From: Lydia Wegman/RTP/USEPA/US To: "Frey, Nathan J." <Nathan_J._Frey@omb.eop.gov> Cc: Joseph
- J Dougherty/DC/USEPA/US@EPA, Karen Martin/RTP/USEPA/US@EPA, John

Hannon/DC/USEPA/US@EPA, Beth Hassett

- Sipple/RTP/USEPA/US@EPA, Erika Sasser/RTP/USEPA/US@EPA Date: 12/14/2012 10:00 AM Subject: Our responses to some of your comments. Nathan, Ple ase find attached our responses to your comments on the primary and secondary standards. Karen's note below explains where the changes are. We are now going to start working on the redline. Work is continuing on the RIA. Thanks Lydia

Karen's note: The attached files include our edits to address OMB's comments on our revisions to sections III (primary standards) and VI (secondary standards) to address OMB's earlier comments. We have generally accepted OMB comments except in a few cases we have modi fied the text of OMB's suggested edit. In section III, our modified text occurs on pp. 101 (last sentence in paragraph), 128, 161, and 183. In section VI, our modified text occurs on p. 465. (For the file on the secondary standards, we compared the file we sent with the file OMB sent back so that we could see what edits OMB suggested.)

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Page 2 of 432 I. Rationale for Final Decisions on the Primary PM 2.5 Standards This section presents the Administrator’s final decision regarding the need to revise the current primary PM 2.5 standards and, more specifically, regarding revisions to the level and form of the existing primary annual PM 2.5 standard in conjunction with retaining the existing primary 24

- hour PM 2.5 standard. As discussed more fully below, the rationale for the final decision is based on a thorough review, in the Integrated Science Assessment, of the latest scientific information, published through mid

- 2009, on human health effects associated with long

- and short

- term exposures to fine particles in the ambient air. Th e final decisions also take into account: (1) staff assessments of the most policy

- relevant information presented and assessed in the Integrated Science Assessment and staff analyses of air quality and human risks presented in the Risk Assessment and the P olicy Assessment, upon which staff conclusions regarding appropriate considerations in this review are based; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the Integrated Science Assessment, Risk Assessment, and Policy Asse ssment at public meetings, in separate written comments, and in CASAC’s letters to the Administrator; (3) the multiple rounds of public comments received during the development of these documents, both in connection with CASAC meetings and separately; and (4) extensive public comments received on the proposal. In developing this final rule, the Administrator recognizes that the CAA requires her to reach a public health policy judgment as to what standards would be requisite

-- neither more nor less stringe nt than necessary

-- to protect public health with an adequate margin of safety, based on scientific evidence and technical assessments that have inherent uncertainties and limitations. This judgment requires making reasoned decisions

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Page 3 of 432 as to what weight to place on various types of evidence and assessments, and on the related uncertainties and limitations. Thus, in selecting the final standards, the Administrator is seeking not only to prevent fine particle concentrations that have been demonstrated to be ha

rmful but also to prevent lower fine particle concentrations that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. As discussed below, as well as in more detail in the proposal, a substantial amou nt of new research has been conducted since the close of the science assessment in the last review of the PM 2.5 NAAQS ( U.S. EPA, 2004), with important new information coming from epidemiological studies, in particular. This body of evidence includes hundre ds of new epidemiological studies conducted in many countries around the world. In its assessment of the evidence judged to be most relevant to making decisions on elements of the primary PM 2.5 standards, the EPA has placed greater weight on U.S. and Canad ian studies using PM 2.5 measurements, since studies conducted in other countries may reflect different demographic and air pollution characteristics. 1 The newly available research studies as well as the earlier body of scientific evidence presented and as sessed in the Integrated Science Assessment have undergone intensive scrutiny through multiple layers of peer review and opportunities for public review and comment. In developing this final rule, the EPA has drawn upon an integrative synthesis of the enti re body of evidence concerning exposure to ambient fine particles and a broad range of health endpoints ( U.S. EPA, 2009a, Chapters 2, 4, 5, 6, 7,

1 Nonetheless, the Administrator recognizes the importance of all studies, including international studies, in the Integrated Science Assessment’s considerations of the weight of the evidence that informs causality determinations.

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Page 4 of 432 and 8) focusing on those health endpoints for which the Integrated Science Assessment concludes that there is a causal or likely causal relationship with long

- or short

- term PM 2.5 exposures. The EPA has also considered health endpoints for which the Integrated Science Assessment concludes there is evidence suggestive of a causal relationship with long

- term PM 2.5 e xposures. The EPA has also drawn upon a quantitative risk assessment based upon the scientific evidence described and assessed in the Integrated Science Assessment. These analyses, discussed in the Risk Assessment ( U.S. EPA, 2010a) and Policy Assessment ( U.S. EPA, 2011a, chapter 2), have also undergone intensive scrutiny through multiple layers of peer review and multiple opportunities for public review and comment. Although important uncertainties remain in the qualitative and quantitative characterizati ons of health effects attributable to ambient fine particles, progress has been made in addressing these uncertainties in this review . The EPA ’s review of this information has been extensive and deliberate. This intensive evaluation of the scientific evide nce and quantitative assessments has provided a comprehensive and adequate basis for regulatory decision making at this time. This section describes the integrative synthesis of the evidence and technical information contained in the Integrated Science As sessment , the Risk Assessment , and the Policy Assessment with regard to the current and alternative standards. The EPA notes that the final decision for retaining or revising the current primary PM 2.5 standards is a public health policy judgment made by th e Administrator. The Administrator’s final decision draws upon scientific information and analyses related to health effects and

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Page 5 of 432 risks; judgments about uncertainties that are inherent in the scientific evidence and analyses; CASAC advice; and comments rece ived in response to the proposal. In presenting the rationale for the final decisions on the primary PM 2.5 standards, this section begins with a summary of the approaches used in setting the initial primary PM 2.5 NAAQS in 1997 and in reviewing and revising those standards in 2006 (section III.A.1). The D.C. Circuit Court of Appeals remand of the primary annual PM 2.5 standard in 2009 is discussed in section III.A.2. Taking into consideration this history, section I II.A.3 describes the EPA’s general approach used in the current review for considering the need to retain or revise the current suite of fine particle standards, taking into account public comment on the proposed approach. The scientific evidence and quantitative risk assessment were presented in

s ections III.B and III.C of the proposal, respectively (77 FR 38906 to 38917, June 29, 2012) and are outlined in sections III.B and III.C below. Subsequent sections of this preamble provide a more complete discussion of the Administrator’s rationale, in lig ht of key issues raised in public comments, for concluding that it is appropriate to revise the suite of current primary PM 2.5 standards (section III.D), as well as a more complete discussion of the Administrator’s rationale for retaining or revising the s pecific elements of the primary PM 2.5 standards, namely the indicator (section III.E.1); averaging time (section III.E.2); form (section III.E.3); and level (section III.E.4). A summary of the final decisions to revise the suite of primary PM 2.5 standards is presented in section III.F. A. Background There are currently two primary PM 2.5 standards providing public health protection from effects associated with fine particle exposures. The annual standard is set

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Page 6 of 432 at 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 monitors sited to represent community

- wide air quality. The 24

- hour standard is set at a level of 35 μg/m 3 , based on the 3

- year average of the 98 th percentile of 24

- hour PM 2.5 c oncentrations at each population

- oriented monitor within an area. The past and current approaches for reviewing the primary PM 2.5 standards described below are all based most fundamentally on using information from epidemiological studies to inform the se lection of PM 2.5 standards that, in the Administrator’s judgment, protect public health with an adequate margin of safety. Such information can be in the form of air quality distributions over which health effect associations have been observed in scientif ic studies or in the form of concentration

response functions that support quantitative risk assessment. However, evidence

- and risk

- based approaches using information from epidemiological studies to inform decisions on PM 2.5 standards are complicated by t he recognition that no population threshold, below which it can be concluded with confidence that PM 2.5 - related effects do not occur, can be discerned from the available evidence. 2 As a result, any general approach to reaching decisions on what standards a re appropriate necessarily requires judgments about how to translate the information available from the epidemiological studies into a basis for appropriate standards. This includes consideration of how to weigh the uncertainties in the reported associatio ns across the distributions of PM 2.5

2 The term "evidence

- based" approach or consideration generally refers to using the information in the scientific evidence to inform judgments on the need to retain or revise the NAAQS. The term "risk

- based" generally refers to using the quantitative inform ation in the Risk Assessment to inform such judgments.

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Page 7 of 432 concentrations in the studies and the uncertainties in quantitative estimates of risk, in the context of the entire body of evidence before the Agency. Such approaches are consistent with setting standards that are neit her more nor less stringent than necessary, recognizing that a zero

- risk standard is not required by the CAA. 1. General Approach Used in Previous Reviews The general approach used to translate scientific information into standards in the previous PM NAAQS r eviews focused on consideration of alternative standard levels that were somewhat below the long

- term mean PM 2.5 concentrations reported in key epidemiological studies ( U.S. EPA, 2011a, section 2.1.1). This approach recognized that the strongest evidence o f PM 2.5 - related associations occurs where the bulk of the data exists, which is over a range of concentrations around the long

- term (i.e., annual) mean. In setting primary PM 2.5 annual and 24

- hour standards for the first time in 1997, the Agency relied pr imarily on an evidence

- based approach that focused on epidemiological evidence, especially from short

- term exposure studies of fine particles judged to be the strongest evidence at that time ( U.S. EPA, 2011a, section 2.1.1.1). The EPA selected a level for the annual standard that was at or below the long

- term mean PM 2.5 concentrations in studies providing evidence of associations with short

- term PM 2.5 exposures, placing greatest weight on those short

- term exposure studies that reported clearly statistically significant associations with mortality and morbidity effects. Further consideration of long

- term mean PM 2.5 concentrations associated with mortality and respiratory effects in children did not provide a basis for establishing a lower annual standard leve l. The EPA did not place much weight on quantitative risk estimates from the very limited risk assessment conducted, but did conclude that the risk assessment

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Page 8 of 432 results confirmed the general conclusions drawn from the epidemiological evidence that a serious public health problem was associated with ambient PM levels allowed under the then current PM 10 standards (62 FR 38665/1, July 18, 1997). The EPA considered the epidemiological evidence and data on air quality relationships to set an annual PM 2.5 standard that was intended to be the “generally controlling” standard; i.e., the primary means of lowering both long

- and short

- term ambient concentrations of PM 2.5 . 3 In conjunction with the annual standard, the EPA also established a 24

- hour PM 2.5 standa rd to provide supplemental protection against days with high peak concentrations, localized “hotspots,” and risks arising from seasonal emissions that might not be well controlled by an annual standard (62 FR 38669/3). In 2006, the EPA used a different ev idence

- based approach to assess the appropriateness of the levels of the 24

- hour and annual PM 2.5 standards ( U.S. EPA, 2011a, section 2.1.1.2). Based on an expanded body of epidemiological evidence that was stronger and more robust than that available in t he 1997 review, including additional studies of both short

- and long

- term exposures, the EPA decided that using evidence of effects associated with periods of exposure that were most closely matched to the

3 In so doing, the EPA noted that because an annual standard would focus control programs on annual average PM 2.5 concentrations, it would not only control long

- term exposure levels, but would also gen erally control the overall distribution of 24

- hour exposure levels, resulting in fewer and lower 24

- hour peak concentrations. Alternatively, a 24

- hour standard that focused controls on peak concentrations could also result in lower annual average concentra tions. Thus, the EPA recognized that either standard could provide some degree of protection from both short

- and long

- term exposures, with the other standard serving to address situations where the daily peaks and annual averages are not consistently corr elated (62 FR 38669, July 18, 1997). In the circumstances presented in that review, the EPA determined that it was appropriate to focus on the annual standard as the standard best suited to control both annual and daily air quality distributions (62 FR 386 70).

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Page 9 of 432 averaging time of each standard was the most appro priate public health policy approach for evaluating the scientific evidence to inform selecting the level of each standard. Thus, the EPA relied upon evidence from the short

- term exposure studies as the principal basis for revising the level of the 24

- hour PM 2.5 standard from 65 to 35 μg/m 3 to protect against effects associated with short

- term exposures. The EPA relied upon evidence from long

- term exposure studies as the principal basis for retaining the level of the annual PM 2.5 standard at 15 μg/m 3 to pro tect against effects associated with long

- term exposures. This approach essentially took the view that short

- term studies were not appropriate to inform decisions relating to the level of the annual standard, and long

- term studies were not appropriate to i nform decisions relating to the level of the 24

- hour standard. With respect to quantitative risk

- based considerations, the EPA determined that the estimates of risks likely to remain upon attainment of the 1997 suite of PM 2.5 standards were indicative of r isks that could be reasonably judged important from a public health perspective and, thus, supported revision of the standards. However, the EPA judged that the quantitative risk assessment had important limitations and did not provide an appropriate basis for selecting the levels of the revised standards in 2006 (71 FR 61174/1

- 2, October 17, 2006). 2. Remand of Primary Annual PM 2.5 Standard As noted above in section II.B.2, several parties filed petitions for review in the U.S. Court of Appeals for the Distr ict of Columbia Circuit following promulgation of the revised PM NAAQS in 2006. These petitions challenged several aspects of the final rule including the level of the primary PM 2.5 annual standard. The primary 24

- hour PM 2.5

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Page 10 of 432 standard was not challenged by any of the litigants and, thus, was not considered in the court’s review and decision. On judicial review, the D.C. Circuit remanded the primary annual PM 2.5 NAAQS to the EPA on grounds that the Agency failed to adequately explain why the annual standard provided the requisite protection from both short

- and long

- term exposures to fine particles including protection for at

- risk populations. American Farm Bureau Federation v. EPA , 559 F. 3d 512 (D.C. Cir. 2009). With respect to human health protection from short

- term PM 2.5 exposures, the court considered the different approaches used by the EPA in the 1997 and 2006 PM NAAQS decisions, as summarized in section III.A.1 above. The court found that the EPA failed to adequately exp lain why a primary 24

- hour PM 2.5 standard by itself would provide the protection needed from short

- term exposures and remanded the primary annual PM 2.5 standard to the EPA “for further consideration of whether it is set at a level requisite to protect the public health while providing an adequate margin of safety from the risk of short

- term exposures to PM 2.5 .” American Farm Bureau Federation v. EPA , 559 F. 3d at 520

- 24. With respect to protection from long

- term exposure to fine particles, the court found that the EPA failed to adequately explain how the primary annual PM 2.5 standard provided an adequate margin of safety for children and other at

- risk populations. The court found that the EPA did not provide a reasonable explanation of why certain morbidity studies, including a study of children in Southern California showing lung damage associated with long

- term PM 2.5 exposure (Gauderman et.al, 2000) and a multi

city study (24

- Cities Study) evaluating decreased lung function in children associated

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Page 11 of 432 with long

- term PM 2.5 exposures (Raizenne et al., 1996), did not warrant a more stringent annual PM 2.5 standard. Id. at 522

- 23. Specifically, the court found that: EPA was unreasonably confident that, even though it relied solely upon long

- term mortality studies, th e revised standard would provide an adequate margin of safety with respect to morbidity among children. Notably absent from the final rule, moreover, is any indication of how the standard will adequately reduce risk to the elderly or to those with certain heart or lung diseases despite (a) the EPA’s determination in its proposed rule that those subpopulations are at greater risk from exposure to fine particles and (b) the evidence in the record supporting that determination. Id. at 525. In addition, the cou rt held that the EPA had not adequately explained its decision to base the level of the annual standard essentially exclusively on the results of long

- term studies and the 24

- hour standard level essentially exclusively on the results of short

- term studies. See 559 F. 3d at 522 (“[e]ven if the long

- term studies available today are useful for setting an annual standard,..., it is not clear why the EPA no longer believes it useful to look as well to short

- term studies in order to design the suite of standards th at will most effectively reduce the risks associated with short

- term exposure”); see also I d. at 523

- 24 (holding that the EPA had not adequately explained why a standard based on levels in short

- term exposure studies alone provided appropriate protection f rom health effects associated with short

- term PM 2.5 exposures given the stated need to lower the entire air quality distribution, and not just peak concentrations, in order to control against short

- term effects). In remanding the primary annual PM 2.5 stan dard for reconsideration, the court did not vacate the standard, Id

. at 530, so the standard remains in effect and is therefore the standard considered by the EPA in this review. 3. General Approach Used in the Policy Assessment for the Current Review

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Page 12 of 432 This review is based on an assessment of a much expanded body of scientific evidence, more extensive air quality data and analyses, and a more comprehensive quantitative risk assessment relative to the information available in past reviews, as presented an d assessed in the Integrated Science Assessment and Risk Assessment and discussed in the Policy Assessment . As a result, the EPA’s general approach to reaching conclusions about the adequacy of the current suite of PM 2.5 standards and potential alternative standards that are appropriate to consider was broader and more integrative than in past reviews. Our general approach also reflected consideration of the issues raised by the court in its remand of the primary annual PM 2.5 standard as discussed in sectio n III.A.2 above , since decisions made in this review, and the rationales for those decisions, will comprise the Agency’s response to the remand. The EPA’s general approach took into account both evidence

- based and risk

based considerations and the uncertai nties related to both types of information, as well as advice from CASAC (Samet, 2010c,d) and public comments on the first and second draft Policy Assessments ( U.S. EPA, 2010c,f). In so doing, the EPA staff developed a final Policy Assessment ( U.S. EPA, 20 11a) which provided as broad an array of policy options as was supported by the available information, recognizing that the selection of a specific approach to reaching final decisions on the primary PM 2.5 standards will reflect the judgments of the Admini strator as to what weight to place on the various approaches and types of information available in the current review. The Policy Assessment concluded it was most appropriate to consider the protection against PM 2.5 - related mortality and morbidity effects, associated with both long

- and short

- term exposures, afforded by the annual and 24

- hour standards taken

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Page 13 of 432 together, as was done in the 1997 review, rather than to consider each standard separately, as was done in the 2006 review ( U.S. EPA, 2011a, section 2. 1.3). 4 As the EPA recognized in 1997, there are various ways to combine two standards to achieve an appropriate degree of public health protection. The extent to which these two standards are interrelated in any given area depends in large part on the rela tive levels of the standards, the peak

- to

- mean ratios that characterize air quality patterns in an area, and whether changes in air quality designed to meet a given suite of standards are likely to be of a more regional or more localized nature. In consid ering the combined effect of annual and 24

- hour standards, the Policy Assessment recognized 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 an d lower peak 24

- hour PM 2.5 concentrations. The Policy Assessment also recognized that changes designed to meet a 24

- hour standard would result not only in fewer and lower peak 24

- hour PM 2.5 concentrations but also in lower annual average PM 2.5 concentratio ns. Thus, either standard could be viewed as providing protection from effects associated with both short

- and long

- term exposures, with the other standard serving to address situations where the daily peak and annual average concentrations are not consist ently correlated.

4 By utilizing this approach, the Agency also is responsive to the remand of the 2006 standard. As noted in section III.A.2, the D.C. Circuit, in remanding the 2006 primary annual PM 2.5 standard, concluded that the Administrator had failed to adequate ly explain why an annual standard was sufficiently protective in the absence of consideration of the long

- term mean PM 2.5 concentrations in short

- term exposure studies as well, and likewise had failed to explain why a 24

- hour standard was sufficiently prot ective in the absence of consideration of the effect of an annual standard on reducing the overall distribution of 24

- hour average PM 2.5 concentrations. 559 F. 3d at 520

- 24.

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Page 14 of 432 In considering the currently available evidence, the Policy Assessment recognized that the short

- term exposure studies were primarily drawn from epidemiological studies that associated variations in area

- wide health effects with monitor( s) that measured the variation in daily PM 2.5 concentrations over the course of several years. The strength of the associations in these data was demonstrably in the numerous “typical” days within the air quality distribution, not in the peak days. See als o 71 FR 61168, October 17, 2006 and American Farm Bureau Federation v EPA , 559 F. 3d at 523, 524 (making the same point). The quantitative risk assessments conducted for this and previous reviews demonstrated the same point ; that is, much, if not most of t he aggregate risk associated with short

- term exposures results from the large number of days during which the 24

- hour average concentrations are in the low

- to mid

- range, below the peak 24

- hour concentrations ( U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a , section 3.1.2.2). In addition, there was no evidence suggesting that risks associated with long

- term exposures were likely to be disproportionately driven by peak 24

- hour concentrations. 5 For these reasons, the Policy Assessment concluded that strategie s that focused primarily on reducing peak days were less likely to achieve reductions in the PM 2.5 concentrations that were most strongly associated with the observed health effects. Furthermore, the Policy Assessment concluded that a policy approach that focused on reducing peak exposures would most likely result in more uneven public health protection across the U.S. by either providing inadequate protection in some areas or

5 In confirmation, a number of studies have presented analyses excluding higher P M concentration days and reported a limited effect on the magnitude of the effect estimates or statistical significance of the association (e.g., Dominici, 2006b; Schwartz et al, 1996; Pope and Dockery, 1992).

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Page 15 of 432 overprotecting in other areas ( U.S. EPA, 2011a, p. 2

- 9; U.S. EPA, 2010a, section 5.2.3). This is because, as discussed above, reductions based on control of peak days are less likely to control the bulk of the air quality distribution. The Policy Assessment concluded that a policy goal of setting a “generally controlling” annual stand ard that will lower a wide range of ambient 24

- hour PM 2.5 concentrations, as opposed to focusing on control of peak 24

- hour PM 2.5 concentrations, was the most effective and efficient way to reduce total population risk and so provide appropriate protection . This approach, in contrast to one focusing on a generally controlling 24

- hour standard, would likely reduce aggregate risks associated with both long

- and short

- term exposures with more consistency and would likely avoid setting national standards that c ould result in relatively uneven protection across the country, due to setting standards that are either more or less stringent than necessary in different geographical areas (U.S. EPA, 2011a, p. 2

- 9)

. The Policy Assessment also concluded that an annual st andard intended to serve as the primary means for providing protection from effects associated with both long

- and short

- term PM 2.5 exposures cannot be expected to offer sufficient protection against the effects of all short

- term PM 2.5 exposures. As a result, in conjunction with a generally controlling annual standard, the Policy Assessment concluded it was appropriate to consider setting a 24

- hour standard to provide supplemental protection, particularly for areas with high peak

- to

- mea n ratios possibly associated with strong local or seasonal sources, or PM 2.5 - related effects that may be associated with shorter

- than

- daily exposure periods ( U.S. EPA, 2011a, p. 2

- 10). The Policy Assessment ’s consideration of the protection afforded by the current

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Page 16 of 432 and alternative suites of standards focused on PM 2.5 - related health effects associated with long

- term exposures for which the magnitude of quantitative estimates of risks to public health generated in the risk assessment was appreciably larger in terms of overall incidence and percent of total mortality or morbidity effects than for short

- term PM 2.5 - related effects. Nonetheless, the EPA also considered health effects and estimated risks associated with short

- term exposures. In both cases, the Polic y Assessment placed greatest weight on health effects that had been judged in the Integrated Science Assessment to have a causal or likely causal relationship with PM 2.5 exposures, while also considering health effects judged to be suggestive of a causal r elationship or evidence that focused on specific at

- risk populations. The Policy Assessment placed relatively greater weight on statistically significant associations that yielded relatively more precise effect estimates and that were judged to be robust t o confounding by other air pollutants. In the case of short

- term exposure studies, the Policy Assessment placed greatest weight on evidence from large multi

- city studies, while also considering associations in single

- city studies. In translating informati on from epidemiological studies into the basis for reaching staff conclusions on the adequacy of the current suite of standards, the Policy Assessment considered a number of factors. As an initial matter, the Policy Assessment considered the extent to whic h the currently available evidence and related uncertainties strengthens or calls into question conclusions from the last review regarding associations between fine particle exposures and health effects. The Policy Assessment also considered evidence of he alth effects in at

- risk populations and the potential impacts on such populations. Further, the Policy Assessment explored the extent to which PM 2.5 - related

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Page 17 of 432 health effects had been observed in areas where air quality distributions extend to lower
concentra tions than previously reported or in areas that would likely have met the current suite of standards. In translating information from epidemiological studies into the basis for reaching staff conclusions on standard levels for consideration ( U.S. EPA, 201 1a, sections 2.1.3 and 2.3.4), the Policy Assessment first recognized the absence of discernible thresholds in the concentration

- response functions from long

- and short

- term PM 2.5 exposure studies ( U.S. EPA, 2011a, section 2.4.3). 6 In the absence of any di scernible thresholds, the Agency’s general approach for identifying appropriate standard levels for consideration involved characterizing the range of PM 2.5 concentrations over which we have the most confidence in the associations reported in epidemiologic al studies. In so doing, the Policy Assessment recognized that there is no single factor or criterion that comprises the “correct” approach, but rather there are various approaches that are reasonable to consider for characterizing the confidence in the as sociations and the limitations and uncertainties in the evidence. Identifying the implications of various approaches for reaching conclusions on the range of alternative standard levels that is appropriate to consider can help inform the final decisions to either retain or revise the standards.

6 The epidemiological studies evaluated in the Integrated Science Assessment that examined the shape of concentration

- response relationships and the potential presence of a threshold focused on cardiovascular

- related hospital admissions and emergency department visits associated with short

- term PM 10 e xposures and premature mortality associated with long

- term PM 2.5 exposure (U.S. EPA, 2009a, sections 6.5, 6.2.10.10 and 7.6). Overall, the Integrated Science Assessment concluded that the studies evaluated support the use of a no

- threshold, log

- linear mode l but recognized that “additional issues such as the influence of heterogeneity in estimates between cities, and the effect of seasonal and regional differences in PM on the concentration

- response relationship still require further investigation” (U.S. EPA , 2009a, section 2.4.3).

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Page 18 of 432 Today’s final decisions also take into account public health policy judgments as to the degree of health protection that is to be achieved. In reaching staff conclusions on the range of annual standard levels that w as appropriate to consider, the Policy Assessment focused on identifying an annual standard that provided requisite protection from effects associated with both long

- and short

- term exposures. In so doing, the Policy Assessment explored different approache s for characterizing the range of PM 2.5 concentrations over which our confidence in the nature of the associations for both long

- and short

- term exposures is greatest, as well as the extent to which our confidence is reduced at lower PM 2.5 concentrations. First, the Policy Assessment recognized that the approach that most directly addressed this issue considered studies that analyzed confidence intervals around concentration

- response relationships and in particular, analyses that averaged across multiple concentration

- response models rather than considering a single concentration

response model. 7 The Policy Assessment explored the extent to which such analyses had been published for studies of health effects associated with long

- or short

- term PM 2.5 exposu res. Such analyses could potentially be used to characterize a concentration below which uncertainty in a concentration

- response relationship substantially increases or is judged to be indicative of an unacceptable degree of uncertainty about the existence of a continuing concentration

- response relationship. The Policy Assessment concluded that

7 This is distinct from confidence intervals around concentration

- response relationships that are related to the magnitude of effect estimates generated at specific PM 2.5 concentrations (i.e., point

- wise confidence intervals) and t hat are relevant to the precision of the effect estimate across the air quality distribution, rather than to our confidence in the existence of a continuing concentration

- response relationship across the entire air quality distribution on which a reported association was based.

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Page 19 of 432 identifying this area of uncertainty in the concentration

- response relationship could be used to inform identification of alternative standard levels that are approp riate to consider. Further, the Policy Assessment explored other approaches that considered different statistical metrics from epidemiological studies. The Policy Assessment first took into account the general approach used in previous PM reviews which fo cused on consideration of alternative standard levels that were somewhat below the long

- term mean PM 2.5 concentrations reported in epidemiological studies using air quality distributions based on composite monitor concentrations. 8 This approach recognized that the strongest evidence of PM 2.5 - related associations occurs at concentrations around the long

- term (i.e., annual) mean. In using this approach, the Policy Assessment placed greatest weight on those long

- and short

- term exposure studies that reported s tatistically significant associations with mortality and morbidity effects. In extending this approach, the Policy Assessment also considered information beyond a single statistical metric of PM 2.5 concentrations (i.e., the mean) to the extent such information was available. Pursuant to an express comment from CASAC (Samet 2010d, p. 2), the Policy Assessment utilized distributional statistics (i.e., statistical characterization of an entire distribu tion of data) to identify the broader range of PM 2.5
8 Using the term “composite monitor” does not imply that the EPA can identify one monitor that represents the air quality evaluated in a specific study area. Rather, the composite monitor concentration represents the average concent ration across monitors within each area with more than one monitor included in a given study as typically reported in epidemiological studies. For multi

- city studies, this metric reflects concentrations averaged across multiple monitors or from single moni tors within each area and then averaged across study areas for an overall study mean PM 2.5 concentration. This is consistent with the epidemiological evidence considered in other NAAQS reviews.

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Page 20 of 432 concentrations that had the most influence on the calculation of relative risk estimates in both long

- and short

- term exposure epidemiological studies. Thus, the Policy Assessment considered the part of the distribution of PM 2.5 concentrations in which the data analyzed in the study (i.e., air quality and population

- level data, as discussed below) w ere most concentrated, specifically, the range of PM 2.5 concentrations around the long

- term mean over which our confidence in the magnitude and significance of associations observed in the epidemiological studies was greatest. The Policy Assessment then focused on the lower part of th e distribution to characterize where the data became appreciably more
sparse an d, thus, where our understanding of the magnitude and significance of the associations correspondingly became more uncertain. The Policy Assessment recognized there was no single percentile value within a given distribution that was most appropriate or “co rrect” to use to characterize where our confidence in the associations becomes appreciably lower. The Policy Assessment concluded that the range from the 25 th to 10 th percentiles is a reasonable range to consider as a region where we had appreciably less

c onfidence in the associations observed in epidemiological studies. 9 In considering distributional statistics from epidemiological studies, the final Policy Assessment focused on two types of population

- level metrics that CASAC advised were most useful to consider in identifying the PM 2.5 concentrations most influential in

9 In the PM NAAQS review completed in 2006, the Staff Paper si milarly recognized that the evidence of an association in any epidemiological study is “strongest at and around the long

- term average where the data in the study are most concentrated. For example, the interquartile range of long

- term average concentration s within a study [with a lower bound of the 25 th percentile] or a range within one standard deviation around the study mean, may reasonably be used to characterize the range over which the evidence of association is strongest” (U.S. EPA, 2005, p. 5

- 22). A range of one standard deviation around the mean represents approximately 68 percent of normally distributed data, and below the mean falls between the 25 th and 10 th percentiles.

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Page 21 of 432 generating the health effect estimates reported in the epidemiological studies. 10 Consistent with CASAC advice, the most relevant information was the distribution of health events (e.g., d eaths, hospitalizations) occurring within a study population in relation to the distribution of PM 2.5 concentrations. However, in recognizing that access to health event data can be restricted, the Policy Assessment also considered the number of study part icipants within each study area as an appropriate surrogate for health event data. The Policy Assessment recognize d that an approach considering analyses of confidence intervals around concentration

- response functions wa s intrinsically related to

an approach that consider ed different distributional statistics. Both of these approaches could be employed to understand the broader distribution of PM 2.5 concentrations which correspond to the health events reported in epidemiological studies. In applying t hese approaches, the Policy Assessment, consistent with CASAC advice (Samet, 2010d, p.3), considered PM 2.5 concentrations from long

- and short

- term PM 2.5 exposure studies using composite monitor distributions. In reaching staff conclusions on alternative standard levels that were appropriate

10 The second draft Policy Assessment focused on the distributions of ambient P M 2.5 concentrations and associated population data across areas included in several multi

- city studies for which such data were available in seeking to identify the most influential range of concentrations (U.S. EPA, 2010f, section 2.3.4.1). In its review of the second draft Policy Assessment, CASAC advised that it “would be preferable to have information on the concentrations that were most influential in generating the health effect estimates in individual studies” (Samet, 2010d, p.2). Therefore, in the f inal Policy Assessment, the EPA considered population

- level data (i.e., area

- specific health event data and study area population data) along with corresponding PM 2.5 concentrations to generate a cumulative distribution of the population

- level data relativ e to long

- term mean PM 2.5 concentrations to determine the most influential part of the air quality distribution (U.S. EPA, 2011a, Figure 2

- 7 and associated text).

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Page 22 of 432 to consider, the Policy Assessment also included a broader consideration of the uncertainties and limitations of the current scientific evidence. Most notably, these uncertainties are related to the heterogeneity obser ved in the epidemiological studies in the eastern versus western parts of the U.S., the relative toxicity of PM 2.5 components, and the potential role of co

- pollutants ( U.S. EPA, 2011a, pp. 2

- 25 to 2

- 26). The limitations and uncertainties associated with th e currently available scientific evidence, including the availability of fewer studies toward the lower range of alternative annual standard levels being considered in this proposal, are summarized in section III.B below and further discussed in section II I.B.2 of the propos al

. The Policy Assessment recognized that the level of protection afforded by the NAAQS relies both on the level and the form of the standard. The Policy Assessment concluded that a policy approach that used data based on composite monit or distributions to identify alternative standard levels, and then compared those levels to concentrations at maximum monitors to determine whether an area meets a given standard, inherently has the potential to build in some margin of safety ( U.S. EPA, 20 11a, p. 2

- 14). 11 This conclusion was consistent with CASAC’s comments on the second draft Policy

11 Statistical metrics (e.g., means) based on composite monitor distributions may be identical to or below the same statistical metrics based on maximum monitor distributions. For example, some areas may have only one monitor, in which case the composite and maximum monitor distributions will be identical in those areas. Other areas may have multip le monitors that may be very close to the monitor measuring the highest concentrations, in which case the composite and maximum monitor distributions could be similar in those areas. As noted in Hassett

- Sipple et al. (2010), for studies involving a large n umber of areas, the composite and maximum concentrations are generally within 5 percent of each other (77 FR 38905, fn. 30). Still other areas may have multiple monitors that may be separately impacted by local sources in which case the composite and maxim

um monitor distributions could be quite different (U.S. EPA, 2011a, p. 2

- 14). See further discussion of this issue in section III.E.4.c.i below.

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Page 23 of 432 Assessment , in which CASAC expressed its preference for focusing on an approach using composite monitor distributions “because of its stability, and for the add itional margin of safety it provides” when “compared to the maximum monitor perspective” (Samet, et al., 2010d, pp. 2 to 3). In reaching staff conclusions on alternative 24

- hour standard levels that are appropriate to consider for setting a 24

- hour standa rd intended to supplement the protection afforded by a generally controlling annual standard, the Policy Assessment considered currently available short

- term PM 2.5 exposure studies. The evidence from these studies informed our understanding of the protecti on afforded by the suite of standards against effects associated with short

- term exposures. In considering the short

term exposure studies, the Policy Assessment evaluated both the distributions of 24

- hour PM 2.5 concentrations, with a focus on the 98 th per centile concentrations (to the extent such data were available) to match the form of the current 24

- hour PM 2.5 standard, as well as the long

- term mean PM 2.5 concentrations reported in these studies. In addition to considering the epidemiological evidence, the Policy Assessment also considered air quality information based on county

- level 24

- hour and annual design values 12 to understand the policy implications of the alternative standard levels supported by the underlying science. In particular, the Policy Assessment considered the extent to which different combinations of alternative annual and 24

- hour standards would support the policy goal of focusing on a ge nerally controlling annual standard in conjunction with a 24

- hour standard that would provide supplemental protection. In so doing, the Policy

12 Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to determine complianc e.

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Page 24 of 432 Assessment discussed the roles that each standard might be expected to play in the protection afforded by alterna tive suites of standards. Beyond these evidence

- based considerations, the Policy Assessment also considered the quantitative risk estimates and the key observations presented in the Risk Assessment. This assessment included an evaluation of 15 urban case s tudy areas and estimated risk associated with a number of health endpoints associated with long

- and short

- term PM 2.5 exposures ( U.S. EPA, 2010a). As part of the risk

- based considerations, the Policy Assessment considered estimates of the magnitude of PM 2. 5 - related risks associated with recent air quality levels and air quality simulated to just meet the current and alternative suites of standards using alternative simulation approaches. The Policy Assessment also characterized the risk reductions, relative to the risks remaining upon just meeting the current standards, associated with just meeting alternative suites of standards. In so doing, the Policy Assessment recognized the uncertainties inherent in such risk estimates, and took such uncertainties into account by considering the sensitivity of the “core” risk estimates to alternative assumptions and methods likely to have substantial impact on the estimates. In addition, the Policy Assessment considered additional analyses characterizing the representat iveness of the urban study areas within a broader national context to understand the roles that the annual and 24

- hour standards may play in affording protection against effects related to both long

- and short

- term PM 2.5 exposures. Based on the approach di scussed above, the Policy Assessment reached conclusions related to the primary PM 2.5 standards that reflected an understanding of both evidence

- based and risk

- based considerations to inform two overarching questions

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Page 25 of 432 related to: (1) the adequacy of the cur rent suite of PM 2.5 standards and (2) revisions to the standards that were appropriate to consider in this review to protect against health effects associated with both long

- and short

- term exposures to fine particles. When evaluating the health protection afforded by the current or any alternative suites of standards considered, the Policy Assessment took into account the four basic elements of the NAAQS: the indicator, averaging time, form, and level. The general approach for reviewing the primary PM 2.5 s tandards described above provided a comprehensive basis that helped to inform the Administrator’s judgments in reaching her proposed and final decisions to revise the current suite of primary fine particle NAAQS and in responding to the remand of the 2006 primary annual PM 2.5 standard. B. Overview of Health Effects Evidence This section outlines the key information presented in section III.B of the proposal (77 FR 38906 to 38911, June 29, 2012) and discussed more fully in the Integrated Science Assessment (Cha pters 2, 4, 5, 6, 7, and 8) and the Policy Assessment (Chapter 2) related to health effects associated with fine particle exposures. Section III.B. of the proposal discusses available information on the health effects associated with exposures to PM 2.5 , in cluding the nature of such health effects (section III.B.1) and associated limitations and uncertainties (section III.B.2), at

- risk populations (section III.B.3), and potential PM 2.5 - related impacts on public health (section III.B.4). As was true in the la st two reviews, evidence from epidemiologic al , controlled human exposure and animal toxicological studies played a key role in the Integrated Science Assessment ’s evaluation of the scientific evidence.

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Page 26 of 432 The 2006 PM NAAQS review concluded that there was “st rong epidemiological evidence” for linking long

- term PM 2.5 exposures with cardiovascular

- related and lung cancer mortality and respiratory

- related morbidity and for linking short

- term PM 2.5 exposures with cardiovascular

- related and respiratory

- related mort ality and morbidity ( U.S. EPA, 2004, p. 9

- 46; U.S. EPA, 2005, p. 5

- 4). Overall, the evidence from epidemiological, toxicological, and controlled human exposure studies supported “likely causal associations” between PM 2.5 and both mortality and morbidity fr om cardiovascular and respiratory diseases, based on “an assessment of strength, robustness, and consistency in results” ( U.S. EPA, 2004, p. 9

- 48). 13 In this review, based on the expanded body of evidence, the EPA finds that: (1) In looking across the extensive new scientific evidence available in this review, our overall understanding of health effects associated with fine particle exposures has been greatly expanded. The currently available evidence is largely consistent with evi dence available in the last review and substantially strengthens what is known about the effects associated with fine particle exposures. (2) A number of large multi

- city epidemiological studies have been conducted throughout the U.S., including extended anal yses of long

- term exposure studies that were important to inform decision

- making in the last review. The body of currently available scientific evidence has also been expanded greatly by the publication of a number of new multi

- city, time

- series studies th at have used uniform methodologies to investigate the effects of short

- term PM 2.5 exposures on public health. This body of evidence provides a more expansive data base and considers multiple locations representing varying regions and seasons that provide e vidence of the influence of different air pollution mixes on PM 2.5 - associated health effects. These studies provide more precise estimates of the magnitude of effects associated with short

- term PM 2.5 exposure than most smaller

- scale single

city studies tha t were more commonly available in the last review. These studies

13 The term “likely causal association” was used in the 2004 Criteria Document to summarize the strength of the available evidence available in the last review for PM 2.5 . However, this terminology was not based on a formal framework for evaluating eviden ce for inferring causation. Since the last review, the EPA has developed a more formal framework for reaching causal determinations with standardized language to express evaluation of the evidence (U.S. EPA, 2009a, section 1.5).

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Page 27 of 432 have reported consistent increases in morbidity and/or premature mortality related to ambient PM 2.5 concentrations, with the strongest evidence reported for cardiovascular

- related effects. (3) In addition, the findings of new toxicological and controlled human exposure studies greatly expand and provide stronger support for a number of potential biologic al mechanisms or pathways for cardiovascular and respiratory effects associated with long

- and short

- term PM exposures. These studies provide coherence and biological plausibility for the effects observed in epidemiological studies. (4) Using a more formal framework for reaching causal determinations than used in prior reviews , 14 the EPA concludes that a causal relationship exists between both long

- and short

- term exposures to PM 2.5 and premature mortality and cardiovascular effects and a likely causal relationship exists between long

- and short

- term PM 2.5 exposures and respiratory effects. Further, there is evidence suggestive of a causal relationship between long

- term PM 2.5 exposures and other health effects, including developmental and reproductive effects (e.g., low birth weight, infant mortality) and carcinogenic, mutagenic, and genotoxic effects (e.g ., lung cancer mortality). 15 (5) The newly available evidence significantly strengthens the link between long

- and short

- term exposure to PM 2.5 and premature mortality, while providing indications that the magnitude of the PM 2.5 - mortality association with long

- term exposures may be larger than previously estimated. The strongest evidence comes from recent studies investigating long

- term exposure to PM 2.5 and cardiovascular

related mortality. The evidence supporting a causal relationship between long

-

14 The causal framework draw s upon the assessment and integration of evidence from across epidemiological, controlled human exposure, and toxicological studies, and the related uncertainties that ultimately influence our understanding of the evidence. This framework employs a five

- le vel hierarchy that classifies the overall weight of evidence and causality using the following categorizations: causal relationship, likely to be causal relationship, suggestive of a causal relationship, inadequate to infer a causal relationship, and not l ikely to be a causal relationship (U.S. EPA, 2009a, Table 1

- 3). The development of the causal framework reflects considerable input from CASAC and the public, with CASAC concluding that, “The five

- level classification of strength of evidence for causal inf erence has been systemically applied [for PM]; this approach has provided transparency and a clear statement of the level of confidence with regard to causation, and we recommend its continued use in future ISAs” (Samet, 2009f, p. 1). 15 These causal infere nces are based not only on the more expansive epidemiological evidence available in this review but also reflect consideration of important progress that has been made to advance our understanding of a number of potential biologic modes of action or pathwa ys for PM

- related cardiovascular and respiratory effects (U.S. EPA, 2009a, chapter 5).

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Page 28 of 432 term PM 2.5 exposure and mortality also includes consideration of new studies that demonstrated an improvement in community health following reductions in ambient fine particles. (6) Several new studies have examined the association between cardiovascular effects and long

- term PM 2.5 exposures in multi

- city studies conducted in the U.S. and Europe. While studies were not available in the last review with regard to long

term exposure and cardiovascular

- related morbidity, recent studies have provided new evidence linking long

- term exposure to PM 2.5 with an array of cardiovascular effects such as heart attacks, congestive heart failure, stroke, and mortality. This evidence is coherent with studies of short

- term exposure to PM 2.5 that have observed associations with a continuum of effects ranging from subtle changes in indicators of cardiovascular health to serious clinical events, such as increased hospitalizations and emergency department visits due to cardiovascular disease and cardiovascular mortality. (7) Extended analyses of st udies available in the last review as well as new epidemiological studies conducted in the U.S. and abroad provide stronger evidence of respiratory

- related morbidity effects associated with long

- term PM 2.5 exposure. The strongest evidence for respiratory

- r elated effects is from studies that evaluated decrements in lung function growth, increased respiratory symptoms, and asthma development. The strongest evidence from short

- term PM 2.5 exposure studies has been observed for increased respiratory

- related emer gency department visits and hospital admissions for chronic obstructive pulmonary disease (COPD) and respiratory infections. (8) The body of scientific evidence is somewhat expanded from the 2006 review but is still limited with respect to associations between long

- term PM 2.5 exposures and developmental and reproductive effects as well as cancer, mutagenic, and genotoxic effects. The strongest evidence for an association between PM 2.5 and developmental and reproductive effects comes from epidemiological studies of low birth weight and infant mortality, especially due to respiratory causes during the post

- neonatal period (i.e., 1 month

- 12 months of age). With regard to cancer effects, “[m]ultiple epidemiologic studies have shown a consistent positive asso ciation between PM 2.5 and lung cancer mortality, but studies have generally not reported associations between PM 2.5 and lung cancer incidence” (U.S. EPA 2009a p. 2

- 13 )

. (9) Efforts to evaluate the relationships between PM composition and health effects contin ue to evolve. While many constituents of PM 2.5 can be linked with differing health effects, the evidence is not yet sufficient to allow differentiation of those constituents or sources that may be more closely related to specific health outcomes nor to exclude any individual component or group of components associated with any source categories from the fine particle mixture of concern.

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Page 29 of 432 (10) Specific groups within the general population are at increased risk for experiencing adverse health effects related to PM exposures . The currently available evidence expands our understanding of previously identified at

- risk populations (i.e., children, older adults, and individuals with pre

- existing heart and lung disease) and supports the identification of additional at

- risk populations (e.g., persons with lower socioeconomic status, genetic differences). Evidence for PM

- related effects in these at

- risk populations has expanded and is stronger than previously observed. There is emerging, though still limited, evidence for additional potentially at

- risk populations, such as those with diabetes, people who are obese, pregnant women, and the developing fetus. (11) The population potentially affected by PM 2.5 is large. In addition, large subgroups of the U.S. population have be en identified as at

- risk populations. While individual effect estimates from epidemiological studies may be small in size, the public health impact of the mortality and morbidity associations can be quite large given the extent of exposure. Taken together, this suggests that exposure to ambient PM 2.5 concentrations can have substantial public health impacts. (12) While the currently available scientific evidence is stronger and more consistent than in previous reviews, providing a strong basis for decision maki ng in this review, the EPA recognizes that important uncertainties and limitations in the health effects evidence remain. Epidemiological studies evaluating health effects associated with long

- and short

- term PM 2.5 exposures have reported heterogeneity in responses between cities and geographic regions within the U.S. This heterogeneity may be attributed, in part, to differences in the fine particle composition or related to exposure measurement error, which can introduce bias and increased uncertainty in a ssociated health effect estimates. Variability in the associations observed across PM 2.5 epidemiological studies may be due in part to exposure error related to measurement

- related issues, the use of central fixed

- site monitors to represent population expo sure to PM 2.5 , models used in lieu of or to supplement ambient measurements, and our limited understanding of factors that may influence exposures (e.g., topography, the built environment, weather, source characteristics, ventilation usage, personal activi ty patterns, photochemistry). In addition, where PM 2.5 and other pollutants (e.g., ozone, nitrogen dioxide, and carbon monoxide) are correlated, it can be difficult to distinguish the effects of the various pollutants in the ambient mixture (i.e., co

- pollu tant confounding). 16 While uncertainties and limitations still remain in the available health effects evidence, the Administrator judges the currently available scientific data base to be

16 A copollutant meets the criteria for potential confounding in PM

- health associations if: (1) it is a potential risk factor for the health effect under study; (2) it is correlated with PM; and (3) it does not act as an intermediate step in the pathway between PM exposure and the health effect under study (U.S. EPA, 2004, p. 8

- 10).

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Page 30 of 432 stronger and more consistent than in previous reviews providing a st rong basis for decision making in this review. C. Overview of Quantitative Characterization of Health Risks In addition to a comprehensive evaluation of the health effects evidence available in this review, the EPA conducted an expanded quantitative risk ass essment for selected health endpoints to provide additional information and insights to inform decisions on the primary PM 2.5 NAAQS. 17 As discussed in section III.C of the proposal, the approach used to develop quantitative risk estimates associated with PM 2.5 exposures was built on the approach used and lessons learned in the last review and focused on improving the characterization of the overall confidence in the risk estimates, including related uncertainties, by incorporating a number of enhancements, i n terms of both the methods and data used in the analyses. The goals of this quantitative risk assessment were largely the same as those articulated in the risk assessment conducted for the last review. These goals included: (1) to provide estimates of th e potential magnitude of premature mortality and/or selected morbidity effects in the population associated with recent ambient levels of PM 2.5 and with simulating just meeting the current and alternative suites of PM 2.5 standards in 15

17 The quantitative risk assessment conducted for this review is more fully described and presented in the Risk Assessment (U.S. EPA, 2010a) and summarized in detail in the Policy Assessment (U.S. EPA, 2011a, sections 2.2.2. and 2.3.4.2). The scope and metho dology for this risk assessment were developed over the last few years with considerable input from CASAC and the public as described in section II.B.3 above

.

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Page 31 of 432 selected urban stud y areas , 18 including, where data were available, consideration of impacts on at

- risk populations; (2) to develop a better understanding of the influence of various inputs and assumptions on the risk estimates to more clearly differentiate among alternative suites of standards; and (3) to gain insights into the distribution of risks and patterns of risk reductions and the variability and uncertainties in those risk estimates. In addition, the quantitative risk assessment included nationwide estimates of the potential magnitude of premature mortality associated with long

- term exposure to recent ambient PM 2.5 concentrations to more broadly characterize this risk on a national scale and to support the interpretation of the more detailed risk estimates generated for select ed urban study areas. The expanded and updated risk assessment conducted in this review included estimates of risk for: (1) all

- cause, ischemic heart disease

- related, cardiopulmonary

related, and lung cancer

- related mortality associated with long

- term PM 2 .5 exposure; (2)

non - accidental, cardiovascular

- related, and respiratory

- related mortality associated with short

- term PM 2.5 exposure; and (3) cardiovascular

- related and respiratory

- related hospital admissions and asthma

- related emergency department visits associated with short

- term

18 The Risk Assessment concluded that these 15 urban study areas were generally representative of urban areas in the U.S. likely to experience relatively elevated levels of risk related to ambient PM 2.5 exposure with the potential for better characterization at the higher end of that distribution (U.S. EPA, 2011a, p. 2

- 42; U.S. EPA, 2010a, section 4.4, Figure 4

- 17). The representativeness analysis also showed that the 15 urban study areas do not capture areas with the highest baseline morality risks or the ol dest populations (both of which can result in higher PM 2.5 - related mortality estimates). However, some of the areas with the highest values for these attributes had relatively low PM 2.5 concentrations (e.g., urban areas in Florida) and, consequently, the R isk Assessment concluded failure to include these areas in the set of urban study areas was unlikely to exclude high PM 2.5 - risk locations (U.S. EPA, 2010a, section 4.4.1).

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Page 32 of 432 PM 2.5 exposure. 19 The Risk Assessment included a core set of risk estimates supplemented by an alternative set of risk results generated using single

- factor and multi

- factor sensitivity analyses. The core set of risk estimates wa s developed using the combination of modeling elements and input data sets identified in the Risk Assessment as having higher confidence relative to inputs used in the sensitivity analyses. The results of the sensitivity analyses provided information to ev aluate and rank the potential impacts of key sources of uncertainty on the core risk estimates. In addition, the sensitivity analyses represented a set of reasonable alternatives to the core set of risk estimates that fell within an overall set of plausibl e risk estimates surrounding the core estimates. The EPA recognized that there were many sources of variability and uncertainty inherent in the inputs to its quantitative risk assessment. 20 The design of the risk assessment included a number of elements to address these issues in order to increase the overall confidence in the risk estimates generated for the 15 urban study areas, including using guidance from the World Health Organization (WHO, 2008) as a framework for characterizing uncertainty in the ana lyses. 21 With respect to the sources of variability, the Risk Assessment considered those

19 The evidence available for these selected health effect endpoints generally focuse d on the entire population, although some information was available to support analyses that considered differences in estimated risk for at

- risk populations including older adults and persons with pre

- existing cardiovascular and respiratory diseases. 20 Va riability refers to the heterogeneity of a variable of interest within a population or across different populations. Uncertainty refers to the lack of knowledge regarding the actual values of inputs to an analysis (U.S. EPA, 2010a, p. 3

- 63). 21 The extent to which key sources of potential variability were (or were not) fully captured in the design of the risk assessment are discussed in section 3.5.2 of the Risk Assessment (U.S. EPA, 2010a, pp. 3

- 67 to 3

- 69).

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Page 33 of 432 that contributed to differences in risk across urban study areas, but did not directly affect the degree of risk reduction associated with the simulation of just meet ing current or alternative standard levels (e.g., differences in baseline incidence rates, demographics and population behavior). The Risk Assessment also focused on factors that not only introduced variability into risk estimates across study areas, but a lso played an important role in determining the magnitude of risk reductions upon simulation of just meeting current or alternative standard levels (e.g., peak

- to

- mean ratios of ambient PM 2.5 concentrations within individual urban study areas and the natur e of the rollback approach used to simulate just meeting the current or alternative standards). Key sources of potential variability that were likely to affect population risks included the following : (1) intra

- urban variability in ambient PM 2.5 concentrat ions, including PM 2.5 composition; (2) variability in the patterns of reductions in PM 2.5 concentrations associated with different rollback approaches when simulating just meeting the current or alternative standards; co

- pollutant exposures; (3) factors re lated to demographic and socioeconomic status; (4) behavioral differences across urban study areas (e.g., time spent outdoors); (5) baseline incidence rates; and (6) longer

- term temporal variability in ambient PM 2.5 concentrations reflecting meteorological trends as well as future changes in the mix of PM 2.5 sources, including changes in air quality related to future regulatory actions. With regard to uncertainties, single and multi

- factor sensitivity analyses were combined with a qualitative analysis to a ssess the impact of potential sources of uncertainty on the core risk estimates. Key sources of uncertainty included: (1) characterizing intra

- urban population exposure in the context of epidemiolog ical studies linking PM 2.5 to specific health effects; (2) statistical fit of the concentration

- response

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Page 34 of 432 functions for short

- term exposure

- related health endpoints; (3) shape of the concentration

- response functions; (4) specifying the appropriate lag structure for short

term exposure studies; (5) transferability of concentration

- response functions from study locations to urban study area locations for long

- term exposure

- related health endpoints; (6) use of single

- city versus multi

- city studies in the derivation of concentration

- response functions; (7) impact of hi storical air quality on estimates of health risk associate with long

- term PM 2.5 exposures; and (8) potential variation in effect estimates reflecting compositional differences in PM 2.5 . Beyond characterizing uncertainty and variability, a number of design elements were included in the risk assessment to increase the overall confidence in the risk estimates generated for the 15 urban study areas ( U.S. EPA, 2011a, pp. 2

- 38 to 2

- 41). These elements included: (1) use of a deliberative process for specifying com ponents of the risk model that reflects consideration of the latest research on PM 2.5 exposure and risk ( U.S. EPA, 2010a, section 5.1.1); (2) integration of key sources of variability into the design as well as the interpretation of risk estimates (U.S.EPA, 2010a, section 5.1.2); (3) assessment of the degree to which the urban study areas are representative of ar eas in the U.S. experiencing higher PM 2.5 - related risk (U.S.EPA, 2010a, section 5.1.3); and (4) identification and assessment of important sources of uncertainty and the impact of these uncertainties on the core risk estimates (U.S.EPA, 2010a, section 5.1. 4). Further, additional analyses examined potential bias and overall confidence in the risk estimates. Greater confidence is associated with risk estimates based on simulated annual mean PM 2.5 concentrations that are within the region of the air quality di stribution used in deriving the concentration

- response functions where the bulk of the data reside (e.g.,

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Page 35 of 432 within one standard deviation around the long

- term mean PM 2.5 concentration) ( U.S. EPA, 2011a, p. 2

- 38). Key observations and insights from the PM 2.5 risk assessment, together with important caveats and limitations , were discussed in section III.C.3 of the proposal. In general, in considering the set of quantitative risk estimates and related uncertainties and limitations related to long

- and short

- term PM 2.5 exposure together with consideration of the health endpoints which could not be quantified, the Policy Assessment concluded this information provided strong evidence that risks estimated to remain upon simulating just meeting the current suite of PM 2.5 standards are important from a public health perspective, both in terms of severity and magnitude of effects. Patterns of increasing estimated risk reductions were generally observed as either the annual or 24

- hour standard level, or both, were reduced over the ranges considered in the Risk Assessment. The magnitude of both long

- and short

- term exposure

- related risk estimated to remain upon just meeting the current suite of standards as well as alternative standard levels w as strongly associated with th e simulated change in annual mean PM 2.5 concentrations. Although long

- and short

- term exposure

- related mortality rates have similar patterns in terms of the subset of urban study areas experiencing risk reductions for the current suite of standard levels, the magnitude of risk remaining is higher for long

term exposure

- related mortality and substantially lower for short

- term exposure

- related mortality. Short

- term exposure

- related morbidity risk estimates w ere greater for cardiovascular

- related than respirat ory - related events and emergency department visits for asthma

- related events were significant: Furthermore, most of the aggregate risk associated with short

- term exposures was not primarily driven by the small number of

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Page 36 of 432 days with PM 2.5 concentrations in th e upper tail of the air quality distribution, but rather by the large number of days with PM 2.5 concentrations at and around the mean of the distribution, that is, the 24

- hour average concentrations that are in the low

- to mid

- range, well below the peak 24

- hour concentrations (U.S. EPA, 2011a, p. 2

- 3)

. With regard to characterizing estimates of PM 2.5 - related risk associated with simulation of alternative standards, the Policy Assessment recognized that greater overall confidence was associated with estimates of risk reduction than for estimates of absolute risk remaining ( U.S. EPA, 2011a, p. 2

- 94). Furthermore, the Policy Assessment recognized that estimates of absolute risk remaining for each of the alternative standard levels considered, particularly in the context of long

- term exposure

- related mortality, may be underestimated 22 . In addition, the Policy Assessment observed that in considering the overall confidence associated with the quantitative analyses, the Risk Assessment recognized that: (1) substantial variability existed in the magnitude of risk remaining across urban study areas and (2) in general, higher confidence was associated with risk estimates based on PM 2.5 concentrations near the mean PM 2.5 concentrations in the underlying epidemiological stu dies providing the concentration

- response functions (e. g. ,

22 Based on the consideration of both the qualita tive and quantitative assessments of uncertainty, the Risk Assessment concluded that it is unlikely that the estimated risks are over - stated, particularly for premature mortality related to long

- term PM 2.5 exposures. In fact, the Policy Assessment and the Risk Assessment concluded that the core risk estimates for this category of health effects may well be biased low based on consideration of alternative model specifications evaluated in the sensitivity analyses (U.S. EPA, 2011a, p. 2

- 41; U.S. EPA, 2010a, p . 5

- 16; Figures 4

- 7 and 4

- 8). In addition, the Policy Assessment recognized that the currently available scientific information included evidence for a broader range of health endpoints and at

- risk populations beyond those included in the quantitative risk assessment, including decrements in lung function growth and respiratory symptoms in children as well as reproductive and developmental effects (U.S. EPA, 2011a, section 2.2.1).

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Page 37 of 432 within one standard deviation of the mean PM 2.5 concentration reported). Furthermore, although the Risk Assessment estimated that the alternative 24

- hour standard levels considered (when controllin g) would result in additional estimated risk reductions beyond those estimated for alternative annual standard levels alone, these additional estimated reductions were highly variable. Conversely, the Risk Assessment recognized that alternative annual stan dard levels, when controlling, resulted in more consistent risk reductions across urban study areas, thereby potentially providing a more consistent degree of public health protection ( U.S. EPA, 2010a, p. 5

- 17). D. Conclusions on the Adequacy of the Current P rimary PM 2.5 Standards 1. Introduction The initial issue to be addressed in the current review of the primary PM 2.5 standards is whether, in view of the advances in scientific knowledge and other information reflected in the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment, the existing standards should be retained or revised. In considering the adequacy of the current suite of PM 2.5 standards, the Administrator has considered the large body of evidence presented and assessed in the Integrated Science Assessment ( U.S. EPA, 2009a), the quantitative assessment of risks, staff conclusions and associated rationales presented in the Policy Assessment , views expressed by CASAC, and public comments. The Administrator has taken into acco unt both evidence

- and risk

- based considerations 23 in developing final conclusions on the adequacy of the current primary

23 Evidence

- based considerations include the assessment of epidemiological, to xicological, and controlled human exposure studies evaluating long

- or short

- term exposures to PM 2.5 , with supporting evidence related to dosimetry and potential pathways/modes of action, as well as the integration of evidence across each of these

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Page 38 of 432 PM 2.5 standards.

a. Evidence

- and Risk

- based Considerations in the Policy Assessment In considering the available epidemiologic al evid ence in this review, the Policy Assessment took a broader approach than was used in the last review. This approach reflected the more extensive and stronger body of evidence available since the last review on health effects related to both long

- and short

- term exposure to PM 2.5 . As discussed in section III.A.3 above, this broader approach focused on setting the annual standard as the “generally controlling” standard for lowering both short

- and long

- term PM 2.5 concentrations and so providing requisite prote ction to public health . In conjunction with such an annual standard, this approach focused on setting the 24

- hour standard to provide supplemental protection against days with high peak PM 2.5 concentrations. In addressing the question whether the evidence now available in this review supports consideration of standards that are more protective than the current PM 2.5 standards, the Policy Assessment considered whether : (1) statistically significant health effects associations with long

- or short

- term exposur es to fine particles occur in areas that would likely have met the current PM 2.5 standards [ see American Trucking Ass ociatio n , 283 F. 3d at 369, 376 (revision of level of PM NAAQS justified when health effects are observed in areas meeting the existing st andard) ] , and (2) associations with long

- term exposures to fine particles extend down to lower air quality concentrations than had
discipli nes, as assessed in the Integrated Science Assessment (U.S. EPA, 2009a) and focus on the policy

- relevant considerations as discussed in section III.B above and in the Policy Assessment (U.S. EPA, 2011a, section 2.2.1). Risk

- based considerations draw from t he results of the quantitative analyses presented in the Risk Assessment (U.S. EPA, 2010a) and focus on the policy

- relevant considerations as discussed in section III.C above and in the Policy Assessment (U.S. EPA, 2011a, section 2.2.2).

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Page 39 of 432 previously been observed. With regard to associations observed in long

- term PM 2.5 exposure studies, the Policy Assessment rec ognized that extended follow

- up analyses of the ACS and Harvard Six Cities studies provided consistent and stronger evidence of an association with mortality at lower air quality distributions than had previously been observed ( U.S. EPA, 2011a, pp. 2

- 31 to 2

- 32). The original and reanalysis of the ACS study reported positive and statistically significant effects associated with a long

- term mean PM 2.5 concentration of 18.2 μg/m 3 across 50 metropolitan areas for 1979 to 1983 (Pope et al., 1995; Krewski et al. , 2000). 24 In extended analyses, positive and statistically significant effects of approximately similar magnitude were associated with declining PM 2.5 concentrations, from an aggregate long

- term mean in 58 metropolitan areas of 21.2 μg/m 3 in the original monitoring period (1979 to 1983) to 14.0 μg/m 3 for 116 metropolitan areas in the most recent years evaluated (1999

- 2000), with an overall average across the two study periods in 51 metropolitan areas of 17.7 μg/m 3 (Pope et al., 2002; Krews ki et al., 2009). With regard to the Harvard Six Cities Study, the original and reanalysis reported positive and statistically significant effects associated with a long

term mean PM 2.5 concentration of 18.0 μg/m 3 for 1980 to 1985 (Dockery et al., 1993; Kr ewski et al., 2000). In an extended follow

- up of this study, the aggregate long

- term mean concentration across all years evaluated was 16.4 μg/m 3 for 1980 to 1988 25 (Laden et al., 2006). In an additional analysis of the extended follow

- up of the Harvard Six Cities

24 The study period s referred to in the Policy Assessment (U.S. EPA, 2011a) and in this final rule reflect the years of air quality data that were included in the analyses, whereas the study periods identified in the Integrated Science Assessment (U.S. EPA, 2009a) reflect th e years of health event data that were included. 25 Aggregate mean concentration provided by study author (personal communication from Dr. Francine Laden, 2009).

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Page 40 of 432 study, investigators reported that the concentration

- response relationship was linear and “clearly continuing below the level” of the current annual standard ( U.S. EPA, 2009a, p. 7

- 92; Schwartz et al., 2008). Cohort studies conducted since the las t review provided additional evidence of mortality associated with air quality distributions that are generally lower than those reported in the ACS and Harvard Six Cities studies, with effect estimates that were similar or, in some studies, significantly greater in magnitude than in the ACS and Harvard Six Cities studies (see also, section III.D.1.a of the proposal, 77 FR 38918 to 28919; U.S. EPA, 2011a, pp. 2

- 32 to 2

- 33). The Women’s Health Initiative (WHI) study reported positive and most often statistic ally significant associations between long

- term PM 2.5 exposure and cardiovascular

- related mortality as well as morbidity effects, with much larger relative risk estimates for mortality than in the ACS and Harvard Six Cities studies, at an aggregate long

- te rm mean PM 2.5 concentration of 12.9 μg/m 3 for 2000 (Miller et al., 2007). 26 Using the Medicare cohort, Eftim et al. (2008) reported somewhat higher effect estimates than in the ACS and Harvard Six Cities studies with aggregate long

- term mean concentrations of 13.6 μg/m 3 and 14.1 μg/m 3 , respectively, for 2000 to 2002. Zeger et al. (2008) reported associations between long

- term PM 2.5 exposure and mortality for the eastern region of the U.S. at an aggregated long

- term PM 2.5 median concentration of 14.0

26 The Policy Assessment noted that in comparison to other long

- term exposure studies, the Mille r et al. (2007) study was more limited in that it was based on only one year of air quality data (U.S. EPA, 2011a, p. 2

- 82). The proposal further noted that the air quality data considered were extrapolated from that one single year of air quality data (20 00) to the whole study, and that the air quality data post

- dated the years of health events considered (i.e., 1994 to 1998) (77 FR 38918, fn 62).

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Page 41 of 432 μg/m 3 , although no association was reported for the western region with an aggregate long

- term PM 2.5 median concentration of 13.1 μg/m 3 ( U.S. EPA, 2009a, p. 7

- 88). 27 Premature mortality in children reported in a national infant mortality study as well as mortalit y in a cystic fibrosis cohort including both children and adults reported positive but statistically nonsignificant effects associated with long

- term aggregate mean concentrations of 14.8 μg/m 3 and 13.7 μg/m 3 , respectively (Woodruff et al., 2008; Goss
et a l., 2004). With respect to respiratory morbidity effects associated with long

- term PM 2.5 exposure, the across

- city mean of 2

- week average PM 2.5 concentrations reported in the initial Southern California Children’s Health Study was approximately 15.1 μg/m 3 (Peters et al., 1999). These results were found to be consistent with results of cross

sectional analyses of the 24

- Cities Study (Dockery et al., 1996; Raizenne et al., 1996), which reported a long

- term cross

- city mean PM 2.5 concentration of 14.5 μg/m 3

. 28 I n this review, extended analyses o f the Southern California Children’s Health Study provide stronger evidence of PM 2.5 - related respiratory effects, at lower air quality concentrations than had previously been reported, with a four

- year aggregate mean concentration of 13.8 μg/m 3 across the 12 study communities (McConnell et al., 2003; Gauderman et al., 2004,

27 Zeger et al. (2008) also reported positive and statistically significant effects for the central region, wit

h an aggregate long

- term mean PM 2.5 concentration of 10.7 μg/m 3

. However, in contrast to the eastern and western risk estimates, the central risk estimate increased with adjustment for COPD (used as a proxy for smoking status). Due to the potential for con founding bias influencing the risk estimate for the central region, the Policy Assessment did not focus on the results reported in the central region to inform the adequacy of the current suite of standards or alternative annual standard levels (U.S. EPA, 2011a, p. 2

- 32). 28 See American Farm Bureau Federation v. EPA , 559 F. 3d at 525 (noting the importance of these studies, as well as EPA’s failure to properly take them into account).

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Page 42 of 432 U.S. EPA, 2009a, Figure 7

- 4). In also considering health effects for which the Integrated Science Assessment concluded evidence was suggestive of a causal relationship, the Policy Assessment noted a limited number of birth outcome studies that reported positive and statistically significant effects related to aggregate long

- term mean PM 2.5 concentrations down to approximately 12 μg/m 3 ( U.S. EPA, 201 1a, p. 2

- 33). Collectively, the Policy Assessment concluded that currently available evidence provided support for associations between long

- term PM 2.5 exposure and mortality and morbidity effects that extend to distributions of PM 2.5 concentrations that a re lower than those that had previously been associated with such effects, with aggregate long

- term mean PM 2.5 concentrations extending to well below the level of the current annual standard. The Policy Assessment also considered the long

- term mean PM 2.5 c oncentrations in short

- term exposure studies in assessing the appropriateness of the level of the current annual standard. See American Farm Bureau Federation v. EPA , 559 F. 3d at 522, 523

- 24 (remanding 2006 standard because the EPA had not adequately expl ained its choice not to consider long

- term means of short

- term exposure studies in assessing adequacy of primary annual PM 2.5 standard). In light of the mixed findings reported in single

- city, short

- term exposure studies, the Policy Assessment placed compa ratively greater weight on the results from multi

- city studies in considering the adequacy of the current suite of standards ( U.S. EPA, 2011a, pp. 2

- 34 to 2

- 35). With regard to associations reported in short

- term PM 2.5 exposure studies, the Policy Assessm ent recognized that long

- term mean concentrations reported in new multi

-

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Page 43 of 432 city U.S. and Canadian studies provided evidence of associations between short

- term PM 2.5 exposure and mortality at similar air quality distributions t o those that had previously been observed in an 8

- cities Canadian study (Burnett and Goldberg, 2003; aggregate long

- term mean PM 2.5 concentration of 13.3 μg/m 3 ). In a multi

- city time

- series analysis of 112 U.S. cities, Zanobetti and Schwartz (2009) reported a positive and statistically si gnificant association with all

- cause, cardiovascular

- related (e.g., heart attacks, stroke), and respiratory

- related mortality and short

- term PM 2.5 exposure, in which the aggregate long

- term mean PM 2.5 concentration was 13.2 μg/m 3 ( U.S. EPA, 2009a, Figure 6

- 24). Furthermore, city

- specific effect estimates indicated the association between short

- term exposure to PM 2.5 and total mortality and cardiovascular

- and respiratory

- related mortality was consistently positive for an overwhelming majority (99 percent) o f the 112 cities across a wide range of air quality concentrations (long

- term mean concentrations ranging from 6.6 μg/m 3 to 24.7 μg/m 3 ; U.S. EPA, 2009a, Figure 6

- 24, p. 6

- 178 to 179). The EPA staff noted that for all

- cause mortality, city

- specific effect

e stimates were statistically significant for 55 percent of the 112 cities, with long

- term city

- mean PM 2.5 concentrations ranging from 7.8 μg/m 3 to 18.7 μg/m 3 and 24

- hour PM 2.5 city

- mean 98 th percentile concentrations ranging from 18.4 to 64.9 μg/m 3 (personal communication with Dr. Antonella Zanobetti, 2009). 29 With regard to cardiovascular and respiratory morbidity effects, in the first analysis of the Medicare cohort conducted by Dominici et al. (2006a) across 204 U.S. counties, investigators repor ted a statistically significant association with hospitalizations

29 Single

- city Bayes

- adjusted effect estimates for the 112 cities analyzed in Zanobetti and Schwartz (2009) were provided by the study authors (personal communication with Dr. Antonella Zanobetti, 2009; see also U.S. EPA, 2009a, Figure 6

- 24).

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Page 44 of 432 for cardiovascular and respiratory diseases and short

- term PM 2.5 exposure, in which the aggregate long

- term mean PM 2.5 concentration was 13.4 μg/m 3 . Furthermore, a sub

analysis restricted to days with 24

- hour average concentrations of PM 2.5 at or below 35 μg/m 3 indicated that, in spite of a reduced statistical power from a smaller number of study days, statistically significant associations were still observed between short

- term exposure to P M 2.5 and hospital admissions for cardiovascular and respiratory diseases (Dominici, 2006b). 30 In an extended analysis of this cohort, Bell et al (2008) reported a positive and statistically significant increase in cardiovascular hospitalizations associated with short

- term PM 2.5 exposure, in which the aggregate long

- term mean PM 2.5 concentration was 12.9 μg/m 3 . These results, along with the observation that approximately 50 percent of the 204 county

- specific mean 98 th percentile PM 2.5 concentrations in the st

udy aggregated across all years were below the 24

- hour standard of 35 μg/m 3 , not only indicated that effects are occurring in areas that would meet the current standards but also suggested that the overall health effects observed across the U.S. are not pr imarily driven by the higher end of the PM 2.5 air quality distribution (Bell, 2009a, personal communication from Dr. Michelle Bell regarding air quality data for Bell et al., 2008 and Dominici et al., 2006a). Collectively, the Policy Assessment concluded that the findings from short

- term PM 2.5 exposure studies provided evidence of PM 2.5 - associated health effects occurring in

30 This sub

- analysis was not included in the original publication (Dominici et al., 200 6a). The study authors provided sub

- analysis results for the Administrator’s consideration as a letter to the docket following publication of the proposed rule in January 2006 (personal communication with Dr. Francesca Dominici, 2006b). As noted in section III.A.3, this study is part of the basis for the conclusion that there is no evidence suggesting that risks associated with long

- term exposures are likely to be disproportionately driven by peak 24

- hour concentrations.

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Page 45 of 432 areas that would likely have met the current suite of PM 2.5 standards ( U.S. EPA, 2011a, p. 2

- 35). These findings were further bolster ed by evidence of statistically significant PM 2.5 - related health effects occurring in analyses restricted to days in which 24

- hour average PM 2.5 concentrations were below 35 μg/m 3 (Dominici, 2006b). In evaluating the currently available scientific evidenc e, as summarized in section III.B of the proposal, the Policy Assessment first concluded that there was stronger and more consistent and coherent support for associations between long

- and short

- term PM 2.5 exposures and a broad range of health outcomes tha n was available in the last review, providing the basis for fine particle standards at least as protective as the current PM 2.5 standards ( U.S. EPA, 2011a, p. 2

- 26). Having reached this initial conclusion, the Policy Assessment addressed the question of whether the available evidence supported consideration of standards that were more protective than the current standards. In so doing, the Policy Assessment considered whether there was now evidence that health effect associations have been observed in areas that likely met the current suite of PM 2.5 standards. As discussed above, long

- and short

- term PM 2.5 exposure studies provided evidence of ass ociations with mortality and cardiovascular and respiratory effects both at lower ambient PM 2.5 concentrations than had been observed in the previous review and at concentrations allowed by the current standards ( U.S. EPA, 2011a, p. 2

- 35). In reviewing th is information, the Policy Assessment recognized that important limitations and uncertainties associated with this expanded body of scientific evidence, as discussed in section III.B.2 of the proposal, needed to be carefully considered in determining the w eight to be placed on the body of studies available in this review. Taking these limitations and uncertainties into consideration, the Policy Assessment

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Page 46 of 432 concluded that the currently available evidence clearly calls into question whether the current suite o f primary PM 2.5 standards protects public health with an adequate margin of safety from effects associated with long

- and short

- term exposures. Furthermore, the Policy Assessment concluded this evidence provides strong support for considering fine particle standards that would afford increased protection beyond that afforded by the current standards ( U.S. EPA, 2011a, p. 2

- 35). In addition to evidence

- based consideration, the Policy Assessment also considered the extent to which health risks estimated to occ ur upon simulating just meeting the current PM 2.5 standards may be judged to be important from a public health perspective, taking into account key uncertainties associated with the quantitative health risk estimates. In so doing, the Policy Assessment fir st noted that the quantitative risk assessment addresses: (1) the core PM 2.5 - related risk estimates; (2) the related uncertainty and sensitivity analyses, including additional sets of reasonable risk estimates generated to supplement the core analysis; (3) an assessment of the representativeness of the urban study areas within a national context; 31 and (4) consideration of patterns in design values and air quality monitoring data to inform interpretation of the risk estimates, as discussed in section III.C a

bove. In considering the health risks estimated to remain upon simulation of just meeting the current suite of standards and considering both the qualitative and quantitative assessment of uncertainty completed as part of the assessment, the Policy

31 Based on analyses of the represent ativeness of the 15 urban study areas in the broader national context, the Policy Assessment concludes that these study areas are generally representative of urban areas in the U.S. likely to experience relatively elevated levels of risk related to ambient PM 2.5 exposures (U.S. EPA, 2011a, p. 2

- 42).

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Page 47 of 432 Assess ment concluded these risks are important from a public health standpoint and provided strong support for consideration of alternative standards that would provide increased protection beyond that afforded by the current PM 2.5 ( U.S. EPA, 2011a, pp. 2

- 47 to 2

- 48). This conclusion reflected consideration of both the severity and the magnitude of the effects. For example, the Risk Assessment indicated the possibility that premature deaths related to ischemic heart disease associated with long

- term PM 2.5 exposur e alone would likely be on the order of thousands of deaths per year in the 15 urban study areas upon simulating just meeting the current standards 32 ( U.S. EPA, 2011a, pp. 2

- 46 to 2

- 47). Moreover, additional risks were anticipated for premature mortality

re lated to cardiopulmonary effects and lung cancer associated with long

- term PM 2.5 exposure as well as mortality and cardiovascular

- and respiratory

- related morbidity effects (e.g., hospital admissions, emergency department visits) associated with short

term PM 2.5 exposures. Based on the consideration of both qualitative and quantitative assessments of uncertainty completed as part of the quantitative risk assessment, the Risk Assessment concluded that it was unlikely that the estimated risks are over

- stated, particularly for mortality related to long

- term PM 2.5 exposure, and may well be biased low based on consideration of alternative model specifications evaluated in the sensitivity analyses ( U.S. EPA, 2010a, p. 5

- 16; U.S. EPA, 2011a, p. 2

- 41). Furthermore, the currently available scientific information summarized in section III.B of the proposal provided evidence for a broader range of health endpoints and at

- risk populations beyond those included in the quantitative risk assessment ( U.S. EPA, 2011a, p. 2

- 47 ).

32 Premature mortality for all causes attributed to PM 2.5 exposure was estimated to be on the order of tens of thousands of deaths per year on a national scale based on 2005 air quality data (U.S. EPA, 2010a, Ap pendix G, Table G

- 1).

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Page 48 of 432 b. CASAC Advice The CASAC, based on its review of drafts of the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment, provided an array of advice both with regard to interpreting the scientific evidence and quantitative risk ass essment, as well as with regard to consideration of the adequacy of the current PM 2.5 standards (Samet, 2009a b,c,d,e,f; Samet 2010a,b,c,d). With regard to the adequacy of the current standards, CASAC concluded that the “currently available information cle arly calls into question the adequacy of the current standards” (Samet, 2010d, p. i) and that the current standards are “not protective” (Samet, 2010d, p. 1). Further, in commenting on the first draft Policy Assessment, CASAC noted: With regard to the inte gration of evidence

- based and risk

- based considerations, CASAC concurs with EPA’s conclusion that the new data strengthens the evidence available on associations previously considered in the last round of the assessment of the PM 2.5 standard. CASAC also ag rees that there are significant public health consequences at the current levels of the standard that justify consideration of lowering the PM 2.5 NAAQS further (Samet, 2010c, p.12).

c. Administrator's Proposed Conclusions Concerning the Adequacy of the Current Primary PM 2.5 Standards At the time of the proposal, in considering the body of scientific evidence, the Administrator concluded there was stronger and more consistent and coheren t support for associations between long

- and short

- term PM 2.5 exposure and a broader range of health outcomes than was available in the last review, providing the basis for fine particle standards at least as protective as th e current PM 2.5 standards. In p articular, the Administrator recognized in section III.D.4 of the proposal that the Integrated Science Assessment concluded that the results of epidemiological and experimental studies form a plausible and coherent data set that supports a causal relations hip between long

- and

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Page 49 of 432 short

- term PM 2.5 exposures and mortality and cardiovascular effects and a likely causal relationship between long

- and short

- term PM 2.5 exposures and respiratory effects. Furthermore, the Administrator reflected that effects had been observed at lower ambient PM 2.5 concentrations than what had been observed in the last review, including at ambient PM 2.5 concentrations in areas that likely met the current PM 2.5 NAAQS. With regard to the results of the quantitative risk assessment, the Administrator note d that the Risk Assessment conclude d that the risks estimated to remain upon simulation of just meeting the current standards we re important from a public health standpoint in terms of both the severity and magnitude of the effects. At the time of the proposal, in considering whether the current suite of PM 2.5 standards should be revised to provide requisite public health protection, the Administrator carefully co nsidered the staff conclusions and rationales presented in the Policy Assessment, the advice and recommendations from CASAC, and public comments to date on this issue. In so doing, the Administrator placed primary consideration on the evidence obtained fro m the epidemiological studies and provisionally found the evidence of serious health effects reported in long

- and short

- term exposure studies conducted in areas that would have met the current standards to be compelling, especially in light of the extent to which such studies are part of an overall pattern of positive and frequently statistically significant associations across a broad range of studies that collectively represent a strong and robust body of evidence. As discussed in the Integrated Science Assessment and Policy Assessment, the Administrator recognized that much progress has been made since the last review in addressing some of the key uncertainties that were important considerations in

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Page 50 of 432 establishing the current suite of PM 2.5 standards. For example, progress made since the last review provides increased confidence in the long

- and short

- term exposure studies as a basis for considering whether any revision of the annual standard is appropriate and increased confidence in the short

- term exposur e studies as a basis for considering whether any revision of the 24

- hour standard is appropriate. 33 Based on her consideration of these conclusions, as well as consideration of CASAC’s conclusion that the evidence and risk assessment clearly call ed into qu estion the adequacy of the public health protection provided by the current PM 2.5 NAAQS and public comments on the proposal, the Administrator provisionally concluded that the current primary PM 2.5 standards, taken together, we re not requisite to protect p ublic health with an adequate margin of safety and that revision wa s needed to provide increased public health protection. The Administrator provisionally concluded that the scientific evidence and information on risk provide d strong support for considerat ion of alternative standards that would provide increased public health protection beyond that afforded by the current PM 2.5 standards. 2. Comments on the Need for Revision This section addresses general comments based on relevant facts that either support or oppose any change to the current suite of primary PM 2.5 standards. Comments on specific long

- and short

- term exposure studies that relate to consideration of the

33 The EPA notes that this increased confidence in the long

- and short

- term associations generally reflects less uncertainty as to the likely causal nature of such associations, but does not address directly the question of the extent to which such associati ons remain toward the lower end of the range of ambient PM 2.5 concentrations. This question is central to the Agency’s evaluation of the relevant evidence to determine appropriate standards levels, as discussed below in section III.E.4.

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Page 51 of 432 appropriate levels of the annual and 24

- hour standards are addressed in section III.E.4 below . Many public comments asserted that the current PM 2.5 standards are insufficient to protect public health with an adequate margin of safety and that revisions to the standards are therefore appropriate, indeed necessitated. Among those calling for revisi ons to the current standards were the Children’s Health Protection Advisory Committee (CHPAC); major medical and public health groups including the American Heart Association (AHA), American Lung Association (ALA), American Public Health Association (APHA) , American Thoracic Society (ATS); the Physicians for Social Responsibility (PSR); major environmental groups such as the Clean Air Council, Clean Air Task Force, Earthjustice, Environmental Defense Fund (EDF), National Resources Defense Council (NRDC), an d Sierra Club; many environmental justice organizations as well as medical doctors, academic researchers, health professionals, and many private citizens. For example, the American Heart Association and other major national public health and medical organi zations stated that, “[o]ur organizations are keenly aware of the public health and medical threats from particulate matter” and called on the EPA to “significantly strengthen” both the annual and 24

- hour PM 2.5 standards “to help us protect the health of o ur patients and our nation” (AHA et al., 2012, pp 1 and 13). AHA et al. and ALA et al., as well as a group of more than 350 physicians, environmental health researchers, and public health and medical professionals articulated similar comments on the availa ble evidence: Ample scientific evidence supports adopting tighter standards to protect the health of people who are most susceptible to the serious health effects of these pollutants. More than 10,000 peer

- reviewed scientific studies have been published si nce 1997 when EPA adopted the current annual standard. These studies validate and extend earlier epidemiologic research linking both acute and chronic fine

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Page 52 of 432 particle pollution with serious morbidity and mortality. The newer research has also expanded our un derstanding of the range of health outcomes associated with PM and has identified adverse respiratory and cardiovascular health effects at lower exposure levels than previously reported. As discussed and interpreted in the EPA’s 2009 Integrated Science Ass essment for Particulate Matter, the new evidence reinforces already strong existing studies and supports the conclusion that PM 2.5 is causally associated with numerous adverse health effects in humans at exposure levels far below the current standard. Such a conclusion demands prompt action to protect human health. (AHA et al., 2012, pp. 1 to 2; ALA et al., pp. 4 to 5; similar comment submitted by Rom et al., 2012, p. 1). All of these medical and public health commenters stated that the current PM 2.5 standa rds need to be revised, and that even more protective standards than those proposed by the EPA are needed to adequately protect public health, particularly for at

- risk populations. Many environmental justice organizations and individual commenters also
exp ressed such views. The National Association of Clean Air Agencies (NACAA), the Northeast States for Coordinated Air Use Management (NESCAUM), and many State and local air agencies and health departments who commented on the PM 2.5 standards supported revisi on of the suite of current PM 2.5 standards, as did five s tate a ttorneys g eneral (Schneiderman et al., 2012) and the National Tribal Air Association (NTAA). These commenters based their views chiefly on the body of evidence and technical analyses presented and discussed in the Integrated Science Assessment, the Risk Assessment, and the Policy Assessment finding the available scientific information to be stronger and more compelling than in the last review. These commenters generally placed much weight on CASAC’s recommendation to revise the PM 2.5 standards to provide increased public health protection and on the EPA staff conclusions presented in the final Policy Assessment.

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Page 53 of 432 Some of these commenters specifically mentioned extended analyses of seminal long

- term exposure studies

- the ACS (Krewski et al., 2009), Harvard Six Cities (Laden et al., 2006), and Southern California Children’s Health (Gauderman et al., 2004) studies. These commenters also highlighted the availability of additional long

- term exposure studies in this review, specifically a study of premature mortality in older adults (Eftim et al., 2008) and the WHI study of cardiovascular morbidity and mortality effects in women (Miller et al., 2007) providing stronger evidence of mortality and morbid ity effects associated with long

- term PM 2.5 exposures at lower concentrations than had previously been observed, including studies of effects in at

- risk populations. For example, some commenters asserted: Evidence during the last review showed clearly that the annual average standard needed to be much lower than the standard of 15 μg/m 3 that was first set in 1997. The evidence has only grown since then. Multiple, multi

- city studies over long periods of time have shown clear evidence of premature death, card iovascular and respiratory harm as well as reproductive and developmental harm at contemporary concentrations far below the level of the current (annual) standard (ALA et al., 2012, p. 39; AHA et al., 2012, p. 10). These commenters also highlighted the av ailability of a number of short

- term PM 2.5 exposure studies as providing evidence of mortality and morbidity effects at concentration s below the level of the current 24

- hour PM 2.5 standard. Specifically, these commenters made note of multi

- city studies of premature mortality (Zanobetti and Schwartz, 2009) and increased hospitalizations for cardiovascular and respiratory

- related effects in older adults (Bell et al., 2008). These commenters also asserted the importance of many of the single

- city studies, argu ing that these studies “provide valuable information regarding impacts on susceptible populations and on health risk in areas with high peak to mean concentration ratios” (ALA, et al., 2012, p. 65). Collectively,

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Page 54 of 432 considering the multi

- and single

- city shor t

- term exposure studies, these commenters asserted “the record clearly supports a more stringent 24

- hour standard of 25 μg/m 3 to provide uniform protection in all regions of the country particularly from short

- term spikes in pollution and from the sub

- dail y exposures that trigger heart attacks and strokes” (ALA et al., 2012, p. 62). A group of more than 350 physicians, environmental health researchers, and public health and medical professionals argued, “[s]tudies of short

- term exposure demonstrate that PM 2 .5 air pollution increases the risk of hospital admissions for heart and lung problems even when you exclude days with pollution concentrations at or above the current daily standard of 35 μg/m 3 . Daily concentrations must be capped at lower levels to prote ct against peak exposure days that occur due to local and seasonal sources of emissions” (Rom et al., 2012, p. 2). In addition, many of these commenters generally concluded that progress had been made in reducing many of the uncertainties identified in the last review, in better understanding mechanisms by which PM 2.5 may be causing the observed health effects, and in improving our understanding of at

- risk populations. Further, a number of commenters argued that by making the standards more protective, the PM 2.5 NAAQS would be more consistent with other existing standards (e.g., California's annual average standard of 12 μg/m 3 ) (CARB, 2012; CA OEHHA, 2012). Other commenters argued that revising the primary PM 2.5 standards would be more consistent with the

re commendations of the World Health Organization (WHO) and/or Canada (e.g., ALA et al., 2012, p. 62; ISEE, 2012, p. 2 ; MOE

- Ontario, 2012, p. 1). With regard to the scope of the literature reviewed for PM 2.5 - related health effects, some commenters asserted that the EPA inappropriately narrowed the scope of

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Page 55 of 432 the review by excluding a number of categories of relevant studies, specifically related to studies of diesel pollution and traffic

- related pollution (ALA, et al., 2012, p. 17). These commenters argued that, based upon the exclusion of these types of studies, the Integrated Science Assessment “came to the erroneous conclusion that the causal relationship between PM and cancer is merely suggestive. This concl usion does not square with the International Agency Research on Cancer (IARC) finding that diesel emissions are a known human carcinogen nor with the conclusions of the extended analyses of the [Harvard] Six Cities and ACS cohort studies that report positi ve and statistically significant associations between PM 2.5 and lung cancer.” Id. Some of these commenters also noted the results of the EPA’s quantitative risk assessment, concluding that it showed that the risks estimated to remain when the current
stan dards are met are large and important from a public health perspective and warrant increased protection. For example, ALA et al., noted that the Risk Assessment indicated the quantitative risk analyses likely underestimated PM 2.5 - related mortality ( U.S. EP A, 2010a, p. 5

- 16) and argued that “the measurements of risk should be treated conservatively” (ALA, et al., 2012, p. 73). These commenters also summarized an expanded analysis of alternative PM 2.5 standard levels that they argued documented the need for m ore protective standards (McCubbin, 2011). In general, all of these commenters agreed on the importance of results from the large body of scientific studies reviewed in the Integrated Science Assessment and on the need to revise the suite of PM 2.5 standar ds as articulated in the EPA’s proposal, while generally differing with the EPA’s proposed judgments about the extent to which the standards should be revised based on this evidence, specifically for providing protection

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Page 56 of 432 for at

- risk populations. The EPA g enerally agrees with these commenters’ conclusion regarding the need to revise the current suite of PM 2.5 standards. The scientific evidence noted by these commenters was generally the same as that assessed in the Integrated Science Assessment and the Poli cy Assessment, and the EPA agrees that this evidence provides a strong basis for concluding that the current PM 2.5 standards, taken together, are not requisite to protect public health with an adequate margin of safety, and they need to be revised to provi de increased protection. For reasons discussed in section III.E.4.c below, however, the EPA disagrees with aspects of these commenters’ views on the level of protection that is appropriate

. T he EPA disagrees with these commenters’ views that diesel exhaust studies were excluded from the Integrated Science Assessment and were not considered when making the causality determination for cancer, mutagenicity, and genotoxicity. As discussed in

s ecti on 7.5 of the Integrated Science Assessment, diesel exhaust studies were integrated within the broader body of scientific evidence that was considered in reaching the causality determination for these health endpoints. Additionally, as discussed in section 1.5.3 of the Integrated Science Assessment, the evidence from diesel exhaust studies was also considered as part of the collective evidence evaluated when making determinations for other, noncancer health outcomes (e.g., cardiovascular and respiratory eff ects). 34

34 In developing the second draft Integrated Science Assessment, the EPA reexamined the controlled human exposure and toxicological studies of fresh diesel and gasoline exhaust This information, in addition to other considerations, supported a change in the causal determinati ons for ultrafine particles. Specifically, in reevaluating the causal determinations for short

- term ultrafine particle exposures and cardiovascular and respiratory effects, the EPA changed the classification from “inadequate” to “suggestive ”

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Page 57 of 432 Specifically, when evaluating this evidence , the focus was on understanding the effects of diesel exhaust particles. It is important to recognize that the Integrated Science Assessment focused on experimental studies of diesel exhaust that evaluat ed exposures that were relevant to ambient concentrations, i.e., “within one or two orders of magnitude of ambient PM concentrations” ( U.S. EPA, 2009a, section 1.3). The causal determination for cancer, mutagenicity, and genotoxicity presented in the Integ rated Science Assessment represents an integration of experimental and observational evidence of exposures to ambient PM concentrations. T he EPA fully considered the findings of studies that assessed the se and other health effects associated with exposure to diesel particles in reaching causality determinations regarding health outcomes associated with PM 2.5 exposures. Furthermore, CASAC supported the EPA’s change to the causal determination for cancer and long

term PM 2.5 concentrations from “inadequate” to “suggestive” (Samet, 2009f, p. 2). With regard to traffic studies, the EPA disagrees with the commenters’ views that traffic studies that focused on exposure indicators such as distance to roadways should have been included in the Integrated Science Asses sment. These studies were excluded from consideration because they did not measure ambient concentrations of specific air pollutants, including PM 2.5 , but instead we re studies evaluating exposure to the undifferentiated “traffic related air pollution” mixture (ALA et al., 2012, p.17) ( U.S. EPA, 2009a, section 1.3). As a result, these studies do not add to the collective body of evidence on the relationship between long

- or short

- term exposure to ambient
for both categ ories of health outcomes (Vandenberg, 2009, p. 3). CASAC agreed with the EPA’s rationale for revising these causal determinations (Samet, 2009f, p. 10).

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Page 58 of 432 concentrations of PM 2.5 and health effects. Some o f these commenters also identified “new” studies that were not included in the Integrated Science Assessment as providing further support for the need to revise the primary PM 2.5 standards. As discussed in section II.B.3 above , the EPA notes that, as in
pa st NAAQS reviews, the Agency is basing the final decisions in this review on the studies and related information included in the PM air quality criteria that have undergone CASAC and public review and will consider the “new” studies for purposes of decisio n making in the next PM NAAQS review. Nonetheless, in provisionally evaluating commenters’ arguments (see Response to Comments document), the EPA notes that its provisional assessment of “new” science found that such studies did not materially change the c onclusions in the Integrated Science Assessment (U.S. EPA, 2012b)

. Another group of commenters opposed revising the current PM 2.5 standards. These views were most extensively presented in comments from the Utility Air Regulatory Group (UARG), representing a group of electric generating companies and organizations and several national trade associations; the American Petroleum Institute (API) representing more than 500 oil and natural gas companies; the National Association of Manufacturers (NAM), the Americ an Chemistry Council (ACC), the American Fuel & Petroleum Manufacturers (AFPM), the Alliance of Automobile Manufacturers (AAM) , and other manufacturing associations; the Electric Power Research Institute (EPRI); and the Texas Commission on Environmental Qu ality (T exas CEQ). These commenters generally mentioned many of the same studies that were cited by the commenters who supported revising the standards, as well as other studies, but

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Page 59 of 432 highlighted different aspects of these studies in reaching substantially different conclusions about their strength and the extent to which progress has been made in reducing uncertainties in the evidence since the last review. Furthermore, they asserted that the evidence that has become available since the last review does not establish a more certain risk or a risk of effects that are significantly different in character to those that provided a basis for the current standards, nor does the evidence demonstrate that the risk to public health upon attainment of the current stan dards would be greater than was understood when the EPA established the current standards in 2006. These commenters generally expressed the view that the current standards provide the requisite degree of public health protection. In supporting their view, these commenters generally argued that the EPA’s conclusions are inconsistent with the current state of the science and questioned the underlying scientific evidence including the causal determinations reached in the Integrated Science Assessment. More sp ecifically, this group of commenters argued that: (1) the EPA did not apply its framework for causal determination consistently across studies or health outcomes and, in the process, the EPA relied on a selective group of long

- and short

- term exposure stud ies to reach conclusions regarding causality; (2) toxicological and controlled human exposure studies do not provide supportive evidence that the health effects observed in epidemiological studies are biologically plausible; (3) uncertainties in the underl ying health science are as great or greater than in 2006; ( 4 ) there is no evidence of greater risk since the last review to justify tightening the current annual PM 2.5 standard; and ( 5 ) “new” studies not included in the Integrated Science Assessment contin ue to increase uncertainty about possible health risks associated with exposure to PM 2.5 . These comments are discussed in turn below.

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Page 60 of 432 (l) Some of these commenters asserted that the EPA did not apply its framework for causal determinations consistently acro ss studies or health outcomes ( ACC, 2012, Attachment A, pp. 1 to 2; API, 2012, Attachment 1, p. 30; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3 ). 35 These commenters argued that the EPA downplayed epidemiological studies with null or inconsist ent results, inappropriately used the Hill criteria when evaluating the epidemiologic al evidence, and used the same study and the same underlying database to conclude that there was a causal association between mortality and multiple criteria pollutants

. T he EPA disagrees with these commenters’ views. First, the EPA recognizes that the evaluation of the scientific evidence and its application of the causal framework used in the current PM NAAQS review was the subject of exhaustive and detailed review by CAS AC and the public. As summarized in section II.B.3 above, p rior to finalizing the Integrated Science Assessment, two drafts were released for CASAC and public review to evaluate the scientific integrity of the documents. Evidence related to the substantive issues raised by CASAC and public commenters with regard to the content of the first and second draft Integrated Science Assessments were discussed at length during these public CASAC meetings and considered in developing the final Integrated Science Asse ssment. CASAC supported the development of the EPA’s causality framework and its use in the current PM NAAQS review and concluded: The five

- level classification of strength of evidence for causal inference has been

35 The EPA notes that the same concerns about the causal determinations presented in the Integrated Scie nce Assessment were raised in comments to CASAC on the draft Integrated Science Assessments (e.g., UARG, 2009; API, 2009; ACC, 2012, Appendix B). CASAC, therefore, had the opportunity to consider these comments in reaching consensus conclusions on this iss ue.

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Page 61 of 432 systematically applied; this approach has provided transparency and a clear statement of the level of confidence with regard to causation, and we recommend its continued use in future Integrated Science Assessments (Samet 2009f, p. 1). These commenters asserted that during the application of the causal framework the EPA inappropriately relied on a selective group of long

- and short

- term exposure studies in reaching causal inferences (API, 2012, pp 12 to 17; ACC, 2012, Attachment A, pp 1 to 2; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). Additionally, these commenters expressed the view that the EPA focused on a subset of epidemiological studies that reported positive and statistically significan t results while ignoring other studies, especially those that reported no statistically significan t associations, those that reported potential thresholds, or those that highlighted uncertainties and limitations in study design or results. Furthermore, some of these commenters argued that epidemiological studies are observational in nature and cannot provide evidence of a causal association. The EPA disagrees with these commenters’ views on assessing the health effects evidence and on the conclusions regardi ng the causal ity determinations reached in the Integrated Science Assessment

. In conducting a comprehensive evaluation of the evidence in the Integrated Science Assessment, the EPA recognized the distinction between the evaluation of the relative scientifi c quality of individual study results and the evaluation of the pattern of results within the broader body of scientific evidence and considered both in reaching causality determinations. The more detailed characterizations of individual studies included a n assessment of the quality of the study based on specific criteria as described in the Integrated Science Assessment ( U.S. EPA, 2009a, section 1.5.3).

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Page 62 of 432 In developing an integrated assessment of the health effects evidence for PM, the EPA emphasized the im

portance of examining the pattern of results across various studies and did not focus solely on statistical significance 36 as a criterion of study strength. This approach is consistent with views clearly articulated throughout the epidemiological and causal inference literature, specifically, that it is important not to focus on results of statistical tests to the exclusion of other information. 37 The concepts underlying the EPA’s approach to evaluating statistical associations have been discussed in numerous publications, including a report by the U.S. Surgeon General on the health consequences of smoking (Centers for Disease Control and Prevention, 2004). This report cautions against over

- reliance on statistical significance in evaluating the overall evidenc e for an exposure

- response relationship. Criteria characterized by Hill (1965) also addressed the value, or lack thereof, of statistical tests in the determination of cause: No formal tests of significance can answer those [causal] questions. Such tests c an, and should, remind us of the effects the play of chance can create, and they will instruct us in the likely magnitude of those effects. Beyond that, they contribute nothing to the ‘proof’ of our hypothesis (Hill, 1965, p. 299). The statistical signifi cance of individual study findings has played an important

36 Statistical significance is an indicator of the precision of a study’s results, which is influenced by a variety of factors including, but not limited to, the size of the study, exposure and measurement error, and statistical model specifications. S tudies typically calculate “p

- values” to determine whether the study results are statistically significant or whether the study results are likely to occur simply by chance. In general practice, effects are considered statistically significant if p values are less than 0.05. 37 For example, Rothman (1998) stated, “Many data analysts appear to remain oblivious to the qualitative nature of significance testing [and that]... statistical significance is itself only a dichotomous indicator. As it has only two value s, significant or not significant ....Nevertheless, p

- values still confound effect size with study size, the two components of estimation that we believe need to be reported separately.” As a result, Rothman recommended that p

- values be omitted as long as po int and interval estimates are available.

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Page 63 of 432 role in the EPA’s evaluation of the study’s results and the EPA has placed greater emphasis on studies reporting statistically significant results. However, in the broader evaluation of the evidenc e from many epidemiological studies, and subsequently during the process of forming causality determinations in integrating evidence across epidemiological, controlled human exposure, and toxicological studies, the EPA has emphasized the pattern of results across epidemiological studies, and whether the effects observed were coherent across the scientific disciplines for drawing conclusions on the relationship between PM 2.5 and different health outcomes. Thus, the EPA did not limit its focus or consideratio n to just studies that reported positive associations or where the results were statistically significant. In addition, some commenters asserted that the EPA inappropriately used the Hill criteria by failing to consider the limitations of studies with wea k associations, thereby overstating the consistency of the observed associations (API, 2012, Attachment 1, pp

. 30 to 35). These commenters argued that risk estimates greater than 3 to 4 reflect strong associations supportive of a causal link, while smaller risk estimates (i.e., 1.5 to 3) are considered to be weak and require other lines of evidence to demonstrate causality. As discussed in section 1.5.3 of the Integrated Science Assessment, the EPA thoroughly considered the limitations of all studies durin g its evaluation of the scientific literature (U.S. EPA,, 2009a, p. 1

- 14). This collective body of evidence, including known uncertainties and limitations of the studies evaluated, were considered during the process of forming causality determinations as d iscussed in chapters 6 and 7 of the Integrated Science Assessment. For example, the EPA concluded that “a causal relationship exists between short

- term PM 2.5 exposure and cardiovascular effects , ” however, in reaching this

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Page 64 of 432 conclusion, the Agency recognized and considered limitations of the current evidence that still requires further examination (U.S. EPA, 2009a

. , in s ection 6.2.12.1 ). Therefore,
t he EPA disagrees with these commenters’ views that the Hill criteria were inappropriately used in that the limitations of studies were not considered. The EPA also disagrees with the commenters ’ assertion that the magnitude of the association must be large to support a determination of causality. As discussed in the Integrated Science Assessment, the strength of the observed association is an important aspect to aid in judging causality and “while large effects support causality, modest effects therefore do not preclude it” ( U.S. EPA, 2009a, Table 1

- 2, section 1.5.4). The weight of evidence approach used by the EPA encompasses a multitude of factors of which the magnitude of the association is only one component ( U.S. EPA, 2009a, Table 1

- 3). An evaluation of the association across multiple investigato rs and locations supports the “reproducibility of findings [which] constitutes one of the strongest arguments for causality” ( U.S. EPA, 2009a, Table 1

- 2). Even though the risk estimates for air pollution studies may be modest, the associations are consiste nt across hundreds of studies as demonstrated in the Integrated Science Assessment. Furthermore, the causality determinations rely on different lines of evidence, by integrating evidence across disciplines, including animal toxicological studies and contro lled human exposure studies. Furthermore, as summarized in section III.B above and discussed more fully in section III.B.3 of the proposal , the EPA recognizes that the population potentially affected by PM 2.5 is considerable, including large subgroups of t he U.S. population that have been identified as at

- risk populations (e.g., children, older adults, persons with

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Page 65 of 432 underlying cardiovascular or respiratory disease). While individual effect estimates from epidemiological studies may be modest in size, the pub lic health impact of the mortality and morbidity associations can be quite large given that air pollution is ubiquitous. Indeed, with the large population exposed, exposure to a pollutant causally associated at a population level with mortality and serious illness has significant public health consequences, virtually regardless of the relative risk. Taken together, th is information indicates that exposure to ambient PM 2.5 concentrations has substantial public health impacts. In addition, these commenters be lieved that the EPA downplayed null or inconsistent findings in numerous long

- term mortality studies with reported PM 2.5 concentrations above and below the level of the current annual standard. The EPA disagrees that studies with null or inconsistent findi ngs were not accurately presented and considered in the Integrated Science Assessment. For example, as discussed throughout section 7.6 and depicted in Figures 7

- 6 and 7

- 7 of the Integrated Science Assessment, the EPA presented the collective evidence from all studies that examined the association between long

- term PM 2.5 exposure and mortality. Overall, across these studies there was evidence of consistent positive associations in different cohorts. That evidence, in combination with the biological plausibi lity provided by experimental and toxicological studies evaluated in sections 7.1 and 7.2 of the Integrated Science Assessment, supported a causal relationship exists between long

- term PM 2.5 exposure and mortality

. Lastly, some of these commenters argued that in some cases, the EPA used the same study and the same underlying database to conclude that there is a causal association between mortality and multiple criteria pollutants. These commenter s argued,

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Page 66 of 432 “[i]n doing so, EPA attributes the cause of the mor tality effects observed to whichever criteria pollutant it is reviewing at the time” (API, 2012, pp. 14 to 16). The EPA strongly disagrees that the Agency “attributes the cause of mortality effects observed to whichever criteria pollutant it is reviewing at the time.” The EPA consistently recognizes that other pollutants are also associated with health outcomes, as is reflected in the fact that the EPA has established regulations to limit emissions of particulate criteria pollutants as well as other gaseou s criteria pollutants. Epidemiologic al studies often examine the association between short

- and long

- term exposures to multiple air pollutants and mortality within a common dataset in an attempt to identify the air pollutant(s) of the complex mixture most strongly associated with mortality. In evaluating these studies , the EPA employs specific study selection criteria to identify those studies most relevant to the review of the NAAQS. In its assessment of the health evidence regarding PM 2.5 , the EPA has car efully evaluated the potential for confounding, effect measure modification , and the role of PM 2.5 as a component of a complex mixture of air pollutants ( U.S. EPA, 2009, p. 1

- 9). The EPA use d a rigorous weight of evidence approach to inform causality that evaluate d consistency across studies within a discipline, evidence for coherence across disciplines, and biological plausibility. Additionally, during this process, the EPA assesse d the limitations of each study in the context of the collective body of evi dence. It wa s the collective evidence, not one individual study that ultimately determine d whether a causal relationship exists between a pollutant and health outcome. In the Integrated Science Assessment , the combination of epidemiologic al and experimental evidence formed the basis for the Agency concluding for the first time that a causal relationship exists between short

- or long

- term exposure to a criteria pollutant

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Page 67 of 432 and mortality ( U.S. EPA, 2009, s ection s 2.3.1.1 and 2.3.1.2). Additiona lly, while the EPA has evaluated some of the studies used to inform the causality determination for PM in the Integrated Science Assessments for other criteria air pollutants, the Agency has done so in the context of examining the collective body of eviden ce for each of the respective criteria air pollutants. As such, the body of evidence to inform causality has varied from pollutant to pollutant resulting in the association between each criteria air pollutant and mortality being classified at a different l evel of the five

- level hierarchy used to inform causation ( e.g., U.S. EPA, 2008 e , U.S. EPA, 2008 f , U.S. EPA, 2010 k ). The EPA notes that the final causality determinations presented in the Integrated Science Assessment reflected CASAC’s recommendations on the second draft Integrated Science Assessment (Samet, 2009f, pp. 2 to 3). Specifically, CASAC supported the EPA’s changes (in the second versus first draft Integrated Science Assessment) from “likely causal” to “causal” for long

- term exposure to PM 2.5 and cardiovascular effects and for cancer and PM 2.5 (from “inadequate” to “suggestive”)

. Id . Furthermore, CASAC recommended “upgrading” the causal classification for PM 2.5 and total mortality to “causal” for both the short

- and long

- term timeframes. Id. W i th regard to mortality, the “EPA carefully reevaluated the body of evidence, including the collective evidence for biological plausibility for mortality effects, and determined that a causal relationship exists for short

- and long

- term exposure to PM 2.5 and m ortality, consistent with the CASAC comments” (Jackson, 2012). (2) With regard to toxicological and controlled human exposure studies, these commenters argued that the available evidence does not provide coherence or biological

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Page 68 of 432 plausibility for health eff

ects observed in epidemiological studies (API, 2012, pp. 21 to 22, Attachment 1, pp. 25 to 29; AAM, 2012, pp. 15 to 16; T exas CEQ, 2012, p. 3). With regard to the issue of mechanisms, these commenters noted that although the EPA recognizes that new evidenc e is now available on potential mechanisms and plausible biological pathways, the evidence provided by toxicological and controlled human exposure studies still does not resolve all questions about how PM 2.5 at ambient concentrations could produce the mort ality and morbidity effects observed in epidemiological studies. More specifically, for example, some of these commenters argued that: A review of the Integrated Science Assessment, however, suggests that the experimental evidence is inconsistent and not c oherent with findings in epidemiology studies. Specifically, the findings of mild and reversible effects in most experimental studies conducted at elevated exposures are not consistent with the more serious associations described in epidemiology studies (e .g., hospital admissions and mortality). Also, both animal studies and controlled human exposure studies have identified no effect levels for acute and chronic exposure to PM and PM constituents at concentrations considerably above ambient levels. EPA shou ld consider the experimental findings in light of these higher exposure levels and what the relevance may be for ambient exposures (API, 2012, Attachment 1, p. 25). The EPA notes that in the review completed in 1997 , the Agency considered the lack of demon strated biologic al mechanisms for the varying effects observed in epidemiological studies to be an important caution in its integrated assessment of the health evidence upon which the standards were based ( 71 FR 61157, October 17, 2006 ). In the review completed in 2006, the EPA recognized the findings from additional research that indicated that different health responses were linked with different particle characteristics and that both individual components and complex particle mixtures

a ppeared to be responsible for many biologic responses relevant to fine particle

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Page 69 of 432 exposures. Id. Since that review, there has been a great deal of research directed toward advancing our understanding of biologic mechanisms. While this research has not resolv ed all questions, and further research is warranted ( U.S. EPA, 2011a, section 2.5), it has provided important insights as discussed in section III.B.1 of the proposal (77 FR at 38906 to 38909) and discussed more fully in the Integrated Science Assessment ( U.S. EPA, 2009a, C hapter 5). As noted in the proposal, toxicological studies provide evidence to support the biologic al plausibility of cardiovascular and respiratory effects associated with long

- and short

- term PM exposures observed in epidemiological st udies (77 FR 38906) and provide supportive mechanistic evidence that the cardiovascular morbidity effects observed in long

- term exposure epidemiological studies are coherent with studies of cardiovascular

related mortality (77 FR 38907). The Integrated Sci ence Assessment concluded that t he new evidence available in this review “greatly expands” upon the evidence available in the last review “particularly in providing greater understanding of the underlying mechanisms for PM 2.5 induced cardiovascular and res piratory effects for both short

- and long

- term exposures ” ( U.S. EPA, 2009a, p. 2

- 17). The mechanistic evidence now available, taken together with newly available epidemiological evidence, increases the Agency’s confidence that a causal relationship exists between long

- and short

- term

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Page 70 of 432 exposure to PM 2.5 and cardiovascular effects and mortality. 38 In addition, CASAC supported the Integrated Science Assessment approach and characterization of potential mechanisms or modes of action (Samet, 2009e, pp. 7 to 8; Sam et, 2009f, p. 11), as well as the findings of a causal relationship at the population level between exposure to PM 2.5 and mortality and cardiovascular effects (Samet, 2009f, pp. 2 to 3). 39 Additionally, the EPA disagrees with commenters that the mild and r eversible effects observed in controlled human exposure studies are inconsistent with the more serious effects observed in epidemiologic al studies. Ethical considerations regarding the types of studies that can be performed with human subjects generally li mit the effects that can be evaluated to those that are transient, reversible, and of limited short

- term consequence. The relatively small number of subjects recruited for controlled exposure studies should also be expected to have less variability in heal th status and risk factors

38 See American Trucking Ass ociation v. EPA, 175 F. 3d 1027, 1055

- 56 (D.C. Cir. 1999) rev erse d in part and affirmed in part sub nom , Whitman v. American Trucking Assn’s, 531 U.S. 457 (2001) holding that the EPA co uld establish NAAQS without identifying a biological mechanism (“ To begin with, the statute itself requires no such proof. The Administrator may regulate air pollutants "emissions of which, in his judgment, cause or contribute to air pollution which may re asonably be anticipated to endanger public health or welfare ." ( emphasis added by the court). Moreover, this court has never required the type of explanation petitioners seek from EPA. In fact, we have expressly held that EPA's decision to adopt and set ai r quality standards need only be based on ‘reasonable extrapolations from some reliable evidence’.... Indeed, were we to accept petitioners' view, EPA (or any agency for that matter) would be powerless to act whenever it first recognizes clear trends of mort ality or morbidity in areas dominated by a particular pathogen.”).

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Page 71 of 432 than occurring in the general population. 40 Consequently, the severity of health effects observed in controlled human exposure studies evaluating the effects of PM should be expected to be less than observed in epidemiologic studi es. Nonetheless, that effects are observed in relatively healthy individuals participating in controlled exposure studies serves as an indicator that PM is initiating health responses and that more severe responses may reasonably be expected in a more dive rse population. It should also be noted that there is a small body of toxicological evidence demonstrating mortality in rodents exposed to PM (e.g., Killingsworth et al. 1997; Godleski et al. 1996). Overall it is not surprising that lethality is not induced in more toxicological research, as these types of studies do not readily lend themselves to this endpoint. Epidemiologic al studies have observed associations between PM and mortality in communities with populations in the range of many thousands to millions of people. Clearly, it is not feasible to expose hundreds (if not thousands) of animals to ambient PM (potentially over many years) in a laboratory setting to induce enough lethalities to

40 For example, the EPA excludes from its controlled human exposure studies involving exposure to PM 2.5 any individual with a significant risk factor for experiencing adverse effects from s uch exposure. Thus, the EPA excludes a priori the following categories of persons: those with a history of angina, cardiac arrhythmias, and ischemic myocardial infarction or coronary bypass surgery; those with a cardiac pacemaker; those with uncontrolled h ypertension (greater than 150 systolic and 90 diastolic); those with neurogenetive diseases; those with a history of bleeding diathesis; those taking beta

blockers; those using oral anticoagulants; those who are pregnant, attempting to become pregnant, or breastfeeding; those who have experienced a respiratory infection within four weeks of exposure; those experiencing eye or abdominal surgery within six weeks of exposure; those with active allergies; those with a history of chronic illnesses such as diabet es, cancer, rheumatologic diseases, immunodeficiency state, known cardiovascular disease, or chronic respiratory diseases; smokers. The EPA “Application for Independent Review Board Approval of Human Subjects Research: Cardiopulmonary Effects of healthy Ol der GSTM1 Null and Sufficient individuals to Concentrated Ambient Air Particles (CAPTAIN)”, Nov. 9, 2011, p. 9.

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Page 72 of 432 distinguish between natural deaths and those attributable to PM. Furthermore, the heterogeneous human populations sampled in epidemiological studies are comprised of individuals with different physical, genetic, health, and socioeconomic backgrounds which may impact the outcome. However, in toxicological studies, the rodent groups are typically inbred, such that inter

- individual variability is minimized. Thus, if the rodent strain used is quite robust, PM

- induced effects may not be observed at low exposure concentrations. (3) In asserting that the uncertainties in the underlying health science are as great or greater than in the last review and therefore do not support revision to the standards at this time, commenters in this group variously discussed a number of issues related to: (a) confounding, (b) heterogenei ty in risk estimates, (c) exposure measurement error, (d) model specification , (e) the concentration

- response relationship, and ( f ) understanding the relative toxicity of components within the mixture of fine particles. Each of these issues is addressed be low and some are discussed in more detail in the Response to Comments document

. In summary, these commenters concluded that the substantial uncertainties present in the last review have not been resolved and/or that the uncertainty about the possible heal th risks associated with PM 2.5 exposure has not diminished. As discussed below, the EPA believes that the overall uncertainty about possible health risks associated with both long

- and short

- term PM 2.5 exposure has diminished to an important degree since t

he last review. While the EPA agrees that important uncertainties remain, and that future research directed toward addressing these uncertainties is warranted, the EPA disagrees with commenters’ views that the remaining uncertainties in the scientific

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Page 73 of 43 2 evid ence are too great to warrant revising the current PM 2.5 NAAQS. ( a ) Confounding Some commenters have criticized the EPA for not adequately addressing the issue of confounding in both long

- and short

- term exposure studies of mortality and morbidity. This includes confounding due to copollutants, as well as unmeasured confounding. 41 With regard to copollutant confounding, these commenters asserted that the EPA has not adequately interpreted the results from studies that examined the effect of copollutants o n the relationship between long

- and short

- term PM 2.5 exposures and mortality and morbidity outcomes. These commenters contend that the EPA has inappropriately concluded that PM 2.5 - related mortality and morbidity associations are generally robust to confou nding. The commenters stated that statistically significant PM 2.5 associations in single

- pollutant models in epidemiologic al studies do not remain statistically significant in copollutant models. The loss of statistical significance or the reduction in th e magnitude of the effect estimate when a co

- pollutant model is used may be the result of factors other than confounding. These changes do not prove either the existence or absence of confounding. These impacts must be evaluated in a broader context that c onsiders the entire body of

41 The Integrated Science Assessment defines confounding as “a confusion of effects. Specifically, the apparent effect of the exposure of interes t is distorted because the effect of an extraneous factor is mistaken for or mixed with the actual exposure effect (which may be null) (Rothman and Greenland, 1998)” (U.S. EPA, 2009a, p. 1

- 16). Epidemiological analyses attempt to adjust or control for thes e characteristics (i.e., potential confounders) that differ between exposed and non

- exposed individuals (U.S. EPA, 2009a, section 1.5.3). Not all risk factors can be controlled for within a study design/model and are termed “unmeasured confounders.” An unm easured confounder is a confounder that has not previously been measured and therefore is not included in the study design/model.

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Page 74 of 432 evidence. The broader examination of this issue in the Integrated Science Assessment included a focus on evaluating the stability of the size of the effect estimates in epidemiological studies conducted by a number of research g roups using single

- and copollutant models ( U.S. EPA, 2009a, sections 6.2.10.9, 6.3.8.5, and 6.5, Figures 6

- 5, 6

- 9, and 6

- 15). This examination found that, for most epidemiological studies, there was little change in effect estimates based on single

- and c opollutant models, although the Integrated Science Assessment recognized that in some cases, the PM 2.5 effect estimates were markedly reduced in size and lost statistical significance. Additionally, the EPA notes that these comments do not adequately refle ct the complexities inherent in assessing the issue of copollutant confounding. As discussed in the proposal (77 FR 38907, 38909, and 38910) and more fully in the Integrated Science Assessment ( U.S. EPA, 2009a, sections 6.2, 6.3, and 6.5), although copollut ant models may be useful tools for assessing whether gaseous copollutants may be potential confounders, such models alone cannot determine whether copollutants are in fact confounders. Interpretation of the results of copollutant models is complicated by c orrelations that often exist among air pollutants, by the fact that some pollutants play a role in the atmospheric reactions that form other pollutants such as secondary fine particles, and by the statistical power of the studies in question inherent in th e study methodology. For example, the every

- third or sixth

- day sampling schedule often employed for PM 2.5 measurements compared to daily measurements of gaseous copollutants drastically reduces the overall sample size to assess the effect of copollutants o n the PM 2.5 - morbidity or mortality relationship, such that the reduced sample size can lead to less precise effect estimates (e.g., wider confidence intervals).

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Page 75 of 432 The EPA recognizes that when PM 2.5 is correlated with gaseous pollutants it can be difficult to identify the effect of individual pollutants in the ambient mixture (77 FR 38910). However, based on the available evidence, the EPA concludes epidemiological studies continue to support the conclusion that PM 2.5 associations with mortality and morbidity outcomes are robust to the inclusion of gaseous copollutants in statistical models. The EPA evaluated the potential confounding effects of gaseous copollutants and, although it is recognized that uncertainties and limitations still remain, the Agency concl uded the collective body of scientific evidence is “stronger and more consistent than in previous reviews providing a strong basis for decision making in this review" (77 FR 38910/1). Several commenters offered detailed comments on the long

- term PM 2.5 exp osure studies arguing that associations from mortality studies are subjected to unmeasured confounding and as a result are not appropriately characterized as providing evidence of a causal relationship between long

- term PM 2.5 exposure and mortality (e.g., UARG, 2012, pp. 10 to 11, Attachment A, pp. 17 to 23; API, 2012, pp. 13 to 14, Attachment 1, pp. 11 to 14, Attachment 7, pp. 2

- 10; ACC, 2012, p. 18 to 21; AFPM, 2012, p. 8; Texas CEQ, 2012, p. 4). Specifically, commenters cited two studies (i.e., Janes et al. , 2007 and Greven et al. , 2011) that used a new type of statistical analysis to examine associations between annual (long

- term) and monthly (sub

- chronic) PM 2.5 exposure and mortality. The commenters interpreted the results of these analyses as evidence of unmeasured confounding in the long

- term PM 2.5 exposure - mortality relationship. These commenters interpreted these studies as raising fundamental questions rega rding the EPA’s determination that a causal relationship exists between long

- term PM 2.5 exposure and

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Page 76 of 432 mortality. In addition to the commenters mentioned above, all of the authors of the publications by Janes et al. (2007) and Greven et al. (2011) (i.e., Fra ncesca Dominici, Scott Zeger, Holly Janes, and Sonja Greven) submitted a joint comment to the public docket in order to clarify specific points regarding these two studies (Dominici et al., 2012). The first study, Janes et al. ( 2007 ) , was evaluated in the Integrated Science Assessment (U.S. EPA, 2009a, p. 7

- 88). The second study, Greven et al. (2011), an extension of the Janes et al. (2007) study adding three more years of data, is a “new” study discussed in the Provisional Science Assessment (U.S. EPA, 201 2). Both studies used nationwide Medicare mortality data to examine the association between monthly average PM 2.5 concentrations over the preceding 12 months and monthly mortality rates in 113 U.S. counties and examined whether community

- specific trends in monthly PM 2.5 concentrations and mortality declined at the same rate as the national rate. The investigators examined this by decomposing the association between PM 2.5 and mortality into two components: (1) national trends , defined as the association betw een the national average trend in monthly PM 2.5 concentrations averaged over the previous 12 months and the national average trend in monthly mortality rates , and (2) local trends, defined as county

- specific deviations in monthly PM 2.5 concentrations and m onthly mortality rates from national trends. The EPA does not question the results of the national trends analyses conducted by Janes et al. (2007) and Greven et al. (2011). 42 Both Janes et al. (2007) and Greven et

42 In its evaluation of Janes et al. (2007) in the I ntegrated S cience A ssessment , the EPA did not identify limitations in the statistical methods used per se (U.S. EPA, 2009a, p.

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Page 77 of 432 al. (2011) observed positive and statisti cally significant associations between long

- term exposure to PM 2.5 and mortality in their national analyses. However, Janes et al. (2007) and Greven et al. (2011) eliminated all of the spatial variation in air pollution and mortality in their data set when estimating the national effect, focusing instead on both chronic (yearly) and sub

- chronic (monthly) temporal differences in the data (Dominici et al. 2012). Janes et al. (2007) (Table 1) highlight ed that over 90 percent of the variance in the data set use d for the analyses conducted by both Janes et al. (2007) and Greven et al. (2011) was attributable to spatial variability, which the authors chose to discard. As noted above, the focus of the analyses by Janes et al. (2007) and Greven et al. (2011) was on two components: (1) a temporal or time component, i.e., the “national” trends analysis, which examined the association between the national average trend in monthly PM 2.5 concentrations averaged over the previous 12 months and the national average trend in monthly mortality rates and (2) a space

- by

- time component, i.e., the “local” trends analysis, which examined county

- specific deviations in monthly PM 2.5 concentrations and monthly mortality rates from national trends. These two components combined compris ed less than 10 percent of the variance in the data set. The authors included a focus on the space

- by

- time component, which represented approximately 5 percent of the variance in the data set, in an attempt to identify, absent confounding, if PM 2.5 was associated with mortality at this unique exposure window. Thus, these studies are not directly comparable to other cohort studies investigating the relationship between long

term exposure to PM 2.5 and mortality, which make use of spatial variability i n air 7

- 88) and included the results of the national

- scale analyses in that study in the body of evidence that supported the determination that there is a causal r elationship between long

- term PM 2.5 exposure and mortality

.

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Page 78 of 432 pollution and mortality data. 43 This point was highlighted by the study authors who stated that “when one considers that this wealth of information is not accounted for in [Janes 2007], it is not as surprising that... vastly different estimates of the P M 2.5 /mortality relationship [were observed] than in other studies that do exploit that variability” (Dominici et al., 2012, p. 2 ). The EPA notes that the results of the local trends analyses conducted by Janes et al. (2007) and Greven et al. (2011) are li mited by the monthly timescale used in these analyses. This view is consistent with comments on the Janes et al. (2007) study articulated in Pope and Burnett (2007), 44 which noted that an important limitation of the local scale analysis conducted by Janes e t al. (2007) and subsequently by Greven et al. (2011) was the subchronic exposure window considered in these analyses. Both studies used annual average PM 2.5 concentrations to characterize long

- term national trends which was consistent with exposure window s considered in other studies of long

- term exposure to PM 2.5 and mortality. 45 However, the local scale analyses used monthly average PM 2.5

43 Though not directly comparable, the national effect estimates for mortality reported by Janes et al. (2007) and Greven et al. (2011) are coincidentally similar in magnitude to those previously r eported. It is important to note that previous cohort studies have focused on identifying spatial differences in PM 2.5 concentrations between cities, while Janes et al. (2007) and Greven et al. (2011) focus primarily on temporal differences in PM 2.5 concen trations. In fact, Greven et al. (2011) state “We do not focus here on a third type [of statistical approach] used in cohort studies, measuring the association between average PM 2.5 levels and average age

- adjusted mortality rates across cities (purely spat ial or cross - sectional association).” 44 S ome commenters argued that there were flaws in the criticisms offered by Pope and Burnett (2007) on the paper by Janes et al. (2007) (UARG, 2012, Attachment A, pp. 19 to 23). The EPA responds to each of these specif ic comments in the Response to Comments document. 45 As noted above, however, Janes et al. (2007) and Greven et al. (2011) focus ed on temporal variability and other studies of long

- term exposure to PM 2.5 and mortality focus on spatial variability

.

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Page 79 of 432 concentrations to characterize county

- specific deviations from national trends (the local scale). The use of monthly a verage data likely does not provide enough exposure contrast to observe temporal changes in mortality at the local scale. It also represents a different exposure window than considered in the large body of evidence of health effects related to short

- term ( from less than one day to up to several days) and chronic (one or more years) measures of PM 2.5 . Furthermore, the EPA disagrees with commenters that studies by Janes et al. (2007) and Greven et al. (2011) provide evidence that other studies of long

- term e xposure to PM 2.5 and mortality are affected by unmeasured confounding. As noted above, the design of the studies conducted by Janes et al (2007) and Greven et al (2011) are fundamentally different than those used in other studies of long

- term exposure to P M 2.5 and mortality, including the ACS cohort and the Harvard Six Cities study. Studies, such as the ACS and Harvard Six Cities studies, used the spatial variation between cities to measure the effect of long

- term (annual) exposures to PM 2.5 on mortality ri sk, and did not conduct any analyses relying on the temporal variation in PM 2.5 . The opposite is true of the Janes et al. (2007) and Greven et al. (2011) studies which first removed the spatial variability in PM 2.5 and then examined the temporal variation at both the national and local scale to measure the effects of temporal differences in PM 2.5 on mortality risk. Janes et al. (2007) and Greven et al. (2011) focus on changes in PM 2.5 concentrations over time and , therefore , control for confounders would be based on including variables that vary over time rather than over space. As a result, any evidence of potential confounding of the PM 2.5 - mortality risk relationship derived from Janes et al. (2007) and Greven et al. (2011) cannot be extrapolated to draw c onclusions related to potential spatial

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Page 80 of 432 confounding in studies based on the spatial variation in PM 2.5 concentrations. As detailed in the Integrated Science Assessment (U.S. EPA, 2009a, section 7.6) , and recognized by the authors of Janes et al. (2007) an d Greven et al. (2011), the cohort studies that informed the causality determination for long

- term PM 2.5 exposure and mortality “have developed approaches to adjust for measured and unmeasured confounders” (Dominici et al., 2012, p. 2). These approaches were specifically designed to adjust for spatial confounding. The hypothesis that the authors of Janes et al. (2007) and Greven et al. (2011) chose to examine was that differences in the local and national effects indicated unmeasured temporal confounding in either the local or national effect estimate. This hypothesis was specific to these two studies that ex amined temporal variability in exposure to air pollution and did not include known potential confounders at either the national or local scale as time

- varying covariates in the statistical model. The authors acknowledge d that the interpretation of either t he national or local estimates needs to occur with an appreciation of the potential confounding effects of national and local scale covariates that were omitted from the model ( Dominici et al., 2012 ). It is important to recognize that because Janes et al. (2007) and Greven et al. (2011) focus ed on variations in PM 2.5 over time and not space, the results from these two studies do not provide any indication that other studies of long

- term exposure to PM 2.5 and mortality exhibit spatial confounding, or that PM 2.5 does not cause mortality. 46 The authors of Janes et al. (2007) and Greven et al. (2011) recognized that “it is entirely possible that these papers are looking for an association at a timescale for which no

46 Further, the EPA notes that Janes et al. (2007) and Greve n et al. (2011) provide no information relevant to examining confounding in studies of short

- term exposure to PM 2.5 .

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Page 81 of 432 association truly exists” (Dominici et al.,. 20 12, p. 3). Furthermore, as highlighted in the Integrated Science Assessment and discussed by Pope and Burnett (2007), the conclusions of Janes et al. (2007) “are overstated. . . [T]heir analysis tells us little or nothing about unmeasured confounding in th ose and related studies because the methodology of Janes et al. largely excludes the sources of variability that are exploited in those other studies. By using monthly mortality counts and lagged 12

- month average pollution concentrations, the authors elimi nate the opportunity to exploit short

- term or day

- to

- day variability.” In conclusion, the EPA interprets the results of the analyses conducted by Janes et al. (2007) and Greven et al. (2011) as being consistent with prior knowledge of examining associations with long

- term exposure to PM 2.5 at the national scale using long

term average PM 2.5 concentrations. For the reasons presented above and discussed in more detail in the Response to Comments document, the Agency disagrees with the commenters’ a ssumption that the results of Janes et al. (2007) and Greven et al. (2011) indicate unmeasured confounding in the results of other cohort studies of long

- term exposure to PM 2.5 and mortality . Therefore, the EPA concludes that these studies do not invalidat e the large body of epidemiological evidence that supports the EPA’s determination that a causal relationship exists between long

- term PM 2.5 exposure and mortality. 47 ( b ) Heterogeneity in Risk Estimates

47 The EPA notes that the EPA’s conclusion with regard to interpretation of the re sults from Janes et al. (2007) and Greven et al. (2012) is supported by the study authors ’ conclusion that “[ o]ur results do not invalidate previous epidemiologic al studies ” (Dominici, 2012, p. 1 (emphasis original)).

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Page 82 of 432 Some commenters argued that the heterogeneity in risk estimates observed in multi

- city epidemiological studies and the lack of statistical significance in many regional or seasonal estimates highlights a potential bias associated with combined multi

- city epide miological study results (e.g., API, 2012, Attachment 1, pp. 15 to 19). These commenters further argued that more refined intra

- urban exposure estimates conducted for two of the largest cities included in the ACS study, Los Angeles and New York City, based on land

- use regression models and/or kriging methods (Krewski et al., 2009) “underscore the importance of considering city

- specific health estimates, which may account for heterogeneity in PM 2.5 concentrations or other differences among cities, rather tha n relying on pooled nationwide results from multi

- city studies” (API, 2012, Attachment 1, p. 17). With respect to understanding the nature and magnitude of PM 2.5 - related risks, the EPA agrees that epidemiological studies evaluating health effects associate d with long

and short

- term PM 2.5 exposures have reported heterogeneity in responses between cities and effect estimates across geographic regions of the U.S. ( U.S. EPA, 2009a, sections 6.2.12.1, 6.3.8.1, 6.5.2, and 7.6.1; U.S. EPA, 2011a, p. 2

- 25). For ex ample, when focusing on short

- term PM 2.5 exposure, the Integrated Science Assessment found that multi

- city studies that examined associations with mortality and cardiovascular and respiratory hospital admissions and emergency department visits demonstrated greater cardiovascular effects in the eastern versus the western U.S. (Dominici, et al., 2006; Bell et al., 2008; Franklin et al. (2007, 2008)). In addition, the Integrated Science Assessment evaluated studies that provided some evidence for seasonal dif ferences in PM 2.5 risk estimates, specifically in the

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Page 83 of 432 northeast. The I ntegrated S cience A ssessment found evidence indicating that individuals may be at greater risk of dying from higher exposures to PM 2.5 in the warmer months, and at greater risk of PM 2.5 associated hospitalization for cardiovascular and respiratory diseases during colder months of the year. The limited influence of seasonality on PM risk estimates in other regions of the U.S. may be due to a number of factors including varying PM compositi on by season, exposure misclassification due to regional tendencies to spend more or less time outdoors and air conditioning usage, and the prevalence of infectious diseases during the winter months ( U.S. EPA, 2009a, p. 3

- 182). Overall, the EPA took note in the proposal that uncertainties still remain regarding various factors that contribute to heterogeneity observed in epidemiological studies (77 FR 38909/3). Nonetheless, the EPA recognizes that this heterogeneity could be attributed, at least in part, t o differences in PM 2.5 composition across the U.S., as well as to exposure differences that vary regionally such as personal activity patterns, microenvironmental characteristics, and the spatial variability of PM 2.5 concentrations in urban areas ( U.S. EPA , 2009a, section 2.3.2; 77 FR 38910). As recognized in the Policy Assessment, the current epidemiological evidence and the limited amount of city

- specific speciated PM 2.5 data do not allow conclusions to be drawn that specifically differentiate effects of PM 2.5 in different locations ( U.S. EPA, 2011a, p. 2

- 25). Furthermore, the Integrated Science Assessment concluded “that many constituents of PM 2.5 can be linked with multiple health effects, and 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, 2009a, p. 2

- 17). CASAC thoroughly reviewed the EPA’s presentation of the scientific evidence indicating

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Page 84 of 432 heterogeneity in PM 2.5 effect estimates in epidem iological studies and concurred with the overall conclusions presented in the Integrated Science Assessment. (c) Exposure Measurement Error Some commenters argued that the EPA did not adequately consider exposure measurement error, which they asserted is an important source of bias in epidemiological studies that can bias effect estimates in either direction (e.g., API, 2012, Attachment 1, pp. 19 to 20). The EPA agrees that exposure measurement error is an important source of uncertainty and that the vari ability in risk estimates observed in multi

- city studies could be attributed, in part, to exposure error due to measurement

- related issues (77 FR 38910). However, the Agency disagrees with the commenters’ assertion that exposure
measurement error was not a dequately considered in this review. The Integrated Science Assessment included an extensive discussion that addresses issues of exposure measurement error ( U.S. EPA, 2009a, sections 2.3.2 and 3.8.6). Exposure measurement error may lead to bias in effect e stimates in epidemiological studies. A number of studies evaluated in the last review ( U.S. EPA, 2004, section 8.4.5) and in the current review ( U.S. EPA, 2009a, section 3.8.6) have discussed the direction and magnitude of bias resulting from specified pat terns of exposure measurement error (Armstrong 1998; Thomas et al. 1993; Carroll et al. 1995) and have generally concluded “classical” (i.e., random, within

- person) exposure measurement error can bias effect estimates towards the null. Therefore, consisten t with conclusions reached in the last review, the Integrated Science Assessment concluded “in most circumstances, exposure error tends to bias a health effect estimate downward ” ( U.S. EPA, 2009a, sections 2.3.2 and 3.8.6) (emphasis

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Page 85 of 432 added). Thus, the EPA has both considered and accounted for the possibility of exposure measurement error, and the possible bias would make it more difficult to detect true associations, not less difficult. ( d ) Model Specification Commenters contend ed that the EPA did not account for the fact that “selecting an appropriate statistical model for epidemiologic studies of air pollution involves several choices that involve much ambiguity, scant biological evidence, and a profound impact on analytic results, given that many es timated associations are weak” (ACC, 2012, p. 5) For short

- term exposure studies , the EPA recognizes, as summarized in the HEI review panel commentary that selecting a level of control to adjust for time

- varying factors, such as temperature, in time

- series epidemiologic al studies involves a trade

- off (HEI, 2003). For example, if the model does not sufficiently adjust for the relationship between the health outcome and temperature, some effects of temperature could be falsely ascribed to the pollution variab le. Conversely, if an overly aggressive approach is used to control for temperature, the result would possibly underestimate the pollution

- related effect and compromise the ability to detect a small but true pollution effect ( U.S. EPA, 2004, p. 8

- 236; HEI, 2003, p. 266). The selection of approaches to address such variables depends in part on prior knowledge and judgments made by the investigators, for example, about weather patterns in the study area and expected relationships between weather and other tim e - varying factors and health outcomes considered in the study. As demonstrated in

s ection 6.5 of the Integrated Science Assessment, the EPA thoroughly considered each of these issues and the overall effect of different model specifications on the associati on between short

- term PM 2.5 exposure and mortality. Regardless of the model employed,

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Page 86 of 432 consistent positive associations were observed across studies that controlled for the potential confounding effects of time and weather using different approaches (U

. S

. E PA 2009, Figure 6

- 27). The EPA also considered the influence of model specification in the examination of long

- term PM 2.5 exposure studies. For example, in s ection 7.6 of the Integrated Science Assessment , Figures 7

- 6 and 7

- 7 summarize the collective evide nce that evaluated the association between long

- term PM 2.5 exposure and mortality. Regardless of the model used, these studies collectively found evidence of consistent positive associations between long

- term PM 2.5 exposure and mortality. The EPA, therefo re, disagrees with commenters that model specification was not considered when evaluating the epidemiologic al evidence used to form causality determinations. The EPA specifically points out that the process of assessing the scientific quality and relevance of epidemiologic al studies includes examining “important methodological issues (e.g., lag or time period between exposure and effects, model specifications, thresholds, mortality displacement) related to interpretation of the health evidence (U

. S

. EPA, 20 09, p. 1

- 9).” Consistent with the conclusions of the 2004 PM A ir Q uality C riteria D ocument , the EPA recognizes that there is still no clear consensus at this time as to what constitutes appropriate control of weather and temporal trends in short

- term expos ure studies, and that no single statistical modeling approach is likely to be most appropriate in all cases ( U.S. EPA, 2004, p. 8

- 238). However, the EPA believes that the available evidence interpreted in light of these remaining uncertainties does provide increased confidence relative to the last review in the reported associations between short

- and long

- term PM 2.5 exposures and mortality and morbidity effects, alone and in combination with other pollutants.

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Page 87 of 432 ( e ) Concentration

- response Relationship Additio nally, commenters questioned the interpretation of the shape of the concentration

- response relationship, specifically stat ing that multiple studies have demonstrated that there is a threshold in the PM

- health effect relationship and that the log

- linear model is not biologically plausible (API, 2012, Attachment 9; ACC, 2012, Appendix A, pp. 7 to 8). The EPA disagrees with this assertion due to the number of studies evaluated in the Integrated Science Assessment that continue to support the use of

a no - threshold, log

- linear model to most appropriately represent the PM concentration

response relationship ( U.S. EPA, 2009a, section 2.4.3). W hile recognizing that uncertainties rem ain, the EPA believes that our understanding of this issue for both long

and short

- term exposure studies has advanced since the last review. As discussed in the Integrated Science Assessment, both long

- and short

- term exposure studies have employed a varie ty of statistical approaches to examine the shape of the concentration

response function and whether a threshold exists. While the EPA recognizes that there likely are individual biologic al thresholds for specific health responses, the Integrated Science A ssessment concluded the overall evidence from existing epidemiological studies do es not support the existence of thresholds at the population level, for effects associated with either long

- term or short

- term PM exposures within the ranges of air quality ob served in these studies (U.S. EPA, 2009a, section 2.4.3)

. 48 The Integrated Science Assessment concluded that this evidence collectively supported the conclusion

48 While epidemiological analyses have not identified a population threshold in the range of air quality concentrations evaluated in these studies, the EPA recognizes that it is possible that such thresholds exist towards the lower end of these ranges (or below these ranges).

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Page 88 of 432 that a no

- threshold, log

- linear model is most appropriate ( U.S. EPA, 2009a, sections 6.2.10.10, 6.5.2.7, and 7.6.4). CASAC likewise advised that “[a]lthough there is increasing uncertainty at lower levels, there is no evidence of a threshold (Samet, 2010d, p. ii). The EPA recognizes that some short

- term exposure studies have examined the PM 2.5 concen tration

- response relationship in individual cities or on a city

- to

- city basis and observed heterogeneity in the shape of the concentration

- response curve across cities. As discussed in (b) above, these findings are a source of uncertainty that the EPA agre es requires further investigation. Nonetheless, the Integrated Science Assessment concluded that “the studies evaluated further support the use of a no

- threshold, log

- linear model, but additional issues such as the influence of heterogeneity in estimates b etween cities and the effects of seasonal and regional differences in PM on the concentration

- response relationship still require further investigation” ( U.S. EPA, 2009a, p. 2

- 25). (f) Relative Toxicity of PM 2.5 Components Some commenters highlighted un certainties in understanding the role of individual constituents with in the mix of fine particles. These commenters asserted that a

mass - based standard may not be appropriate due to the growing body of evidence indicating that certain PM 2.5 components may be more closely related to specific health outcomes (e.g., EC and OC) (EPRI, 2012, p. 2). With regard to questions about the role of individual constituents within the mix of fine particle s , as a general matter, the EPA recognizes that although new resear ch directed toward this question has been conducted since the last review, important questions remain and the issue remains an important element in the Agency’s ongoing

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Page 89 of 432 research program. At the time of the last review, the Agency determined that it was
app ropriate to continue to control fine particles as a group, as opposed to singling out any particular component or class of fine particles (71 FR 61162 to 61164, October 17, 2006). This distinction was based largely on epidemiological evidence of health eff ects using various indicators of fine particles in a large number of areas that had significant contributions of differing components or sources of fine particles, together with some limited experimental studies that provided some evidence suggestive of he alth effects associated with high concentrations of numerous fine particle components. In this review, as discussed in the proposal (77 FR 38922 to 38923) and in section III.E.1 below, while most epidemiological studies continue to be indexed by PM 2.5 mass, several recent epidemiological studies included in the Integrated Science Assessment have used PM 2.5 speciation data to evaluate health effects associated with fine particle exposures. In the Integrated Science Assessment, the EPA thoroughly evaluat ed the scientific evidence that examined the effect of different PM 2.5 components and sources on a variety of health outcomes ( U.S. EPA, 2009a, section 6.6) and observed that the available information continues to suggest that many different chemical compo nents of fine particles and a variety of different types of source categories are all associated with, and probably contribute to, effects associated with PM 2.5 . The Integrated Science Assessment concluded that the current body of scientific evidence indic ated that “many constituents of PM can be linked with differing health effects and 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, 2009a, p. 2

- 26 and 6

- 212). Furthermore, the Policy Assessment concluded that the evidence is not sufficient to support eliminating

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Page 90 of 432 any component or group of components associated with any specific source categories from the mix of fine particles included in the PM 2. 5 indicator ( U.S. EPA, 2009a, p. 2

- 56). CASAC agreed that it was reasonable to retain PM 2.5 as an indicator for fine particles in this review as “[t]here was insufficient peer

- reviewed literature to support any other indicator at this time” (Samet, 2010c, p. 12). This information is relevant to the Agency’s decision to retain PM 2.5 as the indicator for fine particles as discussed in section III.E.1 below. The EPA also believes that it is relevant to the Agency’s conclusion as to whether revision of the suit e of primary PM 2.5 standards is appropriate. While there remain uncertainties about the role and relative toxicity of various components of fine PM, the current evidence continues to support the view that fine particles should be addressed as a group for p urposes of public health protection. In summary, in considering the above issues related to uncertainties in the underlying health science, on balance, the EPA believes that the available evidence interpreted in light of these remaining uncertainties does provide increased confidence relative to the last review in the reported associations between long

- and short

- term PM 2.5 exposures and mortality and morbidity effects, alone and in combination with other pollutants, and supports stronger inferences as to the causal nature of the associations. The EPA also believes that this increased confidence, when taken in context of the entire body of available health effects evidence and in light of the evidence from epidemiological studies of associations observed i n areas meeting the current primary PM 2.5 standards, specifically in areas meeting the current primary annual PM 2.5 standard, adds support to its conclusion that the current suite of PM 2.5 standards needs to be revised

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Page 91 of 432 to provide increased public health pr otection. (4) In asserting that there is no evidence of greater risk since the 2006 review to justify lowering the current annual PM 2.5 standard, some commenters argued that, “if the current primary PM 2.5 annual standard of 15 μg/m 3 was considered to be adequately protective of public health in 2006, given relative risk estimates that EPA was using at that time, then that standard would surely still be adequately protective of the public health if relative risk estimates remain at th e same level (or lower)” (UARG, 2012, Attachment 1, p. 24). These commenters compared risk coefficients used for mortality in the EPA’s risk assessment done in the last review with those from the Agency’s core risk assessment done as part of this review, a nd they concluded that “the entire range of the core relative risk for long

- term mortality is lower now than it was in the prior review” (UARG, 2012, Attachment 1, p. 24). These commenters used this conclusion as the basis for a claim that there is no reas on to revise the current annual PM 2.5 standard. The EPA believes that this claim is fundamentally flawed. In comparing the scientific understanding of the risk presented by exposure to PM 2.5 between the last and current reviews, one must examine not only t he quantitative estimate of risk from those exposures (e.g., the numbers of premature deaths or increased hospital admissions at various concentrations), but also the degree of confidence that the Agency has that the observed health effects are causally li nked to PM 2.5 exposure at those concentrations. As documented in the Integrated Science Assessment and in the recommendations and conclusions of CASAC, the EPA recognizes significant advances in our understanding of the health effects of PM 2.5 , based on ev idence that is stronger than in the last review. As a result of these advances, the EPA is now more certain that fine particles, alone or in

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Page 92 of 432 combination with other pollutants, present a significant risk to public health at concentrations allowed by the cur rent primary PM 2.5 standards. From this more comprehensive perspective, since the risks presented by PM 2.5 are more certain, similar or even somewhat lower relative risk estimates would not be a basis to conclude that no revision to the suite of PM 2.5 stan dards is “requisite” to protect public health with an adequate margin of safety. This also ignores that the relative risk estimate is only one factor considered by the Administrator, e.g. it ignores that epidemiological studies since the last review indica

te associations between PM 2.5 and mor t ality and morbidity in areas meeting the current annual standard. In any case, the commenters’ reliance on the flawed 2006 review is misplaced. As discussed in section III.A.2 above, the D.C. Circuit remanded Administr ator Johnson’s 2006 decision to retain the primary annual PM 2.5 standard because the Agency failed to adequately explain why the annual standard provided the requisite protection from both short

- and long

- term exposure to fine particles including protectio n for at

- risk populations. The 2006 standard was also at sharp odds with CASAC advice and recommendations as to the requisite level of protection (Henderson, 2006a,b). In other words, the judgment of the then

- Administrator that the 2006 primary annual PM 2. 5 standard was requisite to protect public health with an adequate margin of safety is not an appropriate benchmark for comparison. (5) Some of these commenters also identified “new” as well as older studies that had been included in prior reviews as prov iding additional evidence that the causality determinations presented in the Integrated Science Assessment did not consider the totality of the scientific literature , further supporting the ir view that a revision of the

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Page 93 of 432 PM 2.5 is unwarranted. As discussed i n section II.B.3 above, the EPA notes that, as in past NAAQS reviews, the Agency is basing the final decisions in this review on the studies and related information included in the I ntegrated S cience A ssessment that have undergone CASAC and public review, and will consider newly published studies for purposes of decision making in the next PM NAAQS review. In provisionally evaluating commenters’ arguments (see Response to Comments document), the EPA notes that its provisional assessment of “new” science fou nd that such studies did not materially change the conclusions reached in the Integrated S cience Assessment (U.S. EPA, 2012b)

. 3. Administrator’s Final Conclusions Concerning the Adequacy of the Current Primary PM 2.5 Standards Having carefully considered the public comments, as discussed above, the Administrator believes the fundamental scientific conclusions on the effects of PM 2.5 reached in the Integrated Science Assessment, and discussed in the Policy Assessment, are valid. In considering whether the suit e of primary PM 2.5 standards should be revised, the Administrator places primary consideration on the evidence obtained from the epidemiological studies. The Administrator believes that this literature, combined with the other scientific evidence discussed in the I ntegrated S cience A ssessment , collectively represents a strong and generally robust body of evidence of serious health effects associated with both long

- and short

- term exposures to PM 2.5 . As discussed in the Integrated Science Assessment and Policy Assessment, the EPA believes that much progress has been made since the last review in reducing some of the major uncertainties that were important considerations in establishing the current suite of PM 2.5 standards. In that context, the Administrat or finds the evidence of serious health effects reported in

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Page 94 of 432 exposure studies conducted in areas with long

- term mean concentration s ranging from approximately at or above the level of the annual standard to long

- term mean concentration s significantly below the level of the annual standard to be compelling, especially in light of the extent to which such studies are part of an overall pattern of positive and frequently statistically significant associations across a broad range of studies. The information in the quantitative risk assessment lends support to this conclusion. There has been extensive critical review of this body of evidence, the quantitative risk assessment, and related uncertainties, including review by CASAC and the public. The public comments on the basis for the EPA’s proposed decision to revise the suite of primary PM 2.5 standards have identified a number of issues about which different parties disagree including issues for which additional research is warranted. Having weighed all
comments and the advice of CASAC, the Administrator believes that since the last review the overall uncertainty about the public health risks associated with both long

- and short

- term exposure to PM 2.5 has been diminished to an important degree. The remaining

uncer tainties in the available evidence do not diminish confidence in the associations between exposure to fine particles and mortality and serious morbidity effects. Based on her increased confidence in the association between exposure to PM 2.5 and serious pub lic health effects, combined with evidence of such an association in areas that would meet the current standards, the Administrator agrees with CASAC that revision of the current suite of PM 2.5 standards to provide increased public health protection is nec essary. Based on these considerations, the Administrator concludes that the current suite of primary PM 2.5 standards is not sufficient, and thus not requisite, to protect public health with an

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Page 95 of 432 adequate margin of safety, and that revision is needed to incre ase public health protection. It is important to note that this conclusion, and the reasoning on which it is based, do not resolve the question of what specific revisions are appropriate. That requires looking specifically at the current 24

- hour and annua l PM 2.5 standards, including their indicator, averaging times, forms, and levels, and evaluating the scientific evidence and other information relevant to determining the appropriate revision of the suite of standards. E. Conclusions on the Elements of the Pr imary Fine Particle Standards 1. Indicator In initially setting standards for fine particles in 1997, the EPA concluded it was appropriate to control fine particles as a group, rather than singling out any particular component or class of fine particles. The EPA noted that community health studies had found significant associations between various indicators of fine particles, and that health effects in a large number of areas had significant mass contributions of differing components or sources of fine partic les. In addition, a number of toxicological and controlled human exposure studies had reported health effects associations with high concentrations of numerous fine particle components. It was also not possible to rule out any component within the mix of f ine particles as not contributing to the fine particle effects found in the epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a size

- based indicator in 1977 to distinguish fine particles from particles in the coarse mode, the EPA noted th at the available epidemiological studies of fine particles were based largely on PM 2.5 and also considered monitoring technology that was generally

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Page 96 of 432 available. The selection of a 2.5 μm size cut reflected the regulatory importance of defining an indicator t

hat would more completely capture fine particles under all conditions likely to be encountered across the U.S., especially when fine particle concentrations and humidity are likely to be high, while recognizing that some small coarse particles would also b e captured by current methods to monitor PM 2.5 (62 FR 38666 to 38668, July 18, 1997). In the last review, based on the same considerations, the EPA again recognized that the available information supported retaining the PM 2.5 indicator and remained too lim ited to support a distinct standard for any specific PM 2.5 component or group of components associated with any source categories of fine particles (71 FR 61162 to 61164, October 17, 2006). In this current review, the same considerations continue to apply for selection of an appropriate indicator for fine particles. As an initial matter, the Policy Assessment recognizes that the available epidemiological studies linking mortality and morbidity effects with long

- and short

- term exposures to fine particles co ntinue to be largely indexed by PM 2.5 . For the same reasons discussed in the last two reviews, the Policy Assessment concluded that it was appropriate to consider retaining a PM 2.5 indicator to provide protection from effects associated with long

- and shor t

- term fine particle exposures ( U.S. EPA, 2011, p. 2

- 50). The Policy Assessment also considered the expanded body of evidence available in this review to consider whether there was sufficient evidence to support a separate

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Page 97 of 432 standard for ultrafine particles 49 or whether there was sufficient evidence to establish distinct standards focused on regulating specific PM 2.5 components or a group of components associated with any source categories of fine particles ( U.S. EPA, 2011a, section 2.3.1). A number of studies available in this review have evaluated potential health effects associated with short

- term exposures to ultrafine particles. As noted in the Integrated Science Assessment, the enormous number and larger, collective surface area of ultrafine particles are important considerations for focusing on this particle size fraction in assessing potential public health impacts ( U.S. EPA, 2009a, p. 6

- 83). Per unit mass, ultrafine particles may have more opportunity to interact with cell surfaces due to their greater surface area and their greater particle number compared with larger particles ( U.S. EPA, 2009a, p. 5

- 3). Greater surface area also increases the potential for soluble components (e.g., transition metals, organics) to adsorb to ultrafine particles and poten tially cross cell membranes and epithelial barriers ( U.S. EPA, 2009a, p. 6

- 83). In addition, evidence available in this review suggests that the ability of particles to enhance allergic sensitization is associated more strongly with particle number and surface area than with particle mass ( U.S. EPA, 2009a, p. 6

- 127). New evidence, primarily from controlled human exposure and toxicological studies, expands our understanding of cardiovascular and respiratory effects related to short

- term ultrafine particle exposures. However, the Policy Assessment concluded that
t his evidence was still very limited and largely focused on exposure to diesel exhaust, for

49 Ultrafine partic les, generally including particles with a mobility diameter less than or equal to 0.1 μm, are emitted directly to the atmosphere or are formed by nucleation of gaseous constituents in the atmosphere (U.S. EPA, 2009a, p. 3

- 3).

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Page 98 of 432 which the Integrated Science Assessment concluded it was unclear whether the effects observed are due to ultrafine particles, larger particles within the PM 2.5 mixtu re, or the gaseous components of diesel exhaust ( U.S. EPA, 2009a, p. 2

- 22). In addition, the Integrated Science Assessment noted uncertainties associated with the controlled human exposure studies using concentrated ambient particle systems which have been shown to modify the composition of ultrafine particles ( U.S. EPA, 2009a, p. 2

- 22, see also section 1.5.3). The Policy Assessment recognized that there are relatively few epidemiological studies that have examined potential cardiovascular and respiratory effects associated with short

- term exposures to ultrafine particles ( U.S. EPA, 2011a, p. 2

- 51). These studies have reported inconsistent and mixed results ( U.S. EPA, 2009a, section 2.3.5). Collectively, in considering the body of scientific evidence availa ble in this review, the Integrated Science Assessment concluded that the currently available evidence was suggestive of a causal relationship between short

- term exposures to ultrafine particles and cardiovascular and respiratory effects. Furthermore, the I ntegrated Science Assessment concluded that evidence was inadequate to infer a causal relationship between short

- term exposure to ultrafine particles and mortality as well as long

- term exposure to ultrafine particles and all outcomes evaluated ( U.S. EPA, 2 009a, sections 2.3.5, 6.2.12.3, 6.3.10.3, 6.5.3.3, 7.2.11.3, 7.3.9, 7.4.3.3, 7.5.4.3, and 7.6.5.3; Table 2

- 6). With respect to our understanding of ambient ultrafine particle concentrations, at present, there is no national network of ultrafine particle s amplers; thus, only episodic and/or site

- specific data sets exist ( U.S. EPA, 2009a, p. 2

- 2). Therefore, the Policy Assessment recognized a national characterization of concentrations, temporal and spatial

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Page 99 of 432 patterns, and trends was not possible at this time, and the availability of ambient ultrafine measurements to support health studies was extremely limited ( U.S. EPA, 2011a, p. 2

- 51). In general, measurements of ultrafine particles are highly dependent on monitor location and, therefore, more subject to exp osure error than accumulation mode particles ( U.S. EPA, 2009a, p. 2

- 22). Furthermore, the number of ultrafine particles generally decreases sharply downwind from sources, as ultrafine particles may grow into the accumulation mode by coagulation or condensa tion ( U.S. EPA, 2009a, p. 3

- 89). Limited studies of ambient ultrafine particle measurements have suggested that these particles exhibit a high degree of spatial and temporal heterogeneity driven primarily by differences in nearby source characteristics ( U. S. EPA, 2009a, p. 3

- 84). Internal combustion engines and, therefore, roadways are a notable source of ultrafine particles, so concentrations of these particles near roadways are generally expected to be elevated ( U.S. EPA, 2009a, p. 2

- 3). Concentrations of ultrafine particles have been reported to drop off much more quickly with distance from roadways than fine particles ( U.S. EPA, 2009a, p. 3

- 84). In considering both the currently available health effects evidence and the air quality data, the Policy Asses sment concluded that this information was still too limited to provide support for consideration of a distinct PM standard for ultrafine particles ( U.S. EPA, 2011a, p. 2

- 52). In addressing the issue of particle composition, the Integrated Science Assessmen t concluded that, “[f]rom a mechanistic perspective, it is highly plausible that the chemical composition of PM would be a better predictor of health effects than particle size” ( U.S. EPA, 2009a, p. 6

- 202). Heterogeneity of ambient concentrations of PM 2.5 constituents

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Page 100 of 432 (e.g., elemental carbon, organic carbon, sulfates, nitrates) observed in different geographical regions as well as regional heterogeneity in PM 2.5 - related health effects reported in a number of epidemiological studies are consistent with this hypothesis ( U.S. EPA, 2009a, section 6.6). With respect to the availability of ambient measurement data for fine particle components in this review, the Policy Assessment noted that there were now more extensive ambient PM 2.5 speciation measurement data a vailable through the Chemical Speciation Network (CSN) than in previous reviews ( U.S. EPA, 2011a, section 1.3.2 and Appendix B, section B.1.3). The Integrated Science Assessment observed that data from the CSN provided further evidence of spatial and seaso nal variation in both PM 2.5 mass and composition among cities and geographic regions ( U.S. EPA, 2009a, pp. 3

- 50 to 3

- 60; Figures 3

- 12 to 3

- 18; Figure 3

- 47). Some of this variation may be related to regional differences in meteorology, sources, and topograp hy ( U.S. EPA, 2009a, p. 2

- 3). The currently available epidemiological, toxicological, and controlled human exposure studies evaluated in the Integrated Science Assessment on the health effects associated with ambient PM 2.5 constituents and categories of f ine particle sources used a variety of quantitative methods applied to a broad set of PM 2.5 constituents, rather than selecting a few constituents a priori ( U.S. EPA, 2009a, p. 2

- 26). Epidemiological studies have used measured ambient PM 2.5 speciation data , including monitoring data from the CSN, while all of the controlled human exposure and most of the toxicological studies have used concentrated ambient particles and analyzed the constituents therein ( U.S.

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Page 101 of 432 EPA, 2009a, p. 6

- 203). 50 The CSN provides PM 2.5 s peciation measurements generally on

a one - in

- three or one

- in

- six day sampling schedule and, thus, do es not capture data every day at most sites. 51 The Policy Assessment recognized that several new multi

- city studies evaluating short

- term exposures to fine particle constituents are now available. These studies continued to show an association between mortality and cardiovascular and/or respiratory morbidity effects and short

- term exposures to various PM 2.5 components including nickel, vanadium, elemental car bon, organic carbon, nitrates, and sulfates ( U.S. EPA, 2011a, section 2.3.1; U.S. EPA, 2009a, sections 6.5.2.5 and 6.6). Limited evidence is available to evaluate the health effects associated with long

term exposures to PM 2.5 components ( U.S. EPA, 2009a, section 7.6.2). The Policy Assessment noted the most significant new evidence was provided by a study that evaluated multiple PM 2.5 components and an indicator of traffic density in an assessment of health effects related to long

- term exposure to PM 2.5 (L ipfert et al., 2006). Using health data from a cohort of U.S. military veterans and PM 2.5 measurement data from the CSN, Lipfert et al. (2006) reported positive associations between mortality and long

- term
50 Most studies considered betw een 7 to 20 ambient PM 2.5 constituents, with elemental carbon, organic carbon, sulfates, nitrates, and metals most commonly measured. Many of the studies grouped the constituents with various factorization or source apportionment techniques to examine the relationship between the grouped constituents and various health effects. However, not all studies labeled the constituent groupings according to their presumed source and a small number of controlled human exposure and toxicological studies did not use an y constituent grouping. These differences across studies substantially limit any integrative interpretation of these studies (U.S. EPA, 2009a, p. 6

- 203). 51 To expand our understanding of the role of specific PM 2.5 components and sources with respect to th e observed health effects, researchers have expressed a strong interest in having access to PM 2.5 speciation measurements collected more frequently (U.S. EPA, 2011a, p. 2

- 53, including footnote 47).

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Page 102 of 432 exposures to nitrates, elemental carbon, nickel, and vanadium as well as traffic density and peak ozone concentrations ( U.S. EPA, 2011a, p. 2

- 54; U.S. EPA, 2009a, pp. 7

- 89 to 7

- 90). With respect to source categories of fine particles potentially associated with a range of health endpoints, the Integrated Science Assessment reported that the currently available evidence suggests associations between cardiovascular effects and a number of specific PM 2.5 –

related source categories, including specifically oil combustion, wood or biomass burning, motor vehicle emissions, and crustal or road dust sources ( U.S. EPA, 2009a, section 6.6; Table 6

- 18). In addition, a few studies have evaluated associations between PM 2.5 - related source categories and mortality. For example, o O ne stud y reported an association between mortality and a PM 2.5 coal combustion factor (Laden et al., 2000), while other studies linke d mortality to a secondary sulfate long

- range transport PM 2.5 source (Ito et al., 2006; Mar et al., 2006) ( U.S. EPA, 2009a, section 6.6.2.1). Other studies have looked at different components of particulate matter. There was less consistency in association s observed between selected sources of fine particles and respiratory health endpoints effects , which may be partially due to the fact that fewer studies have evaluated respiratory

- related outcomes and measures. However, there was some evidence for PM 2.5 - re lated associations with secondary sulfate and decrements in lung function in asthmatic and healthy adults ( U.S. EPA, 2009a, p. 6

- 211; Gong et al., 2005; Lanki et al., 2006). A couple of studies have observed an association between

r R espiratory e ndpoints i n children and adults with asthmatic s and ffects relating
to surrogates for the crustal/soil/road dust and traffic sources of PM have been observed in asthmatic children and adults ( U.S. EPA, 2009a, p. 6

- 205; Gent et al., 2009; Penttinen et

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Page 103 of 432 al., 2006). Recent studies have shown that source apportionment methods have the potential to add useful insights into which sources and/or PM constituents may contribute to different health effects. Of particular interest are several epidemiological studies that
comp ared source apportionment methods and reported consistent results across research groups ( U.S. EPA, 2009a, p. 6

- 211; Hopke et al., 2006; Ito et al., 2006; Mar et al., 2006; Thurston et al., 2005). These studies reported associations between total mortality and secondary sulfate in two cities for two different lag times. The sulfate effect was stronger for total mortality in Washington D.C. and for cardiovascular

- related mor t ality in Phoenix ( U.S. EPA, 2009a, p. 6

- 204). These studies also found some evidence for associations with mortality and a number of source categories (e.g., biomass/wood combustion, traffic, copper smelter, coal combustion, sea salt) at various lag times ( U.S. EPA, 2009a, p. 6

- 204). Sarnat et al. (2008) compared three different source
ap portionment methods and reported consistent associations between emergency department visits for cardiovascular diseases with mobile sources and biomass combustion as well as increased respiratory

- related emergency department visits associated with seconda

ry sulfate ( U.S. EPA, 2009a, pp. 6

- 204 and 6

- 211). Collectively, in considering the currently available evidence for health effects associated with specific PM 2.5 components or groups of components associated with any source categories of fine particles as presented in the Integrated Science Assessment, the Policy Assessment concluded that additional information available in this review continues to provide evidence that many different constituents of the fine particle mixture as well as groups of component s associated with specific source categories of fine

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Page 104 of 432 particles are linked to adverse health effects ( U.S. EPA, 2011a, p. 2

- 55). However, as noted in the Integrated Science Assessment, while “[t]here is some evidence for trends and patterns that link partic ular ambient PM constituents or sources with specific health outcomes...there is insufficient evidence to determine whether these patterns are consistent or robust” ( U.S. EPA, 2009a, p. 6

- 210). Assessing this information, the Integrated Science Assessment co ncluded 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, 2009a, pp. 2

- 26 and 6

- 212). Therefore, the Policy Assessment concluded that the currently available evidence is not sufficient to support consideration of a separate indicator for a specific PM 2.5 component or group of components associated with any source category of fine particles. Furthermore, the Policy Assessment concluded that the evidence is not sufficient to support eliminating any component or group of components associated with any source categories of fine particles from the mix of fine particles included in the PM 2.5 indicator ( U.S. EPA, 2011a, p. 2

- 56). The CASAC agreed with the EPA staff conclusions presented in the Policy Assessmen t and concluded that it is appropriate to consider retaining PM 2.5 as the indicator for fine particles and further asserted, “There [is] insufficient peer

- reviewed literature to support any ot her indicator at this time” (Samet, 2010c, p. 12). CASAC expressed a strong desire for the EPA to “look ahead to future review cycles and reinvigorate support for the development of evidence that might lead to newer indicators that may correlate better wit h the health effects associated with ambient air concentrations of PM ...” (Samet, 2010c, p 2).

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Page 105 of 432 Consistent with the staff conclusions presented in the Policy Assessment and CASAC advice, the Administrator proposed to retain PM 2.5 as the indicator for fine
p articles. Further, the Administrator provisionally concluded that currently available scientific information does not provide a sufficient basis for supplementing mass

- based, primary fine particle standards with standards using a separate indicator for ult rafine particles or a separate indicator for a specific PM 2.5 component or group of components associated with any source categories of fine particles. In addition, the Administrator also provisionally concluded that the currently available scientific info rmation did not provide a sufficient basis for eliminating any individual component or group of components associated with any source categories from the mix of fine particles included in the PM 2.5

mass - based indicator. The EPA received comparatively few public comments on issues related to the indicator for fine particles. 52 Some commenters emphasized the need to conduct additional research to more fully understand the effect of specific PM 2.5 components and/or sources on public health. These commenters ex pressed views about the importance of evaluating health effect associations with various fine particle components and types of source categories as a basis for focusing ongoing and future research to reduce uncertainties in this area and for considering wh ether alternative indicator(s) may be appropriate to consider in future PM NAAQS reviews for standards intended to protect against the array of health effects that have been associated with fine particles as indexed by PM 2.5 . For example, the PSR encourage d more research and monitoring related to

52 No public comments were submitted regarding the use of a different size cut for fine particles.

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Page 106 of 432 PM 2.5 components and noted the importance of components associated with coal combustion (PSR, 2012, pp. 5 to 6). EPRI asserted that “new” studies support focusing on EC and OC and encouraged the EPA to seriously co nsider the mass

- based approach (EPRI, 2012, p. 2). Likewise, Georgia Mining Association supported additional monitoring and research efforts related to PM 2.5 composition and specifically encouraged the evaluation of using particle number (e.g., particle co unt) (GMA, 2012, pp. 2 to 3). The Administrator agrees with CASAC as well as these commenters that the results of additional research and monitoring efforts will be helpful for informing future PM NAAQS reviews. Information from such studies could also he lp inform the development of strategies that emphasize control of specific types of emission sources so as to address particles of greatest concern to public health. However, based upon the scientific information considered in the Integrated Science Assess ment as well as the public comments summarized above, the Administrator continues to take note there is evidence that many different constituents of the fine particle mixture as well as groups of components associated with specific sources of fine particle s are linked to adverse health effects. Furthermore, she recognizes that the evidence is not yet sufficient to differentiate those constituents or sources that are most closely related to specific health outcomes nor to exclude any PM 2.5 components or sour ces of fine particles from the mix of particles included in the PM 2.5 indicator. Having considered the public comments on this issue, the Administrator concurs with the Policy Assessment conclusions and CASAC recommendations and concludes that it is appro priate to retain PM 2.5 as the indicator for fine particles. 2. Averaging Time

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Page 107 of 432 In 1997, the EPA initially set both an annual standard, to provide protection from health effects associated with both 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 last review, the EPA retained both annual and 24

- hour averaging times (71 FR 61164, October 17, 2006). These decisions were based, in part, on evidence of health effects related to both long

- term (from a year to several years) and short

- term (from less than one day to up to several days) measures of PM 2.5 . The overwhelming majority of studies conducted since the last review continue to utiliz e annual (or multi

- year) and 24

- hour averaging times, reflecting the averaging times of the current PM 2.5 standards. These studies continue to provide evidence that health effects are associated with annual and 24

- hour averaging times. Therefore, the Policy Assessment concluded it is appropriate to retain the current annual and 24

- hour averaging times to prov ide protection from effects associated with both long

- and short

- term PM 2.5 exposures ( U.S. EPA, 2011a, p. 2

- 57). In considering whether the information available in this review supports consideration of different averaging times for PM 2.5 standards specif ically with regard to considering a standard with an averaging time less than 24 hours to address health effects associated with sub

- daily PM 2.5 exposures, the Policy Assessment noted there continues to be a growing body of studies that provide additional evidence of effects associated with exposure periods less than 24

- hours ( U.S. EPA, 2011a, p. 2

- 57). Relative to information available in the last review, recent studies provide additional evidence for cardiovascular effects associated with sub

- daily (e.g., one to several hours) exposure to PM, especially effects related to cardiac ischemia, vasomotor function, and more subtle

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Page 108 of 432 changes in markers of systemic inflammation, hemostasis, thrombosis and coagulation ( U.S. EPA, 2009a, section 6.2). Because these stu dies have used different indicators (e.g., PM 2.5 , PM 10 , PM 10

- 2.5 , ultrafine particles), averaging times (e.g., 1, 2, and 4 hours), and health outcomes, it is difficult to draw conclusions about cardiovascular effects associated specifically with sub

- daily exposures to PM 2.5 . With regard to respiratory effects associated with sub

- daily PM 2.5 exposures, the currently available evidence was much sparser than for cardiovascular effects and continues to be very limited. The Integrated Science Assessment conclud ed that for several studies of hospital admissions or medical visits for respiratory diseases, the strongest associations were observed with 24

- hour average or longer exposures, not with less than 24

- hour exposures ( U.S. EPA, 2009a, section 6.3). Collecti vely, the Policy Assessment concluded that this information, when viewed as a whole, is too unclear, with respect to the indicator, averaging time and health outcome, to serve as a basis for consideration of establishing a primary PM 2.5 standard with an av eraging time shorter than 24

- hours at this time ( U.S. EPA, 2011a, p. 2

- 57). With regard to health effects associated with PM 2.5 exposure across varying seasons in this review, Bell et al. (2008) reported higher PM 2.5 risk estimates for hospitalization for cardiovascular and respiratory diseases in the winter compared to other seasons. In comparison to the winter season, smaller statistically significant associations were also reported between PM 2.5 and cardiovascular morbidity for spring and autumn, and a positive, but statistically non

- significant association was observed for the summer months. In the case of mortality, Zanobetti and Schwartz (2009) reported a 4

- fold higher effect estimate for PM 2.5 - associated mortality for the spring as compared to

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Page 109 of 432 the wi nter. Taken together, these results provided emerging but limited evidence that individuals may be at greater risk of dying from higher exposures to PM 2.5 in the warmer months and may be at greater risk of PM 2.5 - associated hospitalization for cardiovascula r and respiratory diseases during colder months of the year ( U.S. EPA, 2011a, p. 2

- 58). Overall, the Policy Assessment observed that there are few studies presently available to deduce a general pattern in PM 2.5 - related risk across seasons. In addition,
t hese studies utilized 24

- hour exposure periods within each season to assess the PM 2.5 - associated health effects and do not provide information on health effects associated with a season

- long exposure to PM 2.5 . Due to these limitations in the currently avai lable evidence, the Policy Assessment concluded that there was no basis to consider a seasonal averaging time separate from a 24

- hour averaging time. Based on the above considerations, the Policy Assessment concluded that the currently available informatio n provided strong support for consideration of retaining the current annual and 24

- hour averaging times but does not provide support for considering alternative averaging times ( U.S. EPA, 2011a, p. 2

- 58). In addition, CASAC considered it appropriate to ret ain the current annual and 24

- hour averaging times for the primary PM 2.5 standards (Samet, 2010c, pp. 2 to 3). At the time of the proposal, the Administrator concurred with the staff conclusions and CASAC advice and proposed that the averaging times for th e primary PM 2.5 standards should continue to include annual and 24

- hour averages to protect against health effects associated with long

- and short

- term exposures. Furthermore, the Administrator provisionally concluded, consistent with conclusions reached i n the Policy Assessment and by CASAC, that the currently available information was too limited to support consideration of alternative averaging times to

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Page 110 of 432 establish a national standard with a shorter

- than 24

- hour averaging time or with a seasonal averaging time. The EPA received no significant public comments on the issue of averaging time for the PM 2.5 primary standards. The Administrator concurs with recommendations made by CASAC and the staff conclusions presented in the Policy Assessment and concludes, as proposed, that it is 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. 3. Form The “form” of a standard defines the air quality statistic that is to be compared to the level of the standard in determining whether an area attains the standard. In this review, the EPA considers whether currently available information supports retaining or revising the forms for the annual or 24

- hour PM 2.5 standards.

a. Annual Standard 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. This form was intended to represent a relatively stable measure of air quality and to characterize longer

- term area

- wide PM 2.5 concentrations, in conjunction with a 24

- hour standard designed to provide adequate protection against localized peak or seasonal PM 2.5 concentrations. The level of the 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

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Page 111 of 432 averaged (i.e., spatia l averaging) 53 (62 FR 38671 to 38672, July 18, 1997). The constraints were intended to ensure that spatial averaging would not result in inequities in the level of protection provided by the standard (62 FR 38672, July 18, 1997). This approach was consisten t with the epidemiological studies on which the PM 2.5 standard was primarily based, in which air quality data were generally averaged across multiple monitors in an area or were taken from a single monitor that was selected to represent community

- wide expo sures. In the last review, the EPA tightened the criteria for use of spatial averaging to
pr ovide increased protection for vulnerable populations exposed to PM 2.5 . This change was based in part on an analysis of the potential for disproportionate impacts on potentially at

- risk populations, which found that the highest concentrations in an area tend to be measured at monitors located in areas where the surrounding population is more likely to have lower education and income levels and higher percentages of minority populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section 5.3.6.1). In this review, as outlined in section III.B above and discussed more fully in section III.B.3 of the proposal, there now exist more health data such that the Integrat ed Science Assessment has identified persons from lower socioeconomic strata as an at

- risk population ( U.S. EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover, there now exist more years of PM 2.5 air quality data than were available in th e last review. Consideration in the Policy Assessment of the spatial variability across urban

53 Spatial averaging as part of the form of the annual PM 2.5 standard is unique to this standard and is not used with other PM standards nor with other NAAQS.

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Page 112 of 432 areas that was revealed by this expanded data base has raised questions as to whether an annual standard that allows for spatial averaging, even within specified constraints as narrowed in 2006 (71 FR 61165 to 61167, October 17, 2006) , would provide appropriate public health protection. In considering the potential for disproportionate impacts on at

- risk populations, the Policy Assessment considered an update of a n air quality analysis conducted for the last review ( U.S. EPA, 2011a, pp. 2

- 59 to 60; Schmidt, 2011, Analysis A). This analysis focused on determining whether the spatial averaging provisions, as modified in 2006, could introduce inequities in protection for at

- risk populations exposed to PM 2.5 . Specifically, the Policy Assessment considered whether persons of lower socioeconomic
status , minority groups, or different age groups (i.e., children or older adults) are more likely than the general population to live in areas in which the monitors recording the highest air quality values in an area are located. Data used in this analysis included demographic parameters measured at the Census Block or Census Block Group level, including percent minority population , percent minority subgroup population, percent of persons living below the poverty level, percent of persons 18 years of age or older, and percent of persons 65 years of age and older. In each candidate geographic area, data from the Census Block(s) or Ce nsus Block Group(s) surrounding the location of the monitoring site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) in which the highest air quality value was monitored were compared to the average of monitored values in the area. This ana lysis looked beyond areas that would meet the current spatial averaging criteria and considered all urban areas (i.e., Core Based Statistical Areas or CBSAs) with at least two valid annual desig n value monitors (Schmidt, 2011 , Analysis

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Page 113 of 432 A). Recognizing the limitations of such cross

- sectional analyses, the Policy Assessment observed that the highest concentrations in an area tend to be measured at monitors located in areas where the surrounding populations are more likely to live below the poverty line and to have higher percentage of minorities ( U.S. EPA, 2011a, p. 2

- 60). Based upon the analysis described above, the Policy Assessment concluded that the existing constraints on spatial averaging, as modified in 2006, may be inadequate to avoid substantially gr eater exposures in some areas, potentially resulting in disproportionate impacts on at

- risk populations of persons with lower SES levels as well as minorities. Therefore, the Policy Assessment 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. In doing so, the level of the annual PM 2.5 standard would be compared to measurements made at the monitoring site that represents

area - wide air quali ty recording the highest PM 2.5 concentrations 54 ( U.S. EPA, 2011a, p. 2

- 60). The CASAC agreed with staff conclusions that it was “reasonable” for the EPA to eliminate the spatial averaging provisions (Samet, 2010d, p. 2). Further, in CASAC’s comments on the first draft Policy Assessment, it noted, “Given mounting evidence showing that persons with lower SES levels are a susceptible group for PM

- related health risks, CASAC recommends that the provisions that allow for spatial averaging across monitors be elim inated for the reasons cited in the (first draft) Policy Assessment” (Samet, 2010c, p. 13). In its review of the second draft Policy Assessment, CASAC

54 As discussed in section VIII.B.1 below, the EPA is revising several terms associated with PM 2.5 monitor placement. Specifically, the EPA is revoking the term “community

oriented” and replacing it with the term “area

- wide” monitoring.

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Page 114 of 432 recognized “although much of the epidemiological research has been conducted using community

- wide average s, several key studies reference the nearest measurement site, so that some risk estimates are not necessarily biased by the averaging process. Further, the number of such studies is likely to expand in the future” (Samet, 2010d, pp. 1 to 2). Only two area s in the country used the initial spatial averaging provisions for demonstrating attainment with the primary annual PM 2.5 standard set in 1997 ( 70 FR 19847, April 14, 2005; U.S. EPA, 2006c ). Since these provisions were tightened in 2006 , no area has used spatial averaging to demonstrate attainment. No areas in the country are currently using the spatial averaging provisions to demonstrate attainment with the current primary annual PM 2.5 standard. In considering the Policy Assessment’s con clusions based on the results of the analysis discussed above and concern over the evidence of potential disproportionate impacts on at

- risk populations as well as CASAC advice, the Administrator proposed to revise the form of the annual PM 2.5 standard to eliminate the use of spatial averaging. Thus, the Administrator proposed revising the form of the annual PM 2.5 standard to compare the level of the standard with measurements from each “appropriate” monitor in

an area 55 with no allowance for spatial averagi ng. Thus, for an area with multiple monitors, the appropriate reporting monitor with the highest design value would determine the attainment status for that area. Of the commenters noted in section III.D.2 above who supported a more stringent

55 As discussed in se ction VIII.B.2.b below, the EPA concludes that PM 2.5 monitoring sites at micro

- and middle

- scale locations should be comparable to the annual standard unless the monitoring site has been approved by the Regional Administrator as a “relatively unique micro

- scale, or localized hot

- spot, or unique middle

- scale site.”

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Page 115 of 432 annual PM 2.5 standard, those who commented on the form of the annual PM 2.5 standard supported the EPA’s proposal to eliminat e the spatial averaging provisions. These commenters contended that the EPA ’s analyses of the potential impacts of spatial averaging, discussed a

bove and in the proposal (77 FR 38924) , demonstrate d that the current form results in uneven public health protection leading to disproportionate impacts on at

- risk populations. Specifically, the ALA and other environmental and public health commenters con tended that “ spatial averaging allows exposure of people to unhealthy levels of pollution at specific locales even within an area meeting the standard” (ALA et al., 2012, p. 23 ). These commenters particularly focused on the importance for low

- income and mi nority populations of eliminating the spatial averaging provisions. They concluded that spatial averaging “is an environmental justice concern because poor people are more likely to live near roads, depots, factories, ports, and other pollution sources.” I

d. p. 24. Other commenters (e.g., AAM , 2012 ; Dow , 2012 ) also supported the elimination of spatial averaging in order to “avoid potential disproportionate impacts on at

- risk populations” and to maximize “the benefits to public health of reducing the annual PM 2.5 standard.” However, these groups expressed conc ern that the elimination of spatial averaging, in combination with the requirement for near road monitors (as discussed in section VIII.B.3.b.i of the proposal) , would effectively and inappropriately inc rease the stringency of the annual PM 2.5 standard. This concern was also shared by other commenters who disagreed with the elimination of spatial averaging. For example, the Class of ’85 RRG emphasized concerns about increasing the stringency of the stand ard while providing few health

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Page 116 of 432 benefits if spatial averaging is eliminated, particularly in combination with the requirement for near

- road monitors. These commenters contended that “[b]ecause EPA proposes to use the readings from the highest single worst c ase monitor (rather than the average of all community area monitors), and since roadway monitoring locations will likely be worst case monitors, the proposed NAAQS will become more stringent without targeting the PM 2.5 species most harmful to human health” (Class of ’85 RRG, 2012, p. 6). Several commenters also ma intained that because spatial averaging is consistent with how air quality data are considered in the underlying epidemiological studies, such averaging should not be eliminated. Specifically, commenters including NAM et al. , AFPM, and ACC pointed out that PM 2.5 epidemiological studies use spatially averaged multi

- monitor concentrations, rather than the single highest monitor, when evaluating health effects. The refore, these commenters contended that allowing spatial averaging would make the PM 2.5 standard more consistent with the approaches used in the epidemiological studies upon which the standard is based. In addition, some commenters also contended that the EPA failed to consider whether modifying, rather than eliminating, the constraints on spatial averaging would have been sufficient to protect the public health. If so, th ese commenter s argued that “elimination of spatial averaging would go beyond what is r equisite to protect the public health” (NAM et al., 2012, p. 20)

. In considering the public comments on the form of the annual standard, the EPA recognizes a number of commenters agreed with the basis for the EPA’s proposal to eliminate spatial averaging. While other commenters expressed disagreement or concern with the proposed decision to eliminate the spatial averaging provisions, the A gency

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Page 117 of 432 notes that these comment er s did not challenge the analyses or considerations that provided the fundamental basis f or the Administrator’s propos ed decision

. Rather, these commenters generally raised concerns that eliminating the option for spatial averaging would increase the stringency of the standard, especially in light of additional monitoring sites in near

- road en vironments (as discussed in section VIII.B.3.b.1 below). The EPA does not agree with the comment that siting some monitors in near roadway environments makes the standard more stringent or impermissibly more stringent. As discussed in section VIII.B.3.b.i below, a significant fraction of the population lives in proximity to major roads, and these exposures occur in locations that represent ambient air. Monitoring in such areas does not make the standard more stringent than warranted, but rather affords the intended protection to the exposed populations, among them at

- risk populations, exposed to fine particles in these areas. Thus, the EPA has made a determination that monitors representative of area

- wide air quality may be compared to the annual standard. The 24

- hour and annual NAAQS are designed to protect the public with an adequate margin of safety, and this siting provision is fully consistent with providing the protection the standard is designed to provide and does not make the standard more stringent or more stringent than necessary. Monitors that are representative of area

- wide air quality may be compared to the annual standard. This is consistent with the use of monitoring data in the epidemiological studies that provide the primary basis for dete rmining the level of the annual standard. In addition, the EPA notes that the annual standard is designed to protect against both long

and short

- term exposures through controlling the broad distribution of air quality across

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Page 118 of 432 an area over time. 56 It is fully consistent with the protection the standard is designed to provide for near road monitors to be compared to the annual standard if the monitor is representative of area

- wide air quality. This does not make the standard either more stringent or impermissibly more stringent. In further considering these comments, the EPA notes that the stringency or level of protection provided by each NAAQS is not based solely on the form of the standard; rather, the four elements of the standard that together s erve to define each standard (i.e., indicator, averaging time, form, and level) must be considered collectively in evaluating the protection afforded by each standard. Therefore, the EPA considers these comments

are also appropriate to discuss collectively with other issues related to the appropriate level for annual standard, and are discussed below in section s III.E.4.c

- d. In reaching a final decision on the form of the annual standard, the Administrator considers the available analyses, CASAC advice, an d public comments on form as discussed above . She also considers related issues in the public comments on the level of the annual standard as discussed in section III.E.4.c below . She notes that even when the annual PM 2.5 standard was first set in 1997, th e spatial averaging provisions included constraints intended to ensure that inequities in the level of protection would not result. These constraints on spatial averaging were tightened in the last review, based on an analysis showing the potential for spa tial averaging to allow higher PM 2.5 concentrations in locations where subgroups within the general population we re potentially

56 This is in contrast to the 24

- hour standard which is designed to provide supplemental protection, addressing peak exposures that might not otherwise be addressed by the annual standard. Consiste nt with this, monitors are not required to be representative of area

- wide air quality to be compared to the 24

- hour standard. Formatted: Not Superscript/ Subscript

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Page 119 of 432 disproportionately exposed and hence, at disproportionate risk (e.g., low income and minority communities). The Administrator no tes that in proposing to eliminate spatial averaging altogether in this review, she has relied on further analyses in the current review (Schmidt, 2011 , Analysis A ). As discussed above and in the proposal (77 FR 38924) , these analyses showed that the curre nt constraints on spatial averaging may be inadequate in some areas to avoid substantially greater exposures for people living near monitors recording the highest PM 2.5 concentrations. Such exposures could result in disproportionate impacts to at

- risk popu lations, including low

- income populations as well

as minority groups. On this basis, t

he Administrator concludes that public health would not be protected with an adequate margin of safety in all locations , as required by law, if disproportionately higher exposure concentrations in at

- risk populations such as low income communities as well as minority communities were averaged together with lower concentrations measured at other sites in a large urban area. See ALA v. EPA , 134 F. 3d 388, 389 (D.C. Cir., 19 98) (“this court has held that ‘NAAQS must protect not only average healthy individuals, but also sensitive citizens such as children,’ and ‘if a pollutant adversely affects the health of these sensitive individuals, EPA must strengthen the entire national standard’”) and Coalition of Battery Recyclers Association v. EPA, 604 F 3d. 613, 617 (D.C. Cir., 2010) (“Petitioners’ assertion that the revised lead NAAQS is overprotective because it is more stringent than necessary to protect the entire population of young U.S. children ignores that the Clean Air Act allows protection of sensitive subpopulations.”) In reaching this conclusion, the Administrator further notes that her concern over possible disproportionate PM 2.5 - related health impacts in at

- risk populat ions

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Page 120 of 43 2 extends to populations living near important sources of PM 2.5 , including the large populations that live near major roadways. 57 In light of all of the above considerations, including consideration of available analyses, CASAC advice, and public comments, the Administrator concludes that the current form of the annual PM 2.5 standard should be revised to eliminate spatial averaging provisions. Thus, the level of the revised annual PM 2.5 standard established with this notice will be compared with me asurements from each appropriate monitor in an area, with no allowance for spatial averaging. The Administrator’s conclusions with regard to the appropriate level of the annual PM 2.5 standard to set in conjunction with this form are discussed below in sect ion III.E.4.d. b. 24

- Hour Standard In 1997, the EPA established the form of the 24

- hour PM 2.5 standard as the 98 th percentile of 24

- hour concentrations at each population

- oriented monitor within an area, averaged over three years (62 FR at 38671 to 38674, Jul y 18, 1997). The Agency selected the 98 th percentile as an appropriate balance between adequately limiting the occurrence of peak concentrations and providing increased stability which, when averaged over 3 years, facilitated effective health protection th rough the development of more stable implementation programs. By basing the form of the standard on concentrations measured at population

- oriented monitoring sites, the EPA intended to provide protection for people residing in or near localized areas of el evated concentrations. In the last review, in conjunction with lowering the level of the 24

- hour standard, the EPA retained

57 Section VIII.B.3.b.i below discusses public comments specifically related to the proposed requirement for near

- road monitors.

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Page 121 of 432 this form based in part on a comparison with the 99 th percentile form. 58 In revisiting the stability of a 98 th versus 99 th percentil e form for a 24

- hour standard intended to provide supplemental protection for a generally controlling annual standard, an analysis presented in the Policy Assessment considered air quality data reported in 2000 to 2008 to update our understanding of the ra tio between peak

- to

- mean PM 2.5 concentrations. This analysis provided evidence that the 98 th percentile value was a more stable metric than the 99 th percentile ( U.S. EPA, 2011a, Figure 2

- 2, p. 2

- 62). At the time of the proposal, the Agency recognized that the selection of the appropriate form of the 24

- hour standard includes maintaining adequate protection against peak 24

- hour concentrations while also providing a stable target for risk management programs, which serves to provide for the most effective pu blic health protection in the long run. 59 As in previous reviews, the EPA recognized that a concentration

- based form, compared to an exceedance

- based form, was more reflective of the health risks posed by elevated pollutant concentrations because such a for m gives proportionally greater weight to days when concentrations are well above the level of the standard than to days when the concentrations are just above the level of the standard. Further, the Agency provisionally concluded that a concentration

- based form, when

58 In reaching this final decision, the EPA recognized a technical problem associated with a potential bias in the method used to calculate the 98 th percentile concentration for this form. The EPA adjusted the sampling frequency requirement in order to reduce this bias. Accordingly, the Agency modified the final monitoring requirements such that areas that are within 5 percent of the standards are required to increase the sampling frequency to every day (71 FR 61164 to 61165, October 17, 2006). 59 See ATA III , 283 F.3d at 374

- 376 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.

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Page 122 of 432 averaged over three years, provided an appropriate balance between limiting peak pollutant concentrations and providing a stable regulatory target, thus facilitating the development of more stable implementation programs. In considering the information provided in the Policy Assessment and recognizing that the degree of public health protection likely to be afforded by a standard is a result of the combination of the form and the level of the standard, the Administrator proposed to retain the 98 th percentile form of the 24

- hour standard. The Administrator provisionally concluded that the 98 th percentile form represents an appropriate balance between adequately limiting the occurrence of peak concentrations and providing increased stability rel ative to an alternative 99 th percentile form. Few public commenters commented specifically on the form of the 24

- hour standard. None of the public commenters raised objections to continuing the use of a concentration

- based form for the 24

- hour standard. M any of the individuals and groups who supported a more stringent 24

- hour PM 2.5 standard noted in section III.D.2 above, however, recommended a more restrictive concentration

- based percentile form, specifically a 99 th percentile form. The limited number of these commenters who provided a specific rationale for this recommendation generally expressed their concern that the 98 th percentile form could allow too many days where concentrations exceeded the level of the standard, and thus fail to adequately protec t public health. Other public commenters representing s tate and local air agencies and industry groups generally supported retaining the current 98 th percentile form. In most cases, these groups expressed the overall view that the current 24

- hour PM 2.5 sta ndard, including the form of the current standard, should be retained.

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Page 123 of 432 The EPA notes that the viewpoints represented in this review are similar to comments submitted in the last review and through various NAAQS reviews. The EPA recognizes that the selecti on of the appropriate form includes maintaining adequate protection against peak 24

- hour values while also providing a stable target for risk management programs, which serves to provide for the most effective public health protection in the long run. 60 Not hing in the commenters’ views has provided a reason to change the Administrator’s previous conclusion regarding the appropriate balance represented in the proposed form of the 24

- hour PM 2.5 standard. Therefore, the Administrator concurs with staff conclusi ons presented in the Policy Assessment and CASAC recommendations and concludes that it is appropriate to retain the 98 th percentile form for the 24

- hour PM 2.5 standard. 4. Level In the last review, the EPA selected levels for the annual and the 24

- hour PM 2.5 standards using evidence of effects associated with periods of exposure that were most closely matched to the averaging time of each standard. Thus, as discussed in section III.A.1, the EPA relied upon evidence from long

- term exposure studies as the princi pal basis for selecting the level of the annual PM 2.5 standard that would protect against effects associated with long

- term exposures. The EPA relied upon evidence from the short

- term exposure studies as the principal basis for selecting the level of the 2 4

- hour

60 As just noted above, it is legitimate for the EPA to consider promotion of overall effectiveness of risk management programs designed to attain the NAAQS, including their overall stability, in setting a standard that is requisite to protect the public health. The context for the court’s discussion in ATA III is identical to that here; whether to adopt a 98 th percentile form for a 24

- hour standard intended to provide supplemental protection for a generally controlling annual standard.

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Page 124 of 432 PM 2.5 standard that would protect against effects associated with short

- term exposures. As summarized in section III.A.2 above, the 2006 decision to retain the level of the annual PM 2.5 standard at 15 μg/m 3 61 was challenged and on judicial review, the D.C. Circuit remanded the primary annual PM 2.5 standard to the EPA, finding that EPA’s explanation for its approach to setting the level of the annual standard was inadequate.

a. General Approach for Considering St andard Levels Building upon the lessons learned in the previous PM NAAQS reviews, in considering alternative standard levels supported by the currently available scientific information, the Policy Assessment used an approach that integrated evidence

- based and risk

- based considerations, took into account CASAC advice, and considered the issues raised by the court in remanding the primary annual PM 2.5 standard. Following the general approach outlined in section III.A.3 above, for the reasons discussed below, the Policy Assessment concluded it was appropriate to consider the protection afforded by the annual and 24

- hour standards taken together against mortality and morbidity effects associated with both long

- and short

- term PM 2.5 exposures. This was consistent with the approach taken in the review completed in 1997 rather than considering each standard separately, as was done in the review completed in 2006. Beyond looking directly at the relevant epidemiologic evidence, the Policy Assessment considered the ex tent to which specific alternative PM 2.5 standard levels were likely to reduce the nature and magnitude of both long

- term exposure

- related mortality

61 Throughou t this section, the annual standard levels are denoted as integer values for simplicity, although, as noted above in section II.B.1, Table 1, the annual standard level is defined to one decimal place, such that the current annual standard level is 15.0 μg/ m 3

. Alternative annual standard levels discussed in this section are similarly defined to one decimal place.

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Page 125 of 432 risk and short

- term exposure

- related mortality and morbidity risk ( U.S. EPA, 2011a, section 2.3.4.2; U.S.EP A, 2010a, section 4.2.2). As noted in section III.C.3 above, patterns of increasing estimated risk reductions were generally observed as either the annual or 24

- hour standard, or both, were reduced below the level of the current standards ( U.S. 2011a, Figu res 2

- 11 and 2

- 12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 5.2.3). Based on the quantitative risk assessment, the Policy Assessment observed, as discussed in section III.A.3, that analyses conducted for this and previous reviews demonstrated that much , if not most, of the aggregate risk associated with short

- term exposures results from the large number of days during which the 24

- hour average concentrations are in the low

- to mid

- range, below the peak 24

- hour concentrations ( U.S. EPA, 2011a, p. 2

- 9). Fu rthermore, as discussed in section III.C above and in section III.C.3 of the proposal, the Risk Assessment observed that alternative annual standard levels, when controlling, resulted in more consistent risk reductions across urban study areas, thereby pot entially providing a more consistent degree of public health protection ( U.S. EPA, 2010a, pp. 5

- 15 to 5

- 16). In contrast, the Risk Assessment noted that the results of simulating alternative suites of PM 2.5 standards including different combinations of alt ernative annual and 24

- hour standard levels suggested that an alternative 24

- hour standard level can produce additional estimated risk reductions beyond that provided by an alternative annual standard alone. However, the degree of estimated risk reduction provided by alternative 24

- hour standard levels was highly variable, in part due to the choice of rollback approached used ( U.S. EPA, 2010a, p. 5

- 17).

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Page 126 of 432 Based on its review of the second draft Policy Assessment, CASAC agreed with the EPA staff’s general app roach for translating the available epidemiological evidence, risk information, and air quality information into the basis for reaching conclusions on alternative standards for consideration. Furthermore, CASAC agreed “that it is appropriate to return to t

he strategy used in 1997 that considers the annual and the short

term standards together, with the annual standard as the controlling standard, and the short

- term standard supplementing the protection afforded by the annual standard” and “considers it appr opriate to place the greatest emphasis” on health effects judged to have evidence supportive of a causal or likely causal relationship as presented in the Integrated Science Assessment (Samet, 2010d, p. 1). Therefore, the Policy Assessment concluded, cons istent with specific CASAC advice, that it was appropriate to set a “generally controlling” annual standard that will lower a wide range of ambient 24

- hour concentrations. The Policy Assessment concluded this approach would likely reduce aggregate risks as sociated with both long

- and short

term exposures with more consistency than a generally controlling 24

- hour standard and would be the most effective and efficient way to reduce total PM 2.5 - related population risk and so provide appropriate protection. The staff believed this approach, in contrast to one focusing on a generally controlling 24

- hour standard, would likely reduce aggregate risks associated with both long

- and short

- term exposures with more consistency and would likely avoid setting national st andards that could result in relatively uneven protection across the country due to setting standards that were either more or less stringent than necessary in different geographical areas. The Policy Assessment recognized that an annual standard intended to serve as

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Page 127 of 432 the primary means for providing protection against effects associated with both long

- and short

- term PM 2.5 exposures cannot be expected to offer an adequate margin of safety against the effects of all short

- term PM 2.5 exposures. As a result, i n conjunction with a generally controlling annual standard, the Policy Assessment concluded it was appropriate to consider setting a 24

- hour standard to provide supplemental protection, particularly for areas with high peak

- to

- mean ratios possibly associat ed with strong local or seasonal sources, or PM 2.5 - related effects that may be associated with shorter

- than

daily exposure periods. At the time of the proposal, the Administrator agreed with the approach discussed in the Policy Assessment as summarized in section III.A.3 above, and supported by CASAC, of considering the protection afforded by the annual and 24

- hour standards taken together for mortality and morbidity effects associated with both long

- and short

term exposures to PM 2.5 . Furthermore, based on the evidence and quantitative risk assessment, the Administrator provisionally concluded it was appropriate to set a “generally controlling” annual standard that will lower a wide range of ambient 24

- hour concentrations, with a 24

- hour standard focused on providing supplemental protection, particularly for areas with high peak

- to

- mean ratios possibly associated with strong local or seasonal sources, or PM 2.5 - related effects that may be associated with shorter

- than daily exposure periods. The Administrator provisionally concluded this approach would likely reduce aggregate risks associated with both long

- and short

- term exposures more consistently than a generally controlling 24

- hour standard and would be the most effective and efficient way to reduce total PM 2

- 5 - related population risk. The Administrator is mindful that considering what standards are requisite to

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Page 128 of 432 protect public health with an adequate margin of safety requires pub lic health policy judgments that neither overstate nor understate the strength and limitations of the evidence or the appropriate inferences to be drawn from the evidence. At the time of the proposal, in considering how to translate the available information into appropriate standard levels, the Administrator weighed the available sc ientific information and associated uncertainties and limitations. For the purpose of determining what standard levels were appropriate to propose, the Administrator recognized, as did the EPA staff in the Policy Assessment, that there was no single factor or criterion that comprised the “correct” approach to weighing the various types of available evidence and information, but rather there were various approaches that were appropriate to consider. The Administrator further recognized that different evaluat ions of the evidence and other information before the Administrator could reflect placing different weight on the relative strengths and limitations of the scientific information, and different judgments could be made as to how such information should appr opriately be used in making public health policy decisions on standard levels. This recognition led the Administrator to consider various approaches to weighing the evidence so as to identify appropriate standard levels to propose. In so doing, the Adminis trator encouraged extensive public comment on alternative approaches to weighing the evidence and other information so as to inform her public health policy judgments before reaching final decisions on appropriate standard levels. b. Proposed Decisions on Sta ndard Levels i. Consideration of the Alternative Standard Levels in the Policy Assessment In recognizing the absence of a discernible population threshold below which

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Page 129 of 432 effects would not occur, the Policy Assessment’s general approach for identifying alternative annual standard levels that were appropriate to consider focused on characterizing th e part of the distribution of PM 2.5 concentrations in which we had the most confidence in the associations reported in the epidemiological studies and conversely where our confidence in the association became appreciably lower. The most direct approach to address this issue, consistent with CASAC advice (Samet, 2010c, p.10), was to consider epidemiological studies reporting confidence intervals around concentration

- response relationships ( U.S. EPA, 2011a, p. 2

- 63). Based on a thorough search of the availabl e evidence, the Policy Assessment identified only one study (Schwartz et al., 2008) that conducted a multi

- model analysis to characterize confidence intervals around the estimated concentration

- response relationship. The Policy Assessment concluded that th is single relevant analysis was too limited to serve as the principal basis for identifying alternative standard levels in this review ( U.S. EPA, 2011a, p. 2

- 70). The Policy Assessment explored other approaches that considered different statistical metrics to identify heuristic approaches to characteriz ing e the part of the distributions of long

- term mean PM 2.5 concentrations that were most influential in generating health effect estimates in long

- and short

- term epidemiological studies, a nd placed greatest weight on those studies that reported positive and statistically significant associations ( U.S. EPA, 2011a, p. 2

- 63). First, as discussed in section III.A.3 above, the Policy Assessment considered the statistical metric used in previous reviews. This approach recognized our assumption the EPA’s view that the strongest evidence of associations occurs at concentrations around the long

- term mean concentration. Thus, in

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Page 130 of 432 earlier reviews, the EPA focused on identifying standard levels that were somewhat below the long

- term mean concentrations reported in PM 2.5 e pidemiological xposure studies. The long

- term mean concentrations represented air quality data typically used in epidemiological analyses and provided a direct link between PM 2.5 concentrat ions and the observed health effects. Further, these data were available for all long

- and short

- term exposure studies analyzed and, therefore, represented the data set available for the broadest set of epidemiological studies. However, consistent with CA SAC’s comments on the second draft Policy Assessment 62 (Samet, 2010d, p. 2), in preparing the final Policy Assessment, the EPA staff explored ways to take into account additional information from epidemiological studies, when available (Rajan et al., 2011). These analyses focused on evaluating different statistical metrics, beyond the long

- term mean concentration, to characterize the part of the distribution of PM 2.5 concentrations in which staff continued to have confidence in the association s observed in epidemiological studies and below which there was a comparative lack of data such that the staff’s confidence in the relationship was appreciably less. This would also be the part of the distribution of PM 2.5 concentrations which had the most influence on generating the health effect estimates reported in epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment recognized there was no one percentile value within a given distribution that was the most appropriate or “correct” way to characterize where our confidence in the

62 While CASAC expressed the view that it would be most desirable to have information on concentration

- response relationships, they recognized that it would also be “preferable to have information on the concentrations that were most influential in generating the health effect estimates in individual studies” (Samet, 2010d, p. 2).

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Page 131 of 432 associations becomes appreciably lower. The Policy Assessment concluded that focusing on concentrations within the lower quartile of a distribution, such as the range from the 25 th to the 10 th perc entile, was reasonable to consider as a region within which we begin to have appreciably less confidence in the associations observed in epidemiological studies. 63 In the EPA staff’s view, considering lower PM 2.5 concentrations, down to the lowest concentra tion observed in a study, would be a highly uncertain basis for selecting alternative standard levels ( U.S. EPA, 2009a, p. 2

- 71). As outlined in section III.A.3 above, the Policy Assessment recognized that there were two types of population

- level informati on to consider in identifying the range of PM 2.5 concentrations which have the most influence on generating the health effect estimates reported in epidemiological studies. The most relevant information to consider was the number of health events (e.g., de aths, hospitalizations) occurring within a study population in relation to the distribution of PM 2.5 concentrations likely experienced by study participants. However, in recognizing that access to health event data may be restricted, and consistent with ad vice from CASAC (Samet 2010d, p.2), EPA staff also considered the number of participants within each study area, in relation to the

63 In the last review, staff believed it was appropriate to consider a level for an annual PM 2.5 standard that was somewhat below the averages of the long

- term concentrations across the cities in each of the key long

- term exposures studies, recognizing that the evidence of an association in any such study was strongest at and around the long

- term average where the data in the study are most concentrated. For example, the interquartile range of long

- term average concentrations within a study and a range within one standard deviation around the study mean were considered reasona ble approaches for characterizing the range over which the evidence of association is strongest (U.S. EPA, 2005, pp. 5

- 22 to 5

- 23). In this review, the Policy Assessment noted the interrelatedness of the distributional statistics and a range of one standar d deviation around the mean which contains approximately 68 percent of normally distributed data, in that one standard deviation below the mean falls between the 25 th and 10 th percentiles (U.S. EPA, 2011a, p. 2

- 71).

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Page 132 of 432 distribution of PM 2.5 concentrations (i.e., study population data), as a surrogate for health event data. In applying this approach, the Policy Assessment focused on identifying the part of the distribution of PM 2.5 concentrations which had the most influence on generating health effect estimates in epidemiological studies, as discussed in section III.A.3 above. As discussed b elow, in working with study investigators, the EPA staff was able to obtain health event data for three large multi

- city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et al., 2008) and population data for the same three studies and one additional long

- term exposure study (Miller et al., 2007) , as documented in a staff memorandum (Rajan et al., 2011). 64 For the three studies for which both health event and study population data were available, the EPA staff analyzed the reliability of usin g study population data as a surrogate for health event data. Based on these analyses, the EPA staff recognized that the 10 th and 25 th percentiles of the health event and study population distributions are nearly identical and concluded that the distributi on of population data can be a useful surrogate for event data, providing support for consideration of the study population data for Miller et al. (2007), for which health event data were not available (Rajan et al., 2011, Analysis 1 and Analysis 2, in par ticular, Table 1 and Figures 1 and 2). With regard to the long

- term mean PM 2.5 concentrations which are relevant to the first approach, Figures 1 through 3 ( U.S. EPA, 2011a, Figures 2

- 4, 2

- 5, 2

- 6, and 2

- 8) summarize data available for multi

- city, long

- and short

- term exposure studies that

64 The distributional statistical analys is of population

- level data built upon an earlier analysis that evaluated the distributions of air quality and associated population data for three long

- term exposure studies and three short

- term exposure studies (Schmidt et al., 2010, Analysis 2).

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Page 133 of 432 evaluated endpoints classified in the Integrated Science Assessment as having evidence of a causal or likely causal relationship or evidence suggestive of a causal relationship, showing the studies with long

- term mean PM 2. 5 concentrations below 17 μg/m 3

. 65 As discussed in more detail in section III.E.4.b of the proposal, Figures 1 and 3 summarize the health outcomes evaluated, relative risk estimates, air quality data, and geographic scope for long

- and short

- term exposure s tudies, respectively, that evaluated mortality (evidence of a causal relationship); cardiovascular effects (evidence of a causal relationship); and respiratory effects (evidence of a likely causal relationship) in the general population, as well as in olde r adults, an at

- risk population. Figure 2 provides this same summary information for long

- term exposure studies that evaluated respiratory effects (evidence of a likely causal relationship) in children, an at

- risk population, as well as developmental effec ts (evidence suggestive of a causal relationship).

65 Addi tional studies presented and assessed in the Integrated Science Assessment report effects at higher long

- term mean PM 2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2

- 1, 2

- 2, 7

- 6, and 7

- 7).

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Page 134 of 432 

### Figure 1. Summary of Effect Estimates (per 10 μg/m 3 

### ) and Air Quality Distributions for Multi - City, Long - term PM 2.5 

### Exposure Studies of the General Population and Older Adults 66 , 67 , 68

66 

### The air quality data for the Miller et al. (2007) study is discussed more fully in the Response to Comments document. 67 

### The Goss et al. (2004) study patients over the age of 6 years enrolled in the Cystic Fibrosis Foundation National Patient Reg istry in 1 999 and 2000. The average age of the study participants was 18.4 (SD 10). 68 The air quality data reported in Figure 1 of the proposal incorrectly identified the long

- term mean PM 2.5 concentration for this study as 14.3 μg/m 3 , which was the long

term mean PM 2.5 concentration for Lipfert et al. (2006b). This mistake was also made in the Figure 2

- 2 of the ISA (U.S. EPA, 2009a, p. 2

- 15). The EPA notes that footnote a for Figure 4 of Lipfert et al. (2006a) identifies that the morta lity risks for PM 2.5 are based on one year of air quality data (1999). However, this study does not report the long

- term mean PM 2.5 concentration for 1999. Rather, in Table 1 of Lipfert et al. (2006a), the study authors reported long

- term mean PM 2 .5 concen trations of 14.3 μg/m 3 f

or the years 1989 to 1996 and 14.6 μg/m 3 for the years 1997 to 2001.

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Page 135 of 432 

### Source: U.S. EPA, 2011a, Figure 2

- 4

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Page 136 of 432 

### Figure 2. Summary of Effect Estimates (per 10 μg/m 3 

### ) and Air Quality Distributions for Multi - City, Long - term PM 2.5 

### Exposure Studies of Children

Source: U.S. EPA, 2011a, Figure 2

- 5

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Page 137 of 432 

### Figure 3. Summary of Effect Estimates (per 10 μg/m 3 

### ) and Air Quality Distributions for Multi - City, Short - term PM 2.5 

### Exposure Studies of the General Population and Older Adults

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Page 138 of 432 

### Source: U.S. EPA, 2011a, Figure 2

- 6

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Page 139 of 432 With regard to consideration of additional information from epidemiological studies which was relevant to the second approach, the EPA staff compiled a summary of the range of PM 2.5 concentrations corresponding with the 25 th to 10 th percentiles of health event or study population data from the four multi

- city studies, for which distributional statistics are a vailable 69 ( U.S. EPA, 2011a, Figure 2

- 7; Rajan et al., 2011, Table 1). By considering this approach, one could focus on the range of PM 2.5 concentrations below the long

- term mean ambient concentrations over which we continue to have confidence in the associ ations observed in epidemiological studies (e.g., above the 25 th percentile) where commensurate public health protection could be obtained for PM 2.5 - related effects and, conversely, identify the range in the distribution below which our confidence in the a ssociations is appreciably less, to identify alternative annual standard levels. The mean PM 2.5 concentrations associated with the studies summarized in Figures 1, 2, and 3 and with the distributional statistics analyses (Rajan et al., 2011) are based on concentrations averaged across ambient monitors within each area included in a given study and then averaged across study areas to calculate an overall study mean concentration, as discussed above. Figure 4, discussed in more detail in section III.E.4.a of the proposal, summarizes statistical metrics for those key studies 70 included in

69 The EPA staff obtained health event data (e.g., number of deaths, hospitalizations) occurring in a study population for three multi

- city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et al., 2008) and study population data were obtained for the same three studies and one additional study (Miller et al., 2007) (U.S. EPA, 2011a, p.2

- 71). If health event or study population data were available for additional studies, the EPA could employ distributional statistics to identify the broader range of PM 2.5 concentrations that were most influential in generating health effect estimates in those studies. 70 Long

- and short

- term exposure studies considered “key” studies for consideration are summarized in Figure 4 and include those studies observing effects for which the evidence supported a causal or likely causal association. This figure represents the subset of multi

- city studies included in Figures 1 th rough 3 that provided evidence of positive and generally statistically significant effects associated in whole or in part with more recent air quality data, generally representing health effects associated with lower PM 2.5 concentrations than had previousl y been considered in the last review. The EPA notes that many of these studies evaluated multiple health endpoints, and not all of the effects evaluated provided evidence of positive and statistically significant effects. For purposes of informing the Admi nistrator’s decision on the appropriate standard levels, the Agency considers the full body of scientific evidence and focuses on

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Page 141 of 432 

### Figure 4. Translating Epidemiological Evidence from Multi - City Exposure Studies into an Annual PM 2.5 

### Standard 71

71 

### The a ir quality data for the Miller et al. (2007) study is discussed more fully in the Response to Comments document.

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Page 142 of 432 

### Source: U.S. EPA, 2011a, Fig ure 2

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Page 143 of 432 Figures 1, 2, and 3 that provide evidence of positive and generally statistically significant PM 2.5 - related effects, which are relevant to the two approaches for translating epidemiological evidence into potential standard levels as discussed above. The top of Figure 4 includes information for long

- term exposure studies evaluating health outcomes classified as having evidence of a cau s al or likely cau s al relationship with PM 2.5 exposures (long

- term mean PM 2.5 concentrations indicated by diamond symbols). The middle of Figure 4 includes information for short

- term exposure studies evaluating health outcomes classified as having evidence of a cau s al or likely cau s al relationship with PM 2.5 exposures (long

- term me an PM 2.5 concentrations indicated by triangle symbols). The bottom of Figure 4 includes information for long

- term exposures studies evaluating health outcomes classified as having evidence suggestive of a causal relationship (long

term mean PM 2.5 concentra tions indicated by square symbols). Figure 4 also summarizes the range of PM 2.5 concentrations corresponding with the 25 th (indicated by solid circles) to 10 th (indicated by open circles) percentiles of the health event or study population data from the fo ur multi

- city studies (highlighted in bold text) for which distributional statistics are available. In considering the evidence, the Policy Assessment recognized that NAAQS are standards set so as to provide requisite protection, neither more nor less stri ngent than necessary to protect public health, with an adequate margin of safety. This judgment, ultimately made by the Administrator, involves weighing the strength of the evidence and the inherent uncertainties and limitations of that evidence. Therefore , depending on the weight placed on different aspects of the evidence and inherent uncertainties, consideration of different alternative standard levels could be supported.***E.O. 12866 Review

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Page 144 of 432 Given the currently available evidence discussed in more detail in section III.E.4 .b of the proposal and considering the various approaches discussed above, the Policy Assessment concluded it was appropriate to focus on an annual standard level within a range of about 12 to 11 μg/m 3 ( U.S. EPA, 2011a, pp. 2

- 82, 2

- 101, and 2

- 106). As illu strated in Figure 4, the Policy Assessment recognized that a standard level of 12 μg/m 3 , at the upper end of this range, was somewhat below the long

- term mean PM 2.5 concentrations reported in all the multi

- city, long

- and short

- term exposure studies that
p rovided evidence of positive and statistically significant associations with health effects classified as having evidence of a causal or likely causal relationship, including premature mortality and hospitalizations and emergency department visits for card iovascular and respiratory effects as well as respiratory effects in children. Further, a level of 12 μg/m 3 would reflect consideration of additional population

- level information from such epidemiological studies in that it generally corresponded with appr oximately the 25 th percentile of the available distributions of health events data in the studies for which population

- level information was available. In addition, a level of 12 μg/m 3 would reflect some consideration of studies that provided more limited evidence of reproductive and developmental effects, which were suggestive of a causal relationship, in that it was about at the same level as the lowest long

- term mean PM 2.5 concentrations reported in such studies (see Figure 4). Alternatively the Policy Assessment recognized that an annual standard level of 11 μg/m 3 , at the lower end of this range, was well below the lowest long

- term mean PM 2.5 concentrations reported in all multi

- city long

- and short

- term exposure studies that provide evidence of positiv e and statistically significant associations with health effects***E.O. 12866 Review

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Page 145 of 432 classified as having evidence of a causal or likely causal relationship. A level of 11 μg/m 3 would reflect placing more weight on the distributions of health event and population data, in tha t this level was within the range of PM 2.5 concentrations corresponding to the 25 th and 10 th percentiles of all the available distributions of such data. In addition, a level of 11 μg/m 3 was somewhat below the lowest long

- term mean PM 2.5 concentrations reported in reproductive and developmental effects studies that are suggestive of a causal relationship. Thus, a level of 11 μg/m 3 would reflect an approach to translating the available evidence that places relatively more emphasis on margi n of safety considerations and less certain causal relationships than would a standard set at a higher level. Such a policy approach would tend to weigh uncertainties in the evidence in such a way as to avoid potentially underestimating PM 2.5 - related risks to public health. Further, recognizing the uncertainties inherent in identifying any particular point at which our confidence in reported associations becomes appreciably less, the Policy Assessment concluded that the available evidence did not provide a sufficient basis to consider alternative annual standard levels below 11 μg/m 3 ( U.S. EPA, 2011a, p. 2

- 81). The Policy Assessment also considered the extent to which the available evidence provided a basis for considering alternative annual standard levels above 12 μg/m 3 . As discussed below, the Policy Assessment concluded that it could be reasonable to consider a standard level up to 13 μg/m 3 based on a policy approach that weighed uncertainties in the evidence in such a way as to avoid potentially overest imating PM 2.5 - related risks to public health, especially to the extent that primary emphasis was placed on long

- term exposure studies as a basis for an annual standard level. A level of 13 μg/m 3 was somewhat below the long

- term mean PM 2.5 concentrations re ported in all but one of the***E.O. 12866 Review

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Page 146 of 432 long

- term exposure studies providing evidence of positive and statistically significant associations with PM 2.5 - related health effects classified as having a causal or likely causal relationship. As shown in Figure 4, the one l ong - term exposure study with a long

- term mean PM 2.5 concentration just below 13 μg/m 3 was the Miller et al., ( 2007) study

. However, as noted in section III.D.1.a of the proposal and discussed in more detail in the Response to Comments document , the Policy Assessment observed that in comparison to other long

- term exposure studies, the Miller et al. study was more limited in that it was based on only one year of air quality data and the one year was after the health outcomes were reported ( U.S. EPA, 2011a, pp . 2

- 81 to 2

- 82). Thus, to the extent that less weight was placed on the Miller et al. study than on other long

- term exposure studies with more robust air quality data, a level of 13 μg/m 3 could be considered as being protective of long

- term exposure relate d effects classified as having a causal or likely causal relationship. In also considering short

- term exposure studies, however, the Policy Assessment noted that a level of 13 μg/m 3 was below the long

- term mean PM 2.5 concentrations reported in most but not all such studies . In particular , two studies

-- Burnett et al. (2004) and Bell et al. (2008)

-- reported long

- term mean PM 2.5 concentration s of 12.8 and 12.9 μg/m 3 , respectively. In considering these studies, the Policy Assessment found no basis to conclu de that these two studies were any more limited or uncertain than the other short

- term exposure studies shown in Figures 3 and 4 ( U.S. EPA, 2011a, p. 2

- 82). On this basis, as discussed below, the Policy Assessment concluded that consideration of an annual standard level of 13 μg/m 3 would have implications for the degree of protection that would need to be provided by the 24

- hour standard, in order that the suite of PM 2.5 standards, taken together, would provide***E.O. 12866 Review

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Page 147 of 432 appropriate protection from effects on public health related to short

- term exposure to PM 2.5 ( U.S. EPA, 2011a, p. 2

- 82). The Policy Assessment also noted that a standard level of 13 μg/m 3 would reflect a judgment that the uncertainties in the epidemiological evidence as summarized in section III.B above and discussed in more detail in section III.B.2 of the proposal, including uncertainties related to the heterogeneity observed in the epid emiological studies in the eastern versus western parts of the U.S., the relative toxicity of PM 2.5 components, and the potential role of co

- pollutants, are too great to warrant placing any weight on the distributions of health event and population data th at extend down below the long

- term mean concentrations into the lower quartile of the data. This level would also reflect a judgment that the evidence from reproductive and developmental effects studies that is suggestive of a causal relationship was too u ncertain to support consideration of any lower level. Beyond evidence

- based considerations, the Policy Assessment also considered the extent to which the quantitative risk assessment supported consideration of these alternative standard levels or provided support for lower levels. In considering simulations of just meeting alternative annual standard levels within the range of 13 to 11 μg/m 3 (in conjunction with the current 24

- hour standard level of 35 μg/m 3 ), the Policy Assessment concluded that important public health improvements are associated with risk reductions estimated for standard levels of 13 and 12 μg/m 3 and noted that the level of 11 μg/m 3 was not included in the quantitative risk assessment. The Policy Assessment noted that the overall confiden ce in the quantitative risk estimates varied for the different alternative standard levels evaluated and was stronger for the higher levels and***E.O. 12866 Review

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Page 148 of 432 substantially lower for the lowest level evaluated (i.e., 10 μg/m 3 ). Based on the above considerations, the Poli cy Assessment concluded that the quantitative risk assessment provided support for considering alternative annual standard levels within a range of 13 to 11 μg/m 3 , but did not provide strong support for considering lower alternative standard levels ( U.S. E PA, 2011a, pp. 2

- 102 to 2

- 103). Taken together, the Policy Assessment concluded that consideration of alternative annual standard levels in the range of 13 to 11 μg/m 3 may be appropriate. Furthermore, the Policy Assessment concluded that the currently avai lable evidence most strongly supported consideration of an alternative annual standard level in the range of 12 to 11 μg/m 3 ( U.S. EPA, 2011a, p. 2

- 82). The Policy Assessment concluded that an alternative level within the range of 12 to 11 μg/m 3 would more fully take into consideration the available information from all long

- and short

- term PM 2.5 exposure studies, including studies of at

- risk populations, than would a higher level. This range also reflected placing weight on information from studies that hel ped to characterize the range of PM 2.5 concentrations over which we continue to have confidence in the associations observed in epidemiological studies, as well as the extent to which our confidence in the associations was appreciably less at lower concent rations. As recognized in sections III.A.3 and III.E.4.a above, an annual standard intended to serve as the primary means for providing protection from effects associated with both long

- and short

- term PM 2.5 exposures is not expected to provide appropriat e protection against the effects of all short

- term PM 2.5 exposures (unless established at a level so low as to undoubtedly provide more protection than necessary for long

- term exposures). Of particular concern are areas with high peak

- to

- mean ratios possib ly associated with***E.O. 12866 Review

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Page 149 of 432 strong local or seasonal sources, or PM 2.5 - related effects that may be associated with shorter

- than

- daily exposure periods. As a result, the Policy Assessment concluded that it was appropriate to consider alternative 24

- hour PM 2.5 standa rd levels that would supplement the protection provided by an annual standard. As outlined in section III.A.3 above, the Policy Assessment considered the available evidence from short

- term PM 2.5 exposure studies, as well as the uncertainties and limitatio ns in that evidence, to assess the degree to which alternative annual and 24

- hour PM 2.5 standards can be expected to reduce the estimated risks attributed to short

term fine particle exposures. In considering the available epidemiological evidence, the Pol icy Assessment took into account information from multi

- city studies as well as single

- city studies. The Policy Assessment considered the distributions of 24

- hour PM 2.5 concentrations reported in short

- term exposure studies, focusing on the 98 th percentile concentrations to match the form of the 24

- hour standard as discussed in section III.E.3.b above. In recognizing that the annual and 24

- hour standards work together to provide protection from effects associated with short

- term PM 2.5 exposures, the Policy Assessment also considered information on the long

- term mean PM 2.5 concentrations from these studies. In addition to considering the epidemiological evidence, the Policy Assessment considered air quality information, specifically peak

- to

- mean ratios using county

- level 24

- hour and annual design values, to characterize air quality patterns in areas possibly associated with strong local or seasonal sources. These patterns helped in understanding the extent to which different combinations of annual and 24

- hour standards would be consistent with the policy goal of setting a generally controlling annual standard with a***E.O. 12866 Review

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Page 150 of 432 24

- hour standard that provides supplemental protection especially for areas with high peak

- to

- mean ratios ( U.S. EPA, 2011a, p. 2

- 14). In consider ing the information provided by the short

- term exposure studies, the Policy Assessment recognized that to the extent these studies were conducted in areas that likely did not meet one or both of the current standards, such studies did not help inform the c haracterization of the potential public health improvements of alternative standards set at lower levels. Therefore, in considering the short

- term exposure studies to inform staff conclusions regarding levels of the 24

- hour standard that are appropriate to consider, the Policy Assessment placed greatest weight on studies conducted in areas that likely met both the current annual and 24

- hour standards. With regard to multi

- city studies that evaluated effects associated with short

- term PM 2.5 exposures, as sum marized in Figure 3 above and discussed in more detail in section III.E.4.c of the proposal, the Policy Assessment noted that, to the extent air quality distributions were reduced to reflect just meeting the current 24

- hour standard, additional protection would be anticipated for the effects observed in the three multi

- city studies with 98 th percentile values greater than 35 μg/m 3 (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98 th percentile values below 35 μg/m 3 , specifically 98 th percentile concentrations of 34.2, 34.3, and 34.8 μg/m 3 , the Policy Assessment noted that these studies reported long

- term mean PM 2.5 concentrations of 12.9, 13.2, and 13.4 μg/m 3 , respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). To the extent that consideration was given to revising the level of the annual standard, as discussed in section III.E.4.b of the proposal, the Policy Assessment recognized that potential changes assoc iated with***E.O. 12866 Review

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Page 151 of 432 meeting such an alternative annual standard would result in lowering risks associated with both long

- and short

- term PM 2.5 exposures. Consequently, in considering a 24

- hour standard that would operate in conjunction with an annual standard to pr ovide appropriate public health protection, the Policy Assessment noted that to the extent that the level of the annual standard was revised to within a range of 13 to 11 μg/m 3 , in particular in the range of 12 to 11 μg/ m 3 , additional protection would be p rovided for the long

- term effects observed in these multi

- city studies ( U.S. EPA, 2011a, p. 2

- 84). Based on this information, the Policy Assessment concluded that the multi

- city, short

- term exposure studies generally provided support for retaining the 24

- hour standard level at 35 μg/m 3 so long as the standard is in conjunction with an annual standard level revised to within a range of 12 to 11 μg/m 3 ( U.S. EPA, 2011a, p. 2

- 84). Alternatively, in conjunction with an annual standard level of 13 μg/m 3 , the Pol icy Assessment concluded that the multi

- city studies provided limited support for revising the 24

- hour standard level somewhat below 35 μg/m 3 , such as down to 30 μg/m 3 , based on one study (Bell et al., 2008) that reported positive and statistically signifi cant effects with an overall 98 th percentile value below the level of the current 24

- hour standard and an overall long

- term mean concentration slightly less than 13 μg/m 3 (Figure 3; U.S. EPA, 2011a, p. 2

- 84). In reaching staff conclusions regarding alter native 24

- hour standard levels that were appropriate to consider, the Policy Assessment also took into account relevant information from single

- city studies that evaluated effects associated with short

- term PM 2.5 exposures. The Policy Assessment recognized that these studies may provide additional insights regarding impacts on at

- risk populations and/or on areas with isolated peak concentrations.***E.O. 12866 Review

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Page 152 of 432 As discussed in more detail in section III.E.4.c of the proposal, although a number of single

- city studies repo rted effects at appreciably lower PM 2.5 concentrations than multi

- city short

- term exposure studies, the uncertainties and limitations associated with the single

- city studies were considerably greater than those associated with the multi

- city studies and, t

hus, the Policy Assessment concluded there was less confidence in using these studies as a basis for setting the level of a standard. Therefore, the Policy Assessment concluded that the multi

- city short

- term exposure studies provided the strongest evidence to inform decisions on the level of the 24

- hour standard, and the single

- city studies did not warrant consideration of 24

- hour standard levels different from those supported by the multi

- city studies ( U.S. EPA, 2011a, p. 2

- 88). In addition to considering the epidemiological evidence, the Policy Assessment took into account air quality information based on county

- level 24

- hour and annual design values to understand the public health implications of the alternative standard levels supported by the currently available scientific evidence, as discussed in section III.E.4.b above. Consistent with the general approach discussed in section III.A.3 above, the Policy Assessment considered the extent to which different combinations of alternative annual and 24

- hour standard levels based on the evidence would support the policy goal of lowering annual and 24

- hour air quality distributions by using the annual standard to be the “generally controlling” standard in conjunction with setting the 24

- hour standard to provid e supplemental protection ( U.S. EPA, 2011a, pp 2

- 88 to 2

- 91, Figure 2

- 10). Using information on the relationship of the 24

- hour and annual design values, the Policy Assessment examined the implications of three alternative suites of PM 2.5***E.O. 12866 Review

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Page 153 of 432 standards identi fied as appropriate to consider based on the currently available scientific evidence, as discussed above. The Policy Assessment concluded that an alternative suite of PM 2.5 standards that would include an annual standard level of 11 or 12 μg/m 3 and a 24

- ho ur standard with a level of 35 μg/m 3 (i.e., 11/35 or 12/35) would result in the annual standard being the generally controlling standard in most areas although the 24

- hour standard would continue to be the generally controlling standard in the Northwest ( U .S. EPA, 2011a, pp. 2

- 89 to 2

- 91 and Figure 2

- 10). These Northwest counties generally represented areas where the annual mean PM 2.5 concentrations have historically been low but where relatively high 24

- hour concentrations occur, often related to seasonal wood smoke emissions. Alternatively, combining an alternative annual standard of 13 μg/m 3 with a 24

- hour standard of 30 μg/m 3 would result in many more areas across the country in which the 24

- hour standard would likely become the controlling standard (the standard driving air quality distributions lower) than if an alternative annual standard of 12 or 11 μg/m 3 were paired with the current level of the 24

- hour standard (i.e., 35 μg/m 3 ). The Policy Assessment concluded that consideration of retaining the 24

- hour standard level at 35 μg/m 3 would reflect placing greatest weight on evidence from multi

city studies that reported positive and statistically significant associations with health effects classified as having a causal or likely causal relationship. In conjunction with lowering the annual standard level, especially within a range of 12 to 11 μg/m 3 , this alternative recognized additional public health protection against effects associated with short

- term PM 2.5 exposures which would be provided by lowerin g the annual standard such that revision to the 24

- hour standard would not be warranted ( U.S. EPA, 2011a, p. 2

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Page 154 of 432 Beyond evidence

- based considerations, the Policy Assessment also considered the extent to which the quantitative risk assessment supported consideration of retaining the current 24

- hour standard level or provided support for lower standard levels. In considering simulations of just meeting the current 24

- hour standard level of 35 μg/m 3 or alternative levels of 30 or 25 μg/m 3 (in conjunction w ith alternative annual standard levels within a range of 13 to 11 μg/m 3 ), the Policy Assessment noted that the overall confidence in the quantitative risk estimates varied for the different standard levels evaluated and was stronger for the higher levels a nd substantially lower for the lowest level evaluated (i.e., 25 μg/m 3 ). Based on this information, the Policy Assessment concluded that the quantitative risk assessment provided support for considering a 24

- hour standard level of 35 or 30 μg/m 3 (in conjunction with an alternative standard level within a range of 13 to 11 μg/m 3 ) but did not provide strong support for considering lower alternative 24

- hour standard levels ( U.S. EPA, 2011a, pp. 2

- 102 to 2

- 103). Taken together, the Policy Assessment concluded that while it was appropriate to consider an alternative 24

- hour standard level within a range of 35 to 30 μg/m 3 , the currently available evidence most strongly supported consideration for retaining the current 24

- hour standard level at 35 μg/m 3 in conjunction with lowering the level of the annual standard within a range of 12 to 11 μg/m 3 ( U.S. EPA, 2011a, p. 2

- 92). ii. CASAC Advice Based on its review of the second draft Policy Assessment, CASAC agreed with the general approach for translating the av ailable epidemiological evidence, risk information, and air quality information into the basis for reaching conclusions on alternative standards for consideration. Furthermore, CASAC agreed “that it is***E.O. 12866 Review

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Page 155 of 432 appropriate to return to the strategy used in 1997 tha t considers the annual and the short

term standards together, with the annual standard as the controlling standard, and the short

- term standard supplementing the protection afforded by the annual standard” and “considers it appropriate to place the greates t emphasis” on health effects judged to have evidence supportive of a causal or likely causal relationship as presented in the Integrated Science Assessment (Samet, 2010d, p. 1). CASAC concluded that the range of levels presented in the second draft Polic y Assessment (i.e., alternative annual standard levels within a range of 13 to 11 μg/m 3 and alternative 24

- hour standard levels within a range of 35 to 30 μg/m 3 ) “are supported by the epidemiological and toxicological evidence, as well as by the risk and a ir quality information compiled” in the Integrated Science Assessment, Risk Assessment, and second draft Policy Assessment. CASAC further noted that “[a]lthough there is increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a l evel below which there is no risk for adverse health effects)” (Samet, 2010d, p. ii). Although CASAC supported the alternative standard level ranges presented in the second draft Policy Assessment, it did not express support for any specific levels or

com binations of standards. Rather, CASAC encouraged the EPA to develop a clearer rationale in the final Policy Assessment for staff conclusions regarding annual and 24

- hour standards that were appropriate to consider, including consideration of the combinatio n of these standards supported by the available information (Samet, 2010d, p. ii). Specifically, in commenting on a distributional statistical analysis of air quality and associated population data presented in the second draft Policy Assessment, CASAC

enc ouraged staff to focus on information related to the concentrations that were most***E.O. 12866 Review

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Page 156 of 432 influential in generating the health effect estimates in individual studies to inform alternative standard levels. CASAC urged that the EPA redo that analysis using health

e vent or study population data (Samet, 2010d, p. 2). CASAC also commented that the approach presented in the second draft Policy Assessment to identify alternative 24

- hour standard levels which focused on peak

- to

- mean ratios was not relevant for informing t he actual level (Samet 2010d, p. 4). Further, they expressed the concern that the combinations of annual and 24

- hour standard levels discussed in the second draft Policy Assessment (i.e., in the range of 13 to 11 μg/m 3 for the annual standard, in conjuncti on with retaining the current 24

- hour PM 2.5 standard level of 35 μg/m 3 ; alternatively, revising the level of the 24

- hour standard to 30 μg/m 3 in conjunction with an annual standard level of 11 μg/m 3 ) “may not be adequately inclusive” and “[i]t was not clea r why, for example a daily standard of 30 μg/m 3 should only be considered in combination with an annual level of 11 μg/m 3 ” (Samet, 2010d, p. ii). CASAC encouraged the EPA to more clearly explain its rationale for identifying the 24

- hour/annual combinations that are appropriate for consideration (Samet 2010d, p. ii). In considering CASAC’s advice as well as public comment on the second draft Policy Assessment, the EPA staff conducted additional analyses and modified their conclusions regarding alternative s tandard levels that were appropriate to consider. The staff conclusions in the final Policy Assessment ( U.S. EPA, 2011a, section 2.3.4.4) differed somewhat from the alternative standard levels discussed in the second draft Policy Assessment ( U.S. EPA, 2010 f, section 2.3.4.3), upon which CASAC based its advice. Changes made in the final Policy Assessment were primarily focused on improving and clarifying the approach for translating the epidemiological evidence into a***E.O. 12866 Review

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Page 157 of 432 basis for staff conclusions on the broad est range of alternative standard levels supported by the available scientific information and more clearly articulating the rationale for the staff’s conclusions (Wegman, 2011, pp. 1 to 2). Consistent with CASAC’s advice to consider more information from epidemiological studies, as discussed in section III.E.4.b.1 above, the EPA analyzed additional population

- level data obtained from several study authors (Rajan et al., 2011). In transmitting the final Policy Assessment to CASAC, the Agency notified CASAC that the final staff conclusions reflected consideration of CASAC’s advice and that those staff conclusions were based, in part, on the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 2011, p.2). Thus, CASAC had an opportu nity to comment on the final Policy Assessment, but chose not to provide any additional comments or advice after receiving it. iii. Administrator’s Proposed Decisions on the Primary PM 2.5 Standard Levels In reaching her conclusions regarding appropriate altern ative standard levels to consider, the Administrator considered the epidemiological and other scientific evidence, estimates of risk reductions associated with just meeting alternative annual and/or 24

- hour standards, air quality analyses, related limitati ons and uncertainties, staff conclusions as presented in the Policy Assessment, and the advice of CASAC. As an initial matter, the Administrator agreed with the general approach discussed in the Policy Assessment as summarized in sections III.A.3 and III.E .4.a above, and supported by CASAC, of considering the protection afforded by the annual and 24

- hour standards taken together for mortality and morbidity effects associated with both long

- and short

term exposures to PM 2.5 ( 77 FR 38939 ) . Furthermore, based on the evidence and***E.O. 12866 Review

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Page 158 of 432 quantitative risk assessment, the Administrator provisionally concluded it is appropriate to set a “generally controlling” annual standard that will lower a wide range of ambient 24

- hour concentrations, with a 24

- hour standard focused on providing supplemental protection, particularly for areas with high peak

- to

- mean ratios possibly associated with strong local or seasonal sources, or PM 2.5 - related effects that may be associated with shorter

- than daily exposure periods. The Administrato r provisionally concluded this approach would likely reduce aggregate risks associated with both long

- and short

- term exposures more consistently than a generally controlling 24

- hour standard and would be the most effective and efficient way to reduce tota l PM 2

- 5 - related population risk. Id. In reaching decisions on alternative standard levels to propose, the Administrator judged that it was most appropriate to examine where the evidence of associations observed in the epidemiological studies was strongest and, conversely, where she had appreciably less confidence in the associations observed in the epidemiological studies. Based on the characterization and assessment of the epidemiological and other studies presented and assessed in the Integrated Science A ssessment and the Policy Assessment, the Administrator recognized the substantial increase in the number and diversity of studies available in this review including extended analyses of the seminal studies of long

- term PM 2.5 exposures (i.e., ACS and Harvar d Six Cities studies) as well as important new long

- term exposure studies (as summarized in Figures 1 and 2). Collectively, the Administrator noted that these studies, along with evidence available in the last review, provided consistent and stronger evide nce of an association with premature mortality, with the strongest evidence related to cardiovascular

- related mortality, at lower ambient concentrations than previously observed. The Administrator also recognized the***E.O. 12866 Review

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Page 159 of 432 availability of stronger evidence of mo rbidity effects associated with long

- term PM 2.5 exposures, including evidence of cardiovascular effects from the WHI study and respiratory effects, including decreased lung function growth, from the extended analyses for the Southern California Children’s Health Study. Furthermore, the Administrator recognized new U.S. multi

- city studies that greatly expanded and reinforced our understanding of mortality and morbidity effects associated with short

- term PM 2.5 exposures, providing stronger evidence of associations at ambient concentrations similar to those previously observed (as summarized in Figure 3). Id. at 38939

- 40. The newly available scientific evidence built upon the previous scientific data base to pro vide evidence of generally robust associations and to provide a basis for greater confidence in the reported associations than in the last review. The Administrator recognized that the weight of evidence, as evaluated in the Integrated Science Assessment, was strongest for health endpoints classified as having evidence of a causal relationship. These relationships included those between long

- and short

- term PM 2.5 exposures and mortality and cardiovascular effects. She recognized that the weight of evidence was also strong for health endpoints classified as having evidence of a likely causal relationship, which included those between long

- and short

- term PM 2.5 exposures and respiratory effects. In addition, the Administrator made note of the much more limited evidence for health endpoints classified as having evidence suggestive of a causal relationship, including developmental, reproductive and carcinogenic effects. Id . at 38940. Based on information discussed and presented in the Integrated Science Assessmen t, the Administrator recognized that health effects may occur over the full***E.O. 12866 Review

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Page 160 of 432 range of concentrations observed in the long

- and short

- term epidemiological studies and that no discernible threshold for any effects can be identified based on the currently avai lable evidence ( U.S. EPA, 2009a, section 2.4.3). She also recognized, in taking note of CASAC advice and the distributional statistics analysis discussed in section III.E.4.b.i above and in the Policy Assessment, that there was significantly greater confid ence in observed associations over certain parts of the air quality distributions in the studies, and conversely, that there was significantly diminished confidence in ascribing effects to concentrations toward the lower part of the distributions. Consiste nt with the general approach summarized in section III.A.3 above, and supported by CASAC as discussed in section III.E.4.a above, the Administrator generally agreed that it was appropriate to consider a level for an annual standard that was somewhat below the long

- term mean PM 2.5 concentrations reported in long

- and short

term exposure studies. In recognizing that the evidence of an association in any such study was strongest at and around the long

- term average where the data in the study are
most concentra ted, she understood that this approach did not provide a bright line for reaching decisions about appropriate standard levels. The Administrator noted that long

term mean PM 2.5 concentrations were available for each study considered and, therefore,
represe nted the most robust data set to inform her decisions on appropriate annual standard levels. She also noted that the overall study mean PM 2.5 concentrations were generally calculated based on monitored concentrations averaged across monitors in each study area with multiple monitors, referred to as a composite monitor concentration, in contrast to the highest concentration monitored in each study area, referred to as a maximum monitor concentration, which are used to determine whether an area meets a***E.O. 12866 Review

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Page 161 of 432 given standard. In considering such long

- term mean concentrations, the Administrator understood that it was appropriate to consider the weight of evidence for the health endpoints evaluated in such studies in giving weight to this information. Id

. Based on the i nformation summarized in Figure 4 above and presented in more detail in the Policy Assessment ( U.S. EPA, 2011a, chapter 2) for effects classified in the Integrated Science Assessment as having a causal or likely causal relationship with PM 2.5 exposures, th e Administrator observed an overall pattern of statistically significant associations reported in studies of long

- term PM 2.5 exposures with long

- term mean concentrations ranging from somewhat above the current standard level of 15 μg/m 3 down to the lowest mean concentration in such studies of 12.9 μg/m 3 (in Miller et al., 2007). 72 She observed a similar pattern of statistically significant associations in studies of short

- term PM 2.5 exposures with long

- term mean concentrations ranging from around 15 μg/m 3 do wn to 12.8 μg/m 3 (in Burnett et al., 2004). With regard to effects classified as providing evidence suggestive of a causal relationship, the Administrator observed a small number of long

- term exposure studies related to developmental and reproductive effec ts that reported statistically significant associations with overall study mean PM 2.5

72 The EPA notes that the Miller et al., (2007) study provides strong evidence of cardiovascular related effects associated with long

- term PM 2.5 exposures. At the time of the proposal, the EPA recognized the limited nature of the air quality data considered in this study (77 FR 38918, fn. 62). The EPA has reviewed those limitations, in conjunction with consideration of public comments received on the proposal as discussed in section III.E.4.c, in conjunction with reaching a final decision on the level of the annual standard.***E.O. 12866 Review

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Page 162 of 432 concentrations down to 11.9 μg/m 3 (in Bell et al., 2007). 73 Id. The Administrator also considered additional information from epidemiological studies, consistent with CASAC advice, to take into account the broader distribution of PM 2.5 concentrations and the degree of confidence in the observed associations over the broader air quality distribution. In considering this additional information, she understood that the Pol icy Assessment presented information on the 25 th and 10 th percentiles of the distributions of PM 2.5 concentrations available from four multi

- city studies to provide a general frame of reference as to the part of the distribution in which the data become
ap preciably more sparse and, thus, where her confidence in the associations observed in epidemiological studies would become appreciably less. As summarized in Figure 4 above, the Administrator took note of additional population

- level data that were availab le for four studies (Krewski et al., 2009; Miller et al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009), each of which reported statistically significant associations with health endpoints classified as having evidence of a causal relationship. In considering the long

- term PM 2.5 concentrations associated with the 25 th percentile values of the population

- level data for these four studies, she observed that these values ranged from somewhat above to somewhat below 12 μg/m 3 . The Administrator recogniz ed that three of these four studies represented include some of the strongest evidence available within the overall body of scientific evidence and noted that three of these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz,
73 With respect to suggestive evidence related to cancer, mutagenic, and genotoxic effects, the PM 2.5 concentrations reported in studies 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, 2009a, section 7.5), such that in selecting alt ernative standard levels that provide protection from mortality and cardiovascular and respiratory effects, it is reasonable to anticipate that protection will also be provided for carcinogenic effects.***E.O. 12866 Review

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Page 163 of 432 2009) were used as the basis for concentration

- response functio ns used in the quantitative risk assessment ( U.S. EPA, 2010a, section 3.3.3). In considering this information, the Administrator noted that CASAC advised that information about the long

- term PM 2.5 concentrations that were most influential in generating th e health effect estimates in epidemiological studies can help to inform selection of an appropriate annual standard level. However, t he Administrator also recognized that additional population

- level data were available for only these four studies and, ther efore, she believed that these studies comprised a more limited data set than one based on long

- term mean PM 2.5 concentrations for which data were available for all studies considered, as discussed above. The Administrator recognized, as summarized in sec tion III.B.2 above, that important uncertainties remain in the evidence and information considered in this review of the primary fine particle standards. These uncertainties are generally related to understanding the relative toxicity of the different comp onents in the fine particle mixture, the role of PM 2.5 in the complex ambient mixture, exposure measurement errors inherent in epidemiological studies based on concentrations measured at fixed monitor sites, and the nature, magnitude, and confidence in est imated risks related to increasingly lower ambient PM 2.5 concentrations. Furthermore, the Administrator noted that epidemiological studies have reported heterogeneity in responses both within and between cities and geographic regions across the U.S. She re cognized that this heterogeneity may be attributed, in part, to differences in fine particle composition in different regions and cities. The Administrator also recognized that t here are additional limitations associated with evidence for reproductive and developmental effects,***E.O. 12866 Review

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Page 164 of 432 identified as being suggestive of a causal relationship with long

- term PM 2.5 exposures, including: the limited number of studies evaluating such effects; uncertainties related to identifying the relevant exposure time periods of conc ern; 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. Id . at 38941. The Administrator was mindful that consi dering what standards were requisite to protect public health with an adequate margin of safety required public health policy judgments that neither overstated nor understated the strength and limitations of the evidence or the appr opriate inferences to be drawn from the evidence. In considering how to translate the available information into appropriate standard levels, the Administrator weighed the available scientific information and associated uncertainties and limitations. For the purpose of determinin g what standard levels were appropriate to propose, the Administrator recognized, as did EPA staff in the Policy Assessment, that there was no single factor or criterion that comprised the sole “correct” approach to weighing the various types of available evidence and information, but rather there were various approaches that are appropriate to consider. The Administrator further recognized that different evaluations of the evidence and other information before the Administrator could reflect placing differ ent weight on the relative strengths and limitations of the scientific information, and different judgments could be made as to how such information should appropriately be used in making public health policy decisions on standard levels. This recognition led the Administrator to consider various approaches to weighing the evidence so as to identify appropriate standard levels to propose. In so doing, the***E.O. 12866 Review

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Page 165 of 432 Administrator encouraged extensive public comment on alternative approaches to weighing the evidence a

nd other information so as to inform her public health policy judgments before reaching final decisions on appropriate standard levels. In considering the available information, the Administrator noted the advice of CASAC that the currently available scien tific information, including epidemiological and toxicological evidence as well as risk and air quality information, provided support for considering an annual standard level within a range of 13 to 11 μg/m 3 and a 24

- hour standard level within a range of 3 5 to 30 μg/m 3 . In addition, the Administrator recognized that the Policy Assessment concluded that the available evidence and risk

- based information support consideration of annual standard levels in the range of 13 to 11 μg/m 3 , and that the Policy Assessm ent also concluded that the evidence most strongly supported consideration of an annual standard level in the range of 12 to 11 μg/m 3 . In considering how the annual and 24

- hour standards work together to provide appropriate public health protection, the Administrator observed that CASAC did not express support for any specific levels or combinations of standards within these ranges . Nor did CASAC choose to comment on additional information and analyses presented in the final Policy Assessment prepared in response to CASAC’s recommendations on the second draft Policy Assessment (Wegman, 2011)

. In considering the extent to which the curre ntly available evidence and information provided support for specific standard levels within the ranges identified by CASAC and the Policy Assessment as appropriate for consideration, the Administrator initially considered standard levels within the range of 13 to 11 μg/m 3 for the annual standard. In so doing, the Administrator first considered the long

- term mean PM 2.5***E.O. 12866 Review

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Page 166 of 432 concentrations reported in studies of effects classified as having evidence of a causal or likely causal relationship, as summarized in Figu re 4 above and discussed more broadly above. She noted that a level at the upper end of this range would be below most but not all the overall study mean concentrations from the multi

- city studies of long

- and short

term exposures, whereas somewhat lower l evels within this range would be below all such overall study mean concentrations. In considering the appropriate weight to place on this information, the Administrator again noted that the evidence of an association in any such study was strongest at and around the long

- term average where the data in the study are most concentrated, and that long

- term mean PM 2.5 concentrations were available for each study considered and, therefore, represented the most robust data set to inform her decisions on appropriat e annual standard levels. Further, she was mindful that this approach did not provide a bright line for reaching decisions about appropriate standard levels. Id. In considering the long

- term mean PM 2.5 concentrations reported in studies of effects classified as having evidence suggestive of a causal relationship, as summarized in Figure 4 for reproductive and developmental effects, the Administrator noted that a level at the upper end of this range woul d be below the overall study mean concentration in one of the three studies, while levels in the mid

- to lower part of this range would be below the overall study mean concentrations in two or three of these studies. In considering the appropriate weight t o place on this information, the Administrator noted the very limited nature of this evidence of such effects and the additional uncertainties in these epidemiological studies relative to the studies that provide evidence of causal or likely causal relatio nships.***E.O. 12866 Review

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Page 167 of 432 The Administrator also considered the distributional analyses of population

- level information that were available from four of the epidemiological studies that provide evidence of effects identified as having a causal relationship with long

- or sho rt

- term PM 2.5 concentrations for annual standard levels within the same range of 13 to 11 μg/m 3

. In so doing, the Administrator first noted that a level in the mid

- part of this range generally corresponds with approximately the 25 th percentile of the distr ibutions of health events data available in three of these studies. The Administrator also noted that standard levels toward the upper part of this range would reflect placing substantially less weight on this information, whereas standard levels toward th e lower part of this range would reflect placing substantially more weight on this information. In considering this information, the Administrator noted that there was no bright line that delineates the part of the distribution of PM 2.5 concentrations with in which the data become appreciably more sparse and, thus, where her confidence in the associations observed in epidemiological studies became appreciably less. In considering mean PM 2.5 concentrations and distributional analyses from the various sets of epidemiological studies noted above, the Administrator was mindful, as noted above, that such studies typically report concentrations based on composite monitor distributions, in which concentrations may be averaged across multiple ambient monitors that ma y be present within each area included in a given study. Thus, a policy approach that used data based on composite monitors to identify potential alternative standard levels would inherently build in a margin of safety of some degree relative to an alterna tive standard level based on measurements at the monitor within an area that records the highest concentration, or the maximum monitor, since once a standard was***E.O. 12866 Review

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Page 168 of 432 set, concentrations at appropriate maximum monitors within an area were generally used to dete rmine whether an area meets a given standard. The Administrator also recognized that judgments about the appropriate weight to place on any of the factors discussed above should reflect consideration not only of the relative strength of the evidence but al so on the important uncertainties that remained in the evidence and information being considered in this review. The Administrator noted that the extent to which these uncertainties influenced judgments about appropriate annual standard levels within the r ange of 13 to 11 μg/m 3 would likely be greater for standard levels in the lower part of this range which would necessarily be based on fewer available studies than would higher levels within this range. Based on the above considerations, the Administrator concluded that it was appropriate to propose to set a level for the primary annual PM 2.5 standard within the range of 12 to 13 μg/m 3 . The Administrator provisionally concluded that a standard set within this range would reflect alternative approaches to ap propriately placing the most weight on the strongest available evidence, while placing less weight on much more limited evidence and on more uncertain analyses of information available from a relatively small number of studies. Further, she provisionally c oncluded that a standard level within this range would reflect alternative approaches to appropriately providing an adequate margin of safety for the populations at risk for the serious health effects classified as having evidence of a causal or likely cau sal relationship, depending in part on the emphasis placed on margin of safety considerations. The Administrator recognized that setting an annual standard level at the lower end of this range would reflect an approach that placed more emphasis on the entire body of the evidence, including the***E.O. 12866 Review

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Page 169 of 432 analysis of the distribution of air quality concentrations most influential in generating health effect estimates in the studies, and on margin of safety considerations, than would setting a level at the upper end of the range. Conversely, an approach that would support a level at the upper end of this range would place more emphasis on the remaining uncertainties in the evidence to avoid potentially overe stimating public health improvements, and would generally support a view that the uncertainties remaining in the evidence are such that the evidence does not too great to warrant setting a lower annual standard level. Id . at 38942

. At the time of the propos al, while the Administrator recognized that CASAC advised, and the Policy Assessment concluded, that the available scientific information provided support for considering a range that extended down to 11 μg/m 3 , she concluded that proposing such an extended range would reflect a public health policy approach that placed more weight on relatively limited evidence and more uncertain information and analyses than she considered appropriate at this time. Nonetheless, the Administrator solicited comment on a leve l down to 11 μg/m 3 as well as on approaches for translating scientific evidence and rationales that would support such a level. Such an approach might reflect a view that the uncertainties associated with the available scientific information warrant a high ly precautionary public health policy response that would incorporate a large margin of safety. The Administrator recognized that potential air quality changes associated with meeting an annual standard set at a level within the range of 12 to 13 μg/m 3 wil l result in lowering risks associated with both long

- and short

- term PM 2.5 exposures. However, the Administrator recognized that such an annual standard intended to serve as the primary***E.O. 12866 Review

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Page 170 of 432 means for providing protection from effects associated with both long

- and short

- term PM 2.5 exposures would not by itself be expected to offer requisite protection with an adequate margin of safety against the effects of all short

- term PM 2.5 exposures. As a result, in conjunction with proposing an annual standard level in th e range of 12 to 13 μg/m 3 , the Administrator provisionally concluded that it was appropriate to continue to provide supplemental protection by means of a 24

- hour standard set at the appropriate level, particularly for areas with high peak

- to

- mean ratios po ssibly associated with strong local or seasonal sources, or for PM 2.5 - related effects that may be associated with shorter

than

- daily exposure periods. Based on the approach discussed in section III.A.3 above, at the time of the proposal the Administrator relied upon evidence from the short

- term exposure studies as the principal basis for selecting the level of the 24

- hour standard. In considering these studies as a basis for the level of a 24

- hour standard, and having selected a 98 th percentile form for th e standard, the Administrator agreed with the focus in the Polic y Assessment of looking at the 98 th percentile values, as well as at the long

- term mean PM 2.5 concentrations in these studies. In considering the information provided by the short

- term exposur e studies, the Administrator recognized that to the extent these studies were conducted in areas that likely did not meet one or both of the current standards, such studies did not help inform the characterization of the potential public health improvement s of alternative standards set at lower levels. By reducing the PM 2.5 concentrations in such areas to just meet the current standards, the Administrator anticipated that additional public health protection would occur. Therefore, the Administrator focused on studies that reported positive and***E.O. 12866 Review

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Page 171 of 432 statistically significant assoc iations in areas that would likely have met both the current 24

- hour and annual standards. She also considered whether or not these studies were conducted in areas that would likely have met an annual standard level of 12 to 13 μg/m 3 to inform her decision regarding an appropriate 24

- hour standard level. As discussed in section III.E.4.a, consistent with the Policy Assessment, the Administrator concluded that multi

- city, short

- term exposure studies provided the strongest data set for informing her decisions on appropriate 24

- hour standard levels. The Administrator viewed the single

city, short

- term exposure studies as a much more limited data set providing mixed results and, therefore, she had less confidence in using those studies as a basis for setting the level of a 24

- hour standard. With regard to the limited number of single

- city studies that reported positive and statistically significant associations for a range of health endpoints related to short

- term PM 2.5 concentrations in areas that would likely h ave met the current suite of PM 2.5 standards, the Administrator recognized that many of those studies had significant limitations (e.g., limited statistical power, limited exposure data) or equivocal results (mixed results within the same study area) that made them unsuitable to form the basis for setting the level of a 24

- hour standard. With regard to multi

- city studies that evaluated effects associated with short

- term PM 2.5 exposures, the Administrator observed an overall pattern of positive and statisti cally significant associations in studies with 98 th percentile values averaged across study areas in the range of 45.8 to 34.2 μg/m 3 (Burnett et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The Administrator noted that, to the extent air quality distributions were reduced to reflect just meeting the current 24

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Page 172 of 432 would be anticipated for the effects observed in the three multi

- city st udies with 98 th percentile values greater than 35 μg/m 3 (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98 th percentile values below 35 μg/m 3 , specifically 98 th percentile concentrations of 34 .2, 34.3, and 34.8 μg/m 3 , the Administrator noted that these studies reported long

- term mean PM 2.5 concentrations of 12.9, 13.2, and 13.4 μg/m 3 , respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In proposing to revise the level of the annual standard to within the range of 12 to 13 μg/m 3 , as discussed above, the Administrator recognized that additional protection would be provided for the short

- term effects observed in these multi

- city studies in conjunction with an ann ual standard level of 12 μg/m 3 , and in two of these three studies in conjunction with an annual standard level of 13 μg/m 3 . She noted that the study

- wide mean concentrations were based on averaging across monitors within study areas and that compliance wit

h the standard would be based on concentrations measured at the monitor reporting the highest concentration within each area. The Administrator believed it would be reasonable to conclude that revision to the 24

- hour standard would not be appropriate in co njunction with an annual standard within this range. Based on the above considerations related to the epidemiological evidence, the Administrator provisionally concluded that it was appropriate to retain the level of the 24

- hour standard at 35 μg/m 3 , in co njunction with a revised annual standard level in the proposed range of 12 to 13 μg/m 3

. In addition to considering the epidemiological evidence, the Administrator also took into account air quality information based on county

- level 24

- hour and annual***E.O. 12866 Review

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Page 173 of 432 desig n values to understand the public health implications of retaining the 24

- hour standard level at 35 μg/m 3 in conjunction with an annual standard level within the proposed range of 12 to 13 μg/m 3 . She considered whether these suites of standards would meet a public health policy goal which included setting the annual standard to be the “generally controlling” standard in conjunction with setting the 24

- hour standard to provide supplemental protection to the extent that additional protection is warranted. As discussed above, the Administrator provisionally concluded that this approach was the most effective and efficient way to reduce total population risk associated with both long

and short

- term PM 2.5 exposures, resulting in more uniform protection across th e U.S. than the alternative of setting the 24

- hour standard to be the controlling standard. In considering the air quality information, the Administrator first recognized that there was no annual standard within the proposed range of levels, when combined with a 24

- hour standard at the proposed level of 35 μg/m 3 , for which the annual standard would be the generally controlling standard in all areas of the country. She further observed that such a suite of PM 2.5 standards with an annual standard level of 12 μg/m 3 would result in the annual standard as the generally controlling standard in most regions across the country, except for certain areas in the Northwest, where the annual mean PM 2.5 concentrations have historically been low but where relatively high 24

- hour concentrations occur, often related to seasonal wood smoke emissions ( U.S. EPA, 2011a, pp. 2

- 89 to 2

- 91, Figure 2

- 10). Although not explicitly delineated on Figure 2

- 10 in the Policy Assessment, an annual standard of 13 μg/m 3 would be somewhat less likely to be the generally controlling standard in some regions of the U.S. outside the Northwest in conjunction with a 24

- hour standard level of 35 μg/m 3

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Page 174 of 432 Taking the above considerations into account, the Administrator proposed to revise the level of the primary annual PM 2.5 standard from 15.0 μg/m 3 to within the range of 12.0 to 13.0 μg/m 3 and to retain the 24

- hour standard level at 35 μg/m 3 . In the Administrator’s judgment, such a suite of primary PM 2.5 standards and the rationale supporting such levels could reasonably be judged to reflect alternative approaches to the appropriate consideration of the strength of the available evidence and other information and their associated uncertainties and the advice of CASAC. The Administra tor recognized that the final suite of standards selected from within the proposed range of annual standard levels, or the broader range of annual standard levels on which public comment was solicited, must be clearly responsive to the issues raised by the D.C. Circuit’s remand of the 2006 primary annual PM 2.5 standard. Furthermore, at the time of the proposal, she recognized that the final suite of standards will reflect her ultimate judgment in the final rulemaking as to the suite of primary PM 2.5 standar ds that would be requisite to protect the public health with an adequate margin of safety from effects associated with fine particle exposures. The final judgment to be made by the Administrator will appropriately consider the requirement for a standard th at is neither more nor less stringent than necessary and will recognize that the CAA does not require that primary standards be set at a zero

- risk level, but rather at a level that reduces risk sufficiently so as to protect public health with an adequate m argin of safety. At the time of the proposal, having reached her provisional judgment to propose revising the annual standard level from 15.0 to within a range of 12.0 to 13.0 μg/m 3 and to propose retaining the 24

- hour standard level at 35 μg/m 3 , the Admi nistrator solicited public comment on this range of levels and on approaches to considering the available***E.O. 12866 Review

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Page 175 of 432 evidence and information that would support the choice of levels within this range. The Administrator also solicited public comment on alternative ann ual standard levels down to 11 μg/m 3 and on the combination of annual and 24

- hour standards that commenters may believe is appropriate, along with the approaches and rationales used to support such levels. In addition, given the importance the evidence fro m epidemiologic studies played in considering the appropriate annual and 24

- hour levels, the Administrator solicited public comment on issues related to translating epidemiological evidence into standards, including approaches for addressing the uncertaint ies and limitations associated with this evidence.

c. Comments on Standard Levels This section addresses comments that relate to consideration of the appropriate levels of the primary annual and 24

- hour PM 2.5 standards , including c omments on the general appro ach used by the EPA to translate the available scientific information into standard levels and how specific PM 2.5 exposure studies should be considered as a basis for the standard level s. These comments on standard levels expand upon the more general comments that either support ed or oppose d any change to the current suite of primary PM 2.5 standards , which are addressed above in section III.D.2. 74 As explained there, one group of commenters generally opposed any change to the current primary PM 2.5 stan dards and more specifically disagreed with the basis for the EPA’s proposal to revise the annual standard level. Another group of commenters supported revising the current suite of primary PM 2.5 standards to provide increased public health protection. Some commenters in this second group argued that both the annual and 24

- hour standard levels

74 Specific comments on the forms of the annual and 24

- hour standards are addressed in section III.E.3.a and III.E.3.b, respectively.***E.O. 12866 Review

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Page 176 of 432 should be lowered while other commenters in this group agreed with the EPA’s proposal to retain the level of the 24

- hour standard in conjunction with revising the leve l of the annual standard. While generally supporting the EPA’s proposal to lower the level of the annual standard, many commenters in this group disagreed that a level within the EPA’s proposed range was adequately protective and supported a level of 11 μg /m 3 or below. i. Annual Standard Level The group of commenters opposed to any change to the current suite of primary PM 2.5 standards generally raised questions regarding the underlying scientific evidence, including the causal determinations reached in the Integrated Science Assessment, and focused strongly on the uncertainties they saw in the scientific evidence as a basis f or their conclusion that no changes to the current standard levels were warranted. In commenting on the proposed standard levels, these commenters typically relied on the arguments summarized and addressed above in section III.D.2 as to why they believed i t was inappropriate for the EPA to make any revisions to the suite of primary PM 2.5 standards. That is, they asserted that the EPA’s causal determinations were not adequately supported by the underlying scientific information; the biological plausibility

o f health effects observed in epidemiologic al studi es has not been demonstrated in controlled human exposure and toxicological studies ; uncertainties in the underlying health science are as great or greater than in 2006; there is no evidence of greater risk since the last review to justify tightening the current annual PM 2.5 standard; and “new” studies not included in the Integrated Science Assessment continue to increase uncertainty about possible health risks associated with exposure to PM 2.5 . With regard to the level of the annual standard, these commenters strongly***E.O. 12866 Review

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Page 177 of 432 disagreed with the Agency’s proposed decision to revise the level to within a range of 12 to 13 μg/m 3 and argued that the current standard level of 15 μg/m 3 should be retained

. For example, UARG, API, and other commenters in this group raised a number of issues that they asserted called into question the EPA’s interpretation of the epidemiological evidence to support revising the annual standard level. These commenters raised specifi c questions related to the general approach used by the EPA to translate the air quality and other information from specific epidemiological studies into standard levels, including : (1) the EPA’s approach for using composite monitor air quality distributio ns reported in epidemiological studies to select a standard level that would be compared to measurements at the monitor recording the highest value in an area to determine compliance with the standard; (2) the appropriate exposure period for effects observ ed in long

- term exposure mortality studies; and (3) the use of the EPA’s analysis of distribution s of underlying population

- level data (i.e., health event and study population data) for those epidemiological studies for which such information was available . These commenters also raised questions regarding the EPA’s consideration of specific scientific evidence as a basis for setting a standard level, including : (4) evidence of respiratory morbidity effects in long

- term exposure studies and (5) more limited evidence of health effects which have been categorized in the Integrated Science Assessment as suggestive of a causal relationship (i.e., developmental and reproductive outcomes ). These comments are discussed in turn below. (1) Some commenters in this grou p argued that one reason why they believe there is no basis for setting a standard level below 15 μg/m 3 is that the air quality metric from epidemiological studies that the EPA relied on in the proposal is not the same metric that***E.O. 12866 Review

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Page 178 of 432 will be compared to the l evel of the standard to determine compliance with the standard. That is, commenters noted that the long

- term mean PM 2.5 concentrations that the EPA considered, shown in Figure 4 above, are composite monitor mean concentrations (i.e., concentrations averaged across multiple monitors within areas with more than one monitor), whereas the PM 2.5 concentrations that will be compared to the level of the standard are maximum monitor concentrations (i.e., the concent ration measured by the monitor within an area reporting the highest concentration). This comment was presented most specifically in UARG’s comments (UARG, 2012, Attachment 1, pp. 2 to 6), which raised two overarching issues as discussed below. First, the c ommenter noted that the EPA’s approach of considering composite monitor mean PM 2.5 concentrations in selecting a standard level, and then comparing the maximum monitor mean PM 2.5 concentration in each area to the standard level when the standard is impleme nted, was characterized in the proposal as inherently having the potential to build in a margin of safety (UARG, 2012, Attachment 1, p. 4, citing 77 FR 38905). The commenter asserted that the Administrator is ignoring this distinction between composite and maximum monitor concentrations, and that this approach creates an unwarranted case for lowering the standard level, since in the commenter’s view, it would result in a margin of safety that would be arbitrary, not based on evidence, and unquantified (UARG , 2012, Attachment 1, p. 4). In support of this view, the commenter asserted that there is a significant difference between composite monitor mean PM 2.5 concentrations and maximum monitor mean PM 2.5 concentrations. The commenter asserted that the maximum m onitor value will always be higher than the composite monitor value (except in areas that contain only a single monitor), such that when an area***E.O. 12866 Review

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Page 179 of 432 just attains the NAAQS, that area’s composite monitor long

- term mean PM 2.5 concentration will be lower than the level of the standard (UARG, 2012, Attachment 1, p. 3). Second, the commenter asserted that a more “reasoned and consistent approach would be to decide on a mean composite monitor PM 2.5 level that should be achieved and then identify the maximum monitor l evel that would result in that composite value” (UARG, 2012, Attachment 1, p. 4). The commenter conducted an analysis of maximum monitor versus composite monitor annual mean PM 2.5 concentrations using monitoring data 75 from 2006 to 2008 and presented result s averaged across areas within two groups (i.e., those with design values 76 above the current standard level and those with design values just below the current standard level) to illustrate their suggested alternative approach. The commenter interpreted th is analysis as showing that the composite monitor long

- term mean PM 2.5 concentrations from the subset of the epidemiological studies shown in Figure 4 (of the proposal and above) that the commenter considered to be an appropriate focus for this analysis wo uld be achieved across the U.S. if the current annual NAAQS of 15 μg/m 3 is retained and attained

. The commenter considered the subset of epidemiological studies that included only long

- term exposures studies of effects for which the evidence is categorized as causal or likely causal, but did not consider short

term exposure studies. On this basis, the commenter asserted that attaining the current annual PM 2.5 standard would result in composite monitor long

- term mean concentrations in all areas that would be generally within or below the range of the composite monitor

75 The commenter indicated that this analysis was based on monitoring data for every core based statistical area (CBSA) in the EPA’s Air Quality System (AQS) database. 76 The des ign value is the air quality statistic that is compared to the level of the NAAQS to determine the attainment status of a given area.***E.O. 12866 Review

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Page 180 of 432 long

- term mean concentrations from such studies and, as a result, there is no reason to lower the level of the current annual NAAQS. In considering the first issue related to the EPA’s approach, the EPA notes that in proposing to revise both the form and level of the annual standard, the Administrator clearly took into account the distinction between the composite monitor long

- term mean PM 2.5 concentrations from the epidemiological studies, consi dered as a basis for selecting an annual standard level, and maximum monitor long

- term mean PM 2.5 concentrations. In deciding to focus on the composite monitor long

- term mean concentrations in selecting the standard level, and on the maximum monitor concen trations in selecting the form of the standard (i.e., consistent with proposing to eliminate the option for spatial averaging across monitors within an area when implementing the standard 77 ), the Administrator reasonably considered the distinction between t hese metrics in a manner that was consistent with advice from CASAC (Samet et al., 2010d, pp. 2 to 3). As noted above in section III.A.3, the EPA recognizes that a statistical metric (e.g., the mean of a distribution) based on maximum monitor concentration s may be identical to or above the same statistical metric based on composite monitor concentrations. More specifically, many areas have only one monitor, in which case the composite and maximum monitor concentrations are identical. Based on the most recen t data from the EPA’s AQS from 2009 to 2011 in the 331 CBSAs in which valid PM 2.5 data are available, as discussed in Frank (2012a, Table 5), there were 20 8 such areas (with design values ranging up to about 15 μg/m 3 ). Frank (2012a) also observed that othe r areas have multiple monitors with composite and maximum monitor mean PM 2.5 concentrations that were the same or relatively close, with 5 7 areas in which the
77 As discussed above in section III.E.3.a.***E.O. 12866 Review

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Page 181 of 432 maximum monitor mean concentration was no more than 0.5 μg/m 3 higher than the composite monitor m ean concentration and 56 areas in which the difference was between 0.6 and 2 μg/m 3 . Further, there were only a few other areas in which the maximum monitor mean concentration was appreciably higher than the composite monitor mean concentration, such as areas in which some monitors may be separately impacted by local sources. There were only 10 such areas in the country in which the maximum monitor mean concentration was between 2 to 6 μg/m 3 higher than the composite monitor concentration (Frank, 2012a, Table 4). 78 Thus, the EPA does not agree that there is a significant difference between composite monitor mean PM 2.5 concentrations and maximum monitor mean PM 2.5 concentrations in the large majority of areas across the country. In proposing to revise the form of the annual PM 2.5 standard, as discussed above in section III.E.3.a, the EPA not ed that when an annual PM 2.5 standard was first set in 1997, the form of the standard included the option for averaging across measurements at appropriate monitoring sites within an area, generally consistent with the composite monitor approach used in epi demiological studies, with some constraints intended to ensure that spatial averaging would not result in inequities in the level of protection for communities within large metropolitan areas. In the last review the EPA tightened the constraints on spatial averaging, and in this review has eliminated the option altogether, on the basis of analyses in each review that showed that such constraints may be inadequate to avoid substantially greater exposures for people living in locations around

78 The average difference between the maximum and composite design value among the 123 CBSAs with two or more monitors is 0.8 μg/m 3 and the median difference is 0.6 μg/m 3 . The 25 th and 75 th percentiles are 0.3 and 1.0 μg/m 3 , respectively (Frank, 2012a, p. 4)

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Page 182 of 432 the monitors rec ording the highest PM 2.5 concentrations in some areas, potentially resulting in disproportionate impacts on at

- risk populations of persons with lower SES levels as well as minorities . In light of these analyses, and consistent with the Administrator’s deci sion to revise the form of the annual PM 2.5 standard by eliminating the option for spatial averaging, the EPA continues to conclude that a standard level based on consideration of long

- term mean concentrations from composite monitors, and applied at each m onitor within an area including the monitor measuring the highest concentration, is the appropriate approach to use in setting a standard that will protect public health, including the health of at

- risk populations, with an adequate margin of safety, as re quired by the CAA. The EPA acknowledges that at proposal, the Agency characterized the approach of using maximum monitor concentrations to determine compliance with the standard, while selecting the standard level based on consideration of composite monito r concentrations, as one that inherently had the potential to build in a margin of safety (77 FR 38905), and CASAC reiterated that view in supporting the EPA’s approach (Samet, 2010d, p. 3). Nonetheless, in light of the discussion above, the EPA more speci fically recognizes that this approach does not build in any margin of safety in the large number of areas across the country with only one monitor. Further, based on the analyses done to inform consideration of the form of the standard (Schmidt, 2011, Anal ysis A), the EPA concludes that this approach does not provide a margin of safety for the at

- risk populations that live around the monitor measuring the highest concentration, such as in those few areas in which the maximum monitor concentration is appreci ably higher than the composite monitor concentration. Rather, this approach properly treats those at

- risk***E.O. 12866 Review

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Page 183 of 432 populations the same way it does the broader populations that live in areas with only one monitor, by providing the same degree of protection for thos e at

- risk populations that would otherwise be disproportionately impacted as it does for the broader populations in other areas, While the EPA recognizes that this approach can result in some additional margin of safety for the subset of areas with multipl e monitors in which at

- risk populations may not be disproportionately represented in areas around the maximum monitor, which may be the case in areas with relatively small differences between the maximum and composite monitor concentrations, the EPA notes that this margin would be relatively small in such areas. Based on the above considerations, the EPA does not agree that the Agency’s approach of using maximum monitor concentrations to determine compliance with the standard, while selecting the standard l evel based on consideration of composite monitor concentrations creates an unwarranted case for lowering the standard level based on a margin of safety that would be arbitrary, not based on evidence, or lack quantification. The EPA recognizes that setting a standard to protect public health, including the health of at

- risk populations, with an adequate margin of safety, depends upon selecting a standard level sufficiently below where the EPA has found the strongest evidence of health effects so as to provid e such protection, and that the EPA’s approach regarding consideration of composite and maximum monitor concentrations is intended to, and does, serve to address this requirement as part of and not separate from the selection of an appropriate standard lev el based on the health effects evidence. In considering the second issue related to the commenter’s suggested alternative approach, the EPA strongly disagrees with the commenter’s view that a more “reasoned***E.O. 12866 Review

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Page 1 84 of 432 and consistent approach would be to decide on a m ean composite monitor PM 2.5 level that should be achieved and then identify the maximum monitor level that would result in that composite value” (UARG, 2012, Attachment 1, p. 4). As discussed above, the EPA notes that for areas with only one monitor, or wi th multiple monitors that measure concentrations that are very close in magnitude, the maximum monitor level that would limit the composite monitor PM 2.5 level to be no greater than the level that should be achieved to protect public health with an adequat e margin of safety, would essentially be the same as that composite monitor level. Further, as discussed above, even for areas in which the maximum monitor concentration is appreciably higher than other monitor concentrations within the same area, public h ealth would not be protected with an adequate margin of safety if the disproportionately higher exposures of at

- risk, susceptible populations around the monitor measuring the highest concentration were in essence averaged away with measurements from monito rs in other locations within large urban areas. Further, the commenter’s suggested approach would be based on annual average PM 2.5 concentrations that have been measured over some past time period. Such an approach would reflect the air quality that existe d in the past, but it would not necessarily provide appropriate constraints on the range of concentrations that would be allowed by such a standard in the future, when relationships between maximum and composite monitor concentrations in areas across the c ountry may be different. For these reasons, the EPA fundamentally rejects the commenter’s suggested approach because in the EPA’s view it would results in a standard that is inconsistent with EPA’s reasons for concluding that a standard of 15ug/m3 demonstrably would does not protect public health, including providing protection for at

- risk populations, with an adequate margin of***E.O. 12866 Review

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Page 185 of 432 safety in areas across the country. More specifically, in further considering the commenter’s analysis of design values b ased on maximum versus composite monitor annual mean PM 2.5 concentrations using monitoring data from 2006 to 2008 which they assert supports retaining the current standard level of 15 μg/m 3 , the EPA finds flaws with the numerical results and the scope of t he analysis, as well as flaws in the commenter’s translation of the analysis results into the basis for selecting an annual standard level. In considering the commenter’s analysis, the EPA notes that the analysis compared maximum versus composite monitor a nnual mean PM 2.5 concentrations, averaged over 3 years, for two groups of areas: (1) areas with design values that exceed the current annual standard level (i.e., greater than 15.0 μg/m 3 ) and (2) areas with design values that are just attaining the current annual standard (i.e., between 14.5 and 15.0 μg/m 3 ). 79 The commenter indicated that they used the full body of PM 2.5 monitoring data from the EPA’s AQS database (UARG, 2012, Attachment 1, p. 4), In attempting to reproduce the commenter’s results, the EPA r epeated the calculations using only valid air quality data (i.e., data that meet data completeness and monitor siting criteria) from the AQS database for the same time period (Frank, 2012a). 80 Based on this corrected analysis, the EPA finds that the composi te monitor concentrations averaged across the
79 For the first group of areas (which included 33 areas), this analysis calculated an average across the areas of maximum monitor annual mean PM 2.5 concentrations, averaged over 3 years, of 17.2 μg/m 3 compared to an average of composite monitor concentra tions of 14.3 μg/m 3 . For the second group of areas (which included 11 areas), this analysis calculated an average across the areas of maximum monitor annual mean concentrations, averaged over 3 years, of 14.8 μg/m 3 compared to an average of composite monit or concentrations of 13.6 μg/m 3 (UARG, 2012, Attachment 1, Table 1). 80 The EPA notes that the Frank (2012a) analysis is similar to an earlier EPA staff analysis (Hassett

- Sipple et al., 2010), which used air quality data from EPA’s AQS database to compare m aximum versus composite monitor long

- term mean PM 2.5 concentrations across the study areas in six selected multi

- city epidemiological studies.***E.O. 12866 Review

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Page 186 of 432 areas within each group are somewhat higher than those calculated by the commenter, and the average differences between the maximum and composite monitor concentrations are somewhat smaller (Frank, 2012a, Table 3). 81 Notably, the difference between the maximum and composite monitor average concentrations for the second group of areas is substantially reduced in the corrected analysis, such that the difference (averaged across the 10 areas with valid data in the s econd group) is approximately 0.5 μg/m 3 , not 1.2 μg/m 3 as in the commenter’s analysis. In addition, the commenter’s analysis compared the average of the composite monitors to the average of the maximum monitors for each subset of areas. This comparison of averages across all the areas in each subset masks the fact that the large majority of areas across the country have only one monitor, with the composite monitor and maximum monitor values the same for such areas, and many other areas have a maximum monito r value that is close to the composite monitor value. As discussed above, these circumstances have a major impact on the protection that would be achieved by the approach suggested by the commenter. With regard to the scope of the commenter’s analysis, the EPA finds that by limiting the scope to a small subset of areas with design values above or just below the current annual standard level of 15 μg/m 3 , the analysis ignores the large number of areas across the country with lower design values that are relev ant to consider in light of the epidemiological evidence of serious health effects at lower concentrations, well below the
81 The EPA’s analysis was intended to repeat the commenter’s analysis, but using only valid air quality data (from 2006 to 2008). For the first group of areas (which included 21 areas with valid data), the EPA’s analysis calculated an average across the areas of maximum monitor annual mean concentrations, averaged of 3 years, of 16.8 μg/m 3 compared to an average of co mposite monitor concentrations of 14.8 μg/m 3 . For the second group of areas (which included 10 areas with valid data), the EPA’s analysis calculated an average across the areas of maximum monitor annual mean concentrations, averaged over 3 years, of 14.8 μ g/m 3 compared to an average of composite monitor concentrations of 14.2 μg/m 3 (Frank, 2012a, Table 3).***E.O. 12866 Review

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Page 187 of 432 level of the current standard. In translating the analysis results into the basis for selecting an annual standard level, the comment er’s translation is premised on the view that the “natural focal point” for setting an annual PM 2.5 standard level should be somewhere within the range of the long

- term mean PM 2.5 concentrations from the subset of epidemiological studies that included only long

- term exposure studies of effects for which the evidence is categorized as causal or likely causal, but not for effects categorized as suggestive of causality, nor did it include short

- term exposure studies (which are included in Figure 4 of the propo sal notice and above). Such a view is not consistent with setting a standard that would provide sufficient protection from the serious health effects reported even in the limited subset of studies considered by the commenter, including protecting public he alth with an adequate margin of safety. As discussed below, the EPA does not agree with the commenter’s view as to the appropriate focal point for selecting the level of an annual PM 2.5 standard, or with the limited set of studies considered by the comment er as a basis for selecting the level of the annual PM 2.5 standard. Regarding an appropriate focal point for selecting the level of the annual standard, as discussed in the proposal and as advised by CASAC, the EPA has focused on PM 2.5 concentrations somew hat below the lowest long

- term mean concentrations from each of the key studies of both long

- and short

- term exposures of effects for which the evidence is causal or likely causal, as considered by the EPA (i.e., the first two sets of studies shown in Figure 4). If the level of the annual standard was set just some where within the range of the long

- term mean concentrations from the various long

- term exposure studies, then one or more of the stud ie s would have a long

- term mean concentration below the***E.O. 12866 Review

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Page 188 of 432 selec ted level of the standard. Absent some reason to ignore or dis count these studies, which the commenter does not provide (and of which the EPA is unaware) , setting such a standard would allow that level of air quality , where the evidence of health effects is strongest, and its associated risk of PM 2.5 - related mortalit y and /or morbidity effects to continue. Selecting such a standard level could not be considered sufficient to protect the public health with an adequate margin of safety. Further, focusing on just the long

- term mean PM 2.5 concentrations in the key epidemi ological studies

– even the lowest long

- term mean concentration from the set of key studies

-- is not appropriate. Concentrations at and around the long

- term mean concentration s represent the part of the air quality distribution where the data in any given study are most concentrated and, thus, where the confidence in the magnitude and significance of an associati on in such study is strongest. However , the evidence of an association with adverse health effects in the studies is not limited to the PM 2.5

conc entrations just at and around the long

- term mean, but rather extends more broadly to a lower part of the distribution, recognizing that no discernible population

- level threshold for any such effects can be identified based on the available evidence . This broader region of the distribution of PM 2.5 concentrations should be considered to the extent relevant information is available , recognizing that the degree of confidence in the association identified in a study would become lower as one moves below concen trations at and around the long

- term mean concentration in any given study. The commenter’s approach ignores this fundamental consideration. Regarding the set of studies that is appropriate to inform the selection of the level of the annual PM 2.5 standard, the EPA finds that limiting consideration only to the long

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Page 189 of 432 term exposure studies, as this commenter suggests, would be tantamount to ignoring the short

- term exposure studies , 82 which provide some of the strongest evidence from the entire body of ep idemiolo gical studies. Thus, s e lecting a n annual standard level using the limited set of studies suggested by the commenter would fail to provide a degree of protection that would be sufficient to protect public health with an adequate margin of safety. For all th e reasons discussed above, the EPA finds the commenter’s concerns with the EPA’s approach to considering composite and maximum monitor PM 2.5 concentrations in selecting the level of the annual PM 2.5 standard to be without merit. Further, the EPA finds no s upport in the commenter’s analysis for their suggested alternative approach. (2) With respect to the appropriate exposure period for mortality effects observed in long

- term exposure studies, some commenters in this group generally expressed views consisten t with comments from UARG that argued that these studies “are most likely detecting health risk from earlier, higher PM 2.5 levels and misattributing those risks to more recent, lower PM 2.5 levels” (UARG, 2012, Attachment 1, p 7). Further, this
commenter as serted that “there is no knowledge or evidence indicating whether
82 The commenter suggests that the EPA should not place significant reliance on the long

term mean concentrations from short

- term exposure studies because “[T]he short

‐ term studies did not use the annual average of PM 2.5 to develop their associations; they used the daily 24

‐ hour averages of PM 2.5 . Thus, short

‐ term studies do not provide a natural indicator for the appropriate level of an annual standard....” (UARG, 2012, Attachment 1, p. 3). The EPA finds this argument unpersuasive. Quite simply, effects were observed in these studies with an air quality distribution that can meaningfully be characterized by these long

- term mean concentrations. Indeed, in rema nding the 2006 standard, the D.C. Circuit discussed at length the interrelationship of the long

- and short

- term standards and studies, and remanded the 2006 standard to the EPA, in part, for ignoring those relationships without adequate explanation. Americ an Farm Bureau Federation v. EPA. 559 F. 3d at 522

- 24.***E.O. 12866 Review

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Page 190 of 432 premature deaths are the result of PM 2.5 exposures in the most recent year; or due to physical damages incurred from PM 2.5 exposures much earlier in life (with the impact on lifespan only emerging later in life); or due to total accumulated PM 2.5 exposure over many years.” Id . Further, the commenter asserted that the long

- term exposure studies of mortality are central to the EPA’s basis for proposing to set a lower annual standard level, since most of the estimated benefits associated with a lower annual PM 2.5 standard are based on reductions in mortality related to long

- term exposures to PM 2.5 . As an initial matter, the EPA has recognized t he challenge in distinguishing between PM 2.5 - associated effects due to past and recent long

- term exposures , and in identif ying the relevant latency period for long

- term exposure to PM and resultant health effects (U.S. EPA, 2009a, section 7 .6.4 ; 77 FR 38941/1). While the EPA has acknowledged that there remain important uncertainties related to characterizing the most relevant exposure periods in long

- term exposure studies, the assertion that there is “no knowledge or evidence” that helps to inform this issue is not correct, as discussed below. Both in the last review and in the current review , the EPA has assessed studies that used different air quality periods for estimating long

- term exposure and tested associations with mortality for the d ifferent exposure periods ( U.S. EPA, 2004, section 8 .2.3.5; U.S. EPA 2009a, section 7.6.4)

. In this review, the Integrate d Science Assessment discussed studies available since the last review that have assessed the relationship between long

- term exposure t o PM 2.5 and mortality to explore the issue of the latency period between exposure to PM 2.5 and death ( U.S. EPA, 2009 a , s ection 7.6.4 ). Notably, i n a recent analys i s of the extended Harvard Six Cities Study, Schwartz et al. (2008) used model averaging (i.e., multiple models were averaged and weighted by***E.O. 12866 Review

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Page 191 of 432 probability of accuracy) to assess exposure periods prospectively (77 FR 38907/1

- 2) . The exposure periods were estimated across a range of unconstrained distributed lag models (i.e., same ye ar, one year prior, two years prior to death ). In comparing lags , the authors reported that the effects of changes in exposure to PM 2.5 on mortality we re strongest within a 2

- year period prior to death ( U.S. EPA, 2009a, p. 7

- 92, Figure 7

- 9 )

. Similarly, a large multi

- ci ty study of the elderly found that the mortality risk associated with long

term exposure to PM 10 reported cumulative effect s that extended over the year s that deaths were observed in the study population (i.e., the follow

- up period) and for the 3

- year peri od prior to death (Zanobetti et al., 2008). Further, i n a study of two locations that experienced an abrupt decline in PM 2.5 concentrations (i.e. , Utah Steel Strike, coal ban in Ireland), Röösli et al. (2005) reported that approximately 75 percent of heal th benefits were observed in the first 5 years ( U.S. EPA, 2009a , Table 7

- 9). Schwartz et al. (2008) and Puett et al. (2008) found, in a comparison of exposure periods ranging from 1 month to 48 months prior to death , that exposure to PM 10 24 months prior t o death exhibited the strongest association, and the weakest association was reported for exposure in the time period of 1 month prior to death. Overall, the EPA notes that the available evidence for determining the exposure period that is causally relate d to the mortality effects of long

- term PM 2.5 exposures , as discussed above, cannot specifically disentangle the effects observed in long

- term exposure studies associated with more recent air quality measurements from effects that may have been associated with earlier, and most likely higher, PM 2.5 exposures. While the evidence suggests that a latency period of up to five years would account for the***E.O. 12866 Review

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Page 192 of 432 majority of deaths , it does not provide a basis for concluding that it is solely recent PM 2.5 concentrations that account for the mortality risk observed in such studies. Nonetheless, the more recent air quality data does well at explaining the relationships observed between long

- term exposures to PM 2.5 and mortality, with the strongest associatio n observed in the two years prior to death. Further, the EPA recognizes that there is no discernible population

- level threshold below which effects would not occur, such that it is reasonable to consider that health effects may occur over the full range of concentrations observed in the epidemiological studies, including the lower concentrations in the latter years. In light of this evidence and these considerations, the EPA concludes that it is appropriate to consider air quality concentrations that are ge nerally contemporaneous with the collection of health event data (i.e., collected over the same time period) as being causally associated with at least some proportion of the deaths assessed in a long

- term exposure study. This would include long

- term mean PM 2.5 concentrations from most of the key long

- term exposure studies of effects with causal or likely causal evidence shown in Figure 4 above, which reported long

- term mean PM 2.5 concentrations ranging from 13.6 μg/m 3 to 14.3 μg/m 3

. These studies include s tudies of mortality by Eftim et al. (2008), which separately analyzed the ACS and Harvard Six City sites, Zeger et al. (2008), and Lipfert et al. (2006), as well as studies of morbidity endpoints by Goss et al. (2004), McConnell et al. (2003) and Gauderman et al. (2004), and Dockery et al. (1996) and Razienne et al. (1996). The EPA acknowledges that uncertainty in the relevant exposure period is most notable in two other long

- term exposure studies of mortality. The Miller et al. (2007) reported a long

- term mean PM 2.5 concentration for a 1

- year exposure***E.O. 12866 Review

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Page 193 of 432 period that post

- dated the follow

- up period in which health event data were collected by two years. Also, the Krewski et al. (2009) study reported a long

- term mean PM 2.5 concentration for an exposure period th at included only the last two years of the 18

- year follow

- up period. Based on these considerations, the EPA does not now consider it appropriate to put weight on the reported long

- term mean concentrations from these two studies for the purpose of translati ng the information from the long

- term mortality studies into a basis for selecting the level of the annual PM 2.5 standard. 83 In addition, the EPA acknowledges that exposure periods that extend at least a couple years prior to the follow

- up period in which h ealth event data were collected would likely more fully capture the PM

- related deaths in such studies. To explore how much higher the long

- term mean PM 2.5 concentrations would likely have been had air quality data prior to the follow

- up years of the studie s been included, the EPA conducted a sensitivity analysis of long

- term mean PM 2.5 concentrations (Schmidt, 2012 a ) particularly considering studies that only included deaths from a relatively recent follow

up period. As examples of such studies, this analys is considered the Eftim et al. (2008) study of mortality in the ACS sites and the Harvard Six Cities sites, as well as sites in the eastern region in the Zeger et al. (2008) study. Using data from the EPA’s AQS database, the analysis added the two years of air quality data just prior to the follow

- up period in each study, which was 2000 to 2002 in Eftim et al. (2008) and 2000 to 2005 in Zeger et al. (2008). The analysis then calculated the extended long

- term mean PM 2.5 concentration for each study. As discussed in Schmidt (2012 a ), in each case the long

- term mean PM 2.5

83 Nonetheless, the EPA notes that the Krewski et al. (2009) and Miller et al. (2007) studies provide strong evidence of mortality and cardiovascular

- related effects associated with long

- term PM 2.5 exp osures to inform causality determinations reached in the Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).***E.O. 12866 Review

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Page 194 of 432 concentration averaged over the exten ded exposure period was less than 0. 4 μg/m 3 higher than the long

- term mean PM 2.5 concentration averaged over the follow

- up period. The EPA finds it reasonable to conclude that such a relatively small difference in long

- term mean PM 2.5 concentrations would likely apply for other long

- term exposure studies that used similarly recent follow

- up periods as well (e.g., Goss et al., 2004; Lipfert et al., 2006). Based on the above considerations, the EPA concludes that it is appropriate to consider the available ai r quality information from the long

- term exposure studies, while taking into account the uncertainties in the relevant long

- term exposure periods in weighing the information from these studies. The EPA recognizes that considering such information in select

ing an appropriate annual standard level has the potential to build in some margin of safety. The EPA further concludes that it is appropriate to consider the air quality information from the set of long

- term exposure studies discussed above in the
context of the broader array of epidemiological studies that inform the EPA’s consideration of the level of the annual PM 2.5 standard. The EPA also notes that while the long

- term exposure studies are an important component of the epidemiological evidence that inf orms the Agency’s consideration of the level of the annual standard, they do not provide the only relevant information, nor are they the set of studies for which the relevant long

- term mean PM 2.5 concentrations are the lowest. As discussed in the proposal, the EPA also considers the long

- term mean PM 2.5 concentrations from the short

- term mortality and morbidity studies as providing important information in considering the level of the annual standard. As discussed above, a large proportion of the aggregate risk associated with short

- term exposures***E.O. 12866 Review

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Page 195 of 432 results from the large number of days during which the 24

- hour average concentrations are in the low

- to mid

- range of the concentrations observed in the studies. Thus, setting the level of the annual standard based on long

- term mean concentrations, as well as the distribution of concentrations below the mean, in the short

- term exposure studies is the most effective and efficient way to reduce total PM 2.5 - related risk from the broad array of mortality and morbidity e ffects associated with short

- term exposures. Further, the EPA notes that the relevant exposure period for the short

- term exposure studies is the period contemporaneous with the collection of health event data, and that this exposure period is not subject t o the uncertainties discussed above related to the long

- term exposure studies. Recognizing that the long

- term mean PM 2.5 concentrations from several of the multi

- city short

- term exposure studies shown in Figure 4 are below the long

- term mean PM 2.5 concentr ations from the long

- term exposure studies (with the exception of Miller et al., 2007). 84 it is reasonable that in selecting the level of the annual standard primary consideration should be given to the information from this set of short

- term exposure studi es. There is no reasonable basis to discount the long

- term mean concentrations of the short

- term exposure studies for purposes of setting the level of the annual standard. Thus, the commenter is incorrect in asserting that the long

- term exposure studies, n ot the short

- term exposure studies, would be central in the Administrator’s decision on the level of the annual standard. The standard is ultimately intended to protect not just against the single type of effect that contributes the most to quantitative es timates of risk to public health, but rather to the broad array of effects,
84 As noted above, the EPA is not placing weight on the reported long

- term mean concentrations from the Miller et al. (2007) s tudy for the purpose of translating the information from the long

- term mortality studies into a basis for selecting the level of the annual PM 2.5 standard.***E.O. 12866 Review

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Page 196 of 432 including mortality and morbidity effects from long

- and short

- term exposures across the range of at

- risk populations impacted by PM 2.5 - related effects. (3) With regard to the EPA ’s analysis of distribution s of underlying population

level data (i.e., health event and study population data) and corresponding air quality data from each study area in certain key multi

- city epidemiological studies (Rajan et al., 2011), some commenters in this group raised a number of issues related to this analysis (API, 2012, Attachment 1 pp. 5 to 6; McClennan, 2012, p p.2 to 4 ) . Some commenters noted the limited number of studies for which health event and study population data were available, and questioned whether these distributions would apply to other studies. Commenters expressed concerns that this analysis had not been formally reviewed by CASAC and was not published in the peer

- review literature. Based on such concerns,
some commenters asser ted that the EPA should not consider this information as a basis for selecting a standard level. As an initial matter, as discussed in section III.E.4.b above, the EPA agrees with CASAC’s advice that it is appropriate to consider additional data beyond the mean PM 2.5 concentrations in key multi

- city studies to help inform selection of the level of the annual PM 2.5 standard. As both the EPA and CASAC recognize, in the absence of a discernible threshold, health effects may occur over the full range of concent rations observed in the epidemiological studies . Nonetheless , the EPA recognizes that confidence in the magnitude and significance of an association is highest at and around the long

- term mean PM 2.5 concentrations reported in the studies and the degree of confidence becomes lower at lower concentrations within any given study . Following CASAC’s advice (Samet, 2010d, p.2), the EPA used additional population

- level and air quality data made available***E.O. 12866 Review

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Page 197 of 432 by study authors to conduct an analysis of the distributions of such data, to help inform consideration of how the degree of confidence in the magnitude and significance of observed associations varies across the range of long

- term mean PM 2.5 concentrations in study areas within key multi

- city epidemiological studi es. In the EPA’s view, such consideration is important in selecting a level for an annual standard that will protect public health with an adequate margin of safety. With regard to the number of multi

- city studies for which an analysis of the distributions of population

- level data across the study areas and the corresponding annual mean PM 2.5 concentrations was done, the EPA noted at proposal that data for such an analysis were made available from study authors for four studies, including two long

term expo sure studies and two short

- term exposure studies. 85 The EPA recognized that access to health event data can be restricted due to confidentiality issues, such that it is not reasonable to expect that such information could be made available from all studies. In considering the information from these four studies, the EPA has further taken into consideration uncertainties discussed in response to the above comment related to the appropriate exposure period for long

- term exposure studies. Based on these conside rations, as noted above, the EPA concludes that such uncertainties are an important factor in evaluating the usefulness of the air quality information from the two long

- term exposure studies in this analysis (Krewski et al., 2009; Miller et al., 2007) and that it would not be appropriate to place weight on the distributional analysis of health event and air quality data from these two studies specifically for the purpose of

85 Health event data and study population data were available from two short

- term exposure studies (Bell et al. 2008; Zanobetti and Schwartz, 2009) and one long

- term exposure study (Krewski et al., 2009). Only study population data were available from anothe r long

- term exposure study (Miller et al., 2007).***E.O. 12866 Review

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Page 198 of 432 translating the information from the long

- term mortality studies into a basis for se lecting the level of the annual PM 2.5 standard. Such uncertainties are not relevant to the short

term exposure studies, and thus, the Agency focuses on the two short

- term exposure studies in this analysis (Bell et al., 2008; Zanobetti and Schwartz, (2009). In focusing on these two short

- term exposure studies, the EPA first notes that these studies are key multi

- city studies that reported positive and statistically significant associations between mortality and cardiovascular

- related hospital admissions acro ss a large number of areas throughout the U.S. (112 U.S. cities in Zanobetti and Schwartz, 2009; 202 U.S. counties in Bell et al., 2008) using relatively recent air quality and health event data (i.e., 1999 through 2005 in both studies). The EPA considers this to be a modest but important data set to use for this distributional analysis to help inform consideration of how much below the long

- term mean PM 2.5 concentrations in key multi

city long

- and short

- term exposure studies the annual PM 2.5 standard leve l should be set. While the EPA acknowledges that having such data available from more studies would have been useful, the Agency finds the information from this limited set of studies to be an important consideration in selecting an annual standard level, consistent with CASAC advice to consider such information. In considering t

he results of this distributional analysis , as discussed more fully in the Response to Comments document, the EPA considers PM 2.5 concentrations between the 25 th and 10 th percentile s of the distribution of health events to be a reasonable range for providing a general frame of reference for that part of the distribution in which confidence in the magnitude and significance of the association may be appreciably lower than confidence a t and around the long

- term mean concentration

. For the two short

- term***E.O. 12866 Review

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Page 199 of 432 exposure studies included in this analysis, the EPA notes that the PM 2.5 concentrations corresponding to the 25 th percentiles of the distributions of health events were 12.5 μg/m 3 and 11.5 μg/m 3 , respectively, for Zanobetti and Schwartz (2009) and for Bell et al. (2008) , with the 10 th percentiles being lower by approximately 2 μg/m 3 in each study (Rajan et al., 2011, Table 1)

. In considering this information, t he EPA recognizes, however , that there is no clear dividing line or single percentile within a given distribution (including both above and below the 25 th percentile ) provided by the scientific evidence that is most appropriate or ‘correct’ to use to characterize where the degree o f confidence in the associations warrants setting the annual standard level. The decision as to the appropriate standard level below the long

- term mean concentrations of the key studies is largely a public health policy judgment to be made by the Administr ator, taking into account all of the evidence and its related uncertainties, as discussed in section III.E.4.d below. In response to concerns that this analysis was not reviewed by CASAC nor published in the peer

- reviewed literature, the EPA notes that thi s analysis was conducted to directly respond to advice from CASA C , as discussed in section III.E.4.b.1 above, in conjunction with their review of the Policy Assessment. The EPA notes that the same type of distributional analysis was presented in the second draft Policy Assessment based on air quality data, as well as population

- weighted air quality data, rather than health event or study population data. In considering that distributional information, CASAC urged that the EPA redo the analysis using health event or study population data, which is exactly what the EPA did and presented in the final Policy Assessment. The EPA provided CASAC with the final Policy Assessment and communicated how the final staff***E.O. 12866 Review

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Page 200 of 432 conclusions reflected consideration of its advice a nd that those staff conclusions were based in part on the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 2011, Attachment p. 2). CASAC did not choose to provide any additional comments or advice after receiving the final Policy Assessment. The EPA considers this distributional analysis to be the product of the peer review conducted by CASAC of the Policy Assessment, and thus does not agree with commenters’ characterization that the analysis lacked appropriate peer review. The EPA’s final analysis was based on the comments provided by CASAC, the pe e r review committee established pursuant to the CAA , on the draft analysis, such that the final analysis stems directly from CASAC’s advice and the EPA’s response to its comments. Based on the above considerations, the EPA continues to conclude that its analysis of distributions of health event and air quality data from two key multi

- city epidemiological studies provides important information related to understanding the association s between health events observed in each city (e.g., deaths, hospitalizations) and the corresponding long

- term mean PM 2.5 concentrations observed in the studies. While recognizing that this is a relatively modest data set, the EPA further concludes that

su ch information can appropriately help to inform the selection of the level of an annual standard that will protect public health with an adequate margin of safety from these types of health effects which are causally related to long

- and short

- term exposur es to PM 2.5 . (4) Some commenters in this group asserted there were limitations in the long

term exposure studies of morbidity, including studies evaluating respiratory effects in children. For example, one commenter (UARG, 2012, p. 12, Attachment 1, pp. 14 to 16)***E.O. 12866 Review

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Page 201 of 432 asserted there were serious limitations in the long

- term exposure studies of respiratory morbidity in each of the studies considered by the EPA (including McConnell et al., 2003; Gauderman et al., 2004; Dockery et al., 1996; Raizenne et al., 1996; and Goss et al., 2004) and argued that this evidence provides only a “weak association” with PM 2.5 exposures. This commenter asserted that many of these long

- term exposure studies evaluating respiratory effects were considered at the time the EPA reaffirme d the current annual standard level of 15 μg/m 3 in 2006, that the Administrator in the last review determined that the information they provided “was too limited to serve as the basis for setting a level of a national standard,” and that they should be giv en little weight in setting the level of the annual standard in this review (UARG, 2012, Attachment 1, p. 14). More specifically, this commenter asserted that the McConnell et al. (2003) and Gauderman et al. (2004) stud ies reported mixed results for assoc iations with PM 2.5 and stronger associations with NO 2 (API, 2012, Attachment 1, pp. 14 to 15). Similarly, this commenter argued that the Dockery et al. (1 996) and Raizenne et al. (1996) studies show ed stronger associations with acidity than with fine parti cles (measured as PM 2.1 ). Id. pp. 15 to 16. With regard to the cystic fibrosis study, this commenter noted that the association between pulmonary exacerbations and PM 2.5 in this study was no longer statistically significant when the model adjusted for eac h individual’s baseline lung function. The commenters referred to the data on lung function as an “important explanatory variable,” and suggested that the EPA should rely on results from the model that included individual baseline lung function information

. Id . p. 16. For the reasons discussed below and in more detail in the Response to Comments document, t he EPA***E.O. 12866 Review

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Page 202 of 432 disagrees with the commenters’ interpretation of these studies

. As an initial matter, the EPA notes that three of these studies (McConnell et al., 2003; Dockery et al., 1996; Raizenne et al., 1996) as well as the initial studies from the Southern California Children’s Health Study ( Peters et al., 1999; McConnell et al. , 1999’ Gauderman et al., 2000, 2002 ; Avol et al., 2001 ) were discussed and considered in the 2004 Air Quality Criteria Document ( U.S. EPA, 2004) and , thus, considered within the air quality criteria supporting the EPA’s final decisions in the review compl eted in 2006. Two additional studies (Gauderman et al., 2004; Goss et al., 2004) were discussed and considered in the provisional science assessment conducted for the last review ( U.S. EPA, 2006a). The EPA concluded that “new” studies considered in the pro visional assessment completed in 2006 did not materially change any of the broad scientific conclusions regarding the health effects of PM exposure made in the Criteria Document (71 FR 61148 to 61149, October 17, 2006). All of these studies were considered in the Integrated Science Assessment that informs the current review ( U.S. EPA, 2009a). With regard to the Southern California Children’s Health Study, extended analyses considered in the Integrated Science Assessment provided evidence that clinically important deficits in lung function 86 associated with long

- term exposure to PM 2.5 persist into early adulthood (U.S. EPA, 2009a, p. 7

- 27; Gauderman et al., 2004). These effects remained positive in copollutant models. 87 Additional analyses of the

86 Clinical significance was defined as an FEV 1 below 80 percent of the predicted value, a criterion commonly used in clinical settings to identify persons at increased risk for adverse respiratory condition s (U.S. EPA, 2009a, p. 7

- 29 to 7

- 30). The primary NAAQS for sulfur dioxide (SO 2 ) also included this interpretation for FEV 1 (75 FR 35525, June 22, 2010). 87 Gauderman et al. (2004) clearly stated throughout their analysis that NO 2 was one component of a hig hly correlated mixture that contains PM 2.5 . Gauderman et al. (2004) did not present the results from copollutants models but stated “two

- pollutant models for***E.O. 12866 Review

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Page 203 of 432 Southern Ca lifornia Children’s Health Study cohort reported an association between long

- term PM 2.5 exposure and bronchitic symptoms ( U.S. EPA, 2009a, p. 7

- 23 to 7

- 24; McConnell et al., 2003, long

- term mean concentration of 13.8 μg/m 3 ) that remained positive in co

- pol lutant models, with the PM 2.5 effect estimates increasing in magnitude in some models and decreasing in others, and a strong modifying effect of PM 2.5 on the association between lung function and asthma incidence ( U.S. EPA, 2009 a , 7

- 24; Islam et al., 2007)

. T he outcomes observed in the more recent reports from the Southern California Children’s Health Study, including evaluation of a broader range of endpoints and longer follow

- up periods, were larger in magnitude and more precise than reported in the initi al version of the study . Supporting these results were new longitudinal cohort studies conducted by other researchers in varying locations using different methods ( U.S. EPA, 2009a, section 7.3.9.1). The EPA, therefore, disagrees with the commenters that th e studies by McConnell et al. (2003) and Gauderman et al. (2004) are flawed and should not be used in the PM NAAQS review process. The 24

- City study 88 by Dockery et al. (1996) (long

- term mean concentration of 14.5 μg/m 3 ) was considered in the current as well as two previous reviews ( U.S. EPA, 2009a; U.S. EPA, 2004; U.S. EPA, 1996). This study observed that PM, specifically “particle strong acidity” and sulfate particles (indicators of fine particles), were associated with reports of bronchitis in the prev ious year. Similarly, the magnitude of the associations between bronchitis and PM 10 and PM 2.1 were similar to those for acidi c aerosols and sulfate particles, though the confidence intervals for the PM 10 and PM 2.1 any pair of pollutants did not provide a significantly better fit to the data than the correspondi ng single

- pollutant models.” 88 The 24

- City study conducted by Dockery et al. (1996) included 18 sites in the U.S. and 6 sites in Canada. The Raizenne et al. (1996) study considered 22 of these 24 study areas. Athens, OH and South Brunswick, NJ were not included in this study.***E.O. 12866 Review

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Page 204 of 432 associations were slightly wider and the a ssociations were not statistically significant. Acid aerosols , sulfate, and fine particles are formed in secondary reactions of the emissions from incomplete combustion and these pollutants have similar regional and temporal distributions . As noted by the study authors, “the strong correlations of several pollutants in this study, especially particle strong acidity with sulfate (r=0.90) and PM 2.1 (r=0.82), make it difficult to di stinguish the agent of interest ” (Dockery et al., 1996, p. 505). Overall, Docke ry et al. (1996) (and, similarly, Raizenne et al., 1996) observed similar associations between respiratory health effects and acid aerosols , sulfate, PM 10 and PM 2.1 concentrations. The commenters noted that the associations with particle acidity were sensi tive to the inclusion of the six Canadian sites. The EPA notes that none of these Canadian cities were in the “sulfate belt” where particle strong acidity was highest. Thus, the change in the effect estimate when the six Canadian cities were excluded from the analysis is likely due to the lower prevalence of bronchitis and the lower concentrations of acid aerosols in these cities, and not due to some difference in susceptibility to bronchitis between the U.S. and Canadian populations that is not due to air pollution, as suggested by the commenters (UARG, 2012, Attachment 1, p. 15). In fact, contrary to the statements made by the commenters, the authors did not observe any subgroups that appeared to be markedly more susceptible to the risk of bronchitis. The Goss et al. (2004) study considered a U.S. cohort of cystic fibrosis patients and provided evidence of association between long

- term PM 2.5 exposures and exacerbations of respiratory symptoms resulting in hospital admissions or use of home intravenous anti biotics ( U.S. EPA, 2009a, p. 7

- 25; long

- term mean concentration of 13.7 μg/m 3 ). The commenters noted that the association between pulmonary exacerbations and***E.O. 12866 Review

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Page 205 of 432 PM 2.5 in this study was no longer statistically significant when the model adjusted for each indiv idual’s baseline lung function. The commenters referred to the data on lung function as an “important explanatory variable,” and suggested that the EPA should rely on results from the model that included individual baseline lung function information. The E PA disagrees with the commenters’ interpretation of this study. The Agency concludes it is unlikely that lung function is a potential confounder or an important explanatory variable in this study. In fact, the authors noted that “it is more likely that lun g function decline may be intimately associated with chronic exposure to air pollutants and may be part of the causal pathway in worsening prognosis in CF [cystic fibrosis]; in support of this explanation, we found both cross

- sectional and longitudinal str ong inverse relationships between FEV 1 and PM levels” (Goss et al., 2004, p. 819). The EPA notes that adjusting for a variable that is on the causal pathway can lead to overadjustment bias, which is likely to attenuate the association (Schisterman et al. 2 009); this is likely what was observed by the authors. Thus, the EPA continues to believe it is appropriate to focus on the results reported in Goss et al. (2004) that did not include individual baseline lung function in the model. In addition, the EPA dis agrees with commenters’ reliance solely on statistical significance when interpreting the study results from individual study results and the collective evidence across studies. As discussed in section III.D.2 above, statistical significance of individual study findings has played an important role in the EPA’s evaluation of the study’s results and the EPA has placed greater emphasis on studies reporting statistically significant results. However, in the broader evaluation of the evidence from many epidemio logical studies, and subsequently during the process of***E.O. 12866 Review

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Page 206 of 432 forming causality determinations in the Integrated Science Assessment by integrating evidence from across epidemiological, controlled human exposure, and toxicological studies, the EPA has emphasized the pattern of results across epidemiological studies and whether the effects observed were coherent across the scientific disciplines for drawing conclusions on the relationship between PM 2.5 and different health outcomes. As noted in section III.B.1.a of the proposal, with regard to respiratory effects, the Integrated Science Assessment concluded that extended analyses of studies available in the last review as well as new epidemiological studies conducted in the U.S. and abroad provided stronger eviden ce of respiratory

- related morbidity associated with long

- term PM 2.5 exposure (77 FR 38918). The strongest evidence for respiratory

- related effects available in this review was from epidemiological studies that evaluated decrements in lung function growth i n children and increased respiratory symptoms and disease incidence in adults (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1). In considering the collective evidence from epidemiological, toxicological, and controlled human exposure studies, incl uding the studies discussed above, the EPA recognizes that the Integrated Science Assessment concluded that a causal relationship is likely to exist between long

- term PM 2.5 exposures and respiratory effects ( U.S. EPA, 2009a, p. 2

- 12, pp. 7

- 42 to 7

- 43). CAS AC concurred with this causality determination (Samet, 2009f, p.9). The commenter’s assertion that the EPA should adhere to its assessment of these studies as it did in the review completed in 2006 is significantly mistaken. Most obviously, the EPA’s final decision in the last review was held to be deficient by the D.C. Circuit in remanding the 2006 primary annual PM 2.5 standard. As discussed in section***E.O. 12866 Review

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Page 207 of 432 III.A.2 above, the D.C. Circuit specifically held that the EPA did not provide a reasonable explanation o f why certain morbidity studies, including an earlier study from the Southern California Children’s Health Study (Gauderman et al., 2000 , long

- term mean PM 2.5 concentration approximately 15 μg/m 3 ) and the 24

- Cities Study (Raizenne et al., 1996 , long

- term m ean concentrations approximately 14.5 μg/m 3 ) did not warrant a more stringent annual PM 2.5 standard when the long

- term mean PM 2.5 concentrations reported in those studies were at or lower than the level of the annual standard. American Farm Bureau Federati on v. EPA . 559 F. 3d at 525. Indeed, the court found that, viewed together, the Gauderman et al. (2000) and Raizenne et al., (1996) studies “are related and together indicate a significant public health risk...On this record, therefore, it appears the EPA to o hastily discounted the Gauderman and 24

- Cities studies as lacking in significance.” Id. In this review, the EPA recognizes a significant amount of evidence beyond these two studies that expands our understanding of respiratory effects associated with lo ng term PM 2.5 exposures. This body of scientific evidence includes an extended and new analyses from the Southern California Children’s Health Study (Gauderman et al., 2004; Islam et al., 2007; Stanojevic et al., 2008) as well as additional studies that ex amined these health effects (Kim et al., 2004; Goss et al., 2004). Thus, even more so than in the last review, the evidence indicates a “significant public health risk” to children from long

- term PM 2.5 exposures at concentrations below the level of the cur rent annual standard. A standard that does not reflect appropriate consideration of this evidence would not be requisite to protect public health with an adequate margin of safety. (5) With regard to the use of studies of health effects for which the EPA f inds the***E.O. 12866 Review

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Page 208 of 432 evidence to be “suggestive” of a causal relationship, some commenters argued that such studies “do not merit any weight in the setting of the annual NAAQS” (e.g., UARG, 2012, Appendix 1, p. 3). The EPA disagrees with the commenter’s view that stud ies of health effects for which the evidence is suggestive of a causal relationship, rather than studies of health effects for which the evidence supports a causal or likely causal relationship, merit no weight at all in setting the NAAQS. To place no weig ht at all on such evidence would in essence treat such evidence as though it had been categorized as “not likely to be a causal relationship.” To do so would ignore the important distinctions in the nature of the evidence supporting these different causali ty determinations in the Integrated Science Assessment. It would also ignore the CAA requirement that primary standards are to be set to provide protection with an adequate margin of safety, including providing protection for at

- risk populations. Thus, ign oring this information in making decisions on the appropriate standard level would not be appropriate. 89 Nonetheless, in considering studies of health effects for which the evidence is suggestive of a causal relationship, the EPA does believe that it is app ropriate to place less weight on such studies than on studies of health effects for which there is evidence of a causal or likely causal relationship. A second group of commenters supported revising the suite of primary PM 2.5 standards to provide increased public health protection. These commenters found the

89 As dis cussed in section II.A above, 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. I w as also intended to provide a reasonable degree of protection against hazards that research has not yet identified. This certainly encompasses consideration of effects for which there is evidence suggestive of a causal relationship.***E.O. 12866 Review

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Page 209 of 432 available scientific information and technical analyses to be stronger and more compelling than in the last review. These commenters generally placed substantial weight on CASAC advice and on the EPA st aff analyses presented in the final Policy Assessment, which concluded that the evidence most strongly supported an annual standard level within a range of 11 to 12 μg/m 3 ( U.S. EPA, 2011a, p. 2

- 206). While some of these commenters felt that the level shoul d be set within the proposed range (12 to 13 μg/m 3 ), most of these commenters advocated a level of 11 μg/m 3

. 90 For example, ALA et al., asserted: The EPA’s proposed PM 2.5 standards, while a step in the right direction are insufficient to protect public health, including the health of susceptible populations, with an adequate margin of safety as required by the Clean Air Act...we will discuss the enormous gap in public heal th protection afforded by an annual standard of 13 μg/m 3 , at the upper end of the proposed range, compared to the more protective 11 μg/m 3 , as advocated by our organizations (ALA et al., 2012, p. 6). In general, these commenters expressed the view that gi ven the strength of the available scientific evidence, the serious nature of the health effects associated with PM 2.5 exposures, the large size of the at

- risk populations, the risks associated with long

- and short

- term PM 2.5 exposures, and the important un certainties inherently present in the evidence, the EPA should follow a highly precautionary policy response by selecting an annual standard level that incorporates a large margin of safety. More specifically, these commenters offered a range of comments related to the general approach used by the EPA to select standard levels, including: (1) the EPA’s approach for setting a generally controlling annual standard; (2) the importance of the greatly expanded and stronger overall scientific data base; (3) cons ideration of the

90 As discussed in secti on III.E.4.c.ii, many of these commenters also supported lowering the level of the primary 24

- hour PM 2.5 standard.***E.O. 12866 Review

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Page 210 of 432 distributional statistical analysis conducted by the EPA and other approaches for translating the air quality information from specific epidemiological studies into standard levels; and (4) the significance of the PM 2.5 - related public heal th impacts, especially potential impacts on at

- risk populations, including children, in reaching judgments on setting standards that provide protection with an adequate margin of safety. These comments are discussed in turn below. (1) Some of these comment ers disagreed with the EPA’s approach for setting a “generally controlling” annual standard in conjunction with a 24

- hour standard providing supplemental protection particularly for areas with high peak

- to

- mean ratios. These commenters argued this approach would lead to “regional inequities” as demonstrated in the EPA’s analyses contained in Appendix C of the Policy Assessment (ALA et al., pp. 26 to 27). Specifically, these commenters argued: There is no basis in the Clean Air Act for such a determination. The Clean Air Act requires only that the NAAQS achieve public health protection with an adequate margin of safety. It is well

- documented that both long

- and short

- term exposures to PM 2.5 have serious and sometimes irreversible health impacts. There is no h ealth protection reason to argue tha t one standard should be “controlling” as a matter of policy without regard to the health consequences of such a policy. To adopt such a policy ignores the obligation to provide equal protection under the law to all Amer icans because it would result in uneven protection from air pollution in different localities and regions of the country (ALA et al., 2012, p. 26). The EPA believes these commenters misunderstood the basis for the EPA’s policy goal of setting a “generally controlling” annual standard. This approach relates exclusively to setting standards that will provide requisite protection against effects associated with both long

- and short

- term PM 2.5 exposures. It does so by lowering the overall air quality distributi ons across an area, recognizing that changes in PM 2.5 air quality designed to meet an annual standard would likely result not only in lower annual***E.O. 12866 Review

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Page 211 of 432 mean PM 2.5 concentrations but also in fewer and lower peak 24

- hour PM 2.5 concentrations. As discussed in sect ion III.A.3 in the proposal and above, the EPA recognizes that there are various ways to combine the two primary PM 2.5 standards to achieve an appropriate degree of public health protection. Furthermore, the extent to which these two standards are interrel ated in any given area depends in large part on the relative levels of the standards, the peak

- to

- mean ratios that characterize air quality patterns in an area, and whether changes in air quality designed to meet a given suite of standards are likely to be of a more regional or more localized nature. In focusing on a

n approach of setting a generally controlling annual standard, the EPA’s intent is in fact to avoid the potential “regional inequities” that are of concern to the commenters. The EPA judges tha t the most appropriate way to set standards that provide more consistent public health protection is by using the approach of setting a generally controlling annual standard. This judgment builds upon information presented in the Policy Assessment as discu ssed in section III.A.3 above. More specifically, the Policy Assessment recognized that the short

- term exposure studies primarily evaluated daily variations in health effects with monitor(s) that measured the variation in daily PM 2.5 concentrations over th e course of several years. The strength of the associations observed in these epidemiological studies was demonstrably in the numerous “typical” days within the air quality distribution, not in the peak days ( U.S. EPA, 2011a, p. 2

- 9). In addition, the quan titative risk assessments conducted for this and previous reviews demonstrated the same point, that is, much, if not most , of the aggregate risk associated with short

- term PM 2.5 exposures results from the large number of days during which the 24

- hour avera ge concentrations are in the low

- to mid

- range, below the peak 24

- hour***E.O. 12866 Review

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Page 212 of 432 concentrations ( U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In addition, there was no evidence suggesting that risks associated with long

- term exposures were likel y to be disproportionately driven by peak 24

- hour concentrations. 91 For these reasons, the Policy Assessment concluded that strategies that focused primarily on reducing peak days were less likely to achieve reductions in the PM 2.5 concentrations that were most strongly associated with the observed health effects. Furthermore, the Policy Assessment concluded that a n approach that focused on reducing peak exposures would most likely result in more uneven public health protection acro ss the U.S. by either providing inadequate protection in some areas or overprotecting in other areas ( U.S. EPA, 2011a, p. 2

- 9; U.S. EPA, 2010a, section 5.2.3). This is because reductions based on control of peak days are less likely to control the bulk of the air quality distribution. As a result, the EPA believes a n approach that focuses on a generally controlling annual standard would likely reduce aggregate risks associated with both long

- and short

term exposures more consistently than a generally cont rolling 24

- hour standard and, therefore, would be the most effective and efficient way to reduce total PM 2

- 5 - related population risk. The CASAC agreed with this approach and considered it was “appropriate to return to the strategy used in 1997 that conside rs the annual and the short

term standards together, with the annual standard as the controlling standard, and the short

- term standard supplementing the protection afforded by the annual standard” (Samet, 2010d, p. 1). For the reasons discussed above, the EPA disagrees with the

91 In confirmation, a number of studies have presented analyses excluding higher PM concentration days and reported a limited effect on the ma gnitude of the effect estimates or statistical significance of the association (e.g., Dominici, 2006b; Schwartz et al., 1996; Pope and Dockery, 1992).***E.O. 12866 Review

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Page 213 of 432 comments that this approach will result in the concerns raised by the commenters; rather the EPA concludes that this approach will help to address these concerns. (2) Many of these commenters asserted that the currently available sci entific information is greatly expanded and stronger compared to the last review. Some of these commenters highlighted the availability of multiple, multi

- city long

- and short

- term exposure studies providing “repeated, consistent evidence of effects below the current annual standard level” (ALA et al., 2012, pp. 39 to 49) and, more specifically, “significant evidence of harm with strong confidence well below EPA’s proposed annual standard range of 12

- 13 μg/m 3 ” (AHA et al., 2012, pp. 10 to 12). The EPA reco gnizes that in setting standards that are requisite to protect public health with an adequate margin of safety, the Administrator must weigh the various types of available scientific information in reaching public health policy judgments that neither

overs tate nor understate the strength and limitations of this information or the appropriate inferences to be drawn from the available science. In general, the EPA agrees with these commenters’ views that the currently available scientific evidence is stronger “because of its breadth and the substantiation of previously observed health effects” (77 FR 38906/2) and provides “greater confidence in the reported associations than in the last review” (77 FR 38940/1). The EPA also agrees with the commenters’ position that it is appropriate to consider the regions within the broader air quality distributions where we have the strongest confidence in the associations reported in epidemiological studies in setting the level of the annual standard. However, as discussed i n section III.E.4.d below, in weighing the available evidence and technical analyses, as well as the associated uncertainties and limitations in***E.O. 12866 Review

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Page 214 of 432 that information, the EPA disagrees with the commenters’ views regarding the extent to which the available scie ntific information provides support for considering an annual standard level below the proposed range (i.e., below 12 to 13 μg/m 3 ). In particular, the EPA disagrees with the degree to which these commenters place more weight on the relatively more uncertai n evidence that is suggestive of a causal relationship (e.g., low birth weight). Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency concludes it is appropriate and reasonable to place the greatest emphasis on health effects for which the Integra ted Science Assessment concluded there is evidence of a causal or likely causal relationship and to place less weight on the health effects that provide evidence that is only suggestive of a causal relationship.

. (3) With regard to us ing the air quality i nformation from epidemiological studies to inform decisions on standard levels, commenters in this group generally supported the EPA’s efforts to explore different statistical metrics from epidemiological studies to inform the Administrator’s decisions. Th ese commenters argued that by considering different analytic measures

-- either concentrations one standard deviation below the long

- term means reported in the epidemiological studies or the EPA’s d istributional statistical analysis of population

- level dat a that extends the approach used in previous PM NAAQS reviews to consider information beyond a single statistical metric

-- “the annual standard must be significantly lower than EPA has proposed” (ALA et al., 2012, pp. 50 to 61). Furthermore, with regard t o characterizing the PM 2.5 air quality at which associations have been observed, some of these commenters highlighted CASAC’s recommendation that “[f]urther consideration should be given to using the 10 th percentile as a level for assessing various scenari os of levels for the PM NAAQS” (Samet, 2010c, p. 11) (ALA et***E.O. 12866 Review

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Page 215 of 432 al., 2012, p. 55). Other commenters urged that the EPA extend the distributional analysis to include additional studies. For example, CHPAC urged the EPA to also conduct distributional analysis f

or children’s health studies to better inform standards that would protect both children and adults from adverse health outcomes (CHPAC, 2012, p. 3) The EPA agrees with these commenters’ views that it is appropriate to take into account different statistic al metrics from epidemiological studies to inform the decisions on standard levels that are appropriate to consider in setting a standard that will protect public health with an adequate margin of safety . In the development of the Policy Assessment, the EP A staff explored various approaches for us ing information from epidemiological studies in setting the standards. The general approach used in the final Policy Assessment, discussed in sections III.A.3 and III.E.4.a above, reflects consideration of CASAC ad vice (Samet, 2010c, d) and public comments on multiple drafts of the Policy Assessment

. With regard to using the distributional statistical analysis to characterize the confidence in the associations , the EPA emphasizes that there is no clear dividing line provided by the scientific evidence, and that choosing how far below the long

- term mean concentrations from the epidemiological studies is appropriate to identify a standard level that will provide protection for the public health with an adequate margin of safety is largely a public health policy judgment. The EPA considers the region from approximately the 25 th to 10 th percentiles to be a reasonable range for providing a general frame of reference as to the part of the distribution over which our confide nce in the magnitude and significance of the associations observed in epidemiological studies is appreciably lower. Based on these considerations, t he EPA concludes that it is not***E.O. 12866 Review

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Page 216 of 432 appropriate to place as much confidence in the magnitude and significance of the

assoc i ation s over the lower percentiles of the distributions in each study as at and around the long

- term mean concentrations. Thus, the EPA disagrees with the commenters’ vi ews that this analysis compels placing more emphasis on the lower part of this range in selecting a level for an annual standard that will protect public health with an adequate margin of safety

. The EPA recognizes that this information comes primarily fro m two short

- term exposure studies, a relatively modest data set. In light of the limited nature of this information, and in recognition of more general uncertainties inherent in the epidemiological evidence, the Administrator deems it reasonable not to pla ce more emphasis on concentrations in the lower part of this range, as discussed below in section III.E.4.d. With regard to the scope of the distributional statistical analysis, the EPA requested additional population

- level data from the study authors for a group of six multi

city studies for which previous air quality analyses had been conducted (Hassett

- Sipple et al., 2010; Schmidt et al., 2010, Analysis 2). These six studies were originally selected because they considered multiple locations representing varying geographic regions across multiple years. Thus, these studies provided evidence on the influence of different particle mixtures on health effects associated with long

- and short

- term PM 2.5 exposures. In addition, these multi

- city studies considere d relatively more recent health events and air quality conditions (1999 to 2005). As discussed in section III.E.4.b.i above, the EPA received and analyzed population

- level data for four of the six studies (Rajan et al., 2011). T hree of these four studies ( Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) served as the basis for the concentration

- response functions used to***E.O. 12866 Review

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Page 217 of 432 develop the core risk estimates ( U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees that it would be useful to h ave such data from more studies, the Agency believes that the additional data that was requested and received from study authors provide useful information to help inform the Administrator’s selection of the annual standard level. (4) Many commenters in this group highlighted PM 2.5 - related impacts on at

- risk populations, including potential impacts on children, older adults, persons with pre

existing heart and lung disease, and low

- income populations, to support their views that the annual standard should be revised to a level of 11 μg/m 3 or lower (CHPAC, 2012; AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al., 2012; Air Alliance Houston, et al, 2012). These commenters urged the EPA to adopt a policy approach that placed less weight on the remaining un certainties and limitations in the evidence and placed more emphasis on margin of safety considerations, including providing protection against effects for which there is more limited scientific evidence. For example, CHPAC urged the EPA “to place the same weight on studies examining impacts on children’s health as that of adult studies. ... The fact that there may be stronger evidence from adult studies does not mean that standards based on adult studies will be protective for children and consequently will meet the standard requisite to protect public health with an adequate margin of safety” (CHPAC, 2012 p. 3). Furthermore, with regard to the EPA’s approach for weighing uncertainties, some of these commenters stated that “we find no justification in the pre amble for an annual standard level as high as 13 μg/m 3 , other than the vague assertion that uncertainties increase at lower concentrations. Further, the final proposal completely failed to address the Policy Assessment recommendations that if 13 μg/m 3

was proposed, the 24

- hour standard should be strengthened as well” (ALA et al., p. 7).***E.O. 12866 Review

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Page 218 of 432 The EPA has carefully evaluated and considered evidence of effects in at

- risk populations. With regard to effects classified as having evidence of a causal or likely causal relationship with long

- or short

- term PM 2.5 exposures (i.e., premature mortality, cardiovascular effects, and respiratory effects), the Agency takes note that it considered the full range of studies evaluating these effects, including studies of at

- risk po pulations, to inform its review of the primary PM 2.5 standards. Specific multi

- city studies summarized in Figures 1, 2, and 3 above highlight evidence of effects observed in two different lifestages

– children and older adults

– that have been identified a s at

- risk populations. Thus, the EPA places as much weight on studies that explored effects in children for which the evidence is causal or likely causal in nature as on studies of such effects in adults, including older adults

. . As discussed above in resp onses to commenters supporting the retention of the current standards, in setting the standard, the EPA has focused on considering PM 2.5 concentrations somewhat below the lowest long

- term mean concentrations from each of the key studies of both long

- and s hort

- term exposures of effects for which the evidence supports a causal or likely causal relationship (i.e., the first two sets of studies shown in Figure 4). Absent some reason to ignore or discount these studies, which the commenter does not provide (and of which the EPA is unaware) , the EPA considers the available evidence of effects in children as well as other at

- risk populations. With respect to the EPA’s consideration of more limited studies providing evidence suggestive of a causal relationship (e.g ., developmental and reproductive effects), as noted above in responding to comments from the first group of commenters, the Agency agrees that it is important to place some weight on this body of evidence in***E.O. 12866 Review

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Page 219 of 432 setting standards that provide protection for a t

- risk populations, as required by the CAA. However, the Agency does not agree that the same weight must be placed on this information as on the body of scientific information for which there is evidence of a causal or likely causal relationship. To do so would ignore the difference in the breadth and strength of the evidence supporting the different causality determinations reached in the Integrated Science Assessment. With regard to weighing the uncertainties and limitations remaining in the evidence and technical analyses, as discussed in section II.A above, the EPA recognizes that in setting a primary NAAQS that provides an adequate margin of safety, the Administrator must consider a number of factors including the nature and severity of the health effe cts involved, the size of sensitive population(s) at risk, and the kind and degree of the uncertainties that remain. As discussed in section III.E.4.d below, the Agency agrees with these commenters that, in weighing the available evidence and technical

ana lyses including the uncertainties and limitations in this scientific information, there is no justification for setting a primary PM 2.5 annual standard level as high as 13 μg/m 3

. Finally, some commenters in both groups also identified “new” studies that were not included in the Integrated Science Assessment as providing further support for their views on the level of the annual standard. As discussed in section II.B.3 above, the EPA completed a provisional review and assessment of “new” studies published since the close of the Integrated Science Assessment, including “new” studies submitted by commenters (U.S. EPA, 2012a). The provisional assessment found that the “new” studies expand the scientific information considered in the Integrated Science Assessme nt and provide important insights on the relationship between PM 2.5 exposure and health effects***E.O. 12866 Review

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Page 220 of 432 of PM (U.S. EPA, 2012b) . However, the EPA notes that the provisional assessment found that the “new” science did not materially change the conclusions reached i n the Integrated Science Assessment. The EPA notes that, as in past NAAQS reviews, the Agency is basing the final decisions in this review on the studies and related information included in the Integrated Science Assessment that have undergone CASAC and pu blic review, and will consider newly published studies for purposes of decision making in the next PM NAAQS review. ii. 24

- Hour Standard Level With respect to the level of the 24

- hour standard, the EPA received comments on the proposal from two distinct gr oups of commenters. One group that included virtually all commenters representing industry associations , businesses , and many States agreed with the Agency’s proposed decision to retain the level of the 24

- hour PM 2.5 standard. The other group of commenters included many medical groups, numerous physicians and academic researchers, many public health organizations, some State and local agencies, five State Attorneys General, and a large number of individual commenters. These commenters disagreed with the Age ncy’s proposed decision and argued that EPA should lower the level of the 24

- hour standard to 30 or 25 μg/m 3 . Comments from these groups

on the level of the 24

- hour PM 2.5 standard are a ddressed below and in the Response to Comments Document

. As noted above , o f the public commenters who addressed the level of the 24

- hour PM 2.5 standard, all industry commenters and most State and l ocal commenters supported the proposed decision to retain the current level of 35 g/m 3 . In many cases, these groups agreed with t

he rationale supporting the Administrator’s proposed decision to retain the***E.O. 12866 Review

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Page 221 of 432 current 24

- hour PM 2.5 standard, including her emphasis on the annual standard as the generally controlling standard with the 24

- hour standard providing supplementary protection, an d her conclusion that multi

- city, short

- term exposure studies provide the strongest data set for informing decisions on the appropriate 24

- hour standard level. Many of these commenters agreed with the Administrator’s view that the single

- city, short

- term studies provided a much more limited data set (e.g., limited statistical power, limited exposure data) and more equivocal results (e.g., mixed results within the same study area), making them an unsuitable basis for setting the leve l of the 24

- hour standard. While these commenters agreed with the EPA’s proposed decision to retain the current 24

- hour PM 2.5 standard, some did not agree with the EPA’s approach to considering the evidence from short

- term multi

- city studies. For example, a commenter representing UARG pointed out that the 98 th percentile concentrations reported in the proposal for multi

- city studies reflect the averages of 98 th percentile concentrations across the cities included in those studies (UARG, 2012; Attachment 1; p. 25). This commenter contended that such averaged 98 th percentile PM 2.5 concentrations do not provide information that can appropriately inform a decision on the adequacy of the public health protection provided by the current or alternative 24

- hour sta ndards. While the EPA agrees that there is uncertainty in linking effects reported in multi

city studies to specific air quality concentrations (U.S. EPA, 2011, section 2.3.4.1), the EPA disagrees with this commenter’s view that such uncertainty preclude s the use of averaged 98 th percentile PM 2.5 concentrations to inform a decision on the appropriateness of the protection provided by the 24

- hour PM 2.5 standard. In particular, the EPA notes that averaged 98 th percentile concentrations do provide informatio n on the extent to***E.O. 12866 Review

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Page 222 of 432 which study cities contributing to reported associations would likely have met or violated the current 24

- hour PM 2.5 standard during the study period. As evidence of this, the EPA notes the three multi

- city studies specifically highlight ed by this commenter as having averaged 98 th percentile 24

- hour PM 2.5 concentrations below 35 g/m 3 (Dominici et al., 2006; Bell et al., 2008; Zanobetti and Schwartz, 2009). Based on the 98 th percentiles of 24

- hour PM 2.5 concentrations in the individual ci ties evaluated in these studies, the EPA notes that the majority of these study cities would likely have met the current standard during the study periods (Hassett

- Sipple et al., 2010). Therefore, regardless of whether the averaged 98 th percentile concentr ations or the 98 th percentile concentrations in each city are considered, these studies provide evidence for associations between short

- term PM 2.5 and mortality or morbidity across a large number of U.S. cities, the majority of which would likely have met the current 24

- hour PM 2.5 standard during study periods. In their review of the PM Policy Assessment, CASAC endorsed the conclusions drawn from analyses of averaged 98 th percentile 24

- hour PM 2.5 concentrations, and the EPA continues to conclude that this t ype of information can appropriately inform the Administrator’s decision on the current 24

- hour PM 2.5 standard. 92
A nother group of commenters argued that the 24

- hour standard level should be lowered . Many of these commenters supported setting the level of the 24

- hour PM 2.5 standard at either 25 or 30 g/m 3 . In support of their position, the ALA et. al. , AHA et al.,

92 This is not to say that the EPA’s decision on whether to revise the 24

- hour PM 2.5 standard should be bas ed on or only be informed by considerations of whether studies reported associations with mortality or morbidity in areas with averaged 98 th percentile PM 2.5 concentrations less than 35 mg/m 3 . As discussed below, in reaching a decision in this final notice on the most appropriate approach to strengthen the suite of PM 2.5 standards, the Administrator considers the degree of public health protection provided by the combination of the annual and 24

- hour standards together.***E.O. 12866 Review

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Page 223 of 432 five state Attorneys General, and a number of additional groups pointed to 98 th percentile PM 2.5 concentrations in locations of multi

- c ity and single

- city epidemiological studies. For example, the ALA and others pointed to multi

- city studies by Dominici et al. (2006), Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al. (2008) as providing evidence for associations with m ortality and morbidity in study locations with averaged (i.e., averaged across cities) 98 th percentile 24

- hour PM 2.5 concentrations below 35 g/m 3 . These commenters also pointed to several single

- city and panel studies reporting associations between short

- term PM 2.5 and mortality or morbidity in locations with relatively low 24

- hour PM 2.5 concentrations. Because some of these multi

- and single

- city studies have reported associations with health effects in locations with 98 th percentile PM 2.5 concentrations below 35 g/m 3 , commenters maintained that the current 24

- hour
PM 2.5 standard (i.e., with its level of 35 g/m 3 ) does not provide an appropriate degree of protection in all areas. In further support of their position that the level of the current 24

- hour standard should be lowered , these commenters pointed out the variability across the U.S. in ratios of 24

- hour to annual PM 2.5 concentrations. They noted that some locations, including parts of the northwestern U.S., e xperience relatively low annual PM 2.5 concentrations but can experience relatively high 24

- hour concentrations at certain times of the year. In order to provide protection against effects associated with short

- term PM 2.5 exposures, especially in locations with high ratios of 24

- hour to annual PM 2.5 concentrations, these commenters advocated setting a lower level for the 24

- hour standard. The EPA agrees with these commenters that it is appropriate to maintain a 24

- hour PM 2.5 standard in order to supplement the protection provided by the revised annual***E.O. 12866 Review

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Page 224 of 432 standard, particularly in locations with relatively high ratios of 24

- hour to annual PM 2.5 concentrations. However, in highlighting 98 th percentile PM 2.5 concentrations in study locations without also consideri ng the impact of a revised annual standard on short

- term concentrations, these commenters ignore the fact that many areas would be expected to experience decreasing short

- and long

- term PM 2.5 concentrations in response to a revised annual standard. In con sidering the specific multi

- city studies highlighted by public commenters who advocated a more stringen t 24

- hour standard, the EPA notes that such studies have reported consistently positive and statistically significant associations with short

- term PM 2.5 exposures in locations with averaged 98 th percentile PM 2.5 concentrations ranging from 45.8 to 34.2 μg/m 3 and long

- term mean PM 2.5 concentrations rang ing from 13.4 to 12.9 (Burnett and Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006 a ; Bell et al., 2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009). 93 The EPA notes that to the extent air quality distributions are reduced to meet the current 24

- hour standard with its level of 35 g/m 3 and/or the revised annual standard with its level of 12 g/m 3 , additional protection would be anticipated against the effects reported in these short

- term , multi

- city studies. Put another way, to attain an annual standard with a level below the long

- term

93 Commenters also highlighted associa tions with short

- term PM 2.5 concentrations reported in sub

- analyses restricted to days with 24

- hour concentrations at or below 35 3 (Dominici, 2006b). These sub

- analyses were not included in the original publication by Dominici et al. (2006a). Authors provided results of sub

- analyses for the Administrator’s consideration in a letter to the docket following publication of the proposed rule in January 2006 (personal communication with Dr. Francesca Dominici, 2006b). As noted in section III.A.3, these sub

- analyses are part of the basis for the conclusion that there is no evidence suggesting that risks associated with long

- term exposures are likely to be disproportionately driven by peak 24

- hour concentrations. Because the sub

- analyses did not present long

- t erm average PM 2.5 concentrations, it is not clear whether they reflected PM 2.5 air quality that would have been allowed by the revised annual PM 2.5 standard being established in this notice.***E.O. 12866 Review

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Page 225 of 432 means in the locations of these short

- term studies (as E PA is adopting here), the overall air quality distributions in the majority of study cities will necessarily be reduced, resulting in lower daily PM 2.5 ambient concentrations. We therefore expect that the revised annual standard will result in 98 th percentile PM 2.5 concentrations in these cities that are lower than those measured in the studies, and that the overall distributions of PM 2.5 concentrations will be lower than those reported to be associated with health effects. Thus, even for effects re ported in multi

- city studies with averaged 98 th percentile concentrations below 35 μ g/m 3 , additional protection from the risks associated with short

- term exposures is anticipated from the revised annual standard, without revising the 24

- hour standard, beca

use long

- term average PM 2.5 concentrations in multi

- city study locations were above the level of the revised annual standard (i.e., 12 μ g/m 3 ). 94 As discussed above, reducing the annual standard is the most efficient way to reduce the risks from short

- term e xposures identified in these studies, as the bulk of the risk comes from the large number of days across the bulk of the air quality distribution, not the relatively small number of days with peak concentrations. In considering the single

- city studies hig hlighted by public commenters who advocated a more stringen t 24

- hour standard, the EPA first notes that, overall, these single

- city studies reported mixed results. Specifically, some studies reported positive and statistically significant associations with PM 2.5 , some studies reported positive but

non - significant associations, and several studies reported negative associations or a mix

94 It is also the case that additional protection is anticipated i n locations with 98 th percentile 24

- hour PM 2.5 concentrations above 35 μg/m 3 , even if long

- term concentrations are below 12 μg/m 3 . As noted in the proposal and in the Policy Assessment (U.S. EPA, 2011, Figure 2

- 10), parts of the northwestern U.S. are more likely than other parts of the country to violate the 24

- hour standard and meet the revised annual standard.***E.O. 12866 Review

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Page 226 of 43 2 of positive and negative associations with PM 2.5 . In light of these inconsistent results, the proposal noted that the overa ll body of evidence from single

- city studies is mixed, particularly in locations with 98 th percentile s of 24

- hour concentrations below 35 μg/m 3

. Therefore, a lthough some single

- city studies report ed effects at appreciably lower PM 2.5 concentrations than short

- term multi

- city studies, the uncertainties and limitations associated with the single

- city studies were noted to be greater . In light of these greater uncertainties and limitations, the Administrator concluded in the proposal that she had less confid ence in using these studies as a basis for setting the level of the standard (77 FR 38943). Given the considerations and conclusions noted above, in the proposal the Administrator concluded that the short

- term multi

- city studies provide the strongest evid ence to inform decisions on the level of the 24

- hour stan dard. F urther , she viewed single

- city, short

- term exposure studies as a much more limited data set providing mixed results, and she had less confidence in using these studies as a basis for setting t he level of a 24

- hour standard (77 FR 38942)

. . In highlighting specific single

- city studies, public health, environmental, and State and local commenters appear to have selectively focused on studies reporting associations with PM 2.5 and to have overlooked studies that reported more equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004; Slaughter et al., 2005; Villeneuve et al., 2006) (U.S. EPA, 2011, Figure 2

- 9). As such, these commenters have not presented new information that causes the E PA to reconsider its decision to emphasize multi

- city studies over single

- city studies when identifying the appropriate level of the 24

- hour PM 2.5 standard. In further considering the single

- city studies highlighted by public commenters,***E.O. 12866 Review

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Page 227 of 432 the EPA notes that some commenters advocating for a lower level for the 24

- hour PM 2.5 standard also discussed short

- term studies that have been published since the close of the Integrated Science Assessment. These recent studies were conducted in single cities or in small p anels of volunteers. As in prior NAAQS reviews and as discussed above in more detail (section II.B.3) , the EPA is basing its decisions in this review on studies and related information assessed in the Integrated Science Assessment. The studies assessed in the Integrated Science Assessment, and the conclusions based on those studies, have undergone extensive critical review by the EPA, CASAC, and the public. The rigor of that review makes the studies assessed in the Integrated Science Assessment, and the

con clusions based on those studies, the most reliable source of scientific information on which to base decisions on the NAAQS. However, as discussed above (section II.B.3) , the EPA recognizes that ‘‘new studies” may sometimes be of such significance that it is appropriate to delay a decision on revision of a NAAQS and to supplement the pertinent air quality criteria so the studies can be taken into account. In the present case, the EPA’s provisional consideration of ‘‘new studies” concludes that, taken in context, the “new” information and findings do not materially change any of the broad scientific conclusions made in the air quality criteria regarding the health effects of PM 2.5 ( U.S. EPA , 2012a). For this reason, reopening the air quality criteria review would not be warranted , even if there were time to do so under the court order governing the schedule for completing this review . Accordingly, the EPA is basing its final decisio ns in this review on the studies and related information included in the PM Integrated Science Assessment (i.e., the air quality criteria) that has undergone CASAC and public review. The EPA will***E.O. 12866 Review

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Page 228 of 432 consider the “new studies” in the next periodic review of th e PM NAAQS, which will provide an opportunity to fully assess these studies through a more rigorous review process involving the EPA, CASAC, and the public. Some p ublic health, medical , and env ironmental commenters also criticized the EPA’s interpretation of PM 2.5 risk results. These commenters presented risk estimates for combinations of annual and 24

- hour standards using more recent air quality data than that used in the EPA’s Risk Assessment (U.S. EPA, 2010a). Based on these additional risk analyses, th e ALA and other commenters contended that public health benefits could continue to increase as annual and 24

- hour standard levels decrease below 13 g/m 3 and 35 g/m 3 , respectively

. The EPA agrees with commenters that important public health benefits are expected as a result of revising the level of the annual standard to 12 g/m 3 , as is done in this notice, rather than 13 g/m 3 . The Agency also acknowledges that estimated PM 2.5 -

assoc iated health risks continue to decrease with annual standard levels below 12 g/m 3
and/or with 24

- hour standard levels below 35 g/m 3 . However, the EPA disagrees with the commenters’ views regarding the extent to which risk estimates support setting standa rd levels below 12 μg/m 3 (annual standard) and 35 g/m 3 (24

- hour standard). 95 The CAA charges the Administrator with setting NAAQS that are “requisite” (i.e., neither more nor less stringent than necessary) to protect public health with an adequate margin of safety . In setting such standards the Administrator must weigh the available scientific evidence and information, including associated uncertainties and limitations . As described above, in reaching her proposed decisions on the PM 2.5
95 This section focus es on the 24

- hour standard. Section III.E.4.c.i above also discusses these commenters’ recommendations within the context of the annual PM 2.5 standard.***E.O. 12866 Review

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Page 229 of 432 standards that would provide “requisite” protection , the Administrator carefully considered the available scientific evidence and risk information , making public health policy judgments that , in her view, neither overstate d nor understate d the streng ths and limitations of that evidence and information. In contrast, a s discussed more fully above , public health, medical, and environmental commenters who recommended levels below 35 g/m 3 for the 24

- hour PM 2.5 standard have not provided new information or analyses to suggest that such standard levels are appropriate, given the uncertainties and limitations in the available health evidence , particularly uncertainties in studies conducted in locations with 98 th percentile 24

- hour PM 2.5 concentrations below 3 5 g/m 3 and long

- term average concentrations below 12 g/m 3

. d. Administrator’s Final Conclusions on the Primary PM 2.5 Standard Levels In reaching her conclusions regarding appropriate standard levels, the Administrator has considered the epidemiological and other scientific evidence, estimates of risk reductions associated with just meeting alternative annual and/or 24

- hour standards, air quality analyses, related limitations and uncertainties, the advice of CASAC, and extensive public comments on the propos al. After careful consideration of all of these, the Administrator has decided to revise the level of the primary annual PM 2.5 standard from 15.0 μg/m 3 to 12.0 μg/m 3 and to retain the level of the primary 24

- hour standard at 35 μg/m 3

. As an initial matter, the Administrator agrees with the approach supported by CASAC and discussed in the Policy Assessment as summarized in sections III.A.3 and III.E.4.a above, of considering the annual and 24

- hour standards together in determining the protection afforded aga inst mortality and morbidity effects associated with both long

-***E.O. 12866 Review

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Page 230 of 432 and short

- term exposures to PM 2.5 . This approach is consistent with the approach taken in the review completed in 1997, in contrast to the approach used in the review completed in 2006 where e ach standard was considered independently of the other (i.e., only data from long

- term exposure studies were used to inform the level of the annual standard and only data from short

- term exposure studies were used to inform the level of the 24

- hour standar d). 96 Based on the evidence and quantitative risk assessment, the Administrator concludes that it is appropriate to set an annual standard that is generally controlling, which will lower the broad distribution of 24

- hour average concentrations in an area a s well as the annual average concentration, so as to provide protection from both long

- and short

- term PM 2.5 exposures. In conjunction with this, it is appropriate to set a 24

- hour standard focused on providing supplemental protection, particularly for are as with high peak

- to

- mean ratios of 24

- hour concentrations, possibly associated with strong local or seasonal sources, and for PM 2.5 - related effects that may be associated with shorter

- than daily exposure periods. The Administrator concludes this approach will reduce aggregate risks associated with both long

- and short

- term exposures more consistently than a generally controlling 24

- hour standard and is the most effective and efficient way to reduce total PM 2

- 5 - related population risk and to protect public health with an adequate margin of safety

. In selecting the level of the annual PM 2.5 standard, b ased on the characterization and assessment of the epidemiological and other studies presented and assessed in the Integrated Science Assessment and the Policy Assessment, the Administrator recognizes the substantial increase in the number and diversity of studies available in this review

. 96 See 71 FR 61148 and 61168, October 17, 2006.***E.O. 12866 Review

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Page 231 of 432 This expanded body of evidence includes extended analyses of the seminal studies of long

- term PM 2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important new long

- term exposure studies (as summarized in Figures 1 and 2). Collectively, the Administrator notes that these studies, along with evidence available in the last review, provide consistent and stronger evidence than previously observed of an association between long

- term PM 2.5 exposures and premature mortality in areas with lower long

term ambient concentrations than previously observed, with the strongest evidence related to cardiovascular

- related mort ality. The Administrator also recognizes the availability of stronger evidence of morbidity effects associated with long

- term PM 2.5 exposures, including evidence of respiratory effects such as decreased lung function growth, from the extended analyses for the Southern California Children’s Health Study and evidence of cardiovascular effects from the WHI study. Furthermore, the Administrator recognizes new U.S. multi

- city studies that greatly expand and reinforce our understanding of mortality and morbidity effects associated with short

- term PM 2.5 exposures, providing stronger evidence of associations in areas with ambient concentrations similar to those previously observed in short

- term exposure studies considered in the previous review (as summarized in Fig ure 3). The Administrator recognizes the strength of the scientific evidence for evaluating health effects associated with fine particles, noting that the newly available scientific evidence builds upon the previous scientific data base to provide evidenc e of generally robust associations and a basis for greater confidence in the reported associations than in the last review. She notes the conclusion of the Integrated Science Assessment that this body of evidence supports a causal relationship between long

- and short

- term PM 2.5***E.O. 12866 Review

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Page 232 of 432 exposures and mortality and cardiovascular effects and a likely causal relationship between long

- and short

- term PM 2.5 exposures and respiratory effects. In addition, the Administrator notes additional, but more limited evidence, for a broader range of health endpoints including evidence suggestive of a causal relationship for developmental and reproductive effects as well as for carcinogenic effects. Based on information discussed and presented in the Integrated Science Assessment, t

he Administrator recognizes that health effects may occur over the full range of concentrations observed in the epidemiological studies of both long

- term and short

- term exposures, since no discernible population

- level threshold for any such effects can be identified based on the currently available evidence (U.S. EPA, 2009a, section 2.4.3). To inform her decisions on an appropriate level for the annual standard that will protect public health with an adequate margin of safety, in the absence of any discerni ble population

- level thresholds, the Administrator judges that it is appropriate to consider the relative degree of confidence in the magnitude and significance of the associations observed in epidemiological stud ies across the range of long

- term PM 2.5 con centrations in such studies . Further, she recognizes, in taking note of CASAC advice and the distributional statistics analysis discussed in the Policy Assessment and in section III.E.4.a above, that there is significantly greater confidence in the magnitu de and significance of observed associations for the part of the air quality distribution corresponding to where the bulk of the health events evaluated in each study have been observed , generally at and around the long

- term mean concentrations . Conversely , she also recognizes that there is significantly diminished confidence in the magnitude and significance of observed associations in the low er part of the air quality distribution***E.O. 12866 Review

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Page 233 of 432 corresponding to where a relatively small proportion of the health events w ere observed. F urther, the Administrator recognizes that the long

- term mean concentrations, or any other specific point in the air quality distribution of each study, do not represent a “bright line” at and above which effects have been observed and below which effects have not been observed. I n considering the long

- term mean concentrations reported in epidemiological studies, the Administrator recognizes that in selecting a level of the annual standard that will protect public health with an adequate margin of safety , it is not sufficient to focus on a concentration generally somewhere within the range of long

- term mean concentrations from the key long

- term and short

- term exposure studies that reported lower concentrations than had been observed in ear lier reviews. These key studies provide information for various types of serious health endpoints (including mortality and morbidity effects), different study populations (which may include at

- risk populations such as children and older adults), and differ ent air quality distributions that are specific to each study. A level somewhere within the range of long

- term mean concentrations of the full set of key studies would be higher than the long

- term mean of at least one of the studies being considered and th erefore would not provide a sufficient degree of protection against the health effects observed in that study. Absent some reasoned basis to place less weight on the evidence in the epidemiological study with the lowest long

- term mean concentration among t

hese key studies, this approach would not be consistent with the requirement to set a standard that will protect public health with an adequate margin of safety. 97 Thus, the Administrator recognizes it is important to protect against the serious

97 See American Farm Bureau Federation v. EPA , 559 F. 3d 512, 525

- 26 (D.C. Cir. 2009).***E.O. 12866 Review

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Page 234 of 432 effects obs erved in each of these studies so as to protect public health with an adequate margin of safety . In so doing, she looks to identify the study with the lowest long

- term mean concentration within the full set of key studies to help inform her decision of the appropriate standard level which will provide protection for the broad array of health outcomes observed in all of the studies, including effects observed in at

- risk populations. Further, consistent with the general approach summarized in section III.E.4. a above and supported by CASAC as discussed in section III.E.4.b.ii above, the Administrator recognizes that it is appropriate to consider a level for an annual standard that is not just at but rather is somewhat below the long

- term mean PM 2.5 concentratio ns reported in each of the key long

- and short

- term exposure studies. In so doing, she focuses especially on multi

- city studies that evaluated health endpoints for which the associations are causal or likely causal (i.e., mortality and cardiovascular and r espiratory effects associated with both long

- and short

- term PM 2.5 exposures). As discussed above, the importance of considering a level somewhat below the lowest long

- term mean concentrations in this set of key studies is to establish a standard that woul d be protective against the observed effects in all of the studies, and that takes into account the relative degree of confidence in the magnitude and significance of observed associations across the air quality distributions in these studies. The Adminis trator recognizes that there is no clear way to identify how much below the long

- term mean concentrations of key studies to set a standard that would provide requisite protection with an adequate margin of safety. She therefore must use her judgment to wei gh the available scientific and technical information, and associated uncertainties, to reach a final decision on the appropriate standard level. In considering***E.O. 12866 Review

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Page 235 of 432 the information in Figures 1

- 4 for effects classified as having evidence of a causal or likel y causal relationship with long

- or short

- term PM 2.5 exposures, she observes a cluster of short

- term exposure studies with long

- term mean concentrations within a range of 13.4 μg/m 3 down to 12.8 μg/m 3 (Dominici et al., 2006; Burnett and Goldberg, 2003; Zano betti and Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also observes a cluster of long

- term exposure studies with long

- term mean concentrations within a range of 14.5 μg/m 3 to 13.6 μg/m 3 (Dockery et al., 1996; Lipfert et al., 2006; Zeger e t al., 2008; McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the reasons discussed in response to public comments in section III.E.4.c above, the Administrator is less influenced by the long

- term mean PM 2.5 concentrations from the Mille r et al. (2007) and Krewski et al. (2009) studies with reported long

- term mean PM 2.5 concentrations of 12.9 and 14.0 μg/m 3 , respectively. In each case, the most relevant exposure periods would likely have had higher mean PM 2.5 concentrations than those rep orted in the studies. 98 Thus, the Administrator considers the long

- term mean PM 2.5 concentrations from these two studies to be a highly uncertain basis for informing her selection of the annual standard level. 99 To help guide her judgment of the appropriate level below the long

- term mean concentrations in the epidemiological studies at which to set the standard, the Administrator considered additional information from epidemiological studies

98 In the case of Miller et al. (2007), the mean concentration is based on a single year of air quality data which post

- dated by two years the period for which the health events data were collected. In the case of Krewski et a l. (2009),the air quality data were based on the last two years of the 18

- year period for which the health event data were collected. 99 Nonetheless, as noted above, the EPA notes that the Krewski et al. (2009) and Miller et al. (2007) studies provide stro ng evidence of mortality and cardiovascular

- related effects associated with long

- term PM 2.5 exposures to inform causality determinations reached in the Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).***E.O. 12866 Review

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Page 236 of 432 concerning the broader distribution of PM 2.5 concentrations which c orrespond to the health events observed in these studies (e.g., deaths, hospitalizations). The Administrator observes that the development and use of this information in considering standard levels is consistent with CASAC’s advice, as discussed in section III.E.4.b.ii above, to focus on understanding the concentrations that were most influential in generating the health effect estimates in individual studies (Samet, 2010d, p. 2). In considering this additional population

- level information, the Administrat or recognizes that, in general, the confidence in the magnitude and significance of an association identified in a study is strongest at and around the long

- term mean concentration for the air quality distribution, as this represents the part of the distri bution in which the data in any given study are generally most concentrated. She also recognizes that the degree of confidence decreases as one moves towards the lower part of the distribution. Consistent with the approach used in the Policy Assessment, th e Administrator believes that the range from approximately the 25 th to 10 th percentiles is a reasonable range for providing a general frame of reference as to the part of the distribution in which her confidence in the associations observed in epidemiologi cal studies is appreciably lower. However, as noted above, it is important to emphasize that there is no clear dividing line or single percentile within a given distribution provided by the scientific evidence that is most appropriate or ‘correct’ to use to characterize where the degree of confidence in the associations warrants setting the annual standard level. The decision of the appropriate standard level below the long

- term mean concentrations of the key studies , which in conjunction with the other el ements of the standard would protect public health with an adequate margin of safety, is largely a public health policy***E.O. 12866 Review

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Page 237 of 432 judgment, taking into account all of the evidence and its related uncertainties. As discussed in section III.E.4.b, the Administrator t akes note of additional population

- level data that were made available to the EPA by study authors. 100 In considering this information, the Administrator particularly focuses on the analysis of the distributions of the health event data for each area within these studies and the corresponding air quality data for the two short

- term exposure studies (Zanobetti and Schwartz, 2009; Bell et al., 2008). These short

- term exposure studies evaluate the relationship between daily changes (one or more days) in PM 2.5 co ncentrations and daily changes in health events (e.g., deaths, hospitalizations), such that the air quality concentrations that comprise the most relevant exposure periods in these studies are contemporaneous with the health event data. In addition, these studies considered more recent air quality data, representing generally lower PM 2.5 concentrations, in a large number of study areas across the U.S. Thus, such studies provide the most useful evidence for an analysis evaluating the distribution of health e vent data and the corresponding long

- term mean PM 2.5 concentrations across the areas included in each multi

- city study. The Administrator also considered the additional population

- level data that were made available to EPA for two long

- term exposure studi es (Krewski et al., 2009; Miller

100 As summarized in section II I.E.4.a, population

- level data were provided to the EPA for four studies. These four studies represent some of the strongest evidence showing associations between health effects and PM 2.5 within the overall body of scientific evidence and include three stu dies (Krewski et al., 2009; Bell et al., 2008; and Zanobetti and Schwartz, 2009) that were used as the basis for concentration

- response functions in the quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). T he Administrator recognizes that the ad ditional population

- level data available for these four multi

- city studies represents a more limited data set compared to the set of long

- term mean PM 2.5 concentrations which were available in the published literature for all studies considered in the Inte grated Science Assessment.***E.O. 12866 Review

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Page 238 of 432 et al., 2007). She re cognizes that in long

- term exposure studies investigators follow a specific group of study participants (i.e., cohort) over time and across urban study areas, and evaluate how PM 2.5 concentrations avera ged over a period of years are associated with specific health endpoints (e.g., deaths) across cities. As discussed in response to public comments in section III.E.4.c, disentangling the effects observed in long

- term exposure studies associated with more r ecent air quality measurements from effects that may have been associated with earlier, and most likely higher, PM 2.5 exposures introduces some uncertainty with regard to understanding the appropriate exposure window associated with the observed effects. T his is in contrast to the short

- term exposure studies where the relevant exposure period is contemporaneous to the period for which the health data were collected. In light of these considerations, as noted above, the Administrator considers the analysis o f air quality concentrations that correspond to the distribution of population

- level data in these two studies to be a highly uncertain basis for informing her selection of the annual standard level. Based on the above considerations, the Administrator vi ews the additional population

- level data for the two short

- term exposure studies as appropriate to help inform her judgment of how much below the long

- term mean concentrations to set the level of the annual standard. The Administrator notes that the long

- t erm mean PM 2.5 concentrations corresponding with study areas contributing to the 25 th percentiles of the distribution of deaths and cardiovascular

- related hospitalizations in these two short

- term exposure studies were 12.5 μg/m 3 and 11.5 μg/m 3 , respectivel y, for Zanobetti and Schwartz (2009) and for Bell et al. (2008) , with the 10 th percentiles being lower by approximately 2 μg/m 3 in each study.

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Page 239 of 432 The Administrator recognizes, as summarized in section III.B above and discussed more fully in section III.B.2 of the proposal, that important uncertainties remain in the evidence and information considered in this review of the primary fine particle standards. These uncertainties are generally related to understanding the relative toxicity of the different compone nts in the fine particle mixture, the role of PM 2.5 in the complex ambient mixture, exposure measurement errors, and the nature and magnitude of estimated risks related to increasingly lower ambient PM 2.5 concentrations. Furthermore, the Administrator note s that epidemiological studies have reported heterogeneity in responses both within and between cities and geographic regions across the U.S. She recognizes that this heterogeneity may be attributed, in part, to differences in fine particle composition in different regions and cities. 101 With regard to evidence for reproductive and developmental effects identified as being suggestive of a causal relationship with long

- term PM 2.5 exposures, the Administrator recognizes that t here are a number of limitations as sociated with this body of evidence including: 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. Nonetheless, the Administrator believes that this more limited body of evidence provides some support for considering that seriou s effects may be occurring in a
101 Nonetheless, as explained in section III.E.1, the currently available evidence is not sufficient to support replacing or supplementing the PM 2.5 indicator with any other indicator defined in terms of a specific fine particle c omponent or group of components associated with any source categories of fine particles. Furthermore, the evidence is not sufficient to support eliminating any component or group of components associated with any source categories of fine particles from th e mix of fine particles included in the PM 2.5 indicator.***E.O. 12866 Review

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Page 240 of 432 susceptible population at concentrations lower than those associated with effects classified as having a causal or likely causal relationship with long

- term PM 2.5 exposures (i.e., mortality, cardiovascular, and respiratory e ffects). Overall, the Administrator believes that the available evidence interpreted in light of the remaining uncertainties, as summarized above and discussed more fully in the Integrated Science Assessment and the Policy Assessment, provides increased co nfidence relative to information available in the last review and provides a strong basis for informing her final decisions in the current review. The Administrator is mindful that considering what standards are requisite to protect public health with an a dequate margin of safety requires public health policy judgments that neither overstate nor understate the strength and limitations of the evidence or the appropriate inferences to be drawn from the evidence. In considering how to translate the available i nformation into appropriate standard levels, the Administrator weighs the available scientific information and associated uncertainties and limitations. For the purpose of determining what annual standard level is appropriate the Administrator recognizes t hat there is no single factor or criterion that comprises the ‘‘correct’’ approach to weighing the various types of available evidence and information. In considering this information, the Administrator notes the advice of CASAC that “there are significant public health consequences at the current levels of the standards that justify consideration of lowering the PM 2.5 NAAQS further” (Samet, 2010c, p. 12). In addition, she recognizes that CASAC concluded, “although there is increasing uncertainty at lower l evels, there is no evidence of a threshold (i.e., a level below which there is no risk for adverse effects)” (Samet, 2010, p.ii) and that the final***E.O. 12866 Review

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Page 241 of 432 decisions on standard levels must reflect a judgment of the available scientific information with respect to her interpretation of the CAA’s requirement to set primary standards that provide requisite protection to public health with an adequate margin of safety (Samet, 2010d, p. 4). The Administrator recognizes CASAC’s advice that the currently available scient ific information provided support for considering an annual standard level within a range of 13 to 11 μg/m 3 and a 24

- hour standard level within a range of 35 to 30 μg/m 3 . In considering how the annual and 24

- hour standards work together to provide appropri ate public health protection, the Administrator observes that CASAC did not express support for any specific levels or combinations of standards within these ranges. She also notes that CASAC encouraged the EPA staff to consider additional data from epidem iological studies to help quantify the characterization of the PM 2.5 concentrations that were most influential in generating the health effect estimates in these studies (Samet, 2010d, p. 2). In response to CASAC’s advice, the Administrator recognizes tha t the EPA staff acquired additional data from authors of key epidemiological studies and analyzed these data to characterize the distribution of PM 2.5 concentrations in relation to health events data to better understand the degree of confidence in the ass ociations observed in the studies as discussed above. The Administrator recognizes that the final Policy Assessment included consideration of these additional analyses in reaching final staff conclusions with regard to the broadest range of alternative sta ndard levels supported by the science. She takes note that the final Policy Assessment concluded that while alternative standard levels within the range of 13 to 11 μg/m 3 were appropriate to consider, the evidence most strongly supported consideration of a n annual standard level***E.O. 12866 Review

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Page 242 of 432 in the range of 12 to 11 μg/m 3 . The Administrator is aware that, in transmitting the final Policy Assessment to CASAC, the Agency notified CASAC that the final staff conclusions reflected consideration of CASAC’s advice and that tho se staff conclusions were based, in part, on the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on the final Policy Assessment, but chose not to provide any additional comm ents or advice after receiving it. In selecting the annual standard level, the Administrator has considered many factors including the nature and severity of the health effects involved, the strength of the overall body of scientific evidence as considere d in reaching causality determinations, the size of the at

- risk populations, and the estimated public health impacts. She has also considered the kind and degree of the uncertainties that remain in the available scientific information. She recognizes that the association between PM 2.5 and serious health effects is well established, including at concentrations below those allowed by the current standard. Further, she recognizes the CAA requirement that requires primary standards to provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information as well as to provide a reasonable degree of protection against hazards that research has not yet identified. In considering the currently av ailable evidence, as summarized and discussed more broadly above, the information on risk, CASAC advice, the conclusions of the Policy Assessment, and public comments on the proposal, the Administrator strongly believes that a lower annual standard level i s needed to protect public health with an adequate margin of safety. In reaching her final decision on the appropriate annual standard level to set, the***E.O. 12866 Review

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Page 243 of 432 Administrator is mindful that the CAA does not require that primary standards be set at a

zero - risk level, but rather at a level that reduces risk sufficiently so as to pro tect public health, including the health of at

- risk populations, with an adequate margin of safety. On balance, the Administrator concludes that an annual standard level of 12 μg/m 3 would be requisite to protect the public health with an adequate margin of safety from effects associated with long

- and short

- term PM 2.5 exposures, while still recognizing that uncertainties remain in the scientific information. In the Administrator’s judgment, an annual standard of 12 μg/m 3 appropriately reflects placing grea test weight on evidence of effects for which the Integrated Science Assessment determined there is a causal or likely causal relationship with long

- and short

- term PM 2.5 exposures. An annual standard level of 12 μg/m 3 is below the long

- term mean PM 2.5 conc entrations reported in each of the key multi

- city, long

- and short

- term exposures studies providing evidence of an array of serious health effects (e.g., premature mortality, increased hospitalization for cardiovascular and respiratory effects). As noted above, the importance of cons idering a level somewhat below the lowest long

- term mean concentration in the full set of studies considered is to set a standard that would provide appropriate protecti on against the observed effects in all such studies. In reaching her decision, the Adm inistrator has taken into account that at and around the mean PM 2.5 concentration in any given study represents a part of the air quality distribution in which the health event data in that study are generally most concentrated. Furthermore, in identifying an appropriate annual standard level below the long

- term mean concentrations, she recognizes that there is no evidence to support the existence of any discernible threshold, and, therefore, she has a high degree of confidence***E.O. 12866 Review

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Page 244 of 432 that the observed effects are associated with concentrations not just at but extending somewhat below the long

- term mean concentration. To further inform her judgment in setting the annual standard level so as to protect public health with an adequate margin of safety , the Administrat or has placed weight on additional population

- level information available from a subset of these epidemiological studies, consistent with CASAC advice. In particular, she has drawn from two short

- term exposure studies, which provide the most relevant infor mation for evaluating the distribution of health events and corresponding long

- term PM 2.5 concentrations. As explained above, this helps inform her judgment as to the degree of confidence in the observed associations in the epidemiological studies. In this regard, the Administrator generally judges the region around the 25 th percentile as a reasonable part of the distribution to help guide her decision on the appropriate standard level. Since this evidence comes primarily from two studies, a relatively mode st data set, the Administrator deems it reasonable not to draw further inferences from air quality and health event data in the lower part of the distribution for the purpose of setting a standard level. The Administrator notes that the long

- term mean PM 2. 5 concentrations around the 25 th percentile of the distributions of deaths and cardiovascular

- related hospitalizations were approximately around 12 μg/m 3 in these two studies . The Administrator views this information as helpful in guiding her determination as to where her confidence in the magnitude and significance of the associations is reduced to such a degree that a standard set at a lower level would not be
warrant ed to provide requisite protection that is neither more nor less than needed to provide a n adequate margin of safety

. The Administrator also recognizes that a level of 12 μg/m 3 places some weight on***E.O. 12866 Review

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Page 245 of 432 studies which provide evidence of reproductive and developmental effects (e.g., infant mortality, low birth weight). These studies were identified in the Integrated Science Assessment as having evidence suggestive of a causal relationship with long

- term PM 2.5 concentrations. A level of 12 μg/m 3 is approximately the same level as the lowest long

term mean concentration reported in such studies (Figur es 2 and 4; 11.9 μg/m 3 for Bell et al., 2007). 102 While the Administrator acknowledges that this evidence is limited, she believes it is appropriate to place some weight on these studies in order to set a standard that provides protection with an adequate ma rgin of safety, including providing protection for at

- risk populations, as required by the CAA. Due to the limited nature of this evidence, she has determined it is not necessary to set a standard below the lowest long

- term mean concentration in these stud ies. In reflecting on extensive public comments received on the proposal as discussed in section III.E.4.c above, the Administrator recognizes that some commenters have offered different evaluations of the evidence and other information available in this review and would make different judgments about the weight to place on the relative strengths and limitations of the scientific information and about how such information could be used in making public health policy decisions on the annual standard level. One group of such commenters who supported a higher annual standard level (e.g., above 13 μg/m 3 ) would place greater weight on the remaining uncertainties in the evidence as a

102 With respect to cancer, mutagenic, and genotoxic effects, the Administrator observes that the PM 2.5 concentrations reported in studies evaluating these effects generally included ambient concentra tions that are equal to or greater than ambient concentrations observed in studies that reported mortality and cardiovascular and respiratory effects (U.S. EPA, 2009a, section 7.5). Therefore, the Administrator concludes that in selecting alternative stand ard levels that provide protection from mortality and cardiovascular and respiratory effects, it is reasonable to anticipate that protection will also be provided for carcinogenic effects.***E.O. 12866 Review

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Page 246 of 432 basis for supporting a higher standard discounting the overall strength of the evidence than the Administrator judges to be appropriate. Such an approach is based on these commenters’ judgment that the uncertainties remaining in the evidence are too great to warrant setting a n lower annual standard below the current level. The Admini strator does not agree. As an initial matter, an annual standard level of 13 μg/m 3 or higher would be above the long

- term mean concentrations reported in two well

- conducted, multi

- city short

- term exposure studies reporting positive and statistically signi ficant associations of serious effects (Burnett et al., 2004 and Bell et al., 2008). These important studies are fully consistent with the pattern of evidence presented by the large body of evidence in this review. As the Administrator recognized in the pr oposal, and as advised by CASAC, the appropriate focus for selecting the level of the annual PM 2.5 standard is on concentrations somewhat below the lowest long

- term mean concentrations from the set of key studies of both long

- term and short

- term PM 2.5 expo sures considered by the EPA (i.e., as shown in Figure 4). Thus, a standard level set at 13 μg/m 3 or higher would clearly not provide protection for the effects observed in the full set epidemiological studies and, therefore, this standard level could not b e judged to be requisite with an adequate margin of safety. 103 In addition, as noted above, in recognizing that there is no evidence to support the

103 The Administrator is mindful that, in reviewing the 2006 final PM NAAQS decisions, the D.C. Circuit Court of Appeals concluded that the EPA failed to adequately explain why that annual standard provided requisite protection from effects associated with both long

- and short

- term exposures or from morbidity effects in chi ldren and other at

- risk populations when long

- term means of important short

- term studies were below the level the Administrator selected for the annual standard. See American Farm Bureau v. EPA. 559 F. 3d 512, 524

- 26. There is no reasonable basis to discou nt these two studies for purposes of setting the level of the annual standard.***E.O. 12866 Review

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Page 247 of 432 existence of a discernible threshold below which an effect would not occur, the Administrator is mindful that effects occur around and below the long

- term mean concentrations reported in both the short

- term and long

- term the epidemiological studies. A standard level of 13 μg/m 3 or higher would not appropriately take into account evidence from the two well

- conduct ed, multi

- city, short

- term exposure studies reporting serious effects with long

- term mean concentrations below 13 μg/m 3 noted above (Burnett et al, 2004; Bell et al., 2008). Such a standard level would also not appropriately take into
account additional po pulation

- level data from a limited number of epidemiological studies. This approach would ignore CASAC’s advice to consider such information in order to better understand the concentrations over which there is a high degree of confidence regarding the magn itude and significance of the associations observed in individual epidemiological studies and where there is appreciably less confidence

. Furthermore, a standard level of 13 μg/m 3 or higher would not appropriately take into account the more limited eviden ce of effects in some at

- risk populations (e.g., low birth weight). In the Administrator’s view, a standard set at this level would not provide protection with an adequate margin of safety, including providing protection for at

- risk populations. The Admini strator is mindful that the CAA requirement that primary standards provide an adequate margin of safety, discussed in section II.A above, was intended to address uncertainties associated with inconclusive scientific and technical information available at t he time of standard setting as well as to provide a reasonable degree of protection against hazards that research has not yet identified. In light of the entire body of evidence as discussed above, the Administrator judges that an annual standard level se t above 12 μg/m 3 would not be sufficient to protect***E.O. 12866 Review

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Page 248 of 432 public health with an adequate margin of safety from the serious health effects associated with long

- and short

- term exposure to PM 2.5. The Administrator also recognizes that a second group of commenters supported a lower annual standard level (e.g., no higher than 11 μg/m 3 ). Such a standard level would reflect placing essentially as much weight on the relatively more limited data providing evidence suggestive of a causal relationship for effects observed in some at

- risk populations (e.g., low birth weight) as on more certain evidence of effects classified as having a causal or likely causal relationship with PM 2.5 exposures. In the Administrator’s view, while it is important to place some weight on such suggestive evidence, it would not be appropriate to place as much weight on it as the commenters would do. An annual standard level of 11 μg/m 3 would also reflect these commenters’ judgment that it is appropriate to focus on a lower part of the distributions of health event data from the small number of epidemiological studies for which this information was made available than the Administrator believes is warranted . In the Administrator’s view, using this type of information to set a standard level of 11 μg/m 3 or below would assume too high a degree of confidence in the magnitude and significance of the associations observed in the lower part of the distributions of health events observed in these studies. Given the uncertainties in the evidence and the limited set of studies for which the EPA has information on the distribution of health event data and corresponding air quality data, the Administrator believes it is not appropriate to focus on the lower part of the distributions of health events data. On balance, the Administrator finds that the available evidence interpreted in light of the remaining uncertainties does not justify a standard level set below 12 μg/m 3 as***E.O. 12866 Review

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Page 249 of 432 necessary to protect public health with an adequate margin of safety. After carefully considering the above considerations and the public comments summarized in section III.E.4.c above, the Administrator has decided to set the level of the primary an nual PM 2.5 standard at 12 μg/m 3 . In her judgment, a standard set at this level provides the requisite degree of public health protection, including the health of at

risk populations, with an adequate margin of safety and is neither more nor less stringent than necessary for this purpose. As discussed above, the Administrator concludes that an approach that focuses on setting a generally controlling annual standard is the most effective and efficient way to reduce total population risk associated with both l ong - and short

- term PM 2.5 exposures. Such an approach would result in more uniform protection across the U.S. than the alternative of setting the levels of the 24

- hour and annual standard such that the 24

- hour standard would generally be the controlling st andard in areas across the country (see section III.A.3). The Administrator recognizes that potential air quality changes associated with meeting an annual standard level of 12.μg/m 3 will result in lowering risks associated with both long

- and short

- term PM 2.5 exposures by lowering the overall air quality distribution. However, the Administrator recognizes that such an annual standard alone would not be expected to offer sufficient protection with an adequate margin of safety against the effects of all sho rt

- term PM 2.5 exposures in all parts of the country . As a result, in conjunction with an annual standard level of 12 μg/m 3 , the Administrator concludes that it is appropriate to continue to provide supplemental protection by means of a 24

- hour standard set at the appropriate level, particularly for areas with high peak

- to

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Page 250 of 432 mean ratios possibly associated with strong local or seasonal sources and for areas with PM 2.5 - related effects that may be associated with shorter

- than

- daily exposure periods. In selectin g the level of a 24

- hour standard meant to provide such supplemental protection, the Administrator relies upon evidence and air quality information from key short

- term exposure studies. In considering these studies, the Administrator notes that to the exte nt air quality distributions in the study areas considered are reduced to meet the current 24

- hour standard (at a level of 35 μg/m 3 ) or to meet the revised annual standard discussed above (at a level of 12 μg/m 3 ), additional protection would be anticipated against the effects observed in these studies. In light of this, when selecting the appropriate level for the 24

- hour standard, the Administrator considers both the 98 th percentiles of 24

- hour PM 2.5 concentrations and the long

- term mean PM 2.5 concentratio ns in the locations of the short

- term exposure studies. She notes that such consideration of both short

- and long

- term PM 2.5 concentrations can inform her decision on the extent to which a given 24

- hour standard, in combination with the revised annual stan dard established in this notice, would provide protection against the health effects reported in short

- term studies. As discussed in section III.E.4.a above, the Administrator concludes that multi

city short

- term exposure studies provide the strongest dat a set for informing her decisions on appropriate 24

- hour standard levels. With regard to the limited number of single

- city studies that reported positive and statistically significant associations for a range of health endpoints related to short

- term PM 2.5 concentrations in areas that would likely have met the current suite of PM 2.5 standards, the Administrator recognizes that many of these studies had significant limitations (e.g., limited statistical power, limited exposure data)***E.O. 12866 Review

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Page 251 of 432 or equivocal results (mix ed results within the same study area) that make them unsuitable to form the basis for setting the level of a 24

- hour standard. With regard to multi

- city studies that evaluated effects associated with short

- term PM 2.5 exposures, the Administrator observes an overall pattern of positive and statistically significant associations in studies with 98 th percentile 24

- hour values averaged across study areas within the range of 45.8 to 34.2 μg/m 3 (Burnett et al., 2004; Zanobe tti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The Administrator notes that, to the extent air quality distributions are reduced to reflect just meeting the current 24

- hour standard,

a dditional protection would be provided for the effects observed in the three multi

- city studies with 98 th percentile values greater than 35 μg/m 3 (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin et al., 2008). In the three additional multi

- city studies with 98 th percentile values below 35 μg/m 3 , specifically 98 th percentile concentrations of 34.2, 34.3, and 34.8 μg/m 3 , the Administrator notes that these studies reported long

- term mean PM 2.5 concentrations of 12.9, 13.2, and 13.4 μg/m 3 , respective ly (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In revising the level of the annual standard to 12 μg/m 3 , as discussed above, the Administrator recognizes that additional protection would be provided for the short

- term effects observed in these multi

- city studies such that revision to the 24

- hour standard would not be warranted. That is, by lowering the level of the annual standard to 12 μg/m 3 , the 98 th percentile of the distribution would be lowered as well such that additiona l protection from effects associated with short

- term exposures would be afforded. Therefore, the epidemiological evidence supports a conclusion that it is appropriate to retain the level of the 24

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Page 252 of 432 standard at 35 μg/m 3 , in conjunction with a revised an nual standard level of 12 μg/m 3

. In addition to considering the epidemiological evidence, the Administrator also has taken into account air quality information based on county

- level 24

- hour and annual design values to understand the implications of revisi ng the annual standard level from 15 to 12 μg/m 3 in conjunction with retaining the 24

- hour standard level at 35 μg/m 3 . She has considered this information to evaluate the public health protection provided by the two standards in combination and to evaluate the most appropriate means of developing a suite of standards providing requisite public health protection with an adequate margin of safety. In considering the air quality information, the Administrator observes that a suite of PM 2.5 standards that incl udes an annual standard level of 12 μg/m 3 and a 24

- hour standard level of 35 μg/m 3 would result in the annual standard as the generally controlling standard in most regions across the country, except for certain areas in the Northwest, where the annual mea n PM 2.5 concentrations have historically been low but where relatively high 24

- hour concentrations occur, often related to seasonal wood smoke emissions (U.S. EPA, 2011a, pp. 2

- 89 to 2

- 91, Figure 2

- 10). In fact, these are the type of areas for which the su pplemental protection afforded by the 24

- hour standard is intended, such that the two standards together provide the requisite degree of protection. The Administrator concludes the current 24

- hour standard at a level of 35 μg/m 3 , in conjunction with a revi sed annual standard level of 12 μg/m 3 , will provide appropriate protection from effects observed in studies in such areas in which the long

- term mean concentrations were below 12 μg/m 3 and the 98 th percentile 24

- hour concentrations were above 35 μg/m 3 (e.g ., areas in the Northwest U.S.).***E.O. 12866 Review

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Page 253 of 432 After carefully taking the public comments and above considerations into account, the Administrator has decided to retain the current level of the primary PM 2.5 24

- hour standard at 35 μg/m 3 in conjunction with revising the annual standard level from 15.0 μg/m 3 to 12.0 μg/m 3

. 104 In the Administrator’s judgment, this suite of primary PM 2.5 standards and the rationale supporting these levels appropriately reflects consideration of the strength of the available evidence and other information and its associated uncertainties as well as the advice of CASAC and consideration of public comments. In the Administrator’s judgment, this suite of primary PM 2.5 standards is sufficient but not more protective than necessary to protect the pu blic health, including at

- risk populations, with an adequate margin of safety from effects associated with long

- and short

- term exposures to fine particles. This suite of standards will provide significant protection from serious health effects including p remature mortality and cardiovascular and respiratory morbidity effects that are causally or likely causally related to long

- and short

term PM 2.5 exposures. These standards will also provide an appropriate degree of protection against other health effects for which there is more limited evidence of effects and causality, such as reproductive and developmental effects. This judgment by the Administrator appropriately considers the requirement for a standard that is requisite to protect public health but is neither more nor less stringent than necessary. 105
D. Administrator’s Final Decisions on Primary PM 2.5 Standards For the reasons discussed above, and taking into account the information and

104 As noted in section II.B.1, Table 1 and section III.E.4.a above, the annual standard level is defined to one decimal place. Throughout this section, the annual standard levels discussed have been denoted as integer values (e.g., 12 μg/m 3 ) for simplicity. 105 The Administrator also judges that this suite of standards addresses the issues raised by the D.C. Circuit’s remand of the 2006 primary annual PM 2.5 standard by appropriately revising that standard.***E.O. 12866 Review

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Page 254 of 432 assessments presented in the Integrated Science Assessment, Risk Assessment, and Policy Assessment, the advice and recommendations of CASAC, and public comments to date, the Administrator revises the current suite of primary PM 2.5 standards. Specifically, the Administrator revises: (1) the level of the primary annual P M 2.5 standard to 12.0 μg/m 3 and (2) the form of the primary annual PM 2.5 standard to one based on the highest appropriate area

- wide monitor in an area, with no option for spatial averaging. In conjunction with revising the primary annual PM 2.5 standard to provide protection from effects associated with long

- and short

- term PM 2.5 exposures, the Administrator retains the level of 35 μg/m 3 and the 98 th percentile form of the primary 24

- hour PM 2.5 standard to continue to provide supplemental protection for area s with high peak PM 2.5 concentrations. The Administrator is not revising the current PM 2.5 indicator or the annual and 24

- hour averaging times for the primary PM 2.5 standards. The Administrator concludes that this suite of standards would be requisite to p rotect public health with an adequate margin of safety against health effects potentially associated with long

- and short

- term PM 2.5 exposures.

VI. Rationale for Final Decisions on the Secondary PM Standards This section presents the Administrator’s final decisions regarding the need to revise the current suite of secondary PM 2.5 and PM 10 standards to address visibility impairment and other welfare effects considered in this review. Specifically, t his section describes the Administrator’s final decision to retain the current suite of secondary PM standards to address PM

- related visibil ity impairment as well as other PM

- related welfare effects, including ecological effects, effects on materials, and climate impacts. This suite***E.O. 12866 Review

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Page 255 of 432 of standards includes an annual PM 2.5 standard of 15 μg/m 3 , a 24

- hour PM 2.5 standard of 35 μg/m 3 , and a 24

- hour PM 10 standard of 150 μg/m 3

. The Administrator is revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. C ontrary to what was proposed, the Administrator has decided not to establish a distinct standard to address PM

- related visibility impairment . The rationale for this decision is presented below

. T he Administrator’s final decision s on the secondary standards are based on a thorough revie w of the latest scientific information published through mid

- 2009 on welfare effects associated with fine and coarse particles in the ambient air , as presented in the Integrated Science Assessment . The final decisions also take into account: (1) staff

asse ssments of the most policy

- relevant information presented and assessed in the Integrated Science Assessment and staff analyses of air quality and visibility effects presented in the Visibility Assessment and the Policy Assessment, upon which staff conclusi ons regarding appropriate considerations in this review are based; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the Integrated Science Assessment, Visibility Assessment, and Policy Assessment at public meetings, in separat e written comments, and in CASAC’s letters to the Administrator; (3) the multiple rounds of public comments received during the development of these documents, both in connection with CASAC meetings and separately; and (4) public comments received on the p roposal. In particular, this section presents background information on the EPA’s previous and current reviews of the secondary PM standards (section VI.A), a summary of the***E.O. 12866 Review

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Page 256 of 432 proposed decisions regarding the secondary PM standards (section VI.B), a discussi on of significant public comments received on those proposed decisions (section VI.C), and the Administrator’s final decisions on the secondary PM standards (section VI.D). A. Background The current suite of secondary PM standards is identical to the suite of primary PM standards set in 2006 , including 24

- hour and annual PM 2.5 standards and a 24

- hour PM 10 standard. The current secondary PM 2.5 standards are intended to provide protection from PM

- related visibility impairment, whereas the entire suite of secondary PM standards is intended to provide protection from other PM

- related effects on public welfare, including effects on sensitive ecosys tems, materials damage and soiling, and climatic and radiative processes. The approach used for reviewing the current suite of secondary PM standards built upon and broadened the approaches used in previous PM NAAQS reviews. The following discussion focuse s particularly on the current secondary PM 2.5 standards related to visibility impairment and provides a summary of the approaches used to review and establish secondary PM 2.5 standards in the last two reviews (section VI.A.1); judicial review of the 2006 s tandards that resulted in the remand of the secondary annual and 24

- hour PM 2.5 NAAQS to the EPA (section VI.A.2); and the approach used in this review for evaluating the secondary PM 2.5 standards (section VI.A.3). 1. Approaches Used in Previous Reviews The original secondary PM 2.5 standards were established in 1997, and a revision to the 24

- hour standard was made in 2006. The approaches used in making final decisions on secondary standards in those reviews, as well as the current review, utilized***E.O. 12866 Review

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Page 257 of 432 different ways to consider the underlying body of scientific evidence. They also reflected an evolution in EPA’s understanding of the nature of the effect on public welfare from PM

- related visibility impairment, from an approach that focus ed only on Federal Class I area visibility impacts to a more multifaceted approach that also consider ed PM

- related impacts on visibility in non - Federal Class I area s , such as in urban areas. This evolution occurred in conjunction with the expansion of available PM data and informati on from visibility

- related studies of public perception, valuation, and personal comfort and well

being. In 1997, the EPA revised the PM NAAQS in part by establishing new identical primary and secondary PM 2.5 standards. In revising the secondary standards , the EPA recognized that PM produces adverse effects on visibility and that impairment of visibility was being experienced throughout the U.S., in multi

- state regions, urban areas, and remote mandatory Federal Class I areas alike. However, in considering an appropriate level for a secondary standard to address adverse effects of PM 2.5 on visibility, the EPA concluded that the determination of a single national level was complicated by important regional differences influenced by factors such as background and current levels of PM 2.5 , composition of PM 2.5 , and average relative humidity. Variations in these factors across regions could thus result in situations where attaining an appropriately protective concentration of fine particles in one region might or might not provide adequate protection in a different region. The EPA also determined that there was insufficient information at that time to establish a level for a national secondary standard that would represent a threshold above which visibility conditi ons would always be adverse and below which visibility conditions would always be acceptable.***E.O. 12866 Review

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Page 258 of 432 Based on an assessment of the potential visibility improvements that would result from reaching attainment with the new primary standards for PM 2.5 , the EPA concl uded that attainment of the annual and 24

- hour PM 2.5 primary standards would lead to visibility improvements in the eastern U.S. at both urban and regional scales, but little or no change in the western U.S., except in and near certain urban areas. The EPA also considered the potential effectiveness of a regional haze program, required by sections 169A and 169B of the CAA 106 to address those effects of PM on visibility that would not be addressed through attainment of the primary PM 2.5 standards. The regional haze program would be designed to address the widespread, regionally uniform type of haze caused by a multitude of sources. The structure and requirements of sections 169A and 169B of the CAA provide for visibility protection programs that can be more res ponsive to the factors contributing to regional differences in visibility than can programs addressing the kinds of nationally applicable secondary NAAQS considered in the 1997 review. The regional haze visibility goal is more protective than a secondary N AAQS since the goal is to eliminate any anthropogenic impairment rather than to provide a level of protection from visibility impairment that is requisite to protect the public welfare. Thus, an important factor considered in the 1997 review was whether

a regional haze program, in conjunction with secondary standards set identical to the suite of PM 2.5 primary standards, would provide appropriate protection for visibility in non

Federal Class I areas. The EPA concluded that the two programs and associated c ontrol strategies should provide such protection due to the regional approaches needed to
106 In 1977, Congress established as a national goal ‘‘the prevention of any future, and the remedying of any existing, impairment of visibility in mandatory Federal Class I areas which impairment results from manmade air pollution,” section 169A(a)(1) of the CAA. The EPA is required by section 169A(a)(4) of the C AA to promulgate regulations to ensure that ‘‘reasonable progress’’ is achieved toward meeting the national goal.***E.O. 12866 Review

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Page 259 of 432 manage emissions of pollutants that impair visibility in many of these areas. For these reasons, in 1997 the EPA concluded that a national regional haze program, comb ined with a nationally applicable level of protection achieved through secondary PM 2.5 standards set identical to the primary PM 2.5 standards, would be more effective for addressing regional variations in the adverse effects of PM 2.5 on visibility than wou ld be national secondary standards for PM with levels lower than the primary PM 2.5 standards. The EPA further recognized that people living in certain urban areas may place a high value on unique scenic resources in or near these areas and as a result migh t experience visibility problems attributable to sources that would not necessarily be addressed by the combined effects of a regional haze program and PM 2.5 secondary standards. The EPA concluded that in such cases, state or local regulatory approaches,

s uch as past action in Colorado to establish a local visibility standard for the City of Denver, would be more appropriate and effective in addressing these special situations because of the localized and unique characteristics of the problems involved. Vis ibility in an urban area located near a mandatory Federal Class I area could also be improved through state implementation of the then

- current visibility regulations, by which emission limitations can be imposed on a source or group of sources found to be contributing to ‘‘reasonably attributable’’ impairment in the mandatory Federal Class I area. Based on these considerations, in 1997 the EPA set secondary PM 2.5 standards identical to the primary PM 2.5 standards, that would work in conjunction with the R egional H aze P rogram to be established under sections 169A and 169B of the CAA, as the most appropriate and effective means of addressing the public welfare effects associated with visibility impairment. Together, the two programs and associated control***E.O. 12866 Review

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Page 260 of 432 strategies were expected to provide appropriate protection against PM

- related visibility impairment and enable all regions of the country to make reasonable progress toward the national visibility goal. In 2006, the EPA revised the suite of secondary PM 2.5 standards to address visibility impairment by making the suite of secondary standards identical to the revised suite of primary PM 2.5 standards. The EPA’s decision regarding the need to revise the suite of secondary PM 2.5 standards reflected a number of n ew developments that had occurred and sources of information that had become available following the 1997 review. First, the EPA promulgated a Regional Haze Program in 1999 (65 FR 35713, July 1, 1999) which required states to establish goals for improving visibility in Federal Class I areas and to adopt control strategies to achieve these goals. Second, extensive new information from visibility and fine particle monitoring networks had become available, allowing for updated characterizations of visibility t rends and PM concentrations in urban areas, as well as Federal Class I areas. These new data allowed the EPA to better characterize visibility impairment in urban areas and the relationship between visibility and PM 2.5 concentrations. Finally, additional s tudies in the U.S. and abroad provided the basis for the establishment of standards and programs to address specific visibility concerns in a number of local areas. These studies (Denver, Phoenix, and British Columbia) utilized photographic representations of visibility impairment and produced reasonably consistent results in terms of the visual ranges found to be generally acceptable by study participants. The EPA considered the information generated by these studies useful in characterizing the nature of particle

- induced haze and for informing judgments about the acceptability of various levels of visual air quality in urban areas***E.O. 12866 Review

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Page 261 of 432 across the U.S. Based largely on this information, the Administrator concluded that it was appropriate to revise the secondary PM 2.5 standards to provide increased protection from visibility impairment principally in urban areas, in conjunction with the regional haze program for protection of visual air quality in Federal Class I areas. In so doing, the Administrator recognized t hat PM

- related visibility impairment is principally related to fine particle concentrations and that perception of visibility impairment is most directly related to short

- term, nearly instantaneous levels of visual air quality. Thus, in considering whether the then

- current suite of secondary standards would provide the appropriate degree of protection, he concluded that it was appropriate to focus on just the 24

- hour secondary PM 2.5 standard to provide requisite protection. The Administrator then considered whether PM 2.5 mass remained the appropriate indicator for a secondary standard to protect visibility, primarily in urban areas. The Administrator noted that PM

- related visibility impairment is principally related to fine particle levels. H ygroscopic compo nents of fine particles, in particular sulfates and nitrates, contribute disproportionately to visibility impairment under high humidity conditions. Particles in the coarse mode generally contribute only marginally to visibility impairment in urban areas. With the substantial addition to the air quality and visibility data made possible by the national urban PM 2.5 monitoring networks, an analysis conducted for the 2006 review found that, in urban areas, visibility levels showed far less difference between e astern and western regions on a 24

- hour or shorter time basis than implied by the largely non

- urban data available in the 1997 review. In analyzing how well PM 2.5 concentrations correlated with visibility in urban locations across the U.S., the 2005 Staff Paper concluded that clear correlations existed between 24

- hour average PM 2.5***E.O. 12866 Review

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Page 262 of 432 concentrations and calculated (i.e., reconstructed) light extinction, which is directly related to visual range (U.S. EPA, 2005, p. 7

- 6). These correlations were similar in the

e astern and western regions of the U.S. These correlations were less influenced by relative humidity and more consistent across regions when PM 2.5 concentrations were averaged over shorter, daylight time periods (e.g., 4 to 8 hours) when relative humidity i n eastern urban areas was generally lower and thus more similar to relative humidity in western urban areas. The 2005 Staff Paper noted that a standard set at any specific PM 2.5 concentration would necessarily result in visual ranges that vary somewhat in urban areas across the country, reflecting the variability in the correlations between PM 2.5 concentrations and light extinction. The 2005 Staff Paper concluded that it was appropriate to use PM 2.5 as an indicator for standards to address visibility impair ment in urban areas, especially when the indicator is defined for a relatively short period (e.g., 4 to 8 hours) of daylight hours (U.S. EPA, 2005, p. 7

- 6). Based on their review of the Staff Paper, most CASAC Panel members also endorsed such a PM 2.5 indic ator for a secondary standard to address visibility impairment (Henderson, 2005a, p. 9). Based on the above considerations, the Administrator concluded that PM 2.5 should be retained as the indicator for fine particles as part of a secondary standard to add ress visibility protection, in conjunction with averaging times from 4 to 24 hours. In considering what level of protection against PM

- related visibility impairment would be appropriate, the Administrator took into account the results of the public percept ion and attitude surveys regarding the acceptability of various degrees of visibility impairment in the U.S. and Canada, state and local visibility standards within the U.S., and visual inspection of photographic representations of several urban areas acro ss the***E.O. 12866 Review

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Page 263 of 432 U.S. In the Administrator’s judgment, these sources provided useful but still quite limited information on the range of levels appropriate for consideration in setting a national visibility standard primarily for urban areas, given the generally sub jective nature of the public welfare effect involved. Based on photographic representations of varying levels of visual air quality, public perception studies, and local and state visibility standards, the 2005 Staff Paper had concluded that 30 to 20 μg/m 3 PM 2.5 represented a reasonable range for a national visibility standard primarily for urban areas, based on a sub

- daily averaging time (U.S. EPA, 2005, p. 7

- 13). The upper end of this range was below the levels at which illustrative scenic views are signi ficantly obscured, and the lower end was around the level at which visual air quality generally appeared to be good based on observation of the illustrative views. This concentration range generally corresponded to median visual ranges in urban areas withi n regions across the U.S. of approximately 25 to 35 km, a range that was bounded above by the visual range targets selected in specific areas where state or local agencies placed particular emphasis on protecting visual air quality. In considering a reason able range of forms for a PM 2.5 standard within this range of levels, the 2005 Staff Paper had concluded that a concentration

- based percentile form was appropriate, and that the upper end of the range of concentration percentiles for consideration should b e consistent with the 98 th percentile used for the primary standard and that the lower end of the range should be the 92 nd percentile, which represented the mean of the distribution of the 20 percent most impaired days, as targeted in the regional haze pro gram (U.S. EPA, 2005 pp. 7

- 11 to 7

- 13). While recognizing that it was difficult to select any specific level and form based on then

- currently available information (Henderson, 2005a, p. 9), the CASAC Panel was generally in agreement with the ranges***E.O. 12866 Review

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Page 264 of 432 of leve ls and forms presented in the 2005 Staff Paper. The Administrator also considered the level of protection that would be afforded by the proposed suite of primary PM 2.5 standards (71 FR 2681, January 17, 2006), on the basis that although significantly more information was available than in the 1997 review concerning the relationship between fine PM levels and visibility across the country, there was still little available information for use in making the relatively subjective value judgment needed in selec ting the appropriate degree of protection to be afforded by such a standard. In so doing, the Administrator compared the extent to which the proposed suite of primary standards would require areas across the country to improve visual air quality with the e xtent of increased protection likely to be afforded by a standard based on a sub

- daily averaging time. Based on such an analysis, the Administrator observed that the predicted percent of counties with monitors not likely to meet the proposed suite of prima ry PM 2.5 standards was actually somewhat greater than the predicted percent of counties with monitors not likely to meet a sub

- daily secondary standard with an averaging time of 4 daylight hours, a level toward the upper end of the range recommended in the 2005 Staff Paper, and a form within the recommended range. Based on this comparison, the Administrator tentatively concluded that revising the secondary 24

- hour PM 2.5 standard to be identical to the proposed revised primary PM 2.5 standard (and retaining the then

- current annual secondary PM 2.5 standard) was a reasonable policy approach to addressing visibility protection primarily in urban areas. In proposing this approach, the Administrator also solicited comment on a sub

- daily (4

- to 8

- hour averaging time) secondary PM 2.5 standard (71 FR 2675 to 2781, January 17, 2006). In commenting on the proposed decision, the CASAC requested that a sub

- daily***E.O. 12866 Review

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Page 265 of 432 standard to protect visibility “be favorably reconsidered” (Henderson, 2006a, p.6). T he CASAC noted three cautions regarding the proposed reliance on a secondary PM 2.5 standard identical to the proposed 24

- hour primary PM 2.5 standard: (1) PM 2.5 mass measurement is a better indicator of visibility impairment during daylight hours, when rela tive humidity is generally low; the sub

- daily standard more clearly matches the nature of visibility impairment, whose adverse effects are most evident during the daylight hours; using a 24

- hour PM 2.5 standard as a proxy introduces error and uncertainty in protecting visibility; and sub

- daily standards are used for other NAAQS and should be the focus for visibility; (2) CASAC and its monitoring subcommittees had repeatedly commended EPA’s initiatives promoting the introduction of continuous and near

continu ous PM monitoring and recognized that an expanded deployment of continuous PM 2.5 monitors would be consistent with setting a sub

- daily standard to protect visibility; and (3) the analysis showing a similarity between percentages of counties not likely to

m eet what the CASAC Panel considered to be a lenient 4

- to 8

- hour secondary standard and a secondary standard identical to the proposed 24

- hour primary standard was a numerical coincidence that was not indicative of any fundamental relationship between visi bility and health. The CASAC Panel further stated that “visual air quality is substantially impaired at PM 2.5 concentrations of 35 μg/m 3 ” and that “[i]t is not reasonable to have the visibility standard tied to the health standard, which may change in
ways that make it even less appropriate for visibility concerns” (Henderson, 2006a, pp. 5 to 6). In reaching a final decision, the Administrator focused on the relative protection provided by the proposed primary standards based on the above

- mentioned similar ities in***E.O. 12866 Review

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Page 266 of 432 percentages of counties meeting alternative standards and on the limitations in the information available concerning studies of public perception and attitudes regarding the acceptability of various degrees of visibility impairment in urban areas, as well as on the subjective nature of the judgment required. In so doing, the Administrator concluded that caution was warranted in establishing a distinct secondary standard for visibility impairment and that the available information did not warrant ad opting a secondary standard that would provide either more or less protection against visibility impairment in urban areas than would be provided by secondary standards set equal to the proposed primary PM 2.5 standards. 2. Remand of 2006 Secondary PM 2.5 St andards As noted above in section II.B.2 above, several parties filed petitions for review challenging EPA’s decision to set the secondary NAAQS for fine PM identical to the primary NAAQS. On judicial review, the D.C. Circuit remanded to the EPA for

recon sideration the secondary NAAQS for fine PM because the Agency’s decision was unreasonable and contrary to the requirements of section 109(b)(2). American Farm Bureau Federation v. EPA , 559 F. 3d 512 (D.C. Cir., 2009). The petitioners argued that the EPA’s decision lacked a reasoned basis. First, they asserted that the EPA never determined what level of visibility was “requisite to protect the public welfare.” They argued that the EPA unreasonably rejected the target level of protection recommended by its st aff, while failing to provide a target level of its own. The court agreed, stating that “the EPA’s failure to identify such a level when deciding where to set the level of air quality required by the revised secondary fine PM NAAQS is contrary to the statu te and therefore unlawful. Furthermore, the failure to set any target***E.O. 12866 Review

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Page 267 of 432 level of visibility protection deprived the EPA’s decision

- making of a reasoned basis.” 559 F. 3d at 530. Second, the petitioners challenged EPA’s method of comparing the protection
expe cted from potential standards. They contended that the EPA relied on a meaningless numerical comparison, ignored the effect of humidity on the usefulness of a standard using a daily averaging time, and unreasonably concluded that the primary standards woul d achieve a level of visibility roughly equivalent to the level the EPA staff and CASAC deemed “requisite to protect the public welfare.” The court found that the EPA’s equivalency analysis based on the percentages of counties exceeding alternative standar ds “failed on its own terms.” The same table showing the percentages of counties exceeding alternative secondary standards, used for comparison to the percentages of counties exceeding alternative primary standards to show equivalency, also included six ot her alternative secondary standards within the recommended CASAC range that would be more “protective” under EPA’s definition than the adopted primary standards. Two

- thirds of the potential secondary standards within the CASAC’s recommended range would be substantially more protective than the adopted primary standards. The court found that the EPA failed to explain why it looked only at one of the few potential secondary standards that would be less protective, and only slightly less so, than the primary

s tandards. More fundamentally, however, the court found that the EPA’s equivalency analysis based on percentages of counties demonstrated nothing about the relative protection offered by the different standards, and that the tables offered no valid informat ion about the relative visibility protection provided by the standards. 559 F. 3d at 530

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Page 268 of 432 Finally, the Staff Paper had made clear that a visibility standard using PM 2.5 mass as the indicator in conjunction with a daily averaging time would be confounded by regional differences in humidity. The court noted that the EPA acknowledged this problem, yet did not address this issue in concluding that the primary standards would be sufficiently protective of visibility. 559 F. 3d at 530. Therefore, the court gra nted the petition for review and remanded for reconsideration the secondary PM 2.5 NAAQS. 3. General Approach Used in the Policy Assessment for the Current Review The approach used in this review broadened the general approaches used in the last two PM NAAQ S reviews by utilizing, to the extent available, enhanced tools, methods, and data to more comprehensively characterize visibility impacts. As such, the EPA took into account considerations based on both the scientific evidence (“evidence

based”) and a qua ntitative analysis of PM

- related impacts on visibility (“impact

- based”) to inform conclusions related to the adequacy of the current secondary PM 2.5 standards and alternative standards that were appropriate for consideration in this review. As in past revi ews, the EPA also considered that the secondary NAAQS should address PM

- related visibility impairment in conjunction with the Regional Haze Program, such that the secondary NAAQS would focus on protection from visibility impairment principally in urban are as in conjunction with the Regional Haze Program that is focused on improving visibility in Federal Class I areas. The EPA again recognized that such an approach remains the most appropriate and effective means of addressing the public welfare effects

asso ciated with visibility impairment in areas across the country. The Policy Assessment drew from the qualitative evaluation of all studies discussed in the Integrated Science Assessment (U.S. EPA, 2009a). Specifically, the***E.O. 12866 Review

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Page 269 of 432 Policy Assessment considered the ex tensive new air quality and source apportionment information available from the regional planning organizations, long

- standing evidence of PM effects on visibility, and limited public preference stud y information from four urban areas (U.S. EPA, 2009a, cha pter 9), as well as the integration of evidence across disciplines (U.S. EPA, 2009a, chapter 2). In addition, limited information that had become available regarding the characterization of public preferences in urban areas provided some new perspectives o n the usefulness of this information in informing the selection of target levels of urban visibility protection. On these bases, the Policy Assessment again focused assessments on visibility conditions in urban areas. The conclusions in the Policy Assessme nt reflected EPA staff’s understanding of both evidence

- based and impact

- based considerations to inform two overarching questions related to (1) the adequacy of the current suite of PM 2.5 standards and (2) what potential alternative standards, if any, shou ld be considered in this review to provide appropriate protection from PM

- related visibility impairment. In addressing these broad questions, the discussions in the Policy Assessment were organized around a series of more specific questions reflecting diff erent aspects of each overarching question (U.S. EPA, 2011a, Figure 4

- 1). When evaluating the visibility protection afforded by the current or any alternative standards considered, the Policy Assessment took into account the four basic elements of the NAAQ S: indicator, averaging time, level, and form. B. Proposed Decisions on Secondary PM Standards At the time of proposal, the Administrator proposed to revise the suite of secondary PM standards by adding a distinct standard for PM 2.5 to address PM

- related visibility impairment, focused primarily on visibility in urban areas. This proposed***E.O. 12866 Review

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Page 270 of 432 standard was to be defined in terms of a PM 2.5 visibility index, which would use measured PM 2.5 mass concentration, in combination with speciation a nd relative humidity data, to calculate PM 2.5 light extinction, translated into the deciview (dv) scale; a 24

- hour averaging time; a 90 th percentile form, averaged over 3 years; and a level of 28

- 30 dv. To address other non

- visibility welfare effects, the Administrator proposed to retain the current suite of secondary PM standards generally, while revising only the form of the secondary ann ual PM 2.5 standard to remove the option for spatial averaging consistent with this proposed change to the primary annual PM 2.5 standard. Each of these proposed decisions is described in more detail in the proposal and below. 1. PM

- Related Visibility Impair ment As discussed in Section VI.B. of the proposal, the Administrator’s proposed decision regarding a distinct secondary standard to provide protection from visibility impairment reflected careful consideration of the following: (1) the latest scientific i nformation on visibility effects associated with PM as described in the Integrated Science Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of correlations between ambient PM 2.5 and visibility impairment prepared by EPA staff in the Visib ility Assessment (U.S. EPA, 2010b); and (3) specific conclusions regarding the need for revisions to the current standards (i.e., indicator, averaging time, form, and level) that, taken together, would be requisite to protect the public welfare from advers e effects on visual air quality. This section summarizes key information from the proposal regarding the nature of visibility impairment, including the relationship between ambient PM and visibility, temporal variations in light extinction, periods during the day of interest for assessing visibility conditions, and exposure durations of interest (section***E.O. 12866 Review

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Page 271 of 432 VI.B.1.a); limited public perceptions and attitudes about visibility impairment and the impacts of visibility impairment on public welfare (section VI.B.1. b); CASAC advice regarding the need for, and design of, secondary standards to protect visibility (section VI.B.1.c); and the Administrator’s proposed conclusions regarding setting a distinct standard to address visibility impairment (section VI.B.1.d).

a. Nature of PM

- related Visibility Impairment As noted at the time of proposal, the fundamental science characterizing the contribution of PM, especially fine particles, to visibility impairment is well understood. This science provides the basis for the Int egrated Science Assessment designation of the relationship between PM and visibility impairment as causal. New research available in this review, discussed in chapter 9 of the Integrated Science Assessment, continues to support and refine EPA’s understandi ng of the effect of PM on visibility and the source contributions to that effect in rural and remote locations. This research provides new insights regarding the regional source contributions to urban visibility impairment and better characterization of th e increment in PM concentrations and visibility impairment that occur in many cities (i.e., the urban excess) relative to conditions in the surrounding rural areas (i.e., regional background). Ongoing urban PM 2.5 speciated and aggregated mass monitoring ha s produced new information that has allowed for updated characterization of current visibility levels in urban areas. i. Relationship between Ambient PM and Visibility Visibility impairment is caused by the scattering and absorption of light by suspended particles and gases in the atmosphere. When PM is present in the air, its contribution to light extinction typically greatly exceeds that of gases. The combined***E.O. 12866 Review

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Page 272 of 432 effect of light scattering and absorption by both particles and gases is characterized as light extinction, i.e., the fraction of light that is scattered or absorbed in the atmosphere. Light extinction can be quantified by a light extinction coefficient with units of 1/distance, which is often expressed as 1/(1 million meters) or inverse megameters (abbreviated Mm - 1 ) or in terms of an alternative scale known as the deciview scale, defined by the following equation: 107 Deciview (dv) = 10 ln ( b ext / 10 Mm - 1 ) The deciview scale is frequently used in the scientific literature on visibility, as well as in the Regional Haze Program. In particular, the deciview scale is used in the public perception studies that were considered in the past and current reviews to inform judgments about an appropriate degree of protection to be provided by a secondary NAAQS. Th e amount of light extinction contributed by PM depends on the particle concentration as well as on the particle size distribution and composition and also on the relative humidity. As described in detail in section VI.B.1.a of the proposal, visibility scie ntists have developed an algorithm, known as the IMPROVE algorithm, 108 to estimate light extinction using routinely monitored fine particle (PM 2.5 ) speciation and coarse particle mass (PM 10

– 2.5 ) data, as well as data on relative humidity. There is both an
107 As used in the Regional Haze Program, the term b ext refers to light extinction due to PM 2.5 , PM 10

- 2.5 , and “clean” atmospheric gases. In the Policy Assessment, in focusing on light extinction due to PM 2.5 , the deciview values include only the effects of PM 2.5 and the gases. The “Rayleigh” term associated with clean atmospheric gases is represented by the constant value of 10 Mm - 1 . Omission of the Rayleigh term would create the possibility of negative deciview values when the PM 2.5 concentration is very low. 108 The algorithm is referred to as the IMPROVE algorithm because it was developed specifically to use the aerosol monitoring data generated at network sites and with equipment specifically designed to support the IMPROVE program and was evaluated using IMPROVE optical measurements at the subset of sites that make those measurements (Malm et al., 1994).***E.O. 12866 Review

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Page 273 of 432 or iginal and a revised version of the IMPROVE algorithm (Pitchford et al., 2007). The revised version was developed to address observed biases in the predictions using the original algorithm under very low and very high light extinction conditions. 109 These IM PROVE algorithms are routinely used to calculate light extinction levels on a 24

- hour basis in Federal Class I areas under the Regional Haze Program. In either version of the IMPROVE algorithm, the concentration of each of the major aerosol components is m ultiplied by a dry extinction efficiency value and, for the hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also multiplied by an additional factor to account for the water growth to estimate these components’ contribution to light extinction. Summing the contribution of each component gives the estimate of total light extinction per unit distance denoted as the light extinction coefficient ( b ext ), as shown below for the original IMPROVE algorithm. b ext ≈ 3 x f

(RH) x [Sulfate] + 3 x f (RH) x [Nitrate] + 4 x [Organic Mass] + 10 x [Elemental Carbon] + 1 x [Fine Soil] + 0.6 x [Coarse Mass] + 10 Light extinction ( b ext ) is in units of Mm - 1 , the mass concentrations of the components indicated in brackets are in units of μg/m 3 , and f (RH) is the unitless water growth term that depends on relative humidity. The final term of 10 Mm - 1 is known as the Rayleigh scattering term and accounts for light scattering by the natural gases in unpolluted air. Despite the simplicity of this alg orithm, it performs reasonably well and permits the contributions to light extinction from each of the major components

109 These biases were detected by comparing light extinction estimates generated from the IMPROVE algorithm to direct optical measurements in a number of rural Federal Class I areas.***E.O. 12866 Review

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Page 274 of 432 (including the water associated with the sulfate and nitrate compounds) to be separately approximated. Inspection of the PM component

- sp ecific terms in the simple original IMPROVE algorithm shows that most of the PM 2.5 components contribute 5 times or more light extinction than a similar concentration of PM 10

– 2.5 . The f (RH ) term in the original algorithm reflects the increase in light sca ttering caused by particulate sulfate and nitrate under conditions of high relative humidity. Particles with hygroscopic components (e.g., particulate sulfate and nitrate) contribute more light extinction at higher relative humidity than at lower relative humidity because they change size in the atmosphere in response to ambient relative humidity conditions

. For relative humidity below 40 percent the f (RH) value is 1, but it increases to 2 at approximately 66 percent, 3 at approximately 83 percent, 4 at app roximately 90 percent, 5 at approximately 93 percent, and 6 at approximately 95 percent relative humidity. The result is that both particulate sulfate and nitrate are more efficient per unit mass in light extinction than any other aerosol component for rel ative humidity above approximately 85 percent where their total light extinction efficiency exceeds the 10 m 2 /g associated with elemental carbon (EC). PM containing elemental or black carbon (BC) absorbs light as well as scattering it, making it the compon ent with the greatest light extinction contributions per unit of mass concentration, except for the hygroscopic components under these high relative humidity conditions. 110 As noted above, s ubsequent to the development of the original IMPROVE algorithm, an alternative algorithm (variously referred to as the “revised algorithm” or the “new algorithm” in the literature) w as developed. The revised IMPROVE algorithm is

110 The IMPROVE algorithm does not explicitly separate the light

- scattering and light

absorbing effects of elemental c arbon.***E.O. 12866 Review

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Page 275 of 432 different from the original algorithm in several important ways

. First, the revised algorithm employs a more complex split

- component mass extinction efficiency to correct biases believed to be related to particle size distributions

. 111 Specifically, t he revised algorithm incorporat es terms to account for particles representing the different dry extinction and water uptake from two size modes of sulfate, nitrate and organic mass

. 112 Second, the revised algorithm uses a different multiplier for organic carbon for purposes of estimating org anic carbonaceous material to better represent aged aerosol found in
remote areas
. 113 In addition, the revised algorithm includes a term for hygroscopic sea salt that can be important for remote coastal areas, and site

- specific Rayleigh light scattering term s in place of a universal Rayleigh light scattering value. As noted in section VI.B.1.a of the proposal, the revised IMPROVE algorithm can yield higher estimates of current light extinction levels in urban areas on days with relatively poor visibility as c ompared to the original algorithm (Pitchford, 2010). This difference is primarily attributable to the split

- component mass extinction efficiency treatment in the
111 In either version of the IMPROVE algorithm, the concentration of each of the major aerosol components is multiplied by a dry extinction efficiency value and, for the hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also multiplied by an additional factor to account for the water growth to estimate these components’ contribution to light extinction. Bo th the dry extinction efficiency and water growth terms have been developed by a combination of empirical assessment and theoretical calculation using typical particle size distributions associated with each of the major aerosol components. 112 The relative contributions of sulfate, nitrate and organic mass concentrations to visibility impairment with the revised algorithm, are different than with the original algorithm due to the combination of the dry extinction coefficient and f(RH) functions for derived c oncentrations of small and large particles. The apportionment of the total fine particle concentration of each of the three PM 2.5 components into the concentrations of the small and large size fractions was empirically developed for remote areas. The fract ion of the fine particle component that is in the large mode is estimated by dividing the total concentration of the component by 20 ug/m 3 . If the total concentration of a component exceeds 20 ug/m 3 , all of it is assumed to be in the large mode

. 113 The revised IMPROVE algorithm uses a multiplier of 1.8 for rural areas instead of 1.4 as used in the original algorithm for the mean ratio of organic mass to organic carbon.***E.O. 12866 Review

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Page 276 of 432 revised algorithm. Th is revised algorithm was evaluated at 21 remote locations and is generall y used by RPOs and States for implementation of the Regional Haze Rule

. ii. Temporal Variations of Light Extinction Particulate matter concentrations and light extinction in urban environments vary from hour to hour throughout the 24

- hour day due to a com bination of diurnal changes in meteorological conditions and systematic changes in emissions activity (e.g., rush hour traffic). Various factors combine to make early morning the most likely time for peak urban light extinction; although the net effects of the systematic urban

- and larger

- scale variations mean that peak daytime PM light extinction levels can occur any time of day, in many areas they occur most often in early morning hours (U.S. EPA, 2010b, sections 3.4.2 and 3.4.3; Figures 3

- 9, 3

- 10, and 3

- 12). This temporal pattern in urban areas contrasts with the general lack of a strong diurnal pattern in PM concentrations and light extinction in most Federal Class I areas, reflective of a relative lack of local sources as compared to urban areas. The us e in the Regional Haze Program of 24

- hour average concentrations in the IMPROVE algorithm is consistent with this general lack of a strong diurnal pattern in Federal Class I areas. iii. Periods during the Day of Interest for Assessment of Visibility As n oted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime visibility has dominated the attention of those who have studied the visibility effects of air pollution, particularly in urban areas. The EPA recognizes, however, that physically PM light extinction behaves the same at night as during the day and can contribute to nighttime visibility effects by enhancing the scattering of anthropogenic light, contributing to the “skyglow” within and ov er populated areas, adding to the total sky brightness , and***E.O. 12866 Review

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Page 277 of 432 contributing to the reduction in contrast of stars against the background. However, l ittle research has been conducted on nighttime visibility, and the state of the science is not comparable to th at associated with daytime visibility impairment, particularly in terms of the impact on human welfare. T he Policy Assessment notes that the science is not available at this time to support adequate characterization specifically of nighttime PM light extin ction conditions and the related effects on public welfare (U.S. EPA, 2011a, p. 4

- 18). Therefore the EPA has focused its assessments of PM visibility impacts in urban areas on daylight hours during this review. iv. Exposure Durations of Interest As noted in section VI.B.1.d of the proposal, the roles that exposure duration and variations in visual air quality within any given exposure period play in determining the acceptability or unacceptability of a given level of visual air quality have not been invest igated via preference studies. In the preference studies available for this review, subjects were simply asked to rate the acceptability or unacceptability of each image of a haze

- obscured scene, without being provided any suggestion of assumed duration or of assumed conditions before or after the occurrence of the scene presented. Preference and/or valuation studies show that atmospheric visibility conditions can be quickly assessed and preferences determined. The EPA is unaware of any studies that charact erize the extent to which different frequencies and durations of exposure to visibility conditions contribute to the degree of public welfare impact that occurs. The Policy Assessment considered a variety of circumstances that are commonly expected to occ ur in evaluating the potential impact of visibility impairment on the public welfare based on available information (U.S. EPA, 2011a, pp. 4

- 19 to 4

- 20). In some***E.O. 12866 Review

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Page 278 of 432 circumstances, such as infrequent visits to scenic vistas in natural or urban environments,
peo ple are motivated specifically to take the opportunity to view a valued scene and are likely to do so for many minutes to hours to appreciate various aspects of the vista they choose to view. However, the public has many more opportunities to notice visibi lity conditions on a daily basis in settings associated with performing daily routines (e.g., during commutes and while working, exercising, or recreating outdoors). As noted in the Policy Assessment, information regarding the fraction of the public that h as only one or a few opportunities to experience visibility during the day, or on the role the duration of the observed visibility conditions has on wellbeing effects associated with those visibility conditions, is not available (U.S. EPA, 2011a, p. 4

- 20). However, it is possible that people with limited opportunities to experience visibility conditions on a daily basis would receive the entire impact of the day’s visual air quality based on the visibility conditions that occur during the short time period when they can see it. Since this group could be affected on the basis of observing visual air quality conditions for periods as short as one hour or less, and because during each daylight hour there are some people outdoors, commuting, or near windows, the Policy Assessment judged that it would be appropriate to use the maximum hourly value of PM light extinction during daylight hours for each day for purposes of evaluating the adequacy of the current suite of secondary standards. Other observers may have a ccess to visibility conditions throughout the day. For this group, it might be that an hour with poor or “unacceptable” visibility can be offset by one or more other hours with clearer conditions. Therefore, the proposal acknowledged that it might also be appropriate to consider a multi

- hour daylight exposure period.***E.O. 12866 Review

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Page 279 of 432 v. Periods of Fog and Rain As discussed in section VI.C of the proposal, the EPA also recognized that it is appropriate to give special treatment to periods of fog and rain when considering wh ether current PM 2.5 standards adequately protect public welfare from PM

- related visibility impairment. Visibility impairment occurs during periods with fog or precipitation irrespective of the presence or absence of PM. Therefore, it is logical that period s with naturally impaired visibility due to fog or precipitation should not be treated as having PM

- impaired visibility. There are multiple ways to adjust visibility data to reduce the effects of fog and precipitation. In the Visibility Assessment, followi ng the advice of CASAC, the EPA evaluated the effect of excluding daylight hours for which relative humidity was greater than 90 percent from analyses in order to avoid precipitation and fog confounding estimates of PM visibility impairment. For the 15 urb an areas included in the Visibility Assessment, the EPA found that a 90 percent relative humidity cutoff criterion was effective in that on average less than 6 percent of the daylight hours were removed from consideration, yet those hours had on average te n times the likelihood of rain, six times the likelihood of snow/sleet, and 34 times the likelihood of fog compared with hours with 90 percent or lower relative humidity. In the Regional Haze program, the EPA utilizes monthly average relative humidity valu es based on 10 years of climatological data to reduce the effect of fog and precipitation. This approach focuses on longer

- term averages for each monitoring site and thereby eliminates the effect of very high humidity conditions on visibility at those loca tions. b. Public Perception of Visibility Impairment As described in section VI.B.2 of the proposal, there are two main types of studies***E.O. 12866 Review

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Page 280 of 432 that evaluate the public perception of urban visibility impairment: urban visibility preference studies and urban visi bility valuation studies. As noted in the Integrated Science Assessment, “[b]oth types of studies are designed to evaluate individuals’ desire (or demand) for good visual air quality (VAQ) where they live, using different metrics to evaluate demand. Urban visibility preference studies examine individuals’ demand by investigating what amount of visibility degradation is unacceptable while economic studies examine demand by investigating how much one would be willing to pay to improve visibility” (U.S. EPA, 2 009a, p. 9

- 66). Because of the limited number of new studies on urban visibility valuation, the Integrated Science Assessment cites to the discussion in the 2004 Criteria Document of the various methods one can use to determine the economic valuation of ch anges in visibility, which include hedonic valuation, contingent valuation and contingent choice, and travel cost. Contingent valuation studies are a type of stated preference study that measures the strength of preferences and expresses that preference in dollar values. Contingent valuation studies often include payment vehicles that require respondents to consider implementation costs and their ability to pay for visibility improvements in their responses. This study design aspect is critical because the EPA cannot consider implementations costs in setting either primary or secondary NAAQS. Therefore in considering the information available to help inform the standard

- setting process, the EPA has focused on the public perception studies that do not embed c onsideration of implementation costs. Nonetheless, the EPA recognizes that valuation studies do provide additional evidence that the public is experiencing losses in welfare due to visibility***E.O. 12866 Review

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Page 281 of 432 impairment. 114 The public perception studies are described in deta il below. In order to identify levels of visibility impairment appropriate for consideration in setting secondary PM NAAQS to protect the public welfare, the Visibility Assessment comprehensively examined information that was available in this review regar ding people’s stated preferences regarding acceptable and unacceptable visual air quality. Light extinction is an atmospheric property that by itself does not directly translate into a public welfare effect. Instead, light extinction becomes meaningful in the context of the impact of differences in visibility on the human observer. This has been studied in terms of the acceptability or unacceptability expressed for the visibility impact of a given level of light extinction by a human observer. The percepti on of the visibility impact of a given level of light extinction occurs in conjunction with the associated characteristics and lighting conditions of the viewed scene. 115 Thus, a given level of light extinction may be perceived differently by observers looki ng at different scenes or the same scene with different lighting characteristics. Likewise, different observers looking at the same scene with the same lighting may have different preferences regarding the
114 In the regulatory impact analysis (RIA) accompanying this rulemaking, the EPA d escribes a revised approach to estimate urban residential visibility benefits that applies the results of several contingent valuation studies. The EPA is unable to apply the public perception studies to estimate benefits because they do not provide suffic ient information on which to develop monetized benefits estimates. Specifically, the public perception studies do not provide preferences expressed in dollar values, even though they do provide additional evidence that the benefits associated with improvin g residential visibility are not zero. As previously noted in this preamble, the RIA is done for informational purposes only, and the proposed decisions on the NAAQS in this rulemaking are not in any way based on consideration of the information or analyse s in the RIA. 115 By “characteristics of the scene” the EPA means the distance(s) between the viewer and the object(s) of interest, the shapes and colors of the objects, the contrast between objects and the sky or other background, and the inherent interest of the objects to the viewer. Distance is particularly important because at a given value of l ight extinction, which is a property of air at a given point(s) in space, more light is actually absorbed and scattered when light passes through more air between the object and the viewer.***E.O. 12866 Review

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Page 282 of 432 associated visual air quality. When scene and ligh ting characteristics are held constant, the perceived appearance of a scene (i.e., how well the scenic features can be seen and the amount of visible haze) depends only on changes in light extinction. This has been demonstrated using the WinHaze model (Mol enar et al., 1994) that uses image processing technology to apply user

- specified changes in light extinction values to the same base photograph with set scene and lighting characteristics. Much of what is known about the acceptability of levels of visibil ity comes from survey studies in which participants were asked questions about their preference or the value they place on various visibility levels as displayed to them in scenic photographs and/or WinHaze images with a range of known light extinction lev els. The Visibility Assessment (U.S. EPA, 2010b, chapter 2) revi ew ed the limited number of u rban visibility preference studies currently available (i.e., four studies) to assess the light extinction levels judged by the participant to have acceptable visib ility for those particular scenes. The reanalysis of urban preference studies conducted in the Visibility Assessment for this review included three completed western urban visibility preference survey studies plus a pair of smaller focus studies designed to explore and further develop urban visibility survey instruments. The three western studies included one in Denver, Colorado (Ely et al., 1991), one in the lower Fraser River valley near Vancouver, British Columbia (BC), Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research & Consulting, 2003). A pilot focus group study was also conducted for Washington, DC (Abt Associates Inc., 2001). In response to an EPA request for public comment on the Scope and Methods Plan (74 FR 11580, March 18, 2009), c omments were received (Smith, 2009) about the results of a new focus group study of scenes from Washington, DC, that***E.O. 12866 Review

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Page 283 of 432 had been conducted on subjects from both Houston, Texas, and Washington, DC, using scenes, methods and approaches similar to the method and approach employed in the EPA pilot study (Smith and Howell, 2009). When taken together, these studies from the four different urban areas included a total of 852 individuals, with each individual responding to a series of questions while viewing a set of images of various urban visual air quality conditions. The approaches used in the four studies were similar and were all derived from the method first developed for the Denver urban visibility study. In particular, the studies all used a similar group int erview type of survey to investigate the level of visibility impairment that participants described as “acceptable.” In each preference study, participants were initially given a set of “warm up” exercises to familiarize them with how the scene in the phot ograph or image appears under different VAQ conditions. The participants next were shown 25 randomly ordered photographs (images), and asked to rate each one based on a scale of 1 (poor) to 7 (excellent). They were then shown the same photographs or images again, in the same order, and asked to judge whether each of the photographs (images) would violate what they would consider to be an appropriate urban visibility standard (i.e. whether the level of impairment was “acceptable” or “unacceptable”). The term “acceptable” was not defined, so that each person’s response was based on his/her own values and preferences for VAQ. However, when answering this question, participants were instructed to consider the following three factors: (1) The standard would be fo r their own urban area, not a pristine national park area where the standards might be stricter; (2) The level of an urban visibility standard violation should be set at a VAQ level considered to be unreasonable, objectionable, and unacceptable***E.O. 12866 Review

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Page 284 of 432 visually; a nd (3) Judgments of standards violations should be based on visibility only, not on health effects. While the results differed among the four urban areas, results from a rating exercise show that within each preference study, individual survey participants consistently distinguish between photos or images representing different levels of light extinction, and that more participants rate as acceptable images representing lower levels of light extinction than they do images representing higher levels. Given the similarities in the approaches used, the EPA staff concluded that it was reasonable to compare the results to identify overall trends in the study findings and to conclude that this comparison can usefully inform the selection of a range of levels for use in further analyses . However, the staff also noted that variations in the specific materials and methods used in each study introduce uncertainties that should also be considered when interpreting the results of these comparisons. Key differences betwe en the studies include the following: 1) scene characteristics; 2) image presentation methods (e.g., projected slides of actual photos, projected images generated using WinHaze (a significant technical advance in the method of presenting visual air quality conditions), or use of a computer monitor screen; 3) number of participants in each study; 4) participant representativeness of the general population of the relevant metropolitan area; and 5) specific wording used to frame the questions used in the group interview process. In the Visibility Assessment, each study was evaluated separately and figures developed to display the percentage of participants that rated the visual air quality depicted in each photograph as “acceptable.” Ely et al. (1991) introduc ed a “50% acceptability” criterion analysis of the Denver preference study results. The 50 percent acceptability criterion is designed to identify the visual air quality level (defined in terms***E.O. 12866 Review

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Page 285 of 432 of deciviews or light extinction) that best divides the photog raphs into two groups: those with a visual air quality rated as acceptable by the majority of the participants, and those rated not acceptable by the majority of participants. The Visibility Assessment adopted this criterion as a useful index for compariso n between studies. The results of each analysis were then combined graphically to allow for visual comparison. This information was then carried forward into the Policy Assessment. Figure 6 presents the graphical summary of the results of the studies in th e four cities and draws on results previously presented in Figures 2

- 3, 2

- 5, 2

- 7, and 2

- 11 of chapter 2 in the Visibility Assessment. Figure 6 also contains lines at 20 dv and 30 dv that generally identify a range where the 50 percent acceptance criteria o ccur across all four of the urban preference studies (U.S. EPA, 2011a, p. 4

- 24). Out of the 114 data points shown in Figure 6, only one photograph (or image) with a visual air quality below 20 dv was rated as acceptable by less than 50 percent of the parti cipants who rated that photograph. 116 Similarly, only one image with a visual air quality above 30 dv was rated acceptable by more than 50 percent of the participants who viewed it. 117

116 Only 47 percent of the British Columbia participants rated a 19.2 dv photograph as acceptable. 117 In the 2001 Washington, DC study, a 30.9 dv image was used as a repeated slide. The first time it was shown 56 percent of the par ticipants rated it as acceptable, but only 11 percent rated it as acceptable the second time it was shown. The same visual air quality level was rated as acceptable by 4 percent of the participants in the 2009 study (Test 1). All three points are shown in Figure 6.***E.O. 12866 Review

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Page 286 of 432 Figure 6. Summary of Results of Urban Visibility Studies in Four Cities, Showing the Identified

Range of the 50% Acceptance Criteria 118

Source: U.S. EPA, 2011a, Figure 4

- 2; U.S. EPA 2010b, Figure 2

- 16 As Figure 6 above shows, each urban area has a separate and unique response curve that appears to indicate that it is distinct from the others. 119 These curves are the result of a logistical regression analysis using a logit model of the greater than 19,000 ratings of haze images as acceptable or unacceptable. The model results can be used to estimate the visual air quality in term s of dv values where the estimated response functions cross the 50 percent acceptability level, as well as any alternative criteria levels. Selected examples of these are shown in Table 4

- 1 of the Policy Assessment (U.S. EPA, 2011a; U.S. EPA, 2010b, Table 2

- 4). This table shows that the logit model results also
118 Top scale shows light extinction in inverse megameter units; bottom scale in deciviews. Logit analysis estimated response functions are shown as the color

- coded curved lines for each of the four urban areas. 119 At present , data is only available for four urban areas, as presented in Figure 6 and discussed throughout this section. Additional research could help inform whether the range identified by combining the results of the studies depicted in Figure 6 is more broadly representative

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Page 287 of 432 support the upper and lower ends of the range of 50 th percentile acceptability values (e.g., near 20 dv for Denver and near 30 dv for Washington, DC) already identified in Figure 6. Based on the com

posite results and the effective range of 50 th percentile acceptability across the four urban preference studies shown in Figure 6 and Table 4

- 1 of the Policy Assessment, benchmark levels of (total) light extinction were selected in a range from 20 dv to 3 0 dv (75 to 200 Mm - 1 ) 120 for the purpose of provisionally assessing whether visibility conditions would be considered acceptable (i.e., less than the low end of the range), unacceptable (i.e., greater than the high end of the range), or potentially acceptabl e (within the range) based on the very limited public preference information . A midpoint of 25 dv (120 Mm - 1 ) was also selected for use in the assessment. This level is also very near to the 50 th percentile criterion value from the Phoenix study (i.e., 24.2 dv), which is by far the best of the four studies in terms of the fit of the data to the response curve and the representativeness of study participants. Based on the currently available information, the Policy Assessment concluded that the use of 25 dv t o represent the middle of the distribution of results seemed well supported (U.S. EPA, 2011a, p. 4

- 25). These three benchmark values provide a low, middle, and high set of light extinction conditions that are used to provisionally define daylight hours wit h urban haze conditions that have been judged unacceptable by at least 50 percent of the participants in one or more of these preference studies. As discussed above, PM light extinction is taken

120 These v alues were rounded from 74 Mm - 1 and 201 Mm - 1 to avoid an implication of greater precision than is warranted. Note that th e middle value of 25 dv when converted to light extinction is 122 Mm - 1 is rounded to 120 Mm - 1 for the same reason. Assessments conducted for the Visibility Assessment and the first and second drafts of the Policy Assessment used the unrounded values. The Policy Assessment considered the results of assessment using unrounded values to be sufficient ly representative of what would result if the rounded values were used that it was unnecessary to redo the assessments. That is why some tables and figures in the Policy Assessment reflected the unrounded values.***E.O. 12866 Review

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Page 288 of 432 to be (total) light extinction minus the Rayleigh scatter, 121 s uch that the low, middle, and high levels correspond to PM light extinction levels of about 65 Mm - 1 , 110 Mm - 1 , and 190 Mm - 1 . In the Visibility Assessment, these three light extinction levels were called Candidate Protection Levels (CPLs). This term was als o used in the Policy Assessment and in the proposal notice. It is important to note, however, that the degree of protection provided by a secondary NAAQS is not determined solely by any one component of the standard but by all the components (i.e., indicat or, averaging time, form, and level) being applied together. Therefore, the Policy Assessment noted that the term CPL is meant only to indicate target levels of visibility within a range that the EPA staff felt appropriate for consideration that could, in conjunction with other elements of the standard, including indicator, averaging time, and form, potentially provide an appropriate degree of visibility protection. In characterizing the Policy Assessment’s confidence in each CPL and across the
range, a num

ber of issues were considered (U.S. EPA, 2011a, p. 4

- 26). Looking first at the two studies that define the upper and lower bounds of the range, the Policy Assessment considered whether they represent a true regional distinction in preferences for urban vis ibility conditions between western and eastern U.S. There was little information available to help evaluate the possibility of a regional distinction especially given that there have been preference studies in only one eastern urban area. Smith and Howell (2009) found little difference in preference response to Washington, DC, haze photographs between the study participants from Washington, DC, and those from

121 Rayleigh scatter is light scattering by a tmospheric gases which is on average about 10 Mm - 1

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Page 289 of 432 Houston, Texas. 122 This provides some limited evidence that the value judgment of the public in diffe rent areas of the country may not be an important factor in explaining the differences in these study results. In further considering what factors could explain the observed differences in preferences across the four urban areas, the Policy Assessment not ed that the urban scenes used in each study had different characteristics (U.S. EPA, 2011a, p. 4

- 26). For example, each of the western urban visibility preference study scenes included mountains in the background while the single eastern urban study did no t. It is also true that each of the western scenes included objects at greater distances from the camera location than in the eastern study. There is no question that objects at a greater distance have a greater sensitivity to perceived visibility changes as light extinction is changed compared to otherwise similar scenes with objects at a shorter range. This alone might explain the difference between the results of the eastern study and those from the western urban studies. Having scenes with the object of greatest intrinsic value nearer and hence less sensitive in the eastern urban area compared with more distant objects of greatest intrinsic value in the western urban areas could further explain the difference in preference results. Another question cons idered was whether the high CPL value that is based on the eastern preference results is likely to be generally representative of urban areas that do not have associated mountains or other valued objects visible in the distant background. Such areas would include the middle of the country, many areas in the eastern U.S., and
122 The first preference study using WinHaze images of a scenic vista from Washington, DC was conducted in 2001 using subjects who were residents of Washington, DC. More recently, Smith and Howell (2009) i nterviewed additional subjects using the same images and interview procedure. The additional subjects included some residents of the Washington, DC area and some residents of the Houston, Texas area.***E.O. 12866 Review

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Page 290 of 432 possibly some areas in the western U.S. as well. 123 Based on the currently available information, the Policy Assessment concluded that the high end of the CPL range (30 dv) is an appropri ate level to consider ( U.S. EPA, 2011a, p. 4

- 27). With respect to the low end of the range, the Policy Assessment considered factors that might further refine its understanding of the robustness of this level. The Policy Assessment concluded that additiona l urban preference studies, especially with a greater variety in types of scenes, could help evaluate whether the lower CPL value of 20 dv is generally supportable (U.S. EPA, 2011a, p. 4

- 27). Further, the reason for the noisiness in data points around the curves apparent in both the Denver and British Columbia results compared to the smoother curve fit of Phoenix study results could be explored. One possible explanation discussed in the Policy Assessment is that these older studies use photographs taken at different times of day and on different days to capture the range of light extinction levels needed for the preference studies. In contrast, the use of WinHaze in the Phoenix (and Washington, DC) study reduced variations that affect scene appearance prefer ence rating and avoided the uncertainty inherent in using ambient measurements to represent sight path

- averaged light extinction values. Reducing these sources of noisiness and uncertainty in the results of future studies of sensitive urban scenes could pr ovide more confidence in the selection of a low CPL value. Based on the above considerations, and recognizing the limitations in the currently available information, the Policy Assessment concluded that it is reasonable to

123 In order to examine this issue, an effort would have to be made to see if scenes in such areas could be found that would be generally comparable to the western scenes (e.g., scenes that contain valued scenic elements at more sensitive distances than that used in the eastern study). This is only one of a fami ly of issues concerning how exposure to urban scenes of varying sensitivity affects public perception for which no preference study information is currently available.***E.O. 12866 Review

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Page 291 of 432 consider a range of CPL values in cluding a high value of 30 dv, a mid

- range value of 25 dv, and a low value of 20 dv (U.S. EPA, 2011a, p. 4

- 27). Based on its review of the second draft Policy Assessment, CASAC also supported this set of CPLs for consideration by the EPA in this review. CA SAC noted that these CPL values were based on all available visibility preference data and that they bound the study results as represented by the 50 percent acceptability criteria. While recommending that further visibility preference studies be conducted to reduce remaining uncertainties, 124 CASAC concluded that this range of levels was “adequately supported by the evidence presented” (Samet, 2010d, p. iii).

c. Summary of Proposed Conclusions i. Adequacy of the Current Standards for PM

- Related Visibility Impairment At the time of proposal, the Administrator provisionally concluded that the current suite of secondary PM standards is not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary standar d that would provide additional protection against PM

- related visibility impairment, with a focus primarily in urban areas. This proposed conclusion was based on the information presented in the proposal with regard to the nature of PM

- related visibility i mpairment, the results of public perception surveys on the acceptability of varying degrees of visibility impairment in urban areas, analyses of the number of days that are estimated to exceed a range of candidate protection levels under conditions simulat ed to just meet the current standards, and the advice of CASAC. This section summarizes key points from
124 “CASAC has also identified needs for the next review cycle in terms of further researc h on a number of topics related to urban visibility; .... In particular, there is a need for the Agency to conduct additional urban visibility preference studies over a broad range of urban areas and viewing conditions, to further evaluate and refine the ran ge of visibility levels considered to be acceptable in the current assessment.” (Samet, 2010a)***E.O. 12866 Review

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Page 292 of 432 section VI.C of the proposal regarding visibility under current conditions, the degree of protection afforded by the current standards, and CASAC’s advi ce regarding the adequacy of the current standards. As discussed in section VI.C.1 of the proposal, to evaluate visibility under current conditions the Visibility Assessment and Policy Assessment estimated PM

- related light extinction 125 levels for 15 urban areas 126 in the United States. Consistent with the emphasis in this review on the hourly or multi

- hour time periods that might reasonably characterize the visibility effects experienced by various segments of the population, these analyses focused on using m aximum 1

- hour and 4

- hour values of PM light extinction during daylight hours for purposes of evaluating the degree of visibility impairment. Hourly average PM

- related light extinction was analyzed in terms of both PM 10 and PM 2.5 light extinction. For reaso ns discussed above, hours with relative humidity greater than 90 percent were excluded from consideration. Recent visibility conditions in these urban areas were then compared to the CPLs identified above. The Visibility Assessment, which focused on PM 10 l ight extinction in 14 of the 15 urban areas during the 2005 to 2007 time period, 127 found that all 14 areas had daily maximum hourly

125 PM

- related light extinction is used here to refer to the light extinction caused by PM regardless of particle size; PM 10 light extinction refers to the contrib ution by particles sampled through an inlet with a particle size 50 percent cutpoint of 10 μm diameter; and PM 2.5 light extinction refers to the contribution by particles sampled through an inlet with a particle size 50 percent cutpoint of 2.5 μm diameter. 126 The 15 urban areas are Tacoma, Fresno, Los Angeles, Phoenix, Salt Lake City, Dallas, Houston, St. Louis, Birmingham, Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, and New York. 127 Comments on the second draft Visibility Assessment from those fa miliar with the monitoring sites in St. Louis indicated that the site selected to provide continuous PM 10 monitoring, although less than a mile from the site of the PM 2.5 data, was not representative of the urban area and resulted in unrealistically large PM 10

- 2.5 values. The EPA staff considered these comments credible and set aside the St. Louis assessment results for PM 10 light extinction. Thus, results and statements in the Policy Assessment***E.O. 12866 Review

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Page 293 of 432 PM 10 light extinction values estimated to exceed even the highest CPL some of the days. Except for the two Texas areas and th e non - California western urban areas, all of the other urban areas were estimated to have maximum hourly PM 10 concentrations that exceeded the high CPL on about 20 percent to over 60 percent of the days. All 14 of the urban areas were estimated to have max imum hourly PM 10 concentrations that exceeded the low CPL on about 40 percent to over 90 percent of the days. In general, areas in the East and in California tend to have a higher frequency of hourly visibility conditions estimated to be above the high CPL compared with those in the western U.S. The Policy Assessment repeated the Visibility Assessment

- type modeling based on PM 2.5 light extinction and data from the more recent 2007 to 2009 time period for the same 15 study areas (including St. Louis). While the estimates of the percentage of daily maximum hourly PM 2.5 light extinction values exceeding the CPLs were somewhat lower than for PM 10 light extinction, the patterns of these estimates across the study areas was found to be similar. More specifically, except for the two Texas and the non

- California western urban areas, all of the other urban areas were estimated to have maximum hourly PM 2.5 concentrations that exceeded the high CPL on about 10 percent up to about 50 percent of the days based on PM 2.5 l ight extinction, while all 15 areas were estimated to have maximum hourly PM 2.5 concentrations that exceeded the low CPL on over 10 percent to over 90 percent of the days. To evaluate how PM

- related visibility would be affected by just meeting the
current suite of PM 2.5 secondary standards, t he Policy Assessment applied the proportional rollback approach described in section VI.C.2 of the proposal to all the regarding PM 10 light extinction applied to only the other 14 a reas. However, results regarding PM 2.5 light extinction in most cases applied to all 15 study areas because the St. Louis estimates for PM 2.5 light extinction were not affected by the PM 10 monitoring issue.***E.O. 12866 Review

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Page 294 of 432 PM 2.5 monitoring sites in each study area. 128 After adjusting for composition, the Policy Assessment applied the original IMPROVE algorithm to calculate the PM 10 light extinction, using “rolled back” PM 2.5 component concentrations, the current conditions PM 10

- 2.5 concentration for the day and hour, and relative humidity for the day and hour. In these analyses, the Policy Assessment estimated both PM 2.5 and PM 10 light extinction in terms of both daily maximum 1

- hour average values and multi

- hour (i.e., 4

- hour) average values for daylight hours. Figure 4

- 7 and Table 4

- 6 of the Policy Assessment displayed the results of the rollback procedures as a box and whisker plot of daily maximum daylight 1

- hour PM 2.5 light extinction and the percentage of daily maximum hourly PM 2.5 light extinction values estimated to exceed the CPLs when just meeting the current su ite of PM 2.5 secondary standards for all 15 areas considered in the Visibility Assessment (including St. Louis) (excluding hours with relative humidity greater than 90 percent). These displays showed that the daily maximum 1

- hour average PM 2.5 light extinc tion values in all of the study areas other than the three western non

California areas were estimated to exceed the high CPL on about 8 percent up to over 30 percent of the days and to exceed the middle CPL on about 30 percent up to about 70 percent of th e days, while all areas except Phoenix were estimated to have daily maximum 1

- hour average PM 2.5 light extinction values that exceeded the low CPL on over 15 percent to about 90 percent of the days. Figure 4

- 8 and Table 4

- 7 of the Policy Assessment present results based on daily maximum 4

- hour average values. These displays show that the daily maximum 4

- hour average PM 2.5 light extinction values in all of the study areas other than the three western non

- California areas and the two areas in

128 Phoenix and Salt Lake City met the current PM 2. 5 NAAQS under current conditions and required no reduction.***E.O. 12866 Review

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Page 295 of 432 Texas were estim ated to exceed the high CPL on about 4 percent up to over 15 percent of the days and to exceed the middle CPL on about 15 percent up to about 45 percent of the days, while all areas except Phoenix were estimated to have daily maximum 4

- hour average PM 2.5 l ight extinction values that exceeded the low CPL on over 10 percent to about 75 percent of the days. A similar set of figures and tables were developed in terms of PM 10 light extinction (U.S. EPA, 2011a, Figures 4

- 5 and 4

- 6, Tables 4

- 4 and 4

- 5). Taking th e results of these analyses focusing on 1

- hour and 4

- hour maximum light extinction values into account, the Policy Assessment concluded that the available information in this review clearly called into question the adequacy of the current suite of PM 2.5 st andards in the context of public welfare protection from visibility impairment, primarily in urban areas, and supported consideration of alternative standards to provide appropriate protection (U.S. EPA, 2011a, p. 4

- 39). This conclusion was based in part o n the large percentage of days, in many urban areas, that were estimated to have maximum 1

- hour or 4

- hour light extinction values that exceed the range of CPLs identified for consideration under simulations of conditions that would just meet the current su ite of PM 2.5 secondary standards. In particular, for air quality that was simulated to just meet the current PM 2.5 standards, greater than 10 percent of the days were estimated to have peak light extinction values that exceed the highest, least protective CPL of 30 dv in terms of PM 2.5 light extinction for 9 of the 15 urban areas, based on 1

- hour average values, and would thus likely fail to meet a 90 th percentile

- based standard at that level. For these areas, the percent of days estimated to have maximum 1

- hour values that exceed the highest CPL ranged from over 10 percent to over 30 percent. Similarly, when the middle CPL of 25 dv was considered, greater than 30 percent up to approximately 70***E.O. 12866 Review

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Page 296 of 432 percent of the days were estimated to have peak light extinction that exceeded that CPL in terms of PM 2.5 light extinction, for 11 of the 15 urban areas, based on 1

- hour average values. Based on a 4

- hour averaging time, 5 of the areas were estimated to have at least 10 percent of the days with peak light extinction exc eeding the highest CPL in terms of PM 2.5 light extinction, and 8 of the areas were estimated to have at least 30 percent of the days with peak light extinction exceeding the middle CPL in terms of PM 2.5 light extinction. For the lowest CPL of 20 dv, the pe rcentages of days with 4

- hour maximum light extinction estimated to exceed that CPL are even higher for all cases considered. Based on all of the above, the Policy Assessment concluded that PM light extinction estimated to be associated with just meeting t he current suite of PM 2.5 secondary standards in many areas across the country exceeded levels and percentages of days that could reasonably be considered to be important from a public welfare perspective (U.S. EPA, 2011a, p. 4

- 40). Further, the Policy As sessment concluded that use of the current indicator of PM 2.5 mass, in conjunction with the current 24

- hour and annual averaging times, is clearly called into question for a national standard intended to protect public welfare from PM

- related visibility im

pairment (U.S. EPA, 2011a, p. 4

- 40). This is because such a standard is inherently variable in the degree of protection provided because of regional differences in relative humidity and species composition of PM 2.5 , which are critical factors in the relati onship between the mix of fine particles in the ambient air and the associated impairment of visibility. The Policy Assessment noted that this concern was one of the important elements in the court’s decision to remand the PM 2.5 secondary standards set in 2006 to the Agency.***E.O. 12866 Review

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Page 297 of 432 Thus, in addition to concluding that the available information clearly calls into question the adequacy of the protection against PM

- related visibility impairment afforded by the current suite of PM 2.5 standards, the Policy Assessment also concluded that it clearly calls into question the appropriateness of each of the current standard elements: indicator, averaging time, form, and level (U.S. EPA, 2011a, p. 4

- 40). After reviewing the information and analysis in the second draft Policy Assessment, CASAC concluded that the “currently available information clearly calls into question the adequacy of the current standards and that consideration should be given to revising the suite of standards to provide increased public welfare protection ” (Samet, 2010d, p. iii). CASAC noted that the detailed estimates of hourly PM light extinction associated with just meeting the current standards “clearly demonstrate that current standards do not protect against levels of visual air quality which have be en judged to be unacceptable in all of the available urban visibility preference studies.” Further, CASAC stated, with respect to the current suite of secondary PM 2.5 standards, that “[T]he levels are too high, the averaging times are too long, and the PM 2 .5 mass indicator could be improved to correspond more closely to the light scattering and absorption properties of suspended particles in the ambient air” (Samet, 2010d, p. 9). After considering the available evidence and the advice of CASAC, the Administ rator concluded at the time of proposal that such information did provide an appropriate basis to inform a conclusion as to whether the current standards afford adequate protection against PM

- related visibility impairment in urban areas. The Administrator took into account the information discussed above with regard to the nature of PM

- related visibility impairment, the results of public perception surveys on the***E.O. 12866 Review

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Page 298 of 432 acceptability of varying degrees of visibility impairment in urban areas, analyses of the numbe r of days on which peak 1

- hour or 4

- hour light extinction values are estimated to exceed a range of candidate protection levels under conditions simulated to just meet the current standards, and the advice of CASAC. She noted the clear causal relationship between PM in the ambient air and impairment of visibility, the evidence from the visibility preference studies, and the rationale for determining a range of candidate protection levels based on those studies. She also noted the relatively large number of days when maximum 1

- hour or 4

- hour light extinction values were estimated to exceed the three candidate protection levels, including the highest level of 30 dv, under the current standards. While recognizing the limitations in the available information on public perceptions of the acceptability of varying degree of visibility impairment and the information on the number of days estimated to exceed the CPLs, she concluded that such information provided an appropriate basis to inform a conclusion as to whethe r the current standards provide adequate protection against PM

- related visibility impairment in urban areas. Based on these considerations, and placing great importance on the advice of CASAC, the Administrator provisionally concluded that the current stan dards are not sufficiently protective of visual air quality, and that consideration should be given to an alternative secondary standard that would provide additional protection against PM

related visibility impairment, with a focus primarily in urban area s. Having reached this conclusion, the Administrator also stated at the time of proposal that the current indicator of PM 2.5 mass, in conjunction with the current 24

- hour and annual averaging times, is not well suited for a national standard intended to pr otect public welfare from PM

- related visibility impairment. As noted in the proposal, the***E.O. 12866 Review

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Page 299 of 432 current standards do not incorporate information on the concentrations of various species within the mix of ambient particles, nor do they incorporate information on relative humidity, both of which play a central role in determining the relationship between the mix of PM in the ambient air and impairment of visibility. Such considerations were reflected both in CASAC’s advice to set a distinct secondary standard that would more directly reflect the relationship between ambient PM and visibility impairment and in the court’s remand of the current secondary PM 2.5 standards. Based on the above considerations, at the time of proposal the Administrator provisionally conclud ed that the current secondary PM 2.5 standards, taken together, are neither sufficiently protective nor suitably structured to provide an appropriate degree of public welfare protection from PM

- related visibility impairment, primarily in urban areas. This l ed the EPA to consider alternative standards by looking at each of the elements of the standards

— indicator, averaging time, form, and level

— as discussed below. ii. Indicator At the time of proposal, the EPA considered three alternative indicators for a P M 2.5 standard designed to protect against visibility impairment: the current PM 2.5 mass indicator; directly measured PM 2.5 light extinction; and calculated PM 2.5 light extinction. Directly measured PM 2.5 light extinction is a measurement (or combination of measurements) of the light absorption and scattering caused by PM 2.5 under ambient conditions. Calculated PM 2.5 light extinction uses the IMPROVE algorithm to calculate PM 2.5 light extinction using measured PM 2.5 mass, speciated PM 2.5 mass, and measured relative humidity. The Policy Assessment evaluated each of these alternatives, finally concluding that consideration should be given to establishing a new calculated PM 2.5 light***E.O. 12866 Review

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Page 300 of 432 extinction indicator (U.S. EPA, 2011a, p. 4

- 51). As discus sed in section VI.D.1 of the proposal, the Policy Assessment concluded that consideration of the use of either directly measured PM 2.5 light extinction or calculated PM 2.5 light extinction as an indicator is justified because light extinction is a physical ly meaningful measure of the characteristic of ambient PM 2.5 that is most relevant and directly related to PM

- related visibility effects (U.S. EPA, 2011a, p. 4

- 41). Further, as noted above, PM 2.5 is the component of PM responsible for most of the visibilit y impairment in most urban areas. In these areas, the contribution of PM 10

– 2.5 is a minor contributor to visibility impairment most of the time. The Policy Assessment also indicated that the available evidence demonstrated a strong correspondence between

c alculated PM 2.5 light extinction and PM

- related visibility impairment, as well as the significant degree of variability in visibility protection across the U.S. allowed by a PM 2.5 mass indicator. The Policy Assessment recognized that while in the future it would be appropriate to consider a direct measurement of PM 2.5 light extinction it was not an appropriate option in this review because a suitable specification of the equipment and associated performance verification procedures cannot be developed in the time frame for this review. (a) PM 2.5 Mass In terms of utilizing a PM 2.5 mass indicator, the proposal noted that PM 2.5 mass monitoring methods are in widespread use, including the FRM involving the collection of periodic (usually 1

- day

- in

- 6 or 1

- day

- in

- 3) 24

- hour filter samples. However, these routine monitoring activities do not include measurement of the full water content of the ambient PM 2.5 that contributes, often significantly, to visibility impacts. Further, the PM 2.5 mass***E.O. 12866 Review

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Page 301 of 432 concentration monitors d o not provide information on the composition of the ambient PM 2.5 , which plays a central role in the relationship between PM

- related visibility impairment and ambient PM 2.5 mass concentrations. Additional analyses discussed in the proposal that looked at t

he contribution of PM 2.5 to total PM

- related light extinction (defined in terms of hourly PM 10 calculated light extinction ) indicate that there is a poor correlation between hourly PM 10 light extinction and hourly PM 2.5 mass principally due to the impact o f the water content of the particles on light extinction, which depends on both the composition of the PM 2.5 and the ambient relative humidity. Both composition and especially relative humidity vary during a single day, as well as from day

- to

- day, at
any s ite and time of year. Also, there are systematic regional and seasonal differences in the distribution of ambient humidity and PM 2.5 composition conditions that make it impossible to select a PM 2.5 concentration that generally would correspond to the same PM

- related light extinction levels across all areas of the nation. Analyses discussed in the proposal quantify the projected uneven protection that would result from the use of 1

- hour average PM 2.5 mass as the indicator. (b) Directly Measured PM 2.5 Light Extinction PM light extinction has a nearly one

- to

- one relationship to light extinction, unlike PM 2.5 mass concentration. As explained above, PM 2.5 is the component responsible for the large majority of PM light extinction in most places and times. PM 2.5 light extinction can be directly measured using several instrumental methods, some of which have been used for decades to routinely monitor the two components of PM 2.5 light extinction (light scattering and absorption) or to jointly measure both as total light extinction (from which Rayleigh scattering is subtracted to get PM 2.5 light extinction). As noted at the time of***E.O. 12866 Review

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Page 302 of 432 proposal, there are a number of advantages to direct measurements of light extinction for use in a secondary standard relative to estimat es of PM 2.5 light extinction calculated using PM 2.5 mass and speciation data. These include greater accuracy of direct measurements with shorter averaging times and overall greater simplicity when compared to the need for measurements of multiple parameter s to calculate PM light extinction. In evaluating whether direct measurement of PM 2.5 or PM 10 light extinction is appropriate to consider in the context of this PM NAAQS review, the EPA solicited comment from the Ambient Air Monitoring and Methods Subcommi ttee (AAMMS) of CASAC. The CASAC AAMMS recommended that consideration of direct measurement should be limited to PM 2.5 light extinction, and that although instruments suitable for this purpose are commercially available at present, research is expected to produce even better instruments in the near term. The CASAC AAMMS advised against choosing any currently available commercial instrument, or even a general measurement approach, as an FRM because to do so could discourage development of other potentially s uperior approaches. Instead, the CASAC AAMMS recommended that the EPA develop performance

- based approval criteria for direct measurement methods in order to put all approaches on a level playing field. At the present time, the EPA has not undertaken to dev elop and test such performance

- base approval criteria. The EPA anticipates that if an effort were begun it would take at least several years before such criteria would be ready for regulatory use. Thus, the Policy Assessment concluded that while in the fut ure it would be appropriate to consider a direct measurement of PM 2.5 light extinction, or the sum of separate measurements of light scattering and light absorption, as the indicator for the secondary***E.O. 12866 Review

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Page 303 of 432 PM 2.5 standard, this is not an appropriate option in th is review because a suitable specification of the equipment or appropriate performance

- based verification procedures cannot be developed in the time frame for this review (U.S. EPA, 2011a, p. 4

- 51,

- 52). (c) Calculated PM 2.5 Light Extinction For the reasons discussed above, the Policy Assessment concluded that a calculated PM 2.5 light extinction indicator would be the preferred approach. PM 2.5 light extinction can be calculated from PM 2.5 mass, combined with speciated PM 2.5 mass concentration data plu s relative humidity data, as is presently routinely done on a 24

- hour average basis under the Regional Haze Program using data from the rural IMPROVE monitoring network. This same calculation procedure, using a 24

- hour average basis, could be used for a NA AQS focused on protecting against PM

- related visibility impairment primarily in urban areas. This approach would use the type of data that is routinely collected from the urban CSN 129 in combination with monthly average relative humidity data based on long

- t erm climatological means as used in the Regional Haze Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal discussed the complex approach utilized in the Visibility Assessment for calculating hourly PM 2.5 light extinction 130 and discussed various simplified approaches for calculating these hourly values that were analyzed in the Policy Assessment. The Policy Assessment concluded that each of these simplified approaches provided reasonably good estimates of PM 2.5

129 About 200 sites in the CSN routinely measure 24

- hour average PM 2.5 chemical components using filter

- based samplers and chemical analysis in a laboratory, on either a 1

- day

- in

- 3 or 1

- day

- in

- 6 sch edule (U.S. EPA, 2011a, Appendix B, section B.1.3). 130 As noted at the time of proposal, the sheer size of the ambient air quality, meteorological, and chemical transport modeling data files involved with the Visibility Assessment approach would make it very difficult for state agencies or any interested part y to consistently apply such an app roach on a routine basis for the purpose of implementing a national standard defined in terms of the Visibility Assessment approach.***E.O. 12866 Review

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Page 304 of 432 light extinction and each would be appropriate to consider as the indicator for a distinct hourly or multi

- hour secondary standard (U.S. EPA, 2011a, p. 4

- 48). The proposal also recognized that the Policy Assessment identified a number of variations on these simplified approaches that it wou ld be appropriate to consider, including: (1) the use of the split

- component mass extinction efficiency approach from the revised IMPROVE algorithm 131 (2) the use of more refined value(s) for the organic carbon multiplier 132 (3) the use of the reconstructed 24

- hour PM 2.5 mass (i.e., the sum of the five PM 2.5 components from speciated monitoring) as a normalization value for the hourly measurements from the PM 2.5 instrument as a way of better reflecting ambient nitrate concentrations (4) the use of historical monthly or season al, or regional, speciation averages Overall, the analyses conducted for the Visibility Assessment and Policy Assessment indicate d that the use of a calculated PM 2.5 light extinction indicator would provide a much higher degree of uniformity in terms of th e degree of protection from visibility impairment across the country than a PM 2.5 mass indicator, because a calculated PM 2.5 light extinction indicator would directly incorporate the effects of humidity and PM 2.5 composition differences between various reg ions. Further, the proposal noted that the Policy Assessment concluded that consideration could be given to defining a calculated PM 2.5 light extinction indicator on either a 24

- hour or a sub

- daily basis (U.S.

131 If the revised IMPROVE algorithm were used to define the calculated PM 2.5 mass based i ndicator, it would not be possible to algebraically reduce the revised algorithm to a two

- factor version as described above and in Appendix F of the Policy Assessment for the simplified approaches. Instead, five component fractions would be determined from each day of speciated sampling, and then either applied to hourly measurements of PM 2.5 mass on the same day or averaged across a month and then applied to measurements of PM 2.5 mass on each day of the month. 132 An organic carbon (OC)

- to

- organic mass (OM) multiplier of 1.6 was used for the assessment, which was found to produce a value of OM comparable to the one derived with the original, albeit more complex Visibility Assessment method.***E.O. 12866 Review

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Page 305 of 432 EPA, 2011a, p. 4

- 52). However, the Policy Asse ssment noted that approval of continuous FEM monitors has been based only on 24

- hour average, not hourly, PM 2.5 mass. In addition, there are mixed results of data quality assessments on a 24

- hour basis for these monitors, as well as the near absence of per formance data for sub

- daily averaging periods. Thus, while it is possible to utilize data from PM 2.5 continuous FEMs on a 1

- hour or multi

- hour (e.g., 4

- hour) basis, these factors increase the uncertainty of utilizing continuous methods to support 1

- hour or 4

- hour PM 2.5 mass measurements as an input to the light extinction calculation. Therefore, as noted at the time of proposal, until issues regarding the comparability of 24

- hour PM 2.5 mass values derived from continuous FEMs and filter

- based FRMs 133 are reso lved, there is reason to be cautious about relying on a calculation procedure that uses hourly PM 2.5 mass values reported by continuous FEMs in combination with speciated PM 2.5 mass values from 24

- hour filter

- based samplers. (d) CASAC Advice In reviewing the second draft Policy Assessment, CASAC stated that it “overwhelmingly ... would prefer the direct measurement of light extinction,” recognizing it as the property of the atmosphere that most directly relates to visibility effects (Samet, 2010d, p. iii). C ASAC noted that “[I]t has the advantage of relating directly to the demonstrated harmful welfare effect of ambient PM on human visual perception.” However, CASAC also concluded that the calculated PM 2.5 light extinction indicator “appears to be a reasonable approach for estimating hourly light extinction” (Samet, 2010d, p. 11). Further, based on CASAC’s understanding of the time that would be required to develop an FRM for this indicator, CASAC agreed wi th the staff
133 Filter

- based FRMs are designed to adequately quantify the amount o f PM 2.5 collected over 24

- hours. Th ey cannot be presumed to be appropriate for quantifying average concentrations over 1

- hour or 4

- hour periods.***E.O. 12866 Review

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Page 306 of 432 preference presented in the second draft Policy Assessment for a calculated PM 2.5 light extinction indicator. CASAC noted that “[I]ts reliance on procedures that have already been implemented in the CSN and routinely collected continuous PM 2.5 data suggest that it could be implemented much sooner than a directly measured indicator” (Samet, 2010d, p. iii). 134 (e) Administrator's Proposed Conclusions on Indicator At the time of proposal, while agreeing with CASAC that a directly measured PM light extinction indicator would provide the most direct link between PM in the ambient air and PM

- related light extinction, the Administrator provisionally concluded that this
was not an appropriate option in this review because a suitable specification of curr ently available equipment or performance

- based verification procedures cannot be developed in the time frame of this review. Taking all of the above considerations and CASAC advice into account, the Administrator provisionally concluded that a new calculat ed PM 2.5 light extinction indicator, similar to that used in the Regional Haze Program (i.e., using an IMPROVE algorithm as translated into the deciview scale), was the appropriate indicator to replace the current PM 2.5 mass indicator. Such an indicator, r eferred to as a PM 2.5 visibility index, would appropriately reflect the relationship between ambient PM and PM

- related light extinction, based on the analyses discussed in the proposal and incorporation of factors based on measured PM 2.5 speciation concent rations and relative humidity data. In addition, selection of this type of indicator would address, in part, the issues raised in the court’s remand of the 2006 PM 2.5 standards. The Administrator also

134 In commenting on the second draft Policy Assessment, CASAC did not have an opportunity to review the assessme nt of continuous PM 2.5 FEMs compared to collocated FRMs (Hanley and Reff, 2011) as presented and discussed in the final Policy Assessment (U.S. EPA, 2011a, p. 4

- 50).***E.O. 12866 Review

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Page 307 of 432 noted that such a PM 2.5 visibility index would afford a relatively high degree of uniformity of visual air quality protection in areas across the country by virtue of directly incorporating the effects of differences in PM 2.5 composition and relative humidity across the country. Based on these above considera tions, the Administrator proposed to set a distinct secondary standard for PM 2.5 defined in terms of a PM 2.5 visibility index (i.e., a calculated PM 2.5 light extinction indicator, translated into the deciview scale) to protect against PM

- related visibility impairment primarily in urban areas. The Administrator proposed that such an index be based on the original IMPROVE algorithm in conjunction with monthly average relative humidity data based on long

- term climatological means as used in the Regional Haze P rogram. The EPA solicited comment on all aspects of the proposed indicator, especially: (1) the proposed use of a PM 2.5 visibility index rather than a PM 10 visibility index which would include an additional term for coarse particles; (2) using the revised IMPROVE algorithm rather than the original IMPROVE algorithm; (3) the use of alternative values for the organic carbon multiplier in conjunction with either the original or revised IMPROVE algorithm; (4) the use of historical monthly, seasonal, or regional speciation aver ages; (5) alternative approaches to determining relative humidity; and (6) simplified approaches to generating hourly PM 2.5 light extinction values for purposes of calculating an hourly or multi

- hour indicator. iii. Averaging Times In this review, as discussed in section VI.D.2 of the proposal, consideration of appropriate averaging times for use in conjunction with a PM 2.5 visibility index was informed by information related to the nature of PM visibility effects and the nature of***E.O. 12866 Review

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Page 308 of 432 inputs to the calculation of PM 2. 5 light extinction, as discussed above. The EPA considered both sub

- daily (1

- and 4

- hour averaging times) and 24

- hour averaging times. In considering sub

- daily averaging times, the EPA has also considered what diurnal periods and ambient relative humidity conditions would be appropriate to consider in conjunction with such an averaging time. As an initial matter, the Policy Assessment considered sub

- daily averaging times. Taking into account what is known from available studies concerning how quickly people experience and judge visibility conditions, the possibility that some fraction of the public experiences infrequent or short periods of exposure to ambient visibility conditions, and the typical rate of change of the path

- averaged PM light extinction over urban areas, the initial analyses conducted as part of the Visibility Assessment focused on a 1

- hour averaging time. In its review of the first draft Policy Assessment, CASAC agreed that a 1

- hour averaging time would be appropriate to consider, noting tha t PM effects on visibility can vary widely and rapidly over the course of a day and such changes are almost instantaneously perceptible to human observers (Samet, 2010c, p. 19). The Policy Assessment noted that this view related specifically to a standard defined in terms of a directly measured PM light extinction indicator, in that CASAC also noted that a 1

- hour averaging time is well within the instrument response times of the various currently available and developing optical monitoring methods. However , CASAC also advised that if a PM 2.5 mass indicator were to be used, it would be appropriate to consider “somewhat longer averaging times

– 2 to 4 hours

– to assure a more stable instrumental response” (Samet, 2010c, p. 19). In considering this advice, the Policy Assessment concluded that since a calculated PM 2.5 light extinction***E.O. 12866 Review

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Page 309 of 432 indicator relies in part on measured PM 2.5 mass, it would be appropriate to consider a multi

- hour averaging time on the order of a few hours (e.g. 4

- hours). A multi

- hour averaging time might reasonably characterize the visibility effects experienced by the segment of the population who have access to visibility conditions often or continuously throughout the day. For this segment of the population, it may be that their perception of visual air quality reflects some degree of offsetting an hour with poor visual air quality with one or more hours of clearer visual conditions. Further, the Policy Assessment recognized that a multi

- hour averaging time would have the effect of averaging a way peak hourly visibility impairment, which can change significantly from one hour to the next (U.S. EPA, 2011a, p. 4

- 53; U.S. EPA, 2010b, Figure 3

- 12). In considering either 1

- hour or multi

- hour averaging times, the Policy Assessment recognized that no data are available with regard to how the duration and variation of time a person spends outdoors during the daytime impacts his or her judgment of the acceptability of different degrees of visibility impairment. As a consequence, it is not clear to what d egree, if at all, the protection levels found to be acceptable in the public preference studies would change for a multi

- hour averaging time as compared to a 1

- hour averaging time. Thus, the Policy Assessment concluded that it is appropriate to consider a 1

- hour or multi

- hour (e.g., 4

- hour) averaging time as the basis for a sub

- daily standard defined in terms of a calculated PM 2.5 light extinction indicator (U.S. EPA, 2011a, p. 4

- 53). In addition, as discussed above, some data quality uncertainties have bee n observed with regard to hourly data collected by FEMs. Specifically, as part of the review of data from all continuous FEM PM 2.5 instruments operating at state/local monitoring***E.O. 12866 Review

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Page 310 of 432 sites, the Policy Assessment noted that the occurrence of questionable outlie rs in 1

- hour data submitted to AQS from continuous FEM PM 2.5 instruments had been observed at some of these sites (Evangelista, 2011). Some of these outliers were questionable simply by virtue of their extreme magnitude, as high as 985 μg/m 3 , whereas other values were questionable because they were isolated to single hours with much lower values before and after, a pattern that is much less plausible than if the high concentrations were more sustained. 135 The Policy Assessment noted that any current data qual ity problems might be resolved in the normal course of monitoring program evolution as operators become more adept at instrument operation and maintenance and data validation or by improving the approval criteria and testing requirements for continuous ins truments. Regardless, the Policy Assessment noted that multi

- hour averaging of FEM data could serve to reduce the effects of such outliers relative to the use of a 1

- hour averaging time. The Policy Assessment noted that there are significant reasons to co nsider using PM 2.5 light extinction calculated on a 24

- hour basis to reduce the various data quality concerns described above with respect to relying on continuous PM 2.5 monitoring data. However, the Policy Assessment recognized that 24 hours is far longer than the hourly or multi

- hour time periods that might reasonably characterize the visibility effects experienced by various segments of the population, including both those who do and do not have access to visibility conditions often or continuously throu ghout the day. Thus, the Policy Assessment concluded that the appropriateness of considering a 24

- hour
135 Similarly questionable hourly data were not observed in the 2005 to 2007 continuous PM 2.5 data used in the Visibility Assessment, all of which came from early

- generation continuous instruments that had not been approved as FEMs. However, only 15 sites and instruments were involved in the Visibility Assessment analyses, versus about 180 curr ently operating FEM instruments submitting data to AQS. Therefore, there were more opportunities for very infrequent measurement errors to be observed in the larger FEM data set.***E.O. 12866 Review

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Page 311 of 432 averaging time would depend upon the extent to which PM

- related light extinction calculated on a 24

- hour average basis would be a reasonable and appropri ate surrogate for PM

- related light extinction calculated on a sub

- daily basis. To examine this relationship, the EPA conducted comparative analyses of 24

- hour and 4

- hour averaging times in conjunction with a calculated PM 2.5 indicator. For these analyses, 4

- hour average PM 2.5 light extinction was calculated based on using the Visibility Assessment approach. The 24

- hour average PM 2.5 light extinction was calculated using the original IMPROVE algorithm and long

- term relative humidity conditions to calculate PM 2.5 light extinction. Based on these analyses, 136 which are presented and discussed in Appendix G of the Policy Assessment, scatte r plots comparing 24

- hour and 4

- hour calculated PM 2.5 light extinction were constructed for each of the 15 cities included in the Visibility Assessment and for all 15 cities pooled together (U.S. EPA, 2011a, Figures G

- 4 and G

- 5). Though there was some scat ter around the regression line for each city because the calculated 4

- hour light extinction values included day

specific and hour

- specific influences that are not captured by the simpler 24

- hour approach, these analyses generally showed good correlation be tween 24

- hour and 4

- hour average PM 2.5 light extinction, as evidenced by reasonably high city

- specific and pooled R 2 values, generally in the range of over 0.6 to over 0.8. 137 This suggested that PM 2.5 light extinction calculated on a 24

- hour basis is a reas onable and appropriate surrogate to PM 2.5 light extinction calculated on a sub

- daily basis. Taking the above considerations and CASAC’s advice into account, the Policy
136 These analyses are also based on the use of a 90 th percentile form, averag ed over 3 years, as discussed below i n section VI.D.3 and in section 4.3.3 of the Policy Assessment (U.S. EPA, 2011a). 137 The EPA staff noted that the R 2 value (0.44) for Houston was notably lower than for the other cities.***E.O. 12866 Review

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Page 3 12 of 432 Assessment concluded that it would be appropriate to consider a 24

- hour averaging time, in conjunction with a calculated PM 2.5 light extinction indicator and an appropriately specified standard level, as discussed below. By using site

- specific daily data on PM 2.5 composition and site

- specific long

- term relative humidity conditions, this 24

- h our average indicator would provide more consistent protection from PM 2.5 - related visibility impairment than would a secondary PM 2.5 NAAQS based only on 24

- hour or annual average PM 2.5 mass. In particular, this approach would account for the systematic dif ference in humidity conditions between most eastern states and most western states. The Policy Assessment also concluded that it would also be appropriate to consider a multi

- hour, sub

- daily averaging time, for example a period of 4 hours, in conjunction

w ith a calculated PM 2.5 light extinction indicator and with further consideration of the data quality issues discussed above. Such an averaging time, to the extent that data quality issues can be appropriately addressed, would be more directly related to th e short

term nature of the perception of visibility impairment, short

- term variability in PM

related visual air quality, and the short

- term nature (hourly to multiple hours) of relevant exposure periods for segments of the viewing public. Such an averaging time would still result in an indicator that is less sensitive than a 1

- hour averaging time to short

- term instrument variability with respect to PM 2.5 mass measurement. In conjunction with consideration of a multi

- hour, sub

- daily averaging time, the Polic y Assessment concluded that consideration should be given to including daylight hours only and to applying a relative humidity screen of approximately 90 percent to remove hours in which fog or precipitation is much more likely to contribute to the observe d visibility impairment (U.S. EPA, 2011a, p. 4

- 58). Recognizing that a 1

- hour averaging time would be even more***E.O. 12866 Review

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Page 313 of 432 sensitive to data quality issues, including short

- term variability in hourly data from currently available continuous monitoring methods, the Po licy Assessment concluded that it would not be appropriate to consider a 1

- hour averaging time in conjunction with a calculated PM 2.5 light extinction indicator in this review (U.S. EPA, 2011a, p. 4

- 58). As noted above, in its review of the first draft Po licy Assessment, CASAC concluded that PM effects on visibility can vary widely and rapidly over the course of a day and such changes are almost instantaneously perceptible to human observers (Samet, 2010c, p. 19). Based in part on this consideration, CASAC agreed that a 1

- hour averaging time would be appropriate to consider in conjunction with a directly measured PM light extinction indicator, noting that a 1

- hour averaging time is well within the instrument response times of the various currently available and developing optical monitoring methods. At that time, CASAC also advised that if a PM 2.5 mass indicator were to be used, it would be appropriate to consider “somewhat longer averaging times

– 2

- to 4

- hours

– to assure a more stable instrumental respons e” (Samet, 2010c, p. 19). Thus, CASAC’s advice on averaging times that would be appropriate for consideration was predicated in part on the capabilities of monitoring methods that were available for the alternative indicators discussed in the draft Policy Assessment. CASAC’s views on a multi

- hour averaging time would also apply to the calculated PM 2.5 light extinction indicator since hourly PM 2.5 mass measurements are also required for this indicator when calculated on a sub

- daily basis. It is important to note that at the time it provided advice on suitable averaging times, CASAC did not have the benefit of EPA’s subsequent assessment of the data quality issues associated with the use of continuous FEMs as the basis for hourly PM 2.5***E.O. 12866 Review

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Page 314 of 432 mass measurements. Furth ermore, since CASAC only commented on the first and second drafts of the Policy Assessment, neither of which included discussion of a calculated PM 2.5 indicator based on a 24

- hour averaging time, CASAC did not have a basis to offer advice regarding a 24

- ho ur averaging time. In addition, the 24

- hour averaging time is not based on consideration of 24

- hours as a relevant exposure period, but rather as a surrogate for a sub

- daily period of 4 hours, which is consistent with CASAC’s advice concerning an averaging time associated with the use of a PM 2.5 mass indicator. Taking into account the information discussed above with regard to analyses and conclusions presented in the final Policy Assessment the Administrator recognize d that hourly or sub

- daily, multi

- hour averaging times, within daylight hours and excluding hours with relative humidity above approximately 90 percent, are more directly related than a 24

- hour averaging time to the short

- term nature of the perception of PM

- related visibility impairment and the relevant exposure periods for segments of the viewing public. On the other hand, she recognize d that data quality uncertainties have recently been associated with currently available instruments that would be used to provide the hourly PM 2.5 mass measurem ents that would be needed in conjunction with an averaging time shorter than 24

- hours. As a result, while the Administrator recognize d the desirability of a sub

- daily averaging time, she ha d strong reservations about proposing to set a standard at this tim e in terms of a sub

- daily averaging time. In considering the information and analyses related to consideration of a 24

- hour averaging time, the Administrator recognized that the Policy Assessment concluded that PM 2.5 light extinction calculated on a 24

- ho ur averaging basis is a reasonable and appropriate surrogate for sub

- daily PM 2.5 light extinction calculated on a 4

- hour average***E.O. 12866 Review

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Page 315 of 432 basis. In light of this finding and the views of CASAC based on its reviews of the first and second drafts of the Policy Assess ment, the Administrator proposed to set a distinct secondary standard with a 24

- hour averaging time in conjunction with a PM 2.5 visibility index. iv. Form As discussed in section VI.D.3 of the proposal, the “form” of a standard defines the air quality sta tistic that is to be compared to the level of the standard in determining whether the standard is achieved. The form of the current 24

- hour PM 2.5 NAAQS is such that the level of the standard is compared to the 3

- year average of the annual 98 th percentile v alue of the measured indicator. The purpose in averaging for three years is to provide stability from the occasional effects of inter

- annual meteorological variability that can result in unusually high pollution levels for a particular year. The use of a m ulti

year percentile form, among other things, makes the standard less subject to the possibility of transient violations caused by statistically unusual indicator values, thereby providing more stability to the air quality management process that may enha nce the practical effectiveness of efforts to implement the NAAQS. Also, a percentile form can be used to take into account the number of times an exposure might occur as part of the judgment on protectiveness in setting a NAAQS. For all of these reasons, the Policy Assessment concluded it would be appropriate to consider defining the form of a distinct secondary standard in terms of a 3

- year average of a specified percentile air quality statistic (U.S. EPA, 2011a, p. 4

- 58). The urban visibility preference studies that provided results leading to the range of CPLs being considered in this review offer no information that addresses the***E.O. 12866 Review

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Page 316 of 432 frequency of time that visibility levels should be below those values. Given this lack of information, and recognizing that t he nature of the public welfare effect is one of aesthetics and/or feelings of well

- being, the Policy Assessment concluded that it would not be appropriate to consider eliminating all exposures above the level of the standard and that allowing some number of hours/days with reduced visibility can reasonably be considered (U.S. EPA, 2011a, p. 4

- 59). In the Visibility Assessment, 90 th , 95 th , and 98 th percentile forms were assessed for alternative PM light extinction standards (U.S. EPA, 2010b, section 4.3.3). In considering these alternative percentiles, the Policy Assessment noted that the Regional Haze Program targets the 20 percent most impaired da ys for improvements in visual air quality in Federal Class I areas. If improvement in the 20 percent most impaired days were similarly judged to be appropriate for protecting visual air quality in urban areas, a percentile well above the 80 th percentile wo uld be appropriate to increase the likelihood that all days in this range would be improved by control strategies intended to attain the standard. A focus on improving the 20 percent most impaired days suggests that the 90 th percentile, which represents th e median of the distribution of the 20 percent worst days, would be an appropriate form to consider. Strategies that are implemented so that 90 percent of days have visual air quality that is at or below the level of the standard would reasonably be expect ed to lead to improvements in visual air quality for the 20 percent most impaired days. Higher percentile values within the range assessed could have the effect of limiting the occurrence of days with peak PM

- related light extinction in urban areas to a gr eater degree. In considering the limited information available from the public preference studies, the Policy Assessment found no basis to conclude that it would be appropriate to consider limiting the***E.O. 12866 Review

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Page 317 of 432 occurrence of days with peak PM

- related light extincti on in urban areas to a greater degree. Another aspect of the form discussed in the proposal for a sub

- daily averaging time was whether to include all daylight hours or only the maximum daily daylight hour(s). The maximum daily daylight 1

- hour or multi

- hour form would be most directly protective of the welfare of people who have limited, infrequent or intermittent exposure to visibility during the day (e.g., during commutes), but spend most of their time without an outdoor view. For such people a view of poo r visibility during their morning commute may represent their perception of the day’s visibility conditions until the next time they venture outside during daylight, which may be hours later or perhaps the next day. Other people have exposure to visibility conditions throughout the day. For those people, it might be more appropriate to include every daylight hour in assessing compliance with a standard, since it is more likely that each daylight hour could affect their welfare. The Policy Assessment did no t have information regarding the fraction of the public that has only one or a few opportunities to experience visibility during the day, nor did it have information on the role the duration of the observed visibility conditions has on wellbeing effects as sociated with those visibility conditions. However, it is logical to conclude that people with limited opportunities to experience visibility conditions on a daily basis would experience the entire impact associated with visibility based on their short

- ter m exposure. The impact of visibility for those who have access to visibility conditions often or continuously during the day may be based on varying conditions throughout the day. In light of these considerations, the analyses conducted as part of the Vis ibility***E.O. 12866 Review

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Page 318 of 432 Assessment analyses included both the maximum daily hour and the all daylight hours forms. The Policy Assessment noted that there is a close correspondence between the level of protection afforded for all 15 urban areas by a maximum daily daylight 1

- hour approach using the 90 th percentile form and an all daylight hours approach combined with the 98 th percentile form (U.S. EPA, 2010b, section 4.1.4). This suggest ed that reductions in visibility impairment required to meet either form of the standard would provide protection to both fractions of the public (i.e., those with limited opportunities and those with greater opportunities to view PM

- related visibility conditions). CASAC generally supported consideration of both types of forms without expressi ng a preference based on its review of information presented in the second draft Policy Assessment (Samet, 2010d, p. 11). In conjunction with a calculated PM 2.5 light extinction indicator and alternative 24

- hour or sub

- daily (e.g., 4

- hour) averaging times, based on the above considerations, and given the lack of information on and the high degree of uncertainty over the impact on public welfare of the number of days with visibility impairment over a year, the Policy Assessment concluded that it would be app ropriate to give primary consideration to a 90 th percentile form, averaged over three years (U.S. EPA, 2011a, p. 4

- 60). Further, in the case of a multi

- hour, sub

- daily alternative standard, the Policy Assessment concluded that it would be appropriate to gi ve primary consideration to a form based on the maximum daily multi

- hour period in conjunction with the 90 th percentile form (U.S. EPA, 2011a, p. 4

- 60). This sub

- daily form would be expected to provide appropriate protection for various segments of the pop ulation, including those with limited opportunities during a day and those with more extended opportunities over the daylight hours to experience***E.O. 12866 Review

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Page 319 of 432 PM

- related visual air quality. Though CASAC did not provide advice as to a specific form that would be
appropr iate, it took note of the alternative forms considered in that document and encouraged further analyses in the final Policy Assessment that might help to clarify a basis for selecting from within the range of forms identified. In considering the available information and the conclusions in the final Policy Assessment in light of CASAC’s comments, at the time of proposal the Administrator concluded that a 90 th percentile form, averaged over 3 years, is appropriate, and proposed such a form in conjunction wit h a PM 2.5 visibility index and a 24

- hour averaging time.

v. Level As discussed in section VI.D.4 of the proposal, in considering appropriate levels for a 24

- hour standard defined in terms of a PM 2.5 visibility index and an 90 th percentile form, averaged ov er 3 years, the Policy Assessment took into account the evidence

- and impact

- based considerations discussed above, with a focus on the results of public perception and attitude surveys related to the acceptability of various levels of visual air quality an d on the important limitations in the design and scope of such available studies

. The Policy Assessment considered a variety of approaches for identifying appropriate levels for such a standard, including utilizing both adjusted and unadjusted CPLs derived from the visibility preference studies. The Policy Assessment interpreted the results from the visibility preferences studies conducted in four urban areas to define a range of low, middle, and high CPLs for a sub

- daily standard (e.g., 1

- to 4

- hour avera ging time) of 20, 25, and 30 dv, which are approximately equivalent to PM 2.5 light extinction of values of 65, 110, and 190 Mm - 1

.***E.O. 12866 Review

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Page 320 of 432 The CASAC generally supported this approach, noting that the “ EPA staff’s approach for translating and presenting the technica l evidence and assessment results is logically conceived and clearly presented. The 20

- 30 deciview range of levels chosen by EPA staff as “Candidate Protection Levels” is adequately supported by the evidence presented” (Samet, 2010d, p. 11). 138 The Policy As sessment also recognized that to define a range of alternative levels that would be appropriate to consider for a 24

- hour calculated PM 2.5 light extinction standard, it would be appropriate to consider whether some adjustment to these CPLs is warranted sin ce these preference studies cannot be directly interpreted as applying to a 24

- hour exposure period (as noted above and in Policy Assessment section 4.3.1). Considerations related to such adjustments are more specifically discussed below. In considering al ternative levels for a sub

- daily standard based directly on the four preference study results, the Policy Assessment noted that the individual low and high CPLs are in fact generally reflective of the results from the Denver and Washington, DC studies resp ectively, and the middle CPL is very near to the 50 th percentile criteria result from the Phoenix study, which was by far the best of the studies, providing somewhat more support for the middle CPL. In considering the results from the four visibility prefe rence studies, the Policy Assessment recognized that currently available studies are limited in that they were conducted in only four areas, three in the U.S. and one in Canada. Further, the Policy Assessment recognized that available studies provide no in formation on how the duration

138 In 2009, the D.C. Circuit reman ded the secondary PM 2.5 standards to the EPA in part because the Agency failed to identify a target level of protection, even though EPA staff and CASAC had identified a range of target levels of protection that were appropriate for consideration. The cour t determined that the Agency’s failur e to identify a target level of protection as part of its final decision was contrary to the statute and therefore unlawful, and that it deprived EPA’s decision

- making of a reasoned basis. See 559F. 3d at 528

- 31; see al so section VI.A.2 above and the Policy Assessment, section 4.1.2.***E.O. 12866 Review

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Page 321 of 432 and variation of time a person spends outdoors during the daytime may impact their judgment of the acceptability of different degrees of visibility impairment

. As such, there is a relatively high degree of uncertainty associat ed with using the results of these studies to inform consideration of a national standard for any specific averaging time. Nonetheless, the Policy Assessment concluded, as did CASAC, that these studies are appropriate to use for this purpose (U.S. EPA, 201 1a, p. 4

- 61). Using approaches described in section VI.C.4 of the proposal, the Policy Assessment explored various approaches to adjusting the CPLs derived from the preference studies to inform alternative levels for a 24

- hour standard. These various
appro aches, based on analyses of 2007

– 2009 data from the 15 urban areas assessed in the Visibility Assessment, focused on estimating CPLs for a 24

- hour standard that would provide generally equivalent protection as that provided by a 4

- hour standard with CPLs of 20, 25, and 30 dv. In conducting these analyses, staff initially expected that the values of 24

- hour average PM 2.5 light extinction and daily maximum daylight 4

- hour average PM 2.5 light extinction would differ on any given day, with the shorter term peak value generally being larger. This would mean that, in concept, the level of a 24

- hour standard should include a downward adjustment compared to the level of a 4

- hour standard to
p rovide generally equivalent protection. As discussed more fully in section G.5 of Appendix G and summarized below, this initial expectation was not found to be the case across the range of CPLs considered. In fact, as shown in Table s G

- 7 and G

- 8 of Appendi x G and in the corrected version of Table G

- 6 found in Frank et al. (2012b) , 139 in

139 Note that the city

- specific ranges shown in Table G

- 6, Appendix G of the Policy Assessment are incorrectly stated for Approaches C and E. Drawing from the more detailed and correct results for Approaches C and E presented in T ables G

- 7 and G

- 8, respectively, the city

- specific ranges in Table G

- 6 for Approach C should be 17

– 21 dv***E.O. 12866 Review

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Page 322 of 432 considering estimates aggregated or averaged over all 15 cities as well as the range of city

- specific estimates for the various approaches considered, these analyses indicated that the generally equivalent 24

- hour levels ranged from somewhat below the 4

- hour level to just above the 4

- hour level for each of the CPLs. 140 In all cases, the range of city

specific estimates of generally equivalent 24

- hour levels included the 4

- hour le vel for each of the CPLs of 20, 25, and 30 dv. As noted in the proposal, looking more broadly at these results could support consideration of using the same CPL for a 24

- hour standard as for a 4

- hour standard, recognizing that there is no one approach that can most closely identify a generally equivalent 24

- hour standard level in each urban area for each CPL. The use of such an unadjusted CPL for a 24

- hour standard would place more emphasis on the relatively high degree of spatial and temporal variability i n relative humidity and fine particle composition observed in urban areas across the country, so as to reduce the potential of setting a 24

- hour standard level that would require more than the intended degree of protection in some areas. In considering the appropriate level of a secondary standard focused on protection from PM

- related urban visibility impairment based on either a 24

- hour or a multi

- hour,
for the CPL of 20 dv; 21

– 25 dv for the CPL of 25 dv; and 24

– 30 dv for the CPL of 30 dv; the city

- specific ra nges in Table G

- 6 for Approach E should be 17

– 21 dv for the CPL of 20 dv; 21

– 26 dv for the CPL of 25 dv; and 25

– 31 dv for the CPL of 30 dv. In the EPA’s reanalysis comparing 4

- vs. 24

- hour values, Frank et al. (2012b) recreated Table G

- 6 using the co rrect values from Tables G

- 7 and G

- 8. 140 As discussed in more detail in Appendix G of the Policy Assessment, some days have higher values for 24

- hour average light extinction than for daily maximum 4

- hour daylight light extinction, and consequently an adjust ed "equivalent" 24

- hour CPL can be gr eater than the original 4

- hour CPL. This can happen for two reasons. First, the use of monthly average historical RH data will lead to cases in which the f (RH) values used for the calculation of 24

- hour average light ex tinction are higher than all or some of the four hourly values of f

(RH) used to determine daily maximum 4

- hour daylight light extinction on the same day. Second, PM 2.5 concentrations may be greater during non

- daylight periods than during daylight hours.***E.O. 12866 Review

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Page 323 of 432 sub

- daily (e.g., 4

- hour) averaging time, the EPA has been mindful of the important limitations in the av ailable evidence from public preference studies. These uncertainties and limitations are due in part to the small number of stated preference studies available for this review; the relatively small number of study participants and the extent to which the s tudy participants may not be representative of the broader study area population in some of the studies; and the variations in the specific materials and methods used in each study such as scene characteristics, the range of VAQ levels presented to study
p articipants, image presentation methods and specific wording used to frame the questions used in the group interviews. In addition the EPA has noted that the scenic vistas available on a daily basis in many urban areas across the country generally may not have the inherent visual interest or the distance between viewer and object of greatest intrinsic value as in the Denver and Phoenix preference studies, and that there is the possibility that there could be regional differences in individual preferences fo r VAQ. It is also important to note that as in past reviews, the EPA is considering a national visibility standard in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM

- related visibility impairme nt in urban, non

- urban, and Federal Class I areas across the country. The EPA recognizes that programs implemented to meet a national standard focused primarily on the visibility problems in urban areas can be expected to improve visual air quality in surr ounding non

urban areas as well, as would programs now being developed to address the requirements of the Regional Haze Program established for protection of visual air quality in Federal Class I areas. The EPA also believes that the development of local p rograms, such as those in Denver and Phoenix, can continue to be an effective and appropriate approach to***E.O. 12866 Review

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Page 324 of 432 provide additional protection, beyond that afforded by a national standard, for unique scenic resources in and around certain urban areas that are par ticularly highly valued by people living in those areas. The Policy Assessment concluded that it is appropriate to give primary consideration to alternative standard levels toward the upper end of the ranges identified above for 24

- hour and sub

- daily standards, respectively (U.S. EPA, 2011a, p. 4

- 63). Thus, the Policy Assessment concluded it is appropriate to consider the following alternative levels: a level of 28 dv or somewhat below, down to 25 dv, for a standard defined in terms of a calculated PM 2 .5 light extinction indicator, a 90 th percentile form, and a 24

- hour averaging time; and a standard level of 30 dv or somewhat below, down to 25 dv, for a similar standard but with a 4

- hour averaging time (U.S. EPA, 2011a, p. 4

- 63). The Policy Assessment j udged that such standards would provide appropriate protection against PM

- related visibility impairment primarily in urban areas. The Policy Assessment noted that CASAC generally supported consideration of the 20

– 30 dv range as CPLs and, more specificall y, that support for consideration of the upper part of the range of the CPLs derived from the public preference studies was expressed by some CASAC Panel members during the public meeting on the second draft Policy Assessment. The Policy Assessment conclud ed that such a standard would be appropriate in conjunction with the Regional Haze Program to achieve appropriate levels of protection against PM

- related visibility impairment in areas across the country (U.S. EPA, 2011a, p. 4

- 63). Based on the considerati ons discussed above and in section VI.D.4 of the proposal, and taking into account the advice of CASAC, at the time of proposal the***E.O. 12866 Review

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Page 325 of 432 Administrator concluded that it would be appropriate to establish a target level of protection

– for a standard defined in t erms of a PM 2.5 visibility index; a 90 th percentile form averaged over 3 years; and a 24

- hour averaging time

– equivalent to the protection afforded by such a sub

- daily (i.e., 4

- hour) standard at a level of 30 dv, which is the upper end of the range of CPL s identified in the Policy Assessment and generally supported by CASAC. More specifically, the Administrator provisionally concluded that a 24

- hour level of either 30 dv or 28 dv could be construed as providing such a degree of protection, and that either level was supported by the available information and was generally consistent with the advice of CASAC. Thus, the EPA proposed two options for the level of a new 24

- hour standard (defined in terms of a PM 2.5 visibility index and a 90 th percentile form, ave raged over 3 years) to provide appropriate protection from PM

related visibility impairment: either 30 dv or 28 dv. As noted in the proposal, the option of setting such a 24

- hour standard at a level of 30 dv would reflect recognition that there is consider able spatial and temporal variability in the key factors that determine the value of the PM 2.5 visibility index in any given urban area, such that there is a relatively high degree of uncertainty as to the most appropriate approach to use in selecting a 24

- hour standard level that would be generally equivalent to a specific 4

- hour standard level. Selecting a 24

- hour standard level of 30 dv would reflect a judgment that such substantial degrees of variability and uncertainty should be reflected in a higher s tandard level than would be appropriate if the underlying information were more consistent and certain. Alternatively, the option of setting such a 24

- hour standard at a level of 28 dv would reflect placing more weight on statistical analyses of aggregated data from across the study cities and not placing as much emphasis on the city

- to

- city variability as a basis for***E.O. 12866 Review

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Page 326 of 432 determining an appropriate degree of protection on a national scale. The information available for the Administrator to consider when setting the secondary PM standard raises a number of uncertainties. While CASAC supported moving forward with a new standard on the basis of the available information, CASAC also recognized these uncertainties, referencing the discussion of key uncertainties and areas for future research in the second draft of the Policy Assessment. In discussing areas of future research, CASAC stated that: “The range of 50% acceptability values discussed as possible standards are based on just four studies (Figure 4

- 2), which, gi ven the large spread in values, provide only limited confidence that the benchmark candidate protection levels cover the appropriate range of preference values. Studies using a range of urban scenes (including, but not limited to, iconic scenes

– “valued s cenic elements” such as those in the Washington, DC study), should also be considered” (Samet, 2010d, p. 12). The EPA solicited comment on how the Administrator should weigh those uncertainties as well as any additional comments and information to inform h er consideration of these uncertainties. In addition, the EPA solicited comment on a number of other issues related to the level of the standard, including: (1) Both of the proposed levels and the various approaches to identifying generally equivalent levels upon which the alternative proposed levels are based. (2) A broader range of levels down to 25 dv in conjunction with a 24

- hour averaging time. (3) A range of alternative levels from 30 to 25 dv in conjunction with a sub

- daily (e.g., 4

- hour) averaging time. (4) The strengths and limitations associated with the public preference studies and the use of these studies to inform the selection of a range of levels that could be used to provide an appropriate degree of public welfare protection when combined with the other elements of the standard (i.e. indicator, form and averaging time).***E.O. 12866 Review

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Page 327 of 432 (5) Specific aspects of the public preference studies, including the extent to which the 50 percent acceptability criterion is an appropriate basis for establishing target protection levels in the context of establishing a distinct secondary NAAQS to address PM

- related visibility impairment in urban areas; how the variability among preference studies in the extent to which study participants may be representative of the broader study area popul ation should be weighed in the context of considering these studies in reaching proposed conclusions on a distinct secondary NAAQS; and the extent to which the ranges of VAQ levels presented to participants in each of the studies may have influenced study results and on how this aspect of the study designs should appropriately be weighed in the context of considering these studies in the context of this review. vi. Administrator’s Proposed Conclusions Regarding PM Standards to Protect Visibility At the t ime of proposal, based on the considerations described above, the Administrator proposed to revise the suite of secondary PM standards by adding a distinct standard for PM 2.5 to address PM

- related visibility impairment, focused primarily on visibility in u rban areas. This proposed visibility standard was to be defined in terms of a PM 2.5 visibility index, which would use measured PM 2.5 mass, combined with PM 2.5 speciation data and relative humidity data, to calculate PM 2.5 light extinction, translated into the deciview (dv) scale; a 24

- hour averaging time; a 90 th percentile form, averaged over 3 years; and a level of 28

- 30 dv. vii. Related Technical Analysis At the time of proposal, the EPA conducted a two

- pronged technical analysis of the relationships be tween the proposed PM 2.5 visibility index standard and the current 24

- hour PM 2.5 mass - based standard (Kelly, et al., 2012a). This analysis was designed to provide technical information to inform key issues related to implementing a distinct secondary stand ard for visibility as proposed. Specifically, the EPA recognized that significant technical issues were likely to arise for new or modified emissions sources***E.O. 12866 Review

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Page 328 of 432 conducting air quality analyses for purposes of demonstrating that they would not cause or contrib ute to a violation of the visibility standard under the Prevention of Significant Deterioration (PSD) program. Such a demonstration for the proposed secondary PM 2.5 visibility index standard could require each PSD applicant to predict, via air quality mode ling, the increase in visibility impairment, in terms of the proposed PM 2.5 visibility index, that would result from the proposed source's emissions in conjunction with an assessment of existing air quality (visibility impairment) conditions in terms of th e proposed PM 2.5 visibility index. The EPA noted that if this demonstration were to be attempted using the six

- step procedure that the EPA proposed to use for calculating PM 2.5 visibility index design values from monitored air concentrations of PM 2.5 components, significant technical issues with the modeling procedures could arise. To address these technical issues, the EPA sought to explore whether sources that met the requirements pertaining to the 24

- hour mass

- based standard of 35 μg/m 3 would also meet the requirements pertaining to the proposed visibility index standard. As described in Kelly et al. (2012a), the first prong of the analysis addressed aspects of a PSD significant impact analysis by evaluating whether an individual source’s impact

res ulting in a small increase in the ambient PM 2.5 concentration would produce a comparably small increase in visibility impairment. This analysis included estimates of PM 2.5 speciation profiles based on direct PM 2.5 emission profiles for a broad range of

sou rce categories and for theoretical upper and lower bound scenarios. The second prong of the analysis addressed aspects of a PSD cumulative impact analysis by exploring the relationship between the three

- year design values for the existing 24

- hour PM 2.5 sta ndard and coincident design values for the proposed PM 2.5***E.O. 12866 Review

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Page 329 of 432 visibility index standard based on recent air quality data. This aspect of the analysis indicated that increases in 24

- hour PM 2.5 design values generally correspond to increases in visibility index design values, and vice

- versa. The analysis further explored the appropriateness of using a demonstration that a source does not cause or contribute to a violation of the 24

- hour PM 2.5 standard as a surrogate for a demonstration that a source does not caus e or contribute to a violation of the proposed secondary PM 2.5 visibility index standard . This analysis wa s based on 2008 to 2010 air quality data, and compare d the proposed level of 35 μg/m 3 for the 24

- hour PM 2.5 standard and for illustrative
purposes an alternative standard level of 12 μg/m 3 for the annual PM 2.5 standard with the proposed levels of 28 or 30 dv for the secondary PM 2.5 visibility index standard with a 24

- hour averaging time and a 90th percentile form. The results indicate d that all (for the 30 dv level) or nearly all (for the 28 dv level) areas in attainment of the 24

- hour PM 2.5 standard would also have been in attainment of the proposed secondary PM 2.5 visibility index standard. Based on this technical analysis, the EPA proposed that there is sufficient evidence that a demonstration that a source does not cause or contribute to a violation of the mass

- based 24

- hour PM 2.5 standard serves as a suitable surrogate for demonstrating that a source does not cause or contribute to a violation of the proposed secondary 24

- hour PM 2.5 visibility index standard under the PSD program. As such, the EPA proposed to conclude that many or all sources undergoing PSD review for PM 2.5 could rely upon their analysis for demonstrating that they do not cause or con tribute to a violation of the

mass - based 24

- hour PM 2.5 standard to also show that they do not cause or contribute to a violation of the proposed secondary PM 2.5 visibility index standard , if a distinct visibility***E.O. 12866 Review

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Page 330 of 432 standard were finalized. Although this pro posed “surrogacy policy” was designed to address an implementation

- related issue, the second prong of the technical analysis addresses the broader technical question of the relationship between the existing 24

- hour PM 2.5 standard and the proposed PM 2.5 visibility index standard in terms of the degree of protection likely to be afforded by each standard. Specifically, the analysis indicated that depending on the level of the proposed PM 2.5 visibility index standard, the existing 24

- hour PM 2.5 mass - based standard would be as protective or in some areas more protective of visibility than a distinct secondary standard set within the range of levels proposed. Commenters on the proposed PM 2.5 visibility index explored the implications of this analysis at lengt

h, as discussed further below in section VI.C.1.f. For this reason, the analysis is described in some detail here. Kelly et al. (2012a) noted that the relationship between design values for the 24

- hour PM 2.5 standard and the proposed secondary visibility i ndex standard is not obvious a priori because of differences in design value calculations for the standards. However, closer examination of this relationship indicated that increases or decreases in 24

- hour PM 2.5 design values correspond, respectively, to increases or decreases in visibility index values. Specifically, based on measurements from 102 sites with complete data from 2008

- 2010, Kelly et al. (2012a) found linear correlations between the 24

- hour PM 2.5 design values and the visibility index design values with r 2 values ranging from 0.65 to 0.98 across these sites, with an average r 2 value of 0.75 across all U.S. sites. Moreover, the data indicated that no design value existed where the visibility index design value exceeded 30 dv, but the 24

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Page 331 of 432 index design values for certain sites in the Industrial Midwest were shown to exceed 28 dv despite the fact that the 24

- hour PM 2.5 design values for these sites were below 35 μg/m 3 . This was attribute d to the combination of high nitrate and sulfate fractions, substantial RH adjustment factors, and PM 2.5 distribution characteristics that led to relatively high visibility index design values for a given 24

- hour PM 2.5 design value for counties in the Indu strial Midwest. 141 Kelly et al. (2012a) concluded that the “overall, design values based on 2008

- 2010 data suggest that counties that attain 24

- hour PM 2.5 NAAQS level of 35 μg/m 3 would attain the proposed secondary PM 2.5 visibility index NAAQS level of 30 dv and generally attain the level of 28 dv” (pp. 17

- 18). In addition, the Kelly et al. analysis indicated that at sites that violated both the 24

- hour PM 2.5 level and the proposed visibility index 30 dv level, the proposed level of 30 dv would likely be
atta ined if PM 2.5 concentrations were reduced such that the 24

- hour PM 2.5 level of 35 μg/m 3 was attained ( Kelly et al. , 2012a, p.15 )

. 142 A key implication of this analysis, therefore, was that within the range of levels proposed by the EPA for a visibility index standard (28

- 30 dv), the 24

- hour PM 2.5 standard of 35 μg/m 3 would be controlling in almost all (at 28 dv) or all (at 30 dv) instances. 2. Other (Non

- Visibility) PM

- related Welfare Effects In the 2006 review, the EPA concluded that there was insufficient i nformation to consider a distinct secondary standard based on PM

- related impacts to ecosystems,
141 Kelly et al. (2012a) also noted that “Regional reductions in sulfate PM 2.5 due to emission controls planned as part of national rules as well as emission reductions associated with potential annual standard violations are expected to improve visibility in this region” (p. 17). 142 The analysis also showed that attaining the 24

- hou r PM 2.5 standard level of 35 μg/m 3 would result in achieving a lower PM 2.5 visibility index level in certain areas of the country, largely western areas, than would be achieved in other areas of the country. This is due to differences in the composition of ambient PM 2.5 and the lower relative humidity in those areas.***E.O. 12866 Review

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Page 332 of 432 materials damage and soiling, and climatic and radiative processes (71 FR 61144, October 17, 2006). Specifically, there was a lack of evidence linking various n on visibility welfare effects to specific levels of ambient PM. In that review, to provide a level of protection for these welfare

- related effects, the secondary standards were set equal to the revised primary standards to directionally improve the level o f protection afforded vegetation, ecosystems, and materials (71 FR 61210, October 17, 2006). This section briefly outlines key conclusions discussed more fully in section VI.E of the proposal regarding the non

- visibility welfare effects of PM. These conclu sions relate to the climate, ecological (including effects on plants, soil and nutrient cycling, wildlife and water) and materials damage effects of PM. For all of these effects, the Policy Assessment concluded that there is insufficient information at thi s time to revise the current suite of secondary standards. It is important to note that the Policy Assessment explicitly excluded discussion of the effects associated with deposited particulate matter components of NO x and SO x and their transformation prod ucts which are addressed fully in the joint review of the secondary NO 2 and SO 2 NAAQS.

a. Evidence of Other Welfare Effects Related to PM With regard to the role of PM in climate, the proposal noted that there is considerable ongoing research focused on u nderstanding aerosol contributions to changes in global mean temperature and precipitation patterns. The Integrated Science Assessment concluded “that a causal relationship exists between PM and effects on climate, including both direct effects on radiativ e forcing and indirect effects that involve cloud feedbacks that influence precipitation formation and cloud lifetimes” (U.S. EPA, 2009a, section 9.3.10). These effects are discussed in more detail in section VI.E.1 of the***E.O. 12866 Review

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Page 333 of 432 proposal, which provides informat ion on the major aerosol components of interest for climate processes, including black carbon (BC), organic carbon (OC), sulfates, nitrates, and mineral dusts, and the nature, magnitude, and direction (e.g., cooling vs. warming) of various aerosol impacts on climate. 143 The Policy Assessment concluded that aerosols alter climate processes directly through radiative forcing and by indirect effects on cloud brightness, changes in precipitation, and possible changes in cloud lifetimes (U.S. EPA, 2011a, p. 5

- 10). Further, the Policy Assessment noted that the major aerosol components that contribute to climate processes (i.e. BC, OC, sulfate, nitrate and mineral dusts) vary in their reflectivity, forcing efficiencies and even in the direction of climate forcing, th ough there is an overall net climate cooling associated with aerosols in the global atmosphere (U.S. EPA, 2009a, section 9.2.10). The Policy Assessment concluded that the
current mass

- based PM 2.5 and PM 10 secondary standards were not an appropriate or effe ctive means of focusing protection against PM

- associated climate effects due to these differences in components (U.S. EPA, 2011a, p. 5

- 11). In addition, in light of the significant uncertainties in current scientific information and the lack of sufficient data, the Policy Assessment concluded it is not currently feasible to conduct a quantitative analysis for the purpose of informing revisions of the current secondary PM standards based on climate (U.S. EPA, 2011a, p. 5

- 11). Overall the Policy Assessment co ncluded that there is insufficient information at this time to base a national ambient standard on climate impacts associated with current ambient concentrations of PM or its constituents (U.S. EPA, 2011a, p. 5

- 11,

- 12). 144

143 Atmospheric PM is referred to as aerosols in the remainder of this section to be consistent with the Integrated Science Assessment. 144 This conclusion would apply for both the secondary (welfa re - based) and the primary (health

- based) standards.***E.O. 12866 Review

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Page 334 of 432 With regard to ecological effects, the proposal noted that several ecosystem components (e.g., plants, soils and nutrient cycling, wildlife and water) are impacted by PM air pollution, which may alter the services provided by affected ecosystems. Ecological effects include both dir ect effects due to deposition (e.g., wet, dry or occult) to vegetation surfaces and indirect effects occurring via deposition to ecosystem soils or surface waters where the deposited constituents of PM then interact with biological organisms. Some of the e cological effects considered in this review include direct effects to metabolic processes of plant foliage; contribution to total metal loading resulting in alteration of soil biogeochemistry and microbiology, and plant and animal growth and reproduction; and contribution to total organics loading resulting in bioaccumulation and biomagnification across trophic levels. Section VI.E.2 of the proposal summarizes key findings related to: (1) Impacts on plants and the ecosystem services they provide due to depositi on of PM to vegetative surfaces, which alters the radiation received by the plant, and uptake of deposited PM components by plants from soil or foliage, which can lead to stress and decreased photosynthesis; (2) Impacts on ecosystem support services such as nu trient cycling, products such as crops and the regulation of flooding and water quality; (3) Impacts on wildlife, especially due to biomagnification of heavy metals (especially Hg) up the food chain and bioconcentration of POPs and PBDEs; and (4) Impacts of deposi ted PM, especially metals and organics, on the ecosystem services provided by water bodies, including primary production, provision of fresh water, regulation of climate and floods, recreational fishing and water purification. The proposal noted that the I ntegrated Science Assessment had concluded that ecological evidence is sufficient to conclude that a causal relationship is likely to exist between deposition of PM and a variety of effects on individual organisms and ecosystems (U.S. EPA, 2009a, sections 2.5.3 and 9.4.7), and also noted that vegetation***E.O. 12866 Review

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Page 335 of 432 and other ecosystem components are affected more by particulate chemistry than size fraction. However, the proposal also pointed to the Integrated Science Assessment conclusion that it is generally difficult to characterize the nature and magnitude of effects and to quantify relationships between ambient concentrations of PM and ecosystem response due to significant data gaps and uncertainties as well as considerable variability that exists in the components of PM and their various ecological effects. There are few studies that link ambient PM concentrations to observed effect. Most direct ecosystem effects associated with particulate pollution occur in severely polluted areas near industrial point sources (qu arries, cement kilns, metal smelting) (U.S. EPA, 2009a, sections 9.4.3 and 9.4.5.7). Based on the evidence available at this time, the proposal noted the following key conclusions in the Policy Assessment: (1) A number of significant environmental effects that either have already occurred or are currently occurring are linked to deposition of chemical constituents found in ambient PM. (2) Ecosystem services can be adversely impacted by PM in the environment, including supporting, provisioning, regulating and cultur al services. (3) The lack of sufficient information to relate specific ambient concentrations of particulate metals and organics to a degree of impairment of a specific ecological endpoint hinders the identification of a range of appropriate indicators, levels , forms and averaging times of a distinct secondary standard to protect against associated effects. (4) Data from regionally

- based ecological studies can be used to establish probable local, regional and/or global sources of deposited PM components and their c oncurrent effects on ecological receptors. The proposal noted that the Policy Assessment had concluded that the currently available information is insufficient for purposes of assessing the adequacy of the protection for ecosystems afforded by the current suite of PM secondary standards or***E.O. 12866 Review

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Page 336 of 432 establishing a distinct national standard for ambient PM based on ecosystem effects of particulates not addressed in the NO x /SO x secondary review (e.g., metals, organics) (U.S. EPA, 2011a, p. 5

- 24). Furthermore, the Policy Assessment had concluded that in the absence of information providing a basis for specific standards in terms of particle composition, the observations continue to support retaining an appropriate degree of control on both fine and coarse particles to help address effects to ecosystems and ecosystem components associated with PM (U.S. EPA, 2011a, p. 5

- 24). With regard to materials damage, the proposal discussed ef fects associated with deposition of PM, including both physical damage (materials damage effects) and impaired aesthetic qualities (soiling effects). As with the other categories of welfare effects discussed above, the Integrated Science Assessment conclud ed that evidence is sufficient to support a causal relationship between PM and effects on materials (U.S. EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition of PM can physically affect materials, adding to the effects of natural weathering processes, by potentially promoting or accelerating the corrosion of metals, by degrading paints and by deteriorating building materials such as stone, concrete and marble (U.S. EPA, 2009a, section 9.5). In addition, the deposition of ambient PM can reduce the aestheti c appeal of buildings and objects through soiling. The Policy Assessment made the following observations: (1) Materials damage and soiling that occur through natural weathering processes are enhanced by exposure to atmospheric pollutants, most notably sulfur d ioxide and particulate sulfates. (2) While ambient particles play a role in the corrosion of metals and in the weathering of materials, no quantitative relationships between ambient particle concentrations and rates of damage have been established. (3) While soil ing associated with fine and course particles can result in increased cleaning frequency and repainting of surfaces, no quantitative relationships between particle characteristics and the frequency of cleaning or repainting have***E.O. 12866 Review

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Page 337 of 432 been established. (4) Limited n ew data on the role of microbial colonizers in biodeterioration processes and contributions of black crust to soiling are not sufficient for quantitative analysis. (5) While several studies in the PM Integrated Science Assessment and NO x /SO x Integrated Science Assessment suggest that particles can promote corrosion of metals there remains insufficient evidence to relate corrosive effects to specific particulate levels or to establish a quantitative relationship between ambient PM and metal de gradation. With respect to damage to calcareous stone, numerous studies suggest that wet or dry deposition of particles and dry deposition of gypsum particles can enhance natural weathering processes. The Policy Assessment concluded that none of the new ev idence in this review called into question the adequacy of the current standards for protecting against material damage effects, that such effects could play no quantitative role in determining whether revisions to the secondary PM NAAQS are appropriate at this time, and that observations continue to support retaining an appropriate degree of control on both fine and coarse particles to help address materials damage and soiling associated with PM (U.S. EPA, 2011a, p. 5

- 29). b. CASAC Advice In advising the EPA regarding the non

- visibility welfare effects, CASAC stated that it “ concurs with the Policy Assessment’s conclusions that while these effects are important, and should be the focus of future research efforts, there is not currently a strong technical basis to support revisions of the current standards to protect against these other welfare effects” (Samet, 2010c). More specifically, with regard to climate impacts, CASAC concluded that while there is insufficient information on which to base a national standard, the causal relationship is established and the risk of impacts is high, so further research on a regional basis is urgently needed (Samet, 2010c, p. 5). CASAC also noted that reducing certain aerosol components could lead to increased radiative f orcing***E.O. 12866 Review

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Page 338 of 432 and regional climate warming while having a beneficial effect on PM

- related visibility. As a consequence, CASAC noted that a secondary standard directed toward reducing PM

- related visibility impairment has the potential to be accompanied by regional warming if light scattering aerosols are preferentially targeted. With regard to ecological effects, CASAC concluded that the published literature is insufficient to support a national standard for PM effects on ecosystem services (Samet, 2010c, p.23) . CA SAC noted that the best

- established effects are related to particles containing nitrogen and sulfur, which are being considered in the EPA’s ongoing review of the secondary NAAQS for NO x /SO x . With regard to PM

- related effects on materials, CASAC concluded that the published literature, including literature published since the last review, is insufficient either to call into question the current level of the standard or to support any specific national standard for PM effects on materials (Samet, 2010c, p.23 ) . Nonetheless, with regard to both types of effects, CASAC noted the importance of maintaining an appropriate degree of control of both fine and coarse particles to address such effects, even in the current absence of sufficient information to develop a s tandard.

c. Summary of Proposed Decisions Regarding Other Welfare Effects Based on the above considerations and the advice of CASAC, at the time of proposal the Administrator provisionally concluded that it would not be appropriate to establish any distin ct secondary PM standards to address other non

- visibility PM

- related welfare effects, including ecological effects, effects on materials, and climate impacts. Nonetheless, the Administrator concurred with the conclusions of the Policy Assessment and CASAC advice that it is important to maintain an appropriate degree of control of***E.O. 12866 Review

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Page 339 of 432 both fine and coarse particles to address such effects. Noting that there is an absence of information that would support any different standards, the Administrator proposed
genera lly to retain the current suite of secondary PM standards 145 to address non

- visibility welfare effects. Specifically, the Administrator proposed to retain all aspects of the current secondary 24

- hour PM 2.5 and PM 10 standards. With regard to the secondary

ann ual PM 2.5 standard, the Administrator proposed to retain the level of the current standard and to revise the form of the standard by removing the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard

. C. Public Comments on Proposed Decisions regarding Secondary PM Standards The EPA received a large number of comments on its proposed decisions with regard to secondary PM standards, with the large majority of those comments focusing on the proposal to set a distinct standa rd to protect against visibility impairment, discussed below in section VI.C.1. Very few commenters addressed the proposal to retain the existing secondary standards for non

- visibility welfare effects, discussed below in section VI.C.2

. As di s cussed in section VI.D. below, the Administrator has decided to retain the current suite of secondary PM standards generally, while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with thi s change to the primary annual PM 2.5 standard

. The Administrator has also decided, contrary to what was proposed, not to establish a distinct secondary standard to address PM

- related visibility impairment. T his section discusses EPA’s responses to the

com ments EPA received on its proposal, and t he rationale behind the Administrator’s final decisions is discussed in section VI. D below

.

145 As summarized in section VI.A and Table 1 above, the current suite of secondary PM standards includes annual and 24

- hour PM 2.5 standards and a 24

- hour PM 10 standard.***E.O. 12866 Review

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Page 340 of 432 1. Comments on Proposed Secondary Standard for Visibility Protection

a. Overview of Comments Among those commenting on t he proposal to set a distinct secondary PM 2.5 visibility index standard, a large majority of commenters, including more than 25 state and local agencies; regional organizations such as NACAA, NESCAUM, and WESTAR; and industry commenters, such as ACC, API, BP, EPRI, NCBA, NEDA

- CAP, NMA, NSSGA, and UARG, opposed setting a distinct secondary standard for visibility at this time. Many commenters in this group expressed the view that such a standard was not needed, primarily on the basis that adequate protection was provided by the existing 24

- hour secondary PM 2.5 standard. Some of these commenters also expressed legal concerns with the nature of the proposed standard. Other commenters in this group supported a distinct secondary standard for visibility in concep t, but expressed the view that it was premature to set such a standard pending collection of additional visibility preference study data and the resolution of a number of key technical issues. Support for setting such a distinct secondary standard for visi bility at this time came from a second group of commenters, including the Department of the Interior (National Park Service), several states, the Mid

- Atlantic/Northeast Visibility Union (MANE

- VU), the National Tribal Air Association (NTAA), environmental o rganizations such as the Appalachian Mountain Club, National Parks Conservation Association, Earthjustice (AMC, et al.) and the League of Women Voters of Texas. These commenters argued that the existing secondary standards are not sufficiently protective o f visual air quality, and that a distinct secondary standard similar to the proposed visibility index standard is both necessary and appropriate to ensure adequate protection of visibility.***E.O. 12866 Review

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Page 341 of 432 Commenters in both groups expressed concerns about various aspects of the proposed distinct secondary standard, including the indicator, averaging time, level, and form. In addition, a large number of commenters, including commenters from both groups, expressed concern and/or confusion over the relationship between the R egional Haze Program and the proposed distinct secondary standard for visibility, raising issues such as analytical differences in methods between the programs, monitoring issues, and other implementation challenges. A discussion of the significant comment s outlined above, including EPA’s responses to the comments, is presented here, with more detailed discussion in the Response to Comments document. Comments relating to the specific elements of the proposed standard

– indicator, averaging time, form and le vel

– are discussed in sections VI.C.1.b

- e, respectively. Comments related to the need for a distinct secondary standard at this time are discussed in section VI.C.f. Legal issues raised by commenters opposed to setting a secondary standard based on the pro posed visibility index are discussed in section VI.C.g. Finally, comments related to the relationship between a distinct secondary standard and the Regional Haze Program are discussed in section VI.C.h. 146 While the EPA concludes in section VI.D below to ret ain the current suite of secondary PM 2.5 standards, the appropriateness of the protection that would be provided by the proposed PM 2.5 visibility index standard, and the relationship between this degree of protection and that provided by the current second ary 24

- hour secondary PM 2.5 standard, are key elements in the Administrator's decision, and are discussed below.

146 Comments pertaining to implement ation issues, which the Administrator may not consider in making decisions about setting national ambient air quality standards, are discussed in the Response to Comments document, as a re comments regarding monitoring issues related to the proposed distinc t visibility index standard.***E.O. 12866 Review

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Page 342 of 432 b. Indicator Numerous commenters, both those supporting a distinct secondary standard and those opposed to setting such a standard, expressed views on the suitability of utilizing a PM 2.5 calculated light extinction indicator for the standard as proposed. While these groups of commenters differed in terms of their views on the appropriateness of using calculated PM 2.5 light extinction as the bas is for the indicator rather than relying on direct measurements of PM 2.5 light extinction, commenters from both groups expressed concern over specific elements of the proposed method of calculating PM 2.5 light extinction. In particular, commenters expresse d differing views on which IMPROVE algorithm should be utilized; whether it is appropriate to exclude coarse particles from the indicator; and whether the proposed protocols for incorporating data on relative humidity and PM 2.5 species are appropriate. 147
i

. Comments on Calculated vs. Directly Measured Light Extinction The majority of commenters in both groups noted the uncertainties associated with relying on a calculated light extinction indicator and stated a preference for utilizing direct light extincti on measurements. However, recognizing the limitations on applying direct measurements at present, commenters supporting the proposal to set a distinct standard argued that relying on “calculated light extinction is a reasonable first approach” (DOI, p. 2). These commenters pointed to the advice of CASAC, which had acknowledged that it was not possible for the EPA to develop an FRM for direct measurement of light extinction within the time frame of this review and had concluded that relying on a calculated P M 2.5 light extinction indicator represented a reasonable
147 Some commenters expressed concern about the omission of other contributors to visibility impairment from the visibility index, as discussed in the Respons e to Comments document.***E.O. 12866 Review

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Page 343 of 432 approach that could be implemented sooner than a directly measured indicator. These commenters generally supported the proposal to adopt a calculated PM 2.5 light extinction indicator, at least as an interim approach. Commenters opposed to setting a distinct standard generally argued that it was inappropriate to rely on a calculated light extinction indicator rather than direct measurements. Some of these commenters argued that the proposed calculated light extinction indictor is ill suited for a bright line standard because the method uses average humidity and a reconstructed visibility measurement calculated from PM 2.5 speciation filter analysis, rather than measuring what is actually observed by indi viduals. A number of commenters advocated postponing setting a distinct standard until an approach based on direct light extinction measurements can be adopted. Many of these commenters stated that relying on direct light extinction measurements would enab le a standard to be based on a shorter averaging time, either 1

- hour or sub

- daily (4 to 6 hours), consistent with the more instantaneous nature of perceptions of visual air quality and the advice of CASAC in this review. The EPA generally agrees with comme nters that an indicator based on directly measured light extinction would provide the most direct link between PM in the ambient air and PM

- related light extinction. However, as noted at the time of proposal and in accordance with the advice of CASAC, the EPA has concluded that this is not an appropriate option in this review because a suitable specification of currently available equipment or performance

- based verification procedures could not be developed in the time frame of this review. Moreover, CASAC concluded that relying on a calculated PM 2.5 light extinction indicator based on PM 2.5 chemical speciation and relative humidity***E.O. 12866 Review

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Page 344 of 432 data represented a reasonable approach . The inputs that are necessary include measurements that are available through existing monitoring networks and approved protocols. Thus, the EPA remains confident that the available evidence demonstrates that a strong correspondence exists between calculated PM 2.5 light extinction and PM

- related visibility impairment. Furthermore, CASAC agre ed, noting that the proposed calculated PM 2.5 light extinction indicator based on the original IMPROVE algorithm “appears to be a reasonable approach for estimating hourly light extinction” (Samet, 2010d, p. 11) and “its reliance on procedures that have al ready been implemented in the CSN and routinely collected continuous PM 2.5 data suggest that it could be implemented much sooner than a directly measured indicator” (Samet, 2010d, p. iii). Thus it would not be appropriate to postpone setting a distinct sec ondary standard until an approach based on direct light extinction measurements could be adopted. ii. Comments on Specific Aspects of Calculated Light Extinction Indicator Some commenters, even those supporting the adoption of a calculated light extinct ion indicator, also expressed concern over specific aspects of the proposed indicator. First, a number of commenters expressed concern over the proposal to use the original IMPROVE algorithm as the basis for the calculated light extinction indicator. These commenters noted that the original IMPROVE algorithm has been shown to have consistent biases at both low and high levels of light extinction. In particular, these commenters expressed concern with the algorithm’s bias at higher levels of light extinction , which they pointed out were the conditions that might be encountered on hazier days in urban areas. Some commenters supported use of the revised IMPROVE algorithm. These***E.O. 12866 Review

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Page 345 of 432 commenters noted that the revised equation has been through a peer review which confirmed that it is based on the best science and corrects the biases inherent in the original algorithm. Commenters also noted that this revised algorithm has been widely incorporated into Regional Haze plans, and urged the EPA to use this same equation in the visibility index for the sake of consistency: “EPA approved this approach for regional haze and doe s not dispute its greater accuracy. Therefore, a national secondary ambient air quality standard based on criteria that accurately reflect the latest scientific knowledge logically should not revert to the original IMPROVE algorithm” (Oklahoma DEQ, p. 2). Other commenters noted that both the original and the revised IMPROVE algorithms were designed in support of the Regional Haze Program which is focused on largely rural Class I areas, and that neither algorithm is necessarily suitable for urban areas. Noti ng that the EPA has not thoroughly evaluated the applicability of either IMPROVE algorithm in urban areas, these commenters urged additional research to evaluate the suitability of either algorithm (or an alternative approach) in urban areas. Second, a num

ber of commenters argued that exclusion of coarse PM from the calculated light extinction indicator was inappropriate. These commenters noted that coarse particulate matter is an important contributor to visibility impairment in many areas, particularly in the western U.S., and that the levels of “acceptable” visual air quality derived from the visibility preference studies reflected total light extinction due to the full mix of particles (including coarse PM) in ambient air. A few commenters noted that due to the exclusion of coarse particles, a “deciview” calculated for purposes of the proposed PM 2.5 visibility index is inconsistent with the unit as conventionally defined under the Regional Haze Program. Other commenters, however, supported the proposal***E.O. 12866 Review

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Page 346 of 432 to exclude coarse PM from the calculated light extinction indicator, noting the important role that PM 2.5 plays in urban visibility and arguing it would be more difficult to control the contribution of coarse particle sources such as wind

- blown dust to urban visibility impairment. Third, some commenters questioned why the EPA was proposing to rely on monthly average relative humidity (f(RH)) values when hourly humidity data are widely available, particularly in urban areas. One commenter argued that the EPA’ s proposed approach involves “guessing relative humidity” rather than relying on accurate, readily available measurements (Oklahoma DEQ, p. 1). The commenter stated that since relative humidity is highly variable and weather dependent, the proposed approac h “effectively undermines the capacity of the prescribed monitoring regime to identify periods when PM 2.5 adversely affects visibility.” Other commenters supported this view, noting that relative humidity can vary substantially even within a 24

- hour period , and that light extinction can be very sensitive to these changes. These commenters recommended that hourly or daily humidity measurements should be utilized in place of the proposed monthly average f(RH) values. Some commenters also recommended that the EPA should utilize a 90 percent relative humidity screen rather than 95 percent cap for purposes of eliminating periods in which visibility impairment is due to rain or fog. These commenters claimed that under a 95 percent cap, both the average f(RH) valu es and the PM 2.5 visibility index values could be inflated in locations frequently affected by fog and/or precipitation. These commenters preferred the approach of excluding hours with relative humidity above 90 percent on the grounds that this approach wo uld eliminate foggy/rainy hours irrespective of the***E.O. 12866 Review

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Page 347 of 432 frequency of occurrence. The EPA does not agree with commenters who advocated using the revised IMPROVE algorithm. Both the original and the revised IMPROVE algorithms have been evaluated by comparing th e calculated estimates of light extinction with coincident optical measurements. As discussed above in section VI.B.1.a.i, t he revised algorithm was developed to address observed biases in the predictions using the original algorithm under very low and ver y high light extinction conditions , with further modifications and additions to better account for differences in particle composition and aging in remote

areas . 148 However, the EPA does not believe that these same modifications and additions would necessari ly be appropriate for calculating light extinction in urban areas. Instead, the EPA considers the original algorithm to be suitable for purposes of calculating urban light

- extinction, although some adjustments may be appropriate for urban environments

as w ell. The reasons why the original algorithm is suited to urban environments are discussed further below, along with adjustments that the EPA believes are likely appropriate based on the current (limited) state of knowledge. First, the EPA considers that th e multiplier of 1.8 used to convert OC to OM in the revised IMPROVE algorithm is too high for urban environments. The EPA is aware that there has been considerable debate within the research community about the appropriate multiplier to use to best represe nt urban environments. As discussed in Appendix F of the Policy Assessment (U.S. EPA, 2011a) , the EPA used the SANDW ICH

148 Specifically, the revised IMPROVE algorithm i ncorporates additional terms to account for particles representing the different dry extinction and water uptake (f(RH)) from two size modes of sulfate, nitrate and organic mass, as well as adding a term for hygroscopic sea salt. There are also adjustments for the calculation of OM as 1.8*OC compared to 1.4*OC in the original algorithm to better account for the more aged PM organic components found in remote areas.***E.O. 12866 Review

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Page 348 of 432 mass closure approach in the Urban Focused Visibility Assessment (U.S. EPA, 2010b) for purposes of calculating maximum daylight hourly PM 2.5 light extinction and evaluated which multiplier would produce 24

- hour results most similar to the SANDWICH approach using 24

- hour PM 2.5 organic carbon derived from the new Chemical Speciation Netwo rk (CSN) carbon monitoring protocol established in 2007

. 149 Analyses presented in Appendix F of the Policy Assessment indicate that a multiplier of 1.6 is most appropriate for purposes of comparing the hourly PM 2.5 light extinction with calculated 24

- hour ex tinction (see Appendix F, section F.6 for a full explanation). The EPA also considers this higher multiplier to be a better approach for urban CSN monitoring sites where the new measurements of organic carbon tend to be lower than those produced by the old er NIOSH

- type monitoring protocol (Malm, 2011). A multiplier of 1.6 is now used to calculate OM from OC measurements at CSN sites. At the time of proposal, the EPA proposed to use the original IMPROVE algorithm with its 1.4 multiplier for converting OC to OM, but requested comment on whether this value was appropriate

. C omments received by the Agency generally indicate that t he OC

- to

- OM multiplier of 1.4 used in the original IMPROVE algorithm is too low for urban areas. Based on the analyses presented in Appendix F of the Policy Assessment, the EPA agrees with these commenters. However, the EPA also believes that it would be inappropriate to use a multiplier as high as 1.8 to convert OC to OM in urban areas. As noted by commenters, the organic mass contribution to visibility impairment can be large, and generally OM is significantly larger in urban areas compared to surrounding rural
149 Starting in 2007, the CSN adopted the IMPROVE monitoring protocol for the measurement of or ganic and elemental carbon using the IMPROVE analytical method and an IMPROVE

- like sampler. The transition was completed in 2009. ( “ Modification of Carbon Procedures in the Speciation Network,” http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbo n.pdf)***E.O. 12866 Review

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Page 349 of 432 areas. 150 Because a large portion of the organic component of urban PM results from nearby emissions sources, the total OM mass is generally closer to the measured OC from which it is derived. This means it is appropriate to use a smaller multiplier to convert OC to OM in urban areas as compared to the value of 1.8 used in the revised algorithm, which is tailored to remote areas. The CASAC noted that urban OM includes fresh emissions and the EPA concluded in the Visibility Assessment that “the original version is considered more representative of urban situations when emissions are still fresh ra ther than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3

- 19). Although the revised algorithm represents the best science of estimating extinction in remote areas with its aged aerosol, the commenters did not address how the EPA should modify the r evised algorithm to best represent the more complex and different urban aerosol, particularly for OM. In light of all of these considerations, in particular the analyses the EPA conducted for Appendix F of the Policy Assessment and the fact that the monito ring method for organic carbon has recently changed in the CSN network, the EPA judges that a multiplier of 1.6 for urban areas would be most appropriate for purposes of calculating PM 2.5 light extinction in urban areas. 151 In formulating this judgment, the EPA recognizes that neither the original nor the revised IMPROVE algorithm has been tested for suitability in urban areas and that additional research is necessary to reduce the uncertainties about the most appropriate va lue for th e OC to OM multiplier in urban environments

. With regard to other changes between the original and revised IMPROVE

150 The difference between higher PM 2.5 mass in urban areas compared to surrounding regions, known as the urban excess, is largely attributed to organic mass (U.S. EPA Particle Pollution Report, 2004). 151 The implications of this shift to a 1.6 multipl ier for OC in urban areas for decisions about averaging time, level, and need for a distinct secondary standard are discussed further below in sections VI.C.1.c, VI.C.1.e, and VI.C.1.f, respectively.***E.O. 12866 Review

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Page 350 of 432 algorithms, the EPA also does not believe that it would be appropriate to include a term for hygroscopic sea salt for urban light e xtinction, or to differentiate between different size modes of sulfate, nitrate, and organic mass as empirically defined by the revised IMPROVE algorithm. Unlike in some remote coastal locations, sea salt is not major contribut or to light extinction in urb an areas . Mor e over, urban sources of salt include sanding of roads during the winter and those re - entrained particles are mostly in the coarse size range

. Like in remote areas, small and large size modes of sulfate, nitrate and organic mass would exist in the urban environment. However, the apportionment of the total fine particle concentration of each of the three P M 2.5 components into the concentrations of the small and large size fractions would likely need a different approach than that used for remote areas. This is because of the closer proximity of urban sources to their emissions

. This is a particular concern not only for organic mass, which as explained previously has a large contribution from nearby urban emission sources, but also for P M 2.5 nitra te whose concentrations are also higher in urban areas compared to the surrounding regions. Thus, a higher portion of the total urban concentration may be in the small mode compared to remote areas and thus a different apportionment algorithm would be need ed. Finally, the EPA does not consider it necessary to employ site

- specific Rayleigh light scattering terms in place of a universal Rayleigh light scattering value for purposes of calculating light extinction in urban areas for purposes of calculating the 90 th percentile values. The site

- specific Rayleigh value is most important to accurately***E.O. 12866 Review

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Page 351 of 432 estimate extinction on the best visibility days which is an essential metric for the regional haze program. For all of these reasons, the EPA considers the original IM PROVE algorithm better suited to the task of calculating urban light extinction than the revised IMPROVE algorithm. However, the EPA does consider it appropriate to make certain adjustments to th e original algorithm for purposes of calculating urban light extinction. As discussed above, the EPA believes it is appropriate to use a 1.6 multiplier to convert OC to OM in urban areas . In addition, the EPA believes it is appropriate to exclude the term for coarse particles from the equation. The EPA does not agre e with commenters who suggested that coarse particles should be included in the calculated light extinction indicator. As noted in the proposal, PM 2.5 is the component of PM responsible for most of the visibility impairment in most urban areas. Currently available data suggest that PM 10

– 2.5 is a minor contributor to visibility impairment most of the time, although at some locations (U.S. EPA, 2010b, Fig ure 3

- 13 for Phoenix) PM 10

– 2.5 can be a major contributor to urban visibility effects. While it is reasonable to assume that other urban areas in the desert southwestern region of the country may have conditions similar to the conditions shown for Phoenix, in fact few urban areas conduct continuous PM 10

- 2.5 monitoring. This significantly increases the difficulty of assessing the role of coarse particles in urban visibility impairment. For example, among the 15 urban areas assessed in this review, only four areas had collocated continuous PM 10 data allowing calculation of hourly PM 10

-
2.5 data for 2005 to 2007. In addition, PM 10

- 2.5 is generally less homogenous in urban areas than PM 2.5 in that coarse particle concentrations exhibit greater temporal variabilit y and a steeper gradient across urban areas than fine particles (U.S. EPA, 2009a, p. 3

- 72).***E.O. 12866 Review

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Page 352 of 432 This makes it more challenging to select sites that would adequately represent urban visibility conditions. Thus, while it would be possible to include a PM 10

– 2.5 l ight extinction term in a calculated light extinction indicator, as was done in the Visibility Assessment, there is insufficient information available at this time to assess the impact and effectiveness of such a refinement in providing public welfare prot ection in areas across the country (U.S. EPA, 2011a, pp. 4

- 41 to 4

- 42). Therefore, the EPA concludes that it is not appropriate to set a standard based on a calculated light extinction indicator that includes coarse particles at this time, and the calculat ed indicator should be based on PM 2.5 light extinction. With regard to the suggestion by some commenters that the calculated light extinction indicator should be calculated using hourly humidity data, the EPA disagrees that concurrent humidity measurements should be used. The use of longer

- term averages for each monitoring site adequately captures the seasonal variability of relative humidity and its effects of visibility impairment, and this approach focuses more on the underlying aerosol contributions to visibility impairment and less on the day

- to

- day variations in humidity. This provides a more stable indicator for comparison to the NAAQS and one that is more directly related to the underlying emissions that contribute to visibility impairment. With reg ard to the comments advocating the use of a 90 percent humidity screen as opposed to a 95 percent humidity cap, the EPA believes that relying on monthly average relative humidity values based on 10 years of climatological data appropriately reduces the eff

ect of fog and precipitation. Although the approach of using a 95 percent humidity cap, as in the Regional Haze Program, includes some hours with relative***E.O. 12866 Review

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Page 353 of 432 humidity between 90

- 95 percent, the general approach of using a longer

- term average for each monitori ng site effectively eliminates the effect of very high humidity conditions on visibility at those locations. Therefore, taking all of the above considerations and CASAC advice into account, the EPA continues to conclude that a calculated PM 2.5 light extinc tion indicator, similar to that used in the Regional Haze Program (i.e., using an IMPROVE algorithm as translated into the deciview scale), would be the most appropriate indicator to replace the current PM 2.5 mass indicator for a distinct secondary standar d. Moreover, the EPA continues to conclude that this calculated indicator should based on the original IMPROVE algorithm , adjusted to use a 1.6 OC multiplier and exclude the term for coarse particles, in conjunction with monthly average relative humidity d ata (i.e., f(RH) values) based on long

- term climatological means as used in the Regional Haze Program. A PM 2.5 visibility index defined in this way would appropriately reflect the relationship between ambient PM and PM

- related light extinction, based on th e analyses discussed in the proposal and reflecting the aerosol and relative humidity contributions to visibility impairment by incorporation of factors based on measured PM 2.5 speciation concentrations and climatological average relative humidity data. In addition, this type of indicator would address, in part, the issues raised in the court’s remand of the 2006 PM 2.5 standards. Such a PM 2.5 visibility index would afford a relatively high degree of uniformity of visual air quality protection in areas acros s the country by virtue of directly incorporating the effects of differences in PM 2.5 composition and relative humidity across the country.

c. Averaging time***E.O. 12866 Review

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Page 354 of 43 2 Few commenters specifically addressed the issue of averaging time. Those who did generally expre ssed the view that an hourly or sub

- daily averaging time would be the most appropriate approach, as supported by CASAC and the EPA’s own analyses in this review. These comments were generally consistent with the emphasis among all commenters on the desirab ility of adopting a directly measured light extinction indicator that could be measured on an hourly or sub

- daily time scale. Some commenters noted that a standard based on a 4

- 6 hour averaging time would better capture peak daily light extinction while al lowing stable signal quality; others urged EPA to adopt a 1

- hour averaging time in conjunction with direct measurements. Commenters pointed to significant limitations associated with using a 24

- hour averaging time, including the uncertainties in translatin g hourly or sub

- daily visibility index values into 24

- hour equivalent values. Some commenters criticized the analysis presented in the Policy Assessment comparing the 24

- hour calculated light extinction values to the maximum daylight 4

- hour calculated ligh t extinction values. These commenters stated that the scatter plots and regressions presented in the Policy Assessment indicate there is considerable variation in the 24

- hour vs. 4

- hour relationship, and interpreted this to mean that 24

- hour light extincti on values are a poor surrogate for 4

- hour values. For example, several industry commenters cited an analysis which noted that the correlation coefficient between the 24

- hour and 4

- hour values was as low as r 2 = 0.42 in Houston, and stated that the EPA was being overly “optimistic” in concluding that city

- specific and pooled r 2 values in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2, p. 27). In addition, some commenters expressed concern over potential bias and greater uncertainty intro duced by the inclusion of nighttime hours, noting that because relative***E.O. 12866 Review

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Page 355 of 432 humidity tends to be higher at night, inclusion of these hours could cause areas to “record NAAQS exceedances that have no corresponding visibility impairment value” (UARG, p. 36). Com menters also emphasized the poor fit of a 24

- hour averaging time with the near instantaneous judgments about visibility impairment reflected in the visibility preference studies. Commenters also noted that there is greater hourly variation in PM
concentrat ions and resulting visibility conditions in urban areas than in Class I areas; thus, while the Regional Haze Program uses 24

- hour IMPROVE data, the commenters stated that a shorter averaging time is needed for an urban

- focused PM 2.5 visibility standard. So me commenters objected to a 24

- hour averaging time as unsupported by the record in this review: “Because the science the Administrator relies on for the other elements of the proposed visibility standard is tied to short

- term exposures to visibility impair ment, the EPA has no basis for promulgating a standard that uses a 24

- hour averaging time” (API, p. 43). These commenters claimed that while the EPA may not have the information or infrastructure in place to allow the Agency to set a standard based on a 1

- hour or other sub

- daily averaging time, this does not justify moving to a 24

- hour averaging time. Among commenters supporting the proposed distinct secondary standard for visibility, many commenters recognized the limitations on monitoring methods and

cur rently available data that led to the EPA’s proposal to adopt a standard based on a 24

- hour averaging time. Most of these commenters acknowledged that the lack of reliable hourly speciation data means that a 24

- hour averaging time is the only workable
appr oach for a standard based on calculated light extinction. Commenters advocating a distinct secondary standard for visibility therefore generally supported the proposal to***E.O. 12866 Review

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Page 356 of 432 adopt a 24

- hour averaging time, at least as an interim approach until a directly meas ured light extinction indicator could be adopted in the future. This approach was also supported by a few industry commenters who noted that since a visibility index standard would be based on data from the IMPROVE and CSN monitors, which operate on a 24

- h our basis with 1

- in

- 3 (or 1

- in

- 6) day sampling, “it is imperative that EPA retain a 24

- hour averaging time if a secondary visibility standard is promulgated” (API, Attachment 2, p. 9). In response to comments supporting a 1

- hour or sub

- daily (4

- to 6

- hour ) averaging time in conjunction with a direct light extinction measurements, the EPA notes that, as discussed above in the response to comments on indicator, the Agency has concluded that a directly measured light extinction indicator is not an appropriate option in this review, independent of the decision on averaging time. Having reached the conclusion that a calculated PM 2.5 light extinction indicator would be most appropriate, the EPA has next considered what averaging time would be most desirable for s uch an indicator. As noted in the proposal, the EPA has recognized that hourly or sub

- daily (4

- to 6

- hour) averaging times, within daylight hours and excluding hours with high relative humidity, are more directly related than a 24

- hour averaging time to th e short

- term nature of the perception of PM

- related visibility impairment and the relevant exposure periods for segments of the viewing public. Thus, the Agency agrees with commenters’ general point that, as a starting premise, a sub

- daily averaging time w ould generally be preferable. However, as noted at the time of proposal and discussed above in section VI.B.1.c, important data quality uncertainties have recently been identified in association***E.O. 12866 Review

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Page 357 of 432 with currently available instruments that would be used to p rovide the hourly PM 2.5 mass measurements that would be needed in conjunction with an averaging time shorter than 24 hours. As a result, at this time the Agency has strong technical reservations about a secondary standard that would be defined in terms of a sub

- daily averaging time. The data quality issues which have been identified, including short

- term variability in hourly data from currently available continuous monitoring methods, effectively preclude adoption of a 1

- hour averaging time in this review, given the sensitivity of a 1

- hour averaging time to these data quality limitations. Even with regard to multi

- hour averaging times, the EPA continues to conclude that the data quality concerns preclude adoption of a sub

- daily averaging time. Moreover, ana lyses conducted for the Policy Assessment indicate that PM 2.5 light extinction calculated on a 24

- hour average basis would be a reasonable and appropriate surrogate for PM 2.5 light extinction calculated on a 4

- hour basis. The scatter plots comparing 24

- hou r and 4

- hour calculated PM 2.5 light extinction in the Policy Assessment (U.S. EPA, 2011a, Figures G

- 4 and G

- 5) do show some scatter around the regression line for each city. This was to be expected, since the calculated 4

- hour light extinction includes day

- specific and hour

- specific influences that are not captured by the simpler 24

- hour approach. Overall, however, in the EPA’s view, both the city

- specific and pooled 15

- city 24

- hour vs. 4

- hour comparisons show strong correlation between the two averaging ti mes. Moreover, the 90th percentile design values calculated for 4

- hour vs. 24

- hour light extinction are much more closely correlated than are the values for individual days in particular urban areas calculated using these two approaches. Thus, while the EP A agrees with commenters who pointed out the relatively low correlation***E.O. 12866 Review

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Page 358 of 432 between 4

- and 24

- hour values in cities such as Houston, the Agency points out that the correlations of 90 th percentile values are much higher, particularly when one considers the aver age values across urban areas. In general, the 90th percentile values line up better and demonstrate closer to a one

- to

- one relationship. The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships between estimated 24

- hour and 4

- hour vi sibility impairment based on the variety of metrics discussed in Appendix G of the Policy Assessment that further supports this finding. The reanalysis more appropriately considered the uncertainty of the calculated 4

- hour values. It also considered the ef fect of changing the OC to OM multiplier used in urban areas with the new CSN monitoring protocol from 1.4 to 1.6. The revised analysis shows that the 24

- hour values are generally closer to the 4

- hour values than originally estimated. Since conclusions in the proposal about the relationship between 4

- hour and 24

- hour values were drawn not just on the basis of the city

- specific results but also on the more robust 90 th percentile values, the EPA disagrees with commenters who state that th e Agency was overly optimistic in considering 24

- hour values an appropriate surrogate for 4

- hour values. Also, it is appropriate to focus on the 90 th percentile design value comparison since the design values would determine attainment status and the degre e of improvement in air quality that could be expected in areas instituting controls to meet the NAAQS. Therefore the EPA does not agree with commenters who state that a 24

- hour averaging time cannot serve as an appropriate surrogate for sub

- daily periods of visibility impairment. On the contrary, the EPA continues to conclude, on the basis of this analysis, that PM 2.5 light extinction calculated on a 24

- hour basis is a reasonable and appropriate***E.O. 12866 Review

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Page 359 of 432 surrogate for sub

- daily PM 2.5 light extinction calculated on a 4

- hour basis. The EPA recognizes that the effect of adopting a 24

- hour averaging time may be to smooth out some of the hour

- by

- hour variability in visibility index values. (Indeed, this is true if we compare a 4

- hour averaging time to a 1

- hour averaging time as well.) Hour

specific influences which would be evident if an hourly or sub

- daily averaging time were to be used will be masked to some extent when those hours are averaged together with other hours. This means, in part, that a 24

- hour averaging tim e may effectively reduce peak values by means of averaging them together with other hours, which may have lower values. However, given the well documented variability in hourly visibility conditions, especially in urban areas, as noted by commenters, it is reasonable to assume that in some cases peak hours may be significantly influenced by atypical conditions, making it appropriate to adopt an averaging time that is sufficiently long to ensure that hour

- specific influences are balanced against more typical conditions. Perhaps even more important is the concern that many peak hourly measurements may be significantly influenced by atypical instrument performance; this reinforces the conclusion that it is appropriate to adopt a longer averaging time, to ensure that hour

- specific uncertainties are balanced against more robust measurements. Thus, in agreement with commenters who supported a daily averaging time, the EPA concludes that a 24

- hour averaging time would be appropriate for a distinct secondary standard based on a calculated PM 2.5 light extinction indicator. d. Form The EPA received very few comments with regard to the proposal to adopt a 90 th percentile form, averaged over 3

- years, in conjunction with a PM 2.5 visibility index and a***E.O. 12866 Review

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Page 360 of 432 24

- hour averaging t ime. One commenter stated that it was inappropriate to use a 90 th percentile form, noting that this would result in the exclusion of a minimum of 36 days of data annually. The commenter expressed particular concern that this proposed approach, in combinati on with a 24

- hour standard based on an unadjusted CPL, would not capture the worst visibility impairment and that this would undermine “the intent of setting a meaningful secondary visibility standard” (AMC, et al., p. 2). Another commenter argued that the EPA had provided no scientific basis for why the 90 th percentile form was suitable, and claimed that the Agency was making “a somewhat arbitrary judgment that people’s welfare would be affected only if adverse urban visibility were to occur more than 10 p ercent of the time” (API, Attachment 2, p. 4). On other hand, a few commenters who appeared to generally support the proposal to use a 90 th percentile form advocated averaging the 90 th percentile values over longer time periods, arguing that averaging over only 3 years would not provide a stable assessment of visual air quality in the West because this time period is insufficient to properly account for western drought and fire cycles. These commenters pointed to the approach in the Regional Haze Program of averaging visibility impairment over 5 years, and noted that even within this longer time period data can be significantly influenced by high emissions during significant fire years. The EPA disagrees with all of these comments. With regard to the comment opposing the 90 th percentile form as inappropriately excluding the worst visibility days, the EPA notes that there is a significant lack of information on, and a high degree of
uncert ainty regarding, the impact on public welfare of the number of days with visibility impairment over the course of a year. For example, the visibility preference studies used***E.O. 12866 Review

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Page 361 of 432 to derive the range of CPLs considered in this review offered no information regar ding the frequency of time that visibility levels should be below those values. Based on this limitation, the EPA concluded in the Policy Assessment that it would not be appropriate to consider eliminating all exposures above the level of the standard and that it was reasonable to consider allowing some number of days with reduced visibility. Recognizing that the Regional Haze Program focuses attention on the 20 percent worst visibility days (i.e., those at or above the 80 th percentile of visibility impairm ent), the EPA continues to believe, as noted in the proposal, that a percentile well above the 80 th percentile would be appropriate to increase the likelihood that all days in this range would be improved by control strategies intended to help areas attain the standard. Focusing on the 90 th percentile, which represents the median of the distribution of the 20 percent worst visibility days, could be reasonably expected to lead to improvements in visual air quality on the 20 percent most impaired days. Thus, the EPA has made a reasoned judgment based on a full consideration of the upper end of the distribution of visibility impairment conditions and continues to conclude that it is appropriate to focus on the 90 th percentile of visibility impairment values. Wi th regard to comments requesting the EPA adopt a longer multi

- year averaging period for the 90 th percentile values, the EPA disagrees that it would be appropriate to average the 90 th percentile values over periods longer than 3 years. The EPA recognizes th at a multi

- year percentile form offers greater stability to the air quality management process by reducing the possibility that statistically unusual indicator values will lead to transient violations of the standard. Utilizing a 3

- year average form provid es stability from the occasional effects of inter

- annual meteorological variability that can result in***E.O. 12866 Review

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Page 362 of 432 unusually high pollution levels for a particular year. The Agency h as adopted this approach in other NAAQS, including the current secondary 24

- hour PM 2.5 NAAQS, which has a 98 th percentile form averaged over 3 years

. However, adopting a multi

- year averaging period longer than 3 years would increase the number of days with visibility impairment above the target level of protection and would therefore reduce the protectiveness of the standard. Based on this the EPA does not believe it would be appropriate to average 90 th percentile values over a period as long as five years. Therefore, the EPA continues to conclude that a 90 th percentile form, averaged over 3 years, would be appropriate, in conjunction with a calculated PM 2.5 light extinction indicator and a 24

- hour averaging time.

e. Level With regard to level, commenters focused on two main themes. First, a large number of commenters addressed the informat ion available from the public preference studies with regard to the acceptability of various levels of visual air quality. These comments, which are discussed in subsection VI.C.1.e.i below, address the EPA’s use of visibility preference studies as the bas is for the selection of a range of appropriate levels for the Administrator to consider. Many commenters challenged the use of these studies as the basis for setting a distinct secondary standard, arguing that limitations in these studies rendered them an unsuitable and insufficient basis on which to establish such a standard. Second, commenters expressed different views as to what level(s) of a distinct secondary standard would be appropriate, if the EPA were to set such a standard. These comments reflecte d consideration of the results of the public preference studies as well as analyses conducted in the Visibility Assessment and the Policy Assessment, as discussed***E.O. 12866 Review

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Page 363 of 432 in the proposal. Comments addressing the appropriateness of specific levels are discussed in subsection VI.C.1.e.ii below. i. Comments on Visibility Preference Studies A majority of commenters expressed the view that the existing preference studies provide an insufficient basis for selection by the Administrator of an appropriate level of public we lfare visibility protection for a national standard. These commenters highlighted a number of limitations and uncertainties (enumerated below) associated with these studies as support for this view. In contrast, other commenters felt that despite certain l imitations, these studies do provide a sufficient basis on which the Administrator can select an appropriate level of a standard to provide national public welfare visibility protection. The remainder of this section organizes and discusses these comments under four broad topic areas, including: (a) limitations and uncertainties associated with the visibility preference studies; (b) preference study methods and design; (c) use of preference study results for determining adversity; (d) the appropriateness of using regionally varying preference study results to select a single level for a national standard. (a) Preference Study Limitations and Uncertainties A large and diverse number of limitations and uncertainties associated with the visibility preference studi es have been identified and discussed in the public comments. Many of these same limitations and uncertainties were also identified and discussed by the EPA in the various documents developed throughout this review. The most important and fundamental limit ations and uncertainties will be discussed here in the preamble, while more specific, unique or detailed comments will be addressed in the Response to Comments document. The primary or most frequent limitation cited by many commenters relate s to the***E.O. 12866 Review

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Page 364 of 432 small number of preference studies that are available in this review. In particular, some commenters note that these preference studies cover just four locations, only three of which occur in the U.S., that the two studies conducted in Washington, DC were pilot studies, not full preference studies, and/or that three of the preference studies were conducted in the West, while only one was conducted in the East, providing only limited geographic coverage. Typically, these same commenters also pointed out that take n together, these limited studies only included a total of 852 participants, which they claimed was too small a sample size and unrepresentative nationally. These commenters thus concluded that there is insufficient information, both geographically and dem ographically, upon which to select a national level of a visibility index for purposes of visibility protection. In contrast, several commenters stated support for using the preference studies, concluding they provide an adequate basis, in spite of their limited nature. In particular, AMC et al. state: We believe that these studies provide sufficient results to inform setting a national visibility standard. While the number of studies is small, they do incorporate spatial variation and, in the case of Denv er and Phoenix, varied populations.... EPA should have confidence, rather than uncertainty, in the fact that these studies used different methods and respondents and yield a range of 20

– 24 dv, with one outlier of 29. (AMC, et al., pp. 6

- 7) Regarding the f

irst group of commenters , the EPA notes that it is well aware of the limited nature of the information, which it has described in great detail in the Integrated Science Assessment, Visibility Assessment, and Policy Assessment, as well as in section VI.B.2 of the proposed rule (77 FR 3897 3 ). The EPA further notes, however, that limited information does not preclude the Administrator from making judgments based on the best available science, taking into account the existing uncertainties and***E.O. 12866 Review

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Page 365 of 432 limitations assoc iated with that available science. Thus, in reaching judgments based on the science, the Administrator appropriately weighs the associated uncertainties. The CASAC supported this view and concluded that the available information provided a sufficient basis on which the Administrator could form a judgment about requisite PM

related public welfare visibility protection. Specifically, CASAC stated “[t]he 20

- 30 deciview range of levels chosen by EPA staff as ‘Candidate Protection Levels’ is adequately supported by the evidence presented ” (Samet, 2 010b, p. iii). As discussed in the proposed rule (77 FR 38990), the Administrator recognized and explicitly took into account the uncertainties and limitations in the science in determining an appropriate degree of protection when she proposed a level at the upper end o f the recommended
range. As discussed below, the Administrator continue s to be mindful of these uncertainties and limitations in reaching her final determination regarding what constitutes an appropriate degree of protection with respect to visibility imp airment

. With respect to the comments of AMC et al., the EPA agrees that these studies provide a sufficient basis to inform the Administrator’s judgments regarding an appropriate level of protection from PM

- related visibility impairment , but she recogniz es that these studies, which are the only studies before her, are a limited source of information . However, the EPA does not agree that the Washington, DC , results represent an outlier and thus the EPA believes that these results are appropriately included in the range identified for the Administrator to consider. Some commen ters made the point that the EPA relied on much of this same evidence to reach the conclusion in 2006 that the information was too limited to allow selection of a national standard. For example, API stated:***E.O. 12866 Review

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Page 366 of 432 [T]he bulk of the VAQ preference studies were av ailable during the previous PM NAAQS review and were considered by the Agency in its establishment of the 2006 PM secondary NAAQS.... The Proposed Rule does not mention this fact and does not explain why many of these same studies now compel EPA to propose t his new secondary NAAQS....The Proposed Rule notes in passing that, since the last review of the PM NAAQS, ‘limited information that has become available regarding the characterization of public preferences in urban areas has provided some new perspectives o n the usefulness of this information in informing the selection of target levels of urban visibility protection.’ 77 Fed. Reg. at 38969/2. It is a serious oversight that the Proposed Rule makes no attempt to explain what that information is or how it affec ts the interpretation of the VAQ preference studies. This ‘limited information’ is an apparent reference to information provided by Dr. Anne Smith. (API, p. 37) The EPA disagrees with these commenters. First, the EPA disagrees that it failed to distinguis h between studies that were available in the previous review and the current review. The discussion in section VI .A.1 of the proposal specifically identifies the studies from Denver, Phoenix and British Columbia (77 FR 38967/2) as being considered in the l ast review. The EPA further disagrees with the implication that it is being circumspect about identifying the “limited information that has become available regarding the characterization of public preferences in urban areas.” Beginning in section VI .A.3 of the proposed rule (77 FR 38969), the EPA was clear about what information, both preexisting and new, it relied upon in this review to inform its views and provide the basis for its proposal. In section VI.B.2, the EPA elaborates on the specific informa tion, tools, methods and data which are considered in relation to the public preference studies, including the new information available since the last review. As noted above and in the proposal, in addition to the substantial PM urban air quality informa tion and analyses new to this review, there are three other sources of information that have specifically “provided some new perspectives on the usefulness of” the preference studies “in informing the selection of target levels of urban visibili ty***E.O. 12866 Review

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Page 367 of 432 protection” (77 FR 38969). They include: (1) results from additional urban visibility preference study experiments conducted for Washington, DC by Smith and Howell (2009) which added to the preference data for that location and shed light on the role of location in preference responses 152 ; (2) a review and reanalysis of the urban visibility public preference studies from the four urban areas, including the newly available Smith and Howell (2009) experiments which examined the similarities and differences b etween the studies and evaluated the potential significance of those differences on the study results 153 ; and (3) additional analyses , including most importantly a logit analysis (discussed in Chapter 2 and Appendix J of the Visibility Assessment) , which was requested and reviewed by CASAC, which showed that each city’s responses represented unique and statistically different curves. 154 Taken together, these sources contributed to the EPA’s current knowledge and understanding of each survey study ’s results, the appropriateness of comparing each study’s results to the others, and the key uncertainties relevant to data interpretation. In addition, in the last review the decision to not adopt a distinct secondary standard was remanded as contrary to law and failing to provide a reasoned explanation for the decision. As such it is not appropriate for purposes of comparison with the Administrator's judgment and reasoning in this review. (b) Preference Study Methods and Design In addition to the limitations and uncertainties noted above, many comments also

152 Smith, AE; Howell S (2009). An assessment of the robustness of visual air quality preference study results. CRA International. Washington, DC. http://yosemite.epa.gov/sab/sabproduct.nsf/B55911DF9796E5E385257592006FB737/$F ile/CRA+VAQ+Pref+Robustness+Study+3+30+09+final.pdf. 153 Stratus Consulting, 2009

. Rev iew of Urban Visibility Public Preference Studies: Final Report

. EPA

- HQ

- OAR

- 2007

- 0492

- 0042. 154 Deck L., Lawson M. 2010

. Statistical Analysis of Existing Urban Visibility Preference Studies

. EPA

- HQ

- OAR

- 2007

- 0492

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Page 368 of 432 asserted the methodologies used in the preference studies are fundamentally flawed. Many commenters cited some of the same issues that have already been identified by the EPA as sources of uncer tainty and potential factors in producing the statistically different study results (see section VI.B.1.b above). As noted above, the EPA is well aware of the issues raised regarding the adequacy of the preference studies to serve as a basis for a secondar y NAAQS (see 77 FR 38975) and solicited comment on how these uncertainties should be considered (see 77 FR 38990). Most of these same commenters also pointed to an assessment of the preference studies methodology provided by Smith and Howell (2009) as the basis for their views, as indicated by the following comments: Smith and Howell (2009) show that VAQ preference study outcomes are malleable and depend entirely on the design of the study. Accordingly, such studies do not identify any meaningful threshold of acceptable visibility conditions. Despite Smith and Howell’s conclusions, EPA continues to assert that the VAQ preference studies can be used to identify minimally acceptable visibility conditions even though the Agency has never provided any valid scie ntific basis for discounting the Smith and Howell (2009) results. (API, p. 38) Well

- controlled preference studies discussed by Anne Smith of Charles River Associates at the March 2010 CASAC meeting demonstrated that the judgment of panel members was affec ted by the order in which photographs were presented and tendency to identify the middle of the range of visibility degredation as a threshold of acceptability. This points to a potential flaw in these studies and that artifacts caused by these tendencies may have influenced study results. Dismissing these inherent flaws in the existing preference studies and then using these studies to set a secondary NAAQS is arbitrary and capricious. (API, Attachment 2, p. 12) EPA also fails to acknowledge that the onl y study conducted since the last review rebuts the validity of the VAQ preference studies previously conducted. (UARG, Attachment 2, p. 28) As is explained in a more detailed discussion in the Response to Comments document, the EPA disagrees that the stu dy conducted by Smith and Howell (2009) supports the conclusion that the preference study methodologies were fundamentally***E.O. 12866 Review

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Page 369 of 432 flawed ; however , the EPA notes that their experiments do identify areas where additional research would be useful to further inform o ur limited understanding of public preferences in urban areas

. T h e EPA views the Smith and Howell experiments as increasing the EPA’s knowledge and understanding of the findings of the 2001 Washington, DC focus group pilot study (Abt, 2001) in several impo rtant ways , although this information still remains limited overall . Specifically, the Smith and Howell results suggest: 1) the 2001 s results, while based on a small sample size of 9, were consistent with results from a larger sample of the general Washin gton, DC population; 2) an individual’s preferences for visibility in one location may not depend on whether they live in that location; and 3) presentation method (i.e., changing from slide projection to computer monitor) did not appear to affect the reported preferences. (c) Preference Study Results and Adversity A number of comments were received regarding the EPA’s use of preference study results to make the determination that adverse PM 2.5 - related visibility effects on the public welfare are occurring . In this context, several commenters questioned whether the EPA had made the case that unacceptable levels of visual air quality based on preference study results alone can be equated with an adverse public welfare effect. These commenters suggested that unless preference study information is linked to personal comfort and well

- being or other associated welfare effects, it cannot form the basis of a determination of adversity. For example, Kennecott Utah Copper LLC stated that: Thus, EPA seemingly was bui lding the foundation for a determination of what constitutes an adverse effect on visibility in the context of public welfare. However ... EPA subsequently veered toward an oversimplified focus on public acceptance of visibility conditions.... EPA’s discussion of visibility in the Policy Assessment and its proposed rule in the Federal Register focuses entirely on “acceptable” and “unacceptable” visual air quality and make no mention of an “adverse effect” in the context of visibility. EPA’s reliance on only 3 u rban***E.O. 12866 Review

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Page 370 of 432 preference studies represents a paucity of data and a wholesale abandonment of any effort to seek a scientifically measurable adverse effect. (Kennecott Utah Copper LLC, p. 26) In response, the EPA first notes that the definition of effects on welfare included in section 302(h) of the CAA identifies both visibility and the broader category of effects on personal comfort and well

- being as effects on welfare. In setting a secondary standard to address visibility impairment, the EPA considers the effect o n the public from impairment of visibility as a separate and distinct welfare effect in its own right. The EPA is not required to translate this into terms of personal comfort and well

- being, as visibility impairment is designated explicitly by Congress as an effect on welfare. While there may be a large degree of overlap among these different welfare effects, the EPA properly focuses on evaluating all of the information before the Agency on the effect visibility impairment has on the public, whether or not this impairment would also be categorized as having an adverse effect on personal comfort and well

- being. It is in the context of all of this information that the EPA makes the judgment as to the appropriate degree of protection from known and anticipated adverse effects on the public from visibility impairment. The EPA recognizes that there is uncertainty about the degree of adversity to the public welfare associated with PM

- related visibility impairment. However a secondary standard is designed to provid e protection from "known or anticipated" adverse effects, and a bright line determination of adversity is not required in judging the requisite degree of protection under section 109(b)(2). Furthermore, the EPA disagrees that it has abandoned its considerat ion of visibility

- related impacts on the welfare effect of personal comfort and well

- being, as is made clear in the following quote: Research has demonstrated that people are emotionally affected by low visual air quality, that perception of pollution is c orrelated with stress, annoyance, and symptoms of depression, and that visual air quality is deeply intertwined with a***E.O. 12866 Review

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Page 371 of 432 ‘‘sense of place,’’ affecting people’s sense of the desirability of a neighborhood (U.S. EPA, 2009a, section 9.2.4). Though it is not kno wn to what extent these emotional effects are linked to different periods of exposure to poor visual air quality, providing additional protection against short - term exposures to levels of visual air quality considered unacceptable by subjects in the contex t of the preference studies would be expected to provide some degree of protection against the risk of loss in the public’s ‘‘sense of well - being.’’ (77 FR 38973/1, emphasis added) The approach taken to address such qualitative, but policy

- relevant, infor mation in this review is the same as in other NAAQS reviews. The review is initiated with a comprehensive assessment of all possible public health and welfare effects associated with PM in the Integrated Science Assessment. Then policy

- relevant effects for which there is sufficient quantitative information to allow a determination of the change in risks associated with incremental changes in air quality are assessed (in this review, in the Visibility Assessment) and used to provide a quantitative basis to i nform the selection of an appropriate range of levels for further consideration in the Policy Assessment. In the Policy Assessment, the EPA considers all important policy

- relevant evidence and information, both quantitative and qualitative, in making recom mendations regarding the range of policy options appropriate for the Administrator to consider. It is in the context of all of this information that the Administrator makes her final judgment as to the appropriate degree of protection from known and antici pated adverse effects on the public from visibility impairment. Another issue raised in the comments regarding adversity is the EPA’s decision to use the 50 percent acceptability criterion from the public preference studies in determining candidate protect ion levels of visibility impairment for the selection of a national level of visibility protection. For example, AMC et al. recommended “a 75% acceptability criterion as a target that is in line with protecting the broader public from***E.O. 12866 Review

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Page 372 of 432 the negative effects of visibility impairment” (AMC, et al., p. 9). In the Visibility Assessment, the EPA noted that the use of the 50 percent acceptance level for urban visibility was first presented in Ely et al. (1991) (U.S. EPA, 2010b, p. 2

- 5). Ely discussed the use of the 50 percent acceptability criterion as a reasonable basis for setting an urban visibility standard. The standard was determined based on a 50% acceptability criterion, that is, the standard was set at the level of extinction that would divide the slides in to two groups: those judged acceptable and those judged unacceptable by a majority of the people in the study. The criterion is politically reasonable because it defines the point where a majority of the study participants begin to judge slides as represen ting unacceptable visibility. It is also consistent with psychological scaling theory which indicates that a “true score” exceeds a standard when more than 50% of the “observed scores” exceed that standard. (Ely et al., 1991, p. 11) As Ely described, the 50 percent acceptability criterion and the preference study conducted by Ely were used as the basis for setting the level of the Denver Visibility Standard in 1990. Th at same criterion was judged appropriate and selected for use in the Phoenix preference s tudy (BBC research, 2003) and as the basis for setting the level of the Phoenix Visibility Standard in 2003. Most recently, the 50 percent acceptability criterion has been recommended by the British Columbia Visibility Coordinating Committee as the basis f

or the visibility standard currently under consideration by British Columbia, Canada. Furthermore, CASAC supported this approach, while recognizing the uncertainty associated with this issue. Specifically, CASAC agreed that “the 50 th percentile for the acc eptability criteria is logical, given the noted similarities in methodologies employed in the 4 study areas. ... In terms of choosing a specific percentile from the preference studies, we note that there may not be a “preferred” one, but in assessing prefere nce studies to propose a PM secondary NAAQS, the 50 th percentile is sufficient, as it is the basis for existing visibility indexes used in the Denver/Colorado Front Range and***E.O. 12866 Review

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Page 373 of 432 Phoenix metropolitan areas” (Samet, 2009c, pp. 8

- 9). Therefore, after considering the information that served as the original basis for its selection as described in Ely et al., 1991, and given its acceptance and use in existing visibility programs , the EPA continues to conclude , consistent with the advice of CASAC, that it is reasonab le to use the 50 percent acceptability criterion in determining target levels of protection from visibility impairment. (d) Appropriateness of using regionally varying preference study results to select a single level for a national standard. A number of commenters raised concerns regarding the bases for and implications of the differences observed in the preference study results, concluding that these results were due to regionally varying factors and thus could not be used to set a national

s tandard. For example , some commenters asserted that because the confidence intervals around the four 50 percent acceptability levels do not overlap at all, and because there are variations in preference study designs and inherent differences in the visual setting among cities and panels, the four preference curves and their associated 50 percent dv values are city

- specific and statistically different. The commenters conclud ed , therefore, that it was inappropriate to aggregate the 50 th percentile dv values from multiple studies and that they should instead be evaluated individually. Other commenters expressed the related view that the preference study results cannot be used to set a national standard for visibility impairment because t he results show that visibility preferences vary regionally. For example, API stated that: The ‘one

- size

- fits

- all’ approach...is not viable because it does not account for regional and city

- specific factors that have been made evident in the disparity of pr eference study data.... It is well known, for example, that the level of light extinction to which people in different areas of the country are accustomed, as well as the urban setting, are the primary factors that affect a person’s visual perception of an u rban vista. Thus, the degree to which extinction threshold can***E.O. 12866 Review

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Page 374 of 432 be related to human welfare is inevitably regionally

- dependent. (API, Attachment 2, p. 4) Some of these commenters argued that because acceptable visual air quality is regionally dependent, it would be more appropriate to develop distinct visibility standards at the state or local level. Others pointed out that areas which lack “important visibility vistas” might not need standards at all, since flat areas without significant terrain have a lim ited maximum visual range (NEDA/CAP, p. 3). Other commenters stated that due to regionally varying factors, such as relative humidity, it is not possible to select a single level for a national standard to protect visibility across the United States

. In pa rticular, these commenters pointed to differences between Eastern and Western areas, arguing that a single national standard could not offer the appropriate degree of protection in locations with distinct characteristics. For example: [T]he proposed method falls short because it is not temporally or geographically representative enough to have any meaning.... The uncertainty evidenced in these studies and the non

- uniformity between the western and eastern vistas makes it impossible at this time to set an acce ptable light extinction value that would appropriately address visibility concerns in non

- Class I areas. (New York DOH/DEC, pp. 5

- 6) The EPA agrees that the preference curves and the 50 percent dv levels are separate and distinct data points representing f our different VAQ preference curves for four unique urban scenes. However, the EPA does not consider the fact that the four curves are distinct as a weakness of the approach or a reason that the results cannot be compared. In addition, the EPA does not agr ee that the study results necessarily support a conclusion that preferences are regionally dependent

. In particular, the EPA notes that the results of Smith and Howell (2009) which show that participants in Houston and Washington, DC did not have significa ntly different views on acceptable air quality in***E.O. 12866 Review

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Page 375 of 432 Washington, DC, provide limited support for the conclusion that people’s preferences differ less because of where they live and more because of the scene they are viewing. On the other hand, the existing l iterature indicates that people’s preferences for VAQ depend in large part on the characteristics and sensitivity of the scene being viewed. The EPA understands there is a wide variety or range of urban scenes within the United States. These sensitive urba n scenes include those with natural vistas such as the Colorado Rocky Mountains as well as those with iconic man

- made urban structures like the Washington Monument. The EPA believes that the scenes presented in the four urban areas include important types of sensitive valued urban scenes and therefore, when considered together, can inform the selection of a level of acceptable urban VAQ at the national scale, taking into account the variation across the country evidenced in the studies. This is discussed fu rther in the next section, below. The EPA does agree with commenters that there are regionally varying factors that are important to take into account when setting a national standard for visibility protection. Section VI.A above regarding the history of t he secondary PM NAAQS review discusses the evolution of the EPA’s understanding regarding the regional differences in PM concentrations, relative humidity and other factors. As a result, the current review has gone to great lengths to address these factors , leading to the EPA’s proposal to use the IMPROVE algorithm to calculate light extinction in order to take into account the varying effects of relative humidity and speciated PM. While this approach does not result in a uniform level of ambient PM 2.5 , it does ensure a nationally uniform level of visibility protection. The EPA refers the reader to other sections of the final rule, including sections VI.B.1.a, VI.B.1.c, VI.C.1.b and VI.C.1.f, and the Response to***E.O. 12866 Review

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Page 376 of 432 Comments document for a more detailed response as to how it is taking these variables into account. ii. Specific Comments on Level The EPA received relatively few comments endorsing a specific level for a distinct secondary standard for visibility. In general, commenters who opposed setting a distinc t secondary standard at this time did not address the question of what level would be appropriate if the EPA were to set a distinct secondary standard for visibility; similarly, commenters who supported adopting a distinct secondary standard at this time
g enerally did not recommend a specific level. However, a few commenters did provide comments in support of a specific level or range of levels, with some commenters advocating standards at the upper end of the range of proposed levels (i.e., 30 dv), while

o thers supported levels below the lower end of the proposed range (i.e., below 28 dv). As discussed above, a large number of commenters argued that the currently available data are insufficient to determine what constitutes a standard that would be neither more nor less protective than necessary and that no standard should be set at this time. These commenters pointed to the limitations and uncertainties in the preference studies discussed above as the basis for this claim. These commenters pointed to signi ficant variation in the results of the preference studies in support of their arguments that the studies should not be used to derive a level for a distinct secondary standard for visibility. For example, one consultant cited by several industry commenters argued that the proposed level of 28 or 30 dv did not reflect the substantial difference in visibility preferences between the East and the West reflected in the preference studies (UARG, Attachment 2, p. 11), and that it did not reflect the full range of preferences (i.e., potential***E.O. 12866 Review

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Page 377 of 432 50 percent acceptability levels) likely to exist nationwide (UARG, Attachment 2, p. 19). This commenter further objected to the EPA’s proposal for a level of 28 or 30 dv on the grounds that the EPA had inaccurately adjusted 4

- hour values into 24

- hour values. Based on his analysis, the consultant concluded that “a range of adjusted values from 28 to 32 dv is needed” to account for the majority of the spread between the 4

- hour vs. 24

- hour equivalent values at the upper end of the distribution of values. A number of commenters questioned whether the proposed range of levels was appropriate. One industry commenter claimed that the EPA had not explicitly justified why a standard within the proposed range was requisite, stating that “EPA makes no attempt to explain how the proposed level of the standard is neither lower nor higher than necessary to protect public welfare” (NSSGA, p. 15). Arizona DEQ noted that since the proposed calculated light extinction indicator excluded coarse pa rticles and Rayleigh scattering, the proposed levels of 28 or 30 dv were inconsistent with the visibility preference studies, which considered total light extinction. Noting these perceived problems with the proposed range of levels, a few commenters noted that if the EPA were to set a distinct secondary standard, the level should be set no lower than 30 dv, “to account for inconsistent value judgments, a great deal of spatial and temporal variability, and a very high level of uncertainty” (Texas CEQ, p. 7)

. In contrast, some commenters supporting the EPA’s proposal for a distinct secondary standard for visibility stated that the proposed range of levels from 28

- 30 dv was insufficiently protective based on a 24

- hour averaging time, and recommended a lower l evel for the visibility index standard. These commenters expressed the view that the proposed levels of 28 or 30 dv represented neither adequate surrogates for equivalent***E.O. 12866 Review

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Page 378 of 432 4

- hour values, as the EPA claimed, nor sufficiently protective levels based on recent air quality data. Several commenters stated that the EPA’s own analyses suggested that a standard set at a level of 28 or 30 dv was insufficiently protective based on a 24

- hour averaging time. One commenter emphasized that the Policy Assessment had indica ted a level between 25

- 28 dv was appropriate for a standard calculated on a 24

- hour average, and encouraged the EPA to adopt a standard level of 25 dv. Several environmental groups provided comments stating that a 24

- hour average would underestimate a 4

- ho ur value by 13

- 42 percent and certain areas of the country

— particularly the Northeast

— would be affected disproportionately. These commenters suggested that a 24

- hour PM 2.5 visibility index standard should be set at a level of 18.6

- 20 dv. The Department of the Interior pointed to recent air quality data indicating that visibility on the 20% worst days in several large metropolitan areas, including Birmingham, Fresno, New York City, Phoenix, and Washington, DC was below 29 dv. While noting that these calculat ions were based on IMPROVE calculations which include contributions from coarse PM mass, DOI expressed the view that the proposed level of 28 to 30 dv would not provide adequate visibility protection compared to the current 24

- hour PM 2.5 standard of 35 μg/ m 3 and recommended that the standard be set at a level of 25 dv consistent with the results of the Phoenix visibility preference study. In contrast, the states of Arizona and Colorado submitted comments arguing that the visibility preference studies condu cted in Phoenix and Denver, respectively, were designed to address a specific local problem and that the results of these studies were not an appropriate basis for selecting the level of a national standard. For example, Arizona DEQ noted:***E.O. 12866 Review

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Page 379 of 432 The cited studi es were conducted considering total light extinction; including extinction resulting from particulate matter and Rayleigh scattering. Visibility impairment due to coarse particulate matter can be an important contributor in Arizona, specifically in the Pho enix area where ongoing measurements have been made. Therefore, ADEQ believes that the proposed levels of the secondary visibility standard are inconsistent with applicable urban studies. (Arizona DEQ, p. 2) Similarly, the Colorado Department of Public Hea lth and the Environment noted that the Denver visibility standard was designed to address “brown clouds”, i.e., strong inversions that occur in the Denver metropolitan area, and that this standard “is based on a specific view of Denver” associated with par ticular sight paths and direct measurement methods. The commenter stated that this standard “is applicable only to this location,” and that these limitations make it potentially unsuitable for application as “a national secondary standard, particularly a p roposed standard that does not use a direct measurement method” (Colorado DPHE, p. 2). While acknowledging the uncertainties and limitations associated with the visibility preference studies as discussed above, the EPA continues to conclude, as did CASAC, that the preference studies are appropriate to use as the basis for selecting a target level of protection from visibility impairment. However, the EPA agrees with commenters who emphasize the high degree of variability in visibility conditions and the
po tential variability in visibility preferences across different parts of the country. In light of the associated uncertainty, as noted in the proposal, the Administrator judged it appropriate to establish a target level of protection equivalent to the upper end of the range of Candidate Protection Levels (CPLs) identified in the Policy Assessment and generally supported by CASAC. Thus, the EPA proposed to set a 24

- hour visibility index standard that would provide protection equivalent to the protection affor ded by a 4

- hour standard set at a level of 30 dv. In light of the comments received on the proposal, in***E.O. 12866 Review

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Page 380 of 432 particular comments emphasizing the uncertainty and variability in the results of the public preference studies, the EPA continues to conclude that this approach is warranted, and that it is appropriate to set a target level of protection equivalent to the protection that would be afforded by a 4

- hour, 30 dv visibility index standard. Moreover, t

he EPA disagrees with commenters who argued that the EPA’s

a pproach for translating 4

- hour CPLs into equivalent 24

- hour values was inappropriate. In adjusting 4

- hour values for purposes of defining an appropriate level for a 24

- hour standard, the EPA noted at the time of proposal that there were multiple approaches for estimating generally equivalent levels on a city

- specific or national basis . While expressing the view that it was appropriate to consider the two approaches with the highest r 2 values (Approaches A and B in Appendix G of the Policy Assessment), 155 which used regressions of 90 th percentile light extinction values, the EPA determined it would also be appropriate to consider the city

- specific estimates resulting from Approaches C and E which showed greater variability than the aggregated estimates. Ap proaches C and E generated a range of city

- specific estimates of generally equivalent 24

- hour levels that encompassed the range of levels considered appropriate for 4

- hour CPLs, including the CPL of 30 dv at the upper end of that range. This information
pr ovided support for using the same CPL for a 24

- hour standard as for a 4

- hour standard , since no single approach could generate an equivalent 24

- hour standard level in each urban area for each CPL. The EPA continues to conclude, as it did at the time of

155 In particular, EPA staff expressed a pr eference for Approach B in the Policy Assessment. However, in light of the additional information provided by the other approaches explored in Appendix G of the Po licy Assessment and the reanalysis in Frank, et al. (2012b), the EPA judges it more appropria te to consider the range of values resulting from all five analytical approaches for purposes of informing decisions about the equivalent level of a 24

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Page 381 of 432 pro posal, that using an unadjusted 4

- hour CPL for purposes of establishing a target level of protection for a 24

- hour standard is appropriate because this approach places more emphasis on the relatively high degree of spatial and temporal variability in relat ive humidity and fine particle composition observed in urban areas across the country , consistent with EPA’s reanalysis discussed below

. The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships between estimated 24

- hour and 4

- hour vis ibility impairment based on the variety of metrics discussed in Appendix G of the Policy Assessment. The reanalysis has more appropriately considered the uncertainty of the calculated 4

- hour values. The revised analysis shows that the 24

- hour equivalent le vel is generally closer to the 4

- hour value at the upper end of the range of CPLs than originally estimated , as can be seen in the results for Approaches B, C, and D. 156 For example, the reanalysis indicates that Approach B yields an adjusted 24

- hour CPL of 29 dv 157 as generally being equivalent to a 4

- hour CPL of 30 dv , while Approach C yields a 24

- hour equivalent CPL of 29 averaged across cities and a range of city

- specific values from 25

- 36 dv

. 158 , 159 Not only are the 90 th percentile and pooled average values closer to the 4

- hour CP L of 30 dv , the range of city

- specific results shows a wider spread that clearly encompasses the unadjusted 4

- hour value of 30

156 Approach E as presented in the Policy Assessment is based on the median values for each city ; these results are not affected by the regression analyses. Therefore, Approach E was not included in the reanalysis, and the results remain unchanged from those reported in the corrected Table G

- 6 as reported in Frank, et al., 2012b. 157 In Appendix G of the Policy Assessment, a 24

- hour ad justed CPL of 28 was estimated to be equivalent to a 4

- hour value of 30 under Approach B

. 158 In Appendix G of the Policy Assessment, a 24

- hour adjusted CPL of 27 dv was estimated to be equivalent to a 4

- hour CPL of 30 dv when averaged across cities, while city

- specific values were estimated to range from 24

- 30 dv. 159 In the reanalysis, Approach D (all days pooled regression) generated results of 28 dv for the 24

- hour CPL equivalent to a 4

- hour value of 30 dv as compared to a value of 27 dv in the original analysis described in Appendix G.***E.O. 12866 Review

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Page 382 of 432 dv near the midpoint of the city

- specific range. This provides suppo rt for concluding that the EPA’s approach to translating of 4

- hour CPLs into equivalent 24

- hour values was appropriate , and that it is appropriate to use unadjusted 4

- hour values for purposes of selecting a level for a standard based on a 24

- hour averaging time

. 160 Moreover, the EPA disagrees with commenters who argue that the currently available evidence is sufficient to justify establishing a target level of protection at 25 dv or below. The EPA recognizes that 25 dv represents the middle of the range of 50 percent acceptability levels from the 4 cities studied, and represents the 50 percent acceptability level from the Phoenix study, which the Agency has acknowledged as the best of the four studies in terms of having the least noise in the preference study results and the most representative selection of participants. The EPA also notes the caveats discussed in the proposal regarding whether it would be appropriate to interpret results from the western studies as generally representative of a broader range o f scenic vistas in urban areas across the country. The Policy Assessment noted significant differences in the characteristics of the urban scenes used in each study, with western urban visibility preference study scenes including mountains in the backgroun d and objects at greater distances, while scenes in the eastern study did not. Since objects at a greater distance
160 The analysis in Appendix G of the Policy Assessment used the 4

- hour light extinction value treated as the independent (x

- axis) variable in an ordinary least squares regression. T he EPA now concludes that this regression approach was inappropriate because that variable has error and in fact may be more uncertain than the calculated 24

- hour extinction values. The Frank et al. (2012b) reanalysis uses an orthogonal regression instead of ord inary least squares regression and results in slopes closer to the 1:1 line for all the results, particularly for Dallas, TX. Furthermore, consistent with the EPA’s conclusion that a higher multiplier for converting OC to OM would be appropriate (see secti on VI.C.1.b.ii above), the reanalysis substitutes a 1.6 multiplier for converting OC to OM in the calculation of 24

- hour values instead of the value of 1.4 that was used in calculating 24

- hour values for Appendix G. The higher multiplier is more consistent with the SANDWICH approach used to calculate the 4

- hour values found in Appendix G. See Frank et al. (2012b) for a more detailed explanation.***E.O. 12866 Review

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Page 383 of 432 have a greater sensitivity to perceived visibility changes as light extinction changes compared to otherwise similar scenes with objects at a shorter range, this likely explains part of the difference between the results of the eastern study and results of the western studies. In the proposal, the EPA noted that the scenic vistas available on a daily basis in many urban areas across the country generally do not have the inherent visual interest or the distance between viewer and object of greatest intrinsic value as in the Denver and Phoenix preference studies. Also, the Agency takes note of the caution expressed by Colorado and Arizona about us ing the results of the Denver and Phoenix preference studies, which were aimed at addressing specific local visibility problems, to inform the choice of level for a national standard. Therefore, the Agency considers it reasonable to conclude, especially in light of the significant uncertainties, that it is appropriate to place less weight on the western preference results and that the high CPL value (30 dv) that is based on the eastern preference results is likely to be more representative of urban areas th at do not have associated mountains or other valued objects visible in the distant background. These areas would include the middle of the country and many areas in the eastern U.S., as well as some western areas. As a result, the EPA concludes that it is more appropriate to establish a target level of protection at the upper end of the range of 24

- hour CPLs considered, recognizing that no one level will be "correct" for every urban area in the country. In considering the upper end of this range, the EPA must identify a target level of protection that is considered requisite to protect public welfare from a national perspective, recognizing that the same target level would apply in all locations. Making this judgment requires a balancing of the risks to th e public welfare and the substantial***E.O. 12866 Review

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Page 384 of 432 uncertainties surrounding appropriate levels of visibility protection. As acknowledged in the proposal, the EPA recognizes that setting a target level of protection for a 24

- hour standard at 30 dv would reflect a judgme nt that the current substantial degrees of variability and uncertainty inherent in the public preference studies should be reflected in a higher target protection level than would be appropriate if the underlying information were more consistent and certai n. Also, a 24

- hour visibility index at a level of 30 dv would reflect recognition that there is considerable spatial and temporal variability in the key factors that determine the value of the PM 2.5 visibility index in any given urban area, such that there is a relatively high degree of uncertainty as to the most appropriate approach to use in selecting a 24

- hour standard level that would be generally equivalent to a specific 4

- hour standard level. In light of these uncertainties, the EPA continues to belie ve that it is appropriate to establish a target level of protection for visual air quality of 30 dv, averaged over 24

- hours, with a form as discussed above. In reaching this conclusion, the EPA notes that any national ambient air quality standard for visi bility would be designed to work in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM

- related visibility impairment in all areas of the country, including urban, non

- urban, and Federal Class I ar eas. While the Regional Haze Program is focused on improving visibility in Federal Class I areas and a secondary visibility index NAAQS would focus on protecting visual air quality principally in urban areas, both programs could be expected to provide bene fits in surrounding areas. In addition, the development of local programs, such as those in Denver and Phoenix, can continue to be an effective and appropriate approach to provide additional protection, beyond that afforded by a national standard, for uniq ue***E.O. 12866 Review

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Page 385 of 432 scenic resources in and around certain urban areas that are particularly highly valued by people living in those areas. With regard to comments from the Department of Interior noting that many large metropolitan areas have 24

- hour IMPROVE values below 3 0 dv on the worst 20 percent of days already, the EPA notes that the purpose of establishing NAAQS is to ensure adequate protection of public welfare everywhere, not to mandate continuous improvements in areas that may already be relatively clean. In fact, the evidence from the IMPROVE program that many urban areas have total 24

- hour PM

related light extinction below 29 dv on the 20 percent worst visibility days suggests that many areas have relatively good visual air quality already. f. Need for a Distinct Secondary Standard to Protect Visibility Numerous commenters questioned whether a distinct secondary standard for visibility is necessary in light of the analysis described in section VI.B.1.c.vii above (Kelly et al., 2012a) which indicated that a 24

- hour mass - based PM 2.5 standard of 35 μg/m 3 would protect against visibility impacts exceeding the range of levels considered in the proposal (28

- 30 dv). While this analysis was conducted in support of proposed implementation requirements for a distinct seconda ry standard (specifically, the modeling demonstrations that would be required under the PSD program), the second prong of the analysis showed that within the range of levels proposed by the EPA for the visibility index NAAQS (28

- 30 dv), the 24

- hour PM 2.5 s tandard of 35 μg/m 3 would generally be controlling. Kelly et al. (2012a) concluded that “overall, design values based on 2008

- 2010 data suggest that counties that attain 24

- hour PM 2.5 NAAQS level of 35 μg/m 3 would attain the proposed secondary PM 2.5 visibility index NAAQS level of 30 dv and generally attain the level of 28 dv” (pp. 17

- 18).***E.O. 12866 Review

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Page 386 of 432 Citing this conclusion, many state and local agencies and industry commenters argued that a visibility index standard in the range proposed (28

- 30 dv) would provi de no additional protection beyond that afforded by the existing secondary 24

- hour PM 2.5 NAAQS, and therefore no distinct visibility standard was necessary. These commenters advocated retaining the current 24

- hour PM 2.5 mass - based standard to protect again st visibility effects. “Since the 24

- hour PM 2.5 standard already protects the welfare the 24

- hour PM 2.5 visibility standard is designed to protect, the new standard is duplicative and unnecessary” (South Dakota DENR, p. 2). Furthermore, a number of state c ommenters objected to the additional resource burden associated with implementing a standard which had, in their view, no practical effect: “If the 24

- hour PM 2.5 mass standard has the same effect as the visibility standard, crafting complex regulations to implement another standard seems redundant” (South Carolina DHEC, p. 3). Other states agreed: “A PM 2.5 - related Visibility Index appears redundant since the benefits achieved from the current primary and secondary annual and 24

- hour PM 2.5 standards already provide reductions that would improve visibility. Establishing a new PM 2.5 secondary standard for visibility would be an additional complication and burden to the states that is not warranted” (Indiana DEM, p. 5). In addition, several commenters submitted additional analyses supporting their position that a 35 μg/m 3 24

- hour PM 2.5 standard would provide at least equivalent protection to a distinct 24

- hour visibility standard within the range of levels proposed (API, Attachment 2, p. 8 and Attachment 3, p. 1 ). In responding to these comments stating that a distinct visibility standard is not needed, the EPA notes as an initial matter that the Administrator provisionally concluded***E.O. 12866 Review

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Page 387 of 432 at the time of proposal that the current PM standards were not sufficiently pro tective of visual air quality, and that consideration should be given to an alternative secondary standard that would provide additional protection against PM

- related visibility impairment, especially in urban areas. This provisional conclusion was based o n the results of public preference surveys on the acceptability of varying degrees of visibility impairment in urban areas, analyses of the number of days on which peak 1

- hour or 4

- hour light extinction values were estimated to exceed a range of CPLs under conditions simulated to just meet the current standards, and the advice of CASAC. The Administrator also noted that the current indicator of PM 2.5 mass, in conjunction with the current 24

- hour and annual averaging times, was not well suited for purposes o f protecting visibility, since it does not incorporate species composition or relative humidity, both of which play a central role in determining the impact of ambient PM on visibility. Taking into account the advice of CASAC and the court’s remand of the current secondary PM 2.5 standards, the Administrator provisionally concluded that the current secondary standards were neither sufficiently protective nor suitably structured to provide an appropriate degree of public welfare protection from PM

- related vis ibility impairment. As a result, the EPA proposed a new, distinct secondary standard that was designed to address these deficiencies. The EPA notes that in critiquing the proposed secondary standard, commenters generally did not advocate that the form of t he existing mass

- based PM 2.5 standards was better suited scientifically to the task of protecting against visibility impairment. Rather, the commenters’ position that a distinct secondary standard was not needed for purposes of protecting visibility was ba sed almost entirely on the relative degree of protection***E.O. 12866 Review

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Page 388 of 432 likely to be afforded by the existing standards (in particular, the existing 24

- hour PM 2.5 standard) as compared to the proposed visibility index, along with the relatively large uncertainties associ ated with the latter. Thus, for all the reasons discussed in the proposal with regard to the scientific appropriateness of an indicator that takes into account both species composition and relative humidity, the EPA continues to conclude that the proposed standard based on a visibility index would be appropriate scientifically to provide targeted protection of visibility, since it would provide a measure of PM

- related light extinction that directly takes into account the factors (i.e., species composition a nd relative humidity) that influence the relationship between PM 2.5 in the ambient air and PM

- related visibility impairment. Furthermore, the EPA disagrees with commenters who stated that implementation concerns, in particular the additional resource burd en associated with implementing a distinct secondary standard, should alter the Agency’s decision making with regard to a standard to protect visibility. The EPA may not take the costs of implementation into account in setting or revising the NAAQS. Howev er, in light of the results of the Kelly et al. (2012a) analysis and the views expressed by commenters on the implications of this analysis for conclusions regarding the adequacy of the current secondary 24

- hour PM 2.5 standard, the EPA has reconsidered

som e of the conclusions drawn at the time of proposal, in particular with regard to the degree of protection that would be provided by the current secondary standard. Based on a review of comments related to indicator, averaging time, form and level, the Agen cy has concluded that (as described in sections VI.C.1b

- e above) a standard defined in terms of a PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative***E.O. 12866 Review

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Page 389 of 432 humidity data to calculate PM 2.5 light extinction), a 24

- hour averaging time , and a 90 th percentile form, averaged over 3 years, and a level of 30 dv, would provide sufficient but not more than necessary protection of the public welfare with regard to visual air quality. Having identified this target level of protection, the EPA i s now in a position to compare it specifically to the existing secondary 24

- hour PM 2.5 standard of 35 μg/m 3 for purposes of determining whether it would provide more, the same, or less protection from visibility impairment. The EPA must consider both whether the existing secondary 24

- hour PM 2.5 standard of 35 μg/m 3 is sufficient (i.e. not under

- protective) and whether it is more stringent than necessary (i.e. over

- protective). W ith regard to the degree to which the existing secondary 24

- hour PM 2.5 standard provides sufficient but not more than necessary protection for visibility, the EPA first notes that the kind of area

- specific analysis conducted in Kelly et al. (2012a) is esse ntial for addressing the court remand of the 2006 secondary standards. In the case of the 2006 secondary standards , the EPA had argued that the 35 μg/m 3 24

- hour PM 2.5 standard was requisite because one part of the proposed range for a distinct secondary st andard the Agency had considered would affect the attainment status of a somewhat fewer counties than the 35 μg/m 3 24

- hour PM 2.5 standard. The court rejected this kind of rough balancing, finding that the EPA’s equivalency analysis based on percentages of counties demonstrated nothing about the relative protection offered by the different standards. Based on this, an area

- by

- area evaluation of the relative degree of protection offered by different standards should be conducted to the extent air quality data is available. Kelly et al. (2012a) performed such an evaluation. Based on 2008

- 2010 data, there are no areas that would have exceeded a 30 dv, 24

- hour visibility index standard***E.O. 12866 Review

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Page 390 of 432 that would not also have exceeded a 24

- hour PM 2.5 standard of 35 μg/m 3 . State d another way, all areas that met the 24

- hour PM 2.5 standard of 35 μg/m 3 would have had visual air quality at least as good as 30 dv (24

- hour average, based on 90 th percentile form averaged over 3 years). The Kelly (2012a) analysis also showed that for som e areas, particularly in the West, areas that would have met a 24

- hour PM 2.5 standard of 35 μg/m 3 would have had visual air quality better than 30 dv for the PM 2.5 visibility index standard , and that at sites that violated both the 24

- hour level and the vi sibility index 30 dv level, the visibility index level of 30 dv would likely be attained if PM 2.5 concentrations were reduced such that the 24

- hour PM 2.5 level of 35 μg/m 3 was attained. The EPA has conducted a reanalysis (Kelly et al., 2012b) to update the area

- by

area analysis in the original Kelly et al. (2012a) analysis in t hree respects. First, noting that the original Kelly at al. (2012a) analysis used a 1.4 multiplier to convert OC to OM at those monitors not using the new CSN monitoring protocol , the EPA recalculated the visibility index design values for 2008

- 2010 using a higher multiplier for converting OC to OM at monitors not already using the new CSN monitoring protocol SANDWICH approach, consistent with the Agency’s view that it is more appro priate to use a multiplier of 1.6 at such monitors as compared to 1.4, as described in section VI.C.1.a.ii, above. 161 The recomputed visibility design index values for 2008

- 2010 show the same overall relationship to 24

- hour PM 2.5 design values as presented i n Kelly et al., 2012a. Second, the EPA repeated the calculations comparing visibility index design values with 24

- hour PM 2.5 design values using 2009

- 2011 data, the most recent three

161 Some of the OC measurements were produced with CSN’s newer monitoring protocol and did not require a change in the computed OM.***E.O. 12866 Review

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Page 391 of 432 years of air quality information currently available. 162 Third, the EPA mo dified the area

by

- area evaluation to ensure consistency with the data completeness criteria of 40 CFR
p art 50, Appendix N , including the removal of data approved by EPA as exceptional
events , for the current 24

– hour PM 2.5 standard and the proposed visibility index standard

. The results of this re analysis, as presented in Kelly et al. (2012b), show a similar pattern to that described in the original Kelly memo. Specifically, the analysis indicates that there were no areas with visibi lity impairment above 30 dv that d id not also exceed the 24

- hour PM 2.5 standard of 35 μg/m 3 . The updated memo concludes that the results for 2009

- 2011 corroborate the findings for 2008

- 2010. Based on these analyses (Kelly et al., 2012a; 2012b), the EPA con cludes with a high degree of confidence that having air quality that meets the 24

- hour PM 2.5 standard of 35 μg/m 3 would be sufficient to ensure areas would not exceed 30 dv. The EPA notes that this conclusion from Kelly et al. (2012a) is supported by two a nalyses submitted by industry commenters (API, Attachments 2 and 3). At the time of proposal, the EPA had reached a different conclusion, specifically that the 35 μg/m 3 24

- hour PM 2.5 standard was not sufficiently protective. This conclusion was based, in part, on the analyses conducted for the Visibility Assessment and Policy Assessment regarding 1

- to 4

- hour peak light extinction values based on 2007

- 2009 data. For the reasons outlined above in sections VI.B.1.c and VI.C.1.c, the EPA originally focused on hourly or sub

- daily timeframes for evaluating visibility conditions. However, data quality concerns effectively precluded adoption of a 1

- hour or sub

- daily averaging time in this review, and ultimately the EPA has concluded that a 24

- hour averaging time

c an serve as an appropriate surrogate. In reaching this conclusion, the EPA has
162 The 2011 air quality data were not yet available at the time of proposal.***E.O. 12866 Review

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Page 392 of 432 recognized that adopting a 24

- hour averaging time will likely smooth out some of the hour

- by

- hour variability in visibility index values, and will effectively reduce peak values by averaging them together with other hours. In concluding it is appropriate to adopt a 24

- hour averaging time, which limits the impact of hour

- specific influences, the Agency is now placing less weight on the results of the 1

- hour and 4

- hour analyses pre sented in the Visibility Assessment and the Policy Assessment which focused on identifying the percent of days with peak hourly light extinction above various CPLs. In light of the Agency’s conclusion that a 24

- hour averaging time would be appropriate, the Agency has determined to place more weight on analyses of visibility conditions over a 24

- hour time period, especially the results in Kelly et al. (2012a and 2012b). In addition, the EPA notes that the Kelly et al. analyses reflects updated air quality in formation from more recent years of data (2008

- 2010 for Kelly et al., 2012a; 2009

- 2011 for Kelly et al. 2012b) as compared to the air quality information used in the Visibility Assessment and Policy Assessment

. In light of all of these considerations, inc luding the results of the Kelly et al. (2012a; 2012b) analyses, and the supporting comments provided by a broad range of public commenters, the EPA now concludes that the 24

- hour PM 2.5 standard of 35 μg/m 3 provides sufficient protection in all areas agains t the effects of visibility impairment. The EPA concludes that the existing 24

- hour PM 2.5 standard would provide at least the target level of protection for visual air quality defined by a visibility index set at 30 dv, as described above, which the EPA judges appropriate. However, the EPA also recognizes that it is important to evaluate whether such a standard would be over

- protective (i.e. more stringent than necessary to protect public***E.O. 12866 Review

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Page 393 of 432 welfare). The analyses presented in Kelly et al. (2012a; 2012b) indicates that the 24

- hour PM 2.5 standard of 35 μg/m 3 would achieve more than the target level of protection of visual air qua lity (30 dv) in some areas. That is, when meeting a mass

- based standard of 35 μg/m 3 , some areas would have levels of PM

- related visibility impairment far below 30 dv. Thus, when considered by itself and without consideration of the secondary standards adop ted for purposes of non

- visibility welfare effects, the 24

- hour PM 2.5 standard of 35 μg/m 3 would be over

- protective of visibility in some areas. However, it is important to note that as long as the current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 remai ns in effect, this overprotection for visibility would occur, regardless of whether a distinct secondary standard based on a visibility index set at 30 dv were adopted. These issues are discussed more fully in section VI.D, which outlines the Administrator ’s final conclusions on the secondary PM standards, below.
g. Legal Issues Some commenters opposed the proposal to establish a distinct secondary standard that would be defined in terms of a PM 2.5 visibility index. The proposed standard would use measure d PM 2.5 mass concentration, in combination with speciated PM 2.5 mass concentration and relative humidity data, to calculate PM 2.5 light extinction, translated to the deciview (dv) scale. The standard would also be defined in terms of a specified averaging time and form, and a level for the PM 2.5 visibility index set at one of two options

— either 30 dv or 28 dv. The commenters argued that the entire approach proposed by the EPA is inconsistent with the requirements of CAA section 109(b). They pointed to a num

ber of different aspects of the proposal which in their view made it incompatible with the CAA. For example, the Utility Air Resources Group (UARG)***E.O. 12866 Review

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Page 394 of 432 stated: In the past, EPA has always used a measure of PM mass as the indicator for both primary and secondar y PM NAAQS. Such a standard is, as a general matter, consistent with the directive in the CAA that the NAAQS “specify a level of air quality” and targets for control the listed criteria air pollutant. CAA § 109(b)(2). The standard contained in EPA’s propos ed rule does neither of these things. Instead, it would (1) regulate relative humidity, which is not a criteria pollutant; (2) fail to “specify a level of air quality” as required by section 109(b)(2) of the CAA; and (3) result in a standard necessitating nationally variable PM concentrations instead of a standard establishing a nationally uniform, minimally acceptable PM concentration. (UARG, p. 22

- 23) Other commenters raised similar or related issues, arguing that the EPA improperly set a visibility stan dard, and not a PM 2.5 standard, and that NAAQS can only be set in terms of a level or concentration of the air pollutant. Commenters also argued that an endangerment finding and air quality criteria would be needed before the EPA could set a standard based on PM components. Each of these comments is discussed below. As an initial matter, the commenters argued that the proposed standard is unlawful because it is “not a PM 2.5 standard at all, but rather a visibility standard, and visibility is neither an air pollutant nor a criteria pollutant for which a NAAQS may be promulgated” (NMA/NCBA, p. 21). According to these commenters, the CAA requires that NAAQS be established as limits on the concentration of an air pollutant in ambient air, not limits on the “iden tifiable effects” caused by that air pollutant. These commenters claimed that reduced visibility due to light extinction is not an air pollutant but instead is an effect, noting that “the Act’s definition of ‘air pollutant’ speaks in terms of specific subs tances or matter in the ambient air” (NSSGA, p. 8). The commenters pointed to the use of the term “air pollutant” in sections 109(a)(1)(A) and (b)(2) as support for their argument, as these provisions refer to setting standards for the “air pollutant” to a ddress***E.O. 12866 Review

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Page 395 of 432 the effects associated with the presence of the air pollutant in the ambient air. They likewise pointed to section 108(a)(2)’s reference to the presence of the air pollutant in the ambient air. Since reduced visibility is not an air pollutant, they argue the EPA cannot set a NAAQS that is a standard for visibility. They argue that the proposed secondary standard it is not a PM 2.5 standard as it does not limit the concentration of PM 2.5 or any other fraction of particulate matter in the ambient air an d therefore is not an “ambient air quality standard” for any pollutant. One commenter argued that the EPA is required to “specify a level of air quality” under section 109(b)(2), which Congress intended as an acceptable concentration level of the air pollutant in the ambient air, noting that specification of acceptable visibil ity conditions is not the same as an acceptable air pollution concentration level. Citing American Farm Bureau v. EPA , 559 F.3d at 516, one commenter claimed that the court had affirmed that “the NAAQS

— whether primary or secondary

— is a mass

- based standard” (Nevada DEP, p. 5). Commenters also refer to the legislative history of the 1970 amendments, referring to NAAQS as setting the “ maximum permissible ambient air level” for an air pollutant. T he commenters argue that the proposed standard is improper becaus e it does not limit the concentration of PM 2.5 or any fraction of PM in ambient air, but improperly sets a limit on visibility effects. With regard to humidity, these commenters argued that the proposed standard improperly regulates relative humidity bec ause it is included in the calculation to determine the value of the visibility index. According to these commenters, the CAA allows the EPA to control criteria air pollutants through the NAAQS program, but not other various substances. The commenters stat ed that the EPA recognized this in the last***E.O. 12866 Review

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Page 396 of 432 review, treating humidity as a confounding factor and considering addressing it by measuring PM 2.5 mass - based concentration over the midday hours, when humidity would have the least effect. This would target the effects caused by PM, and not by humidity. Referring to American Farm Bureau v. EPA , 559 F.3d 512, 528 (D.C. Cir. 2009) and 77 Fed. Reg. at 38979 n.153. UARG contested the proposed calculated visibility index as it does not approach relative humidity as a confounding factor but instead “embraces it and treats it as if it were a PM effect” (UARG p. 24). The commenters also stated that the use of a calculated visibility index, and the failure to exclude the effects of humidity, would result in acceptable PM concentrations that vary across the nation. These commenters claimed that such a standard is inconsistent with the requirements of the CAA because the proposed approach fails to establish a nationally uniform PM concentration standard. For example, API arg ued that the proposed visibility index approach is “essentially specifying levels

— not a level

— of air quality” (API, p. 29). UARG agreed, and stated that the Act “requires that criteria pollutant concentrations throughout the nation reach, at the least, a s ingle , specified ambient concentration level” (UARG, p. 25, emphasis in original). The commenters argue that a PM 2.5 visibility index standard cannot provide equal protection nationwide due to geographic variation in key factors such as relative humidity t hat affect level of particles allowed in different areas. The commenters noted that establishing a single national level for the PM 2.5 visibility index would necessarily result in unequal acceptable PM 2.5 levels in different areas of the country, with lowest allowable PM 2.5 levels in urban areas in the Southeast and highest allowable levels in the arid West. UARG recognized that under section 108 the air quality criteria are to “address those variable fact ors***E.O. 12866 Review

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Page 397 of 432 (including atmospheric conditions) which of themselves or in combination with other factors may alter the effects on public health or welfare of such air pollutant,” but stated that while section 108 “allows” this, it has no bearing on this issue. Inst ead, the commenter stated that the EPA may take such information into account in setting a permissible concentration of the pollutant that is uniform and national (UARG, p. 25). In addition, some commenters opposed to the proposed distinct secondary stand ard argued that in order to base a standard on measured levels of several speciated substances, the EPA must first make an endangerment finding and issue air quality criteria for each of the speciated substances included in the calculation of PM 2.5 light

e xtinction. According to these commenters, “EPA cannot use NAAQS to indirectly regulate multiple substances which are not criteria pollutants under the guise of establishing a visibility standard” (NMA/NCBA, p. 21). Noting that air quality criteria for part iculate matter were issued in 1969, NMA/NCBA argued that the 1969 Criteria Document “did not establish air quality criteria for individual constituents that occur in particle form, instead it established criteria for particulate matter as a whole” (p. 27). In light of the fact that criteria have never been issued for “individual speciated components of particulate matter,” these commenters argued, “if EPA wishes to promulgate a rule such as its secondary visibility NAAQS, it first must make a finding that t he speciated components listed in Appendix N endanger public health or welfare and then issue an air quality criteria document for those components” (NMA/NCBA, p. 29). According to these commenters, the approach the EPA adopted in promulgating a NAAQS for lead supports this view: When EPA promulgated a NAAQS for lead, an individual substance in particle form, it did not assert that an endangerment finding or criteria document for lead***E.O. 12866 Review

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Page 398 of 432 was unnecessary because lead was already covered by the PM Criteria Docu ment. Instead, EPA complied with the Section 108 and 109 NAAQS prerequisites for lead, just as it must do for Appendix N substances if it intends to promulgate a NAAQS for those substances. ... [In 1976], EPA listed lead as an air pollutant that adversely af

fected public health or welfare, issued an air quality criteria document for lead, and promulgated a NAAQS for lead. 43 Fed. Reg. 46246 (Oct. 5, 1976). (NMA/NCBA, p. 29) Finally, UARG argued that the EPA has in the past recognized that the secondary NAAQS is an inappropriate vehicle for regulating PM

- related visibility, referring to 62 Fed. Reg. at 38680, including fn 49. UARG claimed the same situation continues, and the EPA has not provided a valid basis for changing this conclusion. The EPA disagrees wi th the points raised by these commenters. While the EPA is not adopting the proposed secondary standard, as explained below, this decision is not based on concern over the EPA’s authority to adopt a secondary standard such as the one proposed. The propose d distinct secondary standard is a standard for PM 2.5 , and is not a “visibility standard.” The proposed secondary standard is based on the mass concentration of PM 2.5 in the ambient air. The standard is defined in terms of calculated PM 2.5 light extinction which is based on the measurement of the mass concentration of ambient PM 2.5 over a 24

- hour period. The measured mass concentration is adjusted based on information on the speciated mass components of the PM 2.5 and the relative humidity, resulting in a ca lculated visibility index. The level of the visibility index, combined with the form of the standard and averaging time, identifies whether a level of ambient mass concentration of PM 2.5 achieves the standard or not. Given any specific mass concentration o f ambient PM 2.5 , combined with information on speciation and relative humidity, it can be determined whether the specific mass concentration of ambient PM 2.5 achieved the NAAQS. Hence, the proposed secondary NAAQS specifies acceptable***E.O. 12866 Review

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Page 399 of 432 levels of ambient mas s concentration of PM 2.5 . The combination of indicator, averaging time, form, and level of the proposed NAAQS is designed to provide the appropriate degree of protection from visibility impairment caused by ambient levels of PM 2.5 . It does this by calculat ing the light extinction associated with ambient concentrations of PM 2.5 and specifying the level of acceptable PM 2.5 mass concentration in terms of this calculation. However this does not change the fact that the standard is for the air pollutant PM 2.5 , a nd defines acceptable ambient levels of this air pollutant. It does not transform the standard into a “visibility standard” and not a standard for PM 2.5 . While the commenters had additional concerns over the use of relative humidity in the calculation, and the variation around the country of acceptable mass concentrations, those issues are separate and do not change the fact that the proposed standard defined in terms of calculated PM 2.5 light extinction is based on measurement of PM 2.5 concentration in the ambient air, and is a NAAQS for PM 2.5 . With regard to the contention that section 109(b) limits the EPA to setting a standard that is based on the concentration of the pollutant in the ambient air, we note that the term “concentration” typically means so me measure of relative content. For example, this would include relative measures such as mass per unit of volume or parts per million. The EPA has often used such metrics to define the NAAQS, largely because the scientific evidence of health or welfare ef fects supporting the NAAQS typically use such metrics in air pollution studies. For example, the current secondary standards for PM are defined in terms of the concentration of PM 2.5 and PM 10 in the ambient air, measured as the dried mass of the particulat e matter per unit of air. However section 109(b) does not require that a NAAQS be defined this way.***E.O. 12866 Review

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Page 400 of 432 Sections 109(a) and (b) both use the general term “air quality” when discussing the EPA’s obligation to set NAAQS. The NAAQS are clearly national ambient “ air quality” standards under section 109(b), which specifies that the primary NAAQS “shall be ambient air quality standards” and the secondary NAAQS “shall specify a level of air quality.” Both the primary and secondary NAAQS are to be based on the “air qu ality criteria,” which are to accurately reflect the latest scientific knowledge on the effects on public health and welfare associated with “the presence of such air pollutant in the ambient air, in varying quantities.” Section 109(b), 108(a)(2). Congress spoke in broad terms, tasking the EPA with assessing the latest scientific knowledge about the public health and welfare associated with the presence of the pollutant in the air, without limiting this to consideration of only those effects associated with one or more measures of concentration of the air pollutant. Congress referred to any and all effects associated with the presence of the pollutant in the ambient air, not just the effects associated with the concentration of the pollutant in the ambient a ir. Based on this knowledge, the EPA is required to set standards for the quality of the air that will provide the appropriate degree of protection from these health and welfare effects, without limitation on how to measure or define air quality. While con centration in the air has typically been an appropriate way to set a standard to achieve these requirements, the more general terms used in section 108(a) and 109(b) do not limit the EPA to using concentration as the only way to measure air quality for pur poses of setting a NAAQS. The EPA is charged with setting air quality standards, and has the discretion under section 109(b) to choose the metric for defining air quality that is appropriate to address the health or welfare effect at issue. Congress did re fer to “concentration” in certain situations. In section 109(c)***E.O. 12866 Review

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Page 401 of 432 Congress required the EPA to set a primary NAAQS for NO 2 concentration over 3 hours. This addressed Congress’ concern over whether the then current NO 2 standard, which used concentration as a metric, provided adequate protection. Congress also called on CASAC to advise the Administrator on the relative contribution to “air pollution concentrations” of natural and anthropogenic sources, under section 109(d)(2)(C)(iii). This information is in add ition to the advice CASAC is required to provide concerning appropriate revisions to the “air quality criteria” and to the NAAQS under section 109(d)(2)(B). 163 While these provisions refer to ambient concentrations of pollutants, this reflects the EPA’s stan dard practice to date in setting NAAQS, and none of them change or limit the range of discretion provided under section 109(b) in setting NAAQS. They do not change the fact that the EPA is to set “air quality” standards, and is not limited to “air
concentr ation” standards. The reference in the legislative history to a maximum permissible ambient air level for the pollutant also does not limit the EPA to a level of air pollutant concentration, as compared to a different metric for specifying the level of air quality, if that is judged to be appropriate. The text of sections 108 and 109 does not support the limited interpretation commenters suggest. Instead these provisions provide the EPA with significant discretion in determining the metric for air quality that is appropriate to achieve the required degree of protection of public welfare. The commenters’ interpretation would improperly limit this discretion, interfering with achieving the goals of section 109(b).
163 In a provision that is not part of the CAA, in 1990 Congress required EPA to request a report from the National Academy of Sciences on the role of secondary nati onal ambient air quality standards, including information on the “effects on welfare and the environment which are caused by ambient concentrations of pollutants” listed under section 108, and the “ambient concentrations of each such pollutant which would be adequate to protect welfare and the environment from such effects.” Section 817(a) of the CAA Amendments of 1990, P.L. 101

- 549.***E.O. 12866 Review

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Page 402 of 432 For example, in this review the EPA considere d whether it would be appropriate to base a secondary NAAQS on direct measurement of the light extinction caused by PM 2.5 . See 77 FR 38890, 38980

- 1 (June 29, 2012). There are several instrumental methods that directly measure PM 2.5 light extinction

– the a mount of light extinction caused by the presence of PM 2.5 in the ambient air. This is not a measure of the concentration of PM 2.5 in the air, but a measure of the light extinction caused by PM 2.5 . This is clearly an effect associated with the presence of P M 2.5 in the ambient air, and this atmospheric property is directly related to visibility effects. Unlike PM 2.5 mass concentration, there is a close scientific relationship between directly measured PM 2.5 light extinction and visibility effects. It would ap pear straightforward to say that PM 2.5 light extinction is a quality of the ambient air, and a secondary NAAQS that specified an acceptable level of PM 2.5 based on directly measured PM 2.5 light extinction would be an “ambient air quality standard” for the air pollutant that specifies a “level of air quality” designed to provide protection against visibility impairment. Unlike directly measured PM 2.5 light extinction, the mass concentration of PM 2.5 does not have the same direct relationship to light extinct ion, and specifying an acceptable level of mass concentration of PM 2.5 would be a more indirect and less effective way to provide protection from visibility impairment caused by the presence of PM 2.5 in the ambient air. Under the commenters’ interpretation, the EPA would be precluded from specifying a level of air quality in terms of directly measured PM 2.5 light extinction, the more scientifically appropriate and direct measure of the effect PM 2.5 ha s on visibility. Instead the EPA would be limited to the more indirect and less effective specification of a level of concentration of PM 2.5 .***E.O. 12866 Review

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Page 403 of 432 The commenters also objected to the inclusion of relative humidity as an adjustment factor in the calculation of P M 2.5 light extinction. Contrary to the claims of these commenters, the use of calculated PM 2.5 light extinction does not regulate relative humidity. The proposed secondary standard would define acceptable levels of ambient PM 2.5 , not acceptable levels of r elative humidity. In addition, section 108 explicitly requires that the air quality criteria include information on the atmospheric conditions that can alter the effects of the air pollutant on public health or welfare, and relative humidity certainly has this kind of impact. Section 109(b) requires that the standard be based on the air quality criteria, indicating that this information can and should be taken into account in setting the standard. Including relative humidity as an adjustment factor in the c alculation of PM 2.5 light extinction is a reasonable and straightforward way to use the scientific information in the air quality criteria in establishing a standard to provide protection from visibility impairment. 164 Some commenters pointed to the EPA’s p osition in the last review, stating that the EPA properly treated relative humidity as a confounding factor, and in this review improperly moves away from that position. See 77 FR at 38979, 71 FR 61144, 61205 (October 17, 2006). In the last review the EPA considered a distinct PM 2.5 mass - based secondary standard. In that context, limiting the measurement of PM 2.5 mass concentration to the mid

- day hours when relative humidity had the least impact would promote the correlation between measured PM 2.5 mass conc entration and light extinction, which would promote achievement of a relatively consistent degree of visibility protection across the country. However in this rulemaking the proposed calculated PM 2.5

164 UARG recognizes these provisions, but argues, as above, that this is limited by the requirement that EPA set a NAAQS based solely on ambient concentration.***E.O. 12866 Review

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Page 404 of 432 light extinction standard achieves a consistent degree o f visibility protection by directly accounting for humidity, in a scientifically defensible manner. The goal has not changed

– achieving the desired degree of protection across the country. What has changed is that calculated PM 2.5 light extinction is a mo re direct and scientifically appropriate way to achieve that result. Finally, it should be made clear that water is not a separate compound from PM 2.5 that confounds the impact PM 2.5 has on light extinction. As described in the Integrated Science Assessmen t, “P M is the generic term for a broad class of chemically and physically diverse substances that exist as discrete particles (liquid droplets or solids) over a wide range of sizes” (U.S. EPA, 2009a, p. 1

- 4). “ Particles composed of water soluble inorganic salts (i.e., ammoniated sulfate, ammonium nitrate, sodium chloride, etc.) are hygroscopic in that they absorb water as a function of relative humidity to form a liquid solution droplet. Aside from the chemical consequences of this water growth, the droplet s become larger when relative humidity increases, resulting in increased light scattering. Hence, the same PM dry concentration produces more haze” ( U.S. EPA, 2009a, p. 9

- 6). Thus water is not a compound that is separate and apart from the particle that ac ts as an extraneous confounding factor. 165 The effect of relative humidity occurs after the water becomes part of the particle. Certain water soluble salts absorb water and the resulting particle is larger in size and scatters more light, increasing the visi bility impact of the particle. But the particle is still a PM 2.5 particle. The fact that the PM NAAQS traditionally uses a measurement of the dried mass of the particles as the metric
165 According to the Integrated Science Assessment, “ Confounding is ‘...a confusion of effects. Specifically, the apparent effect of the exposure of interest is distorted because the effect of an extraneous factor is mistaken for or mixed with the actual ex posure effect (which may be null)’ (Rothman and Greenland, 1998, 086599)” (U.S. EPA, 2009a, p. 1

- 16).***E.O. 12866 Review

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Page 405 of 432 for the standard does not change the fact that the particles in the air include liquid droplets and particles that have increased in size because of absorption of water. These ambient PM 2.5 particles are what is in the air and impacting visibility, not just the dried mass of PM 2.5 that is measured in the laboratory and is curr ently used as the indicator for the PM NAAQS. Thus the commenters improperly claimed that the proposed secondary standard regulates water or relative humidity, and not PM 2.5 , when in fact the proposed secondary standard accounts in a scientific manner for the fact that some PM 2.5 particles are larger in size and have a greater impact on light extinction when the relative humidity increases. The commenters also raised concerns that a standard based on calculated PM 2.5 light extinction, compared to a standar d based on just PM 2.5 mass concentration, improperly results in variable levels of acceptable PM 2.5 mass concentrations across the country. This stems from the adjustments in the calculation for speciated components of PM 2.5 and relative humidity. Accordin g to commenters, this is improper as section 109(b) requires that the NAAQS set a single, specified ambient concentration that is nationally uniform across the country. As discussed above, the text of section 109(b) does not specify this limitation of a si ngle national acceptable concentration. Instead the secondary NAAQS is to specify a level of air quality that achieves the appropriate degree of protection. The proposed secondary standard would do just that

– specify a level of air quality, defined in ter ms of calculated PM 2.5 light extinction, that would achieve the desired degree of protection. The fact that this results in varying allowable levels of PM 2.5 mass concentrations is not inconsistent with the Act. The D.C. Circuit recently approved such a re sult. In the last***E.O. 12866 Review

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Page 406 of 432 review of the PM 10 primary NAAQS, the court approved the EPA’s choice of an indicator that was designed to allow varying levels of acceptable coarse PM. The court stated that: The industry petitioners next argue that the 150 μg/m 3 standar d for PM 10 will result in arbitrarily varying levels of coarse PM, and that the agency should instead have used a PM 10

- 2.5 indicator. The EPA does not dispute that using the PM 10 indicator will result in coarse PM levels that vary within the limit of 150 μ g/m 3 . As the EPA explains: "Because the PM 10 indicator includes both coarse PM (PM 10

- 2.5 ) and fine PM (PM 2.5 ), the concentration of PM 10

- 2.5 allowed by a PM 10 standard set at a single level declines as the concentration of PM 2.5 increases. Thus, the level of coarse particles allowed varies depending on the level of fine particles present." Id. at 61,195

. Although the EPA acknowledges that a PM 10 indicator will result in varying coarse PM levels, it does not agree that the variance will be a rbitrary. The EPA agrees with the industry petitioners that protection from coarse particles should be targeted at urban areas, where coarse particles have been shown to pose the greatest danger. Id. at 61,194 . But the agency argues that targeting of urban areas is effectively accomplished by using an indicator that permits the varying levels that the industry petitioners challenge. ... Id. at 61,195

- 96 (citations omitted). In other words: "The varying levels of coarse particles allowed by a PM 10 indicator wi ll therefore target protection in urban and industrial areas where the evidence of adverse health effects associated with exposure to coarse particles is strongest." Id

. The EPA also offers a further rationale for tying the stringency of coarse PM regulati on to increases in the level of PM 2.5 . ... EPA argues that it is "logical to allow lower levels of coarse particles when fine particle concentrations are high. . . . [I]nclusion of PM 2.5 in the PM 10 indicator for purposes of coarse particle protection would appropriately reflect the contribution that contaminants emitted in fine particle form can make to the overall health risk posed by coarse particles." Id

. In sum, we find that the EPA has provided a reasonable explanation for its decision[ ] ... to utilize a standard that allows targeted variance in coarse PM levels in an inverse relationship to the amount of fine PM in the air. American Farm Bureau v. EPA , 559 F.3d 512, 534

- 5 (D.C. Cir. 2009). A similar result applies here. Under the proposed secondary sta ndard there would be a single level of air quality specified for the NAAQS. The standard would apply across the nation; it would not be a regional standard. The proposed standard would be the same***E.O. 12866 Review

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Page 407 of 432 standard everywhere

— the acceptable level of mass concentrat ion of PM 2.5 would be defined the same way across the nation, using the same method of calculating the allowable concentration of PM 2.5 . The same degree of protection from visibility impairment would apply across the country. While the allowable amount of PM 2.5 could vary, this would be a reasoned way to achieve the desired degree of protection from visibility impairment. The requirements of section 109(b) would be satisfied. Commenters also objected that the EPA could not set a NAAQS for the separate components of PM 2.5 without listing the components of PM 2.5 under section 108, based on an endangerment finding, and issuing air quality criteria for these components. They argued that the issuance of air quality criteria for particulate matter starting in 1969 did not provide a lawful basis for a proposed secondary standard that is based on components of PM, as the 1969 air quality was for particulate matter “as a whole,” defining PM as particles smaller than 500 micrometers (NMA/NCBA, p. 27). However, as discussed above, the proposed standard sets the allowable limit on ambient concentrations of PM 2.5 . Information on both the speciated components of PM 2.5 and the relative humidity affect how much light extinction is associated with any specific level of PM 2.5 , but the standard is for PM 2.5 . The D.C. Circuit has made it clear that PM 2.5 , just like PM 10 and TSP before that, is an appropriate subset of PM for the EPA to focus on in setting the NAAQS based on the scientific evidence before the EPA. This focus o f the NAAQS does not make the subset a new pollutant that requires listing and new air quality criteria under section 108 before setting a NAAQS. American Trucking Association et al. v. EPA , 175 F.3d 1027, 1055 (D.C. Cir. 1999). Commenters’ interpretation would apply to PM 2.5 as well as to components of PM 2.5 , and is inconsistent with the ATA decision. In addition, it is clear***E.O. 12866 Review

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Page 408 of 432 that the current air quality criteria do address the scientific basis for calculating PM 2.5 light extinction as the EPA proposed (U. S. EPA, 2009a, pp. 9

- 5 to 9

- 8). Finally, at least one commenter argued that the EPA has concluded in prior reviews that the secondary NAAQS program is an inappropriate vehicle for regulating PM related visibility impairment (UARG, p. 26). UARG mischaracte rized the EPA’s past decision

- making. In past reviews the EPA has been clear that the EPA should take into account the existence of the visibility program under section 169A, the regional haze program, when considering a secondary NAAQS and should not trea t the secondary NAAQS as the sole mechanism to address visibility impairment across the country. That is the approach the EPA has taken in this and prior reviews. See 77 FR at 38990. h. Relationship with Regional Haze Program A large number of commenters expressed confusion and concern over differences between the proposed visibility index standard and the Regional Haze Program. This included commenters who supported setting a distinct secondary standard to protect visibility as well as those opposed to se tting such a standard. A number of these commenters noted that visibility impairment would be assessed differently under the two approaches due to differences in the way light extinction is calculated, including different IMPROVE equations and differences in the inclusion and weighting of specific species and components. The commenters argued it would be inappropriate to have two different regimes for managing visibility impairment in the exact same location. These commenters claimed that since data from th e IMPROVE monitoring network would inform nonattainment designations, as well as an area’s obligations under the Regional Haze Program, there could be considerable confusion over how to draw nonattainment***E.O. 12866 Review

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Page 409 of 432 boundaries and what requirements would affect large sources in rural areas. These commenters also noted the resource burden associated with maintaining two different programs aimed at protecting visibility in the same geographic area. Some commenters argued that a visibility NAAQS should not apply to rural areas. The Department of the Interior requested that the EPA clearly define the geographic area to which the visibility index standard would be applicable, and suggested that Class I and Class II areas should generally be excluded from the standard. As di scussed above, commenters questioned the need for a distinct visibility standard, arguing that the existing primary PM standards combined with the Regional Haze Program ensured adequate protection of visibility, even in urban areas. In response to these co mments relating to the overlap between the Regional Haze program and a distinct secondary standard designed to protect visibility principally in urban areas, the EPA notes that the objectives of each program are distinct. While the Regional Haze program is designed to eliminate man

- made impairment of visibility in Federal Class I areas over the course of several decades, a distinct secondary standard for PM

- related visibility impairment would be focused on providing a nationally applicable level of protecti on for all areas, particularly urban areas which do not receive targeted protection under the Regional Haze Program. Moreover, the metric used to assess visibility impairment differs between the two programs precisely because each program is aimed at a dif

ferent aspect of the problem. Recognizing the importance of fresh emissions for urban visibility, the Visibility Assessment focused on visibility impairment as measured by the original IMPROVE equation because “the original version is considered more repre sentative of urban situations when emissions are still fresh rather***E.O. 12866 Review

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Page 410 of 432 than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3

- 19). The Regional Haze Program, on the other hand, has shifted to a revised IMPROVE algorithm more suited to remote locations. While this difference is discussed in more detail in section VI.C.1.b above, the result is that each program would appropriately measure those aspects of visibility impairment most closely related to the problem the program is trying to prevent. Since the same data can be used to calculate both visibility impairment under the Regional Haze approach and the proposed visibility index, the additional calculation burden for state and local agencies would be light. Also, to the extent that there is any differenc e in terms of the emissions control obligations the two different programs would impose upon state and local areas, this is likely appropriate given the extent and nature of visibility impairment in those areas. The EPA notes that in general, there is like ly to be substantial overlap in the control strategies a state or local area would pursue under either program. Thus, the EPA disagrees with commenters who stated that a distinct visibility standard as proposed would inherently conflict with the Regional H aze Program or that it would be appropriate to draw geographical distinctions that would explicitly exclude some areas (e.g., Class I areas) from the NAAQS. The EPA notes that the CAA requires that NAAQS be national in scope, and that the specific requirem ents laid out in the proposal for the distinct secondary standard would ensure that the protection it afforded would be appropriately targeted toward urban areas so that it could work in conjunction with

— not be in conflict with

— the Regional Haze Program un der sections 169A and 169B of the CAA. 2. Comments on the Proposed Decision Regarding Non

- Visibility Welfare Effects Relatively few commenters addressed the proposal to retain the existing suite of***E.O. 12866 Review

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Page 411 of 432 secondary PM standards to address non

- visibility welfare e ffects. A couple of states, including Mississippi and South Dakota, offered brief endorsements of the proposal. A few other commenters offered more extensive comments on the proposal to retain the existing secondary standards, and these commenters opposed this aspect of the proposal for one of two reasons. First, some commenters opposed the proposal to retain the current secondary annual PM 2.5 standard of 15 μg/m 3 in light of the proposal to revise the level of the primary annual PM 2.5 standard to a level between 12

- 13 μg/m 3 . Expressing concern over the implications of this decision for the air quality planning obligations of states, these commenters argued that the EPA should revise the secondary PM 2.5 standards to be equivalent in all respects to the primary PM 2.5 standards. For example, the American Association of State Highway and Transportation Officials (AASHTO) supported “retaining secondary standards that are consistent with the primary standards in order to reduce the complexity of the transportation and air quality planning processes, as well as the transportation conformity process” (AASHTO, p. 3). Thus, if the EPA were to adopt a lower level for the primary annual PM 2.5 standard, the commenters recommended that the EPA adopt t

his same lower level for the primary secondary PM 2.5 standard as well. In response to these comments, the EPA notes that the Agency lacks an appropriate scientific basis for revising the level of the secondary annual PM 2.5 standard. As noted above in sect

ion VI.B.2, there is an absence of information that would support any different secondary standards for PM. Comments related to the implementation challenges associated with distinct primary and secondary standards are not relevant to the Administrator’s f

inal decisions regarding what standards are requisite to protect the public welfare. Therefore, the EPA continues to conclude that it would be appropriate to***E.O. 12866 Review

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Page 412 of 432 retain the current suite of secondary PM standards 166 to address non

- visibility welfare effects, whi le revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard, as proposed. Other commenters focused on the impacts of particulate matter on clim ate. One commenter cited a number of recent studies that considered mobile source black carbon emissions and associated climate impacts, and urged the EPA to protect the public welfare by setting “higher standards for gasoline quality” (Urban Air Initiativ e, p. 4). This commenter did not, however, advocate specific secondary NAAQS to address climate impacts of PM. More extensive comments on this same subject were provided by the Center for Biological Diversity (CBD), which urged the EPA to “set a separate l imit for black carbon within the overall PM 2.5 standard” to ensure that public welfare is fully protected “from the serious climate impacts of black carbon” (CBD, p. 2). This commenter argued that “[p]recaution is required for secondary NAAQS,” citing Amer ican Trucking Associations, Inc. v. EPA , 283 F.3d 355, 369 (D.C. Cir. 2002): [N]othing in the Clean Air Act requires EPA to wait until it has perfect information before adopting a protective secondary NAAQS. Rather, the Act mandates promulgation of seconda ry standards requisite to protect public welfare from any “ anticipated adverse effects associated with” regulated pollutants, 42 U.S.C. § 7409(b)(2) (emphasis added), suggesting that EPA must act as soon as it has enough information (even if crude) to “ant icipate[]” such effects[.] The commenter stressed the growing scientific evidence regarding the impacts of black carbon on climate, and argued that the EPA’s proposal ignores important research studies published within the last five years which provide imp roved estimates of the radiative forcing associated with black carbon, and the effects of black carbon on snow and ice, the

166 As summarized in section VI.A and Table 1 above, the current suite of secondary PM standards includes annual and 24

- hour PM 2.5 standards and a 24

- hour PM 10 standard.***E.O. 12866 Review

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Page 413 of 432 Arctic climate, water availability and climate “tipping points.” The commenter also noted that reductions in cooling aerosol species , particularly sulfate, due to pollution control programs are leading to an “unmasking” of the true extent of warming due to the accumulation of greenhouse gases in the atmosphere. The commenter argued that this unmasking effect can be offset by ensuring “ that sufficient black carbon reductions accompany reductions in overall aerosol pollution” (CBD, p. 10). The commenter also argued that the EPA did not consider the negative impacts of climate change on public health adequately in the proposal. The comment er stated that the EPA had an obligation to address the impacts of black carbon in the PM NAAQS, despite the remaining uncertainties. The commenter pointed to the U.S. EPA’s report to Congress on Black Carbon (U.S. EPA, 2012c), stating that the “report sho ws that EPA is aware of the climate science and public health information that point to the importance of addressing black carbon pollution. EPA must use this information in its relevant decisionmaking” (CBD, p. 13). The commenter also noted that the U

. S

. participates in a number of international forums that have recognized the need to take action on black carbon, and argued that the U.S. has “an obligation under the Gothenburg Protocol to address black carbon pollution.” The commenter challenged the uncert ainties cited by EPA with regard to the climate impacts of aerosols generally, arguing that they “do not apply to the regulation of black carbon” (CBD, p. 14). Specifically, the commenter stated that “there are significant anthropogenic sources of black ca rbon that contribute a large proportion of total black carbon emissions”; that “there is enough information related to black carbon’s impact to know that global temperatures will rise due to black carbon emissions”; that spatial and temporal***E.O. 12866 Review

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Page 414 of 432 heterogeneity in black carbon emissions do not matter for estimating likely climate effects; that “[b]lack carbon’s negative climate impacts do not depend upon details of cloud interactions with aerosols”; and that the EPA does not need to be able to quantify the health or climate benefits precisely to know that it is appropriate to control black carbon as a specific component of PM under the CAA (CBD, pp. 14

- 15). As a result, the commenter concluded that the current size

- based PM mass standard “is insufficient to fully protect health and welfare,” and that the EPA was obligated to establish a specific limit on black carbon as a component of PM. The commenter argued that “Black carbon must be regulated separately and in addition to PM 2.5 because absent separate standards sulfates and nitrates may be more likely to be mitigated than the black carbon component of PM” (CBD, p. 17). To support this point, the commenter cited the conclusion in the Policy Assessment that: The current standards that are defined in terms of aggre gate size mass cannot be expected to appropriately target controls on components of fine and coarse particles that are related to climate forcing effects. Thus, the current mass

- based PM 2.5 and PM 10 secondary standards are not an appropriate or effective means of focusing protection against PM

- associated climate effects due to these differences in components. (U.S. EPA, 2011a, p. 5

- 11) The commenter also noted that existing regulations on diesel engi nes, which are the largest source of black carbon in the United States, do not affect existing engines and vehicles, and stated that “The NAAQS program is one of the few opportunities to reduce black carbon from existing engines, industrial and biofuel sou rces within the United States and rapidly reduce emissions from this pollutant” (CBD, p. 18). The EPA agrees with the commenters’ assertion that the scientific information about the impacts of aerosol species on climate is developing rapidly, and that unde rstanding of the magnitude of aerosol effects on climate and the contribution of***E.O. 12866 Review

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Page 415 of 432 individual aerosol components to those effects has improved substantially over the past decade. The EPA also agrees that certain species, in particular black carbon, play a si gnificant role in multiple aspects of climate. The Policy Assessment recognized that “Aerosols can impact glaciers, snowpack, regional water supplies, precipitation and climate patterns,” and may contribute to the melting of ice and snow, a decrease in

sur face albedo, and climate impacts in the Arctic and other locations (U.S. EPA, 2011a, p. 5

- 9). The contribution of black carbon to these effects is discussed in detail in the EPA’s recent Report to Congress on Black Carbon (U.S. EPA, 2012c). In particular, black carbon plays an important role in heating the lower atmosphere by absorbing incoming solar radiation and outgoing terrestrial radiation, i.e. via “direct” radiative forcing. However, the EPA disagrees that there is sufficient information available at this time to establish a NAAQS to protect against the climate impacts associated with current ambient concentrations of black carbon or other PM constituents. While the Integrated Science Assessment concluded that “a causal relationship exists between PM and effects on climate, including both direct effects on radiative forcing and indirect effects that involve cloud feedbacks that influence precipitation formation and cloud lifetime” (U.S. EPA, 2009a, section 9.3.10), it also identified substantial remain ing uncertainties with regard to the contribution of individual aerosol species to these climate effects. The contribution of individual aerosol components to total aerosol direct radiative forcing is more uncertain than the global average (U.S. EPA, 2009a , section 9.3.6.6), and the indirect effects of aerosols and aerosol components remain highly uncertain, in particular with regard to their complex interactions with clouds.***E.O. 12866 Review

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Page 416 of 432 With regard to black carbon, for example, the EPA disagrees with CBD’s claims that “black carbon’s negative climate impacts do not depend upon details of cloud interactions with aerosols” and that the uncertainties associated with climate impacts of aerosols generally do not apply to black carbon. In fact, the EPA has pointed to cloud i nteractions as the area of greatest uncertainty with regard to black carbon: recognizing that black carbon affects cloud reflectivity (albedo), lifetime, and stability as well as precipitation, the Report to Congress on Black Carbon noted that “few quantit ative estimates of these effects are available, and significant uncertainty remains. Due to all of the remaining gaps in scientific knowledge, it is difficult to place quantitative bounds on the forcing attributable to [black carbon] impacts on clouds at p resent” (U.S. EPA, 2012c, p. 4). The Report acknowledged that “most estimates of the forcing from aerosol indirect effects are based on all aerosol species (e.g. total PM) and are not estimated for individual species (e.g, BC alone)” (U.S. EPA, 2012c, p. 4 0). The Report concluded that it remains unclear the extent to which black carbon contributes to the overall aerosol indirect effect, and did not assign any central estimate or even a range of possible values to the role of black carbon in the overall aero sol indirect effect. With regard to black carbon’s net contribution to climate, therefore, the Report concluded: The direct and snow/ice albedo effects of BC are widely understood to lead to climate warming. However, the globally averaged net climate effec t of BC also includes the effects associated with cloud interactions, which are not well quantified and may cause either warming or cooling. Therefore, though most estimates indicate that BC has a net warming influence, a net cooling effect cannot be ruled out. It is also important to note that the net radiative effect of all aerosols combined (including sulfates, nitrates, BC and OC) is widely understood to be negative (cooling) on a global average basis. (U.S. EPA, 2012c, p. 3) Given the remaining uncertainties about the impact of aerosols on climate, there is even greater uncertainty with regard to how aerosol

- induced climate change will affect public***E.O. 12866 Review

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Page 417 of 432 health. At this time, it is not possible to estimate the extent to which aerosols in general, let alone particular aerosol components, contribute to the occurrence or exacerbation of adverse health outcomes due to climate change. The EPA therefore disagrees with CBD’s claim that the EPA should pursue black carbon reductions for purposes of reducing the impacts of climate change on public health. The Report to Congress on Black Carbon also stressed the importance of considering co

- emitted PM species, such as SO 2 and NO x , in evaluating the benefits of black carbon mitigation options. Noting that many of t hese co

- emitted particles and gases have a cooling influence on climate, the Report noted the difficulty of estimating the net effect of various mitigation measures on net radiative forcing or other climate variables. The EPA concluded that the location an d timing of emissions reductions would be critically important for achieving climate benefits, and that “more research is needed on the benefits of individual control measures in specific locations to support policy decisions made at the national level” (U .S. EPA, 2012c, p. 140). Thus, the EPA disagrees with CBD’s claim that spatial and temporal heterogeneity in black carbon emissions do not matter for estimating likely climate effects, and continues to believe that being able to quantify the climate impact s of various aerosol species, alone and in combination, is essential for informing any possible revisions to the current secondary PM standards based on climate. Furthermore, while the EPA agrees with the commenter that a large percentage of black carbon e missions come from anthropogenic sources, including diesel engines and vehicles, the EPA notes that existing regulations on mobile diesel engines are already reducing these emissions substantially. Between 1990 and 2005, new engine***E.O. 12866 Review

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Page 418 of 432 requirements resulted in a 32 percent reduction in black carbon emissions from mobile sources, and a further 86 percent reduction from 2005 levels is projected to occur by 2030 as vehicles and engines meeting existing regulations are phased into the fleet (U.S. EPA, 2012c, p. 175 ). Long

- term historic data indicate that there has been a dramatic overall decline in black carbon emissions over the past century, due to changes in fuel use, more efficient combustion practices, and implementation of PM controls. Therefore, the EPA disag rees with CBD’s claim that a distinct black carbon NAAQS is necessary to achieve reductions in black carbon emissions. Clearly, U.S. emissions of black carbon are already declining substantially, suggesting that the existing mass

- based PM standards, though not targeting black carbon specifically, have been effective in achieving black carbon emissions reductions in practice. As acknowledged in the Report to Congress on Black Carbon , “While [black carbon] is not the direct target of existing programs, it has been reduced through controls aimed at reducing ambient PM 2.5 concentrations and/or direct particle emissions” (U.S. EPA, 2012c, p. 161). The EPA has acknowledged the need to encourage PM mitigation strategies that focus on reducing directly emitted PM 2.5 for purposes of reducing black carbon, and this is reflected in U.S. commitments under the Gothenburg Protocol: the new provisions in the Protocol pertaining to PM encourage parties to develop national inventories and projections for black carbon, and to “give priority” to black carbon when implementing measures to control PM. However, the EPA notes that the U.S. has not yet ratified the PM amendments to the Gothenburg Protocol, and furthermore, these amendments do not require action specifically to reduce black carbon, but rather encourage countries to take such actions voluntarily within the context of their broader PM reduction strategies. Thus the EPA disagrees with the commenter***E.O. 12866 Review

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Page 419 of 432 that the U.S. has an “obligation” to reduce black carbon under the Gothenb urg Protocol, or that it has “agree[d] to choose mitigation options for particulate matter that focus on black carbon reductions” under the Protocol (CBD, p. 13). In sum, the EPA notes the substantial remaining the uncertainties and gaps with regard to the climate impacts of PM components, including black carbon. These include the uncertainties associated with the spatial and temporal heterogeneity of PM components that contribute to climate forcing; the uncertainties associated with measurement of aerosol components; the inadequate consideration of aerosol impacts in climate modeling; and the currently insufficient data on local and regional microclimate variations and the heterogeneity of cloud formations. As a result, the EPA continues to conclude that it is not currently feasible to conduct a quantitative analysis for the purpose of informing revisions of the current secondary PM standards based on climate, and that there is insufficient information at this time to base a national ambient standard on clim ate impacts associated with current ambient concentrations of PM or any of its constituents. 167
D. Conclusions on Secondary PM Standards This section describes the Administrator’s conclusions regarding the secondary PM standards and the rationale leading to th e Administrator’s final decision to retain the current suite of secondary PM standards , including an annual PM 2.5 standard of 15 μg/m 3
a 24

- hour PM 2.5 standard of 35 μg/m 3 , and a 24

- hour PM 10 standard of 150 μg/m 3 , to address PM

- related visibility impairment as well as other PM

- related welfare effects, including ecological effects, effects on materials, and climate impacts. Specifically, this

167 This conclusion applies for both the secondary (welfare

- based) and the primary (health

- based) standards.***E.O. 12866 Review

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Page 420 of 432 section explains the Administrator’s decision, consistent with the proposal, to retain the

curren t suite of secondary PM standards generally, while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. It also explains the Administrator ’s decision, contrary to what was proposed, not to establish a distinct standard to address PM

- related visibility impairment. In reaching conclusions regarding the need to revise the secondary PM standards for both visibility and non

- visibility welfare ef fects, the Administrator has taken into account several key factors, including: (1) The latest scientific information on both visibility and non

- visibility welfare effects associated with PM, as previously described; 2) the advice of CASAC; and 3) the comm ents received during the public comment period, as discussed above. Based on this information, the Administrator has reached final conclusions about the secondary PM standards and made final decisions about those standards, as outlined below. Because the A dministrator’s final conclusions with regard to the need to establish a distinct secondary standard to protect against visibility impairment reflect, in part, her conclusions on secondary PM standard s for non

- visibility welfare effects, section VI.D.1 firs t outlines her conclusions regarding secondary PM standards to address non

- visibility welfare effects. This is followed by section VI.D.2 which outlines her conclusions regarding a secondary PM standard to address PM

- related visibility impairment . Finally, section VI.D.3 summarizes the Administrator’s final decisions with regard to the secondary PM standards for both visibility and non

- visibility welfare effects. 1. Conclusions Regarding Secondary PM Standards to Address Non

- Visibility Welfare***E.O. 12866 Review

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Page 421 of 432 Effects With reg ard to the secondary PM standards to address non

- visibility welfare effects, the Administrator concludes that it is generally appropriate to retain the existing secondary standards and that it is not appropriate to establish any distinct secondary PM stand ards to address non

- visibility PM

- related welfare effects. This conclusion is based on the considerations discussed above in section VI.B.2, including the latest scientific information and the advice of CASAC, and the public comments received on the
propos al, as discussed above in section VI.C.2. The Administrator concurs with the advice of CASAC and the conclusions expressed at the time of proposal that it is important to maintain an appropriate degree of control of both fine and coarse particles to addres s non - visibility welfare effects, including ecological effects, effects on materials, and climate impacts. In the absence of information that would support any different standards the Administrator concludes that it is appropriate to retain the existing su ite of secondary standards to address non

- visibility welfare effects, as proposed. More specifically, the Administrator concludes it is appropriate to retain all aspects of the current 24

- hour PM 2.5 and PM 10 standards. With regard to the secondary annual P M 2.5 standard, the Administrator concludes that it is appropriate to retain a level of 15.0 μg/m 3 for this standard while revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change t o the primary annual PM 2.5 standard. In reaching this conclusion, the Administrator notes that no areas in the country are currently using the option for spatial averaging to demonstrate attainment with the secondary annual PM 2.5 standard. 2. Conclusions Reg arding Secondary PM Standards for Visibility Protection***E.O. 12866 Review

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Page 422 of 432 Having reached the conclusion that it is generally appropriate to retain the existing secondary standards to protect against non

- visibility welfare effects, the Administrator next considered the targe t level of protection that would be requisite to protect public welfare with regard to visual air quality. The Administrator then determined whether to adopt a distinct secondary standard to achieve this target level of protection. In making this decision, the Administrator compared the degree of protection for visibility that would be provided by such a distinct secondary standard to the degree of protection provided by the existing secondary standards, focusing specifically on the secondary 24

- hour PM 2.5 standard of 35 μg/m 3

. 168 Based on the considerations discussed above in section VI.B and VI.C, the Administrator first concludes that a target level of protection for a secondary standard is most appropriately defined in terms of a PM 2.5 visibility index as proposed, since it would provide a measure of PM

- related light extinction that directly takes into account the factors (i.e., species composition and relative humidity) that influence the relationship between PM 2.5 in the ambient air and PM

- related visibil ity impairment. Such a PM 2.5 visibility index standard would afford a relatively high degree of uniformity of visual air quality protection in areas across the country by virtue of directly incorporating the effects of differences in PM 2.5 composition and relative humidity across the country. In defining a target level of protection based on a PM 2.5 visibility index, the Administrator has considered specific aspects of such an index, including the appropriate indicator, averaging time, level, and form. Fir st, with regard to indicator, the

168 This focus on the 24

- hour PM 2.5 standard reflects the Administrator’s judgments that PM

- related visi bility impairment is principally related to fine particle concentrations and that perception of visibility impairment is most directly related to short

- term levels of visual air quality.***E.O. 12866 Review

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Page 423 of 432 Administrator notes the conclusion of CASAC that relying on a calculated PM 2.5 light extinction indicator based on PM 2.5 chemical speciation and relative humidity data represented a reasonable approach . Based on the analyses conducted in support of this rulemaking, as described above, as well as the advice of CASAC, the Administrator concludes that a calculated PM 2.5 light extinction indicator that utilizes the original IMPROVE algorithm , adjusted to us e a 1.6 OC multiplier and exclude the term for coarse particles, in conjunction with monthly average relative humidity data (i.e., f(RH) values) based on long

- term climatological means would be the most appropriate indicator for a PM 2.5 visibility index st andard. With regard to averaging time, the Administrator notes that both CASAC and EPA staff have concluded that hourly or sub

- daily (4

- to 6

- hour) averaging times, within daylight hours and excluding hours with high relative humidity, are more directly r elated than a 24

- hour averaging time to the short

- term nature of the perception of PM

- related visibility impairment and the relevant exposure periods for segments of the viewing public. However, in light of the important data quality uncertainties that hav e recently been identified in association with currently available instruments that would be used to provide the hourly PM 2.5 mass measurements that would be needed in conjunction with an averaging time shorter than 24 hours, the Administrator concludes it would not be appropriate at this time to set a standard based on a sub

- daily averaging time. Moreover, the Administrator notes that analyses conducted by the EPA during this review clearly indicate that PM 2.5 light extinction calculated on a 24

- hour avera ge basis would be a reasonable and appropriate surrogate for PM 2.5 light extinction calculated on a 4

- hour basis. Thus, the Administrator concludes that a 24

- hour averaging time would be***E.O. 12866 Review

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Page 424 of 432 appropriate for a PM 2.5 visibility index. The Administrator recognize s that a 24

- hour averaging time would effectively reduce the influence of peak hours of visibility impairment on visibility index values, but concludes that in light of the concern that peak hourly measurements may be significantly influenced by atypical c onditions and/or atypical instrument performance, it is appropriate to adopt a longer averaging time to ensure that hour

- specific influences and uncertainties are balanced against more robust
measurements. With regard to form, the Administrator notes that consistent with the approach taken in other NAAQS, including the current 24

- hour PM 2.5 NAAQS, a multi

- year percentile form offers greater stability to the air quality management process by reducing the possibility that statistically unusual indicator valu es will lead to transient violations of the standard. Utilizing a three

- year average form provides stability from the occasional effects of inter

- annual meteorological variability that can result in unusually high pollution levels for a particular year. Mo reover, considering the lack of information on and the high degree of uncertainty regarding the impact on public welfare of the number of days with visibility impairment over the course of a year, the Administrator considers it reasonable to focus on the 9 0 th percentile, which represents the median of the distribution of the 20 percent worst visibility days, a key focus of the Regional Haze program. The Administrator concludes that ensuring that 90 percent of days have visual air quality that is at or below the target level of protection could be reasonably expected to lead to improvements in visual air quality on the 20 percent most impaired days, and that the limited information available in this review provides no basis for adopting a different form which would limit the occurrence of days with peak PM

- related light***E.O. 12866 Review

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Page 425 of 432 extinction in urban areas to a greater degree. Therefore, the Administrator concludes that a 90 th percentile form, averaged over 3 years, is appropriate, for purposes of establishing a target l evel of protection in terms of a 24

- hour PM 2.5 visibility index. With regard to level, the Administrator concludes that in light of the uncertainty associated with the high degree of variability in visibility conditions and the potential variability in vis ibility preferences across different parts of the country, it is appropriate to establish a target level of protection based on the upper end of the range of Candidate Protection Levels (CPLs) identified in the Policy Assessment (i.e., 20

- 30 dv) and
genera lly supported by CASAC. Thus, the Administrator concludes that it would be appropriate to set a target level of protection in terms of a PM 2.5 visibility index with a 24

- hour averaging time that would provide protection equivalent to the protection afforde d by a 4

- hour PM 2.5 visibility index with a level of 30 dv. Furthermore, the Administrator notes that the approaches used to estimat e generally equivalent levels for a 24

- hour PM 2.5 visibility index generated 90 th percentile 24

- hour values similar to the 4

- hour values and a range of city

- specific estimates of generally equivalent 24

- hour levels that encompassed the range of levels considered appropriate for 4

- hour CPLs, including the CPL of 30 dv at the upper end of that range. The Administrator thus conclu des that it would be appropriate to use an unadjusted 4

- hour CPL for purposes of establishing a target level of protection in terms of a 24

- hour PM 2.5 visibility index. In considering the alternative levels proposed for a 24

- hour standard, either 28 dv

or 30 dv, the Administrator concludes that the current substantial degrees of variability and uncertainty inherent in the public preference studies should be reflected in a higher target protection level than would be appropriate if the underlying informatio n were more***E.O. 12866 Review

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Page 426 of 432 consistent and certain. In addition, she concludes that, in light of the significant uncertainties, it is appropriate to place less weight on the results of western visibility preference studies and that the CPL value (30 dv) that is based on t he eastern preference study results is likely to be more representative of urban areas that do not have associated mountains or other valued objects visible in the distant background For all of these reasons, the Administrator concludes that it is appropri ate to set a target level of protection in terms of a 24

- hour PM 2.5 visibility index at 30 dv. In summary, in light of all the information available in this review, the Administrator concludes that the protection provided by a standard defined in terms of a PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction), a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, set at a level of 30 dv, would be requisit e to protect public welfare with regard to visual air quality. In reaching this conclusion, the Administrator notes that any national ambient air quality standard to address PM

- related visibility impairment would be designed to work in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM

- related visibility impairment in all areas of the country, including urban, non

- urban, and Federal Class I areas. Wh ile the Regional Haze Program is focused on improving visibility in Federal Class I areas and a secondary NAAQS to address PM

- related visibility impairment would focus on protecting visual air quality principally in urban areas, both programs could be expe cted to provide benefits in surrounding areas. In addition, the development of local programs, such as those in Denver and Phoenix, could continue to be an effective and appropriate approach to***E.O. 12866 Review

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Page 427 of 432 provide additional protection, beyond that afforded by a natio nal standard, for unique scenic resources in and around certain urban areas that are particularly highly valued by people living in those areas. Having concluded that the protection provided by a standard defined in terms of a PM 2.5 visibility index, with a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, set at a level of 30 dv, would be requisite to protect public welfare with regard to visual air quality, the Administrator next has to determine whether to adopt such a visibility index as a distinct secondary standard. This determination requires considering such a secondary standard not in isolation but in the context of the full suite of secondary standards. As discussed above, the Administrator has determined to retain the curr ent suite of secondary PM standards to address non

- visibility welfare effects (except for the form of the annual standard). A distinct secondary standard to address visibility impairment is properly considered in a context where there is also a 24

- hour PM 2 .5 standard of 35 μg/m 3

. In this context, the Administrator has considered the degree of protection from visibility impairment afforded by the existing secondary PM 2.5 standards. The Administrator has considered both whether the existing 24

- hour PM 2.5 st andard of 35 μg/m 3 is sufficient (i.e. not under

- protective) and whether it is not more stringent than necessary (i.e. not over

- protective). As discussed above in section VI.C.1. f , the results of the Kelly et al. (2012 a; 2012b ) analys es indicate that base d on 2008

- 2010 and 2009

- 2011 data, all areas meeting the 24

- hour PM 2.5 standard of 35 μg/m 3 had visual air quality at least as good as 30 dv (24

- hour average, based on 90 th percentile form averaged over 3 years). This means that***E.O. 12866 Review

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Page 428 of 432 it is highly likely that th e secondary 24

- hour PM 2.5 standard of 35 μg/m 3 would be controlling relative to a 24

- hour standard based on a PM 2.5 visibility index set at a level of 30 dv, and highly unlikely that areas would exceed the target level of protection for visibility of 30 dv without also exceeding the existing secondary 24

- hour standard. On the basis of this evidence, and the supporting public comments, the Administrator judges that the 24

- hour PM 2.5 standard of 35 μg/m 3 provides sufficient protection in all areas against the effects of visibility impairment

— i.e., that the existing 24

- hour PM 2.5 standard would provide at least the target level of protection for visual air quality of 30 dv which the Administrator judges appropriate. The Administrator also recognizes that the analyses presented in Kelly et al. (2012a; 2012b) indicate that the 24

- hour PM 2.5 standard of 35 μg/m 3 also would likely achieve more than the target level of protection of visual air quality (30 dv) in some areas. That is, when meeting a mass

- based standard of 35 μg/m 3 , some areas would have levels of PM

- related visibility impairment below 30 dv. Thus, the 24

- hour PM 2.5 standard of 35 μg/m 3 would be over

- protective in some areas (i.e. more stringent than necessary) relative to the target level of protection for visibility. This is not surprising, as the current

mass - based standard does not account for variation in particle species and relati ve humidity. The 24

- hour PM 2.5 standard of 35 μg/m 3 would provide more than the necessary protection in the areas where this would be expected, for example western areas with lower relative humidity. In light of the Administrator’s conclusion that it is a ppropriate to retain the
current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 for non

- visibility welfare effects, the Administrator notes that this standard will remain in place regardless of whether she***E.O. 12866 Review

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Page 429 of 432 elects to set a distinct secondary standard in terms of a PM 2.5 visibility index. The issue is not whether to adopt a PM 2.5 visibility index standard when viewed in isolation, but whether such a distinct secondary standard should be adopted in addition to the current secondary 24

- hour PM 2.5 standard of 35 μ g/m 3 . The EPA notes that adoption of such a distinct secondary standard is not needed to provide sufficient protection from visibility impairment with respect to the target level of protection determined above. In addition, adoption of such a distinct seco ndary standard would not change the fact that the current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 would result in over

- protection from visibility impairment in certain areas of the country. Such over

- protection will occur whether or not such a distinc t secondary standard is adopted. In effect, adopting such a distinct secondary standard would have no impact on the degree of protection provided from visibility impairment. Since sufficient protection from visibility impairment would be provided for all a

reas of the country without adoption of a distinct secondary standard, and adoption of a distinct secondary standard will not change the degree of over

protection provided for some areas of the country, the Administrator judges that adoption of such a dist inct secondary standard is not needed to provide requisite protection for both visibility and non

- visibility related welfare effects. It is important to note that this conclusion is based on the specific target level of protection determined above, and th e specific set of current secondary standards. The Administrator’s conclusion with regard to the sufficiency of the protection provided by the current suite of secondary standards is based on comparing the a 30 dv target level of protection for a PM 2.5 vis ibility index standard against the degree of protection provided by the current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 . It is the combination of the***E.O. 12866 Review

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Page 430 of 432 specific target level of protection and the current suite of secondary standards that is the basis fo r the decision not to adopt a distinct secondary standard in terms of a PM 2.5 visibility index at this time. The EPA recognizes that, as in the last review, the final decision is to not adopt a distinct secondary standard to address visibility impairment. While the D.C. Circuit remanded the decision on a secondary standard in the last review, the EPA’s decision in this review has addressed the issues raised in the court’s remand. Here the EPA has clearly identified the target degree of protection (defined in terms of a PM 2.5 visibility index at a level of 30 dv based on a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years) that would be requisite to protect public welfare with regard to visual air quality. The EPA has carefully compar ed this degree of protection with that provided by the current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 , based on an area specific analysis of recent air quality data and concluded that the degree of protection from visibility impairment provided by th e current secondary standard is sufficient to protect public welfare consistent with section 109(b)(2). This provides a clear basis for judging that the current secondary 24

- hour PM 2.5 standard of 35 μg/m 3 would provide sufficient protection. The analysis also shows that the current secondary 24

- hour PM 2.5 standard would provide more protection than is needed in some areas, largely because it does not take into account variable factors such as relative humidity. However, the EPA has recognized that adoption of a distinct secondary standard to address visibility, in addition to retaining the current secondary standard, would not change this result. The EPA has therefore concluded that adoption of such a distinct secondary standard, in addition to the current suite of secondary PM standards, is not needed to provide requisite***E.O. 12866 Review

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Page 431 of 432 protection for both visibility and non

- visibility related welfare effects. Thus the EPA’s decision has carefully considered and accounted for the views of the court in the remand of the 20 06 NAAQS. E. Administrator’s Final Decisions on Secondary PM Standards To address PM

- related welfare effects, including ecological effects, effects on materials, climate impacts, and visibility impairment, the Administrator is retaining the current suite of secondary PM standards, except for a change to the form of the annual standard. Specifically, to address PM

- related non

- visibility welfare effects including ecological effects, effects on materials, and climate impacts , the EPA is retaining the current secondary 24

- hour PM 2.5 and PM 10 standard and is revising only the form of the secondary annual PM 2.5 standard to remove the option for spatial averaging consistent with this change to the primary annual PM 2.5 standard. With respect to PM

- related visibility impairment, the Administrator has identified a target degree of protec tion, defined in terms of a PM 2.5 visibility index (based on speciated PM 2.5 mass concentrations and relative humidity data to calculate PM 2.5 light extinction), a 24

- hour averaging time, and a 90 th percentile form, averaged over 3 years, and a level of 30 deciviews (dv), which she judges to be requisite to protect public welfare with regard to visual air quality. The EPA’s analysis of monitoring data provides the basis for concluding that the current secondary 24

- hour PM 2.5 standard would provide sufficien t protection, and in some areas greater protection, relative to this target protection level. Adding a distinct secondary standard to address PM

- related visibility impairment would not affect this protection. Since sufficient protection from visibility imp airment will be provided for all areas of the country without adoption of a distinct secondary standard,***E.O. 12866 Review

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Page 432 of 432 and adoption of a distinct secondary standard will not change the degree of over

protection of visual air quality provided for some areas of the countr y by the secondary 24

- hour PM 2.5 standard, the Administrator judges that adoption of a distinct secondary standard, in addition to the current suite of secondary standards, is not needed to provide requisite protection for both visibility and non

- visibilit y related welfare effects.