 

expected variability in gradients across 

locations and over time, most standard 

levels within the range considered in 

the proposal with this option (i.e., 50 to 

75 ppb) would not be expected to 

consistently limit the occurrence of NO2

concentrations that have been reported 

to increase airway responsiveness in 

asthmatics. 

With regard to expected area-wide 

concentrations, the Administrator notes 

the following: 

• The extent to which a standard 

reflecting the maximum allowable NO2

concentration anywhere in an area 

would be expected to limit area-wide 

NO2 concentrations would vary across 

locations, e.g., depending on the NO2

concentration gradient around roads. 

However, in conjunction with a 

standard level from anywhere within 

the proposed range (i.e., 80–100 ppb), 

such an approach would be expected to 

maintain area-wide NO2 concentrations 

below those measured in locations 

where key U.S. epidemiologic studies 

have reported associations between 

ambient NO2 and respiratory-related 

hospital admissions and emergency 

department visits (based on available 

information regarding the NO2

concentration gradient around roads as 

discussed below). 

• A standard reflecting the maximum 

allowable area-wide NO2 concentration 

would provide a relatively high degree 

of certainty regarding the extent to 

which area-wide NO2 concentrations are 

limited. In conjunction with a standard 

level from anywhere within the range of 


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levels discussed in the proposal (i.e., 

50–75 ppb) with this alternative 

approach, such a standard would be 

expected to maintain area-wide NO2

concentrations below those measured in 

locations where key U.S. epidemiologic 

studies have reported associations 

between ambient NO2 and respiratoryrelated hospital admissions and 

emergency department visits. 

Given the above considerations, the 

Administrator concludes that both 

approaches, in conjunction with 

appropriate standard levels, would be 

expected to maintain area-wide NO2

concentrations below those measured in 

locations where key U.S. epidemiologic 

studies have reported associations 

between ambient NO2 and respiratoryrelated hospital admissions and 

emergency department visits. In 

contrast, the Administrator concludes 

that only a standard reflecting the 

maximum allowable NO2 concentration 

anywhere in an area, in conjunction 

with an appropriate standard level, 

would be expected to consistently limit 

exposures, across locations and over 

time, to NO2 concentrations reported to 

increase airway responsiveness in 

asthmatics. After considering the 

evidence and uncertainties, and the 

advice of the CASAC Panel, the 

Administrator judges that the most 

appropriate approach to setting a 1-hour 

standard to protect against the 

distribution of short-term NO2

concentrations across an area, including 

the higher concentrations that can occur 

around roads and result in elevated 

exposure concentrations, is to set a 

standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area. 

In considering the level of a 1-hour 

NO2 standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area, the Administrator notes that 

there is no bright line clearly directing 

the choice of level. Rather, the choice of 

what is appropriate is a public health 

policy judgment entrusted to the 

Administrator. This judgment must 

include consideration of the strengths 

and limitations of the evidence and the 

appropriate inferences to be drawn from 

the evidence and the exposure and risk 

assessments. Specifically, the 

Administrator notes the following: 

• Controlled human exposure studies 

have reported that various NO2

exposure concentrations increased 

airway responsiveness in mostly mild 

asthmatics (section II above and II.B.1.d 

in proposal). 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. 

Because concentrations evaluated in 

controlled human exposure studies are 

at the high end of the distribution of 

ambient NO2 concentrations (ISA, 

section 5.3.2.1), these studies most 

directly inform consideration of the 

risks associated with exposure to peak 

short-term NO2 concentrations. 

• Epidemiologic studies (section 

II.B.1.a and b) conducted in the United 

States 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 

are not sited in locations where 

localized peak concentrations are likely 

to occur. Thus, they do not measure the 

full range of ambient NO2

concentrations across the area. Rather, 

the area-wide NO2 concentrations 

measured by these monitors are used as 

surrogates for the distribution of 

ambient NO2 concentrations across the 

area, a distribution that includes NO2

concentrations both higher than (e.g., 

around major roadways) and lower than 

the area-wide concentrations measured 

in study locations. Epidemiologic 

studies evaluate whether area-wide NO2

concentrations are associated with the 

risk of respiratory morbidity. Available 

information on NO2 concentration 

gradients around roadways can inform 

estimates of the relationship between 

the area-wide NO2 concentrations 

measured in epidemiologic study 

locations and the higher NO2

concentrations likely to have occurred 

around roads in those locations, which 

can then inform the decision on the 

level of a standard reflecting the 

maximum allowable NO2 concentration 

anywhere in an area. 

• The risk and exposure analyses 

presented in the REA provide 

information on the potential public 

health implications of setting standards 

that limit area-wide NO2 concentrations 

to specific levels. While 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, she judges that these analyses are 

informative for considering the relative 

levels of public health protection that 

could be provided by different 

standards. 

The Administrator’s consideration of 

the controlled human exposure 

evidence, epidemiologic evidence, and 

exposure/risk information are discussed 

below specifically with regard to a 

decision on the level of a standard that 

reflects the maximum allowable NO2

concentration anywhere in an area. 

In considering the potential for 

controlled human exposure studies of 

NO2 and airway responsiveness to 

inform a decision on standard level, the 

Administrator notes the following: 

• NO2-induced increases in airway 

responsiveness, as reported in 

controlled human exposure studies, are 

logically linked to the adverse 

respiratory effects that have been 

reported in NO2 epidemiologic studies. 

• The meta-analysis of controlled 

human exposure data in the ISA 

reported increased airway 

responsiveness in a large percentage of 

asthmatics at rest following exposures 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. 

Therefore, it is possible that exposure to 

NO2 concentrations below 100 ppb 

could increase airway responsiveness in 

some asthmatics. 

In considering the evidence, the 

Administrator recognizes that the NO2- 

induced increases in airway 

responsiveness reported for exposures 

to NO2 concentrations at or above 100 

ppb could be adverse for some 

asthmatics. However, she also notes that 

important uncertainties exist with 

regard to the extent to which NO2- 

induced increases in airway 

responsiveness are adverse. Specifically, 

she notes the following with regard to 

these uncertainties: 

• The magnitude of the NO2-induced 

increase in airway responsiveness, and 

the extent to which it is adverse, cannot 

be quantified from the ISA metaanalysis (REA, section 10.3.2.1). 

• The NO2-induced increase in 

airway responsiveness in resting 

asthmatics was typically not 

accompanied by increased respiratory 

symptoms, even following exposures to 

NO2 concentrations well above 100 ppb 

(ISA, section 3.1.3.3). 

• The increase in airway 

responsiveness that was reported for 

resting asthmatics was not present in 

exercising asthmatics (ISA, Table 3.1–3). 

Taking into consideration all of the 

above, the Administrator concludes that 

existing evidence supports the 

conclusion that the NO2-induced 

increase in airway responsiveness at or 

above 100 ppb presents a risk of adverse 


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20Some of these studies also included susceptible 

and vulnerable populations (e.g., children in Peel 

et al. (2005); poor and minority populations in Ito 

et al., 2007). 

effects for some asthmatics, especially 

those with more serious (i.e., more than 

mild) asthma. The Administrator notes 

that the risks associated with increased 

airway responsiveness cannot be fully 

characterized by these studies, and thus 

she is not able to determine whether the 

increased airway responsiveness 

experienced by asthmatics in these 

studies is an adverse health effect. 

However, based on these studies the 

Administrator concludes that 

asthmatics, particularly those suffering 

from more severe asthma, warrant 

protection from the risk of adverse 

effects associated with the NO2-induced 

increase in airway responsiveness. 

Therefore, the Administrator concludes 

that the controlled human exposure 

evidence supports setting a standard 

level no higher than 100 ppb to reflect 

a cautious approach to the uncertainty 

regarding the adversity of the effect. 

However, those uncertainties lead her to 

also conclude that this evidence does 

not support setting a standard level 

lower than 100 ppb. 

In considering the more serious health 

effects reported in NO2 epidemiologic 

studies, as they relate to the level of a 

standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area, the Administrator notes the 

following: 

• A cluster of 5 key U.S. 

epidemiologic studies (Ito et al., 2007; 

Jaffe et al., 2003; Peel et al., 2005; 

Tolbert et al., 2007; and a study by the 

New York State Department of Health, 

2006) provide evidence for associations 

between NO2 and respiratory-related 

emergency department visits and 

hospital admissions in locations where 

98th percentile 1-hour daily maximum 

NO2 concentrations measured at areawide monitors ranged from 85 to 94 

ppb. The Administrator judges it 

appropriate to place substantial weight 

on this cluster of key U.S. epidemiologic 

studies in selecting a standard level, as 

they are a group of studies that reported 

positive, and often statistically 

significant, associations between NO2

and respiratory morbidity in multiple 

cities across the United States.20

• A single study (Delfino et al., 2002) 

provides mixed evidence for NO2 effects 

(i.e., respiratory symptoms) in a location 

with a 98th percentile 1-hour daily 

maximum NO2 concentration, as 

measured by an area-wide monitor, of 

50 ppb. In that study, most of the 

reported NO2 effect estimates were 

positive, but not statistically significant. 

Given the variability in the NO2 effect 

estimates in this study, as well as the 

lack of studies in other locations with 

similarly low NO2 concentrations, the 

Administrator judges it appropriate to 

place limited weight on this study, 

compared to the cluster of 5 studies as 

noted above. 

Given these considerations, the 

Administrator concludes that the 

epidemiologic evidence provides strong 

support for setting a standard that limits 

the 98th percentile of the distribution of 

1-hour daily maximum area-wide NO2

concentrations to below 85 ppb. 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 the types 

of respiratory morbidity effects reported 

in these studies. Given the 

considerations discussed above, the 

Administrator judges that it is not 

necessary, based on existing evidence, 

to set a standard that maintains peak 

area-wide NO2 concentrations to below 

50 ppb. 

In considering specific standard levels 

supported by the epidemiologic 

evidence, the Administrator notes that a 

level of 100 ppb, for a standard 

reflecting the maximum allowable NO2

concentration anywhere in the area, 

would be expected to maintain areawide NO2 concentrations well below 85 

ppb, which is the lowest 98th percentile 

concentration in the cluster of 5 studies. 

With regard to this, she specifically 

notes the following: 

• If NO2 concentrations near roads are 

100% higher than concentrations away 

from roads, a standard level of 100 ppb 

would limit area-wide concentrations to 

approximately 50 ppb. 

• If NO2 concentrations near roads are 

30% higher than concentrations away 

from roads, a standard level of 100 ppb 

would limit area-wide concentrations to 

approximately 75 ppb. 

The Administrator has also 

considered the NO2 exposure and risk 

information within the context of the 

above conclusions on standard level. 

Specifically, she notes that the results of 

exposure and risk analyses were 

interpreted as providing support for 

limiting area-wide NO2 concentrations 

to no higher than 100 ppb. Specifically, 

these analyses estimated that a standard 

that limits area-wide NO2

concentrations to approximately 100 

ppb or below would be expected to 

result in important reductions in 

respiratory risks, relative to the level of 

risk permitted by the current annual 

standard alone. As discussed above, a 

standard reflecting the maximum 

allowable NO2 concentration with a 

level of 100 ppb would be expected to 

maintain area-wide NO2 concentrations 

to within a range of approximately 50 to 

75 ppb. Given this, the Administrator 

concludes that a standard level of 100 

ppb is consistent with conclusions 

based on the NO2 exposure and risk 

information. 

Finally, the Administrator notes that 

a standard level of 100 ppb is consistent 

with the consensus recommendation of 

CASAC. 

Given the above considerations and 

the comments received on the proposal, 

the Administrator determines that the 

appropriate judgment, based on the 

entire body of evidence and information 

available in this review, and the related 

uncertainties, is a standard level of 100 

ppb (for a standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area). She concludes 

that such a standard, with the averaging 

time and form discussed above, will 

provide a significant increase in public 

health protection compared to that 

provided by the current annual standard 

alone and would be expected to protect 

against the respiratory effects that have 

been linked with NO2 exposures in both 

controlled human exposure and 

epidemiologic studies. Specifically, she 

concludes that such a standard will 

limit exposures at and above 100 ppb 

for the vast majority of people, 

including those in at-risk groups, and 

will maintain maximum area-wide NO2

concentrations well below those in 

locations where key U.S. epidemiologic 

studies have reported that ambient NO2

is associated with clearly adverse 

respiratory health effects, as indicated 

by increased hospital admissions and 

emergency department visits. 

In setting the standard level at 100 

ppb rather than a lower level, the 

Administrator notes that a 1-hour 

standard with a level lower than 100 

ppb would only result in significant 

further public health protection if, in 

fact, there is a continuum of serious, 

adverse health risks caused by exposure 

to NO2 concentrations below 100 ppb 

and/or associated with area-wide NO2

concentrations well-below those in 

locations where key U.S. epidemiologic 

studies have reported associations with 

respiratory-related emergency 

department visits and hospital 

admissions. Based on the available 

evidence, the Administrator does not 

believe that such assumptions are 

warranted. Taking into account the 

uncertainties that remain in interpreting 

the evidence from available controlled 

human exposure and epidemiologic 

studies, the Administrator notes that the 

likelihood of obtaining benefits to 

public health with a standard set below 


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100 ppb decreases, while the likelihood 

of requiring reductions in ambient 

concentrations that go beyond those that 

are needed to protect public health 

increases. 

Therefore, the Administrator judges 

that a standard reflecting the maximum 

allowable NO2 concentration anywhere 

in an area set at 100 ppb is sufficient to 

protect public health with an adequate 

margin of safety, including the health of 

at-risk populations, from adverse 

respiratory effects that have been linked 

to short-term exposures to NO2 and for 

which the evidence supports a likely 

causal relationship with NO2 exposures. 

The Administrator does not believe that 

a lower standard level is needed to 

provide this degree of protection. These 

conclusions by the Administrator 

appropriately consider the requirement 

for a standard that is neither more nor 

less stringent than necessary for this 

purpose and recognizes that the CAA 

does not require that primary standards 

be set at a zero-risk level or to protect 

the most sensitive individual, but rather 

at a level that reduces risk sufficiently 

so as to protect the public health with 

an adequate margin of safety. 

G. Annual Standard 

In the proposal, the Administrator 

noted that some evidence supports a 

link between long-term exposures to 

NO2 and adverse respiratory effects and 

that CASAC recommended in their 

comments prior to the proposal that, in 

addition to setting a new 1-hour 

standard to increase public health 

protection, the current annual standard 

be retained. CASAC’s recommendation 

was based on the scientific evidence 

and on their conclusion that a 1-hour 

standard might not provide adequate 

protection against exposure to long-term 

NO2 concentrations (Samet, 2008b). 

With regard to an annual standard, 

CASAC and a large number of public 

commenters (e.g., NACAA, NESCAUM; 

agencies from States including CA, IN, 

MO, NC, NY, SC, TX, VA; Tribal 

organizations including Fon du Lac and 

the National Tribal Air Organization; 

environmental/medical/public health 

groups including ACCP, ALA, AMA, 

ATS, CAC, EDF, EJ, GASP, NACPR, 

NAMDRC, NRDC) agreed with the 

proposed decision to maintain an 

annual standard, though their 

recommendations with regard to the 

level of that annual standard differed 

(see below). 

As noted above, CASAC 

recommended ‘‘retaining the current 

standard based on the annual average’’ 

based on the ‘‘limited evidence related 

to potential long-term effects of NO2

exposure and the lack of strong 

evidence of no effect’’ and 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). A number of State 

agencies and organizations also 

recommended maintaining the current 

level of the annual standard (i.e., 53 

ppb). This recommendation was based 

on the conclusion that, while some 

evidence supports a link between longterm NO2 exposures and adverse 

respiratory effects, that evidence is not 

sufficient to support a standard level 

either higher or lower than the current 

level. In addition, a number of industry 

groups (e.g., AAM, API, Dow, INGAA, 

UARG) recommended retaining the 

level of the current annual standard but, 

as described above, did so within the 

context of a recommendation that EPA 

should not set a new 1-hour standard. 

In contrast, some environmental 

organizations and medical/public health 

organizations as well as a small number 

of States (e.g., ALA, EDF, EJ, NRDC, and 

organizations in CA) recommended 

setting a lower level for the annual 

standard. These commenters generally 

supported their recommendation by 

pointing to the State of California’s 

annual standard of 30 ppb and to 

studies where long-term ambient NO2

concentrations have been associated 

with adverse respiratory effects such as 

impairments in lung function growth. 

As discussed above (II.B.3), the 

evidence relating long-term NO2

exposures 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 the case of 

respiratory morbidity, the ISA (section 

5.3.2.4) concluded that ‘‘The high 

correlation among traffic-related 

pollutants made it difficult to accurately 

estimate the independent effects in 

these long-term exposure studies.’’ 

Given these uncertainties associated 

with the role of long-term NO2

exposures in causing the reported 

effects, the Administrator concluded in 

the proposal that, consistent with the 

CASAC recommendation, existing 

evidence is not sufficient to justify 

setting an annual standard with either a 

higher or lower level than the current 

standard. Commenters have not 

submitted any new analyses or 

information that would change this 

conclusion. Therefore, the 

Administrator does not agree with the 

commenters who recommended a lower 

level for the annual standard. 

The Administrator judges that her 

conclusions in the proposal regarding 

the annual standard remain appropriate. 

Specifically, she continues to agree with 

the conclusion that, though some 

evidence does support the need to limit 

long-term exposures to NO2, the existing 

evidence for adverse health effects 

following long-term NO2 exposures does 

not support either increasing or 

decreasing the level of the annual 

standard. In light of this and 

considering the recommendation from 

CASAC to retain the current level of the 

annual standard, the Administrator 

judges it appropriate to maintain the 

level of the annual standard at 53 ppb. 

H. Summary of Final Decisions on the 

Primary NO2 Standard 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the ISA and 

REA, the advice and recommendations 

of the CASAC, and public comments, 

the Administrator has decided to revise 

the existing primary NO2 standard. 

Specifically, the Administrator has 

determined that the current annual 

standard by itself is not requisite to 

protect public health with an adequate 

margin of safety. In order to provide 

protection for asthmatics and other atrisk populations against an array of 

adverse respiratory health effects related 

to short-term NO2 exposure, the 

Administrator is establishing a shortterm NO2 standard defined by the 3-year 

average of the 98th percentile of the 

yearly distribution of 1-hour daily 

maximum NO2 concentrations. She is 

setting the level of this standard at 100 

ppb, which is to reflect the maximum 

allowable NO2 concentration anywhere 

in an area. In addition to setting a new 

1-hour standard, the Administrator 

retains the current annual standard with 

a level of 53 ppb. The new 1-hour 

standard, in combination with the 

annual standard, will provide protection 

for susceptible groups against adverse 

respiratory health effects associated 

with short-term exposures to NO2 and 

effects potentially associated with longterm exposures to NO2. 

III. Amendments to Ambient 

Monitoring and Reporting 

Requirements 

The EPA is finalizing several changes 

to the ambient air monitoring, reporting, 

and network design requirements for the 

NO2 NAAQS. This section discusses the 

changes we are finalizing which are 

intended to support the proposed 1- 


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21A list of approved FRM and FEMs is 

maintained by EPA’s Office of Research and 

Development, and can be found at: http:// 

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

reference-equivalent-methods-list.pdf. 

hour NAAQS and retention of the 

current annual NAAQS as discussed 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 

We are finalizing the proposed 

changes regarding the NO2 Federal 

Reference Method (FRM) or Federal 

Equivalent Method (FEM) analyzers. 

Specifically, we are continuing to use 

the NO2 chemiluminescence FRM and 

are finalizing the requirement that any 

NO2 FRM or FEM used for making 

primary NAAQS decisions must be 

capable of providing hourly averaged 

concentration data. The following 

paragraphs provide background and 

rationale for the continued use of the 

chemiluminescence FRM and the 

decision to finalize the proposed 

changes. 

1. Chemiluminescence FRM and 

Alternative Methods 

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. EPA did not 

propose to discontinue using the 

chemiluminescence FRM, although we 

received some comments from industry 

(Alliance of Automobile Manufacturers, 

Edison Electric, and the National 

Petrochemical and Refiners Association) 

raising concerns about using a method 

that is subject to known interferences 

from certain species of oxides of 

nitrogen known as NOZ. Important 

components of ambient NOZ include 

nitrous acid (HNO2), nitric acid (HNO3), 

and the peroxyacetyl nitrates (PANs). 

The issue of concern in public 

comments is that the reduction of NO2

to NO on the MoOX converter substrate 

used in chemiluminescence FRMs 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 overestimate 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). Further, as noted in the ISA 

(ISA Section 2.3), 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. 

The EPA acknowledges that the NOZ

interference in the reported NO2

concentrations collected well 

downwind of NOX source areas and in 

relatively remote areas away from 

concentrated point, area, or mobile 

sources is significantly larger than the 

NOZ interference in NO2 measurements 

taken in urban cores or other areas with 

fresh NOX emissions. To meet the 

primary objective of monitoring 

maximum NO2 concentrations in an 

area, the EPA is requiring NO2 monitors 

to be placed in locations of the expected 

highest concentrations, not in relatively 

remote areas away from NOX sources. 

The required monitors resulting from 

the network design discussed below in 

Section III.B will require monitors to be 

placed near fresh NOX sources or in 

areas of dense NOX emissions, where 

NO2 concentrations are expected to be at 

a maximum, and interference from NOZ

species is at a minimum. Therefore, EPA 

believes that the positive artifact issue, 

although present, is small, relative to 

the actual NO2 being measured. As a 

result EPA believes the 

chemiluminescence FRM is suitable for 

continued use in the ambient NO2

monitoring network, as the potential 

positive bias from NOZ species is not 

significant enough to discontinue using 

the chemiluminescence FRM. 

EPA also received support from some 

industry groups (e.g. Savannah River 

Nuclear Solutions, Teledyne API, and 

the Utility Air Regulatory Groups) and 

States (e.g., MODEQ and NCDENR) to 

further the development of alternative 

methods in determining NO2

concentrations. Such alternative 

methods include the photolyticchemiluminescence method and cavity 

ring-down spectroscopy. As a result, 

EPA will continue working with 

commercial and industrial vendors, to 

identify and evaluate such new 

technologies. These efforts may include 

field testing instruments and further 

characterizing methods in a laboratory 

setting to assess their potential as future 

reference or equivalent methods, and 

their role in more directly measuring 

NO2. 

2. Allowable FRM and FEMs for 

Comparison to the NAAQS 

The current CFR language does not 

prohibit the use of any particular NO2

FRM or FEM to be used in comparison 

to the standard.21 There are designated 

wet chemical methods that are only able 

to report ambient concentration values 

averaged across multiple hours. With 

the establishment of a 1-hour NAAQS, 

any FRM or FEM which is a wet 

chemical based method would not be 

appropriate for use in determining 

compliance of the 1-hour NAAQS 

because they are unable to report hourly 

data. EPA addressed this issue by 

proposing and finalizing that only those 

methods capable of providing 1-hour 

measurements will be comparable to the 

NAAQS. 

a. Proposed Changes to FRM and FEMs 

That May Be Compared to the NAAQS 

EPA proposed that only those FRMs 

or FEMs that are capable of providing 

hourly averaged concentration data may 

be used for comparison to the NAAQS. 

b. Comments 

EPA received comments from some 

State and industry groups (e.g. Missouri, 

North Carolina, and Air Quality 

Research and Logistics) supporting the 

proposed approach to only allowing 

those FRMs or FEMs that are capable of 

providing hourly averaged 

concentration data may be used for 

comparison to both the annual and 1- 

hour NAAQS, and did not receive any 

public comments that objected to the 

proposed approach. 

c. Decisions on Allowable FRM and 

FEMs for Comparison to the NAAQS 

Accordingly, EPA is finalizing the 

proposed changes to 40 CFR Part 58 

Appendix C to allow only data from 

FRM or FEMs that are capable of 

providing hourly data to be used for 

comparison to both the annual and 1- 

hour NAAQS. 

B. Network Design 

With the establishment of a 1-hour 

NO2 NAAQS intended to limit exposure 

to maximum concentrations that may 

occur anywhere in an area, EPA 

recognizes that the data from the current 

NO2 network is inadequate to fully 

assess compliance with the revised 


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NAAQS. As a result, EPA is 

promulgating new NO2 network design 

requirements. The following sections 

provide background, rationale, and 

details for the final changes to the NO2

network design requirements. 

1. Two-Tiered Network Design 

A two-tiered monitoring network is 

appropriate for the NO2 NAAQS 

because one tier (the near-road network) 

reflects the much higher NO2

concentrations that occur near-road and 

the second-tier (area-wide) characterizes 

the NO2 concentrations that occur in a 

larger area such as neighborhood or 

urban areas. The ISA (Section 2.5.4 and 

4.3.6) stated that NO2 concentrations in 

heavy traffic or on freeways ‘‘can be 

twice the residential outdoor or 

residential/arterial road level,’’ that 

‘‘exposure in traffic can dominate 

personal exposure to NO2,’’ and that 

‘‘NO2 levels are strongly associated with 

distance from major roads (i.e., the 

closer to a major road, the higher the 

NO2 concentration).’’ The exposure 

assessment presented in the REA 

estimated that roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations 

(REA, Figures 8–17, 8–18). Monitoring 

studies suggest that NO2 concentrations 

near roads can be considerably higher 

than those in the same area but away 

from the road (e.g., by 30–100%, see 

section II.A.2), where pollutants 

typically display peak concentrations on 

or immediately adjacent to roads, 

producing a gradient in pollutant 

concentrations where concentrations 

decrease with increasing distance from 

roads. Since the intent of the revised 

NAAQS is to limit exposure to peak 

NO2 concentrations that occur anywhere 

in an area, monitors intended to 

measure the maximum allowable NO2

concentration in an area should include 

measurements of the peak 

concentrations that occur on and near 

roads due to on-road mobile sources. 

The first tier of the network design, 

which focuses monitoring near highly 

trafficked roads in urban areas where 

peak NO2 concentrations are likely to 

occur, is intended to measure maximum 

concentrations anywhere in an area, 

particularly those due to on-road mobile 

sources since roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations. 

The basis for the second tier of the 

network design is to measure the 

highest area-wide concentrations to 

characterize the wider area impact of a 

variety of NO2 sources on urban 

populations. Area-wide monitoring of 

NO2 also serves to maintain continuity 

in collecting data to inform long-term 

pollutant concentration trends analysis 

and support ongoing health and 

scientific research. 

This section discusses the two-tier 

network design approach compared to 

the alternative network design which 

was also presented for comment in 

conjunction with a solicitation for 

comment on an alternative NAAQS. The 

alternative network design concept was 

based entirely on requiring only 

monitors that would be considered areawide, while not requiring any near-road 

monitoring sites. The details of the twotier network design, including how 

many monitors are required, where they 

are to be located, and the related siting 

criteria are discussed in subsequent 

sections. 

a. Proposed Two-Tier Network Design 

EPA proposed a two-tier network 

design composed of (1) near-road 

monitors which would be placed in 

locations of expected maximum 1-hour 

NO2 concentrations near heavily 

trafficked roads in urban areas and (2) 

monitors located to characterize areas 

with the highest expected NO2

concentrations at the neighborhood and 

larger spatial scales (also referred to as 

‘‘area-wide’’ monitors). As an alternative, 

and in conjunction with a solicitation 

for comment on an alternative NAAQS, 

EPA solicited comment on a network 

comprised of only area-wide monitors. 

b. Comments 

EPA received many comments on the 

overall two-tier network design, with 

those who made statements with a 

relatively clear position on the issue 

generally falling into four categories: (1) 

Those who support the adoption of the 

proposed two-tier design approach, (2) 

those who support the adoption of the 

two-tier concept, but with 

modifications, (3) those who only 

supported the adoption of the 

alternative network design, and (4) 

those who encourage EPA to commit to 

further research of the near-road 

environment by monitoring near-roads, 

but not to use near-road data for 

regulatory purposes, and therefore 

support the alternative network design 

in which EPA solicited comment on a 

network design composed only of areawide monitors. 

Those commenters who generally 

supported the proposed two-tier 

network, included CASAC (while there 

was not a consensus, a majority were in 

support of the proposed network 

design), public health organizations 

(e.g., AACPR, ACCP, AMA, ATA, and 

NAMDRC), several State groups (e.g., 

the New York City Law Department and 

the Metropolitan Washington Air 

Quality Committee), and some industry 

commenters (e.g., American Chemistry 

Council, The Clean Energy Group, and 

Dow Chemical). 

Those commenters who supported the 

adoption of the two-tier network design 

concept, but suggested modifications to 

the actual design included some health 

and environmental organizations (e.g., 

ALA, EDF, EJ, and the NRDC), some 

States (e.g., California, the Central 

Pennsylvania Clean Air Board, Harris 

County (Texas), Iowa, New York, San 

Joaquin Air Pollution Control District, 

Spokane Regional Clean Air Agency 

(SRCAA), the Texas Commission on 

Environmental Quality, and Wisconsin), 

and some industry commenters, 

including the American Petroleum 

Institute and the Utility Air Regulatory 

Group, who are cited by other industry 

commenters. We believe that although 

these commenters made suggestions to 

modify the proposed two-tier network 

design, they are indicating that it is an 

acceptable approach. Their comments 

and suggestions are discussed in greater 

detail in the following sections. 

Those commenters who only 

supported the adoption of the 

alternative network design included 

State and industry groups (e.g., Indiana 

Department of Environmental 

Management, the New York Department 

of Transportation (NYSDOT), Alliance 

of Automobile Manufacturers, and the 

Engine Manufacturers Association). 

These commenters typically made 

comments on the two-tier network 

design, but did not do so in a way that 

clearly supported near-road research. 

EPA received comments from some 

States or State organizations (e.g., 

National Association of Clean Air 

Agencies (NACAA), the Northeast States 

for Coordinated Air Use Management 

(NESCAUM), and 10 other individual 

States or State groups) and industry 

commenters (e.g., Consumers Energy, 

Edison Electric, and the National 

Association of Manufacturers) that 

encouraged EPA to further research the 

near-road environment, opposing use of 

near-road monitoring data for regulatory 

purposes, and supported the adoption of 

the alternative network design for 

regulatory purposes. For example, with 

regard to implementing the two-tier 

network design that includes near-road 

regulatory monitoring, NACAA stated 

that ‘‘* * *a major new network— 

particularly one that is inherently 

complicated and untried—should not be 

rolled out without the benefit of an 

effective near-road monitoring research 

program that can address many of the 

relevant data questions, and inform the 

specific siting requirements of the rule.’’ 

The NAM stated that ‘‘conducting such 


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a near road [research] monitoring 

program would allow EPA to collect 

necessary data that can be used to better 

understand the health impacts 

associated with short term NO2

exposures.’’ 

The EPA notes that the existing 

scientific research referenced in the 

proposal and throughout this final rule 

show that there are on- and near-road 

peaks of NO2 concentrations, relative to 

upwind or background levels, which 

exist due to on-road mobile source 

emissions. This research, as a body of 

evidence, also identifies the multiple 

local factors that affect how, where, and 

when peak NO2 concentrations occur on 

or near a particular road segment. These 

factors include traffic volume, fleet mix, 

roadway design, congestion patterns, 

terrain, and meteorology. The EPA and 

States have access to such data typically 

through Federal, State, and/or local 

departments of transportation or other 

government organizations, and, as a 

result, are in a position to implement a 

near-roar monitoring network that is 

intended to measure maximum 

expected NO2 concentrations resulting 

from on-road mobile source emissions. 

Further, EPA notes that near-road 

monitoring is not a new objective for the 

ambient air monitoring community as 

near-road carbon monoxide monitoring 

has been a part of ongoing, long-term, 

routine networks for nearly three 

decades. As a result, there is experience 

within EPA (both OAR and ORD) and 

State and local agencies on conducting 

ambient monitoring near-roads. In 

addition, EPA intends to develop 

guidance with input from all 

stakeholders to assist with 

implementation of the monitoring 

requirements, which is discussed in 

section III.B.5. EPA believes that the 

existing science and research provide a 

sufficient base of information to require 

a near-road monitoring network and that 

the collective experience that exists in 

the ambient monitoring community will 

allow for successful implementation of 

that network. EPA also believes that 

through adherence of requirements for 

near-road site selection and siting 

criteria discussed in sections III.B.6 and 

III.B.7, respectively, that the two-tier 

network design will provide a network 

that has a reasonable degree of 

similarity across the country where the 

required near-road monitors are 

targeting the maximum NO2

concentrations in an area attributable to 

on-road mobile sources. 

Some industry commenters (e.g., 

Engine Manufacturers Association, the 

South Carolina Chamber of Commerce, 

and the South Carolina Manufacturers 

Alliance) who supported the adoption 

of the alternative network design 

suggested that monitoring in the nearroad environment would not be 

indicative of exposure for general 

populations, and that EPA should not 

focus on the near-road environment 

when requiring monitoring. For 

example, the South Carolina Chamber of 

Commerce and the South Carolina 

Manufacturers Alliance both state that 

‘‘it appears the proposed monitoring 

network will result in a collection of 

microscale data, which is not at all 

representative of air quality relevant to 

population exposure.’’ 

The EPA notes that the intent of a 

near-road monitoring is to support the 

revised NAAQS by assessing peak NO2

concentrations that may occur anywhere 

in an area. EPA recognizes that there is 

variability in the properties (such as 

traffic counts, fleet mix, and localized 

features) among the road segments that 

may exist in an area, but on the whole, 

roads are ubiquitous, particularly in 

urban environments. Consequently, a 

substantial fraction of the population is 

potentially exposed to relatively higher 

concentrations of NO2 that can occur in 

the near-road environment. The 2007 

American Housing Survey (http:// 

www.census.gov/hhes/www/housing/ 

ahs/ahs07/ahs07.html) estimates that 

over 20 million housing units are within 

300 feet (91 meters) of a 4-lane highway, 

airport, or railroad. Using the same 

survey, and considering that the average 

number of residential occupants in a 

housing unit is approximately 2.25, it is 

estimated that at least 45 million 

American citizens live near 4-lane 

highways, airports, or railroads. 

Although that survey includes airports 

and railroads, roads are the most 

pervasive of the three, indicating that a 

significant amount of the general 

population live near roads. 

Furthermore, the 2008 American Time 

Use Survey (http://www.bls.gov/tus/) 

reported that the average U.S. civilian 

spent over 70 minutes traveling per day. 

Accordingly, EPA concludes that 

monitors near major roads will address 

a component of exposure for a 

significant portion of the general 

population that would otherwise not be 

addressed. 

The majority of State commenters, 

regardless of their position on the 

proposed network design, along with 

some industry commenters, observed 

that there was a need for funding the 

monitoring network. These comments 

urged EPA to provide the resources 

needed to implement and operate the 

required monitoring network. EPA notes 

that it has historically funded part of the 

cost of the installation and operation of 

monitors used to satisfy Federal 

monitoring requirements. EPA 

understands these concerns, although 

the CAA requirements from which this 

final rule derives (CAA sections 110, 

310(a) and 319) are not contingent on 

EPA providing funding to States to 

assist in meeting monitoring 

requirements. However, EPA intends to 

work with NACAA and the State and 

local air agencies in identifying 

available State and Tribal Air Grant 

(STAG) funds and consider the 

increased resource needs that may be 

needed to plan, implement, and operate 

this revised set of minimum 

requirements. 

c. Conclusions Regarding the Two-Tier 

Network Design 

The EPA believes that requiring nearroad monitors in urban areas as part of 

the network design are necessary to 

protect against risks associated with 

exposures to peak concentrations of NO2

anywhere in an area. The combination 

of increased mobile source emissions 

and increased urban population 

densities can lead to increased 

exposures and associated risks, 

therefore urban areas are the appropriate 

areas to concentrate required near-road 

monitoring efforts. The EPA also 

recognizes the need to have monitors in 

neighborhood and larger spatial scale 

locations away from roads that represent 

area-wide concentrations. These types 

of monitors serve multiple important 

monitoring objectives including 

comparison to the NAAQS, 

photochemical pollutant assessment, 

ozone forecasting, characterization of 

point and area source impacts, and by 

providing historical trends data for 

current and future epidemiological 

health research. In some situations, 

when coupled with data from near-road 

monitors, area-wide monitors may also 

assist in the determination of spatial 

variation of NO2 concentrations across a 

given area and provide insight to the 

gradients that exist between near-road 

or stationary source oriented 

concentrations and area-wide 

concentration levels. 

After considering the scientific data 

and the public comments regarding the 

proposed network design, the 

Administrator concludes that a two-tier 

network design composed of (1) nearroad monitors which would be placed 

in locations of expected maximum 1- 

hour NO2 concentrations near heavily 

trafficked roads in urban areas and (2) 

monitors located to characterize areas 

with the maximum expected NO2

concentrations at the neighborhood and 

larger spatial scales (also referred to as 

‘‘area-wide’’ monitors) are needed to 

implement the 1-hour NO2 NAAQS and 


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22Of the 24 additional sites, 22 are estimated to 

be triggered due to a population of 2,500,000 while 

2 (Las Vegas, NV and Sacramento, CA) are 

estimated to be triggered by the presence of one or 

more road segments with 250,000 AADT since they 

do not have a population of 2,500,000 people. 

23AASHTO, NESCAUM, and NYDOT did not 

support the two-tier network design; however they 

provided suggestions on how the network design 

might be modified if the EPA were to finalize 

requirements for near-road monitors. In the case of 

AASHTO and NYDOT, their suggestions were made 

with the suggestion that EPA use a separate 

rulemaking process to require monitors. 

support the annual NAAQS. The details 

of this two-tier network design are 

discussed in the following eight 

sections. 

2. First Tier (Near-Road Monitoring 

Component) of the NO2 Network Design 

This section provides background, 

rationale, and details for the final 

changes to the first tier of the two-tier 

NO2 network design. In particular, this 

section will focus on the thresholds that 

trigger monitoring requirements. Nearroad site selection and siting criteria 

details will be discussed in subsequent 

sections. 

a. Proposed First Tier (Near-Road 

Monitoring Component) of the Network 

Design 

EPA proposed that the first tier of the 

two-tier NO2 monitoring network design 

focus monitors in locations of expected 

maximum 1-hour concentrations near 

major roads in urban areas. As noted in 

the previous section, the exposure 

assessment presented in the REA 

estimated that roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations 

(REA, Figures 8–17, 8–18). Since the 

combination of increased mobile source 

emissions and increased urban 

population densities leads to increased 

exposures and associated risks, the 

Administrator judges that urban areas 

are the appropriate areas in which to 

concentrate required near-road 

monitoring efforts. Therefore, we 

proposed that a minimum of one nearroad NO2 monitor be required in Core 

Based Statistical Areas (CBSAs) with a 

population greater than or equal to 

350,000 persons. Based on 2008 Census 

Bureau statistics, EPA estimated this 

would result in approximately 143 

monitoring sites in as many CBSAs. 

We also proposed 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 

Annual Average Daily Traffic (AADT) 

count greater than or equal to 250,000. 

Based on 2008 Census Bureau statistics 

and data from the 2007 Highway 

Performance Monitoring System 

(HPMS) maintained by the U.S. 

Department of Transportation (DOT) 

Federal Highway Administration 

(FHWA), this particular element of the 

minimum monitoring requirements 

would have added approximately 24 22

sites to the approximate 143 near-road 

sites in CBSAs that already would have 

had one near-road monitor required due 

to the 350,000 population threshold. 

Overall, the first tier of the proposed 

network design was estimated to require 

167 near-road sites in 143 CBSAs. 

b. Comments 

The EPA received comments from 

some industry and public health 

organizations (e.g. Dow Chemical, ATS, 

and the AMA) supporting the proposed 

approach to use population thresholds 

for triggering minimum near-road 

monitoring requirements. For example, 

Dow Chemical Company stated that 

‘‘Dow comments that the proposed 

population thresholds are reasonable for 

implementation of the new network 

design and that we don’t see a need to 

establish a threshold lower than 350,000 

people for the lower bound.’’ 

The EPA received comments from 

some States and State groups suggesting 

that a combination of population and 

AADT counts or just AADT counts 

should be used to trigger minimum 

near-road monitoring requirements. For 

example, the San Joaquin Air Pollution 

Control District in California suggested 

that we modify minimum monitoring 

requirements so that one near-road NO2

monitor is required for any CBSA with 

a population of 350,000 people which 

also had one or more road segments 

with AADT counts of 125,000 or more. 

In another example, Harris County 

Public Health and Environmental 

Services (HCPHES) suggested that 

‘‘* * * rather than specifying 

population limits for the monitoring, 

HCPHES supports a metric like the 

Annual Average Daily Traffic (AADT) as 

a threshold for requiring a near-road 

monitor. An initial focus on an AADT 

in excess of 250,000 is acceptable as a 

starting point but EPA should revisit 

that level and consider lowering it to 

100,000 in five years.’’ AASHTO 23 and 

NYDOT23 suggested that EPA could set 

a threshold at 140,000 AADT for 

requiring near-road monitors rather than 

using population thresholds. 

EPA is finalizing the population-only 

threshold approach to trigger near-road 

monitoring, as the first step in the 

process of establishing the first-tier of 

near-road monitors, and for identifying 

the appropriate number and locations 

for siting these monitors. EPA believes 

that the uncertainty in defining specific 

national AADT counts is too great to 

support use in this first step of the 

alternative approaches suggested by the 

commenters. EPA notes that, in general, 

roads with higher AADT counts have 

relatively higher amounts of mobile 

source emissions, leading to an 

increased potential for relatively higher 

on-road and roadside NO2

concentrations. This concept is 

supported, for example, by Gilbert et al., 

2007, who state that the NO2

concentrations analyzed in their study 

are significantly associated with traffic 

counts. In part, these suggestions by 

commenters to include AADT counts as 

part of, or independently as, a threshold 

for requiring monitors appears to be 

aimed at increasing the focus of the 

near-road network to locations where 

NO2 concentrations are expected to be 

highest. However these suggestions 

would also, in effect, reduce the size of 

the required network compared to the 

network that EPA had proposed. The 

differences in fleet mix, roadway design, 

congestion patterns, terrain, and local 

meteorology amongst road segments that 

may have identical AADTs are quite 

variable and affect the NO2

concentrations on and near those 

segments. The available data and related 

technical and scientific quantification of 

what particular AADT count might be 

expected to contribute to some specific 

NO2 concentration is insufficient to 

establish a specific, nationally 

applicable AADT count threshold that 

could be used as part of a populationAADT combination, or a distinct AADT 

count, to require all near-road monitors. 

Therefore, EPA chose not to utilize a 

population-AADT or an AADT-only 

threshold to trigger all minimally 

required near-road monitoring because 

of the lack of a quantitative, nationally 

applicable relationship between a 

certain AADT threshold and an 

expected NO2 concentration. Instead, 

EPA is finalizing the proposed 

population-only threshold approach to 

trigger a minimum of one monitor in a 

CBSA. In larger CBSAs, EPA does 

require, at a minimum, a second 

monitor based on either an AADT count 

of 250,000 or a population threshold of 

2,500,00 or more persons in a CBSA as 

described more fully below. EPA 

believes this approach for siting nearroad monitoring provides a greater 

degree of certainty in covering a large 

segment of the total population (66%, 

which is explained below) and will 

provide data on exposure from 

geographically and spatially diverse 

areas where a larger number of people 


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are likely to be exposed to peak NO2

concentrations. 

Some commenters (e.g., AASHTO,23

NESCAUM,23 NYDEC, NYDOT23) 

suggested focusing multiple near-road 

monitors only in relatively larger CBSAs 

than those which were proposed. For 

example, NYDEC suggested that EPA 

require, at minimum, two near-road 

monitors in any CBSA of 2,500,000 

people or more, but not in CBSAs below 

that population threshold. In their 

comments, they point out the variety of 

near-road environments that exist in the 

larger CBSAs such as New York City. 

EPA notes that the larger CBSAs, such 

as those with a population of 2,500,000 

or more persons, 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 in different settings 

(evidenced in the U.S. Department of 

Transportation (U.S. DOT) Federal 

Highway Administration (FHWA) 

‘‘Status of the Nation’s Highways, 

Bridges, and Transit: 2006 Conditions 

and Performance’’ document which is 

discussed below). This is the primary 

reasoning behind the requirement for 

two monitors in CBSAs with more than 

2,500,000 people. EPA also believes that 

having multiple monitors in the largest 

CBSAs will allow better understanding 

of the differences that may exist 

between roads in the same CBSA due to 

fleet mix, congestion patterns, terrain, or 

geographic locations. However, EPA 

believes that a network with 

substantially fewer monitors in 

correspondingly fewer CBSAs, as the 

commenters suggested, would lead to an 

insufficient monitoring network lacking 

a balanced approach needed for a 

regulatory network intended to support 

the revised NAAQS on a national basis. 

On a related note to those comments 

that suggested focusing more near-road 

monitors only in the larger CBSAs, EPA 

proposed that any CBSAs with one or 

more road segments with an Annual 

Average Daily Traffic (AADT) count 

greater than or equal to 250,000 must 

have a second monitor if they do not 

already have two near-road monitors 

because of the population threshold. 

Such an AADT-triggered monitor would 

account for situations where a relatively 

less populated area has a very highly 

trafficked road. In this case, EPA notes 

that because those road segments with 

250,000 AADT have been identified by 

U.S. DOT FHWA (http:// 

www.fhwa.dot.gov/policyinformation/ 

tables/02.cfm) as being the top 0.03 

percent of the most traveled public road 

segments, that they are the most heavily 

trafficked roads in the country. Again 

noting that NO2 concentrations are 

significantly associated with traffic 

counts (Gilbert et al. 2007), these roads 

segments likely have the greatest 

potential for high exposures directly 

connected to motor vehicle emissions in 

the entire country. Typically, these very 

highly trafficked roads are in the largest 

populated CBSAs, such as those with 

2,500,000 people or more, and are 

somewhat atypical for CBSAs with less 

than 2,500,000 people. As a result, EPA 

believes it is appropriate to require a 

second monitor in a CBSA that has one 

or more road segments with 250,000 

AADT counts or more if they do not 

already have two near-road monitors 

required due their population. 

EPA received comments requesting 

that EPA explain the rationale for the 

selection of the population thresholds 

that trigger minimum monitoring 

requirements and also to reconsider the 

size of the network. For example, 

NYDOT suggested that this final rule 

explain the basis for the 350,000 and 

2,500,000 population thresholds that 

will establish near-road monitors. In 

another comment, the Clean Air Council 

(CAC) questioned the selected 

population thresholds, noting that they 

believe that the population thresholds 

that were proposed were too high. 

Specifically, CAC stated that ‘‘at 350,000 

persons, numerous metro areas in the 

mid-Atlantic and Northeastern States 

with urban cores and highways running 

through will likely be exempted from 

the new monitors.’’ The Spokane 

Regional Clean Air Agency stated that 

they ‘‘do not believe it is necessary to 

require air quality monitoring for NO2

near major roadways in every 

metropolitan area. It is our [SRCAA’s] 

view that EPA could establish a 

statistically significant number of air 

quality monitoring stations near 

roadways and develop a correlation 

between traffic density and ambient 

NO2 levels.’’ Further, the EPA received 

many State comments suggesting 

reductions to the overall size of the 

near-road network; however the 

commenters did not provide very 

specific suggestions on how EPA should 

accomplish that reduction in size. For 

example, the Regional Air Pollution 

Control Agency, which represents a 

portion of Ohio, stated ‘‘given the fairly 

standard fleet of vehicles on the nation’s 

major highways, we urge EPA to 

consider the need for 142 near-roadway 

monitors. Perhaps a limited number of 

monitors across the country would 

suffice to sufficiently characterize nearroadway NO2 levels.’’ These State 

commenters provided various reasons 

which are discussed throughout this 

document suggesting that the network 

be reduced in size, including funding 

concerns (section III.B.1.b), the 

perceived need to implement a smaller 

near-road research network in lieu of a 

regulatory network (section III.B.1.b), 

safety issues (section III.B.7.b), and 

problems with State implementation 

plans (section VI. D) and designation 

issues (section V). 

EPA notes that the intent of the first 

tier of the network design is to support 

the revised NAAQS in measuring peak 

NO2 exposures in an area by including 

a minimum number of monitors 

resulting in a sufficiently sized national 

near-road monitoring network that will 

provide data from a geographically and 

spatially diverse array of areas, in terms 

of population, potential fleet mixes, 

geographic extent, and geographic 

setting, from across the country. The 

U.S. Department of Transportation (U.S. 

DOT) Federal Highway Administration 

(FHWA) ‘‘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 (U.S.) 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 (U.S.) 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. The 

basis for the selection of the proposed 

CBSA population level of 350,000 to 

trigger the requirement of one near-road 

monitor was chosen in an attempt to 

provide near-road monitoring data from 

a diverse array of areas, as noted above. 

However, in response to the significant 

number of comments discussed above, 

which in various ways encouraged at 

least a reduction of the size of the 

required near-road network or the 

implementation of a relatively smaller 

research network, EPA reconsidered the 

population threshold that will require 

one near-road NO2 monitor in a CBSA. 

EPA reviewed the data, such as 

population, geographic, and spatial 

distribution, associated with particular 

CBSA areas that would and would not 

be included in particular CBSA 

population thresholds. According to the 

2008 U.S. Census Bureau estimates 

(http://www.census.gov) there are 143 

CBSAs with 350,000 or more persons 

(including territories) which contain 

approximately 71% of the total 

population (excluding territories). These 


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CBSAs collectively represent territory in 

44 States, the District of Columbia, and 

Puerto Rico. For comparison, there are 

391 CBSAs with 100,000 or more 

persons, which contain approximately 

86% of the total population (excluding 

territories). These particular CBSAs 

collectively represent territory in 49 

States, the District of Columbia, and 

Puerto Rico. Further, there are 102 

CBSAs with 500,000 or more persons, 

which contain approximately 66% of 

the total population (excluding 

territories). These 102 CBSAs 

collectively represent territory in 43 

States, the District of Columbia, and 

Puerto Rico. Finally, there are 22 CBSAs 

with 2,500,000 or more persons, which 

contain approximately 39% of the total 

population, collectively representing 

territory in 19 States, the District of 

Columbia, and Puerto Rico. In 

comparison to the CBSA population 

threshold of 350,000, the 500,000 

population threshold has 41 less CBSAs. 

However, the percentage of the total 

U.S. population residing in these two 

sets of CBSAs differs by only 

approximately 5 percent of the total 

population (e.g., 71% in CBSAs of 

350,000 or more versus 66% in CBSAs 

of 500,000 or more persons). Also, when 

comparing the number of States that 

have some amount of their territory 

included in these CBSAs, the difference 

between the two sets of CBSAs differs 

by only 1 State (Alaska). 

Further, EPA notes that the REA Air 

Quality Analysis, (REA, section 7.3.2) 

estimated the exceedences of health 

benchmark levels across the United 

States, including explicit consideration 

of on- or near- roadway exceedances in 

17 urban areas associated with CBSA 

populations ranging from approximately 

19,000,000 to 540,000. The analysis 

indicated that all 17 of the areas under 

explicit consideration were estimated to 

experience NO2 concentrations on or 

near roads that exceeded health 

benchmark levels. 

c. Conclusions Regarding the First Tier 

(Near-Road Monitoring Component) of 

the Network Design 

After consideration of public 

comments, and in light of the 

information discussed above, the 

Administrator has chosen to finalize the 

CBSA population threshold for 

requiring a minimum of one near-road 

monitor in CBSAs with a population of 

500,000 or more persons. The 

Administrator is finalizing the other 

thresholds that will trigger a second 

near-road monitor as proposed. 

Accordingly, one near-road NO2

monitor is required in CBSAs with a 

population greater than or equal to 

500,000 persons and a second near-road 

monitor is 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 

Annual Average Daily Traffic (AADT) 

count greater than or equal to 250,000. 

The Administrator has concluded that 

using a population threshold of 500,000 

to require a minimum of one near-road 

monitor in a CBSA provides a 

sufficiently sized, national network of 

near-road monitors that will provide 

data from a geographically and spatially 

diverse set of CBSAs that supports the 

intent of the revised NAAQS and 

continues to meet the monitoring 

objectives of the network. Combined 

with the forty additional monitors that 

the Regional Administrators are 

required to site, discussed below, the 

monitoring network would cover an 

additional percentage of the total 

population. 

EPA believes that selecting a lower 

population threshold, such as 100,000 

or, to a lesser degree, 350,000, as 

discussed in the above examples, would 

create a much larger network of required 

near-road monitors but would provide 

diminished population coverage per 

monitor, compared to that provided by 

the 500,000 threshold. EPA notes that if 

a particular area, such as one with a 

population less than 500,000 people, 

might warrant a near-road monitor, the 

Regional Administrator has the 

authority to require additional monitors. 

The Regional Administrators’ authority 

is discussed in section III.B.4. Further, 

States have the right to conduct 

additional monitoring above the 

minimum requirements on their own 

initiative. In the Administrator’s 

judgment, selecting a higher threshold, 

such as 2,500,000, as was suggested by 

some commenters, does not provide a 

sufficient geographical and spatially 

diverse near-road network, compared to 

that provided by the 500,000 threshold. 

The selection of the 2,500,000 

population threshold to trigger a second 

near-road monitor, as noted earlier in 

this section, is based on the fact that the 

larger urban areas in the country are 

likely to have a greater number of major 

roads across a potentially larger 

geographic area, and have a 

corresponding increase in potential for 

population exposure to elevated levels 

in different settings. 

Changing the CBSA population 

threshold 350,000 to 500,000 results in 

a near-road monitoring network 

requiring approximately 126 monitors 

distributed within 102 CBSAs. 

Compared to the total number of 

required near-road monitors that would 

have resulted from the proposed CBSA 

population threshold of 350,000 (167 

monitors), an estimated 41 fewer 

monitors are required. EPA has also 

recognized that susceptible and 

vulnerable populations, which include 

asthmatics and disproportionately 

exposed groups, (as discussed in 

sections II.B.4 and II.F.4.d) are at 

particular risk of NO2-related health 

effects. The Administrator is therefore 

requiring the Regional Administrators, 

working in collaboration with States, to 

site forty monitors in appropriate 

locations, focusing primarily on 

protecting such susceptible and 

vulnerable communities. This decision 

is discussed in detail in section III.B.4. 

3. Second Tier (Area-Wide Monitoring 

Component) of the Network Design 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the second tier of 

the two-tier NO2 network design. In 

particular, this section will focus on the 

threshold that triggers area-wide 

monitoring requirements. Area-wide site 

selection and siting criteria details will 

be discussed in a subsequent section. 

a. Proposed Second Tier (Area-Wide 

Monitoring Component) of the Network 

Design 

As the second tier of the proposed 

two-tier network design, EPA proposed 

to require monitors to characterize the 

expected maximum NO2 concentrations 

at the neighborhood and larger (areawide) spatial scales in an area. This 

component of the two-tier network 

design provides information on areawide exposures that may occur due to 

an individual or a group of point, area, 

on-road, and/or non-road sources. 

Further, area-wide sites serve multiple 

monitoring objectives aside from 

NAAQS comparison to both the 1-hour 

and the annual NAAQS, including 

photochemical pollutant assessment, 

aiding in ozone forecasting, aiding in 

particulate matter precursor analysis 

and particulate matter forecasting. We 

proposed to require one area-wide 

monitoring site in each CBSA with a 

population greater than or equal to 

1,000,000. We proposed that these areawide sites were to be sited to represent 

an area of highest concentration at the 

neighborhood or larger spatial scales. 

Based on 2008 Census Bureau statistics, 

there are 52 CBSAs with 1,000,000 

people or more, which would result in 

an estimated 52 area-wide monitors in 

as many CBSAs being minimally 

required. EPA also proposed to allow 

any current photochemical assessment 

monitoring station (PAMS) sites that are 

sited where the highest NO2

concentrations occur in an urban area 


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and represent a neighborhood or urban 

scale to satisfy the area-wide monitoring 

requirement. 

b. Comments 

Most commenters who commented on 

area-wide monitoring supported the 

adoption of the alternative area-wide 

network design and did not specifically 

comment on the area-wide monitoring 

component of the proposed two-tier 

network design. However, EPA did 

receive comments from public health 

organizations on area-wide monitoring 

in the context of the proposed network 

design. The public health group 

commenters, including the ALA, EJ, 

EDF, and the NRDC, stated they ‘‘oppose 

the proposed requirement to retain only 

52 air monitors to measure area-wide 

concentrations of NO2.’’ 

EPA understands the perceived 

concern to be that with this provision, 

EPA is actively reducing the number of 

required area-wide monitors. Prior to 

this rulemaking, the Ambient Air 

Monitoring Regulations, 71 FR 61236 

(Oct. 17, 2006) (2006 monitoring rule) 

removed minimum monitoring 

requirements for NO2, and the rationale 

for that action is explained in that rule; 

however, the 2006 Monitoring rule has 

had a limited impact to date, evidenced 

by the fact that the size of the NO2

network has remained relatively steady 

at around 400 monitors, a majority of 

which are area-wide monitors, that were 

operating in 2008 (Watkins and 

Thompson, 2008). The stability of the 

NO2 network is due in large part to the 

fact that area-wide monitors serve 

multiple monitoring objectives, 

including photochemical pollutant 

assessment, pollutant forecasting, and in 

some cases, support to ongoing health 

research. However, considering the 

objective of this two-tier network 

design, particularly the first tier, of 

supporting the revised NAAQS to 

protect against peak NO2 exposures, 

some shrinkage in the area-wide 

network is appropriate and likely. EPA 

believes that the actual number of areawide monitors that will operate in the 

NO2 network will be greater than the 

minimally required 52 sites, but likely 

less than the current number. States and 

Regional Administrators will work 

together on which area-wide sites may 

warrant retention above the minimum 

required if States request existing areawide sites to be shut down or relocated. 

c. Conclusions on the Second Tier 

(Area-Wide Monitoring Component) of 

the Network Design 

Area-wide monitoring sites serve 

multiple monitoring objectives aside 

from NAAQS comparison to both the 1- 

hour and the annual NAAQS, including 

photochemical pollutant assessment, 

ozone forecasting, particulate matter 

precursor analysis and particulate 

matter forecasting. EPA recognizes that 

a significant portion of the existing NO2

monitoring network can be 

characterized as area-wide monitors and 

that these monitoring sites serve 

multiple monitoring objectives, as noted 

above. In order to ensure that a 

minimum number of area-wide 

monitors continue operating into the 

future, we are finalizing the proposed 

minimum monitoring requirements for 

area-wide monitors, where one areawide monitor is required in any CBSA 

with 1,000,000 people or more. Since 

there were no adverse comments 

received with regard to allowing PAMS 

stations that meet siting criteria to 

satisfy minimum monitoring 

requirements for area-wide monitors, we 

are finalizing that allowance as 

proposed. EPA encourages States to use 

the upcoming 2010 network assessment 

process to review existing area-wide 

NO2 sites to help determine what 

monitors might meet minimum 

monitoring requirements and whether 

or not other existing monitors warrant 

continued operation. 

4. Regional Administrator Authority 

The following paragraphs provide 

background, rationale, and details for 

the final changes to Regional 

Administrator authority to use 

discretion in requiring additional NO2

monitors beyond the minimum network 

requirements. The proposed rule 

estimated that approximately 167 nearroad monitors would be required within 

CBSAs having populations of 350,000 or 

more persons. As discussed above in 

section III.B.2, in response to public 

comments, particularly from States, EPA 

is changing the population threshold for 

siting a minimum of one near-road NO2

monitor from CBSAs with 350,000 or 

more persons to CBSAs with 500,000 or 

more persons. EPA estimates that this 

change in the population threshold will 

result in a reduction in the number of 

minimally required near-road NO2

monitors by approximately forty 

monitors. EPA has also recognized that 

susceptible and vulnerable populations, 

which include asthmatics and 

disproportionately exposed groups (as 

discussed in sections II.B.4 and II.F.4.d) 

are at particular risk of NO2-related 

health effects. The Administrator is 

therefore requiring the Regional 

Administrators, working in 

collaboration with States, to site these 

forty monitors in appropriate locations, 

focusing primarily on protecting 

susceptible and vulnerable 

communities. In addition, the Regional 

Administrators, working with States, 

may take into account other 

considerations described below in using 

their discretion to require additional 

monitors. 

a. Proposed Regional Administrator 

Authority 

EPA proposed that Regional 

Administrators have the authority to 

require monitoring at their discretion in 

particular instances. First, EPA 

proposed that the Regional 

Administrator 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. Second, EPA proposed to allow 

Regional Administrators the discretion 

to require additional near-road 

monitoring sites to address 

circumstances where minimum 

monitoring requirements are not 

sufficient to meet monitoring objectives, 

such as where exposures to NO2

concentrations vary across an area 

because of varied fleet mixes, 

congestion patterns, terrain, or 

geographic areas within a CBSA. And 

third, EPA proposed that Regional 

Administrators have the discretion to 

require additional area-wide NO2

monitoring sites above the minimum 

requirements for area-wide monitors 

where the minimum requirements are 

not sufficient to meet monitoring 

objectives. 

b. Comments 

EPA received comments from the 

Center on Race, Poverty and 

Environment expressing concern that 

the proposed monitoring provisions fail 

to consider ‘‘disproportionately 

impacted communities’’ which include 

people of color and of lower 

socioeconomic status. The commenter 

argues that this is ‘‘a gaping hole’’ in the 

proposed monitoring system and 

disproportionately impacts minority 

and low income populations in rural 

communities. In addition, the National 

Tribal Air Association stated that 

‘‘Indian Tribes and Alaska Natives are 

highly susceptible to health impacts as 

a result of NO2 exposure’’ and ‘‘the 

prevalence and severity of asthma is 

higher among certain ethnic or racial 

groups such as Indian Tribes and Alaska 

Natives,’’ which is also discussed in 

section II.B.4 and the ISA (ISA, section 

4.4). 

The proposed rule provided the 

Regional Administrators with the 

authority to use their discretion and 


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consider certain factors to require 

monitors above the minimum number in 

a CBSA. The proposal described one 

example where a Regional 

Administrator might require an 

additional near-road monitor where ‘‘a 

particular community or neighborhood 

is significantly or uniquely affected by 

road emissions.’’ EPA recognizes that 

susceptible and vulnerable populations, 

which include asthmatics and 

disproportionately exposed groups, as 

noted in section II.F.4.d, are at 

particular risk of NO2-related health 

effects, both because of increased 

exposure and because these groups have 

a higher prevalence of asthma and 

higher hospitalization rates for asthma. 

As noted above, in conjunction with 

raising the threshold for requiring one 

near-road NO2 monitor in CBSAs with 

500,000 persons or more, EPA is 

requiring the Regional Administrators, 

under their discretionary authority, to 

work with States to site an additional 

forty monitors, nationally, focusing 

primarily on communities where 

susceptible and vulnerable populations 

are located. To address the risks of 

increased exposure to these 

populations, the Administrator has 

determined that it is appropriate and 

necessary, under this provision, to 

ensure these additional forty monitors 

are sited primarily in communities 

where susceptible and vulnerable 

populations are exposed to NO2

concentrations that have the potential to 

exceed the NAAQS (due to emissions 

from motor vehicles, point sources, or 

area sources). As a result of this action, 

the total number of monitors required 

through this rulemaking is generally 

equivalent to the proposed number of 

minimally required monitors. 

EPA received comments from public 

health groups (e.g., ALA, Center on 

Race, Poverty, and the Environment, 

EDF, EJ, NRDC) and the Swinomish 

Tribe, who suggested that EPA expand 

monitoring coverage to address impacts 

from stationary sources outside of urban 

areas. For example, ALA, EDF, EJ, and 

NRDC, stated that ‘‘EPA should require 

States and local offices to review 

inventory data to identify any potential 

NO2 hotspots outside of those large 

metropolitan areas. For instance, if a 

large power plant or any other source is 

creating elevated NO2 levels in 

proximity to homes, schools or other 

sensitive sites, in an area of less than 

one million people, EPA should 

consider requiring a monitor.’’ 

EPA recognizes that there are major 

NO2 sources outside of CBSAs that have 

the potential to contribute to NO2

concentrations approaching or 

exceeding the NAAQS. The issue is 

whether such monitoring should be 

addressed through a more extensive set 

of minimum requirements that might 

include monitoring near all large 

stationary sources such as airports, 

seaports, and power plants, which could 

lead to deploying a large number of 

monitors. EPA believes that a more 

reasonable approach to address 

monitoring needs related to the diverse 

set of point, area, and non-road mobile 

NO2 sources, whether inside or outside 

of CBSAs, is to provide Regional 

Administrators the authority to require 

additional monitoring in areas where 

these impacts could occur. While the 

proposal did not specifically state that 

Regional Administrators could require 

non-area-wide monitors outside of 

CBSAs, EPA believes that it is important 

that Regional Administrators have the 

authority to require NO2 monitoring in 

locations where NO2 concentrations 

may be approaching or exceeding the 

NAAQS, whether located inside or 

outside of CBSAs. Therefore, in the final 

rule, EPA is not limiting the Regional 

Administrators’ discretionary authority 

to require NO2 monitoring only inside 

CBSAs; instead, the EPA is providing 

Regional Administrators the authority to 

site monitors in locations where NO2

concentrations may be approaching or 

exceeding the NAAQS, both inside or 

outside of CBSAs. 

The EPA also received comments 

from some State groups (e.g. the New 

York Department of Environmental 

Conservation (NYSDEC), New York 

Department of Transportation 

(NYSDOT), and the New York City Law 

Department) and an industry group (the 

Council of Industrial Boiler Operators) 

requesting greater clarification on the 

way in which Regional Administrators 

may use their authority to require 

additional monitors above the minimum 

requirements. For example, the Council 

of Industrial Boiler Operators stated that 

‘‘this [Regional Administrator authority] 

unreasonably vests an unbounded 

amount of discretion in EPA to 

determine when ‘‘minimum monitoring 

requirements are not sufficient’’ and 

which neighborhoods are ‘‘uniquely 

affected,’’ and impose additional 

monitoring requirements where all 

applicable monitoring requirements are 

already met by the State and local 

agency.’’ 

The authority of Regional 

Administrators to require additional 

monitoring above the minimum 

required is not unique to NO2. For 

example, Regional Administrators have 

or are proposed to have the authority to 

use their discretion to require additional 

Pb monitors (40 CFR Part 58 Appendix 

D section 4.5), and have the discretion 

to work with States or local agencies in 

designing and/or maintaining an 

appropriate ozone network, per 40 CFR 

Part 58 Appendix D section 4.1. EPA 

believes that while the NO2 monitoring 

network is sufficiently sized and 

focused, a nationally applicable network 

design may not account for all locations 

in which potentially high 

concentrations approaching or 

exceeding the NAAQS exist. Therefore, 

EPA believes it is important for Regional 

Administrators to have the ability to 

address possible gaps in the minimally 

required monitoring network, by 

granting them authority to require 

monitoring above the minimum 

requirements. 

One case in which the Regional 

Administrator may exercise discretion 

in requiring a monitor might be a 

location or community affected by a 

stationary source where the required 

near-road NO2 monitor site is not the 

location of the maximum hourly 

concentration in a CBSA. For any given 

CBSA, there is the possibility that the 

maximum NO2 concentrations could be 

attributed to impacts from one, or a 

combination of, multiple sources that 

could include point, area, and non-road 

source emissions in addition to on-road 

mobile source emissions. As a result, 

the Regional Administrator may choose 

to require monitoring in such a location. 

In addition, there is the possibility that 

a single source or group of sources 

exists which may contribute to 

concentrations approaching or 

exceeding the NAAQS at locations 

inside or outside CBSAs, including rural 

communities. In such cases, Regional 

Administrators, working with States, 

may require a monitor in these 

locations. Further, if there are NO2

sources responsible for producing more 

widespread impacts on a community or 

relatively larger area, Regional 

Administrators may require an areawide monitor to assess wider 

population exposures, or to support 

other monitor objectives served by areawide monitors such as photochemical 

pollutant assessment or pollutant 

forecasting. 

Regional Administrators may also 

require additional monitoring where a 

State or local agency is fulfilling its 

minimum monitoring requirements with 

an appropriate number of near-road 

monitors, but an additional location is 

identified where near-road population 

exposure exists at concentrations 

approaching or exceeding the NAAQS. 

In this case, the exposure may be due to 

differences in fleet mix, congestion 

patterns, terrain, or geographic area, 

relative to any minimally required 

monitoring site(s) in that area. We note 


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that such areas might exist in CBSAs 

with populations less than 500,000 

persons. 

EPA recognizes that high 

concentrations of NO2 that approach or 

exceed the NAAQS could potentially 

occur in a variety of locations in an area, 

and we believe that Regional 

Administrators should have the 

discretion to require additional 

monitoring when a location is identified 

based on the factors discussed in the 

paragraph above. In such situations, 

State or EPA Regional staff is likely to 

have identified these locations through 

data analysis, such as the evaluation of 

existing ambient data and/or emissions 

data, or through air quality modeling. 

Such information may indicate that an 

area has NO2 concentrations that may 

approach or exceed the NAAQS, and 

that there is potential for population 

exposure to those high concentrations. 

The Regional Administrator would 

use this authority in collaboration with 

State agencies. We expect Regional 

Administrators to work with State and 

local agencies to design and/or maintain 

the most appropriate NO2 network to 

meet the needs of a given area. For all 

the situations where a Regional 

Administrator may require additional 

monitoring, including the forty 

additional monitors the Regional 

Administrators are required to site, EPA 

expects Regional Administrators to 

work on a case-by-case basis with 

States. Further, for the forty additional 

monitors that will focus primarily on 

protecting susceptible and vulnerable 

communities, EPA intends to work with 

States to develop criteria to guide site 

selection for those monitors. 

c. Conclusions on Regional 

Administrator Authority 

EPA is requiring Regional 

Administrators to work with States to 

site forty NO2 monitors, above the 

minimum number required in the twotier network design, focused primarily 

in susceptible and vulnerable 

communities exposed to NO2

concentrations that have the potential to 

approach or exceed NAAQS. In 

addition, recognizing that a nationally 

applicable monitoring network design 

will not include all sites with 

potentially high concentrations due to 

variations across locations, and in 

response to public comments, the 

Administrator is providing Regional 

Administrators with the discretion to 

require additional monitors above the 

minimum requirements. 

Regional Administrators may also use 

their discretionary authority to require 

monitoring above the minimum 

requirements as necessary to address 

situations inside or outside of CBSAs in 

which (1) The required near-road 

monitors do not represent all locations 

of expected maximum hourly NO2

concentrations in an area and NO2

concentrations may be approaching or 

exceeding the NAAQS in that area; (2) 

areas that are not required to have a 

monitor in accordance with the 

monitoring requirements and NO2

concentrations may be approaching or 

exceeding the NAAQS; or (3) the 

minimum monitoring requirements for 

area-wide monitors are not sufficient to 

meet monitoring objectives. In all cases 

in which a Regional Administrator may 

consider the need for additional 

monitoring, EPA expects that Regional 

Administrators will work with the State 

or local agencies to evaluate evidence 

that suggests an area may warrant 

additional monitoring. EPA also notes 

that if additional monitoring should be 

required, as negotiated between the 

Regional Administrator and the State, 

the State will modify the information in 

its Annual Monitoring Network Plan to 

include any potential new sites prior to 

approval by the EPA Regional 

Administrator. 

5. Monitoring Network Implementation 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the approach for the 

monitoring network implementation. 

a. Proposed Monitoring Network 

Implementation Approach 

EPA proposed that State and, when 

appropriate, local air monitoring 

agencies provide a plan for deploying 

monitors in accordance with the 

proposed network design by July 1, 

2011. EPA also proposed that the 

proposed NO2 network be physically 

established no later than January 1, 

2013. 

b. Comments 

Most environmental and public health 

group commenters suggested that EPA 

change the implementation date from 

the proposed January 1, 2013 to a date 

that would require the minimum 

required NO2 network to be deployed 

sooner than proposed. Most States and 

State group commenters, along with 

industry group commenters, 

recommended that EPA keep the 

network implementation date as January 

1, 2013, or move it later than proposed. 

Those commenters who suggested 

moving it later noted that issues with 

monitoring site identification, site 

development, and overall lack of 

experience working in the near-road 

environment would make 

implementation difficult under the 

proposed implementation deadline. 

EPA recognizes the challenges 

involved with deploying the two-tier 

network design by the January 1, 2013 

date. We recognize the need for 

additional information and plan to aid 

State agencies in the network 

implementation process, particularly by 

developing guidance in partnership 

with affected stakeholders, ideally 

including at a minimum NACAA and 

the States. EPA agrees with NACAA’s 

suggestion that the CASAC Ambient Air 

Monitoring and Methods subcommittee 

should be consulted as part of 

developing any guidance developed for 

near-road monitoring, and has already 

begun the process by scheduling 

meetings with them regarding near-road 

monitoring. Further, EPA believes that 

collaboration with the States and State 

groups in developing guidance will be 

highly beneficial to the implementation 

process. This would allow for those 

States that do have increased experience 

in near-road monitoring to support the 

guidance development process and 

provide a conduit for sharing 

experiences amongst all stakeholders. 

In perspective, EPA believes that the 

approximate 2 years and 11 months 

between promulgation of this 

rulemaking and the mandated January 1, 

2013 network implementation date 

includes extra time relative to what is 

traditionally allowed for network 

implementation following rulemakings. 

We are also cognizant of the time 

needed to collect complete data that 

would allow data from the two-tier 

network to be considered for 

designations and for use in the next NO2

NAAQS review data from the 2013, 

2014, and 2015 years would provide 

critical information in the next NAAQS 

review, intended to occur on a 5-year 

cycle, and for use in subsequent 

designations. Even with complete data 

from 2013, 2014, and 2015 years 

designations would not occur until 

2017, at the earliest. 

c. Conclusions on Monitoring Network 

Implementation 

EPA is finalizing the date by which 

State and, when appropriate, local air 

monitoring agencies shall establish the 

required NO2 monitoring network as 

January 1, 2013, as was proposed. We 

believe that the allotted time for 

implementation will allow for the 

development of guidance 

documentation, particularly allowing 

for interactions with CASAC and 

NACAA/States, and for the processes 

that will be involved in deploying this 

network. However, EPA recognizes that 

the network implementation process, 


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particularly for near-road monitors, will 

include the assessment of road segments 

in CBSAs to identify locations of 

maximum expected hourly NO2

concentrations, identifying and working 

with other State and local agencies, 

such as transportation officials, as 

needed on issues regarding access and 

safety, and the exchange of information 

and feedback on potential sites with 

EPA, prior to any commitment to 

selecting and presenting new sites in an 

annual monitoring plan. As a result, 

based on feedback received through 

public comments, and to allow for more 

time to process guidance information, to 

carry out the deployment processes, and 

to allow for information exchanges to 

occur, we are changing the date by 

which State and, when appropriate, 

local air monitoring agencies shall 

provide a plan for deploying monitors 

in accordance with required network 

design, including the monitors required 

under the Regional Administrators’ 

discretional authority which are to be 

primarily focused on providing 

protection to susceptible and vulnerable 

populations, as discussed in section 

III.B.4, from July 1, 2011 to July 1, 2012. 

EPA strongly encourages State and local 

air agencies to supply as much 

information as possible on the NO2 sites 

they may be considering, including 

possible site coordinates if available, or 

have possibly selected, to satisfy the 

minimum NO2 network monitoring 

requirements in their Annual 

Monitoring Network Plan submitted 

July 1, 2011. 

6. Near-Road Site Selection 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the approach and 

criteria by which required near-road 

sites shall be selected. 

a. Proposed Near-Road Site Selection 

Criterion 

EPA proposed that the required nearroad 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 that proposed for 40 

CFR Part 58 Appendix E (discussed in 

III.B.7). 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, we proposed that 

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. Comments 

EPA received many comments from 

CASAC, public health groups, States 

and State groups, and industry groups 

on the proposed process by which 

States will select near-road sites. 

CASAC, along with some health group 

and State commenters questioned how 

States should select a site near the road 

with the highest ranked AADT possible, 

noting that EPA did not appear to 

require States to account for other 

factors. For example, one CASAC panel 

member noted that siting monitors 

based on traffic counts alone might miss 

locations where maximum NO2

concentrations would occur. They 

proceeded to recommend the use of 

modeling to assist in the site selection 

process. In another example, the ALA, 

EDJ, EJ, and NRDC, stated that ‘‘Nearroad monitor placement should be 

determined not only by the highest 

AADT volumes in a given CBSA, but 

also by the highest heavy-duty truck 

volumes.’’ NACAA also expressed 

concerns on ‘‘* * * basing monitor 

locations on the annual average daily 

traffic (AADT) without regard to vehicle 

mix or dispersion characteristics 

***’’. 

EPA does not intend for AADT counts 

to be the sole basis for choosing a nearroad site. As noted earlier in section 

III.B.2, there is a general relationship 

between AADT and mobile source 

pollution, where higher traffic counts 

correspond to higher mobile source 

emissions. The use of AADT counts is 

intended to be a mechanism for focusing 

on identifying the locations of expected 

maximum NO2 concentrations due to 

mobile sources. There are other factors 

that can influence which road segment 

in a CBSA may be the actual location 

where the maximum NO2

concentrations could occur. These 

factors include vehicle fleet mix, 

roadway design, congestion patterns, 

terrain, and meteorology. When States 

identify their top-ranked road segments 

by AADT, EPA intends for States to 

evaluate all of the factors listed above in 

their site selection process, due to their 

influence on where the location of 

expected maximum NO2 concentration 

may occur. As a result of the comments 

indicating a need for clarification, EPA 

will specifically list the factors that 

must be considered by States in their 

site selection process once a State has 

identified the most heavily trafficked 

roads in a CBSA based on AADT counts. 

In addition, EPA proposed that States 

consider these factors when they are 

required to place two near-road 

monitors in a CBSA, i.e., CBSAs with a 

population of 2,500,000 persons or 

more. EPA notes that these factors will 

be used in differentiating the two 

monitoring sites from each other, 

providing further characterization of 

near-road environments in 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 in different settings. Finally, 

EPA notes that air quality models, 

which were noted by the CASAC panel 

member to be considered for use in 

near-road site selection, are tools that 

EPA believes will be useful, and likely 

used by some States to inform where 

near-road sites need to be placed. 

EPA received comments from some 

State and industry commenters (e.g. 

Iowa, NY DEC, Edison Electric Institute, 

and Savannah River Nuclear Solutions) 

who suggested that potential population 

exposure should be a first-level metric 

in the near-road monitoring site 

selection process, instead of a secondlevel metric as EPA had proposed. 

EPA notes that the intent of the 

revised primary NO2 NAAQS is to 

protect against the maximum allowable 

NO2 concentration anywhere in an area, 

which includes ambient air on and 

around roads. This would limit 

exposures to peak NO2 concentrations, 

including those due to mobile source 

emissions, across locations (including 

those locations where population 

exposure near roads is greatest) in a 

given CBSA or area, with a relatively 

high degree of confidence. We also note 

the agency’s historical practice has been 

to site ambient air monitors in locations 

of maximum concentration, at the 

appropriate spatial scale. If EPA were to 

allow population, population density, or 

another population weighted metric to 

be a primary factor in the decision on 

where required near-road NO2 monitors 

are to be located, it is possible that the 

required near-road monitors in a CBSA 

would not be located at a site of 

expected maximum hourly near-road 

NO2 concentration. By monitoring in the 

location of expected maximum 1-hour 

concentrations, near-road monitoring 

sites will likely represent the highest 

NO2 concentrations in an area directly 

attributable to mobile sources or a group 

of sources that includes mobile sources. 

The proposed rule did permit, and the 

final rule states, that States are to 


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consider population in the site selection 

process in situations when a State 

identifies multiple candidate sites 

where maximum hourly NO2

concentrations are expected to occur. 

EPA received a comment from 

HCPHES suggesting that required 

monitoring should take into 

consideration the location of other 

major mobile sources for NO2 emissions 

such as airports and seaports. EPA also 

received a comment from the South 

Carolina Department of Health and 

Environmental Control stating that a 

near-road network does not address 

‘‘widespread pollutants from numerous 

and diverse sources.’’ 

EPA recognizes that there are major 

NO2 sources outside of CBSAs that have 

the potential to contribute to NO2

concentrations approaching or 

exceeding the NAAQS. The issue is 

whether such monitoring should be 

addressed through a more extensive set 

of minimum requirements that might 

include monitoring near all large 

stationary sources such as airports, 

seaports, and power plants, which could 

lead to deploying a large number of 

monitors. EPA believes that a more 

reasonable approach to address 

monitoring needs related to the diverse 

set of point, area, and non-road mobile 

NO2 sources, whether inside or outside 

of CBSAs, is to provide Regional 

Administrators the authority to require 

additional monitoring in areas where 

these impacts could occur. Providing 

the Regional Administrators with the 

discretion to require additional 

monitors allows them to effectively 

address such situations, even if that area 

is satisfying minimum monitoring 

requirements. This Regional 

Administrator authority is discussed 

above in section III.B.4. EPA also notes 

that State and local agencies may also 

monitor such locations on their own 

initiative. 

One State commenter, the Wisconsin 

Department of Natural Resources, 

requested that the term ‘‘major road’’ be 

defined and also requested clarification 

on what ‘‘top-ranked’’ means with regard 

to AADT counts on road segments. 

While the term ‘‘major road’’ is widely 

used in literature and can be found to 

be defined differently from one 

scientific study to another, here, EPA is 

using it in its commonly understood 

meaning as a road that is relatively 

heavily trafficked. EPA also does not 

believe it is appropriate to provide a 

bright-line definition for ‘‘top-ranked’’. 

Each CBSA will have a different 

distribution of total road segments and 

corresponding AADT counts on those 

segments. Further, since required nearroad monitors are to be sited in 

locations of expected maximum 

concentrations, a percentile restriction 

on ‘‘top ranked’’ roads is unnecessary. 

The intent of the requirement to rank all 

road segments by AADT counts and 

select a site, considering the other local 

factors noted above, near a ‘‘top-ranked’’ 

road segment is to focus attention on the 

most heavily trafficked roads, around 

which there is higher potential for 

maximum NO2 concentrations to occur. 

c. Conclusions on Near-Road Site 

Selection 

We are finalizing the near-road site 

selection criteria as proposed, and are 

clarifying that the proposal intended the 

selection criteria to include 

consideration of localized factors when 

identifying locations of expected 

maximum concentrations. As a result, 

required 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, considering fleet 

mix, roadway design, congestion 

patterns, terrain, and meteorology, 

where maximum hourly NO2

concentrations are expected to occur 

and siting criteria can be met in 

accordance with 40 CFR Part 58 

Appendix E. As was noted in section 

III.B.5 above, EPA will work with States 

to assist with the near-road site 

selection process through the 

development of guidance material and 

through information exchanges amongst 

the air monitoring community. 

We are also finalizing the 

requirement, as proposed, that when 

one CBSA is required to have two nearroad 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, as was proposed. 

7. Near-Road Siting Criteria 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the siting criteria for 

required near-road monitoring sites. 

a. Proposed Near-Road Siting Criteria 

EPA proposed 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. We solicited comment on, 

but did not propose, having near-road 

sites located on the predominantly 

downwind side of the target roadways. 

EPA proposed that the monitor probe 

shall be located within 2 to 7 meters 

above the ground, as is required for 

microscale PM2.5 and PM10 sites. We 

also proposed 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. 

b. Comments 

EPA received comments from a 

number of States (e.g. Michigan, 

Mississippi, and Tennessee) indicating 

that the near-road network poses 

significant safety issues and a related 

need for increased logistical flexibility 

for installing a monitoring site. For 

example, the Mississippi Department of 

Environmental Quality states that 

‘‘Given the fact that these NO2 sites will 

be required to be housed in shelters that 

are within 50 meters of the road, we 

believe that these buildings could be 

large and pose a serious risk to drivers 

on the road.’’ 

EPA notes that in all instances of field 

work, safety is a top priority. In this 

instance of near-road monitoring, we are 

dealing with the safety of the public 

driving on roads and the monitoring 

staff who may operate the near-road 

monitoring station as well. There are 

various ways to install near-road sites 

while ensuring worker and traffic safety, 

and safety is an important part of the 

logistical considerations that States 

should consider when selecting and 

installing near-road sites. In many cases, 

State and local monitoring agencies may 

be able to work with their State or local 

transportation officials during the site 

selection process to deal with access 

and safety issues. In public comments, 

AASHTO recommended that ‘‘* * * 

State and local air monitoring agencies 

be required to coordinate with State and 

local DOTs for near-road monitoring 

during the establishment of the 

monitoring plan.’’ Although EPA cannot 

require States to coordinate with other 

State or local entities, EPA believes that 

transportation officials would likely be 

able to assist in finding solutions to 

ensure safety while working with 

monitoring agencies in accommodating 

a new near-road monitoring station. An 


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24NESCAUM officially supported the alternative 

network design; however, they made suggestions 

regarding the near-road network in the event EPA 

finalized the proposed two-tier network design. 

25NACAA made a statement containing many 

concerns about the near-road monitoring 

component proposal which included a passage 

regarding the lack of requiring sites to be 

downwind. They expressed concern in ‘‘* * * 

allowing upwind siting of monitors over a wide 

range of horizontal and vertical distances from the 

road * * *’’. 

example of a step that could be taken to 

alleviate safety concerns might be 

purposefully placing a monitoring site 

behind existing barriers like guardrails 

and fencing, or possibly by installing a 

short distance of such barriers to protect 

the site workers, site infrastructure, and 

nearby traffic. In addition, EPA notes 

that the 50m distance proposed is wide 

enough to accommodate a site that 

would satisfy many setback provisions 

that exist for private or commercial 

building permits near roads, and may be 

viewed as a confirmation that our 

proposed siting criteria are safely 

attainable. 

Some State commenters (e.g. 

AASHTO, NYSDOT, and Wisconsin) 

suggested that the allowable maximum 

distance a near-road monitoring probe 

can be from the target road be increased 

from 50 meters to something wider, 

such as 200 meters. Conversely, there 

were some State, environmental, and 

industry commenters (e.g. NESCAUM,24

Group Against Smog and Pollution, and 

Air Quality Research and Logistics) who 

suggested that the proposed range was 

appropriate, or, as suggested by both 

NESCAUM and the Group Against Smog 

and Pollution, the allowable distance 

should be reduced to as close as 30 or 

20 meters to the nearest edge of the 

traffic lanes of the target road segment, 

respectively. 

EPA believes that increasing the 

allowable distance above 50 meters 

would compromise the intent of nearroad monitoring. As was noted in the 

proposal and this document, the ISA 

(2.5.4 and 4.3.6) and REA (7.3.2) 

indicate that on-road, mobile source 

derived NO2 exhibits a peak 

concentration on or very near the source 

road, and those concentrations decay 

over a variable but relatively short 

distance back to near area-wide or 

background (upwind of the target road) 

concentrations. 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, but the peak 

concentrations are occurring on or very 

near the source roadway. The behavior 

of NO2 concentrations and the actual 

distance over which concentrations 

return to near area-wide or background 

levels 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). Therefore, monitor 

probe placement at increasing distances 

from a road, such as 200 meters, will 

correspondingly decrease the potential 

for sampling maximum concentrations 

of NO2 due to the traffic on the target 

road. Baldauf et al. (2009) indicate that 

monitoring probes would ideally be 

situated between 10 and 20 meters from 

the nearest traffic lane for near-road 

pollutant monitoring. 

Regarding the comments suggesting 

required monitor probes be closer than 

50 meters, EPA believes the allowable 

distance of 50 meters that a near-road 

NO2 probe can be from the target road 

provides enough flexibility for the 

logistical issues that can occur on a 

case-by-case basis, which is inherent in 

monitoring site placement, while not 

sacrificing the potential to monitor the 

peak NO2 concentrations. However, in 

light of the information provided here 

on how NO2 peak concentrations can 

decay over relatively short distances 

away from roads, EPA strongly 

encourages States to place near-road 

sites, or at least monitor probes, as close 

as safely possible to target roads to 

increase the probability of measuring 

the peak NO2 concentrations that occur 

in the near-road environment, again 

noting that Baldauf et al. (2009) indicate 

that monitor probes would ideally be 

situated between 10 and 20 meters from 

the nearest traffic lane for near-road 

pollutant monitoring. 

EPA also proposed that required nearroad NO2 monitor probes 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. EPA 

expects that when a State makes a 

measurement in determining whether 

an NO2 inlet probe is no greater than 50 

meters away, horizontally, from the 

outside nearest edge of the traffic lanes 

of the target road segment, that the 

measurement would likely represent a 

path to the monitor probe that is normal 

to the target road. However, EPA notes 

that the monitor probe will likely be 

influenced by various parts of the target 

road segment that are at a relative angle 

compared to the normal transect 

between the road and the monitor 

probe. EPA is not adjusting the wording 

of this requirement, but does intend for 

States to consider more than one linear 

pathway between the target road and the 

monitor probe being clear of 

obstructions when considering 

candidate site locations. 

EPA received comments on the 

solicitation for comment on requiring 

near-road monitoring sites to be placed 

on the downwind side of the target road 

where the commenters (e.g. NACAA,25

NESCAUM, and the Clean Air Council) 

encouraged such a requirement. 

Conversely, other commenters (e.g., Air 

Quality and Logistics and NYSDEC 

suggested that such a requirement may 

be overly restrictive and not necessary. 

For example, NYSDEC stated that ‘‘It is 

important to avoid making the monitor 

siting criteria too restrictive. It is very 

likely that in some CBSAs, finding 

suitable locations near the busiest road 

segments will not be possible. It is also 

important to remember that the NO2

monitoring instrumentation provides 

data continuously. Sites located 

downwind of sources will likely be 

impacted more frequently than the sites 

located upwind particularly when the 

sites are more than 50 meters from the 

source, and are preferred, but either side 

of the road will be downwind some of 

the time. Many of the highest NO2

concentrations are also likely to occur 

during inversion periods and during 

calm meteorological conditions when 

the upwind-downwind designations 

have little meaning.’’ 

EPA noted in its proposal that 

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. Further, 

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, at least in part, to 

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, but also reduces 

the absolute need to be downwind of 

the road. EPA believes that very few, if 

any, near-road sites would be able to be 

situated in a location that was always 

downwind. For example, a hypothetical 


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site may have winds routinely out of 

several different cardinal directions 

throughout the year, without one being 

a dominant direction. As a result, given 

variable meteorology, for some period of 

a year, a given near-road site may not be 

downwind of the target road, no matter 

which side of the road it is on. 

Therefore, EPA is not finalizing a 

requirement that near-road sites must be 

climatologically downwind of the target 

road segment because of the additional 

limitations this introduces to finding 

potential site candidates in exchange for 

what may be a small increase in the 

opportunity to monitor peak NO2

concentrations. However, EPA 

encourages States to place monitors in 

the climatologically downwind 

direction whenever possible, in an 

attempt to measure the peak NO2

concentrations more often than not. One 

way States may identify where the 

predominantly downwind location 

might be for candidate sites could be to 

use portable meteorological devices to 

characterize meteorological tendencies, 

in addition to evaluating other available 

meteorological data sources. 

EPA proposed that required near-road 

NO2 monitor probes be located within 2 

to 7 meters above the ground, as is 

required for microscale PM2.5 and PM10

sites. EPA also proposed 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. NESCAUM 

commented that ‘‘EPA needs to 

reconcile near-roadway NO2 probe 

height requirements with the existing 

micro-scale near-roadway CO probe 

height requirement of 2.5 to 3.5 meters 

above prevailing terrain. NESCAUM 

supports using this existing height for 

all near-roadway pollution monitors, as 

it minimizes probe height effects on 

measurements, and allows for proper 

measurement of collocated particle 

number concentration (which requires a 

very short inlet, i.e., on the order of 

inches) and CO.’’ NYSDEC commented 

that ‘‘The height requirement may not be 

practical for road segments in dense 

urban areas where existing buildings 

heights may exceed 7 meters. The 

requirement to maintain a 1 meter 

clearance from a supporting wall or 

structure may not be adequate for taller 

walls often found in urban areas. These 

walls can create down washing and 

street canyon effects which will make 

the resulting data less representative of 

nearby areas and will make 

interpretation of the resulting data 

difficult. However, there will need to be 

consistency between similar site 

settings.’’ Finally, EPA received 

comments from some health groups 

(e.g., ALA, EJ, EDF, and NRDC) who 

commented that ‘‘the lower end of the 

proposed height of 2 to 7 meters appears 

to capture the highest NO2

concentrations, and more accurately 

represents human exposure at the 

breathing zone.’’ 

In the proposal, EPA noted that nearroad 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). EPA recognizes that 

consistency across sites with regard to 

probe height is desirable, and 

consistency with microscale, urban 

canyon CO sites might also be desirable. 

However, as was noted in the earlier 

discussion on ‘‘downwind’’ site 

placements, it is important to avoid 

making the monitor siting criteria too 

restrictive. An allowable range between 

2 and 7 meters provides more flexibility 

in site installation, which EPA 

considers important because of the 

variety of siting situations each State 

may have to deal with for each 

individual site. While EPA agrees that a 

tighter allowable range such as 2.5 to 3.5 

meters would reduce site to site 

variability and keep probes nearer the 

microscale siting requirements of CO, 

the wider range of 2 to 7 meters still 

provides an adequate amount of site to 

site consistency. EPA may also address 

this issue through forthcoming 

guidance, where an increased 

consistency for probe heights in similar 

situations such as urban canyons may 

be a site implementation goal, within 

the required 2 to 7 meter probe height 

range. Further, EPA believes that 

although certain situations, as noted by 

NYSDEC, may exist where the 1 meter 

clearance from walls or structures may 

be problematic near taller buildings or 

walls, this requirement is consistent 

with similar such clearance 

requirements for microscale CO sites in 

similar such situations that exist in 

urban canyons. 

In the proposed rule, EPA proposed in 

the siting criteria language that the 

subsequent residence time of the 

pollutant in the sample line between the 

inlet probe and the analyzer cannot 

exceed 20 seconds. EPA received 

comments from Air Quality Research 

and Logistics regarding guidelines for 

maximum allowable inlet length and 

sample residence time, where they 

stated that ‘‘* * * the fast 

photodynamic O3-NOX equilibrium may 

occur in darkened sample lines at 

residence times of 10–20 seconds 

(Butcher et al. 1971; Ridley et al. 1988; 

Parrish et al. 1990). EPA should correct 

this apparent error by specifying much 

lower maximum residence times (e.g., 

1–2 seconds) or accounting for this 

effect by reporting ‘corrected’ values in 

error by no more than the allowed 

rounding convention (e.g., ±1 ppb).’’ 

EPA notes that in 40 CFR Part 58 

Appendix E, paragraph (9)(c), states that 

sample probes for reactive gas analyzers, 

particularly NOY monitors, at NCore 

monitoring sites must have a sample 

residence time less than 20 seconds. 

EPA believes this rule is also 

appropriate for NO2 monitors, 

particularly if a monitor inlet manifold 

is extended away from the main 

monitoring shelter. EPA does agree that 

shorter sample residence time in the 

inlet manifold is desirable. Although we 

do not believe it appropriate to require 

residence times on the order of 1 to 2 

seconds, and do not believe correcting 

values is appropriate (which was not a 

concept which was proposed), we do 

encourage States to use best practices in 

selecting non-reactive manifold 

materials, and to install sampling 

manifolds in an efficient manner that 

minimizes sample residence time. 

While EPA proposed this concept in the 

preamble to the proposed rule, we did 

not include it in the proposed regulatory 

text. The final rule includes regulatory 

text on this subject at 40 CFR Part 58 

Appendix E, paragraph (9)(c). 

c. Conclusions on Near-Road Siting 

Criteria 

We are finalizing the near-road NO2

monitor siting criteria, as proposed, 

where (1) required near-road NO2

monitor probes 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, (2) required near-road 

NO2 monitor probes 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, (3) required near-road NO2

monitors are required to have sampler 

inlets between 2 and 7 meters above 

ground level, and (4) residence time of 

NO2 in the sample line between the 


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inlet probe and the analyzer does not 

exceed 20 seconds. 

8. Area-Wide Monitor Site Selection and 

Siting Criteria 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the site selection 

and monitor siting criteria for required 

area-wide monitoring sites. 

a. Proposed Area-Wide Monitor Site 

Selection and Siting Criteria 

EPA proposed that sites required as 

part of the second tier of the NO2

monitoring network design, known as 

the area-wide monitoring component, be 

sited to characterize the highest 

expected NO2 concentrations at the 

neighborhood and larger (area-wide) 

spatial scales in a CBSA. 

b. Comments 

While most commenters who 

supported area-wide monitoring did so 

with regard to the adoption of the 

alternative area-wide network design 

rather than as part of the proposed 

approach, only a few commented on the 

actual sites and siting criteria. The Dow 

Chemical Company suggested that areawide sites should be located at least 

1,000 meters away from any major roads 

or intersections to ensure that the 

concentration of NO2 measured is 

representative of an area-wide 

concentration instead of peak near-road 

concentrations. 

EPA notes that in order for an NO2

monitoring site to be classified as a 

neighborhood (or larger) spatial scale 

site, it must meet the roadway set-back 

requirements in Table E–1 of 40 CFR 

Part 58 Appendix E. EPA believes that 

this existing set-back table is 

appropriate to use to ensure that any 

NO2 site that may be intended as an 

area-wide site will be sufficiently 

distanced from any major road. For 

example, an NO2 monitoring site may be 

considered neighborhood scale if it is 10 

or more meters from the edge of the 

nearest traffic lane of a road with 10,000 

or less AADT counts. 

c. Conclusions on Area-Wide Monitor 

Site Selection and Siting Criteria 

We are finalizing the requirement that 

any sites required as part of the second 

tier of the NO2 monitoring network 

design, known as the area-wide 

monitoring component, be sited to 

characterize the highest expected NO2

concentrations at the neighborhood and 

larger (area-wide) spatial scales in a 

CBSA. 

9. Meteorological Measurements 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the requirement of 

meteorological monitoring at near-road 

monitoring sites. 

a. Proposed Meteorological 

Measurements 

In further support of characterizing 

the peak NO2 concentrations occurring 

in the near-road environment, EPA 

proposed 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. 

b. Comments 

EPA received comments from the 

South Carolina Department of Health 

and Environmental Control commented 

that the recording of air turbulence data 

at near-road monitoring stations should 

be encouraged but not required. Other 

States (e.g., Alaska, North Carolina, and 

Wisconsin) provided comments that did 

not support the proposed meteorological 

measurement requirements, noting 

issues with costs, problems siting the 

probe nearer to structures and to the 

ground than is typically done, and that 

the averaging period required to better 

understand turbulence (through 

anemometry data) in the near-road 

environment requires a much higher 

frequency than what is typically 

reported. 

EPA is removing the proposed 

requirements that would have required 

meteorological monitoring at near-road 

NO2 monitoring stations. However, EPA 

strongly encourages States to do some 

meteorological monitoring to better 

characterize the conditions under which 

they are acquiring NO2 data. The nearroad microscale environment is 

complex, and understanding the 

turbulent dispersion that may be 

affecting NO2 measurements, along with 

having a basic understanding of from 

which direction the measured NO2

concentrations are coming from, which 

are very informative in the effort to fully 

understand the data being collected. At 

a minimum, basic anemometry data 

would be useful in identifying whether 

the site is upwind, downwind, or 

otherwise oriented, relative to the target 

road. 

c. Conclusions on Meteorological 

Measurements 

We are not finalizing the proposal 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. 

C. Data Reporting 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the data reporting 

requirements, data quality objectives, 

and measurement uncertainty. 

1. Proposed Data Quality Objectives and 

Measurement Uncertainty 

In the proposal, EPA noted that 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. We also 

noted that many 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. 

We believe these data reporting 

procedures are appropriate to support 

the revised primary NO2 NAAQS. 

EPA proposed to develop data quality 

objectives (DQOs) for the proposed NO2

network. We proposed 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. 

2. Comments 

EPA received comments from the 

State of Missouri, supporting the 

proposed DQOs and goals for 

measurement uncertainty, and from 

North Carolina, suggesting that 

measurement uncertainty goals match 

those of the NCore multi-pollutant 

network. 

EPA agrees that it is desirable to have 

measurement uncertainty goals that 

match that of other pollutants. EPA 

originally proposed the goals for 

precision and bias under consideration 

that there may be a need to account for 

potential increased uncertainty in 1- 

hour near-road NO2 data. However, we 

agree with the suggestion from the State 

of North Carolina, and are changing the 

goals 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 10 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent. These goals match 

the existing goals for NO2 and are 

consistent with historical measurement 

uncertainty goals. 


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3. Conclusions on Data Quality 

Objectives and Measurement 

Uncertainty 

We are finalizing the approach to 

develop data quality objectives, and are 

changing the proposed goal for 

measurement uncertainty, where the 

goals 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 10 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent. 

IV. Appendix S—Interpretation of the 

Primary NAAQS for Oxides of Nitrogen 

and Revisions to the Exceptional Events 

Rule 

The EPA proposed 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 detailed the 

computations necessary for determining 

when the proposed 1-hour and existing 

annual primary NO2 NAAQS are met. 

The proposed Appendix S also 

addressed data reporting, data 

completeness considerations, and 

rounding conventions. 

Two versions of Appendix S were 

proposed. The first applied to a 1-hour 

primary standard based on the annual 

4th high value form, while the second 

applied to a 1-hour primary standard 

based on the 99th percentile daily value 

form. 

The final version of Appendix S is 

printed at the end of this notice and 

applies to an annual primary standard 

and a 1-hour primary standard based on 

the 98th percentile daily value form. 

Appendix S is based on the 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 1- 

hour standard when making 

nonattainment determinations and other 

findings related to attainment or 

violation of the standard. 

The EPA is amending and moving the 

provisions of 40 CFR 50.11 related to 

data completeness for the existing 

annual primary standard to the new 

Appendix S, and adding provisions for 

the proposed 1-hour primary standard. 

Substantively, the 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 an addition 

of a cross-reference to the Exceptional 

Events Rule, the addition of 

Administrator discretion to consider 

otherwise incomplete data complete, 

and the addition of a provision 

addressing the possibility of there being 

multiple NO2 monitors at one site. The 

procedures for the 1-hour primary 

standard are entirely new. 

The EPA is also making 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 final 

NO2 NAAQS. 

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 pre-existing annual 

NO2 NAAQS, originally adopted in 

1977, contained data interpretation 

instructions only for the issue of data 

completeness. This situation contrasts 

with the situations for ozone, PM2.5, 

PM10, and most recently Pb for which 

there are detailed data interpretation 

appendices in 40 CFR part 50 

addressing more issues that can arise in 

comparing monitoring data to the 

NAAQS. 

A. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen for the Annual 

Primary Standard 

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 regulatory text 

at 40 CFR 50.11, anticipating and 

resolving in advance various future 

situations that could occur. 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 

addressed several issues in ways which 

are specific to the individual primary 

NO2 standards, as described below. 

1. Proposed Interpretation of the Annual 

Standard 

The proposed data interpretation 

provisions for the annual standard are 

consistent with the pre-existing 

instructions included along with the 

statement of the level and form of the 

standard in 40 CFR 50.11. 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). 

In the proposal, EPA noted that 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 also 

incorporated some such features. 

2. Comments on Interpretation of the 

Annual Standard 

We received four comments, all from 

State agencies, on data interpretation for 

the annual NO2 standard. Of the four 

commenters, two recommended the use 

of a weighted annual mean to 

appropriately implement the annual 

primary standard. Two other 

commenters asserted that there is no 

strong seasonality in NO2

concentrations, and that therefore there 

is no need to use a weighted annual 

mean or to require data completeness 

quarter-by-quarter. 


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3. Conclusions on Interpretation of the 

Annual Standard 

Upon investigating the issue of NO2

seasonality using data from AQS as part 

of considering the comments, we have 

found that there are notable variations 

in quarterly mean NO2 concentrations. It 

is therefore quite possible that an 

unweighted annual mean calculated 

without a quarter-by-quarter data 

completeness requirement might not 

represent the true annual mean as well 

as a weighted annual mean calculated 

with a quarter-by-quarter completeness 

requirement. However, the current 

practice of requiring 75% completeness 

of all of the hours in the year and 

calculating the annual mean without 

weighting has been retained in the final 

rule, because of its simplicity and 

because we believe it will not interfere 

with effective implementation of the 

annual NAAQS. No area presently is 

nonattainment for or comes close to 

violating the annual standard. 

Therefore, the choice between the two 

approaches can only have a practical 

effect, if any, on whether at some time 

in the future an area is determined to be 

newly violating the annual standard. If 

a monitor has a complete and valid 

design value below the standard using 

the unweighted mean approach (with 

only an annual data completeness 

requirement) but the design value 

would be considered incomplete and 

invalid under a hypothetical weighted 

mean approach (with a quarterly 

completeness requirement), the monitor 

would in either case be considered not 

to be violating and its data would not be 

the basis for a nonattainment 

designation. If a monitor has a design 

value above the standard using the 

unweighted annual mean approach but 

is incomplete with respect to a 

hypothetical quarterly completeness 

requirement, then the two approaches 

would have different implications for 

the determination of a violation. A 

quarterly completeness requirement 

would make a finding of violation 

impossible, unless the Administrator 

chose to treat the data as if complete 

under another provision of the final 

rule. The unweighted annual mean 

approach would allow but not force a 

finding of violation, because the 

Administrator will have discretion to 

make any such findings because there 

will be no mandatory round of 

designations for the annual standard 

given that the annual standard has not 

been revised in this review. The 

Administrator will be able to consider 

the representativeness of the 

unweighted annual mean when 

deciding whether to make a 

discretionary nonattainment 

redesignation. Given that the annual 

standard requires only one year of 

monitoring data for the calculation of a 

design value, little time will be lost if 

the Administrator chooses to work with 

a State to obtain a new design value 

based on more complete and/or 

seasonally balanced monitoring data. 

B. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen 1-Hour Primary 

Standard 

1. Proposed Interpretation of the 1-Hour 

Standard 

With regard to data completeness for 

the 1-hour primary standard with a 4th 

highest daily value form, the proposed 

Appendix followed 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 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 was proposed to 

be applied at the daily and quarterly 

levels. 

Recognizing that there may be years 

with incomplete data, the proposed text 

provided that a design value derived 

from incomplete data would 

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 high 

values to exceed the standard. In this 

situation, more complete monitoring 

could not possibly have indicated that 

the standard was actually met. 

Second, we proposed a diagnostic 

data substitution test which was 

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. 

It should be noted that one possible 

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. 

Also, we proposed 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. 

The second version of the proposed 

Appendix S contained 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 had 

somewhat different provisions for 

dealing with data incompleteness. One 

difference was 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 provided 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 


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a 99th percentile form for the 1-hour 

primary NO2 standard. The proposed 

Appendix S also provided instructions 

for rounding (not truncating) the average 

of three annual 99th percentile hourly 

concentrations before comparison to the 

level of the primary NAAQS. 

2. Comments on Interpretation of the 1- 

Hour Standard 

Three commenters expressed the view 

that the 75% completion per quarter 

requirement should apply with respect 

to the 1-hour standard. A fourth 

commenter recommended that the 

requirement be increased to 82%. 

Another person commented that the 

requirement of 75% of the hours in a 

day is too stringent. The commenter 

noted that it would be inappropriate not 

to count the day if the maximum 

concentration observed in the hours 

measured is sufficiently high to make a 

difference with regard to compliance 

with the NAAQS. A comment was 

received that the substitution test 

should not be included, on the grounds 

that nonattainment should not be 

declared without irrefutable proof. This 

commenter also said that the same 

completeness requirement as used for 

nonattainment should be used for 

attainment. We received one comment 

that the computation of design values 

where multiple monitors are present at 

a site should be averaged and not taken 

from a designated primary monitor. 

3. Conclusions on Interpretation of the 

1-Hour Standard 

Consistent with the Administrator’s 

decision to adopt a 98th percentile form 

for the 1-hour NAAQS, the final version 

of Appendix S is based on that form. 

Table 1 has been revised from the 

version that was proposed, so that it 

results in the selection of the 98th 

percentile value rather than the 99th 

percentile value. 

We agree with the three comments 

expressing the view that the 

requirement for 75% data completeness 

per quarter should apply with respect to 

the 1-hour standard. A fourth comment 

recommended that the requirement be 

increased to 82%. We believe 82% is 

too stringent because of the number of 

monitors that would not achieve such a 

requirement and we believe that 75% 

captures the season. We agree that an 

incomplete day should be counted if the 

maximum concentration observed in the 

hours measured is sufficiently high to 

make a difference with regard to 

compliance with the NAAQS, and we 

have accounted for that in section 3.2.c.i 

by validating the design value if it is 

above the level of the primary 1-hour 

standard when at least 75 percent of the 

days in each quarter have at least one 

reported hourly value. We agree that 

substitution should not be used for the 

establishment of attainment/ 

nonattainment. The commenter who 

remarked on this issue appears not to 

have understood that the specific 

proposed substitution tests have 

essentially zero probability of making a 

clean area fail the NAAQS, or vice 

versa, because the substituted values are 

chosen to be conservative against such 

an outcome. As noted in section 

3.2(c)(i), when substitution is used, the 

3-year design value based on the data 

actually reported, not the ‘‘test design 

value’’, shall be used as the valid design 

value. 

In the course of considering the above 

comment regarding data substitution 

tests to be used in cases of data 

incompleteness, EPA has realized that 

there could be some cases of data 

incompleteness in which the proposed 

procedure for calculating the 1-hour 

design value might result in an in 

appropriately low design value. As 

proposed, only days with measurements 

for at least 75% of the hours in the day 

would be considered in any way when 

identifying the 99th percentile value 

(99th for purposes of the adopted 

NAAQS). However, there could be 

individual hours in other, incompletely 

monitored days that had measured 

concentrations higher than the 

identified 98th percentile value from the 

complete days. It would be 

inappropriate not to consider those 

hours and days in some way. However, 

if all days with at least one hourly 

concentration were used to identify the 

99th percentile value without any 

regard to their incompleteness, this 

could also result in a design value that 

is biased low because the extra days 

could increase the number of ‘‘annual 

number of days with valid data’’ enough 

to affect which row of Table 1 of 

Appendix S is used. It could, for 

example, result in the 8th highest 

ranked daily maximum concentration 

being identified as the 98th percentile 

value (based on Table 1 of Appendix S) 

rather than a higher ranked 

concentration; this would also be 

inappropriate because days which were 

not monitored intensively enough to 

give a reasonable likelihood of catching 

the maximum hourly concentration 

would in effect be treated as if they had 

such a likelihood. For example, 50 days 

with only one hourly measurement 

during a time of day with lower 

concentrations would ‘‘earn’’ the State 

the right to drop one notch lower in the 

ranking of days when identifying the 

98th percentile day, inappropriately. 

The final version of Appendix S solves 

this problem by providing that two 

procedures be used to identifying the 

98th percentile value, the first based 

only on days with 75% data 

completeness and the second based on 

all days with at least one hourly 

measurement. The final design value is 

the higher of the two values that result 

from these two procedures. 

With regard to situations with 

multiple monitors operating at one site, 

we think as discussed in the proposal, 

that designation of a primary monitor is 

preferable to averaging the data from 

multiple monitors based on 

administrative simplicity and 

transparency for the public, and is 

unbiased with respect to compliance 

outcome provided the State is able to 

make the designation only before any 

data has been collected. 

Finally, as proposed, the final version 

of Appendix S has a cross reference to 

the Exceptional Events Rule (40 CFR 

50.14) with regard to the exclusion of 

data affected by exceptional events. In 

addition, the specific steps for including 

such data in completeness calculations 

while excluding such data from actual 

design value calculations is clarified in 

Appendix S. 

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 


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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.’’ 

EPA proposed revised exceptional 

event data flagging and documentation 

deadlines in FR 34404 [Federal 

Register/Vol. 74, No. 134/Wednesday, 

July 15, 2009/Proposed Rules] and 

invited comments from the public. The 

Agency received no comments related to 

the revised proposed schedule for NO2

exceptional event data flagging and 

documentation deadlines. 

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. 

The final rule text at the end of this 

notice shows the changes that will 

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. 

Table 1 below summarizes the data 

flagging and documentation deadlines 

corresponding to the two year 

designation schedule discussed in this 

section. If the promulgation date for a 

revised NO2 NAAQS occurs on a 

different date than January 22, 2010, 

EPA will revise the final NO2

exceptional event flagging and 

documentation submission deadlines 

accordingly to provide States with 

reasonably adequate opportunity to 

review, identify, and document 

exceptional events that may affect an 

area designation under a revised 

NAAQS. 

TABLE 1—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 

NO2/1-Hour Standard (100 PPB) .............................. 2008 July 1, 2010 a ............................................................ January 22, 2011. 

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

2010 April 1, 2011a ............................................................ July 1, 2011.a

a 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. Designation of Areas 

A. Proposed Process 

The CAA requires EPA and the States 

to take steps to ensure that the new or 

revised NAAQS are met following 

promulgation. The first step is to 

identify areas of the country that do not 

meet the 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 should be designated as 

nonattainment, attainment, or 

unclassifiable for the new NAAQS. 

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.’’ 

No later than 120 days prior to 

promulgating designations, EPA is 

required to notify States of any intended 

modifications to their designations 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, the 

EPA must promulgate the designation 

that it deems appropriate. 

Accordingly, Governors must submit 

their initial NO2 designation 

recommendations to EPA no later than 

January 2011. If the Administrator 

intends to modify any State’s 

recommendation, the EPA will notify 

the Governor no later than 120 days 

prior to 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 

finalized. 

B. Public Comments 

Several industry commenters 

requested that EPA slow the timeline for 

implementing a near-roadway 

monitoring network and designating 

roadway areas because they believe EPA 

lacks significant information about the 

implementation and performance of a 

national, near-roadway monitoring 

network. Two commenters also 

requested that if a near-roadway 

monitoring network is deployed, that 1- 

hour NO2 standards be made more 


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26Since EPA is retaining the annual standard 

without revision, the discussion in this section 

relates to implementation of the proposed 1-hour 

standard, rather than the annual standard. 

lenient until the next review period so 

that more information will be available 

about near-roadway NO2 concentrations 

before a stringent standard is selected. 

A response to commenters’ requests 

that EPA slow the monitoring 

implementation schedule and the 

request that EPA make the 1-hour NO2

standard more lenient until the next 

review period are addressed in sections 

III.B.5 and II.F.4.D, respectively. 

Section 110(d)(1)(B) requires the EPA 

to designate areas no later than 2 years 

following promulgation of a new or 

revised NAAQS (i.e., by January 2012). 

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 

an additional year would not result in 

significant new data to inform the initial 

designations. A near-roadway 

monitoring network is not expected to 

be fully deployed until January 2013 

therefore, EPA must proceed with initial 

designations using air quality data from 

the existing NO2 monitoring network. 

Because none of the current NO2

monitors are sited to measure nearroadway ambient air, we expect that 

most areas in the country with current 

NO2 monitors will not violate the new 

NO2 NAAQS. In the event that a current 

NO2 monitor indicates a violation of the 

revised standards, EPA intends to 

designate such areas ‘‘nonattainment’’ no 

later than 2 years following 

promulgation of the revised standards. 

We intend to designate the rest of the 

country as ‘‘unclassifiable’’ for the 

revised NO2 NAAQS until sufficient air 

quality data is collected from a nearroadway monitoring network. Once the 

near-roadway network is fully deployed 

and 3 years of air quality data are 

available, the EPA has authority under 

the CAA to redesignate areas as 

appropriate from ‘‘unclassifiable’’ to 

‘‘attainment’’ or ‘‘nonattainment.’’ We 

anticipate that sufficient data to conduct 

designations would be available after 

2015. 

A number of commenters, largely 

from industry groups, focused on the 

concern that a near-roadway monitoring 

network would lead to regional 

nonattainment on the basis of high NO2

concentrations found near roadways. 

These commenters requested that any 

future nonattainment areas be limited to 

the area directly surrounding roadways 

found to have above-standard NO2

concentrations. 

The CAA requires that any area that 

does not meet a NAAQS or that 

contributes to a violation in a nearby 

area that does not meet the NAAQS be 

designated ‘‘nonattainment.’’ States and 

EPA will need to determine which 

sources and activities contribute to a 

NAAQS violation in each area. 

Depending on the circumstances in each 

area this may include sources and 

activities in areas beyond the area 

directly surrounding a major roadway. 

EPA intends to issue nonattainment area 

boundary guidance after additional 

information is gathered on the probable 

contributors to violating near-roadway 

NO2 monitors. 

C. Final Designations Process 

The EPA intends to promulgate initial 

NO2 designations by January 2012 (2 

years after promulgation of the revised 

NAAQS). Along with today’s action 

EPA is also promulgating new 

monitoring rules that focus on 

roadways. As noted in section III, States 

must site required NO2 near-roadway 

monitors and have them operational by 

January 1, 2013. States will need an 

additional 3 years thereafter to collect 

air quality data in order to determine 

compliance with the revised NAAQS. 

This means that a full set of air quality 

data from the new network will not be 

available until after 2015. Since we 

anticipate that data from the new 

network will not be available prior to 

the CAA designation deadlines 

discussed above, the EPA intends to 

complete initial NO2 designations by 

January 2012 using the 3 most recent 

years of quality-assured air quality data 

from the current monitoring network, 

which would be for the years 2008– 

2010. The EPA will designate as 

‘‘nonattainment’’ any areas with NO2

monitors recording violations of the 

revised NO2 NAAQS. We intend to 

designate all other areas of the country 

as ‘‘unclassifiable’’ to indicate that there 

is insufficient data to determine 

whether or not they are attaining the 

revised NO2 NAAQS. 

Once the NO2 monitors are positioned 

in locations meeting the near-roadway 

siting requirements and monitoring data 

become available, the Agency has 

authority under section 107(d)(3) of the 

CAA to redesignate areas as appropriate 

from ‘‘unclassifiable’’ to ‘‘attainment’’ or 

‘‘nonattainment.’’ The EPA intends to 

issue guidance on the factors that States 

should consider when determining 

nonattainment boundaries after 

additional information is gathered on 

the probable contributors to violating 

near-roadway NO2 monitors. 

VI. 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.26 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 

help areas meet 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 and nonattainment new source 

review (NSR). See sections 160–169 of 

the CAA. In addition, Federal programs 

provide for nationwide reductions in 

emissions of NO2 and other air 

pollutants under Title II of the Act, 42 

U.S.C. 7521–7574, which involves 

controls for automobiles, trucks, buses, 

motorcycles, nonroad engines, and 

aircraft emissions; the new source 

performance standards (NSPS) for 

stationary sources under section 111 of 

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

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 


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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. Classifications 

1. Proposal 

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 for 

certain pollutants including NO2 in 

sections 191 and 192: 18 months from 

nonattainment designation for the 

submittal of an attainment plan, and as 

expeditiously as possible, but no later 

than 5 years from nonattainment 

designation for areas to attain the 

standard. In the proposal, EPA stated its 

belief that classifications are 

unnecessary in light of these relatively 

short deadlines. 

2. Public Comments 

One commenter stated that they 

disagree with EPA’s decision not to 

impose non-attainment classifications 

on areas with measured near-road NO2

concentrations in excess of the new NO2

standard, and urged EPA to provide a 

graduated non-attainment classification 

system for the new standard. According 

to the commenter, ‘‘a classification 

system defining higher levels of nonattainment with increasingly stringent 

requirements at those levels is one that 

allows for finer calibration of air quality 

regulatory response defined at the 

Federal level.’’ 

As stated in the proposed rule, 

Section 192(a), of part D, of the CAA 

specifically provides an attainment date 

for areas designated as nonattainment 

for the NO2 NAAQS. Therefore, EPA has 

legal authority to classify NO2

nonattainment areas, but the 5-year 

attainment date addressed under section 

192(a) cannot be extended pursuant to 

section 172(a)(2)(D). Based on this 

limitation, EPA proposed not to 

establish classifications within the 5- 

year interval for attaining any new or 

revised NO2 NAAQS. It is also EPA’s 

belief that given the short deadlines that 

States have to develop and submit SIP’s 

and for areas to achieve emissions 

reductions in order to attain the 

standard within the 5 year attainment 

period, a graduated classifications 

system would not be appropriate. 

Therefore, EPA is using it’s discretion 

under the CAA not to establish 

classifications. 

3. Final 

EPA is not making any changes to the 

discussion on classifications in the 

proposed rule. Therefore, there will be 

no classifications for the revised NO2

NAAQS. 

B. Attainment Dates 

The maximum deadline 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 NAAQS for NO2 if revised, 

which will be codified at 40 CFR part 

50, Appendix F. 

1. Attaining the NAAQS 

a. Proposal 

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. 

b. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 


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27Two 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. 

discussion on attaining the NAAQS in 

the proposed rule. 

2. Consequences of Failing To Attain by 

the Statutory Attainment Date 

a. Proposal 

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.’’ 

b. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 

discussion on consequences of failing to 

attain by the statutory attainment date 

in the proposed rule. 

C. Section 110(a)(2) NAAQS 

Infrastructure Requirements 

1. Proposal 

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 

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.27 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 and 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. 

2. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 

discussion on section 110(a)(2) NAAQS 

infrastructure requirements in the 

proposed rule. 

D. Attainment Planning Requirements 

1. Nonattainment Area SIPs 

a. Proposal 

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) 


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6524 Federal Register / Vol. 75, No. 26 / Tuesday, February 9, 2010 / Rules and Regulations 

28The 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. 

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 contains an 

attainment demonstration that 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 

inventories for nonattainment areas. The 

periodic inventory must include 

emissions of NO2 for point, nonpoint, 

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

sources. 

b. Public Comments 

Several commenters indicated that 

EPA should take steps to ensure that 

States actually require mobile source 

emissions reductions in order to attain 

the NO2 NAAQS as opposed to 

controlling point sources. Another 

commenter went further and stated that 

States be required to control on-road 

emissions as opposed to emissions from 

stationary sources and in particular 

EGUs. This commenter also indicated 

that EPA should delay nonattainment 

designations until States had a cost 

effective means of reducing on-road 

emissions of NO2. 

EPA cannot require States to develop 

a SIP that only addresses one type of 

source, in this case on-road mobile 

sources. States may select appropriate 

control measures to attain the NAAQS 

and EPA must approve them if they 

otherwise meet all applicable 

requirements of the Act. See CAA 116. 

EPA expects that States will evaluate a 

range of control measures that will 

reduce NO2 emissions within the time 

allowed to attain the standard. This 

would include the emissions reductions 

attributable to Federal controls on onroad and non-road mobile sources, and 

controls that they have put in place to 

reduce NOX emissions in order to attain 

the 8-hour ozone NAAQS and/or the 

PM2.5 NAAQS. If these existing controls 

are not sufficient for an area to reach 

attainment with the NO2 NAAQS, EPA 

would expect the State to implement 

additional control measures that would 

bring the area into attainment by the 

deadline. For a designation based on 

data from a near roadway monitor EPA 

would expect the States to give primary 

consideration to controlling emissions 

from on-road sources; however, it is 

likely that other types of sources 

contribute to the concentrations that are 

measured at a near roadway monitor 

and a State may decide to implement 

controls on these other contributing 

sources. 

The Clean Air Act requires that EPA 

finalize designations within two years 

after a NAAQS is revised unless the 

available air quality data is insufficient 

to make designations by that time. In 

that case, EPA must finalize 

designations within three years after the 

NAAQS is revised. As discussed 

elsewhere in today’s final rule, EPA 

believes that it has sufficient data to 

make designations within two years and 

that most areas will be designated as 

unclassifiable at that time. Taking the 

additional year provided by the CAA 

would not allow additional data from 

the new near roadway monitors to be 

factored into the designations process in 

any event. Therefore, it is EPA’s 

intention to designate areas within two 

years as required by the Act. EPA 

intends to redesignate areas once it has 

sufficient data from the new monitoring 

network to designate areas as clearly 

attaining or not attaining the standard. 

c. Final 

The EPA is not making any changes 

to the discussion on nonattainment area 

SIPs in the proposed rule. 

2. New Source Review and Prevention 

of Significant Deterioration 

Requirements 

a. Proposal 

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.28 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 


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29 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). 

30Criteria pollutants are those pollutants for 

which EPA has established a NAAQS under section 

109 of the CAA. 

unclassifiable for any criteria pollutant, 

is constructed, or undergoes a major 

modification.29 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 source NSR program addresses 

both major and minor sources which 

undergo construction or modification 

activities that do not qualify as major, 

and it applies, as necessary to assure 

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 assure that 

the [NAAQS] are achieved.’’ Areas 

which are newly designated as 

nonattainment for the NO2 NAAQS as a 

result of any changes made to the 

NAAQS will be required to adopt a 

nonattainment NSR program to address 

major sources of NO2 where the program 

does not currently exist for the NO2

NAAQS 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 may apply. 

b. Public Comments 

One commenter claimed that EPA’s 

setting of a more stringent standard, i.e., 

short-term NO2 NAAQS, could have 

important implications for NSR and 

PSD and title V permits. Another 

commenter indicated that the 

promulgation of a new 1-hr NO2 shortterm standard could create the need for 

a short-term PSD increment. Another 

commenter stated that a 1-hr NO2

Significant Impact Level (SIL) should be 

developed. 

The EPA acknowledges that a 

decision to promulgate a new short-term 

NO2 NAAQS will clearly have 

implications for the air permitting 

process. The full extent of how a new 

short-term NO2 NAAAQS will affect the 

NSR process will need to be carefully 

evaluated. First, major new and 

modified sources applying for NSR/PSD 

permits will initially be required to 

demonstrate that their proposed 

emissions increases of NOX will not 

cause or contribute to a violation of 

either the annual or 1-hour NO2 NAAQS 

and the annual PSD increment. In 

addition, we believe that section 166 of 

the CAA authorizes us to consider the 

need to promulgate a new 1-hour 

increment. Historically, EPA has 

developed increments for each 

applicable averaging period for which a 

NAAQS has been promulgated. 

However, increments for a particular 

pollutant do not necessarily need to 

match the averaging periods that have 

been established for NAAQS for the 

same pollutant. Environmental Defense 

Fund, Inc. v. EPA, 898 F.2d 183, 189– 

190 (DC Cir. 1990) (‘‘ * * * the ‘goals 

and purposes’ of the PSD program, set 

forth in 160, are not identical to the 

criteria on which the ambient standards 

are based.’’) Thus, we would need to 

evaluate the need for a new 1-hour NO2

increment in association with the goals 

and purposes of the statutory PSD 

program requirements. 

We also believe that there may be a 

need to revise the screening tools 

currently used under the NSR/PSD 

program for completing NO2 analyses. 

These screening tools include the 

significant impact levels (SILs), as 

mentioned by one commenter, but also 

include the significant emissions rate 

for emissions of NOX and the significant 

monitoring concentration (SMC) for 

NO2. EPA intends to evaluate the need 

for possible changes or additions to each 

of these important screening tools for 

NOX/NO2 due to the addition of a 

1-hour NO2 NAAQS. If changes or 

additions are deemed necessary, EPA 

will propose any such changes for 

public notice and comment in a separate 

action. 

c. Final 

The EPA is not making any changes 

to the discussion concerning the 

requirements for NSR and PSD as stated 

in the proposed rule. 

3. General Conformity 

a. Proposal 

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: 30

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. 

b. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 

discussion concerning general 

conformity stated in the proposed rule. 


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4. Transportation Conformity 

a. Proposal 

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 51.390, and Part 93, 

Subpart A establishes the criteria and 

procedures for determining whether 

transportation activities conform to the 

SIP. The EPA is not making changes to 

the Transportation Conformity rule in 

this rulemaking. However, in the future, 

EPA will review the need to conduct a 

rulemaking to establish any new or 

revised transportation conformity tests 

that would apply under a revision to the 

NO2 NAAQS for transportation plans, 

TIPs, and applicable highway and 

transit projects. 

b. Public Comments 

Several commenters stated that 

transportation conformity could stop the 

funding of highway and transit projects 

in NO2 nonattainment areas. These 

commenters stated that if an area fails 

to demonstrate conformity, it enters a 

conformity lapse and only certain types 

of projects can be funded during a lapse. 

The commenters further stated that the 

NO2 NAAQS will require more areas to 

determine conformity for the first time. 

The commenters also expressed concern 

that the NO2 NAAQS proposal did not 

contain sufficient information to 

understand to what extent revisions to 

the NAAQS, and the NO2 monitoring 

requirements, will result in 

transportation conformity requirements 

for individual transportation projects 

such as the need for a hot-spot analysis. 

The commenters further stated that hotspot analyses could result in needless 

delays for transportation improvement 

projects. 

With regard to the comment that more 

areas will have to demonstrate 

conformity for the first time due to the 

revisions to the NO2 NAAQS, given that 

today’s final rule is requiring that near 

roadway monitoring be carried out in 

urban areas with populations greater 

than 350K, EPA believes that most areas 

with such populations that would be 

designated nonattainment for NO2 are 

already designated nonattainment or 

maintenance for one or more of the 

other transportation-related criteria 

pollutants (ozone, PM2.5, PM10 and 

carbon monoxide). As such, these areas 

would have experience in making 

transportation conformity 

determinations. If areas with no 

conformity experience are designated 

nonattainment for the NO2 NAAQS, 

EPA and U.S. DOT would be available 

to assist areas in implementing the 

transportation conformity requirements. 

The commenter expressed concern 

that transportation conformity could 

stop highway and transit funding 

because areas could experience a 

conformity lapse and in such cases only 

certain types of projects could be 

funded. A conformity lapse occurs 

when an area misses a deadline for a 

required conformity determination. A 

new nonattainment area must 

demonstrate conformity within one year 

after the effective date of its designation. 

For any areas designated nonattainment 

for the revised NO2 NAAQS in early2012, they would have to determine 

conformity within one year of the 

effective date of that designation which 

would be in early-2013. If that date was 

missed, a lapse would occur and only 

projects exempt from conformity such 

as safety projects, transportation control 

measures in an approved SIP for the 

area and projects or project phases that 

were approved by U.S. DOT before the 

lapse began can proceed during the 

lapse. EPA’s experience in 

implementing the 1997 ozone and PM2.5

NAAQS shows that nearly all areas 

make their initial conformity 

determinations within the one-year 

grace period. Areas can also lapse if 

they fail to determine conformity by an 

applicable deadline such as determining 

conformity within two years after motor 

vehicle emissions budgets are found 

adequate. However, areas that miss one 

of these conformity deadlines have a 

one-year grace period before the lapse 

goes into effect. During the grace period, 

the area can continue to advance 

projects from the transportation plan 

and transportation improvement 

program. EPA’s experience is that areas 

generally are able to make a conformity 

determination before the end of the 

grace period. 

The commenter expressed concern 

that the NO2 NAAQS proposal did not 

contain sufficient detail concerning 

possible project-level requirements for 

transportation projects and that any 

requirements for hot-spot analyses 

could needlessly delay transportation 

projects. As EPA indicated in the 

NPRM, EPA is considering whether to 

revise the transportation conformity rule 

to establish requirements that would 

apply to transportation plans, 

transportation improvement programs 

and/or transportation projects in NO2

nonattainment and maintenance areas. 

If EPA concludes that the conformity 

rule must be revised in light of the final 

NO2 NAAQS, we will conduct notice 

and comment rulemaking to accomplish 

the revisions. At that time interested 

parties will have the opportunity to 

comment on any transportation 

conformity NPRM. This is the same 

course of action that EPA has taken with 

respect to revising the transportation 

conformity rule for the ozone and PM2.5

NAAQS. 

With regard to the commenter’s 

assertion that a requirement for hot-spot 

analyses for individual projects would 

needlessly delay transportation projects, 

EPA disagrees. First, CAA section 

176(c)(1)(B) requires that transportation 

projects not cause new violations or 

make existing violations worse, or delay 

timely attainment or cause an interim 

milestone to be missed. EPA would only 

impose a hot-spot requirement for 

projects in NO2 nonattainment and 

maintenance areas if they are necessary 

to comply with CAA conformity 

requirements and therefore are needed 

to protect public health by reducing 

exposures to unhealthy levels of NO2

that could be created by the 

implementation of a proposed highway 

or transit project. The public would be 

exposed to unhealthy levels of NO2 if a 

highway or transit project caused a new 

violation of the NO2 NAAQS, made an 

existing violation worse, or delayed 

timely attainment or delayed achieving 

an interim emissions milestone. If any 

delay in the project did occur, it would 

not be viewed as needless as it occurred 

for the important purpose of protecting 

the exposed public’s health. Second, 

EPA does not agree that requiring a hotspot analysis would needlessly delay 

projects in NO2 nonattainment areas. 

Such hot-spot analyses, if they are 

eventually required, generally would be 

done as part of the NEPA process, 

which these projects are already subject 

to; therefore, conducting an NO2 hotspot analysis would not be introducing 

a new step to a project’s approval 

process, but rather would add one 

additional analysis which must be 

completed as part of an existing project 

approval process. 


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c. Final 

EPA is not making any changes to the 

discussion concerning transportation 

conformity as stated in the proposed 

rule. 

VII. 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. This 

section describes the conforming 

changes that were proposed, major 

comments received on these changes, 

EPA’s responses to these comments and 

final decisions on the AQI breakpoints. 

Recognizing the importance of revising 

the AQI in a timely manner to be 

consistent with any revisions to the 

NAAQS, EPA proposed conforming 

changes to the AQI in connection with 

the final decision on the NO2 NAAQS 

if revisions to the primary standard 

were 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 did 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, EPA proposed to 

set this value to be between 0.040 and 

0.053 ppm NO2, 1-hour average. EPA 

proposed that the figure 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 noted that 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, and solicited 

comments on this range for an AQI of 

50 and the appropriate basis for 

selecting an AQI of 50 within this range. 

With regard to an AQI value of 150, 

the breakpoint between the unhealthy 

for sensitive groups and unhealthy 

categories, 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 

proposed to set the AQI value of 150 to 

be between 0.360 and 0.370 ppm NO2, 

1-hour average. 

EPA received comments from several 

State environmental agencies and 

organizations of State and local agencies 

that generally expressed the view that 

the AQI was designed to provide the 

public with information about regional 

air quality and therefore it should be 

based on community-wide monitors. 

These commenters went on to state that 

using near-road NO2 monitors for the 

AQI would present problems because 

they would not represent regional NO2

concentrations and it would be difficult 

to communicate this type of information 

to the public using the AQI. Some 

expressed concern that NO2 measured at 

near-roadway monitors could be the 

critical pollutant and could drive the 

AQI even though it may not represent 

air quality across the area. Other 

agencies expressed concern that there is 

currently no way to forecast ambient 

NO2 levels near roadways. One State 

agency commented that the AQI is 

intended to represent air quality where 

people live, work and play. 

EPA agrees with commenters that the 

AQI should represent regional air 

quality, and that measurements that 

apply to a limited area should not be 

used to characterize air quality across 

the region. Community-wide NO2

monitors should be used to characterize 

air quality across the region. However, 

the AQI reporting requirements 

encourage, but do not require, the 

reporting of index values of sub-areas of 

an MSA. We agree with the commenter 

that stated the view that the AQI is 

intended to represent air quality where 

people live, work and play. To the 

extent that near-roadway monitoring 

occurs in areas where people live, work 

or play, EPA encourages reporting of the 

AQI for that specific sub-area of the 

MSA (64 FR 42548, August 4, 1999). We 

also agree that it may be difficult to 

communicate this type of information 

and we plan to work with State and 

local air agencies to figure out the best 

way to present this information to the 

public using the AQI. Air quality 

forecasting is recommended but not 

required (64 FR 42548, August 4, 1999). 

EPA will work with State agencies that 

want to develop a forecasting program. 

With regard to the proposed 

breakpoints, EPA received few 

comments. The National Association of 

Clean Air Agencies commented that it 

would be confusing to the public to 

have an AQI value of 50 set below the 

level of the annual NO2 standard. We 

agree with this comment, and therefore 

have decided that it is appropriate to set 

the AQI value of 50, the breakpoint 

between the good and moderate ranges, 

set at the numerical level of the annual 

standard, 53 ppb NO2, 1-hour average. 

The AQI value of 100, the breakpoint 

between the moderate and unhealthy for 

sensitive groups category, is set at 100 

ppb, 1-hour average, the level of the 

primary NO2 NAAQS. EPA is setting an 

AQI value of 150, the breakpoint 

between the unhealthy for sensitive 

groups and unhealthy categories, at 

0.360 ppm NO2, 1-hour average. 

VIII. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under Executive Order 12866 (58 FR 

51735, October 4, 1993), this action is a 

‘‘significant regulatory action’’ because it 

was deemed to ‘‘raise novel legal or 

policy issues.’’ Accordingly, EPA 

submitted this action to the Office of 

Management and Budget (OMB) for 

review under Executive Order 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 final 

rule. 

B. Paperwork Reduction Act 

The information collection 

requirements in this final 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 

requirements are not enforceable until 

OMB approves them. 

The Information Collection Request 

(ICR) document prepared by EPA for 

these revisions to part 58 has been 

assigned EPA ICR number 2358.02. 

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 


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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 amendments would revise 

the technical requirements for NO2

monitoring sites, require the siting and 

operation of additional NO2 ambient air 

monitors, and the reporting of the 

collected ambient NO2 monitoring data 

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

annual average reporting burden for the 

collection under 40 CFR part 58 

(averaged over the first 3 years of this 

ICR) is $3,261,007. 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. 

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 final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule establishes national standards 

for allowable concentrations of NO2 in 

ambient air as required by section 109 

of the CAA. American Trucking Ass’ns 

v. EPA, 175 F.3d 1027, 1044–45 (DC cir. 

1999) (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

Similarly, the 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. 

D. Unfunded Mandates Reform Act 

This 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. Thus, this rule is not subject to 

the requirements of sections 202 or 205 

of UMRA. 

With regard to implementation 

guidance, the CAA imposes the 

obligation for States to submit SIPs to 

implement the NO2 NAAQS. In this 

final 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. 

This rule is also not subject to the 

requirements of section 203 of UMRA 

because it contains no regulatory 

requirements that might significantly or 

uniquely affect small governments 

because it imposes no enforceable duty 

on any small governments. 

E. Executive Order 13132: Federalism 

This action 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. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have Tribal 

implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 

2000). 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 


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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 action. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

This action is subject to Executive 

Order 13045 (62 FR 19885, April 23, 

1997) because it is an economically 

significant regulatory action as defined 

by Executive Order 12866, and EPA 

believes that the environmental health 

or safety risk addressed by this action 

has a disproportionate effect on 

children. The final 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 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action 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 final rulemaking involves 

technical standards. Therefore the 

Agency conducted a search to identify 

potential applicable voluntary 

consensus standards. However, we 

identified no such standards, and none 

were brought to our attention in 

comments. Therefore, EPA has decided 

to use the technical standard described 

in Section III.A of the preamble. 

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 final 

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 

final rule will establish uniform 

national standards for NO2 in ambient 

air. 

K. Congressional Review Act 

The Congressional Review Act, 5 

U.S.C. 801 et seq., as added by the Small 

Business Regulatory Enforcement 

Fairness Act of 1996, generally provides 

that before a rule may take effect, the 

agency promulgating the rule must 

submit a rule report, which includes a 

copy of the rule, to each House of the 

Congress and to the Comptroller General 

of the United States. EPA will submit a 

report containing this rule and other 

required information to the U.S. Senate, 

the U.S. House of Representatives, and 

the Comptroller General of the United 

States prior to publication of the rule in 

the Federal Register. A Major rule 

cannot take effect until 60 days after it 

is published in the Federal Register. 

This action is a ‘‘major rule’’ as defined 

by 5 U.S.C. 804(2). This rule will be 

effective on April 12, 2010. 

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Ware JH, Spengler JD, Koutrakis P, Speizer 

FE, Ferris BG, Jr. (1994). Acute effects of 

summer air pollution on respiratory 

symptom reporting in children. Am J 

Respir Crit Care Med. 150:1234–1242. 

Singer, B, Hodgson A, Hotchi T, Kim J. 

(2004). Passive measurement of nitrogen 

oxides to assess traffic-related pollutant 

exposure for the East Bay Children’s 

Respiratory Health Study. Atmos Environ. 

38:393–403. 

Thompson, R. (2008). Nitrogen Dioxide (NO2) 

Descriptive Statistics Tables. Memo to the 

NO2 NAAQS docket. Available at http:// 

www.epa.gov/ttn/naaqs/standards/nox/ 

s_nox_cr_rea.html. 

Tolbert, PE, Klein M, Peel JL, Sarnat SE, 

Sarnat JA. (2007). Multipollutant modeling 

issues in a study of ambient air quality and 

emergency department visits in Atlanta. J. 

Exposure Sci. Environ. Epidemiol. 

17(Suppl. 2s): S29–S35. 

Watkins, N. and Thompson, R. (2008). NOX

Network Review and Background. Memo to 

the NO2 NAAQS docket. 

Zhou, Y and Levy JI. (2007). Factors 

influencing the spatial extent of mobile 

source air pollution impacts: a metaanalysis. BMC Public Health. 7:89. 

List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: January 22, 2010. 

Lisa P. Jackson, 

Administrator. 

■ For the reasons stated in the preamble, 

title 40, chapter I of the Code of Federal 

Regulations is amended as follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

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

■ 2. Section 50.11 is revised to read as 

follows: 

§ 50.11 National primary and secondary 

ambient air quality standards for oxides of 

nitrogen (with nitrogen dioxide as the 

indicator). 

(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 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 98th percentile of the daily 

maximum 1-hour average concentration 

is less than or equal to 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 adding 

an entry to the end of table in paragraph 

(c)(2)(vi) to read as follows: 

§ 50.14 Treatment of air quality monitoring 

data influenced by exceptional events. 

* * * * * 

(c) * * * 

(2) * * * 

(vi) * * * 

TABLE 1—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 

* * * * * * * 

NO2/1-Hour Standard (100 PPB) .............................. 2008 July 1, 2010 a ............................................................. January 22, 2011. 

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

2010 April 1, 2011 a ............................................................ July 1, 2011 a. 

a 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 to Part 50 is added to 

read as follows: 

Appendix S to Part 50—Interpretation 

of the Primary National Ambient Air 

Quality Standards for Oxides of 

Nitrogen (Nitrogen Dioxide) 

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 98th 

percentile daily maximum 1-hour values for 

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

primary standard design value’’). 

98th percentile daily maximum 1-hour 

value is the value below which nominally 98 

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 

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 

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, including State-flagged data affected 

by exceptional events which have been 

approved for exclusion by the Administrator, 

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, including Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, 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, 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator; 

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, excluding Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, 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 


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75 percent data capture. If after substituting 

the highest non-excluded 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 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. 

(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 98th 

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 98th percentile value 

is accomplished as follows. 

(i) The annual 98th percentile value for a 

year is the higher of the two values resulting 

from the following two procedures. 

(1) Procedure 1. 

(A) For the year, determine the number of 

days with at least 75 percent of the hourly 

values reported including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

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

at least 75 percent of the hourly values 

reported, select from each day the maximum 

hourly value excluding State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(C) Sort all these daily maximum hourly 

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 98th 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) as determined from step (A). 

The corresponding ‘‘n’’ value in the right 

column identifies the rank of the annual 98th 

percentile value in the descending sorted list 

of daily site values for year y. Thus, P0.98, y

= the nth largest value. 

(2) Procedure 2. 

(A) For the year, determine the number of 

days with at least one hourly value reported 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(B) For the year, from all the days with at 

least one hourly value reported, select from 

each day the maximum hourly value 

excluding State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(C) Sort all these daily maximum 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 98th 

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) as determined from step (A). The 

corresponding ‘‘n’’ value in the right column 

identifies the rank of the annual 98th 

percentile value in the descending sorted list 

of daily site values for year y. Thus, P0.98, y

= the nth largest value. 

(b) The 1-hour primary standard design 

value for a site is mean of the three annual 

98th percentile values, rounded according to 

the conventions in section 4. 


jlentini on DSKJ8SOYB1PROD with RULES2
6534 Federal Register / Vol. 75, No. 26 / Tuesday, February 9, 2010 / Rules and Regulations 

TABLE 1 

Annual number 

of days with 

valid data for 

year ‘‘y’’ (cny) 

P0.98, y is the 

nth maximum 

value of the 

year, where n 

is the listed 

number 

1–50 1 

51–100 2 

101–150 3 

151–200 4 

201–250. 5 

251–300 6 

301–350 7 

351–366 8 

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 

the definitions for ‘‘AADT’’ and ‘‘Nearroad 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(d), 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, 2012. The plan shall provide 

for all required monitoring 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, and 

E 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; 

PM2.5mass 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 and PAMS 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 

by adding paragraph 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 

by adding paragraph 2.1.1 to read as 

follows: 

Appendix C to Part 58—Ambient Air 

Quality Monitoring Methodology 

* * * * * 

2.1.1 Any NO2 FRM or FEM used for 

making primary NAAQS decisions must be 

capable of providing hourly averaged 

concentration data. 

* * * * * 

■ 12. Appendix D to Part 58 is amended 

by revising paragraph 4.3 to read as 

follows: 

Appendix D to Part 58—Network 

Design Criteria for Ambient Air Quality 

Monitoring 

* * * * * 

4.3 Nitrogen Dioxide (NO2) Design Criteria 

4.3.1 General 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 

500,000 or more persons to monitor a 

location of expected maximum hourly 

concentrations sited near a major road with 

high AADT counts as specified in 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 500,000 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, 

considering fleet mix, roadway design, 

congestion patterns, terrain, and 

meteorology, where maximum hourly NO2

concentrations are expected to occur 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 

shall consider 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 


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CBSA; or different route, interstate, or 

freeway designation. 

(b) Measurements at required near-road 

NO2 monitor sites utilizing 

chemiluminescence FRMs must include at a 

minimum: NO, NO2, and NOX. 

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 Administrators, in 

collaboration with States, must require a 

minimum of forty additional NO2 monitoring 

stations nationwide in any area, inside or 

outside of CBSAs, above the minimum 

monitoring requirements, with a primary 

focus on siting these monitors in locations to 

protect susceptible and vulnerable 

populations. The Regional Administrators, 

working with States, may also consider 

additional factors described in paragraph (b) 

below to require monitors beyond the 

minimum network requirement. 

(b) The Regional Administrators may 

require monitors to be sited inside or outside 

of CBSAs in which: 

(i) The required near-road monitors do not 

represent all locations of expected maximum 

hourly NO2 concentrations in an area and 

NO2 concentrations may be approaching or 

exceeding the NAAQS in that area; 

(ii) Areas that are not required to have a 

monitor in accordance with the monitoring 

requirements and NO2 concentrations may be 

approaching or exceeding the NAAQS; or 

(iii) The minimum monitoring 

requirements for area-wide monitors are not 

sufficient to meet monitoring objectives. 

(c) 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 address the data needs for an area, 

and include all monitors under this provision 

in the annual monitoring network plan. 

4.3.5 NO2 Monitoring Spatial Scales 

(a) The most important spatial scale for 

near-road NO2 monitoring stations to 

effectively characterize the maximum 

expected hourly NO2 concentration due to 

mobile source emissions on major roadways 

is the microscale. The most important spatial 

scales for other monitoring stations 

characterizing maximum expected hourly 

NO2 concentrations are the microscale and 

middle scale. The most important spatial 

scale for area-wide monitoring of high NO2

concentrations is the neighborhood scale. 

(1) Microscale—This scale represents 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 represents air quality 

conditions throughout some relatively 

uniform land use areas with dimensions in 

the 0.5 to 4.0 kilometer range. Emissions 

from stationary point and area sources may, 

under certain plume conditions, result in 

high NO2 concentrations at the neighborhood 

scale. Where a neighborhood site is located 

away from immediate NO2 sources, the site 

may be useful in representing typical air 

quality values for a larger residential area, 

and therefore suitable for population 

exposure and trends analyses. 

(4) Urban scale—Measurements in this 

scale would be used to estimate 

concentrations over large portions of an 

urban area with dimensions from 4 to 50 

kilometers. Such measurements would be 

useful for assessing trends in area-wide air 

quality, and hence, the effectiveness of large 

scale air pollution control strategies. Urban 

scale sites may also support other monitoring 

objectives of the NO2 monitoring network 

identified in paragraph 4.3.4 above. 

4.3.6 NOy Monitoring 

(a) NO/NOy measurements are included 

within the NCore multi-pollutant 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. Appendix E to Part 58 is amended 

as follows: 

■ a. By revising paragraphs 2, and 6.1. 

■ b. By adding paragraphs 4(d) and 6.4. 

■ c. By revising paragraphs 9(c), 11 and 

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 

concentration potential for the pollutant 

being measured. 

* * * * * 

4. * * * 

(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 


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path in the area of potential atmospheric 

interference from automobile emissions. 

Therefore, this calculation must include the 

length of the monitoring path over the 

roadway plus any segments of the monitoring 

path that lie in the area between the roadway 

and minimum separation distance, as 

determined from the Table E–1 of this 

appendix. The sum of these distances must 

not be greater than 10 percent of the total 

monitoring path length. 

* * * * * 

6.4 Spacing for Nitrogen Dioxide (NO2) 

Probes and Monitoring Paths 

(a) In siting 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 form the 

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

distances must not be greater than 10 percent 

of the total monitoring path length. 

* * * * * 

9. * * * 

(c) No matter how nonreactive the 

sampling probe material is initially, after a 

period of use reactive particulate matter is 

deposited on the probe walls. Therefore, the 

time it takes the gas to transfer from the 

probe inlet to the sampling device is also 

critical. Ozone in the presence of nitrogen 

oxide (NO) will show significant losses even 

in the most inert probe material when the 

residence time exceeds 20 seconds.26 Other 

studies 27 28 indicate that a 10 second or 

less residence time is easily achievable. 

Therefore, sampling probes for reactive gas 

monitors at NCore and at NO2 sites must 

have a sample residence time less than 20 

seconds. 

* * * * * 

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 structures2

to probe, inlet or 90% 

of monitoring path1

(meters) 

Distance from trees 

to probe, inlet or 90% 

of monitoring path1

(meters) 

Distance from roadways to probe, inlet 

or monitoring path1

(meters) 

SO2 3,4,5,6 ..................... Middle (300 m) 

Neighborhood 

Urban, and Regional (1 km).

2–15 .......................... >1 .............................. >10 ............................ N/A 

CO 4,5,7 ........................ Micro, middle (300 

m), Neighborhood 

(1 km).

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 nearroad 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) 345.

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. 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. 


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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 

as by revising paragraph 9 and Table 2 

to read as follows: 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

* * * * * 

9. How Does the AQI Relate to Air Pollution 

Levels? 

For each pollutant, the AQI transforms 

ambient concentrations to a scale from 0 to 

500. The AQI is keyed as appropriate to the 

national ambient air quality standards 

(NAAQS) for each pollutant. In most cases, 

the index value of 100 is associated with the 

numerical level of the short-term (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 AQIs 

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 0–0.053 0–50 Good. 

0.060–0.075 ..................... 15.5–40.4 55–154 4.5–9.4 0.035–0.144 0.054–0.100 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 0.101–0.360 101–150 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 0.361–0.64 151–200 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-hours 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. 2010–1990 Filed 2–8–10; 8:45 am] 

BILLING CODE 6560–50–P 


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