The risk assessment also includes updates and improvements to input data and modeling approaches, summarized in section II.C above and in section 3.4 of the PA (U.S. EPA, 2022b). As in previous reviews, exposure and risk are estimated from air quality scenarios defined by the highest design value in the study area, which is the monitor location with the highest 3-year average of the annual mean PM2.5 concentrations (e.g., equal to 12.0 ug/m[3] for the current standard scenario) for the annual PM2.5 standard and with the highest 3-year average of the 98[th] percentile 24-hour PM2.5 concentrations (e.g., equal to 35 ug/m[3] for the current standard scenario) for the 24-hour PM2.5 standard. As described in more detail in section II.C above and in section 3.4 of the PA (U.S. EPA, 2022b), air quality modeling was used to simulate just meeting the existing annual and 24-hour standards of 12.0 ug/m[3] and 35 ug/m[3] and to just meeting potential alternative annual and 24-hour standards of 10.0 ug/m[3] and 30 ug/m[3]. In addition to the air quality modeling approach, linear interpolation and extrapolation were used to simulate just meeting alternative annual standards with levels of 11.0 (interpolated between 12.0 and 10.0 g/m[3]), 9.0 g/m[3], and 8.0 g/m[3] (both extrapolated from 12.0 and 10.0 g/m[3]) in the subset of study areas controlled by the annual standard.
In addition to the risk assessment described above, the PA presents quantitative analyses that also assess long-term PM2.5-attributable exposure and mortality risk, stratified by racial/ethnic demographics. As described in more detail in section II.B.2 above, the evidence suggests that different racial and ethnic groups, such as Black and Hispanic populations residing in the study areas, have higher PM2.5 exposures than White and non-Hispanic populations also residing in the study areas, respectively, thus contributing to increased risk of PM-related effects. Of the available studies, Di et al. (2017b) was identified as best characterizing populations potentially at increased risk of long-term exposure-attributable all-cause mortality effects and provides race- and ethnicity-stratified C-R functions for ages 65 and over (U.S. EPA, 2022b, section <IP_ADDRESS> and Appendix C). Risk and exposure are quantitatively assessed within racial and ethnic minority populations of older adults in the full set of 47 areas and the subset of 30 areas controlled by the annual PM2.5 standard. This analysis, when considered alongside estimates of risk across all populations in the 47 study areas, can help to inform conclusions on the annual primary PM2.5 standards that would be requisite to protect the public health of demographic populations potentially at increased risk of long-term PM2.5-related mortality effects.
In considering the risk results, the PA focuses first on estimates for the full set of 47 urban study areas. The risk assessment estimates that the current primary PM2.5 standards could allow a substantial number of deaths in the U.S., with the large majority of those deaths associated with long-term PM2.5 exposures. For example, when air quality in the 47 study areas is adjusted to just meet the current standards, the risk assessment estimates about 41,000 to 45,000 deaths from all-cause mortality in a single year (i.e., for long-term exposures; confidence intervals range from about 30,000 to 59,000) (U.S. EPA, 2022b, section <IP_ADDRESS>). For the 30 study areas where just meeting the current standards is controlled by the annual standard, long-term PM2.5 exposures are estimated to be associated with as many as 39,000 (confidence intervals range from about 26,000 to 51,000) deaths from all-cause mortality in a single year (U.S. EPA, 2022b, section <IP_ADDRESS>). For the 11 study areas where just meeting the current standards is controlled by the daily standard, long-term PM2.5 exposures are estimated to be associated with as many as 2,600 (confidence intervals ranging from 1,700 to 3,400) deaths in a single year (U.S. EPA, 2022b, section <IP_ADDRESS>). The risk assessment estimates far fewer deaths in a single year for short-term PM2.5 exposures as compared to long-term PM2.5 exposures, across all of the study area subsets (U.S. EPA, 2022b, section <IP_ADDRESS>).
While the absolute numbers of estimated deaths vary across exposure durations, populations, and C-R functions, the general magnitude of risk estimates supports the potential for significant public health impacts in locations meeting the current primary PM2.5 standards. This is particularly the case given that the large majority of PM2.5-associated deaths for air quality just meeting the current standards are estimated at annual average PM2.5 concentrations from about 10 to 12 ug/m[3]. These annual average PM2.5 concentrations fall within the range of long-term average concentrations over which key epidemiologic studies provide strong support for reported positive and statistically significant health effect associations (U.S. EPA, 2022b, section <IP_ADDRESS>).
In the 47 urban study areas, when air quality is simulated to just meet alternative standards, the PA notes that there are substantially larger risk reductions associated with lowering the annual standard than  with lowering the 24-hour standard. Risks are estimated to decrease by 13-17% when air quality is adjusted to just meet an alternative annual standard with a level of 10.0 g/m[3] or by 1-2% when adjusted to just meet an alternative 24-hour standard with a level of 30 g/m[3] (U.S. EPA, 2022b, section <IP_ADDRESS>). The percentage decrease when just meeting an alternative annual standard with a level of 10.0 g/m[3] corresponds to approximately 7,400 fewer deaths per year (confidence intervals ranging from about 4,100 to 9,800) attributable to long-term PM2.5 exposures (U.S. EPA, 2022b, section <IP_ADDRESS>).
In the 30 study areas where just meeting the current and alternative standards is controlled by the annual standard, air quality adjusted to meet alternative annual standards with lower levels is associated with reductions in estimated all-cause mortality risk. These reductions in risk for alternative annual levels are as follows: 7-9% reduction for an alternative annual level of 11.0 ug/m[3], 15-19% reduction for a level of 10.0 ug/m[3], 22-28% reduction for a level of 9.0 ug/m[3], and 30-37% reduction for a level of 8.0 ug/m[3] (U.S. EPA, 2022b, section <IP_ADDRESS>). For each of these standards, most of the risk remaining is estimated at annual average PM2.5 concentrations that fall somewhat below the alternative standard levels (U.S. EPA, 2022b, section <IP_ADDRESS>).
In considering the at-risk analysis, the PA notes that across all simulated air quality for both the full set of 47 and the subset of 30 study areas, Blacks experience the highest average PM2.5 concentrations of the demographic groups analyzed. Native Americans experienced the lowest average PM2.5 concentrations, particularly in the full set of 47 study areas. White, Hispanic, and Asian populations were exposed to similar average PM2.5 concentrations. Additionally, as the levels of potential alternative annual PM2.5 standards decrease, there is comparatively less disproportionate exposure between demographic populations (U.S. EPA, 2022b, section <IP_ADDRESS>).
The PA recognizes that the risk estimates can provide additional information beyond the exposure information to inform our understanding of potentially disproportionate impacts, in this instance by including demographic-specific information on baseline incidence and the relationship between exposure and health effect. Across all air quality scenarios and demographic groups evaluated, Black populations in the study areas are associated with the largest PM2.5-attributable mortality risk rate per 100,000 people, while White populations in the study areas are associated with the smallest PM2.5-attributative mortality risk rate (U.S. EPA, 2022b, section <IP_ADDRESS>, Figure 3-20). Generally, as the levels of potential alternative annual PM2.5 standards decrease in the 30 areas controlled by the annual standard, the average reductions in PM2.5 concentration and mortality risk rates increase across all demographic populations (U.S. EPA, 2022b, section <IP_ADDRESS>, Figure 3-21).
In comparing the reductions in average national PM2.5 concentrations and risk rates within each demographic population, the average percent PM2.5 concentrations and risk reductions are slightly greater in the Black population than in the White population for each alternative standard evaluated (11.0 ug/m[3], 10.0 ug/m[3], 9.0 ug/m[3], and 8.0 ug/m[3]), when shifting from the current annual PM2.5 standard (12.0 ug/m[3]) in the full set of 47 areas and the subset of 30 areas controlled by the annual standard. Furthermore, the difference in average percent risk reductions increases slightly more in Blacks than in Whites as the level of the potential alternative annual standard decreases (U.S. EPA, 2022b, section <IP_ADDRESS>, Table 3-19 and Table 3-20).
The PA also recognizes that there are several particularly important uncertainties that affect the quantitative estimates of risk rates and exposure in the at-risk analysis and their interpretation in the context of considering the current primary PM2.5 standards. These include uncertainties related to the modeling and adjustment methods for simulating air quality scenarios; the potential influence of confounders on the relationship between PM2.5 exposure and mortality; and the interpretation of the shapes of C-R functions, particularly at lower concentrations. It is also important to recognize the limited availability of studies to inform the at-risk analysis. As noted in section II.C above and in section 3.4 of the PA, the at-risk analysis included C-R functions from one study, Di et al. (2017b), which reported associations between long-term PM2.5 exposures and mortality, stratified by race/ethnicity, in populations age 65 and older. Of the studies available from the 2019 ISA, Di et al. (2017b) was identified as best characterizing potentially at-risk minority populations across the U.S.  While the at-risk analyses provide additional insight on the estimated exposures and risks for certain demographic groups, it is not clear how the results would vary if: (1)analyses included populations that were younger than 65 years old, (2) the analyses were conducted areas that are demographically different than the 47 study areas included in this analysis, and (3) the air quality adjustments reflected source-specific emissions reduction strategies. Therefore, in light of the limitations and uncertainties associated with the at-risk analyses, the results should be considered within the context of the full risk assessment. The uncertainties associated with the quantitative risk assessment and at-risk analyses are described in more detail in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>, Appendix C) and are summarized in section II.C.2 above.
In considering the public health implications of the risk assessment, the PA notes that the purpose for the study areas is to illustrate circumstances that may occur in areas that just meet the current or potential alternative standards, and not to estimate risk associated with conditions occurring in those specific locations currently. The PA notes that some areas across the U.S. have air quality for PM2.5 that is near or above the existing standards. Risks associated with air quality above the current standards are not informative to decisions about the adequacy of the current standards. This is because the risk assessment uses an approach to adjust air quality to just meet the current standards, which means that areas that have air quality that is above the current standards would be adjusted to just meet the current standards such that the evaluation of changes in risk and risk remaining would be associated with those areas meeting the current standards. The same is true for air quality adjusted to simulate just meeting alternative standard levels as well. Thus, the air quality and exposure circumstances assessed in the study areas in the risk assessment are specifically designed to inform whether the currently available information calls into question the adequacy of the public health protection afforded by the current standards, as well as to provide information regarding potential alternative standard levels.
The risk estimates for the study areas assessed in this reconsideration reflect differences in exposure circumstances among those areas and illustrate the exposures and risks that might be expected to occur in other areas with such circumstances under air quality conditions that just meet the current standards or the alternative standards assessed. Thus, the exposure and risk estimates indicate the magnitude of exposure and risk that might be expected in many areas of the U.S. with PM2.5 concentrations at or near the current or alternative standards. Although the methodologies and data used to estimate risks in this reconsideration differ in several ways from what was used in the 2020 review, the findings and considerations summarized in the PA present a pattern of exposure and risk that is generally similar to that considered in the 2020 review, and indicate a level of protection generally consistent with that described in the 2020 PA.
The PA notes that the considerations related to the potential public health implications of the risk assessment and at-risk analysis are important to informing the Administrator's proposed decisions regarding the public health significance of the risk assessment results. Specifically, the PA notes that available evidence and information suggests that both long- and short-term PM2.5 exposures are associated with adverse health effects, including more severe effects such as mortality. In addition, the PA further notes that such effects impact large segments of the U.S. population, including those populations that may have other factors that influence risk (i.e., lifestage, pre-existing cardiovascular and respiratory diseases, race/ethnicity), as well as disparities in PM2.5 exposures and health risks based on race and ethnicity (U.S. EPA, 2022b, section <IP_ADDRESS>). Therefore, the PA recognizes that the air quality allowed by the current primary PM2.5 standards could be judged to be associated with significant public health risk. The PA also recognizes that such conclusions also depend in part on public health policy judgments that will weigh in the Administrator's decision in this reconsideration with regard to the adequacy of protection afforded by the current standards. Such judgments that are common to NAAQS decisions include those related to public health implications of effects of differing severity. Such judgments also include those concerning the public health significance of effects at exposures for which evidence is limited or lacking, such as effects at lower concentrations than those demonstrated in the key epidemiologic studies and in those population groups for which population-specific information, such as C-R functions, are not available from the epidemiologic literature.
3. Administrator's Proposed Conclusions on the Primary PM2.5 Standards
This section summarizes the Administrator's considerations and proposed conclusions related to the adequacy of the current primary PM2.5 standards and presents his proposed decision to revise the primary annual PM - 2.5 standard and retain the primary 24-hour PM2.5 standard. In establishing primary standards under the Act that are "requisite" to protect public health with an adequate margin of safety, the Administrator is seeking to establish standards that are neither more nor less stringent than necessary for this purpose. He recognizes that the requirement to provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information and to provide a reasonable degree of protection against hazards that research has not yet identified. However, the Act does not require that primary standards be set at a zero-risk level; rather, the NAAQS must be sufficiently protective, but not more stringent than necessary.
      Given these requirements, the Administrator's final decision in this reconsideration will be a public health policy judgment drawing upon scientific and technical information examining the health effects of PM2.5 exposures, including how to consider the range and magnitude of uncertainties inherent in that information. This public health policy judgment will be based on an interpretation of the scientific and technical information that neither overstates nor understates its strengths and limitations, nor the appropriate inferences to be drawn, and will be informed by the Administrator's consideration of advice from the CASAC and public comments received on this proposal notice.
a. Adequacy of the Current Primary PM2.5 Standards
In considering whether the currently available scientific evidence and quantitative risk-based information support or call into question the adequacy of the public health protection afforded by the current primary PM2.5 standards, and as is the case with NAAQS reviews in general, the extent to which the current primary PM2.5 standards are judged to be adequate will depend on a variety of factors, including science policy and public health policy judgments to be made by the Administrator on the strength and uncertainties of the scientific evidence. The factors relevant to judging the adequacy of the standards also include the interpretation of, and decisions as to the weight to place on, different aspects of the results of the risk assessment for the study areas included and the associated uncertainties. Thus, the Administrator's proposed conclusions regarding the adequacy of the current standards will depend in part on judgments regarding aspects of the evidence and risk estimates, and judgments about the degree of protection that is requisite to protect public health with an adequate margin of safety.
i. Proposed Conclusions on the Adequacy of the Current Primary PM2.5 Standards
In reaching proposed conclusions on the adequacy of the current primary PM2.5 standards, the Administrator has considered the scientific evidence, including that assessed in the 2019 ISA and the ISA Supplement. The Administrator has also considered the quantitative estimates of risk developed in this reconsideration, including associated uncertainties and limitations, and the extent to which they indicate differing conclusions regarding the magnitude of risk, as well as level of protection from adverse effects, associated with the current standards. The Administrator has additionally considered the key aspects of the evidence and risk estimates emphasized in establishing the current standards, and the associated public health policy judgments and judgments about the uncertainties inherent in the scientific evidence and quantitative analyses that are integral to the proposed conclusions on the adequacy of the current primary PM2.5 standards.
First, as described above in section II.A.2, the Administrator's approach recognizes that the current annual standard (based on arithmetic mean concentrations) and 24-hour standard (based on 98[th] percentile concentrations), together, are intended to provide public health protection against the full distribution of short- and long-term PM2.5 exposures. In evaluating the adequacy of the current standards, the Administrator focuses on evaluating the public health protection afforded by the annual and 24-hour standards, taken together, against adverse health effects associated with long- or short-term PM2.5 exposures. This approach recognizes that changes in PM2.5 air quality designed to meet either the annual or the 24-hour standard would likely result in changes to both long-term average and short-term peak PM2.5 concentrations.
	In general, the Administrator recognizes that the annual standard is most effective at controlling exposures to "typical" daily PM2.5 concentrations that are experienced over the year, while the 24-hour standard, with its 98th percentile form, is most effective at limiting peak daily or 24-hour PM2.5 concentrations. In considering the combined effects of these standards, the Administrator recognizes that changes in PM2.5 air quality designed to meet an annual standard would likely result not only in lower short- and long-term PM2.5 concentrations near the middle of the air quality distribution, but also in fewer and lower short-term peak PM2.5 concentrations. Additionally, changes designed to meet a lower 24-hour standard, with a 98[th] percentile form, would most effectively result in fewer and lower peak 24-hour PM2.5 concentrations, but also have an effect on lowering the annual average PM2.5 concentrations. Thus, the Administrator acknowledges the focus in evaluating the current primary standards is on the protection provided by the combination of the annual and 24-hour standards against the distribution of both short- and long-term PM2.5 exposures.
The Administrator recognizes the longstanding body of health evidence supporting relationships between PM2.5 exposures (short- and long-term) and mortality or serious morbidity effects. The evidence available in this reconsideration (i.e., that assessed in the 2019 ISA (U.S. EPA, 2019a) and ISA Supplement (U.S. EPA, 2022a) and summarized above in section II.B.1 and section II.D.2.a reaffirms, and in some cases strengthens, the conclusions from the 2009 ISA regarding the health effects of PM2.5 exposures (U.S. EPA, 2009a). As noted above, epidemiologic studies demonstrate generally positive, and often statistically significant, PM2.5 health effect associations. Such studies report associations between estimated PM2.5 exposures and non-accidental, cardiovascular, or respiratory mortality; cardiovascular or respiratory hospitalizations or emergency room visits; and other mortality/morbidity outcomes (e.g., lung cancer mortality or incidence, asthma development). Recent experimental evidence, as well as evidence from panel studies, strengthens support for potential biological pathways through which PM2.5 exposures could lead to the serious effects reported in many population-level epidemiologic studies, including support for pathways that could lead to cardiovascular, respiratory, nervous system, and cancer-related effects. The Administrator also recognizes that the PA notes that while the full body of health effects evidence is considered in this reconsideration of the PM NAAQS, the greatest emphasis in the PA is placed on the health effects for which the evidence has been judged in the 2019 ISA to demonstrate a "causal" or "likely to be causal" relationship with PM2.5 exposures (i.e., mortality, cardiovascular effects, respiratory effects, cancer, and nervous system effects). In considering the available scientific evidence, consistent with approaches employed in past NAAQS reviews, the Administrator places the most weight on evidence supporting "causal" or "likely to be causal" relationship with long or short-term PM2.5 exposures. In addition, the Administrator also takes note of those populations identified to be at greater risk of PM2.5-related health effects, as characterized in the 2019 ISA and ISA Supplement, and the potential public health implications.
In evaluating the public health protection afforded by the current primary PM2.5 standards against long- and short-term PM - 2.5 exposures, the Administrator considers the four basic elements of the NAAQS (indicator, averaging time, form, and level) collectively. With respect to indicator, the Administrator recognizes that the scientific evidence in this reconsideration, as in previous reviews, continues to provide strong support for health effects associated with PM2.5 mass. He notes the PA conclusion that the available information continues to support the PM2.5 mass-based indicator and remains too limited to support a distinct standard for any specific PM2.5 component or group of components, and too limited to support a distinct standard for the ultrafine fraction (U.S. EPA, 2022b, section <IP_ADDRESS>.1). In its advice on the adequacy of the current primary PM2.5 standards, the CASAC reached consensus that the PM2.5 mass-based indicator should be retained, without revision (Sheppard, 2022a, p. 2 of consensus letter). Thus, as in the 2020 review (85 FR 82715, December 18, 2020) and consistent with the advice from the CASAC, the Administrator proposes to conclude that it is appropriate to consider retaining PM2.5 mass as the indicator for the primary standards for fine particles.
With respect to averaging time and form, the Administrator notes that the scientific evidence continues to provide strong support for health effect associations with both long-term (e.g., annual or multi-year) and short-term (e.g., mostly 24-hour exposures to PM2.5) (U.S. EPA, 2022b, section <IP_ADDRESS>.2). In this reconsideration, the epidemiologic and controlled human exposure studies have examined a variety of PM2.5 exposure durations. Epidemiologic studies continue to provide strong support for health effects associated with short-term PM2.5 exposures based on 24-hour PM2.5 averaging periods, and the EPA notes that associations with sub-daily estimates are less consistent and, in some cases, smaller in magnitude (U.S. EPA, 2019a, section <IP_ADDRESS>; U.S. EPA, 2022b, section <IP_ADDRESS>.2). In addition, controlled human exposure and panel-based studies of sub-daily exposures typically examine subclinical effects rather than the more serious population-level effects that have been reported to be associated with 24-hour exposures (e.g., mortality, hospitalizations). Taken together, the 2019 ISA concludes that epidemiologic studies do not indicate that subdaily averaging periods are more closely associated with health effects than the 24-hour average exposure metric (U.S. EPA, 2019a, section <IP_ADDRESS>). Additionally, while recent controlled human exposure studies provide consistent evidence for cardiovascular effects following PM2.5 exposures for less than 24 hours (i.e., < 30 minutes to 5 hours), exposure concentrations in these studies are well-above the ambient concentrations typically measured in locations meeting the current standards (U.S. EPA, 2022b, section <IP_ADDRESS>). Therefore, these studies do not provide support for additional protection against sub-daily PM2.5 exposures, beyond that provided by the current primary standards. In its advice on the adequacy of the current primary PM2.5 standards, the CASAC reached consensus that averaging times for the standards should be retained, without revision (Sheppard, 2022a, p. 2 of consensus letter). Thus, as in the 2020 review (85 FR 82715, December 18, 2020), and consistent with the advice from the CASAC, the Administrator reaches the proposed conclusion that the currently available evidence does not support considering alternatives to the annual and 24-hour averaging times for standards meant to protect against long- and short-term PM2.5 exposures.
With regard to form, the Administrator proposes to conclude that it is appropriate to consider retaining the current form of both the annual and the 24-hour standards. In so doing, he first notes that, in the 1997 review, the EPA set both an annual standard, to provide protection from health effects associated with both long- and short-term exposures to PM2.5, and a 24-hour standard to a supplement the protection afforded by the annual standard (62 FR 38667, July 18, 1997). With regard to the form of the annual standard, the Administrator recognizes that a large majority of the recently available epidemiologic studies continue to report associations between health effects and annual average PM2.5 concentrations. These studies of annual average PM2.5 concentrations provide support for retaining the current form of the annual standard to provide protection against long- and short-term PM2.5 exposures. In its advice on the adequacy of the current standards, the CASAC reached consensus that the form of the annual standard (i.e., annual mean, averaged over 3 years) should be retained, without revision (Sheppard, 2022a, p. 2 of consensus letter). In relation to the form of the 24-hour standard (98th percentile, averaged over three years), the Administrator notes that epidemiologic studies continue to provide strong support for health effect associations with short-term (e.g., mostly 24-hour) PM2.5 exposures (U.S. EPA, 2022b, section <IP_ADDRESS>.3) and that controlled human exposure studies provide evidence for health effects following single short-term "peak" PM2.5 exposures. Thus, the evidence supports retaining a standard focused on providing supplemental protection against short-term peak exposures and supports a 98th percentile form for a 24-hour standard. The Administrator further notes that this form also provides an appropriate balance between limiting the occurrence of peak 24-hour PM2.5 concentrations and identifying a stable target for risk management programs (U.S. EPA, 2022b, section <IP_ADDRESS>.3). While the CASAC provided recommendations regarding the adequacy of the current 24-hour standard conditional on the current form (i.e., 98[th] percentile, averaged over three years), they recommended that in future reviews, the EPA also consider alternative forms for the primary 24-hour PM2.5 standard (Sheppard, 2022a, p. 18 of consensus responses). Furthermore, the Administrator notes that the multi-year percentile form (i.e., averaged over three years) 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. Thus, in considering the information summarized above, and consistent with the advice from the CASAC, the Administrator reaches the preliminary conclusion that it is appropriate to consider retaining the forms of the current annual and 24-hour PM2.5 standards.
With regard to the level of the current standards, the Administrator first considers the scientific evidence evaluated in the 2019 ISA and ISA Supplement, and considerations regarding the evidence as presented in the PA. The Administrator recognizes that the PA places greater weight on epidemiologic studies conducted in the U.S. and Canada, as these studies are more directly applicable for quantitative considerations compared to studies conducted in other countries. Studies conducted in other countries outside of the U.S. and Canada generally reflect different populations, exposure characteristics, air pollution mixtures, and higher PM2.5 concentrations in ambient air than are currently found in the U.S. Therefore, consistent with approaches in previous reviews, the Administrator judges that it is appropriate to place greater weight on the U.S. and Canadian epidemiologic studies in reaching conclusions regarding the adequacy of the current standards. In so doing, the Administrator notes that the epidemiologic studies in the U.S. and Canada report health effect associations with mortality and/or morbidity across multiple cities and in diverse populations, including in studies examining populations and lifestages that may be at increased risk of experiencing a PM2.5-related health effect (e.g., older adults, children, populations with pre-existing cardiovascular and respiratory disease, minority populations, and low SES communities). Further, he notes the epidemiologic studies that use a variety of statistical designs and employ a variety of methods to examine exposure measurement error as well as to control for confounding effects, and he acknowledges that results of these analyses support the robustness of the reported associations. Additionally, the Administrator notes findings from an expanded body of studies that employ alternative methods for confounder control and accountability methods further inform the causal nature of the relationship between long or short-term term PM2.5 exposure and mortality as described in the 2019 ISA and ISA Supplement (U.S. EPA, 2019, sections <IP_ADDRESS>, <IP_ADDRESS>; U.S. EPA, 2022a, sections <IP_ADDRESS>, <IP_ADDRESS>, <IP_ADDRESS>, and <IP_ADDRESS>). These studies, summarized above in II.B.3 above and in Table 3-11 and Table 3-12 of the PA (U.S. EPA, 2022b) examine both short- and long-term PM2.5 exposure and cardiovascular effects and mortality, and, using a variety of statistical methods to control for confounding bias, consistently report positive associations, which further supports the broader body of epidemiologic evidence for both cardiovascular effects and mortality. Moreover, the Administrator notes that recent epidemiologic studies strengthen support for health effect associations at PM2.5 concentrations lower than in those evaluated in epidemiologic studies available at the time of previous reviews. Lastly, the Administrator notes that studies that examine the shape of the C-R relationship over the full distribution of ambient PM2.5 concentrations have not identified a threshold concentration, below which associations no longer exist (U.S. EPA, 2019a, section 1.5.3; U.S. EPA, 2022a, sections <IP_ADDRESS>.1 and <IP_ADDRESS>.2). However, the Administrator also notes that uncertainties remain about the shape of the C-R curve at PM2.5 concentrations < 8 μg/m[3], with some recent studies providing evidence for either a sublinear, linear, or supralinear relationship at these lower concentrations (section II.B.4 above; U.S. EPA, 2019a, section 11.2.4; U.S. EPA, 2022a , section <IP_ADDRESS>).
In considering the available scientific evidence to inform proposed decisions on the adequacy of the current level of the annual standard, the Administrator acknowledges that the evidence available in this reconsideration provides support for adverse health effect associations at lower ambient PM2.5 concentrations than in previous reviews. The Administrator notes that in previous reviews (including 1997, 2006 and 2012 reviews), evidence-based approaches focused on identifying standard levels near or somewhat below long-term mean concentrations reported in key epidemiologic studies. These approaches were supported by the CASAC in previous reviews and are supported in this reconsideration by the current CASAC, who also referenced the potential for considering other lines of epidemiologic evidence. The Administrator notes that in this reconsideration, a large number of key U.S. epidemiologic studies report positive and statistically significant associations for air quality distributions with overall mean PM2.5 concentrations that are well below the current level of the annual standard of 12 μg/m[3] (i.e., Figure 1 and Figure 2 above with concentrations ranging down as low as 9.9 μg/m[3] in U.S.-based monitor-based studies and 9.3 μg/m[3] in U.S.-based hybrid model-based studies). The Administrator also recognizes that, while Canadian studies can be more difficult to directly compare to the annual design value used to determine in compliance in the U.S., the overall mean PM2.5 concentrations from the key Canadian epidemiologic studies are close to, though somewhat lower than, those from the U.S. studies. The range of monitor-based mean PM2.5 concentrations is from 6.9 ug/m[3] to 13.3 ug/m[3] while the range of mean PM2.5 concentrations in studies that use hybrid modeling is 5.9 ug/m[3] to 9.8 ug/m[3].
In assessing the adequacy of the current annual standard, the Administrator also examines additional epidemiologic studies, consistent with CASAC advice, that provide supplementary information for consideration in reaching conclusions regarding the current annual standard. These studies include analyses that restrict annual average PM2.5 concentrations to values below level the annual standard (described above in section II.B.3.b and in Table 3-10 of the PA) and the CASAC advised that "for the purpose of informing the adequacy of the standards" that the EPA evaluate the means from these studies. In this reconsideration, there are two key studies available that restrict average annual PM2.5 concentrations to less than 12 ug/m[3] (Di et al., 2017a and Dominici et al., 2019). These restricted analyses report positive and statistically significant associations with all-cause mortality and report mean PM2.5 concentrations of 9.6 ug/m[3]. Thus, these two epidemiologic studies provide support for positive and statistically significant associations at lower mean PM2.5 concentrations. The Administrator does note that uncertainties exist in these analyses (described in more detail in sections II.B.3.b and II.D.2.a above), including uncertainty in how studies exclude concentrations (e.g., at what spatial resolution are concentrations being excluded), which would make any comparisons of concentrations in restricted analyses difficult to compare directly to design values.
 In considering the available key U.S. epidemiologic studies, the Administrator also notes that CASAC recommended looking at the distribution of concentrations reported in epidemiologic studies for purposes of informing the adequacy of the standards and notes that a small number of studies report PM2.5 concentrations corresponding to the 25[th] and 10[th] percentiles of health data or exposure estimates. He observes that in studies that use monitors to estimate PM2.5 exposures, 25[th] percentiles of health events correspond to PM2.5 concentrations (i.e., averaged over the study period for each study city) at or above 11.5 ug/m[3] and 10th percentiles of health events correspond to PM2.5 concentrations at or above 9.8 ug/m[3] (i.e., 25% and 10% of health events, respectively, occur in study locations with PM2.5 concentrations below these values) (Figure 1 above and U.S. EPA, 2022b, Figure 3-8). The Administrator further observes that of the key U.S. epidemiologic studies that use hybrid modeling approaches to estimate long-term PM2.5 exposures, the ambient PM2.5 concentrations corresponding to 25th percentiles of estimated exposures are 9.1 ug/m[3] (Figure 2 above and U.S. EPA, 2022b, Figure 3-14). In key U.S. epidemiologic studies that use hybrid modeling approaches to estimate short-term PM2.5 exposures, the ambient concentrations corresponding to 25[th] percentiles of estimated exposures, or health events, are 6.7 ug/m[3] and the ambient PM2.5 concentration corresponding to that 10th percentile range from 4.7 ug/m[3] to 7.3 ug/m[3] (Figure 2 above and U.S. EPA, 2022b, Figure 3-14). While the Administrator places less weight on the limited number of studies that report these lower quartiles of the air quality distributions, he notes these concentrations are generally below the level of the annual standard of 12 μg/m[3].
In further assessing the adequacy of the current annual standard, the Administrator also evaluates what the accountability studies may indicate with respect to potential for improvements in public health with improvements in air quality. In so doing, he takes note of three accountability studies (Sanders et al., 2020b; Corrigan et al., 2018; and Henneman et al., 2019a) newly available in this reconsideration with starting concentrations at or below 12.0 ug/m[3] that indicate positive and significant associations with mortality and morbidity and reductions in ambient PM2.5 (described above in section II.B.3.b and in Table 3-12 of the PA) and notes that these studies suggest public health improvements may occur at concentrations below 12 ug/m[3].
Thus, in considering the available scientific evidence to inform proposed decisions on the adequacy of the current primary annual PM2.5 standard, the Administrator recognizes that there is a long-standing body of epidemiologic evidence that provides support for associations between PM2.5 exposures and health effects across a distribution of air quality that includes concentrations near (i.e., at, above, and below) the current standards. As such, the Administrator recognizes that the available scientific evidence, as assessed in the 2019 ISA and ISA Supplement, including the newly available epidemiologic studies and the supplemental information from specific types of epidemiologic studies, provides a strong scientific foundation for consideration of the adequacy of the level of the current annual standard.
In considering the available scientific evidence to inform proposed decisions on the adequacy of the current 24-hour standard, the Administrator finds that there is less information available to support decisions on the 24-hour standard than that summarized above for the annual standard. When looking to the experimental studies, he notes that controlled human exposure studies provide evidence for health effects following single, short-term exposures to PM2.5 concentrations that are greater than those typically present in ambient air. In the controlled human exposure studies, the Administrator observes that results are inconsistent, particularly at lower PM2.5 concentrations, but that studies do report statistically significant effects on one or more indicators of cardiovascular function following 2-hour exposures to PM2.5 concentrations at and above 120 μg/m[3] (and at and above 149 μg/m[3] for vascular impairment, the effect shown to be most consistent across studies). As noted in the 2019 ISA, these studies are important in establishing biological plausibility for PM2.5 exposures causing more serious health effects, such as those seen in short-term exposure epidemiologic studies. However, as noted in the PA, the observed effects in these controlled human exposures studies are ones that signal an intermediate effect in the body, likely due to short-term exposure to PM2.5, and which may provide support that more adverse effects may be experienced following longer exposure durations and/or exposure to higher concentrations but such intermediate effects typically would not, by themselves, be judged as adverse. Additionally, he acknowledges, as noted by the CASAC, that these controlled human exposure studies generally do not include populations with substantially increased risk from exposure to PM2.5, such as children, older adults, or those with more severe underlying illness. So, noting these points and balancing these limitations (i.e., that the health outcomes observed in these controlled human exposure studies are not clearly adverse and that the studies generally do not include those at increased risk from PM2.5 exposure), the Administrator examines the air quality analyses, described in more detail in section II.B.3.a above, to assess whether during recent air quality conditions, areas meeting the current standards would experience the concentrations reported in these controlled human exposure studies. He observes that these air quality analyses demonstrate that the PM2.5 exposures shown to cause consistent effects in the controlled human exposure studies are well-above the ambient concentrations typically measured in locations meeting the current primary standards, thus suggesting that the current primary PM2.5 standards provide protection against these "peak" concentrations. In fact, at air quality monitoring sites meeting the current primary PM2.5 standards (i.e., the 24-hour standard and the annual standard), the 2-hour concentrations generally remain below 10 μg/m[3], and rarely exceed 30 μg/m[3]. Two-hour concentrations are higher at monitoring sites violating the current standards, but generally remain below 16 μg/m[3] and rarely exceed 80 μg/m[3]. Based on this information, the Administrator finds that the current suite of standards maintains sub-daily concentrations far below the current concentrations in controlled human exposure studies where consistent effects have been observed, and notes that while these studies generally do not include the most at-risk individuals, the exposure concentrations in these studies also do not elicit adverse effects.
In addition, the Administrator also notes that the majority of the CASAC provide support for their advice to revise the current daily standard by pointing to "substantial epidemiologic evidence from both morbidity and mortality studies" which "includes three U.S. air pollution studies with analyses restricted to 24-hour concentrations below 25 μg/m[3]" (Sheppard, 2022a, p. 17 consensus responses). In considering this advice from the majority of the CASAC, the Administrator notes that the substantial epidemiologic evidence available in this reconsideration, including the studies that restrict short-term (24-hour average PM2.5 concentrations) PM2.5 exposures below 25 g/m[3], provides support for positive and statistically significant associations between exposure to short-term PM2.5 concentrations and all-cause mortality (Di et al., 2017a) and CVD hospital admissions (deSouza et al., 2021 and Di et al., 2017a). In particular, for the available epidemiologic studies that employ restricted analyses of short-term exposure studies, multicity studies indicate that positive and statistically significant associations with mortality persist in analyses restricted to short-term (24-hour average PM2.5 concentrations) PM2.5 exposures below 35 g/m[3] (), below 30 g/m[3] (Shi et al., 2016), and below 25 g/m[3] (Di et al., 2017a). Thus, the Administrator agrees that these studies help to provide additional support for reaching conclusions on causality in the 2019 ISA. Additionally, when considering these studies, the restricted approach in these short-term studies most clearly indicates that risks associated with short-term PM2.5 exposures are not disproportionately driven by the peaks of the air quality distribution. While this is useful information, it does not help to inform questions on the adequacy of the current 24-hour standard given that the 24-hour standard focuses on reducing "peak" exposures (with its 98[th] percentile form). In further evaluating these studies, the Administrator notes that the fact that there are positive and significant associations in these analyses does not mean that one can conclude that there would be short-term effects occurring in areas that meet a 24-hour standard at these levels. This is true for multiple reasons. First, there are uncertainties with respect to the methodologies used in these studies to exclude concentrations and the specific methodology used (e.g., are individual days with concentrations above the concentration of interest in the restricted analyses excluded at the modeled grid cell level or the ZIP code level rather than removing entire areas with day(s) that exceed that concentration) has direct implications for the resulting air quality scenario(s). This in turn affects how the adjusted air quality scenarios in these studies can be related to air quality distributions and exposures to PM2.5 concentrations in ambient air and thus how the data can be interpreted with regard to the current standard level. Second, given that these studies are only evaluating daily or annual average PM2.5 concentrations that would correspond to the levels of the standards, they do not consider these levels along with the forms and averaging times of the standards. This is quite limiting for use in judging the adequacy of the 24-hour standard given that the study-reported mean concentration is not useful in informing the level of a standard with a 98[th] percentile form that is designed to limit exposures to peak PM2.5 concentrations. Further, as noted in the PA, the study-reported means from these studies, are not useful in identifying a level at which we can say with some confidence that effects are occurring due to impacts from "peak" exposures (i.e., those most closely aligned with the protection provide by the 24-hour standard, with its 98[th] percentile form) but are instead more useful in informing questions about impacts from "typical" or average 24-hour exposures (i.e., those most closely aligned with the protection provided by the annual standard). These uncertainties and lack of information available from these studies are quite limiting and as such, the Administrator concludes that it is unclear how to apply these studies to a decision framework that could inform whether the level of the current 24-hour standard is or is not adequate. However, the Administrator notes this uncertainty may not be quite as limiting for using restricted analyses studies to inform conclusions regarding the adequacy of the annual standard, given that the study-reported means could be evaluated in the context of the decision framework described above for informing proposed decisions on the level of the annual standard. However, in considering the available evidence with regard to the current 24-hour PM2.5 standard, while the Administrator agrees with the majority of the CASAC's comment that the controlled human exposure studies have significant limitations which must be considered when reaching conclusions on the adequacy of the current 24-hour standard, he finds that restricted analyses studies have significant limitations and do not provide a stronger line of evidence with which to inform his proposed decisions on the current 24-hour standard.
In addition to the evidence above, the Administrator also considers what the risk assessment indicates with regard to the adequacy of the current primary annual and 24-hour PM2.5 standards. These analyses provide estimates of PM2.5-attributable mortality which are estimated based on input data that include C-R functions from epidemiologic studies that have no threshold and a linear C-R relationship down to zero, as well an air quality adjustment approach that incorporates proportional decreases in PM2.5 concentrations to meet lower standard levels. The Administrator observes that the risk assessment estimates that the current primary annual PM2.5 standard could allow a substantial number of deaths in the U.S. For example, when air quality in 30 study areas is adjusted to simulate just meeting the current annual standard, the risk assessment estimates long-term PM2.5 exposures to be associated with as many as 39,000 total deaths, with confidence intervals ranging from 26,000-51,000. The Administrator notes that these estimates do not reflect uncertainties in associations of health effects at lower concentrations and simulated air quality improvements will always lead to proportional decreases in risk (i.e., each additional ug/m[3] reduction produces additional benefits with no clear stopping point). Noting these limitations and noting that the absolute numbers of estimated deaths vary across exposure durations, populations, and C-R functions, he also observes that the general magnitude of risk estimates supports the potential for significant public health impacts in locations meeting the current primary annual PM2.5 standard. He observes that this is particularly the case given that the large majority of PM2.5-associated deaths for air quality just meeting the current annual standard are estimated at annual average PM2.5 concentrations from about 10 to 12 ug/m[3], annual average PM2.5 concentrations that fall well within the range of long-term average concentrations over which key epidemiologic studies provide strong support for reported positive and statistically significant PM2.5 health effect associations. With respect to the CASAC's advice on the risk assessment, the Administrator notes that the majority of the CASAC agreed that "[t]he results support the conclusion that the current primary annual PM2.5 standard does not adequately protect public health" (Sheppard, 2022a, p. 2 of consensus letter) and that "[t]he CASAC concurs with the EPA's assessment that meaningful risk reductions will result from lowering the annual PM2.5 standard" (Sheppard, 2022a, p. 3 of consensus letter)..With respect to the CASAC's advice on the risk assessment, the Administrator notes that the majority of the CASAC agreed that "[t]he results support the conclusion that the current primary annual PM2.5 standard does not adequately protect public health" (Sheppard, 2022a, p. 2 of consensus letter).With respect to the CASAC's advice on the risk assessment, the Administrator notes that the majority of the CASAC agreed that "[t]he results support the conclusion that the current primary annual PM2.5 standard does not adequately protect public health" (Sheppard, 2022a, p. 2 of consensus letter) and that "[t]he CASAC concurs with the EPA's assessment that meaningful risk reductions will result from lowering the annual PM2.5 standard" (Sheppard, 2022a, p. 3 of consensus letter). Additionally, the minority of CASAC also agreed that the risk assessment results support revision to the annual standard but commented that there were important uncertainties in the analyses and interpretation of the analyses for annual standard levels below 10 ug/m[3] (Sheppard, 2022a, p. 3 of consensus letter).
The Administrator also recognizes that the risk assessment was able to include a new analysis based on the availability of a new study in this reconsideration that provided mortality risk coefficients for older adults (i.e., 65 years and older) based on PM2.5 exposure and stratified by racial and ethnic demographics. This at-risk analysis provided estimates of potential long-term PM2.5-attributable exposure and mortality risk in older adults, stratified by racial/ethnic demographics, when meeting a revised annual standard with a lower level. The Administrator recognizes that this analysis is subject to the same uncertainties as those associated with the main risk assessment estimates, including being limited to a subset of areas across the U.S. and influenced by air quality adjustment methodologies that may not produce estimates of PM2.5 concentration exposures that match those that can result from control strategies implemented to meet more stringent standards, and that the results are based on the risk coefficients of only one epidemiologic study. Taking into account these uncertainties and limitations, he does judge that the analysis supports that a lower annual standard level (i.e., below 12 ug/m3 and down as low as 8 ug/m[3]) will help to reduce PM2.5 exposure and may also help to mitigate risk disparities. The Administrator notes that what urban areas are included in the risk assessment analysis will greatly influence the results but notes that based on the areas included in the analyses, the results show the largest impact is on reducing exposure and risk in Black populations, who were estimated in the risk assessment case study areas to have the highest levels of exposures and the greatest rates of premature mortality risk.
With respect to the 24-hour standard, the risk assessment indicates that the annual standard is the controlling standard across most of the urban study areas evaluated. When air quality is adjusted to just meet an alternative 24-hour standard level of 30 g/m[3] in the areas where the 24-hour standard is controlling, the risk assessment estimates reductions in PM2.5-associated risks across a more limited population and number of areas compared to when air quality is adjusted to simulate alternative levels for the annual standard, and these predictions are largely confined to areas located in the western U.S., several of which are also likely to experience risk reductions upon meeting a revised annual standard. With respect to CASAC advice, the Administrator notes that the minority of CASAC advised that these results suggest that the annual standard can be used to limit both long- and short-term PM2.5 concentrations and views these risk assessment results as supporting the conclusion that the current 24-hour standard is adequate (Sheppard, 2022a, p. 4 of consensus letter). In contrast, the majority of CASAC members commented that they placed greater weight on the evidence-based considerations than on the values estimated by the risk assessment, noting the potential for uncertainties in how the risk assessment was able to "capture areas with wintertime stagnation and residential wood-burning where the annual standard is less likely to be protective" (Sheppard, 2022a, p. 4 of consensus letter). The majority of the CASAC members further state that "[t]here is also less confidence that the annual standard could adequately protect against health effects of short-term exposures. A range of 25-30 μg/m[3] for the 24-hour PM2.5 standard would be adequately protective" (Sheppard, 2022a, p. 4 of consensus letter). The majority of the CASAC members further state that "[t]here is also less confidence that the annual standard could adequately protect against health effects of short-term exposures. A range of 25-30 μg/m[3] for the 24-hour PM2.5 standard would be adequately protective" (Sheppard, 2022a, p.4 of consensus letter).
In considering the application of the risk assessment in a decision framework assessing the adequacy of the current 24-hour standard, the Administrator again notes that the risk assessment analyses of PM2.5-attributable mortality use input data that include C-R functions from epidemiologic studies that have no threshold and a linear C-R relationship down to zero, as well an air quality adjustment approach that incorporates proportional decreases in PM2.5 concentrations to meet lower standard levels, and that this quantitative approach does not incorporate any elements of uncertainty in associations of health effects at lower concentrations and simulated air quality improvements will always lead to proportional decreases in risk (i.e., each additional ug/m[3] reduction produces additional benefits with no clear stopping point). Therefore, the Administrator recognizes that the risk estimates can help to place the evidence for specific health effects into a broader public health context but should be considered along with the inherent uncertainties and limitations of such analyses when informing judgments about the potential for additional public health protection associated with PM2.5 exposure and related health effects. The Administrator also notes that in the U.S., current air quality shows that the 24-hour standard is controlling in very few areas and thus, it is understandable that there are very few areas that would be included in the study areas in the risk assessment. The Administrator also recognizes that the risk assessment did not provide quantitative information on risk impacts associated with an alternative 24-hour standard level of 25 ug/m[3].
Based on the above considerations, the Administrator reaches the proposed conclusion that the available scientific evidence (summarized above in section II.B) and quantitative risk assessment (summarized above in section II.C), can reasonably be viewed as calling into question the adequacy of the public health protection afforded by the current annual standard. In reaching this conclusion, the Administrator places weight on the extensive epidemiologic evidence available in this reconsideration, strengthened from previous reviews, showing associations between adverse health effects (particularly cardiovascular effects and mortality) and long-term mean PM2.5 concentrations, and notes the number and strength of studies available showing associations with mean PM2.5 concentrations well below the current annual standard of 12.0 ug/m[3]. The Administrator also takes note of the evidence supporting the biological plausibility of these associations, including toxicological studies and controlled human exposure studies. When turning to additional information from the epidemiologic evidence base, he notes the advice from CASAC to also consider the 25[th] percentile of the data that is available and the study reported means from long-term studies that restrict concentrations to below 12 ug/m[3].   When considering the 25[th] percentile of the data, the Administrator notes that it is available from a limited number of epidemiologic studies and that the current level of the annual standard is above most of the 25[th] percentile values reported in the key epidemiologic studies. When looking to the restricted analyses studies, he notes that there are two studies that report positive and statistically significant associations with all-cause mortality, and report a study mean PM2.5 concentration of 9.6 ug/m[3]. While noting the limited nature of these two lines of evidence and the associated uncertainties, the Administrator does judge that these data support the need to revise the annual standard level. Lastly, with respect to the epidemiologic evidence, the Administrator also takes into account accountability studies newly available in this reconsideration with starting concentrations at or below 12.0 ug/m[3] that indicate positive and significant associations with mortality and morbidity and reductions in ambient PM2.5 and notes that these studies suggest public health improvements may occur at concentrations below 12 ug/m[3].
The Administrator also considers the results of the risk assessment in light of the information it provides on risks associated with the current and more stringent levels of the annual standard.  While he recognizes a number of uncertainties and limitations associated with the quantitative estimates of the risk assessment, he judges that the estimated risks remaining under air quality adjusted to just meet the current suite of standards are too high to be considered requisite to protect public health with an adequate margin of safety, noting in particular the large number of premature deaths estimated to remain with air quality that just meets the current annual standard. The Administrator also recognizes that the risk assessment was able to include a new analysis (at-risk analysis) that provided estimates of potential long-term PM2.5-attributable exposure and mortality risk in older adults, stratified by racial/ethnic demographics, when meeting a revised annual standard with a lower level. While the Administrator recognizes that this analysis is subject to multiple uncertainties and limitations (as noted above in sections II.C.2 and II.D.2.b), he does judge that the analysis suggests that a lower annual standard level (i.e., below 12 ug/m3 and down as low as 8 ug/m[3]) will help to reduce PM2.5 exposure and may also help to mitigate exposure and risk disparities. Finally, the Administrator considers the advice from the CASAC, who unanimously recommended revising the annual standard.
	The Administrator finds it is less clear whether the available scientific evidence and quantitative information call into question the adequacy of the public health protection afforded by the current 24-hour standard, particularly when considered in conjunction with the protection provided by the suite of standards and the proposed decision to revise the annual standard. In considering the scientific evidence, he notes that the controlled human exposure studies do not provide a threshold below which no effects occur and they do not include the most at-risk populations. However, the concentrations reported in these studies are for observed effects that signal a change in the body likely due to short-term exposure to PM2.5 and which may be the prelude to more adverse effects following longer duration and/or higher concentration exposures but typically would not, by themselves, be judged as adverse. Balancing this with the observation that the air quality concentrations in areas meeting the current standards are well below the PM2.5 concentrations shown to elicit effects in these studies, the Administrator does not judge that these studies call into question the adequacy of the current 24-hour standard. With respect to the epidemiologic evidence, the Administrator notes that the body of epidemiologic evidence provides limited support for judging adequacy of the level of the 24-hour standard. As discussed in detail above (section II.B.3.b), epidemiologic studies provide the strongest support for reported health effect associations for the part of the air quality distribution corresponding to the bulk of the underlying data (i.e., estimated exposures and/or health events), often around the overall mean concentrations evaluated rather than near the upper end of the distribution. While there are three studies available in this reconsideration that restricted 24-hour concentrations to concentrations below 25 ug/m3 and while some members of CASAC pointed to these studies as the basis for their recommendation to revise the 24-hour standard, the Administrator preliminarily concludes that the results from these studies, particularly in light of the uncertainties associated with these studies (as discussed above), are an inadequate basis for revising the level of the 24-hour PM2.5 standard.
When evaluating the risk assessment information, the Administrator notes that the risk assessment estimates a reduction of 9-13% PM2.5 attributable mortality in areas where the 24-hour standard is controlling when the 24-hour PM2.5 standard is reduced from a level of 35 μg/m[3] to 30 μg/m[3]. The Administrator notes that this estimated reduction in PM2.5-associated risks is across a more limited population and is largely confined to a small number of areas located in the western U.S. Other areas included in the risk assessment were shown to experience risk reductions that were driven primarily by meeting a lower annual standard level (though the associated change in air quality also resulted in lower 24-hour standard concentrations). With respect to CASAC advice, the Administrator notes that , p. 4 of consensus letter), the majority of CASAC advised that less weight be placed, while the minority of CASAC advised that these risk assessment results support the conclusion that the current 24-hour standard is adequate (Sheppard, 2022a, p. 4 of consensus letter), the majority of CASAC advised that less weight be placed on the risk assessment results and noted the potential for uncertainties in how the risk assessment was able to "capture areas with wintertime stagnation and residential wood-burning where the annual standard is less likely to be protective" (Sheppard, 2022a, p. 4 of consensus letter).
Based on the current evidence and quantitative information, as well as consideration of CASAC advice and public comment thus far in this reconsideration, the Administrator proposes to conclude that the current primary PM2.5 standards are not adequate to protect public health with an adequate margin of safety. While he notes that the scientific evidence and quantitative information clearly call into question the adequacy of the public health protection afforded by the current annual standard, the Administrator finds it is less clear whether the available scientific evidence and quantitative information calls into question the adequacy of the public health protection afforded by the current 24-hour standard. In considering how to revise the suite of standards to provide the requisite degree of protection, he recognizes that changes in PM2.5 air quality designed to meet either the annual or the 24-hour standard would likely result in changes to both long-term average and short-term peak PM2.5 concentrations. He also recognizes that the current annual standard and 24-hour standard, together, are intended to provide public health protection against the full distribution of short- and long-term PM2.5 exposures. As noted above, the annual standard is targeted at controlling the typical exposures for which the evidence of adverse health effects is strongest. The Administrator places the most weight on the large number and strength of epidemiologic studies that report positive, and often statistically significant, associations with long-term mean reported PM2.5 concentrations well below the current level of the annual standard of 12.0 ug/m[3], as well as corroborating evidence from U.S. accountability studies with starting concentrations below 12 ug/m3 and studies that found positive and statistically significant associations in analyses restricted to concentrations less than 12 ug/m[3]. In considering the risk assessment information, he notes that, for most of the U.S., the annual standard is the controlling standard and that the risk assessment estimates reductions in PM2.5-associated risks across more of the population and in more areas with alternative annual standard levels compared to estimates for alternative 24-hour standard levels. Moreover, the Administrator notes that a more stringent annual standard has been shown to effectively reduce both average (annual) concentrations and peak (daily) concentrations, ensuring the broadest protection of public health. Finally, the Administrator notes that the CASAC was unanimous in its advice regarding the need to revise the annual standard, although they did not reach consensus on what range of alternative levels would be most appropriate to consider. Thus, in considering how to revise the suite of standards to provide the requisite degree of protection, the Administrator proposes to conclude it is appropriate to focus on revising the annual standard.
b. Consideration of Alternative Primary Annual PM2.5 Standard Levels
This section summarizes the Administrator's conclusions and proposed decisions related to the current primary annual PM2.5 standard and presents his proposed decision to revise the level of the current annual standard within the range of 9.0 to 10.0 ug/m[3], in conjunction with retaining the current indicator, averaging time, and form of that standard. The EPA is also soliciting public comment on alternative annual standard levels down to 8.0 ug/m[3] and up to 11.0 ug/m[3], on an alternative 24-hour standard level as low as 25 ug/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 establishing primary standards under the Act that are "requisite" to protect public health with an adequate margin of safety, the Administrator is seeking to establish standards that are neither more nor less stringent than necessary for this purpose. He recognizes that the requirement to provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information and to provide a reasonable degree of protection against hazards that research has not yet identified. However, the Act does not require that primary standards be set at a zero-risk level; rather, the NAAQS must be sufficiently protective, but not more stringent than necessary.
Having reached the conclusion that the current indicator, averaging time, and form of the standard are appropriate for the reasons outlined above, the Administrator next considers the range of potential alternative standard levels that could be reasonably supported by the available scientific evidence and risk-based information to increase public health protection against short-term and long-term PM2.5 exposures. The evidence available in this reconsideration regarding PM2.5 exposures associated with health effects affirms and strengthens the evidence available at the completion of the 2009 ISA, taking into account studies evaluated in the 2019 ISA and ISA Supplement. The Administrator recognizes that the weight of evidence is strongest for health effects for which the 2019 ISA concludes that the evidence provides support for a causal relationship between PM2.5 exposures and health effects, including those between long- and short-term PM2.5 exposures and mortality and cardiovascular effects. He recognizes that the weight of evidence is also strong for health effects for which the 2019 ISA concludes that the evidence supports a likely to be causal relationship, which include long- and short-term PM2.5 exposures and respiratory effects and long-term PM2.5 exposures and cancer, and nervous system effects.
In considering the available scientific evidence that could inform conclusions regarding potential alternative levels of the annual PM2.5 standard, the Administrator notes that in past reviews, the decision framework used to judge adequacy of the existing PM2.5 standards, and what levels of any potential alternative standards should be considered, placed significant weight on epidemiologic studies that assessed associations between PM2.5 exposure and health outcomes that were most strongly supported by the body of scientific evidence (i.e., causal or likely to be causal determinations). In so doing, the Administrator recognizes that the number of epidemiologic studies has expanded since the completion of the 2009 ISA and the epidemiologic studies evaluated in the 2019 ISA and the ISA Supplement continue to report positive and statistically significant associations between long- and short-term exposure to PM2.5 and mortality and morbidity.
	Additionally, the Administrator recognizes that the available epidemiologic studies enable the examination of the entire population and include, and even focus on, those that may be at comparatively higher risk of experiencing a PM2.5-related health effects. The Administrator notes that the 2019 ISA found that factors that may contribute to increased risk of PM2.5-related health effects include lifestage (children and older adults), pre-existing diseases (cardiovascular disease and respiratory disease), and SES, and that the ISA Supplement noted new evidence that further supported racial and ethnic differences in PM2.5 exposures and PM2.5-related health risks. The Administrator also observes that at-risk populations make up a substantial portion of the U.S. population (section II.B.2 above), including children (22%) and older adults (16%), as well as non-Hispanic Black (12%) and Hispanic populations (18%) and that the prevalence of pre-existing diseases varies by lifestage and race/ethnicity. The Administrator notes that the cohorts examined in the epidemiologic studies available in this reconsideration include diverse populations that are broadly representative of the U.S. population as a whole, and include those populations identified as at-risk (i.e., children and older adults), as well as individuals in the general population with pre-existing disease, such as cardiovascular disease and respiratory disease.
Recent epidemiologic studies also strengthen support for health effect associations at lower ambient PM2.5 concentrations than previous reviews and studies that examine the shapes of C-R functions over the full distribution of ambient PM2.5 concentrations have not identified a threshold concentration, below which associations no longer exist (U.S. EPA, 2019a, section 1.5.3; U.S. EPA, 2022a, sections <IP_ADDRESS> and <IP_ADDRESS>).,Though these analyses are complicated by the relatively sparse data available at the lower end of the air quality distribution (U.S. EPA, 2019a, section 1.5.3), the evidence remains consistent in supporting a no-threshold relationship, and in supporting a linear relationship for PM2.5 concentrations > 8 μg/m[3], though uncertainties remain about the shape of the C-R curve at PM2.5 concentrations < 8 μg/m[3].
With respect to uncertainties in epidemiologic studies, a broad range of approaches have been adopted across studies to examine confounding and the results of those examinations support the robustness of reported associations. Additionally, there is a considerable amount of new epidemiologic evidence in this reconsideration, including a large number of new epidemiologic studies that use varying study designs that reduce uncertainties, including studies that employ alternative methods for confounder control and support associations between exposure and adverse health effects at lower PM2.5 concentrations. Consistent findings from the broad body of epidemiologic studies are supported by studies employing alternative methods for confounder control, which used a variety of statistical methods to control for confounding bias and consistently report positive associations. The results of these studies support the positive and significant effects seen in cohort studies associated with short- and long-term exposure to PM2.5 and mortality. Moreover, epidemiologic studies continue to evaluate the uncertainty related to exposure measurement error, and while none of these approaches eliminates the potential for exposure error in epidemiologic studies, the consistent reporting of PM2.5 health effect associations across exposure estimation approaches, even in the face of exposure error, together with the larger effect estimates reported in some studies that have attempted to reduce exposure error, provides further support for the robustness of associations between PM2.5 exposures and mortality and morbidity. Therefore, given the strength of the available epidemiologic evidence, including the ability of these studies to provide information about impacts on the most at-risk populations, the Administrator concludes that the strongest available evidence for evaluating alternative levels of the annual standard continues to be the epidemiologic studies.
The evidence base available in this reconsideration also consists of experimental studies that include controlled human exposure studies and animal toxicological studies.  These studies demonstrate health outcomes following long-term and short-term exposure to PM2.5 at exposures that are well-above those typically found in ambient air. This body of evidence provides support for the biological mechanisms and the plausibility of the serious health effects associated with ambient PM2.5 exposures in epidemiologic studies. Thus, the Administrator recognizes that while experimental studies may not be as useful in a decision-making framework alone, results from these studies lend further support to the use of the epidemiologic evidence base in informing the level of the annual standard.
In considering the level of the annual standard, the Administrator recognizes that the annual standard, with its form based on the arithmetic mean concentration, is most appropriately meant to limit the "typical" daily and annual exposures that are most strongly associated with the health effects observed in epidemiologic studies. However, the Administrator also recognizes that while epidemiologic studies examine associations between distributions of PM2.5 air quality and health outcomes, they do not identify particular PM2.5 exposures that cause effects. Thus, any approach that uses epidemiologic information in reaching decisions on what standards are appropriate necessarily requires judgments of the Administrator about how to consider the information available from the epidemiologic studies as a basis for appropriate standards. This includes consideration of how to weigh the uncertainties in the reported associations between daily or annual average PM2.5 exposures and mortality or morbidity in the epidemiologic studies. Such an approach is consistent with setting standards that are neither more nor less stringent than necessary, recognizing that a zero-risk standard is not required by the CAA.
Thus, in recognizing the need to weigh these uncertainties in reaching decisions on alternative standard levels to propose, the Administrator judges that it is most appropriate to examine where the evidence of associations observed in the epidemiologic studies is strongest and, conversely, where he has appreciably less confidence in the associations observed in the epidemiologic studies. Based on information evaluated in the 2019 ISA and ISA Supplement, the Administrator recognizes that health effects may occur over the full range of concentrations observed in the long- and short-term epidemiologic studies and that no discernible threshold for any effects can be identified based on the currently available evidence (U.S. EPA, 2019a, section 1.5.3, U.S. EPA, 2022a, section <IP_ADDRESS> and <IP_ADDRESS>). He also recognizes, in taking note of CASAC advice and the distributional statistics analysis discussed in section II.B.3.b above and in the PA, that there is significantly greater confidence in observed associations over certain parts of the air quality distributions in the studies, and conversely, that there is significantly diminished confidence in ascribing effects to concentrations toward the lower part of the distributions.
The Administrator notes that in previous reviews, evidence-based approaches noted that the evidence of an association in any epidemiologic study is "strongest at and around the long-term average where the data in the study are most concentrated" (78 FR 3140, January 15, 2013). Given this, these approaches focused on identifying standard levels near or somewhat below long-term mean concentrations reported in key epidemiologic studies. These approaches were supported by previous CASAC advice. The current CASAC also supported assessing the mean (or median) concentrations, but also suggested additional approaches that could be explored. In utilizing this evidence-based approach, the Administrator looks to study-reported means from the key epidemiologic studies (as shown in Figure 1 and Figure 2) available in this reconsideration. He notes that there have been new approaches to estimating exposure concentrations since the 2012 review, such that many of the available key epidemiologic studies include new approaches that apply hybrid modeling techniques to estimate exposures. In looking at the epidemiologic studies, he considers these studies in two groups: (1) monitor-based studies (epidemiologic studies that used ground-based monitors to estimate exposure, similar to approaches used in past reviews), and (2) hybrid modeling-based studies (epidemiologic studies that used hybrid modeling approaches to estimate exposures). As such, he recognizes that reported mean PM2.5 concentrations in monitor-based studies are 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 the study area, referred to as a maximum monitor concentration (i.e., the "design value" concentration), which is used to determine whether an area meets a given standard. Further, he recognizes that studies that use hybrid modeling approaches employ methods to estimate ambient PM2.5 concentrations across large geographical areas, including those without monitors, and thus, when compared to monitor-based studies, require additional information to inform the relationship between the estimated PM2.5 concentrations across an area to the maximum monitor design values used to assess compliance. For the key U.S. monitor-based epidemiologic studies, the study reported mean concentrations range from 9.9-16.5 μg/m[3] and for the U.S. hybrid modeling based key epidemiologic studies, the mean concentrations range from 9.3-12.2 μg/m[3].
In thinking further about the relationship between mean PM2.5 concentrations in key epidemiologic studies and annual design values, the Administrator specifically notes that in a given area, the area design value is determined by the monitor in an area with the highest PM2.5 concentrations and is used to determine compliance with the standard. He observes, as detailed above in the air quality analyses in section I.D.5, that the highest PM2.5 concentrations spatially distributed in the area would generally occur at or near the area design value monitor and that PM2.5 concentrations will be equal to or lower at other monitors in the area. Furthermore, since monitoring strategies aim to site monitors in areas with higher concentrations, monitored areas will generally have higher concentrations than areas without monitors. Thus, when a study reports a mean that reflects the average of annual average measured concentrations for an area, the area design value will generally be higher. Similarly, when a study reports a mean that reflects the average of annual average concentrations estimated at various points across an area using a hybrid modeling approach, the area design value will generally be higher. More specifically, the Administrator observes that the additional air quality analyses (described in section I.D.5) suggest that the area annual design value is greater than the study-reported mean values by 10-20% for monitor-based studies and 15-18% for hybrid modeling with population weighting applied. As such, the Administrator observes that a policy approach for setting a standard level that requires the design value monitor to meet study-reported means will generally result in lower concentrations of PM2.5 across the entire area, such that even those people living near an area design value monitor (where PM2.5 concentrations are generally highest) will be exposed to PM2.5 concentrations below the air quality conditions reported in the epidemiologic studies where there is the highest confidence of an association. In addition, he specifically notes that an annual standard level that is no more than 10-20% higher than the study-reported means in the U.S. monitor-based studies (i.e., for the lowest study reported mean value of 9.9 ug/m[3], this means an annual standard level of approximately 10.9-11.9 ug/m[3]) and no more than 15-18% higher for the U.S. hybrid modeling with population weighting applied (i.e., for the lowest study reported mean value of 9.3 ug/m[3], this means an annual standard level of approximately 10.7-11.0 ug/m[3]), would generally maintain air quality exposures at or below those associated with the study-reported mean PM2.5 concentrations, exposures for which we have the strongest support for adverse health effects occurring. Based on this, the Administrator concludes that a revised standard level of 9.0 to 10.0 ug/m[3] would generally limit air quality exposures to levels well below those associated with the study-reported mean PM2.5 concentrations in the key epidemiologic studies. A revised standard level of 11.0 ug/m[3] would maintain air quality exposures to below those associated with most of these study-reported means, and a revised standard level of 8.0 ug/m[3] would maintain air quality exposures to far below all of these study-reported means. The Administrator notes that every member of the CASAC found that the information on study-reported means supported revising the annual standard level to 10.0 ug/m[3], with the minority of the CASAC advising that these data also supported a revised annual standard level of 10.0-11.0 ug/m[3] and the majority of the CASAC advising that these study-reported means, in conjunction with additional bodies of evidence, supported a revised annual standard level of 8.0-10.0 ug/m[3].
The Administrator also considers additional information from epidemiologic studies, consistent with CASAC advice, to take into account the broader distribution of PM2.5 concentrations, including the 25[th] percentiles of the distributions, and the degree of confidence in the observed associations over the broader air quality distribution. In considering this additional information, he understands that the PA presented information on the distributions of PM2.5 concentrations, when available, from key epidemiologic studies to provide a general frame of reference as to the part of the distribution within which the data become appreciably more sparse and, thus, where his confidence in the associations observed in epidemiologic studies would become appreciably less. As discussed in section II.B.3.b above and presented in Figure 1 and Figure 2 above, he observes that most studies do not report such data and the conclusions that can be drawn from such information across the full body of evidence are quite limited. However, the Administrator takes note of additional population-level data that are available and in considering the long-term PM2.5 concentrations associated with the 25[th] percentile values of the population-level data for the studies for which such data are available, he observes that for the three key U.S. epidemiologic studies that use hybrid modeling approaches that apply population weighting and report these data, the values reported were 6.7 ug/m[3], 9.1 ug/m[3] and 9.1 ug/m[3]. For the U.S.-based studies that use ground-based monitors, the 25[th] percentiles ranged from 11.5 ug/m[3] to just below 13.0 ug/m[3].
The Administrator notes that there are substantial uncertainties associated with using 25[th] percentile data for purposes of setting this standard and these uncertainties are heightened by the relatively few studies which report such data and the fact that, by definition, this data is relatively less common even within a study for which it is reported. At the same time, the Administrator is conscious of his obligation to set primary standards with an adequate margin of safety and recognizes that some members of the CASAC advised that these data indicate that effects are occurring below the reported means of studies. Balancing these concerns about the need to provide some protection against uncertain risks with the obligation to not set standards that are more stringent than necessary, the Administrator preliminarily concludes that a revised standard should limit exposures to ambient concentrations near the 25[th] percentile of reported studies. Given this consideration, the Administrator recognizes that a standard level of 8.0-10.0 ug/m[3] is generally within the range of these values, while a standard level of 11.0 ug/m[3] is above the 25[th] percentile values reported in the hybrid model-based studies but below the 25[th] percentile values in studies that use ground-based monitors. Based on this, the Administrator recognizes that a standard within the range of 8.0-11.0 ug/m[3] would limit exposures to ambient concentrations near the 25[th] percentile reported in the available studies, with the lower end of this range further limiting those exposures.
	The Administrator also takes into consideration the long-term mean PM2.5 concentrations reported in Canadian epidemiologic studies that, in the context of the larger body of available evidence, provided support for causal or likely to be causal determinations between PM2.5 exposure and health effects, as summarized in the 2019 ISA and ISA Supplement. He notes that the study-reported means from these Canadian studies tend to be somewhat lower than those reported from the key epidemiologic studies in the U.S. ranging from 6.9-13.3 ug/m[3] for the monitor-based studies and 5.9-9.8 ug/m[3] for the hybrid model-based studies. However, the Administrator is also mindful that there are important differences between the exposure environments in the U.S. and Canada and that interpreting the data (e.g., mean concentrations) from the Canadian studies in the context of a U.S.-based standard may present challenges in directly and quantitatively informing decisions regarding potential alternative levels of the annual standard, as detailed above. He additionally notes that the majority of the CASAC pointed to the Canadian studies as supporting their recommendation to revise the annual standard level to within the range of 8.0-10.0 ug/m[3]. Based on this, the Administrator is not excluding Canadian studies from his consideration in this reconsideration, but he is considering them in light of the limitations and challenges presented.
The Administrator also notes that the CASAC recommended looking at the studies that included analyses that restrict annual average PM2.5 concentrations to concentrations below the level of the current annual standard in evaluating an appropriate range of levels for a revised annual standard. In this reconsideration, there are two key studies available (Di et al., 2017b and Dominici et al., 2019) that restrict annual average PM2.5 concentrations to less than 12 ug/m[3]. These restricted analyses report positive and statistically significant associations with all-cause mortality, and both report mean PM2.5 concentrations of 9.6 ug/m[3]. The Administrator does note that uncertainties exist in these analyses (described in more detail in sections II.B.3.b and II.D.2.a above), including uncertainty in how the studies exclude concentrations (e.g., at what spatial resolution are concentrations being excluded), which would make it difficult to compare concentrations in restricted analyses directly to design values. However, he does note that an annual standard level of 9.0-10.0 ug/m3 would be close to these reported mean values, while a standard level of 11.0 ug/m3 would be above and a standard level of 8.0 ug/m3 would be much further below.
The Administrator additionally considers recent U.S. accountability studies, which assess the health effects associated with actions that improve air quality (e.g., air quality policies or implementation of an intervention). The Administrator notes that there are three studies available in this reconsideration (Henneman et al. (2019b), Corrigan et al. (2018), and Sanders et al. (2020a)) that account for changes in PM2.5 concentrations due to implementation of policies and assess whether there was evidence of changes in associations with mortality or cardiovascular morbidity due to changes in annual PM2.5 concentrations. The Administrator notes that in each of these studies, prior to implementation of the policies, mean PM2.5 concentrations were below the level of the current annual standard level (12.0 ug/m[3]) and ranged from 10.0 ug/m[3] to 11.1 ug/m[3].  The Administrator notes that these studies report positive and significant associations between mortality and cardiovascular morbidity and reductions in ambient PM2.5 (described above in section II.B.3.b and in Table 3-12 of the PA) and notes that these studies suggest public health improvements may occur following the implementation of a policy that reduces annual average PM2.5 concentrations below the level of the current standard of 12.0 ug/m[3]. The Administrator notes that a revised annual standard level of 9.0-10.0 ug/m[3] would be at or below the lowest starting concentration of these accountability studies (i.e., 10.0 ug/m[3]).
	In addition to the evidence, the Administrator also considers the results of the risk assessment. The PA includes a risk assessment that estimates PM2.5-attributable mortality risk associated with PM2.5 air quality that has been adjusted to simulate "just meeting" the current standards, as well as potential alternative standards. These analyses of PM2.5-attributable mortality use input data that include C-R functions from epidemiologic studies that have no-threshold and a linear C-R relationship down to zero, as well an air quality adjustment approach that incorporates proportional decreases in PM2.5 concentrations to meet lower standard levels. Such an approach does not incorporate any elements of uncertainty in associations of health effects at lower concentrations and simulated air quality improvements will always lead to proportional decreases in risk (i.e., each additional ug/m[3] reduction produces additional benefits with no clear stopping point). Therefore, the Administrator recognizes that the risk estimates can help to place the evidence for specific health effects into a broader public health context, but should be considered along with the inherent uncertainties and limitations of such analyses when informing judgments about the potential for additional public health protection associated with PM2.5 exposure and related health effects.
	The risk assessment estimates that the current primary PM2.5 standards could allow a substantial number of PM2.5-associated deaths in the U.S. Additionally, compared to the current annual standard, meeting a revised annual standard with a lower level is estimated to reduce PM2.5-associated health risks in the 30 study areas controlled by the annual standard by about 7-9% for a level of 11.0 ug/m[3], 15-19% for a level of 10.0 ug/m[3], 22-28% for a level of 9.0 ug/m[3], and 30-37% for a level of 8.0 ug/m[3]) (U.S. EPA, 2022a, Table 3-17). The CASAC concurred with the PA's assessment that meaningful risk reductions will result from lowering the annual PM2.5 standard (Sheppard, 2022a, p. 16 of consensus responses).
	The PA also provides information on the distribution of concentrations associated with the estimated mortality risk at each alternative standard level assessed (U.S. EPA, 2022a, sections <IP_ADDRESS> and <IP_ADDRESS>, Figure 3-18 and 3-19). Further evaluating these results can help clarify the percentage of the exposure reductions that fall within the range of concentrations in which there is the most confidence in the associations and thus, confidence that estimated risk reductions will actually occur. When meeting a standard level of 11.0 ug/m[3], the risk is estimated to be associated with exposure concentrations that are generally greater than 10.0 ug/m[3], while for a standard level of 10.0 ug/m[3], the majority of the days contributing to the risk estimates are estimated to be below 10.0 ug/m[3]. When meeting an annual standard or 9.0 ug/m[3], the majority of the exposure concentrations are estimated to be 8.0-9.0 ug/m[3], while for a standard level of 8.0 ug/m[3], most of the days are below 8.0 ug/m[3]. The Administrator notes that the evidence suggests that majority of the study-reported means are above 10.0 ug/m[3] (concentrations at which the evidence is the strongest in supporting an association between exposure to PM2.5 and adverse health effects observed in the key epidemiologic studies available in this reconsideration) and that at PM2.5 concentrations less than 8.0 μg/m[3], the 2019 ISA notes that uncertainties remain in the shape of the C-R curve. He thus recognizes that there is increasing uncertainty in quantitative estimates of PM2.5-associated mortality risk for alternative standard levels at the lower end of the range of 8.0-11.0 μg/m[3].
	As discussed more above, the Administrator also recognizes that the risk assessment was able to include an at-risk analysis that estimated the potential long-term PM2.5-attributable exposure and mortality risk in older adults, stratified by racial/ethnic demographics, when meeting a revised annual standard with a lower level. While the Administrator recognizes that this analysis is subject to the multiple uncertainties and limitations (sections II.C.2 and II.D.2.b), he does note that the analysis suggests that a revised annual standard level within the range of 8.0 to 11.0 ug/m[3] is estimated to reduce PM2.5 exposure and may also help to mitigate risks. Based on the case study areas included in the analysis, The Administrator notes that what urban areas are included in the risk assessment analysis will greatly influence the results but notes that based on the areas included in the analyses, the results show the largest impact is on reducing exposure and risk in Black populations, who were estimated in the risk assessment case study areas to have the highest levels of exposures and the greatest rates of premature mortality risk. The Administrator also notes that, similar to the main risk estimates discussed above, there is increasing uncertainty in quantitative estimates of stratified risk estimates at the lower end of the range of standard levels assessed.
	The Administrator recognizes 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 also of the important uncertainties that remain in the evidence and the quantitative information being considered in this reconsideration. The Administrator also recognizes that the CAA requires him to set standards that in his judgment are neither more stringent nor less stringent than necessary to protect public health with an adequate margin of safety. Based on the above considerations, the Administrator concludes that it is appropriate to propose to set a level for the primary annual PM2.5 standard within the range of 9.0 to 10.0 ug/m[3], while also taking comment on a level for the primary annual PM2.5 standard as low as 8.0 ug/m[3] and as high as 11.0 ug/m[3]. The Administrator provisionally concludes that a standard level within the range of 9.0 to 10.0 ug/m[3] would reflect appropriate approaches to 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. He notes that a standard set at 9.0 to 10.0 ug/m[3] would be at or below the study-reported mean PM2.5 concentrations in the key U.S. epidemiologic studies, exposures for which we have the strongest support for adverse health effects occurring. Further, in considering margin of safety, he notes that an annual standard level that is no more than 10-20% higher than the study-reported means in the U.S. monitor-based studies (i.e., for the lowest study reported mean value of 9.9 ug/m[3], this means an annual standard level of approximately 10.9-11.9 ug/m[3]) and no more than 15-18% higher for the U.S. hybrid modeling with population weighting (i.e., for the lowest study reported mean value of 9.3 ug/m[3], this means an annual standard level of approximately 10.7-11.0 ug/m[3]), would generally maintain air quality exposures at or below those associated with the study-reported mean PM2.5 concentrations. Additionally, the Administrator also notes that these key U.S. epidemiologic studies utilize cohorts that include populations identified as at-risk, including children and older adults, as well as individuals in the general population with pre-existing disease, like cardiovascular disease and respiratory disease. Based on this information, he concludes that a revised standard level of 9.0-10.0 ug/m[3] would limit air quality exposures to concentrations well below those associated with the study reported mean, studies which include and assess impacts on the most at-risk populations. Thus, the Administrator provisionally concludes that a standard level within this range would appropriately provide an adequate margin of safety for the populations most at risk for adverse health effects associated with exposure to PM2.5.
	The Administrator also considers other lines of evidence, including the study reported means from epidemiologic studies that restrict concentrations to levels below 12 ug/m[3], the 25[th] percentiles values reported by a subset of epidemiologic studies, and the information from the accountability studies. He notes that a standard in the range of 9.0 to 10.0 ug/m[3] would limit exposures to ambient concentrations near the 25[th] percentile reported in the available studies, with a standard level of 9.0 ug/m[3] limiting those exposures somewhat more than a standard level of 10.0 ug/m[3]. He also notes that a standard in the range of 9.0 to 10.0 ug/m[3] would be near the value of the study reported means from the two available long-term restricted analyses studies (i.e., 9.6 ug/m[3]). The Administrator notes a standard level of 9.0-10.0 ug/m[3], would also be at or below the lowest starting concentration of the newest available accountability studies (i.e., 10.0-11.1 ug/m[3]). The Administrator also considers the results from the risk assessment. He recognizes that the risk estimates should be considered along with the inherent uncertainties and limitations of such analyses when informing judgments about the potential for additional public health protection associated with PM2.5 exposure and related health effects. When looking at the risk assessment results, he notes that an annual standard level of 9.0-10.0 ug/m[3] is estimated to reduce exposure concentrations such that those remaining risks are associated with exposure concentrations that are below most of the study-reported means in the key U.S. epidemiologic studies, where we have the strongest support for adverse health effects occurring, and below PM2.5 concentrations (i.e., 8 μg/m[3]) where the 2019 ISA notes that uncertainties remain in the shape of the C-R curve, particularly for a standard level as low as 9.0 μg/m[3]. Lastly, the Administrator also notes that every member of the CASAC found that the available scientific evidence and information supported revising the annual standard level to a level of 10.0 ug/m[3]. Additionally, the majority of the CASAC also recommended that the available evidence and information supported revision to a level of 9.0 ug/m[3]. Thus, recognizing the uncertainties in the evidence and the necessity of providing requisite protection, with an adequate margin of safety, the Administrator is proposing to set the level of the annual standard in the range of 9.0-10.0 ug/m[3], and solicits comments on the appropriate standard level within that range.
	While the Administrator recognizes that some members of the CASAC advised, and the PA concluded, that the available scientific information provides support for considering a range that extends up to 11.0 ug/m[3] and down to 8.0 ug/m[3], he provisionally concludes that proposing such an extended range would not be appropriate at this time. More specifically, the Administrator provisionally concludes that proposing to revise the annual standard level to above 10.0 ug/m3 and as high as 11.0 ug/m[3] would reflect a public health policy approach that would place less weight on setting a standard level at or below the study-reported means from a number of key U.S. epidemiologic studies and less weight on the risk assessment results. Such an approach would also place little or no weight on the study reported means from epidemiologic studies that restrict concentrations to below 12 ug/m[3] and the 25[th] percentile concentrations reported by a subset of epidemiologic studies. The Administrator notes that such an approach may fail to provide an adequate margin of safety in light of the evidence available in this reconsideration. In considering revision to the annual standard level to below 9.0 ug/m3 and as low as 8.0 ug/m[3], the Administrator notes that such a level would be substantially below the study-reported means and would not recognize the controlling nature of the design value monitor with respect to the concentration gradients consistently occurring across urban areas. The Administrator also recognizes that the evidence and uncertainties for public health benefits of lower standards exists on a continuum across the range of possible standard levels.  He preliminarily judges that the evidence is sufficient to support standards in the range of 9.0-10.0 ug/m[3], recognizing that the selection of a final standard level will depend on judgments about the relative weight to place on various aspects of the evidence and how to provide for an adequate margin of safety. However, the Administrator preliminarily judges that the available information and evidence are not sufficient to warrant revising the level of the annual standard below 9.0 ug/m[3]. He finds the uncertainties as to the public health risks and benefits associated with such a standard to be too great at this time. Nonetheless, while the Administrator notes these considerations above, he solicits comment on revising the annual standard down to a level below 9.0 ug/m[3] and as low as 8.0 ug/m[3], as well as to above 10.0 ug/m[3] and as high as 11.0 ug/m[3], and on approaches for interpreting the scientific evidence and rationales that would support such a level.
E. Proposed Decisions on the Primary PM2.5 Standards
Taking the above considerations into account, upon reconsidering the current primary PM2.5 standards in light of the currently available scientific evidence and quantitative information, the Administrator proposes to revise the level of the primary annual PM2.5 standard from 12.0 ug/m[3] to within the range of 9.0 to 10.0 ug/m[3] and to retain the 24-hour standard level at 35 ug/m[3]. In the Administrator's judgment, such a suite of primary PM2.5 standards and the rationale supporting such levels could reasonably be judged to reflect the appropriate consideration of the strength of the available evidence and other information and their associated uncertainties and the advice of the CASAC.
The Administrator recognizes that the final suite of standards will reflect the Administrator's ultimate judgments in the final rulemaking as to the suite of primary PM2.5 standards that are requisite to protect the public health with an adequate margin of safety from effects associated with PM2.5 exposures. The final judgments to be made by the Administrator will appropriately consider the requirement for standards that are 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 margin of safety.
Having reached his provisional judgment to propose revising the annual standard level from 12.0 to within a range of 9.0 to 10.0 ug/m[3] and to propose retaining the 24-hour standard level at 35 ug/m[3], the Administrator solicits public comment on this range of levels and on approaches to considering the available evidence and information that would support the choice of levels within this range. The Administrator also solicits public comment on alternative annual standard levels down to 8.0 ug/m[3] and up to 11.0 ug/m[3], on an alternative 24-hour standard level as low as 25 ug/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.
III. Rationale for Proposed Decisions on the Primary PM10 Standard
This section presents the rationale for the Administrator's proposed decision to retain the existing primary PM10 standard. This decision is based on a thorough review of the latest scientific information, published through January 2018, and evaluated in the 2019 ISA, on human health effects associated with PM10 - 2.5 in ambient air. As described in section 1.2 of the ISA Supplement, the scope of the updated scientific evaluation of the health effects evidence is based on those PM size fractions, exposure durations, and health effects category combinations where the 2019 ISA concluded a causal relationship exists (U.S. EPA, 2019a, U.S. EPA, 2022a). Therefore, because the 2019 ISA did not conclude a causal relationship for PM10-2.5 for any exposure durations or health effect categories, the ISA Supplement does not include an evaluation of additional studies for PM10-2.5. As a result, the 2019 ISA continues to serve as the scientific foundation for assessing the adequacy of the primary PM10 standard in this reconsideration of the 2020 final decision (U.S. EPA, 2019a, section 1.7; U.S. EPA, 2022a). The Administrator's rationale also takes into account: (1) the PA evaluation of the policy-relevant information in the 2019 ISA; (2) CASAC advice and recommendations, as reflected in discussions of the draft of the PA at public meetings and in the CASAC's letter dated March 18, 2022, to the Administrator; and (3) public comments received during the development of the PA.
In presenting the rationale for the Administrator's proposed decision and its foundations, section III.A provides background and introductory information for this reconsideration of the primary PM10 standard. It includes background on the 2020 final decision to retain the primary PM10 standard (section III.A.1) and also describes the general approach for this reconsideration (section III.A.2) Section III.B summarizes the key aspects of the currently available scientific evidence for PM10-2.5-related health effects. Section III.C presents the Administrator's proposed conclusions regarding the adequacy of the primary PM10 standard (section III.C.3), drawing on evidence-based considerations (section III.C.2) and advice from the CASAC (section III.C.1).
A. General Approach
The current primary PM10 standard was affirmed in 2020 based on the scientific information available at that time, as well as the Administrator's judgments regarding the available public health effects evidence, and the appropriate degree of public health protection for the existing standards (85 FR 82725, December 18, 2020). With the 2020 decision, the Administrator retained the existing 24-hour primary PM10 standard, with its level of 150 ug/m[3] and its one-expected-exceedance form on average over three years, to continue to provide public health protection against short-term exposures to PM10-2.5 (85 FR 82725, December 18, 2020). The subsection below focuses on the key considerations, and the prior Administrator's conclusions, for PM10-2.5-related health effects and the adequacy of the primary PM10 standard in the 2020 review.
1. Background on the Current Standard
In the 2019 ISA, the strongest evidence for PM10-2.5-related health effects was for cardiovascular effects, respiratory effects, and premature mortality following short-term exposures. For each of these categories of effects, the 2019 ISA concludes that the evidence was "suggestive of, but not sufficient to infer, a causal relationship". Specifically, the health effects evidence evaluated in the 2019 ISA included an expanded body of scientific evidence that has become available since the completion of the 2009 ISA linking short-term PM10-2.5 to health outcomes such as premature death and hospital visits (U.S. EPA, 2009a; U.S. EPA, 2019a). This evidence base  evaluated the causal relationships between short-term exposure to PM10-2.5 and a broad range of health effects (U.S. EPA, 2019a, section 1.4.2). These effects associated with short-term exposure ranged from hospital admissions and emergency department visits for cardiovascular effects (documented in epidemiologic studies that reported PM10-2.5 associations with cardiovascular hospital admissions and emergency department visits in study locations with mean 24-hour average PM10-2.5 concentrations ranging from 7.4 to 13 ug/m[3]) and respiratory effects (documented in epidemiologic studies that reported PM10-2.5 associations with respiratory hospital admissions and emergency department visits in study locations with mean 24-hour average concentrations ranging from 5.6 to 16.2 ug/m[3]) to mortality (documented in epidemiologic studies that reported PM10-2.5 associations with mortality in study areas with mean 24-hour average concentrations ranging from 6.1 to 16.4 ug/m[3]). In addition to the epidemiologic studies, the evidence base included a small number of controlled human exposure studies and animal toxicological studies that provided insight into the biological plausibility of these effects. Collectively, the epidemiologic studies, controlled human exposure, and animal toxicological studies, with their inherent uncertainties, contributed to the causality determinations of "suggestive of, but not sufficient to infer, a causal relationship" between short-term exposures to PM10-2.5 and cardiovascular effects, respiratory effects, cancer, and mortality (U.S. EPA, 2019a, section 1.4.2). The 2019 ISA includes expanded evidence for the relationships between long-term exposures and cardiovascular effects, metabolic effects, nervous system effects, cancer, and mortality. While the evidence available in the 2019 ISA included additional health outcomes, including those associated with long-term PM10-2.5 -  exposure, key limitations in the evidence that were identified in the 2009 ISA persist in studies evaluated in the 2019 ISA.
In considering the available body of evidence, it was noted in the 2020 review there were considerable uncertainties and limitations associated with the experimental evidence for PM2.5 exposures and health effects, and as such more weight was placed on the available epidemiologic evidence. Therefore, the primary focus in the 2020 review was on multi-city and single-city epidemiologic studies that evaluated associations between short-term PM10-2.5 and mortality, cardiovascular effects (hospital admissions and emergency department visits, as well as blood pressure and hypertension), and respiratory effects. Despite differences in the approaches used to estimate ambient PM10-2.5 concentrations, the majority of the studies reported positive, though often not statistically significant, associations with short-term PM10-2.5 exposures. Most PM10-2.5 effect estimates remained positive in copollutant models that included either gaseous pollutants or other particulate matter size fractions (e.g., PM2.5). In U.S. study locations likely to have met the PM10 standard during the study period, a few studies reported positive associations between PM10-2.5 and mortality that were statistically significant and remained so in copollutant models (U.S. EPA, 2019a). In addition to the epidemiologic studies, there were a small number of controlled human exposure studies evaluated in the 2019 ISA that reported alterations in heart rate variability or increased pulmonary inflammation following short-term exposure to PM10-2.5, providing some support for the associations in the epidemiologic studies. Animal toxicological studies examined the effect of short-term PM10-2.5 exposures using non-inhalation (e.g., intratracheal instillation) route. Therefore, these studies provided limited evidence for the biological plausibility of PM10-2.5-induced effects (U.S. EPA, 2019a). Although the scientific evidence available in the 2019 ISA expanded the understanding of health effects associated with PM10-2.5 exposures, a number of important uncertainties remained. These uncertainties, and their implications for interpreting the scientific evidence, include the following:
The potential for confounding by copollutants, notably PM2.5, was addressed with copollutant models in a relatively small number of PM10-2.5 epidemiologic studies (U.S. EPA, 2019a). This was particularly important given the relatively small body of experimental evidence (i.e., controlled human exposure and animal toxicological studies) available to support the independent effect of PM10-2.5 on human health. This increases the uncertainty regarding the extent to which PM10-2.5 itself, rather than one or more copollutants, is responsible for the mortality and morbidity effects reported in epidemiologic studies.
There was greater spatial variability in PM10-2.5 concentrations than PM2.5 concentrations, resulting in the potential for increased exposure error for PM10-2.5 (U.S. EPA, 2019a). Available measurements did not provide sufficient information to adequately characterize the spatial distribution of PM10-2.5 concentrations (U.S. EPA, 2019a). The limitations in estimates of ambient PM10-2.5 concentrations "would tend to increase uncertainty and make it more difficult to detect effects of PM10-2.5 in epidemiologic studies" (U.S. EPA, 2019a).
Estimation of PM10-2.5 concentrations over which reported health outcomes occur remain highly uncertain. When compared with PM2.5, there is uncertainty spanning all epidemiologic studies examining associations with PM10−2.5 including deficiencies in the existing monitoring networks, the lack of a systematic evaluation of the various methods used to estimate PM10−2.5 concentrations and the resulting uncertainty in the spatial as well as the temporal variability in PM10−2.5 concentration (U.S. EPA, 2019a). Given these limitations in routine monitoring, epidemiologic studies employed a number of different approaches for estimating PM10-2.5 concentrations, including (1) calculating the difference between PM10 and PM2.5 at co-located monitors, (2) calculating the difference between county-wide averages of monitored PM10 and PM2.5 based on monitors that are not necessarily co-located, and (3) direct measurement of PM10-2.5 using a dichotomous sampler (U.S. EPA, 2019a, section 1.4.2). Given the relatively small number of PM10-2.5 monitoring sites, the relatively large spatial variability in ambient PM10-2.5 concentrations, the use of different approaches to estimating ambient PM10-2.5 concentrations across epidemiologic studies, and the limitations inherent in such estimates, the distributions of PM10-2.5 concentrations over which reported health outcomes occur remain highly uncertain (U.S. EPA, 2019a).
There was relatively little information available to characterize the apparent variability in associations between short-term PM10-2.5 exposures and health effects across study locations (U.S. EPA, 2019a). Specifically, the relative lack of information on the chemical and biological composition of PM10-2.5 as well as potential spatial and temporal variability in PM10-2.5 exposures complicates the interpretation of results between study locations (U.S. EPA, 2009b, U.S. EPA, 2019a).
Consistent with the general approach routinely employed in NAAQS reviews, the initial consideration in the 2020 review of the primary PM10 standard was with regard to the adequacy of protection provided by the then-existing standard. Key aspects of that consideration are summarized below.
i. Considerations Regarding the Adequacy of the Existing Standard in the 2020 Review
In the 2020 final decision, the EPA retained the existing 24-hour primary PM10 standard with its level of 150 ug/m[3] and its one-expected-exceedance form on average over three years to continue to provide public health protection against exposures to PM10-2.5 (85 FR 82727, December 18, 2020). In reaching his decision, the Administrator specifically noted that, while the health effects evidence was somewhat expanded since the prior reviews, the overall conclusions in the 2019 ISA, including uncertainties and limitations, were generally consistent with what was considered in the 2012 review (85 FR 82725, December 18, 2020). In addition, the Administrator recognized that there were still a number of uncertainties and limitations associated with the available evidence. With regard to the evidence on PM10-2.5-related health effects, the Administrator noted that epidemiologic studies continued to report positive associations with mortality and morbidity in cities across North America, Europe, and Asia, where PM10-2.5 sources and composition were expected to vary widely. While significant uncertainties remained in the 2020 review, the Administrator recognized that this expanded body of evidence had broadened the range of effects that have been linked with PM10-2.5 exposures. The studies evaluated in the 2019 ISA expanded the scientific foundation presented in the 2009 ISA and led to revised causality determinations (and new determinations) for long-term PM10-2.5 exposures and mortality, cardiovascular effects, metabolic effects, nervous system effects, and cancer (85 FR 82726, December 18, 2020). Drawing from his consideration of this evidence, the Administrator concluded that the scientific information available since the time of the last review supported a decision to maintain a primary PM10 standard to provide public health protection against PM10-2.5 exposures, regardless of location, source of origin, or particle composition (85 FR 82726, December 18, 2020). With regard to uncertainties in the available evidence, the Administrator first noted that a number of limitations were identified in the 2012 review related to: (1) estimates of ambient PM10-2.5 concentrations used in epidemiologic studies; (2) limited evaluation of copollutant models to address the potential for confounding; and (3) limited experimental studies supporting biological plausibility for PM10-2.5-related effects. Despite the expanded body of evidence for PM10-2.5 exposures and health effects, the Administrator recognized that uncertainties in the 2020 review continued to include those associated with the exposure estimates used in epidemiologic studies, the independence of the PM10-2.5 health effect associations, and the biologically plausible pathways for PM10-2.5 health effects (85 FR 82726, December 18, 2020). These uncertainties contributed to the 2019 ISA determinations that the evidence is at most "suggestive of, but not sufficient to infer" causal relationships (85 FR 82726, December 18, 2020). In considering the available evidence in his basis for the proposed decision, the Administrator emphasized evidence supporting "causal" and "likely to be causal" relationships, and therefore, judged that the PM10-2.5-related health effects evidence provided an uncertain scientific foundation for making standard-setting decisions. He further judged limitations in the evidence raised questions as to whether additional public health improvements would be achieved by revising the existing PM10 standard (85 FR 24126, April 30, 2020). In the 2020 decision, for all of the reasons discussed above and recognizing the CASAC conclusion that the evidence provided support for retaining the current standard, the Administrator concluded that it was appropriate to retain the existing primary PM10 standard, without revision. His decision was consistent with the CASAC advice related to the primary PM10 standard. Specifically, the CASAC agreed with the 2020 PA conclusions that, while these effects are important, the "evidence does not call into question the adequacy of the public health protection afforded by the current primary PM10 standard" and "supports consideration of retaining the current standard in this review" (Cox, 2019b, p. 3 of letter). Thus, the Administrator concluded that the primary PM10 standard (in all of its elements) was requisite to protect public health with an adequate margin of safety against effects that have been associated with PM10-2.5. In light of this conclusion, the EPA retained the existing PM10 standard.
2. General Approach and Key Issues in this Reconsideration of the 2020 Final Decision
	To evaluate whether it is appropriate to consider retaining the current primary PM10 standard, or whether consideration of revision is appropriate, the EPA has adopted an approach in this reconsideration that builds upon the general approach used in past reviews and reflects the body of evidence and information now available, as well as the assessments and evaluations performed in those reviews. As summarized above, the Administrator's decision in the 2020 review was based on an integration of PM10-2.5-related health effects information with the judgments on the public health significance of key effects, policy judgments as to when the standard is requisite, consideration of CASAC advice, and consideration of public comments.
Similarly, in this reconsideration, information is drawn from recent studies of PM10-2.5-related health effects. In so doing, the PA considers information critically analyzed and characterized in the 2019 ISA, as well as consideration of the associated uncertainties and limitations for the available evidence.
B. Overview of the Health Effects Evidence
The information summarized here is based on the scientific assessment of the health effects evidence available in this reconsideration; this evaluation is documented in the 2019 ISA and its policy implications are discussed further in the PA. As noted above, the ISA Supplement does not include an evaluation of studies for PM10-2.5 and the 2019 ISA continues to serve as the scientific foundation for this reconsideration.
1. Nature of Effects