16436 Federal Register / Vol. 73, No. 60 / Thursday, March 27, 2008 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50 and 58 

[EPA–HQ–OAR–2005–0172; FRL–8544–3] 

RIN 2060–AN24 

National Ambient Air Quality 

Standards for Ozone 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on its review of the air 

quality criteria for ozone (O3) and 

related photochemical oxidants and 

national ambient air quality standards 

(NAAQS) for O3, EPA is making 

revisions to the primary and secondary 

NAAQS for O3 to provide requisite 

protection of public health and welfare, 

respectively. With regard to the primary 

standard for O3, EPA is revising the 

level of the 8-hour standard to 0.075 

parts per million (ppm), expressed to 

three decimal places. With regard to the 

secondary standard for O3, EPA is 

revising the current 8-hour standard by 

making it identical to the revised 

primary standard. EPA is also making 

conforming changes to the Air Quality 

Index (AQI) for O3, setting an AQI value 

of 100 equal to 0.075 ppm, 8-hour 

average, and making proportional 

changes to the AQI values of 50, 150 

and 200. 

DATES: This final rule is effective on 

May 27, 2008. 

ADDRESSES: EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2005–0172. All 

documents in the docket are listed on 

the www.regulations.gov Web site. 

Although listed in the index, some 

information is not publicly available, 

e.g., confidential business information 

or other information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

is not placed on the Internet and will be 

publicly available only in hard copy 

form. Publicly available docket 

materials are available either 

electronically through 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., 

NW., Washington, DC. This Docket 

Facility is open from 8:30 a.m. to 4:30 

p.m., Monday through Friday, excluding 

legal holidays. The Docket telephone 

number is 202–566–1742. The 

telephone number for the Public 

Reading Room is 202–566–1744. 

FOR FURTHER INFORMATION CONTACT: Dr. 

David J. McKee, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail Code C504–06, Research Triangle 

Park, NC 27711; telephone: 919–541– 

5288; fax: 919–541–0237; e-mail: 

mckee.dave@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in this 

preamble: 

I. Background 

A. Summary of Revisions to the O3 NAAQS 

B. Legislative Requirements 

C. Review of Air Quality Criteria and 

Standards for O3

D. Summary of Proposed Revisions to the 

O3 NAAQS 

E. Organization and Approach to Final 

Decision on O3 NAAQS 

II. Rationale for Final Decision on the 

Primary O3 Standard 

A. Introduction 

1. Overview 

2. Overview of Health Effects 

3. Overview of Human Exposure and 

Health Risk Assessments 

B. Need for Revision of the Current 

Primary O3 Standard 

1. Introduction 

2. Comments on the Need for Revision 

3. Conclusions Regarding the Need for 

Revision 

C. Conclusions on the Elements of the 

Primary O3 Standard 

1. Indicator 

2. Averaging Time 

3. Form 

4. Level 

D. Final Decision on the Primary O3

Standard 

III. Communication of Public Health 

Information 

IV. Rationale for Final Decision on the 

Secondary O3 Standard 

A. Introduction 

1. Overview 

2. Overview of Vegetation Effects Evidence 

3. Overview of Biologically Relevant 

Exposure Indices 

4. Overview of Vegetation Exposure and 

Risk Assessments 

B. Need for Revision of the Current 

Secondary O3 Standard 

1. Introduction 

2. Comments on the Need for Revision 

3. Conclusions Regarding the Need for 

Revision 

C. Conclusions on the Secondary O3

Standard 

1. Staff Paper Evaluation 

2. CASAC Views 

3. Administrator’s Proposed Conclusions 

4. Comments on the Secondary Standard 

Options 

5. Administrator’s Final Conclusions 

D. Final Decision on the Secondary O3

Standard 

V. Creation of Appendix P—Interpretation of 

the NAAQS for O3

A. General 

B. Data Completeness 

C. Data Reporting and Handling and 

Rounding Conventions 

VI. Ambient Monitoring Related to Revised 

O3 Standards 

VII. Implementation and Related Control 

Requirements 

A. Future Implementation Steps 

1. Designations 

2. State Implementation Plans 

3. Trans-boundary Emissions 

4. Monitoring Requirements 

B. Related Control Requirements 

VIII. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

to Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Congressional Review Act 

References 

I. Background 

A. Summary of Revisions to the O3

NAAQS 

Based on its review of the air quality 

criteria for O3 and related 

photochemical oxidants and national 

ambient air quality standards (NAAQS) 

for O3, EPA is making revisions to the 

primary and secondary NAAQS for O3

to provide protection of public health 

and welfare, respectively, that is 

appropriate under section 109, and is 

making corresponding revisions in data 

handling conventions for O3. 

With regard to the primary standard 

for O3, EPA is revising the level of the 

8-hour standard to a level of 0.075 parts 

per million (ppm), to provide increased 

protection for children and other ‘‘at 

risk’’ populations against an array of O3- 

related adverse health effects that range 

from decreased lung function and 

increased respiratory symptoms to 

serious indicators of respiratory 

morbidity including emergency 

department visits and hospital 

admissions for respiratory causes, and 

possibly cardiovascular-related 

morbidity as well as total nonaccidental 

and cardiorespiratory mortality. EPA is 

specifying the level of the primary 

standard to the nearest thousandth ppm. 

With regard to the secondary standard 

for O3, EPA is revising the standard by 

making it identical to the revised 

primary standard. 


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1The 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)]. 

2Welfare 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, manmade 

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 wellbeing.’’

3 In considering whether the CAA allowed for 

economic considerations to play a role in the 

promulgation of the NAAQS, the Supreme Court 

rejected arguments that because many more factors 

than air pollution might affect public health, EPA 

should consider compliance costs that produce 

health losses in setting the NAAQS. 531 U.S. at 466. 

Thus, EPA may not take into account possible 

public health impacts from the economic cost of 

implementation. Id. 

B. Legislative Requirements 

Two sections of the Clean Air Act 

(CAA) govern the establishment and 

revision of the NAAQS. Section 108 (42 

U.S.C. 7408) directs the Administrator 

to identify and list ‘‘air pollutants’’

emissions of which ‘‘in his judgment, 

cause or contribute to air pollution 

which may reasonably be anticipated to 

endanger public health or welfare,’’

whose ‘‘presence * * * in the ambient 

air results from numerous or diverse 

mobile or stationary sources,’’ and for 

which the Administrator plans to issue 

air quality criteria, and to issue air 

quality criteria for those that are listed. 

Air quality criteria are to ‘‘accurately 

reflect the latest scientific knowledge 

useful in indicating the kind and extent 

of identifiable effects on public health 

or welfare which may be expected from 

the presence of [a] pollutant in ambient 

air, in varying quantities * * *.’’

Section 109 (42 U.S.C. 7409) directs the 

Administrator to propose and 

promulgate ‘‘primary’’ and ‘‘secondary’’

NAAQS for pollutants listed under 

section 108. 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.’’ 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 

requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

[the] pollutant in the ambient air.’’ 2

The requirement that primary 

standards provide an adequate margin 

of safety was intended to address 

uncertainties associated with 

inconclusive scientific and technical 

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. Lead Industries 

Association v. EPA, 647 F.2d 1130, 1154 

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

1042 (1980); American Petroleum 

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

(DC Cir. 1981), cert. denied, 455 U.S. 

1034 (1982). Both kinds of uncertainties 

are components of the risk associated 

with pollution at levels below those at 

which human health effects can be said 

to occur with reasonable scientific 

certainty. Thus, in selecting primary 

standards that provide 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 Association 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. 

The selection of any particular 

approach to providing an adequate 

margin of safety is a policy choice left 

specifically to the Administrator’s 

judgment. Lead Industries Association 

v. EPA, 647 F.2d at 1161–62. In 

addressing the requirement for an 

adequate margin of safety, EPA 

considers such factors as the nature and 

severity of the health effects involved, 

the size of the population(s) at risk, and 

the kind and degree of the uncertainties 

that must be addressed. 

In setting standards that are 

‘‘requisite’’ to protect public health and 

welfare, as provided in section 109(b), 

EPA’s task is to establish standards that 

are neither more nor less stringent than 

necessary for these purposes. Whitman 

v. America Trucking Associations, 531 

U.S. 457, 473. Further the Supreme 

Court ruled that ‘‘[t]he text of § 109(b), 

interpreted in its statutory and historical 

context and with appreciation for its 

importance to the CAA as a whole, 

unambiguously bars cost considerations 

from the NAAQS–setting process 

***’’ Id. at 472.3

Section 109(d)(1) of the CAA 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 in accordance with 

section 108 and [109(b)].’’ 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 revisions 

of existing criteria and standards as may 

be appropriate under section 108 and 

[section 109(b)].’’ This independent 

review function is performed by the 

Clean Air Scientific Advisory 

Committee (CASAC) of EPA’s Science 

Advisory Board. 

C. Review of Air Quality Criteria and 

Standards for O3

Ground-level O3 is 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, specifically NOX and 

volatile organic compounds (VOC), 

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. 

The last review of the O3 NAAQS was 

completed on July 18, 1997, based on 

the 1996 O3 Air Quality Criteria 

Document (EPA, 1996a) and 1996 O3

Staff Paper (EPA, 1996b). EPA revised 

the primary and secondary O3 standards 

on the basis of the then latest scientific 

evidence linking exposures to ambient 

O3 to adverse health and welfare effects 

at levels allowed by the 1-hour average 

standards (62 FR 38856). The O3

standards were revised by replacing the 

existing primary 1-hour average 

standard with an 8-hour average O3

standard set at a level of 0.08 ppm, 

which is equivalent to 0.084 ppm using 

the standard rounding conventions. The 

form of the primary standard was 

changed to the annual fourth-highest 

daily maximum 8-hour average 

concentration, averaged over 3 years. 

The secondary O3 standard was changed 

by making it identical in all respects to 

the revised primary standard. 

EPA initiated this current review in 

September 2000 with a call for 

information (65 FR 57810) for the 

development of a revised Air Quality 


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4The CASAC O3 Review Panel includes the seven 

members of the chartered CASAC, supplemented by 

fifteen subject-matter experts appointed by the 

Administrator to provide additional scientific 

expertise relevant to this review of the O3 NAAQS. 

5EPA made available corrected versions of the 

final Staff Paper (EPA, 2007b, henceforth, ‘‘Staff 

Paper’’) and the human exposure and health risk 

assessment technical support documents on July 31, 

2007 on the EPA Web site http://www.epa.gov/ttn/ 

naaqs. 

6American Lung Association v. Whitman (No. 

1:03CV00778, D.D.C. 2003). 

7For ease of reference, these studies will be 

referred to as ‘‘new’’ studies or ‘‘new’’ science, 

using quotation marks around the word new. 

Referring to studies that were published too 

recently to have been included in the 2004 Criteria 

Document as ‘‘new’’ studies is intended to clearly 

differentiate such studies from those that have been 

published since the last review and are included in 

the 2004 Criteria Document (these studies are 

sometimes referred to as new (without quotation 

marks) or more recent studies, to indicate that they 

were not included in the 1996 Criteria Document 

and thus are newly available in this review. 

Criteria Document for O3 and Other 

Photochemical Oxidants (henceforth the 

‘‘Criteria Document’’). A project work 

plan (EPA, 2002) for the preparation of 

the Criteria Document was released in 

November 2002 for CASAC O3 Panel 4

(henceforth, ‘‘CASAC Panel’’) and 

public review. EPA held a series of 

workshops in mid-2003 on several draft 

chapters of the Criteria Document to 

obtain broad input from the relevant 

scientific communities. These 

workshops helped to inform the 

preparation of the first draft Criteria 

Document (EPA, 2005a), which was 

released for CASAC Panel and public 

review on January 31, 2005; a CASAC 

Panel meeting was held on May 4–5, 

2005 to review the first draft Criteria 

Document. A second draft Criteria 

Document (EPA, 2005b) was released for 

CASAC Panel and public review on 

August 31, 2005, and was discussed 

along with a first draft Staff Paper (EPA, 

2005c) at a CASAC Panel meeting held 

on December 6–8, 2005. In a February 

16, 2006 letter to the Administrator, the 

CASAC Panel offered final comments on 

all chapters of the Criteria Document 

(Henderson, 2006a), and the final 

Criteria Document (EPA, 2006a) was 

released on March 21, 2006. In a June 

8, 2006 letter (Henderson, 2006b) to the 

Administrator, the CASAC Panel offered 

additional advice to the Agency 

concerning chapter 8 of the final Criteria 

Document (Integrative Synthesis) to 

help inform the second draft Staff Paper. 

A second draft Staff Paper (EPA, 

2006b) was released on July 17, 2006 

and reviewed by the CASAC Panel on 

August 24 and 25, 2006. In an October 

24, 2006 letter to the Administrator, 

CASAC Panel provided advice and 

recommendations to the Agency 

concerning the second draft Staff Paper 

(Henderson, 2006c). A final Staff Paper 

(EPA, 2007a) was released on January 

31, 2007. Around the time of the release 

of the final Staff Paper in January 2007, 

EPA discovered a small error in the 

exposure model that when corrected 

resulted in slight increases in the 

human exposure estimates. Since the 

exposure estimates are an input to the 

lung function portion of the health risk 

assessment, this correction also resulted 

in slight increases in the lung function 

risk estimates as well. The exposure and 

risk estimates discussed in this final 

rule reflect the corrected estimates, and 

thus are slightly different than the 

exposure and risk estimates cited in the 

January 31, 2007 Staff Paper.5 In a 

March 26, 2007 letter (Henderson, 

2007), the CASAC Panel offered 

additional advice to the Administrator 

with regard to recommendations and 

revisions to the primary and secondary 

O3 NAAQS. 

The schedule for completion of this 

review has been governed by a consent 

decree resolving a lawsuit filed in 

March 2003 by a group of plaintiffs 

representing national environmental 

and public health organizations, 

alleging that EPA had failed to complete 

the current review within the period 

provided by statute.6 The modified 

consent decree that currently governs 

this review provides that EPA sign for 

publication notices of proposed and 

final rulemaking concerning its review 

of the O3 NAAQS no later than June 20, 

2007 and March 12, 2008, respectively. 

The proposed decision (henceforth 

‘‘proposal’’) was signed on June 20, 

2007 and published in the Federal 

Register on July 11, 2007. 

A large number of comments were 

received from various commenters on 

the proposed revisions to the O3

NAAQS. Significant issues raised in the 

public comments are discussed 

throughout the preamble of this final 

action. A comprehensive summary of all 

significant comments, along with EPA’s 

responses (henceforth ‘‘Response to 

Comments’’), can be found in the docket 

for this rulemaking. 

Various commenters have referred to 

and discussed a number of new 

scientific studies on the health effects of 

O3 that had been published recently and 

therefore were not included in the 

Criteria Document (EPA, 2006a, 

henceforth ‘‘Criteria Document).7 EPA 

has provisionally considered any 

significant ‘‘new’’ studies, including 

those submitted during the public 

comment period. The purpose of this 

effort was to ensure that the 

Administrator was fully aware of the 

‘‘new’’ science before making a final 

decision on whether to revise the 

current O3 NAAQS. EPA provisionally 

considered these studies to place their 

results in the context of the findings of 

the Criteria Document. 

As in prior NAAQS reviews, EPA is 

basing its decision in this review on 

studies and related information 

included in the Criteria Document and 

Staff Paper, which have undergone 

CASAC and public review. The studies 

assessed in the Criteria Document, and 

the integration of the scientific evidence 

presented in that document, have 

undergone extensive critical review by 

EPA, CASAC, and the public during the 

development of the Criteria Document. 

The rigor of that review makes these 

studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 

NAAQS decisions can have profound 

impacts on public health and welfare, 

and NAAQS decisions should be based 

on studies that have been rigorously 

assessed in an integrative manner not 

only by EPA but also by the statutorily 

mandated independent advisory 

committee, as well as the public review 

that accompanies this process. As 

described above, EPA’s provisional 

consideration of these studies did not 

and could not provide that kind of indepth critical review. 

This decision is consistent with EPA’s 

practice in prior NAAQS reviews. Since 

the 1970 amendments, the EPA has 

taken the view that NAAQS decisions 

are to be based on scientific studies and 

related information that have been 

assessed as a part of the pertinent air 

quality criteria, and has consistently 

followed this approach. See 71 FR 

61144, 61148 (October 17, 2006) (final 

decision on review of PM NAAQS) for 

a detailed discussion of this issue and 

EPA’s past practice. 

As discussed in EPA’s 1993 decision 

not to revise the NAAQS for O3 ‘‘new’’

studies may sometimes be of such 

significance that it is appropriate to 

delay a decision on revision of a 

NAAQS and to supplement the 

pertinent air quality criteria so the 

studies can be taken into account (58 FR 

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

present case, EPA’s provisional 

consideration of ‘‘new’’ studies 

concludes that, taken in context, the 

‘‘new’’ information and findings do not 

materially change any of the broad 

scientific conclusions regarding the 

health effects of O3 exposure made in 

the Criteria Document. For this reason, 

reopening the air quality criteria review 

would not be warranted even if there 

were time to do so under the court order 


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Federal Register / Vol. 73, No. 60 / Thursday, March 27, 2008 / Rules and Regulations 16439 

8The word ‘‘evidence’’ is used in this notice to 

refer to studies that provide information relevant to 

an area of inquiry, which can include studies that 

report positive or negative results or that provide 

interpretative information. 

governing the schedule for this 

rulemaking. Accordingly, EPA is basing 

the final decisions in this review on the 

studies and related information 

included in the O3 air quality criteria 

that have undergone CASAC and public 

review. EPA will consider the newly 

published studies for purposes of 

decision making in the next periodic 

review of the O3 NAAQS, which will 

provide the opportunity to fully assess 

them through a more rigorous review 

process involving EPA, CASAC, and the 

public. Further discussion of these 

‘‘new’’ studies can be found in the 

Response to Comments document. 

This action presents the 

Administrator’s final decisions on the 

review of the current primary and 

secondary O3 standards. Throughout 

this preamble a number of conclusions, 

findings, and determinations made by 

the Administrator are noted. They 

identify the reasoning that supports this 

final decision and are intended to be 

final and conclusive. 

D. Summary of Proposed Revisions to 

the O3 NAAQS 

For reasons discussed in the proposal, 

the Administrator proposed to revise the 

current primary and secondary O3

standards. With regard to the primary 

O3 standard, the Administrator 

proposed to revise the level of the 8- 

hour O3 standard to a level within the 

range of 0.070 ppm to 0.075 ppm, based 

on a 3-year average of the fourth-highest 

maximum 8-hour average concentration. 

Related revisions for O3 data handling 

conventions and for the reference 

method for monitoring O3 were also 

proposed. These revisions were 

proposed to provide increased 

protection for children and other ‘‘at 

risk’’ populations against an array of O3- 

related adverse health effects that range 

from decreased lung function and 

increased respiratory symptoms to 

serious indicators of respiratory 

morbidity, including emergency 

department visits and hospital 

admissions for respiratory causes, and 

possibly cardiovascular-related 

morbidity, as well as total nonaccidental 

and cardiorespiratory mortality. EPA 

also proposed to specify the level of the 

primary standard to the nearest 

thousandth ppm. EPA solicited 

comment on alternative levels down to 

0.060 ppm and up to and including 

retaining the current 8-hour standard of 

0.08 ppm (effectively 0.084 ppm using 

current data rounding conventions). 

With regard to the secondary standard 

for O3, EPA proposed to revise the 

current 8-hour standard with one of two 

options to provide increased protection 

against O3-related adverse impacts on 

vegetation and forested ecosystems. One 

option was to replace the current 

standard with a cumulative, seasonal 

standard expressed as an index of the 

annual sum of weighted hourly 

concentrations, cumulated over 12 

hours per day (8 am to 8 pm) 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-hours. The other option 

was to make the secondary standard 

identical to the proposed primary 8- 

hour standard. EPA solicited comment 

on specifying a cumulative, seasonal 

standard in terms of a 3-year average of 

the annual sums of weighted hourly 

concentrations; on the range of 

alternative 8-hour standard levels for 

which comment was being solicited for 

the primary standard, including 

retaining the current secondary 

standard, which is identical to the 

current primary standard; and on an 

alternative approach to setting a 

cumulative, seasonal secondary 

standard. 

E. Organization and Approach to Final 

O3 NAAQS Decisions 

This action presents the 

Administrator’s final decisions 

regarding the need to revise the current 

primary and secondary O3 standards. 

Revisions to the primary standard for O3

are addressed below in section II, and a 

discussion on communication of public 

health information regarding revisions 

to the primary O3 standard is presented 

in section III. The secondary O3

standard is addressed below in section 

IV. Related data completeness and data 

handling and rounding conventions are 

addressed in section V, and federal 

reference methods for monitoring O3 are 

addressed below in section VI. Future 

implementation steps and related 

control requirements are discussed in 

section VII. A discussion of statutory 

and executive order reviews is provided 

in section VIII. 

Today’s final decisions are based on 

a thorough review in the Criteria 

Document of scientific information on 

known and potential human health and 

welfare effects associated with exposure 

to O3 at levels typically found in the 

ambient air. These final decisions also 

take into account: (1) Staff assessments 

in the Staff Paper of the most policyrelevant information in the Criteria 

Document as well as quantitative 

exposure and risk assessments based on 

that information; (2) CASAC Panel 

advice and recommendations, as 

reflected in its letters to the 

Administrator, its discussions of drafts 

of the Criteria Document and Staff Paper 

at public meetings, and separate written 

comments prepared by individual 

members of the CASAC Panel; (3) public 

comments received during the 

development of these documents, either 

in connection with CASAC Panel 

meetings or separately; and (4) extensive 

public comments received on the 

proposed rulemaking. 

II. Rationale for Final Decisions on the 

Primary O3 Standard 

A. Introduction 

1. Overview 

This section presents the 

Administrator’s final decisions 

regarding the need to revise the current 

primary O3 NAAQS, and the 

appropriate revision to the level of the 

8-hour standard. As discussed more 

fully below, the rationale for the final 

decision on appropriate revisions to the 

primary O3 NAAQS includes 

consideration of: (1) Evidence of health 

effects related to short-term exposures to 

O3; (2) insights gained from quantitative 

exposure and health risk assessments; 

(3) public and CASAC Panel comments 

received during the development and 

review of the Criteria Document, Staff 

Paper, exposure and risk assessments 

and on the proposal notice. 

In developing this rationale, EPA has 

drawn upon an integrative synthesis of 

the entire body of evidence 8 relevant to 

examining associations between 

exposure to ambient O3 and a broad 

range of health endpoints (EPA, 2006a, 

Chapter 8), focusing on those health 

endpoints for which the Criteria 

Document concluded that the 

associations are causal or likely to be 

causal. This body of evidence includes 

hundreds of studies conducted in many 

countries around the world. In its 

assessment of the evidence judged to be 

most relevant to decisions on elements 

of the primary O3 standards, EPA has 

placed greater weight on U.S. and 

Canadian studies, since studies 

conducted in other countries may well 

reflect different demographic and air 

pollution characteristics. 

As discussed below, a significant 

amount of new research has been 

conducted since the last review, with 

important new information coming from 

epidemiological, toxicological, 

controlled human exposure, and 

dosimetric studies. Moreover, the newly 

available research studies evaluated in 

the Criteria Document have undergone 

intensive scrutiny through multiple 

layers of peer review, with extended 


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16440 Federal Register / Vol. 73, No. 60 / Thursday, March 27, 2008 / Rules and Regulations 

opportunities for review and comment 

by CASAC Panel and the public. As 

with virtually any policy-relevant 

scientific research, there is uncertainty 

in the characterization of health effects 

attributable to exposure to ambient O3, 

most generally with regard to whether 

observed health effects and associations 

are causal or likely causal in nature and, 

if so, the certainty of causal associations 

at various exposure levels. While 

important uncertainties remain, the 

review of the health effects information 

has been extensive and deliberate. In the 

judgment of the Administrator, this 

intensive evaluation of the scientific 

evidence provides an adequate basis for 

regulatory decision making at this time. 

This review also provides important 

input to EPA’s research plan for 

improving our future understanding of 

the relationships between exposures to 

ambient O3 and health effects. 

The health effects information and 

quantitative exposure and health risk 

assessment were summarized in 

sections II.A and II.B of the proposal (72 

FR at 37824–37862) and are only briefly 

outlined below in sections II.A.2 and 

II.A.3. Subsequent sections of this 

preamble provide a more complete 

discussion of the Administrator’s 

rationale, in light of key issues raised in 

public comments, for concluding that 

the current standard is not requisite to 

protect public health with an adequate 

margin of safety, and it is appropriate to 

revise the current primary O3 standards 

to provide additional public health 

protection (section II.B), as well as a 

more complete discussion of the 

Administrator’s rationale for retaining 

or revising the specific elements of the 

primary O3 standards (section II.C), 

namely the indicator (section II.C.1); 

averaging time (section II.C.2); form 

(section II.C.3); and level (section II.C.4). 

A summary of the final decisions on 

revisions to the primary O3 standards is 

presented in section II.D. 

2. Overview of Health Effects 

This section outlines the information 

presented in Section II.A of the proposal 

on known or potential effects on public 

health which may be expected from the 

presence of O3 in ambient air. The 

decision in the last review focused 

primarily on evidence from short-term 

(e.g., 1 to 3 hours) and prolonged ( 6 to 

8 hours) controlled-exposure studies 

reporting lung function decrements, 

respiratory symptoms, and respiratory 

inflammation in humans, as well as 

epidemiology studies reporting excess 

hospital admissions and emergency 

department visits for respiratory causes. 

The Criteria Document prepared for this 

review emphasizes a large number of 

epidemiological studies published since 

the last review with these and 

additional health endpoints, including 

the effects of acute (short-term and 

prolonged) and chronic exposures to O3

on lung function decrements and 

enhanced respiratory symptoms in 

asthmatic individuals, school absences, 

and premature mortality. It also 

emphasizes important new information 

from toxicology, dosimetry, and 

controlled human exposure studies. 

Highlights of the evidence include: 

(1) Two new controlled humanexposure studies are now available that 

examine respiratory effects associated 

with prolonged O3 exposures at levels at 

and below 0.080 ppm, which was the 

lowest exposure level that had been 

examined in the last review. 

(2) Numerous recent controlled 

human-exposure studies have examined 

indicators of O3-induced inflammatory 

response in both the upper respiratory 

tract (URT) and lower respiratory tract 

(LRT), while other studies have 

examined changes in host defense 

capability following O3 exposure of 

healthy young adults and increased 

airway responsiveness to allergens in 

subjects with allergic asthma and 

allergic rhinitis exposed to O3. 

(3) New evidence from controlled 

human exposure studies showing that 

asthmatics have greater respiratoryrelated physiological responses than 

healthy subjects and new evidence from 

epidemiological studies showing 

associations between O3 exposure and 

lung function and respiratory symptom 

responses; these findings differ from the 

presumption in the last review that 

people with asthma had generally the 

same magnitude of respiratory 

responses to O3 as those experienced by 

healthy individuals. 

(4) Animal toxicology studies provide 

new information regarding potential 

mechanisms of action, increased 

susceptibility to respiratory infection, 

and biological plausibility of acute 

effects as well as chronic, irreversible 

respiratory damage observed in animals. 

(5) Numerous epidemiological studies 

published during the past decade offer 

added evidence of associations between 

acute ambient O3 exposures and lung 

function decrements and respiratory 

symptoms in physically active healthy 

subjects and asthmatic subjects, as well 

as new evidence regarding additional 

health endpoints, including 

relationships between ambient O3

concentrations and school absenteeism 

and between ambient O3 and cardiacrelated physiological endpoints. 

(6) Several additional studies have 

been published over the last decade 

examining the temporal associations 

between acute O3 exposures and both 

emergency department visits for 

respiratory diseases and respiratoryrelated hospital admissions. 

(7) A large number of newly available 

epidemiological studies have examined 

the effects of acute exposure to PM and 

O3 on premature mortality, notably 

including large multi-city studies that 

provide much more robust information 

than was available in the last review, as 

well as recent meta-analyses that have 

evaluated potential sources of 

heterogeneity in O3-mortality 

associations. 

Section II.A of the proposal provides 

a detailed summary of key information 

contained in the Criteria Document 

(chapters 4–8) and in the Staff Paper 

(chapter 3), on the known and potential 

effects of O3 exposure and information 

on the effects of O3 exposure in 

combination with other pollutants that 

are routinely present in the ambient air 

(72 FR 37824–37851). The information 

there summarizes: 

(1) New information available on 

potential mechanisms for morbidity and 

mortality effects associated with 

exposure to O3, including potential 

mechanisms or pathways related to 

direct effects on the respiratory system, 

systemic effects that are secondary to 

effects in the respiratory system (e.g., 

cardiovascular effects); 

(2) The nature of effects that have 

been associated directly with exposure 

to O3 or indirectly with the presence of 

O3 in ambient air, including premature 

mortality, aggravation of respiratory and 

cardiovascular disease (as indicated by 

increased hospital admissions and 

emergency department visits), changes 

in lung function and increased 

respiratory symptoms, as well as new 

evidence for more subtle indicators of 

cardiovascular health; 

(3) An integrative interpretation of the 

health effects evidence, focusing on the 

biological plausibility and coherence of 

the evidence and key issues raised in 

interpreting epidemiological studies, 

along with supporting evidence from 

experimental (e.g., dosimetric and 

toxicological) studies as well as the 

limitations of the evidence; and 

(4) Considerations in characterizing 

the public health impact of O3, 

including the identification of sensitive 

and vulnerable subpopulations that are 

potentially at risk to such effects, 

including active people, people with 

pre-existing lung and heart diseases, 

children and older adults, and people 

with increased responsiveness to O3. 


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9For informational purposes only, modeling that 

projects how areas might attain alternative 

standards in a future year as a result of Federal, 

State, local, and Tribal efforts is presented in the 

final Regulatory Impact Analysis being prepared in 

connection with this decision. 

3. Overview of Human Exposure and 

Health Risk Assessments 

To put judgments about health effects 

that are adverse for individuals into a 

broader public health context, EPA 

developed and applied models to 

estimate human exposures and health 

risks. This broader public health context 

included consideration of the size of 

particular population groups at risk for 

various effects, the likelihood that 

exposures of concern would occur for 

individuals in such groups under 

varying air quality scenarios, estimates 

of the number of people likely to 

experience O3-related effects, the 

variability in estimated exposures and 

risks, and the kind and degree of 

uncertainties inherent in assessing the 

exposures and risks involved. 

As discussed in more detail in section 

II.B of the proposal, there are a number 

of important uncertainties that affect the 

exposure and health risk estimates. It is 

also important to note that there have 

been significant improvements since the 

last review in both the exposure and 

health risk models. The CASAC Panel 

expressed the view that the exposure 

analysis represents a state-of-the-art 

modeling approach and that the health 

risk assessment was ‘‘well done, 

balanced and reasonably 

communicated’’ (Henderson, 2006c). 

In modeling exposures and health 

risks associated with just meeting the 

current and alternative O3 standards, 

EPA simulated air quality just meeting 

these standards based on O3 air quality 

patterns in several recent years and on 

how the shape of the O3 air quality 

distributions has changed over time 

based on historical trends in monitored 

O3 air quality data. As discussed in the 

proposal notice and in the Staff Paper 

(section 4.5.8), recent O3 air quality 

distributions were statistically adjusted 

to simulate just meeting the current and 

selected alternative standards. 

Specifically, the exposure and risk 

assessment included estimates for a 

recent year of air quality and for air 

quality adjusted to simulate just meeting 

the current and alternative standards 

based on O3 season data from a recent 

three-year period (2002–2004). The O3

season in each area included the period 

of the year for which routine hourly O3

monitoring data are available. Typically 

this period spans from March or April 

through September or October, although 

in some areas it includes the entire year. 

Three years were modeled to reflect the 

substantial year-to-year variability that 

occurs in O3 levels and related 

meteorological conditions, and because 

the standard is specified in terms of a 

three-year period. The year-to-year 

variability observed in O3 levels is due 

to a combination of different weather 

patterns and the variation in emissions 

of O3 precursors. Nationally, 2002 was 

a relatively high year with respect to the 

4th highest daily maximum 8-hour O3

levels observed in urban areas across the 

U.S. (see Staff Paper, Figure 2–16), with 

the mean of the distribution of annual 

4th highest daily maximum 8-hour O3

levels for urban monitors nationwide 

being in the upper third among the 

years 1990 through 2004. In contrast, on 

a national basis, 2004 was the lowest 

year on record with respect to the mean 

of the distribution of annual 4th highest 

daily maximum 8-hour O3 levels for this 

same 15 year period. The 4th highest 

daily maximum 8-hour levels observed 

in most, but not all of the 12 urban areas 

included in the exposure and risk 

assessment, were relatively low in 2004 

compared to other recent years. The 4th 

highest daily maximum 8-hour O3 levels 

observed in 2003 in the 12 urban areas 

and nationally generally were between 

those observed in 2002 and 2004. As a 

result of the variability in air quality, 

the exposure and risk estimates 

associated with just meeting the current 

or any alternative standard also will 

vary depending on the year chosen for 

the analysis. Thus, exposure and risk 

estimates based on 2002 air quality 

generally show relatively higher 

numbers of children affected and the 

estimates based on 2004 air quality 

generally show relatively fewer numbers 

of children affected. 

These simulations do not reflect any 

consideration of specific control 

programs or strategies designed to 

achieve the reductions in emissions 

required to meet the specified 

standards. Further, these simulations do 

not represent predictions of when, 

whether, or how areas might meet the 

specified standards.9 Instead these 

simulations represent a projection of the 

kind of air quality levels that would be 

likely to occur in areas just attaining 

various alternative standards, when 

historical patterns of air quality, 

reflecting averages over many areas, are 

applied in the urban areas examined. 

a. Exposure Analyses 

As discussed in section II.B.1 of the 

proposal, EPA conducted human 

exposure analyses using a simulation 

model to estimate O3 exposures for the 

general population, school age children 

(ages 5–18), and school age children 

with asthma living in 12 U.S. 

metropolitan areas representing 

different regions of the country where 

the current 8-hour O3 standard is not 

met. The emphasis on children reflected 

the finding of the last review that 

children are an important at-risk group. 

Exposure estimates were developed 

using a probabilistic exposure model 

that is designed to explicitly model the 

numerous sources of variability that 

affect people’s exposures. This exposure 

assessment is more fully described and 

presented in the Staff Paper and in a 

technical support document, Ozone 

Population Exposure Analysis for 

Selected Urban Areas (EPA, 2007c; 

henceforth ‘‘Exposure Analysis TSD’’). 

As noted in the proposal, the scope and 

methodology for this exposure 

assessment were developed over the last 

few years with considerable input from 

the CASAC Panel and the public. 

As discussed in the proposal notice 

and in greater detail in the Staff Paper 

(chapter 4) and Exposure Analysis TSD, 

EPA recognized that there are many 

sources of variability and uncertainty 

inherent in the input to this assessment 

and that there was uncertainty in the 

resulting O3 exposure estimates. In 

EPA’s judgment, the most important 

uncertainties affecting the exposure 

estimates are related to the modeling of 

human activity patterns over an O3

season, the modeling of variations in 

ambient concentrations near roadways, 

and the modeling of air exchange rates 

that affect the amount of O3 that 

penetrates indoors. Another important 

uncertainty that affects the estimation of 

how many exposures are associated 

with moderate or greater exertion is the 

characterization of energy expenditure 

for children engaged in various 

activities. As discussed in more detail in 

the Staff Paper (section 4.3.4.7), the 

uncertainty in energy expenditure 

values carries over to the uncertainty of 

the modeled breathing rates, which are 

important since they are used to classify 

exposures occurring at moderate or 

greater exertion. These are the relevant 

exposures since O3-related effects 

observed in clinical studies only are 

observed when individuals are engaged 

in some form of exercise. The 

uncertainties in the exposure model 

inputs and the estimated exposures 

have been assessed using quantitative 

uncertainty and sensitivity analyses. 

Details are discussed in the Staff Paper 

(section 4.6) and in a technical 

memorandum describing the exposure 

modeling uncertainty analysis 

(Langstaff, 2007). 

The exposure assessment, which 

provided estimates of the number of 

people exposed to different levels of 


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10As discussed in section II.A of the proposal, O3

health responses observed in controlled human 

exposure studies are associated with exposures 

while subjects are engaged in moderate or greater 

exertion on average over the exposure period 

(hereafter referred to as ‘‘elevated exertion’’) and, 

therefore, these are the exposures of interest. 

11While the proposal notice stated in the text that 

‘‘approximately 2 to 4 percent of all and asthmatic 

children’’ were estimated to experience exposures 

of concern at and above the 0.070 ppm benchmark 

level for standards in the range of 0.070 to 0.075 

ppm (72 FR 37879), the correct range is about 1 to 

5 perecent consistent with the estimates provided 

in Table 1 of the proposal (72 FR 37855). 

12The methodology, scope, and results from the 

risk assessment conducted in the last review are 

described in Chapter 6 of the 1996 Staff Paper (EPA, 

1996) and in several technical reports (Whitfield et 

al., 1996; Whitfield, 1997) and publication 

(Whitfield et al., 1998). 

ambient O3 while at elevated exertion 10, 

served two purposes. First, the entire 

range of modeled personal exposures to 

ambient O3 was an essential input to the 

portion of the health risk assessment 

based on exposure-response functions 

from controlled human exposure 

studies, discussed in the next section. 

Second, estimates of personal exposures 

to ambient O3 concentrations at and 

above specified benchmark levels while 

at elevated exertion provided some 

perspective on the public health 

impacts of health effects that we cannot 

currently evaluate in quantitative risk 

assessments but that may occur at 

current air quality levels, and the extent 

to which such impacts might be reduced 

by meeting the current and alternative 

standards. In the proposal, we referred 

to exposures at and above these 

benchmark levels while at elevated 

exertion as ‘‘exposures of concern.’’

Based on the observation from the 

exposure analyses conducted in the 

prior review that children represented 

the population subgroup with the 

greatest exposure to ambient O3, EPA 

chose to model 8-hour exposures at 

elevated exertion for all school age 

children, and separately for asthmatic 

school age children, as well as for the 

general population in the current 

exposure assessment. While outdoor 

workers and other adults who engage in 

moderate or greater exertion for 

prolonged periods while outdoors 

during the day in areas experiencing 

elevated O3 concentrations also are at 

risk for O3-related health effects, EPA 

did not focus on developing quantitative 

exposure estimates for these population 

subgroups due to the lack of information 

about the number of individuals who 

regularly work or exercise outdoors. 

Thus, as presented in the proposal and 

in the Staff Paper the exposure estimates 

are most useful for making relative 

comparisons of estimated exposures in 

school age children across alternative 

air quality scenarios. This assessment 

does not provide information on 

exposures for adult subgroups within 

the general population associated with 

the air quality scenarios. 

EPA noted in the proposal key 

observations that were important to 

consider in comparing exposure 

estimates associated with just meeting 

the current NAAQS and alternative 

standards considered. These included: 

(1) As shown in Table 6–1 of the Staff 

Paper, the patterns of exposures in 

terms of percentages of the population 

exceeding given exposure levels were 

very similar for the general population 

and for asthmatic and all school age (5–

18) children, although children were 

about twice as likely as the general 

population to be exposed at any given 

level. 

(2) As shown in Table 1 in the 

proposal (72 FR 37855), the number and 

percentage of asthmatic and all school 

age children aggregated across the 12 

urban areas estimated to experience 1 or 

more exposures of concern declined 

from simulations of just meeting the 

current standard to simulations of 

alternative 8-hour standards by varying 

amounts, depending on the benchmark 

level, the population subgroup 

considered, and the air quality year 

chosen.11

(3) Substantial year-to-year variability 

in exposure estimates was observed over 

the three-year modeling period. 

(4) There was substantial variability 

observed across the 12 urban areas in 

the percent of the population subgroups 

estimated to experience exposures at 

and above specified benchmark levels 

while at elevated exertion. 

(5) Of particular note, there is high 

inter-individual variability in 

responsiveness such that only a subset 

of individuals who were exposed at and 

above a given benchmark level while at 

elevated exertion would actually be 

expected to experience any such 

potential adverse health effects. 

(6) In considering these observations, 

it was important to take into account the 

variability, uncertainties, and 

limitations associated with this 

assessment, including the degree of 

uncertainty associated with a number of 

model inputs and uncertainty in the 

model itself. 

b. Quantitative Health Risk Assessment 

As discussed in section II.B.2 of the 

proposal, the approach used to develop 

quantitative risk estimates associated 

with exposures to O3 builds upon the 

risk assessment conducted during the 

last review.12 The expanded and 

updated assessment conducted in this 

review includes estimates of (1) risks of 

lung function decrements in all and 

asthmatic school age children, 

respiratory symptoms in asthmatic 

children, respiratory-related hospital 

admissions, and non-accidental and 

cardiorespiratory-related mortality 

associated with recent short-term 

ambient O3 levels; (2) risk reductions 

and remaining risks associated with just 

meeting the current 8-hour O3 NAAQS; 

and (3) risk reductions and remaining 

risks associated with just meeting 

various alternative 8-hour O3 NAAQS in 

a number of example urban areas. The 

health risk assessment was discussed in 

the Staff Paper (chapter 5) and 

presented more fully in a technical 

support document, Ozone Health Risk 

Assessment for Selected Urban Areas 

(Abt Associates, 2007a). As noted in the 

proposal, the scope and methodology 

for this risk assessment was developed 

over several years with considerable 

input from the CASAC Panel and the 

public. 

EPA recognized that there were many 

sources of uncertainty and variability 

inherent in the inputs to these 

assessments and that there was a high 

degree of uncertainty in the resulting O3

risk estimates. Such uncertainties 

generally relate to a lack of clear 

understanding of a number of important 

factors, including, for example, the 

shape of exposure-response and 

concentration-response functions, 

particularly when, as here, effect 

thresholds can neither be discerned nor 

determined not to exist; issues related to 

selection of appropriate statistical 

models for the analysis of the 

epidemiologic data; the role of 

potentially confounding and modifying 

factors in the concentration-response 

relationships; and issues related to 

simulating how O3 air quality 

distributions will likely change in any 

given area upon attaining a particular 

standard, since strategies to reduce 

emissions are not yet fully defined. 

While some of these uncertainties were 

addressed quantitatively in the form of 

estimated confidence ranges around 

central risk estimates, other 

uncertainties and the variability in key 

inputs were not reflected in these 

confidence ranges, but rather were 

partially characterized through separate 

sensitivity analyses or discussed 

qualitatively. 

Key observations and insights from 

the O3 risk assessment, together with 

important caveats and limitations, were 

discussed in section II.B of the proposal. 

In general, estimated risk reductions 

associated with going from current O3

levels to just meeting the current and 


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13PRB O3 concentrations used in the O3 risk 

assessment were defined in chapter 2 of the Staff 

Paper (EPA, 2007, pp. 2–48, 2–54) as the O3

concentrations that would be observed in the U.S. 

in the absence of anthropogenic emissions of 

precursors (e.g., VOC, NOX, and CO) in the U.S., 

Canada, and Mexico. Based on runs of the GEOS–

CHEM model (a global tropospheric O3 model) 

applied for the 2001 warm season (i.e., April to 

September), monthly background daily diurnal 

profiles for each of the 12 urban areas for each 

month of the O3 season were simulated using 

meteorology for the year 2001. Based on these 

model runs, the Criteria Document states that 

current estimates of PRB O3 concentrations are 

generally in the range of 0.015 to 0.035 ppm in the 

afternoon, and they are generally lower under 

conditions conducive to high O3 episodes. They are 

highest during spring due to contributions from 

hemispheric pollution and stratospheric intrusions. 

The Criteria Document states that the GEOS–CHEM 

model applied for the 2001 warm season reports 

PRB O3 concentrations for afternoon surface air over 

the United States that are likely 10 ppbv too high 

in the southeast in summer, and accurate within 5 

ppbv in other regions and seasons. 

14Sensitivity analyses examining the impact of 

alternative assumptions about PRB were only 

conducted for lung function decrements and nonaccidental mortality. 

alternative 8-hour standards show 

patterns of increasing estimated risk 

reductions associated with just meeting 

the lower alternative 8-hour standards 

considered. Furthermore, the estimated 

percentage reductions in risk were 

strongly influenced by the baseline air 

quality year used in the analysis (see 

Staff Paper, Figures 6–1 through 6–6) 

Key observations important in 

comparing estimated health risks 

associated with attainment of the 

current NAAQS and alternative 

standards included: 

(1) As discussed in the Staff paper 

(section 5.4.5), EPA has greater 

confidence in relative comparisons in 

risk estimates between alternative 

standards than in the absolute 

magnitude of risk estimates associated 

with any particular standard. 

(2) Significant year-to-year variability 

in O3 concentrations combined with the 

use of a 3-year design value to 

determine the amount of air quality 

adjustment to be applied to each year 

analyzed, results in significant year-toyear variability in the annual health risk 

estimates upon just meeting the current 

and potential alternative standards. 

(3) There is noticeable city-to-city 

variability in estimated O3-related 

incidence of morbidity and mortality 

across the 12 urban areas analyzed for 

both recent years of air quality and for 

air quality adjusted to simulate just 

meeting the current and selected 

potential alternative standards. This 

variability is likely due to differences in 

air quality distributions, differences in 

estimated exposure related to many 

factors including varying activity 

patterns and air exchange rates, 

differences in baseline incidence rates, 

and differences in susceptible 

populations and age distributions across 

the 12 urban areas. 

(4) With respect to the uncertainties 

about estimated policy-relevant 

background (PRB) concentrations,13 as 

discussed in the Staff Paper (section 

5.4.3), alternative assumptions about 

background levels had a variable impact 

depending on the health effect 

considered and the location and 

standard analyzed in terms of the 

absolute magnitude and relative changes 

in the risk estimates. There was 

relatively little impact on either 

absolute magnitude or relative changes 

in lung function risk estimates due to 

alternative assumptions about 

background levels.14 With respect to O3- 

related non-accidental mortality, while 

notable differences (i.e., greater than 50 

percent) were observed in some areas, 

particularly for more stringent 

standards, the overall pattern of 

estimated reductions, expressed in 

terms of percentage reduction relative to 

the current standard, was significantly 

less impacted. 

(5) Concerning the part of the risk 

assessment based on effects reported in 

epidemiological studies, important 

uncertainties include uncertainties (1) 

surrounding estimates of the O3

coefficients for concentration-response 

relationships used in the assessment, (2) 

involving the shape of the 

concentration-response relationship and 

whether or not a population threshold 

or non-linear relationship exists within 

the range of concentrations examined in 

the studies, (3) related to the extent to 

which concentration-response 

relationships derived from studies in a 

given location and time when O3 levels 

were higher or behavior and /or housing 

conditions were different provide 

accurate representations of the 

relationships for the same locations 

with lower air quality distributions and/ 

or different behavior and/or housing 

conditions, and (4) concerning the 

possible role of co-pollutants which also 

may have varied between the time of the 

studies and the current assessment 

period. An important additional 

uncertainty for the mortality risk 

estimates is the extent to which the 

associations reported between O3 and 

non-accidental and cardiorespiratory 

mortality actually reflect causal 

relationships. 

As discussed in the proposal, some of 

these uncertainties have been addressed 

quantitatively in the form of estimated 

confidence ranges around central risk 

estimates; others are addressed through 

separate sensitivity analyses (e.g., the 

influence of alternative estimates for 

policy-relevant background levels) or 

are characterized qualitatively. For both 

parts of the health risk assessment, 

statistical uncertainty due to sampling 

error has been characterized and is 

expressed in terms of 95 percent 

credible intervals. EPA recognizes that 

these credible intervals do not reflect all 

of the uncertainties noted above. 

B. Need for Revision of the Current 

Primary O3 Standard 

1. Introduction 

The initial issue to be addressed in 

this review of the primary O3 standard 

is whether, in view of the advances in 

scientific knowledge reflected in the 

Criteria Document and Staff Paper, the 

current standard should be revised. As 

discussed in section II.C of the proposal, 

in evaluating whether it was appropriate 

to propose to retain or revise the current 

standard, the Administrator built upon 

the last review and reflected the broader 

body of evidence and information now 

available. In the proposal, EPA 

presented information, judgments, and 

conclusions from the last review, which 

revised the level, averaging time, and 

form of the standard, from the Staff 

Paper’s evaluation of the adequacy of 

the current primary standard, including 

both evidence- and exposure/risk-based 

considerations, as well as from the 

CASAC Panel’s advice and 

recommendations. The Staff Paper 

evaluation, CASAC Panel’s views, and 

the Administrator’s proposed 

conclusions on the adequacy of the 

current primary standard are presented 

below. 

a. Staff Paper Evaluation 

The Staff Paper considered the 

evidence presented in the Criteria 

Document as a basis for evaluating the 

adequacy of the current O3 standard, 

recognizing that important uncertainties 

remain. The extensive body of human 

clinical, toxicological, and 

epidemiological evidence, highlighted 

above in section II.A.2 and discussed in 

section II.A of the proposal, serves as 

the basis for judgments about O3-related 

health effects, including judgments 

about causal relationships with a range 

of respiratory morbidity effects, 

including lung function decrements, 

increased respiratory symptoms, airway 

inflammation, increased airway 

responsiveness, and respiratory-related 

hospitalizations and emergency 

department visits in the warm season, 

and about the evidence being highly 

suggestive that O3 directly or indirectly 

contributes to non-accidental and 

cardiorespiratory-related mortality. 


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These judgments take into account 

important uncertainties that remain in 

interpreting this evidence. For example, 

with regard to the utility of time-series 

epidemiological studies to inform 

judgments about a NAAQS for an 

individual pollutant, such as O3, within 

a mix of highly correlated pollutants, 

such as the mix of oxidants produced in 

photochemical reactions in the 

atmosphere, the Staff Paper noted that 

there are limitations especially at 

ambient O3 concentrations below levels 

at which O3-related effects have been 

observed in controlled human exposure 

studies. The Staff Paper also recognized 

that the available epidemiological 

evidence neither supports nor refutes 

the existence of thresholds at the 

population level for effects such as 

increased hospital admissions and 

premature mortality. There are 

limitations in epidemiological studies 

that make discerning thresholds in 

populations difficult, including low 

data density in the lower concentration 

ranges, the possible influence of 

exposure measurement error, and 

variability in susceptibility to O3-related 

effects in populations. 

While noting these limitations in the 

interpretation of the findings from the 

epidemiological studies, the Staff Paper 

concluded that if a population threshold 

level does exist, it would likely be well 

below the level of the current O3

standard and possibly within the range 

of background levels. This conclusion is 

supported by several epidemiological 

studies that have explored the question 

of potential thresholds either by using a 

statistical curve-fitting approach to 

evaluate whether linear or non-linear 

models fit the data better using, or by 

analyzing, sub-sets of the data where 

days over or under a specific cutpoint 

(e.g., 0.080 ppm or even lower O3 levels) 

were excluded and then evaluating the 

association for statistical significance. In 

addition to consideration of the 

epidemiological studies, findings from 

controlled human exposure studies 

indicate that prolonged exposures 

produced statistically significant group 

mean FEV1 decrements and symptoms 

in healthy adult subjects at levels down 

to at least 0.060 ppm, with a small 

percentage of subjects experiencing 

notable effects (e.g., >10 percent FEV1

decrement, pain on deep inspiration). 

Controlled human exposure studies 

evaluated in the last review also found 

significant responses in indicators of 

lung inflammation and cell injury at 

0.080 ppm in healthy adult subjects. 

The effects in these controlled human 

exposure studies were observed in 

healthy young adult subjects, and it is 

likely that more serious responses, and 

responses at lower levels, would occur 

in people with asthma and other 

respiratory diseases. These 

physiological effects can lead to 

aggravation of asthma and increased 

susceptibility to respiratory infection. 

The observations provide support for 

the conclusion in the Staff Paper that 

the associations observed in the 

epidemiological studies, particularly for 

respiratory-related effects such as 

increased medication use, increased 

school and work absences, increased 

visits to doctors’ offices and emergency 

departments, and increased hospital 

admissions, extend down to O3 levels 

well below the current standard (i.e., 

0.084 ppm) (p. 6–7). 

The newly available information 

reinforces the judgments in the Staff 

Paper from the last review about the 

likelihood of causal relationships 

between O3 exposures and respiratory 

effects and broadens the evidence of O3- 

related associations to include 

additional respiratory-related endpoints, 

newly identified cardiovascular-related 

health endpoints, and mortality. Newly 

available evidence also led the Staff 

Paper to conclude that people with 

asthma are likely to experience more 

serious effects than people who do not 

have asthma. The Staff Paper also 

concluded that substantial progress has 

been made since the last review in 

advancing the understanding of 

potential mechanisms by which ambient 

O3, alone and in combination with other 

pollutants, is causally linked to a range 

of respiratory-related health endpoints, 

and may be causally linked to a range 

of cardiovascular-related health 

endpoints. Thus, the Staff Paper found 

strong support in the evidence available 

since the last review, for consideration 

of an O3 standard that is at least as 

protective as the current standard and 

finds no support for consideration of an 

O3 standard that is less protective than 

the current standard. This conclusion is 

consistent with the advice and 

recommendations of the CASAC Panel 

and with the views expressed by all 

interested parties who provided 

comments on drafts of the Staff Paper. 

While the CASAC Panel and some 

commenters on drafts of the Staff Paper 

supported revising the current standard 

to provide increased public health 

protection and other such commenters 

supported retaining the current 

standard, no one who provided 

comments on drafts of the Staff Paper 

supported a standard that would be less 

protective than the current standard. 

i. Evidence-Based Considerations 

In looking more specifically at the 

controlled human exposure and 

epidemiological evidence, the Staff 

Paper first noted that controlled human 

exposure studies provide the clearest 

and most compelling evidence for an 

array of human health effects that are 

directly attributable to acute exposures 

to O3 per se. Evidence from such human 

studies, together with animal 

toxicological studies, help to provide 

biological plausibility for health effects 

observed in epidemiological studies. In 

considering the available evidence, the 

Staff Paper focused on studies that 

examined health effects that have been 

demonstrated to be caused by exposure 

to O3, or for which the Criteria 

Document judges associations with O3

to be causal or likely causal, or for 

which the evidence is highly suggestive 

that O3 contributes to the reported 

effects. 

In considering the epidemiological 

evidence as a basis for reaching 

conclusions about the adequacy of the 

current standard, the Staff Paper 

focused on studies reporting effects in 

the warm season, for which the effect 

estimates are more consistently positive 

and statistically significant than those 

from all-year studies. The Staff Paper 

considered the extent to which such 

studies provide evidence of associations 

that extend down to ambient O3

concentrations below the level of the 

current standard, which would thereby 

call into question the adequacy of the 

current standard. In so doing, the Staff 

Paper noted that if a population 

threshold level does exist for an effect 

observed in such studies, it would likely 

be at a level well below the level of the 

current standard. The Staff Paper also 

attempted to characterize whether the 

area in which a study was conducted 

likely would or would not have met the 

current standard during the time of the 

study, although it recognizes that the 

confidence that would appropriately be 

placed on the associations observed in 

any given study, or on the extent to 

which the association would likely 

extend down to relatively low O3

concentrations, is not dependent on this 

distinction. Further, the Staff Paper 

considered studies that examined 

subsets of data that include only days 

with ambient O3 concentrations below 

the level of the current O3 standard, or 

below even lower O3 concentrations, 

and continue to report statistically 

significant associations. The Staff Paper 

judged that such studies are directly 

relevant to considering the adequacy of 

the current standard, particularly in 

light of reported responses to O3 at 


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levels below the current standard found 

in controlled human exposure studies. 

The Staff Paper evaluation of such 

studies is discussed below and in 

section II.C.2.a of the proposal, focusing 

in turn on studies of (1) lung function, 

respiratory symptoms and other 

respiratory-related physiological effects, 

(2) respiratory hospital admissions and 

emergency department visits, and (3) 

mortality. 

(1) Lung function, respiratory 

symptoms and other respiratory-related 

physiological effects. Health effects for 

which the Criteria Document continued 

to find clear evidence of causal 

associations with short-term O3

exposures include lung function 

decrements, respiratory symptoms, 

pulmonary inflammation, and increased 

airway responsiveness. In the last 

review, these O3-induced effects were 

demonstrated with statistical 

significance down to the lowest level 

tested in controlled human exposure 

studies at that time (i.e., 0.080 ppm). 

Two new studies are notable in that 

they are the only controlled human 

exposure studies that examined 

respiratory effects, including lung 

function decrements and respiratory 

symptoms, in healthy adults at lower 

exposure levels than had previously 

been examined. EPA’s reanalysis of the 

data from the most recent study shows 

small group mean decrements in lung 

function responses to be statistically 

significant at the 0.060 ppm exposure 

level, while the author’s analysis did 

not yield statistically significant lung 

function responses but did yield some 

statistically significant respiratory 

symptom responses toward the end of 

the exposure period. These studies 

report a small percentage of subjects 

experiencing lung function decrements 

(≥ 10 percent) at the 0.060 ppm 

exposure level. These studies provide 

very limited evidence of O3-related lung 

function decrements and respiratory 

symptoms at this lower exposure level. 

The Staff Paper noted that evidence 

from controlled human exposures 

studies indicates that people with 

moderate-to-severe asthma have 

somewhat larger decreases in lung 

function in response to O3 relative to 

healthy individuals. In addition, lung 

function responses in people with 

asthma appear to be affected by baseline 

lung function (i.e., magnitude of 

responses increases with increasing 

disease severity). This newer 

information expands our understanding 

of the physiological basis for increased 

sensitivity in people with asthma and 

other airway diseases, recognizing that 

people with asthma present a different 

response profile for cellular, molecular, 

and biochemical responses than people 

who do not have asthma. New evidence 

indicates that some people with asthma 

have increased occurrence and duration 

of nonspecific airway responsiveness, 

which is an increased 

bronchoconstrictive response to airway 

irritants. Controlled human exposure 

studies also indicate that some people 

with allergic asthma and rhinitis have 

increased airway responsiveness to 

allergens following O3 exposure. 

Exposures to O3 exacerbated lung 

function decrements in people with preexisting allergic airway disease, with 

and without asthma. Ozone-induced 

exacerbation of airway responsiveness 

persists longer and attenuates more 

slowly than O3-induced lung function 

decrements and respiratory symptom 

responses and can have important 

clinical implications for asthmatics. 

The Staff Paper also concluded that 

newly available human exposure 

studies suggest that some people with 

asthma also have increased 

inflammatory responses, relative to nonasthmatic subjects, and that this 

inflammation may take longer to 

resolve. The new data on airway 

responsiveness, inflammation, and 

various molecular markers of 

inflammation and bronchoconstriction 

indicate that people with asthma and 

allergic rhinitis (with or without 

asthma) comprise susceptible groups for 

O3-induced adverse effects. This body of 

evidence qualitatively informs the Staff 

Paper’s evaluation of the adequacy of 

the current O3 standard in that it 

indicates that controlled human 

exposure and epidemiological panel 

studies of lung function decrements and 

respiratory symptoms that evaluate only 

healthy, non-asthmatic subjects likely 

underestimate the effects of O3 exposure 

on asthmatics and other susceptible 

populations. 

The Staff Paper noted that in addition 

to the experimental evidence of lung 

function decrements, respiratory 

symptoms, and other respiratory effects 

in healthy and asthmatic populations 

discussed above, epidemiological 

studies have reported associations of 

lung function decrements and 

respiratory symptoms in several 

locations. Two large U.S. panel studies 

which together followed over 1,000 

asthmatic children on a daily basis 

(Mortimer et al., 2002, the National 

Cooperative Inner-City Asthma Study, 

or NCICAS; and Gent et al., 2003), as 

well as several smaller U.S. and 

international studies, have reported 

robust associations between ambient O3

concentrations and measures of lung 

function, daily respiratory symptoms 

(e.g., chest tightness, wheeze, shortness 

of breath), and increased asthma 

medication use in children with 

moderate to severe asthma. Mortimer et 

al. (2002) found that of the pollutants 

measured (including O3, NO2, SO2 and 

PM10), O3 was the only one that had a 

statistically significant effect on lung 

function. (Mortimer et al. 2002) also 

found associations between NO2, SO2

and PM10 and respiratory symptoms that 

were stronger than those between O3

and respiratory symptoms. Gent et al. 

(2003) found that in co-pollutant 

models, O3 but not PM2.5 significantly 

predicted increased risk of respiratory 

symptoms and rescue medication use 

among children using asthma 

maintenance medication. Overall, the 

multi-city NCICAS (Mortimer et al., 

2002), (Gent et al. 2003), and several 

other single-city studies indicate a 

robust positive association between 

ambient O3 concentrations and 

increased respiratory symptoms and 

increased medication use in asthmatic 

children. 

In considering the large number of 

single-city epidemiological studies 

reporting lung function or respiratory 

symptoms effects in healthy or 

asthmatic populations, the Staff Paper 

noted that most such studies that 

reported positive and often statistically 

significant associations in the warm 

season were conducted in areas that 

likely would not have met the current 

standard. In considering the large multicity NCICAS (Mortimer et al., 2002), the 

Staff Paper noted that the 98th 

percentile 8-hour daily maximum O3

concentrations at the monitor reporting 

the highest O3 concentrations in each of 

the study areas ranged from 0.084 ppm 

to > 0.10 ppm. However, the authors 

indicate that less than 5 percent of the 

days in the eight urban areas had 8-hour 

daily O3 concentrations exceeding 0.080 

ppm. Moreover, the authors observed 

that when days with 8-hour average O3

levels greater than 0.080 ppm were 

excluded, similar effect estimates were 

seen compared to estimates that 

included all of the days. There are also 

a few other studies in which the 

relevant air quality statistics provide 

some indication that lung function and 

respiratory symptom effects may be 

occurring in areas that likely would 

have met the current standard (EPA, 

2007b, p. 6–12). 

(2) Respiratory hospital admissions 

and emergency department visits. At the 

time of the last review, many time-series 

studies indicated positive associations 

between ambient O3 and increased 

respiratory hospital admissions and 

emergency room visits, providing strong 

evidence for a relationship between O3

exposure and increased exacerbations of 


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preexisting lung disease extending 

below the level of the then current 1- 

hour O3 standard (EPA 2007b, section 

3.3.1.1.6). Analyses of data from studies 

conducted in the northeastern U.S. 

indicated that O3 air pollution was 

consistently and strongly associated 

with summertime respiratory hospital 

admissions. 

Since the last review, new 

epidemiological studies have evaluated 

the association between short-term 

exposures to O3 and unscheduled 

hospital admissions for respiratory 

causes. Large multi-city studies, as well 

as many studies from individual cities, 

have reported positive and often 

statistically significant O3 associations 

with total respiratory hospitalizations as 

well as asthma- and chronic obstructive 

pulmonary disease (COPD)-related 

hospitalizations, especially in studies 

analyzing the O3 effect during the 

summer or warm season. Analyses using 

multipollutant regression models 

generally indicate that copollutants do 

not confound the association between 

O3 and respiratory hospitalizations and 

that the O3 effect estimates were robust 

to PM adjustment in all-year and warmseason only data. The Criteria Document 

concluded that the evidence supports a 

causal relationship between acute O3

exposures and increased respiratoryrelated hospitalizations during the 

warm season. 

In looking specifically at U.S. and 

Canadian respiratory hospitalization 

studies that reported positive and often 

statistically significant associations (and 

that either did not use GAM or were 

reanalyzed to address GAM-related 

problems), the Staff Paper noted that 

many such studies were conducted in 

areas that likely would not have met the 

current O3 standard, with many 

providing only all-year effect estimates, 

and with some reporting a statistically 

significant association in the warm 

season. Of the studies that provide some 

indication that O3-related respiratory 

hospitalizations may be occurring in 

areas that likely would have met the 

current standard, the Staff Paper noted 

that some are all-year studies, whereas 

others reported statistically significant 

warm-season associations. 

Emergency department visits for 

respiratory causes have been the focus 

of a number of new studies that have 

examined visits related to asthma, 

COPD, bronchitis, pneumonia, and 

other upper and lower respiratory 

infections, such as influenza, with 

asthma visits typically dominating the 

daily incidence counts. Among studies 

with adequate controls for seasonal 

patterns, many reported at least one 

significant positive association 

involving O3. However, inconsistencies 

were observed which were at least 

partially attributable to differences in 

model specifications and analysis 

approach among various studies. In 

general, O3 effect estimates from 

summer-only analyses tended to be 

positive and larger compared to results 

from cool season or all-year analyses. 

Almost all of the studies that reported 

statistically significant effect estimates 

were conducted in areas that likely 

would not have met the current 

standard. The Criteria Document 

concluded that analyses stratified by 

season generally supported a positive 

association between O3 concentrations 

and emergency department visits for 

asthma in the warm season. These 

studies provide evidence of effects in 

areas that likely would not have met the 

current standard and evidence of 

associations that likely extend down to 

relatively low ambient O3

concentrations. 

(3) Mortality. The 1996 Criteria 

Document concluded that an association 

between daily mortality and O3

concentrations for areas with high O3

levels (e.g., Los Angeles) was suggested. 

However, due to inconsistencies in the 

results from the very limited number of 

studies available at that time, there was 

insufficient evidence to determine 

whether the observed association was 

likely causal, and thus the possibility 

that O3 exposure may be associated with 

mortality was not relied upon in the 

1997 decision on the O3 primary 

standard. 

Since the last review, the body of 

evidence with regard to O3-related 

health effects has been expanded by 

animal, controlled human exposure, and 

epidemiological studies and now 

identifies biologically plausible 

mechanisms by which O3 may affect the 

cardiovascular system. In addition, 

there is stronger information linking O3

to serious morbidity outcomes, such as 

hospitalization, that are associated with 

increased mortality. Thus, there is now 

a coherent body of evidence that 

describes a range of health outcomes 

from lung function decrements to 

hospitalization and premature mortality. 

Newly available large multi-city 

studies and related analyses (Bell et al., 

2004; Huang et al., 2005; and Schwartz, 

2005) designed specifically to examine 

the effect of O3 and other pollutants on 

mortality have provided much more 

robust and credible information. 

Together these studies have reported 

significant associations between O3 and 

mortality that were robust to adjustment 

for PM and different adjustment 

methods for temperature and suggest 

that the effect of O3 on mortality may be 

immediate but may also persist for 

several days. Further analysis of one of 

these multi-city studies (Bell et al., 

2006) examined the shape of the 

concentration-response function for the 

O3-mortality relationship in 98 U.S. 

urban communities for the period 1987 

to 2000 specifically to evaluate whether 

a threshold level exists. Results from 

various analytic methods all indicated 

that any threshold, if it exists, would 

likely occur at very low concentrations, 

far below the level of the current O3

NAAQS and nearing background levels. 

New data are also available from 

several single-city studies conducted 

worldwide, as well as from several 

meta-analyses that have combined 

information from multiple studies. 

Three recent meta-analyses evaluated 

potential sources of heterogeneity in O3- 

mortality associations. All three 

analyses reported common findings, 

including effect estimates that were 

statistically significant and larger in 

warm season analyses. Reanalysis of 

results using default GAM criteria did 

not change the effect estimates, and 

there was no strong evidence of 

confounding by PM. 

Overall, the Criteria Document (p. 8–

78) found that the results from U.S. 

multi-city time-series studies, along 

with the meta-analyses, provide 

relatively strong evidence for 

associations between short-term O3

exposure and all-cause mortality even 

after adjustment for the influence of 

season and PM. The results of these 

analyses of studies considered in this 

review indicate that copollutants 

generally do not appear to substantially 

confound the association between O3

and mortality. In addition, several 

single-city studies observed positive 

associations of ambient O3

concentrations with total nonaccidental 

and cardiorespiratory mortality. 

Finally, from those studies that 

included assessment of associations 

with specific causes of death, it appears 

that effect estimates for associations 

with cardiovascular mortality are larger 

than those for total mortality; effect 

estimates for respiratory mortality are 

less consistent in size, possibly due to 

reduced statistical power in this 

subcategory of mortality. For 

cardiovascular mortality, the Criteria 

Document (p. 7–106) suggested that 

effect estimates are consistently positive 

and more likely to be larger and 

statistically significant in warm season 

analyses. The Criteria Document (p. 8–

78) concluded that these findings are 

highly suggestive that short-term O3

exposure directly or indirectly 

contributes to nonaccidental and 

cardiorespiratory-related mortality, but 


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15 In commenting on the Criteria Document, the 

CASAC Ozone Panel raised questions about the 

implications of these time-series results in a policy 

context, emphasizing that ‘‘* * * while the timeseries study design is a powerful tool to detect very 

small effects that could not be detected using other 

designs, it is also a blunt tool’’ (Henderson, 2006b). 

They note that ‘‘* * * not only is the interpretation 

of these associations complicated by the fact that 

the day-to-day variation in concentrations of these 

pollutants is, to a varying degree, determined by 

meteorology, the pollutants are often part of a large 

and highly correlated mix of pollutants, only a very 

few of which are measured’’ (Henderson, 2006b). 

Even with these uncertainties, the CASAC Ozone 

Panel, in its review of the Staff Paper, found ‘‘* * * 

premature total non-accidental and 

cardiorespiratory mortality for inclusion in the 

quantitative risk assessment to be appropriate.’’

(Henderson, 2006b) 

additional research is needed to more 

fully establish underlying mechanisms 

by which such effects occur.15

ii. Exposure- and Risk-Based 

Considerations 

In evaluating the adequacy of the 

current standard, the Staff Paper also 

considered estimated quantitative 

exposures and health risks, and 

important uncertainties and limitations 

in those estimates, which are 

highlighted above in section II.A.3 and 

discussed in section II.B of the proposal. 

These estimates are derived from an 

EPA assessment of exposures and health 

risks associated with recent air quality 

levels and with air quality simulated to 

just meet the current standard to help 

inform judgments about whether or not 

the current standard provides adequate 

protection of public health. 

The Staff Paper (and the CASAC 

Panel) recognized that the exposure and 

risk analyses could not provide a full 

picture of the O3 exposures and O3- 

related health risks posed nationally. 

The Staff Paper did not have sufficient 

information to evaluate all relevant atrisk groups (e.g., outdoor workers, 

children under age 5) or all O3-related 

health outcomes (e.g., increased 

medication use, school absences, and 

emergency department visits that are 

part of a broader pyramid of effects), 

and the scope of the Staff Paper analyses 

was generally limited to estimating 

exposures and risks in 12 urban areas 

across the U.S., and to only five or just 

one area for some health effects 

included in the risk assessment. Thus, 

due to the limited geographic scope of 

the exposure and risk assessments, EPA 

recognizes that national-scale public 

health impacts of ambient O3 exposures 

would be much larger than the 

quantitative exposure and risk estimates 

associated with recent air quality or air 

quality that just meets the current or 

alternative standards in the 12 urban 

areas analyzed. On the other hand, 

inter-individual variability in 

responsiveness means that only a subset 

of individuals in each group estimated 

to experience exposures at and above a 

given benchmark level while at elevated 

exertion would actually be expected to 

experience such adverse health effects. 

The Staff Paper estimated exposures 

and risks for the three most recent years 

(2002–2004) for which data were 

available at the time of the analyses. As 

discussed above in section II.A.3.a, 

within this 3-year period, 2002 was a 

year with relatively higher O3 levels in 

most, but not all, areas and simulation 

of just meeting the current standard 

based on 2002 air quality data provides 

a generally higher-end estimate of 

exposures and risks, while 2004 was a 

year with relatively lower O3 levels in 

most, but not all, areas and simulation 

of just meeting the current standard 

using 2004 air quality data provides a 

generally lower-end estimate of 

exposures and risks. 

The Staff Paper consideration of such 

exposure and risk analyses is discussed 

below and in section II.C.2.b of the 

proposal, focusing on both the exposure 

analyses and the human health risk 

assessment. 

(1) Exposure analyses. EPA’s exposure 

analysis estimated personal exposures 

to ambient O3 levels at and above 

specific benchmark levels while at 

elevated exertion to provide some 

perspective on the potential public 

health impacts of respiratory symptoms 

and respiratory-related physiological 

effects that cannot currently be 

evaluated in quantitative risk 

assessments but that may occur at 

current air quality levels, and the extent 

to which such impacts might be reduced 

by meeting the current and alternative 

standards. As noted above in section 

II.A.3, the Staff Paper referred to 

exposures at and above these 

benchmark levels as ‘‘exposures of 

concern.’’ The Staff Paper noted that 

potential public health impacts likely 

occur across a range of O3 exposure 

levels, such that there is no one 

exposure level that addresses all 

relevant public health impacts. 

Therefore, with the concurrence of the 

CASAC Panel, the Staff Paper estimated 

exposures of concern not only at 0.080 

ppm O3, a level at which there are 

demonstrated effects, but also at 0.070 

and 0.060 ppm O3. The Staff Paper 

recognized that there will be varying 

degrees of concern about exposures at 

each of these levels, based in part on the 

population subgroups experiencing 

them. Given that there is clear evidence 

of inflammation, increased airway 

responsiveness, and changes in host 

defenses in healthy people exposed to 

0.080 ppm O3 and reason to infer that 

such effects will continue at lower 

exposure levels, but with increasing 

uncertainty about the extent to which 

such effects occur at lower O3

concentrations, the Staff Paper focused 

on exposures at or above benchmark 

levels of 0.070 and 0.060 ppm O3 while 

at elevated exertion for purposes of 

evaluating the adequacy of the current 

standard. 

Exposure estimates were presented in 

the Staff Paper and in section II.B (Table 

1) of the proposal for the number and 

percent of all school age children and 

asthmatic school age children exposed, 

and the number of person-days 

(occurrences) of exposures, with daily 8- 

hour maximum exposures at or above 

several benchmark levels while at 

intermittent moderate or greater 

exertion. The percent of population 

exposed at any given level is very 

similar for all and asthmatic school age 

children. Substantial year-to-year 

variability in exposure estimates is 

observed, ranging to over an order of 

magnitude at the current standard level, 

in estimates of the number of children 

and the number of occurrences of 

exposures at both of these benchmark 

levels while at elevated exertion. The 

Staff Paper stated that it is appropriate 

to consider not just the average 

estimates across all years, but also to 

consider public health impacts in years 

with relatively higher O3 levels. The 

Staff Paper also noted that there is 

substantial city-to-city variability in 

these estimates, and notes that it is 

appropriate to consider not just the 

aggregate estimates across all cities, but 

also to consider the public health 

impacts in cities where these estimates 

are higher than the average upon 

meeting the current standard. 

About 50 percent of asthmatic of all 

school age children, representing nearly 

1.3 million asthmatic children and 

about 8.5 million school age children in 

the 12 urban areas examined, are 

estimated to experience exposures at or 

above the 0.070 ppm benchmark level 

while at elevated exertion (i.e., these 

individuals are estimated to experience 

8-hour O3 exposures at or above 0.070 

ppm while engaged in moderate or 

greater exertion 1 or more times during 

the O3 season) associated with 2002 O3

air quality levels. In contrast, about 17 

percent of asthmatic and all school age 

children are estimated to experience 

exposures at or above the 0.070 ppm 

benchmark level while at elevated 

exertion associated with 2004 O3 air 

quality levels. Just meeting the current 

standard results in an aggregate estimate 

of about 20 percent of asthmatic or 18 

percent of all school age children likely 

to experience exposures at or above the 


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0.070 ppm benchmark level while at 

elevated exertion using the 2002 

simulation. The exposure estimates for 

this benchmark level range up to about 

40 percent of asthmatic or all school age 

children in the single city with the 

highest estimate among the cities 

analyzed. Just meeting the current 

standard based on the 2004 simulation, 

results in an aggregate estimate of about 

1 percent of asthmatic or all school age 

children experiencing exposures 

exceeding the 0.070 ppm benchmark 

level while at elevated exertion. 

At the benchmark level of 0.060 ppm, 

about 70 percent of all or asthmatic 

school age children are estimated to 

experience exposures at or above this 

benchmark level while at elevated 

exertion for the aggregate of the 12 

urban areas associated with 2002 O3

levels. Just meeting the current standard 

would result in an aggregate estimate of 

about 45 percent of asthmatic or all 

school age children likely to experience 

exposures at or above the 0.060 ppm 

benchmark level while at elevated 

exertion using the 2002 simulation. The 

exposure estimates for this benchmark 

level range up to nearly 70 percent of all 

or asthmatic school age children in the 

single city with the highest estimate 

among the cities analyzed associated 

with just meeting the current standard 

using the 2002 simulation. The Staff 

Paper indicated an aggregate estimate of 

about 10 percent of asthmatic or all 

school age children would experience 

exposures at or above the 0.060 ppm 

benchmark level while at elevated 

exertion associated with just meeting 

the current standard using the 2004 

simulation. 

(2) Risk assessment. The health risk 

assessment estimated risks for several 

important health endpoints, including: 

(1) Lung function decrements (i.e., ≥ 15 

percent and ≥ 20 percent reductions in 

FEV1) in all school age children for 12 

urban areas; (2) lung function 

decrements (i.e., ≥ 10 percent and ≥ 20 

percent reductions in FEV1) in 

asthmatic school age children for 5 

urban areas (a subset of the 12 urban 

areas); (3) respiratory symptoms (i.e., 

chest tightness, shortness of breath, 

wheeze) in moderate to severe asthmatic 

children for the Boston area; (4) 

respiratory-related hospital admissions 

for 3 urban areas; and (5) nonaccidental 

and cardiorespiratory mortality for 12 

urban areas for three recent years (2002 

to 2004) and for just meeting the current 

standard using a 2002 simulation and a 

2004 simulation. 

With regard to estimates of moderate 

lung function decrements, meeting the 

current standard substantially reduces 

the estimated number of school age 

children experiencing one or more 

occurrences of FEV1 decrements ≥ 15 

percent for the 12 urban areas, going 

from about 1.3 million children (7 

percent of children) under 2002 air 

quality to about 610,000 (3 percent of 

children) based on the 2002 simulation, 

and from about 620,000 children (3 

percent of children) to about 230,000 (1 

percent of children) using the 2004 

simulation. In asthmatic children, the 

estimated number of children 

experiencing one or more occurrences of 

FEV1 decrements ≥ 10 percent for the 5 

urban areas goes from about 250,000 

children (16 percent of asthmatic 

children) under 2002 air quality to 

about 130,000 (8 percent of asthmatic 

children) using the 2002 simulation, 

and from about 160,000 (10 percent of 

asthmatic children) to about 70,000 (4 

percent of asthmatic children) using the 

2004 simulation. Thus, even when the 

current standard is met, about 4 to 8 

percent of asthmatic school age children 

are estimated to experience one or more 

occurrences of moderate lung function 

decrements, resulting in about 1 million 

occurrences (using the 2002 simulation) 

and nearly 700,000 occurrences (using 

the 2004 simulation) in just 5 urban 

areas. Moreover, the estimated number 

of occurrences of moderate or greater 

lung function decrements per child is 

on average approximately 6 to 7 in all 

children and 8 to 10 in asthmatic 

children in an O3 season, even when the 

current standard is met, depending on 

the year used to simulate meeting the 

current standard. In the 1997 review of 

the O3 standard a general consensus 

view of the adversity of such moderate 

responses emerged as the frequency of 

occurrences increases, with the 

judgment that repeated occurrences of 

moderate responses, even in otherwise 

healthy individuals, may be considered 

adverse since they may well set the 

stage for more serious illness. 

With regard to estimates of large lung 

function decrements, the Staff Paper 

noted that FEV1 decrements > 20 

percent would likely interfere with 

normal activities in many healthy 

individuals, therefore single 

occurrences would be considered to be 

adverse. In people with asthma, large 

lung function responses would likely 

interfere with normal activities for most 

individuals and would also increase the 

likelihood that these individuals would 

use additional medication or seek 

medical treatment. Single occurrences 

would be considered to be adverse to 

asthmatic individuals under the ATS 

definition. They also would be cause for 

medical concern in some individuals. 

While the current standard reduces the 

occurrences of large lung function 

decrements in all children and 

asthmatic children from about 60 to 

70%, in a year with relatively higher O3

levels (2002), there are estimated to be 

about 500,000 occurrences in all school 

children across the entire 12 urban 

areas, and about 40,000 occurrences in 

asthmatic children across just 5 urban 

areas. As noted above, it is clear that 

even when the current standard is met 

over a three-year period, O3 levels in 

each year can vary considerably, as 

evidenced by relatively large differences 

between risk estimates based on 2002 to 

2004 air quality. The Staff Paper 

expressed the view that it was 

appropriate to consider this yearly 

variation in O3 levels allowed by the 

current standard in judging the extent to 

which impacts on members of at-risk 

groups in a year with relatively higher 

O3 levels remain of concern from a 

public health perspective. 

With regard to other O3-related health 

effects, the estimated risks of respiratory 

symptom days in moderate to severe 

asthmatic children, respiratory-related 

hospital admissions, and non-accidental 

and cardiorespiratory mortality, 

respectively, are not reduced to as great 

an extent by meeting the current 

standard as are lung function 

decrements. For example, just meeting 

the current standard reduces the 

estimated average incidence of chest 

tightness in moderate to severe 

asthmatic children living in the Boston 

urban area by 11 to 15%, based on 2002 

and 2004 simulations, respectively, 

resulting in an estimated incidence of 

about 23,000 to 31,000 per 100,000 

children attributable to O3 exposure 

(Table 6–4). Just meeting the current 

standard is estimated to reduce the 

incidence of respiratory-related hospital 

admissions in the New York City urban 

area by about 16 to 18%, based on 2002 

and 2004 simulations, respectively, 

resulting in an estimated incidence per 

100,000 population of 4.6 to 6.4, 

respectively. Across the 12 urban areas, 

the estimates of non-accidental 

mortality incidence per 100,000 relevant 

population range from 0.4 to 2.6 (for 

2002) and 0.5 to 1.5 (for 2004). Meeting 

the current standard results in a 

reduction of the estimated incidence per 

100,000 population to a range of 0.3 to 

2.4 based on the 2002 simulation and a 

range of 0.3 to 1.2 based on the 2004 

simulation. Estimates for 

cardiorespiratory mortality show similar 

patterns. 

In considering the estimates of the 

proportion of population affected and 

the number of occurrences of the health 

effects that are included in the risk 

assessment, the Staff Paper noted that 


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16 In defining at-risk groups this way we are 

including both groups with greater inherent 

sensitivity and those more likely to be exposed. 

these limited estimates are indicative of 

a much broader array of potential O3- 

related health endpoints that we 

consider part of a ‘‘pyramid of effects’’

that include various indicators of 

morbidity that could not be included in 

the risk assessment (e.g., school 

absences, increased medication use, 

emergency department visits) and 

which primarily affect members of atrisk groups. While the Staff Paper had 

sufficient information to estimate and 

consider the number of symptom days 

in children with moderate to severe 

asthma, it recognized that there are 

many other effects that may be 

associated with symptom days, such as 

increased medication use, school and 

work absences, or visits to doctors’

offices, for which there was not 

sufficient information to estimate risks 

but which are important to consider in 

assessing the adequacy of the current 

standard. The same is true for more 

serious, but less frequent effects. The 

Staff Paper estimated hospital 

admissions, but there was not sufficient 

information to estimate emergency 

department visits in a quantitative risk 

assessment. Consideration of such 

unquantified risks in the Staff Paper 

reinforced the Staff Paper conclusion 

that consideration should be given to 

revising the standard so as to provide 

increased public health protection, 

especially for at-risk groups such as 

people with asthma or other lung 

diseases, as well as children and older 

adults, particularly those active 

outdoors, and outdoor workers. 

iii. Summary of Staff Paper 

Considerations 

The Staff Paper concluded that the 

overall body of evidence clearly calls 

into question the adequacy of the 

current standard in protecting at-risk 

groups against an array of adverse 

health effects that range from decreased 

lung function and respiratory symptoms 

to serious indicators of respiratory 

morbidity including emergency 

department visits and hospital 

admissions for respiratory causes, 

nonaccidental mortality, and possibly 

cardiovascular effects. These at-risk 

groups notably include asthmatic 

children and other people with lung 

disease, as well as all children and older 

adults, especially those active outdoors, 

and outdoor workers.16 The available 

information provides strong support for 

consideration of an O3 standard that 

would provide increased health 

protection for these at-risk groups. The 

Staff Paper also concluded that risks 

projected to remain upon meeting the 

current standard are indicative of risks 

to at-risk groups that can be judged to 

be important from a public health 

perspective. This information reinforced 

the Staff Paper conclusion that 

consideration should be given to 

revising the level of the standard so as 

to provide increased public health 

protection. 

b. CASAC Views 

The CASAC Panel unanimously 

concluded in a letter to the 

Administrator that there is ‘‘no 

scientific justification for retaining’’ the 

current primary O3 standard, and the 

current standard ‘‘needs to be 

substantially reduced to protect human 

health, particularly in sensitive 

subpopulations’’ (Henderson, 2006c, pp. 

1–2). In its rationale for this conclusion, 

the CASAC Panel concluded that ‘‘new 

evidence supports and builds-upon key, 

health-related conclusions drawn in the 

1997 O3 NAAQS review’’ (id., p. 3). The 

Panel noted that several new single-city 

studies and large multi-city studies have 

provided more evidence for adverse 

health effects at concentrations lower 

than the current standard, and that these 

epidemiological studies are backed-up 

by evidence from controlled human 

exposure studies. The Panel specifically 

noted evidence from the recent Adams 

(2006) study that reported statistically 

significant decrements in the lung 

function of healthy, moderately 

exercising adults at a 0.080 ppm 

exposure level, and importantly, also 

reported adverse lung function effects in 

some healthy individuals at 0.060 ppm. 

The CASAC Panel concluded that these 

results indicate that the current 

standard ‘‘is not sufficiently healthprotective with an adequate margin of 

safety,’’ noting that while similar 

studies in sensitive groups such as 

asthmatics have yet to be conducted, 

‘‘people with asthma, and particularly 

children, have been found to be more 

sensitive and to experience larger 

decrements in lung function in response 

to O3 exposures than would healthy 

volunteers (Mortimer et al., 2002)’’

(Henderson, 2006c, p. 4). 

The CASAC Panel also highlighted a 

number of O3-related adverse health 

effects that are associated with exposure 

to ambient O3, below the level of the 

current standard based on a broad range 

of epidemiological studies (Henderson, 

2006c). These adverse health effects 

include increases in school absenteeism, 

respiratory hospital emergency 

department visits among asthmatics and 

patients with other respiratory diseases, 

hospitalizations for respiratory illnesses, 

symptoms associated with adverse 

health effects (including chest tightness 

and medication usage), and premature 

mortality (nonaccidental, 

cardiorespiratory deaths) reported at 

exposure levels well below the current 

standard. ‘‘The CASAC considers each 

of these findings to be an important 

indicator of adverse health effects’’

(Henderson, 2006c). 

The CASAC Panel expressed the view 

that more emphasis should be placed on 

the subjects in controlled human 

exposure studies with FEV1 decrements 

greater than 10 percent, which can be 

clinically significant, rather than on the 

relatively small average decrements. 

The Panel also emphasized significant 

O3-related inflammatory responses and 

markers of injury to the epithelial lining 

of the lung that are independent of 

spirometric responses. Further, the 

Panel expressed the view that the Staff 

Paper did not place enough emphasis on 

serious morbidity (e.g., hospital 

admissions) and mortality observed in 

epidemiological studies. On the basis of 

the large amount of recent data 

evaluating adverse health effects at 

levels at and below the current O3

standard, it was the unanimous opinion 

of the CASAC Panel that the current 

primary O3 standard is not adequate to 

protect human health, that the relevant 

scientific data do not support 

consideration of retaining the current 

standard, and that the current standard 

needs to be substantially reduced to be 

protective of human health, particularly 

in sensitive subpopulations (Henderson, 

2006c, pp. 4–5). 

Further, the CASAC letter noted that 

‘‘there is no longer significant scientific 

uncertainty regarding the CASAC’s 

conclusion that the current 8-hour 

primary NAAQS must be lowered’’

(Henderson, 2006c, p. 5). The Panel 

noted that a ‘‘large body of data clearly 

demonstrates adverse human health 

effects at the current level’’ of the 

standard, such that ‘‘[R]etaining this 

standard would continue to put large 

numbers of individuals at risk for 

respiratory effects and/or significant 

impact on quality of life including 

asthma exacerbations, emergency room 

visits, hospital admissions and 

mortality’’ (Henderson, 2006c). 

c. Administrator’s Proposed 

Conclusions 

At the time of proposal, in 

considering whether the current 

primary standard should be revised, the 

Administrator carefully considered the 

conclusions contained in the Criteria 

Document, the rationale and 

recommendations contained in the Staff 

Paper, the advice and recommendations 


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17As discussed in section II.A.3 above, 

‘‘exposures of concern’’ are estimates of personal 

exposures while at moderate or greater exertion to 

8-hour average ambient O3 levels at and above 

specific benchmark levels which represent 

exposure levels at which O3-related health effects 

are known or can with varying degrees of certainty 

be inferred to occur in some individuals. Estimates 

of exposures of concern provide some perspective 

on the public health impacts of health effects that 

may occur in some individuals at recent air quality 

levels but cannot be evaluated in quantitative risk 

assessments, and the extent to which such impacts 

might be reduced by meeting the current and 

alternative standards. 

18As noted above in section II.A.3, recent O3 air 

quality distributions have been statistically adjusted 

to simulate just meeting the current and selected 

alternative standards. These simulations do not 

represent predictions of when, whether, or how 

areas might meet the specified standards. 

from CASAC, and public comments to 

date on this issue. In so doing, the 

Administrator noted the following: (1) 

That evidence of a range of respiratoryrelated morbidity effects seen in the last 

review has been considerably 

strengthened, both through toxicological 

and controlled human exposure studies 

as well as through many new panel and 

epidemiological studies; (2) that new 

evidence from controlled human 

exposure and epidemiological studies 

identifies people with asthma 

(including children with asthma) as an 

important susceptible population for 

which estimates of respiratory effects in 

the general population likely 

underestimate the magnitude or 

importance of these effects; (3) that new 

evidence about mechanisms of toxicity 

further contributes to the biological 

plausibility of O3-induced respiratory 

effects and is beginning to suggest 

mechanisms that may link O3 exposure 

to cardiovascular effects; (4) that there is 

now relatively strong evidence for 

associations between O3 and total 

nonaccidental and cardiopulmonary 

mortality, even after adjustment for the 

influence of season and PM; and (5) the 

limits of the available evidence. Relative 

to the information that was available to 

inform the Agency’s 1997 decision to set 

the current standard, the newly 

available evidence increased the 

Administrator’s confidence that 

respiratory morbidity effects such as 

lung function decrements and 

respiratory symptoms are causally 

related to O3 exposures, that indicators 

of respiratory morbidity such as 

emergency department visits and 

hospital admissions are causally related 

to O3 exposures, and that the evidence 

is highly suggestive that O3 exposures 

during the O3 season contribute to 

premature mortality. 

The Administrator judged that there is 

important new evidence demonstrating 

that exposures to O3 at levels below the 

level of the current standard are 

associated with a broad array of adverse 

health effects, especially in at-risk 

populations that include people with 

asthma or other lung diseases who are 

likely to experience more serious effects 

from exposure to O3, children and older 

adults with increased susceptibility, as 

well as those who are likely to be 

vulnerable as a result of spending a lot 

of time outdoors engaged in physical 

activity, especially active children and 

outdoor workers. Examples of this 

important new evidence include 

demonstration of O3-induced lung 

function effects and respiratory 

symptoms in some healthy individuals 

down to the previously observed 

exposure level of 0.080 ppm, as well as 

very limited new evidence at exposure 

levels well below the level of the 

current standard. In addition, there is 

now epidemiological evidence of 

statistically significant O3-related 

associations with lung function and 

respiratory symptom effects, respiratoryrelated emergency department visits and 

hospital admissions, and increased 

mortality, in areas that likely would 

have met the current standard. There are 

also many epidemiological studies done 

in areas that likely would not have met 

the current standard but which 

nonetheless report statistically 

significant associations that generally 

extend down to ambient O3

concentrations that are below the level 

of the current standard. Further, there 

are a few studies that have examined 

subsets of data that include only days 

with ambient O3 concentrations below 

the level of the current standard, or 

below even much lower O3

concentrations, and continue to report 

statistically significant associations with 

respiratory morbidity outcomes and 

mortality. The Administrator recognized 

that the evidence from controlled 

human exposure studies, together with 

animal toxicological studies, provides 

considerable support for the biological 

plausibility of the respiratory morbidity 

associations observed in the 

epidemiological studies and for 

concluding that the associations extend 

below the level of the current standard. 

However, the Administrator recognized 

that in the body of epidemiological 

evidence, many studies reported 

positive and statistically significant 

associations, while others reported 

positive results that were not 

statistically significant, and a few did 

not report any positive O3-related 

associations. In addition, the 

Administrator judged that evidence of a 

causal relationship between adverse 

health outcomes and O3 exposures 

became increasingly uncertain at lower 

levels of exposure. 

Based on the strength of the currently 

available evidence of adverse health 

effects, and on the extent to which the 

evidence indicates that such effects 

likely result from exposures to ambient 

O3 concentrations below the level of the 

current standard, the Administrator 

judged that the current standard does 

not protect public health with an 

adequate margin of safety and that the 

standard should be revised to provide 

such protection, especially for at-risk 

groups, against a broad array of adverse 

health effects. 

In reaching this judgment, the 

Administrator had also considered the 

results of both the exposure and risk 

assessments conducted for this review, 

to provide some perspective on the 

extent to which at-risk groups would 

likely experience ‘‘exposures of 

concern’’ 17 and on the potential 

magnitude of the risk of experiencing 

various adverse health effects when 

recent air quality data (from 2002 to 

2004) are used to simulate meeting the 

current standard and alternative 

standards in a number of urban areas in 

the U.S.18 In considering the results of 

the health risk assessment, as discussed 

in the proposal notice (section II.C.2), 

the Administrator noted that there were 

important uncertainties and 

assumptions inherent in the risk 

assessment and that this assessment was 

most appropriately used to simulate 

trends and patterns that could be 

expected, as well as providing informed, 

but still imprecise, estimates of the 

potential magnitude of risks. 

In considering the exposure 

assessment results at the time of 

proposal, the Administrator considered 

analyses that define ‘‘exposures of 

concern’’ by three benchmark exposure 

levels: 0.080, 0.070, and 0.060 ppm. 

Estimates of exposures in at-risk groups 

at and above these benchmark levels 

while at elevated exertion, using O3 air 

quality data in 2002 and 2004, provide 

some indication of the potential 

magnitude of the incidence of health 

outcomes that cannot currently be 

evaluated in a quantitative risk 

assessment, such as increased airway 

responsiveness, increased pulmonary 

inflammation, increased cellular 

permeability, and decreased pulmonary 

defense mechanisms. These respiratoryrelated physiological effects have been 

demonstrated to occur in healthy people 

at O3 exposures as low as 0.080 ppm, 

the lowest level tested for these effects. 

These physiological effects provide 

plausible mechanisms underlying 

observed associations with aggravation 

of asthma, increased medication use, 

increased school and work absences, 


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increased susceptibility to respiratory 

infection, increased visits to doctors’

offices and emergency departments, and 

increased admissions to hospitals. In 

addition, these physiological effects, if 

repeated over time, have the potential to 

lead to chronic effects such as chronic 

bronchitis or long-term damage to the 

lungs that can lead to reduced quality of 

life. 

In considering these various 

benchmark levels for exposures of 

concern at the time of proposal, the 

Administrator focused primarily on 

estimated exposures at and above the 

0.070 ppm benchmark level while at 

elevated exertion as an important 

surrogate measure for potentially more 

serious health effects in at-risk groups 

such as people with asthma. This 

judgment was based on the strong 

evidence of effects in healthy people at 

the 0.080 ppm exposure level and the 

new evidence that people with asthma 

are likely to experience larger and more 

serious effects than healthy people at 

the same level of exposure. In the 

Administrator’s view at the time of 

proposal, this evidence did not support 

a focus on exposures at and above the 

benchmark level of 0.080 ppm O3, as it 

would not adequately account for the 

increased risk of harm from exposure for 

members of at-risk groups, especially 

people with asthma. The Administrator 

also judged that the evidence of 

demonstrated effects is too limited to 

support a primary focus on exposures 

down to the lowest benchmark level 

considered of 0.060 ppm. The 

Administrator particularly noted that 

although the analysis of ‘‘exposures of 

concern’’ was conducted to estimate 

exposures at and above three discrete 

benchmark levels (0.080, 0.070, and 

0.060 ppm) while at elevated exertion, 

the concept is appropriately viewed as 

a continuum. In so doing, the 

Administrator sought to balance 

concern about the potential for health 

effects and their severity with the 

increasing uncertainty associated with 

our understanding of the likelihood of 

such effects at lower O3 exposure levels. 

The Administrator observed that 

based on the aggregate exposure 

estimates for the 2002 simulation 

(summarized in section II.B.1, Table 1, 

of the proposal) for the 12 U.S. urban 

areas included in the exposure analysis, 

upon just meeting the current standard 

up to about 20 percent of asthmatic or 

all school age children are likely to 

experience one or more exposures at 

and above the 0.070 ppm benchmark 

level while at elevated exertion; the 

2004 simulation yielded an estimate of 

about 1 percent of such children. The 

Administrator noted from this 

comparison that there is substantial 

year-to-year variability, ranging up to an 

order of magnitude or more in estimates 

of the number of people and the number 

of occurrences of exposures at and 

above this benchmark level while at 

elevated exertion. Moreover, within any 

given year, the exposure assessment 

indicates that there is substantial cityto-city variability in the estimates of the 

children exposed or the number of 

occurrences of exposure at and above 

this benchmark level while at elevated 

exertion. For example, city-specific 

estimates of the percent of asthmatic or 

all school age children likely to 

experience exposures at and above the 

benchmark level of 0.070 ppm while at 

elevated exertion ranges from about 1 

percent up to about 40 percent across 

the 12 urban areas upon just meeting the 

current standard based on the 2002 

simulation; the 2004 simulation yielded 

estimates that range from about 0 up to 

about 7 percent. The Administrator 

judged that it was important to 

recognize the substantial year-to-year 

and city-to-city variability in 

considering these estimates. 

With regard to the results of the risk 

assessment, the Administrator focused 

on the risks estimated to remain upon 

just meeting the current standard. Based 

on the aggregate risk estimates 

(summarized in section II.B.2, Table 2, 

of the proposal), the Administrator 

observed that upon just meeting the 

current standard based on the 2002 

simulation, approximately 8 percent of 

asthmatic school age children across 5 

urban areas (ranging up to about 11 

percent in the city with the highest 

estimate among the cities analyzed) 

would still be estimated to experience 

moderate or greater lung function 

decrements one or more times within an 

O3 season. These estimated percentages 

would be approximately 3 percent of all 

school age children across 12 urban 

areas (ranging up to over 5 percent in 

the city with the highest estimate among 

the cities analyzed). The Administrator 

recognized that, as with the estimates of 

exposures of concern, there is 

substantial year-to-year and city-to-city 

variability in these risk estimates. 

In addition to the percentage of 

asthmatic or all children estimated to 

experience one or more occurrences of 

an effect, the Administrator recognized 

that some individuals are estimated to 

have multiple occurrences. For 

example, across all the cities in the 

assessment, approximately 6 to 7 

occurrences of moderate or greater lung 

function decrements per child are 

estimated to occur in all children and 

approximately 8 to 10 occurrences are 

estimated to occur in asthmatic children 

in an O3 season, even upon just meeting 

the current standard. In the last review, 

a general consensus view of the 

adversity of such responses emerged as 

the frequency of occurrences increases, 

with the judgment that repeated 

occurrences of moderate responses, 

even in otherwise healthy individuals, 

may be considered adverse since they 

may well set the stage for more serious 

illness. The Administrator continued to 

support this view. 

Large lung function decrements (i.e., 

≥ 20 percent FEV1 decrement) would 

likely interfere with normal activities in 

many healthy individuals, therefore 

single occurrences would be considered 

to be adverse. In people with asthma, 

large lung function responses (i.e., ≥ 20 

percent FEV1 decrement), would likely 

interfere with normal activities for most 

individuals and would also increase the 

likelihood that these individuals would 

use additional medication or seek 

medical treatment. Not only would 

single occurrences be considered to be 

adverse to asthmatic individuals under 

the ATS definition, but they also would 

be cause for medical concern for some 

individuals. Upon just meeting the 

current standard based on the 2002 

simulation, close to 1 percent of 

asthmatic and all school age children 

are estimated to experience one or more 

occurrences of large lung function 

decrements in the aggregate across 5 and 

12 urban areas, respectively, with close 

to 2 percent of both asthmatic and all 

school age children estimated to 

experience such effects in the city that 

receives relatively less protection from 

this standard. These estimates translate 

into approximately 500,000 occurrences 

of large lung function decrements in all 

children across 12 urban areas, and 

about 40,000 occurrences in asthmatic 

children across 5 urban areas upon just 

meeting the current standard based on 

the 2002 simulation; the 2004 

simulation yielded estimates that 

translate into approximately 160,000 

and 10,000 such occurrences in all 

children and asthmatic children, 

respectively. 

Upon just meeting the current 

standard based on the 2002 simulation, 

the estimate of the O3-related risk of 

respiratory symptom days in moderate 

to severe asthmatic children in the 

Boston area is about 8,000 symptom 

days; the 2004 simulation yielded an 

estimate of about 6,000 such symptoms 

days. These estimates translate into as 

many as one symptom day in six, and 

one symptom day in eight, respectively, 

that are attributable to O3 exposure 

during the O3 season of the total number 

of symptom days associated with all 


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causes of respiratory symptoms in 

asthmatic children during those years. 

The estimated O3-related risk of 

respiratory-related hospital admissions 

upon just meeting the current standard 

based on the 2002 simulation is greater 

than 500 hospital admissions in the 

New York City area alone, or about 1.5 

percent of the total incidence of 

respiratory-related admissions 

associated with all causes; the 2004 

simulation yielded an estimate of 

approximately 400 such hospital 

admissions. For nonaccidental 

mortality, just meeting the current 

standard based on the 2002 simulation 

results in an estimated incidence of 

from 0.3 to 2.4 per 100,000 population; 

the 2004 simulation resulted in an 

estimated incidence of from 0.3 to 1.2 

per 100,000 population. Estimates for 

cardiorespiratory mortality show similar 

patterns (Abt Associates, 2007a, Table 

4–26). 

The Administrator recognized that in 

considering the estimates of the 

proportion of population affected and 

the number of occurrences of those 

specific health effects that are included 

in the risk assessment, these limited 

estimates based on 2002 and 2004 

simulations are indicative of a much 

broader array of O3-related health 

endpoints that are part of a ‘‘pyramid of 

effects’’ (discussed in section II.A.4.d of 

the proposal) that include various 

indicators of morbidity that could not be 

included in the risk assessment (e.g., 

school absences, increased medication 

use, emergency department visits) and 

which primarily affect members of atrisk groups. Moreover, the 

Administrator noted that the CASAC 

Panel supported a qualitative 

consideration of the much broader array 

of O3-related health endpoints, and 

specifically referred to respiratory 

emergency department visits in 

asthmatics and people with other lung 

diseases, increased medication use, and 

increased respiratory symptoms 

reported at exposure levels well below 

the current standard. 

The Administrator expressed the view 

in the proposal that the exposure and 

risk estimates discussed in the Staff 

Paper and summarized above are 

important from a public health 

perspective and indicative of potential 

exposures and risks to at-risk groups. In 

reaching this proposed judgment, the 

Administrator considered the following 

factors: (1) The estimates of numbers of 

persons exposed at and above the 0.070 

ppm benchmark level; (2) the risk 

estimates of the proportion of the 

population and number of occurrences 

of various health effects in areas upon 

just meeting the current standard; (3) 

the year-to-year and city-to-city 

variability in both the exposure and risk 

estimates; (4) the uncertainties in these 

estimates; and (5) recognition that there 

is a broader array of O3-related adverse 

health outcomes for which risk 

estimates could not be quantified (that 

are part of a broader ‘‘pyramid of 

effects’’) and that the scope of the 

assessment was limited to just a sample 

of urban areas and to some but not all 

at-risk populations, leading to an 

incomplete estimation of public health 

impacts associated with O3 exposures 

across the country. The Administrator 

also noted that it was the unanimous 

conclusion of the CASAC Panel that 

there is no scientific justification for 

retaining the current primary O3

standard, that the current standard is 

not sufficiently health-protective with 

an adequate margin of safety, and that 

the standard needs to be substantially 

reduced to protect human health, 

particularly in at-risk subpopulations. 

Based on all of these considerations, 

the Administrator proposed that the 

current O3 standard is not requisite to 

protect public health with an adequate 

margin of safety because it does not 

provide sufficient protection and that 

revision would result in increased 

public health protection, especially for 

members of at-risk groups. 

2. Comments on the Need for Revision 

The above section outlines the health 

effects evidence and assessments used 

by the Administrator to inform his 

proposed judgments about the adequacy 

of the current O3 primary standard. 

General comments received on the 

proposal that either supported or 

opposed the proposed decision to revise 

the current O3 primary standard are 

addressed in this section. Comments on 

the health effects evidence, which 

includes evidence from controlled 

human exposure and epidemiological 

studies, are considered in section 

II.B.2.a below. Comments on human 

exposure and health risk assessments 

are considered in section II.B.2.b, and 

comments on other policy-related issues 

are considered in section II.B.2.c, below. 

Comments on specific issues, health 

effects evidence, or the human exposure 

and health risk assessments that relate 

to consideration of the appropriate 

averaging time, form, or level of the O3

standard are addressed below in 

sections II.C.3 and II.C.4. General 

comments based on implementationrelated factors that are not a permissible 

basis for considering the need to revise 

the current standard are noted in the 

Response to Comments document. 

a. Consideration of Health Effects 

Evidence 

With regard to the need to revise the 

current primary O3 standard, sharply 

divergent comments were received from 

two general sets of commenters. Many 

public comments received on the 

proposal asserted that the current O3

standard is insufficient to protect public 

health, especially the health of sensitive 

groups, with an adequate margin of 

safety and revisions to the standard are 

appropriate. Among those calling for 

revisions to the current primary 

standard were medical groups, 

including for example, the American 

Medical Association (AMA), the 

American Thoracic Society (ATS), the 

American Academy of Pediatrics (AAP), 

and the American College of Chest 

Physicians (ACCP), as well as medical 

doctors and academic researchers. For 

example, the ATS stated: 

We believe that the Administrator has 

correctly stated that, beyond any degree of 

scientific uncertainty, convincing and 

compelling evidence has demonstrated that 

exposure to ozone at levels below the current 

standard is responsible for measurable and 

significant adverse health effects, both in 

terms of morbidity and mortality. * * * The 

known respiratory, cardiac and perinatal 

effects of ozone pollution are each in their 

own right major public health issues. In 

combination they provide immediate, 

actionable information and require a 

meaningful public health policy response 

from the EPA. [ATS et al. pp. 1, 11] 

Similar conclusions were also reached 

in comments by many national, State, 

and local public health organizations, 

including, for example, the American 

Lung Association (ALA) in a joint set of 

comments with several environmental 

groups, the American Heart Association 

(AHA), the American Nurses 

Association (ANA), the American Public 

Health Association (APHA), and the 

National Association of County and City 

Health Officials (NACCHO), as well as 

in letters to the Administrator from 

EPA’s advisory panel on children’s 

environmental health (Children’s Health 

Protection Advisory Committee; Marty 

et al., 2007a, 2007b). Environmental 

groups also commented in support of 

revising the standard, including the 

Sierra Club, Environmental Defense, the 

Natural Resources Defense Council 

(NRDC), Earthjustice, and the U.S. 

Public Interest Research Group (US 

PIRG). All of these medical, 

environmental and public health 

commenters stated that the current O3

standard needs to be revised and that an 

even more protective standard than 

proposed by EPA is needed to protect 

the health of sensitive population 


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groups. Many individual commenters 

also expressed such views. 

The majority of State and local air 

pollution control authorities who 

commented on the O3 standard 

supported revision of the current O3

standard, as did the National Tribal Air 

Association (NTAA). Environmental 

agencies that supported revising the 

standard include agencies from: 

Arkansas; California; Delaware; Iowa; 

Illinois; Michigan; North Carolina; New 

Mexico; New York; Oklahoma; Oregon; 

Pennsylvania; Utah; Wisconsin; and 

Washington, DC. State organizations, 

including the National Association of 

Clean Air Agencies (NACAA), Northeast 

States for Coordinated Air Use 

Management (NESCAUM), and the 

Ozone Transport Commission (OTC) 

urged that EPA revise the O3 standard. 

All of these commenters supported 

revisions to the current standard, with 

most supporting a standard consistent 

with CASAC’s recommendations. 

In general, the commenters noted 

above primarily based their views on 

the body of evidence assessed in the 

Criteria Document, finding it to be 

stronger and more compelling than in 

the last review. Some specifically agreed 

with the weight of evidence approach 

taken by the Criteria Document. These 

commenters generally placed much 

weight on CASAC’s interpretation of the 

body of available evidence and the 

results of EPA’s exposure and risk 

assessments, both of which formed the 

basis for CASAC’s recommendation to 

revise the O3 standard to provide 

increased public health protection. 

In recent years, a broad scientific 

consensus has emerged that EPA’s current air 

quality standards for ozone are not sufficient 

to protect public health, and that the levels 

and form must be greatly tightened. This 

consensus is evidenced by the by the strong 

unanimous comments of the CASAC, which 

was backed by the endorsement of over 100 

leading independent air quality scientists, 

EPA’s Children’s Health Protection Advisory 

Committee, and many others. In the face of 

this strong consensus, it is untenable to cite 

‘‘uncertainty’’ as a rationale for failing to 

propose tighter standards. [ALA et al., p. 15] 

Medical and public health commenters 

also expressed the view that EPA must 

not use uncertainty in the scientific 

evidence as justification for retaining 

the current O3 standard. 

EPA generally agrees with these 

commenters’ conclusion regarding the 

need to revise the current primary O3

standard. The scientific evidencerelated health effects to O3 exposure 

noted by these commenters was 

generally the same as that assessed in 

the Criteria Document and the proposal. 

EPA agrees that this information 

provides a basis for concluding that the 

current O3 standard is not adequately 

protective of public health. For reasons 

discussed below in sections II.C.3 and 

II.C.4, however, EPA disagrees with 

aspects of these commenters’ views on 

the level of protection that is 

appropriate and supported by the 

available scientific information. 

Another group of commenters 

representing industry associations and 

businesses opposed revising the current 

primary O3 standard. These views were 

extensively presented in comments from 

the Utility Air Regulatory Group 

(UARG), representing a group of electric 

generating companies and organizations 

and several national trade associations, 

and in comments from other industry 

and business associations including, for 

example: Exxon Mobil Corporation; the 

Alliance of Automobile Manufacturers 

(AAM); the National Association of 

Manufacturers (NAM), the American 

Petroleum Institute (API). The API 

sponsored a workshop at the University 

of Rochester in June 2007 to review the 

scientific information and health risk 

assessment considered by EPA during 

the review of the O3 NAAQS. Although 

the report (hereafter, ‘‘Rochester 

Report’’) from this workshop does not 

offer judgments on the specific elements 

of the current or proposed standard, it 

has been cited in a number of public 

comments that opposed revision of the 

current 8-hour standard. The Annapolis 

Center for Science-Based Public Policy 

issued a report (hereafter, ‘‘Annapolis 

Center’’) on the science and health 

effects of O3, which explicitly opposed 

revising the current O3 primary 

standard. Several State environmental 

agencies also opposed revising the 

current O3 primary standard, including 

agencies from: Georgia; Indiana; 

Kentucky; Louisiana; Nevada; and 

Texas. 

As discussed more fully below in 

sections dealing with specific 

comments, these and other commenters 

in this group generally mentioned many 

of the same studies from the body of 

evidence in the Criteria Document that 

were cited by the commenters who 

supported revising the standards, but 

highlighted different aspects of these 

studies in reaching substantially 

different conclusions about their 

strength and the extent to which 

progress has been made in reducing 

uncertainties in the evidence since the 

last review. They then considered 

whether the evidence that has become 

available since the last review has 

established a more certain risk or a risk 

of effects that is significantly different in 

character from those that provided a 

basis for the current standards, or 

whether the evidence demonstrates that 

the risk to public health upon 

attainment of the current standards 

would be greater than was understood 

when EPA established the current O3

standard in 1997. These commenters 

generally expressed the view that the 

current standard provides the requisite 

degree of public health protection. 

In supporting their view that the 

present primary O3 standard continues 

to provide the requisite public health 

protection and should not be revised, 

UARG and others generally stated: That 

the effects of concern have not changed 

significantly since 1997; that the 

uncertainties in the underlying health 

science are as great or greater than in 

1997; that the estimated number of 

exposures of concern and health risks 

upon attainment of the current O3

standard has not changed or decreased 

since 1997; and that ‘‘new’’ studies not 

included in the Criteria Document 

continue to demonstrate uncertainties 

about possible health risks associated 

with exposure to O3 at levels below the 

current standard. As noted above, EPA 

disagrees with this general assessment, 

and agrees with the general position that 

the available information provides a 

basis for concluding that the current O3

standard is not adequately protective of 

public health. The rationale for this 

position is discussed more fully in the 

responses to specific comments that are 

presented below. 

More specific comments on the 

evidence and EPA’s responses are 

discussed below. Section II.B.2.a.i 

contains comments on evidence from 

controlled human exposure studies; 

section II.B.2.a.ii contains comments on 

evidence from epidemiological studies, 

including interpretation of the evidence 

and specific methodological issues. 

Comments on evidence pertaining to atrisk subgroups for O3-related effects can 

be found in section II.B.2.a.iii below. 

EPA notes here that most of the issues 

and concerns raised by commenters 

concerning the health effects evidence, 

including both the interpretation of the 

evidence and specific technical or 

methodological issues, were essentially 

restatements of issues raised during the 

review of the Criteria Document and the 

Staff Paper. Most of these issues were 

highlighted and thoroughly discussed 

during the review of these documents 

by the CASAC. More detailed responses 

related to the interpretation of the 

health effects evidence and its role in 

the decision on the O3 NAAQS are 

contained in the Response to Comments 

document. 


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i. Evidence from Controlled Human 

Exposure Studies 

As noted in the overview of health 

effects evidence, section II.A.2 above, 

two new controlled human-exposure 

studies (Adams 2002, 2006) are now 

available that examine respiratory 

effects associated with prolonged O3

exposures at levels at and below 0.080 

ppm, which was the lowest exposure 

level that had been examined in the last 

review. One group of commenters that 

included national medical (e.g., ATS, 

AMA, ACCP) and national 

environmental and public health 

organizations (e.g., ALA in a joint set of 

comments with Environmental Defense, 

Sierra Club), agreed with EPA’s 

reanalysis of the Adams’ data while 

disagreeing with EPA’s characterization 

of the evidence from the Adams studies 

as ‘‘very limited’’ (72 FR 37870). These 

commenters expressed the view that the 

Adams studies provide evidence of 

effects at lower concentrations than had 

previously been reported. They noted 

that Adams, while finding small group 

mean changes at 0.060 ppm, reported 

total subjective symptom scores reached 

statistical significance (relative to preexposure) at 5.6 and 6.6 hours, with the 

triangular exposure scenario, and that 

pain on deep inspiration values 

followed a similar pattern to total 

subjective symptoms scores. In addition, 

Adams (2002) reports that ‘‘some 

sensitive subjects experience notable 

effects at 0.060 ppm,’’ based on a greater 

than 10% reduction in FEV1. These 

commenters made the point that the 

responses of individuals are more 

important than group mean responses 

and that when the Adams (2002, 2006) 

study data are corrected for the effects 

of exercise in clean air, 7 percent of 

subjects experience FEV1 decrements 

greater than 10% at the 0.040 and 0.060 

ppm exposure levels. They expressed 

the view that while 2 of 30 tested 

subjects responding at the 0.060 ppm 

level may seem like a small number, a 

7 percent response rate is far from 

trivial. Seven percent of the U.S. 

population is 21.2 million people (ALA 

et al., p. 51). Noting that the subjects in 

the Adams’ studies were all healthy 

adults, these groups expressed concern 

that ‘‘in some vulnerable populations 

the magnitude of the response would be 

greater and the exposure level at which 

responses are observed to occur would 

be lower’’ (ATS, p. 4). 

These commenters generally 

supported EPA’s reanalysis of the 

Adams’ data, stating that EPA has 

undertaken a careful reanalysis of the 

underlying data in the Adams studies to 

assess the change in FEV1 following 

exposure to 0.060 ppm O3 and filtered 

air, and concluding that ‘‘the reanalysis 

employs the standard approach used by 

other researchers, and supported by 

CASAC’’ (ALA et al., p. 49), and ‘‘we 

believe that the Adams study shows 

significant health effects at 0.06 ppm 

exposure levels’’ (ATS, p. 5). The 

American Thoracic Society, AMA and 

other medical organizations conclude: 

The Adams study confirms our 

understanding that in healthy populations, 

an important fraction of the population will 

experience larger-than-average decrements in 

FEV1 when exposed to low levels of ozone. 

It is reasonable to assume that these effects 

would be even greater when extrapolated to 

other populations known to have sensitivities 

to ozone (children, asthmatics, COPD 

patients). We feel the correct conclusion to 

draw from the Adams study is that there is 

a significant fraction of the population that 

will express significant responses to low 

levels of ozone. [ATS, p. 5] 

EPA generally agrees with most of the 

comments summarized above, while 

placing more emphasis on the limited 

nature of the evidence addressing O3- 

related lung function and respiratory 

symptom responses at the 0.060 and 

0.040 ppm exposure levels. As 

characterized in the proposal notice, 

EPA’s reanalysis of the data from the 

most recent Adams study shows small 

group mean decrements in lung 

function responses to be statistically 

significant at the 0.060 ppm exposure 

level, while acknowledging that the 

author’s analysis did not yield 

statistically significant lung function 

responses. The Adams studies report a 

small percentage of subjects 

experiencing lung function decrements 

(≥10 percent) at the 0.060 ppm exposure 

level. EPA disagrees with these 

commenters that the percent of subjects 

that experienced FEV1 decrements 

greater than 10% in this study of 30 

subjects can appropriately be 

generalized to the U.S. population. The 

Administrator concludes that these 

studies provide very limited evidence of 

O3-related lung function decrements 

and respiratory symptoms at this lower 

exposure level. 

The second group of commenters, 

who opposed revision of the standard, 

raised many concerns about the role of 

the Adams studies and EPA’s reanalysis 

of the Adams data in the decision. With 

regard to the results reported by Adams, 

these commenters expressed the view 

that the group mean FEV1 decrement 

measured at 0.060 ppm was small, less 

than 3%, which is within the 3 to 5% 

range of normal measurement variability 

for an individual (UARG, p. 12). 

Moreover even the reported group mean 

FEV1 decrements in Adams subjects 

when exposed to an O3 concentration of 

0.080 ppm were described as quite 

minimal, likely non-detectable by the 

subjects and within the range that the 

EPA would consider to be normal or 

mild (UARG, p. 13); With respect to the 

larger decrements in FEV1 (≥ 10%) 

experienced by some subjects in the 

Adams studies, these commenters stated 

the view that such decrements would 

not be considered adverse in healthy 

individuals, and that ‘‘reliance on the 

individual responses of such a 

miniscule number of subjects (2 of 30) 

is woefully inadequate as any basis for 

a nationwide O3 standard’’ (UARG, 

p.14). Some of these commenters put 

the results of the Adams studies (2002, 

2006) in the context of the 1997 

decision on the O3 standard to reach the 

conclusion that there is no basis for 

revising that standard. They stated that 

the data from Adams (2002, 2006) on O3

levels below 0.080 ppm was too limited 

to support a revised standard, and noted 

that responses reported in the Adams 

studies at 0.080 ppm were similar to 

responses reported previously 

(Horstmann et al., 1990 and McDonnell 

et al., 1991), and therefore, provided no 

new information on O3 that was not 

known at the time of EPA’s last review 

(Exxon Mobil, pp. 5–6). 

These commenters raised one or more 

of the following concerns about EPA’s 

reanalysis of the Adams data: (1) EPA’s 

re-analysis was not published or peerreviewed, and therefore neither the 

scientific community nor the public was 

afforded opportunity to appropriately 

review the analysis (Exxon Mobil, p. 6); 

(2) EPA has misinterpreted the studies 

of Dr. Adams, and over his objections 

used a different analytical methodology 

to reach a different conclusion; (3) 

EPA’s reanalysis did not employ an 

appropriate statistical test; the ANOVA 

statistical test employed by Adams was 

preferred over the statistical test used in 

EPA’s reanalysis (paired t-test); and (4) 

the reanalysis of the Adams data is 

evidence that EPA interpreted and 

presented scientific information in a 

systematically biased manner, reflecting 

purposeful bias because the reanalysis 

supported staff policy recommendations 

and Adams’ own analysis did not, and 

the 10% decrement in FEV1 was a posthoc threshold chosen for compatibility 

with EPA staff policy recommendations 

(NAM, p. 19). 

First, EPA agrees that the group mean 

lung function decrement observed in the 

Adams study at the 0.060 ppm exposure 

level is relatively small. However, EPA 

and the CASAC Panel observed that the 

study showed some individuals 

experienced lung function decrements 

≥ 10 percent, which is the most 


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important finding from this study in 

terms of public health implications. The 

magnitude of changes in the group mean 

do not address whether a subset of the 

population is at risk of health effects. 

The clinical evidence to date makes it 

clear that there is significant variability 

in responses across individuals, so it is 

important to look beyond group mean to 

the response of subsets of the group to 

evaluate the potential impact for 

sensitive or susceptible parts of the 

population. The Administrator also 

agrees with both EPA staff and CASAC’s 

views that this level of response may 

not represent an adverse health effect in 

healthy individuals but does represent a 

level that should be considered adverse 

for asthmatic individuals. 

Second, EPA notes that its reanalysis 

of the Adams (2006) study was prepared 

in response to the issues and analysis 

raised by a public commenter who made 

a presentation to the CASAC Panel at its 

March 5, 2007 teleconference. EPA 

replicated the analysis and addressed 

issues raised in these public comments 

concerning the statistical significance of 

0.060 ppm O3 exposure on lung 

function response in the Adams (2006) 

publication. EPA documented its 

response in a technical memorandum 

(Brown, 2007), which was placed in the 

rulemaking docket prior to publication 

of the proposal. EPA has clearly stated 

that the additional statistical analyses 

conducted by both the public 

commenter and by EPA staff do not 

contradict or undercut the statistical 

analysis presented by Dr. Adams in his 

published study, as EPA and the author 

were addressing different questions. 

While the author of the original study 

was focused on determining whether 

the changes observed on an hour-byhour basis were statistically significant 

for different exposure protocols, EPA’s 

reanalysis was focused on the different 

question of whether there was a 

statistically significant difference in 

lung function decrement before and 

after the entire 6.6 hour exposure period 

between the 0.060 ppm exposure 

protocol and filtered air. 

Third, with respect to the concerns 

raised by Dr. Adams and other 

commenters that EPA had used an 

inappropriate statistical approach to 

address the question regarding 

statistical significance of the average 

lung function response at 0.060 ppm, 

members of the CASAC Panel noted on 

the March 5, 2007 teleconference the 

very conservative nature of the 

approach used by Adams to evaluate the 

research questions posed by the author. 

These same CASAC Panel members also 

supported the use of the statistical 

approach (i.e., paired-t test) used in the 

analysis prepared by the public 

commenter, which was the same 

approach later used in EPA’s reanalysis, 

as the preferred method for analyzing 

the pre-minus post-exposure lung 

function responses reported in this 

study. EPA agrees with the 

characterization of the Adams (2006) 

study in the Rochester Report, which 

stated, ‘‘Although these findings have 

not been confirmed or replicated, the 

responses to 0.06 ppm ozone in this 

[Adams] study are consistent with the 

presence of an exposure-response curve 

with responses that do not end abruptly 

below 0.08 ppm.’’ This same report also 

concluded, 

The statistical test used in Adams (2006) 

did not identify the response of the 0.06 ppm 

exposure as statistically different from that of 

the filtered air exposure. However, 

alternative statistical tests suggest that the 

observed small group mean response in FEV1

induced by exposure to 0.06 ppm compared 

to filtered air is not the result of chance 

alone. [Rochester Report, p. 56]. 

Fourth, EPA rejects the contention 

that the conduct and presentation of its 

reanalysis of the Adams (2006) study to 

address issues raised by public 

commenters represents purposeful bias 

and was developed only to support a 

pre-determined policy position. As 

discussed above, EPA’s reanalysis 

addressed a different question than the 

author’s analysis contained in the 

publication. Other controlled human 

exposure studies had routinely 

examined the same question EPA’s 

reanalysis addressed, whether or not 

there was a statistically significant 

group mean response for the entire 

exposure period compared to filtered 

air. 

ii Evidence from Epidemiological 

Studies 

This section contains major comments 

on EPA’s assessment of epidemiological 

studies in the proposal and the Agency’s 

general responses to those comments. 

Many of the issues discussed below are 

addressed in more detail in the 

Response to Comments document. 

Comments on EPA’s interpretation and 

assessment of the body of 

epidemiological evidence are discussed 

first and then comments on 

methodological issues and particular 

study designs are discussed. EPA notes 

here that most of the issues and 

concerns raised by commenters on the 

interpretation of the epidemiological 

evidence and methodological issues are 

essentially restatements of issues raised 

during the review of the Criteria 

Document and Staff Paper. EPA 

presented and the CASAC Panel 

reviewed the interpretation of the 

epidemiological evidence in the Criteria 

Document and the integration of the 

evidence with policy considerations in 

the development of the policy options 

presented in the Staff Paper for 

consideration by the Administrator. 

CASAC reviewed both the O3 Criteria 

Document and O3 Staff Paper and 

approved of the scientific content and 

accuracy of both documents. The 

CASAC chairman sent to the 

Administrator one letter (Henderson, 

2006a) for the O3 Criteria Document and 

another letter for the O3 Staff Paper 

(Henderson, 2006c) indicating that these 

documents provided an appropriate 

basis for use in regulatory decision 

making regarding the O3 NAAQS. 

As with evidence from controlled 

human exposure studies, sharply 

divergent comments were received on 

the evidence from epidemiological 

studies, including EPA’s interpretation 

of the evidence. One group of 

commenters from medical, public health 

and environmental organizations, in 

general, supported EPA’s interpretation 

of the epidemiological evidence (72 FR 

37838, section II.a.3.a–c) with regard to 

whether the evidence for associations is 

consistent and coherent and whether 

there is biological plausibility for 

judging whether exposure to O3 is 

causally related to respiratory and 

cardiovascular morbidity and mortality 

effects. Comments of public health and 

environmental groups, including a joint 

set of comments from ALA and several 

environmental groups, note that more 

than 250 new epidemiological studies, 

published from 1996 to 2005, were 

included in the Criteria Document and 

point to a figure from the Staff Paper 

and proposal (72 FR 37842, Figure 1) of 

short-term O3 exposures and respiratory 

health outcome showing consistency in 

an array of positive effects estimates and 

health endpoints observed in multiple 

locations in Canada and the U.S. 

Medical commenters, including ATS 

and AMA, stated that these ‘‘real world’’

studies support the findings of chamber 

studies to show adverse respiratory 

health effects at levels below the current 

8-hour O3 standard. These commenters 

generally expressed agreement with the 

weight of evidence approach taken by 

the Criteria Document and the 

conclusions reached, which were 

reviewed by CASAC, that the effects of 

O3 on respiratory symptoms, lung 

function changes, emergency 

department visits for respiratory and 

cardiovascular effects, and hospital 

admissions can be considered causal. 

EPA generally agrees with this 

interpretation of the epidemiological 

evidence. The Criteria Document 

concludes that positive and robust 


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19The Hill criteria, published by Sir Bradford Hill 

(1965), are commonly used criteria for reaching 

judgments about causality from observed 

associations, and these criteria were the basis for 

the critical assessment of the epidemiological 

evidence presented in the Criteria Document (pp. 

7–3–7–4). 

associations were found between 

ambient O3 concentrations and various 

respiratory disease hospitalization 

outcomes and emergency department 

visits for asthma, when focusing 

particularly on results of warm-season 

analyses. These positive and robust 

associations are supported by the 

human clinical, animal toxicological, 

and epidemiological evidence for lung 

function decrements, increased 

respiratory symptoms, airway 

inflammation, and increased airway 

responsiveness. Taken together, the 

overall evidence supports a causal 

relationship between acute ambient O3

exposures and increased respiratory 

morbidity outcomes resulting in 

increased emergency department visits 

and hospitalizations during the warm 

season (EPA, 2006a, p. 8–77). 

However, in contrast with EPA, these 

commenters from ALA and other 

environmental, medical and public 

health groups asserted that the causal 

associations extend down to the lowest 

ambient O3 concentrations reported in 

these studies. These commenters also 

expressed the view that the respiratory 

and cardiovascular system effects are 

well-supported by the Hill criteria19 of 

judging causality: strength of 

association, consistency between 

studies, coherence among studies, and 

biological plausibility (ALA et al., pp. 

51–52). They also noted that recent 

studies provide compelling evidence 

that exposure to O3 results in adverse 

cardiovascular health effects (ATS, 

p. 6–7). 

EPA disagrees with the assertion of 

these commenters that the causal 

associations extend down to the lowest 

ambient O3 concentrations reported in 

these studies. The biological plausibility 

of the epidemiological associations is 

generally supported by controlled 

human exposure and toxicological 

evidence of respiratory morbidity effects 

for levels at and below 0.080 ppm, but 

that biological plausibility becomes 

increasingly uncertain at much lower 

levels. Further, at much lower levels, it 

becomes increasingly uncertain as to 

whether the reported associations are 

related to O3 alone rather than to the 

broader mix of air pollutants present in 

the ambient air. With regard to 

cardiovascular health outcomes, the 

Criteria Document concludes that the 

generally limited body of evidence from 

animal toxicology, human controlled 

exposure, and epidemiologic studies is 

suggestive that O3 can directly and/or 

indirectly contribute to cardiovascularrelated morbidity, and that for 

cardiovascular mortality the Criteria 

Document suggests that effects estimates 

are more consistently positive and 

statistically significant in warm season 

analyses but that additional research is 

needed to more fully establish the 

underlying mechanisms by which such 

mortality effects occur (EPA, 2006a, pp. 

8–77–78). 

The second group of commenters, 

mostly representing industry 

associations and some businesses 

opposed to revising the primary O3

standard, disagreed with EPA’s 

interpretation of the epidemiological 

evidence. These commenters expressed 

the view that while many new 

epidemiological studies have been 

published since the current primary O3

standard was promulgated, the 

inconsistencies and uncertainties 

inherent in these studies as a whole 

should preclude any reliance on them as 

justification for a more stringent 

primary O3 NAAQS. They contend that 

the purported consistency is the result 

of inappropriate selectivity in focusing 

on specific studies and specific results 

within those studies (UARG, p. 15). 

With regard to daily mortality, the 

proposal emphasizes the multi-city 

studies, suggesting that they have the 

statistical power to allow the authors to 

reliably distinguish even weak 

relationships from the null hypothesis 

with statistical confidence. However, 

these commenters note that these 

studies are not consistent, with regard to 

the findings concerning individual 

cities analyzed in the multi-city 

analyses. One commenter asserted that 

each of the multi-city studies and metaanalyses cited by EPA involves cities for 

which the city-specific estimates of O3

effects have been observed to vary over 

a wide range that includes negative [i.e., 

beneficial] effects (API, p. 15). To 

illustrate this point, many commenters 

point to EPA’s use of the study by Bell 

et al., 2004. They note that in focusing 

on the national estimate from Bell of the 

association between 24-hour average O3

levels and daily mortality, the 

Administrator overlooks the very 

significant and heterogeneous 

information of the individual analyses 

of the 95 cities used to produce the 

national estimate and, based on this 

inconsistency, question whether what is 

being seen is actually an O3 mortality 

association at all (UARG, p. 16). 

EPA has accurately characterized the 

inconsistencies and uncertainties in the 

epidemiological evidence and strongly 

denies that it has inappropriately 

focused on specific positive studies or 

specific positive results within those 

studies. EPA’s assessment of the health 

effects evidence in the Criteria 

Document has been reviewed by the 

CASAC Panel. EPA has appropriately 

characterized the heterogeneity in O3

health effects in assessing the results of 

the single-city and multi-city studies 

and the meta-analyses, as discussed in 

section 7.6.6 of the Criteria Document. 

In general, in the proposal, the 

Administrator recognized that in the 

body of epidemiological evidence, many 

studies reported positive and 

statistically significant associations, 

while others reported positive results 

that were not statistically significant, 

and a few did not report any positive 

O3-related associations. In addition, the 

Administrator judged that evidence of a 

causal relationship between adverse 

health outcomes and O3 exposures 

became increasingly uncertain at lower 

levels of exposure. 

More specifically, the Bell et al. 

(2004) study observed a statistically 

significant, positive association between 

short-term O3 concentrations (24-hour 

average) and all-cause mortality using 

data from 95 U.S. National Morbidity, 

Mortality, and Air Pollution Study 

(NMMAPS) communities. The objective 

of the NMMAPS was to develop an 

overall national effect estimate using 

multi-city time-series analyses, by 

drawing on information from all of the 

individual cities. The strength of this 

approach is the use of a uniform 

analytic methodology, avoidance of 

selection bias, and larger statistical 

power. Significant intercity 

heterogeneity was noted in the Bell et 

al. and other multi-city studies, 

probably due to many factors, including 

city-specific differences in pollution 

characteristics, the use of air 

conditioning, time spent indoors versus 

outdoors, and socioeconomic factors. 

Levy et al. (2005) found suggestive 

evidence that air conditioning 

prevalence was a predictor of 

heterogeneity in O3 risk estimates in 

their meta-analysis. 

Several commenters argued that EPA 

overstates the probability of causal links 

between health effects and exposure to 

O3, especially at the lower 

concentrations examined, and that the 

statistical associations found in the 

cited epidemiological studies do not 

automatically imply that a causal 

relationship exists. These commenters 

expressed the view that the correlation 

between health effects and O3 exposure 

must be rigorously evaluated according 

to a standard set of criteria before 

concluding that there is a causal link 

and that EPA fails to articulate and 


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follow the weight of the evidence or 

established causality criteria for 

evaluating epidemiological studies in 

drawing conclusion regarding causality 

(Exxon Mobil, pp. 10–11). 

In the proposal, EPA explicitly stated 

that epidemiological studies are not 

themselves direct evidence of a causal 

link between exposure to O3 and the 

occurrence of effects (72 FR 37879). 

Throughout the O3 review, a standard 

set of criteria have been used to evaluate 

evidence of a causal link. The critical 

assessment of epidemiological evidence 

presented in the Criteria Document was 

conceptually based upon consideration 

of salient aspects of the evidence of 

associations so as to reach fundamental 

judgments as to the likely causal 

significance of the observed associations 

in accordance with the Hill criteria 

(Criteria Document, pp. 7–3—7–4). 

Moreover, consistent with the proposal 

the Administrator has specifically 

considered evidence from 

epidemiological studies in the context 

of all the other available evidence in 

evaluating the degree of certainty that 

O3-related adverse health effects occur 

at various levels at and below 0.080 

ppm, including the strong evidence 

from controlled human exposure studies 

and the toxicological studies that 

demonstrate biological plausibility and 

mechanisms for effects. More detailed 

discussion of the criteria used to 

evaluate evidence with regard to 

judgments about causality can be found 

in the Response to Comments 

document. 

Several commenters made the point 

that the results of the new 

epidemiological studies included in this 

review are not coherent. They state that 

although EPA notes that estimates of 

risk from cardiovascular mortality are 

higher than those for total mortality and 

indicates that these findings are highly 

suggestive that short-term O3 exposure 

directly or indirectly contributes to 

cardiovascular mortality, the Agency 

fails to contrast the mortality studies to 

studies of hospital admissions for 

cardiovascular causes. Most studies of 

cardiovascular causes have not found 

statistically significant associations with 

O3 exposures (UARG, pp. 16–17). 

EPA strongly disagrees that it has 

failed to appropriately characterize the 

association between O3 exposure and 

potential cardiovascular morbidity and 

mortality effects. As noted above, the 

Criteria Document characterizes the 

overall body of evidence as limited, but 

highly suggestive, and concludes that 

much needs to be done to more fully 

integrate links between ambient O3

exposures and adverse cardiovascular 

outcomes (EPA, 2006a, p. 8–77). Some 

field/panel studies that examined 

associations between O3 and various 

cardiac physiologic endpoints have 

yielded limited epidemiological 

evidence suggestive of a potential 

association between acute O3 exposure 

and altered HRV, ventricular 

arrhythmias, and incidence of 

myocardial infarction (Criteria 

Document, section 7.2.7). In addition, 

there were approximately 20 single-city 

studies of emergency department visits 

and hospital admissions for all 

cardiovascular diseases or specific 

diseases (i.e., myocardial infarction, 

congestive heart failure, ischemic heart 

disease, dysrhythmias). In the studies 

using all year data, many showed 

positive results but few were 

statistically significant. Given the strong 

seasonal variations in O3 concentrations 

and the changing relationship between 

O3 and other copollutants by season, 

inadequate adjustment for seasonal 

effects might have masked or 

underestimated the associations. In the 

limited number of studies that analyzed 

data by season (6 studies), statistically 

significant associations were observed 

in all but one study (Criteria Document, 

section 7.3.4). Newly available animal 

toxicology data provide some 

plausibility for the observed 

associations between O3 and 

cardiovascular outcomes. EPA believes 

that its characterization of the evidence 

for O3-related cardiovascular system 

effects is appropriate. It is clear that 

coherence is stronger in the much larger 

body of evidence of O3-related 

respiratory morbidity and mortality 

effects. 

Many commenters who did not 

support revising the current O3 primary 

standard also submitted comments on 

specific methodological issues related to 

the epidemiological evidence, 

including: The adequacy of exposure 

data; confounding by copollutants; 

model selection; evidence of mortality; 

and, new studies not included in the 

Criteria Document. Some of the major 

comments on methodological issues 

raised by these commenters are 

discussed below. The Response to 

Comments document contains more 

detailed responses to many of these 

comments, as well as responses to other 

comments not considered here. 

(1) Adequacy of exposure data. Many 

commenters expressed concern about 

the adequacy of exposure data both for 

time-series and panel studies. These 

commenters argued that almost all of 

the epidemiological studies on which 

EPA relies in recommending a more 

stringent O3 standard are based on data 

from ambient monitors for which there 

is a poor correlation with the actual 

personal exposure subjects receive 

during their daily activities. They 

questioned the Administrator’s 

conclusion that in the absence of 

available data on personal O3 exposure, 

the use of routinely monitored ambient 

O3 concentrations as a surrogate for 

personal exposures is not generally 

expected to change the principal 

conclusions from epidemiological 

studies. These commenters also note 

that, in its June 2006 letter, the CASAC 

Panel raised the issue of exposure error, 

concluding that it called into question 

whether observed associations could be 

attributed to O3 alone (API, p. 17). One 

of these commenters cited studies (e.g., 

Sarnat et al., 2001; Sarnat et al., 2005) 

that show a lack of correlation between 

personal exposures and ambient 

concentrations (NAM, p. 22). Another 

cited studies (Sarnat et al., 2001, 2005, 

and 2006; and Koutrakis et al., 2005) 

that have found that the ability of 

ambient gas monitors to represent 

personal exposure to such gases is 

similarly quite limited, including: (1) 

Most personal exposures are so low as 

to be not detectable at a level of 5 parts 

per billion (ppb), resulting in very low 

correlation between concentrations 

reported from central ambient monitors 

and personal monitors; (2) O3

measurements from ambient monitors 

are a better surrogate for personal 

exposure to PM2.5 than to O3; and (3) 

populations expected to be potentially 

susceptible to O3, including children, 

the elderly, and those with COPD, are at 

the low end of the population exposure 

distribution (Exxon Mobil, pp. 15–16). 

These commenters contended that 

without such a correlation there is no 

legitimate way for EPA to conclude that 

O3 exposure has caused the reported 

health effects, or to conclude that use of 

routinely monitored ambient O3

concentrations as a surrogate for 

personal exposures is adequate. Some of 

these commenters also contended that 

EPA incorrectly concludes that the 

exposure error in epidemiological 

studies results in an underestimate of 

risk (Exxon Mobil, p. 20). 

With regard to the views on exposure 

measurement error expressed by 

CASAC, while the commenter is correct 

that the CASAC Panel raised the 

question of exposure error and whether 

observed associations could be 

attributed to O3 alone, the commenter 

failed to note that CASAC’s comment 

was focused on the association between 

O3 and mortality, at very low O3

concentrations and in the group of 

people most susceptible to premature 

mortality. The CASAC Panel in its June 

2006 letter stated: 


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The population that would be expected to 

be potentially susceptible to dying from 

exposure to ozone is likely to have ozone 

exposures that are at the lower end of the 

ozone population distribution, in which case 

the population would be exposed to very low 

ozone concentrations, and especially so in 

winter. Therefore it seems unlikely that the 

observed associations between short-term 

ozone concentrations and daily mortality are 

due solely to ozone itself. [Henderson 2006b, 

pp. 3–4] 

This section of the quote, which was 

not addressed in the comment 

submitted by API, together with the 

conclusions in the final CASAC letter 

(Henderson, 2007), leads EPA to 

conclude that contrary to the 

commenters’ assertion, the CASAC 

Panel was not calling into question the 

association between O3 exposure and 

the full range of morbidity effects found 

in panel or time-series studies that rely 

on ambient monitoring data as a 

surrogate for personal exposure data. It 

is important to note that EPA agrees that 

the evidence is only highly suggestive 

that O3 directly or indirectly contributes 

to mortality, as compared to the stronger 

evidence of causality for respiratory 

morbidity effects. 

EPA agrees that exposure 

measurement error may result from the 

use of stationary ambient monitors as an 

indicator of personal exposure in 

population studies. There is a full 

discussion of measurement error and its 

effect on the estimates of relative risk in 

section 7.1.3.1 of the Criteria Document. 

However, the possibility of 

measurement error does not preclude 

the use of ambient monitoring data as a 

surrogate for personal exposure data in 

time-series or panel studies. It simply 

means that in some situations where the 

likelihood of measurement error is 

greatest, effects estimates must be 

evaluated carefully and that caution 

must be used in interpreting the results 

from these studies. Throughout this 

review, EPA has recognized this 

concern. The Criteria Document states 

that there is supportive evidence that 

ambient O3 concentrations from central 

monitors may serve as valid surrogate 

measures for mean personal O3

exposures experienced by the 

population, which is of most relevance 

to time-series studies, in which 

individual variations in factors affecting 

exposure tend to average out across the 

study population. This is especially true 

for respiratory hospital admission 

studies for which much of the response 

is attributable to O3 effects on 

asthmatics. In children, for whom 

asthma is more prevalent than for 

adults, ambient monitors are more likely 

to correlate reasonably well with 

personal exposure to O3 of ambient 

origin because children tend to spend 

more time outdoors than adults in the 

warm season. EPA does not agree that 

the correlation between personal 

exposure and ambient monitoring data 

is necessarily poor, especially in 

children. Moreover, the CASAC Panel 

supported this view as they noted that 

‘‘[p]ersonal exposures most likely 

correlate better with central site values 

for those subpopulations that spend a 

good deal of time outdoors, which 

coincides, for example, with children 

actively engaged in outdoor activities, 

and which happens to be a group that 

the ozone risk assessment focuses 

upon.’’ (Henderson, 2006c. p. 10). 

However, the Criteria Document notes 

that there is some concern in 

considering certain mortality and 

hospitalization time-series studies 

regarding the extent to which ambient 

O3 concentrations are representative of 

personal O3 exposures in another 

particularly susceptible group of 

individuals, the debilitated elderly, as 

the correlation between the two 

measurements has not been examined in 

this population. A better understanding 

of the relationship between ambient 

concentrations and personal exposures, 

as well as of the factors that affect the 

relationship, will improve the 

interpretation of observed associations 

between ambient concentration and 

population health response. 

With regard to the specific comments 

that reference the findings of studies by 

Sarnat et al. (2001, 2005, 2006) and 

Koutrakis et al. (2005), the fact that 

personal exposure monitors cannot 

detect O3 levels of 5 ppb and below may 

in part explain why there was a poor 

correlation between personal exposure 

measurements and ambient monitoring 

data in the winter relative to the 

correlation in the warm season, along 

with differences in activity patterns and 

building ventilation. In one study 

conducted in Baltimore, Sarnat et al. 

(2001) observed that ambient O3

concentrations showed stronger 

associations with personal exposure to 

PM2.5 than to O3; however, in a later 

study conducted in Boston (Sarnat et al., 

2005), ambient O3 concentrations and 

personal O3 exposures were found to be 

significantly associated in the summer. 

Another study cited by the commenter, 

but not included in the Criteria 

Document, conducted in Steubenville 

(Sarnat et al., 2006), also observed 

significant associations between 

ambient O3 concentrations and personal 

O3. The authors noted that the cityspecific discrepancy in the results may 

be attributable to differences in 

ventilation. Though the studies by 

Sarnat et al. (2001, 2005, and 2006) 

included senior citizens, the study 

selection criteria required them to be 

nonsmoking and physically healthy. 

EPA is not relying on studies that are 

not in the Criteria Document, such as 

Sarnat et al. (2006), to refute the 

commenters. However, EPA notes that 

Sarnat et al. (2006) does not support the 

conclusion drawn by the commenters 

that this study shows very limited 

associations between ambient O3

concentrations and personal exposures. 

Existing epidemiologic models may 

not fully take into consideration all the 

biologically relevant exposure history or 

reflect the complexities of all the 

underlying biological processes. Using 

ambient concentrations to determine 

exposure generally overestimates true 

personal O3 exposures (by 

approximately 2- to 4-fold in the various 

studies described in the Criteria 

Document, section 3.9), which assuming 

the relationship is causal, would result 

in biased descriptions of underlying 

concentration-response relationships 

(i.e., in attenuated effect estimates). 

From this perspective, the implication is 

that the effects being estimated in 

relationship to ambient levels occur at 

fairly low personal exposures and the 

potency of O3 is greater than these effect 

estimates indicate. On the other hand, 

as very few studies evaluating O3 health 

effects with personal O3 exposure 

measurements exist in the literature, 

effect estimates determined from 

ambient O3 concentrations must be 

evaluated and used with caution to 

assess the health risks of O3 (Criteria 

Document, pp. 7–8 to 7–10). 

Nonetheless, as noted in section II.C.3 of 

the proposal, the use of routinely 

monitored ambient O3 concentrations as 

a surrogate for personal exposures is not 

generally expected to change the 

principal conclusions from O3

epidemiologic studies. Therefore, 

population risk estimates derived using 

ambient O3 concentrations from 

currently available observational 

studies, with appropriate caveats about 

personal exposure considerations, 

remain useful (72 FR 37839). 

(2) Confounding by copollutants. 

Many commenters argued that known 

confounders are inadequately controlled 

in the epidemiological studies of O3 and 

various health outcomes and that the 

health effects of O3 are often not 

statistically significant when 

epidemiological studies consider the 

effects of confounding air pollutants 

(e.g., PM2.5, CO, nitrogen dioxide (NO2) 

in multi-pollutant models. For example, 

Mortimer et al. (2002), a large multi-city 

asthma panel study, found that when 


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other pollutants, i.e., sulfur dioxide 

(SO2), NO2, and particles with an 

aerodynamic diameter less than or equal 

to a nominal 10 micrometers (PM10), 

were placed in a multi-pollutant model 

with O3, the O3-related associations 

with respiratory symptoms and lung 

function became non-significant. 

The National Cooperative Inner-City 

Asthma Study (Mortimer et al., 2002) 

evaluated air pollution health effects in 

846 asthmatic children in 8 urban areas. 

The pollutants evaluated included O3, 

PM10, SO2, and NO2. Three effects were 

evaluated: (1) Daily percent change in 

lung function, measured as peak 

expiratory flow rate (PEFR); (2) 

incidence of (≥ 10% reduction in lung 

function (PEFR); and, (3) incidence of 

symptoms (i.e., cough, chest tightness, 

and wheeze). EPA notes that in this 

study, O3 was the only pollutant 

associated with reduction in lung 

function. Nitrogen dioxide had the 

strongest effect on morning symptoms, 

and the authors concluded it ‘‘* * * 

may be a better marker for the summerpollutant mix in these cities’’ but had no 

association with morning lung function. 

In a two-pollutant model with NO2, the 

O3 effect on morning symptoms 

remained relatively unchanged. Sulfur 

dioxide had statistically significant 

effects on morning symptoms but no 

association with morning lung function. 

Particulate matter (PM10), which was 

measured daily in 3 cities, had no 

statistically significant effect on 

morning lung function. In a twopollutant model with O3, the PM10

estimate for morning symptoms was 

slightly reduced and there was a larger 

reduction in the O3 estimate, which 

remained positive but not statistically 

significant. A more general discussion 

and response to this issue concerning 

confounding by copollutants is 

presented in the Response to Comments 

document. 

(3) Model selection. Commenters who 

did not support revision of the primary 

O3 standard raised issues regarding the 

adequacy of model specification 

including control of temporal and 

weather variables in the time-series 

epidemiological studies that EPA has 

claimed support the finding of O3- 

related morbidity and mortality health 

outcomes. Specifically, concerns were 

expressed regarding the following 

issues: (i) Commenters noted that recent 

meta-analyses have confirmed the 

important effects of model selection in 

the results of the time-series studies, 

including the choice of models to 

address weather and the degree of 

smoothing, in direct contradiction of the 

Staff Paper’s conclusion on the 

robustness of the models used in the O3

time-series studies (Exxon Mobil, p. 41); 

(ii) commenters contended that there 

were no criteria for how confounders 

such as temperature or other factors 

were to be addressed, resulting in 

arbitrary model selection potentially 

impacting the resulting effect estimates; 

and (iii) commenters expressed the view 

that to appropriately address concerns 

about model selection in the O3 timeseries studies, EPA should rely on an 

alternative statistical approach, 

Bayesian model averaging, that 

incorporates a range of models 

addressing confounding variables, 

pollutants, and lags rather than a single 

model. 

In response to the first issue, EPA 

agrees that the results of the metaanalyses do support the conclusion that 

there are important effects of model 

selection and that, for example, 

alternative models to address weather 

might make a difference of a factor of 

two in the effect estimates. However, as 

noted in the Criteria Document, one of 

the meta-analyses (Ito et al., 2005) 

suggested that the stringent weather 

model used in the Bell et al. (2004) 

NMMAPS study may tend to yield 

smaller effect estimates than those used 

in other studies (Criteria Document, p. 

7–96), and, thus concerns about 

appropriate choice of models could 

result in either higher or lower effect 

estimates than reported. In addressing 

this issue, the Criteria Document 

concluded, 

Considering the wide variability in 

possible study designs and statistical model 

specification choices, the reported O3 risk 

estimates for the various health outcomes are 

in reasonably good agreement. In the case of 

O3-mortality time-series studies, 

combinations of choices in model 

specifications * * * alone may explain the 

extent of difference in O3 risk estimates 

across studies. (Criteria Document, p. 7–174) 

Second, the issues surrounding 

sensitivity to model specifications were 

thoroughly discussed in the Criteria 

Document (see section 7.1.3.6) and 

evaluated in some of the meta-analyses 

reviewed in the Criteria Document and 

Staff Paper. As stated in the Criteria 

Document, O3 effect estimates ‘‘were 

generally more sensitive to alternative 

weather models than to varying degrees 

of freedom for temporal trend 

adjustment’’ (Criteria Document, p. 7–

176). The Criteria Document also 

concluded that ‘‘although there is some 

concern regarding the use of 

multipollutant models * * * results 

generally suggest that the inclusion of 

copollutants into the models do not 

substantially affect O3 risk estimates’’

and the results of the time-series studies 

are ‘‘robust and independent of the 

effects of other copollutants’’ (Criteria 

Document, p. 7–177). Overall, EPA 

continues to believe that based on its 

integrated assessment, the time-series 

studies provide strong support for 

concluding there are O3-related 

morbidity effects, including respiratoryrelated hospital admissions and 

emergency department visits during the 

warm season, and that the time-series 

studies provide findings that are highly 

suggestive that short-term O3 exposure 

directly or indirectly contributes to nonaccidental and cardiorespiratory-related 

mortality. 

The Administrator acknowledges that 

uncertainties concerning appropriate 

model selection are an important source 

of uncertainty affecting the specific risk 

estimates included in EPA’s risk 

assessment and that these quantitative 

risk estimates must be used with 

appropriate caution, keeping in mind 

these important uncertainties, as 

discussed above in section II.A.3. As 

discussed later in this notice, the 

Administrator is not relying on any 

specific quantitative effect estimates 

from the time-series studies or any risk 

estimates based on the time-series 

studies in reaching his judgment about 

the need to revise the current 8-hour O3

standard. 

Third, in response to commenters 

who suggested that EPA adopt an 

alternative statistical approach, i.e., 

Bayesian model averaging, to address 

concerns about potential arbitrary 

selection of models, the Criteria 

Document evaluated the strengths and 

weaknesses of such methods in the 

context of air pollution epidemiology. 

The Criteria Document noted several 

limitations, especially where there are 

many interaction terms and 

meteorological variables and where 

variables are highly correlated, as is the 

case for air pollution studies, which 

makes it very difficult to interpret the 

results using this alternative approach. 

EPA believes further research is needed 

to address concerns about model 

selection and to develop appropriate 

methods addressing these concerns. 

(4) Evidence of mortality. Many 

commenters, including those that 

argued for revising the current O3

standard as well as those that argued 

against revisions, focused on the new 

evidence from multi-city time-series 

analyses and meta-analyses linking O3

exposure with mortality. Again, the 

comments were highly polarized. One 

set of commenters, including medical, 

public health, and environmental 

organizations argued that recent 

published research has provided more 

robust, consistent evidence linking O3 to 

cardiovascular and respiratory 


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mortality. The ATS, AMA, and others 

stated that data from single-city studies, 

multiple-city studies, and meta-analyses 

show a consistent relationship between 

O3 exposure and mortality from 

respiratory and cardiovascular causes. 

These commenters noted that this effect 

was observed after controlling for copollutants and seasonal impacts. These 

commenters stated that research has 

demonstrated that exposure to O3

pollution is causing premature deaths, 

and has also provided clues on the 

possible mechanisms that lead to 

premature mortality (ATS, p. 4). These 

commenters noted that people may die 

from O3 exposure even when the 

concentrations are well below the 

current standard. They pointed to a 

study (Bell et al., 2006) in which the 

authors followed up on their 2004 

multi-city study to estimate the 

exposure-response curve for O3 and the 

risk of mortality and to evaluate 

whether a threshold exists below which 

there is no effect. The authors applied 

several statistical models to data on air 

pollution, weather, and mortality for 98 

U.S. urban communities for the period 

1987 to 2000. The study reported that 

O3 and mortality results did not appear 

to be confounded by temperature or PM 

and showed that any threshold, if it 

existed, would have to be at very low 

concentrations, far below the current 

standard (ALA et al., p. 74). Another 

approach also indicated that the 

mortality effect is unlikely to be 

confounded by temperature. A casecrossover study (Schwartz 2005) of over 

one million deaths in 14 U.S. cities, 

designed to control for the effect of 

temperature on daily deaths attributable 

to O3, found that the association 

between O3 and mortality risk reported 

in the multi-city studies is unlikely to 

be due to confounding by temperature 

(ALA et al., p. 76). These commenters 

argue that meta-analyses also provide 

compelling evidence that the O3- 

mortality findings are consistent. They 

point to three independent analyses 

conducted by separate research groups 

at Johns Hopkins University, Harvard 

University and New York University, 

using their own methods and study 

criteria, which reported a remarkably 

consistent link between daily O3 levels 

and total mortality. 

In response, EPA notes that the 

Criteria Document states that the results 

from the U.S. multi-city time-series 

studies provide the strongest evidence 

to date for O3 effects on acute mortality. 

Recent meta-analyses also indicate 

positive risk estimates that are unlikely 

to be confounded by PM; however, 

future work is needed to better 

understand the influence of model 

specifications on the risk coefficient 

(EPA, 2006a, p. 7–175). The Criteria 

Document concludes that these findings 

are highly suggestive that short-term O3

exposure directly or indirectly 

contributes to non-accidental and 

cardiorespiratory-related mortality but 

that additional research is needed to 

more fully establish the underlying 

mechanisms by which such effects 

occur (72 FR 37836). Thus while EPA 

generally agrees with the direction of 

the comment, EPA believes the evidence 

supports a view as noted above. In 

addition, it must be noted that the 

Administrator did not focus on 

mortality as a basis for proposing that 

the current O3 standard was not 

adequate. In the proposal, the 

Administrator focused on the very 

strong evidence of respiratory morbidity 

effects in healthy people at the 0.080 

ppm exposure level and new evidence 

that people with asthma are likely to 

experience larger and more serious 

effects than healthy people at the same 

level of exposure (72 FR 37870). With 

regard to the ambient concentrations at 

which O3-related mortality effects may 

be occurring, EPA recognized in the 

proposal that evidence of a causal 

relationship between adverse health 

effects and O3 exposures becomes 

increasingly uncertain at lower levels of 

exposure (72 FR 37880). This is 

discussed more fully in section (b) 

below. 

Several industry organizations argued 

against placing any reliance on the timeseries epidemiological studies, 

especially those studies related to 

mortality effects. The Annapolis Center 

(p. 46) makes the point that although 

there may be somewhat more positive 

associations than negative associations, 

there is so much noise or variability in 

the data that identifying which positive 

associations may be real health effects 

and which are not is beyond the 

capability of current methods. They cite 

the view that the CASAC Panel 

expressed in a June 2006 letter 

(Henderson, 2006b), noting that 

‘‘Because results of time-series studies 

implicate all of the criteria pollutants, 

findings of mortality time-series studies 

do not seem to allow us to confidently 

attribute observed effects specifically to 

individual pollutants.’’

Because of the importance of the O3

mortality multi-city studies in EPA’s 

analysis of this issue, several of these 

commenters focused on them in 

particular, arguing that, although these 

studies have the statistical power to 

distinguish weak relationships between 

daily O3 and mortality, they do not 

provide reliable or consistent evidence 

implicating O3 exposures as a cause of 

mortality. Several reasons were given, 

including: (a) The multi-city studies 

cited by EPA involve a wide range of 

city-specific effects estimates, including 

some large cities that have very slight or 

negligible effects (e.g., Los Angeles) 

(Bell et al., 2004), thus causing several 

commenters to question the relevance of 

a ‘‘national’’ effect of O3 on mortality 

and argue that a single national O3

concentration-mortality coefficient 

should be used and interpreted with 

caution (Rochester Report p. 4); (b) the 

multi-city mortality studies did not 

sufficiently account for other pollutants, 

for example, Bell et al. (2004) adjusted 

for PM10 but did not have the necessary 

air quality data to adequately adjust for 

PM2.5, which EPA has concluded also 

causes mortality and is correlated with 

O3, especially in the summer months 

(Annapolis Center, p. 42); and (c) these 

studies contain several findings that are 

inconsistent or implausible, such as 

premature mortality reported at such 

low levels as to imply that O3-related 

mortality is occurring at levels well 

within natural background, which is not 

biologically plausible (Annapolis 

Center, p. 42). 

Evidence supporting an association 

between short-term O3 exposure and 

premature mortality is not limited to 

multi-city time-series studies. Most 

single-city studies show elevated risk of 

total, non-accidental mortality, 

cardiorespiratory, and respiratory 

mortality (> 20 studies), including one 

study in an area that would have met 

current standard (Vedal et al., 2003). 

Three large meta-analyses, which pool 

data from many single-city studies to 

increase statistical power, reported 

statistically significant associations and 

examined sources of heterogeneity in 

those associations (Bell et al., 2005; Ito 

et al., 2005; Levy et al. 2005). These 

studies found: (1) Larger and more 

significant effects in the warm season 

than in the cool season or all year; (2) 

no strong evidence of confounding by 

PM; and (3) suggestive evidence of 

publication bias, but significant 

associations remain even after 

adjustment for the publication bias. 

Moreover, EPA asserts that the 

biological plausibility of the 

epidemiological mortality associations 

is generally supported by controlled 

human exposure and toxicological 

evidence of respiratory morbidity effects 

for levels at and below 0.080 ppm, but 

that biological plausibility becomes 

increasingly uncertain especially below 

0.060 ppm, the lowest level at which 

effects were observed in controlled 

human exposure studies. Further, at 

lower levels, it becomes increasingly 


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uncertain as to whether the reported 

associations are related to O3 alone 

rather than to the broader mix of air 

pollutants present in the ambient air. 

EPA agrees that the multi-city times 

series studies evaluated in this review 

do not completely resolve this issue. It 

also becomes increasingly uncertain as 

to whether effect thresholds exist but 

cannot be clearly discerned by statistical 

analyses. Thus, when considering the 

epidemiological evidence in light of the 

other available information, it is 

reasonable to judge that at some point 

the epidemiological associations cannot 

be interpreted with confidence as 

providing evidence that the observed 

health effects can be attributed to O3

alone. 

In the letter cited, the CASAC Panel 

did raise the issue of the utility of timeseries studies in the standard setting 

process with regard to time-series 

mortality studies. Nevertheless, in a 

subsequent letter to the Administrator, 

CASAC noted these mortality studies as 

evidence to support a recommendation 

to revise the current primary O3

standard. ‘‘Several new single-city 

studies and large multi-city studies 

designed specifically to examine the 

effects of ozone and other pollutants on 

both morbidity and mortality have 

provided more evidence for adverse 

health effects at concentrations lower 

than the current standard (Henderson, 

2006c, p. 3).’’

With regard to the specific issues 

raised in the comments as to why the 

times-series mortality studies do not 

provide reliable or consistent evidence 

implicating O3 exposure as a cause of 

mortality, EPA has the following 

responses: 

(a) The purpose of the NMMAPS 

approach is not to single out individual 

city results but rather to estimate the 

overall effect from the 95 communities. 

It was designed to provide a general, 

nationwide estimate. With regard to the 

very slight or negligible effects estimates 

for some large cities (e.g., Los Angeles), 

an important factor to consider is that 

the Bell et al. (2004) study used all 

available data in their analyses. Bell et 

al., reported that the effect estimate for 

all available (including 55 cities with all 

year data) and warm season (April–

October) analyses for the 95 U.S. cities 

were similar in magnitude; however, in 

most other studies, larger excess 

mortality risks were reported in the 

summer season (generally June–August 

when O3 concentrations are the highest) 

compared to all year or the cold season. 

Though the effect estimate for Los 

Angeles is small compared to some of 

the other communities, it should be 

noted that all year data (combined warm 

and cool seasons) was used in the 

analyses for this city, which likely 

resulted in a smaller effect estimate. 

Because all year data was used for Los 

Angeles, the median O3 concentration 

for Los Angeles is fairly low compared 

to the other communities, ranked 23rd 

out of 95 communities. The median 24- 

hour average O3 concentration for Los 

Angeles in this dataset was 22 ppb, with 

a 10th percentile of 8 ppb to a 90th 

percentile of 38 ppb. The importance of 

seasonal differences in O3-related health 

outcomes has been well documented. 

(b) In section 7.4.6, O3 mortality risk 

estimates adjusting for PM exposure, the 

Criteria Document states that the main 

confounders of interest for O3, 

especially for the northeast U.S., are 

‘‘summer haze-type’’ pollutants such as 

acid aerosols and sulfates. Since very 

few studies included these chemical 

measurements, PM (especially PM2.5) 

data, may serve as surrogates. However, 

due to the expected high correlation 

among the constituents of the ‘‘summer 

haze mix,’’ multipollutant models 

including these pollutants may result in 

unstable coefficients; and, therefore, 

interpretation of such results requires 

some caution. 

In this section, Figure 7–22 shows the 

O3 risk estimates with and without 

adjustment for PM indices using all-year 

data in studies that conducted twopollutant analyses. Approximately half 

of the O3 risk estimates increased 

slightly, whereas the other half 

decreased slightly with the inclusion of 

PM in the models. In general, the O3

mortality risk estimates were robust to 

adjustment for PM in the models. 

The U.S. 95 communities study by 

Bell et al. (2004) examined the 

sensitivity of acute O3-mortality effects 

to potential confounding by PM10. 

Restricting analysis to days when both 

O3 and PM10 data were available, the 

community-specific O3-mortality effect 

estimates as well as the national average 

results indicated that O3 was robust to 

adjustment for PM10 (Bell et al., 2004). 

As commenters noted, there were 

insufficient data available to examine 

potential confounding by PM2.5. One 

study (Lipfert et al., 2000) reported O3

risk estimates with and without 

adjustment for sulfate, a component of 

PM2.5. Lipfert et al. (2000) calculated O3

risk estimates based on mean (45 ppb) 

less background (not stated) levels of 1- 

hour max O3 in seven counties in 

Pennsylvania and New Jersey. The O3

risk estimate was not substantially 

affected by the addition of sulfate in the 

model (3.2% versus 3.0% with sulfate) 

and remained statistically significant. 

Several O3 mortality studies examined 

the effect of confounding by PM indices 

in different seasons (Figure 7–23, 

section 7.4.6, Criteria Document). In 

analyses using all-year data and warmseason only data, O3 risk estimates were 

once again fairly robust to adjustment 

for PM indices, with values showing 

both slight increases and decreases with 

the inclusion of PM in the model. In the 

analyses using cool season data only, 

the O3 risk estimates all increased 

slightly with the adjustment of PM 

indices, although none reached 

statistical significance. 

The three recent meta-analyses (Bell 

et al., 2005; Ito et al., 2005; Levy et al., 

2005) all examined the influence of PM 

on O3 risk estimates. No substantial 

influence was observed in any of these 

studies. In the analysis by Bell et al. 

(2005), the combined estimate without 

PM adjustment was 1.75% (95% PI: 

1.10, 2.37) from 41 estimates, and the 

combined estimate with PM adjustment 

was 1.95% (95% PI: ¥0.06, 4.00) from 

11 estimates per 20 ppb increase in 24- 

hour average O3. In the meta-analysis of 

15 cities by Ito et al. (2005), the 

combined estimate was 1.6% (95% CI: 

1.1, 2.2) and 1.5% (95% CI: 0.8, 2.2) per 

20 ppb in 24-hour average O3 without 

and with PM adjustment, respectively. 

The additional time-series analysis of 

six cities by Ito et al. found that the 

influence of PM by season varied across 

alternative weather models but was 

never substantial. Levy et al. (2005) 

examined the regression relationships 

between O3 and PM indices (PM10 and 

2.5) with O3-mortality effect estimates for 

all year and by season. Positive slopes, 

which might indicate potential 

confounding, were observed for PM2.5

on O3 risk estimates in the summer and 

all-year periods, but the relationships 

were weak. The effect of one causal 

variable (i.e., O3) is expected to be 

overestimated when a second causal 

variable (e.g., PM) is excluded from the 

analysis, if the two variables are 

positively correlated and act in the same 

direction. However, EPA notes that the 

results from these meta-analyses, as well 

as several single- and multiple-city 

studies, indicate that copollutants, 

including PM, generally do not appear 

to substantially confound the 

association between O3 and mortality. 

(c) With regard to the biological 

plausibility of O3-related mortality 

occurring at levels well within natural 

background, EPA concluded in the 

proposal that additional research is 

needed to more fully establish 

underlying mechanisms by which 

mortality effects occur (72 FR 37836). 

Such research would likely also help 

determine whether it is plausible that 

mortality would occur at such low 

levels. As noted above, the multi-city 


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times series studies evaluated in this 

review can not resolve the issue of 

whether the reported associations at 

such low levels are related to O3 alone 

rather than to the broader mix of air 

pollutants present in the ambient air. 

(5) ‘‘New’’ studies not included in the 

Criteria Document. Many commenters 

identified ‘‘new’’ studies that were not 

included in the Criteria Document that 

they stated support arguments both for 

and against the revision of the current 

O3 standard. Commenters who 

supported revising the current O3

standard identified new studies that 

generally supported EPA’s conclusions 

about the associations between O3

exposure and a range of respiratory and 

cardiovascular health outcomes. These 

commenters also identified new studies 

that provide evidence for associations 

with health outcomes that EPA has not 

linked to O3 exposure, such as cancer, 

and populations that EPA has not 

identified as being susceptible or 

vulnerable to O3 exposure, including 

African-American men and women. 

Commenters who did not support 

revision of the current O3 standard often 

submitted the same ‘‘new’’ studies, but 

focused on different aspects of the 

findings. Commenters who did not 

support revision of the current O3

standard stated that these ‘‘new’’ studies 

provide inconsistent and sometimes 

conflicting findings that do little to 

resolve uncertainties regarding whether 

O3 has a causal role in the reported 

associations with adverse health 

outcomes, including premature 

mortality and various morbidity 

outcomes. More detail about the topic 

areas covered in the ‘‘new’’ studies can 

be found in the Response to Comments 

document. 

To the extent that these commenters 

included ‘‘new’’ scientific studies, 

studies that were published too late to 

be considered in the Criteria Document, 

in support of their arguments for 

revising or not revising the standards, 

EPA notes, as discussed in section I 

above, that as in past NAAQS reviews, 

it is basing the final decisions in this 

review on the studies and related 

information included in the O3 air 

quality criteria that have undergone 

CASAC and public review and will 

consider newly published studies for 

purposes of decision making in the next 

O3 NAAQS review. In provisionally 

evaluating commenters’ arguments, as 

discussed in the Response to Comments 

document, EPA notes that its 

provisional consideration of ‘‘new’’

science found that such studies did not 

materially change the conclusions in the 

Criteria Document. 

iii. Evidence Pertaining to At-Risk 

Subgroups for O3-Related Effects 

This section contains major comments 

on EPA’s assessment of the body of 

evidence, including controlled human 

exposure and epidemiological studies, 

related to the effects of O3 exposure on 

sensitive subpopulations. Since new 

information about the increased 

responsiveness of people with lung 

disease, especially children and adults 

with asthma, was an important 

consideration in the Administrator’s 

proposed decision that the current O3

standard is not adequate, many of the 

comments focused on this information 

and the conclusions drawn from it. 

There were also comments on other 

sensitive groups identified by EPA, as 

well as comments suggesting that 

additional groups should be considered 

at increased risk from O3 exposure. 

Many of the issues discussed below, as 

well as other related issues, are 

addressed in more detail in the 

Response to Comments document. 

As with the comments on controlled 

human exposure and epidemiological 

studies, upon which judgments about 

sensitive subpopulations were based, 

the comments about EPA’s delineation 

of these groups were highly polarized. 

In general, one group of commenters 

who supported revising the current O3

primary standard, including medical 

associations, public health and 

environmental groups, agreed in part 

with EPA’s assessment of the 

subpopulations that are at increased risk 

from O3 exposure, but commented that 

there are additional groups that need to 

be considered. A comment from ATS, 

AMA and other medical associations 

noted: 

Within this population exists a number of 

individuals uniquely at much higher risk for 

adverse health effects from ozone exposures, 

including children, people with respiratory 

illness, the elderly, outdoor workers and 

healthy children and adults who exercise 

outdoors. [ATS, p. 2] 

These commenters agreed with EPA 

that, based on evidence from controlled 

human exposure and epidemiology 

studies, people with asthma, especially 

children, are likely to have greater lung 

function decrements and respiratory 

symptoms in response to O3 exposure 

than people who do not have asthma, 

and are likely to respond at lower levels. 

Because of this, these commenters make 

the point that controlled human 

exposure studies that employ healthy 

subjects will underestimate the effects 

of O3 exposures in people with asthma. 

These commenters agreed with EPA’s 

assessment that epidemiological studies 

provide evidence of increased morbidity 

effects, including lung function 

decrements, respiratory symptoms, 

emergency department visits and 

hospital admissions, in people with 

asthma and that controlled human 

exposure studies provide biological 

plausibility for these morbidity 

outcomes. Further, the Rochester 

Report, funded by API, evaluated some 

of the same the studies that EPA did and 

found similar results with regard to the 

increased inflammatory responses and 

increased airway responsiveness of 

people with asthma when exposed to 

O3. The Rochester Report reached the 

same conclusion that EPA did, that this 

increased responsiveness provides 

biological plausibility for the respiratory 

morbidity effects found in 

epidemiological studies. 

Several new studies have demonstrated 

that exposure of individuals with atopic 

asthma to sufficient levels of ozone produces 

an increase in specific airway responsiveness 

to inhaled allergens* * * These findings, in 

combination with previously observed effects 

of ozone on nonspecific airway 

responsiveness and airway inflammation, 

supports the idea that ambient ozone 

exposure could result in exacerbation of 

asthma several days following exposure, and 

provides biological plausibility for the 

epidemiologic studies in which ambient 

ozone concentration has been associated with 

increased asthma symptoms, medication use, 

emergency room visits, and hospitalizations 

for asthma. [Rochester Report, pp. 57–58] 

Commenters also often mentioned the 

increased susceptibility of people with 

COPD, and in this case cited new 

studies not considered in the Criteria 

Document. 

They identify one potentially 

susceptible subpopulation that EPA did 

not focus on in the proposal is infants. 

Commenters from medical associations, 

and environmental and public health 

groups expressed the view that O3

exposure can have important effects on 

infants, including reduced birth weight, 

pre-term birth, and increased respiratory 

morbidity effects in infants. Exposure to 

O3 during pregnancy, especially during 

the second and third trimesters, was 

associated with reduced birth weight in 

full-term infants. Although this effect 

was noted at relatively low O3 exposure 

levels, the ATS notes that, ‘‘* * * the 

reduced birth weight in infants in the 

highest ozone exposures communities 

equaled the reduced birth weight 

observed in pregnant women who 

smoke’’ (ATS, p. 7). 

In general, EPA agrees with comments 

that there is very strong evidence from 

controlled human exposure and 

epidemiological studies that people 

with lung disease, especially children 

and adults with asthma, are susceptible 

to O3 exposure and are likely to 


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experience more serious effects than 

those people who do not have lung 

disease. This means that controlled 

human exposure studies that employ 

subjects who do not have lung disease 

will likely underestimate effects in 

those people that do have asthma or 

other lung diseases. 

In summarizing the epidemiological 

evidence related to birth-related health 

outcomes, the Criteria Document (p. 7–

133) concludes that O3 was not an 

important predictor of several birthrelated outcomes including premature 

births and low birth weight. Birthrelated outcomes generally appeared to 

be associated with air pollutants that 

tend to peak in the winter and are 

possibly traffic-related. However, given 

that most of these studies did not 

analyze the data by season, seasonal 

confounding may have therefore 

influenced the reported associations. 

One study reported some results 

suggestive of associations between 

exposures to O3 in the second month of 

pregnancy and birth defects, but further 

evaluation of such potential associations 

is needed. With regard to comments 

about effect in infants, EPA notes that 

some of the studies cited by commenters 

were not considered in the Criteria 

Document. More detailed responses to 

studies submitted by commenters but 

not considered in the Criteria Document 

can be found in the Response to 

Comments document. 

The second group of commenters, 

mostly representing industry 

associations and some businesses 

opposed to revising the primary O3

standard, asserted that EPA is wrong to 

claim that new evidence indicates that 

the current standard does not provide 

adequate health public health protection 

for people with asthma. In support of 

this position, these commenters made 

the following major comments: (1) Lung 

function decrements and respiratory 

symptoms observed in controlled 

human exposure studies of asthmatics 

are not clinically important; (2) EPA 

postulates that asthmatics would likely 

experience more serious responses and 

responses at lower levels than the 

subjects of controlled human exposure 

experiments, but that hypothesis is not 

supported by scientific evidence; and, 

(3) EPA recognized asthmatics as a 

sensitive subpopulation in 1997, and 

new information does not suggest 

greater susceptibility than was 

previously believed. 

With regard to the first point, these 

commenters expressed the view that 

asthmatics are not likely to experience 

medically significant lung function 

changes or respiratory symptoms at 

ambient O3 concentrations at or even 

above the level of the current standard. 

Many of these commenters cited the 

opinion of one physician who was 

asked on behalf of a group of trade 

associations and companies to provide 

his views on the health significance for 

asthmatics of the types of responses that 

have been reported in controlled human 

exposure studies of O3. This commenter 

(McFadden) reviewed earlier controlled 

human exposure studies of asthmatics 

(from the last review) as well as the 

recent controlled human exposure 

studies of healthy individuals (Adams 

2002, 2003a,b, and 2006) at 0.12, 0.08, 

0.06, and 0.04 ppm and expressed the 

view that ‘‘* * * these studies on 

asthmatics indicate that ozone 

exposures at ∼ 0.12 ppm do not produce 

medically significant functional changes 

and are right around the inflection point 

where one begins to see an increase in 

symptoms; however, that increase is 

small’’ (McFadden, p. 3). This 

commenter went on to express the view 

that responses to O3 exposure at levels 

< 0 .08 ppm would be even less and that 

the available data are not sufficiently 

robust to indicate that such exposures 

would present a significant health 

concern even to sensitive people like 

asthmatics. 

EPA notes that this commenter based 

his comment on the group mean 

functional and respiratory symptom 

changes in the studies he reviewed. EPA 

agrees that group mean changes at these 

levels are relatively small and has 

described them as such in both the 

previous review and this one (72 FR 

37828). The importance of group mean 

changes is to evaluate the statistical 

significance of the association between 

the exposures and the observed effects, 

to try to determine if the observed 

effects are likely due to O3 exposure 

rather than chance. In the previous 

review as well as in this one, EPA has 

also focused on the fact that some 

individuals experience more severe 

effects that may be clinically significant. 

With regard to the significance of 

individual responses, this commenter 

(McFadden, p. 2) states ‘‘* * * transient 

decreases in FEV1 of 10–20% are not by 

themselves significant or meaningful to 

asthmatics* * *. It has been my 

experience from examining and 

studying thousands of patients for both 

clinical and research purposes that 

asthmatics typically will not begin to 

sense bronchoconstriction until their 

FEV1 falls about 50% from normal.’’

EPA strongly disagrees with this 

assessment. As stated in the Criteria 

Document (Table 8–3, p. 8–68) for 

people with lung disease, even 

moderate functional responses (e.g., 

FEV1 decrements ≥ 10% but < 20%) 

would likely interfere with normal 

activities for many individuals, and 

would likely result in more frequent 

medication use. EPA notes that in the 

context of standard setting, CASAC 

indicated (Henderson, 2006c) that a 

focus on the lower end of the range of 

moderate functional responses (e.g., 

FEV1 decrements ≥ 10%) is most 

appropriate for estimating potentially 

adverse lung function decrements in 

people with lung disease. 

With regard to the second point, 

whether asthmatics would likely 

experience more serious responses and 

responses at lower levels than the 

subjects of controlled human exposure 

experiments and EPA’s discussion of 

the relationship of increased airway 

responsiveness and inflammation 

experienced by asthmatics to 

exacerbation of asthma, this commenter 

stated that ‘‘there simply are no data to 

support the sequence described’’ and 

that ‘‘the assumption that these 

responses would lead to clinical 

manifestations in terms of exacerbations 

of asthma or other adverse health effects 

remains unproven theory’’ (McFadden, 

p. 3). 

In these sections of the proposal (72 

FR 37826 and 37846–37847), EPA 

describes the evidence indicating that 

people with asthma are as sensitive as, 

if not more sensitive than, normal 

subjects in manifesting O3-induced 

pulmonary function decrements. 

Controlled human exposure studies 

show that asthmatics present a 

differential response profile for cellular, 

molecular, and biochemical parameters 

that are altered in response to acute O3

exposure. Asthmatics have greater O3- 

induced inflammatory responses and 

increased O3-induced airway 

responsiveness (both incidence and 

duration) that could have important 

clinical implications. 

There are two ways to interpret these 

comments. One way to interpret them is 

that because these controlled human 

exposure studies have not produced 

exacerbations of asthma in study 

subjects resulting in the need for 

medical attention, there are no data to 

support the clinical significance of the 

results. EPA rejects this interpretation 

because it would be unethical to 

knowingly conduct a controlled human 

exposure study that would lead to 

exacerbation of asthma. Controlled 

human exposure studies are specifically 

designed to avoid these types of 

responses. The other interpretation is 

that the commenter does not agree that 

the differences in lung function, 

inflammation and increased airway 

responsiveness found in these 


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controlled human exposure studies 

support the inference that asthmatics 

are likely to have more serious 

responses than healthy subjects, and 

that these responses could have 

important clinical implications. EPA 

rejects this interpretation as well. EPA 

did not base its increased concern for 

asthmatics solely on the results of the 

controlled human exposure studies, but 

has appropriately used a weight of 

evidence approach, integrating evidence 

from animal toxicological, controlled 

human exposure and epidemiological 

studies as a basis for this concern. The 

Criteria Document concludes that the 

positive and robust epidemiological 

associations between O3 exposure and 

emergency department visits and 

hospitalizations in the warm season are 

supported by the human clinical, 

animal toxicological and 

epidemiological evidence for lung 

function decrements, increased 

respiratory symptoms, airway 

inflammation, and increased airway 

responsiveness (72 FR 37832). The 

CASAC Panel itself expressed the view 

that people with asthma, especially 

children, have been found to be more 

sensitive to O3 exposure, and indicated 

that EPA should place more weight on 

inflammatory responses and serious 

morbidity effects, such as increased 

respiratory-related emergency 

department visits and hospitalizations 

(Henderson, p. 4). Moreover, the 

Rochester Report, cited above, reaches 

essentially the same conclusions as EPA 

did, that the evidence from controlled 

human exposure studies provides 

biological plausibility for the 

epidemiological studies in which 

ambient O3 concentrations have been 

associated with increased asthma 

symptoms, medication use, emergency 

room visits, and hospitalizations for 

asthma. Therefore, EPA continues to 

assert that there is strong evidence that 

asthmatics likely have more serious 

responses to O3 exposure than people 

without asthma, and that these 

responses have the potential to lead to 

exacerbation of asthma as indicated by 

the serious morbidity effects, such as 

increased respiratory-related emergency 

department visits and hospitalizations 

found in epidemiological studies. 

With regard to the third point, 

commenters expressed the view that 

there is no significant new evidence 

establishing greater risk to asthmatics 

than was accepted in 1997, when EPA 

concluded that the existing NAAQS was 

sufficiently stringent to protect public 

health—including asthmatics—with an 

adequate margin of safety (UARG, pp. 

22–23). To support this view, these 

commenters noted the points made 

above and expressed the view that 

epidemiological studies of asthmatics 

that provide new evidence of respiratory 

symptoms and medication use in 

asthmatic children are subject to the 

limitations of epidemiological studies 

discussed above (e.g., confounding by 

co-pollutants, heterogeneity of results). 

In addition, these commenters 

identified a new, large multi-city panel 

study, not included in the Criteria 

Document, by Schildcrout et al. (2006), 

which the commenters characterize as 

reporting no association between O3

concentrations and exacerbation of 

asthma. 

At the time of the last review, EPA 

concluded that people with asthma 

were at greater risk because the impact 

of O3-induced responses on alreadycompromised respiratory systems would 

noticeably impair an individual’s ability 

to engage in normal activity or would be 

more likely to result in increased selfmedication or medical treatment. At 

that time there was little evidence that 

people with pre-existing disease were 

more responsive than healthy 

individuals in terms of the magnitude of 

pulmonary function decrements or 

symptomatic responses. The new results 

from controlled exposure and 

epidemiologic studies indicate that 

individuals with preexisting lung 

disease, especially people with asthma, 

are likely to have more serious 

responses than people who do not have 

lung disease and therefore are at greater 

risk for O3 health effects than previously 

judged in the 1997 review. EPA notes 

that comments on the limitations of 

epidemiological studies and evidence 

from ‘‘new’’ studies (not in the Criteria 

Document) have been addressed above. 

As with other ‘‘new’’ studies, this study 

by Schildcrout et al. (2006) is 

specifically discussed in the Response 

to Comments document. 

b. Consideration of Human Exposure 

and Health Risk Assessments 

Section II.A.3 above provides a 

summary overview of the exposure and 

risk assessment information used by the 

Administrator to inform judgments 

about exposure and health risk 

estimates associated with attainment of 

the current and alternative standards. 

EPA notes here that most of the issues 

and concerns raised by commenters 

concerning the methods used in the 

exposure and risk assessments are 

essentially restatements of concerns 

raised during the review of the Criteria 

Document and the development and 

review of these quantitative assessments 

as part of the preparation and review of 

the Staff Paper and the associated 

analyses. EPA presented and the 

CASAC Panel reviewed in detail the 

approaches used to assess exposure and 

health risk, the studies and health effect 

categories selected for which exposureresponse and concentration-response 

relationships were estimated, and the 

presentation of the exposure and risk 

results summarized in the Staff Paper. 

As stated in the proposal notice, EPA 

believes and CASAC Panel concurred, 

that the model selected to estimate 

exposure represent the state of the art 

and that the risk assessment was ‘‘well 

done, balanced and reasonably 

communicated’’ and that the selection 

of health endpoints for inclusion in the 

quantitative risk assessment was 

appropriate (Henderson, 2006c). EPA 

does not believe that the exposure or 

risk assessments are fundamentally 

biased in one direction or the other as 

claimed in some of the comments. 

Comments received after proposal 

related to the development of exposure 

and health risk assessments, 

interpretation of exposure and risk 

results, and the role of the quantitative 

human exposure and health risk 

assessments in considering the need to 

revise the current 8-hour O3 standard 

generally fell into two groups. One 

group of commenters that included 

national environmental and public 

health organizations (e.g., joint set of 

comments by ALA and several 

environmental groups including 

Environmental Defense and Sierra 

Club), NESCAUM, and some State and 

local health and air pollution agencies 

argued that the exposure and health risk 

assessments underestimated exposure 

and risks for several reasons including: 

(1) The geographic scope was limited to 

at most only 12 urban areas and thus 

underestimates national public health 

impacts due to exposures to O3; (2) the 

assessments did not include all relevant 

at risk population groups and excluded 

populations such as pre-school 

children, outdoor workers, adults who 

exercise outdoors; and (3) the risk 

assessment did not include all of the 

health effect endpoints for which there 

is evidence that there are O3-related 

health effects (e.g., increased medicine 

use by asthmatics, lung function 

decrements and respiratory symptoms 

in adults, increased doctors’ visits, 

emergency department visits, school 

absences, inflammation, and decreased 

resistance to infection among children 

and adults); and (4) EPA’s exposure 

assessment underestimates exposures 

since it considers average children, not 

active children who spend more time 

outdoors and repeated exposures are 

also underestimated. The joint set of 


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comments from ALA and several 

environmental groups contended that 

the ‘‘exposures of concern’’ metric 

presented in the Staff Paper and 

proposal is ‘‘an inappropriate basis for 

decisionmaking’’ and urged EPA to set 

the standard based on the 

concentrations shown by health studies 

to cause adverse effects, not on how 

much O3 Americans inhale. This same 

set of commenters stated that if 

exposures of concern were to be 

considered then the benchmark level of 

0.060 ppm should be the focus, and not 

higher benchmark levels. These same 

commenters also stated that EPA should 

have estimated and considered total risk 

without excluding risks associated with 

PRB levels because there is no rational 

basis for excluding natural and 

anthropogenic sources from outside 

North America and that the NAAQS 

must protect against total exposure. 

While disagreeing with EPA’s approach 

of estimating risks only above PRB, 

these same commenters supported the 

use of the GEOS–CHEM model as the 

‘‘best tool available to derive 

background concentrations’’ should 

EPA continue to pursue this approach. 

These comments are discussed in turn 

below. 

EPA agrees that the exposure and 

health risk assessments are limited to 

certain urban areas and do not capture 

all of the populations at risk for O3- 

related effects, and that the risk 

assessment does not include all 

potential O3-related health effects. The 

criteria and rationale for selecting the 

populations and health outcomes 

included in the quantitative assessments 

were presented in the draft Health 

Assessment Plan, Staff Paper, and 

technical support documents for the 

exposure and health risk assessments 

that were reviewed by the CASAC Panel 

and the public. The CASAC Panel 

indicated in its letter that the health 

outcomes included in the quantitative 

risk assessment were appropriate, while 

recognizing that other health outcomes 

such as emergency department visits 

and increased doctors’ visits should be 

addressed qualitatively (Henderson, 

2006c). The Staff Paper (and the CASAC 

Panel) clearly recognized that the 

exposure and risk analyses could not 

provide a full picture of the O3

exposures and O3-related health risks 

posed nationally. The proposal notice 

made note of this important point and 

stated that ‘‘national-scale public health 

impacts of ambient O3 exposures are 

clearly much larger than the 

quantitative estimates of O3-related 

incidences of adverse health effects and 

the numbers of children likely to 

experience exposures of concern 

associated with recent air quality or air 

quality that just meets the current or 

alternative standards’’ (72 FR 37866). 

However, as stated in the proposal 

notice, EPA also recognizes that interindividual variability in responsiveness 

to O3 shown in controlled human 

exposure studies for a variety of effects 

means that only a subset of individuals 

in any population group estimated to 

experience exposures exceeding a given 

benchmark exposure of concern level 

would actually be expected to 

experience such adverse health effects. 

The Administrator continues to 

recognize that there is a broader array of 

O3-related adverse health outcomes for 

which risk estimates could not be 

quantified (that are part of a broader 

‘‘pyramid of effects’’) and that the scope 

of the assessment was limited to just a 

sample of urban areas and to some but 

not all at-risk populations, leading to an 

incomplete estimation of public health 

impacts associated with O3 exposures 

across the country. The Administrator is 

fully mindful of these limitations, along 

with the uncertainties in these 

estimates, in reaching his conclusion 

that observations from the exposure and 

health risk assessments provide 

additional support for his judgment that 

the current 8-hour standard does not 

protect public health with an adequate 

margin of safety and must be revised. 

For reasons discussed below in section 

II.C.4, however, the Administrator 

disagrees with aspects of these 

commenters’ views on the level of the 

standard that is appropriate and 

supported by the available health effects 

evidence and quantitative assessments 

associated with just meeting alternative 

standards. 

EPA does not agree that consideration 

of exposure estimates is not permitted 

or is somehow inappropriate in 

decisions concerning the primary 

standard. EPA has considered 

population exposure estimates as a 

consideration in prior NAAQS review 

decisions, including the 1997 revision 

of the O3 primary standard and the 1994 

decision on the carbon monoxide (CO) 

standard. As indicated in the proposal, 

estimating exposures of concern is 

important because it provides some 

indication of potential public health 

impacts of a range of O3-related health 

outcomes, such as lung inflammation, 

increased airway responsiveness, and 

changes in host defenses. These 

particular health effects have been 

demonstrated to occur in some 

individuals in controlled human 

exposure studies at levels as low as 

0.080 ppm O3 but have not been 

evaluated at lower levels. While there is 

very limited evidence addressing lung 

function and respiratory symptom 

responses at 0.060 ppm, this evidence 

does not address these other health 

effects. 

As noted in the proposal, EPA 

emphasized that although the analysis 

of ‘‘exposures of concern’’ was 

conducted using three discrete 

benchmark levels (0.080, 0.070, 0.060 

ppm), the concept was more 

appropriately viewed as a continuum, 

with greater confidence and less 

uncertainty about the existence of 

health effects at the upper end and less 

confidence and greater uncertainty as 

one considers increasingly lower O3

exposure levels. EPA recognized that 

there was no sharp breakpoint within 

the continuum ranging from at and 

above 0.080 ppm down to 0.060 ppm. In 

considering the concept of exposures of 

concern, the proposal noted that it was 

important to balance concerns about the 

potential for health effects and their 

severity with the increasing uncertainty 

associated with our understanding of 

the likelihood of such effects at lower 

levels. 

As noted above, environmental and 

public health group comments 

expressed the view that if exposures of 

concern were considered, then the 

Administrator should focus only on the 

0.060 ppm benchmark based on the 

contention that adverse health effects 

had been demonstrated down to this 

level. In contrast, other commenters, 

primarily industry and business groups 

focused on comparisons of the 

exposures of concern at the 0.080 ppm 

benchmark level based on their view 

that there was no convincing evidence 

demonstrating adverse health effects at 

levels below this benchmark. In view of 

the comments received related to the 

definition and use of the term ‘‘exposure 

of concern’’ at the time of proposal, the 

Administrator recognizes that that there 

is a risk for confusion, as it could be 

read to imply a determination that a 

certain benchmark level of exposure has 

been shown to be causally associated 

with adverse health effects. As a 

consequence, the Administrator believes 

that it is more appropriate to consider 

such exposure estimates in the context 

of a continuum rather than focusing on 

any one discrete benchmark level, as 

was done at the time of proposal, since 

the Administrator does not believe that 

the underlying scientific evidence is 

certain enough to support a focus on 

any single bright-line benchmark level. 

Thus, the Administrator believes it is 

appropriate to consider a range of 

benchmark levels from 0.080 down to 

0.060 ppm, recognizing that exposures 

of concern must be considered in the 


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context of a continuum of the potential 

for health effects of concern, and their 

severity, with increasing uncertainty 

associated with the likelihood of such 

effects at lower O3 exposure levels. 

EPA recognizes that the 0.080 ppm 

benchmark level represents a level at 

which several health outcomes 

including lung inflammation, increased 

airway responsiveness, and decreased 

resistance to infection have been shown 

to occur in healthy adults. The 

Administrator places relatively great 

weight on the public health significance 

of exposures at and above this 

benchmark level given these 

physiological effects measured in 

healthy adults at O3 exposures of 0.080 

ppm and the evidence from controlled 

human exposure studies showing that 

people with asthma have more serious 

responses than people without asthma. 

However, the Administrator does not 

agree with those commenters who 

would only consider this single 

benchmark level. While the 

Administrator places less weight on 

exposures at and above the 0.070 pm 

benchmark level, given the increased 

uncertainty about the fraction of the 

population and severity of the health 

responses that might occur associated 

with exposures at and above this level, 

he believes that it is appropriate to 

consider exposures at and above this 

benchmark as well in judging the 

adequacy of the current standard to 

protect public health. Considering 

exposures at and above the 0.070 ppm 

benchmark level provides some 

consideration for the fact that the effects 

observed at 0.080 ppm were in healthy 

adult subjects but sensitive population 

groups such as asthmatics are likely to 

respond at lower O3 levels than healthy 

individuals. The Administrator 

considered but placed very little weight 

on exposures at and above the 0.060 

ppm benchmark given the very limited 

scientific evidence supporting a 

conclusion that O3 is causally related to 

various health outcomes at this 

exposure level. 

EPA does not agree that it is 

inappropriate or impermissible to assess 

risks that are in excess of PRB or that 

EPA must focus on total risks when 

using a risk assessment to inform 

decisions on the primary standard. 

Consistent with the approach used in 

the risk assessment for the prior O3

standard review and consistent with the 

approach used in risk assessments for 

other prior NAAQS reviews, estimating 

risks in excess of PRB is judged to be 

more relevant to policy decisions 

regarding the ambient air quality 

standard than risk estimates that 

include effects potentially attributable 

to uncontrollable background O3

concentrations. EPA also notes that with 

respect to the adequacy of the current 

standard taking total risks into account 

would not impact the Administrator’s 

decision, since he judges that the 

current standard is not adequate even 

when risks in excess of current PRB 

estimates are considered. In addition, 

EPA notes that consideration of the 

evidence itself, as well as exposures at 

and above benchmark levels in the 

range of 0.060 to 0.080 ppm, are not 

impacted at all by consideration of 

current PRB estimates. 

EPA does agree with the ALA and 

environmental groups comment that the 

GEOS–CHEM model represents the best 

tool currently available to estimate PRB 

as recognized in the Criteria Document 

evaluation of this issue and the CASAC 

Panel support expressed during the 

review of the Criteria Document. 

The second group of commenters 

mostly representing industry 

associations, businesses, and some State 

and local officials opposed to revising 

the 8-hour standard, and most 

extensively presented in comments from 

UARG, API, Exxon-Mobil, AAM, and 

NAM, raised one or more of the 

following concerns: (1) That exposures 

of concern and health risk estimates 

have not changed significantly since the 

prior review in 1997; (2) that 

uncertainties and limitations underlying 

the exposure and risk assessments make 

them too speculative to be used in 

supporting a decision to revise the 

standard; (3) that EPA should have 

defined PRB differently and that EPA 

underestimated PRB levels which 

results in health risk reductions 

associated with more stringent 

standards being overestimated; (4) that 

exposures are overestimated based on 

specific methodological choices made 

by EPA including, for example, O3

measurements at fixed-site monitors can 

be higher than other locations where 

individuals are exposed, the exposure 

estimates do not account for O3

avoidance behaviors, and the exposure 

model overestimates elevated breathing 

rates; and (5) that health risks are 

overestimated based on specific 

methodological choices made by EPA 

including, for example, selection of 

inappropriate effect estimates from 

health effect studies and EPA’s 

approach to addressing the shape of 

exposure-response relationships and 

whether or not to incorporate thresholds 

into its models for the various health 

effects analyzed. These comments are 

discussed in turn below. Additional 

detailed comments related to the 

development, presentation, and 

interpretation of EPA’s exposure and 

health risk assessments, along with 

EPA’s responses to the specific issues 

raised by these commenters can be 

found in the Response to Comments 

document. 

(1) In asserting that the estimated 

exposures and risks associated with air 

quality just meeting the current 

standard have not appreciably changed 

since the prior review, comments from 

Exxon-Mobil, the Annapolis Center and 

others have compared results of EPA’s 

lung function risk assessment done in 

the last review with those from the 

Agency’s risk assessment done as part of 

this review and have concluded that 

lung function risks upon attainment of 

the current O3 standard are below those 

that were predicted in 1997 and that 

uncertainties about other health effects 

based on epidemiological studies 

remain the same. These commenters 

used this conclusion as the basis for a 

claim that there is no reason to depart 

from the Administrator’s 1997 decision 

that the current 8-hour standard is 

requisite to protect public health. 

EPA believes that this claim is 

fundamentally flawed for three reasons, 

as discussed in turn below: (i) It is 

factually inappropriate to compare the 

quantitative risks estimated in 1997 

with those estimated in the current 

rulemaking; (ii) it fails to take into 

account that with similar risks, 

increased certainty in the risks 

presented by O3 implies greater concern 

than in the last review, and (iii) it fails 

to recognize that the Administrator has 

used these estimates in a supportive 

role, in light of significant uncertainties 

in the exposure and risk estimates, to 

inform the conclusions drawn primarily 

from integrative assessment of the 

controlled human exposure and 

epidemiological evidence on whether 

ambient O3 levels allowed under the 

current standard present a serious 

public health problem warranting 

revision of the O3 standard. 

With respect to the first point, the 

1997 risk estimates, or any comparison 

of the 1997 risk estimates to the current 

estimates, are irrelevant for the purpose 

of judging the adequacy of the current 

8-hour standard, as the 1997 estimates 

reflect outdated analyses that have been 

updated in this review to reflect the 

current science. Just comparing the 

results for lung function decrements 

ignores these differences. In particular, 

as discussed in section 4.6.1 of the Staff 

Paper, there have been significant 

improvements to the exposure model 

and the model inputs since the last 

review that make comparisons 

inappropriate between the prior and 

current review. For example, the 

geographic areas modeled are larger 


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than in the previous review and when 

modeling a larger area, extending well 

beyond the urban core, there will be 

more people exposed, but a smaller 

percentage of the modeled population 

will be exposed at high levels, if O3

concentrations are lower in the 

extended areas. In the prior review, only 

typical years, in terms of O3 air quality 

were modeled, while the current review 

used the most recent three-year period 

(i.e., 2002–2004). Also, the prior review 

estimated exposures for children who 

spent more time outdoors, while the 

assessment for the current review 

included all school age and all 

asthmatic school age children. 

Therefore, the population groups 

examined in the exposure assessment 

are different between those considered 

in the 1997 and current review, making 

comparison of the resulting estimates 

inappropriate. Another important 

difference making comparison between 

the 1997 health risk assessment and the 

current assessment inappropriate is that 

a number of additional health effects 

were included in the current review 

(e.g., respiratory symptoms in moderate/ 

severe asthmatic children, nonaccidental and cardiorespiratory 

mortality) based on health effects 

observed in epidemiological studies that 

were not included in the risk 

assessment for the prior review. These 

commenters only compare the risk 

estimates with respect to lung function 

decrement, and fail to account for 

differences in additional and more 

severe health endpoints not covered in 

the 1997 assessment, as well as the fact 

that there are somewhat different and 

more urban areas included in the 

current assessment. 

Second, it is important to take into 

account EPA’s increased level of 

confidence in the associations between 

short-term O3 exposures and morbidity 

and mortality effects. In comparing the 

scientific understanding of the risk 

presented by exposure to O3 between 

the last and current reviews, one must 

examine not only the quantitative 

estimate of risk from those exposures 

(e.g. the numbers of increased hospital 

admissions at various levels) but also 

the degree of confidence that the 

Agency has that the observed health 

effects are causally linked to O3

exposure at those levels. As 

documented in the Criteria Document 

and the recommendations and 

conclusions of CASAC, EPA recognizes 

significant advances in our 

understanding of the health effects of O3

based on new epidemiological studies, 

new human and animal studies 

documenting effects, new laboratory 

studies identifying and investigating 

biological mechanisms of O3 toxicity, 

and new studies addressing the utility 

of using ambient monitors to assess 

population exposures to ambient O3. As 

a result of these advances, EPA is now 

more certain that ambient O3 presents a 

significant risk to public health at levels 

at or above the range of levels that the 

Agency had considered for these 

standards in 1997. From this more 

comprehensive perspective, since the 

risks presented by O3 are more certain 

and the current quantitative risk 

estimates include additional important 

health effects, O3-related risks for a 

wider range of health effects are now of 

greater concern at the current level of 

the standard than in the last review. 

Third, quantitative risk estimates 

were not the only basis for EPA’s 

decision in setting a level for the O3

standard in 1997, and they do not set 

any quantified ‘‘benchmark’’ for the 

Agency’s decision to revise the O3

standard at this time. While EPA 

believes that confidence in the causal 

relationships between short-term 

exposures to O3 and various health 

effects reported in epidemiological 

studies has increased markedly since 

1997, the Administrator also recognizes 

that the risk estimates for these effects 

must be considered in the light of 

uncertainties about whether or not these 

O3-related effects occur at very low O3

concentrations. The Administrator 

continues to believe that the exposure 

and risk estimates associated with just 

meeting the current standard discussed 

in the Staff Paper and summarized in 

the proposal notice are important from 

a public health perspective and are 

indicative of potential exposures and 

risks to at-risk groups. In considering 

the exposure and risk estimates, the 

Administrator has considered the yearto-year and city-to-city variability in 

both the exposure and risk estimates, 

the uncertainties in these estimates, and 

recognition that there is a broader array 

of O3-related adverse health outcomes 

for which risk estimates could not be 

quantified (that are part of a broader 

‘‘pyramid of effects’’) and that the scope 

of the assessment was limited to just a 

sample of urban areas and to some, but 

not all, at-risk populations, leading to an 

incomplete estimation of public health 

impacts associated with O3 exposures 

across the country. 

(2) In asserting that uncertainties and 

limitations associated with the exposure 

and health risk assessments make them 

too speculative to be used in supporting 

a decision to revise the standard, 

comments from industry associations 

and others cited a number of issues 

including: (i) Uncertainties about the air 

quality adjustment approach used to 

simulate just meeting the current and 

alternative standards; (ii) uncertainties 

and limitations associated with the 

definition and estimation of PRB 

concentrations; (iii) uncertainties about 

whether the respiratory symptoms, 

hospital admissions, and non-accidental 

and cardiorespiratory mortality effects 

included in the health risk assessment 

are actually causally related to ambient 

O3 concentrations, particularly at levels 

well below the current standard; and 

(iv) uncertainties about the shape of the 

exposure-response relationships for 

lung function responses and 

concentration-response relationships for 

the health effects based on findings from 

epidemiological studies and the 

assumption of a linear non-threshold 

relationship for these responses. In 

summary, these commenters contend 

that the substantial uncertainties 

present in the exposure and risk 

assessments preclude the Administrator 

from using any of the results to support 

a conclusion that the current 8-hour 

standard does not adequately protect 

public health. 

Several of the issues raised, including 

whether EPA’s judgments about 

causality for the effects included in the 

risk assessment are appropriate, the 

shape of concentration-response 

relationships, and use of a linear nonthreshold relationship for the health 

outcomes based on the epidemiological 

evidence, have been discussed in the 

previous section on health effects 

evidence. Concerns expressed about the 

definition and estimation of PRB levels 

for O3 and the role of PRB in the risk 

assessment are addressed as a separate 

item below. These issues also are 

addressed in more detail in the 

Response to Comments document. 

With respect to the air quality 

adjustment approach used in the current 

review to simulate air quality just 

meeting the current and alternative O3

standards, as discussed in the Staff 

Paper (section 4.5.6) and in more detail 

in a staff memorandum (Rizzo, 2006), 

EPA concluded that the quadratic air 

quality adjustment approach generally 

best represented the pattern of 

reductions across the O3 air quality 

distribution observed over the last 

decade in areas implementing control 

programs designed to attain the O3

NAAQS. While EPA recognizes that 

future changes in air quality 

distributions are area-specific, and will 

be affected by whatever specific control 

strategies are implemented in the future 

to attain a revised NAAQS, there is no 

empirical evidence to suggest that future 

reductions in ambient O3 will be 

significantly different from past 


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reductions with respect to impacting the 

overall shape of the O3 distribution. 

As discussed in the proposal notice, 

EPA recognizes that the exposure and 

health risk assessments necessarily 

contain many sources of uncertainty 

including those noted by these 

commenters, and EPA has accounted for 

such uncertainties to the extent 

possible. EPA developed and presented 

an uncertainty analysis addressing the 

most significant uncertainties affecting 

the exposure estimates. With respect to 

the health risk assessment, EPA 

conducted and presented sensitivity 

analyses addressing the impact on risk 

estimates of different assumptions about 

the shape of the exposure-response 

relationship for lung function 

decrements and alternative assumptions 

about PRB levels. EPA notes that most 

of the comments summarized above 

concerning limitations and uncertainties 

in these assessments are essentially 

restatements of concerns raised during 

the development and review of these 

quantitative assessments as part of the 

preparation and review of the Staff 

Paper and assessments. The CASAC 

Panel reviewed in detail the approaches 

used to assess exposure and health risks 

and the presentation of the results in the 

Staff Paper. EPA believes, and the 

CASAC Panel concurred, that the model 

used to estimate exposures represents a 

state-of-the-art approach and that ‘‘there 

is an explicit discussion of the 

limitations of the APEX model in terms 

of variability and quality of the input 

data, which is appropriate and fine’’

(Henderson, 2006c, p. 11). The CASAC 

Panel also found the risk chapter in the 

Staff Paper and the risk assessment ‘‘to 

be well done, balanced, and reasonably 

communicated’’ (Henderson, 2006c, p. 

12). Although EPA agrees that important 

limitations and uncertainties remain, 

and that future research directed toward 

addressing these uncertainties is 

warranted, EPA believes that overall 

uncertainties about population exposure 

and possible health risks associated 

with short-term O3 exposure have 

diminished since the last review. The 

Administrator has carefully considered 

the limitations and uncertainties 

associated with these quantitative 

assessments but continues to believe 

that they provide general support for 

concluding that exposures and health 

risks associated with meeting the 

current 8-hour standard are important 

from a public health perspective and 

that the 8-hour standard needs to be 

revised to provide additional protection 

in order to protect public health with an 

adequate margin of safety. 

(3) Comments from several industry 

organizations, businesses, and others 

related to PRB included: (i) That EPA 

should have defined PRB differently so 

as to include anthropogenic emissions 

from Canada and Mexico; (ii) that EPA 

underestimated PRB levels by relying on 

estimates from the GEOS–CHEM model 

using 2001 meteorology and EPA should 

instead rely on O3 levels observed at 

remote monitoring locations or sites that 

represent PRB conditions; and (iii) that 

the use of underestimated PRB levels in 

the risk assessment results in 

overestimated health risks associated 

with air quality just meeting the current 

standard. Finally, some commenters 

cited concerns expressed by the CASAC 

Panel that ‘‘the current approach to 

determining PRB is the best method to 

make this estimation’’ (Henderson, 

2007, p. 2). Each of these concerns is 

addressed below and in more detail in 

the Response to Comments document. 

First, the U.S. government has 

influence over emissions at our borders 

that affect ambient O3 concentrations 

entering the U.S. from Canada and 

Mexico through either regulations or 

international agreements, and therefore 

EPA does not agree that these emissions 

are uncontrollable. PRB is designed to 

identify O3 levels that result from 

emissions that are considered 

uncontrollable because the U.S. has 

little if any influence on their control, 

and in that context anthropogenic 

emissions from Mexico or Canada 

should be excluded from PRB. EPA has 

consistently defined PRB as excluding 

anthropogenic emissions from Canada 

and Mexico in NAAQS reviews over 

more than two decades and sees no 

basis in the comments to alter this 

definition. 

Second, the criticisms raised 

concerning the use of a modeling 

approach (GEOS–CHEM using 2001 

meteorology) and the alternative 

approach of using remote monitoring 

data to estimate PRB were considered by 

EPA’s scientific staff and the CASAC 

Panel during the course of reviewing the 

Criteria Document. Both EPA’s experts 

and CASAC endorsed the use of the 

peer-reviewed, thoroughly evaluated 

modeling approach (GEOS–CHEM) 

described in the Criteria Document as 

the best current approach for estimating 

PRB levels. The Criteria Document 

reviewed detailed evaluations of GEOS–

CHEM with O3 observations at U.S. 

surface sites (Fiore et al., 2002, 2003) 

and comparisons of GEOS–CHEM 

predictions with observations at 

Trinidad Head, CA (Goldstein et al., 

2004) and found no significant 

differences between the model 

predictions and observations for all 

conditions, including those reflecting 

those given in the current PRB 

definition. The Criteria Document states 

that the current model estimates 

indicate that PRB in the U.S. is 

generally 0.015 to 0.035 ppm that 

declines from spring to summer and is 

generally < 0.025 ppm under conditions 

conducive to high O3 episodes. The 

Criteria Document acknowledges that 

PRB can be higher, especially at 

elevated sites in the spring due to 

stratospheric exchange. However, 

unusually high springtime O3 episodes 

tied to stratospheric intrusion are rare 

and generally occur at elevated 

locations and these can be readily 

identified and excluded under EPA’s 

exceptional events rule (72 FR 13560) to 

avoid any impact on attainment/nonattainment status of an area. 

Third, many of the commenters who 

raised the concern that EPA’s estimates 

of PRB were too low and had the impact 

of exaggerating the risks associated with 

the current standard ignored the fact 

that the risk assessment included a 

sensitivity analysis which showed the 

potential impact of both lower and 

higher estimates of PRB or only focused 

on the impact of higher estimates of 

PRB. The choices of lower and higher 

estimates of PRB included in the risk 

assessment sensitivity analyses were 

based on the peer-reviewed evaluation 

of the accuracy of GEOS–CHEM model. 

The Criteria Document states ‘‘in 

conclusion, we estimate that the PRB O3

values reported by Fiore et al. (2003) for 

afternoon surface air over the United 

States are likely 10 parts per billion by 

volume (ppbv) too high in the southeast 

in summer, and accurate within 5 ppbv 

in other regions and seasons.’’ These 

error estimates are based on comparison 

of model output with observations for 

conditions which most nearly reflect 

those given in the PRB definition, i.e., 

at the lower end of the probability 

distribution. As discussed in the Criteria 

Document and Staff Paper, it can be 

seen that GEOS–CHEM overestimates O3

for the southeast and underestimates it 

by a small amount for the northeast. 

These commenters generally ignored the 

scientific conclusion presented in the 

Criteria Document that for some regions 

of the country the evidence suggests that 

the model actually overestimates PRB. 

Thus, the influence of alternative 

estimates of PRB on risks in excess of 

PRB associated with meeting the current 

standard can be to lower or increase the 

risk estimates. While the choice of 

estimates for PRB contributes to the 

uncertainty in the risk estimates, EPA 

does not agree that the approach used is 

biased since peer-reviewed evaluations 

of the model have shown relatively good 


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agreement (i.e., generally within 5 ppb 

for most regions of the country). 

Finally, EPA believes that some 

commenters have misread the CASAC 

Panel concern ‘‘that the current 

approach to determining PRB is the best 

method to make this estimation’’

(Henderson, 2007, p. 2) as a criticism of 

the use of the GEOS–CHEM modeling 

approach and/or support for primary 

reliance on estimates based on remote 

monitoring sites. However, the CASAC 

Panel went on to state that one reason 

for its concern was that the contribution 

to PRB from beyond North America was 

uncontrollable by EPA and that ‘‘a better 

scientific understanding of 

intercontinental transport of air 

pollutants could serve as the basis for a 

more concerted effort to control its 

growth . . .’’ (Henderson, 2007, p. 3). 

Hence, CASAC’s concern appeared to be 

more with defining what emissions to 

include in defining PRB, and the role 

that PRB should play, as compared to 

the technical question of the best way to 

estimate PRB levels. In reviewing the 

Staff Paper, the atmospheric modeling 

expert on the CASAC Panel in his 

comments on how PRB had been 

estimated using the GEOS–CHEM model 

concluded that the ‘‘current approach 

has been peer-reviewed, and is 

appropriate’’ (Henderson, 2006b, p. 

D–48). 

(4) Some commenters raised concerns 

about aspects of the exposure modeling 

that they felt resulted in overestimates 

of modeled exposures, including: (i) O3

measurements at downwind monitors 

are usually higher than the overall area 

and may not reflect the overall outdoor 

exposures in the area; (ii) O3 exposures 

near roadways will be below that 

measured at the monitor due to titration 

of O3 from automobile emissions of NO; 

(iii) O3 concentrations are lower at a 

person’s breathing height compared to 

measurement height, (iv) exposure 

estimates do not account for O3

avoidance behaviors; and (v) the APEX 

model over predicts elevated ventilation 

rate occurrences, which results in an 

overestimation of the number of 

exposures of concern and risk estimates 

for lung function decrements. 

The concern raised in the first point 

is unfounded since all O3 monitors in 

each area are used to take into account 

the spatial variations of O3

concentrations. The geographic 

variation of O3 concentration is 

accounted for by using measurements 

from the closest O3 monitor to represent 

concentrations in a neighborhood and 

the measurements at downwind 

monitors are applied only to the 

downwind areas. 

Second, the reduction in O3

concentrations near roadways due to 

titration of O3 from automobile 

emissions of NO is accounted for and 

explicitly modeled in APEX and thus 

does not bias estimates of exposures. 

This phenomenon was modeled through 

the use of ‘‘proximity factors,’’ which 

adjust the monitored concentrations to 

account for the titration of O3 by NO 

emissions (the monitored 

concentrations are multiplied by the 

proximity factors). Three proximity 

factor distributions were developed, one 

for local roads, one for urban roads, and 

one for interstates, with mean factors of 

0.75, 0.75, and 0.36 respectively (section 

3.10.2, Exposure Analysis TSD). 

Furthermore, the uncertainty of these 

proximity factor distributions was 

included in the exposure uncertainty 

analysis. 

Third, as discussed in the exposure 

uncertainty analysis, data were not 

available to quantify the potential biases 

of differences between O3

concentrations at a person’s breathing 

height compared to the heights of 

nearby monitors. EPA believes that 

these biases, to the extent that they 

exist, are relatively small during warm 

summer afternoons when O3

concentrations tend to be higher. 

Fourth, behavior changes in response 

to O3 pollution or in response to AQI 

notification alerts (‘‘avoidance 

behavior’’) is not explicitly taken into 

account in the exposure modeling. 

There is not much information about the 

extent to which people currently modify 

their activities in response to O3 alerts. 

However, under the scenarios modeled 

for just meeting alternative standards, 

O3 alerts would be infrequent relative to 

the number of alerts that currently occur 

in the nonattainment areas modeled. 

Consequently, EPA does not feel that 

this is an influential factor in the 

estimation of exposure for the scenarios 

simulating just meeting the current or 

proposed standards. 

Fifth, a comparison of ventilation 

rates predicted by APEX to 

measurements showed APEX 

overpredicting ventilation rates for ages 

5 to 10, underpredicting ventilation 

rates for ages 11 to 29 and greater than 

39, and in close agreement for ages 30 

to 39. The overall agreement was judged 

favorable, and the errors of the 

predicted ventilation rates were 

partially incorporated into the overall 

uncertainty analysis with the 

uncertainties of the metabolic 

equivalents (METs), which are the 

primary drivers of ventilation rates. 

(5) Comments from a number of 

industry organizations, businesses, and 

others contended that EPA’s health risk 

assessment was biased and that the 

resulting risk assessment is ‘‘much 

higher than would have been obtained 

using objective methods’’ (NAM), and 

commenters raised one or more of the 

following points in support of this view: 

(i) EPA inappropriately based its risk 

assessment for respiratory symptoms, 

hospital admissions, and non-accidental 

and cardiorespiratory mortality on 

positive studies with high risk 

coefficients while ignoring negative 

studies and studies with lower 

coefficients; (ii) EPA focused on 

combined ‘‘national’’ effect estimates 

from multi-city studies when it should 

have relied on individual city effect 

estimates from these studies in its risk 

assessment; (iii) the risk assessment 

presented single-pollutant model results 

that overstate the likely impact of O3

when co-pollutant model results were 

available which should have been used; 

(iv) the risk assessment used linear 

concentration-response relationships for 

the health endpoints based on 

epidemiological studies when nonlinear or threshold models should have 

been used; and (v) the lung function 

portion of the risk assessment should 

not rely on what they characterized as 

‘‘outlier’’ information to define 

exposure-response relationships, with 

reference to the data from the Adams 

(2006) study, but rather should focus on 

group central tendency response levels. 

Each of these issues is discussed below 

and in more detail in the Response to 

Comments document. 

First, several commenters asserted 

that the results of time-series studies 

should not be used at all in quantitative 

risk assessments, that risk estimates 

from single-city time-series studies 

should not be used since they are highly 

heterogeneous and influenced by 

publication bias, and that the panel 

study which served as the basis for the 

concentration-response relationships for 

respiratory symptoms in asthmatic 

children suffered from various 

weaknesses and was contradicted by a 

more recent study. EPA notes that the 

selection of specific studies and effect 

estimates was based on a careful 

evaluation of the evidence evaluated in 

the Criteria Document and that the 

criteria and rationale for selection of 

studies and effect estimates were 

presented and extensively reviewed and 

discussed by the CASAC Panel and in 

public comments presented to the 

CASAC Panel. EPA notes that the 

CASAC Panel judged the selection of 

the endpoints based on the 

epidemiological studies for inclusion in 

the quantitative risk assessment to be 

‘‘appropriate’’ and that the risk 


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assessment chapter of the Staff Paper 

and its accompanying risk assessment 

were ‘‘well done, balanced and 

reasonably communicated’’ (Henderson, 

2006c, p. 12). 

While EPA notes that two of the metaanalyses, Bell et al. (2005) and Ito et al. 

(2005), provided suggestive evidence of 

publication bias, O3-mortality 

associations remained after accounting 

for that potential bias. The Criteria 

Document (p. 7–97) concludes that the 

‘‘positive O3 effects estimates, along 

with the sensitivity analyses in these 

three meta-analyses, provide evidence 

of a robust association between ambient 

O3 and mortality.’’ Concerns about the 

heterogeneity of responses observed 

across different urban areas, particularly 

for O3-related mortality are addressed in 

the section above on health effect 

considerations. 

Second, as discussed in more detail in 

the Staff Paper (section 5.3.2.3), there 

are different advantages associated with 

use of single-city and multi-city effect 

estimates as the basis for estimating 

health risks in specific urban areas. 

Therefore, the risk assessment included 

estimates based on both types of effect 

estimates where such information was 

available. 

Third, the risk assessment included 

risk estimates based on both single 

pollutant and multi-pollutant 

concentration-response relationships 

where such information was available 

for the health outcomes included in the 

assessment. Issues related to the 

consideration of single versus multipollutant models have been addressed 

in the section above on health effects 

evidence. 

Fourth, EPA’s approach of using 

linear concentration-response 

relationships for the health outcomes 

based on epidemiological studies and 

whether or not to include any nonlinear models or assumed threshold 

were reviewed and discussed by the 

CASAC Panel during the development 

of the Staff Paper and risk assessment, 

and the Panel concurred with the 

approach used. As discussed in the 

proposal notice, Staff Paper (section 

3.4.5), and above in the prior section on 

health effects evidence, EPA recognizes 

that the available epidemiological 

evidence neither supports or refutes the 

existence of thresholds at the 

population level for effects such as 

increased hospital admissions and 

premature mortality. Noting the 

limitations of epidemiological evidence 

to address such questions, EPA 

concluded that if a population threshold 

does exist, it would likely be well below 

the level of the current O3 standard. The 

Administrator is very mindful of the 

uncertainties related to whether the 

observed associations between O3

concentrations at levels well below 

0.080 ppm and the health outcomes 

reported in the epidemiological studies 

reflect actual causal relationships, and 

has taken this into account in 

considering the risk assessment 

estimates in his decision. 

Fifth, consistent with the prior 

review, the lung function component of 

the risk assessment has focused on the 

number and percentage of children that 

are estimated to experience a degree of 

lung function decrement that represents 

an adverse health effect. EPA does not 

agree that the focus of the quantitative 

risk assessment should be on the 

average lung function response in the 

population, since such an assessment 

would not address the public health 

policy question concerning to extent to 

which a portion of the population 

would likely experience health effects of 

concern. Looking at just the average for 

the population would ignore the 

evidence of health effects for sensitive 

subpopulations, an important aspect of 

public health impact in this and other 

O3 reviews. EPA believes that it is 

appropriate to include all of the 

individual data from the series of 

controlled human exposure studies that 

address lung function responses 

associated with 6.6-hour exposures to 

O3 and which were reviewed and 

included in the final Criteria Document, 

and this includes the Adams (2006) 

study. EPA notes that the CASAC Panel 

clearly did not judge the responses 

observed in this study to be an 

‘‘outlier.’’ Rather, CASAC stated in its 

comments on the Staff Paper’s 

discussion of this study, ‘‘there were 

clearly a few individuals who 

experienced declines in lung function at 

these lower concentrations. These were 

healthy subjects so the percentage of 

asthmatic subjects, if they had been 

studied, would most likely be 

considerably greater’’ (Henderson, 

2006c, p. 10). 

Having considered comments on the 

quantitative exposure and health risk 

assessments from both groups of 

commenters, the Administrator finds no 

basis to change his position on these 

quantitative assessments that was taken 

at the time of proposal. That is, as 

discussed above, while the 

Administrator recognizes that the 

assessments rest on a more extensive 

body of data and is more comprehensive 

in scope than the assessment conducted 

in the last review, he is mindful that 

significant uncertainties continue to 

underlie the resulting quantitative 

exposure and risk estimates. 

Nevertheless, the Administrator 

concludes that the exposure and risk 

estimates are sufficiently reliable to 

inform his judgment about the 

significance of the exposures and risk of 

health effects in susceptible and 

vulnerable populations at O3 levels 

associated with just meeting the current 

8-hour standard. However, the 

Administrator disagrees with aspects of 

these commenters’ views on the level of 

the standard that is appropriate and 

supported by the available health effects 

evidence and quantitative assessments 

associated with just meeting alternative 

standards. 

3. Conclusions Regarding the Need for 

Revision 

Having carefully considered the 

public comments, as discussed above, 

the Administrator believes the 

fundamental scientific conclusions on 

the effects of O3 reached in the Criteria 

Document and Staff Paper, briefly 

summarized above in section II.A.2 and 

discussed more fully in section II.A of 

the proposal, remain valid. In 

considering whether the primary O3

standard should be revised, the 

Administrator places primary 

consideration on the body of scientific 

evidence available in this review on the 

health effects associated with O3

exposure, as summarized above in 

section II.B.1. The Administrator notes 

that there is much new evidence that 

has become available since the last 

review, including an especially large 

number of new epidemiological studies. 

The Administrator believes that this 

body of scientific evidence is very 

robust, recognizing that it includes large 

numbers of various types of studies, 

including toxicological studies, 

controlled human exposure studies, 

field panel studies, and community 

epidemiological studies, that provide 

consistent and coherent evidence of an 

array of O3-related respiratory morbidity 

effects and possibly cardiovascularrelated morbidity as well as total 

nonaccidental and cardiorespiratory 

mortality. The Administrator observes 

that (1) the evidence of a range of 

respiratory-related morbidity effects 

seen in the last review has been 

considerably strengthened, both through 

toxicological and controlled human 

exposure studies as well as through 

many new panel and epidemiological 

studies; (2) newly available evidence 

from controlled human exposure and 

epidemiological studies identifies 

people with asthma as an important 

susceptible population for which 

estimates of respiratory effects in the 

general population likely underestimate 

the magnitude or importance of these 

effects; (3) newly available evidence 


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20As noted above, such health outcomes include 

increased airway responsiveness, increased 

pulmonary inflammation, increased cellular 

permeability, and decreased pulmonary defense 

mechanisms. These physiological effects provide 

plausible mechanisms underlying observed 

associations with aggravation of asthma, increased 

medication use, increased school and work 

absences, increased susceptibility to respiratory 

infection, increased visits to doctors’ offices and 

emergency departments, and increased admissions 

to hospitals. In addition, these physiological effects, 

if repeated over time, have the potential to lead to 

chronic effects such as chronic bronchitis or longterm damage to the lungs that can lead to reduced 

quality of life. 

about mechanisms of toxicity more 

completely explains the biological 

plausibility of O3-induced respiratory 

effects and is beginning to suggest 

mechanisms that may link O3 exposure 

to cardiovascular effects; and (4) there is 

now relatively strong evidence for 

associations between O3 and total 

nonaccidental and cardiopulmonary 

mortality, even after adjustment for the 

influence of season and PM. The 

Administrator believes that this very 

robust body of evidence, taken together, 

enhances our understanding of O3- 

related effects relative to what was 

known at the time of the last review. 

Further, he believes that the available 

evidence provides increased confidence 

that respiratory morbidity effects such 

as lung function decrements and 

respiratory symptoms are causally 

related to O3 exposures, that indicators 

of respiratory morbidity such as 

emergency department visits and 

hospital admissions are causally related 

to O3 exposures, and that the evidence 

is highly suggestive that O3 exposures 

during the warm O3 season contribute to 

premature mortality. 

Further, the Administrator judges that 

there is important new evidence 

demonstrating that exposures to O3 at 

levels below the level of the current 

standard are associated with a broad 

array of adverse health effects. This is 

especially true in at-risk populations 

that include people with asthma or 

other lung diseases, who are likely to 

experience more serious effects from 

exposure to O3, children, and older 

adults with increased susceptibility, as 

well as those who are likely to be 

vulnerable as a result of spending a lot 

of time outdoors engaged in physical 

activity, especially active children and 

outdoor workers. The Administrator 

notes that this important new evidence 

demonstrates O3-induced lung function 

effects and respiratory symptoms in 

some healthy individuals down to the 

previously observed exposure level of 

0.080 ppm, as well as very limited new 

evidence at exposure levels well below 

the level of the current standard. In 

addition, the Administrator notes that 

(1) there is now epidemiological 

evidence of statistically significant O3- 

related associations with lung function 

and respiratory symptom effects, 

respiratory-related emergency 

department visits and hospital 

admissions, and increased mortality, in 

areas that likely would have met the 

current standard; (2) there are also many 

epidemiological studies done in areas 

that likely would not have met the 

current standard but which nonetheless 

report statistically significant 

associations that generally extend down 

to ambient O3 concentrations that are 

below the level of the current standard; 

(3) there are a few studies that have 

examined subsets of data that include 

only days with ambient O3

concentrations below the level of the 

current standard, or below even much 

lower O3 concentrations, and continue 

to report statistically significant 

associations with respiratory morbidity 

outcomes and mortality; and (4) the 

evidence from controlled human 

exposure studies, together with animal 

toxicological studies, provides 

considerable support for the biological 

plausibility of the respiratory morbidity 

associations observed in the 

epidemiological studies and for 

concluding that the associations extend 

below the level of the current standard. 

Based on the available evidence, the 

Administrator agrees with the CASAC 

Panel and the majority of public 

commenters that the current standard is 

not requisite to protect public health 

with an adequate margin of safety 

because it is does not provide sufficient 

protection and that revision of the 

current O3 standard is needed to 

provide increased public health 

protection. The Administrator notes that 

extensive critical review of this body of 

evidence and related uncertainties 

during the criteria and standard review 

process, including review by the 

CASAC Panel and the public of the 

basis for EPA’s proposed decision to 

revise the primary O3 standard, has 

identified a number of issues about 

which different reviewers disagree and 

for which additional research is 

warranted. Nonetheless, on balance, the 

Administrator believes that the 

remaining uncertainties in the available 

evidence do not diminish confidence in 

the causal relationships between O3

exposures and indicators of serious 

respiratory morbidity effects, or the 

highly suggestive evidence of 

associations between O3 exposures and 

premature mortality, nor do they 

diminish confidence in the conclusion 

that the associations extend below the 

level of the current standard. 

Beyond a primary consideration of the 

available evidence, the Administrator 

has also taken into consideration the 

Agency’s exposure and risk assessments 

to help inform his evaluation of the 

adequacy of the current standard. As at 

the time of proposal, the Administrator 

believes the results of those assessments 

inform his judgment on the adequacy of 

the current standard to protect against 

health effects of concern. In considering 

the exposure analysis results at this 

time, the Administrator recognizes that 

that there is a risk for confusion in the 

term ‘‘exposure of concern’’ that was 

used at the time of proposal, as it could 

be read to imply a determination that a 

certain benchmark level of exposure has 

been shown to be causally associated 

with adverse health effects. As a 

consequence, the Administrator believes 

that it is more appropriate to consider 

such exposure estimates in the context 

of a continuum rather than focusing on 

any one discrete benchmark level, as 

was done at the time of proposal, since 

the Administrator does not believe that 

the underlying scientific evidence is 

certain enough to support a focus on 

any bright-line benchmark level. In so 

doing, the Administrator recognizes that 

associations between O3 exposures and 

health effects of concern become 

increasingly uncertain at lower O3

exposure levels. Thus, the 

Administrator has taken into 

consideration the pattern of such 

exposure estimates across the range of 

discrete benchmark levels considered in 

EPA’s exposure assessment to provide 

some indication of the potential 

magnitude of the incidence of health 

outcomes that could not be evaluated in 

the Agency’s quantitative risk 

assessment but which have been 

demonstrated to occur in healthy people 

at O3 exposures as low as 0.080 ppm, 

the lowest level at which such health 

outcomes have been tested.20

More specifically, the Administrator 

has considered the pattern of reductions 

in such exposures across the benchmark 

levels of 0.080, 0.070, and 0.060 ppm, 

which span the level at which there is 

strong evidence of effects in healthy 

people down to a level at which the 

Administrator judges the evidence of 

effects to be very limited. The 

Administrator observes that based on 

the aggregated exposure estimates for 

the 2002 simulation for the 12 urban 

areas included in the exposure analysis, 

upon just meeting the current standard, 

the percentages of asthmatic or all 

school age children likely to experience 

one of more exposures at and above 

these benchmark levels of 0.080, 0.070, 

and 0.060 ppm (while at moderate or 

greater exertion) are approximately 4%, 


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20%, and 45%, respectively. As noted at 

the time of proposal, the Administrator 

recognizes that there is substantial yearto-year and city-to-city variability in 

these estimates and that it is important 

to recognize this variability in 

considering these estimates. For 

example, for the 0.080, 0.070, and 0.060 

ppm benchmark levels, these 

percentages are estimated to range from 

approximately 1 to 10%, 1 to 40%, and 

7 to 65%, respectively, across each of 

the 12 urban areas based on the 2002 

simulation, and from approximately 0 to 

1%, 0 to 7%, and 1 to 25%, 

respectively, based on the 2004 

simulation. 

With regard to the results of the risk 

assessment, the Administrator again 

considered the risks estimated to remain 

upon just meeting the current standard. 

The Administrator takes note of the 

estimated magnitudes of such risks, 

which are presented above in section 

II.B.1.c for a range of health effects 

including moderate and large lung 

function decrements (including 

percentages of children and number of 

occurrences), respiratory symptom days, 

respiratory-related hospital admissions, 

and nonaccidental and cardiorespiratory 

mortality, as well as year-to-year and 

city-to-city variability, and the 

uncertainties in these estimates. 

Further, the Administrator recognizes 

that these estimated risks for the 

specific health effects that could be 

analyzed in the Agency’s risk 

assessment are indicative of a much 

broader array of O3-related health 

endpoints that are part of a ‘‘pyramid of 

effects’’ that include various indicators 

of morbidity that could not be included 

in the risk assessment (e.g., school 

absences, increased medication use, 

emergency department visits) and 

which primarily affect members of atrisk groups. 

In considering these quantitative 

exposure and risk estimates, as well as 

the broader array of O3-related health 

endpoints that could not be quantified, 

the Administrator believes that they are 

important from a public health 

perspective and indicative of potential 

exposures and risks to at-risk groups. 

The Administrator thus finds that the 

exposure and risk estimates provide 

additional support to the evidencebased conclusion, reached above, that 

the current standard needs to be revised. 

Based on these considerations, and 

consistent with CASAC Panel’s 

unanimous conclusion that there is no 

scientific justification for retaining the 

current standard, the Administrator 

concludes that the current primary O3

standard is not sufficient and thus not 

requisite to protect public health with 

an adequate margin of safety, and that 

revision is needed to provide increased 

public health protection. It is important 

to note that this conclusion, and the 

reasoning on which it is based, does not 

address the question of what specific 

revisions are appropriate. That requires 

looking specifically at the current 

indicator, averaging time, form, and 

level of the O3 standard, and evaluating 

the evidence relevant to determining 

whether and to what extent any of these 

elements should be revised, as is 

discussed in the following section. 

C. Conclusions on the Elements of the 

Primary O3 Standard 

1. Indicator 

In the last review of the air quality 

criteria for O3 and other photochemical 

oxidants and the O3 standard, as in 

other prior reviews, EPA focused on a 

standard for O3 as the most appropriate 

surrogate for ambient photochemical 

oxidants. In this review, while the 

complex atmospheric chemistry in 

which O3 plays a key role has been 

highlighted, no alternatives to O3 have 

been advanced as being a more 

appropriate surrogate for ambient 

photochemical oxidants. 

The Staff Paper (section 2.2.2) noted 

that it is generally recognized that 

control of ambient O3 levels provides 

the best means of controlling 

photochemical oxidants. Among the 

photochemical oxidants, the acute 

exposure chamber, panel, and field 

epidemiological human health database 

provides specific evidence for O3 at 

levels commonly reported in the 

ambient air, in part because few other 

photochemical oxidants are routinely 

measured. However, recent 

investigations on copollutant 

interactions have used simulated urban 

photochemical oxidant mixes. These 

investigations suggest the need for 

similar studies to help in understanding 

the biological basis for effects observed 

in epidemiological studies that are 

associated with air pollutant mixtures, 

where O3 is used as the surrogate for the 

mix of photochemical oxidants. Meeting 

the O3 standard can be expected to 

provide some degree of protection 

against potential health effects that may 

be independently associated with other 

photochemical oxidants but which are 

not discernable from currently available 

studies indexed by O3 alone. Since the 

precursor emissions that lead to the 

formation of O3 generally also lead to 

the formation of other photochemical 

oxidants, measures leading to 

reductions in population exposures to 

O3 can generally be expected to lead to 

reductions in population exposures to 

other photochemical oxidants. 

The Staff Paper noted that while the 

new body of time-series epidemiological 

evidence cannot resolve questions about 

the relative contribution of other 

photochemical oxidant species to the 

range of morbidity and mortality effects 

associated with O3 in these types of 

studies, control of ambient O3 levels is 

generally understood to provide the best 

means of controlling photochemical 

oxidants in general, and thus of 

protecting against effects that may be 

associated with individual species and/ 

or the broader mix of photochemical 

oxidants, independent of effects 

specifically related to O3. No public 

comments specifically suggested 

changing the indicator for the O3

NAAQS. 

In its letter to the Administrator, the 

CASAC Panel noted that O3 is ‘‘the key 

indicator of the extent of oxidative 

chemistry and serves to integrate 

multiple pollutants.’’ The CASAC also 

stated that ‘‘although O3 itself has direct 

effects on human health and 

ecosystems, it can also be considered as 

indicator of the mixture of 

photochemical oxidants and of the 

oxidizing potency of the atmosphere’’

(Henderson, 2006c, p. 9). 

Based on the available information, 

and consistent with the views of EPA 

staff and the CASAC, the Administrator 

concludes that it is appropriate to 

continue to use O3 as the indicator for 

a standard that is intended to address 

effects associated with exposure to O3, 

alone or in combination with related 

photochemical oxidants. In so doing, 

the Administrator recognizes that 

measures leading to reductions in 

population exposures to O3 will also 

reduce exposures to other 

photochemical oxidants. 

2. Averaging Time 

a. Short-Term and Prolonged (1 to 8 

Hours) 

The current 8-hour averaging time for 

the primary O3 NAAQS was set in 1997. 

At that time, the decision to revise the 

averaging time of the primary standard 

from 1 hour to 8 hours was supported 

by the following key observations and 

conclusions: 

(1) The 1-hour averaging time of the 

previous NAAQS was originally 

selected primarily on the basis of health 

effects associated with short-term (i.e., 

1- to 3-hour) exposures. 

(2) Substantial health effects 

information was available for the 1997 

review that demonstrated associations 

between a wide range of health effects 

(e.g., moderate to large lung function 


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decrements, moderate to severe 

respiratory symptoms and pulmonary 

inflammation) and prolonged (i.e., 6- to 

8-hour) exposures below the level of the 

then current 1-hour NAAQS. 

(3) Results of the quantitative risk 

analyses showed that reductions in risks 

from both short-term and prolonged 

exposures could be achieved through a 

primary standard with an averaging 

period of either 1 hour or 8 hours. Thus 

establishing both a 1-hour and an 8-hour 

standard would not be necessary to 

reduce risks associated with the full 

range of observed health effects. 

(4) The 8-hour averaging time was 

more directly associated with health 

effects of concern at lower O3

concentrations than the 1-hour 

averaging time. It was thus the 

consensus of the CASAC ‘‘that an 8- 

hour standard was more appropriate for 

a human health-based standard than a 1- 

hour standard.’’ (Wolff, 1995) 

(5) An 8-hour averaging resulted in a 

significantly more uniformly protective 

national standard than the then current 

1-hour standard. 

(6) An 8-hour averaging time 

effectively limits both 1- and 8-hour 

exposures of concern. 

In looking at the new information that 

is discussed in section 7.6.2 of the 

current Criteria Document, the Staff 

Paper noted that epidemiological 

studies have used various averaging 

periods for O3 concentrations, most 

commonly 1-hour, 8-hour and 24-hour 

averages. As described more specifically 

in sections 3.3 and 3.4 of the Staff 

Paper, in general the results presented 

from U.S. and Canadian studies showed 

no consistent difference for various 

averaging times in different studies. 

Because the 8-hour averaging time 

continues to be more directly associated 

with health effects of concern from 

controlled human exposure studies at 

lower concentrations than do shorter 

averaging periods, the Staff Paper did 

not evaluate alternative averaging times 

in this review and did not conduct 

exposure or risk assessments for 

standards with averaging times other 

than 8 hours. 

The Staff Paper discussed an analysis 

of a recent three-year period of air 

quality data (2002 to 2004) which was 

conducted to determine whether the 

comparative 1- and 8-hour air quality 

patterns that were observed in the last 

review continue to be observed based on 

more recent air quality data. This 

updated air quality analysis (McCluney, 

2007) was very consistent with the 

analysis done in the last review in that 

it indicated that only two urban areas of 

the U.S. have such ‘‘peaky’’ air quality 

patterns such that the ratio of 1-hour to 

8-hour design values is greater than 1.5. 

This suggested that based on recent air 

quality data, it was again reasonable to 

conclude that an 8-hour average 

standard at or below the current level 

would generally be expected to provide 

protection equal to or greater than the 

previous 1-hour standard of 0.12 ppm in 

almost all urban areas. Thus, the Staff 

Paper again concluded that setting a 

standard with an 8-hour averaging time 

can effectively limit both 1- and 8-hour 

exposures of concern and is appropriate 

to provide adequate and more uniform 

protection of public health from both 

short-term and prolonged exposures to 

O3 in the ambient air. In its letter to the 

Administrator, the CASAC Panel 

unanimously supported the continued 

use of an 8-hour averaging time for the 

primary O3 standard (Henderson 2007, 

p. 2). 

With respect to comments received on 

the proposal, most public commenters 

did not address the issue of whether 

EPA should consider additional or 

alternative averaging time standards. A 

few commenters, most notably the CA 

EPA and joint comments by ALA and 

several environmental groups, 

expressed the view that consideration 

should be given to setting or reinstating 

a 1-hour standard, in addition to 

maintaining the use of an 8-hour 

averaging time, to protect people in 

those parts of the country with 

relatively more ‘‘peaky’’ exposure 

profiles (e.g., Los Angeles). These 

commenters pointed out that when 

controlled exposure studies using 

triangular exposure patterns (with 

relatively higher 1-hour peaks) have 

been compared to constant exposure 

patterns with the same aggregate O3

dose (in terms of concentration 

multiplied by time), ‘‘peaky’’ exposure 

patterns are seen to lead to higher risks. 

The CA EPA made particular note of 

this point, expressing the view that a 1- 

hour standard would more closely 

represent actual exposures, in that many 

people spend only 1 to 2 hours a day 

outdoors, and that it would be better 

matched to O3 concentration profiles 

along the coasts where O3 levels are 

typically high for shorter averaging 

periods than 8 hours. 

For the reasons discussed in the Staff 

Paper and summarized above and 

considering the unanimous views of the 

CASAC Panel supporting the continued 

use of an 8-hour averaging time for the 

primary O3 standard, the Administrator 

finds that, in combination with the 

decisions on form and level described 

below, the 8-hour standard provides 

adequate protection from both shortterm (1 to 3 hours) and prolonged (6 to 

8 hours) exposures to O3 in the ambient 

air and that it is appropriate to continue 

use of the 8-hour averaging time for the 

O3 NAAQS. 

b. Long-term 

During the last review, there was a 

large animal toxicological database for 

consideration that provided clear 

evidence of associations between longterm (e.g., from several months to years) 

exposures and lung tissue damage, with 

additional evidence of reduced lung 

elasticity and accelerated loss of lung 

function. However, there was no 

corresponding evidence for humans, 

and the state of the science had not 

progressed sufficiently to allow 

quantitative extrapolation of the animal 

study findings to humans. For these 

reasons, consideration of a separate 

long-term primary O3 standard was not 

judged to be appropriate at that time, 

recognizing that the 8-hour standard 

would act to limit long-term exposures 

as well as short-term and prolonged 

exposures. 

Taking into consideration the 

currently available evidence on longterm O3 exposures, discussed above in 

section II.A.2.a.ii, the Staff Paper 

concluded that a health-based standard 

with a longer-term averaging time than 

8 hours is not warranted at this time. 

The Staff Paper noted that while 

potentially more serious health effects 

have been identified as being associated 

with longer-term exposure studies of 

laboratory animals and in epidemiology 

studies, there remains substantial 

uncertainty regarding how these data 

could be used quantitatively to develop 

a basis for setting a long-term health 

standard. Because long-term air quality 

patterns would be improved in areas 

coming into attainment with an 8-hour 

standard, the potential risk of health 

effects associated with long-term 

exposures would be reduced in any area 

meeting an 8-hour standard. Thus, the 

Staff Paper did not recommend 

consideration of a long-term, healthbased standard at this time. 

In its final letter to the Administrator, 

the CASAC Panel offered no views on 

the long-term exposure evidence, nor 

did it suggest that consideration of a 

primary O3 standard with a long-term 

averaging time was appropriate, and 

instead the CASAC Panel agreed with 

the choice of an 8-hour averaging time 

for the primary O3 NAAQS suggested by 

Agency staff (Henderson, 2007). 

Similarly, no public commenters 

expressed support for considering such 

a long-term standard. Taking into 

account the evidence, the CASAC 

Panel’s views, and the public 

comments, the Administrator finds that 

there is not a sufficient basis for setting 


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21The 1-expected-exceedance form essentially 

requires that the fourth-highest air quality value in 

3 years, based on adjustments for missing data, be 

less than or equal to the level of the standard for 

the standard to be met at an air quality monitoring 

site. 

a long-term primary O3 NAAQS at this 

time. 

c. Administrator’s Conclusions on 

Averaging Time 

In considering the information 

discussed above, the CASAC Panel’s 

views and public comments, the 

Administrator concludes that a standard 

with an 8-hour averaging time can 

effectively limit both 1- and 8-hour 

exposures of concern and that an 8-hour 

averaging time is appropriate to provide 

adequate and more uniform protection 

of public health from both short-term (1- 

to 3-hour) and prolonged (6- to 8-hour) 

exposures to O3 in the ambient air. This 

conclusion is based on the observations 

summarized above, particularly: (1) The 

fact that the 8-hour averaging time is 

more directly associated with health 

effects of concern at lower O3

concentrations than are averaging times 

of shorter duration and (2) results from 

quantitative risk analyses showing that 

attaining an 8-hour standard reduces the 

risk of experiencing health effects 

associated with both 8-hour and shorter 

duration exposures. Furthermore, the 

Administrator observes that the CASAC 

Panel agreed with the choice of 

averaging time (Henderson, 2007). 

Therefore, the Administrator finds it 

appropriate to retain the 8-hour 

averaging time and to not set a separate 

1-hour standard. The Administrator also 

concludes that a standard with a longterm averaging time is not warranted at 

this time. 

3. Form 

In 1997, the primary O3 NAAQS was 

changed from a ‘‘1-expectedexceedance’’ form per year over three 

years 21 to a concentration-based 

statistic, specifically the 3-year average 

of the annual fourth-highest daily 

maximum 8-hour concentrations. The 

principal advantage of the 

concentration-based form is that it is 

more directly related to the ambient O3

concentrations that are associated with 

health effects of concern. With a 

concentration-based form, days on 

which higher O3 concentrations occur 

would weigh proportionally more than 

days with lower concentrations, since 

the actual concentrations are used in 

determining whether the standard is 

attained. That is, given that there is a 

continuum of effects associated with 

exposures to varying levels of O3, the 

extent to which public health is affected 

by exposure to ambient O3 is related to 

the actual magnitude of the O3

concentration, not just whether the 

concentration is above a specified level. 

During the 1997 review, consideration 

was given to a range of alternative 

forms, including the second-, third-, 

fourth- and fifth-highest daily maximum 

8-hour concentrations in an O3 season, 

recognizing that the public health risks 

associated with exposure to a pollutant 

without a clear, discernable threshold 

can be appropriately addressed through 

a standard that allows for multiple 

exceedances to provide increased 

stability, but that also significantly 

limits the number of days on which the 

level may be exceeded and the 

magnitude of such exceedances. 

Consideration was given to setting a 

standard with a form that would 

provide a margin of safety against 

possible, but uncertain, chronic effects 

and would also provide greater stability 

to ongoing control programs. The 

fourth-highest daily maximum was 

selected because it was decided that the 

differences in the degree of protection 

against potential chronic effects 

afforded by the alternatives within the 

range were not well enough understood 

to use any such differences as a basis for 

choosing the most restrictive forms. On 

the other hand, the relatively large 

percentage of sites that would 

experience O3 peaks well above 0.08 

ppm and the number of days on which 

the level of the standard may be 

exceeded even when attaining a fifthhighest 0.08 ppm concentration-based 

standard, argued against choosing that 

form. 

As an initial matter, the Staff Paper 

considered whether it is appropriate to 

continue to specify the level of the O3

standard to the nearest hundredth (two 

decimal places) ppm, or whether the 

precision with which ambient O3

concentrations are measured supports 

specifying the standard level to the 

thousandth (three decimal places) ppm 

(i.e., to the part per billion (ppb)). The 

Staff Paper discussed an analysis 

conducted by EPA staff to determine the 

impact of ambient O3 measurement 

error on calculated 8-hour average O3

design value concentrations, which are 

compared to the level of the standard to 

determine whether the standard is 

attained (Cox and Camalier, 2006). The 

results of this analysis suggested that 

instrument measurement error, or 

possible instrument bias, contribute 

very little to the uncertainty in design 

values. More specifically, measurement 

imprecision was determined to 

contribute less than 1 ppb to design 

value uncertainty, and a simulation 

study indicated that randomly occurring 

instrument bias could contribute 

approximately 1 ppb. EPA staff 

interpreted this analysis as being 

supportive of specifying the level of the 

standard to the thousandth ppm. If the 

current standard were to be specified to 

this degree of precision, the current 

standard would effectively be at a level 

of 0.084 ppm, reflecting the data 

rounding conventions that are part of 

the definition of the current 0.08 ppm 

8-hour standard. This information was 

provided to the CASAC Panel and made 

available to the public. 

In evaluating alternative forms for the 

primary standard in conjunction with 

specific standard levels, the Staff Paper 

considered the adequacy of the public 

health protection provided by the 

combination of the level and form to be 

the foremost consideration. In addition, 

the Staff Paper recognized that it is 

important to have a form of the standard 

that is stable and insulated from the 

impacts of extreme meteorological 

events that are conducive to O3

formation. Such instability can have the 

effect of reducing public health 

protection, because frequent shifting in 

and out of attainment due of 

meteorological conditions can disrupt 

an area’s ongoing implementation plans 

and associated control programs. 

Providing more stability is one of the 

reasons that EPA moved to a 

concentration-based form in 1997. 

The Staff Paper considered two 

concentration-based forms of the 

standard: the nth-highest maximum 

concentration and a percentile-based 

form. A percentile-based statistic is 

useful for comparing datasets of varying 

length because it samples approximately 

the same place in the distribution of air 

quality values, whether the dataset is 

several months or several years long. 

However, a percentile-based form would 

allow more days with higher air quality 

values in locations with longer O3

seasons relative to places with shorter 

O3 seasons. An nth-highest maximum 

concentration form would more 

effectively ensure that people who live 

in areas with different length O3 seasons 

receive the same degree of public health 

protection. For this reason, the exposure 

and risk analyses were based on a form 

specified in terms of an nth-highest 

concentration, with n ranging from 3 to 

5. 

The results of some of these analyses 

are shown in the Staff Paper (Figures 6–

1 through 6–4) and specifically 

discussed in chapter 6. These figures 

illustrate the estimated percent change 

in risk estimates for the incidence of 

moderate or greater decrements in lung 

function (≥ 15 percent FEV1) in all 

school age children and moderate or 


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greater lung function decrements (≥ 10 

percent FEV1) in asthmatic school age 

children, associated with going from 

meeting the current standard to meeting 

alternative standards with alternative 

forms based on the 2002 and 2004 

simulations. Figures 6–5 and 6–6 

illustrate the estimated percent change 

in the estimated incidence of nonaccidental mortality, associated with 

going from meeting the current standard 

to meeting alternative standards, based 

on the 2002 and 2004 simulations. 

These results are generally 

representative of the patterns found in 

all of the analyses. The estimated 

reductions in risk associated with 

different forms of the standard, ranging 

from third- to fourth-highest daily 

maximum concentrations at 0.084 ppm, 

and from third- to fifth-highest daily 

maximum concentrations at 0.074 ppm, 

are generally less than the estimated 

reductions associated with the different 

levels that were analyzed. As seen in 

these figures, there is much city-to-city 

variability, particularly in the percent 

changes associated with going from a 

fourth-highest to third-highest form at 

the current level of 0.084 ppm, and with 

estimated reductions associated with 

the fifth-highest form at a 0.074 ppm 

level. In most cities, there are generally 

only small differences in the estimated 

reductions in risks associated with the 

third- to fifth-highest forms at a level of 

0.074 ppm simulated using 2002 and 

2004 O3 monitoring data. 

The Staff Paper noted that there is not 

a clear health-based rationale for 

selecting a particular nth-highest daily 

maximum form of the standard from 

among the ones analyzed. It also noted 

that the changes in the form considered 

in the analyses result in only small 

differences in the estimated reductions 

in risks in most cities, although in some 

cities larger differences are estimated. 

The Staff Paper concluded that a range 

of concentration-based forms from the 

third-to the fifth-highest daily maximum 

8-hour average concentration is 

appropriate for consideration in setting 

the standard. Given that there is a 

continuum of effects associated with 

exposures to varying levels of O3, the 

extent to which public health is affected 

by exposure to ambient O3 is related to 

the actual magnitude of the O3

concentration, not just whether the 

concentration is above a specified level. 

The principal advantage of a 

concentration-based form is that it is 

more directly related to the ambient O3

concentrations that are associated with 

health effects. Robust, concentrationbased forms, in the range of the thirdto fifth-highest daily maximum 8-hour 

average concentration, including the 

current 4th-highest daily maximum 

form, minimize the inherent lack of 

year-to-year stability of exceedancebased forms and provide insulation 

from the impacts of extreme 

meteorological events. Such instability 

can have the effect of reducing public 

health protection by disrupting ongoing 

implementation plans and associated 

control programs. 

With regard to the precision of the 

standard, in its letter to the 

Administrator, the CASAC concluded 

that current monitoring technology 

‘‘allows accurate measurement of O3

concentrations with a precision of parts 

per billion’’ (Henderson, 2006c). The 

CASAC recommended that the 

specification of the level of the O3

standard should reflect this degree of 

precision (Henderson, 2006c). While the 

CASAC Panel unanimously supported 

specifying the level of the standard to 

this degree of precision, public 

comments were mixed. Environmental 

organizations (e.g., ALA et al.) and some 

State/regional agencies (e.g., 

NESCAUM, PA Department of 

Environmental Protection) supported 

the proposed increased precision and 

but did not support truncating to the 

third decimal. However, several 

industry associations (e.g., API, EMA, 

AAAM) suggested that there is not 

sufficient evidence to modify the 1997 

decision to round to two decimal places. 

These comments are addressed in the 

Response to Comments document. 

The Administrator concludes that the 

level of the standard should be specified 

to the thousandth ppm (three decimal 

places), based on the staff’s analysis and 

conclusions discussed in the Staff Paper 

that current monitoring technology 

allows accurate measurement of O3 to 

support specifying the 8-hour standard 

to this degree of precision, and on the 

CASAC Panel’s reasoning and 

recommendation with respect to this 

aspect of the standard. 

With regard to the form of the 

standard, in its letter to the 

Administrator prior to proposal, the 

CASAC recommended that ‘‘a range of 

concentration-based forms from the 

third-to the fifth-highest daily maximum 

8-hour average concentration’’ be 

considered (Henderson, 2006c, p. 5). 

Several commenters supported 

maintaining the current form of the 

standard because it strikes an 

appropriate balance between stability 

and protection, as well as because EPA 

used this form in their analyses (e.g., 

EMA, NESCAUM, and Pennsylvania 

Department of Environmental 

Protection). Some public commenters 

that expressed the view that the current 

primary O3 standard is not adequate 

also submitted comments that 

supported a more health-protective form 

of the standard than the current form 

(e.g., a second-or third-highest daily 

maximum form) (e.g., ALA et al.). Most 

commenters who expressed the view 

that the current standard should not be 

revised did not provide any views on 

alternative forms that would be 

appropriate for consideration should the 

Administrator consider revisions to the 

standard. A few industry association 

and business commenters supported 

changing to a 5th highest form (e.g., 

Dow Chemical, AAM). One commenter 

(Oklahoma Department of 

Transportation) suggested the use of a 

6th or 7th highest daily maximum form. 

The Administrator recognizes that 

there is not a clear health-based 

threshold for selecting a particular nthhighest daily maximum form of the 

standard from among the ones analyzed 

in the Staff Paper and that the current 

form of the standard provides a stable 

target for implementing programs to 

improve air quality. The Administrator 

also agrees that the adequacy of the 

public health protection provided by the 

combination of the level and form is a 

foremost consideration. Based on this, 

the Administrator finds that the form of 

the current standard, 4th-highest daily 

maximum 8-hour average concentration, 

should be retained, recognizing that the 

public health protection that would be 

provided by this standard is based on 

combining this form with the increased 

health protection provided by the lower 

level of the standard discussed in the 

section below. 

4. Level 

a. Proposed Range 

For the reasons discussed below, and 

taking into account information and 

assessments presented in the Criteria 

Document and Staff Paper, the advice 

and recommendations of the CASAC, 

and the public comments received prior 

to proposal, the Administrator proposed 

to revise the existing 8-hour primary O3

standard. Specifically, the 

Administrator proposed to revise the 

level of the primary O3 standard to 

within a range from 0.070 to 0.075 ppm. 

The Administrator’s consideration of 

alternative levels of the primary O3

standard builds on his proposal, 

discussed above, that the overall body of 

evidence indicates that the current 8- 

hour O3 standard is not requisite to 

protect public health with an adequate 

margin of safety because it does not 

provide sufficient protection, and that 

revision would result in increased 

public health protection, especially for 


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members of at-risk groups, notably 

including asthmatic children and other 

people with lung disease, as well as all 

children and older adults, especially 

those active outdoors, and outdoor 

workers, against an array of adverse 

health effects. These effects range from 

health outcomes that could be 

quantified in the risk assessment, 

including decreased lung function, 

respiratory symptoms, serious 

indicators of respiratory morbidity such 

as hospital admissions for respiratory 

causes, and nonaccidental mortality, to 

health outcomes that could not be 

directly estimated, including pulmonary 

inflammation, increased medication 

use, emergency department visits, and 

possibly cardiovascular-related 

morbidity effects. In reaching a 

proposed decision about the level of the 

O3 primary standard, the Administrator 

considered: the evidence-based 

considerations from the Criteria 

Document and the Staff Paper; the 

results of the exposure and risk 

assessments discussed above and in the 

Staff Paper, giving weight to the 

exposure and risk assessments as judged 

appropriate; CASAC advice and 

recommendations, as reflected in 

discussions of drafts of the Criteria 

Document and Staff Paper at public 

meetings, in separate written comments, 

and in CASAC’s letters to the 

Administrator; EPA staff 

recommendations; and public 

comments received during the 

development of these documents, either 

in connection with CASAC meetings or 

separately. In considering what 8-hour 

standard is requisite to protect public 

health with an adequate margin of 

safety, the Administrator noted at the 

time of proposal that he was mindful 

that this choice requires judgment based 

on an interpretation of the evidence and 

other information that neither overstates 

nor understates the strength and 

limitations of the evidence and 

information nor the appropriate 

inferences to be drawn. 

The Administrator noted that the 

most certain evidence of adverse health 

effects from exposure to O3 comes from 

the clinical studies and that the large 

bulk of this evidence derives from 

studies of exposures at levels of 0.080 

and above. At those levels, there is 

consistent evidence of lung function 

decrements and respiratory symptoms 

in healthy young adults, as well as 

evidence of inflammation and other 

medically significant airway responses. 

Moreover, there is no evidence that the 

0.080 ppm level is a threshold for these 

effects. Although the Administrator took 

note of the very limited new evidence 

of lung function decrements and 

respiratory symptoms in some healthy 

individuals at the 0.060 ppm exposure 

level, he judged this evidence too 

limited to support a primary focus at 

this level. The Administrator also noted 

that clinical studies, supported by 

epidemiological studies, provide 

important new evidence that people 

with asthma were likely to experience 

larger and more serious effects than 

healthy people from exposure to O3. 

There were also epidemiological studies 

that provide evidence of statistically 

significant associations between shortterm O3 exposures and more serious 

health effects, such as emergency 

department visits, hospital admissions, 

and premature mortality, in areas that 

likely would have met the current 

standard. The Administrator also took 

note of the many epidemiological 

studies done in areas that likely would 

not have met the current standard but 

which nonetheless report statistically 

significant associations that generally 

extend down to ambient O3

concentrations that were below the level 

of the current standard. Further, there 

were a few studies that have examined 

subsets of data that include only days 

with ambient O3 concentrations below 

the level of the current standard, or 

below even much lower O3

concentrations, and continued to report 

statistically significant associations with 

respiratory morbidity outcomes and 

mortality. In considering this evidence, 

the Administrator noted that the extent 

to which these studies provide evidence 

of causal relationships with exposures 

to O3 alone, down to the lowest levels 

observed, remains uncertain. EPA 

sought comment on the degree to which 

associations observed in 

epidemiological studies reflect causal 

relationships between important health 

endpoints and exposure to O3 alone at 

ambient O3 levels below the current 

standard. 

Therefore, the Administrator judged 

at the time of proposal, and continues 

to judge as discussed in section II.B.3, 

that revising the current standard to 

protect public health with an adequate 

margin of safety is warranted and would 

reduce risk to public health, based on: 

(1) The strong body of clinical evidence 

in healthy people at exposure levels of 

0.080 and above of lung function 

decrements, respiratory symptoms, 

pulmonary inflammation, and other 

medically significant airway responses, 

as well as some indication of lung 

function decrements and respiratory 

symptoms at lower levels; (2) the 

substantial body of clinical and 

epidemiological evidence indicating 

that people with asthma are likely to 

experience larger and more serious 

effects than healthy people; and (3) the 

body of epidemiological evidence 

indicating associations are observed for 

a wide range of serious health effects, 

including respiratory emergency 

department visits, hospital admissions, 

and premature mortality, at and below 

0.080 ppm. The Administrator also 

judged at the time of proposal and 

continues to conclude that the estimates 

of exposures of concern and risks 

remaining upon just meeting the current 

standard or a standard at the 0.080 ppm 

level provide additional support for this 

view. For the same reasons stated in the 

proposal notice and discussed above in 

section II.B on the adequacy of the 

current standard, the Administrator 

judges that the standard should be set 

below 0.080 ppm, a level at which the 

evidence provides a high degree of 

certainty about the adverse effects of O3

exposure even in healthy people. 

The Administrator next considered 

what standard level below 0.080 ppm 

would be requisite to protect public 

health with an adequate margin of safety 

that is sufficient, but not more than 

necessary, to achieve that result, 

recognizing that such a standard would 

result in increased public health 

protection. The assessment of a standard 

level calls for consideration of both the 

degree of additional protection that 

alternative levels of the standard might 

be expected to provide as well as the 

certainty that any specific level will in 

fact provide such protection. In the 

circumstances present in this review, 

there is no evidence-based bright line 

that indicates a single appropriate level. 

Instead there is a combination of 

scientific evidence and other 

information that needs to be considered 

holistically in making this public health 

policy judgment and selecting a 

standard level from a range of 

reasonable values. 

The Administrator noted that at 

exposure levels below 0.080 ppm there 

is only a very limited amount of 

evidence from clinical studies, 

indicating effects in some healthy 

individuals at levels as low as 0.060 

ppm. The great majority of the evidence 

concerning effects below 0.080 ppm is 

from epidemiological studies. The 

epidemiological studies do not identify 

any bright-line threshold level for 

effects. At the same time, the 

epidemiological studies are not in and 

of themselves direct evidence of a 

causal link between exposure to O3 and 

the occurrence of the effects. The 

Administrator considers these studies in 

the context of all the other available 

evidence in evaluating the degree of 


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certainty that O3-related adverse health 

effects would occur at various ambient 

levels below 0.080 ppm, including the 

strong human clinical studies and the 

toxicological studies that demonstrate 

the biological plausibility and 

mechanisms for the effects of O3 on 

airway inflammation and increased 

airway responsiveness at exposure 

levels of 0.080 ppm and above. 

Based on consideration of the entire 

body of evidence and information 

available at this time, as well as the 

recommendations of the CASAC, the 

Administrator proposed that a standard 

within the range of 0.070 to 0.075 ppm 

would be requisite to protect public 

health with an adequate margin of 

safety. As noted at the time of proposal, 

a standard level within this range is 

estimated to reduce the risk of a variety 

of health effects associated with 

exposure to O3, including the 

respiratory symptoms and lung function 

effects demonstrated in clinical studies, 

and in emergency department visits, 

hospital admissions, and mortality 

effects indicated in the epidemiological 

studies. All of these effects are 

indicative of a much broader array of 

O3-related health endpoints, as 

represented by the pyramid of effects, 

such as school absences and increased 

medication use that are plausibly linked 

to these observed effects. 

The Administrator also considered 

the degree of improvements in public 

health that potentially could be 

achieved by a standard of 0.070 to 0.075 

ppm, giving weight to the exposure and 

risk assessments as he judged 

appropriate. As discussed in the 

proposal notice (section II.D.4) in 

considering the results of the exposure 

assessment, the Administrator primarily 

focused on exposures at and above the 

0.070 ppm benchmark level as an 

important surrogate measure for 

potentially more serious health effects 

for at-risk groups, including people with 

asthma. In so doing, the Administrator 

noted that although the analysis of 

‘‘exposures of concern’’ was conducted 

to estimate exposures at and above three 

discrete benchmark levels, the concept 

is appropriately viewed as a continuum. 

As discussed above, the Administrator 

strives to balance concern about the 

potential for health effects and their 

severity with the increasing uncertainty 

associated with our understanding of 

the likelihood of such effects at lower 

O3 exposure levels. In focusing on this 

benchmark, the Administrator noted 

that upon just meeting a standard 

within the range of 0.070 to 0.075 ppm 

based on the 2002 simulation, the 

number of school age children likely to 

experience exposures at and above this 

benchmark level in aggregate (for the 12 

cities in the assessment) was estimated 

to be approximately 2 to 4 percent of all 

and asthmatic children and generally 

less than 10 percent of children even in 

cities that receive the least degree of 

protection from such a standard in a 

recent year with relatively high O3

levels. A standard within the 0.070 to 

0.075 ppm range would thus 

substantially reduce exposures of 

concern by about 90 to 80 percent, 

respectively, from those estimated to 

occur upon just meeting the current 

standard. While placing less weight on 

the results of the risk assessment, in 

light of the important uncertainties 

inherent in the assessment, the 

Administrator noted that the results 

indicated that a standard set within this 

range would likely reduce risks to atrisk groups from the O3-related health 

effects considered in the risk 

assessment, and by inference across the 

much broader array of O3-related health 

effects that could only be considered 

qualitatively, relative to the level of 

protection afforded by the current 

standard. This lent support to the 

proposed range. 

The Administrator judged that a 

standard set within the range of 0.070 to 

0.075 ppm would provide a degree of 

reduction in risk that is important from 

a public health perspective and that a 

standard within this range would be 

requisite to protect public health, 

including the health of at-risk groups, 

with an adequate margin of safety. 

EPA’s evaluation of the body of 

scientific evidence and quantitative 

estimates of exposures and risks 

indicated that substantial reductions in 

public health risks would occur 

throughout this range. As noted in the 

proposal notice, because there is no 

bright line clearly directing the choice 

of level within this reasonable range, the 

choice of what is appropriate, 

considering the strengths and 

limitations of the evidence, and the 

appropriate inference to be drawn from 

the evidence and the exposure and risk 

assessments is a public health policy 

judgment. To further inform this 

judgment, EPA sought public comment 

on the extent to which the 

epidemiological and clinical evidence 

provide guidance as to the level of a 

standard that would be requisite to 

protect public health with an adequate 

margin of safety, especially for at-risk 

groups. 

In considering the available 

information, the Administrator also 

judged that a standard level below 0.070 

ppm would not be appropriate. In 

reaching this judgment, the 

Administrator noted that there was only 

quite limited evidence from clinical 

studies at exposure levels below 0.080 

ppm O3. Moreover, the Administrator 

recognized that in the body of 

epidemiological evidence, many studies 

reported positive and statistically 

significant associations, while others 

reported positive results that were not 

statistically significant, and a few did 

not report any positive O3-related 

associations. In addition, the 

Administrator judged that evidence of a 

causal relationship between adverse 

health outcomes and O3 exposures 

became increasingly uncertain at lower 

levels of exposure. 

The Administrator also considered 

the results of the exposure assessments 

in reaching his judgment that a standard 

level below 0.070 ppm would not be 

appropriate. The Administrator noted 

that in considering the results from the 

exposure assessment, a standard set at 

the 0.070 ppm level, with the same form 

as the current standard, was estimated 

to provide substantial reductions in 

exposures of concern (i.e., 

approximately 90 to 92 percent 

reductions in the numbers of school age 

children and 94 percent reduction in the 

total number of occurrences) for both all 

and asthmatic school age children 

relative to just meeting the current 

standard based on a simulation of a 

recent year with relatively high O3

levels (2002). Thus, a 0.070 ppm 

standard would be expected to provide 

protection from the exposures of 

concern that the Administrator had 

primarily focused on for over 98 percent 

of all and asthmatic school age children 

even in a year with relatively high O3

levels, increasing to over 99.9 percent of 

children in a year with relatively low O3

levels (2004). 

In considering the results of the 

health risk assessment, as discussed in 

the proposal notice (section II.C.2), the 

Administrator noted that there were 

important uncertainties and 

assumptions inherent in the risk 

assessment and that this assessment was 

most appropriately used to simulate 

trends and patterns that could be 

expected, as well as providing informed, 

but still imprecise, estimates of the 

potential magnitude of risks. The 

Administrator particularly noted that as 

lower standard levels were modeled, 

including a standard set at a level below 

0.070 ppm, the risk assessment 

continued to assume a causal link 

between O3 exposures and the 

occurrence of the health effects 

examined, such that the assessment 

continued to indicate reductions in O3- 

related risks upon meeting a lower 

standard level. As discussed above, 

however, the Administrator recognized 


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that evidence of a causal relationship 

between adverse health effects and O3

exposures becomes increasingly 

uncertain at lower levels of exposure. 

Given all of the information available to 

him at the time of the proposal, the 

Administrator judged that the increasing 

uncertainty of the existence and 

magnitude of additional public health 

protection that standards below 0.070 

ppm might provide suggested that such 

lower standard levels would likely be 

below what is necessary to protect 

public health with an adequate margin 

of safety. 

In addition, the Administrator judged 

that a standard level higher than 0.075 

ppm would also not be appropriate. 

This judgment took into consideration 

the information discussed in the 

proposal notice (sections II.A and B) 

and was based on the strong body of 

clinical evidence in healthy people at 

exposure levels of 0.080 ppm and above, 

the substantial body of clinical and 

epidemiological evidence indicating 

that people with asthma are likely to 

experience larger and more serious 

effects than healthy people, the body of 

epidemiological evidence indicating 

that associations are observed for a wide 

range of more serious health effects at 

levels below 0.080 ppm, and the 

estimates of exposure and risk 

remaining upon just meeting a standard 

set at 0.080 ppm. The much greater 

certainty of the existence and magnitude 

of additional public health protection 

that such levels would forego provides 

the basis for judging that levels above 

0.075 ppm would be higher than what 

is requisite to protect public health, 

including the health of at-risk groups, 

with an adequate margin of safety. 

For the reasons discussed in more 

detail in the proposal notice and 

summarized above, the Administrator 

proposed to revise the level of the 

primary O3 standard to within the range 

of 0.070 to 0.075 ppm. 

At the time of proposal, the 

Administrator recognized that sharply 

divergent views on the appropriate level 

of this standard had been presented to 

EPA as part of the NAAQS review 

process, and he solicited comment on a 

wide range of standard levels and 

alternative approaches to characterizing 

and addressing scientific uncertainties. 

One such alternative view focused very 

strongly on the uncertainties inherent in 

the controlled human exposure and 

epidemiological studies and 

quantitative exposure and health risk 

assessments as the basis for concluding 

that no change to the current 8-hour O3

standard of 0.084 ppm was warranted. 

In sharp contrast, others viewed the 

controlled human exposure and 

epidemiological studies as strong and 

robust, and generally placed more 

weight on the results of the quantitative 

exposure and risk assessments and the 

unanimous CASAC recommendations as 

a basis for concluding that an 8-hour 

standard at or below 0.070 ppm was 

warranted. As discussed below, the 

same sharply divergent views were 

generally repeated in comments on the 

proposal by the two distinct groups of 

commenters identified in II.B.2 above. 

b. Comments on Level 

i. Health Evidence Considerations 

With regard to the evaluation and 

consideration of the health effects 

evidence and how such information 

should be considered in the decision on 

the standard level, EPA notes that the 

commenters fell into the same two 

groups discussed above in section II.B.2. 

The two groups often cited the same 

studies and evidence, but they reached 

sharply divergent conclusions as to 

what standard level is supported by the 

health effects evidence. The general 

views of both groups on the 

interpretation and use of the health 

effects evidence are presented above in 

section II.B.2.a, with most comments 

from one group arguing that this 

evidence supports a decision to revise 

the 8-hour standard to 0.060 ppm or 

below, and the other group arguing that 

it supports a decision not to revise the 

current 8-hour standard. 

With regard to the evidence from 

controlled human exposure studies, 

commenters that included public health 

and environmental groups who 

supported revising the current standard 

expressed the view that the large body 

of evidence available at the time of the 

last review, demonstrating an array of 

adverse health effects (i.e., reduced lung 

function, respiratory symptoms, 

increased airway responsiveness, 

inflammation, and increased 

susceptibility to respiratory infection), 

at concentrations of 0.080 ppm O3, 

indicated that the standard should have 

been set at a lower level. These 

commenters noted that standards must 

be set below the level shown to cause 

effects in healthy subjects in order to 

protect sensitive populations with an 

adequate margin of safety. As discussed 

in section II.B.2.a above, these 

commenters focused on the results of 

the Adams studies (2002, 2006) as 

evidence that exposure to 0.060 ppm O3

will result in a significant proportion 

(i.e., 7%) of the adult population who 

do not have asthma or other lung 

diseases experiencing notable lung 

function decrements (FEV1 decrement 

(≥10%), and furthermore that larger 

decrements in FEV1 would be expected 

in more susceptible populations. This 

evidence caused these commenters to 

reject EPA’s proposed range: 

Clearly, EPA’s proposed standard of 0.070 

to 0.075 ppm cannot be considered protective 

of public health in light of experimental 

evidence demonstrating adverse respiratory 

effects in healthy individuals exposed to 

0.060 ppm, and the legal requirements to 

protect sensitive populations with an 

adequate margin of safety. [ALA et al., p. 51] 

The second group of commenters, 

who opposed revision of the standard, 

expressed the view that the group mean 

changes reported in the Adams studies 

(2002, 2006) were small, that such 

decrements should not be considered to 

be adverse, and that the individuals 

who experienced larger responses were 

too few to serve as a basis for a revised 

O3 standard. This group included 

virtually all commenters representing 

industry associations and businesses. 

These general comments are addressed 

above in section II.B.2.a and in more 

detail in the Response to Comments 

document. 

In considering comments received on 

controlled human exposure studies, and 

how these studies support a focus on 

particular standard levels, the 

Administrator observes that in general 

the comments support his original view 

that these studies provide the most 

certain evidence of adverse health 

effects, and that the large bulk of 

evidence derives from studies of 

exposures at levels of 0.080 ppm and 

above. The Administrator notes that 

since the last review important new 

evidence includes demonstration of O3- 

induced lung function effects and 

respiratory symptoms in some healthy 

adults down to the previously observed 

exposure level of 0.080 ppm, as well as 

very limited new evidence of the same 

effects at exposure levels well below the 

level of the current standard (Adams, 

2002, 2006). EPA disagrees with these 

commenters that the percent of subjects 

that experienced FEV1 decrements 

greater than 10% in this study of 30 

subjects can appropriately be 

generalized to the U.S. population. 

Based on careful consideration of the 

comments, the Administrator again 

concludes that while the Adams studies 

provide evidence that some healthy 

individuals will experience lung 

function decrements and respiratory 

symptoms at the 0.060 ppm exposure 

level, this evidence is too limited to 

support a primary focus at this level. 

Moreover, the Administrator notes that 

while the CASAC Panel supported a 

level of 0.060 ppm, they also supported 

a level above 0.060, indicating that they 

disagree with the commenters’ view that 


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the results of Adams studies mean that 

the level of the standard has to be set 

at 0.060 ppm. 

With regard to the information from 

epidemiological studies, commenters 

representing public health, 

environmental, and medical 

organizations generally asserted that the 

large body of new epidemiological 

studies provides evidence of causal 

associations between O3 exposures and 

a wide array of respiratory and 

cardiovascular morbidity effects, 

including emergency department visits 

and hospital admissions. They 

expressed the view that a significant 

body of strong, consistent evidence 

links short-term exposures to premature 

mortality and noted that this evidence is 

supported by new research that 

provides biological plausibility for such 

effects. These commenters noted that 

various approaches, including air 

quality assessments which show that 

statistically significant associations 

occurred in areas that likely would have 

met the current standard, or statistical 

approaches that examined subsets of the 

data which indicate that statistically 

significant associations remain down to 

very low ambient O3 levels, show effects 

well below the level of the current 

standard. Moreover they identified 

particular studies, including some 

‘‘new’’ studies not considered in the 

Criteria Document, that indicated there 

are additional sub-populations that are 

likely to be sensitive to O3, including 

infants, women, and African-Americans, 

that should be considered in deciding 

the requisite level of protection. They 

asserted that this information supports a 

standard set at a level no higher than 

0.060 ppm O3. 

With regard to the information from 

epidemiological studies, the second 

group of commenters focused strongly 

on EPA’s interpretation of the 

epidemiological evidence and the 

uncertainties they saw in this evidence 

as a basis for concluding that no change 

to the current level of the 8-hour O3

standard is warranted. In commenting 

on the proposed range of levels, these 

commenters generally relied on the 

same arguments presented above in 

section II.B.2.a as to why they believed 

it would be inappropriate for EPA to 

make any revisions to the primary O3

standard. That is, they asserted that the 

health effects of concern associated with 

short-term or prolonged exposures to O3

have not changed significantly since 

1997; that the inconsistencies and 

uncertainties inherent in these studies 

as a whole should preclude any reliance 

on them as justification for a more 

stringent standard; and that ‘‘new’’

science not included in the Criteria 

Document continues to increase 

uncertainty about possible health risks 

associated with exposure to O3. Specific 

methodological issues cited as 

additional support for their conclusions 

included: adequacy of exposure data; 

potential confounding by copollutants; 

model selection; inconsistent evidence 

relating O3 exposure to mortality, and 

‘‘new’’ studies that provide additional 

evidence of inconsistencies. These 

general comments are addressed above 

in section II.B.2.a, and in greater detail 

in the Response to Comments 

document. 

In considering these comments on the 

epidemiological evidence with regard to 

the interpretation of the epidemiological 

evidence and methodological issues, the 

Administrator notes that in general, 

most of the issues and concerns raised 

by those who do not support any 

revisions to the primary O3 standard 

with regard to the interpretation of the 

epidemiological evidence and 

methodological issues, are essentially 

restatements if issues raised during the 

review of the Criteria Document and 

Staff Paper. The same is true of the 

views of commenters who supported a 

level of the standard no higher than 

0.060 ppm O3. EPA presented and the 

CASAC Panel reviewed the 

interpretation of the epidemiological 

evidence in the Criteria Document and 

the integration of the evidence with 

policy considerations in the 

development of the policy options 

presented in the Staff Paper for 

consideration by the Administrator. 

CASAC reviewed the scientific content 

of both the Criteria Document and Staff 

Paper and advised the Administrator 

that these documents provided an 

appropriate basis for use in regulatory 

decision making. Therefore, these 

comments do not provide a basis for the 

Administrator to reach fundamentally 

different conclusions than he reached at 

the time of proposal. 

Moreover, the Administrator notes 

that epidemiological evidence is most 

appropriately evaluated in the context 

of all available evidence, including 

evidence from controlled human 

exposure and toxicological studies. In 

general, the Administrator agrees with 

the weight of evidence approach used in 

the Criteria Document and believes that 

this body of scientific evidence across 

all types of studies is very robust, 

recognizing that it includes a large 

number of various types of studies that 

provide consistent and coherent 

evidence of an array of O3-related 

respiratory morbidity effects and 

possibly cardiovascular-related 

morbidity as well as total nonaccidental 

and cardiorespiratory mortality. More 

specifically, the Administrator judges 

that the body of epidemiological 

evidence indicating associations with a 

wide range of serious health effects, 

including respiratory emergency 

department visits and hospital 

admissions and premature mortality, at 

and below 0.080 ppm supports revising 

the current standard to protect public 

health. While the great majority of 

evidence concerning effects below 0.080 

ppm was from epidemiological studies, 

the epidemiological studies do not 

identify any bright-line threshold level 

for effects. At the same time, the 

epidemiological studies are not 

themselves direct evidence of a causal 

link between exposure to O3 and the 

occurrence of the effects. Therefore, 

Administrator has considered these 

studies in the context of all the other 

available evidence in evaluating the 

degree of certainty that O3-related 

adverse health effects would occur at 

various ambient levels below 0.080 

ppm. In that context, there is only quite 

limited evidence from controlled human 

exposure studies at exposure levels 

below 0.080 ppm O3. The Administrator 

recognizes that in the body of 

epidemiological evidence, many studies 

reported positive and statistically 

significant associations, while others 

reported positive results that were not 

statistically significant, and a few did 

not report any positive O3-related 

associations. In addition, the 

Administrator judged that evidence of a 

causal relationship between adverse 

health outcomes and O3 exposures 

became increasingly uncertain at lower 

levels of exposure. Based on this the 

Administrator continues to believe that 

the body of epidemiological evidence 

does not support setting a standard as 

low as 0.060 as suggested by some 

commenters. 

The Administrator also notes the 

many epidemiological studies done in 

areas that likely would not have met the 

current standard but which nonetheless 

report statistically significant 

associations that generally extend down 

to ambient O3 concentrations that were 

below the level of the current standard. 

Further, there were a few studies that 

have examined subsets of data that 

include only days with ambient O3

concentrations below the level of the 

current standard, or below even much 

lower O3 concentrations, and continued 

to report statistically significant 

associations with respiratory morbidity 

outcomes and mortality. In the context 

of the strong clinical evidence of 

adverse effect in healthy adults at 0.080, 

the Administrator finds that the body of 

epidemiological evidence does not 


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support retaining a standard of 0.080, as 

suggested by commenters. 

Both groups of commenters also 

considered evidence from controlled 

human exposure and epidemiological 

studies of increased susceptibility in 

people with lung disease, especially 

people with asthma, but they reached 

sharply divergent conclusions about 

what standard level is supported by this 

evidence. As discussed above in section 

II.B.2.a, medical organizations and 

public health and environmental groups 

agreed with EPA that, based on 

evidence from controlled human 

exposure and epidemiological studies, 

people with asthma, especially children, 

are likely to have greater lung function 

decrements and respiratory symptoms 

in response to O3 exposure than people 

who do not have asthma, and are likely 

to respond at lower levels. Furthermore, 

these commenters noted that 

epidemiological studies have identified 

other potentially sensitive 

subpopulations, including for example, 

infants, women and African-Americans, 

and that effects in these groups should 

be part of the consideration in providing 

an adequate margin of safety. These 

commenters concluded that the 

appropriate level for the primary O3

standard is 0.060 ppm, to provide 

protection for members of sensitive 

groups, especially people with asthma, 

who are likely to have more serious 

responses and to respond at lower levels 

that healthy people. They also 

contended that a standard set at this 

level also would provide protection 

against anticipated, but as yet unproven 

effects in the additional groups cited. 

The Administrator agrees with these 

commenters that important new 

evidence shows that asthmatics have 

more serious responses, and are more 

likely to respond at lower O3 levels, 

than healthy individuals. Moreover, he 

agrees that this evidence supports a 

standard set at a level below 0.080 ppm 

O3, based on the strong evidence from 

human clinical studies in healthy adults 

at this level. However, for the reasons 

described above, he does not agree that 

the controlled human exposure and 

epidemiological evidence provide 

support for a standard set at 0.060 ppm, 

for the reasons discussed above. 

In contrast, industry association and 

business commenters asserted that EPA 

is wrong to claim that new evidence 

indicates that the current standard does 

not provide adequate health public 

health protection for people with 

asthma. In support of this position, 

these commenters made the following 

major comments: (1) The lung function 

decrements and respiratory symptoms 

observed in clinical studies of 

asthmatics are not clinically important; 

(2) EPA postulates that asthmatics 

would likely experience more serious 

responses and responses at lower levels 

than the subjects of controlled human 

exposure experiments, but that 

hypothesis is not supported by scientific 

evidence; and, (3) EPA recognized 

asthmatics as a sensitive subpopulation 

in 1997, and new information does not 

suggest greater susceptibility than was 

previously believed. EPA has generally 

responded to these comments and those 

summarized in the paragraph above in 

section II.B.2.a above, and in greater 

detail in the Response to Comments 

document. 

After careful consideration of these 

comments, the Administrator continues 

to judge that there is important new 

evidence demonstrating that exposures 

to O3 at levels below the level of the 

current standard are associated with a 

broad array of adverse health effects, 

especially in at-risk populations that 

include people with asthma or other 

lung diseases who are likely to 

experience more serious effects from 

exposure to O3, as well as children and 

older adults with increased 

susceptibility, and those who are likely 

to be vulnerable as a result of spending 

a lot of time outdoors engaged in 

physical activity, especially active 

children and outdoor workers. The 

Administrator notes that this important 

new evidence demonstrates O3-induced 

lung function effects and respiratory 

symptoms in some healthy individuals 

down to the previously observed 

exposure level of 0.080 ppm, as well as 

very limited new evidence at exposure 

levels well below the level of the 

current standard. In addition, there are 

many epidemiological studies done in 

areas that likely would not have met the 

current standard but which nonetheless 

report statistically significant 

associations that generally extend down 

to ambient O3 concentrations that were 

below the level of the current standard. 

Further, there were a few studies that 

have examined subsets of data that 

include only days with ambient O3

concentrations below the level of the 

current standard, or below even much 

lower O3 concentrations, and continued 

to report statistically significant 

associations with respiratory morbidity 

outcomes and mortality. The 

Administrator recognizes that in the 

body of epidemiological evidence, many 

studies reported positive and 

statistically significant associations, 

while others reported positive results 

that were not statistically significant, 

and a few did not report any positive 

O3-related associations. In addition, the 

Administrator judged that evidence of a 

causal relationship between adverse 

health outcomes and O3 exposures 

became increasingly uncertain at lower 

levels of exposure. This body of 

evidence provides a strong basis for the 

Administrator’s judgment that the 

standard needs to be revised to provide 

more protection, and that a revised 

standard must be set at a level 

appreciably below 0.080 ppm, the level 

at which there is considerable evidence 

of effects in healthy people. At the same 

time, for the reasons discussed above 

the Administrator judges that this body 

of evidence does not support setting a 

standard as low as 0.060, as suggested 

by other commenters. 

ii. Exposure and Risk Considerations 

With regard to considering how the 

quantitative exposure and health risk 

assessments should factor into a 

decision on the standard level, EPA 

notes that both groups of commenters 

generally consider these assessments in 

their comments on the standard level, 

but they reach sharply divergent 

conclusions as to what standard level is 

supported by these assessments. The 

general views of both groups on the 

implications of the exposure and risk 

assessment are presented above in 

section II.B.2.b, with one group arguing 

that it supports a decision to revise the 

8-hour standard to 0.060 ppm or below, 

and the other group arguing that it 

supports a decision not to revise the 

current 8-hour standard. 

A joint set of comments from ALA 

and several environmental groups 

expressed the view that EPA cannot use 

exposures of concern to justify a 

standard in the range of 0.070 to 0.075 

ppm. These commenters contended that 

standards in the proposed range would 

continue to expose too many asthmatic 

children, as well as other at risk groups 

such as outdoor workers and preschool 

children, to ‘‘demonstrably unhealthy 

levels of ozone pollution’’ in only 12 

cities which does not represent a 

national estimate (ALA et al., p. 106). 

These same commenters asserted that if 

EPA were to consider exposures of 

concern, then the benchmark level must 

be defined as 0.060 ppm based on the 

considerable evidence of adverse health 

effects occurring at this level. As 

discussed in section II.B.2.b above, they 

also cited various reasons why the 

exposure estimates were 

underestimated, including: only 12 

cities were included in the assessment, 

various at risk groups including outdoor 

workers and preschool children were 

not included in the assessment, and 

EPA’s exposure assessment 

underestimated exposures since it 


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considers average children, not active 

children who spend more time outdoors 

and repeated exposures also were 

underestimated. 

In contrast, industry association and 

business group commenters expressed 

the view that the concept of exposures 

of concern should not be considered as 

a basis for revising the level of the 

standard because it provided no 

indication of the probability that 

individuals would actually experience 

an adverse health effect. These same 

commenters also provided various 

reasons why the exposure estimates 

were overestimated based on specific 

methodological choices made by EPA 

including, for example, O3

measurements at fixed-site monitors can 

be higher than other locations where 

individuals are exposed, the exposure 

estimates do not account for O3

avoidance behaviors, and the exposure 

model overestimates elevated breathing 

rates. Finally, these commenters also 

contended that the estimates of 

exposures of concern associated with 

just meeting the current standard, using 

the 0.080 ppm benchmark levels, have 

not appreciably changed since the prior 

review and, thus provide no support for 

revising the current standard. 

EPA has responded to the criticisms 

from both groups of commenters related 

to concerns that the exposure estimates 

are either underestimated or 

overestimated in section II.B.2.b above 

and in more detail in the Response to 

Comments document. EPA also has 

addressed the issues raised by both 

groups of commenters concerning the 

appropriateness of considering 

exposures at and above various 

benchmark levels as an element in the 

decision on the adequacy of the current 

standard in section II.B.2.b. 

As discussed in section II.B.2b, the 

Administrator believes that it is 

appropriate to consider such exposure 

estimates in the context of a continuum 

rather than focusing on any one discrete 

benchmark level, as was done at the 

time of proposal, since the 

Administrator does not believe that the 

underlying evidence is certain enough 

to support a focus on any single brightline benchmark level. Thus, the 

Administrator believes it is appropriate 

to consider a range of benchmark levels 

from 0.080 down to 0.060 ppm, 

recognizing that exposures at and above 

these benchmark levels must be 

considered in the context of a 

continuum of the potential for health 

effects of concern, and their severity, 

with increasing uncertainty associated 

with the likelihood of such effects at 

lower O3 exposure levels. 

The Administrator recognizes that the 

0.080 ppm benchmark level represents a 

level at which several health outcomes, 

including lung inflammation, increased 

airway responsiveness, and decreased 

resistance to infection have been shown 

to occur in healthy adults. The 

Administrator places great weight on 

the public health significance of 

exposures at and above this benchmark 

level given the greater certainty that 

these adverse health responses are likely 

to be observed in a significant fraction 

of the at-risk population. With respect to 

his decision on the level of the 8-hour 

standard, the Administrator notes that 

upon just meeting a standard within the 

range of 0.070 to 0.075 ppm based on 

the 2002 simulation, the number of 

school age asthmatic children likely to 

experience exposures at and above the 

0.080 ppm benchmark level in aggregate 

(for the 12 cities in the assessment) is 

estimated to range from 0.1 to 0.4 

percent of asthmatic school age 

children. Based on the 2004 simulation, 

the estimates are even lower, with no 

asthmatic children estimated to 

experience exposures at and above the 

0.080 ppm benchmark level. Similar 

patterns are observed for all school age 

children. Recognizing the uncertainties 

inherent in the exposure assessment, the 

Administrator concludes that the 

exposure assessment suggests that 

exposures at and above the 0.080 ppm 

level, where several health effects have 

been shown to occur in healthy 

individuals, are eliminated or nearly 

eliminated depending on the modeling 

year upon just meeting a standard 

within the range of 0.070 to 0.075 ppm. 

The Administrator does not agree 

with those commenters who would only 

consider the single benchmark level of 

0.080 ppm. While the Administrator 

places less weight on exposures at and 

above the 0.070 pm benchmark level, 

given the increased uncertainty about 

the fraction of the population and 

severity of the health responses that 

might occur associated with exposures 

above this level, he believes that it is 

appropriate to consider exposures at 

this benchmark as well in judging the 

adequacy of the current standard to 

protect public health. Consideration of 

the 0.070 ppm benchmark level 

recognizes that the effects observed at 

0.080 ppm were in healthy adult 

subjects and sensitive population 

groups, such as asthmatics, are expected 

to respond at lower O3 levels than 

healthy individuals. The Administrator 

notes that upon just meeting a standard 

within the range of 0.070 to 0.075 ppm 

based on the 2002 simulation, the 

number of asthmatic school age children 

likely to experience exposures at and 

above the 0.070 ppm benchmark level in 

aggregate (for the 12 cities in the 

assessment) is estimated to range from 

about 2 to 5 percent of asthmatic school 

age children. Based on the 2004 

simulation, the estimates are 

substantially lower, with 0 to 0.6 

percent of asthmatic children estimated 

to experience exposures at and above 

the 0.070 ppm benchmark level upon 

just meeting a standard within the range 

of 0.070 to 0.075 ppm. 

Finally, the Administrator has 

considered but places very little weight 

on the benchmark level of 0.060 ppm 

given the very limited scientific 

evidence supporting a conclusion that 

O3 is causally related to various health 

outcomes at this exposure level. 

Nevertheless, the Administrator 

observes that there is a similar pattern 

of reductions in exposures of concern 

for all and asthmatic school age children 

at this benchmark level as well when 

comparing the 0.070 ppm and 0.075 

ppm 8-hour standards. 

Given the degree of uncertainty 

associated with the exposure assessment 

discussed in the Staff Paper and 

uncertainty assessment (Langstaff, 

2007), the Administrator judges that for 

each specific benchmark level examined 

there is not an appreciable difference, 

from a public health perspective, in the 

estimates of exposures associated with 

air quality just meeting an 8-hour 

standard at 0.075 ppm versus an 8-hour 

standard set at 0.070 ppm. For example, 

given the uncertainty in the exposure 

estimates, the difference between an 

estimate of 2 percent and 5 percent of 

asthmatic children for the exposure 

benchmark of 0.070 is not an 

appreciable difference from a public 

health perspective. While directionally 

there are likely to be fewer exposures at 

and above this benchmark for a standard 

of 0.070 than a standard of 0.075 ppm, 

given the uncertainty in the exposure 

assessment it is not at all clear that the 

actual difference is large enough to 

present a public health concern. 

With regard to considering how the 

quantitative risk assessment should 

factor into a decision on the standard 

level, as noted above both groups of 

commenters generally considered the 

risk assessment in their comments on 

the standard level, but they reached 

sharply divergent conclusions as to 

what standard level is supported by the 

risk assessment. More specifically, the 

environmental, public health, and most 

medical organizations, and some State 

and regional air pollution agencies (e.g., 

California, NESCAUM) contended that 

EPA’s proposed range of 0.070 to 0.075 

ppm would result in significant residual 


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public health risks. As articulated most 

fully in the joint set of comments from 

ALA and several environmental 

organizations, these commenters 

expressed the view that EPA’s risk 

assessment clearly demonstrates that a 

more stringent 8-hour O3 standard of 

0.065 ppm, the most stringent standard 

analyzed by EPA, would significantly 

decrease O3-related lung function 

decrements, respiratory symptoms, 

hospital admissions, and mortality and 

that ‘‘EPA must adopt a more stringent 

ozone standard of 0.060 ppm or below—

a level that incorporates a more 

adequate margin of safety’’ (ALA et al., 

p. 108). These same commenters also 

cited various reasons for asserting that 

the risk assessment likely 

underestimates health risks to a 

substantial degree, including the limited 

nature of the assessment with respect to 

number of cities, populations covered, 

and health endpoints analyzed. EPA has 

responded to the comments concerning 

the scope of the risk assessment and 

assertion that health risks are likely 

underestimated both in section II.B.2.b 

above and in more detail in the 

Response to Comments document. The 

Administrator’s reasoning and 

conclusions regarding the weight he 

places on the health risk assessment in 

reaching a judgment about the 

appropriate level for the primary 

standard are discussed below in section 

II.C.4.c. 

In contrast, industry association and 

business group commenters who 

supported not revising the level of the 

current 8-hour standard generally 

asserted the following points: (1) That 

risk estimates have not changed 

significantly since the prior review in 

1997; (2) that uncertainties and 

limitations underlying the risk 

assessment make it too speculative to be 

used in supporting a decision to revise 

the standard; (3) that EPA should have 

defined PRB differently and that EPA 

underestimated PRB levels, which 

results in health risk reductions 

associated with more stringent 

standards being overestimated; and (4) 

that health risks are overestimated based 

on specific methodological choices 

made by EPA including, for example, 

selection of inappropriate effect 

estimates from health effect studies, 

EPA’s approach to addressing the shape 

of exposure-response relationships, and 

whether or not to incorporate thresholds 

into its models for the various health 

effects analyzed. EPA has responded to 

these comments both in section II.B.2.b 

above and in more detail in the 

Response to Comments document. 

In summary, the Administrator 

concludes that the exposure assessment 

suggests that exposures at and above the 

0.080 ppm benchmark level, where 

several health effects have been shown 

to occur in healthy individuals, are 

essentially eliminated for standards in 

the range of 0.070 to 0.075 ppm. He also 

concludes that at the 0.070 ppm 

benchmark level, the exposures are 

substantially reduced and eliminated for 

the vast majority of people in at-risk 

groups, and that the very low estimates 

of such exposures are not appreciably 

different, from a public health 

perspective, between those exposures 

associated with just meeting a standard 

set at 0.070 ppm or 0.075 ppm. Further, 

the Administrator places relatively little 

weight on the exposures using the 0.060 

ppm benchmark level given the very 

limited scientific evidence supporting a 

conclusion that O3 is causally related to 

health outcomes at this exposure level. 

Considering the uncertainties associated 

with the exposure assessment, the 

Administrator concludes that the 

exposure estimates associated with each 

of the benchmark levels are not 

appreciably different, between a 0.070 

or 0.075 ppm standard, and therefore, 

the exposure assessment does not 

provide a basis for choosing a level 

within the proposed range. 

While the Administrator places less 

weight on the results of the risk 

assessment, he notes that the results 

indicate that a standard set within the 

proposed range would likely reduce 

risks to at-risk groups from the O3- 

related health effects considered in the 

assessment, and by inference across the 

much broader array of O3-related health 

effects that can only be considered 

qualitatively, relative to the level of 

protection afforded by the current 

standard. Moreover, he notes that the 

results of the assessment suggest a 

gradual reduction in risks with no clear 

breakpoint as increasingly lower 

standard levels are considered. In light 

of this continuum and the important 

uncertainties inherent in the assessment 

discussed above and in the proposal, the 

Administrator concludes that the risk 

assessment does not provide a basis for 

choosing a level within the proposed 

range. 

c. Conclusions on Level 

Having carefully considered the 

public comments on the appropriate 

level of the O3 standard, as discussed 

above, the Administrator believes the 

fundamental scientific conclusions on 

the effects of O3 reached in the Criteria 

Document and Staff Paper, briefly 

summarized above in section II.A.2 and 

discussed more fully in section II.A of 

the proposal, remain valid. In 

considering the level at which the 

primary O3 standard should be set, the 

Administrator continues to place 

primary consideration on the body of 

scientific evidence available in this 

review on the health effects associated 

with O3 exposure, as summarized above 

in section II.C.4.a, while viewing the 

results of exposure and risk assessment, 

discussed above in section II.C.4.b, as 

providing information in support of his 

decision. In considering the available 

scientific evidence he judges that, as at 

the proposal, a focus on the proposed 

range of 0.070 to 0.075 ppm is 

appropriate in light of the large body of 

controlled human exposure and 

epidemiological and other scientific 

evidence. As discussed above, this body 

of evidence does not support retaining 

the current standard, as suggested by 

some commenters. Nor does it support 

setting a level just below 0.080 ppm 

because, based on the entire body of 

evidence, such a level would not 

provide a significant increase in 

protection compared to the current 

standard. Further, such a level would 

not be appreciably below the level in 

controlled human exposure studies at 

which adverse effects have been 

demonstrated (i.e., 0.080 ppm). This 

body of evidence also does not support 

setting a level of 0.060 ppm or below, 

as suggested by other commenters. The 

Administrator has also evaluated the 

information from the exposure 

assessment and the risk assessment, and 

judges that this evidence does not 

provide a clear enough basis for 

choosing a specific level within the 

range of 0.075 to 0.070 ppm. In making 

a final judgment about the level of the 

O3 standard, the Administrator notes 

that the level of 0.075 ppm is above the 

range recommended by the CASAC (i.e., 

0.070 to 0.060 ppm). Placing great 

weight on the views of CASAC, the 

Administrator has carefully considered 

its stated views and the scientific basis 

and policy views for the range it 

recommended. In so doing, the 

Administrator notes that he fully agrees 

that the scientific evidence supports the 

conclusion that the current standard is 

not adequate and must be revised. 

With respect to CASAC’s 

recommended range of standard levels, 

the Administrator observes that the 

basis for its recommendation appears to 

be a mixture of scientific and policy 

considerations. The Administrator notes 

that he is in general agreement with 

CASAC’s views concerning the 

interpretation of the scientific evidence. 

The Administrator also notes that there 

is no bright line clearly directing the 

choice of level, and the choice of what 

is appropriate is clearly a public health 


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policy judgment entrusted to the 

Administrator. This judgment must 

include consideration of the strengths 

and limitations of the evidence and the 

appropriate inferences to be drawn from 

the evidence and the exposure and risk 

assessments. In reviewing the basis for 

the CASAC Panel’s recommendations 

for the range of the O3 standard, the 

Administrator observes that he reaches 

a different policy judgment than the 

CASAC Panel based on apparently 

placing different weight in two areas: 

the role of the evidence from the Adams 

studies and the relative weight placed 

on the results from the exposure and 

risk assessments. While he found the 

evidence reporting effects at the 0.060 

ppm level from the Adams studies to be 

too limited to support a primary focus 

at this level, the Administrator observes 

that the CASAC Panel appears to place 

greater weight on this evidence, as 

indicated by its recommendation of a 

range down to 0.060 ppm. The 

Administrator also observes that while 

the CASAC Panel supported a level of 

0.060 ppm, they also supported a level 

above 0.060, indicating that they do not 

believe that the results of Adams studies 

mean that the level of the standard has 

to be set at 0.060 ppm. The 

Administrator also observes that the 

CASAC Panel appeared to place greater 

weight on the results of the risk 

assessment as a basis for its 

recommended range. In referring to the 

results of the risk assessment results for 

lung function, respiratory symptoms, 

hospital admissions and mortality, the 

CASAC Panel concluded that: 

‘‘beneficial effects in terms of reduction 

of adverse health effects were calculated 

to occur at the lowest concentration 

considered (i.e., 0.064 ppm)’’

(Henderson, 2006c, p. 4). However, the 

Administrator more heavily weighs the 

implications of the uncertainties 

associated with the Agency’s 

quantitative human exposure and health 

risk assessments, as discussed above in 

section II.A.3. Given these uncertainties, 

the Administrator does not agree that 

these assessment results appropriately 

serve as a primary basis for concluding 

that levels at or below 0.070 ppm are 

required for the 8-hour O3 standard. 

After carefully taking the above 

comments and considerations into 

account, and fully considering the 

scientific and policy views of the 

CASAC, the Administrator has decided 

to revise the level of the primary 8-hour 

O3 standard to 0.075 ppm. In the 

Administrator’s judgment, based on the 

currently available evidence, a standard 

set at this level would be requisite to 

protect public health with an adequate 

margin of safety, including the health of 

sensitive subpopulations, from serious 

health effects including respiratory 

morbidity, that is judged to be causally 

associated with short-term and 

prolonged exposures to O3, and 

premature mortality. A standard set at 

this level provides a significant increase 

in protection compared to the current 

standard, and is appreciably below 

0.080 ppm, the level in controlled 

human exposure studies at which 

adverse effects have been demonstrated. 

At a level of 0.075, exposures at and 

above the benchmark of 0.080 ppm are 

essentially eliminated, and exposures at 

and above the benchmark of 0.070 are 

substantially reduced or eliminated for 

the vast majority of people in at-risk 

groups. A standard set at a level lower 

than 0.075 would only result in 

significant further public health 

protection if, in fact, there is a 

continuum of health risks in areas with 

8-hour average O3 concentrations that 

are well below the concentrations 

observed in the key controlled human 

exposure studies and if the reported 

associations observed in 

epidemiological studies are, in fact, 

causally related to O3 at those lower 

levels. Based on the available evidence, 

the Administrator is not prepared to 

make these assumptions. Taking into 

account the uncertainties that remain in 

interpreting the evidence from available 

controlled human exposure and 

epidemiological studies at very low 

levels, the Adminisitrator notes that the 

likelihood of obtaining benefits to 

public health with a standard set below 

0.075 ppm O3 decreases, while the 

likelihood of requiring reductions in 

ambient concentrations that go beyond 

those that are needed to protect public 

health increases. The Administrator 

judges that the appropriate balance to be 

drawn, based on the entire body of 

evidence and information available in 

this review, is a standard set at 0.075. 

The Administrator believes that a 

standard set at 0.075 ppm would be 

sufficient to protect public health with 

an adequate margin of safety, and does 

not believe that a lower standard is 

needed to provide this degree of 

protection. This judgment by the 

Administrator appropriately considers 

the requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognizes that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

D. Final Decision on the Primary O3

Standard 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the Criteria 

Document and Staff Paper, the advice 

and recommendations of the CASAC 

Panel, and the public comments to date, 

the Administrator has decided to revise 

the existing 8-hour primary O3 standard. 

Specifically, the Administrator is 

revising (1) the level of the primary O3

standard to 0.075 ppm and (2) the 

degree of precision to which the level of 

the standard is specified to the 

thousandth ppm. The revised 8-hour 

primary standard, with a level of 0.075 

ppm, would be met at an ambient air 

monitoring site when the 3-year average 

of the annual fourth-highest daily 

maximum 8-hour average O3

concentration is less than or equal to 

0.075 ppm. Data handling conventions 

are specified in the new Appendix P 

that is adopted, as discussed in section 

V below. 

At this time, EPA is also promulgating 

revisions to the Air Quality Index for O3

to be consistent with the revisions to the 

primary O3 standard. These revisions 

are discussed below in section III. Issues 

related to the monitoring requirements 

for the revised O3 primary standard are 

discussed below in section VI. 

III. Communication of Public Health 

Information 

Information on the public health 

implications of ambient concentrations 

of criteria pollutants is currently made 

available primarily through EPA’s Air 

Quality Index (AQI) program (40 CFR 

58.50). The current Air Quality Index 

has been in use since its inception in 

1999 (64 FR 42530). It provides 

accurate, timely, and easily 

understandable information about daily 

levels of pollution. The AQI establishes 

a nationally uniform system of indexing 

pollution levels for O3, CO, NO2, PM 

and SO2. The AQI converts pollutant 

concentrations in a community’s air to 

a number on a scale from 0 to 500. 

Reported AQI values enable the public 

to know whether air pollution levels in 

a particular location are characterized as 

good (0–50), moderate (51–100), 

unhealthy for sensitive groups (101–

150), unhealthy (151–200), very 

unhealthy (201–300), or hazardous 

(301–500). The AQI index value of 100 

typically corresponds to the level of the 

short-term NAAQS for each pollutant. 

For the 1997 O3 NAAQS, an 8-hour 

average concentration of 0.084 ppm 

corresponds to an AQI value of 100. An 

AQI value greater than 100 means that 

a pollutant is in one of the unhealthy 


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categories (i.e., unhealthy for sensitive 

groups, unhealthy, very unhealthy, or 

hazardous) on a given day; an AQI value 

at or below 100 means that a pollutant 

concentration is in one of the 

satisfactory categories (i.e., good or 

moderate). Decisions about the pollutant 

concentrations at which to set the 

various AQI breakpoints, that delineate 

the various AQI categories, draw 

directly from the underlying health 

information that supports the NAAQS 

review. 

The Agency recognized the 

importance of revising the AQI in a 

timely manner to be consistent with any 

revisions to the NAAQS. Therefore, EPA 

proposed to finalize conforming changes 

to the AQI, in connection with the 

Agency’s final decision on the O3

NAAQS if revisions to the primary 

standard were promulgated. These 

conforming changes would include 

setting the 100 level of the AQI at the 

same level as the revised primary O3

NAAQS, and also making proportional 

adjustments to AQI breakpoints at the 

lower end of the range (i.e., AQI values 

of 50, 150 and 200). EPA did not 

propose to change breakpoints at the 

higher end of the range (from 301 to 

500), which would apply to State 

contingency plans or the Significant 

Harm Level (40 CFR 51.16), because the 

information from this review does not 

inform decisions about breakpoints at 

those higher levels. 

EPA received relatively few 

comments on the proposed changes to 

the AQI. Three major issues came up in 

the comments, including: (1) Whether 

the AQI should be revised at all, even 

if the primary standard is revised; (2) 

whether the AQI should be revised in 

conjunction with this rulemaking, or in 

a separate rulemaking; and, (3) whether 

an AQI value of 100 should be set equal 

to or lower than the level of the shortterm primary O3 standard, and the other 

breakpoints adjusted accordingly. 

UARG asserted that EPA should not 

revise the AQI at all, even if EPA does 

revise the primary O3 standard. In 

support of this view, UARG noted that 

there is no requirement for EPA to set 

an AQI value of 100 equal to the level 

of the short-term standard, and cited the 

1999 decision to set an AQI value of 100 

for PM2.5 equal to 40 μg/m3, when the 

level of the short-term standard was 

then 65 μg/m3. UARG also expressed the 

view that lowering the ambient 

concentrations associated with different 

AQI values would confuse and mislead 

the public about actual trends in air 

quality, which UARG asserted are 

improving. ALA and other 

environmental groups in a joint set of 

comments did not support revising the 

AQI in conjunction with this 

rulemaking. ALA et al. expressed the 

view that since EPA did not propose 

specific breakpoints in its proposed 

revisions to the AQI, EPA should 

conduct a separate rulemaking, 

specifying the proposed breakpoints to 

allow the public an opportunity to 

comment on them. Several State 

agencies, including agencies from 

Pennsylvania, Wisconsin and 

Oklahoma, and State organizations, 

including NACAA and NESCAUM, 

supported revising the AQI at the same 

time that the standard is revised. 

NACAA expressed the view that: ‘‘The 

effectiveness of the AQI as a public 

health tool will be undermined if EPA 

undertakes regulatory changes to the 

ozone NAAQS without simultaneously 

revising the AQI.’’ (NACAA, p. 5) The 

Wisconsin Department of Natural 

Resources (WI DNR) further noted that: 

‘‘* * * when the 24-hour PM2.5 standard 

was revised, EPA missed an opportunity to 

adopt conforming changes to the AQI. The 

Administrator signed the Federal Register 

notice promulgating a revised fine-particle 

standard in September 2006, but EPA still 

has not changed the AQI to reflect the revised 

standard. We recommend that the AQI be 

amended to be consistent with the revised 

ozone and PM2.5 standards.’’ [WI DNR, p. 3] 

Finally, ALA et al. and NESCAUM 

expressed the view that an AQI value of 

100 should be set at an ambient 

concentration below the range for the 

proposed primary standard. These 

commenters cited the health evidence 

showing adverse health effects below 

the proposed range of the standard, the 

recommended range of CASAC, and also 

cited the 1999 decision to set an AQI 

value of 100 for PM2.5 equal to 40 μg/ 

m3 when the level of the short-term 

standard was 65 μg/m3, as support for 

this view. Most other State commenters 

supported setting an AQI value of 100 

equal to the level of the primary O3

standard. 

Recognizing the importance of the 

AQI as a communication tool that 

allows the public to take exposure 

reduction measures when air quality 

may pose health risks, EPA agrees with 

State agencies and organizations that 

favored revising the AQI at the same 

time as the primary standard. EPA 

agrees with State agency commenters 

that its historical approach of setting an 

AQI value of 100 equal to the level of 

the revised primary standard is 

appropriate, both from a public health 

and a communication perspective. 

Both UARG and ALA et al. cite the 

1999 AQI rulemaking, which set an AQI 

value of 100 for PM2.5 equal to 40 μg/ 

m3, a lower level than the level of the 

short-term PM2.5 standard, as support 

for their view that an AQI value of 100 

does not need to be set at the level of 

the revised O3 standard. However, the 

sub-index for PM2.5 was developed 

using an approach that was 

conceptually consistent with past 

practice for selecting the air quality 

concentrations associated with the AQI 

breakpoints. The Agency’s historical 

approach to selecting index breakpoints 

had been to simply set the AQI value of 

100 at the level of the short-term 

standard (e.g., 24 hours) for a pollutant. 

This method of structuring the index is 

appropriate in the case where a shortterm standard is set to protect against 

the health effects associated with shortterm exposures and/or an annual 

standard is set to protect against health 

effects associated with long-term 

exposures. In such cases, the short-term 

standard in effect defines a level of 

health protection provided against 

short-term risks and thus can be a useful 

benchmark against which to compare 

daily air quality concentrations. 

In the case of the 1997 PM2.5

standards, EPA took a different 

approach to protecting against the 

health risks associated with short-term 

exposures. The intended level of 

protection against short-term risk was 

not defined by the 24-hour standard (set 

at a level of 65 μg/m3) but by the 

combination of the 24-hour and the 

annual standards working in concert. In 

fact, the annual standard (set at a level 

of 15 μg/m3) was intended to serve as 

the principal vehicle for protecting 

against both long-term and short-term 

PM2.5 exposures by lowering the entire 

day-by-day distribution of PM2.5

concentrations in an area throughout the 

year. See generally 62 FR at 38668–70 

(July 18, 1997). Because the 24-hour 

standard served to provide additional 

protection against very high short-term 

concentrations, localized ‘‘hotspots,’’ or 

risks arising from seasonal emissions 

that would not be well-controlled by a 

national annual standard, EPA 

consequently concluded that it would 

be appropriate to caution members of 

sensitive groups exposed to 

concentrations below the level of the 24- 

hour standard. EPA also concluded that 

it would be inappropriate to compare 

daily air quality concentrations directly 

with the level of the annual standard by 

setting an AQI value of 100 at that level. 

EPA wanted to set the AQI value of 100 

to reflect the general level of health 

protection against short-term risks 

offered by the annual and 24-hour 

standards combined, consistent with the 

underlying logic of the historical 

approach to establishing AQI 100 levels. 

Therefore EPA set the AQI value of 100 


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at the midpoint of the range between the 

annual and the 24-hour PM2.5 standards 

(i.e., 40 μg/m3) in order to reflect the 

combined role of the 24-hour and the 

annual PM2.5 standards in protecting 

against short-term risks. Therefore, this 

approach for defining an AQI value of 

100 is conceptually consistent with the 

proposed decision to set an AQI value 

of 100 equal to the level of the primary 

O3 standard. 

Therefore, EPA is revising the AQI for 

O3 by setting an AQI value of 100 equal 

to 0.075 ppm, 8-hour average, the level 

of the revised primary O3 standard. EPA 

is also revising the following 

breakpoints: An AQI value of 50 is set 

at 0.059 ppm, an AQI value of 150 is set 

at 0.095 ppm, and an AQI value of 200 

is set at 0.115 ppm. All these levels are 

averaged over 8 hours. As indicated in 

the proposal, these levels were 

developed by making proportional 

adjustments to the other AQI 

breakpoints (i.e., AQI values of 50, 150 

and 200). The proportional adjustments 

were modified slightly to allow for each 

category to span at least a 0.015 ppm 

range to allow for more accurate 

forecasting. So, for example, simply 

making a proportional adjustment to the 

level of an AQI value of 150 (0.104 ppm) 

would result in a level of about 0.092 

ppm. Since most of these ranges are 

rounded to the nearest 5 thousandths of 

a ppm, that rounding would have 

resulted in a 0.014 ppm range (i.e., 

0.076 to 0.090 ppm). So, the number 

was rounded upward to the nearest 5 

thousandths of a ppm, to allow for at 

least a 0.015 ppm range for forecasting. 

The same principle applies to the 

calculation of an AQI value for 200 

(0.115 ppm). EPA believes that the 

finalized breakpoints provide a balance 

between proportional adjustments to 

reflect the revised O3 standard and 

providing category ranges that are large 

enough to be forecasted accurately, so 

that the new AQI for O3 can be 

implemented more easily in the public 

forum for which the AQI ultimately 

exists. 

IV. Rationale for Final Decision on 

Secondary O3 Standard 

A. Introduction 

1. Overview 

This section presents the rationale for 

the Administrator’s final decisions 

regarding the need to revise the current 

secondary O3 NAAQS, and the 

appropriate revisions to the standard. 

As discussed more fully below, the 

rationale for the final decisions on 

appropriate revisions to the secondary 

O3 NAAQS is based on a thorough 

review of the latest scientific 

information on vegetation effects 

associated with exposure to ambient 

levels of O3, as assessed in the Criteria 

Document. This rationale also takes into 

account: (1) Staff assessments of the 

most policy-relevant information in the 

Criteria Document regarding the 

evidence of adverse effects of O3 to 

vegetation and ecosystems, information 

on biologically-relevant exposure 

metrics, and staff analyses of air quality, 

vegetation exposure and risks, presented 

in the Staff Paper and described in 

greater detail in the associated 

Technical Report on Ozone Exposure, 

Risk, and Impact Assessments for 

Vegetation (Abt, 2007), upon which staff 

recommendations for revisions to the 

secondary O3 standard were based; (2) 

CASAC Panel advice and 

recommendations as reflected in 

discussion of drafts of the Criteria 

Document and Staff Paper at public 

meetings, in separate written comments, 

and in CASAC’s letters to the 

Administrator (Henderson, 2006a, b, c; 

2007); (3) public comments received 

during development of these documents 

either in conjunction with CASAC 

meetings or separately and on the 

proposal notice; (4) consideration of the 

degree of protection to vegetation 

potentially afforded by the revised 8- 

hour primary standard; and (5) the 

limits of the available evidence. 

In developing this rationale, EPA has 

again focused on direct O3 effects on 

vegetation, specifically drawing upon an 

integrative synthesis of the entire body 

of evidence, published through early 

2006, on the broad array of vegetation 

effects associated with exposure to 

ambient levels of O3 (EPA, 2006a, 

chapter 9). In addition, because O3 can 

also indirectly affect other ecosystem 

components such as soils, water, and 

wildlife, and their associated ecosystem 

goods and services, through its effects 

on vegetation, a qualitative discussion 

of these other indirect impacts is also 

included, though these effects are not 

quantifiable at this time. As was 

concluded in the 1997 review, and 

based on the body of scientific literature 

assessed in the current Criteria 

Document, the Administrator believes 

that it is reasonable to conclude that a 

secondary standard protecting the 

public welfare from known or 

anticipated adverse effects to trees, 

native vegetation and crops would also 

afford increased protection from adverse 

effects to other environmental 

components relevant to the public 

welfare, including ecosystem services 

and function. The peer-reviewed 

literature includes studies conducted in 

the U.S., Canada, Europe, and many 

other countries around the world. In its 

assessment of the evidence judged to be 

most relevant to making decisions on 

the level of the O3 secondary standard, 

however, EPA has placed greater weight 

on U.S. studies, due to the often 

species-, site- and climate-specific 

nature of O3-related vegetation response. 

As with virtually any policy-relevant 

vegetation effects research, there is 

uncertainty in the characterization of 

vegetation effects attributable to 

exposure to ambient O3. As discussed 

below, however, research conducted 

since the last review provides important 

information coming from field-based 

exposure studies, including free air, 

gradient and biomonitoring surveys, in 

addition to the more traditional 

controlled open top chamber (OTC) 

studies. Moreover, the newly available 

studies evaluated in the Criteria 

Document have undergone intensive 

scrutiny through multiple layers of peer 

review and many opportunities for 

public review and comment. While 

important uncertainties remain, the 

review of the vegetation effects 

information has been extensive and 

deliberate. In the judgment of the 

Administrator, the intensive evaluation 

of the scientific evidence that has 

occurred in this review has provided an 

adequate basis for regulatory decisionmaking at this time. This review also 

provides important input to EPA’s 

research plan for improving our future 

understanding of the effects of ambient 

O3 at lower levels. 

Information related to vegetation and 

ecosystem effects, biologically relevant 

exposure indices, and quantitative 

vegetation exposure and risk 

assessments were summarized in 

sections IV.A through IV.C of the 

proposal (72 FR at 37883–37895), 

respectively, and are only briefly 

outlined below in sections IV.A.2 

through IV.A.4. Subsequent sections of 

this preamble provide a more complete 

discussion of the Administrator’s 

rationale, in light of key issues raised in 

public comments, for concluding that 

the current standard is not requisite to 

protect public welfare from known or 

anticipated adverse effects, and it is 

appropriate to revise the current 

secondary O3 standard to provide 

additional public welfare protection 

(section IV.B) by making the secondary 

standard identical to the revised 

primary standard (section IV.C). A 

summary of the final decisions on 

revisions to the secondary O3 standard 

is presented in section IV.D. 


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2. Overview of Vegetation Effects 

Evidence 

This section outlines the information 

presented in section IV.A of the 

proposal on known or potential effects 

on public welfare which may be 

expected from the presence of O3 in 

ambient air. Exposures to O3 have been 

associated quantitatively and 

qualitatively with a wide range of 

vegetation effects. The decision in the 

last review to set a more protective 

secondary standard primarily reflected 

consideration of the quantitative 

information on vegetation effects 

available at that time, particularly 

growth impairment (e.g., biomass loss) 

in sensitive forest tree species during 

the seedling growth stage and yield loss 

in important commercial crops. This 

information, derived mainly using the 

OTC exposure method, found 

cumulative, seasonal O3 exposures were 

most strongly associated with observed 

vegetation response. The Criteria 

Document prepared for this review 

discussed a number of additional 

studies that support and strengthen key 

conclusions regarding O3 effects on 

vegetation and ecosystems found in the 

previous Criteria Document (EPA, 

1996a, 2006a), including further 

clarification of the underlying 

mechanistic and physiological processes 

at the subcellular, cellular, and whole 

system levels within the plant. More 

importantly, however, in the context of 

this review, new quantitative 

information is now available across a 

broader array of vegetation effects (e.g., 

growth impairment during seedlings, 

saplings and mature tree growth stages, 

visible foliar injury, and yield loss in 

annual crops) and across a more diverse 

set of exposure methods, including 

chamber, free air, gradient, model, and 

field-based observation. These nonchambered, field-based study results 

begin to address one of the key data 

gaps cited by the Administrator in the 

last review. 

Section IV.A of the proposal provides 

a detailed summary of key information 

contained in the Criteria Document 

(EPA, 2006, chapter 9) and in the Staff 

Paper (EPA, 2007, chapter 7) on known 

or potential effects on public welfare 

which may be expected from the 

presence of O3 in ambient air (72 FR 

37883–37890). The information in that 

section summarized: 

(1) New information available on 

potential mechanisms for vegetation 

effects associated with exposure to O3, 

including information on plant uptake 

of O3, cellular to systemic responses, 

compensation and detoxification 

responses, changes to plant metabolism, 

and plant responses to chronic O3

exposures; 

(2) The nature of effects on vegetation 

that have been associated with exposure 

to O3 including effects related to 

carbohydrate production and allocation, 

growth effects on trees and yield 

reductions in crops, visible foliar injury, 

and reduced plant vigor, as well as 

consequent potential impacts on 

ecosystems including potential 

alteration of ecosystem structure and 

function and effects on ecosystem 

services and carbon sequestration; and 

(3) Considerations in characterizing 

what constitutes an adverse welfare 

impact of O3, including an approach 

that expands the consideration of 

adversity beyond the species level by 

making explicit the linkages between 

stress-related effects such as O3

exposure at the species level and at 

higher levels within an ecosystem 

hierarchy. 

3. Overview of Biologically Relevant 

Exposure Indices 

This section outlines the information 

presented in section IV.B of the 

proposal on biologically relevant 

exposure indices that relate known or 

potential effects on vegetation to 

exposure to O3 in ambient air. The 

Criteria Document concluded that O3

exposure indices that cumulate 

differentially weighted hourly 

concentrations are the best candidates 

for relating exposure to plant growth 

responses (EPA, 2006a). This conclusion 

followed from the extensive evaluation 

of the relevant studies in the 1996 

Criteria Document (EPA, 1996a) and the 

recent evaluation of studies that have 

been published since that time (EPA, 

2006a). The depth and strength of these 

conclusions are illustrated by the 

following observations that are drawn 

from the 1996 Criteria Document (EPA, 

1996a, section 5.5): 

(1) Specifically, with respect to the 

importance of taking into account 

exposure duration, ‘‘when O3 effects are 

the primary cause of variation in plant 

response, plants from replicate studies 

of varying duration showed greater 

reductions in yield or growth when 

exposed for the longer duration’’ and 

‘‘the mean exposure index of 

unspecified duration could not account 

for the year-to-year variation in 

response’’ (EPA, 1996a, pg. 5–96). 

(2) ‘‘[B]ecause the mean exposure 

index treats all concentrations equally 

and does not specifically include an 

exposure duration component, the use 

of a mean exposure index for 

characterizing plant exposures appears 

inappropriate for relating exposure with 

vegetation effects’’ (EPA, 1996a, pg. 5–

88). 

(3) Regarding the relative importance 

of higher concentrations than lower in 

determining plant response, ‘‘the 

ultimate impact of long-term exposures 

to O3 on crops and seedling biomass 

response depends on the integration of 

repeated peak concentrations during the 

growth of the plant’’ (EPA, 1996a, pg. 5–

104). 

(4) ‘‘[A]t this time, exposure indices 

that weight the hourly O3

concentrations differentially appear to 

be the best candidates for relating 

exposure with predicted plant 

response’’ (EPA, 1996a, pgs. 5–136). 

At the conclusion of the last review, 

the biological basis for a cumulative, 

seasonal form was not in dispute. There 

was general agreement between the EPA 

staff, CASAC, and the Administrator, 

based on their review of the air quality 

criteria, that a cumulative, seasonal 

form was more biologically relevant 

than the previous 1-hour and new 8- 

hour average forms (61 FR 65716). 

The Staff Paper prepared for this 

review evaluated the most appropriate 

choice of a cumulative, seasonal form 

for a secondary standard to protect the 

public welfare from known and 

anticipated adverse vegetation effects in 

light of the new information available in 

this review. Specifically, the Staff Paper 

considered: (1) The continued lack of 

evidence within the vegetation effects 

literature of a biological threshold for 

vegetation exposures of concern and (2) 

new estimates of PRB that are lower 

than in the last review. The form 

commonly called W126 was evaluated 

in the last review and was compared 

with the form called SUM06, which 

incorporates a threshold level above 

which exposures are summed, that was 

proposed in the last review. The 

concentration-weighted form commonly 

called W126 is defined as the sum of 

sigmoidally weighted hourly O3

concentrations over a specified period, 

where the daily sigmoidal weighting 

function is defined in the Staff Paper 

(EPA, 2007a, p. 7–16.) as: 


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22The CMAQ model is a multi-pollutant, 

multiscale air quality model that contains state-ofthe-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-hour average). 

W wCi 126 1

1 4403 == = +

 C where C hourly O at hour i, and w ii 3 Ci , e C i

i PM

− t

=

<

∑ 126 8

8

 AM

Regarding the first consideration, the 

Staff Paper noted that the W126 form, 

by its incorporation of a continuous 

sigmoidal weighting scheme, does not 

create an artificially imposed 

concentration threshold, yet also gives 

proportionally more weight to the 

higher and typically more biologically 

potent concentrations, as supported by 

the scientific evidence. Second, the 

index value is not significantly 

influenced by O3 concentrations within 

the range of estimated PRB, as the 

weights assigned to concentrations in 

this range are very small. Thus, the Staff 

Paper concluded that it would provide 

a more appropriate target for air quality 

management programs designed to 

reduce emissions from anthropogenic 

sources contributing to O3 formation. 

On the basis of these considerations, the 

Staff Paper and the CASAC Panel 

concluded that the W126 form is the 

most biologically-relevant cumulative, 

seasonal form appropriate to consider in 

the context of the secondary standard 

review. 

4. Overview of Vegetation Exposure and 

Risk Assessments 

This section outlines the information 

presented in section IV.C of the 

proposal on the vegetation exposure and 

risk assessments conducted for this 

review, which improved and built upon 

similar analyses performed in the last 

review. The vegetation exposure 

assessment was performed using 

interpolation and included information 

from ambient monitoring networks and 

results from air quality modeling. The 

vegetation risk 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 recent monitored O3 air quality for 

the years 2001–2004; (2) estimates of 

seedling growth loss under 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 a single 

western species (ponderosa pine) and 

two eastern species (red maple and tulip 

poplar). The crop analysis includes 

estimates of the risks to crop yields from 

current and alternative O3 exposure 

conditions and the associated change in 

economic benefits expected to accrue in 

the agriculture sector upon meeting the 

levels of various alternative standards. 

Each element of the assessment is 

outlined below, together with key 

observations from this assessment. 

a. Exposure Characterization 

The exposure analyses examined O3

air quality patterns in the U.S. relative 

to the location of O3 sensitive species 

that have a known concentrationresponse in order to predict whether 

adverse effects are occurring at current 

levels of air quality, and whether they 

are likely to occur under alternative 

standard forms and levels. The most 

important information about exposure 

to vegetation comes from the O3

monitoring data that are available from 

two national networks: (1) Air Quality 

System (AQS; http://www.epa.gov/ttn/ 

airs/airsaqs) and (2) Clean Air Status 

and Trends Network (CASTNET; 

http://www.epa.gov/castnet/). In order 

to characterize exposures to vegetation 

at the national scale, however, the Staff 

Paper concluded that it could not rely 

solely on limited site-specific 

monitoring data, and that it was 

necessary to use an interpolation 

method to characterize O3 air quality 

over broad geographic areas. The 

analyses used the O3 outputs from the 

EPA/NOAA Community Multi-scale Air 

Quality (CMAQ) 22 model system 

(http://www.epa.gov/asmdnerl/CMAQ, 

Byun and Ching, 1999; Arnold et al. 

2003, Eder and Yu, 2005) to improve 

spatial interpolations based solely on 

existing monitoring networks. 

Based on the significant difference in 

monitor network density between the 

eastern and western U.S., the Staff Paper 

concluded that it was appropriate to use 

separate interpolation techniques in 

these two regions: AQS and CASTNET 

monitoring data were solely used for the 

eastern interpolation, and in the western 

U.S., where rural monitoring is more 

sparse, O3 values generated by the 

CMAQ model were used to develop 

scaling factors to augment the 

interpolation. In order to characterize 

uncertainty in the interpolation method, 

monitored O3 concentrations were 

systematically compared to interpolated 

O3 concentrations in areas where 

monitors were located. In general, the 

interpolation method used in the 

current review performed well in many 

areas in the U.S., although it underpredicted higher 12-hour W126 

exposures in rural areas. Due to the 

important influence of higher exposures 

in determining risks to plants, this 

feature of the interpolated surface could 

result in an under-estimation of risks to 

vegetation in some areas. Taking these 

uncertainties into account, and given 

the absence of more complete rural 

monitoring data, this approach was used 

in developing national vegetation 

exposure and risk assessments that 

estimate relative changes in risk for the 

various alternative standards analyzed. 

To evaluate changing vegetation 

exposures and risks under selected air 

quality scenarios, the Staff Paper 

utilized adjusted 2001 base year O3 air 

quality distributions with a rollback 

method (Horst and Duff, 1995; Rizzo, 

2005, 2006) to reflect meeting the 

current and alternative secondary 

standard options. The following key 

observations were drawn from 

comparing predicted changes in 

interpolated air quality under each 

alternative standard form and level 

scenario analyzed: 

(1) The results of the exposure 

assessment indicate that current air 

quality levels could result in significant 

impacts to vegetation in some areas. For 

example, for the base year (2001), a large 

portion of California had 12-hr W126 O3

levels above 31 ppm-hour, which has 

been associated with approximately up 

to 14 percent biomass loss in 50 percent 

of tree seedling cases studies. Broader 

multi-state regions in the east (NC, TN, 

KY, IN, OH, PA, NJ, NY, DE, MD, VA) 

and west (CA, NV, AZ, OK, TX) are 

predicted to have levels of air quality 

above the W126 level of 21 ppm-hour, 

which is approximately equal to the 

secondary standard proposed in 1996 

and is associated with approximately up 

to 10 percent biomass loss in 50 percent 

of tree seedling cases studied. Much of 

the east and Arizona and California 

have 12-hour W126 O3 levels above 13 

ppm-hour which has been associated 

with approximately up to 10 percent 

biomass loss in 75 percent of tree 

seedling cases studied. 

(2) When 2001 air quality is rolled 

back to meet the current 8-hour 


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23The Staff Paper presented this analysis using 

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. 

secondary standard, the overall 3-month 

12-hour W126 O3 levels were somewhat 

improved, but not substantially. Under 

this scenario, there were still many 

areas in California with 12-hour W126 

O3 levels above 31 ppm-hour. A broad 

multi-state region in the east (NC, TN, 

KY, IN, OH, PA, MD) and west (CA, NV, 

AZ, OK, TX) were still predicted to have 

O3 levels above the W126 level of 21 

ppm-hour. 

(3) Exposures generated for just 

meeting a 0.070 ppm, 4th-highest 

maximum 8-hour average alternative 

standard (the lower end of the proposed 

range for the primary O3 standard) 

showed substantially improved O3 air 

quality when compared to just meeting 

the current 0.08 ppm, 8-hour standard. 

Most areas were predicted to have O3

levels below the W126 level of 21 ppmhr, although some areas in the east (KY, 

TN, MI, AR, MO, IL) and west (CA, NV, 

AZ, UT, NM, CO, OK, TX) were still 

predicted to have O3 levels above the 

W126 level of 13 ppm-hour. 

(4) While these results suggest that 

meeting a proposed 0.070 ppm, 8-hour 

secondary standard would provide 

substantially improved protection in 

some areas, the Staff Paper recognized 

that other areas could continue to have 

elevated seasonal exposures, including 

forested park lands and other natural 

areas, and Class I areas which are 

federally mandated to preserve certain 

air quality related values. The proposal 

notes that this is especially important in 

the high elevation forests in the Western 

U.S. where there are few O3 monitors 

and where air quality patterns can result 

in relatively low 8-hour averages while 

still experiencing relatively high 

cumulative exposures (72 FR 37892). 

To further characterize O3 air quality 

in terms of current and alternative 

secondary standard forms, an analysis 

was performed in the Staff Paper to 

evaluate the extent to which countylevel O3 air quality measured in terms 

of various levels of the current 8-hour 

average form overlapped with that 

measured in terms of various levels of 

the 12-hour W126 cumulative, seasonal 

form.23 This analysis was limited by the 

lack of monitoring in rural areas where 

important vegetation and ecosystems are 

located, especially at higher elevation 

sites. This is because O3 air quality 

distributions at high elevation sites 

often do not reflect the typical urban 

and near-urban pattern of low morning 

and evening O3 concentrations with a 

high mid-day peak, but instead maintain 

relatively flat patterns with many 

concentrations in the mid-range (e.g., 

0.05–0.09 ppm) for extended periods. 

These conditions can lead to relatively 

low daily maximum 8-hour averages 

concurrently with high cumulative 

values so that there is potentially less 

overlap between an 8-hour average and 

a cumulative, seasonal form at these 

sites. The Staff Paper concluded that it 

is reasonable to anticipate that 

additional unmonitored rural high 

elevation areas important for vegetation 

may not be adequately protected even 

with a lower level of the 8-hour form. 

The Staff Paper indicated that it 

further remains uncertain as to the 

extent to which air quality 

improvements designed to reduce 8- 

hour 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-hour O3

concentrations in relation to plant 

response; and (3) the effects of 

attainment of a 8-hour 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. These factors are important 

considerations in determining whether 

the current 8-hour form can 

appropriately provide requisite 

protection for vegetation. 

b. Assessment of Risk to Vegetation 

The Staff Paper presented results from 

quantitative and qualitative risk 

assessments of O3 risks to vegetation. In 

the last review, crop yield and seedling 

biomass loss OTC data provided the 

basis for staff analyses, conclusions, and 

recommendations (EPA, 1996b). Since 

then, several additional lines of 

evidence have progressed sufficiently to 

provide a basis for a more complete and 

coherent picture of the scope of O3- 

related vegetation risks, especially those 

currently faced by seedling, sapling and 

mature tree species growing in field 

settings, and indirectly, forested 

ecosystems. Specifically, new research 

reflects an increased emphasis on fieldbased exposure methods (e.g., free air 

exposure and ambient gradient), 

improved field survey biomonitoring 

techniques, and mechanistic tree 

process models. Key observations and 

insights from the vegetation risk 

assessment, together with important 

caveats and limitations, were discussed 

in section IV.C of the proposal. 

Highlights from the analyses that 

addressed visible foliar injury, seedling 

and mature tree biomass loss, and 

effects on crops are summarized below: 

(1) Visible foliar injury. Recent 

systematic injury surveys continue to 

document visible foliar injury 

symptoms diagnostic of phytotoxic O3

exposures on sensitive bioindicator 

plants. These surveys produced more 

expansive evidence than that available 

at the time of the last review that visible 

foliar injury is occurring in many areas 

of the U.S. under current ambient 

conditions. The Staff Paper presented an 

assessment combining 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 

current 0.08 ppm 8-hour standard. Of 

the counties that met an 8-hour level of 

0.07 ppm in those years, 11 to 30 

percent of the counties still had 

incidence of visible foliar injury. The 

magnitude of these percentages suggests 

that phytotoxic exposures sufficient to 

induce visible foliar injury would still 

occur in many areas after meeting the 

level of the current secondary standard 

or alternative 0.07 ppm 8-hour standard. 

While the data show that visible foliar 

injury occurrence is geographically 

widespread and is occurring on a 

variety of plant species in forested and 

other natural systems, linking visible 

foliar injury to other plant effects is still 

problematic. However, its presence 

indicates that other O3-related 

vegetation effects might also be present. 

(2) Seedling and mature tree biomass 

loss. In the last review, analyses of the 

effects of O3 on trees were limited to 11 

tree species for which C-R functions for 

the seedling growth stage had been 

developed from OTC studies. Important 

tree species such as quaking aspen, 

ponderosa pine, black cherry, and tulip 

poplar were found to be sensitive to 

cumulative seasonal O3 exposures. 

Work done since the last review 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) 

have confirmed the detrimental effects 

of O3 exposure on tree growth in field 

studies without chambers and beyond 

the seedling stage (King et al., 2005). To 

update the seedling biomass loss 

analysis, C-R functions for biomass loss 


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24Maps of these biomass loss projections were 

presented in the Staff Paper (chapter 7). 

25TREGRO is a process-based, individual tree 

growth simulation model (Weinstein et al. 1991) 

and has been used to evaluate the effects of a 

variety of O3 scenarios and linked with concurrent 

climate data to account for O3 and climate/ 

meteorology interactions 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). 

for available seedling tree species taken 

from the Criteria Document 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 of the seedling 

tree species individually.24 In summary, 

these analyses showed that biomass loss 

still occurred in many tree species when 

O3 air quality was adjusted to meet the 

current 8-hour standard. For instance, 

black cherry, ponderosa pine, eastern 

white pine, and aspen had estimated 

median seedling biomass losses over 

portions of their growing range as high 

as 24, 11, 6, and 6 percent, respectively, 

when O3 air quality was rolled back to 

just meet the current 8-hour standard. 

The Staff Paper noted that these results 

are for tree seedlings and that mature 

trees of the same species may have more 

or less of a response to O3 exposure. Due 

to the potential for compounding effects 

over multiple years, a consensus 

workshop on O3 effects reported that a 

biomass loss greater than 2 percent 

annually can be significant (Heck and 

Cowling, 1997). Decreased seedling root 

growth and survivability could affect 

overall stand health and composition in 

the long term. 

Recent work has also enhanced our 

understanding of risks beyond the 

seedling stage. In order to better 

characterize the potential O3 effects on 

mature tree growth, a tree growth model 

(TREGRO) was used to evaluate the 

effect of changing O3 air quality 

scenarios from just meeting alternative 

O3 standards on the growth of mature 

trees.25 The model integrates 

interactions between O3 exposure, 

precipitation and temperature as they 

affect vegetation, thus providing an 

internal consistency for comparing 

effects in trees under different exposure 

scenarios and climatic conditions. The 

TREGRO model was used to assess O3- 

related impacts on the growth of 

Ponderosa pine in the San Bernardino 

Mountains of California (Crestline) and 

the growth of yellow poplar and red 

maple in the Appalachian mountains of 

Virginia and North Carolina, 

Shenandoah National Park (Big 

Meadows) and Linville Gorge 

Wilderness Area (Cranberry), 

respectively. Ponderosa pine is one of 

the most widely distributed pines in 

western North America, a major source 

of timber, important as wildlife habitat, 

and valued for aesthetics (Burns and 

Honkala, 1990). Red maple is one of the 

most abundant species in the eastern 

U.S. and is important for its brilliant fall 

foliage and highly desirable wildlife 

browse food (Burns and Honkala, 1990). 

Yellow poplar is an abundant species in 

the southern Appalachian forest. It is 10 

percent of the cove hardwood stands in 

southern Appalachians which are 

widely viewed as some of the country’s 

most treasured forests because the 

protected, rich, moist set of conditions 

permit trees to grow the largest in the 

eastern U.S. The wood has high 

commercial value because of its 

versatility and as a substitute for 

increasingly scarce softwoods in 

furniture and framing construction. 

Yellow poplar is also valued as a honey 

tree, a source of wildlife food, and a 

shade tree for large areas (Burns and 

Honkala, 1990). 

The 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. This is based on model 

outputs that estimate that as O3 levels 

are reduced below those of the current 

standard, significant improvements in 

growth would occur. Though there is 

uncertainty associated with the above 

analyses, it is important to note that 

new evidence from experimental studies 

that go beyond the seedling growth stage 

continues 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 

must be considered since the 

accumulation of such ‘‘carry-over’’

effects over time may affect long-term 

survival and reproduction of 

individuals and ultimately the 

abundance of sensitive tree species in 

forest stands. 

(3) Crops. Similar to the tree seedling 

analysis, an analysis that combined C-R 

information on crops, crop growing 

regions, and interpolated exposures 

during each crop growing season was 

conducted for commodity crops, fruits 

and vegetables. NCLAN crop functions 

developed in the 1980s were used for 

commodity crops, including 9 

commodity crop species (i.e., cotton, 

field corn, grain sorghum, peanut, 

soybean, winter wheat, lettuce, kidney 

bean, potato) that accounted for 69 

percent of 2004 principal crop acreage 

planted in the U.S. in 2004. The C-R 

functions for six fruit and vegetable 

species (tomatoes-processing, grapes, 

onions, rice, cantaloupes, Valencia 

oranges) were identified from the 

California fruit and vegetable analysis 

from the last review (Abt, 1995). The 

risk assessment estimated that just 

meeting the current 8-hour standard 

would still allow O3-related yield loss to 

occur in some commodity crop species 

and fruit and vegetable species currently 

grown in the U.S. For example, based on 

median C-R function response, in 

counties with the highest O3 levels, 

potatoes and cotton had estimated yield 

losses of 9–15 percent and 5–10 percent, 

respectively, when O3 air quality just 

met the level of the current standard. 

Estimated yield improved in these 

counties when the alternative W126 

standard levels were met. The very 

important soybean crop had generally 

small yield losses throughout the 

country under just meeting the current 

standard (0–4 percent). 

The Staff Paper also presented 

estimates of monetized benefits for 

crops associated with the 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 current and alternative 

standard levels were met. Meeting the 

various alternative standards did show 

some significant benefits beyond the 

current 8-hour standard. However, the 

Staff Paper recognized that the modeled 

economic benefits from AGSIM had 

many associated uncertainties which 

limited the usefulness of these 

estimates. 

B. Need for Revision of the Current 

Secondary O3 Standard 

1. Introduction 

The initial issue to be addressed in 

this review of the O3 standard is 

whether, in view of the advances in 

scientific knowledge reflected in the 

Criteria Document and Staff Paper, the 

current standard should be revised. As 

discussed in section IV.D of the 

proposal, in evaluating whether it was 

appropriate to propose to retain or 

revise the current standard, the 

Administrator built upon the last review 

and reflected the broader body of 

evidence and information now 

available. In the proposal, EPA 

presented information, judgments, and 

conclusions from the last review, which 

revised the secondary O3 standard by 


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setting it identical to the revised 

primary O3 standard, and from the 

current review’s evaluation of the 

adequacy of the current secondary 

standard, including both evidence- and 

exposure/risk-based considerations in 

the Staff Paper, as well as from the 

CASAC Panel’s advice and 

recommendations. The Staff Paper 

evaluation, the CASAC Panel’s views, 

and the Administrator’s proposed 

conclusions on the adequacy of the 

current secondary standard are 

presented below. 

a. Staff Paper Evaluation 

The Staff Paper considered the 

evidence presented in the Criteria 

Document as a basis for evaluating the 

adequacy of the current O3 standard, 

recognizing that important uncertainties 

remain. The Staff Paper concluded that 

the new evidence available in this 

review as described in the Criteria 

Document continues to support and 

strengthen key policy-relevant 

conclusions drawn in the previous 

review. Based on this new evidence, the 

current Criteria Document once more 

concluded that: (1) A plant’s response to 

O3 depends upon the cumulative nature 

of ambient exposure as well as the 

temporal dynamics of those 

concentrations; (2) current ambient 

concentrations in many areas of the 

country are sufficient to impair growth 

of numerous common and economically 

valuable plant and tree species; (3) the 

entrance of O3 into the leaf through the 

stomata is the critical step in O3 effects; 

(4) effects can occur with only a few 

hourly concentrations above 0.08 ppm; 

(5) other environmental biotic and 

abiotic factors are also influential to the 

overall impact of O3 on plants and trees; 

and (6) a high degree of uncertainty 

remains in our ability to assess the 

impact of O3 on ecosystem services. 

In light of the new evidence, as 

described in the Criteria Document, the 

Staff Paper evaluated the adequacy of 

the current standard based on 

assessments of both the most policyrelevant vegetation effects evidence and 

exposure and risk-based information, 

highlighted above in section IV.A and 

discussed in sections IV.A–C of the 

proposal. In evaluating the strength of 

this information, the Staff Paper took 

into account the uncertainties and 

limitations in the scientific evidence 

and analyses as well as the views of 

CASAC. The Staff Paper concluded that 

progress has been made since the last 

review and generally found support in 

the available effects- and exposure/riskbased information for consideration of 

an O3 standard that is more protective 

than the current standard. The Staff 

Paper further concluded that there is no 

support for consideration of an O3

standard that is less protective than the 

current standard. This general 

conclusion is consistent with the advice 

and recommendations of CASAC. 

i. Evidence-Based Considerations 

In the last review, crop yield and tree 

seedling biomass loss data obtained in 

OTC studies provided the basis for the 

Administrator’s judgment that the then 

current 1-hour, 0.12 ppm secondary 

standard was inadequate (EPA, 1996b). 

Since then, several additional lines of 

evidence have progressed sufficiently to 

provide a more complete and coherent 

picture of the scope of O3-related 

vegetation risks, especially those 

currently faced by sensitive seedling, 

sapling and mature growth stage tree 

species growing in field settings, and 

their associated forested ecosystems. 

Specifically, new research reflects an 

increased emphasis on field-based 

exposure methods (e.g., free air, ambient 

gradient, and biomonitoring surveys). In 

reaching conclusions regarding the 

adequacy of the current standard, the 

Staff Paper considered the combined 

information from all these areas 

together, along with associated 

uncertainties, in an integrated, weightof-evidence approach. 

Regarding the O3-induced effect of 

visible foliar injury, observations for the 

years 2001 to 2004 at USDA FIA 

biomonitoring sites showed widespread 

O3-induced leaf injury occurring in the 

field, including in forested ecosystems, 

under current ambient O3 conditions. 

For a few studied species, it has been 

shown that the presence of visible foliar 

injury is further linked to the presence 

of other vegetation effects (e.g., reduced 

plant growth and impaired below 

ground root development) (EPA, 2006), 

though for most species, this linkage has 

not been specifically studied or where 

studied, has not been found. 

Nevertheless, when visible foliar injury 

is present, the possibility that other O3- 

induced vegetation effects could also be 

present for some species should be 

considered. Likewise, the absence of 

visible foliar injury should not be 

construed to demonstrate the absence of 

other O3-induced vegetation effects. The 

Staff Paper concluded that it is not 

possible at this time to quantitatively 

assess the degree of visible foliar injury 

that should be judged adverse in all 

settings and across all species, and that 

other environmental factors can mitigate 

or exacerbate the degree of O3-induced 

visible foliar injury expressed at any 

given concentration of O3. However, the 

Staff Paper also concluded that the 

presence of visible foliar injury alone 

can be adverse to the public welfare, 

especially when it occurs in protected 

areas such as national parks and 

wilderness areas. Thus, on the basis of 

the available information on the 

widespread distribution of O3-sensitive 

species within the U.S. including in 

areas, such as national parks, which are 

afforded a higher degree of protection, 

the Staff Paper concluded that the 

current standard continues to allow 

levels of visible foliar injury in some 

locations that could reasonably be 

considered to be adverse from a public 

welfare perspective. Additional 

monitoring of both O3 air quality and 

foliar injury levels are needed in these 

areas of national significance to more 

fully characterize the spatial extent of 

this public welfare impact. 

With respect to O3-induced biomass 

loss in trees, the Staff Paper concluded 

that the new body of field-based 

research on trees strengthens the 

conclusions drawn on tree seedling 

biomass loss from earlier OTC work by 

documenting similar seedling responses 

in the field. For example, recent 

empirical studies conducted on quaking 

aspen at the AspenFACE site in 

Wisconsin have confirmed the 

detrimental effects of O3 exposure on 

tree growth in a field setting without 

chambers (Isebrands et al., 2000, 2001). 

In addition, results from an ambient 

gradient study (Gregg et al., 2003), 

which evaluated biomass loss in 

cottonwood along an urban-to-rural 

gradient at several locations, found that 

conditions in the field were sufficient to 

produce substantial biomass loss in 

cottonwood, with larger impacts 

observed in downwind rural areas due 

to the presence of higher O3

concentrations. These gradients from 

low urban to higher rural O3

concentrations occur when O3

precursors generated in urban areas are 

transported to downwind sites and are 

transformed into O3. In addition, O3

concentrations typically fall to near 0 

ppm at night in urban areas due to 

scavenging of O3 by NOX and other 

compounds. In contrast, rural areas, due 

to a lack of nighttime scavenging, tend 

to maintain elevated O3 concentrations 

for longer periods. On the basis of such 

key studies, the Staff Paper concluded 

that the expanded body of field-based 

evidence, in combination with the 

substantial corroborating evidence from 

OTC data, provides stronger evidence 

than that available in the last review 

that ambient levels of O3 are sufficient 

to produce visible foliar injury 

symptoms and biomass loss in sensitive 

vegetative species growing in natural 

environments. Further, the Staff Paper 


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judged that the consistency in response 

in studied species/genotypes to O3

under a variety of exposure conditions 

and methodologies demonstrates that 

these sensitive genotypes and 

populations of plants are susceptible to 

adverse impacts from O3 exposures at 

levels known to occur in the ambient 

air. Due to the potential for 

compounded risks from repeated insults 

over multiple years in perennial species, 

the Staff Paper concluded that these 

sensitive subpopulations are not 

afforded adequate protection under the 

current secondary O3 standard. Despite 

the fact that only a relatively small 

portion of U.S. plant species have been 

studied with respect to O3 sensitivity, 

those species/genotypes shown to have 

O3 sensitivity span a broad range of 

vegetation types and public use 

categories, including direct-use 

categories like food production for 

human and domestic animal 

consumption; fiber, materials, and 

medicinal production; urban/private 

landscaping. Many of these species also 

contribute to the structure and 

functioning of natural ecosystems (e.g., 

the EEAs) and thus, to the goods and 

services those ecosystems provide 

(Young and Sanzone, 2002), including 

non-use categories such as relevance to 

public welfare based on their aesthetic, 

existence or wildlife habitat value. 

The Staff Paper therefore concluded 

that the current secondary standard is 

inadequate to protect the public welfare 

against the occurrence of adverse levels 

of visible foliar injury and tree seedling 

biomass loss occurring in tree species 

(e.g., ponderosa pine, aspen, black 

cherry, cottonwood) that are sensitive 

and clearly important to the public 

welfare. 

ii. Exposure- and Risk-Based 

Considerations 

In evaluating the adequacy of the 

current standard, the Staff Paper also 

presented the results of exposure and 

risk assessments, which are highlighted 

above in section IV.A.3 and discussed in 

section IV.C of the proposal. Due to 

multiple sources of uncertainty, both 

known and unknown, that continue to 

be associated with these analyses, the 

Staff Paper put less weight on this 

information in drawing conclusions on 

the adequacy of the current standard. 

However, the Staff Paper also 

recognized that some progress has been 

made since the last review in better 

characterizing some of these associated 

uncertainties and, therefore concluded 

that the results of the exposure and risk 

assessments continue to provide 

information useful to informing 

judgments as to the relative changes in 

risks predicted to occur under exposure 

scenarios associated with the different 

standard alternatives considered. 

Importantly, with respect to two key 

uncertainties, the uncertainty associated 

with continued reliance on C–R 

functions developed from OTC exposure 

systems to predict plant response in the 

field and the potential for changes in 

tree seedling and crop sensitivities in 

the intervening period since the C–R 

functions were developed, the Staff 

Paper concluded that recent research 

has provided information useful in 

judging how much weight to put on 

these concerns. Specifically, new fieldbased studies, conducted on a limited 

number of tree seedling and crop 

species to date, demonstrate plant 

growth and visible foliar injury 

responses in the field that are similar in 

nature and magnitude to those observed 

previously under OTC exposure 

conditions, lending qualitative support 

to the conclusion that OTC conditions 

do not fundamentally alter the nature of 

the O3-plant response. Second, nothing 

in the recent literature suggests that the 

O3 sensitivity of crop or tree species 

studied in the last review and for which 

C–R functions were developed has 

changed significantly in the intervening 

period. Indeed, in the few recent studies 

where this is examined, O3 sensitivities 

were found to be as great as or greater 

than those observed in the last review. 

The Staff Paper consideration of such 

exposure and risk analyses is discussed 

below and in section IV.D.2.b of the 

proposal, focusing on seedling and 

mature tree biomass loss, qualitative 

ecosystem risks, and crop yield loss. 

(1) Seedling and mature tree biomass 

loss. Biomass loss in sensitive tree 

seedlings is predicted to occur under O3

exposures that meet the level of the 

current secondary standard. For 

instance, black cherry, ponderosa pine, 

eastern white pine, and aspen had 

estimated median seedling biomass 

losses as high as 24, 11, 6, and 6 

percent, respectively, over some 

portions of their growing ranges when 

air quality was rolled back to meet the 

current 8-hr standard with the 10 

percent downward adjustment for the 

potential O3 gradient between monitor 

height and short plant canopies applied. 

The Staff Paper noted that these results 

are for tree seedlings and that mature 

trees of the same species may have more 

or less of a response to O3 exposure. 

Decreased root growth associated with 

biomass loss has the potential to 

indirectly affect the vigor and 

survivability of tree seedlings. If such 

effects occur on a sufficient number of 

seedlings within a stand, overall stand 

health and composition can be affected 

in the long term. Thus, the Staff Paper 

concluded that these levels of estimated 

tree seedling growth reduction should 

be considered significant and 

potentially adverse, given that they are 

well above the 2 percent level of 

concern identified by the 1997 

consensus workshop (Heck and 

Cowling, 1997). 

Though there is significant 

uncertainty associated with this 

analysis, the Staff Paper recommended 

that this information should be given 

careful consideration in light of several 

other pieces of evidence. Specifically, 

limited evidence from experimental 

studies that go beyond the seedling 

growth stage continues to show 

decreased growth under elevated O3

levels (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). 

The potential for effects to ‘‘carry over’’

to the following year or cumulate over 

multiple years, including the potential 

for compounding, must be considered 

(see 72 FR 37885; Andersen et al., 1997; 

Hogsett et al., 1989; Sasek et al., 1991; 

Temple et al., 1993; EPA, 1996). The 

accumulation of such ‘‘carry-over’’

effects over time may affect long-term 

survival and reproduction of individual 

trees and ultimately the abundance of 

sensitive tree species in forest stands. 

(2) Qualitative Ecosystem Risks. In 

addition to the quantifiable risk 

categories discussed above, the Staff 

Paper presented qualitative discussions 

on a number of other public welfare 

effects categories. In so doing, the Staff 

Paper concluded that the quantified 

risks to vegetation estimated to be 

occurring under current air quality or 

upon meeting the current secondary 

standard likely represent only a portion 

of actual risks that may be occurring for 

a number of reasons. 

First, as mentioned above, out of the 

over 43,000 plant species catalogued as 

growing within the U.S. (USDA 

PLANTS database, USDA, NRCS, 2006), 

only a small percentage have been 

studied with respect to O3 sensitivity. 

Most of the studied species were 

selected because of their commercial 

importance or observed O3-induced 

visible foliar injury in the field. Given 

that O3 impacts to vegetation also 

include less obvious but often more 

significant impacts, such as reduced 

annual growth rates and below ground 

root loss, the paucity of information on 

other species means the number of O3- 

sensitive species that exists within the 

U.S. is likely greater than what is now 

known. Since no state in the lower 48 

states has less than seven known O3- 


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26One CASAC Panel member reached different 

conclusions from those of the broader Panel 

regarding certain aspects of the vegetation effects 

information and the appropriate degree of emphasis 

that should be placed on the associated 

uncertainties. These concerns related to how the 

results of O3/vegetation exposure experiments 

carried out in OTC can be extrapolated to the 

ambient environment and how C–R functions 

developed in the 1980s can be used today given that 

he did not expect that current crop species/cultivars 

in use in 2002 would have the same O3 sensitivity 

as those studied in NCLAN (Henderson, 2007, pg. 

C–18). 

sensitive plant species, with the 

majority of states having between 11 

and 30 (see Appendix 7J–2 in Staff 

Paper), protecting O3-sensitive 

vegetation is clearly important to the 

public welfare at the national scale. 

Second, the Staff Paper also took into 

consideration the possibility that more 

subtle and hidden risks to ecosystems 

are potentially occurring in areas where 

vegetation is being significantly 

impacted. Given the importance of these 

qualitative and anticipated risks to 

important public welfare effects 

categories such as ecosystem impacts 

leading to potential losses or shifts in 

ecosystem goods and services (e.g., 

carbon sequestration, hydrology, and 

fire disturbance regimes), the Staff Paper 

concluded that any secondary standard 

set to protect against the known and 

quantifiable adverse effects to vegetation 

should also consider the anticipated, 

but currently unquantifiable, potential 

effects on natural ecosystems. 

(3) Crop Yield Loss. Exposure and risk 

assessments in the Staff Paper estimated 

that meeting the current 8-hour standard 

would still allow O3-related yield loss to 

occur in several fruit and vegetable and 

commodity crop species currently 

grown in the U.S. These estimates of 

crop yield loss are substantially lower 

than those estimated in the last review 

as a result of several factors, including 

adjusted exposure levels to reflect the 

presence of a variable O3 gradient 

between monitor height and crop 

canopies, and use of a different 

econometric agricultural benefits model 

updated to reflect more recent 

agricultural policies (EPA, 2006b). 

Though these sources of uncertainty 

associated with the crop risk and 

benefits assessments were better 

documented in this review, the Staff 

Paper concluded that the presence of 

these uncertainties make the risk 

estimates suitable only as a basis for 

understanding potential trends in 

relative yield loss and economic 

benefits. The Staff Paper further 

recognized that actual conditions in the 

field and management practices vary 

from farm to farm, that agricultural 

systems are heavily managed, and that 

adverse impacts from a variety of other 

factors (e.g., weather, insects, disease) 

can be orders of magnitude greater than 

that of yield impacts predicted for a 

given O3 exposure. Thus, the relevance 

of such estimated impacts on crop 

yields to the public welfare are 

considered highly uncertain and less 

useful as a basis for assessing the 

adequacy of the current standard. The 

Staff Paper noted, however, that in some 

experimental cases, exposure to O3 has 

made plants more sensitive or 

vulnerable to some of these other 

important stressors, including disease, 

insect pests, and harsh weather (EPA, 

2006a). The Staff Paper therefore 

concluded that this remains an 

important area of uncertainty and that 

additional research to better 

characterize the nature and significance 

of these interactions between O3 and 

other plant stressors would be useful. 

iii. Summary of Staff Paper 

Considerations 

In summary, the Staff Paper 

concluded that the current secondary O3

standard is inadequate. This conclusion 

was based on the extensive vegetation 

effects evidence, in particular the recent 

empirical field-based evidence on 

biomass loss in seedlings, saplings and 

mature trees, and foliar injury incidence 

that has become available in this review, 

which demonstrates the occurrence of 

adverse vegetation effects at ambient 

levels of recent O3 air quality, as well as 

evidence and exposure- and risk-based 

analyses indicating that adverse effects 

would be predicted to occur under air 

quality scenarios that meet the current 

standard. 

b. CASAC Views 

In a letter to the Administrator 

(Henderson, 2006c), the CASAC O3

Panel, with full endorsement of the 

chartered CASAC, unanimously 

concluded that ‘‘despite limited recent 

research, it has become clear since the 

last review that adverse effects on a 

wide range of vegetation including 

visible foliar injury are to be expected 

and have been observed in areas that are 

below the level of the current 8-hour 

primary and secondary ozone 

standards.’’ Therefore, ‘‘based on the 

Ozone Panel’s review of Chapters 7 and 

8 [of the Staff Paper], the CASAC 

unanimously agrees that it is not 

appropriate to try to protect vegetation 

from the substantial, known or 

anticipated, direct and/or indirect, 

adverse effects of ambient O3 by 

continuing to promulgate identical 

primary and secondary standards for O3. 

Moreover, the members of the 

Committee and a substantial majority of 

the Ozone Panel agree with EPA staff 

conclusions and encourage the 

Administrator to establish an alternative 

cumulative secondary standard for O3

and related photochemical oxidants that 

is distinctly different in averaging time, 

form and level from the currently 

existing or potentially revised 8-hour 

primary standard’’ (Henderson, 

2006c).26

c. Administrator’s Proposed 

Conclusions 

At the time of proposal, in 

considering whether the current 

secondary standard should be revised, 

the Administrator carefully considered 

the conclusions contained in the 

Criteria Document, the rationale and 

recommendations contained in the Staff 

Paper, the advice and recommendations 

from CASAC, and public comments to 

date on this issue. In so doing, the 

Administrator recognized that the 

secondary standard is to protect against 

‘‘adverse’’ O3 effects, as discussed in 

section IV.A.3 of the proposal. In 

considering what constitutes a 

vegetation effect that is also adverse to 

the public welfare, the Administrator 

took into account the Staff Paper 

conclusions regarding the nature and 

strength of the vegetation effects 

evidence, the exposure and risk 

assessment results, the degree to which 

the associated uncertainties should be 

considered in interpreting the results, 

and the views of CASAC and members 

of the public. On these bases, the 

Administrator proposed that the current 

secondary standard is inadequate to 

protect the public welfare from known 

and anticipated adverse O3-related 

effects on vegetation and ecosystems. 

Ozone levels that would be expected to 

remain after meeting the current 

secondary standard were judged to be 

sufficient to cause visible foliar injury, 

seedling and mature tree biomass loss, 

and crop yield reductions to degrees 

that could be considered adverse 

depending on the intended use of the 

plant and its significance to the public 

welfare, and the current secondary 

standard does not provide adequate 

protection from such effects. Other O3- 

induced effects described in the 

literature, including an impaired ability 

of many sensitive species and genotypes 

within species to adapt to or withstand 

other environmental stresses, such as 

freezing temperatures, pest infestations 

and/or disease, and to compete for 

available resources, would also be 

anticipated to occur. In the long run, the 

result of these impairments (e.g., loss in 

vigor) could lead to premature plant 

death in O3 sensitive species. Though 

effects on other ecosystem components 


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have only been examined in isolated 

cases, effects such as those described 

above could have significant 

implications for plant community and 

associated species biodiversity and the 

structure and function of whole 

ecosystems. These considerations also 

support the proposed conclusion that 

the current secondary standard is not 

adequate and that revision is needed to 

provide additional public welfare 

protection. 

2. Comments on the Need for Revision 

The above section outlines the 

vegetation and ecosystem effects 

evidence and assessments used by the 

Administrator to inform his proposed 

judgments about the adequacy of the 

current O3 secondary standard. General 

comments received on the proposal that 

either supported or opposed the 

proposed decision to revise the current 

O3 secondary standard are addressed in 

this section. Comments related to the 

vegetation and ecosystem effects 

evidence and information related to 

exposure indices are considered in 

section IV.B.2.a below, and comments 

on vegetation exposure and risk 

assessments are considered in section 

IV.B.2.b. Comments on specific issues, 

vegetation and ecosystem effects 

evidence, information on exposure 

indices, or the vegetation exposure and 

risk assessments that relate to 

consideration of the appropriate form, 

averaging time, or level of the O3

standard are addressed below in section 

IV.C. General comments based on 

implementation-related factors that are 

not a permissible basis for considering 

the need to revise the current standard 

are noted in the Response to Comments 

document. 

a. Evidence of Effects and Exposure 

Indices 

Sections IV.A.2 and IV.A.3 above 

provide a summary overview of the 

information on vegetation and 

ecosystem effects and exposure indices 

used by the Administrator to inform his 

proposed judgments about the adequacy 

of the current O3 secondary standard. As 

discussed more fully below, comments 

received on the proposal regarding the 

nature and strength of the vegetation 

and ecosystem effects information, 

information on exposure indices, and 

the conclusions that could appropriately 

be drawn from such information fell 

generally into two groups. 

One group of commenters that 

included national and local 

environmental organizations (e.g., 

Environmental Defense, Appalachian 

Mountain Club, Rocky Mountain Clean 

Air Action), NESCAUM, NACAA, 

individual States, Tribal Associations, 

and the National Park Service (NPS) 

argued that the available science clearly 

showed that O3-induced vegetation and 

ecosystem effects are occurring at and 

below levels that meet the current 8- 

hour standard, and therefore provides a 

strong basis and support for the 

conclusion that the current secondary 

standard is inadequate. In support of 

their view, these commenters relied on 

the entire body of evidence available for 

consideration in this review, including 

evidence assessed previously in the last 

review. These commenters pointed to 

the information and analyses in the Staff 

Paper and the conclusions and 

recommendations of CASAC as 

providing a clear basis for concluding 

that the current standard does not 

adequately protect vegetation from an 

array of O3-related effects. For example, 

the NPS noted that ‘‘[w]idespread foliar 

injury has been documented in areas 

meeting the current standard; field and 

chamber studies indicate that O3- 

induced significant growth reductions 

are also occurring at levels below the 

current standard’’ (NPS, p. 3). 

In addition to the body of information 

already considered by EPA in this 

review, these same commenters also 

presented new information for the 

Administrator’s consideration, 

including a number of ‘‘new’’ studies 

published after completion of the 

Criteria Document, as well as additional 

information on air quality and 

vegetation exposures and effects 

pertaining to local conditions within 

their State, Tribal or federal lands, as 

additional support for their views that 

the current standard is inadequate. For 

example, NESCAUM, NY, PA, and NPS 

all provided air quality information 

describing typical O3 concentrations in 

areas that rarely, if ever, exceeded the 

level of the current 8-hour standard in 

areas that still showed O3-related 

vegetation effects, particularly visible 

foliar injury. 

Building on EPA’s qualitative 

discussions of the potential linkage 

between O3 vegetation effects and 

effects on ecosystems, a number of these 

commenters expressed concern that the 

possible impact of O3-related reductions 

in plant productivity could result in a 

reduced capacity of vegetation to serve 

as a carbon sink to mitigate the impacts 

of rising CO2 in a changing climate, 

citing to a ‘‘new’’ study on that topic 

(Sitch et al., 2007). Many of these same 

commenters also cited to ‘‘new’’ fieldbased studies in the Great Smoky 

Mountain National Park that find a 

relationship between O3 exposure, tree 

stem growth loss, tree water use and 

stream flow as evidence that current 

ambient O3 levels can impact 

ecosystems and that ecosystems should 

be afforded protection from such 

potential effects. For example, some of 

these commenters note that ‘‘new’’

studies in the Great Smoky Mountain 

National Park (McLaughlin, et al., 

2007a, b) have found that (1) ambient O3

caused substantial growth reductions in 

mature trees in a mixed deciduous 

forest, which was due in part to 

increased O3-induced water loss and led 

to seasonal losses in stem growth of 30–

50 percent for most species in a highozone year; (2) increasing ambient O3

levels also resulted in depletion of soil 

moisture in the rooting zone and 

reduced late-season streamflow in the 

watershed; and (3) O3 may amplify the 

adverse effects of increasing 

temperature on forest growth and forest 

hydrology and may exacerbate the 

effects of drought on forest growth and 

stream health. Other ‘‘new’’ research 

noted by these commenters as 

supporting EPA’s findings that current 

O3 exposures cause significant biomass 

losses in sensitive seedlings of various 

tree species include a study that 

predicted up to 31 percent growth loss 

in aspen in certain areas of its North 

American range in 2001–2003 (Percy, et 

al., 2007). These commenters 

encouraged the Administrator to 

consider these ‘‘new’’ studies in making 

his final decision. 

This group of commenters strongly 

supported revising the current standard, 

not only because in their view the 

available evidence conclusively 

demonstrates that the current standard 

is inadequate to protect sensitive 

vegetation, but also because the Staff 

Paper provides abundant evidence that 

it is appropriate to establish an 

alternative cumulative, seasonal 

secondary standard that is distinctly 

different in form from the current or 

revised primary standard. For example, 

NESCAUM states that ‘‘[i]n light of the 

EPA Staff and CASAC 

recommendations, and the extensive 

body of historical and recent monitoring 

and research data upon which these 

recommendations were based, the 

option of equating the ozone secondary 

NAAQS with the 8-hour primary is 

inappropriate and clearly not supported 

by the weight of scientific evidence.’’

EPA agrees with these commenters 

that when evaluated as a whole, the 

entire body of vegetation and ecosystem 

effects information available in this 

review supports the need to revise the 

current standard to provide increased 

protection from an array of O3-related 

effects on sensitive vegetation and 

ecosystems. EPA also agrees that the 

available evidence indicates that a 


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cumulative, seasonal form better reflects 

the scientific information on 

biologically relevant exposures for 

vegetation. For reasons discussed below 

in sections IV.C, however, EPA 

disagrees with aspects of these 

commenters’ views as to whether a 

standard defined in terms of a 

cumulative, seasonal form is requisite to 

protect public welfare based on the 

available scientific information. 

To the extent that these and other 

commenters whose comments are 

discussed below included ‘‘new’’

scientific studies, studies that were 

published too late to be considered in 

the Criteria Document, in support of 

their arguments for revising or not 

revising the standards, EPA notes, as 

discussed in section I above, that as in 

past NAAQS reviews, it is basing the 

final decisions in this review on the 

studies and related information 

included in the O3 air quality criteria 

that have undergone CASAC and public 

review and will consider newly 

published studies for purposes of 

decision making in the next O3 NAAQS 

review. In provisionally evaluating 

commenters’ arguments, as discussed in 

the Response to Comments document, 

EPA notes that its provisional 

consideration of ‘‘new’’ science found 

that such studies did not materially 

change the conclusions in the Criteria 

Document. 

The other main group of commenters, 

which included Exxon-Mobil, UARG, 

API, other industry groups, The 

Annapolis Center for Science Based 

Public Policy, individual States and 

other organizations representing local 

energy, agriculture or business interests, 

expressed the contrasting view that the 

limited number of studies published 

since the last review and addressed in 

the Criteria Document provided 

insufficient evidence to support a 

conclusion different than what was 

reached in the last review. In particular, 

they asserted that the types of vegetation 

effects evaluated in the last review have 

not changed, and that the Criteria 

Document, Staff Paper, and CASAC 

have acknowledged that the information 

that has become available since the last 

review does not fundamentally change 

the conclusions reached in the last 

review. As a result, they argued that the 

currently available evidence fails to 

show that revision to the standard is 

requisite to provide additional 

protection from these effects. In 

particular, Exxon-Mobil stated that 

‘‘EPA is incorrect in concluding 

vegetation impacts [occur] at or below 

the level of the current standard’’ * * * 

and that the ‘‘newer field-based 

evidence EPA cites for ozone impacts on 

seedlings, saplings and mature trees 

indicates ozone impacts but at 

exposures that are likely in exceedence 

of the current secondary standard.’’ This 

commenter concluded that while these 

studies provide additional support for 

O3-related impacts on vegetation, 

including observing effects in field 

settings without chambers, they do not 

provide support for the conclusion that 

ambient levels in compliance with the 

current standard would result in 

significant O3 impact. In addition, these 

commenters also generally asserted that 

the evidence that has become available 

since the last review does not materially 

reduce the uncertainties that were 

present and cited by the Administrator 

in the last review as important factors in 

her decision to set the secondary 

identical to the revised primary. Those 

aspects of these comments that include 

uncertainties associated with the 

exposure, risk and benefits assessments 

are addressed below in section IV.2.b 

and in the Response to Comments 

document. 

EPA disagrees with the commenters’

assertion that the currently available 

evidence has not materially reduced key 

uncertainties present in the last review 

that factored into the Administrator’s 

decision. For example, there is an 

expansion of field-based evidence 

across a broad array of vegetation effects 

categories, as discussed in the Criteria 

Document, Staff Paper, and highlighted 

above in section IV.A.2. Though in some 

such studies (e.g., the FACE studies) the 

O3 exposures are indeed at or above 

ambient levels, the observed vegetation 

response is similar to that observed in 

OTC studies at similar levels of 

exposure. Though these studies are still 

limited in scope, it is nevertheless 

EPA’s view that such field-based 

evidence reduces the uncertainties 

associated with the C–R functions 

generated in OTC studies that were 

noted by the Administrator in the last 

review. Thus, the current body of 

evidence increases EPA’s confidence in 

the results from the OTC studies which 

demonstrate O3-related effects below the 

level of the current standard. EPA has 

also considered this evidence in 

conjunction with USDA FIA foliar 

injury survey data and the Gregg et al. 

(2003) tree seedling biomass loss 

gradient study showing effects on a 

sensitive tree species occurring in the 

field across a range of exposure levels 

including levels of air quality at to well 

below the level of the current secondary 

standard. Taken together, EPA 

concludes that these studies form a 

coherent body of evidence that 

significantly strengthens EPA’s 

confidence that such effects are 

currently occurring in the field and 

would continue to be anticipated at and 

below the level of the current secondary 

standard. A more detailed discussion of 

these issues can be found in the 

Response to Comments document. 

b. Vegetation Exposure and Risk 

Assessments 

Section IV.A.4 above provides a 

summary overview of the vegetation 

exposure and risk assessment 

information used by the Administrator 

to help inform judgments about 

vegetation exposure and risk estimates 

associated with attainment of the 

current and alternative standards. As an 

initial matter, EPA notes that at the time 

of proposal, the Administrator primarily 

based his conclusion on whether 

revision of the secondary standard was 

needed primarily on evidence-based 

considerations, while using the more 

uncertain exposure and risk assessments 

in a supportive role. As discussed more 

fully below, comments received on the 

proposal regarding these assessments 

and the conclusions that could 

appropriately be drawn from them fell 

generally into two groups. One group of 

commenters generally included those 

noted above who supported revising the 

current secondary standard, while the 

other group of commenters were those 

noted above who expressed the view 

that no revision was appropriate. 

The first group of commenters 

primarily focused on evidence-based 

considerations in their support of a 

revised standard, while some also 

referenced EPA’s findings from the 

exposure and risk assessments in 

supporting their view that the standard 

needed to be revised to provide 

increased protection for sensitive 

vegetation. A few of these commenters 

also provided additional exposure, risk 

and benefits information from localized 

assessments conducted by themselves or 

others in their behalf in support of their 

view that the standard needed to be 

revised. In so doing, these commenters 

have generally shown support for using 

such assessments to help inform a final 

decision on the need to revise. 

The other group of commenters 

expressed a number of concerns with 

these assessments and generally 

asserted that these assessments do not 

support revision of the current standard. 

These commenters’ concerns generally 

focused on (1) the method used by EPA 

to estimate PRB, (2) the lack of new 

information since the last review that 

would, in their judgment, materially 

reduce the uncertainties present in the 

assessments conducted for the last 

review, and (3) EPA’s interpretation and 


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use of the results in making a judgment 

about the adequacy of the current 

standard. These comments are 

addressed below. 

(1) Regarding concerns related to the 

method used by EPA to estimate PRB, 

EPA notes that this issue has been 

raised repeatedly throughout the review 

in the context of both the primary and 

secondary standards. Most generally, 

these commenters asserted that EPA 

used unrealistically low levels of PRB 

that resulted in an overestimate of risks 

and benefits associated with just 

meeting alternative standards. EPA 

disagrees with this view, for the reasons 

discussed above in section II.B.2.b, 

which addresses this and other 

comments related to EPA’s approach to 

estimating PRB and its role in exposure 

and risk assessments related to the 

primary standard. 

(2) Another concern posed by these 

commenters was the lack of any new 

information that, in their judgment, 

would materially reduce the 

uncertainties present in the exposure, 

risk and benefits assessments conducted 

for the last review. For example, the 

Annapolis Center asserted that ‘‘[s]ome 

of the most important caveats and 

uncertainties concerning the exposure 

and risk assessments for crop yield that 

were listed in the [1996] proposal 

included (1) extrapolating from 

exposure-response functions generated 

in open-top chambers to ambient 

conditions; (2) the lack of a performance 

evaluation of the national air quality 

extrapolation; (3) the methodology to 

adjust modeled air quality to reflect 

attainment of various alternative 

standard options; and (4) inherent 

uncertainties in models to estimate 

economic values associated with 

attainment of alternative standard. 

* * * Because of the lack of new data 

or substantive improvements in the risk 

assessment, these same issues remain 

today, contributing a similar degree of 

uncertainty, as was the case in the prior 

review.’’ EPA recognizes that important 

uncertainties remain in estimates of 

vegetation exposure and O3-related risk 

to vegetation, especially with regard to 

O3-related effects on crop yields. 

However, EPA disagrees with comments 

that assert that uncertainties have not 

been reduced since the last review, as 

discussed below. 

With regard to the uncertainties 

associated with using the OTC C–R 

functions, the Annapolis Center further 

stated that ‘‘ten years have now elapsed, 

and the same concentration-response 

functions from the OTC studies of the 

1980’s are still the only viable data to 

use to estimate crop loss. * * * The 

1996 CASAC Panel agreed that the 

estimates of crop loss at that time were 

highly uncertain.’’ While EPA agrees 

that important uncertainties continue to 

be associated with the use of the C–R 

functions generated many years ago 

using OTC studies for crop yield loss, 

EPA does not agree that the new 

information available in this review 

does nothing to reduce such 

uncertainties identified in the last 

review. As described above and in the 

Staff Paper and proposal, results from 

the new SoyFACE and AspenFACE 

studies provide qualitative support that 

the levels of vegetation response that 

have been observed in the field are of 

similar magnitude as those predicted at 

similar exposure levels using the OTC 

generated C–R functions. Therefore, 

EPA believes that the uncertainties cited 

in the last review regarding the 

appropriateness of using OTC generated 

C–R functions to predict vegetation 

response in the field have been reduced. 

Providing some further support in this 

regard is the limited information 

available in this review on some 

sensitive crop species (e.g., soybean) 

suggesting that O3 sensitivity has not 

changed significantly in the intervening 

years. Taking all the above into account, 

EPA’s level of confidence in the 

applicability of the OTC generated C–R 

functions to represent ambient 

conditions in the field has increased. 

With regard to the lack of a 

performance evaluation of the national 

air quality extrapolation, EPA notes that 

there have been advancements in the 

tools and methods used for such 

extrapolations since the last review. 

With respect to the generation of 

interpolated O3 exposure surfaces, EPA 

employed a different approach than that 

used in the last review and undertook 

a quantitative assessment of the 

uncertainties associated with the use of 

this method. This uncertainty 

assessment was accomplished by 

sequentially dropping out of the 

interpolation each monitoring site, and 

then recalculating the exposure surface 

using the remaining monitoring sites. As 

discussed in the Staff Paper, this 

method of evaluation may result in a 

slight overestimation of error and bias 

for the exposure surface, since dropping 

out monitors loses information that the 

interpolation uses in that local area. As 

another point of comparison, EPA also 

examined the subset of rural CASTNET 

sites to illustrate how the interpolation 

technique predicted air quality in that 

rural monitoring network. For this 

subset, the evaluation indicated that in 

general, the interpolation technique 

slightly overestimated W126 exposures 

at relatively low levels and 

underestimated W126 exposure at 

relatively high levels. This aspect of the 

estimation method potentially resulted 

in an underestimation of the more 

important risks associated with higher 

cumulative exposures in some areas. 

Based on this evaluation, EPA reiterates 

the conclusion in the Staff Paper that 

‘‘the calculation of error and bias 

metrics for the interpolation represents 

a notable improvement over the 1996 

assessment which did not have such an 

evaluation.’’ EPA further concludes that 

in general, the sources and likely 

direction of uncertainties associated 

with the exposure and risk assessments 

have been better accounted for and 

characterized than in the last review. 

With regard to criticisms of the 

methodology used to adjust modeled air 

quality to reflect attainment of various 

alternative standard options, EPA notes 

that this issue has been raised in the 

context of both the primary and 

secondary standards. As noted above in 

section II.B.2.b, based on information in 

the Staff Paper (section 4.5.6) and in 

more detail in a staff memorandum 

(Rizzo, 2006), EPA concluded that the 

quadratic air quality adjustment 

approach used in this assessment 

generally best represented the pattern of 

reductions across the O3 air quality 

distribution observed over the last 

decade in areas implementing control 

programs designed to attain the O3

NAAQS. While EPA recognizes that 

future changes in air quality 

distributions are area-specific, and will 

be affected by whatever specific control 

strategies are implemented in the future 

to attain a revised NAAQS, there is no 

empirical evidence to suggest that future 

reductions in ambient O3 will be 

significantly different from past 

reductions with respect to impacting the 

overall shape of the O3 distribution. 

With regard to comments that asserted 

that inherent uncertainties in models to 

estimate economic values of crop loss 

have not been reduced since the last 

review, EPA acknowledges that while 

an updated state of the art model, the 

AGSIM benefits model, was used in this 

review, substantial uncertainties remain 

in these estimates of economic crop 

loss. Further, EPA notes that these 

estimates were not relied on as a basis 

for reaching a decision on the need to 

revise the current standard. 

(3) Some commenters also asserted 

that the estimated exposures and risks 

associated with air quality just meeting 

the current standard have not 

appreciably changed since the last 

review. These commenters used this 

conclusion as the basis for a claim that 

there is no reason to depart from the 

Administrator’s 1997 decision that the 


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27The Administrator also recognizes that other 

aspects of public welfare, as welfare is defined in 

the CAA, may rely on concepts other than 

‘‘intended use.’’

current secondary standard is requisite 

to protect public welfare. EPA believes 

that this claim is fundamentally flawed 

for three reasons. First, it is 

inappropriate to compare quantitative 

vegetation risks estimated in the last 

review with those estimated in the 

current review. The 1997 risk estimates, 

or any comparison of the 1997 risks 

estimates to the current estimates, are 

irrelevant for the purpose of judging the 

adequacy of the current standard, as the 

1997 estimates reflect outdated analyses 

that have been updated in this review to 

reflect the current science and as there 

have been significant improvements to 

the modeling approaches and model 

inputs. Second, it is important to take 

into account EPA’s increased 

confidence in some of the model inputs, 

as discussed above, since in judging the 

weight to place on quantitative risk 

estimates it is important to examine not 

only the magnitude of the estimated 

risks but also the degree of confidence 

in those estimates. Third, quantitative 

vegetation risk estimates were not the 

main basis for EPA’s decision in setting 

a level for the secondary standard in 

1997, and they do not set any quantified 

‘‘benchmark’’ for the Agency’s decision 

to revise the current standard at this 

time. The proposal notice made clear 

that decisions about the need to revise 

the current standard are mainly based 

on an integrated evaluation of evidence 

available across a broad array of 

vegetation effects, while the more 

uncertain exposure, risk and benefits 

estimates were used in a supportive 

role. Both the Staff Paper and proposal 

clearly distinguished the roles that these 

different types of information played in 

informing the Administrator’s proposed 

decision. The proposal states that ‘‘due 

to multiple sources of uncertainty, both 

known and unknown, that continue to 

be associated with these analyses, the 

Staff Paper put less weight on this 

information in drawing conclusions on 

the adequacy of the current standard. 

However, the Staff Paper also recognizes 

that some progress has been made since 

the last review in better characterizing 

some of these associated uncertainties 

and, therefore, concluded that the 

results of the exposure and risk 

assessments continue to provide 

information useful to informing 

judgments as to the relative changes in 

risks predicted to occur under exposure 

scenarios associated with the different 

standard alternatives considered.’’ In 

determining the requisite level of 

protection, the Staff Paper recognized 

that it is appropriate to weigh the 

importance of the predicted risks of 

these effects in the overall context of 

public welfare protection, along with a 

determination as to the appropriate 

weight to place on the associated 

uncertainties and limitations of this 

information. Thus, while the 

Administrator is fully mindful of the 

uncertainties associated with the 

estimates of exposure, risk and benefits, 

as discussed above, he judges that these 

estimates are still useful in providing 

additional support for his judgment that 

the current 8-hour secondary standard 

does not adequately protect sensitive 

vegetation. 

3. Conclusions Regarding the Need for 

Revision 

Having carefully considered the 

public comments, discussed above, the 

Administrator believes the fundamental 

scientific conclusions on the effects of 

O3 on vegetation and sensitive 

ecosystems reached in the Criteria 

Document and Staff Paper, as discussed 

above in section IV.A, remain valid. In 

considering whether the secondary O3

standard should be revised, the 

Administrator finds that evidence that 

has become available in this review 

demonstrates the occurrence of adverse 

vegetation effects at ambient levels of 

recent O3 air quality, and that evidence 

and exposure- and risk-based analyses 

indicate that adverse effects would be 

predicted to occur under air quality 

scenarios that meet the current 

standard, taking into consideration both 

the level and form of the current 

standard. Ozone exposures that would 

be expected to remain after meeting the 

current secondary standard are 

sufficient to cause visible foliar injury 

and seedling and mature tree biomass 

loss in O3-sensitive vegetation. The 

Administrator believes that the degree 

to which such effects should be 

considered to be adverse depends on the 

intended use of the vegetation and its 

significance to the public welfare. Other 

O3-induced effects described in the 

literature, including an impaired ability 

of many sensitive species and genotypes 

within species to adapt to or withstand 

other environmental stresses, such as 

freezing temperatures, pest infestations 

and/or disease, and to compete for 

available resources, would also be 

anticipated to occur. In the long run, the 

result of these impairments (e.g., loss in 

vigor) could lead to premature plant 

death in O3 sensitive species. Though 

effects on other ecosystem components 

have only been examined in isolated 

cases, effects such as those described 

above could have significant 

implications for plant community and 

associated species biodiversity and the 

structure and function of whole 

ecosystems. 

The Administrator recognizes that the 

secondary standard is not meant to 

protect against all known observed or 

anticipated O3-related effects, but only 

those that can reasonably be judged to 

be adverse to the public welfare. In 

considering what constitutes a 

vegetation effect that is adverse from a 

public welfare perspective, the 

Administrator believes it is appropriate 

to continue to rely on the definition of 

‘‘adverse,’’ discussed in section IV.A.3 

of the proposal, that imbeds the concept 

of ‘‘intended use’’ of the ecological 

receptors and resources that are 

affected, and applies that concept 

beyond the species level to the 

ecosystem level.27 In so doing, the 

Administrator has taken note of a 

number of actions taken by Congress to 

establish public lands that are set aside 

for specific uses that are intended to 

provide benefits to the public welfare, 

including lands that are to be protected 

so as to conserve the scenic value and 

the natural vegetation and wildlife 

within such areas, and to leave them 

unimpaired for the enjoyment of future 

generations. Such public lands that are 

protected areas of national interest 

include national parks and forests, 

wildlife refuges, and wilderness areas. 

Because O3-sensitive species are 

generally found in such areas, and 

because levels of O3 allowed by the 

current secondary standard are 

sufficient to cause known or anticipated 

impairment that the Administrator 

judges to be adverse to sensitive 

vegetation and ecosystems in such 

areas, the Administrator concludes that 

it is appropriate to revise the secondary 

standard, in part, to provide increased 

protection against O3-caused 

impairment to such protected vegetation 

and ecosystems. 

The Administrator further recognizes 

that States, Tribes and public interest 

groups also set aside areas that are 

intended to provide similar benefits to 

the public welfare, for residents on State 

and Tribal lands, as well as for visitors 

to those areas. Given the clear public 

interest in and value of maintaining 

these areas in a condition that does not 

impair their intended use, and the fact 

that many of these areas contain O3- 

sensitive vegetation, the Administrator 

further concludes that it is appropriate 

to revise the secondary standard in part 

to provide increased protection against 

O3-caused impairment to vegetation and 

ecosystems in such specially designated 

areas. 


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The Administrator also recognizes 

that O3-related effects on sensitive 

vegetation occur in areas that have not 

been afforded such special protections, 

ranging from vegetation used for 

residential or commercial ornamental 

purposes, such as urban/suburban 

landscaping, to land use categories that 

are heavily managed for commercial 

production of commodities such as 

agricultural crops, timber, and 

ornamental vegetation. For vegetation 

used for residential or commercial 

ornamental purposes, such as urban/ 

suburban landscaping, there are 

indications that impairment to the 

intended use of such vegetation can 

occur from O3 exposures allowed by the 

current standard. While the 

Administrator believes that there is not 

adequate information at this time to 

establish a secondary standard based 

specifically on impairment of urban/ 

suburban landscaping and other uses of 

ornamental vegetation, he notes that a 

secondary standard revised to provide 

protection for sensitive natural 

vegetation and ecosystems may also 

provide some degree of protection for 

such ornamental vegetation. 

With respect to commercial 

production of commodities, however, 

the Administrator notes that judgments 

about the extent to which O3-related 

effects on commercially managed 

vegetation are adverse from a public 

welfare perspective are particularly 

difficult to reach, given that what is 

known about the relationship between 

O3 exposures and agricultural crop yield 

response derives largely from data 

generated almost 20 years ago. The 

Administrator recognizes that there is 

substantial uncertainty at this time as to 

whether these data remain relevant to 

the majority of species and cultivars of 

crops being grown in the field today. In 

addition, the extensive management of 

such vegetation may to some degree 

mitigate potential O3-related effects. The 

management practices used on these 

lands are highly variable and are 

designed to achieve optimal yields, 

taking into consideration various 

environmental conditions. Thus, while 

the Administrator believes that a 

secondary standard revised to provide 

protection for sensitive natural 

vegetation and ecosystems may also 

provide some degree of additional 

protection for heavily managed 

commercial vegetation, the need for 

such additional protection is uncertain. 

Based on these considerations, and 

taking into consideration the advice and 

recommendations of CASAC, the 

Administrator concludes that the 

protection afforded by the current 

secondary O3 standard is not sufficient 

and that the standard needs to be 

revised to provide additional protection 

from known and anticipated adverse 

effects on sensitive natural vegetation 

and sensitive ecosystems, and that such 

a revised standard could also be 

expected to provide additional 

protection to sensitive ornamental 

vegetation. The Administrator also 

concludes that there is not adequate 

information to establish a separate 

secondary standard based on other 

effects of O3 on public welfare. It is 

important to note that these 

conclusions, and the reasoning on 

which they are based, do not address 

the question of what specific revisions 

to the current secondary standard are 

appropriate. Addressing that question 

requires looking specifically at the two 

proposed options: establishing a new 

standard defined in terms of a 

cumulative, seasonal form, or revising 

the current secondary standard by 

making it identical to the revised 

primary standard. These alternative 

secondary standards are discussed in 

the following section. 

As highlighted below, the discussion 

of public comments above indicates that 

deciding the appropriate secondary 

standard involves making a difficult 

choice between two possible 

alternatives, each with their strengths 

and weaknesses. EPA’s decision, and 

the reasons for it, are described in detail 

above. In reaching this decision, there 

has been a robust discussion within the 

Administration of these same strengths 

and weaknesses. As part of that process 

EPA received a Memorandum on March 

6, 2008 from Susan Dudley, 

Administrator, Office of Information 

and Regulatory Affairs, Office of 

Management and Budget, indicating 

various concerns over adopting a 

cumulative, seasonal secondary 

standard. Deputy Administrator Marcus 

Peacock responded with a 

Memorandum dated March 7, 2008 

stating EPA’s views supporting adoption 

of a cumulative, seasonal secondary 

standard. On March 11, 2008, the 

President ‘‘concluded that, consistent 

with Administration policy, added 

protection should be afforded to public 

welfare by strengthening the secondary 

ozone standard and setting it to be 

identical to the new primary standard, 

the approach adopted when ozone 

standards were last promulgated. This 

policy thus recognizes the 

Administrator’s judgment that the 

secondary standard needs to be adjusted 

to provide increased protection to 

public welfare and avoids setting a 

standard lower or higher than is 

necessary.’’ EPA’s decision therefore 

also reflects the view of the 

Administration as to the most 

appropriate secondary standard. While 

the Administrator fully considered the 

President’s views, the Administrator’s 

decision, and the reasons for it, are 

based on and supported by the record in 

this rulemaking. 

C. Conclusions on the Secondary O3

Standard 

As an initial matter, EPA has 

considered the indicator for a secondary 

O3 standard. As discussed above in 

section II.C.1 on the primary standard, 

in the last review, EPA focused on a 

standard for O3 as the most appropriate 

surrogate for ambient photochemical 

oxidants. In this review, while the 

complex atmospheric chemistry in 

which O3 plays a key role has been 

highlighted, no alternatives to O3 have 

been advanced as being a more 

appropriate surrogate for ambient 

photochemical oxidants and their 

effects on vegetation. Thus, as is the 

case for the primary standard, the 

Administrator concludes that it is 

appropriate to continue to use O3 as the 

indicator for a standard that is intended 

to address effects associated with 

exposure to O3, alone and in 

combination with related 

photochemical oxidants. In so doing, 

the Administrator recognizes that 

measures leading to reductions in 

vegetation exposures to O3 will also 

reduce exposures to other 

photochemical oxidants. 

1. Staff Paper Evaluation 

The current Criteria Document and 

Staff Paper concluded that the recent 

vegetation effects literature evaluated in 

this review strengthens and reaffirms 

conclusions made in the last review that 

the use of a cumulative exposure index 

that differentially weights ambient 

concentrations is best able to relate 

ambient exposures to vegetation 

response at this time (EPA, 2006a, b). 

The last review focused in particular on 

two of these cumulative forms, the 

SUM06 and W126 (EPA, 1996). Given 

that the data available at that time were 

unable to distinguish between these 

forms, the Administrator, based on the 

policy consideration of not including O3

concentrations considered to be within 

the PRB, estimated to be between 0.03 

and 0.05 ppm, concluded that the 

SUM06 form would be the more 

appropriate choice for a cumulative, 

exposure index for a secondary 

standard, though a cumulative form was 

not adopted at that time. 

In this review, the Staff Paper 

evaluated the continued 

appropriateness of the SUM06 form in 


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light of two key pieces of information: 

new estimates of PRB that are lower 

than in the last review, and continued 

lack of evidence within the vegetation 

effects literature of a biological 

threshold for vegetation exposures of 

concern. On the basis of those policy 

and science-related considerations, the 

Staff Paper concluded that the W126 

form was more appropriate in the 

context of this review. Specifically, the 

W126, by its incorporation of a 

sigmoidal weighting function, does not 

create an artificially imposed 

concentration threshold, gives 

proportionally more weight to the 

higher and typically more biologically 

potent concentrations, and is not 

significantly influenced by O3

concentrations within the range of 

estimated PRB. 

The Staff Paper also considered that 

in the 1997 final rule, the decision was 

made, on the basis of both science and 

policy considerations, to make the 

secondary standard identical to the 

primary standard (62 FR 38876). On the 

basis of that history, the current Staff 

Paper analyzed the degree of overlap 

expected between alternative 8-hour 

and cumulative seasonal secondary 

standards using recent air quality 

monitoring data. Based on the results, 

the Staff Paper concluded that the 

degree to which the current 8-hour 

standard form and level would overlap 

with areas of concern for vegetation 

expressed in terms of the 12-hour W126 

standard is inconsistent from year to 

year and would depend greatly on the 

level of the 12-hour W126 and 8-hour 

standards selected and the distribution 

of hourly O3 concentrations within the 

annual and/or 3-year average period. 

Thus, though the Staff Paper 

recognized again that meeting the 

current or alternative levels of the 8- 

hour average standard could result in air 

quality improvements that would 

potentially benefit vegetation in some 

areas, it urged caution be used in 

evaluating the likely vegetation impacts 

associated with a given level of air 

quality expressed in terms of the 8-hour 

average form in the absence of parallel 

W126 information. This caution is due 

to the concern that the analysis in the 

Staff Paper may not be an accurate 

reflection of the true situation in nonmonitored, rural counties due to the 

lack of more complete monitor coverage 

in many rural areas. Further, of the 

counties that did not show overlap 

between the two standard forms, most 

were located in rural/remote high 

elevation areas which have O3 air 

quality patterns that are typically 

different from those associated with 

urban and near urban sites at lower 

elevations. Because the majority of such 

areas are currently not monitored, it is 

believed there are likely to be additional 

areas that have similar air quality 

distributions that would lead to the 

same disconnect between forms. Thus, 

the Staff Paper concluded that it 

remains problematic to determine the 

appropriate level of protection for 

vegetation using an 8-hour average form. 

2. CASAC Views 

The CASAC, based on its assessment 

of the same vegetation effects science, 

agreed with the Criteria Document and 

Staff Paper and unanimously concluded 

that protection of vegetation from the 

known or anticipated adverse effects of 

ambient O3 ‘‘requires a secondary 

standard that is substantially different 

from the primary standard in averaging 

time, level, and form,’’ i.e. not identical 

to the primary standard for O3

(Henderson, 2007). Moreover, the 

members of CASAC and a substantial 

majority of the CASAC Panel agreed 

with Staff Paper conclusions and 

encouraged the Administrator to 

establish an alternative cumulative 

secondary standard for O3 and related 

photochemical oxidants that is 

distinctly different in averaging time, 

form and level from the current or 

potentially revised 8-hour primary 

standard (Henderson, 2006c). The 

CASAC Panel also stated that ‘‘the 

recommended metric for the secondary 

ozone standard is the (sigmoidally 

weighted) W126 index’’ (Henderson, 

2007). 

3. Administrator’s Proposed 

Conclusions 

In EPA’s proposal, the Administrator 

agreed with the conclusions drawn in 

the Criteria Document, Staff Paper and 

by CASAC that the scientific evidence 

available in the current review 

continues to demonstrate the 

cumulative nature of O3-induced plant 

effects and the need to give greater 

weight to higher concentrations. Thus, 

the Administrator proposed that a 

cumulative exposure index that 

differentially weights O3 concentrations 

could represent a reasonable policy 

choice for a seasonal secondary 

standard to protect against the effects of 

O3 on vegetation. The Administrator 

further agreed with both the Staff Paper 

and CASAC that the most appropriate 

cumulative, concentration-weighted 

form to consider in this review is the 

sigmoidally weighted W126 form, due 

to his recognition that there is no 

evidence in the literature for an 

exposure threshold that would be 

appropriate across all O3-sensitive 

vegetation and that this form is unlikely 

to be significantly influenced by O3 air 

quality within the range of PRB levels 

identified in this review. Thus, the 

Administrator proposed as one option to 

replace the current 8-hour average 

secondary standard form with the 

cumulative, seasonal W126 form. 

The Administrator also proposed to 

revise the current secondary standard by 

making it identical to the proposed 8- 

hour primary standard, which was 

proposed to be within the range of 0.070 

to 0.075 ppm. For this option, EPA also 

solicited comment on a wider range of 

8-hour standard levels, including levels 

down to 0.060 ppm and up to the 

current standard (i.e., effectively 0.084 

ppm with the current rounding 

convention). In putting forward such a 

proposal, the Administrator focused on 

the decision made in the last review, 

and the rationale for that decision that 

made the revised secondary standard 

identical to the revised primary 

standard. 

4. Comments on the Secondary 

Standard Options 

Comments received following 

proposal regarding revising the 

secondary standard either to reflect a 

new, cumulative form or by remaining 

equal to a revised primary standard 

generally fell into two groups. These 

comments were similar to those raised 

prior to the proposal during earlier 

phases of the NAAQS review, as 

summarized in the proposal notice and 

highlighted below. 

One group of commenters, including 

the National Park Service, 

Environmental Defense, NESCAUM, 

NACAA, individual States, Tribal 

Associations, and local environmental 

organizations, asserted that the weight 

of scientific evidence was unambiguous 

with regard to the need for a cumulative 

form, and specifically supported the 

proposed W126 exposure index. For 

example, New York State DEC 

explained that ‘‘scientific research 

recognizes that exposure-based indices 

considering seasonal time period, 

exposure duration, diurnal dynamics, 

peak hourly ozone concentrations, and 

cumulative effects are important when 

assessing vegetation effects of ozone 

exposure (Musselman et al., 2006). The 

W126 exposure index has long been 

recognized as a biologically meaningful 

and useful way to summarize hourly 

ozone data as a measure of ozone 

exposure to vegetation (Lefohn et al., 

1989)’’. Similarly, Environmental 

Defense stated ‘‘[f]or reasons amply 

explained by CASAC and the Staff, 

neither the existing secondary standard 

for ozone nor the proposed primary 

standards are requisite to protect against 


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28EPA has done further analysis of the degree of 

overlap, and that analysis is in the docket. 

adverse welfare effects on vegetation 

and forested ecosystems. CASAC and 

Staff further amply justified the need for 

a separate cumulative seasonal welfare 

standard to protect against these effects, 

rather than relying solely on the primary 

standards to provide such protection.’’

The National Park Service (NPS) 

comment provided additional support 

to this view and more specifically stated 

that ‘‘the NPS supports both the 

conclusion that a seasonal, cumulative 

metric is needed to protect vegetation, 

and that the W126 is a more appropriate 

metric than the SUM06.’’ EPA agrees 

with these comments for the reasons 

discussed above in sections IV.A.3 and 

IV.B.2.a). 

In addition to expressing strong 

support for the W126 cumulative 

seasonal form, commenters in this group 

also expressed serious concerns with 

EPA’s other proposed option of setting 

the secondary standard equal to a 

revised primary standard. For example, 

NPS agreed with CASAC that ‘‘retaining 

the current form of the 8-hour standard 

for the secondary NAAQS is 

inappropriate and inadequate for 

characterizing ozone exposures to 

vegetation.’’ NESCAUM stated ‘‘we also 

strongly encourage EPA to avoid the 

flawed rationale employed in the 

previous 1997 ozone NAAQS review, 

i.e., that many of the benefits of a 

secondary NAAQS would be achieved if 

the primary NAAQS were attained. This 

rationale is flawed in at least two ways: 

first, ozone damage to vegetation 

persists in areas that attain the primary 

NAAQS; and second, the relationship 

between short-term 8-hour peak 

concentrations and longer-term seasonal 

aggregations is not constant, but varies 

over space and time * * * as EPA notes 

at 72 FR 37904. * * * EPA should set 

a secondary NAAQS on its own 

independent merits based on adverse 

welfare effects. Real or perceived 

relationships between primary and 

secondary nonattainment areas are 

irrelevant to setting the appropriate 

form and level of the secondary 

NAAQS.’’ Environmental Defense made 

the argument that ‘‘[b]ecause there is no 

rational connection between the 

proposed primary standards and the 

level of protection needed to protect 

vegetation against adverse ozoneinduced welfare effects, any EPA 

finding that the primary standards 

would be sufficient for secondary 

standards purposes would be 

arbitrary.* * * The mere fact that the 

primary might provide ancillary welfare 

benefits does not satisfy the statute and 

does not provide a rational basis for 

concluding that the primary standards 

are also requisite to protect to [sic] any 

adverse welfare effects.’’

The other set of commenters, 

including UARG, API, Exxon-Mobil, 

The Annapolis Center, ASL and 

Associates, and AAM, did not support 

adopting an alternative, cumulative 

form for the secondary standard. Some 

of these commenters, while agreeing 

that ‘‘directionally a cumulative form of 

the standard may better match the 

underlying data,’’ believe that further 

work is needed to determine whether a 

cumulative exposure index for the form 

of the secondary standard is requisite to 

protect public welfare. These 

commenters also restated concerns that 

have been described above in section 

IV.B.2 regarding the remaining 

uncertainties associated with the 

vegetation effects evidence and/or the 

exposure, risk and benefits assessments. 

They point to the uncertainties cited by 

the Administrator in the 1997 review as 

part of her rationale for deciding it was 

not appropriate to move forward with a 

seasonal secondary, and state that these 

same uncertainties have not been 

materially reduced in the current 

review. These commenters also asserted 

that EPA’s analysis of the impact of the 

nation’s O3 control program for the 8- 

hour standard on W126 exposures is not 

scientifically sound due to the use of 

low estimates of PRB and an arbitrary 

rollback method that is uninformed by 

atmospheric chemistry from 

photochemical models. They argue that 

EPA must first realistically evaluate the 

total O3 reductions that would occur by 

using a state-of-the-art photochemical 

model and perform an analysis of the 

exposure-response data to determine if 

effects are observed for exposures which 

do not exceed the 8-hour standard. 

These commenters also stated that 

without producing C–R functions for the 

8-hour form of the standard, EPA has 

failed to show that the current 8-hour 

standard would provide less than 

requisite protection. These commenters 

asserted that substantial uncertainties 

remain in this review, and that the 

benefits of changing to a W126 form are 

too uncertain to warrant revising the 

form of the standard at this time. 

This group of commenters also 

addressed limitations associated with 

selection of the W126 cumulative form. 

Commenters asserted that: (1) The W126 

form lacks a biological basis, since it is 

merely a mathematical expression of 

exposure that has been fit to specific 

responses in OTC studies, such that its 

relevance for real world biological 

responses is unclear; (2) a flux-based 

model would be a better choice than a 

cumulative metric because it is an 

improvement over the many limitations 

and simplifications associated with the 

cumulative form; however, there is 

insufficient data to apply such a model 

at present; (3) the European experience 

with cumulative O3 metrics has been 

disappointing and now Europeans are 

working on their second level approach, 

which will be flux-based; and (4) the 

W126 form cannot provide nationally 

uniform protection, as the same value of 

an exposure index may relate to 

different vegetation responses; some 

commenters support adding a second 

index that reflects the accumulation of 

peaks at or above 0.10 ppm (called 

N100). 

5. Administrator’s Final Conclusions 

In considering the appropriateness of 

establishing a new standard defined in 

terms of a cumulative, seasonal form, or 

revising the current secondary standard 

by making it identical to the revised 

primary standard, the Administrator 

took into account the approach used by 

the Agency in the last review, the 

conclusions of the Staff Paper, CASAC 

advice, and the views of public 

commenters. In giving careful 

consideration to the approach taken in 

the last review, the Administrator first 

considered the Staff Paper analysis of 

the projected degree of overlap between 

counties with air quality expected to 

meet the revised 8-hour primary 

standard, set at a level of 0.075 ppm, 

and alternative levels of a W126 

standard based on currently monitored 

air quality data. This analysis showed 

significant overlap between the revised 

8-hour primary standard and selected 

levels of the W126 standard form being 

considered, with the degree of overlap 

between these alternative standards 

depending greatly on the W126 level 

selected and the distribution of hourly 

O3 concentrations within the annual 

and/or 3-year average period.28 On this 

basis, as an initial matter, the 

Administrator recognizes that a 

secondary standard set identical to the 

proposed primary standard would 

provide a significant degree of 

additional protection for vegetation as 

compared to that provided by the 

current secondary standard. In further 

considering the significant uncertainties 

that remain in the available body of 

evidence of O3-related vegetation effects 

and in the exposure and risk analyses 

conducted for this review, and the 

difficulty in determining at what point 

various types of vegetation effects 

become adverse for sensitive vegetation 

and ecosystems, the Administrator 

focused his consideration on a level for 


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an alternative W126 standard at the 

upper end of the proposed range (i.e., 21 

ppm-hours). The Staff Paper analysis 

shows that at that W126 standard level, 

there would be essentially no counties 

with air quality that would be expected 

both to exceed such an alternative W126 

standard and to meet the revised 8-hour 

primary standard—that is, based on this 

analysis of currently monitored 

counties, a W126 standard would be 

unlikely to provide additional 

protection in any areas beyond that 

likely to be provided by the revised 

primary standard. 

The Administrator also recognizes 

that the general lack of rural monitoring 

data makes uncertain the degree to 

which the revised 8-hour standard or an 

alternative W126 standard would be 

protective, and that there would be the 

potential for not providing the 

appropriate degree of protection for 

vegetation in areas with air quality 

distributions that result in a high 

cumulative, seasonal exposure but do 

not result in high 8-hour average 

exposures. While this potential for 

under-protection is clear, the number 

and size of areas at issue and the degree 

of risk is hard to determine. However, 

such a standard would also tend to 

avoid the potential for providing more 

protection than is necessary, a risk that 

would arise from moving to a new form 

for the secondary standard despite 

significant uncertainty in determining 

the degree of risk for any exposure level 

and the appropriate level of protection, 

as well as uncertainty in predicting 

exposure and risk patterns. 

The Administrator also considered 

the views and recommendations of 

CASAC, and agrees that a cumulative, 

seasonal standard is the most 

biologically relevant way to relate 

exposure to plant growth response. 

However, as reflected in the public 

comments, the Administrator also 

recognizes that there remain significant 

uncertainties in determining or 

quantifying the degree of risk 

attributable to varying levels of O3

exposure, the degree of protection that 

any specific cumulative, seasonal 

standard would produce, and the 

associated potential for error in 

determining the standard that will 

provide a requisite degree of 

protection—i.e., sufficient but not more 

than what is necessary. Given these 

significant uncertainties, the 

Administrator concludes that 

establishing a new secondary standard 

with a cumulative, seasonal form at this 

time would result in uncertain benefits 

beyond those afforded by the revised 

primary standard and therefore may be 

more than necessary to provide the 

requisite degree of protection. 

Based on his consideration of the full 

range of views as described above, the 

Administrator judges that the 

appropriate balance to be drawn is to 

revise the secondary standard to be 

identical in every way to the revised 

primary standard. The Administrator 

believes that such a standard would be 

sufficient to protect public welfare from 

known or anticipated adverse effects, 

and does not believe that an alternative 

cumulative, seasonal standard is needed 

to provide this degree of protection. 

This judgment by the Administrator 

appropriately considers the requirement 

for a standard that is neither more nor 

less stringent than necessary for this 

purpose. 

D. Final Decision on the Secondary O3

Standard 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the Criteria 

Document and Staff Paper, the advice 

and recommendations of the CASAC 

Panel, and the public comments to date, 

the Administrator has decided to revise 

the existing 8-hour secondary standard. 

Specifically, the Administrator is 

revising the current standard by making 

it identical to the revised primary 

standard. Data handling conventions for 

the secondary standard are the same as 

for the primary standard, and are 

specified in the new Appendix P that is 

adopted, as discussed in section V 

below. Issues related to the monitoring 

requirements for the revised O3

secondary standard are discussed below 

in section VI. 

V. Creation of Appendix P— 

Interpretation of the NAAQS for O3

This section presents EPA’s final 

decisions regarding the addition of 

Appendix P to 40 CFR part 50 on 

interpreting the primary and secondary 

NAAQS for O3. EPA did not propose to 

address revocation of the existing 8- 

hour standard in this rulemaking. 

Therefore, EPA is retaining Appendix I 

to 40 CFR part 50 in its current form. A 

new Appendix P explains the 

computations necessary for determining 

when the new 8-hour primary and 

secondary standards are met. More 

specifically, Appendix P addresses data 

completeness requirements, data 

reporting and handling conventions, 

and rounding conventions, and provides 

example calculations. 

In the proposal, two alternative 

secondary standards were proposed: a 3- 

month secondary standard expressed as 

a cumulative peak-weighted index form; 

or a standard set to be identical to the 

primary standard. For reasons stated 

above, the Administrator has decided to 

set the secondary standard to be 

identical in all respects to the primary 

standard. Therefore, the portions of the 

proposed Appendix P providing data 

handling procedures for a non-identical 

secondary standard are not included in 

the final rule. 

Key elements of Appendix P are 

outlined below. 

A. General 

As proposed, EPA is adding several 

new definitions to section 1.0 and using 

these definitions throughout Appendix 

P. 

B. Data Completeness 

EPA proposed data completeness 

requirements for the new Appendix P 

for the revised 8-hour primary standard 

that would be the same as those in 

Appendix I applicable to the preexisting standard. To satisfy the data 

completeness requirement, Appendix P 

as proposed would require 90% data 

completeness, on average, for the 3-year 

period at a monitoring site, with no 

single year within the period having less 

than 75% data completeness. This data 

completeness requirement applies only 

during the required O3 monitoring 

season and must be satisfied in order to 

determine that the standard has been 

met at a monitoring site. A site could be 

found to violate the standard with less 

than complete data. EPA concluded in 

adopting these same data completeness 

requirements in Appendix I in 1997 that 

these proposed requirements are 

reasonable based on its earlier analysis 

of available air quality data that showed 

that 90% of all monitoring sites that are 

operated on a continuous basis 

routinely meet this objective. EPA 

received no comments on these 

requirements, and the final Appendix P 

includes them as proposed. 

Appendix I and the proposed 

Appendix P allow missing days to be 

counted for the purpose of meeting the 

data completeness requirements if 

meteorological conditions on these 

missing days were not conducive to 

concentrations above the level of the 

standard. Such determinations under 

Appendix I and the proposed Appendix 

P would be made on a case-by-case basis 

using available evidence. In the 

proposal, EPA specifically requested 

comment on whether meteorological 

data could provide an objective basis for 

determining, for a day for which there 

is missing data, that the meteorological 

conditions were not conducive to high 

O3 concentrations, and therefore, that 

the day could be assumed to have an O3

concentration less than the level of the 


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NAAQS. Further, the proposal 

requested comments on whether days 

assumed less than the level of the 

standard should be counted as nonmissing when computing whether the 

data completeness requirements have 

been met at the site. The proposal 

pointed out that this could allow a 

determination of attainment which 

would otherwise be precluded by the 

75% and/or 90% completeness tests. 

Most commenters supported the use of 

meteorological data to establish that 

missing days could be assumed to have 

low O3 levels. However, no commenter 

suggested any particular objective 

criteria or formula for making such 

determinations. Based on these 

comments, EPA will continue to use the 

current case-by-case approach as 

proposed in Appendix P, as is the 

current approach in Appendix I, to 

count missing days when computing 

whether the data completeness 

requirement has been met for the 

primary standard. 

As noted above, because the 

Administrator has decided to set the 

secondary standard identical in all 

respects to the primary standard, the 

final Appendix P provides that its data 

completeness requirements apply to 

both standards. 

C. Data Reporting and Handling and 

Rounding Conventions 

For reasons discussed above, the 

Administrator has set the level of the 

revised 8-hour primary and secondary 

standards at 0.075 ppm. As explained in 

the proposal, the level of the 8-hour 

standard is expressed to the third 

decimal place. Almost all State agencies 

now report hourly O3 concentrations to 

three decimal places, in ppm, or in a 

format easily convertible to ppm, since 

the typical incremental sensitivity of 

currently used O3 monitors is 0.001 

ppm. Consistent with the current 

approach for computing 8-hour 

averages, in calculating 8-hour average 

O3 concentrations from hourly data, any 

calculated digits beyond the third 

decimal place would be truncated, 

preserving the number of digits in the 

reported data. In calculating 3-year 

averages of the fourth highest maximum 

8-hour average concentrations, digits to 

the right of the third decimal place 

would also be truncated, preserving the 

number of digits in the reported data. 

Analyses discussed in the Staff Paper 

demonstrated that taking into account 

the precision and bias in 1-hour O3

measurements, the 8-hour design value 

has an uncertainty of approximately 

0.001 ppm. Truncating both the 

individual 8-hour averages used to 

determine the annual fourth maximum 

as well as the 3-year average of the 

fourth maxima to the third decimal 

place is consistent with the approach 

used in Appendix I for the previous 8- 

hour O3 standard. In the proposal, EPA 

sought comment on the appropriateness 

of rounding rather than truncating to the 

third decimal place as well as the 

scientific validity of truncating the 3- 

year average and the policy reasons 

behind either truncating or rounding the 

3-year average to the third decimal 

place. Many of the comments EPA 

received on the rounding/truncation 

issue in effect were comments that 

supported expressing the level of the 

NAAQS to either the second or third 

decimal place. These comments are 

addressed in the Response to Comment 

document. EPA continues to believe the 

conclusions from the Staff paper 

regarding monitor precision and error 

propagation when calculating 8-hour O3

averages are appropriate. EPA has 

decided to continue to truncate, as done 

in Appendix I, and this approach is 

included in the final Appendix P. 

As discussed above in section II.C.3, 

EPA is setting an 8-hour standard 

extending to three decimal places. 

Given that both the standard and the 

calculated value of the 3-year average of 

the fourth highest maximum 8-hour O3

concentration are expressed to three 

decimal places, the two values can be 

compared directly. 

As noted above, because the 

Administrator has decided to set the 

secondary standard identical in all 

respects to the primary standard, the 

same data reporting and handling and 

rounding conventions will apply to 

both. 

VI. Ambient Monitoring Related to 

Revised O3 Standards 

As noted in the O3 NAAQS proposal 

(see 72 FR 37906), EPA did not propose 

any specific changes to existing 

requirements for monitoring of O3 in the 

ambient air. However, comment was 

invited on a number of specific issues 

which were expected to be of 

significance in the event that one or 

more of the O3 NAAQS was revised. 

Comments were received from Federal 

agencies, State monitoring agencies, 

State organizations, environmental 

organizations, and industrial trade 

associations. As noted elsewhere in this 

rulemaking, EPA is finalizing changes to 

both the primary and secondary O3

NAAQS. In light of these revisions, EPA 

intends to issue a monitoring rule to 

address the issues identified in the 

proposal, as well as other issues raised 

in the comments. EPA intends to issue 

a proposed monitoring rule in June 2008 

and a final rule by March 2009. In 

recognition of the comments received 

on the proposed O3 standards and to 

provide EPA’s initial thinking on O3

specific monitoring rule amendments, 

we offer the following observations. The 

following paragraphs also point out one 

way in which some State/local 

monitoring agencies might need to make 

changes to their O3 monitoring network 

as a result of the revision to the primary 

and secondary O3 NAAQS, based on the 

existing minimum monitoring 

requirements including a factor based 

on the comparison of design value to the 

O3 NAAQS (see 71 FR 61318). The 

following text explains why an 

amendment to the monitoring 

regulations is not required to trigger 

these increased O3 monitoring 

requirements. 

Presently, States (including the 

District of Columbia, Puerto Rico, and 

the Virgin Islands, and including local 

agencies when so delegated by the State) 

are required to operate minimum 

numbers of EPA-approved O3 monitors 

based on the population of each of their 

Metropolitan Statistical Areas (MSA) 

and the most recently measured O3

levels in each area. These requirements 

are contained in 40 CFR part 58 

Appendix D, Network Design Criteria 

for Ambient Air Quality Monitoring, 

Table D–2. These requirements were last 

revised on October 17, 2006 as part of 

a comprehensive review of ambient 

monitoring requirements for all criteria 

pollutants. (See 71 FR 61236). 

The minimum number of monitors 

required in an MSA ranges from zero 

(for an area with population under 

350,000 and no recent history of an O3

design value greater than 85 percent of 

the NAAQS) to four (for an area with 

population greater than 10 million and 

an O3 design value greater than 85 

percent of the NAAQS). Because these 

requirements apply at the MSA level, 

large urban areas consisting of multiple 

MSAs can require more than four 

monitors. In total, about 400 monitors 

are required in MSAs, but about 1100 

are actually operating in MSAs because 

most States operate more than the 

minimum required number of monitors. 

As noted above, the requirements 

listed in Table D–2 of 40 CFR part 58 

Appendix D are based on the percentage 

of the O3 NAAQS, with a design value 

breakpoint at 85 percent of the NAAQS. 

For an MSA of a given population size, 

there are a greater number of required 

monitors when the design value is 

greater than or equal to 85 percent of the 

O3 NAAQS compared with MSAs that 

have a design value of less than 85 

percent of the O3 NAAQS. At the preexisting level of 0.084 ppm for the 8- 

hour primary and secondary standards, 


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29Calculated as 85 percent of 0.08 ppm, per the 

stated level of the pre-existing 8-hour primary and 

secondary standards. 

30Approximately 16 MSAs that are subject to 

minimum monitoring requirements have 8-hour 

design values between 0.064 ppm and 0.067 ppm 

based on an analysis of 2004–2006 ambient O3 data. 

an 8-hour O3 design value of 0.068 ppm 

would trigger such increased minimum 

monitoring requirements for an MSA.29

With the decision to revise the 8-hour 

primary and secondary standards to a 

level of 0.075 ppm, the 8-hour O3 design 

value that will trigger increased 

minimum monitoring requirements for 

an MSA has decreased from 0.068 ppm 

to 0.064 ppm. Therefore, MSAs with 8- 

hour design values between 0.064 ppm 

and 0.067 ppm are now required to 

increase the number of monitors 

operating to meet minimum 

requirements based on existing 

monitoring requirements.30 In practice, 

however, virtually all of these areas 

already are operating at least as many 

monitors as required based on the 

revised primary standard, so the number 

of new monitors that are needed (or 

needed to be moved from a location of 

excess monitors) is negligible to meet 

the existing minimum requirements. 

About 100 MSAs with populations 

less than 350,000 presently are without 

any O3 monitors, and hence they do not 

have an O3 design value for use with 

Table D–2. These unmonitored MSAs 

are not required to add monitors. 

Commenters from State monitoring 

agencies and State organizations 

expressed concern that these current 

requirements ignore the needs that 

States and localities will have for 

additional monitors to measure O3

levels in currently under-monitored 

areas and, in particular, in unmonitored 

areas with populations under 350,000. 

They stated that unless this deficiency 

is corrected, the health benefits of EPA’s 

O3 NAAQS revision would likely be 

limited to those living in Metropolitan 

Statistical Areas (MSAs) having 

populations of more than 350,000. Other 

commenters noted the difficulty in 

defining the boundaries of new 

attainment/non-attainment areas 

without additional monitoring in the 

MSAs below 350,000. 

EPA recognizes that the issues raised 

by the commenters are important. EPA 

intends to address these issues as part 

of its proposed monitoring rule. 

In relation to the proposed secondary 

standard options, EPA invited comment 

on whether, where, and how monitoring 

in rural areas specifically focused on the 

secondary NAAQS should be required. 

As noted in the O3 NAAQS proposal 

and described earlier in this section, 

existing O3 monitoring requirements 

and current State monitoring practices 

are primarily oriented towards 

protecting against health effects in 

people and therefore the primary 

NAAQS. This accounts for the current 

focus of the monitoring requirements on 

urban areas, where large populations 

reside, in which significant emissions of 

O3 -forming precursors are found, and 

where O3 concentrations of concern are 

likely to occur. 

There are no EPA requirements for O3

monitoring in less populated areas 

outside of MSA boundaries or in rural 

areas. However, at present there are 

about 250 O3 monitors in counties that 

are not part of MSAs. These monitors 

are operated by State, local, and tribal 

monitoring agencies for a variety of 

objectives including the assessment of 

O3 transport and the support of research 

programs including studies of 

atmospheric chemistry and ecosystem 

impacts. Additionally, EPA operates a 

network of about 56 O3 monitors as part 

of its Clean Air Status and Trends 

Network (CASTNET). The National Park 

Service (NPS) operates about 27 

monitors at other CASTNET sites. On an 

overall basis, the spatial density of nonurban O3 monitors is relatively high in 

the eastern one-third of the U.S. and in 

California, with significant gaps in 

coverage elsewhere across the country. 

Some commenters expressed concern 

about the quality assurance practices at 

CASTNET sites with regard to certain 

aspects of O3 monitoring. They 

recommended that EPA upgrade such 

practices to meet the 40 CFR part 58 

Appendix A quality assurance 

requirements already followed by the 

States so that the resulting data could be 

used in assessing compliance with the 

revised secondary standard. EPA notes 

that such upgrades have been completed 

at some of the CASTNET sites, and that 

such upgrades will be completed at all 

CASTNET sites by 2009. EPA notes that 

the resulting O3 ambient data from the 

upgraded sites will meet Appendix A 

requirements as is presently the case for 

O3 data from State operated monitors 

and NPS monitors. These data will be 

deemed acceptable for NAAQScomparison objectives and available in 

the AQS database beginning in 2008. 

Most commenters noted the relative 

lack of rural O3 monitors, stating that 

EPA should consider adding monitoring 

requirements that support a revised 

secondary O3 standard by requiring O3

monitors in locations that contain O3- 

sensitive plants or ecosystems. These 

commenters also noted that the 

placement of current O3 monitors may 

not be appropriate for evaluating 

vegetation exposure since many of these 

monitors were likely located to meet 

other objectives. 

In light of the Administrator’s 

decision to revise the 8-hour secondary 

standard, EPA believes that it is 

appropriate to consider whether the 

existing urban-based monitoring 

requirements described elsewhere in 

this section are adequate and 

appropriate to characterize the exposure 

in more rural areas where O3-sensitive 

plant species and more sensitive 

ecosystems exist and where resulting 

vegetation damage would adversely 

affect land usage. Such areas would 

likely include public lands that are 

protected areas of national interest (e.g., 

national parks, wilderness areas). 

In consideration of the spatial gaps 

that currently exist in the rural ozone 

monitoring network, and to the extent 

that the existence of such gaps has 

contributed to the overall uncertainty 

that exists in the level of protection that 

would be provided by the revised 

secondary standard, EPA believes that 

there is merit in considering whether 

additional monitoring requirements in 

certain rural areas would help support 

ongoing ecosystem research studies as 

well as future reviews of the O3 NAAQS 

by providing a more robust data set with 

which to assess the relationship of 

vegetation damage to O3 concentrations. 

Accordingly, as part of its separate 

monitoring rulemaking, EPA intends to 

consider specific requirements for a 

minimum number of rural monitors per 

State, with detailed rule language to 

ensure that States locate such monitors 

in appropriate areas. For example, these 

areas could include Federal, State, or 

Tribal lands characterized by areas of 

sensitive vegetation species subject to 

visible foliar injury, seedling and 

mature tree biomass loss, and other 

adverse impacts to a degree that could 

be considered adverse depending on the 

intended use of the plant and its 

significance to the public welfare. EPA 

is also considering recommending that 

States and Tribes employ other 

quantitative tools, such as 

photochemical modeling and/or the 

spatial interpolation of ambient data 

from existing O3 monitors, to determine 

the adequacy of existing locations of 

rural monitors and to inform the 

locations of new or relocated monitors 

that might be required to meet revised 

rural minimum monitoring 

requirements. 

Finally, EPA solicited comment on 

the issue of O3 monitoring seasons. 

Unlike the year-round monitoring 

required for other criteria pollutants, the 


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31See 40 CFR Part 58 Appendix D, section 2.5 for 

a table of required O3 seasons. 

32American Petroleum Institute v. Costle, 609 

F.2d 20 (D.C. Cir. 1979). 

33Memorandum of March 28, 2000 from John 

Seitz, ‘‘Boundary Guidance on Air Quality 

Designations for the 8-Hour Ozone National 

Ambient Air Quality Standards (NAAQS or 

Standard).’’

required O3 monitoring seasons 31 vary 

in length due to the inter-relationship of 

O3-forming photochemical activity with 

ambient temperature, strength of solar 

insolation, and length of day. For 

example, in States with colder climates 

such as Montana and South Dakota, the 

O3 season has a length of 4 months. In 

States with warmer climates such as 

California, Nevada, and Arizona, the O3

season has a length of 12 months. 

With the decision to revise the 8-hour 

primary standard to a level of 0.075 

ppm, and to set the secondary standard 

identical in all respects to the primary 

standard, the issue arises of whether in 

some areas the required O3 monitoring 

season should be made longer. EPA 

notes that under the existing 

regulations, the Regional Administrator 

may approve State-requested deviations 

from the established O3 monitoring 

season, but EPA may not increase the 

length of the season for an area at EPA’s 

own initiative other than by notice and 

comment rulemaking. 

EPA has done a preliminary analysis 

of 2004–2006 ambient data to address 

the issue of whether extensions of 

currently required O3 monitoring 

seasons are appropriate in light of the 

revised level for the primary and 

secondary O3 standards and the revised 

breakpoints for the AQI. The results of 

the analysis demonstrated that out-ofseason exceedances of the revised level 

occurred in eight States during the 

study period. Additionally, the 

frequency of days with O3

concentrations that reached the revised 

Moderate AQI category (based on a 

breakpoint of 0.060 ppm) was much 

greater compared with the frequency of 

days with concentrations that reached 

the pre-existing Moderate AQI category 

(based on a breakpoint of 0.065 ppm). 

This increased frequency of days with 

Moderate AQI levels was noted to occur 

during periods before and after the 

currently required O3 seasons. 

Based on these preliminary analyses, 

EPA intends to consider changes to the 

length of the required O3 season for the 

coming monitoring rulemaking. Such 

changes could be based solely on the 

frequency of exceedances of the revised 

primary and secondary standards, or 

could also consider the frequency of 

concentrations in the Moderate category 

of the AQI. 

VII. Implementation and Related 

Control Requirements 

A. Future Implementation Steps 

In today’s rule, EPA is replacing the 

existing (1997) standards with revised 

primary and secondary O3 standards. 

However, the 1997 standards—and the 

implementation rules for those 

standards—will remain in place for 

implementation purposes as EPA 

undertakes rulemaking to address the 

transition from the 1997 O3 standards to 

the 2008 O3 standards. States are 

required to continue to develop and 

implement their State Implementation 

Plans (SIPs) for the 1997 standards as 

they begin the process of recommending 

designations for the 2008 standards. 

1. Designations 

After EPA establishes or revises a 

NAAQS, the CAA requires EPA and 

States to begin taking steps to ensure 

that the new or revised standards are 

met. The first step is to identify areas of 

the country that do not attain the new 

or revised standards, or that contribute 

to violations of the new or revised 

standards. Section 107(d)(1) provides 

‘‘By such date as the Administrator may 

reasonably require, but not later than 1 

year after promulgation of a new or 

revised national ambient air quality 

standard for any pollutant under section 

109, the Governor of each State shall 

* * * submit to the Administrator a list 

of all areas (or portions thereof) in the 

State’’ that designates those areas as 

non-attainment, attainment, or 

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

further provides, ‘‘Upon promulgation 

or revision of a national ambient air 

quality standard, the Administrator 

shall promulgate the designations of all 

areas (or portions thereof) * * * as 

expeditiously as practicable, but in no 

case later than 2 years from the date of 

promulgation. Such period may be 

extended for up to one year in the event 

the Administrator has insufficient 

information to promulgate the 

designations.’’

The term ‘‘promulgation’’ has been 

interpreted by the courts to be signature 

and dissemination of a rule.32 As noted 

above, the CAA requires EPA to 

establish a deadline for the States’

submission of the designation 

recommendations, but under the CAA, 

it can be no later than March 12, 2009, 

one year after the promulgation of this 

rule. Therefore, Governors of States 

should submit their designation 

recommendations to EPA no later than 

March 12, 2009. EPA’s promulgation of 

designations must occur no later than 

March 12, 2010, although that date may 

be extended by up to one year under the 

CAA (no later than March 12, 2011) if 

EPA has insufficient information to 

promulgate the designations. 

EPA intends to provide additional 

guidance to the States concerning the 

technical considerations for establishing 

boundaries for designated areas. For the 

revised primary and secondary 

standards, we anticipate relying on past 

O3 designation guidance issued by EPA 

prior to the designations for the 1997 O3

standards.33 We anticipate working 

closely with State air agencies and 

Tribes on establishing new guidance on 

designations, if needed. 

2. State Implementation Plans 

CAA section 110 provides the general 

requirements for SIPs. Within 3 years 

after the promulgation of new or revised 

NAAQS (or such shorter period as the 

Administrator may prescribe) each State 

must adopt and submit ‘‘infrastructure’’

SIPs to EPA to address the requirements 

of section 110(a)(1). Thus, States should 

submit these SIPs no later than March 

12, 2011. These ‘‘infrastructure SIPs’’

provide assurances of State resources 

and authorities, and establish the basic 

State programs, to implement, maintain, 

and enforce new or revised standards. 

In addition to the infrastructure SIPs, 

which apply to all States, CAA title I, 

part D outlines the State requirements 

for achieving clean air in designated 

nonattainment areas. These 

requirements include timelines for 

when designated nonattainment areas 

must attain the standards, deadlines for 

developing SIPs that demonstrate how 

the State will ensure attainment of the 

standards, and specific emissions 

control requirements. EPA plans to 

address how these requirements, such 

as attainment demonstrations and 

attainment dates, reasonable further 

progress, new source review, 

conformity, and other implementation 

requirements, apply to the revised O3

NAAQS in a proposed rulemaking in 

Fall 2008. Also in that rulemaking EPA 

will establish deadlines for submission 

of nonattainment area SIPs but 

anticipates that the deadlines will be no 

later than 3 years after final designation. 

Depending on the classification of an 

area, the SIP must provide for 

attainment within 3 years (for areas 

classified marginal) to 20 years (for 

areas classified extreme) after final 

designations. 

3. Trans-boundary Emissions 

Cross border O3 contributions from 

within North America (Canada and 

Mexico) entering the U.S. are generally 

thought to be small. Section 179B of the 


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34National Emission Inventory posted at the 

following Web site: http://www.epa.gov/ttn/chief/ 

trends/index.html. 

35 In some cases natural emissions may cause or 

significantly contribute to violations of the ozone 

standard. EPA has issued rules that address how 

these ‘‘exceptional events’’ can be discounted in 

regulatory determinations. The Exceptional Events 

Rule (72 FR 13560 (March 22, 2007) implements 

CAA section 319(b)(3)(B) and section 107(d)(3) 

authority to exclude air quality monitoring data 

from regulatory determinations related to 

exceedances or violations of the National Ambient 

Air Quality Standards (NAAQS). If an event is 

determined by EPA to be a qualifying exceptional 

event, the affected area may avoid being designated 

as nonattainment, being redesignated as 

nonattainment, or being reclassifed to a higher 

classification. The requirements for demonstrating 

that elevated ozone levels are the result of a 

qualifying exceptional event are provided in the 

Exceptional Events Rule. 

Clean Air Act allows designated 

nonattainment areas to petition EPA to 

consider whether such a locality might 

have met a clean air standard ‘‘but for’’

cross border contributions. To date, few 

areas have petitioned EPA under this 

authority. The impact of foreign 

emissions on domestic air quality in the 

United States is a challenging and 

complex problem to assess. EPA is 

engaged in a number of activities to 

improve our understanding of 

international transport. As work 

progresses on these activities, EPA will 

be able to better address the 

uncertainties associated with transboundary flows of air pollution and 

their impacts. 

4. Monitoring Requirements 

As discussed more fully in section VI, 

EPA intends, in light of the revisions of 

the O3 standards, to issue a monitoring 

rule to address a variety of monitoringrelated issues identified in the preamble 

to the proposed rule or in comments 

received by the Agency on the proposal. 

EPA intends to issue a proposed 

monitoring rule in June 2008 and a final 

rule by March 2009. 

B. Related Control Requirements 

The man-made oxides of nitrogen 

(NOX) and volatile organic carbon (VOC) 

emissions that contribute to O3

formation in the United States come 

from a variety of source categories, 

including mobile sources, industrial 

processes, area-wide sources (which 

include consumer and commercial 

products), and the electric power 

industry.34 Emissions from natural 

sources, such as trees and wildfires can 

also constitute a significant portion of 

total VOC emissions in certain regions 

of the country, especially during the O3

season. Natural sources such as 

wildfires, lightning, and soils also emit 

NOX. Emissions of VOCs and NOX from 

these sources are considered natural 

background emissions.35

EPA has developed new emissions 

standards for many types of stationary 

sources and for nearly every class of 

mobile sources in the last decade to 

reduce O3 by decreasing emissions of 

NOX and VOC. These programs 

complement State and local efforts to 

improve air quality and to meet the 

national O3 standards. Under the 

Federal Motor Vehicle Control Program 

(FMVCP, see title II of the CAA, 42 

U.S.C. 7521–7574), EPA has established 

new emissions standards for nearly 

every type of automobile, truck, bus, 

motorcycle, earth mover, and aircraft 

engine, and for the fuels used to power 

these engines. Also, EPA established 

new standards for the smaller engines 

used in small watercraft, lawn and 

garden equipment. Recently, EPA 

proposed new standards for locomotive 

and marine diesel engines. Vehicles and 

engines are replaced over time with 

newer, cleaner models. In time, these 

programs will yield substantial 

emissions reductions. Emissions 

reductions associated with fuel 

programs generally begin as soon as a 

new fuel is available. 

The reduction of VOC emissions from 

industrial processes and consumer and 

commercial product categories has been 

achieved either directly or indirectly 

through implementation of control 

technology standards, including 

reasonably available control technology, 

best available control technology, and 

maximum achievable control 

technology standards; or is anticipated 

due to proposed or upcoming proposals 

based on generally available control 

technology or best available controls 

under provisions related to consumer 

and commercial products. These 

standards have resulted in VOC 

emissions reductions of almost a million 

tons per year accumulated starting in 

1997 from a variety of sources including 

combustion sources, coating categories, 

and chemical manufacturing. In 2006 

and 2007, EPA issued national rules and 

control techniques guidelines for 

control of VOC emissions from 10 

categories of consumer and commercial 

products. EPA is currently working to 

finalize new Federal rules, or 

amendments to existing rules, intended 

to establish new nationwide VOC 

content limits for several categories of 

consumer and commercial products, 

including aerosol coatings, architectural 

and industrial maintenance coatings, 

and household and institutional 

commercial products. EPA anticipates 

that final rules addressing emissions 

from these sources will take effect in 

2009. 

Fuel combustion is one of the largest 

anthropogenic sources of emissions of 

NOX in the United States. Power 

industry emission sources include large 

electric generating units and some large 

industrial boilers and turbines. The 

EPA’s landmark Clean Air Interstate 

Rule (CAIR), issued on March 10, 2005, 

permanently caps power industry 

emissions of NOX in the eastern United 

States. The first phase of the cap begins 

in 2009, and a lower second phase cap 

begins in 2015. By 2015, EPA projects 

that the CAIR and other programs in the 

Eastern U.S. will reduce power industry 

annual NOX emissions in that region by 

about 60 percent from 2003 levels. 

With respect to agricultural sources, 

the U.S. Department of Agriculture 

(USDA) has recommended conservation 

systems and activities that can reduce 

agricultural emissions of NOX and VOC. 

Current practices that may reduce 

emissions of NOX and VOC include 

engine replacement programs, 

management of pesticide applications, 

and manure management techniques. 

The EPA recognizes that USDA has been 

working with the agricultural 

community to plan conservation 

systems and activities to manage 

emissions of O3 precursors. 

These conservation systems and 

activities can be voluntarily adopted in 

areas where mitigation of O3 precursors 

have been identified as an air quality 

concern through the use of incentives 

provided to the agricultural producer. In 

cases where the States need these 

measures to attain the O3 standards, 

agricultural producers could choose to 

adopt these measures. The EPA will 

continue to work with USDA on 

planning the implementation of these 

conservation systems and activities in 

order to identify and/or improve 

mitigation efficiencies, prioritize their 

adoption, and ensure that appropriate 

criteria are used for identifying the most 

effective application of conservation 

systems and activities. 

The EPA will work together with 

USDA and with States to identify 

appropriate measures to meet the 

primary and secondary standards, 

including site-specific conservation 

systems and activities. Based on prior 

experience identifying conservation 

measures and practices to meet the PM 

NAAQS requirements, the EPA will use 

a similar process to identify measures 

that could meet the O3 requirements. 

The EPA anticipates that certain USDAapproved conservation systems and 

activities that reduce agricultural 

emissions of NOX and VOC may be able 

to satisfy the requirements for 


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applicable sources to implement 

reasonably available control measures 

for purposes of attaining the primary 

and secondary O3 NAAQS. 

VIII. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under section 3(f)(1) of Executive 

Order (EO) 12866 (58 FR 51735, October 

4, 1993), this action is an ‘‘economically 

significant regulatory action’’ because it 

is likely to have an annual effect on the 

economy of $100 million or more. 

Accordingly, EPA submitted this action 

to the Office of Management and Budget 

(OMB) for review under EO 12866 and 

any changes made in response to OMB 

recommendations have been 

documented in the docket for this 

action. In addition, EPA prepared an 

analysis of the potential costs and 

benefits associated with this action. 

This analysis is contained in the Final 

Ozone NAAQS Regulatory Impact 

Analysis, March 2008 (henceforth, 

‘‘RIA’’). A copy of the analysis is 

available in the RIA docket (EPA–HQ–

OAR–2007–0225) and the analysis is 

briefly summarized here. The RIA 

estimates the costs and monetized 

human health and welfare benefits of 

attaining three alternative O3 NAAQS 

nationwide. Specifically, the RIA 

examines the alternatives of 0.079 ppm, 

0.075 ppm, 0.070 ppm, and 0.065 ppm. 

The RIA contains illustrative analyses 

that consider a limited number of 

emissions control scenarios that States 

and Regional Planning Organizations 

might implement to achieve these 

alternative O3 NAAQS. However, the 

CAA and judicial decisions make clear 

that the economic and technical 

feasibility of attaining ambient 

standards are not to be considered in 

setting or revising NAAQS, although 

such factors may be considered in the 

development of State plans to 

implement the standards. Accordingly, 

although a RIA has been prepared, the 

results of the RIA have not been 

considered in issuing this final rule. 

B. Paperwork Reduction Act 

This action does not impose an 

information collection burden under the 

provisions of the Paperwork Reduction 

Act, 44 U.S.C. 3501 et seq. There are no 

information collection requirements 

directly associated with the 

establishment of a NAAQS under 

section 109 of the CAA. 

Burden means the total time, effort, or 

financial resources expended by persons 

to generate, maintain, retain, or disclose 

or provide information to or for a 

Federal agency. This includes the time 

needed to review instructions; develop, 

acquire, install, and utilize technology 

and systems for the purposes of 

collecting, validating, and verifying 

information, processing and 

maintaining information, and disclosing 

and providing information; adjust the 

existing ways to comply with any 

previously applicable instructions and 

requirements; train personnel to be able 

to respond to a collection of 

information; search data sources; 

complete and review the collection of 

information; and transmit or otherwise 

disclose the information. 

An agency may not conduct or 

sponsor, and a person is not required to 

respond to a collection of information 

unless it displays a currently valid OMB 

control number. The OMB control 

numbers for EPA’s regulations in 40 

CFR are listed in 40 CFR part 9. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of this rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of this final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule establishes national standards 

for allowable concentrations of O3 in 

ambient air as required by section 109 

of the CAA. American Trucking Ass’ns 

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

cir. 1999) (NAAQS do not have 

significant impacts upon small entities 

because NAAQS themselves impose no 

regulations upon small entities). 

D. Unfunded Mandates Reform Act 

Title II of the Unfunded Mandates 

Reform Act of 1995 (UMRA), Public 

Law 104–4, establishes requirements for 

Federal agencies to assess the effects of 

their regulatory actions on State, local, 

and Tribal governments and the private 

sector. Under section 202 of the UMRA, 

EPA generally must prepare a written 

statement, including a cost-benefit 

analysis, for proposed and final rules 

with ‘‘Federal mandates’’ that may 

result in expenditures to State, local, 

and Tribal governments, in the 

aggregate, or to the private sector, of 

$100 million or more in any one year. 

Before promulgating an EPA rule for 

which a written statement is needed, 

section 205 of the UMRA generally 

requires EPA to identify and consider a 

reasonable number of regulatory 

alternatives and to adopt the least 

costly, most cost-effective or least 

burdensome alternative that achieves 

the objectives of the rule. The 

provisions of section 205 do not apply 

when they are inconsistent with 

applicable law. Moreover, section 205 

allows EPA to adopt an alternative other 

than the least costly, most cost-effective 

or least burdensome alternative if the 

Administrator publishes with the final 

rule an explanation why that alternative 

was not adopted. Before EPA establishes 

any regulatory requirements that may 

significantly or uniquely affect small 

governments, including Tribal 

governments, it must have developed 

under section 203 of the UMRA a small 

government agency plan. The plan must 

provide for notifying potentially 

affected small governments, enabling 

officials of affected small governments 

to have meaningful and timely input in 

the development of EPA regulatory 

proposals with significant Federal 

intergovernmental mandates, and 

informing, educating, and advising 

small governments on compliance with 

the regulatory requirements. 

This final rule contains no Federal 

mandates (under the regulatory 

provisions of Title II of the UMRA) for 

State, local, or Tribal governments or 

the private sector. The rule imposes no 

new expenditure or enforceable duty on 

any State, local or Tribal governments or 

the private sector, and EPA has 

determined that this rule contains no 

regulatory requirements that might 

significantly or uniquely affect small 

governments. Furthermore, as indicated 

previously, in setting a NAAQS EPA 

cannot consider the economic or 

technological feasibility of attaining 

ambient air quality standards, although 

such factors may be considered to a 

degree in the development of State 


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plans to implement the standards. See 

also American Trucking Ass’ns v. EPA, 

175 F. 3d at 1043 (noting that because 

EPA is precluded from considering costs 

of implementation in establishing 

NAAQS, preparation of a Regulatory 

Impact Analysis pursuant to the 

Unfunded Mandates Reform Act would 

not furnish any information which the 

court could consider in reviewing the 

NAAQS). Thus, this rule is not subject 

to the requirements of sections 202 and 

205 of the UMRA. EPA has determined 

that this rule contains no regulatory 

requirements that might significantly or 

uniquely affect small governments. 

E. Executive Order 13132: Federalism 

Executive Order 13132, entitled 

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

1999), requires EPA to develop an 

accountable process to ensure 

‘‘meaningful and timely input by State 

and local officials in the development of 

regulatory policies that have federalism 

implications.’’ ‘‘Policies that have 

federalism implications’’ is defined in 

the Executive Order to include 

regulations that have ‘‘substantial direct 

effects on the States, on the relationship 

between the national government and 

the States, or on the distribution of 

power and responsibilities among the 

various levels of government.’’

This final rule does not have 

federalism implications. It will not have 

substantial direct effects on the States, 

on the relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. The rule does 

not alter the relationship between the 

Federal government and the States 

regarding the establishment and 

implementation of air quality 

improvement programs as codified in 

the CAA. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, CAA section 116 preserves the 

rights of States to establish more 

stringent requirements if deemed 

necessary by a State. Furthermore, this 

rule does not impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. Finally, 

as noted in section E (above) on UMRA, 

this rule does not impose significant 

costs on State, local, or Tribal 

governments or the private sector. Thus, 

Executive Order 13132 does not apply 

to this rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

Executive Order 13175, entitled 

‘‘Consultation and Coordination with 

Indian Tribal Governments’’ (65 FR 

67249, November 9, 2000), requires EPA 

to develop an accountable process to 

ensure ‘‘meaningful and timely input by 

tribal officials in the development of 

regulatory policies that have tribal 

implications.’’ This final rule does not 

have Tribal implications, as specified in 

Executive Order 13175. It does not have 

a substantial direct effect on one or 

more Indian Tribes, since Tribes are not 

obligated to adopt or implement any 

NAAQS. Thus, Executive Order 13175 

does not apply to this rule. 

Although Executive Order 13175 does 

not apply to this rule, EPA contacted 

Tribal environmental professionals 

during the development of this rule. 

EPA staff participated in the regularly 

scheduled Tribal Air call sponsored by 

the National Tribal Air Association 

during the spring of 2007 as the 

proposal was under development. EPA 

specifically solicited additional 

comment on the proposed rule from 

Tribal officials. Comments from Tribal 

officials on the proposed rule are 

summarized in the Response to 

Comments document. 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

Executive Order 13045, ‘‘Protection of 

Children from Environmental Health 

Risks and Safety Risks’’ (62 FR 19885, 

April 23, 1997) applies to any rule that: 

(1) Is determined to be ‘‘economically 

significant’’ as defined under Executive 

Order 12866, and (2) concerns an 

environmental health or safety risk that 

EPA has reason to believe may have a 

disproportionate effect on children. If 

the regulatory action meets both criteria, 

the Agency must evaluate the 

environmental health or safety effects of 

the planned rule on children, and 

explain why the planned regulation is 

preferable to other potentially effective 

and reasonably feasible alternatives 

considered by the Agency. 

This final rule is subject to Executive 

Order 13045 because it is an 

economically significant regulatory 

action as defined by Executive Order 

12866, and we believe that the 

environmental health risk addressed by 

this action may have a disproportionate 

effect on children. Accordingly, we have 

evaluated the environmental health or 

safety effects of exposure to O3 pollution 

among children. These effects and the 

size of the population affected are 

summarized in section 8.7 of the 

Criteria Document and section 3.6 of the 

Staff Paper, and the results of our 

evaluation of the effects of O3 pollution 

on children are discussed in sections 

II.A–C of this preamble. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

Executive Order 13211, ‘‘Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

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

22, 2001)), requires EPA to prepare and 

submit a Statement of Energy Effects to 

the Administrator of the Office of 

Information and Regulatory Affairs, 

Office of Management and Budget, for 

certain actions identified as ‘‘significant 

energy actions.’’ Section 4(b) of 

Executive Order 13211 defines 

‘‘significant energy actions’’ as ‘‘any 

action by an agency (normally 

published in the Federal Register) that 

promulgates or is expected to lead to the 

promulgation of a final rule or 

regulation, including notices of inquiry, 

advance notices of proposed 

rulemaking, and notices of proposed 

rulemaking: (1)(i) That is a significant 

regulatory action under Executive Order 

12866 or any successor order, and (ii) is 

likely to have a significant adverse effect 

on the supply, distribution, or use of 

energy; or (2) that is designated by the 

Administrator of the Office of 

Information and Regulatory Affairs as a 

significant energy action.’’ The U.S. 

Office of Management and Budget has 

designated this rulemaking as a 

significant energy action. Accordingly, 

EPA has prepared a Statement of Energy 

Effects for this action which appears in 

Chapter 9 of the RIA conducted for this 

rulemaking. A copy of the RIA is 

available in the RIA docket (EPA–HQ–

OAR–2007–0225) and the energy 

analysis is briefly summarized here. The 

analysis estimates potential impacts of 

an illustrative control strategy for the 

0.070 ppm primary standard alternative 

on the production of coal, crude oil, 

natural gas, and electricity; on energy 

prices; on control technologies adopted 

by the electricity generating sector; and 

on the mix of electricity generation. EPA 

believes that the energy impacts 

estimated for this illustrative control 

strategy for the 0.070 ppm primary 

standard alternative are higher than 

those that would be estimated for an 

illustrative control strategy for the 

primary standard level of 0.075 ppm 

which was selected by the 

Administrator. However, due to 

modeling limitations, EPA did not 

generate separate estimates of the energy 

impacts associated specifically with an 


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illustrative control strategy designed for 

a primary standard of 0.075 ppm. It is 

important to note that the CAA make 

clear that the economic impacts 

associated with attaining ambient 

standards are not to be considered in 

setting or revising the NAAQS. 

Accordingly, although the Statement of 

Energy Effects has been prepared, the 

results of EPA’s energy analysis have 

not been considered in issuing this final 

rule. 

I. National Technology Transfer and 

Advancement Act 

As noted in the proposed rule, section 

12(d) of the National Technology 

Transfer and Advancement Act of 1995 

(NTTAA), Public Law 104–113, section 

12(d) (15 U.S.C. 272 note) directs EPA 

to use voluntary consensus standards in 

its regulatory activities unless to do so 

would be inconsistent with applicable 

law or otherwise impractical. Voluntary 

consensus standards are technical 

standards (e.g., materials specifications, 

test methods, sampling procedures, and 

business practices) that are developed or 

adopted by voluntary consensus 

standards bodies. The NTTAA directs 

EPA to provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

This action does not involve technical 

standards. Therefore, EPA did not 

consider the use of any voluntary 

consensus standards. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629; 

Feb. 16, 1994) establishes Federal 

executive policy on environmental 

justice. Its main provision directs 

Federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this final 

rule will not have disproportionately 

high and adverse human health or 

environmental effects on minority or 

low-income populations because it 

increases the level of environmental 

protection for all affected populations 

without having any disproportionately 

high and adverse human health or 

environmental effects on any 

population, including any minority or 

low-income population. This final rule 

will establish uniform national 

standards for O3 air pollution. 

K. Congressional Review Act 

The Congressional Review Act, 5 

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

Business Regulatory Enforcement 

Fairness Act of 1996, generally provides 

that before a rule may take effect, the 

agency promulgating the rule must 

submit a rule report, which includes a 

copy of the rule, to each House of the 

Congress and to the Comptroller General 

of the United States. EPA submitted a 

report containing this rule and other 

required information to the U.S. Senate, 

the U.S. House of Representatives, and 

the Comptroller General of the United 

States prior to publication of the rule in 

the Federal Register. A major rule 

cannot take effect until 60 days after it 

is published in the Federal Register. 

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

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

effective May 27, 2008. 

References 

Abt Associates Inc. (2007a) Ozone Health 

Risk Assessment for Selected Urban Areas. 

Prepared for Office of Air Quality Planning 

and Standards, U.S. Environmental 

Protection Agency, Research Triangle Park, 

NC. July 2007; EPA report no. EPA–452/R–

07–009. Available online at: http:// 

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

s_o3_cr_td.html. 

Abt Associates Inc. (2007b) Technical Report 

on Ozone Exposure, Risk, and Impacts 

Assessments for Vegetation: Final Report. 

Prepared for Office of Air Quality Planning 

and Standards, U.S. Environmental 

Protection Agency, Research Triangle Park, 

NC. January 2007; EPA report no. EPA–

452/R–07–002. Available online at: http:// 

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

s_o3_cr_td.html. 

Adams, W. C. (2002) Comparison of chamber 

and face-mask 6.6-hour exposures to ozone 

on pulmonary function and symptoms 

responses. Inhalation Toxicol. 14: 745–764. 

Adams, W. C. (2003a) Comparison of 

chamber and face mask 6.6-hour exposure 

to 0.08 ppm ozone via square-wave and 

triangular profiles on pulmonary 

responses. Inhalation Toxicol. 15: 265–281. 

Adams, W. C. (2003b) Relation of pulmonary 

responses induced by 6.6 hour exposures 

to 0.08 ppm ozone and 2-hour exposures 

to 0.30 ppm via chamber and face-mask 

inhalation. Inhalation Toxicol. 15: 745–

759. 

Adams, W. C. (2006) Comparison of chamber 

6.6 hour exposures to 0.04–0.08 ppm ozone 

via square-wave and triangular profiles on 

pulmonary responses. Inhalation Toxicol. 

18: 127–136. 

Alliance of Automobile Manufacturers 

(AAM) (2007) Letter and Document Sent to 

Stephen L. Johnson re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4191. October 9, 2007. 

American Academy of Pediatrics (2007) 

Letter and Comments Sent to Docket No. 

OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4570. October 10, 2007. 

American Association of State Highway and 

Transportation Officials (AASHTO) (2007) 

Letter and Comments Sent to Docket No. 

OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4325. October 9, 2007. 

American Chemistry Council (ACC) (2007) 

Letter and Comments Sent to Docket No. 

OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4159. October 9, 2007. 

American Enterprise Institute (2007) Letter 

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2005–0172 re: Proposed Rule—National 

Ambient Air Quality Standards for Ozone. 

Docket No. OAR–2005–0172–4312. 

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American Electric Power (2007) Letter and 

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American Heart Association (2007) Letter 

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American Lung Association/Environmental 

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American Nurses Association (2007) Letter 

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American Thoracic Society/American 

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Arizona Department of Environmental 

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Arkansas Department of Environmental 

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Delaware, State of (2007) Letter and 

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2007. 


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New York State Department of 

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New Mexico Environment Department Air 

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North Carolina Department of Environment 

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Northeast States for Coordinated Air Use 

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Oklahoma Department of Transportation 

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National Ambient Air Quality Standards 

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4365. October 5, 2007. 

Oregon Department of Environmental Quality 

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4014. September 20, 2007. 

Ozone Transport Commission (OTC) (2007) 

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4135. October 5, 2007. 

Pennsylvania Department of Environmental 

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Proposed Rule—National Ambient Air 

Quality Standards for Ozone. Docket No. 

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Percy, K. E.; Nosal, M.; Heilman, W.; Dann, 

T; Sober, J.; Legge, A. H.; Karnosky, D. F. 

(2007) New exposure-based metric 

approach for evaluating O3 risk to North 

American aspen forests. Environmental 

Pollution 147:3 554–566. 

Retzlaff, W. A.; Arthur, M. A.; Grulke, N. E.; 

Weinstein, D. A.; Gollands, B. (2000) Use 

of a single-tree simulation model to predict 

effects of ozone and drought on growth of 

a white fir tree. Tree Physiol. 20: 195–202. 

Rizzo, M (2005). Evaluation of a quadratic 

approach for adjusting distributions of 

hourly ozone concentrations to meet air 

quality standards. November 7, 2005. 

Available online at: http://www.epa.gov/ 

ttn/naaqs/standards/ozone/ 

s_o3_cr_td.html. 

Rizzo, M. (2006). A distributional 

comparison between different rollback 

methodologies applied to ambient ozone 

concentrations. August 23, 2006. Available 

online at: http://www.epa.gov/ttn/naaqs/ 

standards/ozone/s_o3_cr_td.html. 

Rochester Report (2007) Critical 

Considerations in Evaluating Scientific 

Evidence of Health Effects of Ambient 

Ozone: Report of a Working Conference 

held in Rochester, NY, June 5, 2007. Sent 

as an attachment by Roger McClellan to 

Docket No. OAR–2005–0172 re: Proposed 

Rule—National Ambient Air Quality 

Standards for Ozone. Docket No. OAR–

2005–0172–4727. October 9 2007. 

Rocky Mountain Clean Air Action (2007) 

Letter and Comments Sent to Docket No. 

OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4428. October 9 2007. 

Sarnat, J. A.; Schwartz, J.; Catalano, P. J.; Suh, 

H. H. (2001) Gaseous pollutants in 

particulate matter epidemiology: 

confounders or surrogates? Environ. Health 

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Coull, B. A.; Koutrakis, P. (2005) Ambient 

gas concentrations and personal particulate 

matter exposures: implications for studying 

the health effects of particles. 

Epidemiology 16: 385–395. 

Sarnat, J. A.; Coull, B. A.; Schwartz, J; Gold, 

D. R.; Suh, H. H. (2006) Factors affecting 

the association between ambient 

concentrations and personal exposure to 

particles and gases. Environ. Health 

Perspect. 114(5):649–654. 

Sasek, T. W.; Richardson, C. J.; Fendick, E. 

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Carryover effects of acid rain and ozone on 

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Schwartz, J. (2005) How sensitive is the 

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deaths to control for temperature? Am. J. 

Respir. Crit. Care Med. 171: 627–631. 

Schildcrout, J. S.; Sheppard, L.; Lumley, T.; 

Slaughter, J. C.; Koenig, J. Q.; Shapiro, G. 

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National Ambient Air Quality Standards 

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Federal Register / Vol. 73, No. 60 / Thursday, March 27, 2008 / Rules and Regulations 16511 

December 2006). National Plant Data 

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OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4768. October 9, 2007. 

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Air Quality Standards for Ozone. Docket 

No. OAR–2005–0172–4157. October 8, 

2007. 

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No. OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4267. October 9, 2007. 

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(1991) ‘‘Modeling changes in red spruce 

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to interacting ozone and nutrient stress.’’

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J.S. Kern, E.H. Lee, W.E. Hogsett, J. Weber 

(2005) Predicting the effects of 

tropospheric ozone on regional 

productivity of ponderosa pine and white 

fir. Forest Ecology and Management 205: 

73–89. 

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(2007) Letter and Comments Sent to Docket 

No. OAR–2005–0172 re: Proposed Rule—

National Ambient Air Quality Standards 

for Ozone. Docket No. OAR–2005–0172–

4358. October 9, 2007. 

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Keisler, J. (1996) A Probabilistic 

Assessment of Health Risks Associated 

with Short- and Long-Term Exposure to 

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Assessment of Health Risks Associated 

with Short-term Exposure to Tropospheric 

Ozone: A Supplement. Argonne National 

Laboratory, Argonne, IL. 

Whitfield, R.G.; Richmond, H.M.; and 

Johnson, T.R. (1998) ‘‘Overview of Ozone 

Human Exposure and Health Risk Analyses 

Used in the U.S. EPA’s Review of the 

Ozone Air Quality Standard,’’ pp.483–516 

in: T. Schneider, ed. Air Pollution in the 

21st Century: Priority Issues and Policy 

Elsevier; Amsterdam. 

Wolff, G.T. (1995) Letter from Chairman of 

Clean Air Scientific Advisory Committee to 

the EPA Administrator, dated November 

30, 1995. EPA–SAB–CASAC–LTR–96–002. 

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Clean Air Scientific Advisory Committee to 

the EPA Administrator, dated April 4, 

1996. EPA–SAB–CASAC–LTR–96–006. 

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framework for assessing and reporting on 

ecological condition: an SAB report. 

Washington, DC: U.S. Environmental 

Protection Agency, Science Advisory 

Board; report no. EPA–SAB–EPEC–02–009. 

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yosemite.epa.gov/sab/sabproduct.nsf/ 

C3F89E598D843B58852570CA0075717E/ 

$File/epec02009a.pdf. 

List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 58 

Environmental protection, Air 

pollution control, Reporting and 

recordkeeping requirements. 

Dated: March 12, 2008. 

Stephen L. Johnson, 

Administrator. 

■ For the reasons stated in the preamble, 

title 40, chapter I of the code of Federal 

regulations is to be amended as follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

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

■ 2. Section 50.15 is added to read as 

follows: 

§ 50.15 National primary and secondary 

ambient air quality standards for ozone. 

(a) The level of the national 8-hour 

primary and secondary ambient air 

quality standards for ozone (O3) is 0.075 

parts per million (ppm), daily maximum 

8-hour average, measured by a reference 

method based on Appendix D to this 

part and designated in accordance with 

part 53 of this chapter or an equivalent 

method designated in accordance with 

part 53 of this chapter. 

(b) The 8-hour primary and secondary 

O3 ambient air quality standards are met 

at an ambient air quality monitoring site 

when the 3-year average of the annual 

fourth-highest daily maximum 8-hour 

average O3 concentration is less than or 

equal to 0.075 ppm, as determined in 

accordance with Appendix P to this 

part. 

■ 3. Appendix P is added to read as 

follows: 

Appendix P to Part 50—Interpretation 

of the Primary and Secondary National 

Ambient Air Quality Standards for 

Ozone 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining whether the 

national 8-hour primary and secondary 

ambient air quality standards for ozone (O3) 

specified in § 50.15 are met at an ambient O3

air quality monitoring site. Ozone is 

measured in the ambient air by a reference 

method based on Appendix D of this part, as 

applicable, and designated in accordance 

with part 53 of this chapter, or by an 

equivalent method designated in accordance 

with part 53 of this chapter. Data reporting, 

data handling, and computation procedures 

to be used in making comparisons between 

reported O3 concentrations and the levels of 

the O3 standards are specified in the 

following sections. Whether to exclude, 

retain, or make adjustments to the data 

affected by exceptional events, including 

stratospheric O3 intrusion and other natural 

events, is determined by the requirements 

under §§ 50.1, 50.14 and 51.930. 

(b) The terms used in this appendix are 

defined as follows: 

8-hour average is the rolling average of 

eight hourly O3 concentrations as explained 

in section 2 of this appendix. 

Annual fourth-highest daily maximum 

refers to the fourth highest value measured at 

a monitoring site during a particular year. 

Daily maximum 8-hour average 

concentration refers to the maximum 

calculated 8-hour average for a particular day 

as explained in section 2 of this appendix. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

shown in section 3 of this appendix. 

O3 monitoring season refers to the span of 

time within a calendar year when individual 

States are required to measure ambient O3

concentrations as listed in part 58 Appendix 

D to this chapter. 

Year refers to calendar year. 

2. Primary and Secondary Ambient Air 

Quality Standards for Ozone 

2.1 Data Reporting and Handling 

Conventions 

Computing 8-hour averages. Hourly 

average concentrations shall be reported in 

parts per million (ppm) to the third decimal 

place, with additional digits to the right of 

the third decimal place truncated. Running 8- 

hour averages shall be computed from the 

hourly O3 concentration data for each hour 

of the year and shall be stored in the first, 

or start, hour of the 8-hour period. An 8-hour 

average shall be considered valid if at least 

75% of the hourly averages for the 8-hour 

period are available. In the event that only 6 

or 7 hourly averages are available, the 8-hour 

average shall be computed on the basis of the 

hours available using 6 or 7 as the divisor. 

8-hour periods with three or more missing 

hours shall be considered valid also, if, after 

substituting one-half the minimum detectable 

limit for the missing hourly concentrations, 

the 8-hour average concentration is greater 


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than the level of the standard. The computed 

8-hour average O3 concentrations shall be 

reported to three decimal places (the digits to 

the right of the third decimal place are 

truncated, consistent with the data handling 

procedures for the reported data). 

Daily maximum 8-hour average 

concentrations. (a) There are 24 possible 

running 8-hour average O3 concentrations for 

each calendar day during the O3 monitoring 

season. The daily maximum 8-hour 

concentration for a given calendar day is the 

highest of the 24 possible 8-hour average 

concentrations computed for that day. This 

process is repeated, yielding a daily 

maximum 8-hour average O3 concentration 

for each calendar day with ambient O3

monitoring data. Because the 8-hour averages 

are recorded in the start hour, the daily 

maximum 8-hour concentrations from two 

consecutive days may have some hourly 

concentrations in common. Generally, 

overlapping daily maximum 8-hour averages 

are not likely, except in those non-urban 

monitoring locations with less pronounced 

diurnal variation in hourly concentrations. 

(b) An O3 monitoring day shall be counted 

as a valid day if valid 8-hour averages are 

available for at least 75% of possible hours 

in the day (i.e., at least 18 of the 24 averages). 

In the event that less than 75% of the 8-hour 

averages are available, a day shall also be 

counted as a valid day if the daily maximum 

8-hour average concentration for that day is 

greater than the level of the standard. 

2.2 Primary and Secondary Standardrelated Summary Statistic 

The standard-related summary statistic is 

the annual fourth-highest daily maximum 8- 

hour O3 concentration, expressed in parts per 

million, averaged over three years. The 3-year 

average shall be computed using the three 

most recent, consecutive calendar years of 

monitoring data meeting the data 

completeness requirements described in this 

appendix. The computed 3-year average of 

the annual fourth-highest daily maximum 8- 

hour average O3 concentrations shall be 

reported to three decimal places (the digits to 

the right of the third decimal place are 

truncated, consistent with the data handling 

procedures for the reported data). 

2.3 Comparisons with the Primary and 

Secondary Ozone Standards 

(a) The primary and secondary O3 ambient 

air quality standards are met at an ambient 

air quality monitoring site when the 3-year 

average of the annual fourth-highest daily 

maximum 8-hour average O3 concentration is 

less than or equal to 0.075 ppm. 

(b) This comparison shall be based on three 

consecutive, complete calendar years of air 

quality monitoring data. This requirement is 

met for the 3-year period at a monitoring site 

if daily maximum 8-hour average 

concentrations are available for at least 90% 

of the days within the O3 monitoring season, 

on average, for the 3-year period, with a 

minimum data completeness requirement in 

any one year of at least 75% of the days 

within the O3 monitoring season. When 

computing whether the minimum data 

completeness requirements have been met, 

meteorological or ambient data may be 

sufficient to demonstrate that meteorological 

conditions on missing days were not 

conducive to concentrations above the level 

of the standard. Missing days assumed less 

then the level of the standard are counted for 

the purpose of meeting the data completeness 

requirement, subject to the approval of the 

appropriate Regional Administrator. 

(c) Years with concentrations greater than 

the level of the standard shall be included 

even if they have less than complete data. 

Thus, in computing the 3-year average fourth 

maximum concentration, calendar years with 

less than 75% data completeness shall be 

included in the computation if the 3-year 

average fourth-highest 8-hour concentration 

is greater than the level of the standard. 

(d) Comparisons with the primary and 

secondary O3 standards are demonstrated by 

examples 1 and 2 in paragraphs (d)(1) and 

(d)(2) respectively as follows: 

EXAMPLE 1.—AMBIENT MONITORING SITE ATTAINING THE PRIMARY AND SECONDARY O3 STANDARDS

Year 

Percent valid 

days (within 

the required 

monitoring 

season) 

1st Highest 

daily max 8- 

hour Conc. 

(ppm) 

2nd Highest 

daily max 8- 

hour Conc. 

(ppm) 

3rd Highest 

daily max 8- 

hour Conc. 

(ppm) 

4th Highest 

daily max 8- 

hour Conc. 

(ppm) 

5th Highest 

daily max 8- 

hour Conc. 

(ppm) 

2004 ......................................................... 100 0.092 0.090 0.085 0.079 0.078 

2005 ......................................................... 96 0.084 0.083 0.075 0.072 0.070 

2006 ......................................................... 98 0.080 0.079 0.077 0.076 0.060 

Average ............................................. 98 ........................ ........................ ........................ 0.075 ........................

(1) As shown in Example 1, this 

monitoring site meets the primary and 

secondary O3 standards because the 3-year 

average of the annual fourth-highest daily 

maximum 8-hour average O3 concentrations 

(i.e., 0.075666 * * * ppm, truncated to 0.075 

ppm) is less than or equal to 0.075 ppm. The 

data completeness requirement is also met 

because the average percent of days within 

the required monitoring season with valid 

ambient monitoring data is greater than 90%, 

and no single year has less than 75% data 

completeness. In Example 1, the individual 

8-hour averages used to determine the annual 

fourth maximum have also been truncated to 

the third decimal place. 

EXAMPLE 2.—AMBIENT MONITORING SITE FAILING TO MEET THE PRIMARY AND SECONDARY O3 STANDARDS

Year 

Percent valid 

days (within 

the required 

monitoring 

season) 

1st Highest 

daily max 8- 

hour Conc. 

(ppm) 

2nd Highest 

daily max 8- 

hour Conc. 

(ppm) 

3rd Highest 

daily max 8- 

hour Conc. 

(ppm) 

4th Highest 

daily max 8- 

hour Conc. 

(ppm) 

5th Highest 

daily max 8- 

hour Conc. 

(ppm) 

2004 ......................................................... 96 0.105 0.103 0.103 0.103 0.102 

2005 ......................................................... 74 0.104 0.103 0.092 0.091 0.088 

2006 ......................................................... 98 0.103 0.101 0.101 0.095 0.094 

Average ............................................. 89 ........................ ........................ ........................ 0.096 ........................

As shown in Example 2, the primary and 

secondary O3 standards are not met for this 

monitoring site because the 3-year average of 

the fourth-highest daily maximum 8-hour 

average O3 concentrations (i.e., 0.096333 

* * * ppm, truncated to 0.096 ppm) is 

greater than 0.075 ppm, even though the data 

capture is less than 75% and the average data 

capture for the 3 years is less than 90% 

within the required monitoring season. In 

Example 2, the individual 8-hour averages 

used to determine the annual fourth 

maximum have also been truncated to the 

third decimal place. 


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3. Design Values for Primary and Secondary 

Ambient Air Quality Standards for Ozone 

The air quality design value at a 

monitoring site is defined as that 

concentration that when reduced to the level 

of the standard ensures that the site meets the 

standard. For a concentration-based standard, 

the air quality design value is simply the 

standard-related test statistic. Thus, for the 

primary and secondary standards, the 3-year 

average annual fourth-highest daily 

maximum 8-hour average O3 concentration is 

also the air quality design value for the site. 

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

■ 4. The authority citation of part 58 

continues to read as follows: 

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

7611, and 7619. 

■ 5. Appendix G to Part 58 is amended 

as follows: 

■ a. By revising section 9. 

■ b. By revising section 10. 

■ c. By revising section 12. 

■ d. By revising section 13. 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

* * * * * 

9. How Does the AQI Relate to Air Pollution 

Levels? 

For each pollutant, the AQI transforms 

ambient concentrations to a scale from 0 to 

500. The AQI is keyed as appropriate to the 

national ambient air quality standards 

(NAAQS) for each pollutant. In most cases, 

the index value of 100 is associated with the 

numerical level of the short-term standard 

(i.e., averaging time of 24-hours or less) for 

each pollutant. A different approach is taken 

for NO2, for which no short-term standard 

has been established. The index value of 50 

is associated with the numerical level of the 

annual standard for a pollutant, if there is 

one, at one-half the level of the short-term 

standard for the pollutant, or at the level at 

which it is appropriate to begin to provide 

guidance on cautionary language. Higher 

categories of the index are based on 

increasingly serious health effects and 

increasing proportions of the population that 

are likely to be affected. The index is related 

to other air pollution concentrations through 

linear interpolation based on these levels. 

The AQI is equal to the highest of the 

numbers corresponding to each pollutant. 

For the purposes of reporting the AQI, the 

sub-indexes for PM10 and PM2.5 are to be 

considered separately. The pollutant 

responsible for the highest index value (the 

reported AQI) is called the ‘‘critical’’

pollutant. 

10. What Monitors Should I Use To Get the 

Pollutant Concentrations for Calculating the 

AQI? 

You must use concentration data from 

population-oriented State/Local Air 

Monitoring Station (SLAMS) or parts of the 

SLAMS required by 40 CFR 58.10 for each 

pollutant except PM. For PM, calculate and 

report the AQI on days for which you have 

measured air quality data (e.g., from 

continuous PM2.5 monitors required in 

Appendix D to this part). You may use PM 

measurements from monitors that are not 

reference or equivalent methods (for 

example, continuous PM10 or PM2.5

monitors). Detailed guidance for relating nonapproved measurements to approved 

methods by statistical linear regression is 

referenced in section 13 below. 

* * * * * 

12. How Do I Calculate the AQI? 

i. The AQI is the highest value calculated 

for each pollutant as follows: 

a. Identify the highest concentration among 

all of the monitors within each reporting area 

and truncate the pollutant concentration to 

one more than the significant digits used to 

express the level of the NAAQS for that 

pollutant. This is equivalent to the rounding 

conventions used in the NAAQS. 

b. Using Table 2, find the two breakpoints 

that contain the concentration. 

c. Using Equation 1, calculate the index. 

d. Round the index to the nearest integer. 

TABLE 2.—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQI’s 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour 1

PM2.5

(μg/m3) 

PM10

(μg/m3) CO (ppm) SO2 (ppm) NO2 (ppm) AQI Category 

0.000–0.059 .... ........................ 0.0–15.4 0–54 0.0–4.4 0.000–0.034 (3) 0–50 Good. 

0.060–0.075 .... ........................ 15.5–40.4 55–154 4.5–9.4 0.035–0.144 (3) 51–100 Moderate. 

0.076–0.095 .... 0.125–0.164 40.5–65.4 155–254 9.5–12.4 0.145–0.224 (3) 101–150 Unhealthy for 

Sensitive 

Groups. 

0.096–0.115 .... 0.165–0.204 4 65.5–150.4 255–354 12.5–15.4 0.225–0.304 (3) 151–200 Unhealthy. 

0.116–0.374 .... 0.205–0.404 4 150.5–250.4 355–424 15.5–30.4 0.305–0.604 0.65–1.24 201–300 Very 

Unhealthy. 

(2) .................... 0.405–0.504 4 250.5–350.4 425–504 30.5–40.4 0.605–0.804 1.25–1.64 301–400 

(2) .................... 0.505–0.604 4 350.5–500.4 505–604 40.5–50.4 0.805–1.004 1.65–2.04 401–500 Hazardous. 

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

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

ozone index value may be calculated, and the maximum of the two values reported. 2 8-hour O3 values do not define higher AQI values (≥ 301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations. 3 NO2 has no short-term NAAQS, and can generate an AQI only above the value of 200. 4 If a different SHL for PM2.5 is promulgated, these numbers will change accordingly. 

ii. If the concentration is equal to a 

breakpoint, then the index is equal to the 

corresponding index value in Table 2. 

However, Equation 1 can still be used. The 

results will be equal. If the concentration is 

between two breakpoints, then calculate the 

index of that pollutant with Equation 1. You 

must also note that in some areas, the AQI 

based on 1-hour O3 will be more 

precautionary than using 8-hour values (see 

footnote 1 to Table 2). In these cases, you 

may use 1-hour values as well as 8-hour 

values to calculate index values and then use 

the maximum index value as the AQI for O3. 

I

I I

BP BP C BP I Equation p

Hi Lo

Hi Lo

= p Lo Lo

−

− ( ) − + ( ) 1

Where: Ip = the index value for pollutantp Cp = the truncated concentration of 

pollutantp


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16514 Federal Register / Vol. 73, No. 60 / Thursday, March 27, 2008 / Rules and Regulations 

BPHi = the breakpoint that is greater 

than or equal to Cp

BPLo = the breakpoint that is less than 

or equal to Cp

IHi = the AQI value corresponding to 

BPHi

Ilo = the AQI value corresponding to 

BPLo. 

iii. If the concentration is larger than the 

highest breakpoint in Table 2 then you may 

use the last two breakpoints in Table 2 when 

you apply Equation 1. 

Example 

iv. Using Table 2 and Equation 1, calculate 

the index value for each of the pollutants 

measured and select the one that produces 

the highest index value for the AQI. For 

example, if you observe a PM10 value of 210 

μg/m3, a 1-hour O3 value of 0.156 ppm, and 

an 8-hour O3 value of 0.130 ppm, then do 

this: 

a. Find the breakpoints for PM10 at 210 μg/ 

m3 as 155 μg/m3 and 254 μg/m3, 

corresponding to index values 101 and 150; 

b. Find the breakpoints for 1-hour O3 at 

0.156 ppm as 0.125 ppm and 0.164 ppm, 

corresponding to index values 101 and 150; 

c. Find the breakpoints for 8-hour O3 at 

0.130 ppm as 0.116 ppm and 0.374 ppm, 

corresponding to index values 201 and 300; 

d. Apply Equation 1 for 210 μg/m3, PM10: 

150 101

254 155

210 155 101 128 −

− ( ) − + =

e. Apply Equation 1 for 0.156 ppm, 1-hour 

O3: 

150 101

0 164 0 125

0 156 0 125 101 140 −

− ( ) − + = . . . .

f. Apply Equation 1 for 0.130 ppm, 8-hour 

O3: 

300 201

0 374 0 116

0 130 0 116 201 206 −

− ( ) − + = . . . .

g. Find the maximum, 206. This is the AQI. 

The minimal AQI report would read: 

v. Today, the AQI for my city is 206 which 

is Very Unhealthy, due to ozone. Children 

and people with asthma are the groups most 

at risk. 

13. What Additional Information Should I 

Know? 

The EPA has developed a computer 

program to calculate the AQI for you. The 

program prompts for inputs, and it displays 

all the pertinent information for the AQI (the 

index value, color, category, sensitive group, 

health effects, and cautionary language). The 

EPA has also prepared a brochure on the AQI 

that explains the index in detail (The Air 

Quality Index), Reporting Guidance 

(Guideline for Public Reporting of Daily Air 

Quality) that provides associated health 

effects and cautionary statements, and 

Forecasting Guidance (Guideline for 

Developing an Ozone Forecasting Program) 

that explains the steps necessary to start an 

air pollution forecasting program. You can 

download the program and the guidance 

documents at www.airnow.gov. Reference for 

relating non-approved PM measurements to 

approved methods (Eberly, S., T. FitzSimons, T. Hanley, L. Weinstock., T. 

Tamanini, G. Denniston, B. Lambeth, E. 

Michel, S. Bortnick. Data Quality Objectives 

(DQOs) For Relating Federal Reference 

Method (FRM) and Continuous PM2.5

Measurements to Report an Air Quality Index 

(AQI). U.S. Environmental Protection 

Agency, research Triangle Park, NC. EPA–

454/B–02–002, November 2002) can be found 

on the Ambient Monitoring Technology 

Information Center (AMTIC) Web site, http:// 

www.epa.gov/ttnamti1/. 

[FR Doc. E8–5645 Filed 3–26–08; 8:45 am] 

BILLING CODE 6560–50–P 


pwalker on PROD1PC71 with RULES2ER27MR08.002</MATH> ER27MR08.003</MATH> ER27MR08.004</MATH>