From: Lydia Wegman/RTP/USEPA/US

To: "Frey, Nathan J." <Nathan_J._Frey@omb.eop.gov>, Margo_Schwab@omb.eop.gov

Cc: Karen Martin/RTP/USEPA/US@EPA, Beth Hassett-Sipple/RTP/USEPA/US@EPA, 

Joseph-J Dougherty/DC/USEPA/US@EPA, John Hannon/DC/USEPA/US@EPA, David 

Orlin/DC/USEPA/US@EPA

Date: 12/13/2012 12:26 PM

Subject: Revisions to section III of the PM NAAQS NFR to address your comments on the annual 

standard

Nathan and Margo, 

The attached redline file shows changes made to the Nov. 20 OMB review draft of section III. 

These changes primarily address your comments as discussed on our 11 AM call yesterday. 

They begin around p. 88, then mostly appear on pp. 175-191 and 207-239. 

In addition, you will see redline in other parts of the section, including our response to comments 

regarding the Janes et al. and Greven et al. studies (around pp. 72-78) which we have previously 

sent to you. There are also edits to our response to comments regarding spatial averaging and 

near-road monitoring related to the form of the annual standard, which are unrelated to your 

comments of yesterday, but which we have incorporated into the section to strengthen it. You will 

also see other minor edits throughout this section.

Please let us know as soon as possible if you have any questions. We would like to incorporate 

these revisions into the package later this afternoon. Thanks.
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I. Rationale for Final Decisions on the Primary PM2.5 Standards

This section presents the Administrator’s final decision regarding the need to 

revise the current primary PM2.5 standards and, more specifically, regarding revisions to 

the level and form of the existing primary annual PM2.5 standard in conjunction with 

retaining the existing primary 24-hour PM2.5 standard. As discussed more fully below, the 

rationale for the final decision is based on a thorough review, in the Integrated Science 

Assessment, of the latest scientific information, published through mid-2009, on human 

health effects associated with long- and short-term exposures to fine particles in the 

ambient air. The final decisions also take into account: (1) staff assessments of the most 

policy-relevant information presented and assessed in the Integrated Science Assessment 

and staff analyses of air quality and human risks presented in the Risk Assessment and 

the Policy Assessment, upon which staff conclusions regarding appropriate 

considerations in this review are based; (2) CASAC advice and recommendations, as 

reflected in discussions of drafts of the Integrated Science Assessment, Risk Assessment, 

and Policy Assessment at public meetings, in separate written comments, and in 

CASAC’s letters to the Administrator; (3) the multiple rounds of public comments 

received during the development of these documents, both in connection with CASAC 

meetings and separately; and (4) extensive public comments received on the proposal. 

In developing this final rule, the Administrator recognizes that the CAA requires 

her to reach a public health policy judgment as to what standards would be requisite --

neither more nor less stringent than necessary -- to protect public health with an adequate 

margin of safety, based on scientific evidence and technical assessments that have 

inherent uncertainties and limitations. This judgment requires making reasoned decisions 
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as to what weight to place on various types of evidence and assessments, and on the 

related uncertainties and limitations. Thus, in selecting the final standards, the 

Administrator is seeking not only to prevent fine particle concentrations that have been 

demonstrated to be harmful but also to prevent lower fine particle concentrations that 

may pose an unacceptable risk of harm, even if the risk is not precisely identified as to 

nature or degree. 

As discussed below, as well as in more detail in the proposal, a substantial amount 

of new research has been conducted since the close of the science assessment in the last 

review of the PM2.5 NAAQS (U.S. EPA, 2004), with important new information coming 

from epidemiological studies, in particular. This body of evidence includes hundreds of 

new epidemiological studies conducted in many countries around the world. In its 

assessment of the evidence judged to be most relevant to making decisions on elements 

of the primary PM2.5 standards, the EPA has placed greater weight on U.S. and Canadian 

studies using PM2.5 measurements, since studies conducted in other countries may reflect 

different demographic and air pollution characteristics.1

The newly available research studies as well as the earlier body of scientific 

evidence presented and assessed in the Integrated Science Assessment have undergone 

intensive scrutiny through multiple layers of peer review and opportunities for public 

review and comment. In developing this final rule, the EPA has drawn upon an 

integrative synthesis of the entire body of evidence concerning exposure to ambient fine 

particles and a broad range of health endpoints (U.S. EPA, 2009a, Chapters 2, 4, 5, 6, 7, 

and 8) focusing on those health endpoints for which the Integrated Science Assessment 

 

1 Nonetheless, the Administrator recognizes the importance of all studies, including 

international studies, in the Integrated Science Assessment’s considerations of the weight 

of the evidence that informs causality determinations.
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concludes that there is a causal or likely causal relationship with long- or short-term 

PM2.5 exposures. The EPA has also considered health endpoints for which the Integrated 

Science Assessment concludes there is evidence suggestive of a causal relationship with 

long-term PM2.5 exposures. 

The EPA has also drawn upon a quantitative risk assessment based upon the 

scientific evidence described and assessed in the Integrated Science Assessment. These 

analyses, discussed in the Risk Assessment (U.S. EPA, 2010a) and Policy Assessment 

(U.S. EPA, 2011a, chapter 2), have also undergone intensive scrutiny through multiple 

layers of peer review and multiple opportunities for public review and comment. 

Although important uncertainties remain in the qualitative and quantitative 

characterizations of health effects attributable to ambient fine particles, progress has been 

made in addressing these uncertainties in this review. The EPA’s review of this 

information has been extensive and deliberate. This intensive evaluation of the scientific 

evidence and quantitative assessments has provided a comprehensive and adequate basis 

for regulatory decision making at this time. 

This section describes the integrative synthesis of the evidence and technical 

information contained in the Integrated Science Assessment, the Risk Assessment, and 

the Policy Assessment with regard to the current and alternative standards. The EPA 

notes that the final decision for retaining or revising the current primary PM2.5 standards 

is a public health policy judgment made by the Administrator. The Administrator’s final 

decision draws upon scientific information and analyses related to health effects and 

risks; judgments about uncertainties that are inherent in the scientific evidence and 

analyses; CASAC advice; and comments received in response to the proposal.
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In presenting the rationale for the final decisions on the primary PM2.5 standards, 

this section begins with a summary of the approaches used in setting the initial primary 

PM2.5 NAAQS in 1997 and in reviewing and revising those standards in 2006 (section 

III.A.1). The D.C. Circuit Court of Appeals remand of the primary annual PM2.5 standard 

in 2009 is discussed in section III.A.2. Taking into consideration this history, section 

IIIII.A.3 describes the EPA’s general approach used in the current review for considering 

the need to retain or revise the current suite of fine particle standards, taking into account 

public comment on the proposed approach. 

The scientific evidence and quantitative risk assessment were presented in

sections III.B and III.C of the proposal, respectively (77 FR 38906 to 38917, June 29, 

2012) and are outlined in sections III.B and III.C below. Subsequent sections of this 

preamble provide a more complete discussion of the Administrator’s rationale, in light of 

key issues raised in public comments, for concluding that it is appropriate to revise the 

suite of current primary PM2.5 standards (section III.D), as well as a more complete 

discussion of the Administrator’s rationale for retaining or revising the specific elements 

of the primary PM2.5 standards, namely the indicator (section III.E.1); averaging time 

(section III.E.2); form (section III.E.3); and level (section III.E.4). A summary of the 

final decisions to revise the suite of primary PM2.5 standards is presented in section III.F. 

A. Background

There are currently two primary PM2.5 standards providing public health 

protection from effects associated with fine particle exposures. The annual standard is set 

at a level of 15.0 μg/m3, based on the 3-year average of annual arithmetic mean PM2.5

concentrations from single or multiple monitors sited to represent community-wide air 

quality. The 24-hour standard is set at a level of 35 μg/m3, based on the 3-year average of 
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the 98th percentile of 24-hour PM2.5 concentrations at each population-oriented monitor 

within an area. 

The past and current approaches for reviewing the primary PM2.5 standards 

described below are all based most fundamentally on using information from 

epidemiological studies to inform the selection of PM2.5 standards that, in the 

Administrator’s judgment, protect public health with an adequate margin of safety. Such 

information can be in the form of air quality distributions over which health effect 

associations have been observed in scientific studies or in the form of concentrationresponse functions that support quantitative risk assessment. However, evidence- and 

risk-based approaches using information from epidemiological studies to inform 

decisions on PM2.5 standards are complicated by the recognition that no population 

threshold, below which it can be concluded with confidence that PM2.5-related effects do 

not occur, can be discerned from the available evidence.2 As a result, any general 

approach to reaching decisions on what standards are appropriate necessarily requires 

judgments about how to translate the information available from the epidemiological 

studies into a basis for appropriate standards. This includes consideration of how to 

weigh the uncertainties in the reported associations across the distributions of PM2.5

concentrations in the studies and the uncertainties in quantitative estimates of risk, in the 

context of the entire body of evidence before the Agency. Such approaches are consistent 

with setting standards that are neither more nor less stringent than necessary, recognizing 

that a zero-risk standard is not required by the CAA. 

 

2

The term "evidence-based" approach or consideration generally refers to using the 

information in the scientific evidence to inform judgments on the need to retain or revise 

the NAAQS. The term "risk-based" generally refers to using the quantitative information 

in the Risk Assessment to inform such judgments.
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1. General Approach Used in Previous Reviews

The general approach used to translate scientific information into standards in the 

previous PM NAAQS reviews focused on consideration of alternative standard levels that 

were somewhat below the long-term mean PM2.5 concentrations reported in key 

epidemiological studies (U.S. EPA, 2011a, section 2.1.1). This approach recognized that 

the strongest evidence of PM2.5-related associations occurs where the bulk of the data 

exists, which is over a range of concentrations around the long-term (i.e., annual) mean. 

In setting primary PM2.5 annual and 24-hour standards for the first time in 1997, 

the Agency relied primarily on an evidence-based approach that focused on 

epidemiological evidence, especially from short-term exposure studies of fine particles 

judged to be the strongest evidence at that time (U.S. EPA, 2011a, section 2.1.1.1). The 

EPA selected a level for the annual standard that was at or below the long-term mean 

PM2.5 concentrations in studies providing evidence of associations with short-term PM2.5

exposures, placing greatest weight on those short-term exposure studies that reported 

clearly statistically significant associations with mortality and morbidity effects. Further 

consideration of long-term mean PM2.5 concentrations associated with mortality and 

respiratory effects in children did not provide a basis for establishing a lower annual 

standard level. The EPA did not place much weight on quantitative risk estimates from 

the very limited risk assessment conducted, but did conclude that the risk assessment 

results confirmed the general conclusions drawn from the epidemiological evidence that a 

serious public health problem was associated with ambient PM levels allowed under the 

then current PM10 standards (62 FR 38665/1, July 18, 1997).
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The EPA considered the epidemiological evidence and data on air quality 

relationships to set an annual PM2.5 standard that was intended to be the “generally 

controlling” standard; i.e., the primary means of lowering both long- and short-term 

ambient concentrations of PM2.5.

3

In conjunction with the annual standard, the EPA also 

established a 24-hour PM2.5 standard to provide supplemental protection against days 

with high peak concentrations, localized “hotspots,” and risks arising from seasonal 

emissions that might not be well controlled by an annual standard (62 FR 38669/3). 

In 2006, the EPA used a different evidence-based approach to assess the 

appropriateness of the levels of the 24-hour and annual PM2.5 standards (U.S. EPA, 

2011a, section 2.1.1.2). Based on an expanded body of epidemiological evidence that was 

stronger and more robust than that available in the 1997 review, including additional 

studies of both short- and long-term exposures, the EPA decided that using evidence of 

effects associated with periods of exposure that were most closely matched to the 

averaging time of each standard was the most appropriate public health policy approach 

for evaluating the scientific evidence to inform selecting the level of each standard. Thus, 

the EPA relied upon evidence from the short-term exposure studies as the principal basis 

for revising the level of the 24-hour PM2.5 standard from 65 to 35 μg/m3to protect 

 

3

In so doing, the EPA noted that because an annual standard would focus control 

programs on annual average PM2.5 concentrations, it would not only control long-term 

exposure levels, but would also generally control the overall distribution of 24-hour 

exposure levels, resulting in fewer and lower 24-hour peak concentrations. Alternatively, 

a 24-hour standard that focused controls on peak concentrations could also result in lower 

annual average concentrations. Thus, the EPA recognized that either standard could 

provide some degree of protection from both short- and long-term exposures, with the 

other standard serving to address situations where the daily peaks and annual averages 

are not consistently correlated (62 FR 38669, July 18, 1997). In the circumstances 

presented in that review, the EPA determined that it was appropriate to focus on the 

annual standard as the standard basedbest suited to control both annual and daily air 

quality distributions [(62 FR 38670].).
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against effects associated with short-term exposures. The EPA relied upon evidence from 

long-term exposure studies as the principal basis for retaining the level of the annual 

PM2.5 standard at 15 μg/m3to protect against effects associated with long-term exposures. 

This approach essentially took the view that short-term studies were not appropriate to 

inform decisions relating to the level of the annual standard, and long-term studies were 

not appropriate to inform decisions relating to the level of the 24-hour standard. With 

respect to quantitative risk-based considerations, the EPA determined that the estimates 

of risks likely to remain upon attainment of the 1997 suite of PM2.5 standards were 

indicative of risks that could be reasonably judged important from a public health 

perspective and, thus, supported revision of the standards. However, the EPA judged that 

the quantitative risk assessment had important limitations and did not provide an 

appropriate basis for selecting the levels of the revised standards in 2006 (71 FR 61174/1-

2, October 17, 2006). 

2. Remand of Primary Annual PM2.5 Standard

As noted above in section II.B.2, several parties filed petitions for review in the 

U.S. Court of Appeals for the District of Columbia Circuit following promulgation of the 

revised PM NAAQS in 2006. These petitions challenged several aspects of the final rule 

including the level of the primary PM2.5 annual standard. The primary 24-hour PM2.5

standard was not challenged by any of the litigants and, thus, was not considered in the 

court’s review and decision. 

On judicial review, the D.C. Circuit remanded the primary annual PM2.5 NAAQS 

to the EPA on grounds that the Agency failed to adequately explain why the annual 

standard provided the requisite protection from both short- and long-term exposures to 
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fine particles including protection for at-risk populations. American Farm Bureau 

Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). With respect to human health 

protection from short-term PM2.5 exposures, the court considered the different approaches 

used by the EPA in the 1997 and 2006 PM NAAQS decisions, as summarized in section 

III.A.1 above. The court found that the EPA failed to adequately explain why a primary 

24-hour PM2.5 standard by itself would provide the protection needed from short-term 

exposures and remanded the primary annual PM2.5 standard to the EPA “for further 

consideration of whether it is set at a level requisite to protect the public health while 

providing an adequate margin of safety from the risk of short-term exposures to PM2.5.” 

American Farm Bureau Federation v. EPA, 559 F. 3d at 520-24. 

With respect to protection from long-term exposure to fine particles, the court 

found that the EPA failed to adequately explain how the primary annual PM2.5 standard 

provided an adequate margin of safety for children and other at-risk populations. The 

court found that the EPA did not provide a reasonable explanation of why certain 

morbidity studies, including a study of children in Southern California showing lung 

damage associated with long-term PM2.5 exposure (Gauderman et.al, 2000) and a multicity study (24-Cities Study) evaluating decreased lung function in children associated 

with long-term PM2.5 exposures (Raizenne et al., 1996), did not warrant a more stringent 

annual PM2.5 standard. Id. at 522-23. Specifically, the court found that:

EPA was unreasonably confident that, even though it relied solely upon long-term 

mortality studies, the revised standard would provide an adequate margin of 

safety with respect to morbidity among children. Notably absent from the final 

rule, moreover, is any indication of how the standard will adequately reduce risk 

to the elderly or to those with certain heart or lung diseases despite (a) the EPA’s 

determination in its proposed rule that those subpopulations are at greater risk 

from exposure to fine particles and (b) the evidence in the record supporting that 

determination. Id. at 525.
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In addition, the court held that the EPA had not adequately explained its decision 

to base the level of the annual standard essentially exclusively on the results of long-term 

studies and the 24-hour standard level essentially exclusively on the results of short-term 

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

for setting an annual standard,..., it is not clear why the EPA no longer believes it useful 

to look as well to short-term studies in order to design the suite of standards that will 

most effectively reduce the risks associated with short-term exposure”); see also Id. at 

523-24 (holding that the EPA had not adequately explained why a standard based on 

levels in short-term exposure studies alone provided appropriate protection from health 

effects associated with short-term PM2.5 exposures given the stated need to lower the 

entire air quality distribution, and not just peak concentrations, in order to control against 

short-term effects). 

In remanding the primary annual PM2.5 standard for reconsideration, the court did 

not vacate the standard, Id. at 530, so the standard remains in effect and is therefore the 

standard considered by the EPA in this review.

3. General Approach Used in the Policy Assessment for the Current Review

This review is based on an assessment of a much expanded body of scientific 

evidence, more extensive air quality data and analyses, and a more comprehensive 

quantitative risk assessment relative to the information available in past reviews, as 

presented and assessed in the Integrated Science Assessment and Risk Assessment and 

discussed in the Policy Assessment. As a result, the EPA’s general approach to reaching 

conclusions about the adequacy of the current suite of PM2.5 standards and potential 

alternative standards that are appropriate to consider was broader and more integrative 
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than in past reviews. Our general approach also reflected consideration of the issues 

raised by the court in its remand of the primary annual PM2.5 standard as discussed in 

section III.A.2 above, since decisions made in this review, and the rationales for those 

decisions, will comprise the Agency’s response to the remand.

The EPA’s general approach took into account both evidence-based and riskbased considerations and the uncertainties related to both types of information, as well as 

advice from CASAC (Samet, 2010c,d) and public comments on the first and second draft 

Policy Assessments (U.S. EPA, 2010c,f). In so doing, the EPA staff developed a final 

Policy Assessment (U.S. EPA, 2011a) which provided as broad an array of policy options 

as was supported by the available information, recognizing that the selection of a specific 

approach to reaching final decisions on the primary PM2.5 standards will reflect the 

judgments of the Administrator as to what weight to place on the various approaches and 

types of information available in the current review.

The Policy Assessment concluded it was most appropriate to consider the 

protection against PM2.5-related mortality and morbidity effects, associated with both 

long- and short-term exposures, afforded by the annual and 24-hour standards taken 

together, as was done in the 1997 review, rather than to consider each standard 

separately, as was done in the 2006 review (U.S. EPA, 2011a, section 2.1.3).4 As the 

EPA recognized in 1997, there are various ways to combine two standards to achieve an 

 

4 By utilizing this approach, the Agency also is responsive to the remand of the 2006 

standard. As noted in section III.A.2, the D.C. Circuit, in remanding the 2006 primary 

annual PM2.5 standard, concluded that the Administrator had failed to adequately explain 

why an annual standard was sufficiently protective in the absence of consideration of the 

long-term mean PM2.5 concentrations in short-term exposure studies as well, and likewise 

had failed to explain why a 24-hour standard was sufficiently protective in the absence of 

consideration of the effect of an annual standard on reducing the overall distribution of 

24-hour average PM2.5 concentrations. 559 F. 3d at 520-24. 
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appropriate degree of public health protection. The extent to which these two standards 

are interrelated in any given area depends in large part on the relative levels of the 

standards, the peak-to-mean ratios that characterize air quality patterns in an area, and 

whether changes in air quality designed to meet a given suite of standards are likely to be 

of a more regional or more localized nature. 

In considering the combined effect of annual and 24-hour standards, the Policy 

Assessment recognized that changes in PM2.5 air quality designed to meet an annual 

standard would likely result not only in lower annual average PM2.5 concentrations but 

also in fewer and lower peak 24-hour PM2.5 concentrations. The Policy Assessment also 

recognized that changes designed to meet a 24-hour standard would result not only in 

fewer and lower peak 24-hour PM2.5 concentrations but also in lower annual average 

PM2.5 concentrations. Thus, either standard could be viewed as providing protection from 

effects associated with both short- and long-term exposures, with the other standard 

serving to address situations where the daily peak and annual average concentrations are 

not consistently correlated. 

In considering the currently available evidence, the Policy Assessment recognized

that the short-term exposure studies were primarily drawn from epidemiological studies 

that associated variations in area-wide health effects with monitor(s) that measured the 

variation in daily PM2.5 concentrations over the course of several years. The strength of 

the associations in these data was demonstrably in the numerous “typical” days within the 

air quality distribution, not in the peak days. See also 71 FR 61168, October 17, 2006 and 

American Farm Bureau Federation v EPA, 559 F. 3d at 523, 524 (making the same 

point). The quantitative risk assessments conducted for this and previous reviews 
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demonstrated the same point; that is, much, if not most of the aggregate risk associated 

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

hour average concentrations are in the low-to mid-range, below the peak 24-hour 

concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In 

addition, there was no evidence suggesting that risks associated with long-term exposures 

were likely to be disproportionately driven by peak 24-hour concentrations.5

For these reasons, the Policy Assessment concluded that strategies that focused 

primarily on reducing peak days were less likely to achieve reductions in the PM2.5

concentrations that were most strongly associated with the observed health effects. 

Furthermore, the Policy Assessment concluded that a policy approach that focused on 

reducing peak exposures would most likely result in more uneven public health 

protection across the U.S. by either providing inadequate protection in some areas or 

overprotecting in other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). 

This is because, as discussed above, reductions based on control of peak days are less 

likely to control the bulk of the air quality distribution. 

The Policy Assessment concluded that a policy goal of setting a “generally 

controlling” annual standard that will lower a wide range of ambient 24-hour PM2.5

concentrations, as opposed to focusing on control of peak 24-hour PM2.5 concentrations, 

was the most effective and efficient way to reduce total population risk and so provide 

appropriate protection. This approach, in contrast to one focusing on a generally 

controlling 24-hour standard, would likely reduce aggregate risks associated with both 

 

5

In confirmation, a number of studies have presented analyses excluding higher PM 

concentration days and reported a limited effect on the magnitude of the effect estimates 

or statistical significance of the association (e.g., Dominici, 2006b; Schwartz et al, 1996; 

Pope and Dockery, 1992).
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long- and short-term exposures with more consistency and would likely avoid setting 

national standards that could result in relatively uneven protection across the country, due 

to setting standards that are either more or less stringent than necessary in different 

geographical areas (U.S. EPA, 2011a, p. 2-9).

The Policy Assessment also concluded that an annual standard intended to serve 

as the primary means for providing protection from effects associated with both long- and 

short-term PM2.5 exposures cannot be expected to offer sufficient protection against the 

effects of all short-term PM2.5 exposures. As a result, in conjunction with a generally 

controlling annual standard, the Policy Assessment concluded it was appropriate to 

consider setting a 24-hour standard to provide supplemental protection, particularly for 

areas with high peak-to-mean ratios possibly associated with strong local or seasonal 

sources, or PM2.5-related effects that may be associated with shorter-than-daily exposure 

periods (U.S. EPA, 2011a, p. 2-10).

The Policy Assessment’s consideration of the protection afforded by the current 

and alternative suites of standards focused on PM2.5-related health effects associated with 

long-term exposures for which the magnitude of quantitative estimates of risks to public 

health generated in the risk assessment was appreciably larger in terms of overall 

incidence and percent of total mortality or morbidity effects than for short-term 

PM2.5-related effects. Nonetheless, the EPA also considered health effects and estimated 

risks associated with short-term exposures. In both cases, the Policy Assessment placed 

greatest weight on health effects that had been judged in the Integrated Science 

Assessment to have a causal or likely causal relationship with PM2.5 exposures, while 

also considering health effects judged to be suggestive of a causal relationship or 
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evidence that focused on specific at-risk populations. The Policy Assessment placed 

relatively greater weight on statistically significant associations that yielded relatively 

more precise effect estimates and that were judged to be robust to confounding by other 

air pollutants. In the case of short-term exposure studies, the Policy Assessment placed 

greatest weight on evidence from large multi-city studies, while also considering 

associations in single-city studies. 

In translating information from epidemiological studies into the basis for reaching 

staff conclusions on the adequacy of the current suite of standards, the Policy Assessment

considered a number of factors. As an initial matter, the Policy Assessment considered 

the extent to which the currently available evidence and related uncertainties strengthens 

or calls into question conclusions from the last review regarding associations between 

fine particle exposures and health effects. The Policy Assessment also considered 

evidence of health effects in at-risk populations and the potential impacts on such 

populations. Further, the Policy Assessment explored the extent to which PM2.5-related 

health effects had been observed in areas where air quality distributions extend to lower 

concentrations than previously reported or in areas that would likely have met the current 

suite of standards. 

In translating information from epidemiological studies into the basis for reaching 

staff conclusions on standard levels for consideration (U.S. EPA, 2011a, sections 2.1.3 

and 2.3.4), the Policy Assessment first recognized the absence of discernible thresholds 

in the concentration-response functions from long- and short-term PM2.5 exposure studies 

(U.S. EPA, 2011a, section 2.4.3).6In the absence of any discernible thresholds, the 

 

6

The epidemiological studies evaluated in the Integrated Science Assessment that 

examined the shape of concentration-response relationships and the potential presence of 
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Agency’s general approach for identifying appropriate standard levels for consideration 

involved characterizing the range of PM2.5 concentrations over which we have the most 

confidence in the associations reported in epidemiological studies. In so doing, the Policy 

Assessment recognized that there is no single factor or criterion that comprises the 

“correct” approach, but rather there are various approaches that are reasonable to 

consider for characterizing the confidence in the associations and the limitations and 

uncertainties in the evidence. Identifying the implications of various approaches for 

reaching conclusions on the range of alternative standard levels that is appropriate to 

consider can help inform the final decisions to either retain or revise the standards. 

Today’s final decisions also take into account public health policy judgments as to the 

degree of health protection that is to be achieved. 

In reaching staff conclusions on the range of annual standard levels that was 

appropriate to consider, the Policy Assessment focused on identifying an annual standard 

that provided requisite protection from effects associated with both long- and short-term 

exposures. In so doing, the Policy Assessment explored different approaches for 

characterizing the range of PM2.5 concentrations over which our confidence in the nature 

of the associations for both long- and short-term exposures is greatest, as well as the 

extent to which our confidence is reduced at lower PM2.5 concentrations. 

First, the Policy Assessment recognized that the approach that most directly 

 

a threshold focused on cardiovascular-related hospital admissions and emergency 

department visits associated with short-term PM10 exposures and premature mortality 

associated with long-term PM2.5 exposure (U.S. EPA, 2009a, sections 6.5, 6.2.10.10 and 

7.6). Overall, the Integrated Science Assessment concluded that the studies evaluated 

support the use of a no-threshold, log-linear model but recognized that “additional issues 

such as the influence of heterogeneity in estimates between cities, and the effect of 

seasonal and regional differences in PM on the concentration-response relationship still 

require further investigation” (U.S. EPA, 2009a, section 2.4.3).
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addressed this issue considered studies that analyzed confidence intervals around 

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

multiple concentration-response models rather than considering a single concentrationresponse model.7

The Policy Assessment explored the extent to which such analyses had 

been published for studies of health effects associated with long- or short-term PM2.5

exposures. Such analyses could potentially be used to characterize a concentration below 

which uncertainty in a concentration-response relationship substantially increases or is 

judged to be indicative of an unacceptable degree of uncertainty about the existence of a 

continuing concentration-response relationship. The Policy Assessment concluded that 

identifying this area of uncertainty in the concentration-response relationship could be 

used to inform identification of alternative standard levels that are appropriate to 

consider. 

Further, the Policy Assessment explored other approaches that considered 

different statistical metrics from epidemiological studies. The Policy Assessment first 

took into account the general approach used in previous PM reviews which focused on 

consideration of alternative standard levels that were somewhat below the long-term 

mean PM2.5 concentrations reported in epidemiological studies using air quality 

distributions based on composite monitor concentrations.8This approach recognized that 

 

7

This is distinct from confidence intervals around concentration-response relationships 

that are related to the magnitude of effect estimates generated at specific PM2.5

concentrations (i.e., point-wise confidence intervals) and that are relevant to the precision 

of the effect estimate across the air quality distribution, rather than to our confidence in 

the existence of a continuing concentration-response relationship across the entire air 

quality distribution on which a reported association was based. 

8 Using the term “composite monitor” does not imply that the EPA can identify one 

monitor that represents the air quality evaluated in a specific study area. Rather, the 

composite monitor concentration represents the average concentration across monitors 

within each area with more than one monitor included in a given study as typically 
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the strongest evidence of PM2.5-related associations occurs at concentrations around the 

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

greatest weight on those long- and short-term exposure studies that reported statistically 

significant associations with mortality and morbidity effects.

In extending this approach, the Policy Assessment also considered information 

beyond a single statistical metric of PM2.5 concentrations (i.e., the mean) to the extent 

such information was available. Pursuant to an express comment from CASAC (Samet 

2010d, p. 2), the Policy Assessment utilized distributional statistics (i.e., statistical 

characterization of an entire distribution of data) to identify the broader range of PM2.5

concentrations that had the most influence on the calculation of relative risk estimates in 

both long- and short-term exposure epidemiological studies. Thus, the Policy Assessment

considered the part of the distribution of PM2.5 concentrations in which the data analyzed 

in the study (i.e., air quality and population-level data, as discussed below) were most 

concentrated, specifically, the range of PM2.5 concentrations around the long-term mean 

over which our confidence in the magnitude and significance of associations observed in 

the epidemiological studies was greatest. The Policy Assessment then focused on the 

lower part of the distribution to characterize where the data became appreciably more 

sparse and, thus, where our understanding of the magnitude and significance of the 

associations correspondingly became more uncertain. The Policy Assessment recognized 

there was no single percentile value within a given distribution that was most appropriate 

or “correct” to use to characterize where our confidence in the associations becomes 

 

reported in epidemiological studies. For multi-city studies, this metric reflects 

concentrations averaged across multiple monitors or from single monitors within each 

area and then averaged across study areas for an overall study mean PM2.5 concentration. 

This is consistent with the epidemiological evidence considered in other NAAQS 

reviews.
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appreciably lower. The Policy Assessment concluded that the range from the 25th to 10th

percentiles is a reasonable range to consider as a region where we had appreciably less 

confidence in the associations observed in epidemiological studies.9

In considering distributional statistics from epidemiological studies, the final 

Policy Assessment focused on two types of population-level metrics that CASAC advised 

were most useful to consider in identifying the PM2.5 concentrations most influential in 

generating the health effect estimates reported in the epidemiological studies.10

Consistent with CASAC advice, the most relevant information was the distribution of 

health events (e.g., deaths, hospitalizations) occurring within a study population in 

relation to the distribution of PM2.5 concentrations. However, in recognizing that access 

to health event data can be restricted, the Policy Assessment also considered the number 

of study participants within each study area as an appropriate surrogate for health event 

data.

 

9

In the PM NAAQS review completed in 2006, the Staff Paper similarly recognized that 

the evidence of an association in any epidemiological study is “strongest at and around 

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

the interquartile range of long-term average concentrations within a study [with a lower 

bound of the 25th percentile] or a range within one standard deviation around the study 

mean, may reasonably be used to characterize the range over which the evidence of 

association is strongest” (U.S. EPA, 2005, p. 5-22). A range of one standard deviation 

around the mean represents approximately 68 percent of normally distributed data, and 

below the mean falls between the 25th and 10th percentiles.

10 The second draft Policy Assessment focused on the distributions of ambient PM2.5

concentrations and associated population data across areas included in several multi-city 

studies for which such data were available in seeking to identify the most influential 

range of concentrations (U.S. EPA, 2010f, section 2.3.4.1). In its review of the second 

draft Policy Assessment, CASAC advised that it “would be preferable to have 

information on the concentrations that were most influential in generating the health 

effect estimates in individual studies” (Samet, 2010d, p.2). Therefore, in the final Policy 

Assessment, the EPA considered population-level data (i.e., area-specific health event 

data and study area population data) along with corresponding PM2.5 concentrations to 

generate a cumulative distribution of the population-level data relative to long-term mean 

PM2.5 concentrations to determine the most influential part of the air quality distribution 

(U.S. EPA, 2011a, Figure 2-7 and associated text).
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The Policy Assessment recognized that an approach considering analyses of 

confidence intervals around concentration-response functions was intrinsically related to 

an approach that considered different distributional statistics. Both of these approaches 

could be employed to understand the broader distribution of PM2.5 concentrations which 

correspond to the health events reported in epidemiological studies. In applying these 

approaches, the Policy Assessment, consistent with CASAC advice (Samet, 2010d, p.3),

considered PM2.5 concentrations from long- and short-term PM2.5 exposure studies using 

composite monitor distributions. 

In reaching staff conclusions on alternative standard levels that were appropriate 

to consider, the Policy Assessment also included a broader consideration of the 

uncertainties and limitations of the current scientific evidence. Most notably, these 

uncertainties are related to the heterogeneity observed in the epidemiological studies in 

the eastern versus western parts of the U.S., the relative toxicity of PM2.5 components, 

and the potential role of co-pollutants (U.S. EPA, 2011a, pp. 2-25 to 2-26). The 

limitations and uncertainties associated with the currently available scientific evidence, 

including the availability of fewer studies toward the lower range of alternative annual 

standard levels being considered in this proposal, are summarized in section III.B below 

and further discussed in section III.B.2 of the preamble to the proposed ruleproposal.

The Policy Assessment recognized that the level of protection afforded by the 

NAAQS relies both on the level and the form of the standard. The Policy Assessment 

concluded that a policy approach that used data based on composite monitor distributions 

to identify alternative standard levels, and then compared those levels to concentrations at 

maximum monitors to determine whether an area meets a given standard, inherently has 
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the potential to build in some margin of safety (U.S. EPA, 2011a, p. 2-14).11 This 

conclusion was consistent with CASAC’s comments on the second draft Policy 

Assessment, in which CASAC expressed its preference for focusing on an approach 

using composite monitor distributions “because of its stability, and for the additional 

margin of safety it provides” when “compared to the maximum monitor perspective” 

(Samet, et al., 2010d, pp. 2 to 3). 

In reaching staff conclusions on alternative 24-hour standard levels that are 

appropriate to consider for setting a 24-hour standard intended to supplement the 

protection afforded by a generally controlling annual standard, the Policy Assessment

considered currently available short-term PM2.5 exposure studies. The evidence from 

these studies informed our understanding of the protection afforded by the suite of 

standards against effects associated with short-term exposures. In considering the shortterm exposure studies, the Policy Assessment evaluated both the distributions of 24-hour 

PM2.5 concentrations, with a focus on the 98th percentile concentrations (to the extent 

such data were available) to match the form of the current 24-hour PM2.5 standard, as 

well as the long-term mean PM2.5 concentrations reported in these studies. In addition to 

considering the epidemiological evidence, the Policy Assessment also considered air 

 

11 Statistical metrics (e.g., means) based on composite monitor distributions may be 

identical to or below the same statistical metrics based on maximum monitor 

distributions. For example, some areas may have only one monitor, in which case the 

composite and maximum monitor distributions will be identical in those areas. Other 

areas may have multiple monitors that may be very close to the monitor measuring the 

highest concentrations, in which case the composite and maximum monitor distributions 

could be similar in those areas. As noted in Hassett-Sipple et al. (2010), for studies 

involving a large number of areas, the composite and maximum concentrations are 

generally within 5 percent of each other (77 FR 38905, fn. 30). Still other areas may have 

multiple monitors that may be separately impacted by local sources in which case the 

composite and maximum monitor distributions could be quite different (U.S. EPA, 

2011a, p. 2-14). See further discussion of this issue in section III.E.4.c.i below.
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quality information based on county-level 24-hour and annual design values12 to 

understand the policy implications of the alternative standard levels supported by the 

underlying science. In particular, the Policy Assessment considered the extent to which 

different combinations of alternative annual and 24-hour standards would support the 

policy goal of focusing on a generally controlling annual standard in conjunction with a 

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

Assessment discussed the roles that each standard might be expected to play in the 

protection afforded by alternative suites of standards.

Beyond these evidence-based considerations, the Policy Assessment also 

considered the quantitative risk estimates and the key observations presented in the Risk 

Assessment. This assessment included an evaluation of 15 urban case study areas and 

estimated risk associated with a number of health endpoints associated with long- and 

short-term PM2.5 exposures (U.S. EPA, 2010a). As part of the risk-based considerations, 

the Policy Assessment considered estimates of the magnitude of PM2.5-related risks 

associated with recent air quality levels and air quality simulated to just meet the current 

and alternative suites of standards using alternative simulation approaches. The Policy 

Assessment also characterized the risk reductions, relative to the risks remaining upon 

just meeting the current standards, associated with just meeting alternative suites of 

standards. In so doing, the Policy Assessment recognized the uncertainties inherent in 

such risk estimates, and took such uncertainties into account by considering the 

sensitivity of the “core” risk estimates to alternative assumptions and methods likely to 

have substantial impact on the estimates. In addition, the Policy Assessment considered 

 

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

determine compliance.
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additional analyses characterizing the representativeness of the urban study areas within a 

broader national context to understand the roles that the annual and 24-hour standards 

may play in affording protection against effects related to both long- and short-term 

PM2.5 exposures.

Based on the approach discussed above, the Policy Assessment reached 

conclusions related to the primary PM2.5 standards that reflected an understanding of both 

evidence-based and risk-based considerations to inform two overarching questions 

related to: (1) the adequacy of the current suite of PM2.5 standards and (2) revisions to the 

standards that were appropriate to consider in this review to protect against health effects 

associated with both long- and short-term exposures to fine particles. When evaluating 

the health protection afforded by the current or any alternative suites of standards 

considered, the Policy Assessment took into account the four basic elements of the 

NAAQS: the indicator, averaging time, form, and level.

The general approach for reviewing the primary PM2.5 standards described above 

provided a comprehensive basis that helped to inform the Administrator’s judgments in 

reaching her proposed and final decisions to revise the current suite of primary fine 

particle NAAQS and in responding to the remand of the 2006 primary annual PM2.5

standard.

B. Overview of Health Effects Evidence

This section outlines the key information presented in section III.B of the 

proposal (77 FR 38906 to 38911, June 29, 2012) and discussed more fully in the 

Integrated Science Assessment (Chapters 2, 4, 5, 6, 7, and 8) and the Policy Assessment

(Chapter 2) related to health effects associated with fine particle exposures. Section III.B. 
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of the proposal discusses available information on the health effects associated with 

exposures to PM2.5, including the nature of such health effects (section III.B.1) and 

associated limitations and uncertainties (section III.B.2), at-risk populations (section 

III.B.3), and potential PM2.5-related impacts on public health (section III.B.4). As was 

true in the last two reviews, evidence from epidemiological, controlled human exposure 

and animal toxicological studies played a key role in the Integrated Science Assessment’s 

evaluation of the scientific evidence. 

The 2006 PM NAAQS review concluded that there was “strong epidemiological 

evidence” for linking long-term PM2.5 exposures with cardiovascular-related and lung 

cancer mortality and respiratory-related morbidity and for linking short-term PM2.5

exposures with cardiovascular-related and respiratory-related mortality and morbidity

(U.S. EPA, 2004, p. 9-46; U.S. EPA, 2005, p. 5-4). Overall, the evidence from 

epidemiological, toxicological, and controlled human exposure studies supported “likely 

causal associations” between PM2.5 and both mortality and morbidity from cardiovascular 

and respiratory diseases, based on “an assessment of strength, robustness, and 

consistency in results” (U.S. EPA, 2004, p. 9-48).13

In this review, based on the expanded body of evidence, the EPA finds that:

(1) In looking across the extensive new scientific evidence available in this review, 

our overall understanding of health effects associated with fine particle exposures 

has been greatly expanded. The currently available evidence is largely consistent 

with evidence available in the last review and substantially strengthens what is 

known about the effects associated with fine particle exposures. 

 

13 The term “likely causal association” was used in the 2004 Criteria Document to 

summarize the strength of the available evidence available in the last review for PM2.5. 

However, this terminology was not based on a formal framework for evaluating evidence 

for inferring causation. Since the last review, the EPA has developed a more formal 

framework for reaching causal determinations with standardized language to express 

evaluation of the evidence (U.S. EPA, 2009a, section 1.5).
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(2) A number of large multi-city epidemiological studies have been conducted 

throughout the U.S., including extended analyses of long-term exposure studies 

that were important to inform decision-making in the last review. The body of 

currently available scientific evidence has also been expanded greatly by the 

publication of a number of new multi-city, time-series studies that have used 

uniform methodologies to investigate the effects of short-term PM2.5 exposures on 

public health. This body of evidence provides a more expansive data base and 

considers multiple locations representing varying regions and seasons that provide 

evidence of the influence of different air pollution mixes on PM2.5-associated 

health effects. These studies provide more precise estimates of the magnitude of 

effects associated with short-term PM2.5 exposure than most smaller-scale singlecity studies that were more commonly available in the last review. These studies 

have reported consistent increases in morbidity and/or premature mortality related 

to ambient PM2.5 concentrations, with the strongest evidence reported for 

cardiovascular-related effects.

(3) In addition, the findings of new toxicological and controlled human exposure 

studies greatly expand and provide stronger support for a number of potential 

biological mechanisms or pathways for cardiovascular and respiratory effects 

associated with long- and short-term PM exposures. These studies provide 

coherence and biological plausibility for the effects observed in epidemiological 

studies.

(4) Using a more formal framework for reaching causal determinations than used in 

prior reviews14,,

15 the EPA concludes that a causal relationship exists between 

 

14 The causal framework draws upon the assessment and integration of evidence from 

across epidemiological, controlled human exposure, and toxicological studies, and the 

related uncertainties that ultimately influence our understanding of the evidence. This 

framework employs a five-level hierarchy that classifies the overall weight of evidence 

and causality using the following categorizations: causal relationship, likely to be causal 

relationship, suggestive of a causal relationship, inadequate to infer a causal relationship, 

and not likely to be a causal relationship (U.S. EPA, 2009a, Table 1-3). The development 

of the causal framework reflects considerable input from CASAC and the public, with 

CASAC concluding that, “The five-level classification of strength of evidence for causal 

inference has been systemically applied [for PM]; this approach has provided 

transparency and a clear statement of the level of confidence with regard to causation, 

and we recommend its continued use in future ISAs” (Samet, 2009f, p. 1).

15 The causal framework draws upon the assessment and integration of evidence from 

across epidemiological, controlled human exposure, and toxicological studies, and the 

related uncertainties that ultimately influence our understanding of the evidence. This 

framework employs a five-level hierarchy that classifies the overall weight of evidence 

and causality using the following categorizations: causal relationship, likely to be causal 

relationship, suggestive of a causal relationship, inadequate to infer a causal relationship, 

and not likely to be a causal relationship (U.S. EPA, 2009a, Table 1-3). The development 

of the causal framework reflects considerable input from CASAC and the public, with 

CASAC concluding that, “The five-level classification of strength of evidence for causal 
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both long- and short-term exposures to PM2.5 and premature mortality and 

cardiovascular effects and a likely causal relationship exists between long- and 

short-term PM2.5 exposures and respiratory effects. Further, there is evidence 

suggestive of a causal relationship between long-term PM2.5 exposures and other 

health effects, including developmental and reproductive effects (e.g., low birth 

weight, infant mortality) and carcinogenic, mutagenic, and genotoxic effects (e.g., 

lung cancer mortality).16

(5) The newly available evidence significantly strengthens the link between long- and 

short-term exposure to PM2.5 and premature mortality, while providing indications 

that the magnitude of the PM2.5-mortality association with long-term exposures 

may be larger than previously estimated. The strongest evidence comes from 

recent studies investigating long-term exposure to PM2.5 and cardiovascularrelated mortality. The evidence supporting a causal relationship between longterm PM2.5 exposure and mortality also includes consideration of new studies that 

demonstrated an improvement in community health following reductions in 

ambient fine particles.

(6) Several new studies have examined the association between cardiovascular effects 

and long-term PM2.5 exposures in multi-city studies conducted in the U.S. and 

Europe. While studies were not available in the last review with regard to longterm exposure and cardiovascular-related morbidity, recent studies have provided 

new evidence linking long-term exposure to PM2.5 with an array of cardiovascular 

effects such as heart attacks, congestive heart failure, stroke, and mortality. This 

evidence is coherent with studies of short-term exposure to PM2.5 that have 

observed associations with a continuum of effects ranging from subtle changes in 

indicators of cardiovascular health to serious clinical events, such as increased 

hospitalizations and emergency department visits due to cardiovascular disease 

and cardiovascular mortality.

(7) Extended analyses of studies available in the last review as well as new 

epidemiological studies conducted in the U.S. and abroad provide stronger 

evidence of respiratory-related morbidity effects associated with long-term PM2.5

exposure. The strongest evidence for respiratory-related effects is from studies 

that evaluated decrements in lung function growth, increased respiratory 

symptoms, and asthma development. The strongest evidence from short-term 

PM2.5 exposure studies has been observed for increased respiratory-related 

 

inference has been systemically applied [for PM]; this approach has provided 

transparency and a clear statement of the level of confidence with regard to causation, 

and we recommend its continued use in future ISAs” (Samet, 2009f, p. 1).

16 These causal inferences are based not only on the more expansive epidemiological 

evidence available in this review but also reflect consideration of important progress that 

has been made to advance our understanding of a number of potential biologic modes of 

action or pathways for PM-related cardiovascular and respiratory effects (U.S. EPA, 

2009a, chapter 5).
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emergency department visits and hospital admissions for chronic obstructive 

pulmonary disease (COPD) and respiratory infections. 

(8) The body of scientific evidence is somewhat expanded from the 2006 review but 

is still limited with respect to associations between long-term PM2.5 exposures and 

developmental and reproductive effects as well as cancer, mutagenic, and 

genotoxic effects. The strongest evidence for an association between PM2.5 and 

developmental and reproductive effects comes from epidemiological studies of 

low birth weight and infant mortality, especially due to respiratory causes during 

the post-neonatal period (i.e., 1 month -12 months of age). With regard to cancer 

effects, “[m]ultiple epidemiologic studies have shown a consistent positive 

association between PM2.5 and lung cancer mortality, but studies have generally 

not reported associations between PM2.5 and lung cancer incidence” (U.S. EPA 

2009a p. 2-13 ).

(9) Efforts to evaluate the relationships between PM composition and health effects 

continue to evolve. While many constituents of PM2.5 can be linked with differing 

health effects, the evidence is not yet sufficient to allow differentiation of those 

constituents or sources that may be more closely related to specific health 

outcomes nor to exclude any individual component or group of components 

associated with any source categories from the fine particle mixture of concern.

(10) Specific groups within the general population are at increased risk for 

experiencing adverse health effects related to PM exposures. The currently 

available evidence expands our understanding of previously identified at-risk 

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

and lung disease) and supports the identification of additional at-risk populations 

(e.g., persons with lower socioeconomic status, genetic differences). Evidence for 

PM-related effects in these at-risk populations has expanded and is stronger than 

previously observed. There is emerging, though still limited, evidence for 

additional potentially at-risk populations, such as those with diabetes, people who 

are obese, pregnant women, and the developing fetus.

(11) The population potentially affected by PM2.5 is large. In addition, large subgroups 

of the U.S. population have been identified as at-risk populations. While 

individual effect estimates from epidemiological studies may be small in size, the 

public health impact of the mortality and morbidity associations can be quite large 

given the extent of exposure. Taken together, this suggests that exposure to 

ambient PM2.5 concentrations can have substantial public health impacts. 

(12) While the currently available scientific evidence is stronger and more consistent 

than in previous reviews, providing a strong basis for decision making in this 

review, the EPA recognizes that important uncertainties and limitations in the 

health effects evidence remain. Epidemiological studies evaluating health effects 

associated with long- and short-term PM2.5 exposures have reported heterogeneity 

in responses between cities and geographic regions within the U.S. This 

heterogeneity may be attributed, in part, to differences in the fine particle 
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composition or related to exposure measurement error, which can introduce bias 

and increased uncertainty in associated health effect estimates. Variability in the 

associations observed across PM2.5 epidemiological studies may be due in part to 

exposure error related to measurement-related issues, the use of central fixed-site 

monitors to represent population exposure to PM2.5, models used in lieu of or to 

supplement ambient measurements, and our limited understanding of factors that 

may influence exposures (e.g., topography, the built environment, weather, source 

characteristics, ventilation usage, personal activity patterns, photochemistry). In 

addition, where PM2.5 and other pollutants (e.g., ozone, nitrogen dioxide, and 

carbon monoxide) are correlated, it can be difficult to distinguish the effects of the 

various pollutants in the ambient mixture (i.e., co-pollutant confounding).17

While uncertainties and limitations still remain in the available health effects 

evidence, the Administrator judges the currently available scientific data base to be 

stronger and more consistent than in previous reviews providing a strong basis for 

decision making in this review. 

C. Overview of Quantitative Characterization of Health Risks

In addition to a comprehensive evaluation of the health effects evidence available 

in this review, the EPA conducted an expanded quantitative risk assessment for selected 

health endpoints to provide additional information and insights to inform decisions on the 

primary PM2.5 NAAQS.18 As discussed in section III.C of the proposal, the approach used 

to develop quantitative risk estimates associated with PM2.5 exposures was built on the 

approach used and lessons learned in the last review and focused on improving the 

characterization of the overall confidence in the risk estimates, including related 

 

17 A copollutant meets the criteria for potential confounding in PM-health associations if: 

(1) it is a potential risk factor for the health effect under study; (2) it is correlated with 

PM; and (3) it does not act as an intermediate step in the pathway between PM exposure 

and the health effect under study (U.S. EPA, 2004, p. 8-10). 

18 The quantitative risk assessment conducted for this review is more fully described and

presented in the Risk Assessment (U.S. EPA, 2010a) and summarized in detail in the 

Policy Assessment (U.S. EPA, 2011a, sections 2.2.2. and 2.3.4.2). The scope and 

methodology for this risk assessment were developed over the last few years with 

considerable input from CASAC and the public as described in section I.C.2 of the 

proposal. II.B.3 above. 
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uncertainties, by incorporating a number of enhancements, in terms of both the methods 

and data used in the analyses. 

The goals of this quantitative risk assessment were largely the same as those 

articulated in the risk assessment conducted for the last review. These goals included: (1) 

to provide estimates of the potential magnitude of premature mortality and/or selected 

morbidity effects in the population associated with recent ambient levels of PM2.5 and 

with simulating just meeting the current and alternative suites of PM2.5 standards in 15 

selected urban study areas19,,

20 including, where data were available, consideration of 

impacts on at-risk populations; (2) to develop a better understanding of the influence of 

various inputs and assumptions on the risk estimates to more clearly differentiate among 

alternative suites of standards; and (3) to gain insights into the distribution of risks and 

patterns of risk reductions and the variability and uncertainties in those risk estimates. In 

 

19 The Risk Assessment concluded that these 15 urban study areas were generally 

representative of urban areas in the U.S. likely to experience relatively elevated levels of 

risk related to ambient PM2.5 exposure with the potential for better characterization at the 

higher end of that distribution (U.S. EPA, 2011a, p. 2-42; U.S. EPA, 2010a, section 4.4, 

Figure 4-17). The representativeness analysis also showed that the 15 urban study areas 

do not capture areas with the highest baseline morality risks or the oldest populations 

(both of which can result in higher PM2.5-related mortality estimates). However, some of 

the areas with the highest values for these attributes had relatively low PM2.5

concentrations (e.g., urban areas in Florida) and, consequently, the Risk Assessment 

concluded failure to include these areas in the set of urban study areas was unlikely to 

exclude high PM2.5-risk locations (U.S. EPA, 2010a, section 4.4.1).

20 The Risk Assessment concluded that these 15 urban study areas were generally 

representative of urban areas in the U.S. likely to experience relatively elevated levels of 

risk related to ambient PM2.5 exposure with the potential for better characterization at the 

higher end of that distribution (U.S. EPA, 2011a, p. 2-42; U.S. EPA, 2010a, section 4.4, 

Figure 4-17). The representativeness analysis also showed that the 15 urban study areas 

do not capture areas with the highest baseline morality risks or the oldest populations 

(both of which can result in higher PM2.5-related mortality estimates). However, some of 

the areas with the highest values for these attributes had relatively low PM2.5

concentrations (e.g., urban areas in Florida) and, consequently, the Risk Assessment 

concluded failure to include these areas in the set of urban study areas was unlikely to 

exclude high PM2.5-risk locations (U.S. EPA, 2010a, section 4.4.1).
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addition, the quantitative risk assessment included nationwide estimates of the potential 

magnitude of premature mortality associated with long-term exposure to recent ambient 

PM2.5 concentrations to more broadly characterize this risk on a national scale and to 

support the interpretation of the more detailed risk estimates generated for selected urban 

study areas. 

The expanded and updated risk assessment conducted in this review included 

estimates of risk for: (1) all-cause, ischemic heart disease-related, cardiopulmonaryrelated, and lung cancer-related mortality associated with long-term PM2.5 exposure; (2) 

non-accidental, cardiovascular-related, and respiratory-related mortality associated with 

short-term PM2.5 exposure; and (3) cardiovascular-related and respiratory-related hospital 

admissions and asthma-related emergency department visits associated with short-term 

PM2.5 exposure.

21

The Risk Assessment included a core set of risk estimates supplemented by an 

alternative set of risk results generated using single-factor and multi-factor sensitivity 

analyses. The core set of risk estimates was developed using the combination of modeling 

elements and input data sets identified in the Risk Assessment as having higher 

confidence relative to inputs used in the sensitivity analyses. The results of the sensitivity 

analyses provided information to evaluate and rank the potential impacts of key sources 

of uncertainty on the core risk estimates. In addition, the sensitivity analyses represented 

a set of reasonable alternatives to the core set of risk estimates that fell within an overall 

set of plausible risk estimates surrounding the core estimates. 

 

21 The evidence available for these selected health effect endpoints generally focused on 

the entire population, although some information was available to support analyses that 

considered differences in estimated risk for at-risk populations including older adults and 

persons with pre-existing cardiovascular and respiratory diseases.
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The EPA recognized that there were many sources of variability and uncertainty 

inherent in the inputs to its quantitative risk assessment.22 The design of the risk 

assessment included a number of elements to address these issues in order to increase the 

overall confidence in the risk estimates generated for the 15 urban study areas, including 

using guidance from the World Health Organization (WHO, 2008) as a framework for 

characterizing uncertainty in the analyses.23

With respect to the sources of variability, the Risk Assessment considered those 

that contributed to differences in risk across urban study areas, but did not directly affect 

the degree of risk reduction associated with the simulation of just meeting current or 

alternative standard levels (e.g., differences in baseline incidence rates, demographics and 

population behavior). The Risk Assessment also focused on factors that not only 

introduced variability into risk estimates across study areas, but also played an important 

role in determining the magnitude of risk reductions upon simulation of just meeting 

current or alternative standard levels (e.g., peak-to-mean ratios of ambient PM2.5

concentrations within individual urban study areas and the nature of the rollback 

approach used to simulate just meeting the current or alternative standards). Key sources 

of potential variability that were likely to affect population risks included the following: 

(1) intra-urban variability in ambient PM2.5 concentrations, including PM2.5 composition;

(2) variability in the patterns of reductions in PM2.5 concentrations associated with 

different rollback approaches when simulating just meeting the current or alternative 

 

22 Variability refers to the heterogeneity of a variable of interest within a population or 

across different populations. Uncertainty refers to the lack of knowledge regarding the 

actual values of inputs to an analysis (U.S. EPA, 2010a, p. 3-63). 

23 The extent to which key sources of potential variability were (or were not) fully 

captured in the design of the risk assessment are discussed in section 3.5.2 of the Risk 

Assessment (U.S. EPA, 2010a, pp. 3-67 to 3-69).
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standards; co-pollutant exposures; (3) factors related to demographic and socioeconomic 

status; (4) behavioral differences across urban study areas (e.g., time spent outdoors); (5) 

baseline incidence rates; and (6) longer-term temporal variability in ambient PM2.5

concentrations reflecting meteorological trends as well as future changes in the mix of 

PM2.5 sources, including changes in air quality related to future regulatory actions. 

With regard to uncertainties, single and multi-factor sensitivity analyses were 

combined with a qualitative analysis to assess the impact of potential sources of 

uncertainty on the core risk estimates. Key sources of uncertainty included: (1) 

characterizing intra-urban population exposure in the context of epidemiologyical studies 

linking PM2.5 to specific health effects; (2) statistical fit of the concentration-response 

functions for short-term exposure-related health endpoints; (3) shape of the 

concentration-response functions; (4) specifying the appropriate lag structure for shortterm exposure studies; (5) transferability of concentration-response functions from study 

locations to urban study area locations for long-term exposure-related health endpoints; 

(6) use of single-city versus multi-city studies in the derivation of concentration-response 

functions; (7) impact of historical air quality on estimates of health risk associate with 

long-term PM2.5 exposures; and (8) potential variation in effect estimates reflecting 

compositional differences in PM2.5.

Beyond characterizing uncertainty and variability, a number of design elements 

were included in the risk assessment to increase the overall confidence in the risk 

estimates generated for the 15 urban study areas (U.S. EPA, 2011a, pp. 2-38 to 2-41). 

These elements included: (1) use of a deliberative process for specifying components of 

the risk model that reflects consideration of the latest research on PM2.5 exposure and risk 
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(U.S. EPA, 2010a, section 5.1.1); (2) integration of key sources of variability into the 

design as well as the interpretation of risk estimates (U.S.EPA, 2010a, section 5.1.2); (3) 

assessment of the degree to which the urban study areas are representative of areas in the 

U.S. experiencing higher PM2.5-related risk (U.S.EPA, 2010a, section 5.1.3); and (4) 

identification and assessment of important sources of uncertainty and the impact of these 

uncertainties on the core risk estimates (U.S.EPA, 2010a, section 5.1.4). Further, 

additional analyses examined potential bias and overall confidence in the risk estimates. 

Greater confidence is associated with risk estimates based on simulated annual mean 

PM2.5 concentrations that are within the region of the air quality distribution used in 

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

within one standard deviation around the long-term mean PM2.5 concentration) (U.S.

EPA, 2011a, p. 2-38).

Key observations and insights from the PM2.5 risk assessment, together with 

important caveats and limitations, were discussed in section III.C.3 of the proposal. In 

general, in considering the set of quantitative risk estimates and related uncertainties and 

limitations related to long- and short-term PM2.5 exposure together with consideration of 

the health endpoints which could not be quantified, the Policy Assessment concluded this 

information provided strong evidence that risks estimated to remain upon simulating just 

meeting the current suite of PM2.5 standards are important from a public health 

perspective, both in terms of severity and magnitude of effects. Patterns of increasing 

estimated risk reductions were generally observed as either the annual or 24-hour 

standard level, or both, were reduced over the ranges considered in the Risk Assessment.

The magnitude of both long- and short-term exposure-related risk estimated to 
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remain upon just meeting the current suite of standards as well as alternative standard 

levels was strongly associated with the simulated change in annual mean PM2.5

concentrations. Although long- and short-term exposure-related mortality rates have 

similar patterns in terms of the subset of urban study areas experiencing risk reductions 

for the current suite of standard levels, the magnitude of risk remaining is higher for longterm exposure-related mortality and substantially lower for short-term exposure-related 

mortality. Short-term exposure-related morbidity risk estimates were greater for 

cardiovascular-related than respiratory-related events and emergency department visits 

for asthma-related events were significant: Furthermore, most of the aggregate risk 

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

days with PM2.5 concentrations in the upper tail of the air quality distribution, but rather 

by the large number of days with PM2.5 concentrations at and around the mean of the 

distribution, that is, the 24-hour average concentrations that are in the low-to mid-range, 

well below the peak 24-hour concentrations (U.S. EPA, 2011a, p. 2-3).

With regard to characterizing estimates of PM2.5-related risk associated with 

simulation of alternative standards, the Policy Assessment recognized that greater overall 

confidence was associated with estimates of risk reduction than for estimates of absolute 

risk remaining (U.S. EPA, 2011a, p. 2-94). Furthermore, the Policy Assessment 

recognized that estimates of absolute risk remaining for each of the alternative standard 

levels considered, particularly in the context of long-term exposure-related mortality, 
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may be underestimated24. In addition, the Policy Assessment observed that in considering 

the overall confidence associated with the quantitative analyses, the Risk Assessment 

recognized that: (1) substantial variability existed in the magnitude of risk remaining 

across urban study areas and (2) in general, higher confidence was associated with risk 

estimates based on PM2.5 concentrations near the mean PM2.5 concentrations in the 

underlying epidemiological studies providing the concentration-response functions (i.e.g., 

within one standard deviation of the mean PM2.5 concentration reported). Furthermore, 

although the Risk Assessment estimated that the alternative 24-hour standard levels 

considered (when controlling) would result in additional estimated risk reductions beyond 

those estimated for alternative annual standard levels alone, these additional estimated 

reductions were highly variable. Conversely, the Risk Assessment recognized that 

alternative annual standard levels, when controlling, resulted in more consistent risk 

reductions across urban study areas, thereby potentially providing a more consistent 

degree of public health protection (U.S. EPA, 2010a, p. 5-17).

D. Conclusions on the Adequacy of the Current Primary PM2.5 Standards

1. Introduction

The initial issue to be addressed in the current review of the primary PM2.5

 

24 Based on the consideration of both the qualitative and quantitative assessments of 

uncertainty, the Risk Assessment concluded that it is unlikely that the estimated risks are 

over-stated, particularly for premature mortality related to long-term PM2.5 exposures. In 

fact, the Policy Assessment and the Risk Assessment concluded that the core risk 

estimates for this category of health effects may well be biased low based on 

consideration of alternative model specifications evaluated in the sensitivity analyses 

(U.S. EPA, 2011a, p. 2-41; U.S. EPA, 2010a, p. 5-16; Figures 4-7 and 4-8). In addition, 

the Policy Assessment recognized that the currently available scientific information 

included evidence for a broader range of health endpoints and at-risk populations beyond 

those included in the quantitative risk assessment, including decrements in lung function 

growth and respiratory symptoms in children as well as reproductive and developmental 

effects (U.S. EPA, 2011a, section 2.2.1).
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standards is whether, in view of the advances in scientific knowledge and other 

information reflected in the Integrated Science Assessment, the Risk Assessment, and the

Policy Assessment, the existing standards should be retained or revised. In considering 

the adequacy of the current suite of PM2.5 standards, the Administrator has considered the 

large body of evidence presented and assessed in the Integrated Science Assessment

(U.S. EPA, 2009a), the quantitative assessment of risks, staff conclusions and associated 

rationales presented in the Policy Assessment, views expressed by CASAC, and public 

comments. The Administrator has taken into account both evidence- and risk-based 

considerations25 in developing final conclusions on the adequacy of the current primary 

PM2.5 standards.

a. Evidence- and Risk- based Considerations in the Policy Assessment

In considering the available epidemiological evidence in this review, the Policy 

Assessment took a broader approach than was used in the last review. This approach 

reflected the more extensive and stronger body of evidence available since the last review 

on health effects related to both long- and short-term exposure to PM2.5. As discussed in

section III.A.3 above, this broader approach focused on setting the annual standard as the 

“generally controlling” standard for lowering both short- and long-term PM2.5

concentrations and so providing requisite protection to public health. In conjunction with 

 

25 Evidence-based considerations include the assessment of epidemiological, 

toxicological, and controlled human exposure studies evaluating long- or short-term 

exposures to PM2.5, with supporting evidence related to dosimetry and potential 

pathways/modes of action, as well as the integration of evidence across each of these 

disciplines, as assessed in the Integrated Science Assessment (U.S. EPA, 2009a) and 

focus on the policy-relevant considerations as discussed in section III.B above and in the 

Policy Assessment (U.S. EPA, 2011a, section 2.2.1). Risk-based considerations draw 

from the results of the quantitative analyses presented in the Risk Assessment (U.S. EPA, 

2010a) and focus on the policy-relevant considerations as discussed in section III.C 

above and in the Policy Assessment (U.S. EPA, 2011a, section 2.2.2).
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such an annual standard, this approach focused on setting the 24-hour standard to provide 

supplemental protection against days with high peak PM2.5 concentrations.

In addressing the question whether the evidence now available in this review 

supports consideration of standards that are more protective than the current PM2.5

standards, the Policy Assessment considered whether: (1) statistically significant health 

effects associations with long- or short-term exposures to fine particles occur in areas that 

would likely have met the current PM2.5 standards ([see American Trucking 

Ass’nsAssociation, 283 F. 3d at 369, 376 (revision of level of PM NAAQS justified 

when health effects are observed in areas meeting the existing standard)),)], and (2) 

associations with long-term exposures to fine particles extend down to lower air quality 

concentrations than had previously been observed. With regard to associations observed 

in long-term PM2.5 exposure studies, the Policy Assessment recognized that extended 

follow-up analyses of the ACS and Harvard Six Cities studies provided consistent and 

stronger evidence of an association with mortality at lower air quality distributions than 

had previously been observed (U.S. EPA, 2011a, pp. 2-31 to 2-32). The original and 

reanalysis of the ACS study reported positive and statistically significant effects 

associated with a long-term mean PM2.5 concentration of 18.2 μg/m3across 50 

metropolitan areas for 1979 to 1983 (Pope et al., 1995; Krewski et al., 2000).26 In 

extended analyses, positive and statistically significant effects of approximately similar 

magnitude were associated with declining PM2.5 concentrations, from an aggregate longterm mean in 58 metropolitan areas of 21.2 μg/m3

in the original monitoring period (1979 

 

26 The study periods referred to in the Policy Assessment (U.S. EPA, 2011a) and in this 

final rule reflect the years of air quality data that were included in the analyses, whereas 

the study periods identified in the Integrated Science Assessment (U.S. EPA, 2009a) 

reflect the years of health statusevent data that were included.
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to 1983) to 14.0 μg/m3for 116 metropolitan areas in the most recent years evaluated 

(1999-2000), with an overall average across the two study periods in 51 metropolitan 

areas of 17.7 μg/m3(Pope et al., 2002; Krewski et al., 2009). With regard to the Harvard 

Six Cities Study, the original and reanalysis reported positive and statistically significant 

effects associated with a long-term mean PM2.5 concentration of 18.0 μg/m3for 1980 to 

1985 (Dockery et al., 1993; Krewski et al., 2000). In an extended follow-up of this study, 

the aggregate long-term mean concentration across all years evaluated was 16.4 μg/m3

for 1980 to 198827 (Laden et al., 2006). In an additional analysis of the extended followup of the Harvard Six Cities study, investigators reported that the concentration-response 

relationship was linear and “clearly continuing below the level” of the current annual 

standard (U.S. EPA, 2009a, p. 7-92; Schwartz et al., 2008). 

Cohort studies conducted since the last review provided additional evidence of 

mortality associated with air quality distributions that are generally lower than those 

reported in the ACS and Harvard Six Cities studies, with effect estimates that were 

similar or, in some studies, significantly greater in magnitude than in the ACS and 

Harvard Six Cities studies (see also, section III.D.1.a of the proposal, 77 FR 38918 to 

28919; U.S. EPA, 2011a, pp. 2-32 to 2-33). The Women’s Health Initiative (WHI) study 

reported positive and most often statistically significant associations between long-term 

PM2.5 exposure and cardiovascular-related mortality as well as morbidity effects, with 

much larger relative risk estimates for mortality than in the ACS and Harvard Six Cities 

studies, at an aggregate long-term mean PM2.5 concentration of 12.9 μg/m3for 2000 

 

27 Aggregate mean concentration provided by study author (personal communication 

from Dr. Francine Laden, 2009).
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(Miller et al., 2007).28

Using the Medicare cohort, Eftim et al. (2008) reported somewhat higher effect 

estimates than in the ACS and Harvard Six Cities studies with aggregate long-term mean 

concentrations of 13.6 μg/m3and 14.1 μg/m3, respectively, for 2000- to 2002. Zeger et al. 

(2008) reported associations between long-term PM2.5 exposure and mortality for the 

eastern region of the U.S. at an aggregated long-term PM2.5 median concentration of 14.0 

μg/m3, although no association was reported for the western region with an aggregate 

long-term PM2.5 median concentration of 13.1 μg/m3(U.S. EPA, 2009a, p. 7-88).29

Premature mortality in children reported in a national infant mortality study as 

well as mortality in a cystic fibrosis cohort including both children and adults reported 

positive but statistically nonsignificant effects associated with long-term aggregate mean 

concentrations of 14.8 μg/m3and 13.7 μg/m3, respectively (Woodruff et al., 2008; Goss 

et al., 2004).

With respect to respiratory morbidity effects associated with long-term PM2.5

exposure, the across-city mean of 2-week average PM2.5 concentrations reported in the 

initial Southern California Children’s Health Study was approximately 15.1 μg/m3

 

28 The Policy Assessment noted that in comparison to other long-term exposure studies, 

the Miller et al. (2007) study was more limited in that it was based on only one year of air 

quality data (U.S. EPA, 2011a, p. 2-82). The proposal further noted that the air quality 

data considered were extrapolated from that one single year of air quality data (2000) to 

the whole study, and that the air quality data post-dated the years of health events 

considered (i.e., 1994 to 1998) (77 FR 38918, fn 62).

29 Zeger et al. (2008) also reported positive and statistically significant effects for the 

central region, with an aggregate long-term mean PM2.5 concentration of 10.7 μg/m3. 

However, in contrast to the eastern and western risk estimates, the central risk estimate 

increased with adjustment for COPD (used as a proxy for smoking status). Due to the 

potential for confounding bias influencing the risk estimate for the central region, the 

Policy Assessment did not focus on the results reported in the central region to inform the 

adequacy of the current suite of standards or alternative annual standard levels (U.S. 

EPA, 2011a, p. 2-32).
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(Peters et al., 1999). These results were found to be consistent with results of crosssectional analyses of the 24-Cities Study (Dockery et al., 1996; Raizenne et al., 1996), 

which reported a long-term cross-city mean PM2.5 concentration of 14.5 μg/m3.

30 In this 

review, extended analyses of the Southern California Children’s Health Study provide 

stronger evidence of PM2.5-related respiratory effects, at lower air quality concentrations 

than had previously been reported, with a four-year aggregate mean concentration of 13.8 

μg/m3across the 12 study communities (McConnell et al., 2003; Gauderman et al., 2004, 

U.S. EPA, 2009a, Figure 7-4). 

In also considering health effects for which the Integrated Science Assessment 

concluded evidence was suggestive of a causal relationship, the Policy Assessment noted 

a limited number of birth outcome studies that reported positive and statistically 

significant effects related to aggregate long-term mean PM2.5 concentrations down to 

approximately 12 μg/m3(U.S. EPA, 2011a, p. 2-33).

Collectively, the Policy Assessment concluded that currently available evidence 

provided support for associations between long-term PM2.5 exposure and mortality and 

morbidity effects that extend to distributions of PM2.5 concentrations that are lower than 

those that had previously been associated with such effects, with aggregate long-term 

mean PM2.5 concentrations extending to well below the level of the current annual 

standard.

The Policy Assessment also considered the long-term mean PM2.5 concentrations 

in short-term exposure studies in assessing the appropriateness of the level of the current 

annual standard. See American Farm Bureau Federation v. EPA, 559 F. 3d at 522, 523-

 

30 See American Farm Bureau Federation v. EPA, 559 F. 3d at 525 (noting the 

importance of these studies, as well as EPA’s failure to properly take them into account).
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24 (remanding 2006 standard because the EPA had not adequately explained its choice 

not to consider long-term means of short-term exposure studies in assessing adequacy of 

primary annual PM2.5 standard). In light of the mixed findings reported in single-city, 

short-term exposure studies, the Policy Assessment placed comparatively greater weight 

on the results from multi-city studies in considering the adequacy of the current suite of 

standards (U.S. EPA, 2011a, pp. 2-34 to 2-35). 

With regard to associations reported in short-term PM2.5 exposure studies, the 

Policy Assessment recognized that long-term mean concentrations reported in new multicity U.S. and Canadian studies provided evidence of associations between short-term 

PM2.5 exposure and mortality at similar air quality distributions to those that had 

previously been observed in an 8-cities Canadian study (Burnett and Goldberg, 2003; 

aggregate long-term mean PM2.5 concentration of 13.3 μg/m3). In a multi-city time-series 

analysis of 112 U.S. cities, Zanobetti and Schwartz (2009) reported a positive and 

statistically significant association with all-cause, cardiovascular-related (e.g., heart 

attacks, stroke), and respiratory-related mortality and short-term PM2.5 exposure, in 

which the aggregate long-term mean PM2.5 concentration was 13.2 μg/m3(U.S. EPA, 

2009a, Figure 6-24). Furthermore, city-specific effect estimates indicated the association 

between short-term exposure to PM2.5 and total mortality and cardiovascular- and 

respiratory-related mortality was consistently positive for an overwhelming majority (99 

percent) of the 112 cities across a wide range of air quality concentrations (long-term 

mean concentrations ranging from 6.6 μg/m3to 24.7 μg/m3; U.S. EPA, 2009a, Figure 6-

24, p. 6-178 to 179). The EPA staff noted that for all-cause mortality, city-specific effect 

estimates were statistically significant for 55 percent of the 112 cities, with long-term 
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city-mean PM2.5 concentrations ranging from 7.8 μg/m3to 18.7 μg/m3and 24-hour PM2.5

city-mean 98th percentile concentrations ranging from 18.4 to 64.9 μg/m3(personal 

communication with Dr. Antonella Zanobetti, 2009).31

With regard to cardiovascular and respiratory morbidity effects, in the first 

analysis of the Medicare cohort conducted by Dominici et al. (2006a) across 204 U.S. 

counties, investigators reported a statistically significant association with hospitalizations 

for cardiovascular and respiratory diseases and short-term PM2.5 exposure, in which the 

aggregate long-term mean PM2.5 concentration was 13.4 μg/m3. Furthermore, a subanalysis restricted to days with 24-hour average concentrations of PM2.5 at or below 35 

μg/m3indicated that, in spite of a reduced statistical power from a smaller number of 

study days, statistically significant associations were still observed between short-term 

exposure to PM2.5 and hospital admissions for cardiovascular and respiratory diseases 

(Dominici, 2006b).32 In an extended analysis of this cohort, Bell et al (2008) reported a 

positive and statistically significant increase in cardiovascular hospitalizations associated 

with short-term PM2.5 exposure, in which the aggregate long-term mean PM2.5

concentration was 12.9 μg/m3. These results, along with the observation that 

approximately 50 percent of the 204 county-specific mean 98th percentile PM2.5

concentrations in the study aggregated across all years were below the 24-hour standard 

 

31 Single-city Bayes-adjusted effect estimates for the 112 cities analyzed in Zanobetti and 

Schwartz (2009) were provided by the study authors (personal communication with Dr. 

Antonella Zanobetti, 2009; see also U.S. EPA, 2009a, Figure 6-24). 

32 This sub-analysis was not included in the original publication (Dominici et al., 2006a). 

The study authors provided sub-analysis results for the Administrator’s consideration as a 

letter to the docket following publication of the proposed rule in January 2006 (personal 

communication with Dr. Francesca Dominici, 2006b). As noted in section III.A.3, this 

study is part of the basis for the conclusion that there is no evidence suggesting that risks 

associated with long-term exposures are likely to be disproportionately driven by peak 

24-hour concentrations.
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of 35 μg/m3, not only indicated that effects are occurring in areas that would meet the 

current standards but also suggested that the overall health effects observed across the 

U.S. are not primarily driven by the higher end of the PM2.5 air quality distribution (Bell, 

2009a, personal communication from Dr. Michelle Bell regarding air quality data for Bell 

et al., 2008 and Dominici et al., 2006a). 

Collectively, the Policy Assessment concluded that the findings from short-term 

PM2.5 exposure studies provided evidence of PM2.5-associated health effects occurring in 

areas that would likely have met the current suite of PM2.5 standards (U.S. EPA, 2011a, p. 

2-35). These findings were further bolstered by evidence of statistically significant 

PM2.5-related health effects occurring in analyses restricted to days in which 24-hour 

average PM2.5 concentrations were below 35 μg/m3(Dominici, 2006b). 

In evaluating the currently available scientific evidence, as summarized in section 

III.B of the proposal, the Policy Assessment first concluded that there was stronger and 

more consistent and coherent support for associations between long- and short-term 

PM2.5 exposures and a broad range of health outcomes than was available in the last 

review, providing the basis for fine particle standards at least as protective as the current 

PM2.5 standards (U.S. EPA, 2011a, p. 2-26). Having reached this initial conclusion, the 

Policy Assessment addressed the question of whether the available evidence supported 

consideration of standards that were more protective than the current standards. In so 

doing, the Policy Assessment considered whether there was now evidence that health 

effect associations have been observed in areas that likely met the current suite of PM2.5

standards. As discussed above, long- and short-term PM2.5 exposure studies provided 

evidence of associations with mortality and cardiovascular and respiratory effects both at 
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lower ambient PM2.5 concentrations than had been observed in the previous review and at 

concentrations allowed by the current standards (U.S. EPA, 2011a, p. 2-35). 

In reviewing this information, the Policy Assessment recognized that important 

limitations and uncertainties associated with this expanded body of scientific evidence, as 

discussed in section III.B.2 of the proposal, needed to be carefully considered in 

determining the weight to be placed on the body of studies available in this review. 

Taking these limitations and uncertainties into consideration, the Policy Assessment 

concluded that the currently available evidence clearly calls into question whether the 

current suite of primary PM2.5 standards protects public health with an adequate margin 

of safety from effects associated with long- and short-term exposures. Furthermore, the 

Policy Assessment concluded this evidence provides strong support for considering fine 

particle standards that would afford increased protection beyond that afforded by the 

current standards (U.S. EPA, 2011a, p. 2-35).

In addition to evidence-based consideration, the Policy Assessment also 

considered the extent to which health risks estimated to occur upon simulating just 

meeting the current PM2.5 standards may be judged to be important from a public health 

perspective, taking into account key uncertainties associated with the quantitative health 

risk estimates. In so doing, the Policy Assessment first noted that the quantitative risk 

assessment addresses: (1) the core PM2.5-related risk estimates; (2) the related uncertainty 

and sensitivity analyses, including additional sets of reasonable risk estimates generated 

to supplement the core analysis; (3) an assessment of the representativeness of the urban 

study areas within a national context;33 and (4) consideration of patterns in design values 

 

33 Based on analyses of the representativeness of the 15 urban study areas in the broader 

national context, the Policy Assessment concludes that these study areas are generally 
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and air quality monitoring data to inform interpretation of the risk estimates, as discussed 

in section III.C above. 

In considering the health risks estimated to remain upon simulation of just 

meeting the current suite of standards and considering both the qualitative and 

quantitative assessment of uncertainty completed as part of the assessment, the Policy 

Assessment concluded these risks are important from a public health standpoint and 

provided strong support for consideration of alternative standards that would provide 

increased protection beyond that afforded by the current PM2.5 (U.S. EPA, 2011a, pp. 2-

47 to 2-48). This conclusion reflected consideration of both the severity and the 

magnitude of the effects. For example, the Risk Assessment indicated the possibility that 

premature deaths related to ischemic heart disease associated with long-term PM2.5

exposure alone would likely be on the order of thousands of deaths per year in the 15 

urban study areas upon simulating just meeting the current standards34 (U.S. EPA, 2011a, 

pp. 2-46 to 2-47). Moreover, additional risks were anticipated for premature mortality 

related to cardiopulmonary effects and lung cancer associated with long-term PM2.5

exposure as well as mortality and cardiovascular- and respiratory-related morbidity 

effects (e.g., hospital admissions, emergency department visits) associated with shortterm PM2.5 exposures. Based on the consideration of both qualitative and quantitative 

assessments of uncertainty completed as part of the quantitative risk assessment, the Risk 

Assessment concluded that it was unlikely that the estimated risks are over-stated, 

particularly for mortality related to long-term PM2.5 exposure, and may well be biased 

 

representative of urban areas in the U.S. likely to experience relatively elevated levels of 

risk related to ambient PM2.5 exposures (U.S. EPA, 2011a, p. 2-42). 

34 Premature mortality for all causes attributed to PM2.5 exposure was estimated to be on 

the order of tens of thousands of deaths per year on a national scale based on 2005 air 

quality data (U.S. EPA, 2010a, Appendix G, Table G-1).
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low based on consideration of alternative model specifications evaluated in the sensitivity 

analyses (U.S. EPA, 2010a, p. 5-16; U.S. EPA, 2011a, p. 2-41). Furthermore, the 

currently available scientific information summarized in section III.B of the proposal 

provided evidence for a broader range of health endpoints and at-risk populations beyond 

those included in the quantitative risk assessment (U.S. EPA, 2011a, p. 2-47).

b. CASAC Advice

The CASAC, based on theirits review of drafts of the Integrated Science 

Assessment, the Risk Assessment, and the Policy Assessment, provided an array of 

advice both with regard to interpreting the scientific evidence and quantitative risk 

assessment, as well as with regard to consideration of the adequacy of the current PM2.5

standards (Samet, 2009a b,c,d,e,f; Samet 2010a,b,c,d). With regard to the adequacy of the 

current standards, CASAC concluded that the “currently available information clearly 

calls into question the adequacy of the current standards” (Samet, 2010d, p. i) and that the 

current standards are “not protective” (Samet, 2010d, p. 1). Further, in commenting on 

the first draft Policy Assessment, CASAC noted:

With regard to the integration of evidence-based and risk-based considerations, 

CASAC concurs with EPA’s conclusion that the new data strengthens the 

evidence available on associations previously considered in the last round of the 

assessment of the PM2.5 standard. CASAC also agrees that there are significant 

public health consequences at the current levels of the standard that justify 

consideration of lowering the PM2.5 NAAQS further (Samet, 2010c, p.12). 

c. Administrator's Proposed Conclusions Concerning the Adequacy of the Current 

Primary PM2.5 Standards

At the time of the proposal, in considering the body of scientific evidence, the 

Administrator concluded there was stronger and more consistent and coherent support for 

associations between long- and short-term PM2.5 exposure and a broader range of health 

outcomes than was available in the last review, providing the basis for fine particle 
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standards at least as protective as the current PM2.5 standards. In particular, the 

Administrator recognized in section III.D.4 of the proposal that the Integrated Science 

Assessment concluded that the results of epidemiological and experimental studies form 

a plausible and coherent data set that supports a causal relationship between long- and 

short-term PM2.5 exposures and mortality and cardiovascular effects and a likely causal 

relationship between long- and short-term PM2.5 exposures and respiratory effects. 

Furthermore, the Administrator reflected that effects had been observed at lower ambient 

PM2.5 concentrations than what had been observed in the last review, including at 

ambient PM2.5 concentrations in areas that likely met the current PM2.5 NAAQS. With 

regard to the results of the quantitative risk assessment, the Administrator noted that the 

Risk Assessment concludesd that the risks estimated to remain upon simulation of just 

meeting the current standards arewere important from a public health standpoint in terms 

of both the severity and magnitude of the effects. 

At the time of the proposal, in considering whether the current suite of PM2.5

standards should be revised to provide requisite public health protection, the 

Administrator carefully considered the staff conclusions and rationales presented in the 

Policy Assessment, the advice and recommendations from CASAC, and public comments 

to date on this issue. In so doing, the Administrator placed primary consideration on the 

evidence obtained from the epidemiological studies and provisionally found the evidence 

of serious health effects reported in long- and short-term exposure studies conducted in 

areas that would have met the current standards to be compelling, especially in light of 

the extent to which such studies are part of an overall pattern of positive and frequently 

statistically significant associations across a broad range of studies that collectively 
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represent a strong and robust body of evidence. 

As discussed in the Integrated Science Assessment and Policy Assessment, the 

Administrator recognized that much progress has been made since the last review in 

addressing some of the key uncertainties that were important considerations in 

establishing the current suite of PM2.5 standards. For example, progress made since the 

last review provides increased confidence in the long- and short-term exposure studies as 

a basis for considering whether any revision of the annual standard is appropriate and 

increased confidence in the short-term exposure studies as a basis for considering 

whether any revision of the 24-hour standard is appropriate.35

Based on her consideration of these conclusions, as well as consideration of 

CASAC’s conclusion that the evidence and risk assessment clearly called into question 

the adequacy of the public health protection provided by the current PM2.5 NAAQS and 

public comments on the proposal, the Administrator provisionally concluded that the 

current primary PM2.5 standards, taken together, arewere not requisite to protect public 

health with an adequate margin of safety and that revision iswas needed to provide 

increased public health protection. The Administrator provisionally concluded that the 

scientific evidence and information on risk provided strong support for consideration of 

alternative standards that would provide increased public health protection beyond that 

afforded by the current PM2.5 standards.

2. Comments on the Need for Revision

 

35 The EPA notes that this increased confidence in the long- and short-term associations 

generally reflects less uncertainty as to the likely causal nature of such associations, but 

does not address directly the question of the extent to which such associations remain 

toward the lower end of the range of ambient PM2.5 concentrations. This question is 

central to the Agency’s evaluation of the relevant evidence to determine appropriate 

standards levels, as discussed below in section III.E.4.
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This section addresses general comments based on relevant facts that either 

support or oppose any change to the current suite of primary PM2.5 standards. Comments 

on specific long- and short-term exposure studies that relate to consideration of the 

appropriate levels of the annual and 24-hour standards are addressed in section III.E.4 

below. Many public comments asserted that the current PM2.5 standards are insufficient 

to protect public health with an adequate margin of safety and that revisions to the 

standards are therefore appropriate, indeed necessitated. 

Among those calling for revisions to the current standards were the Children’s 

Health Protection Advisory Committee (CHPAC); major medical and public health 

groups including the American Heart Association (AHA), American Lung Association 

(ALA), American Public Health Association (APHA), American Thoracic Society 

(ATS); the Physicians for Social Responsibility (PSR); major environmental groups such 

as the Clean Air Council, Clean Air Task Force, Earthjustice, Environmental Defense 

Fund (EDF), National Resources Defense Council (NRDC), and Sierra Club; many 

environmental justice organizations as well as medical doctors, academic researchers, 

health professionals, and many private citizens. For example, the American Heart 

Association and other major national public health and medical organizations stated that, 

“[o]ur organizations are keenly aware of the public health and medical threats from 

particulate matter” and called on the EPA to “significantly strengthen” both the annual 

and 24-hour PM2.5 standards “to help us protect the health of our patients and our nation” 

(AHA et al., 2012, pp 1 and 13). AHA et al. and ALA et al., as well as a group of more 

than 350 physicians, environmental health researchers, and public health and medical 

professionals articulated similar comments on the available evidence:
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Ample scientific evidence supports adopting tighter standards to protect the health 

of people who are most susceptible to the serious health effects of these 

pollutants. More than 10,000 peer-reviewed scientific studies have been published 

since 1997 when EPA adopted the current annual standard. These studies validate 

and extend earlier epidemiologic research linking both acute and chronic fine 

particle pollution with serious morbidity and mortality. The newer research has 

also expanded our understanding of the range of health outcomes associated with 

PM and has identified adverse respiratory and cardiovascular health effects at 

lower exposure levels than previously reported. As discussed and interpreted in 

the EPA’s 2009 Integrated Science Assessment for Particulate Matter, the new 

evidence reinforces already strong existing studies and supports the conclusion 

that PM2.5 is causally associated with numerous adverse health effects in humans 

at exposure levels far below the current standard. Such a conclusion demands 

prompt action to protect human health. (AHA et al., 2012, pp. 1 to 2; ALA et al., 

pp. 4 to 5; similar comment submitted by Rom et al., 2012, p. 1).

All of these medical and public health commenters stated that the current PM2.5

standards need to be revised, and that even more protective standards than those proposed 

by the EPA are needed to adequately protect public health, particularly for at-risk 

populations. Many environmental justice organizations and individual commenters also 

expressed such views.

The National Association of Clean Air Agencies (NACAA), the Northeast States 

for Coordinated Air Use Management (NESCAUM), and many State and local air 

agencies and health departments who commented on the PM2.5 standards supported 

revision of the suite of current PM2.5 standards, as did five state attorneys general 

(Schneiderman et al., 2012) and the National Tribal Air Association (NTAA). 

These commenters based their views chiefly on the body of evidence and 

technical analyses presented and discussed in the Integrated Science Assessment, the 

Risk Assessment, and the Policy Assessment finding the available scientific information 

to be stronger and more compelling than in the last review. These commenters generally 

placed much weight on CASAC’s recommendation to revise the PM2.5 standards to 

provide increased public health protection and on the EPA staff conclusions presented in 
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the final Policy Assessment.

Some of these commenters specifically mentioned extended analyses of seminal 

long-term exposure studies - the ACS (Krewski et al., 2009), Harvard Six Cities (Laden 

et al., 2006), and Southern California Children’s Health (Gauderman et al., 2004) studies. 

These commenters also highlighted the availability of additional long-term exposure 

studies in this review, specifically a study of premature mortality in older adults (Eftim et 

al., 2008) and the WHI study of cardiovascular morbidity and mortality effects in women 

(Miller et al., 2007) providing stronger evidence of mortality and morbidity effects 

associated with long-term PM2.5 exposures at lower concentrations than had previously 

been observed, including studies of effects in at-risk populations. For example, some 

commenters asserted:

Evidence during the last review showed clearly that the annual average standard 

needed to be much lower than the standard of 15 μg/m3that was first set in 1997. 

The evidence has only grown since then. Multiple, multi-city studies over long 

periods of time have shown clear evidence of premature death, cardiovascular and 

respiratory harm as well as reproductive and developmental harm at contemporary 

concentrations far below the level of the current (annual) standard (ALA et al., 

2012, p. 39; AHA et al., 2012, p. 10). 

These commenters also highlighted the availability of a number of short-term 

PM2.5 exposure studies as providing evidence of mortality and morbidity effects at 

levelsconcentrations below the level of the current 24-hour PM2.5 standard. Specifically, 

these commenters made note of multi-city studies of premature mortality (Zanobetti and 

Schwartz, 2009) and increased hospitalizations for cardiovascular and respiratory-related 

effects in older adults (Bell et al., 2008). These commenters also asserted the importance 

of many of the single-city studies, arguing that these studies “provide valuable 

information regarding impacts on susceptible populations and on health risk in areas with 

high peak to mean concentration ratios” (ALA, et al., 2012, p. 65). Collectively, 
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considering the multi- and single-city short-term exposure studies, these commenters 

asserted “the record clearly supports a more stringent 24-hour standard of 25 μg/m3to 

provide uniform protection in all regions of the country particularly from short-term 

spikes in pollution and from the sub-daily exposures that trigger heart attacks and 

strokes” (ALA et al., 2012, p. 62). A group of more than 350 physicians, environmental 

health researchers, and public health and medical professionals argued, “[s]tudies of 

short-term exposure demonstrate that PM2.5 air pollution increases the risk of hospital 

admissions for heart and lung problems even when you exclude days with pollution 

concentrations at or above the current daily standard of 35 μg/m3. Daily concentrations 

must be capped at lower levels to protect against peak exposure days that occur due to 

local and seasonal sources of emissions” (Rom et al., 2012, p. 2).

In addition, many of these commenters generally concluded that progress had 

been made in reducing many of the uncertainties identified in the last review, in better 

understanding mechanisms by which PM2.5 may be causing the observed health effects, 

and in improving our understanding of at-risk populations.36 Further, these Further, a 

number of commenters argued that by making the standards more protective, the PM2.5

NAAQS would be more consistent with other existing standards (e.g., California's annual 

average standard of 12 μg/m3) (CARB, 2012; CA OEHHA, 2012). Other commenters 

argued that revising the primary PM2.5 standards would be more consistent with the 

recommendations of the World Health Organization (WHO) and/or Canada (e.g., ALA et 

al., 2012, p. 62; ISEE, 2012, p. 2; MOE-Ontario, 2012, p. 1; ISEE, 2012, p. 2). 

 

36 In addition, the EPA notes that CASAC found that the Integrated Science Assessment 

clearly presented and discussed the current scientific information related to potential 

modes of action (Chapter 5) and at-risk populations (Chapter 8) (Samet, 2009e, pp. 7 to 

8; Samet, 2009f, pp. 2 and 11 to 12).
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With regard to the scope of the literature reviewed for PM2.5-related health 

effects, some commenters asserted that the EPA inappropriately narrowed the scope of 

the review by excluding a number of categories of relevant studies, specifically related to 

studies of diesel pollution and traffic-related pollution (ALA, et al., 2012, p. 17). These 

commenters argued that, based upon the exclusion of these types of studies, the 

Integrated Science Assessment “came to the erroneous conclusion that the causal 

relationship between PM and cancer is merely suggestive. This conclusion does not 

square with the International Agency Research on Cancer (IARC) finding that diesel 

emissions are a known human carcinogen nor with the conclusions of the extended 

analyses of the [Harvard] Six Cities and ACS cohort studies that report positive and 

statistically significant associations between PM2.5 and lung cancer.” Id.

Some of these commenters also noted the results of the EPA’s quantitative risk 

assessment, concluding that it showed that the risks estimated to remain when the current 

standards are met are large and important from a public health perspective and warrant 

increased protection. For example, ALA et al., noted that the Risk Assessment indicated 

the quantitative risk analyses likely underestimated PM2.5-related mortality (U.S. EPA, 

2010a, p. 5-16) and argued that “the measurements of risk should be treated 

conservatively” (ALA, et al., 2012, p. 73). These commenters also summarized an 

expanded analysis of alternative PM2.5 standard levels that they argued documented the 

need for more protective standards (McCubbin, 2011). 

In general, all of these commenters agreed on the importance of results from the 

large body of scientific studies reviewed in the Integrated Science Assessment and on the 

need to revise the suite of PM2.5 standards as articulated in the EPA’s proposal, while 
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generally differing with the EPA’s proposed judgments about the extent to which the 

standards should be revised based on this evidence, specifically for providing protection 

for at-risk populations. 

The EPA generally agrees with these commenters’ conclusion regarding the need 

to revise the current suite of PM2.5 standards. The scientific evidence noted by these 

commenters was generally the same as that assessed in the Integrated Science 

Assessment and the Policy Assessment, and the EPA agrees that this evidence provides a 

strong basis for concluding that the current PM2.5 standards, taken together, are not 

requisite to protect public health with an adequate margin of safety, and they need to be 

revised to provide increased protection. For reasons discussed in section III.E.4.c below, 

however, the EPA disagrees with aspects of these commenters’ views on the level of 

protection that is appropriate.

The EPA disagrees with these commenters’ views that diesel exhaust studies were 

excluded from the Integrated Science Assessment and were not considered when making 

the causality determination for cancer, mutagenicity, and genotoxicity. As discussed in 

section 7.5 of the Integrated Science Assessment, diesel exhaust studies were integrated 

within the broader body of scientific evidence that was considered in reaching the 

causality determination for these health endpoints. Additionally, as discussed in section

1.5.3 of the Integrated Science Assessment, the evidence from diesel exhaust studies was 

also considered as part of the collective evidence evaluated when making determinations 

for other, noncancer health outcomes (e.g., cardiovascular and respiratory effects).37

 

37 In developing the second draft Integrated Science Assessment, the EPA reexamined the 

controlled human exposure and toxicological studies of fresh diesel and gasoline exhaust 

and considered This information, in addition to other considerations, supported a change 

in the causal determinations for ultrafine particles. Specifically, in reevaluating the causal 
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Specifically, when evaluating this evidence, the focus was on understanding the effects of 

diesel exhaust particles. 

It is important to recognize that the Integrated Science Assessment focused on 

experimental studies of diesel exhaust that evaluated exposures that were relevant to 

ambient concentrations, i.e., “within one or two orders of magnitude of ambient PM 

concentrations” (U.S. EPA, 2009a, section 1.3). The causal determination for cancer, 

mutagenicity, and genotoxicity presented in the Integrated Science Assessment represents 

an integration of experimental and observational evidence of exposures to ambient PM 

concentrations. The EPA fully considered the findings of studies that assessed these and 

other health effects associated with exposure to diesel particles in reaching causality 

determinations regarding health outcomes associated with PM2.5 exposures. Furthermore, 

CASAC supported the EPA’s change to the causal determination for cancer and longterm PM2.5 concentrations from “inadequate” to “suggestive” (Samet, 2009f, p. 2).

With regard to traffic studies, the EPA disagrees with the commenters’ views that 

traffic studies that focused on exposure indicators such as distance to roadways should 

have been included in the Integrated Science Assessment. These studies were excluded 

from consideration because they did not measure ambient concentrations of specific air 

pollutants, including PM2.5, but instead were studies evaluating exposure to the 

undifferentiated “traffic related air pollution” mixture (ALA et al., 2012, p.17) (U.S.

EPA, 2009a, section 1.3). As a result, these studies do not add to the collective body of 

evidence on the relationship between long- or short-term exposure to ambient 

 

determinations for short-term ultrafine particle exposures and cardiovascular and 

respiratory effects, the EPA changed the classification from “inadequate” to “suggestive”

for both categories of health outcomes (Vandenberg, 2009, p. 3). CASAC agreed with the 

EPA’s rationale for revising these causal determinations (Samet, 2009f, p. 10).
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concentrations of PM2.5 and health effects.

Some of these commenters also identified “new” studies that were not included in 

the Integrated Science Assessment as providing further support for the need to revise the 

primary PM2.5 standards. As discussed above in section II.B.3 above, the EPA notes that, 

as in past NAAQS reviews, the Agency is basing the final decisions in this review on the 

studies and related information included in the PM air quality criteria that have 

undergone CASAC and public review and will consider the “new” studies for purposes of 

decision making in the next PM NAAQS review. Nonetheless, in provisionally 

evaluating commenters’ arguments (see Response to Comments document), the EPA 

notes that its provisional assessment of “new” science found that such studies did not 

materially change the conclusions in the Integrated Science Assessment (U.S. EPA, 

2012b).

Another group of commenters opposed revising the current PM2.5 standards. 

These views were most extensively presented in comments from the Utility Air 

Regulatory Group (UARG), representing a group of electric generating companies and 

organizations and several national trade associations; the American Petroleum Institute 

(API) representing more than 500 oil and natural gas companies; the National 

Association of Manufacturers (NAM), the American Chemistry Council (ACC), the 

American Fuel & Petroleum Manufacturers (AFPM), the Alliance of Automobile 

Manufacturers, (AAM), and other manufacturing associations; the Electric Power 

Research Institute (EPRI); and the Texas Commission on Environmental Quality (Texas 

CEQ). These commenters generally mentioned many of the same studies that were cited 

by the commenters who supported revising the standards, as well as other studies, but 
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highlighted different aspects of these studies in reaching substantially different 

conclusions about their strength and the extent to which progress has been made in 

reducing uncertainties in the evidence since the last review. Furthermore, they asserted 

that the evidence that has become available since the last review does not establish a 

more certain risk or a risk of effects that are significantly different in character to those 

that provided a basis for the current standards, nor does the evidence demonstrate that the 

risk to public health upon attainment of the current standards would be greater than was 

understood when the EPA established the current standards in 2006. 

These commenters generally expressed the view that the current standards provide 

the requisite degree of public health protection. In supporting their view, these 

commenters generally argued that the EPA’s conclusions are inconsistent with the current 

state of the science and questioned the underlying scientific evidence including the causal 

determinations reached in the Integrated Science Assessment. More specifically, this 

group of commenters argued that: (1) the EPA did not apply its framework for causal 

determination consistently across studies or health outcomes and, in the process, the EPA 

relied on a selective group of long- and short-term exposure studies to reach conclusions 

regarding causality; (2) toxicological and controlled human exposure studies do not 

provide supportive evidence that the health effects observed in epidemiological studies 

are biologically plausible; (3) uncertainties in the underlying health science are as great 

or greater than in 2006; (4) there is no evidence of greater risk since the last review to 

justify tightening the current annual PM2.5 standard; and (5) “new” studies not included in 

the Integrated Science Assessment continue to increase uncertainty about possible health 

risks associated with exposure to PM2.5. These comments are discussed in turn below.
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(l) Some of these commenters asserted that the EPA did not apply its framework 

for causal determinations consistently across studies or health outcomes (ACC, 2012, 

Attachment A, pp. 1 to 2; API, 2012, Attachment 1, p. 30; NAM et al., 2012, pp. 22 to 

25; Texas CEQ, 2012, pp 2 to 3).38 These commenters argued that the EPA downplayed 

epidemiological studies with null or inconsistent results, inappropriately used the Hill 

criteria when evaluating the epidemiological evidence, and used the same study and the 

same underlying database to conclude that there was a causal association between 

mortality and multiple criteria pollutants.

The EPA disagrees with these commenters’ views. First, the EPA recognizes that 

the evaluation of the scientific evidence and its application of the causal framework used 

in the current PM NAAQS review was the subject of exhaustive and detailed review by 

CASAC and the public. As summarized in section II.B.3 above, prior to finalizing the 

Integrated Science Assessment, two drafts were released for CASAC and public review 

to evaluate the scientific integrity of the documents. Evidence related to the substantive 

issues raised by CASAC and public commenters with regard to the content of the first 

and second draft Integrated Science Assessments were discussed at length during these 

public CASAC meetings and considered in developing the final Integrated Science 

Assessment. CASAC supported the development of the EPA’s causality framework and 

its use in the current PM NAAQS review and concluded:

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

systematically applied; this approach has provided transparency and a clear 

statement of the level of confidence with regard to causation, and we recommend 

 

38 The EPA notes that the same concerns about the causal determinations presented in the 

Integrated Science Assessment were raised in comments to CASAC on the draft 

Integrated Science Assessments (e.g., UARG, 2009; API, 2009; ACC, 2012, Appendix 

B). CASAC, therefore, had the opportunity to consider these comments in reaching 

consensus conclusions on this issue. 
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its continued use in future Integrated Science Assessments (Samet 2009f, p. 1). 

These commenters asserted that during the application of the causal framework

the EPA inappropriately relied on a selective group of long- and short-term exposure 

studies in reaching causal inferences (API, 2012, pp 12 to 17; ACC, 2012, Attachment A, 

pp 1 to 2; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). Additionally, 

these commenters expressed the view that the EPA focused on a subset of 

epidemiological studies that reported positive and statistically significant results while 

ignoring other studies, especially those that reported no statistically significant

associations, those that reported potential thresholds, or those that highlighted 

uncertainties and limitations in study design or results. Furthermore, some of these 

commenters argued that epidemiological studies are observational in nature and cannot 

provide evidence of a causal association.

The EPA disagrees with these commenters’ views on assessing the health effects 

evidence and on the conclusions regarding the causality determinations reached in the 

Integrated Science Assessment. In conducting a comprehensive evaluation of the 

evidence in the Integrated Science Assessment, the EPA recognized the distinction 

between the evaluation of the relative scientific quality of individual study results and the 

evaluation of the pattern of results within the broader body of scientific evidence and 

considered both in reaching causality determinations. The more detailed characterizations 

of individual studies included an assessment of the quality of the study based on specific 

criteria as described in the Integrated Science Assessment (U.S. EPA, 2009a, section 

1.5.3). 

In developing an integrated assessment of the health effects evidence for PM, the 

EPA emphasized the importance of examining the pattern of results across various 
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studies and did not focus solely on statistical significance39 as a criterion of study 

strength. This approach is consistent with views clearly articulated throughout the 

epidemiological and causal inference literature, specifically, that it is important not to 

focus on results of statistical tests to the exclusion of other information.40 The concepts 

underlying the EPA’s approach to evaluating statistical associations have been discussed 

in numerous publications, including a report by the U.S. Surgeon General on the health 

consequences of smoking (Centers for Disease Control and Prevention, 2004). This 

report cautions against over-reliance on statistical significance in evaluating the overall 

evidence for an exposure-response relationship. Criteria characterized by Hill (1965) also 

addressed the value, or lack thereof, of statistical tests in the determination of cause: 

No formal tests of significance can answer those [causal] questions. Such tests 

can, and should, remind us of the effects the play of chance can create, and they 

will instruct us in the likely magnitude of those effects. Beyond that, they 

contribute nothing to the ‘proof’ of our hypothesis (Hill, 1965, p. 299). 

The statistical significance of individual study findings has played an important 

role in the EPA’s evaluation of the study’s results and the EPA has placed greater 

emphasis on studies reporting statistically significant results. However, in the broader 

evaluation of the evidence from many epidemiological studies, and subsequently during 

 

39 Statistical significance is an indicator of the precision of a study’s results, which is 

influenced by a variety of factors including, but not limited to, the size of the study, 

exposure and measurement error, and statistical model specifications. Studies typically 

calculate “p-values” to determine whether the study results are statistically significant or 

whether the study results are likely to occur simply by chance. In general practice, effects 

are considered statistically significant if p values are less than 0.05.

40 For example, Rothman (1998) stated, “Many data analysts appear to remain oblivious 

to the qualitative nature of significance testing [and that]... statistical significance is itself 

only a dichotomous indicator. As it has only two values, significant or not significant 

....Nevertheless, p-values still confound effect size with study size, the two components 

of estimation that we believe need to be reported separately.” As a result, Rothman 

recommended that p-values be omitted as long as point and interval estimates are 

available. 
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the process of forming causality determinations in integrating evidence across 

epidemiological, controlled human exposure, and toxicological studies, the EPA has 

emphasized the pattern of results across epidemiological studies, and whether the effects 

observed were coherent across the scientific disciplines for drawing conclusions on the 

relationship between PM2.5 and different health outcomes. Thus, the EPA did not limit its 

focus or consideration to just studies that reported positive associations or where the 

results were statistically significant. 

In addition, some commenters asserted that the EPA inappropriately used the Hill 

criteria by failing to consider the limitations of studies with weak associations, thereby 

overstating the consistency of the observed associations (API, 2012, Attachment 1, pp. 30 

to 35). These commenters argued that risk estimates greater than 3 to 4 reflect strong 

associations supportive of a causal link, while smaller risk estimates (i.e., 1.5 to 3) are 

considered to be weak and require other lines of evidence to demonstrate causality. 

As discussed in section 1.5.3 of the Integrated Science Assessment, the EPA 

thoroughly considered the limitations of all studies during its evaluation of the scientific 

literature (U.S. EPA,, 2009a, p. 1-14). This collective body of evidence, including known 

uncertainties and limitations of the studies evaluated, were considered during the process 

of forming causality determinations as discussed in chapters 6 and 7 of the Integrated 

Science Assessment. For example, the EPA concluded that “a causal relationship exists 

between short-term PM2.5 exposure and cardiovascular effects,” however, in reaching this 

conclusion, the Agency recognized and considered limitations of the current evidence 

that still requires further examination (U.S. EPA, 2009a., in section 6.2.12.1). Therefore, 

the EPA disagrees with these commenters’ views that the Hill criteria were 
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inappropriately used in that the limitations of studies were not considered. 

The EPA also disagrees with the commenterscommenters’ assertion that the 

magnitude of the association must be large to support a determination of causality. As 

discussed in the Integrated Science Assessment, the strength of the observed association 

is an important aspect to aid in judging causality and “while large effects support 

causality, modest effects therefore do not preclude it” (U.S. EPA, 2009a, Table 1-2, 

section 1.5.4). The weight of evidence approach used by the EPA encompasses a 

multitude of factors of which the magnitude of the association is only one component 

(U.S. EPA, 2009a, Table 1-3). An evaluation of the association across multiple 

investigators and locations supports the “reproducibility of findings [which] constitutes 

one of the strongest arguments for causality” (U.S. EPA, 2009a, Table 1-2). Even though 

the risk estimates for air pollution studies may be modest, the associations are consistent 

across hundreds of studies as demonstrated in the Integrated Science Assessment. 

Furthermore, the causality determinations rely on different lines of evidence, by 

integrating evidence across disciplines, including animal toxicological studies and 

controlled human exposure studies.

Furthermore, as summarized in section III.B above and discussed more fully in 

section III.B.3 of the proposal, the EPA recognizes that the population potentially 

affected by PM2.5 is considerable, including large subgroups of the U.S. population that 

have been identified as at-risk populations (e.g., children, older adults, persons with 

underlying cardiovascular or respiratory disease). While individual effect estimates from 

epidemiological studies may be modest in size, the public health impact of the mortality 

and morbidity associations can be quite large given that air pollution is ubiquitous. 
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Indeed, with the large population exposed, exposure to a pollutant causally associated at a 

population level with mortality and serious illness has significant public health 

consequences, virtually regardless of the relative risk. Taken together, this information 

indicates that exposure to ambient PM2.5 concentrations has substantial public health 

impacts.

In addition, these commenters believed that the EPA downplayed null or 

inconsistent findings in numerous long-term mortality studies with reported PM2.5

concentrations above and below the level of the current annual standard. The EPA 

disagrees that studies with null or inconsistent findings were not accurately presented and 

considered in the Integrated Science Assessment. For example, as discussed throughout 

section 7.6 and depicted in Figures 7-6 and 7-7 of the Integrated Science Assessment, the 

EPA presented the collective evidence from all studies that examined the association 

between long-term PM2.5 exposure and mortality. Overall, across these studies there was 

evidence of consistent positive associations in different cohorts. That evidence, in 

combination with the biological plausibility provided by experimental and toxicological 

studies evaluated in sections 7.1 and 7.2 of the Integrated Science Assessment, supported 

a causal relationship exists between long-term PM2.5 exposure and mortality. 

Lastly, some of these commenters argued that in some cases, the EPA used the 

same study and the same underlying database to conclude that there is a causal 

association between mortality and multiple criteria pollutants. These commenters argued, 

“[i]n doing so, EPA attributes the cause of the mortality effects observed to whichever 

criteria pollutant it is reviewing at the time” (API, 2012, pp. 14 to 16). 

The EPA strongly disagrees that the Agency “attributes the cause of mortality 
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effects observed to whichever criteria pollutant it is reviewing at the time.” The EPA 

consistently recognizes that other pollutants are also associated with health outcomes, as 

is reflected in the fact that the EPA has established regulations to limit emissions of the 

particulate criteria pollutants as well as other gaseous criteria pollutants. Epidemiological

studies often examine the association between short- and long-term exposures to multiple 

air pollutants and mortality within a common dataset in an attempt to identify the air 

pollutant(s) of the complex mixture most strongly associated with mortality. In evaluating 

these studies, the EPA employs specific study selection criteria to identify those studies 

most relevant to the review of the NAAQS. In its assessment of the health evidence 

regarding PM2.5, the EPA has carefully evaluated the potential for confounding, effect 

measure modification, and the role of PMPM2.5 as a component of a complex mixture of 

air pollutants (U.S. EPA, 2009, p. 1-9). The EPA used a rigorous weight of evidence 

approach to inform causality that evaluated consistency across studies within a discipline, 

evidence for coherence across disciplines, and biological plausibility. Additionally, 

during this process, the EPA assessed the limitations of each study in the context of the 

collective body of evidence. It was the collective evidence, not one individual study that 

ultimately determined whether a causal relationship exists between a pollutant and health 

outcome. In the Integrated Science Assessment, the combination of epidemiological and 

experimental evidence formed the basis for the Agency concluding for the first time that 

a causal relationship exists between short- or long-term exposure to a criteria pollutant 

and mortality (U.S. EPA, 2009, sections 2.3.1.1 and 2.3.1.2). 

Additionally, while the EPA has evaluated some of the studies used to inform the 

causality determination for PM in the Integrated Science Assessments for the other 
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criteria air pollutants, the Agency has done so in the context of examining the collective 

body of evidence for each of the respective criteria air pollutants. As such, the body of 

evidence to inform causality has varied from pollutant to pollutant resulting in the 

association between each criteria air pollutant and mortality being classified at a different 

level of the five-level hierarchy used to inform causation (Seee.g., U.S. EPA, 2008e, U.S.

EPA, 2008f, U.S. EPA, 2010k). 

The EPA notes that the final causality determinations presented in the Integrated 

Science Assessment reflected CASAC’s recommendations on the second draft Integrated 

Science Assessment (Samet, 2009f, pp. 2 to 3). Specifically, CASAC supported the 

EPA’s changes (in the second versus first draft Integrated Science Assessment) from 

“likely causal” to “causal” for long-term exposure to PM2.5 and cardiovascular effects and 

for cancer and PM2.5 (from “inadequate” to “suggestive”). Id. Furthermore, CASAC 

recommended “upgrading” the causal classification for PM2.5 and total mortality to 

“causal” for both the short- and long-term timeframes. Id. With regard to mortality, the 

“EPA carefully reevaluated the body of evidence, including the collective evidence for 

biological plausibility for mortality effects, and determined that a causal relationship 

exists for short- and long-term exposure to PM2.5 and mortality, consistent with the 

CASAC comments” (Jackson, 2012). 

(2) With regard to toxicological and controlled human exposure studies, these 

commenters argued that the available evidence does not provide coherence or biological 

plausibility for health effects observed in epidemiological studies (API, 2012, pp. 21 to 

22, Attachment 1, pp. 25 to 29; AAM, 2012, pp. 15 to 16; Texas CEQ, 2012, p. 3). With 

regard to the issue of mechanisms, these commenters noted that although the EPA 
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recognizes that new evidence is now available on potential mechanisms and plausible 

biological pathways, the evidence provided by toxicological and controlled human 

exposure studies still does not resolve all questions about how PM2.5 at ambient 

concentrations could produce the mortality and morbidity effects observed in 

epidemiological studies. More specifically, for example, some of these commenters 

argued that:

A review of the Integrated Science Assessment, however, suggests that the 

experimental evidence is inconsistent and not coherent with findings in 

epidemiology studies. Specifically, the findings of mild and reversible effects in 

most experimental studies conducted at elevated exposures are not consistent with 

the more serious associations described in epidemiology studies (e.g., hospital 

admissions and mortality). Also, both animal studies and controlled human 

exposure studies have identified no effect levels for acute and chronic exposure to 

PM and PM constituents at concentrations considerably above ambient levels. 

EPA should consider the experimental findings in light of these higher exposure 

levels and what the relevance may be for ambient exposures (API, 2012, 

Attachment 1, p. 25).

The EPA notes that in the review completed in 1997, the Agency considered the 

lack of demonstrated biological mechanisms for the varying effects observed in 

epidemiological studies to be an important caution in its integrated assessment of the 

health evidence upon which the standards were based (71 FR 61157, October 17, 2006). 

In the review completed in 2006, the EPA recognized the findings from additional 

research that indicated that different health responses were linked with different particle 

characteristics and that both individual components and complex particle mixtures 

appeared to be responsible for many biologic responses relevant to fine particle 

exposures. Id. Since that review, there has been a great deal of research directed toward 

advancing our understanding of biologic mechanisms. While this research has not 

resolved all questions, and further research is warranted (U.S. EPA, 2011a, section 2.5), it 

has provided important insights as discussed in section III.B.1 of the proposal (77 FR at 
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38906 to 38909) and discussed more fully in the Integrated Science Assessment (U.S.

EPA, 2009a, Chapter 5). 

As noted in the proposal, toxicological studies provide evidence to support the 

biological plausibility of cardiovascular and respiratory effects associated with long- and 

short-term PM exposures observed in epidemiological studies (77 FR 38906) and provide 

supportive mechanistic evidence that the cardiovascular morbidity effects observed in 

long-term exposure epidemiological studies are coherent with studies of cardiovascularrelated mortality (77 FR 38907). The Integrated Science Assessment concluded that the 

new evidence available in this review “greatly expands” upon the evidence available in 

the last review “particularly in providing greater understanding of the underlying 

mechanisms for PM2.5 induced cardiovascular and respiratory effects for both short- and 

long-term exposures” (U.S. EPA, 2009a, p. 2-17). The mechanistic evidence now 

available, taken together with newly available epidemiological evidence, increases the 

Agency’s confidence that a causal relationship exists between long- and short-term 

exposure to PM2.5 and cardiovascular effects and mortality.41 In addition, CASAC 

supported the Integrated Science Assessment approach and characterization of potential 

mechanisms or modes of action (Samet, 2009e, pp. 7 to 8; Samet, 2009f, p. 11), as well 

 

41 See American Trucking Assn’sAssociation v. EPA, 175 F. 3d 1027, 1055-56 (D.C. Cir. 

1999) rev’ersed in part and affirmed in part sub nom, Whitman v. American Trucking 

Assn’s, 531 U.S. 457 (2001) holding that the EPA could establish NAAQS without 

identifying a biological mechanism (“To begin with, the statute itself requires no such 

proof. The Administrator may regulate air pollutants "emissions of which, in his 

judgment, cause or contribute to air pollution which may reasonably be anticipated to 

endanger public health or welfare." (emphasis added by the court). Moreover, this court 

has never required the type of explanation petitioners seek from EPA. In fact, we have 

expressly held that EPA's decision to adopt and set air quality standards need only be 

based on ‘reasonable extrapolations from some reliable evidence’.... Indeed, were we to 

accept petitioners' view, EPA (or any agency for that matter) would be powerless to act 

whenever it first recognizes clear trends of mortality or morbidity in areas dominated by a 

particular pathogen.”).
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as the findings of a causal relationship at the population level between exposure to PM2.5

and mortality and cardiovascular effects (Samet, 2009f, pp. 2 to 3).42

Additionally, the EPA disagrees with commenters that the mild and reversible 

effects observed in controlled human exposure studies are inconsistent with the more 

serious effects observed in epidemiological studies. Ethical considerations regarding the 

types of studies that can be performed with human subjects generally limit the effects that 

can be evaluated to those that are transient, reversible, and of limited short-term 

consequence. The relatively small number of subjects recruited for controlled exposure 

studies should also be expected to have less variability in health status and risk factors 

than occurring in the general population.43 Consequently, the severity of health effects 

observed in controlled human exposure studies evaluating the effects of PM should be 

expected to be less than observed in epidemiologic studies. Nonetheless, that effects are 

observed in relatively healthy individuals participating in controlled exposure studies 

serves as an indicator that PM is initiating health responses and that more severe 

responses may reasonably be expected in a more diverse population. 

 

43 For example, the EPA excludes from its controlled human exposure studies involving 

exposure to PM2.5 any individual with a significant risk factor for experiencing adverse 

effects from such exposure. Thus, the EPA excludes a priori the following categories of 

persons: those with a history of angina, cardiac arrhythmias, and ischemic myocardial 

infarction or coronary bypass surgery; those with a cardiac pacemaker; those with 

uncontrolled hypertension (greater than 150 systolic and 90 diastolic); those with 

neurogenetive diseases; those with a history of bleeding diathesis; those taking betablockers; those using oral anticoagulants; those who are pregnant, attempting to become 

pregnant, or breastfeeding; those who have experienced a respiratory infection within 

four weeks of exposure; those experiencing eye or abdominal surgery within six weeks of 

exposure; those with active allergies; those with a history of chronic illnesses such as 

diabetes, cancer, rheumatologic diseases, immunodeficiency state, known cardiovascular 

disease, or chronic respiratory diseases; smokers. The EPA “Application for Independent 

Review Board Approval of Human Subjects Research: Cardiopulmonary Effects of 

healthy Older GSTM1 Null and Sufficient individuals to Concentrated Ambient Air

Particles (CAPTAIN)”, Nov. 9, 2011, p. 9.
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It should also be noted that there is a small body of toxicological evidence 

demonstrating mortality in rodents exposed to PM (e.g., Killingsworth et al. 1997; 

Godleski et al. 1996). Overall it is not surprising that lethality is not induced in more 

toxicological research, as these types of studies do not readily lend themselves to this 

endpoint. Epidemiological studies have observed associations between PM and mortality 

in communities with populations in the range of many thousands to millions of people. 

Clearly, it is not feasible to expose hundreds (if not thousands) of animals to ambient PM 

(potentially over many years) in a laboratory setting to induce enough lethalities to 

distinguish between natural deaths and those attributable to PM. Furthermore, the 

heterogeneous human populations sampled in epidemiological studies are comprised of 

individuals with different physical, genetic, health, and socioeconomic backgrounds 

which may impact the outcome. However, in toxicological studies, the rodent groups are 

typically inbred, such that inter-individual variability is minimized. Thus, if the rodent 

strain used is quite robust, PM-induced effects may not be observed at low exposure 

concentrations.

(3) In asserting that the uncertainties in the underlying health science are as great 

or greater than in the last review and therefore do not support revision to the standards at 

this time, commenters in this group variously discussed a number of issues related to: (a) 

confounding, (b) heterogeneity in risk estimates, (c) exposure measurement error, (d) 

model specification, (e) the concentration-response relationship, and (f) understanding 

the relative toxicity of components within the mixture of fine particles. Each of these 

issues is addressed below and some are discussed in more detail in the Response to 

Comments document. 
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In summary, these commenters concluded that the substantial uncertainties 

present in the last review have not been resolved and/or that the uncertainty about the 

possible health risks associated with PM2.5 exposure has not diminished. As discussed 

below, the EPA believes that the overall uncertainty about possible health risks 

associated with both long- and short-term PM2.5 exposure has diminished to an important 

degree since the last review. While the EPA agrees that important uncertainties remain, 

and that future research directed toward addressing these uncertainties is warranted, the 

EPA disagrees with commenters’ views that the remaining uncertainties in the scientific 

evidence are too great to warrant revising the current PM2.5 NAAQS.

(a) Confounding 

Some commenters have criticized the EPA for not adequately addressing the issue 

of confounding in both long- and short-term exposure studies of mortality and morbidity. 

This includes confounding due to copollutants, as well as unmeasured confounding.44

With regard to copollutant confounding, these commenters asserted that the EPA 

has not adequately interpreted the results from studies that examined the effect of 

copollutants on the relationship between long- and short-term PM2.5 exposures and 

mortality and morbidity outcomes. These commenters contend that the EPA has

inappropriately concluded that PM2.5-related mortality and morbidity associations are 

 

44 The Integrated Science Assessment defines confounding as “a confusion of effects. 

Specifically, the apparent effect of the exposure of interest is distorted because the effect 

of an extraneous factor is mistaken for or mixed with the actual exposure effect (which 

may be null) (Rothman and Greenland, 1998)” (U.S. EPA, 2009a, p. 1-16). 

Epidemiological analyses attempt to adjust or control for these characteristics (i.e., 

potential confounders) that differ between exposed and non-exposed individuals (U.S. 

EPA, 2009a, section 1.5.3). Not all risk factors can be controlled for within a study 

design/model and are termed “unmeasured confounders.” An unmeasured confounder is a 

confounder that has not previously been measured and therefore is not included in the 

study design/model. 
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generally robust to confounding. The commenters stated that statistically significant 

PM2.5 associations in single-pollutant models in epidemiological studies do not remain 

statistically significant in copollutant models. 

The loss of statistical significance or the reduction in the magnitude of the effect 

estimate when a co-pollutant model is used may be the result of factors other than 

confounding. These changes do not prove either the existence or absence of confounding. 

These impacts must be evaluated in a broader context that considers the entire body of 

evidence. The broader examination of this issue in the Integrated Science Assessment 

included a focus on evaluating the stability of the size of the effect estimates in 

epidemiological studies conducted by a number of research groups using single- and 

copollutant models (U.S. EPA, 2009a, sections 6.2.10.9, 6.3.8.5, and 6.5, Figures 6-5, 6-

9, and 6-15). This examination found that, for most epidemiological studies, there was 

little change in effect estimates based on single- and copollutant models, although the 

Integrated Science Assessment recognized that in some cases, the PM2.5 effect estimates 

were markedly reduced in size and lost statistical significance. Additionally, the EPA 

notes that these comments do not adequately reflect the complexities inherent in 

assessing the issue of copollutant confounding. As discussed in the proposal (77 FR 

38907, 38909, and 38910) and more fully in the Integrated Science Assessment 

(U.S.EPA, 2009a, sections 6.2, 6.3, and 6.5), although copollutant models may be useful 

tools for assessing whether gaseous copollutants may be potential confounders, such 

models alone cannot determine whether copollutants are in fact confounders. 

Interpretation of the results of copollutant models is complicated by correlations that 

often exist among air pollutants, by the fact that some pollutants play a role in the 
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atmospheric reactions that form other pollutants such as secondary fine particles, and by 

the statistical power of the studies in question inherent in the study methodology. For 

example, the every-third or sixth-day sampling schedule often employed for PM2.5

measurements compared to daily measurements of gaseous copollutants drastically 

reduces the overall sample size to assess the effect of copollutants on the PM2.5-morbidity 

or mortality relationship, such that the reduced sample size can lead to less precise effect 

estimates (e.g., wider confidence intervals).

The EPA recognizes that when PM2.5 is correlated with gaseous pollutants it can 

be difficult to identify the effect of individual pollutants in the ambient mixture (77 FR 

38910). However, based on the available evidence, the EPA concludes epidemiological 

studies continue to support the conclusion that PM2.5 associations with mortality and 

morbidity outcomes are robust to the inclusion of gaseous copollutants in statistical 

models. The EPA evaluated the potential confounding effects of gaseous copollutants 

and, although it is recognized that uncertainties and limitations still remain, the Agency 

concluded the collective body of scientific evidence is “stronger and more consistent than 

in previous reviews providing a strong basis for decision making in this review" (77 FR 

38910/1). 

Several commenters offered detailed comments on the long-term PM2.5 exposure 

studies arguing that associations from mortality studies are subjected to unmeasured 

confounding and as a result are not appropriately characterized as being causal (providing 

evidence of a causal relationship between long-term PM2.5 exposure and mortality (e.g., 

UARG, 2012, pp. 10 to 11, Attachment A, pp. 17 to 23; API, 2012, pp. 13 to 14, 

Attachment 1, pp. 11 to 14, Attachment 7, pp. 2-10; ACC, 2012, p. 18 to 21; AFPM, 
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2012, p. 8; TCEQTexas CEQ, 2012, p. 4). Specifically, these commenters cited two 

studies (i.e., Janes et al., 2007 and Greven et al., 2011) that used a new type of statistical 

analysis, in which the study authors to examine associations between annual (long-term) 

and monthly (sub-chronic) PM2.5 exposure and mortality. The commenters interpreted the 

results of the analysisthese analyses as evidence of unmeasured confounding in the longterm PM2.5 exposure-mortality relationship. These commenters interpreted these studies 

as raising fundamental questions regarding the EPA’s determination that a causal 

relationship exists between long-term PM2.5 exposure and mortality. The first study, 

Janes et al., 2007, In addition to the commenters mentioned above, all of the authors of 

the publications by Janes et al. (2007) and Greven et al. (2011) (i.e., Francesca Dominici, 

Scott Zeger, Holly Janes, and Sonja Greven) submitted a joint comment to the public 

docket in order to clarify specific points regarding these two studies (Dominici et al., 

2012).

The first study, Janes et al. (2007), was evaluated in the Integrated Science 

Assessment (U.S. EPA, 2009a, p. 7-88). The second study, Greven et al., . (2011,), an 

extension of the Janes et al. (2007) study adding three more years of data, is a “new” 

study discussed in the Provisional Science Assessment (U.S. EPA, 2012) and the 

Response to Comments document.). Both studies used nationwide Medicare mortality 

data to examine the association between monthly average PM2.5 concentrations over the 

preceding 12 months and monthly mortality rates in 113 U.S. counties. and examined 

whether community-specific trends in monthly PM2.5 concentrations and mortality 

declined at the same rate as the national rate. The investigators decomposedexamined this 

by decomposing the association between PM2.5 and mortality into two components: (1) 
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“national trends”national trends, defined as the association between the national average 

trend in monthly PM2.5 concentrations averaged over the previous 12 months and the 

national average trend in monthly mortality rates, and (2) “local trends,”, defined as 

county-specific deviations in monthly PM2.5 concentrations and monthly mortality rates

from national trends. 

The EPA does not question the results of the national trends analyses conducted 

by Janes et al. (2007) and Greven et al. (2011). These analyses demonstrated associations 

similar in magnitude and precision to other studies of long-term PM2.5 exposure and 

mortality. However, the authors hypothesized that the results from the “local” and 

“national” analyses should be similar in magnitude, and that any difference between the 

two results would be due to unmeasured confounding.45 The authors provided no 

references to support their assumptions that associations between PM2.5 and mortality 

should be similar across temporal and spatial scales nor why any differences in effect 

estimate size would be due to unmeasured confounding.46 Both Janes et al. (2007) and 

Greven et al. (2011) observed positive and statistically significant associations between 

long-term exposure to PM2.5 and mortality in their national analyses. However, Janes et 

al. (2007) and Greven et al. (2011) eliminated all of the spatial variation in air pollution 

and mortality in their data set when estimating the national effect, focusing instead on 

both chronic (yearly) and sub-chronic (monthly) temporal differences in the data 

 

45 Unmeasured confounders need to be factors that are associated with both the exposure 

and outcome of interest, but in the case of the Janes and Greven studies this would also 

need to hold true over multiple dimensions of time and space as stated by Pope and 

Burnett, 2007.

46 In its evaluation of Janes et al. (2007) in the Integrated Science Assessment, the EPA 

did not identify limitations in the statistical methods used per se (U.S. EPA, 2009a, p. 

7-88) and included the results of the national-scale analyses in that study in the body of 

evidence that supported the determination that there is a causal relationship between 

long-term PM2.5 exposure and mortality.
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(Dominici et al. 2012). Janes et al. (2007) (Table 1) highlighted that over 90 percent of 

the variance in the data set used for the analyses conducted by both Janes et al. (2007) 

and Greven et al. (2011) was attributable to spatial variability, which the authors chose to 

discard. As noted above, the focus of the analyses by Janes et al. (2007) and Greven et al. 

(2011) was on two components: (1) a temporal or time component, i.e., the “national” 

trends analysis, which examined the association between the national average trend in 

monthly PM2.5 concentrations averaged over the previous 12 months and the national 

average trend in monthly mortality rates and (2) a space-by-time component, i.e., the 

“local” trends analysis, which examined county-specific deviations in monthly PM2.5

concentrations and monthly mortality rates from national trends. These two components 

combined comprised less than 10 percent of the variance in the data set. The authors 

included a focus on the space-by-time component, which represented approximately 5 

percent of the variance in the data set, in an attempt to identify, absent confounding, if 

PM2.5 was associated with mortality at this unique exposure window. Thus, these studies 

are not directly comparable to other cohort studies investigating the relationship between 

long-term exposure to PM2.5 and mortality, which make use of spatial variability in air 

pollution and mortality data.47 This point was highlighted by the study authors who stated 

that “when one considers that this wealth of information is not accounted for in [Janes 

2007], it is not as surprising that... vastly different estimates of the PM2.5/mortality 

 

47 Though not directly comparable, the national effect estimates for mortality reported by 

Janes et al. (2007) and Greven et al. (2011) are coincidentally similar in magnitude to 

those previously reported. It is important to note that previous cohort studies have 

focused on identifying spatial differences in PM2.5 concentrations between cities, while 

Janes et al. (2007) and Greven et al. (2011) focus primarily on temporal differences in 

PM2.5 concentrations. In fact, Greven et al. (2011) state “We do not focus here on a third 

type [of statistical approach] used in cohort studies, measuring the association between 

average PM2.5 levels and average age-adjusted mortality rates across cities (purely spatial 

or cross-sectional association).”
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relationship [were observed] than in other studies that do exploit that variability”

(Dominici et al., 2012, p. 2).

The EPA notes that the results of the local trends analyses conducted by Janes et 

al. (2007) and Greven et al. (2011) are limited by the monthly timescale used in these 

analyses. This view is consistent with comments on the Janes et al. (2007) study 

articulated in Pope and Burnett (2007), the EPA notes48 which noted that an important 

limitation of the local scale analysis conducted by Janes et al. (2007) and subsequently by 

Greven et al. (2011) iswas the subchronic exposure window considered in these analyses. 

Both studies used annual average PM2.5 concentrations to characterize long-term national 

trends which iswas consistent with exposure windows considered in other studies of longterm exposure to PM2.5 and mortality studies..

49 However, the local scale analyses used 

monthly average PM2.5 concentrations to characterize county-specific deviations from 

national trends (the “local” scale). The use of monthly average data likely does not 

provide enough exposure contrast to observe temporal changes in mortality at the local 

scale. It also represents a different exposure window than considered in the large body of 

evidence of health effects related to short-term (from less than one day to up to several 

days) and chronic (one or more years) measures of PM2.5. 

As discussed by Pope and Burnett (2007) and highlighted in the Integrated 

Science Assessment, the conclusions of Janes et al.Furthermore, the EPA disagrees with 

commenters that studies by Janes et al. (2007) and Greven et al. (2011) provide evidence 

 

48 Some commenters argued that there were flaws in the criticisms offered by Pope and 

Burnett (2007) on the paper by Janes et al. (2007) (UARG, 2012, Attachment A, pp. 19 to 

23). The EPA responds to each of these specific comments in the Response to Comments 

document.

49 As noted above, however, Janes et al. (2007) and Greven et al. (2011) focused on 

temporal variability and other studies of long-term exposure to PM2.5 and mortality focus 

on spatial variability.
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that other studies of long-term exposure to PM2.5 and mortality are affected by 

unmeasured confounding. As noted above, the design of the studies conducted by Janes 

et al (2007) and Greven et al (2011) are fundamentally different than those used in other 

studies of long-term exposure to PM2.5 and mortality, including the ACS cohort and the 

Harvard Six Cities study. Studies, such as the ACS and Harvard Six Cities studies, used 

the spatial variation between cities to measure the effect of long-term (annual) exposures 

to PM2.5 on mortality risk, and did not conduct any analyses relying on the temporal 

variation in PM2.5. The opposite is true of the Janes et al. (2007) and Greven et al. (2011) 

studies which first removed the spatial variability in PM2.5 and then examined the 

temporal variation at both the national and local scale to measure the effects of temporal 

differences in PM2.5 on mortality risk. Janes et al. (2007) and Greven et al. (2011) focus 

on changes in PM2.5 concentrations over time and, therefore, control for confounders 

would be based on including variables that vary over time rather than over space. As a 

result, any evidence of potential confounding of the PM2.5-mortality risk relationship 

derived from Janes et al. (2007) and Greven et al. (2011) cannot be extrapolated to draw 

conclusions related to potential spatial confounding in studies based on the spatial 

variation in PM2.5 concentrations.

As detailed in the Integrated Science Assessment (U.S. EPA, 2009a, section 7.6), 

and recognized by the authors of Janes et al. (2007) and Greven et al. (2011), the cohort 

studies that informed the causality determination for long-term PM2.5 exposure and 

mortality “have developed approaches to adjust for measured and unmeasured 

confounders” (Dominici et al., 2012, p. 2). These approaches were specifically designed 

to adjust for spatial confounding. The hypothesis that the authors of Janes et al. (2007) 
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and Greven et al. (2011) chose to examine was that differences in the local and national 

effects indicated unmeasured temporal confounding in either the local or national effect 

estimate. This hypothesis was specific to these two studies that examined temporal 

variability in exposure to air pollution and did not include known potential confounders 

at either the national or local scale as time-varying covariates in the statistical model. The 

authors acknowledged that the interpretation of either the national or local estimates 

needs to occur with an appreciation of the potential confounding effects of national and

local scale covariates that were omitted from the model (Dominici et al., 2012).

It is important to recognize that because Janes et al. (2007) and Greven et al. 

(2011) focused on variations in PM2.5 over time and not space, the results from these two 

studies do not provide any indication that other studies of long-term exposure to PM2.5

and mortality exhibit spatial confounding, or that PM2.5 does not cause mortality.50 The 

authors of Janes et al. (2007) and Greven et al. (2011) recognized that “it is entirely 

possible that these papers are looking for an association at a timescale for which no 

association truly exists” (Dominici et al.,. 2012, p. 3). Furthermore, as highlighted in the 

Integrated Science Assessment and discussed by Pope and Burnett (2007), the 

conclusions of Janes et al. (2007) “are overstated. . . [T]heir analysis tells us little or 

nothing about unmeasured confounding in those and related studies because the 

methodology of Janes et al. largely excludes the sources of variability that are exploited 

in those other studies. By using monthly mortality counts and lagged 12-month average 

pollution concentrations, the authors eliminate the opportunity to exploit short-term or 

day-to-day variability.” Some commenters argued that there were flaws in the criticisms 

 

50 Further, the EPA notes that Janes et al. (2007) and Greven et al. (2011) provide no 

information relevant to examining confounding in studies of short-term exposure to 

PM2.5.
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offered by Pope and Burnett (2007) on the paper by Janes et al. (2007) (UARG, 2012, 

Attachment A, pp. 19 to 23). The EPA responds to each of these specific comments in the 

Response to Comments document. In its evaluation of Janes et al. (2007), the EPA did 

not identify limitations in the statistical methods used per se (U.S. EPA, 2009a, p. 7-88) 

and the results of the national-scale analyses were included in the body of evidence that 

supported the determination that there is a causal relationship between long-term PM2.5

exposure and mortality

In conclusion, the EPA believesinterprets the results of the analyses conducted by 

Janes et al. (2007) and Greven et al. (2011) areas being consistent with prior knowledge 

of examining associations with long-term exposure to PM2.5 at the national scale using 

long-term average PM2.5 concentrations. However, For the reasons presented above and 

discussed in more detail in the Response to Comments document, the Agency disagrees 

with the commenters’ assumption that their “national” and “local” scale analyses should 

produce similar the results and their conclusion that differing results is evidence of Janes 

et al. (2007) and Greven et al. (2011) indicate unmeasured confounding. in the results of 

other cohort studies of long-term exposure to PM2.5 and mortality. Therefore, the EPA 

concludes that these studies do not bring into questioninvalidate the large body of 

epidemiological evidence indicatingthat supports the EPA’s determination that a causal 

associationrelationship exists between long-term PM2.5 exposure and mortality.51

(b) Heterogeneity in Risk Estimates

IndustrySome commenters argued that the heterogeneity in risk estimates 

 

51 The EPA notes that the EPA’s conclusion with regard to interpretation of the results 

from Janes et al. (2007) and Greven et al. (2012) is supported by the study authors’ 

conclusion that “[o]ur results do not invalidate previous epidemiological studies”

(Dominici, 2012, p. 1 (emphasis original)).
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observed in multicitymulti-city epidemiological studies and the lack of statistical 

significance in many regional or seasonal estimates highlights a potential bias associated 

with combined multi-city epidemiological study results (e.g., API, 2012, Attachment 1, 

pp. 15 to 19). These commenters further argued that more refined intra-urban exposure 

estimates conducted for two of the largest cities included in the ACS study, Los Angeles 

and New York City, based on land-use regression models and/or kriging methods 

(Krewski et al., 2009) “underscore the importance of considering city-specific health 

estimates, which may account for heterogeneity in PM2.5 concentrations or other 

differences among cities, rather than relying on pooled nationwide results from multi-city 

studies” (API, 2012, Attachment 1, p. 17).

With respect to understanding the nature and magnitude of PM2.5-related risks, the 

EPA agrees that epidemiological studies evaluating health effects associated with longand short-term PM2.5 exposures have reported heterogeneity in responses between cities 

and effect estimates across geographic regions of the U.S. (U.S. EPA, 2009a, sections 

6.2.12.1, 6.3.8.1, 6.5.2, and 7.6.1; U.S. EPA, 2011a, p. 2-25). For example, when 

focusing on short-term PM2.5 exposure, the Integrated Science Assessment found that 

multi-city studies that examined associations with mortality and cardiovascular and 

respiratory hospital admissions and emergency department visits demonstrated greater 

cardiovascular effects in the eastern versus the western U.S. (Dominici, et al., 2006; Bell 

et al., 2008; Franklin et al. (2007, 2008)). 

In addition, the Integrated Science Assessment evaluated studies that provided 

some evidence for seasonal differences in PM2.5 risk estimates, specifically in the 

northeast. The PM ISAIntegrated Science Assessment found evidence indicating that 
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individuals may be at greater risk of dying from higher exposures to PM2.5 in the warmer 

months, and at greater risk of PM2.5 associated hospitalization for cardiovascular and 

respiratory diseases during colder months of the year. The limited influence of 

seasonality on PM risk estimates in other regions of the U.S. may be due to a number of 

factors including varying PM composition by season, exposure misclassification due to 

regional tendencies to spend more or less time outdoors and air conditioning usage, and 

the prevalence of infectious diseases during the winter months (U.S. EPA, 2009a, p. 3-

182). 

Overall, the EPA took note in the proposal that uncertainties still remain 

regarding various factors that contribute to heterogeneity observed in epidemiological 

studies (77 FR 38909/3). Nonetheless, the EPA recognizes that this heterogeneity could 

be attributed, at least in part, to differences in PM2.5 composition across the U.S., as well 

as to exposure differences that vary regionally such as personal activity patterns, 

microenvironmental characteristics, and the spatial variability of PM2.5 concentrations in 

urban areas (U.S. EPA, 2009a, section 2.3.2; 77 FR 38910). 

As recognized in the Policy Assessment, the current epidemiological evidence 

and the limited amount of city-specific speciated PM2.5 data do not allow conclusions to 

be drawn that specifically differentiate effects of PM2.5 in different locations (U.S. EPA, 

2011a, p. 2-25). Furthermore, the Integrated Science Assessment concluded “that many 

constituents of PM2.5 can be linked with multiple health effects, and the evidence is not 

yet sufficient to allow differentiation of those constituents or sources that are more 

closely related to specific health outcomes” (U.S. EPA, 2009a, p. 2-17). CASAC 

thoroughly reviewed the EPA’s presentation of the scientific evidence indicating 
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heterogeneity in PM2.5 effect estimates in epidemiological studies and concurred with the 

overall conclusions presented in the Integrated Science Assessment.

(c) Exposure Measurement Error 

IndustrySome commenters argued that the EPA did not adequately consider 

exposure measurement error, which they asserted is an important source of bias in 

epidemiological studies that can bias effect estimates in either direction (e.g., API, 2012, 

Attachment 1, pp. 19 to 20). 

The EPA agrees that exposure measurement error is an important source of 

uncertainty and that the variability in risk estimates observed in multi-city studies could 

be attributed, in part, to exposure error due to measurement-related issues (77 FR 38910). 

However, the Agency disagrees with the commenters’ assertion that exposure 

measurement error was not adequately considered in this review. The Integrated Science 

Assessment included an extensive discussion that addresses issues of exposure 

measurement error (U.S. EPA, 2009a, sections 2.3.2. and 3.8.6). Exposure measurement 

error may lead to bias in effect estimates in epidemiological studies. A number of studies 

evaluated in the last review (U.S. EPA, 2004, section 8.4.5) and in the current review 

(U.S. EPA, 2009a, section 3.8.6) have discussed the direction and magnitude of bias 

resulting from specified patterns of exposure measurement error (Armstrong 1998; 

Thomas et al. 1993; Carroll et al. 1995) and have generally concluded “classical” (i.e., 

random, within-person) exposure measurement error can bias effect estimates towards the 

null. Therefore, consistent with conclusions reached in the last review, the Integrated 

Science Assessment concluded “in most circumstances, exposure error tends to bias a 

health effect estimate downward” (U.S. EPA, 2009a, sections 2.3.2 and 3.8.6) (emphasis 
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added). Thus, the EPA has both considered and accounted for the possibility of exposure 

measurement error, and the possible bias would make it more difficult to detect true 

associations, not less difficult. 

(d) Model Specification 

Commenters contendcontended that the EPA did not account for the fact that 

“selecting an appropriate statistical model for epidemiologic studies of air pollution 

involves several choices that involve much ambiguity, scant biological evidence, and a 

profound impact on analytic results, given that many estimated associations are weak” 

(ACC, 2012, p. 5) For short-term exposure studies, the EPA recognizes, as summarized 

in the HEI review panel commentary that selecting a level of control to adjust for timevarying factors, such as temperature, in time-series epidemiological studies involves a 

trade-off (HEI, 2003). For example, if the model does not sufficiently adjust for the 

relationship between the health outcome and temperature, some effects of temperature 

could be falsely ascribed to the pollution variable. Conversely, if an overly aggressive 

approach is used to control for temperature, the result would possibly underestimate the 

pollution-related effect and compromise the ability to detect a small but true pollution 

effect (U.S. EPA, 2004, p. 8--236; HEI, 2003, p. 266). The selection of approaches to 

address such variables depends in part on prior knowledge and judgments made by the 

investigators, for example, about weather patterns in the study area and expected 

relationships between weather and other time-varying factors and health outcomes 

considered in the study. As demonstrated in section 6.5 of the Integrated Science 

Assessment, the EPA thoroughly considered each of these issues and the overall effect of 

different model specifications on the association between short-term PM2.5 exposure and 

mortality. Regardless of the model employed, consistent positive associations were 
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observed across studies that controlled for the potential confounding effects of time and 

weather using different approaches (USU.S. EPA 2009, Figure 6-27). The EPA also 

considered the influence of model specification in the examination of long-term PM2.5

exposure studies. For example, in section 7.6 of the Integrated Science Assessment, 

Figures 7-6 and 7-7 summarize the collective evidence that evaluated the association 

between long-term PM2.5 exposure and mortality. Regardless of the model used, these 

studies collectively found evidence of consistent positive associations between long-term 

PM2.5 exposure and mortality. 

The EPA, therefore, disagrees with commenters that model specification was not 

considered when evaluating the epidemiological evidence used to form causality 

determinations. The EPA specifically points out that the process of assessing the 

scientific quality and relevance of epidemiological studies includes examining “important 

methodological issues (e.g., lag or time period between exposure and effects, model 

specifications, thresholds, mortality displacement) related to interpretation of the health 

evidence (USU.S. EPA, 2009, p. 1-9).” Consistent with the conclusions of the 2004 PM 

AQCDAir Quality Criteria Document, the EPA recognizes that there is still no clear 

consensus at this time as to what constitutes appropriate control of weather and temporal 

trends in short-term exposure studies, and that no single statistical modeling approach is 

likely to be most appropriate in all cases (U.S. EPA, 2004, p. 8-238). However, the EPA 

believes that the available evidence interpreted in light of these remaining uncertainties 

does provide increased confidence relative to the last review in the reported associations 

between short- and long-term PM2.5 exposures and mortality and morbidity effects, alone 

and in combination with other pollutants.
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(e) Concentration-response Relationship

Additionally, commenters questioned the interpretation of the shape of the 

concentration-response relationship, specifically stating that multiple studies have 

demonstrated that there is a threshold in the PM-health effect relationship and that the 

log-linear model is not biologically plausible (API, 2012, Attachment 9; ACC, 2012, 

Appendix A, pp. 7 to 8). The EPA disagrees with this assertion due to the number of 

studies evaluated in the Integrated Science Assessment that continue to support the use of 

a no-threshold, log-linear model to most appropriately represent the PM concentrationresponse relationship (U.S. EPA, 2009a, section 2.4.3). While recognizing that 

uncertainties remain, the EPA believes that our understanding of this issue for both longand short-term exposure studies has advanced since the last review. As discussed in the 

Integrated Science Assessment, both long- and short-term exposure studies have 

employed a variety of statistical approaches to examine the shape of the concentrationresponse function and whether a threshold exists. While the EPA recognizes that there 

likely are individual biological thresholds for specific health responses, the Integrated 

Science Assessment concluded the overall evidence from existing epidemiological 

studies does not support the existence of thresholds at the population level, for effects 

associated with either long-term or short-term PM exposures within the ranges of air 

quality observed in these studies (U.S. EPA, 2009a, section 2.4.3).

52 The Integrated 

Science Assessment concluded that this evidence collectively supported the conclusion 

that a no-threshold, log-linear model is most appropriate (U.S. EPA, 2009a, sections 

 

52 While epidemiological analyses have not identified a population threshold in the range 

of air quality concentrations evaluated in these studies, the EPA recognizes that it is 

possible that such thresholds exist towards the lower end of these ranges (or below these 

ranges).
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6.2.10.10, 6.5.2.7, and 7.6.4). CASAC likewise advised that “[a]lthough there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (Samet, 2010d, 

p. ii).

The EPA recognizes that some short-term exposure studies have examined the 

PMPM2.5 concentration-response relationship in individual cities or on a city-to-city basis 

and observed heterogeneity in the shape of the concentration-response curve across cities. 

As discussed in (b) above, these findings are a source of uncertainty that the EPA agrees 

requires further investigation. Nonetheless, the Integrated Science Assessment concluded 

that “the studies evaluated further support the use of a no-threshold, log-linear model, but 

additional issues such as the influence of heterogeneity in estimates between cities and 

the effects of seasonal and regional differences in PM on the concentration-responserelationship still require further investigation” (U.S. EPA, 2009a, p. 2-25). 

(f) Relative Toxicity of PM2.5 Components 

Some commenters highlighted uncertainties in understanding the role of 

individual constituents within the mix of fine particles. These commenters asserted that a 

mass-based standard may not be appropriate due to the growing body of evidence 

indicating that certain PM2.5 components may be more closely related to specific health 

outcomes (e.g., EC and OC) (EPRI, 2012, p. 2). 

With regard to questions about the role of individual constituents within the mix 

of fine particles, as a general matter, the EPA recognizes that although new research 

directed toward this question has been conducted since the last review, important 

questions remain and the issue remains an important element in the Agency’s ongoing 

research program. At the time of the last review, the Agency determined that it was 
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appropriate to continue to control fine particles as a group, as opposed to singling out any 

particular component or class of fine particles (71 FR 61162 to 61164, October 17, 2006). 

This distinction was based largely on epidemiological evidence of health effects using 

various indicators of fine particles in a large number of areas that had significant 

contributions of differing components or sources of fine particles, together with some 

limited experimental studies that provided some evidence suggestive of health effects 

associated with high concentrations of numerous fine particle components. 

In this review, as discussed in the proposal (77 FR 38922 to 38923) and in section 

III.E.1 below, while most epidemiological studies continue to be indexed by PM2.5 mass, 

several recent epidemiological studies included in the Integrated Science Assessment 

have used PM2.5 speciation data to evaluate health effects associated with fine particle 

exposures. In the Integrated Science Assessment, the EPA thoroughly evaluated the 

scientific evidence that examined the effect of different PM2.5 components and sources on 

a variety of health outcomes (U.S. EPA, 2009a, section 6.6) and observed that the 

available information continues to suggest that many different chemical components of 

fine particles and a variety of different types of source categories are all associated with, 

and probably contribute to, effects associated with PM2.5. The Integrated Science 

Assessment concluded that the current body of scientific evidence indicated that “many 

constituents of PM can be linked with differing health effects and the evidence is not yet 

sufficient to allow differentiation of those constituents or sources that are more closely 

related to specific health outcomes” (U.S. EPA, 2009a, p. 2-26 and 6-212). Furthermore, 

the Policy Assessment concluded that the evidence is not sufficient to support eliminating 

any component or group of components associated with any specific source categories 
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from the mix of fine particles included in the PM2.5 indicator (U.S. EPA, 2009a, p. 2-56). 

CASAC agreed that it was reasonable to retain PM2.5 as an indicator for fine particles in 

this review as “[t]here was insufficient peer-reviewed literature to support any other 

indicator at this time” (Samet, 2010c, p. 12).

This information is relevant to the Agency’s decision to retain PM2.5 as the 

indicator for fine particles as discussed in section III.E.1 below. The EPA also believes 

that it is relevant to the Agency’s conclusion as to whether revision of the suite of 

primary PM2.5 standards is appropriate. While there remain uncertainties about the role 

and relative toxicity of various components of fine PM, the current evidence continues to 

support the view that fine particles should be addressed as a group for purposes of public 

health protection. 

In summary, in considering the above issues related to uncertainties in the 

underlying health science, on balance, the EPA believes that the available evidence 

interpreted in light of these remaining uncertainties does provide increased confidence 

relative to the last review in the reported associations between long- and short- term 

PM2.5 exposures and mortality and morbidity effects, alone and in combination with other 

pollutants, and supports stronger inferences as to the causal nature of the associations. 

The EPA also believes that this increased confidence, when taken in context of the entire 

body of available health effects evidence and in light of the evidence from 

epidemiological studies of associations observed in areas meeting the current primary 

PM2.5 standards, specifically in areas meeting the current primary annual PM2.5 standard, 

adds support to its conclusion that the current suite of PM2.5 standards needs to be revised 

to provide increased public health protection. 
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(4) In asserting that there is no evidence of greater risk since the 2006 review to 

justify lowering the current annual PM2.5 standard, some commenters argued that, “if the 

current primary PM2.5 annual standard of 15 μg/m3 was considered to be adequately 

protective of public health in 2006, given relative risk estimates that EPA was using at 

that time, then that standard would surely still be adequately protective of the public 

health if relative risk estimates remain at the same level (or lower)” (UARG, 2012, 

Attachment 1, p. 24). These commenters compared risk coefficients used for mortality in 

the EPA’s risk assessment done in the last review with those from the Agency’s core risk 

assessment done as part of this review, and they concluded that “the entire range of the 

core relative risk for long-term mortality is lower now than it was in the prior review” 

(UARG, 2012, Attachment 1, p. 24). These commenters used this conclusion as the basis 

for a claim that there is no reason to revise the current annual PM2.5 standard.

The EPA believes that this claim is fundamentally flawed as it fails to take into 

account that exposure to a pollutant causally associated at a population-level with

premature mortality and serious illness has significant public health consequences, 

virtually regardless of the relative risk.. In comparing the scientific understanding of the 

risk presented by exposure to PM2.5 between the last and current reviews, one must 

examine not only the quantitative estimate of risk from those exposures (e.g., the 

numbers of premature deaths or increased hospital admissions at various concentrations), 

but also the degree of confidence that the Agency has that the observed health effects are 

causally linked to PM2.5 exposure at those concentrations. As documented in the 

Integrated Science Assessment and in the recommendations and conclusions of CASAC, 

the EPA recognizes significant advances in our understanding of the health effects of 
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PM2.5, based on evidence that is stronger than in the last review because of its breadth 

and the substantiation of previously observed health effects.. As a result of these 

advances, the EPA is now more certain that fine particles, alone or in combination with 

other pollutants, present a significant risk to public health at concentrations allowed by 

the current primary PM2.5 standards. From this more comprehensive perspective, since 

the risks presented by PM2.5 are more certain, similar or even somewhat lower relative 

risk estimates would not be a basis to conclude that no revision to the suite of PM2.5

standards is “requisite” to protect public health with an adequate margin of safety. This 

also ignores that the relative risk estimate is only one factor considered by the 

Administrator, e.g. it ignores that epidemiological studies since the last review indicate 

associations between PM2.5 and mortality and morbidity in areas meeting the current 

annual standard.

In any case, the commenters’ reliance on the flawed 2006 review is misplaced. As 

discussed in section III.A.2 above, the D.C. Circuit remanded Administrator Johnson’s 

2006 decision to retain the primary annual PM2.5 standard because the Agency failed to 

adequately explain why the annual standard provided the requisite protection from both 

short- and long-term exposure to fine particles including protection for at-risk 

populations. The 2006 standard was also at sharp odds with CASAC advice and 

recommendations as to the requisite level of protection (Henderson, 2006a,b). In other 

words, the judgment of the then-Administrator that the 2006 primary annual PM2.5

standard was requisite to protect public health with an adequate margin of safety is not an 

appropriate benchmark for comparison. 

(5) Some of these commenters also identified “new” as well as older studies that 
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had been included in prior reviews as providing additional evidence that the causality 

determinations presented in the Integrated Science Assessment did not consider the 

totality of the scientific literature, further supporting their view that a revision of the 

PM2.5 is unwarranted. As discussed in section II.B.3 above, the EPA notes that, as in past 

NAAQS reviews, the Agency is basing the final decisions in this review on the studies 

and related information included in the PM ISAIntegrated Science Assessment that have 

undergone CASAC and public review, and will consider newly published studies for 

purposes of decision making in the next PM NAAQS review. In provisionally evaluating 

commenters’ arguments (see Response to Comments document), the EPA notes that its 

provisional assessment of “new” science found that such studies did not materially 

change the conclusions reached in the Integrated Science Assessment (U.S. EPA, 2012b). 

3. Administrator’s Final Conclusions Concerning the Adequacy of the Current 

Primary PM2.5 Standards

Having carefully considered the public comments, as discussed above, the 

Administrator believes the fundamental scientific conclusions on the effects of PM2.5

reached in the Integrated Science Assessment, and discussed in the Policy Assessment, 

are valid. In considering whether the suite of primary PM2.5 standards should be revised, 

the Administrator places primary consideration on the evidence obtained from the 

epidemiological studies. The Administrator believes that this literature, combined with 

the other scientific evidence discussed in the ISAIntegrated Science Assessment, 

collectively represents a strong and generally robust body of evidence of serious health 

effects associated with both long- and short-term exposures to PM2.5. As discussed in the 

Integrated Science Assessment and Policy Assessment, the EPA believes that much 

progress has been made since the last review in reducing some of the major uncertainties 
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that were important considerations in establishing the current suite of PM2.5 standards. In 

that context, the Administrator finds the evidence of serious health effects reported in 

exposure studies conducted in areas with long-term meansmean concentrations ranging 

from approximately at or above the level of the annual standard to long-term meansmean

concentrations significantly below the level of the annual standard to be compelling, 

especially in light of the extent to which such studies are part of an overall pattern of 

positive and frequently statistically significant associations across a broad range of 

studies. The information in the quantitative risk assessment lends support to this 

conclusion.

There has been extensive critical review of this body of evidence, the quantitative 

risk assessment, and related uncertainties, including review by CASAC and the public. 

The public comments on the basis for the EPA’s proposed decision to revise the suite of 

primary PM2.5 standards have identified a number of issues about which different parties 

disagree including issues for which additional research is warranted. Having weighed all 

comments and the advice of CASAC, the Administrator believes that since the last 

review the overall uncertainty about the public health risks associated with both long- and 

short-term exposure to PM2.5 has been diminished to an important degree. The remaining 

uncertainties in the available evidence do not diminish confidence in the associations 

between exposure to fine particles and mortality and serious morbidity effects. Based on 

her increased confidence in the association between exposure to PM2.5 and serious public 

health effects, combined with evidence of such an association in areas that would meet 

the current standards, the Administrator agrees with CASAC that revision of the current 

suite of PM2.5 standards to provide increased public health protection is necessary. Based 
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on these considerations, the Administrator concludes that the current suite of primary 

PM2.5 standards is not sufficient, and thus not requisite, to protect public health with an

adequate margin of safety, and that revision is needed to increase public health 

protection. 

It is important to note that this conclusion, and the reasoning on which it is based, 

do not resolve the question of what specific revisions are appropriate. That requires 

looking specifically at the current 24-hour and annual PM2.5 standards, including their 

indicator, averaging times, forms, and levels, and evaluating the scientific evidence and 

other information relevant to determining the appropriate revision of the suite of 

standards.

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator

In initially setting standards for fine particles in 1997, the EPA concluded it was 

appropriate to control fine particles as a group, rather than singling out any particular 

component or class of fine particles. The EPA noted that community health studies had 

found significant associations between various indicators of fine particles, and that health 

effects in a large number of areas had significant mass contributions of differing 

components or sources of fine particles. In addition, a number of toxicological and 

controlled human exposure studies had reported health effects associations with high 

concentrations of numerous fine particle components. It was also not possible to rule out 

any component within the mix of fine particles as not contributing to the fine particle 

effects found in the epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a 

size-based indicator in 1977 to distinguish fine particles from particles in the coarse 
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mode, the EPA noted that the available epidemiological studies of fine particles were 

based largely on PM2.5 and also considered monitoring technology that was generally 

available. The selection of a 2.5 μm size cut reflected the regulatory importance of 

defining an indicator that would more completely capture fine particles under all 

conditions likely to be encountered across the U.S., especially when fine particle 

concentrations and humidity are likely to be high, while recognizing that some small 

coarse particles would also be captured by current methods to monitor PM2.5 (62 FR 

38666 to 38668, July 18, 1997). In the last review, based on the same considerations, the 

EPA again recognized that the available information supported retaining the PM2.5

indicator and remained too limited to support a distinct standard for any specific PM2.5

component or group of components associated with any source categories of fine 

particles (71 FR 61162 to 61164, October 17, 2006).

In this current review, the same considerations continue to apply for selection of 

an appropriate indicator for fine particles. As an initial matter, the Policy Assessment 

recognizes that the available epidemiological studies linking mortality and morbidity 

effects with long- and short-term exposures to fine particles continue to be largely 

indexed by PM2.5. For the same reasons discussed in the last two reviews, the Policy 

Assessment concluded that it was appropriate to consider retaining a PM2.5 indicator to 

provide protection from effects associated with long- and short-term fine particle 

exposures (U.S. EPA, 2011, p. 2-50).

The Policy Assessment also considered the expanded body of evidence available 

in this review to consider whether there was sufficient evidence to support a separate 
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standard for ultrafine particles53 or whether there was sufficient evidence to establish 

distinct standards focused on regulating specific PM2.5 components or a group of 

components associated with any source categories of fine particles (U.S. EPA, 2011a, 

section 2.3.1).

A number of studies available in this review have evaluated potential health 

effects associated with short-term exposures to ultrafine particles. As noted in the 

Integrated Science Assessment, the enormous number and larger, collective surface area 

of ultrafine particles are important considerations for focusing on this particle size 

fraction in assessing potential public health impacts (U.S. EPA, 2009a, p. 6-83). Per unit 

mass, ultrafine particles may have more opportunity to interact with cell surfaces due to 

their greater surface area and their greater particle number compared with larger particles 

(U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the potential for soluble 

components (e.g., transition metals, organics) to adsorb to ultrafine particles and 

potentially cross cell membranes and epithelial barriers (U.S. EPA, 2009a, p. 6-83). In 

addition, evidence available in this review suggests that the ability of particles to enhance 

allergic sensitization is associated more strongly with particle number and surface area 

than with particle mass (U.S. EPA, 2009a, p. 6-127). 

New evidence, primarily from controlled human exposure and toxicological 

studies, expands our understanding of cardiovascular and respiratory effects related to 

short-term ultrafine particle exposures. However, the Policy Assessment concluded that 

this evidence was still very limited and largely focused on exposure to diesel exhaust, for 

which the Integrated Science Assessment concluded it was unclear whether the effects 

 

53 Ultrafine particles, generally including particles with a mobility diameter less than or 

equal to 0.1 μm, are emitted directly to the atmosphere or are formed by nucleation of 

gaseous constituents in the atmosphere (U.S. EPA, 2009a, p. 3-3).
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observed are due to ultrafine particles, larger particles within the PM2.5 mixture, or the 

gaseous components of diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition, the 

Integrated Science Assessment noted uncertainties associated with the controlled human 

exposure studies using concentrated ambient particle systems which have been shown to 

modify the composition of ultrafine particles (U.S. EPA, 2009a, p. 2-22, see also section 

1.5.3). 

The Policy Assessment recognized that there are relatively few epidemiological 

studies that have examined potential cardiovascular and respiratory effects associated 

with short-term exposures to ultrafine particles (U.S. EPA, 2011a, p. 2-51). These studies 

have reported inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).

Collectively, in considering the body of scientific evidence available in this 

review, the Integrated Science Assessment concluded that the currently available 

evidence was suggestive of a causal relationship between short-term exposures to 

ultrafine particles and cardiovascular and respiratory effects. Furthermore, the Integrated 

Science Assessment concluded that evidence was inadequate to infer a causal relationship 

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

exposure to ultrafine particles and all outcomes evaluated (U.S. EPA, 2009a, sections 

2.3.5, 6.2.12.3, 6.3.10.3, 6.5.3.3, 7.2.11.3, 7.3.9, 7.4.3.3, 7.5.4.3, and 7.6.5.3; Table 2-6). 

With respect to our understanding of ambient ultrafine particle concentrations, at 

present, there is no national network of ultrafine particle samplers; thus, only episodic 

and/or site-specific data sets exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy 

Assessment recognized a national characterization of concentrations, temporal and spatial 

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

51). In general, measurements of ultrafine particles are highly dependent on monitor 

location and, therefore, more subject to exposure error than accumulation mode particles 

(U.S. EPA, 2009a, p. 2-22). Furthermore, the number of ultrafine particles generally 

decreases sharply downwind from sources, as ultrafine particles may grow into the 

accumulation mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited 

studies of ambient ultrafine particle measurements have suggested that these particles 

exhibit a high degree of spatial and temporal heterogeneity driven primarily by 

differences in nearby source characteristics (U.S. EPA, 2009a, p. 3-84). Internal 

combustion engines and, therefore, roadways are a notable source of ultrafine particles, 

so concentrations of these particles near roadways are generally expected to be elevated 

(U.S. EPA, 2009a, p. 2-3). Concentrations of ultrafine particles have been reported to 

drop off much more quickly with distance from roadways than fine particles (U.S. EPA, 

2009a, p. 3-84).

In considering both the currently available health effects evidence and the air 

quality data, the Policy Assessment concluded that this information was still too limited 

to provide support for consideration of a distinct PM standard for ultrafine particles (U.S.

EPA, 2011a, p. 2-52).

In addressing the issue of particle composition, the Integrated Science Assessment 

concluded that, “[f]rom a mechanistic perspective, it is highly plausible that the chemical 

composition of PM would be a better predictor of health effects than particle size” (U.S.

EPA, 2009a, p. 6-202). Heterogeneity of ambient concentrations of PM2.5 constituents 

(e.g., elemental carbon, organic carbon, sulfates, nitrates) observed in different 
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geographical regions as well as regional heterogeneity in PM2.5-related health effects 

reported in a number of epidemiological studies are consistent with this hypothesis (U.S.

EPA, 2009a, section 6.6). 

With respect to the availability of ambient measurement data for fine particle 

components in this review, the Policy Assessment noted that there were now more 

extensive ambient PM2.5 speciation measurement data available through the Chemical 

Speciation Network (CSN) than in previous reviews (U.S. EPA, 2011a, section 1.3.2 and 

Appendix B, section B.1.3). The Integrated Science Assessment observed that data from 

the CSN provided further evidence of spatial and seasonal variation in both PM2.5 mass 

and composition among cities and geographic regions (U.S. EPA, 2009a, pp. 3-50 to 3-

60; Figures 3-12 to 3-18; Figure 3-47). Some of this variation may be related to regional 

differences in meteorology, sources, and topography (U.S. EPA, 2009a, p. 2-3). 

The currently available epidemiological, toxicological, and controlled human 

exposure studies evaluated in the Integrated Science Assessment on the health effects 

associated with ambient PM2.5 constituents and categories of fine particle sources used a 

variety of quantitative methods applied to a broad set of PM2.5 constituents, rather than 

selecting a few constituents a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies 

have used measured ambient PM2.5 speciation data, including monitoring data from the 

CSN, while all of the controlled human exposure and most of the toxicological studies 

have used concentrated ambient particles and analyzed the constituents therein (U.S.

EPA, 2009a, p. 6-203).54 The CSN provides PM2.5 speciation measurements generally on 

 

54 Most studies considered between 7 to 20 ambient PM2.5 constituents, with elemental 

carbon, organic carbon, sulfates, nitrates, and metals most commonly measured. Many of 

the studies grouped the constituents with various factorization or source apportionment 

techniques to examine the relationship between the grouped constituents and various 
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a one-in-three or one-in-six day sampling schedule and, thus, does not capture data every 

day at most sites.55

The Policy Assessment recognized that several new multi-city studies evaluating 

short-term exposures to fine particle constituents are now available. These studies 

continued to show an association between mortality and cardiovascular and/or respiratory 

morbidity effects and short-term exposures to various PM2.5 components including 

nickel, vanadium, elemental carbon, organic carbon, nitrates, and sulfates (U.S. EPA, 

2011a, section 2.3.1; U.S. EPA, 2009a, sections 6.5.2.5 and 6.6). 

Limited evidence is available to evaluate the health effects associated with longterm exposures to PM2.5 components (U.S. EPA, 2009a, section 7.6.2). The Policy 

Assessment noted the most significant new evidence was provided by a study that 

evaluated multiple PM2.5 components and an indicator of traffic density in an assessment 

of health effects related to long-term exposure to PM2.5 (Lipfert et al., 2006). Using 

health data from a cohort of U.S. military veterans and PM2.5 measurement data from the 

CSN, Lipfert et al. (2006) reported positive associations between mortality and long-term 

exposures to nitrates, elemental carbon, nickel, and vanadium as well as traffic density 

and peak ozone concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a, pp. 7-89 to 

7-90). 

With respect to source categories of fine particles associated with a range of 

 

health effects. However, not all studies labeled the constituent groupings according to 

their presumed source and a small number of controlled human exposure and 

toxicological studies did not use any constituent grouping. These differences across 

studies substantially limit any integrative interpretation of these studies (U.S. EPA, 

2009a, p. 6-203). 

55 To expand our understanding of the role of specific PM2.5 components and sources 

with respect to the observed health effects, researchers have expressed a strong interest in 

having access to PM2.5 speciation measurements collected more frequently (U.S. EPA, 

2011a, p. 2-53, including footnote 47). 
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health endpoints, the Integrated Science Assessment reported that the currently available 

evidence suggests associations between cardiovascular effects and a number of specific

PM2.5–related source categories, specifically oil combustion, wood or biomass burning, 

motor vehicle emissions, and crustal or road dust sources (U.S. EPA, 2009a, section 6.6; 

Table 6-18). In addition, a few studies have evaluated associations between PM2.5-related 

source categories and mortality. One study reported an association between mortality and 

a PM2.5 coal combustion factor (Laden et al., 2000), while other studies linked mortality 

to a secondary sulfate long-range transport PM2.5 source (Ito et al., 2006; Mar et al., 

2006) (U.S. EPA, 2009a, section 6.6.2.1). There was less consistency in associations 

observed between sources of fine particles and respiratory health effects, which may be 

partially due to the fact that fewer studies have evaluated respiratory-related outcomes 

and measures. However, there was some evidence for PM2.5-related associations with 

secondary sulfate and decrements in lung function in asthmatic and healthy adults (U.S.

EPA, 2009a, p. 6-211; Gong et al., 2005; Lanki et al., 2006). Respiratory effects relating 

to the crustal/soil/road dust and traffic sources of PM have been observed in asthmatic 

children and adults (U.S. EPA, 2009a, p. 6-205; Gent et al., 2009; Penttinen et al., 2006). 

Recent studies have shown that source apportionment methods have the potential 

to add useful insights into which sources and/or PM constituents may contribute to 

different health effects. Of particular interest are several epidemiological studies that 

compared source apportionment methods and reported consistent results across research 

groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al., 2006; Mar et al., 2006; 

Thurston et al., 2005). These studies reported associations between total mortality and 

secondary sulfate in two cities for two different lag times. The sulfate effect was stronger 
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for total mortality in Washington D.C. and for cardiovascular-related mortality in 

Phoenix (U.S. EPA, 2009a, p. 6-204). These studies also found some evidence for 

associations with mortality and a number of source categories (e.g., biomass/wood 

combustion, traffic, copper smelter, coal combustion, sea salt) at various lag times (U.S.

EPA, 2009a, p. 6-204). Sarnat et al. (2008) compared three different source 

apportionment methods and reported consistent associations between emergency 

department visits for cardiovascular diseases with mobile sources and biomass 

combustion as well as increased respiratory-related emergency department visits 

associated with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-211).

Collectively, in considering the currently available evidence for health effects 

associated with specific PM2.5 components or groups of components associated with any 

source categories of fine particles as presented in the Integrated Science Assessment, the 

Policy Assessment concluded that additional information available in this review 

continues to provide evidence that many different constituents of the fine particle mixture 

as well as groups of components associated with specific source categories of fine 

particles are linked to adverse health effects (U.S. EPA, 2011a, p. 2-55). However, as 

noted in the Integrated Science Assessment, while “[t]here is some evidence for trends 

and patterns that link particular ambient PM constituents or sources with specific health 

outcomes...there is insufficient evidence to determine whether these patterns are 

consistent or robust” (U.S. EPA, 2009a, p. 6-210). Assessing this information, the 

Integrated Science Assessment concluded that “the evidence is not yet sufficient to allow 

differentiation of those constituents or sources that are more closely related to specific 

health outcomes” (U.S. EPA, 2009a, pp. 2-26 and 6-212). Therefore, the Policy 
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Assessment concluded that the currently available evidence is not sufficient to support 

consideration of a separate indicator for a specific PM2.5 component or group of 

components associated with any source category of fine particles. Furthermore, the 

Policy Assessment concluded that the evidence is not sufficient to support eliminating 

any component or group of components associated with any source categories of fine 

particles from the mix of fine particles included in the PM2.5 indicator (U.S. EPA, 2011a, 

p. 2-56). 

The CASAC agreed with the EPA staff conclusions presented in the Policy 

Assessment and concluded that it is appropriate to consider retaining PM2.5 as the 

indicator for fine particles and further asserted, “There [is] insufficient peer-reviewed 

literature to support any other indicator at this time” (Samet, 2010c, p. 12). CASAC 

expressed a strong desire for the EPA to “look ahead to future review cycles and 

reinvigorate support for the development of evidence that might lead to newer indicators 

that may correlate better with the health effects associated with ambient air 

concentrations of PM ...” (Samet, 2010c, p 2). 

Consistent with the staff conclusions presented in the Policy Assessment and 

CASAC advice, the Administrator proposed to retain PM2.5 as the indicator for fine 

particles. Further, the Administrator provisionally concluded that currently available 

scientific information does not provide a sufficient basis for supplementing mass-based, 

primary fine particle standards with standards using a separate indicator for ultrafine 

particles or a separate indicator for a specific PM2.5 component or group of components 

associated with any source categories of fine particles. In addition, the Administrator also 

provisionally concluded that the currently available scientific information did not provide 
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a sufficient basis for eliminating any individual component or group of components 

associated with any source categories from the mix of fine particles included in the PM2.5

mass-based indicator. 

The EPA received comparatively few public comments on issues related to the 

indicator for fine particles.56 Some commenters emphasized the need to conduct 

additional research to more fully understand the effect of specific PM2.5 components 

and/or sources on public health. These commenters expressed views about the importance 

of evaluating health effect associations with various fine particle components and types 

of source categories as a basis for focusing ongoing and future research to reduce 

uncertainties in this area and for considering whether alternative indicator(s) may be 

appropriate to consider in future PM NAAQS reviews for standards intended to protect 

against the array of health effects that have been associated with fine particles as indexed 

by PM2.5. For example, the PSR encouraged more research and monitoring related to 

PM2.5 components and noted the importance of components associated with coal 

combustion (PSR, 2012, pp. 5 to 6). EPRI asserted that “new” studies support focusing 

on EC and OC and encouraged the EPA to seriously consider the mass-based approach 

(EPRI, 2012, p. 2). Likewise, Georgia Mining Association supported additional 

monitoring and research efforts related to PM2.5 composition and specifically encouraged 

the evaluation of using particle number (e.g., particle count) (GMA, 2012, pp. 2 to 3). 

The Administrator agrees with CASAC as well as these commenters that the 

results of additional research and monitoring efforts will be helpful for informing future 

PM NAAQS reviews. Information from such studies could also help inform the 

 

56 No public comments were submitted regarding the use of a different size cut for fine 

particles.
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development of strategies that emphasize control of specific types of emission sources so 

as to address particles of greatest concern to public health. However, based upon the 

scientific information considered in the Integrated Science Assessment as well as the 

public comments summarized above, the Administrator continues to take note there is 

evidence that many different constituents of the fine particle mixture as well as groups of 

components associated with specific sources of fine particles are linked to adverse health 

effects. Furthermore, she recognizes that the evidence is not yet sufficient to differentiate 

those constituents or sources that are most closely related to specific health outcomes nor 

to exclude any PM2.5 components or sources of fine particles from the mix of particles 

included in the PM2.5 indicator. 

Having considered the public comments on this issue, the Administrator concurs 

with the Policy Assessment conclusions and CASAC recommendations and concludes 

that it is appropriate to retain PM2.5 as the indicator for fine particles. 

2. Averaging Time

In 1997, the EPA initially set both an annual standard, to provide protection from 

health effects associated with both long- and short-term exposures to PM2.5, and a 24-

hour standard to supplement the protection afforded by the annual standard (62 FR 38667 

to 38668, July, 18, 1997). In the last review, the EPA retained both annual and 24-hour 

averaging times (71 FR 61164, October 17, 2006). These decisions were based, in part, 

on evidence of health effects related to both long-term (from a year to several years) and 

short-term (from less than one day to up to several days) measures of PM2.5.

The overwhelming majority of studies conducted since the last review continue to 

utilize annual (or multi-year) and 24-hour averaging times, reflecting the averaging times 
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of the current PM2.5 standards. These studies continue to provide evidence that health 

effects are associated with annual and 24-hour averaging times. Therefore, the Policy 

Assessment concluded it is appropriate to retain the current annual and 24-hour averaging 

times to provide protection from effects associated with both long- and short-term PM2.5

exposures (U.S. EPA, 2011a, p. 2-57).

In considering whether the information available in this review supports 

consideration of different averaging times for PM2.5 standards specifically with regard to 

considering a standard with an averaging time less than 24 hours to address health effects 

associated with sub-daily PM2.5 exposures, the Policy Assessment noted there continues 

to be a growing body of studies that provide additional evidence of effects associated 

with exposure periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to 

information available in the last review, recent studies provide additional evidence for 

cardiovascular effects associated with sub-daily (e.g., one to several hours) exposure to 

PM, especially effects related to cardiac ischemia, vasomotor function, and more subtle 

changes in markers of systemic inflammation, hemostasis, thrombosis and coagulation 

(U.S. EPA, 2009a, section 6.2). Because these studies have used different indicators (e.g., 

PM2.5, PM10, PM10-2.5, ultrafine particles), averaging times (e.g., 1, 2, and 4 hours), and 

health outcomes, it is difficult to draw conclusions about cardiovascular effects 

associated specifically with sub-daily exposures to PM2.5. 

With regard to respiratory effects associated with sub-daily PM2.5 exposures, the 

currently available evidence was much sparser than for cardiovascular effects and 

continues to be very limited. The Integrated Science Assessment concluded that for 

several studies of hospital admissions or medical visits for respiratory diseases, the 
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strongest associations were observed with 24-hour average or longer exposures, not with 

less than 24-hour exposures (U.S. EPA, 2009a, section 6.3). 

Collectively, the Policy Assessment concluded that this information, when viewed 

as a whole, is too unclear, with respect to the indicator, averaging time and health 

outcome, to serve as a basis for consideration of establishing a primary PM2.5 standard 

with an averaging time shorter than 24-hours at this time (U.S. EPA, 2011a, p. 2-57). 

With regard to health effects associated with PM2.5 exposure across varying 

seasons in this review, Bell et al. (2008) reported higher PM2.5 risk estimates for 

hospitalization for cardiovascular and respiratory diseases in the winter compared to 

other seasons. In comparison to the winter season, smaller statistically significant 

associations were also reported between PM2.5 and cardiovascular morbidity for spring 

and autumn, and a positive, but statistically non-significant association was observed for 

the summer months. In the case of mortality, Zanobetti and Schwartz (2009) reported a 

4-fold higher effect estimate for PM2.5-associated mortality for the spring as compared to 

the winter. Taken together, these results provided emerging but limited evidence that 

individuals may be at greater risk of dying from higher exposures to PM2.5 in the warmer 

months and may be at greater risk of PM2.5-associated hospitalization for cardiovascular 

and respiratory diseases during colder months of the year (U.S. EPA, 2011a, p. 2-58). 

Overall, the Policy Assessment observed that there are few studies presently 

available to deduce a general pattern in PM2.5-related risk across seasons. In addition, 

these studies utilized 24-hour exposure periods within each season to assess the 

PM2.5-associated health effects and do not provide information on health effects 

associated with a season-long exposure to PM2.5. Due to these limitations in the currently 
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available evidence, the Policy Assessment concluded that there was no basis to consider a 

seasonal averaging time separate from a 24-hour averaging time.

Based on the above considerations, the Policy Assessment concluded that the 

currently available information provided strong support for consideration of retaining the 

current annual and 24-hour averaging times but does not provide support for considering 

alternative averaging times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC considered 

it appropriate to retain the current annual and 24-hour averaging times for the primary 

PM2.5 standards (Samet, 2010c, pp. 2 to 3). At the time of the proposal, the Administrator 

concurred with the staff conclusions and CASAC advice and proposed that the averaging 

times for the primary PM2.5 standards should continue to include annual and 24-hour 

averages to protect against health effects associated with long- and short-term exposures. 

Furthermore, the Administrator provisionally concluded, consistent with conclusions 

reached in the Policy Assessment and by CASAC, that the currently available 

information was too limited to support consideration of alternative averaging times to 

establish a national standard with a shorter-than 24-hour averaging time or with a 

seasonal averaging time. 

The EPA received no significant public comments on the issue of averaging time 

for the PM2.5 primary standards. The Administrator concurs with recommendations made 

by CASAC and the staff conclusions presented in the Policy Assessment and concludes, 

as proposed, that it is appropriate to retain the current annual and 24-hour averaging 

times for the primary PM2.5 standards to protect against health effects associated with 

long- and short-term exposure periods.

3. Form
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The “form” of a standard defines the air quality statistic that is to be compared to 

the level of the standard in determining whether an area attains the standard. In this 

review, the EPA considers whether currently available information supports retaining or 

revising the forms for the annual or 24-hour PM2.5 standards. 

a. Annual Standard

In 1997, the EPA established the form of the annual PM2.5 standard as an annual 

arithmetic mean, averaged over 3 years, from single or multiple community-oriented 

monitors. This form was intended to represent a relatively stable measure of air quality 

and to characterize longer-term area-wide PM2.5 concentrations, in conjunction with a 

24-hour standard designed to provide adequate protection against localized peak or 

seasonal PM2.5 concentrations. The level of the standard was to be compared to 

measurements made at each community-oriented monitoring site, or, if specific criteria 

were met, measurements from multiple community-oriented monitoring sites could be 

averaged (i.e., spatial averaging)57 (62 FR 38671 to 38672, July 18, 1997). The 

constraints were intended to ensure that spatial averaging would not result in inequities in 

the level of protection provided by the standard (62 FR 38672, July 18, 1997). This 

approach was consistent with the epidemiological studies on which the PM2.5 standard 

was primarily based, in which air quality data were generally averaged across multiple 

monitors in an area or were taken from a single monitor that was selected to represent 

community-wide exposures. 

In the last review, the EPA tightened the criteria for use of spatial averaging to 

provide increased protection for vulnerable populations exposed to PM2.5. This change 

 

57 Spatial averaging as part of the form of the annual PM2.5 standard is unique to this 

standard and is not used with other PM standards nor with other NAAQS.
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was based in part on an analysis of the potential for disproportionate impacts on 

potentially at-risk populations, which found that the highest concentrations in an area 

tend to be measured at monitors located in areas where the surrounding population is 

more likely to have lower education and income levels and higher percentages of 

minority populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section 

5.3.6.1).

In this review, as outlined in section III.B above and discussed more fully in 

section III.B.3 of the proposal, there now exist more health data such that the Integrated 

Science Assessment has identified persons from lower socioeconomic strata as an at-risk 

population (U.S. EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover, 

there now exist more years of PM2.5 air quality data than were available in the last 

review. Consideration in the Policy Assessment of the spatial variability across urban 

areas that was revealed by this expanded data base has raised questions as to whether an 

annual standard that allows for spatial averaging, even within specified constraints as 

narrowed in 2006 (71 FR 61165 to 61167, October 17, 2006), would provide appropriate 

public health protection. 

In considering the potential for disproportionate impacts on at-risk populations, 

the Policy Assessment considered an update of an air quality analysis conducted for the 

last review (U.S. EPA, 2011a, pp. 2-59 to 60; Schmidt, 2011a, Analysis A). This analysis 

focused on determining whether the spatial averaging provisions, as modified in 2006, 

could introduce inequities in protection for at-risk populations exposed to PM2.5. 

Specifically, the Policy Assessment considered whether persons of lower socioeconomic 

status, minority groups, or different age groups (i.e., children or older adults) are more 
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likely than the general population to live in areas in which the monitors recording the 

highest air quality values in an area are located. Data used in this analysis included 

demographic parameters measured at the Census Block or Census Block Group level, 

including percent minority population, percent minority subgroup population, percent of 

persons living below the poverty level, percent of persons 18 years of age or older, and 

percent of persons 65 years of age and older. In each candidate geographic area, data 

from the Census Block(s) or Census Block Group(s) surrounding the location of the 

monitoring site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) in which the 

highest air quality value was monitored were compared to the average of monitored 

values in the area. This analysis looked beyond areas that would meet the current spatial 

averaging criteria and considered all urban areas (i.e., Core Based Statistical Areas or 

CBSAs) with at least two valid annual design value monitors (Schmidt, 2011a, Analysis 

A). Recognizing the limitations of such cross-sectional analyses, the Policy Assessment 

observed that the highest concentrations in an area tend to be measured at monitors 

located in areas where the surrounding populations are more likely to live below the 

poverty line and to have higher percentage of minorities (U.S. EPA, 2011a, p. 2-60). 

Based upon the analysis described above, the Policy Assessment concluded that 

the existing constraints on spatial averaging, as modified in 2006, may be inadequate to 

avoid substantially greater exposures in some areas, potentially resulting in 

disproportionate impacts on at-risk populations of persons with lower SES levels as well 

as minorities. Therefore, the Policy Assessment concluded that it was appropriate to 

consider revising the form of the annual PM2.5 standard such that it did not allow for the 

use of spatial averaging across monitors. In doing so, the level of the annual PM2.5
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standard would be compared to measurements made at the monitoring site that represents 

area-wide air quality recording the highest PM2.5 concentrations58 (U.S. EPA, 2011a, p. 

2-60). 

The CASAC agreed with staff conclusions that it was “reasonable” for the EPA to 

eliminate the spatial averaging provisions (Samet, 2010d, p. 2). Further, in CASAC’s 

comments on the first draft Policy Assessment, it noted, “Given mounting evidence 

showing that persons with lower SES levels are a susceptible group for PM-related health 

risks, CASAC recommends that the provisions that allow for spatial averaging across 

monitors be eliminated for the reasons cited in the (first draft) Policy Assessment” 

(Samet, 2010c, p. 13). In its review of the second draft Policy Assessment, CASAC 

recognized “although much of the epidemiological research has been conducted using 

community-wide averages, several key studies reference the nearest measurement site, so 

that some risk estimates are not necessarily biased by the averaging process. Further, the 

number of such studies is likely to expand in the future” (Samet, 2010d, pp. 1 to 2).

Only two areas in the country used the initial spatial averaging provisions for 

demonstrating attainment with the primary annual PM2.5 standard set in 1997 (70 FR 

19847, April 14, 2005; U.S. EPA, 2006c). Since these provisions were tightened in 2006,

no area has used spatial averaging to demonstrate attainment. No areas in the country are 

currently using the spatial averaging provisions to demonstrate attainment with the 

current primary annual PM2.5 standard.

In considering the Policy Assessment’s conclusions based on the results of the 

analysis discussed above and concern over the evidence of potential disproportionate 

 

58 As discussed in section VIII.B.1 below, the EPA is revising several terms associated 

with PM2.5 monitor placement. Specifically, the EPA is revoking the term “communityoriented” and replacing it with the term “area-wide” monitoring. 
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impacts on at-risk populations as well as CASAC advice, the Administrator proposed to 

revise the form of the annual PM2.5 standard to eliminate the use of spatial averaging. 

Thus, the Administrator proposed revising the form of the annual PM2.5 standard to 

compare the level of the standard with measurements from each “appropriate” monitor in 

an area

59 with no allowance for spatial averaging. Thus, for an area with multiple 

monitors, the appropriate reporting monitor with the highest design value would 

determine the attainment status for that area.

Of the commenters noted in section III.D.2 above who supported a more stringent 

annual PM2.5 standard, those who commented on the form of the annual PM2.5 standard 

supported the EPA’s proposal to eliminate the spatial averaging provisions. These 

commenters contended that the EPA’s analyses of the potential impacts of spatial 

averaging, discussed above and in the proposal, (77 FR 38924), demonstrated that the 

current form results in uneven public health protection leading to disproportionate 

impacts on at-risk populations. Specifically, the ALA and other environmental and public 

health commenters contended that “spatial averaging allows exposure of people to 

unhealthy levels of pollution at specific locales even within an area meeting the standard” 

(ALA et al., 2012, p. 23). These commenters particularly focused on the importance for 

low-income and minority populations of eliminating the spatial averaging provisions. 

They concluded that spatial averaging “is an environmental justice concern because poor 

people are more likely to live near roads, depots, factories, ports, and other pollution 

sources.” Id. p. 24.

 

59 As discussed in section VIII.B.2.b below, the EPA concludes that PM2.5 monitoring 

sites at micro- and middle-scale locations should be comparable to the annual standard 

unless the monitoring site has been approved by the Regional Administrator as a 

“relatively unique micro-scale, or localized hot-spot, or unique middle-scale site.”
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Other commenters (e.g., AAM, 2012; Dow, 2012) also supported the elimination 

of spatial averaging in order to “avoid potential disproportionate impacts on at-risk 

populations” and to maximize “the benefits to public health of reducing the annual PM2.5

standard.” However, these groups expressed concern that the elimination of spatial 

averaging, in combination with the requirement for near road monitors (as discussed in 

section VIII.B.3.b.i of the proposal), would effectively and inappropriately increase the 

stringency of the annual PM2.5 standard. 

This concern was also shared by other commenters who disagreed with the 

elimination of spatial averaging. For example, the Class of ’85 RRG emphasized 

concerns about increasing the stringency of the standard while providing few health 

benefits if spatial averaging is eliminated, particularly in combination with the 

requirement for near-road monitors. These commenters contended that “[b]ecause EPA 

proposes to use the readings from the highest single worst case monitor (rather than the 

average of all community area monitors), and since roadway monitoring locations will 

likely be worst case monitors, the proposed NAAQS will become more stringent without 

targeting the PM2.5 species most harmful to human health” (Class of ’85 RRG, 2012, p. 

6).

Several commenters also maintained that because spatial averaging is consistent 

with how air quality data are considered in the underlying epidemiological studies, such 

averaging should not be eliminated. Specifically, commenters including NAM et al., 

AFPM, and ACC pointed out that PM2.5 epidemiological studies use spatially averaged 

multi-monitor concentrations, rather than the single highest monitor, when evaluating 

health effects. Therefore, these commenters contended that allowing spatial averaging 
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would make the PM2.5 standard more consistent with the approaches used in the

epidemiological studies upon which the standard is based. In addition, some commenters 

also contended that the EPA failed to consider whether modifying, rather than 

eliminating, the constraints on spatial averaging would have been sufficient to protect the 

public health. If so, these commenters argued that “elimination of spatial averaging 

would go beyond what is requisite to protect the public health” (NAM et al., 2012, p. 

20)).

In considering the public comments on the form of the annual standard, the EPA 

recognizes a number of commenters agreed with the basis for the EPA’s proposal to 

eliminate spatial averaging. While other commenters expressed disagreement or concern 

with the proposed decision to eliminate the spatial averaging provisions, the Agency 

notes that these commenters did not challenge the analyses or considerations that 

provided the fundamental basis for the Administrator’s proposed decision. Rather, these 

commenters generally raised concerns that eliminating the option for spatial averaging 

would increase the stringency of the standard, especially in light of additional monitoring 

sites in near-road environments (as discussed in section VIII.B.3.b.1 below). In

The EPA does not agree with the comment that siting some monitors in near 

roadway environments makes the standard more stringent or impermissibly more 

stringent. As discussed in section VIII.B.3.b.i below, a significant fraction of the 

population lives in proximity to major roads, and these exposures occur in locations that 

represent ambient air. Monitoring in such areas does not make the standard more 

stringent than warranted, but rather affords the intended protection to the exposed 

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

quality may be compared to the annual standard. The 24-hour and annual NAAQS are 

designed to protect the public with an adequate margin of safety, and this siting provision 

is fully consistent with providing the protection the standard is designed to provide and 

does not make the standard more stringent or more stringent than necessary. 

Monitors that are representative of area-wide air quality may be compared to the 

annual standard. This is consistent with the use of monitoring data in the epidemiological 

studies that provide the primary basis for determining the level of the annual standard. In 

addition, the EPA notes that the annual standard is designed to protect against both longand short-term exposures through controlling the broad distribution of air quality across 

an area over time.60 It is fully consistent with the protection the standard is designed to 

provide for near road monitors to be compared to the annual standard if the monitor is 

representative of area-wide air quality. This does not make the standard either more 

stringent or impermissibly more stringent.

In further considering these comments, the EPA notes that the stringency or level 

of protection provided by each NAAQS is not based solely on the form of the standard; 

rather, the four elements of the standard that together serve to define each standard (i.e., 

indicator, averaging time, form, and level) must be considered collectively in evaluating 

the protection afforded by each standard. Therefore, the EPA considers these comments 

are morealso appropriate to discuss collectively with other issues related to the 

appropriate level for annual standard, and are discussed below in sections III.E.4.c-d. 

 

60 This is in contrast to the 24-hour standard which is designed to provide supplemental protection, 

addressing peak exposures that might not otherwise be addressed by the annual standard. Consistent with 

this, monitors are not required to be representative of area-wide air quality to be compared to the 24-hour 

standard.
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In reaching a final decision on the form of the annual standard, the Administrator 

considers the available analyses, CASAC advice, and public comments on form as 

discussed above. She also considers related issues in the public comments on the level of 

the annual standard as discussed in section III.E.4.c below. She notes that even when the 

annual PM2.5 standard was first set in 1997, the spatial averaging provisions included 

constraints intended to ensure that inequities in the level of protection would not result. 

These constraints on spatial averaging were tightened in the last review, based on an 

analysis showing the potential for spatial averaging to allow higher PM2.5 concentrations 

in locations where subgroups within the general population were potentially 

disproportionately exposed and hence, at disproportionate risk (e.g., low income and 

minority communities). The Administrator notes that in proposing to eliminate spatial 

averaging altogether in this review, she has relied on further analyses in the current 

review (Schmidt, 2011a, Analysis A). As discussed above and in the proposal (77 FR 

38924), these analyses showed that the current constraints on spatial averaging may be 

inadequate in some areas to avoid substantially greater exposures for people living near 

monitors recording the highest PM2.5 concentrations. Such exposures could result in 

disproportionate impacts to at-risk populations, including low-income populations as well 

as minority groups. 

On this basis, the Administrator concludes that public health would not be 

protected with an adequate margin of safety in all locations, as required by law, if 

disproportionately higher exposure concentrations in at-risk populations such as low 

income communities as well as minority communities were averaged together with lower 

concentrations measured at other sites in a large urban area. See ALA v. EPA, 134 F. 3d 
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388, 389 (D.C. Cir., 1998) (“this court has held that ‘NAAQS must protect not only 

average healthy individuals, but also sensitive citizens such as children,’ and ‘if a 

pollutant adversely affects the health of these sensitive individuals, EPA must strengthen 

the entire national standard’”) and Coalition of Battery Recyclers Association v. EPA, 604 

F 3d. 613, 617 (D.C. Cir., 2010) (“Petitioners’ assertion that the revised lead NAAQS is 

overprotective because it is more stringent than necessary to protect the entire population 

of young U.S. children ignores that the Clean Air Act allows protection of sensitive 

subpopulations.”) In reaching this conclusion, the Administrator further notes that her 

concern over possible disproportionate PM2.5-related health impacts in at-risk populations 

extends to populations living near important sources of PM2.5, including the large 

populations that live near major roadways.61

In light of all of the above considerations, including consideration of available 

analyses, CASAC advice, and public comments, the Administrator concludes that the 

current form of the annual PM2.5 standard should be revised to eliminate spatial averaging 

provisions. Thus, the level of the revised annual PM2.5 standard established with this 

notice will be compared with measurements from each appropriate monitor in an area, 

with no allowance for spatial averaging. The Administrator’s conclusions with regard to 

the appropriate level of the annual PM2.5 standard to set in conjunction with this form are 

discussed below in section III.E.4.d.

b. 24-Hour Standard

In 1997, the EPA established the form of the 24-hour PM2.5 standard as the 98th

percentile of 24-hour concentrations at each population-oriented monitor within an area, 

 

61Section VIII.B.3.b.i below discusses public comments specifically related to the 

proposed requirement for near-road monitors. 
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averaged over three years (62 FR at 38671 to 38674, July 18, 1997). The Agency selected 

the 98th percentile as an appropriate balance between adequately limiting the occurrence 

of peak concentrations and providing increased stability which, when averaged over 3 

years, facilitated effective health protection through the development of more stable 

implementation programs. By basing the form of the standard on concentrations 

measured at population-oriented monitoring sites, the EPA intended to provide protection 

for people residing in or near localized areas of elevated concentrations. In the last 

review, in conjunction with lowering the level of the 24-hour standard, the EPA retained 

this form based in part on a comparison with the 99th percentile form.62

In revisiting the stability of a 98thversus 99th percentile form for a 24-hour 

standard intended to provide supplemental protection for a generally controlling annual 

standard, an analysis presented in the Policy Assessment considered air quality data 

reported in 2000 to 2008 to update our understanding of the ratio between peak-to-mean 

PM2.5 concentrations. This analysis provided evidence that the 98th percentile value was a 

more stable metric than the 99th percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62). 

At the time of the proposal, the Agency recognized that the selection of the 

appropriate form of the 24-hour standard includes maintaining adequate protection 

against peak 24-hour concentrations while also providing a stable target for risk 

management programs, which serves to provide for the most effective public health 

 

62 In reaching this final decision, the EPA recognized a technical problem associated with 

a potential bias in the method used to calculate the 98th percentile concentration for this 

form. The EPA adjusted the sampling frequency requirement in order to reduce this bias. 

Accordingly, the Agency modified the final monitoring requirements such that areas that 

are within 5 percent of the standards are required to increase the sampling frequency to 

every day (71 FR 61164 to 61165, October 17, 2006).
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protection in the long run.63 As in previous reviews, the EPA recognized that a 

concentration-based form, compared to an exceedance-based form, was more reflective 

of the health risks posed by elevated pollutant concentrations because such a form gives 

proportionally greater weight to days when concentrations are well above the level of the 

standard than to days when the concentrations are just above the level of the standard. 

Further, the Agency provisionally concluded that a concentration-based form, when 

averaged over three years, provided an appropriate balance between limiting peak 

pollutant concentrations and providing a stable regulatory target, thus facilitating the 

development of more stable implementation programs.

In considering the information provided in the Policy Assessment and recognizing 

that the degree of public health protection likely to be afforded by a standard is a result of 

the combination of the form and the level of the standard, the Administrator proposed to 

retain the 98th percentile form of the 24-hour standard. The Administrator provisionally 

concluded that the 98th percentile form represents an appropriate balance between 

adequately limiting the occurrence of peak concentrations and providing increased 

stability relative to an alternative 99th percentile form. 

Few public commenters commented specifically on the form of the 24-hour 

standard. None of the public commenters raised objections to continuing the use of a 

concentration-based form for the 24-hour standard. Many of the individuals and groups 

who supported a more stringent 24-hour PM2.5 standard noted in section III.D.2 above, 

however, recommended a more restrictive concentration-based percentile form, 

 

63 See ATA III, 283 F.3d at 374-376 which concludes that it is legitimate for the EPA to 

consider overall stability of the standard and its resulting promotion of overall 

effectiveness of NAAQS control programs in setting a standard that is requisite to protect 

the public health. 
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specifically a 99th percentile form. The limited number of these commenters who 

provided a specific rationale for this recommendation generally expressed their concern 

that the 98th percentile form could allow too many days where concentrations exceeded 

the level of the standard, and thus fail to adequately protect public health. Other public 

commenters representing state and local air agencies and industry groups generally 

supported retaining the current 98th percentile form. In most cases, these groups 

expressed the overall view that the current 24-hour PM2.5 standard, including the form of 

the current standard, should be retained. 

The EPA notes that the viewpoints represented in this review are similar to 

comments submitted in the last review and through various NAAQS reviews. The EPA 

recognizes that the selection of the appropriate form includes maintaining adequate 

protection against peak 24-hour values while also providing a stable target for risk 

management programs, which serves to provide for the most effective public health 

protection in the long run.64 Nothing in the commenters’ views has provided a reason to 

change the Administrator’s previous conclusion regarding the appropriate balance 

represented in the proposed form of the 24-hour PM2.5 standard. Therefore, the 

Administrator concurs with staff conclusions presented in the Policy Assessment and 

CASAC recommendations and concludes that it is appropriate to retain the 98th percentile 

form for the 24-hour PM2.5 standard.

4. Level

 

64 As just noted above, it is legitimate for the EPA to consider promotion of overall 

effectiveness of risk management programs designed to attain the NAAQS, including 

their overall stability, in setting a standard that is requisite to protect the public health. 

The context for the court’s discussion in ATA III is identical to that here; whether to adopt 

a 98th percentile form for a 24-hour standard intended to provide supplemental protection 

for a generally controlling annual standard.
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In the last review, the EPA selected levels for the annual and the 24-hour PM2.5

standards using evidence of effects associated with periods of exposure that were most 

closely matched to the averaging time of each standard. Thus, as discussed in section 

III.A.1, the EPA relied upon evidence from long-term exposure studies as the principal 

basis for selecting the level of the annual PM2.5 standard that would protect against 

effects associated with long-term exposures. The EPA relied upon evidence from the 

short-term exposure studies as the principal basis for selecting the level of the 24-hour 

PM2.5 standard that would protect against effects associated with short-term exposures. 

As summarized in section III.A.2 above, the 2006 decision to retain the level of the 

annual PM2.5 standard at 15 μg/m3 65 was challenged and on judicial review, the D.C. 

Circuit remanded the primary annual PM2.5 standard to the EPA, finding that EPA’s 

explanation for its approach to setting the level of the annual standard was inadequate. 

a. General Approach for Considering Standard Levels

Building upon the lessons learned in the previous PM NAAQS reviews, in 

considering alternative standard levels supported by the currently available scientific 

information, the Policy Assessment used an approach that integrated evidence-based and 

risk-based considerations, took into account CASAC advice, and considered the issues 

raised by the court in remanding the primary annual PM2.5 standard. Following the 

general approach outlined in section III.A.3 above, for the reasons discussed below, the 

Policy Assessment concluded it was appropriate to consider the protection afforded by 

the annual and 24-hour standards taken together against mortality and morbidity effects 

 

65 Throughout this section, the annual standard levels are denoted as integer values for 

simplicity, although, as noted above in section II.B.1, Table 1, the annual standard level 

is defined to one decimal place, such that the current annual standard level is 15.0 μg/m3. 

Alternative annual standard levels discussed in this section are similarly defined to one 

decimal place.
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associated with both long- and short-term PM2.5 exposures. This was consistent with the 

approach taken in the review completed in 1997 rather than considering each standard 

separately, as was done in the review completed in 2006. 

Beyond looking directly at the relevant epidemiologic evidence, the Policy 

Assessment considered the extent to which specific alternative PM2.5 standard levels were 

likely to reduce the nature and magnitude of both long-term exposure-related mortality 

risk and short-term exposure-related mortality and morbidity risk (U.S. EPA, 2011a, 

section 2.3.4.2; U.S.EPA, 2010a, section 4.2.2). As noted in section III.C.3 above, 

patterns of increasing estimated risk reductions were generally observed as either the 

annual or 24-hour standard, or both, were reduced below the level of the current 

standards (U.S. 2011a, Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 

5.2.3). 

Based on the quantitative risk assessment, the Policy Assessment observed, as 

discussed in section III.A.3, that analyses conducted for this and previous reviews 

demonstrated that much, if not most, of the aggregate risk associated with short-term 

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

concentrations are in the low-to mid-range, below the peak 24-hour concentrations (U.S.

EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C above and in section 

III.C.3 of the proposal, the Risk Assessment observed that alternative annual standard 

levels, when controlling, resulted in more consistent risk reductions across urban study 

areas, thereby potentially providing a more consistent degree of public health protection 

(U.S. EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk Assessment noted that the 

results of simulating alternative suites of PM2.5 standards including different 
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combinations of alternative annual and 24-hour standard levels suggested that an 

alternative 24-hour standard level can produce additional estimated risk reductions 

beyond that provided by an alternative annual standard alone. However, the degree of 

estimated risk reduction provided by alternative 24-hour standard levels was highly 

variable, in part due to the choice of rollback approached used (U.S. EPA, 2010a, p. 5-

17). 

Based on its review of the second draft Policy Assessment, CASAC agreed with 

the EPA staff’s general approach for translating the available epidemiological evidence, 

risk information, and air quality information into the basis for reaching conclusions on 

alternative standards for consideration. Furthermore, CASAC agreed “that it is 

appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have 

evidence supportive of a causal or likely causal relationship as presented in the Integrated 

Science Assessment (Samet, 2010d, p. 1). 

Therefore, the Policy Assessment concluded, consistent with specific CASAC 

advice, that it was appropriate to set a “generally controlling” annual standard that will 

lower a wide range of ambient 24-hour concentrations. The Policy Assessment concluded 

this approach would likely reduce aggregate risks associated with both long- and shortterm exposures with more consistency than a generally controlling 24-hour standard and 

would be the most effective and efficient way to reduce total PM2.5-related population 

risk and so provide appropriate protection. The staff believed this approach, in contrast to 
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one focusing on a generally controlling 24-hour standard, would likely reduce aggregate 

risks associated with both long- and short-term exposures with more consistency and 

would likely avoid setting national standards that could result in relatively uneven 

protection across the country due to setting standards that were either more or less 

stringent than necessary in different geographical areas. 

The Policy Assessment recognized that an annual standard intended to serve as 

the primary means for providing protection against effects associated with both long- and 

short-term PM2.5 exposures cannot be expected to offer an adequate margin of safety 

against the effects of all short-term PM2.5 exposures. As a result, in conjunction with a 

generally controlling annual standard, the Policy Assessment concluded it was 

appropriate to consider setting a 24-hour standard to provide supplemental protection, 

particularly for areas with high peak-to-mean ratios possibly associated with strong local 

or seasonal sources, or PM2.5-related effects that may be associated with shorter-thandaily exposure periods.

At the time of the proposal, the Administrator agreed with the approach discussed 

in the Policy Assessment as summarized in section III.A.3 above, and supported by 

CASAC, of considering the protection afforded by the annual and 24-hour standards 

taken together for mortality and morbidity effects associated with both long- and shortterm exposures to PM2.5. Furthermore, based on the evidence and quantitative risk 

assessment, the Administrator provisionally concluded it was appropriate to set a 

“generally controlling” annual standard that will lower a wide range of ambient 24-hour 

concentrations, with a 24-hour standard focused on providing supplemental protection, 

particularly for areas with high peak-to-mean ratios possibly associated with strong local 
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or seasonal sources, or PM2.5-related effects that may be associated with shorter-than 

daily exposure periods. The Administrator provisionally concluded this approach would 

likely reduce aggregate risks associated with both long- and short-term exposures more 

consistently than a generally controlling 24-hour standard and would be the most 

effective and efficient way to reduce total PM2-5-related population risk.

The Administrator is mindful that considering what standards are requisite to 

protect public health with an adequate margin of safety requires public health policy 

judgments that neither overstate nor understate the strength and limitations of the 

evidence or the appropriate inferences to be drawn from the evidence. At the time of the 

proposal, in considering how to translate the available information into appropriate 

standard levels, the Administrator weighed the available scientific information and 

associated uncertainties and limitations. For the purpose of determining what standard 

levels were appropriate to propose, the Administrator recognized, as did the EPA staff in 

the Policy Assessment, that there was no single factor or criterion that comprised the 

“correct” approach to weighing the various types of available evidence and information, 

but rather there were various approaches that were appropriate to consider. The 

Administrator further recognized that different evaluations of the evidence and other 

information before the Administrator could reflect placing different weight on the relative 

strengths and limitations of the scientific information, and different judgments could be 

made as to how such information should appropriately be used in making public health 

policy decisions on standard levels. This recognition led the Administrator to consider 

various approaches to weighing the evidence so as to identify appropriate standard levels 

to propose. In so doing, the Administrator encouraged extensive public comment on 
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alternative approaches to weighing the evidence and other information so as to inform her 

public health policy judgments before reaching final decisions on appropriate standard 

levels.

b. Proposed Decisions on Standard Levels

i. Consideration of the Alternative Standard Levels in the Policy Assessment

In recognizing the absence of a discernible population threshold below which 

effects would not occur, the Policy Assessment’s general approach for identifying 

alternative annual standard levels that were appropriate to consider focused on 

characterizing the part of the distribution of PM2.5 concentrations in which we had the 

most confidence in the associations reported in the epidemiological studies and 

conversely where our confidence in the association became appreciably lower. The most 

direct approach to address this issue, consistent with CASAC advice (Samet, 2010c, 

p.10), was to consider epidemiological studies reporting confidence intervals around 

concentration-response relationships (U.S. EPA, 2011a, p. 2-63). Based on a thorough 

search of the available evidence, the Policy Assessment identified only one study 

(Schwartz et al., 2008) that conducted a multi-model analysis to characterize confidence 

intervals around the estimated concentration-response relationship. The Policy 

Assessment concluded that this single relevant analysis was too limited to serve as the 

principal basis for identifying alternative standard levels in this review (U.S. EPA, 2011a, 

p. 2-70).

The Policy Assessment explored other approaches that considered different 

statistical metrics to identify the part of the distributions of long-term mean PM2.5

concentrations that were most influential in generating health effect estimates in long-
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and short-term epidemiological studies, and placed greatest weight on those studies that 

reported positive and statistically significant associations (U.S. EPA, 2011a, p. 2-63). 

First, as discussed in section III.A.3 above, the Policy Assessment considered the 

statistical metric used in previous reviews. This approach recognized that the strongest 

evidence of associations occurs at concentrations around the long-term mean 

concentration. Thus, in earlier reviews, the EPA focused on identifying standard levels 

that were somewhat below the long-term mean concentrations reported in PM2.5 exposure 

studies. The long-term mean concentrations represented air quality data typically used in 

epidemiological analyses and provided a direct link between PM2.5 concentrations and the 

observed health effects. Further, these data were available for all long- and short-term 

exposure studies analyzed and, therefore, represented the data set available for the 

broadest set of epidemiological studies. 

However, consistent with CASAC’s comments on the second draft Policy 

Assessment66 (Samet, 2010d, p. 2), in preparing the final Policy Assessment, the EPA 

staff explored ways to take into account additional information from epidemiological 

studies, when available (Rajan et al., 2011). These analyses focused on evaluating 

different statistical metrics, beyond the long-term mean concentration, to characterize the 

part of the distribution of PM2.5 concentrations in which staff continued to have 

confidence in the associations observed in epidemiological studies and below which there 

was a comparative lack of data such that the staff’s confidence in the relationship was 

appreciably less. This would also be the part of the distribution of PM2.5 concentrations 

 

66 While CASAC expressed the view that it would be most desirable to have information 

on concentration-response relationships, they recognized that it would also be “preferable 

to have information on the concentrations that were most influential in generating the 

health effect estimates in individual studies” (Samet, 2010d, p. 2). 
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which had the most influence on generating the health effect estimates reported in 

epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment 

recognized there was no one percentile value within a given distribution that was the 

most appropriate or “correct” way to characterize where our confidence in the 

associations becomes appreciably lower. The Policy Assessment concluded that focusing 

on concentrations within the lower quartile of a distribution, such as the range from the 

25th to the 10th percentile, was reasonable to consider as a region within which we begin 

to have appreciably less confidence in the associations observed in epidemiological 

studies.67 In the EPA staff’s view, considering lower PM2.5 concentrations, down to the 

lowest concentration observed in a study, would be a highly uncertain basis for selecting 

alternative standard levels (U.S. EPA, 2009a, p. 2-71).

As outlined in section III.A.3 above, the Policy Assessment recognized that there 

were two types of population-level information to consider in identifying the range of 

PM2.5 concentrations which have the most influence on generating the health effect 

estimates reported in epidemiological studies. The most relevant information to consider 

was the number of health events (e.g., deaths, hospitalizations) occurring within a study 

population in relation to the distribution of PM2.5 concentrations likely experienced by 

 

67 In the last review, staff believed it was appropriate to consider a level for an annual 

PM2.5 standard that was somewhat below the averages of the long-term concentrations 

across the cities in each of the key long-term exposures studies, recognizing that the 

evidence of an association in any such study was strongest at and around the long-term 

average where the data in the study are most concentrated. For example, the interquartile 

range of long-term average concentrations within a study and a range within one standard 

deviation around the study mean were considered reasonable approaches for 

characterizing the range over which the evidence of association is strongest (U.S. EPA, 

2005, pp. 5-22 to 5-23). In this review, the Policy Assessment noted the interrelatedness 

of the distributional statistics and a range of one standard deviation around the mean 

which contains approximately 68 percent of normally distributed data, in that one 

standard deviation below the mean falls between the 25th and 10th percentiles (U.S. EPA, 

2011a, p. 2-71). 
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study participants. However, in recognizing that access to health event data may be 

restricted, and consistent with advice from CASAC (Samet 2010d, p.2), EPA staff also 

considered the number of participants within each study area, in relation to the 

distribution of PM2.5 concentrations (i.e., study population data), as a surrogate for health 

event data. 

In applying this approach, the Policy Assessment focused on identifying the part 

of the distribution of PM2.5 concentrations which had the most influence on generating 

health effect estimates in epidemiological studies, as discussed in section III.A.3 above. 

As discussed below, in working with study investigators, the EPA staff was able to obtain 

health event data for three large multi-city studies (Krewski et al., 2009; Zanobetti and 

Schwartz, 2009; Bell et al., 2008) and population data for the same three studies and one 

additional long-term exposure study (Miller et al., 2007), as documented in a staff 

memorandum (Rajan et al., 2011).68 For the three studies for which both health event and 

study population data were available, the EPA staff analyzed the reliability of using study 

population data as a surrogate for health event data. Based on these analyses, the EPA 

staff recognized that the 10th and 25th percentiles of the health event and study population 

distributions are nearly identical and concluded that the distribution of population data 

can be a useful surrogate for event data, providing support for consideration of the study 

population data for Miller et al. (2007), for which health event data were not available 

(Rajan et al., 2011, Analysis 1 and Analysis 2, in particular, Table 1 and Figures 1 and 2).

With regard to the long-term mean PM2.5 concentrations which are relevant to the 

 

68 The distributional statistical analysis of population-level data built upon an earlier 

analysis that evaluated the distributions of air quality and associated population data for 

three long-term exposure studies and three short-term exposure studies (Schmidt et al., 

2010, Analysis 2). 
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first approach, Figures 1 through 3 (U.S. EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) 

summarize data available for multi-city, long- and short-term exposure studies that 

evaluated endpoints classified in the Integrated Science Assessment as having evidence 

of a causal or likely causal relationship or evidence suggestive of a causal relationship, 

showing the studies with long-term mean PM2.5 concentrations below 17 μg/m3.

69 As 

discussed in more detail in section III.E.4.b of the proposal, Figures 1 and 3 summarize 

the health outcomes evaluated, relative risk estimates, air quality data, and geographic 

scope for long- and short-term exposure studies, respectively, that evaluated mortality 

(evidence of a causal relationship); cardiovascular effects (evidence of a causal 

relationship); and respiratory effects (evidence of a likely causal relationship) in the 

general population, as well as in older adults, an at-risk population. Figure 2 provides this 

same summary information for long-term exposure studies that evaluated respiratory 

effects (evidence of a likely causal relationship) in children, an at-risk population, as well 

as developmental effects (evidence suggestive of a causal relationship). 

 

69 Additional studies presented and assessed in the Integrated Science Assessment report 

effects at higher long-term mean PM2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2-

1, 2-2, 7-6, and 7-7).
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Figure 1. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Long-term PM2.5 Exposure 

Studies of the General Population and Older Adults70, 71, 72

 

70 The air quality data for the Miller et al. (2007) study is discussed more fully in the Response to Comments document.

71 The Goss et al. (2004) study patients over the age of 6 years enrolled in the Cystic Fibrosis Foundation National Patient Registry in 1999 

and 2000. The average age of the study participants was 18.4 (SD 10).

72 The air quality data reported in Figure 1 of the proposal incorrectly identified the long-term mean PM2.5 concentration for this study as 14.3 μg/m3

, which was the longterm mean PM2.5 concentration for Lipfert et al. (2006b). This mistake was also made in the Figure 2-2 of the ISA (U.S. EPA, 2009a, p. 2-15). The EPA notes that 

footnote a for Figure 4 of Lipfert et al. (2006a) identifies that the mortality risks for PM2.5 are based on one year of air quality data (1999). However, this study does not 

report the long-term mean PM2.5 concentration for 1999. Rather, in Table 1 of Lipfert et al. (2006a), the study authors reported long-term mean PM2.5 concentrations of 

14.3 μg/m3for the years 1989 to 1996 and 14.6 μg/m3for the years 1997 to 2001.
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Formatted: Font: 12 pt
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Source: U.S. EPA, 2011a, Figure 2-4

Formatted: Font: 12 pt
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Figure 2. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Long-term PM2.5 Exposure 

Studies of Children

Source: U.S. EPA, 2011a, Figure 2-5
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Figure 3. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Short-term PM2.5 Exposure 

Studies of the General Population and Older Adults

Source: U.S. EPA, 2011a, Figure 2-6
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With regard to consideration of additional information from epidemiological studies which was 

relevant to the second approach, the EPA staff compiled a summary of the range of PM2.5 concentrations 

corresponding with the 25th to 10th percentiles of health event or study population data from the four 

multi-city studies, for which distributional statistics are available73 (U.S. EPA, 2011a, Figure 2-7; Rajan 

et al., 2011, Table 1). By considering this approach, one could focus on the range of PM2.5

concentrations below the long-term mean ambient concentrations over which we continue to have 

confidence in the associations observed in epidemiological studies (e.g., above the 25th percentile) where 

commensurate public health protection could be obtained for PM2.5-related effects and, conversely, 

identify the range in the distribution below which our confidence in the associations is appreciably less, 

to identify alternative annual standard levels. 

The mean PM2.5 concentrations associated with the studies summarized in Figures 1, 2, and 3 

and with the distributional statistics analyses (Rajan et al., 2011) are based on concentrations averaged 

across ambient monitors within each area included in a given study and then averaged across study areas

to calculate an overall study mean concentration, as discussed above. Figure 4, discussed in more detail 

in section III.E.4.a of the proposal, summarizes statistical metrics for those key studies74 included in

 

73 The EPA staff obtained health event data (e.g., number of deaths, hospitalizations) occurring in a 

study population for three multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et 

al., 2008) and study population data were obtained for the same three studies and one additional study 

(Miller et al., 2007) (U.S. EPA, 2011a, p.2-71). If health event or study population data were available 

for additional studies, the EPA could employ distributional statistics to identify the broader range of 

PM2.5 concentrations that were most influential in generating health effect estimates in those studies. 

74 Long- and short-term exposure studies considered “key” studies for consideration are summarized in 

Figure 4 and include those studies observing effects for which the evidence supported a causal or likely 

causal association. This figure represents the subset of multi-city studies included in Figures 1 through 3 

that provided evidence of positive and generally statistically significant effects associated in whole or in 

part with more recent air quality data, generally representing health effects associated with lower PM2.5

concentrations than had previously been considered in the last review. The EPA notes that many of these 

studies evaluated multiple health endpoints, and not all of the effects evaluated provided evidence of 

positive and statistically significant effects. For purposes of informing the Administrator’s decision on 

the appropriate standard levels, the Agency considers the full body of scientific evidence and focuses on 

those aspects of the key studies that provided evidence of positive and generally statistically significant 

effects. 
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Figure 4. Translating Epidemiological Evidence from Multi-City Exposure Studies into an Annual PM2.5 Standard75

Source: U.S. EPA, 2011a, Figure 2-

 

75 The air quality data for the Miller et al. (2007) study is discussed more fully in the Response to Comments document.
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Figures 1, 2, and 3 that provide evidence of positive and generally statistically significant 

PM2.5-related effects, which are relevant to the two approaches for translating 

epidemiological evidence into potential standard levels as discussed above. The top of 

Figure 4 includes information for long-term exposure studies evaluating health outcomes 

classified as having evidence of a causal or likely causal relationship with PM2.5

exposures (long-term mean PM2.5 concentrations indicated by diamond symbols). The 

middle of Figure 4 includes information for short-term exposure studies evaluating health 

outcomes classified as having evidence of a causal or likely causal relationship with 

PM2.5 exposures (long-term mean PM2.5 concentrations indicated by triangle symbols). 

The bottom of Figure 4 includes information for long-term exposures studies evaluating 

health outcomes classified as having evidence suggestive of a causal relationship (longterm mean PM2.5 concentrations indicated by square symbols). Figure 4 also summarizes 

the range of PM2.5 concentrations corresponding with the 25th (indicated by solid circles) 

to 10th (indicated by open circles) percentiles of the health event or study population data 

from the four multi-city studies (highlighted in bold text) for which distributional 

statistics are available.

In considering the evidence, the Policy Assessment recognized that NAAQS are 

standards set so as to provide requisite protection, neither more nor less stringent than 

necessary to protect public health, with an adequate margin of safety. This judgment, 

ultimately made by the Administrator, involves weighing the strength of the evidence and 

the inherent uncertainties and limitations of that evidence. Therefore, depending on the 

weight placed on different aspects of the evidence and inherent uncertainties, 

consideration of different alternative standard levels could be supported. 
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Given the currently available evidence discussed in more detail in section 

III.E.4.b of the proposal and considering the various approaches discussed above, the 

Policy Assessment concluded it was appropriate to focus on an annual standard level 

within a range of about 12 to 11 μg/m3(U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-106). 

As illustrated in Figure 4, the Policy Assessment recognized that a standard level of 12 

μg/m3, at the upper end of this range, was somewhat below the long-term mean PM2.5

concentrations reported in all the multi-city, long- and short-term exposure studies that 

provided evidence of positive and statistically significant associations with health effects 

classified as having evidence of a causal or likely causal relationship, including 

premature mortality and hospitalizations and emergency department visits for 

cardiovascular and respiratory effects as well as respiratory effects in children. Further, a 

level of 12 μg/m

3 would reflect consideration of additional population-level information 

from such epidemiological studies in that it generally corresponded with approximately 

the 25th percentile of the available distributions of health events data in the studies for 

which population-level information was available. In addition, a level of 12 μg/m3 would 

reflect some consideration of studies that provided more limited evidence of reproductive 

and developmental effects, which were suggestive of a causal relationship, in that it was 

about at the same level as the lowest long-term mean PM2.5 concentrations reported in 

such studies (see Figure 4). 

Alternatively the Policy Assessment recognized that an annual standard level of 

11 μg/m3, at the lower end of this range, was well below the lowest long-term mean 

PM2.5 concentrations reported in all multi-city long- and short-term exposure studies that 

provide evidence of positive and statistically significant associations with health effects 
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classified as having evidence of a causal or likely causal relationship. A level of 11 μg/m3

would reflect placing more weight on the distributions of health event and population 

data, in that this level was within the range of PM2.5 concentrations corresponding to the 

25th and 10th percentiles of all the available distributions of such data. In addition, a level 

of 11 μg/m3 was somewhat below the lowest long-term mean PM2.5 concentrations 

reported in reproductive and developmental effects studies that are suggestive of a causal 

relationship. Thus, a level of 11 μg/m3 would reflect an approach to translating the 

available evidence that places relatively more emphasis on margin of safety 

considerations and less certain causal relationships than would a standard set at a higher 

level. Such a policy approach would tend to weigh uncertainties in the evidence in such a 

way as to avoid potentially underestimating PM2.5-related risks to public health. Further, 

recognizing the uncertainties inherent in identifying any particular point at which our 

confidence in reported associations becomes appreciably less, the Policy Assessment 

concluded that the available evidence did not provide a sufficient basis to consider 

alternative annual standard levels below 11 μg/m3(U.S. EPA, 2011a, p. 2-81). 

The Policy Assessment also considered the extent to which the available evidence 

provided a basis for considering alternative annual standard levels above 12 μg/m3. As 

discussed below, the Policy Assessment concluded that it could be reasonable to consider 

a standard level up to 13 μg/m3based on a policy approach that weighed uncertainties in 

the evidence in such a way as to avoid potentially overestimating PM2.5-related risks to 

public health, especially to the extent that primary emphasis was placed on long-term 

exposure studies as a basis for an annual standard level. A level of 13 μg/m3 was 

somewhat below the long-term mean PM2.5 concentrations reported in all but one of the 
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long-term exposure studies providing evidence of positive and statistically significant 

associations with PM2.5-related health effects classified as having a causal or likely causal 

relationship. As shown in Figure 4, the one long-term exposure study with a long-term 

mean PM2.5 concentration just below 13 μg/m3 was the Miller et al.,(., (2007) study. 

However, as noted in section III.D.1.a of the proposal and discussed in more detail in the 

Response to Comments document, the Policy Assessment observed that in comparison to 

other long-term exposure studies, the Miller et al. study was more limited in that it was 

based on only one year of air quality data and the one year was after the health outcomes 

were reported (U.S. EPA, 2011a, pp. 2-81 to 2-82). Thus, to the extent that less weight 

was placed on the Miller et al. study than on other long-term exposure studies with more 

robust air quality data, a level of 13 μg/m3could be considered as being protective of 

long-term exposure related effects classified as having a causal or likely causal 

relationship. In also considering short-term exposure studies, however, the Policy 

Assessment noted that a level of 13 μg/m3 was below the long-term mean PM2.5

concentrations reported in most but not all such studies. In particular, two studies --

Burnett et al. (2004) and Bell et al. (2008) -- reported long-term mean PM2.5

concentrations of 12.8 and 12.9 μg/m3, respectively. In considering these studies, the 

Policy Assessment found no basis to conclude that these two studies were any more 

limited or uncertain than the other short-term exposure studies shown in Figures 3 and 4 

(U.S. EPA, 2011a, p. 2-82). On this basis, as discussed below, the Policy Assessment 

concluded that consideration of an annual standard level of 13 μg/m3 would have 

implications for the degree of protection that would need to be provided by the 24-hour 

standard, in order that the suite of PM2.5 standards, taken together, would provide 
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appropriate protection from effects on public health related to short-term exposure to 

PM2.5 (U.S. EPA, 2011a, p. 2-82).

The Policy Assessment also noted that a standard level of 13 μg/m3 would reflect 

a judgment that the uncertainties in the epidemiological evidence as summarized in 

section III.B above and discussed in more detail in section III.B.2 of the proposal, 

including uncertainties related to the heterogeneity observed in the epidemiological 

studies in the eastern versus western parts of the U.S., the relative toxicity of PM2.5

components, and the potential role of co-pollutants, are too great to warrant placing any 

weight on the distributions of health event and population data that extend down below 

the long-term mean concentrations into the lower quartile of the data. This level would

also reflect a judgment that the evidence from reproductive and developmental effects 

studies that is suggestive of a causal relationship was too uncertain to support 

consideration of any lower level.

Beyond evidence-based considerations, the Policy Assessment also considered the 

extent to which the quantitative risk assessment supported consideration of these 

alternative standard levels or provided support for lower levels. In considering 

simulations of just meeting alternative annual standard levels within the range of 13 to 11 

μg/m3(in conjunction with the current 24-hour standard level of 35 μg/m3), the Policy 

Assessment concluded that important public health improvements are associated with risk 

reductions estimated for standard levels of 13 and 12 μg/m3and noted that the level of 11 

μg/m3 was not included in the quantitative risk assessment. The Policy Assessment noted 

that the overall confidence in the quantitative risk estimates varied for the different 

alternative standard levels evaluated and was stronger for the higher levels and 
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substantially lower for the lowest level evaluated (i.e., 10 μg/m3). Based on the above 

considerations, the Policy Assessment concluded that the quantitative risk assessment 

provided support for considering alternative annual standard levels within a range of 13 

to 11 μg/m3, but did not provide strong support for considering lower alternative standard 

levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that consideration of alternative 

annual standard levels in the range of 13 to 11 μg/m3 may be appropriate. Furthermore, 

the Policy Assessment concluded that the currently available evidence most strongly 

supported consideration of an alternative annual standard level in the range of 12 to 11 

μg/m3(U.S. EPA, 2011a, p. 2-82). The Policy Assessment concluded that an alternative 

level within the range of 12 to 11 μg/m3 would more fully take into consideration the 

available information from all long- and short-term PM2.5 exposure studies, including 

studies of at-risk populations, than would a higher level. This range also reflected placing 

weight on information from studies that helped to characterize the range of PM2.5

concentrations over which we continue to have confidence in the associations observed in 

epidemiological studies, as well as the extent to which our confidence in the associations 

was appreciably less at lower concentrations. 

As recognized in sections III.A.3 and III.E.4.a above, an annual standard intended 

to serve as the primary means for providing protection from effects associated with both 

long- and short-term PM2.5 exposures is not expected to provide appropriate protection 

against the effects of all short-term PM2.5 exposures (unless established at a level so low 

as to undoubtedly provide more protection than necessary for long-term exposures). Of 

particular concern are areas with high peak-to-mean ratios possibly associated with 
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strong local or seasonal sources, or PM2.5-related effects that may be associated with 

shorter-than-daily exposure periods. As a result, the Policy Assessment concluded that it 

was appropriate to consider alternative 24-hour PM2.5 standard levels that would 

supplement the protection provided by an annual standard. 

As outlined in section III.A.3 above, the Policy Assessment considered the 

available evidence from short-term PM2.5 exposure studies, as well as the uncertainties 

and limitations in that evidence, to assess the degree to which alternative annual and 24-

hour PM2.5 standards can be expected to reduce the estimated risks attributed to shortterm fine particle exposures. In considering the available epidemiological evidence, the 

Policy Assessment took into account information from multi-city studies as well as 

single-city studies. The Policy Assessment considered the distributions of 24-hour PM2.5

concentrations reported in short-term exposure studies, focusing on the 98th percentile 

concentrations to match the form of the 24-hour standard as discussed in section III.E.3.b 

above. In recognizing that the annual and 24-hour standards work together to provide 

protection from effects associated with short-term PM2.5 exposures, the Policy 

Assessment also considered information on the long-term mean PM2.5 concentrations 

from these studies. 

In addition to considering the epidemiological evidence, the Policy Assessment 

considered air quality information, specifically peak-to-mean ratios using county-level 

24-hour and annual design values, to characterize air quality patterns in areas possibly 

associated with strong local or seasonal sources. These patterns helped in understanding 

the extent to which different combinations of annual and 24-hour standards would be 

consistent with the policy goal of setting a generally controlling annual standard with a 
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24-hour standard that provides supplemental protection especially for areas with high 

peak-to-mean ratios (U.S. EPA, 2011a, p. 2-14). 

In considering the information provided by the short-term exposure studies, the 

Policy Assessment recognized that to the extent these studies were conducted in areas 

that likely did not meet one or both of the current standards, such studies did not help 

inform the characterization of the potential public health improvements of alternative 

standards set at lower levels. Therefore, in considering the short-term exposure studies to 

inform staff conclusions regarding levels of the 24-hour standard that are appropriate to 

consider, the Policy Assessment placed greatest weight on studies conducted in areas that 

likely met both the current annual and 24-hour standards.

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, as summarized in Figure 3 above and discussed in more detail in section 

III.E.4.c of the proposal, the Policy Assessment noted that, to the extent air quality 

distributions were reduced to reflect just meeting the current 24-hour standard, additional 

protection would be anticipated for the effects observed in the three multi-city studies 

with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett and 

Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98th percentile 

values below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 

μg/m3, the Policy Assessment noted that these studies reported long-term mean PM2.5

concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti 

and Schwartz, 2009; Dominici et al., 2006a). To the extent that consideration was given 

to revising the level of the annual standard, as discussed in section III.E.4.b of the 

proposal, the Policy Assessment recognized that potential changes associated with 
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meeting such an alternative annual standard would result in lowering risks associated 

with both long- and short-term PM2.5 exposures. Consequently, in considering a 24-hour 

standard that would operate in conjunction with an annual standard to provide appropriate 

public health protection, the Policy Assessment noted that to the extent that the level of

the annual standard was revised to within a range of 13 to 11 μg/m3, in particular in the 

range of 12 to 11 μg/m

3

, additional protection would be provided for the long-term 

effects observed in these multi-city studies (U.S. EPA, 2011a, p. 2-84). 

Based on this information, the Policy Assessment concluded that the multi-city, 

short-term exposure studies generally provided support for retaining the 24-hour standard 

level at 35 μg/m3so long as the standard is in conjunction with an annual standard level 

revised to within a range of 12 to 11 μg/m3(U.S. EPA, 2011a, p. 2-84). Alternatively, in 

conjunction with an annual standard level of 13 μg/m3, the Policy Assessment concluded 

that the multi-city studies provided limited support for revising the 24-hour standard level 

somewhat below 35 μg/m3, such as down to 30 μg/m3, based on one study (Bell et al., 

2008) that reported positive and statistically significant effects with an overall 98th

percentile value below the level of the current 24-hour standard and an overall long-term 

mean concentration slightly less than 13 μg/m3(Figure 3; U.S. EPA, 2011a, p. 2-84). 

In reaching staff conclusions regarding alternative 24-hour standard levels that 

were appropriate to consider, the Policy Assessment also took into account relevant 

information from single-city studies that evaluated effects associated with short-term 

PM2.5 exposures. The Policy Assessment recognized that these studies may provide 

additional insights regarding impacts on at-risk populations and/or on areas with isolated 

peak concentrations. 
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As discussed in more detail in section III.E.4.c of the proposal, although a number 

of single-city studies reported effects at appreciably lower PM2.5 concentrations than 

multi-city short-term exposure studies, the uncertainties and limitations associated with 

the single-city studies were considerably greater than those associated with the multi-city 

studies and, thus, the Policy Assessment concluded there was less confidence in using 

these studies as a basis for setting the level of a standard. Therefore, the Policy 

Assessment concluded that the multi-city short-term exposure studies provided the 

strongest evidence to inform decisions on the level of the 24-hour standard, and the 

single-city studies did not warrant consideration of 24-hour standard levels different from 

those supported by the multi-city studies (U.S. EPA, 2011a, p. 2-88). 

In addition to considering the epidemiological evidence, the Policy Assessment 

took into account air quality information based on county-level 24-hour and annual 

design values to understand the public health implications of the alternative standard 

levels supported by the currently available scientific evidence, as discussed in section 

III.E.4.b above. Consistent with the general approach discussed in section III.A.3 above, 

the Policy Assessment considered the extent to which different combinations of 

alternative annual and 24-hour standard levels based on the evidence would support the 

policy goal of lowering annual and 24-hour air quality distributions by using the annual 

standard to be the “generally controlling” standard in conjunction with setting the 24-

hour standard to provide supplemental protection (U.S. EPA, 2011a, pp 2-88 to 2-91, 

Figure 2-10). 

Using information on the relationship of the 24-hour and annual design values, 

the Policy Assessment examined the implications of three alternative suites of PM2.5
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standards identified as appropriate to consider based on the currently available scientific 

evidence, as discussed above. The Policy Assessment concluded that an alternative suite 

of PM2.5 standards that would include an annual standard level of 11 or 12 μg/m3and a 

24-hour standard with a level of 35 μg/m3(i.e., 11/35 or 12/35) would result in the annual 

standard being the generally controlling standard in most areas although the 24-hour 

standard would continue to be the generally controlling standard in the Northwest (U.S.

EPA, 2011a, pp. 2-89 to 2-91 and Figure 2-10). These Northwest counties generally 

represented areas where the annual mean PM2.5 concentrations have historically been low 

but where relatively high 24-hour concentrations occur, often related to seasonal wood 

smoke emissions. Alternatively, combining an alternative annual standard of 13 μg/m3

with a 24-hour standard of 30 μg/m3 would result in many more areas across the country 

in which the 24-hour standard would likely become the controlling standard (the standard 

driving air quality distributions lower) than if an alternative annual standard of 12 or 11 

μg/m3 were paired with the current level of the 24-hour standard (i.e., 35 μg/m3). 

The Policy Assessment concluded that consideration of retaining the 24-hour 

standard level at 35 μg/m3 would reflect placing greatest weight on evidence from multicity studies that reported positive and statistically significant associations with health 

effects classified as having a causal or likely causal relationship. In conjunction with 

lowering the annual standard level, especially within a range of 12 to 11 μg/m3, this 

alternative recognized additional public health protection against effects associated with 

short-term PM2.5 exposures which would be provided by lowering the annual standard 

such that revision to the 24-hour standard would not be warranted (U.S. EPA, 2011a, p. 

2-91). 
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Beyond evidence-based considerations, the Policy Assessment also considered the 

extent to which the quantitative risk assessment supported consideration of retaining the 

current 24-hour standard level or provided support for lower standard levels. In 

considering simulations of just meeting the current 24-hour standard level of 35 μg/m3or 

alternative levels of 30 or 25 μg/m3(in conjunction with alternative annual standard 

levels within a range of 13 to 11 μg/m3), the Policy Assessment noted that the overall 

confidence in the quantitative risk estimates varied for the different standard levels 

evaluated and was stronger for the higher levels and substantially lower for the lowest 

level evaluated (i.e., 25 μg/m3). Based on this information, the Policy Assessment 

concluded that the quantitative risk assessment provided support for considering a 24-

hour standard level of 35 or 30 μg/m3(in conjunction with an alternative standard level 

within a range of 13 to 11 μg/m3) but did not provide strong support for considering 

lower alternative 24-hour standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that while it was appropriate to 

consider an alternative 24-hour standard level within a range of 35 to 30 μg/m3, the 

currently available evidence most strongly supported consideration for retaining the 

current 24-hour standard level at 35 μg/m3in conjunction with lowering the level of the 

annual standard within a range of 12 to 11 μg/m3(U.S. EPA, 2011a, p. 2-92).

ii. CASAC Advice

Based on its review of the second draft Policy Assessment, CASAC agreed with 

the general approach for translating the available epidemiological evidence, risk 

information, and air quality information into the basis for reaching conclusions on 

alternative standards for consideration. Furthermore, CASAC agreed “that it is 
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appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have 

evidence supportive of a causal or likely causal relationship as presented in the Integrated 

Science Assessment (Samet, 2010d, p. 1). 

CASAC concluded that the range of levels presented in the second draft Policy 

Assessment (i.e., alternative annual standard levels within a range of 13 to 11 μg/m3and 

alternative 24-hour standard levels within a range of 35 to 30 μg/m3) “are supported by 

the epidemiological and toxicological evidence, as well as by the risk and air quality 

information compiled” in the Integrated Science Assessment, Risk Assessment, and 

second draft Policy Assessment. CASAC further noted that “[a]lthough there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level 

below which there is no risk for adverse health effects)” (Samet, 2010d, p. ii). 

Although CASAC supported the alternative standard level ranges presented in the 

second draft Policy Assessment, it did not express support for any specific levels or 

combinations of standards. Rather, CASAC encouraged the EPA to develop a clearer 

rationale in the final Policy Assessment for staff conclusions regarding annual and 24-

hour standards that were appropriate to consider, including consideration of the 

combination of these standards supported by the available information (Samet, 2010d, p. 

ii). Specifically, in commenting on a distributional statistical analysis of air quality and 

associated population data presented in the second draft Policy Assessment, CASAC 

encouraged staff to focus on information related to the concentrations that were most 
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influential in generating the health effect estimates in individual studies to inform 

alternative standard levels. CASAC urged that the EPA redo that analysis using health 

event or study population data (Samet, 2010d, p. 2). CASAC also commented that the 

approach presented in the second draft Policy Assessment to identify alternative 24-hour 

standard levels which focused on peak-to-mean ratios was not relevant for informing the 

actual level (Samet 2010d, p. 4). Further, they expressed the concern that the 

combinations of annual and 24-hour standard levels discussed in the second draft Policy 

Assessment (i.e., in the range of 13 to 11 μg/m3for the annual standard, in conjunction 

with retaining the current 24-hour PM2.5 standard level of 35 μg/m3; alternatively, 

revising the level of the 24-hour standard to 30 μg/m3in conjunction with an annual 

standard level of 11 μg/m3) “may not be adequately inclusive” and “[i]t was not clear 

why, for example a daily standard of 30 μg/m3should only be considered in combination 

with an annual level of 11 μg/m3” (Samet, 2010d, p. ii). CASAC encouraged the EPA to 

more clearly explain its rationale for identifying the 24-hour/annual combinations that are 

appropriate for consideration (Samet 2010d, p. ii). 

In considering CASAC’s advice as well as public comment on the second draft 

Policy Assessment, the EPA staff conducted additional analyses and modified their 

conclusions regarding alternative standard levels that were appropriate to consider. The 

staff conclusions in the final Policy Assessment (U.S. EPA, 2011a, section 2.3.4.4) 

differed somewhat from the alternative standard levels discussed in the second draft 

Policy Assessment (U.S. EPA, 2010f, section 2.3.4.3), upon which CASAC based its 

advice. Changes made in the final Policy Assessment were primarily focused on 

improving and clarifying the approach for translating the epidemiological evidence into a 
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basis for staff conclusions on the broadest range of alternative standard levels supported 

by the available scientific information and more clearly articulating the rationale for the 

staff’s conclusions (Wegman, 2011, pp. 1 to 2). Consistent with CASAC’s advice to 

consider more information from epidemiological studies, as discussed in section 

III.E.4.b.1 above, the EPA analyzed additional population-level data obtained from 

several study authors (Rajan et al., 2011). In transmitting the final Policy Assessment to 

CASAC, the Agency notified CASAC that the final staff conclusions reflected 

consideration of CASAC’s advice and that those staff conclusions were based, in part, on 

the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 

2011, p.2). Thus, CASAC had an opportunity to comment on the final Policy 

Assessment, but chose not to provide any additional comments or advice after receiving 

it. 

iii. Administrator’s Proposed Decisions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate alternative standard levels to 

consider, the Administrator considered the epidemiological and other scientific evidence, 

estimates of risk reductions associated with just meeting alternative annual and/or 24-

hour standards, air quality analyses, related limitations and uncertainties, staff 

conclusions as presented in the Policy Assessment, and the advice of CASAC. As an 

initial matter, the Administrator agreed with the general approach discussed in the Policy 

Assessment as summarized in sections III.A.3 and III.E.4.a above, and supported by 

CASAC, of considering the protection afforded by the annual and 24-hour standards 

taken together for mortality and morbidity effects associated with both long- and shortterm exposures to PM2.5 (77 FR 38939). Furthermore, based on the evidence and 
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quantitative risk assessment, the Administrator provisionally concluded it is appropriate 

to set a “generally controlling” annual standard that will lower a wide range of ambient 

24-hour concentrations, with a 24-hour standard focused on providing supplemental 

protection, particularly for areas with high peak-to-mean ratios possibly associated with 

strong local or seasonal sources, or PM2.5-related effects that may be associated with 

shorter-than daily exposure periods. The Administrator provisionally concluded this 

approach would likely reduce aggregate risks associated with both long- and short-term 

exposures more consistently than a generally controlling 24-hour standard and would be 

the most effective and efficient way to reduce total PM2-5-related population risk. Id.

In reaching decisions on alternative standard levels to propose, the Administrator 

judged that it was most appropriate to examine where the evidence of associations 

observed in the epidemiological studies was strongest and, conversely, where she had 

appreciably less confidence in the associations observed in the epidemiological studies. 

Based on the characterization and assessment of the epidemiological and other studies 

presented and assessed in the Integrated Science Assessment and the Policy Assessment, 

the Administrator recognized the substantial increase in the number and diversity of 

studies available in this review including extended analyses of the seminal studies of 

long-term PM2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important 

new long-term exposure studies (as summarized in Figures 1 and 2). Collectively, the 

Administrator noted that these studies, along with evidence available in the last review, 

provided consistent and stronger evidence of an association with premature mortality, 

with the strongest evidence related to cardiovascular-related mortality, at lower ambient 

concentrations than previously observed. The Administrator also recognized the 
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availability of stronger evidence of morbidity effects associated with long-term PM2.5

exposures, including evidence of cardiovascular effects from the WHI study and 

respiratory effects, including decreased lung function growth, from the extended analyses 

for the Southern California Children’s Health Study. Furthermore, the Administrator 

recognized new U.S. multi-city studies that greatly expanded and reinforced our 

understanding of mortality and morbidity effects associated with short-term PM2.5

exposures, providing stronger evidence of associations at ambient concentrations similar 

to those previously observed (as summarized in Figure 3). Id. at 38939-40.

The newly available scientific evidence built upon the previous scientific data 

base to provide evidence of generally robust associations and to provide a basis for 

greater confidence in the reported associations than in the last review. The Administrator 

recognized that the weight of evidence, as evaluated in the Integrated Science 

Assessment, was strongest for health endpoints classified as having evidence of a causal 

relationship. These relationships included those between long- and short-term PM2.5

exposures and mortality and cardiovascular effects. She recognized that the weight of 

evidence was also strong for health endpoints classified as having evidence of a likely 

causal relationship, which included those between long- and short-term PM2.5 exposures 

and respiratory effects. In addition, the Administrator made note of the much more 

limited evidence for health endpoints classified as having evidence suggestive of a causal 

relationship, including developmental, reproductive and carcinogenic effects. Id .at 

38940.

Based on information discussed and presented in the Integrated Science 

Assessment, the Administrator recognized that health effects may occur over the full 
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range of concentrations observed in the long- and short-term epidemiological studies and 

that no discernible threshold for any effects can be identified based on the currently 

available evidence (U.S. EPA, 2009a, section 2.4.3). She also recognized, in taking note 

of CASAC advice and the distributional statistics analysis discussed in section III.E.4.b.i 

above and in the Policy Assessment, that there was significantly greater confidence in 

observed associations over certain parts of the air quality distributions in the studies, and 

conversely, that there was significantly diminished confidence in ascribing effects to 

concentrations toward the lower part of the distributions.

Consistent with the general approach summarized in section III.A.3 above, and 

supported by CASAC as discussed in section III.E.4.a above, the Administrator generally 

agreed that it was appropriate to consider a level for an annual standard that was 

somewhat below the long-term mean PM2.5 concentrations reported in long- and shortterm exposure studies. In recognizing that the evidence of an association in any such 

study was strongest at and around the long-term average where the data in the study are 

most concentrated, she understood that this approach did not provide a bright line for 

reaching decisions about appropriate standard levels. The Administrator noted that longterm mean PM2.5 concentrations were available for each study considered and, therefore, 

represented the most robust data set to inform her decisions on appropriate annual 

standard levels. She also noted that the overall study mean PM2.5 concentrations were 

generally calculated based on monitored concentrations averaged across monitors in each 

study area with multiple monitors, referred to as a composite monitor concentration, in 

contrast to the highest concentration monitored in each study area, referred to as a 

maximum monitor concentration, which are used to determine whether an area meets a 
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given standard. In considering such long-term mean concentrations, the Administrator 

understood that it was appropriate to consider the weight of evidence for the health 

endpoints evaluated in such studies in giving weight to this information. Id.

Based on the information summarized in Figure 4 above and presented in more 

detail in the Policy Assessment (U.S. EPA, 2011a, chapter 2) for effects classified in the 

Integrated Science Assessment as having a causal or likely causal relationship with PM2.5

exposures, the Administrator observed an overall pattern of statistically significant 

associations reported in studies of long-term PM2.5 exposures with long-term mean 

concentrations ranging from somewhat above the current standard level of 15 μg/m3

down to the lowest mean concentration in such studies of 12.9 μg/m3(in Miller et al., 

2007).76 She observed a similar pattern of statistically significant associations in studies 

of short-term PM2.5 exposures with long-term mean concentrations ranging from around 

15 μg/m3down to 12.8 μg/m3(in Burnett et al., 2004). With regard to effects classified as 

providing evidence suggestive of a causal relationship, the Administrator observed a 

small number of long-term exposure studies related to developmental and reproductive 

effects that reported statistically significant associations with overall study mean PM2.5

 

76 The EPA notes that the Miller et al., (2007) study provides strong evidence of 

cardiovascular related effects associated with long-term PM2.5 exposures. At the time of 

the proposal, the EPA recognized the limited nature of the air quality data considered in 

this study (77 FR 38918, fn. 62). The EPA has reviewed those limitations, in conjunction 

with consideration of public comments received on the proposal as discussed in section 

III.E.4.c, in conjunction with reaching a final decision on the level of the annual standard. 
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concentrations down to 11.9 μg/m3(in Bell et al., 2007).77 Id.

The Administrator also considered additional information from epidemiological 

studies, consistent with CASAC advice, to take into account the broader distribution of 

PM2.5 concentrations and the degree of confidence in the observed associations over the 

broader air quality distribution. In considering this additional information, she understood 

that the Policy Assessment presented information on the 25th and 10th percentiles of the 

distributions of PM2.5 concentrations available from four multi-city studies to provide a 

general frame of reference as to the part of the distribution in which the data become 

appreciably more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies would become appreciably less. 

As summarized in Figure 4 above, the Administrator took note of additional 

population-level data that were available for four studies (Krewski et al., 2009; Miller et 

al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009), each of which reported 

statistically significant associations with health endpoints classified as having evidence of 

a causal relationship. In considering the long-term PM2.5 concentrations associated with 

the 25th percentile values of the population-level data for these four studies, she observed 

that these values ranged from somewhat above to somewhat below 12 μg/m3. The 

Administrator recognized that these four studies represented some of the strongest 

evidence available within the overall body of scientific evidence and noted that three of 

these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) were 

 

77 With respect to suggestive evidence related to cancer, mutagenic, and genotoxic 

effects, the PM2.5 concentrations reported in studies generally included ambient 

concentrations that are equal to or greater than ambient concentrations observed in 

studies that reported mortality and cardiovascular and respiratory effects (U.S. EPA, 

2009a, section 7.5), such that in selecting alternative standard levels that provide 

protection from mortality and cardiovascular and respiratory effects, it is reasonable to 

anticipate that protection will also be provided for carcinogenic effects.
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used as the basis for concentration-response functions used in the quantitative risk 

assessment (U.S. EPA, 2010a, section 3.3.3). 

In considering this information, the Administrator noted that CASAC advised that 

information about the long-term PM2.5 concentrations that were most influential in 

generating the health effect estimates in epidemiological studies can help to inform 

selection of an appropriate annual standard level. However, the Administrator also 

recognized that additional population-level data were available for only these four studies 

and, therefore, she believed that these studies comprised a more limited data set than one 

based on long-term mean PM2.5 concentrations for which data were available for all 

studies considered, as discussed above. 

The Administrator recognized, as summarized in section III.B.2 above, that 

important uncertainties remain in the evidence and information considered in this review 

of the primary fine particle standards. These uncertainties are generally related to 

understanding the relative toxicity of the different components in the fine particle 

mixture, the role of PM2.5 in the complex ambient mixture, exposure measurement errors 

inherent in epidemiological studies based on concentrations measured at fixed monitor 

sites, and the nature, magnitude, and confidence in estimated risks related to increasingly 

lower ambient PM2.5 concentrations. Furthermore, the Administrator noted that 

epidemiological studies have reported heterogeneity in responses both within and 

between cities and geographic regions across the U.S. She recognized that this 

heterogeneity may be attributed, in part, to differences in fine particle composition in 

different regions and cities. The Administrator also recognized that there are additional 

limitations associated with evidence for reproductive and developmental effects, 
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identified as being suggestive of a causal relationship with long-term PM2.5 exposures,

including: the limited number of studies evaluating such effects; uncertainties related to 

identifying the relevant exposure time periods of concern; and limited toxicological 

evidence providing little information on the mode of action(s) or biological plausibility 

for an association between long-term PM2.5 exposures and adverse birth outcomes. Id. at 

38941.

The Administrator was mindful that considering what standards were requisite to 

protect public health with an adequate margin of safety required public health policy 

judgments that neither overstated nor understated the strength and limitations of the 

evidence or the appropriate inferences to be drawn from the evidence. In considering how 

to translate the available information into appropriate standard levels, the Administrator 

weighed the available scientific information and associated uncertainties and limitations. 

For the purpose of determining what standard levels were appropriate to propose, the 

Administrator recognized, as did EPA staff in the Policy Assessment, that there was no 

single factor or criterion that comprised the sole “correct” approach to weighing the 

various types of available evidence and information, but rather there were various 

approaches that are appropriate to consider. The Administrator further recognized that 

different evaluations of the evidence and other information before the Administrator 

could reflect placing different weight on the relative strengths and limitations of the 

scientific information, and different judgments could be made as to how such information 

should appropriately be used in making public health policy decisions on standard levels.

This recognition led the Administrator to consider various approaches to weighing the 

evidence so as to identify appropriate standard levels to propose. In so doing, the 
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Administrator encouraged extensive public comment on alternative approaches to 

weighing the evidence and other information so as to inform her public health policy 

judgments before reaching final decisions on appropriate standard levels.

In considering the available information, the Administrator noted the advice of 

CASAC that the currently available scientific information, including epidemiological and 

toxicological evidence as well as risk and air quality information, provided support for 

considering an annual standard level within a range of 13 to 11 μg/m3and a 24-hour 

standard level within a range of 35 to 30 μg/m3. In addition, the Administrator recognized 

that the Policy Assessment concluded that the available evidence and risk-based 

information support consideration of annual standard levels in the range of 13 to 11 

μg/m3, and that the Policy Assessment also concluded that the evidence most strongly 

supported consideration of an annual standard level in the range of 12 to 11 μg/m3. In 

considering how the annual and 24-hour standards work together to provide appropriate 

public health protection, the Administrator observed that CASAC did not express support 

for any specific levels or combinations of standards within these ranges. Nor did CASAC 

choose to comment on additional information and analyses presented in the final Policy 

Assessment prepared in response to CASAC’s recommendations on the second draft 

Policy Assessment (Wegman, 2011). 

In considering the extent to which the currently available evidence and 

information provided support for specific standard levels within the ranges identified by 

CASAC and the Policy Assessment as appropriate for consideration, the Administrator 

initially considered standard levels within the range of 13 to 11 μg/m3for the annual 

standard. In so doing, the Administrator first considered the long-term mean PM2.5
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concentrations reported in studies of effects classified as having evidence of a causal or 

likely causal relationship, as summarized in Figure 4 above and discussed more broadly 

above. She noted that a level at the upper end of this range would be below most but not 

all the overall study mean concentrations from the multi-city studies of long- and shortterm exposures, whereas somewhat lower levels within this range would be below all 

such overall study mean concentrations. In considering the appropriate weight to place on 

this information, the Administrator again noted that the evidence of an association in any 

such study was strongest at and around the long-term average where the data in the study 

are most concentrated, and that long-term mean PM2.5 concentrations were available for 

each study considered and, therefore, represented the most robust data set to inform her 

decisions on appropriate annual standard levels. Further, she was mindful that this 

approach did not provide a bright line for reaching decisions about appropriate standard 

levels. Id.

In considering the long-term mean PM2.5 concentrations reported in studies of 

effects classified as having evidence suggestive of a causal relationship, as summarized 

in Figure 4 for reproductive and developmental effects, the Administrator noted that a 

level at the upper end of this range would be below the overall study mean concentration 

in one of the three studies, while levels in the mid- to lower part of this range would be 

below the overall study mean concentrations in two or three of these studies. In 

considering the appropriate weight to place on this information, the Administrator noted 

the very limited nature of this evidence of such effects and the additional uncertainties in 

these epidemiological studies relative to the studies that provide evidence of causal or 

likely causal relationships.
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The Administrator also considered the distributional analyses of population-level 

information that were available from four of the epidemiological studies that provide 

evidence of effects identified as having a causal relationship with long- or short-term 

PM2.5 concentrations for annual standard levels within the same range of 13 to 11 μg/m3. 

In so doing, the Administrator first noted that a level in the mid-part of this range 

generally corresponds with approximately the 25th percentile of the distributions of health 

events data available in three of these studies. The Administrator also noted that standard 

levels toward the upper part of this range would reflect placing substantially less weight 

on this information, whereas standard levels toward the lower part of this range would 

reflect placing substantially more weight on this information. In considering this 

information, the Administrator noted that there was no bright line that delineates the part 

of the distribution of PM2.5 concentrations within which the data become appreciably 

more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies became appreciably less.

In considering mean PM2.5 concentrations and distributional analyses from the 

various sets of epidemiological studies noted above, the Administrator was mindful, as 

noted above, that such studies typically report concentrations based on composite monitor 

distributions, in which concentrations may be averaged across multiple ambient monitors 

that may be present within each area included in a given study. Thus, a policy approach 

that used data based on composite monitors to identify potential alternative standard 

levels would inherently build in a margin of safety of some degree relative to an 

alternative standard level based on measurements at the monitor within an area that 

records the highest concentration, or the maximum monitor, since once a standard was 
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set, concentrations at appropriate maximum monitors within an area were generally used 

to determine whether an area meets a given standard.

The Administrator also recognized that judgments about the appropriate weight to 

place on any of the factors discussed above should reflect consideration not only of the 

relative strength of the evidence but also on the important uncertainties that remained in 

the evidence and information being considered in this review. The Administrator noted 

that the extent to which these uncertainties influenced judgments about appropriate 

annual standard levels within the range of 13 to 11 μg/m3 would likely be greater for 

standard levels in the lower part of this range which would necessarily be based on fewer 

available studies than would higher levels within this range.

Based on the above considerations, the Administrator concluded that it was 

appropriate to propose to set a level for the primary annual PM2.5 standard within the 

range of 12 to 13 μg/m3. The Administrator provisionally concluded that a standard set 

within this range would reflect alternative approaches to appropriately placing the most 

weight on the strongest available evidence, while placing less weight on much more 

limited evidence and on more uncertain analyses of information available from a 

relatively small number of studies. Further, she provisionally concluded that a standard 

level within this range would reflect alternative approaches to appropriately providing an 

adequate margin of safety for the populations at risk for the serious health effects 

classified as having evidence of a causal or likely causal relationship, depending in part 

on the emphasis placed on margin of safety considerations. The Administrator recognized 

that setting an annual standard level at the lower end of this range would reflect an 

approach that placed more emphasis on the entire body of the evidence, including the 
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analysis of the distribution of air quality concentrations most influential in generating 

health effect estimates in the studies, and on margin of safety considerations, than would 

setting a level at the upper end of the range. Conversely, an approach that would support 

a level at the upper end of this range would place more emphasis on the remaining 

uncertainties in the evidence to avoid potentially overestimating public health 

improvements, and would generally support a view that the uncertainties remaining in the 

evidence are too great to warrant setting a lower annual standard level. Id. at 38942.

At the time of the proposal, while the Administrator recognized that CASAC 

advised, and the Policy Assessment concluded, that the available scientific information 

provided support for considering a range that extended down to 11 μg/m3, she concluded 

that proposing such an extended range would reflect a public health policy approach that 

placed more weight on relatively limited evidence and more uncertain information and 

analyses than she considered appropriate at this time. Nonetheless, the Administrator 

solicited comment on a level down to 11 μg/m3as well as on approaches for translating 

scientific evidence and rationales that would support such a level. Such an approach 

might reflect a view that the uncertainties associated with the available scientific 

information warrant a highly precautionary public health policy response that would 

incorporate a large margin of safety.

The Administrator recognized that potential air quality changes associated with 

meeting an annual standard set at a level within the range of 12 to 13 μg/m3 will result in 

lowering risks associated with both long- and short-term PM2.5 exposures. However, the 

Administrator recognized that such an annual standard intended to serve as the primary 

means for providing protection from effects associated with both long- and short-term 
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PM2.5 exposures would not by itself be expected to offer requisite protection with an 

adequate margin of safety against the effects of all short-term PM2.5 exposures. As a 

result, in conjunction with proposing an annual standard level in the range of 12 to 13 

μg/m3, the Administrator provisionally concluded that it was appropriate to continue to 

provide supplemental protection by means of a 24-hour standard set at the appropriate 

level, particularly for areas with high peak-to-mean ratios possibly associated with strong 

local or seasonal sources, or for PM2.5-related effects that may be associated with shorterthan-daily exposure periods. 

Based on the approach discussed in section III.A.3 above, at the time of the 

proposal the Administrator relied upon evidence from the short-term exposure studies as 

the principal basis for selecting the level of the 24-hour standard. In considering these 

studies as a basis for the level of a 24-hour standard, and having selected a 98th percentile 

form for the standard, the Administrator agreed with the focus in the Policy Assessment 

of looking at the 98th percentile values, as well as at the long-term mean PM2.5

concentrations in these studies.

In considering the information provided by the short-term exposure studies, the 

Administrator recognized that to the extent these studies were conducted in areas that 

likely did not meet one or both of the current standards, such studies did not help inform 

the characterization of the potential public health improvements of alternative standards 

set at lower levels. By reducing the PM2.5 concentrations in such areas to just meet the 

current standards, the Administrator anticipated that additional public health protection 

would occur. Therefore, the Administrator focused on studies that reported positive and 

statistically significant associations in areas that would likely have met both the current 
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24-hour and annual standards. She also considered whether or not these studies were 

conducted in areas that would likely have met an annual standard level of 12 to 13 μg/m3

to inform her decision regarding an appropriate 24-hour standard level. As discussed in 

section III.E.4.a, consistent with the Policy Assessment, the Administrator concluded that 

multi-city, short-term exposure studies provided the strongest data set for informing her 

decisions on appropriate 24-hour standard levels. The Administrator viewed the singlecity, short-term exposure studies as a much more limited data set providing mixed results 

and, therefore, she had less confidence in using those studies as a basis for setting the 

level of a 24-hour standard. With regard to the limited number of single-city studies that 

reported positive and statistically significant associations for a range of health endpoints 

related to short-term PM2.5 concentrations in areas that would likely have met the current 

suite of PM2.5 standards, the Administrator recognized that many of those studies had 

significant limitations (e.g., limited statistical power, limited exposure data) or equivocal 

results (mixed results within the same study area) that made them unsuitable to form the 

basis for setting the level of a 24-hour standard. 

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, the Administrator observed an overall pattern of positive and 

statistically significant associations in studies with 98th percentile values averaged across 

study areas in the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; Zanobetti and 

Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; 

Franklin et al., 2008). The Administrator noted that, to the extent air quality distributions 

were reduced to reflect just meeting the current 24-hour standard, additional protection

would be anticipated for the effects observed in the three multi-city studies with 98th
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percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett and Goldberg, 

2003; Franklin et al., 2008). In the three additional studies with 98th percentile values 

below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 μg/m3, 

the Administrator noted that these studies reported long-term mean PM2.5 concentrations 

of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti and Schwartz, 

2009; Dominici et al., 2006a).

In proposing to revise the level of the annual standard to within the range of 12 to 

13 μg/m3, as discussed above, the Administrator recognized that additional protection 

would be provided for the short-term effects observed in these multi-city studies in 

conjunction with an annual standard level of 12 μg/m3, and in two of these three studies 

in conjunction with an annual standard level of 13 μg/m3. She noted that the study-wide 

mean concentrations were based on averaging across monitors within study areas and that 

compliance with the standard would be based on concentrations measured at the monitor 

reporting the highest concentration within each area. The Administrator believed it would 

be reasonable to conclude that revision to the 24-hour standard would not be appropriate 

in conjunction with an annual standard within this range. Based on the above 

considerations related to the epidemiological evidence, the Administrator provisionally 

concluded that it was appropriate to retain the level of the 24-hour standard at 35 μg/m3, 

in conjunction with a revised annual standard level in the proposed range of 12 to 13 

μg/m3.

In addition to considering the epidemiological evidence, the Administrator also 

took into account air quality information based on county-level 24-hour and annual 

design values to understand the public health implications of retaining the 24-hour 
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standard level at 35 μg/m3in conjunction with an annual standard level within the 

proposed range of 12 to 13 μg/m3. She considered whether these suites of standards 

would meet a public health policy goal which included setting the annual standard to be 

the “generally controlling” standard in conjunction with setting the 24-hour standard to 

provide supplemental protection to the extent that additional protection is warranted. As 

discussed above, the Administrator provisionally concluded that this approach was the 

most effective and efficient way to reduce total population risk associated with both longand short-term PM2.5 exposures, resulting in more uniform protection across the U.S. than 

the alternative of setting the 24-hour standard to be the controlling standard. 

In considering the air quality information, the Administrator first recognized that 

there was no annual standard within the proposed range of levels, when combined with a 

24-hour standard at the proposed level of 35 μg/m3, for which the annual standard would 

be the generally controlling standard in all areas of the country. She further observed that 

such a suite of PM2.5 standards with an annual standard level of 12 μg/m3 would result in 

the annual standard as the generally controlling standard in most regions across the 

country, except for certain areas in the Northwest, where the annual mean PM2.5

concentrations have historically been low but where relatively high 24-hour 

concentrations occur, often related to seasonal wood smoke emissions (U.S. EPA, 2011a, 

pp. 2-89 to 2-91, Figure 2-10). Although not explicitly delineated on Figure 2-10 in the 

Policy Assessment, an annual standard of 13 μg/m3 would be somewhat less likely to be 

the generally controlling standard in some regions of the U.S. outside the Northwest in 

conjunction with a 24-hour standard level of 35 μg/m3.

Taking the above considerations into account, the Administrator proposed to 
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revise the level of the primary annual PM2.5 standard from 15.0 μg/m3to within the range 

of 12.0 to 13.0 μg/m3and to retain the 24-hour standard level at 35 μg/m3. 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 alternative approaches to the 

appropriate consideration of the strength of the available evidence and other information 

and their associated uncertainties and the advice of CASAC.

The Administrator recognized that the final suite of standards selected from 

within the proposed range of annual standard levels, or the broader range of annual 

standard levels on which public comment was solicited, must be clearly responsive to the 

issues raised by the D.C. Circuit’s remand of the 2006 primary annual PM2.5 standard. 

Furthermore, at the time of the proposal, she recognized that the final suite of standards 

will reflect her ultimate judgment in the final rulemaking as to the suite of primary PM2.5

standards that would be requisite to protect the public health with an adequate margin of 

safety from effects associated with fine particle exposures. The final judgment to be made 

by the Administrator will appropriately consider the requirement for a standard that is 

neither more nor less stringent than necessary and will recognize that the CAA does not 

require that primary standards be set at a zero-risk level, but rather at a level that reduces 

risk sufficiently so as to protect public health with an adequate margin of safety. 

At the time of the proposal, having reached her provisional judgment to propose 

revising the annual standard level from 15.0 to within a range of 12.0 to 13.0 μg/m3and 

to propose retaining the 24-hour standard level at 35 μg/m3, the Administrator solicited 

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 
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Administrator also solicited public comment on alternative annual standard levels down 

to 11 μg/m3and on the combination of annual and 24-hour standards that commenters 

may believe is appropriate, along with the approaches and rationales used to support such 

levels. In addition, given the importance the evidence from epidemiologic studies played 

in considering the appropriate annual and 24-hour levels, the Administrator solicited 

public comment on issues related to translating epidemiological evidence into standards, 

including approaches for addressing the uncertainties and limitations associated with this 

evidence.

c. Comments on Standard Levels

This section addresses comments that relate to consideration of the appropriate 

levels of the primary annual and 24-hour PM2.5 standards, including comments on the 

general approach used by the EPA to translate the available scientific information into 

standard levels and how specific PM2.5 exposure studies should be considered as a basis 

for the standard levels. These comments on standard levels expand upon the more general

comments that either supported or opposed any change to the current suite of primary 

PM2.5 standards, which are addressed above in section III.D.2.78 As explained there, one 

group of commenters generally opposed any change to the current primary PM2.5

standards and more specifically disagreed with the basis for the EPA’s proposal to revise 

the annual standard level. Another group of commenters supported revising the current 

suite of primary PM2.5 standards to provide increased public health protection. Some 

commenters in this second group argued that both the annual and 24-hour standard levels 

should be lowered while other commenters in this group agreed with the EPA’s proposal 

 

78 Specific comments on the forms of the annual and 24-hour standards are addressed in 

section III.E.3.a and III.E.3.b, respectively.
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to retain the level of the 24-hour standard in conjunction with revising the level of the 

annual standard. While generally supporting the EPA’s proposal to lower the level of the 

annual standard, many commenters in this group disagreed that a level within the EPA’s 

proposed range was adequately protective and supported a level of 11 μg/m3or below.

i. Annual Standard Level

The group of commenters opposed to any change to the current suite of primary 

PM2.5 standards generally raised questions regarding the underlying scientific evidence, 

including the causal determinations reached in the Integrated Science Assessment, and 

focused strongly on the uncertainties they saw in the scientific evidence as a basis for 

their conclusion that no changes to the current standard levels were warranted. In 

commenting on the proposed standard levels, these commenters typically relied on the 

arguments summarized and addressed above in section III.D.2 as to why they believed it 

was inappropriate for the EPA to make any revisions to the suite of primary PM2.5

standards. That is, they asserted that the EPA’s causal determinations were not 

adequately supported by the underlying scientific information; the biological plausibility 

of health effects observed in epidemiological studies has not been demonstrated in 

controlled human exposure and toxicological studies; uncertainties in the underlying 

health science are as great or greater than in 2006; there is no evidence of greater risk 

since the last review to justify tightening the current annual PM2.5 standard; and “new” 

studies not included in the Integrated Science Assessment continue to increase 

uncertainty about possible health risks associated with exposure to PM2.5.

With regard to the level of the annual standard, these commenters strongly 

disagreed with the Agency’s proposed decision to revise the level to within a range of 12 
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to 13 μg/m3and argued that the current standard level of 15 μg/m3should be retained. 

For example, UARG, API, and other commenters in this group raised a number of issues 

that they asserted called into question the EPA’s interpretation of the epidemiological 

evidence to support revising the annual standard level. These commenters raised specific 

questions related to the general approach used by the EPA to translate the air quality and 

other information from specific epidemiological studies into standard levels, including:

(1) the EPA’s approach for using composite monitor air quality distributions reported in 

epidemiological studies to select a standard level that would be compared to 

measurements at the monitor recording the highest value in an area to determine 

compliance with the standard; (2) the appropriate exposure period for effects observed in 

long-term exposure mortality studies; and (3) the use of the EPA’s analysis of 

distributions of underlying population-level data (i.e., health event and study population 

data) for those epidemiological studies for which such information was available. These 

commenters also raised questions regarding the EPA’s consideration of specific scientific 

evidence as a basis for setting a standard level, including: (4) evidence of respiratory 

morbidity effects in long-term exposure studies and (5) more limited evidence of health 

effects which have been categorized in the Integrated Science Assessment as suggestive 

of a causal relationship (i.e., developmental and reproductive outcomes). These 

comments are discussed in turn below.

(1) Some commenters in this group argued that one reason why they believe there 

is no basis for setting a standard level below 15 μg/m3is that the air quality metric from 

epidemiological studies that the EPA relied on in the proposal is not the same metric that 

will be compared to the level of the standard to determine compliance with the standard. 
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That is, commenters noted that the long-term mean PM2.5 concentrations that the EPA 

considered, shown in Figure 4 above, are composite monitor mean concentrations (i.e., 

concentrations averaged across multiple monitors within areas with more than one 

monitor), whereas the PM2.5 concentrations that will be compared to the level of the 

standard are maximum monitor concentrations (i.e., the concentration measured by the 

monitor within an area reporting the highest concentration). This comment was presented 

most specifically in UARG’s comments (UARG, 2012, Attachment 1, pp. 2 to 6), which 

raised two overarching issues as discussed below.

First, the commenter noted that the EPA’s approach of considering composite 

monitor mean PM2.5 concentrations in selecting a standard level, and then comparing the 

maximum monitor mean PM2.5 concentration in each area to the standard level when the 

standard is implemented, was characterized in the proposal as inherently having the 

potential to build in a margin of safety (UARG, 2012, Attachment 1, p. 4, citing 77 FR 

38905). The commenter asserted that the Administrator is ignoring this distinction 

between composite and maximum monitor concentrations, and that this approach creates 

an unwarranted case for lowering the standard level, since in the commenter’s view, it 

would result in a margin of safety that would be arbitrary, not based on evidence, and 

unquantified (UARG, 2012, Attachment 1, p. 4). In support of this view, the commenter 

asserted that there is a significant difference between composite monitor mean PM2.5

concentrations and maximum monitor mean PM2.5 concentrations. The commenter 

asserted that the maximum monitor value will always be higher than the composite 

monitor value (except in areas that contain only a single monitor), such that when an area 

just attains the NAAQS, that area’s composite monitor long-term mean PM2.5
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concentration will be lower than the level of the standard (UARG, 2012, Attachment 1, p. 

3).

Second, the commenter asserted that a more “reasoned and consistent approach 

would be to decide on a mean composite monitor PM2.5 level that should be achieved and 

then identify the maximum monitor level that would result in that composite value” 

(UARG, 2012, Attachment 1, p. 4). The commenter conducted an analysis of maximum 

monitor versus composite monitor annual mean PM2.5 concentrations using monitoring 

data79 from 2006 to 2008 and presented results averaged across areas within two groups 

(i.e., those with design values80 above the current standard level and those with design 

values just below the current standard level) to illustrate their suggested alternative 

approach. The commenter interpreted this analysis as showing that the composite monitor 

long-term mean PM2.5 concentrations from the subset of the epidemiological studies 

shown in Figure 4 (of the proposal and above) that the commenter considered to be an 

appropriate focus for this analysis would be achieved across the U.S. if the current annual 

NAAQS of 15 μg/m3is retained and attained. The commenter considered the subset of 

epidemiological studies that included only long-term exposures studies of effects for 

which the evidence is categorized as causal or likely causal, but did not consider shortterm exposure studies. On this basis, the commenter asserted that attaining the current 

annual PM2.5 standard would result in composite monitor long-term mean concentrations 

in all areas that would be generally within or below the range of the composite monitor 

long-term mean concentrations from such studies and, as a result, there is no reason to 

 

79 The commenter indicated that this analysis was based on monitoring data for every 

core based statistical area (CBSA) in the EPA’s Air Quality System (AQS) database.

80 The design value is the air quality statistic that is compared to the level of the NAAQS 

to determine the attainment status of a given area.
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lower the level of the current annual NAAQS.

In considering the first issue related to the EPA’s approach, the EPA notes that in 

proposing to revise both the form and level of the annual standard, the Administrator 

clearly took into account the distinction between the composite monitor long-term mean 

PM2.5 concentrations from the epidemiological studies, considered as a basis for selecting 

an annual standard level, and maximum monitor long-term mean PM2.5 concentrations. In 

deciding to focus on the composite monitor long-term mean concentrations in selecting 

the standard level, and on the maximum monitor concentrations in selecting the form of 

the standard (i.e., consistent with proposing to eliminate the option for spatial averaging 

across monitors within an area when implementing the standard81), the Administrator 

reasonably considered the distinction between these metrics in a manner that was 

consistent with advice from CASAC (Samet et al., 2010d, pp. 2 to 3).

As noted above in section III.A.3, the EPA recognizes that a statistical metric 

(e.g., the mean of a distribution) based on maximum monitor concentrations may be 

identical to or above the same statistical metric based on composite monitor 

concentrations. More specifically, many areas have only one monitor, in which case the 

composite and maximum monitor concentrations are identical. Based on the most recent 

data from the EPA’s AQS from 2009 to 2011 in the 331 CBSAs in which valid PM2.5

data are available, as discussed in Frank (2012a, Table 5), there were 208 such areas 

(with design values ranging up to about 15 μg/m3). Frank (2012a) also observed that 

other areas have multiple monitors with composite and maximum monitor mean PM2.5

concentrations that were the same or relatively close, with 57 areas in which the 

maximum monitor mean concentration was no more than 0.5 μg/m3higher than the 

 

81 As discussed above in section III.E.3.a.
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composite monitor mean concentration and 56 areas in which the difference was between 

0.6 and 2 μg/m3. Further, there were only a few other areas in which the maximum 

monitor mean concentration was appreciably higher than the composite monitor mean 

concentration, such as areas in which some monitors may be separately impacted by local 

sources. There were only 10 such areas in the country in which the maximum monitor 

mean concentration was between 2 to 6 μg/m3higher than the composite monitor 

concentration (Frank, 2012a, Table 4).82 Thus, the EPA does not agree that there is a 

significant difference between composite monitor mean PM2.5 concentrations and 

maximum monitor mean PM2.5 concentrations in the large majority of areas across the 

country.

In proposing to revise the form of the annual PM2.5 standard, as discussed above 

in section III.E.3.a, the EPA noted that when an annual PM2.5 standard was first set in 

1997, the form of the standard included the option for averaging across measurements at 

appropriate monitoring sites within an area, generally consistent with the composite 

monitor approach used in epidemiological studies, with some constraints intended to 

ensure that spatial averaging would not result in inequities in the level of protection for 

communities within large metropolitan areas. In the last review the EPA tightened the 

constraints on spatial averaging, and in this review has eliminated the option altogether, 

on the basis of analyses in each review that showed that such constraints may be 

inadequate to avoid substantially greater exposures for people living in locations around 

the monitors recording the highest PM2.5 concentrations in some areas, potentially 

 

82 The average difference between the maximum and composite design value among the 

123 CBSAs with two or more monitors is 0.8 μg/m3and the median difference is 0.6 

μg/m3. The 25th and 75th percentiles are 0.3 and 1.0 μg/m3, respectively (Frank, 2012a, p. 

4). 
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resulting in disproportionate impacts on at-risk populations of persons with lower SES 

levels as well as minorities. In light of these analyses, and consistent with the 

Administrator’s decision to revise the form of the annual PM2.5 standard by eliminating 

the option for spatial averaging, the EPA continues to conclude that a standard level 

based on consideration of long-term mean concentrations from composite monitors, and 

applied at each monitor within an area including the monitor measuring the highest 

concentration, is the appropriate approach to use in setting a standard that will protect 

public health, including the health of at-risk populations, with an adequate margin of 

safety, as required by the CAA.

The EPA acknowledges that at proposal, the Agency characterized the approach 

of using maximum monitor concentrations to determine compliance with the standard, 

while selecting the standard level based on consideration of composite monitor 

concentrations, as one that inherently had the potential to build in a margin of safety (77 

FR 38905), and CASAC reiterated that view in supporting the EPA’s approach (Samet, 

2010d, p. 3). Nonetheless, in light of the discussion above, the EPA more specifically 

recognizes that this approach does not build in any margin of safety in the large number 

of areas across the country with only one monitor. Further, based on the analyses done to 

inform consideration of the form of the standard (Schmidt, 2011, Analysis A), the EPA 

concludes that this approach does not provide a margin of safety for the at-risk 

populations that live around the monitor measuring the highest concentration, such as in 

those few areas in which the maximum monitor concentration is appreciably higher than 

the composite monitor concentration. Rather, this approach properly treats those at-risk 

populations the same way it does the broader populations that live in areas with only one 
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monitor, by providing the same degree of protection for those at-risk populations that 

would otherwise be disproportionately impacted as it does for the broader populations in 

other areas, While the EPA recognizes that this approach can result in some additional 

margin of safety for the subset of areas with multiple monitors in which at-risk 

populations may not be disproportionately represented in areas around the maximum 

monitor, which may be the case in areas with relatively small differences between the 

maximum and composite monitor concentrations, the EPA notes that this margin would 

be relatively small in such areas.

Based on the above considerations, the EPA does not agree that the Agency’s 

approach of using maximum monitor concentrations to determine compliance with the 

standard, while selecting the standard level based on consideration of composite monitor 

concentrations creates an unwarranted case for lowering the standard level based on a 

margin of safety that would be arbitrary, not based on evidence, or lack quantification. 

The EPA recognizes that setting a standard to protect public health, including the health 

of at-risk populations, with an adequate margin of safety, depends upon selecting a 

standard level sufficiently below where there is clearthe EPA has found the strongest

evidence of health effects so as to provide such protection, and that the EPA’s approach 

regarding consideration of composite and maximum monitor concentrations is intended 

to, and does, serve to address this requirement as part of and not separate from the 

selection of an appropriate standard level based on the health effects evidence.

In considering the second issue related to the commenter’s suggested alternative 

approach, the EPA strongly disagrees with the commenter’s view that a more “reasoned 

and consistent approach would be to decide on a mean composite monitor PM2.5 level 
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that should be achieved and then identify the maximum monitor level that would result in 

that composite value” (UARG, 2012, Attachment 1, p. 4). As discussed above, the EPA 

notes that for areas with only one monitor, or with multiple monitors that measure 

concentrations that are very close in magnitude, the maximum monitor level that would 

limit the composite monitor PM2.5 level to be no greater than the level that should be 

achieved to protect public health with an adequate margin of safety, would essentially be 

the same as that composite monitor level. Further, as discussed above, even for areas in 

which the maximum monitor concentration is appreciably higher than other monitor 

concentrations within the same area, public health would not be protected with an 

adequate margin of safety if the disproportionately higher exposures of at-risk, 

susceptible populations around the monitor measuring the highest concentration were in 

essence averaged away with measurements from monitors in other locations within large 

urban areas. Further, the commenter’s suggested approach would be based on annual 

average PM2.5 concentrations that have been measured over some past time period. Such 

an approach would reflect the air quality that existed in the past, but it would not 

necessarily provide appropriate constraints on the range of concentrations that would be 

allowed by such a standard in the future, when relationships between maximum and 

composite monitor concentrations in areas across the country may be different. For these 

reasons, the EPA fundamentally rejects the commenter’s suggested approach because it 

results in a standard that demonstrably would not protect public health, including 

providing protection for at-risk populations, with an adequate margin of safety in areas 

across the country.

More specifically, in further considering the commenter’s analysis of design 
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values based on maximum versus composite monitor annual mean PM2.5 concentrations 

using monitoring data from 2006 to 2008 which they assert supports retaining the current 

standard level of 15 μg/m3, the EPA finds flaws with the numerical results and the scope 

of the analysis, as well as flaws in the commenter’s translation of the analysis results into 

the basis for selecting an annual standard level.

In considering the commenter’s analysis, the EPA notes that the analysis 

compared maximum versus composite monitor annual mean PM2.5 concentrations, 

averaged over 3 years, for two groups of areas: (1) areas with design values that exceed 

the current annual standard level (i.e., greater than 15.0 μg/m3) and (2) areas with design 

values that are just attaining the current annual standard (i.e., between 14.5 and 15.0 

μg/m3).83 The commenter indicated that they used the full body of PM2.5 monitoring data 

from the EPA’s AQS database (UARG, 2012, Attachment 1, p. 4), In attempting to 

reproduce the commenter’s results, the EPA repeated the calculations using only valid air 

quality data (i.e., data that meet data completeness and monitor siting criteria) from the 

AQS database for the same time period (Frank, 2012a).84 Based on this corrected 

analysis, the EPA finds that the composite monitor concentrations averaged across the 

areas within each group are somewhat higher than those calculated by the commenter, 

and the average differences between the maximum and composite monitor concentrations 

 

83For the first group of areas (which included 33 areas), this analysis calculated an 

average across the areas of maximum monitor annual mean PM2.5 concentrations, 

averaged over 3 years, of 17.2 μg/m3compared to an average of composite monitor 

concentrations of 14.3 μg/m3. For the second group of areas (which included 11 areas), 

this analysis calculated an average across the areas of maximum monitor annual mean 

concentrations, averaged over 3 years, of 14.8 μg/m3compared to an average of 

composite monitor concentrations of 13.6 μg/m3(UARG, 2012, Attachment 1, Table 1).

84 The EPA notes that the Frank (2012a) analysis is similar to an earlier EPA staff 

analysis (Hassett-Sipple et al., 2010), which used air quality data from EPA’s AQS 

database to compare maximum versus composite monitor long-term mean PM2.5

concentrations across the study areas in six selected multi-city epidemiological studies.
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are somewhat smaller (Frank, 2012a, Table 3).85 Notably, the difference between the 

maximum and composite monitor average concentrations for the second group of areas is 

substantially reduced in the corrected analysis, such that the difference (averaged across 

the 10 areas with valid data in the second group) is approximately 0.5 μg/m3, not 1.2 

μg/m3as in the commenter’s analysis. In addition, the commenter’s analysis compared 

the average of the composite monitors to the average of the maximum monitors for each 

subset of areas. This comparison of averages across all the areas in each subset masks the 

fact that the large majority of areas across the country have only one monitor, with the 

composite monitor and maximum monitor values the same for such areas, and many 

other areas have a maximum monitor value that is close to the composite monitor value. 

As discussed above, these circumstances have a major impact on the protection that 

would be achieved by the approach suggested by the commenter.

With regard to the scope of the commenter’s analysis, the EPA finds that by 

limiting the scope to a small subset of areas with design values above or just below the 

current annual standard level of 15 μg/m3, the analysis ignores the large number of areas 

across the country with lower design values that are relevant to consider in light of the 

epidemiological evidence of serious health effects at lower concentrations, well below the 

level of the current standard.

In translating the analysis results into the basis for selecting an annual standard 

 

85 The EPA’s analysis was intended to repeat the commenter’s analysis, but using only 

valid air quality data (from 2006 to 2008). For the first group of areas (which included 21 

areas with valid data), the EPA’s analysis calculated an average across the areas of 

maximum monitor annual mean concentrations, averaged of 3 years, of 16.8 μg/m3

compared to an average of composite monitor concentrations of 14.8 μg/m3. For the 

second group of areas (which included 10 areas with valid data), the EPA’s analysis 

calculated an average across the areas of maximum monitor annual mean concentrations, 

averaged over 3 years, of 14.8 μg/m3compared to an average of composite monitor 

concentrations of 14.2 μg/m3(Frank, 2012a, Table 3).
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level, the commenter’s translation is premised on the view that the “natural focal point” 

for setting an annual PM2.5 standard level should be somewhere within the range of the 

long-term mean PM2.5 concentrations from the subset of epidemiological studies that 

included only long-term exposure studies of effects for which the evidence is categorized 

as causal or likely causal, but not for effects categorized as suggestive of causality, nor 

did it include short-term exposure studies (which are included in Figure 4 of the proposal 

notice and above). Such a view is not consistent with setting a standard that would 

provide sufficient protection from the serious health effects reported even in the limited 

subset of studies considered by the commenter, including protecting public health with an 

adequate margin of safety. As discussed below, the EPA does not agree with the 

commenter’s view as to the appropriate focal point for selecting the level of an annual 

PM2.5 standard, or with the limited set of studies considered by the commenter as a basis 

for selecting the level of the annual PM2.5 standard.

Regarding an appropriate focal point for selecting the level of the annual standard, 

as discussed in the proposal and as advised by CASAC, the EPA has focused on PM2.5

concentrations somewhat below the lowest long-term mean concentrations from each of 

the key studies of both long- and short-term exposures of effects for which the evidence 

is causal or likely causal, as considered by the EPA (i.e., the first two sets of studies 

shown in Figure 4). If the level of the annual standard was set just somewhere within the 

range of the long-term mean concentrations from the various long-term exposure studies, 

then one or more of the studies would have a long-term mean concentration below the 

selected level of the standard. Absent some reason to ignore or discount these studies, 

which the commenter does not provide (and of which the EPA is unaware), setting such a 
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standard would allow that level of air quality, where the evidence of health effects is 

strongest, and its associated risk of PM2.5-related mortality and/or morbidity effects to 

continue. Selecting such a standard level that would explicitly allow such projected 

PM2.5-related mortality and morbidity effects to continue to occur could not be 

considered sufficient to protect the public health with an adequate margin of safety. 

Further, focusing on just the long-term mean PM2.5 concentrations in the key 

epidemiological studies – even the lowest long-term mean concentration from the set of 

key studies -- is not appropriate. Concentrations at and around the long-term mean

concentrations represent the part of the air quality distribution where the data in any 

given study are most concentrated and, thus, where the confidence in the magnitude and 

significance of an association in such study is strongest. However, the evidence of an 

association with adverse health effects in the studies is not limited to the PM2.5

concentrations just at and around the long-term mean, but rather extends more broadly to 

a lower part of the distribution, recognizing that no discernible population-level threshold 

for any such effects can be identified based on the available evidence. This broader 

region of the distribution of PM2.5 concentrations should be considered to the extent 

relevant information is available, recognizing that the degree of confidence in the 

association identified in a study would changebecome lower as one moves away 

frombelow concentrations at and around the long-term mean concentration in any given 

study. The commenter’s approach ignores this fundamental consideration.

Regarding the set of studies that is appropriate to inform the selection of the level 

of the annual PM2.5 standard, the EPA finds that limiting consideration only to the longterm exposure studies, as this commenter suggests, would be tantamount to ignoring the 
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short-term exposure studies,

86 which provide some of the strongest evidence from the 

entire body of epidemiological studies. Thus, selecting an annual standard level using the 

limited set of studies suggested by the commenter would fail to provide a degree of 

protection that would be sufficient to protect public health with an adequate margin of 

safety.

For all the reasons discussed above, the EPA finds the commenter’s concerns with 

the EPA’s approach to considering composite and maximum monitor PM2.5

concentrations in selecting the level of the annual PM2.5 standard to be without merit. 

Further, the EPA finds no support in the commenter’s analysis for their suggested 

alternative approach.

(2) With respect to the appropriate exposure period for mortality effects observed 

in long-term exposure studies, some commenters in this group generally expressed views 

consistent with comments from UARG that argued that these studies “are most likely 

detecting health risk from earlier, higher PM2.5 levels and misattributing those risks to 

more recent, lower PM2.5 levels” (UARG, 2012, Attachment 1, p 7). Further, this 

commenter asserted that “there is no knowledge or evidence indicating whether 

premature deaths are the result of PM2.5 exposures in the most recent year; or due to 

 

86 The commenter suggests that the EPA should not place significant reliance on the longterm mean concentrations from short-term exposure studies because “[T]he short‐term 

studies did not use the annual average of PM2.5 to develop their associations; they used 

the daily 24‐hour averages of PM2.5. Thus, short‐term studies do not provide a natural 

indicator for the appropriate level of an annual standard....” (UARG, 2012, Attachment 

1, p. 3). The EPA finds this argument unpersuasive. Quite simply, effects were observed 

in these studies with an air quality distribution that can meaningfully be characterized by 

these long-term mean concentrations. Indeed, in remanding the 2006 standard, the D.C. 

Circuit discussed at length the interrelationship of the long- and short-term standards and 

studies, and remanded the 2006 standard to the EPA, in part, for ignoring those 

relationships without adequate explanation. American Farm Bureau Federation v. EPA. 

559 F. 3d at 522-24.
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physical damages incurred from PM2.5 exposures much earlier in life (with the impact on 

lifespan only emerging later in life); or due to total accumulated PM2.5 exposure over 

many years.” Id. Further, the commenter asserted that the long-term exposure studies of 

mortality are central to the EPA’s basis for proposing to set a lower annual standard level, 

since most of the estimated benefits associated with a lower annual PM2.5 standard are 

based on reductions in mortality related to long-term exposures to PM2.5.

As an initial matter, the EPA has recognized the challenge in distinguishing 

between PM2.5-associated effects due to past and recent long-term exposures, and in

identifying the relevant latency period for long-term exposure to PM and resultant health 

effects (U.S. EPA, 2009a, section 7.6.4; 77 FR 38941/1). While the EPA has 

acknowledged that there remain important uncertainties related to characterizing the most 

relevant exposure periods in long-term exposure studies, the assertion that there is “no 

knowledge or evidence” that helps to inform this issue is not correct, as discussed below.

Both in the last review and in the current review, the EPA has assessed studies 

that used different air quality periods for estimating long-term exposure and tested 

associations with mortality for the different exposure periods (U.S. EPA, 2004, section 

8.2.3.5; U.S. EPA 2009a, section 7.6.4). In this review, the Integrated Science 

Assessment discussed studies available since the last review that have assessed the 

relationship between long-term exposure to PM2.5 and mortality to explore the issue of 

the latency period between exposure to PM2.5 and death (U.S. EPA, 2009a, section 7.6.4).

Notably, in a recent analysis of the extended Harvard Six Cities Study, Schwartz 

et al. (2008) used model averaging (i.e., multiple models were averaged and weighted by 

probability of accuracy) to assess exposure periods prospectively (77 FR 38907/1-2). The 
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exposure periods were estimated across a range of unconstrained distributed lag models 

(i.e., same year, one year prior, two years prior to death). In comparing lags, the authors 

reported that the effects of changes in exposure to PM2.5 on mortality were strongest 

within a 2-year period prior to death (U.S. EPA, 2009a, p. 7-92, Figure 7-9). Similarly, a 

large multi-city study of the elderly found that the mortality risk associated with longterm exposure to PM10 reported cumulative effects that extended over the years that 

deaths were observed in the study population (i.e., the follow-up period) and for the 3-

year period prior to death (Zanobetti et al., 2008). 

Further, in a study of two locations that experienced an abrupt decline in PM2.5

concentrations (i.e., Utah Steel Strike, coal ban in Ireland), Röösli et al. (2005) reported 

that approximately 75 percent of health benefits were observed in the first 5 years (U.S.

EPA, 2009a, Table 7-9). Schwartz et al. (2008) and Puett et al. (2008) found, in a 

comparison of exposure periods ranging from 1 month to 48 months prior to death, that 

exposure to PM10 24 months prior to death exhibited the strongest association, and the 

weakest association was reported for exposure in the time period of 1 month prior to 

death. 

Overall, the EPA notes that the available evidence for determining the exposure 

period that is causally related to the mortality effects of long-term PM2.5 exposures, as 

discussed above, cannot specifically disentangle the effects observed in long-term 

exposure studies associated with more recent air quality measurements from effects that 

may have been associated with earlier, and most likely higher, PM2.5 exposures. While 

the evidence suggests that a latency period of up to five years would account for the 

majority of deaths, it does not provide a basis for concluding that it is solely recent PM2.5
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concentrations that account for the mortality risk observed in such studies. Nonetheless, 

the more recent air quality data does well at explaining the relationships observed 

between long-term exposures to PM2.5 and mortality, with the strongest association

observed in the two years prior to death. Further, the EPA recognizes that there is no 

discernible population-level threshold below which effects would not occur, such that it 

is reasonable to consider that health effects may occur over the full range of 

concentrations observed in the epidemiological studies, including the lower 

concentrations in the latter years.

In light of this evidence and these considerations, the EPA concludes that it is 

appropriate to consider air quality concentrations that are generally contemporaneous 

with the collection of health event data (i.e., collected over the same time period) as being 

causally associated with at least some proportion of the deaths assessed in a long-term 

exposure study. This would include long-term mean PM2.5 concentrations from most of 

the key long-term exposure studies of effects with causal or likely causal evidence shown 

in Figure 4 above, which reported long-term mean PM2.5 concentrations ranging from 

13.6 μg/m3to 14.3 μg/m3. These studies include studies of mortality by Eftim et al. 

(2008), which separately analyzed the ACS and Harvard Six City sites, Zeger et al. 

(2008), and Lipfert et al. (2006), as well as studies of morbidity endpoints by Goss et al. 

(2004), McConnell et al. (2003) and Gauderman et al. (2004), and Dockery et al. (1996) 

and Razienne et al. (1996). The EPA acknowledges that uncertainty in the relevant 

exposure period is most notable in two other long-term exposure studies of mortality. The 

Miller et al. (2007) reported a long-term mean PM2.5 concentration for a 1-year exposure 

period that post-dated the follow-up period in which health event data were collected by 
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two years. Also, the Krewski et al. (2009) study reported a long-term mean PM2.5

concentration for an exposure period that included only the last two years of the 18-year 

follow-up period. Based on these considerations, the EPA does not now consider it 

appropriate to put weight on the reported long-term mean concentrations from these two 

studies for the purpose of translating the information from the long-term mortality studies 

into a basis for selecting the level of the annual PM2.5 standard.87

In addition, the EPA acknowledges that exposure periods that extend at least a 

couple years prior to the follow-up period in which health event data were collected 

would likely more fully capture the PM-related deaths in such studies. To explore how 

much higher the long-term mean PM2.5 concentrations would likely have been had air 

quality data prior to the follow-up years of the studies been included, the EPA conducted 

a sensitivity analysis of long-term mean PM2.5 concentrations (Schmidt, 2012a) 

particularly considering studies that only included deaths from a relatively recent followup period. As examples of such studies, this analysis considered the Eftim et al. (2008) 

study of mortality in 51the ACS sites and the Harvard Six Cities sites, as well as sites in 

the eastern region in the Zeger et al. (2008) study. Using data from the EPA’s AQS 

database, the analysis added the two years of air quality data just prior to the follow-up 

period in each study, which was 2000 to 2002 in Eftim et al. (2008) and 2000 to 2005 in 

Zeger et al. (2008). The analysis then calculated the extended long-term mean PM2.5

concentration for each study. As discussed in Schmidt (2012a), in each case the longterm mean PM2.5 concentration averaged over the extended exposure period was less than 

 

87 Nonetheless, the EPA notes that the Krewski et al. (2009) and Miller et al. (2007) 

studies provide strong evidence of mortality and cardiovascular-related effects associated 

with long-term PM2.5 exposures to inform causality determinations reached in the 

Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).
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0.34 μg/m3higher than the long-term mean PM2.5 concentration averaged over the 

follow-up period. The EPA finds it reasonable to conclude that such a relatively small 

difference in long-term mean PM2.5 concentrations would likely apply for other long-term 

exposure studies that used similarly recent follow-up periods as well (e.g., Goss et al., 

2004; Lipfert et al., 2006).

Based on the above considerations, the EPA concludes that it is appropriate to 

consider the available air quality information from the long-term exposure studies, while 

taking into account the uncertainties in the relevant long-term exposure periods in 

weighing the information from these studies. The EPA recognizes that considering such 

information in selecting an appropriate annual standard level has the potential to build in 

some margin of safety. The EPA further concludes that it is appropriate to consider the 

air quality information from the set of long-term exposure studies discussed above in the 

context of the broader array of epidemiological studies that inform the EPA’s 

consideration of the level of the annual PM2.5 standard.

The EPA also notes that while the long-term exposure studies are an important 

component of the epidemiological evidence that informs the Agency’s consideration of 

the level of the annual standard, they do not provide the only relevant information, nor 

are they the set of studies for which the relevant long-term mean PM2.5 concentrations are 

the lowest. As discussed in the proposal, the EPA also considers the long-term mean 

PM2.5 concentrations from the short-term mortality and morbidity studies as providing 

important information in considering the level of the annual standard. As discussed 

above, a large proportion of the aggregate risk associated with short-term exposures 

results from the large number of days during which the 24-hour average concentrations 
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are in the low- to mid-range of the concentrations observed in the studies. Thus, setting 

the level of the annual standard based on long-term mean concentrations, as well as the 

distribution of concentrations below the mean, in the short-term exposure studies is the 

most effective and efficient way to reduce total PM2.5-related risk from the broad array of 

mortality and morbidity effects associated with short-term exposures.

Further, the EPA notes that the relevant exposure period for the short-term 

exposure studies is the period contemporaneous with the collection of health event data, 

and that this exposure period is not subject to the uncertainties discussed above related to 

the long-term exposure studies. Recognizing that the long-term mean PM2.5

concentrations from several of the multi-city short-term exposure studies shown in Figure 

4 are below the long-term mean PM2.5 concentrations from the long-term exposure 

studies (with the exception of Miller et al., 2007).88 it is reasonable that in selecting the 

level of the annual standard primary consideration should be given to the information 

from this set of short-term exposure studies. There is no reasonable basis to discount the

long-term mean concentrations of the short-term exposure studies for purposes of setting 

the level of the annual standard. Thus, the commenter is incorrect in asserting that the 

long-term exposure studies, not the short-term exposure studies, would be central in the 

Administrator’s decision on the level of the annual standard. The standard is ultimately 

intended to protect not just against the single type of effect that contributes the most to 

quantitative estimates of risk to public health, but rather to the broad array of effects, 

including mortality and morbidity effects from long- and short-term exposures across the 

 

88 As noted above, the EPA is not placing weight on the reported long-term mean 

concentrations from the Miller et al. (2007) study for the purpose of translating the 

information from the long-term mortality studies into a basis for selecting the level of the 

annual PM2.5 standard.
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range of at-risk populations impacted by PM2.5-related effects.

(3) With regard to the EPA’s analysis of distributions of underlying populationlevel data (i.e., health event and study population data) and corresponding air quality data 

from each study area in certain key multi-city epidemiological studies (Rajan et al., 

2011), some commenters in this group raised a number of issues related to this analysis.

(API, 2012, Attachment 1 pp. 5 to 6; McClennan, 2012, pp.2 to 4). Some commenters 

noted the limited number of studies for which health event and study population data 

were available, and questioned whether these distributions would apply to other studies. 

Commenters expressed concerns that this analysis had not been formally reviewed by 

CASAC and was not published in the peer-review literature. Based on such concerns, 

some commenters asserted that the EPA should not consider this information as a basis 

for selecting a standard level.

As an initial matter, as discussed in section III.E.4.b above, the EPA agrees with 

CASAC’s advice that it is appropriate to consider additional data beyond the mean PM2.5

concentrations in key multi-city studies to help inform selection of the level of the annual 

PM2.5 standard. As both the EPA and CASAC recognize, in the absence of a discernible 

threshold, health effects may occur over the full range of concentrations observed in the 

epidemiological studies,. Nonetheless, the EPA recognizes that confidence in the 

magnitude and theresignificance of an association is a high degree of confidence that the 

observed effects are associated with concentrations that extend belowhighest at and 

around the long-term mean PM2.5 concentrations reported in the studies and the degree of 

confidence becomes lower at lower concentrations within any given study. Following 

CASAC’s advice (Samet, 2010d, p.2), the EPA used additional population-level and air 
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quality data made available by study authors to conduct an analysis of the distributions of 

such data, to help provide information oninform consideration of how the degree of 

confidence in the magnitude and significance of observed associations varies across the 

range of long-term mean PM2.5 concentrations in study areas within key multi-city 

epidemiological studies. In the EPA’s view, such consideration is important in selecting a 

level for an annual standard that will protect public health with an adequate margin of 

safety.

With regard to the number of multi-city studies for which an analysis of the 

distributions of population-level data across the study areas and the corresponding annual 

mean PM2.5 concentrations was done, the EPA noted at proposal that data for such an 

analysis were made available from study authors for four studies, including two longterm exposure studies and two short-term exposure studies.89 The EPA recognized that 

access to health event data can be restricted due to confidentiality issues, such that it is 

not reasonable to expect that such information could be made available from all studies. 

In considering the information from these four studies, the EPA has further taken into 

consideration uncertainties discussed in response to the above comment related to the 

appropriate exposure period for long-term exposure studies. Based on these 

considerations, as noted above, the EPA concludes that such uncertainties are an 

important factor in evaluating the usefulness of the air quality information from the two 

long-term exposure studies in this analysis (Krewski et al., 2009; Miller et al., 2007) and 

that it would not be appropriate to place weight on the distributional analysis of health 

 

89 Health event data and study population data were available from two short-term 

exposure studies (Bell et al. 2008; Zanobetti and Schwartz, 2009) and one long-term 

exposure study (Krewski et al., 2009). Only study population data were available from 

another long-term exposure study (Miller et al., 2007).
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event and air quality data from these two studies specifically for the purpose of 

translating the information from the long-term mortality studies into a basis for selecting 

the level of the annual PM2.5 standard. Such uncertainties are not relevant to the shortterm exposure studies, and thus, the Agency focuses on the two short-term exposure 

studies in this analysis (Bell et al., 2008; Zanobetti and Schwartz, (2009).

In focusing on these two short-term exposure studies, the EPA first notes that 

these studies are key multi-city studies that reported positive and statistically significant 

associations between mortality and cardiovascular-related hospital admissions across a 

large number of areas throughout the U.S. (112 U.S. cities in Zanobetti and Schwartz, 

2009; 202 U.S. counties in Bell et al., 2008) using relatively recent air quality and health 

event data (i.e., 1999 through 2005 in both studies). The EPA considers this to be a 

modest but important data set to use for this distributional analysis to help inform 

consideration of how much below the long-term mean PM2.5 concentrations in key multicity long- and short-term exposure studies the annual PM2.5 standard level should be set. 

While the EPA acknowledges that having such data available from more studies would 

have been useful, the Agency finds the information from this limited set of studies to be 

an important consideration in selecting an annual standard level, consistent with CASAC 

advice to consider such information.

The results of this distributional analysis are shown below in Figure 5 (adapted 

from Figure 3 in Rajan et al., 2011). For each study, this figure shows the cumulative 

frequency of the number of health events in each study area and the corresponding longterm mean PM2.5 concentrations in each area. Consistent with CASAC advice (Samet, 

2010d, p.2), such an analysis helps to inform the EPA’s understanding of the long-term 
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mean PM2.5 concentrations that were most influential in generating the health effect 

estimates in individual studies. In particular, the EPA recognizes that there is 

significantly greater confidence in the magnitude and significance of observed 

associations in that part of the air quality distribution corresponding to where the bulk of 

the health events evaluated in each study have been observed. With regard to the part of 

the distribution in which confidence in the magnitude and significance of associations 

observed may become appreciably lowerIn considering the results of this distributional 

analysis, as discussed more fully in the Response to Comments document, the EPA 

considers PM2.5 concentrations between the 25th and 10th percentiles of the distribution of 

health events to be a reasonable range for providing a general frame of reference for that 

part of the distribution in which confidence may becomein the magnitude and 

significance of the association may be appreciably lower. As highlighted in Figure 5, than 

confidence at and around the long-term mean PM2.5 concentrations corresponding with 

study areas contributing to the 25th and 10th percentiles of the distribution of deaths and 

cardiovascular-related hospitalizations in concentration. For the two short-term exposure 

studies included in this analysis, the EPA notes that the PM2.5 concentrations 

corresponding to the 25th percentiles of the distributions of health events were 12.5 μg/m3

and 10.311.5 μg/m3, respectively, for Zanobetti and Schwartz (2009),) and 11.5 μg/m3

and 9.8 μg/m3, respectively, for Bell et al. (2008). The EPA also), with the 10th

percentiles being lower by approximately 2 μg/m3in each study (Rajan et al., 2011, Table 

1). In considering this information, the EPA recognizes, however, that there is no clear 

dividing line or single percentile within a given distribution (including both above and 

below the 25th percentile) provided by the scientific evidence that is most appropriate or 
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‘correct’ to use to characterize where the degree of confidence in the associations 

warrants setting the annual standard level. The decision as to the appropriate standard 

level below the long-term mean concentrations of the key studies is largely a public 

health policy judgment to be made by the Administrator, taking into account all of the 

evidence and its related uncertainties, as discussed in section III.E.4.d below.

In response to concerns that this analysis was not reviewed by CASAC nor 

Formatted: Font: 12 pt
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published in the peer-reviewed literature, the EPA notes that this analysis was conducted 

to directly respond to advice from CASAC, as discussed in section III.E.4.b.1 above, in 

conjunction with their review of the Policy Assessment. The EPA notes that the same 

type of distributional analysis was presented in the second draft Policy Assessment based 

on air quality data, as well as population-weighted air quality data, rather than health 

event or study population data. In considering that distributional information, CASAC 

urged that the EPA redo the analysis using health event or study population data, which is 

exactly what the EPA did and presented in the final Policy Assessment. The EPA 

provided CASAC with the final Policy Assessment and communicated how the final staff 

conclusions reflected consideration of its advice and that those staff conclusions were 

based in part on the specific distributional analysis that CASAC had urged the EPA to 

conduct (Wegman, 2011, Attachment p. 2). CASAC did not choose to provide any 

additional comments or advice after receiving the final Policy Assessment. The EPA 

considers this distributional analysis to be the product of the peer review conducted by 

CASAC of the Policy Assessment, and thus does not agree with commenters’ 

characterization that the analysis lacked appropriate peer review. The EPA’s final 

analysis was based on the comments provided by CASAC, the peer review committee 

established pursuant to the CAA, on the draft analysis, such that the final analysis stems 

directly from CASAC’s advice and the EPA’s response to its comments.

Based on the above considerations, the EPA continues to conclude that its 

analysis of distributions of health event and air quality data from two key multi-city 

epidemiological studies provides important information related to understanding the 

associations between health events observed in each city (e.g., deaths, hospitalizations) 
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and the corresponding long-term mean PM2.5 concentrations observed in the studies. 

While recognizing that this is a relatively modest data set, the EPA further concludes that 

such information can appropriately help to inform the selection of the level of an annual 

standard that will protect public health with an adequate margin of safety from these 

types of health effects which are causally related to long- and short-term exposures to 

PM2.5.

(4) Some commenters in this group asserted there were limitations in the longterm exposure studies of morbidity, including studies evaluating respiratory effects in 

children. For example, one commenter (UARG, 2012, p. 12, Attachment 1, pp. 14 to 16) 

asserted there were serious limitations in the long-term exposure studies of respiratory 

morbidity in each of the studies considered by the EPA (including McConnell et al., 

2003; Gauderman et al., 2004; Dockery et al., 1996; Raizenne et al., 1996; and Goss et 

al., 2004) and argued that this evidence provides only a “weak association” with PM2.5

exposures. This commenter asserted that many of these long-term exposure studies

evaluating respiratory effects were considered at the time the EPA reaffirmed the current 

annual standard level of 15 μg/m3in 2006, that the Administrator in the last review 

determined that the information they provided “was too limited to serve as the basis for 

setting a level of a national standard,” and that they should be given little weight in 

setting the level of the annual standard in this review (UARG, 2012, Attachment 1, p. 

14). 

More specifically, this commenter asserted that the McConnell et al. (2003) and 

Gauderman et al. (2004) studies reported mixed results for associations with PM2.5 and 

stronger associations with NO2 (API, 2012, Attachment 1, pp. 14 to 15). Similarly, this 
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commenter argued that the Dockery et al. (1996) and Raizenne et al. (1996) studies 

showed stronger associations with acidity than with fine particles (measured as PM2.1). 

Id. pp. 15 to 16. With regard to the cystic fibrosis study, this commenter noted that the 

association between pulmonary exacerbations and PM2.5 in this study was no longer 

statistically significant when the model adjusted for each individual’s baseline lung 

function. The commenters referred to the data on lung function as an “important 

explanatory variable,” and suggested that the EPA should rely on results from the model 

that included individual baseline lung function information. Id. p. 16. For the reasons 

discussed below and in more detail in the Response to Comments document, the EPA 

disagrees with the commenters’ interpretation of these studies.

As an initial matter, the EPA notes that three of these studies (McConnell et al., 

2003; Dockery et al., 1996; Raizenne et al., 1996) as well as the initial studies from the 

Southern California Children’s Health Study (Peters et al., 1999; McConnell et al., 1999’ 

Gauderman et al., 2000, 2002; Avol et al., 2001) were discussed and considered in the 

2004 Air Quality Criteria Document (U.S. EPA, 2004) and, thus, considered within the 

air quality criteria supporting the EPA’s final decisions in the review completed in 2006. 

Two additional studies (Gauderman et al., 2004; Goss et al., 2004) were discussed and 

considered in the provisional science assessment conducted for the last review (U.S.

EPA, 2006a). The EPA concluded that “new” studies considered in the provisional 

assessment completed in 2006 did not materially change any of the broad scientific 

conclusions regarding the health effects of PM exposure made in the Criteria Document 

(71 FR 61148 to 61149, October 17, 2006). All of these studies were considered in the 

Integrated Science Assessment that informs the current review (U.S. EPA, 2009a).
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With regard to the Southern California Children’s Health Study, extended 

analyses considered in the Integrated Science Assessment provided evidence that 

clinically important deficits in lung function90 associated with long-term exposure to 

PM2.5 persist into early adulthood (U.S. EPA, 2009a, p. 7-27; Gauderman et al., 2004). 

These effects remained positive in copollutant models.91 Additional analyses of the 

Southern California Children’s Health Study cohort reported an association between 

long-term PM2.5 exposure and bronchitic symptoms (U.S. EPA, 2009a, p. 7-23 to 7-24; 

McConnell et al., 2003, long-term mean concentration of 13.8 μg/m3) that remained 

positive in co-pollutant models, with the PM2.5 effect estimates increasing in magnitude 

in some models and decreasing in others, and a strong modifying effect of PM2.5 on the 

association between lung function and asthma incidence (U.S. EPA, 2009a, 7-24; Islam et 

al., 2007). The outcomes observed in the more recent reports from the Southern 

California Children’s Health Study, including evaluation of a broader range of endpoints 

and longer follow-up periods, were larger in magnitude and more precise than reported in 

the initial version of the study. Supporting these results were new longitudinal cohort 

studies conducted by other researchers in varying locations using different methods (U.S.

EPA, 2009a, section 7.3.9.1). The EPA, therefore, disagrees with the commenters that the 

studies by McConnell et al. (2003) and Gauderman et al. (2004) are flawed and should 

 

90 Clinical significance was defined as an FEV1 below 80 percent of the predicted value, a 

criterion commonly used in clinical settings to identify persons at increased risk for 

adverse respiratory conditions (U.S. EPA, 2009a, p. 7-29 to 7-30). The primary NAAQS 

for sulfur dioxide (SO2) also included this interpretation for FEV1 (75 FR 35525, June 22, 

2010).

91 Gauderman et al. (2004) clearly stated throughout their analysis that NO2 was one 

component of a highly correlated mixture that contains PM2.5. Gauderman et al. (2004) 

did not present the results from copollutants models but stated “two-pollutant models for 

any pair of pollutants did not provide a significantly better fit to the data than the 

corresponding single-pollutant models.”
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not be used in the PM NAAQS review process.

The 24-City study92 by Dockery et al. (1996) (long-term mean concentration of 

14.5 μg/m3) was considered in the current as well as two previous reviews (U.S. EPA, 

2009a; U.S. EPA, 2004; U.S. EPA, 1996). This study observed that PM, specifically 

“particle strong acidity” and sulfate particles (indicators of fine particles), were 

associated with reports of bronchitis in the previous year. Similarly, the magnitude of the 

associations between bronchitis and PM10 and PM2.1 were similar to those for acidic 

aerosols and sulfate particles, though the confidence intervals for the PM10 and PM2.1

associations were slightly wider and the associations were not statistically significant.

Acid aerosols, sulfate, and fine particles are formed in secondary reactions of the 

emissions from incomplete combustion and these pollutants have similar regional and 

temporal distributions. As noted by the study authors, “the strong correlations of several 

pollutants in this study, especially particle strong acidity with sulfate (r=0.90) and PM2.1

(r=0.82), make it difficult to distinguish the agent of interest” (Dockery et al., 1996, p. 

505). Overall, Dockery et al. (1996) (and, similarly, Raizenne et al., 1996) observed 

similar associations between respiratory health effects and acid aerosols, sulfate, PM10

and PM2.1 concentrations. The commenters noted that the associations with particle 

acidity were sensitive to the inclusion of the six Canadian sites. The EPA notes that none 

of these Canadian cities were in the “sulfate belt” where particle strong acidity was 

highest. Thus, the change in the effect estimate when the six Canadian cities were 

excluded from the analysis is likely due to the lower prevalence of bronchitis and the 

lower concentrations of acid aerosols in these cities, and not due to some difference in 

 

92 The 24-City study conducted by Dockery et al. (1996) included 18 sites in the U.S. and 

6 sites in Canada. The Raizenne et al. (1996) study considered 22 of these 24 study areas. 

Athens, OH and South Brunswick, NJ were not included in this study. 
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susceptibility to bronchitis between the U.S. and Canadian populations that is not due to 

air pollution, as suggested by the commenters (UARG, 2012, Attachment 1, p. 15). In 

fact, contrary to the statements made by the commenters, the authors did not observe any 

subgroups that appeared to be markedly more susceptible to the risk of bronchitis. 

The Goss et al. (2004) study considered a U.S. cohort of cystic fibrosis patients 

and provided evidence of association between long-term PM2.5 exposures and 

exacerbations of respiratory symptoms resulting in hospital admissions or use of home 

intravenous antibiotics (U.S. EPA, 2009a, p. 7-25; long-term mean concentration of 13.7 

μg/m3). The commenters noted that the association between pulmonary exacerbations and 

PM2.5 in this study was no longer statistically significant when the model adjusted for 

each individual’s baseline lung function. The commenters referred to the data on lung 

function as an “important explanatory variable,” and suggested that the EPA should rely 

on results from the model that included individual baseline lung function information. 

The EPA disagrees with the commenters’ interpretation of this study. The Agency 

concludes it is unlikely that lung function is a potential confounder or an important 

explanatory variable in this study. In fact, the authors noted that “it is more likely that 

lung function decline may be intimately associated with chronic exposure to air pollutants 

and may be part of the causal pathway in worsening prognosis in CF [cystic fibrosis]; in 

support of this explanation, we found both cross-sectional and longitudinal strong inverse 

relationships between FEV1 and PM levels” (Goss et al., 2004, p. 819). The EPA notes 

that adjusting for a variable that is on the causal pathway can lead to overadjustment bias, 

which is likely to attenuate the association (Schisterman et al. 2009); this is likely what 

was observed by the authors. Thus, the EPA continues to believe it is appropriate to focus 
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on the results reported in Goss et al. (2004) that did not include individual baseline lung 

function in the model.

In addition, the EPA disagrees with commenters’ reliance solely on statistical 

significance when interpreting the study results from individual study results and the 

collective evidence across studies. As discussed in section III.D.2 above, statistical 

significance of individual study findings has played an important role in the EPA’s 

evaluation of the study’s results and the EPA has placed greater emphasis on studies 

reporting statistically significant results. However, in the broader evaluation of the 

evidence from many epidemiological studies, and subsequently during the process of 

forming causality determinations in the Integrated Science Assessment by integrating 

evidence from across epidemiological, controlled human exposure, and toxicological 

studies, the EPA has emphasized the pattern of results across epidemiological studies and 

whether the effects observed were coherent across the scientific disciplines for drawing 

conclusions on the relationship between PM2.5 and different health outcomes. 

As noted in section III.B.1.a of the proposal, with regard to respiratory effects, the 

Integrated Science Assessment concluded that extended analyses of studies available in 

the last review as well as new epidemiological studies conducted in the U.S. and abroad 

provided stronger evidence of respiratory-related morbidity associated with long-term 

PM2.5 exposure (77 FR 38918). The strongest evidence for respiratory-related effects 

available in this review was from epidemiological studies that evaluated decrements in 

lung function growth in children and increased respiratory symptoms and disease 
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incidence in adults (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1).93

In considering the collective evidence from epidemiological, toxicological, and 

controlled human exposure studies, including the studies discussed above, the EPA 

recognizes that the Integrated Science Assessment concluded that a causal relationship is 

likely to exist between long-term PM2.5 exposures and respiratory effects (U.S. EPA, 

2009a, p. 2-12, pp. 7-42 to 7-43). CASAC concurred with this causality determination 

(Samet, 2009f, p.9).

The commenter’s assertion that the EPA should adhere to its assessment of these 

studies as it did in the review completed in 2006 is significantly mistaken. Most 

obviously, the EPA’s final decision in the last review was held to be deficient by the D.C. 

Circuit in remanding the 2006 primary annual PM2.5 standard. As discussed in section 

III.A.2 above, the D.C. Circuit specifically held that the EPA did not provide a 

reasonable explanation of why certain morbidity studies, including an earlier study from 

the Southern California Children’s Health Study (Gauderman et al., 2000, long-term 

mean PM2.5 concentration approximately 15 μg/m3) and the 24-Cities Study (Raizenne et 

al., 1996, long-term mean concentrations approximately 14.5 μg/m3) did not warrant a 

more stringent annual PM2.5 standard when the long-term mean PM2.5 concentrations

reported in those studies were at or lower than the level of the annual standard. American 

Farm Bureau Federation v. EPA. 559 F. 3d at 525. Indeed, the court found that, viewed 

together, the Gauderman et al. (2000) and Raizenne et al., (1996) studies “are related and 

together indicate a significant public health risk...On this record, therefore, it appears the 

EPA too hastily discounted the Gauderman and 24-Cities studies as lacking in 

 

93 Supporting evidence comes from studies “that observed associations between longterm exposure to PM10 and an increase in respiratory symptoms and reductions in lung 

function growth in areas where PM10 is dominated by PM2.5” (U.S. EPA, 2009a, p. 2-12). 
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significance.” Id.

In this review, the EPA recognizes a significant amount of evidence beyond these 

two studies that expands our understanding of respiratory effects associated with longterm PM2.5 exposures. This body of scientific evidence includes an extended and new 

analyses from the Southern California Children’s Health Study (Gauderman et al., 2004; 

Islam et al., 2007; Stanojevic et al., 2008) as well as additional studies that examined 

these health effects (Kim et al., 2004; Goss et al., 2004). Thus, even more so than in the 

last review, the evidence indicates a “significant public health risk” to children from 

long-term PM2.5 exposures at concentrations below the level of the current annual 

standard. A standard that does not reflect appropriate consideration of this evidence 

would not be requisite to protect public health with an adequate margin of safety.

(5) With regard to the use of studies of health effects for which the EPA finds the 

evidence to be “suggestive” of a causal relationship, some commenters argued that such 

studies “do not merit any weight in the setting of the annual NAAQS” (e.g., UARG, 

2012, Appendix 1, p. 3).

The EPA disagrees with the commenter’s view that studies of health effects for 

which the evidence is suggestive of a causal relationship, rather than studies of health 

effects for which the evidence supports a causal or likely causal relationship, merit no 

weight at all in setting the NAAQS. To place no weight at all on such evidence would in 

essence treat such evidence as though it had been categorized as “not likely to be a causal 

relationship.” To do so would ignore the important distinctions in the nature of the 

evidence supporting these different causality determinations in the Integrated Science 

Assessment. It would also ignore the CAA requirement that primary standards are to be 
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set to provide protection with an adequate margin of safety, including providing 

protection for at-risk populations. Thus, ignoring this information in making decisions on 

the appropriate standard level would not be appropriate.94 Nonetheless, in considering 

studies of health effects for which the evidence is suggestive of a causal relationship, the 

EPA does believe that it is appropriate to place less weight on such studies than on 

studies of health effects for which there is evidence of a causal or likely causal 

relationship.

A second group of commenters supported revising the suite of primary PM2.5

standards to provide increased public health protection. These commenters found the 

available scientific information and technical analyses to be stronger and more 

compelling than in the last review. These commenters generally placed substantial weight 

on CASAC advice and on the EPA staff analyses presented in the final Policy 

Assessment, which concluded that the evidence most strongly supported an annual 

standard level within a range of 11 to 12 μg/m3(U.S. EPA, 2011a, p. 2-206). While some 

of these commenters felt that the level should be set within the proposed range (12 to 13 

μg/m3), most of these commenters advocated a level of 11 μg/m3.

95 For example, ALA et 

al., asserted:

The EPA’s proposed PM2.5 standards, while a step in the right direction are 

insufficient to protect public health, including the health of susceptible 

populations, with an adequate margin of safety as required by the Clean Air 

Act...we will discuss the enormous gap in public health protection afforded by an 

 

94 As discussed in section II.A above, the requirement that primary standards provide an 

adequate margin of safety was intended to address uncertainties associated with 

inconclusive scientific and technical information available at the time of standard setting. 

I was also intended to provide a reasonable degree of protection against hazards that 

research has not yet identified. This certainly encompasses consideration of effects for 

which there is evidence suggestive of a causal relationship.

95 As discussed in section III.E.4.c.ii, many of these commenters also supported lowering 

the level of the primary 24-hour PM2.5 standard.
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annual standard of 13 μg/m3, at the upper end of the proposed range, compared to 

the more protective 11 μg/m3, as advocated by our organizations (ALA et al., 

2012, p. 6). 

In general, these commenters expressed the view that given the strength of the 

available scientific evidence, the serious nature of the health effects associated with PM2.5

exposures, the large size of the at-risk populations, the risks associated with long- and 

short-term PM2.5 exposures, and the important uncertainties inherently present in the 

evidence, the EPA should follow a highly precautionary policy response by selecting an 

annual standard level that incorporates a large margin of safety. 

More specifically, these commenters offered a range of comments related to the 

general approach used by the EPA to select standard levels, including: (1) the EPA’s 

approach for setting a generally controlling annual standard; (2) the importance of the 

greatly expanded and stronger overall scientific data base; (3) consideration of the 

distributional statistical analysis conducted by the EPA and other approaches for 

translating the air quality information from specific epidemiological studies into standard 

levels; and (4) the significance of the PM2.5-related public health impacts, especially 

potential impacts on at-risk populations, including children, in reaching judgments on 

setting standards that provide protection with an adequate margin of safety. These 

comments are discussed in turn below.

(1) Some of these commenters disagreed with the EPA’s approach for setting a 

“generally controlling” annual standard in conjunction with a 24-hour standard providing 

supplemental protection particularly for areas with high peak-to-mean ratios. These 

commenters argued this approach would lead to “regional inequities” as demonstrated in 

the EPA’s analyses contained in Appendix C of the Policy Assessment (ALA et al., pp. 

26 to 27). Specifically, these commenters argued:
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There is no basis in the Clean Air Act for such a determination. The Clean Air 

Act requires only that the NAAQS achieve public health protection with an 

adequate margin of safety. It is well-documented that both long- and short-term 

exposures to PM2.5 have serious and sometimes irreversible health impacts. There 

is no health protection reason to argue that one standard should be “controlling” 

as a matter of policy without regard to the health consequences of such a policy. 

To adopt such a policy ignores the obligation to provide equal protection under 

the law to all Americans because it would result in uneven protection from air 

pollution in different localities and regions of the country (ALA et al., 2012, p. 

26).

The EPA believes these commenters misunderstood the basis for the EPA’s 

policy goal of setting a “generally controlling” annual standard. This approach relates 

exclusively to setting standards that will provide requisite protection against effects 

associated with both long- and short-term PM2.5 exposures. It does so by lowering the 

overall air quality distributions across an area, recognizing that changes in PM2.5 air 

quality designed to meet an annual standard would likely result not only in lower annual 

mean PM2.5 concentrations but also in fewer and lower peak 24-hour PM2.5

concentrations. As discussed in section III.A.3 in the proposal and above, the EPA 

recognizes that there are various ways to combine the two primary PM2.5 standards to 

achieve an appropriate degree of public health protection. Furthermore, the extent to 

which these two standards are interrelated in any given area depends in large part on the 

relative levels of the standards, the peak-to-mean ratios that characterize air quality 

patterns in an area, and whether changes in air quality designed to meet a given suite of 

standards are likely to be of a more regional or more localized nature. 

In focusing on an approach of setting a generally controlling annual standard, the 

EPA’s intent is in fact to avoid the potential “regional inequities” that are of concern to 

the commenters. The EPA judges that the most appropriate way to set standards that 

provide more consistent public health protection is by using the approach of setting a 
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generally controlling annual standard. This judgment builds upon information presented 

in the Policy Assessment as discussed in section III.A.3 above. More specifically, the 

Policy Assessment recognized that the short-term exposure studies primarily evaluated 

daily variations in health effects with monitor(s) that measured the variation in daily 

PM2.5 concentrations over the course of several years. The strength of the associations 

observed in these epidemiological studies was demonstrably in the numerous “typical” 

days within the air quality distribution, not in the peak days (U.S. EPA, 2011a, p. 2-9). In 

addition, the quantitative risk assessments conducted for this and previous reviews 

demonstrated the same point, that is, much, if not most, of the aggregate risk associated 

with short-term PM2.5 exposures results from the large number of days during which the 

24-hour average concentrations are in the low-to mid-range, below the peak 24-hour 

concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In 

addition, there was no evidence suggesting that risks associated with long-term exposures 

were likely to be disproportionately driven by peak 24-hour concentrations.96

For these reasons, the Policy Assessment concluded that strategies that focused 

primarily on reducing peak days were less likely to achieve reductions in the PM2.5

concentrations that were most strongly associated with the observed health effects. 

Furthermore, the Policy Assessment concluded that an approach that focused on reducing 

peak exposures would most likely result in more uneven public health protection across 

the U.S. by either providing inadequate protection in some areas or overprotecting in 

other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because 

 

96 In confirmation, a number of studies have presented analyses excluding higher PM 

concentration days and reported a limited effect on the magnitude of the effect estimates 

or statistical significance of the association (e.g., Dominici, 2006b; Schwartz et al., 1996; 

Pope and Dockery, 1992).
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reductions based on control of peak days are less likely to control the bulk of the air 

quality distribution. 

As a result, the EPA believes an approach that focuses on a generally controlling 

annual standard would likely reduce aggregate risks associated with both long- and shortterm exposures more consistently than a generally controlling 24-hour standard and, 

therefore, would be the most effective and efficient way to reduce total PM2-5-related 

population risk. The CASAC agreed with this approach and considered it was 

“appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” 

(Samet, 2010d, p. 1). For the reasons discussed above, the EPA disagrees with the 

comments that this approach will result in the concerns raised by the commenters; rather 

the EPA concludes that this approach will help to address these concerns.

(2) Many of these commenters asserted that the currently available scientific 

information is greatly expanded and stronger compared to the last review. Some of these 

commenters highlighted the availability of multiple, multi-city long- and short-term 

exposure studies providing “repeated, consistent evidence of effects below the current 

annual standard level” (ALA et al., 2012, pp. 39 to 49) and, more specifically, 

“significant evidence of harm with strong confidence well below EPA’s proposed annual 

standard range of 12-13 μg/m3” (AHA et al., 2012, pp. 10 to 12). 

The EPA recognizes that in setting standards that are requisite to protect public 

health with an adequate margin of safety, the Administrator must weigh the various types 

of available scientific information in reaching public health policy judgments that neither 
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overstate nor understate the strength and limitations of this information or the appropriate 

inferences to be drawn from the available science. 

In general, the EPA agrees with these commenters’ views that the currently 

available scientific evidence is stronger “because of its breadth and the substantiation of 

previously observed health effects” (77 FR 38906/2) and provides “greater confidence in 

the reported associations than in the last review” (77 FR 38940/1). The EPA also agrees 

with the commenters’ position that it is appropriate to consider the regions within the 

broader air quality distributions where we have the strongest confidence in the 

associations reported in epidemiological studies in setting the level of the annual 

standard. However, as discussed in section III.E.4.d below, in weighing the available 

evidence and technical analyses, as well as the associated uncertainties and limitations in 

that information, the EPA disagrees with the commenters’ views regarding the extent to 

which the available scientific information provides support for considering an annual 

standard level below the proposed range (i.e., below 12 to 13 μg/m3). In particular, the 

EPA disagrees with the degree to which these commenters place more weight on the 

relatively more uncertain evidence that is suggestive of a causal relationship (e.g., low 

birth weight). Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency 

concludes it is appropriate and reasonable to place the greatest emphasis on health effects 

for which the Integrated Science Assessment concluded there is evidence of a causal or 

likely causal relationship and to place less weight on the health effects that provide 

evidence that is only suggestive of a causal relationship.. 

(3) With regard to using the air quality information from epidemiological studies 

to inform decisions on standard levels, commenters in this group generally supported the 
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EPA’s efforts to explore different statistical metrics from epidemiological studies to 

inform the Administrator’s decisions. These commenters argued that by considering 

different analytic measures -- either concentrations one standard deviation below the 

long-term means reported in the epidemiological studies or the EPA’s distributional 

statistical analysis of population-level data that extends the approach used in previous PM 

NAAQS reviews to consider information beyond a single statistical metric -- “the annual 

standard must be significantly lower than EPA has proposed” (ALA et al., 2012, pp. 50 to 

61). Furthermore, with regard to characterizing the PM2.5 air quality at which associations 

have been observed, some of these commenters highlighted CASAC’s recommendation 

that “[f]urther consideration should be given to using the 10th percentile as a level for 

assessing various scenarios of levels for the PM NAAQS” (Samet, 2010c, p. 11) (ALA et 

al., 2012, p. 55). Other commenters urged that the EPA extend the distributional analysis 

to include additional studies. For example, CHPAC urged the EPA to also conduct 

distributional analysis for children’s health studies to better inform standards that would 

protect both children and adults from adverse health outcomes (CHPAC, 2012, p. 3)

The EPA agrees with these commenters’ views that it is appropriate to take into 

account different statistical metrics from epidemiological studies to inform the decisions 

on standard levels that are appropriate to consider. in setting a standard that will protect 

public health with an adequate margin of safety. In the development of the Policy 

Assessment, the EPA staff explored various approaches for using information from 

epidemiological studies in setting the standards. The general approach used in the final 

Policy Assessment, discussed in sections III.A.3 and III.E.4.a above, reflects 

consideration of CASAC advice (Samet, 2010c, d) and public comments on multiple 
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drafts of the Policy Assessment.

With regard to using the distributional statistical analysis to characterize the 

confidence in the associations, the EPA emphasizes that there is no clear dividing line 

provided by the scientific evidence, and that choosing how far below the long-term mean 

concentrations from the epidemiological studies is appropriate to identify a standard level 

that will provide protection for the public health with an adequate margin of safety is 

largely a public health policy judgment. The EPA considers the region from 

approximately the 25th to 10th percentiles to be a reasonable range for providing a general 

frame of reference as to the part of the distribution over which our confidence in the 

magnitude and significance of the associations observed in epidemiological studies 

becomesis appreciably lower. However Based on these considerations, the EPA 

concludes that it is not appropriate to place as much confidence in the magnitude and 

significance of the associations over the lower percentiles of the distributions in each 

study as at and around the long-term mean concentrations. Thus, the EPA disagrees with 

the commenters’ views that the available informationthis analysis compels placing more 

emphasis on the lower part of this range in selecting a level for an annual standard that 

will protect public health with an adequate margin of safety. The EPA recognizes that 

this information comes primarily from two short-term exposure studies, a relatively 

modest data set. In light of the limited nature of this information, and in recognition of 

more general uncertainties inherent in the epidemiological evidence, the Administrator

deems it reasonable not to place more emphasis on concentrations in the lower part of 

this range, as discussed below in section III.E.4.d.

With regard to the scope of the distributional statistical analysis, the EPA 
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requested additional population-level data from the study authors for a group of six multicity studies for which previous air quality analyses had been conducted (Hassett-Sipple et 

al., 2010; Schmidt et al., 2010, Analysis 2). These six studies were originally selected 

because they considered multiple locations representing varying geographic regions 

across multiple years. Thus, these studies provided evidence on the influence of different 

particle mixtures on health effects associated with long- and short-term PM2.5 exposures. 

In addition, these multi-city studies considered relatively more recent health events and 

air quality conditions (1999 to 2005). As discussed in section III.E.4.b.i above, the EPA 

received and analyzed population-level data for four of the six studies (Rajan et al., 

2011). Three of these four studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and 

Schwartz, 2009) served as the basis for the concentration-response functions used to 

develop the core risk estimates (U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees 

that it would be useful to have such data from more studies, the Agency believes that the 

additional data that was requested and received from study authors provide useful 

information to help inform the Administrator’s selection of the annual standard level. 

(4) Many commenters in this group highlighted PM2.5-related impacts on at-risk 

populations, including potential impacts on children, older adults, persons with preexisting heart and lung disease, and low-income populations, to support their views that 

the annual standard should be revised to a level of 11 μg/m3or lower (CHPAC, 2012; 

AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al., 2012; Air Alliance Houston, et al, 

2012). These commenters urged the EPA to adopt a policy approach that placed less 

weight on the remaining uncertainties and limitations in the evidence and placed more 

emphasis on margin of safety considerations, including providing protection against 
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effects for which there is more limited scientific evidence. For example, CHPAC urged 

the EPA “to place the same weight on studies examining impacts on children’s health as 

that of adult studies. ... The fact that there may be stronger evidence from adult studies 

does not mean that standards based on adult studies will be protective for children and 

consequently will meet the standard requisite to protect public health with an adequate 

margin of safety” (CHPAC, 2012 p. 3). Furthermore, with regard to the EPA’s approach 

for weighing uncertainties, some of these commenters stated that “we find no justification 

in the preamble for an annual standard level as high as 13 μg/m3, other than the vague 

assertion that uncertainties increase at lower concentrations. Further, the final proposal 

completely failed to address the Policy Assessment recommendations that if 13 μg/m3

was proposed, the 24-hour standard should be strengthened as well” (ALA et al., p. 7).

The EPA has carefully evaluated and considered evidence of effects in at-risk 

populations. With regard to effects classified as having evidence of a causal or likely 

causal relationship with long- or short-term PM2.5 exposures (i.e., premature mortality, 

cardiovascular effects, and respiratory effects), the Agency takes note that it considered 

the full range of studies evaluating these effects, including studies of at-risk populations, 

to inform its review of the primary PM2.5 standards. Specific multi-city studies 

summarized in Figures 1, 2, and 3 above highlight evidence of effects observed in two 

different lifestages – children and older adults – that have been identified as at-risk 

populations. Thus, the EPA places as much weight on studies that explored effects in 

children for which the evidence is causal or likely causal in nature as on studies of such 

effects in adults, including older adults. .As discussed above in responses to commenters 

supporting the retention of the current standards, in setting the standard, the EPA has 
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focused on considering PM2.5 concentrations somewhat below the lowest long-term mean 

concentrations from each of the key studies of both long- and short-term exposures of 

effects for which the evidence supports a causal or likely causal relationship (i.e., the first 

two sets of studies shown in Figure 4). Absent some reason to ignore or discount these 

studies, which the commenter does not provide (and of which the EPA is unaware), the 

EPA considers the available evidence of effects in children as well as other at-risk 

populations.

With respect to the EPA’s consideration of more limited studies providing 

evidence suggestive of a causal relationship (e.g., developmental and reproductive 

effects), as noted above in responding to comments from the first group of commenters, 

the Agency agrees that it is important to place some weight on this body of evidence in 

setting standards that provide protection for at-risk populations, as required by the CAA. 

However, the Agency does not agree that the same weight must be placed on this 

information as on the body of scientific information for which there is evidence of a 

causal or likely causal relationship. To do so would ignore the difference in the breadth 

and strength of the evidence supporting the different causality determinations reached in 

the Integrated Science Assessment. 

With regard to weighing the uncertainties and limitations remaining in the 

evidence and technical analyses, as discussed in section II.A above, the EPA recognizes 

that in setting a primary NAAQS that provides an adequate margin of safety, the 

Administrator must consider a number of factors including the nature and severity of the 

health effects involved, the size of sensitive population(s) at risk, and the kind and degree 

of the uncertainties that remain. As discussed in section III.E.4.d below, the Agency 
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agrees with these commenters that, in weighing the available evidence and technical 

analyses including the uncertainties and limitations in this scientific information, there is 

no justification for setting a primary PM2.5 annual standard level as high as 13 μg/m3.

Finally, some commenters in both groups also identified “new” studies that were 

not included in the Integrated Science Assessment as providing further support for their 

views on the level of the annual standard. As discussed in section II.B.3 above, the EPA 

completed a provisional review and assessment of “new” studies published since the 

close of the Integrated Science Assessment, including “new” studies submitted by 

commenters (U.S. EPA, 2012a). The provisional assessment found that the “new” studies 

expand the scientific information considered in the Integrated Science Assessment and 

provide important insights on the relationship between PM2.5 exposure and health effects 

of PM (U.S. EPA, 2012b). However, the EPA notes that the provisional assessment found 

that the “new” science did not materially change the conclusions reached in the 

Integrated Science Assessment. The EPA notes that, as in past NAAQS reviews, the 

Agency is basing the final decisions in this review on the studies and related information 

included in the Integrated Science Assessment that have undergone CASAC and public 

review, and will consider newly published studies for purposes of decision making in the 

next PM NAAQS review.

ii. 24-Hour Standard Level

With respect to the level of the 24-hour standard, the EPA received comments on 

the proposal from two distinct groups of commenters. One group that included virtually 

all commenters representing industry associations, businesses, and many States agreed 

with the Agency’s proposed decision to retain the level of the 24-hour PM2.5 standard. 
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The other group of commenters included many medical groups, numerous physicians and 

academic researchers, many public health organizations, some State and local agencies, 

five State Attorneys General, and a large number of individual commenters. These 

commenters disagreed with the Agency’s proposed decision and argued that EPA should 

lower the level of the 24-hour standard to 30 or 25 μg/m3. Comments from these groups 

on the level of the 24-hour PM2.5 standard are addressed below and in the Response to 

Comments Document.

As noted above, of the public commenters who addressed the level of the 24-hour 

PM2.5 standard, all industry commenters and most State and local commenters supported 

the proposed decision to retain the current level of 35 g/m3. In many cases, these groups 

agreed with the rationale supporting the Administrator’s proposed decision to retain the 

current 24-hour PM2.5 standard, including her emphasis on the annual standard as the 

generally controlling standard with the 24-hour standard providing supplementary 

protection, and her conclusion that multi-city, short-term exposure studies provide the 

strongest data set for informing decisions on the appropriate 24-hour standard level. 

Many of these commenters agreed with the Administrator’s view that the single-city, 

short-term studies provided a much more limited data set (e.g., limited statistical power, 

limited exposure data) and more equivocal results (e.g., mixed results within the same 

study area), making them an unsuitable basis for setting the level of the 24-hour standard. 

While these commenters agreed with the EPA’s proposed decision to retain the 

current 24-hour PM2.5 standard, some did not agree with the EPA’s approach to 

considering the evidence from short-term multi-city studies. For example, a commenter 

representing UARG pointed out that the 98th percentile concentrations reported in the 
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proposal for multi-city studies reflect the averages of 98th percentile concentrations across 

the cities included in those studies (UARG, 2012; Attachment 1; p. 25). This commenter 

contended that such averaged 98th percentile PM2.5 concentrations do not provide 

information that can appropriately inform a decision on the adequacy of the public health 

protection provided by the current or alternative 24-hour standards. 

While the EPA agrees that there is uncertainty in linking effects reported in multicity studies to specific air quality concentrations (U.S. EPA, 2011, section 2.3.4.1), the 

EPA disagrees with this commenter’s view that such uncertainty precludes the use of 

averaged 98th percentile PM2.5 concentrations to inform a decision on the appropriateness 

of the protection provided by the 24-hour PM2.5 standard. In particular, the EPA notes 

that averaged 98th percentile concentrations do provide information on the extent to 

which study cities contributing to reported associations would likely have met or violated 

the current 24-hour PM2.5 standard during the study period. As evidence of this, the EPA 

notes the three multi-city studies specifically highlighted by this commenter as having 

averaged 98th percentile 24-hour PM2.5 concentrations below 35 g/m3(Dominici et al., 

2006; Bell et al., 2008; Zanobetti and Schwartz, 2009). Based on the 98th percentiles of 

24-hour PM2.5 concentrations in the individual cities evaluated in these studies, the EPA 

notes that the majority of these study cities would likely have met the current standard 

during the study periods (Hassett-Sipple et al., 2010). Therefore, regardless of whether 

the averaged 98th percentile concentrations or the 98th percentile concentrations in each 

city are considered, these studies provide evidence for associations between short-term 

PM2.5 and mortality or morbidity across a large number of U.S. cities, the majority of 

which would likely have met the current 24-hour PM2.5 standard during study periods. In 
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their review of the PM Policy Assessment, CASAC endorsed the conclusions drawn from 

analyses of averaged 98th percentile 24-hour PM2.5 concentrations, and the EPA continues 

to conclude that this type of information can appropriately inform the Administrator’s 

decision on the current 24-hour PM2.5 standard.97

Another group of commenters argued that the 24-hour standard level should be 

lowered. Many of these commenters supported setting the level of the 24-hour PM2.5

standard at either 25 or 30 g/m3. In support of their position, the ALA et. al., AHA et al.,

five state Attorneys General, and a number of additional groups pointed to 98th percentile 

PM2.5 concentrations in locations of multi-city and single-city epidemiological studies. 

For example, the ALA and others pointed to multi-city studies by Dominici et al. (2006), 

Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al. (2008) as providing 

evidence for associations with mortality and morbidity in study locations with averaged 

(i.e., averaged across cities) 98th percentile 24-hour PM2.5 concentrations below 35 

g/m3. These commenters also pointed to several single-city and panel studies reporting 

associations between short-term PM2.5 and mortality or morbidity in locations with 

relatively low 24-hour PM2.5 concentrations. Because some of these multi- and single-city 

studies have reported associations with health effects in locations with 98th percentile 

PM2.5 concentrations below 35 g/m3, commenters maintained that the current 24-hour 

PM2.5 standard (i.e., with its level of 35 g/m3) does not provide an appropriate degree of 

 

97This is not to say that the EPA’s decision on whether to revise the 24-hour PM2.5

standard should be based on or only be informed by considerations of whether studies 

reported associations with mortality or morbidity in areas with averaged 98th percentile 

PM2.5 concentrations less than 35 mg/m3. As discussed below, in reaching a decision in 

this final notice on the most appropriate approach to strengthen the suite of PM2.5

standards, the Administrator considers the degree of public health protection provided by 

the combination of the annual and 24-hour standards together. 
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protection in all areas.

In further support of their position that the level of the current 24-hour standard 

should be lowered, these commenters pointed out the variability across the U.S. in ratios 

of 24-hour to annual PM2.5 concentrations. They noted that some locations, including 

parts of the northwestern U.S., experience relatively low annual PM2.5 concentrations but 

can experience relatively high 24-hour concentrations at certain times of the year. In 

order to provide protection against effects associated with short-term PM2.5 exposures, 

especially in locations with high ratios of 24-hour to annual PM2.5 concentrations, these 

commenters advocated setting a lower level for the 24-hour standard. 

The EPA agrees with these commenters that it is appropriate to maintain a 24-

hour PM2.5 standard in order to supplement the protection provided by the revised annual 

standard, particularly in locations with relatively high ratios of 24-hour to annual PM2.5

concentrations. However, in highlighting 98th percentile PM2.5 concentrations in study 

locations without also considering the impact of a revised annual standard on short-term 

concentrations, these commenters ignore the fact that many areas would be expected to 

experience decreasing short- and long-term PM2.5 concentrations in response to a revised 

annual standard. 

In considering the specific multi-city studies highlighted by public commenters 

who advocated a more stringent 24-hour standard, the EPA notes that such studies have 

reported consistently positive and statistically significant associations with short-term 

PM2.5 exposures in locations with averaged 98th percentile PM2.5 concentrations ranging

from 45.8 to 34.2 μg/m3and long-term mean PM2.5 concentrations ranging from 13.4 to 

12.9 (Burnett and Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006a; Bell et al., 
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2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009).98 The EPA notes that to the 

extent air quality distributions are reduced to meet the current 24-hour standard with its 

level of 35 g/m3and/or the revised annual standard with its level of 12 g/m3, additional 

protection would be anticipated against the effects reported in these short-term, multi-city 

studies. Put another way, to attain an annual standard with a level below the long-term 

means in the locations of these short-term studies (as EPA is adopting here), the overall 

air quality distributions in the majority of study cities will necessarily be reduced, 

resulting in lower daily PM2.5 ambient concentrations. We therefore expect that the 

revised annual standard will result in 98th percentile PM2.5 concentrations in these cities 

that are lower than those measured in the studies, and that the overall distributions of 

PM2.5 concentrations will be lower than those reported to be associated with health 

effects. Thus, even for effects reported in multi-city studies with averaged 98th percentile 

concentrations below 35 μg/m3, additional protection from the risks associated with 

short-term exposures is anticipated from the revised annual standard, without revising the 

24-hour standard, because long-term average PM2.5 concentrations in multi-city study 

 

98Commenters also highlighted associations with short-term PM2.5 concentrations 

reported in sub-analyses restricted to days with 24-hour concentrations at or below 35 

3

(Dominici, 2006b). These sub-analyses were not included in the original 

publication by Dominici et al. (2006a). Authors provided results of sub-analyses for the 

Administrator’s consideration in a letter to the docket following publication of the 

proposed rule in January 2006 (personal communication with Dr. Francesca Dominici, 

2006b). As noted in section III.A.3, these sub-analyses are part of the basis for the 

conclusion that there is no evidence suggesting that risks associated with long-term 

exposures are likely to be disproportionately driven by peak 24-hour concentrations. 

Because the sub-analyses did not present long-term average PM2.5 concentrations, it is 

not clear whether they reflected PM2.5 air quality that would have been allowed by the 

revised annual PM2.5 standard being established in this notice. 
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locations were above the level of the revised annual standard (i.e., 12 μg/m3).99 As 

discussed above, reducing the annual standard is the most efficient way to reduce the 

risks from short-term exposures identified in these studies, as the bulk of the risk comes 

from the large number of days across the bulk of the air quality distribution, not the 

relatively small number of days with peak concentrations. 

In considering the single-city studies highlighted by public commenters who 

advocated a more stringent 24-hour standard, the EPA first notes that, overall, these 

single-city studies reported mixed results. Specifically, some studies reported positive 

and statistically significant associations with PM2.5, some studies reported positive but 

non-significant associations, and several studies reported negative associations or a mix 

of positive and negative associations with PM2.5. In light of these inconsistent results, the 

proposal noted that the overall body of evidence from single-city studies is mixed, 

particularly in locations with 98th percentiles of 24-hour concentrations below 35 μg/m3. 

Therefore, although some single-city studies reported effects at appreciably lower PM2.5

concentrations than short-term multi-city studies, the uncertainties and limitations 

associated with the single-city studies were noted to be greater. In light of these greater 

uncertainties and limitations, the Administrator concluded in the proposal that she had 

less confidence in using these studies as a basis for setting the level of the standard (77 

FR 38943). 

Given the considerations and conclusions noted above, in the proposal the 

 

99It is also the case that additional protection is anticipated in locations with 98th

percentile 24-hour PM2.5 concentrations above 35 μg/m3, even if long-term 

concentrations are below 12 μg/m3. As noted in the proposal and in the Policy 

Assessment (U.S. EPA, 2011, Figure 2-10), parts of the northwestern U.S. are more 

likely than other parts of the country to violate the 24-hour standard and meet the revised 

annual standard. 
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Administrator concluded that the short-term multi-city studies provide the strongest 

evidence to inform decisions on the level of the 24-hour standard. Further, she viewed 

single-city, short-term exposure studies as a much more limited data set providing mixed 

results, and she had less confidence in using these studies as a basis for setting the level 

of a 24-hour standard (77 FR 38942).. In highlighting specific single-city studies, public 

health, environmental, and State and local commenters appear to have selectively focused 

on studies reporting associations with PM2.5 and to have overlooked studies that reported 

more equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004; Slaughter et al., 

2005; Villeneuve et al., 2006) (U.S. EPA, 2011, Figure 2-9). As such, these commenters 

have not presented new information that causes the EPA to reconsider its decision to 

emphasize multi-city studies over single-city studies when identifying the appropriate 

level of the 24-hour PM2.5 standard.

In further considering the single-city studies highlighted by public commenters, 

the EPA notes that some commenters advocating for a lower level for the 24-hour PM2.5

standard also discussed short-term studies that have been published since the close of the 

Integrated Science Assessment. These recent studies were conducted in single cities or in 

small panels of volunteers. As in prior NAAQS reviews and as discussed above in more 

detail (section II.B.3), the EPA is basing its decisions in this review on studies and related 

information assessed in the Integrated Science Assessment. The studies assessed in the 

Integrated Science Assessment, and the conclusions based on those studies, have 

undergone extensive critical review by the EPA, CASAC, and the public. The rigor of 

that review makes the studies assessed in the Integrated Science Assessment, and the 

conclusions based on those studies, the most reliable source of scientific information on 
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which to base decisions on the NAAQS. 

However, as discussed above (section II.B.3), the EPA recognizes that ‘‘new 

studies” may sometimes be of such significance that it is appropriate to delay a decision 

on revision of a NAAQS and to supplement the pertinent air quality criteria so the studies 

can be taken into account. In the present case, the EPA’s provisional consideration of 

‘‘new studies” concludes that, taken in context, the “new” information and findings do 

not materially change any of the broad scientific conclusions made in the air quality 

criteria regarding the health effects of PM2.5 (U.S. EPA, 2012a). 

For this reason, reopening the air quality criteria review would not be warranted, 

even if there were time to do so under the court order governing the schedule for 

completing this review. Accordingly, the EPA is basing its final decisions in this review 

on the studies and related information included in the PM Integrated Science Assessment 

(i.e., the air quality criteria) that has undergone CASAC and public review. The EPA will 

consider the “new studies” in the next periodic review of the PM NAAQS, which will 

provide an opportunity to fully assess these studies through a more rigorous review 

process involving the EPA, CASAC, and the public. 

Some public health, medical, and environmental commenters also criticized the 

EPA’s interpretation of PM2.5 risk results. These commenters presented risk estimates for 

combinations of annual and 24-hour standards using more recent air quality data than that 

used in the EPA’s Risk Assessment (U.S. EPA, 2010a). Based on these additional risk 

analyses, the ALA and other commenters contended that public health benefits could 

continue to increase as annual and 24-hour standard levels decrease below 13 g/m3and 

35 g/m3, respectively. 
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The EPA agrees with commenters that important public health benefits are 

expected as a result of revising the level of the annual standard to 12 g/m3, as is done in 

this notice, rather than 13 g/m3. The Agency also acknowledges that estimated PM2.5-

associated health risks continue to decrease with annual standard levels below 12 g/m3

and/or with 24-hour standard levels below 35 g/m3. However, the EPA disagrees with 

the commenters’ views regarding the extent to which risk estimates support setting 

standard levels below 12 μg/m3(annual standard) and 35 g/m3(24-hour standard).100

The CAA charges the Administrator with setting NAAQS that are “requisite” 

(i.e., neither more nor less stringent than necessary) to protect public health with an 

adequate margin of safety. In setting such standards the Administrator must weigh the 

available scientific evidence and information, including associated uncertainties and 

limitations. As described above, in reaching her proposed decisions on the PM2.5

standards that would provide “requisite” protection, the Administrator carefully 

considered the available scientific evidence and risk information, making public health 

policy judgments that, in her view, neither overstated nor understated the strengths and 

limitations of that evidence and information. In contrast, as discussed more fully above, 

public health, medical, and environmental commenters who recommended levels below 

35 g/m3for the 24-hour PM2.5 standard have not provided new information or analyses 

to suggest that such standard levels are appropriate, given the uncertainties and 

limitations in the available health evidence, particularly uncertainties in studies 

conducted in locations with 98th percentile 24-hour PM2.5 concentrations below 35 g/m

3

and long-term average concentrations below 12 g/m3. 

 

100This section focuses on the 24-hour standard. Section III.E.4.c.i above also discusses 

these commenters’ recommendations within the context of the annual PM2.5 standard. 
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d. Administrator’s Final Conclusions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate standard levels, the 

Administrator has considered the epidemiological and other scientific evidence, estimates 

of risk reductions associated with just meeting alternative annual and/or 24-hour 

standards, air quality analyses, related limitations and uncertainties, the advice of 

CASAC, and extensive public comments on the proposal. After careful consideration of 

all of these, the Administrator has decided to revise the level of the primary annual PM2.5

standard from 15.0 μg/m3to 12.0 μg/m3and to retain the level of the primary 24-hour 

standard at 35 μg/m3.

As an initial matter, the Administrator agrees with the approach supported by 

CASAC and discussed in the Policy Assessment as summarized in sections III.A.3 and 

III.E.4.a above, of considering the annual and 24-hour standards together in determining 

the protection afforded against mortality and morbidity effects associated with both longand short-term exposures to PM2.5. This approach is consistent with the approach taken in 

the review completed in 1997, in contrast to the approach used in the review completed in 

2006 where each standard was considered independently of the other (i.e., only data from 

long-term exposure studies were used to inform the level of the annual standard and only 

data from short-term exposure studies were used to inform the level of the 24-hour 

standard).101

Based on the evidence and quantitative risk assessment, the Administrator 

concludes that it is appropriate to set an annual standard that is generally controlling, 

which will lower the broad distribution of 24-hour average concentrations in an area as 

well as the annual average concentration, so as to provide protection from both long- and 

 

101

 See 71 FR 61148 and 61168, October 17, 2006.
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short-term PM2.5 exposures. In conjunction with this, it is appropriate to set a 24-hour 

standard focused on providing supplemental protection, particularly for areas with high 

peak-to-mean ratios of 24-hour concentrations, possibly associated with strong local or 

seasonal sources, and for PM2.5-related effects that may be associated with shorter-than 

daily exposure periods. The Administrator concludes this approach will reduce aggregate 

risks associated with both long- and short-term exposures more consistently than a 

generally controlling 24-hour standard and is the most effective and efficient way to 

reduce total PM2-5-related population risk and so provide appropriate protectionto protect 

public health with an adequate margin of safety. 

In selecting the level of the annual PM2.5 standard, based on the characterization 

and assessment of the epidemiological and other studies presented and assessed in the 

Integrated Science Assessment and the Policy Assessment, the Administrator recognizes 

the substantial increase in the number and diversity of studies available in this review, 

including. This expanded body of evidence includes extended analyses of the seminal 

studies of long-term PM2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as 

important new long-term exposure studies (as summarized in Figures 1 and 2). 

Collectively, the Administrator notes that these studies, along with evidence available in 

the last review, provide consistent and stronger evidence than previously observed of an 

association between long-term PM2.5 exposures and premature mortality in areas with 

lower long-term ambient concentrations than previously observed, with the strongest 

evidence related to cardiovascular-related mortality. The Administrator also recognizes 

the availability of stronger evidence of morbidity effects associated with long-term PM2.5

exposures, including evidence of respiratory effects such as decreased lung function 
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growth, from the extended analyses for the Southern California Children’s Health Study 

and evidence of cardiovascular effects from the WHI study. Furthermore, the 

Administrator recognizes new U.S. multi-city studies that greatly expand and reinforce 

our understanding of mortality and morbidity effects associated with short-term PM2.5

exposures, providing stronger evidence of associations in areas with ambient 

concentrations similar to those previously observed in short-term exposure studies 

considered in the previous review (as summarized in Figure 3). 

The Administrator recognizes the strength of the scientific evidence for evaluating 

health effects associated with fine particles, noting that the newly available scientific 

evidence builds upon the previous scientific data base to provide evidence of generally 

robust associations and a basis for greater confidence in the reported associations than in 

the last review. She notes the conclusion of the Integrated Science Assessment that this 

body of evidence supports a causal relationship between long- and short-term PM2.5

exposures and mortality and cardiovascular effects and a likely causal relationship 

between long- and short-term PM2.5 exposures and respiratory effects. In addition, the 

Administrator notes additional, but more limited evidence, for a broader range of health 

endpoints including evidence suggestive of a causal relationship for developmental and 

reproductive effects as well as for carcinogenic effects.

Based on information discussed and presented in the Integrated Science 

Assessment, the Administrator recognizes that health effects may occur over the full 

range of concentrations observed in the epidemiological studies of both long-term and 

short-term exposures, since no discernible population-level threshold for any such effects 

can be identified based on the currently available evidence (U.S. EPA, 2009a, section 
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2.4.3). To inform her decisions on an appropriate level for the annual standard levelsthat 

will protect public health with an adequate margin of safety, in the absence of any 

discernible population-level thresholds, the Administrator judges that it is appropriate to 

consider the relative degree to which the evidence of an associationconfidence in the 

magnitude and significance of the associations observed in an epidemiological study is 

supportedstudies across the range of long-term PM2.5 concentrations in the study, and 

therefore the degree to which the study provides confidence in the effects across this 

range of concentrationssuch studies. Further, she recognizes, in taking note of CASAC 

advice and the distributional statistics analysis discussed in the Policy Assessment and in 

section III.E.4.a above, that there is significantly greater confidence in the magnitude and 

significance of observed associations for certain partsthe part of the air quality 

distribution corresponding to where the bulk of the health events evaluated in each study 

have been observed., generally at and around the long-term mean concentrations.

Conversely, she also recognizes that there is significantly diminished confidence in the 

magnitude and significance of observed associations in the lowestr part of the air quality 

distribution corresponding to where a relatively small proportion of the health events 

were observed. Based on the above considerations, the Administrator takes into account 

information that characterizes the part of the air quality distributions that corresponds to 

the large proportion of health event data that was most influential in defining the 

association between PM2.5 and the health endpoints evaluated in each study. 

In so doing Further, the Administrator recognizes that the long-term mean 

concentrations, or any other specific point in the air quality distribution of each study, do 

not represent a “bright line” at and above which effects have been observed and below 
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which effects have not been observed. As just noted, no discernible population-level 

threshold for any such effects can be identified based on the currently available evidence. 

The Administrator notes that the concentrations at and around the long-term mean 

concentrations, for both the long- and short-term exposure studies, represents a part of the 

air quality distribution in which the health event data in any given study are generally 

most concentrated and, thus, where these data have the most influence on the magnitude 

and significance of the association observed in such study. The Administrator notes that 

long-term mean PM2.5 concentrations are generally available for each published study 

and, therefore, represent the most comprehensive data set to inform her decisions on the 

appropriate annual standard level. 

Nonetheless, In considering the long-term mean concentrations reported in 

epidemiological studies, the Administrator recognizes that in setting an selecting a level 

of the annual standard levelthat will protect public health with an adequate margin of 

safety, it is not sufficient to focus on a concentration generally somewhere within the 

range of long-term mean concentrations from the key long-term and short-term exposure 

studies that reported lower concentrations than had been observed in earlier reviews. 

These key studies provide information for various types of serious health endpoints 

(including mortality and morbidity effects), different study populations (which may 

include at-risk populations such as children and older adults), and different air quality 

distributions that are specific to each study. A level somewhere within the range of longterm mean concentrations of the full set of key studies would be higher than the longterm mean of at least one of the studies being considered and therefore would fail to 

protectnot provide a sufficient degree of protection against the health effects associated 
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with the air quality distributionobserved in that study. Absent some reasoned basis to 

place less weight on the evidence in the epidemiological study with the lowest long-term 

mean concentration among these key studies, this approach would fail to provide 

sufficient protectionnot be consistent with the requirement to set a standard that will 

protect public health with an adequate margin of safety.102 Thus, the Administrator 

recognizes it is important to protect against the serious effects observed in each of these 

studies. so as to protect public health with an adequate margin of safety. In so doing, she 

looks to identify the study with the lowest long-term mean concentration within the full

set of key studies to help inform her decision of the appropriate standard level which will 

provide protection for the broad array of health outcomes observed in all of the studies, 

including effects observed in at-risk populations.

Further, consistent with the general approach summarized in section III.E.4.a 

above and supported by CASAC as discussed in section III.E.4.b.ii above, the 

Administrator recognizes that it is appropriate to consider a level for an annual standard 

that is not just at but rather is somewhat below the long-term mean PM2.5 concentrations 

reported in each of the key long- and short-term exposure studies. In so doing, she 

focuses especially on multi-city studies that evaluated health endpoints for which the 

associations are causal or likely causal (i.e., mortality and cardiovascular and respiratory 

effects associated with both long- and short-term PM2.5 exposures). As discussed above, 

the importance of considering a level somewhat below the lowest long-term mean 

concentrations in this set of key studies is to establish a standard that would be protective 

against the observed effects in all of the studies, and that takes into account the relative 

 

102 See American Farm Bureau Federation v. EPA, 559 F. 3d 512, 525-26 (D.C. Cir. 

2009).
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degree of confidence in the magnitude and significance of observed associations across 

the air quality distributions in these studies. 

The Administrator recognizes that there is no clear way to identify how much 

below the long-term mean concentrations of key studies to set a standard that would 

provide requisite protection with an adequate margin of safety. She therefore must use 

her judgment to weigh the available scientific and technical information, and associated 

uncertainties, to reach a final decision on the appropriate standard level. In considering 

the information in Figures 1 - 4 for effects classified as having evidence of a causal or 

likely causal relationship with long- or short-term PM2.5 exposures, she observes a cluster 

of short-term exposure studies with long-term mean concentrations within a range of 13.4 

μg/m3down to 12.8 μg/m3(Dominici et al., 2006; Burnett and Goldberg, 2003; Zanobetti 

and Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also observes a cluster of 

long-term exposure studies with long-term mean concentrations within a range of 14.5 

μg/m

3

to 13.6 μg/m3(Dockery et al., 1996; Lipfert et al., 2006; Zeger et al., 2008; 

McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the reasons discussed in 

response to public comments in section III.E.4.c above, the Administrator is less

influenced by the long-term mean PM2.5 concentrations from the Miller et al. (2007) and 

Krewski et al. (2009) studies with reported long-term mean PM2.5 concentrations of 12.9 

and 14.0 μg/m3, respectively. In each case, the most relevant exposure periods would 

likely have had higher mean PM2.5 concentrations than those reported in the studies.103

Thus, the Administrator considers the long-term mean PM2.5 concentrations from these 

 

103 In the case of Miller et al. (2007), the mean concentration is based on a single year of 

air quality data which post-dated by two years the period for which the health events data 

were collected. In the case of Krewski et al. (2009),the air quality data were based on the 

last two years of the 18-year period for which the health event data were collected. 
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two studies to be a highly uncertain basis for informing her selection of the annual 

standard level.104

To help guide her judgment of the appropriate level below the long-term mean 

concentrations in the epidemiological studies at which to set the standard, the 

Administrator considered additional information from epidemiological studies 

concerning the broader distribution of PM2.5 concentrations which correspond to the 

health events observed in these studies (e.g., deaths, hospitalizations). The Administrator 

observes that the development and use of this information in considering standard levels 

is consistent with CASAC’s advice, as discussed in section III.E.4.b.ii above, to focus on 

understanding the concentrations that were most influential in generating the health effect 

estimates in individual studies (Samet, 2010d, p. 2). 

In considering this additional population-level information, the Administrator 

recognizes that, in general, the confidence in the magnitude and significance of an 

association identified in a study is strongest at and around the long-term mean 

concentration for the air quality distribution, as this represents the part of the distribution 

in which the data in any given study are generally most concentrated. She also recognizes 

that the degree of confidence decreases as one moves towards the lower part of the 

distribution. Consistent with the approach used in the Policy Assessment, the 

Administrator believes that the range from approximately the 25th to 10th percentiles is a 

reasonable range for providing a general frame of reference as to the part of the 

distribution in which her confidence in the associations observed in epidemiological 

 

104 Nonetheless, as noted above, the EPA notes that the Krewski et al. (2009) and Miller 

et al. (2007) studies provide strong evidence of mortality and cardiovascular-related 

effects associated with long-term PM2.5 exposures to inform causality determinations 

reached in the Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6). 
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studies becomesis appreciably lower. However, as noted above, it is important to 

emphasize that there is no clear dividing line or single percentile within a given 

distribution provided by the scientific evidence that is most appropriate or ‘correct’ to use 

to characterize where the degree of confidence in the associations warrants setting the 

annual standard level. The decision of the appropriate standard level below the long-term 

mean concentrations of the key studies, which in conjunction with the other elements of 

the standard would protect public health with an adequate margin of safety, is largely a 

public health policy judgment, taking into account all of the evidence and its related 

uncertainties. 

As discussed in section III.E.4.b, the Administrator takes note of additional 

population-level data that were made available to the EPA by study authors.105 In 

considering this information, the Administrator particularly focuses on the analysis of the 

distributions of the health event data for each area within these studies and the 

corresponding air quality data for the two short-term exposure studies (Zanobetti and 

Schwartz, 2009; Bell et al., 2008). These short-term exposure studies evaluate the 

relationship between daily changes (one or more days) in PM2.5 concentrations and daily 

changes in health events (e.g., deaths, hospitalizations), such that the air quality 

concentrations that comprise the most relevant exposure periods in these studies are 

 

105 As summarized in section III.E.4.a, population-level data were provided to the EPA 

for four studies. These four studies represent some of the strongest evidence showing 

associations between health effects and PM2.5 within the overall body of scientific 

evidence and include three studies (Krewski et al., 2009; Bell et al., 2008; and Zanobetti 

and Schwartz, 2009) that were used as the basis for concentration-response functions in 

the quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). The Administrator 

recognizes that the additional population-level data available for these four multi-city 

studies represents a more limited data set compared to the set of long-term mean PM2.5

concentrations which were available in the published literature for all studies considered 

in the Integrated Science Assessment. 
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contemporaneous with the health event data. In addition, these studies considered more 

recent air quality data, representing generally lower PM2.5 concentrations, in a large 

number of study areas across the U.S. Thus, such studies provide the most useful 

evidence for an analysis evaluating the distribution of health event data and the 

corresponding long-term mean PM2.5 concentrations across the areas included in each 

multi-city study. 

The Administrator also considered the additional population-level data that were 

made available to EPA for two long-term exposure studies (Krewski et al., 2009; Miller 

et al., 2007). She recognizes that in long-term exposure studies investigators follow a 

specific group of study participants (i.e., cohort) over time and across urban study areas, 

and evaluate how PM2.5 concentrations averaged over a period of years are associated 

with specific health endpoints (e.g., deaths) across cities. As discussed in response to 

public comments in section III.E.4.c, disentangling the effects observed in long-term 

exposure studies associated with more recent air quality measurements from effects that 

may have been associated with earlier, and most likely higher, PM2.5 exposures 

introduces some uncertainty with regard to understanding the appropriate exposure 

window associated with the observed effects. This is in contrast to the short-term 

exposure studies where the relevant exposure period is contemporaneous to the period for 

which the health data were collected. In light of these considerations, as noted above, the 

Administrator considers the analysis of air quality concentrations that correspond to the 

distribution of population-level data in these two studies to be a highly uncertain basis for 

informing her selection of the annual standard level. 

Based on the above considerations, the Administrator views the additional 
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population-level data for the two short-term exposure studies as appropriate to help 

inform her judgment of how much below the long-term mean concentrations to set the 

level of the annual standard. The Administrator notes that the long-term mean PM2.5

concentrations corresponding with study areas contributing to the 25th and 10thpercentiles

of the distribution of deaths and cardiovascular-related hospitalizations in these two 

short-term exposure studies were 12.5 μg/m3and 10.311.5 μg/m3, respectively, for 

Zanobetti and Schwartz (2009),) and 11.5 μg/m3and 9.8 μg/m3, respectively, for Bell et 

al. (2008) (Figure 5 above; Rajan et al., 2011, Table 1).), with the 10th percentiles being 

lower by approximately 2 μg/m3in each study..

The Administrator recognizes, as summarized in section III.B above and 

discussed more fully in section III.B.2 of the proposal, that important uncertainties 

remain in the evidence and information considered in this review of the primary fine 

particle standards. These uncertainties are generally related to understanding the relative 

toxicity of the different components in the fine particle mixture, the role of PM2.5 in the 

complex ambient mixture, exposure measurement errors, and the nature and magnitude of 

estimated risks related to increasingly lower ambient PM2.5 concentrations. Furthermore, 

the Administrator notes that epidemiological studies have reported heterogeneity in 

responses both within and between cities and geographic regions across the U.S. She 

recognizes that this heterogeneity may be attributed, in part, to differences in fine particle 

composition in different regions and cities.106

 

106 Nonetheless, as explained in section III.E.1, the currently available evidence is not 

sufficient to support replacing or supplementing the PM2.5 indicator with any other 

indicator defined in terms of a specific fine particle component or group of components 

associated with any source categories of fine particles. Furthermore, the evidence is not 

sufficient to support eliminating any component or group of components associated with 

any source categories of fine particles from the mix of fine particles included in the PM2.5
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With regard to evidence for reproductive and developmental effects identified as 

being suggestive of a causal relationship with long-term PM2.5 exposures, the 

Administrator recognizes that there are a number of limitations associated with this body 

of evidence including: the limited number of studies evaluating such effects; uncertainties 

related to identifying the relevant exposure time periods of concern; and limited 

toxicological evidence providing little information on the mode of action(s) or biological 

plausibility for an association between long-term PM2.5 exposures and adverse birth 

outcomes. Nonetheless, the Administrator believes that this more limited body of 

evidence provides some support for considering that serious effects may be occurring in a 

susceptible population at concentrations lower than those associated with effects 

classified as having a causal or likely causal relationship with long-term PM2.5 exposures 

(i.e., mortality, cardiovascular, and respiratory effects).

Overall, the Administrator believes that the available evidence interpreted in light 

of the remaining uncertainties, as summarized above and discussed more fully in the 

Integrated Science Assessment and the Policy Assessment, provides increased confidence 

relative to information available in the last review and provides a strong basis for 

informing her final decisions in the current review. The Administrator is mindful that 

considering what standards are requisite to protect public health with an adequate margin 

of safety requires public health policy judgments that neither overstate nor understate the 

strength and limitations of the evidence or the appropriate inferences to be drawn from 

the evidence. In considering how to translate the available information into appropriate 

standard levels, the Administrator weighs the available scientific information and 

associated uncertainties and limitations. For the purpose of determining what annual 

 

indicator. 
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standard level is appropriate the Administrator recognizes that there is no single factor or 

criterion that comprises the ‘‘correct’’ approach to weighing the various types of 

available evidence and information.

In considering this information, the Administrator notes the advice of CASAC 

that “there are significant public health consequences at the current levels of the 

standards that justify consideration of lowering the PM2.5 NAAQS further” (Samet, 

2010c, p. 12). In addition, she recognizes that CASAC concluded, “although there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level 

below which there is no risk for adverse effects)” (Samet, 2010, p.ii) and that the final 

decisions on standard levels must reflect a judgment of the available scientific 

information with respect to her interpretation of the CAA’s requirement to set primary 

standards that provide requisite protection to public health with an adequate margin of 

safety (Samet, 2010d, p. 4). The Administrator recognizes CASAC’s advice that the 

currently available scientific information provided support for considering an annual 

standard level within a range of 13 to 11 μg/m3and a 24-hour standard level within a 

range of 35 to 30 μg/m3. In considering how the annual and 24-hour standards work 

together to provide appropriate public health protection, the Administrator observes that 

CASAC did not express support for any specific levels or combinations of standards 

within these ranges. She also notes that CASAC encouraged the EPA staff to consider 

additional data from epidemiological studies to help quantify the characterization of the 

PM2.5 concentrations that were most influential in generating the health effect estimates 

in these studies (Samet, 2010d, p. 2). 

In response to CASAC’s advice, the Administrator recognizes that the EPA staff 
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acquired additional data from authors of key epidemiological studies and analyzed these 

data to characterize the distribution of PM2.5 concentrations in relation to health events 

data to better understand the degree of confidence in the associations observed in the 

studies as discussed above. The Administrator recognizes that the final Policy 

Assessment included consideration of these additional analyses in reaching final staff 

conclusions with regard to the broadest range of alternative standard levels supported by 

the science. She takes note that the final Policy Assessment concluded that while 

alternative standard levels within the range of 13 to 11 μg/m3 were appropriate to 

consider, the evidence most strongly supported consideration of an annual standard level 

in the range of 12 to 11 μg/m3. The Administrator is aware that, in transmitting the final 

Policy Assessment to CASAC, the Agency notified CASAC that the final staff 

conclusions reflected consideration of CASAC’s advice and that those staff conclusions 

were based, in part, on the specific distributional analysis that CASAC had urged the 

EPA to conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on the 

final Policy Assessment, but chose not to provide any additional comments or advice 

after receiving it. 

In selecting the annual standard level, the Administrator has considered many 

factors including the nature and severity of the health effects involved, the strength of the 

overall body of scientific evidence as considered in reaching causality determinations, the 

size of the at-risk populations, and the estimated public health impacts. She has also 

considered the kind and degree of the uncertainties that remain in the available scientific 

information. She recognizes that the association between PM2.5 and serious health effects 

is well established, including at concentrations below those allowed by the current 
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standard. Further, she recognizes the CAA requirement that requires primary standards to 

provide an adequate margin of safety was intended to address uncertainties associated 

with inconclusive scientific and technical information as well as to provide a reasonable 

degree of protection against hazards that research has not yet identified. In considering 

the currently available evidence, as summarized and discussed more broadly above, the 

information on risk, CASAC advice, the conclusions of the Policy Assessment, and 

public comments on the proposal, the Administrator strongly believes that a lower annual 

standard level is needed to protect public health with an adequate margin of safety. In this 

instance, the association between PM2.5 and serious health effects is well established, 

including at concentrations below those allowed by the current standard. 

In reaching her final decision on the appropriate annual standard level to set, the 

Administrator is mindful that the CAA does not require that primary standards be set at a 

zero-risk level, but rather at a level that reduces risk sufficiently so as to protect public 

health, including the health of at-risk populations, with an adequate margin of safety. On 

balance, the Administrator concludes that an annual standard level of 12 μg/m3 would be 

requisite to protect the public health with an adequate margin of safety from effects 

associated with long- and short-term PM2.5 exposures, while still recognizing that 

uncertainties remain in the scientific information. 

In the Administrator’s judgment, an annual standard of 12 μg/m3appropriately 

reflects placing greatest weight on evidence of effects for which the Integrated Science 

Assessment determined there is a causal or likely causal relationship with long- and 

short-term PM2.5 exposures. An annual standard level of 12 μg/m3is below the long-term 

mean PM2.5 concentrations reported in each of the key multi-city, long- and short-term 
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exposures studies providing evidence of an array of serious health effects (e.g., premature 

mortality, increased hospitalization for cardiovascular and respiratory effects). As noted 

above, the importance of considering a level somewhat below the lowest long-term mean 

concentration in the full set of studies considered is to set a standard that would be 

protectiveprovide appropriate protection against the observed effects in all such studies. 

In reaching her decision, the Administrator has taken into account that at and 

around the mean PM2.5 concentration in any given study represents a part of the air 

quality distribution in which the health event data in that study are generally most 

concentrated. Furthermore, in identifying an appropriate annual standard level below the 

long-term mean concentrations, she recognizes that there is no evidence to support the 

existence of any discernible threshold, and, therefore, she has a high degree of confidence 

that the observed effects are associated with concentrations not just at but extending 

somewhat below the long-term mean concentration. To further inform her judgment in 

setting the annual standard level so as to protect public health with an adequate margin of 

safety, the Administrator has placed weight on additional population-level information 

available from a subset of these epidemiological studies, consistent with CASAC advice. 

In particular, she has drawn from two short-term exposure studies, which provide the 

most relevant information for evaluating the distribution of health events and 

corresponding long-term PM2.5 concentrations. As explained above, this provides 

information onhelps inform her judgment as to the degree of confidence in the observed 

associations in the epidemiological studies. In this regard, the Administrator generally 

judges the region around the 25th percentile as a reasonable part of the distribution to help 

guide her decision on the appropriate standard level. Since this evidence comes primarily 
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from two studies, a relatively modest data set, the Administrator deems it reasonable not 

to draw further inferences from air quality and health event data in the lower part of the 

distribution for the purpose of setting a standard level. The Administrator notes that the 

long-term mean PM2.5 concentrations around the 25th percentile of the distributions of 

deaths and cardiovascular-related hospitalizations were approximately around 12.5 μg/m3

and 11.5 μg/m3in Zanobetti and Schwartz (2009) and in Bell et al. (2008), respectively 

(Figure 5; Rajan et al., 2011, Table 1).these two studies. The Administrator views this 

information as helpful in guiding her determination as to where she continues to have 

sufficienther confidence in the magnitude and significance of the associations to warrant 

setting an annualis reduced to such a degree that a standard set at a lower level would not 

be warranted to provide requisite protection that is neither more nor less than needed to 

provide an adequate margin of safety.

The Administrator also recognizes that a level of 12 μg/m3places some weight on 

studies which provide evidence of reproductive and developmental effects (e.g., infant 

mortality, low birth weight). These studies were identified in the Integrated Science 

Assessment as having evidence suggestive of a causal relationship with long-term PM2.5

concentrations. A level of 12 μg/m3is approximately the same level as the lowest longterm mean concentration reported in such studies (Figures 2 and 4; 11.9 μg/m3

for Bell et 

al., 2007).107 While the Administrator acknowledges that this evidence is limited, she 

 

107 With respect to cancer, mutagenic, and genotoxic effects, the Administrator observes 

that the PM2.5 concentrations reported in studies evaluating these effects generally 

included ambient concentrations that are equal to or greater than ambient concentrations 

observed in studies that reported mortality and cardiovascular and respiratory effects 

(U.S. EPA, 2009a, section 7.5). Therefore, the Administrator concludes that in selecting 

alternative standard levels that provide protection from mortality and cardiovascular and 

respiratory effects, it is reasonable to anticipate that protection will also be provided for 

carcinogenic effects.
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believes it is appropriate to place some weight on these studies in order to set a standard 

that provides protection with an adequate margin of safety, including providing 

protection for at-risk populations, as required by the CAA. Due to the limited nature of 

this evidence, she has determined it is not necessary to set a standard below the lowest 

long-term mean concentration in these studies. 

In reflecting on extensive public comments received on the proposal as discussed 

in section III.E.4.c above, the Administrator recognizes that some commenters have 

offered different evaluations of the evidence and other information available in this 

review and would make different judgments about the weight to place on the relative 

strengths and limitations of the scientific information and about how such information 

could be used in making public health policy decisions on the annual standard level. One 

group of such commenters who supported a higher annual standard level (e.g., above 13 

μg/m3) would place greater weight on the remaining uncertainties in the evidence as a 

basis for discounting the overall strength of the evidence than the Administrator judges to 

be appropriate. Such an approach is based on these commenters’ judgment that the 

uncertainties remaining in the evidence are too great to warrant setting a lower annual 

standard level. The Administrator does not agree. 

As an initial matter, an annual standard level of 13 μg/m3or higher would be 

above the long-term mean concentrations reported in two well-conducted, multi-city 

short-term exposure studies reporting positive and statistically significant associations of 

serious effects (Burnett et al., 2004 and Bell et al., 2008). These important studies are 

fully consistent with the pattern of evidence presented by the large body of evidence in 

this review. As the Administrator recognized in the proposal, and as advised by CASAC, 
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the appropriate focus for selecting the level of the annual PM2.5 standard is on 

concentrations somewhat below the lowest long-term mean concentrations from the set of 

key studies of both long-term and short-term PM2.5 exposures considered by the EPA 

(i.e., as shown in Figure 4). Thus, a standard level set at 13 μg/m3or higher would clearly 

not provide protection for the effects observed in the full set epidemiological studies and, 

therefore, this standard level could not be judged to be requisite with an adequate margin 

of safety.108

In addition, as noted above, in recognizing that there is no evidence to support the 

existence of a discernible threshold below which an effect would not occur, the 

Administrator is mindful that effects occur around and below the long-term mean 

concentrations reported in both the short-term and long-term the epidemiological studies. 

A standard level of 13 μg/m3or higher would place essentially no weight onnot 

appropriately take into account evidence from the two well-conducted, multi-city, shortterm exposure studies reporting serious effects with long-term mean concentrations 

below 13 μg/m3noted above (Burnett et al, 2004; Bell et al., 2008). Such a standard level 

would also place essentially no weight onnot appropriately take into account additional 

population-level data from a limited number of epidemiological studies. This approach 

would ignore CASAC’s advice to consider such information in order to better understand 

the concentrations that were most influential in generatingover which there is a high 

 

108 The Administrator is mindful that, in reviewing the 2006 final PM NAAQS decisions, 

the D.C. Circuit Court of Appeals concluded that the EPA failed to adequately explain 

why that annual standard provided requisite protection from effects associated with both 

long- and short-term exposures or from morbidity effects in children and other at-risk 

populations when long-term means of important short-term studies were below the level 

the Administrator selected for the annual standard. See American Farm Bureau v. EPA.

559 F. 3d 512, 524-26. There is no reasonable basis to discount these two studies for 

purposes of setting the level of the annual standard.
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degree of confidence regarding the health effect estimatesmagnitude and significance of 

the associations observed in individual epidemiological studies and where there is 

appreciably less confidence. 

Furthermore, a standard level of 13 μg/m3or higher would place no weight onnot 

appropriately take into account the more limited evidence of effects in some at-risk 

populations (e.g., low birth weight). In the Administrator’s view, placing no weight ona 

standard set at this informationlevel would fail tonot provide protection with an adequate 

margin of safety, including providing protection for at-risk populations. The 

Administrator is mindful that the CAA requirement that primary standards provide an 

adequate margin of safety, discussed in section II.A above, was intended to address 

uncertainties associated with inconclusive scientific and technical information available 

at the time of standard setting as well as to provide a reasonable degree of protection 

against hazards that research has not yet identified. 

In light of the entire body of evidence as discussed above, the Administrator 

judges that an annual standard level set above 12 μg/m3 would not be sufficient to protect 

public health with an adequate margin of safety from the serious health effects associated 

with long- and short-term exposure to PM2.5.

The Administrator also recognizes that a second group of commenters supported a 

lower annual standard level (e.g., no higher than 11 μg/m3). Such a standard level would 

reflect placing essentially as much weight on the relatively more limited data providing 

evidence suggestive of a causal relationship for effects observed in some at-risk 

populations (e.g., low birth weight) as on more certain evidence of effects classified as 

having a causal or likely causal relationship with PM2.5 exposures. In the Administrator’s 
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view, while it is important to place some weight on such suggestive evidence, it would 

not be appropriate to place as much weight on it as the commenters would do. 

An annual standard level of 11 μg/m3 would also reflect these commenters’ 

judgment that it is appropriate to focus on a lower part of the distributions of health event 

data from the small number of epidemiological studies for which this information was 

made available than the Administrator believes is warranted. In the Administrator’s view, 

using this type of information to set a standard level of 11 μg/m3or below would assume 

too high a degree of confidence in the magnitude and significance of the associations 

observed in the lower part of the distributions of health events observed in these studies. 

Given the uncertainties in the evidence and the limited set of studies for which the EPA 

has information on the distribution of health event data and corresponding air quality 

data, the Administrator believes it is not appropriate to focus on the lower part of the 

distributions of health events data. 

On balance, the Administrator finds that the available evidence interpreted in light 

of the remaining uncertainties does not justify a standard level set below 12 μg/m3as 

necessary to protect public health with an adequate margin of safety. 

After carefully considering the above considerations and the public comments 

summarized in section III.E.4.c above, the Administrator has decided to set the level of 

the primary annual PM2.5 standard at 12 μg/m3. In her judgment, a standard set at this 

level provides the requisite degree of public health protection, including the health of atrisk populations, with an adequate margin of safety and is neither more nor less stringent 

than necessary for this purpose.

As discussed above, the Administrator concludes that an approach that focuses on 
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setting a generally controlling annual standard is the most effective and efficient way to 

reduce total population risk associated with both long- and short-term PM2.5 exposures. 

Such an approach would result in more uniform protection across the U.S. than the 

alternative of setting the levels of the 24-hour and annual standard such that the 24-hour 

standard would generally be the controlling standard in areas across the country (see 

section III.A.3). 

The Administrator recognizes that potential air quality changes associated with 

meeting an annual standard level of 12.μg/m3 will result in lowering risks associated with 

both long- and short-term PM2.5 exposures by lowering the overall air quality 

distribution. However, the Administrator recognizes that such an annual standard alone 

would not be expected to offer sufficient protection with an adequate margin of safety 

against the effects of all short-term PM2.5 exposures. As a result, in conjunction with an 

annual standard level of 12 μg/m3, the Administrator concludes that it is appropriate to 

continue to provide supplemental protection by means of a 24-hour standard set at the 

appropriate level, particularly for areas with high peak-to-mean ratios possibly associated 

with strong local or seasonal sources and for areas with PM2.5-related effects that may be 

associated with shorter-than-daily exposure periods. 

In selecting the level of a 24-hour standard meant to provide such supplemental 

protection, the Administrator relies upon evidence and air quality information from key 

short-term exposure studies. In considering these studies, the Administrator notes that to 

the extent air quality distributions in the study areas considered are reduced to meet the 

current 24-hour standard (at a level of 35 μg/m3) or to meet the revised annual standard 

discussed above (at a level of 12 μg/m3), additional protection would be anticipated 
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against the effects observed in these studies. In light of this, when selecting the 

appropriate level for the 24-hour standard, the Administrator considers both the 98th

percentiles of 24-hour PM2.5 concentrations and the long-term mean PM2.5 concentrations 

in the locations of the short-term exposure studies. She notes that such consideration of 

both short- and long-term PM2.5 concentrations can inform her decision on the extent to 

which a given 24-hour standard, in combination with the revised annual standard 

established in this notice, would provide protection against the health effects reported in 

short-term studies. 

As discussed in section III.E.4.a above, the Administrator concludes that multicity short-term exposure studies provide the strongest data set for informing her decisions 

on appropriate 24-hour standard levels. With regard to the limited number of single-city 

studies that reported positive and statistically significant associations for a range of health 

endpoints related to short-term PM2.5 concentrations in areas that would likely have met 

the current suite of PM2.5 standards, the Administrator recognizes that many of these 

studies had significant limitations (e.g., limited statistical power, limited exposure data) 

or equivocal results (mixed results within the same study area) that make them unsuitable 

to form the basis for setting the level of a 24-hour standard. 

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, the Administrator observes an overall pattern of positive and 

statistically significant associations in studies with 98th percentile 24-hour values 

averaged across study areas within the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; 

Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and 

Goldberg, 2003; Franklin et al., 2008). The Administrator notes that, to the extent air 
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quality distributions are reduced to reflect just meeting the current 24-hour standard, 

additional protection would be provided for the effects observed in the three multi-city 

studies with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett 

and Goldberg, 2003; Franklin et al., 2008). In the three additional multi-city studies with 

98th percentile values below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 

34.3, and 34.8 μg/m3, the Administrator notes that these studies reported long-term mean 

PM2.5 concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; 

Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In revising the level of the annual 

standard to 12 μg/m3, as discussed above, the Administrator recognizes that additional 

protection would be provided for the short-term effects observed in these multi-city 

studies such that revision to the 24-hour standard would not be warranted. That is, by 

lowering the level of the annual standard to 12 μg/m3, the 98th percentile of the 

distribution would be lowered as well such that additional protection from effects 

associated with short-term exposures would be afforded. Therefore, the epidemiological 

evidence supports a conclusion that it is appropriate to retain the level of the 24-hour 

standard at 35 μg/m3, in conjunction with a revised annual standard level of 12 μg/m3. 

In addition to considering the epidemiological evidence, the Administrator also 

has taken into account air quality information based on county-level 24-hour and annual 

design values to understand the implications of revising the annual standard level from 15 

to 12 μg/m3in conjunction with retaining the 24-hour standard level at 35 μg/m3. She has 

considered this information to evaluate the public health protection provided by the two 

standards in combination and to evaluate the most appropriate means of developing a 

suite of standards providing requisite public health protection with an adequate margin of 
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safety. 

In considering the air quality information, the Administrator observes that a suite 

of PM2.5 standards that includes an annual standard level of 12 μg/m3and a 24-hour 

standard level of 35 μg/m3 would result in the annual standard as the generally 

controlling standard in most regions across the country, except for certain areas in the 

Northwest, where the annual mean PM2.5 concentrations have historically been low but 

where relatively high 24-hour concentrations occur, often related to seasonal wood smoke 

emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). In fact, these are the type of 

areas for which the supplemental protection afforded by the 24-hour standard is intended, 

such that the two standards together provide the requisite degree of protection. The 

Administrator concludes the current 24-hour standard at a level of 35 μg/m3, in 

conjunction with a revised annual standard level of 12 μg/m3, will provide appropriate 

protection from effects observed in studies in such areas in which the long-term mean 

concentrations were below 12 μg/m3and the 98th percentile 24-hour concentrations were 

above 35 μg/m3(e.g., areas in the Northwest U.S.). 

After carefully taking the public comments and above considerations into 

account, the Administrator has decided to retain the current level of the primary PM2.5

24-hour standard at 35 μg/m3in conjunction with revising the annual standard level from 

15.0 μg/m3to 12.0 μg/m3.

109 In the Administrator’s judgment, this suite of primary PM2.5

standards and the rationale supporting these levels appropriately reflects consideration of 

the strength of the available evidence and other information and its associated 

uncertainties as well as the advice of CASAC and consideration of public comments.In 

 

109As noted in section II.B.1, Table 1 and section III.E.4.a above, the annual standard 

level is defined to one decimal place. Throughout this section, the annual standard levels 

discussed have been denoted as integer values (e.g., 12 μg/m3) for simplicity.
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the Administrator’s judgment, this suite of primary PM2.5 standards is sufficient but not 

more protective than necessary to protect the public health, including at-risk populations, 

with an adequate margin of safety from effects associated with long- and short-term 

exposures to fine particles. This suite of standards will provide significant protection 

from serious health effects including premature mortality and cardiovascular and 

respiratory morbidity effects that are causally or likely causally related to long- and shortterm PM2.5 exposures. These standards will also provide an appropriate degree of 

protection against other health effects for which there is more limited evidence of effects 

and causality, such as reproductive and developmental effects. This judgment by the 

Administrator appropriately considers the requirement for a standard that is requisite to 

protect public health but is neither more nor less stringent than necessary.110

D. Administrator’s Final Decisions on Primary PM2.5 Standards

For the reasons discussed above, and taking into account the information and 

assessments presented in the Integrated Science Assessment, Risk Assessment, and 

Policy Assessment, the advice and recommendations of CASAC, and public comments to 

date, the Administrator revises the current suite of primary PM2.5 standards. Specifically,

the Administrator revises: (1) the level of the primary annual PM2.5 standard to 12.0 

μg/m3and (2) the form of the primary annual PM2.5 standard to one based on the highest 

appropriate area-wide monitor in an area, with no option for spatial averaging. In 

conjunction with revising the primary annual PM2.5 standard to provide protection from 

effects associated with long- and short-term PM2.5 exposures, the Administrator retains 

the level of 35 μg/m3and the 98th percentile form of the primary 24-hour PM2.5 standard 

 

110 The Administrator also judges that this suite of standards addresses the issues raised 

by the D.C. Circuit’s remand of the 2006 primary annual PM2.5 standard by appropriately 

revising that standard.
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to continue to provide supplemental protection for areas with high peak PM2.5

concentrations. The Administrator is not revising the current PM2.5 indicator or the 

annual and 24-hour averaging times for the primary PM2.5 standards. The Administrator 

concludes that this suite of standards would be requisite to protect public health with an 

adequate margin of safety against health effects potentially associated with long- and 

short-term PM2.5 exposures.