From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov

Cc: martin.karen@epa.gov, Beth Hassett-Sipple/RTP/USEPA/US@EPA, hannon.john@epa.gov, Scott 

Jenkins/RTP/USEPA/US@EPA, Joseph-J Dougherty/DC/USEPA/US@EPA

Date: 06/13/2012 03:17 PM

Subject: Revisions to primary standard section of the PM preamble in response to interagency comments

Nathan,

Please find attached revisions made to the primary fine particle standard section of the PM preamble. These changes address 

interagency comments 9-21 which we received last week. Please let me know if you have any questions. Thanks.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 2 of 133

EO12866_PM NAAQS_2060-AO47_NPRM_Preamble_and_Reg_Text_2012052920120613

Includes edits to Section III.E.4 as updated on 20120608

6560-50-P

III. Rationale for Proposed Decisions on the Primary PM2.5 Standards

This section presents the rationale for the Administrator’s proposed decision to revise the level 

and form of the existing primary annual PM2.5 standard and to retain the existing primary 24-hour PM2.5

standard. As discussed more fully below, this rationale 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. This proposal 

also takes 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; and (3) public comments received during the development of these documents, either in 

connection with CASAC meetings or separately.

In developing this proposal, the Administrator recognizes that the CAA requires her to reach a 

public health policy judgment as to what standards would be requisite 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 as to what weight to 

place on various types of evidence and assessments, and on the related uncertainties and limitations. 

Thus, in selecting standards to propose, and subsequently in selecting the final standards, the 

Administrator is seeking not only to prevent fine particle concentrations that have been demonstrated to 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 3 of 133

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, 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 (US 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 well 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 proposed 

rule, the EPA has drawn upon an integrative synthesis of the entire body of evidence between exposure 

to ambient fine particles and a broad range of health endpoints (US EPA, 2009a, Chapters 2, 4, 5, 6, 7, 

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

there is a causal or likely causal relationship with long- or short-term 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 in taking into account 

 

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.

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 4 of 133

potential impacts on susceptibleat-risk populations2and in considering alternative standard levels that 

provide protection with an appropriate margin of safety. 

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 (US EPA, 2010a) and Policy Assessment (US EPA, 2011a, chapter 2), have also undergone 

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

comment. 

Although important uncertainties remain in the qualitative and quantitative characterizations of 

health effects attributable to ambient fine particles, the review of this information has been extensive and 

deliberate. This intensive evaluation of the scientific evidence and quantitative assessments has provided 

an 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 potential 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 will draw 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 this proposal.

In presenting the rationale for the proposed revisions of 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 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 

 

2

In this proposal, the term “at-risk” is the broadly encompassing term used for groups with specific 

factors that increase the risk of PM-related health effects in a population. In the Integrated Science 

Assessment, as discussed in section III.B.3 below, the term “susceptibility” was used broadly to 

recognize populations at greater risk. 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 5 of 133

history, section II.A.3 describes EPA’s general approach used in the current review for considering the 

need to retain or revise the current suite of fine particle standards. Section III.B summarizes the body of 

scientific evidence supporting the rationale for the proposed decisions, including key health endpoints 

associated with long- and short-term exposures to ambient fine particles. This overview includes a 

discussion of susceptibleat-risk populations and potential PM2.5-related impacts on public health. Section 

III.C outlines the approach taken by the EPA to assess health risks associated with exposure to ambient 

PM2.5, including a discussion of key uncertainties and limitations associated with these analyses. Section 

III.D discusses the scientific evidence, air quality, risk-based information; CASAC advice; and the 

Administrator’s proposed decisions related to the adequacy of the current standards. Section III.E 

discusses the scientific evidence, air quality, and risk-based information; CASAC advice; and the 

Administrator’s proposed decisions related to alternative standards. Section III.F summarizes the 

Administrator’s proposed decisions with regard to the primary PM2.5 NAAQS.

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 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 PM 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, or in the form of concentration-response functions that support 

quantitative risk assessment. However, evidence- and risk-based approaches using information from 

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 6 of 133

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. 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. 

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. 

1. General Approach Used in Previous Reviews

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

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

mean PM2.5 concentrations reported in epidemiological studies (US EPA, 2011a, section 2.1.1). This 

approach recognized that the strongest evidence of PM2.5-related associations occurs at concentrations 

near 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 (US 

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 longterm 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 7 of 133

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).

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 a national 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 (US 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 both short- and long-term exposure studies, 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 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 

 

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).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 8 of 133

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 US 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, 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 fine particles including protection for 

susceptibleat-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. 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 

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 9 of 133

requisite to protect the public health while providing an adequate margin of safety from the risk of shortterm 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 susceptibleat-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 multi-city 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.

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

Formatted: Font: Italic, No underline

Formatted: Font: Italic
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 10 of 133

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 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, 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 for consideration to consider is broader and more 

integrative than in past reviews. Our general approach also reflects consideration of the issues raised by 

the court in its remand of the primary annual PM2.5 standard, 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 takes into account both evidence-based and risk-based 

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 (US 

EPA, 2010c,f). In so doing, EPA staff developed a final Policy Assessment (US EPA, 2011a) which 

provides as broad an array of policy options as is supportable 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 presented in this document.

The Policy Assessment concludes it is 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 11 of 133

to consider each standard separately, as was done in the 2006 review (US EPA, 2011a, section 2.1.3).4

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

appropriate degree of public health protection. The extent to which these two standards are interrelated 

in any given area depends in large part on the 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 

recognizes 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 recognizes 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 recognizes that the shortterm exposure studies are 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 is 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 

 

4 By utilizing this approach, the Agency would also be 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. 

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 12 of 133

same point). The quantitative risk assessments conducted for this and previous reviews demonstrate 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 midrange, below the peak 24-hour concentrations (US EPA, 2011a, section 2.2.2; US EPA, 2010a, section 

3.1.2.2). In addition, there is no evidence suggesting that risks associated with long-term exposures are 

likely to be disproportionately driven by peak 24-hour concentrations.5For these reasons, strategies that 

focus primarily on reducing peak days are less likely to achieve reductions in the PM2.5 concentrations 

that are most strongly associated with the observed health effects. 

Furthermore, a policy approach that focuses 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 (US EPA, 2010a, section 5.2.3). This is because reductions 

based on control of peak days are less likely to control the bulk of the air quality distribution, as 

discussed above. 

The Policy Assessment concludes 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, is 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 longand 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 (US EPA, 2011a, p. 2-9).

The Policy Assessment also concludes, however, 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

 

5

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

days 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).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 13 of 133

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 concludes it is 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 (US EPA, 2011a, p. 2-10).

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

suites of standards focuses 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 

is 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 considers effects and estimated risks 

associated with short-term exposures. In both cases, the Policy Assessment places greatest weight on 

health effects that have 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 evidence that focuses on specific susceptibleat-risk populations. The Policy 

Assessment places relatively greater weight on statistically significant associations that yield relatively 

more precise effect estimates and that are judged to be robust to confounding by other air pollutants. In 

the case of short-term exposure studies, the Policy Assessment places 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 considers a number 

of factors (US EPA, 2011a, section 2.2). As an initial matter, the Policy Assessment considers 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 14 of 133

effects. The Policy Assessment also considers evidence on susceptibleat-risk populations and potential 

impacts on such populations. Further, the Policy Assessment explores the extent to which PM2.5-related 

health effects have been observed in areas where air quality distributions extend to lower levels 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 alternative standard levels for consideration (US EPA, 2011a, sections 2.1.3 and 2.3.4), 

the Policy Assessment first recognizes the absence of discernible thresholds in the concentrationresponse functions from long- and short-term PM2.5 exposure studies (US EPA, 2011a, section 2.4.3).6

In the absence of any discernible thresholds, the Agency’s general approach for identifying appropriate 

standard levels for consideration involves 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 recognizes 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 decisions to either retain or revise the standards. Final 

decisions will necessarily 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 is appropriate to 

consider, the Policy Assessment focuses on identifying an annual standard that provides requisite 

 

6

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

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

cardiovascular-related hospital admissions and emergency department visits associated with short-term 

PM10 exposures and premature mortality associated with long-term PM2.5 exposure (US EPA, 2009a, 

sections 6.5, 6.2.10.10 and 7.6). Overall, the Integrated Science Assessment concludes that the studies 

evaluated support the use of a no-threshold, log-linear model but recognizes 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” (US 

EPA, 2009a, section 2.4.3).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 15 of 133

protection from effects associated with both long- and short-term exposures. In so doing, the Policy 

Assessment explores 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. 

The approach that most directly addresses this issue considers studies that present confidence 

intervals around concentration-response relationships, and in particular, analyses that average across 

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

The Policy Assessment explores the extent to which such analyses have 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 concludes 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 explores other approaches that consider different statistical 

metrics from epidemiological studies. The Policy Assessment first takes 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. 8

This approach recognizes that the strongest evidence of PM2.5-related associations occurs at 

 

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

Epidemiological studies typically report PM2.5 concentrations averaged across the available ambient 

monitors. For multi-city studies, this metric reflects concentrations averaged across one or more ambient 

monitors within each area included in a given study 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.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 16 of 133

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

places 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 considers information beyond a single 

statistical metric of PM2.5 concentrations (i.e., the mean) to the extent such information is available. In 

so doing, the Policy Assessment employs distributional statistics (i.e., statistical characterization of an 

entire distribution of data) to identify the broader range of PM2.5 concentrations that were most 

influential in generating health effecthad the most influence on the calculation of relative risk estimates 

in epidemiological studies. Thus, the Policy Assessment considers the range of PM2.5 concentrations 

where the data analyzed in the study (i.e., air quality and population-level data, as discussed below) are 

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

which our confidence in the associations observed in the epidemiological studies is greatest. The Policy 

Assessment then focuses on the lower part of this range to characterize where in the distributions the 

data become appreciably more sparse and, thus, where our understanding of the associations 

correspondingly becomes more uncertain. The Policy Assessment recognizes there is no one percentile 

value within a given distribution that is the most appropriate or “correct” way to characterize where our 

confidence in the associations becomes appreciably lower. The Policy Assessment concludes that the 

range from the 25th to 10th percentiles is a reasonable range to consider as a region where we have 

appreciably less confidence in the associations observed in epidemiological studies.9

In considering distributional statistics from epidemiological studies, the final Policy Assessment

 

9

In the PM NAAQS review completed in 2006, the Staff Paper 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” (US 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.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 17 of 133

recognizes that there are focused on two types of population-level metrics that CASAC advices are most

useful to consider in identifying the PM2.5 concentrations most influential in generating the health effect 

estimates reported in the epidemiological studies. The10 Consistent with CASAC advice, the most 

relevant information is 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, as discussed in section III.E.4.b below, the Policy 

Assessment also considers the number of study participants within each study area as a reasonablean 

appropriate surrogate for health event data.11

The Policy Assessment recognizes that an approach considering analyses of confidence intervals 

around concentration-response functions is intrinsically related to an approach that considers different 

distributional statistics. Both of these approaches could be employed to identify the range of PM2.5

concentrations over which we have the most confidence in the associations reported in epidemiological 

 

10 The second draft Policy Assessment focused on the distributions of PM2.5 concentrations across areas 

included in several multi-city studies for which such data were available in seeking to identify the most 

influential range of concentrations (US 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, EPA considered area-specific health 

event and area-specific population data along with corresponding PM2.5 concentrations to generate a 

cumulative distribution of the population data relative to long-term mean PM2.5 concentrations to 

determine the most influential range (US EPA, 2011a, Figure 2-7 and associated text).

11 In the absence of data on the number of health events or study participants, the distribution of PM2.5

concentrations across study areas in multi-city studies could also be considered to be representative of 

the PM2.5 concentrations likely experienced by study participants, which are integral to the generation of 

effect estimates. This approach is particularly relevant for identifying the range of PM2.5 concentrations 

most influential in generating the effect estimates in short-term exposure studies (e.g., time-series 

studies). In short-term exposure studies, a similar number of events are likely occurring on a daily basis 

and, thus, information on the PM2.5 air quality distributions provides a better approximation of a study 

participant’s likely exposure in relation to the observed health effects as compared to long-term 

exposure studies where individual study participants’ exposures change over the course of the study 

follow-up period (Samet, 2010d, p. 2). However, the EPA was unable to determine from the 

methodologies published for the epidemiological studies how the air quality data were statistically 

weighted to account for variations in the availability of daily PM2.5 measurements by study area (Rajan 

et al., 2011, p. 7). Consequently, the EPA was unable to consider this type of information as part of its 

effort to identify the broader range of PM2.5 concentrations that were most influential in generating the 

health effect estimates in epidemiological studies (US EPA, 2011a, p. 2-71).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 18 of 133

studies. 

In applying these approaches, the Policy Assessment considers PM2.5 concentrations from longand short-term PM2.5 exposure studies using composite monitor distributions.12 For multi-city studies, 

this distribution reflects concentrations averaged across one or more ambient monitors within each area 

included in a given study and then averaged across study areas for an overall study mean PM2.5

concentration. Beyond considering air quality concentrations based on composite monitor distributions, 

the second draft Policy Assessment also considered PM2.5 concentrations based on measurements at the 

monitor within each area that records the highest concentration (i.e., maximum monitor) (US EPA, 

2010f, sections 2.1.3 and 2.3.4.1).13 Although the second draft Policy Assessment discussed whether 

consideration of alternative annual standard levels couldshould be based on either composite or 

maximum monitor distributions, the final Policy Assessment, consistent with CASAC advice (Samet, 

2010d, p. 3), concluded that it is most reasonable to place more weight on an approach based on 

composite monitor distributions, which represent the PM2.5 concentrations typically presented and used 

in epidemiological analyses and which provide a direct link between PM2.5 concentrations and the 

observed health effects reported in both long- and short-term exposure studies (US EPA, 2011a, p. 2-

13). 

In reaching staff conclusions on alternative standard levels that are appropriate to consider, the 

Policy Assessment also includes a broader consideration of the uncertainties related to the 

concentration-response relationships from multi-city, long- and short-term exposure studies. Most 

 

12 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, as noted above, the composite 

monitor concentration represents the average concentration across one or more monitors within each 

area included in a given study and then averaged across study areas for an overall study mean PM2.5

concentration. 

13 The maximum monitor distribution is relevant because it is generally used to determine whether a 

given standard is met in an area and the extent to which ambient PM2.5 concentrations need to be 

reduced in order to bring an area into attainment with the standard. However, maximum monitor 

distributions represent a far less robust metric than composite monitor distributions for consideration of 

alternative annual standard levels in part because they are available for only a few epidemiological 

studies.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 19 of 133

notably, these uncertainties relate to our currently limited understanding of the heterogeneity of relative 

risk estimates in areas across the country. This heterogeneity may be attributed, in part, to the potential 

for different components within the mix of ambient fine particles to differentially contribute to health 

effects observed in the studies and to exposure-related factors (US 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 further discussed in section III.B.2 below.

The Policy Assessment recognizes that the level of protection afforded by the NAAQS relies 

both on the level and the form of the standard. The Policy Assessment concludes that a policy approach 

that uses data based on composite monitor distributions to identify alternative standard levels, and then 

compares those levels to concentrations at maximum monitors to determine if an area meets a given 

standard, inherently has the potential to build in some margin of safety (US EPA, 2011a, p. 2-14).14 This 

conclusion is 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

 

14 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. 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 and the 

composite monitor distributions may be well below the maximum monitor distributions (US EPA, 

2011a, p. 2-14). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 20 of 133

exposure studies. The evidence from these studies informs 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 evaluates both the distributions of 24-hour PM2.5

concentrations, with a focus on the 98th percentile concentrations to match the form of the current 24-

hour PM2.5 standard, to the extent such data were available, 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 considers air quality information based on county-level 24-hour and annual 

design values15 to understand the policy implications of the alternative standard levels supported by the 

underlying science. In particular, the Policy Assessment considers 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. Based on the evidence-based considerations outlined above, the Policy 

Assessment develops integrated conclusions with regard to alternative suites of standards, including 

both annual and 24-hour standards that are appropriate to consider in this review based on the currently 

available evidence and air quality information. In so doing, the Policy Assessment discusses 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 considers the 

quantitative risk estimates and the key observations presented in the Risk Assessment. This assessment 

includes 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 (US EPA, 2010a). As part of the 

risk-based considerations, the Policy Assessment considers 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 

 

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

compliance.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 21 of 133

characterizes 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 

recognizes the uncertainties inherent in such risk estimates, and takes 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 considers 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.

The Policy Assessment conclusions related to the primary PM2.5 standards reflect 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) potential alternative 

standards, if any, that are appropriate to consider in this review to protect against effects associated with 

both long- and short-term exposures to fine particles. In addressing these broad questions, the 

discussions included in the Policy Assessment were organized around a series of more specific questions 

reflecting different aspects of each overarching question (US EPA, 2011a, chapter 2, Figure 2-1). When 

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

the Policy Assessment takes 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 provides a comprehensive basis to help inform the judgments required of the 

Administrator in reaching decisions about the current and potential alternative primary fine particle 

standards and in responding to the remand of the 2006 primary annual PM2.5 standard.

B. Health Effects Related to Exposure to Fine Particles

This section outlines key information contained 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 

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 22 of 133

particle exposures. As was true in the last review, evidence from epidemiologic studies plays a key role 

in the Integrated Science Assessment’s evaluation of the scientific evidence. The following sections 

discuss 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), susceptibleat-risk populations (section III.B.3), and potential PM2.5-related impacts on public 

health (section III.B.4).

1. Nature of Effects

In considering the strength of the associations between long- and short-term exposures to PM2.5

and health effects, the Policy Assessment notes that in the PM NAAQS review completed in 2006 the 

Agency 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 (US EPA, 2004, p. 9-46; US EPA, 2005, p. 5-4). Overall, the epidemiological evidence 

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” (US EPA, 2004, p. 9-48).16

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 (US 

EPA, 2009a, sections 2.3.1 and 2.3.2). The currently available evidence is stronger in comparison to 

evidence available in the last review because of its breadth and the substantiation of previously observed 

health effects. A number of large multi-city epidemiological studies have been conducted throughout the 

U.S., including extended analyses of studies that were important to inform decision-making in the last 

 

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

strength of the available epidemiological 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 (US EPA, 2009a, section 1.5). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 23 of 133

review. These studies have reported consistent increases in morbidity and/or mortality related to ambient 

PM2.5 concentrations, with the strongest evidence reported for cardiovascular-related effects. In addition, 

the findings of new toxicological and controlled human exposure studies greatly expand and provide 

stronger support for a number of potential biologic mechanisms or pathways for cardiovascular and 

respiratory effects associated with long- and short-term PM exposures (US EPA, 2009a, p 2-17; chapter 

5; Figures 5-4 and 5-5). 

With regard to causal inferences described in the Integrated Science Assessment, the Policy

Assessment notes that since the last review, the Agency has developed a more formal framework for 

reaching causal determinations that 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 (US EPA, 2011a, p. 2-18; US EPA, 2009a, 

section 1.5). 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 a causal 

relationship, suggestive of a causal relationship, inadequate to infer a causal relationship, and not likely 

to be a causal relationship (US EPA, 2009a, Table 1-3).17

Using this causal framework, the Integrated Science Assessment concludes that the collective 

evidence is largely consistent with past studies and substantially strengthens what was known about fine 

particles in the last review to reach the conclusion that a causal relationship exists between both longand short-term exposures to PM2.5 and mortality and cardiovascular effects including cardiovascularrelated mortality. The Integrated Science Assessment also concludes that the collective evidence 

continues to support a likely causal relationship between long- and short-term PM2.5 exposures and 

respiratory effects, including respiratory-related mortality. Further, the Integrated Science Assessment

 

17 Causal inferences, as discussed in the Integrated Science Assessment, 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 (US EPA, 2009a, chapter 5). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 24 of 133

concludes that the currently available evidence is 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) (US EPA, 2009a, sections 2.3.1 and 2.6; Table 2-6; US EPA, 2011a, Table 2-1). 

a. Health Effects Associated with Long-term PM2.5 Exposures

With regard to mortality, the Integrated Science Assessment concludes that newly available 

evidence significantly strengthens the link between long-term exposure to PM2.5 and mortality, while 

providing indications that the magnitude of the PM2.5-mortality association may be larger than 

previously estimated (US EPA, 2009a, sections 7.2.10, 7.2.11, and 7.6.1; Figures 7-6 and 7-7). A 

number of large U.S. cohort studies have been published since the last review, including extended 

analyses of the American Cancer Society (ACS) and Harvard Six Cities studies (US EPA, 2009a, pp 7-

84 to 7-85; Figure 7-6; Krewski et al., 2009; Pope et al., 2004; Jerrett et al., 2005; Laden et al., 2006). In 

addition, new long-term PM2.5 exposure studies evaluating mortality impacts in additional cohorts are 

now available (US EPA, 2009a, section 7.6). For example, the Women’s Health Initiative (WHI) 

Observational Study reported effects of PM2.5 on cardiovascular-related mortality in post-menopausal 

women with no previous history of cardiac disease (Miller et al., 2007), while). The PM2.5 effect 

estimate in this study remained positive and statistically significant in a multi-pollutant model that 

included gaseous co-pollutants as well as coarse particles. In addition, multiple studies observed PM2.5-

associated mortality among older adults using Medicare data (Eftim et al., 2008; Zeger et al., 2007, 

2008). Collectively, these new studies, along with evidence available in the last review, provide 

consistent and stronger evidence of associations between long-term exposure to PM2.5 and mortality 

(U.S. EPA, 2009a, sections 2.3.1 and 7.6). 

The strength of the causal relationship between long-term PM2.5 exposure and mortality also 

builds upon new studies providing evidence of improvement in community health following reductions 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 25 of 133

in ambient fine particles. Pope et al. (2009) documented the population health benefits of reducing 

ambient air pollution by correlating past reductions in ambient PM2.5 concentrations with increased life 

expectancy. These investigators reported that reductions in ambient fine particles during the 1980s and 

1990s account for as much as 15 percent of the overall improvement in life expectancy in 51 U.S. 

metropolitan areas, with the fine particle reductions reported to be associated with an estimated increase 

in mean life expectancy of approximately 5 to 9 months (US EPA, 2009a, p. 7-95; Pope et al., 2009). An 

extended analysis of the Harvard Six Cities study found that as cities cleaned up their air, locations with 

the largest reductions in PM2.5 saw the largest improvements in reduced mortality rates, while those with 

the smallest decreases in PM2.5 concentrations saw the smallest improvements (Laden et al., 2006). 

Another extended follow-up to the Harvard Six Cities study investigated the delay between changes in 

ambient PM2.5 concentrations and changes in mortality (Schwartz et al., 2008) and reported that the 

effects of changes in PM2.5 were seen within the 2 years prior to death (US EPA, 2009a, p. 7-92; Figure 

7-9). 

With regard to cardiovascular effects, 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. The Integrated Science Assessment concludes that the strongest evidence comes from 

recent studies investigating cardiovascular-related mortality. This includes evidence from a number of 

large, multi-city U.S. long-term cohort studies including extended follow-up analyses of the ACS and 

Harvard Six Cities studies, as well as the WHI study (US EPA, 2009a, sections 7.2.10 and 7.6.1; 

Krewski et al., 2009; Pope et al., 2004; Laden et al., 2006; Miller et al., 2007). Pope et al. (2004) 

reported a positive association between mortality and long-term PM2.5 exposure for a number of specific 

cardiovascular diseases, including ischemic heart disease, dysrhythmia, heart failure, and cardiac arrest 

(US EPA, 2009a, p. 7-84; Figure 7-7). Krewski et al. (2009) provides further evidence for mortality 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 26 of 133

related to ischemic heart disease associated with long-term PM2.5 exposure (US EPA, 2009a, p. 7-84, 

Figure 7-7). 

With regard to cardiovascular-related morbidity associated with long-term PM2.5 exposures, 

studies were not available in the last review. Recent studies, however, have provided new evidence 

linking long-term exposure to PM2.5 with cardiovascular outcomes that has “expanded upon the 

continuum of effects ranging from the more subtle subclinical measures to cardiopulmonary mortality” 

(US EPA, 2009a, p. 2-17). In the current review, studies are now available that evaluated a number of

endpoints ranging from subtle indicators of cardiovascular health to serious clinical events associated 

with coronary heart disease and cardiovascular and cerebrovascular disease.18 The most important new 

evidence comes from the WHI study which provides evidence of nonfatal cardiovascular events 

including both coronary and cerebrovascular events (Miller et al., 2007; US EPA, 2009a, sections 7.2.9 

and 7.6.1). Toxicological studies provide supportive evidence that the cardiovascular morbidity effects 

observed in long-term exposure epidemiological studies are biologically plausible and coherent with 

studies of cardiovascular-related mortality as well as with studies of cardiovascular-related effects 

associated with short-term exposures to PM2.5, as described below (US EPA, 2009a, p 7-19). 

With regard to respiratory effects, the Integrated Science Assessment concludes that 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 associated with long-term 

PM2.5 exposure. The strongest evidence for respiratory-related effects available in this review is from 

studies that evaluated decrements in lung function growth, increased respiratory symptoms, and asthma 

development (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1).19 Specifically, extended analyses 

of the Southern California Children’s Health Study provide evidence that clinically important deficits in 

 

18Coronary and cerebrovascular events include myocardial infarction, coronary artery revascularization 

(e.g., bypass graft, angioplasty, stent, atherectomy), congestive heart failure and stroke. 

19 Supporting evidence comes from studies “that observed associations between long-term exposure to 

PM10 and an increase in respiratory symptoms and reductions in lung function growth in areas where 

PM10 is dominated by PM2.5” (US EPA, 2009a, p. 2-12). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 27 of 133

lung function20 associated with long-term exposure to PM2.5 persist into early adulthood (U.S. EPA, 

2009a, p. 7-27; Gauderman et al., 2004). Additional analyses of the Southern California Children’s 

Health Study cohort reported an association between long-term PM2.5 exposure and bronchitic 

symptoms (US EPA, 2009a, p. 7-23 to 24; McConnell et al., 2003) 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 (US EPA, 2009, 7-24; Islam et al., 2007). The outcomes observed in these 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 previously 

reported. Supporting these results are new longitudinal cohort studies conducted by other researchers in 

varying locations using different methods (US EPA, 2009a, section 7.3.9.1). New evidence from a U.S. 

cohort of cystic fibrosis patients 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 (US EPA, 2009a, p. 7-25; Goss et al., 2004).

Toxicological studies provide coherence and biological plausibility for the respiratory effects 

observed in epidemiological studies (US EPA, 2009a, p. 7-42). For example, pre- and postnatal 

exposure to ambient levels of urban particles has been found to affect lung development in an animal 

model (US EPA, 2009a, section 7.3.2.2; p. 7-43). This finding is important because impaired lung 

development is one mechanism by which PM exposure may decrease lung function growth in children 

(US EPA, 2009a, p. 2-12; section 7.3). 

With regard to respiratory-related mortality associated with long-term PM2.5 exposure, the 

Integrated Science Assessment concludes that “when deaths due to respiratory causes are separated from 

 

20 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 (US EPA, 2009a, p. 7-29 to 7-30). The primary NAAQS for sulfur dioxide (SO2) also 

includes this interpretation for FEV1 (75 FR 35525, June 22, 2010).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 28 of 133

all-cause (nonaccidental) and cardiopulmonary deaths, there is limited and inconclusive evidence for an 

effect of PM2.5 on respiratory mortality, with one large cohort study finding a reduction in deaths due to 

respiratory causes associated with reduced PM2.5 concentrations, and another large cohort study finding 

no PM2.5 associations with respiratory mortality” (US EPA, 2009a, p. 7-41). The extended follow-up of 

the Harvard Six Cities study reported a positive but statistically non-significant association between 

long-term PM2.5 exposure and respiratory-related mortality (Laden et al., 2006), whereas Pope et al. 

(2004) found no association in the ACS cohort (US EPA, 2009a, p. 7-84). There is emerging but limited 

evidence for an association between long-term PM2.5 exposure and respiratory mortality in post-neonatal 

infants where long-term exposure was considered as approximately one month to one year (US EPA, 

2009a, pp. 7-54 to 7-55). Emerging evidence of short- and long-term exposure to PM2.5 and respiratory 

morbidity and infant mortality provide some support for the weak respiratory-related mortality effects 

observed. 

Beyond effects considered to have causal or likely causal relationships with long-term PM2.5

exposure as discussed above, the following health outcomes are classified in the Integrated Science 

Assessment as having evidence suggestive of a causal relationship with long-term PM2.5 exposure: (1) 

reproductive and developmental effects and (2) cancer, mutagenicity, and genotoxicity effects (US EPA, 

2009a, Table 2-6). With regard to reproductive and developmental effects, the Integrated Science 

Assessment notes, “[e]vidence is accumulating for PM2.5-related effects on low birth weight and infant 

mortality, especially due to respiratory causes during the post-neonatal period” (US EPA, 2009a, p. 

2-13). New evidence available in this review reports significant associations between exposure to PM2.5

during pregnancy and lower birth weight and infant mortality, with less consistent evidence for pre-term 

birth and intrauterine growth restriction. (US EPA, 2009a, section 7.4). The Integrated Science 

Assessment further notes that “[i]nfants and fetal development processes may be particularly vulnerable 

to PM exposure, and although the physical mechanisms are not fully understood, several hypotheses 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 29 of 133

have been proposed involving direct effects on fetal health, altered placenta function, or indirect effects 

on the mother’s health” (US EPA, 2009a, section 7.4.1). Although toxicological studies provide some 

evidence that supports an association between long-term PM2.5 exposure and adverse reproductive and 

developmental outcomes, there is “little mechanistic information or biological plausibility for an 

association between long-term PM exposure and adverse birth outcomes (e.g., low birth weight, infant 

mortality)” (US EPA, 2009a, p. 2-13). 

With regard to cancer, mutagenic and genotoxic 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” (US EPA, 2009a, p. 2-13 

and sections 2.3.1.2 and 7.5; Table 7-7; Figures 7-6 and 7-7). The extended follow-up to the ACS study 

reported an association between long-term PM2.5 exposure and lung cancer mortality (US EPA, 2009a, 

p. 7-71; Krewski et al., 2009) as did the extended follow-up to the Harvard Six Cities study when 

considering the entire 25-year follow-up period (Laden et al., 2006). There is some evidence, primarily 

from in vitro studies, providing biological plausibility for the PM-lung cancer relationships observed in 

epidemiological studies (US EPA, 2009a, p. 7-80), although in vivo toxicological studies of 

carcinogenicity generally reported mixed results (US EPA, 2009a, section 7.5).

b. Health Effects Associated with Short-term PM2.5 Exposures

In considering effects associated with short-term PM2.5 exposure, the body of currently available 

scientific evidence has been expanded greatly by the publication of a number of new multi-city, timeseries studies that have used uniform methodologies to investigate the effects of short-term fine particle 

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 

climate and air pollution mixes on PM2.5-associated health effects. These studies provide more precise 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 30 of 133

estimates of the magnitude of effects associated with short-term PM2.5 exposure than most smaller-scale 

single-city studies that were more commonly available in the last review (US EPA 2009a, chapter 6). 

With regard to mortality, new U.S. and Canadian multi-city and single-city PM2.5 exposure 

studies have found generally consistent positive associations between short-term PM2.5 exposures and 

cardiovascular- and respiratory-related mortality as well as all-cause (non-accidental) mortality (US 

EPA, 2009a, sections 2.3.1.1, 6.2.11 and 6.5.2.2; Figures 6-26, 6-27, and 6-28). In an analysis of the 

National Morbidity, Mortality, and Air Pollution Study (NMMAPS) data, Dominici et al. (2007) 

reported associations between fine particle exposures and all-cause and cardiopulmonary-related 

mortality (US EPA, 2009a, p. 6-175, Figure 6-26). In a study of 112 U.S. cities, Zanobetti and Schwartz

(2009) reported positive associations (in 99 percent of the cities) and frequently statistically significant 

associations (in 55 percent of the cities) between short-term PM2.5 exposure and total (non-accidental) 

mortality (US EPA, 2009a, pp 6-176 to 6-179; Figures 6-23 and 6-24).21 A Canadian 12-city study 

(Burnett et al., 2004) is generally consistent with an earlier Canadian 8-city study (Burnett and 

Goldberg, 2003). Both studies reported a positive and statistically significant association between shortterm PM2.5 exposure and mortality (US EPA, 2009a, p 6-182, Figure 2-1), although the influence of 

nitrogen dioxide (NO2) and limited PM2.5 data for several years during the study period somewhat 

diminished the findings reported in the 12-city study. In addition to these multi-city studies, evidence 

from available single-city studies suggests that gaseous copollutants do not confound the PM2.5-

mortality association (US EPA, 2009a, section 2.3.1.1). Collectively, these studies provide generally 

consistent and much stronger evidence for PM2.5-associated mortality than the evidence available in the 

last review. (US EPA, 2011a, p.2-24). 

With regard to cardiovascular effects, new multi-city as well as single-city short-term PM2.5

exposure studies conducted since the last review support a largely positive and frequently statistically 

 

21 Single-city Bayes-adjusted effect estimates for the 112 cities analyzed in Zanobetti and Schwartz 

(2009) were provided by the study author (personal communication with Dr. Antonella Zanobetti, 2009;

see also US EPA, 2009a, Figure 6-24). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 31 of 133

significant association between short-term exposure to PM2.5 and cardiovascular-related morbidity and 

mortality, substantiating prior findings. For example, among a multi-city cohort of older adults 

participating in the Medicare Air Pollution Study (MCAPS), investigators reported evidence of a 

positive association between short-term PM2.5 exposures and hospital admissions related to 

cardiovascular outcomes (US EPA, 2009a, pp. 6-57 to 58; Dominici et al, 2006a; Bell et al, 2008). The 

strongest evidence for cardiovascular effects has been observed predominantly for hospital admissions 

and emergency department visits for ischemic heart disease and congestive heart failure, and 

cardiovascular-related mortality (US EPA, 2009a, Figure 2-1, p. 6-79, sections 6.2.10.3, 6.2.10.5, and 

6.2.11; Bell et al., 2008; Dominici et al., 2006a; Tolbert et al., 2007; Zanobetti and Schwartz, 2009). In 

studies that evaluated the potential for confounding using co-pollutant models, PM2.5 effect estimates for 

cardiovascular-related hospital admissions and emergency department visits generally remained 

positive, with the magnitude of PM2.5 effect estimates increasing in some models and decreasing in 

others (US EPA, 2009a, Figure 6-5). Furthermore, these findings are supported by a recent study of a 

multi-city cohort of women participating in the WHI study that reported a positive but statistically 

nonsignificant association between short-term exposure to PM2.5 and electrocardiogram measures of 

myocardial ischemia (Zhang et al., 2009). 

In focusing on respiratory effects, the strongest evidence from short-term PM2.5 exposure studies 

has been observed for respiratory-related emergency department visits and hospital admissions for 

chronic obstructive pulmonary disease (COPD) and respiratory infections (US EPA, 2009a, sections 

2.3.1.1 and 6.3.8.3; Figures 2-1 and 6-13; Dominici et al., 2006a). In studies that employed co-pollutant 

models to evaluate the potential for confounding, PM2.5 effect estimates for respiratory-related hospital 

admissions and emergency department visits generally remained positive, with the magnitude of PM2.5

effect estimates increasing in some models and decreasing in others (US EPA, 2009a, Figure 6-15). 

Evidence for PM2.5-related respiratory effects has also been observed in panel studies, which indicate 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 32 of 133

associations with respiratory symptoms, pulmonary function, and pulmonary inflammation among 

asthmatic children (US EPA, 2009a, p. 2-10). Although not consistently observed, some controlled 

human exposure studies have reported small decrements in various measures of pulmonary function 

following controlled exposures to PM2.5 (US EPA, 20009a, p. 2-10). Furthermore, the comparatively 

larger body of toxicological evidence since the last review is coherent with the evidence from 

epidemiological and controlled human exposure studies that examined short-term exposures to PM2.5

and respiratory effects (US EPA, 2009a, section 6.3.10.1). 

c. Summary 

In considering the extent to which newly available scientific evidence strengthens or calls into 

question evidence of associations identified in the last review between ambient fine particle exposures 

and health effects, the Policy Assessment recognizes that much progress has been made in assessing 

some key uncertainties related to our understanding of health effects associated with long- and shortterm exposure to PM2.5. As briefly discussed above as well as in the more complete discussion of the 

evidence as presented and assessed in the Integrated Science Assessment, the Policy Assessment notes 

that the newly available information combined with information available in the last review provides 

substantially stronger confidence in a causal relationship between long- and short-term exposures to 

PM2.5 and mortality and cardiovascular effects. In addition, the newly available evidence reinforces and 

expands the evidence supporting a likely causal relationship between long- and short-term exposure to 

PM2.5 and respiratory effects. The body of scientific evidence is somewhat expanded 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 Integrated Science Assessment 

concludes that these data provides evidence that is suggestive of a causal relationship for these effects. 

Thus, the Policy Assessment concludes there is 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 33 of 133

was available in the last review, providing the basis for fine particle standards at least as protective as 

the current PM2.5 standards.

2. Limitations and Uncertainties Associated with the Currently Available Evidence

With respect to understanding the nature and magnitude of PM2.5-related risks, the Policy 

Assessment recognizes that important uncertainties remain in the current review (US EPA, 2011a, p. 

2-25). Epidemiological studies evaluating health effects associated with long- and short-term PM2.5

exposures have reported heterogeneity in responses both within and between cities and geographic 

regions within the U.S. In particular, the Policy Assessment notes that there are challenges with 

interpreting differences in health effects observed in the eastern versus western parts of the U.S., 

including evaluating effects stratified by seasons.22 This heterogeneity may be attributed, in part, to 

differences in the fine particle composition or related to exposure measurement error. However, the 

currently available evidence and limited availability of city-specific PM2.5 speciation data does not allow 

conclusions to be drawn regarding the relative toxicity of PM2.5 components or group of components 

associated with any source categories of fine particles in different locations (US EPA, 2009a, p. 2-17). 

Furthermore, important uncertainties remain in this review related to understanding the temporal and 

spatial variability in PM2.5 concentrations, including PM2.5 components, and associated health impacts 

across different geographic areas and seasons.

In considering the relationships between PM composition and health effects, the ISA notes that 

the scientific evidence continues to evolve and concludes that, while many constituents of PM 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 (US EPA, 2009a, p. 

2-17). In particular, based on assessing the body of available evidence, the ISA notes that (1) 

cardiovascular effects have been linked with elemental carbon as well as with PM2.5 from crustal 

 

22 Seasonal differences in effects may be related to PM2.5 composition as well as regional differences in 

climate and infrastructure that may affect time spent outdoors or indoors, housing characteristics 

including air conditioning usage, and differences in baseline incidence rates (US EPA, 2009a, p. 3-182).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 34 of 133

sources, traffic, and wood smoke/vegetative burning; (2) respiratory effects have been linked with 

secondary sulfate PM2.5 as well as with PM2.5 from crustal/soil/road dust and traffic sources; and (3) a 

few studies have reported associations between total mortality and secondary sulfate/long-range 

transport, traffic, and salt. While specific PM2.5 constituents have been linked with various PM2.5-related 

health effects in a small number of studies, research continues to focus on the identification of specific 

constituents or sources that may be most closely related to specific PM2.5-related health outcomes.

Exposure measurement error is also an important source of uncertainty (US EPA, 2009a, section 

3.8.6). 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, climate, source characteristics, ventilation usage, personal activity patterns, 

photochemistry). As noted in the Integrated Science Assessment, exposure measurement error can 

introduce bias and increased uncertainty in associated health effect estimates (US EPA, 2009a, p. 2-17). 

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).23 Although many recent U.S. multi-cityAs noted 

above, although short-term studies did not analyze multipollutantof cardiovascular and respiratory 

hospital admissions and emergency department visits generally reported that PM2.5 effect estimates 

remained positive, the magnitude of those effect estimates increased in some models, and decreased in 

others. In addition, although evidence from single-city studies available in the last review 

suggestsuggests that gaseous copollutants do not confound the PM2.5-related mortality association (US 

 

23 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 (US EPA, 

2004, p. 8-10). 

Formatted: Font color: Black
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 35 of 133

EPA, 2004, section 8.4.3.3) which), a conclusion that is further supported by studies that examined the 

PM10-mortality relationship (US EPA, 2009a, p. 6-182 and 6-201). 

), many recent U.S. multi-city studies have not analyzed multipollutant models. 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.

3. SusceptibleAt-Risk Populations

In identifying population groups or lifestages at greatest risk for health risk from a specific 

pollutant, the terms susceptibility, vulnerability, sensitivity, and at-risk are commonly employed. The 

definition for these terms sometimes varies, but in most instances “susceptibility” refers to biological or 

intrinsic factors (e.g., lifestage, gender, preexisting disease/conditions) while “vulnerability” refers to 

nonbiological or extrinsic factors (e.g., socioeconomic factors). Additionally, However, factors included

in some cases, the terms “at-risk” and “sensitive” have been used to encompass both of these concepts. 

At times, however, factors of “susceptibility” and “vulnerability” are intertwined and are difficult to 

distinguish. In the Integrated Science Assessment, the term “susceptibility” has been used broadly to 

recognize populations that have a greater likelihood of experiencing effects related to ambient PM 

exposure

24, such that use of the term “susceptible populations” in the Integrated Science Assessment is 

defined as follows:

Populations that have a greater likelihood of experiencing health effects related to 

exposure to an air pollutant (e.g., PM) due to a variety of factors including, but not 

limited to: genetic or developmental factors, race, gender, lifestage, lifestyle (e.g., 

smoking status and nutrition) or preexisting disease; as well as population-level factors 

that can increase an individual's exposure to an air pollutant (e.g., PM) such as 

socioeconomic status [SES], which encompasses reduced access to health care, low 

educational attainment, residential location, and other factors (US EPA, 2009a, p. 8-1).

 

24 Although studies have primarily used exposures to PM10 or PM2.5, the available evidence suggests that 

the identified factors also increase risk from PM10-2.5 (US EPA, 2009a, section 8.1.8). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 36 of 133

Susceptibleused as a term that encompasses factors related both to susceptibility and 

vulnerability.25 In the development of a more recent Integrated Science Assessment, the Agency is using 

the term “at-risk” groups to more broadly define the populations with characteristics that increase the 

risk of pollutant-related health effects (US EPA, 2011d, p. 8-1). Therefore, in this proposal, the term “atrisk” is the broadly encompassing term used for groups with specific factors that increase the risk of 

PM-related health effects in a population.At-risk populations could exhibit a greater risk of PM-related 

health effects than the general population for a number of reasons including,: being affected by lower 

concentrations of PM, experiencing a larger health impact at a given PM concentration or being exposed 

to higher PM concentrations than the general population. Given the heterogeneity of individual 

responses to PM exposures, the severity of the health effects experienced by a susceptiblean at-risk

population may be much greater than that experienced by the population at large.

As summarized below and presented in more detail in chapter 8 of the Integrated Science 

Assessment and section 2.2.1 of the Policy Assessment, the currently available epidemiological and 

controlled human exposure evidence expands our understanding of previously identified susceptibleatrisk populations (i.e., children, older adults, and individuals with pre-existing heart and lung disease) 

and supports the identification of additional susceptibleat-risk populations (e.g., persons with lower 

socioeconomic status, genetic differences) (US EPA, 2009a, section 2.4.1, Table 8-2). In addition, 

toxicological studies provide underlying support for the biological mechanisms that potentially lead to 

increased susceptibility to PM-related health effects (US EPA, 2009a, sections 2.4.1 and 8.1.8). 

 

25 The term “susceptible population” is defined in the Integrated Science Assessment as “[P]opulations 

that have a greater likelihood of experiencing health effects related to exposure to an air pollutant (e.g., 

PM) due to a variety of factors including, but not limited to: genetic or developmental factors, race, 

gender, lifestage, lifestyle (e.g., smoking status and nutrition) or preexisting disease; as well as 

population-level factors that can increase an individual's exposure to an air pollutant (e.g., PM) such as 

socioeconomic status [SES], which encompasses reduced access to health care, low educational 

attainment, residential location, and other factors (US EPA, 2009a, p. 8-1).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 37 of 133

Two different lifestages have been associated with increased susceptibilityrisk to PM-related 

health effects: childhood (i.e., less than 18 years of age) and older adulthood (i.e., 65 years of age and 

older). Childhood represents a lifestage where susceptibility to PM exposures may be related to the 

following observations: children spend more time outdoors; children have greater activity levels than 

adults; children have exposures resulting in higher doses per body weight and lung surface area; and the 

developing lung is prone to damage, including irreversible effects, from environmental pollutants as it 

continues to develop through adolescence (US EPA, 2009a, section 8.1.1.2). Older adults represent a 

lifestage where susceptibility to PM-associated health effects may be related to the higher prevalence of 

pre-existing cardiovascular and respiratory diseases found in this age group compared to younger age 

groups as well as the gradual decline in physiological processes that occur as part of the aging process

(US EPA, 2009a, section 8.1.1.1). In addition, accumulating evidence suggests that the developing fetus 

may also represent an additional lifestage that is susceptibleat greater risk to PM exposures (US EPA, 

2009a, sections 2.3.1.2 and 7.4). 

With regard to mortality, recent epidemiological studies have continued to find that older adults 

are at greater risk of all-cause (non-accidental) mortality associated with short-term exposure to both 

PM2.5 and PM10, providing consistent and stronger evidence of effects in this susceptibleat-risk

population compared to the last review (US EPA, 2009a, Figure 7-7, section 8.1.1.1, Zeger et al., 2008). 

Evidence is accumulating for PM2.5-related infant mortality, especially due to respiratory causes during 

the post-neonatal period (US EPA, 2009a, sections 2.3.1.2 and 7.4). 

With regard to morbidity effects, currently available studies provide evidence that older adults 

have heightened responses, especially for cardiovascular-related effects, and children have heightened 

responses for respiratory-related effects (US EPA, 2009a, p. 2-23). In considering respiratory-related 

effects in children associated with long-term PM exposures, the Policy Assessment recognizes that our 

understanding of effects on lung development has been strengthened based on newly available evidence 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 38 of 133

that is consistent and coherent across different study designs, locations, and research groups (US EPA, 

2011a, p. 2-28). The strongest evidence comes from the extended follow-up for the Southern California 

Children’s Health Study which includes several new studies that report positive associations between 

long-term exposure to PM2.5 and respiratory morbidity, particularly for such endpoints as lung function 

growth, respiratory symptoms (e.g., bronchitic symptoms), and respiratory disease incidence (US EPA, 

2009a, section 7.3; McConnell et al, 2003; Gauderman et al., 2004; Islam et al., 2007). These analyses 

provide evidence that PM2.5-related effects persist into early adulthood and are more robust and larger in

magnitude than previously reported. With regard to respiratory effects in children associated with shortterm exposures to PM2.5, currently available studies provide stronger evidence of respiratory-related 

hospitalizations with larger effect estimates observed among children. In addition, reductions in lung 

function (i.e., FEV1) and increases in respiratory symptoms and medication use associated with PM 

exposures have been reported among asthmatic children (US EPA, 2009a, sections 6.3.1, 6.3.2.1, and 

8.4.9). 

A number of health conditions have been found to put individuals at greater risk for adverse 

effects following exposure to PM. The currently available evidence confirms and strengthens evidence 

in the last review that individuals with underlying cardiovascular and respiratory diseases are more 

susceptible to PM exposures (US EPA, 2009a, section 8.1.6; US EPA, 2011a, section 2.2.1). There is 

also emerging evidence that people with diabetes, who are at risk for cardiovascular disease, as well as 

obese individuals, may have increased susceptibility to PM exposures (US EPA, 2009a, section 8.1.6.4). 

As discussed in section 8.1.6 of the Integrated Science Assessment, this body of evidence includes 

findings from epidemiological and human clinical studies that associations with mortality or morbidity 

are greater in those with pre-existing conditions, and also includes evidence from toxicological studies 

using animal models of cardiopulmonary disease.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 39 of 133

Stronger evidence is available in this review than the last indicating that people from lower 

socioeconomic strata are a susceptiblean at-risk population relative to PM exposures (US EPA, 2009a, 

section 8.1.7; US EPA, 2011a, section 2.2.1). Persons with lower socioeconomic status (SES)26 have 

been generally found to have a higher prevalence of pre-existing diseases; limited access to medical 

treatment; and increased nutritional deficiencies, which can increase this population’s risk to PM-related 

effects. 

Investigation of potential genetic susceptibility has provided evidence that individuals with 

specific genetic differences are more susceptible to PM-related effects (US EPA, 2009a, pp. 8-7 to 8-9). 

More research is needed to better understand the relationship between genetic effects and potential 

susceptibility to PM-related effects (US EPA, 2011a, p. 2-109). 

In summary, there are several at-risk populations that may be especially susceptible or vulnerable 

to PM-related effects. These groups include those with preexisting heart and lung diseases, specific 

genetic differences, and lower socioeconomic status as well as the lifestages of childhood and older 

adulthood. Evidence for PM-related effects in these susceptibleat-risk populations has expanded and is 

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

potentially susceptibleat-risk populations, such as those with diabetes, people who are obese, pregnant 

women, and the developing fetus. The available evidence does not generally allow distinctions to be 

drawn between the PM indicators in terms of whether populations are more susceptibleat-risk to a 

particular size fraction (i.e., PM2.5 and PM10-2.5). 

4. Potential PM2.5-Related Impacts on Public Health

The population potentially affected by PM2.5 is large. In addition, large subgroups of the U.S. 

population have been identified as susceptibleat-risk populations as described in section III.B.3. While 

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

 

26 Socioeconomic status is a composite measure that usually consists of economic status, measured by 

income; social status measured by education; and work status measured by occupation (US EPA, 2009a, 

p. 8-14).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 40 of 133

of the mortality and morbidity associations can be quite large. In addition, it appears that mortality risks 

are not limited to the very frail. Taken together, these results suggest that exposure to ambient PM2.5

concentrations can have substantial public health impacts.

With regard to susceptibleat-risk populations in the United States, approximately 7 percent of 

adults (approximately 16 million adults) and 9 percent of children (approximately 7 million children) 

have asthma (US EPA 2009a, Table 8-3; CDC, 200827). In addition, approximately 4 percent of adults 

have been diagnosed with chronic bronchitis and approximately 2 percent with emphysema (US EPA, 

2009a, Table 8-3). Approximately 11 percent of adults have been diagnosed with heart disease, 6 

percent with coronary heart disease, 23 percent with hypertension, and 8 percent with diabetes (US EPA, 

2009a, Table 8-3). In addition, approximately 3 percent of the U.S. adult population has suffered a 

stroke (US EPA, 2009a, Table 8-3). Therefore, large portions of the United States population are in 

groups that may be susceptibleat increased risk to health effects associated with exposures to ambient 

PM2.5. The size of the potentially at-risk population suggests that exposure to ambient PM2.5 has 

significant impact on public health in the United States.

C. Quantitative Characterization of Health Risks

1. Overview

In this review, the quantitative risk assessment builds on the approach used and lessons learned 

in the last review and focuses on improving the characterization of the overall confidence in the risk 

estimates, including related 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 are largely the 

same as those articulated in the risk assessment conducted for the last review. These goals include: (1) to 

provide estimates of the potential magnitude of premature mortality and/or selected morbidity effects in 

the population associated with recent ambient level of PM2.5 and with simulating just meeting the 

 

27For percentages, see http://www.cdc.gov/ASTHMA/nhis/06/table4-1.htm. For population estimates, 

see http://www.cdc.gov/ASTHMA/nhis/06/table3-1.htm. 

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 41 of 133

current and alternative suites of PM2.5 standards in 15 selected urban study areas, including, where data 

were available, consideration of impacts on susceptibleat-risk populations; (2) to develop a better 

understanding of the influence of various inputs and assumptions on the risk estimates to more clearly 

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

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

the quantitative risk assessment included nationwide estimates of the potential magnitude of premature 

mortality associated with long-term exposure to recent ambient 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 risk assessment conducted for this review is more fully described and presented in the Risk 

Assessment (US EPA, 2010a) and summarized in detail in the Policy Assessment (US 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.B.3.

2. Summary of Design Aspects

Based on a review of the evidence presented and assessed in the Integrated Science Assessment 

and criteria for selecting specific health effect endpoints discussed in the Risk Assessment (US EPA, 

2010a, section 3.3.1), the following broad categories of health endpoints were included in the 

quantitative risk assessment: (1) all-cause, ischemic heart disease-related, cardiopulmonary-related, 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. 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 susceptibleat-risk
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 42 of 133

populations including older adults and persons with pre-existing cardiovascular and respiratory diseases 

(US EPA, 2010a, p. 3-29). The quantitative risk assessment estimates risks for various health effects in 

15 urban study areas. The selection of urban study areas was based on a number of criteria including: (1) 

consideration of urban study areas evaluated in the last PM risk assessment; (2) consideration of 

locations evaluated in key epidemiological studies; (3) preference for locations with relatively elevated 

annual and/or 24-hour PM2.5 monitored concentrations; and (4) preference for including locations from 

different regions across the country, reflecting potential differences in PM2.5 sources, composition, and 

potentially other factors which might impact PM2.5-related risk (US EPA, 2010a, section 3.3.2). Based 

on the results of several analyses examining the representativeness of these 15 urban study areas in the 

broader national context, the Risk Assessment concludes that these study areas are generally 

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

ambient PM2.5 exposure with the potential for better characterization at the higher end of that 

distribution (US EPA, 2011a, p. 2-42; US EPA, 2010a, section 4.4, Figure 4-17).28

In order to estimate the incidence of a particular health effect associated with recent ambient 

conditions in a specific urban study area attributable to PM2.5 exposures, as well as the change in 

incidence corresponding to a given change in PM2.5 concentrations resulting from simulating just 

meeting current or alternative PM2.5 standards, three elements are required (US EPA, 2010a, section 

3.1.1, Figures 3-2 and 3-3). These elements are: (1) air quality information (including recent air quality 

data for PM2.5 from ambient monitors for the selected location, estimates of background PM2.5

concentrations appropriate for that location, and a method for adjusting the recent data to reflect patterns 

of air quality estimated to occur when the area just meets a given set of PM2.5 standards); (2) relative 

 

28 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 have 

relatively low PM2.5 concentrations (e.g., urban areas in Florida) and, consequently, the Risk Assessment 

concludes failure to include these areas in the set of urban study areas is unlikely to exclude high PM2.5-

risk locations (US EPA, 2010a, section 4.4.1).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 43 of 133

risk-based concentration-response functions that provide an estimate of the relationship between the 

health endpoints of interest and ambient PM2.5 concentrations; and (3) baseline health effects incidence 

rates and population data, which are needed to provide an estimate of the incidence of health effects in 

an area before any changes in PM2.5 air quality.29

The Risk Assessment includes 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 provide information to evaluate and rank the potential impacts of key 

sources of uncertainty on the core risk estimates (US EPA, 2010a, sections 3.5 and 4.3, Table 4-3). In 

addition, the sensitivity analyses represent a set of reasonable alternatives to the core set of risk 

estimates that fall within an overall set of plausible risk estimates surrounding the core estimates (US 

EPA, 2010a, section 4.3.2). 

Recent air quality was characterized for the 15 urban study areas based on 24-hour PM2.5

concentrations measured for 3 years (i.e., 2005, 2006, and 2007) as described in section 3.2.1 of the Risk 

Assessment. Different methodologies were then used to simulate conditions for just meeting the current 

or alternative PM2.5 standards (US EPA, 2010a, section 3.2.3). This included using the single rollback 

approach used in the risk assessment conducted for the last review which reflects a uniform regional 

pattern of reductions in ambient PM2.5 concentrations across monitors (i.e., proportional rollback 

approach). The proportional rollback approach was used in generating the core risk estimates (US EPA, 

 

29 Incidence rates express the occurrence of a disease or event (e.g., death, hospital admission) in a 

specific period of time, usually per year. Rates are expressed either as a value per population group (e.g., 

the number of cases in Philadelphia County) or a value per number of people (e.g., the number of cases 

per 10,000 residents in Philadelphia County), and may be age- and/or sex-specific. Incidence rates vary 

among geographic areas due to differences in populations characteristics (e.g., age distribution) and 

factors promoting illness (e.g., smoking rates, air pollution concentrations). The baseline incidence rate 

provides an estimate of the incidence rate (i.e., number of cases of the health effect per year, usually per 

10,000 or 100,000 general population) in the assessment location unrelated to changes in ambient PM2.5

concentrations in that location (US EPA, 2010a, section 3.4). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 44 of 133

2010a, section 3.2.3.1). In sensitivity analyses, the Risk Assessment also applied two alternative 

rollback approaches (i.e., hybrid and locally-focused rollback approaches)30 to better characterize 

potential variability in the way air quality in urban areas responds to programs put in place to meet the 

current or alternative PM2.5 standards. In considering the three rollback approaches collectively, the 

proportional and locally-focused methods are approaches that are more likely to represent “bounding” 

scenarios related to the spatial pattern of future reductions in ambient PM2.5 concentrations. In contrast, 

the hybrid approach, in principle, reflects a more plausible or representative rollback strategy since it: 

(1) reflects consideration for site-specific information regarding larger PM2.5 sources and their potential 

impact on source-oriented monitors and (2) combines elements of more locally-focused and regionallyfocused patterns of reductions (US EPA, 2010a, section 3.2.3).

The peak-to-mean ratio of ambient PM2.5 concentrations within a study area informs the type of 

rollback approach used to simulate just meeting the current or alternative suites of standards to 

determine the magnitude of the reduction in annual mean PM2.5 concentrations for that study area and 

consequently the degree of risk reduction.31 For example, study areas with relatively high peak-to-mean 

ratios are likely to have greater estimated risk reductions for the current suite of standards (depending on 

the combination of 24-hour and annual design values), and such locations can be especially sensitive to 

the type of rollback approach used, with the proportional rollback approach resulting in notably greater 

estimated risk reduction compared with the locally-focused rollback approach. In contrast, study areas 

with lower peak-to-mean ratios typically experience greater risk reductions when simulating just 

meeting the current or alternative annual-standard level than with simulating just meeting the current or 

 

30 The hybrid rollback approach involves a combination of an initial step of a more localized reduction 

in ambient PM2.5 concentrations at source-oriented monitors followed by a regional pattern of reduction 

across all monitors in a study area (US EPA, 2010a, section 3.2.3.2). The locally-focused rollback 

approach involves a focused reduction of concentrations only at those monitors exceeding the current or 

alternative 24-hour standard levels (US EPA, 2010a, section 3.2.3.3).

31 The peak-to-mean ratio of ambient PM2.5 concentrations also has a direct bearing on whether the 24-

hour or annual standard will be the generally controlling standard for a particular study area, with higher 

peak-to-mean ratios generally being associated with locations where the 24-hour standard is likely the 

controlling standard.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 45 of 133

alternative 24-hour standard level (again depending on the combination of 24-hour and annual design 

values). In addition, the type of rollback approach used will tend to have less of an impact on the 

magnitude of risk reductions for study areas with lower peak-to-mean ratios. Consideration of these two 

factors helps to inform an understanding of the nature and pattern of estimated risk reductions and risk 

remaining upon simulation of just meeting the current and alternative suites of standards across the 

urban study areas (US EPA, 2010a, section 5.2.1). 

The concentration-response functions used in the risk assessment were based on findings from 

epidemiological studies that have relied on fixed-site, population-oriented, ambient monitors as a 

surrogate for actual ambient PM2.5 exposures. The risk assessment addresses risks attributable to 

anthropogenic sources and activities (i.e., risk associated with concentrations above policy-relevant

background).32 This approach of estimating risks in excess of background was judged to be more 

relevant to policy decisions regarding ambient air quality standards than risk estimates that include 

effects potentially attributable to PM2.5 concentrations that are not associated with North American 

anthropogenic emissions. 

In modeling risk associated with long- and short-term PM2.5 exposures, the Risk Assessment 

initially focused on selecting concentration-response functions from multi-city studies.33 Concentrationresponse functions from two single-city studies provided coverage for additional health effect endpoints 

(i.e., emergency department visits for cardiovascular and/or respiratory effects) associated with shortterm PM2.5 exposures (US EPA, 2010a, p. 3-37).

 

32 Policy-relevant background estimates used in the risk assessment model were based on information 

presented in the Integrated Science Assessment (US EPA, 2009a, section 3.7, Table 3-23) and discussed 

in the Risk Assessment (US EPA, 2010a, section 3.2.2). These values were generated based on a 

combination of Community Multiscale Air Quality model (CMAQ) and Goddard Earth Observing 

System (GEOS)-Chem modeling (US EPA, 2009a, section 3.7.1.2; US EPA, 2010a, section 3.2.2). 

33 As noted in section 3.3.3 of the Risk Assessment, multi-city studies have a number of advantages over 

single-city studies including: increased statistical power providing effect estimates with relatively 

greater precision and reduced problems with publication bias (i.e., in which studies with statistically 

insignificant or negative results are less likely to get published than those with positive and/or 

statistically significant results).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 46 of 133

With regard to modeling risks associated with long-term PM2.5 exposure, concentration-response 

functions used in the risk model are all based on cohort studies, in which a cohort of individuals is 

followed over time. In the core analysis, estimated premature mortality risk associated with long-term 

PM2.5 concentrations used concentration-response functions from the extended ACS study (Krewski et 

al., 2009). This study had a number of advantages including: analyses that expanded upon previous 

publications presenting evaluations of the ACS long-term cohort study and extending the follow-up 

period to eighteen years; a rigorous examination of different model forms for estimating effect 

estimates; coverage for a range of ecological variables (e.g., social, economic, and demographic factors) 

which allowed for consideration of whether these factors confound or modify the relationship between 

PM2.5 exposure and mortality; and updated and expanded data sets on incidence and exposure (US EPA, 

2010a, p 2-9 and 3-38). 

As discussed in section III.B.3, persons of lower socioeconomic status have been identified as a 

susceptiblean at-risk population. The ACS study cohort does not provide representative coverage for 

persons of lower-socioeconomic status and, thus, the Risk Assessment concludes that using the 

concentration-response functions from this study may result in risk estimates that are biased low (US 

EPA, 2010a, p. 5-7). Therefore, concentration-response functions from a reanalysis of the Harvard Six 

Cities study (Krewski et al., 2000) were used in a sensitivity analysis to better support generalizing the 

results of the risk assessment across the broader national population.34

While being mindful that the use of concentration-response functions from Krewski et al. (2009) 

introduces potential for low bias in the core risk estimates, the Policy Assessment also recognizes many 

strengths of this study and reasons for its continued use for generating the core risk estimates, including: 

 

34 As noted in the last review, the ACS study population has persons generally representative of a higher 

SES (e.g., higher educational status) relative to the Harvard Six Cities study population (12 percent

versus 28 percent of the cohort had less than a high school education, respectively) (US EPA, 2004, p. 8-

118). The Policy Assessment concludes that the Harvard Six Cities study cohort may provide a more 

representative sample of the broader national population than the ACS study cohort (US EPA, 2011a, p. 

2-40). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 47 of 133

consideration of a large number of metropolitan statistical areas, inclusion of two time periods for the air 

quality data which allowed us to consider different exposure windows, and analysis of a wide range of 

concentration-response function models. Therefore, the Risk Assessment concludes that concentrationresponse functions obtained from this study had the greatest overall support and were appropriate to 

incorporate in the core risk model (US EPA, 2010a, p. 3-38).

In the core analysis, for modeling health endpoints associated with long-term exposure, the Risk 

Assessment concluded that modeling risks down to policy-relevant background would require 

substantial extrapolation of the estimated concentration-response functions below the range of the data 

on which they were estimated (i.e., the lowest measured levels reported in the epidemiological studies 

were substantially above policy-relevant background). Therefore, the Risk Assessment concluded it was 

most appropriate in the core analysis to estimate risk only down to the lowest measured level to avoid 

introducing additional uncertainty into the analysis (US EPA, 2010a, 3-1 to 3-3).35 A sensitivity analysis 

comparing the impact of estimated risks down to policy-relevant background rather than down to the 

lowest measured level (US EPA, 2010a, section 3.5.4.1) used annual estimates of policy-relevant 

background values for specific geographic regions (US EPA, 2010a, section 3.2.2, Table 3-2).

With regard to modeling risks associated with short-term PM2.5 exposure, concentration-response

functions from two time-series studies were selected as the primary studies to support the core analysis. 

Concentration-response functions from Zanobetti and Schwartz (2009) were used in estimating 

premature non-accidental, cardiovascular-related, and respiratory-related mortality. Concentrationresponse functions from Bell et al. (2008) were used in estimating cardiovascular-related and 

respiratory-related hospital admissions. In addition, concentration-response functions from two singlecity studies were used to estimate emergency department visits for cardiovascular and/or respiratory 

 

35 To provide consistency for the different concentration-response functions selected from the long-term

exposure studies, and, in particular, to avoid the choice of lowest measured levels unduly influencing the 

results of the risk assessment, the Risk Assessment concluded it was appropriate to select a single lowest 

measured level – 5.8 μg/m3from the later exposure period evaluated in Krewski et al. (2009) -- to use in 

estimating risks associated with long-term PM2.5 exposures (US EPA, 2010a, p 3-3).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 48 of 133

illnesses associated with short-term PM2.5 exposure (Tolbert et al., 2007; Ito et al., 2007; US EPA, 

2010a, p. 3-37). 

For modeling health endpoints associated with short-term PM2.5 exposure, the Risk Assessment 

estimates risk down to policy-relevant background exclusively using quarterly values to represent the 

appropriate block of days within a simulated year (US EPA, 2010a, section 3.2.2, Table 3-2). 

To estimate the change in incidence of a health endpoint associated with a given change in PM2.5

concentrations, information on the baseline incidence of that endpoint is needed (US EPA, 2010a, 

section 3.4). In calculating a baseline incidence rate to be used with a concentration-response function 

from a given epidemiological study, the Risk Assessment matched the counties, age grouping, and 

International Classification of Diseases (ICD) codes used in that study (US EPA, 2010a, section 3.4.2).

An important component of a population health risk assessment is the characterization of both 

uncertainty and variability.36 The design of the risk assessment includes a number of elements to address 

these issues, including using guidance from the World Health Organization (WHO, 2008) as a

framework for developing the approach used for characterizing uncertainty in the analyses (US EPA, 

2010a, section 3.5).

The Risk Assessment considers key sources of variability that can impact the nature and 

magnitude of risks associated with simulating just meeting current and alternative standard levels across 

the urban study areas (US EPA, 2010a, section 3.5.2). These sources of variability include those that 

contribute to differences in risk across urban study areas, but do 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 focuses on factors that not only introduce variability into risk estimates across study areas, but also 

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

 

36 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 (US EPA, 2010a, p. 3-63). 

Formatted: Font: Italic

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 49 of 133

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 are likely to affect population 

risks and the degree to which they were (or were not) fully captured in the design of the risk assessment 

are discussed in section 3.5.2 of the Risk Assessment. These sources include: PM2.5 composition; intraurban variability in ambient PM2.5 concentrations; variability in the patterns of reductions in PM2.5

concentrations associated with different rollback approaches when simulating just meeting the current or 

alternative standards; co-pollutant exposures; factors related to demographic and socioeconomic status; 

behavioral differences across urban study areas (e.g., time spent outdoors); baseline incidence rates; and 

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 (US EPA, 2010a, pp. 3-67 to 3-69).

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 (US EPA, 2010a, sections 3.5.3 

and 3.5.4). The quantitative sensitivity analyses informed our understanding of sources of uncertainty 

that may have a moderate to large impact on the core risk estimates including: (1) characterizing intraurban population exposure in the context of epidemiology 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 short-term exposure studies; (5) transferability of concentration-response functions from study 

locations to urban study area locations for long-term exposure-related health endpoints; (6) use of 

single-city versus multi-city studies in the derivation of concentration-response functions; (7) impact of 

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 (US EPA, 2011a, 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 50 of 133

section 5.1.4). In addition to identifying sources of uncertainty with a moderate to large impact on the 

core risk estimates, the single and multi-element sensitivity analyses also produced a set of reasonable 

alternative risk estimates that allowed us to place the results of the core analysis in context with regard 

to uncertainty and potential bias (US EPA, 2010a, section 5.1.4). The qualitative uncertainty analysis 

supplemented the quantitative sensitivity analyses by allowing coverage for sources of uncertainty that 

could not be readily included in the sensitivity analysis (US EPA, 2010a, section 3.5.3).

With respect to the long-term exposure-related mortality risk estimates37, the most important 

sources of uncertainty identified in the quantitative sensitivity analyses included: selection of 

concentration-response functions; modeling risk down to policy-relevant background versus lowest 

measured level; and the choice of rollback approach used to simulate just meeting current or alternative 

standards (US EPA, 2011a, p. 2-39). With regard to the qualitative analysis of uncertainty, the following 

sources were identified as potentially having a large impact on the core risk estimates for the long-term 

exposure-related mortality: characterization of intra-urban population exposures; impact of historical air 

quality; and potential variation in effect estimates reflecting differences in PM2.5 composition (US EPA, 

2011a, p. 2-39). 

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 (US 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 (US 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 

 

37 Given increased emphasis placed in this analysis on long-term exposure-related mortality, the 

uncertainty analyses completed for this health endpoint category were more comprehensive than those 

conducted for analyses of short-term exposure-related mortality and morbidity. This reflects, to some 

extent, limitations in the epidemiological data available for addressing uncertainty in the latter categories 

(U.S.EPA, 2010a, section 3.5.4.2).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 51 of 133

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

estimates. The first analysis explored potential bias in the core risk estimates by considering a set of 

alternative reasonable risk estimates generated as part of a sensitivity analysis. The second analysis 

compared the annual mean PM2.5 concentrations associated with simulating just meeting the current and 

alternative suites of standards with the air quality distribution used in deriving the concentrationresponse functions applied in modeling mortality risk. 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) (US EPA, 

2011a, p. 2-38). 

3. Risk Estimates and Key Observations

As discussed below, three factors figure prominently in the interpretation of the risk estimates 

associated with simulating just meeting the current and alternative suites of standards, including: (1) the 

importance of changes in annual mean PM2.5 concentrations for a specific study area in estimating 

changes in risks related to both long- and short-term exposures associated with recent air quality 

conditions and air quality simulated to just meet the current and alternative suites of PM2.5 standards; (2) 

the ratio of peak- to-mean ambient PM2.5 concentrations in a study area; and (3) the spatial pattern of 

ambient PM2.5 reductions that result from using different approaches to simulate just meeting the current 

standard levels (i.e., rollback approaches). The latter two factors are interrelated and influence the 

degree of risk reduction estimated under the current suite of standards. 

The magnitude of both long- and short-term exposure-related risk estimated to remain upon just 

meeting the current suite of standards is strongly associated with the simulated change in annual mean 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 52 of 133

PM2.5 concentrations. The role of annual mean PM2.5 concentrations in driving long-term exposurerelated risk estimates is intuitive given that risks are modeled using the annual mean air quality metric.38

The fact that short-term exposure-related risk estimates are also driven by changes in long-term mean 

PM2.5 concentrations is less intuitive, since changes in mean 24-hour PM2.5 concentrations are used to 

estimate changes in risk for this time period.39 Analyses show that short-term exposure-related risks are 

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 (US EPA, 2010a, section 3.1.2.2). Consequently, the largest part of the 

estimates of short-term exposure-related risk is related to the changes in the portion of the distribution of 

short term PM2.5 exposures that are well represented by changes in the annual mean. Therefore, the 

Policy Assessment focuses on changes in annual mean PM2.5 concentrations to inform our understanding 

of patterns of both long- and short-term exposure-related risk estimates across the set of urban study 

areas evaluated in the quantitative risk assessment (US EPA, 2011a, pp. 2-36 to 2-37). 

In estimating PM2.5-related risks likely to remain upon simulation of just meeting the current 

annual and 24-hour standards in the 15 urban study areas, the Risk Assessment focuses on the 13 areas 

that would likely not have met the current suite of PM2.5 standards based on recent air quality (2005 to 

2007). These 13 areas have annual and/or 24-hour design values that are above the levels of the current 

 

38 As noted in section 3.2.1 of the Risk Assessment (US EPA, 2010a), estimates of long-term exposurerelated mortality are actually based on an annual mean PM2.5 concentration that is the average across 

monitors in a study area (i.e., based on the composite monitor distribution). Therefore, in considering 

changes in long-term exposure-related mortality, it is most appropriate to compare composite monitor 

estimates generated for a study area under each alternative suite of standards considered. The annual 

mean at the highest reporting monitor (i.e., based on the maximum monitor distribution) for a study area 

is the annual design value. The annual design value is used to determine the percent reduction in PM2.5

concentrations required to meet a particular standard. Both types of air quality estimates are provided in 

Table 3-4 of the Risk Assessment (US EPA, 2010a, pp. 3-25 to 3-27).

39 Estimates of short-term PM2.5 exposure-related mortality and morbidity are based on composite 

monitor 24-hour PM2.5 concentrations. However, similar to the case with long-term exposure-related 

mortality, under the current rules, it is the 98th percentile 24-hour concentration estimated at the 

maximum monitor (the 24-hour design value) that will determine the degree of reduction required to 

meet a given 24-hour standard level (US EPA, 2011a, p. 2-37).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 53 of 133

standards (US EPA, 2010a, Table 3-3).40 Based on the core risk estimates for these areas, using the 

proportional rollback approach, the Policy Assessment makes the following key observations regarding 

the magnitude of risk remaining upon simulation of just meeting the current suite of standards: 

1) Long-term exposure-related mortality risk estimated to remain upon just meeting the current 

standards are significant: Premature mortality related to ischemic heart disease attributable to 

long-term PM2.5 exposure was estimated to range from less than 100 to approximately 2,000 

cases per year across the urban study areas. The variability in these estimates reflects, to a great 

extent, differences in the size of study area populations. These estimates represent from 4 to 17% 

of all mortality related to ischemic heart disease in a given year for the urban study areas 

evaluated, representing a measure of risk that takes into account differences in population size 

and baseline mortality rates (US EPA, 2011a, p. 2-43, Table 2-2). These estimates of risk for 

mortality related to ischemic heart disease associated with long-term PM2.5 exposure would 

likely be in a range of thousands of deaths per year for the 15 urban study areas41 (US EPA, 

2011a, pp. 2-46 to 2-47). Based on these risk estimates for premature mortality related to 

ischemic heart disease alone, the Policy Assessment concludes that risks estimated to remain 

upon simulation of just meeting the current suite of standards are important from a public health 

standpoint (US EPA, 2011a, p. 2-47). The Risk Assessment also includes estimated risks for 

premature mortality related to cardiopulmonary effects and lung cancer, which increase the total 

annual incidence of mortality attributable to long-term PM2.5 exposure (see U.S. EPA, 2010a, 

section 4.2.1). 

2) Short-term exposure-related mortality risk estimated to remain upon just meeting the current 

standards are much smaller than long-term exposure-related mortality risks: Cardiovascularrelated mortality associated with short-term PM2.5 exposure was estimated to range from less 

than 10 to 500 cases per year across the urban study areas. These estimates represent 

approximately 1 to 2 percent of total cardiovascular-related mortality in a given year for the 

urban study areas evaluated (US EPA, 2011a, p. 2-43, Table 2-3). 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 substantially lower, up to an order of magnitude smaller, for short-term exposurerelated mortality (US EPA, 2011a, p. 2-47). 

3) Short-term exposure-related morbidity risk estimated to remain upon just meeting the current 

standards indicate hospitalizations are significantly larger for cardiovascular-related rather 

than respiratory-related events and emergency department visits for asthma-related events are 

significant: Cardiovascular-related hospitalizations were estimated to range from approximately 

10 to 800 cases per year across the study areas, which are less than 1 percent of total 

cardiovascular-related hospitalizations (US EPA, 2011a, p. 2-43, Table 2-3). Respiratory-related 

hospital admissions attributable to short-term PM2.5 exposure were significantly smaller than 

 

40 Of the 15 urban study areas, only Dallas and Phoenix have both annual and 24-hour design values 

below the levels of the current standards based on 2005-2007 air quality data (US EPA, 2010a, Table 

3-3).

41 Premature mortality for all causes attributed to PM2.5 exposure was estimated to be in a range of tens 

of thousands of deaths per year on a national scale based on 2005 air quality data (US EPA, 2010a, 

Appendix G, Table G-1).

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 54 of 133

those related to cardiovascular events (US EPA, 2010a, Tables E-102 and E-111). 

Cardiovascular- and respiratory-related hospital admissions together ranged up to approximately 

1,000 admissions per year across the urban study areas. The estimated incidence of asthmarelated emergency department visits is several times larger than the estimates of cardiovascularand respiratory-related hospital admissions (US EPA, 2011a, p. 2-47; US EPA, 2010a, Tables E118 to E-123 

4) Substantial variability exists in the magnitude of risk remaining across urban study areas: 

Estimated risks remaining upon just meeting the current suite of standards vary substantially 

across study areas, even when considering risks normalized for differences in population size and 

baseline incidence rates. This variability is a consequence of the substantial differences in the 

annual mean PM2.5 concentrations across study areas that result from simulating just meeting the 

current standards. This is important because, as discussed above, annual mean concentrations are 

highly correlated with both long- and short-term exposure-related risk. The variability in annual 

mean PM2.5 concentrations occurred primarily in those study areas in which the 24-hour standard 

was the generally controlling standard. In such areas, the variability in estimated risks across 

study areas was largest when regional patterns of reductions in PM2.5 concentrations were 

simulated, using the proportional rollback approach, as was done in the core analysis. Less 

variability was observed when more localized patterns of PM2.5 reductions were simulated using 

the locally-focused rollback approach, as was done in a sensitivity analysis. When simulations 

were done using the locally-focused rollback approach, estimated risks remaining upon just 

meeting the current suite of standards were appreciably larger than those estimated in the core 

analysis (US EPA, 2011a, p. 2-46; US EPA, 2010a, section 4.3.1.1).

5) Simulation of just meeting the current suite of standards results in annual mean PM2.5

concentrations well below the current standard for some study areas: In simulating just meeting 

the current suite of standards, the resulting composite monitor annual mean PM2.5 concentrations 

ranged from about 15 μg/m3(for those study areas in which the annual standard was controlling) 

down to as low as about 8 μg/m3(for those study areas in which the 24-hour standard was the 

generally controlling standard or the annual mean concentration was well below 15 μg/m3based 

on recent air quality) (US EPA, 2011a, p. 2-46). 

Reductions in risk associated with simulating air quality to just meet alternative standard levels 

were also estimated in this review (US EPA, 2010a, sections 4.2.2, 5.2.2, and 5.2.3; US EPA, 2011a, 

section 2.3.4.2). The estimated percent of risk reductions are depicted graphically in the Policy 

Assessment (US 2011a, Figures 2-11 and 2-12), showing patterns of estimated risk reductions associated 

with alternative suites of standards.42 These figures also depict the level of confidence associated with 

 

42 Patterns of risk reduction across alternative annual standard levels, in terms of percent change relative 

to risk estimates upon simulating just meeting the current standards, are similar for all health endpoints 

modeled (i.e., all-cause, ischemic heart disease-related, and cardiopulmonary-related mortality). This 

similarity reflects the fact that the concentration-response functions used in the quantitative risk 

assessment are close to linear across the range of ambient PM2.5 concentrations evaluated. However, 

estimated incidence will vary by health endpoint (US EPA, 2011a, pp. 2-93 to 2-94, footnote 70). 

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 55 of 133

the risk estimates generated for simulating just meeting the current standards as well as alternative 

standard levels considered. As would be expected, patterns of increasing estimated risk reductions are 

generally observed as either the annual or 24-hour standard, or both, are reduced over the ranges 

considered in the Risk Assessment. A number of the key observations regarding the magnitude of risk 

remaining upon simulation of just meeting the alternative suites of standards are analogous to the 

observations identified above for simulation of just meeting the current standards (US EPA, 2011a, pp. 

2-97 to 2-100). 

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

alternative standards, the Policy Assessment recognizes that greater overall confidence is associated 

with estimates of risk reduction than for estimates of absolute risk remaining (US EPA, 2011a, p. 2-94). 

Furthermore, the Policy Assessment recognizes that estimates of absolute risk remaining for each of the 

alternative standard levels considered, particularly in the context of long-term exposure-related 

mortality, may be underestimated (US EPA, 2011a, p. 2-97). In addition, the Policy Assessment 

observes that in considering the overall confidence associated with the quantitative analyses, the Risk 

Assessment recognizes that: (1) substantial variability exists in the magnitude of risk remaining across 

urban study areas and (2) in general, higher confidence is 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. 

The variability in risk is a consequence of the substantial differences in the annual mean PM2.5

concentrations across urban study areas that result from simulating just meeting current or alternative 

standards. As PM2.5 concentrations decrease from the mean PM2.5 concentrations, the Risk Assessment 

concludes there is decreasing confidence in the risk estimates (US EPA, 2010a, p. 5-16). As lower longterm mean PM2.5 concentrations are simulated (i.e., ambient concentrations further from recent air 

quality conditions), the potential variability in such factors as the spatial pattern of ambient PM2.5

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 56 of 133

reductions (i.e., rollback) increases, thereby introducing greater uncertainty into the simulation of 

composite monitor annual mean PM2.5 concentrations, and, consequently, in the risk estimates (US EPA, 

2010a, Appendix J). 

Based on consideration of the composite monitor annual mean PM2.5 concentrations involved in 

estimating long-term exposure-related mortality, the Risk Assessment has higher confidence in using 

those concentrations that generally fall well within the range of ambient PM2.5 concentrations considered 

in fitting the concentration-response functions used (i.e., within one standard deviation of the mean 

PM2.5 concentration reported in Krewski et al. (2009) for 1999-2000) as inputs to the risk model. For 

example, with the exception of one urban study area, those areas estimated to have risk reductions using 

alternative annual standard levels of 13 and 14 μg/m3had simulated composite monitor annual mean 

concentrations ranging from approximately 10.6 to 13.3 μg/m3. With lower alternative annual standard 

levels of 12 μg/m3 and 10 μg/m3, the composite monitor annual mean values ranged from approximately 

9.0 to 11.4 μg/m3and 7.6 and 8.9 μg/m3, respectively. These concentrations are towards the lower end 

of the range of ACS data (in some cases approaching the lowest measured level) used in fitting the 

concentration-response functions, particularly for an annual standard level of 10 μg/m3, and, thus, the 

Policy Assessment concludes there is less confidence in the risk estimates associated with these levels 

compared with those for the higher alternative annual standard levels considered (US EPA, 2011a, p. 2-

99). Thus, while simulation of risks for an alternative annual standard level of 10 μg/m3suggests that 

additional risk reductions could be expected with alternative annual standards below 12 μg/m3, the 

Policy Assessment recognizes that there is potentially greater uncertainty associated with these risk 

estimates compared with estimates generated for the higher alternative annual standard levels considered 

in the quantitative risk assessment, since these estimates required simulation of relatively greater 

reductions in ambient PM2.5 concentrations (US EPA, 2011a, p. 2-98).

The results of simulating alternative suites of PM2.5 standards including a combination of 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 57 of 133

alternative annual and 24-hour standard levels suggest 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 the alternative 24-hour standard is 

highly variable (US EPA, 2010a, section 4.2.2). Thus, the Risk Assessment concludes more consistent 

reductions in estimated risk and consequently degrees of public health protection are estimated to result 

from simulating just meeting the alternative annual standard levels considered (US EPA, 2010a, pp. 

5-15 to 5-16). Furthermore, the Policy Assessment concludes that the urban study areas with the greatest 

degree of estimated reduction associated with simulating just meeting alternative 24-hour standard levels 

of 30 and 25 μg/m3also had the lowest estimated annual mean PM2.5 concentrations, and, therefore, 

there was substantially lower confidence in these risk estimates (US EPA, 2011a, pp. 2-99 to 2-100). 

Based on the consideration of both the qualitative and quantitative assessments of uncertainty, 

the Risk Assessment concludes 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 Risk 

Assessment conclude 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 analyses43

(US EPA, 2011a, p. 2-41; US EPA, 2010a, p. 5-16; Figures 4-7 and 4-8). In addition, the Policy 

Assessment recognizes that the currently available scientific information includes evidence for a broader 

range of health endpoints and susceptibleat-risk populations beyond those included in the quantitative 

risk assessment, including lung function growth and respiratory symptoms in children and reproductive 

and developmental effects (US EPA, 2011a, section 2.2.1). 

In considering the set of quantitative risk estimates and related uncertainties and limitations 

related to long- and short-term PM2.5 exposure discussed above together with consideration of the health 

endpoints which could not be quantified, the Policy Assessment concludes this information provides 

 

43 Most of the alternative model specifications supported by the currently available scientific 

information produced risk estimates that are higher (by up to a factor of 2 to 3) than the core risk 

estimates (US EPA, 2011a, pp. 2-40 and 2-41). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 58 of 133

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 (US 

EPA, 2011a, p. 2-47). Furthermore, while the alternative 24-hour standard levels considered (when 

controlling) did result in additional estimated risk reductions beyond those estimated for alternative 

annual standards alone, these additional estimated reductions are highly variable, in part due to different 

rollback approaches. Conversely, the Risk Assessment recognizes 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 (US EPA, 2010a, p. 5-17).

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

The initial issue to be addressed in the current review of the primary PM2.5 standards is whether, 

in view of the additional information now available, the existing standards should be retained or revised. 

In evaluating whether it is appropriate to retain or revise the current suite of standards, the Administrator 

considered the scientific information from the last review and the broader body of evidence and 

information now available. The Administrator has taken into account both evidence- and risk-based 

considerations in developing conclusions on the adequacy of the current primary PM2.5 standards. 

Evidence-based considerations (section III.D.1) 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 (US EPA, 2009a) and focus on the policy-relevant considerations as discussed in section 

III.B above and in the Policy Assessment (US EPA, 2011a, section 2.2.1). The risk-based considerations 

(section III.D.2) draw from the results of the quantitative analyses presented in the Risk Assessment (US 

EPA, 2010a) and focus on the policy-relevant considerations as discussed in section III.C above and in 

the Policy Assessment (US EPA, 2011a, section 2.2.2). The advice received from CASAC is discussed 

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 59 of 133

in section III.D.3. Finally, the Administrator's proposed conclusion on the adequacy of the current PM2.5

primary standards is provided in section III.D.4. 

1. Evidence-based Considerations in the Policy Assessment

In light of the health evidence described above, specifically with regard to factors contributing to 

greater susceptibility to health effects associated with ambient PM2.5 exposures, the Policy Assessment 

considers the extent to which the currently available scientific evidence reports associations between 

fine particle exposures and health effects that extend to air quality concentrations that are lower than had 

previously been observed or that have been observed in areas that would likely meet the current suite of 

PM2.5 standards (US EPA, 2011a, section 2.2.1). As noted above, the Integrated Science Assessment 

concludes there is no evidence to support the existence of a discernible threshold below which effects 

would not occur (US EPA, 2009a, section 2.4.3).

a. Associations with Long-term PM2.5 Exposures

With regard to associations observed in long-term PM2.5 exposure studies, the Policy Assessment 

recognizes that extended follow-up analyses of the ACS and Harvard Six Cities studies provide 

consistent and stronger evidence of an association with mortality at lower air quality distributions than 

had previously been observed (US 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 1979-1983 (Pope et al., 1995; Krewski et 

al., 2000).44 In extended analyses, positive and statistically significant effects of approximately similar 

magnitude were associated with declining PM2.5 concentrations, from an aggregate long-term mean in 

58 metropolitan areas of 21.2 μg/m3in the original monitoring period (1979-1983) to 14.0 μg/m3for 116 

metropolitan areas in the most recent years evaluated (1999-2000), with an overall average across the 

 

44 The study periods referred to in the Policy Assessment (US EPA, 2011a) and in this proposed rule 

reflect the years of air quality data that were included in the analyses, whereas the study periods 

identified in the Integrated Science Assessment (US EPA, 2009a) reflect the years of health status data 

that were included.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 60 of 133

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 1980-1985 

(Dockery et al., 1993; Krewski et al., 2000). In an extended follow-up of this study, the aggregate longterm mean concentration across all years evaluated was 16.4 μg/m3

for 1980-198845 (Laden et al., 2006). 

In an additional analysis of the extended follow-up 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 (US EPA, 2009a, p. 7-92; Schwartz et al., 2008). 

New cohort studies provide 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 greater in magnitude (US EPA, 2011a, pp. 2-32 to 2-33). The 

WHI study reported positive and most often statistically significant associations between long-term 

PM2.5 exposure and cardiovascular-related mortality, with much larger relative risk estimates than in the 

ACS and Harvard Six Cities studies, as well as morbidity effects at an aggregate long-term mean PM2.5

 

45 Aggregate mean concentration provided by study author (personal communication from Dr. Francine 

Laden, 2009).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 61 of 133

concentration of 12.9 μg/m3for 2000 (Miller et al., 2007).46 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 2000-2002. 

The MCAPS 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(US EPA, 2009a, p. 7-88; Zeger et al., 2008).47 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).

 

46 Miller et al. (2007) studied postmenopausal women without previous cardiovascular disease in 36 

study areas from 1994 to 1998, with a median follow-up period of six years. The ambient PM2.5 monitor 

nearest to a study subject’s residence (within 30 miles or 48 kilometers) was identified and used to 

assign long-term mean PM2.5 concentrations to each subject. The annual average concentration in the 

year 2000 was the primary exposure measure because of the substantially increased network of monitors 

in that year, as compared with previous years. Miller et al. (2007) reported a long-term mean PM2.5

concentration across study areas of 13.5 μg/m3. This concentration was presented in the Integrated 

Science Assessment (US EPA, 2009a, Figure 2-2, Table 7-8) and discussed in the second draft Policy 

Assessment (US EPA, 2010f, Figure 2-4). In response to a request from the EPA for additional 

information on the air quality data used in selected epidemiological studies (Hassett-Sipple and Stanek, 

2009), study investigators provided updated air quality data for the study period. The updated long-term 

mean PM2.5 concentration provided by the study authors was 12.9 μg/m3(personal communication from 

Cynthia Curl, 2009; Stanek et al., 2010). The EPA notes that this updated long-term mean concentration 

matches the composite monitor approach annual mean calculated by staff for the year of air quality data 

(i.e., 2000) considered by the study investigators (Hassett-Sipple et al., 2010, Attachment A, p. 6). The 

updated air quality data for the Women’s Health Initiative study was presented and considered in the 

final Policy Assessment (US EPA, 2011a, p. 2-32). The Policy Assessment notes that in comparison to 

other long-term exposure studies, the WHI study was more limited in that it was based on only one year 

of air quality data (US EPA, 2011a, p. 2-82).

47 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 (US EPA, 

2011a, p. 2-32).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 62 of 133

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(Peters et al., 1999). These results were found to 

be consistent with results of cross-sectional analyses of the 24-Cities Study (Dockery et al., 1996; 

Raizenne et al., 1996), which reported a long-term cross-city mean PM2.5 concentration of 14.5 μg/m3. 

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, US EPA, 2009a, Figure 7-4). 

In also considering health effects for which the Integrated Science Assessment concludes 

evidence is suggestive of a causal relationship, the Policy Assessment notes 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(US EPA, 2011a, p. 2-33).

Collectively, the Policy Assessment concludes that currently available evidence provides support 

for associations between long-term PM2.5 exposure and mortality and morbidity effects that extend to air 

quality concentrations that are lower than had previously been observed, with aggregate long-term mean 

PM2.5 concentrations extending to well below the level of the current annual standard. These studies 

evaluated a broader range of health outcomes in the general population and in susceptibleat-risk

populations than were considered in the last review, and include extended follow-up for prospective 

epidemiological studies that were important in the last review as well as additional evidence in 

important new cohorts. 

b. Associations with Short-term PM2.5 Exposures

In light of the mixed findings reported in single-city, short-term exposure studies, the Policy 

Assessment places comparatively greater weight on the results from multi-city studies in considering the 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 63 of 133

adequacy of the current suite of standards (US EPA, 2011a, pp. 2-34 to 2-35). With regard to 

associations reported in short-term PM2.5 exposure studies, the Policy Assessment recognizes that longterm mean concentrations reported in new multi-city U.S. and Canadian studies provide evidence of 

associations between short-term PM2.5 exposure and mortality at similar air quality distributions than 

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 allcause, cardiovascular-related (e.g., heart attacks, stroke), and respiratory-related mortality and shortterm PM2.5 exposure, in which the aggregate long-term mean PM2.5 concentration was 13.2 μg/m3

(US 

EPA, 2009a, Figure 6-24). Furthermore, city-specific effect estimates indicate the association between 

short-term exposure to PM2.5 and total mortality and cardiovascular- and respiratory-related mortality is 

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; US 

EPA, 2009a, Figure 6-24, p. 6-178 to 179). The EPA staff notes that for all-cause mortality, city-specific 

effect estimates were statistically significant for 55 percent of the 112 cities, with long-term 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).48

With regard to cardiovascular and respiratory morbidity effects, in the first analysis of the 

MCAPS 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 sub-analysis restricted to days with 24-hour average concentrations of PM2.5 at or below 

 

48 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 US EPA, 2009a, Figure 6-24). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 64 of 133

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).49 In an extended 

analysis of the MCAPS study, 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 of 35 μg/m3, not only indicate that 

effects are occurring in areas that would meet the current standards but also suggest 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 concludes that the findings from short-term PM2.5 exposure 

studies provide evidence of PM2.5-associated health effects occurring in areas that would likely have met 

the current suite of PM2.5 standards (US EPA, 2011a, p. 2-35). These findings are 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, the 

Policy Assessment first concludes that there is 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 

 

49 This sub-analysis was not included in the original publication (Dominici et al., 2006a). 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. 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 65 of 133

the current PM2.5 standards (US EPA, 2011a, p. 2-26). Having reached this initial conclusion, the Policy 

Assessment addresses the question of whether the available evidence supports consideration of standards 

that are more protective than the current standards. In so doing, the Policy Assessment considers 

whether there is 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 

provide evidence of associations with mortality and cardiovascular and respiratory effects both at lower 

ambient PM2.5 concentrations than had been observed in the previous review and at concentrations 

allowed by the current standards (US EPA, 2011a, p. 2-35). 

In reviewing this information, the Policy Assessment recognizes that important limitations and 

uncertainties associated with this expanded body of scientific evidence, noted above in section III.B.2, 

need 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 

concludes 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 concludes this evidence 

provides strong support for considering fine particle standards that would afford increased protection 

beyond that afforded by the current standards (US EPA, 2011a, p. 2-35).

2. Summary of Risk-based Considerations in the Policy Assessment

In addition to evidence-based consideration, the Policy Assessment also considers 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 notes 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 66 of 133

supplement the core analysis; (3) an assessment of the representativeness of the urban study areas within 

a national context;50 and (4) consideration of patterns in design values and air quality monitoring data to 

inform interpretation of the risk estimates, as discussed in section III.C 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 concludes these risks are important from a 

public health standpoint (US EPA, 2011a, p. 2-47). This conclusion reflects consideration of both the

severity and the magnitude of the effects. For example, the risk assessment indicates 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 standards51 (US EPA, 2011a, pp. 2-46 to 2-47). Moreover, additional risks are 

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 short-term 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 concludes that it is unlikely 

that the estimated risks are over-stated, particularly for mortality related to long-term PM2.5 exposure, 

and may well be biased low based on consideration of alternative model specifications evaluated in the 

sensitivity analyses (US EPA, 2010a, p. 5-16; US EPA, 2011a, p. 2-41). Furthermore, the currently 

available scientific information summarized in section III.B above provides evidence for a broader range 

of health endpoints and susceptibleat-risk populations beyond those included in the quantitative risk 

 

50 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 representative of urban 

areas in the U.S. likely to experience relatively elevated levels of risk related to ambient PM2.5 exposures 

(US EPA, 2011a, p. 2-42). 

51 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 (US EPA, 2010a, 

Appendix G, Table G-1).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 67 of 133

assessment (US EPA, 2011a, p. 2-47). 

In considering the risks estimated to occur upon simulating just meeting the current PM2.5

standards, the Policy Assessment concludes that these estimated risks can reasonably be judged to be 

important from a public health perspective and provide strong support for consideration of alternative 

standards that would provide increased protection beyond that afforded by the current PM2.5 standards 

(US EPA, 2011a, p. 2-48).

3. CASAC Advice

CASAC, based on their review of drafts of the Integrated Science Assessment, the Risk 

Assessment, and the Policy Assessment, has 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). 

4. Administrator’s Proposed Conclusions Concerning the Adequacy of the Current Primary PM2.5

Standards

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 (US 

EPA, 2009a), the staff conclusions and associated rationales presented in the Policy Assessment, views 

expressed by CASAC, and public comments. In particular, the Administrator recognizes that the 

Integrated Science Assessment concludes that the results of epidemiological and experimental studies 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 68 of 133

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 longand short-term PM2.5 exposures and respiratory effects. Moreover, the Administrator reflects that these 

effects have 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. See American Trucking Associations v. EPA, 283 F. 3d at 369, 376 (revision of level of 

existing standards justified when effects are observed in areas that meet those standards). With regard to 

the results of the quantitative risk assessment, the Administrator notes that the Risk Assessment 

concludes that the risks estimated to remain upon simulation of just meeting the current standards are 

important from a public health standpoint in terms of both the severity and magnitude of the effects. 

Based on her consideration of these conclusions, as well as consideration of CASAC’s 

conclusion that the evidence and risk assessment clearly call into question the adequacy of the public 

health protection provided by the current PM2.5 NAAQS, the Administrator provisionally concludes that 

the current primary PM2.5 standards, taken together, are not requisite to protect public health with an 

adequate margin of safety and that revision is needed to provide increased public health protection. The 

Administrator provisionally concludes that the scientific evidence and information on risk provide 

strong support for consideration of alternative standards that would provide increased public health 

protection beyond that afforded by the current PM2.5 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 

Formatted: Font: Italic, No underline

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 69 of 133

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 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 concludes that it is appropriate to consider retaining a PM2.5 indicator to 

provide protection from effects associated with long- and short-term fine particle exposures (US EPA, 

2011, p. 2-50).

The Policy Assessment also considers the expanded body of evidence available in this review to 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 70 of 133

consider whether there is sufficient evidence to support a separate standard for ultrafine particles52 or 

whether there is 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 (US 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 (US 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 (US 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 (US 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 (US 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 concludes this evidence is still very limited and largely focused on 

exposure to diesel exhaust, for which the Integrated Science Assessment concludes it is unclear if the 

effects observed are due to ultrafine particles, larger particles within the PM2.5 mixture, or the gaseous 

components of diesel exhaust (US EPA, 2009a, p. 2-22). In addition, the Integrated Science Assessment 

notes uncertainties associated with the controlled human exposure studies using concentrated ambient 

particle systems which have been shown to modify the composition of ultrafine particles (US EPA, 

 

52 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 (US EPA, 2009a, p. 3-3).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 71 of 133

2009a, p. 2-22, see also section 1.5.3). 

The Policy Assessment recognizes that there are relatively few epidemiological studies that have 

examined potential cardiovascular and respiratory effects associated with short-term exposures to 

ultrafine particles (US EPA, 2011a, p. 2-51). These studies have reported inconsistent and mixed results 

(US EPA, 2009a, section 2.3.5).

Collectively, in considering the body of scientific evidence available in this review, the 

Integrated Science Assessment concludes that the currently available evidence is suggestive of a causal 

relationship between short-term exposures to ultrafine particles and cardiovascular and respiratory 

effects. Furthermore, the Integrated Science Assessment concludes that evidence is 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 (US 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 

(US EPA, 2009a, p. 2-2). Therefore, the Policy Assessment recognizes a national characterization of 

concentrations, temporal and spatial patterns, and trends is not possible at this time, and the availability 

of ambient ultrafine measurements to support health studies is extremely limited (US 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 (US 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 (US EPA, 

2009a, p. 3-89). Limited studies of ambient ultrafine particle measurements suggest these particles 

exhibit a high degree of spatial and temporal heterogeneity driven primarily by differences in nearby 

source characteristics (US EPA, 2009a, p. 3-84). Internal combustion engines and, therefore, roadways 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 72 of 133

are a notable source of ultrafine particles, so concentrations of these particles near roadways are 

generally expected to be elevated (US 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 (US EPA, 

2009a, p. 3-84).

In considering both the currently available health effects evidence and the air quality data, the 

Policy Assessment concludes that this information is still too limited to provide support for 

consideration of a distinct PM standard for ultrafine particles (US EPA, 2011a, p. 2-52).

In addressing the issue of particle composition, the Integrated Science Assessment concludes 

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” (US EPA, 2009a, p. 6-202). 

Heterogeneity of ambient concentrations of PM2.5 constituents (e.g., elemental carbon, organic carbon, 

sulfates, nitrates) observed in different 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 (US EPA, 2009a, section 6.6). 

With respect to the availability of ambient measurement data for fine particle components in this

review, there are now more extensive ambient PM2.5 speciation measurement data available through the 

Chemical Speciation Network (CSN) than in previous reviews (US EPA, 2011a, section 1.3.2 and 

Appendix B, section B.1.3). Data from the CSN provide further evidence of spatial and seasonal 

variation in both PM2.5 mass and composition among cities and geographic regions (US 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 (US 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 73 of 133

broad set of PM2.5 constituents, rather than selecting a few constituents a priori (US 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 (US EPA, 2009a, p. 

6-203).53 The CSN provides PM2.5 speciation measurements generally on a one-in-three or one-in-six 

day sampling schedule and, thus, do not capture data every day at most sites.54

The Policy Assessment recognizes that several new multi-city studies evaluating short-term 

exposures to fine particle constituents are now available. These studies continue 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 (US EPA, 2011a, section 2.3.1; US EPA, 2009a, sections 6.5.2.5 and 6.6). 

Limited evidence is available to evaluate the health effects associated with long-term exposures 

to PM2.5 components (US EPA, 2009a, section 7.6.2). The Policy Assessment notes the most significant 

new evidence is 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 

(US EPA, 2011a, p. 2-54; US EPA, 2009a, pp. 7-89 to 7-90). 

 

53 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 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 (US EPA, 2009a, p. 6-203). 

54 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 (US EPA, 2011a, p. 2-53, including footnote 47). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 74 of 133

With respect to source categories of fine particles associated with a range of health endpoints, the 

Integrated Science Assessment reports 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 (US 

EPA, 2009a, section 6.6; Table 6-18). In addition, a few studies have evaluated associations between 

PM2.5-related source categories and mortality. These studies include a study that 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) (US 

EPA, 2009a, section 6.6.2.1). There is 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 is some evidence for PM2.5-related 

associations with secondary sulfate and decrements in lung function in asthmatic and healthy adults (US 

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 (US 

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 (US EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et 

al., 2006; Mar et al., 2006; Thurston et al., 2005). These studies reported associations between total 

mortality and secondary sulfate in two cities for two different lag times. The sulfate effect was stronger 

for total mortality in Washington D.C. and for cardiovascular-related morality in Phoenix (US 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) 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 75 of 133

at various lag times (US 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 respiratoryrelated emergency department visits associated with secondary sulfate (US 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 concludes 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 (US 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” (US EPA, 2009a, p. 6-210). 

Assessing this information, the Integrated Science Assessment concludes that “the evidence is not yet 

sufficient to allow differentiation of those constituents or sources that are more closely related to 

specific health outcomes” (US EPA, 2009a, pp. 2-26 and 6-212). Therefore, the Policy Assessment 

concludes 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 concludes 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 (US EPA, 2011a, p. 2-56). 

The CASAC 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 76 of 133

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 proposes to retain PM2.5 as the indicator for fine particles. Further, the Administrator 

provisionally concludes 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. Furthermore, the Administrator also provisionally 

concludes that the currently available scientific information does not provide a sufficient basis for 

eliminating any individual component or group of components associated with any source categories 

from the mix of fine particles included in the PM2.5 mass-based indicator. 

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 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 concludes it is appropriate to retain the 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 77 of 133

current annual and 24-hour averaging times to provide protection from effects associated with both longand short-term PM2.5 exposures (US 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 notes there continues to be a growing body of studies that provide additional 

evidence of effects associated with exposure periods less than 24-hours (US 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 (US 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 is much sparser than for cardiovascular effects and continues to be very limited. The 

Integrated Science Assessment concludes that for several studies of hospital admissions or medical visits 

for respiratory diseases, the strongest associations were observed with 24-hour average or longer 

exposures, not with less than 24-hour exposures (US EPA, 2009a, section 6.3). 

Collectively, the Policy Assessment concludes 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 (US EPA, 2011a, p. 2-57). 

With regard to health effects associated with PM2.5 exposure across varying seasons in this 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 78 of 133

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 provide 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 (US EPA, 

2011a, p. 2-58). 

Overall, the Policy Assessment observes 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 available evidence, the Policy Assessment concludes that there is no basis to consider a 

seasonal averaging time separate from a 24-hour averaging time.

Based on the above considerations, the Policy Assessment concludes that the currently available 

information provides strong support for consideration of retaining current annual and 24-hour averaging 

timers but does not provide support for considering alternative averaging times (US EPA, 2011a, p. 2-

58). In addition, CASAC considers it appropriate to retain the current annual and 24-hour averaging 

times for the primary PM2.5 standards (Samet, 2010c, pp. 2 to 3). The Administrator concurs with the 

staff conclusions and CASAC advice and proposes 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 concludes, 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 79 of 133

consistent with conclusions reached in the Policy Assessment and by CASAC, that the currently 

available information is 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. 

3. Form

The “form” of a standard defines the air quality statistic that is to be compared to the level of the 

standard in determining whether an area attains the standard. In this review, we consider whether 

currently available information supports consideration of alternative 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 (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 was based in part on an analysis of 

the potential for disproportionate impacts on potentially susceptibleat-risk populations, which found that 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 80 of 133

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; US EPA, 2005, section 5.3.6.1).

In this review, as discussed in section III.B.3, there now exist more health data such that the 

Integrated Science Assessment has identified persons from lower socioeconomic strata as a 

susceptiblean at-risk population (US EPA, 2009a, section 8.1.7; US 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 is 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, would provide appropriate public 

health protection. 

In considering the potential for disproportionate impacts on susceptibleat-risk populations, the 

Policy Assessment recognizes an update of an air quality analysis conducted for the last review (US 

EPA, 2011a, pp. 2-59 to 60; Schmidt, 2011a, Analysis A). This analysis focuses on determining if the 

spatial averaging provisions, as modified in 2006, could introduce inequities in protection for 

susceptibleat-risk populations exposed to PM2.5. Specifically, the Policy Assessment considers whether 

persons of lower socioeconomic status are more likely than the general population to live in areas in 

which the monitors recording the highest air quality values in an area are located. Data used in this 

analysis included demographic parameters measured at the Census Block or Census Block Group level, 

including percent minority population, percent minority subgroup population, percent of persons living 

below the poverty level, percent of persons 18 years of age or older, and percent of persons 65 years of 

age and older. In each candidate geographic area, data from the Census Block(s) or 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 81 of 133

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 observes 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 (US EPA, 2011a, p. 2-60). 

Based upon the analysis described above, the Policy Assessment concludes 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 susceptibleat-risk

populations of persons with lower SES levels as well as minorities. Therefore, the Policy Assessment 

concludes that it is appropriate to consider revising the form of the annual PM2.5 standard such that it 

does not allow for the use of spatial averaging across monitors. In doing so, the level of the annual PM2.5

standard would be compared to measurements made at the monitoring site that represents area-wide air 

quality recording the highest PM2.5 concentrations55 (US EPA, 2011a, p. 2-60). 

The CASAC agreed with staff conclusions that it is “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, they 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 considering the Policy Assessment’s conclusions based on the results of the analysis discussed 

above and concern over the evidence of potential disproportionate impacts on susceptibleat-risk

populations as well as CASAC advice, the Administrator proposes to revise the form of the annual PM2.5

 

55 As discussed in section VIII.B.1 below, the EPA is proposing to revise several terms associated with 

PM2.5 monitor placement. Specifically, the EPA is proposing to revoke the term “community-oriented” 

and replace it with the term “area-wide” monitoring. 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 82 of 133

standard to eliminate the use of spatial averaging. Thus, the Administrator proposes revising the form of 

the annual PM2.5 standard to compare the level of the standard with measurements from each 

“appropriate” monitor in an area56 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.

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, 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.57

In revisiting the stability of a 98th versus 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 considers air quality data reported in 2000 to 2008 to update our understanding of the 

ratio between peak-to-mean PM2.5 concentrations. This analysis provides evidence that the 98th

 

56 As discussed in section VIII.B.2.b below, the EPA proposes that PM2.5 monitoring sites at micro- and 

middle-scale locations 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.”

57 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).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 83 of 133

percentile value is a more stable metric than the 99th percentile (US EPA, 2011a, Figure 2-2, p. 2-62). 

The Agency recognizes 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 

protection in the long run.58 As in previous reviews, the EPA recognizes that a concentration-based 

form, compared to an exceedance-based form, is 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 concludes that a concentration-based form, when 

averaged over three years, provides 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 proposes to retain the 98th percentile form of the 

24-hour standard. The Administrator provisionally concludes 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. 

4. Level

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-

 

58 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. The context for the court’s discussion is 

identical to that here; whether to adopt a 98th percentile form for a 24-hour primary PM2.5 standard 

intended to provide supplemental protection for a generally controlling annual standard.

Formatted: Font: Italic, No underline
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 84 of 133

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 exposures 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 59

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. Approach Used in the Policy Assessment

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 uses an approach that integrates evidence-based and risk-based considerations, takes into 

account CASAC advice, and considers the issues raised by the court in remanding the primary annual 

PM2.5 standard. Following the general approach outlined in section III.A.3, for the reasons discussed 

below, the Policy Assessment concludes it is appropriate to consider the protection afforded by the 

annual and 24-hour standards taken together against mortality and morbidity effects associated with both 

long- and short-term PM2.5 exposures. This is 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 considers 

the extent to which specific alternative PM2.5 standard levels are likely to reduce the nature and 

magnitude of both long-term exposure-related mortality risk and short-term exposure-related mortality 

 

59 Throughout this section, the annual standard level is denoted as an integer value for simplicity, 

although, as noted above in section II.B.1, Table 1, footnote 7, the standard level is defined to one 

decimal place, such that the current standard level is 15.0 μg/m3. Alternative standard levels discussed in 

this section are similarly defined to one decimal place.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 85 of 133

and morbidity risk (US 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 are generally observed as either the annual 

or 24-hour standard, or both, are reduced below the level of the current standards (US 2011a, Figures 

2-11 and 2-12; US EPA, 2010a, sections 4.2.2, 5.2.2, and 5.2.3). 

Based on the quantitative risk assessment, the Policy Assessment observes, as discussed in 

section III.A.3, that analyses conducted for this and previous reviews demonstrate 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 (US EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C.3, the Risk 

Assessment observes 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 (US EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk Assessment 

notes that while the results of simulating alternative suites of PM2.5 standards including different 

combinations of alternative annual and 24-hour standard levels suggest 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 is highly variable, in part due to the choice of rollback approached used (US EPA, 

2010a, p. 5-17).

Therefore, the Policy Assessment concludes, consistent with CASAC advice (Samet 2010c, p. 

1), that it is appropriate to set a “generally controlling” annual standard that will lower a wide range of 

ambient 24-hour concentrations. The Policy Assessment concludes this approach would likely reduce 

aggregate risks associated with both long- and short-term exposures with more consistency than a 

generally controlling 24-hour standard and would be the most effective and efficient way to reduce total 

PM2-5-related population risk and so provide appropriate protection. The staff believes this approach, in 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 86 of 133

contrast to one focusing on a generally controlling 24-hour standard, would likely reduce aggregate risks 

associated with both long- and short-term exposures with more consistency and would likely avoid 

setting national standards that 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. 

The Policy Assessment recognizes 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 concludes it is 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.

Based on the above considerations, the approach used in the Policy Assessment to identify 

alternative standard levels that are appropriate for consideration focuses on translating information from 

epidemiological studies into the basis for staff conclusions on levels. This approach is broader and more 

integrative than the general approach used by the EPA in previous reviews (see summary in section 

III.A.3 above; US EPA, 2011a, sections 2.1.3 and 2.3.4.1) and reflects the more extensive and stronger 

body of scientific evidence now available on health effects related to long- and short-term PM2.5

exposures, a more comprehensive quantitative risk assessment, and more extensive PM2.5 air quality 

data. In considering the currently available information, the Policy Assessment focuses on identifying 

levels for an annual standard and a 24-hour standard that, in combination, provide protection against 

health effects associated with both long- and short-term PM2.5 exposures. The Policy Assessment also 

considers the extent to which various combinations of annual and 24-hour standards reflect setting a 

generally controlling annual standard with a 24-hour standard providing supplemental protection (US 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 87 of 133

EPA, 2011a, sections 2.1.3, 2.3.4.1). 

As discussed in the Policy Assessment, EPA staff recognizes that there is no single factor or 

criterion that comprises the “correct” approach for reaching conclusions on alternative standard levels 

for consideration, but rather there are various approaches that are reasonable to consider (US EPA, 

2011a, section 2.3.4.1). In reaching conclusions in the Policy Assessment on the ranges of standard 

levels that are appropriate to consider, staff considered the relative weight to place on different evidence. 

The Policy Assessment initially focuses on long- and short-term PM2.5 exposure studies conducted in the

U.S. and Canada and places the greatest weight on health outcomes judged in the Integrated Science 

Assessment as having evidence to support a causal or likely causal relationship. The Policy Assessment 

also considers the evidence for a broader range of health outcomes judged in the Integrated Science 

Assessment to have evidence suggestive of a causal relationship, specifically studies that focus on 

effects in susceptible populations, to evaluate whether this evidence provides support for considering 

lower alternative standard levels. 

Several factors were taken into account in placing relative weight on the body of available 

epidemiological studies, for example, study characteristics, including study design (e.g., time period of 

air quality monitoring, control for potential confounders); strength of the study (in terms of statistical 

significance and precision of results); and availability of population-level and air quality distribution 

data. As noted above in section III.A.3, the Policy Assessment places greatest weight on information 

from multi-city epidemiological studies to inform staff conclusions regarding alternative annual standard 

levels. These studies have a number of advantages compared to single-city studies60 that include 

providing representation of ambient PM2.5 concentrations and potential health impacts across a range of 

 

60 As discussed in section III.B.1 above, the Policy Assessment recognizes that single-city studies 

provide ancillary evidence to multi-city studies in support of calling into question the adequacy of the 

current suite of standards. However, in light of the mixed findings reported in single-city short-term 

PM2.5 exposure studies, and the likelihood that these results are influenced by localized events and not 

representative of air quality across the country, the Policy Assessment places comparatively greater 

weight on the results from multi-city studies in considering alternative annual and 24-hour standard 

levels (US EPA, 2011a, p. 2-64). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 88 of 133

diverse locations providing spatial coverage for different regions across the country, reflecting 

differences in PM2.5 sources, composition, and potentially other exposure-related factors which might 

impact PM2.5-related risks; lack of ‘publication bias’ (US EPA, 2004, p. 8-30); and consideration of 

larger study populations that afford the possibility of generalizing to the broader national population and 

provide higher statistical power than single-city studies to detect potentially statistically significant 

associations with relatively more precise effect estimates. 

In reaching conclusions in the Policy Assessment regarding alternative 24-hour standard levels 

that are appropriate to consider, staff also considers relevant information from single-city short-term 

PM2.5 exposure studies. Although, as discussed above, multi-city studies have greater power to detect 

associations and provide broader geographic coverage in comparison to single-city studies, the extent to 

which effects reported in multi-city short-term PM2.5 exposure studies are associated with the specific 

short-term air quality in any particular location is unclear, especially when considering short-term 

concentrations at the upper end of the air quality distribution (i.e., at the 98th percentile value) for a 

given study area. In contrast, single-city studies are more limited in terms of power and geographic 

coverage but the link between reported health effects and the air quality in a given study area is more 

straightforward. Therefore, the Policy Assessment considers the results of both multi-city and single-city 

short-term exposure studies to inform staff conclusions regarding alternative levels that are appropriate 

to consider for a 24-hour standard that is intended to provide supplemental protection in areas where the 

annual standard may not offer appropriate protection against the effects of all short-term exposures (US 

EPA, 2011a, pp. 2-62 to 2-65). 

b. Consideration of the Annual Standard 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 

are appropriate to consider focuses on characterizing the range of PM2.5 concentrations over which we 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 89 of 133

have the most confidence in the associations reported in the epidemiological studies, and conversely 

where our confidence in the association becomes appreciably lower. The most direct approach to 

address this issue, consistent with CASAC adviseadvice (Samet, 2010c, p.10), is to consider 

epidemiological studies reporting confidence intervals around concentration-response relationships (US 

EPA, 2011a, p. 2-63). Based on a thorough search of the available evidence, the Policy Assessment 

identified three long-term PM2.5 exposure studies reporting confidence intervals around concentrationresponse functions (i.e., Schwartz et al., 2008; Pope et al., 2002; Miller et al., 2007; US EPA, 2011a, pp. 

2-65 to 2-70 and Figure 2-3).61 In its assessment of these studies, the Policy Assessment places greater 

weight on analyses that averaged across multiple concentration-response models since this approach 

represents a more robust examination of the underlying concentration-response relationship than 

analyses considering a single concentration-response model. Although these analyses of long-term 

exposure to PM2.5 provide information on the lack of any discernible population threshold, only 

Schwartz et al. (2008) conducted a multi-model analysis to characterize confidence intervals around the 

estimated concentration-response relationship that can help inform at what PM2.5 concentrations we have 

appreciably less confidence in the nature of the underlying concentration-response relationship. 

Although analyses of confidence intervals associated with concentration-response relationships can help 

inform consideration of alternative standard levels, the Policy Assessment concludes that the single 

relevant analysis now available is too limited to serve as the principal basis for identifying alternative 

standard levels in this review (US EPA, 2011a, p. 2-70).

The Policy Assessment explores other approaches that considered different statistical metrics to 

identify ranges of long-term mean PM2.5 concentrations that were most influential in generating health 

effect estimates in long- and short-term epidemiological studies, placing greatest weight on those studies 

 

61 The EPA carefully analyzed the published evidence, but was unable to identify any short-term PM2.5

exposure studies that characterized confidence intervals around concentration-response relationships. 

Nor did CASAC or public comments on this issue, as addressed in their comments on the second draft 

Policy Assessment, identify any additional analyses.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 90 of 133

that reported positive and statistically significant associations (US 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 recognizes 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 represent air quality data typically used in 

epidemiological analyses and provide a direct link between PM2.5 concentrations and the observed 

health effects. Further, these data are available for all long- and short-term exposure studies analyzed 

and, therefore, represent the data set available for the broadest set of epidemiological studies. 

However, consistent with CASAC’s comments on the second draft Policy Assessment62 (Samet, 

2010d, p. 2), in preparing the final Policy Assessment, 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 range of PM2.5 concentrations down through which staff continued to have confidence 

in the associations observed in epidemiological studies and below which there is a comparative lack of 

data such that the staff’s confidence in the relationship was appreciably less. This would also be the 

range of PM2.5 concentrations which has the most influence on generating the health effect estimates 

reported in epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment 

recognizes there is no one percentile value within a given distribution that is the most appropriate or 

“correct” way to characterize where our confidence in the associations becomes appreciably lower. The 

Policy Assessment concludes that focusing on concentrations within the lower quartile of a distribution, 

such as the range from the 25th to the 10th percentile, is reasonable to consider as a region within which 

 

62 While CASAC expressed the view that it would be most desirable to have information on 

concentration-response relationships, they recognized that it would also be “preferable to have 

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

individual studies” (Samet, 2010d, p. 2). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 91 of 133

we begin to have appreciably less confidence in the associations observed in epidemiological studies.63

In 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. (US EPA, 2009a, p. 2-

71).

As outlined in section III.A.3 above, the Policy Assessment recognizes that there are 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 is 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 study participants. Although not as directly relevant,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 can be used as a surrogate 

for health event data in relation to the distribution of PM2.5 concentrations (i.e., study population data), 

as an appropriate surrogate for health event data. 

In applying this approach, the Policy Assessment focuses on identifying the broader range 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, 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 

 

63 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 (US 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 

(US EPA, 2011a, p. 2-71).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 92 of 133

three studies and one additional long-term exposure study (Miller et al., 2007); as documented in a staff 

memorandum (Rajan et al., 2011). For the three studies for which both health event and study 

population data were available, EPA staff analyzed the reliability of using study population data as a 

surrogate for health event data. Based on these analyses, EPA staff recognized that the 10thand 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 first approach, 

Figures 1 through 3 (US EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) summarize data available for multicity, 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.

64

Figures 1 and 3 provide summaries ofsummarize 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 (a susceptible, an at-risk population).. Figure 2 provides athis same summary 

ofinformation for long-term exposure studies that evaluated respiratory effects (evidence of a likely 

causal relationship) in children, a susceptiblean at-risk population, as well as long-term exposure studies 

of children that evaluated developmental effects (evidence suggestive of a causal relationship). By 

following the general approach used in previous PM NAAQS reviews, one could consider identifying 

alternative standard levels that are somewhat below the long-term mean PM2.5 concentrations reported in 

 

64 Additional studies presented and assessed in the Integrated Science Assessment report effects at 

higher long-term mean PM2.5 concentrations (e.g., US EPA, 2009a, Figures 2-1, 2-2, 7-6, and 7-7).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 93 of 133

these epidemiological studies.

As outlined above in section III.A.3,With regard to consideration of additional information from

epidemiological studies typically reportwhich is relevant to the second approach, EPA has compiled a 

summary of the range of PM2.5 concentrations based upon a composite monitorcorresponding with the 

25th to 10th percentiles of health event or study population data from the four multi-city studies, for 

which distributional statistics are available65 (US 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; that is,

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 areand then averaged across study 

areas to calculate an overall study mean concentration, as discussed above. As noted above in section 

III.A.3 and discussed in the Policy Assessment, a policy approach that uses data based on composite 

monitor distributions to identify alternative standard levels, and then compares those levels to 

concentrations at appropriate maximum monitors to determine if an area meets a given standard, 

inherently has the potential to build in some margin of safety (US EPA, 2011a, p. 2-14). In analyses 

conducted by EPA staff based on selected long- and short-term exposure studies, the Policy Assessment 

 

65 Health event data (e.g., number of deaths, hospitalizations) occurring in a study population were 

obtained 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) (US 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. 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 94 of 133

notes
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 95 of 133

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 Adults

Source: US EPA, 2011a, Figure 2-4
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 96 of 133

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: US EPA, 2011a, Figure 2-5
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 97 of 133

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: US EPA, 2011a, Figure 2-6

Formatted: Font: 12 pt
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 98 of 133

Figure 4. Translating Epidemiological Evidence from Multi-City Exposure Studies into an Annual PM2.5 Standard

Source: US EPA, 2011a, Figure 2-8

Formatted: Font: 12 pt
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 99 of 133

that the differences between the maximum and composite distributions were greater for studies with 

fewer years of air quality data (i.e., 1 to 3 years) and smaller numbers of study areas (i.e., 36 to 51 study 

areas). The differences in the maximum and composite monitor distribution were much smaller (i.e., 

generally within five percent) for studies with more years of air quality data (i.e., up to 6 years) and 

larger numbers of study areas (i.e., 112 to 204 study areas) (Hassett-Sipple et al., 2010; US EPA, 2010f, 

section 2.3.4.1). Therefore, any margin of safety that may be provided by a policy approach that uses 

data based on composite monitor distributions to identify alternative standard levels, and then compares 

those levels to concentrations at appropriate maximum monitors to determine if an area meets a given 

standard, will vary depending upon the number of monitors and air quality distributions within a given 

area. See also, footnote 27 in section III.A.3 above and associated text.

The Policy Assessment initially considers distributional statistics for both long- and short-term 

exposure studies that evaluated mortality (causal relationship), cardiovascular effects (causal 

relationship) or respiratory effects (likely causal relationship). In looking first at the long-term mean 

concentrations in the long-term exposure studiesFigure 4 summarizes statistical metrics for those studies 

included in Figures 1, 2, and 3 that provide evidence of statistically significant PM2.5-related effects, 

which are relevant to the two approaches for translating epidemiological evidence into standard levels 

discussed above. The top of Figure 4 includes information for long-term exposure studies evaluating 

health outcomes classified as having evidence of a casual or likely casual 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 casual or likely casual relationship with PM2.5 exposures (long-term mean PM2.5

concentrations indicated by triangle symbols). The bottom of Figure 4 includes information for longterm exposures studies evaluating health outcomes classified as having evidence suggestive of a causal 

relationship (long-term mean PM2.5 concentrations indicated by square symbols). Figure 4 also 

Formatted: Indent: First line: 0"
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 100 of 133

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 looking first at the long-term mean PM2.5 concentrations reported in the multi-city long-term 

exposure studies, as summarized at the top of Figure 4, the Policy Assessment observes positive and 

often statistically significant associations at long-term mean PM2.5 concentrations ranging from 16.4 to 

12.9 μg/m3 66 (Laden et al., 2006; Lipfert et al., 2006; Krewski et al., 2009; Goss et al., 2004; Miller et 

al.; 2007; Zeger et al., 2008; Eftim et al., 2008; Dockery et al., 1996; McConnell et al., 2003; see Figures 

1 and 2).). In considering the one long-term PM2.5 exposure study for which health event data are 

available (Krewski et al., 2009), the Policy Assessment observes that the long-term mean PM2.5

concentrations corresponding with study areas contributing to the 25th and 10th percentiles of the 

distribution of mortality data are 12.0 μg/m3and 10.2 μg/m3, respectively (Figure 4; US EPA, 2011a, 

Figure 2-7; Rajan et al., 2011, Table 1). As identified above, although less directly relevant than event 

data, the number of participants within each study area can be used as a surrogate for health event data 

in relation to the distribution of PM2.5 concentrations. The long-term mean PM2.5 concentrations 

corresponding with study areas contributing to the 25th and 10th percentiles of the distribution of study 

participants for Miller et al. (2007) were 11.2 μg/m3and 9.7 μg/m3, respectively (Figure 4; US EPA, 

2011a, Figure 2-7; Rajan et al., 2011, Table 1). 

In then considering information from multi-city, short-term exposure studies reporting positive 

and statistically significant associations with these same broad health effect categories, as summarized in 

the middle of Figure 4, the Policy Assessment observes positive and statistically significant associations 

at long-term mean PM2.5 concentrations in a similar range of 15.6 to 12.8 μg/m3(Franklin et al., 2007, 

 

66 As discussed in footnote 58section III.D.1.a above, the lowest long-term mean PM2.5 concentration 

reported in the long-term exposure studies was based on updated air quality data for Miller et al. (2007). 

As noted in the Policy Assessment, these air quality data were based on only one year of ambient 

measurements (2000) and in comparison to other long-term exposure studies that considered multiple 

years of air quality data, were much more limited (US EPA, 2011a, pp. 2-81 to 2-82).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 101 of 133

2008; Klemm and Mason, 2003; Burnett and Goldberg, 2003; Zanobetti and Schwartz, 2009; Burnett et 

al., 2004; Bell et al., 2008; Dominici et al., 2006a; see Figure 3). In considering the two multi-city, 

short-term PM2.5 exposure studies for which health event data are available, the Policy Assessment 

observes that 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 are 

12.5 μg/m3and 10.3 μg/m3, respectively, for Zanobetti and Schwartz (2009), and 11.5 μg/m3and 9.8 

μg/m3, respectively, for Bell et al. (2008) (Figure 4; US EPA, 2011a, Figure 2-7; Rajan et al., 2011, 

Table 1). 

Taking into consideration additional studies of specific susceptible lifestagesat-risk populations

(i.e., childhoodchildren), the Policy Assessment expands its evaluation of the long-term exposure studies 

to include a broader range of health outcomes judged in the Integrated Science Assessment to have 

evidence suggestive of a causal relationship. This evidence was taken into account to evaluate whether it 

provides support for considering lower alternative levels than if weight were only placed on studies for 

which health effects have been judged in the Integrated Science Assessment to have evidence supporting 

a causal or likely causal relationship. The Policy Assessment makes note of a limited number of studies 

that provide emerging evidence for PM2.5-related low birth weight and infant mortality, especially 

related to respiratory causes during the post-neonatal period. This more limited body of evidence, as 

summarized at the bottom of Figure 4, indicates positive and often statistically significant effects 

associated with long-term PM2.5 mean concentrations in the range of 14.9 to 11.9 μg/m3(Woodruff et 

al., 2008; Liu et al., 2007; Bell et al., 2007; see Figure 2). As illustrated in Figure 2, although Parker and 

Woodruff (2008) did not observe an association between quarterly estimates of exposure to PM2.5 and 

low birth weight in a multi-city U.S. study, other U.S. and Canadian studies did report positive and 

statistically significant associations between PM2.5 and low birth weight at lower ambient concentrations 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 102 of 133

(Bell et al., 2007; Liu et al., 2007).67 There remain significant limitations (e.g., identifying the 

etiologically relevant time period) in the evaluation of evidence on the relationship between PM2.5

exposures and birth outcomes (US EPA, 2009a, pp. 7-48 and 7-56) which should be taken into 

consideration in reaching judgments about how to weigh these studies of potential impacts on specific 

susceptible populations in considering alternative standard levels that provide protection with an 

appropriate margin of safety.

With respect to carcinogenicity, mutagenicity, and genotoxicity (evidence suggestive of a causal 

relationship), the strongest evidence currently available is from long-term prospective cohort studies that 

report positive associations between PM2.5 and lung cancer mortality. At this time, 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 (US EPA, 2009a, section 7.5). Therefore, in selecting alternative 

standard levels appropriate to consider, the Policy Assessment noted that, in providing protection against 

mortality and cardiovascular and respiratory effects it is reasonable to anticipate that protection will also 

be provided for carcinogenicity, mutagenicity, and genotoxicity effects (US EPA, 2011a, p. 2-78). 

In summarizing the currently available evidence and air quality information within the context of 

identifying potential alternative annual standard levels for consideration, the Policy Assessment first 

notes that the Integrated Science Assessment concludes there is no evidence of a discernible population 

threshold below which effects would not occur. Thus, health effects may occur over the full range of 

concentrations observed in the epidemiological studies. In the absence of any discernible thresholds, the 

general approach used in the Policy Assessment for identifying alternative standard levels that would 

provide appropriate protection against effects observed in epidemiological studies has focused on the 

 

67 As noted in section 7.4 of the Integrated Science Assessment, Parker et al. (2005) reported that over a 

9-month exposure period (mean PM2.5 concentration of 15.4 μg/m3) a significant decrease in birth 

weight was associated with infants in the highest quartile of PM2.5 exposure as compared to infants 

exposed in the lowest quartile.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 103 of 133

central question of identifying the range of PM2.5 concentrations below the long-term mean 

concentrations where we continue to have confidence in the associations observed in epidemiological 

studies. 

In considering the evidence, the Policy Assessment recognizes 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, considerations of different alternative standard levels could be supported. 

As discussed in section III.A.3 above, the Policy Assessment recognizes that by applying the 

general policy approach used in previous reviews, one could focus on identifying alternative standard 

levels for the annual standard that are somewhat below the long-term mean PM2.5 concentrations 

reported in the long- and short-term exposure studies. By expanding this approach to consider additional 

population-level information from epidemiological studies, one could also focus on the range of PM2.5

concentrations below the long-term mean 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 is appreciably less at lower concentrations, to identify alternative annual standard levels. 

Figure 4 provides a summary of the long-term mean PM2.5 concentrations for the long- and short-term 

exposure studies presented in Figures 1 through 3 as well as the range of PM2.5 concentrations 

corresponding with the 25th to 10th percentiles of health event or number of study participants data from 

the four multi-city studies, for which distributional statistics are available68 (US EPA, 2011a, Figure 2-7; 

Rajan et al., 2011, Table 1). If health event or study population data were available for additional 

 

68 Health event data (e.g., number of deaths, hospitalizations) occurring in a study population were 

obtained 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) (US EPA, 2011a, p.2-71).
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 104 of 133

studies, the EPA could employ distributional statistics to identify the broader range of PM2.5

concentrations that were most influential in generating effect estimates in those studies. The EPA 

anticipates that, in characterizing where the bulk of the data exist, patterns similar to those presented in 

Figure 4 would occur for the additional long- and short-term PM2.5 exposure studies.

Given the currently available evidence and considering the various approaches discussed above, 

the Policy Assessment concludes it is appropriate to focus on an annual standard level within a range of 

about 12 to 11 μg/m3(US EPA, 2011a, pp. 2-82, 2-101, and 2-106). As illustrated in Figure 4, a 

standard level of 12 μg/m3, at the upper end of this range, is somewhat below the long-term mean PM2.5

concentrations reported in all the multi-city, long- and short-term exposure studies that provide 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/m3 would reflect consideration of additional population-level information from 

such epidemiological studies in that it generally corresponds with approximately the 25th percentile of 

the available distributions of health events data in the studies for which population-level information 

was available.69 In addition, a level of 12 μg/m3 would reflect some consideration of studies that provide 

more limited evidence of reproductive and developmental effects, which are suggestive of a causal 

relationship, in that it is about at the same level as the lowest long-term mean PM2.5 concentrations 

reported in such studies (see Figure 4). 

Alternatively, an annual standard level of 11 μg/m3, at the lower end of this range, is 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 

 

69 As outlined in section III.A.3, the Policy Assessment considers the 25th percentile to be the start of the 

range of PM2.5 concentrations below the mean within which the data become appreciably more sparse 

and, thus, where our confidence in the associations observed in epidemiological studies begins to 

become appreciably less. 

Formatted: Not Superscript/ Subscript
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 105 of 133

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 is within 

the range of PM2.5 concentrations corresponding to the 25th and 10th percentiles of all the available 

distributions of such data.70 In addition, a level of 11 μg/m3is 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 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 concludes that the 

available evidence does not provide a sufficient basis to consider alternative annual standard levels 

below 11 μg/m3(US EPA, 2011a, p. 2-81). 

The Policy Assessment also considers the extent to which the available evidence provides a basis 

for considering alternative annual standard levels above 12 μg/m3. As discussed below, the Policy 

Assessment concludes that it could be reasonable to consider a standard level up to 13 μg/m3based on a 

policy approach that tends to weigh 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 is 

placed on long-term exposure studies as a basis for an annual standard level. A level of 13 μg/m3is 

somewhat below the long-term mean PM2.5 concentrations reported in all but one of the 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 

 

70 As discussed in section III.A.3, the Policy Assessment identifies the range from the 25th to the 10th

percentiles as a reasonable range to consider, in that it is a range where we have appreciably less 

confidence in the associations observed in epidemiological studies (US EPA, 2011a, p. 2-12). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 106 of 133

μg/m3is the WHI study (Miller et al., 2007). As noted in section III.D.1.a above, including footnote 58, 

the Policy Assessment observes that in comparison to other long-term exposure studies, the WHI study 

was more limited in that it was based on only one year of air quality data (US EPA, 2011a, pp. 2-81 to 

2-82). Thus, to the extent that less weight is placed on the WHI 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, the Policy Assessment notes that a level of 13 μg/m3is below 

the long-term mean PM2.5 concentrations reported in most such studies, but is above the long-term 

means of 12.8 and 12.9 μg/m3reported in Burnett et al. (2004) and Bell et al. (2008), respectively. In 

considering these studies, the Policy Assessment finds no basis to conclude that these two studies are 

any more limited or uncertain than the other short-term exposure studies shown in Figures 3 and 4 (US 

EPA, 2011a, p. 2-82). On this basis, as discussed below, the Policy Assessment concludes 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, such that taken together the suite of 

PM2.5 standards would provide appropriate protection from effects on public health related to short-term 

exposure to PM2.5 (US EPA, 2011a, p. 2-82).

The Policy Assessment also notes 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.2 above, 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 is too uncertain to support consideration of any
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 107 of 133

lower level.

Beyond evidence-based considerations, the Policy Assessment also considered the extent to 

which quantitative risk assessment supports consideration of these alternative standard levels or provides 

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/m3, noting that the level of 11 μg/m3 was not 

include 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 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 (US EPA, 

2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concludes that consideration of alternative annual 

standard levels in the range of 13 to 11 μg/m3 may be appropriate. Furthermore, the Policy Assessment 

concludes that the currently available evidence most strongly supports consideration of an alternative 

annual standard level in the range of 12 to 11 μg/m3(US EPA, 2011a, p. 2-82). The Policy Assessment 

concludes 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 susceptibleat-risk populations, than would a higher level. This range would also reflect 

placing weight on information from studies that help 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 is appreciably less at lower concentrations. 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 108 of 133

c. Consideration of the 24-Hour Standard in the Policy Assessment

As recognized in section III.A.3 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 longterm exposures). Of particular concern are 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. As a result, the Policy Assessment concludes that it is 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 considers 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 short-term fine particle exposures. In considering the available 

epidemiological evidence, the Policy Assessment takes into account information from multi-city studies 

as well as single-city studies. The Policy Assessment considers 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 considers information on the long-term mean 

PM2.5 concentrations from these studies. 

In addition to considering the epidemiological evidence, the Policy Assessment also considers 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 109 of 133

sources. These patterns help 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 24-hour standard that provides supplemental protection especially for areas with high 

peak-to-mean ratios (US EPA, 2011a, p. 2-14). 

In considering the information provided by the short-term exposure studies, the Policy 

Assessment recognizes that to the extent these studies were conducted in areas that likely did not meet 

one or both of the current standards, such studies do 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 places 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, the Policy Assessment observes 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 Policy Assessment notes 

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 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). To the extent that 

consideration is given to revising the level of the annual standard, as discussed above in section 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 110 of 133

III.E.4.b, the Policy Assessment recognizes that potential changes associated with 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 work in conjunction with 

an annual standard to provide appropriate public health protection, the Policy Assessment notes that to 

the extent that the level of the annual standard is revised to within a range of 13 to 11 μg/m3, in 

particular in the range of 12 to 11 μg/m3, additional protection would be provided for the effects 

observed in these multi-city studies (US EPA, 2011a, p. 2-84). 

In summary, the Policy Assessment concludes that the multi-city, short-term exposure studies 

generally provide support for retaining the 24-hour standard level at 35 μg/m3in conjunction with an 

annual standard level revised to within a range of 12 to 11 μg/m3(US EPA, 2011a, p. 2-84). 

Alternatively, in conjunction with an annual standard level of 13 μg/m3, the Policy Assessment 

concludes that the multi-city studies provide 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 in conjunction with an overall long-term mean concentration slightly less 

than 13 μg/m3(Figure 3; US EPA, 2011a, p. 2-84). 

In reaching staff conclusions regarding alternative 24-hour standard levels that are appropriate to 

consider, the Policy Assessment also takes into account relevant information from single-city studies 

that evaluated effects associated with short-term PM2.5 exposures. The Policy Assessment recognizes 

that these studies may provide additional insights regarding impacts on susceptible populations and/or 

on areas with isolated peak concentrations. Although, as discussed in section III.E.4.a above, multi-city 

studies have advantages over single-city studies in terms of statistical power to detect associations and 

broader geographic coverage as well as other factors such as less likelihood of publication bias, 

reflecting differences in PM2.5 sources, composition, and potentially other factors that could impact 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 111 of 133

PM2.5-related effects, multi-city studies often present overall effect estimates rather than single-city 

effect estimates. Since short-term air quality can vary considerably across cities, the extent to which 

effects reported in multi-city studies are associated with short-term air quality in any particular location 

is uncertain, especially when considering short-term concentrations at the upper end of the distribution 

of daily PM2.5 concentrations (i.e., at the 98th percentile value). In contrast, single-city studies are more 

limited in terms of power and geographic coverage but the link between reported health effects and the 

air quality in a given study area is more straightforward to establish. Therefore, the Policy Assessment 

also considers evidence from single-city, short-term exposure studies to inform staff conclusions 

regarding alternative levels that are appropriate to consider for a 24-hour standard that is intended to 

provide supplemental protection in areas where the annual standard may not provide an adequate margin 

of safety against the effects of all short-term PM2.5 exposures.

As discussed above for the multi-city studies, the Policy Assessment takes into account both the 

24-hour PM2.5 concentrations in the single-city studies, focusing on the 98th percentile air quality values, 

as well as the long-term mean PM2.5 concentrations. The Policy Assessment considers single-city studies 

conducted in areas that would likely have met the current suite of PM2.5 standards as most useful for 

informing staff conclusions related to the level of the 24-hour standard (US EPA, 2011a, Figure 2-9). 

The Policy Assessment notes that additional single-city studies summarized in that Figure 2-9 were 

conducted in areas that would likely have met one but not both of the current PM2.5 standards. To the 

extent changes in air quality designed to just meet the current suite of PM2.5 standards are undertaken, 

one could reasonably anticipate additional public health protection will occur in these study areas. 

Therefore, the Policy Assessment concludes that these studies are not helpful to inform staff conclusions 

regarding alternative standard levels that are appropriate to consider (US EPA, 2011a, p. 2-87).

With regard to single-city studies that were conducted in areas that would likely have met both 

the current 24-hour and annual standards, the Policy Assessment first considers studies that reported 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 112 of 133

positive and statistically significant associations. In considering this group of studies, the Policy 

Assessment notes Mar et al. (2003) reported a positive and statistically significant association for 

premature mortality in Phoenix with a long-term mean concentration of 13.5 μg/m3in conjunction with 

a 98th percentile value of 32.2 μg/m3(US EPA, 2011a, Figure 2-9). To the extent that consideration is 

given to revising the level of the annual standard, within a range of 13 to 11 μg/m3, as discussed above, 

additional protection would be provided for the effects observed in this study (US EPA, 2011a, p. 2-87). 

Four additional studies reported positive and statistically significant associations with 98th

percentile values within a range of 31.2 to 25.8 μg/m3and long-term mean concentrations within a range 

of 12.1 to 8.5 μg/m3(Delfino et al., 1997; Peters et al., 2001; Stieb et al., 2000; and Mar et al., 2004; US 

EPA, 2011a, Figure 2-9). Delfino et al. (1997) reported statistically significant associations between 

PM2.5 and respiratory emergency department visits for older adults (greater than 64 years old) but not 

young children (less than 2 years old), in one part of the study period (summer 1993) but not the other 

(summer 1992). Peters et al. (2001) reported a positive and statistically significant association between 

short-term exposure to PM2.5 (2-hour and 24-hour averaging times) and onset of acute myocardial 

infarction in Boston. Stieb et al. (2000) reported positive and statistically significant associations with 

cardiovascular- and respiratory-related emergency department visits in Saint John, Canada, in single 

pollutant models but not in multi-pollutant models (US EPA, 2004, pp. 8-154 and 8-252 to 8-253). Mar 

et al. (2004) reported a positive and statistically significant association for short-term PM2.5 exposures in 

relation to respiratory symptoms among children but not adults in Spokane, however, this study had very 

limited statistical power because of the small number of children and adults evaluated. 

The Policy Assessment also considers short-term single-city PM2.5 exposure studies that reported 

positive but nonstatistically significant associations for cardiovascular and respiratory endpoints in areas 

that would likely have met both the current 24-hour and annual standards. The 98th percentile values 

reported in these studies ranged from 31.6 to 17.2 μg/m3and the long-term mean concentrations ranged 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 113 of 133

from 13.0 to 7.0 μg/m3(US EPA, 2011a, Figure 2-9). These studies included consideration of 

cardiovascular-related mortality effects in Phoenix (Wilson et al., 2007), asthma medication use in 

children in Denver (Rabinovitch et al., 2006), hospital admissions for hemorrhagic and ischemic stroke 

in Edmonton, Canada (Villeneuve et al., 2006), and hospital admissions for ischemic stroke/transient 

ischemic attack in Nueces County, TX (Lisabeth et al., 2008). 

Lastly, the Policy Assessment considers single-city studies conducted in areas that would likely 

have met both the current 24-hour and annual standards that reported null findings. The 98th percentile 

values reported in these studies ranged from 29.6 to 24.0 μg/m3and the long-term mean concentrations 

ranged from 10.8 to 8.5 μg/m3(US EPA, 2011a, Figure 2-9). These studies reported no associations with 

short-term PM2.5 exposures and cardiovascular-related hospital admissions and respiratory-related 

emergency department visits (Slaughter et al., 2005) and cardiovascular-related emergency department 

visits (Schreuder et al., 2006) in Spokane; asthma exacerbation in children in Denver (Rabinovitch et al., 

2004); and hospital admissions for transient ischemic attack in Edmonton, Canada (Villeneuve et al., 

2006). 

Viewing the evidence as a whole, the Policy Assessment observes a limited number of singlecity 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. Many of these studies had significant limitations (e.g., limited statistical power, limited 

exposure data) or equivocal results (i.e., mixed results within the same study area) as briefly identified 

above and discussed in more detail in the Policy Assessment (US EPA, 2011a, p. 2-88). Other studies 

reported positive but not statistically significant results or null associations also in areas that would 

likely have met the current suite of PM2.5 standards. Overall, the entire body of results from these singlecity studies is mixed, particularly as 24-hour 98th percentile concentrations go below 35 μg/m3

. 

Although a number of single-city studies report effects at appreciably lower PM2.5 concentrations 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 114 of 133

than multi-city short-term exposure studies, the uncertainties and limitations associated with the singlecity studies were greater and, thus, the Policy Assessment concludes there is less confidence in using 

these studies as a basis for setting the level of a standard. Therefore, the Policy Assessment concludes 

that the multi-city short-term exposure studies provide the strongest evidence to inform decisions on the 

level of the 24-hour standard, and the single-city studies do not warrant consideration of 24-hour 

standard levels different from those supported by the multi-city studies (US EPA, 2011a, p. 2-88). 

In addition to considering the epidemiological evidence, the Policy Assessment takes into 

account air quality information based on county-level 24-hour and annual design values to understand 

the implications of the alternative standard levels supported by the currently available scientific 

evidence, as discussed in section III.E.4.b above. As discussed in section III.A.3 above, the Policy 

Assessment concludes that a policy goal which includes 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, is the most effective and efficient way to 

reduce total population risk associated with both long- and 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. Therefore, the Policy Assessment considers the extent to which different 

combinations of alternative annual and 24-hour standard levels based on the evidence would support this 

policy goal (US 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 examines the implications of three alternative suites of PM2.5 standards identified as 

appropriate to consider based on the currently available scientific evidence, as discussed above. The 

Policy Assessment concludes 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
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 115 of 133

24-hour standard would continue to be the generally controlling standard in the Northwest (US EPA, 

2011a, pp. 2-89 to 2-91 and Figure 2-10). These Northwest counties generally represent 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 

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 concludes that consideration of retaining the 24-hour standard level at 35 

μg/m3 would reflect placing greatest weight on evidence from multi-city studies that reported positive 

and statistically significant associations with health effects classified as having a causal or likely causal 

relationship. In conjunction with lowering the annual standard level, especially within a range of 12 to 

11 μg/m3, this alternative would recognize 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 (US EPA, 2011a, p. 2-91). 

The Policy Assessment also recognizes an alternative approach to considering the evidence that 

provides some support for revising the level below 35 μg/m3, perhaps as low as 30 μg/m3(US EPA, 

2011a, p. 2-92). This alternative 24-hour standard level would be more compatible with an alternative 

annual standard of 13 μg/m3based on placing greater weight on one multi-city short-term exposure 

study (Bell et al., 2008) that reported positive and statistically significant effects at a 98th percentile 

value less than 35 μg/m3(i.e., 34.2 μg/m3) in conjunction with a long-term mean concentration less than 

13 μg/m3(i.e., 12.9 μg/m3). 

Beyond evidence-based considerations, the Policy Assessment also considered the extent to 

which the quantitative risk assessment supports consideration of retaining the current 24-hour standard 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 116 of 133

level or provides 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 provides 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 does not provide strong support 

for considering lower alternative 24-hour standard levels (US EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concludes that while it is appropriate to consider an 

alternative 24-hour standard level within a range of 35 to 30 μg/m3, the currently available evidence 

most strongly supports 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(US EPA, 

2011a, p. 2-92).

d. 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 appropriate to return to the strategy used in 1997 that considers 

the annual and the short-term standards together, with the annual standard as the controlling standard, 

and the short-term standard supplementing the protection afforded by the annual standard” and 

“considers it 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). 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 117 of 133

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 are appropriate to consider, including 

consideration of the combination of these standards supported by the available information (Samet, 

2010d, p. ii). Specifically, CASAC encouraged staff to focus on information related to the 

concentrations that were most influential in generating the health effect estimates in individual studies to 

inform alternative standard levels (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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 118 of 133

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, EPA staff conducted additional analyses and modified their conclusions regarding 

alternative standard levels that are appropriate to consider. The staff conclusions in the final Policy 

Assessment (US EPA, 2011a, section 2.3.4.4) differ somewhat from the alternative standard levels 

discussed in the second draft Policy Assessment (US 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 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, the EPA analyzed 

additional population-level data obtained from several study investigators. In commenting on draft staff 

conclusions in the second draft Policy Assessment, CASAC did not have an opportunity to review the 

staff analyses of distributional statistics conducted by staff to identify the broader range of PM2.5

concentrations that were most influential in generating health effect estimates in epidemiological studies 

(Rajan et al., 2011). In addition, CASAC was not aware of the revised long-term mean PM2.5

concentration in the WHI study as discussed in section III.D.1.a above, specifically footnote 58, or the 

staff’s inclusion of that value in its evaluation of the evidence (i.e., in FigureFigures 1 and 4 above and 

related discussion). The WHI study is the only long-term cohort study that provides information 

regarding effects classified as having evidence of a causal or likely causal relationship associated with a 

long-term PM2.5 concentration below 13 μg/m3. However, as noted in section III.D.1.a above, this study 

was based on only one year of air quality data and linked to effects that occurred in earlier years. 

Furthermore, CASAC did not have an opportunity to review the staff’s revised rationale for the 

combinations of alternative standards suggested in the final Policy Assessment. 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 119 of 133

e. Administrator’s Proposed Conclusions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate alternative standard levels to consider, 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 and the advice of CASAC. As an initial matter, the Administrator 

agrees with the approach discussed in the Policy Assessment as summarized in sections III.A.3 and 

III.E.4.a above, and supported by CASAC, of considering the protection afforded by the annual and 

24-hour standards taken together for mortality and morbidity effects associated with both long- and 

short-term exposures to PM2.5. This is consistent with the approach taken in the review completed in 

1997, in contrast to considering each standard separately, as was done in the review completed in 2006. 

Furthermore, based on the evidence and quantitative risk assessment, the Administrator provisionally 

concludes 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 concludes 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 and so provide appropriate protection. 

In reaching decisions on alternative standard levels to propose, the Administrator judges that it is 

most appropriate to examine where the evidence of associations observed in the epidemiological studies 

is strongest and, conversely, where she has appreciably less confidence in the associations observed in 

the epidemiological studies. 

Based on the characterization and assessment of the epidemiological and other studies presented 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 120 of 133

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 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 takes note that these studies, along with evidence available in the last 

review, provide 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 recognizes the 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 

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 at ambient concentrations similar to those previously observed (as summarized in Figure 3). 

The newly available scientific evidence builds 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 recognizes that the weight of evidence, as 

evaluated in the Integrated Science Assessment, is strongest for health endpoints classified as having 

evidence of a causal relationship. These relationships include those between long- and short-term PM2.5

exposures and mortality and cardiovascular effects. She recognizes that the weight of evidence is also 

strong for health endpoints classified as having evidence of a likely causal relationship, which include 

those between long- and short-term PM2.5 exposures and respiratory effects. In addition, the 

Administrator makes note of the much more limited evidence for health endpoints classified as having 

evidence suggestive of a causal relationship, including developmental, reproductive and carcinogenic 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 121 of 133

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 long- and short-term epidemiological studies and that no discernible threshold for any effects can be 

identified based on the currently available evidence (US EPA, 2009a, section 2.4.3). She also 

recognizes, in taking note of CASAC advice and the distributional statistics analysis discussed in section 

III.E.4.b above and in the Policy Assessment, that there is significantly greater confidence in observed 

associations over certain parts of the air quality distributions in the studies, and conversely, that there is 

significantly diminished confidence in ascribing effects to concentrations toward the lower part of the 

distributions.

Consistent with the general approach summarized in section III.A.3 above, and supported by 

CASAC as discussed in section III.E.4.d above, the Administrator generally agrees that it is appropriate 

to consider a level for an annual standard that is somewhat below the long-term mean PM2.5

concentrations reported in long- and short-term exposure studies, recognizing that the evidence of an 

association in any such study is strongest at and around the long-term average where the data in the 

study are most concentrated. The Administrator notes that long-term mean PM2.5 concentrations are 

available for each study considered and, therefore, represent the most robust data set to inform her 

decisions on appropriate annual standard levels. In considering such long-term mean concentrations, the 

Administrator understands that it is appropriate to consider the weight of evidence for the health 

endpoints evaluated in such studies in giving weight to this information.

Based on the information summarized in Figure 4 and presented in more detail in the Policy 

Assessment (US 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 observes an overall 

pattern of statistically significant associations reported in studies of long-term PM2.5 exposures with 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 122 of 133

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). She 

observes 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 observes a small number of studies related to developmental and 

reproductive effects that reported statistically significant associations for long-term PM2.5 concentrations 

with long-term meansmean concentrations down to 11.9 μg/m3(in Bell et al., 2007).71

The Administrator also considers 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 considers the range from the 25th to the 10th percentiles, consistent with the 

Policy Assessment analyses. More specifically, the Administrator recognizes the 25th percentile as the 

start of the range of PM2.5 concentrations below the mean within which the data become appreciably 

more sparse and, thus, where her confidence in the associations observed in epidemiological studies 

begins to become appreciably less. As discussed in section III.E.4.b above and summarized in Figure 4,

the Administrator takes note of additional population-level data that are available for four studies 

(Krewski et al., 2009; Miller et al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009), each of which 

report statistically significant associations with health endpoints classified as having evidence of a 

causal relationship. In considering the long-term mean PM2.5 concentrations associated with the 25th

percentile values for these four studies, she observes that these values range from somewhat above to 

 

71 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 (US 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.
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 123 of 133

somewhat below 12 μg/m3(Figure 4). The Administrator recognizes that these four studies represent 

some of the strongest evidence available within the overall body of scientific evidence and notes that 

three of these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) were used 

as the basis for concentration-response functions used in the quantitative risk assessment (US EPA, 

2010a, section 3.3.3). However, the Administrator also recognizes that additional population-level data 

are available for only these four studies and, therefore, she believes that these studies comprise a more 

limited data set than one based on long-term mean PM2.5 concentrations for which data are available for 

all studies considered, as discussed above. In considering this information, the Administrator notes 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. 

The Administrator recognizes, 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 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. The Administrator also recognizes that there are additional 

limitations associated with evidence for reproductive and developmental effects, 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 124 of 133

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.

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 standard levels are appropriate to propose, the 

Administrator recognizes, as did EPA staff in the Policy Assessment, that there is no single factor or 

criterion that comprises the “correct” approach to weighing the various types of available evidence and 

information, but rather there are various approaches that are appropriate to consider. The Administrator 

further recognizes 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 leads 

the Administrator to consider various approaches to weighing the evidence so as to identify an 

appropriate range of alternative standard levels, rather than a single level, to propose. In so doing, the 

Administrator encourages 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 notes the advice of CASAC that the 

currently available scientific information, including epidemiological and toxicological evidence, as well 

as the risk and air quality information, provides support for considering an annual standard level within 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 125 of 133

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 recognizes that the Policy Assessment concludes that the available evidence and riskbased information supports consideration of alternative annual standard levels in the range of 13 to 11

μg/m3and further concludes that the evidence most strongly supports 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 observes that the Policy 

Assessment concludes it is appropriate to consider retaining the current 24-hour standard level at 35 

μg/m3in conjunction with an annual standard in the range of 12 to 11 μg/m3, as well as to consider an 

alternative 24-hour standard level of 30 μg/m3, particularly in conjunction with an annual standard level 

of 13 μg/m3. As noted in section III.E.4.d above, the Administrator observes that CASAC did not 

express support for any specific levels or combinations of standards within in these ranges, although she 

recognizes that CASAC did not have an opportunity to review additional information and analyses 

presented in the final Policy Assessment prepared in response to CASAC’s recommendations on the 

second draft Policy Assessment. Nor did CASAC have an opportunity to review the EPA staff’s revised 

rationale for the combinations of alternative standards presented in the final document.

In considering the extent to which the currently available evidence and information provide 

support for specific standard levels within the ranges identified by CASAC and the Policy Assessment 

as appropriate for consideration, the Administrator initially considers standard levels within the range of 

13 to 11 μg/m3for the annual standard. In so doing, the Administrator first considers the long-term mean 

PM2.5 concentrations reported in studies of effects classified as having evidence of a causal or likely 

causal relationship, as summarized in Figure 4 and discussed more broadly above. She notes that a level 

of 13 would be below most but not all the mean concentrations from the multi-city studies of long- and 

short-term exposures, whereas a somewhat lower level, such as 12 μg/m3or lower, would be below all 

such mean concentrations. In considering the appropriate weight to place on this information, the 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 126 of 133

Administrator again notes that the evidence of an association in any such study is 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 are available for each study considered and, therefore, represent the most 

robust data set to inform her decisions on appropriate annual standard levels.

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 notes that a level of 13 would be below the mean concentration 

in one of the three studies, while lower levels such as 12 or 11 μg/m3 would be below the mean 

concentration in two or three of these studies, respectively. In considering the appropriate weight to 

place on this information, the Administrator notes 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.

In considering additional distributional statistics 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, the Administrator notes that a level of 

12 μg/m3is the PM2.5 concentration that generally corresponds with approximately the 25th percentile of 

the distributions of health events data available in three of these studies. The Administrator notes that 

higher standard levels up to 13 μg/m3 would reflect placing significantly less weight on this statistical 

information, whereas lower standard levels down to 11 μg/m3 would reflect placing significantly more 

weight on this information. In considering this information, the Administrator notes that there is no 

bright line that delineates the start of the range of PM2.5 concentrations below the mean within which the 

data become appreciably more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies begins to become appreciably less. 

In considering mean PM2.5 concentrations and distributional statistics from the various sets of 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 127 of 133

epidemiological studies noted above, the Administrator recognizes 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, 

as noted above, a policy approach that uses 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 information from measurements at the monitor within an area that 

records the highest concentration, or the “maximum monitors in an area,monitor”, since, once a standard 

is set, concentrations at appropriate maximum monitors within an area are generally used to determine if 

an area meets a given standard.

The Administrator also recognizes that judgments about the appropriate weight to place on any 

of the factors considereddiscussed above should reflect consideration not only of the relative strength of 

the evidence but also on the important uncertainties that remain in the evidence and information being 

considered in this review. The Administrator notes that the extent to which these uncertainties influence 

judgments about appropriate annual and daily standard levels within the range of 11 to 13 μg/m3 would 

likely be greater for standard levels in the lower part of the range which would necessarily be based on 

fewer available studies than would higher levels within the range.

Based on the above considerations, the Administrator concludes that it is appropriate to propose 

to set a level for the primary annual PM2.5 standard within the range of 12 to 13 μg/m3. The 

Administrator provisionally concludes 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 concludes 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 128 of 133

likely causal relationship, depending, in part, on the emphasis placed on margin of safety considerations. 

The Administrator recognizes that setting an annual standard level at the lower end of this range would 

reflect an approach that places more emphasis on the strength of the evidence, including the analysis of 

the distribution of air quality concentrations most influential for generating health effect estimates in the 

studies, and more emphasis on margin of safety considerations, than would setting a level at the upper 

end. of this range. Conversely, an approach that would support a level at the upper end of this range 

would place greater weight 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.

While the Administrator recognizes that CASAC advised, and the Policy Assessment concluded, 

that the available scientific information provides support for considering a range that extended down to 

11 μg/m3, she concludes that proposing such an extended range would reflect a public health policy 

approach that places more weight on relatively limited evidence and more uncertain information and 

analyses than she considers appropriate at this time. Nonetheless, the Administrator solicits 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 recognizes 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 recognizes that such 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 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 129 of 133

result, in conjunction with proposing an annual standard level in the range of 12 to 13 μg/m3, the 

Administrator provisionally 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, or for PM2.5-related 

effects that may be associated with shorter-than-daily exposure periods. 

Based on the approach discussed in section III.A.3 above, the Administrator has 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 agrees 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 

recognizes that to the extent these studies were conducted in areas that likely did not meet one or both of 

the current standards, such studies do 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 anticipates that additional public health 

protection will occur. Therefore, the Administrator has focused on studies that reported positive and 

statistically significant associations in areas that would likely have met both the current 24-hour and 

annual standards. She has 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/m3to inform her decision regarding an 

appropriate 24-hour standard level. As discussed in section III.E.4.a, the Administrator concludes that 

multi-city, short-term exposure studies provide the strongest data set for informing her decisions on 

appropriate 24-hour standard levels. The Administrator views the single-city, short-term exposure 

studies as a much more limited data set providing mixed results and, therefore, she has less confidence 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 130 of 133

in using these 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 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 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 notes that, to the extent air quality 

distributions are 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 98thpercentile 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 notes that these studies reported longterm 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 recognizes 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. Based 

on this, it would be reasonable to conclude that revision to the 24-hour standard would not be warranted 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 131 of 133

in either case. However, the Administrator has less confidence in judging that a level of 35 μg/m3 would 

be appropriate in conjunction with an annual level at the upper end of the range. Based on the above 

considerations related to the epidemiological evidence, the Administrator provisionally concludes 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 in the proposed range of 12 to 13 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 retaining the 24-hour standard level at 35 μg/m3in conjunction with an annual 

standard level within the proposed range of 12 to 13 μg/m3. She has considered whether this suite of 

standards would meet a public health policy goal which includes 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 concludes that this approach is the most effective and efficient way to 

reduce total population risk associated with both long- and 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 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

(i.e., 12/35) 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 (US 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 132 of 133

Northwest in conjunction with a 24-hour standard level of 35 μg/m3.

Taking the above considerations into account, the Administrator proposes to 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 recognizes 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 

is 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, the final suite of standards will reflect the Administrator’s 

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. 

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 solicits public comment on this range of levels and on approaches to considering the 

available evidence and information that would support the choice of levels within this range. The 

Administrator also solicits public comment on alternative annual standard levels down to 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 
***E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 133 of 133

evidence from epidemiologic studies plays in considering the appropriate annual and 24-hour levels, the 

Administrator solicits public comment on issues related to translating epidemiological evidence into 

standards, including approaches for addressing the uncertainties and limitations associated with this 

evidence.

F. Administrator’s Proposed 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 proposes to revise the 

current primary PM2.5 standards. Specifically, the Administrator proposes to revise: (1) the level of the 

primary annual PM2.5 standard to a level within the range of 12.0 to 13.0 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 allowance 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 proposes to retain the level and form of the primary 24-hour PM2.5 standard to provide 

supplemental protection for areas with high peak PM2.5 concentrations. The Administrator provisionally 

concludes that such a revised suite of standards, including a revised annual standard together with the 

current 24-hour standard, could provide requisite protection against health effects potentially associated 

with long- and short-term PM2.5 exposures. The Administrator is not proposing any revisions to the 

current PM2.5 indicator and the annual and 24-hour averaging times for the primary PM2.5 standards. 

Data handling conventions are specified in proposed revisions to appendix N, as discussed in section VII 

below. The Administrator solicits comment on all aspects of this proposed decision.

Formatted: Font: Italic, No underline

Formatted: Indent: Left: 0", First line: 0.5",

Don't add space between paragraphs of the

same style