[Federal Register: March 27, 2008 (Volume 73, Number 60)] [Rules and Regulations] [Page 16435-16514] From the Federal Register Online via GPO Access [wais.access.gpo.gov] [DOCID:fr27mr08-8] [[Page 16435]] ----------------------------------------------------------------------- Part II Environmental Protection Agency ----------------------------------------------------------------------- 40 CFR Parts 50 and 58 National Ambient Air Quality Standards for Ozone; Final Rule [[Page 16436]] ----------------------------------------------------------------------- ENVIRONMENTAL PROTECTION AGENCY 40 CFR Parts 50 and 58 [EPA-HQ-OAR-2005-0172; FRL-8544-3] RIN 2060-AN24 National Ambient Air Quality Standards for Ozone AGENCY: Environmental Protection Agency (EPA). ACTION: Final rule. ----------------------------------------------------------------------- SUMMARY: Based on its review of the air quality criteria for ozone (O3) and related photochemical oxidants and national ambient air quality standards (NAAQS) for O3, EPA is making revisions to the primary and secondary NAAQS for O3 to provide requisite protection of public health and welfare, respectively. With regard to the primary standard for O3, EPA is revising the level of the 8-hour standard to 0.075 parts per million (ppm), expressed to three decimal places. With regard to the secondary standard for O3, EPA is revising the current 8- hour standard by making it identical to the revised primary standard. EPA is also making conforming changes to the Air Quality Index (AQI) for O3, setting an AQI value of 100 equal to 0.075 ppm, 8- hour average, and making proportional changes to the AQI values of 50, 150 and 200. DATES: This final rule is effective on May 27, 2008. ADDRESSES: EPA has established a docket for this action under Docket ID No. EPA-HQ-OAR-2005-0172. All documents in the docket are listed on the www.regulations.gov Web site. Although listed in the index, some information is not publicly available, e.g., confidential business information or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is not placed on the Internet and will be publicly available only in hard copy form. Publicly available docket materials are available either electronically through www.regulations.gov or in hard copy at the Air and Radiation Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution Ave., NW., Washington, DC. This Docket Facility is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The Docket telephone number is 202-566-1742. The telephone number for the Public Reading Room is 202-566-1744. FOR FURTHER INFORMATION CONTACT: Dr. David J. McKee, Health and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail Code C504-06, Research Triangle Park, NC 27711; telephone: 919-541-5288; fax: 919- 541-0237; e-mail: mckee.dave@epa.gov. SUPPLEMENTARY INFORMATION: Table of Contents The following topics are discussed in this preamble: I. Background A. Summary of Revisions to the O3 NAAQS B. Legislative Requirements C. Review of Air Quality Criteria and Standards for O3 D. Summary of Proposed Revisions to the O3 NAAQS E. Organization and Approach to Final Decision on O3 NAAQS II. Rationale for Final Decision on the Primary O3 Standard A. Introduction 1. Overview 2. Overview of Health Effects 3. Overview of Human Exposure and Health Risk Assessments B. Need for Revision of the Current Primary O3 Standard 1. Introduction 2. Comments on the Need for Revision 3. Conclusions Regarding the Need for Revision C. Conclusions on the Elements of the Primary O3 Standard 1. Indicator 2. Averaging Time 3. Form 4. Level D. Final Decision on the Primary O3 Standard III. Communication of Public Health Information IV. Rationale for Final Decision on the Secondary O3 Standard A. Introduction 1. Overview 2. Overview of Vegetation Effects Evidence 3. Overview of Biologically Relevant Exposure Indices 4. Overview of Vegetation Exposure and Risk Assessments B. Need for Revision of the Current Secondary O3 Standard 1. Introduction 2. Comments on the Need for Revision 3. Conclusions Regarding the Need for Revision C. Conclusions on the Secondary O3 Standard 1. Staff Paper Evaluation 2. CASAC Views 3. Administrator's Proposed Conclusions 4. Comments on the Secondary Standard Options 5. Administrator's Final Conclusions D. Final Decision on the Secondary O3 Standard V. Creation of Appendix P--Interpretation of the NAAQS for O3 A. General B. Data Completeness C. Data Reporting and Handling and Rounding Conventions VI. Ambient Monitoring Related to Revised O3 Standards VII. Implementation and Related Control Requirements A. Future Implementation Steps 1. Designations 2. State Implementation Plans 3. Trans-boundary Emissions 4. Monitoring Requirements B. Related Control Requirements VIII. Statutory and Executive Order Reviews A. Executive Order 12866: Regulatory Planning and Review B. Paperwork Reduction Act C. Regulatory Flexibility Act D. Unfunded Mandates Reform Act E. Executive Order 13132: Federalism F. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments G. Executive Order 13045: Protection of Children From Environmental Health & Safety Risks H. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution or Use I. National Technology Transfer and Advancement Act J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations K. Congressional Review Act References I. Background A. Summary of Revisions to the O3 NAAQS Based on its review of the air quality criteria for O3 and related photochemical oxidants and national ambient air quality standards (NAAQS) for O3, EPA is making revisions to the primary and secondary NAAQS for O3 to provide protection of public health and welfare, respectively, that is appropriate under section 109, and is making corresponding revisions in data handling conventions for O3. With regard to the primary standard for O3, EPA is revising the level of the 8-hour standard to a level of 0.075 parts per million (ppm), to provide increased protection for children and other ``at risk'' populations against an array of O3-related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total nonaccidental and cardiorespiratory mortality. EPA is specifying the level of the primary standard to the nearest thousandth ppm. With regard to the secondary standard for O3, EPA is revising the standard by making it identical to the revised primary standard. [[Page 16437]] B. Legislative Requirements Two sections of the Clean Air Act (CAA) govern the establishment and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list ``air pollutants'' emissions of which ``in his judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare,'' whose ``presence * * * in the ambient air results from numerous or diverse mobile or stationary sources,'' and for which the Administrator plans to issue air quality criteria, and to issue air quality criteria for those that are listed. Air quality criteria are to ``accurately reflect the latest scientific knowledge useful in indicating the kind and extent of identifiable effects on public health or welfare which may be expected from the presence of [a] pollutant in ambient air, in varying quantities * * *.'' Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate ``primary'' and ``secondary'' NAAQS for pollutants listed under section 108. Section 109(b)(1) defines a primary standard as one ``the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health.'' \1\ A secondary standard, as defined in section 109(b)(2), must ``specify a level of air quality the attainment and maintenance of which in the judgment of the Administrator, based on such criteria, is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of [the] pollutant in the ambient air.'' \2\ --------------------------------------------------------------------------- \1\ The legislative history of section 109 indicates that a primary standard is to be set at ``the maximum permissible ambient air level * * * which will protect the health of any [sensitive] group of the population,'' and that for this purpose ``reference should be made to a representative sample of persons comprising the sensitive group rather than to a single person in such a group'' [S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970)]. \2\ Welfare effects as defined in section 302(h) (42 U.S.C. 7602(h)) include, but are not limited to, ``effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility and climate, damage to and deterioration of property, and hazards to transportation, as well as effects on economic values and on personal comfort and well-being.'' --------------------------------------------------------------------------- The requirement that primary standards provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting. It was also intended to provide a reasonable degree of protection against hazards that research has not yet identified. Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (DC Cir 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied, 455 U.S. 1034 (1982). Both kinds of uncertainties are components of the risk associated with pollution at levels below those at which human health effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary standards that provide an adequate margin of safety, the Administrator is seeking not only to prevent pollution levels that have been demonstrated to be harmful but also to prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. The CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels, see Lead Industries Association v. EPA, 647 F.2d at 1156 n. 51, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety. The selection of any particular approach to providing an adequate margin of safety is a policy choice left specifically to the Administrator's judgment. Lead Industries Association v. EPA, 647 F.2d at 1161-62. In addressing the requirement for an adequate margin of safety, EPA considers such factors as the nature and severity of the health effects involved, the size of the population(s) at risk, and the kind and degree of the uncertainties that must be addressed. In setting standards that are ``requisite'' to protect public health and welfare, as provided in section 109(b), EPA's task is to establish standards that are neither more nor less stringent than necessary for these purposes. Whitman v. America Trucking Associations, 531 U.S. 457, 473. Further the Supreme Court ruled that ``[t]he text of Sec. 109(b), interpreted in its statutory and historical context and with appreciation for its importance to the CAA as a whole, unambiguously bars cost considerations from the NAAQS-setting process * * *'' Id. at 472.\3\ --------------------------------------------------------------------------- \3\ In considering whether the CAA allowed for economic considerations to play a role in the promulgation of the NAAQS, the Supreme Court rejected arguments that because many more factors than air pollution might affect public health, EPA should consider compliance costs that produce health losses in setting the NAAQS. 531 U.S. at 466. Thus, EPA may not take into account possible public health impacts from the economic cost of implementation. Id. --------------------------------------------------------------------------- Section 109(d)(1) of the CAA requires that ``not later than December 31, 1980, and at 5-year intervals thereafter, the Administrator shall complete a thorough review of the criteria published under section 108 and the national ambient air quality standards * * * and shall make such revisions in such criteria and standards and promulgate such new standards as may be appropriate in accordance with section 108 and [109(b)].'' Section 109(d)(2) requires that an independent scientific review committee ``shall complete a review of the criteria * * * and the national primary and secondary ambient air quality standards * * * and shall recommend to the Administrator any new * * * standards and revisions of existing criteria and standards as may be appropriate under section 108 and [section 109(b)].'' This independent review function is performed by the Clean Air Scientific Advisory Committee (CASAC) of EPA's Science Advisory Board. C. Review of Air Quality Criteria and Standards for O3 Ground-level O3 is formed from biogenic and anthropogenic precursor emissions. Naturally occurring O3 in the troposphere can result from biogenic organic precursors reacting with naturally occurring nitrogen oxides (NOX) and by stratospheric O3 intrusion into the troposphere. Anthropogenic precursors of O3, specifically NOX and volatile organic compounds (VOC), originate from a wide variety of stationary and mobile sources. Ambient O3 concentrations produced by these emissions are directly affected by temperature, solar radiation, wind speed and other meteorological factors. The last review of the O3 NAAQS was completed on July 18, 1997, based on the 1996 O3 Air Quality Criteria Document (EPA, 1996a) and 1996 O3 Staff Paper (EPA, 1996b). EPA revised the primary and secondary O3 standards on the basis of the then latest scientific evidence linking exposures to ambient O3 to adverse health and welfare effects at levels allowed by the 1-hour average standards (62 FR 38856). The O3 standards were revised by replacing the existing primary 1-hour average standard with an 8-hour average O3 standard set at a level of 0.08 ppm, which is equivalent to 0.084 ppm using the standard rounding conventions. The form of the primary standard was changed to the annual fourth-highest daily maximum 8-hour average concentration, averaged over 3 years. The secondary O3 standard was changed by making it identical in all respects to the revised primary standard. EPA initiated this current review in September 2000 with a call for information (65 FR 57810) for the development of a revised Air Quality [[Page 16438]] Criteria Document for O3 and Other Photochemical Oxidants (henceforth the ``Criteria Document''). A project work plan (EPA, 2002) for the preparation of the Criteria Document was released in November 2002 for CASAC O3 Panel \4\ (henceforth, ``CASAC Panel'') and public review. EPA held a series of workshops in mid-2003 on several draft chapters of the Criteria Document to obtain broad input from the relevant scientific communities. These workshops helped to inform the preparation of the first draft Criteria Document (EPA, 2005a), which was released for CASAC Panel and public review on January 31, 2005; a CASAC Panel meeting was held on May 4-5, 2005 to review the first draft Criteria Document. A second draft Criteria Document (EPA, 2005b) was released for CASAC Panel and public review on August 31, 2005, and was discussed along with a first draft Staff Paper (EPA, 2005c) at a CASAC Panel meeting held on December 6-8, 2005. In a February 16, 2006 letter to the Administrator, the CASAC Panel offered final comments on all chapters of the Criteria Document (Henderson, 2006a), and the final Criteria Document (EPA, 2006a) was released on March 21, 2006. In a June 8, 2006 letter (Henderson, 2006b) to the Administrator, the CASAC Panel offered additional advice to the Agency concerning chapter 8 of the final Criteria Document (Integrative Synthesis) to help inform the second draft Staff Paper. --------------------------------------------------------------------------- \4\ The CASAC O3 Review Panel includes the seven members of the chartered CASAC, supplemented by fifteen subject- matter experts appointed by the Administrator to provide additional scientific expertise relevant to this review of the O3 NAAQS. --------------------------------------------------------------------------- A second draft Staff Paper (EPA, 2006b) was released on July 17, 2006 and reviewed by the CASAC Panel on August 24 and 25, 2006. In an October 24, 2006 letter to the Administrator, CASAC Panel provided advice and recommendations to the Agency concerning the second draft Staff Paper (Henderson, 2006c). A final Staff Paper (EPA, 2007a) was released on January 31, 2007. Around the time of the release of the final Staff Paper in January 2007, EPA discovered a small error in the exposure model that when corrected resulted in slight increases in the human exposure estimates. Since the exposure estimates are an input to the lung function portion of the health risk assessment, this correction also resulted in slight increases in the lung function risk estimates as well. The exposure and risk estimates discussed in this final rule reflect the corrected estimates, and thus are slightly different than the exposure and risk estimates cited in the January 31, 2007 Staff Paper.\5\ In a March 26, 2007 letter (Henderson, 2007), the CASAC Panel offered additional advice to the Administrator with regard to recommendations and revisions to the primary and secondary O3 NAAQS. --------------------------------------------------------------------------- \5\ EPA made available corrected versions of the final Staff Paper (EPA, 2007b, henceforth, ``Staff Paper'') and the human exposure and health risk assessment technical support documents on July 31, 2007 on the EPA Web site http://www.epa.gov/ttn/naaqs. --------------------------------------------------------------------------- The schedule for completion of this review has been governed by a consent decree resolving a lawsuit filed in March 2003 by a group of plaintiffs representing national environmental and public health organizations, alleging that EPA had failed to complete the current review within the period provided by statute.\6\ The modified consent decree that currently governs this review provides that EPA sign for publication notices of proposed and final rulemaking concerning its review of the O3 NAAQS no later than June 20, 2007 and March 12, 2008, respectively. The proposed decision (henceforth ``proposal'') was signed on June 20, 2007 and published in the Federal Register on July 11, 2007. --------------------------------------------------------------------------- \6\ American Lung Association v. Whitman (No. 1:03CV00778, D.D.C. 2003). --------------------------------------------------------------------------- A large number of comments were received from various commenters on the proposed revisions to the O3 NAAQS. Significant issues raised in the public comments are discussed throughout the preamble of this final action. A comprehensive summary of all significant comments, along with EPA's responses (henceforth ``Response to Comments''), can be found in the docket for this rulemaking. Various commenters have referred to and discussed a number of new scientific studies on the health effects of O3 that had been published recently and therefore were not included in the Criteria Document (EPA, 2006a, henceforth ``Criteria Document).\7\ EPA has provisionally considered any significant ``new'' studies, including those submitted during the public comment period. The purpose of this effort was to ensure that the Administrator was fully aware of the ``new'' science before making a final decision on whether to revise the current O3 NAAQS. EPA provisionally considered these studies to place their results in the context of the findings of the Criteria Document. --------------------------------------------------------------------------- \7\ For ease of reference, these studies will be referred to as ``new'' studies or ``new'' science, using quotation marks around the word new. Referring to studies that were published too recently to have been included in the 2004 Criteria Document as ``new'' studies is intended to clearly differentiate such studies from those that have been published since the last review and are included in the 2004 Criteria Document (these studies are sometimes referred to as new (without quotation marks) or more recent studies, to indicate that they were not included in the 1996 Criteria Document and thus are newly available in this review. --------------------------------------------------------------------------- As in prior NAAQS reviews, EPA is basing its decision in this review on studies and related information included in the Criteria Document and Staff Paper, which have undergone CASAC and public review. The studies assessed in the Criteria Document, and the integration of the scientific evidence presented in that document, have undergone extensive critical review by EPA, CASAC, and the public during the development of the Criteria Document. The rigor of that review makes these studies, and their integrative assessment, the most reliable source of scientific information on which to base decisions on the NAAQS, decisions that all parties recognize as of great import. NAAQS decisions can have profound impacts on public health and welfare, and NAAQS decisions should be based on studies that have been rigorously assessed in an integrative manner not only by EPA but also by the statutorily mandated independent advisory committee, as well as the public review that accompanies this process. As described above, EPA's provisional consideration of these studies did not and could not provide that kind of in-depth critical review. This decision is consistent with EPA's practice in prior NAAQS reviews. Since the 1970 amendments, the EPA has taken the view that NAAQS decisions are to be based on scientific studies and related information that have been assessed as a part of the pertinent air quality criteria, and has consistently followed this approach. See 71 FR 61144, 61148 (October 17, 2006) (final decision on review of PM NAAQS) for a detailed discussion of this issue and EPA's past practice. As discussed in EPA's 1993 decision not to revise the NAAQS for O3 ``new'' studies may sometimes be of such significance that it is appropriate to delay a decision on revision of a NAAQS and to supplement the pertinent air quality criteria so the studies can be taken into account (58 FR at 13013-13014, March 9, 1993). In the present case, EPA's provisional consideration of ``new'' studies concludes that, taken in context, the ``new'' information and findings do not materially change any of the broad scientific conclusions regarding the health effects of O3 exposure made in the Criteria Document. For this reason, reopening the air quality criteria review would not be warranted even if there were time to do so under the court order [[Page 16439]] governing the schedule for this rulemaking. Accordingly, EPA is basing the final decisions in this review on the studies and related information included in the O3 air quality criteria that have undergone CASAC and public review. EPA will consider the newly published studies for purposes of decision making in the next periodic review of the O3 NAAQS, which will provide the opportunity to fully assess them through a more rigorous review process involving EPA, CASAC, and the public. Further discussion of these ``new'' studies can be found in the Response to Comments document. This action presents the Administrator's final decisions on the review of the current primary and secondary O3 standards. Throughout this preamble a number of conclusions, findings, and determinations made by the Administrator are noted. They identify the reasoning that supports this final decision and are intended to be final and conclusive. D. Summary of Proposed Revisions to the O3 NAAQS For reasons discussed in the proposal, the Administrator proposed to revise the current primary and secondary O3 standards. With regard to the primary O3 standard, the Administrator proposed to revise the level of the 8-hour O3 standard to a level within the range of 0.070 ppm to 0.075 ppm, based on a 3-year average of the fourth-highest maximum 8-hour average concentration. Related revisions for O3 data handling conventions and for the reference method for monitoring O3 were also proposed. These revisions were proposed to provide increased protection for children and other ``at risk'' populations against an array of O3-related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity, including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity, as well as total nonaccidental and cardiorespiratory mortality. EPA also proposed to specify the level of the primary standard to the nearest thousandth ppm. EPA solicited comment on alternative levels down to 0.060 ppm and up to and including retaining the current 8-hour standard of 0.08 ppm (effectively 0.084 ppm using current data rounding conventions). With regard to the secondary standard for O3, EPA proposed to revise the current 8-hour standard with one of two options to provide increased protection against O3-related adverse impacts on vegetation and forested ecosystems. One option was to replace the current standard with a cumulative, seasonal standard expressed as an index of the annual sum of weighted hourly concentrations, cumulated over 12 hours per day (8 am to 8 pm) during the consecutive 3-month period within the O3 season with the maximum index value, set at a level within the range of 7 to 21 ppm- hours. The other option was to make the secondary standard identical to the proposed primary 8-hour standard. EPA solicited comment on specifying a cumulative, seasonal standard in terms of a 3-year average of the annual sums of weighted hourly concentrations; on the range of alternative 8-hour standard levels for which comment was being solicited for the primary standard, including retaining the current secondary standard, which is identical to the current primary standard; and on an alternative approach to setting a cumulative, seasonal secondary standard. E. Organization and Approach to Final O3 NAAQS Decisions This action presents the Administrator's final decisions regarding the need to revise the current primary and secondary O3 standards. Revisions to the primary standard for O3 are addressed below in section II, and a discussion on communication of public health information regarding revisions to the primary O3 standard is presented in section III. The secondary O3 standard is addressed below in section IV. Related data completeness and data handling and rounding conventions are addressed in section V, and federal reference methods for monitoring O3 are addressed below in section VI. Future implementation steps and related control requirements are discussed in section VII. A discussion of statutory and executive order reviews is provided in section VIII. Today's final decisions are based on a thorough review in the Criteria Document of scientific information on known and potential human health and welfare effects associated with exposure to O3 at levels typically found in the ambient air. These final decisions also take into account: (1) Staff assessments in the Staff Paper of the most policy-relevant information in the Criteria Document as well as quantitative exposure and risk assessments based on that information; (2) CASAC Panel advice and recommendations, as reflected in its letters to the Administrator, its discussions of drafts of the Criteria Document and Staff Paper at public meetings, and separate written comments prepared by individual members of the CASAC Panel; (3) public comments received during the development of these documents, either in connection with CASAC Panel meetings or separately; and (4) extensive public comments received on the proposed rulemaking. II. Rationale for Final Decisions on the Primary O3 Standard A. Introduction 1. Overview This section presents the Administrator's final decisions regarding the need to revise the current primary O3 NAAQS, and the appropriate revision to the level of the 8-hour standard. As discussed more fully below, the rationale for the final decision on appropriate revisions to the primary O3 NAAQS includes consideration of: (1) Evidence of health effects related to short-term exposures to O3; (2) insights gained from quantitative exposure and health risk assessments; (3) public and CASAC Panel comments received during the development and review of the Criteria Document, Staff Paper, exposure and risk assessments and on the proposal notice. In developing this rationale, EPA has drawn upon an integrative synthesis of the entire body of evidence \8\ relevant to examining associations between exposure to ambient O3 and a broad range of health endpoints (EPA, 2006a, Chapter 8), focusing on those health endpoints for which the Criteria Document concluded that the associations are causal or likely to be causal. This body of evidence includes hundreds of studies conducted in many countries around the world. In its assessment of the evidence judged to be most relevant to decisions on elements of the primary O3 standards, EPA has placed greater weight on U.S. and Canadian studies, since studies conducted in other countries may well reflect different demographic and air pollution characteristics. --------------------------------------------------------------------------- \8\ The word ``evidence'' is used in this notice to refer to studies that provide information relevant to an area of inquiry, which can include studies that report positive or negative results or that provide interpretative information. --------------------------------------------------------------------------- As discussed below, a significant amount of new research has been conducted since the last review, with important new information coming from epidemiological, toxicological, controlled human exposure, and dosimetric studies. Moreover, the newly available research studies evaluated in the Criteria Document have undergone intensive scrutiny through multiple layers of peer review, with extended [[Page 16440]] opportunities for review and comment by CASAC Panel and the public. As with virtually any policy-relevant scientific research, there is uncertainty in the characterization of health effects attributable to exposure to ambient O3, most generally with regard to whether observed health effects and associations are causal or likely causal in nature and, if so, the certainty of causal associations at various exposure levels. While important uncertainties remain, the review of the health effects information has been extensive and deliberate. In the judgment of the Administrator, this intensive evaluation of the scientific evidence provides an adequate basis for regulatory decision making at this time. This review also provides important input to EPA's research plan for improving our future understanding of the relationships between exposures to ambient O3 and health effects. The health effects information and quantitative exposure and health risk assessment were summarized in sections II.A and II.B of the proposal (72 FR at 37824-37862) and are only briefly outlined below in sections II.A.2 and II.A.3. Subsequent sections of this preamble provide a more complete discussion of the Administrator's rationale, in light of key issues raised in public comments, for concluding that the current standard is not requisite to protect public health with an adequate margin of safety, and it is appropriate to revise the current primary O3 standards to provide additional public health protection (section II.B), as well as a more complete discussion of the Administrator's rationale for retaining or revising the specific elements of the primary O3 standards (section II.C), namely the indicator (section II.C.1); averaging time (section II.C.2); form (section II.C.3); and level (section II.C.4). A summary of the final decisions on revisions to the primary O3 standards is presented in section II.D. 2. Overview of Health Effects This section outlines the information presented in Section II.A of the proposal on known or potential effects on public health which may be expected from the presence of O3 in ambient air. The decision in the last review focused primarily on evidence from short- term (e.g., 1 to 3 hours) and prolonged ( 6 to 8 hours) controlled- exposure studies reporting lung function decrements, respiratory symptoms, and respiratory inflammation in humans, as well as epidemiology studies reporting excess hospital admissions and emergency department visits for respiratory causes. The Criteria Document prepared for this review emphasizes a large number of epidemiological studies published since the last review with these and additional health endpoints, including the effects of acute (short-term and prolonged) and chronic exposures to O3 on lung function decrements and enhanced respiratory symptoms in asthmatic individuals, school absences, and premature mortality. It also emphasizes important new information from toxicology, dosimetry, and controlled human exposure studies. Highlights of the evidence include: (1) Two new controlled human-exposure studies are now available that examine respiratory effects associated with prolonged O3 exposures at levels at and below 0.080 ppm, which was the lowest exposure level that had been examined in the last review. (2) Numerous recent controlled human-exposure studies have examined indicators of O3-induced inflammatory response in both the upper respiratory tract (URT) and lower respiratory tract (LRT), while other studies have examined changes in host defense capability following O3 exposure of healthy young adults and increased airway responsiveness to allergens in subjects with allergic asthma and allergic rhinitis exposed to O3. (3) New evidence from controlled human exposure studies showing that asthmatics have greater respiratory-related physiological responses than healthy subjects and new evidence from epidemiological studies showing associations between O3 exposure and lung function and respiratory symptom responses; these findings differ from the presumption in the last review that people with asthma had generally the same magnitude of respiratory responses to O3 as those experienced by healthy individuals. (4) Animal toxicology studies provide new information regarding potential mechanisms of action, increased susceptibility to respiratory infection, and biological plausibility of acute effects as well as chronic, irreversible respiratory damage observed in animals. (5) Numerous epidemiological studies published during the past decade offer added evidence of associations between acute ambient O3 exposures and lung function decrements and respiratory symptoms in physically active healthy subjects and asthmatic subjects, as well as new evidence regarding additional health endpoints, including relationships between ambient O3 concentrations and school absenteeism and between ambient O3 and cardiac- related physiological endpoints. (6) Several additional studies have been published over the last decade examining the temporal associations between acute O3 exposures and both emergency department visits for respiratory diseases and respiratory-related hospital admissions. (7) A large number of newly available epidemiological studies have examined the effects of acute exposure to PM and O3 on premature mortality, notably including large multi-city studies that provide much more robust information than was available in the last review, as well as recent meta-analyses that have evaluated potential sources of heterogeneity in O3-mortality associations. Section II.A of the proposal provides a detailed summary of key information contained in the Criteria Document (chapters 4-8) and in the Staff Paper (chapter 3), on the known and potential effects of O3 exposure and information on the effects of O3 exposure in combination with other pollutants that are routinely present in the ambient air (72 FR 37824-37851). The information there summarizes: (1) New information available on potential mechanisms for morbidity and mortality effects associated with exposure to O3, including potential mechanisms or pathways related to direct effects on the respiratory system, systemic effects that are secondary to effects in the respiratory system (e.g., cardiovascular effects); (2) The nature of effects that have been associated directly with exposure to O3 or indirectly with the presence of O3 in ambient air, including premature mortality, aggravation of respiratory and cardiovascular disease (as indicated by increased hospital admissions and emergency department visits), changes in lung function and increased respiratory symptoms, as well as new evidence for more subtle indicators of cardiovascular health; (3) An integrative interpretation of the health effects evidence, focusing on the biological plausibility and coherence of the evidence and key issues raised in interpreting epidemiological studies, along with supporting evidence from experimental (e.g., dosimetric and toxicological) studies as well as the limitations of the evidence; and (4) Considerations in characterizing the public health impact of O3, including the identification of sensitive and vulnerable subpopulations that are potentially at risk to such effects, including active people, people with pre-existing lung and heart diseases, children and older adults, and people with increased responsiveness to O3. [[Page 16441]] 3. Overview of Human Exposure and Health Risk Assessments To put judgments about health effects that are adverse for individuals into a broader public health context, EPA developed and applied models to estimate human exposures and health risks. This broader public health context included consideration of the size of particular population groups at risk for various effects, the likelihood that exposures of concern would occur for individuals in such groups under varying air quality scenarios, estimates of the number of people likely to experience O3-related effects, the variability in estimated exposures and risks, and the kind and degree of uncertainties inherent in assessing the exposures and risks involved. As discussed in more detail in section II.B of the proposal, there are a number of important uncertainties that affect the exposure and health risk estimates. It is also important to note that there have been significant improvements since the last review in both the exposure and health risk models. The CASAC Panel expressed the view that the exposure analysis represents a state-of-the-art modeling approach and that the health risk assessment was ``well done, balanced and reasonably communicated'' (Henderson, 2006c). In modeling exposures and health risks associated with just meeting the current and alternative O3 standards, EPA simulated air quality just meeting these standards based on O3 air quality patterns in several recent years and on how the shape of the O3 air quality distributions has changed over time based on historical trends in monitored O3 air quality data. As discussed in the proposal notice and in the Staff Paper (section 4.5.8), recent O3 air quality distributions were statistically adjusted to simulate just meeting the current and selected alternative standards. Specifically, the exposure and risk assessment included estimates for a recent year of air quality and for air quality adjusted to simulate just meeting the current and alternative standards based on O3 season data from a recent three-year period (2002-2004). The O3 season in each area included the period of the year for which routine hourly O3 monitoring data are available. Typically this period spans from March or April through September or October, although in some areas it includes the entire year. Three years were modeled to reflect the substantial year-to-year variability that occurs in O3 levels and related meteorological conditions, and because the standard is specified in terms of a three-year period. The year-to-year variability observed in O3 levels is due to a combination of different weather patterns and the variation in emissions of O3 precursors. Nationally, 2002 was a relatively high year with respect to the 4th highest daily maximum 8-hour O3 levels observed in urban areas across the U.S. (see Staff Paper, Figure 2-16), with the mean of the distribution of annual 4th highest daily maximum 8-hour O3 levels for urban monitors nationwide being in the upper third among the years 1990 through 2004. In contrast, on a national basis, 2004 was the lowest year on record with respect to the mean of the distribution of annual 4th highest daily maximum 8-hour O3 levels for this same 15 year period. The 4th highest daily maximum 8-hour levels observed in most, but not all of the 12 urban areas included in the exposure and risk assessment, were relatively low in 2004 compared to other recent years. The 4th highest daily maximum 8-hour O3 levels observed in 2003 in the 12 urban areas and nationally generally were between those observed in 2002 and 2004. As a result of the variability in air quality, the exposure and risk estimates associated with just meeting the current or any alternative standard also will vary depending on the year chosen for the analysis. Thus, exposure and risk estimates based on 2002 air quality generally show relatively higher numbers of children affected and the estimates based on 2004 air quality generally show relatively fewer numbers of children affected. These simulations do not reflect any consideration of specific control programs or strategies designed to achieve the reductions in emissions required to meet the specified standards. Further, these simulations do not represent predictions of when, whether, or how areas might meet the specified standards.\9\ Instead these simulations represent a projection of the kind of air quality levels that would be likely to occur in areas just attaining various alternative standards, when historical patterns of air quality, reflecting averages over many areas, are applied in the urban areas examined. --------------------------------------------------------------------------- \9\ For informational purposes only, modeling that projects how areas might attain alternative standards in a future year as a result of Federal, State, local, and Tribal efforts is presented in the final Regulatory Impact Analysis being prepared in connection with this decision. --------------------------------------------------------------------------- a. Exposure Analyses As discussed in section II.B.1 of the proposal, EPA conducted human exposure analyses using a simulation model to estimate O3 exposures for the general population, school age children (ages 5-18), and school age children with asthma living in 12 U.S. metropolitan areas representing different regions of the country where the current 8-hour O3 standard is not met. The emphasis on children reflected the finding of the last review that children are an important at-risk group. Exposure estimates were developed using a probabilistic exposure model that is designed to explicitly model the numerous sources of variability that affect people's exposures. This exposure assessment is more fully described and presented in the Staff Paper and in a technical support document, Ozone Population Exposure Analysis for Selected Urban Areas (EPA, 2007c; henceforth ``Exposure Analysis TSD''). As noted in the proposal, the scope and methodology for this exposure assessment were developed over the last few years with considerable input from the CASAC Panel and the public. As discussed in the proposal notice and in greater detail in the Staff Paper (chapter 4) and Exposure Analysis TSD, EPA recognized that there are many sources of variability and uncertainty inherent in the input to this assessment and that there was uncertainty in the resulting O3 exposure estimates. In EPA's judgment, the most important uncertainties affecting the exposure estimates are related to the modeling of human activity patterns over an O3 season, the modeling of variations in ambient concentrations near roadways, and the modeling of air exchange rates that affect the amount of O3 that penetrates indoors. Another important uncertainty that affects the estimation of how many exposures are associated with moderate or greater exertion is the characterization of energy expenditure for children engaged in various activities. As discussed in more detail in the Staff Paper (section 4.3.4.7), the uncertainty in energy expenditure values carries over to the uncertainty of the modeled breathing rates, which are important since they are used to classify exposures occurring at moderate or greater exertion. These are the relevant exposures since O3-related effects observed in clinical studies only are observed when individuals are engaged in some form of exercise. The uncertainties in the exposure model inputs and the estimated exposures have been assessed using quantitative uncertainty and sensitivity analyses. Details are discussed in the Staff Paper (section 4.6) and in a technical memorandum describing the exposure modeling uncertainty analysis (Langstaff, 2007). The exposure assessment, which provided estimates of the number of people exposed to different levels of [[Page 16442]] ambient O3 while at elevated exertion \10\, served two purposes. First, the entire range of modeled personal exposures to ambient O3 was an essential input to the portion of the health risk assessment based on exposure-response functions from controlled human exposure studies, discussed in the next section. Second, estimates of personal exposures to ambient O3 concentrations at and above specified benchmark levels while at elevated exertion provided some perspective on the public health impacts of health effects that we cannot currently evaluate in quantitative risk assessments but that may occur at current air quality levels, and the extent to which such impacts might be reduced by meeting the current and alternative standards. In the proposal, we referred to exposures at and above these benchmark levels while at elevated exertion as ``exposures of concern.'' --------------------------------------------------------------------------- \10\ As discussed in section II.A of the proposal, O3 health responses observed in controlled human exposure studies are associated with exposures while subjects are engaged in moderate or greater exertion on average over the exposure period (hereafter referred to as ``elevated exertion'') and, therefore, these are the exposures of interest. --------------------------------------------------------------------------- Based on the observation from the exposure analyses conducted in the prior review that children represented the population subgroup with the greatest exposure to ambient O3, EPA chose to model 8- hour exposures at elevated exertion for all school age children, and separately for asthmatic school age children, as well as for the general population in the current exposure assessment. While outdoor workers and other adults who engage in moderate or greater exertion for prolonged periods while outdoors during the day in areas experiencing elevated O3 concentrations also are at risk for O3-related health effects, EPA did not focus on developing quantitative exposure estimates for these population subgroups due to the lack of information about the number of individuals who regularly work or exercise outdoors. Thus, as presented in the proposal and in the Staff Paper the exposure estimates are most useful for making relative comparisons of estimated exposures in school age children across alternative air quality scenarios. This assessment does not provide information on exposures for adult subgroups within the general population associated with the air quality scenarios. EPA noted in the proposal key observations that were important to consider in comparing exposure estimates associated with just meeting the current NAAQS and alternative standards considered. These included: (1) As shown in Table 6-1 of the Staff Paper, the patterns of exposures in terms of percentages of the population exceeding given exposure levels were very similar for the general population and for asthmatic and all school age (5-18) children, although children were about twice as likely as the general population to be exposed at any given level. (2) As shown in Table 1 in the proposal (72 FR 37855), the number and percentage of asthmatic and all school age children aggregated across the 12 urban areas estimated to experience 1 or more exposures of concern declined from simulations of just meeting the current standard to simulations of alternative 8-hour standards by varying amounts, depending on the benchmark level, the population subgroup considered, and the air quality year chosen.\11\ --------------------------------------------------------------------------- \11\ While the proposal notice stated in the text that ``approximately 2 to 4 percent of all and asthmatic children'' were estimated to experience exposures of concern at and above the 0.070 ppm benchmark level for standards in the range of 0.070 to 0.075 ppm (72 FR 37879), the correct range is about 1 to 5 perecent consistent with the estimates provided in Table 1 of the proposal (72 FR 37855). --------------------------------------------------------------------------- (3) Substantial year-to-year variability in exposure estimates was observed over the three-year modeling period. (4) There was substantial variability observed across the 12 urban areas in the percent of the population subgroups estimated to experience exposures at and above specified benchmark levels while at elevated exertion. (5) Of particular note, there is high inter-individual variability in responsiveness such that only a subset of individuals who were exposed at and above a given benchmark level while at elevated exertion would actually be expected to experience any such potential adverse health effects. (6) In considering these observations, it was important to take into account the variability, uncertainties, and limitations associated with this assessment, including the degree of uncertainty associated with a number of model inputs and uncertainty in the model itself. b. Quantitative Health Risk Assessment As discussed in section II.B.2 of the proposal, the approach used to develop quantitative risk estimates associated with exposures to O3 builds upon the risk assessment conducted during the last review.\12\ The expanded and updated assessment conducted in this review includes estimates of (1) risks of lung function decrements in all and asthmatic school age children, respiratory symptoms in asthmatic children, respiratory-related hospital admissions, and non- accidental and cardiorespiratory-related mortality associated with recent short-term ambient O3 levels; (2) risk reductions and remaining risks associated with just meeting the current 8-hour O3 NAAQS; and (3) risk reductions and remaining risks associated with just meeting various alternative 8-hour O3 NAAQS in a number of example urban areas. The health risk assessment was discussed in the Staff Paper (chapter 5) and presented more fully in a technical support document, Ozone Health Risk Assessment for Selected Urban Areas (Abt Associates, 2007a). As noted in the proposal, the scope and methodology for this risk assessment was developed over several years with considerable input from the CASAC Panel and the public. --------------------------------------------------------------------------- \12\ The methodology, scope, and results from the risk assessment conducted in the last review are described in Chapter 6 of the 1996 Staff Paper (EPA, 1996) and in several technical reports (Whitfield et al., 1996; Whitfield, 1997) and publication (Whitfield et al., 1998). --------------------------------------------------------------------------- EPA recognized that there were many sources of uncertainty and variability inherent in the inputs to these assessments and that there was a high degree of uncertainty in the resulting O3 risk estimates. Such uncertainties generally relate to a lack of clear understanding of a number of important factors, including, for example, the shape of exposure-response and concentration-response functions, particularly when, as here, effect thresholds can neither be discerned nor determined not to exist; issues related to selection of appropriate statistical models for the analysis of the epidemiologic data; the role of potentially confounding and modifying factors in the concentration- response relationships; and issues related to simulating how O3 air quality distributions will likely change in any given area upon attaining a particular standard, since strategies to reduce emissions are not yet fully defined. While some of these uncertainties were addressed quantitatively in the form of estimated confidence ranges around central risk estimates, other uncertainties and the variability in key inputs were not reflected in these confidence ranges, but rather were partially characterized through separate sensitivity analyses or discussed qualitatively. Key observations and insights from the O3 risk assessment, together with important caveats and limitations, were discussed in section II.B of the proposal. In general, estimated risk reductions associated with going from current O3 levels to just meeting the current and [[Page 16443]] alternative 8-hour standards show patterns of increasing estimated risk reductions associated with just meeting the lower alternative 8-hour standards considered. Furthermore, the estimated percentage reductions in risk were strongly influenced by the baseline air quality year used in the analysis (see Staff Paper, Figures 6-1 through 6-6) Key observations important in comparing estimated health risks associated with attainment of the current NAAQS and alternative standards included: (1) As discussed in the Staff paper (section 5.4.5), EPA has greater confidence in relative comparisons in risk estimates between alternative standards than in the absolute magnitude of risk estimates associated with any particular standard. (2) Significant year-to-year variability in O3 concentrations combined with the use of a 3-year design value to determine the amount of air quality adjustment to be applied to each year analyzed, results in significant year-to-year variability in the annual health risk estimates upon just meeting the current and potential alternative standards. (3) There is noticeable city-to-city variability in estimated O3-related incidence of morbidity and mortality across the 12 urban areas analyzed for both recent years of air quality and for air quality adjusted to simulate just meeting the current and selected potential alternative standards. This variability is likely due to differences in air quality distributions, differences in estimated exposure related to many factors including varying activity patterns and air exchange rates, differences in baseline incidence rates, and differences in susceptible populations and age distributions across the 12 urban areas. (4) With respect to the uncertainties about estimated policy- relevant background (PRB) concentrations,\13\ as discussed in the Staff Paper (section 5.4.3), alternative assumptions about background levels had a variable impact depending on the health effect considered and the location and standard analyzed in terms of the absolute magnitude and relative changes in the risk estimates. There was relatively little impact on either absolute magnitude or relative changes in lung function risk estimates due to alternative assumptions about background levels.\14\ With respect to O3-related non-accidental mortality, while notable differences (i.e., greater than 50 percent) were observed in some areas, particularly for more stringent standards, the overall pattern of estimated reductions, expressed in terms of percentage reduction relative to the current standard, was significantly less impacted. --------------------------------------------------------------------------- \13\ PRB O3 concentrations used in the O3 risk assessment were defined in chapter 2 of the Staff Paper (EPA, 2007, pp. 2-48, 2-54) as the O3 concentrations that would be observed in the U.S. in the absence of anthropogenic emissions of precursors (e.g., VOC, NOX, and CO) in the U.S., Canada, and Mexico. Based on runs of the GEOS-CHEM model (a global tropospheric O3 model) applied for the 2001 warm season (i.e., April to September), monthly background daily diurnal profiles for each of the 12 urban areas for each month of the O3 season were simulated using meteorology for the year 2001. Based on these model runs, the Criteria Document states that current estimates of PRB O3 concentrations are generally in the range of 0.015 to 0.035 ppm in the afternoon, and they are generally lower under conditions conducive to high O3 episodes. They are highest during spring due to contributions from hemispheric pollution and stratospheric intrusions. The Criteria Document states that the GEOS-CHEM model applied for the 2001 warm season reports PRB O3 concentrations for afternoon surface air over the United States that are likely 10 ppbv too high in the southeast in summer, and accurate within 5 ppbv in other regions and seasons. \14\ Sensitivity analyses examining the impact of alternative assumptions about PRB were only conducted for lung function decrements and non-accidental mortality. --------------------------------------------------------------------------- (5) Concerning the part of the risk assessment based on effects reported in epidemiological studies, important uncertainties include uncertainties (1) surrounding estimates of the O3 coefficients for concentration-response relationships used in the assessment, (2) involving the shape of the concentration-response relationship and whether or not a population threshold or non-linear relationship exists within the range of concentrations examined in the studies, (3) related to the extent to which concentration-response relationships derived from studies in a given location and time when O3 levels were higher or behavior and /or housing conditions were different provide accurate representations of the relationships for the same locations with lower air quality distributions and/or different behavior and/or housing conditions, and (4) concerning the possible role of co-pollutants which also may have varied between the time of the studies and the current assessment period. An important additional uncertainty for the mortality risk estimates is the extent to which the associations reported between O3 and non- accidental and cardiorespiratory mortality actually reflect causal relationships. As discussed in the proposal, some of these uncertainties have been addressed quantitatively in the form of estimated confidence ranges around central risk estimates; others are addressed through separate sensitivity analyses (e.g., the influence of alternative estimates for policy-relevant background levels) or are characterized qualitatively. For both parts of the health risk assessment, statistical uncertainty due to sampling error has been characterized and is expressed in terms of 95 percent credible intervals. EPA recognizes that these credible intervals do not reflect all of the uncertainties noted above. B. Need for Revision of the Current Primary O3 Standard 1. Introduction The initial issue to be addressed in this review of the primary O3 standard is whether, in view of the advances in scientific knowledge reflected in the Criteria Document and Staff Paper, the current standard should be revised. As discussed in section II.C of the proposal, in evaluating whether it was appropriate to propose to retain or revise the current standard, the Administrator built upon the last review and reflected the broader body of evidence and information now available. In the proposal, EPA presented information, judgments, and conclusions from the last review, which revised the level, averaging time, and form of the standard, from the Staff Paper's evaluation of the adequacy of the current primary standard, including both evidence- and exposure/risk-based considerations, as well as from the CASAC Panel's advice and recommendations. The Staff Paper evaluation, CASAC Panel's views, and the Administrator's proposed conclusions on the adequacy of the current primary standard are presented below. a. Staff Paper Evaluation The Staff Paper considered the evidence presented in the Criteria Document as a basis for evaluating the adequacy of the current O3 standard, recognizing that important uncertainties remain. The extensive body of human clinical, toxicological, and epidemiological evidence, highlighted above in section II.A.2 and discussed in section II.A of the proposal, serves as the basis for judgments about O3-related health effects, including judgments about causal relationships with a range of respiratory morbidity effects, including lung function decrements, increased respiratory symptoms, airway inflammation, increased airway responsiveness, and respiratory-related hospitalizations and emergency department visits in the warm season, and about the evidence being highly suggestive that O3 directly or indirectly contributes to non-accidental and cardiorespiratory-related mortality. [[Page 16444]] These judgments take into account important uncertainties that remain in interpreting this evidence. For example, with regard to the utility of time-series epidemiological studies to inform judgments about a NAAQS for an individual pollutant, such as O3, within a mix of highly correlated pollutants, such as the mix of oxidants produced in photochemical reactions in the atmosphere, the Staff Paper noted that there are limitations especially at ambient O3 concentrations below levels at which O3- related effects have been observed in controlled human exposure studies. The Staff Paper also recognized that the available epidemiological evidence neither supports nor refutes the existence of thresholds at the population level for effects such as increased hospital admissions and premature mortality. There are limitations in epidemiological studies that make discerning thresholds in populations difficult, including low data density in the lower concentration ranges, the possible influence of exposure measurement error, and variability in susceptibility to O3-related effects in populations. While noting these limitations in the interpretation of the findings from the epidemiological studies, the Staff Paper concluded that if a population threshold level does exist, it would likely be well below the level of the current O3 standard and possibly within the range of background levels. This conclusion is supported by several epidemiological studies that have explored the question of potential thresholds either by using a statistical curve-fitting approach to evaluate whether linear or non-linear models fit the data better using, or by analyzing, sub-sets of the data where days over or under a specific cutpoint (e.g., 0.080 ppm or even lower O3 levels) were excluded and then evaluating the association for statistical significance. In addition to consideration of the epidemiological studies, findings from controlled human exposure studies indicate that prolonged exposures produced statistically significant group mean FEV1 decrements and symptoms in healthy adult subjects at levels down to at least 0.060 ppm, with a small percentage of subjects experiencing notable effects (e.g., >10 percent FEV1 decrement, pain on deep inspiration). Controlled human exposure studies evaluated in the last review also found significant responses in indicators of lung inflammation and cell injury at 0.080 ppm in healthy adult subjects. The effects in these controlled human exposure studies were observed in healthy young adult subjects, and it is likely that more serious responses, and responses at lower levels, would occur in people with asthma and other respiratory diseases. These physiological effects can lead to aggravation of asthma and increased susceptibility to respiratory infection. The observations provide support for the conclusion in the Staff Paper that the associations observed in the epidemiological studies, particularly for respiratory-related effects such as increased medication use, increased school and work absences, increased visits to doctors' offices and emergency departments, and increased hospital admissions, extend down to O3 levels well below the current standard (i.e., 0.084 ppm) (p. 6-7). The newly available information reinforces the judgments in the Staff Paper from the last review about the likelihood of causal relationships between O3 exposures and respiratory effects and broadens the evidence of O3-related associations to include additional respiratory-related endpoints, newly identified cardiovascular-related health endpoints, and mortality. Newly available evidence also led the Staff Paper to conclude that people with asthma are likely to experience more serious effects than people who do not have asthma. The Staff Paper also concluded that substantial progress has been made since the last review in advancing the understanding of potential mechanisms by which ambient O3, alone and in combination with other pollutants, is causally linked to a range of respiratory-related health endpoints, and may be causally linked to