Federal Register 

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ENVIRONMENTAL PROTECTION
AGENCY

40 CFR Parts 50 and 58 [EPA-HQ-OAR-2005-0172; FRL-8544-3]
RIN 2060-AN24

National Ambient AirQuality Standards for Ozone
AGENCY: Environmental Protection Agency (EPA). ACTION: Final rule.

SUMMARY: Based on its review ofthe air quality criteria for ozone (03)and related photochemical oxidants and national ambient airquality standards (NAAQS) for 03, EPA is making revisions to the primary and secondary NAAQS for 03 to provide requisite protection ofpublic health andwelfare, respectively. With regard to the primary standard for 03, EPA is revising the level ofthe 8-hour standard to 0.075 parts per million (ppm), expressed to three decimal places. With regard to the secondary standard for 03, 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 03, setting an-AQI value of 100 equal to 0.075 ppm, 8-hour average, and making proportional changes to the AQI values of50, 150 and 200. DATES: This final rule is effective on May 27, 2008. ADDRESSES: EPA has established a docket for this actionunder Docket ID No. EPA-HQ-OAR-2005-0172. All documents in the docket are listed on the www.regulations.govWeb site. Although listed in the index, some information is notpublicly 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 Internel and willbe publicly available only in hard copy form. Publicly available docket materials are available either electronically through www.regulations.govor in hard copy at the Air and Radiation Docket and Information Center, EPA/DC, EPAWest, 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. DavidJ. McKee, Health and Environmental Impacts Division, Office of AirQuality 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 ofContents

The following topics are discussed in this preamble:
I.Background

A. Summary ofRevisions to the 03 NAAQS B. Legislative Requirements C. Review ofAir Quality Criteria and

Standards for 03 D. Summary of Proposed Revisions to the

03 NAAQS E. Organization and Approach to Final

Decision on 03 NAAQS II. Rationale for Final Decision on the Primary 03 Standard A. Introduction 1. Overview 2. Overview ofHealth Effects 3. Overview ofHuman Exposure and

Health Risk Assessments B. Need for Revision of the Current

Primary 03 Standard 1. Introduction 2. Comments onthe Need for Revision 3. Conclusions Regarding the Need for

Revision C. Conclusions onthe Elements ofthe

Primary 03 Standard 1. Indicator 2. Averaging Time 3. Form 4. Level D.Final Decision on the Primary 03

Standard IlI. Communication ofPublic Health Information IV. Rationale for Final Decision on the Secondary 03 Standard A. Introduction 1. Overview 2. Overview ofVegetation Effects Evidence 3. Overview ofBiologically Relevant

Exposure Indices 4. Overview ofVegetation Exposure and

Risk Assessments B. Need for Revision ofthe Current

Secondary 03 Standard 1. Introduction 2. Comments on the Need for Revision 3. Conclusion's Regarding theNeed for

Revision C. Conclusions on the Secondary 03

Standard 1. StaffPaper 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 03

Standard V. Creation ofAppendix P-Interpretation of the NAAQS for 03 A. General B. Data Completeness C. Data Reporting and Handling and

Rounding Conventions VI. Ambient Monitoring Related to Revised
03 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 andExecutive Order Reviews

A.Executive'Order 12866: Regulatory

Planning andReview 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 orUse 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 the03 NAAQS

Based on its review ofthe air quality criteria for03 and related photochemical oxidants and national ambient air quality standards (NAAQS) for 03, EPA is making revisions to the primary and secondary NAAQS for 03

to provide protection ofpublic health and welfare, respectively, that is appropriate under section 109,and is making corresponding revisions in data handling conventions for 03.

With regard to the primary standard for 03, EPA is revising the level of the 8-hour standard to a level of0.075 parts
per million (ppm), to provide increased protection for children and other "at risk" populations against an array of 03- related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators ofrespiratory 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 ofthe primary standard to the nearest thousandth ppm.

With regard to the secondary standard for 03, EPA is revising the standard by making it identical to the revised primary standard. 

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B.LegislativeRequirements

Two sections ofthe Clean Air Act (CAA) govern the establishment and revision ofthe NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator
to identify andlist "air pollutants"
emissions ofwhich "in his judgment,

cause or contribute to airpollution which mayreasonably be anticipated to endanger public health orwelfare," whose "presence * * * inthe ambient air results fromnumerous or diverse mobile or stationary sources," and for which the Administrator plans toissue air quality criteria, and to issue air quality criteria for those that arelisted. Air quality criteria are to "accurately reflect the latest scientific knowledge useful in indicating the kind andextent of identifiable effects on publichealth

or welfare which maybe expected from the presence of[a] pollutant in ambient air, in varying quantities * * *." Sedtion 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 ofwhich in the judgment ofthe Administrator, based on such criteria and allowing an adequate margin ofsafety, are requisite to protect the publichealth."' A secondary standard, as defined in section 109(b)(2), must "specify a level ofair quality the attainment and maintenance ofwhich in the judgment ofthe 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 ofsafety was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting. Itwas also intended to provide a reasonable degree of protection againsthazards that research has not yet identified. LeadIndustries

'The legislative history of section 109 indicates that aprimary standard is to be set at "th& maximum permissible ambient air level * * * which willprotect thehealth ofany [sensitive]
group ofthe population," and that for this purpose "reference should be made to arepresentative sample ofpersons comprising the sensiti ,e group rather than to a single person in such agroup" [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, visibilityand climate, damage to and deterioration of property, andhazards to transportation, as well as effects on economic values and on personal comfort and wellbeing." Associationv. EPA, 647 F.2d 1130, 1154 (DC Cir 1980), cert. denied,449 U.S. 1042 (1980);AmericanPetroleum Institutev. Castle,665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied,455 U.S. '1034 (1982). Both kinds ofuncertainties

are components ofthe risk associated with pollution at levels below those at which human health effects canbe said
to occur with reasonable scientific certainty. Thus, in selecting primary standards that provide an adequate margin ofsafety, the Administrator is seeking not onlyto prevent pollution levels that havebeen demonstrated to be harmful but also to prevent lower pollutant levels thatmay pose an unacceptable risk ofharm, even ifthe risk is not precisely identified asto nature or degree. The CAA does not require the Administrator to establish a primary NAAQ$ at a zero-risk level or at background concentration levels, see LeadIndustries Associationv.EPA, 647 F.2d at 1156 n. 51, but rather at a level thatreduces risk sufficiently so as to protect public health with an adequate margin of safety.

The selection ofany particular approach to providing an adequate margin ofsafety is a policy choice left specifically tothe Administrator's judgment. LeadIndustries Association

v. EPA, 647F.2d at 1161-62. In addressing therequirement for an adequate margin ofsafety, EPA considers such factors asthe nature and severity ofthe health effects involved, the size ofthe population(s) at risk, and the kind and degree ofthe 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 norless stringent than
necessary for these purposes. Whitman

v.America TruckingAssociations,531 U.S. 457, 473. Further the Supreme Court ruled that "[t]he text of § 109(b), interpreted in its statutory and historical
context andwith appreciation forits importance tothe CAA as a whole, unambiguously bars cost considerations from theNAAQS-setting process

. * Id. at472.3

Section 109(d)(1) ofthe CAA requires
that "not later than December 31, 1980, and at 5-year intervals thereafter, the Administrator shall complete a

3In considering whether the CAA allowed for economic considerations to play a role in the promulgation ofthe NAAQS, the Supreme Court rejected arguments that because many more factors than airpollution might affect public health, EPA should consider compliance costs thatproduce health losses in setting theNAAQS. 531 U.S. at466. Thus, EPAmay not take intoaccount possible public health impacts from the economic cost of implementation. Id. thoroughreview ofthe criteria publishedunder section 108 andthe 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 ofthe criteria * * *
andthe national primary and secondary ambient air quality standards * * * and shall recommend to the Administrator
any new * * * standards andrevisions ofexisting criteria and standards asmay be appropriate under section 108 and [section 109(b)]." Thisindependent review function isperformed bythe Clean Air Scientific Advisory Committee (CASAC) of EPA's Science Advisory Board.

C.Review of AirQualityCriteria and Standards for03 Grpund-level 03 is formed from biogenic and anthropogenic precursor emissions. Naturally occurring 03 in the troposphere can result from biogenic organicprecursors reacting with naturally occurring nitrogen oxides (NOx) and by stratospheric 03 intrusion into the troposphere. Anthropogenic
precursors of 03, specifically NOx and volatile organic compounds (VOC), originate from a wide variety of stationary and mobile sources. Ambient 03concentrations produced by these emissions are directly affected by temperature, solar radiation, wind speed and other meteorological factors.

The last review ofthe 03 NAAQS was completed on July 18, 1997, based on the 1996 03 Air Quality Criteria Document (EPA, 1996a) and 1996 03 StaffPaper (EPA, 1996b). EPA revised thePprimary and secondary 03 standards

on thebasis ofthethen latest scientific evidence linking exposures to anibient 03 to adverse health and welfare effects
at levels allowed by the 1-hour average standards (62 FR 38856). The 03 standards were revised by replacing the existing primary 1-hour average standard with an 8-hour average 03 standard set at a level of 0.08 ppm, which is equivalent to 0.084 ppm using the standard rounding conventions. The form oftheprimary standard was changed tothe annual fourth-highest daily maximum 8-hour average concentration, averaged over 3 years. The secondary 03 standard was changed by making itidentical in all respects to therevised primary standard.

EPA initiated this current review in September 2000 with a call for information (65 FR 57810) for the development ofarevised Air Quality 

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Criteria Document for 03 and Other Photochemical Oxidants (henceforth the "Criteria Document"). Aproject work plan (EPA, 2002) for the preparation of the Criteria Document was released in November 2002 forCASAC 03 Panel4

-(henceforth, "CASAC Panel") and public review. EPA held a series of workshops in mid-2003 on several draft chapters of the CriteriaDocument to obtain broad input from the relevant scientific communities. These workshops helped to inform the preparation ofthe first draft Criteria Document (EPA, 2005a), which was released for CASAC Panel and public review onJanuary 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 afirst draft StaffPaper (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 ofthe 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 ofthe final Criteria Document (Integrative Synthesis) to help inform the second draft Staff Paper.

A second draft StaffPaper (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 StaffPaper (Henderson, 2006c). A final StaffPaper (EPA, 2007a) was released onJanuary 31, 2007. Around the time ofthe release ofthe final StaffPaper 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 alsoresulted 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

4The CASAC 03 Review Panel includes the seven members ofthe chartered CASAC, supplemented by fifteen subject-matter experts appointed by the Administrator to provide additional scientific expertise relevant to this review ofthe 03 NAAQS. January 31, 2007 StaffPaper.5 In a March 26, 2007 letter (Henderson, 2007), the CASAC Panel offered additional advice to the Administrator with regard to recommendations and revisionsto the primary and secondary 03 NAAQS.

The schedule for completion ofthis review has been governed by a consent decree resolving a lawsuit filed in March 2003by a group ofplaintiffs 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 thisreview provides that EPA sign for publication notices ofproposed and final rulemaking concerning its review ofthe 03 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 onJuly 11, 2007.

A large number of comments were received from various commenters on the proposed revisions to the 03 NAAQS. Significant issues raised in the public comments are discussed throughout the preamble ofthis final action. A comprehensive summary of all significant comments, along with EPA's responses (henceforth "Response to Comments"), canbe found in the docket for this rulemaking.

Various commenters have referred to and discussed anumber of new scientific studies onthe health effects of 03 thathad 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 ofthis effort was to ensure that the Administrator was fully aware ofthe "new" science before makinga final

5EPA made available corrected versions ofthe final Staff Paper (EPA, 2007b, henceforth, "Staff Paper") andthe human exposure and health risk

assessment technical support documents on July 31, 2007 on the EPA Web site http://www.epa.govlttnl

naaqs.

"American LungAssociationv. Whitman (No. 1:03CV00778, D.D.C. 2003).

7 For ease ofreference, 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 fromthose that have been published since the last review andare included in the 2004 Criteria Document (these studies are sometimes referred to as new (without quotation marks) ormore recent studies, to indicate that they

were not included in the 1996 Criteria Document and thus are newly available in this review. decision onwhether to revise the
current 03 NAAQS. EPAprovisionally considered these studies to place their results in the context ofthe findings of the Criteria Document.

As in priorNAAQS reviews, EPA is basing its decision in this review on studies and related information included inthe Criteria Document and StaffPaper, which have undergone CASAC and public review. The studies assessed in the Criteria Document, and the integration ofthe scientific evidence presented in that document, have undergone extensive critical review by EPA, CASAC, and the public during the development ofthe Criteria Document. The rigor ofthat review makesthese studies, andtheir integrative assessment, the most reliable source of scientific information on which to base decisions on the NAAQS, decisions that all parties recognize as ofgreat 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 onlyby EPAbut alsoby the statutorily mandated independent advisory committee, as well as the public review that accompanies this process. As described above, EPA's provisional consideration ofthese studies did not and couldnot 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 thathavebeen assessed as a part ofthe 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 ofthis issue and EPA's past practice.

As discussed inEPA's 1993 decision
not to revise the NAAQS for 03 "new" studies may sometimes be ofsuch significance that it is appropriate to delay a decision onrevision ofa NAAQS and to supplement the pertinent air quality criteria so the studies can be taken into account (58FR
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 ofthe broad scientific conclusions regarding the health effects of 03 exposure made in the Criteria Document. Forthis reason, reopening the air quality criteria review would not be warranted even ifthere

were time to do so under the court order 

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governing the schedule for this rulemaking. Accordingly, EPA isbasing the final decisions in this review onthe studies and related information included in the 03 air quality criteria that have undergone CASAC and public review. EPA will consider the newly published studies forpurposes of decision making in the next periodic review ofthe 03 NAAQS, which will provide the opportunity to fullyassess them through a more rigorous review
process involving EPA, CASAC, and the public. Further discussion ofthese "new" studies can be found in the Response to Comments document.

This action presents the Administrator's final decisions on the review ofthe current primary and secondary 03 standards. Throughout this preamble a number ofconclusions, findings, and determinations madeby the Administrator are noted. They identify thereasoning that supports this final decision and are intended to be final and conclusive.

D.Summary of Proposed Revisions to the 03NAAQS

For reasons discussed inthe proposal, the Administrator proposed torevise the
current primary and secondary 03 standards. With regard to the primary 03 standard, the Administrator proposed to revise the level ofthe 8- hour 03 standard to a level within the
range of 0.070 ppm to 0.075 ppm, based

on a 3-year average ofthe fourth-highest maximum 8-hour average concentration. Related revisions for 03 data handling conventions and for the reference method for monitoring 03 were also proposed. These revisions were proposed to provide increased protection for children and other "at risk" populations against an array of03- related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators ofrespiratory morbidity, including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity, aswell as total nonaccidental and cardiorespiratory mortality. EPA also proposed to specify the level ofthe 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 03, EPA proposed to revise the
current 8-hour standard with one oftwo options to provide increased protection against 0 3-related adverse impacts on vegetation andforested ecosystems. One option. was to replace the current standard with a cumilative, seasonal standard expressed as an index ofthe annual sum ofweighted hourly concentrations, cumulated over 12 hours per day (8 am to 8 pm) during the consecutive 3-month period within the 03 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 theproposed primary 8- hour standard. EPA solicited comment

on specifying a cumulative, seasonal standard interms ofa 3-year average of the annual sums ofweighted hourly concentrations; on therange 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 andApproachtoFinal 03 NAAQS Decisions

This action presents the Administrator's final decisions regarding the need torevise the current primary and secondary 03 standards. Revisions to the primary standard for03

are addressed below in section II, and a discussion oncommunication ofpublic health information regarding revisions
to the primary 03 standard is presented in section III. The secondary 03 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 03 are addressed below in section VI. Future implementation steps andrelated 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 03 at levels typically found in the ambient air. These final decisions also take into account: (1) Staffassessments in the StaffPaper ofthe 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 ofdrafts ofthe Criteria Document and StaffPaper
at public meetings, and separate written comments prepared byindividual members ofthe CASAC Panel; (3) public comments received during the development ofthese documents, either in connection with CASAC Panel meetings or separately; and (4) extensive public comments received onthe proposed rulemaking.

II. Rationale forFinal Decisions onthe Primary 03 Standard

A. Introduction

1.Overview

This section presents the Administrator's final decisions regarding the need torevise the current primary 03 NAAQS, andthe appropriate revision to the level ofthe 8-hour standard. As discussed more fully below,the rationale forthe final decision on appropriate revisions to the primary 03 NAAQS includes consideration of: (1)Evidence ofhealth effects related to short-term exposures to 03; (2)insights gained from quantitative exposure and health risk assessments; (3) public and CASAC Panel comments received during the development and review ofthe 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 8relevant to examining associations between exposure to ambient 03 and abroad
range ofhealth endpoints (EPA, 2006a, Chapter 8), focusing on those health endpoints forwhich the Criteria Document concluded that the associations are causal or likely tobe causal. Thisbody ofevidence includes hundreds ofstudies conducted in many countries around the world. In its
assessment ofthe evidence judged to be
most relevant to decisions on elements ofthe primary 03 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 ofnew 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



### "The word "evidence" is used in this notice to
referto studies thatprovide information relevant to

an area of inquiry, which can include studies that report positive or negativeresults orthatprovide interpretative information. 

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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 ofhealth effects attributable to exposure to ambient 03, most generally withregard to whether observed health effects and associations

are causal or likely causal in nature and, ifso, the certainty ofcausal associations
at various exposure levels. While important uncertainties remain, the review ofthe health effects information has been extensive and deliberate. In the judgment ofthe Administrator, this intensive evaluation ofthe scientific evidence provides an adequate basis for regulatory decision making atthis time. Thisreview also provides important inputto EPA's research plan for improving our future understanding of the relationships between exposures to ambient 03 andhealth effects.

The health effects information and quantitative exposure and health risk
assessment were summarized in sections Il.A and II.B oftheproposal (72 FR at 37824-37862) and are onlybriefly outlined below in sections II.A.2 and II.A.3. Subsequent sections ofthis preamble provide a more.complete discussion ofthe Administrator's rationale, in light of key issues raised in public comments, forconcluding that the current standard is not requisite to protect public health with an adequate margin of safety, and itis appropriate to revise the current primary 03 standards
toprovide additional public health protection (section II.B), as well as a

more complete discussion ofthe Administrator's rationale for retaining

orrevising the specific elements of the primary 03 standards (sectionll.C), namely the indicator (section II.C.1); averaging time (sectionII.C.2); form (sectionII.C.3); andlevel (section II.C.4). A summary ofthe final decisions on revisions to theprimary 03 standards is presented in section ll.D.

2. Overview of Health Effects

This section outlines the information presented in Section II.A ofthe proposal

on known or potential effects onpubliG health which may be expected from the
presence of 03in ambient air. The decision in the last review focused primarily on evidence from short-term (e.g., 1to 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 andemergency department visits for respiratory causes. The Criteria Document prepared for this review emphasizes a large number of, epidemiological studies published since the lastreview with these and additional health endpoints, including the effects of acute (short-term and prolonged) and chronic exposures to 03

on lung function decrements and enhanced respiratory -symptomsin asthmatic individuals, school absences, and premature mortality. It also emphasizes important new information from toxicology, dosimetry, and controlled human exposure studies. Highlights ofthe evidence include:

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

examine respiratory effects associated with prolonged 03exposures 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 0 3-induced inflammatory
response in both the upper respiratory
tract (URT) and lowerrespiratory tract (LRT), while other studies have examined changes inhost defense capability following 03 exposure of healthy young adults and increased airway responsiveness to allergens in subjects with allergic asthma and allergic rhinitis exposed to 03.

(3) New evidence from controlled human exposure studies showing that asthmatics have greater respiratoryrelated physiological responses than healthy subjects andnew evidence from epidemiological studies showing associations between 03 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 ofrespiratory
responses to 03 as those experienced by healthy individuas.

(4) Animal toxicology studies provide

new inforiation regarding potential mechanisms of action, increased susceptibility to respiratory infection, andbiological plausibility ofacute effects as well as chronic, irreversible respiratory damage observed in animals.

(5) Numerous epidemiological studies published during the past decade offer added evidence ofassociations between
acute ambient 03 exposures and lung function decrements and respiratory symptoms in physically active healthy subjects and asthmatic subjects, aswell

as new evidence regarding additional health endpoints, including relationships between ambient 03 concentrations and school absenteeism and between ambient 03 and cardiacrelated physiological endpoints.

(6) Several additional studies have been published over the last decade examining the temporal associations between acute 03 exposures andboth
emergency department visits for respiratory diseases and respiratoryrelated hospital admissions. (7)A large number of newly available epidemiological studies have examined the effects ofacute exposure to PM and
03 on premature mortality, notably including large multi-city studies that provide much more robust information than was available in the lastreview, as well asrecent meta-analyses thathave evaluated potential sources of heterogeneity in 0 3-mortality associations.

Section II.A ofthe proposal provides

a detailed summary ofkey information contained in the Criteria Document (chapters 4-8) and in the StaffPaper (chapter 3), on the known and potential effects of03 exposure and information

on the effects of03 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 03, including potential mechanisms or pathways related to direct effects on the respiratory system, systemic effects that are secondary to effects in therespiratory system (e.g., cardiovascular effects); (2)The nature ofeffects thathave been associated directly with exposure to 03 or indirectly with the presence of 03 in ambient air, including premature mortality, aggravation ofrespiratory 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 ofthe 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 asthe limitations ofthe evidence; and

(4) Considerations incharacterizing the public health impact of03, including the identification of sensitive and vulnerable subpopulations that are potentially atrisk to such effects, including active people, people with pre-existing lung and heart diseases, children and older adults, and people with increased responsiveness to 03. 16440

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3. Overview ofHuman Exposure and Health RiskAssessments

To putjudgments 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 ofthe size 6f 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 ofthe number ofpeople likely to experience 0 3-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 ofthe proposal, there are a number ofimportant 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 inboth 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 thatthe health risk assessment was "well done, balanced and reasonably communicated" (Henderson, 2006c).

In modeling exposures and health risks associated withjustmeeting the 6urrent and alternative 03 standards, EPA simulated air qualityjust meeting these standards based on 03 air quality patterns in several recent years and on how the shape ofthe 03 air quality distributions has changed over time based onhistorical trends inmonitored 03 air quality data. As discussed in the proposal notice and in the StaffPaper (section 4.5.8), recent 03 air quality distributions were statistically adjusted
to simulate justmeeting the current and selected alternative standards. Specifically, the exposure and risk
assessment included estimates for a recent year ofair quality and for air quality adjusted to simulate just meeting the current and alternative standards based on 03 season data from arecent three-year period (2002-2004). The 03

season in each area included the period ofthe year forwhich routine hourly 03 monitoring data are available. Typically this period spans from March or April through September orOctober, although in some areas it includes the entire year. Three years were modeled toreflect the substantial year-to-year variability that

occurs in 03 levels andrelated meteorological conditions, and because the standard is specified in terms ofa three-year period. The year-to-year variability .observed in03 levels is due
to a combination of differentweather patterns and the variation in emissions of03 precursors. Nationally, 2002 was

a relatively highyear with respect tothe 4th highest daily maximum 8-hour 03 levels observed in urban areas across the U.S. (see StaffPaper, Figure 2-16), with the mean ofthe distribution of annual 4th highest daily maximum 8-hour 03 levels for urban monitors nationwide being in the upperthird among the
years 1990 through 2004. In contrast, on

a national basis, 2004 was the lowest " year on record with respect to the mean ofthe distribution of annual 4th highest daily maximum 8-hour 03 levels forthis

same 15 year period. The 4th highest daily maximum 8-hour levels observedin most, but not all ofthe 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 03levels observed in 2003 in the 12 urban areas and nationally generally were between those observed in 2002 and 2004. As a result ofthe variability in air quality, the exposure and risk estimates associated withjust meeting the current

or any alternative standard also will vary depending onthe year chosen for the analysis. Thus, exposure andrisk 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 ofchildren affected.

These simulations donot reflect any consideration ofspecific control
programs or strategies designed to achieve the reductions in emissions required to meet the specified

- standards. Further, these simulations do
not represent predictions ofwhen, whether, or how areas might meet the specified standards." Instead these simulations represent a projection ofthe kind ofairquality levels that would be likely to occur in areasjust attaining various alternative standards, when historical patterns of air quality, reflecting averages over many areas, are applied in theurban areas examined.

a.Exposure Analyses

As discussed in section 11.B.1 ofthe ppoposal, EPA conducted human
exposure analyses using a simulation model to estimate 03exposures for the general population, school age children (ages 5-18), and school age children

5For informational purposes only, modelingthat projects how areas might attain alternative standards in a future year as aresult of Federal, State, local, and Tribal effortsis-presented in the finalRegulatory Impact Analysis being prepared in connection with this decision. with asthma living in 12 U.S. metropolitan areas representing different regions ofthe country where the current 8-hour 03 standard is not met. The emphasis on children reflected the finding ofthe lastreview 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 ofvariability that affect people's exposures. This exposure assessment is more fully described and presented in the StaffPaper and in a technical support document, Ozone Population ExposureAnalysis for Selected Urban Areas (EPA, 2007c; henceforth "Exposure Analysis TSD"). As notedin the proposal, the scope and methodology forthis 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 StaffPaper (chapter 4) and Exposure Analysis TSD, EPA recognized that there aremany

sources ofvariability and uncertainty inherent in the input to this assessment and that there was uncertainty in the resulting 03 exposure estimates. In EPA's judgment, the most important uncertainties affecting the exposure estimates are relatedto the modeling of human activity patterns over an 03 season, the modeling ofvariations in ambient concentrations near roadways, and the modeling ofair exchange rates that affect the amount of 03 that penetrates indoors. Another important uncertainty that affects the estimation of how many exposures are associated with moderate or greater exertion is the characterization ofenergy expenditure for children engaged in various activities. As discussed in more detailin
the StaffPaper (section 4.3.4.7), the uncertainty in energy expenditure values carries over tothe uncertainty of the modeled breathing rates, which are important since they are used to classify
exposures occurring atmoderate or greater exertion. These are the relevarqt
exposures since 0 3-related effects observed in clinical studies only are observed when individuals areengaged in some form ofexercise. The uncertainties in the exposure model inputs and theestimated exposures have been assessed using quantitative uncertainty and sensitivity analyses. Details are discussed in the StaffPaper (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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ambient 03 while at elevated exertion10, served two purposes. First,the entire.
range ofmodeled personal exposures to ambient 03 was an essential input to the portion ofthe health risk assessment based on exposure-response functions from controlled human exposure studies, discussed inthe next section. Second, estimates ofpersonal exposures to ambient 03 concentrations atand above specified benchmark levels while
at elevated exertion provided some perspective onthe public health impacts ofhealth 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 atand above these benchmark levels while at elevated exertion as "exposures ofconcern."

Based on the observation from the
exposure analyses conducted inthe prior review that children represented the population subgroup with the greatest exposure to ambient 03, 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 forthe general population inthe current exposure assessment. While outdoor workers and other adults who engage in moderate orgreater exertion for prolonged periods while outdoors during the day in areas experiencing elevated 03 concentrations also are at risk for 0 3-related health effects, EPA did not focus on developing quantitative
exposure estimates forthese population subgroups due to the lack of information about the number ofindividuals who regularly work or exercise outdoors. Thus, as presented in the proposal and in the StaffPaper the exposure estimates

are most useful for making relative comparisons 9festimated 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 incomparing exposure estimates associated withjust meeting the current NAAQS and alternative standards considered. These included:



### 10As discussed in section lI.A of the proposal, 03 health responses observed in controlled human
exposure studies are associated with exposures while subjects are engaged in moderate or greater exertion on average overthe exposure period (hereafterreferred to as "elevated exertion") and, therefore, these arethe exposures ofinterest. (1) As shown in Table 6-1 ofthe Staff 

### Paper, thepatterns of exposures in
terms ofpercentages ofthe population exceeding given exposure levels were very similar for the general population and forasthmatic and all school age (5- 18) children, although children were about twice as likely as the general population to be exposed atany given level.

(2) As shown in Table 1 inthe 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 ofconcern declined from simulations ofjust meeting the
current standard to simulations of alternative 8-hour standards by varying amounts, depending on the benchmark level, the population subgroup considered, andthe air quality year chosen."

(3) Substantial year-to-year variability in exposure estimates was observed over thethree-year modeling period.

(4) There was substantial variability observed across the 12 urban areas in the percent ofthe population subgroups estimated to experience exposures at and above specified benchmark levels while at elevated exertion.

(5) Ofparticular note, there is high inter-individual variability in responsiveness such that only a subset ofindividuals who were exposed at and above a givenbenchmark 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 anduncertainty in the model itself.

b. Quantitative Health Risk Assessment

As discussed in section II.B.2 ofthe proposal, the approach used to develop quantitative risk estimates associated with exposures to 03builds upon the risk assessment conducted during the last review. 12 The expanded and

"While the proposal notice statedin the textthat "approximately 2 to 4 percent ofall and asthmatic children" were estimated to experience exposures ofconcern at and above the 0.070 ppmbenchmark level for standards inthe range of 0.070 to 0.075
ppm (72 FR 37879), the correctrange is about 1 to 5 perecent consistent with the estimates provided inTable 1 ofthe proposal (72 FR 37855).

12The methodology, scope, and results from the
risk assessment conducted in the last review are described inChapter 6 ofthe 1996 Staff Paper (EPA, 1996) and in several technical reports (Whitfield et al., 1996; Whitfield, 1997) and publication (Whitfield etaL., 1998). updated assessment conducted inthis 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 03 levels; (2) risk reductions and remaining risks associated withjust meeting the current 8-hour03 NAAQS; and (3) risk reductions and remaining risks associated withjust meeting various alternative 8-hour 03 NAAQS in

a number ofexample urban areas. The healthrisk assessment was discussed in the StaffPaper (chapter 5) and presented more fully in a technical support document, OzoneHealthRisk Assessment forSelected UrbanAreas (AbtAssociates, 2007a). As noted inthe proposal, the scope and methodology for this risk assessment was developed over several years with considerable inputfrom the CASAC Panel and the public.

EPA recognized thatthere were many

sources ofuncertainty and variability inherent in the inputs to these
assessments and that there was a high degree ofuncertainty inthe resulting 03 risk estimates. Such uncertainties generally relate to a lack of clear understanding of anumber ofimportant factors, including, for example, the shape ofexposure-response and concentration-response functions, particularly when, as here, effect thresholds can neither be discerned nor determined not to exist; issues related to selection ofappropriate statistical models forthe analysis ofthe epidemiologic data; therole of potentially confounding andmodifying factors in the concentration-response relationships; and issues related to simulating how 03 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 oftheseuncertainties were addressed quantitatively in the form of estimated confidence ranges around central risk estimates, other uncertainties and the variability inkey inputs were not reflected inthese confidence ranges, butrather were partially characterized through separate sensitivity analyses or discussed qualitatively.

Key observations and insights from the 03 risk assessment, together with important caveats andlimitations, were discussed in section H1.B ofthe proposal. In general, estimated risk reductions associated with going from current 03 levels to justmeeting the current and 

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alterhative 8-hour standards show patterns ofincreasing estimated risk reductions associated withjust meeting the lower alternative 8-hour standards considered. Furthermore, the estimated percentage reductions inrisk were strongly influenced bythe baseline air quality year used inthe analysis (see StaffPaper, Figures 6-1 through 6-6)

Key observations important in comparing estimated health risks associated with attainment ofthe current NAAQS and alternative standards included:

(1)As discussed in the Staffpaper (section 5.4.5), EPA has greater confidence in relative comparisons in risk estimates between alternative standards than in the absolute magnitude ofrisk estimates associated with any particular standard.

(2) Significant year-to-year variability in 03 concentrations combined withthe

use of a 3-year designvalue to determine the amount ofair quality adjustment tobe applied to each year analyzed, results in significant year-toyearvariability inthe annual health risk estimates uponjust meeting the current and potential alternative standards.

(3) There-is noticeable city-to-city variability in estimated 0 3-related incidence ofmorbidity and mortality

across the 12 urban areas analyzed for both recent years ofair 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

13PRB 03concentrations used inthe 03 risk

assessment were defined in chapter 2 ofthe Staff Paper (EPA, 2007, pp. 2-48, 2-54) as the 03 concentrations that would beobserved in the U.S. in the absence ofanthropogenic emissions of precursors (e.g., VOC, NOx, and CO) in theU.S., Canada, and Mexico. Based onruns oftheGEOSCHEM model (aglobal tropospheric 03 model) applied for the 2001 warm season (i.e., April to September), monthly background daily diurnal profiles for eachofthe 12 urban areas foreach
month of the 03 season were simulated using meteorology for the year 2001. Based on these model runs, the Criteria Document states that

current estimates ofPRB 03 concentrations are generally in the range of0.015 to 0.035 ppm in the afternoon, and they are generally lower under conditions conducive to high 03 episodes. They are highest during spring dueto contributions from hemispheric pollution andstratospheric intrusions. The Criteria Document states that the GEOS-CHEM model applied for the 2001 warm season reports discussed in the StaffPaper (section 5.4.3), alternative assumptions about background levels had avariable impact depending onthe health effect considered and the location and standard analyzed in terms ofthe 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. 14With respect to 03- related non-accidental mortality, while notable differences (i.e., greater than 50 percent) were observed in some areas, particularly formore stringent standards, the overall pattern of estimated reductions, expressed in terms ofpercentage reduction relative to the current standard, was significantly less impacted. (5) Concerning the part ofthe risk
assessment based on effects reported in epidemiological studies,-important uncertainties include uncertainties (1) surrounding estimates ofthe 03 coefficients for concentration-response relationships used in the assessment, (2) involving the shape ofthe concentration-response relationship and whether or not apopulation threshold

or non-linear relationship exists within the range ofconcentrations examined in the studies, (3) related tothe extent to which concentration-response relationships derived from studies in a given location andtime when 03 levels'

were higher or behavior and/or housing conditions were different provide
accurate representations ofthe relationships for the same locations with lower airquality distributions and/

or differentbehavior 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 03 and non-accidental and cardiorespiratory mortality actually reflect causal relationships.

As discussed in theproposal, some of these uncertainties have been addressed quantitatively inthe form ofestimated confidence ranges around central risk estimatds; others are addressed through separate sensitivity analyses (e.g., the

PRB 03concentrations for afternoon surface air over the United States that are likely 10ppbv too high
in the southeast in summer, and apcurate within 5 ppbv in otherregions and seasons.

14Sensitivity analyses examining the impact of alternative assumptions about PRB were only conducted for lung function decrements andnonaccidental mortality. influence of alternative estimates for policy-relevant background levels) or

are characterized qualitatively. For both parts ofthe health risk assessment, statistical uncertainty dueto sampling

error has been characterized and is expressed in terms of95 percent credible intervals. EPA recognizes that these credible intervals do notreflect all ofthe uncertainties noted above.

B.Need forRevision ofthe Current Primary03 Standard

1.Introduction

The initial issue tobe addressed in this review-of the primary 03 standard is whether, in view ofthe advances in

- scientific knowledge reflected in the Criteria Document and StaffPaper, the current standard shouldbe revised. As discussed in section I.C oftheproposal, in evaluating whether itwas appropriate to propose to retain orrevisethe current standard, the Administrator built upon the last review andreflected thebroader body ofevidence and information now available. Inthe proposal, EPA presented information,judgments, and conclusions from the last review, which revised the level, averaging time, and form ofthe standard, from the Staff Paper's evaluation ofthe adequacy of the current primary standard, including both evidence- and exposure/risk-based considerations, aswell as from the CASAC Panel's advice and recommendations. The StaffPaper evaluation, CASAC Panel's views, and the Administrator's proposed conclusions on the adequacy ofthe
current primary standard are presented below..

a. StaffPaper Evaluation

The StaffPaper considered the evidence presented in the Criteria Document as abasis for evaluating the adequacy ofthe current 03 standard, recognizing that important uncertainties remain. The exten~ive body ofhuman clinical, toxicological, and epidemiological evidence, highlighted above in section II.A.2 and discussed in section I1.A ofthe proposal, serves as the basisforjudgments about 0 3-related health effects, includingjudgments about causal relationships with arange ofrespiratory 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 that03 directly orindirectly contributes to non-accidental and cardiorespiratory-related mortality. 

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Thesejudgments take into account important uncertainties that remain in interpreting this evidence. Forexample, with regard to the utility oftime-series epidemiological studies to inform judgments about a NAAQS for an individual pollutant, such as 03, within

amix of highly correlated pollutants, such as the mix ofoxidants produced in photochemical reactions in the atmosphere, the StaffPaper noted that there are limitations especially at ambient 03 concentrations below levels
atwhich 0 3-related effects havebeen observed in controlled human exposure studies. The StaffPaper also recognized that the available epidemiological evidence neither supports nor refutes the existence ofthresholds atthe population level for effects such as increased hospital admissions and prematuremortality. There are limitations in epidemiological studies thatmake 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 0 3-related effects inpopulations.

While noting these limitations inthe interpretation ofthe findings from the epidemiological studies, the StaffPaper concluded thatif apopulation threshold level does exist, itwould likely be well below the level ofthe current 03 standard and possibly within the range ofbackground levels. This conclusion is supported by several epidemiological studies that have explored the question ofpotential thresholds either by using a statistical curve-fitting approach to evaluate whether linear or non-linear models fitthe data better using, or by analyzing, sub-sets ofthe data where days over or under a specific cutpoint (e.g., 0.080 ppm or even lower 03 levels)

were excluded andthen evaluating the association forstatistical significance. In addition to consideration ofthe 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 atleast 0.060 ppm, with a small percentage ofsubjects experiencing notable effects (e.g., >10 percent FEVI decrement, pain on deep inspiration). Controlled human exposure studies evaluated inthe last review also found significant responses in indicators of lung inflammation and cell injury at 0.080 ppm inhealthy adult subjects. The effects inthese controlled human exposure studies were observed in healthy young adult subjects, and it is likely that more serious responses, and
responses atlowdr levels, would occur in people with asthma and other respiratory diseases. These physiological effects can lead to aggravation ofasthma and increased susceptibility to respiratory infection. The observations provide support for the conclusion in the StaffPaper 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 03 levels well belowthe current standard (i.e., 0.084 ppm) (p. 6-7).

The newly available information reinforces thejudgments in the Staff Paper fromthe lastreview about the likelihood ofcausal relationships between 03 exposures andrespiratory effects andbroadens the evidence of0 3- related associations to include additional respiratory-related endpoints, newly identified cardiovascular-related health endpoints, and mortality. Newly available evidence also led the Staff Paperto conclude that people with asthma are likely to experience more serious effects thanpeople who do not have asthma. The StaffPaper also concluded that substantial progress has been made since the lastreview in advancing the understanding of potential mechanisms by which ambient 03, alone and in combination with other pollutants, is causally linkedto a range ofrespiratory-related health endpoints,

- and maybe causally linked to a range

of cardiovascular-related health endpoints. Thus, the StaffPaper found strong support inthe evidence available since the lastreview, for consideration ofan 03 standard that is at least as protective as the current standard and finds no support forconsideration ofan 03 standard that is lessprotective than the current standard. This conclusion is consistent with the advice and recommendations ofthe CASAC Panel and with the views expressed by all interested parties who provided
comments on drafts ofthe StaffPaper. While the CASAC Panel and some commenters on drafts ofthe StaffPaper 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 ofthe StaffPaper supported a standard thatwould be'less protective than the current standard. i.Evidence-Based Considerations

Inlooking 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 ofhuman health effects that are directly attributable to acute exposures to 03perse.Evidence from such human studies, together with animal toxicological studies, help to provide biological plausibility forhealth effects observed in epidemiological studies. In considering the available evidence, the StaffPaper focused on studies that examined health effects that havebeen demonstrated to be caused by exposure to 03, or for which the Criteria Documentjudges associations with 03 to be causal or likely causal, or for which the evidence is highly suggestive that03 contributes to thb reported effects.

Inconsidering the epidemiological evidence as a basis forreaching conclusions about the adequacy ofthe
current standard, the StaffPaper 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 StaffPaper considered the extent to which such studies provide evidence of associations thatextend down to ambient 03 concentrations below the level ofthe
current standard, which would thereby call-into question the adequacy ofthe
current standard. In so doing, the Staff Paper noted that ifa population threshold level does exist for an effect observed in such studies, it would likely be ata level well below the level ofthe
current standard. The StaffPaper also attempted to characterize whether the

area in which a studywas conducted likely would orwould not havemet the
current standard during the time ofthe study, although itrecognizes thatthe 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 03 concentrations, is not dependent on this distinction. Further, the StaffPaper considered studies that examined subsets ofdata that include only days withambient 03concentrations below the level ofthe current 03 standard, or below even lower 03 concentrations, and continue to report statistically significant associations. The StaffPaper judged that such studies are directly relevant to considering the adequacy of the current standard, particularly in light ofreported responses to 03 at 

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levels below the current standard found incontrolled human exposure studies.

The StaffPaper evaluation ofsuch studies is discussed below andin section II.C.2.a ofthe proposal, focusing in turn on studies of(1) lungfunction, 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 otherrespiratory-related physiological effects. Health effects for which the Criteria Document continued
to find clear evidence ofcausal associations with short-term 03
exposures include lung function decrements, respiratory sympfoms, pulmonary inflammation, and increased airwayresponsiveness. Inthe last review, these 0 3-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 arethe 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 ofthe data from the most recent study shows small group mean decrements in lung functionresponses 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 of0 3-related lung function decrements and respiratory symptoms at this lower exposure level.

The StaffPaper 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 03relative 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 ofthe physiological basis forincreased 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 somepeople 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 andrhinitis have increased airwayresponsiveness to allergens following 03 exposure. Exposures to 03 exacerbated lung function decrements inpeople with preexisting allergic airway disease, with and without asthma. Ozone-induced exacerbation ofairway responsiveness persists longer and attenuates more slowly than 0 3-induced lung function decrements andrespiratory symptom responses andcan have important clinical implications for asthmatics.

The StaffPaper also concluded that newly available human exposure studies suggest that some people with asthma alsohave increased inflammatory responses, relative to nonasthmatic subjects, and that this inflammation maytake 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 orwithout asthma) comprise susceptible groups for 0 3-induced adverse effects. This body of evidence qualitatively informs the Staff Paper's evaluation ofthe adequacy of the current 03 standard inthat it indicates that controlled human
exposure and epidemiological panel studies oflung function decrements and respiratory symptoms that evaluate only healthy, non-asthmatic subjects likely underestimate the effects of03 exposure on asthmatics and other susceptible populations.

The StaffPapernoted that in addition
to the experimental evidence oflung function decrements, respiratory symptoms, and other respiratory effects in healthy and asthmatic populations discussed above, epidemiological

- studies havereported 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 ona daily basis (Mortimer etal., 2002, the National Cooperative Inner-City Asthma Study,

or NCICAS; and Gent etal.,2003), as well as several smallerU.S. and international studies, havereported robust associations between ambient 03 concentrations and measures oflung function, daily respiratory symptoms (e.g., chest tightness, wheeze, shortness ofbreath), andincreased asthma medication use in children with moderate to severe asthma. Mortimer et al. (2002) found that ofthe pollutants measured (including 03, NO2, SO2 and PM10), 03 was the only one thathad a statistically significant effect onlung function. (Mortimer eta." 2002) also found associations between NO2, SO 2 andPM10 and respiratory symptoms that

were stronger than those between 03 andrespiratory symptoms. Gent et al. (2003) found that in co-pollutant models, 03 but not PM2. 5 significantly predicted increased risk ofrespiratory symptoms and rescue medication use among children using asthma maintenance medication. Overall, the multi-city NCICAS (Mortimer etal., 2002), (Gent etal. 2003), and several other single-city studies indicate a robust positive association between ambient 03 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 thatmost such studies that reported positive and often statistically significant associations in the warm

season were donducted in areas that likelywould not have metthe current standard. In considering the large multicity NCICAS (Mortimer etal.,2002), the StaffPaper noted that the 98th percentile 8-hour daily maximum 03 concentrations at the monitor reporting thehighest 03 concentrations in each of the study areas ranged from 0.084 ppm*
to > 0.10 ppm. However, the authors indicate thatless than 5 percent ofthe days in the eighturban areas had 8-hour daily 03 concentrations exceeding 0.080
ppm. Moreover, the authors observed thatwhen days with 8-hour average 03 levels greater than 0.080 ppmwere excluded, similar effect estimates were

seen compared to estimates that included all ofthe days. There are also

a few other studies in which the relevant airquality statistics provide

some indication that lung function and respiratory symptom effects may be occurring in areas thatlikely would have met the current standard (EPA, 2007b, p. 6-12).

(2) Respiratory hospital admissions and emergency department visits. At the time ofthe lastreview, many time-series studies indicated positive associations between ambient 03 and increased respiratory hospital admissions and
emergency roomvisits, providing strong evidence for a relationship between Q3
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preexisting lung disease extending below the level ofthe then current 1- hour 03 standard (EPA 2007b, section 3.3.1.1.6). Analyses ofdata from studies conducted in the northeastern U.S. indicated that 03 airpollution 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 03 and unscheduled hospital admissions forrespiratory

causes. Large multi-city studies, as well

as many studies from individual cities, have reported positive and often statistically significant 03 associations with total respiratory hospitalizations as well as asthma- and chronic obstructive pulmonary disease (COPD)-related hospitalizations, especially in studies analyzing the 03 effect during the

summer or warm season. Analyses using multipollutant regression models generally indicate thatcopollutants do not confound the association between 03 and respiratory hospitalizations and that the 03 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 03 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 orwere reanalyzed to address GAM-related problems), the StaffPaper noted that many such studies were conducted in

areas that likelywould not havemet the
current 03 standard, with many providing only all-year effect estimates, and with some reporting a statistically significant association in the warm

season. Ofthe studies that provide some indication that 0 3-related respiratory hospitalizations may be occurring in

areas that likely would have met the
current standard, the StaffPaper noted that some are all-year studies, whereas others reported statistically significant

warm-season associations.

Emergency department visits for respiratory causes havebeen the focus ofa number ofnew studies that have examined visitsrelated to asthma, COPD, bronchitis, pneumonia, and otherupper andlower 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 03. However, inconsistencies

were observed which were at least partially attributable to differences in model specifications and analysis approach among various studies. In general, 03 effect estimates from summer-only analyses tended to be positive and larger compared to results from cool season or all-year analyses. Almost all ofthe studies that reported statistically significant effect estimates

were conducted in areas that likely wouldnot have met the current standard. The Criteria Document concluded that analyses stratified by

season generally supported a positive association between 03 concentrations andemergency department visits for asthma in the warm season. These studies provide evidence of effects in

areas that likely would not have metthe
current standard and evidence of associations that likely extend down to relatively low ambient 03 concentrations. (3)Mortality. The 1996 Criteria Document concluded that an association between daily mortality and 03 concentrations for areas with high 03 levels (e.g., Los Angeles) was suggested. However, dueto inconsistencies in the results from the very limitednumber of studies available at that time,there was insufficient evidence to determine whether the observed association was likely causal, andthus the possibility that 03 exposure may be associated with mortality was not relied upon in the 1997 decision on the 03primary standard.

Since the last review, the body of evidence with regard to 0 3-related health effects has been expanded by animal, controlled human exposure, and epidemiological studies andnow identifies biologically plausible mechanisms by which 03may affect the cardiovascular system. In addition, there is stronger information linking 03 to serious morbidity outcomes, such as hospitalization, that are associated with increased mortality. Thus, there is now

acoherent body ofevidence that describes arange ofhealth outcomes from lungfunction decrements to hospitalization and premature mortality.

Newly available large multi-city studies and related analyses (Bell etal., 2004; Huang etal., 2005; and Schwartz, 2005) designed specifically to examine the effect of03 and other pollutants on mortality have provided much more robust andcredible information. Together these studies havereported significant associations between 03 and mortality that were robustto adjustment forPM and different adjustment methods fortemperature and suggest that the effect of 03 on mortality may be immediate butmay also persist for several days. Further analysis ofone of these multi-city studies (Bell et

### al., 2006) examined the shape ofthe concentration-response function for the 0 3-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, ifit exists, would likely occur atvery low concentrations, farbelow the level ofthecurrent 03 NAAQS and nearing background levels.

New data are also available from several single-city studies conducted worldwide, as well as from several meta-analyses thathave combined information from multiple studies. Threerecent meta-analyses evaluated potential sources ofheterogeneity in 03- mortality associations. Allthree 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 03 exposure and all-cause mortality even after adjustment for the influence of

season and PM. The results ofthese analyses of studies considered in this review indicate that copollutants generally do not appear to substantially confound the association between 03 and mortality. In addition, several single-city studies observed positive associations ofambient 03 concentrations with total nonaccidental and cardiorespiratory mortality.

Finally, from those studies that included assessment ofassociations with specific causes of death, it appears that effect estimates for associations with cardiovascular mortality are larger than those fortotal mortality; effect estimates for respiratory mortality are less consistent in size, possibly due to reduced statistical power in this subcategory ofmortality. 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 thatthese findings are highly suggestive that short-term 03 exposure directly or indirectly contributes to nonaccidental and cardiorespiratory-related mortality, but

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additional research is needed to more fullyestablish underlying mechanisms by which such effects occur.15
ii.Exposure- and Risk-Based Considerations

In evaluating the adequacy ofthe current standard, the StaffPaper also considered estimated quantitative
exposures andhealth risks, and important uncertainties andlimitations in those estimates, which are highlighted above in section II.A.3 and discussed in section II.B oftheproposal. These estimates are derived from an EPA assessment ofexposures andhealth risks associated with recent airquality levels and with air quality simulated to just meet the current standard to help informjudgments about whether or not the current standard provides adequate protection ofpublic health.

The StaffPaper (andthe CASAC Panel) recognized that the exposure and risk analyses could not provide afull picture ofthe 03exposures and 03- related health risks posed nationally. The Staff Paperdid nothave sufficient information to evaluate allrelevant atrisk groups (e.g., outdoor workers, children under age 5) or all03-related health outcomes (e.g., increased medication use, school absences, and emergency department visits that are part of abroader pyramid ofeffects), and the scope ofthe StaffPaper analyses

was generally limited to estimating exposures and risks in 12 urban areas
across the U.S., and to only five orjust

one area for some health effects included inthe risk assessment. Thus, due to the limited geographic scope of the exposure and risk assessments, EPA recognizes thatnational-scale public health impacts ofambient 03 exposures would be muchlarger than the quantitative exposure and risk estimates associated with recent air quality or air quality thatjust meets the current or alternative standards in the 12 urban

areas analyzed. Onthe other hand, inter-individual variability in

1 5 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 ofthese associations complicated by the fact that the day-to-day variation in concentrations ofthese pollutants is, to a varying degree, determiried by meteorology, the pollutants are often part ofa large and highly correlated mix ofpollutants, only a very few ofwhich are measured" (Henderson, 2006b). Even with these uncertainties, the CASAC Ozone Panel, in itsreview ofthe StaffPaper, found "* * * premature total non-accidental and cardiorespiratory mortality for inclusion in the quantitative risk assessment to be appropriate." (Henderson, 2006b) responsiveness means that only a,subset ofindividuals in each group estimated
to experience exposures at and above a given benchmark level while atelevated exertion would actually be expected to experience such adverse health effects.

The StaffPaper estimated exposures andrisks forthe three most recent years (2002-2004) for which data were available at the time ofthe analyses. As discussed above in sectibn II.A.3.a, within this 3-year period, 2002 was a year with relatively higher 03 levels in most, but not all, areas and simulation ofjust meeting the current standard based on 2002 airquality data provides

a generally higher-end estimate of exposures and risks, while 2004 was a year with relatively lower 03 levels in most, but not all, areas and simulation ofjust meeting the current standard using 2004 air quality data provides a generally lower-end estimate of exposures and risks.

The Staff Paper consideration ofsuch exposure andrisk analyses is discussed below and in section II.C.2.b ofthe proposal, focusing onboth the exposure analyses and the human health risk
assessment.

(1)Exposure analyses. EPA's exposure analysis estimated personal exposures to ambient 03 levels at and above specific benchmark levels while at elevated exertion to provide some perspective on the potential public health impacts ofrespiratory 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 bymeeting the current and alternative standards. As noted above in section II.A.3, theStaffPaper referred to exposures at and above these benchmark levels as "exposures of concern." The StaffPaper noted that potential public health impacts likely•

occur across a range of03 exposure levels, such that there is no one exposure level that addresses all relevant public health impacts. Therefore, with the concurrence ofthe CASAC Panel, the StaffPaper estimated exposures of concern not only at 0.080 ppm 03, a level at which there are demonstrated effects, but also at 0.070 and 0.060 ppm 03. The StaffPaper recognized that there willbe varying degrees ofconcern about exposures at each ofthese levels, based in part onthe population subgroups experiencing them. Given that there is clear evidence ofinflammation, increased airway responsiveness, and changes inhost defenses inhealthy people exposed to 0.080 ppm 03 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 03 concentrations, the StaffPaper focused

on exposures at or above benchmark levels of 0.070 and 0.060 ppm 03 while
at elevated exertion for purposes of evaluating the adequacy ofthe current standard.

Exposure estimates were presented in the Staff Paperand in section H.B (Table 1) ofthe proposal for the number and percent of all school age children and asthmatic school agechildren exposed, andthe number ofperson-days (occurrences) ofexposures, with daily 8- hourmaximum exposures at or above several benchmark levels while at intermittent moderate or greater exertion. The percent ofpopulation 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 atthe current standard level, in estimates ofthe number ofchildren and the number of occurrences of exposures at both ofthese benchmark levels while at elevated exertion. The StaffPaper stated that it is appropriate to consider notjust the average estimates across all years, but also to consider public-healthimpacts in years with relatively higher 03 levels. The StaffPaper also noted thatthere is substantial city-to-city variability in these estimates, and notes that itis appropriate to considernotjust the aggregate estimates across all.cities, but alsoto consider thepublic health impacts in cities where these estimates

arehigher 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 03 exposures at or above 0.070 ppm while engaged in moderate or greater exertion 1 or more times during the 0 3 season) associated with 2002 03 air quality levels. In contrast, about 17 percent of asthmatic and all school age children are estimated to experience
exposures at or above the0.070 ppm benchmark level while at elevated exertion associated with 2004 03 air quality levels.Just meeting the current standard results in an aggregate estimate ofabout 20 percent of asthmatic or 18 percent ofall school age children likely
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0.070 ppm benchmark level while at elevated exertionusing the 2002 simulation. The exposure estimates for this benchmark level range up to about 40 percent ofasthmatic 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 ofabout 1 percent ofasthmatic or all school age children experiencing exposures exceeding the 0.070 ppm benchmark level while at elevated exertion.

Atthe benchmark level of0.060 ppm, about 70percent ofall or asthiatic school age children are estimatedto experience exposures at or above this benchmark level while atelevated exertion forthe aggregate ofthe 12 urban areas associated with 2002 03 levels. Just meeting the current standard would result in an aggregate estimate of about 45 percent ofasthmatic 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 ofall

or asthmatic school age children in the single city with the highest estimate among the cities analyzed associated withjust meeting the current standard usingthe 2002 simulation. The Staff Paper indicated an aggregate estimate of about 10 percent ofasthmatic or all school age children would experience exposures at or above the 0.060 ppm benchmark level while at elevated exertion associated withjust 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 FEVI) in asthmatic school age children for 5 urban areas (a subset ofthe 12 urban areas); (3)respiratory symptoms (i.e., chesttightness, shortness ofbreath, wheeze) in moderate to severe asthmatic children forthe Bostonarea; (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 forjust 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 theestimated number of school age children experiencing one or more

occurrences ofFEV decrements > 15 percent for the 12 urban areas, going from about 1.3 million children (7 percent ofchildren) 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 ofchildren) 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 FEV decrements >10 percent for the 5 urlban areas goes from about 250,000 children (16 percent ofasthmatic children) under 2002 air quality to about 130,000 (8 percent ofasthmatic children) using the 2002 simulation, andfrom about 160,000 (10 percent of asthmatic children) to about 70,000 (4 percent ofasthmatic children) usingthe 2004 simulation. Thus, even when the
current standard is met, about 4 to 8 percent ofasthmatic school age children

are estimated to experience one ormore

occurrences ofmoderate lung function decrements, resulting in about 1million occurrences (using the 2002 simulation) and nearly 700,000 occurrences (using the 2004 simulation) injust 5 urban

areas. Moreover, the estimated number of occurrences ofmoderate orgreater lung function decrements per child is

on average approximately 6 to 7 in all children and 8 to 10 in asthmatic children in an 03 season, even when the
current standard is met, depending on the year used to simulate meeting the
current standard. In the 1997review of the 03 standard a general consensus view ofthe adversity ofsuch moderate respoifses emerged as the frequency.of

occurrences increases, with the judgment that repeated occurrenpes of moderate responses, even in otherwjise healthy individuals, may be considered adverse since they may well setthe stage for more serious illness.

With regard to estimates oflarge lung function decrements, the StaffPaper noted thatFEV decrements > 20 percent would likely interfere with normal activities in many healthy individuals, therefore single

occurrences would be considered tobe 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 underthe ATS definition. They also wouldbe cause for medical concern in some individuals. While the current standard reduces the occurrences of large lung function decrements in allchildren and asthmatic children from about 60 to 70%, in a yearwith relatively higher 03 levels (2002), there are estimated to be about 500,000 occurrences in all school children across the entire 12urban
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 athree-year period, 03 levels in each year can vary considerably, as evidenced by relatively large differences between risk estimates based on 2002 to 2004 airquality. The StaffPaper expressed theview that it was appropriate to consider this yearly variation in 0 3levels allowed by the current standard injudging the extent to which impacts on members ofat-risk
groups in a year with relatively higher 03 levels remain ofconcern from a public health perspective.

With regard to other0 3-related health effects, the estimated risks ofrespiratory symptom days in moderate to severe asthmatic children, respiratory-related hospital admissions, and non-accidental and cardiorespiratory mortality, respectively, arenot reduced to as great

an extent bymeeting the current standard as are lung function decrements. For example, just meeting the current standard reduces the estimated average incidence ofchest 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 03exposure (Table 6-4). Just meeting the current standard is estimated to reduce the incidence ofrespiratory-related hospital admissions in theNew 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 of4.6 to 6.4, respectively. Across the 12 urban areas, the estimates ofnon'-accidental mortality incidence per 100,000 relevant population range from 0.4 to 2.6 (for 2002) and 0.5 to 1.5 (for2004). Meetifig the current standard results in a reduction ofthe estimated incidence per 100,000 population to arange of 0.3 to 2.4 based onthe 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 ofthe proportion'of population affected and the number of occurrences ofthe health effects that are included in the risk assessment, the StaffPapernoted that 

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these limited estimates are indicative of

a much broader array ofpotential 03- related health endpoints that we consider part ofa "pyramid ofeffects" thatinclude 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 ofatrisk groups. While the StaffPaperhad sufficient information to estimate and consider the number ofsymptom 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 doct6rs' offices, for which there was not sufficient information to estimate risks but which are important to consider in assessing the adequacy ofthe current standard. The same is true for more serious, but less frequent effects. The StaffPaper estimated hospital admissions, but there was not sufficient information to estimate emergency department visits in a quantitative risk assessment. Consideration ofsuch unquantified risksin the StaffPaper reinforced the StaffPaper conclusion that consideration should begiven to revising the standard so asto 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 StaffPaper Considerations

The StaffPaper concluded that the overall body ofevidence clearly calls into question the adequacy ofthe
current standard inprotecting at-risk
groups against an array of adverse health effects that range from decreased lung function and respiratory symptoms
to serious indicators ofrespiratory morbidity including emergency department visits and hospital admissions forrespiratory causes, nonaccidental mortality, and possibly cardiovascular effects. These at-risk
groups notably include asthmatic children and other people with lung disease, as well as allchildren and older adults, especially those active outdoors, and outdoor workers.16 The available information provides strong support for consideration ofan 03 standard that would provide increased health protection for these at-risk groups. The

16ln defining at-riskgroups this way we are including bothgroups with greaterinherent sensitivity and those more likely to be exposed. StaffPaper also concluded that risks projected to remain upon meeting-the
current standard are indicative ofrisks
to at-risk groups that can bejudged to be important from a public health perspective. This information reinforced the StaffPaper conclusion that consideration should be given to revising the level ofthe standard so as to provide increased public health protection.

b. CASAC Views

The CASAC Panel unanimously concluded in a letter tothe Administrator that there is "no -scientifi"justification forretaining" the
current primary 03 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 inthe 1997 03 NAAQSxeview" (id., p. 3). The Panel noted that several new single-city studies andlarge 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. ThePanel specifically notedevidence from therecent Adams (2006) study that reported statistically significant decrements in the lung function ofhealthy, moderately exercising adults at a 0.080 ppm exposure level, and importantly, also reported adverse lung function effects in

somehealthy 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 tobe 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 03 exposures than would healthy volunteers (Mortimer etal., 2002)" (Henderson, 2006c, p. 4).

The CASAC Panel also highlighted a number of0 3-related adverse health effects that are associated with exposure
to ambient 03, below the level ofthe
current standard based on a broadrange ofepidemiological studies (Henderson, 2006c). These adverse health effects include increases in school absenteeism, respiratory hospital emergency department visits among asthmatics and patients withother respiratory diseases, hospitalizations forrespiratory 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 ofthese findings to be an important indicator ofadverse 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 canbe clinically significant, rather than on the relatively small average decrements. The Panel also emphasized significant 0 3-related.inflammatory responses and markers ofinjury to the epithelial lining ofthe lung that are independent of spirometric responses. Further, the Panel expressed the view that the Staff Paper did notplace enough emphasis on serious morbidity (e.g., hospital admissions) and mortality observed in epidemiological studies. On thebasis of the large amount ofrecent data evaluating adverse health effects at levels at andbelow the current 03 standard, it was the unanimous opinion ofthe CASAC Panel that the current primary 03 standard is not adequate to protect human health, thatthe relevant scientific data do not support consideration ofretaining the current standard, and that the current standard needs to be substantially reduced to be protective ofhuman 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 ofdata clearly demonstrates adverse human health effects at the current level" ofthe standard, such that "[Rietaining this standard would continue to put large numbers ofindividuals at risk for respiratory effects and/or significant impact onquality oflife including asthma exacerbations, emergency room visits,hospital admissions and mortality" (Henderson, 2006c).

c. Administrator's Proposed Conclusions

At the time ofproposal, in considering whether the current primary standard shouldbe revised, the Administrator carefully considered the conclusions contained in the Criteria Document, the rationale and recommendations contained in the Staff Paper, the advice andrecommendations 

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from CASAC, and public comments to date on this issue. In so doing, the Administrator noted the following: (1) That evidence ofa range ofrespiratoryrelated morbidity effects seen inthe 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) thatnew evidence from controlled human
exposure and epidemiological studies identifies people with asthma (including children with asthma) as an important susceptible population for which estimates ofrespiratory effects in the general population likely underestimate the magnitude or importance ofthese effects; (3) that new evidence about mechanisms oftoxicity further contributes to the biological plausibility of0 3-induced respiratory effects and isbeginning to suggest mechanisms that may link 03 exposure to cardiovascular effects; (4) thatthere is

now relatively strong evidence for associations between 03 and total nonaccidental and cardiopulmonary mortality, even after adjustment forthe influence ofseason and PM; and (5) the limits ofthe 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 03exposures, that indicators ofrespiratory morbidity such as emergency department visits and hospital admissions are causally related
to 03 exposures, andthat the evidence is highly suggestive that 03exposures during the 03 season contribute to premature mortality.

The Administrator judged thatthere is important new evidence demonstrating that exposures to 03 at levelsbelow the level ofthe current standard are associated with a broad array ofadverse 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 03, children and older adults with increased susceptibility, as well asthose who arelikely to be vulnerable as aresult ofspending alot oftime outdoors engaged in physical activity, especially active children and outdoor workers. Examples ofthis important new evidence include demonstration of0 3-induced lung function effects and respiratory symptoms in some healthy individuals down to the previously observed exposure level of0.080 ppm,as well as very limited new evidence at exposure levels well below the level ofthe current standard. In addition, there is

now epidemiological evidence of statistically significant 0 3-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 03 concentrations that are below the level ofthe current standard. Further, there

are a few studies that have examined subsets of datathat include only days with ambient 03 concentrations below the level ofthe current standard, or below even much lower03 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 ofthe respiratory morbidity associations observed in the epidemiological studies and for concluding that the associations extend below the level ofthe current standard. However, the Administrator recognized that inthe body ofepidemiological evidence, many studies reported positive and statistically significant associations, while others reported positive results thatwere not statistically significant, and a few did
not report any positive 0 3-related associations. Inaddition, the Administratorjudged thatevidence ofa causal relationship between adverse health outcomes and 03 exposures became increasingly uncertain atlower levels of exposure.

Based onthe strength ofthecurrently available evidence ofadverse health effects, and on the extent towhich the evidence indicates that such effects likely result from exposures to ambient 03 concentrations below the level ofthe
current standard, the Administrator judged that the current standard does not protectpublic health with an adequate margin of safety andthat the standard shouldbe revised to provide such protection, especially for at-risk
groups, against abroad array ofadverse health effects.

In reaching thisjudgment, the Administrator had also considered the results ofboth the exposure and risk assessments conducted forthis review,
to provide some perspective on the
extentto which at-risk groups would likely experience "exposures of concern" 17 and on the potential magnitude ofthe risk ofexperiencing various adverse health effects when
recent air quality data (from 2002 to 2004) are used to simulate meeting the
current standard and alternative standards ina number ofurban 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, aswell asproviding informed, but still imprecise, estimates ofthe potential magnitude ofrisks.

In considering the exposure assessment results atthe 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 ofexposures in at-risk groups at and above these benchmark levels while at elevated exertion, using 03 air quality data in 2002 and 2004, provide

some indication ofthe potential magnitude.of the incidence ofhealth
outcomes thatcannot 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 03 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 ofasthma, increased medication use, increased school andwork absences,

" ' 7 As discussed in section II.A.3 above, "exposures ofconcern" are estimates ofpersonal exposures while atmoderate or greater exertion to 8-hour average ambient03 levels at and above specificbenchmark levels which represent exposure levels at which0 3-related health effects

are known or can with varying degrees ofcertainty be inferred to occur in some individuals. Estimates
ofexposures ofconcern provide some perspective on-the public health impacts ofhealth effects that may occur in some individuals at recent airquality levelsbut cannot be evaluated in quantitative risk assessments, and the extent to which such impacts mightbe reduced by meeting the current and alternative standards.

18As noted above in section Il.A.3, recent 03 air quality distributions have been statistically adjusted
to simulatejust meeting thecurrent and selected alternative standards. These simulations donot represent predictions ofwhen, whether, or how

areas might meet the specified standards. 

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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.

Inconsidering these various benchmark levels for exposures of

concern atthe time ofproposail, 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 inat-risk groups such as people with asthma. This judgment was based on the strong evidence ofeffects 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 ofexposure. 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 of0.080 ppm 03, asit would not adequately account for the increased risk ofharm from exposure for members ofat-risk groups, especially people with asthma. The Administrator alsojudged-that the evidence of demonstrated effects is too limited to support a primary focus on-exposures down to the lowest benchmark level considered of0.060 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 ofthe likelihood of such effects at lower 03 exposure levels.

The Administrator observed that " based on the aggregate exposure estimates for the 2002 simulation (summarized in section II.B.1, Table 1, ofthe proposal) forthe 12 U.S. urban

areas included in the exposure analysis, uponjust meeting the current standard up to about 20 percent ofasthmatic 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 1percent ofsuch children. The Administrator noted from this comparison that there is substantial year-to-year variability, ranging up to an order of magnitude ormore in estimates ofthe number ofpeople andthe number ofoccurrences ofexposures at and above thisbenchmark level while at elevated exertion. Moreover, within any given year, the exposure assessment indicates that there is substantial cityto-city variability inthe estimates ofthe children exposed orthe number of

occurrences ofexposure atand above this benchmark level while atelevated exertion. For example, city-specific estimates ofthe percent ofasthmatic or all school age children likely to experience exposures atand above the benchmark level of 0.070 ppm while at elevated exertionranges from about 1 percent up to about 40 percent across the 12 urban areas uponjust meeting the current standard based onthe 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 andcity-to-city variability in considering these estimates.

With regard to the results ofthe risk assessment, the Administrator focused

on therisks estimated to remainupon just meeting the current standard. Based

on the aggregate risk estimates (summarized in section II.B.2, Table 2, ofthe proposal), the Administrator obsbrved that uponjust meeting the
current standard based on the 2002 simulation, approximately 8 percent of asthmatic school age children across urban areas (ranging up to about 11 percent in the city with the highest estimate among the cities analyzed) would stillbe estimated to experience moderate orgreater lung function decrements one ormore times within an 03 season. These estimated percentages would be approximately 3percent ofall 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 ofconcern, there is substantial year-to-year and city-to-city variabilityin 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 ofmoderate orgreater 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 03 season, even uponjust meeting the current standard. Inthe lastreview,

a general consensus view ofthe adversity of such responses emerged as the frequency ofoccurrences increases, with the judgment that repeated

occurrences ofmoderate responses,

even in otherwise-healthy individuals, maybe 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 FEV, 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 FEV decrement), would likely interfere with normal activities for most individuals and would also increase the likelihood thatthese individuals would

use additional medication or seek medical treatment. Not only would single occurrences be considered tobe adverse to asthmatic individuals under the ATS definition, but they also would be cause for medical concern forsome individuals. Uponjust meeting the
current standard based onthe 2002 simulation, close to 1percent of asthmatic and all school age children

are estimated to experience one ormore

occurrences oflarge lung function decrements inthe 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 citythat receives relatively lessprotection 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 uponjust 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.

Uponjustmeeting the current standard based on the 2002 simulation, the estimate ofthe 0 3-related riskof respiratory symptom days in moderate
to severe asthmatic children inthe 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 03 exposure during the 03 season ofthe total number ofsymptom days associated with all 

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causes ofrespiratory symptoms in asthmatic children during those years.

The estimated 0 3-related risk of respiratory-related hospital admissions
uponjust meeting the current standard based on the 2002 simulation is greater than 500 hospital admissions inthe New York City area alone, or about 1.5 .percent ofthe total incidence of respiratory-related admissions associated with allcauses; the 2004 simulation yielded an estimate of approximately 400 such hospital admissions. Fornonaccidental mortality, just meeting the current standardbased 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 (AbtAssociates, 2007a, Table 4-26).

The Administrator recognized that in considering the estimates ofthe proportion ofpopulation affected and the number ofoccurrences ofthose specific health effects that are included in the risk assessment, these limited estimates based on 2002 and 2004 simulations are indicative ofamuch broader array of0 3-related health endpoints that are part ofa "pyramid of effects" (discussed in section II.A.4.d of the proposal) thatinclude various indicators ofmorbidity that could notbe 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 ofthe much broader array of 0 3-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 ofpotential
exposures and risks to at-risk groups. In reaching this proposed judgment, the Administrator considered the following factors: (1) The estimates ofnumbers of
persons exposed atand above the 0.070
ppm benchmark level; (2) the risk estimates oftheproportion ofthe. population andnumber ofoccurrences ofvarious health effects in areas upon just meeting the current standard; (3) the year-to-year and city-to-city variability inboth the exposure and risk estimates; ,(4) theuncertainties in these estimates; and (5) recognition that there is abroader array of0 3-related adverse health outcomes forwhich risk estimates could not be quantified (that

are part of abroader "pyramid of effects") and thatthe scope ofthe

assessment was limited tojust a sample ofurban areas andto some but not all at-risk populations, leading to an incomplete estimation ofpublic health impacts associated with 03 exposures

across the country. The Administrator alsonoted that it was the unanimous conclusion ofthe CASAC Panel that there is no scientificjustification for retaining the current primary 03 standard, that the current standard is
not sufficiently heal~th-protective with

an adequate margin of safety, andthat the standard needs to be substantially reduced to protect human health, particularly in at-risk subpopulations.

Based on all ofthese considerations, the Administrator proposed that the
current 03 standard is not requisite to protect public health with an adequate margin of safety because itdoes not provide sufficient protection and that revision would result inincreased public health protection, especially for members ofat-risk groups.

2. Comments on the Need for Revision

The above section outlines thehealth effects evidence and assessments used by the Administrator toinform his proposed judgments about the adequacy ofthe current 03 primary standard. General comments received on the proposal thateither supported or opposed the proposed decision to revise the current 03 primary standard aro 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, orthe human exposure andhealth risk assessments that relate
to consideration of the appropriate averaging time, form, orlevel ofthe 03 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 torevise the current standard are noted inthe Response to Comments document. a. Consideration ofHealth Effects Evidence

With regard to the need to reVise the

currentprimary 03 standard, sharply divergent comments were received from
two general sets ofcommenters. Many public comments received on the proposal asserted thatthe current 03 standard is insufficient to protect public health, especially the health of sensitive
groups, with an adequate margin of safety andrevisions tothe standard are appropriate. Among those calling for revisions tothe current primary standard were medical groups, including for example, the American Medical Association (AMA), the American Thoracic Society (ATS), the American Academy ofPediatrics (AAP), and the American College ofChest Physicians (ACCP), as well asmedical doctors and academic researchers. For example, the ATS stated:

We believe thatthe Administrator has correctly stated that,beyond any degree of scientific uncertainty, convincing and compelling evidence has demonstrated that
exposure toozone at levels below the current standard isresponsible for measurable and significant adverse health effects,both in
terms ofmorbidity and mortality. * * *The known respiratory, cardiac and perinatal effects ofozone pollution are each in their

own rightmajorpublic health issues. In combination they provide immediate, actionable information and require a meaningful publichealth policy response fromthe EPA. tATSetal. 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 ajoint 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 ofCounty 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

etal., 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 ofthese medical, environmental andpublic health
commenters stated that the current 03 standard needs to be revised andthat an

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groups. Many individual commenters also expressed such views.

The majority ofState and local air pollution control authorities who commented on the 03 standard supported revision ofthe current 03 standard, as did the National Tribal Air Association (NTAA). Environmental agencies that supported revisingthe 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 theNational 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 03 standard. All ofthese commenters supported revisions tothe 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 ofevidence assessed inthe Criteria Document, findingit to be stronger andmore compelling than in the last review. Some specifically agreed with the weight ofevidence approach taken by the Criteria Document. These
commenters generally placed much weight on CASAC's interpretation ofthe body of available evidence and the results ofEPA's exposure-and risk assessments, both ofwhich formed the basis for CASAC's recommendation to revise the 03 standard to provide increased public health protection.

In recentyears, abroad scientific

consensus hasemerged thatEPA's current air quality standards for ozone are not sufficient to protectpublic health, and that the levels and form mustbe greatly tightened. This

consensus isevidenced bythebythe strong unanimous comments ofthe CASAC, which

wasbacked bythe endorsement ofover 100' leading independent air quality scientists, EPA'sChildren's Health Protection Advisory Committee, and many others. In theface of this strong consensus, it isuntenable to cite "uncertainty" as arationale forfailing to propose tighter standards. [ALA etql., p. 15]
Medical and public health commenters also ekpressed the view that EPA must not use uncertainty in the scientific evidence asjustification for retaining the current 03 standard.

EPA generally agrees with these commenters' conclusion regarding the' need to revise the current primary 03 standard. The scientific evidencerelated health effects to 03 exposure notedby these commenters was generally the same asthat assessed in the Criteria Document and theproposal. EPA agrees thatthis information provides abasis for concluding thatthe
current 03 standard is not adequately protective ofpublic health. For reasons discussed below in sections II.C.3 and II.C.4, however, EPA disagrees with aspects ofthese commenters' views on the level ofprotection that is appropriate and supported by the available scientific information.

Another group of commenters representing industry aisodiations and businesses opposed revising the current primary 03 standard. These views were extensively presented in comments from the Utility Air Regulatory Group (UARG),.representing agroup ofelectric 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 ofAutomobile Manufacturers (AAM); th6 National Association of Manufacturers (NAM), the American Petroleum Institute (API). The API sponsored a workshop at the University ofRochester in June 2007 to review the scientific information and health risk assessment considered byEPA during the review ofthe 03 NAAQS. Although the report (hereafter, "Rochester Report") fromthis workshop does not offerjudgments onthe specific elements ofthe current or proposed standard, it has been cited in a number ofpublic
comments that opposed revision ofthe current 8-hour standard. The Annapolis Center for Science-Based Public Policy issued a report (hereafter, "Annapolis. Center") on the science andhealth effects of 03, which explicitly opposed revising the current 03 primary standard. Several State environmental agencies also opposed revising the current 03 primary standard, including agencies from: Georgia; Indiana; Kentucky; Louisiana; Nevada; and Texas.

As discussed more fullybelow in sections dealing with specific comments, these and other commenters inthis group generally mentioned many ofthe same studies from the body of evidence in the Criteria Document that

were cited bythe commenters who supported revising the standards, but highlighted different aspects ofthese 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 arisk ofeffects that is significantly different in character from those thatprovided a basis for the current standards, or whether the evidence demonstrates that the risk to public health upon attainment ofthe current standards wouldbe greater than was understood when EPA established the current 03 standard in 1997. These commenters generally expressed the view that the
current standard provides the requisite degree ofpublic health protection.

In supporting their view that the present primary 03 standard continues
to provide the requisite public health protection and should notbe 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; thatthe estimated number of exposures ofconcern and health risks upon attainment ofthe current 03 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 03 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 thatthe current 03 standard is not adequately protective of public health. Therationale forthis position is discussed more fully inthe responses to specific comments that are presented below.

More specific comments on the evidence and EPA's responses are discussed below. Section Il.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 oftheevidence and specific methodological issues. Comments on evidence pertaining to atrisk subgroups for 0 3-related effects can be foundin section II.B.2.a.iii below. EPA notes here thatmost ofthe issues and concernsraised by commenters concerning the health effects evidence, includingboth the interpretation ofthe evidence and specific technical or methodological issues,were essentially
restatements ofissues raised during the review ofthe Criteria Document and the StaffPaper. Most ofthese issues were highlighted andthoroughly discussed during the review ofthese documents by the CASAC. More detailed responses related to the interpretation ofthe health effects evidence and itsrole in the decision onthe 03 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 ofhealth effects evidence, section II.A.2 above,
two new controlled human-exposure studies (Adams 2002, 2006) are now available thatexamine respiratory effects associated with prolonged 03
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 ofcommenters that included national medical (e.g., ATS, AMA, ACCP) and national environmental and public health organizations (e.g., ALA in ajoint set of
comments with Environmental Defense, Sierra Club), agreed with EPA's reanalysis ofthe Adams' data while disagreeing with EPA's characterization ofthe evidence fromthe 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 ofindividuals are more important than group mean responses and that when theAdams (2002, 2006) study data are corrected for the effects ofexercise 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 thatwhile 2 of30 tested subjects responding atthe 0.060 ppm level may seem like a small number, a 7percent response rate is far from trivial. Seven percent ofthe U.S. population is 21.2 million people (ALAetal., p. 51). Noting thatthe 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 ofthe Adams' data, stating thatEPA has undertaken a careful reanalysis ofthe underlying data in the Adams studies to

assess the change in FEV following exposure to 0.060.ppm 03 and filtered air, andconcluding that "the reanalysis employs the standard approach used by otherresearchers, 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 othermedical organizations conclude:

The Adams study confirms our understanding that inhealthy populations,

an important fraction ofthe population will experience larger-than-average decrements in FEVI when exposed to low levels ofozone. It isreasonable to assume thatthese effects would be evengreater When extrapolated to other populations known to have sensitivities
to ozone (children, asthmatics, COPD patients). We feel thecorrect conclusion to drawfrom the Adams study isthat thereis

a significant fraction ofthe population that will express significant responsesto low levels ofozone. [ATS, p. 5]

EPA generally agrees with most ofthe
comments summarized above, while placing more emphasis on the limited
nature of the evidence addressing 03- related lungfunction and respiratory symptom responses atthe 0.060 and 0.040 ppm exposure levels. As characterized in the proposal notice, EPA's reanalysis ofthe data fromthe
mostrecent 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 smallpercentage ofsubjects experiencing lung function decrements (>10percent) atthe 0.060 ppm exposure level. EPA disagrees with these
commenters that the percent of subjects that experienced FEVI decrements greater than 10% inthis study of30 subjects can appropriately be generalized tothe U.S. population. The Administrator concludes that these studies provide very limited evidence of 0 3-related lung function decrements and respiratory symptoms atthis lower
exposure level.

The second group ofcommenters, who opposed revision ofthe standard, raised many concerns about the role of the Adams studies and EPA's reanalysis ofthe Adams data inthe decision. With regard to theresults reported by Adams, these commenters expressed the view that the group mean FEV decrement measured at 0.060 ppm was small, less than 3%, which is within the 3 to 5%
range ofnormal measurement variability for an individual (UARG,p. 12). Moreover even thereported group mean FEVI decrements inAdams subjects when exposed to an 03 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); Withrespect to the larger decrements in FEV (>_ 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 onthe individual responses ofsuch a miniscule number of subjects (2 of30) is woefully inadequate as any basis for

a nationwide 03 standard" (UARG, p.14). Some ofthese commenters put theresults ofthe Adams studies (2002, 2006) in the context ofthe 1997 decision on the 03 standardto reach the conclusion that there is nobasis for revising that standard. They stated that the data from Adams (2002, 2006) on 03 levels below 0.080 ppm was too limited
-tosupport arevised standard, and noted that responses reported in the Adams studies at 0.080 ppm were similar to responses reported previously (Horstmann etal., 1990 and McDonnell
etal., 1991), and therefore, provided no

new information on 03thatwas not known at the tirfle ofEPA's lastreview (ExxonMobil, pp. 5-6).

These commenters raised one ormore ofthe following concerns about EPA's reanalysis ofthe Adams data: (1)EPA's re-analysis was notpublished or peerreviewed, and therefore neither the scientific community nor the public was afforded opportunity to appropriately reviewthe analysis (Exxon Mobil, p. 6); (2) EPA has misinterpreted the studies ofDr. Adams, and overhis objections used a different analytical methodology
to reach adifferent conclusion; (3) EPA's reanalysis did not employ an appropriate statistical test; the ANOVA statistical test employed byAdams was preferred over the statistical test usedin EPA's reanalysis (paired t-test); and (4) thereanalysis ofthe Adams data is evidence thatEPA interpreted and presented scientific information in a systematically biased manner, reflecting purposeful bias because the reanalysis supported staffpolicy recommendations and Adams' own analysis did not, and the 10% decrement in FEV1 was a posthoc threshold chosen for compatibility with EPA staffpolicy recommendations (NAM,p. 19).

First, EPA agrees that the group mean lung fuiction 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 lungfunction decrements

10 percent, which is the most 

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important finding from this study in
terms ofpublic health implications. The magnitude ofchanges in the group mean do not address whether a subset ofthe population is at risk ofhealth effects. The clinical evidence to date makes it clear thatthereis significant variability inresponses across individuals, so itis important to lookbeyond group mean to the response ofsubsets ofthe group to evaluate the potential impact for sensitive or susceptible parts ofthe population. The Administrator also
agrees with bothEPA staffand CASAC's views that this level ofresponse may notrepresent an adverse health effect in healthy individuals but does represent a level that should beconsidered adverse forasthmatic individuals.

Second, EPA notes that its reanalysis ofthe Adams (2006) study was prepared inresponse to the issues and analysis raisedby 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 03 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 priorto publication" ofthe proposal. EPA has clearly stated that the additional statistical analyses conducted by both the public
commenter and by EPA staffdo not contradict or undercut the statistical analysis presented by Dr.Adams in his published study, as EPA andthe author

were addressing different questions. While the author ofthe 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 andfiltered 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 ofthe average lung function response at 0.060 ppm, members ofthe CASAC Panel noted on the March 5, 2007 teleconference the
very conservative nature ofthe approach used by Adams to evaluate the research questions posed bythe author. These same CASAC Panel members also supported the use ofthe statistical approach (i.e., paired-t test)used in the analysis prepared by the public commenter, which was the same approach later used inEPA'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 ofthe Adams (2006) study inthe Rochester Report, which stated, "Although these findings have
not been confirmed orreplicated, 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 usedin Adams (2006) did notidentify theresponse ofthe 0.06 ppm exposure asstatistically different from thatof the filtered air exposure. However, alternative statisticaltests suggest that the observed small group meanresponse in FEVI induced by exposure to0.06 ppm compared to filtered airis not theresult,of chance alone. [Rochester Report, p. 561.

Fourth, EPA rejects the contention that the conduct and presentation ofits reanalysis ofthe Adams (2006) study to address issuesraised bypublic

. 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 majorcomments

on EPA's assessment ofepidemiological studies inthe proposal and the Agency's geneial responses to those comments. Many ofthe issues discussed below are addressed inmore detail in the Response to Comments document. Comments on EPA's interpretation and
assessment ofthe body of epidemiological evidence are discussed first and then comments on methodological issues and particular study designs are discussed. EPA notes here that most ofthe issues and

concerns raised by commenters onthe interpretation ofthe epidemiological evidence and methodological issues are essentially restatements of issues raised during the review ofthe Criteria Document and StaffPaper. EPA presented and the CASAC Panel reviewed the interpretation ofthe epidemiological evidence inthe Criteria Document and the integration ofthe evidence with policy considerations in the development ofthe policy options presented in the StaffPaper for consideration by the Administrator. CASAC reviewed both the 03 Criteria Document and 03 StaffPaper and approved ofthe scientific content and
accuracy ofboth documents. The CASAC chairman sentto the Administrator one letter (Henderson, 2006a) for the 03 Criteria Document and another letter for the 03 StaffPaper (Henderson,2006cl indicating thatthese documents provided an appropriate basis for use in regulatory decision making regarding the 03NAAQS.

As with evidence from controlled human exposure studies, sharply divergent comments were received on the evidence from epidemiological studies, including EPA's interpretation ofthe evidence. One group of
commenters from medical, public health and environmental organizations, in general, supported EPA's interpretation ofthe epidemiological evidence (72 FR 37838, section II.a.3.a-c) with regard to whether the evidence for associations is consistent and coherent andwhethei there isbiological plausibility for judging whether exposure to 03 is causally related torespiratory and cardiovascular morbidity and mortality effects. Comments ofpublic health and environmental groups, including ajoint
set ofcomments fromALA 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 StaffPaper and proposal (72 FR 37842, Figure 1) of short-term 03 exposures and respiratory health outcome showing consistency in

an array ofpositive 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 03 standard. These commenters generally expressed agreement with the weight ofevidence approach taken by the Criteria Document and the conclusions reached, which were reviewed by CASAC, that the effects of 03 on respiratory symptoms, lung function changes, emergency department visits for respiratory and cardiovascular effects, andhospital admissions canbe considered causal.

EPA generally agrees with this interpretation of the epidemiological evidence. The Criteria Document concludesthat positive androbust 

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associations were found between ambient 03 concentrations and various respiratory disease hospitalization
outcomes and emergency department visits for asthma, when focusing particularly on results ofwarm-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 acausal relationship between acute ambient 03
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 03 concentrations reported in these studies. These commenters also expressed the view thatthe respiratory and cardiovascular system effects are well-supported by the Hill criteria'9 of judging causality: strength of association, consistency between studies, coherence among studies, and biological plausibility (ALA etal., pp. 51-52). They also noted that recent studies provide compelling evidence thatexposure to 03 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 03 concentrations reported in these studies. Thebiological plausibility ofthe epidemiological associations is generally supported by controlled human exposure and toxicological evidence of respiratory morbidity effects for levels at and below O.080ppm, but thatbiological plausibility becomes increasingly uncertain atmuch lower levels. Further, at much lower levels, it becomes increasingly uncertain as to whether the reported associations are related to 03 alone rather thanto the broader mix ofair pollutants present in the ambient air. Withregard to cardiovascular health outcomes, the Criteria Document concludes thatthe generally limited body of evidence from animal toxicology, human controlled

1 9 The 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 ofthe epidemiological evidence presented in the CriteriaDocument (pp. 7-3-7-4). exposure, and epidemiologic studies is suggestive that 03 can directly and/or indirectly contribute to cardiovascular related morbidity, andthat 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 bywhich such mortality effects occur (EPA, 2006a, pp. 8-77-78).

The second group ofcommenters, mostly representing industry associations and some businesses opposed torevising the primary 03 standard, disagreed with EPA's interpretation ofthe epidemiological evidence. These commenters expressed the view that while many new epidemiological studies have been published since the current primary 03 standard was promulgated, the inconsistencies and uncertainties inherent in these studies as awhole should preclude any reliance on them as justification for a more stringent primary 03 NAAQS. They contend that the purported consistency is the result ofinappropriate 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 thatthey havethe statistical power to allow the authors to reliably distinguish even weak relationships from the nullhypothesis with statistical confidence. However, these commenters note that these studies are notconsistent, with regard to the findings concerning individual cities analyzed in themulti-city analyses. One commenter asserted that each ofthe multi-city studies and metaanalyses cited by EPA involves cities for' which the city-specific estimates of03 effects havebeen observed to vary over

awide range that includes negative [i.e., beneficial] effects (API, p. 15). To illustrate this point, many commenters point to EPA's use ofthe study by Bell

eta.,2004. They note that in focusing

on the national estimate from Bell ofthe association between 24-hour average 03 levels and daily mortality, the Administrator overlooks the very significant and heterogeneous information ofthe individual analyses ofthe 95 cities used toproduce the national estimate and, based on this inconsistency, question whether what is being seen is actually an 03 mortality association at all (UARG, p. 16).

EPA has accurately characterized the inconsistencies and uncertainties in the epidemiological evidence and strongly denies that ithas inappropriately focused on specific positive studies or specific positive results within those studies. EPA's assessment ofthe health effects evidence inthe Criteria Document has been reviewed by the CASAC Panel. EPA has appropriately characterized the heterogeneity in 03 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 ofthe Criteria Document. In general, in the proposal, the Administrator recognized that in the body ofepidemiological 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 0 3-relatedassociations. In addition, the Administratorjudged that evidence ofa causal relationship between adverse health outcomes and 03 exposures became increasingly uncertain at lower levels ofexposure.

More specifically, the Bell etal. (2004) study observed a statistically significant, positive association between short-term 03 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 df the NMMAPS was to develop an overall national effect estimate using multi-city time-series analyses, by drawing on information from all ofthe individual cities. The strength ofthis approach is the use ofa uniform analytic methodology, avoidance of selection bias, and larger statistical power. Significant intercity heterogeneity was notedin the Bell et 

### al. and other multi-city studies, probably due to many factors, including city-specific differences inpollution characteristics, theuse of air conditioning, time spent indoors versus outdoors, and socioeconomic factors. Levy etal. (2005) found suggestive evidence that air conditioning prevalence was apredictor of heterogeneity in 03 risk estimates in their meta-analysis.

Several commenters argued that EPA
overstates the probability of causal links between health effects and exposure to 03, especially at the lower concentrations examined, and thatthe statistical associations foundin the cited epidemiological studies do not automatically imply that a causal relationship exists. These commenters expressed the view that the correlation between health effects and03 exposure mustbe rigorously evaluated according
to a standard set ofcriteria before concluding thatthere is a causal link and that EPA fails to articulate and I 

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follow the weight ofthe 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 directevidence ofa causal link between exposure to 03 and the

occurrence of effects (72 FR 37879). Throughout the 03 review, a standard
set of criteria have beenused 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 ofthe evidence of associations so asto reach fundamental judgments as tothe likely causal significance ofthe observed associations in accordance with the Hill criteria (Criteria Document, pp. 7-3-7-4). Moreover, consistent with the proposal -theAdministrator has specifically considered evidence from epidemiological studies in the context of all the other available evidence in evaluating the degree ofcertainty that 0 3-related adverse health effects occur at various levels atand below 0.080 ppm, including the strong evidence from controlled human exposure studies and the toxicological studies that demonstrate biological plausibility and mechanisms foreffects. More detailed discussion ofthe 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 thattheresults ofthe new epidemiological studies included in this review are not coherent. They state that although EPA notes that estimates of risk fromcardiovascular mortality are, higher than those for total mortality and indicates that these findings are highly suggestive that short-term 03 exposure directly orindirectly contributes to cardiovascular mortality, the Agency failsto contrast the mortality studies to studies ofhospital admissions for cardiovascular causes. Most studies of cardiovascular causes have not found statistically significant associations with 03 exposures (UARG, pp. 16-17).

EPA strongly disagrees that it has failed to appropriately characterize the association between 03 exposure and potential cardiovascular morbidity and mortality effects. As noted above, the Criteria Document characterizes the overall body ofevidence as limited,but highly suggestive, andconcludes that much needs to be done to more fully integrate links between ambient 03
-exposuresand adverse cardiovascular
outcomes (EPA, 2006a,p. 8-77). Some field/panel studies that examined associations between 03 and various cardiac physiologic endpoints have yielded limited epidemiological evidence suggestive ofa potential association between acute 03 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 ofemergency 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 03 concentrations and the changing relationship between 03 and othercopollutants by season, inadequate adjustment for seasonal effects might have masked or underestimated the associations. In the limited number ofstudies that analyzed data by season (6 studies), statistically significant associations were observed in all but one study (CriteriaDocument, section 7.3.4). Newly available animal toxicology data provide some plausibility for the observed associations between 03 and cardiovascular outcomes. EPAbelieves that its characterization ofthe evidence for 0 3-related cardiovascular system effects is appropriate. Itis clear that coherence is stronger in the much larger body ofevidence of0 3-related respiratory morbidity and mortality effects.

Many commenters who did not support revising the current 03primary standard also submitted comments on specific methodological issues related to the epidemiological evidence, including: The adequacy ofexposure data; confounding bycopollutants; model selection; evidence ofmortality; and, new studies not included in the Criteria Document. Some ofthe major
comments on methodological issues raisedby these commenters are discussedbelow. TheResponse to Comments document contains more detailed responses tomany ofthese comments, as well as responses to other
comments not considered here.

(1)Adequacy of exposure data.Many
commenters expressed concern about ' the adequacy ofexposure 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 03 standard are based on data from ambient monitors for which there is a poor correlation with the actual pers.onal exposure subjects receive during their daily activities. They questioned the Administrator's conclusion'that in the absence of available data on personal 03 exposure, the use ofroutinely monitored ambient 03 concentrations as a surrogate for personal exposures is not generally expected to change theprincipal conclusions from epidemiological studies. These commenters also note that, in its June 2006 letter, the CASAC Panel raised the issue ofexposure error, concluding that it called into question whether observed associations could be attributed to 03 alone (AP5I, p. 17). One ofthese commenters cited studies (e.g., Sarnat etal., 2001; Sarnat etal., 2005) that show a lack ofcorrelation between personal exposures and ambient concentrations (NAM, p. 22). Another cited studies (Sarnat et al., 2001, 2005, and 2006; and Koutrakis etal., 2005) that have found thatthe 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 tobe not detectable at a level of5 parts per billion (ppb), resulting in very low correlation between concentrations reported from central ambient monitors and personal monitors; (2) 03
measurements from ambient monitors

are abetter surrogate for personal

exposure to PM2.5 than to 03; and (3) populations expected to be potentially susceptible to 03, including children, the elderly, and those with COPD, are at the.low end ofthe population exposure distribution (ExxonMobil, pp. 15-16). These commenters contended that without such a correlation there is no legitimate way for EPA to conclude that 03 exposure has caused thereported health effects, or to conclude that use of routinely monitored ambient 03 concentrations as a surrogate for

- personal exposures is adequate. Some of these commenters also contended that EPA incorrectly concludes that the
exposure error inepidemiological studies results in an underestimate of risk (ExxonMobil, p. 20).

With regard to the views on exposure measurement error expressed by CASAC, while the commenter is correct thatthe CASAC Panel raised the question ofexposure error and whether observed associations could be attributed to 03 alone, the commenter failed tonote that CASAC's comment

was focused on the association between 03 and mortality, at very low 03 concentrations and inthe group of people most susceptible topremature mortality. The CASAC Panel in its June 2006 letter stated: 

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Thepopulation thatwouldbe expected to be potentially susceptible todying from exposure toozone is likely to haveozone exposures thatare at the lower end ofthe

ozone population distribution, in which case the population would be exposed to very low

ozone concentrations, and especially so in winter. Therefore itseems unlikelythat the observed associations between short-term

ozone concentrations and dailymortality are due solely to ozone itself. [Henderson 2006b, pp.3-41

This section ofthe quote, which was not addressed in the comment submitted by API, together with the conclusions inthe final CASAC letter (Henderson, 2007), leads EPA to conclude that contrary to the commenters' assertion, the CASAC Panel was notcalling into questionthe association between 03 exposure and the full range ofmorbidity effects found in panel or time-series studies that rely

on ambient monitoring data as a surrogate for personal exposure data. It is important tonote thatEPA agrees that the evidence is onlyhighly suggestive that 03 directly or indirectly contributes
to mortality, as compared to the stronger evidence ofcausality for respiratory morbidity effects.

EPA agrees that exposure measurement error may result from the

use ofstationary ambient monitors as an indicator ofpersonal exposure in population studies. There is a full discussion ofmeasurement error and its effect on the estimates ofrelative risk in section 7.1.3.1 ofthe Criteria Document. However, the possibility of
measurement error does not preclude the use ofambient 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 ofmeasurement error is greatest, effects estimates must be evaluated carefully and thatcaution
mustbe 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 03 concentrations from.central monitors may serve asvalid surrogate

measures for mean personal 03
exposures experienced bythe population, which is ofmost relevance
totime-series studies, inwhich individual variations in factors affecting
exposure tend to average out across the study population. This is especially true forrespiratory hospital admission studies for which much ofthe response is attributable to 03 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 03 ofambient originbecause children tend to spend

more time outdoors than adults in the

warm season. EPA does notagree that the correlation between personal
exposure and ambient monitoring data is necessarily poor, especially in children. Moreover, the CASAC Panel supported thisview as they noted that "[plersonal exposures most likely correlate better with central site values for those subpopulations that spend a good deal oftime outdoors, which coincides, for example, with children actively engaged in outdoor activities, and which happens to be a group that the ozonerisk assessment focuses upon." (Henderson, 2006c. p. 10). However, the Criteria Document notes thatthere is some concern in considering certain mortality and hospitalization time-series studies regarding the extent to which ambient
03 concentrations arerepresentative of personal 03 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. Abetter understanding ofthe relationship between ambient concentrations and personal exposures,

as well as ofthe factors that affect the -relationship,will improve the interpretation ofobserved associations between ambient concentration and population health response.

Withregard tothe specific comments thatreference the findings of studies by Sarnat eta. (2001, 2005, 2006) and Koutrakis eta]. (2005), the fact that personal exposure monitors cannot detect 03 levels of 5 ppb andbelow 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 eta]. (2001) observed that ambient 03 concentrations showed stronger associations with personal exposure to PM2. 5 than to 03; however, ina later study conducted in Boston (Sarnat etal., 2005), ambient 03 concentrations and personal 03 exposures were found tobe significantly associated in the summer. Another study citedbythe commenter, but not included in the Criteria " Document, conducted in Steubenville (Sarnat etaL., 2006), also observed significant associations between ambient 03 concentrations and personal 03. 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 etal.(2006), to refute the
commenters. However, EPA notes that Sarnat etal.(2006) does not support the conclusion drawn by the commenters thatthis study shows very limited associations between ambient 03 concentrations andpersonal exposures.

Existing epidemiologic models may not fully take into consideration all the biologically relevant exposure history or reflectthe complexities of all the underlying biological processes. Using ambient concentrations to determine
exposure generally overestimates true personal 03 exposures (by approximately 2- to4-fold in the various studies described in the Criteria Document, section 3.9), which assuming therelationship is causal, would result inbiased descriptions ofunderlying 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 andthe potency of03 is greater than these effect estimates indicate. On the other hand,

as very few studies evaluating 03 health effects with personal 03 exposure measurements exist in the literature, effect estimates determined from ambient 03 concentrations mustbe evaluated and used with caution to

assess the health risks of03 (Criteria Document, pp. 7-8 to 7-10). Nonetheless, as noted in section II.C.3 of the proposal, the use ofroutinely monitored ambient 03 concentrations as

a surrogate for personal exposures is not generally expected to change the principal conclusions from03 epidemiologic studies. Therefore, population risk estimates derived using ambient 03 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 of03 and various health outcomes and that the health effects of03 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 etal.(2002), a large multi-city. asthma panel study, found that when 

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other pollutants; i.e., sulfur dioxide (SO 2), NO2, andparticles with an aerodynamic diameter less than orequal
to anominal 10 micrometers (PM10),

were placed in a multi-pollutant model with 03, the O3-related associations with respiratory symptoms and lung function became non-significant.

The National Cooperative Inner-City Asthma Study (Mortimer etal., 2002) evaluated air pollution health effects in 846 asthmatic children in 8 urban areas. The pollutants evaluated included 03, PM1 0, SO 2, 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, 03 was the onlypollutant associated with reduction in lung function. Nitrogen dioxide had the strongest effect on morning symptoms, and the authors concluded it "* * *
may be a bettermarker for the summerpollutant mix in these cities" but hadno association with morning lung function. In a two-pollutant model with NO2,the 03 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 (PM1o), which was measured daily in 3 cities, had no statistically significant effect on morning lung function. In a twopollutant model with 03, the PM 1o estimate for morning symptoms was slightlyreduced and there was a larger reduction in the 03 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 ofthe primary 03 standard raised issues.regarding the adequacy of model specification including control oftemporal and weather variables in the time-series epidemiological studies that EPA has claimed support the finding ofO3- 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 ofmodel selection in theresults ofthe time-series studies, including the choice of models to address weather and the degree of smoothing, in direct contradiction ofthe StaffPaper's conclusion onthe robustness ofthe models used in the 03 time-series studies (Exxon Mobil, p. 41); (ii) commenters contended that therewere no criteria forhow confounders such astemperature 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 inthe 03 timeseries studies, EPA should rely on an alternative statistical approach, Bayesian model averaging, that incorporates a range ofmodels addressing confounding variables, pollutants, and lags rather than a single model.

Inresponse to the first issue, EPA agrees that the results ofthe metaanalyses do support the conclusion that there are important effects ofmodel selection and that, for example, alternative models to address weather. mightmake a difference ofa factor of
two in the effect estimates. However, as noted in the Criteria Document, one of the meta-analyses (Ito eta.,2005) suggested that the stringent weather model used in the Bell eta].(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 lowereffect estimates than reported. In addressing this issue, the Criteria Document concluded,

Considering the wide variability in possible study designs and statistical model specification choices, thereported 03 risk estimates for thevarious health outcomes are in reasonably good agreement. In the caseof 0 3-mortality time-series studies, combinations ofchoices inmodel specifications * * * alone may explain the
extent ofdifference in 03 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 ofthe meta-analyses reviewed in the Criteria Document and StaffPaper. As stated in the Criteria Document, 03 effect estimates "were generally more sensitive to alternative weather models than to varying degrees of freedom for temporal trend adjustment" (CriteriaDocument, p. 7- 176). The Criteria Document also concluded that "although there is some

concern regarding the use of multipollutant models * * * results generally suggest thatthe inclusion of copollutants into the models do not substantially affect 03 risk estimates" and theresults ofthe time-series studies

are "robust and independent ofthe effects ofother copollutants" (Criteria Document, p. 7-177). Overall, EPA continues to believe thatbased on its integrated assessment, the time-series studies provide strong support for' concluding there are 03-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 03 exposure directly or indirectly contributes to nonaccidental and cardiorespiratory-related mortality.

The Administrator acknowledges that uncertainties concerning appropriate model selection are an important source ofuncertainty affecting the specific risk estimates included inEPA's risk
assessment and thatthese quantitative risk estimates must be used with appropriate caution, keeping in mind these important uncertainties, as discussed above in section ll.A.3. As discussed later inthis notice, the Administrator is notrelying on any specific quantitative effect estimates from the time-series studies or any risk estimates based onthe time-series studies in reaching hisjudgment about the need torevise the current 8-hour 03, 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 ofmodels, the Criteria Document evaluated the strengths and weaknesses of such methods in the
context ofair pollutionepidemiology. The Criteria Documentnoted several limitations, especially where there are many interaction terms and meteorological variables andwhere variables are highly correlated, as is the

case for airpollution studies, which makes itvery difficult to interpret the results using this alternative approach. EPAbelieves further research is needed
to address concerns about model selection and to develop appropriate methods addressing these concerns.

(4) Evidence ofmortality. Many commenters, including those that argued for revising the current 03 standard as well asthose that argued against revisions, focused on the new evidence from multi-city time-series analyses and meta-analyses linking 03 exposure with mortality. Again, the
comments were highly polarized. One
set ofcommenters, including medical, public health, and environmental organizations argued that recent published research has provided more robust, consistent evidence.linking 03to cardiovascular and respiratory 

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mortality. The ATS, AMA, and others stated that data from single-city studies, multiple-city studies, andmeta-analyses show a consistent relationship between 03 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 thatresearch has demonstrated thatexposure to 03 pollution iscausing premature deaths, and has also provided clues on the possible mechanisms that lead to premature mortality (ATS, p.4). These
commenters notedthat people may die from 03 exposure even when the concentrations are well below the current standard. They pointed to a study (Bell etal., 2006) in which the authors followed up on their 2004 multi-city study to estimate the
exposure-response curve for 03 and the risk of mortality andto evaluate whether athreshold existsbelow 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 theperiod 1987 to 2000. The study reported that 03 and mortalityresults did not appear tobe confounded by temperature or PM and showed that any threshold, ifit existed, would have to be at very low concentrations, farbelow the current standard (ALA etal.,p. 74). Another approach also indicated thatthe mortality effect is unlikely to be confounded bytemperature. A casecrossover study (Schwartz 2005) ofover
one million deaths in 14 U.S. cities, designed to control for the effect of temperature on daily deaths attributable
to 03, found that the association between 03 and mortality risk reported in the multi-city studies is unlikely to be due to confounding bytemperature (ALAet al.,p. 76). These commenters argue thatmeta-analyses also provide compelling evidence thatthe 03- mortality findings are consistent. They point to three independent analyses conducted by separate research groups atJohns Hopkins University, Harvard University and New York University, using their own methods and study criteria, which reported a remarkably consistent link between daily 03 levels andtotal mortality.

In response, EPA notes that the Criteria Document states that theresults from the U.S. multi-city time-series studies provide the strongest evidence
to date for 03 effects on acute mortality. Recent meta-analyses also indicate positive risk estimates that are unlikely
tobe confounded by PM; however, future work is needed to better understand the influence of model specifications onthe risk coefficient (EPA; 2006a, p. 7-175). The Criteria Document concludes that these findings

are highly suggestive that short-term 03 exposure directly or indirectly contributes to non-accidental and cardiorespiratory-related mortality but that additional research is needed to

more fullyestablish theunderlying mechanisms bywhich such effects

occur (72 FR 37836). Thus while EPA generally agrees with the direction of the comment, EPAbelieves the evidence supports aview as noted above. In addition, it must be noted thatthe Administrator did not focus on mortality as a basis for proposing that the current 03 standard was not adequate..In the proposal, the Administrator focused on the very strong evidence ofrespiratory morbidity effects in healthy people at the 0.080
ppm exposure level andnew evidence thatpeople with asthma are likelyto experience larger and-moreserious effects than healthy people at the same level ofexposure (72 FR 37870). With regard to the ambient concentrations at which 0 3-related mortality-effects may be occurring, EPA recognized in the proposal thatevidence ofa causal relationship between adverse health effects and 03 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 thetimeseries epidemiological studies, especially those studies relatedto mortality effects. The Annapolis Center (p.46) makes thepoint that although there may be somewhat more positive associations than negative associations, there is so much noise or variability in the data thatidentifying which positive associations may be real health effects andwhich are not is beyond the capability ofcurrent methods. They cite the view that the CASAC Panel expressed in aJune 2006 letter (Henderson,2006b), noting that "Because results oftime-series studies implicate all ofthe criteria pollutants, findings ofmortality time-series studies do not seem to allow us to confidently attribute observed effects specifically to individual pollutants."

Because ofthe importance ofthe 03 mortality multi-city studies inEPA's analysis ofthis issue, several ofthese
commenters focused on them in particular, arguing that, although these studies havethe statistical power to distinguish weak relationships between daily 03 and mortality, they do not provide reliable orconsistent evidence implicating 03 exposures as a cause of mortality. Several reasons were given, including: (a) The multi-city studies citedbyEPA involve a widerange of city-specific effects estimates, including

some large citiesthat have very slight or negligible effects (e.g., Los Angeles) (Bell etal., 2004), thus causing several
commenters to question the relevance of

a "national" effect of03 on mortality and argue that a single national 03 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 otherpollutants, for example, Bell etal.(2004) adjusted for PM1obut did not have the necessary air quality data to adequately adjust for PM25, which EPA has concluded also

causes mortality and is correlated with 03, especially inthe 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 that0 3-related m6rtality is occurring at levels well within natural background, which is not biologically plausible (Annapolis Center,p. 42).

Evidence supporting ai association between short-term 03 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, andrespiratory mortality (> 20 studies), including one study inan area that would have met current standard (Vedal etal., 2003). Three large meta-analyses, which pool data from many single-city studies to increase statistical power,reported statistically significant associations and examined sources ofheterogeneity in those associations (Bell etal., 2005; Ito
etal., 2005; Levy etal.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 ofconfounding by PM; and (3) suggestive evidence of publication bias, but significant associations remain even after adjustment forthe publication bias.

Moreover, EPA asserts thatthe biological plausibility ofthe epidemiological mortality associations is generally supported by controlled human exposure and toxicological evidence ofrespiratory morbidity effects for levels at and below 0.080 ppm,but thatbiological 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 arerelated to 03 alone rather than to the broader mix ofair pollutants present in the ambient air. EPA agrees that the multi-city times series studies evaluated inthis review donot completely resolve this issue. It also becomes increasingly uncertain as to whether effect thresholds existbut
cannot be clearly discerned by statistical analyses, Thus, when considering the epidemiological evidence in light ofthe other available information, it is reasonable tojudge that at some point the epidemiological associations cannot be interpreted with confidence as providing evidence that the observed health effects can be attributed to03 alone.

In the letter cited, the CASAC Panel did raise the issue ofthe utility oftimeseries 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
torevise the current primary 03 standard. "Several new single-city studies andlarge multi-city studies designed specifically to examine the effects ofozone and other pollutants on both morbidity and mortality have provided more evidence for adverse health effects at concentrations lower thanthe current standard (Henderson, 2006c, p. 3)."

With regard to the specific issues raised in the comments as towhy the times-series mortality studies do not provide reliable or consistent evidence implicating 03 exposure as a cause of mortality, EPA has the following responses: (a) The purpose ofthe NMMAPS approach is not to single out individual city results butrather to estimate the overall effect from the 95 communities. It was designed to provide ageneral, 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 theBell etal.(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 (AprilOctober) analyses forthe 95 U.S. cities

were similar inmagnitude; however, in
most other studies, larger excess mortality risks were reported in the

summer season (generallyJune-August when 03 concentrations arethe highest) compared to all year orthe cold season. Though the effect estimate forLos Angeles is small compared to some of the other communities, it should be noted that all year data (combined warm and cool seasons) was usedin the analyses for this city, which likely resulted in a smaller effect estimate. Because all year data was used for Los Angeles, themedian 03 concentration for Los Angeles is fairly low compared
to the other communities, ranked 23rd
out of95 communities. The median 24- hour average 03 concentration for Los Angeles in this dataset was 22 ppb, with

a 10th percentile of8 ppb to a 90th percentile of38 ppb. The importance of seasonal differences in0 3-related health
outcomes has been well documented.

(b)In section 7.4.6, 03mortality risk estimates adjusting forPM exposure, the Criteria Document states that the main confounders ofinterest for 03, especially forthe 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, dueto the expected high correlation
amongthe constituents ofthe "summer haze mix," multipollutant models including these pollutants may result in unstable coefficients; and, therefore, interpretation ofsuch results requires

some caution.

Inthis section, Figure 7-22 shows the 03risk estimates with and without adjustment forPM indices using all-year datain studies that conducted twopollutant analyses. Approximately half ofthe 03 risk estimates increased slightly, whereas the other half decreased slightly with the inclusion of PM in the models. In general, the 03 mortality risk estimates were robust to adjustment forPM in the models.

The U.S. 95 communities studyby Bellet al.(2004) examined the sensitivity of acute 03-mortality effects
to potential confounding byPM1o. Restricting analysis to days when both 03 and PMo data were available, the community-specific 0 3-mortality effect estimates as well as the national average results indicated that 03 was robust to adjustment forPM10 (Belletal., 2004). As commenters noted, there were insufficient data available to examine potential confounding by PM2. 5. One study (Lipfert etal., 2000) reported 03 risk estimates with and without adjustment for sulfate, a component of PM2.5.Lipfert 

### etal.(2000) calculated 03 risk estimates based onmean (45 ppb) less background (not stated) levels of 1- hour max 03 in seven counties in Pennsylvania andNew Jersey. The 03 risk estimate was not substantially affected by the addition ofsulfate in the model (3.2% versus 3.0% with sulfate) and remained statistically significant.

Several 03 mortality studies examined the effect ofconfounding by PM indices indifferent seasons (Figure 7-23, section 7.4.6, Criteria Document). In analyses using all-year data and warm-
seasononly data, 03 risk estimates were
once again fairlyrobust to adjustment for PM indices, with values showing both slight increases and decreases with the inclusion ofPM in themodel. In the analyses using cool season'data only, the 03 risk estimates all increased slightly withthe adjustment ofPM indices, although none reached statistical significance.

The threerecent meta-analyses (Bell
etal., 2005; Ito etal.,2005; Levy etal., 2005) all examined the influence ofPM

on 03 risk estimates. No substantial influence was observed inany ofthese studies. In the analysis byBell etal. (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 03. In the meta-analysis of 15 cities by Ito etal.(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 03 without and withPM adjustment, respectively. The additional time-series analysis Of six cities by Ito etal.found thatthe influence ofPMby season varied across alternative weather models but was

never substantial. Levy et al.(2005) examined theregression relationships between 03 and PM indices (PM10 and
2.5) with 0 3-mortality effect estimates for all year and by season.Positive slopes, which might indicate potential confounding, were observed for PM2.5 on 03 riskestimates inthe summer and all-year periods, but the relationships

were weak. The effect of one causal variable (i.., 03) is expected to be overestimated when a second causal variable (e.g., PM) is excluded from the analysis, ifthetwo variables are positively correlated and act in the same direction. However, EPAnotes thatthe 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 03 and mortality.

(c)With regard tothe biological plausibility of0 3-related mortality occurring at levels well within natural background, EPA concluded in the proposal that additional research is needed tomore fully establish underlying mechanisms bywhich mortality effects occur (72 FR 37836). Such research would likely alsohelp determine whether it isplausible that mortality would occur at such low levels. As noted above, the multi-city 

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times series studies evaluated inthis review can not resolve the issue of whether the reported associations at such low levels are relatedto 03 alone rather than to the broader mix ofair pollutants present inthe ambient air.

(5) "New" studies not included in the Criteria Docujnent. Many commenters identified "new" studies that were not included in the Criteria Document that they stated support arguments both for and against the revision ofthe current 03 standard. Commenters who supported revising the current 03 standard identified new studies that generally supported EPA's conclusions about the associations between 03
exposure and a range ofrespiratory and cardiovascular health outcomes. These comnienters also identified new studies that provide evidence forassociations with health outcomes thatEPA has not linked to 03 exposure, such as cancer, andpopulations that EPA has not identified as being susceptible or vulnerable to 03 exposure, including African-American men and women. Commenters who did not support revision ofthe current 03 standard often submitted the same "new" studies, but focused on different aspects ofthe findings. Commenters who did not supportrevision ofthe current 03 standard statedthatthese "new" studies provide inconsistent and sometimes conflicting findings that do little to resolve uncertainties regarding whether 03 has a causal role in thereported 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 inthe Response to Comments document.

To the extent thatthese commenters included "new" scientific studies, studies that werepublished too late to be considered in the Criteria Document, in support oftheir arguments for revising or not revising the standards, EPA notes, as discussed in section I above, that as inpast NAAQS reviews, it is basing the final decisions in this review on the studies andrelated information included in the 03 air quality criteria that haveundergone CASAC and publicreview and will consider newly published studies for purposes ofdecision making in the next 03 NAAQS review. In provisionally evaluating commenters' arguments, as discussed in the Response to Comments document, EPA notes that its provisional consideration of "new" science foundthat such studies did not materially change the conclusions in the Criteria Document. iii. Evidence Pertaining to At-Risk Subgroups for0 3-Related Effects

This section contains major comments

onEPA's assessment ofthe bodyof evidence, including controlled human exposure and epidemiological studies, related to the effects of03 exposure on sensitive subpopulations. Since new information about the increased responsiveness ofpeople with lung disease, especially children and adults with asthma, was an important consideration in the Aoministrator's proposed decision that the current 03 standard isnot adequate, many ofthe 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 03 exposure. Many ofthe 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 andepidemiological studies, upon whichjudgments about sensitive subpopulations were based, the comments about EPA's delineation ofthese groups were highly polarized. In general, one group ofcommenters who supported revising the current 03 primary standard, including medical associations, public health and environmental groups, agreed in part with EPA's assessment ofthe subpopulations that are atincreased risk from 03 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 anumber of individuals uniquely at muchhigher 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 03 exposure than people who do not have asthma, and arelikely to respond at lower levels. Because ofthis, these commenters make the point that controlled human
exposure studies that employ healthy subjects will underestimate the effects of 03 exposures in people with asthma.

These commenters agreed with EPA's
assessment that epidemiological studies provide evidence ofincreased 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 byAPI, evaluated some ofthe 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 03. 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 ofindividuals with atopic asthma to sufficient levels of ozoneproduces

an increase inspecific airway responsiveness to inhaled allergens* * *These findings, in combination with previously observed effects ofozone on nonspecific airway responsiveness and airway inflammation, supports the idea thatambient ozone exposure could result in exacerbation of asthma several days following exposure, and provides biological plausibility forthe epidemiologic studies in which ambient

ozone concentration hasbeen associated with increased asthma symptoms, medication use, emergency roomvisits, and hospitalizations for asthma. [Rochester Report, pp. 57-58]

Commenters also often mentioned the increased susceptibility ofpeople with COPD, and in thiscase cited new studies not considered in the Criteria Document.

They identify one potentially susceptible subpopulation thatEPA did
not focus on in theproposal is infants. Commenters from medical associations, andenvironmental and public health groups expressed the view that 03
exposure can have important effects on infants, including reduced birth weight, pre-term birth, and increased respDiratory morbidity effects in infants. Exposure to 03 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 03 exposure levels, the ATS notes that, "* * *the reduced birthweight in infants inthe 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 thatpeople with lung disease, especially children and adults with asthma, are susceptible
to 03 exposure and are likely to 

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experience more serious effects than those people who do nothave 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 tobirth-related health outcomes, the Criteria Document (p. '7- 133) concludes that 03 was not an important predictor ofseveral birthrelated outcomes including premature births and low birth weight. Birthrelated outcomes generally appeared to be associated with air pollutants that tend to peak in theWinter and are possibly traffic-related. However, given that most ofthese studies did not analyze the data by season, seasonal confounding may have therefore influenced the reported associations. One studyreported someresults suggestive of associations between exposures to 03 in the second month of pregnancy andbirth defects, but further evaluation of such potential associations is needed. With regard to comments about effect in infants, EPA notes that

some ofthe studies citedby commenters

were not considered inthe Criteria Document. More detailed responses to studies submitted by commenters but
not considered in the Criteria Document

canbe found in the Response to Comments document.

The second group ofcommenters, mostlyrepresenting industry associations and some businesses opposed torevising the primary 03 standard, asserted that EPA is wrong to claim that newevidence indicates that the current standard does not provide adequate health public health 1rotection for people with asthma. In support of this position, these commenters made the following majorcomments: (1)Lung function decrements andrespiratory symptoms observed in controlled human exposure studies ofasthmatics

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 thathypothesis 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 notlikely to experience medically significant lung function changes or respiratory symptoms at ambient 03 concentrations at or even above the level ofthe current standard. Many ofthese commenters cited the opinion of one physician who was asked on behalfofa group oftrade associations and companies toprovide his views onthe health significance for asthmatics ofthetypes ofresponses that have been reported in controlled human
exposure studies of03. This commenter (McFadden) reviewed earlier controlled human exposure studies of asthmatics (from the last review) as well as the
recent controlled human 'exposure studies ofhealthy 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 donot produce medically significant functional changes and areright 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 03 exposure at levels

<0 .08 ppm would beeven less and that the available data are not sufficiently robust to indicate that such -exposures would present a significant health

concern evento sensitive people like asthmatics.

EPA notes thatthis commenter based his comment on the group mean functional and respiratory symptom changes inthe studies he reviewed. EPA agrees thatgroup mean changes atthese levels are relatively small and has described them as such in both the previous review andthis one (72 FR 37828). The importance ofgroup mean changes is to evaluate the statistical significance ofthe association between the exposures and the observed effects,
to try to determine ifthe observed effects are likely due to 03 exposure ratherthan 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 FEV of 10-20% are notby themselves significant or meaningful to asthmatics* * *. Ithas been my experience from examining and studying thousands ofpatients forboth clinical and research purposes that asthmatics typically will notbegin to

sense bronchoconstriction until their FEV, falls about 50% fromnormal." 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 re.g., FEV decrements _> 10%but < 20%) would likely interfere with normal activities formany individuals, and would likely result inmore frequent medication use.EPA notes that in the
context ofstandard setting, CASAC indicated (Henderson, 2006c) that a focus on the lower end ofthe range of moderate functional responses (e.g., FEVI decrements >10%) is most appropriate for estimating potentially adverse lungfunction decrements in people with lung disease.

-

Withregard to the second point, whether asthmatics would likely experience more serious responses and responses at lower levels than the subjects ofcontrolled human exposure experiments andEPA's discussion of the relationship ofincreased airway responsiveness and inflammation experienced by asthmatics to exacerbation ofasthma, this commenter stated that "there simply are no data to support the sequence described" and that "the assumption thatthese
responses would lead to clinical manifestations in terms ofexacerbations of asthma or other adverse health effects remains unproven theory" (McFadden, p. 3).

In these sections ofthe proposal (72 FR 37826 and 37846-37847), EPA describes the evidence indicating that people with asthma areas sensitive as, ifnotmore sensitive than, normal subjects in manifesting 0 3-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 03 exposure. Asthmatics have greater 03- induced inflammatory responses and increased 0 3-induced airway responsiveness (both incidence and duration) that couldhave 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 ofasthma in study subjects resultingin the need for medical attention, there are no data to support the clinical significance ofthe results. EPA rejects this interpretation because it would be unethical to knowingly conduct a controlled human exposure study that would leadto exacerbation ofasthma. 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 couldhave important clinical implications. EPA rejects this interpretation as well. EPA did not base its increased concern for asthmatics solely onthe results ofthe controlled human exposure studies,but has appropriately used a weight of evidence approach, integrating evidence from animaltoxicological, controlled human exposure and epidemiological studies asa basis forthis concern. The Criteria Document concludes thatthe positive and robust epidemiological associations between 03 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 itselfexpressed the view that people with asthma, especially children, have been found to be more sensitive to 03 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 03 exposure than people without asthma, andthat these
responses have the potential to leadto exacerbation ofasthma as indicated by the serious morbidity effects, such as increased respiratory-related emergency department visits and hospitalizations found in epidemiological studies.

Withregard 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 ofasthmatics that provide new evidence ofrespiratory symptoms and medication use in asthmatic children are subject to the limitations ofepidemiological studies discussed above (e.g., confounding by co-pollutants, heterogeneity ofresults). In addition, these-commenters identified a new, large multi-city panel study, not included inthe Criteria Document, by Schildcrout et al.(2006), which the commenters characterize as reporting no association between 03 concentrations and exacerbation of asthma.

At the time ofthe lastreview, EPA concluded thatpeople with asthma

were atgreater risk because the impact of0 3-induced responses on alreadycompromised respiratory systems would noticeably impair an individual's ability to engage innormal activity orwould be

more likely to result in increased selfmedication ormedical treatment. At that time there was little evidence that people with pre-existing disease were
more responsive than healthy individuals in terms ofthe 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 nothave lung disease and therefore are at greater risk for 03 health effects than previously judged inthe 1997 review. EPA notes that comments on the limitations of epidemiological studies and evidence from "new" studies (notin the Criteria Document) havebeen addressed above. As with other "new" studies, this study by Schildcrout etal. (2006) is specifically discussed in the Response to Comments document.

b. Consideration ofHuman Exposure and Health Risk Assessments

Section ll.A.3 above provides a summary overview ofthe exposure and risk assessment information usedby the Administrator to informjudgments about exposure and health risk estimates associated with attainment of the current and alternative standards. EPA notes here that most ofthe issues andconcerns raised bycommenters concerning the methods used inthe exposure and risk assessments are essentially restatements ofconcerns raised during the review ofthe Criteria Document and the development and review ofthese quantitative assessments

as part ofthe preparation and review of the StaffPaper and the associated analyses. EPA presented and the CASAG Panel reviewed in detail the approaches usedto assess exposure and health risk,the studies andhealth effect categories selected for which exposureresponse and concentration-response relationships were estimated, and the presentation ofthe exposure and risk results summarized in the StaffPaper. As stated in the proposal notice, EPA believes and CASAC Panel concurred, thatthe model selected to estimate
exposure represent the state ofthe art and that the risk assessment was "well done, balanced and reasonably communicated" andthat the selection ofhealth 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 orthe other as claimed in some ofthe comments.

Comments received after proposal relatedto the development ofexposure andhealth risk assessments, interpretation of exposure andrisk results, and therole ofthe quantitative human exposure and health risk
assessments in consideriig the need to revise the current 8-hour 03 standard generally fell intotwo groups. One group ofcommenters that included national environmental and p4lic 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 airpollution agencies argued that the exposure andhealth 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 03; (2) the
assessments did not include all relevant at risk population groups andexcluded populations such as pre-school children, outdoor workers, adults who exercise outdoors; and (3) the risk
assessment did not include all ofthe health effect endpoints for which there is evidence that there are 03-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 alsounderestimated. Thejoint set of 

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comments from ALA and several environmental groups contended that the "exposures ofconcern" metric presented in the StaffPaper and proposal is "an inappropriate basis for decisionmaking" and urgedEPA to set the standard based on the concentrations shown by health studies
to cause adverse effects, not on how much 03 Americans inhale. This same set of commenters stated that if exposures ofconcern were.to be considered then thebenchmark 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 ofestimating risks only above PRB, these same commenters supported the

use ofthe GEOS-CHEM model asthe "best tool available to derive background concentrations" should 'EPA continue to pursue this approach. These comments are discussed inturn below.

EPA agrees that the exposure and health risk assessments are limited to certain urban areas and do notcapture all ofthe populations atrisk for 03- related effects, and that the risk
assessment does not include all potential 0 3-relatedhealth effects. The criteria andrationale for selecting the populations and health outcomes

- included in the quantitative assessments

were presented in the draftHealth Assessment Plan, StaffPaper, 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 thatthe 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 StaffPaper (andthe CASAC Panel) clearly recognized that the exposure andrisk analyses could not provide a full picture ofthe 03- exposures and 0 3-relatedhealth risks posed nationally. The proposal notice made note ofthis important point and stated that "national-scale public health impacts ofambient 03 exposures are clearly much larger than the quantitative estimates of0 3-related incidences of adverse health effects and the numbers of children likely to experience exposures of concern associated with recent air quality orair quality that just meets the current or alternative standards" (72 FR 37866).

However, as stated inthe proposal notice, EPA also recognizes that interindividual variability in responsiveness
to 03 shown in controlled human exposure studies fora variety ofeffects

means that only a subset of individuals in any population group estimated to experience exposures exceeding a given benchmark exposure ofconcern level would actually be expected to experience such adverse health effects. The Administrator continues to recognize that there is abroaddr array of 0 3-related adverse health outcomes for which risk estimates could not be quantified (thatare part ofabroader "pyramid ofeffects") and thatthe scope ofthe assessment was limited tojust a sample ofurban areas and to some but
notall at-risk populations, leading to an incomplete estimation ofpublic health impacts associated with 03 exposures
across the country. The Administrator is fullymindful ofthese limitations, along with the uncertainties in these estimates, in reaching his conclusion that observations from the exposure and health risk assessments provide additional support forhis judgment that the current 8-hour standard does not protect publichealth with an adequate margin ofsafety and must be revised.. Forreasons discussed below in section ll.C.4, however, the Administrator disagrees with aspects ofthese commenters' views on the level ofthe standard that is appropriate and supported by the available health effects evidence and quantitative assessments associated withjust meeting alternative standards.

EPA does not agree that consideration ofexposure 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 ofthe 03 primary standard andthe 1994 decision on the carbon moioxide (CO) standard. As indicated in theproposal, estimating exposures ofconcern is importantbecause itprovides some indication ofpotential public.health impacts ofa range of0 3-related health outcomes, such aslung 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 lowas 0.080 ppm 03 but have notbeen 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 andgreater uncertainty as
one considers increasingly lower 03 exposure levels. EPA recognized that there was no sharpbreikpoint within the continuum ranging from atand above 0.080 ppm down to 0.060 ppm.In considering the concept of exposures of
concern, the proposal notedthat itwas important tobalance 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 ifexposures of

concern were considered, then the Administrator should focus only onthe 0.060 ppm benchmarkbased 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 ofthe exposures of concern at the 0.080 ppm benchmark level based on their view thatthere was no convincing evidence demonstrating adverse health effects at levels below this benchmark. Inview of the comments received related to the definition and use ofthe term "exposure ofconcern" at the time ofproposal, the Administrator recognizes that that there is a risk for confusion, as it could be read to imply a determination that a certainbenchmark level ofexposure has been shown to be causally associated with adverse health effects. As a consequence, the Administrator believes that itis more appropriate to consider such exposure estimates in the context ofa continuum rather than focusing on any one discrete benchmark level, as
was done at thetime ofproposal, since the Administrator does not believe that the underlying scientific evidence is certain enough to support a focus on any singlebright-line benchmark level. Thus, the Administrator believes itis appropriate to consider a range of benchmark levels from 0.080 down to 0.060 ppm, recognizing that exposures ofconcern must be considered in the 16465

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context ofa continuum ofthe potential for health effects ofconcern,,and their severity, with increasing uncertainty associated with the likelihood of such effects at lower 03 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 havebeen 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 03 exposures .of0.080
ppm andthe evidence fromcontrolled human exposure studies showing that people with asthma have more serious
responses thanpeople without asthma. However, the Administrator does not
agree with those commenters who would only consider this single benchmark level. While the Administrator places lessweight on exposures at and above the 0.070 pm benchmark level, given the increased uncertainty about the fraction ofthe population and severity ofthe 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 injudging the adequacy ofthe 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 atlower 03 levels than healthy individuals. The Administrator considered but placed very little weight

onexposures at and above the 0.060
ppm benchmark given the very limited scientific evidence supporting a conclusion that 03 is causally related to various health outcomes at this
exposure level.

EPA does not agree that itis inappropriate or impermissible to assess risks that are in excess ofPRB or that EPA must focus on total risks when using a risk assessment toinform decisions on the primary standard. Consistent with the approach used in the risk assessment forthe prior 03 standard review and consistent with the approach used inrisk assessments for other prior NAAQS reviews, estimating risks in excess ofPRB isjudged tobe

more relevant to policy decisions regarding the ambient air quality standard thanrisk estimates that include effects potentially attributable to uncontrollable background 03 concentrations. EPA also notes that with respect to the adequacy ofthe current standard takingtotal risks into account would not impact the Administrator's decision, since hejudges that the
current standard is notadequate even when risks in excess ofcurrent PRB estimates are considered. In addition, EPA notes that consideration ofthe 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 ofthis issue and the CASAC Panel support expressed during the review ofthe Criteria Document.

The second group ofcommenters mostly representing industry associations, businesses, and some State and local officials opposed torevising 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 ofconcern 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 levelswhich 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, 03
measurements at fixed-site monitors can be higher than otherlocations where individuals are exposed, the exposure estimates do not account for 03 avoidance behaviors, and the exposure model overestimates elevated breathing rates; and (5) That health risks are overestimated based on specific methodological choices made byEPA 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 ornot to incorporate thresholds into its models forthe various health effects analyzed. These comments are discussed in turnbelow. Additional detailed comments related tothe development, presentation, and interpretation of EPA's exposure and healtbivislk assessments, alongwith EPA'stre"p, s 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 qualityjust meeting the current standard have not appreciably changed since the prior review, comments from Exxon-Mobil, the Annapolis Center and others have compared results ofEPA'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 03 standard are below those that were predicted in 1997 andthat uncertainties about other health effects based on epidemiological studies remain the same. These commenters used this conclusion asthe basis for a claim that there is no reason to depart from the Administrator's 1997 decision thatthe current 8-hour standard is requisite to protect public health.

EPAbelieves that this claim is fundamentally flawed for three reasons,

as discussed in turnbelow: (i) It is factually inappropriate to compare the quantitative risks estimated in 1997 with those estimated in the current rulemaking; (ii)it failsto take into
account thatwith similar risks, increased certainty in the risks presented by 03 implies greater concern than in the last review, and (iii) it fails
to recognize thatthe Administrator has used these estimates in a supportive role, in light ofsignificant uncertainties in the exposure and risk estimates, to inform the conclusions drawn primarily from integrative assessment ofthe controlled human exposure and epidemiological evidence on whether ambient 03 levels allowed under the current standard present a serious publichealth problem warranting revision ofthe 03 standard.

With respect to the first point, the 1997 risk estimates, orany comparison of the 1997 risk estimates to the current estimates, are irrelevant for thepurpose of judging the adequacy ofthe current 8-hour standard, asthe 1997 estimates reflect outdated analyses thathave 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 ofthe.Staff Paper, there have been significant improvements tothe 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 inthe previous review andwhen modeling a larger area, extending well beyond the urban core, there willbe

more people exposed, but a smaller percentage ofthe modeled population willbe exposed athigh levels, if03 concentrations are lower inthe extended areas. In the prior review, only typical years, interms of 03 air quality

were modeled, while the current review usedthe most recent three-year period (i.e., 2002-2004). Also, the priorreview estimated exposures for children who spentmore time outdoors, whilethe
assessment forthe 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 ofthe resulting estimates inappropriate. Another important difference making comparison between the 1997 health risk assessment and the
current assessment inappropriate is that

a number ofadditional 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'therisk
assessment for the priorreview. 'hese
commenters only compare the risk estimates with respect to lung function decrement, and failto account for differences in additional andmore

severe healthendpoints not covered in the 1997 assessment, aswell asthe fact thatthere 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 03 exposures and morbidity and mortality effects. Incomparing the scientific understanding ofthe risk presented by exposure to 03 between the last and currentreviews, one must examine not onlythe quantitative estimate ofrisk from those exposures (e.g. the numbers ofincreased hospital admissions at various levels)but also the degree of confidence that the Agency has thatthe observed health effects are causally linked to 03
exposure at those levels. As documented in the Criteria Document. and therecommendations and conclusions ofCASAC, EPA recognizes significant advances in our understanding ofthe health effects of03 based on new epidemiological studies, new human and animal studies documenting effects, new laboratory studies identifying andinvestigating biological mechanisms of03 toxicity, andnew studies addressing the utility ofusing ambient monitoxs t assess population exposures to ambient 03. As

aresult ofthese advances, EPA is now
more certain that ambient 03 presents a significant riskto publichealth at levels
at orabove the range oflevels that the Agency had considered forthese standards in 1997. From this more comprehensive perspective, since the risks presented by 03 are more certain andthe current quantitative risk estimates include additional important health effects, 0 3-relatedrisks for a wider range ofhealth effects are now of greater concern at the current level of the standard than in the lastreview.

Third, quantitative risk estimates

were not the onlybasis for EPA's decision in setting a level forthe 03 standard in 1997, and they do not set any quantified "benchmark" forthe Agency's decision torevise the 03 standard at this time.While EPA believes that confidence inthe causal relationships between short-term exposures to 03 and various health effects reported in epidemiological studies has increased markedly since 1997, the Administrator also recognizes that therisk estimates for these effects
must be considered in the light of uncertainties about whether or not these 

### 0 3-related effects occur at very low 03 concentrations. The Administrator continues tobelieve that the exposure and riskestimates associated with just meeting thecurrent standard discussed in the StaffPaper and summarized in theproposal notice are important from

a public health perspective and are indicative ofpotential exposures and risks to at-risk groups. In considering the exposure andriskestimates, 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 abroader array of0 3-related adverse health outcomes forwhich risk estimates could notbe quantified (thatare part of abroader "pyramid ofeffects") and that the scope ofthe assessment was limited tojusta sample ofurban areas and to some,but
not all, at-risk populations, leading to an incomplete estimation ofpublic health impacts associated with 03 exposures

acrossthe country.

(2) In asserting that uncertainties and limitations associated with the exposure andhealth risk assessments make them
too speculative to be used in supporting

a decision to revisethe standard,
comments from industry associations and others cited a number ofissues including: (i) Uncertainties about the air quality adjustment approach used to simulate justmeeting the current and alternative standards; (ii)uncertainties and limitations assgciated with the definition and estimation ofPRB

- concentrations; (iii)uncertainties about whether the respiratory symptoms, hospital admissions, andnon-accidental and cardiorespiratory mortality effects included in the health risk assessment

are actually causally related to ambient 03 concentrations, particularly at levels

- well below the current standard; and

(iv) uncertainties about the shape ofthe exposure-response relationships for lung function responses and concentration-response relationships for the health effects based on findings from epidemiological studies and the assumption ofa linear non-threshold relationship forthese responses. In
summary, these commenters contend thatthe substantial uncertainties present in the exposure andrisk
assessments preclude the Administrator from using any ofthe results to support

a conclusion thatthe current 8-hour standard does not adequately protect public health.

Several ofthe issues raised, including whether EPA's judgmentsabout causality for the effects included in the risk assessment are appropriate, the shape ofconcentration-response relationships, and use ofa linear nonthreshold relationship forthe health
outcomes based on the epidemiological evidence, have been discussed in the previous section onhealth effects evidence. Concerns expressed about the definition and estimation of PRB levels for 03 andthe role ofPRB in therisk
assessment are addressed as a separate item below. These issues also are addressed inmore detail in the Response to Comments document.

With respect tothe air quality adjtistment approach used in the current review to simulate air qualityjust meeting the current and alternative 03 standards, as discussed-inthe Staff Paper (section4.5.6) and in more detail in a staffmemorandum (Rizzo, 2006), EPA concluded thatthe quadratic air quality adjustment approach generally best represented thepattern of reductions across the 03 aixquality distribution observed over the last decade in areas implementing control programs designed to attain the 03 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 03 will be significantly different from past 

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reductions with respect toimpacting the overall shape ofthe 03 distribution.

As discussed inthe proposal notice, EPA recognizes that the exposure and health risk assessments necessarily contain many sources ofuncertainty includingthose 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. Withrespect to the health risk assessment, EPA conducted and presented sensitivity analyses addressing the impact on risk estimates of different assumptions about the shape ofthe exposure-response relationship for lung function decrements and alternative assumptions about PRB levels. EPA notes that most ofthe comments summarized above concerning limitations and uncertainties in these assessments are essentially
restatements ofconcerns raised during the development and review ofthese quantitative assessments as part ofthe preparation and review ofthe Staff Paper and assessments. The CASAC Panel reviewed in detail the approaches .used to assess exposure and health risks and the presentation ofthe results in the StaffPaper. EPAbelieves, and the CASAC Panel concurred, thatthe model used to estimate exposures represents a state-of-the-art approach and that "there is an explicit discussion ofthe limitations ofthe APEX model in terms ofvariability andquality ofthe input data, which is appropriate and fine" (Henderson, 2006c, p. 11). The CASAC Panel also foundthe risk chapter inthe StaffPaper and the risk assessment "to be well done, balanced, and reasonably communicated" (Henderson, 2006c, p. 12). Although EPA agrees thatimportant limitations and uncertainties remain, and that futureresearch directed toward addressing these uncertainties is warranted, EPAbelieves that overall uncertainties about population exposure andpossible health risks associated with short-term 03exposure have dimidiished since the last review. The Administrator has carefully considered the limitations anduncertainties 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 tobe revised toprovide additional protection in order toprotect 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 byrelying on estimates from the GEOS-CHEM model using 2001 meteorology andEPA should instead rely on 03 levels observed at remote monitoring locations or sites that represent PRB conditions; and (iii)that the use ofunderestimated PRB levels in the risk assessment results in overestimated health risks associated with air qualityjust meetingthe 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 ofthese 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 03 concentrations entering the U.S. from Canada and Mexico through either regulations or international agreements, andtherefore EPA does not agree that these emissions

are uncontrollable. PRB is designed to identify 03 levels that result from emissions that are considered uncontrollable because the U.S. has littleif any influence ontheir control, andin that context anthropogenic emissions from Mexico orCanada should be excluded from PRB. EPA has consistently defined PRB as excluding anthropogenic emissions from Canada and Mexico in NAAQS reviews over

more thantwo decades and sees no basis in the comments to alterthis definition. Second, the criticisms raised concerning the use ofa modeling approach (GEOS-CHEM using 2001 meteorology) and the alternative approach ofusing remote monitoring data to estimate PRB were considered by EPA's scientific staff andthe CASAC Panel during the course ofreviewing the Criteria Document. Both EPA's experts and CASAC endorsed the use ofthe 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 ofGEOSCHEM with 03 observations at U.S. surface sites (Fiore et al., 2002, 2003) and comparisons ofGEOS-CHEM predictions with observations at Trinidad Head, CA (Goldstein etal., 2004) and found no significant differences between the model predictions and observations for all conditions, including those reflecting those given inthe 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 springto summer and is generally < 0.025 ppm under conditions conducive to high 03 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 03 episodes tied to stratospheric intrusion arerare and generally occur at elevated locations and these canbe readily identified and excluded under EPA's exceptional events rule (72 FR 13560) to avoid any impact on attainment/nonattainment status ofan area.

Third, many ofthe commenters who raised the concern that EPA's estimates ofPRB were too low and had the impact ofexaggerating the risks associated with the current standard ignored the fact that therisk assessment included a sensitivity analysis which showed the potential impact ofboth lower and higher estimates ofPRB or only focused

on the impact ofhigher estimates of PRB. The choices oflower and higher estimates ofPRB included in the risk
assessment sensitivity analyses were based on thepeer-reviewed evaluation ofthe accuracy ofGEOS-CHEM model. The Criteria Document states "in conclusion, we estimate that the PRB 03 values reported by Fiore etal. (2003) for afternoon surface air over the United States are likely 10 parts perbillion 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 oncomparison of model outputwith observations for conditions which most nearlyreflect those given in the PRB definition, i.e.,
at the lower end ofthe probability distribution. As discussed inthe Criteria Document and Staff Paper, itcan be

seenthat GEOS-CHEM overestimates 03 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 ofthe country the evidence suggests that the model actually overestimates PRB. Thus, the influence ofalternative estimates ofPRB onrisks in excess of PRB associated with meeting the current standard can be to lower orincrease 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 ofthe model have shown relatively good 

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agreement (i.e., generally within 5 ppb for most regions ofthe country):

Finally, EPA believes that some commenters have misread the CASAC Panel concern "that the current approach to determining PRB is the best method tomake this estimation" (Henderson, 2007, p. 2) as a criticism of the use ofthe GEOS-CHEM modeling approach and/or support for primary reliance on estimates based on remote monitoring sites. However, the CASAC Panel wenton to state that one reason for its concern was that the contribution
to PRB from beyond North America was uncontrollable by EPA andthat "a better scientific understanding of intercontinental transport ofair pollutants could serve as thebasis 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 therole that PRB should play, as compared to 'the technical question ofthebest wayto 7estimate PRB levels. In reviewing the StaffPaper, the atmospheric modeling expert on the CASAC Panel in his * comments onhow PRB had been estimated using the GEOS-CHEM model concluded that the "current approach has been peer-reviewed, andis appropriate" (Henderson, 2006b, p. D-48).

(4) Some commenters raised concerns about aspects ofthe exposure modeling that they felt resulted in overestimates ofmodeled exposures, including: (i 03
measurements at downwind monitors

are usually higher than the overall area and may notreflect the overall outdoor
exposures inthe area; (ii) 03 exposures

near roadvays willbe below that measured at the monitor due to titration of03 from automobile emissions ofNO; (iii)03 concentrations are lower at a person's breathing height compared to measurement height, (iv) exposure estimates donot account for 03 avoidance behaviors; and (v)the APEX model over predicts elevated ventilation
rate occurrences, which results in an overestimation ofthe number of
exposures ofconcern and risk estimates for lungfunction decrements.

The concern raised in the first point is unfounded since all 03monitors in each area are used totake into account the spatial variations of03 concentrations. The geographic variation of 03 concentration is accounted forby using measurements from the closest 03 monitor torepresent concentrations in a neighborhood and the measurements at downwind monitors are applied only tothe downwind areas. Second, the reduction in 03 concentrations near roadways dueto titration of03 from automobile emissions ofNO is accounted for and explicitly modeled inAPEX and thus does not bias estimates ofexposures. Thisphenomenon was modeled through the use of"proximity factors," which adjust the monitored concentrations to

account forthe titration of 03 by NO emissions (the monitored concentrations are multiplied by the proximity factors). Three proximity factor distributions were developed, one for local roads, one forurban 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 ofthese 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 03 concentrations at a person's breathing height compared to the heights of nearby monitors. EPA believes that these biases, tothe extent that they exist, are relatively small during warm

summer afternoons when 03 concentrations tend to be higher.

Fourth, behavior changes in response
to 03 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
extentto which people currently modify
their activities in response to 03 alerts. However, under the scenarios modeled forjust meeting alternative standards, 03 alerts would be infrequent relative to the number ofalerts that currently occur inthe nonattainment areas modeled. Consequently, EPA does not feel that this is an influential factor in the estimation ofexposure for the scenarios simulatingjust meeting the current or proposed standards.

Fifth, a comparison ofventilation
rates predicted byAPEX to measurements showed APEX overpredicting ventilation rates for ages 5 to 10, underpredicting ventilation
rates forages 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 ofthe predicted ventilation rates were partially incorporated into the overall uncertainty analysis with the uncertainties ofthe metabolic equivalents (METs), which are the primary drivers ofventilation rates.

(5) Comments from a number of industry organizations, businesses, and others contended that EPA's health risk assessment was biased and thatthe resulting risk assessment is "much higher than would have been obtained using objective methods" (NAM), and
commenters raised one or more ofthe following points in support ofthis 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 onindividual city effect estimates from these studies in its risk assessment; (iii)the riskassessment presented single-pollutant model results that overstate the likely impact of03 when co-pollutant model results were available which should havebeen used; (iv)therisk assessment used linear concentration-response relationships for the health endpoints based on epidemiological studies when nonlinear or threshold models shouldhave been used; and (v) the lung function portion ofthe risk assessment should
not rely on what they characterized as "outlier" information to define
exposure-response relationships, with reference tothe data from the Adams (2006) study, but rather should focus on group central tendency response levels. Each ofthese issues is discussed below and in more detail inthe Response to Comments document.

First, several commenters asserted thatthe results oftime-series studies should notbe used at all in quantitative risk assessments, that risk estimates from single-city time-series studies should not beused since they arehighly heterogeneous andinfluenced by publication bias, and that -thepanel 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 ofspecific studies and effect estimates was based on a careful evaluation ofthe evidence evaluated in the CriteriaDocument and thatthe criteria and rationale for selection of studies and effect estimates were presented and extensively reviewed and discussed bythe 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 forinclusion in .thequantitative riskassessment to be "appropriate" andthatthe risk 

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assessment chapter ofthe StaffPaper and its accompanying risk assessment

were "well done, balanced and reasonably communicated" (Henderson, 2006c, p. 12).

While EPA notes that two ofthe metaanalyses, Bell et a. (2005) and Ito etof. (2005), provided suggestive evidence of publication bias, 0 3-mortality associations remained after accounting for that potential bias. The Criteria Document (p. 7-97) concludes that the "positive 03effects estimates, along with the sensitivity analyses inthese three meta-analyses, provide evidence ofa robust association between ambient 03 and mortality." Concerns aboutthe heterogeneity ofresponses observed

across different urban areas,'particularly for 0 3-related mortality are addressed in the section above on health effect considerations.

Second, as discussed in more detail in the StaffPaper (section 5.3.2.3), there

are different advantages associated with

use of single-city and multi-city effect estimates as thebasis for estimating health risks in specific urban areas. Therefore, the risk assessment included estimates based onboth types ofeffect estimates where suchinformation 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 ofusing linear concentration-response relationships for the health outcomes based on epidemiological studies and whether or notto include any nonlinear models or assumed threshold

were reviewed and discussed by the CASAC Panel during the development ofthe StaffPaper andrisk assessment, and the Panel concurred with the approach used. As discussed inthe proposal notice, StaffPaper (section 3.4.5), and above in the prior section on health effects evidence, EPA recognizes thatthe available epidemiological evidence neither supports orrefutes the existence ofthresholds at the population level for effects such as increased hospital admissions and premature mortality. Noting the limitations ofepidemiological evidence
to address such questions, EPA concluded that if a population threshold does exist, it would likely be well below the level ofthe current 03 standard. The Administrator is very mindful of the uncertainties related towhether the observed associations between 03 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 therisk assessment estimates inhis decision.

-Fifth, consistent with the prior review, the lung function component of the risk assessment has focused on the number and percentage ofchildren that

are estimated to experience a degree of lung function decrement thatrepresents

an adverse health effect. EPA does not agree that the focus ofthe quantitative risk assessment shouldbe on the
average lung function response in the population, since such an assessment wouldnot address thepublic health policy question concerning to extent to which a portion ofthe population would likely experience health effects of

concern. Looking atjust-the average for the population would ignore the evidence ofhealth effects for sensitive subpopulations, an important aspect of public health impactin this and other 03 reviews. EPAbelieves that it is appropriate to include all ofthe individual data from the series of controlled human exposure studies that address lung function responses associated with 6.6-hour exposures to 03 andwhich were reviewed and included inthe final Criteria Document, and this includes the Adams (2006) study. EPA notes thatthe CASAC Panel clearly did notjudge the responses observed in this study to be an "outlier." Rather, CASAC stated in its
comments on the StaffPaper's discussion ofthis study, "there were clearly a few individuals who experienced declines inlung function at these lower concentrations. These were healthy subjects so the percentage of asthmatic subjects, if they had been studied, would mostlikely be considerably greater" (Henderson, 2006c, p. 10).

Having considered comments onthe quantitative exposure and health risk
assessments from both groups of commenters, the Administrator finds no basis to change his position on these quantitative assessments thatwas taken
at the time ofproposal. That is, as discussed above, while the Administrator recognizes that the
assessments rest on a more extensive body ofdata 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 andrisk estimates. Nevertheless, the Administrator concludes that the exposure and risk estimates are sufficiently reliable to inform hisjudgment about the

- significance ofthe exposures andrisk of health effects in susceptible and vulnerable populations at 03 levels associated withjust meeting the current 8-hour standard. However, the Administrator disagrees with aspects of these commenters' views onthe level of the standard that is appropriate and supported bythe available health effects evidence and quantitative assessments associated withjust meeting alternative standards.

3. Conclusions Regarding theNeed for Revision

Having carefully considered the public comments, as discussed above, the Administrator believes the fundamental scientific conclusions on the effects of03 reached in the Criteria Document and StaffPaper,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 03 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 03
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 ofnew epidemiological studies. The Administrator believes thatthis body ofscientific evidence is very robust, recognizing thatit includes large numbers of various types ofstudies, "includingtoxicological studies, ,controlled human exposure studies, field panel studies, and community epidemiological studies, that provide consistent and coherent evidence ofan array of0 3-related respiratory morbidity effects and possibly cardiovascularrelated morbidity as well as total nonaccidental andcardiorespiratory mortality. The Administrator observes that (1)the evidence ofa 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 asthrough 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 ofrespiratory effects in the general population likelyunderestimate the magnitude orimportance ofthese effects; (3)newly available evidence 

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about mechanisms oftoxicity mole completely explains the biological plausibility of0 3-inducedrespiratory effects and isbeginning to suggest mechanisms that may link03 exposure to cardiovascular effects; and (4) there is

now relatively strong evidence for associations between 03 andtotal nonaccidental and cardiopulmonary mortality, even after adjustment for the influence of season and PM. The Administrator believes that this very robust body ofevidence, taken together, enhances our understanding of03- related effects relative to what was known at the time ofthe 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 03 exposures, that indicators ofrespiratory morbidity suchas emergency department visits and hospital admissions are causally related
to 03 exposures, and that the evidence is highly suggestive that 03 exposures during the warm 03 season contribute to premature mortality.

Further, the Administratorjudges that there is important new evidence demonstrating that exposures to 03 at 'levels below the level ofthe current standard are associated with abroad array ofadverse health effects. This is especially true in at-risk populations that include people with asthma or other lungdiseases, who are likely to experience more serious effects from exposure to 03, children, and older adults with increased susceptibility, as well as those who are likely to be vulnerable as aresult ofspending a lot oftime outdoors engaged inphysical activity, especially active children and outdoor workers. The Administrator
notes that this important new evidence demonstrates 0 3-inducedlung function effects and respiratory symptoms in

some healthy individuals down tothe previously observed exposure level of 0.080 ppm, as well as very limited new evidence at exposure levels well below the level ofthe current standard. In addition, the Administrator notes that (1) there is now epidemiological evidence ofstatistically significant 03- related associations with lungfunction 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 nothave met the
current standard but which nonetheless report statistically significant associations that generally extend down to ambient 03 concentrations that are below the level ofthe current standard; (3) there are afew studies that have examined subsets ofdata that include only days with ambient 03 concentrations below the level ofthe current standard, orbelow even much lower 03 concentrations, and continue
to report statistically significant associations with respiratory morbidity
outcomes andmortality; and (4) the evidence from controlled human exposure studies, together with animal toxicological studies, provides considerable support for the biological plausibility ofthe respiratory morbidity associations observed inthe epidemiological studies and for concluding that the associations extend below the level ofthe current standard.

Based on the available evidence, the Administrator agrees with the CASAC Panel andthe majority ofpublic,
commenters that the current standard is
not requisite to protect public health with an adequate margin ofsafety because itis does notprovide sufficient protection andthat revision ofthe
current 03"standard is needed to provide increased public health protection. The Administrator notes that extensive critical-review of thisbody of evidence and related uncertainties during the criteria and standard review" process, including review by the
CASAC Panel andthe public ofthe basis for EPA's proposed decisfion to revise the primary 03 standard, has identified a number of issues about which different reviewers disagree and for which additional research is Warranted. Nonetheless, onbalance, the Administrator believes that the remaining uncertainties inthe available evidence do notdiminish confidence in the causal relationships between 03 exposures and indicators of serious respiratory morbidity effects, or the highly suggestive evidence of associations between 03 exposures and premature mortality, nor do they diminish confidence in the conclusion thatthe associations extendbelow the level ofthe current standard.

.Beyond aprimary consideration ofthe available evidence, the Administrator has also taken into consideration the Agency's exposure andrisk assessments to help inform his evaluation ofthe adequacy ofthe current standard. As at the time ofproposal, the Administrator' believes the results ofthose assessments inform hisjudgment onthe adequacy of the current standard to protect against health effects ofconcern. In considering the exposure analysis results atthis time,the Administrator recognizes that thatthere is a risk for confusion in the term "exposure ofconcern" that was used at the time ofproposal, asit could be read to imply a determination that a certain benchmark level ofexposure 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 inthe context ofa continuum rather than focusing on any one discrete benichmark level, as

was done at the time ofproposal, 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 03 exposures and health effects ofconcern become increasingly uncertain at lower 03 exposure levels. Thus,the Administrator has taken into consideration the pattern ofsuch exposure estimates across therange of discrete benchmark levels considered in EPA's exposure assessment to provide

some indication ofthe potential magnitude ofthe incidence ofhealth
outcomes that could notbe evaluated in the Agency's quantitative risk
assessment but which have been demonstrated to occur in healthypeople
at 03 exposures as low as 0.080 ppm, the lowest level atwhich suchhealth
outcomes have been tested.20

More specifically, the Administrator has considered the pattern ofreductions in such exposures across the benchmark levels of0.080, 0.070, and 0.060 ppm, which span the level at which there is strong evidence ofeffects inhealthy people down to a level at which the Administrator judges the-evidence of effectsto be very limited. The Administrator observes that based on the aggregated exposure estimates for the 2002 simulation forthe 12 urban

areas included inthe exposure analysis, uponjust meeting the current standard, the percentages ofasthmatic or all school age children likely to experience

one ofmore exposures at and above these benchmark levels of 0.080, 0.070, and 0.060 ppm (while atmoderate or greater exertion) are approximately 4%,

20As noted above, suchhealth 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 withaggravation ofasthma, increased medication use, increased school and workabsences, increased susceptibility to respiratory infection, increased visits to doctors' offices and
emergency departments, and increased admissions
to hospitals. In addition, these physiological effects, ifrepeated 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 oflife. 

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20%, and 45%, respectively. As noted at the time ofproposal, the Administrator recognizes that there is substantial yearto-year and city-to-city variability in these estimates and that itis 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 1to 10%, 1to 40%, and 7 to 65%, respectively, across each of the 12 urban areas based onthe 2002 simulation, and from approximately 0 to 1%, 0 to 7%, and 1to 25%, respectively, based onthe 2004 simulation.

With regard to the results ofthe risk assessment, the Administrator again considered the risks estimated to remain
uponjust meeting the current standard. The Administrator takes note ofthe estimated magnitudes ofsuch risks, which are presented above in section ll.B.1.c for arange ofhealth effects includingmoderate and large lung function decrements (including percentages ofchildren 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, andthe uncertainties in these estimates. Further, the Administrator recognizes that these estimated risks for the specific healtheffects that couldbe

- analyzed in the Agency's risk
assessment are indicative ofa much broader array of0 3-relatedhealth endpoints that are part ofa "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 ofatrisk groups.

In considering these quantitative exposure and risk estimates, as well as the broader array of 0 3-related health endpoints that could not be quantified, the Administrator believes that they are important from a public health perspective and indicative ofpotential 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 onthese considerations, and consistent with CASAC Panel's unanimous conclusion thatthere is no scientific justification for retaining the
current standard, the Administrator concludes that the current primary 03 standard is not sufficient and thus not requisite to protect public health with an adequate margin of safety, and that revision is needed toprovide 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 ofwhat specific revisions are appropriate. That requires looking specifically at the current indicator, averaging time, form, and level ofthe 03 standard, and evaluating the evidence relevant to determining whether and to what extent any ofthese elements should be revised, as is discussed in the following section.

C. Conclusionson theElementsofthe Primary03 Standard

1.Indicator

In the lastreview ofthe air quality criteria for 03 and other photochemical oxidants and the 03 standard, asin other prior reviews, EPA focused on a standard for03 as the most appropriate surrogate for ambient photochemical oxidants. Inthis review, while the complex atmospheric chemistry in which 03 plays a keyrole has been highlighted, no alternatives to 03 have been advanced as being amore appropriate surrogate for ambient photochemical oxidants.

The StaffPaper (section 2.2.2) noted that itis generally recognized that control of ambient 03 levels provides thebest means of controlling photochemical oxidants. Among the photochemical oxidants, the acute
exposure chamber, panel, andfield epidemiological human health database provides specific evidence for03 at levels commonly reported inthe ambient air, in partbecause 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 thebiological basis for effects observed in epidemiological studies that are associated with air pollutant mixtures, where 03 is used as the surrogate for the mix ofphotochemical oxidants. Meeting the 03 standard can be expected to provide some degree ofprotection against potential health effects thatmay be independently associated with other photochemical oxidants but which are not discemable fromcurrently available studies indexed by 03 alone. Since the precursor emissions that lead to the formation of03 generally also lead to the formation of otherphotochemical oxidants, measures leading to reductions in population exposures to 03 can generally be expected tolead to reductions in population exposures to other$ntkhihehmical oxidants.

The StaffPaper 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 ofmorbidity and mortality effects associated with 03 in these types of studies, control ofambient 03 levels is generally understood to provide the best means ofcontrolling photochemical oxidants in general, and thus of protecting against effects that may be associated with individual species and/

orthe broader mix ofphotochemical oxidants, independent of effects specifically related to 03. No public
comments specifically suggested changing the indicator for the 03 NAAQS.

In its letter to the Administrator, the 

### CASAC Panel noted that 03 is "the key indicator ofthe extent ofoxidative chemistry and serves to integrate multiple pollutants." The CASAC also statedthat "although 03 itselfhas direct effects on human health and ecosystems, it can also be considered as indicator ofthe mixture of photochemical oxidants and ofthe oxidizing potency ofthe atmosphere" (Henderson, 2006c, p. 9).

Based on the available information, and consistent with the views ofEPA staffand the CASAC, the Administrator concludes that it is appropriate to continue to use 03 as the indicator for

a standard thatis intended to address effects associated with exposure to 03, alone or incombination with related photochemical oxidants. In sodoing, the Administrator recognizes that measures leading to reductions in population exposures to 03 will also reduce exposures to other photochemical oxidants.

2. Averaging Time

a. Short-Term and Prolonged (1to 8 Hours)

The current 8-hour averaging time for -the primary 03 NAAQS was setin 1997. At thattime, the decision to revise the averaging time ofthe primary standard from 1hour to 8 hours was supported by the following key observations and conclusions: (1)The 1-hour averaging time ofthe previous NAAQS was originally selected primarily on the basis ofhealth 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 awide range ofhealth effects (e.g., moderate to large lungfunction

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decrements, moderate to seve:e respiratory symptoms and pulmonary infiammation) and prolonged (i.e., 6- to 8-hour)exposures below the level ofthe then current 1-hour NAAQS.

(3) Results ofthe quantitative risk analyses showed that reductions in risks fromboth short-term andprolonged
exposures could be achieved through a primary standard with an averaging period ofeither 1hour or 8 hours. Thus establishing both a 1-hour and an 8-hour standard wouldnot be necessary to reduce risks associated with the full
range ofobserved health effects.

(4) The 8-hour averaging timewas

more directly associated with health effects ofconcern at lower 03 concentrations than the 1-hour averaging time. It was thus the

consensus ofthe 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 uniformlyprotective national standard than the then current 1-hour standard.

(6) An 8-hour averaging time effectively limits both 1- and 8-hour
exposures ofconcern.

In looking at the new information that is discussed in section 7.6.2 ofthe
current Criteria Document, the Staff Paper noted that epidemiological studies have used various averaging periods for 03 concentrations, most commonly 1-hour, 8-hour and 24-hour
averages. As described more specifically in sections 3.3 and 3.4 ofthe Staff Paper, in general the results presented from U.S. and Canadian studies showed

no consistent difference forvarious 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 StaffPaper did
not evaluate alternative averaging times in thisreview dnd did not conduct
exposure or risk assessments for standards with averaging times other than 8 hours.

The StaffPaper discussed an analysis ofa recent three-year period ofair quality data (2002 to 2004) which was conducted to determine whether the comparative 1- and 8-hour air quality patterns that were observed inthe last review continue to be observed based on

more recent airquality data. This updated air quality analysis (McCluney,

- 2007) was very consistent with the

analysis done in the last review in that itindicated 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 onrecent air quality data, itwas again reasonable to conclude that an 8-hour average standard at orbelow the current level would generally be expected to provide protection equal to orgreater 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 limitboth 1- and 8-hour
exposures ofconcern and is appropriate
to provide adequate and more uniform protection ofpublic health from both short-term and prolonged exposures to 03 in the ambient air. Inits letterto the Administrator, the CASAC Panel unanimously supported the continued

use of an 8-hour averaging time for the primary 03 standard (Henderson2007,
p. 2).

With respect to comments received on the proposal, mostpublic commenters did not address the issue ofwhether EPA should consider additional or alternative averaging time standards. A few commenters, most notably the CA EPA andjoint comments by ALA and several environmental groups, expressed the view that consideration shouldbe givento setting orreinstating

a 1-hour standard, in addition to maintaining the use of an 8-hour averaging time,to protect people in those parts ofthe country with relatively more "peaky" exposure profiles (e.g., Los Angeles). These
commenters pointed out thatwhen controlled exposure studies using triangular exposure patterns (with relatively higher 1-hourpeaks) have been compared to constant exposure patterns with the same aggregate 03 dose (interms ofconcentration multiplied by time), "peaky" exposure patterns are seen to lead to higher risks. The CA EPA made particular note of thispoint, expressing the view that a 1- hour standard would more closely represent actual exposures, in that many people spend only 1to 2 hours a day outdoors, and that itwould be better matched to 03 concentration profiles along the coasts where 03levels are typically high for shorter averaging periods than 8 hours.

Forthe reasons discussed in the Staff Paper and summarized above and considering the unanimous views ofthe CASAC Panel supporting the continued

use ofan 8-hour averaging time for the primary 03 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 (1to 3 hours) andprolonged (6 to 8 hours) exposures to 03 inthe ambient air and that it is appropriate to continue use ofthe 8-hour averaging time for the 03 NAAQS.

b. Long-term

During the last review, there was a large animal toxicological database for consideration thatprovided clear evidence ofassociations between longterm (e.g., from several months to years)
exposures and lungtissue damage, with additional evidence ofreduced lung. elasticity and accelerated loss of lung function. However, there was no corresponding evidence for humans, andthe state ofthe science had not progressed sufficiently to allow quantitative extrapolation ofthe animal study findings to humans. For these
reasons, consideration ofa separate long-term primary 03 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 03 exposures, discussed above in section II.A.2.a.ii, the StaffPaper concluded that a health-based standard with a longer-term averaging time than 8 hours is not warranted at this time. The StaffPaper noted that while potentially more serious health effects have been identified as being associated with longer-term exposure studies of laboratory animals and inepidemiology 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 ofhealth effects associated with long-term
exposures would be reduced in any area meeting an 8-hofr standard. Thus, the StaffPaper did not recommend consideration of a long-term, healthbased standard at thistime.

In its final letter tothe Administrator, the CASAC Panel offered noviews on the long-term exposure evidence, nor did it suggest that consideration of a primary 03 standard with a long-term averaging timewas appropriate, and instead the CASAC Panel agreed with the choice ofan 8-hour averaging time for theprimary 03 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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a long-term primary 03 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 ayeraging time can effectively limit both 1- and 8-hour
exposures ofconcern and that an 8-hour averaging time is appropriate to provide adequate and more uniform protection ofpublic health from both short-term (1-
to 3-hour) and prolonged (6- to 8-hour)
exposures to 03 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 ofconcern at lower 03 concentrations than are averaging times ofshorter duration and (2) results from quantitative risk analyses showing that attaining an 8-hour standard reduces the risk ofexperiencing 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 seta separate 1-hour standard. TheAdministrator also concludes that a standard with a longterm averaging time is not warranted at this time.

3. Form

In 1997, the primary 03 NAAQS was changed from a "l-expectedexceedance" form peryear over three
years 21 to a concentration-based statistic, specifically the 3-year average ofthe annual fourth-highest daily maximum 8-hour concentrations. The principal advantage ofthe concentration-based form is that it is

more directly related to theambient 03 concentrations that are associated with health effects ofconcern. With a concentration-based form, days on which higher 03 concentrations occur wouldweigh proportionally more than days with lower cortcentrations, since the actual concentrations areused in determining whether the standard is attained. That is, given that there is a continuum ofeffects associated with
exposures tovarying levels of03, the
extent to which public health is affected

21

The 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 ofthe standard for 'the standard to be met at an airquality monitoring
site. by exposure to ambient 03is related to the actual mnagnitde ofthe 03 concentration,.not *istwhether the concentration is above a specified level.

During the 1997review, consideration

was given to a range ofalternative forms, including the second-, third-, fourth- and fifth-highest daily maximum 8-hour concentrations in an 03 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 formultiple exceedances'to provide increased stability, but that also significantly limits the number of days onwhich 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 ofprotection against potential chronic effects afforded by the alternatives within the
range were not well enough understood
to use any such differences as abasis for choosing the most restrictive forms. On the other hand, the relatively large percentage ofsites thatwould experience 03 peaks well above 0.08
ppm andthe number of days on which the level ofthe standardmay be exceeded even when attaining a fifthhighest 0.08 ppm concentration-based standard, argued against choosing that form.

As an initial matter, the StaffPaper considered whether it is appropriate to continue to specify the level ofthe 03 standard to the nearest hundredth (two decimal places) ppm, or whether the precision with which ambient 03 concentrations are measured supports specifying the standard level to the thousandth (three decimal places) ppm (i.e., to the part perbillion (ppb)). The StaffPaperdiscussed an analysis conducted by EPA staffto determine the impact ofambient 03measurement

error on calculated 8-hour average 03 design value concentrations, which are compared to the level ofthe standard to determine whether the standard is attained (Cox and Camalier, 2006). The results of thisanalysis suggested that instrument measurement error, or possible instrument bias, contribute
very littleto theuncertainty in design values. More specifically, measurement imprecision was determined to contribute less than 1ppb to design value uncertainty, and a simulation study indicated that randomly occurring instruiaent bias could contribute apprqxciuately,

,ppb. EPA staff interpreted this analysis as being supportive of specifying the level ofthe standard to the thousandth ppm. Ifthe
current standard were to be specified to this degree ofprecision, the current standard would effectively be at a level of0.084 ppm,reflecting the data rounding conventions that are part of the definition ofthe 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 StaffPaper considered the adequacy ofthe public health protection provided by the combination ofthe level and form to be the foremost consideration. In addition, the StaffPaper recognized that it is important to have a form ofthe standard that is stable and insulated from the impacts ofextreme meteorological
events that are conducive to 03 formation. Such instability can have the effect ofreducing public health protection, because frequent shifting in and out ofattainment dueof meteorological conditions can disrupt

an area's ongoing implementation plans and associated control programs. Providing more stability is one ofthe

reasons thatEPA moved to a concentration-based form in 1997.'

The StaffPaper considered two concentration-based forms ofthe standard: the nth-highest maximum concentration and a percentile-based form. Apercentile-based statistic is useful for comparing datasets of varying length because it samples approximately the same place in the distribution ofair 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 03

seasons relative to places with shorter 03 seasons. An nth-highest maximum concentration form would more effectively ensure that people who live in areas with different length 03 seasons receive the same degree ofpublic health protection. For thisreason, the exposure and risk analyses were based on a form specified interms ofan nth-highest concentration, with nranging from 3 to 5. The results ofsome ofthese analyses

are shown in the StaffPaper (Figures 6- 1through 6-4) and specifically discussed in chapter 6. These figures illustrate the estimated percent change in risk estimates for the incidence of moderate orgreater decrements in lung function (__ 15 percent FEV,) in all school age children and moderate or 

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greater lung function decrements ( - 10 percent FEV) in asthmatic school age children, associated with going from meeting the current standard tomeeting alternative standards with alternative forms based onthe 2002 and 2004 simulations. Figures 6-5 and 6-6 illustrate the estimated percent change in the estimated incidence ofnonaccidental 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 ofthe patterns found in all ofthe analyses. The estimated reductions in risk associated with different forms ofthe 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 of0.084 ppm, andwith estimated reductions associated with the fifth-highest form at a 0.074"pprm level. Inmost cities, there are generally only small differences in the estimated reductions inrisks associated with the third- to fifth-highest forms at a level of 0.074 ppm simulated using 2002 and 2004 03 monitoring data.

The StaffPaper noted thatthere is not

a clear health-based rationale for selecting a particular nth-highest daily maximun form ofthe standard from
among the ones analyzed. Italso noted that the changes in the form considered in the analyses result in only small differences in the estimated reductions in risksin most cities, although in sonde cities larger differences are estimated. The StaffPaper concluded that a range ofconcentration-based forms from the third-to the fifth-highest daily maximum 8-hour average cbncentration is appropriate for 'consideration in setting the standard. Given that there is a continuum ofeffects associated with

exposures to varying levels of03, the extentto which public health is affected by exposure to ambient 03 is related to the actual magnitude ofthe 03 concentration, notjust whether the concentration is above a specified level. The principal advantage ofa concentration-based form is thatit is

more directly related to the ambient 03 concentrations that are associated with health effects. Robust, concentrationbased forms, in the range ofthethirdto 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 ofexceedancebased forms andprovide insulation from the impacts ofextreme meteorological events. Such instability

can have the effect ofreducing public health protection by disrupting ongoing. implementation plans and associated control programs.

With regard to the precision ofthe standard, inits letter to the Administrator, the CASAC concluded thatcurrent monitoring technology "allows accurate measurement of 03 concentrations with a precision ofparts per billion"(Henderson, 2006c). The CASACrecommended that the specification ofthe level ofthe 03 standard should reflect this degree of precision (Henderson, 2006c). While the CASACPanel unanimously supported specifying the level of the standard to this degree ofprecision,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 theproposed increased precision and but did not support truncating to the third decimal. However, several industry associations (e.g., API, EMA, AAAM) suggested thatthere 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 ofthe standard should be specified
tothe thousandth ppm (three decimal places),based onthe staff's analysis and conclusions discussed in the StaffPaper that current monitoring technology allows accurate measurement of 03 to

- support specifying the 8-hour standard
to this degree ofprecision, and on the CASACPanel's reasoning and recommendation with respect to this aspect ofthe standard.

With regard tothe form ofthe standard, inits 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 ofthe standard because it strikes an appropriate balance between stability and protection, as well asbecause EPA used this form in their analyses (e.g., EMA, NESCAUM, and Pennsylvania Department ofEnvironmental Protection). Some public commenters that expressed the view thatthe current primary 03 standard is not adequate also submitted comments that supported a more health-protective form ofthe standard than the current form (e.g., a second-or third-highest daily maximum form) (e.g., ALA etal.). 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 ofa 6th or 7th highest daily maximum form.

The Administrator recognizes that there is not aclear health-based threshold for selecting a particular nthhighest daily maximum form ofthe standard from among the ones analyzed in the Staff Paper and that the current form ofthe standard provides a stable targetfor implementing programs to improve air quality. The Administrator also agrees thatthe adequacy ofthe public health protection provided bythe combination ofthe level and form is a foremost consideration. Based on this, the Administrator finds that the formof the current standard, 4th-highest daily maximum 8-hour average concentration, should be retained,recognizing that the public health protection that wouldbe provided bythis standard is based on combining this form with the increased health protection provided by the lower level ofthe standard discussed inthe sectionbelow.

4. Level

a. Proposed Range

Forthe reasons discussed below, and taking into account information and
assessments presented in the Criteria Document and StaffPaper, the advice and recommendations ofthe CASAC, and thepublic comments received prior
toproposal, the Administrator proposed
torevise the existing 8-hour primary 03 standard. Specifically, the Administrator proposed to revise the level ofthe primary 03 standard to within'a rangefrom 0.070 to 0.075 ppm.

The Administrator's consideration of alternative levels ofthe primary 03 standard builds onhis proposal, discussed above, that the overall body of evidence indicates thatthe current 8- hour 03 standard is not requisite to protect public health with an adequate margin of safetybecause it does not provide sufficient protection, and that revision would resultin increased public health protection, especially for 

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members ofat-risk groups, notably including asthmatic children and other people with lung disease, aswell as all children and older adults, especially those active outdoors, and outdoor workers, against an array ofadverse health effects. These effects range from health outcomes that could be quantified in the risk assessment, including decreased lungfunction, respiratory symptoms, serious indicators ofrespiratory morbidity such

as hospital admissions forrespiratory
causes, and nonaccidental mortality, to health outcomes that couldnot be directly estimated, including pulmonary inflammation, increased medication
use, emergency department visits, and possibly cardiovascular-related morbidity effects. Inreaching a proposed decision about the level ofthe 03 primary standard, the Administrator considered: the evidence-based considerations fromthe Criteria Document and the Staff Paper; the results ofthe exposure and risk
assessments discussed above andin the StaffPaper, givingweight to the exposure and risk assessments asjudged appropriate; CASAC advice and recommendations, as reflected in discussions of drafts ofthe Criteria Document and StaffPaperat public meetings, in separate written comments, and in CASAC's letters to the Administrator; EPA staff recommendations; and public
comments received during the development ofthese documents, either in connection with CASAC meetings or separately. Inconsidering what 8-hour standard is requisite to protect public health with an adequate margin -of safety, the Administrator noted atthe time ofproposal thathe was mindful that this choice requiresjudgment based

on an interpretation ofthe evidence and other information that neither overstates nor understates the strength and limitations ofthe evidence and information northe appropriate inferences tobe drawn.

The Administrator noted that the
mostcertain evidence of adverse health effects from exposure to 03 comes from the clinical studies andthat the large bulk ofthis evidence derives from studies of exposures at levels of 0.080 and above. At those levels, there is consistent evidence oflung function decrements andrespiratory 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 ppmlevel is a threshold for these effects. Although the Administrator took
note ofthe very limited new evidence oflung function decrements and respiratory symptoms in some healthy individuals at the 0.060 ppm exposure level, hejudged this evidence too limited to support a primary focus at this level. The Administrator also noted thatclinical 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 03. There were also epidemiological studies thatprovide evidence of statistically significant associations between shortterm 03exposures and more serious health effects, such as emergency department visits,hospital admissions, and premature mortality, in areas that likely would have metthe current standard. The Administrator also took
note ofthe many epidemiological studies done in areas that likely would
not have metthe current standard but which nonetheless report statistically significant associations thatgenerally extend down to ambient 03 concentrations thatwere below the level ofthe current standard. Further, there

were a few studies that have examined subsets of datathat include only days with ambient 03 concentrations below the level ofthe current standard, or below even much lower 03 concentrations, and continued toreport statistically significant associations with respiratory morbidity outcomes and mortality. In considering this evidence, the Administrator noted thatthe extent
to which these studies provide evidence of causal relationships with exposures to Oialone, down tothe 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 03 alone at ambient 03 levels below the current standard.

Therefore, the Administrator judged
at thetime ofpr6posal, and continues
tojudge as discussed in section II.B.3, thatrevising the current standard to protect public health with an adequate margin of safety is warranted andwould reduce risk to public health, based on: (1) The strongbody ofclinical evidence in healthy people atexposure 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 oflung function decrements and respiratory symptoms atlower levels; (2) the substantial body ofclinical and epidemiological evidence indicating that people with asthma are likely to experience larger and more serious effects than healthy people; and (3) the body ofepidemiological evidence indicating associations are observed for

a wide range ofserious 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 ofproposal and continues to conclude that the estimates ofexposures of concern and risks remaining uponjust meeting the current standard or a standard at the 0.080 ppm level provide additional support forthis view. Forthe same reasons stated in the proposal notice and discussed above in section II.B onthe adequacy ofthe
current standard, the Administrator judges thatthe standard shouldbe set below 0.080 ppm, a level at which the evidence provides a high degree of certainty about the adverse effects of03
exposure even inhealthy people.

The Administrator next considered what standard level below 0.080 ppm would be requisite toprotect public health with an adequate margin ofsafety thatis sufficient, but not more than
necessary, to achieve thatresult, recognizing that such a standard would result-in increased public health protection. The assessment ofa standard level calls for consideration ofboth the degree ofadditional protection that alternative levels ofthe standard might be expected to provide as well asthe certainty thatany 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 policyjudgment and selecting a standard level from a range of reasonable values.

TheAdministrator 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 ofthe evidence concerning effects below 0.080 ppm is from epidemiological studies. The epidemiological studies do notidentify
any bright-line threshold level for effects. At the same time, the epidemiological studies arenot in and ofthemselves direct evidence ofa causal linkbetween exposure to 03 and the occurrence ofthe effects. The Administrator considers these studies in the context ofall the other available evidence in evaluating the degree of 

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certainty that 0 3-related adverse health effects would occur atvarious ambient levels below 0.080 ppm, including the strong human clinical studies and the toxicological studies that demonstrate the biological plausibility and mechanismis forthe effects of 03 on airway inflammation and increased airway responsiveness atexposure levels of0.080 ppm and above.

Based onconsideration ofthe entire body ofevidence and information available at this time, as well as the recommendations ofthe CASAC, the Administrator proposed that a standard within therange of0.070 to 0.07"5 ppm would be requisite to protect public health with an adequate margin of safety. As noted at the time ofproposal,

a standard level within thisrange is estimated to reduce the risk ofa variety ofhealth effects associated with
exposure to 03, includingthe 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 ofthese effects are indicative ofa muchbroader array of 03-related health endpoints, as represented by thepyramid ofeffects, such as school absences and increased medication use that are plausibly linked
to these observed effects.

The Administrator also considered the degree ofimprovements in public health that potentially could be achieved by a standard of0.070 to 0.075
ppm, giving weight to the exposure and risk assessments as hejudged appropriate. As discussed inthe proposal notice (section II.D.4) in considering the results ofthe exposure assessment, the Administrator primarily focused onexposures 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 ofconcern" was conducted

to estimate exposures at and above three discrete benchmark levels, the concept is appropriately viewed asa continuum. As discussed above, the Administrator strives to balance concern about the potential for health effects andtheir severity with the increasing uncertainty associated with our understanding of the likelihood ofsuch effects at lower 03 exposure levels. In focusing on this benchmark, the Administrator noted that uponjust meeting a standard within the range of0.070 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 (forthe 12 cities inthe assessment) was estimated
to be approximately 2 to 4 percent ofall and asthmatic children and generally less than 10 percent ofchildren even in cities that receive theleast degree of protection from such a standard ina recent year with relatively high 03 levels. A standard within the 0.070 to 0.075 ppm rangewould thus substantially reduce exposures of

concern by about 90 to 80 percent, respectively, from those estimated to

occur uponjust meeting the current standard. While placing less weight on the results oftherisk assessment, in light ofthe important uncertainties inherent in the assessment, the Administrator noted that theresults indicated that a standard set within this
range would likely reduce risks to atrisk groups from the 0 3-related health effects considered in the risk assessment, and by inference across the much broader array of0 3-related health effects that could onlybe considered qualitatively, relative to the level of protection afforded by the current standard. This lent support to the proposed range.

The Administratorjudged that a standard set within therange of0.070 to 0.075 ppm would provide a degree of reduction in risk thatis important from

a public health perspective and that a standard within this range would be requisite to protect public health, including the health ofat-risk groups, with an adequate margin ofsafety. EPA's evaluation ofthe body of scientific evidence andquantitative estimates ofexposures andrisks 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 directingthe choice of level within thisreasonable range, the choice of whatis appropriate, considering the strengths and limitations ofthe evidence, and the appropriate inference tobe drawn from the evidence andthe exposure and risk
assessments is a publichealth policy judgment. To further inform this judgment, EPA sought public comment

onthe extent to which the epidemiological and clinical evidence provide guidance as tothe level ofa 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 astandard level below 0.070
ppm would not be appropriate. In reaching thisjudgment, the Administrator noted that there was only quite limited evidence from clinical studies at exposure levels below 0.080
ppm 03. Moreoyer, theAdministrator recognized thatin 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
notreport any positive 03-related associations. Inaddition, the Administratorjudged that evidence ofa causal relationship between adverse health outcomes and 03 exposures became increasingly uncertain at lower levels ofexposure.

The Administrator also considered the results ofthe exposure assessments in reaching his judgment that a standard level below 0.070 ppm would not be appropriate. The Administrator noted thatin considering therdsults fromthe
exposure assessment, a standard set at the 0.070 ppm level, with the same form

as the current standard, was estimated
toprovide substantial reductions in. exposures ofconcern (i.e., approximately 90 to 92 percent reductions in the numbers ofschool age children and 94 percent reduction in the total number ofoccurrences) for both all and asthmatic school age children relative to justmeeting the current

- standard based ona simulation of a recent year with relatively high 03 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 ofall and asthmatic school agechildren

evenin a year with relatively high 03 levels, increasing to over 99.9 percent of children in ayear with relatively low03 levels (2004).

In considering theresults ofthehealth riskassessment, as discussed in the proposal notice (section II.C.2), the Administrator noted thatthere were important uncertainties and assumptions inherent in therisk
assessment and that this assessment was most appropriately used to simulate trends andpatterns that could be expected, as well asproviding informed, but still imprecise, estimates ofthe potential magnitude ofrisks. The Administrator particularly noted that as lower standard levels were modeled, including a standard set at a level below 0.070 ppm, therisk assessment continued to assume a causal link between 03 exposures and the

occurrence ofthe health effects examined, such thatthe assessment continued to indicate reductions in 03- related risks upon meeting a lower standard level.As discussed above, however, the Administrator recognized 

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that evidence ofa causal relationship between adverse health effects and 03 exposures becomes increasingly uncertain at lower levels ofexposure. Given all ofthe information available to him atthe time oftheproposal, the Administratorjudged that the increasing uncertainty ofthe existence and magnitude ofadditional public health protection that standards below 0.070
ppm might provide suggested that such lower standard levels would likely be below whatis necessary to protect public health with an adequate margin ofsafety.

In addition, the Administrator judged that a standard level higher than 0.075
ppm would also not be appropriate. Thisjudgment took into consideration the information discussed in the proposal notice (sections IL.A and B) and was based on the strongbody of clinical evidence in healthy people at exposure levels of0.080 ppm and above, the substantial body ofclinical and epidemiological evidence indicating that people with asthma are likely to experience larger and more serious' effects than healthy people, thebody of epidemiological evidence indicating that associations are observed for a wide range ofmore serious health effects at levelsbelow 0.080 ppm, and the estimates ofexposure and risk remaining upon just meeting a standard
set at 0.080 ppm.The muchgreater certainty ofthe existence and magnitude of additional public health protection that such levels would forego provides the basis forjudging that levels above 0.075 ppm wouldbe higherthan what is requisite to protect public health, including the health ofat-risk groups, with an adequate margin of safety.

Forthe reasons discussed in more detail in the proposal notice and summarized above, the Administrator proposed torevise the level ofthe primary 03 standard to within the range of0.070 to 0.075 ppm.

Atthe time ofproposal, the Administrator recognized that sharply divergent views on the appropriate level ofthis standard hadbeen presented to EPA as part ofthe NAAQS review
process, and he solicited comment ona wide range ofstandard levels and alternative approaches to characterizing and addressing scientific uncertainties. One such alternative view focused very strongly onthe uncertainties inherent in the controlled human exposure and epidemiological studies and quantitative exposure andhealth risk
assessments as the basis for concluding that no

### change to the current 8-hour 03 standard of 0.084 ppm was warranted. In sharp contrast, other viewed the controlled human exposure and epidemiological studies as strong and robust, and generally placed more weight on the results ofthe 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

-samesharply 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 ofthe health effects evidence and how such information shouldbe considered inthe decision on the standard level, EPA notes thatthe
commenters fell into the same two groups discussed above in section II.B.2. The two groups often cited the same studies andevidence, butthey 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 anduse ofthe 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 be14w, and the other group arguing that it supports a decision notto revisethe
current 8-hour standard.

With regard to the evidence from controlled human exposure studies,
commenters that included publichealth and environmental groups who supported revising the current standard expressed the view that the large body of evidence available atthe time ofthe last review, demonstrating an array of adverse health effects (i.e., reduced lung function, respiratory symptoms, increased airway responsiveness, inflammation, and increased susceptibility torespiratory infection),
at concentrations of0.080 ppm 03, indicated that the standard shouldhave been set at alower level. These
commenters noted that standards must be setbelow 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 03 willresult in a significant proportion (i.e., 7%) ofthe adult population who do not have asthma or other lung diseases experiencing notable lung function decrements (FEV decrement (>10%), andfurthermore that larger decrements inFEVI would be expected in more susceptible populatiohs. This evidence caused these commenters to reject EPA's proposed range:

Clearly, EPA's proposed standard of0.070 to 0.075 ppm cannot be considered protective ofpublic health in light ofexperimental evidence demonstrating adverse respiratory effects in healthy individuals exposedto 0.060 ppm, and the legal requirements to protect sensitive populations withan adequate margin ofsafety. [ALA et al., p. 511
The second group ofcommenters, who opposed revision ofthe 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 thatthe individuals who experienced largerresponses were - too few to serve as abasis for a revised 03 standard. This group included virtually all commenters representing industry associations and businesses. These general comments are addressed above in section ll.B.2.a and in more detail in theResponse 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 ingeneral the comments support his original view thatthese studies provide the mpst 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 03- 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 ofthe same effects at exposure levels wellbelow the level ofthe current standard (Adams, 2'002, 2006). EPA disagrees with these
comrnmenters thatthe percent ofsubjects that experienced FEV decrements greater than 10% in this study of 30 subjects can appropriately be generalized'to the U.S. population. Based oncareful consideration ofthe comments, the Administrator again concludes thatwhile the Adams studies provide evidence that some healthy individuals will experience lung function decrements and respiratory symptoms atthe 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 thatI 

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the resi;lt ofAdams studies mean that.,, the level,qthe standard has to be set-
, at 0.060 ppm.

With regard tothe information from epidemiological studies, commenters representing public health, environmental, and medical organizations generally asserted that the large body ofnew epidemiological studies provides evidence ofcausal associations between 03 exposures and

a wide array ofrespiratory and cardiovascular morbidity effects, including emergency department visits and hospital admissions. They expressed the view that a significant body ofstrong, consistent evidence links short-term exposures to premature mortality and noted that this evidence is supported bynew research that provides biological plausibility for such effects. These conmenters 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 ofthe data which indicate that statistically significant associations remain down to very low ambient 03 levels, show effects well below the level ofthe 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 03, including infants, women, and African-Americans, that should be considered in deciding therequisite level ofprotection. They asserted that this information supports a standard set at alevel nohigher than 0.060 ppm 03.

With regard to the information from epidemiological studies, the second
group ofcommenters focused strongly

on EPA's interpretation ofthe epidemiological evidence and the uncertainties they saw in this evidence

as abasis for concluding that no change
to the currentlevel ofthe 8-hour 03 standard is warranted. Incommenting

on the proposed range oflevels, these
commenters generally relied on the

same arguments presented above in section II.B.2.a as to why theybelieved it wouldbe inappropriate for EPA to make any revisions to the primary 03 standard. That is, they asserted that the health effects of concern associated with short-term or prolonged exposures to 03 have not changed significantly since 1997; that the inconsistencies and uncertainties inherent in these studies

as a whole should preclude any reliance

on them asjustification for amore stringent standard; and that "new" science not included in the Criteria Doqvment continues to increase uncertainty about possible health risks associated with exposure to 03. Specific methodological issues cited as additional support for their conclusions included: adequacy of exposure data; potential confounding by copollutants; model selection; inconsistent evidence relating 03 exposure to mortality, and "new" studies that provide additional evidence ofinconsistencies. These. general comments are addressed above in section IIB.2.a, andin greater detail in the Response to Comments document.

In considering these comments on the epidemiological evidence with regard to the interpretation ofthe epidemiological evidence and methodological issues, the Administrator notes that ingeneral,
most ofthe issues and concerns raised by those who do not support any revisions tothe primary 03 standard with regard to the interpretation ofthe epidemiological evidence fid

. I

. methodological issues" 'e§ii Wtii": restatements ifissues rii '& ing th& review ofthe Criteria Documen and StaffPaper. The same is true ofthe views ofcommenters who supported a level ofthe standard nohigher than 0.060 ppm 03. EPApresented and the CASAC Panel reviewed the interpretation ofthe epidemiological evidence in the Criteria Document and the integration ofthe evidencewith policy considerations in the development ofthepolicy options presented in the Staff Paperfor consideration bythe Administrator. CASAC reviewed the scientific content ofboth the Criteria Document and'Staff Paper and advised the Administrator that these documents provided an appropriate basis for use inregulatory decision making. Therefore, these
comments do not provide a basisfor the Administrator toreach fundamentally different conclusions thanhe reached at the time ofproposal.

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 ofevidence approach used in the Criteria Document and believes that this body of scientific evidence across all types of studies is veryrobust, recognizing that itincludes a large number ofvarious types of studies that provide consistent and coherent evidence ofan array of0 3-related respiratory morbidity effects and possibly cardiovascular-related morbidity as well as total nonaccidental and cardiorespiratory mortality. More specifically, the Administratorjudges thatthe body ofepidemiological evidence indicating associations with a wide range ofserious health effects, including respiratory emergency department visits and hospital admissions and premature mortality, at and below 0.080 ppm suppprts revising the current standard to-protect public health. While the great majority of evidence concerning effects below 0.080
ppm was fromepidemiological studies, the epidemiological studies do not identify any bright-line threshold level for effects. Atthe sametime, the epidemiological studies are nbt themselves direct evidence of a causal link between exposure to 03 and the

occurrence ofthe effects. Therefore, Administrator has considered these studies inthe context of allthe other available evidence in evaluating the degree of certainty that 0 3-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 03. The Administrator recognizes that in the body of epidemiological evidence, many studies reported positive and statistically significant associations, while others reported positive results thatwere not statistically significant, and a few did
not report any positive 0 3-related associations. In addition, the Administratorjudged that evidence ofa causal relationship between adverse health outcomes and 03 exposures' became increasingly uncertain atlower levels ofexposure. Based onthis the Administrator continues tobelieve 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 03 concentrations thatwere below the level ofthe current standard. Further, there were a few studies that have examined subsets ofdata that include only days with ambient 03 concentrations below the level ofthe
current standard, orbelow even much lower 03 concentrations, and continued
to report statistically significant associations with respiratory morbidity
outcomes and mortality. In the context ofthe strong clinical evidence of adverse effectin healthy adults at0.080, the Administrator finds thatthe body of epidemiological evidence does not 

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support retaining a standard of 0.080, as suggested by commenters.

Both groups ofcommenters also considered evidence fromcontrolled human exposure and epidemiological studies ofincreased susceptibility in people with lung disease, especially people with asthma,but theyreached sharply divergent conclusions about what standard level is supported bythis 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 03 exposure than people who do nothave 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 andAfrican-Americans, and that effects inthese groups should be part ofthe consideration in providing

an adequate margin ofsafety. These
commenters concluded thatthe appropriate level for theprimary 03 standard is 0.060 ppm, toprovide protection for members ofsensitive groups, especially people with asthma, who are likely to have more serious
responses andto respond at lower levels thathealthy people. They also contended that a standard set atthis level also would provide protection against anticipated, but asyet 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 O3leVels, than healthy individuals. Moreover, he
agrees thatthis evidence supports a standard set at a level below 0.080 ppm 03,based onthe strong evidence from human clinical studies in healthy adults
at this level. However, for the reasons described above, he does notagree that the controlled human exposure and epidemiological evidence provide support for astandard set at 0.060 ppm, for thereasons 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 ofthis 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 . -1 would likely experience more serious
responses andresponses at lower levels than the subjects ofcontrolled human
exposure experiments, but that hypothesis is not supported by scientific evidence; and, (3) EPA recognized asthmatics as a sensitive subpopulation in 1997, andnew 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 inthe Response to Comments document.

After careful consideration ofthese comments, the Administrator continues
to judge that there is important new evidence demonstrating that exposures to 03 at levels below the level ofthe
current standard are associated with a broad array ofadverse health effects, especially in at-risk populations that include people with asthina or other lung diseases who are likely to experience more serious effects from
exposure to 03, as well as children and older adults with increased susceptibility, and those who are likely
to be vulnerable as a resultof 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 0 3-induced lung function effects andrespiratory symptoms in some healthy individuals down to thepreviously observed
exposure level of 0.080 ppm, aswell as very limited new evidence at exposure levels wellbelow the level ofthe
current standard. In addition, there are many epidemiological studies done in

areas thatlikely would not have metthe
current standard but which nonetheless report statistically significant associations that generally extend down
to ambient 03concentrations that were below the level ofthe current standard. Further, there were a few studies that have examined subsets of datathat include only days with ambient03 concentrations below the level ofthe
current standard, orbelow evenmuch lower 03 concentrations, and continued
to report statistically significant associations with respiratory morbidity
outcomes and mortality.The Administrator recognizes that inthe body ofepidemiological evidence, many studies reported positive and statistically significant-associations, while others reported positive results that were not statistically significant, and a few did notreport any positive 0 3-relatedassociations. In addition,the Administratorjudged that evidence ofa causal relationship between adverse health outcomes and 03 exposures became increasingly uncertain at lower levels ofexposure. This body of evidence provides a strongbasis forthe Administrator's judgment that the standard needs tobe revised to provide

more protection, and that a revised standard mustbe setat a level appreciably below 0.080 ppm, the level
atwhich there is considerable evidence ofeffects inhealthy people. At the same time, for the reasons discussed above the Administratorjudges thatthis body ofevidence does not support setting a standard as low as 0.060, as suggestedby other commenters.

ii.Exposure and Risk Considerations

With regard to considering how the quantitative exposure andhealth risk
assessments should factor into.a decision on the standard level,EPA
notes that both groups ofcommenters generally consider these assessments in their comments onthe standard level, but they reach sharply divergent conclusions asto what standard level is supported bythese assessments. The general views ofboth groups on the implications ofthe exposure andrisk
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 torevise the
current 8-hour standard.

A joint set ofcomments from ALA and several environmental groups expressed the view that EPA cannot use exposures ofconcern tojustify a standard in the range of0.070 to 0.075 ppm.These commenters contended that standards inthe 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 etal., p, 106). 'Thesesame commenters asserted thatif EPA were to consider exposures of
concern, then thebenchmark level must be defined as 0.060 ppm based on the considerable evidence of adverse health effects occurring at this level.As disoussed 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 riskgroups 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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In contrast, industry association and business group commenters expressed the view that theconcept of exposures ofconcern should notbe considered as

a basis forrevising the level ofthe standard because it provided no indication ofthe 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 byEPA including, for example, 03
measurements at fixed-site monitors can be higherthan other locations where individuals are exposed, the exposure estimates do not account for 03 avoidance behaviors, and the exposure model overestimates elevated breathing
rates. Finally, these commenters also contended that the estimates of
exposures ofconcern associated with just meeting the current standard, using the 0.080 ppni 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 ofcommenters related
to concerns that the exposute estimates

are either underestimated or overestimated in section II.B.2.b above and inmore detail in the Response to Comments document. EPA alsohas addressed the issues raised by both
groups ofcommenters concerning the appropriateness ofconsidering exposures atand above various benchmark levels as an element in the decision on the adequacy ofthe current standard in section ll.B.2.b.

As discussed in section H.B.2b, the Administrator believes that it is appropriate to consider such exposure estimate-in the context ofa continuum rather than focusing on any one discrete benchmark level, as was done at the time ofproposal, 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 arange ofbenchmark 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 ofthe potential forhealth effects ofconcern, and their severity, with increasing uncertainty associated With the likelihood ofsuch effects at lower 03 exposure levels. The Administrator recognizes thatthe 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 healthresponses are likely to be observed in asignificant fraction ofthe at-risk population. With respect to his decision on the level ofthe 8-hour standard, the Administrator notes that
uponjustmeeting a standard within the
range of0.070 to 0.075 ppm based on the 2002 simulation, the number of school age asthmatic children likely to experience exposuresat 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 scbqc,age. children. Based onthe,2P0jiljqWrA- ethe estimates are even lower, withno asthmatic children estimated to experience exposures at and above the 0.080 ppm benchmark level. Similar patterns are observed forall 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 ornearly eliminated depending on the modeling
year uponjustmeeting a standard within therange of0.070 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 pmbenchmark level, given the increased uncertainty about the fraction ofthe population and severity ofthe 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 injudging the adequacy ofthe current standard to protect public health. Consideration of the 0.070 ppm benchmark level recognizes thatthe effects observed at 0.080 ppm were in healthy adult subjects and sensitive population groups, such as asthmatics, are expected
torespond at lower 03 levels than healthy individuals. The Administrator
notes that uponjust meeting a standard withinthe range of0.070 to 0.075 ppm based onthe 2002 simulation, the number of asthmatic school age children likely to experience exposures atand above the 0.070 ppm benchmark level in aggregate (forthe 12 cities in the assessment) is estimated to range from about 2 to 5 percent ofasthmatic school age children. Based onthe 2004 simulation, the estimates are substantially lower, with 0 to 0.6 percent ofasthmatic children estimated
to experience exposures at and above the 0.070 ppm benchmark level upon just meeting astandard within the range of0.070 to 0.075 ppm.

Finally, the Administrator has considered but places very littleweight

onthe benchmark level of0.060 ppm given the very limited scientific evidence supporting a conclusion that 03 is causally relatedto various health
outcomes at this exposure level. Nevertheless, the Administrator observes that there is a similar pattern ofreductions inexposures ofconcern 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. Giventhe degree ofuncertainty associated withthe exposure assessment discussed in the StaffPaper 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 ofexposures associated with airqualityjust 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 of2 percent and 5 percent of asthmatic children forthe exposure benchmark of 0.070 is not'an appreciable difference from apublic health perspective. While directionally there are likely to be fewer exposures at and above this benchmark for a standard of0.070 than a standard of0.075 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, andmost medical organizations, and some State andregional airpollution agencies (e.g., California, NESCAUM) contended that EPA's proposed range of0.070 to 0.075 ppm would result in significant residual 

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public health risks. As articulated most fully in thejoint 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 03 standard of 0.065 ppm, the most stringent standard analyzed byEPA, would significantly decrease 0 3-related lung function decrements, respiratory symptoms, hospital admissions, and mortality and that "EPA must adopt a more stringent

ozone standard of0.060 ppm or belowa level thatincorporates a more adequate margin ofsafety" (ALA etaL.,
p. 108). These same commenters also cited various reasons forasserting that the risk assessment likely underestimates health risks to a substantial degree, including the limited
nature ofthe assessment with respectto number ofcities, populations covered, and health endpoints analyzed. EPA has responded to the comments concerning the scope ofthe risk assessment and assertion that health risks are likely underestimated both in section ll.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 onthe healthrisk assessment in reaching ajudgment about the appropriate level forthe 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 ofthe
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 riskreductions 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 ofinappropriate effect estimates fi-Rm health effect studies, EPA's appro ch to addressing the shape of exposure-response relationships, and whether or notto incorporate thresholds into its models forthe various health effects analyzed. EPA has responded to these comments bothin 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 abovethe 0.080 ppm benchmark level, where 1 - ,- several health effects have been shown
to occur in healthy individuals, are essentially eliminated for standards in therange 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 ofpeople inat-risk
groups, and thatthe 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 onthe exposures using the 0.060
ppm benchmark level given the very limited scientific evidence supporting a conclusion that 03 is causally related to health outcomes at this exposure level. Considering the uncertainties associated with the exposure assessment, the Ad.pinstrat roncludes that the
exposure esa es associated with each ofthe b6UBiML levels are not appret bly different, between a0.070

or 0.075 ppm standard, andtherefore, 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 ofthe 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 03- related health effects considered in the assessment, and by inference across the muchbroader array of0 3-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 ofthe assessment suggest a gradual reduction in risks with no clear breakpoint as increasingly lower standard levels are considered. In light ofthis continuum and the important uncertainties inherent in the assessment discussed above and intheproposal, the Administrator concludes that the risk
assessment does not provide a basisfor choosing a level*within the proposed
range.

c. Conclusions on Level

Having carefully considered the public comments on the appropriate level ofthe 03 standard, as discussed above, the Administrator believes the fundamental scientific conclusions on the effects of03 reached inthe Criteria Document and StaffPaper, briefly summarized above in section II.A.2 and discussed more fullyin section ll.Aof the proposal, remain valid. In considering the level at which the primary 03 standard should be'set, the Administrator continues to pla~e primary consideration onthebody of scientific evidence available in this review on the health effects associated with 03 exposure, as summarized above in section II.C.4.a, while viewing the results ofexposure andrisk assessment, discussed above in section II.C.4.b, as providing information in support ofhis decision. In considering the available scientific evidence he judges that, as at the proposal, a focus on the proposed
range of0.070 to 0.075 ppm is appropriate in light ofthe large body of controlled human exposure and epidemiological and other scientific evidence!As discussed above, this body ofevidence does not supportretaining the current standard, as suggested by

some commqnters. Nor does it support setting aleveljust 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
notbe appreciably below the level in controlled human exposure studies at which adverse effects have been demonstrated (i.e., 0.080 ppm). This body ofevidence also does not support setting alevel of0.060 ppm or below,

as suggested by other commenters. The Administrator has also evaluated the information fromthe exposure assessment andthe 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. Inmaking

a finaljudgment about the level ofthe
03 standard, the Administrator notes thatthe level of 0.075 ppm is above the range recommended by the CASAC (i.e., 0.070 to 0.060 ppm). Placing great weight onthe views of CASAC,the Administrator has carefully considered its statedviews and the scientific basis and policy views for the range it recommended. In so doing, the Administrator notes that he fully agrees thatthe scientific evidence supportsthe conclusion thatthe 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 ofscientific and policy considerations. The Administrator notes thathe is in general agreement with CASAC's views concerning the interpretation ofthe scientific evidence. TheAdministrator also notes thatthere is no bright line clearly directing the choice oflevel, and the choice of what is appropriate is clearly apublic health | I ll I I I hll " T I I I I I 

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policy judgment entrusted to the Administrator. Thisjudgment must include consideration ofthe strengths and limitations ofthe evidence and the appropriate inferences to be drawn from the evidence and the exposure andrisk
assessments. Inreviewing the basis for the CASAC Panel's recommendations for the range ofthe 03 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 ofthe 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 indiqated by its recommendation ofa 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 ofAdams studies

mean that the level ofthe standard has
tobe set at 0.060 ppm. The. Administrator also observes that the CASAC Panel appeared to place greater weight onthe results ofthe risk
assessment as abasis for its recommended range. In referring to the results ofthe risk assessmentresults for lungfunction, respiratory. symptoms, hospital admissions and mortality, the CASAC Panel concluded that: "beneficial effects in terms ofreduction ofadverse 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 ofthe uncertainties associated with the Agency's quantitative human exposure andhealth risk assessments, as discussed above in section H.A.3. Giventhese uncertainties, the Administrator does not agree that these assessment results appropriately

serve as a primary basis forconcluding that levels at orbelow 0.070 ppm are required for the 8-hour 03 standard.

After carefully taking the above
comments and considerations into account, and fully considering the scientific and policy views ofthe CASAC, the Administrator has decided
to revise the level ofthe primary 8-hour 03 standard to 0.075 ppm. In the Administrator's judgment, based on the currently available evidence, a standard
set at this level would berequisite to protect public health with an adequate margin ofsafety, 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 03, 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 of0.075, exposures at and above the benchmark of0.080 ppm are essentially eliminated, and exposures at and above the benchmark of0.070 are substantially reduced or eliminated for the vast-majority of people in at-risk
groups. A standard setat a level lower than 0.075 would onlyresult in significant further public health protection if,in fact,there is a continuum ofhealth risks in areas with 8-hour average 03 concentrations that

are wellbelow the concentrations observed inthe key controlled human exposure studies andif the reported associations observed in epidemiological studies are, in fact, causally related to 03at those lower levels. Based on the available evidence, the Administrator is notprepared 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 thatthe likelihood ofobtaining benefits to public health with astandard set below 0.075 ppm 03 decreases, while the likelihood ofrequiring reductions in ambient concentrations that gobeyond thosethat are needed to protect public health increases. The Administrator judges thatthe appropriate balance to be drawn, based on the entirebody 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 ofsafety, 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 doesnot 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 ofsafety. D.Final Decisionon thePrimary03 Standard

Forthe reasons discussed above, and taking into account information and
assessments presented in the Criteria Document and Staff Paper, the advice and recommendations ofthe CASAC Panel, and the public comments to date, the Administrator has decided to revise the existing 8-hourprimary 03 standard. Specifically, the Administrator is revising (1)the level ofthe primary 03 standard to 0.075 ppm and (2) the degree ofprecision towhich the level of the standard is specified to the thousandth ppT. The revised 8-hour primary standard, with a level of0.075 ppm, would be met at an ambient air monitoring site when the 3-year average ofthe annual fourth-highest daily maximum 8-hour average 03 concentration is less than or equal to 0.075 ppm. Data handling conventions

are specified in the newAppendix P that is adopted, as discussed in section V below.

At this time,EPA is also promulgating revisions to the Air Quality Index for 03 to be consistent with the revisions to the primary 03 standard. These revisions

are discussed below in section III. Issues related to themonitoring requirements forthe revised 03 primary standard are discussed below in section VI. 

## mI. Communication ofPublic Health Information

Information on the public health implications ofambient concentrations ofcriteria 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 ofpollution. The AQI establishes

a nationally uniform system ofindexing pollution levels for 03, 

### CO, NO2, PM and SO 2.The AQI converts pollutant concentrations in a community's air to

anumber on a scale from 0 to 500. Reported AQI values enable thepublic
to know whether airpollution 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 ofthe short-term NAAQS for each pollutant. For the 1997 03NAAQS, an 8-hour
average concentration of 0.084 ppm corresponds to an AQI value of100. An AQI value greater than 100 means that

a pollutant is in one ofthe unhealthy 

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categories (i.e., unhealthy for sensitive
groups, unhealthy, very unhealthy, or hazardous) on agiven day; an AQI value
ator below 100 means -thata pollutant concentration is in one ofthe satisfactory categories (i.e., good or moderate). Decisions about the pollutant concentrations at which to setthe 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 ofrevising 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 onthe 03 NAAQS if revisions to the primary standard were promulgated. These conforming changes would include setting the 100 level ofthe AQI at the

same level as therevised primary 03 NAAQS, and also making proportional adjustments to AQI breakpoints at the * lower end ofthe range (i.e., AQI values of 50, 150 and 200). EPA did not propose to change breakpoints at the higher end ofthe range (from 301 to 500), which would apply to State contingency plans orthe 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 onthe proposed changes to the AQI. Three major issues came up in the comments, including: (1)Whether theAQI should be revised at all, even if the primary standard is revised; (2) whether the AQI should be revised in conjunction with thisrulemaking, orin

a separate rulemaking; and, (3) whether

an AQI value of 100 shouldbe set equal to orlower than the level ofthe shortterm primary 03 standard, and the other breakpoints adjusted accordingly. UARG asserted that EPA should not revise the AQI at all, even ifEPA does revise the primary 03 standard. In support ofthis view, UARGnoted that there is no requirement forEPA to set

an AQI value of 100 equal to the level ofthe short-term standard, and cited the 1999 decision to set an AQI value of 100 forPM2. 5 equal to 40 gig/m 3

,when the level ofthe short-term standard was then 65 gg/m 3

. 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 ajoint set of
comments did not support revising the AQI in conjunction with this rulemaking. ALA et a1. expressed the view that since EPA did not propose specific breakpoints in its proposed revisions tothe 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 atthe same time that the standard is revised. NACAA expressed the view that: "The effectiveness oftheAQI as apublic health tool willbe undermined ifEPA undertakes regulatory changes to the

ozone NAAQS without simultaneously revising theAQI." (NACAA,p. 5) The Wisconsin Department ofNatural Resources (WI DNR) further noted that:

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

wasrevised, EPA missed an opportunity to adopt-conforming changes to theAQI. The Administrator signed the Federal Register noticepromulgating arevised fine-particle standard in September2006, butEPA still hqsnot changed theAQI to reflect therevised standard. We recommend that the AQI be amendedto beconsistent with therevised

ozone andPM2. 5 standards." [WIDNR, p. 3] Finally, ALA etal.and NESCAUM expressed the view that an AQI value of 100 should be setat 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 ofthe standard, the recommended range of.CASAC, and also cited the 1999 decision to set an AQI value of 100 forPM2. 5 equal to 40 gg/

m3 when the level ofthe short-term standard was 65 ig/m3, as support for this view. Most other State commenters supported setting an AQI value of100 equal -tothe level ofthe primary 03 standard.

Recognizing the importance ofthe AQIas a communication tool that allows the public totake exposure reduction measures when air quality
may pose health risks, EPA agrees with State agencies and organizations that favored revising the AQI atthe same time asthe primary standard. EPA agrees with State agency commenters thatits historical approach ofsetting an AQI value of 100 equal to the level of the revised primary standard is appropriate, both from apublic health and acommunication perspective.

Both UARG and ALA et

### al. cite the 1999 AQI rulemaking, which set an AQI value of100 for PM2. 5 equal to 40jig/

m

3

,a lower level than the level ofthe short-term PM2. 5 standard, as support for their view that an AQI value of 100 does not need to be setat the level of the revised 03 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 setthe AQI value of 100 at the level ofthe short-term standard (e.g., 24 hours) for apollutant. This method of structuring the index is appropriate in the case where a shortterm standard is setto protect against the health effects associated with shortterm exposures and/or an annual standard is set to protect againsthealth effects associated with long-term
exposures. In such cases, the short-term standard in effect defines alevel 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 ofthe 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 bythe 24-hour 9tandard (set

at a level of 65 gg/m 3 )but by the combination ofthe 24-hour and the annual standards working in concert. In fact, the annual standard (set 'ta level
of15 gg/m 3 ) was intended to serve as the principal vehicle for protecting against both long-term and short-term
PM 2 .

5 exposures by lowering the entire day-by-day distribution ofPM2. 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 bya national annual standard, EPA consequently concluded that it would be appropriate to caution members of sensitive groups exposed to concentrations below the level ofthe 24- hour standard. EPA also concluded that it would be inappropriate to compare daily airquality concentrations directly with the level ofthe annual standard by setting an AQI value of 100 atthat level. EPA wanted to set the AQI value of 100
toreflect the general level ofhealth protection against short-term risks offered by the annual and 24-hour standards combined, consistent with the underlying logic ofthe historical approach to establishing AQI 100 levels. Therefore EPA set the AQI value of 100 

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at the midpoint oftherange between the annual and the 24-hour PM2. 5 standards (i.e., 40 gg/m3) in order toreflect the .combined role ofthe 24-hour andthe annual PM2.5 standards in protecting against short-terjn 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 oftheprimary 03 standard.

Therefore, EPA is revising the AQI for 03 by setting an AQI value of100 equal
to 0.075 ppm, 8-hour average, the level ofthe revised primary 03 standard. EPA is alsorevising the following breakpoints: AnAQI valde of50 is set
at 0.059 ppm, an AQI value of 150 is set at 0.095 ppm, and an AQI value of200 is set at 0.115 ppm.All these levels are averaged over 8 hours. As indicated in theproposal, these levels were developed bymaking 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 atleast a 0.015 ppm range to allow for more accurate forecasting. So, for example, simply making a proportional adjustment to the level ofan AQI value of 150 (0.104 ppm) would result in a level ofabout 0.092
ppm. Since most of these ranges are rounded to the nearest 5 thousandths of

a ppm, that rounding would have

- resulted in a0.014 ppm range (i.e., 0.076 to 0.090 ppm). So, the number

was rounded upward to the nearest 5 thousandths ofa 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 thatthe finalized breakpoints provide abalance between proportional adjustments to reflect therevised 03 standard and providing category ranges that are large enough to be forecasted accurately, so thatthe new AQI for 03 can be implemented more easily in the public forum for which the AQI ultimately exists.

IV. Rationale for Final Decision on Secondary 03 Standard

A. Introduction

1. Overview

This section presents the rationale for the Administrator's final decisions regarding the need to revise the current secondary 03 NAAQS, and the appropriate revisions to the standard. As discussed more fully below, the rationale forthe final decisions on appropriate revisions to the secondary 03NAAQS is based ona thorough review ofthe latest scientific information on vegetation effects associated with exposure to ambient levels of03, as assessed in the Criteria Document. This rationale also takes into
account: (1) Staffassessments ofthe
most policy-relevant information in the Criteria Document regarding the evidence ofadverse effects of03 to vegetation and ecosystems, information ohbiologically-relevant exposure metrics, and staffanalyses ofair quality, vegetation exposure and risks, presented in the StaffPaper 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 03 standard were based; (2) CASAC Panel advice and recommendations as reflected in discussion ofdrafts ofthe Criteria Document and StaffPaper at public meetings, in separate written comments, and inCASAC's letters tothe Administrator (Henderson,2006a, b, c; 2007); (3) public comments received during development ofthese documents either in conjunction with CASAC meetings or separately and on the proposal notice; (4) consideration ofthe degree ofprotection to vegetation potentially afforded bythe revised 8- hour primary standard; and (5) the limits ofthe available evidence.

In developing this rationale, EPA has again focused on direct 03 effects on vegetation, specifically drawing upon an integrative synthesis ofthe entire body ofevidence, published through early 2006, on the broad array ofvegetation effects associated with exposure to ambient levels of03 (EPA, 2006a, chapter 9). In addition, because 03 can also indirectly affect otherecosystem components such as soils,water, and wildlife, and their associated ecosystem goods and services, through its effects

on vegetation, a qualitative discussion ofthese other indirect impacts is also included, though these effects are not quantifiable atthis time. As was concluded in the 1997 review, and based onthe body of scientific-literature assessed in the current Criteria Document, the Administrator believes thatit is reasonable to conclude that a secondary standard protecting the public welfare fromknown 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 *othercountries around the world. In its
assessment ofthe evidencejudged to be most relevant to making decisions on the level ofthe 03 secondary standard, however, EPA has placed greater weight

on U.S. studies, due to the often species-, site-and climate-specific
nature of0 3-related vegetation response.

As with virtually any policy-relevant vegetation effects research, there is uncertainty inthe characterization of vegetation effects attributable to exposure to ambient 03. 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 tothe more traditional controlled open top chamber (OTC) studies. Moreover, the newly available studies evaluated inthe 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 ofthe vegetation effects information has been extensive and deliberate. Inthe judgment ofthe. Administrator, the intensive evaluation ofthe scientific evidence thathas occurred in thisreview has provided an adequate basis for regulatory decisionmaking at this time. Thisreview also provides important input to EPA's research plan for improving ourfuture understanding ofthe effects ofambient 03 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 ofthe proposal (72 FR at 37883-37895), respectively, and are only briefly outlined below insections IV.A.2 through IV.A.4. Subsequent sections of this preamble provide amore complete discussion ofthe Administrator's rationale, in light ofkeyissues raised in public comments, for concluding that the current standard is not requisite to protect public welfare from known or anticipated adverse effects, and itis appropriate to revise the current secondary 03 standard to provide additional public welfare protection (section IV.B) bymaking the secondary standard identical tothe revised primary standard (sectionIV.C). A
summary ofthe final decisions on revisions to the secondary 03 standard is presented in section IV.D. 

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2. Overview ofVegetation Effects Evidence

This section outlines the information presented in section IV.A ofthe proposal on known or potential effects

on public welfare which may be expected from thepresence of 03 in ambient air. Exposures to 03 havebeen 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 ofthe quantitative information on vegetation effects available at thattime, 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 usingthe OTC exposure method, found cumulative, seasonal 03 exposures were most strongly associated with observed vegetation response. The Criteria Document prepared for this review discussed a number ofadditional studies that support and strengthen key conclusions regarding 03 effects on vegetation and ecosystems found inthe previous Criteria Document (EPA, 1996a, 2006a), including further clarification ofthe underlying mechanistic and physiological processes atthe subcellular, cellular, andwhole system levels within the plant. More importantly, however, in the context of this review, new quantitative information is now available across a broader array ofvegetation 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 ofexposure methods, including chamber, free air, gradient, model, and field-based observation. These nonchambered, field-based study results begin to address one ofthe key data gaps citedby the Administrator in the last review.

Section IV.A ofthe proposal provides

a detailed summary ofkey information contained in the Criteria Document (EPA, 2006, chapter 9) and in the Staff Paper (EPA, 2007, chapter 7) onknown

or potential effects on public welfare which may be expected from the
presence of03 in ambient air (72 FR 37883-37890). The information inthat section summarized:

(1) New information available on potential mechanisms for vegetation effects associated with exposure to 03, including information onplant uptake of03, cellularto systemic responses, compensation and detoxification
responses, changes to plant metabolism, andplant responses to chronic 03 exposures;

(2) The nature ofeffects onvegetation that have been associated with exposure to 03 including effects related to carbohydrate production and allocation, growth effects on trees andyield reductions in crops, visible foliar injury, andreduced plant vigor, as well as consequent potential impacts on ecosystems including potential alteration ofecosystem structure and function and effects on ecosystem services and carbon sequestration; and

(3) Considerations in characterizing what constitutes an adverse welfare impact of03, including an approach that expands the consideration of adversity beyond the species level by making explicit the linkages between stress-related effects such as 03

ex~osure at the species level and at higher levels within an ecosystem hierarchy.

3. Overview ofBiologically Relevant Exposure Indices

This section outlines the information presented in section IV.B ofthe proposal on biologically relevant
exposure indices thatrelate known or potential effects on vegetation to exposure to 03 in ambient air. The Criteria Document concluded that 03
exposure indices thatcumulate differentially weighted hourly concentrations are thebest candidates forrelating exposure toplant growth
responses (EPA, 2006a). This conclusion followed fromthe extensive evaluation ofthe relevant studies in the 1996 Criteria Document (EPA, 1996a) andthe
recent evaluation of studies that have been published since that time (EPA, 2006a). The depth and strength of these conclusions are illustrated bythe following observations that are drawn from the 1996 Criteria Document (EPA, 1996a, section 5.5):

(1) Specifically, with respect to the importance oftaking into account exposure duration, "when 03 effects are the primary cause ofvariation in plant
response, plants from replicate studies ofvarying duration showed greater reductions in yield orgrowth when exposed for the longer duration" and "the mean exposure index of unspecified duration could not account for the year-to-year variation in respoAse" (EPA, 1996a, pg. 5-96).

(2) "[Blecause the mean exposure index treats allconcentrations equally and does not specifically include an exposure duration component, theuse ofa mean exposure index for characterizing plant exposures appears inappropriate forrelating exposure with vegetation effects" (EPA, 1996a, pg. 5- 88).,

(3) Regarding the relative importance ofhigher concentrations than lower in determining plantresponse, "the ultimate impact oflong-term exposures to 03 on crops and seedling biomass
response depends onthe integration of repeated peak concentrations during the growth ofthe plant" (EPA, 1996a, pg. 5- 104).

(4) "[Alt this time, exposure indices that weight the hourly 03 concentrations differentially appear to bethe best candidates forrelating
exposure with predicted plant response" (EPA, 1996a, pgs. 5-136).

Atthe conclusion ofthe last review, the biological basis fora cumulative, seasonal form was not in dispute. There

was general agreement between the EPA staff,CASAC, and the Administrator, based on their review ofthe air quality criteria, that a cumulative, seasonal form was more biologically relevant than the previous 1-hour andnew 8- hour average forms (61 FR 65716).

The StaffPaper prepared for this review evaluated the most appropriate choice ofa cumulative, seasonal form for a secondary standard to protect the public welfare fromknown and anticipated adverse vegetation effects in light ofthe new information available in this review. Specifically, the StaffPaper considered: (1)The continued lack of evidence within the vegetation effects literature of a biological threshold for vegetation exposures ofconcern and (2)

new estimates ofPRB that are lower than inthe last review. The form commonly called W126 was evaluated inthe last review and was compared with the form called SUM06, which incorporates a threshold level above which exposures are summed, thatwas proposed in the lastreview. The concentration-weighted formcommonly called W126 is defined as the sum of sigmoidally weighted hourly 03 concentrations over a specified period, where the daily sigmoidal weighting function is defined inthe StaffPaper (EPA, 2007a, p. 7-16.) as: 

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W126

i=8 AM• Ci,.,'wfre Ci = hourly 03 athour i, and wc, 1+ 4403e- 21C

.

Regarding the firstconsideration, the StaffPaper noted that theW126 form, by its incorporation ofa continuous sigmoidal weighting scheme, does not create an artificially imposed concentration threshold, yet also gives proportionally more weight tothe higher and typically more biologically potent concentrations, as supportedby the scientific evidence. Second, the index value is not significantly influenced by 03 concentrations within therange ofestimated PRB, as the weights assigned to concentrations in this range are very small. Thus, the Staff Paper concluded that itwould provide

a more appropriate target for air quality management programs designed to reduce emissions from anthropogenic

sources contributing to 03 formation. Onthe basis ofthese considerations, the StaffPaper andthe CASAC Panel concluded thatthe W126 form is the
most biologically-relevant cumulative, seasonal form appropriate to consider in the context ofthe secondary standard review.

4. Overview ofVegetation Exposure and Risk Assessments

This section outlines the information presented in section IV.C ofthe 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 airquality modeling. The vegetation risk assessment included both tree andcrop analyses. The tree risk analysis included three distinct lines ofevidence: (1) Observations of visible foliar injury in the field linked
to recent monitored 03 air quality for the years 2001-2004; (2) estimates of seedling growth loss under current and alternative 03 exposure conditions; and (3) simulated mature tree growth reductions using the TREGRO model to simulate the effect ofmeeting alternative airquality standards onthe predicted annual growth ofa single
western species (ponderosa pine) and
two eastern species (redmaple and tulip poplar). The crop analysis includes estimates ofthe risks to crop yields from current and alternative 03 exposure conditions andthe associated change in economic benefits expected to accrue in the agriculture sector upon meeting the levels ofvarious alternative standards. Each element ofthe assessment is outlined below, together withkey observations from this assessment.

a. Exposure Characterization

The exposure analyses examined 03 air quality patterns in the U.S. relative
to the location of 03 sensitive species that have aknown concentrationresponse in order topredict whether adverse effects are occurring at current levels ofair quality, and whether they

are likely to occur under alternative standard forms andlevels. The most important information about exposure to vegetation comes from the 03 monitoring data that are available from
two national networks: (1)Air Quiality System (AQSI http://www.epa.gov/ttnairs/airsaqs) and (2) Cletq Statt4jfii and Trends Network- (,', I

- ¢($ t Irl, .:-
htt

:/www.epa.gov/cstnqt) 

## ~~

to characterize exposures tovegtation
at the national scale, however, the Staff Paper concluded that itcould not rely solely on limited site-specific monitoring data, and thatit was - necessary to use an interpolation method to characterize 03 air qualityover broad geographic areas. The analyses used the 03 outputs from the EPA/NOAA Community Multi-scale Air Quality (CMAQ) 2 2 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 StaffPaper concluded that it was appropriate to use separate interpolation techniques in these two regions: AQS and CASTNET monitoring datawere solelyused forthe
eastern interpolation, and inthe western U.S., where rural monitoring is more sparse, 03 values generated bythe CMAQ model Were used to develop scaling factors to augment the interpolation. In order to characterize

22TheCMAQ model isamulti-pollutant,
multiscale airquality model that contains state-ofthe-science techniques for simulatingall 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 forhandling many major pollutants (including photochemical oxidants/0 3 , particulate matter, and nutrient deposition) holistically. The CMAQ model can generate estimates ofhourly 03 concentrations for the contiguous U.S., making it possible to express model outputs in terms ofa variety of exposure indices (e.g., W126, 8-hour average). uncertainty in the interpolation method, monitored 03 concentrations were systematically compared to interpolated 03 concentrations in areas where monitors were located. In general, the interpolation method used inthe
current review performed well in many

areas in theU.S., although it underpredicted higher 12-hour W126 exposures inrural areas. Due to the important influence ofhigher exposures in determining risks to plants, this feature ofthe interpolated surface could result in an under-estimation ofrisks to vegetation in some areas. Taking these uncertainties into account, and given the absence ofmore complete rural monitoring data, this approach was used in developing national vegetation
exposure andrisk 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 StaffPaper utilized adjusted 2001 base year 03 air quality distributions with a rollback method (Horstand 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 ofthe exposure assessment indicate thatcurrent air quality levels could result in significant impacts to vegetation in some areas. For example, for the base year (2001), a large portion ofCalifornia had 12-hrW126 03 levels above 31 ppm-hour, whichhas been associated with approximately up to 14 percent biomass loss in 50 percent oftree 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 ofair quality above theW126 level of 21 ppm-hour, which is approximately equalto the secondary standard proposed in 1996 and is associated with approximately up to 10 percent biomass loss in 50 percent oftree seedling cases studied.Much of the east and Arizona and California have 12-hour W126 03 levels above 13 ppm-hour which has been associated with approximately up to 10 percent biomass loss in 75 percent oftree seedling cases studied.

(2) When 2001 air quality is rolled backto meet the current 8-hour 

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### Federal Register/ Vol. 73, No. 60/Thursday, March 27, 2008/Rules and Regulationssecondary standard, the overall 3-month 12-hour W126 03 levels were somewhat improved, but notsubstantially. Under this scenario, there were still many

areas in California with 12-hour W126 03 levels above 31 ppm-hour. Abroad multi-state region inthe east.(NC, TN, KY, IN, OH, PA, MD) and west (CA, NV,

- AZ, OK, TX) were still predicted to have 03 levels above the W126 level of21 ppm-hour.

(3) Exposures generated forjust meeting a 0.070 ppm, 4th-highest maximum 8-hour average alternative standard (thelower end ofthe proposed
range for the primary 03 standard) showed substantially improved 03 air quality when compared to just meeting the current 0.08 ppm, 8-hour standard. Most areas were predicted to have 03 levels below the W126 level of 21 ppmhr, although some areas in the east (KY, TN, MI, AR, MO, IL) andwest (CA,NV, AZ,UT,NM, CO, OK, TX) were still predicted to have 03 levels above the W126 level of 13 ppm-hour. (4) While these results suggestthat meeting a proposed 0.070 ppm, 8-hour secondary standard would provide substantially improved protection in

some areas, the StaffPaper 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 highelevation forests inthe Western U.S. where there are few 03 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 03 airquality interms ofcurrent and alternative secondary standard forms, an analysis

was performed in the StaffPaper to evaluate the e-ktent towhich countylevel 03 air quality measured in terms ofvarious levels ofthe current 8-hour average form overlapped with that measured interms ofvarious levels of the 12-hour W126 cumulative, seasonal form. 23 This analysis was limited by the lack ofmonitoring inrural areas where important vegetation and ecosystems are lbcated, especially at higher elevation sites. This isbecause 03 air quality distributions at high elevation sites often do not reflect thetypical urban and near-urban pattern oflowmorning and evening 03 concentrations with a

23The Staff Paperpresented this analysis using recent-(2002-2004) county-level 03 air quality data (using3-year average data as wellas datafrom each individual year) from AQS sites and the subset of CASTNET sites having the highest 03 levels for the counties in which they are located. high mid-day peak, but instead maiatain relatively flat patterns with many , i concentrations inthe 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 StaffPaper concluded that it isreasonable to anticipate that additional unmonitored rural high elevation areas important for vegetation may notbe adequately protected even with a lower level ofthe 8-hour form.

The StaffPaper indicated that it furtherremains uncertain asto the
extent to which air quality improvements designed toreduce 8- hour 03 average concentrations would reduce 03 exposures measured by a seasonal, cumulative W126 index. The Staff Paper indicated this tobe an important consideration because: (1) Thebiological database stresses the importance ofcumulative, seasonal
exposures in determining plant response; (2) plants have not been specifically tested for the importance of daily maximum 8-hour 03 concentrations inrelation to plant response; and (3) the effects of attainment of a 8-hour standard in upwind urban areas onrural 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 ofRisktoVegetation

The StaffPaper presented results from quantitative andqualitative risk
assessments of03 risks to vegetation. In the last review, crop yield and seedling biomass loss OTC data provided the basis forstaff analyses, conclusions, and recommendations (EPA, 1996b). Since then, several additional lines of evidence have progressed sufficiently to provide abasis for amore complete and coherent picture ofthe scope ofQ3- 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 onfieldbased exposure methods (e.g., free air exposure and ambient gradient), improved field survey biomonitoring techniques, andmechanistic tree process models. Key observations and insights fromthe vegetation risk assessment, together with important caveats and limitations, were dscussed in section IV.C oftheproposal. , Highlights fromthe 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 ofphytotoxic 03 exposures on sensitive bioindicator plants. These surveys produced more expansive evidence than that available
at thetime ofthe last review that visible foliar injury is occurring in many areas ofthe U.S. under current ambient conditions. The StaffPaper 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 theFIA biomonitoring sites. This assessment showed thatincidence ofvisible foliar injury ranged from 21 to 39 percent of the counties during the four-yearperiod (2001-2004) across all counties with air quality levels at or below that ofthe
current 0.08ppm 8-hour standard. Of the counties that met an 8-hour level of 0.07 ppm inthose years, 11 to 30 percent ofthe counties still had incidence ofvisible foliar injury. The magnitude ofthese percentages suggests that phytotoxic exposures sufficient to induce visible foliar injury would still

occur in many areas aftermeeting the level ofthe current secondary standard

or alternative 0.07 ppm 8-hour standard. While the data show thatvisible foliar injury occurrence is geographically widespread and is occurring on a variety ofplant species in forested and other natural systems, linking visible foliar injury to other plant effects is still problematic. However, its presence indicates that other 0 3-related vegetation effects might alsobe present.

(2) Seedling andmature treebiomass loss. Inthe last review, analyses ofthe effects of03 ontrees 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 apen, ponderosa pine, black cherry, and tulip poplar were foundto be sensitive to cumulative seasonal 03exposures. Work done since the last review at the AspenFACE site in Wisconsin on quaking aspen (Karnosky etal., 2005) and a gradient study performed in the New York City area (Gregg etal., 2003) have confirmed the detrimental effects of 03exposure on tree growth in field studies without chambers andbeyond the seedling stage (King etal., 2005). To update the seedling biomass loss analysis, C-R functions for biomass loss 

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for availgble seedling tree species taken fromthe CsiteriaDocument and informatioiY ontree growing regions derived from the U.S. Department of Agriculture's Atlas ofUnited States Trees were combined with projections ofair quality based on 2001 interpolated
exposures, to produce estimated biomass loss for each ofthe seedling
tree species individually.24 In summary, these analyses showed that biomass loss still occurred inmany tree species when 03 air quality was adjusted tomeet the
current 8-hour standard. Forinstance, black cherry, ponderosa pine, eastern white pine, and aspen had estimated median seedling biomass losses over portions oftheir growing range ashigh

as 24, 11, 6, and 6 percent, respectively, when 03 air quality was rolled backto just meet the current 8-hour standard. The StaffPapernoted that these results

are fortreo seedlings and that mature trees ofthe same species may have more or less of a response to 03 exposure. Due to the potential for compounding effects

over multiple years, a consensus workshop on03 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 03 effects on mature tree growth, a tree growth model (TREGRO) was usedto evaluate the effect ofchanging 03 air quality scenarios fromjust meeting alternative 03 standards onthe growth ofmature trees. 25 The model integrates interactions between 03 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 usedto assess 03- related impacts onthe growth of Ponderosa pine in the San Bernardino Mountains ofCalifornia (Crestline) and the growth ofyellow poplar andred maple in the Appalachian mountains of Virginia and North Carolina, Shenandoah National Park (Big

2 4 Maps ofthese biomass loss projections were presented in the StaffPaper (chapter 7).

Z TREGRO is a process-based, individual.tree growth simulation model (Weinstein eta). 1991) and has been used to evaluate the effects of a variety of 03 scenarios and linked with concurrent climate data to account for03 and climate/ meteorology interactions on several species of trees
in different regions ofthe U.S. (Tingey etal., 20.01; Weinstein etal., 1991; Retzlaffetal., 2000;
Laurence etal., 1993; Laurence et al., 2001; Weinstein at al., 2005). Madows) and Linville Gorge Wilderness Area (Cranberry), respectively. Ponderosa pine is one of the most widely distributed pines in
western North America, a major source oftimber, important aswildlife habitat, and valued foraesthetics (Burns and Honkala, 1990). Red maple is one ofthe
most abundant species inthe eastern U.S. and is important for itsbrilliant 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 ofthe cove hardwood stands in southern Appalachians which are widely viewed as some ofthe country's
mosttreasured forests because the protected, rich, moist set ofconditions permit trees to grow the largest in the
eastern U.S. The wood has high commercial value because ofits versatility and as a substitute for increasingly scarce softwoods in furniture and framing construction. 41- Yellow poplar is also val~ad,ai[l hb 1r tree, a source ofwildlife fod, tidJh;'" shade tree for large areas (Burns and Honkala, 1990).

The StaffPaper analyses found that just meeting the current standard would likely continue to allow 0 3-related reductions in annual netbiomass gain in these species. This is based on model outputs that estimate that as 03 levels

are reduced below those ofthe 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 03 (King et al., 2005);

some mature trees such as red oak have shown an even greater sensitivity of photosynthesis to 03 than seedlings of the same species (Hanson etal., 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 timemay affect long-term survival and reproduction of individuals and ultimately the abundance ofsensitive tree species in forest stands.

(3) Crops: Similar to the tree seedling analysis, an analysis thatcombined 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 of2004 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 thatjust meeting the current 8-hour standard would still allow 0 3-related yield loss to occur in some commodity crop species and fruit andvegetable species currently
grown inthe U.S. For example, based on median C-R function response, in counties with the highest 03 levels, potatoes and cotton had estimated yield losses of 9-15 percent and 5-10 percent, respectively, when 03 air qualityjust
met the level ofthe 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 underjust meeting the current standard (0-4 percent).

The StaffPaper also presented estimates ofmonetized 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 StaffPaper recognized that the modeled economic benefits fromAGSIM had

-many associated uncertainties which limited theusefulness ofthese estimates.

B.Need forRevision ofthe Current Secondary03 Standard

1.Introduction

The initial issue to be addressed in this review ofthe 03 standard is whether, inview ofthe advances in scientific knowledge reflected in the Criteria Document and StaffPaper,the
current standard should be revised. As discussed in section IV.D ofthe proposal, in evaluating whether it was appropriate to piropose to retain or revise the current standard, the Administrator built upon the lastreview and reflected the broader body of evidence and information now available. Inthe proposal, EPA presented information, judgments, and conclusions fromthe lastreview, which revised the secondary 03 standard by 

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setting itidentical to the revised primary 03 standard, and from the
current review's evaluation ofthe adequacy ofthe current secondary standard, including both evidence- and exposure/risk-based considerations in the StaffPaper, as well as from the CASAC Panel's advice and recommendations. The StaffPaper evaluation, the CASAC Panel's views, and the Administrator's proposed conclusions onthe adequacy ofthe
current secondary standard are presented below.

-

a. StaffPaper Evaluation

The StaffPaper considered the evidence presented inthe Criteria Document as a basis for evaluating the adequacy ofthe current 03 standard, recognizing thatimportant uncertainties remain. The StaffPaper 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 Documentonce more concluded that: (1)A plant's response to 03 depends upon the cumulative nature of ambient exposure as well as the temporal dynamics ofthose concentrations; (2) current ambient concentrations in many areas ofthe country are sufficient to impairgrowth ofnumerous common and economically valuable plant and tree species; (3) the
entrance of03 intothe leafthrough the
stomata is the critical step in 03 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 03 onplants and trees; and (6) a high degree ofuncertainty remains in our ability to assess the impact of03 onecosystem services.

In light ofthe new evidence, as described inthe Criteria Document, the StaffPaper evaluated the adequacy of the current standard based on assessments ofboth the mostpolicyrelevant vegetation effects evidence and exposure and risk-based information, highlighted above in section IV.A and discussed in sections 1V.A-C ofthe proposal. In evaluating the strength of this information, the StaffPaper took into account the uncertainties and limitations in the scientific evidence and analyses as well as the views of CASAC. The StaffPaper concluded that progress has beenmade since the last review and generally found support in the available effects- and exposure/riskbased information for consideration of

an 03 standard that is more protective than the current standard. The Staff Paper further concluded that tiere is no support for consideration of an 03 ' : " ' standard thatis less protective thanthe
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 thebasis 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 ofthe scope of0 3-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 ekpEsumrxndsto& (e.g., free air, ambient gradieiiamiihbiomonitoring surveys). In reaching conclusions regarding the adequacy ofthe current standard, the Staff Paper considered the combined information from allthese areas together, along with associated uncertainties, in an integrated, weightof-evidence approach.

Regarding the 0 3-induced effect of visible foliar injury, observations for the years 2001 to 2004 at USDA FIA biomonitoring sites showed widespread 0 3-induced leaf injury occurring in the field, including inforested ecosystems, under current ambient 03 conditions. For a few studied species, ithas been shown thatthe presenc4 ofvisible foliar injury is further linked to the presence ofother vegetation effects (e.g., reduced plantgrowth and impaired below ground root developrhent) (EPA, 2006), though for most species, this linkage has
notbeen specifically studied orwhere studied,has not been found.. Nevertheless, when visible foliar injury is present, the possibility that other 03- induced vegetation effects could also be present for some species should be considered. Likewise, the absence of visible foliar injury should notbe construed to demonstrate the absence of other 0 3-induced vegetation effects. The Staff Paper concluded that itis not possible at this timeto quantitatively

assess the degree ofvisible foliar injury that should bejudged adverse in all settings and across all species, and that other environmental factors can mitigate

or exacerbate the degree ofO3-induced visible foliar injury expressed at any given concentration of 03. However, the StaffPaper also concluded that the
presence ofvisible foliar injury alone can be adverse to thepublic welfare, especially when itoccurs incpirttected

areas such as national parks and wilderness areas. Thus, on the basis of the available information on the widespread distribution ofO3-sensitive species within the U.S. including in
areas, such as national parks, which are afforded ahigher degree ofprotection, the Staff Paper concluded that the
current standard continues to allow levels of visible foliar injury in some locations thatcould reasonably be considered to be adverse from a public welfare perspective. Additional monitoring ofboth 03 air quality and foliar injury levels are needed inthese

areas ofnational significance to more fully characterize the spatial extent of this public welfare impact.

With respect to 03-induced biomass loss in trees, the StaffPaper concluded that the new body offield-based research ontrees strengthens the conclusions drawn on tree seedling biomass loss from earlier OTC workby documenting similar seedling responses in the field. For example, recent empirical studies conducted on quaking
aspen atthe AspenFACE site in Wisconsin have confirmed the detrimental effects of 03 exposure on tree growth in a field setting without chambers (Isebrands etal., 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 ofhigher 03 concentrations. These gradients from low urban to higher rural 03 concentrations occur when 03 precursors generated in urban areas are transported to downwind sites and are transformed into 03. In addition, 03 concentrations typically fall tonear 0 ppm at night in urban areas due to scavenging of 03by NOx and other compounds. In contrast, rural areas, due
to a lack ofnighttime scavenging, tend
to maintain elevated 03 concentrations for longer periods. Onthe basis ofsuch key studies, the StaffPaper concluded that the expanded body offield-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 03 are sufficient
to produce visible foliar injury symptoms and biomass loss in sensitive vegetative species growing innatural environments. Further, the StaffPaper

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judged that the consistency in response in studied species/genotypes to 03 under a variety ofexposure conditions andmethodologies demonstrates that these sensitive genotypes and populations ofplants are susceptible to adverse impacts from 03 exposures at levels known to occur in the ambient air. Due to the potential for compounded risks fromrepeated insults

over multiple years in perennial species, the StaffPaper concluded that these sensitive subpopulations are not afforded adequate protection underthe
current secondary 03 standard. Despite the fact that only a relatively small portion ofU.S. plant species havebeen studiedwith respectto 03 sensitivity, those species/genotypes shown to have 03 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., theEEAs) andthus, to the goods and services those ecosystems provide (Young and Sanzone, 2002), including

non-use categories such asrelevance to public welfare based on their aesthetic, existence orwildlife habitat value.

The StaffPaper therefore concluded that the current secondary standard is inadequate to protect the public welfare against the occurrence ofadverse levels ofvisible foliar injury arfd 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 ofthe
current standard, the StaffPaper also presented the results ofexposure and risk assessments, which are highlighted above in section IV.A.3 and discussed in section IV.C ofthe proposal. Due to multiple sources ofuncertainty, both known and unknown, that continue to be associated with these analyses, the StaffPaperput less weight on this information in drawing conclusions on the adequacy ofthe current standard. However, the Staff Paperalso recognized that some progress has been made since the last review in better characterizing some ofthese associated uncertainties and, therefore concluded that the results ofthe exposure and risk
assessments continue toprovide 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 fromOTC exposure systems to predict plant response in the field and thepotential 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 alimited number oftree seedling and crop species to date, demonstrate plant growth and visible foliar injury
responses in the field that are similar in
nature and magnitude tothose observed previously under OTC exposure conditions, lending qualitative support
to the conclusion that OTC conditions do not fundamentally alter the nature of the 03-plant response. Second, nothing in the recent literature suggests that the 03 sensitivity ofcrop or tree species studied in the last review andfor which C-R functions were developed has changed significantly in the intervening period. Indeed, in the few recent studies where this is examined, 03 sensitivities

were found to be as great as orgreater than those observed in the lastreview.

The StaffPaper consideration ofsuch
exposure and risk analyses is discussed below and in section IV.D.2.b ofthe 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 03
exposures that meet the level ofthe
current secondary standard. For instance, black cherry, ponderosapine,
eastern white pine, and aspen had estimated median seedling biomass losses as high as 24, 11, 6, and 6 percent, respectively, over some portions oftheirgrowing ranges when air quality was rolled back to meet the
current 8-hr standard with the 10 percent downward adjustment forthe potential 03 gradient between monitor height and short plant canopies applied. The Staff Paper noted thatthese results are for tree seedlings and that mature trees ofthe same species may have more

or less ofa response to 03 exposure. Decreased root growth associated with biomass loss has the potential to indirectly affect th6 vigor and survivability oftree 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 ofestimated
tree seedling growth reduction should be considered significant and potentially adverse, given thatthey 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 StaffPaper recommended that this information should be given careful consideration in lightof several other pieces of evidence. Specifically, limited evidence from experimental studies thatgo beyond the seedling growth stage continues to show decreased growth under elevated 03 levels (King eta1., 2005). Some mature trees such as red oak have shown an

even greater sensitivity of photosynthesis to 03than seedlings of the same species (Hansoneta., 1994). The potential for effects to "carry over"
to the following year or cumulate over multiple years, including thepotential for compounding, must be considered (see 72 FR 37885; Andersen etal., 1997; Hogsett etel., 1989; Sasek etal., 1991; Temple et al., 1993; EPA, 1996). The accumulation of such "carry-over" effects over time may affect long-term survival and reproduction ofindividual
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 otherpublic welfare effects categories. In so doing, the Staff Paper concluded that the quantified risks to vegetation estimated tobe occurring under current air quality or upon meeting the current secondary standard likely represent only a portion of actual risks thatmay be occurring for

a number ofreasons.

First, as mentioned above, out ofthe

over 43,000 plant species catalogued as growing within theU.S. (USDA PLANTS database, USDA, NRCS, 2006), only a small percentage havebeen studied with respect to 03 sensitivity. Most ofthe studied species were selected because oftheir commercial importance orobserved 0 3-induced visible foliar injury in the field. Given that 03 impacts to vegetation also include less obvious but often more significant impacts, such as reduced "annualgrowth rates and below ground
root loss, the paucity ofinformation on other species means the number of03- sensitive species that exists within the U.S. is likely greater than what is now known. Since no state in the lower 48
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sensitive plant species, with the majority ofstates havingbetween 11 and 30 (see Appendix 7J-2 in Staff Paper), protecting 0 3-sensitive vegetation is clearly importantto the public welfare atthe national scale.

Second, the StaffPaper 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 ofthese 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 onnatural ecosystems.

(3) Crop YieldLoss. Exposure and risk
assessments in the StaffPaper estimated that meeting the current 8-hour standard would still allow 0 3-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 lastreview

as a result of several factors, including adjusted exposure levels to reflect the presence of avariable 03 gradient between monitor height and crop canopies, and use of a different econometric agricultural benefits model updated toreflect more recent agricultural policies (EPA, 2006b). Though these sources .ofuncertainty associated with the croprisk 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 StaffPaper 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 avariety ofother factors (e.g., weather, insects, disease) can be orders ofmagnitude greater than that ofyield impacts predicted for a given 03 expbsure. Thus, therelevance ofsuch estimated impacts oncrop yields to the public welfare are considered highly uncertain and less useful as a basis for assessing the adequacy ofthecurrent standard. The StaffPaper noted, however, that in some experimental cases, exposure to 03 has made plantsmore sensitive or vulnerable to some ofthese other important stressors, including disease, insect pests, and harsh weather (EPA, 2006a). The StaffPapertherefore concluded that this remains an important area ofuncertainty andthat additional research to better characterize the nature and significance ofthese interactions between 03 and other plant stressors would be useful.

iii. Summary of StaffPaper Considerations

In summary, the StaffPaper concluded that the current secondary 03 standard is inadequate. This conclusion

was based on the extensive vegetation effects evidence, fn particular the recent empirical field-based evidence on biomass loss in seedlings, saplings and
mature trees, and foliar injury incidence thathas become available inthis review, which demonstrates the occurrence of adverse vegetation effects at ambient levels ofrecent 03 air quality, aswell as evidence and exposure- andrisk-based analyses indicating that adverse effects wouldbe predicted to occur under air quality scenarios that meet the current standard.

b. CASAC Views

In a letter tothe Administrator (Henderson, 2006c), the CASACO 3 Panel,"with full endorsement ofthe chartered CASAC, unanimously concluded that "despite limited recent research, ithas become clear since the last review that adverse effects on a widerange ofvegetation including visible foliar injury are to be expected and havebeen observed inareas thatare below the level ofthe current 8-hour primary and secondary ozone standards." Therefore, "based on the Ozone Panel's review ofChapters 7 and 8 [ofthe StaffPaper], the CASAC unanimously agrees that itis not appropriate to try toprotect vegetation from the substantial, known or anticipated, direct and/or indirect, adverse effects of ambient 03 by continuing topromulgate identical primary and secondary standards for 03. Moreover, the members ofthe 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 03 andrelated photochemical oxidants that is distinctly different inaveraging time, form and level from the currently existing orpotentially revised 8-hour primary standard" (Henderson, 2006c).26

26

One CASAC Panel member reached different conclusions from those ofthe broader Panel c. Administrator's Proposed Conclusions

At thetime ofproposal, in considering whether the current secondary standard should berevised, 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" 03 effects, as discussed in section IV.A.3 ofthe proposal. In considering what constitutes a vegetation effect that is also adverse to the public welfare, the Administrator took into account the StaffPaper conclusions regarding the nature and strength ofthe vegetation effects evidence, the exposure and risk
assessment results, the degree to which the associateduncertainties should be considered in interpreting the results, and the views ofCASAC andmembers 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 0 3-related effects onvegetation and ecosystems. Ozone levels that would be expected to remain after meeting the current secondary standard werejudged 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 ofthe plant and its significance tothepublic welfare, andthe current secondary standard does not provide adequate protection from such effects. Other 03- 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 temperatuves, pest infestations and/or disease, and to compete for available resources, would also be anticipated to occur. Inthe long run,the result ofthese impairments (e.g., loss in vigor)could lead to premature plant death in 03 sensitive species. Though effects on other ecosystem components

regarding certain aspects ofthe vegetation effects information and the appropriate degree ofemphasis that shouldbe placed on the associated uncertainties. These concerns related to how the results of 0 3/vegetation exposure experiments carried outin OTC can be extrapolated to the ambient environment and how C--R functions developed in the 1980s can be used today giventhat he did not expect that current crop species/cultivars inuse in 2002 would have the same 03 sensitivity

as those studied inNCLAN (Henderson, 2007, pg. C-18). 

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have onlybeen examined in isolated
cases, effects such as those described above could have significant implications forplant community and. associated species biodiversity and the
structure and function ofwhole ecosystems. These considerations also support theproposed conclusion that the current secondary standard is not adequate and that revision is needed to provide additional public welfare protection.

2. Comments on theNeed for Revision

The above section outlines the vegetation and ecosystem effects evidence and assessments usedby the Administrator toinform his proposed judgments about the adequacy ofthe
current 03 secondary standard. General
comments received on theproposal that either supported or opposed the proposed decision torevise the current 03 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.abelow, and comments

onvegetation 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 thatrelate to consideration ofthe appropriate form, averaging time, or level ofthe 03 standard are addressed below in section IV.C. General comments based on implementation-related factors that are not a permissible basis for considering the needto revise the current standard

are noted inthe Response to Comments document.

a. Evidence ofEffects and Exposure Indices

Sections IV.A.2 andIV.A.3 above provide a summary overview ofthe information on vegetation and ecosystem effects and exposure indices usedby the Administrator toinform his proposedjudgments about the adequacy ofthe current 03 secondary standard. As discussed more fully below, comments received onthe proposal regarding the
nature and strength ofthe 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 ofcommenters.that included national andlocal 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 thatthe available science clearly .showed that0 3-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 ofevidence available for consideration in thisreview, including evidence assessed previously in the last review. These commenters pointed to the information and analyses inthe Staff Paper and the conclusions and recommendations of CASAC as providing a clear basis for concluding that the current standard does not adequately protectvegetation from an array of0 3-related effects. For example, the NPS noted that "wlidespread foliar injury has been documented in areas meeting the current standard; field and chamber studies indicate that 03- induced significant growth reductions

are also occurring at levels below the
current standard" (NPS, p. 3).

In addition tothe body ofinformation already considered by EPA in this review, these same commenters also presented new information forthe Administrator's consideration, including a number of"new" studies published aftercompletion ofthe Criteria Document, aswell as additional information on air quality and vegetation exposures and effects pertaining tolocal conditions within their State, Tribal orfederal 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 03 concentrations in

areas that rarely, ifever, exceeded the level ofthe current 8-hour standard in

areas that still showed 0 3-related vegetation effects, particularly visible foliar injury.

Building on EPA's qualitative discussions ofthe potential linkage between 03 vegetation effects and effects on ecosystems, anumber ofthese
commenters expressed concern that the possible impact of03-related reductions in plant productivity could result in a reduced capacity ofvegetation to serve

as a carbon sinkto mitigate the impacts ofrising CO 2 ina changing climate, citing to a "new" study on that topic (Sitch etal., 2007). Many of these same commenters also cited to "new" fieldbased studies, in the Great Smoky Mountain National Park that find a relationship between 03 exposure, tree stem growth loss, tree water use and
stream flow as evidence that current ambient 03 levels can impact ecosystems andthat 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, eta]., 2007a, b) have found that (1) ambient03 caused substantial growth reductions in
mature trees in a-mixed deciduous forest, which was due in partto increased 0 3-induced water loss and led
to seasonal losses in stem growth of 30- 50 percent formost species in ahighozone year; (2) increasing ambient 03 levels also resulted in depletion ofsoil moisture in the rooting zone and reduced late-season stieamflow in the watershed; and (3) 03 may amplify the adverse effects of increasing temperature on forest growth and forest hydrology and may exacerbate the effects ofdrought onforest growth and
stream health. Other "new" research noted bythese commenters as supporting EPA's findings that current 03 exposures cause significant biomass losses in sensitive seedlings ofvarious tree species include a studythat predicted up to 31 percent growth loss in aspen in certain areas ofits North American rangein 2001-2003 (Percy, et

a.,2007). These commenters encouraged the Administrator to consider these "new" studies in making his final decisionThis group of commenters strongly supported revising the current standard,
not onlybecause in theirview the available evidence conclusively demonstrates that the current standard is inadequate toprotect sensiti~e vegetation, but also because the Staff Paperprovides abundant evidence that itis appropriate to establish an alternative cumulative, seasonal secondary standard thatis distinctly different in form from the current or revised primary standard. For example, NESCAUM statesthat "[in light ofthe EPA Staffand CASAC recommendations, and the extensive body ofhistorical and recent monitoring andresearch data uponwhich these recommendations were based, the option ofequating the ozone secondary NAAQS with the 8-hour primary is inappropriate and clearly not supported by the weight ofscientific evidence."

EPA agrees with these commenters that when evaluated as a whole, the entire body ofvegetation and ecosystem effects information available in this review supportsthe need torevise th
current standard to provide increased protection from an array of 03:related effects on sensitive vegetation and ecosystems. EPA also agrees that the available evidence indicates that a 

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cumulative, seasonal form betterreflects the scientific information on biologically relevant exposures for vegetation. Forreasons discussed below in sections IV.C, however, EPA disagrees with aspects ofthese coimenters' views as to whether a standard defined in terms ofa cumulative, seasonal form is requisite to protect public welfare based onthe available scientific information.

To the extent thatthese and other
commenters whose comments arediscussed 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 inthis review onthe studies andrelated information included in the 03 air quality criteria that have undergone CASAC and public review and will consider newly published studies for purposes of decision making inthe next 03 NAAQS review. Inprovisionally evaluating commenters' arguments, as discussed in the Response to Comments document, EPAnotes that its provisional consideration of "new" science found that such studies did notmaterially change the conclusions inthe Criteria Document.

The othermain group ofcommenters, which included Exxon-Mobil, UARG, API, other industry groups, The Annapolis Center for Science Based Public Policy,individual States and otherorganizations 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 assertedthat the types of vegetation effects evaluated inthe last review have
not changed, andthat the Criteria Document, StaffPaper, and CASAC have acknowledged that the information thathas 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 thatrevision to the standard is requisite to provide additional protection fromthese effects. In particular, Exxon-Mobil stated that "EPAis incorrect in concluding vegetation impacts [occur] at or below the level ofthe current standard" * * * and that the "newer field-based evidence EPA cites forozone impacts on seedlings, saplings and mature trees indicates ozone impacts but at exposures that are likely in exceedence ofthe current secondary standard." Thiscommenter concluded that while these studies provide additional support for 0 3-related impacts on vegetation, including observing effects in field settings without chambers, they do not provide support forthe conclusion that ambient levels in compliance with the
current standard would result in significant 03 impact. In addition, these
commenters also generally asserted that the evidence that has become available since the last review does notmaterially 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 tothe revised primary. Those aspects ofthese comments that include uncertainties associated with the exposure, risk andbenefits assessments

are addressed below in section IV.2.b and inthe Response to Comments document.

EPA disagrees with the commenters' assertion thatthe 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 offield-based evidence

across a broad array ofvegetation effects categories, as-discussedin the Criteria Document, StaffPaper, andhighlighted above in section IV.A.2. Though in some such studies (e.g., the FACE studies)the 03 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, itis nevertheless EPA's view that such field-based evidence reduces the uncertainties associated with the C-R functions generated in OTC studies thatwere notedby 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 0 3-relatedeffects below the level ofthe current standard. EPA has also considered this evidence in conjunction with USDA FIA foliar injury survey data andthe Gregg et al. (2003) tree seedling biomass loss gradient study showing effects ona sensitive tree species occurring in the field across a range ofexposure levels including levels ofair quality at to well below the level ofthe current secondary standard. Taken together, EPA concludes thatthese studies fprm a coherent body ofevidence that significantly strengthens EPA's confidence that such effects are currently occurring inthe field and would cofitinue to be anticipated at and below the level ofthe current secondary standard. Amore detailed discussion of these issues can be found inthe Response to Comments document.

b. Vegetation Exposure and Risk Assessments

Section IV.A.4 above provides a summary overview ofthe vegetation exposure and risk assessment information usedby the Administrator
to help informjudgments about vegetation exposure and risk estimates associated with attainment ofthe
current and alternative standards. As an initial matter, EPA notes that at the time ofproposal, the Administrator primarily based his conclusion on whether revision ofthe 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 ofcommenters were those noted above who expressed the view thatno revision was appropriate.

The firstgroup ofcommenters 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 thatthe standard needed to be revised to provide increased protection for sensitive vegetation. A few ofthese commenters also provided additional exposure, risk and benefits information from localized
assessments conducted bythemselves or others in theirbehalfin support oftheir view thatthe standard needed to be revised. In so doing, these commenters have generally shown support forusing such assessments to help inform a final decision on the need to revise..

The other group of commenters expressed a number ofconcerns with these assessments and generally assertedthat these assessments do not support revision ofthe current standard. These commenters' concerns generally focused on (1)the method used by EPA
to estimate PRB, (2) the lack ofnew information sincethe last review that would, intheirjudgment, materially reduce the uncertainties present in the
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use ofthe results in making ajudgment about the adequacy ofthe current standard. These comments are addressed below.

(1)Regarding concerns related tothe method used by EPA to estimate PRB, EPA notes that this issue has been raised repeatedly throughout the review in the context ofboth the primary and secondary standards. Most generally, these commenters asserted that EPA used unrealistically low levels ofPRB that resulted in an overestimate ofrisks and benefits associated withjust meeting alternative standards. EPA disagrees with this view, forthe 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 tothe primary standard.

(2) Another concern posed bythese
commenters was the lack of any new information that, in theirjudgment, would materially reduce the uncertainties present in the exposure, risk andbenefits assessments conducted for the last review. For example, the Annapolis Center asserted that "[s]ome ofthe most important caveats and uncertainties concerning the exposure and risk assessments for crop yield that

were listed in the [19961 proposal included (1)extrapolating from exposure-response functions generated in open-top chambers to ambient conditions; (2) the lack ofa performance evaluation ofthe national air quality extrapolation; (3) the methodology to adjust modeled airquality to reflect attainment ofvarious alternative standard options; and (4)inherent uncertainties in modelsto estimate economic values associated with attainment of alternative standard. * * * Because ofthe lack ofnew data

or substantive improvements in therisk assessment, these same issues remain today, contributing a similar degree of uncertainty, as was the case inthe prior review." EPA recognizes that important uncertainties remain in estimates of vegetation exposure and 0 3-related risk
to vegetation, especially with regard to 0 3-related effects on crop yields. However, EPA disagrees with comments that assert that uncertainties have not been reduced since the lastreview, as discussed below.

With regard to the uncertainties associated with using the OTC C-R functions, the Annapolis Centerfurther stated that "ten years have now elapsed, and the same concentration-response functions from the OTC studies ofthe 1980's are still the only viable data to

useto estimate crop loss. * * * The 1996 CASAC Panel agreed that the estimates ofcrop loss at that timewere highly uncertain." While EPA agrees that important uncertainties continue to be associated with the use ofthe C-R functions geneiated many years ago using OTC studies for crop yield loss, EPA does not agree that the new information available inthis review does nothing to reduce such uncertainties identified in the last review. As described above and in the StaffPaper and proposal, results from the new SoyFACE and AspenFACE studies provide qualitative support that the levels ofvegetation response that havebeen observed in the field are of similar magnitude asthose predicted at similar exposure levels using the OTC generated C-R functions. Therefore, EPA believes thatthe uncertainties cited inthe last review regarding the appropriateness ofusing OTC generated C-R functions to predict vegetation
response in the field have been reduced. Providing some further support in this regard isthe limited information available in this review on some sensitive crop species (e.g., soybean) suggesting that 03 sensitivityhas not changed significantly in the intervening years. Taking all the above into account, EPA's level of confidence in the applicability ofthe OTC generated C-R functions torepresent ambient conditions in the field has increased.

With regard to the lack of a performance evaluation ofthe national air quality extrapolation, EPA notes that there have been advancements inthe tools and methods used for such extrapolations since the last review. With respect to the generation of interpolated 03 exposure surfaces, EPA employed a different approach than that used in the last review andundertook

a quantitative assessment ofthe uncertainties associated with the use of this method. This uncertainty
assessment was accomplished by sequentially dropping out ofthe interpolation each monitoring site, and then recalculating the exposure surface using the remaining monitoring sites. As discussed in the StaffPaper, this method ofevaluation mayresult in a slight overestimation oferror and bias for the exposure surface, since dropping
out monitors loses information that the interpolation uses in thatlocal area. As another point ofcomparison, EPA also examined the subset ofrural CASTNET sites toillustrate 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 ofthe estimation method potentially resulted in an underestimation ofthe more important risks associated with higher cumulative exposures in some areas. Based on this evaluation, EPA reiterates the conclusion in the StaffPaper that "the calculation of error and bias metrics forthe interpolation represents

anotable improvement over the 1996
assessment which did nothave such an evaluation." EPA further concludes that ingeneral, the sources andlikely direction ofuncertainties associated with ihe exposure and risk assessments have been better accounted for and characterized than in the last review.

WithTegard to criticisms ofthe methodology used to adjust modeled air quality toreflect attainment ofvarious alternative standard options, EPA notes that this issue has been raised in the
context ofboth theprimary and secondary standards. As noted above in section ll.B.2.b,based on information in the StaffPaper (section4.5.6) and in

more detail in a staff memorandum (Rizzo, 2006), EPA concluded that the quadratic air quality adjustment approach used in this assessment generally bestrepresented the pattern of reductions across the 03 airquality distribution observed over the last decade in areas implementing control
programs designed to attain the 03 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 03 will be significantly different from past reductions with respect to impacting the overall shape ofthe 03 distribution.

Withregard to comments that asserted that inherent uncertainties in models to estimate ecdnomic values ofcrop loss have not been reduced since the last review, EPA acknowledges that while

an updated state ofthe art model, the AGSIM benefits model, was used in this review, substantial uncertainties remain in these estimates ofeconomic crop loss. Further, EPA notes that these estimates were not relied on as abasis 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 usedthis conclusion as thebasis for a claim that there is no reason to depart from the Administrator's 1997 decision thatthe 

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current secondary standard is requisite
to protect public welfare. EPAbelieves 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 ofthe 1997 risks estimates to the current estimates, are irrelevant for the purpose ofjudging the adequacy ofthe current standard, as the 1997 estimates reflect outdated analyses thathavebeen updated in thisreview to reflect the current science and as there have.been significant improvements to the modeling approaches and model inputs. Second, itis important to take into account EPA's increased confidence in some ofthe model inputs,

as discussed above, since injudging the weight to place on quantitative risk -estimates it is important to examine not only the magnitude ofthe estimated risksbut also the degree ofconfidence inthose estimates. Third, quantitative vegetation risk estimates were notthe mainbasis 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 ofevidence available across a broad array of vegetation effects, while the more uncertain exposure, risk andbenefits -estimates were used in a supportive role. Both the StaffPaper and proposal clearly distinguished the roles that these different types ofinformation played in informing the Administrator's proposed decision. The proposal states that "due *tomultiple sources ofuncertainty,both known andunknown, that continue to be associated with these analyses, the StaffPaper put less weight on this information in drawing conclusions on the adequacy ofthe current standard. However, the StaffPaper also recognizes that some progress has been made since the last review in better characterizing some ofthese associated uncertainties and, therefore, concluded that the results ofthe exposure andrisk
assessments continue to provide information useful to informing judgments asto therelative changes in riskspredicted to occur underexposure scenarios associated withthe different standard alternatives considered." In determining the requisite level of protection,the StaffPaper recognized that itis appropriate toweigh the importance ofthe predicted risks of these effects in the overall context of public welfare protection, alongwith a determination as to the appropriate weight toplace onthe associated uncertainties and limitations ofthis information. Thus, while the Administrator is fullymindful ofthe uncertainties associated with the estimates ofexposure, riskand benefits,

as discussed above, hejudges thatthese 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 03 on vegetation and sensitive ecosystems reached in the Criteria Document and StaffPaper, as discussed above in section IV.A, remain valid. In considering whether the secondary 03 standard shouldbe revised, the Administrator finds that evidence that has become available in this review demonstrates the occurrence ofadverse vegetation effects at ambient levels of
recent 03 air quality, and that evidence and exposure- and risk-based analyses indicate that adverse effects would be predicted to occur under air quality scenarios thatmeet the current standard, taking into consideration both the level and form ofthe current standard. Ozone exposures thatwould be expected to remain aftermeeting the
current secondary standard are sufficient to cause visible foliar injury and seedling and maturetree biomass loss in 0 3-sensitive vegetation. The Administrator believes that-the degree
towhich such effects should be considered to be adverse depends on the intended use ofthe vegetation and its significance to the public welfare. Other 0 3-induced effects described in the literature, including an impaired ability ofmany 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 resultofthese impairments (e.g., loss in vigor)could lead topremature plant death in 03 sensitive species. Though effects on other ecosystem components have only been examined inisolated
cases, effects such asthose described above could have significant implications for plantcommunity and associated species biodiversity and the
structure and function ofwhole ecosystems. The Administrator recognizes that the secondary standard is notmeant to protect against all known observed or anticipated 0 3-related effects,but only those thatcan reasonably bejudged 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 onthe definition of "adverse," discussed in section IV.A.3 oftheproposal, that imbeds the concept of "intended use" ofthe 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 ofa number ofactions taken byCongress to establish public lands that are set aside for specific uses that areintended 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 andwildlife within such areas, and toleave them unimpaired for the enjoyment offuture generations. Such public lands that are protected areas ofnational interest include national parks and forests, wildlife refuges, and-wilderness areas. Because 0 3-sensitive species are generally foundin such areas, and because levels of03 allowedby 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 0 3-caused impairment to such protected vegetation and ecosystems.

TheAdministrator further recognizes that States, Tribes and public interest groups also setaside areas that are intended to provide similar benefits to thepublic welfare, for residents on State and Tribal lands, as well as for visitors
to those areas. Giventhe clear public interest in and value of maintaining these areas in a condition that does not impair their intended use, andthe fact that many of these areas contain 03- sensitive vegetation, the Administrator further concludes that it is appropriate
to revise the secondary standard in part
toprovide increased protection against 0 3-caused impairment to vegetation and ecosystems in such specially designated

areas.

2 7 The Administrator alsorecognizes that other aspects of public welfare, as welfare is defined in the CAA, may rely on concepts other than "intended use." I t I1 il i lill IiillffTT i n 

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The Administrator also recognizes that 0 3-related effects on sensitive vegetation occur inareas that havenot been afforded such special protections, ranging from vegetation used for residential or commercial drnamental purposes, such asurban/suburban landscaping, to land use categories that

are heavily managed for commercial production ofcommodities such as agricultural crops, timber, and ornamental vegetation. For vegetation used forresidential or commercial ornamental purposes, such as urban/ suburban landscaping, there are indications that impairment to the intended use ofsuch vegetation can

occurfrom 03 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 ofurban/ suburban landscaping and other uses of ornamental vegetation, he notes that a secondary standard revised to provide protection for sensitive natural vegetationand ecosystems mayalso provide some degree ofprotection for such ornamental vegetation.

With respect to commercial production of commodities, however, the Administrator notes thatjudgments about the extentto which 0 3-related effects on commercially managed vegetation are adverse from a public welfare perspective are particularly difficultto reach, given that what is known about the relationship between 03 exposures .andagricultural crop yield response derives largely from data generated almost 20 years ago. The Administrator recognizes that there is substantial uncertainty at this time asto whether these dataremain relevant to the majority ofspecies and cultivars of cropsbeing grown in the field today. In addition, the extensive management of suchvegetation may to some degree mitigate potential 0 3-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 ofadditional 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 ofCASAC, the Administrator concludes that the protection afforded bythe current secondary 03 standard is not sufficient andthat the standard needs to be revised to provide additional protection fromknown and anticipated adverse effects on sensitive natural vegetation and sensitive ecosystems, and that such

arevised standard could also be expected toprovide additional protection to sensitive ornamental vegetation. The Administrator also concludes thatthere is not adequate information to establish a separate secondary standard based on other effects of 03 on public welfare. It is important to note that these conclusions, and the reasoning on which they are based, donot address the question ofwhat specific revisions
tothe current secondary standard are appropriate. Addressing that question requires looking specifically at thetwo proposed options: establishing a new standard defined interms ofa cumulative, seasonal form, or revising the current secondary standard by making it identical to therevised primary standard. These alternative secondary standards are discussed in the following section.

As highlighted below, the discussion ofpublic 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 arobust discussion within the Administration ofthese same strengths and weaknesses. As part ofthat process EPAreceived a Memorandum on March 6, 2008 from Susan Dudley, Administrator, Office ofInformation 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 ofa cumulative, seasonal secondary standard. On March 11, 2008, the President "concluded that, consistent with Administration policy, added protection shouldbe afforded to public welfare by strengthening the secondary

ozone standard and setting itto 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 orhigher thanis necessary." EPA's decision therefore alsoreflects the view ofthe Administration as to the most appropriate secondary standard. While the Administrator fully considered the President's views, the Administrator's decision, and thereasons for it, are based on and supportedby the record in this rulemaking.

C. Conclusionson theSecondary03 Standard

As an initial matter, EPA has considered the indicator for a secondary 03 standard. As discussed above in section ll.C.1 on the primary standard, in the last review, EPA focused on a standard for 03 as the most appropriate surrogate for ambient photochemical oxidants. In this review, whilethe complex atmospheric chemistry in which 03 plays akey role has been highlighted, no alternatives to 03 have been advanced asbeing a more appropriate surrogate forambient photochemical oxidants and their effects on vegetation. Thus, asis the case for theprimary standard, the Administrator concludes that itis appropriate to continue to use 03 as the indicator for a standardthat is intended
to address effects associated with

exposure to 03, alone and in combination with related photochemical oxidants. In so doing, the Administrator recognizes that

measures leading to reductions in vegetation exposures to 03 will also redude exposures to other photochemical oxidants.

1. StaffPaper Evaluation

The current Criteria Document and StaffPaper concluded that therecent vegetation effects literature evaluated in this review strengthens andreaffirms conclusions made in the last review that the use ofa cumulative exposure index that differentially weights ambient concentrations is best able torelate ambient exposures tovegetation
response at this time (EPA, 2006a, b). The last review focused in particular on two ofthese 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 ofnotincluding 03 concentrations considered to be within the PRB, estimated tobe 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 formwas not adopted at thattime.

In this review, the StaffPaper evaluated the continued appropriateness ofthe SUM06 form in 

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light oftwo key pieces ofinformation:

new estimates ofPRB that are lower than in the last review, and continued lack ofevidence within the vegetation effects literature ofa biological threshold for vegetation exposures of

concern. On thebasis ofthose policy and science-related considerations, the StaffPaper concluded that the W126 form was more appropriate inthe
context ofthis review. Specifically, the W126, by its incorporation ofa 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 03 concentrations within therange of estimated PRB.

The StaffPaper also considered that in the 1997 final rule, the decision was made, on the basis ofboth science and policy considerations, tomake the secondary standard identical to the primary standard (62 FR 38876). Onthe basis ofthat history, the current Staff Paper analyzed the degree ofoverlap expected between alternative 8-hour and cumulative seasonal secondary standards using recent air quality monitoring data. Based on theresults, the StaffPaper concluded thatthe degree to which the current 8-hour standard form and level would overlap with areas ofconcern for vegetation expressed in terms ofthe 12-hour W126 standard is inconsistent from yearto year and would depend greatly onthe level ofthe 12-hour W126 and 8-hour standards selected andthe distribution ofhourly 03 concentrations within the annual and/or 3-year average period.

Thus, though the StaffPaper

- recognized again thatmeeting the
current or alternative levels ofthe 8- hour average standard could result in air quality improvements that would potentially benefit vegetation in some areas, iturged caution be used in evaluating the likelyvegetation impacts associated with a given level ofair quality expressed in terms ofthe 8-hour
average form in the absence ofparallel W126 information. This caution is due
to the concern thatthe analysis inthe StaffPaper may not be an accurate reflection ofthetrue situation in nonmonitored, rural counties due to the lack ofmore complete monitor coverage in many rural areas. Further, ofthe counties that did not show overlap between the two standard forms, most

were located in rural/remote high elevation areas whichhave 03 air quality patterns that are typically different from those associated with urban and near urban sites at lower elevations. Because the majority ofsuch

areas are currently not monitored, it is believed there are likely to be additional

areas that have similar air quality distributions thatwould leadto the

same disconnect between forms. Thus, the StaffPaper concluded that it remains problematic to determine the appropriate level ofprotection for vegetation using an 8-hour average form.

2. CASAC Views

The CASAC,based onits assessment ofthe same vegetation effects science, agreed with the Criteria Document and StaffPaper and unanimously concluded that protection of vegetation from the known or anticipated adverse effects of ambient 03 "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 03 (Henderson, 2007). Moreover, the members ofCASAC and a substantial majority ofthe CASAC Panel agreed with StaffPaper conclusions and encouraged the Administrator to establish an alternative cumulative secondary standard for 03 andrelated photochemical oxidants that is distinctly different in averaging time, form"and level fromthe 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 CriteriaDocument, StaffPaper and by CASAC that the scientific evidence available in the current review continues to demonstrate the cumulative nature of0 3-induced plant effects andthe need to give greater weight to higher concentrations. Thus, the Administrator proposed that a cumulative exposure index that differentially weights 03 concentrations could represent a reasonable policy choice for a seasonal secondary standard toprotect against the effects of 03 onvegetation. The Administrator further agreed with boththe StaffPaper and CASAC that the most appropriate cumulative, concentration-weighted form to consider inthis review is the sigmoidally weighted W126 form, due
tohis recognition that there is no evidence inthe literature for an exposure threshold thatwould be appropriate across all 03-sensitive vegetation and that this form is unlikely to be significantly influenced by 03 air quality within the range ofPRB 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 of0.070
to 0.075 ppm.For this option, EPA also solicited comment on a wider range of 8-hour standard levels, including levels downto 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 therationale forthat decision that made the revised secondary standard identical to the revised primary standard.

4. Comments onthe Secondary Standard Options

Comments received following proposal regarding revising the secondary standard either to reflect a new, cumulative form or byremaining equal to a revised primary standard generally fell into two groups. These
comments were similarto those raised prior to theproposal during earlier phases ofthe NAAQS review, as summarized inthe proposal notice and highlighted below.

One group ofcommenters, 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 fora cumulative form, and specifically supportedthe 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 ofozone exposure (Musselman etal., 2006). The W126 exposure index has long been recognized as a biologically meaningful and useful way to summarize hourly

ozone data as a measure ofozone exposure to vegetation (Lefohn etaL., 1989)". Similarly, Environmental Defense stated "[for reasons amply explained by CASAC andthe Staff, neither the existing secondary standard for ozone nor the proposed primary standards are requisite to protect against 

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adverse welfare effects on vegetation and forested ecosystems. CASAC and Stafffurther amplyjustified 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 sptecifically 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 thereasons 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 agreedwith CASAC that "retaining the current form ofthe 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 ofthe benefits ofa secondary NAAQS would be achieved if the primary NAAQS were attained. This rationale is flawed in atleast two ways: first, ozone damage to vegetation persists inareas 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 ofthe secondary NAAQS." Environmental Defense made the argument that "[b]ecause there is no rational connection between the proposed primary standards and the level ofprotection needed to protect vegetation against adverse ozoneinduced welfare effects, any EPA finding that the primary standards would be sufficient forsecondary standards purposes wouldbe arbitrary.* * * The mere factthat 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 ofcommenters, 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 ofthese cornmenters, 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 forthe form ofthe 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 andbenefits assessments. They point to the uncertainties cited by the Administrator inthe 1997 review as part ofherrationale 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 ofthe impact ofthe nation's 03 control program forthe 8- hour standard onW126 exposures is not scientifically sound dueto the use of low estimates ofPRB and an arbitrary rollback method that is uninformed by atmospheric chemistry from photochemical models. They argue that EPA must first realistically evaluate the total 03 reductions that would occur byusing a state-of-the-art photochemical model and perform an analysis ofthe 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 forthe 8-hour form ofthe standard, EPA has failed to show that the current 8-hour standard wouldprovide less than requisite protection. These commenters asserted that substantial uncertainties remain in this review, and that the benefits ofchanging to a W126 form are
toouncertain to warrantrevising the form ofthe standard at thistime.

This group of commenters also addressed limitations associated with selection ofthe 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 fitto specific responses in OTC studies, such that its relevance for realworld biological
responses is unclear; (2) aflux-based model would be abetter 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) theEuropean experience with cumulative 03 metrics has been disappointing and now Europeans are working ontheir second level approach, which willbe flux-based; and (4) the W126 form 6annot provide nationally uniform protection, as the same valueof

an exposure index may relate to different vegetation responses; some commenters support adding a second index thatreflects 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 ofa cumulative, seasonal form, or revising the current secondary standard by makingit identical to the revised primary standard, the Administrator took into account the approach usedby the Agency in the last review, the conclusions ofthe StaffPaper, CASAC advice, and the views ofpublic
commenters. In giving careful consideration to the approach taken in the lastreview, the Administrator first considered the StaffPaper analysis of the projected degree-ofoverlap between counties with air quality expected to meet the revised 8-hourprimary standard, set at a level of0.075 ppm, and alternative levels ofa W126 standard based on currently monitored air quality data. This analysis showed significant overlap between the revised 8-hour primary standard and selected levels ofthe W126 standardform being considered, with the degree of overlap between these alternative standards depending greatly on the W126 level selected and the distribution ofhourly 03 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 tothat provided by the
current secondary standard. In further considering the significant uncertainties thatremain in the available body'of evidence of0 3-related vegetation effects and in the exposure andrisk analyses conducted for this review, and the difficulty in determining at what point various types ofvegetation effects become adverse for sensitive vegetation and ecosystems, the Administrator focused his consideration on a level for

28EPA has done further analysis ofthe degree of overlap, and that analysis is in the docket. 

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an alternative W126 standard atthe
upper end ofthe proposed range (i.e., 21 ppm-hours). The StaffPaper analysis shows that atthat 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 9-hour primary standard-that is,based onthis analysis ofcurrently monitored counties, a W126 standard wouldbe unlikely to provide additional protection inany areas beyond that likely to be provided by the revised primary standard.

The Administrator alsorecognizes that the general lack ofrural monitoring data makes uncertain the degree to which therevised 8-hour standard oran alternativiW126 standard would be protective, and that there wouldbe the potential for not providing the appropriate degree ofprotection for vegetation in areas with air quality distributions that result in ahigh cumulative, seasonal exposure but do
not result inhigh 8-hour average exposures. While this potential for under-protection is clear, the number and size ofareas atissue andthe degree ofrisk is hard to determine. However, such a standard would also tend to avoid thepotential for providing more protection than is necessary, a riskthat would arise from moving to a new form forthe secondary standard despite significant uncertainty in determining the degree of risk forany exposure level and the appropriate level ofprotection,

as well as uncertainty in predicting exposure and risk patterns.

The Administrator also considered the views and recommendations of CASAC, and agrees that acumulative, 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 ofrisk attributable to varying levels of03 exposure, the degree ofprotection that
any specific cumulative, seasonal standard would produce, and the associated potential for error in determining the standard thak'will provide arequisite degree of protection-i.e., sufficient butnot more than what is necessary. Given these significant uncertainties, the Administrator concludes that establishing anew secondary standard with a cumulative, seasonal form at this time would resultin uncertain benefits beyond those afforded by the revised primary standard andtherefore may be more than necessary to provide the requisite degree ofprotection. Based onhis consideration ofthe full
range ofviews as described above, the Administrator judges that the appropriate balance to be drawn is to revise the secondary standard to be identical inevery way to the revised primary standard. The Administrator believes that such a standard would be sufficient to protect public welfare from known oranticipated adverse effects, and doesnot believe that an alternative cumulative, seasonal standard is needed
to provide this degree ofprotection. This judgment bythe Administrator appropriately considers the requirement for a standard that is neither more nor less stringent than necessary for this purpose.

D.Final Decisionon theSecondary03 Standard

For thereasons discussed above, and taking into account information and
assessments presented in the Criteria Document and StaffPaper, the advice and recommendations ofthe 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 standardby making it identical tothe 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 therevised 03 secondary standard are discussed below in section VI.

V. Creation ofAppendix PInterpretation ofthe NAAQS for03

This section presents EPA's final decisions regarding the addition of Appendix P to40 CFR part 50 on interpreting the primary and secondary NAAQS for 03. EPA did not propose to address revocation ofthe 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 tothe primary standard. Forreasons stated above, the Administrator has decided to
set the secondary standard to be identical in all respects to the primary standard. Therefore, the portions ofthe proposed Appendix P providing data handling procedures for a non-identical secondary standard are not included in the final rule.

Key elements ofAppendix 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.DataCompleteness

EPA proposed data completeness requirements for the new Appendix P for the revised 8-hour primary standard that wouldbe 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, forthe 3-year period at amonitoring site, with no single year withinthe period having less than 75% data completeness. This data completeness requirement applies only during therequired 03 monitoring

season and mustbe satisfied inorder 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 Iin 1997 that these proposed requirements are reasonable based on its earlier analysis ofavailable air quality datathat showed that90% ofall 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 tobe counted for the purpose ofmeeting the data completeness requirements if meteorological conditions onthese missing days were not conducive to concentrations above the level ofthe standard. Such determinations under Appendix Iand theproposed Appendix P would be made on acase-by-case basis using available evidence. In the proposal, EPA specifically requested
comment onwhether meteorological data could provide an objectivebasis for determining, for a day forwhich there is missing data, thatthe meteorological conditions were not conducive to high 03 concentrations, and therefore, that the day could be assumed to have an 03 concentration less than the level ofthe 

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NAAQS. Further, theproposal requested comments onwhether days assumed less than the level ofthe standard should be counted as nonmissing when computing whetherthe data completeness requirements have beenmet at the site. The proposal pointed outthat this could allow a determination ofattainment which would otherwise be precluded by the 75% and/or 90%completeness tests. Mostcommenters supported the use of meteorological data to establish that missing days could be assumed to have low 03 levels. However, no commenter suggested any particular objective criteria or formula for making such determinations. Based on these comments, EPA will continue touse 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 forthe primary standard.

As noted above, because the Administrator has decided to set the secondary standard identical in all respects tothe primary standard, the final Appendix P provides that its data completeness requirements apply to both standards.

C.Data ReportingandHandlingand RoundingConventions

Forreasons discussed above, the Administrator has setthe level ofthe revised 8-hour primary and secondary standards at 0.075 ppm.As explained in theproposal, the level ofthe 8-hour standard is expressed to thethird decimal place. Almost all State agencies

now report hourly 03 concentrations to three decimal places, in ppm, or in a format easily convertible to ppm, since the typical incremental sensitivity of currently used 03monitors is 0.001
ppm. Consistent with the current approach for computing 8-hour averages, in calculating 8-hour average 03 concentrations from hourly data, any calculated digits beyond the third decimal place would be truncated, preserving the number ofdigits inthe reported data. In calculating 3-year
averages ofthe fourth highest maximum 8-hour average concentrations, digitsto theright ofthe third decimal place would also be truncated, prdserving the number ofdigits in the reported data. Analyses discussed in the StaffPaper demonstrated thattaking into account the precision and bias in 1-hour 03 measurements, the 8-hour design value has an uncertainty ofapproximately 0.001 ppm.Truncating both the individual 8-hour averages used to determine the annual fourth maximum as well asthe 3-year average ofthe fourth maxima to the third decimal place is consistent with the approach used in Appendix Ifor theprevious 8- hour 03 standard. Inthe proposal, EPA sought comment on the appropriateness ofrounding rather thantruncating to the third decimal place as well asthe scientific validity oftruncating the 3- year average and the policy reasons behind either truncating or roundifig the 3-year average to the third decimal place.Many ofthe comments EPA received on the rounding/truncation issue in effect were comments that supported expressing the level ofthe NAAQS to eitherthe second orthird decimal place. These comments are addressed in the Response to Comment document. EPA continues to believe the conclusions fromthe Staffpaper regarding monitor precision and error propagation when calculating 8-hour 03 averages are appropriate. EPA has decided to continue totruncate, as done in Appendix I, and this approach is included inthe final Appendix P.

As discussed above in section II.C.3, EPA is setting an 8-hour standard extending to three decimal places. Given thatboth the standard and the calculated value ofthe 3-year average of the fourth highest maximum 8-hour 03 concentration areexpressed to three decimal places, the two values can be compared directly.

As noted above, because the Administrator has decided to setth& secondary standard identical in all respects tothe primary standard, the

same data reporting and handlingand rounding conventions will apply to both.

VI. Ambient Monitoring Related to Revised 03 Standards

As noted in the 03 NAAQS proposal (see 72 FR 37906), EPA did not propose any specific changes to existing requirements formonitoring of03 inthe ambient air. However, comment was invited on a number ofspecific issues which were expected.to be of significance in the event that one or

more ofthe 03 NAAQS was revised. Comments werereceived from Federal agencies, Statemonitoring 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 03 NAAQS. In light ofthese revisions, EPA intends to issue amonitoring rule to address the issues identified inthe proposal, as well as otherissues raised in the comments. EPA intends to issue

a proposed monitoring rule inJune 2008 and a final rule by March 2009. In recognition ofthe comments received

on the proposed 03 standards and to provide EPA's initial thinking on03 specific monitoring rule amendments,

we offer the following observations. The following paragraphs also point out one way inwhich some State/local monitoring agencies might need tomake changes to their 03 monitoring network

as a result ofthe revision tothe primary and secondary 03NAAQS, based on the existing minimum monitoring requirements including a factorbased

on the comparison ofdesign value to the 03 NAAQS (see 71 FR 61318). The following text explains why an amendment to the monitoring regulations is notrequired to trigger these increased 03 monitoring requirements.

Presently, States (including the District ofColumbia, 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 03 monitors based onthe population of each oftheir Metropolitan Statistical Areas (MSA) and themostrecently measured 03 levels in each area. These requirements

are contained in 40 CFR part 58 Appendix D, Network Design Criteria forAmbient Air Quality Monitoring, Table D-2. Theserequirements were last revised on October 17, 2006 aspart of

acomprehensive review ofambient monitoring requirements for all criteria pollutants. (See 71 FR 61236).

The minimum number ofmonitors required in an MSA ranges from zero (foran area with population under 350,000 andno recent history ofan 03 design value greaterthan 85 percent of the NAAQS) tofour (for an area with population greater than 10 million and

an 03 design value greater than 85 percent ofthe NAAQS). Because these requirements apply atthe MSA level, large urban areas consisting ofmultiple MSAs canrequire 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 ofmonitors.

As noted above, the requirements listed inTable D-2 of40 CFR part 58 Appendix D are based on thepercentage ofthe 03 NAAQS, with a design value breakpoint at 85 percent ofthe NAAQS. For an MSA ofa given population size, there are a greater number ofrequired monitors when the design value is greater than or equal to 85 percent ofthe 03 NAAQS compared with MSAs that have a design value ofless than 85 percent ofthe 03 NAAQS. Atthe preexisting level of0.084 ppm for the 8- hour primary and secondary standards, 

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an 8-hour 03 design value of 0.068 ppm would trigger such increased minimum monitoring requirements for inMSA. 2 9

With the decision torevise the 8-hour primary and secondary standards to a level of0.075 ppm, the 8-hour 03design value thatwill 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 and0.067 ppm are nowrequired to increase the number of monitors operating to meet minimum requirements based on existing monitoring requirements. 30 In practice, however, virtually all ofthese areas already are operating at least asmany monitors as required based onthe revised primary standard, so the number ofnew 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 03 monitors, andhence they do not have an 03 design value for use with Table D-2. These unmonitored MSAs

are not required to add monitors. Commenters from Statemonitoring agencies and State organizations expressed concern thatthese current requirements ignore the needs that States and localities will have for additional monitors to measure 03 levels incurrently under-monitored

areas and, in particular, in unmonitored

areas with populations under 350,000. They statedthat unless this deficiency is corrected, the health benefits ofEPA's 03 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 ofnew attainment/non-attainment areas without additional monitoring inthe MSAsbelow 350,000.

EPA recognizes that the issues raised by the commenters are important. EPA intends to address these issues as part ofits 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 03 NAAQS proposal and described earlier in this section, existing 03 monitoring requirements

29Calculated as 85 percent of0.08 ppm,perthe
statedlevel ofthe pre-existing 8-hour primary and secondary standards.

30Approximately 16 MSAs that are subject to minimum monitoringrequirements have 8-hour design values between 0.064 ppmand 0.067 ppm based onan analysis of2004-2006 ambient 03 data. 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 ofthe monitoring requirements on urban areas, where large populations reside,in which significant emissions of 03 -forming precursors are found, and where 03 concentrations ofconcern are likely to occur.

There are noEPA requirements for 03 monitoring in lesspopulated areas outside ofMSAboundaries or in rural

areas. However, at present there are about 250 03 monitors in counties that

are notpart ofMSAs. These monitors

are operated by State, local, andtribal monitoring agencies for a variety of objectives including the assessment of
03 transport and the support ofresearch
programs including studies of atmospheric chemistry and ecosystem impacts. Additionally, EPA operates a network of about 56 03 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 ofnonurban 03 monitors is relatively highin the eastern one-third ofthe 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 withregard to certain aspects of03 monitoring. They recommended that EPA upgrade such practices to meet the40 CFR part 58 Appendix Aquality assurance requirements already followed bythe 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 ofthe CASTNET sites, andthat such upgrades will be completed atall CASTNET sites by 2009. EPA notes that theresulting 03 ambient data from the upgraded sites will meet Appendix A requirements as is presently the casefor 03 data from State operated monitors and NPS monitors. These data will be deemed acceptable forNAAQS-

. comparison objectives andavailable in the AQS database beginning in 2008.

Mostcommenters noted the relative lackofrural 03 monitors, stating that EPA should consider adding monitoring requirements that support a revised secondary 03 standard byrequiring 03 monitors inlocations thatcontain 03- sensitive plants orecosystems. These
commenters also noted thatthe placement ofcurrent 03 monitors may not be appropriate forevaluating vegetation exposure since many ofthese monitors were likely located to meet other objectives.

Inlight ofthe Administrator's decision to revise the 8-hour secondary standard, EPAbelieves 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 0 3-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 ofnational interest (e.g., national parks, wilderness areas).

In consideration ofthe spatial gaps thatcurrently existin the rural ozone monitoring network, and tothe extent thatthe existence ofsuch gaps has contributed to the overall uncertainty that exists in the level ofprotection that wouldbe 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 ofthe 03NAAQS by providing a more robust data setwith which to assess the relationship of vegetation damage to 03 concentrations.

Accordingly, as part ofits separate monitoring rulemaking, EPA intends to ,consider specific requirements for a minimum number ofrural 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 degreethat could be considered adverse depending on the intended use oftheplant and its significance to thepublic welfare. EPA is also considering recommending that States andTribes employ other quantitative tools, such as photochemical modeling and/or the spatial interpolation of ambient data from existing 03 monitors, to determine the adequacy ofexisting locations of rural monitors and to inform the locations of new orrelocated monitors that might be required to meet revised rural minimum monitoring requirements.

Finally,EPA solicited comment on the issue of03 monitoring seasons. Unlike the year-round monitoring required for other criteria pollutants, the 

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required 03 monitoring seasons 31 vary in length due tothe inter-relationship of 0 3-forming photochemical activity with ambient temperature, strength ofsolar insolation, and length of day.For example, in States with colder climates such as Montana and South Dakota, the 03 season has q length of4 months. In States with warmer climates such as California; Nevada, and Arizona, the 03

season has a length of 12 months.

With the decision to revise the 8-hour primary standard to a level of0.075
ppm, and to set the secondary standard identical in all respects to theprimary standard, the issue arises ofwhether in

some areas the required 03 monitoring

season should be made longer.EPA
notes thatunder the existing regulations, the Regional Administrator
may approve State-requested deviations from the established 03 monitoring
season,but EPA may notincrease the length ofthe season for an area at EPA's

own initiative other than bynotice and
comment rulemaking.

EPA has done a preliminary analysis of2004-2006 ambient datato address the issue ofwhether extensions of currently required 03monitoring.

seasons are appropriate in light ofthe revised level for the primary and secondary 03 standards andthe revised breakpoints for theAQI. The results of the analysis demonstrated thatout-ofseason exceedances ofthe revised level occurred in eight States during the study period. Additionally, the frequency of days with 03 concentrations that reached the revised Moderate AQI category (based on a breakpoint of0.060 ppm)was much greater compared withthe frequency of days with concentrations thatreached the pre-existing Moderate AQI category (based on a breakpoint of0.065 ppm). This increased frequency ofdays with Moderate AQI levels was noted to occur during periods before and afterthe currently required 03 seasons.

Based on these preliminary analyses, EPA intends to consider changes tothe length ofthe required 03 season for the coming monitoring rulemaking. Such changes couldbe based solely onthe frequency of exceedances ofthe revised primary and secondary standards, or could also consider the frequency of concentrations in the Moderate category ofthe AQI.
VII. Implementation andRelated ControlRequirements

A. Future ImplementationSteps

In today's rule,EPA is replacing the existing (1997) standards with revised

31 See40 CFRPart 58Appendix D, section 2.5 for

a table ofrequired 03 seasons. primary and secondary 03 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 03 standards to the 2008 03 standards. States are required to continue to develop and implement their State Implementation Plans (SIPs) for the 1997 standards as theybegin theprocess ofrecommending designations for the 2008 standards.

1. Designations

AfterEPA establishes orrevises a NAAQS, the CAA requires EPA and States to begin taking steps to ensure that the new or revised standards are met. The firststep is to identify areas of the country that do not attain the new

or revised standards, orthat contribute
toviolations ofthe new or revised standards. Section 107(d)(1) provides "By such date as the Administrator may reasonably require, but not laterthan 1 year'after promulgation ofa new or revised national ambient air quality standard for any pollutant under section 109, the Governor ofeach State shall * * * submit to the Administrator.a list ofall areas (or portions thereof) inthe State".that designates those areas as non-attainment, attainment, or unclassifiable. Section 107(d)(f)(B)(i) furtherprovides, "Upon promulgation

or revision of a national ambient air quality standard, the Administrator shall promulgate the designations ofall

areas (orportions thereof) * * * as expeditiousl as practicable, but in no

case laterthan 2 years fromthe 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 ofa rule.3 2 As noted above, the CAA requires EPA to establish a deadline for the States' submission ofthe designation recommendations, but under the CAA, it can be no later than March 12, 2009, one year afterthe promulgation ofthis rule. Therefore, Governors ofStates should submittheir designation recommendations to EPA no later than March 12, 2009. EPA's promulgation of designations must occur no later than March 12, 2010, although thatdate may be extended byup to one year Linder the CAA (no later than-March 12, 2011) if EPA has insufficient information to promulgate the designations.

32American PetroleumInstitutev. Costle,609

F.2d 20 (D.C. Cir. 1979). EPA intends to provide additional guidance to the States concerning the technical considerations for establishing boundaries for designated areas. Forthe revised primary and secondary standards, we anticipate relying on past 03 designation guidance issued by EPA prior to the designations for the 1997 03 standards.3 3 We anticipate working closely with State air agencies and Tribes on establishing new guidance on designations, ifneeded.

2. State Implementation Plans

CAA sectioi 110 provides the general requirements for SIPs. Within 3 years after the promulgation ofnew orrevised NAAQS (orsuch shorter period as the Administrator may prescribe) each State
must adopt and submit "infrastructure" SIPs toEPA to address the requirements ofsection 110(a)(1). Thus, States should submitthese SIPs no later than March 12, 2011. These "infrastructure SIPs" provide assurances ofState resources and authorities, andestablish 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 Staterequirements 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 ofthe 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 03 NAAQS in a proposed rulemaking in Fall 2008. Also in thatrulemaking EPA will establish deadlines for submission ofnonattainment area SIPs but anticipates that the deadlines will be no later than 3 years after final designation. Depending on the classification ofan area, the SIP must provide for attainment within 3 years (for areas 6lassified marginal) to 20 years (for

areas classified extreme) after final designations.

3. Trans-boundary Emissions

Cross border 03 contributions from within North America (Canada and Mexico) entering the U.S. are generally thought to be small. Section 179B ofthe

33Memorandum ofMarch 28,2000 frwom John Seitz, "Boundary Guidance on Air Quality Designations for the 8-Hour Ozone National Ambient Air Quality Standards (NAAQS or Standard).- 

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Clean Air Act allows designated nonattainment areas to petitionEPA 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 airquality inthe United States is a challenging and complex problem to assess. EPAis engaged in a number ofactivities to improve our understanding of international transport. As work
progresses onthese 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 ofthe revisions of the 03 standards, to issue a monitoring rule to address a variety ofmonitoringrelated issues identified in the preamble
to theproposed rule or incomments received by the Agency onthe proposal. EPA intends to issue a proposed monitoring rule inJune 2008 and a final rule byMarch 2009.

B. RelatedControlRequirements

The man-made oxides of nitrogen (NOx) and volatile organic carbon (VOC) emissions that contribute to 03 formation in the United States come from a variety ofsource 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 astrees and wildfires can also constitute a significant portion of total VOC emissions in certain regions ofthe country, especially during the 03

season. Natural sources such as wildfires, lightning, and soils also emit NOx. Emissions ofVOCs and NOx from these sources are considered natural background emissions. 35

34National Emission Inventory posted at the following Web site: http://www.epa.gov/ttn/chieftrends/index.html.

35In some cases natural emissions maycause or significantly contribute to violations ofthe ozone standard.EPA has issued rules thataddress 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 qualitymonitoring data from regulatory determinations related to exceedances or violations ofthe National Ambient Air Quality Standards (NAAQS). Ifan event is determined byEPA to be a qualifying exceptional event, theaffected area may avoidbeing designated

asnonattainment, being redesignated as. nonattainment, orbeing reclassifed to ahigher classification, Therequirements for demonstrating that elevated ozone levels arethe result ofa EPA has developed new emissions standards for many types ofstationary

sources and for nearly every class of mobile sources in the last decade to reduce 03by decreasing emissions of NOx and VOC. These programs complement State and local efforts to improve air quality and to meet the national 03 standards. Under the Federal Motor Vehicle ControlProgram (FMVCP, see title H ofthe CAA, 42 U.S.C. 7521-7574), EPA has established

new emissions standards for nearly
every type ofautomobile, 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 willyield substantial emissions reductions. Emissionsreductions associated with fuel
programs generally begin as soon as a
new fuel is available.

The reduction ofVOC emissions from industrial processes and consumer and commercial product categories has been achieved either directly or indirectly through implementation ofcontrol 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 ongenerally available control technology orbest available controls underprovisions related to consumer and commercial products. These standards haveresulted inVOC emissions reductions of almost a million tonsper year accumulated starting in 1997 from a variety of sources including combustion sources, coating categories, andchemical manufacturing. In 2006 and 2007, EPAissued national rules and control techniques guidelines for control ofVOC emissions from 10 categories ofconsumer and commercial products. EPA is currently working to finalize new Federal rules, or amendments to existingrules, intended
to establish new nationwide VOC content limits for several categories of

consumer and commercial products, including aerosol coatings, architectural andindustrial maintenance coatings, andhousehold and institutional commercial products. EPA anticipates that final rules addressing emissions

qualifying exceptional event are provided in the Exceptional Events Rule. from these sources will take effect in 2009.

Fuel combustion is one ofthe 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 AirInterstate Rule (CAIR), issued on March 10, 2005, permanently caps power industry emissions ofNOx in the eastern United States. The first phase ofthe cap begins in 2009, and a lower second phase cap begins in 2015. By 2015, EPA projects that the CAIR and other programs inthe Eastern U.S. willreduce power industry annual NOx emissions in that region by about 60 percent from 2003 levels.

With respect to agricultural sources, the U.S. Department ofAgriculture (USDA) has recommended conservation systems and activities that canreduce agricultural emissions ofNOx and VOC. Current practices that may reduce emissions ofNOx andVOC include enginereplacement programs, management ofpesticide applications, and manure management techniques. The EPA recognizes that USDA has been working with the agricultural community toplan conservation systems and activities to manage emissions of03 precursors.

These conservation systems and activities canbe voluntarily adopted in

areas where mitigation of03 precursors have been identified as an air quality

concern through the use ofincentives provided to the agricultural producer. In

cases where the States need these

measures to attain the 03 standards, agricultural producers could choose to adopt these measures. The EPA will continue to work with USDA on planning the implementation ofthese conservation systems and activities in order toidentify and/or improve mitigation efficiencies, prioritize their adoption, and ensure that appropriate criteria are used for identifying the most effective application ofconservation systems and activities.

The EPA will work together with USDA andwith States to identify appropriate measures to meet the primary and secondary standards, including site-specific conservation systems and activities. Based onprior experience identifying conservation

measures andpractices to meet the PM NAAQS requirements, the EPAwill use

a similar process toidentify measures thatcould meet the 03 requirements. The EPA anticipates that certain USDAapproved conservation systems and activities thatreduce agricultural emissions ofNOx and VOC may be able
to satisfy therequirements for 16504

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applicable sources to implement ' reasonably available control measures forpurposes ofattaining the primary and secondary 03 NAAQS.

VIII. Statutory andExecutive Order Reviews

A. Executive Order12866:Regulatory Planning andReview
Under section 3(f)(1) ofExecutive 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
tothe Office ofManagement and Budget (OMB) forreview under EO 12866 and
any changes made in response to OMB recommendations havebeen documented in the docket forthis action. In addition, EPA prepared an analysis ofthe potential costs and benefits associated with this action. Thisanalysis is contained in the Final Ozone NAAQS RegulatoryImpact Analysis,March2008 (henceforth, "RIA"). A copy ofthe analysis is available in the RIA docket (EPA-HQOAR-2007-0225) and the analysis is briefly summarized here. The RIA estimates the costs and monetized human health and welfare b~nefits of attaining three alternative 03 NAAQS nationwide. Specifically, the RIA examines the alternatives of0.079 ppm, 0.075 ppm, 0.070 ppm, and 0.065 ppm. The RIA contains illustrative analyses that consider alimited number of emissions control scenarios that States and Regional Planning Organizations might implement to achieve these alternative 03 NAAQS. However, the CAA and judicial decisions make clear that the economic and technical feasibility ofattaining ambient standards are notto be considered in setting orrevising NAAQS, although such factors may be considered in the development ofState plans to implement the standards. Accordingly, although aRIA has beenprepared, the results ofthe RIA have notbeen considered inissuing this final rule.

B. Paperwork ReductionAct
This action does not impose an information collection burden under the provisions ofthe Paperwork Reduction Act, 44 U.S.C. 3501 etseq.There are no information collection requirements directly associated with the establishment of aNAAQS under section 109 ofthe CAA.

Burden means the total time, effort, or financial resources expended bypersons 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 thepurposes of collecting, validating, andverifying information, processing and maintaining information, and disclosing and providing information; adjust the existingways to comply with any previously applicable instructions and requirements; train personnel tobe able
to respond to a coll~ctionof 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 aperson is not required to respond to a collection ofinformation unless it displays a currently valid OMB control number. The OMB control numbers for EPA's regulations in40 CFR are listed in40 CFR part 9.

C.Regulatory Flexibility Act
The Regulatory Flexibility Act (RFA) generally requires an agency to prepare

a regulatory flexibility analysis ofany rule subject to notice and comment rulemaking requirements under the Administrative Procedure Act orany other statute unless the agency certifies that the rule willnot have a significant economic impact on a substantial number of smallentities. Small entities include small businesses, small organizations, and small governmental jurisdictions.

Forpurposes of assessing the impacts ofthis rule on small entities, small entity is defined as: (1)A smallbusiness that is a small industrial entity as defined bythe Small Business Administration's (SBA)regulations at 13 CFR 121.201; (2) a small governmental jurisdiction that is a government ofa city, county, town, school district or special district with apopulation ofless than 50,000; and (3) a small organization that is any not-for-profit enterprise which is independently owned and operated andis not dominant in its field.

After considering the economic impacts ofthis final rule on small entities, Icertify that this action willnot have a significant economic impact on

a substantial number ofsmall entities. This final rulewill not impose any requirements on small entities.-Rather, this rule establishes national standards for allowable concentrations of03 in ambient air as required by section'109 ofthe 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. UnfundedMandatesReformAct
Title Hofthe Unfunded Mandates Reform Act of 1995 (UMvRA), Publir. 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 ofthe 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 tothe private sector, of $100 million or more in any one year. Before promulgating an EPA rule for which awritten statement is needed, section 205 oftheUMRA generally requires EPA to identify and consider a reasonable number ofregulatory alternatives and to adopt the least costly, most cost-effective or least burdensome alternative that.achieves the objectives ofthe rule. The provisions ofsection 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 ifthe 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 ofthe UMRA a small government agency plan.The planmust provide for notifying potentially affected small governments, enabling. officials of affected small governments
to have meaningful and timely input in the development ofEPAregulatory proposals with significant Federal intergovernmental mandates, and informing, educating, and advising small governments on compliance with theregulatory requirements.

This final rule contains no Federal mandates (underthe regulatory provisions ofTitle II ofthe 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 maybe considered to a degree in the development of State 

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plans to implementthe standards. See also American TruckingAss'nsv. EPA, 175 F. 3d at 1043 (notingthatbecause EPA is precluded from considering costs ofimplementation in establishing NAAQS, preparation ofa 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
tothe requirements of sections 202 and 205 ofthe UMRA. EPA has determined that this rule contains no regulatory requirements that might significantly or uniquely affect small governments.

E. Executive Order13132:Federalisnr

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 haye federalism implications" is defined in theExecutive Order to include regulations thathave "substantial direct effects onthe States, on the relationship between the national government and the States, or on the distribution of
power and responsibilities amongthe various levels ofgovernment."

This final rule does not have federalism implications. Itwill not have substantial direct effects on the States,

on the relationship between the national government and the States, or on the distribution ofpower 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 ofair quality improvement programs as codified in the CAA. Under section 109 ofthe CAA, EPA is mandated to establish NAAQS; however, CAA section 116 preserves the rights ofStates to establish more stringent requirements ifdeemed
necessary by a State. Furthermore, this rule doesnot impact CAA section.107 which establishes thatthe States have primary responsibility for implementation oftheNAAQS. Finally,

asnoted 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. ExecutiveOrder13175:Consultation andCoordinationWith 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 andtimely 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 ofthis rule. EPA staffparticipated in the regularly scheduled Tribal Air call sponsored by the National TribalAir Association during the spring of2007 asthe proposal was under development. EPA specifically solicited additional
comment on the proposed rule from Tribal officials. Comments from Tribal officials onthe proposed rule are summarized in the Response to Comments document.

G. Executive Order13045:Protectionof ChildrenFromEnvironmentalHealth& SafetyRisks

Executive Order 13045, "Protection of Children fromEnvironmental 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 onchildren, 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 byExecutive Order 12866, and we believe thatthe environmental health risk addressed by this action may have a disproportionate effect on children. Accordingly, we have evaluated the environmental health or safety effects ofexposure to 03 pollution
among children. These effects andthe size ofthe population affected are summarized in section 8.7 ofthe Criteria Document and section 3.6 ofthe StaffPaper, andthe results of our evaluation ofthe effects of03 pollution

on children qre discussed in sections II.A-C ofthis preamble.

H.Executive Order13211:Actions That Significantly Affect EnergySupply, Distribution orUse
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 ofEnergy Effects to the Administrator of the Office of Information and Regulatory Affairs, Office ofManagement and Budget, for certain actions identified as "significant
energy actions." Section4(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 ofinquiry, advance notices of proposed rulemaking, andnotices ofproposed rulemaking: (1)(i) That is a significant regulatory action under Executive Order 12866 orany 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 ofthe Office of Information and Regulatory Affairs as a significant energy action." The U.S. Office ofManagement and Budget has designated this rulemaking as a significant energy action. Accordingly, EPA has prepared aStatement ofEnergy Effects for this action which appears in Chapter 9 ofthe RIA conducted for this rulemaking. A copy ofthe RIA is available in the RIA docket (EPA-HQOAR-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; onenergy prices; on control technologies adopted bythe electricity generating sector; and

onthe mix ofelectricity 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 forthe primary standard level of 0.075 ppm which was selected bythe Administrator. However, due to modeling limitations, EPA did not generate separate estimates ofthe 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 tonote that the CAA make clear that the economic impacts associated with attaining ambient standards are not to be considered in setting orrevising the NAAQS. Accordingly, although the Statement of Energy Effects has been prepared, the results ofEPA's energy analysis have
notbeen considered in issuingthis final rule.

LNationalTechnology Transferand AdvancementAct

As noted inthe proposed rule, section 12(d) ofthe National Technology Transfer and Advancement Act of 1995 (NTTAA), Public Law 104-113, section 12(d) (15 U.S.C. 272 note) directs EPA
touse 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
notto use available and applicable voluntary consensus standards.

This action does not involve technical standards. Therefore, EPA did not consider the use ofany voluntary

consensus standards.

1.Executive Order12898:Federal Actions ToAddressEnvironmental Justicein MinorityPopulations and Low-IncomePopulations
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 oftheir mission by-identifying and addressing,

as appropriate, disproportionately high and adverse human health or environmental effects oftheir 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 onminority or low-income populations because it increases the level ofenvironmental 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 03 air pollution.

K. Congressional ReviewAct The Congressional Review Act, 5 U.S.C. 801 etseq., as added by the Small Business Regulatory Enforcement Fairness Act of1996, generally provides that before a rule may take effect, the
agency promulgating the rulemust submit arule report, which includes a copy ofthe rule,to each House ofthe Congress and to the Comptroller General ofthe United States. EPA submitted a report containing thisrule and other required information to theU.S. Senate, the U.S. House ofRepresentatives, and the Comptroller General ofthe United States prior to.publication ofthe rule in the Federal Register. A majorrule

cannot take effect until 60 days after it ispublished in the Federal Register. This action is a "major rule" as defined by 5 U.S.C. 804(2). This rule willbe effective May 27, 2008.
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Transportation Officials (AASHTO) (2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4325. October 9, 2007. American Chemistry Council (ACC) (2007)

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Quality (2007) Letter and Comments Sent
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function at 0.06 ppm in healthy adults. June 14, 2007. Memo to the Ozone NAAQS Review Docket. EPA-HQ-OAR-2005- 0172-0175. Available online at: http:// www.epa.gov/ttn/naaqs/standards/ozone/ so3_cr td.html. Burns, R. M., Honkala, B. H., tech. coords.

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evaluation ofthe 2004 release ofModels3 CMAQ, Atmos. Environ. 40: 4811-4824. Special issue on Model Eialuation: Evaluation of Urban and Regional Eulerian Air Quality Models. Engine Manufacturers Association (EMA)

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Air Quality Criteria for Ozone and Related Photochemical Oxidants (SecondExternal Review Draft) Washington, DC: National Center for Environmental Assessment; EPA report no. EPAI600/R-05/004aB--cB. Available online at: http://cfpub.epa.gov/ ncea/cfmlrecordisplay.cfm?deid=137307. Environmental Protection Agency (2005c)

Review ofthe national ambient air quality standards for ozone: assessment of scientific and technical information. OAQPS staffpaper (First Draft). Research Triangle Park,NC: Office ofAir Quality Planning and Standards; EPA report no. EPA-452/D-05-002. Available online at: http://epa.gov/ttn/naaqslstondards/ozone/ so3_crsp.html. Environmental Protection Agency (2006a) Air

Quality Criteria for Ozone and Related Photochemical Oxidants. (Final) Washington, DC: National Center for Environmental Assessment; EPA report no. EPA/600/R-05/OO4aB-cB. Available online
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Review ofthe national ambient air quality standards for ozone: assessment of scientific andtechnical information. OAQPS staffpaper. (Second Draft). ResearchTriangle Park, NC: Office ofAir Quality Planning and Standards; EPA report no. EPA-452/D1-05-002. Available online at: http://epa.gov/ttn/naaqs/ standards/ozone/so3_.crsp.html. Environmental Protection Agency (2007a)

Review ofthe national ambient air quality standards for ozone: assessment of scientific and technical information. OAQPS staffpaper. (Final) January 2007. Research Triangle Park,NC: Office ofAir Quality Planning and Standards; EPA report no. EPA-452/R-07-003. Available online at: http://epa.gov/ttn/naaqsl standardsozone/s o3_-cr._sp.html. Environmental Protection Agency (2007b)

Review of the national ambient air quality standards for ozone: assessment of scientific and technical information. OAQPS staffpaper. (Updated Final) July 2007. Research Triangle Park,NC: Office of Air Quality Planning and Standards; EPA report no. EPA-452/R-07-007. Available online at: http://epa.gov/ttn/naaqs/ standards/ozone/so3_.crsp.html. Environmental Protection Agency (2007c)

Ozone Population Exposure Analysis for Selected Urban Areas. (Updated Final)July 2007. Research Triangle Park, NC: Office of Air Quality Planning and Standards; EPA report no. EPA-452/R-07-010. Available online at: http://www.epa.gov/ttn/naaqsl standardslozone/s-o3crtd.html. Exxon Mobil Corporation (2007) Letter and

Comments Sent to Docket No. OAR-2005- 0172 re:'Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4163. October 9, 2007. 

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Fiore, A. M.; Jacob, D.J.; Bey, I.; Yantosca, R. M.; Field, B. D.; Fusco, A. C.; Wilkinson, J. G.(2002) Background ozone ovef the

. United States insummer: origin, trend, and contribution to pollution episodes. J. Geophys. Res. (Atmos.) 107(D15): 10.1029/ 2001JD000982. Fiore, A. M.; Jacob, D. J.; Liu, H.; Yantosca,

R. M.; Fairlie, T. D.; Fusco, A. C.; Li, Q. (2003) Variability in surface ozone background over the United States: implications for Air Quality Policy.J. of Geophysical Research, 108(D24)19-1-19- 12. Geflt, J. F.; Triche, E. W.; Holford, T. R.;

Belanger, K.; Bracken, M. B.; Beckett, W. S.; Leaderer, B. P. (2003) Association of low-level ozone and fine particles with respiratory symptoms in children with asthma. JAMA J. Am. Med. Assoc. 290: 1859-1867. Georgia Department of Natural Resources,

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Jaegle, L.; Horowitz, L.; Cooper, 0.; Hudman, R.; Jacob, D; Oltmans, S; Clarke, A. (2004) Impact ofAsian emissions on observations atTrinidad Head, California, during ITCT 2K2. J. ofGeophysical Research, 109(D23S17), doi: 10.1029/ 2003JD004406. Gregg, J. W.; Jones, C. G.; Dawson, T. E.

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for a longterm cumulative secondary

ozone standard--an ecological perspective. EM Qanuary): 23-33. Henderson, R. (2006a)Letter from CASAC

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Abdul-Salaan, S.; McDonnell, W. F. (1990) Ozone concentration and pulmonary response relationships for 6.6-hr exposures with five hours ofmoderate exercise to 0.08, 0.10, and 0.12 ppm. Am. Rev. Respir. Dis. 142: 1158-1163. Huang, Y.; Dominici, F.; Bell,M. L. (2005)

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Management (2007) Letter and Comments Sentto StephenJohnson, Administrator re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4777. October 9, 2007. Iowa Department of Natural Resources (2007)

Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4078. October 1,2007. Isebrands, J. G.; Dickson, R.E.; Rebbeck, J.;

Karnosky, D. F.'(2000) Interacting effects of multiple stresses on growth and physiological processes innorthern forest
trees. In: Mickler, R. A.; Birsdey,R. A.; Hom, J., eds.Responses ofnorthern U.S. forests to environmental change. New. York, NY: Springer-Verlag; pp. 149-180. (Ecologicalstudies: v. 139). Isebrands, J. G.; McDonald, E. P.; Kruger, E.;

Hendrey, G.; Percy, K.; Pregitzer, K.; Sober, J.; Karnosky, D. F. (2001) Growth responses ofPopulus tremuloides clones to interacting carbon dioxide and tropospheric ozone. Environ. Pollut. 115: 359-371. Ito, K.; De Leon, S. F.; Lippmann, M. (2005)

Associations between ozone and daily mortality, analysis and meta-analysis. Epidemiology 16: 446-457. Karnosky, D.F., Pregitzer, K.S., Zak, D.R.,

Kubiske, M.E., Hendrey, G.R., Weinstein, D., Nosal, M. &Percy, K.E. (2005) Scaling

ozone responses offorest trees to the ecosystem level in a changing climate. Plant Cell Environ. 28, 965-981. Kentucky Environmental and Public

Protection Cabinet (20071" Letter and Comments Sent to Docket No. OAR-2005- 0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4186. October 9, 2007. King, J.S., M. E. Kubiske, K. S. Pregitzer, G.

R. Hendrey, E. P. McDonald, C. P. Giardina, V. S. Quinn, D. F. Karnosky. (2005) Tropospheric 03 compromises het primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2. New Phytologist. 168:623-636. Koutrakis, P.; Suh, H.H.; Sarnat, J. A.; Brown,

K. W.; Coull, B.A; Schwartz, J. (2005) Characterization ofparticulate andgas exposures of sensitive subpopulations living in Baltimore andBoston. HEI Research Report 131. Langstaff, J.(2007) Analysis ofUnceftainty in

Ozone Population Exposure Modeling. January 31, 2007. Memo to the Ozone NAAQS Review Docket. EPA-HQ-OAR2005-0172-0174. Available online at: http://www.epa.gov/ttnlnaaqs/standards/ ozone/s o3_crtd.html. Laurence, J.A., Kohut, R.J., Amundson, R.G.,

(1993). Use ofTREGRO to simulate the effects ofozone on the growth ofred spruce seedlings. Forest Science. 39: 453- 464. Laurence, J.A.; Retzlaff, W. A.; Kern, J.S.;

Lee, E. H.; Hogsett, W.E.; Weinstein, D. A. (2001) Predicting the regional impact of

ozone and precipitation on the growth of loblolly pine and yellow poplar using linked TREGRO and ZELG models.For. Ecol. Manage. 146: 247-263. Lefohn, A.S.; Runeckles, V.C.; Krupa, S.V.;

Shadwick, D.S. (1989) Important considerations for establishing a secondary

ozone standard to protect vegetation. JAPCA 39, pp. 1039-1045. Levy, J. L; Chemerynski, S. M.; Sarnat,J.A.

(2005) Ozone exposure and mortality, an empiric Bayes metaregression analysis. Epidemiology 16: 458-468. Lipfert, F. W.; Perry, H. M., Jr.; Miller, J. P.;

Baty,J. D.; Wyzga, R.E.; Carmody, S. E. (2000) The Washington University EPRI veterans' cohort mortality study: preliminary results. In: Grant, L. D., ed. PM2000: particulate matter and health. Inhalation Toxicol. 12(suppl. 4): 41-73. Louisiana Department ofEnvironmental

Quality (2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4096. October 3, 2007. McCluney, L. (2007) Ozone 1-Hour to 8-Hour

Ratios forthe 2002-2004 Design Value Period.January 18, 2007. Memo to the Ozone NAAQS Review Docket. EPA-HQOAR-0172-0073. McDonnell, W.F.; Kehrl, H.R.; AbdulSalaam, S.; Ives, P.J.; Folinsbee, L.J.; Devlin, R.B.; O'Neil, J.J.; Horstman, D. H. (1991) Respiratory response ofhumans exposed to low levels ofozone for 6.6 hours. Arch. Environ. Health 46: 145-150. Marty, M. (2007a) Letter from CHPAC Chair

to the Administrator. March 23. EPA-HQOAR-2005-0172-0105. Marty, M. (2007b)Letter from CHPAC Chair

to the Administrator. September 4. EPAHQ-OAR-2005-0172-2031. McLaughlin, S.B., Nosal, M., Wullschleger,

S.D., Sun, G. (2007a) Interactive effects of

ozone and climate on treegrowth and water use in a southern Appalachian forest in theUSA. New Phytologist 174:109-124. McLaughlin, S.B., Wullschleger, S.D., Sun,G.

and Nosal, M. (2007b) Interactive effects of

ozone and climate on water use, soil moisture content and streamflow in a 

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southern Appalachian forest in the USA. New Phytologist 174: 125-136. Mortimer, K. M.; Neas, L. M.; Dockery, D. W.;

Redline, S.; Tager, I. B. (2002) The effect ofair pollution on inner-city children with asthma.Eur. Respir. J. 19: 699-705. Musselman, R.C.; Lefohn, A.S.; Massman,

W.J.; Heath, R.L. (2006) A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects. Atmos. Environ. 40:1869-1888. National Association ofClean Air Agencies

(NACAA)(2007) Letter and Comments Sentto Docket No. OAR-2005-0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. DocketNo. OAR-2005-0172-4274. October 9, 2007. National Association ofRegional Councils

(2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4504. October 9, 2007. National Association ofManufacturers

(NAM) (2007) Letter and Comments Sentto Molly A. O'Neill (Assistant Administrator, Office ofEnvironmental Information and ChiefInformation Officer) re: Proposed Rule-National Ambient Air Quality Standards for Ozone. DocketNo. OAR2005-0172-4275. October 9, 2007. National Park Service (NPS) Letter and

Comments Sentto Docket No. OAR-2005- 0172 re: Proposed Rule-National -Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4934. September 27, 2007. National Tribal Air Association (NTAA)

(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- 4169. October 9, 2007. Nevada Department ofConservation &

Natural Resources (2007)Letter and Comments Sent to Docket No. OAR-2005- 0172re: Proposed Rule--National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4173. October 9, 2007. New Jersey Clean Air Council (2007) Letter

and Comments Sent to Docket No. OAR2005-0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4119. October 9, 2007. New York StateDepartment of

Environmental Conservation (2007) Letter and Comments Sent to Docket No. OAR2005-0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4099. October 9, 2007. New York State Department of

Transportation (2007)Letter and Comments Sent to Docket No. OAR-2005- 0172 re: Proposed Rule--National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4865. October 9, 2007. New Mexico Environment Department Air

Quality Bureau (2007) Letter and Comments Sent to Docket No. OAR-2005- 0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4195. October 5, 2007. North Carolina Department ofEnvironment

and Natural Resources (NCDENR) (2007)
Letter Sent to Stephen L. Johnson re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4497. October 9, 2007. Northeast States for Coordinated Air Use

Management (NESCAUM) (2007) Letter and Comments Sent to Stephen Johnson, Administrator re: Proposed Rule.-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4165. October 4, 2007. Oklahoma Department ofTransportation

(2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4365. October 5, 2007. Oregon Department ofEnvironmental Quality

(2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4014. September 20, 2007. Ozone Transport Commission (OTC) (2007)

Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4135. October 5, 2007. Pennsylvania Department ofEnvironmental

Protection (2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4130. October 9 2007. Percy, K. E.; Nosal, M.; Heilman, W.; Dann,

T; Sober, J.; Legge, A. H.; Kamosky, D. F. (2007) New exposure-based metric approach for evaluating 03 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 ofa 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 ofa 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/standords/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.govlttn/naaqsl standards/ozone/s_o3_cr td.html. Rochester Report (2007) Critical

Considerations in Evaluating Scientific Evidence ofHealth Effects ofAmbient Ozone: Report ofa 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. OAR2005-0172-4727. October 9 2007. Rocky Mountain Clean Air Action (2007)

Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4428. October 4 2007. Sarnat, J. A.; Schwartz, J.; Catalano, P. J.; Suh,

H. H. (2001) Gaseous pollutants in particulate matter epidemiology: confounders orsurrogates? Environ. Health Perspect. 109: 1053-1061. Sarnat, J. A.; Brown, K. W.; Schwartz, J.;

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.; Sub, 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.

A.; Bevington, S. R.; Kress, L. W. (1991) Carryover effects ofacid iain and ozone on the physiology ofmultiple flushes of loblolly pine seedlings. For. Sci. 37: 1078- 1098. Schwartz, J. (2005) How sensitive is the

association between ozone and daily 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. G. (2006) Ambient air pollution and asthma exacerbations in children: an eight city analysis. Am. J. Epidemiol. 164(5):505-517. Sitch, S.; Cox, P.M.; Collins, W. J.;

Huntingford, C. (2007) Indirect radiative forcing of climate change through ozone effects on the land-carbon sink.Nature (London, U.K.) 448: 791-794. Taylor, C.R. "AGSIM: Model Description and

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stochastic evaluation ofthe aggregate economic effects ofprice support prograims" Agricultural Systems 44: 461- 473. Temple, P. J.; Riechers, G. H.; Miller, P. R.;

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ozone, wet and dry acidic deposition, and drought. Can.J. For. Res. 23: 59-66. Texas Commission on Environmental Quality

(2007) Letter and Comments Sentto Docket No. OAR-2005-0172 re: Proposed Rule--- National Ambient Air Quality Standards for Ozone.Docket No. OAR-2005-0172- 4435. October 9, 2007. TexasDepartment ofTransportation (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- 4409. October 9, 2007. Tingey, D. T.; Laurence, J. A.; Weber, J. A.;

Greene, J.; Hogsett, W. E.; Brown, S.; Lee, E. H. (2001) Elevated CO2 and temperature alter theresponse of Pinus ponderosa to

ozone: A simulation analysis. Ecol. Appl.11: 1412-1424. U.S. Department ofAgriculture, 2006. The

PLANTS Database (http://plants.usda.gov,

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December 2006). National Plant Data Center, Baton Rouge, LA. Utah Department ofEnvironmental Quality,

Division ofAir Quality (2007) Letter and Comments Sent toDocket No. OAR-2005- 0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4138. October 9, 2007. Utility Air Regulatory Group (2007) Letter

and Comments Sent to Docket No. OAR2005-0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4183. October 9, 2007. Union ofConcerned Scientists (UCS) (2007)

Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4768. October 9, 2007. Vedal, S.; Brauer, M.; White, R.; Petkau, J.

(2003) Air pollution and daily mortality in

a city with low levels ofpollution. Environ. Health Perspect. 111: 45-51. Washington State Department of

Transportation (2007) Letter and Comments Sent to Docket No. OAR-2005- 0172 re: Proposed Rule-National Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172-4157. October 8, 2007. Washington State Department ofEcology

(2007) Letter and Comments Sent to Docket No. OAR-2005-0172 re: Proposed RuleNational AmbientAir Quality Standards for Ozone. DocketNo. OAR-2005-0172- 4267. October 9, 2007. Weinstein, D.A., Beloin, R.M., R.D. Yanai

(1991) "Modeling changes in red spruce carbon balance and allocation in response
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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 ofponderosa pine and white fir. Forest Ecology and Management 205: 73-89. Wisconsin Department ofNatural Resources

(2007) Letter and Comments Sentto Docket No. OAR-2005-0172 re: Proposed RuleNational Ambient Air Quality Standards for Ozone. Docket No. OAR-2005-0172- 4358. October 9, 2007. Whitfield, R.; Biller, W.; Jusko, M.; and

Keisler, J. (1996) A Probabilistic Assessment ofHealth Risks Associated with Short-and Long-Term Exposure to Tropospheric Ozone. Argonne National Laboratory, Argonne, IL. Whitfield, R. (1997) A Probabilistic

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Clean Air Scientific Advisory Committee to theEPA Administrator, dated November 30, 1995. EPA-SA.B-CASAC-LTR-96-002. Wolff, G.T. (1996) Letter from Chairman of

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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. Available online at: http:// yosemite.epa.govlsab/sabproduct.nsfl C3F89E598D843B58852570CA007571 7E/ $File/epecO2009a.pdf.

List ofSubjects

40 CFR Part 50

Environmental protection, Air pollution control, Carbon monoxide, Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

40 CFRPart58

Environmental protection, Air pollution control, Reporting and recordkeeping requirements.

Dated: March 12, 2008. Stephen L. Johnson,
Administrator.

mFor the reasoni stated in the preamble, title 40, chapter I ofthe code ofFederal regulations is to be amended as follows:

PART 50-NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY

STANDARDS

a 1. The authority citation forpart 50 continues to read as follows:

Authority: 42 U.S.C. 7401, etseq.
* 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 ofthe national 8-hour primary and secondary ambient air quality standards for ozone (03) is 0.075 parts permillion (ppm), daily maximum 8-hour average, measured by areference method based onAppendix D to this part and designated in accordance with part 53 ofthis chapter or an equivalent method designated in accordance with part 53 ofthis chapter.

(b) The 8-hour primary and secondary 03 ambient air quality standards are met
at an ambient air quality monitoring site when the 3-year average ofthe annual fourth-highest daily maximum 8-hour
average 03 concentration is less than or equal to 0.075 ppm, as determined in accordance with Appendix P to this part.

m 3. Appendix Pis added to read as follows: Appendix P to Part 50-Interpretation ofthe 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 (03) specified in § 50.15 are met at an ambient 03 air quality monitoring site. Ozone is measured in the ambient air by a reference method based on Appendix D ofthis part, as applicable, and designated in accordance with part 53 ofthis chapter, orby an equivalent method designated in accordance with part 53 ofthischapter. Data reporting, data handling, and computation procedures
to be used in making comparisons between reported 03 concentrations and the levels of the 03 standards are specified in the following sections. Whether to exclude, retain, or make adjustments tothe data affectedby exceptional events, including stratospheric 03 intrusion and other natural events, is determined by the requirements under §§50.1, 50.14 and 51.930.

(b)The terms used inthis appendix are defined as follows:

8-houraverageis therolling average of eight hourly 03 concentrations as explained in section 2of this appendix.

Annual fourth-highestdailymaximum refers to the fourth highest value measured at

a monitoring site during a particular year.

Dailymaximum 8-houraverage concentrationrefers to the maximum calculated 8-hour average for a particular day

as explained in section 2 of this appendix.

Designvaluesare the metrics (i.e., statistics) that are compared to the NAAQS levels to determine compliance, calculated as shown in section 3 ofthis appendix.

03 monitoringseasonrefers to the span of timewithin a calendar year when individual States are required to measure ambient 03 concentrations as listed in part 58 Appendix D to this chapter.

Yearrefers to calendar year. 2. Primary and Secondary Ambient Air Quality Standards for Ozone

2.1 Data Reporting and Handling Conventions

Computing 8-houraverages.Hourly average concentrations shall be reported in parts per million (ppm)to the third decimal place, with additional digits to the right of thethird decimal place truncated. Running 8- hour averages shall be computed from the hourly 03 concentration data for each hour ofthe year and shall be stored inthe first,

or start, hour of the 8-hour period. An 8-hour
average shall be considered valid if at least 75% ofthehourly 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 shallbe considered valid also, if,after substituting one-halfthe minimum detectable limit for themissing hourly concentrations, the 8-hour average concentration is greater 16511

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than the level of the standard. The computed 8-hour average 03 concentrationsshall be reported to three decimal places (the digits to the right of thethird decimal place are truncated,consistent with the data handling procedures for the reported data).

Dailymaximum 8-houraverage concentrations. (a) There are 24possible running 8-hour average 03 concentrations for eachcalendar day during the 03 monitoring

season. The daily maximum 8-hour concentration for a given calendar day is the highest ofthe 24 possible 8-houraverage concentrations computed for that day. This process is repeated, yieldinga daily

. maximum 8-hour average 03 concentration for each calendarday with ambient 03 monitoring data. Because the 8-houraverages
are recorded inthe 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 hourlyconcentrations.

(b) An 03 monitoring day shall be counted

as a valid day if valid 8-hour averages are available for at least 75% ofpossible hours in the day (i.e., at least 18 ofthe 24 averages). In the event that lessthan 75% ofthe 8-hour averages are available, a day shall also be counted asa valid day if the daily maximum 8-hour average concentration for that day is greater thanthe levelofthe standard.
2.2 Primary and Secondary Standardrelated Summary Statistic

The standard-related summary statistic is theannual fourth-highest daily maximum 8- hour 03 concentration, expressed in parts per million, averaged over three years. The 3-year average shall becomputed usingthe 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 03 concentrations shallbe reported to three decimalplaces (the digitsto the right ofthe third decimal placeare 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 03 ambient air quality standards are met at an ambient air quality monitoring sitewhen the 3-year average ofthe annual fourth-highest daily maximum 8-hour average 03 concentration is less than or equal to 0.075 ppm.

(b) This comparison shall be based onthree consecutive, complete calendar years of air quality monitoring data. This requirement is met for the 3-year period ata monitoring site if daily maximum 8-hour average concentrations are availablefor at least 90% ofthe dayswithin the03 monitoring season,
onaverage, for the 3-year period, with a minimum data completeness requirement in any one yearofat least 75% ofthe days within the 03 monitoring season. When computing whether the minimum data completeness requirements have been met, meteorological orambient data may be sufficientto demonstrate that meteorological conditions on missing days were not conduciveto concentrations above the level ofthe standard.Missing days assumed less then.the level ofthe standard are counted for the purpose ofmeeting the data completeness requirement, subjectto the approval of the appropriate Regional Administrator.

(c) Yearswith concentrations greaterthan the levelofthe standard shall be included

even if they have lessthan 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 ofthe standard.

(d) Comparisons with the primary and secondary 03 standards are demonstrated by examples 1 and 2in paragraphs (d)(1) and (d)(2) respectively asfollows:

EXAMPLE 1.-AMBIENT MONITORING SITE ATTAINING THE PRIMARY AND SECONDARY 03,STANDARDS

Percent valid days (within 1st Highest 2nd Highest 3rd Highest 4th Highest 5th Highest

Year the required daily max 8- daily max 8- daily max 8- daily max 8- daily max 8- monitoring hour Conc. hour Conc. hour Conc. hour Conc. hour Conc. season) (ppm) (ppm) (ppm) (ppm) (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............................................0075............

(1)As shown in Example 1,this ppm) is lessthan or equal to 0.075 ppm. The Icompleteness. In Example 1,the individual monitoring site meets the primary and datacompleteness reguirement-is also met 8-hour averages used to determinethe annual secondary 03 standards because the 3-year becausethe average percent of dayswithin fourth maximum have alsobeen truncated to average ofthe annual fourth-highest daily the required monitoring seasonwith valid thethird decimal place. maximum 8-hour average 03 concentrations ambient monitoring data is greater than 90%, (i.e., 0.075666 * * * ppm, truncated to 0.075 and no single year has lessthan 75% data

EXAMPLE 2.-AMBIENT MONITORING SITE FAILING TO MEET THE PRIMARY AND SECONDARY 03 STANDARDS

Percent valid 1st Highest 2nd Highest 3rd Highest 4th Highest 5th Highest days (within 1tHget 2i ihs r ihs t ihs t ihs Year the required daily max 8- daily max 8- daily max 8- daily max 8- daily max 8- monitoring hour Conc. hour Conc. hour Conc. hour Conc. hour Conc.

season) (ppm) (ppm) (ppm) (ppm) (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

.............................. ..................... 0.096

............

As shown in Example 2, the primary and secondary 03 standards are not met forthis monitoring sitebecause the 3-yearaverage of thefourth-highest daily maximum 8-hour average 03 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 3years is less than90% within the required monitoring season. In Example 2,the individual 8-hour averages used to determine the annual fourth maximum havealso 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 ofthe standard ensures that the site meets the standard. Fora 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 03 concentration is also the air quality design value for the site.

PART 58-AMBIENT AIR QUALITY SURVEILLANCE

* 4. The authority citation ofpart 58 continues to read as follows:

Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.

a S. Appendix Gto Part 58 isamended

as follows:
" a. By revising section 9.
" b. Byrevising section 10.

mc. 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 theAQI Relate to Air Pollution Levels?

For each pollutant, theAQI transforms ambient concentrations to a scale from 0 to 500. The AQI iskeyed as appropriate to the national ambient air quality standards (NAAQS) for each pollutant. Inmost cases, the index value of 100 is associated with the .numerical level ofthe short-term standard (i.e., averaging time of24-hours orless) for each pollutant. A different approach is taken for NO2, for which no short-term standard has been established. The index value of50 is associated with the numerical level ofthe annual standard for a pollutant, ifthere is

one, at one-halfthe level of the short-term standard for the pollutant, or atthe level at which it is appropriate to begin to provide guidance on cautionary language. Higher categories of theindex are based on increasingly serious health effects and increasing proportions ofthe population that

are likely to be affected. Theindex isrelated
to other air pollution concentrations through linear interpolation based on these levels. The AQI is equal to thehighest of the numbers corresponding to each pollutant. Forthe purposes ofreporting the AQI, the sub-indexes for PM10and 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 forCalculating 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. ForPM, calculate and report theAQI 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 mayuse PM
measurements from monitors that are not reference or equivalent methods (for example, continuous PM,o or PM2.5 monitors). Detailed guidance for relating nonapproved measurements to approved methods by statistical linear regression is referenced in section 13below.

12. How Do ICalculate the AQI?

i. TheAQI is thehighest value calculated for each pollutant as follows:

a.Identify the highestconcentration among all of the monitors within eachreporting area and truncate the pollutant concentration to

one more than the significant digits used to express the level ofthe NAAQS for that pollutant. Thisis equivalent to the rounding conventions used in theNAAQS.

b. Using Table 2, find thetwo 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

03 (ppm) 03 (ppm) PM 2. 5 PM10 Sp 8-hour 1-hour1 (uIg/m 3) (jig/m 3) CO (ppm) S 2 (PPM) N 2 (PPM) AQl 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

.... 0.165-0.204 465.5-150.4 255-354 12.5-15.4 0.225-0.304 (3) 151-200 Unhealthy. 0.116-0.374

.... 0.205-0.404 4150.5-250.4 355-424 15&5-.30.4 0.305-0.604 0.65-1.24 201-300 Very Unhealthy. (2)

.................... 0.405-0.504 4250.5-350.4 425-504 30.5-40.4 0.605-0.804 1.25-1.64 301-400 (2)

.................... 0.505-0.604 4350.5-500.4 505-604- 40.5-50.4 0.805-1.004 1.65-2.04 401-500 Hazardous..

Areas are generally required to report the A0l based on 8-hour ozone values. However, there are a small number of areas where an AQl 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.

28-hour 03 values do not define higher AQI values (2- 301). AQI values of 301 or greater are calculated with 1 -hour 03 concentrations. 3N02 has no short-term NAAQS, and can generate an AQl only above the value of 200.. 4 Ifa different SHLfor PM 2.5 is promulgated, these numbers will change accordingly.

ii.Ifthe concentration is equal to a breakpoint, then the index is equal to the corresponding index value in Table 2. However, Equation 1can still be used. The results willbe equal. Ifthe concentration is between two breakpoints, then calculate the index of that pollutant with Equation 1.You must alsonote that in some areas, theAQI based on 1-hour 03 willbe more precautionary than using 8-hour values (see footnote 1 to Table 2). Inthese cases, you may use 1-hour values as well as 8-hour values to calculate-index values and then use the maximum index value asthe AQI for 03.

lp=IIE- ILO (C,

- BP, ) + 1,
°



### BP,

- 8P,

Ip = the index value for pollutantp Where: (Equation 1)

Cp = the truncated concentration of pollutantp 

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BPHi = 

### thebreakpoint that isgreater than or equal to C,
BP = thebreakpoint that is less than

or equal to C,

=Hi

the AQI value corresponding to BPi I,, = the AQI value corresponding to BPL. iii. Ifthe concentration is larger than the highest breakpoint inTable 2then you may

use the last two breakpoints inTable 2 when
you applyEquation 1. Example

iv. Using Table 2 and Equation 1, calculate
the index value for each of thepollutants measured and select the one that produces the highest index value for the AQI. For example, ifyou observe a PM1ovalue of 210 jtg/m 3

,a 1-hour 03 value of0.156 ppm, and

an 8-hour 03 value of0.130 ppm, then do this:

a. Find the breakpoints for PM10 at 210jgg/

m3as 155 gg/m 3 and 254 gg/m 3

, corresponding to index values 101 and 150; b. Find the breakpoints for 1-hour 03 at 0.156 ppm as 0.125 ppm and0.164 ppm, corresponding to index values 101 and 150;

c. Find thebreakpoints for 8-hour 03 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 gg/m 3

,PMo:

150

- 101 254

- 155 (210

- 155) + 101 = 128

e. Apply Equation 1for 0.156 ppm, 1-hour 03:

150-101 

### 06

- 10125 (0.156

- 0.125) + 101 = 140 0.164

- 0.125

'

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

300- 201 (0.130

- 0.116) +201 = 206

0.374

- 0.116

g. Find themaximum, 206. Thisis theAQI. The minimal AQ report would read:

v. Today, the AQI for mycity is 206 which is Very Unhealthy, due to ozone. Children and people with asthma are the groups most
at risk.

13. What-Additional Information ShouldI 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 indexvalue, 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 ofDaily 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 PMmeasurements to approved methods (Eberly, S., T.Fitz- .Simons, T. Hanley, L. Weinstock., T. Tamanini, G. Denniston, B. Lambeth, E. Michel, S. Bortnick. DataQuality 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. EPA454/B-02-002, November 2002) can be found

on the Ambient Monitoring Technology Information Center (AMTIC) Web site, http:// www.epa.gov/ttnamtil/.
[FRDoc. E8-5645 Filed 3-26-08; 8:45 am]

BILUNG CODE 6560-50-P 

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