# Review of the National Ambient Air Quality Standards for Particulate Matter > Briefs, arguments, decisions, and more. URL: https://www.frixlaw.com/law-library/documents/fr%3A2020-08143 ## Record - **Collection:** Federal Register - **Document type:** Proposed Rule - **Published:** April 30, 2020 - **Citation:** 85 FR 24094 ## Text ENVIRONMENTAL PROTECTION AGENCY 40 CFR Part 50 [EPA-HQ-OAR-2015-0072; FRL-10008-31-OAR] RIN 2060-AS50 Review of the National Ambient Air Quality Standards for Particulate Matter AGENCY: Environmental Protection Agency (EPA). ACTION: Proposed action. SUMMARY: Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria and the national ambient air quality standards (NAAQS) for particulate matter (PM), the Administrator has reached proposed decisions on the primary and secondary PM NAAQS. With regard to the primary standards meant to protect against fine particle exposures ( i.e., annual and 24-hour PM 2.5 standards), the primary standard meant to protect against coarse particle exposures ( i.e., 24-hour PM 10 standard), and the secondary PM 2.5 and PM 10 standards, the EPA proposes to retain the current standards, without revision. DATES: Comments must be received on or before June 29, 2020. Public Hearings: The EPA will hold one or more virtual public hearings on this proposed rule. These will be announced in a separate Federal Register notice that provides details, including specific dates, times, and contact information for these hearings. ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-OAR-2015-0072, by any of the following means: • Federal eRulemaking Portal: https://www.regulations.gov (our preferred method). Follow the online instructions for submitting comments. • Email: a-and-r-Docket@epa.gov. Include the Docket ID No. EPA-HQ-OAR-2015-0072 in the subject line of the message. Instructions: All submissions received must include the Docket ID No. for this document. Comments received may be posted without change to https://www.regulations.gov, including any personal information provided. For detailed instructions on sending comments, see the SUPPLEMENTARY INFORMATION section of this document. Out of an abundance of caution for members of the public and our staff, the EPA Docket Center and Reading Room was closed to public visitors on March 31, 2020, to reduce the risk of transmitting COVID-19. Our Docket Center staff will continue to provide remote customer service via email, phone, and webform. We encourage the public to submit comments via https://www.regulations.gov or email, as there is a temporary suspension of mail delivery to EPA, and no hand deliveries are currently accepted. For further information of EPA Docket Center services and the current status, please visit us online at https://www.epa.gov/dockets. FOR FURTHER INFORMATION CONTACT: Dr. Scott Jenkins, Health and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail Code C504-06, Research Triangle Park, NC 27711; telephone: (919) 541-1167; fax: (919) 541-5315; email: jenkins.scott@epa.gov. SUPPLEMENTARY INFORMATION: General Information Written Comments: Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2015-0072, at https://www.regulations.gov (our preferred method), or the other methods identified in the ADDRESSES section. Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written submission. The written submission is considered the official submission and should include discussion of all points you wish to make. The EPA will generally not consider submissions or submission content located outside of the primary submission ( i.e., on the web, cloud, or other file sharing system). For additional submission methods, the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets. The EPA is temporarily suspending its Docket Center and Reading Room for public visitors to reduce the risk of transmitting COVID-19. Written comments submitted by mail are temporarily suspended and no hand deliveries will be accepted. Our Docket Center staff will continue to provide remote customer service via email, phone, and webform. We encourage the public to submit comments via https://www.regulations.gov. For further information and updates on EPA Docket Center services, please visit us online at https://www.epa.gov/dockets. The EPA continues to carefully and continuously monitor information from the Centers for Disease Control and Prevention (CDC), local area health departments, and our Federal partners so that we can respond rapidly as conditions change regarding COVID-19. Availability of Information Related to This Action A number of the documents that are relevant to this proposed decision are available through the EPA's website at https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards. These documents include the Integrated Review Plan for the National Ambient Air Quality Standards for Particulate Matter (U.S. EPA, 2016), available at https://www3.epa.gov/ttn/naaqs/standards/pm/data/201612-final-integrated-review-plan.pdf, the Integrated Science Assessment for Particulate Matter (U.S. EPA, 2019), available at https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534, and the Policy Assessment for the Review of the National Ambient Air Quality Standards for Particulate Matter (U.S. EPA, 2020), available at https://www.epa.gov/naaqs/particulate-matter-pm-standards-policy-assessments-current-review-0. These and other related documents are also available for inspection and copying in the EPA docket identified above. Table of Contents The following topics are discussed in this preamble: Executive Summary I. Background A. Legislative Requirements B. Related PM Control Programs C. Review of the Air Quality Criteria and Standards for Particulate Matter 1. Reviews Completed in 1971 and 1987 2. Review Completed in 1997 3. Review Completed in 2006 4. Review Completed in 2012 5. Current Review D. Air Quality Information 1. Distribution of Particle Size in Ambient Air 2. Sources and Emissions Contributing to PM in the Ambient Air 3. Monitoring of Ambient PM 4. Ambient Concentrations and Trends a. PM 2.5 mass b. PM 2.5 components c. PM 10 d. PM 10-2.5 a. UFP 5. Background PM II. Rationale for Proposed Decisions on the Primary PM 2.5 Standards A. General Approach 1. Approach Used in the Last Review a. Indicator b. Averaging Time c. Form d. Level 2. Approach in the Current Review B. Health Effects Related to Fine Particle Exposures 1. Nature of Effects a. Mortality b. Cardiovascular Effects c. Respiratory Effects d. Cancer e. Nervous System Effects 2. Populations at Risk of PM 2.5 -Related Health Effects 3. CASAC Advice C. Proposed Conclusions on the Current Primary PM 2.5 Standards 1. Evidence- and Risk-Based Considerations in the Policy Assessment a. Evidence-Based Considerations b. Risk-Based Considerations 2. CASAC Advice 3. Administrator's Proposed Decision on the Current Primary PM 2.5 Standards III. Rationale for Proposed Decisions on the Primary PM 10 Standard A. General Approach 1. Approach Used in the Last Review 2. Approach in the Current Review B. Health Effects Related to Thoracic Coarse Particle Exposures 1. Mortality a. Long-Term Exposures b. Short-Term Exposures 2. Cardiovascular Effects a. Long-Term Exposures b. Short-Term Exposures 3. Respiratory Effects—Short-Term Exposures 4. Cancer—Long-Term Exposures 5. Metabolic Effects—Long-Term Exposures 6. Nervous System Effects—Long-Term Exposures C. Proposed Conclusions on the Current Primary PM 10 Standard 1. Evidence-Based Considerations in the Policy Assessment 2. CASAC Advice 3. Administrator's Proposed Decision on the Current Primary PM 10 Standard IV. Rationale for Proposed Decisions on the Secondary PM Standards A. General Approach 1. Approach Used in the Last Review a. Non-Visibility Effects b. Visibility Effects 2. Approach for the Current Review B. PM-Related Visibility Impairment 1. Nature of PM-Related Visibility Impairment 2. Relationship between Ambient PM and Visibility 3. Public Perception of Visibility Impairment C. Other PM-Related Welfare Effects 1. Climate 2. Materials D. Proposed Conclusions on the Current Secondary PM Standards 1. Evidence- and Quantitative Information-Based Considerations in the Policy Assessment 2. CASAC Advice 3. Administrator's Proposed Decision on the Current Secondary PM Standards V. Statutory and Executive Order Reviews A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review B. Executive Order 13771: Reducing Regulations and Controlling Regulatory Costs C. Paperwork Reduction Act (PRA) D. Regulatory Flexibility Act (RFA) E. Unfunded Mandates Reform Act (UMRA) F. Executive Order 13132: Federalism G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks I. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution or Use J. National Technology Transfer and Advancement Act (NTTAA) K. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations L. Determination Under Section 307(d) References Executive Summary This document presents the Administrator's proposed decisions on the primary (health-based) and secondary (welfare-based) National Ambient Air Quality Standards (NAAQS) for particulate matter (PM). In ambient air, PM is a mixture of substances suspended as small liquid and/or solid particles. Particles in the atmosphere range in size from less than 0.01 to more than 10 micrometers (μm) in diameter. Particulate matter and its precursors are emitted from both anthropogenic sources ( e.g., electricity generating units, cars and trucks, agricultural operations) and natural sources ( e.g., sea salt, wildland fires, biological aerosols). When describing PM, subscripts are used to denote particle size. For example, PM 2.5 includes particles with diameters generally less than or equal to 2.5 μm and PM 10 includes particles with diameters generally less than or equal to 10 μm. The EPA has established primary (health-based) and secondary (welfare-based) NAAQS for PM 2.5 and PM 10 . This includes two primary PM 2.5 standards, an annual average standard with a level of 12.0 μg/m 3 and a 24-hour standard with a 98th percentile form and a level of 35 μg/m 3 . It also includes a primary PM 10 standard with a 24-hour averaging time, a 1-expected exceedance form, and a level of 150 μg/m 3 . Secondary PM standards are set equal to the primary standards, except that the level of the secondary annual PM 2.5 standard is 15.0 μg/m 3 . In reaching proposed decisions on these PM standards in the current review, the Administrator has considered the available scientific evidence assessed in the Integrated Science Assessment (ISA), analyses in the Policy Assessment (PA), and advice from the Clean Air Scientific Advisory Committee (CASAC). For the primary PM 2.5 standards, the Administrator proposes to conclude that there are important uncertainties in the evidence for adverse health effects below the current standards and in the potential public health impacts of reducing ambient PM 2.5 concentrations below those standards. As a result, he proposes to conclude that the available evidence and information do not call into question the adequacy of the current primary PM 2.5 standards, and he proposes to retain those standards ( i.e., both the annual and 24-hour standards) without revision in this review. For the primary PM 10 standard, the Administrator observes that, while the available health effects evidence has expanded, recent studies are subject to the same types of uncertainties that were judged important in the last review. He proposes to conclude that the newly available evidence does not call into question the adequacy of the current primary PM 10 standard, and he proposes to retain that standard without revision in this review. For the secondary standards, the Administrator observes that the expanded evidence for non-ecological welfare effects is consistent with the last review 1 and that updated quantitative analyses show results similar to those in the last review. Therefore, he proposes to conclude that the newly available evidence and updated analyses do not call into question the adequacy of the current secondary PM standards, and he proposes to retain those standards without revision in this review. 1 The welfare effects considered in this review include visibility impairment, climate effects, and materials effects. Ecological effects associated with PM, and the adequacy of protection provided by the secondary PM standards for those effects, are being addressed in the separate review of the secondary NAAQS for oxides of nitrogen, oxides of sulfur and PM. Information on the current review of these secondary NAAQS can be found at https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards. These proposed decisions are consistent with the CASAC's consensus advice on the primary 24-hour PM 2.5 standard, the primary PM 10 standard, and the secondary standards. The CASAC did not reach consensus on the primary annual PM 2.5 standard, with some committee members recommending that EPA retain the current standard and other members recommending revision of that standard. I. Background A. 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 certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator is to list those pollutants “emissions of which, in his judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare”; “the presence of which in the ambient air results from numerous or diverse mobile or stationary sources”; and for which he “plans to issue air quality criteria . . . . ” (42 U.S.C. 7408(a)(1)). Air quality criteria are intended to “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [a] pollutant in the ambient air . . . . ” (42 U.S.C. 7408(a)(2)). Section 109 [42 U.S.C. 7409] directs the Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutants for which air quality criteria are issued [42 U.S.C. 7409(a)]. Section 109(b)(1) defines primary standards as ones “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.” 2 Under section 109(b)(2), a secondary standard 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.” 3 2 The legislative history of section 109 indicates that a primary standard is to be set at “the maximum permissible ambient air level . . . which will protect the health of any [sensitive] group of the population,” and that for this purpose “reference should be made to a representative sample of persons comprising the sensitive group rather than to a single person in such a group.” S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970). 3 Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on welfare 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.” In setting primary and secondary standards that are “requisite” to protect public health and welfare, respectively, as provided in section 109(b), the EPA's task is to establish standards that are neither more nor less stringent than necessary. In so doing, the EPA may not consider the costs of implementing the standards. See generally Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant considerations in the promulgation of national ambient air quality standards.” American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corporation v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). 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. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). Both kinds of uncertainties are components of the risk associated with pollution at levels below those at which human health effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary standards that include an adequate margin of safety, the Administrator is seeking not only to prevent pollution levels that have been demonstrated to be harmful but also to prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. The CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels, see Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety. In addressing the requirement for an adequate margin of safety, the EPA considers such factors as the nature and severity of the health effects involved, the size of the sensitive population(s), and the kind and degree of uncertainties. The selection of any particular approach to providing an adequate margin of safety is a policy choice left specifically to the Administrator's judgment. See Lead Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353. Section 109(d)(1) of the Act requires the review every five years of existing air quality criteria and, if appropriate, the revision of those criteria to reflect advances in scientific knowledge on the effects of the pollutant on public health and welfare. Under the same provision, the EPA is also to review every five years and, if appropriate, revise the NAAQS, based on the revised air quality criteria. Section 109(d)(2) addresses the appointment and advisory functions of an independent scientific review committee. Section 109(d)(2)(A) requires the Administrator to appoint this committee, which is to be composed of “seven members including at least one member of the National Academy of Sciences, one physician, and one person representing State air pollution control agencies.” Section 109(d)(2)(B) provides that the 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. . . .” Since the early 1980s, this independent review function has been performed by the Clean Air Scientific Advisory Committee (CASAC) of the EPA's Science Advisory Board. A number of other advisory functions are also identified for the committee by section 109(d)(2)(C), which reads: Such committee shall also (i) advise the Administrator of areas in which additional knowledge is required to appraise the adequacy and basis of existing, new, or revised national ambient air quality standards, (ii) describe the research efforts necessary to provide the required information, (iii) advise the Administrator on the relative contribution to air pollution concentrations of natural as well as anthropogenic activity, and (iv) advise the Administrator of any adverse public health, welfare, social, economic, or energy effects which may result from various strategies for attainment and maintenance of such national ambient air quality standards. As previously noted, the Supreme Court has held that section 109(b) “unambiguously bars cost considerations from the NAAQS-setting process.” Whitman v. Am. Trucking Associations, 531 U.S. 457, 471 (2001). Accordingly, while some of these issues regarding which Congress has directed the CASAC to advise the Administrator are ones that are relevant to the standard setting process, others are not. Issues that are not relevant to standard setting may be relevant to implementation of the NAAQS once they are established. 4 4 Some aspects of the CASAC's advice may not be relevant to the EPA's process of setting primary and secondary standards that are requisite to protect public health and welfare. Indeed, were EPA to consider costs of implementation when reviewing and revising the standards “it would be grounds for vacating the NAAQS.” Whitman, 531 U.S. at 471 n.4. At the same time, the CAA directs the CASAC to provide advice on “any adverse public health, welfare, social, economic, or energy effects which may result from various strategies for attainment and maintenance” of the NAAQS to the Administrator under section 109(d)(2)(C)(iv). In Whitman, the Court clarified that most of that advice would be relevant to implementation but not standard setting, as it “enable[s] the Administrator to assist the States in carrying out their statutory role as primary implementers of the NAAQS.” Id. at 470 (emphasis in original). However, the Court also noted that the CASAC's “advice concerning certain aspects of `adverse public health . . . effects' from various attainment strategies is unquestionably pertinent” to the NAAQS rulemaking record and relevant to the standard setting process. Id. at 470 n.2. B. Related PM Control Programs States are primarily responsible for ensuring attainment and maintenance of ambient air quality standards once the EPA has established them. Under section 110 and 171-190 of the CAA, and related provisions and regulations, states are to submit, for EPA's approval, state implementation plans (SIPs) that provide for the attainment and maintenance of such standards through control programs directed to sources of the pollutants involved. The states, in conjunction with the EPA, also administer the Prevention of Significant Deterioration (PSD) program (CAA sections 160 to 169). In addition, Federal programs provide for nationwide reductions in emissions of PM and other air pollutants through the Federal motor vehicle and motor vehicle fuel control program under title II of the Act (CAA sections 202 to 250), which involves controls for emissions from mobile sources and controls for the fuels used by these sources, and new source performance standards for stationary sources under section 111 of the CAA. C. Review of the Air Quality Criteria and Standards for Particulate Matter 1. Reviews Completed in 1971 and 1987 The EPA first established NAAQS for PM in 1971 (36 FR 8186, April 30, 1971), based on the original Air Quality Criteria Document (AQCD) (DHEW, 1969). 5 The federal reference method (FRM) specified for determining attainment of the original standards was the high-volume sampler, which collects PM up to a nominal size of 25 to 45 µm (referred to as total suspended particulates or TSP). The primary standards were set at 260 µg/m 3 , 24-hour average, not to be exceeded more than once per year, and 75 µg/m 3 , annual geometric mean. The secondary standards were set at 150 µg/m 3 , 24-hour average, not to be exceeded more than once per year, and 60 µg/m 3 , annual geometric mean. 5 Prior to the review initiated in 2007 (see below), the AQCD provided the scientific foundation ( i.e., the air quality criteria) for the NAAQS. Beginning in that review, the Integrated Science Assessment (ISA) has replaced the AQCD. In October 1979 (44 FR 56730, October 2, 1979), the EPA announced the first periodic review of the air quality criteria and NAAQS for PM. Revised primary and secondary standards were promulgated in 1987 (52 FR 24634, July 1, 1987). In the 1987 decision, the EPA changed the indicator for particles from TSP to PM 10 , in order to focus on the subset of inhalable particles small enough to penetrate to the thoracic region of the respiratory tract (including the tracheobronchial and alveolar regions), referred to as thoracic particles. 6 The level of the 24-hour standards (primary and secondary) was set at 150 µg/m 3 , and the form was one expected exceedance per year, on average over three years. The level of the annual standards (primary and secondary) was set at 50 µg/m 3 , and the form was annual arithmetic mean, averaged over three years. 6 PM 10 refers to particles with a nominal mean aerodynamic diameter less than or equal to 10 µm. More specifically, 10 µm is the aerodynamic diameter for which the efficiency of particle collection is 50 percent. 2. Review Completed in 1997 In April 1994, the EPA announced its plans for the second periodic review of the air quality criteria and NAAQS for PM, and in 1997 the EPA promulgated revisions to the NAAQS (62 FR 38652, July 18, 1997). In the 1997 decision, the EPA determined that the fine and coarse fractions of PM 10 should be considered separately. This determination was based on evidence that serious health effects were associated with short- and long-term exposures to fine particles in areas that met the existing PM 10 standards. The EPA added new standards, using PM 2.5 as the indicator for fine particles (with PM 2.5 referring to particles with a nominal mean aerodynamic diameter less than or equal to 2.5 µm). The new primary standards were as follows: (1) An annual standard with a level of 15.0 µg/m 3 , based on the 3-year average of annual arithmetic mean PM 2.5 concentrations from single or multiple community-oriented monitors; 7 and (2) a 24-hour standard with a level of 65 µg/m 3 , based on the 3-year average of the 98th percentile of 24-hour PM 2.5 concentrations at each monitor within an area. Also, the EPA established a new reference method for the measurement of PM 2.5 in the ambient air and adopted rules for determining attainment of the new standards. To continue to address the health effects of the coarse fraction of PM 10 (referred to as thoracic coarse particles or PM 10-2.5 ; generally including particles with a nominal mean aerodynamic diameter greater than 2.5 µm and less than or equal to 10 µm), the EPA retained the primary annual PM 10 standard and revised the form of the primary 24-hour PM 10 standard to be based on the 99th percentile of 24-hour PM 10 concentrations at each monitor in an area. The EPA revised the secondary standards by setting them equal in all respects to the primary standards. 7 The 1997 annual PM 2.5 standard was compared with measurements made at the community-oriented monitoring site recording the highest concentration or, if specific constraints were met, measurements from multiple community-oriented monitoring sites could be averaged ( i.e., “spatial averaging”). In the last review (completed in 2012) the EPA replaced the term “community-oriented” monitor with the term “area-wide” monitor. Area-wide monitors are those sited at the neighborhood scale or larger, as well as those monitors sited at micro- or middle-scales that are representative of many such locations in the same core-based statistical area (CBSA) (78 FR 3236, January 15, 2013). Following promulgation of the 1997 PM NAAQS, petitions for review were filed by several parties, addressing a broad range of issues. In May 1999, the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit) upheld the EPA's decision to establish fine particle standards, holding that “the growing empirical evidence demonstrating a relationship between fine particle pollution and adverse health effects amply justifies establishment of new fine particle standards.” American Trucking Associations, Inc. v. EPA, 175 F. 3d 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found “ample support” for the EPA's decision to regulate coarse particle pollution, but vacated the 1997 PM 10 standards, concluding that the EPA had not provided a reasonable explanation justifying use of PM 10 as an indicator for coarse particles. American Trucking Associations v. EPA, 175 F. 3d at 1054-55. Pursuant to the D.C. Circuit's decision, the EPA removed the vacated 1997 PM 10 standards, and the pre-existing 1987 PM 10 standards remained in place (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld the EPA's determination not to establish more stringent secondary standards for fine particles to address effects on visibility. American Trucking Associations v. EPA, 175 F. 3d at 1027. The D.C. Circuit also addressed more general issues related to the NAAQS, including issues related to the consideration of costs in setting NAAQS and the EPA's approach to establishing the levels of NAAQS. Regarding the cost issue, the court reaffirmed prior rulings holding that in setting NAAQS the EPA is “not permitted to consider the cost of implementing those standards.” American Trucking Associations v. EPA, 175 F. 3d at 1040-41. Regarding the levels of NAAQS, the court held that the EPA's approach to establishing the level of the standards in 1997 ( i.e., both for PM and for the ozone NAAQS promulgated on the same day) effected “an unconstitutional delegation of legislative authority.” American Trucking Associations v. EPA, 175 F. 3d at 1034-40. Although the court stated that “the factors EPA uses in determining the degree of public health concern associated with different levels of ozone and PM are reasonable,” it remanded the rule to the EPA, stating that when the EPA considers these factors for potential non-threshold pollutants “what EPA lacks is any determinate criterion for drawing lines” to determine where the standards should be set. The D.C. Circuit's holding on the cost and constitutional issues were appealed to the United States Supreme Court. In February 2001, the Supreme Court issued a unanimous decision upholding the EPA's position on both the cost and constitutional issues. Whitman v. American Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional issue, the Court held that the statutory requirement that NAAQS be “requisite” to protect public health with an adequate margin of safety sufficiently guided the EPA's discretion, affirming the EPA's approach of setting standards that are neither more nor less stringent than necessary. The Supreme Court remanded the case to the D.C. Circuit for resolution of any remaining issues that had not been addressed in that court's earlier rulings. Id. at 475-76. In a March 2002 decision, the D.C. Circuit rejected all remaining challenges to the standards, holding that the EPA's PM 2.5 standards were reasonably supported by the administrative record and were not “arbitrary and capricious.” American Trucking Associations v. EPA, 283 F. 3d 355, 369-72 (D.C. Cir. 2002). 3. Review Completed in 2006 In October 1997, the EPA published its plans for the third periodic review of the air quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997). After the CASAC and public review of several drafts, the EPA's National Center for Environmental Assessment (NCEA) finalized the AQCD in October 2004 (U.S. EPA, 2004). The EPA's Office of Air Quality Planning and Standards (OAQPS) finalized a Risk Assessment and Staff Paper in December 2005 (Abt Associates, 2005, U.S. EPA, 2005). 8 On December 20, 2005, the EPA announced its proposed decision to revise the NAAQS for PM and solicited public comment on a broad range of options (71 FR 2620, January 17, 2006). On September 21, 2006, the EPA announced its final decisions to revise the primary and secondary NAAQS for PM to provide increased protection of public health and welfare, respectively (71 FR 61144, October 17, 2006). With regard to the primary and secondary standards for fine particles, the EPA revised the level of the 24-hour PM 2.5 standards to 35 µg/m 3 , retained the level of the annual PM 2.5 standards at 15.0 µg/m 3 , and revised the form of the annual PM 2.5 standards by narrowing the constraints on the optional use of spatial averaging. With regard to the primary and secondary standards for PM 10 , the EPA retained the 24-hour standards, with levels at 150 µg/m 3 , and revoked the annual standards. 9 The Administrator judged that the available evidence generally did not suggest a link between long-term exposure to existing ambient levels of coarse particles and health or welfare effects. In addition, a new reference method was added for the measurement of PM 10-2.5 in the ambient air in order to provide a basis for approving federal equivalent methods (FEMs) and to promote the gathering of scientific data to support future reviews of the PM NAAQS. 8 Prior to the review initiated in 2007, the Staff Paper presented the EPA staff's considerations and conclusions regarding the adequacy of existing NAAQS and, when appropriate, the potential alternative standards that could be supported by the evidence and information. More recent reviews present this information in the Policy Assessment. 9 In the 2006 proposal, the EPA proposed to revise the 24-hour PM 10 standard in part by establishing a new PM 10-2.5 indicator for thoracic coarse particles ( i.e., particles generally between 2.5 and 10 µm in diameter). The EPA proposed to include any ambient mix of PM 10-2.5 that was dominated by resuspended dust from high density traffic on paved roads and by PM from industrial sources and construction sources. The EPA proposed to exclude any ambient mix of PM 10-2.5 that was dominated by rural windblown dust and soils and by PM generated from agricultural and mining sources. In the final decision, the existing PM 10 standard was retained, in part due to an “inability . . . to effectively and precisely identify which ambient mixes are included in the [PM 10-2.5 ] indicator and which are not” (71 FR 61197, October 17, 2006). Several parties filed petitions for review following promulgation of the revised PM NAAQS in 2006. These petitions addressed the following issues: (1) Selecting the level of the primary annual PM 2.5 standard; (2) retaining PM 10 as the indicator of a standard for thoracic coarse particles, retaining the level and form of the 24-hour PM 10 standard, and revoking the PM 10 annual standard; and (3) setting the secondary PM 2.5 standards identical to the primary standards. On February 24, 2009, the D.C. Circuit issued its opinion in the case American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). The court remanded the primary annual PM 2.5 NAAQS to the EPA because the Agency had failed to adequately explain why the standards provided the requisite protection from both short- and long-term exposures to fine particles, including protection for at-risk populations. Id. at 520-27. With regard to the standards for PM 10, the court upheld the EPA's decisions to retain the 24-hour PM 10 standard to provide protection from thoracic coarse particle exposures and to revoke the annual PM 10 standard. Id. at 533-38. With regard to the secondary PM 2.5 standards, the court remanded the standards to the EPA because the Agency failed to adequately explain why setting the secondary PM standards identical to the primary standards provided the required protection for public welfare, including protection from visibility impairment. Id. at 528-32. The EPA responded to the court's remands as part of the next review of the PM NAAQS, which was initiated in 2007 (discussed below). 4. Review Completed in 2012 In June 2007, the EPA initiated the fourth periodic review of the air quality criteria and the PM NAAQS by issuing a call for information (72 FR 35462, June 28, 2007). Based on the NAAQS review process, as revised in 2008 and again in 2009, 10 the EPA held science/policy issue workshops on the primary and secondary PM NAAQS (72 FR 34003, June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released the planning and assessment documents that comprise the review process ( i.e., IRP (U.S. EPA, 2008), ISA (U.S. EPA, 2009c), REA planning documents for health and welfare (U.S. EPA, 2009b, U.S. EPA, 2009a), a quantitative health risk assessment (U.S. EPA, 2010a) and an urban-focused visibility assessment (U.S. EPA, 2010a), and PA (U.S. EPA, 2011)). In June 2012, the EPA announced its proposed decision to revise the NAAQS for PM (77 FR 38890, June 29, 2012). 10 The history of the NAAQS review process, including revisions to the process, is discussed at https://www.epa.gov/naaqs/historical-information-naaqs-review-process. In December 2012, the EPA announced its final decisions to revise the primary NAAQS for PM to provide increased protection of public health (78 FR 3086, January 15, 2013). With regard to primary standards for PM 2.5 , the EPA revised the level of the annual PM 2.5 standard 11 to 12.0 µg/m 3 and retained the 24-hour PM 2.5 standard, with its level of 35 µg/m 3 . For the primary PM 10 standard, the EPA retained the 24-hour standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles ( i.e., PM 10-2.5 ). With regard to the secondary PM standards, the EPA generally retained the 24-hour and annual PM 2.5 standards 12 and the 24-hour PM 10 standard to address visibility and non-visibility welfare effects. 11 The EPA also eliminated the option for spatial averaging. 12 Consistent with the primary standard, the EPA eliminated the option for spatial averaging with the annual standard. As with previous reviews, petitioners challenged the EPA's final rule. Petitioners argued that the EPA acted unreasonably in revising the level and form of the annual standard and in amending the monitoring network provisions. On judicial review, the revised standards and monitoring requirements were upheld in all respects. NAM v. EPA, 750 F.3d 921 (D.C. Cir. 2014). 5. Current Review In December 2014, the EPA announced the initiation of the current periodic review of the air quality criteria for PM and of the PM 2.5 and PM 10 NAAQS and issued a call for information (79 FR 71764, December 3, 2014). On February 9 to 11, 2015, the EPA's NCEA and OAQPS held a public workshop to inform the planning for the current review of the PM NAAQS (announced in 79 FR 71764, December 3, 2014). Workshop participants, including a wide range of external experts as well as EPA staff representing a variety of areas of expertise ( e.g., epidemiology, human and animal toxicology, risk/exposure analysis, atmospheric science, visibility impairment, climate effects), were asked to highlight significant new and emerging PM research, and to make recommendations to the Agency regarding the design and scope of this review. This workshop provided for a public discussion of the key science and policy-relevant issues around which the EPA has structured the current review of the PM NAAQS and of the most meaningful new scientific information that would be available in this review to inform understanding of these issues. The input received at the workshop guided the EPA staff in developing a draft IRP, which was reviewed by the CASAC Particulate Matter Panel and discussed on public teleconferences held in May 2016 (81 FR 13362, March 14, 2016) and August 2016 (81 FR 39043, June 15, 2016). Advice from the CASAC, supplemented by the Particulate Matter Panel, and input from the public were considered in developing the final IRP (U.S. EPA, 2016). The final IRP discusses the approaches to be taken in developing key scientific, technical, and policy documents in this review and the key policy-relevant issues. In May 2018, the Administrator issued a memorandum describing a “back-to-basics” process for reviewing the NAAQS (Pruitt, 2018). This memo announced the Agency's intention to conduct the current review of the PM NAAQS in such a manner as to ensure that any necessary revisions are finalized by December 2020. Following this memo, on October 10, 2018 the Administrator additionally announced that the role of reviewing the key assessments developed as part of the ongoing review of the PM NAAQS ( i.e., drafts of the ISA and PA) would be performed by the seven-member chartered CASAC ( i.e., rather than the CASAC Particulate Matter Panel that reviewed the draft IRP). 13 13 The CASAC charter is available at: https://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/2019casaccharter/$File/CASAC%202019%20Renewal%20Charter%203.21.19%20-%20final.pdf. The Administrator's announcement is available at: https://archive.epa.gov/epa/newsreleases/acting-administrator-wheeler-announces-science-advisors-key-clean-air-act-committee.html. The EPA released the draft ISA in October 2018 (83 FR 53471, October 23, 2018). The draft ISA was reviewed by the chartered CASAC at a public meeting held in Arlington, VA in December 2018 (83 FR 55529, November 6, 2018) and was discussed on a public teleconference in March 2019 (84 FR 8523, March 8, 2019). The CASAC provided its advice on the draft ISA in a letter to the EPA Administrator dated April 11, 2019 (Cox, 2019b). In that letter, the CASAC's recommendations address both the draft ISA's assessment of the science for PM-related effects and the process under which this review of the PM NAAQS is being conducted. Regarding the assessment of the evidence, the CASAC letter states that “the Draft ISA does not provide a sufficiently comprehensive, systematic assessment of the available science relevant to understanding the health impacts of exposure to particulate matter (PM)” (Cox, 2019b, p. 1 of letter). The CASAC recommended that this and other limitations ( i.e., “[i]nadequate evidence for altered causal determinations” and the need for a “[c]learer discussion of causality and causal biological mechanisms and pathways”) be remedied in a revised ISA (Cox, 2019b, p. 1 of letter). Given the Administrator's timeline for this review, as noted above (Pruitt, 2018), the EPA did not prepare a second draft ISA. Rather, the EPA has taken steps to address the CASAC's comments in the Final PM ISA (U.S. EPA, 2019). In particular, the final ISA includes additional text and a new appendix to clarify the comprehensive and systematic process employed by the EPA to develop the PM ISA. In addition, several causality determinations were re-examined and, consistent with the CASAC advice, the final ISA reflects a revised causality determination for long-term ultrafine particle (UFP) exposures and nervous system effects ( i.e., from “likely to be causal” to “suggestive of, but not sufficient to infer, a causal relationship”). 14 The final ISA also contains additional text to clarify the evidence for biological pathways of particular PM-related effects and the role of that evidence in causality determinations. 14 Based on the CASAC's comments, the EPA also re-examined the causality determinations for cancer and for nervous system effects following long-term PM 2.5 exposures. The EPA's consideration of these comments in the final ISA is discussed below in sections II.B.1.d and II.B.1.e. Among its comments on the process, the chartered CASAC recommended “that the EPA reappoint the previous CASAC PM panel (or appoint a panel with similar expertise)” (Cox, 2019b). The Agency's response to this advice was provided in a letter from the Administrator to the CASAC chair dated July 25, 2019. 15 In that letter, the Administrator announced his intention to identify a pool of non-member subject matter expert consultants to support the CASAC's review activities for the PM and ozone NAAQS. A Federal Register notice requesting the nomination of scientists from a broad range of disciplines “with demonstrated expertise and research in the field of air pollution related to PM and ozone” was published in August 2019 (84 FR 38625, August 7, 2019). The Administrator selected consultants from among those nominated, and input from members of this pool of consultants informed the CASAC's review of the draft PA. 15 Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/0/6CBCBBC3025E13B4852583D90047B352/$File/EPA-CASAC-19-002_Response.pdf. The EPA released the draft PA in September 2019 (84 FR 47944, September 11, 2019). The draft PA drew from the assessment of the evidence in the draft ISA. It was reviewed by the chartered CASAC and discussed in October 2019 at a public meeting held in Cary, NC. Public comments were received via a separate public teleconference (84 FR 51555, September 30, 2019). A public meeting to discuss the chartered CASAC letter and response to charge questions on the draft PA was held in Cary, NC in December 2019 (84 FR 58713, November 1, 2019), and the CASAC provided its advice on the draft PA, including its advice on the current primary and secondary PM standards, in a letter to the EPA Administrator dated December 16, 2019 (Cox, 2019a). With regard to the primary standards, the CASAC recommended retaining the current 24-hour PM 2.5 and PM 10 standards but did not reach consensus on the adequacy of the current annual PM 2.5 standard. With regard to the secondary standards, the CASAC recommended retaining the current standards. The CASAC's advice on the primary and secondary PM standards, and the Administrator's consideration of that advice in reaching proposed decisions, is discussed in detail in sections II.C.2 and II.C.3 (primary PM 2.5 standards), III.C.2 and III.C.3 (primary PM 10 standards), and IV.D.2 and IV.D.3 (secondary standards) of this document. The CASAC additionally made a number of recommendations regarding the information and analyses presented in the draft PA. Specifically, the CASAC recommended that a revised PA include (1) additional discussion of the current CASAC and NAAQS review process; (2) additional characterization of PM-related emissions, monitoring and air quality information, including uncertainties in that information; (3) additional discussion and examination of uncertainties in the PM 2.5 health evidence and the risk assessment; (4) updates to reflect changes in the ISA's causality determinations; and (5) additional discussion of the evidence for PM-related welfare effects, including uncertainties (Cox, 2019a, pp. 2-3 in letter). In response to the CASAC's comments, the final PA 16 incorporated a number of changes, including the following (U.S. EPA, 2020): 16 Given the Administrator's timeline for this review, as noted above (Pruitt, 2018), the EPA did not prepare a second draft PA. Rather, the CASAC's advice was considered in developing the final PA (U.S. EPA, 2020). • Text was added to Chapter 1 to clarify the process followed for this review of the PM NAAQS, including how the process has evolved since the initiation of the review. • Text and figures were added to Chapter 2 on emissions of PM and PM precursors, and a section discussing uncertainty in emissions estimates was added. A discussion of measurement uncertainty for FRM, FEM, CSN, and IMPROVE monitors was also added. • Chapter 3 and Appendices B and C include a number of changes, including: ○ An expanded characterization and discussion of the evidence related to exposure measurement error, the potential confounders examined by key studies, the shapes of concentration-response functions, and the results of causal inference and quasi-experimental studies. ○ An expanded and clarified discussion of uncertainties in the risk assessment, and additional air quality model performance evaluations for each of the urban study areas included in the risk assessment. ○ Additional detail on the procedure used to derive concentration-response functions used in the risk assessment. ○ Changes in the text to reflect the change in the final ISA's causality determination from “likely to be causal” to “suggestive of, but not sufficient to infer, a causal relationship.” • Throughout the document (Chapters 3, 4 and 5), summaries of the CASAC advice on the PM standards are included, and expanded discussions of data gaps and areas for future research in the health and welfare effects evidence are presented. D. Air Quality Information This section provides a summary of basic information related to PM ambient air quality. It summarizes information on the distribution of particle size in ambient air (I.D.1), sources and emissions contributing to PM in the ambient air (I.D.2), monitoring of ambient PM in the U.S. (I.D.3), ambient PM concentrations and trends in the U.S. (I.D.4), and background PM (I.D.5). Additional detail on PM air quality can be found in Chapter 2 of the Policy Assessment (U.S. EPA, 2020; PA). 1. Distribution of Particle Size in Ambient Air In ambient air, PM is a mixture of substances suspended as small liquid and/or solid particles (U.S. EPA, 2019, section 2.2). Particle size is an important consideration for PM, as distinct health and welfare effects have been linked with exposures to particles of different sizes. Particles in the atmosphere range in size from less than 0.01 to more than 10 μm in diameter (U.S. EPA, 2019, section 2.2). When describing PM, subscripts are used to denote the aerodynamic diameter 17 of the particle size range, in µm, of 50% cut points of sampling devices. The EPA defines PM 2.5 , also referred to as fine particles, as particles with aerodynamic diameters generally less than or equal to 2.5 μm. The size range for PM 10-2.5 , also called coarse or thoracic coarse particles, includes those particles with aerodynamic diameters generally greater than 2.5 μm and less than or equal to 10 μm. PM 10 , which is comprised of both fine and coarse fractions, includes those particles with aerodynamic diameters generally less than or equal to 10 μm. In addition, UFP are often defined as particles with a diameter of less than 0.1 μm based on physical size, thermal diffusivity or electrical mobility (U.S. EPA, 2019, section 2.2). 17 Aerodynamic diameter is the size of a sphere of unit density ( i.e., 1 g/cm 3 ) that has the same terminal settling velocity as the particle of interest (U.S. EPA, 2019, section 4.1.1). Atmospheric distributions of particle size generally exhibit distinct modes that roughly align with the PM size fractions defined above. The nucleation mode is made up of freshly generated particles, formed either during combustion or by atmospheric reactions of precursor gases. The nucleation mode is especially prominent near sources like heavy traffic, industrial emissions, biomass burning, or cooking (Vu et al., 2015). While nucleation mode particles are only a minor contributor to overall ambient PM mass and surface area, they are the main contributors to ambient particle number (U.S. EPA, 2019, section 2.2). By number, most nucleation mode particles fall into the UFP size range, though some fraction of the nucleation mode number distribution can extend above 0.1 μm in diameter. Nucleation mode particles can grow rapidly through coagulation or uptake of gases by particle surfaces, giving rise to the accumulation mode. The accumulation mode is typically the predominant contributor to PM 2.5 mass, though only a minor contributor to particle number (U.S. EPA, 2019, section 2.2). PM 2.5 sampling methods measure most of the accumulation mode mass, although a small fraction of particles that make up the accumulation mode are greater than 2.5 μm in diameter. Coarse mode particles are formed by mechanical generation, and through processes like dust resuspension and sea spray formation (Whitby et al., 1972). Most coarse mode mass is captured by PM 10-2.5 sampling, but small fractions of coarse mode mass can be smaller than 2.5 μm or greater than 10 μm in diameter (U.S. EPA, 2019, section 2.2). Most particles are found in the lower troposphere, where they can have residence times ranging from a few hours to weeks. Particles are removed from the atmosphere by wet deposition, such as when they are carried by rain or snow, or by dry deposition, when particles settle out of suspension due to gravity. Atmospheric lifetimes are generally longest for PM 2.5 , which often remains in the atmosphere for days to weeks (U.S. EPA, 2019, Table 2-1) before being removed by wet or dry deposition. In contrast, atmospheric lifetimes for UFP and PM 10-2.5 are shorter. Within hours, UFP can undergo coagulation and condensation that lead to formation of larger particles in the accumulation mode, or can be removed from the atmosphere by evaporation, deposition, or reactions with other atmospheric components. PM 10-2.5 are also generally removed from the atmosphere within hours, through wet or dry deposition (U.S. EPA, 2019, Table 2-1). 2. Sources and Emissions Contributing to PM in the Ambient Air PM is composed of both primary (directly emitted particles) and secondary particles. Primary PM is derived from direct particle emissions from specific PM sources while secondary PM originates from gas-phase chemical compounds present in the atmosphere that have participated in new particle formation or condensed onto existing particles (U.S. EPA, 2019, section 2.3). As discussed further in the ISA (U.S. EPA, 2019, section 2.3.2.1), secondary PM is formed in the atmosphere by photochemical oxidation reactions of both inorganic and organic gas-phase precursors. Precursor gases include sulfur dioxide (SO 2 ), nitrogen oxides (NO X ), and volatile organic compounds (VOC) (U.S. EPA, 2019, section 2.3.2.1). Ammonia also plays an important role in the formation of nitrate PM by neutralizing sulfuric acid and nitric acid. Sources and emissions of PM are discussed in more detail in section 2.1.1 of the PA (U.S. EPA, 2020). Direct emissions of PM have remained relatively unchanged in recent years, while emissions of some precursor gases have declined substantially. 18 From 1990 to 2014, SO 2 emissions have undergone the largest declines while NH 3 emissions have undergone the smallest change. Declining SO 2 emissions during this time period are primarily a result of reductions at stationary sources such as EGUs, with substantial reductions also from mobile sources (U.S. EPA, 2019, section 2.3.2.1). 19 18 More information on these trends, including details on methods and explanations on the noted changes over time is available at https://gispub.epa.gov/neireport/2014/. 19 State-specific emission trends data for 1990 to 2014 can be found at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data. 3. Monitoring of Ambient PM To promote uniform enforcement of the air quality standards set forth under the CAA and to achieve the degree of public health and welfare protection intended for the NAAQS, the EPA established PM Federal Reference Methods (FRMs) 20 for both PM 10 and PM 2.5 (40 CFR appendix J and L to Part 50) and performance requirements for approval of Federal Equivalent Methods (FEMs) (40 CFR part 53). Amended following the 2006 and 2012 p.m. NAAQS reviews, the current PM monitoring network relies on FRMs and automated continuous FEMs, in part to support changes necessary for implementation of the revised PM standards. The requirements for measuring ambient air quality and reporting ambient air quality data and related information are the basis for 40 CFR appendices A through E to Part 58. More information on PM ambient monitoring networks is available in section 2.2 of the PA (U.S. EPA, 2020). 20 FRMs provide the methodological basis for comparison to the NAAQS and also serve as the “gold-standard” for the comparison of other methods being reviewed for potential approval as equivalent methods. The EPA keeps a complete list of designated reference and equivalent methods available on its Ambient Monitoring Technology Information Center (AMTIC) website ( https://www.epa.gov/amtic/air-monitoring-methods-criteria-pollutants ). 4. Ambient Concentrations and Trends This section summarizes available information on recent ambient PM concentrations in the U.S. and on trends in PM air quality. Sections I.D.4.a and I.D.4.b summarize information on PM 2.5 mass and components, respectively. Section I.D.4.c summarizes information on PM 10 . Sections I.D.4.d and I.D.4.e summarize the more limited information on PM 10-2.5 and UFP, respectively. Additional detail on PM air quality and trends can be found in section 2.3 of the PA (U.S. EPA, 2020). a. PM 2.5 Mass At monitoring sites in the U.S., annual PM 2.5 concentrations from 2015 to 2017 averaged 8.0 μg/m 3 (and ranged from 3.0 to 18.2 μg/m 3 ) and the 98th percentiles of 24-hour concentrations averaged 20.9 μg/m 3 (and ranged from 9.2 to 111 μg/m 3 ) (U.S. EPA, 2020, section 2.3.2.1). The highest ambient PM 2.5 concentrations occur in the west, particularly in California and the Pacific northwest (U.S. EPA, 2020, Figure 2-8). Much of the eastern U.S. has lower ambient concentrations, with annual average concentrations generally at or below 12.0 μg/m 3 and 98th percentiles of 24-hour concentrations generally at or below 30 μg/m 3 (U.S. EPA, 2020, section 2.3.2). Recent ambient PM 2.5 concentrations reflect the substantial reductions that have occurred across much of the U.S. (U.S. EPA, 2020, section 2.3.2.1). From 2000 to 2017, national annual average PM 2.5 concentrations have declined from 13.5 μg/m 3 to 8.0 μg/m 3 , a 41% decrease (U.S. EPA, 2020, section 2.3.2.1). 21 These declines have occurred at urban and rural monitoring sites, although urban PM 2.5 concentrations remain consistently higher than those in rural areas (Chan et al., 2018) due to the impact of local sources in urban areas. Analyses at individual monitoring sites indicate that declines in ambient PM 2.5 concentrations have been most consistent across the eastern U.S. and in parts of coastal California, where both annual average and 98th percentiles of 24-hour concentrations have declined significantly (U.S. EPA, 2020, section 2.3.2.1). In contrast, trends in ambient PM 2.5 concentrations have been less consistent over much of the western U.S., with no significant changes since 2000 observed at some sites in the Pacific northwest, the northern Rockies and plains, and the southwest, particularly for 98th percentiles of 24-hour concentrations (U.S. EPA, 2020, section 2.3.2.1). 21 See https://www.epa.gov/air-trends/particulate-matter-pm25-trends and https://www.epa.gov/air-trends/particulate-matter-pm25-trends#pmnat for more information. The recent deployment of PM 2.5 monitors near major roads in large urban areas provides information on PM 2.5 concentrations near an important emissions source. Of the 25 CBSAs with valid design values at near-road monitoring sites, 22 52% measured the highest annual design value at the near-road site while 24% measured the highest 24-hour design value at the near-road site (U.S. EPA, 2020, section 2.3.2.2). Of the CBSAs with highest annual design values at near-road sites, those design values were, on average, 0.7 μg/m 3 higher than at the highest measuring non-near-road sites (range is 0.1 to 2.0 μg/m 3 higher at near-road sites). Although most near-road monitoring sites do not have sufficient data to evaluate long-term trends in near-road PM 2.5 concentrations, analyses of the data at one near-road-like site in Elizabeth, NJ, 23 show that the annual average near-road increment has generally decreased between 1999 and 2017 from about 2.0 μg/m 3 to about 1.3 μg/m 3 (U.S. EPA, 2020, section 2.3.2.2). 22 A design value is considered valid if it meets the data handling requirements given in 40 CFR appendix N to Part 50. Several large CBSAs such as Chicago-Naperville-Elgin, IL-IN-WI and Houston-The Woodlands-Sugar Land, TX had near-road sites that did not have valid PM 2.5 design values for the 2015-2017 period. 23 The Elizabeth Lab site in Elizabeth, NJ is situated approximately 30 meters from travel lanes of the Interchange 13 toll plaza of the New Jersey Turnpike and within 200 meters of travel lanes for Interstate 278 and the New Jersey Turnpike. b. PM 2.5 Components Based on recent air quality data, the major chemical components of PM 2.5 have distinct spatial distributions. Sulfate concentrations tend to be highest in the eastern U.S., while in the Ohio Valley, Salt Lake Valley, and California nitrate concentrations are highest, and relatively high concentrations of organic carbon are widespread across most of the continental U.S. (U.S. EPA, 2020, section 2.3.2.3). Elemental carbon, crustal material, and sea-salt are found to have the highest concentrations in the northeast U.S., southwest U.S., and coastal areas, respectively. An examination of PM 2.5 composition trends can provide insight into the factors contributing to overall reductions in ambient PM 2.5 concentrations. The biggest change in PM 2.5 composition that has occurred in recent years is the reduction in sulfate concentrations due to reductions in SO 2 emissions. Between 2000 and 2015, the nationwide annual average sulfate concentration decreased by 17% at urban sites and 20% at rural sites. This change in sulfate concentrations is most evident in the eastern U.S. and has resulted in organic matter or nitrate now being the greatest contributor to PM 2.5 mass in many locations (U.S. EPA, 2019, Figure 2-19). The overall reduction in sulfate concentrations has contributed substantially to the decrease in national average PM 2.5 concentrations as well as the decline in the fraction of PM 10 mass accounted for by PM 2.5 (U.S. EPA, 2019, section 2.5.1.1.6; U.S. EPA, 2020, section 2.3.1). c. PM 10 At monitoring sites in the U.S., the 2015-2017 average of 2nd highest 24-hour PM 10 concentration was 56 μg/m 3 (ranging from 18 to 173 μg/m 3 ) (U.S. EPA, 2020, section 2.3.2.4). 24 The highest PM 10 concentrations tend to occur in the western U.S. Seasonal analyses indicate that ambient PM 10 concentrations are generally higher in the summer months than at other times of year, though the most extreme high concentration events are more likely in the spring (U.S. EPA, 2019, Table 2-5). This is due to fact that the major PM 10 emission sources, dust and agriculture, are more active during the warmer and drier periods of the year. 24 The form of the current 24-hour PM 10 standard is one-expected-exceedance, averaged over three years. Recent ambient PM 10 concentrations reflect reductions that have occurred across much of the U.S. (U.S. EPA, 2020, section 2.3.2.4). From 2000 to 2017, annual second highest 24-hour PM 10 concentrations have declined by about 30% (U.S. EPA, 2020, section 2.3.2.4). 25 These PM 10 concentrations have generally declined in the eastern U.S., while concentrations in the much of the midwest and western U.S. have remained unchanged or increased since 2000 (U.S. EPA, 2020, section 2.3.2.4). Analyses at individual monitoring sites indicate that annual average PM 10 concentrations have also declined at most sites across the U.S., with much of the decrease in the eastern U.S. associated with reductions in PM 2.5 concentrations. 25 For more information, see https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat. d. PM 10-2.5 Since the last review, the availability of PM 10-2.5 ambient concentration data has greatly increased. As illustrated in the PA (U.S. EPA, 2020, section 2.3.2.5), annual average and 98th percentile PM 10-2.5 concentrations exhibit less distinct differences between the eastern and western U.S. than for either PM 2.5 or PM 10 . Additionally, compared to PM 2.5 and PM 10 , changes in PM 10-2.5 concentrations have been small in magnitude and inconsistent in direction (U.S. EPA, 2020, section 2.3.2.5). e. UFP Compared to PM 2.5 mass, there is relatively little data on U.S. particle number concentrations, which are dominated by UFP. Based on measurements in two urban areas (New York City, Buffalo) and at a background site (Steuben County) in New York, urban particle number counts were several times higher than at the background site (U.S. EPA, 2020, section 2.3.2.6; U.S. EPA, 2019, Figure 2-18). The highest particle number counts in an urban area with multiple sites (Buffalo) were observed at a near-road location. Long-term trends in UFP are not routinely available at U.S. monitoring sites. At one site in Illinois with long-term data available, the annual average particle number concentration declined between 2000 and 2017, closely matching the reductions in annual PM 2.5 mass over that same period (U.S. EPA, 2020, section 2.3.2.6). In addition, a small number of published studies have examined UFP trends over time. While limited, these studies also suggest that UFP number concentrations have declined over time along with decreases in PM 2.5 (U.S. EPA, 2020, section 2.3.2.6). 5. Background PM In this review, background PM is defined as all particles that are formed by sources or processes that cannot be influenced by actions within the jurisdiction of concern. U.S. background PM is defined as any PM formed from emissions other than U.S. anthropogenic ( i.e. manmade) emissions. Potential sources of U.S. background PM include both natural sources ( i.e., PM that would exist in the absence of any anthropogenic emissions of PM or PM precursors) and transboundary sources originating outside U.S. borders. Background PM is discussed in more detail in section 2.4 of the PA (U.S. EPA, 2020). At annual and national scales, estimated background PM concentrations in the U.S. are small compared to contributions from domestic anthropogenic emissions. For example, based on zero-out modeling in the last review of the PM NAAQS, annual background PM 2.5 concentrations were estimated to range from 0.5-3 µg/m 3 across the sites examined. In addition, speciated monitoring data from IMPROVE sites can provide some insights into how contributions from different PM sources, including sources of background PM, may have changed over time. As discussed further in the PA (U.S. EPA, 2020, section 2.4), such data suggests that estimates of background concentrations at IMPROVE monitors are around 1-3 µg/m 3 , and have not changed significantly since the last PM NAAQS Review. As discussed further in the PA (U.S. EPA, 2020, section 2.4), sources that contribute to natural background PM include dust from the wind erosion of natural surfaces, sea salt, wildland fires, primary biological aerosol particles such as bacteria and pollen, oxidation of biogenic hydrocarbons such as isoprene and terpenes to produce secondary organic aerosols (SOA), and geogenic sources such as sulfate formed from volcanic production of SO 2 and oceanic production of dimethyl-sulfide. While most of these sources release or contribute predominantly to fine aerosol, some sources including windblown dust, and sea salt also produce particles in the coarse size range (U.S. EPA, 2019, section 2.3.3). The magnitude and sources of background PM can vary widely by region and time of year. Coastal sites may experience a consistent contribution of PM from sea spray aerosol, while other areas covered with dense vegetation may be impacted by biogenic aerosol production during the summertime. Sources of background PM also operate across a range of time scales. While some sources like biogenic aerosol vary at monthly to seasonal scales, many sources of background PM are episodic in nature. These episodic sources ( e.g., large wildfires) can be characterized by infrequent contributions to high-concentration events occurring over shorter periods of time ( e.g., hours to several days). Such episodic events are sporadic and do not necessarily occur in all years. While these exceptional episodes can lead to exceedances of the 24-hour PM 2.5 standard (35 µg/m 3 ) in some cases (Schweizer et al., 2017), such events are routinely screened for and usually identifiable in the monitoring data. As described further in the PA (U.S. EPA, 2020, section 2.4), contributions to background PM in the U.S. result mainly from sources within North America. Contributions from intercontinental events have also been documented ( e.g., transport from dust storms occurring in deserts in North Africa and Asia), but these events are less frequent and represent a relatively small fraction of background PM in most places. II. Rationale for Proposed Decisions on the Primary PM 2.5 Standards This section provides the rationale supporting the Administrator's proposed decisions on the primary PM 2.5 standards. Section II.A describes the Agency's approach to reaching decisions on the primary PM 2.5 standards in the last review and summarizes the general approach to reaching proposed decisions in this review. Section II.B summarizes the scientific evidence for PM 2.5 -related health effects. Section II.C presents the Administrator's proposed conclusions regarding the adequacy of the current primary PM 2.5 standards and his proposed decision to retain those standards in this review. 26 26 Sections III and IV provide the rationales supporting the Administrator's proposed decisions on the primary PM 10 standard and secondary standards, respectively. A. General Approach 1. Approach Used in the Last Review The last review of the primary PM NAAQS was completed in 2012 (78 FR 3086, January 15, 2013). As noted above (section 1.3), in the last review the EPA lowered the level of the primary annual PM 2.5 standard from 15.0 to 12.0 μg/m 3 , 27 and retained the existing 24-hour PM 2.5 standard with its level of 35 μg/m 3 . The 2012 decision to strengthen the suite of primary PM 2.5 standards was based on the prior Administrator's consideration of the extensive body of scientific evidence assessed in the 2009 ISA (U.S. EPA, 2009c); the quantitative risk analyses presented in the 2010 health risk assessment (U.S. EPA, 2010a); the advice and recommendations of the CASAC (Samet, 2009; Samet, 2010c; Samet, 2010b); and public comments on the proposed rule (78 FR 3086, January 15, 2013; U.S. EPA, 2012a). She particularly noted the “strong and generally robust body of evidence of serious health effects associated with both long- and short-term exposures to PM 2.5 ” (78 FR 3120, January 15, 2013). This included epidemiologic studies reporting health effect associations based on long-term average PM 2.5 concentrations ranging from about 15.0 μg/m 3 or above ( i.e., at or above the level of the then-existing annual standard) to concentrations “significantly below the level of the annual standard” (78 FR 3120, January 15, 2013). Based on her “confidence in the association between exposure to PM 2.5 and serious public health effects, combined with evidence of such an association in areas that would meet the current standards” (78 FR 3120, January 15, 2013), the prior Administrator concluded that revision of the suite of primary PM 2.5 standards was necessary in order to provide increased public health protection. 27 The Agency also eliminated spatial averaging provisions as part of the form of the annual standard. The prior Administrator next considered what specific revisions to the existing primary PM 2.5 standards were appropriate, given the available evidence and quantitative risk information. She considered both the annual and 24-hour PM 2.5 standards, focusing on the basic elements of those standards ( i.e., indicator, averaging time, form, and level). These considerations, and the prior Administrator's conclusions, are summarized in sections II.A.1.a to II.A.1.d below. a. Indicator In the last review, the EPA considered issues related to the appropriate indicator for fine particles, with a focus on evaluating support for the existing PM 2.5 mass-based indicator and for potential alternative indicators based on the UFP fraction or on fine particle composition (78 FR 3121, January 15, 2013). 28 With regard to PM 2.5 mass, as in the 1997 and 2006 reviews, the health studies available during the last review continued to link adverse health outcomes ( e.g., premature mortality, hospital admissions, emergency department visits) with long- and short-term exposures to fine particles indexed largely by PM 2.5 mass (78 FR 3121, January 15, 2013). With regard to the ultrafine fraction of ambient PM, the 2011 PA noted the limited body of health evidence assessed in the 2009 ISA (summarized in U.S. EPA, 2009c, section 2.3.5 and Table 2-6) and the limited monitoring information available to characterize ambient concentrations of UFP (U.S. EPA, 2011, section 1.3.2). With regard to PM composition, the 2009 ISA concluded that “the evidence is not yet sufficient to allow differentiation of those constituents or sources that are more closely related to specific health outcomes” (U.S. EPA, 2009c, pp. 2-26 and 6-212; 78 FR 3123, January 15, 2013). The 2011 PA further noted that “many different constituents of the fine particle mixture as well as groups of components associated with specific source categories of fine particles are linked to adverse health effects” (U.S. EPA, 2011, p. 2-55; 78 FR 3123, January 15, 2013). Consistent with the considerations and conclusions in the 2011 PA, the CASAC advised that it was appropriate to consider retaining PM 2.5 as the indicator for fine particles. In light of the evidence and the CASAC's advice, the prior Administrator concluded that it was “appropriate to retain PM 2.5 as the indicator for fine particles” (78 FR 3123, January 15, 2013). 28 In the last review, the ISA defined UFP as generally including particles with a mobility diameter less than or equal to 0.1 µm. Mobility diameter is defined as the diameter of a particle having the same diffusivity or electrical mobility in air as the particle of interest, and is often used to characterize particles of 0.5 µm or smaller (U.S. EPA, 2009c, pp. 3-2 to 3-3). b. Averaging Time In 1997, the EPA set an annual PM 2.5 standard to provide protection from health effects associated with long- and short-term exposures to PM 2.5 , and a 24-hour standard to supplement the protection afforded by the annual standard (62 FR 38667 to 38668, July 18, 1997). In the 2006 review, the EPA retained both annual and 24-hour averaging times (71 FR 61164, October 17, 2006). In the last review, the EPA again considered issues related to the appropriate averaging times for PM 2.5 standards, with a focus on evaluating support for the existing annual and 24-hour averaging times and for potential alternative averaging times based on sub-daily or seasonal metrics. Based on the evidence assessed in the ISA, the 2011 PA noted that the overwhelming majority of studies that had been conducted since the 2006 review continued to utilize annual (or multi-year) or 24-hour PM averaging periods (U.S. EPA, 2011, section 2.3.2). Given this, and limitations in the data for alternatives, the 2011 PA reached the overall conclusions that the available information provided strong support for considering retaining the current annual and 24-hour averaging times (U.S. EPA, 2011, p. 2-58). The CASAC agreed that these conclusions were reasonable (Samet, 2010a, p. 13). The prior Administrator concurred with the PA conclusions and with the CASAC's advice. Specifically, she judged that it was “appropriate to retain the current annual and 24-hour averaging times for the primary PM 2.5 standards to protect against health effects associated with long- and short-term exposure periods” (78 FR 3124, January 15, 2013). c. Form In 1997, the EPA established the form of the annual PM 2.5 standard as an annual arithmetic mean, averaged over 3 years, from single or multiple community-oriented monitors. 29 That is, the level of the annual standard was to be compared to measurements made at each community-oriented monitoring site or, if specific criteria were met, measurements from multiple community-oriented monitoring sites could be averaged together ( i.e., spatial averaging) 30 (62 FR 38671 to 38672, July 18, 1997). In the 1997 review, the EPA also established the form of the 24-hour PM 2.5 standard as the 98th percentile of 24-hour concentrations at each monitor within an area ( i.e., no spatial averaging), averaged over three years (62 FR at 38671 to 38674, July 18, 1997). In the 2006 review, the EPA retained these standard forms but tightened the criteria for using spatial averaging with the annual standard (71 FR 61167, October 17, 2006). 31 29 In the last review, the EPA replaced the term “community-oriented” monitor with the term “area-wide” monitor (U.S. EPA, 2020, section 1.3). Area-wide monitors are those sited at the neighborhood scale or larger, as well as those monitors sited at micro- or middle scales that are representative of many such locations in the same core-based statistical area (CBSA; 78 FR 3236, January 15, 2013). CBSAs are required to have at least one area-wide monitor sited in the area of expected maximum PM 2.5 concentration. 30 The original criteria for spatial averaging included: (1) The annual mean concentration at each site shall be within 20% of the spatially averaged annual mean, and (2) the daily values for each monitoring site pair shall yield a correlation coefficient of at least 0.6 for each calendar quarter (62 FR 38671 to 38672, July 18, 1997). 31 Specifically, the Administrator revised spatial averaging criteria such that “(1) [t]he annual mean concentration at each site shall be within 10 percent of the spatially averaged annual mean, and (2) the daily values for each monitoring site pair shall yield a correlation coefficient of at least 0.9 for each calendar quarter (71 FR 61167, October 17, 2006). In the last review, the EPA's consideration of the form of the annual PM 2.5 standard again included a focus on the issue of spatial averaging. An analysis of air quality and population demographic information indicated that the highest PM 2.5 concentrations in a given area tended to be measured at monitors in locations where the surrounding populations were more likely to live below the poverty line and to include larger percentages of racial and ethnic minorities (U.S. EPA, 2011, p. 2-60). Based on this analysis, the 2011 PA concluded that spatial averaging could result in disproportionate impacts in at-risk populations, including minority populations and populations with lower socioeconomic status (SES). Therefore, the PA concluded that it was appropriate to consider revising the form of the annual PM 2.5 standard such that it did not allow for the use of spatial averaging across monitors (U.S. EPA, 2011, p. 2-60). The CASAC agreed with the PA conclusions that it was “reasonable” for the EPA to eliminate the spatial averaging provisions (Samet, 2010c, p. 2). The prior Administrator concluded that public health would not be protected with an adequate margin of safety in all locations, as required by law, if disproportionately higher PM 2.5 concentrations in low income and minority communities were averaged together with lower concentrations measured at other sites in a large urban area. Therefore, she concluded that the form of the annual PM 2.5 standard should be revised to eliminate spatial averaging provisions (78 FR 3124, January 15, 2013). In the last review, the EPA also considered the form of the 24-hour PM 2.5 standard. The Agency recognized that the existing 98th percentile form for the 24-hour standard was originally selected to provide a balance between limiting the occurrence of peak 24-hour PM 2.5 concentrations and identifying a stable target for risk management programs. 32 Updated air quality analyses in the last review provided additional support for the increased stability of the 98th percentile PM 2.5 concentration, compared to the 99th percentile (U.S. EPA, 2011, Figure 2-2, p. 2-62). Consistent with the PA conclusions based on this analysis, the prior Administrator concluded that it was appropriate to retain the 98th percentile form for the 24-hour PM 2.5 standard (78 FR 3127, January 15, 2013). 32 See ATA III, 283 F.3d at 374-76 which concludes that it is legitimate for the EPA to consider overall stability of the standard and its resulting promotion of overall effectiveness of NAAQS control programs in setting a standard that is requisite to protect the public health. d. Level The EPA's approach to considering alternative levels of the PM 2.5 standards in the last review was based on evaluating the public health protection afforded by the annual and 24-hour standards, taken together, against mortality and morbidity effects associated with long-term or short-term PM 2.5 exposures. This approach recognized that it is appropriate to consider the protection provided by attaining the air quality needed to meet the suite of standards, and that there is no bright line clearly directing the choice of levels. Rather, the choice of what is appropriate is a public health policy judgment entrusted to the Administrator. See Mississippi, 744 F.3d at 1358, Lead Industries Ass'n, 647 F.2d at 1147. In selecting the levels of the annual and 24-hour PM 2.5 standard, the prior Administrator placed the greatest emphasis on health endpoints for which the evidence was strongest, based on the assessment of the evidence in the ISA and on the ISA's causality determinations (U.S. EPA, 2009c, section 2.3.1). She particularly noted that the evidence was sufficient to conclude a causal relationship exists between PM 2.5 exposures and mortality and cardiovascular effects ( i.e., for both long- and short-term exposures) and that the evidence was sufficient to conclude a causal relationship is “likely” to exist between PM 2.5 exposures and respiratory effects ( i.e., for both long- and short-term exposures). She also noted additional, but more limited, evidence for a broader range of health endpoints, including evidence “suggestive of a causal relationship” between long-term exposures and developmental and reproductive effects as well as carcinogenic effects (78 FR 3158, January 15, 2013). To inform her decisions on an appropriate level for the annual standard, the prior Administrator considered the degree to which epidemiologic studies indicate confidence in the reported health effect associations over distributions of ambient PM 2.5 concentrations. She noted that a level of 12.0 µg/m 3 was below the long-term mean PM 2.5 concentrations reported in key epidemiologic studies that provided evidence of an array of serious health effects (78 FR 3161, January 15, 2013). She further noted that 12.0 µg/m 3 generally corresponded to the lower portions ( i.e., about the 25th percentile) of distributions of health events in the limited number of epidemiologic studies for which population-level information was available. A level of 12.0 µg/m 3 also reflected placing some weight on studies of reproductive and developmental effects, for which the evidence was more uncertain (78 FR 3161-3162, January 15, 2013). 33 33 With respect to cancer, mutagenic, and genotoxic effects, the Administrator observed that the PM 2.5 concentrations reported in studies evaluating these effects generally included ambient concentrations that are equal to or greater than ambient concentrations observed in studies that reported mortality and cardiovascular and respiratory effects (U.S. EPA, 2009c, section 7.5). Therefore, the Administrator concluded that, in selecting a standard level that provides protection from mortality and cardiovascular and respiratory effects, it is reasonable to anticipate that protection will also be provided for carcinogenic effects (78 FR 3161-3162, January 15, 2013). Given the uncertainties remaining in the scientific information, the prior Administrator judged that an annual standard level below 12.0 µg/m 3 was not supported. She specifically noted uncertainties related to understanding the relative toxicity of the different components in the fine particle mixture, the role of PM 2.5 in the complex ambient mixture, exposure measurement errors in epidemiologic studies, and the nature and magnitude of estimated risks at relatively low ambient PM 2.5 concentrations. Furthermore, she noted that epidemiologic studies had reported heterogeneity in responses both within and between cities and in geographic regions across the U.S. She recognized that this heterogeneity may be attributed, in part, to differences in fine particle composition in different regions and cities. With regard to evidence for reproductive and developmental effects, the prior Administrator recognized that there were a number of limitations associated with this body of evidence, including the following: The limited number of studies evaluating such effects; uncertainties related to identifying the relevant exposure time periods of concern; and limited toxicological evidence providing little information on the mode of action(s) or biological plausibility for an association between long-term PM 2.5 exposures and adverse birth outcomes. On balance, she found that the available evidence, interpreted in light of these remaining uncertainties, did not justify an annual standard level set below 12.0 µg/m 3 as being “requisite” to protect public health with an adequate margin of safety ( i.e., a standard with a lower level would have been more stringent than necessary). In conjunction with a revised annual standard with a level of 12.0 µg/m 3 , the prior Administrator concluded that the evidence supported retaining the 35 µg/m 3 level of the 24-hour PM 2.5 standard. She noted that the existing 24-hour standard, with its 35 µg/m 3 level and 98th percentile form, would provide supplemental protection, particularly for areas with high peak-to-mean ratios possibly associated with strong seasonal sources and for areas with PM 2.5 -related effects that may be associated with shorter than daily exposure periods (78 FR 3163, January 15, 2013). Thus, she concluded that the available evidence and information, interpreted in light of remaining uncertainties, supported an annual standard with a level of 12.0 µg/m 3 combined with a 24-hour standard with a level of 35 µg/m 3 . 2. Approach in the Current Review The EPA's approach to reaching proposed decisions on the primary PM 2.5 standards in the current review builds on the decisions made in the last review. Consistent with that review, the approach focuses on evaluating the public health protection afforded by the annual and 24-hour standards, taken together, against mortality and morbidity associated with long-term or short-term PM 2.5 exposures. As discussed in the PA (U.S. EPA, 2020, section 3.1.2), in adopting this approach the EPA recognizes that changes in PM 2.5 air quality designed to meet an annual standard would likely result not only in lower annual average PM 2.5 concentrations, but also in fewer and lower short-term peak PM 2.5 concentrations. Additionally, changes designed to meet a 24-hour standard, with a 98th percentile form, would result not only in fewer and lower peak 24-hour PM 2.5 concentrations, but also in lower annual average PM 2.5 concentrations. Thus, the EPA's approach recognizes that it is appropriate to consider the protection provided by attaining the air quality needed to meet the suite of standards. This approach to reviewing the primary PM 2.5 standards is based most fundamentally on considering the available scientific evidence and technical information as assessed and discussed in the ISA (U.S. EPA, 2019) and PA (U.S. EPA, 2020), including the uncertainties inherent in that evidence and information, and on consideration of advice received from the CASAC in this review (Cox, 2019a). The EPA emphasizes the health outcomes for which the ISA determines that the evidence supports either a “causal” or a “likely to be causal” relationship with PM 2.5 exposures (U.S. EPA, 2019). This approach focuses proposed decisions on the health outcomes for which the evidence is strongest. Such a focus, which is supported by the CASAC (Cox, 2019a, p. 12 of consensus responses), recognizes that standards set based on evidence supporting “causal” and “likely to be causal” health outcomes will also provide some measure of protection against the broader range of PM 2.5 -associated outcomes, including those for which the evidence is less certain. As in past reviews, the EPA's approach recognizes that there is no bright line clearly directing the choice of standards. Rather, the choice of what is appropriate is a public health policy judgment entrusted to the Administrator. Specifically, the CAA requires primary standards that, in the judgment of the Administrator, are requisite to protect public health with an adequate margin of safety. In setting primary standards that are “requisite” to protect public health, the EPA's task is to establish standards that are neither more nor less stringent than necessary for this purpose. Thus, as discussed above (I.A), the CAA does not require that primary standards be set at a zero-risk level, but rather at a level that, in the judgment of the Administrator, limits risk sufficiently so as to protect public health with an adequate margin of safety. As in previous reviews, this judgment includes consideration of the strengths and limitations of the scientific and technical information, and the appropriate inferences to be drawn from that information. B. Health Effects Related to Fine Particle Exposures This section draws from the EPA's synthesis and assessment of the scientific evidence presented in the ISA (U.S. EPA, 2019) and the summary of that evidence in the PA (U.S. EPA, 2020, section 3.2.1). The ISA uses a weight-of-evidence framework for characterizing the strength of the available scientific evidence for health effects attributable to PM exposures (U.S. EPA, 2015, Preamble, Section 5). As in the last review (U.S. EPA, 2009c), the ISA for this review has adopted a five-level hierarchy to classify the overall weight-of-evidence into one of the following categories: Causal relationship; a likely to be causal relationship; suggestive of, but not sufficient to infer, a causal relationship; 34 inadequate to infer the presence or absence of a causal relationship; and not likely to be a causal relationship (U.S. EPA, 2015, Preamble Table II). In using the weight-of-evidence approach to inform judgments about the likelihood that various health effects are caused by PM exposures, evidence is evaluated for major outcome categories or groups of related outcomes ( e.g., respiratory effects), integrating evidence from across disciplines, including epidemiologic, controlled human exposure, and animal toxicological studies and evaluating the coherence of evidence across a spectrum of related endpoints as well as biological plausibility of the effects observed (U.S. EPA, 2015, Preamble, Section 5.c.). Based on application of this approach, the EPA believes that the final ISA “accurately reflects the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [PM] in the ambient air, in varying quantities” as required by the CAA (42 U.S.C. 7408(a)(2)). 34 As noted in the 2019 p.m. ISA (U.S. EPA, 2019, p. ES-15), this causality determination language has been updated since the last review. In this review of the NAAQS, the EPA considers the full body of health evidence, placing the greatest emphasis on the health effects for which the evidence has been judged in the ISA to demonstrate a “causal” or a “likely to be causal” relationship with PM exposures. The ISA defines these causality determinations as follows (U.S. EPA, 2019, p. p-20): • Causal relationship: The pollutant has been shown to result in health effects at relevant exposures based on studies encompassing multiple lines of evidence and chance, confounding, and other biases can be ruled out with reasonable confidence. • Likely to be a causal relationship: There are studies in which results are not explained by chance, confounding, or other biases, but uncertainties remain in the health effects evidence overall. For example, the influence of co-occurring pollutants is difficult to address, or evidence across scientific disciplines may be limited or inconsistent. The sections below briefly summarize the health effects evidence determined in the ISA to support either a “causal” or a “likely to be causal” relationship with fine particle exposures (II.B.1), the populations potentially at increased risk for PM-related effects (II.B.2), and the CASAC's advice on the draft ISA (II.B.3). Additional detail on these topics can be found in the ISA (U.S. EPA, 2019) and in the PA (U.S. EPA, 2020, section 3.2). 1. Nature of Effects Drawing from the assessment of the evidence in the ISA (U.S. EPA, 2019), and the summaries of that assessment in the PA (U.S. EPA, 2020), the sections below summarize the evidence for relationships between long- or short-term PM 2.5 exposures and mortality (II.B.1.a), cardiovascular effects (II.B.1.b), respiratory effects (II.B.1.c), cancer (II.B.1.d), and nervous system effects (II.B.1.e). For these outcomes, the ISA concludes that the evidence supports either a “causal” or a “likely to be causal” relationship with PM 2.5 exposures. a. Mortality i. Long-term PM 2.5 exposures In the last review, the 2009 PM ISA reported that the evidence was “sufficient to conclude that the relationship between long-term PM 2.5 exposures and mortality is causal” (U.S. EPA, 2009c, p. 7-96). The strongest evidence supporting this conclusion was provided by epidemiologic studies, particularly those examining two seminal cohorts, the American Cancer Society (ACS) cohort and the Harvard Six Cities cohort. Analyses of the Harvard Six Cities cohort included demonstrations that reductions in ambient PM 2.5 concentrations are associated with reduced mortality risk (Laden et al., 2006) and with increases in life expectancy (Pope et al., 2009). Further support was provided by other cohort studies conducted in North America and Europe that also reported positive associations between long-term PM 2.5 exposures and risk of mortality (U.S. EPA, 2009c). Recent cohort studies, which have become available since the 2009 ISA, continue to provide consistent evidence of positive associations between long-term PM 2.5 exposures and mortality. These studies add support for associations with total and non-accidental mortality, 35 as well as with specific causes of death, including cardiovascular disease and respiratory disease (U.S. EPA, 2019, section 11.2.2). Many of these recent studies have extended the follow-up periods originally evaluated in the ACS and Harvard Six Cities cohort studies and continue to observe positive associations between long-term PM 2.5 exposures and mortality (U.S. EPA, 2019, section 11.2.2.1; Figures 11-18 and 11-19). Adding to recent evaluations of the ACS and Six Cities cohorts, studies conducted with other cohorts also show consistent, positive associations between long-term PM 2.5 exposure and mortality across various demographic groups ( e.g., age, sex, occupation), spatial and temporal extents, exposure assessment metrics, and statistical techniques (U.S. EPA, 2019, sections 11.2.2.1, 11.2.5). This includes some of the largest cohort studies conducted to date, with analyses of the U.S. Medicare cohort that include nearly 61 million enrollees (Di et al., 2017b) and studies that control for a range of individual and ecological covariates. 35 The majority of these studies examined non-accidental mortality outcomes, though some Medicare studies lack cause-specific death information and, therefore, examine total mortality. A recent series of retrospective studies has additionally tested the hypothesis that past reductions in ambient PM 2.5 concentrations have been associated with increased life expectancy or a decreased mortality rate (U.S. EPA, 2019, section 11.2.2.5). Pope et al. (2009) conducted a cross-sectional analysis using air quality data from 51 metropolitan areas across the U.S., beginning in the 1970s through the early 2000s, and found that a 10 µg/m 3 decrease in long-term PM 2.5 concentration was associated with a 0.61-year increase in life expectancy. In a subsequent analysis, the authors extended the period of analysis to include 2000 to 2007 (Correia et al., 2013), a time period with lower ambient PM 2.5 concentrations. In this follow-up study, a decrease in long-term PM 2.5 concentration continued to be associated with an increase in life expectancy, though the magnitude of the increase was smaller than during the earlier time period ( i.e., a 10 µg/m 3 decrease in long-term PM 2.5 concentration was associated with a 0.35-year increase in life expectancy). Additional studies conducted in the U.S. or Europe similarly report that reductions in ambient PM 2.5 are associated with improvements in longevity (U.S. EPA, 2019, section 11.2.2.5). The 2019 ISA specifically evaluates the degree to which recent studies that examine the relationship between long-term PM 2.5 exposure and mortality have addressed key policy-relevant issues and/or previously identified data gaps in the scientific evidence. For example, based on its assessment of the evidence, the ISA concludes that positive associations between long-term PM 2.5 exposures and mortality are robust across analyses examining a variety of study designs ( e.g., U.S. EPA, 2019, section 11.2.2.4), approaches to estimating PM 2.5 exposures (U.S. EPA, 2019, section 11.2.5.1), approaches to controlling for confounders (U.S. EPA, 2019, sections 11.2.3 and 11.2.5), geographic regions and populations, and temporal periods (U.S. EPA, 2019, sections 11.2.2.5 and 11.2.5.3). Recent evidence further demonstrates that associations with mortality remain robust in copollutant analyses (U.S. EPA, 2019, section 11.2.3), and that associations persist in analyses restricted to long-term exposures below 12 μg/m 3 (Di et al., 2017b) or 10 μg/m 3 (Shi et al., 2016). An additional important consideration in characterizing the public health impacts associated with PM 2.5 exposure is whether concentration-response relationships are linear across the range of concentrations or if nonlinear relationships exist along any part of this range. Several recent studies examine this issue, and continue to provide evidence of linear, no-threshold relationships between long-term PM 2.5 exposures and all-cause and cause-specific mortality (U.S. EPA, 2019, section 11.2.4). However, interpreting the shapes of these relationships, particularly at PM 2.5 concentrations near the lower end of the air quality distribution, can be complicated by relatively low data density in the lower concentration range, the possible influence of exposure measurement error, and variability among individuals with respect to air pollution health effects. These sources of variability and uncertainty tend to smooth and “linearize” population-level concentration-response functions, and thus could obscure the existence of a threshold or nonlinear relationship (U.S. EPA, 2015, Preamble section 6.c). The biological plausibility of PM 2.5 -attributable mortality is supported by the coherence of effects across scientific disciplines ( i.e., animal toxicological, controlled human exposure studies, and epidemiologic), including in recent studies evaluating the morbidity effects that are the largest contributors to total (nonaccidental) mortality. The ISA outlines the available evidence for plausible pathways by which inhalation exposure to PM 2.5 could progress from initial events ( e.g., respiratory tract inflammation, autonomic nervous system modulation) to endpoints relevant to population outcomes, particularly those related to cardiovascular diseases such as ischemic heart disease, stroke and atherosclerosis (U.S. EPA, 2019, section 6.2.1), and to metabolic disease and diabetes (U.S. EPA, 2019, section 7.2.1). The ISA notes “more limited evidence from respiratory morbidity” (U.S. EPA, 2019, p. 11-101) to support the biological plausibility of mortality due to long-term PM 2.5 exposures (U.S. EPA, 2019, section 11.2.1). Taken together, recent studies reaffirm and further strengthen the body of evidence from the 2009 ISA for the relationship between long-term PM 2.5 exposure and mortality. Recent epidemiologic studies consistently report positive associations with mortality across different geographic locations, populations, and analytic approaches. Such studies reduce key uncertainties identified in the last review, including those related to potential copollutant confounding, and provide additional information on the shape of the concentration-response curve. Recent experimental and epidemiologic evidence for cardiovascular effects, and respiratory effects to a more limited degree, supports the plausibility of mortality due to long-term PM 2.5 exposures. The 2019 ISA concludes that, “collectively, this body of evidence is sufficient to conclude that a causal relationship exists between long-term PM 2.5 exposure and total mortality” (U.S. EPA, 2019, section 11.2.7; p. 11-102). ii. Short-term PM 2.5 exposures The 2009 PM ISA concluded that “a causal relationship exists between short-term exposure to PM 2.5 and mortality” (U.S. EPA, 2009c). This conclusion was based on the evaluation of both multi- and single-city epidemiologic studies that consistently reported positive associations between short-term PM 2.5 exposure and non-accidental mortality. These associations were strongest, in terms of magnitude and precision, primarily at lags of 0 to 1 days. Examination of the potential confounding effects of gaseous copollutants was limited, though evidence from single-city studies indicated that gaseous copollutants have minimal effect on the PM 2.5 -mortality relationship ( i.e., associations remain robust to inclusion of other pollutants in copollutant models). The evaluation of cause-specific mortality found that effect estimates were larger in magnitude, but also had larger confidence intervals, for respiratory mortality compared to cardiovascular mortality. Although the largest mortality risk estimates were for respiratory mortality, the interpretation of the results was complicated by the limited coherence from studies of respiratory morbidity. However, the evidence from studies of cardiovascular morbidity provided both coherence and biological plausibility for the relationship between short-term PM 2.5 exposure and cardiovascular mortality. Recent multicity studies evaluated since the 2009 ISA continue to provide evidence of primarily positive associations between daily PM 2.5 exposures and mortality, with percent increases in total mortality ranging from 0.19% (Lippmann et al., 2013) to 2.80% (Kloog et al., 2013) 36 at lags of 0 to 1 days in single-pollutant models. Whereas most studies rely on assigning exposures using data from ambient monitors, associations are also reported in recent studies that employ hybrid modeling approaches using additional PM 2.5 data ( i.e., from satellites, land use information, and modeling, in addition to monitors), allowing for the inclusion of more rural locations in analyses (Kloog et al., 2013, Shi et al., 2016, Lee et al., 2015). 36 As detailed in the Preface to the ISA, risk estimates are for a 10 µg/m 3 increase in 24-hour avg PM 2.5 concentrations, unless otherwise noted (U.S. EPA, 2019). Some recent studies have expanded the examination of potential confounders ( e.g., U.S. EPA, 2019, section 11.1.5.1), including copollutants. Associations between short-term PM 2.5 exposures and mortality remain positive and relatively unchanged in copollutant models with both gaseous pollutants and PM 10-2.5 (U.S. EPA, 2019, Section 11.1.4). Additionally, the low ( r < 0.4) to moderate correlations (r = 0.4-0.7) between PM 2.5 and gaseous pollutants and PM 10-2.5 increase the confidence in PM 2.5 having an independent effect on mortality (U.S. EPA, 2019, section 11.1.4). The generally positive associations reported with mortality are supported by a small group of studies employing causal inference or quasi-experimental statistical approaches (U.S. EPA, 2019, section 11.1.2.1). For example, a recent study examines whether a specific regulatory action in Tokyo, Japan ( i.e., a diesel emission control ordinance) resulted in a subsequent reduction in daily mortality (Yorifuji et al., 2016). The authors report a reduction in mortality in Tokyo due to the ordinance, compared to Osaka, which did not have a similar diesel emission control ordinance in place. The positive associations for total mortality reported across the majority of studies evaluated are further supported by analyses reporting generally consistent, positive associations with both cardiovascular and respiratory mortality (U.S. EPA, 2019, section 11.1.3). For both cardiovascular and respiratory mortality, there has been only limited assessment of potential copollutant confounding, though initial evidence indicates that associations remain positive and relatively unchanged in models with gaseous pollutants and PM 10-2.5 . This evidence further supports the copollutant analyses conducted for total mortality. The evidence for ischemic events and heart failure, as detailed in the assessment of cardiovascular morbidity (U.S. EPA, 2019, Chapter 6), provides biological plausibility for PM 2.5 -related cardiovascular mortality, which comprises the largest percentage of total mortality ( i.e., ~33%) (U.S. National Institutes of Health, 2013). Although there is evidence for exacerbations of chronic obstructive pulmonary disease (COPD) and asthma, the collective body of evidence, particularly from controlled human exposure studies of respiratory effects, provides only limited support for the biological plausibility of PM 2.5 -related respiratory mortality (U.S. EPA, 2019, Chapter 5). In the 2009 ISA, one of the main uncertainties identified was the regional and city-to-city heterogeneity in PM 2.5 -mortality associations. Recent studies examine both city-specific as well as regional characteristics to identify the underlying contextual factors that could contribute to this heterogeneity (U.S. EPA, 2019, section 11.1.6.3). Analyses focusing on effect modification of the PM 2.5 -mortality relationship by PM 2.5 components, regional patterns in PM 2.5 components and city-specific differences in composition and sources indicate some differences in the PM 2.5 composition and sources across cities and regions, but these differences do not fully explain the observed heterogeneity. Additional studies find that factors related to potential exposure differences, such as housing stock and commuting, as well as city-specific factors ( e.g., land-use, port volume, and traffic information), may explain some of the observed heterogeneity (U.S. EPA, 2019, section 11.1.6.3). Collectively, recent studies indicate that the heterogeneity in PM 2.5 -mortality risk estimates cannot be attributed to one factor, but instead a combination of factors including, but not limited to, PM composition and sources as well as community characteristics that could influence exposures (U.S. EPA, 2019, section 11.1.12). A number of recent studies conducted systematic evaluations of the lag structure of associations for the PM 2.5 -mortality relationship by examining either a series of single-day or multiday lags and these studies continue to support an immediate effect ( i.e., lag 0 to 1 days) of short-term PM 2.5 exposures on mortality (U.S. EPA, 2019, section 11.1.8.1). Recent studies also conducted analyses comparing the traditional 24-hour average exposure metric with a sub-daily metric ( i.e., 1-hour max). These initial studies provide evidence of a similar pattern of associations for both the 24-hour average and 1-hour max metric, with the association larger in magnitude for the 24-hour average metric. Recent multicity studies indicate that positive and statistically significant associations with mortality persist in analyses restricted to short-term PM 2.5 exposures below 35 μg/m 3 (Lee et al., 2015), 37 below 30 μg/m 3 (Shi et al., 2016), and below 25 μg/m 3 (Di et al., 2017a). Additional studies examine the shape of the concentration-response relationship and whether a threshold exists specifically for PM 2.5 (U.S. EPA, 2019, section 11.1.10). These studies have used various statistical approaches and consistently found linear relationships with no evidence of a threshold. Recent analyses provide initial evidence indicating that PM 2.5 -mortality associations persist and may be stronger ( i.e., a steeper slope) at lower concentrations ( e.g., Di et al., 2017a; Figure 11-12 in U.S. EPA, 2019). However, given the limited data available at the lower end of the distribution of ambient PM 2.5 concentrations, the shape of the concentration-response curve remains uncertain at these low concentrations and, to date, studies have not conducted extensive analyses exploring alternatives to linearity when examining the shape of the PM 2.5 -mortality concentration-response relationship. 37 Lee et al. (2015) also report that positive and statistically significant associations between short-term PM 2.5 exposures and mortality persist in analyses restricted to areas with long-term concentrations below 12 μg/m 3 . Overall, recent epidemiologic studies build upon and extend the conclusions of the 2009 ISA for the relationship between short-term PM 2.5 exposures and total mortality. Supporting evidence for PM 2.5 -related cardiovascular morbidity, and more limited evidence from respiratory morbidity, provides biological plausibility for mortality due to short-term PM 2.5 exposures. The primarily positive associations observed across studies conducted in diverse geographic locations is further supported by the results from co-pollutant analyses indicating robust associations, along with evidence from analyses of the concentration-response relationship. The 2019 ISA states that, collectively, “this body of evidence is sufficient to conclude that a causal relationship exists between short-term PM 2.5 exposure and total mortality” (U.S. EPA, 2019, pp. 11-58). b. Cardiovascular Effects i. Long-Term PM 2.5 Exposures The scientific evidence reviewed in the 2009 PM ISA was “sufficient to infer a causal relationship between long-term PM 2.5 exposure and cardiovascular effects” (U.S. EPA, 2009c). The strongest line of evidence comprised findings from several large epidemiologic studies of U.S. cohorts that consistently showed positive associations between long-term PM 2.5 exposure and cardiovascular mortality (Pope et al., 2004, Krewski et al., 2009, Miller et al., 2007, Laden et al., 2006). Studies of long-term PM 2.5 exposure and cardiovascular morbidity were limited in number. Biological plausibility and coherence with the epidemiologic findings were provided by studies using genetic mouse models of atherosclerosis demonstrating enhanced atherosclerotic plaque development and inflammation, as well as changes in measures of impaired heart function, following 4- to 6-month exposures to PM 2.5 concentrated ambient particles (CAPs), and by a limited number of studies reporting CAPs-induced effects on coagulation factors, vascular reactivity, and worsening of experimentally induced hypertension in mice (U.S. EPA, 2009c). Studies conducted since the last review continue to support the relationship between long-term exposure to PM 2.5 and cardiovascular effects. As discussed above, results from recent U.S. and Canadian cohort studies consistently report positive associations between long-term PM 2.5 exposure and cardiovascular mortality (U.S. EPA, 2019, Figure 6-19) in evaluations conducted at varying spatial scales and employing a variety of exposure assessment and statistical methods (U.S. EPA, 2019, section 6.2.10). Positive associations between long-term PM 2.5 exposures and cardiovascular mortality are generally robust in copollutant models adjusted for ozone, NO 2 , PM 10-2.5 , or SO 2 . In addition, most of the results from analyses examining the shape of the concentration-response relationship for cardiovascular mortality support a linear relationship with long-term PM 2.5 exposures and do not identify a threshold below which effects do not occur (U.S. EPA, 2019, section 6.2.16; Table 6-52). 38 38 As noted above for mortality, uncertainty in the shape of the concentration-response relationship increases near the upper and lower ends of the concentration distribution where the data are limited. The body of literature examining the relationship between long-term PM 2.5 exposure and cardiovascular morbidity has greatly expanded since the 2009 PM ISA, with positive associations reported in several cohorts (U.S. EPA, 2019, section 6.2). Though results for cardiovascular morbidity are less consistent than those for cardiovascular mortality (U.S. EPA, 2019, section 6.2), recent studies provide some evidence for associations between long-term PM 2.5 exposures and the progression of cardiovascular disease. Positive associations with cardiovascular morbidity ( e.g., coronary heart disease, stroke) and atherosclerosis progression ( e.g., coronary artery calcification) are observed in several epidemiologic studies (U.S. EPA, 2019, sections 6.2.2. to 6.2.9). Associations in such studies are supported by toxicological evidence for increased plaque progression in mice following long-term exposure to PM 2.5 collected from multiple locations across the U.S. (U.S. EPA, 2019, section 6.2.4.2). A small number of epidemiologic studies also report positive associations between long-term PM 2.5 exposure and heart failure, changes in blood pressure, and hypertension (U.S. EPA, 2019, sections 6.2.5 and 6.2.7). Associations with heart failure are supported by animal toxicological studies demonstrating decreased cardiac contractility and function, and increased coronary artery wall thickness following long-term PM 2.5 exposure (U.S. EPA, 2019, section 6.2.5.2). Similarly, a limited number of animal toxicological studies demonstrating a relationship between long-term exposure to PM 2.5 and consistent increases in blood pressure in rats and mice are coherent with epidemiologic studies reporting positive associations between long-term exposure to PM 2.5 and hypertension. Longitudinal epidemiologic analyses also report positive associations with markers of systemic inflammation (U.S. EPA, 2019, section 6.2.11), coagulation (U.S. EPA, 2019, section 6.2.12), and endothelial dysfunction (U.S. EPA, 2019, section 6.2.13). These results are coherent with animal toxicological studies generally reporting increased markers of systemic inflammation, oxidative stress, and endothelial dysfunction (U.S. EPA, 2019, section 6.2.12.2 and 6.2.14). In summary, the 2019 ISA concludes that there is consistent evidence from multiple epidemiologic studies illustrating that long-term exposure to PM 2.5 is associated with mortality from cardiovascular causes. Associations with CHD, stroke and atherosclerosis progression were observed in several additional epidemiologic studies providing coherence with the mortality findings. Results from copollutant models generally support an independent effect of PM 2.5 exposure on mortality. Additional evidence of the independent effect of PM 2.5 on the cardiovascular system is provided by experimental studies in animals, which support the biological plausibility of pathways by which long-term exposure to PM 2.5 could potentially result in outcomes such as CHD, stroke, CHF and cardiovascular mortality. The combination of epidemiologic and experimental evidence results in the ISA conclusion that “a causal relationship exists between long-term exposure to PM 2.5 and cardiovascular effects” (U.S. EPA, 2019, p. 6-222). ii. Short-Term PM 2.5 Exposures The 2009 PM ISA concluded that “a causal relationship exists between short-term exposure to PM 2.5 and cardiovascular effects” (U.S. EPA, 2009c). The strongest evidence in the 2009 PM ISA was from epidemiologic studies of emergency department visits and hospital admissions for ischemic heart disease (IHD) and heart failure (HF), with supporting evidence from epidemiologic studies of cardiovascular mortality (U.S. EPA, 2009c). Animal toxicological studies provided coherence and biological plausibility for the positive associations reported with myocardial ischemia, emergency department visits, and hospital admissions. These included studies reporting reduced myocardial blood flow during ischemia and studies indicating altered vascular reactivity. In addition, effects of PM 2.5 exposure on a potential indicator of ischemia ( i.e., ST segment depression on an electrocardiogram) were reported in both animal toxicological and epidemiologic panel studies. 39 Key uncertainties from the last review resulted from inconsistent results across disciplines with respect to the relationship between short-term exposure to PM 2.5 and changes in blood pressure, blood coagulation markers, and markers of systemic inflammation. In addition, while the 2009 PM ISA identified a growing body of evidence from controlled human exposure and animal toxicological studies, uncertainties remained with respect to biological plausibility. 39 Some animal studies included in the 2009 PM ISA examined exposures to mixtures, such as motor vehicle exhaust or woodsmoke. In these studies, it was unclear if the resulting cardiovascular effects could be attributed specifically to the particulate components of the mixture. A large body of recent evidence confirms and extends the evidence from the 2009 ISA supporting the relationship between short-term PM 2.5 exposure and cardiovascular effects. This includes generally positive associations observed in multicity epidemiologic studies of emergency department visits and hospital admissions for IHD, HF, and combined cardiovascular-related endpoints. In particular, nationwide studies of older adults (65 years and older) using Medicare records report positive associations between PM 2.5 exposures and hospital admissions for HF (U.S. EPA, 2019, section 6.1.3.1). Additional multicity studies conducted in the northeast U.S. report positive associations between short-term PM 2.5 exposures and emergency department visits or hospital admissions for IHD (U.S. EPA, 2019, section 6.1.2.1) while studies conducted in the U.S. and Canada reported positive associations between short-term PM 2.5 exposures and emergency department visits for HF. Epidemiologic studies conducted in single cities contribute some support, though associations reported in single-city studies are less consistentl [Text truncated at 120,000 characters. The full text is on the page linked above.] --- Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2020-08143. Public record. Not legal advice.