1

6560-50-P 

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50 and 58

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

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 [insert date 60 days after date of publication in the 

Federal Register]. 

ADDRESSES EPA has established a docket for this action under Docket ID No. EPA-HQOAR-2006-0922. All documents in the docket are listed on the www.regulations.gov website. 
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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 

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; email: 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 short-term exposure to NO2
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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 
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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 (Area-wide 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 

a. Proposed Area-wide Monitor Site Selection and Siting Criteria 

b. Comments 

c. Conclusions on Area-wide Monitor Site Selection and Siting 

Criteria 
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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 
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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 
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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 
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“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 (D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); American 

Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert. denied, 455 U.S. 1034 

(1982). Both kinds of uncertainties are components of the risk associated with pollution at levels 

below those at which human health effects can be said to occur with reasonable scientific 

certainty. Thus, in selecting primary standards that include an adequate margin of safety, the 

Administrator is seeking not only to prevent pollution levels that have been demonstrated to be 

 

1

 EPA 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. 

2

 The legislative history of section 109 indicates that a primary standard is to be set at “the 

maximum permissible ambient air level . . . which will protect the health of any [sensitive] group 

of the population,” and that for this purpose “reference should be made to a representative 

sample of persons comprising the sensitive group rather than to a single person in such a group.” 

S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970) 

3

 EPA is currently conducting a separate review of the secondary NO2 NAAQS jointly with a 

review of the secondary SO2 NAAQS. 
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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 
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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 non-attainment. 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

). 

 

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 
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While NOX is emitted from a wide variety of source types, the top three categories of 

sources of NOX emissions are on-road mobile sources, electricity generating units, and non-road 

mobile sources. EPA anticipates that NOX emissions will decrease substantially over the next 20 

years as a result of the ongoing implementation of mobile source emissions standards. In 

particular, Tier 2 NOX emission standards for light-duty vehicle emissions began phasing into the 

fleet beginning with model year 2004, in combination with low-sulfur gasoline fuel standards. 

For heavy-duty engines, new NOX standards are phasing in between the 2007 and 2010 model 

years, following the introduction of ultra-low sulfur diesel fuel. Lower NOX standards for 

nonroad diesel engines, locomotives, and certain marine engines are becoming effective 

throughout the next decade. In future decades, these lower- NOX vehicles and engines will 

become an increasingly large fraction of in-use mobile sources, effecting large NOX emission 

reductions. 

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 

 

community for the sum of all forms of oxidized nitrogen compounds including those listed in 

Section 108(c) is NOY. 
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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 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 
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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, form5

, 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 

 

5

 The “form” of a standard defines the air quality statistic that is to be compared to the level of 

the standard in determining whether an area attains the standard. 
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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 
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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 policyrelevant 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) 
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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 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 
17

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. 
18

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 
19

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 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 

 

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. 
20

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 area-wide exposures on the neighborhood, urban, and 

regional scales, as would be expected for a network sited to support the current annual NO2

standard and PAMS objectives. The current network does not include monitors placed near 

major roadways and, therefore, monitors in the current network do not necessarily measure the 
21

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 traffic-related exposures can dominate personal exposures to NO2 (ISA 

section 2.5.4). While driving, personal exposure concentrations in the cabin of a vehicle could 

be substantially higher than ambient concentrations measured nearby (ISA, section 2.5.4). For 

example, 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 roadway-associated NO2

concentrations, individuals who spend time on and/or near major roadways could experience 

NO2 concentrations that are considerably higher than indicated by monitors in the current areawide NO2 monitoring network. 

Research suggests that the concentrations of on-road mobile source pollutants such as 

NOX, carbon monoxide (CO), directly emitted air toxics, and certain size distributions of 

particulate matter (PM), such as ultrafine PM, typically display peak concentrations on or 

immediately adjacent to roads (ISA, section 2.5). This situation typically produces a gradient in 

pollutant concentrations, with concentrations decreasing with increasing distance from the road, 

and concentrations generally decreasing to near area-wide ambient levels, or typical upwind 
22

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 
23

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 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 
24

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 co-occurring pollutants, and studies 

conducted in areas where the whole distribution of ambient 24-hour average NO2 concentrations 

was below the current NAAQS level of 53 ppb (annual average). With regard to this evidence, 

the ISA concluded that NO2 epidemiologic studies provide “little evidence of any effect 

threshold” (ISA, section 5.3.2.9, p. 5-15). In studies that have evaluated concentration-response 

relationships, they appear linear within the observed range of data (ISA, section 5.3.2.9). 

Overall, the epidemiologic evidence for respiratory effects has been characterized in the 

ISA as consistent, in that associations are reported in studies conducted in numerous locations 

with a variety of methodological approaches, and coherent, in that the studies report associations 

with respiratory health outcomes that are logically linked together. In addition, a number of 
25

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 co-occurring pollutants 

such as PM, O3, CO, and/or SO2. The evidence summarized in the ISA indicates that NO2

associations generally remain robust in these multi-pollutant models and supports a direct effect 

of short-term NO2 exposure on respiratory morbidity (see ISA Figures 3.1-7, 3.1-10, 3.1-11). 

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-
26

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, may contribute to 
27

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 
28

The ISA concluded that overall, the epidemiologic and experimental evidence is 

suggestive, but not sufficient, to infer a causal relationship between long-term NO2 exposure and 

respiratory morbidity (ISA, section 5.3.2.4). The available database evaluating the relationship 

between respiratory illness in children and long-term exposures to NO2 has increased since the 

1996 review of the NO2 NAAQS (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 
29

inadequate to infer the presence or absence of a causal relationship for carcinogenic, 

cardiovascular, and reproductive and developmental effects related to long-term NO2 exposure 

(ISA, section 5.3.2.5). Epidemiologic studies conducted in Europe have shown an association 

between long-term NO2 exposure and increased incidence of cancer (ISA, section 5.3.2.5). 

However, the animal toxicological studies have provided no clear evidence that NO2 acts as a 

carcinogen (ISA, section 5.3.2.5). The very limited epidemiologic and toxicological evidence do 

not suggest that long-term exposure to NO2 has cardiovascular effects (ISA, section 5.3.2.5). 

The epidemiologic evidence is not consistent for associations between NO2 exposure and fetal 

growth retardation; however, some evidence is accumulating for effects on preterm delivery 

(ISA, section 5.3.2.5). Scant animal evidence supports a weak association between NO2

exposure and adverse birth outcomes and provides little mechanistic information or biological 

plausibility for the epidemiologic findings. 

4. NO2-related impacts on public health 

Specific groups within the general population are likely at increased risk 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; 
30

race/ethnicity; genetic factors; and pre-existing disease/condition (e.g., obesity, diabetes, 

respiratory disease, asthma, chronic obstructive pulmonary disease (COPD), cardiovascular 

disease, airway hyperresponsiveness, respiratory infection, adverse birth outcome) (ISA, sections 

4.3.1, 4.3.5, and 5.3.2.8). In addition, certain groups may experience relatively high exposure to 

NO2, thus forming a potentially vulnerable population (ISA, section 4.3.6). Factors that may 

influence 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 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 

 

7

 The most current American Housing Survey 

(http://www.census.gov/hhes/www/housing/ahs/ahs.html) is from 2007 and lists a higher fraction 

of housing units within the 300 foot boundary than do prior surveys. According to Table 1A-6 

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

128,203,000 total housing units in the United States, 20,016,000 were reported by the surveyed 

occupant or landlord as being within 300 feet of a 4-or-more lane highway, railroad, or airport. 

That constitutes 15.613% of the total housing units in the U.S. Assuming equal distributions, 
31

than 150,000 students were located within approximately 500 feet of high-traffic roads, with a 

higher proportion of non-white and economically disadvantaged students attending those schools 

(ISA, section 4.4). Of this population, asthmatics and members of other susceptible groups 

discussed above will have even greater risks of experiencing health effects related to NO2

exposure. In the United States, approximately 10% of adults and 13% of children 

(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 

 

with a current population of 306,330,199, that means that there would be 47.8 million people 

meeting the 300 foot criteria. 
32

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 
33

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 fixed-site monitors and simulated concentrations on/near roadways were 

compared to potential health effect benchmark values derived from the controlled human 

exposure literature. In the second approach, modeled estimates of 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 

 

8

 The 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. 
34

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 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 area-wide 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
35

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-/risk-based 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: 
36

• 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: 
37

• 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 
38

CASAC. These considerations are discussed in detail in the proposal (II.E.) and are summarized 

in this section. In the proposal, the 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 
39

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 
40

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 
41

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 shortterm 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 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 
42

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 Science-Based 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 
43

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 co-pollutants 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 co-pollutants in the models. 

Similarly, associations between short-term NO2 exposure and asthma symptoms are generally 

robust to adjustment for co-pollutants 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 co-pollutants. 

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). 
44

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. 
45

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 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 co-pollutants 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 
46

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 
47

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 
48

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, self-administered 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 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 
49

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 meta-analysis 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 metaanalysis reflects a change in the way the data are presented and does not reflect a substantive 
50

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 meta-analysis 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 meta-analysis. 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 
51

 

Many of these same industry groups also referred in their comments to a recent metaanalysis 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. 

 

9

 EPA 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. 
52

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 meta-analysis to 

assess only the fraction of asthmatics experiencing increased or decreased airway responsiveness 

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 
53

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 meta-analysis 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. 
54

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

 

10 Once 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. 
55

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 shortterm 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). 
56

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 
57

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 co-occurring pollutants by employing multi-pollutant models. 

When evaluated as a whole, NO2 effect estimates in these models generally remained robust 

when co-pollutants were included. Therefore, despite uncertainties associated with separating 

the effects of NO2 from those of co-occurring pollutants, the ISA (section 5.4, p. 5-16) concluded 

that “the evidence summarized in this assessment indicates that NO2 associations generally 

remain robust in multi-pollutant models and supports a direct effect of short-term NO2 exposure 

on respiratory morbidity at ambient concentrations below the current NAAQS.” With regard to 

indoor studies, the ISA noted that these studies can test hypotheses related to NO2 specifically 

(ISA, section 3.1.4.1). Although confounding by indoor combustion sources is a concern, indoor 

studies are not confounded by the same mix of co-pollutants present in the ambient air or by the 

contribution of NO2 to the formation of secondary particles or O3 (ISA, section 3.1.4.1). The 

ISA noted that the findings of indoor NO2 studies are consistent with those of studies using 
58

ambient concentrations from central site monitors and concluded that indoor studies provide 

evidence of coherence for respiratory effects (ISA, section 3.1.4.1). With regard to experimental 

studies, the REA noted that they have the advantage of providing information on health effects 

that are specifically associated with exposure to NO2 in the absence of co-pollutants. The ISA 

concluded that the NO2 epidemiologic literature is supported by 1) evidence from controlled 

human exposure studies of airway hyperresponsiveness in asthmatics, 2) controlled human 

exposure and animal toxicological studies of impaired host-defense systems and increased risk of 

susceptibility to viral and bacterial infection, and 3) controlled human exposure and animal 

toxicological studies of airway inflammation (ISA, section 5.3.2.1 and 5.4). 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 risk-based 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 
59

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 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 
60

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 at-risk 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 short-term 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 
61

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 
62

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 
63

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 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 one-hour 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. 
64

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. 
65

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 short-term to annual 

average NO2 concentrations. Ratios of 1-hour daily maximum concentrations (98th and 99th

percentile11) to annual average concentrations across 14 locations ranged from 2.5 to 8.7 while 

 

11 As discussed below, 98th and 99th percentile forms were evaluated in the REA. A 99th

percentile form corresponds approximately to the 4th highest 1-hour concentration in a year while 

a 98th percentile form corresponds approximately to the 7th or 8th highest 1-hour concentration in 
66

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 number of epidemiologic studies have detected positive associations between 

 

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. 
67

respiratory morbidity and 1-hour (daily maximum) and/or 24-hour NO2 concentrations. A few 

epidemiologic studies have considered both 1-hour and 24-hour averaging times, allowing 

comparisons to be made. The ISA reported that such comparisons in studies that evaluate 

asthma emergency department visits failed to reveal differences between effect estimates based 

on a 1-hour averaging time and those based on a 24-hour averaging time (ISA, section 5.3.2.7). 

Therefore, the ISA concluded that it is not possible, from the available epidemiologic evidence, 

to discern whether effects observed are attributable to average daily (or multi-day) 

concentrations (24-hour average) or high, peak exposures (1-hour maximum) (ISA, section 

5.3.2.7). 

As noted in the policy assessment chapter of the REA, given the above conclusions, the 

experimental evidence provides support for an averaging time of shorter duration than 24 hours 

(e.g., 1-h) while the epidemiologic evidence provides support for both 1-hour and 24-hour 

averaging times. 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 
68

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 
69

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 pollutant-associated 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. 
70

In considering specific concentration-based 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 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 
71

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 
72

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 area-wide 

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 
73

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 area-wide 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 short-term ambient NO2 concentrations across an area, including higher shortterm (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 
74

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 roadway-associated 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 

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, 

 

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 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). 
75

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 short-term 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. 
76

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-
77

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 areawide 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 area-wide 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. 

 

13 The 98th percentile concentrations in these study locations ranged from 85 to 94 ppb. 

14 For 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). 
78

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 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 
79

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 ppb15 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 areawide 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 

 

15 This 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. 
80

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 area-wide 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 roadwayassociated 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 area-wide 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 
81

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 area-wide 

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 members16 favored the proposed approach of 

setting a 1-hour standard that reflects the maximum allowable NO2 concentration anywhere in an 

area and linking such a standard with a 2-tiered monitoring network that would include both 

 

16 CASAC 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. 
82

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 near-road monitoring 

data for criteria pollutants in general but favored retention of EPA's current areawide 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 area-wide 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 
83

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. 
84

 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 area-wide monitoring network should be used to identify initial 

nonattainment areas.” Other State commenters also concluded that the most appropriate 

approach would be to base non-attainment determinations only on area-wide 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 
85

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 roadway-associated 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 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 
86

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 

 

17 To 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). 
87

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 80ppb 

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 
88

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 
89

level below 50 ppb based on mean area-wide NO2 concentrations in epidemiologic study 

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 area-wide 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 

 

18 As discussed above, the Administrator has selected the 98th percentile as the form for the new 

1-hour NO2 standard. 
90

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 multi-pollutant 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 
91

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 dose-response 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 
92

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 short-term ambient NO2 concentrations across 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: 
93

• 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 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 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 
94

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 high-traffic 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

 

19 The most current American Housing Survey 

(http://www.census.gov/hhes/www/housing/ahs/ahs.html) is from 2007 and lists a higher fraction 

of housing units within the 300 foot boundary. 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. 
95

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 area-wide 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 
96

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 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 respiratory-related 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 
97

reported associations between ambient NO2 and respiratory-related 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 
98

(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 areawide 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 
99

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 
100

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 meta-analysis (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 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 
101

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 area-wide 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 

 

20 Some 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). 
102

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 areawide 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 area-wide 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 
103

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. 
104

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 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. 
105

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 long-term NO2

exposures and adverse respiratory effects, that evidence is not sufficient to support a standard 
106

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. 
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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 at-risk populations against an array of adverse respiratory 

health effects related to short-term NO2 exposure, the Administrator is establishing a short-term 

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 shortterm exposures to NO2 and effects potentially associated with long-term exposures to NO2. 

III. Amendments to Ambient Monitoring and Reporting Requirements 
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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-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). 
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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 over-estimate in the reported measurement 

of the actual ambient NO2 concentration. This interference by NOZ compounds has long been 

known and evaluated (Fehsenfeld et al., 1987; Nunnermacker et al., 1998; Parrish and 

Fehsenfeld, 2000; McClenny et al., 2002; U.S. Environmental Protection Agency, 1993, 2006a). 

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 
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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 photolytic- chemiluminescence method and cavity ringdown 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 standard21. 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. 

 

21 A 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/referenceequivalent-methods-list.pdf. 
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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 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 
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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 areawide 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 
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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 area-wide, while not requiring any nearroad monitoring sites. The details of the two-tier 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 
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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 area-wide 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, 
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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 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, 
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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 near-road 

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 
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“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 
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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 near-road 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. 
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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) 

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 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 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. Near-road 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 near-road 

NO2 monitor be required in Core Based Statistical Areas (CBSAs) with a population greater than 
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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 near-road monitor be required in CBSAs with a 

population greater than or equal to 2,500,000 persons, or in any CBSAs with one or more road 

segments with an 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 US Department of Transportation (DOT) Federal 

Highway Administration (FHWA), this particular element of the minimum monitoring 

requirements would have added approximately 2422 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 

 

22 Of 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. 
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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.” AASHTO23 and NYSDOT22

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, 

 

23 AASHTO, 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. 
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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 population-AADT 

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 near-road 

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 are likely to be 

exposed to peak NO2 concentrations. 
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Some commenters (e.g., AASHTO22, NESCAUM22, NYSDEC, NYSDOT22) suggested 

focusing multiple near-road monitors only in relatively larger CBSAs than those which were 

proposed. For example, NYSDEC 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 US Department of Transportation (US 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 
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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 US 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, NYSDOT 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 
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area. It is our [SRCAA’s] view that EPA could establish a statistically significant number of air 

quality monitoring station 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 near-roadway 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 US 

Department of Transportation (US 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 (US) mileage, these roads carried 64.1 percent of the 3 
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trillion vehicles miles (VMT) travelled in the United States in 2004.” The document also states 

that “urban interstate highways made up only 0.4 percent of total (US) mileage but carried 15.5 

percent of total VMT.” These statements indicate how much more traffic volume exists on roads 

in urban areas versus the more rural areas that have significant amounts mileage of the total 

public road inventory. 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 (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 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 
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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 
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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 
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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 (area-wide) spatial scales in an area. This component of the two-tier network design 

provides information on area-wide 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 
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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 area-wide 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 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; 
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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 area-wide 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 area-wide sites to be shutdown 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 area-wide monitor is required in any CBSA with 
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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 near-road 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. 
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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). 
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The proposed rule provided the Regional Administrators with the authority to use their 

discretion and 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 a 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 
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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 
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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 
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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 area-wide monitor to assess wider population exposures, or to support other monitor 

objectives served by area-wide 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 nearroad 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 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 
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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 two-tier 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 
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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 
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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 nearroad 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 
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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, 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
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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 near-road NO2 monitoring stations shall be selected by 

ranking all road segments within a CBSA by AADT and then identifying a location or locations 

adjacent to those highest ranked road segments where maximum hourly NO2 concentrations are 

expected to be highest and siting criteria can be met in accordance with 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 
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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 near-road 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 
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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 second-level 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 nearroad 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, 
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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 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. 
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 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 near-road monitors are 

to be sited in location 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 
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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 near-road NO2 monitoring stations, the sites shall be differentiated from each other by 

one or more of the following factors: fleet mix; congestion patterns; terrain; geographic area 

within the CBSA; or different route, interstate, or freeway designation, 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. 
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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 can not 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 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 
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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. NESCAUM24, 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 reduced to as close as 30 or20 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 near-road 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 

 

24 NESCAUM 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. 
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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 nearroad pollutant monitoring. 

EPA also proposed that required near-road 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 
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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. NACAA25, 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 

 

25 NACAA made a statement containing many concerns about the near-road monitoring 

component proposal which including 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...”. 
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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 

near-road pollutant concentrations on the predominantly upwind side of their study sites dropped 

off to near background levels within the first 50 meters, but were above background in this short 

and variable upwind range, which could be due, 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, nearroad sites would be able to be situated in a location that was always downwind. For example, a 

hypothetical 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 use portable meteorological devices to characterize meteorological tendencies, in 

addition to evaluating other available meteorological data sources. 
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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 nearroadway 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 near-road monitoring sites will be adjacent to a variety of 

road types, where some target roads will be on an even plane with the monitoring station, while 
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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 
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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 
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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 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 area-wide 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 
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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 nearroad 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 
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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 near-road 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 
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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.

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 
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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 near-roadway approach to the setting the level of the 1-hour 

standard and to siting monitors. As such, these versions place no geographical restrictions on 

which monitoring sites’ concentration data can and will be compared to the 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 
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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 
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 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 
163

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. 

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 
164

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 
165

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 
166

data based on case-specific factors, either at the request of a state or at her own initiative. 

Similar provisions exist already for some other NAAQS. 

 The 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 
167

year. The proposed table is similar to one used now for the 24-hour PM2.5 NAAQS, which is 

based on a 98th percentile form, but adjusted to reflect a 99th percentile form for the 1-hour 

primary NO2 standard. The proposed Appendix S also 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. 
168

 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 
169

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 
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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 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 
171

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 
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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. 
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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/(Leve

l)/ 

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, 2010a January 22, 

2011 

2009 July 1, 2010 January 22, 

2011 

2010 April 1, 2011a July 1, 2011a

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. 
174

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. 
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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, nearroadway monitoring network. Two commenters also requested that if a near-roadway 

monitoring network is deployed, that 1-hour NO2 standards be made more 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 near-roadway 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 
176

intend to designate the rest of the country as “unclassifiable” for the revised NO2 NAAQS until 

sufficient air quality data is collected from a near-roadway monitoring network. Once the nearroadway 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 abovestandard 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 
177

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. 

 

26 Since 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. 
178

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 
179

regulations – known as the Tribal Authority Rule or TAR – at 40 CFR Part 49. See 63 FR 7254 

(February 12, 1998). The TAR establishes the process for Indian tribes to seek TAS eligibility 

and sets forth the CAA functions for which TAS will be available. Under the TAR, eligible 

tribes may seek approval for all CAA and regulatory purposes other than a small number of 

functions enumerated at section 49.4. Implementation plans under section 110 are included 

within the scope of CAA functions for which eligible tribes may obtain approval. Section 110(o) 

also specifically describes tribal roles in submitting implementation plans. Eligible Indian tribes 

may thus submit implementation plans covering their reservations and other areas under their 

jurisdiction. 

 Under the CAA and TAR, tribes are not, however, required to apply for TAS or 

implement any CAA program. In promulgating the TAR EPA explicitly determined that it was 

not appropriate to treat tribes similarly to states for purposes of, among other things, specific 

plan submittal and implementation deadlines for NAAQS-related requirements. 40 CFR 49.4(a). 

In addition, where tribes do seek approval of CAA programs, including section 110 

implementation plans, the TAR provides flexibility and allows them to submit partial program 

elements, so long as such elements are reasonably severable – i.e., “not integrally related to 

program elements that are not included in the plan submittal, and are consistent with applicable 

statutory and regulatory requirements.” 40 CFR 49.7. 

 To date, very few tribes have sought TAS for purposes of section 110 implementation 

plans. However, some tribes may be interested in pursuing such plans to implement today’s 

proposed standard. As noted above, such tribes may seek approval of partial, reasonably 

severable plan elements, or they may seek to implement all relevant components of an air quality 

program for purposes of meeting the requirements of the Act. In several sections of this 
180

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 
181

these relatively short deadlines. 

2. Public comments 

One commenter stated that they disagree with EPA’s decision not to impose nonattainment 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 non-attainment 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 
182

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 discussion on attaining the NAAQS in the proposed rule. 

2. Consequences of Failing to Attain by the Statutory Attainment Date 

a. Proposal 
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 Any NO2 nonattainment area that fails to attain by its statutory attainment date would be 

subject to the requirements of sections 179(c) and (d) of the CAA. EPA is required to make a 

finding of failure to attain no later than 6 months after the specified attainment date and publish a 

notice in the Federal Register. The state would be required to submit an implementation plan 

revision, no later than one year following the effective date of the Federal Register notice making 

the determination of the area’s failure to attain, which demonstrates that the standard will be 

attained as expeditiously as practicable, but no later than 5 years from the effective date of 

EPA’s finding that the area failed to attain. In addition, section 179(d)(2) provides that the SIP 

revision must include any specific additional measures as may be reasonably prescribed by EPA, 

including “all measures that can be feasibly implemented in the area in light of technological 

achievability, costs, and any nonair quality and other air quality-related health and environmental 

impacts.” 

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 
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areas in any other state, or which interfere with programs under part C to prevent significant 

deterioration of air quality or to achieve reasonable progress toward the national visibility goal 

for Federal class I areas (national parks and wilderness areas). 

 Under section 110(a)(1) and (2) of the CAA, all states are required to submit SIPs to EPA 

which demonstrate that basic program elements have been addressed within 3 years of the 

promulgation of any new or revised NAAQS. Subsections (A) through (M) of section 110(a)(2) 

listed below, set forth the elements that a state’s program must contain in the SIP.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 

 

27 Two elements identified in section 110(a)(2) are not listed below because, as EPA interprets 

the CAA, SIPs incorporating any necessary local nonattainment area controls would not be due 

within 3 years, but rather are due at the time the nonattainment area planning requirements are 

due. These elements are: (1) Emission limits and other control measures, section 110(a)(2)(A), 

and (2) Provisions for meeting part D, section 110(a)(2)(I), which requires areas designated as 

nonattainment to meet the applicable nonattainment planning requirements of part D, title I of 

the CAA. 
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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 
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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) 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.” 
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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 
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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 on-road 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 
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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 
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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 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 

 

28 The terms “major” and “minor” define the size of a stationary source, for applicability 

purposes, in terms of an annual emissions rate (tons per year, tpy) for a pollutant. Generally, a 

minor source is any source that is not “major.” “Major” is defined by the applicable 

regulations—PSD or nonattainment NSR. 

29 In addition, the PSD program applies to non-criteria pollutants subject to regulation under the 

Act, except those pollutants regulated under section 112 and pollutants subject to regulation only 

under section 211(o). 
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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 
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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 short-term 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 shortterm 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 
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established for NAAQS for the same pollutant. Environmental Defense Fund, Inc. v. EPA, 898 

F.2d 183, 189-190 (D.C. 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 
194

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. 

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 

 

30 Criteria pollutants are those pollutants for which EPA has established a NAAQS under section 

109 of the CAA. 
195

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 hot-spot 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 
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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 transportationrelated 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 US 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 early-2012, they would have to determine 

conformity within one year of the effective date of that designation which would be in early2013. 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 US 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 
197

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 
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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 hot-spot 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 hot-spot 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. 

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 
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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 shortterm 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
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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 
201

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. 
202

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 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. 
203

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 (D.C. 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 
204

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 
205

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 
206

does not alter the relationship between the Federal government and tribes as established in the 

CAA and the TAR. Under section 109 of the CAA, EPA is mandated to establish NAAQS; 

however, this rule does not infringe existing tribal authorities to regulate air quality under their 

own programs or under programs submitted to EPA for approval. Furthermore, this rule does 

not affect the flexibility afforded to tribes in seeking to implement CAA programs consistent 

with the TAR, nor does it impose any new obligation on tribes to adopt or implement any 

NAAQS. Finally, as noted in section E (above) on UMRA, this rule does not impose significant 

costs on tribal governments. Thus, Executive Order 13175 does not apply to this 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. 
207

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 
208

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 lowincome 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 
209

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 [insert date 60 days after date of publication in the 

Federal Register]. 

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List of Subjects 

40 CFR Part 50 

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

dioxide, Ozone, Particulate matter, Sulfur oxides. 

40 CFR Part 58 

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

Intergovernmental relations, Reporting and recordkeeping requirements 

Dated: January 22, 2010. 

Lisa P. Jackson, 

Administrator. 
216

For the reasons stated in the preamble, title 40, chapter I of the Code of Federal Regulations is 

amended as follows: 
217

PART 50-NATIONAL PRIMARY AMBIENT AIR QUALITY STANDARDS 

1. The authority citation for part 50 continues to read as follows: 

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

Subpart A—General Provisions 

2. Section 50.11 is revised to read as follows: 

§50.11 National primary and secondary ambient air quality standards for oxides of nitrogen 

(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. 
218

(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 
219

NAAQS 

Pollutant/ 

Standard/(Leve

l)/ 

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, 2010a January 22, 

2011 

2009 July 1, 2010 January 22, 

2011 

2010 April 1, 2011a July 1, 2011a

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. 

 ***** 

4. Appendix S 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 
220

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”). 
221

(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 
222

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. 
223

(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. 
224

(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 state-flagged 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 stateflagged data affected by exceptional events which have been approved for exclusion by the 
225

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 state-flagged 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 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 
226

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 
227

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. 
228

(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. 
229

(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. 
230

(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. 

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 
231

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 following definitions in alphabetical order to read as 

follows: 

§58.1 Definitions 

* * * * * 

AADT means the annual average daily traffic. 

*** 

Near-road NO2 Monitor means any NO2 monitor meeting the specifications in 4.3.2 of Appendix 

D and paragraphs 2, 4(b), 6.1, and 6.4 of Appendix E of this part.

* * * * * 

Subpart B [AMENDED] 
232

7. Section 58.10, is amended by adding paragraph (a)(5) and adding paragraph (b)(12) to read as 

follows: 

§58.10 Annual monitoring network plan and periodic network assessment. 

* * * * * 

 (a) * * * 

(5) A plan for establishing NO2 monitoring sites in accordance with the requirements of 

appendix D to this part shall be submitted to the Administrator by July 1, 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. 

* * * * * 
233

(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 filter-based PM2.5FRM/FEM the field blank mass, 

sampler-generated average daily temperature, and sampler-generated average daily pressure; 

chemically speciated PM2.5 mass concentration data; PM10–2.5 mass concentration; chemically 

speciated PM10–2.5 mass concentration data; meteorological data from NCore 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 
234

information must be submitted directly to the AQS via electronic transmission on the specified 

quarterly schedule described in paragraph (b) of this section. 

* * * * * 

10. Appendix A to Part 58 is amended as by adding 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 replacing paragraph 4.3 to read as follows: 
235

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 
236

population exposure in the criteria utilized to select the final site location. Where one CBSA is 

required to have two near-road NO2 monitoring stations, the sites shall be differentiated from 

each other by one or more of the following factors: fleet mix; congestion patterns; terrain; 

geographic area within the CBSA; or different route, interstate, or freeway designation. 

 (b) Measurements at required near-road NO2 monitor sites 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. 
237

(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 
238

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. 
239

4.3.6 NOy Monitoring. (a) NO/ NOy measurements are included within the NCore multipollutant site requirements and the PAMS program. These NO/ NOy measurements will produce 

conservative estimates for NO2 that can be used to ensure tracking continued compliance with 

the NO2 NAAQS. NO/ NOy monitors are used at these sites because it is important to collect 

data on total reactive nitrogen species for understanding O3 photochemistry. 

* * * * * 

13. Appendix E to Part 58 is amended by revising paragraph 2, 4(b), 6.1, adding paragraph 6.4 

and revising paragraph 9(c) and Table E-4 to read as follows: 

Appendix E to Part 58—Probe and Monitoring Path Siting Criteria for Ambient Air 

Quality Monitoring 

* * * * * 

2. Horizontal and Vertical Placement 

The probe or at least 80 percent of the monitoring path must be located between 2 and 15 meters 

above ground level for all ozone and sulfur dioxide monitoring sites, and for neighborhood or 

larger spatial scale Pb, PM10, PM10-2.5, PM2.5, NO2 and carbon monoxide sites. Middle scale 

PM10-2.5 sites are required to have sampler inlets between 2 and 7 meters above ground level. 

Microscale Pb, PM10, PM10-2.5 and PM2.5 sites are required to have sampler inlets between 2 and 7 

meters above ground level. Microscale near-road NO2 monitoring sites are required to have 

sampler inlets between 2 and 7 meters above ground level. The inlet probes for microscale 

carbon monoxide monitors that are being used to measure concentrations near roadways must be 

3+ 1⁄2 meters above ground level. The probe or at least 90 percent of the monitoring path must be 
240

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. Spacing from Obstructions 

(a) * * * 

(b) * * * 

(c) * * * 

(d) For near-road NO2 monitoring stations, the monitor probe shall have an unobstructed 

air flow, where no obstacles exist at or above the height of the monitor probe, between the 

monitor probe and the outside nearest edge of the traffic lanes of the target road segment. 

* * * * * 

6. * * * 

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 
241

ranges of daily roadway traffic. A sampling site having a point analyzer probe located closer to a 

roadway than allowed by the Table E-1 requirements should be classified as microscale or 

middle scale, rather than neighborhood or urban scale, since the measurements from such a site 

would more closely represent the middle scale. If an open path analyzer is used at a site, the 

monitoring path(s) must not cross over a roadway with an average daily traffic count of 10,000 

vehicles per day or more. For those situations where a monitoring path crosses a roadway with 

fewer than 10,000 vehicles per day, monitoring agencies must consider the entire segment of the 

monitoring path in the area of potential atmospheric interference from automobile emissions. 

Therefore, this calculation must include the length of the monitoring path over the roadway plus 

any segments of the monitoring path that lie in the area between the roadway and minimum 

separation distance, as determined from the Table E-1 of this appendix. The sum of these 

distances must not be greater than 10 percent of the total monitoring path length. 

* * * * * 

 6.4 Spacing for Nitrogen Dioxide (NO2) Probes and Monitoring Paths (a) In siting nearroad NO2 monitors as required in paragraph 4.3.2 of appendix D of this part, the monitor probe 

shall be as near as practicable to the outside nearest edge of the traffic lanes of the target road 

segment; but shall not be located at a distance greater than 50 meters, in the horizontal, from the 

outside nearest edge of the traffic lanes of the target road segment. 

(b) In siting NO2 monitors for neighborhood and larger scale monitoring, it is important to 

minimize near-road influences. Table E-1 of this appendix provides the required minimum 

separation distances between a roadway and a probe or, where applicable, at least 90 percent of a 

monitoring path for various ranges of daily roadway traffic. A sampling site having a point 
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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. Probe Material and Pollutant Sample Residence Time 

(a) * * * 

(b) * * * 

(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 
243

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. 
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Table E-4 of Appendix E to Part 58. Summary of Probe and Monitoring Path Siting Criteria 

Pollutant Scale (maximum 

monitoring path 

length, meters) 

Height from 

ground to 

probe, inlet or 

80% of 

monitoring 

path1

Horizontal and 

vertical distance 

from supporting 

structures2

 to 

probe, inlet or 

90% of 

monitoring path1

(meters) 

Distance from 

trees to probe, 

inlet or 90% of 

monitoring 

path1

 (meters) 

Distance from 

roadways to probe, 

inlet or monitoring 

path1

 (meters) 

SO2

3,4,5,6 Middle 

(300 m) 

Neighborhood 

Urban, and 

Regional 

(1 km) 

2-15 > 1 > 10 N/A 

CO 4,5,7 Micro, middle 

(300 m), 

Neighborhood (1 

km) 

31⁄2: 2-15 > 1 > 10 2-10; see Table E-2 

of this appendix for 

middle and 

neighborhood 

scales. 

O3

3,4,5

 

Middle (300 m) 

Neighborhood, 

Urban, and 

Regional (1 km) 

2-15 > 1 > 10 See Table E-1 of 

this appendix for 

all scales. 

NO2 

3,4,5 Micro (Near-road 

[50-300]) 

Middle(300m) 

Neighborhood, 

Urban, and 

Regional (1 km) 

2-7 (micro); 

2-15(all other 

scales) 

> 1 > 10 < 50 meters for 

near-road 

microscale; 

See Table E-1 of 

this appendix for 

all other scales 

Ozone 

precursors 

(for 

PAMS)3,4,5

Neighborhood and 

Urban (1 km) 

2-15 > 1 > 10 See Table E-4 of 

this appendix for 

all scales. 

PM,Pb 3,4,5,6,8 Micro: Middle, 

Neighborhood, 

Urban and 

Regional 

2-7 (micro); 

2-7 (middle 

PM10-2.5); 

2-15 (all other 

scales) 

> 2 (all scales, 

horizontal 

distance only) 

> 10 (all scales) 2-10 (micro); see 

Figure E-1 of this 

appendix for all 

other scales. 

N/A--Not applicable. 

1

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

middle, neighborhood, urban, and regional scale NO2 monitoring, and all applicable scales for monitoring SO2,O3, 

and O3 precursors. 2

 When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses 

located on roof. 

3

 Should be >20 meters from the dripline of tree(s) and must be 10 meters from the dripline when the tree(s) act as 
245

an obstruction. 

4

 Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the 

height the obstacle protrudes above the sampler, probe, or monitoring path. Sites not meeting this criterion may be 

classified as middle scale (see text). 

5

 Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of 

a building or a wall. 

6

 The probe, sampler, or monitoring path should be away from minor sources, such as furnace or incineration flues. 

The separation distance is dependent on the height of the minor source's emission point (such as a flue), the type of 

fuel or waste burned, and the quality of the fuel (sulfur, ash, or lead content). This criterion is designed to avoid 

undue influences from minor sources. 

7

 For microscale CO monitoring sites, the probe must be >10 meters from a street intersection and preferably at a 

midblock location. 

8

 Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 

200 liters/min or at least 1 meter apart for samplers having flow rates less than 200 liters/min to preclude airflow 

interference. 

* * * * * 

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” 
246

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 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour1

PM2.5

(μg/m3) 

PM10

(μg/m3) 

CO (ppm) SO2 (ppm) NO2 (ppm) 

1-hour 

A

0.000-0.059 .................. 0.0-15.4 0-54 0.0-4.4 0.000-0.034 0 – 0.053 0-0.060-0.075 .................. 15.5-40.4 55-154 4.5-9.4 0.035-0.144 0.054 – 0.100 51-0.076-0.095 0.125-0.164 40.5-65.4 155-254 9.5-12.4 0.145-0.224 0.101 – 0.360 1010.096-0.115 0.165-0.204 365.5-150.4 255-354 12.5-15.4 0.225-0.304 0.361– 0.64 1510.116-0.374 0.205-0.404 3150.5-250.4 355-424 15.5-30.4 0.305-0.604 0.65 – 1.24 201

(2)............... 0.405-0.504 3250.5-350.4 425-504 30.5-40.4 0.605-0.804 1.25 – 1.64 301(2)............... 0.505-0.604 3350.5-500.4 505-604 40.5-50.4 0.805-1.004 1.65 – 2.04 401

1 Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small numb

AQI based on 1-hour ozone values would be more precautionary. In these cases, in addition to calculating the 8-value, the 1-hour ozone index value may be calculated, and the maximum of the two values reported. 

2 8-hour O3 values do not define higher AQI values (≥ 301). AQI values of 301 or greater are calculated with 1-h

concentrations. 

3 If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.