United States

Environmental 

Protection Agency

Draft Integrated Review 

Plan for the National 

Ambient Air Quality 

Standards for Particulate 

EPA United States

Environmental 

Protection Agency

Integrated Review Plan for 

the National Ambient Air 

Quality Standards for 

Particulate Matter

EPA United States

Environmental 

Protection Agency

Integrated Review Plan for 

the National Ambient Air 

Quality Standards for 

Particulate Matter

EPA United States

Environmental 

Protection Agency

Draft Integrated 

Review Plan for the 

National Ambient Air 

Quality Standards for 

EPA United States

Environmental 

Protection Agency

Integrated Review Plan for 

the National Ambient Air 

Quality Standards for 

Particulate Matter

EPA United States

Environmental 

Protection Agency

Integrated Review Plan for the 

Ozone National Ambient Air 

Quality Standards 
(This page intentionally left blank)
EPA 452/R-11-006 

April 2011 

Integrated Review Plan for the 

Ozone National Ambient Air 

Quality Standards

U. S. Environmental Protection Agency

National Center for Environmental Assessment 

Office of Research and Development

and

Office of Air Quality Planning and Standards 

Office of Air and Radiation

Research Triangle Park, North Carolina 27711 
DISCLAIMER

This integrated review plan for the national ambient air quality standards (NAAQS) for 

ozone (O3) serves as a public information document and a management tool for the United States 

Environmental Protection Agency's National Center for Environmental Assessment and the 

Office of Air Quality Planning and Standards. The approach described in this plan may be 

modified to reflect information developed during the review of the O3 NAAQS and to address 

advice and comments received from the Clean Air Scientific Advisory Committee and the public 

throughout this review. Mention of trade names or commercial products does not constitute 

endorsement or recommendation for use. 
ii

TABLE OF CONTENTS

LIST OF ACRONYMS/ABBREVIATIONS ....................................................................................IV

1 INTRODUCTION .....................................................................................................................1-1

1.1 LEGISLATIVE REQUIREMENTS ................................................................................1-2

1.2 OVERVIEW OF THE NAAQS REVIEW PROCESS .....................................................1-4

1.3 HISTORY OF O3 NAAQS REVIEWS ............................................................................1-8

1.4 RECONSIDERATION OF THE 2008 OZONE NAAQS ................................................1-11

2 STATUS AND SCHEDULE FOR NEW REVIEW ................................................................2-1

3 KEY POLICY-RELEVANT ISSUES .......................................................................................3-1

3.1 ISSUES RELATED TO THE PRIMARY OZONE NAAQS ...........................................3-1

3.2 ISSUES RELATED TO THE SECONDARY OZONE NAAQS.....................................3-3

4 SCIENCE ASSESSMENT .......................................................................................................4-1

4.1 SCOPE AND ORGANIZATION .....................................................................................4-1

4.2 ASSESSMENT APPROACH ..........................................................................................4-2

4.2.1 Introduction....................................................................................................................4-2

4.2.2 Literature Search and Identification of Relevant Studies ..............................................4-2

4.2.3 Criteria for Study Selection ...........................................................................................4-4

4.2.4 Quality Assurance ..........................................................................................................4-6

4.3 CONTENT AND ORGANIZATION OF THE ISA .........................................................4-6

4.4 CAUSAL DETERMINATIONS ......................................................................................4-13

4.5 SUPPLEMENTARY MATERIALS ................................................................................4-14

4.6 SCIENTIFIC AND PUBLIC REVIEW ...........................................................................4-14

5 HUMAN HEALTH RISK AND EXPOSURE ASSESSMENTS ............................................5-1

5.1 OVERVIEW ....................................................................................................................5-1

5.2 EXPOSURE AND HEALTH RISK ASSESSMENTS FROM THE LAST REVIEW ....5-2

5.3 AIR QUALITY CONSIDERATIONS .............................................................................5-4

5.4 POPULATION EXPOSURE ASSESSMENT APPROACH ...........................................5-5

5.5 HUMAN HEALTH RISK ASSESSMENTS ...................................................................5-6

5.5.1 Approach to Health Risk Assessment Based On Epidemiologic Studies .......................5-7

5.5.2 Approach to Health Risk Assessment Based on Controlled Human Exposure Studies .5-8

5.5.3 Uncertainty and Variability ...........................................................................................5-9

5.5.4 Broader Risk Characterization ......................................................................................5-9

5.6 SCIENTIFIC AND PUBLIC REVIEW ...........................................................................5-10

6 VEGETATION AND OTHER WELFARE-RELATED ASSESSMENTS ............................6-1

6.1 OVERVIEW ....................................................................................................................6-1

6.2 EXPOSURE, RISK, AND BENEFITS ASSESSMENTS FROM THE LAST REVIEW 6-1

6.2.1 Exposure Assessment .....................................................................................................6-2

6.2.2 Risk Assessment .............................................................................................................6-4

6.2.3 Benefits Assessment .......................................................................................................6-5

6.3 AIR QUALITY CONSIDERATIONS .............................................................................6-6

6.4 EXPOSURE ASSESSMENT APPROACH .....................................................................6-7

6.5 RISK ASSESSMENT APPROACH ................................................................................6-7
iii

6.6 BENEFIT ASSESSMENT APPROACH .........................................................................6-8

6.7 UNCERTAINTY AND VARIABILITY .........................................................................6-9

6.8 SCIENTIFIC AND PUBLIC REVIEW ...........................................................................6-9

7 POLICY ASSESSMENT/RULEMAKING ..............................................................................7-1

7.1 POLICY ASSESSMENT ........................................................................................................7-1

7.2 RULEMAKING ....................................................................................................................7-3

8 AMBIENT AIR MONITORING ..............................................................................................8-1

8.1 OVERVIEW ....................................................................................................................8-1

8.2 CURRENT O3 NETWORK STATUS .............................................................................8-1

8.3 MONITORING ISSUES RELATED TO THE O3 NAAQS .............................................8-3

8.3.1 Monitoring Methods.......................................................................................................8-4

8.3.2 Network Design ..............................................................................................................8-4

8.3.3 Data Reporting and Assessments ...................................................................................8-5

9 REFERENCES .........................................................................................................................9-1
iv

LIST OF ACRONYMS/ABBREVIATIONS

A/C Air conditioning 

APEX EPA’s Air Pollutants Exposure model, version 4 

AQI Air Quality Index 

AQS EPA’s Air Quality System

CAA Clean Air Act

CASAC Clean Air Scientific Advisory Committee 

CASTNET Clean Air Status and Trends Network 

CFR Code of Federal Regulations 

CHAD EPA’s Consolidated Human Activity Database 

CMAQ Community Multiscale Air Quality 

CO Carbon Monoxide 

COPD Chronic obstructive pulmonary disease 

C-R Concentration-response relationship 

CRP C-reactive protein

CSA Consolidated Statistical Area

CV Cardiovascular

CVD Cardiovascular disease

ED Emergency department

EPA Environmental Protection Agency 

FACE Free-air exposures

FEM Federal Equivalent Method 

FEV1 Change in forced expiratory volume in one second 

FIP Federal Implementation Plan

FR Federal Register

FRM Federal Reference Method

HA Hospital admissions

HEI Health Effects Institute

HRV Heart rate variability

IHD Ischemic heart disease

IRP Integrated Review Plan

ISA Integrated Science Assessment

Km Kilometer 

ME Microenvironment 

MI Myocardial infarction

MOA Mode(s) or mechanism(s) of action 

N Nitrogen

NAAQS National Ambient Air Quality Standards

NCEA-RTP National Center for Environmental Assessment in Research Triangle Park

NCLAN National Crop Loss Assessment Network

NEM NAAQS Exposure Model 

NERL National Exposure Research Laboratory 

NOAA National Oceanic and Atmospheric Administration

NO2 Nitrogen dioxide
v 

NO3

- Nitrate

NOx Nitrogen oxides

NPS National Park Service

NRC National Research Council

O3 Ozone 

OAQPS Office of Air Quality Planning and Standards 

OAR Office of Air and Radiation

OMB Office of Management and Budget

ORD Office of Research and Development

OTC Open Top Chambers 

PA Policy Assessment

PAMS Photochemical Assessment Monitoring Station

PM Particulate matter

PRB Policy-Relevant Background

QA Quality assurance

REA Risk and Exposure Assessment 

RR Relative risk

SAB Science Advisory Board 

SoyFACE Free air field based soybean study

SO2 Sulfur Dioxide 

SO4

2- Sulfate

SOx Sulfur Oxides 

TB Tracheobronchial

TREGRO Tree growth model

TSP Total suspended particulate 

UV Ultraviolet

VOC Volatile organic compounds 

W126 Cumulative, seasonal secondary ozone standard 
vi 
1-1 

1 INTRODUCTION

The U.S. Environmental Protection Agency (EPA) last completed a review of the primary 

(health-based) and secondary (welfare-based) national ambient air quality standards (NAAQS) 

for ozone (O3) in March 2008 (73 FR 16436), resulting in revisions to both standards. In May 

2008, states, environmental groups and industry groups filed petitions with the D.C. Circuit 

Court of Appeals for review of the 2008 O3 standards. In March 2009, the court granted EPA’s 

request to stay the litigation so the new administration could review the standards and determine 

whether they should be reconsidered. On September 16, 2009, the Administrator announced her 

decision to reconsider the 2008 primary and secondary O3 standards to ensure they are 

scientifically sound and protective of public health and the environment as required by the Clean 

Air Act (CAA). The EPA published a proposed rule on the reconsideration of the 2008 O3

NAAQS on January 19, 2010 (75 FR 2938-2999). Prior to the decision to reconsider the 2008 

O3 standards, EPA had initiated a new periodic review of the existing air quality criteria and

standards for O3 in September 2008. 

This Integrative Review Plan (IRP) contains the plans for the new periodic review of the 

air quality criteria for O3-related effects on public health and public welfare and the current O3

standards or any revised standards that may result from the reconsideration of the 2008 O3

standards. This review will provide an integrative assessment of relevant scientific information 

for O3 and related photochemical oxidants and will focus on the basic elements of the NAAQS: 

the indicator,1 averaging time, form,2

This IRP is organized into eight chapters. Chapter 1 presents the legislative requirements 

for the review of the NAAQS, an overview of the NAAQS review process, a history of past 

reviews of the O3 NAAQS, and the Agency’s plans to reconsider the 2008 O3 NAAQS. Chapters 

2 through 8 outline the Agency’s plans for the new periodic review of the existing air quality 

criteria and the O3 standards that result from the reconsideration of the 2008 standards. Chapter 

2 presents the status and schedule for the new review. Chapter 3 presents a set of policy-relevant 

questions that will serve to focus the new review on the critical scientific and policy issues. 

Chapters 4 through 6 discuss the planned scope and organization of the key science and 

 and level. These elements, which together serve to define 

each ambient air quality standard, must be considered collectively in evaluating the protection to 

public health and public welfare afforded by the standards. 

 

1 The “indicator” of a standard defines the chemical species or mixture that is to be measured in determining 

whether an area attains the standard.

2 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. 
1-2 

risk/exposure assessment documents, the planned approaches for preparing the documents, and 

plans for scientific and public review of the documents for the new review. Chapter 7

summarizes the policy assessment and rulemaking process for the new O3 NAAQS review. 

Finally, chapter 8 discusses the current ambient air monitoring network and monitoring issues 

related to the O3 NAAQS. 

1.1 LEGISLATIVE REQUIREMENTS 

Two sections of the Clean Air Act (CAA) govern the establishment and revision of the 

NAAQS. Section 108 (42 U.S.C. section 7408) directs the Administrator to identify and list 

certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator 

is to list those air pollutants that in her “judgment, cause or contribute to air pollution which may 

reasonably be anticipated to endanger public health or welfare;” “the presence of which in the 

ambient air results from numerous or diverse mobile or stationary sources;” and “for which . . . 

[the Administrator] plans to issue air quality criteria . . .” Air quality criteria are intended to 

“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 the ambient air . . .” 42 U.S.C. § 7408(b). Section 109 (42 U.S.C. 7409) directs the 

Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutants for 

which air quality criteria are issued. 42 U.S.C. § 7409 (a). Section 109(b) (1) defines a primary 

standard as one “the attainment and maintenance of which in the judgment of the Administrator, 

based on such criteria and allowing an adequate margin of safety, are requisite to protect the 

public health.”3

 42 U.S.C. § 7409(b)(1). A secondary standard, as defined in section 109(b)(2), 

must “specify a level of air quality the attainment and maintenance of which, in the judgment of 

the Administrator, based on such criteria, is required to protect the public welfare from any 

known or anticipated adverse effects associated with the presence of [the] pollutant in the 

ambient air.”4

The requirement that primary standards provide an adequate margin of safety was 

intended to address uncertainties associated with inconclusive scientific and technical 

 42 U.S.C. § 7409(b)(2). 

 

3 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)]. 4 Welfare effects as defined in section 302(h) [42 U.S.C. 7602(h)] include, but are not limited to, “effects on soils, 

water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to and 

deterioration of property, and hazards to transportation, as well as effects on economic values and on personal 

comfort and well-being.”
1-3 

information available at the time of standard setting. It was also intended to provide a reasonable 

degree of protection against hazards that research has not yet identified. See 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); American Farm Bureau Federation v. EPA, 559 F. 3d 512, 533 (D.C. Cir. 

2009); Association of Battery Recyclers v. EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both 

kinds of uncertainties are components of the risk associated with pollution at levels below those 

at which human health effects can be said to occur with reasonable scientific certainty. Thus, in 

selecting primary standards that include an adequate margin of safety, the Administrator is 

seeking not only to prevent pollution levels that have been demonstrated to be harmful but also 

to prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is 

not precisely identified as to nature or degree. The CAA does not require the Administrator to 

establish a primary NAAQS at a zero-risk level or at background concentration levels, see Lead

Industries v. EPA, 647 F.2d at 1156 n.51, but rather at a level that reduces risk sufficiently so as 

to protect public health with an adequate margin of safety. 

In addressing the requirement for an adequate margin of safety, the EPA considers such 

factors as the nature and severity of the health effects involved, the size of sensitive population(s) 

at risk, 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. See Lead Industries Association v. EPA, 647 F.2d at 1161-62;

Whitman v. American Trucking Associations, 531 U.S. 457, 495 (2001). 

In setting primary and secondary standards that are “requisite” to protect public health and 

welfare, respectively, as provided in section 109(b), EPA’s task is to establish standards that are 

neither more nor less stringent than necessary. In so doing, EPA may not consider the costs of 

implementing the standards. See generally Whitman v. American Trucking Associations, 531 

U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not 

relevant considerations in the promulgation of national ambient air quality standards.” American 

Petroleum Institute v. Costle, 665 F. 2d at 1185. 

Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year 

intervals thereafter, the Administrator shall complete a thorough review of the criteria 

published under section 108 and the national ambient air quality standards . . . and shall make 

such revisions in such criteria and standards and promulgate such new standards as may be 

appropriate . . . .” Section 109(d)(2) requires that an independent scientific review committee 

“shall complete a review of the criteria . . . and the national primary and secondary ambient air 

quality standards . . . and shall recommend to the Administrator any new . . . standards and 
1-4 

revisions of existing criteria and standards as may be appropriate . . . .” Since the early 1980's, 

this independent review function has been performed by the Clean Air Scientific Review 

Committee (CASAC).5

1.2 OVERVIEW OF THE NAAQS REVIEW PROCESS

Since completion of the last O3 NAAQS review, the Agency has made a number of 

changes to the process for reviewing the NAAQS. The current process, which is being applied to 

this review of the NAAQS for O3, has four major phases: (1) planning, (2) science assessment, 

(3) risk/exposure assessment, and (4) policy assessment and rulemaking. An overview of the 

process is illustrated in Figure 1-1 below and each of these phases is described in this section.6 

The Agency maintains a web site on which key documents developed for NAAQS reviews are 

made available (http://www.epa.gov/ttn/naaqs/). 

The planning phase of the NAAQS review process begins with a science policy 

workshop, which is intended to identify issues and questions to frame the review. Drawing from 

the workshop discussions, a draft IRP is prepared jointly by EPA’s National Center for 

Environmental Assessment – Research Triangle Park (NCEA-RTP), within the Office of 

Research and Development (ORD), and EPA’s Office of Air Quality Planning and Standards 

(OAQPS), within the Office of Air and Radiation (OAR). The draft IRP is made available for 

consultation with CASAC and for public comment. The final IRP is prepared in consideration of 

CASAC and public comments. This document presents the current plan and specifies the 

schedule for the entire review, the process for conducting the review, and the key policy-relevant 

science issues that will guide the review. 

The second phase of the review, science assessment, involves the preparation of an 

Integrated Science Assessment (ISA) and supplementary materials. The ISA, prepared by 

NCEA-RTP, provides a concise review, synthesis, and evaluation of the most policy-relevant 

science, including key science judgments that are important to the design and scope of exposure 

and risk assessments, as well as other aspects of the NAAQS review. The ISA and its 

supplementary materials provide a comprehensive assessment of the current scientific literature 

pertaining to known and anticipated effects on public health and welfare associated with the 

presence of the pollutant in the ambient air, emphasizing information that has become available 

since the last air quality criteria review in order to reflect the current state of knowledge. As 

such, the ISA forms the scientific foundation for each NAAQS review and is intended to provide 

 

5 Lists of CASAC members and of members of the CASAC O3 Review Panel are available at: 

http://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/CommitteesandMembership?OpenDocument. 6 Information on changes to the NAAQS review process since the last O3 NAAQS review is available at: 

http://www.epa.gov/ttn/naaqs/review.html. 
1-5 

information useful in forming judgments about air quality indicator(s), form(s), averaging 

time(s) and level(s) for the NAAQS. Hence, the ISA and its associated materials function in the 

current NAAQS review process as the Air Quality Criteria Document (AQCD) did in the 

previous review process. The current review process generally includes production of a first and 

second draft ISA, both of which undergo CASAC and public review prior to completion of the 

final ISA. Section 4 below provides a more detailed description of the planned scope, 

organization and assessment approach for the ISA and its supporting materials. 

In the third phase, the risk/exposure assessment phase, OAQPS staff considers 

information and conclusions presented in the ISA, with regard to support provided for the 

development of quantitative assessments of the risks and/or exposures for health and/or welfare 

effects. As an initial step, staff prepares one or more planning documents that consider the 

extent to which newly available scientific evidence and tools/methodologies warrant the conduct 

of quantitative risk and exposure assessments. To the extent warranted, this document(s)

outlines a general plan, including scope and methods, for conducting the assessments. This 

planning document(s) is generally prepared in conjunction with the first draft ISA and presented 

for consultation with CASAC and for public comment. As discussed in chapters 5 and 6 below, 

these planning documents for the current O3 NAAQS review will focus on consideration of the 

newly available data, methods and tools in light of areas of uncertainty in the assessments 

conducted for the last review and of the potential for new or updated assessments to provide 

notably different exposure and risk estimates with lower associated uncertainty. Comments 

received on the planning document(s) are considered in the Agency’s decision as to whether to

conduct such assessments. When an assessment is performed, one or more drafts of each risk 

and exposure assessment document (REA) undergoes CASAC and public review, with the initial 

draft REA(s) generally being reviewed in conjunction with review of the second draft ISA, prior 

to completion of final REA(s). The REA provides concise presentations of methods, key results, 

observations, and related uncertainties. Chapters 5 and 6 discuss possible approaches being 

considered with regard to human health- and welfare-related assessments, respectively, for this 

review.

The review process ends with a policy assessment and rulemaking phase. Under the 

current NAAQS review process (Jackson, 2009), the EPA Administrator has reinstated the use of 

a Policy Assessment (PA). The PA, like the previous OAQPS Staff Paper, is a document that 

provides a transparent OAQPS staff analysis of the adequacy of the current standards and 

potential alternatives that are appropriate to consider prior to the issuance of proposed and final 

rules. The PA integrates and interprets the information from the ISA and REA(s) to frame policy 

options for consideration by the Administrator. Such an evaluation of policy implications is 
1-6 

intended to help ‘‘bridge the gap’’ between the Agency’s scientific assessments, presented in the 

ISA and REA(s), and the judgments required of the EPA Administrator in determining whether it 

is appropriate to retain or revise the NAAQS. In so doing, the PA is also intended to facilitate 

CASAC’s advice to the Agency and recommendations to the Administrator on the adequacy of 

the existing standards or revisions that may be appropriate to consider, as provided for in the 

CAA. In evaluating the adequacy of the current standards and, as appropriate, a range of 

alternative standards, the PA considers the available scientific evidence and, as available, 

quantitative risk-based analyses, together with related limitations and uncertainties. The PA 

focuses on the information that is most pertinent to evaluating the basic elements of national 

ambient air quality standards: indicator, averaging time, form, and level. One or more drafts of 

a PA are released for CASAC review and public comment prior to completion of the final PA. 

Following issuance of the final PA and consideration of conclusions presented therein, 

the Agency develops and publishes a notice of proposed rulemaking that communicates the 

Administrator’s proposed decisions regarding the standards review. A draft notice undergoes 

interagency review involving other federal agencies prior to publication.7

 Materials upon which 

this decision is based, including the documents described above, are made available to the public 

in the regulatory docket for the review. A public comment period, during which public hearings 

are generally held, follows publication of the notice of proposed rulemaking. Taking into 

account comments received on the proposed rule,8 the Agency issues a final rule to complete the 

rulemaking process. Chapter 7 discusses the development of the PA and the rulemaking steps 

for this review.

 

7 Where implementation of the proposed decision would necessitate the implementation of emissions controls, EPA 

develops and releases a draft regulatory impact analysis (RIA) concurrent with the notice of proposed rulemaking. 

This activity is conducted under Executive Order 12866. The RIA is conducted completely independent of and, by 

statute, is not considered in decisions regarding the NAAQS.

8 In the notice of final rulemaking, and generally also through the use of an accompanying document, the Agency 

responds to all significant comments on the proposed rule. 
1-7 

Figure 1.1. Overview of NAAQS Review Process
1-8 

1.3 HISTORY OF O3 NAAQS REVIEWS 

Tropospheric (ground-level) O3 is the indicator for the mix of photochemical oxidants 

(e.g., peroxyacetyl nitrate, hydrogen peroxide) formed from biogenic and anthropogenic 

precursor emissions. Naturally occurring O3 in the troposphere can result from biogenic organic 

precursors reacting with naturally occurring nitrogen oxides (NOx) and by stratospheric O3

intrusion into the troposphere. Anthropogenic precursors of O3, especially NOx and volatile 

organic compounds (VOCs), originate from a wide variety of stationary and mobile sources. 

Ambient O3 concentrations produced by these emissions are directly affected by temperature, 

solar radiation, wind speed, and other meteorological factors. 

Table 1-1 summarizes the O3 NAAQS that have been promulgated to date. In each 

review, the secondary standard has been set to be identical to the primary standard. These 

reviews are briefly described below.

Table 1-1. Summary of Primary and Secondary O3 NAAQS Promulgated During the 

Period 1971 - 2008.

Final Rule Indicator Averaging

Time

Level

(ppm) Form

1971

(36 FR 8186) 

Total 

photochemical 

oxidants 

1-hr 0.08 Not to be exceeded more than one 

hr per year

1979

(44 FR 8202) O3 1-hr 0.12 

Attainment is defined when the 

expected number of days per 

calendar year, with maximum 

hourly average concentration 

greater than 0.12 ppm, is equal to 

or less than 1

1993

(58 FR 13008) EPA decided that revisions to the standards were not warranted at the time. 

1997

(62 FR 38856) O3 8-hr 0.08 

Annual fourth-highest daily 

maximum 8-hr concentration, 

averaged over 3 years

2008

(73 FR 16483) O3 8-hr 0.075 

Form of the standards remained 

unchanged relative to the 1997 

standard
1-9 

The EPA first established primary and secondary NAAQS for photochemical oxidants in 

1971 (36 FR 8186, April 30, 1971). Both primary and secondary standards were set at a level of 

0.08 parts per million (ppm), 1-hr average, total photochemical oxidants, not to be exceeded 

more than one hour per year. The standards were based on scientific information contained in 

the 1970 Air Quality Criteria for Photochemical Oxidants (U.S. DHEW, 1970). 

The first periodic review of the NAAQS for photochemical oxidants was initiated in 

1977. Based on the 1978 Air Quality Criteria for Ozone and Other Photochemical Oxidants 

(U.S. EPA, 1978), EPA published proposed revisions to the original NAAQS in 1978 (43 FR 

16962) and final revisions in 1979 (44 FR 8202). The level of the primary and secondary 

standards was revised from 0.08 to 0.12 ppm; the indicator was revised from photochemical 

oxidants to O3; and the form of the standards was revised from a deterministic to a statistical 

form, which defined attainment of the standards as occurring when the expected number of days 

per calendar year with maximum hourly average concentration greater than 0.12 ppm is equal to 

or less than one. 

In 1982, EPA announced plans to revise the 1978 Air Quality Criteria document (47 FR 

11561), and in 1983 EPA initiated the second periodic review of the O3 NAAQS (48 FR 38009). 

EPA subsequently published the 1986 Air Quality Criteria for Ozone and Other Photochemical 

Oxidants (U.S. EPA, 1986) and 1989 Staff Paper (U.S. EPA, 1989). Following publication of 

the 1986 Air Quality Criteria,document (AQCD) a number of scientific abstracts and articles 

were published that appeared to be of sufficient importance concerning potential health and 

welfare effects of O3 to warrant preparation of a Supplement (U.S. EPA, 1992). Under the terms 

of a court order, on August 10, 1992 EPA published a proposed decision stating that revisions to 

the existing primary and secondary standards were not appropriate at the time (57 FR 35542). 

The notice explained that the proposed decision would complete EPA’s review of information on 

health and welfare effects of O3 assembled over a 7-year period and contained in the 1986 

AQCD and its 1992 Supplement. The proposal also announced EPA’s intention to proceed as 

rapidly as possible with the next review of the air quality criteria and standards for O3 in light of 

emerging evidence of health effects related to 6- to 8-hour O3 exposures. On March 9, 1993, 

EPA concluded the review by deciding that revisions to the standards were not warranted at that 

time (58 FR 13008). 

In August 1992 EPA announced plans to initiate the third periodic review of the air 

quality criteria and O3 NAAQS (57 FR 35542). On the basis of the scientific evidence contained 

in the 1996 Air Quality Criteria for Ozone and related Photochemical Oxidants (U.S. EPA, 

1996a), the 1996 Staff Paper (U.S. EPA, 1996b), and related technical support documents, 

linking exposures to ambient O3 to adverse health and welfare effects at levels allowed by the 
1-10 

then existing standards, EPA proposed to revise the primary and secondary O3 standards on 

December 13, 1996 (61 FR 65716). The EPA proposed to replace the then existing 1-hour 

primary and secondary standards with 8-hour average O3 standards set at a level of 0.08 ppm 

(equivalent to 0.084 ppm using standard rounding conventions). The EPA also proposed, in the 

alternative, to establish a new distinct secondary standard using a biologically based cumulative, 

seasonal form. The EPA completed the review on July 18, 1997 (62 FR 38856) by setting the 

primary standard at a level of 0.08 ppm, based on the annual fourth-highest daily maximum 8-hr 

average concentration, averaged over three years, and setting the secondary standard identical to 

the revised primary standard. 

On May 14, 1999, in response to challenges to EPA’s 1997 decision by industry and 

others, the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit Court) 

remanded the O3 NAAQS to EPA, finding that section 109 of the Act, as interpreted by EPA, 

effected an unconstitutional delegation of legislative authority. In addition, the D.C. Circuit 

Court directed that, in responding to the remand, EPA should consider the potential beneficial 

health effects of O3 pollution in shielding the public from the effects of solar ultraviolet (UV) 

radiation, as well as adverse health effects. On January 27, 2000, EPA petitioned the U.S. 

Supreme Court for certiorari on the constitutional issue (and two other issues) but did not request 

review of the D.C. Circuit Court ruling regarding the potential beneficial health effects of O3. 

On February 27, 2001, the U.S. Supreme Court unanimously reversed the judgment of the D.C. 

Circuit Court on the constitutional issue, holding that section 109 of the CAA does not delegate 

legislative power to the EPA in contravention of the Constitution, and remanded the case to the 

D.C. Circuit Court to consider challenges to the O3 NAAQS that had not been addressed by that 

Court’s earlier decisions. On March 26, 2002, the D.C. Circuit Court issued its final decision, 

finding the 1997 O3 NAAQS to be “neither arbitrary nor capricious,” and denied the remaining 

petitions for review. In response to the D.C. Circuit Court remand to consider the potential 

beneficial health effects of O3 pollution in shielding the public from effects of solar (ultraviolet

or UV) radiation, on November 14, 2001, EPA proposed to leave the 1997 8-hour NAAQS 

unchanged (66 FR 52768). After considering public comment on the proposed decision, EPA 

published its final response to this remand on January 6, 2003, reaffirming the 8-hour O3

NAAQS set in 1997 (68 FR 614). Finally, on April 30, 2004, EPA announced the decision to 

make the 1-hour O3 NAAQS no longer applicable to areas one year after the effective date of the 

designation of those areas for the 8-hour NAAQS (69 FR 23966). For most areas, the date that 

the 1-hour NAAQS no longer applied was June 15, 2005. 

The EPA initiated the next periodic review of the air quality criteria and O3 standards in 

September 2000 with a call for information (65 FR 57810). The schedule for completion of that
1-11 

rulemaking later became governed by a consent decree resolving a lawsuit filed in March 2003 

by a group of plaintiffs representing national environmental and public health organizations. 

Based on the Air Quality Criteria for Ozone and Other Photochemical Oxidants (US EPA, 2006) 

published in March 2006 and the Staff Paper (U.S EPA, 2007a) and related technical support 

documents published in July 2007, the proposed decision was published in the Federal Register

on July 11, 2007 (72 FR 37818). The EPA proposed to revise the level of the primary standard 

to a level within the range of 0.075 to 0.070 ppm. Two options were proposed for the secondary 

standard: (1) replacing the current standard with a cumulative, seasonal standard, expressed as 

an index of the annual sum of weighted hourly concentrations cumulated over 12 daylight hours 

during the consecutive 3-month period within the O3 season with the maximum index value, set 

at a level within the range of 7 to 21 ppm-hrs, and (2) setting the secondary standard identical to 

the revised primary standard. The EPA completed the review with publication of a final decision 

on March 27, 2008 (73 FR 16436), revising the level of the 8-hour primary O3 standard from 

0.08 ppm to 0.075 ppm and revising the secondary standard to be identical to the revised primary

standard. 

1.4 RECONSIDERATION OF THE 2008 OZONE NAAQS

In May 2008, state, public health, environmental, and industry petitioners filed suit 

against EPA regarding the 2008 final decision on the O3 NAAQS, and on December 23, 2008, 

the Court set a briefing schedule in the consolidated cases. On March 10, 2009, EPA requested 

that the Court vacate the briefing schedule and hold the consolidated cases in abeyance. This 

request for extension was made to allow time for appropriate EPA officials appointed by the new 

Administration to review the O3 NAAQS to determine whether the standards established in the 

March 2008 O3 NAAQS decision should be maintained, modified or otherwise reconsidered. In 

granting EPA’s request, the Court directed EPA to notify the Court by September 16, 2009 of the 

action it will be taking with respect to the 2008 O3 NAAQS rule and the Agency’s schedule for 

undertaking such action. 

The EPA notified the Court on September 16, 2009 of its decision to reconsider the 

primary and secondary O3 NAAQS set in March 2008 to ensure they are scientifically sound and 

protective of public health and the environment.9

 

9 The EPA also separately announced that it will move quickly to implement any new standards that might result 

from the reconsideration. To reduce the workload for states during the interim period of reconsideration, the 

Agency proposed to stay the 2008 standards for the purpose of attainment and nonattainment area designations. 

EPA will work with states, local governments and tribes to ensure that air quality is protected during that time. 

 The EPA is basing this reconsideration on the 

scientific record from the 2008 review, including public comments and CASAC advice and 

recommendations. During the 2008 review, CASAC unanimously recommended a more health 
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protective primary standard than was eventually set in 2008. The CASAC also recommended a 

new cumulative, seasonal secondary standard, distinct from the primary standard, while the 2008 

rule made the secondary standard identical to the primary standard. Following the 2008 

decision, CASAC offered unsolicited advice that reiterated its previous recommendations and 

urged the Agency to reconsider its advice in future action on the O3 standards. The EPA’s notice 

to the Court specifically stated that the Agency had concerns regarding whether the revisions to 

the primary and secondary NAAQS adopted in the 2008 O3 NAAQS rule satisfy the 

requirements of the Clean Air Act. 

The EPA is basing the reconsideration of the 2008 O3 NAAQS decision on the scientific 

and technical information that was assessed during the 2008 review, including information in the 

2006 Air Quality Criteria document (U.S. EPA, 2006), the 2007 OAQPS Staff Paper (U.S. EPA, 

2007a), and related technical support documents including the 2007 REAs (U.S. EPA, 2007b; 

Abt Associates, 2007a,b). Scientific and technical information developed since the 2006 Air 

Quality Criteria document is being considered in the new review, not in the reconsideration 

rulemaking, allowing the new information to receive careful and comprehensive review by 

CASAC and the public before it is used as a basis in a rulemaking that determines whether to 

revise the NAAQS. As in prior NAAQS rulemakings, EPA has also conducted a provisional 

assessment (U.S. EPA, 2009c) of such “new” scientific information (published since review of 

the 2006 Air Quality Criteria document) and has concluded that the scientific literature would 

not materially change the conclusions reached in the 2006 Air Quality Criteria document, 

providing support for the determination that it was appropriate to proceed with the 

reconsideration rulemaking. The provisional assessment was subjected to internal EPA peer 

review, and the final provisional assessment (U.S. EPA, 2009c) was made available to CASAC 

and the public prior to the proposal (75 FR 2938). Consistent with EPA’s approach in other 

NAAQS reviews, the Agency has not based its proposed decisions in the reconsideration on the 

new science but will instead review and consider the new science in the new review covered by 

this IRP. 

Consistent with EPA’s notice to the Court, this reconsideration of the 2008 O3 NAAQS

rule is being conducted through notice and comment rulemaking, with the notice of proposed 

rulemaking signed by the Administrator on January 6, 2010.10

 

10 This reconsideration will include review of the Air Quality Index (AQI) for O3, such that changes to the AQI will 

be proposed if the reconsideration results in a proposed change to the 2008 primary O3 standard.

 Following publication of the 

proposed rule (75 FR 2938) on January 19, 2010, the Agency provided for a 60-day public 

comment period, held public hearings in Arlington, Virginia and Houston, Texas on February 2, 

2010 and in Sacramento, California on February 4, 2010, and solicited CASAC review of the 
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proposed rule on January 25, 2010. To ensure that the final decision on the reconsideration of 

the 2008 O3 primary standard would be based on the most appropriate interpretation of the 

scientific evidence and exposure/risk information that was available in the 2008 review, in 

December 2010 the Administrator decided to ask the CASAC Ozone Reconsideration Panel to 

provide further advice about the strengths and limitations of the scientific evidence and the 

results of the exposure and health risk assessments to aid in her interpretation of this information. 

Public teleconferences with the CASAC Ozone Reconsideration Panel were held on February 18, 

March 3, and March 23, 2011 to respond to charge questions prepared by EPA. CASAC 

provided its response to the Administrator on March 29, 2011. EPA intends to issue a final 

decision on the reconsideration by July 29, 2011. 
2-1 

2 STATUS AND SCHEDULE FOR NEW REVIEW 

On September 29, 2008, the EPA’s NCEA-RTP announced the initiation of a new 

periodic review of the air quality criteria for O3 and related photochemical oxidants and issued a 

call for information in the Federal Register (73 FR 56581). A wide range of external experts as 

well as EPA staff, representing a variety of areas of expertise (e.g., epidemiology, human and 

animal toxicology, statistics, risk/exposure analysis, atmospheric science, ecology, biology, plant 

science, benefits analysis) participated in a “kick-off” workshop, held by EPA on October 28-29, 

2008 in Researach Triangle Park, NC. The proceedings of that workshop have been considered 

and the issues discussed at the workshop have been incorporated into this IRP. 

The development of this IRP was extended while the Agency reviewed the 2008 O3

NAAQS rule for the purpose of determining whether it would reconsider the 2008 standards, as 

discussed above in section 1.4. A draft of this IRP was released on September 30, 2009, for the 

purpose of conducting a public teleconference consultation with CASAC, which was held on 

November 16, 2009, in order to discuss the Agency’s plans for the continuation of this new 

review. This IRP reflects consideration of comments received from CASAC and the public in 

presenting plans for the new review of the air quality criteria and standards for O3-related effects 

on public health and public welfare. This involved updating the assessments presented in the 

2006 Air Quality Criteria document (U.S. EPA, 2006) and the 2007 Staff Paper (U.S. EPA, 

2007a) and REAs (U.S. EPA, 2007b; Abt Associates, 2007a,b). Recognizing that the 

reconsideration of the 2008 standards will be completed early in this new review, this new 

review will involve reviewing any O3 standards that may be set in the July 2011 final rule that 

results from the reconsideration of the 2008 O3 standards. While the Agency is reconsidering the 

2008 O3 standards, NCEA-RTP will continue the development of the ISA, the first draft of 

which was released to CASAC and the public in March 2011. 

The schedule for the entire new review of the air quality criteria and standards is shown 

below in Table 2-1. 
2-2 

Table 2-1. Schedule for the New Periodic O3 NAAQS Review 

Stage of Review Major Milestone Target Dates

Integrated Review 

Plan (IRP)

Literature Search Ongoing

Federal Register Call for Information September 29, 2008

Workshop on Science/Policy Issues October 29-30, 2008

Draft IRP September 30, 2009

CASAC Consultation on Draft IRP November 16, 2009

Final IRP March 2011

Integrated Science 

Assessment (ISA)

First Draft ISA March 2011

CASAC/Public Review of First Draft ISA May 19-20, 2011

Second Draft ISA September 2011

CASAC/Public Review of Second Draft ISA November 2011

Final ISA February 2012

Risk/Exposure 

Assessments (REAs)

Scope and Methods Plans April 2011

CASAC Consultation on Scope and Methods Plans May 19-20, 2011

First Draft REAs October 2011

CASAC/Public Review of First Draft REAs November 2011

Second Draft REAs May 2012

CASAC/Public Review of Second Draft REAs July 2012

Final REAs October 2012

Policy Assessment

(PA)/ Rulemaking

First Draft PA for CASAC/Public Review June 2012

CASAC/Public Review of First Draft PA July 2012

Second Draft PA for CASAC/Public Review November 2012

CASAC/Public Review of Second Draft PA January 2013

Final PA March 2013

Proposed Rulemaking September 2013

Final Rulemaking June 2014
3-1 

3 KEY POLICY-RELEVANT ISSUES

The key policy-relevant issues to be addressed in this new review are presented below as a 

series of policy-relevant questions that will frame our approach to determining whether the 

primary and secondary NAAQS for O3 that result from the Agency’s reconsideration of the 2008 

O3 standards should be retained or revised. The ISA, REAs, and PA to be developed in this new 

review will provide the basis for addressing these questions and will inform the Agency’s 

decisions as to whether to retain or revise those primary and secondary standards for O3. 

3.1 ISSUES RELATED TO THE PRIMARY OZONE NAAQS

The first step in reviewing the adequacy of the primary O3 standard is to consider whether 

the available body of scientific evidence, assessed in the ISA and used as a basis for the analyses 

presented in the public health-related REA, supports or calls into question the scientific 

conclusions reached in the last review regarding health effects related to exposure to O3 and 

other photochemical oxidants (e.g., peroxyacetyl nitrate, hydrogen peroxide) in ambient air, with 

a particular emphasis on exposures and health risks in susceptible populations, such as children. 

This evaluation of the available scientific evidence will focus on key policy-relevant issues by 

addressing a series of questions including the following: 

 To what extent has new scientific information become available that alters or 

substantiates our understanding of the health effects associated with various time 

periods of exposure to ambient O3, including short-term (1 to 3 hrs), prolonged (6 to 8 

hrs), and chronic (months to years) exposures? 

 To what extent has new scientific information become available that alters or 

substantiates our understanding of the health effects of O3 on at-risk populations, 

including those with potentially increased susceptibility such as children and 

disadvantaged populations? 11

 To what extent has new scientific information become available that alters or 

substantiates conclusions from previous reviews regarding the plausibility of adverse 

health effects caused by O3 exposure? 

 At what levels of O3 exposure are health effects observed? Is there evidence of 

effects at exposure levels lower than those previously observed, and what are the 

important uncertainties associated with that evidence? What is the nature of the 

exposure-response relationships of O3 for the various health effects evaluated? 

 

11 Susceptibility refers to innate (e.g., genetic or developmental) or acquired (e.g., age, disease, or smoking) factors 

that make individuals more likely to experience effects with exposure to O3. Vulnerability refers to O3-related 

effects due to factors including socioeconomic status (e.g., reduced access to health care) or particularly elevated 

exposure levels.
3-2 

 To what extent has new scientific information become available that alters or 

substantiates our understanding of non-O3-exposure factors that might influence the 

associations between O3 levels and health effects being considered (e.g., weatherrelated factors; behavioral factors such as heating/air conditioning use; driving 

patterns; and time-activity patterns)?

 To what extent do risk and/or exposure analyses suggest that exposures of concern for 

O3-related health effects are likely to occur with current ambient levels of O3 or with 

levels that just meet the O3 standard? Are these risks/exposures of sufficient 

magnitude such that the health effects might reasonably be judged to be important 

from a public health perspective? What are the important uncertainties associated 

with these risk/exposure estimates?

 To what extent have important uncertainties identified in the last review been 

addressed and/or have new uncertainties emerged?

 To what extent does newly available information reinforce or call into question any of 

the basic elements of the current O3 standard? 

Drawing upon the evidence and analyses presented in the ISA and REA, EPA will 

evaluate whether revisions to the primary O3 standard might be appropriate, and, if so, how this 

standard might be revised. Specifically, we will evaluate how the scientific information and 

assessments inform decisions regarding the basic elements of the NAAQS: indicator, averaging 

time, level, and form. These elements will be considered collectively in evaluating the health 

protection afforded by the current standard or any alternative standards considered. Specific 

policy-relevant questions that will be addressed include: 

 To what extent is there any new information that would support consideration of a 

different indicator for photochemical oxidants? 

 To what extent does the health effects evidence evaluated in the ISA, air quality 

analyses, and the REA provide support for considering different averaging times? 

 To what extent do air quality analyses and other information provide support for 

consideration of alternative forms? 

 What range of alternative standard levels should be considered based on the scientific 

evidence evaluated in the ISA, air quality analyses, and the REA? 

 In considering alternative standards, to what extent do alternative levels, averaging 

times and forms reduce estimated exposures and risks of concern attributable to O3

and other photochemical oxidants, and what are the uncertainties associated with the 

estimated exposure and risk reductions? What conclusions can be drawn regarding 

the health protection afforded susceptible populations? 

 What are the important uncertainties and limitations in the evidence and assessments 

and how might those uncertainties and limitations be taken into consideration in 

identifying alternative standards for consideration? 
3-3 

3.2 ISSUES RELATED TO THE SECONDARY OZONE NAAQS

As with the review of the primary NAAQS, the first step in reviewing the adequacy of the 

secondary O3 standard is to consider whether the available body of scientific evidence, assessed 

in the ISA and used as a basis for the analyses presented in the public welfare-related REA, 

supports or calls into question the scientific conclusions reached in the last review regarding 

welfare effects related to exposure to O3 and other photochemical oxidants (e.g., peroxyacetyl 

nitrate, hydrogen peroxide) in ambient air. This evaluation of the available scientific evidence 

will focus on key policy-relevant issues by addressing a series of questions including the 

following: 

 To what extent has new scientific information become available that alters or 

substantiates our understanding of the effects on vegetation and other welfare effects 

following exposures to levels of O3 found in the ambient air?

 To what extent has new scientific information become available to inform our 

understanding of the nature of the exposures that are associated with such effects in 

terms of biologically relevant cumulative, seasonal exposure indices? 

 To what extent has new scientific information become available that alters or 

substantiates our understanding of the effects of O3 on sensitive plant species, 

ecological receptors, or ecosystem processes?

 To what extent has new scientific information become available that alters or 

substantiates our understanding of exposure factors other than O3 that might influence 

the associations between O3 levels and welfare effects being considered (e.g., site 

specific features such as elevation, soil moisture level, presence of co-occurring 

competitors, pests, pathogens, other pollutant stressors, weather-related factors)? 

 To what extent has new scientific information become available that alters or 

substantiates conclusions regarding the occurrence of adverse welfare effects at levels 

of O3 as low as or lower than those observed previously? What is the nature of the 

exposure-response relationships of O3 for the various welfare effects evaluated? 

 Given recognition in the last review that the significance of O3-induced effects to the 

public welfare depends in part on the intended use of the plants or ecosystems on 

which those effects occurred, to what extent has new scientific evidence become 

available to suggest additional locations where the vulnerability of sensitive species 

or ecosystems would have special significance to the public welfare and should be 

given increased focus in this review? 

 To what extent do risk and/or exposure analyses suggest that exposures of concern for 

O3-related welfare effects are likely to occur with current ambient levels of O3 or with 

levels that just meet the O3 standard? Are these risks/exposures of sufficient 

magnitude such that the welfare effects might reasonably be judged to be important 

from a public welfare perspective? What are the important uncertainties associated 

with these risk/exposure estimates? 
3-4 

 To what extent have important uncertainties identified in the last review been 

addressed and/or have new uncertainties emerged?

 To what extent does newly available information reinforce or call into question any of 

the basic elements of the current O3 standard? 

Drawing upon the information and assessments presented in the ISA and REA, EPA 

will evaluate whether revisions to the secondary O3 standard might be appropriate, and, if 

so, how this standard might be revised. Specifically, EPA will evaluate how the scientific 

information and assessments inform decisions regarding the basic elements of the NAAQS: 

indicator, averaging time, level, and form. These elements will be considered collectively in 

evaluating the welfare protection afforded by the current or any alternative standards 

considered. Specific policy-relevant questions that will be addressed include: 

 To what extent is there any new information that would support consideration of a 

different indicator for photochemical oxidants? 

 To what extent do the welfare effects evidence evaluated in the ISA, air quality 

analyses, and the REA provide support for considering different averaging times and 

forms that reflect biologically relevant exposure indices? 

 What range of alternative standard levels should be considered based on the scientific 

information evaluated in the ISA, air quality analyses, and the REA? 

 In considering alternative standards, to what extent do alternative levels, averaging 

times, and forms reduce estimated exposures and risks of concern attributable to O3

and other photochemical oxidants, and what are the uncertainties associated with the 

estimated exposure and risk reductions? 

 What are the important uncertainties and limitations in the evidence and assessments 

and how might those uncertainties and limitations be taken into consideration in 

identifying alternative standards for consideration?
4-1

4 SCIENCE ASSESSMENT

4.1 SCOPE AND ORGANIZATION

As noted in chapter 1, the Integrated Science Assessment (ISA) is a concise review, 

synthesis, and evaluation of the most policy-relevant science that communicates critical science 

judgments relevant to the NAAQS review. The current ISA serves to update and revise the 

scientific information available at the time of the last review of the air quality criteria. As such, 

the ISA forms the scientific foundation for the new review of the primary (health-based) and 

secondary (welfare-based) NAAQS. The judgments and conclusions drawn in the ISA are 

intended to support risk, exposure and policy analyses as well as decisions to retain or revise the 

NAAQS.

The science assessment will consist of an ISA and supplementary materials which are 

discussed in more detail in subsequent sections. In brief, the ISA critically evaluates and 

integrates the scientific information on the health and welfare effects associated with exposure to 

O3 and related photochemical oxidants in ambient air. This document and its supplementary 

materials will not provide a detailed literature review; but, rather, will discuss the current state of 

knowledge on the most relevant scientific literature on issues pertinent to the review of the 

NAAQS for O3. Discussions in the ISA will primarily focus on scientific evaluations that can 

inform the key policy questions described in chapter 3 of this document. Although emphasis is 

placed on discussion of health and welfare effects information, other scientific data are presented 

and evaluated in order to provide a better understanding of the nature, sources, measurement, and 

concentration distribution of O3 and related photochemical oxidants in ambient air, as well as the 

measurement of population exposure to these pollutants. 

The ISA will build on the conclusions of the 2006 Air Quality Criteria document (U.S. 

EPA, 2006) and focus on peer reviewed literature published since the previous review of the air 

quality criteria for O3. The 2006 Air Qyality Criteria document primarily evaluated literature

published through December 2004. Major legal and historical aspects of prior review documents 

as well as key milestones and procedures for document preparation will be briefly summarized at 

the beginning of the ISA. In subsequent chapters the results of recent scientific studies will be 

integrated with previous findings. Important older studies may be more specifically discussed in 

detail to reinforce key concepts and conclusions and/or if they are open to reinterpretation in 

light of newer data. Older studies also may be the primary focus in some areas of the document 

where research efforts have subsided and these older studies remain the definitive works 

available in the literature. Emphasis will be placed on studies conducted at or near O3
4-2

concentrations found in ambient air. Other studies are included if they contain unique data, such 

as a previously unreported effect or mechanism for an observed effect, or examine multiple 

concentrations to elucidate exposure-response relationships. 

4.2 ASSESSMENT APPROACH

4.2.1 Introduction

The EPA’s NCEA-RTP is responsible for preparing the ISA and its supplementary 

materials for O3. Expert authors include EPA staff with extensive knowledge in their respective 

fields and extramural scientists solicited by EPA for their expertise in specific fields. A diagram 

showing the standard protocol for development of an ISA is shown in Figure 4.1. A description 

of the NAAQS review process is addressed in section 1.2. 

4.2.2 Literature Search and Identification of Relevant Studies

The NCEA-RTP will use a systematic approach to identify relevant studies for 

consideration. The EPA has already published a Federal Register notice (73 FR 56581, 

September 29, 2008) to announce the initiation of this review and request information from the 

public. In addition to the call for information, publications will be identified through an ongoing 

literature search process that includes extensive computer database mining on specific topics. 

Additional publications will be identified by EPA scientists in a variety of disciplines by 

combing through relevant, peer reviewed scientific literature obtained through these ongoing 

literature searches, reviewing previous EPA reports, and a review of reference lists from key 

publications; studies are also identified in the course of CASAC and public review. 

Relevant epidemiologic, human clinical, and animal toxicological studies, including those 

related to exposure response relationships, mode(s) of action (MOA), susceptible populations, 

and ecological or welfare effects studies published since the last air quality criteria review will 

be considered. Additionally, air quality and emissions data, studies on atmospheric chemistry, 

transport, and fate of these emissions, as well as issues related to O3 exposure are considered. 

Further information will be acquired from consultation with content and area experts and the 

public. The studies identified will include research published or accepted for publication 

approximately a month prior to the release of the second external review draft of the ISA (see 

Table 2-1). Some additional studies, published after that date, may also be included if they 

provide new information that impacts one or more key scientific issues. The combination of 

these approaches should produce the comprehensive collection of pertinent studies needed to 

form the basis of the ISA. 
4-3

Figure 4.1. Standard steps in the development of Integrated Science Assessments (ISAs)
4-4

4.2.3 Criteria for Study Selection

In selecting epidemiologic studies for the present assessment, EPA will consider studies 

containing information on (1) short- or long-term exposures at or near ambient levels of O3; (2) 

health endpoints that repeat or extend findings from earlier assessments as well as those not 

previously extensively researched; (3) populations that are susceptible to O3 exposures; (4) 

issues related to potential confounding, and modification of effects; and/or (5) important 

methodological issues (e.g., lag of effects, model specifications, thresholds, mortality 

displacement) related to O3 exposure effects. Among the epidemiologic studies, emphasis will 

be focused on those relevant to standard setting in the United States. Specifically, studies 

conducted in the U.S. or Canada will be generally accorded more emphasis than those from other 

geographic regions, as the potential impacts of different health care systems and the underlying 

health status of populations need to be accounted for in the assessment. However, informative 

studies conducted in other countries will be included, as appropriate. In addition, emphasis will 

be placed on discussion of (1) new, multi-city studies that employ standardized methodological 

analyses for evaluating O3 effects, provide overall estimates for effects based on combined 

analyses of information pooled across cities, and examine results for consistency across cities; 

(2) new studies that provide quantitative effect estimates for populations of interest; and (3) 

studies that evaluate O3 as a component of a complex mixture of air pollutants and thus give 

consideration to the levels of other co-pollutants. 

The selection of research evaluating controlled exposures of laboratory animals will focus 

primarily on those studies conducted at or near ambient O3 concentrations and those studies that 

approximate expected human dose conditions in terms of concentration and duration, which will 

depend on the toxicokinetics and biological sensitivity of the particular laboratory animals 

examined. Studies will be sought that reveal site-specific effects of O3 exposure within the 

respiratory tract. Consideration will be given mainly to animal studies conducted at less than 1 

ppm O3. The necessity of such upper concentrations limits may be illustrated by rats, a key 

species used in O3 toxicological studies, but a species having both behavioral and physiological 

mechanisms that can lower core temperature in response to acute exposures, thus limiting 

extrapolation of data to human responses. However, in recognition of the fact that toxicological 

studies using near ambient concentrations of O3 or other pollutants do not necessarily reflect 

effects in the most susceptible populations, studies at higher exposure levels may be included 

when they provide information relevant to previously unreported effects, evidence of potential 

mechanisms for an observed effect, information on exposure-response relationships, or otherwise 

improve our understanding of interspecies differences or susceptible populations. Additionally, 

in vitro studies may provide information on related to mechanisms of O3 uptake and effect or the 
4-5

influence of photochemical oxidation processes that would otherwise be unavailable through in 

vivo studies. The appropriateness of O3 concentrations will be evaluated as necessary. 

For research evaluating controlled human exposures to O3, emphasis will be placed on 

studies that: (1) investigate effects in healthy populations and/or potentially susceptible 

populations such as those with preexisting respiratory diseases; (2) include appropriate control 

(or sham) exposures such as filtered air so that subjects serve as their own control as well as the 

use of age-matched healthy controls in studies of susceptible individuals; (3) address issues such 

as dose-response or time-course of responses; (4) investigate exposure to O3 separately and in 

combination with other pollutants such as PM and NO2; and (5) have sufficient sample size and 

statistical power to assess findings adequately. Due to the limited amount of recently published 

controlled human exposure studies, much of the available scientific information is expected to 

come from literature that has been included in prior reviews. This older literature will be 

reevaluated and discussed in light of more recent epidemiologic findings and mechanistic 

toxicological data, as well as new controlled human exposure studies. 

For research evaluating welfare effects, emphasis will be placed on recent studies that: (1) 

evaluate effects at realistic ambient levels and (2) investigate effects on cultivated and noncultivated vegetation and ecosystems that occur in the U.S. Studies conducted in other 

geographical areas will be included in the assessment when they contribute to the general 

knowledge of the effects of O3 irrespective of species or locality. As in the evaluation of healthrelated scientific studies, the evaluation of welfare-related studies will assess advances in our 

understanding of mechanisms of direct O3 effects on vegetation and the resulting consequences 

on growth and yield. These mechanisms will inform our considerations of O3 effects on larger 

scale ecosystem structure, function and services. These and other welfare effects will be 

addressed in the ISA for both short- and long-term O3 exposures. Evaluations of research 

methodologies will be integrated into the discussion to allow for comparisons between 

methodologies and to allow characterization of the uncertainties associated with estimating 

exposure of vegetation using different types of experimental systems. 

These criteria provide generalized benchmarks for evaluating various studies and for 

focusing on the highest quality studies in assessing the body of health and welfare effects 

evidence. Detailed critical analysis of all O3 health and welfare effects studies, especially in 

relation to the above considerations, is beyond the scope of this document. Of most relevance 

for evaluation of studies is whether they provide useful qualitative or quantitative information on 

exposure-effect or exposure-response relationships for effects associated with current ambient air 

concentrations of O3 likely to be encountered in the United States. Since the last scientific 

review was completed relatively recently, i.e., within the past four years, it is expected that a 
4-6

considerable portion of the current ISA could reasonably be devoted to reiterating the basis for 

scientific conclusions reached in last review. 

4.2.4 Quality Assurance

NCEA participates in the Agency-wide Quality Management System, which requires the 

development of a Quality Management Plan (QMP). Implementation of the NCEA QMP 

ensures that all data generated or used by NCEA scientists are “of the type and quality needed 

and expected for their intended use” and that all information disseminated by NCEA adheres to a 

high standard for quality including objectivity, utility and integrity. 

The NCEA QA staff is responsible for the review and approval of quality-related 

documentation. NCEA scientists are responsible for the evaluation of all inputs to the ISA, 

including primary (new) and secondary (existing) data, to ensure their quality is appropriate for 

their intended purpose. NCEA follows the Data Quality Objectives, which identify the most 

appropriate inputs to the science assessment, and provides QA instruction for researchers citing 

secondary information. The approaches utilized to search the literature and criteria for study 

selection were detailed in the two preceding subsections. Generally, NCEA scientists rely on 

scientific information found in peer-reviewed journal articles, books, and government reports. 

Where information is integrated or reduced from multiple sources to create new figures, tables, 

or summation, the data generated are considered to be new and subject to rigorous quality 

assurance measures to ensure their accuracy. 

4.3 CONTENT AND ORGANIZATION OF THE ISA

The organization of the ISA for O3 will be consistent with that used in the ISA for 

particulate matter completed in December 2009 (U.S. EPA, 2009a). The ISA will contain 

information relevant to considering whether it is appropriate to retain or revise the current 

standards. Taking into consideration the broad policy-relevant questions outlined in chapter 3, 

the policy-relevant questions that will guide development of the ISA are related to two 

overarching issues. The first issue is the extent to which new scientific evidence has become 

available that alters or substantiates the scientific evidence presented and evaluated in the last O3

NAAQS review. The second issue is whether uncertainties from the last air quality criteria 

review have been addressed and/or whether new uncertainties have emerged. Specific questions 

related to the review of the scientific literature for O3 that stem from these issues will guide the 

content of the ISA. These questions were derived from the last O3 NAAQS review, as well as 

from discussions of new scientific evidence that occurred at the EPA kickoff workshop (October 
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29-30, 2008) for this review of O3 and related photochemical oxidants. These questions are 

listed below by topic area.

Source to Exposure

Air Quality and Atmospheric Science

The assessment will also include information about the distribution of monitors in the 

regulatory O3 network relevant for the interpretation of health and ecosystem effects and new 

studies dealing with the precision and accuracy of the Federal Reference and Federal Equivalent 

Methods (FRM and FEM, respectively) for O3. New information on the distribution of ambient

O3 concentrations from in situ instruments, satellites, and other remote sensing tools will also be 

considered. 

: The ISA will present and evaluate data related to: 

ambient concentration distributions of O3, and its potential associations with other photochemical 

oxidants and with other relevant atmospheric pollutants. New information concerning the 

mechanisms of formation of O3 and other photochemical oxidants, such as peroxyacyl nitrates, 

and hydrogen peroxide and organic peroxides, and the physical properties governing their 

transport and lifetimes in the atmosphere will be considered. The ISA will evaluate relevant data 

concerning the origin, transformation and transport, and fate of atmospheric oxidants in addition 

to O3. 

Since a key component of quantitative risk assessments has been the distribution of the 

policy-relevant background (PRB)12

 

12 "Policy-relevant background” has been defined by EPA historically as the O3 concentrations that would be 

observed in the U.S. in the absence of anthropogenic emissions of O3 precursors (e.g., VOC and NOx) in the U.S., 

Canada, and Mexico. Under this definition, PRB concentrations include contributions from natural sources 

everywhere in the world and from anthropogenic sources outside continental North America (U.S. EPA, 2006). 

 concentration of O3 in the U.S., the ISA will evaluate 

methods for producing these concentrations and characterize O3 PRB concentrations. In the last

review, O3 PRB concentrations were generated using the global scale, three dimensional, 

chemical transport model GEOS-CHEM, based on contributions from natural sources 

everywhere in the world and from anthropogenic sources outside continental North America. 

The ISA will evaluate relevant new studies and information on background concentrations. The 

assessment will include estimation of O3 PRB concentrations based on the definition of PRB 

used in previous reviews, and it will also evaluate additional approaches to estimation of O3 PRB 

concentrations, including consideration of alternative definitions of PRB. This may include 

evaluation of the use of models such as MOZART or CMAQ and approaches to the use of these 

models. 
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The ISA will include evaluation of data on the effects of tropospheric O3 on climate, 

including its role as a constituent greenhouse gas, i.e., in its role as a radiative forcing agent and 

its effects as an absorber of UV-B radiation in the troposphere. The potential effects of climate 

change on regional air quality are being assessed elsewhere by the Agency

(http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=203459). 

Exposure

Human Health Effects

: The ISA will compile and evaluate information new since the last assessment that 

helps characterize the variability and uncertainty in the relationships between ambient O3

concentrations and exposures to O3 of humans and ecosystems relevant to the primary and 

secondary standards. Regarding the primary standard for human health, this means assessing 

data concerning the range of measured O3 concentrations in various human microenvironments 

including indoors, outdoors near roadways, in vehicles, etc. and its relationship with 

concentrations measured by ambient monitors. EPA will also assess data concerning errors in 

measurement or estimation of human exposures as well as the possibly differential exposures of 

some populations. 

The ISA will evaluate the literature related to respiratory, cardiovascular, and other health 

effects associated with short and/or long term exposures to O3. Building upon the last air quality 

criteria review, EPA plans to continue to review the available scientific evidence related to these 

health endpoints and to integrate the previous findings with the results of new studies on these 

health endpoints and, to the extent data are available, on additional endpoints of concern (e.g., 

developmental, inflammatory, carcinogenic/mutagenic, and cellular outcomes). Health effects 

that occur following short- (including sub-daily) and/or long-term exposures to O3 will be 

evaluated in epidemiologic, human clinical, and toxicological studies. The ISA will also 

integrate previous information on susceptible populations (e.g., asthmatics, children, outdoor 

workers) with new evidence for these and possibly other susceptible populations (e.g., fetuses, 

neonates, genetically susceptible populations). 

 For a given type of health outcome, the ISA will evaluate the strength, robustness and 

consistency of the findings from the different disciplines. The health findings will be further 

integrated, using the toxicological and human clinical studies to assess biological plausibility and 

mechanistic evidence for the epidemiologic findings. Efforts will be directed at identifying the 

lower levels at which effects are observed and at determining concentration-response 

relationships. Concentration-response relationships among these studies will be evaluated for 

coherence. The ISA will evaluate the scientific evidence on the occurrence of health effects 

from short-term or long-term exposure to O3 at ambient levels. The ISA will also assess the 
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evidence for uncertainties related to these associations and information on the public health 

implications related to ambient O3 exposure. The evaluation will also focus on which exposure 

durations and developmental time periods of exposure are most strongly associated with effects, 

for both short-term and long-term exposures. Grouped by topic area, some of the scientific 

questions that EPA will seek to address in the ISA follow. 

Health Effects from Exposure:

 How do results of recent studies expand our understanding of the relationship 

between short-term exposure to O3 and respiratory effects, such as lung function 

changes, airways hyperresponsiveness, lung inflammation, and host defense against 

infectious disease? What new evidence is available on the potential clinical relevance 

of these effects? Do recent studies expand the current understanding of adaptation to 

repeated short-term O3 exposures? 

The ISA will evaluate health effects evidence for a multitude of 

outcomes from epidemiologic, toxicological, and human clinical studies. 

 Do long-term exposures to O3 result in chronic effects manifested as permanent lung 

tissue damage, altered lung development, or accelerated decline in lung function with 

age? To what extent does long-term O3 exposure promote development of asthma or 

chronic lung or cardiovascular disease? 

 Does new evidence from studies of hospital admissions or emergency department 

visits support previous findings regarding respiratory or cardiovascular effects of O3? 

Is there evidence of coherence and plausibility for such effects?

 What new evidence is available on associations between O3 and mortality (total, 

respiratory or cardiovascular)? 

 To what extent is key evidence becoming available that could inform the 

understanding of populations and life stages that are particularly susceptible to O3

exposures? What is known about genetic traits, metabolic syndrome, or other factors

that affect susceptibility? What is known about susceptibility at various ages and 

developmental stages, including critical windows of exposure that result in different 

effects and/or effects at lower exposures? Are new animal models becoming available 

to better characterize susceptible populations? 

 What O3-induced health effects are sufficiently characterized to be quantitatively 

compared across species?

 To what extent does exposure to O3 contribute to health effects in other organ 

systems?

 What new evidence has become available to help discern health effects of 

multipollutant exposures (containing O3) versus O3 alone (e.g., additive, synergistic, 

or antagonistic effects)?
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Uncertainties

 How do meteorological factors and co-exposure to other criteria pollutants (e.g., PM, 

NO2, SO2, and CO) influence the uncertainty of the evidence base for both short- and 

long-term O3 exposures? 

: The ISA will evaluate uncertainty in the scientific data, particularly in relation to 

observed epidemiologic findings and their consistency with toxicological and controlled human 

exposure studies in terms of observed effects and biological pathways. 

 To what extent are the observed health effects attributable to O3 versus other oxidants 

that are associated with O3? 

 What are the uncertainties due to other factors in epidemiologic studies (e.g., 

demographic and lifestyle attributes, socioeconomic status, genetic susceptibility 

factors, occupational exposure, and medical care)? 

 What is the nature and shape of the concentration-response models (e.g., linear, nonlinear, threshold models) based on O3 studies? 

 What uncertainties surround the evidence for long-term effects such as life shortening 

and development/progression of disease? 

 How do the findings of the available studies improve our understanding of exposure 

error? What evidence is newly available on the uncertainties related to statistical 

model specification and how can it be used to assess the influence of these 

uncertainties on the outcome of epidemiologic studies? 

Biological Mechanism(s) or Modes of Action

 Is there new information related to the pathways and underlying biological 

mechanism(s) or modes of action for O3? 

: The ISA will evaluate the data examining 

mechanisms for the health outcomes associated with exposure to O3. 

 What are the inherent interspecies differences in sensitivity to O3 and in O3 dosimetry 

in different regions of the respiratory tract? Are these likely to vary across age 

groups? Are there site-specific responses to O3 in the respiratory tract that would 

better explain local and systemic effects of O3 exposure? 

 What are the interspecies differences in basic mechanisms of lung injury and repair 

and cardiovascular responses? What are the implications of interspecies differences 

for extrapolation of results to humans? 

 What are the mechanisms and time-courses of O3-induced cellular and tissue injury, 

repair, and remodeling? 

Susceptible Populations: The ISA will examine health outcome data to identify specific groups 

that have a greater likelihood of experiencing health effects related to O3 exposure due to a 

variety of factors including, but not limited to: genetic or developmental factors, race, gender, 
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lifestage (e.g., children), lifestyle (e.g., smoking status, nutrition, outdoor work) or preexisting 

disease; as well as, population-level factors that can affect an individual's exposure to O3 such as 

socioeconomic status (SES), which encompasses reduced access to health care, low educational 

attainment, residential location, and other factors. The host and environmental factors that are 

responsible for differential susceptibility to O3 will be investigated.

 What do controlled human exposure, animal toxicological, and epidemiologic studies 

indicate regarding the relationship between acute exposures to O3 and health effects 

of concern in healthy individuals and those with preexisting diseases (e.g., asthma, 

COPD, cardiovascular diseases)? What other medical conditions (e.g., diabetes, 

metabolic syndrome) are identified as increasing susceptibility to O3 effects? What 

are the pathways and mechanisms through which O3 may be acting for these groups? 

What is the nature and time-course of the development of effects in healthy persons 

and in persons with pre-existing disease (e.g., asthma, heart disease)?

 How has new evidence increased our understanding of the potential susceptibility of 

children and older adults to effects from O3 exposure? With regard to the 

interpretation of epidemiologic results and exposure-response characteristics of 

populations, to what extent are these findings driven by effects in susceptible 

populations? 

 What evidence is available regarding susceptibility to O3-induced responses in 

population groups due to age, race, gender, or genetic makeup? How do the nature of 

the effects and exposure-response relationships differ between groups? Is there 

evidence of differing biological modes of action related to observed susceptibility? 

 To what extent is susceptibility to the effects of short-term O3 exposure is associated 

with long-term O3 susceptibility?

 What factors (e.g., demographic and socioeconomic) affect susceptibility to shortand long-term O3 exposures? Are there new data regarding population groups, such 

as disadvantaged populations with potentially greater susceptibility to effects of O3? 

Public Health Implications:

Vegetation, Ecosystems and other Welfare Effects

 The ISA will present concepts to define potential health outcomes 

and their implications on public health. This will include estimates of the numbers of people in 

specific at-risk populations groups (e.g., asthmatics, diabetics, older adults, and children). 

The ISA will evaluate the literature related to O3 exposures on the growth of vegetation, 

visible foliar injury, ecosystem services and other welfare effects. This will include evaluation 

of O3 exposures on productivity of ecosystems and crops systems and potential effects on 

ecosystem services. Other effects that will be evaluated include O3 effects on materials and 
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climate. Grouped by topic area, some scientific questions that EPA will seek to address in the 

ISA follow. 

Vegetation

 Past reviews have highlighted evidence from O3 exposure experiments performed in 

open-top chambers (OTCs). More recent studies have also utilized other techniques 

such as Free-air exposures (FACE) and gradient studies. In what ways does the more 

recent literature inform our understanding of O3 exposure on vegetation? For 

example, topics may include: comparing OTC results to other studies and differences 

between small and large trees. 

: Scientific studies have previously reported concentration response functions for the 

relationship between O3 exposure and plant response for a range of endpoints. The ISA will 

consider key uncertainties identified in the last air quality criteria review and the extent to which 

new scientific evidence may be available to substantially inform our ability to characterize 

and/or reduce these uncertainties.

 Though there is a large, historic body of research on O3 effects on vegetation, there 

has been no common metric used across studies to describe the relationship between 

O3 exposures and plant response. How can O3 studies which use various O3 metrics, 

plant species and methodologies be appropriately quantitatively synthesized and 

assessed?

Ecosystem Services

 What is the nature of the information linking O3 pollution and ecosystem services? 

What are the existing studies that make direct or indirect linkages between O3

exposure and ecosystem services? How can studies at smaller scales be used to 

address ecosystem services issues?

: Some recent research has examined further how O3 effects are potentially 

linked to ecosystem services and processes related to these services. Such linked ecosystem 

services and processes identified in recent studies include water supply and quality, N-cycling, 

bee pollination, and CO2 sequestration. 

 Can information available in the older literature be re-examined in light of these 

broader linkages? 

 What new information is available on potential effects of O3 on CO2 sequestration in 

ecosystems?

 How does O3 influence the biodiversity of ecological systems? 

 Has O3 altered nutritional content of forage for domestic animals or wildlife 

populations? 
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Materials Damage: Ozone and other photochemical oxidants react with many economically 

important man-made materials, decreasing their useful life and aesthetic appearance. Materials 

damaged by O3 include elastomers; textiles and fibers; dyes, pigments, and inks; and paints and 

other surface coatings. The new scientific literature will be evaluated in this area to determine 

the extent to which new scientific evidence may inform the standard.

Climate: The ISA will include evaluation of data relevant to the issue of tropospheric O3

as a constituent greenhouse gas, i.e., in its role as a radiative forcing agent and its effects as an 

absorber of UV-B radiation in the troposphere. The potential effects of climate change on 

regional air quality are being assessed elsewhere by the Agency 

(http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=203459). 

4.4 CAUSAL DETERMINATIONS 

 Since the last O3 NAAQS review, EPA has developed a framework that is intended to 

provide a consistent and transparent basis for drawing conclusions on causal relationships 

between pollutant exposures and health or environmental effects. Use of this framework in the 

recent science assessment for particulate matter is described in chapter 1 of that ISA (EPA, 

2009a). Briefly, the framework includes the following considerations for drawing conclusions of 

causality for specific endpoints: consistency of findings for an endpoint across studies in which it 

was examined, coherence of the results related to a specific endpoint among different study types 

or disciplines, the coherence of results with characterized mechanisms of action (biological 

plausibility), and evidence of a concentration- or dose-response relationship for an endpoint. in 

drawing judgments regarding causality for the criteria air pollutants, EPA focuses on evidence of 

effects at relevant pollutant exposures. 

In the current ISA for O3, EPA will assess the results of recent relevant publications, 

building upon evidence available during the previous O3 NAAQS review, to draw conclusions 

regarding the strength of the evidence for causal relationships between health and environmental 

effects associated with short- and long-term exposure periods. Evaluations of causality for 

ecological and welfare effects generally consider the probability of quantitative changes in 

response to exposure. Exposure-response relationships are often determined for a specific 

ecological system and scale, rather than at the national or even regional scale. In making 

determinations of causality for human health effects, evidence will be evaluated for broad health 

outcome categories, such as respiratory effects, and then conclusions drawn based upon the 

integration of evidence from across disciplines (e.g., epidemiology, clinical studies and 

toxicology) and also across the suite of related individual health outcomes. EPA will focus on 
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health outcome categories, rather than very specific endpoints, since the coherence of evidence 

across a spectrum of related endpoints (e.g., effects ranging from inflammatory effects to 

respiratory mortality) is an important aspect for drawing conclusions regarding causality.

4.5 SUPPLEMENTARY MATERIALS

Previous science assessments conducted to support NAAQS reviews included 

supplementary materials, which were designed to provide detailed supporting information and 

more comprehensive coverage of the research areas summarized in the ISA. NCEA intends to 

change the form, while maintaining the relevant content, of the materials that were formerly 

contained within the Annexes to the ISA.

As discussed previously, studies included in the text of the ISA will be those deemed 

informative to the NAAQS review process (e.g. policy-relevant) and of adequate quality. The 

ISA text, tables and figures will highlight and summarize key study details that are needed to 

understand and interpret the results of a study. This information, which was described in the text 

as well as reiterated in the annex tables of previous documents, includes: (1) the chemistry, 

physics, sources, emissions, and measurement of O3; (2) environmental concentrations and 

human exposure to O3; (3) dosimetry; (4) toxicological studies of O3 health effects in laboratory 

animals and in vitro systems; (5) human clinical studies examining health effects following 

controlled exposure to O3; (6) epidemiologic studies of health effects from short- and long-term 

exposure to O3; (7) environmental studies on vegetation and ecosystem effects; and (8) materials 

damage and climate change related to O3. In addition, supplementary materials will be provided 

in the form of output from the Health and Environmental Research Online (HERO) database. A 

key function of the HERO output will be to document the base of evidence containing 

publications evaluated for the O3 review, including any publications considered but not included 

in the ISA. This information will be presented as links to lists of references in the HERO 

database, which include bibliographic information and abstracts.

4.6 SCIENTIFIC AND PUBLIC REVIEW

Drafts of the ISA will be reviewed by the CASAC O3 Review Panel and made available 

for public comment. The CASAC O3 Review Panel will review the first draft ISA and discuss 

their comments in a public meeting announced in the Federal Register. Based on CASAC’s past 

practice, EPA anticipates that key CASAC advice and recommendations for revision of the first 

draft ISA will be summarized by the CASAC Chair in a letter to the EPA Administrator. In 

revising the first draft ISA, EPA will take into account any such recommendations. EPA will 

also consider comments received from CASAC or from the public at the meeting itself and any 
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written public comments. EPA will prepare a second draft ISA for CASAC review and public 

comment. The CASAC O3 Review Panel will review the second draft ISA and discuss their 

comments in a public meeting announced in the Federal Register. Again, based on CASAC’s 

past practice, EPA anticipates that key CASAC advice and recommendations for revision of the 

second draft ISA will be summarized by the CASAC Chair in a letter to the EPA Administrator. 

In finalizing the ISA, EPA will take into account any such recommendations. EPA will also 

consider comments received from CASAC or from the public at the meeting itself and any 

written public comments. After appropriate revision, the final document will be made publicly 

available on an EPA website and in hard copy. A notice announcing the availability of the final 

ISA will be published in the Federal Register. In addition, the final ISA will be placed in the 

ozone ISA docket (Docket ID: EPA-HQ-ORD-2011-0050). 
5-1

5 HUMAN HEALTH RISK AND EXPOSURE 

ASSESSMENTS

5.1 OVERVIEW

Characterizing health risks for the new periodic review of the primary NAAQS for O3 will 

include conducting air quality analyses to support quantitative exposure and risk assessments in 

specific locations to the extent warranted by new information, taking into consideration available 

resources. The results of such assessments will be put into a broader public health perspective, 

with a particular emphasis on exposures and health risks in susceptible populations, such as 

children. These assessments will be designed to estimate human exposures and to characterize 

the potential health risks that are associated with current ambient levels, with ambient levels 

simulated to just meet the current standard, and with ambient levels simulated to just meet 

alternative standards that may be considered. The EPA is planning to focus the quantitative 

exposure/risk assessments on O3, but recognizes that O3 serves as an indicator of the broader 

photochemical oxidant mix. Therefore, health effects reported to be associated with exposure to 

O3 may not be due to O3 only, but to the broader mix of photochemical oxidants. 

An important issue associated with conducting exposure and human health risk 

assessments is the treatment of variability and the characterization of uncertainty. Variability

refers to the inherent heterogeneity in a population or variable of interest (e.g., residential air 

exchange rates) and cannot be reduced through further research, only better characterized with 

additional measurement. Uncertainty refers to the lack of knowledge regarding both the actual 

values of model input variables (i.e., parameter uncertainty) and the physical systems or 

relationships (i.e., model uncertainty – e.g., the shapes of concentration-response relationships). 

As part of such analyses, variability and uncertainty will be explicitly addressed, where feasible,

in the planned air quality, exposure, and health risk assessments. 

The major components of the risk characterization (e.g., air quality analyses, quantitative 

exposure assessment, quantitative health risk assessment, broad health risk characterization) are 

outlined below and will be described in more detail in a Scope and Methods Plan. Preparation of 

this plan will coincide with the development of the first draft ISA to facilitate the integration of 

policy-relevant science into both documents. In particular, the availability of air quality, 

exposure-response, concentration-response, and baseline incidence data will impact the type of 

risk and exposure assessments that will be developed. 
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5.2 EXPOSURE AND HEALTH RISK ASSESSMENTS FROM THE LAST 

REVIEW

 The exposure and health risk assessments conducted in the last review developed 

exposure and health risk estimates for 12 urban areas across the U.S. which were chosen based 

on the location of O3 epidemiologic studies and to represent a range of geographic areas, 

population demographics, and O3 climatology. These analysies were in part based upon the 

exposure and health risk assessments done as part of the review completed in 1997. The 

exposure and risk assessment incorporated air quality data (i.e., 2002 through 2004) and 

estimated annual or O3 season-specific exposure and risk estimates for these recent years of air 

quality and for air quality scenarios simulating just meeting the existing 8-hr O3 standard and 

several alternative 8-hr O3 standards. Exposure estimates were used as an input to the risk 

assessment for lung function responses (i.e., a health endpoint for which exposure-response 

functions were available from controlled human exposure studies). Exposures were estimated 

for the general population and identified populations, including school age children with asthma 

as well as all school age children. The modeled exposures were also used to estimate the number 

of persons having exposures above potential health effect benchmark levels. Staff identified the 

benchmark levels using the occurrence of observed health effect endpoints (e.g., lung 

inflammation, increased airway responsiveness, and decreased resistance to infection) that were 

associated with 6-8 hour exposures to O3 while engaged in moderate exertion that were observed 

in several controlled human exposure studies. 

The exposure analysis took into account several important factors including the 

magnitude and duration of exposures, frequency of repeated high exposures, and breathing rate 

of individuals at the time of exposure. Estimates were developed for several indicators of 

exposure to various levels of O3 air quality, including counts of people exposed one or more 

times to a given O3 concentration while at a specified breathing rate, and counts of personoccurrences which accumulate occurrences of specific exposure conditions over all people in the 

population groups of interest over an O3 season. 

As discussed in the Staff Paper and in section II.A of the 2008 O3 NAAQS final rule (73 

FR 16440 to 16442), of the uncertainties identified and evaluated, the most important

uncertainties affecting the exposure estimates were related to modeling human activity patterns 

over an O3 season, modeling of variations in ambient concentrations near roadways, and 

modeling of air exchange rates that affect the amount of O3 that penetrates indoors. Another 

important uncertainty, discussed in more detail in the Staff Paper (section 4.3.4.7), was the 

uncertainty in energy expenditure values which directly affect the modeled breathing rates. 

These were important since they were used to classify exposures occurring when children were 
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engaged in moderate or greater exertion and health effects observed in the controlled human 

exposure studies generally occurred under these exertion levels for 6 to 8-hr exposures to O3

concentrations at or near 0.08 ppm. 

 The human health risk assessment conducted in the last review was designed to estimate 

population risks in a number of urban areas across the U.S., consistent with the scope of the 

exposure analysis described above. The risk assessment included risk estimates based on both 

controlled human exposure studies and epidemiologic and field studies. Ozone-related risk 

estimates for lung function decrements were generated based on probabilistic exposure-response 

relationships developed based on data from controlled human exposure studies, together with 

probabilistic exposure estimates from the exposure analysis. For several other health endpoints, 

O3-related risk estimates were generated based on concentration-response relationships reported 

in epidemiologic or field studies, together with ambient air quality concentrations, baseline 

health incidence rates, and population data for the various locations included in the assessment. 

Health endpoints included in the assessment based on epidemiologic or field studies included: 

hospital admissions for respiratory illness in 4 urban areas, premature mortality in 12 urban 

areas, and respiratory symptoms in asthmatic children in 1 urban area. 

In the previous health risk assessment, EPA recognized that there were many sources of 

uncertainty and variability in the inputs to the assessment and that there was a high degree of 

uncertainty in the resulting risk estimates. The statistical uncertainty surrounding the estimated 

O3 coefficients in concentration-response functions as well as the shape of the exposure-response 

relationship chosen were addressed quantitatively. Additional uncertainties were addressed 

through sensitivity analyses and/or qualitatively. The previous risk assessment incorporated 

some of the variability in key inputs to the assessment by using location-specific inputs (e.g., 

location-specific concentration-response function, baseline incidence rates and population data, 

and air quality data for epidemiologic –based endpoints, location specific air quality data and 

exposure estimates for the lung function risk assessment). In the previous health risk assessment, 

twelve urban areas were included to provide some sense of the variability in the risk estimates 

across the U.S. Sensitivity analysis was carried out for two sources of uncertainties. The first 

analysis investigated the impact of alternative estimates for policy-relevant background (PRB) 

levels in 3 of the 12 urban areas. The second sensitivity analysis looked at the impact of 

different assumptions around the shape of the exposure-response function. 

Key observations and insight from the O3 risk assessment, in addition to important caveats 

and limitations, were addressed in section II.B of the 2008 O3 NAAQS final rule notice (73 FR 

16440 to 16443). In general, estimated risk reductions associated with going from current O3 

levels to just meeting the current and alternative 8-hr standards showed patterns of increasing 
5-4

estimated risk reductions associated with just meeting the lower alternative 8-hr standards 

considered. Furthermore, the estimated percentage reductions in risk were strongly influenced 

by the baseline air quality year used in the analysis, which was due to significant year-to-year 

variability in O3 concentrations. There was also noticeable city-to-city variability in estimated 

O3-related incidence of morbidity and mortality across the 12 urban areas. Uncertainties 

associated with estimated PRB concentrations were also addressed and revealed differential 

impacts on the risk estimates depending on the health effect considered as well as the location. 

The EPA also acknowledged that there were considerable uncertainties surrounding estimates of 

O3 coefficients and the shape for concentration-response relationships and whether or not a 

population threshold or non-linear relationship exists within the range of concentrations 

examined in the epidemiologic studies.

5.3 AIR QUALITY CONSIDERATIONS 

Air quality analyses are required to conduct both exposure and health risk assessments for 

NAAQS reviews. Air quality inputs to the exposure and/or health risk assessment include: (1) 

provision of ambient air quality data from the fixed-site ambient monitoring network for the 

period 2008-2010 for the urban areas included in the exposure and risk assessments, (2) 

estimates of PRB concentrations for the specific urban areas included in the risk assessment, and 

(3) ambient air quality scenario data sets that are obtained from simulation procedures that adjust 

recent air quality data to reflect changes in the distribution of air quality estimated to occur at 

some unspecified time in the future when an area just meets a given set of NAAQS. Broader 

national scale air quality analyses may also be conducted to place the results of the quantitative 

risk and exposure assessments into a broader public health context. 

While incremental risk reductions may not require estimates of PRB, estimates of the 

risks in excess of PRB remaining upon meeting the current or potential alternative standards, do 

require EPA to estimate PRB. Both types of risk estimates are considered relevant to inform the 

EPA Administrator’s decision on the adequacy of a given standard. 

Historically, PRB has been defined as the "the distribution of O3 concentrations that 

would be observed in the U.S. in the absence of anthropogenic (man-made) emissions of 

precursor emissions (e.g., VOC, NOx, and CO) in the U.S., Canada, and Mexico" (US EPA, 

2007, p.2-48). This has been referred to as PRB, since this definition of background facilitates 

separating pollution levels that can be controlled by U.S. regulations (or through international 

agreements with neighboring countries) from levels that are not generally controllable in this 

manner. Thus, PRB in past reviews included: (1) O3 generated in the U.S. that arises from 

natural (biogenic) sources of emissions in the U.S., Canada, and Mexico and (2) O3 in the U.S. 
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from the transport of O3 or the transport of precursor emissions from both natural and man-made 

sources, from outside of the U.S. and its neighboring countries. As discussed in chapter 4, the 

ISA will include an assessment of methods for estimating PRB concentrations, including 

alternative approaches for defining PRB, and will produce O3 PRB concentrations for use in the 

risk assessment. In this new review, EPA plans to place greater emphasis on understanding the 

contribution of the different components that contribute to PRB (e.g., what portion of PRB is due 

to natural emissions alone and what is the contribution of transport from outside the North 

American continent, as well as the contribution of Canadian and Mexican anthropogenic 

emissions to O3 levels observed in the U.S.) and to the extent warranted, evaluate alternative 

definitions of PRB and their implications on exposures and risk. This additional information will 

help inform policy considerations for this review of the O3 NAAQS as well as more broadly 

inform efforts related to international efforts to reduce trans-boundary O3 air pollution. 

As part of the exposure and risk assessments, it will be necessary to adjust recent O3 air 

quality data to simulate just meeting the current standard and any alternative O3 standards that 

might be considered. In the last review, EPA used a quadratic air quality rollback approach 

(U.S. EPA, 2007a, section 4.5.8). EPA will consider this approach and alternative air quality 

simulation procedures for use in this current review. Staff will evaluate candidate procedures to 

adjust air quality by analyzing historical changes in measured O3 levels and by analyzing 

changes in O3 levels predicted by air quality models. In this new review, EPA also will examine 

techniques that may be used to assess the variability and uncertainty of the simulated change in 

concentrations likely to result from just meeting the current or alternative standards. 

5.4 POPULATION EXPOSURE ASSESSMENT APPROACH 

Population exposure to O3 will be evaluated using EPA’s Air Pollutants Exposure model 

(APEX), a model that simulates microenvironmental personal exposures using temporally and 

spatially variable ambient concentrations and personal time-location-activity patterns. One 

objective is to provide exposure estimates as an input to the portion of the health risk assessment 

that uses exposure-response relationships from controlled human exposure studies. The 

exposure analysis will also provide estimates of population exposure exceeding potential health 

effect benchmarks, values identified based on O3 exposure concentrations and associated health 

effects observed in controlled human exposure studies. 

The approach to the current exposure assessment will build upon the methods developed 

and insights gained from the exposure assessment conducted for the last review. Staff will 

consider performing the exposure assessment for the same 12 urban areas (i.e., Atlanta, Boston, 

Chicago, Cleveland, Detroit, Houston, Los Angeles, New York, Philadelphia, Sacramento, St. 
5-6

Louis, and Washington D.C). Several key considerations in planning for the exposure 

assessment are discussed below.

The most current version of the APEX model (also referred to as the Total Risk 

Integrated Methodology/Exposure (TRIM.Expo) model) will be used to estimate population 

exposures for the various air quality scenarios of interest. APEX simulates the movement of 

individuals through time and space and their exposure to O3 in indoor, outdoor, and in-vehicle 

microenvironments. APEX is a probabilistic model that will be used to simulate a large number 

of randomly sampled individuals within each urban area (e.g., 200,000) to represent area-wide 

population exposures. 

As in the previous exposure assessment, human activity data needed for the analysis will 

be drawn from the Consolidated Human Activity Database (CHAD) developed and maintained 

by ORD’s National Exposure Research Laboratory (NERL). A number of additional activity

diaries have been added to the database and will be used in this exposure assessment. This 

expanded database will likely improve the representation of the simulated exposure population of 

interest because there are increases in the numbers of data diaries available, in particular for 

children, and much of the added data are from studies conducted within the past decade. One 

key issue in this analysis regarding time-location activity patterns is the further evaluation and 

possible modification of the approach used for creating O3-season or year-long activity 

sequences for individuals from primarily cross-sectional activity data diaries. 

As done in the last O3 NAAQS review and other recent NAAQS reviews (e.g., US EPA, 

2007a,b; US EPA, 2008; US EPA, 2009b) and where possible, staff will identify, incorporate, 

and describe any observed variability in input data sets and estimated parameters within the 

analyses performed. In addition, consistent with other NAAQS reviews, the exposure 

assessment will include an uncertainty characterization of the model inputs and model 

formulation. 

5.5 HUMAN HEALTH RISK ASSESSMENTS 

The goals of the O3 health risk assessment are: (1) to provide estimates of the potential 

magnitude of selected morbidity and mortality health effects in the population associated with 

recent ambient O3 levels and with just meeting the current O3 standard and any alternative 

standards that might be considered in specific urban areas, (2) to develop a better understanding 

of the influence of various inputs and assumptions on the risk estimates; and (3) to gain insights 

into the distribution of risks and patterns of risk reduction and uncertainties in those risk 

estimates. The approach to the current health risk assessment will build upon the methods 

developed and insights gained from the risk assessment conducted in the last review. Staff will 
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consider performing the assessment for the same 12 urban areas (i.e., Atlanta, Boston, Chicago, 

Cleveland, Detroit, Houston, Los Angeles, New York, Philadelphia, Sacramento, St. Louis, and 

Washington D.C). Several key considerations in planning for the health risk assessment are 

discussed below. 

Staff is planning to focus the quantitative risk assessments on the most important health 

effect categories and endpoints from the standpoint of public health significance and for which 

the weight of the evidence supports the judgment that the effect category and specific health 

effects endpoints are judged sufficiently causal with respect to O3 either alone and/or in 

combination with other pollutants to be included in the quantitative risk assessment. An 

important additional consideration in deciding which health effect endpoints to include in the 

risk assessment is the availability of sufficient information to conduct a quantitative assessment 

(e.g., characterization of exposure- or concentration-response relationship, information on 

baseline incidence). 

The risk and exposure assessments will draw upon the information presented in the ISA 

and its supplementary materials. This includes information on atmospheric chemistry, air 

quality, human exposure, and health effects of concern. In particular, the availability of air 

quality, concentration- and exposure-response relationships, and baseline incidence rate data will 

impact the type of risk assessments that will be performed. 

As described in section 5.3 above, air quality inputs required to conduct the health risk 

assessment include: (1) recent O3 air quality data from suitable monitors for each selected 

location, (2) estimates of PRB concentrations for each location, and (3) simulated air quality that 

reflects changes in the distribution of O3 air quality estimated to occur when an area just meets a 

given O3 standard. 

5.5.1 Approach to Health Risk Assessment Based On Epidemiologic Studies

 As noted above, the health risk assessment conducted in this review will build on the 

approach developed and applied in the last review. Staff plans to rely on a weight-of-evidence 

approach, as provided in the ISA, based on evaluation of new and prior epidemiologic studies 

including identification of relevant concentration-response functions that characterize the 

relationships between O3 exposures and health outcomes, particularly those conducted at or near 

current ambient concentrations. Quantitative relationships provided in the specific studies or 

derived from the data presented in the epidemiologic studies describe the change in 

concentration (generally based on ambient fixed-site monitors) associated with a change in 

health response. These concentration-response relationships will be combined with air quality 

data, baseline incidence data, and population data to develop population health risk estimates. 
5-8

Epidemiologic studies typically provide estimated concentration-response relationships 

based on data collected in real-world settings. Ambient O3 concentrations are typically measured 

as the area-wide average of monitor-specific measurements, although personal exposures are 

occasionally measured. Health responses for O3 included in the prior risk assessment were: 

respiratory symptoms in asthmatic children, asthma and other respiratory-related hospital 

admissions, and premature mortality. Staff will consider the type of health response function(s) 

available and the availability of ambient O3 concentration data to characterize public health risks. 

We consider that these analyses are most appropriately applied in areas where the specific 

epidemiologic studies were performed. It should be noted that a risk characterization based on 

epidemiologic studies also requires baseline incidence rates and population data for the specific 

locations evaluated in the risk assessment. 

 The inclusion of any particular health endpoint depends in part on the extent to which the 

O3 ISA infers the likelihood of a causal relationship between O3 exposure and a given health 

effect category and the weight of the evidence for concluding that O3 exposures are related to the 

specific health effect endpoint. A number of issues related to the selection and application of 

appropriate concentration-response functions for use in the assessment will be addressed in the 

Scope and Methods Plan. For example, consideration will be given to the appropriate use of 

functions based on single- and multi-city studies, single- and multi-pollutant concentrationresponse models, and alternative lags. 

5.5.2 Approach to Health Risk Assessment Based on Controlled Human 

Exposure Studies

 As noted above, the health risk assessment conducted in this new review will build on the 

approach developed and applied in the last review. In that previous assessment, risk estimates for 

lung function responses associated with 8-hr exposures while engaged in moderate exertion were 

developed. These estimates were based in part on exposure-response relationships estimated from 

the combined data sets from multiple O3 controlled human exposure studies. Data from the studies 

by Folinsbee et al. (1988), Horstman et al. (1990), and McDonnell et al. (1991) in addition to more 

recent data from Adam (2002, 2003, 2006) were used to estimate exposure-response relationships for 

≥10, 15, and 20% decrements in FEV1. In this new review, staff intends to investigate the possibility 

of using a model (McDonnell et al., 2007) that estimates FEV1 responses associated with O3 shortterm exposures. This model is based on the controlled human exposure data included in the prior 

lung function risk assessment as well as additional data sets for different averaging times and 

breathing rates. We will also consider whether there is sufficient evidence to consider adding other 

health endpoints observed in controlled human exposure studies to the quantitative risk assessment 

based on the information contained in the draft ISA. 
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5.5.3 Uncertainty and Variability

For the health risk assessment to be conducted for the new review, we will include both a 

qualitative characterization of uncertainty and variability, and where feasible, a quantitative 

characterization of uncertainty and/or sensitivity analyses for those aspects of the assessment judged 

most influential. Following the same general approach described above in section 5.4, and 

adapted from WHO (2008), staff plans to perform a succinct qualitative characterization of the 

components contributing to uncertainty in estimated health risks. This qualitative 

characterization will be performed early in the process of developing the risk assessment to 

inform and prioritize potential health risk model development activities and to identify additional 

uncertainties that were not previously evaluated. 

5.5.4 Broader Risk Characterization

For this new review, staff is considering extending the risk assessment to a broader range 

of urban areas, beyond the 12 urban areas included in the previous assessment, in light of newly 

available data to provide greater coverage of additional regions of the country where significant 

O3 exposures are likely to occur. We also will consider the feasibility of developing 

concentration-response relationships that can be applied on a regional basis. It is very likely that 

the geographic (and population) coverage will vary for different health endpoint categories due 

to data limitations (e.g., the availability of emergency department and hospital admission 

baseline incidence data is more limited than mortality baseline incidence data). However, we 

recognize that there have been noticeable improvements in the availability of baseline incidence 

data for emergency department and hospital admissions since the last review. 

Beyond the quantitative risk and exposure assessments conducted for this review, staff will 

consider ways to put the results of those assessments into a broader context. Specifically, we

will explore analyses that would complement quantitative risk and exposure assessments 

conducted for a limited number of locations and selected health endpoints to better characterize 

the nature, magnitude, extent, variability, and uncertainty of the public health impacts associated 

with O3 exposures on a broader scale. We will consider how additional analyses can be used to 

inform our understanding of: 

 Additional health endpoints not considered in the quantitative risk assessment; 

 Additional locations not evaluated in the quantitative risk/exposure assessment to inform 

a broader understanding of public health impacts including non-urban environments; 

 Regional differences in O3 risks taking into consideration the following factors: 

- variations in individual and/or population susceptibility; 

- population demographics; 
5-10

- variations in exposures; and 

- impacts of potential effect modifiers (e.g., weather, co-pollutants). 

5.6 SCIENTIFIC AND PUBLIC REVIEW 

A Scope and Methods Plan for the risk/exposure assessment will be submitted to CASAC 

for consultation and will be provided to the public for comment subsequent to the release of the 

1st draft ISA. The CASAC O3 Review Panel will discuss its comments on the Scope and 

Methods Plan in a public meeting that will be announced in the Federal Register. In conducting 

the risk/exposure assessment, staff will take into account comments received from CASAC and 

from the public at the meeting itself and in any written comments. Staff plans to prepare two 

drafts of the risk/exposure assessment for CASAC review and public comment. The CASAC O3

Review Panel will review each draft risk/exposure assessment and discuss their comments in two 

public meetings to be announced in the Federal Register. Based on CASAC’s past practice, staff

anticipates that key CASAC advice and recommendations for revision of the draft risk/exposure 

assessment will be presented in letters to the EPA Administrator. Staff will also consider 

comments received from CASAC and from the public at the meetings themselves and any 

written public comments. In finalizing the risk/exposure assessment, we will take into account 

any such comments and recommendations. After appropriate revision, the final risk/exposure 

assessment document will be made publicly available on an EPA website and in hard copies. A 

notice announcing the availability of the final document will be published in the Federal 

Register. In addition, the final risk/exposure assessment document will be placed in the 

rulemaking docket.
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6 VEGETATION AND OTHER WELFARE-RELATED 

ASSESSMENTS

6.1 OVERVIEW 

Assessments being considered for this new review of the secondary O3 NAAQS would 

focus on new information that has become available since the review completed in 2008. Keypolicy relevant findings from the ISA integrated with information from previous reviews will 

inform policy judgments in regard to the adequacy of the current indicators, averaging times, 

levels and forms of the O3 standard. New information and methods available in this review have 

the potential to improve characterization of O3 exposures and associated impacts, especially in 

non-urban areas, forests, and Class I protected lands. Recent information regarding direct O3

effects on plants, including emerging evidence that O3 alters the chemical signature and 

longevity of scents released by plants to attract pollinators, and the indirect impacts that can 

occur in associated ecological processes that can lead to ecosystem level effects and shifts in or 

loss of ecosystem services (e.g., carbon sequestration, water balance, pollination and/or 

biodiversity) will be considered and evaluated using qualitative and/or quantitative exposure, risk 

and benefits assessments, where feasible.

As in the last review, information regarding the interaction between O3, local 

meteorological conditions, and climate will be reviewed, although we do not anticipate sufficient 

information being available for quantitative analyses of this complex relationship in this review. 

Ozone-related damage to certain manmade materials (e.g., elastomers, textile fibers, dyes, paints 

and pigments) will not be re-assessed, as the scientific literature contains very little new 

information to adequately quantify these effects.

A more detailed description of assessment methods and approaches being considered for 

the exposure, risk and benefits assessments will be provided in a subsequent Scope and Methods 

Plan. Preparation of this plan will coincide with the development of the first draft ISA to 

facilitate the integration of policy-relevant science. 

6.2 EXPOSURE, RISK, AND BENEFITS ASSESSMENTS FROM THE 

LAST REVIEW

The exposure, risk and benefits assessments conducted as part of the last review focused 

on O3-related impacts to sensitive vegetation and their associated ecosystems. The vegetation 

exposure assessment was performed using an interpolation approach that included information 

from ambient monitoring networks and results from air quality modeling. The vegetation risk 
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assessment included both tree and crop analyses. The tree risk analysis included three distinct 

lines of evidence: (1) observations of visible foliar injury in the field linked to monitored O3 air 

quality for the years 2001 – 2004; (2) estimates of seedling growth loss under then current and 

alternative O3 exposure conditions; and (3) simulated mature tree growth reductions using the 

TREGRO model to simulate the effect of meeting alternative air quality standards on the 

predicted annual growth of mature trees from three different species. The crop risk analysis 

included estimates of crop yields under current and alternative O3 exposure conditions. The 

associated change in economic benefits expected to accrue to the agriculture sector upon meeting 

the levels of various alternative standards were analyzed using an agricultural benefits model. 

Key elements and observations from these exposure and risk assessments are outlined in the 

following sections. 

6.2.1 Exposure Assessment

In many rural and remote areas where sensitive species of vegetation can occur, 

monitoring coverage remained limited. Thus, the Staff Paper concluded that it was necessary to 

use an interpolation method in order to better characterize O3 air quality over broad geographic 

areas and at the national scale. Based on the significant difference in monitor network density 

between the eastern and western U.S., the Staff Paper further concluded that it was appropriate to 

use separate interpolation techniques in these two regions: The Air Quality System (AQS; 

http://www.epa.gov/ttn/airs/airsaqs) and Clean Air Status and Trends Network (CASTNET; 

http://www.epa.gov/castnet/) monitoring data were solely used for the eastern interpolation, and 

in the western U.S., where rural monitoring is more sparse, O3 outputs from the EPA/NOAA 

Community Multi-scale Air Quality (CMAQ)13

 

13 The CMAQ model is a multi-pollutant, multi-scale air quality model that contains state-of-the-science techniques 

for simulating all atmospheric and land processes that affect the transport, transformation, and deposition of 

atmospheric pollutants and/or their precursors on both regional and urban scales. It is designed as a science-based

modeling tool for handling many major pollutants (including photochemical oxidants/O3, particulate matter, and 

nutrient deposition) holistically. The CMAQ model can generate estimates of hourly O3 concentrations for the 

contiguous U.S., making it possible to express model outputs in terms of a variety of exposure indices (e.g., W126, 

8-hr average).

model system 

(http://www.epa.gov/asmdnerl/CMAQ, Byun and Ching, 1999) were used to develop scaling 

factors to augment the monitor interpolation. In order to characterize uncertainty associated with 

the exposure estimates generated using the interpolation method, monitored O3 concentrations 

were systematically compared to interpolated O3 concentrations in areas where monitors were 

located. In general, the interpolation method performed well in many areas in the U.S. This 

approach was used to develop a national vegetation O3 exposure surface. 
6-3

To evaluate changing vegetation exposures under selected air quality scenarios, a number 

of analyses were conducted. One analysis adjusted 2001 base year O3 air quality distributions 

using a rollback method (Horst and Duff, 1995; Rizzo, 2005, 2006) to reflect meeting the current 

and alternative secondary standard options. For “just meet” and alternative 8-hr average 

standard scenarios, the associated maps of estimated 12-hr, W126 exposures were generated. 

Based on these comparisons, the following observations were drawn: (1) current O3 air quality 

levels could result in significant O3 exposures to vegetation in some areas; (2) overall 3-month 

12-hr W126 O3 levels were somewhat but not substantially improved under the “just meet” 

current scenario; (3) exposures generated for just meeting a 0.070 ppm, 4th-highest maximum 8-

hr average alternative standard (the lower end of the then proposed range for the primary O3

standard) showed substantially improved O3 air quality when compared to just meeting the 

current 0.08 ppm, 8-hr standard. 

A second analysis described in the Staff Paper was performed to evaluate the extent to 

which county-level O3 air quality measured in terms of various levels of the current 8- 

hr average form overlapped with that measured in terms of various levels of the 12- 

hr W126 cumulative, seasonal form.14

 

14 The Staff Paper presented this analysis using then recent (2002-2004) county-level O3 air quality data (using 3-

year average data as well as data from each individual year) from AQS sites and the subset of CASTNET sites 

having the highest O3 levels for the counties in which they are located.

 While these results also suggested that meeting a 

proposed 0.070 ppm, 8-hr secondary standard would provide substantially improved vegetation 

protection in some areas, the Staff Paper recognized that this analysis had several important 

limitations. In particular, the lack of monitoring in rural areas where sensitive vegetation and 

ecosystems are located, especially at higher elevation sites could have resulted in an inaccurate 

characterization of the degree of potential overlap at sites which have air quality patterns that can 

result in relatively low 8-hr averages while still experiencing relatively high cumulative 

exposures (72 FR 37892). Thus, the Staff Paper concluded that it is reasonable to anticipate that 

additional unmonitored rural high elevation areas with sensitive vegetation may not be 

adequately protected even with a lower level of the 8-hr form. The Staff Paper further indicated 

that it remained uncertain as to the extent to which air quality improvements designed to reduce 

8-hr O3 average concentrations would reduce O3 exposures measured by a seasonal, cumulative 

W126 index. The Staff Paper indicated this to be an important consideration because: (1) the 

biological database stresses the importance of cumulative, seasonal exposures in determining 

plant response; (2) plants have not been specifically tested for the importance of daily maximum 

8-hr O3 concentrations in relation to plant response; and (3) the effects of attainment of a 8-hr 
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standard in upwind urban areas on rural air quality distributions cannot be characterized with 

confidence due to the lack of monitoring data in rural and remote areas. 

6.2.2 Risk Assessment

The risk assessments in the last review reflected the availability of several additional 

lines of evidence that provided a basis for a more complete and coherent picture of the scope of 

O3-related vegetation risks, especially those faced by seedling, sapling and mature tree species 

growing in field settings, and indirectly, forested ecosystems. Specifically, new research 

available at the time reflected an increased emphasis on field-based exposure methods (e.g., free 

air exposure and ambient gradient), improved field survey biomonitoring techniques, and 

mechanistic tree process models. Highlights from the analyses that addressed visible foliar 

injury, seedling and mature tree biomass loss, and effects on crops are summarized below. 

With regard to visible foliar injury, the Staff Paper presented an assessment that 

combined recent U.S. Forest Service Forest Inventory and Analysis (FIA) biomonitoring site 

data with the county level air quality data for those counties containing the FIA biomonitoring 

sites. This assessment showed that incidence of visible foliar injury ranged from 21 to 39 

percent of the counties during the four-year period (2001-2004) across all counties with air 

quality levels at or below that of the then current 0.08 ppm 8-hr standard. Of the counties that 

met an 8-hr level of 0.07 ppm in those years, 11 to 30 percent of the counties still had incidence 

of visible foliar injury. 

 With respect to tree seedling biomass loss, concentration-response (C-R) functions 

developed from OTC data for biomass loss for available seedling tree species and information on 

tree growing regions derived from the U.S. Department of Agriculture's Atlas of United States 

Trees were combined with projections of air quality based on 2001 interpolated exposures, to 

produce estimated biomass loss for each individual seedling tree species. These analyses 

predicted that biomass loss could still occur in many tree species when O3 air quality was 

adjusted to meet the current 8-hr standard. Though this type of analysis was not new to this 

review, the context for understanding these results had changed due to recent field work at the 

AspenFACE site in Wisconsin on quaking aspen (Karnosky et al., 2005) and a gradient study 

performed in the New York City area (Gregg et al., 2003), which confirmed the detrimental 

effects of O3 exposure on tree growth in field studies without chambers and beyond the seedling 

stage (King et al., 2005). 

With respect to risk of mature tree growth reductions, a tree growth model (TREGRO) 

was used to evaluate the effect of changing O3 air quality scenarios from just meeting alternative 
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O3 standards on the growth of mature trees.15

With respect to risks of yield loss in agricultural crops and fruit and vegetable species, 

little new information was available beyond that of the previous review. However, limited 

information from a free air field based soybean study (SoyFACE) and information on then 

current cultivar sensitivities, led to the conclusion that C-R functions developed in OTCs under 

the National Crop Loss Assessment Network (NCLAN) program could still be usefully applied. 

The crop risk assessment, like the tree seedling assessment, combined NCLAN C-R information 

on commodity crops, fruits and vegetables, crop growing regions, and interpolated exposures 

during each crop growing season. The risk assessment estimated that just meeting the 0.08 ppm, 

8-hr standard would still allow O3–related yield loss to occur in some sensitive commodity crops 

and fruit and vegetable species growing at that time in the U.S. 

 The model was run for a single western species 

(ponderosa pine) and two eastern species (red maple and tulip poplar). Staff Paper analyses 

found that just meeting the current standard would likely continue to allow O3-related reductions 

in annual net biomass gain in these species. Though there was uncertainty associated with the 

above analyses, it was important to note that new evidence from experimental studies that go 

beyond the seedling growth stage continued to show decreased growth under elevated O3 (King 

et al., 2005); some mature trees such as red oak have shown an even greater sensitivity of 

photosynthesis to O3 than seedlings of the same species (Hanson et al., 1994); and the potential 

for cumulative “carry over” effects as well as compounding should be considered (Andersen, et 

al, 1997). 

6.2.3 Benefits Assessment

The Staff Paper also presented estimates of monetized benefits for crops associated with 

the then current and alternative standards. The Agriculture Simulation Model (AGSIM) (Taylor, 

1994; Taylor, 1993) was used to calculate annual average changes in total undiscounted 

economic surplus for commodity crops and fruits and vegetables when then current and 

alternative standard levels were met. Meeting the various alternative standards did show some 

significant benefits beyond the 0.08 ppm, 8-hr standard. However, the Staff Paper recognized 

that the modeled economic benefits from AGSIM had many associated uncertainties which 

limited the usefulness of these estimates.

 

15 TREGRO is a process-based, individual tree growth simulation model (Weinstein et al, 1991) that is linked with 

concurrent climate data to account for O3 and climate/meteorology interactions on tree growth. TREGRO has been 

used to evaluate the effects of a variety of O3 scenarios on several species of trees in different regions of the U.S. 

(Tingey et al., 2001; Weinstein et al., 1991; Retzlaff et al., 2000; Laurence et al., 1993; Laurence et al., 2001; 

Weinstein et al., 2005).
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6.3 AIR QUALITY CONSIDERATIONS 

Air quality analyses are necessary to inform and support welfare-related exposure, risk, 

and benefits assessments. The air quality analyses being considered for this review would build 

upon those of the ISA and would include consideration of: (1) summaries of recent ambient air 

quality data, (2) estimation approaches to extrapolate air quality values for rural areas without 

monitors as well as federally designated Class I natural areas important to welfare effects 

assessment, (3) estimates of PRB concentrations, (4) air quality simulation procedures that 

modify recent air quality data to reflect changes in the distribution of air quality estimated to 

occur at some unspecified time in the future when an area just meets a given set of NAAQS. In 

this review, air quality analyses would support quantitative exposure and risk assessments that

may be considered in light of the new scientific information available for specific locations, as 

well as at regional and national scales. 

In addition to updating air quality summaries since the last review, these air quality 

analyses may include summaries of the most currently available ambient measurements for the 

current 8-hr average standard form, the cumulative concentration-weighted W126 form, and 

comparisons of these two types of forms. Such air quality analyses would use monitor data from 

the AQS data base (which includes National Park Service monitors) and the CASTNET network. 

In addition, staff may explore the suitability of using other sources of O3 concentration 

information that might be available, such as from portable monitors or satellites.

In the last review, the vegetation exposure analysis used a spatial interpolation technique 

to create an interpolated air quality surface to fill in the gaps in ambient monitoring data, 

especially those left by a sparse rural monitoring network in the western United States. In this 

review, additional approaches that potentially could be used to fill in the gaps in the rural 

monitoring network, as well as opportunities for enhancing the fusion of monitoring and 

modeled O3 data, may be explored. 

Estimates of the risks in excess of PRB remaining upon meeting the current or potential 

alternative standards require EPA to estimate PRB. As noted above in sections 4.2 and 5.3, EPA 

will be evaluating alternative definitions of PRB and the implications of those definitions for 

estimates of risk. This type of risk estimate is considered relevant to inform the EPA 

Administrator’s decision on the adequacy of a given standard. The current approach to 

estimating O3 PRB for use in conducting the welfare risk assessment is the same as that outlined 

in section 5.3 above. 

As part of the air quality analyses that would support any exposure, risk and benefits 

assessments that may be conducted, it would be necessary to adjust recent O3 air quality data to 
6-7

simulate just meeting the current standard and any alternative O3 standards. In the last review, 

EPA used a quadratic air quality rollback approach (U.S. EPA, 2007a, section 4.5.8). Staff may

consider alternative air quality simulation procedures for use in this current review as previously 

characterized in section 5.3. 

6.4 EXPOSURE ASSESSMENT APPROACH 

Since the last review, little has changed in terms of the extent of monitoring coverage in 

non-urban areas. We are planning to consider both past and alternative approaches for 

generating estimates of national O3 exposures in an effort to continue enhancing our ability to 

characterize exposures in these non-monitored areas. It is expected that any vegetation exposure 

assessments that may be conducted will again include assessments of recent air quality, air 

quality associated with just meeting the current standard and any alternative standards that might 

be considered. 

In addition, given the importance of providing protection for sensitive vegetation in areas 

afforded special protections, such as in federally designated Class I natural areas, we may also 

consider alternative sources of O3 exposure information for those types of sites. For example, 

portable O3 monitors are being deployed in some national parks and a current exploratory study 

is underway to measure O3 concentration variations with gradients in elevation. Information 

from these monitors could potentially inform our understanding of uncertainties associated with 

assessing O3 distribution patterns in complex terrain and high elevations. 

6.5 RISK ASSESSMENT APPROACH 

Since the last review, new scientific information on the direct and indirect effects of O3 on 

vegetation and ecosystems, respectively, has become available. With respect to mature trees and 

forests, the information regarding O3 impacts to forest ecosystems has continued to expand, 

including limited new evidence that implicates O3 as an indirect contributor to decreases in 

stream flow through direct impacts on whole tree level water use. Long-term FACE (Free Air 

CO2 enrichment) studies are continuing to provide additional evidence regarding chronic O3

exposures in closed forest canopy scenarios including interspecies interactions such as decreased 

growth of branches and root mass in sensitive species. Also, lichen and moss communities on 

trees monitored in FACE sites have been shown to undergo species shifts when exposed to O3. 

In addition, it is expected that as in the previous review, recent available data from annual field 

surveys conducted by the USFS to assess foliar damage to selected tree species will again be 

available. In light of this new scientific information, we will consider whether additional 

analyses are warranted, such as combining the USFS data with recent county level air quality 
6-8

data to determine the incidence of visible O3 damage occurring across the U.S. at air quality 

levels that meet or are below the current standard, as was done in the last review. To the extent 

warranted, based on new information regarding O3 effects on forest trees, both qualitative and 

quantitative assessments may be considered in an effort to place both the estimates of risk from 

more recent long-term studies and historic shorter-term studies in the context of Essential 

Services. 

Additional information relevant to both tree and crop risk assessments expected to be 

available includes that regarding the interactions between elevated O3 and CO2 with respect to 

plant growth and how these interactions might be expected to be modified under different 

climatic conditions, and potential reactions of O3 with chemicals released by plants to attract 

pollinators that could decrease the distance the floral “scent trail” travels and potentially 

changing the distance pollinators have to travel to find flowers. To the extent warranted, staff 

also plans to consider any available information regarding potential risks to threatened or 

endangered species. 

6.6 BENEFIT ASSESSMENT APPROACH

 To the extent warranted, qualitative and/or quantitative benefits assessments of ecosystem 

services impacted by O3 will be considered to inform the current review. For example, benefits 

assessments in this review may include tree biomass and crop analyses, and when possible, may 

include impacts on ecosystem services such as impacts on biodiversity, biological community 

composition, health of forest ecosystems, aesthetic values of trees and plants and the nutritive 

quality of forage crops. The impact of O3 on limiting potential CO2 sequestration is another 

important ecosystem services. New preliminary evidence of O3 effects on the ability of 

pollinators to find their target is also of special interest with respect to the possible implication 

for benefits assessment of ecosystem service. Impairment of the ability of pollinators to locate 

flowers could have broad implications for agriculture, horticulture and forestry. 

A new benefits model, the Forest and Agricultural Sector Optimization Model (FASOM), 

is being considered for this assessment. This model jointly assesses the economic impacts of O3

damage to forests and agricultural crops. FASOM is a dynamic, non-linear programming model 

designed for use by the EPA to evaluate welfare and market effects of carbon sequestration in 

trees, understory, forest floor, wood products and landfills that would occur under different 

agricultural and forestry scenarios. It may be possible to use FASOM to model damage by O3 to 

the agriculture and forestry sectors and quantify how O3-exposed vegetation impacts the 

ecosystem service of carbon sequestration. 
6-9

 A conclusion in the last review was that the science continued to support the use of an 

exposure index that reflects the effects of cumulative, seasonal O3 exposures on plants. In light 

of new information on exposures, risks, non-plant effects, and ecosystem services, we will 

consider whether additional analyses are warranted to evaluate the relative risks associated with 

alternative cumulative, seasonal forms. In addition, we plan to consider the impact of using 

different length diurnal windows (e.g., 12, 16 or 24 hrs), different seasonal periods (e.g., 3, 5, or 

7 months), and annual vs. three-year averages.

6.7 UNCERTAINTY AND VARIABILITY 

For exposure, risk and benefits assessments that are being considered for this review, staff is 

considering a similar approach to that used in the previous review to characterize uncertainty and 

variability associated with these assessments. In addition, we are considering the feasibility of 

conducting additional analyses to better characterize the uncertainties and variability associated with 

these assessments.

Many of the sources of uncertainty and variability that were present in the last assessments are 

expected to remain in this review. In particular, uncertainties associated with the use of various 

models such as CMAQ and FASOM would be characterized and where possible, sensitivity analyses 

performed to test the impact of various assumptions imbedded in the models. The uncertainty 

associated with the monitor probe height is expected to remain due to lack of definitive information 

becoming available. Where information exists, staff plans to consider the impact of using different 

adjustment factors. As in the last review, every effort will be made to provide information on the 

uncertainties and variability associated with whatever exposure and risk assessments may be

conducted. Uncertainties associated with empirical evidence due to exposure or research methods 

would be described.

6.8 SCIENTIFIC AND PUBLIC REVIEW 

A Scope and Methods Plan for the vegetation and other welfare-related assessments will 

be submitted to CASAC for consultation and will be provided to the public for comment. The 

CASAC O3 Review Panel will discuss their comments on the Scope and Methods Plan in a 

public meeting that will be announced in the Federal Register. In conducting the welfare-related 

assessments, staff will take into account comments received from CASAC and from the public at 

the meeting itself and in any written comments. Staff plans to prepare two drafts of the 

vegetation and other welfare-related assessments for CASAC review and public comment. The 

CASAC O3 Review Panel will review each draft welfare-related assessment and discuss their 

comments in two public meetings to be announced in the Federal Register. Based on CASAC’s 

past practice, we anticipate that key CASAC advice and recommendations for revision of the 
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draft risk/exposure assessment will be presented in letters to the EPA Administrator. Staff will 

also consider comments received from CASAC or from the public at the meetings themselves 

and any written public comments. In finalizing the vegetation and welfare-related assessments, 

we will take into account any such comments and recommendations. After appropriate revision, 

the final welfare-related assessment document will be made publicly available on an EPA 

website and in hard copy. A notice announcing the availability of the final document will be 

published in the Federal Register. In addition, the final welfare-related assessment document 

will be placed in the rulemaking docket.
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7 POLICY ASSESSMENT/RULEMAKING 

7.1 POLICY ASSESSMENT 

The Policy Assessment (PA), like the previous OAQPS Staff Paper, is a document that 

provides a transparent OAQPS staff analysis and staff conclusions regarding the adequacy of the 

current standards and potential alternatives that are appropriate to consider prior to the issuance 

of proposed and final rules. The PA integrates and interprets the information from the ISA and 

REA(s) to frame policy options for consideration by the Administrator. The PA is also intended 

to facilitate CASAC’s advice to the Agency and recommendations to the Administrator on the 

adequacy of the existing standards or revisions that may be appropriate to consider, as provided 

for in the Clean Air Act. Staff conclusions will be based on the information contained in the 

ISA, and, as available, the REAs, and any additional staff evaluations and assessments discussed 

in the PA. In so doing, the discussion in the PA will be framed by consideration of a series of 

the policy-relevant questions drawn from those outlined in chapter 3, including the fundamental 

questions associated with the adequacy of the current standards and, as appropriate, 

consideration of alternative standards in terms of the specific elements of the standards: 

indicator, averaging time, level, and form. 

The PA will identify conceptual evidence-based and risk/exposure-based approaches for 

reaching public health and welfare policy judgments. It will discuss the implications of the 

science and quantitative assessments for the adequacy of the current standards, and for any 

alternative standards under consideration. The PA will also describe a broad range of policy 

options for standard setting, identifying the broadest range for which the staff identifies support 

within the available information. In so doing, the PA will describe the underlying interpretations 

of the scientific evidence and risk/exposure information that might support such alternative 

policy options that could be considered by the Administrator in making decisions for the O3

standards. 

In identifying a range of primary standard options for the Administrator to consider, it is 

recognized that the final decision will be largely a public health policy judgment. A final 

decision must draw upon scientific information and analyses about health effects and risks, as 

well as judgments about how to deal with the range of uncertainties that are inherent in the 

scientific evidence and analyses. Staff’s approach to informing these judgments is based on a 

recognition that the available health effects evidence generally reflects a continuum consisting of 

ambient levels 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 
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uncertain. This approach is consistent with the requirements of the NAAQS provisions of the 

Act and with how EPA and the courts have historically interpreted the Act. These provisions 

require the Administrator to establish primary standards that are requisite to protect public health 

and are neither more nor less stringent than necessary for this purpose. The provisions do not 

require that primary standards be set at a zero-risk level, but rather at a level that avoids 

unacceptable risks to public health, including the health of susceptible populations.16

In identifying a range of secondary standard options for the Administrator to consider, 

staff recognizes that the final decision will be largely a public policy judgment. A final decision 

must draw upon scientific evidence and analyses about effects on public welfare, as well as 

judgments about how to deal with the range of uncertainties that are inherent in the relevant 

information. This approach is consistent with the requirements of the NAAQS provisions of the 

Act and with how EPA and the courts have historically interpreted the Act. These provisions 

require the Administrator to establish secondary standards that are requisite to protect public 

welfare from any known or anticipated adverse effects associated with the presence of the 

pollutant in the ambient air. In so doing, the Administrator seeks to establish standards that are 

neither more nor less stringent than necessary for this purpose. The provisions do not require 

that secondary standards be set to eliminate all welfare effects, but rather at a level that protects 

public welfare from those effects that are judged to be adverse.

Staff will prepare at least one draft of the PA document for CASAC review and public 

comment. The draft PA document will be distributed to the CASAC O3 Review Panel for their 

consideration and provided to the public for review and comment. Review by the CASAC O3

Review Panel will be discussed at public meetings that will be announced in the Federal 

Register. Based on past practice by CASAC, EPA expects that key advice and recommendations 

for revision of the document would be summarized by the CASAC in a letter to the EPA 

Administrator. In revising the draft PA document, OAQPS will take into account any such 

recommendations, and also consider comments received, from CASAC and from the public, at 

the meeting itself, and any written comments received. The final document will be made 

available on an EPA website, with its public availability announced in the Federal Register. 

 

16 The susceptible populations identified in a NAAQS review may be comprised of low income or minority groups. 

Where low income/minority groups are among the susceptible populations, the rulemaking decision will be based on 

providing protection for these and other susceptible populations. To the extent that low income/minority groups are 

not among the susceptible populations, a decision based on providing protection of the susceptible populations 

would be expected to provide protection for the low income/minority groups (as well as any other less sensitive 

population groups).
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7.2 RULEMAKING

Following issuance of the final PA and EPA management consideration of staff analyses 

and conclusions presented therein, and taking into consideration CASAC advice and 

recommendations, the Agency will develop a notice of proposed rulemaking. The proposed 

rulemaking notice conveys the Administrator’s proposed conclusions regarding the adequacy of 

the current standards and any revision that may be appropriate. A draft notice of proposed 

rulemaking will be submitted to the Office of Management and Budget (OMB) for interagency 

review, in which OMB and other federal agencies are provided the opportunity for review and 

comment. After the completion of interagency review, EPA will publish the notice of proposed 

rulemaking in the Federal Register. Monitoring rule changes associated with review of the O3

standards, and drawing from considerations outlined in chapter 8 below, will be developed and 

proposed, as appropriate, in conjunction with this NAAQS rulemaking.

At the time of publication of the notice of proposed rulemaking, all materials on which the 

proposal is based are made available in the public docket for the rulemaking.17

 

17 The rulemaking docket for the current O3 review is identified as EPA-HQ-OAR-2008-0699. This docket has 

incorporated the ISA docket (EPA-HQ-ORD-2011-0050) by reference. Both dockets are publicly accessible at 

 Publication of 

the proposal notice is followed by a public comment period, generally lasting 60 to 90 days, 

during which the public is invited to submit comments on the proposal to the rulemaking docket. 

Taking into account comments received on the proposed rule, the Agency will then develop a 

notice of final rulemaking, which again undergoes OMB-coordinated interagency review prior to 

issuance by EPA of the final rule. In the notice of final rulemaking, and generally also through 

the use of an accompanying document, the Agency responds to all significant comments on the 

proposed rule. Publication of the final rule in the Federal Register completes the rulemaking 

process.

www.regulations.gov. 
8-1 

8 AMBIENT AIR MONITORING

8.1 OVERVIEW

The O3 monitoring network provides data to meet a wide variety of objectives. They 

include ensuring the public has access to clean air by comparing data to the NAAQS, providing 

the public with reports and forecasts of their exposure to O3 through the Air Quality Index, 

providing input to health and welfare studies utilized as part of the NAAQS review process, 

evaluating the performance of regional air quality models used in developing emission strategies, 

tracking trends in air pollution abatement control measures impact on improving air quality, and 

supporting research studies on atmospheric chemistry and transport or O3. 

To meet these multiple objectives, national O3 sites are deployed in a variety of locations 

to determine the following information: highest concentrations in an area, typical concentrations 

in areas of high population density, the impacts of significant sources or source categories on O3

precursors and formation processes, general background concentration levels, the extent of 

regional pollutant transport among populated areas, assessment impacts on visibility, vegetation 

damage, or other welfare-based effects. 

Federal rules that regulate ambient air monitoring programs are found in 40 CFR Parts 50, 

53 and 58. During the last review completed in 2008, EPA followed a complementary process in 

which changes to monitoring regulations that were required to support the revised NAAQS were 

proposed in a separate rulemaking.18

8.2 CURRENT O3 NETWORK STATUS

 During this review, EPA intends to include any monitoring 

rule changes as part of the NAAQS rule, potentially reducing the time necessary to institute 

monitoring changes that might be required by a decision to revise the NAAQS. 

Presently, states and local air quality management agencies operate minimum numbers of 

EPA-approved O3 monitors based on the population of each of their Metropolitan Statistical 

Area (MSA) and the most recently measured O3 levels for each area. Currently, there are 369 

MSAs in the U.S. subject to minimum O3 monitoring requirements. In these areas, a total of 

392 monitors are required to meet the minimum requirements. In actuality, 992 monitors were in 

operation during 2005 to 2007 representing these MSAs. This monitor count indicates the 

typical practice of operating more than the minimum required number of monitors to support the 

 

18 The proposed rule, Ambient Ozone Monitoring Regulations: Revisions to Network Design Requirements, was 

published on July 16, 2009 (74 FR 34525). 

 
8-2 

basic monitoring objectives described above. In addition, state and local agencies operated 55 

monitors during 2005 to 2007 in MSAs that were not required to have monitors. 

Many of these O3 monitors that were operated in excess of minimum requirements were 

sited to characterize the O3 concentrations in metropolitan areas and in downwind areas that were

potentially impacted by transport from MSAs. As noted in the current monitoring regulations 

described in Part 58, O3 minimum requirements do not account for the full breadth of additional 

factors that would be considered in designing a complete O3 monitoring program for an area. 

Some of these additional factors include geographic size, population density, complexity of 

terrain and meteorology, presence of nearby O3 monitoring sites operated by adjacent State 

programs, air pollution transport from neighboring areas, and measured air quality in comparison 

to all forms of the O3 NAAQS (i.e., 8-hr and 1-hr forms). States and EPA Regional 

Administrators work together to design and/or maintain the most appropriate O3 network to 

service the variety of data needs in an area. The results of these negotiations are documented in 

annual monitoring network plans that are made available for public inspection and then approved 

by the EPA Regional Administrator, and the O3 monitoring requirements in approved plans 

become the basis for state O3 monitoring requirements for the 1-year period following plan 

approval. 

Although there are currently no EPA requirements for O3 monitoring other than in or 

adjacent to MSAs , there are at present about 200 state-operated O3 monitors in counties that are 

not part of MSAs, and these monitors can be categorized in several ways. States commonly 

locate O3 monitors both upwind and downwind of major urban areas to evaluate the spatial 

gradient or extent of transported O3 pollution and the lag time typically associated with 

photochemical production. In some cases, these O3 monitors are located in non-urban or rural 

areas within MSAs or physically outside the MSA boundary if the expected location of 

maximum downwind O3 concentration is outside the MSA. 

As part of the Clean Air Status and Trends Network (CASTNET), the EPA operates 57 

O3 monitors, and the National Park Service (NPS) operates 23 monitors across the eastern and 

western U.S. The NPS also operates additional O3 monitors independent of CASTNET stations. 

CASTNET O3 monitors operate year-round and are primarily located in rural areas; siting 

criteria require distances of at least 40 kilometers from cities of greater than 50,000 population as 

well as other separation requirements from air pollution sources. 

Taking into account both state and EPA/NPS-operated non-urban O3 monitors, an 

analysis of the distribution of these monitors indicates a relatively uniform spatial density in the 

eastern one-third of the U.S. and in California, with significant gaps in coverage elsewhere 

across the country. Virtually all states east of the Mississippi River have at least two to four non-
8-3 

urban O3 monitors, while many large mid-western and western states have one or no non-urban 

monitors. 

Section 182(c)(1) of the Clean Air Act required EPA to promulgate rules requiring 

enhanced monitoring of O3, NO, and VOC in O3 nonattainment areas classified as serious, 

severe, or extreme. On February 12, 1993, EPA promulgated requirements for State and local 

monitoring agencies to establish Photochemical Assessment Monitoring Stations (PAMS) as part 

of their SIP monitoring networks in O3 nonattainment areas classified as serious, severe, or 

extreme. Design criteria for the PAMS network are based on locations relative to O3 precursor 

source areas and predominant wind directions associated with high O3 events. Specific 

monitoring objectives are associated with each location. The overall design supports the 

characterization of precursor emission sources within an area, transport of O3 and its precursors, 

and the photochemical processes related to O3 nonattainment. EPA reduced PAMS requirements 

as part of the October 17, 2006 rulemaking. Current requirements include site-specific 

measurements for speciated VOC, carbonyls, NOx, NOy, CO, O3, surface meteorology, and 

upper air meteorology. 

Unlike the ambient monitoring requirements for other criteria pollutants that mandate 

year-round monitoring, O3 monitoring is currently only required during the seasons of the year 

that are conducive to O3 formation. These seasons vary in length from place to place as the 

conditions that determine the likely O3 formation (i.e., seasonally-dependent factors such as 

ambient temperature, strength of solar insolation, and length of day) differ by location. In some 

locations, conditions conducive to O3 formation are limited to a few summer months of the year. 

For example, in states with colder climates such as Montana and South Dakota, the currently 

required O3 monitoring season has a length of 4 months. However, in other states with warmer 

climates such as California, Nevada, and Arizona, the currently required O3 monitoring season 

for most sites continues all 12 months of the year. 

8.3 MONITORING ISSUES RELATED TO THE O3 NAAQS

This new review of the O3 NAAQS will explore a number of policy-relevant issues 

associated with measuring and characterizing O3 levels in ambient air. The EPA will draw upon 

the information presented in the ISA to inform the evaluation of appropriate ambient monitoring 

methods and network design for O3, including consideration of the available information on 

probe and siting criteria that could best support the current or alternative standards. 
8-4 

8.3.1 Monitoring Methods

 The nation’s O3 monitoring data currently being reported to AQS are obtained 

exclusively with ultraviolet (UV) absorption spectrometry based methods. These methods are 

approved Federal Equivalent Methods (FEMs) per 40 CFR Part 53; a number of commercial 

manufacturers supply such FEM instruments for use in the national network. The use of the 

Federal Reference Method (FRM) in ambient monitoring (a chemiluminescence-based method) 

has become basically non-existent with the adoption of FEMs. States utilize calibration and 

quality assurance procedures that relate their own calibrators to a network of Standard Reference 

Photometers (SRPs) that are maintained and operated by EPA. 

 Previous reviews of the O3 NAAQS have considered the implications of interferences in 

the response of UV and chemiluminescence-based instruments due the effects of water vapor, 

VOC’s, aromatic compounds and their oxidation products, and other organic and inorganic 

compounds. 

Issues that will be considered in this review to inform the selection of monitoring 

methods are reflected in the following questions: 

 To what extent is new information available to judge the adequacy of the current 

methodologies that are approved by EPA for use in judging compliance with the O3

NAAQS and meeting other objectives? 

 Has new information become available that supports the need for alternative 

methodologies to supplement the currently approved FRM and FEM’s? 

 What other technologies (e.g., portable monitors, passive or personal sampling) might 

be appropriate to consider where methods do not have to be EPA-approved, such as 

in the support of ecosystem or epidemiologic studies? 

8.3.2 Network Design 

Monitoring sites must represent ambient air (i.e., that portion of the atmosphere, external 

to buildings, to which the general public has access). The minimum number of required 

monitors for O3 is stated in 40 CFR Part 58, Appendix D, Network Design Criteria for Ambient 

Air Quality Monitoring. The EPA negotiates with States to determine the total number of 

monitors needed to represent an area’s air quality. It should be noted that although monitors are 

often sited with the intention to represent an area of a certain geographic scale, in general, a 

monitor need not be representative of the ambient air quality across an area of any specific size 

to be eligible for comparison to most NAAQS. 

Network design issues that will be considered in this review are reflected in the following 

questions: 
8-5 

 Are revisions to urban O3 monitoring requirements necessary to improve 

characterization of O3 concentrations in metropolitan areas? If so, what specific 

changes are needed? 

 Are there situations where fewer monitors could be utilized in urban areas without 

increasing the uncertainty surrounding data analysis? If so, what criteria should be 

considered when monitors are evaluated for potential termination or relocation? 

 Are revisions to non-urban O3 monitoring requirements necessary to improve 

characterization of O3 concentrations outside of metropolitan areas? If so, what 

specific objectives should be considered in any proposed changes to these 

requirements?

 What new information is available to inform network design options and technologies 

that are utilized in the PAMS network? What specific changes, if any, should be 

considered in PAMS requirements? 

 O3 monitoring sites are typically located to meet very specific probe and monitor 

siting criteria described in 40 CFR Part 58, Appendix E (e.g., acceptable probe 

height). Are there situations where a different set of monitor placement criteria 

would be appropriate to consider depending on the specific objective being 

characterized? For example, would a different set of probe height criteria be 

appropriate for monitors deployed in ecosystems with O3-sensitive vegetation versus 

monitors deployed in cities for NAAQS compliance objectives? What changes, if 

any, should be considered? 

 Is the length of the currently required O3 monitoring seasons adequate to characterize 

concentrations in urban and non-urban areas? What changes, if any, should be 

considered? 

8.3.3 Data Reporting and Assessments

The data interpretation of the primary and secondary NAAQS appendix describes the 

computations necessary for determining when the primary and secondary standards are met. The 

appendix addresses in detail, data completeness requirements, data reporting and handling 

conventions, the form of the standard, averaging times, and provides examples. As part of this

review, the data interpretation appendix may need further revisions to ensure that EPA is 

providing the best protection of public health and welfare. This review will provide the 

opportunity to take advantage of the insights and newer concepts that have arisen in the recent 

review of other NAAQS pollutants.
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http://www.epa.gov/ttn/naaqs/standards/nox/data/20081121_NO2_REA_final.pdf . 

U.S. Environmental Protection Agency (2009a). Integrated Science Assessment for 

Particulate Matter (Second External Review Draft). Research Triangle Park, NC: 

Office of Research and Development; report nos. EPA/600/R-08/139B and 

EPA/600/R-08/139BA. Available electronically on the internet at: 

http://cfint.rtpnc.epa.gov/ncea/prod/recordisplay.cfm?deid=210586 . 

U.S. Environmental Protection Agency (2009b). Risk and Exposure Assessment to 

Support the Review of the SO2 Primary National Ambient Air Quality Standard. 

EPA- EPA-452/R-09-007. Available electronically on the internet at:

http://www.epa.gov/ttn/naaqs/standards/so2/data/200908SO2REAFinalReport.pdf. 
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U.S. Environmental Protection Agency (2009c). Provisional Assessment of Recent 

Studies on Health and Ecological Effects of Ozone Exposure. EPA/600/R-09/101. 

Available electronically at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=214003. 

Weinstein, D.A.; Beloin, R.M.; Yanai, R.D. (1991). Modeling changes in red spruce 

carbon balance and allocation in response to interacting ozone and nutrient stresses. 

Tree Phys. 9:127-146. 

Weinstein, D.A.; Laurence, J.A.; Retzlaff, W.A.; Kern J.S.; Lee, E.H.; Hogsett, W.E.; 

Weber, J. (2005). Predicting the effects of tropospheric ozone on regional 

productivity of ponderosa pine and white fir. Forest Ecol Manag. 205:73-89. 

WHO (2008). Harmonization Project Document No. 6. Part 1: Guidance Document on 

Characterizing and Communicating Uncertainty in Exposure Assessment. 

Available electronically on the internet at: 

http://www.who.int/ipcs/methods/harmonization/areas/exposure/en /.
United States

Environmental Protection 

Agency 

Office of Air Quality Planning and Standards

Health & Environmental Impacts Division

Research Triangle Park, NC 

Publication No. EPA-452/R-11-006 

April 2011