From: Lydia Wegman/RTP/USEPA/US

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

Cc: Joseph-J Dougherty/DC/USEPA/US@EPA, Karen Martin/RTP/USEPA/US@EPA, John 

Hannon/DC/USEPA/US@EPA, Beth Hassett-Sipple/RTP/USEPA/US@EPA, Erika 

Sasser/RTP/USEPA/US@EPA

Date: 12/14/2012 10:00 AM

Subject: Our responses to some of your comments. 

Nathan, 

Please find attached our responses to your comments on the primary and secondary standards. 

Karen's note below explains where the changes are. We are now going to start working on the 

redline. Work is continuing on the RIA. Thanks 

Lydia

Karen's note:

The attached files include our edits to address OMB's comments on our revisions to sections III 

(primary standards) and VI (secondary standards) to address OMB's earlier comments. We have 

generally accepted OMB comments except in a few cases we have modified the text of OMB's 

suggested edit. In section III, our modified text occurs on pp. 101 (last sentence in paragraph), 

128, 161, and 183. In section VI, our modified text occurs on p. 465. (For the file on the 

secondary standards, we compared the file we sent with the file OMB sent back so that we could 

see what edits OMB suggested.)
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I. Rationale for Final Decisions on the Primary PM2.5 Standards

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

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

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

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

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

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

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

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

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

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

the Policy Assessment, upon which staff conclusions regarding appropriate 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

nature or degree. 

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

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

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

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

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

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

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

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

different demographic and air pollution characteristics.1

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

evidence presented and assessed in the Integrated Science Assessment have undergone 

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

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

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

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

 

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

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

of the evidence that informs causality determinations.
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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. 

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

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

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

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

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

Although important uncertainties remain in the qualitative and quantitative 

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

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

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

evidence and quantitative assessments has provided a comprehensive and adequate basis 

for regulatory decision making at this time. 

This section describes the integrative synthesis of the evidence and technical 

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

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

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

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

decision draws upon scientific information and analyses related to health effects and 
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risks; judgments about uncertainties that are inherent in the scientific evidence and 

analyses; CASAC advice; and comments received in response to the proposal.

In presenting the rationale for the final decisions on the primary PM2.5 standards, 

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

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

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

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

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

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

public comment on the proposed approach. 

The scientific evidence and quantitative risk assessment were presented in 

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

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

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

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

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

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

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

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

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

A. Background

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

protection from effects associated with fine particle exposures. The annual standard is set 
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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 PM2.5 standards that, in the 

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

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

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

risk-based approaches using information from epidemiological studies to inform 

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

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

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

approach to reaching decisions on what standards are appropriate necessarily requires 

judgments about how to translate the information available from the epidemiological 

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

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

 

2

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

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

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

in the Risk Assessment to inform such judgments.
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concentrations in the studies and the uncertainties in quantitative estimates of risk, in the 

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

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

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

1. General Approach Used in Previous Reviews

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

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

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

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

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

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

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

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

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

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

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

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

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

clearly statistically significant associations with mortality and morbidity effects. Further 

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

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

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

the very limited risk assessment conducted, but did conclude that the risk assessment 
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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 an annual standard (62 FR 38669/3). 

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

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

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

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

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

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

 

3

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

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

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

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

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

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

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

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

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

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

annual standard as the standard best suited to control both annual and daily air quality 

distributions (62 FR 38670).
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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 annual 

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

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

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

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

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

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

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

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

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

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

2, October 17, 2006). 

2. Remand of Primary Annual PM2.5 Standard

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

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

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

including the level of the primary PM2.5 annual standard. The primary 24-hour PM2.5
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standard was not challenged by any of the litigants and, thus, was not considered in the 

court’s review and decision. 

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

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

standard provided the requisite protection from both short- and long-term exposures to 

fine particles including protection for at-risk populations. American Farm Bureau 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

short-term effects). 

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

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

standard considered by the EPA in this review.

3. General Approach Used in the Policy Assessment for the Current Review
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This review is based on an assessment of a much expanded body of scientific 

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

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

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

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

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

alternative standards that are appropriate to consider was broader and more integrative 

than in past reviews. Our general approach also reflected consideration of the issues 

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

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

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

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

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

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

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

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

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

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

types of information available in the current review.

The Policy Assessment concluded it was most appropriate to consider the 

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

long- and short-term exposures, afforded by the annual and 24-hour standards taken 
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together, as was done in the 1997 review, rather than to consider each standard 

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

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

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

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

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

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

of a more regional or more localized nature. 

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

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

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

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

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

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

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

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

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

not consistently correlated. 

 

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

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

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

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

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

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

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

24-hour average PM2.5 concentrations. 559 F. 3d at 520-24. 
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In considering the currently available evidence, the Policy Assessment recognized 

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

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

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

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

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

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

point). The quantitative risk assessments conducted for this and previous reviews 

demonstrated the same point; that is, much, if not most of the aggregate risk associated 

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

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

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

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

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

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

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

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

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

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

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

 

5

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

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

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

Pope and Dockery, 1992).
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overprotecting in other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). 

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

likely to control the bulk of the air quality distribution. 

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

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

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

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

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

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

long- and short-term exposures with more consistency and would likely avoid setting 

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

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

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

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

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

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

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

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

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

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

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

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

The Policy Assessment’s consideration of the protection afforded by the current 
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and alternative suites of standards focused on PM2.5-related health effects associated with 

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

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

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

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

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

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

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

also considering health effects judged to be suggestive of a causal relationship or 

evidence that focused on specific at-risk populations. The Policy Assessment placed 

relatively greater weight on statistically significant associations that yielded relatively 

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

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

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

associations in single-city studies. 

In translating information from epidemiological studies into the basis for reaching 

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

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

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

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

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

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

populations. Further, the Policy Assessment explored the extent to which PM2.5-related 
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health effects had been observed in areas where air quality distributions extend to lower 

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

suite of standards. 

In translating information from epidemiological studies into the basis for reaching 

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

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

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

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

Agency’s general approach for identifying appropriate standard levels for consideration 

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

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

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

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

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

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

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

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

 

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 (U.S. EPA, 2009a, sections 6.5, 6.2.10.10 and 

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

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

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

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

require further investigation” (U.S. EPA, 2009a, section 2.4.3).
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Today’s final decisions also take into account public health policy judgments as to the 

degree of health protection that is to be achieved. 

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

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

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

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

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

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

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

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

addressed this issue considered studies that analyzed confidence intervals around 

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

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

The Policy Assessment explored the extent to which such analyses had 

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

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

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

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

continuing concentration-response relationship. The Policy Assessment concluded that 

 

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

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

consider. 

Further, the Policy Assessment explored other approaches that considered 

different statistical metrics from epidemiological studies. The Policy Assessment first 

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

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

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

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

the strongest evidence of PM2.5-related associations occurs at concentrations around the 

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

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

significant associations with mortality and morbidity effects.

In extending this approach, the Policy Assessment also considered information 

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

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

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

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

 

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

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

composite monitor concentration represents the average concentration across monitors 

within each area with more than one monitor included in a given study as typically 

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

concentrations averaged across multiple monitors or from single monitors within each 

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

This is consistent with the epidemiological evidence considered in other NAAQS 

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

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

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

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

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

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

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

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

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

associations correspondingly became more uncertain. The Policy Assessment recognized 

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

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

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

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

confidence in the associations observed in epidemiological studies.9

In considering distributional statistics from epidemiological studies, the final 

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

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

 

9

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

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

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

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

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

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

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

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

below the mean falls between the 25th and 10th percentiles.
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generating the health effect estimates reported in the epidemiological studies.10

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

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

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

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

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

data.

The Policy Assessment recognized that an approach considering analyses of 

confidence intervals around concentration-response functions was intrinsically related to 

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

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

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

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

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

composite monitor distributions. 

In reaching staff conclusions on alternative standard levels that were appropriate 

 

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

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

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

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

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

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

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

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

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

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

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

(U.S. EPA, 2011a, Figure 2-7 and associated text).
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to consider, the Policy Assessment also included a broader consideration of the 

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

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

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

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

limitations and uncertainties associated with the currently available scientific evidence, 

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

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

and further discussed in section III.B.2 of the proposal.

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

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

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

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

maximum monitors to determine whether an area meets a given standard, inherently has 

the potential to build in some margin of safety (U.S. EPA, 2011a, p. 2-14).11 This 

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

 

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

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

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

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

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

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

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

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

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

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

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

2011a, p. 2-14). See further discussion of this issue in section III.E.4.c.i below.
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Assessment, in which CASAC expressed its preference for focusing on an approach 

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

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

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

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

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

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

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

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

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

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

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

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

considering the epidemiological evidence, the Policy Assessment also considered air 

quality information based on county-level 24-hour and annual design values12 to 

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

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

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

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

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

 

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

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

protection afforded by alternative suites of standards.

Beyond these evidence-based considerations, the Policy Assessment also 

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

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

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

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

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

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

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

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

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

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

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

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

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

additional analyses characterizing the representativeness of the urban study areas within a 

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

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

PM2.5 exposures.

Based on the approach discussed above, the Policy Assessment reached 

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

evidence-based and risk-based considerations to inform two overarching questions 
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related to: (1) the adequacy of the current suite of PM2.5 standards and (2) revisions to the 

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

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

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

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

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

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

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

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

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

standard.

B. Overview of Health Effects Evidence

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

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

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

(Chapter 2) related to health effects associated with fine particle exposures. Section III.B. 

of the proposal discusses available information on the health effects associated with 

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

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

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

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

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

evaluation of the scientific evidence. 
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The 2006 PM NAAQS review concluded that there was “strong epidemiological 

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

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

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

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

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

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

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

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

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

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

our overall understanding of health effects associated with fine particle exposures 

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

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

known about the effects associated with fine particle exposures. 

(2) A number of large multi-city epidemiological studies have been conducted 

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

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

currently available scientific evidence has also been expanded greatly by the 

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

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

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

considers multiple locations representing varying regions and seasons that provide 

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

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

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

 

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

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

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

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

framework for reaching causal determinations with standardized language to express 

evaluation of the evidence (U.S. EPA, 2009a, section 1.5).
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have reported consistent increases in morbidity and/or premature mortality related 

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

cardiovascular-related effects.

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

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

biological mechanisms or pathways for cardiovascular and respiratory effects 

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

coherence and biological plausibility for the effects observed in epidemiological 

studies.

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

prior reviews,

14 the EPA concludes that a causal relationship exists between both 

long- and short-term exposures to PM2.5 and premature mortality and 

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

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

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

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

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

lung cancer mortality).15

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

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

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

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

recent studies investigating long-term exposure to PM2.5 and cardiovascularrelated mortality. The evidence supporting a causal relationship between long-

 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2009a, chapter 5).
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term PM2.5 exposure and mortality also includes consideration of new studies that 

demonstrated an improvement in community health following reductions in 

ambient fine particles.

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

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

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

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

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

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

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

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

hospitalizations and emergency department visits due to cardiovascular disease 

and cardiovascular mortality.

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

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

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

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

that evaluated decrements in lung function growth, increased respiratory 

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

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

emergency department visits and hospital admissions for chronic obstructive 

pulmonary disease (COPD) and respiratory infections. 

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

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

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

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

developmental and reproductive effects comes from epidemiological studies of 

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

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

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

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

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

2009a p. 2-13 ).

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

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

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

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

outcomes nor to exclude any individual component or group of components 

associated with any source categories from the fine particle mixture of concern.
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(10) Specific groups within the general population are at increased risk for 

experiencing adverse health effects related to PM exposures. The currently 

available evidence expands our understanding of previously identified at-risk 

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

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

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

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

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

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

are obese, pregnant women, and the developing fetus.

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

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

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

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

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

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

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

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

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

health effects evidence remain. Epidemiological studies evaluating health effects 

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

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

heterogeneity may be attributed, in part, to differences in the fine particle 

composition or related to exposure measurement error, which can introduce bias 

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

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

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

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

supplement ambient measurements, and our limited understanding of factors that 

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

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

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

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

various pollutants in the ambient mixture (i.e., co-pollutant confounding).16

While uncertainties and limitations still remain in the available health effects 

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

 

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

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

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

and the health effect under study (U.S. EPA, 2004, p. 8-10). 
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stronger and more consistent than in previous reviews providing a strong basis for 

decision making in this review. 

C. Overview of Quantitative Characterization of Health Risks

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

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

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

primary PM2.5 NAAQS.17 As discussed in section III.C of the proposal, the approach used 

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

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

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

uncertainties, by incorporating a number of enhancements, in terms of both the methods 

and data used in the analyses. 

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

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

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

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

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

 

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

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

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

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

considerable input from CASAC and the public as described in section II.B.3 above. 
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selected urban study areas,

18 including, where data were available, consideration of 

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

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

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

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

addition, the quantitative risk assessment included nationwide estimates of the potential 

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

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

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

study areas. 

The expanded and updated risk assessment conducted in this review included 

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

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

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

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

 

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

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

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

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

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

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

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

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

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

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

exclude high PM2.5-risk locations (U.S. EPA, 2010a, section 4.4.1).
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PM2.5 exposure.19

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

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

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

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

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

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

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

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

set of plausible risk estimates surrounding the core estimates. 

The EPA recognized that there were many sources of variability and uncertainty 

inherent in the inputs to its quantitative risk assessment.20 The design of the risk 

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

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

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

characterizing uncertainty in the analyses.21

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

 

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

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

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

persons with pre-existing cardiovascular and respiratory diseases.

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

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

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

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

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

Assessment (U.S. EPA, 2010a, pp. 3-67 to 3-69).
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that contributed to differences in risk across urban study areas, but did not directly affect 

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

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

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

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

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

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

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

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

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

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

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

different rollback approaches when simulating just meeting the current or alternative 

standards; co-pollutant exposures; (3) factors related to demographic and socioeconomic 

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

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

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

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

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

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

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

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

linking PM2.5 to specific health effects; (2) statistical fit of the concentration-response 
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functions for short-term exposure-related health endpoints; (3) shape of the 

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

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

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

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

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

compositional differences in PM2.5.

Beyond characterizing uncertainty and variability, a number of design elements 

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

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

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

the risk model that reflects consideration of the latest research on PM2.5 exposure and risk 

(U.S. EPA, 2010a, section 5.1.1); (2) integration of key sources of variability into the 

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

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

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

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

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

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

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

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

deriving the concentration-response functions where the bulk of the data reside (e.g., 
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within one standard deviation around the long-term mean PM2.5 concentration) (U.S.

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

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

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

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

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

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

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

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

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

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

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

The magnitude of both long- and short-term exposure-related risk estimated to 

remain upon just meeting the current suite of standards as well as alternative standard 

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

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

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

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

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

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

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

associated with short-term exposures was not primarily driven by the small number of 
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days with PM2.5 concentrations in the upper tail of the air quality distribution, but rather 

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

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

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

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

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

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

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

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

levels considered, particularly in the context of long-term exposure-related mortality, 

may be underestimated22. In addition, the Policy Assessment observed that in considering 

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

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

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

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

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

 

22 Based on the consideration of both the qualitative and quantitative assessments of 

uncertainty, the Risk Assessment concluded that it is unlikely that the estimated risks are 

over-stated, particularly for premature mortality related to long-term PM2.5 exposures. In 

fact, the Policy Assessment and the Risk Assessment concluded that the core risk 

estimates for this category of health effects may well be biased low based on 

consideration of alternative model specifications evaluated in the sensitivity analyses 

(U.S. EPA, 2011a, p. 2-41; U.S. EPA, 2010a, p. 5-16; Figures 4-7 and 4-8). In addition, 

the Policy Assessment recognized that the currently available scientific information 

included evidence for a broader range of health endpoints and at-risk populations beyond 

those included in the quantitative risk assessment, including decrements in lung function 

growth and respiratory symptoms in children as well as reproductive and developmental 

effects (U.S. EPA, 2011a, section 2.2.1).
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within one standard deviation of the mean PM2.5 concentration reported). Furthermore, 

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

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

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

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

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

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

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

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

1. Introduction

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

standards is whether, in view of the advances in scientific knowledge and other 

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

Policy Assessment, the existing standards should be retained or revised. In considering 

the adequacy of the current suite of PM2.5 standards, the Administrator has considered the 

large body of evidence presented and assessed in the Integrated Science Assessment

(U.S. EPA, 2009a), the quantitative assessment of risks, staff conclusions and associated 

rationales presented in the Policy Assessment, views expressed by CASAC, and public 

comments. The Administrator has taken into account both evidence- and risk-based 

considerations23 in developing final conclusions on the adequacy of the current primary 

 

23 Evidence-based considerations include the assessment of epidemiological, 

toxicological, and controlled human exposure studies evaluating long- or short-term 

exposures to PM2.5, with supporting evidence related to dosimetry and potential 

pathways/modes of action, as well as the integration of evidence across each of these 
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PM2.5 standards.

a. Evidence- and Risk- based Considerations in the Policy Assessment

In considering the available epidemiological evidence in this review, the Policy 

Assessment took a broader approach than was used in the last review. This approach 

reflected the more extensive and stronger body of evidence available since the last review 

on health effects related to both long- and short-term exposure to PM2.5. As discussed in 

section III.A.3 above, this broader approach focused on setting the annual standard as the 

“generally controlling” standard for lowering both short- and long-term PM2.5

concentrations and so providing requisite protection to public health. In conjunction with 

such an annual standard, this approach focused on setting the 24-hour standard to provide 

supplemental protection against days with high peak PM2.5 concentrations.

In addressing the question whether the evidence now available in this review 

supports consideration of standards that are more protective than the current PM2.5

standards, the Policy Assessment considered whether: (1) statistically significant health 

effects associations with long- or short-term exposures to fine particles occur in areas that 

would likely have met the current PM2.5 standards [see American Trucking Association, 

283 F. 3d at 369, 376 (revision of level of PM NAAQS justified when health effects are 

observed in areas meeting the existing standard)], and (2) associations with long-term 

exposures to fine particles extend down to lower air quality concentrations than had 

 

disciplines, as assessed in the Integrated Science Assessment (U.S. EPA, 2009a) and 

focus on the policy-relevant considerations as discussed in section III.B above and in the 

Policy Assessment (U.S. EPA, 2011a, section 2.2.1). Risk-based considerations draw 

from the results of the quantitative analyses presented in the Risk Assessment (U.S. EPA, 

2010a) and focus on the policy-relevant considerations as discussed in section III.C 

above and in the Policy Assessment (U.S. EPA, 2011a, section 2.2.2).
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previously been observed. With regard to associations observed in long-term PM2.5

exposure studies, the Policy Assessment recognized that extended follow-up analyses of 

the ACS and Harvard Six Cities studies provided consistent and stronger evidence of an 

association with mortality at lower air quality distributions than had previously been 

observed (U.S. EPA, 2011a, pp. 2-31 to 2-32). The original and reanalysis of the ACS 

study reported positive and statistically significant effects associated with a long-term 

mean PM2.5 concentration of 18.2 μg/m3across 50 metropolitan areas for 1979 to 1983 

(Pope et al., 1995; Krewski et al., 2000).24 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 to 1983) to 14.0 μg/m3for 116 

metropolitan areas in the most recent years evaluated (1999-2000), with an overall 

average across the two study periods in 51 metropolitan areas of 17.7 μg/m3(Pope et al., 

2002; Krewski et al., 2009). With regard to the Harvard Six Cities Study, the original and 

reanalysis reported positive and statistically significant effects associated with a longterm mean PM2.5 concentration of 18.0 μg/m3

for 1980 to 1985 (Dockery et al., 1993; 

Krewski et al., 2000). In an extended follow-up of this study, the aggregate long-term 

mean concentration across all years evaluated was 16.4 μg/m3for 1980 to 198825 (Laden 

et al., 2006). In an additional analysis of the extended follow-up of the Harvard Six Cities 

 

24 The study periods referred to in the Policy Assessment (U.S. EPA, 2011a) and in this 

final rule reflect the years of air quality data that were included in the analyses, whereas 

the study periods identified in the Integrated Science Assessment (U.S. EPA, 2009a) 

reflect the years of health event data that were included.

25 Aggregate mean concentration provided by study author (personal communication 

from Dr. Francine Laden, 2009).
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study, investigators reported that the concentration-response relationship was linear and 

“clearly continuing below the level” of the current annual standard (U.S. EPA, 2009a, p. 

7-92; Schwartz et al., 2008). 

Cohort studies conducted since the last review provided additional evidence of 

mortality associated with air quality distributions that are generally lower than those 

reported in the ACS and Harvard Six Cities studies, with effect estimates that were 

similar or, in some studies, significantly greater in magnitude than in the ACS and 

Harvard Six Cities studies (see also, section III.D.1.a of the proposal, 77 FR 38918 to 

28919; U.S. EPA, 2011a, pp. 2-32 to 2-33). The Women’s Health Initiative (WHI) study 

reported positive and most often statistically significant associations between long-term 

PM2.5 exposure and cardiovascular-related mortality as well as morbidity effects, with 

much larger relative risk estimates for mortality than in the ACS and Harvard Six Cities 

studies, at an aggregate long-term mean PM2.5 concentration of 12.9 μg/m3for 2000 

(Miller et al., 2007).26

Using the Medicare cohort, Eftim et al. (2008) reported somewhat higher effect 

estimates than in the ACS and Harvard Six Cities studies with aggregate long-term mean 

concentrations of 13.6 μg/m3and 14.1 μg/m3, respectively, for 2000 to 2002. Zeger et al. 

(2008) reported associations between long-term PM2.5 exposure and mortality for the 

eastern region of the U.S. at an aggregated long-term PM2.5 median concentration of 14.0 

 

26 The Policy Assessment noted that in comparison to other long-term exposure studies, 

the Miller et al. (2007) study was more limited in that it was based on only one year of air 

quality data (U.S. EPA, 2011a, p. 2-82). The proposal further noted that the air quality 

data considered were extrapolated from that one single year of air quality data (2000) to 

the whole study, and that the air quality data post-dated the years of health events 

considered (i.e., 1994 to 1998) (77 FR 38918, fn 62).
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μg/m3, although no association was reported for the western region with an aggregate 

long-term PM2.5 median concentration of 13.1 μg/m3(U.S. EPA, 2009a, p. 7-88).27

Premature mortality in children reported in a national infant mortality study as 

well as mortality in a cystic fibrosis cohort including both children and adults reported 

positive but statistically nonsignificant effects associated with long-term aggregate mean 

concentrations of 14.8 μg/m3and 13.7 μg/m3, respectively (Woodruff et al., 2008; Goss 

et al., 2004).

With respect to respiratory morbidity effects associated with long-term PM2.5

exposure, the across-city mean of 2-week average PM2.5 concentrations reported in the 

initial Southern California Children’s Health Study was approximately 15.1 μg/m3

(Peters et al., 1999). These results were found to be consistent with results of crosssectional analyses of the 24-Cities Study (Dockery et al., 1996; Raizenne et al., 1996), 

which reported a long-term cross-city mean PM2.5 concentration of 14.5 μg/m3.

28 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, 

 

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

central region, with an aggregate long-term mean PM2.5 concentration of 10.7 μg/m3. 

However, in contrast to the eastern and western risk estimates, the central risk estimate 

increased with adjustment for COPD (used as a proxy for smoking status). Due to the 

potential for confounding bias influencing the risk estimate for the central region, the 

Policy Assessment did not focus on the results reported in the central region to inform the 

adequacy of the current suite of standards or alternative annual standard levels (U.S. 

EPA, 2011a, p. 2-32).

28 See American Farm Bureau Federation v. EPA, 559 F. 3d at 525 (noting the 

importance of these studies, as well as EPA’s failure to properly take them into account).
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U.S. EPA, 2009a, Figure 7-4). 

In also considering health effects for which the Integrated Science Assessment 

concluded evidence was suggestive of a causal relationship, the Policy Assessment noted 

a limited number of birth outcome studies that reported positive and statistically 

significant effects related to aggregate long-term mean PM2.5 concentrations down to 

approximately 12 μg/m3(U.S. EPA, 2011a, p. 2-33).

Collectively, the Policy Assessment concluded that currently available evidence 

provided support for associations between long-term PM2.5 exposure and mortality and 

morbidity effects that extend to distributions of PM2.5 concentrations that are lower than 

those that had previously been associated with such effects, with aggregate long-term 

mean PM2.5 concentrations extending to well below the level of the current annual 

standard.

The Policy Assessment also considered the long-term mean PM2.5 concentrations 

in short-term exposure studies in assessing the appropriateness of the level of the current 

annual standard. See American Farm Bureau Federation v. EPA, 559 F. 3d at 522, 523-

24 (remanding 2006 standard because the EPA had not adequately explained its choice 

not to consider long-term means of short-term exposure studies in assessing adequacy of 

primary annual PM2.5 standard). In light of the mixed findings reported in single-city, 

short-term exposure studies, the Policy Assessment placed comparatively greater weight 

on the results from multi-city studies in considering the adequacy of the current suite of 

standards (U.S. EPA, 2011a, pp. 2-34 to 2-35). 

With regard to associations reported in short-term PM2.5 exposure studies, the 

Policy Assessment recognized that long-term mean concentrations reported in new multi-
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city U.S. and Canadian studies provided evidence of associations between short-term 

PM2.5 exposure and mortality at similar air quality distributions to those that had 

previously been observed in an 8-cities Canadian study (Burnett and Goldberg, 2003; 

aggregate long-term mean PM2.5 concentration of 13.3 μg/m3). In a multi-city time-series 

analysis of 112 U.S. cities, Zanobetti and Schwartz (2009) reported a positive and 

statistically significant association with all-cause, cardiovascular-related (e.g., heart 

attacks, stroke), and respiratory-related mortality and short-term PM2.5 exposure, in 

which the aggregate long-term mean PM2.5 concentration was 13.2 μg/m3(U.S. EPA, 

2009a, Figure 6-24). Furthermore, city-specific effect estimates indicated the association 

between short-term exposure to PM2.5 and total mortality and cardiovascular- and 

respiratory-related mortality was consistently positive for an overwhelming majority (99 

percent) of the 112 cities across a wide range of air quality concentrations (long-term 

mean concentrations ranging from 6.6 μg/m3to 24.7 μg/m3; U.S. EPA, 2009a, Figure 6-

24, p. 6-178 to 179). The EPA staff noted that for all-cause mortality, city-specific effect 

estimates were statistically significant for 55 percent of the 112 cities, with long-term 

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).29

With regard to cardiovascular and respiratory morbidity effects, in the first 

analysis of the Medicare cohort conducted by Dominici et al. (2006a) across 204 U.S. 

counties, investigators reported a statistically significant association with hospitalizations 

 

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

Schwartz (2009) were provided by the study authors (personal communication with Dr. 

Antonella Zanobetti, 2009; see also U.S. EPA, 2009a, Figure 6-24). 
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for cardiovascular and respiratory diseases and short-term PM2.5 exposure, in which the 

aggregate long-term mean PM2.5 concentration was 13.4 μg/m3. Furthermore, a subanalysis restricted to days with 24-hour average concentrations of PM2.5 at or below 35 

μg/m3indicated that, in spite of a reduced statistical power from a smaller number of 

study days, statistically significant associations were still observed between short-term 

exposure to PM2.5 and hospital admissions for cardiovascular and respiratory diseases 

(Dominici, 2006b).30 In an extended analysis of this cohort, Bell et al (2008) reported a 

positive and statistically significant increase in cardiovascular hospitalizations associated 

with short-term 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 indicated that effects are occurring in areas that would meet the 

current standards but also suggested that the overall health effects observed across the 

U.S. are not primarily driven by the higher end of the PM2.5 air quality distribution (Bell, 

2009a, personal communication from Dr. Michelle Bell regarding air quality data for Bell 

et al., 2008 and Dominici et al., 2006a). 

Collectively, the Policy Assessment concluded that the findings from short-term 

PM2.5 exposure studies provided evidence of PM2.5-associated health effects occurring in 

 

30 This sub-analysis was not included in the original publication (Dominici et al., 2006a). 

The study authors provided sub-analysis results for the Administrator’s consideration as a 

letter to the docket following publication of the proposed rule in January 2006 (personal 

communication with Dr. Francesca Dominici, 2006b). As noted in section III.A.3, this 

study is part of the basis for the conclusion that there is no evidence suggesting that risks 

associated with long-term exposures are likely to be disproportionately driven by peak 

24-hour concentrations.
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areas that would likely have met the current suite of PM2.5 standards (U.S. EPA, 2011a, p. 

2-35). These findings were further bolstered by evidence of statistically significant 

PM2.5-related health effects occurring in analyses restricted to days in which 24-hour 

average PM2.5 concentrations were below 35 μg/m3(Dominici, 2006b). 

In evaluating the currently available scientific evidence, as summarized in section 

III.B of the proposal, the Policy Assessment first concluded that there was stronger and 

more consistent and coherent support for associations between long- and short-term 

PM2.5 exposures and a broad range of health outcomes than was available in the last 

review, providing the basis for fine particle standards at least as protective as the current 

PM2.5 standards (U.S. EPA, 2011a, p. 2-26). Having reached this initial conclusion, the 

Policy Assessment addressed the question of whether the available evidence supported 

consideration of standards that were more protective than the current standards. In so 

doing, the Policy Assessment considered whether there was now evidence that health 

effect associations have been observed in areas that likely met the current suite of PM2.5

standards. As discussed above, long- and short-term PM2.5 exposure studies provided 

evidence of associations with mortality and cardiovascular and respiratory effects both at 

lower ambient PM2.5 concentrations than had been observed in the previous review and at 

concentrations allowed by the current standards (U.S. EPA, 2011a, p. 2-35). 

In reviewing this information, the Policy Assessment recognized that important 

limitations and uncertainties associated with this expanded body of scientific evidence, as 

discussed in section III.B.2 of the proposal, needed to be carefully considered in 

determining the weight to be placed on the body of studies available in this review. 

Taking these limitations and uncertainties into consideration, the Policy Assessment 
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concluded that the currently available evidence clearly calls into question whether the 

current suite of primary PM2.5 standards protects public health with an adequate margin 

of safety from effects associated with long- and short-term exposures. Furthermore, the 

Policy Assessment concluded this evidence provides strong support for considering fine 

particle standards that would afford increased protection beyond that afforded by the 

current standards (U.S. EPA, 2011a, p. 2-35).

In addition to evidence-based consideration, the Policy Assessment also 

considered the extent to which health risks estimated to occur upon simulating just 

meeting the current PM2.5 standards may be judged to be important from a public health 

perspective, taking into account key uncertainties associated with the quantitative health 

risk estimates. In so doing, the Policy Assessment first noted that the quantitative risk 

assessment addresses: (1) the core PM2.5-related risk estimates; (2) the related uncertainty 

and sensitivity analyses, including additional sets of reasonable risk estimates generated 

to supplement the core analysis; (3) an assessment of the representativeness of the urban 

study areas within a national context;31 and (4) consideration of patterns in design values 

and air quality monitoring data to inform interpretation of the risk estimates, as discussed 

in section III.C 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 

 

31 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 (U.S. EPA, 2011a, p. 2-42). 
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Assessment concluded these risks are important from a public health standpoint and 

provided strong support for consideration of alternative standards that would provide 

increased protection beyond that afforded by the current PM2.5 (U.S. EPA, 2011a, pp. 2-

47 to 2-48). This conclusion reflected consideration of both the severity and the 

magnitude of the effects. For example, the Risk Assessment indicated the possibility that 

premature deaths related to ischemic heart disease associated with long-term PM2.5

exposure alone would likely be on the order of thousands of deaths per year in the 15 

urban study areas upon simulating just meeting the current standards32 (U.S. EPA, 2011a, 

pp. 2-46 to 2-47). Moreover, additional risks were anticipated for premature mortality 

related to cardiopulmonary effects and lung cancer associated with long-term PM2.5

exposure as well as mortality and cardiovascular- and respiratory-related morbidity 

effects (e.g., hospital admissions, emergency department visits) associated with shortterm PM2.5 exposures. Based on the consideration of both qualitative and quantitative 

assessments of uncertainty completed as part of the quantitative risk assessment, the Risk 

Assessment concluded that it was unlikely that the estimated risks are over-stated,

particularly for mortality related to long-term PM2.5 exposure, and may well be biased 

low based on consideration of alternative model specifications evaluated in the sensitivity 

analyses (U.S. EPA, 2010a, p. 5-16; U.S. EPA, 2011a, p. 2-41). Furthermore, the 

currently available scientific information summarized in section III.B of the proposal 

provided evidence for a broader range of health endpoints and at-risk populations beyond 

those included in the quantitative risk assessment (U.S. EPA, 2011a, p. 2-47).

 

32 Premature mortality for all causes attributed to PM2.5 exposure was estimated to be on 

the order of tens of thousands of deaths per year on a national scale based on 2005 air 

quality data (U.S. EPA, 2010a, Appendix G, Table G-1).
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b. CASAC Advice

The CASAC, based on its review of drafts of the Integrated Science Assessment, 

the Risk Assessment, and the Policy Assessment, provided an array of advice both with 

regard to interpreting the scientific evidence and quantitative risk assessment, as well as 

with regard to consideration of the adequacy of the current PM2.5 standards (Samet, 

2009a b,c,d,e,f; Samet 2010a,b,c,d). With regard to the adequacy of the current standards, 

CASAC concluded that the “currently available information clearly calls into question 

the adequacy of the current standards” (Samet, 2010d, p. i) and that the current standards 

are “not protective” (Samet, 2010d, p. 1). Further, in commenting on the first draft Policy 

Assessment, CASAC noted:

With regard to the integration of evidence-based and risk-based considerations, 

CASAC concurs with EPA’s conclusion that the new data strengthens the 

evidence available on associations previously considered in the last round of the 

assessment of the PM2.5 standard. CASAC also agrees that there are significant 

public health consequences at the current levels of the standard that justify 

consideration of lowering the PM2.5 NAAQS further (Samet, 2010c, p.12). 

c. Administrator's Proposed Conclusions Concerning the Adequacy of the Current 

Primary PM2.5 Standards

At the time of the proposal, in considering the body of scientific evidence, the 

Administrator concluded there was stronger and more consistent and coherent support for 

associations between long- and short-term PM2.5 exposure and a broader range of health 

outcomes than was available in the last review, providing the basis for fine particle 

standards at least as protective as the current PM2.5 standards. In particular, the 

Administrator recognized in section III.D.4 of the proposal that the Integrated Science 

Assessment concluded that the results of epidemiological and experimental studies form 

a plausible and coherent data set that supports a causal relationship between long- and 
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short-term PM2.5 exposures and mortality and cardiovascular effects and a likely causal 

relationship between long- and short-term PM2.5 exposures and respiratory effects. 

Furthermore, the Administrator reflected that effects had been observed at lower ambient 

PM2.5 concentrations than what had been observed in the last review, including at 

ambient PM2.5 concentrations in areas that likely met the current PM2.5 NAAQS. With 

regard to the results of the quantitative risk assessment, the Administrator noted that the 

Risk Assessment concluded that the risks estimated to remain upon simulation of just 

meeting the current standards were important from a public health standpoint in terms of 

both the severity and magnitude of the effects. 

At the time of the proposal, in considering whether the current suite of PM2.5

standards should be revised to provide requisite public health protection, the 

Administrator carefully considered the staff conclusions and rationales presented in the 

Policy Assessment, the advice and recommendations from CASAC, and public comments 

to date on this issue. In so doing, the Administrator placed primary consideration on the 

evidence obtained from the epidemiological studies and provisionally found the evidence 

of serious health effects reported in long- and short-term exposure studies conducted in 

areas that would have met the current standards to be compelling, especially in light of 

the extent to which such studies are part of an overall pattern of positive and frequently 

statistically significant associations across a broad range of studies that collectively 

represent a strong and robust body of evidence. 

As discussed in the Integrated Science Assessment and Policy Assessment, the 

Administrator recognized that much progress has been made since the last review in 

addressing some of the key uncertainties that were important considerations in 
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establishing the current suite of PM2.5 standards. For example, progress made since the 

last review provides increased confidence in the long- and short-term exposure studies as 

a basis for considering whether any revision of the annual standard is appropriate and 

increased confidence in the short-term exposure studies as a basis for considering 

whether any revision of the 24-hour standard is appropriate.33

Based on her consideration of these conclusions, as well as consideration of 

CASAC’s conclusion that the evidence and risk assessment clearly called into question 

the adequacy of the public health protection provided by the current PM2.5 NAAQS and 

public comments on the proposal, the Administrator provisionally concluded that the 

current primary PM2.5 standards, taken together, were not requisite to protect public 

health with an adequate margin of safety and that revision was needed to provide 

increased public health protection. The Administrator provisionally concluded that the 

scientific evidence and information on risk provided strong support for consideration of 

alternative standards that would provide increased public health protection beyond that 

afforded by the current PM2.5 standards.

2. Comments on the Need for Revision

This section addresses general comments based on relevant facts that either 

support or oppose any change to the current suite of primary PM2.5 standards. Comments 

on specific long- and short-term exposure studies that relate to consideration of the 

 

33 The EPA notes that this increased confidence in the long- and short-term associations 

generally reflects less uncertainty as to the likely causal nature of such associations, but 

does not address directly the question of the extent to which such associations remain 

toward the lower end of the range of ambient PM2.5 concentrations. This question is 

central to the Agency’s evaluation of the relevant evidence to determine appropriate 

standards levels, as discussed below in section III.E.4.
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appropriate levels of the annual and 24-hour standards are addressed in section III.E.4 

below. Many public comments asserted that the current PM2.5 standards are insufficient 

to protect public health with an adequate margin of safety and that revisions to the 

standards are therefore appropriate, indeed necessitated. 

Among those calling for revisions to the current standards were the Children’s 

Health Protection Advisory Committee (CHPAC); major medical and public health 

groups including the American Heart Association (AHA), American Lung Association 

(ALA), American Public Health Association (APHA), American Thoracic Society 

(ATS); the Physicians for Social Responsibility (PSR); major environmental groups such 

as the Clean Air Council, Clean Air Task Force, Earthjustice, Environmental Defense 

Fund (EDF), National Resources Defense Council (NRDC), and Sierra Club; many 

environmental justice organizations as well as medical doctors, academic researchers, 

health professionals, and many private citizens. For example, the American Heart 

Association and other major national public health and medical organizations stated that, 

“[o]ur organizations are keenly aware of the public health and medical threats from 

particulate matter” and called on the EPA to “significantly strengthen” both the annual 

and 24-hour PM2.5 standards “to help us protect the health of our patients and our nation” 

(AHA et al., 2012, pp 1 and 13). AHA et al. and ALA et al., as well as a group of more 

than 350 physicians, environmental health researchers, and public health and medical 

professionals articulated similar comments on the available evidence:

Ample scientific evidence supports adopting tighter standards to protect the health 

of people who are most susceptible to the serious health effects of these 

pollutants. More than 10,000 peer-reviewed scientific studies have been published 

since 1997 when EPA adopted the current annual standard. These studies validate 

and extend earlier epidemiologic research linking both acute and chronic fine 
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particle pollution with serious morbidity and mortality. The newer research has 

also expanded our understanding of the range of health outcomes associated with 

PM and has identified adverse respiratory and cardiovascular health effects at 

lower exposure levels than previously reported. As discussed and interpreted in 

the EPA’s 2009 Integrated Science Assessment for Particulate Matter, the new 

evidence reinforces already strong existing studies and supports the conclusion 

that PM2.5 is causally associated with numerous adverse health effects in humans 

at exposure levels far below the current standard. Such a conclusion demands 

prompt action to protect human health. (AHA et al., 2012, pp. 1 to 2; ALA et al., 

pp. 4 to 5; similar comment submitted by Rom et al., 2012, p. 1).

All of these medical and public health commenters stated that the current PM2.5

standards need to be revised, and that even more protective standards than those proposed 

by the EPA are needed to adequately protect public health, particularly for at-risk 

populations. Many environmental justice organizations and individual commenters also 

expressed such views.

The National Association of Clean Air Agencies (NACAA), the Northeast States 

for Coordinated Air Use Management (NESCAUM), and many State and local air 

agencies and health departments who commented on the PM2.5 standards supported 

revision of the suite of current PM2.5 standards, as did five state attorneys general 

(Schneiderman et al., 2012) and the National Tribal Air Association (NTAA). 

These commenters based their views chiefly on the body of evidence and 

technical analyses presented and discussed in the Integrated Science Assessment, the 

Risk Assessment, and the Policy Assessment finding the available scientific information 

to be stronger and more compelling than in the last review. These commenters generally 

placed much weight on CASAC’s recommendation to revise the PM2.5 standards to 

provide increased public health protection and on the EPA staff conclusions presented in 

the final Policy Assessment.
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Some of these commenters specifically mentioned extended analyses of seminal 

long-term exposure studies - the ACS (Krewski et al., 2009), Harvard Six Cities (Laden 

et al., 2006), and Southern California Children’s Health (Gauderman et al., 2004) studies. 

These commenters also highlighted the availability of additional long-term exposure

studies in this review, specifically a study of premature mortality in older adults (Eftim et 

al., 2008) and the WHI study of cardiovascular morbidity and mortality effects in women 

(Miller et al., 2007) providing stronger evidence of mortality and morbidity effects 

associated with long-term PM2.5 exposures at lower concentrations than had previously 

been observed, including studies of effects in at-risk populations. For example, some 

commenters asserted:

Evidence during the last review showed clearly that the annual average standard 

needed to be much lower than the standard of 15 μg/m3that was first set in 1997. 

The evidence has only grown since then. Multiple, multi-city studies over long 

periods of time have shown clear evidence of premature death, cardiovascular and 

respiratory harm as well as reproductive and developmental harm at contemporary 

concentrations far below the level of the current (annual) standard (ALA et al., 

2012, p. 39; AHA et al., 2012, p. 10). 

These commenters also highlighted the availability of a number of short-term 

PM2.5 exposure studies as providing evidence of mortality and morbidity effects at 

concentrations below the level of the current 24-hour PM2.5 standard. Specifically, these 

commenters made note of multi-city studies of premature mortality (Zanobetti and 

Schwartz, 2009) and increased hospitalizations for cardiovascular and respiratory-related 

effects in older adults (Bell et al., 2008). These commenters also asserted the importance 

of many of the single-city studies, arguing that these studies “provide valuable 

information regarding impacts on susceptible populations and on health risk in areas with 

high peak to mean concentration ratios” (ALA, et al., 2012, p. 65). Collectively, 
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considering the multi- and single-city short-term exposure studies, these commenters 

asserted “the record clearly supports a more stringent 24-hour standard of 25 μg/m3to 

provide uniform protection in all regions of the country particularly from short-term 

spikes in pollution and from the sub-daily exposures that trigger heart attacks and 

strokes” (ALA et al., 2012, p. 62). A group of more than 350 physicians, environmental 

health researchers, and public health and medical professionals argued, “[s]tudies of 

short-term exposure demonstrate that PM2.5 air pollution increases the risk of hospital 

admissions for heart and lung problems even when you exclude days with pollution 

concentrations at or above the current daily standard of 35 μg/m3. Daily concentrations 

must be capped at lower levels to protect against peak exposure days that occur due to 

local and seasonal sources of emissions” (Rom et al., 2012, p. 2).

In addition, many of these commenters generally concluded that progress had 

been made in reducing many of the uncertainties identified in the last review, in better 

understanding mechanisms by which PM2.5 may be causing the observed health effects, 

and in improving our understanding of at-risk populations. Further, a number of 

commenters argued that by making the standards more protective, the PM2.5 NAAQS 

would be more consistent with other existing standards (e.g., California's annual average 

standard of 12 μg/m3) (CARB, 2012; CA OEHHA, 2012). Other commenters argued that 

revising the primary PM2.5 standards would be more consistent with the 

recommendations of the World Health Organization (WHO) and/or Canada (e.g., ALA et 

al., 2012, p. 62; ISEE, 2012, p. 2; MOE-Ontario, 2012, p. 1). 

With regard to the scope of the literature reviewed for PM2.5-related health 

effects, some commenters asserted that the EPA inappropriately narrowed the scope of 
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the review by excluding a number of categories of relevant studies, specifically related to 

studies of diesel pollution and traffic-related pollution (ALA, et al., 2012, p. 17). These 

commenters argued that, based upon the exclusion of these types of studies, the 

Integrated Science Assessment “came to the erroneous conclusion that the causal 

relationship between PM and cancer is merely suggestive. This conclusion does not 

square with the International Agency Research on Cancer (IARC) finding that diesel 

emissions are a known human carcinogen nor with the conclusions of the extended 

analyses of the [Harvard] Six Cities and ACS cohort studies that report positive and 

statistically significant associations between PM2.5 and lung cancer.” Id.

Some of these commenters also noted the results of the EPA’s quantitative risk 

assessment, concluding that it showed that the risks estimated to remain when the current 

standards are met are large and important from a public health perspective and warrant 

increased protection. For example, ALA et al., noted that the Risk Assessment indicated 

the quantitative risk analyses likely underestimated PM2.5-related mortality (U.S. EPA, 

2010a, p. 5-16) and argued that “the measurements of risk should be treated 

conservatively” (ALA, et al., 2012, p. 73). These commenters also summarized an 

expanded analysis of alternative PM2.5 standard levels that they argued documented the 

need for more protective standards (McCubbin, 2011). 

In general, all of these commenters agreed on the importance of results from the 

large body of scientific studies reviewed in the Integrated Science Assessment and on the 

need to revise the suite of PM2.5 standards as articulated in the EPA’s proposal, while 

generally differing with the EPA’s proposed judgments about the extent to which the 

standards should be revised based on this evidence, specifically for providing protection 
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for at-risk populations. 

The EPA generally agrees with these commenters’ conclusion regarding the need 

to revise the current suite of PM2.5 standards. The scientific evidence noted by these 

commenters was generally the same as that assessed in the Integrated Science 

Assessment and the Policy Assessment, and the EPA agrees that this evidence provides a 

strong basis for concluding that the current PM2.5 standards, taken together, are not 

requisite to protect public health with an adequate margin of safety, and they need to be 

revised to provide increased protection. For reasons discussed in section III.E.4.c below, 

however, the EPA disagrees with aspects of these commenters’ views on the level of 

protection that is appropriate.

The EPA disagrees with these commenters’ views that diesel exhaust studies were 

excluded from the Integrated Science Assessment and were not considered when making 

the causality determination for cancer, mutagenicity, and genotoxicity. As discussed in 

section 7.5 of the Integrated Science Assessment, diesel exhaust studies were integrated 

within the broader body of scientific evidence that was considered in reaching the 

causality determination for these health endpoints. Additionally, as discussed in section

1.5.3 of the Integrated Science Assessment, the evidence from diesel exhaust studies was 

also considered as part of the collective evidence evaluated when making determinations 

for other, noncancer health outcomes (e.g., cardiovascular and respiratory effects).34

 

34 In developing the second draft Integrated Science Assessment, the EPA reexamined the 

controlled human exposure and toxicological studies of fresh diesel and gasoline exhaust 

This information, in addition to other considerations, supported a change in the causal 

determinations for ultrafine particles. Specifically, in reevaluating the causal 

determinations for short-term ultrafine particle exposures and cardiovascular and 

respiratory effects, the EPA changed the classification from “inadequate” to “suggestive”
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Specifically, when evaluating this evidence, the focus was on understanding the effects of 

diesel exhaust particles. 

It is important to recognize that the Integrated Science Assessment focused on 

experimental studies of diesel exhaust that evaluated exposures that were relevant to 

ambient concentrations, i.e., “within one or two orders of magnitude of ambient PM 

concentrations” (U.S. EPA, 2009a, section 1.3). The causal determination for cancer, 

mutagenicity, and genotoxicity presented in the Integrated Science Assessment represents 

an integration of experimental and observational evidence of exposures to ambient PM 

concentrations. The EPA fully considered the findings of studies that assessed these and 

other health effects associated with exposure to diesel particles in reaching causality 

determinations regarding health outcomes associated with PM2.5 exposures. Furthermore, 

CASAC supported the EPA’s change to the causal determination for cancer and longterm PM2.5 concentrations from “inadequate” to “suggestive” (Samet, 2009f, p. 2).

With regard to traffic studies, the EPA disagrees with the commenters’ views that 

traffic studies that focused on exposure indicators such as distance to roadways should 

have been included in the Integrated Science Assessment. These studies were excluded 

from consideration because they did not measure ambient concentrations of specific air 

pollutants, including PM2.5, but instead were studies evaluating exposure to the 

undifferentiated “traffic related air pollution” mixture (ALA et al., 2012, p.17) (U.S.

EPA, 2009a, section 1.3). As a result, these studies do not add to the collective body of 

evidence on the relationship between long- or short-term exposure to ambient 

 

for both categories of health outcomes (Vandenberg, 2009, p. 3). CASAC agreed with the 

EPA’s rationale for revising these causal determinations (Samet, 2009f, p. 10).
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concentrations of PM2.5 and health effects.

Some of these commenters also identified “new” studies that were not included in 

the Integrated Science Assessment as providing further support for the need to revise the 

primary PM2.5 standards. As discussed in section II.B.3 above, the EPA notes that, as in 

past NAAQS reviews, the Agency is basing the final decisions in this review on the 

studies and related information included in the PM air quality criteria that have 

undergone CASAC and public review and will consider the “new” studies for purposes of 

decision making in the next PM NAAQS review. Nonetheless, in provisionally 

evaluating commenters’ arguments (see Response to Comments document), the EPA 

notes that its provisional assessment of “new” science found that such studies did not 

materially change the conclusions in the Integrated Science Assessment (U.S. EPA, 

2012b).

Another group of commenters opposed revising the current PM2.5 standards. 

These views were most extensively presented in comments from the Utility Air 

Regulatory Group (UARG), representing a group of electric generating companies and 

organizations and several national trade associations; the American Petroleum Institute 

(API) representing more than 500 oil and natural gas companies; the National 

Association of Manufacturers (NAM), the American Chemistry Council (ACC), the 

American Fuel & Petroleum Manufacturers (AFPM), the Alliance of Automobile 

Manufacturers (AAM), and other manufacturing associations; the Electric Power 

Research Institute (EPRI); and the Texas Commission on Environmental Quality (Texas 

CEQ). These commenters generally mentioned many of the same studies that were cited 

by the commenters who supported revising the standards, as well as other studies, but 
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highlighted different aspects of these studies in reaching substantially different 

conclusions about their strength and the extent to which progress has been made in 

reducing uncertainties in the evidence since the last review. Furthermore, they asserted 

that the evidence that has become available since the last review does not establish a 

more certain risk or a risk of effects that are significantly different in character to those 

that provided a basis for the current standards, nor does the evidence demonstrate that the 

risk to public health upon attainment of the current standards would be greater than was 

understood when the EPA established the current standards in 2006. 

These commenters generally expressed the view that the current standards provide 

the requisite degree of public health protection. In supporting their view, these 

commenters generally argued that the EPA’s conclusions are inconsistent with the current 

state of the science and questioned the underlying scientific evidence including the causal 

determinations reached in the Integrated Science Assessment. More specifically, this 

group of commenters argued that: (1) the EPA did not apply its framework for causal 

determination consistently across studies or health outcomes and, in the process, the EPA 

relied on a selective group of long- and short-term exposure studies to reach conclusions 

regarding causality; (2) toxicological and controlled human exposure studies do not 

provide supportive evidence that the health effects observed in epidemiological studies 

are biologically plausible; (3) uncertainties in the underlying health science are as great 

or greater than in 2006; (4) there is no evidence of greater risk since the last review to 

justify tightening the current annual PM2.5 standard; and (5) “new” studies not included in 

the Integrated Science Assessment continue to increase uncertainty about possible health 

risks associated with exposure to PM2.5. These comments are discussed in turn below.
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(l) Some of these commenters asserted that the EPA did not apply its framework 

for causal determinations consistently across studies or health outcomes (ACC, 2012, 

Attachment A, pp. 1 to 2; API, 2012, Attachment 1, p. 30; NAM et al., 2012, pp. 22 to 

25; Texas CEQ, 2012, pp 2 to 3).35 These commenters argued that the EPA downplayed 

epidemiological studies with null or inconsistent results, inappropriately used the Hill 

criteria when evaluating the epidemiological evidence, and used the same study and the 

same underlying database to conclude that there was a causal association between 

mortality and multiple criteria pollutants.

The EPA disagrees with these commenters’ views. First, the EPA recognizes that 

the evaluation of the scientific evidence and its application of the causal framework used 

in the current PM NAAQS review was the subject of exhaustive and detailed review by 

CASAC and the public. As summarized in section II.B.3 above, prior to finalizing the 

Integrated Science Assessment, two drafts were released for CASAC and public review 

to evaluate the scientific integrity of the documents. Evidence related to the substantive

issues raised by CASAC and public commenters with regard to the content of the first 

and second draft Integrated Science Assessments were discussed at length during these 

public CASAC meetings and considered in developing the final Integrated Science 

Assessment. CASAC supported the development of the EPA’s causality framework and 

its use in the current PM NAAQS review and concluded:

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

 

35 The EPA notes that the same concerns about the causal determinations presented in the 

Integrated Science Assessment were raised in comments to CASAC on the draft 

Integrated Science Assessments (e.g., UARG, 2009; API, 2009; ACC, 2012, Appendix 

B). CASAC, therefore, had the opportunity to consider these comments in reaching 

consensus conclusions on this issue. 
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systematically applied; this approach has provided transparency and a clear 

statement of the level of confidence with regard to causation, and we recommend 

its continued use in future Integrated Science Assessments (Samet 2009f, p. 1). 

These commenters asserted that during the application of the causal framework

the EPA inappropriately relied on a selective group of long- and short-term exposure 

studies in reaching causal inferences (API, 2012, pp 12 to 17; ACC, 2012, Attachment A, 

pp 1 to 2; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). Additionally, 

these commenters expressed the view that the EPA focused on a subset of 

epidemiological studies that reported positive and statistically significant results while 

ignoring other studies, especially those that reported no statistically significant

associations, those that reported potential thresholds, or those that highlighted 

uncertainties and limitations in study design or results. Furthermore, some of these 

commenters argued that epidemiological studies are observational in nature and cannot 

provide evidence of a causal association.

The EPA disagrees with these commenters’ views on assessing the health effects 

evidence and on the conclusions regarding the causality determinations reached in the 

Integrated Science Assessment. In conducting a comprehensive evaluation of the 

evidence in the Integrated Science Assessment, the EPA recognized the distinction 

between the evaluation of the relative scientific quality of individual study results and the 

evaluation of the pattern of results within the broader body of scientific evidence and 

considered both in reaching causality determinations. The more detailed characterizations 

of individual studies included an assessment of the quality of the study based on specific 

criteria as described in the Integrated Science Assessment (U.S. EPA, 2009a, section 

1.5.3). 
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In developing an integrated assessment of the health effects evidence for PM, the 

EPA emphasized the importance of examining the pattern of results across various 

studies and did not focus solely on statistical significance36 as a criterion of study 

strength. This approach is consistent with views clearly articulated throughout the 

epidemiological and causal inference literature, specifically, that it is important not to 

focus on results of statistical tests to the exclusion of other information.37 The concepts 

underlying the EPA’s approach to evaluating statistical associations have been discussed 

in numerous publications, including a report by the U.S. Surgeon General on the health 

consequences of smoking (Centers for Disease Control and Prevention, 2004). This 

report cautions against over-reliance on statistical significance in evaluating the overall 

evidence for an exposure-response relationship. Criteria characterized by Hill (1965) also 

addressed the value, or lack thereof, of statistical tests in the determination of cause: 

No formal tests of significance can answer those [causal] questions. Such tests 

can, and should, remind us of the effects the play of chance can create, and they 

will instruct us in the likely magnitude of those effects. Beyond that, they 

contribute nothing to the ‘proof’ of our hypothesis (Hill, 1965, p. 299). 

The statistical significance of individual study findings has played an important 

 

36 Statistical significance is an indicator of the precision of a study’s results, which is 

influenced by a variety of factors including, but not limited to, the size of the study, 

exposure and measurement error, and statistical model specifications. Studies typically 

calculate “p-values” to determine whether the study results are statistically significant or 

whether the study results are likely to occur simply by chance. In general practice, effects 

are considered statistically significant if p values are less than 0.05.

37 For example, Rothman (1998) stated, “Many data analysts appear to remain oblivious 

to the qualitative nature of significance testing [and that]... statistical significance is itself 

only a dichotomous indicator. As it has only two values, significant or not significant 

....Nevertheless, p-values still confound effect size with study size, the two components 

of estimation that we believe need to be reported separately.” As a result, Rothman 

recommended that p-values be omitted as long as point and interval estimates are 

available. 
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role in the EPA’s evaluation of the study’s results and the EPA has placed greater 

emphasis on studies reporting statistically significant results. However, in the broader 

evaluation of the evidence from many epidemiological studies, and subsequently during 

the process of forming causality determinations in integrating evidence across 

epidemiological, controlled human exposure, and toxicological studies, the EPA has 

emphasized the pattern of results across epidemiological studies, and whether the effects 

observed were coherent across the scientific disciplines for drawing conclusions on the 

relationship between PM2.5 and different health outcomes. Thus, the EPA did not limit its 

focus or consideration to just studies that reported positive associations or where the 

results were statistically significant. 

In addition, some commenters asserted that the EPA inappropriately used the Hill 

criteria by failing to consider the limitations of studies with weak associations, thereby 

overstating the consistency of the observed associations (API, 2012, Attachment 1, pp. 30 

to 35). These commenters argued that risk estimates greater than 3 to 4 reflect strong 

associations supportive of a causal link, while smaller risk estimates (i.e., 1.5 to 3) are 

considered to be weak and require other lines of evidence to demonstrate causality. 

As discussed in section 1.5.3 of the Integrated Science Assessment, the EPA 

thoroughly considered the limitations of all studies during its evaluation of the scientific 

literature (U.S. EPA,, 2009a, p. 1-14). This collective body of evidence, including known 

uncertainties and limitations of the studies evaluated, were considered during the process 

of forming causality determinations as discussed in chapters 6 and 7 of the Integrated 

Science Assessment. For example, the EPA concluded that “a causal relationship exists 

between short-term PM2.5 exposure and cardiovascular effects,” however, in reaching this 
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conclusion, the Agency recognized and considered limitations of the current evidence 

that still requires further examination (U.S. EPA, 2009a., in section 6.2.12.1). Therefore, 

the EPA disagrees with these commenters’ views that the Hill criteria were 

inappropriately used in that the limitations of studies were not considered. 

The EPA also disagrees with the commenters’ assertion that the magnitude of the 

association must be large to support a determination of causality. As discussed in the 

Integrated Science Assessment, the strength of the observed association is an important 

aspect to aid in judging causality and “while large effects support causality, modest 

effects therefore do not preclude it” (U.S. EPA, 2009a, Table 1-2, section 1.5.4). The 

weight of evidence approach used by the EPA encompasses a multitude of factors of 

which the magnitude of the association is only one component (U.S. EPA, 2009a, Table 

1-3). An evaluation of the association across multiple investigators and locations supports 

the “reproducibility of findings [which] constitutes one of the strongest arguments for 

causality” (U.S. EPA, 2009a, Table 1-2). Even though the risk estimates for air pollution 

studies may be modest, the associations are consistent across hundreds of studies as 

demonstrated in the Integrated Science Assessment. Furthermore, the causality 

determinations rely on different lines of evidence, by integrating evidence across 

disciplines, including animal toxicological studies and controlled human exposure 

studies.

Furthermore, as summarized in section III.B above and discussed more fully in 

section III.B.3 of the proposal, the EPA recognizes that the population potentially 

affected by PM2.5 is considerable, including large subgroups of the U.S. population that 

have been identified as at-risk populations (e.g., children, older adults, persons with 
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underlying cardiovascular or respiratory disease). While individual effect estimates from 

epidemiological studies may be modest in size, the public health impact of the mortality 

and morbidity associations can be quite large given that air pollution is ubiquitous. 

Indeed, with the large population exposed, exposure to a pollutant causally associated at a 

population level with mortality and serious illness has significant public health 

consequences, virtually regardless of the relative risk. Taken together, this information 

indicates that exposure to ambient PM2.5 concentrations has substantial public health 

impacts.

In addition, these commenters believed that the EPA downplayed null or 

inconsistent findings in numerous long-term mortality studies with reported PM2.5

concentrations above and below the level of the current annual standard. The EPA 

disagrees that studies with null or inconsistent findings were not accurately presented and 

considered in the Integrated Science Assessment. For example, as discussed throughout 

section 7.6 and depicted in Figures 7-6 and 7-7 of the Integrated Science Assessment, the 

EPA presented the collective evidence from all studies that examined the association 

between long-term PM2.5 exposure and mortality. Overall, across these studies there was 

evidence of consistent positive associations in different cohorts. That evidence, in 

combination with the biological plausibility provided by experimental and toxicological 

studies evaluated in sections 7.1 and 7.2 of the Integrated Science Assessment, supported 

a causal relationship exists between long-term PM2.5 exposure and mortality. 

Lastly, some of these commenters argued that in some cases, the EPA used the 

same study and the same underlying database to conclude that there is a causal 

association between mortality and multiple criteria pollutants. These commenters argued, 
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“[i]n doing so, EPA attributes the cause of the mortality effects observed to whichever 

criteria pollutant it is reviewing at the time” (API, 2012, pp. 14 to 16). 

The EPA strongly disagrees that the Agency “attributes the cause of mortality 

effects observed to whichever criteria pollutant it is reviewing at the time.” The EPA 

consistently recognizes that other pollutants are also associated with health outcomes, as 

is reflected in the fact that the EPA has established regulations to limit emissions of 

particulate criteria pollutants as well as other gaseous criteria pollutants. Epidemiological

studies often examine the association between short- and long-term exposures to multiple 

air pollutants and mortality within a common dataset in an attempt to identify the air 

pollutant(s) of the complex mixture most strongly associated with mortality. In evaluating 

these studies, the EPA employs specific study selection criteria to identify those studies 

most relevant to the review of the NAAQS. In its assessment of the health evidence 

regarding PM2.5, the EPA has carefully evaluated the potential for confounding, effect 

measure modification, and the role of PM2.5 as a component of a complex mixture of air 

pollutants (U.S. EPA, 2009, p. 1-9). The EPA used a rigorous weight of evidence 

approach to inform causality that evaluated consistency across studies within a discipline, 

evidence for coherence across disciplines, and biological plausibility. Additionally, 

during this process, the EPA assessed the limitations of each study in the context of the 

collective body of evidence. It was the collective evidence, not one individual study that 

ultimately determined whether a causal relationship exists between a pollutant and health 

outcome. In the Integrated Science Assessment, the combination of epidemiological and 

experimental evidence formed the basis for the Agency concluding for the first time that 

a causal relationship exists between short- or long-term exposure to a criteria pollutant 
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and mortality (U.S. EPA, 2009, sections 2.3.1.1 and 2.3.1.2). 

Additionally, while the EPA has evaluated some of the studies used to inform the 

causality determination for PM in the Integrated Science Assessments for other criteria 

air pollutants, the Agency has done so in the context of examining the collective body of 

evidence for each of the respective criteria air pollutants. As such, the body of evidence 

to inform causality has varied from pollutant to pollutant resulting in the association 

between each criteria air pollutant and mortality being classified at a different level of the 

five-level hierarchy used to inform causation (e.g., U.S. EPA, 2008e, U.S. EPA, 2008f, 

U.S. EPA, 2010k). 

The EPA notes that the final causality determinations presented in the Integrated 

Science Assessment reflected CASAC’s recommendations on the second draft Integrated 

Science Assessment (Samet, 2009f, pp. 2 to 3). Specifically, CASAC supported the 

EPA’s changes (in the second versus first draft Integrated Science Assessment) from 

“likely causal” to “causal” for long-term exposure to PM2.5 and cardiovascular effects and 

for cancer and PM2.5 (from “inadequate” to “suggestive”). Id. Furthermore, CASAC 

recommended “upgrading” the causal classification for PM2.5 and total mortality to 

“causal” for both the short- and long-term timeframes. Id. With regard to mortality, the 

“EPA carefully reevaluated the body of evidence, including the collective evidence for 

biological plausibility for mortality effects, and determined that a causal relationship 

exists for short- and long-term exposure to PM2.5 and mortality, consistent with the 

CASAC comments” (Jackson, 2012). 

(2) With regard to toxicological and controlled human exposure studies, these 

commenters argued that the available evidence does not provide coherence or biological 
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plausibility for health effects observed in epidemiological studies (API, 2012, pp. 21 to 

22, Attachment 1, pp. 25 to 29; AAM, 2012, pp. 15 to 16; Texas CEQ, 2012, p. 3). With 

regard to the issue of mechanisms, these commenters noted that although the EPA 

recognizes that new evidence is now available on potential mechanisms and plausible 

biological pathways, the evidence provided by toxicological and controlled human 

exposure studies still does not resolve all questions about how PM2.5 at ambient 

concentrations could produce the mortality and morbidity effects observed in 

epidemiological studies. More specifically, for example, some of these commenters 

argued that:

A review of the Integrated Science Assessment, however, suggests that the 

experimental evidence is inconsistent and not coherent with findings in 

epidemiology studies. Specifically, the findings of mild and reversible effects in 

most experimental studies conducted at elevated exposures are not consistent with 

the more serious associations described in epidemiology studies (e.g., hospital 

admissions and mortality). Also, both animal studies and controlled human 

exposure studies have identified no effect levels for acute and chronic exposure to 

PM and PM constituents at concentrations considerably above ambient levels. 

EPA should consider the experimental findings in light of these higher exposure 

levels and what the relevance may be for ambient exposures (API, 2012, 

Attachment 1, p. 25).

The EPA notes that in the review completed in 1997, the Agency considered the 

lack of demonstrated biological mechanisms for the varying effects observed in 

epidemiological studies to be an important caution in its integrated assessment of the 

health evidence upon which the standards were based (71 FR 61157, October 17, 2006). 

In the review completed in 2006, the EPA recognized the findings from additional 

research that indicated that different health responses were linked with different particle 

characteristics and that both individual components and complex particle mixtures 

appeared to be responsible for many biologic responses relevant to fine particle 
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exposures. Id. Since that review, there has been a great deal of research directed toward 

advancing our understanding of biologic mechanisms. While this research has not 

resolved all questions, and further research is warranted (U.S. EPA, 2011a, section 2.5), it 

has provided important insights as discussed in section III.B.1 of the proposal (77 FR at 

38906 to 38909) and discussed more fully in the Integrated Science Assessment (U.S.

EPA, 2009a, Chapter 5). 

As noted in the proposal, toxicological studies provide evidence to support the 

biological plausibility of cardiovascular and respiratory effects associated with long- and 

short-term PM exposures observed in epidemiological studies (77 FR 38906) and provide 

supportive mechanistic evidence that the cardiovascular morbidity effects observed in 

long-term exposure epidemiological studies are coherent with studies of cardiovascularrelated mortality (77 FR 38907). The Integrated Science Assessment concluded that the 

new evidence available in this review “greatly expands” upon the evidence available in 

the last review “particularly in providing greater understanding of the underlying 

mechanisms for PM2.5 induced cardiovascular and respiratory effects for both short- and 

long-term exposures” (U.S. EPA, 2009a, p. 2-17). The mechanistic evidence now 

available, taken together with newly available epidemiological evidence, increases the 

Agency’s confidence that a causal relationship exists between long- and short-term 
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exposure to PM2.5 and cardiovascular effects and mortality.38 In addition, CASAC 

supported the Integrated Science Assessment approach and characterization of potential 

mechanisms or modes of action (Samet, 2009e, pp. 7 to 8; Samet, 2009f, p. 11), as well 

as the findings of a causal relationship at the population level between exposure to PM2.5

and mortality and cardiovascular effects (Samet, 2009f, pp. 2 to 3).39

Additionally, the EPA disagrees with commenters that the mild and reversible 

effects observed in controlled human exposure studies are inconsistent with the more 

serious effects observed in epidemiological studies. Ethical considerations regarding the 

types of studies that can be performed with human subjects generally limit the effects that 

can be evaluated to those that are transient, reversible, and of limited short-term 

consequence. The relatively small number of subjects recruited for controlled exposure 

studies should also be expected to have less variability in health status and risk factors 

 

38 See American Trucking Association v. EPA, 175 F. 3d 1027, 1055-56 (D.C. Cir. 1999) 

reversed in part and affirmed in part sub nom, Whitman v. American Trucking Assn’s, 

531 U.S. 457 (2001) holding that the EPA could establish NAAQS without identifying a 

biological mechanism (“To begin with, the statute itself requires no such proof. The 

Administrator may regulate air pollutants "emissions of which, in his judgment, cause or 

contribute to air pollution which may reasonably be anticipated to endanger public health 

or welfare." (emphasis added by the court). Moreover, this court has never required the 

type of explanation petitioners seek from EPA. In fact, we have expressly held that EPA's 

decision to adopt and set air quality standards need only be based on ‘reasonable 

extrapolations from some reliable evidence’.... Indeed, were we to accept petitioners' 

view, EPA (or any agency for that matter) would be powerless to act whenever it first 

recognizes clear trends of mortality or morbidity in areas dominated by a particular 

pathogen.”).
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than occurring in the general population.40 Consequently, the severity of health effects 

observed in controlled human exposure studies evaluating the effects of PM should be 

expected to be less than observed in epidemiologic studies. Nonetheless, that effects are 

observed in relatively healthy individuals participating in controlled exposure studies 

serves as an indicator that PM is initiating health responses and that more severe 

responses may reasonably be expected in a more diverse population. 

It should also be noted that there is a small body of toxicological evidence 

demonstrating mortality in rodents exposed to PM (e.g., Killingsworth et al. 1997; 

Godleski et al. 1996). Overall it is not surprising that lethality is not induced in more 

toxicological research, as these types of studies do not readily lend themselves to this 

endpoint. Epidemiological studies have observed associations between PM and mortality 

in communities with populations in the range of many thousands to millions of people. 

Clearly, it is not feasible to expose hundreds (if not thousands) of animals to ambient PM 

(potentially over many years) in a laboratory setting to induce enough lethalities to 

 

40 For example, the EPA excludes from its controlled human exposure studies involving 

exposure to PM2.5 any individual with a significant risk factor for experiencing adverse 

effects from such exposure. Thus, the EPA excludes a priori the following categories of 

persons: those with a history of angina, cardiac arrhythmias, and ischemic myocardial 

infarction or coronary bypass surgery; those with a cardiac pacemaker; those with 

uncontrolled hypertension (greater than 150 systolic and 90 diastolic); those with 

neurogenetive diseases; those with a history of bleeding diathesis; those taking betablockers; those using oral anticoagulants; those who are pregnant, attempting to become 

pregnant, or breastfeeding; those who have experienced a respiratory infection within 

four weeks of exposure; those experiencing eye or abdominal surgery within six weeks of 

exposure; those with active allergies; those with a history of chronic illnesses such as 

diabetes, cancer, rheumatologic diseases, immunodeficiency state, known cardiovascular 

disease, or chronic respiratory diseases; smokers. The EPA “Application for Independent 

Review Board Approval of Human Subjects Research: Cardiopulmonary Effects of 

healthy Older GSTM1 Null and Sufficient individuals to Concentrated Ambient Air 

Particles (CAPTAIN)”, Nov. 9, 2011, p. 9.
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distinguish between natural deaths and those attributable to PM. Furthermore, the 

heterogeneous human populations sampled in epidemiological studies are comprised of 

individuals with different physical, genetic, health, and socioeconomic backgrounds 

which may impact the outcome. However, in toxicological studies, the rodent groups are 

typically inbred, such that inter-individual variability is minimized. Thus, if the rodent 

strain used is quite robust, PM-induced effects may not be observed at low exposure 

concentrations.

(3) In asserting that the uncertainties in the underlying health science are as great 

or greater than in the last review and therefore do not support revision to the standards at 

this time, commenters in this group variously discussed a number of issues related to: (a) 

confounding, (b) heterogeneity in risk estimates, (c) exposure measurement error, (d) 

model specification, (e) the concentration-response relationship, and (f) understanding 

the relative toxicity of components within the mixture of fine particles. Each of these 

issues is addressed below and some are discussed in more detail in the Response to 

Comments document. 

In summary, these commenters concluded that the substantial uncertainties 

present in the last review have not been resolved and/or that the uncertainty about the 

possible health risks associated with PM2.5 exposure has not diminished. As discussed 

below, the EPA believes that the overall uncertainty about possible health risks 

associated with both long- and short-term PM2.5 exposure has diminished to an important 

degree since the last review. While the EPA agrees that important uncertainties remain, 

and that future research directed toward addressing these uncertainties is warranted, the 

EPA disagrees with commenters’ views that the remaining uncertainties in the scientific 
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evidence are too great to warrant revising the current PM2.5 NAAQS.

(a) Confounding 

Some commenters have criticized the EPA for not adequately addressing the issue 

of confounding in both long- and short-term exposure studies of mortality and morbidity. 

This includes confounding due to copollutants, as well as unmeasured confounding.41

With regard to copollutant confounding, these commenters asserted that the EPA 

has not adequately interpreted the results from studies that examined the effect of 

copollutants on the relationship between long- and short-term PM2.5 exposures and 

mortality and morbidity outcomes. These commenters contend that the EPA has 

inappropriately concluded that PM2.5-related mortality and morbidity associations are 

generally robust to confounding. The commenters stated that statistically significant 

PM2.5 associations in single-pollutant models in epidemiological studies do not remain 

statistically significant in copollutant models. 

The loss of statistical significance or the reduction in the magnitude of the effect 

estimate when a co-pollutant model is used may be the result of factors other than 

confounding. These changes do not prove either the existence or absence of confounding. 

These impacts must be evaluated in a broader context that considers the entire body of 

 

41 The Integrated Science Assessment defines confounding as “a confusion of effects. 

Specifically, the apparent effect of the exposure of interest is distorted because the effect 

of an extraneous factor is mistaken for or mixed with the actual exposure effect (which 

may be null) (Rothman and Greenland, 1998)” (U.S. EPA, 2009a, p. 1-16). 

Epidemiological analyses attempt to adjust or control for these characteristics (i.e., 

potential confounders) that differ between exposed and non-exposed individuals (U.S. 

EPA, 2009a, section 1.5.3). Not all risk factors can be controlled for within a study 

design/model and are termed “unmeasured confounders.” An unmeasured confounder is a 

confounder that has not previously been measured and therefore is not included in the 

study design/model. 
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evidence. The broader examination of this issue in the Integrated Science Assessment 

included a focus on evaluating the stability of the size of the effect estimates in 

epidemiological studies conducted by a number of research groups using single- and 

copollutant models (U.S. EPA, 2009a, sections 6.2.10.9, 6.3.8.5, and 6.5, Figures 6-5, 6-

9, and 6-15). This examination found that, for most epidemiological studies, there was 

little change in effect estimates based on single- and copollutant models, although the 

Integrated Science Assessment recognized that in some cases, the PM2.5 effect estimates 

were markedly reduced in size and lost statistical significance. Additionally, the EPA 

notes that these comments do not adequately reflect the complexities inherent in 

assessing the issue of copollutant confounding. As discussed in the proposal (77 FR 

38907, 38909, and 38910) and more fully in the Integrated Science Assessment 

(U.S.EPA, 2009a, sections 6.2, 6.3, and 6.5), although copollutant models may be useful 

tools for assessing whether gaseous copollutants may be potential confounders, such 

models alone cannot determine whether copollutants are in fact confounders. 

Interpretation of the results of copollutant models is complicated by correlations that 

often exist among air pollutants, by the fact that some pollutants play a role in the 

atmospheric reactions that form other pollutants such as secondary fine particles, and by 

the statistical power of the studies in question inherent in the study methodology. For 

example, the every-third or sixth-day sampling schedule often employed for PM2.5

measurements compared to daily measurements of gaseous copollutants drastically 

reduces the overall sample size to assess the effect of copollutants on the PM2.5-morbidity 

or mortality relationship, such that the reduced sample size can lead to less precise effect 

estimates (e.g., wider confidence intervals).
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The EPA recognizes that when PM2.5 is correlated with gaseous pollutants it can 

be difficult to identify the effect of individual pollutants in the ambient mixture (77 FR 

38910). However, based on the available evidence, the EPA concludes epidemiological 

studies continue to support the conclusion that PM2.5 associations with mortality and 

morbidity outcomes are robust to the inclusion of gaseous copollutants in statistical 

models. The EPA evaluated the potential confounding effects of gaseous copollutants 

and, although it is recognized that uncertainties and limitations still remain, the Agency 

concluded the collective body of scientific evidence is “stronger and more consistent than 

in previous reviews providing a strong basis for decision making in this review" (77 FR 

38910/1). 

Several commenters offered detailed comments on the long-term PM2.5 exposure 

studies arguing that associations from mortality studies are subjected to unmeasured 

confounding and as a result are not appropriately characterized as providing evidence of a 

causal relationship between long-term PM2.5 exposure and mortality (e.g., UARG, 2012, 

pp. 10 to 11, Attachment A, pp. 17 to 23; API, 2012, pp. 13 to 14, Attachment 1, pp. 11 

to 14, Attachment 7, pp. 2-10; ACC, 2012, p. 18 to 21; AFPM, 2012, p. 8; Texas CEQ, 

2012, p. 4). Specifically, commenters cited two studies (i.e., Janes et al., 2007 and 

Greven et al., 2011) that used a new type of statistical analysis to examine associations 

between annual (long-term) and monthly (sub-chronic) PM2.5 exposure and mortality. 

The commenters interpreted the results of these analyses as evidence of unmeasured 

confounding in the long-term PM2.5 exposure-mortality relationship. These commenters 

interpreted these studies as raising fundamental questions regarding the EPA’s 

determination that a causal relationship exists between long-term PM2.5 exposure and 
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mortality. In addition to the commenters mentioned above, all of the authors of the 

publications by Janes et al. (2007) and Greven et al. (2011) (i.e., Francesca Dominici, 

Scott Zeger, Holly Janes, and Sonja Greven) submitted a joint comment to the public 

docket in order to clarify specific points regarding these two studies (Dominici et al., 

2012).

The first study, Janes et al. (2007), was evaluated in the Integrated Science 

Assessment (U.S. EPA, 2009a, p. 7-88). The second study, Greven et al. (2011), an 

extension of the Janes et al. (2007) study adding three more years of data, is a “new” 

study discussed in the Provisional Science Assessment (U.S. EPA, 2012). Both studies 

used nationwide Medicare mortality data to examine the association between monthly 

average PM2.5 concentrations over the preceding 12 months and monthly mortality rates 

in 113 U.S. counties and examined whether community-specific trends in monthly PM2.5

concentrations and mortality declined at the same rate as the national rate. The 

investigators examined this by decomposing the association between PM2.5 and mortality 

into two components: (1) national trends, defined as the association between the national 

average trend in monthly PM2.5 concentrations averaged over the previous 12 months and 

the national average trend in monthly mortality rates, and (2) local trends, defined as 

county-specific deviations in monthly PM2.5 concentrations and monthly mortality rates 

from national trends. 

The EPA does not question the results of the national trends analyses conducted 

by Janes et al. (2007) and Greven et al. (2011).42 Both Janes et al. (2007) and Greven et 

 

42 In its evaluation of Janes et al. (2007) in the Integrated Science Assessment, the EPA 

did not identify limitations in the statistical methods used per se (U.S. EPA, 2009a, p. 
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al. (2011) observed positive and statistically significant associations between long-term 

exposure to PM2.5 and mortality in their national analyses. However, Janes et al. (2007) 

and Greven et al. (2011) eliminated all of the spatial variation in air pollution and 

mortality in their data set when estimating the national effect, focusing instead on both 

chronic (yearly) and sub-chronic (monthly) temporal differences in the data (Dominici et 

al. 2012). Janes et al. (2007) (Table 1) highlighted that over 90 percent of the variance in 

the data set used for the analyses conducted by both Janes et al. (2007) and Greven et al. 

(2011) was attributable to spatial variability, which the authors chose to discard. As noted 

above, the focus of the analyses by Janes et al. (2007) and Greven et al. (2011) was on 

two components: (1) a temporal or time component, i.e., the “national” trends analysis, 

which examined the association between the national average trend in monthly PM2.5

concentrations averaged over the previous 12 months and the national average trend in

monthly mortality rates and (2) a space-by-time component, i.e., the “local” trends 

analysis, which examined county-specific deviations in monthly PM2.5 concentrations and 

monthly mortality rates from national trends. These two components combined 

comprised less than 10 percent of the variance in the data set. The authors included a 

focus on the space-by-time component, which represented approximately 5 percent of the 

variance in the data set, in an attempt to identify, absent confounding, if PM2.5 was 

associated with mortality at this unique exposure window. Thus, these studies are not 

directly comparable to other cohort studies investigating the relationship between longterm exposure to PM2.5 and mortality, which make use of spatial variability in air 

 

7-88) and included the results of the national-scale analyses in that study in the body of 

evidence that supported the determination that there is a causal relationship between 

long-term PM2.5 exposure and mortality.
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pollution and mortality data.43 This point was highlighted by the study authors who stated 

that “when one considers that this wealth of information is not accounted for in [Janes 

2007], it is not as surprising that... vastly different estimates of the PM2.5/mortality 

relationship [were observed] than in other studies that do exploit that variability”

(Dominici et al., 2012, p. 2). 

The EPA notes that the results of the local trends analyses conducted by Janes et 

al. (2007) and Greven et al. (2011) are limited by the monthly timescale used in these 

analyses. This view is consistent with comments on the Janes et al. (2007) study 

articulated in Pope and Burnett (2007),44 which noted that an important limitation of the 

local scale analysis conducted by Janes et al. (2007) and subsequently by Greven et al. 

(2011) was the subchronic exposure window considered in these analyses. Both studies 

used annual average PM2.5 concentrations to characterize long-term national trends which 

was consistent with exposure windows considered in other studies of long-term exposure 

to PM2.5 and mortality.45 However, the local scale analyses used monthly average PM2.5

 

43 Though not directly comparable, the national effect estimates for mortality reported by 

Janes et al. (2007) and Greven et al. (2011) are coincidentally similar in magnitude to 

those previously reported. It is important to note that previous cohort studies have 

focused on identifying spatial differences in PM2.5 concentrations between cities, while 

Janes et al. (2007) and Greven et al. (2011) focus primarily on temporal differences in 

PM2.5 concentrations. In fact, Greven et al. (2011) state “We do not focus here on a third 

type [of statistical approach] used in cohort studies, measuring the association between 

average PM2.5 levels and average age-adjusted mortality rates across cities (purely spatial 

or cross-sectional association).”

44 Some commenters argued that there were flaws in the criticisms offered by Pope and 

Burnett (2007) on the paper by Janes et al. (2007) (UARG, 2012, Attachment A, pp. 19 to 

23). The EPA responds to each of these specific comments in the Response to Comments 

document.

45 As noted above, however, Janes et al. (2007) and Greven et al. (2011) focused on 

temporal variability and other studies of long-term exposure to PM2.5 and mortality focus 

on spatial variability.
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concentrations to characterize county-specific deviations from national trends (the local 

scale). The use of monthly average data likely does not provide enough exposure contrast 

to observe temporal changes in mortality at the local scale. It also represents a different 

exposure window than considered in the large body of evidence of health effects related 

to short-term (from less than one day to up to several days) and chronic (one or more 

years) measures of PM2.5. 

Furthermore, the EPA disagrees with commenters that studies by Janes et al. 

(2007) and Greven et al. (2011) provide evidence that other studies of long-term exposure 

to PM2.5 and mortality are affected by unmeasured confounding. As noted above, the 

design of the studies conducted by Janes et al (2007) and Greven et al (2011) are 

fundamentally different than those used in other studies of long-term exposure to PM2.5

and mortality, including the ACS cohort and the Harvard Six Cities study. Studies, such 

as the ACS and Harvard Six Cities studies, used the spatial variation between cities to 

measure the effect of long-term (annual) exposures to PM2.5 on mortality risk, and did not 

conduct any analyses relying on the temporal variation in PM2.5. The opposite is true of 

the Janes et al. (2007) and Greven et al. (2011) studies which first removed the spatial 

variability in PM2.5 and then examined the temporal variation at both the national and 

local scale to measure the effects of temporal differences in PM2.5 on mortality risk. Janes 

et al. (2007) and Greven et al. (2011) focus on changes in PM2.5 concentrations over time 

and, therefore, control for confounders would be based on including variables that vary 

over time rather than over space. As a result, any evidence of potential confounding of 

the PM2.5-mortality risk relationship derived from Janes et al. (2007) and Greven et al. 

(2011) cannot be extrapolated to draw conclusions related to potential spatial 
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confounding in studies based on the spatial variation in PM2.5 concentrations.

As detailed in the Integrated Science Assessment (U.S. EPA, 2009a, section 7.6), 

and recognized by the authors of Janes et al. (2007) and Greven et al. (2011), the cohort 

studies that informed the causality determination for long-term PM2.5 exposure and 

mortality “have developed approaches to adjust for measured and unmeasured 

confounders” (Dominici et al., 2012, p. 2). These approaches were specifically designed 

to adjust for spatial confounding. The hypothesis that the authors of Janes et al. (2007) 

and Greven et al. (2011) chose to examine was that differences in the local and national 

effects indicated unmeasured temporal confounding in either the local or national effect 

estimate. This hypothesis was specific to these two studies that examined temporal 

variability in exposure to air pollution and did not include known potential confounders 

at either the national or local scale as time-varying covariates in the statistical model. The 

authors acknowledged that the interpretation of either the national or local estimates 

needs to occur with an appreciation of the potential confounding effects of national and 

local scale covariates that were omitted from the model (Dominici et al., 2012).

It is important to recognize that because Janes et al. (2007) and Greven et al. 

(2011) focused on variations in PM2.5 over time and not space, the results from these two 

studies do not provide any indication that other studies of long-term exposure to PM2.5

and mortality exhibit spatial confounding, or that PM2.5 does not cause mortality.46 The 

authors of Janes et al. (2007) and Greven et al. (2011) recognized that “it is entirely 

possible that these papers are looking for an association at a timescale for which no 

 

46 Further, the EPA notes that Janes et al. (2007) and Greven et al. (2011) provide no 

information relevant to examining confounding in studies of short-term exposure to 

PM2.5.
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association truly exists” (Dominici et al.,. 2012, p. 3). Furthermore, as highlighted in the 

Integrated Science Assessment and discussed by Pope and Burnett (2007), the 

conclusions of Janes et al. (2007) “are overstated. . . [T]heir analysis tells us little or 

nothing about unmeasured confounding in those and related studies because the 

methodology of Janes et al. largely excludes the sources of variability that are exploited 

in those other studies. By using monthly mortality counts and lagged 12-month average 

pollution concentrations, the authors eliminate the opportunity to exploit short-term or 

day-to-day variability.” 

In conclusion, the EPA interprets the results of the analyses conducted by Janes et 

al. (2007) and Greven et al. (2011) as being consistent with prior knowledge of 

examining associations with long-term exposure to PM2.5 at the national scale using longterm average PM2.5 concentrations. For the reasons presented above and discussed in 

more detail in the Response to Comments document, the Agency disagrees with the 

commenters’ assumption that the results of Janes et al. (2007) and Greven et al. (2011) 

indicate unmeasured confounding in the results of other cohort studies of long-term 

exposure to PM2.5 and mortality. Therefore, the EPA concludes that these studies do not 

invalidate the large body of epidemiological evidence that supports the EPA’s 

determination that a causal relationship exists between long-term PM2.5 exposure and 

mortality.47

(b) Heterogeneity in Risk Estimates

 

47 The EPA notes that the EPA’s conclusion with regard to interpretation of the results 

from Janes et al. (2007) and Greven et al. (2012) is supported by the study authors’ 

conclusion that “[o]ur results do not invalidate previous epidemiological studies”

(Dominici, 2012, p. 1 (emphasis original)).
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Some commenters argued that the heterogeneity in risk estimates observed in 

multi-city epidemiological studies and the lack of statistical significance in many regional 

or seasonal estimates highlights a potential bias associated with combined multi-city 

epidemiological study results (e.g., API, 2012, Attachment 1, pp. 15 to 19). These 

commenters further argued that more refined intra-urban exposure estimates conducted 

for two of the largest cities included in the ACS study, Los Angeles and New York City, 

based on land-use regression models and/or kriging methods (Krewski et al., 2009) 

“underscore the importance of considering city-specific health estimates, which may 

account for heterogeneity in PM2.5 concentrations or other differences among cities, 

rather than relying on pooled nationwide results from multi-city studies” (API, 2012, 

Attachment 1, p. 17).

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

EPA agrees that epidemiological studies evaluating health effects associated with longand short-term PM2.5 exposures have reported heterogeneity in responses between cities 

and effect estimates across geographic regions of the U.S. (U.S. EPA, 2009a, sections 

6.2.12.1, 6.3.8.1, 6.5.2, and 7.6.1; U.S. EPA, 2011a, p. 2-25). For example, when 

focusing on short-term PM2.5 exposure, the Integrated Science Assessment found that 

multi-city studies that examined associations with mortality and cardiovascular and 

respiratory hospital admissions and emergency department visits demonstrated greater 

cardiovascular effects in the eastern versus the western U.S. (Dominici, et al., 2006; Bell 

et al., 2008; Franklin et al. (2007, 2008)). 

In addition, the Integrated Science Assessment evaluated studies that provided 

some evidence for seasonal differences in PM2.5 risk estimates, specifically in the 
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northeast. The Integrated Science Assessment found evidence indicating that individuals 

may be at greater risk of dying from higher exposures to PM2.5 in the warmer months, 

and at greater risk of PM2.5 associated hospitalization for cardiovascular and respiratory 

diseases during colder months of the year. The limited influence of seasonality on PM

risk estimates in other regions of the U.S. may be due to a number of factors including 

varying PM composition by season, exposure misclassification due to regional tendencies 

to spend more or less time outdoors and air conditioning usage, and the prevalence of 

infectious diseases during the winter months (U.S. EPA, 2009a, p. 3-182). 

Overall, the EPA took note in the proposal that uncertainties still remain 

regarding various factors that contribute to heterogeneity observed in epidemiological 

studies (77 FR 38909/3). Nonetheless, the EPA recognizes that this heterogeneity could 

be attributed, at least in part, to differences in PM2.5 composition across the U.S., as well 

as to exposure differences that vary regionally such as personal activity patterns, 

microenvironmental characteristics, and the spatial variability of PM2.5 concentrations in 

urban areas (U.S. EPA, 2009a, section 2.3.2; 77 FR 38910). 

As recognized in the Policy Assessment, the current epidemiological evidence 

and the limited amount of city-specific speciated PM2.5 data do not allow conclusions to 

be drawn that specifically differentiate effects of PM2.5 in different locations (U.S. EPA, 

2011a, p. 2-25). Furthermore, the Integrated Science Assessment concluded “that many 

constituents of PM2.5 can be linked with multiple health effects, and the evidence is not 

yet sufficient to allow differentiation of those constituents or sources that are more 

closely related to specific health outcomes” (U.S. EPA, 2009a, p. 2-17). CASAC 

thoroughly reviewed the EPA’s presentation of the scientific evidence indicating 
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heterogeneity in PM2.5 effect estimates in epidemiological studies and concurred with the 

overall conclusions presented in the Integrated Science Assessment.

(c) Exposure Measurement Error 

Some commenters argued that the EPA did not adequately consider exposure 

measurement error, which they asserted is an important source of bias in epidemiological 

studies that can bias effect estimates in either direction (e.g., API, 2012, Attachment 1, 

pp. 19 to 20). 

The EPA agrees that exposure measurement error is an important source of 

uncertainty and that the variability in risk estimates observed in multi-city studies could 

be attributed, in part, to exposure error due to measurement-related issues (77 FR 38910). 

However, the Agency disagrees with the commenters’ assertion that exposure 

measurement error was not adequately considered in this review. The Integrated Science 

Assessment included an extensive discussion that addresses issues of exposure 

measurement error (U.S. EPA, 2009a, sections 2.3.2 and 3.8.6). Exposure measurement 

error may lead to bias in effect estimates in epidemiological studies. A number of studies 

evaluated in the last review (U.S. EPA, 2004, section 8.4.5) and in the current review 

(U.S. EPA, 2009a, section 3.8.6) have discussed the direction and magnitude of bias 

resulting from specified patterns of exposure measurement error (Armstrong 1998; 

Thomas et al. 1993; Carroll et al. 1995) and have generally concluded “classical” (i.e., 

random, within-person) exposure measurement error can bias effect estimates towards the 

null. Therefore, consistent with conclusions reached in the last review, the Integrated 

Science Assessment concluded “in most circumstances, exposure error tends to bias a 

health effect estimate downward” (U.S. EPA, 2009a, sections 2.3.2 and 3.8.6) (emphasis 
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added). Thus, the EPA has both considered and accounted for the possibility of exposure 

measurement error, and the possible bias would make it more difficult to detect true 

associations, not less difficult. 

(d) Model Specification 

Commenters contended that the EPA did not account for the fact that “selecting 

an appropriate statistical model for epidemiologic studies of air pollution involves several 

choices that involve much ambiguity, scant biological evidence, and a profound impact 

on analytic results, given that many estimated associations are weak” (ACC, 2012, p. 5) 

For short-term exposure studies, the EPA recognizes, as summarized in the HEI review 

panel commentary that selecting a level of control to adjust for time-varying factors, such 

as temperature, in time-series epidemiological studies involves a trade-off (HEI, 2003). 

For example, if the model does not sufficiently adjust for the relationship between the 

health outcome and temperature, some effects of temperature could be falsely ascribed to 

the pollution variable. Conversely, if an overly aggressive approach is used to control for 

temperature, the result would possibly underestimate the pollution-related effect and 

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

8-236; HEI, 2003, p. 266). The selection of approaches to address such variables depends 

in part on prior knowledge and judgments made by the investigators, for example, about 

weather patterns in the study area and expected relationships between weather and other 

time-varying factors and health outcomes considered in the study. As demonstrated in 

section 6.5 of the Integrated Science Assessment, the EPA thoroughly considered each of 

these issues and the overall effect of different model specifications on the association 

between short-term PM2.5 exposure and mortality. Regardless of the model employed, 
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consistent positive associations were observed across studies that controlled for the 

potential confounding effects of time and weather using different approaches (U.S. EPA 

2009, Figure 6-27). The EPA also considered the influence of model specification in the 

examination of long-term PM2.5 exposure studies. For example, in section 7.6 of the 

Integrated Science Assessment, Figures 7-6 and 7-7 summarize the collective evidence 

that evaluated the association between long-term PM2.5 exposure and mortality. 

Regardless of the model used, these studies collectively found evidence of consistent 

positive associations between long-term PM2.5 exposure and mortality. 

The EPA, therefore, disagrees with commenters that model specification was not 

considered when evaluating the epidemiological evidence used to form causality 

determinations. The EPA specifically points out that the process of assessing the 

scientific quality and relevance of epidemiological studies includes examining “important 

methodological issues (e.g., lag or time period between exposure and effects, model 

specifications, thresholds, mortality displacement) related to interpretation of the health 

evidence (U.S. EPA, 2009, p. 1-9).” Consistent with the conclusions of the 2004 PM Air 

Quality Criteria Document, the EPA recognizes that there is still no clear consensus at 

this time as to what constitutes appropriate control of weather and temporal trends in 

short-term exposure studies, and that no single statistical modeling approach is likely to 

be most appropriate in all cases (U.S. EPA, 2004, p. 8-238). However, the EPA believes 

that the available evidence interpreted in light of these remaining uncertainties does 

provide increased confidence relative to the last review in the reported associations 

between short- and long-term PM2.5 exposures and mortality and morbidity effects, alone 

and in combination with other pollutants.
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(e) Concentration-response Relationship

Additionally, commenters questioned the interpretation of the shape of the 

concentration-response relationship, specifically stating that multiple studies have 

demonstrated that there is a threshold in the PM-health effect relationship and that the 

log-linear model is not biologically plausible (API, 2012, Attachment 9; ACC, 2012, 

Appendix A, pp. 7 to 8). The EPA disagrees with this assertion due to the number of 

studies evaluated in the Integrated Science Assessment that continue to support the use of 

a no-threshold, log-linear model to most appropriately represent the PM concentrationresponse relationship (U.S. EPA, 2009a, section 2.4.3). While recognizing that 

uncertainties remain, the EPA believes that our understanding of this issue for both longand short-term exposure studies has advanced since the last review. As discussed in the 

Integrated Science Assessment, both long- and short-term exposure studies have 

employed a variety of statistical approaches to examine the shape of the concentrationresponse function and whether a threshold exists. While the EPA recognizes that there 

likely are individual biological thresholds for specific health responses, the Integrated 

Science Assessment concluded the overall evidence from existing epidemiological 

studies does not support the existence of thresholds at the population level, for effects 

associated with either long-term or short-term PM exposures within the ranges of air 

quality observed in these studies (U.S. EPA, 2009a, section 2.4.3).

48 The Integrated 

Science Assessment concluded that this evidence collectively supported the conclusion 

 

48 While epidemiological analyses have not identified a population threshold in the range 

of air quality concentrations evaluated in these studies, the EPA recognizes that it is 

possible that such thresholds exist towards the lower end of these ranges (or below these 

ranges).
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that a no-threshold, log-linear model is most appropriate (U.S. EPA, 2009a, sections 

6.2.10.10, 6.5.2.7, and 7.6.4). CASAC likewise advised that “[a]lthough there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (Samet, 2010d, 

p. ii).

The EPA recognizes that some short-term exposure studies have examined the 

PM2.5 concentration-response relationship in individual cities or on a city-to-city basis 

and observed heterogeneity in the shape of the concentration-response curve across cities. 

As discussed in (b) above, these findings are a source of uncertainty that the EPA agrees 

requires further investigation. Nonetheless, the Integrated Science Assessment concluded 

that “the studies evaluated further support the use of a no-threshold, log-linear model, but 

additional issues such as the influence of heterogeneity in estimates between cities and 

the effects of seasonal and regional differences in PM on the concentration-responserelationship still require further investigation” (U.S. EPA, 2009a, p. 2-25). 

(f) Relative Toxicity of PM2.5 Components 

Some commenters highlighted uncertainties in understanding the role of 

individual constituents within the mix of fine particles. These commenters asserted that a 

mass-based standard may not be appropriate due to the growing body of evidence 

indicating that certain PM2.5 components may be more closely related to specific health 

outcomes (e.g., EC and OC) (EPRI, 2012, p. 2). 

With regard to questions about the role of individual constituents within the mix 

of fine particles, as a general matter, the EPA recognizes that although new research 

directed toward this question has been conducted since the last review, important 

questions remain and the issue remains an important element in the Agency’s ongoing 
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research program. At the time of the last review, the Agency determined that it was 

appropriate to continue to control fine particles as a group, as opposed to singling out any 

particular component or class of fine particles (71 FR 61162 to 61164, October 17, 2006). 

This distinction was based largely on epidemiological evidence of health effects using 

various indicators of fine particles in a large number of areas that had significant 

contributions of differing components or sources of fine particles, together with some 

limited experimental studies that provided some evidence suggestive of health effects 

associated with high concentrations of numerous fine particle components. 

In this review, as discussed in the proposal (77 FR 38922 to 38923) and in section 

III.E.1 below, while most epidemiological studies continue to be indexed by PM2.5 mass, 

several recent epidemiological studies included in the Integrated Science Assessment 

have used PM2.5 speciation data to evaluate health effects associated with fine particle 

exposures. In the Integrated Science Assessment, the EPA thoroughly evaluated the 

scientific evidence that examined the effect of different PM2.5 components and sources on 

a variety of health outcomes (U.S. EPA, 2009a, section 6.6) and observed that the 

available information continues to suggest that many different chemical components of 

fine particles and a variety of different types of source categories are all associated with, 

and probably contribute to, effects associated with PM2.5. The Integrated Science 

Assessment concluded that the current body of scientific evidence indicated that “many 

constituents of PM can be linked with differing health effects and the evidence is not yet 

sufficient to allow differentiation of those constituents or sources that are more closely 

related to specific health outcomes” (U.S. EPA, 2009a, p. 2-26 and 6-212). Furthermore, 

the Policy Assessment concluded that the evidence is not sufficient to support eliminating 
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any component or group of components associated with any specific source categories 

from the mix of fine particles included in the PM2.5 indicator (U.S. EPA, 2009a, p. 2-56). 

CASAC agreed that it was reasonable to retain PM2.5 as an indicator for fine particles in 

this review as “[t]here was insufficient peer-reviewed literature to support any other 

indicator at this time” (Samet, 2010c, p. 12).

This information is relevant to the Agency’s decision to retain PM2.5 as the 

indicator for fine particles as discussed in section III.E.1 below. The EPA also believes 

that it is relevant to the Agency’s conclusion as to whether revision of the suite of 

primary PM2.5 standards is appropriate. While there remain uncertainties about the role 

and relative toxicity of various components of fine PM, the current evidence continues to 

support the view that fine particles should be addressed as a group for purposes of public 

health protection. 

In summary, in considering the above issues related to uncertainties in the 

underlying health science, on balance, the EPA believes that the available evidence 

interpreted in light of these remaining uncertainties does provide increased confidence 

relative to the last review in the reported associations between long- and short- term 

PM2.5 exposures and mortality and morbidity effects, alone and in combination with other 

pollutants, and supports stronger inferences as to the causal nature of the associations. 

The EPA also believes that this increased confidence, when taken in context of the entire 

body of available health effects evidence and in light of the evidence from 

epidemiological studies of associations observed in areas meeting the current primary 

PM2.5 standards, specifically in areas meeting the current primary annual PM2.5 standard, 

adds support to its conclusion that the current suite of PM2.5 standards needs to be revised 
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to provide increased public health protection. 

(4) In asserting that there is no evidence of greater risk since the 2006 review to 

justify lowering the current annual PM2.5 standard, some commenters argued that, “if the 

current primary PM2.5 annual standard of 15 μg/m3 was considered to be adequately 

protective of public health in 2006, given relative risk estimates that EPA was using at 

that time, then that standard would surely still be adequately protective of the public 

health if relative risk estimates remain at the same level (or lower)” (UARG, 2012, 

Attachment 1, p. 24). These commenters compared risk coefficients used for mortality in 

the EPA’s risk assessment done in the last review with those from the Agency’s core risk 

assessment done as part of this review, and they concluded that “the entire range of the 

core relative risk for long-term mortality is lower now than it was in the prior review” 

(UARG, 2012, Attachment 1, p. 24). These commenters used this conclusion as the basis 

for a claim that there is no reason to revise the current annual PM2.5 standard.

The EPA believes that this claim is fundamentally flawed. In comparing the 

scientific understanding of the risk presented by exposure to PM2.5 between the last and 

current reviews, one must examine not only the quantitative estimate of risk from those 

exposures (e.g., the numbers of premature deaths or increased hospital admissions at 

various concentrations), but also the degree of confidence that the Agency has that the 

observed health effects are causally linked to PM2.5 exposure at those concentrations. As 

documented in the Integrated Science Assessment and in the recommendations and 

conclusions of CASAC, the EPA recognizes significant advances in our understanding of 

the health effects of PM2.5, based on evidence that is stronger than in the last review. As a 

result of these advances, the EPA is now more certain that fine particles, alone or in 
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combination with other pollutants, present a significant risk to public health at 

concentrations allowed by the current primary PM2.5 standards. From this more 

comprehensive perspective, since the risks presented by PM2.5 are more certain, similar or 

even somewhat lower relative risk estimates would not be a basis to conclude that no 

revision to the suite of PM2.5 standards is “requisite” to protect public health with an 

adequate margin of safety. This also ignores that the relative risk estimate is only one 

factor considered by the Administrator, e.g. it ignores that epidemiological studies since 

the last review indicate associations between PM2.5 and mortality and morbidity in areas 

meeting the current annual standard.

In any case, the commenters’ reliance on the flawed 2006 review is misplaced. As 

discussed in section III.A.2 above, the D.C. Circuit remanded Administrator Johnson’s 

2006 decision to retain the primary annual PM2.5 standard because the Agency failed to 

adequately explain why the annual standard provided the requisite protection from both 

short- and long-term exposure to fine particles including protection for at-risk 

populations. The 2006 standard was also at sharp odds with CASAC advice and 

recommendations as to the requisite level of protection (Henderson, 2006a,b). In other 

words, the judgment of the then-Administrator that the 2006 primary annual PM2.5

standard was requisite to protect public health with an adequate margin of safety is not an 

appropriate benchmark for comparison. 

(5) Some of these commenters also identified “new” as well as older studies that 

had been included in prior reviews as providing additional evidence that the causality 

determinations presented in the Integrated Science Assessment did not consider the 

totality of the scientific literature, further supporting their view that a revision of the 
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PM2.5 is unwarranted. As discussed in section II.B.3 above, the EPA notes that, as in past 

NAAQS reviews, the Agency is basing the final decisions in this review on the studies 

and related information included in the Integrated Science Assessment that have 

undergone CASAC and public review, and will consider newly published studies for 

purposes of decision making in the next PM NAAQS review. In provisionally evaluating 

commenters’ arguments (see Response to Comments document), the EPA notes that its 

provisional assessment of “new” science found that such studies did not materially 

change the conclusions reached in the Integrated Science Assessment (U.S. EPA, 2012b). 

3. Administrator’s Final Conclusions Concerning the Adequacy of the Current 

Primary PM2.5 Standards

Having carefully considered the public comments, as discussed above, the 

Administrator believes the fundamental scientific conclusions on the effects of PM2.5

reached in the Integrated Science Assessment, and discussed in the Policy Assessment, 

are valid. In considering whether the suite of primary PM2.5 standards should be revised, 

the Administrator places primary consideration on the evidence obtained from the 

epidemiological studies. The Administrator believes that this literature, combined with 

the other scientific evidence discussed in the Integrated Science Assessment, collectively 

represents a strong and generally robust body of evidence of serious health effects 

associated with both long- and short-term exposures to PM2.5. As discussed in the 

Integrated Science Assessment and Policy Assessment, the EPA believes that much 

progress has been made since the last review in reducing some of the major uncertainties 

that were important considerations in establishing the current suite of PM2.5 standards. In 

that context, the Administrator finds the evidence of serious health effects reported in 
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exposure studies conducted in areas with long-term mean concentrations ranging from 

approximately at or above the level of the annual standard to long-term mean

concentrations significantly below the level of the annual standard to be compelling, 

especially in light of the extent to which such studies are part of an overall pattern of 

positive and frequently statistically significant associations across a broad range of 

studies. The information in the quantitative risk assessment lends support to this 

conclusion.

There has been extensive critical review of this body of evidence, the quantitative 

risk assessment, and related uncertainties, including review by CASAC and the public. 

The public comments on the basis for the EPA’s proposed decision to revise the suite of 

primary PM2.5 standards have identified a number of issues about which different parties 

disagree including issues for which additional research is warranted. Having weighed all 

comments and the advice of CASAC, the Administrator believes that since the last 

review the overall uncertainty about the public health risks associated with both long- and 

short-term exposure to PM2.5 has been diminished to an important degree. The remaining 

uncertainties in the available evidence do not diminish confidence in the associations 

between exposure to fine particles and mortality and serious morbidity effects. Based on 

her increased confidence in the association between exposure to PM2.5 and serious public 

health effects, combined with evidence of such an association in areas that would meet 

the current standards, the Administrator agrees with CASAC that revision of the current 

suite of PM2.5 standards to provide increased public health protection is necessary. Based 

on these considerations, the Administrator concludes that the current suite of primary 

PM2.5 standards is not sufficient, and thus not requisite, to protect public health with an 
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adequate margin of safety, and that revision is needed to increase public health 

protection. 

It is important to note that this conclusion, and the reasoning on which it is based, 

do not resolve the question of what specific revisions are appropriate. That requires 

looking specifically at the current 24-hour and annual PM2.5 standards, including their 

indicator, averaging times, forms, and levels, and evaluating the scientific evidence and 

other information relevant to determining the appropriate revision of the suite of 

standards.

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator

In initially setting standards for fine particles in 1997, the EPA concluded it was 

appropriate to control fine particles as a group, rather than singling out any particular 

component or class of fine particles. The EPA noted that community health studies had 

found significant associations between various indicators of fine particles, and that health 

effects in a large number of areas had significant mass contributions of differing 

components or sources of fine particles. In addition, a number of toxicological and 

controlled human exposure studies had reported health effects associations with high 

concentrations of numerous fine particle components. It was also not possible to rule out 

any component within the mix of fine particles as not contributing to the fine particle 

effects found in the epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a 

size-based indicator in 1977 to distinguish fine particles from particles in the coarse 

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 
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available. The selection of a 2.5 μm size cut reflected the regulatory importance of 

defining an indicator that would more completely capture fine particles under all 

conditions likely to be encountered across the U.S., especially when fine particle 

concentrations and humidity are likely to be high, while recognizing that some small 

coarse particles would also be captured by current methods to monitor PM2.5 (62 FR 

38666 to 38668, July 18, 1997). In the last review, based on the same considerations, the 

EPA again recognized that the available information supported retaining the PM2.5

indicator and remained too limited to support a distinct standard for any specific PM2.5

component or group of components associated with any source categories of fine 

particles (71 FR 61162 to 61164, October 17, 2006).

In this current review, the same considerations continue to apply for selection of 

an appropriate indicator for fine particles. As an initial matter, the Policy Assessment 

recognizes that the available epidemiological studies linking mortality and morbidity 

effects with long- and short-term exposures to fine particles continue to be largely 

indexed by PM2.5. For the same reasons discussed in the last two reviews, the Policy 

Assessment concluded that it was appropriate to consider retaining a PM2.5 indicator to 

provide protection from effects associated with long- and short-term fine particle 

exposures (U.S. EPA, 2011, p. 2-50).

The Policy Assessment also considered the expanded body of evidence available 

in this review to consider whether there was sufficient evidence to support a separate 
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standard for ultrafine particles49 or whether there was sufficient evidence to establish 

distinct standards focused on regulating specific PM2.5 components or a group of 

components associated with any source categories of fine particles (U.S. EPA, 2011a, 

section 2.3.1).

A number of studies available in this review have evaluated potential health 

effects associated with short-term exposures to ultrafine particles. As noted in the 

Integrated Science Assessment, the enormous number and larger, collective surface area 

of ultrafine particles are important considerations for focusing on this particle size 

fraction in assessing potential public health impacts (U.S. EPA, 2009a, p. 6-83). Per unit 

mass, ultrafine particles may have more opportunity to interact with cell surfaces due to 

their greater surface area and their greater particle number compared with larger particles 

(U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the potential for soluble 

components (e.g., transition metals, organics) to adsorb to ultrafine particles and 

potentially cross cell membranes and epithelial barriers (U.S. EPA, 2009a, p. 6-83). In 

addition, evidence available in this review suggests that the ability of particles to enhance 

allergic sensitization is associated more strongly with particle number and surface area 

than with particle mass (U.S. EPA, 2009a, p. 6-127). 

New evidence, primarily from controlled human exposure and toxicological 

studies, expands our understanding of cardiovascular and respiratory effects related to 

short-term ultrafine particle exposures. However, the Policy Assessment concluded that 

this evidence was still very limited and largely focused on exposure to diesel exhaust, for 

 

49 Ultrafine particles, generally including particles with a mobility diameter less than or 

equal to 0.1 μm, are emitted directly to the atmosphere or are formed by nucleation of 

gaseous constituents in the atmosphere (U.S. EPA, 2009a, p. 3-3).
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which the Integrated Science Assessment concluded it was unclear whether the effects 

observed are due to ultrafine particles, larger particles within the PM2.5 mixture, or the 

gaseous components of diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition, the 

Integrated Science Assessment noted uncertainties associated with the controlled human 

exposure studies using concentrated ambient particle systems which have been shown to 

modify the composition of ultrafine particles (U.S. EPA, 2009a, p. 2-22, see also section 

1.5.3). 

The Policy Assessment recognized that there are relatively few epidemiological 

studies that have examined potential cardiovascular and respiratory effects associated 

with short-term exposures to ultrafine particles (U.S. EPA, 2011a, p. 2-51). These studies 

have reported inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).

Collectively, in considering the body of scientific evidence available in this 

review, the Integrated Science Assessment concluded that the currently available 

evidence was suggestive of a causal relationship between short-term exposures to 

ultrafine particles and cardiovascular and respiratory effects. Furthermore, the Integrated 

Science Assessment concluded that evidence was inadequate to infer a causal relationship 

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

exposure to ultrafine particles and all outcomes evaluated (U.S. EPA, 2009a, sections 

2.3.5, 6.2.12.3, 6.3.10.3, 6.5.3.3, 7.2.11.3, 7.3.9, 7.4.3.3, 7.5.4.3, and 7.6.5.3; Table 2-6). 

With respect to our understanding of ambient ultrafine particle concentrations, at 

present, there is no national network of ultrafine particle samplers; thus, only episodic 

and/or site-specific data sets exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy 

Assessment recognized a national characterization of concentrations, temporal and spatial 
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patterns, and trends was not possible at this time, and the availability of ambient ultrafine 

measurements to support health studies was extremely limited (U.S. EPA, 2011a, p. 2-

51). In general, measurements of ultrafine particles are highly dependent on monitor 

location and, therefore, more subject to exposure error than accumulation mode particles 

(U.S. EPA, 2009a, p. 2-22). Furthermore, the number of ultrafine particles generally 

decreases sharply downwind from sources, as ultrafine particles may grow into the 

accumulation mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited 

studies of ambient ultrafine particle measurements have suggested that these particles 

exhibit a high degree of spatial and temporal heterogeneity driven primarily by 

differences in nearby source characteristics (U.S. EPA, 2009a, p. 3-84). Internal 

combustion engines and, therefore, roadways are a notable source of ultrafine particles, 

so concentrations of these particles near roadways are generally expected to be elevated 

(U.S. EPA, 2009a, p. 2-3). Concentrations of ultrafine particles have been reported to 

drop off much more quickly with distance from roadways than fine particles (U.S. EPA, 

2009a, p. 3-84).

In considering both the currently available health effects evidence and the air 

quality data, the Policy Assessment concluded that this information was still too limited 

to provide support for consideration of a distinct PM standard for ultrafine particles (U.S.

EPA, 2011a, p. 2-52).

In addressing the issue of particle composition, the Integrated Science Assessment 

concluded that, “[f]rom a mechanistic perspective, it is highly plausible that the chemical 

composition of PM would be a better predictor of health effects than particle size” (U.S.

EPA, 2009a, p. 6-202). Heterogeneity of ambient concentrations of PM2.5 constituents 
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(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 (U.S.

EPA, 2009a, section 6.6). 

With respect to the availability of ambient measurement data for fine particle 

components in this review, the Policy Assessment noted that there were now more 

extensive ambient PM2.5 speciation measurement data available through the Chemical 

Speciation Network (CSN) than in previous reviews (U.S. EPA, 2011a, section 1.3.2 and 

Appendix B, section B.1.3). The Integrated Science Assessment observed that data from 

the CSN provided further evidence of spatial and seasonal variation in both PM2.5 mass 

and composition among cities and geographic regions (U.S. EPA, 2009a, pp. 3-50 to 3-

60; Figures 3-12 to 3-18; Figure 3-47). Some of this variation may be related to regional 

differences in meteorology, sources, and topography (U.S. EPA, 2009a, p. 2-3). 

The currently available epidemiological, toxicological, and controlled human 

exposure studies evaluated in the Integrated Science Assessment on the health effects 

associated with ambient PM2.5 constituents and categories of fine particle sources used a 

variety of quantitative methods applied to a broad set of PM2.5 constituents, rather than 

selecting a few constituents a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies 

have used measured ambient PM2.5 speciation data, including monitoring data from the 

CSN, while all of the controlled human exposure and most of the toxicological studies 

have used concentrated ambient particles and analyzed the constituents therein (U.S.
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EPA, 2009a, p. 6-203).50 The CSN provides PM2.5 speciation measurements generally on 

a one-in-three or one-in-six day sampling schedule and, thus, does not capture data every 

day at most sites.51

The Policy Assessment recognized that several new multi-city studies evaluating 

short-term exposures to fine particle constituents are now available. These studies 

continued to show an association between mortality and cardiovascular and/or respiratory 

morbidity effects and short-term exposures to various PM2.5 components including 

nickel, vanadium, elemental carbon, organic carbon, nitrates, and sulfates (U.S. EPA, 

2011a, section 2.3.1; U.S. EPA, 2009a, sections 6.5.2.5 and 6.6). 

Limited evidence is available to evaluate the health effects associated with longterm exposures to PM2.5 components (U.S. EPA, 2009a, section 7.6.2). The Policy 

Assessment noted the most significant new evidence was provided by a study that 

evaluated multiple PM2.5 components and an indicator of traffic density in an assessment 

of health effects related to long-term exposure to PM2.5 (Lipfert et al., 2006). Using 

health data from a cohort of U.S. military veterans and PM2.5 measurement data from the 

CSN, Lipfert et al. (2006) reported positive associations between mortality and long-term 

 

50 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 (U.S. EPA, 

2009a, p. 6-203). 

51 To expand our understanding of the role of specific PM2.5 components and sources 

with respect to the observed health effects, researchers have expressed a strong interest in 

having access to PM2.5 speciation measurements collected more frequently (U.S. EPA, 

2011a, p. 2-53, including footnote 47). 
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exposures to nitrates, elemental carbon, nickel, and vanadium as well as traffic density 

and peak ozone concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a, pp. 7-89 to 

7-90). 

With respect to source categories of fine particles potentially associated with a 

range of health endpoints, the Integrated Science Assessment reported that the currently 

available evidence suggests associations between cardiovascular effects and a number of 

specific PM2.5–related source categories, includingspecifically oil combustion, wood or 

biomass burning, motor vehicle emissions, and crustal or road dust sources (U.S. EPA, 

2009a, section 6.6; Table 6-18). In addition, a few studies have evaluated associations 

between PM2.5-related source categories and mortality. For example, oOne study reported 

an association between mortality and a PM2.5 coal combustion factor (Laden et al., 2000), 

while other studies linked mortality to a secondary sulfate long-range transport PM2.5

source (Ito et al., 2006; Mar et al., 2006) (U.S. EPA, 2009a, section 6.6.2.1). Other 

studies have looked at different components of particulate matter. There was less 

consistency in associations observed between selected sources of fine particles and 

respiratory health endpointseffects, which may be partially due to the fact that fewer 

studies have evaluated respiratory-related outcomes and measures. However, there was 

some evidence for PM2.5-related associations with secondary sulfate and decrements in 

lung function in asthmatic and healthy adults (U.S. EPA, 2009a, p. 6-211; Gong et al., 

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

rRespiratory endpoints in children and adults with asthmatics and ffects relating 

tosurrogates for the crustal/soil/road dust and traffic sources of PM have been observed in 

asthmatic children and adults (U.S. EPA, 2009a, p. 6-205; Gent et al., 2009; Penttinen et 
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al., 2006). 

Recent studies have shown that source apportionment methods have the potential 

to add useful insights into which sources and/or PM constituents may contribute to 

different health effects. Of particular interest are several epidemiological studies that 

compared source apportionment methods and reported consistent results across research 

groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al., 2006; Mar et al., 2006; 

Thurston et al., 2005). These studies reported associations between total mortality and 

secondary sulfate in two cities for two different lag times. The sulfate effect was stronger 

for total mortality in Washington D.C. and for cardiovascular-related mortality in 

Phoenix (U.S. EPA, 2009a, p. 6-204). These studies also found some evidence for 

associations with mortality and a number of source categories (e.g., biomass/wood 

combustion, traffic, copper smelter, coal combustion, sea salt) at various lag times (U.S.

EPA, 2009a, p. 6-204). Sarnat et al. (2008) compared three different source 

apportionment methods and reported consistent associations between emergency 

department visits for cardiovascular diseases with mobile sources and biomass 

combustion as well as increased respiratory-related emergency department visits 

associated with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-211).

Collectively, in considering the currently available evidence for health effects 

associated with specific PM2.5 components or groups of components associated with any 

source categories of fine particles as presented in the Integrated Science Assessment, the 

Policy Assessment concluded that additional information available in this review 

continues to provide evidence that many different constituents of the fine particle mixture 

as well as groups of components associated with specific source categories of fine 
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particles are linked to adverse health effects (U.S. EPA, 2011a, p. 2-55). However, as 

noted in the Integrated Science Assessment, while “[t]here is some evidence for trends 

and patterns that link particular ambient PM constituents or sources with specific health 

outcomes...there is insufficient evidence to determine whether these patterns are 

consistent or robust” (U.S. EPA, 2009a, p. 6-210). Assessing this information, the 

Integrated Science Assessment concluded that “the evidence is not yet sufficient to allow 

differentiation of those constituents or sources that are more closely related to specific 

health outcomes” (U.S. EPA, 2009a, pp. 2-26 and 6-212). Therefore, the Policy 

Assessment concluded that the currently available evidence is not sufficient to support 

consideration of a separate indicator for a specific PM2.5 component or group of 

components associated with any source category of fine particles. Furthermore, the 

Policy Assessment concluded that the evidence is not sufficient to support eliminating 

any component or group of components associated with any source categories of fine 

particles from the mix of fine particles included in the PM2.5 indicator (U.S. EPA, 2011a, 

p. 2-56). 

The CASAC agreed with the EPA staff conclusions presented in the Policy 

Assessment and concluded that it is appropriate to consider retaining PM2.5 as the 

indicator for fine particles and further asserted, “There [is] insufficient peer-reviewed 

literature to support any other indicator at this time” (Samet, 2010c, p. 12). CASAC 

expressed a strong desire for the EPA to “look ahead to future review cycles and 

reinvigorate support for the development of evidence that might lead to newer indicators 

that may correlate better with the health effects associated with ambient air 

concentrations of PM ...” (Samet, 2010c, p 2). 
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Consistent with the staff conclusions presented in the Policy Assessment and 

CASAC advice, the Administrator proposed to retain PM2.5 as the indicator for fine 

particles. Further, the Administrator provisionally concluded that currently available 

scientific information does not provide a sufficient basis for supplementing mass-based, 

primary fine particle standards with standards using a separate indicator for ultrafine 

particles or a separate indicator for a specific PM2.5 component or group of components 

associated with any source categories of fine particles. In addition, the Administrator also 

provisionally concluded that the currently available scientific information did not provide 

a sufficient basis for eliminating any individual component or group of components 

associated with any source categories from the mix of fine particles included in the PM2.5

mass-based indicator. 

The EPA received comparatively few public comments on issues related to the 

indicator for fine particles.52 Some commenters emphasized the need to conduct 

additional research to more fully understand the effect of specific PM2.5 components 

and/or sources on public health. These commenters expressed views about the importance 

of evaluating health effect associations with various fine particle components and types 

of source categories as a basis for focusing ongoing and future research to reduce 

uncertainties in this area and for considering whether alternative indicator(s) may be 

appropriate to consider in future PM NAAQS reviews for standards intended to protect 

against the array of health effects that have been associated with fine particles as indexed 

by PM2.5. For example, the PSR encouraged more research and monitoring related to 

 

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

particles.
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PM2.5 components and noted the importance of components associated with coal 

combustion (PSR, 2012, pp. 5 to 6). EPRI asserted that “new” studies support focusing 

on EC and OC and encouraged the EPA to seriously consider the mass-based approach 

(EPRI, 2012, p. 2). Likewise, Georgia Mining Association supported additional 

monitoring and research efforts related to PM2.5 composition and specifically encouraged 

the evaluation of using particle number (e.g., particle count) (GMA, 2012, pp. 2 to 3). 

The Administrator agrees with CASAC as well as these commenters that the 

results of additional research and monitoring efforts will be helpful for informing future 

PM NAAQS reviews. Information from such studies could also help inform the 

development of strategies that emphasize control of specific types of emission sources so 

as to address particles of greatest concern to public health. However, based upon the 

scientific information considered in the Integrated Science Assessment as well as the 

public comments summarized above, the Administrator continues to take note there is 

evidence that many different constituents of the fine particle mixture as well as groups of 

components associated with specific sources of fine particles are linked to adverse health 

effects. Furthermore, she recognizes that the evidence is not yet sufficient to differentiate 

those constituents or sources that are most closely related to specific health outcomes nor 

to exclude any PM2.5 components or sources of fine particles from the mix of particles 

included in the PM2.5 indicator. 

Having considered the public comments on this issue, the Administrator concurs 

with the Policy Assessment conclusions and CASAC recommendations and concludes 

that it is appropriate to retain PM2.5 as the indicator for fine particles. 

2. Averaging Time
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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 concluded it is appropriate to retain the current annual and 24-hour averaging 

times to provide protection from effects associated with both long- and short-term PM2.5

exposures (U.S. EPA, 2011a, p. 2-57).

In considering whether the information available in this review supports 

consideration of different averaging times for PM2.5 standards specifically with regard to 

considering a standard with an averaging time less than 24 hours to address health effects 

associated with sub-daily PM2.5 exposures, the Policy Assessment noted there continues 

to be a growing body of studies that provide additional evidence of effects associated 

with exposure periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to 

information available in the last review, recent studies provide additional evidence for 

cardiovascular effects associated with sub-daily (e.g., one to several hours) exposure to 

PM, especially effects related to cardiac ischemia, vasomotor function, and more subtle 
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changes in markers of systemic inflammation, hemostasis, thrombosis and coagulation 

(U.S. EPA, 2009a, section 6.2). Because these studies have used different indicators (e.g., 

PM2.5, PM10, PM10-2.5, ultrafine particles), averaging times (e.g., 1, 2, and 4 hours), and 

health outcomes, it is difficult to draw conclusions about cardiovascular effects 

associated specifically with sub-daily exposures to PM2.5. 

With regard to respiratory effects associated with sub-daily PM2.5 exposures, the 

currently available evidence was much sparser than for cardiovascular effects and 

continues to be very limited. The Integrated Science Assessment concluded that for 

several studies of hospital admissions or medical visits for respiratory diseases, the 

strongest associations were observed with 24-hour average or longer exposures, not with 

less than 24-hour exposures (U.S. EPA, 2009a, section 6.3). 

Collectively, the Policy Assessment concluded that this information, when viewed 

as a whole, is too unclear, with respect to the indicator, averaging time and health 

outcome, to serve as a basis for consideration of establishing a primary PM2.5 standard 

with an averaging time shorter than 24-hours at this time (U.S. EPA, 2011a, p. 2-57). 

With regard to health effects associated with PM2.5 exposure across varying 

seasons in this review, Bell et al. (2008) reported higher PM2.5 risk estimates for 

hospitalization for cardiovascular and respiratory diseases in the winter compared to 

other seasons. In comparison to the winter season, smaller statistically significant 

associations were also reported between PM2.5 and cardiovascular morbidity for spring 

and autumn, and a positive, but statistically non-significant association was observed for 

the summer months. In the case of mortality, Zanobetti and Schwartz (2009) reported a 

4-fold higher effect estimate for PM2.5-associated mortality for the spring as compared to 
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the winter. Taken together, these results provided emerging but limited evidence that 

individuals may be at greater risk of dying from higher exposures to PM2.5 in the warmer 

months and may be at greater risk of PM2.5-associated hospitalization for cardiovascular 

and respiratory diseases during colder months of the year (U.S. EPA, 2011a, p. 2-58). 

Overall, the Policy Assessment observed that there are few studies presently 

available to deduce a general pattern in PM2.5-related risk across seasons. In addition, 

these studies utilized 24-hour exposure periods within each season to assess the 

PM2.5-associated health effects and do not provide information on health effects 

associated with a season-long exposure to PM2.5. Due to these limitations in the currently 

available evidence, the Policy Assessment concluded that there was no basis to consider a 

seasonal averaging time separate from a 24-hour averaging time.

Based on the above considerations, the Policy Assessment concluded that the 

currently available information provided strong support for consideration of retaining the 

current annual and 24-hour averaging times but does not provide support for considering 

alternative averaging times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC considered 

it appropriate to retain the current annual and 24-hour averaging times for the primary 

PM2.5 standards (Samet, 2010c, pp. 2 to 3). At the time of the proposal, the Administrator 

concurred with the staff conclusions and CASAC advice and proposed that the averaging 

times for the primary PM2.5 standards should continue to include annual and 24-hour 

averages to protect against health effects associated with long- and short-term exposures. 

Furthermore, the Administrator provisionally concluded, consistent with conclusions 

reached in the Policy Assessment and by CASAC, that the currently available 

information was too limited to support consideration of alternative averaging times to 
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establish a national standard with a shorter-than 24-hour averaging time or with a 

seasonal averaging time. 

The EPA received no significant public comments on the issue of averaging time 

for the PM2.5 primary standards. The Administrator concurs with recommendations made 

by CASAC and the staff conclusions presented in the Policy Assessment and concludes, 

as proposed, that it is appropriate to retain the current annual and 24-hour averaging 

times for the primary PM2.5 standards to protect against health effects associated with 

long- and short-term exposure periods.

3. Form

The “form” of a standard defines the air quality statistic that is to be compared to 

the level of the standard in determining whether an area attains the standard. In this 

review, the EPA considers whether currently available information supports retaining or 

revising the forms for the annual or 24-hour PM2.5 standards. 

a. Annual Standard

In 1997, the EPA established the form of the annual PM2.5 standard as an annual 

arithmetic mean, averaged over 3 years, from single or multiple community-oriented 

monitors. This form was intended to represent a relatively stable measure of air quality 

and to characterize longer-term area-wide PM2.5 concentrations, in conjunction with a 

24-hour standard designed to provide adequate protection against localized peak or 

seasonal PM2.5 concentrations. The level of the standard was to be compared to 

measurements made at each community-oriented monitoring site, or, if specific criteria 

were met, measurements from multiple community-oriented monitoring sites could be 
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averaged (i.e., spatial averaging)53 (62 FR 38671 to 38672, July 18, 1997). The 

constraints were intended to ensure that spatial averaging would not result in inequities in 

the level of protection provided by the standard (62 FR 38672, July 18, 1997). This 

approach was 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 at-risk populations, which found that the highest concentrations in an area 

tend to be measured at monitors located in areas where the surrounding population is 

more likely to have lower education and income levels and higher percentages of 

minority populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section 

5.3.6.1).

In this review, as outlined in section III.B above and discussed more fully in 

section III.B.3 of the proposal, there now exist more health data such that the Integrated 

Science Assessment has identified persons from lower socioeconomic strata as an at-risk 

population (U.S. EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover, 

there now exist more years of PM2.5 air quality data than were available in the last 

review. Consideration in the Policy Assessment of the spatial variability across urban 

 

53 Spatial averaging as part of the form of the annual PM2.5 standard is unique to this 

standard and is not used with other PM standards nor with other NAAQS.
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areas that was revealed by this expanded data base has raised questions as to whether an 

annual standard that allows for spatial averaging, even within specified constraints as 

narrowed in 2006 (71 FR 61165 to 61167, October 17, 2006), would provide appropriate 

public health protection. 

In considering the potential for disproportionate impacts on at-risk populations, 

the Policy Assessment considered an update of an air quality analysis conducted for the 

last review (U.S. EPA, 2011a, pp. 2-59 to 60; Schmidt, 2011, Analysis A). This analysis 

focused on determining whether the spatial averaging provisions, as modified in 2006, 

could introduce inequities in protection for at-risk populations exposed to PM2.5. 

Specifically, the Policy Assessment considered whether persons of lower socioeconomic 

status, minority groups, or different age groups (i.e., children or older adults) are more 

likely than the general population to live in areas in which the monitors recording the 

highest air quality values in an area are located. Data used in this analysis included 

demographic parameters measured at the Census Block or Census Block Group level, 

including percent minority population, percent minority subgroup population, percent of 

persons living below the poverty level, percent of persons 18 years of age or older, and 

percent of persons 65 years of age and older. In each candidate geographic area, data 

from the Census Block(s) or Census Block Group(s) surrounding the location of the 

monitoring site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) in which the 

highest air quality value was monitored were compared to the average of monitored 

values in the area. This analysis looked beyond areas that would meet the current spatial 

averaging criteria and considered all urban areas (i.e., Core Based Statistical Areas or 

CBSAs) with at least two valid annual design value monitors (Schmidt, 2011, Analysis 
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A). Recognizing the limitations of such cross-sectional analyses, the Policy Assessment 

observed that the highest concentrations in an area tend to be measured at monitors 

located in areas where the surrounding populations are more likely to live below the 

poverty line and to have higher percentage of minorities (U.S. EPA, 2011a, p. 2-60). 

Based upon the analysis described above, the Policy Assessment concluded that 

the existing constraints on spatial averaging, as modified in 2006, may be inadequate to 

avoid substantially greater exposures in some areas, potentially resulting in 

disproportionate impacts on at-risk populations of persons with lower SES levels as well 

as minorities. Therefore, the Policy Assessment concluded that it was appropriate to 

consider revising the form of the annual PM2.5 standard such that it did not allow for the 

use of spatial averaging across monitors. In doing so, the level of the annual PM2.5

standard would be compared to measurements made at the monitoring site that represents 

area-wide air quality recording the highest PM2.5 concentrations54 (U.S. EPA, 2011a, p. 

2-60). 

The CASAC agreed with staff conclusions that it was “reasonable” for the EPA to 

eliminate the spatial averaging provisions (Samet, 2010d, p. 2). Further, in CASAC’s 

comments on the first draft Policy Assessment, it noted, “Given mounting evidence 

showing that persons with lower SES levels are a susceptible group for PM-related health 

risks, CASAC recommends that the provisions that allow for spatial averaging across 

monitors be eliminated for the reasons cited in the (first draft) Policy Assessment” 

(Samet, 2010c, p. 13). In its review of the second draft Policy Assessment, CASAC 

 

54 As discussed in section VIII.B.1 below, the EPA is revising several terms associated 

with PM2.5 monitor placement. Specifically, the EPA is revoking the term “communityoriented” and replacing it with the term “area-wide” monitoring. 
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recognized “although much of the epidemiological research has been conducted using 

community-wide averages, several key studies reference the nearest measurement site, so 

that some risk estimates are not necessarily biased by the averaging process. Further, the 

number of such studies is likely to expand in the future” (Samet, 2010d, pp. 1 to 2).

Only two areas in the country used the initial spatial averaging provisions for 

demonstrating attainment with the primary annual PM2.5 standard set in 1997 (70 FR 

19847, April 14, 2005; U.S. EPA, 2006c). Since these provisions were tightened in 2006,

no area has used spatial averaging to demonstrate attainment. No areas in the country are 

currently using the spatial averaging provisions to demonstrate attainment with the 

current primary annual PM2.5 standard.

In considering the Policy Assessment’s conclusions based on the results of the 

analysis discussed above and concern over the evidence of potential disproportionate 

impacts on at-risk populations as well as CASAC advice, the Administrator proposed to 

revise the form of the annual PM2.5 standard to eliminate the use of spatial averaging. 

Thus, the Administrator proposed revising the form of the annual PM2.5 standard to 

compare the level of the standard with measurements from each “appropriate” monitor in 

an area

55 with no allowance for spatial averaging. Thus, for an area with multiple 

monitors, the appropriate reporting monitor with the highest design value would 

determine the attainment status for that area.

Of the commenters noted in section III.D.2 above who supported a more stringent 

 

55 As discussed in section VIII.B.2.b below, the EPA concludes that PM2.5 monitoring 

sites at micro- and middle-scale locations should be comparable to the annual standard 

unless the monitoring site has been approved by the Regional Administrator as a 

“relatively unique micro-scale, or localized hot-spot, or unique middle-scale site.”
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annual PM2.5 standard, those who commented on the form of the annual PM2.5 standard 

supported the EPA’s proposal to eliminate the spatial averaging provisions. These 

commenters contended that the EPA’s analyses of the potential impacts of spatial 

averaging, discussed above and in the proposal (77 FR 38924), demonstrated that the 

current form results in uneven public health protection leading to disproportionate 

impacts on at-risk populations. Specifically, the ALA and other environmental and public 

health commenters contended that “spatial averaging allows exposure of people to 

unhealthy levels of pollution at specific locales even within an area meeting the standard” 

(ALA et al., 2012, p. 23). These commenters particularly focused on the importance for 

low-income and minority populations of eliminating the spatial averaging provisions. 

They concluded that spatial averaging “is an environmental justice concern because poor 

people are more likely to live near roads, depots, factories, ports, and other pollution 

sources.” Id. p. 24.

Other commenters (e.g., AAM, 2012; Dow, 2012) also supported the elimination 

of spatial averaging in order to “avoid potential disproportionate impacts on at-risk 

populations” and to maximize “the benefits to public health of reducing the annual PM2.5

standard.” However, these groups expressed concern that the elimination of spatial 

averaging, in combination with the requirement for near road monitors (as discussed in 

section VIII.B.3.b.i of the proposal), would effectively and inappropriately increase the 

stringency of the annual PM2.5 standard. 

This concern was also shared by other commenters who disagreed with the 

elimination of spatial averaging. For example, the Class of ’85 RRG emphasized 

concerns about increasing the stringency of the standard while providing few health 
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benefits if spatial averaging is eliminated, particularly in combination with the 

requirement for near-road monitors. These commenters contended that “[b]ecause EPA 

proposes to use the readings from the highest single worst case monitor (rather than the 

average of all community area monitors), and since roadway monitoring locations will 

likely be worst case monitors, the proposed NAAQS will become more stringent without 

targeting the PM2.5 species most harmful to human health” (Class of ’85 RRG, 2012, p. 

6).

Several commenters also maintained that because spatial averaging is consistent 

with how air quality data are considered in the underlying epidemiological studies, such 

averaging should not be eliminated. Specifically, commenters including NAM et al., 

AFPM, and ACC pointed out that PM2.5 epidemiological studies use spatially averaged 

multi-monitor concentrations, rather than the single highest monitor, when evaluating 

health effects. Therefore, these commenters contended that allowing spatial averaging 

would make the PM2.5 standard more consistent with the approaches used in the

epidemiological studies upon which the standard is based. In addition, some commenters 

also contended that the EPA failed to consider whether modifying, rather than 

eliminating, the constraints on spatial averaging would have been sufficient to protect the 

public health. If so, these commenters argued that “elimination of spatial averaging 

would go beyond what is requisite to protect the public health” (NAM et al., 2012, p. 20).

In considering the public comments on the form of the annual standard, the EPA 

recognizes a number of commenters agreed with the basis for the EPA’s proposal to 

eliminate spatial averaging. While other commenters expressed disagreement or concern 

with the proposed decision to eliminate the spatial averaging provisions, the Agency 
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notes that these commenters did not challenge the analyses or considerations that 

provided the fundamental basis for the Administrator’s proposed decision. Rather, these 

commenters generally raised concerns that eliminating the option for spatial averaging 

would increase the stringency of the standard, especially in light of additional monitoring 

sites in near-road environments (as discussed in section VIII.B.3.b.1 below). 

The EPA does not agree with the comment that siting some monitors in near 

roadway environments makes the standard more stringent or impermissibly more 

stringent. As discussed in section VIII.B.3.b.i below, a significant fraction of the 

population lives in proximity to major roads, and these exposures occur in locations that 

represent ambient air. Monitoring in such areas does not make the standard more 

stringent than warranted, but rather affords the intended protection to the exposed 

populations, among them at-risk populations, exposed to fine particles in these areas. 

Thus, the EPA has made a determination that monitors representative of area-wide air 

quality may be compared to the annual standard. The 24-hour and annual NAAQS are 

designed to protect the public with an adequate margin of safety, and this siting provision 

is fully consistent with providing the protection the standard is designed to provide and 

does not make the standard more stringent or more stringent than necessary. 

Monitors that are representative of area-wide air quality may be compared to the 

annual standard. This is consistent with the use of monitoring data in the epidemiological 

studies that provide the primary basis for determining the level of the annual standard. In 

addition, the EPA notes that the annual standard is designed to protect against both longand short-term exposures through controlling the broad distribution of air quality across 
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an area over time.56 It is fully consistent with the protection the standard is designed to 

provide for near road monitors to be compared to the annual standard if the monitor is 

representative of area-wide air quality. This does not make the standard either more 

stringent or impermissibly more stringent.

In further considering these comments, the EPA notes that the stringency or level 

of protection provided by each NAAQS is not based solely on the form of the standard; 

rather, the four elements of the standard that together serve to define each standard (i.e., 

indicator, averaging time, form, and level) must be considered collectively in evaluating 

the protection afforded by each standard. Therefore, the EPA considers these comments 

are also appropriate to discuss collectively with other issues related to the appropriate 

level for annual standard, and are discussed below in sections III.E.4.c-d. 

In reaching a final decision on the form of the annual standard, the Administrator 

considers the available analyses, CASAC advice, and public comments on form as 

discussed above. She also considers related issues in the public comments on the level of 

the annual standard as discussed in section III.E.4.c below. She notes that even when the 

annual PM2.5 standard was first set in 1997, the spatial averaging provisions included 

constraints intended to ensure that inequities in the level of protection would not result. 

These constraints on spatial averaging were tightened in the last review, based on an 

analysis showing the potential for spatial averaging to allow higher PM2.5 concentrations 

in locations where subgroups within the general population were potentially 

 

56 This is in contrast to the 24-hour standard which is designed to provide supplemental protection, 

addressing peak exposures that might not otherwise be addressed by the annual standard. Consistent with 

this, monitors are not required to be representative of area-wide air quality to be compared to the 24-hour 

standard.

Formatted: Not Superscript/ Subscript
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disproportionately exposed and hence, at disproportionate risk (e.g., low income and 

minority communities). The Administrator notes that in proposing to eliminate spatial 

averaging altogether in this review, she has relied on further analyses in the current 

review (Schmidt, 2011, Analysis A). As discussed above and in the proposal (77 FR 

38924), these analyses showed that the current constraints on spatial averaging may be 

inadequate in some areas to avoid substantially greater exposures for people living near 

monitors recording the highest PM2.5 concentrations. Such exposures could result in 

disproportionate impacts to at-risk populations, including low-income populations as well 

as minority groups. 

On this basis, the Administrator concludes that public health would not be 

protected with an adequate margin of safety in all locations, as required by law, if 

disproportionately higher exposure concentrations in at-risk populations such as low 

income communities as well as minority communities were averaged together with lower 

concentrations measured at other sites in a large urban area. See ALA v. EPA, 134 F. 3d 

388, 389 (D.C. Cir., 1998) (“this court has held that ‘NAAQS must protect not only 

average healthy individuals, but also sensitive citizens such as children,’ and ‘if a 

pollutant adversely affects the health of these sensitive individuals, EPA must strengthen 

the entire national standard’”) and Coalition of Battery Recyclers Association v. EPA, 604 

F 3d. 613, 617 (D.C. Cir., 2010) (“Petitioners’ assertion that the revised lead NAAQS is 

overprotective because it is more stringent than necessary to protect the entire population 

of young U.S. children ignores that the Clean Air Act allows protection of sensitive 

subpopulations.”) In reaching this conclusion, the Administrator further notes that her 

concern over possible disproportionate PM2.5-related health impacts in at-risk populations 
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extends to populations living near important sources of PM2.5, including the large 

populations that live near major roadways.57

In light of all of the above considerations, including consideration of available 

analyses, CASAC advice, and public comments, the Administrator concludes that the 

current form of the annual PM2.5 standard should be revised to eliminate spatial averaging 

provisions. Thus, the level of the revised annual PM2.5 standard established with this 

notice will be compared with measurements from each appropriate monitor in an area, 

with no allowance for spatial averaging. The Administrator’s conclusions with regard to 

the appropriate level of the annual PM2.5 standard to set in conjunction with this form are 

discussed below in section III.E.4.d.

b. 24-Hour Standard

In 1997, the EPA established the form of the 24-hour PM2.5 standard as the 98th

percentile of 24-hour concentrations at each population-oriented monitor within an area, 

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 

 

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

proposed requirement for near-road monitors. 
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this form based in part on a comparison with the 99th percentile form.58

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 considered air quality data 

reported in 2000 to 2008 to update our understanding of the ratio between peak-to-mean 

PM2.5 concentrations. This analysis provided evidence that the 98th percentile value was a 

more stable metric than the 99th percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62). 

At the time of the proposal, the Agency recognized that the selection of the 

appropriate form of the 24-hour standard includes maintaining adequate protection 

against peak 24-hour concentrations while also providing a stable target for risk 

management programs, which serves to provide for the most effective public health 

protection in the long run.59 As in previous reviews, the EPA recognized that a 

concentration-based form, compared to an exceedance-based form, was more reflective 

of the health risks posed by elevated pollutant concentrations because such a form gives 

proportionally greater weight to days when concentrations are well above the level of the 

standard than to days when the concentrations are just above the level of the standard. 

Further, the Agency provisionally concluded that a concentration-based form, when 

 

58 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).

59 See ATA III, 283 F.3d at 374-376 which concludes that it is legitimate for the EPA to 

consider overall stability of the standard and its resulting promotion of overall 

effectiveness of NAAQS control programs in setting a standard that is requisite to protect 

the public health. 
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averaged over three years, provided an appropriate balance between limiting peak 

pollutant concentrations and providing a stable regulatory target, thus facilitating the 

development of more stable implementation programs. 

In considering the information provided in the Policy Assessment and recognizing 

that the degree of public health protection likely to be afforded by a standard is a result of 

the combination of the form and the level of the standard, the Administrator proposed to 

retain the 98th percentile form of the 24-hour standard. The Administrator provisionally 

concluded that the 98th percentile form represents an appropriate balance between 

adequately limiting the occurrence of peak concentrations and providing increased 

stability relative to an alternative 99th percentile form. 

Few public commenters commented specifically on the form of the 24-hour 

standard. None of the public commenters raised objections to continuing the use of a 

concentration-based form for the 24-hour standard. Many of the individuals and groups 

who supported a more stringent 24-hour PM2.5 standard noted in section III.D.2 above, 

however, recommended a more restrictive concentration-based percentile form, 

specifically a 99th percentile form. The limited number of these commenters who 

provided a specific rationale for this recommendation generally expressed their concern 

that the 98th percentile form could allow too many days where concentrations exceeded 

the level of the standard, and thus fail to adequately protect public health. Other public 

commenters representing state and local air agencies and industry groups generally 

supported retaining the current 98th percentile form. In most cases, these groups 

expressed the overall view that the current 24-hour PM2.5 standard, including the form of 

the current standard, should be retained. 
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The EPA notes that the viewpoints represented in this review are similar to 

comments submitted in the last review and through various NAAQS reviews. The EPA 

recognizes that the selection of the appropriate form includes maintaining adequate 

protection against peak 24-hour values while also providing a stable target for risk 

management programs, which serves to provide for the most effective public health 

protection in the long run.60 Nothing in the commenters’ views has provided a reason to 

change the Administrator’s previous conclusion regarding the appropriate balance 

represented in the proposed form of the 24-hour PM2.5 standard. Therefore, the 

Administrator concurs with staff conclusions presented in the Policy Assessment and 

CASAC recommendations and concludes that it is appropriate to retain the 98th percentile 

form for the 24-hour PM2.5 standard.

4. Level

In the last review, the EPA selected levels for the annual and the 24-hour PM2.5

standards using evidence of effects associated with periods of exposure that were most 

closely matched to the averaging time of each standard. Thus, as discussed in section 

III.A.1, the EPA relied upon evidence from long-term exposure studies as the principal 

basis for selecting the level of the annual PM2.5 standard that would protect against 

effects associated with long-term exposures. The EPA relied upon evidence from the 

short-term exposure studies as the principal basis for selecting the level of the 24-hour 

 

60 As just noted above, it is legitimate for the EPA to consider promotion of overall 

effectiveness of risk management programs designed to attain the NAAQS, including 

their overall stability, in setting a standard that is requisite to protect the public health. 

The context for the court’s discussion in ATA III is identical to that here; whether to adopt 

a 98th percentile form for a 24-hour standard intended to provide supplemental protection 

for a generally controlling annual standard.
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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 61 was challenged and on judicial review, the D.C. 

Circuit remanded the primary annual PM2.5 standard to the EPA, finding that EPA’s 

explanation for its approach to setting the level of the annual standard was inadequate. 

a. General Approach for Considering Standard Levels

Building upon the lessons learned in the previous PM NAAQS reviews, in 

considering alternative standard levels supported by the currently available scientific 

information, the Policy Assessment used an approach that integrated evidence-based and 

risk-based considerations, took into account CASAC advice, and considered the issues 

raised by the court in remanding the primary annual PM2.5 standard. Following the 

general approach outlined in section III.A.3 above, for the reasons discussed below, the 

Policy Assessment concluded it was appropriate to consider the protection afforded by 

the annual and 24-hour standards taken together against mortality and morbidity effects 

associated with both long- and short-term PM2.5 exposures. This was consistent with the 

approach taken in the review completed in 1997 rather than considering each standard 

separately, as was done in the review completed in 2006. 

Beyond looking directly at the relevant epidemiologic evidence, the Policy 

Assessment considered the extent to which specific alternative PM2.5 standard levels were 

likely to reduce the nature and magnitude of both long-term exposure-related mortality 

 

61 Throughout this section, the annual standard levels are denoted as integer values for 

simplicity, although, as noted above in section II.B.1, Table 1, the annual standard level 

is defined to one decimal place, such that the current annual standard level is 15.0 μg/m

3

. 

Alternative annual standard levels discussed in this section are similarly defined to one 

decimal place.
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risk and short-term exposure-related mortality and morbidity risk (U.S. EPA, 2011a, 

section 2.3.4.2; U.S.EPA, 2010a, section 4.2.2). As noted in section III.C.3 above, 

patterns of increasing estimated risk reductions were generally observed as either the 

annual or 24-hour standard, or both, were reduced below the level of the current 

standards (U.S. 2011a, Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 

5.2.3). 

Based on the quantitative risk assessment, the Policy Assessment observed, as 

discussed in section III.A.3, that analyses conducted for this and previous reviews 

demonstrated that much, if not most, of the aggregate risk associated with short-term 

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

concentrations are in the low-to mid-range, below the peak 24-hour concentrations (U.S.

EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C above and in section 

III.C.3 of the proposal, the Risk Assessment observed that alternative annual standard 

levels, when controlling, resulted in more consistent risk reductions across urban study 

areas, thereby potentially providing a more consistent degree of public health protection 

(U.S. EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk Assessment noted that the 

results of simulating alternative suites of PM2.5 standards including different 

combinations of alternative annual and 24-hour standard levels suggested that an 

alternative 24-hour standard level can produce additional estimated risk reductions 

beyond that provided by an alternative annual standard alone. However, the degree of 

estimated risk reduction provided by alternative 24-hour standard levels was highly 

variable, in part due to the choice of rollback approached used (U.S. EPA, 2010a, p. 5-

17). 
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Based on its review of the second draft Policy Assessment, CASAC agreed with 

the EPA staff’s general approach for translating the available epidemiological evidence, 

risk information, and air quality information into the basis for reaching conclusions on 

alternative standards for consideration. Furthermore, CASAC agreed “that it is 

appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have 

evidence supportive of a causal or likely causal relationship as presented in the Integrated 

Science Assessment (Samet, 2010d, p. 1). 

Therefore, the Policy Assessment concluded, consistent with specific CASAC 

advice, that it was appropriate to set a “generally controlling” annual standard that will 

lower a wide range of ambient 24-hour concentrations. The Policy Assessment concluded 

this approach would likely reduce aggregate risks associated with both long- and shortterm exposures with more consistency than a generally controlling 24-hour standard and 

would be the most effective and efficient way to reduce total PM2.5-related population 

risk and so provide appropriate protection. The staff believed this approach, in contrast to 

one focusing on a generally controlling 24-hour standard, would likely reduce aggregate 

risks associated with both long- and short-term exposures with more consistency and 

would likely avoid setting national standards that could result in relatively uneven 

protection across the country due to setting standards that were either more or less 

stringent than necessary in different geographical areas. 

The Policy Assessment recognized that an annual standard intended to serve as 
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the primary means for providing protection against effects associated with both long- and 

short-term PM2.5 exposures cannot be expected to offer an adequate margin of safety 

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

generally controlling annual standard, the Policy Assessment concluded it was 

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

particularly for areas with high peak-to-mean ratios possibly associated with strong local 

or seasonal sources, or PM2.5-related effects that may be associated with shorter-thandaily exposure periods.

At the time of the proposal, the Administrator agreed with the approach discussed 

in the Policy Assessment as summarized in section III.A.3 above, and supported by 

CASAC, of considering the protection afforded by the annual and 24-hour standards 

taken together for mortality and morbidity effects associated with both long- and shortterm exposures to PM2.5. Furthermore, based on the evidence and quantitative risk 

assessment, the Administrator provisionally concluded it was appropriate to set a 

“generally controlling” annual standard that will lower a wide range of ambient 24-hour 

concentrations, with a 24-hour standard focused on providing supplemental protection, 

particularly for areas with high peak-to-mean ratios possibly associated with strong local 

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

daily exposure periods. The Administrator provisionally concluded this approach would 

likely reduce aggregate risks associated with both long- and short-term exposures more 

consistently than a generally controlling 24-hour standard and would be the most 

effective and efficient way to reduce total PM2-5-related population risk.

The Administrator is mindful that considering what standards are requisite to 
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protect public health with an adequate margin of safety requires public health policy 

judgments that neither overstate nor understate the strength and limitations of the 

evidence or the appropriate inferences to be drawn from the evidence. At the time of the 

proposal, in considering how to translate the available information into appropriate 

standard levels, the Administrator weighed the available scientific information and 

associated uncertainties and limitations. For the purpose of determining what standard 

levels were appropriate to propose, the Administrator recognized, as did the EPA staff in 

the Policy Assessment, that there was no single factor or criterion that comprised the 

“correct” approach to weighing the various types of available evidence and information, 

but rather there were various approaches that were appropriate to consider. The 

Administrator further recognized that different evaluations of the evidence and other 

information before the Administrator could reflect placing different weight on the relative 

strengths and limitations of the scientific information, and different judgments could be 

made as to how such information should appropriately be used in making public health 

policy decisions on standard levels. This recognition led the Administrator to consider 

various approaches to weighing the evidence so as to identify appropriate standard levels 

to propose. In so doing, the Administrator encouraged extensive public comment on 

alternative approaches to weighing the evidence and other information so as to inform her 

public health policy judgments before reaching final decisions on appropriate standard 

levels.

b. Proposed Decisions on Standard Levels

i. Consideration of the Alternative Standard Levels in the Policy Assessment

In recognizing the absence of a discernible population threshold below which 
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effects would not occur, the Policy Assessment’s general approach for identifying 

alternative annual standard levels that were appropriate to consider focused on 

characterizing the part of the distribution of PM2.5 concentrations in which we had the 

most confidence in the associations reported in the epidemiological studies and 

conversely where our confidence in the association became appreciably lower. The most 

direct approach to address this issue, consistent with CASAC advice (Samet, 2010c, 

p.10), was to consider epidemiological studies reporting confidence intervals around 

concentration-response relationships (U.S. EPA, 2011a, p. 2-63). Based on a thorough 

search of the available evidence, the Policy Assessment identified only one study 

(Schwartz et al., 2008) that conducted a multi-model analysis to characterize confidence 

intervals around the estimated concentration-response relationship. The Policy 

Assessment concluded that this single relevant analysis was too limited to serve as the 

principal basis for identifying alternative standard levels in this review (U.S. EPA, 2011a, 

p. 2-70).

The Policy Assessment explored other approaches that considered different 

statistical metrics to identify heuristic approaches to characterizinge the part of the 

distributions of long-term mean PM2.5 concentrations that were most influential in 

generating health effect estimates in long- and short-term epidemiological studies, and 

placed greatest weight on those studies that reported positive and statistically significant 

associations (U.S. EPA, 2011a, p. 2-63). First, as discussed in section III.A.3 above, the 

Policy Assessment considered the statistical metric used in previous reviews. This 

approach recognized our assumptionthe EPA’s view that the strongest evidence of 

associations occurs at concentrations around the long-term mean concentration. Thus, in 
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earlier reviews, the EPA focused on identifying standard levels that were somewhat 

below the long-term mean concentrations reported in PM2.5 epidemiologicalxposure

studies. The long-term mean concentrations represented air quality data typically used in 

epidemiological analyses and provided a direct link between PM2.5 concentrations and the 

observed health effects. Further, these data were available for all long- and short-term 

exposure studies analyzed and, therefore, represented the data set available for the 

broadest set of epidemiological studies. 

However, consistent with CASAC’s comments on the second draft Policy 

Assessment62 (Samet, 2010d, p. 2), in preparing the final Policy Assessment, the EPA 

staff explored ways to take into account additional information from epidemiological 

studies, when available (Rajan et al., 2011). These analyses focused on evaluating 

different statistical metrics, beyond the long-term mean concentration, to characterize the 

part of the distribution of PM2.5 concentrations in which staff continued to have 

confidence in the associations observed in epidemiological studies and below which there 

was a comparative lack of data such that the staff’s confidence in the relationship was 

appreciably less. This would also be the part of the distribution of PM2.5 concentrations 

which had the most influence on generating the health effect estimates reported in 

epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment 

recognized there was no one percentile value within a given distribution that was the 

most appropriate or “correct” way to characterize where our confidence in the 

 

62 While CASAC expressed the view that it would be most desirable to have information 

on concentration-response relationships, they recognized that it would also be “preferable 

to have information on the concentrations that were most influential in generating the 

health effect estimates in individual studies” (Samet, 2010d, p. 2). 
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associations becomes appreciably lower. The Policy Assessment concluded that focusing 

on concentrations within the lower quartile of a distribution, such as the range from the 

25th to the 10thpercentile, was reasonable to consider as a region within which we begin 

to have appreciably less confidence in the associations observed in epidemiological 

studies.63 In the EPA staff’s view, considering lower PM2.5 concentrations, down to the 

lowest concentration observed in a study, would be a highly uncertain basis for selecting 

alternative standard levels (U.S. EPA, 2009a, p. 2-71).

As outlined in section III.A.3 above, the Policy Assessment recognized that there 

were two types of population-level information to consider in identifying the range of 

PM2.5 concentrations which have the most influence on generating the health effect 

estimates reported in epidemiological studies. The most relevant information to consider 

was the number of health events (e.g., deaths, hospitalizations) occurring within a study 

population in relation to the distribution of PM2.5 concentrations likely experienced by 

study participants. However, in recognizing that access to health event data may be 

restricted, and consistent with advice from CASAC (Samet 2010d, p.2), EPA staff also 

considered the number of participants within each study area, in relation to the 

 

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 (U.S. EPA, 

2005, pp. 5-22 to 5-23). In this review, the Policy Assessment noted the interrelatedness 

of the distributional statistics and a range of one standard deviation around the mean 

which contains approximately 68 percent of normally distributed data, in that one 

standard deviation below the mean falls between the 25th and 10th percentiles (U.S. EPA, 

2011a, p. 2-71). 
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distribution of PM2.5 concentrations (i.e., study population data), as a surrogate for health 

event data. 

In applying this approach, the Policy Assessment focused on identifying the part 

of the distribution of PM2.5 concentrations which had the most influence on generating 

health effect estimates in epidemiological studies, as discussed in section III.A.3 above. 

As discussed below, in working with study investigators, the EPA staff was able to obtain 

health event data for three large multi-city studies (Krewski et al., 2009; Zanobetti and 

Schwartz, 2009; Bell et al., 2008) and population data for the same three studies and one 

additional long-term exposure study (Miller et al., 2007), as documented in a staff 

memorandum (Rajan et al., 2011).64 For the three studies for which both health event and 

study population data were available, the EPA staff analyzed the reliability of using study 

population data as a surrogate for health event data. Based on these analyses, the EPA 

staff recognized that the 10th and 25th percentiles of the health event and study population 

distributions are nearly identical and concluded that the distribution of population data 

can be a useful surrogate for event data, providing support for consideration of the study 

population data for Miller et al. (2007), for which health event data were not available 

(Rajan et al., 2011, Analysis 1 and Analysis 2, in particular, Table 1 and Figures 1 and 2).

With regard to the long-term mean PM2.5 concentrations which are relevant to the 

first approach, Figures 1 through 3 (U.S. EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) 

summarize data available for multi-city, long- and short-term exposure studies that 

 

64 The distributional statistical analysis of population-level data built upon an earlier 

analysis that evaluated the distributions of air quality and associated population data for 

three long-term exposure studies and three short-term exposure studies (Schmidt et al., 

2010, Analysis 2). 
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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.

65 As 

discussed in more detail in section III.E.4.b of the proposal, Figures 1 and 3 summarize 

the health outcomes evaluated, relative risk estimates, air quality data, and geographic 

scope for long- and short-term exposure studies, respectively, that evaluated mortality 

(evidence of a causal relationship); cardiovascular effects (evidence of a causal 

relationship); and respiratory effects (evidence of a likely causal relationship) in the 

general population, as well as in older adults, an at-risk population. Figure 2 provides this 

same summary information for long-term exposure studies that evaluated respiratory 

effects (evidence of a likely causal relationship) in children, an at-risk population, as well 

as developmental effects (evidence suggestive of a causal relationship). 

 

65 Additional studies presented and assessed in the Integrated Science Assessment report 

effects at higher long-term mean PM2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2-

1, 2-2, 7-6, and 7-7).
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Figure 1. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Long-term PM2.5 Exposure 

Studies of the General Population and Older Adults66, 67, 68

 

66 The air quality data for the Miller et al. (2007) study is discussed more fully in the Response to Comments document.

67 The Goss et al. (2004) study patients over the age of 6 years enrolled in the Cystic Fibrosis Foundation National Patient Registry in 1999 

and 2000. The average age of the study participants was 18.4 (SD 10).

68 The air quality data reported in Figure 1 of the proposal incorrectly identified the long-term mean PM2.5 concentration for this study as 14.3 μg/m3

, which was the longterm mean PM2.5 concentration for Lipfert et al. (2006b). This mistake was also made in the Figure 2-2 of the ISA (U.S. EPA, 2009a, p. 2-15). The EPA notes that 

footnote a for Figure 4 of Lipfert et al. (2006a) identifies that the mortality risks for PM2.5 are based on one year of air quality data (1999). However, this study does not 

report the long-term mean PM2.5 concentration for 1999. Rather, in Table 1 of Lipfert et al. (2006a), the study authors reported long-term mean PM2.5 concentrations of 

14.3 μg/m3for the years 1989 to 1996 and 14.6 μg/m3for the years 1997 to 2001.
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Source: U.S. EPA, 2011a, Figure 2-4
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Figure 2. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Long-term PM2.5 Exposure 

Studies of Children

Source: U.S. EPA, 2011a, Figure 2-5
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Figure 3. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Short-term PM2.5 Exposure 

Studies of the General Population and Older Adults
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Source: U.S. EPA, 2011a, Figure 2-6
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With regard to consideration of additional information from epidemiological studies which was 

relevant to the second approach, the EPA staff compiled a summary of the range of PM2.5 concentrations 

corresponding with the 25th to 10th percentiles of health event or study population data from the four 

multi-city studies, for which distributional statistics are available69 (U.S. EPA, 2011a, Figure 2-7; Rajan 

et al., 2011, Table 1). By considering this approach, one could focus on the range of PM2.5

concentrations below the long-term mean ambient concentrations over which we continue to have 

confidence in the associations observed in epidemiological studies (e.g., above the 25th percentile) where 

commensurate public health protection could be obtained for PM2.5-related effects and, conversely, 

identify the range in the distribution below which our confidence in the associations is appreciably less, 

to identify alternative annual standard levels. 

The mean PM2.5 concentrations associated with the studies summarized in Figures 1, 2, and 3 

and with the distributional statistics analyses (Rajan et al., 2011) are based on concentrations averaged 

across ambient monitors within each area included in a given study and then averaged across study areas 

to calculate an overall study mean concentration, as discussed above. Figure 4, discussed in more detail 

in section III.E.4.a of the proposal, summarizes statistical metrics for those key studies70 included in

 

69 The EPA staff obtained health event data (e.g., number of deaths, hospitalizations) occurring in a 

study population for three multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et 

al., 2008) and study population data were obtained for the same three studies and one additional study 

(Miller et al., 2007) (U.S. EPA, 2011a, p.2-71). If health event or study population data were available 

for additional studies, the EPA could employ distributional statistics to identify the broader range of 

PM2.5 concentrations that were most influential in generating health effect estimates in those studies. 

70 Long- and short-term exposure studies considered “key” studies for consideration are summarized in 

Figure 4 and include those studies observing effects for which the evidence supported a causal or likely 

causal association. This figure represents the subset of multi-city studies included in Figures 1 through 3 

that provided evidence of positive and generally statistically significant effects associated in whole or in 

part with more recent air quality data, generally representing health effects associated with lower PM2.5

concentrations than had previously been considered in the last review. The EPA notes that many of these 

studies evaluated multiple health endpoints, and not all of the effects evaluated provided evidence of 

positive and statistically significant effects. For purposes of informing the Administrator’s decision on 

the appropriate standard levels, the Agency considers the full body of scientific evidence and focuses on 
Summary of Interagency Working Comments on Draft Language under 

EO12866/13563 Interagency Review. Subject to Further Policy Review.
Summary of Interagency Working Comments on Draft Language under EO12866/13563 Interagency Review. 

Subject to Further Policy Review.

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Figure 4. Translating Epidemiological Evidence from Multi-City Exposure Studies into an Annual PM2.5 Standard71

 

71 The air quality data for the Miller et al. (2007) study is discussed more fully in the Response to Comments document.
Summary of Interagency Working Comments on Draft Language under EO12866/13563 Interagency Review. 

Subject to Further Policy Review.

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Source: U.S. EPA, 2011a, Figure 2-
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Figures 1, 2, and 3 that provide evidence of positive and generally statistically significant 

PM2.5-related effects, which are relevant to the two approaches for translating 

epidemiological evidence into potential standard levels as discussed above. The top of 

Figure 4 includes information for long-term exposure studies evaluating health outcomes 

classified as having evidence of a causal or likely causal relationship with PM2.5

exposures (long-term mean PM2.5 concentrations indicated by diamond symbols). The 

middle of Figure 4 includes information for short-term exposure studies evaluating health 

outcomes classified as having evidence of a causal or likely causal relationship with 

PM2.5 exposures (long-term mean PM2.5 concentrations indicated by triangle symbols). 

The bottom of Figure 4 includes information for long-term exposures studies evaluating 

health outcomes classified as having evidence suggestive of a causal relationship (longterm mean PM2.5 concentrations indicated by square symbols). Figure 4 also summarizes 

the range of PM2.5 concentrations corresponding with the 25th (indicated by solid circles) 

to 10th (indicated by open circles) percentiles of the health event or study population data 

from the four multi-city studies (highlighted in bold text) for which distributional 

statistics are available.

In considering the evidence, the Policy Assessment recognized that NAAQS are 

standards set so as to provide requisite protection, neither more nor less stringent than 

necessary to protect public health, with an adequate margin of safety. This judgment, 

ultimately made by the Administrator, involves weighing the strength of the evidence and 

the inherent uncertainties and limitations of that evidence. Therefore, depending on the 

weight placed on different aspects of the evidence and inherent uncertainties, 

consideration of different alternative standard levels could be supported. 
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Given the currently available evidence discussed in more detail in section 

III.E.4.b of the proposal and considering the various approaches discussed above, the 

Policy Assessment concluded it was appropriate to focus on an annual standard level 

within a range of about 12 to 11 μg/m3(U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-106). 

As illustrated in Figure 4, the Policy Assessment recognized that a standard level of 12 

μg/m3, at the upper end of this range, was somewhat below the long-term mean PM2.5

concentrations reported in all the multi-city, long- and short-term exposure studies that 

provided evidence of positive and statistically significant associations with health effects 

classified as having evidence of a causal or likely causal relationship, including 

premature mortality and hospitalizations and emergency department visits for 

cardiovascular and respiratory effects as well as respiratory effects in children. Further, a 

level of 12 μg/m3 would reflect consideration of additional population-level information 

from such epidemiological studies in that it generally corresponded with approximately 

the 25th percentile of the available distributions of health events data in the studies for 

which population-level information was available. In addition, a level of 12 μg/m3 would 

reflect some consideration of studies that provided more limited evidence of reproductive 

and developmental effects, which were suggestive of a causal relationship, in that it was 

about at the same level as the lowest long-term mean PM2.5 concentrations reported in 

such studies (see Figure 4). 

Alternatively the Policy Assessment recognized that an annual standard level of 

11 μg/m3, at the lower end of this range, was well below the lowest long-term mean 

PM2.5 concentrations reported in all multi-city long- and short-term exposure studies that 

provide evidence of positive and statistically significant associations with health effects 
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classified as having evidence of a causal or likely causal relationship. A level of 11 μg/m3

would reflect placing more weight on the distributions of health event and population 

data, in that this level was within the range of PM2.5 concentrations corresponding to the 

25th and 10th percentiles of all the available distributions of such data. In addition, a level 

of 11 μg/m3 was somewhat below the lowest long-term mean PM2.5 concentrations 

reported in reproductive and developmental effects studies that are suggestive of a causal 

relationship. Thus, a level of 11 μg/m3 would reflect an approach to translating the 

available evidence that places relatively more emphasis on margin of safety 

considerations and less certain causal relationships than would a standard set at a higher 

level. Such a policy approach would tend to weigh uncertainties in the evidence in such a 

way as to avoid potentially underestimating PM2.5-related risks to public health. Further, 

recognizing the uncertainties inherent in identifying any particular point at which our 

confidence in reported associations becomes appreciably less, the Policy Assessment 

concluded that the available evidence did not provide a sufficient basis to consider 

alternative annual standard levels below 11 μg/m3(U.S. EPA, 2011a, p. 2-81). 

The Policy Assessment also considered the extent to which the available evidence 

provided a basis for considering alternative annual standard levels above 12 μg/m3. As 

discussed below, the Policy Assessment concluded that it could be reasonable to consider 

a standard level up to 13 μg/m3based on a policy approach that weighed uncertainties in 

the evidence in such a way as to avoid potentially overestimating PM2.5-related risks to 

public health, especially to the extent that primary emphasis was placed on long-term 

exposure studies as a basis for an annual standard level. A level of 13 μg/m3 was 

somewhat below the long-term mean PM2.5 concentrations reported in all but one of the 
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long-term exposure studies providing evidence of positive and statistically significant 

associations with PM2.5-related health effects classified as having a causal or likely causal 

relationship. As shown in Figure 4, the one long-term exposure study with a long-term 

mean PM2.5 concentration just below 13 μg/m3 was the Miller et al., (2007) study. 

However, as noted in section III.D.1.a of the proposal and discussed in more detail in the 

Response to Comments document, the Policy Assessment observed that in comparison to 

other long-term exposure studies, the Miller et al. study was more limited in that it was 

based on only one year of air quality data and the one year was after the health outcomes 

were reported (U.S. EPA, 2011a, pp. 2-81 to 2-82). Thus, to the extent that less weight 

was placed on the Miller et al. study than on other long-term exposure studies with more 

robust air quality data, a level of 13 μg/m3could be considered as being protective of 

long-term exposure related effects classified as having a causal or likely causal 

relationship. In also considering short-term exposure studies, however, the Policy 

Assessment noted that a level of 13 μg/m3 was below the long-term mean PM2.5

concentrations reported in most but not all such studies. In particular, two studies --

Burnett et al. (2004) and Bell et al. (2008) -- reported long-term mean PM2.5

concentrations of 12.8 and 12.9 μg/m3, respectively. In considering these studies, the 

Policy Assessment found no basis to conclude that these two studies were any more 

limited or uncertain than the other short-term exposure studies shown in Figures 3 and 4 

(U.S. EPA, 2011a, p. 2-82). On this basis, as discussed below, the Policy Assessment 

concluded that consideration of an annual standard level of 13 μg/m3 would have 

implications for the degree of protection that would need to be provided by the 24-hour 

standard, in order that the suite of PM2.5 standards, taken together, would provide 
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appropriate protection from effects on public health related to short-term exposure to 

PM2.5 (U.S. EPA, 2011a, p. 2-82).

The Policy Assessment also noted that a standard level of 13 μg/m3 would reflect 

a judgment that the uncertainties in the epidemiological evidence as summarized in 

section III.B above and discussed in more detail in section III.B.2 of the proposal, 

including uncertainties related to the heterogeneity observed in the epidemiological 

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

components, and the potential role of co-pollutants, are too great to warrant placing any 

weight on the distributions of health event and population data that extend down below 

the long-term mean concentrations into the lower quartile of the data. This level would 

also reflect a judgment that the evidence from reproductive and developmental effects 

studies that is suggestive of a causal relationship was too uncertain to support 

consideration of any lower level.

Beyond evidence-based considerations, the Policy Assessment also considered the 

extent to which the quantitative risk assessment supported consideration of these 

alternative standard levels or provided support for lower levels. In considering 

simulations of just meeting alternative annual standard levels within the range of 13 to 11 

μg/m3(in conjunction with the current 24-hour standard level of 35 μg/m3), the Policy 

Assessment concluded that important public health improvements are associated with risk 

reductions estimated for standard levels of 13 and 12 μg/m3and noted that the level of 11 

μg/m3 was not included in the quantitative risk assessment. The Policy Assessment noted 

that the overall confidence in the quantitative risk estimates varied for the different 

alternative standard levels evaluated and was stronger for the higher levels and 
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substantially lower for the lowest level evaluated (i.e., 10 μg/m3). Based on the above 

considerations, the Policy Assessment concluded that the quantitative risk assessment 

provided support for considering alternative annual standard levels within a range of 13 

to 11 μg/m3, but did not provide strong support for considering lower alternative standard 

levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that consideration of alternative 

annual standard levels in the range of 13 to 11 μg/m3 may be appropriate. Furthermore, 

the Policy Assessment concluded that the currently available evidence most strongly 

supported consideration of an alternative annual standard level in the range of 12 to 11 

μg/m3(U.S. EPA, 2011a, p. 2-82). The Policy Assessment concluded that an alternative 

level within the range of 12 to 11 μg/m3 would more fully take into consideration the 

available information from all long- and short-term PM2.5 exposure studies, including 

studies of at-risk populations, than would a higher level. This range also reflected placing 

weight on information from studies that helped to characterize the range of PM2.5

concentrations over which we continue to have confidence in the associations observed in 

epidemiological studies, as well as the extent to which our confidence in the associations 

was appreciably less at lower concentrations. 

As recognized in sections III.A.3 and III.E.4.a above, an annual standard intended 

to serve as the primary means for providing protection from effects associated with both 

long- and short-term PM2.5 exposures is not expected to provide appropriate protection 

against the effects of all short-term PM2.5 exposures (unless established at a level so low 

as to undoubtedly provide more protection than necessary for long-term exposures). Of 

particular concern are areas with high peak-to-mean ratios possibly associated with 
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strong local or seasonal sources, or PM2.5-related effects that may be associated with 

shorter-than-daily exposure periods. As a result, the Policy Assessment concluded that it 

was appropriate to consider alternative 24-hour PM2.5 standard levels that would 

supplement the protection provided by an annual standard. 

As outlined in section III.A.3 above, the Policy Assessment considered the 

available evidence from short-term PM2.5 exposure studies, as well as the uncertainties 

and limitations in that evidence, to assess the degree to which alternative annual and 24-

hour PM2.5 standards can be expected to reduce the estimated risks attributed to shortterm fine particle exposures. In considering the available epidemiological evidence, the 

Policy Assessment took into account information from multi-city studies as well as 

single-city studies. The Policy Assessment considered the distributions of 24-hour PM2.5

concentrations reported in short-term exposure studies, focusing on the 98th percentile

concentrations to match the form of the 24-hour standard as discussed in section III.E.3.b 

above. In recognizing that the annual and 24-hour standards work together to provide 

protection from effects associated with short-term PM2.5 exposures, the Policy 

Assessment also considered information on the long-term mean PM2.5 concentrations 

from these studies. 

In addition to considering the epidemiological evidence, the Policy Assessment 

considered air quality information, specifically peak-to-mean ratios using county-level 

24-hour and annual design values, to characterize air quality patterns in areas possibly 

associated with strong local or seasonal sources. These patterns helped in understanding 

the extent to which different combinations of annual and 24-hour standards would be 

consistent with the policy goal of setting a generally controlling annual standard with a 
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24-hour standard that provides supplemental protection especially for areas with high 

peak-to-mean ratios (U.S. EPA, 2011a, p. 2-14). 

In considering the information provided by the short-term exposure studies, the 

Policy Assessment recognized that to the extent these studies were conducted in areas 

that likely did not meet one or both of the current standards, such studies did not help 

inform the characterization of the potential public health improvements of alternative 

standards set at lower levels. Therefore, in considering the short-term exposure studies to 

inform staff conclusions regarding levels of the 24-hour standard that are appropriate to

consider, the Policy Assessment placed greatest weight on studies conducted in areas that 

likely met both the current annual and 24-hour standards.

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, as summarized in Figure 3 above and discussed in more detail in section 

III.E.4.c of the proposal, the Policy Assessment noted that, to the extent air quality 

distributions were reduced to reflect just meeting the current 24-hour standard, additional 

protection would be anticipated for the effects observed in the three multi-city studies 

with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett and 

Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98th percentile 

values below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 

μg/m3, the Policy Assessment noted that these studies reported long-term mean PM2.5

concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti 

and Schwartz, 2009; Dominici et al., 2006a). To the extent that consideration was given 

to revising the level of the annual standard, as discussed in section III.E.4.b of the 

proposal, the Policy Assessment recognized that potential changes associated with 
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meeting such an alternative annual standard would result in lowering risks associated 

with both long- and short-term PM2.5 exposures. Consequently, in considering a 24-hour 

standard that would operate in conjunction with an annual standard to provide appropriate 

public health protection, the Policy Assessment noted that to the extent that the level of 

the annual standard was revised to within a range of 13 to 11 μg/m3, in particular in the 

range of 12 to 11 μg/m

3

, additional protection would be provided for the long-term 

effects observed in these multi-city studies (U.S. EPA, 2011a, p. 2-84). 

Based on this information, the Policy Assessment concluded that the multi-city, 

short-term exposure studies generally provided support for retaining the 24-hour standard 

level at 35 μg/m3so long as the standard is in conjunction with an annual standard level 

revised to within a range of 12 to 11 μg/m3(U.S. EPA, 2011a, p. 2-84). Alternatively, in 

conjunction with an annual standard level of 13 μg/m3, the Policy Assessment concluded 

that the multi-city studies provided limited support for revising the 24-hour standard level 

somewhat below 35 μg/m3, such as down to 30 μg/m3, based on one study (Bell et al., 

2008) that reported positive and statistically significant effects with an overall 98th

percentile value below the level of the current 24-hour standard and an overall long-term 

mean concentration slightly less than 13 μg/m3(Figure 3; U.S. EPA, 2011a, p. 2-84). 

In reaching staff conclusions regarding alternative 24-hour standard levels that 

were appropriate to consider, the Policy Assessment also took into account relevant 

information from single-city studies that evaluated effects associated with short-term 

PM2.5 exposures. The Policy Assessment recognized that these studies may provide 

additional insights regarding impacts on at-risk populations and/or on areas with isolated 

peak concentrations. 
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As discussed in more detail in section III.E.4.c of the proposal, although a number 

of single-city studies reported effects at appreciably lower PM2.5 concentrations than 

multi-city short-term exposure studies, the uncertainties and limitations associated with 

the single-city studies were considerably greater than those associated with the multi-city 

studies and, thus, the Policy Assessment concluded there was less confidence in using 

these studies as a basis for setting the level of a standard. Therefore, the Policy 

Assessment concluded that the multi-city short-term exposure studies provided the 

strongest evidence to inform decisions on the level of the 24-hour standard, and the 

single-city studies did not warrant consideration of 24-hour standard levels different from 

those supported by the multi-city studies (U.S. EPA, 2011a, p. 2-88). 

In addition to considering the epidemiological evidence, the Policy Assessment 

took into account air quality information based on county-level 24-hour and annual 

design values to understand the public health implications of the alternative standard 

levels supported by the currently available scientific evidence, as discussed in section 

III.E.4.b above. Consistent with the general approach discussed in section III.A.3 above, 

the Policy Assessment considered the extent to which different combinations of 

alternative annual and 24-hour standard levels based on the evidence would support the 

policy goal of lowering annual and 24-hour air quality distributions by using the annual 

standard to be the “generally controlling” standard in conjunction with setting the 24-

hour standard to provide supplemental protection (U.S. EPA, 2011a, pp 2-88 to 2-91, 

Figure 2-10). 

Using information on the relationship of the 24-hour and annual design values, 

the Policy Assessment examined the implications of three alternative suites of PM2.5
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standards identified as appropriate to consider based on the currently available scientific 

evidence, as discussed above. The Policy Assessment concluded that an alternative suite 

of PM2.5 standards that would include an annual standard level of 11 or 12 μg/m3and a 

24-hour standard with a level of 35 μg/m3(i.e., 11/35 or 12/35) would result in the annual 

standard being the generally controlling standard in most areas although the 24-hour 

standard would continue to be the generally controlling standard in the Northwest (U.S.

EPA, 2011a, pp. 2-89 to 2-91 and Figure 2-10). These Northwest counties generally 

represented areas where the annual mean PM2.5 concentrations have historically been low 

but where relatively high 24-hour concentrations occur, often related to seasonal wood 

smoke emissions. Alternatively, combining an alternative annual standard of 13 μg/m3

with a 24-hour standard of 30 μg/m3 would result in many more areas across the country 

in which the 24-hour standard would likely become the controlling standard (the standard 

driving air quality distributions lower) than if an alternative annual standard of 12 or 11 

μg/m3 were paired with the current level of the 24-hour standard (i.e., 35 μg/m3). 

The Policy Assessment concluded that consideration of retaining the 24-hour 

standard level at 35 μg/m3 would reflect placing greatest weight on evidence from multicity studies that reported positive and statistically significant associations with health 

effects classified as having a causal or likely causal relationship. In conjunction with 

lowering the annual standard level, especially within a range of 12 to 11 μg/m3, this 

alternative recognized additional public health protection against effects associated with 

short-term PM2.5 exposures which would be provided by lowering the annual standard 

such that revision to the 24-hour standard would not be warranted (U.S. EPA, 2011a, p. 

2-91). 
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Beyond evidence-based considerations, the Policy Assessment also considered the 

extent to which the quantitative risk assessment supported consideration of retaining the 

current 24-hour standard level or provided support for lower standard levels. In 

considering simulations of just meeting the current 24-hour standard level of 35 μg/m3or 

alternative levels of 30 or 25 μg/m3(in conjunction with alternative annual standard 

levels within a range of 13 to 11 μg/m3), the Policy Assessment noted that the overall 

confidence in the quantitative risk estimates varied for the different standard levels 

evaluated and was stronger for the higher levels and substantially lower for the lowest 

level evaluated (i.e., 25 μg/m3). Based on this information, the Policy Assessment 

concluded that the quantitative risk assessment provided support for considering a 24-

hour standard level of 35 or 30 μg/m3(in conjunction with an alternative standard level 

within a range of 13 to 11 μg/m3) but did not provide strong support for considering 

lower alternative 24-hour standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that while it was appropriate to 

consider an alternative 24-hour standard level within a range of 35 to 30 μg/m3, the 

currently available evidence most strongly supported consideration for retaining the 

current 24-hour standard level at 35 μg/m3in conjunction with lowering the level of the 

annual standard within a range of 12 to 11 μg/m3(U.S. EPA, 2011a, p. 2-92).

ii. CASAC Advice

Based on its review of the second draft Policy Assessment, CASAC agreed with 

the general approach for translating the available epidemiological evidence, risk 

information, and air quality information into the basis for reaching conclusions on 

alternative standards for consideration. Furthermore, CASAC agreed “that it is 
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appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have 

evidence supportive of a causal or likely causal relationship as presented in the Integrated 

Science Assessment (Samet, 2010d, p. 1). 

CASAC concluded that the range of levels presented in the second draft Policy 

Assessment (i.e., alternative annual standard levels within a range of 13 to 11 μg/m3and 

alternative 24-hour standard levels within a range of 35 to 30 μg/m3) “are supported by 

the epidemiological and toxicological evidence, as well as by the risk and air quality 

information compiled” in the Integrated Science Assessment, Risk Assessment, and 

second draft Policy Assessment. CASAC further noted that “[a]lthough there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level 

below which there is no risk for adverse health effects)” (Samet, 2010d, p. ii). 

Although CASAC supported the alternative standard level ranges presented in the 

second draft Policy Assessment, it did not express support for any specific levels or 

combinations of standards. Rather, CASAC encouraged the EPA to develop a clearer 

rationale in the final Policy Assessment for staff conclusions regarding annual and 24-

hour standards that were appropriate to consider, including consideration of the 

combination of these standards supported by the available information (Samet, 2010d, p. 

ii). Specifically, in commenting on a distributional statistical analysis of air quality and 

associated population data presented in the second draft Policy Assessment, CASAC 

encouraged staff to focus on information related to the concentrations that were most 
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influential in generating the health effect estimates in individual studies to inform 

alternative standard levels. CASAC urged that the EPA redo that analysis using health 

event or study population data (Samet, 2010d, p. 2). CASAC also commented that the 

approach presented in the second draft Policy Assessment to identify alternative 24-hour 

standard levels which focused on peak-to-mean ratios was not relevant for informing the 

actual level (Samet 2010d, p. 4). Further, they expressed the concern that the 

combinations of annual and 24-hour standard levels discussed in the second draft Policy 

Assessment (i.e., in the range of 13 to 11 μg/m3for the annual standard, in conjunction 

with retaining the current 24-hour PM2.5 standard level of 35 μg/m3; alternatively, 

revising the level of the 24-hour standard to 30 μg/m3in conjunction with an annual 

standard level of 11 μg/m3) “may not be adequately inclusive” and “[i]t was not clear 

why, for example a daily standard of 30 μg/m3should only be considered in combination 

with an annual level of 11 μg/m3” (Samet, 2010d, p. ii). CASAC encouraged the EPA to 

more clearly explain its rationale for identifying the 24-hour/annual combinations that are 

appropriate for consideration (Samet 2010d, p. ii). 

In considering CASAC’s advice as well as public comment on the second draft 

Policy Assessment, the EPA staff conducted additional analyses and modified their 

conclusions regarding alternative standard levels that were appropriate to consider. The 

staff conclusions in the final Policy Assessment (U.S. EPA, 2011a, section 2.3.4.4) 

differed somewhat from the alternative standard levels discussed in the second draft 

Policy Assessment (U.S. EPA, 2010f, section 2.3.4.3), upon which CASAC based its 

advice. Changes made in the final Policy Assessment were primarily focused on 

improving and clarifying the approach for translating the epidemiological evidence into a 
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basis for staff conclusions on the broadest range of alternative standard levels supported 

by the available scientific information and more clearly articulating the rationale for the 

staff’s conclusions (Wegman, 2011, pp. 1 to 2). Consistent with CASAC’s advice to 

consider more information from epidemiological studies, as discussed in section 

III.E.4.b.1 above, the EPA analyzed additional population-level data obtained from 

several study authors (Rajan et al., 2011). In transmitting the final Policy Assessment to 

CASAC, the Agency notified CASAC that the final staff conclusions reflected 

consideration of CASAC’s advice and that those staff conclusions were based, in part, on 

the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 

2011, p.2). Thus, CASAC had an opportunity to comment on the final Policy 

Assessment, but chose not to provide any additional comments or advice after receiving 

it. 

iii. Administrator’s Proposed Decisions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate alternative standard levels to 

consider, the Administrator considered the epidemiological and other scientific evidence, 

estimates of risk reductions associated with just meeting alternative annual and/or 24-

hour standards, air quality analyses, related limitations and uncertainties, staff 

conclusions as presented in the Policy Assessment, and the advice of CASAC. As an 

initial matter, the Administrator agreed with the general approach discussed in the Policy 

Assessment as summarized in sections III.A.3 and III.E.4.a above, and supported by 

CASAC, of considering the protection afforded by the annual and 24-hour standards 

taken together for mortality and morbidity effects associated with both long- and shortterm exposures to PM2.5 (77 FR 38939). Furthermore, based on the evidence and 
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quantitative risk assessment, the Administrator provisionally concluded it is appropriate 

to set a “generally controlling” annual standard that will lower a wide range of ambient 

24-hour concentrations, with a 24-hour standard focused on providing supplemental 

protection, particularly for areas with high peak-to-mean ratios possibly associated with 

strong local or seasonal sources, or PM2.5-related effects that may be associated with 

shorter-than daily exposure periods. The Administrator provisionally concluded this 

approach would likely reduce aggregate risks associated with both long- and short-term 

exposures more consistently than a generally controlling 24-hour standard and would be 

the most effective and efficient way to reduce total PM2-5-related population risk. Id.

In reaching decisions on alternative standard levels to propose, the Administrator 

judged that it was most appropriate to examine where the evidence of associations 

observed in the epidemiological studies was strongest and, conversely, where she had 

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

Based on the characterization and assessment of the epidemiological and other studies 

presented and assessed in the Integrated Science Assessment and the Policy Assessment, 

the Administrator recognized the substantial increase in the number and diversity of 

studies available in this review including extended analyses of the seminal studies of 

long-term PM2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important 

new long-term exposure studies (as summarized in Figures 1 and 2). Collectively, the 

Administrator noted that these studies, along with evidence available in the last review, 

provided consistent and stronger evidence of an association with premature mortality, 

with the strongest evidence related to cardiovascular-related mortality, at lower ambient 

concentrations than previously observed. The Administrator also recognized the 
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availability of stronger evidence of morbidity effects associated with long-term PM2.5

exposures, including evidence of cardiovascular effects from the WHI study and 

respiratory effects, including decreased lung function growth, from the extended analyses 

for the Southern California Children’s Health Study. Furthermore, the Administrator 

recognized new U.S. multi-city studies that greatly expanded and reinforced our 

understanding of mortality and morbidity effects associated with short-term PM2.5

exposures, providing stronger evidence of associations at ambient concentrations similar 

to those previously observed (as summarized in Figure 3). Id. at 38939-40.

The newly available scientific evidence built upon the previous scientific data 

base to provide evidence of generally robust associations and to provide a basis for 

greater confidence in the reported associations than in the last review. The Administrator 

recognized that the weight of evidence, as evaluated in the Integrated Science 

Assessment, was strongest for health endpoints classified as having evidence of a causal 

relationship. These relationships included those between long- and short-term PM2.5

exposures and mortality and cardiovascular effects. She recognized that the weight of 

evidence was also strong for health endpoints classified as having evidence of a likely 

causal relationship, which included those between long- and short-term PM2.5 exposures 

and respiratory effects. In addition, the Administrator made note of the much more 

limited evidence for health endpoints classified as having evidence suggestive of a causal 

relationship, including developmental, reproductive and carcinogenic effects. Id .at 

38940.

Based on information discussed and presented in the Integrated Science 

Assessment, the Administrator recognized that health effects may occur over the full 
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range of concentrations observed in the long- and short-term epidemiological studies and 

that no discernible threshold for any effects can be identified based on the currently 

available evidence (U.S. EPA, 2009a, section 2.4.3). She also recognized, in taking note 

of CASAC advice and the distributional statistics analysis discussed in section III.E.4.b.i 

above and in the Policy Assessment, that there was significantly greater confidence in 

observed associations over certain parts of the air quality distributions in the studies, and 

conversely, that there was significantly diminished confidence in ascribing effects to 

concentrations toward the lower part of the distributions.

Consistent with the general approach summarized in section III.A.3 above, and 

supported by CASAC as discussed in section III.E.4.a above, the Administrator generally 

agreed that it was appropriate to consider a level for an annual standard that was 

somewhat below the long-term mean PM2.5 concentrations reported in long- and shortterm exposure studies. In recognizing that the evidence of an association in any such 

study was strongest at and around the long-term average where the data in the study are 

most concentrated, she understood that this approach did not provide a bright line for 

reaching decisions about appropriate standard levels. The Administrator noted that longterm mean PM2.5 concentrations were available for each study considered and, therefore, 

represented the most robust data set to inform her decisions on appropriate annual 

standard levels. She also noted that the overall study mean PM2.5 concentrations were 

generally calculated based on monitored concentrations averaged across monitors in each 

study area with multiple monitors, referred to as a composite monitor concentration, in 

contrast to the highest concentration monitored in each study area, referred to as a 

maximum monitor concentration, which are used to determine whether an area meets a 
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given standard. In considering such long-term mean concentrations, the Administrator 

understood that it was appropriate to consider the weight of evidence for the health 

endpoints evaluated in such studies in giving weight to this information. Id.

Based on the information summarized in Figure 4 above and presented in more 

detail in the Policy Assessment (U.S. EPA, 2011a, chapter 2) for effects classified in the 

Integrated Science Assessment as having a causal or likely causal relationship with PM2.5

exposures, the Administrator observed an overall pattern of statistically significant 

associations reported in studies of long-term PM2.5 exposures with long-term mean 

concentrations ranging from somewhat above the current standard level of 15 μg/m3

down to the lowest mean concentration in such studies of 12.9 μg/m3(in Miller et al., 

2007).72 She observed a similar pattern of statistically significant associations in studies 

of short-term PM2.5 exposures with long-term mean concentrations ranging from around 

15 μg/m3down to 12.8 μg/m3(in Burnett et al., 2004). With regard to effects classified as 

providing evidence suggestive of a causal relationship, the Administrator observed a 

small number of long-term exposure studies related to developmental and reproductive 

effects that reported statistically significant associations with overall study mean PM2.5

 

72 The EPA notes that the Miller et al., (2007) study provides strong evidence of 

cardiovascular related effects associated with long-term PM2.5 exposures. At the time of 

the proposal, the EPA recognized the limited nature of the air quality data considered in 

this study (77 FR 38918, fn. 62). The EPA has reviewed those limitations, in conjunction 

with consideration of public comments received on the proposal as discussed in section 

III.E.4.c, in conjunction with reaching a final decision on the level of the annual standard. 
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concentrations down to 11.9 μg/m3(in Bell et al., 2007).73 Id.

The Administrator also considered additional information from epidemiological 

studies, consistent with CASAC advice, to take into account the broader distribution of 

PM2.5 concentrations and the degree of confidence in the observed associations over the 

broader air quality distribution. In considering this additional information, she understood 

that the Policy Assessment presented information on the 25th and 10th percentiles of the 

distributions of PM2.5 concentrations available from four multi-city studies to provide a 

general frame of reference as to the part of the distribution in which the data become 

appreciably more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies would become appreciably less. 

As summarized in Figure 4 above, the Administrator took note of additional 

population-level data that were available for four studies (Krewski et al., 2009; Miller et 

al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009), each of which reported 

statistically significant associations with health endpoints classified as having evidence of 

a causal relationship. In considering the long-term PM2.5 concentrations associated with 

the 25th percentile values of the population-level data for these four studies, she observed 

that these values ranged from somewhat above to somewhat below 12 μg/m3. The 

Administrator recognized that three of these four studies representedinclude some of the 

strongest evidence available within the overall body of scientific evidence and noted that 

three of these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 

 

73 With respect to suggestive evidence related to cancer, mutagenic, and genotoxic 

effects, the PM2.5 concentrations reported in studies generally included ambient 

concentrations that are equal to or greater than ambient concentrations observed in 

studies that reported mortality and cardiovascular and respiratory effects (U.S. EPA, 

2009a, section 7.5), such that in selecting alternative standard levels that provide 

protection from mortality and cardiovascular and respiratory effects, it is reasonable to 

anticipate that protection will also be provided for carcinogenic effects.
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2009) were used as the basis for concentration-response functions used in the quantitative 

risk assessment (U.S. EPA, 2010a, section 3.3.3). 

In considering this information, the Administrator noted that CASAC advised that 

information about the long-term PM2.5 concentrations that were most influential in 

generating the health effect estimates in epidemiological studies can help to inform 

selection of an appropriate annual standard level. However, the Administrator also 

recognized that additional population-level data were available for only these four studies 

and, therefore, she believed that these studies comprised a more limited data set than one 

based on long-term mean PM2.5 concentrations for which data were available for all 

studies considered, as discussed above. 

The Administrator recognized, as summarized in section III.B.2 above, that 

important uncertainties remain in the evidence and information considered in this review 

of the primary fine particle standards. These uncertainties are generally related to 

understanding the relative toxicity of the different components in the fine particle 

mixture, the role of PM2.5 in the complex ambient mixture, exposure measurement errors 

inherent in epidemiological studies based on concentrations measured at fixed monitor 

sites, and the nature, magnitude, and confidence in estimated risks related to increasingly 

lower ambient PM2.5 concentrations. Furthermore, the Administrator noted that 

epidemiological studies have reported heterogeneity in responses both within and 

between cities and geographic regions across the U.S. She recognized that this 

heterogeneity may be attributed, in part, to differences in fine particle composition in 

different regions and cities. The Administrator also recognized that there are additional 

limitations associated with evidence for reproductive and developmental effects, 
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identified as being suggestive of a causal relationship with long-term PM2.5 exposures,

including: the limited number of studies evaluating such effects; uncertainties related to 

identifying the relevant exposure time periods of concern; and limited toxicological 

evidence providing little information on the mode of action(s) or biological plausibility 

for an association between long-term PM2.5 exposures and adverse birth outcomes. Id. at 

38941.

The Administrator was mindful that considering what standards were requisite to 

protect public health with an adequate margin of safety required public health policy 

judgments that neither overstated nor understated the strength and limitations of the 

evidence or the appropriate inferences to be drawn from the evidence. In considering how 

to translate the available information into appropriate standard levels, the Administrator 

weighed the available scientific information and associated uncertainties and limitations. 

For the purpose of determining what standard levels were appropriate to propose, the 

Administrator recognized, as did EPA staff in the Policy Assessment, that there was no 

single factor or criterion that comprised the sole “correct” approach to weighing the 

various types of available evidence and information, but rather there were various 

approaches that are appropriate to consider. The Administrator further recognized that 

different evaluations of the evidence and other information before the Administrator 

could reflect placing different weight on the relative strengths and limitations of the 

scientific information, and different judgments could be made as to how such information 

should appropriately be used in making public health policy decisions on standard levels. 

This recognition led the Administrator to consider various approaches to weighing the 

evidence so as to identify appropriate standard levels to propose. In so doing, the 
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Administrator encouraged extensive public comment on alternative approaches to 

weighing the evidence and other information so as to inform her public health policy 

judgments before reaching final decisions on appropriate standard levels.

In considering the available information, the Administrator noted the advice of 

CASAC that the currently available scientific information, including epidemiological and 

toxicological evidence as well as risk and air quality information, provided support for 

considering an annual standard level within a range of 13 to 11 μg/m3and a 24-hour 

standard level within a range of 35 to 30 μg/m3. In addition, the Administrator recognized 

that the Policy Assessment concluded that the available evidence and risk-based 

information support consideration of annual standard levels in the range of 13 to 11 

μg/m3, and that the Policy Assessment also concluded that the evidence most strongly 

supported consideration of an annual standard level in the range of 12 to 11 μg/m3. In 

considering how the annual and 24-hour standards work together to provide appropriate 

public health protection, the Administrator observed that CASAC did not express support 

for any specific levels or combinations of standards within these ranges. Nor did CASAC 

choose to comment on additional information and analyses presented in the final Policy 

Assessment prepared in response to CASAC’s recommendations on the second draft 

Policy Assessment (Wegman, 2011). 

In considering the extent to which the currently available evidence and 

information provided support for specific standard levels within the ranges identified by 

CASAC and the Policy Assessment as appropriate for consideration, the Administrator 

initially considered standard levels within the range of 13 to 11 μg/m3for the annual 

standard. In so doing, the Administrator first considered the long-term mean PM2.5
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concentrations reported in studies of effects classified as having evidence of a causal or 

likely causal relationship, as summarized in Figure 4 above and discussed more broadly 

above. She noted that a level at the upper end of this range would be below most but not 

all the overall study mean concentrations from the multi-city studies of long- and shortterm exposures, whereas somewhat lower levels within this range would be below all 

such overall study mean concentrations. In considering the appropriate weight to place on 

this information, the Administrator again noted that the evidence of an association in any 

such study was strongest at and around the long-term average where the data in the study 

are most concentrated, and that long-term mean PM2.5 concentrations were available for 

each study considered and, therefore, represented the most robust data set to inform her 

decisions on appropriate annual standard levels. Further, she was mindful that this 

approach did not provide a bright line for reaching decisions about appropriate standard 

levels. Id.

In considering the long-term mean PM2.5 concentrations reported in studies of 

effects classified as having evidence suggestive of a causal relationship, as summarized 

in Figure 4 for reproductive and developmental effects, the Administrator noted that a 

level at the upper end of this range would be below the overall study mean concentration 

in one of the three studies, while levels in the mid- to lower part of this range would be 

below the overall study mean concentrations in two or three of these studies. In 

considering the appropriate weight to place on this information, the Administrator noted 

the very limited nature of this evidence of such effects and the additional uncertainties in 

these epidemiological studies relative to the studies that provide evidence of causal or 

likely causal relationships.
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The Administrator also considered the distributional analyses of population-level 

information that were available from four of the epidemiological studies that provide 

evidence of effects identified as having a causal relationship with long- or short-term 

PM2.5 concentrations for annual standard levels within the same range of 13 to 11 μg/m3. 

In so doing, the Administrator first noted that a level in the mid-part of this range 

generally corresponds with approximately the 25th percentile of the distributions of health 

events data available in three of these studies. The Administrator also noted that standard 

levels toward the upper part of this range would reflect placing substantially less weight 

on this information, whereas standard levels toward the lower part of this range would 

reflect placing substantially more weight on this information. In considering this 

information, the Administrator noted that there was no bright line that delineates the part 

of the distribution of PM2.5 concentrations within which the data become appreciably 

more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies became appreciably less.

In considering mean PM2.5 concentrations and distributional analyses from the 

various sets of epidemiological studies noted above, the Administrator was mindful, as 

noted above, that such studies typically report concentrations based on composite monitor 

distributions, in which concentrations may be averaged across multiple ambient monitors 

that may be present within each area included in a given study. Thus, a policy approach 

that used data based on composite monitors to identify potential alternative standard 

levels would inherently build in a margin of safety of some degree relative to an 

alternative standard level based on measurements at the monitor within an area that 

records the highest concentration, or the maximum monitor, since once a standard was 
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set, concentrations at appropriate maximum monitors within an area were generally used 

to determine whether an area meets a given standard.

The Administrator also recognized that judgments about the appropriate weight to 

place on any of the factors discussed above should reflect consideration not only of the 

relative strength of the evidence but also on the important uncertainties that remained in 

the evidence and information being considered in this review. The Administrator noted 

that the extent to which these uncertainties influenced judgments about appropriate 

annual standard levels within the range of 13 to 11 μg/m3 would likely be greater for 

standard levels in the lower part of this range which would necessarily be based on fewer 

available studies than would higher levels within this range.

Based on the above considerations, the Administrator concluded that it was 

appropriate to propose to set a level for the primary annual PM2.5 standard within the 

range of 12 to 13 μg/m3. The Administrator provisionally concluded that a standard set 

within this range would reflect alternative approaches to appropriately placing the most 

weight on the strongest available evidence, while placing less weight on much more 

limited evidence and on more uncertain analyses of information available from a 

relatively small number of studies. Further, she provisionally concluded that a standard 

level within this range would reflect alternative approaches to appropriately providing an 

adequate margin of safety for the populations at risk for the serious health effects 

classified as having evidence of a causal or likely causal relationship, depending in part 

on the emphasis placed on margin of safety considerations. The Administrator recognized 

that setting an annual standard level at the lower end of this range would reflect an 

approach that placed more emphasis on the entire body of the evidence, including the 
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analysis of the distribution of air quality concentrations most influential in generating 

health effect estimates in the studies, and on margin of safety considerations, than would 

setting a level at the upper end of the range. Conversely, an approach that would support 

a level at the upper end of this range would place more emphasis on the remaining 

uncertainties in the evidence to avoid potentially overestimating public health 

improvements, and would generally support a view that the uncertainties remaining in the 

evidence are such that the evidence does nottoo great to warrant setting a lower annual 

standard level. Id. at 38942.

At the time of the proposal, while the Administrator recognized that CASAC 

advised, and the Policy Assessment concluded, that the available scientific information 

provided support for considering a range that extended down to 11 μg/m3, she concluded 

that proposing such an extended range would reflect a public health policy approach that 

placed more weight on relatively limited evidence and more uncertain information and 

analyses than she considered appropriate at this time. Nonetheless, the Administrator 

solicited comment on a level down to 11 μg/m3as well as on approaches for translating 

scientific evidence and rationales that would support such a level. Such an approach 

might reflect a view that the uncertainties associated with the available scientific 

information warrant a highly precautionary public health policy response that would 

incorporate a large margin of safety.

The Administrator recognized that potential air quality changes associated with 

meeting an annual standard set at a level within the range of 12 to 13 μg/m3 will result in 

lowering risks associated with both long- and short-term PM2.5 exposures. However, the 

Administrator recognized that such an annual standard intended to serve as the primary 
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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 

result, in conjunction with proposing an annual standard level in the range of 12 to 13 

μg/m3, the Administrator provisionally concluded that it was appropriate to continue to 

provide supplemental protection by means of a 24-hour standard set at the appropriate 

level, particularly for areas with high peak-to-mean ratios possibly associated with strong 

local or seasonal sources, or for PM2.5-related effects that may be associated with shorterthan-daily exposure periods. 

Based on the approach discussed in section III.A.3 above, at the time of the 

proposal the Administrator relied upon evidence from the short-term exposure studies as 

the principal basis for selecting the level of the 24-hour standard. In considering these 

studies as a basis for the level of a 24-hour standard, and having selected a 98th percentile 

form for the standard, the Administrator agreed with the focus in the Policy Assessment 

of looking at the 98th percentile values, as well as at the long-term mean PM2.5

concentrations in these studies.

In considering the information provided by the short-term exposure studies, the 

Administrator recognized that to the extent these studies were conducted in areas that 

likely did not meet one or both of the current standards, such studies did not help inform 

the characterization of the potential public health improvements of alternative standards 

set at lower levels. By reducing the PM2.5 concentrations in such areas to just meet the 

current standards, the Administrator anticipated that additional public health protection 

would occur. Therefore, the Administrator focused on studies that reported positive and 
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statistically significant associations in areas that would likely have met both the current 

24-hour and annual standards. She also considered whether or not these studies were 

conducted in areas that would likely have met an annual standard level of 12 to 13 μg/m3

to inform her decision regarding an appropriate 24-hour standard level. As discussed in 

section III.E.4.a, consistent with the Policy Assessment, the Administrator concluded that 

multi-city, short-term exposure studies provided the strongest data set for informing her 

decisions on appropriate 24-hour standard levels. The Administrator viewed the singlecity, short-term exposure studies as a much more limited data set providing mixed results 

and, therefore, she had less confidence in using those studies as a basis for setting the

level of a 24-hour standard. With regard to the limited number of single-city studies that 

reported positive and statistically significant associations for a range of health endpoints 

related to short-term PM2.5 concentrations in areas that would likely have met the current 

suite of PM2.5 standards, the Administrator recognized that many of those studies had 

significant limitations (e.g., limited statistical power, limited exposure data) or equivocal 

results (mixed results within the same study area) that made them unsuitable to form the 

basis for setting the level of a 24-hour standard. 

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, the Administrator observed an overall pattern of positive and 

statistically significant associations in studies with 98th percentile values averaged across 

study areas in the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; Zanobetti and 

Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003;

Franklin et al., 2008). The Administrator noted that, to the extent air quality distributions 

were reduced to reflect just meeting the current 24-hour standard, additional protection 
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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 Administrator noted that these studies reported long-term mean PM2.5 concentrations 

of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti and Schwartz, 

2009; Dominici et al., 2006a).

In proposing to revise the level of the annual standard to within the range of 12 to 

13 μg/m3, as discussed above, the Administrator recognized that additional protection 

would be provided for the short-term effects observed in these multi-city studies in 

conjunction with an annual standard level of 12 μg/m3, and in two of these three studies 

in conjunction with an annual standard level of 13 μg/m3. She noted that the study-wide 

mean concentrations were based on averaging across monitors within study areas and that 

compliance with the standard would be based on concentrations measured at the monitor 

reporting the highest concentration within each area. The Administrator believed it would 

be reasonable to conclude that revision to the 24-hour standard would not be appropriate 

in conjunction with an annual standard within this range. Based on the above 

considerations related to the epidemiological evidence, the Administrator provisionally 

concluded that it was appropriate to retain the level of the 24-hour standard at 35 μg/m3, 

in conjunction with a revised annual standard level in the proposed range of 12 to 13 

μg/m3.

In addition to considering the epidemiological evidence, the Administrator also 

took into account air quality information based on county-level 24-hour and annual 
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design values to understand the public health 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 considered whether these suites of standards 

would meet a public health policy goal which included setting the annual standard to be 

the “generally controlling” standard in conjunction with setting the 24-hour standard to 

provide supplemental protection to the extent that additional protection is warranted. As 

discussed above, the Administrator provisionally concluded that this approach was the 

most effective and efficient way to reduce total population risk associated with both longand short-term PM2.5 exposures, resulting in more uniform protection across the U.S. than 

the alternative of setting the 24-hour standard to be the controlling standard. 

In considering the air quality information, the Administrator first recognized that 

there was no annual standard within the proposed range of levels, when combined with a 

24-hour standard at the proposed level of 35 μg/m3, for which the annual standard would 

be the generally controlling standard in all areas of the country. She further observed that 

such a suite of PM2.5 standards with an annual standard level of 12 μg/m3 would result in 

the annual standard as the generally controlling standard in most regions across the 

country, except for certain areas in the Northwest, where the annual mean PM2.5

concentrations have historically been low but where relatively high 24-hour 

concentrations occur, often related to seasonal wood smoke emissions (U.S. EPA, 2011a, 

pp. 2-89 to 2-91, Figure 2-10). Although not explicitly delineated on Figure 2-10 in the 

Policy Assessment, an annual standard of 13 μg/m3 would be somewhat less likely to be 

the generally controlling standard in some regions of the U.S. outside the Northwest in 

conjunction with a 24-hour standard level of 35 μg/m3.
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Taking the above considerations into account, the Administrator proposed 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 recognized that the final suite of standards selected from 

within the proposed range of annual standard levels, or the broader range of annual 

standard levels on which public comment was solicited, must be clearly responsive to the 

issues raised by the D.C. Circuit’s remand of the 2006 primary annual PM2.5 standard. 

Furthermore, at the time of the proposal, she recognized that the final suite of standards 

will reflect her ultimate judgment in the final rulemaking as to the suite of primary PM2.5

standards that would be requisite to protect the public health with an adequate margin of 

safety from effects associated with fine particle exposures. The final judgment to be made 

by the Administrator will appropriately consider the requirement for a standard that is 

neither more nor less stringent than necessary and will recognize that the CAA does not 

require that primary standards be set at a zero-risk level, but rather at a level that reduces 

risk sufficiently so as to protect public health with an adequate margin of safety. 

At the time of the proposal, having reached her provisional judgment to propose 

revising the annual standard level from 15.0 to within a range of 12.0 to 13.0 μg/m3and 

to propose retaining the 24-hour standard level at 35 μg/m3, the Administrator solicited 

public comment on this range of levels and on approaches to considering the available 
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evidence and information that would support the choice of levels within this range. The 

Administrator also solicited public comment on alternative annual standard levels down 

to 11 μg/m3and on the combination of annual and 24-hour standards that commenters 

may believe is appropriate, along with the approaches and rationales used to support such 

levels. In addition, given the importance the evidence from epidemiologic studies played 

in considering the appropriate annual and 24-hour levels, the Administrator solicited 

public comment on issues related to translating epidemiological evidence into standards, 

including approaches for addressing the uncertainties and limitations associated with this 

evidence.

c. Comments on Standard Levels

This section addresses comments that relate to consideration of the appropriate 

levels of the primary annual and 24-hour PM2.5 standards, including comments on the 

general approach used by the EPA to translate the available scientific information into 

standard levels and how specific PM2.5 exposure studies should be considered as a basis 

for the standard levels. These comments on standard levels expand upon the more general

comments that either supported or opposed any change to the current suite of primary 

PM2.5 standards, which are addressed above in section III.D.2.74 As explained there, one 

group of commenters generally opposed any change to the current primary PM2.5

standards and more specifically disagreed with the basis for the EPA’s proposal to revise 

the annual standard level. Another group of commenters supported revising the current 

suite of primary PM2.5 standards to provide increased public health protection. Some

commenters in this second group argued that both the annual and 24-hour standard levels 

 

74 Specific comments on the forms of the annual and 24-hour standards are addressed in 

section III.E.3.a and III.E.3.b, respectively.
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should be lowered while other commenters in this group agreed with the EPA’s proposal 

to retain the level of the 24-hour standard in conjunction with revising the level of the 

annual standard. While generally supporting the EPA’s proposal to lower the level of the 

annual standard, many commenters in this group disagreed that a level within the EPA’s 

proposed range was adequately protective and supported a level of 11 μg/m3or below.

i. Annual Standard Level

The group of commenters opposed to any change to the current suite of primary 

PM2.5 standards generally raised questions regarding the underlying scientific evidence, 

including the causal determinations reached in the Integrated Science Assessment, and 

focused strongly on the uncertainties they saw in the scientific evidence as a basis for 

their conclusion that no changes to the current standard levels were warranted. In 

commenting on the proposed standard levels, these commenters typically relied on the 

arguments summarized and addressed above in section III.D.2 as to why they believed it 

was inappropriate for the EPA to make any revisions to the suite of primary PM2.5

standards. That is, they asserted that the EPA’s causal determinations were not 

adequately supported by the underlying scientific information; the biological plausibility 

of health effects observed in epidemiological studies has not been demonstrated in 

controlled human exposure and toxicological studies; uncertainties in the underlying 

health science are as great or greater than in 2006; there is no evidence of greater risk

since the last review to justify tightening the current annual PM2.5 standard; and “new” 

studies not included in the Integrated Science Assessment continue to increase 

uncertainty about possible health risks associated with exposure to PM2.5.

With regard to the level of the annual standard, these commenters strongly 
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disagreed with the Agency’s proposed decision to revise the level to within a range of 12 

to 13 μg/m3and argued that the current standard level of 15 μg/m3should be retained. 

For example, UARG, API, and other commenters in this group raised a number of issues 

that they asserted called into question the EPA’s interpretation of the epidemiological 

evidence to support revising the annual standard level. These commenters raised specific 

questions related to the general approach used by the EPA to translate the air quality and 

other information from specific epidemiological studies into standard levels, including:

(1) the EPA’s approach for using composite monitor air quality distributions reported in 

epidemiological studies to select a standard level that would be compared to 

measurements at the monitor recording the highest value in an area to determine 

compliance with the standard; (2) the appropriate exposure period for effects observed in 

long-term exposure mortality studies; and (3) the use of the EPA’s analysis of 

distributions of underlying population-level data (i.e., health event and study population 

data) for those epidemiological studies for which such information was available. These 

commenters also raised questions regarding the EPA’s consideration of specific scientific 

evidence as a basis for setting a standard level, including: (4) evidence of respiratory 

morbidity effects in long-term exposure studies and (5) more limited evidence of health 

effects which have been categorized in the Integrated Science Assessment as suggestive 

of a causal relationship (i.e., developmental and reproductive outcomes). These 

comments are discussed in turn below.

(1) Some commenters in this group argued that one reason why they believe there 

is no basis for setting a standard level below 15 μg/m3is that the air quality metric from 

epidemiological studies that the EPA relied on in the proposal is not the same metric that 
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will be compared to the level of the standard to determine compliance with the standard. 

That is, commenters noted that the long-term mean PM2.5 concentrations that the EPA 

considered, shown in Figure 4 above, are composite monitor mean concentrations (i.e., 

concentrations averaged across multiple monitors within areas with more than one 

monitor), whereas the PM2.5 concentrations that will be compared to the level of the 

standard are maximum monitor concentrations (i.e., the concentration measured by the 

monitor within an area reporting the highest concentration). This comment was presented 

most specifically in UARG’s comments (UARG, 2012, Attachment 1, pp. 2 to 6), which 

raised two overarching issues as discussed below.

First, the commenter noted that the EPA’s approach of considering composite 

monitor mean PM2.5 concentrations in selecting a standard level, and then comparing the 

maximum monitor mean PM2.5 concentration in each area to the standard level when the 

standard is implemented, was characterized in the proposal as inherently having the 

potential to build in a margin of safety (UARG, 2012, Attachment 1, p. 4, citing 77 FR 

38905). The commenter asserted that the Administrator is ignoring this distinction 

between composite and maximum monitor concentrations, and that this approach creates 

an unwarranted case for lowering the standard level, since in the commenter’s view, it 

would result in a margin of safety that would be arbitrary, not based on evidence, and 

unquantified (UARG, 2012, Attachment 1, p. 4). In support of this view, the commenter 

asserted that there is a significant difference between composite monitor mean PM2.5

concentrations and maximum monitor mean PM2.5 concentrations. The commenter 

asserted that the maximum monitor value will always be higher than the composite 

monitor value (except in areas that contain only a single monitor), such that when an area 
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just attains the NAAQS, that area’s composite monitor long-term mean PM2.5

concentration will be lower than the level of the standard (UARG, 2012, Attachment 1, p. 

3).

Second, the commenter asserted that a more “reasoned and consistent approach 

would be to decide on a mean composite monitor PM2.5 level that should be achieved and 

then identify the maximum monitor level that would result in that composite value” 

(UARG, 2012, Attachment 1, p. 4). The commenter conducted an analysis of maximum 

monitor versus composite monitor annual mean PM2.5 concentrations using monitoring 

data75 from 2006 to 2008 and presented results averaged across areas within two groups 

(i.e., those with design values76 above the current standard level and those with design 

values just below the current standard level) to illustrate their suggested alternative 

approach. The commenter interpreted this analysis as showing that the composite monitor 

long-term mean PM2.5 concentrations from the subset of the epidemiological studies 

shown in Figure 4 (of the proposal and above) that the commenter considered to be an 

appropriate focus for this analysis would be achieved across the U.S. if the current annual 

NAAQS of 15 μg/m3is retained and attained. The commenter considered the subset of 

epidemiological studies that included only long-term exposures studies of effects for 

which the evidence is categorized as causal or likely causal, but did not consider shortterm exposure studies. On this basis, the commenter asserted that attaining the current 

annual PM2.5 standard would result in composite monitor long-term mean concentrations 

in all areas that would be generally within or below the range of the composite monitor 

 

75 The commenter indicated that this analysis was based on monitoring data for every 

core based statistical area (CBSA) in the EPA’s Air Quality System (AQS) database.

76 The design value is the air quality statistic that is compared to the level of the NAAQS 

to determine the attainment status of a given area.
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long-term mean concentrations from such studies and, as a result, there is no reason to 

lower the level of the current annual NAAQS.

In considering the first issue related to the EPA’s approach, the EPA notes that in 

proposing to revise both the form and level of the annual standard, the Administrator 

clearly took into account the distinction between the composite monitor long-term mean 

PM2.5 concentrations from the epidemiological studies, considered as a basis for selecting 

an annual standard level, and maximum monitor long-term mean PM2.5 concentrations. In 

deciding to focus on the composite monitor long-term mean concentrations in selecting 

the standard level, and on the maximum monitor concentrations in selecting the form of 

the standard (i.e., consistent with proposing to eliminate the option for spatial averaging 

across monitors within an area when implementing the standard77), the Administrator 

reasonably considered the distinction between these metrics in a manner that was 

consistent with advice from CASAC (Samet et al., 2010d, pp. 2 to 3).

As noted above in section III.A.3, the EPA recognizes that a statistical metric 

(e.g., the mean of a distribution) based on maximum monitor concentrations may be 

identical to or above the same statistical metric based on composite monitor 

concentrations. More specifically, many areas have only one monitor, in which case the 

composite and maximum monitor concentrations are identical. Based on the most recent 

data from the EPA’s AQS from 2009 to 2011 in the 331 CBSAs in which valid PM2.5

data are available, as discussed in Frank (2012a, Table 5), there were 208 such areas 

(with design values ranging up to about 15 μg/m3). Frank (2012a) also observed that 

other areas have multiple monitors with composite and maximum monitor mean PM2.5

concentrations that were the same or relatively close, with 57 areas in which the 

 

77 As discussed above in section III.E.3.a.
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maximum monitor mean concentration was no more than 0.5 μg/m3higher than the 

composite monitor mean concentration and 56 areas in which the difference was between 

0.6 and 2 μg/m3. Further, there were only a few other areas in which the maximum 

monitor mean concentration was appreciably higher than the composite monitor mean 

concentration, such as areas in which some monitors may be separately impacted by local 

sources. There were only 10 such areas in the country in which the maximum monitor 

mean concentration was between 2 to 6 μg/m3higher than the composite monitor 

concentration (Frank, 2012a, Table 4).78 Thus, the EPA does not agree that there is a 

significant difference between composite monitor mean PM2.5 concentrations and 

maximum monitor mean PM2.5 concentrations in the large majority of areas across the 

country.

In proposing to revise the form of the annual PM2.5 standard, as discussed above 

in section III.E.3.a, the EPA noted that when an annual PM2.5 standard was first set in 

1997, the form of the standard included the option for averaging across measurements at 

appropriate monitoring sites within an area, generally consistent with the composite 

monitor approach used in epidemiological studies, with some constraints intended to 

ensure that spatial averaging would not result in inequities in the level of protection for 

communities within large metropolitan areas. In the last review the EPA tightened the 

constraints on spatial averaging, and in this review has eliminated the option altogether, 

on the basis of analyses in each review that showed that such constraints may be 

inadequate to avoid substantially greater exposures for people living in locations around 

 

78 The average difference between the maximum and composite design value among the 

123 CBSAs with two or more monitors is 0.8 μg/m3and the median difference is 0.6 

μg/m3. The 25th and 75th percentiles are 0.3 and 1.0 μg/m3, respectively (Frank, 2012a, p. 

4). 
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the monitors recording the highest PM2.5 concentrations in some areas, potentially 

resulting in disproportionate impacts on at-risk populations of persons with lower SES 

levels as well as minorities. In light of these analyses, and consistent with the 

Administrator’s decision to revise the form of the annual PM2.5 standard by eliminating 

the option for spatial averaging, the EPA continues to conclude that a standard level 

based on consideration of long-term mean concentrations from composite monitors, and 

applied at each monitor within an area including the monitor measuring the highest 

concentration, is the appropriate approach to use in setting a standard that will protect 

public health, including the health of at-risk populations, with an adequate margin of 

safety, as required by the CAA.

The EPA acknowledges that at proposal, the Agency characterized the approach 

of using maximum monitor concentrations to determine compliance with the standard, 

while selecting the standard level based on consideration of composite monitor 

concentrations, as one that inherently had the potential to build in a margin of safety (77 

FR 38905), and CASAC reiterated that view in supporting the EPA’s approach (Samet, 

2010d, p. 3). Nonetheless, in light of the discussion above, the EPA more specifically 

recognizes that this approach does not build in any margin of safety in the large number 

of areas across the country with only one monitor. Further, based on the analyses done to 

inform consideration of the form of the standard (Schmidt, 2011, Analysis A), the EPA 

concludes that this approach does not provide a margin of safety for the at-risk 

populations that live around the monitor measuring the highest concentration, such as in 

those few areas in which the maximum monitor concentration is appreciably higher than 

the composite monitor concentration. Rather, this approach properly treats those at-risk 
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populations the same way it does the broader populations that live in areas with only one 

monitor, by providing the same degree of protection for those at-risk populations that 

would otherwise be disproportionately impacted as it does for the broader populations in 

other areas, While the EPA recognizes that this approach can result in some additional 

margin of safety for the subset of areas with multiple monitors in which at-risk 

populations may not be disproportionately represented in areas around the maximum 

monitor, which may be the case in areas with relatively small differences between the 

maximum and composite monitor concentrations, the EPA notes that this margin would 

be relatively small in such areas.

Based on the above considerations, the EPA does not agree that the Agency’s 

approach of using maximum monitor concentrations to determine compliance with the 

standard, while selecting the standard level based on consideration of composite monitor 

concentrations creates an unwarranted case for lowering the standard level based on a 

margin of safety that would be arbitrary, not based on evidence, or lack quantification. 

The EPA recognizes that setting a standard to protect public health, including the health 

of at-risk populations, with an adequate margin of safety, depends upon selecting a 

standard level sufficiently below where the EPA has found the strongest evidence of 

health effects so as to provide such protection, and that the EPA’s approach regarding 

consideration of composite and maximum monitor concentrations is intended to, and 

does, serve to address this requirement as part of and not separate from the selection of an 

appropriate standard level based on the health effects evidence.

In considering the second issue related to the commenter’s suggested alternative 

approach, the EPA strongly disagrees with the commenter’s view that a more “reasoned 
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and consistent approach would be to decide on a mean composite monitor PM2.5 level 

that should be achieved and then identify the maximum monitor level that would result in 

that composite value” (UARG, 2012, Attachment 1, p. 4). As discussed above, the EPA 

notes that for areas with only one monitor, or with multiple monitors that measure 

concentrations that are very close in magnitude, the maximum monitor level that would 

limit the composite monitor PM2.5 level to be no greater than the level that should be 

achieved to protect public health with an adequate margin of safety, would essentially be 

the same as that composite monitor level. Further, as discussed above, even for areas in 

which the maximum monitor concentration is appreciably higher than other monitor 

concentrations within the same area, public health would not be protected with an 

adequate margin of safety if the disproportionately higher exposures of at-risk, 

susceptible populations around the monitor measuring the highest concentration were in 

essence averaged away with measurements from monitors in other locations within large 

urban areas. Further, the commenter’s suggested approach would be based on annual 

average PM2.5 concentrations that have been measured over some past time period. Such 

an approach would reflect the air quality that existed in the past, but it would not 

necessarily provide appropriate constraints on the range of concentrations that would be 

allowed by such a standard in the future, when relationships between maximum and 

composite monitor concentrations in areas across the country may be different. For these 

reasons, the EPA fundamentally rejects the commenter’s suggested approach because in 

the EPA’s view it would results in a standard that is inconsistent with EPA’s reasons for 

concluding that a standard of 15ug/m3 demonstrably would does not protect public 

health, including providing protection for at-risk populations, with an adequate margin of 
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safety in areas across the country.

More specifically, in further considering the commenter’s analysis of design 

values based on maximum versus composite monitor annual mean PM2.5 concentrations 

using monitoring data from 2006 to 2008 which they assert supports retaining the current 

standard level of 15 μg/m3, the EPA finds flaws with the numerical results and the scope 

of the analysis, as well as flaws in the commenter’s translation of the analysis results into 

the basis for selecting an annual standard level.

In considering the commenter’s analysis, the EPA notes that the analysis 

compared maximum versus composite monitor annual mean PM2.5 concentrations, 

averaged over 3 years, for two groups of areas: (1) areas with design values that exceed 

the current annual standard level (i.e., greater than 15.0 μg/m3) and (2) areas with design 

values that are just attaining the current annual standard (i.e., between 14.5 and 15.0 

μg/m3).79 The commenter indicated that they used the full body of PM2.5 monitoring data 

from the EPA’s AQS database (UARG, 2012, Attachment 1, p. 4), In attempting to 

reproduce the commenter’s results, the EPA repeated the calculations using only valid air 

quality data (i.e., data that meet data completeness and monitor siting criteria) from the 

AQS database for the same time period (Frank, 2012a).80 Based on this corrected 

analysis, the EPA finds that the composite monitor concentrations averaged across the 

 

79For the first group of areas (which included 33 areas), this analysis calculated an 

average across the areas of maximum monitor annual mean PM2.5 concentrations, 

averaged over 3 years, of 17.2 μg/m3compared to an average of composite monitor 

concentrations of 14.3 μg/m3. For the second group of areas (which included 11 areas), 

this analysis calculated an average across the areas of maximum monitor annual mean 

concentrations, averaged over 3 years, of 14.8 μg/m3compared to an average of 

composite monitor concentrations of 13.6 μg/m3(UARG, 2012, Attachment 1, Table 1).

80 The EPA notes that the Frank (2012a) analysis is similar to an earlier EPA staff 

analysis (Hassett-Sipple et al., 2010), which used air quality data from EPA’s AQS 

database to compare maximum versus composite monitor long-term mean PM2.5

concentrations across the study areas in six selected multi-city epidemiological studies.
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areas within each group are somewhat higher than those calculated by the commenter, 

and the average differences between the maximum and composite monitor concentrations 

are somewhat smaller (Frank, 2012a, Table 3).81 Notably, the difference between the 

maximum and composite monitor average concentrations for the second group of areas is 

substantially reduced in the corrected analysis, such that the difference (averaged across 

the 10 areas with valid data in the second group) is approximately 0.5 μg/m3, not 1.2 

μg/m3as in the commenter’s analysis. In addition, the commenter’s analysis compared 

the average of the composite monitors to the average of the maximum monitors for each 

subset of areas. This comparison of averages across all the areas in each subset masks the 

fact that the large majority of areas across the country have only one monitor, with the 

composite monitor and maximum monitor values the same for such areas, and many 

other areas have a maximum monitor value that is close to the composite monitor value. 

As discussed above, these circumstances have a major impact on the protection that 

would be achieved by the approach suggested by the commenter.

With regard to the scope of the commenter’s analysis, the EPA finds that by 

limiting the scope to a small subset of areas with design values above or just below the 

current annual standard level of 15 μg/m3, the analysis ignores the large number of areas 

across the country with lower design values that are relevant to consider in light of the 

epidemiological evidence of serious health effects at lower concentrations, well below the 

 

81 The EPA’s analysis was intended to repeat the commenter’s analysis, but using only 

valid air quality data (from 2006 to 2008). For the first group of areas (which included 21 

areas with valid data), the EPA’s analysis calculated an average across the areas of 

maximum monitor annual mean concentrations, averaged of 3 years, of 16.8 μg/m3

compared to an average of composite monitor concentrations of 14.8 μg/m3. For the 

second group of areas (which included 10 areas with valid data), the EPA’s analysis 

calculated an average across the areas of maximum monitor annual mean concentrations, 

averaged over 3 years, of 14.8 μg/m3compared to an average of composite monitor 

concentrations of 14.2 μg/m3(Frank, 2012a, Table 3).
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level of the current standard.

In translating the analysis results into the basis for selecting an annual standard 

level, the commenter’s translation is premised on the view that the “natural focal point” 

for setting an annual PM2.5 standard level should be somewhere within the range of the 

long-term mean PM2.5 concentrations from the subset of epidemiological studies that 

included only long-term exposure studies of effects for which the evidence is categorized 

as causal or likely causal, but not for effects categorized as suggestive of causality, nor 

did it include short-term exposure studies (which are included in Figure 4 of the proposal 

notice and above). Such a view is not consistent with setting a standard that would 

provide sufficient protection from the serious health effects reported even in the limited 

subset of studies considered by the commenter, including protecting public health with an 

adequate margin of safety. As discussed below, the EPA does not agree with the 

commenter’s view as to the appropriate focal point for selecting the level of an annual 

PM2.5 standard, or with the limited set of studies considered by the commenter as a basis 

for selecting the level of the annual PM2.5 standard.

Regarding an appropriate focal point for selecting the level of the annual standard, 

as discussed in the proposal and as advised by CASAC, the EPA has focused on PM2.5

concentrations somewhat below the lowest long-term mean concentrations from each of 

the key studies of both long- and short-term exposures of effects for which the evidence 

is causal or likely causal, as considered by the EPA (i.e., the first two sets of studies 

shown in Figure 4). If the level of the annual standard was set just somewhere within the 

range of the long-term mean concentrations from the various long-term exposure studies, 

then one or more of the studies would have a long-term mean concentration below the 
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selected level of the standard. Absent some reason to ignore or discount these studies, 

which the commenter does not provide (and of which the EPA is unaware), setting such a 

standard would allow that level of air quality, where the evidence of health effects is 

strongest, and its associated risk of PM2.5-related mortality and/or morbidity effects to 

continue. Selecting such a standard level could not be considered sufficient to protect the 

public health with an adequate margin of safety. 

Further, focusing on just the long-term mean PM2.5 concentrations in the key 

epidemiological studies – even the lowest long-term mean concentration from the set of 

key studies -- is not appropriate. Concentrations at and around the long-term mean

concentrations represent the part of the air quality distribution where the data in any 

given study are most concentrated and, thus, where the confidence in the magnitude and 

significance of an association in such study is strongest. However, the evidence of an 

association with adverse health effects in the studies is not limited to the PM2.5

concentrations just at and around the long-term mean, but rather extends more broadly to 

a lower part of the distribution, recognizing that no discernible population-level threshold 

for any such effects can be identified based on the available evidence. This broader 

region of the distribution of PM2.5 concentrations should be considered to the extent 

relevant information is available, recognizing that the degree of confidence in the 

association identified in a study would become lower as one moves below concentrations

at and around the long-term mean concentration in any given study. The commenter’s 

approach ignores this fundamental consideration.

Regarding the set of studies that is appropriate to inform the selection of the level 

of the annual PM2.5 standard, the EPA finds that limiting consideration only to the long-
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term exposure studies, as this commenter suggests, would be tantamount to ignoring the 

short-term exposure studies,

82 which provide some of the strongest evidence from the 

entire body of epidemiological studies. Thus, selecting an annual standard level using the 

limited set of studies suggested by the commenter would fail to provide a degree of 

protection that would be sufficient to protect public health with an adequate margin of 

safety.

For all the reasons discussed above, the EPA finds the commenter’s concerns with 

the EPA’s approach to considering composite and maximum monitor PM2.5

concentrations in selecting the level of the annual PM2.5 standard to be without merit. 

Further, the EPA finds no support in the commenter’s analysis for their suggested 

alternative approach.

(2) With respect to the appropriate exposure period for mortality effects observed 

in long-term exposure studies, some commenters in this group generally expressed views 

consistent with comments from UARG that argued that these studies “are most likely 

detecting health risk from earlier, higher PM2.5 levels and misattributing those risks to 

more recent, lower PM2.5 levels” (UARG, 2012, Attachment 1, p 7). Further, this 

commenter asserted that “there is no knowledge or evidence indicating whether 

 

82 The commenter suggests that the EPA should not place significant reliance on the longterm mean concentrations from short-term exposure studies because “[T]he short‐term 

studies did not use the annual average of PM2.5 to develop their associations; they used 

the daily 24‐hour averages of PM2.5. Thus, short‐term studies do not provide a natural 

indicator for the appropriate level of an annual standard....” (UARG, 2012, Attachment 

1, p. 3). The EPA finds this argument unpersuasive. Quite simply, effects were observed 

in these studies with an air quality distribution that can meaningfully be characterized by 

these long-term mean concentrations. Indeed, in remanding the 2006 standard, the D.C. 

Circuit discussed at length the interrelationship of the long- and short-term standards and 

studies, and remanded the 2006 standard to the EPA, in part, for ignoring those 

relationships without adequate explanation. American Farm Bureau Federation v. EPA. 

559 F. 3d at 522-24.
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premature deaths are the result of PM2.5 exposures in the most recent year; or due to 

physical damages incurred from PM2.5 exposures much earlier in life (with the impact on 

lifespan only emerging later in life); or due to total accumulated PM2.5 exposure over 

many years.” Id. Further, the commenter asserted that the long-term exposure studies of 

mortality are central to the EPA’s basis for proposing to set a lower annual standard level, 

since most of the estimated benefits associated with a lower annual PM2.5 standard are 

based on reductions in mortality related to long-term exposures to PM2.5.

As an initial matter, the EPA has recognized the challenge in distinguishing 

between PM2.5-associated effects due to past and recent long-term exposures, and in

identifying the relevant latency period for long-term exposure to PM and resultant health 

effects (U.S. EPA, 2009a, section 7.6.4; 77 FR 38941/1). While the EPA has 

acknowledged that there remain important uncertainties related to characterizing the most 

relevant exposure periods in long-term exposure studies, the assertion that there is “no 

knowledge or evidence” that helps to inform this issue is not correct, as discussed below.

Both in the last review and in the current review, the EPA has assessed studies 

that used different air quality periods for estimating long-term exposure and tested 

associations with mortality for the different exposure periods (U.S. EPA, 2004, section 

8.2.3.5; U.S. EPA 2009a, section 7.6.4). In this review, the Integrated Science 

Assessment discussed studies available since the last review that have assessed the 

relationship between long-term exposure to PM2.5 and mortality to explore the issue of 

the latency period between exposure to PM2.5 and death (U.S. EPA, 2009a, section 7.6.4).

Notably, in a recent analysis of the extended Harvard Six Cities Study, Schwartz 

et al. (2008) used model averaging (i.e., multiple models were averaged and weighted by 
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probability of accuracy) to assess exposure periods prospectively (77 FR 38907/1-2). The 

exposure periods were estimated across a range of unconstrained distributed lag models 

(i.e., same year, one year prior, two years prior to death). In comparing lags, the authors 

reported that the effects of changes in exposure to PM2.5 on mortality were strongest 

within a 2-year period prior to death (U.S. EPA, 2009a, p. 7-92, Figure 7-9). Similarly, a 

large multi-city study of the elderly found that the mortality risk associated with longterm exposure to PM10 reported cumulative effects that extended over the years that 

deaths were observed in the study population (i.e., the follow-up period) and for the 3-

year period prior to death (Zanobetti et al., 2008). 

Further, in a study of two locations that experienced an abrupt decline in PM2.5

concentrations (i.e., Utah Steel Strike, coal ban in Ireland), Röösli et al. (2005) reported 

that approximately 75 percent of health benefits were observed in the first 5 years (U.S.

EPA, 2009a, Table 7-9). Schwartz et al. (2008) and Puett et al. (2008) found, in a 

comparison of exposure periods ranging from 1 month to 48 months prior to death, that 

exposure to PM10 24 months prior to death exhibited the strongest association, and the 

weakest association was reported for exposure in the time period of 1 month prior to 

death. 

Overall, the EPA notes that the available evidence for determining the exposure 

period that is causally related to the mortality effects of long-term PM2.5 exposures, as 

discussed above, cannot specifically disentangle the effects observed in long-term 

exposure studies associated with more recent air quality measurements from effects that 

may have been associated with earlier, and most likely higher, PM2.5 exposures. While 

the evidence suggests that a latency period of up to five years would account for the 
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majority of deaths, it does not provide a basis for concluding that it is solely recent PM2.5

concentrations that account for the mortality risk observed in such studies. Nonetheless, 

the more recent air quality data does well at explaining the relationships observed 

between long-term exposures to PM2.5 and mortality, with the strongest association

observed in the two years prior to death. Further, the EPA recognizes that there is no 

discernible population-level threshold below which effects would not occur, such that it 

is reasonable to consider that health effects may occur over the full range of

concentrations observed in the epidemiological studies, including the lower 

concentrations in the latter years.

In light of this evidence and these considerations, the EPA concludes that it is 

appropriate to consider air quality concentrations that are generally contemporaneous 

with the collection of health event data (i.e., collected over the same time period) as being 

causally associated with at least some proportion of the deaths assessed in a long-term 

exposure study. This would include long-term mean PM2.5 concentrations from most of 

the key long-term exposure studies of effects with causal or likely causal evidence shown 

in Figure 4 above, which reported long-term mean PM2.5 concentrations ranging from 

13.6 μg/m3to 14.3 μg/m3. These studies include studies of mortality by Eftim et al. 

(2008), which separately analyzed the ACS and Harvard Six City sites, Zeger et al. 

(2008), and Lipfert et al. (2006), as well as studies of morbidity endpoints by Goss et al. 

(2004), McConnell et al. (2003) and Gauderman et al. (2004), and Dockery et al. (1996) 

and Razienne et al. (1996). The EPA acknowledges that uncertainty in the relevant 

exposure period is most notable in two other long-term exposure studies of mortality. The 

Miller et al. (2007) reported a long-term mean PM2.5 concentration for a 1-year exposure 
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period that post-dated the follow-up period in which health event data were collected by 

two years. Also, the Krewski et al. (2009) study reported a long-term mean PM2.5

concentration for an exposure period that included only the last two years of the 18-year 

follow-up period. Based on these considerations, the EPA does not now consider it 

appropriate to put weight on the reported long-term mean concentrations from these two 

studies for the purpose of translating the information from the long-term mortality studies 

into a basis for selecting the level of the annual PM2.5 standard.83

In addition, the EPA acknowledges that exposure periods that extend at least a 

couple years prior to the follow-up period in which health event data were collected 

would likely more fully capture the PM-related deaths in such studies. To explore how 

much higher the long-term mean PM2.5 concentrations would likely have been had air 

quality data prior to the follow-up years of the studies been included, the EPA conducted 

a sensitivity analysis of long-term mean PM2.5 concentrations (Schmidt, 2012a) 

particularly considering studies that only included deaths from a relatively recent followup period. As examples of such studies, this analysis considered the Eftim et al. (2008) 

study of mortality in the ACS sites and the Harvard Six Cities sites, as well as sites in the 

eastern region in the Zeger et al. (2008) study. Using data from the EPA’s AQS database, 

the analysis added the two years of air quality data just prior to the follow-up period in 

each study, which was 2000 to 2002 in Eftim et al. (2008) and 2000 to 2005 in Zeger et 

al. (2008). The analysis then calculated the extended long-term mean PM2.5 concentration 

for each study. As discussed in Schmidt (2012a), in each case the long-term mean PM2.5

 

83 Nonetheless, the EPA notes that the Krewski et al. (2009) and Miller et al. (2007) 

studies provide strong evidence of mortality and cardiovascular-related effects associated 

with long-term PM2.5 exposures to inform causality determinations reached in the 

Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).
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concentration averaged over the extended exposure period was less than 0.4 μg/m3higher

than the long-term mean PM2.5 concentration averaged over the follow-up period. The 

EPA finds it reasonable to conclude that such a relatively small difference in long-term 

mean PM2.5 concentrations would likely apply for other long-term exposure studies that 

used similarly recent follow-up periods as well (e.g., Goss et al., 2004; Lipfert et al., 

2006).

Based on the above considerations, the EPA concludes that it is appropriate to 

consider the available air quality information from the long-term exposure studies, while 

taking into account the uncertainties in the relevant long-term exposure periods in 

weighing the information from these studies. The EPA recognizes that considering such 

information in selecting an appropriate annual standard level has the potential to build in 

some margin of safety. The EPA further concludes that it is appropriate to consider the 

air quality information from the set of long-term exposure studies discussed above in the 

context of the broader array of epidemiological studies that inform the EPA’s 

consideration of the level of the annual PM2.5 standard.

The EPA also notes that while the long-term exposure studies are an important 

component of the epidemiological evidence that informs the Agency’s consideration of 

the level of the annual standard, they do not provide the only relevant information, nor 

are they the set of studies for which the relevant long-term mean PM2.5 concentrations are 

the lowest. As discussed in the proposal, the EPA also considers the long-term mean 

PM2.5 concentrations from the short-term mortality and morbidity studies as providing 

important information in considering the level of the annual standard. As discussed 

above, a large proportion of the aggregate risk associated with short-term exposures 
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results from the large number of days during which the 24-hour average concentrations 

are in the low- to mid-range of the concentrations observed in the studies. Thus, setting 

the level of the annual standard based on long-term mean concentrations, as well as the 

distribution of concentrations below the mean, in the short-term exposure studies is the 

most effective and efficient way to reduce total PM2.5-related risk from the broad array of 

mortality and morbidity effects associated with short-term exposures.

Further, the EPA notes that the relevant exposure period for the short-term 

exposure studies is the period contemporaneous with the collection of health event data, 

and that this exposure period is not subject to the uncertainties discussed above related to 

the long-term exposure studies. Recognizing that the long-term mean PM2.5

concentrations from several of the multi-city short-term exposure studies shown in Figure 

4 are below the long-term mean PM2.5 concentrations from the long-term exposure 

studies (with the exception of Miller et al., 2007).84 it is reasonable that in selecting the 

level of the annual standard primary consideration should be given to the information 

from this set of short-term exposure studies. There is no reasonable basis to discount the

long-term mean concentrations of the short-term exposure studies for purposes of setting 

the level of the annual standard. Thus, the commenter is incorrect in asserting that the 

long-term exposure studies, not the short-term exposure studies, would be central in the 

Administrator’s decision on the level of the annual standard. The standard is ultimately 

intended to protect not just against the single type of effect that contributes the most to 

quantitative estimates of risk to public health, but rather to the broad array of effects, 

 

84 As noted above, the EPA is not placing weight on the reported long-term mean 

concentrations from the Miller et al. (2007) study for the purpose of translating the 

information from the long-term mortality studies into a basis for selecting the level of the 

annual PM2.5 standard.
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including mortality and morbidity effects from long- and short-term exposures across the 

range of at-risk populations impacted by PM2.5-related effects.

(3) With regard to the EPA’s analysis of distributions of underlying populationlevel data (i.e., health event and study population data) and corresponding air quality data 

from each study area in certain key multi-city epidemiological studies (Rajan et al., 

2011), some commenters in this group raised a number of issues related to this analysis

(API, 2012, Attachment 1 pp. 5 to 6; McClennan, 2012, pp.2 to 4). Some commenters 

noted the limited number of studies for which health event and study population data 

were available, and questioned whether these distributions would apply to other studies. 

Commenters expressed concerns that this analysis had not been formally reviewed by 

CASAC and was not published in the peer-review literature. Based on such concerns, 

some commenters asserted that the EPA should not consider this information as a basis 

for selecting a standard level.

As an initial matter, as discussed in section III.E.4.b above, the EPA agrees with 

CASAC’s advice that it is appropriate to consider additional data beyond the mean PM2.5

concentrations in key multi-city studies to help inform selection of the level of the annual 

PM2.5 standard. As both the EPA and CASAC recognize, in the absence of a discernible 

threshold, health effects may occur over the full range of concentrations observed in the 

epidemiological studies. Nonetheless, the EPA recognizes that confidence in the 

magnitude and significance of an association is highest at and around the long-term mean 

PM2.5 concentrations reported in the studies and the degree of confidence becomes lower 

at lower concentrations within any given study. Following CASAC’s advice (Samet, 

2010d, p.2), the EPA used additional population-level and air quality data made available 
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by study authors to conduct an analysis of the distributions of such data, to help inform 

consideration of how the degree of confidence in the magnitude and significance of 

observed associations varies across the range of long-term mean PM2.5 concentrations in 

study areas within key multi-city epidemiological studies. In the EPA’s view, such 

consideration is important in selecting a level for an annual standard that will protect 

public health with an adequate margin of safety.

With regard to the number of multi-city studies for which an analysis of the 

distributions of population-level data across the study areas and the corresponding annual 

mean PM2.5 concentrations was done, the EPA noted at proposal that data for such an 

analysis were made available from study authors for four studies, including two longterm exposure studies and two short-term exposure studies.85 The EPA recognized that 

access to health event data can be restricted due to confidentiality issues, such that it is 

not reasonable to expect that such information could be made available from all studies.

In considering the information from these four studies, the EPA has further taken into 

consideration uncertainties discussed in response to the above comment related to the 

appropriate exposure period for long-term exposure studies. Based on these 

considerations, as noted above, the EPA concludes that such uncertainties are an 

important factor in evaluating the usefulness of the air quality information from the two 

long-term exposure studies in this analysis (Krewski et al., 2009; Miller et al., 2007) and 

that it would not be appropriate to place weight on the distributional analysis of health 

event and air quality data from these two studies specifically for the purpose of 

 

85 Health event data and study population data were available from two short-term 

exposure studies (Bell et al. 2008; Zanobetti and Schwartz, 2009) and one long-term 

exposure study (Krewski et al., 2009). Only study population data were available from 

another long-term exposure study (Miller et al., 2007).
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translating the information from the long-term mortality studies into a basis for selecting 

the level of the annual PM2.5 standard. Such uncertainties are not relevant to the shortterm exposure studies, and thus, the Agency focuses on the two short-term exposure 

studies in this analysis (Bell et al., 2008; Zanobetti and Schwartz, (2009).

In focusing on these two short-term exposure studies, the EPA first notes that 

these studies are key multi-city studies that reported positive and statistically significant 

associations between mortality and cardiovascular-related hospital admissions across a 

large number of areas throughout the U.S. (112 U.S. cities in Zanobetti and Schwartz, 

2009; 202 U.S. counties in Bell et al., 2008) using relatively recent air quality and health 

event data (i.e., 1999 through 2005 in both studies). The EPA considers this to be a 

modest but important data set to use for this distributional analysis to help inform 

consideration of how much below the long-term mean PM2.5 concentrations in key multicity long- and short-term exposure studies the annual PM2.5 standard level should be set. 

While the EPA acknowledges that having such data available from more studies would 

have been useful, the Agency finds the information from this limited set of studies to be 

an important consideration in selecting an annual standard level, consistent with CASAC 

advice to consider such information.

In considering the results of this distributional analysis, as discussed more fully in 

the Response to Comments document, the EPA considers PM2.5 concentrations between 

the 25th and 10th percentiles of the distribution of health events to be a reasonable range 

for providing a general frame of reference for that part of the distribution in which 

confidence in the magnitude and significance of the association may be appreciably lower

than confidence at and around the long-term mean concentration. For the two short-term 
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exposure studies included in this analysis, the EPA notes that the PM2.5 concentrations 

corresponding to the 25th percentiles of the distributions of health events were 12.5 μg/m3

and 11.5 μg/m3, respectively, for Zanobetti and Schwartz (2009) and for Bell et al. 

(2008), with the 10th percentiles being lower by approximately 2 μg/m3in each study

(Rajan et al., 2011, Table 1). In considering this information, the EPA recognizes, 

however, that there is no clear dividing line or single percentile within a given 

distribution (including both above and below the 25th percentile) provided by the 

scientific evidence that is most appropriate or ‘correct’ to use to characterize where the 

degree of confidence in the associations warrants setting the annual standard level. The 

decision as to the appropriate standard level below the long-term mean concentrations of 

the key studies is largely a public health policy judgment to be made by the 

Administrator, taking into account all of the evidence and its related uncertainties, as 

discussed in section III.E.4.d below.

In response to concerns that this analysis was not reviewed by CASAC nor 

published in the peer-reviewed literature, the EPA notes that this analysis was conducted 

to directly respond to advice from CASAC, as discussed in section III.E.4.b.1 above, in 

conjunction with their review of the Policy Assessment. The EPA notes that the same 

type of distributional analysis was presented in the second draft Policy Assessment based 

on air quality data, as well as population-weighted air quality data, rather than health 

event or study population data. In considering that distributional information, CASAC 

urged that the EPA redo the analysis using health event or study population data, which is 

exactly what the EPA did and presented in the final Policy Assessment. The EPA 

provided CASAC with the final Policy Assessment and communicated how the final staff 
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conclusions reflected consideration of its advice and that those staff conclusions were 

based in part on the specific distributional analysis that CASAC had urged the EPA to 

conduct (Wegman, 2011, Attachment p. 2). CASAC did not choose to provide any 

additional comments or advice after receiving the final Policy Assessment. The EPA 

considers this distributional analysis to be the product of the peer review conducted by 

CASAC of the Policy Assessment, and thus does not agree with commenters’ 

characterization that the analysis lacked appropriate peer review. The EPA’s final 

analysis was based on the comments provided by CASAC, the peer review committee 

established pursuant to the CAA, on the draft analysis, such that the final analysis stems 

directly from CASAC’s advice and the EPA’s response to its comments.

Based on the above considerations, the EPA continues to conclude that its 

analysis of distributions of health event and air quality data from two key multi-city 

epidemiological studies provides important information related to understanding the 

associations between health events observed in each city (e.g., deaths, hospitalizations) 

and the corresponding long-term mean PM2.5 concentrations observed in the studies. 

While recognizing that this is a relatively modest data set, the EPA further concludes that 

such information can appropriately help to inform the selection of the level of an annual 

standard that will protect public health with an adequate margin of safety from these 

types of health effects which are causally related to long- and short-term exposures to 

PM2.5.

(4) Some commenters in this group asserted there were limitations in the longterm exposure studies of morbidity, including studies evaluating respiratory effects in 

children. For example, one commenter (UARG, 2012, p. 12, Attachment 1, pp. 14 to 16) 
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asserted there were serious limitations in the long-term exposure studies of respiratory 

morbidity in each of the studies considered by the EPA (including McConnell et al., 

2003; Gauderman et al., 2004; Dockery et al., 1996; Raizenne et al., 1996; and Goss et 

al., 2004) and argued that this evidence provides only a “weak association” with PM2.5

exposures. This commenter asserted that many of these long-term exposure studies

evaluating respiratory effects were considered at the time the EPA reaffirmed the current 

annual standard level of 15 μg/m3in 2006, that the Administrator in the last review 

determined that the information they provided “was too limited to serve as the basis for 

setting a level of a national standard,” and that they should be given little weight in 

setting the level of the annual standard in this review (UARG, 2012, Attachment 1, p. 

14). 

More specifically, this commenter asserted that the McConnell et al. (2003) and 

Gauderman et al. (2004) studies reported mixed results for associations with PM2.5 and 

stronger associations with NO2 (API, 2012, Attachment 1, pp. 14 to 15). Similarly, this 

commenter argued that the Dockery et al. (1996) and Raizenne et al. (1996) studies 

showed stronger associations with acidity than with fine particles (measured as PM2.1). 

Id. pp. 15 to 16. With regard to the cystic fibrosis study, this commenter noted that the 

association between pulmonary exacerbations and PM2.5 in this study was no longer 

statistically significant when the model adjusted for each individual’s baseline lung 

function. The commenters referred to the data on lung function as an “important 

explanatory variable,” and suggested that the EPA should rely on results from the model 

that included individual baseline lung function information. Id. p. 16. For the reasons 

discussed below and in more detail in the Response to Comments document, the EPA 
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disagrees with the commenters’ interpretation of these studies.

As an initial matter, the EPA notes that three of these studies (McConnell et al., 

2003; Dockery et al., 1996; Raizenne et al., 1996) as well as the initial studies from the 

Southern California Children’s Health Study (Peters et al., 1999; McConnell et al., 1999’ 

Gauderman et al., 2000, 2002; Avol et al., 2001) were discussed and considered in the 

2004 Air Quality Criteria Document (U.S. EPA, 2004) and, thus, considered within the 

air quality criteria supporting the EPA’s final decisions in the review completed in 2006. 

Two additional studies (Gauderman et al., 2004; Goss et al., 2004) were discussed and 

considered in the provisional science assessment conducted for the last review (U.S.

EPA, 2006a). The EPA concluded that “new” studies considered in the provisional 

assessment completed in 2006 did not materially change any of the broad scientific 

conclusions regarding the health effects of PM exposure made in the Criteria Document 

(71 FR 61148 to 61149, October 17, 2006). All of these studies were considered in the 

Integrated Science Assessment that informs the current review (U.S. EPA, 2009a).

With regard to the Southern California Children’s Health Study, extended 

analyses considered in the Integrated Science Assessment provided evidence that 

clinically important deficits in lung function86 associated with long-term exposure to 

PM2.5 persist into early adulthood (U.S. EPA, 2009a, p. 7-27; Gauderman et al., 2004). 

These effects remained positive in copollutant models.87 Additional analyses of the 

 

86 Clinical significance was defined as an FEV1 below 80 percent of the predicted value, a 

criterion commonly used in clinical settings to identify persons at increased risk for 

adverse respiratory conditions (U.S. EPA, 2009a, p. 7-29 to 7-30). The primary NAAQS 

for sulfur dioxide (SO2) also included this interpretation for FEV1 (75 FR 35525, June 22, 

2010).

87 Gauderman et al. (2004) clearly stated throughout their analysis that NO2 was one 

component of a highly correlated mixture that contains PM2.5. Gauderman et al. (2004) 

did not present the results from copollutants models but stated “two-pollutant models for 
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Southern California Children’s Health Study cohort reported an association between 

long-term PM2.5 exposure and bronchitic symptoms (U.S. EPA, 2009a, p. 7-23 to 7-24; 

McConnell et al., 2003, long-term mean concentration of 13.8 μg/m3) that remained 

positive in co-pollutant models, with the PM2.5 effect estimates increasing in magnitude 

in some models and decreasing in others, and a strong modifying effect of PM2.5 on the 

association between lung function and asthma incidence (U.S. EPA, 2009a, 7-24; Islam et 

al., 2007). The outcomes observed in the more recent reports from the Southern 

California Children’s Health Study, including evaluation of a broader range of endpoints 

and longer follow-up periods, were larger in magnitude and more precise than reported in 

the initial version of the study. Supporting these results were new longitudinal cohort 

studies conducted by other researchers in varying locations using different methods (U.S.

EPA, 2009a, section 7.3.9.1). The EPA, therefore, disagrees with the commenters that the 

studies by McConnell et al. (2003) and Gauderman et al. (2004) are flawed and should 

not be used in the PM NAAQS review process.

The 24-City study88 by Dockery et al. (1996) (long-term mean concentration of 

14.5 μg/m3) was considered in the current as well as two previous reviews (U.S. EPA, 

2009a; U.S. EPA, 2004; U.S. EPA, 1996). This study observed that PM, specifically 

“particle strong acidity” and sulfate particles (indicators of fine particles), were 

associated with reports of bronchitis in the previous year. Similarly, the magnitude of the 

associations between bronchitis and PM10 and PM2.1 were similar to those for acidic 

aerosols and sulfate particles, though the confidence intervals for the PM10 and PM2.1

 

any pair of pollutants did not provide a significantly better fit to the data than the 

corresponding single-pollutant models.”

88 The 24-City study conducted by Dockery et al. (1996) included 18 sites in the U.S. and 

6 sites in Canada. The Raizenne et al. (1996) study considered 22 of these 24 study areas. 

Athens, OH and South Brunswick, NJ were not included in this study. 
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associations were slightly wider and the associations were not statistically significant.

Acid aerosols, sulfate, and fine particles are formed in secondary reactions of the 

emissions from incomplete combustion and these pollutants have similar regional and 

temporal distributions. As noted by the study authors, “the strong correlations of several 

pollutants in this study, especially particle strong acidity with sulfate (r=0.90) and PM2.1

(r=0.82), make it difficult to distinguish the agent of interest” (Dockery et al., 1996, p. 

505). Overall, Dockery et al. (1996) (and, similarly, Raizenne et al., 1996) observed 

similar associations between respiratory health effects and acid aerosols, sulfate, PM10

and PM2.1 concentrations. The commenters noted that the associations with particle 

acidity were sensitive to the inclusion of the six Canadian sites. The EPA notes that none 

of these Canadian cities were in the “sulfate belt” where particle strong acidity was 

highest. Thus, the change in the effect estimate when the six Canadian cities were 

excluded from the analysis is likely due to the lower prevalence of bronchitis and the 

lower concentrations of acid aerosols in these cities, and not due to some difference in 

susceptibility to bronchitis between the U.S. and Canadian populations that is not due to 

air pollution, as suggested by the commenters (UARG, 2012, Attachment 1, p. 15). In 

fact, contrary to the statements made by the commenters, the authors did not observe any 

subgroups that appeared to be markedly more susceptible to the risk of bronchitis. 

The Goss et al. (2004) study considered a U.S. cohort of cystic fibrosis patients 

and provided evidence of association between long-term PM2.5 exposures and 

exacerbations of respiratory symptoms resulting in hospital admissions or use of home 

intravenous antibiotics (U.S. EPA, 2009a, p. 7-25; long-term mean concentration of 13.7 

μg/m3). The commenters noted that the association between pulmonary exacerbations and 
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PM2.5 in this study was no longer statistically significant when the model adjusted for 

each individual’s baseline lung function. The commenters referred to the data on lung 

function as an “important explanatory variable,” and suggested that the EPA should rely 

on results from the model that included individual baseline lung function information. 

The EPA disagrees with the commenters’ interpretation of this study. The Agency 

concludes it is unlikely that lung function is a potential confounder or an important 

explanatory variable in this study. In fact, the authors noted that “it is more likely that 

lung function decline may be intimately associated with chronic exposure to air pollutants 

and may be part of the causal pathway in worsening prognosis in CF [cystic fibrosis]; in 

support of this explanation, we found both cross-sectional and longitudinal strong inverse 

relationships between FEV1 and PM levels” (Goss et al., 2004, p. 819). The EPA notes 

that adjusting for a variable that is on the causal pathway can lead to overadjustment bias, 

which is likely to attenuate the association (Schisterman et al. 2009); this is likely what 

was observed by the authors. Thus, the EPA continues to believe it is appropriate to focus 

on the results reported in Goss et al. (2004) that did not include individual baseline lung 

function in the model.

In addition, the EPA disagrees with commenters’ reliance solely on statistical 

significance when interpreting the study results from individual study results and the 

collective evidence across studies. As discussed in section III.D.2 above, statistical 

significance of individual study findings has played an important role in the EPA’s 

evaluation of the study’s results and the EPA has placed greater emphasis on studies 

reporting statistically significant results. However, in the broader evaluation of the 

evidence from many epidemiological studies, and subsequently during the process of 
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forming causality determinations in the Integrated Science Assessment by integrating 

evidence from across epidemiological, controlled human exposure, and toxicological 

studies, the EPA has emphasized the pattern of results across epidemiological studies and 

whether the effects observed were coherent across the scientific disciplines for drawing 

conclusions on the relationship between PM2.5 and different health outcomes. 

As noted in section III.B.1.a of the proposal, with regard to respiratory effects, the 

Integrated Science Assessment concluded that extended analyses of studies available in 

the last review as well as new epidemiological studies conducted in the U.S. and abroad 

provided stronger evidence of respiratory-related morbidity associated with long-term 

PM2.5 exposure (77 FR 38918). The strongest evidence for respiratory-related effects 

available in this review was from epidemiological studies that evaluated decrements in 

lung function growth in children and increased respiratory symptoms and disease 

incidence in adults (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1).

In considering the collective evidence from epidemiological, toxicological, and 

controlled human exposure studies, including the studies discussed above, the EPA 

recognizes that the Integrated Science Assessment concluded that a causal relationship is 

likely to exist between long-term PM2.5 exposures and respiratory effects (U.S. EPA, 

2009a, p. 2-12, pp. 7-42 to 7-43). CASAC concurred with this causality determination 

(Samet, 2009f, p.9).

The commenter’s assertion that the EPA should adhere to its assessment of these 

studies as it did in the review completed in 2006 is significantly mistaken. Most 

obviously, the EPA’s final decision in the last review was held to be deficient by the D.C. 

Circuit in remanding the 2006 primary annual PM2.5 standard. As discussed in section 
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III.A.2 above, the D.C. Circuit specifically held that the EPA did not provide a 

reasonable explanation of why certain morbidity studies, including an earlier study from 

the Southern California Children’s Health Study (Gauderman et al., 2000, long-term 

mean PM2.5 concentration approximately 15 μg/m3) and the 24-Cities Study (Raizenne et 

al., 1996, long-term mean concentrations approximately 14.5 μg/m3) did not warrant a 

more stringent annual PM2.5 standard when the long-term mean PM2.5 concentrations

reported in those studies were at or lower than the level of the annual standard. American 

Farm Bureau Federation v. EPA. 559 F. 3d at 525. Indeed, the court found that, viewed 

together, the Gauderman et al. (2000) and Raizenne et al., (1996) studies “are related and 

together indicate a significant public health risk...On this record, therefore, it appears the 

EPA too hastily discounted the Gauderman and 24-Cities studies as lacking in 

significance.” Id.

In this review, the EPA recognizes a significant amount of evidence beyond these 

two studies that expands our understanding of respiratory effects associated with longterm PM2.5 exposures. This body of scientific evidence includes an extended and new 

analyses from the Southern California Children’s Health Study (Gauderman et al., 2004; 

Islam et al., 2007; Stanojevic et al., 2008) as well as additional studies that examined 

these health effects (Kim et al., 2004; Goss et al., 2004). Thus, even more so than in the 

last review, the evidence indicates a “significant public health risk” to children from 

long-term PM2.5 exposures at concentrations below the level of the current annual 

standard. A standard that does not reflect appropriate consideration of this evidence 

would not be requisite to protect public health with an adequate margin of safety.

(5) With regard to the use of studies of health effects for which the EPA finds the 
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evidence to be “suggestive” of a causal relationship, some commenters argued that such 

studies “do not merit any weight in the setting of the annual NAAQS” (e.g., UARG, 

2012, Appendix 1, p. 3).

The EPA disagrees with the commenter’s view that studies of health effects for 

which the evidence is suggestive of a causal relationship, rather than studies of health 

effects for which the evidence supports a causal or likely causal relationship, merit no 

weight at all in setting the NAAQS. To place no weight at all on such evidence would in 

essence treat such evidence as though it had been categorized as “not likely to be a causal 

relationship.” To do so would ignore the important distinctions in the nature of the 

evidence supporting these different causality determinations in the Integrated Science 

Assessment. It would also ignore the CAA requirement that primary standards are to be 

set to provide protection with an adequate margin of safety, including providing 

protection for at-risk populations. Thus, ignoring this information in making decisions on 

the appropriate standard level would not be appropriate.89 Nonetheless, in considering 

studies of health effects for which the evidence is suggestive of a causal relationship, the 

EPA does believe that it is appropriate to place less weight on such studies than on 

studies of health effects for which there is evidence of a causal or likely causal 

relationship.

A second group of commenters supported revising the suite of primary PM2.5

standards to provide increased public health protection. These commenters found the 

 

89 As discussed in section II.A above, the requirement that primary standards provide an 

adequate margin of safety was intended to address uncertainties associated with 

inconclusive scientific and technical information available at the time of standard setting. 

I was also intended to provide a reasonable degree of protection against hazards that 

research has not yet identified. This certainly encompasses consideration of effects for 

which there is evidence suggestive of a causal relationship.
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available scientific information and technical analyses to be stronger and more 

compelling than in the last review. These commenters generally placed substantial weight 

on CASAC advice and on the EPA staff analyses presented in the final Policy 

Assessment, which concluded that the evidence most strongly supported an annual 

standard level within a range of 11 to 12 μg/m3(U.S. EPA, 2011a, p. 2-206). While some 

of these commenters felt that the level should be set within the proposed range (12 to 13 

μg/m3), most of these commenters advocated a level of 11 μg/m3.

90 For example, ALA et 

al., asserted:

The EPA’s proposed PM2.5 standards, while a step in the right direction are 

insufficient to protect public health, including the health of susceptible 

populations, with an adequate margin of safety as required by the Clean Air 

Act...we will discuss the enormous gap in public health protection afforded by an 

annual standard of 13 μg/m3, at the upper end of the proposed range, compared to 

the more protective 11 μg/m3, as advocated by our organizations (ALA et al., 

2012, p. 6). 

In general, these commenters expressed the view that given the strength of the 

available scientific evidence, the serious nature of the health effects associated with PM2.5

exposures, the large size of the at-risk populations, the risks associated with long- and 

short-term PM2.5 exposures, and the important uncertainties inherently present in the 

evidence, the EPA should follow a highly precautionary policy response by selecting an 

annual standard level that incorporates a large margin of safety. 

More specifically, these commenters offered a range of comments related to the 

general approach used by the EPA to select standard levels, including: (1) the EPA’s 

approach for setting a generally controlling annual standard; (2) the importance of the 

greatly expanded and stronger overall scientific data base; (3) consideration of the 

 

90 As discussed in section III.E.4.c.ii, many of these commenters also supported lowering 

the level of the primary 24-hour PM2.5 standard.
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distributional statistical analysis conducted by the EPA and other approaches for 

translating the air quality information from specific epidemiological studies into standard 

levels; and (4) the significance of the PM2.5-related public health impacts, especially 

potential impacts on at-risk populations, including children, in reaching judgments on 

setting standards that provide protection with an adequate margin of safety. These 

comments are discussed in turn below.

(1) Some of these commenters disagreed with the EPA’s approach for setting a 

“generally controlling” annual standard in conjunction with a 24-hour standard providing 

supplemental protection particularly for areas with high peak-to-mean ratios. These 

commenters argued this approach would lead to “regional inequities” as demonstrated in 

the EPA’s analyses contained in Appendix C of the Policy Assessment (ALA et al., pp. 

26 to 27). Specifically, these commenters argued:

There is no basis in the Clean Air Act for such a determination. The Clean Air 

Act requires only that the NAAQS achieve public health protection with an 

adequate margin of safety. It is well-documented that both long- and short-term 

exposures to PM2.5 have serious and sometimes irreversible health impacts. There 

is no health protection reason to argue that one standard should be “controlling” 

as a matter of policy without regard to the health consequences of such a policy. 

To adopt such a policy ignores the obligation to provide equal protection under 

the law to all Americans because it would result in uneven protection from air 

pollution in different localities and regions of the country (ALA et al., 2012, p. 

26).

The EPA believes these commenters misunderstood the basis for the EPA’s 

policy goal of setting a “generally controlling” annual standard. This approach relates 

exclusively to setting standards that will provide requisite protection against effects 

associated with both long- and short-term PM2.5 exposures. It does so by lowering the 

overall air quality distributions across an area, recognizing that changes in PM2.5 air 

quality designed to meet an annual standard would likely result not only in lower annual 
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mean PM2.5 concentrations but also in fewer and lower peak 24-hour PM2.5

concentrations. As discussed in section III.A.3 in the proposal and above, the EPA 

recognizes that there are various ways to combine the two primary PM2.5 standards to 

achieve an appropriate degree of public health protection. Furthermore, the extent to 

which these two standards are interrelated in any given area depends in large part on the 

relative levels of the standards, the peak-to-mean ratios that characterize air quality 

patterns in an area, and whether changes in air quality designed to meet a given suite of 

standards are likely to be of a more regional or more localized nature. 

In focusing on an approach of setting a generally controlling annual standard, the 

EPA’s intent is in fact to avoid the potential “regional inequities” that are of concern to 

the commenters. The EPA judges that the most appropriate way to set standards that 

provide more consistent public health protection is by using the approach of setting a 

generally controlling annual standard. This judgment builds upon information presented 

in the Policy Assessment as discussed in section III.A.3 above. More specifically, the 

Policy Assessment recognized that the short-term exposure studies primarily evaluated 

daily variations in health effects with monitor(s) that measured the variation in daily 

PM2.5 concentrations over the course of several years. The strength of the associations 

observed in these epidemiological studies was demonstrably in the numerous “typical” 

days within the air quality distribution, not in the peak days (U.S. EPA, 2011a, p. 2-9). In 

addition, the quantitative risk assessments conducted for this and previous reviews 

demonstrated the same point, that is, much, if not most, of the aggregate risk associated 

with short-term PM2.5 exposures results from the large number of days during which the 

24-hour average concentrations are in the low-to mid-range, below the peak 24-hour 
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concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In 

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

were likely to be disproportionately driven by peak 24-hour concentrations.91

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

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

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

Furthermore, the Policy Assessment concluded that an approach that focused on reducing 

peak exposures would most likely result in more uneven public health protection across 

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

other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because 

reductions based on control of peak days are less likely to control the bulk of the air 

quality distribution. 

As a result, the EPA believes an approach that focuses on a generally controlling 

annual standard would likely reduce aggregate risks associated with both long- and shortterm exposures more consistently than a generally controlling 24-hour standard and, 

therefore, would be the most effective and efficient way to reduce total PM2-5-related 

population risk. The CASAC agreed with this approach and considered it was 

“appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” 

(Samet, 2010d, p. 1). For the reasons discussed above, the EPA disagrees with the 

 

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

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

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

Pope and Dockery, 1992).
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comments that this approach will result in the concerns raised by the commenters; rather 

the EPA concludes that this approach will help to address these concerns.

(2) Many of these commenters asserted that the currently available scientific 

information is greatly expanded and stronger compared to the last review. Some of these 

commenters highlighted the availability of multiple, multi-city long- and short-term 

exposure studies providing “repeated, consistent evidence of effects below the current 

annual standard level” (ALA et al., 2012, pp. 39 to 49) and, more specifically, 

“significant evidence of harm with strong confidence well below EPA’s proposed annual 

standard range of 12-13 μg/m3” (AHA et al., 2012, pp. 10 to 12). 

The EPA recognizes that in setting standards that are requisite to protect public 

health with an adequate margin of safety, the Administrator must weigh the various types 

of available scientific information in reaching public health policy judgments that neither 

overstate nor understate the strength and limitations of this information or the appropriate 

inferences to be drawn from the available science. 

In general, the EPA agrees with these commenters’ views that the currently 

available scientific evidence is stronger “because of its breadth and the substantiation of 

previously observed health effects” (77 FR 38906/2) and provides “greater confidence in 

the reported associations than in the last review” (77 FR 38940/1). The EPA also agrees 

with the commenters’ position that it is appropriate to consider the regions within the 

broader air quality distributions where we have the strongest confidence in the 

associations reported in epidemiological studies in setting the level of the annual 

standard. However, as discussed in section III.E.4.d below, in weighing the available 

evidence and technical analyses, as well as the associated uncertainties and limitations in 
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that information, the EPA disagrees with the commenters’ views regarding the extent to 

which the available scientific information provides support for considering an annual 

standard level below the proposed range (i.e., below 12 to 13 μg/m3). In particular, the 

EPA disagrees with the degree to which these commenters place more weight on the 

relatively more uncertain evidence that is suggestive of a causal relationship (e.g., low 

birth weight). Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency 

concludes it is appropriate and reasonable to place the greatest emphasis on health effects 

for which the Integrated Science Assessment concluded there is evidence of a causal or 

likely causal relationship and to place less weight on the health effects that provide 

evidence that is only suggestive of a causal relationship.. 

(3) With regard to using the air quality information from epidemiological studies 

to inform decisions on standard levels, commenters in this group generally supported the 

EPA’s efforts to explore different statistical metrics from epidemiological studies to 

inform the Administrator’s decisions. These commenters argued that by considering 

different analytic measures -- either concentrations one standard deviation below the 

long-term means reported in the epidemiological studies or the EPA’s distributional 

statistical analysis of population-level data that extends the approach used in previous PM 

NAAQS reviews to consider information beyond a single statistical metric -- “the annual 

standard must be significantly lower than EPA has proposed” (ALA et al., 2012, pp. 50 to 

61). Furthermore, with regard to characterizing the PM2.5 air quality at which associations 

have been observed, some of these commenters highlighted CASAC’s recommendation 

that “[f]urther consideration should be given to using the 10th percentile as a level for 

assessing various scenarios of levels for the PM NAAQS” (Samet, 2010c, p. 11) (ALA et 
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al., 2012, p. 55). Other commenters urged that the EPA extend the distributional analysis 

to include additional studies. For example, CHPAC urged the EPA to also conduct 

distributional analysis for children’s health studies to better inform standards that would 

protect both children and adults from adverse health outcomes (CHPAC, 2012, p. 3)

The EPA agrees with these commenters’ views that it is appropriate to take into 

account different statistical metrics from epidemiological studies to inform the decisions 

on standard levels that are appropriate to consider in setting a standard that will protect 

public health with an adequate margin of safety. In the development of the Policy 

Assessment, the EPA staff explored various approaches for using information from 

epidemiological studies in setting the standards. The general approach used in the final 

Policy Assessment, discussed in sections III.A.3 and III.E.4.a above, reflects 

consideration of CASAC advice (Samet, 2010c, d) and public comments on multiple 

drafts of the Policy Assessment.

With regard to using the distributional statistical analysis to characterize the 

confidence in the associations, the EPA emphasizes that there is no clear dividing line

provided by the scientific evidence, and that choosing how far below the long-term mean 

concentrations from the epidemiological studies is appropriate to identify a standard level 

that will provide protection for the public health with an adequate margin of safety is 

largely a public health policy judgment. The EPA considers the region from 

approximately the 25th to 10th percentiles to be a reasonable range for providing a general 

frame of reference as to the part of the distribution over which our confidence in the 

magnitude and significance of the associations observed in epidemiological studies is 

appreciably lower. Based on these considerations, the EPA concludes that it is not
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appropriate to place as much confidence in the magnitude and significance of the 

associations over the lower percentiles of the distributions in each study as at and around 

the long-term mean concentrations. Thus, the EPA disagrees with the commenters’ views 

that this analysis compels placing more emphasis on the lower part of this range in 

selecting a level for an annual standard that will protect public health with an adequate 

margin of safety. The EPA recognizes that this information comes primarily from two 

short-term exposure studies, a relatively modest data set. In light of the limited nature of 

this information, and in recognition of more general uncertainties inherent in the 

epidemiological evidence, the Administrator deems it reasonable not to place more 

emphasis on concentrations in the lower part of this range, as discussed below in section 

III.E.4.d.

With regard to the scope of the distributional statistical analysis, the EPA 

requested additional population-level data from the study authors for a group of six multicity studies for which previous air quality analyses had been conducted (Hassett-Sipple et 

al., 2010; Schmidt et al., 2010, Analysis 2). These six studies were originally selected 

because they considered multiple locations representing varying geographic regions 

across multiple years. Thus, these studies provided evidence on the influence of different 

particle mixtures on health effects associated with long- and short-term PM2.5 exposures. 

In addition, these multi-city studies considered relatively more recent health events and 

air quality conditions (1999 to 2005). As discussed in section III.E.4.b.i above, the EPA 

received and analyzed population-level data for four of the six studies (Rajan et al., 

2011). Three of these four studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and 

Schwartz, 2009) served as the basis for the concentration-response functions used to 
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develop the core risk estimates (U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees 

that it would be useful to have such data from more studies, the Agency believes that the 

additional data that was requested and received from study authors provide useful 

information to help inform the Administrator’s selection of the annual standard level. 

(4) Many commenters in this group highlighted PM2.5-related impacts on at-risk 

populations, including potential impacts on children, older adults, persons with preexisting heart and lung disease, and low-income populations, to support their views that 

the annual standard should be revised to a level of 11 μg/m3or lower (CHPAC, 2012; 

AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al., 2012; Air Alliance Houston, et al, 

2012). These commenters urged the EPA to adopt a policy approach that placed less 

weight on the remaining uncertainties and limitations in the evidence and placed more 

emphasis on margin of safety considerations, including providing protection against 

effects for which there is more limited scientific evidence. For example, CHPAC urged 

the EPA “to place the same weight on studies examining impacts on children’s health as 

that of adult studies. ... The fact that there may be stronger evidence from adult studies 

does not mean that standards based on adult studies will be protective for children and 

consequently will meet the standard requisite to protect public health with an adequate 

margin of safety” (CHPAC, 2012 p. 3). Furthermore, with regard to the EPA’s approach 

for weighing uncertainties, some of these commenters stated that “we find no justification 

in the preamble for an annual standard level as high as 13 μg/m3, other than the vague 

assertion that uncertainties increase at lower concentrations. Further, the final proposal 

completely failed to address the Policy Assessment recommendations that if 13 μg/m3

was proposed, the 24-hour standard should be strengthened as well” (ALA et al., p. 7).
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The EPA has carefully evaluated and considered evidence of effects in at-risk 

populations. With regard to effects classified as having evidence of a causal or likely 

causal relationship with long- or short-term PM2.5 exposures (i.e., premature mortality, 

cardiovascular effects, and respiratory effects), the Agency takes note that it considered 

the full range of studies evaluating these effects, including studies of at-risk populations, 

to inform its review of the primary PM2.5 standards. Specific multi-city studies 

summarized in Figures 1, 2, and 3 above highlight evidence of effects observed in two 

different lifestages – children and older adults – that have been identified as at-risk 

populations. Thus, the EPA places as much weight on studies that explored effects in 

children for which the evidence is causal or likely causal in nature as on studies of such 

effects in adults, including older adults. .As discussed above in responses to commenters 

supporting the retention of the current standards, in setting the standard, the EPA has 

focused on considering PM2.5 concentrations somewhat below the lowest long-term mean 

concentrations from each of the key studies of both long- and short-term exposures of 

effects for which the evidence supports a causal or likely causal relationship (i.e., the first 

two sets of studies shown in Figure 4). Absent some reason to ignore or discount these 

studies, which the commenter does not provide (and of which the EPA is unaware), the 

EPA considers the available evidence of effects in children as well as other at-risk 

populations.

With respect to the EPA’s consideration of more limited studies providing 

evidence suggestive of a causal relationship (e.g., developmental and reproductive 

effects), as noted above in responding to comments from the first group of commenters, 

the Agency agrees that it is important to place some weight on this body of evidence in 
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setting standards that provide protection for at-risk populations, as required by the CAA. 

However, the Agency does not agree that the same weight must be placed on this 

information as on the body of scientific information for which there is evidence of a 

causal or likely causal relationship. To do so would ignore the difference in the breadth 

and strength of the evidence supporting the different causality determinations reached in 

the Integrated Science Assessment. 

With regard to weighing the uncertainties and limitations remaining in the 

evidence and technical analyses, as discussed in section II.A above, the EPA recognizes 

that in setting a primary NAAQS that provides an adequate margin of safety, the 

Administrator must consider a number of factors including the nature and severity of the 

health effects involved, the size of sensitive population(s) at risk, and the kind and degree 

of the uncertainties that remain. As discussed in section III.E.4.d below, the Agency 

agrees with these commenters that, in weighing the available evidence and technical 

analyses including the uncertainties and limitations in this scientific information, there is 

no justification for setting a primary PM2.5 annual standard level as high as 13 μg/m3.

Finally, some commenters in both groups also identified “new” studies that were 

not included in the Integrated Science Assessment as providing further support for their 

views on the level of the annual standard. As discussed in section II.B.3 above, the EPA 

completed a provisional review and assessment of “new” studies published since the 

close of the Integrated Science Assessment, including “new” studies submitted by 

commenters (U.S. EPA, 2012a). The provisional assessment found that the “new” studies 

expand the scientific information considered in the Integrated Science Assessment and 

provide important insights on the relationship between PM2.5 exposure and health effects 
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of PM (U.S. EPA, 2012b). However, the EPA notes that the provisional assessment found 

that the “new” science did not materially change the conclusions reached in the 

Integrated Science Assessment. The EPA notes that, as in past NAAQS reviews, the 

Agency is basing the final decisions in this review on the studies and related information 

included in the Integrated Science Assessment that have undergone CASAC and public 

review, and will consider newly published studies for purposes of decision making in the 

next PM NAAQS review.

ii. 24-Hour Standard Level

With respect to the level of the 24-hour standard, the EPA received comments on 

the proposal from two distinct groups of commenters. One group that included virtually 

all commenters representing industry associations, businesses, and many States agreed 

with the Agency’s proposed decision to retain the level of the 24-hour PM2.5 standard. 

The other group of commenters included many medical groups, numerous physicians and 

academic researchers, many public health organizations, some State and local agencies, 

five State Attorneys General, and a large number of individual commenters. These 

commenters disagreed with the Agency’s proposed decision and argued that EPA should 

lower the level of the 24-hour standard to 30 or 25 μg/m3. Comments from these groups 

on the level of the 24-hour PM2.5 standard are addressed below and in the Response to 

Comments Document.

As noted above, of the public commenters who addressed the level of the 24-hour 

PM2.5 standard, all industry commenters and most State and local commenters supported 

the proposed decision to retain the current level of 35 g/m3. In many cases, these groups 

agreed with the rationale supporting the Administrator’s proposed decision to retain the 
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current 24-hour PM2.5 standard, including her emphasis on the annual standard as the 

generally controlling standard with the 24-hour standard providing supplementary 

protection, and her conclusion that multi-city, short-term exposure studies provide the 

strongest data set for informing decisions on the appropriate 24-hour standard level. 

Many of these commenters agreed with the Administrator’s view that the single-city, 

short-term studies provided a much more limited data set (e.g., limited statistical power, 

limited exposure data) and more equivocal results (e.g., mixed results within the same 

study area), making them an unsuitable basis for setting the level of the 24-hour standard. 

While these commenters agreed with the EPA’s proposed decision to retain the 

current 24-hour PM2.5 standard, some did not agree with the EPA’s approach to 

considering the evidence from short-term multi-city studies. For example, a commenter 

representing UARG pointed out that the 98th percentile concentrations reported in the 

proposal for multi-city studies reflect the averages of 98th percentile concentrations across 

the cities included in those studies (UARG, 2012; Attachment 1; p. 25). This commenter 

contended that such averaged 98th percentile PM2.5 concentrations do not provide 

information that can appropriately inform a decision on the adequacy of the public health 

protection provided by the current or alternative 24-hour standards. 

While the EPA agrees that there is uncertainty in linking effects reported in multicity studies to specific air quality concentrations (U.S. EPA, 2011, section 2.3.4.1), the 

EPA disagrees with this commenter’s view that such uncertainty precludes the use of 

averaged 98th percentile PM2.5 concentrations to inform a decision on the appropriateness 

of the protection provided by the 24-hour PM2.5 standard. In particular, the EPA notes 

that averaged 98th percentile concentrations do provide information on the extent to 
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which study cities contributing to reported associations would likely have met or violated 

the current 24-hour PM2.5 standard during the study period. As evidence of this, the EPA 

notes the three multi-city studies specifically highlighted by this commenter as having 

averaged 98th percentile 24-hour PM2.5 concentrations below 35 g/m3(Dominici et al., 

2006; Bell et al., 2008; Zanobetti and Schwartz, 2009). Based on the 98th percentiles of 

24-hour PM2.5 concentrations in the individual cities evaluated in these studies, the EPA 

notes that the majority of these study cities would likely have met the current standard 

during the study periods (Hassett-Sipple et al., 2010). Therefore, regardless of whether 

the averaged 98th percentile concentrations or the 98th percentile concentrations in each 

city are considered, these studies provide evidence for associations between short-term 

PM2.5 and mortality or morbidity across a large number of U.S. cities, the majority of 

which would likely have met the current 24-hour PM2.5 standard during study periods. In 

their review of the PM Policy Assessment, CASAC endorsed the conclusions drawn from 

analyses of averaged 98th percentile 24-hour PM2.5 concentrations, and the EPA continues 

to conclude that this type of information can appropriately inform the Administrator’s 

decision on the current 24-hour PM2.5 standard.92

Another group of commenters argued that the 24-hour standard level should be 

lowered. Many of these commenters supported setting the level of the 24-hour PM2.5

standard at either 25 or 30 g/m3. In support of their position, the ALA et. al., AHA et al.,

 

92This is not to say that the EPA’s decision on whether to revise the 24-hour PM2.5

standard should be based on or only be informed by considerations of whether studies 

reported associations with mortality or morbidity in areas with averaged 98th percentile 

PM2.5 concentrations less than 35 mg/m3. As discussed below, in reaching a decision in 

this final notice on the most appropriate approach to strengthen the suite of PM2.5

standards, the Administrator considers the degree of public health protection provided by 

the combination of the annual and 24-hour standards together. 
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five state Attorneys General, and a number of additional groups pointed to 98th percentile 

PM2.5 concentrations in locations of multi-city and single-city epidemiological studies. 

For example, the ALA and others pointed to multi-city studies by Dominici et al. (2006), 

Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al. (2008) as providing 

evidence for associations with mortality and morbidity in study locations with averaged 

(i.e., averaged across cities) 98th percentile 24-hour PM2.5 concentrations below 35 

g/m3. These commenters also pointed to several single-city and panel studies reporting 

associations between short-term PM2.5 and mortality or morbidity in locations with 

relatively low 24-hour PM2.5 concentrations. Because some of these multi- and single-city 

studies have reported associations with health effects in locations with 98th percentile 

PM2.5 concentrations below 35 g/m3, commenters maintained that the current 24-hour 

PM2.5 standard (i.e., with its level of 35 g/m3) does not provide an appropriate degree of 

protection in all areas.

In further support of their position that the level of the current 24-hour standard 

should be lowered, these commenters pointed out the variability across the U.S. in ratios 

of 24-hour to annual PM2.5 concentrations. They noted that some locations, including 

parts of the northwestern U.S., experience relatively low annual PM2.5 concentrations but 

can experience relatively high 24-hour concentrations at certain times of the year. In 

order to provide protection against effects associated with short-term PM2.5 exposures, 

especially in locations with high ratios of 24-hour to annual PM2.5 concentrations, these 

commenters advocated setting a lower level for the 24-hour standard. 

The EPA agrees with these commenters that it is appropriate to maintain a 24-

hour PM2.5 standard in order to supplement the protection provided by the revised annual 
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standard, particularly in locations with relatively high ratios of 24-hour to annual PM2.5

concentrations. However, in highlighting 98th percentile PM2.5 concentrations in study 

locations without also considering the impact of a revised annual standard on short-term 

concentrations, these commenters ignore the fact that many areas would be expected to 

experience decreasing short- and long-term PM2.5 concentrations in response to a revised 

annual standard. 

In considering the specific multi-city studies highlighted by public commenters 

who advocated a more stringent 24-hour standard, the EPA notes that such studies have 

reported consistently positive and statistically significant associations with short-term 

PM2.5 exposures in locations with averaged 98th percentile PM2.5 concentrations ranging

from 45.8 to 34.2 μg/m3and long-term mean PM2.5 concentrations ranging from 13.4 to 

12.9 (Burnett and Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006a; Bell et al., 

2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009).93 The EPA notes that to the 

extent air quality distributions are reduced to meet the current 24-hour standard with its 

level of 35 g/m3and/or the revised annual standard with its level of 12 g/m3, additional 

protection would be anticipated against the effects reported in these short-term, multi-city 

studies. Put another way, to attain an annual standard with a level below the long-term 

 

93Commenters also highlighted associations with short-term PM2.5 concentrations 

reported in sub-analyses restricted to days with 24-hour concentrations at or below 35 

3

(Dominici, 2006b). These sub-analyses were not included in the original 

publication by Dominici et al. (2006a). Authors provided results of sub-analyses for the 

Administrator’s consideration in a letter to the docket following publication of the 

proposed rule in January 2006 (personal communication with Dr. Francesca Dominici, 

2006b). As noted in section III.A.3, these sub-analyses are part of the basis for the 

conclusion that there is no evidence suggesting that risks associated with long-term 

exposures are likely to be disproportionately driven by peak 24-hour concentrations. 

Because the sub-analyses did not present long-term average PM2.5 concentrations, it is 

not clear whether they reflected PM2.5 air quality that would have been allowed by the 

revised annual PM2.5 standard being established in this notice. 
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means in the locations of these short-term studies (as EPA is adopting here), the overall 

air quality distributions in the majority of study cities will necessarily be reduced, 

resulting in lower daily PM2.5 ambient concentrations. We therefore expect that the 

revised annual standard will result in 98th percentile PM2.5 concentrations in these cities 

that are lower than those measured in the studies, and that the overall distributions of 

PM2.5 concentrations will be lower than those reported to be associated with health 

effects. Thus, even for effects reported in multi-city studies with averaged 98th percentile 

concentrations below 35 μg/m3, additional protection from the risks associated with 

short-term exposures is anticipated from the revised annual standard, without revising the 

24-hour standard, because long-term average PM2.5 concentrations in multi-city study 

locations were above the level of the revised annual standard (i.e., 12 μg/m3).94 As 

discussed above, reducing the annual standard is the most efficient way to reduce the 

risks from short-term exposures identified in these studies, as the bulk of the risk comes 

from the large number of days across the bulk of the air quality distribution, not the 

relatively small number of days with peak concentrations. 

In considering the single-city studies highlighted by public commenters who 

advocated a more stringent 24-hour standard, the EPA first notes that, overall, these 

single-city studies reported mixed results. Specifically, some studies reported positive 

and statistically significant associations with PM2.5, some studies reported positive but 

non-significant associations, and several studies reported negative associations or a mix 

 

94It is also the case that additional protection is anticipated in locations with 98th

percentile 24-hour PM2.5 concentrations above 35 μg/m3, even if long-term 

concentrations are below 12 μg/m3. As noted in the proposal and in the Policy 

Assessment (U.S. EPA, 2011, Figure 2-10), parts of the northwestern U.S. are more 

likely than other parts of the country to violate the 24-hour standard and meet the revised 

annual standard. 
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of positive and negative associations with PM2.5. In light of these inconsistent results, the 

proposal noted that the overall body of evidence from single-city studies is mixed, 

particularly in locations with 98th percentiles of 24-hour concentrations below 35 μg/m3. 

Therefore, although some single-city studies reported effects at appreciably lower PM2.5

concentrations than short-term multi-city studies, the uncertainties and limitations 

associated with the single-city studies were noted to be greater. In light of these greater 

uncertainties and limitations, the Administrator concluded in the proposal that she had 

less confidence in using these studies as a basis for setting the level of the standard (77 

FR 38943). 

Given the considerations and conclusions noted above, in the proposal the 

Administrator concluded that the short-term multi-city studies provide the strongest 

evidence to inform decisions on the level of the 24-hour standard. Further, she viewed 

single-city, short-term exposure studies as a much more limited data set providing mixed 

results, and she had less confidence in using these studies as a basis for setting the level 

of a 24-hour standard (77 FR 38942).. In highlighting specific single-city studies, public 

health, environmental, and State and local commenters appear to have selectively focused 

on studies reporting associations with PM2.5 and to have overlooked studies that reported 

more equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004; Slaughter et al., 

2005; Villeneuve et al., 2006) (U.S. EPA, 2011, Figure 2-9). As such, these commenters 

have not presented new information that causes the EPA to reconsider its decision to 

emphasize multi-city studies over single-city studies when identifying the appropriate 

level of the 24-hour PM2.5 standard.

In further considering the single-city studies highlighted by public commenters, 
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the EPA notes that some commenters advocating for a lower level for the 24-hour PM2.5

standard also discussed short-term studies that have been published since the close of the 

Integrated Science Assessment. These recent studies were conducted in single cities or in 

small panels of volunteers. As in prior NAAQS reviews and as discussed above in more 

detail (section II.B.3), the EPA is basing its decisions in this review on studies and related 

information assessed in the Integrated Science Assessment. The studies assessed in the 

Integrated Science Assessment, and the conclusions based on those studies, have 

undergone extensive critical review by the EPA, CASAC, and the public. The rigor of 

that review makes the studies assessed in the Integrated Science Assessment, and the 

conclusions based on those studies, the most reliable source of scientific information on 

which to base decisions on the NAAQS. 

However, as discussed above (section II.B.3), the EPA recognizes that ‘‘new 

studies” may sometimes be of such significance that it is appropriate to delay a decision 

on revision of a NAAQS and to supplement the pertinent air quality criteria so the studies 

can be taken into account. In the present case, the EPA’s provisional consideration of 

‘‘new studies” concludes that, taken in context, the “new” information and findings do 

not materially change any of the broad scientific conclusions made in the air quality 

criteria regarding the health effects of PM2.5 (U.S. EPA, 2012a). 

For this reason, reopening the air quality criteria review would not be warranted, 

even if there were time to do so under the court order governing the schedule for 

completing this review. Accordingly, the EPA is basing its final decisions in this review 

on the studies and related information included in the PM Integrated Science Assessment 

(i.e., the air quality criteria) that has undergone CASAC and public review. The EPA will 
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consider the “new studies” in the next periodic review of the PM NAAQS, which will 

provide an opportunity to fully assess these studies through a more rigorous review 

process involving the EPA, CASAC, and the public. 

Some public health, medical, and environmental commenters also criticized the 

EPA’s interpretation of PM2.5 risk results. These commenters presented risk estimates for 

combinations of annual and 24-hour standards using more recent air quality data than that 

used in the EPA’s Risk Assessment (U.S. EPA, 2010a). Based on these additional risk 

analyses, the ALA and other commenters contended that public health benefits could 

continue to increase as annual and 24-hour standard levels decrease below 13 g/m3and 

35 g/m3, respectively. 

The EPA agrees with commenters that important public health benefits are 

expected as a result of revising the level of the annual standard to 12 g/m3, as is done in 

this notice, rather than 13 g/m3. The Agency also acknowledges that estimated PM2.5-

associated health risks continue to decrease with annual standard levels below 12 g/m3

and/or with 24-hour standard levels below 35 g/m3. However, the EPA disagrees with 

the commenters’ views regarding the extent to which risk estimates support setting 

standard levels below 12 μg/m3(annual standard) and 35 g/m3(24-hour standard).95

The CAA charges the Administrator with setting NAAQS that are “requisite” 

(i.e., neither more nor less stringent than necessary) to protect public health with an 

adequate margin of safety. In setting such standards the Administrator must weigh the 

available scientific evidence and information, including associated uncertainties and 

limitations. As described above, in reaching her proposed decisions on the PM2.5

 

95This section focuses on the 24-hour standard. Section III.E.4.c.i above also discusses 

these commenters’ recommendations within the context of the annual PM2.5 standard. 
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standards that would provide “requisite” protection, the Administrator carefully 

considered the available scientific evidence and risk information, making public health 

policy judgments that, in her view, neither overstated nor understated the strengths and 

limitations of that evidence and information. In contrast, as discussed more fully above, 

public health, medical, and environmental commenters who recommended levels below 

35 g/m3for the 24-hour PM2.5 standard have not provided new information or analyses 

to suggest that such standard levels are appropriate, given the uncertainties and 

limitations in the available health evidence, particularly uncertainties in studies 

conducted in locations with 98th percentile 24-hour PM2.5 concentrations below 35 g/m3

and long-term average concentrations below 12 g/m3. 

d. Administrator’s Final Conclusions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate standard levels, the 

Administrator has considered the epidemiological and other scientific evidence, estimates 

of risk reductions associated with just meeting alternative annual and/or 24-hour 

standards, air quality analyses, related limitations and uncertainties, the advice of 

CASAC, and extensive public comments on the proposal. After careful consideration of 

all of these, the Administrator has decided to revise the level of the primary annual PM2.5

standard from 15.0 μg/m3to 12.0 μg/m3and to retain the level of the primary 24-hour 

standard at 35 μg/m3.

As an initial matter, the Administrator agrees with the approach supported by 

CASAC and discussed in the Policy Assessment as summarized in sections III.A.3 and 

III.E.4.a above, of considering the annual and 24-hour standards together in determining 

the protection afforded against mortality and morbidity effects associated with both long-
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and short-term exposures to PM2.5. This approach is consistent with the approach taken in 

the review completed in 1997, in contrast to the approach used in the review completed in 

2006 where each standard was considered independently of the other (i.e., only data from 

long-term exposure studies were used to inform the level of the annual standard and only 

data from short-term exposure studies were used to inform the level of the 24-hour 

standard).96

Based on the evidence and quantitative risk assessment, the Administrator 

concludes that it is appropriate to set an annual standard that is generally controlling, 

which will lower the broad distribution of 24-hour average concentrations in an area as 

well as the annual average concentration, so as to provide protection from both long- and 

short-term PM2.5 exposures. In conjunction with this, it is appropriate to set a 24-hour 

standard focused on providing supplemental protection, particularly for areas with high 

peak-to-mean ratios of 24-hour concentrations, possibly associated with strong local or 

seasonal sources, and for PM2.5-related effects that may be associated with shorter-than 

daily exposure periods. The Administrator concludes this approach will reduce aggregate 

risks associated with both long- and short-term exposures more consistently than a 

generally controlling 24-hour standard and is the most effective and efficient way to 

reduce total PM2-5-related population risk and to protect public health with an adequate 

margin of safety. 

In selecting the level of the annual PM2.5 standard, based on the characterization 

and assessment of the epidemiological and other studies presented and assessed in the 

Integrated Science Assessment and the Policy Assessment, the Administrator recognizes 

the substantial increase in the number and diversity of studies available in this review. 

 

96

 See 71 FR 61148 and 61168, October 17, 2006.
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This expanded body of evidence includes extended analyses of the seminal studies of 

long-term PM2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important 

new long-term exposure studies (as summarized in Figures 1 and 2). Collectively, the 

Administrator notes that these studies, along with evidence available in the last review, 

provide consistent and stronger evidence than previously observed of an association 

between long-term PM2.5 exposures and premature mortality in areas with lower longterm ambient concentrations than previously observed, with the strongest evidence 

related to cardiovascular-related mortality. The Administrator also recognizes the 

availability of stronger evidence of morbidity effects associated with long-term PM2.5

exposures, including evidence of respiratory effects such as decreased lung function 

growth, from the extended analyses for the Southern California Children’s Health Study 

and evidence of cardiovascular effects from the WHI study. Furthermore, the 

Administrator recognizes new U.S. multi-city studies that greatly expand and reinforce 

our understanding of mortality and morbidity effects associated with short-term PM2.5

exposures, providing stronger evidence of associations in areas with ambient 

concentrations similar to those previously observed in short-term exposure studies 

considered in the previous review (as summarized in Figure 3). 

The Administrator recognizes the strength of the scientific evidence for evaluating 

health effects associated with fine particles, noting that the newly available scientific 

evidence builds upon the previous scientific data base to provide evidence of generally 

robust associations and a basis for greater confidence in the reported associations than in 

the last review. She notes the conclusion of the Integrated Science Assessment that this 

body of evidence supports a causal relationship between long- and short-term PM2.5
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exposures and mortality and cardiovascular effects and a likely causal relationship 

between long- and short-term PM2.5 exposures and respiratory effects. In addition, the 

Administrator notes additional, but more limited evidence, for a broader range of health 

endpoints including evidence suggestive of a causal relationship for developmental and 

reproductive effects as well as for carcinogenic effects.

Based on information discussed and presented in the Integrated Science 

Assessment, the Administrator recognizes that health effects may occur over the full 

range of concentrations observed in the epidemiological studies of both long-term and 

short-term exposures, since no discernible population-level threshold for any such effects 

can be identified based on the currently available evidence (U.S. EPA, 2009a, section 

2.4.3). To inform her decisions on an appropriate level for the annual standard that will 

protect public health with an adequate margin of safety, in the absence of any discernible 

population-level thresholds, the Administrator judges that it is appropriate to consider the 

relative degree of confidence in the magnitude and significance of the associations 

observed in epidemiological studies across the range of long-term PM2.5 concentrations in 

such studies. Further, she recognizes, in taking note of CASAC advice and the 

distributional statistics analysis discussed in the Policy Assessment and in section 

III.E.4.a above, that there is significantly greater confidence in the magnitude and 

significance of observed associations for the part of the air quality distribution 

corresponding to where the bulk of the health events evaluated in each study have been 

observed, generally at and around the long-term mean concentrations. Conversely, she 

also recognizes that there is significantly diminished confidence in the magnitude and 

significance of observed associations in the lower part of the air quality distribution 
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corresponding to where a relatively small proportion of the health events were observed.

Further, the Administrator recognizes that the long-term mean concentrations, or any 

other specific point in the air quality distribution of each study, do not represent a “bright 

line” at and above which effects have been observed and below which effects have not 

been observed. 

In considering the long-term mean concentrations reported in epidemiological 

studies, the Administrator recognizes that in selecting a level of the annual standard that 

will protect public health with an adequate margin of safety, it is not sufficient to focus 

on a concentration generally somewhere within the range of long-term mean 

concentrations from the key long-term and short-term exposure studies that reported 

lower concentrations than had been observed in earlier reviews. These key studies 

provide information for various types of serious health endpoints (including mortality and 

morbidity effects), different study populations (which may include at-risk populations 

such as children and older adults), and different air quality distributions that are specific 

to each study. A level somewhere within the range of long-term mean concentrations of 

the full set of key studies would be higher than the long-term mean of at least one of the 

studies being considered and therefore would not provide a sufficient degree of protection

against the health effects observed in that study. Absent some reasoned basis to place less 

weight on the evidence in the epidemiological study with the lowest long-term mean 

concentration among these key studies, this approach would not be consistent with the 

requirement to set a standard that will protect public health with an adequate margin of 

safety.97 Thus, the Administrator recognizes it is important to protect against the serious 

 

97 See American Farm Bureau Federation v. EPA, 559 F. 3d 512, 525-26 (D.C. Cir. 

2009).
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effects observed in each of these studies so as to protect public health with an adequate 

margin of safety. In so doing, she looks to identify the study with the lowest long-term 

mean concentration within the full set of key studies to help inform her decision of the

appropriate standard level which will provide protection for the broad array of health 

outcomes observed in all of the studies, including effects observed in at-risk populations.

Further, consistent with the general approach summarized in section III.E.4.a 

above and supported by CASAC as discussed in section III.E.4.b.ii above, the 

Administrator recognizes that it is appropriate to consider a level for an annual standard 

that is not just at but rather is somewhat below the long-term mean PM2.5 concentrations 

reported in each of the key long- and short-term exposure studies. In so doing, she 

focuses especially on multi-city studies that evaluated health endpoints for which the 

associations are causal or likely causal (i.e., mortality and cardiovascular and respiratory 

effects associated with both long- and short-term PM2.5 exposures). As discussed above, 

the importance of considering a level somewhat below the lowest long-term mean 

concentrations in this set of key studies is to establish a standard that would be protective 

against the observed effects in all of the studies, and that takes into account the relative 

degree of confidence in the magnitude and significance of observed associations across 

the air quality distributions in these studies. 

The Administrator recognizes that there is no clear way to identify how much 

below the long-term mean concentrations of key studies to set a standard that would 

provide requisite protection with an adequate margin of safety. She therefore must use 

her judgment to weigh the available scientific and technical information, and associated 

uncertainties, to reach a final decision on the appropriate standard level. In considering 
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the information in Figures 1 - 4 for effects classified as having evidence of a causal or 

likely causal relationship with long- or short-term PM2.5 exposures, she observes a cluster 

of short-term exposure studies with long-term mean concentrations within a range of 13.4 

μg/m3down to 12.8 μg/m3(Dominici et al., 2006; Burnett and Goldberg, 2003; Zanobetti 

and Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also observes a cluster of 

long-term exposure studies with long-term mean concentrations within a range of 14.5 

μg/m3to 13.6 μg/m3(Dockery et al., 1996; Lipfert et al., 2006; Zeger et al., 2008; 

McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the reasons discussed in 

response to public comments in section III.E.4.c above, the Administrator is less 

influenced by the long-term mean PM2.5 concentrations from the Miller et al. (2007) and 

Krewski et al. (2009) studies with reported long-term mean PM2.5 concentrations of 12.9 

and 14.0 μg/m3, respectively. In each case, the most relevant exposure periods would 

likely have had higher mean PM2.5 concentrations than those reported in the studies.98

Thus, the Administrator considers the long-term mean PM2.5 concentrations from these 

two studies to be a highly uncertain basis for informing her selection of the annual 

standard level.99

To help guide her judgment of the appropriate level below the long-term mean 

concentrations in the epidemiological studies at which to set the standard, the 

Administrator considered additional information from epidemiological studies 

 

98 In the case of Miller et al. (2007), the mean concentration is based on a single year of 

air quality data which post-dated by two years the period for which the health events data 

were collected. In the case of Krewski et al. (2009),the air quality data were based on the 

last two years of the 18-year period for which the health event data were collected. 

99 Nonetheless, as noted above, the EPA notes that the Krewski et al. (2009) and Miller et 

al. (2007) studies provide strong evidence of mortality and cardiovascular-related effects 

associated with long-term PM2.5 exposures to inform causality determinations reached in 

the Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6). 
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concerning the broader distribution of PM2.5 concentrations which correspond to the 

health events observed in these studies (e.g., deaths, hospitalizations). The Administrator 

observes that the development and use of this information in considering standard levels 

is consistent with CASAC’s advice, as discussed in section III.E.4.b.ii above, to focus on 

understanding the concentrations that were most influential in generating the health effect 

estimates in individual studies (Samet, 2010d, p. 2). 

In considering this additional population-level information, the Administrator 

recognizes that, in general, the confidence in the magnitude and significance of an 

association identified in a study is strongest at and around the long-term mean 

concentration for the air quality distribution, as this represents the part of the distribution 

in which the data in any given study are generally most concentrated. She also recognizes 

that the degree of confidence decreases as one moves towards the lower part of the 

distribution. Consistent with the approach used in the Policy Assessment, the 

Administrator believes that the range from approximately the 25th to 10th percentiles is a 

reasonable range for providing a general frame of reference as to the part of the 

distribution in which her confidence in the associations observed in epidemiological 

studies is appreciably lower. However, as noted above, it is important to emphasize that 

there is no clear dividing line or single percentile within a given distribution provided by 

the scientific evidence that is most appropriate or ‘correct’ to use to characterize where 

the degree of confidence in the associations warrants setting the annual standard level. 

The decision of the appropriate standard level below the long-term mean concentrations 

of the key studies, which in conjunction with the other elements of the standard would 

protect public health with an adequate margin of safety, is largely a public health policy 
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judgment, taking into account all of the evidence and its related uncertainties. 

As discussed in section III.E.4.b, the Administrator takes note of additional 

population-level data that were made available to the EPA by study authors.100 In 

considering this information, the Administrator particularly focuses on the analysis of the 

distributions of the health event data for each area within these studies and the 

corresponding air quality data for the two short-term exposure studies (Zanobetti and 

Schwartz, 2009; Bell et al., 2008). These short-term exposure studies evaluate the 

relationship between daily changes (one or more days) in PM2.5 concentrations and daily 

changes in health events (e.g., deaths, hospitalizations), such that the air quality 

concentrations that comprise the most relevant exposure periods in these studies are 

contemporaneous with the health event data. In addition, these studies considered more 

recent air quality data, representing generally lower PM2.5 concentrations, in a large 

number of study areas across the U.S. Thus, such studies provide the most useful 

evidence for an analysis evaluating the distribution of health event data and the 

corresponding long-term mean PM2.5 concentrations across the areas included in each 

multi-city study. 

The Administrator also considered the additional population-level data that were 

made available to EPA for two long-term exposure studies (Krewski et al., 2009; Miller 

 

100 As summarized in section III.E.4.a, population-level data were provided to the EPA 

for four studies. These four studies represent some of the strongest evidence showing 

associations between health effects and PM2.5 within the overall body of scientific 

evidence and include three studies (Krewski et al., 2009; Bell et al., 2008; and Zanobetti 

and Schwartz, 2009) that were used as the basis for concentration-response functions in 

the quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). The Administrator 

recognizes that the additional population-level data available for these four multi-city 

studies represents a more limited data set compared to the set of long-term mean PM2.5

concentrations which were available in the published literature for all studies considered 

in the Integrated Science Assessment. 
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et al., 2007). She recognizes that in long-term exposure studies investigators follow a 

specific group of study participants (i.e., cohort) over time and across urban study areas, 

and evaluate how PM2.5 concentrations averaged over a period of years are associated 

with specific health endpoints (e.g., deaths) across cities. As discussed in response to 

public comments in section III.E.4.c, disentangling the effects observed in long-term 

exposure studies associated with more recent air quality measurements from effects that 

may have been associated with earlier, and most likely higher, PM2.5 exposures 

introduces some uncertainty with regard to understanding the appropriate exposure 

window associated with the observed effects. This is in contrast to the short-term 

exposure studies where the relevant exposure period is contemporaneous to the period for 

which the health data were collected. In light of these considerations, as noted above, the 

Administrator considers the analysis of air quality concentrations that correspond to the 

distribution of population-level data in these two studies to be a highly uncertain basis for 

informing her selection of the annual standard level. 

Based on the above considerations, the Administrator views the additional 

population-level data for the two short-term exposure studies as appropriate to help 

inform her judgment of how much below the long-term mean concentrations to set the 

level of the annual standard. The Administrator notes that the long-term mean PM2.5

concentrations corresponding with study areas contributing to the 25th percentiles of the 

distribution of deaths and cardiovascular-related hospitalizations in these two short-term 

exposure studies were 12.5 μg/m3and 11.5 μg/m3, respectively, for Zanobetti and 

Schwartz (2009) and for Bell et al. (2008), with the 10th percentiles being lower by 

approximately 2 μg/m3in each study..
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The Administrator recognizes, as summarized in section III.B above and 

discussed more fully in section III.B.2 of the proposal, that important uncertainties 

remain in the evidence and information considered in this review of the primary fine 

particle standards. These uncertainties are generally related to understanding the relative 

toxicity of the different components in the fine particle mixture, the role of PM2.5 in the 

complex ambient mixture, exposure measurement errors, and the nature and magnitude of 

estimated risks related to increasingly lower ambient PM2.5 concentrations. Furthermore, 

the Administrator notes that epidemiological studies have reported heterogeneity in 

responses both within and between cities and geographic regions across the U.S. She 

recognizes that this heterogeneity may be attributed, in part, to differences in fine particle 

composition in different regions and cities.101

With regard to evidence for reproductive and developmental effects identified as 

being suggestive of a causal relationship with long-term PM2.5 exposures, the 

Administrator recognizes that there are a number of limitations associated with this body 

of evidence including: the limited number of studies evaluating such effects; uncertainties 

related to identifying the relevant exposure time periods of concern; and limited 

toxicological evidence providing little information on the mode of action(s) or biological 

plausibility for an association between long-term PM2.5 exposures and adverse birth 

outcomes. Nonetheless, the Administrator believes that this more limited body of 

evidence provides some support for considering that serious effects may be occurring in a 

 

101 Nonetheless, as explained in section III.E.1, the currently available evidence is not 

sufficient to support replacing or supplementing the PM2.5 indicator with any other 

indicator defined in terms of a specific fine particle component or group of components 

associated with any source categories of fine particles. Furthermore, the evidence is not 

sufficient to support eliminating any component or group of components associated with 

any source categories of fine particles from the mix of fine particles included in the PM2.5

indicator. 
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susceptible population at concentrations lower than those associated with effects 

classified as having a causal or likely causal relationship with long-term PM2.5 exposures 

(i.e., mortality, cardiovascular, and respiratory effects).

Overall, the Administrator believes that the available evidence interpreted in light 

of the remaining uncertainties, as summarized above and discussed more fully in the 

Integrated Science Assessment and the Policy Assessment, provides increased confidence 

relative to information available in the last review and provides a strong basis for 

informing her final decisions in the current review. The Administrator is mindful that 

considering what standards are requisite to protect public health with an adequate margin 

of safety requires public health policy judgments that neither overstate nor understate the 

strength and limitations of the evidence or the appropriate inferences to be drawn from 

the evidence. In considering how to translate the available information into appropriate 

standard levels, the Administrator weighs the available scientific information and 

associated uncertainties and limitations. For the purpose of determining what annual 

standard level is appropriate the Administrator recognizes that there is no single factor or 

criterion that comprises the ‘‘correct’’ approach to weighing the various types of 

available evidence and information.

In considering this information, the Administrator notes the advice of CASAC 

that “there are significant public health consequences at the current levels of the 

standards that justify consideration of lowering the PM2.5 NAAQS further” (Samet, 

2010c, p. 12). In addition, she recognizes that CASAC concluded, “although there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level 

below which there is no risk for adverse effects)” (Samet, 2010, p.ii) and that the final 
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decisions on standard levels must reflect a judgment of the available scientific 

information with respect to her interpretation of the CAA’s requirement to set primary 

standards that provide requisite protection to public health with an adequate margin of 

safety (Samet, 2010d, p. 4). The Administrator recognizes CASAC’s advice that the 

currently available scientific information provided support for considering an annual 

standard level within a range of 13 to 11 μg/m3and a 24-hour standard level within a 

range of 35 to 30 μg/m3. In considering how the annual and 24-hour standards work 

together to provide appropriate public health protection, the Administrator observes that 

CASAC did not express support for any specific levels or combinations of standards 

within these ranges. She also notes that CASAC encouraged the EPA staff to consider 

additional data from epidemiological studies to help quantify the characterization of the 

PM2.5 concentrations that were most influential in generating the health effect estimates 

in these studies (Samet, 2010d, p. 2). 

In response to CASAC’s advice, the Administrator recognizes that the EPA staff 

acquired additional data from authors of key epidemiological studies and analyzed these 

data to characterize the distribution of PM2.5 concentrations in relation to health events 

data to better understand the degree of confidence in the associations observed in the 

studies as discussed above. The Administrator recognizes that the final Policy 

Assessment included consideration of these additional analyses in reaching final staff 

conclusions with regard to the broadest range of alternative standard levels supported by 

the science. She takes note that the final Policy Assessment concluded that while 

alternative standard levels within the range of 13 to 11 μg/m3 were appropriate to 

consider, the evidence most strongly supported consideration of an annual standard level 
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in the range of 12 to 11 μg/m3. The Administrator is aware that, in transmitting the final 

Policy Assessment to CASAC, the Agency notified CASAC that the final staff 

conclusions reflected consideration of CASAC’s advice and that those staff conclusions 

were based, in part, on the specific distributional analysis that CASAC had urged the 

EPA to conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on the 

final Policy Assessment, but chose not to provide any additional comments or advice 

after receiving it. 

In selecting the annual standard level, the Administrator has considered many 

factors including the nature and severity of the health effects involved, the strength of the 

overall body of scientific evidence as considered in reaching causality determinations, the 

size of the at-risk populations, and the estimated public health impacts. She has also 

considered the kind and degree of the uncertainties that remain in the available scientific 

information. She recognizes that the association between PM2.5 and serious health effects 

is well established, including at concentrations below those allowed by the current 

standard. Further, she recognizes the CAA requirement that requires primary standards to 

provide an adequate margin of safety was intended to address uncertainties associated 

with inconclusive scientific and technical information as well as to provide a reasonable 

degree of protection against hazards that research has not yet identified. In considering 

the currently available evidence, as summarized and discussed more broadly above, the 

information on risk, CASAC advice, the conclusions of the Policy Assessment, and 

public comments on the proposal, the Administrator strongly believes that a lower annual 

standard level is needed to protect public health with an adequate margin of safety.

In reaching her final decision on the appropriate annual standard level to set, the 
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Administrator is mindful that the CAA does not require that primary standards be set at a 

zero-risk level, but rather at a level that reduces risk sufficiently so as to protect public 

health, including the health of at-risk populations, with an adequate margin of safety. On 

balance, the Administrator concludes that an annual standard level of 12 μg/m3 would be 

requisite to protect the public health with an adequate margin of safety from effects 

associated with long- and short-term PM2.5 exposures, while still recognizing that 

uncertainties remain in the scientific information. 

In the Administrator’s judgment, an annual standard of 12 μg/m3appropriately 

reflects placing greatest weight on evidence of effects for which the Integrated Science 

Assessment determined there is a causal or likely causal relationship with long- and 

short-term PM2.5 exposures. An annual standard level of 12 μg/m3is below the long-term 

mean PM2.5 concentrations reported in each of the key multi-city, long- and short-term 

exposures studies providing evidence of an array of serious health effects (e.g., premature 

mortality, increased hospitalization for cardiovascular and respiratory effects). As noted 

above, the importance of considering a level somewhat below the lowest long-term mean 

concentration in the full set of studies considered is to set a standard that would provide 

appropriate protection against the observed effects in all such studies. 

In reaching her decision, the Administrator has taken into account that at and 

around the mean PM2.5 concentration in any given study represents a part of the air 

quality distribution in which the health event data in that study are generally most 

concentrated. Furthermore, in identifying an appropriate annual standard level below the 

long-term mean concentrations, she recognizes that there is no evidence to support the 

existence of any discernible threshold, and, therefore, she has a high degree of confidence 
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that the observed effects are associated with concentrations not just at but extending 

somewhat below the long-term mean concentration. To further inform her judgment in 

setting the annual standard level so as to protect public health with an adequate margin of 

safety, the Administrator has placed weight on additional population-level information 

available from a subset of these epidemiological studies, consistent with CASAC advice. 

In particular, she has drawn from two short-term exposure studies, which provide the 

most relevant information for evaluating the distribution of health events and 

corresponding long-term PM2.5 concentrations. As explained above, this helps inform her 

judgment as to the degree of confidence in the observed associations in the 

epidemiological studies. In this regard, the Administrator generally judges the region 

around the 25th percentile as a reasonable part of the distribution to help guide her 

decision on the appropriate standard level. Since this evidence comes primarily from two 

studies, a relatively modest data set, the Administrator deems it reasonable not to draw 

further inferences from air quality and health event data in the lower part of the 

distribution for the purpose of setting a standard level. The Administrator notes that the 

long-term mean PM2.5 concentrations around the 25th percentile of the distributions of 

deaths and cardiovascular-related hospitalizations were approximately around 12 μg/m3

in these two studies. The Administrator views this information as helpful in guiding her 

determination as to where her confidence in the magnitude and significance of the 

associations is reduced to such a degree that a standard set at a lower level would not be

warranted to provide requisite protection that is neither more nor less than needed to 

provide an adequate margin of safety.

The Administrator also recognizes that a level of 12 μg/m3places some weight on 
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studies which provide evidence of reproductive and developmental effects (e.g., infant 

mortality, low birth weight). These studies were identified in the Integrated Science 

Assessment as having evidence suggestive of a causal relationship with long-term PM2.5

concentrations. A level of 12 μg/m3is approximately the same level as the lowest longterm mean concentration reported in such studies (Figures 2 and 4; 11.9 μg/m3

for Bell et 

al., 2007).102 While the Administrator acknowledges that this evidence is limited, she 

believes it is appropriate to place some weight on these studies in order to set a standard 

that provides protection with an adequate margin of safety, including providing 

protection for at-risk populations, as required by the CAA. Due to the limited nature of 

this evidence, she has determined it is not necessary to set a standard below the lowest 

long-term mean concentration in these studies. 

In reflecting on extensive public comments received on the proposal as discussed 

in section III.E.4.c above, the Administrator recognizes that some commenters have 

offered different evaluations of the evidence and other information available in this 

review and would make different judgments about the weight to place on the relative 

strengths and limitations of the scientific information and about how such information 

could be used in making public health policy decisions on the annual standard level. One 

group of such commenters who supported a higher annual standard level (e.g., above 13 

μg/m3) would place greater weight on the remaining uncertainties in the evidence as a 

 

102 With respect to cancer, mutagenic, and genotoxic effects, the Administrator observes 

that the PM2.5 concentrations reported in studies evaluating these effects generally 

included ambient concentrations that are equal to or greater than ambient concentrations 

observed in studies that reported mortality and cardiovascular and respiratory effects 

(U.S. EPA, 2009a, section 7.5). Therefore, the Administrator concludes that in selecting 

alternative standard levels that provide protection from mortality and cardiovascular and 

respiratory effects, it is reasonable to anticipate that protection will also be provided for 

carcinogenic effects.
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basis for supporting a higher standarddiscounting the overall strength of the evidence

than the Administrator judges to be appropriate. Such an approach is based on these 

commenters’ judgment that the uncertainties remaining in the evidence are too great to 

warrant setting an lower annual standard below the current level. The Administrator does 

not agree. 

As an initial matter, an annual standard level of 13 μg/m3or higher would be 

above the long-term mean concentrations reported in two well-conducted, multi-city 

short-term exposure studies reporting positive and statistically significant associations of 

serious effects (Burnett et al., 2004 and Bell et al., 2008). These important studies are 

fully consistent with the pattern of evidence presented by the large body of evidence in 

this review. As the Administrator recognized in the proposal, and as advised by CASAC, 

the appropriate focus for selecting the level of the annual PM2.5 standard is on 

concentrations somewhat below the lowest long-term mean concentrations from the set of 

key studies of both long-term and short-term PM2.5 exposures considered by the EPA 

(i.e., as shown in Figure 4). Thus, a standard level set at 13 μg/m3or higher would clearly 

not provide protection for the effects observed in the full set epidemiological studies and, 

therefore, this standard level could not be judged to be requisite with an adequate margin 

of safety.103

In addition, as noted above, in recognizing that there is no evidence to support the 

 

103 The Administrator is mindful that, in reviewing the 2006 final PM NAAQS decisions, 

the D.C. Circuit Court of Appeals concluded that the EPA failed to adequately explain 

why that annual standard provided requisite protection from effects associated with both 

long- and short-term exposures or from morbidity effects in children and other at-risk 

populations when long-term means of important short-term studies were below the level 

the Administrator selected for the annual standard. See American Farm Bureau v. EPA.

559 F. 3d 512, 524-26. There is no reasonable basis to discount these two studies for 

purposes of setting the level of the annual standard.
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existence of a discernible threshold below which an effect would not occur, the 

Administrator is mindful that effects occur around and below the long-term mean 

concentrations reported in both the short-term and long-term the epidemiological studies. 

A standard level of 13 μg/m3or higher would not appropriately take into account

evidence from the two well-conducted, multi-city, short-term exposure studies reporting 

serious effects with long-term mean concentrations below 13 μg/m3noted above (Burnett 

et al, 2004; Bell et al., 2008). Such a standard level would also not appropriately take into 

account additional population-level data from a limited number of epidemiological 

studies. This approach would ignore CASAC’s advice to consider such information in 

order to better understand the concentrations over which there is a high degree of 

confidence regarding the magnitude and significance of the associations observed in 

individual epidemiological studies and where there is appreciably less confidence. 

Furthermore, a standard level of 13 μg/m3or higher would not appropriately take 

into account the more limited evidence of effects in some at-risk populations (e.g., low 

birth weight). In the Administrator’s view, a standard set at this level would not provide 

protection with an adequate margin of safety, including providing protection for at-risk 

populations. The Administrator is mindful that the CAA requirement that primary 

standards provide an adequate margin of safety, discussed in section II.A above, was 

intended to address uncertainties associated with inconclusive scientific and technical 

information available at the time of standard setting as well as to provide a reasonable 

degree of protection against hazards that research has not yet identified. 

In light of the entire body of evidence as discussed above, the Administrator 

judges that an annual standard level set above 12 μg/m3 would not be sufficient to protect 
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public health with an adequate margin of safety from the serious health effects associated 

with long- and short-term exposure to PM2.5.

The Administrator also recognizes that a second group of commenters supported a 

lower annual standard level (e.g., no higher than 11 μg/m3). Such a standard level would 

reflect placing essentially as much weight on the relatively more limited data providing 

evidence suggestive of a causal relationship for effects observed in some at-risk 

populations (e.g., low birth weight) as on more certain evidence of effects classified as 

having a causal or likely causal relationship with PM2.5 exposures. In the Administrator’s 

view, while it is important to place some weight on such suggestive evidence, it would 

not be appropriate to place as much weight on it as the commenters would do. 

An annual standard level of 11 μg/m3 would also reflect these commenters’ 

judgment that it is appropriate to focus on a lower part of the distributions of health event 

data from the small number of epidemiological studies for which this information was 

made available than the Administrator believes is warranted. In the Administrator’s view, 

using this type of information to set a standard level of 11 μg/m3or below would assume 

too high a degree of confidence in the magnitude and significance of the associations 

observed in the lower part of the distributions of health events observed in these studies. 

Given the uncertainties in the evidence and the limited set of studies for which the EPA 

has information on the distribution of health event data and corresponding air quality 

data, the Administrator believes it is not appropriate to focus on the lower part of the 

distributions of health events data. 

On balance, the Administrator finds that the available evidence interpreted in light 

of the remaining uncertainties does not justify a standard level set below 12 μg/m3as 
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necessary to protect public health with an adequate margin of safety. 

After carefully considering the above considerations and the public comments 

summarized in section III.E.4.c above, the Administrator has decided to set the level of 

the primary annual PM2.5 standard at 12 μg/m3. In her judgment, a standard set at this 

level provides the requisite degree of public health protection, including the health of atrisk populations, with an adequate margin of safety and is neither more nor less stringent 

than necessary for this purpose.

As discussed above, the Administrator concludes that an approach that focuses on 

setting a generally controlling annual standard is the most effective and efficient way to 

reduce total population risk associated with both long- and short-term PM2.5 exposures. 

Such an approach would result in more uniform protection across the U.S. than the 

alternative of setting the levels of the 24-hour and annual standard such that the 24-hour 

standard would generally be the controlling standard in areas across the country (see 

section III.A.3). 

The Administrator recognizes that potential air quality changes associated with 

meeting an annual standard level of 12.μg/m3 will result in lowering risks associated with 

both long- and short-term PM2.5 exposures by lowering the overall air quality 

distribution. However, the Administrator recognizes that such an annual standard alone 

would not be expected to offer sufficient protection with an adequate margin of safety 

against the effects of all short-term PM2.5 exposures in all parts of the country. As a 

result, in conjunction with an annual standard level of 12 μg/m3, the Administrator 

concludes that it is appropriate to continue to provide supplemental protection by means 

of a 24-hour standard set at the appropriate level, particularly for areas with high peak-to-
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mean ratios possibly associated with strong local or seasonal sources and for areas with 

PM2.5-related effects that may be associated with shorter-than-daily exposure periods. 

In selecting the level of a 24-hour standard meant to provide such supplemental 

protection, the Administrator relies upon evidence and air quality information from key 

short-term exposure studies. In considering these studies, the Administrator notes that to 

the extent air quality distributions in the study areas considered are reduced to meet the 

current 24-hour standard (at a level of 35 μg/m3) or to meet the revised annual standard 

discussed above (at a level of 12 μg/m3), additional protection would be anticipated

against the effects observed in these studies. In light of this, when selecting the 

appropriate level for the 24-hour standard, the Administrator considers both the 98th

percentiles of 24-hour PM2.5 concentrations and the long-term mean PM2.5 concentrations 

in the locations of the short-term exposure studies. She notes that such consideration of 

both short- and long-term PM2.5 concentrations can inform her decision on the extent to 

which a given 24-hour standard, in combination with the revised annual standard 

established in this notice, would provide protection against the health effects reported in 

short-term studies. 

As discussed in section III.E.4.a above, the Administrator concludes that multicity short-term exposure studies provide the strongest data set for informing her decisions 

on appropriate 24-hour standard levels. With regard to the limited number of single-city 

studies that reported positive and statistically significant associations for a range of health 

endpoints related to short-term PM2.5 concentrations in areas that would likely have met 

the current suite of PM2.5 standards, the Administrator recognizes that many of these 

studies had significant limitations (e.g., limited statistical power, limited exposure data) 
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or equivocal results (mixed results within the same study area) that make them unsuitable 

to form the basis for setting the level of a 24-hour standard. 

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, the Administrator observes an overall pattern of positive and 

statistically significant associations in studies with 98th percentile 24-hour values 

averaged across study areas within the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; 

Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and 

Goldberg, 2003; Franklin et al., 2008). The Administrator notes that, to the extent air 

quality distributions are reduced to reflect just meeting the current 24-hour standard, 

additional protection would be provided for the effects observed in the three multi-city 

studies with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett 

and Goldberg, 2003; Franklin et al., 2008). In the three additional multi-city studies with 

98th percentile values below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 

34.3, and 34.8 μg/m3, the Administrator notes that these studies reported long-term mean 

PM2.5 concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; 

Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In revising the level of the annual 

standard to 12 μg/m3, as discussed above, the Administrator recognizes that additional 

protection would be provided for the short-term effects observed in these multi-city 

studies such that revision to the 24-hour standard would not be warranted. That is, by 

lowering the level of the annual standard to 12 μg/m3, the 98th percentile of the 

distribution would be lowered as well such that additional protection from effects 

associated with short-term exposures would be afforded. Therefore, the epidemiological 

evidence supports a conclusion that it is appropriate to retain the level of the 24-hour 
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standard at 35 μg/m3, in conjunction with a revised annual standard level of 12 μg/m3. 

In addition to considering the epidemiological evidence, the Administrator also 

has taken into account air quality information based on county-level 24-hour and annual 

design values to understand the implications of revising the annual standard level from 15 

to 12 μg/m3in conjunction with retaining the 24-hour standard level at 35 μg/m3. She has 

considered this information to evaluate the public health protection provided by the two 

standards in combination and to evaluate the most appropriate means of developing a 

suite of standards providing requisite public health protection with an adequate margin of 

safety. 

In considering the air quality information, the Administrator observes that a suite 

of PM2.5 standards that includes an annual standard level of 12 μg/m3and a 24-hour 

standard level of 35 μg/m3 would result in the annual standard as the generally 

controlling standard in most regions across the country, except for certain areas in the 

Northwest, where the annual mean PM2.5 concentrations have historically been low but 

where relatively high 24-hour concentrations occur, often related to seasonal wood smoke 

emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). In fact, these are the type of 

areas for which the supplemental protection afforded by the 24-hour standard is intended, 

such that the two standards together provide the requisite degree of protection. The 

Administrator concludes the current 24-hour standard at a level of 35 μg/m3, in 

conjunction with a revised annual standard level of 12 μg/m3, will provide appropriate 

protection from effects observed in studies in such areas in which the long-term mean 

concentrations were below 12 μg/m3and the 98th percentile 24-hour concentrations were 

above 35 μg/m3(e.g., areas in the Northwest U.S.). 
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After carefully taking the public comments and above considerations into 

account, the Administrator has decided to retain the current level of the primary PM2.5

24-hour standard at 35 μg/m3in conjunction with revising the annual standard level from 

15.0 μg/m3to 12.0 μg/m3.

104 In the Administrator’s judgment, this suite of primary PM2.5

standards and the rationale supporting these levels appropriately reflects consideration of 

the strength of the available evidence and other information and its associated 

uncertainties as well as the advice of CASAC and consideration of public comments.In 

the Administrator’s judgment, this suite of primary PM2.5 standards is sufficient but not 

more protective than necessary to protect the public health, including at-risk populations, 

with an adequate margin of safety from effects associated with long- and short-term 

exposures to fine particles. This suite of standards will provide significant protection 

from serious health effects including premature mortality and cardiovascular and 

respiratory morbidity effects that are causally or likely causally related to long- and shortterm PM2.5 exposures. These standards will also provide an appropriate degree of 

protection against other health effects for which there is more limited evidence of effects 

and causality, such as reproductive and developmental effects. This judgment by the 

Administrator appropriately considers the requirement for a standard that is requisite to 

protect public health but is neither more nor less stringent than necessary.105

D. Administrator’s Final Decisions on Primary PM2.5 Standards

For the reasons discussed above, and taking into account the information and 

 

104As noted in section II.B.1, Table 1 and section III.E.4.a above, the annual standard 

level is defined to one decimal place. Throughout this section, the annual standard levels 

discussed have been denoted as integer values (e.g., 12 μg/m3) for simplicity.

105 The Administrator also judges that this suite of standards addresses the issues raised 

by the D.C. Circuit’s remand of the 2006 primary annual PM2.5 standard by appropriately 

revising that standard.
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assessments presented in the Integrated Science Assessment, Risk Assessment, and 

Policy Assessment, the advice and recommendations of CASAC, and public comments to 

date, the Administrator revises the current suite of primary PM2.5 standards. Specifically, 

the Administrator revises: (1) the level of the primary annual PM2.5 standard to 12.0 

μg/m3and (2) the form of the primary annual PM2.5 standard to one based on the highest 

appropriate area-wide monitor in an area, with no option for spatial averaging. In 

conjunction with revising the primary annual PM2.5 standard to provide protection from 

effects associated with long- and short-term PM2.5 exposures, the Administrator retains 

the level of 35 μg/m3and the 98th percentile form of the primary 24-hour PM2.5 standard 

to continue to provide supplemental protection for areas with high peak PM2.5

concentrations. The Administrator is not revising the current PM2.5 indicator or the 

annual and 24-hour averaging times for the primary PM2.5 standards. The Administrator 

concludes that this suite of standards would be requisite to protect public health with an 

adequate margin of safety against health effects potentially associated with long- and 

short-term PM2.5 exposures.

VI. Rationale for Final Decisions on the Secondary PM Standards

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

revise the current suite of secondary PM2.5 and PM10 standards to address visibility 

impairment and other welfare effects considered in this review. Specifically, this section 

describes the Administrator’s final decision to retain the current suite of secondary PM 

standards to address PM-related visibility impairment as well as other PM-related welfare 

effects, including ecological effects, effects on materials, and climate impacts. This suite 
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of standards includes an annual PM2.5 standard of 15 μg/m3, a 24-hour PM2.5 standard of 

35 μg/m3, and a 24-hour PM10 standard of 150 μg/m3. The Administrator is revising only 

the form of the secondary annual PM2.5 standard to remove the option for spatial 

averaging consistent with this change to the primary annual PM2.5 standard. Contrary to 

what was proposed, the Administrator has decided not to establish a distinct standard to 

address PM-related visibility impairment. The rationale for this decision is presented 

below.

The Administrator’s final decisions on the secondary standards are based on a 

thorough review of the latest scientific information published through mid-2009 on 

welfare effects associated with fine and coarse particles in the ambient air, as presented in 

the Integrated Science Assessment. The final decisions also take into account: (1) staff 

assessments of the most policy-relevant information presented and assessed in the 

Integrated Science Assessment and staff analyses of air quality and visibility effects 

presented in the Visibility Assessment and the Policy Assessment, upon which staff 

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, Visibility Assessment, and Policy Assessment at public meetings, in 

separate written comments, and in CASAC’s letters to the Administrator; (3) the multiple 

rounds of public comments received during the development of these documents, both in 

connection with CASAC meetings and separately; and (4) public comments received on 

the proposal.

In particular, this section presents background information on the EPA’s previous 

and current reviews of the secondary PM standards (section VI.A), a summary of the 
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proposed decisions regarding the secondary PM standards (section VI.B), a discussion of 

significant public comments received on those proposed decisions (section VI.C), and the 

Administrator’s final decisions on the secondary PM standards (section VI.D).

A. Background

The current suite of secondary PM standards is identical to the suite of primary 

PM standards set in 2006, including 24-hour and annual PM2.5 standards and a 24-hour 

PM10 standard. The current secondary PM2.5 standards are intended to provide protection 

from PM-related visibility impairment, whereas the entire suite of secondary PM 

standards is intended to provide protection from other PM-related effects on public 

welfare, including effects on sensitive ecosystems, materials damage and soiling, and 

climatic and radiative processes.

The approach used for reviewing the current suite of secondary PM standards 

built upon and broadened the approaches used in previous PM NAAQS reviews. The 

following discussion focuses particularly on the current secondary PM2.5 standards 

related to visibility impairment and provides a summary of the approaches used to review 

and establish secondary PM2.5 standards in the last two reviews (section VI.A.1); judicial 

review of the 2006 standards that resulted in the remand of the secondary annual and 24-

hour PM2.5 NAAQS to the EPA (section VI.A.2); and the approach used in this review 

for evaluating the secondary PM2.5 standards (section VI.A.3).

1. Approaches Used in Previous Reviews

The original secondary PM2.5 standards were established in 1997, and a revision 

to the 24-hour standard was made in 2006. The approaches used in making final 

decisions on secondary standards in those reviews, as well as the current review, utilized 
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different ways to consider the underlying body of scientific evidence. They also reflected 

an evolution in EPA’s understanding of the nature of the effect on public welfare from 

PM-related visibility impairment, from an approach that focused only on Federal Class I 

area visibility impacts to a more multifaceted approach that also considered PM-related 

impacts on visibility in non-Federal Class I areas, such as in urban areas. This evolution 

occurred in conjunction with the expansion of available PM data and information from 

visibility-related studies of public perception, valuation, and personal comfort and wellbeing. 

In 1997, the EPA revised the PM NAAQS in part by establishing new identical 

primary and secondary PM2.5 standards. In revising the secondary standards, the EPA 

recognized that PM produces adverse effects on visibility and that impairment of 

visibility was being experienced throughout the U.S., in multi-state regions, urban areas, 

and remote mandatory Federal Class I areas alike. However, in considering an 

appropriate level for a secondary standard to address adverse effects of PM2.5 on 

visibility, the EPA concluded that the determination of a single national level was 

complicated by important regional differences influenced by factors such as background 

and current levels of PM2.5, composition of PM2.5, and average relative humidity. 

Variations in these factors across regions could thus result in situations where attaining 

an appropriately protective concentration of fine particles in one region might or might 

not provide adequate protection in a different region. The EPA also determined that there 

was insufficient information at that time to establish a level for a national secondary 

standard that would represent a threshold above which visibility conditions would always 

be adverse and below which visibility conditions would always be acceptable.
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Based on an assessment of the potential visibility improvements that would result 

from reaching attainment with the new primary standards for PM2.5, the EPA concluded 

that attainment of the annual and 24-hour PM2.5 primary standards would lead to 

visibility improvements in the eastern U.S. at both urban and regional scales, but little or 

no change in the western U.S., except in and near certain urban areas.

The EPA also considered the potential effectiveness of a regional haze program, 

required by sections 169A and 169B of the CAA106 to address those effects of PM on 

visibility that would not be addressed through attainment of the primary PM2.5 standards. 

The regional haze program would be designed to address the widespread, regionally 

uniform type of haze caused by a multitude of sources. The structure and requirements of 

sections 169A and 169B of the CAA provide for visibility protection programs that can 

be more responsive to the factors contributing to regional differences in visibility than 

can programs addressing the kinds of nationally applicable secondary NAAQS 

considered in the 1997 review. The regional haze visibility goal is more protective than a 

secondary NAAQS since the goal is to eliminate any anthropogenic impairment rather 

than to provide a level of protection from visibility impairment that is requisite to protect 

the public welfare. Thus, an important factor considered in the 1997 review was whether 

a regional haze program, in conjunction with secondary standards set identical to the suite 

of PM2.5 primary standards, would provide appropriate protection for visibility in nonFederal Class I areas. The EPA concluded that the two programs and associated control 

strategies should provide such protection due to the regional approaches needed to 

 

106 In 1977, Congress established as a national goal ‘‘the prevention of any future, and the 

remedying of any existing, impairment of visibility in mandatory Federal Class I areas 

which impairment results from manmade air pollution,” section 169A(a)(1) of the CAA. 

The EPA is required by section 169A(a)(4) of the CAA to promulgate regulations to 

ensure that ‘‘reasonable progress’’ is achieved toward meeting the national goal.
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manage emissions of pollutants that impair visibility in many of these areas.

For these reasons, in 1997 the EPA concluded that a national regional haze 

program, combined with a nationally applicable level of protection achieved through 

secondary PM2.5 standards set identical to the primary PM2.5 standards, would be more 

effective for addressing regional variations in the adverse effects of PM2.5 on visibility 

than would be national secondary standards for PM with levels lower than the primary 

PM2.5 standards. The EPA further recognized that people living in certain urban areas 

may place a high value on unique scenic resources in or near these areas and as a result 

might experience visibility problems attributable to sources that would not necessarily be 

addressed by the combined effects of a regional haze program and PM2.5 secondary 

standards. The EPA concluded that in such cases, state or local regulatory approaches, 

such as past action in Colorado to establish a local visibility standard for the City of 

Denver, would be more appropriate and effective in addressing these special situations 

because of the localized and unique characteristics of the problems involved. Visibility in 

an urban area located near a mandatory Federal Class I area could also be improved 

through state implementation of the then-current visibility regulations, by which emission 

limitations can be imposed on a source or group of sources found to be contributing to 

‘‘reasonably attributable’’ impairment in the mandatory Federal Class I area.

Based on these considerations, in 1997 the EPA set secondary PM2.5 standards 

identical to the primary PM2.5 standards, that would work in conjunction with the 

Regional Haze Program to be established under sections 169A and 169B of the CAA, as 

the most appropriate and effective means of addressing the public welfare effects 

associated with visibility impairment. Together, the two programs and associated control 
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strategies were expected to provide appropriate protection against PM-related visibility 

impairment and enable all regions of the country to make reasonable progress toward the 

national visibility goal.

In 2006, the EPA revised the suite of secondary PM2.5 standards to address 

visibility impairment by making the suite of secondary standards identical to the revised 

suite of primary PM2.5 standards. The EPA’s decision regarding the need to revise the 

suite of secondary PM2.5 standards reflected a number of new developments that had 

occurred and sources of information that had become available following the 1997 

review. First, the EPA promulgated a Regional Haze Program in 1999 (65 FR 35713, 

July 1, 1999) which required states to establish goals for improving visibility in Federal 

Class I areas and to adopt control strategies to achieve these goals. Second, extensive new 

information from visibility and fine particle monitoring networks had become available, 

allowing for updated characterizations of visibility trends and PM concentrations in urban 

areas, as well as Federal Class I areas. These new data allowed the EPA to better 

characterize visibility impairment in urban areas and the relationship between visibility 

and PM2.5 concentrations. Finally, additional studies in the U.S. and abroad provided the 

basis for the establishment of standards and programs to address specific visibility 

concerns in a number of local areas. These studies (Denver, Phoenix, and British 

Columbia) utilized photographic representations of visibility impairment and produced 

reasonably consistent results in terms of the visual ranges found to be generally 

acceptable by study participants. The EPA considered the information generated by these 

studies useful in characterizing the nature of particle-induced haze and for informing 

judgments about the acceptability of various levels of visual air quality in urban areas 
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across the U.S. Based largely on this information, the Administrator concluded that it was 

appropriate to revise the secondary PM2.5 standards to provide increased protection from 

visibility impairment principally in urban areas, in conjunction with the regional haze 

program for protection of visual air quality in Federal Class I areas. 

In so doing, the Administrator recognized that PM-related visibility impairment is 

principally related to fine particle concentrations and that perception of visibility 

impairment is most directly related to short-term, nearly instantaneous levels of visual air 

quality. Thus, in considering whether the then-current suite of secondary standards would 

provide the appropriate degree of protection, he concluded that it was appropriate to 

focus on just the 24-hour secondary PM2.5 standard to provide requisite protection.

The Administrator then considered whether PM2.5 mass remained the appropriate 

indicator for a secondary standard to protect visibility, primarily in urban areas. The 

Administrator noted that PM-related visibility impairment is principally related to fine 

particle levels. Hygroscopic components of fine particles, in particular sulfates and 

nitrates, contribute disproportionately to visibility impairment under high humidity 

conditions. Particles in the coarse mode generally contribute only marginally to visibility 

impairment in urban areas. With the substantial addition to the air quality and visibility 

data made possible by the national urban PM2.5 monitoring networks, an analysis 

conducted for the 2006 review found that, in urban areas, visibility levels showed far less 

difference between eastern and western regions on a 24-hour or shorter time basis than 

implied by the largely non-urban data available in the 1997 review. In analyzing how 

well PM2.5 concentrations correlated with visibility in urban locations across the U.S., the 

2005 Staff Paper concluded that clear correlations existed between 24-hour average PM2.5
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concentrations and calculated (i.e., reconstructed) light extinction, which is directly 

related to visual range (U.S. EPA, 2005, p. 7-6). These correlations were similar in the 

eastern and western regions of the U.S. These correlations were less influenced by 

relative humidity and more consistent across regions when PM2.5 concentrations were 

averaged over shorter, daylight time periods (e.g., 4 to 8 hours) when relative humidity in 

eastern urban areas was generally lower and thus more similar to relative humidity in 

western urban areas. The 2005 Staff Paper noted that a standard set at any specific PM2.5

concentration would necessarily result in visual ranges that vary somewhat in urban areas 

across the country, reflecting the variability in the correlations between PM2.5

concentrations and light extinction. The 2005 Staff Paper concluded that it was 

appropriate to use PM2.5 as an indicator for standards to address visibility impairment in 

urban areas, especially when the indicator is defined for a relatively short period (e.g., 4 

to 8 hours) of daylight hours (U.S. EPA, 2005, p. 7-6). Based on their review of the Staff 

Paper, most CASAC Panel members also endorsed such a PM2.5 indicator for a secondary 

standard to address visibility impairment (Henderson, 2005a, p. 9). Based on the above 

considerations, the Administrator concluded that PM2.5 should be retained as the indicator 

for fine particles as part of a secondary standard to address visibility protection, in 

conjunction with averaging times from 4 to 24 hours.

In considering what level of protection against PM-related visibility impairment 

would be appropriate, the Administrator took into account the results of the public 

perception and attitude surveys regarding the acceptability of various degrees of visibility 

impairment in the U.S. and Canada, state and local visibility standards within the U.S., 

and visual inspection of photographic representations of several urban areas across the 
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U.S. In the Administrator’s judgment, these sources provided useful but still quite limited

information on the range of levels appropriate for consideration in setting a national 

visibility standard primarily for urban areas, given the generally subjective nature of the 

public welfare effect involved. Based on photographic representations of varying levels 

of visual air quality, public perception studies, and local and state visibility standards, the 

2005 Staff Paper had concluded that 30 to 20 μg/m3PM2.5 represented a reasonable range 

for a national visibility standard primarily for urban areas, based on a sub-daily averaging 

time (U.S. EPA, 2005, p. 7-13). The upper end of this range was below the levels at 

which illustrative scenic views are significantly obscured, and the lower end was around 

the level at which visual air quality generally appeared to be good based on observation 

of the illustrative views. This concentration range generally corresponded to median 

visual ranges in urban areas within regions across the U.S. of approximately 25 to 35 km, 

a range that was bounded above by the visual range targets selected in specific areas 

where state or local agencies placed particular emphasis on protecting visual air quality. 

In considering a reasonable range of forms for a PM2.5 standard within this range of 

levels, the 2005 Staff Paper had concluded that a concentration-based percentile form 

was appropriate, and that the upper end of the range of concentration percentiles for 

consideration should be consistent with the 98th percentile used for the primary standard 

and that the lower end of the range should be the 92nd percentile, which represented the 

mean of the distribution of the 20 percent most impaired days, as targeted in the regional 

haze program (U.S. EPA, 2005 pp. 7-11 to 7-13). While recognizing that it was difficult 

to select any specific level and form based on then-currently available information 

(Henderson, 2005a, p. 9), the CASAC Panel was generally in agreement with the ranges 
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of levels and forms presented in the 2005 Staff Paper. 

The Administrator also considered the level of protection that would be afforded 

by the proposed suite of primary PM2.5 standards (71 FR 2681, January 17, 2006), on the 

basis that although significantly more information was available than in the 1997 review 

concerning the relationship between fine PM levels and visibility across the country, 

there was still little available information for use in making the relatively subjective value 

judgment needed in selecting the appropriate degree of protection to be afforded by such 

a standard. In so doing, the Administrator compared the extent to which the proposed 

suite of primary standards would require areas across the country to improve visual air 

quality with the extent of increased protection likely to be afforded by a standard based 

on a sub-daily averaging time. Based on such an analysis, the Administrator observed 

that the predicted percent of counties with monitors not likely to meet the proposed suite 

of primary PM2.5 standards was actually somewhat greater than the predicted percent of 

counties with monitors not likely to meet a sub-daily secondary standard with an 

averaging time of 4 daylight hours, a level toward the upper end of the range 

recommended in the 2005 Staff Paper, and a form within the recommended range. Based 

on this comparison, the Administrator tentatively concluded that revising the secondary 

24-hour PM2.5 standard to be identical to the proposed revised primary PM2.5 standard 

(and retaining the then-current annual secondary PM2.5 standard) was a reasonable policy 

approach to addressing visibility protection primarily in urban areas. In proposing this 

approach, the Administrator also solicited comment on a sub-daily (4- to 8-hour 

averaging time) secondary PM2.5 standard (71 FR 2675 to 2781, January 17, 2006). 

In commenting on the proposed decision, the CASAC requested that a sub-daily 
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standard to protect visibility “be favorably reconsidered” (Henderson, 2006a, p.6). The 

CASAC noted three cautions regarding the proposed reliance on a secondary PM2.5

standard identical to the proposed 24-hour primary PM2.5 standard: (1) PM2.5 mass 

measurement is a better indicator of visibility impairment during daylight hours, when 

relative humidity is generally low; the sub-daily standard more clearly matches the nature 

of visibility impairment, whose adverse effects are most evident during the daylight 

hours; using a 24-hour PM2.5 standard as a proxy introduces error and uncertainty in

protecting visibility; and sub-daily standards are used for other NAAQS and should be 

the focus for visibility; (2) CASAC and its monitoring subcommittees had repeatedly 

commended EPA’s initiatives promoting the introduction of continuous and nearcontinuous PM monitoring and recognized that an expanded deployment of continuous 

PM2.5 monitors would be consistent with setting a sub-daily standard to protect visibility; 

and (3) the analysis showing a similarity between percentages of counties not likely to 

meet what the CASAC Panel considered to be a lenient 4- to 8-hour secondary standard 

and a secondary standard identical to the proposed 24-hour primary standard was a 

numerical coincidence that was not indicative of any fundamental relationship between 

visibility and health. The CASAC Panel further stated that “visual air quality is 

substantially impaired at PM2.5 concentrations of 35 μg/m3” and that “[i]t is not 

reasonable to have the visibility standard tied to the health standard, which may change in 

ways that make it even less appropriate for visibility concerns” (Henderson, 2006a, pp. 5 

to 6). 

In reaching a final decision, the Administrator focused on the relative protection 

provided by the proposed primary standards based on the above-mentioned similarities in 
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percentages of counties meeting alternative standards and on the limitations in the 

information available concerning studies of public perception and attitudes regarding the 

acceptability of various degrees of visibility impairment in urban areas, as well as on the 

subjective nature of the judgment required. In so doing, the Administrator concluded that 

caution was warranted in establishing a distinct secondary standard for visibility 

impairment and that the available information did not warrant adopting a secondary 

standard that would provide either more or less protection against visibility impairment in 

urban areas than would be provided by secondary standards set equal to the proposed 

primary PM2.5 standards.

2. Remand of 2006 Secondary PM2.5 Standards 

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

challenging EPA’s decision to set the secondary NAAQS for fine PM identical to the 

primary NAAQS. On judicial review, the D.C. Circuit remanded to the EPA for 

reconsideration the secondary NAAQS for fine PM because the Agency’s decision was 

unreasonable and contrary to the requirements of section 109(b)(2). American Farm 

Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir., 2009).

The petitioners argued that the EPA’s decision lacked a reasoned basis. First, they 

asserted that the EPA never determined what level of visibility was “requisite to protect 

the public welfare.” They argued that the EPA unreasonably rejected the target level of 

protection recommended by its staff, while failing to provide a target level of its own. 

The court agreed, stating that “the EPA’s failure to identify such a level when deciding 

where to set the level of air quality required by the revised secondary fine PM NAAQS is 

contrary to the statute and therefore unlawful. Furthermore, the failure to set any target 
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level of visibility protection deprived the EPA’s decision-making of a reasoned basis.” 

559 F. 3d at 530.

Second, the petitioners challenged EPA’s method of comparing the protection 

expected from potential standards. They contended that the EPA relied on a meaningless 

numerical comparison, ignored the effect of humidity on the usefulness of a standard 

using a daily averaging time, and unreasonably concluded that the primary standards 

would achieve a level of visibility roughly equivalent to the level the EPA staff and 

CASAC deemed “requisite to protect the public welfare.” The court found that the EPA’s 

equivalency analysis based on the percentages of counties exceeding alternative standards 

“failed on its own terms.” The same table showing the percentages of counties exceeding 

alternative secondary standards, used for comparison to the percentages of counties 

exceeding alternative primary standards to show equivalency, also included six other 

alternative secondary standards within the recommended CASAC range that would be 

more “protective” under EPA’s definition than the adopted primary standards. Two-thirds 

of the potential secondary standards within the CASAC’s recommended range would be 

substantially more protective than the adopted primary standards. The court found that 

the EPA failed to explain why it looked only at one of the few potential secondary 

standards that would be less protective, and only slightly less so, than the primary 

standards. More fundamentally, however, the court found that the EPA’s equivalency 

analysis based on percentages of counties demonstrated nothing about the relative 

protection offered by the different standards, and that the tables offered no valid 

information about the relative visibility protection provided by the standards. 559 F. 3d at 

530-31.
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Finally, the Staff Paper had made clear that a visibility standard using PM2.5 mass 

as the indicator in conjunction with a daily averaging time would be confounded by 

regional differences in humidity. The court noted that the EPA acknowledged this 

problem, yet did not address this issue in concluding that the primary standards would be 

sufficiently protective of visibility. 559 F. 3d at 530. Therefore, the court granted the 

petition for review and remanded for reconsideration the secondary PM2.5 NAAQS.

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

The approach used in this review broadened the general approaches used in the 

last two PM NAAQS reviews by utilizing, to the extent available, enhanced tools, 

methods, and data to more comprehensively characterize visibility impacts. As such, the 

EPA took into account considerations based on both the scientific evidence (“evidencebased”) and a quantitative analysis of PM-related impacts on visibility (“impact-based”) 

to inform conclusions related to the adequacy of the current secondary PM2.5 standards 

and alternative standards that were appropriate for consideration in this review. As in past 

reviews, the EPA also considered that the secondary NAAQS should address PM-related 

visibility impairment in conjunction with the Regional Haze Program, such that the 

secondary NAAQS would focus on protection from visibility impairment principally in 

urban areas in conjunction with the Regional Haze Program that is focused on improving 

visibility in Federal Class I areas. The EPA again recognized that such an approach 

remains the most appropriate and effective means of addressing the public welfare effects 

associated with visibility impairment in areas across the country.

The Policy Assessment drew from the qualitative evaluation of all studies 

discussed in the Integrated Science Assessment (U.S. EPA, 2009a). Specifically, the 
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Policy Assessment considered the extensive new air quality and source apportionment 

information available from the regional planning organizations, long-standing evidence 

of PM effects on visibility, and limited public preference study information from four 

urban areas (U.S. EPA, 2009a, chapter 9), as well as the integration of evidence across 

disciplines (U.S. EPA, 2009a, chapter 2). In addition, limited information that had 

become available regarding the characterization of public preferences in urban areas 

provided some new perspectives on the usefulness of this information in informing the 

selection of target levels of urban visibility protection. On these bases, the Policy 

Assessment again focused assessments on visibility conditions in urban areas.

The conclusions in the Policy Assessment reflected EPA staff’s understanding of 

both evidence-based and impact-based considerations to inform two overarching 

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

potential alternative standards, if any, should be considered in this review to provide 

appropriate protection from PM-related visibility impairment. In addressing these broad 

questions, the discussions in the Policy Assessment were organized around a series of 

more specific questions reflecting different aspects of each overarching question (U.S. 

EPA, 2011a, Figure 4-1). When evaluating the visibility protection afforded by the 

current or any alternative standards considered, the Policy Assessment took into account 

the four basic elements of the NAAQS: indicator, averaging time, level, and form.

B. Proposed Decisions on Secondary PM Standards

At the time of proposal, the Administrator proposed to revise the suite of 

secondary PM standards by adding a distinct standard for PM2.5 to address PM-related 

visibility impairment, focused primarily on visibility in urban areas. This proposed 
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standard was to be defined in terms of a PM2.5 visibility index, which would use 

measured PM2.5 mass concentration, in combination with speciation and relative humidity 

data, to calculate PM2.5 light extinction, translated into the deciview (dv) scale; a 24-hour 

averaging time; a 90th percentile form, averaged over 3 years; and a level of 28-30 dv. To 

address other non-visibility welfare effects, the Administrator proposed to retain the 

current suite of secondary PM standards generally, while revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this proposed change to the primary annual PM2.5 standard. Each of these proposed 

decisions is described in more detail in the proposal and below.

1. PM-Related Visibility Impairment

As discussed in Section VI.B. of the proposal, the Administrator’s proposed 

decision regarding a distinct secondary standard to provide protection from visibility 

impairment reflected careful consideration of the following: (1) the latest scientific 

information on visibility effects associated with PM as described in the Integrated 

Science Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of 

correlations between ambient PM2.5 and visibility impairment prepared by EPA staff in 

the Visibility Assessment (U.S. EPA, 2010b); and (3) specific conclusions regarding the 

need for revisions to the current standards (i.e., indicator, averaging time, form, and 

level) that, taken together, would be requisite to protect the public welfare from adverse 

effects on visual air quality. This section summarizes key information from the proposal 

regarding the nature of visibility impairment, including the relationship between ambient 

PM and visibility, temporal variations in light extinction, periods during the day of 

interest for assessing visibility conditions, and exposure durations of interest (section 
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VI.B.1.a); limited public perceptions and attitudes about visibility impairment and the 

impacts of visibility impairment on public welfare (section VI.B.1.b); CASAC advice 

regarding the need for, and design of, secondary standards to protect visibility (section 

VI.B.1.c); and the Administrator’s proposed conclusions regarding setting a distinct 

standard to address visibility impairment (section VI.B.1.d).

a. Nature of PM-related Visibility Impairment

As noted at the time of proposal, the fundamental science characterizing the 

contribution of PM, especially fine particles, to visibility impairment is well understood. 

This science provides the basis for the Integrated Science Assessment designation of the 

relationship between PM and visibility impairment as causal. New research available in 

this review, discussed in chapter 9 of the Integrated Science Assessment, continues to 

support and refine EPA’s understanding of the effect of PM on visibility and the source 

contributions to that effect in rural and remote locations. This research provides new 

insights regarding the regional source contributions to urban visibility impairment and 

better characterization of the increment in PM concentrations and visibility impairment 

that occur in many cities (i.e., the urban excess) relative to conditions in the surrounding 

rural areas (i.e., regional background). Ongoing urban PM2.5 speciated and aggregated 

mass monitoring has produced new information that has allowed for updated 

characterization of current visibility levels in urban areas.

i. Relationship between Ambient PM and Visibility

Visibility impairment is caused by the scattering and absorption of light by 

suspended particles and gases in the atmosphere. When PM is present in the air, its 

contribution to light extinction typically greatly exceeds that of gases. The combined 
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effect of light scattering and absorption by both particles and gases is characterized as 

light extinction, i.e., the fraction of light that is scattered or absorbed in the atmosphere. 

Light extinction can be quantified by a light extinction coefficient with units of 

1/distance, which is often expressed as 1/(1 million meters) or inverse megameters 

(abbreviated Mm-1) or in terms of an alternative scale known as the deciview scale, 

defined by the following equation:107

Deciview (dv) = 10 ln (bext / 10 Mm-1) 

The deciview scale is frequently used in the scientific literature on visibility, as well as in 

the Regional Haze Program. In particular, the deciview scale is used in the public 

perception studies that were considered in the past and current reviews to inform 

judgments about an appropriate degree of protection to be provided by a secondary 

NAAQS.

The amount of light extinction contributed by PM depends on the particle 

concentration as well as on the particle size distribution and composition and also on the 

relative humidity. As described in detail in section VI.B.1.a of the proposal, visibility 

scientists have developed an algorithm, known as the IMPROVE algorithm,108 to 

estimate light extinction using routinely monitored fine particle (PM2.5) speciation and 

coarse particle mass (PM10–2.5) data, as well as data on relative humidity. There is both an 

 

107 As used in the Regional Haze Program, the term bext refers to light extinction due to 

PM2.5, PM10-2.5, and “clean” atmospheric gases. In the Policy Assessment, in focusing on 

light extinction due to PM2.5, the deciview values include only the effects of PM2.5 and 

the gases. The “Rayleigh” term associated with clean atmospheric gases is represented by 

the constant value of 10 Mm-1. Omission of the Rayleigh term would create the 

possibility of negative deciview values when the PM2.5 concentration is very low.

108 The algorithm is referred to as the IMPROVE algorithm because it was developed 

specifically to use the aerosol monitoring data generated at network sites and with 

equipment specifically designed to support the IMPROVE program and was evaluated 

using IMPROVE optical measurements at the subset of sites that make those 

measurements (Malm et al., 1994).
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original and a revised version of the IMPROVE algorithm (Pitchford et al., 2007). The 

revised version was developed to address observed biases in the predictions using the 

original algorithm under very low and very high light extinction conditions.109 These 

IMPROVE algorithms are routinely used to calculate light extinction levels on a 24-hour 

basis in Federal Class I areas under the Regional Haze Program.

In either version of the IMPROVE algorithm, the concentration of each of the 

major aerosol components is multiplied by a dry extinction efficiency value and, for the 

hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also 

multiplied by an additional factor to account for the water growth to estimate these 

components’ contribution to light extinction. Summing the contribution of each 

component gives the estimate of total light extinction per unit distance denoted as the 

light extinction coefficient (bext), as shown below for the original IMPROVE algorithm. 

bext ≈ 3 x f(RH) x [Sulfate]

+ 3 x f(RH) x [Nitrate]

+ 4 x [Organic Mass]

+ 10 x [Elemental Carbon]

+ 1 x [Fine Soil]

+ 0.6 x [Coarse Mass]

+ 10 

Light extinction (bext) is in units of Mm-1, the mass concentrations of the 

components indicated in brackets are in units of μg/m3, and f(RH) is the unitless water 

growth term that depends on relative humidity. The final term of 10 Mm-1 is known as the 

Rayleigh scattering term and accounts for light scattering by the natural gases in 

unpolluted air. Despite the simplicity of this algorithm, it performs reasonably well and 

permits the contributions to light extinction from each of the major components 

 

109 These biases were detected by comparing light extinction estimates generated from the 

IMPROVE algorithm to direct optical measurements in a number of rural Federal Class I 

areas.
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(including the water associated with the sulfate and nitrate compounds) to be separately 

approximated. Inspection of the PM component-specific terms in the simple original 

IMPROVE algorithm shows that most of the PM2.5 components contribute 5 times or 

more light extinction than a similar concentration of PM10–2.5. 

The f(RH) term in the original algorithm reflects the increase in light scattering 

caused by particulate sulfate and nitrate under conditions of high relative humidity. 

Particles with hygroscopic components (e.g., particulate sulfate and nitrate) contribute 

more light extinction at higher relative humidity than at lower relative humidity because 

they change size in the atmosphere in response to ambient relative humidity conditions. 

For relative humidity below 40 percent the f(RH) value is 1, but it increases to 2 at 

approximately 66 percent, 3 at approximately 83 percent, 4 at approximately 90 percent, 

5 at approximately 93 percent, and 6 at approximately 95 percent relative humidity. The 

result is that both particulate sulfate and nitrate are more efficient per unit mass in light 

extinction than any other aerosol component for relative humidity above approximately 

85 percent where their total light extinction efficiency exceeds the 10 m2/g associated 

with elemental carbon (EC). PM containing elemental or black carbon (BC) absorbs light 

as well as scattering it, making it the component with the greatest light extinction 

contributions per unit of mass concentration, except for the hygroscopic components 

under these high relative humidity conditions.110

As noted above, subsequent to the development of the original IMPROVE 

algorithm, an alternative algorithm (variously referred to as the “revised algorithm” or the 

“new algorithm” in the literature) was developed. The revised IMPROVE algorithm is 

 

110 The IMPROVE algorithm does not explicitly separate the light-scattering and lightabsorbing effects of elemental carbon.
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different from the original algorithm in several important ways. First, the revised 

algorithm employs a more complex split-component mass extinction efficiency to correct 

biases believed to be related to particle size distributions.

111 Specifically, the revised

algorithm incorporates terms to account for particles representing the different dry 

extinction and water uptake from two size modes of sulfate, nitrate and organic mass.

112

Second, the revised algorithm uses a different multiplier for organic carbon for purposes 

of estimating organic carbonaceous material to better represent aged aerosol found in 

remote areas.

113 In addition, the revised algorithm includes a term for hygroscopic sea 

salt that can be important for remote coastal areas, and site-specific Rayleigh light 

scattering terms in place of a universal Rayleigh light scattering value. As noted in 

section VI.B.1.a of the proposal, the revised IMPROVE algorithm can yield higher 

estimates of current light extinction levels in urban areas on days with relatively poor 

visibility as compared to the original algorithm (Pitchford, 2010). This difference is 

primarily attributable to the split-component mass extinction efficiency treatment in the 

 

111 In either version of the IMPROVE algorithm, the concentration of each of the major 

aerosol components is multiplied by a dry extinction efficiency value and, for the 

hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also 

multiplied by an additional factor to account for the water growth to estimate these 

components’ contribution to light extinction. Both the dry extinction efficiency and water 

growth terms have been developed by a combination of empirical assessment and 

theoretical calculation using typical particle size distributions associated with each of the 

major aerosol components.

112 The relative contributions of sulfate, nitrate and organic mass concentrations to 

visibility impairment with the revised algorithm, are different than with the original 

algorithm due to the combination of the dry extinction coefficient and f(RH) functions for 

derived concentrations of small and large particles. The apportionment of the total fine 

particle concentration of each of the three PM2.5 components into the concentrations of 

the small and large size fractions was empirically developed for remote areas. The 

fraction of the fine particle component that is in the large mode is estimated by dividing 

the total concentration of the component by 20 ug/m3. If the total concentration of a 

component exceeds 20 ug/m3, all of it is assumed to be in the large mode. 

113 The revised IMPROVE algorithm uses a multiplier of 1.8 for rural areas instead of 1.4 

as used in the original algorithm for the mean ratio of organic mass to organic carbon.
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revised algorithm. This revised algorithm was evaluated at 21 remote locations and is 

generally used by RPOs and States for implementation of the Regional Haze Rule.

ii. Temporal Variations of Light Extinction

Particulate matter concentrations and light extinction in urban environments vary 

from hour to hour throughout the 24-hour day due to a combination of diurnal changes in 

meteorological conditions and systematic changes in emissions activity (e.g., rush hour 

traffic). Various factors combine to make early morning the most likely time for peak 

urban light extinction; although the net effects of the systematic urban- and larger-scale 

variations mean that peak daytime PM light extinction levels can occur any time of day, 

in many areas they occur most often in early morning hours (U.S. EPA, 2010b, sections 

3.4.2 and 3.4.3; Figures 3-9, 3-10, and 3-12). This temporal pattern in urban areas 

contrasts with the general lack of a strong diurnal pattern in PM concentrations and light 

extinction in most Federal Class I areas, reflective of a relative lack of local sources as 

compared to urban areas. The use in the Regional Haze Program of 24-hour average 

concentrations in the IMPROVE algorithm is consistent with this general lack of a strong 

diurnal pattern in Federal Class I areas. 

iii. Periods during the Day of Interest for Assessment of Visibility

As noted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime visibility has 

dominated the attention of those who have studied the visibility effects of air pollution, 

particularly in urban areas. The EPA recognizes, however, that physically PM light 

extinction behaves the same at night as during the day and can contribute to nighttime 

visibility effects by enhancing the scattering of anthropogenic light, contributing to the 

“skyglow” within and over populated areas, adding to the total sky brightness, and 
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contributing to the reduction in contrast of stars against the background. However, little 

research has been conducted on nighttime visibility, and the state of the science is not 

comparable to that associated with daytime visibility impairment, particularly in terms of 

the impact on human welfare. The Policy Assessment notes that the science is not 

available at this time to support adequate characterization specifically of nighttime PM 

light extinction conditions and the related effects on public welfare (U.S. EPA, 2011a, p. 

4-18). Therefore the EPA has focused its assessments of PM visibility impacts in urban 

areas on daylight hours during this review. 

iv. Exposure Durations of Interest

As noted in section VI.B.1.d of the proposal, the roles that exposure duration and 

variations in visual air quality within any given exposure period play in determining the 

acceptability or unacceptability of a given level of visual air quality have not been 

investigated via preference studies. In the preference studies available for this review, 

subjects were simply asked to rate the acceptability or unacceptability of each image of a 

haze-obscured scene, without being provided any suggestion of assumed duration or of 

assumed conditions before or after the occurrence of the scene presented. Preference 

and/or valuation studies show that atmospheric visibility conditions can be quickly 

assessed and preferences determined. The EPA is unaware of any studies that 

characterize the extent to which different frequencies and durations of exposure to 

visibility conditions contribute to the degree of public welfare impact that occurs. 

The Policy Assessment considered a variety of circumstances that are commonly 

expected to occur in evaluating the potential impact of visibility impairment on the public 

welfare based on available information (U.S. EPA, 2011a, pp. 4-19 to 4-20). In some 
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circumstances, such as infrequent visits to scenic vistas in natural or urban environments, 

people are motivated specifically to take the opportunity to view a valued scene and are 

likely to do so for many minutes to hours to appreciate various aspects of the vista they 

choose to view. However, the public has many more opportunities to notice visibility 

conditions on a daily basis in settings associated with performing daily routines (e.g., 

during commutes and while working, exercising, or recreating outdoors). As noted in the 

Policy Assessment, information regarding the fraction of the public that has only one or a 

few opportunities to experience visibility during the day, or on the role the duration of the 

observed visibility conditions has on wellbeing effects associated with those visibility 

conditions, is not available (U.S. EPA, 2011a, p. 4-20). However, it is possible that 

people with limited opportunities to experience visibility conditions on a daily basis 

would receive the entire impact of the day’s visual air quality based on the visibility 

conditions that occur during the short time period when they can see it. Since this group 

could be affected on the basis of observing visual air quality conditions for periods as 

short as one hour or less, and because during each daylight hour there are some people 

outdoors, commuting, or near windows, the Policy Assessment judged that it would be 

appropriate to use the maximum hourly value of PM light extinction during daylight 

hours for each day for purposes of evaluating the adequacy of the current suite of 

secondary standards. Other observers may have access to visibility conditions throughout 

the day. For this group, it might be that an hour with poor or “unacceptable” visibility can 

be offset by one or more other hours with clearer conditions. Therefore, the proposal 

acknowledged that it might also be appropriate to consider a multi-hour daylight 

exposure period.
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v. Periods of Fog and Rain

As discussed in section VI.C of the proposal, the EPA also recognized that it is 

appropriate to give special treatment to periods of fog and rain when considering whether 

current PM2.5 standards adequately protect public welfare from PM-related visibility 

impairment. Visibility impairment occurs during periods with fog or precipitation 

irrespective of the presence or absence of PM. Therefore, it is logical that periods with 

naturally impaired visibility due to fog or precipitation should not be treated as having 

PM-impaired visibility. There are multiple ways to adjust visibility data to reduce the 

effects of fog and precipitation. In the Visibility Assessment, following the advice of 

CASAC, the EPA evaluated the effect of excluding daylight hours for which relative 

humidity was greater than 90 percent from analyses in order to avoid precipitation and 

fog confounding estimates of PM visibility impairment. For the 15 urban areas included 

in the Visibility Assessment, the EPA found that a 90 percent relative humidity cutoff 

criterion was effective in that on average less than 6 percent of the daylight hours were 

removed from consideration, yet those hours had on average ten times the likelihood of 

rain, six times the likelihood of snow/sleet, and 34 times the likelihood of fog compared 

with hours with 90 percent or lower relative humidity. In the Regional Haze program, the 

EPA utilizes monthly average relative humidity values based on 10 years of 

climatological data to reduce the effect of fog and precipitation. This approach focuses on 

longer-term averages for each monitoring site and thereby eliminates the effect of very 

high humidity conditions on visibility at those locations.

b. Public Perception of Visibility Impairment

As described in section VI.B.2 of the proposal, there are two main types of studies 
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that evaluate the public perception of urban visibility impairment: urban visibility 

preference studies and urban visibility valuation studies. As noted in the Integrated 

Science Assessment, “[b]oth types of studies are designed to evaluate individuals’ desire 

(or demand) for good visual air quality (VAQ) where they live, using different metrics to 

evaluate demand. Urban visibility preference studies examine individuals’ demand by 

investigating what amount of visibility degradation is unacceptable while economic 

studies examine demand by investigating how much one would be willing to pay to 

improve visibility” (U.S. EPA, 2009a, p. 9-66). Because of the limited number of new 

studies on urban visibility valuation, the Integrated Science Assessment cites to the 

discussion in the 2004 Criteria Document of the various methods one can use to 

determine the economic valuation of changes in visibility, which include hedonic 

valuation, contingent valuation and contingent choice, and travel cost.

Contingent valuation studies are a type of stated preference study that measures 

the strength of preferences and expresses that preference in dollar values. Contingent 

valuation studies often include payment vehicles that require respondents to consider 

implementation costs and their ability to pay for visibility improvements in their 

responses. This study design aspect is critical because the EPA cannot consider 

implementations costs in setting either primary or secondary NAAQS. Therefore in 

considering the information available to help inform the standard-setting process, the 

EPA has focused on the public perception studies that do not embed consideration of 

implementation costs. Nonetheless, the EPA recognizes that valuation studies do provide 

additional evidence that the public is experiencing losses in welfare due to visibility 
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impairment.114 The public perception studies are described in detail below.

In order to identify levels of visibility impairment appropriate for consideration in 

setting secondary PM NAAQS to protect the public welfare, the Visibility Assessment 

comprehensively examined information that was available in this review regarding 

people’s stated preferences regarding acceptable and unacceptable visual air quality.

Light extinction is an atmospheric property that by itself does not directly 

translate into a public welfare effect. Instead, light extinction becomes meaningful in the 

context of the impact of differences in visibility on the human observer. This has been 

studied in terms of the acceptability or unacceptability expressed for the visibility impact 

of a given level of light extinction by a human observer. The perception of the visibility 

impact of a given level of light extinction occurs in conjunction with the associated 

characteristics and lighting conditions of the viewed scene.115 Thus, a given level of light 

extinction may be perceived differently by observers looking at different scenes or the 

same scene with different lighting characteristics. Likewise, different observers looking 

at the same scene with the same lighting may have different preferences regarding the 

 

114 In the regulatory impact analysis (RIA) accompanying this rulemaking, the EPA 

describes a revised approach to estimate urban residential visibility benefits that applies 

the results of several contingent valuation studies. The EPA is unable to apply the public 

perception studies to estimate benefits because they do not provide sufficient information 

on which to develop monetized benefits estimates. Specifically, the public perception 

studies do not provide preferences expressed in dollar values, even though they do 

provide additional evidence that the benefits associated with improving residential 

visibility are not zero. As previously noted in this preamble, the RIA is done for 

informational purposes only, and the proposed decisions on the NAAQS in this 

rulemaking are not in any way based on consideration of the information or analyses in 

the RIA.

115 By “characteristics of the scene” the EPA means the distance(s) between the viewer 

and the object(s) of interest, the shapes and colors of the objects, the contrast between 

objects and the sky or other background, and the inherent interest of the objects to the 

viewer. Distance is particularly important because at a given value of light extinction, 

which is a property of air at a given point(s) in space, more light is actually absorbed and 

scattered when light passes through more air between the object and the viewer.
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associated visual air quality. When scene and lighting characteristics are held constant, 

the perceived appearance of a scene (i.e., how well the scenic features can be seen and 

the amount of visible haze) depends only on changes in light extinction. This has been 

demonstrated using the WinHaze model (Molenar et al., 1994) that uses image processing 

technology to apply user-specified changes in light extinction values to the same base 

photograph with set scene and lighting characteristics. 

Much of what is known about the acceptability of levels of visibility comes from 

survey studies in which participants were asked questions about their preference or the 

value they place on various visibility levels as displayed to them in scenic photographs 

and/or WinHaze images with a range of known light extinction levels. The Visibility 

Assessment (U.S. EPA, 2010b, chapter 2) reviewed the limited number of urban visibility 

preference studies currently available (i.e., four studies) to assess the light extinction 

levels judged by the participant to have acceptable visibility for those particular scenes.

The reanalysis of urban preference studies conducted in the Visibility Assessment 

for this review included three completed western urban visibility preference survey 

studies plus a pair of smaller focus studies designed to explore and further develop urban 

visibility survey instruments. The three western studies included one in Denver, Colorado 

(Ely et al., 1991), one in the lower Fraser River valley near Vancouver, British Columbia 

(BC), Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research & Consulting, 

2003). A pilot focus group study was also conducted for Washington, DC (Abt 

Associates Inc., 2001). In response to an EPA request for public comment on the Scope 

and Methods Plan (74 FR 11580, March 18, 2009), comments were received (Smith, 

2009) about the results of a new focus group study of scenes from Washington, DC, that 
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had been conducted on subjects from both Houston, Texas, and Washington, DC, using 

scenes, methods and approaches similar to the method and approach employed in the 

EPA pilot study (Smith and Howell, 2009). When taken together, these studies from the 

four different urban areas included a total of 852 individuals, with each individual 

responding to a series of questions while viewing a set of images of various urban visual 

air quality conditions. 

The approaches used in the four studies were similar and were all derived from 

the method first developed for the Denver urban visibility study. In particular, the studies 

all used a similar group interview type of survey to investigate the level of visibility 

impairment that participants described as “acceptable.” In each preference study, 

participants were initially given a set of “warm up” exercises to familiarize them with 

how the scene in the photograph or image appears under different VAQ conditions. The 

participants next were shown 25 randomly ordered photographs (images), and asked to 

rate each one based on a scale of 1 (poor) to 7 (excellent). They were then shown the 

same photographs or images again, in the same order, and asked to judge whether each of 

the photographs (images) would violate what they would consider to be an appropriate 

urban visibility standard (i.e. whether the level of impairment was “acceptable” or 

“unacceptable”). The term “acceptable” was not defined, so that each person’s response 

was based on his/her own values and preferences for VAQ. However, when answering 

this question, participants were instructed to consider the following three factors: (1) The 

standard would be for their own urban area, not a pristine national park area where the 

standards might be stricter; (2) The level of an urban visibility standard violation should 

be set at a VAQ level considered to be unreasonable, objectionable, and unacceptable 
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visually; and (3) Judgments of standards violations should be based on visibility only, not 

on health effects. While the results differed among the four urban areas, results from a 

rating exercise show that within each preference study, individual survey participants

consistently distinguish between photos or images representing different levels of light 

extinction, and that more participants rate as acceptable images representing lower levels 

of light extinction than they do images representing higher levels. 

Given the similarities in the approaches used, the EPA staff concluded that it was

reasonable to compare the results to identify overall trends in the study findings and to 

conclude that this comparison can usefully inform the selection of a range of levels for 

use in further analyses. However, the staff also noted that variations in the specific 

materials and methods used in each study introduce uncertainties that should also be 

considered when interpreting the results of these comparisons. Key differences between 

the studies include the following: 1) scene characteristics; 2) image presentation methods 

(e.g., projected slides of actual photos, projected images generated using WinHaze (a 

significant technical advance in the method of presenting visual air quality conditions), or 

use of a computer monitor screen; 3) number of participants in each study; 4) participant 

representativeness of the general population of the relevant metropolitan area; and 5) 

specific wording used to frame the questions used in the group interview process. 

In the Visibility Assessment, each study was evaluated separately and figures 

developed to display the percentage of participants that rated the visual air quality 

depicted in each photograph as “acceptable.” Ely et al. (1991) introduced a “50% 

acceptability” criterion analysis of the Denver preference study results. The 50 percent 

acceptability criterion is designed to identify the visual air quality level (defined in terms 
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of deciviews or light extinction) that best divides the photographs into two groups: those 

with a visual air quality rated as acceptable by the majority of the participants, and those 

rated not acceptable by the majority of participants. The Visibility Assessment adopted 

this criterion as a useful index for comparison between studies. The results of each 

analysis were then combined graphically to allow for visual comparison. This 

information was then carried forward into the Policy Assessment. Figure 6 presents the 

graphical summary of the results of the studies in the four cities and draws on results 

previously presented in Figures 2-3, 2-5, 2-7, and 2-11 of chapter 2 in the Visibility 

Assessment. Figure 6 also contains lines at 20 dv and 30 dv that generally identify a 

range where the 50 percent acceptance criteria occur across all four of the urban 

preference studies (U.S. EPA, 2011a, p. 4-24). Out of the 114 data points shown in 

Figure 6, only one photograph (or image) with a visual air quality below 20 dv was rated 

as acceptable by less than 50 percent of the participants who rated that photograph.116

Similarly, only one image with a visual air quality above 30 dv was rated acceptable by 

more than 50 percent of the participants who viewed it.117

 

116 Only 47 percent of the British Columbia participants rated a 19.2 dv photograph as 

acceptable.

117 In the 2001 Washington, DC study, a 30.9 dv image was used as a repeated slide. The 

first time it was shown 56 percent of the participants rated it as acceptable, but only 11 

percent rated it as acceptable the second time it was shown. The same visual air quality 

level was rated as acceptable by 4 percent of the participants in the 2009 study (Test 1). 

All three points are shown in Figure 6.
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Figure 6. Summary of Results of Urban Visibility Studies in Four Cities, Showing the Identified 

Range of the 50% Acceptance Criteria118

Source: U.S. EPA, 2011a, Figure 4-2; U.S. EPA 2010b, Figure 2-16

As Figure 6 above shows, each urban area has a separate and unique response 

curve that appears to indicate that it is distinct from the others.119 These curves are the 

result of a logistical regression analysis using a logit model of the greater than 19,000 

ratings of haze images as acceptable or unacceptable. The model results can be used to 

estimate the visual air quality in terms of dv values where the estimated response 

functions cross the 50 percent acceptability level, as well as any alternative criteria levels. 

Selected examples of these are shown in Table 4-1 of the Policy Assessment (U.S. EPA, 

2011a; U.S. EPA, 2010b, Table 2-4). This table shows that the logit model results also 

 

118 Top scale shows light extinction in inverse megameter units; bottom scale in 

deciviews. Logit analysis estimated response functions are shown as the color-coded 

curved lines for each of the four urban areas.

119 At present, data is only available for four urban areas, as presented in Figure 6 and 

discussed throughout this section. Additional research could help inform whether the 

range identified by combining the results of the studies depicted in Figure 6 is more 

broadly representative.
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support the upper and lower ends of the range of 50th percentile acceptability values (e.g., 

near 20 dv for Denver and near 30 dv for Washington, DC) already identified in Figure 6. 

Based on the composite results and the effective range of 50th percentile 

acceptability across the four urban preference studies shown in Figure 6 and Table 4-1 of 

the Policy Assessment, benchmark levels of (total) light extinction were selected in a 

range from 20 dv to 30 dv (75 to 200 Mm-1)

120 for the purpose of provisionally assessing 

whether visibility conditions would be considered acceptable (i.e., less than the low end 

of the range), unacceptable (i.e., greater than the high end of the range), or potentially 

acceptable (within the range) based on the very limited public preference information. A 

midpoint of 25 dv (120 Mm-1) was also selected for use in the assessment. This level is 

also very near to the 50th percentile criterion value from the Phoenix study (i.e., 24.2 dv), 

which is by far the best of the four studies in terms of the fit of the data to the response 

curve and the representativeness of study participants. Based on the currently available 

information, the Policy Assessment concluded that the use of 25 dv to represent the 

middle of the distribution of results seemed well supported (U.S. EPA, 2011a, p. 4-25).

These three benchmark values provide a low, middle, and high set of light 

extinction conditions that are used to provisionally define daylight hours with urban haze 

conditions that have been judged unacceptable by at least 50 percent of the participants in 

one or more of these preference studies. As discussed above, PM light extinction is taken 

 

120 These values were rounded from 74 Mm-1

and 201 Mm-1to avoid an implication of 

greater precision than is warranted. Note that the middle value of 25 dv when converted 

to light extinction is 122 Mm-1is rounded to 120 Mm-1for the same reason. Assessments 

conducted for the Visibility Assessment and the first and second drafts of the Policy 

Assessment used the unrounded values. The Policy Assessment considered the results of 

assessment using unrounded values to be sufficiently representative of what would result 

if the rounded values were used that it was unnecessary to redo the assessments. That is 

why some tables and figures in the Policy Assessment reflected the unrounded values.
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to be (total) light extinction minus the Rayleigh scatter,121such that the low, middle, and 

high levels correspond to PM light extinction levels of about 65 Mm-1, 110 Mm-1, and 

190 Mm-1. In the Visibility Assessment, these three light extinction levels were called 

Candidate Protection Levels (CPLs). This term was also used in the Policy Assessment 

and in the proposal notice. It is important to note, however, that the degree of protection 

provided by a secondary NAAQS is not determined solely by any one component of the 

standard but by all the components (i.e., indicator, averaging time, form, and level) being 

applied together. Therefore, the Policy Assessment noted that the term CPL is meant only 

to indicate target levels of visibility within a range that the EPA staff felt appropriate for 

consideration that could, in conjunction with other elements of the standard, including 

indicator, averaging time, and form, potentially provide an appropriate degree of 

visibility protection.

In characterizing the Policy Assessment’s confidence in each CPL and across the 

range, a number of issues were considered (U.S. EPA, 2011a, p. 4-26). Looking first at 

the two studies that define the upper and lower bounds of the range, the Policy 

Assessment considered whether they represent a true regional distinction in preferences 

for urban visibility conditions between western and eastern U.S. There was little 

information available to help evaluate the possibility of a regional distinction especially 

given that there have been preference studies in only one eastern urban area. Smith and 

Howell (2009) found little difference in preference response to Washington, DC, haze 

photographs between the study participants from Washington, DC, and those from 

 

121 Rayleigh scatter is light scattering by atmospheric gases which is on average about 10 

Mm-1.
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Houston, Texas.122 This provides some limited evidence that the value judgment of the 

public in different areas of the country may not be an important factor in explaining the 

differences in these study results.

In further considering what factors could explain the observed differences in 

preferences across the four urban areas, the Policy Assessment noted that the urban 

scenes used in each study had different characteristics (U.S. EPA, 2011a, p. 4-26). For 

example, each of the western urban visibility preference study scenes included mountains 

in the background while the single eastern urban study did not. It is also true that each of 

the western scenes included objects at greater distances from the camera location than in 

the eastern study. There is no question that objects at a greater distance have a greater 

sensitivity to perceived visibility changes as light extinction is changed compared to 

otherwise similar scenes with objects at a shorter range. This alone might explain the 

difference between the results of the eastern study and those from the western urban 

studies. Having scenes with the object of greatest intrinsic value nearer and hence less 

sensitive in the eastern urban area compared with more distant objects of greatest 

intrinsic value in the western urban areas could further explain the difference in 

preference results. 

Another question considered was whether the high CPL value that is based on the 

eastern preference results is likely to be generally representative of urban areas that do 

not have associated mountains or other valued objects visible in the distant background. 

Such areas would include the middle of the country, many areas in the eastern U.S., and 

 

122 The first preference study using WinHaze images of a scenic vista from Washington, 

DC was conducted in 2001 using subjects who were residents of Washington, DC. More 

recently, Smith and Howell (2009) interviewed additional subjects using the same images 

and interview procedure. The additional subjects included some residents of the 

Washington, DC area and some residents of the Houston, Texas area. 
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possibly some areas in the western U.S. as well.123 Based on the currently available 

information, the Policy Assessment concluded that the high end of the CPL range (30 dv) 

is an appropriate level to consider (U.S. EPA, 2011a, p. 4-27).

With respect to the low end of the range, the Policy Assessment considered 

factors that might further refine its understanding of the robustness of this level. The 

Policy Assessment concluded that additional urban preference studies, especially with a 

greater variety in types of scenes, could help evaluate whether the lower CPL value of 20 

dv is generally supportable (U.S. EPA, 2011a, p. 4-27). Further, the reason for the 

noisiness in data points around the curves apparent in both the Denver and British 

Columbia results compared to the smoother curve fit of Phoenix study results could be 

explored. One possible explanation discussed in the Policy Assessment is that these older 

studies use photographs taken at different times of day and on different days to capture 

the range of light extinction levels needed for the preference studies. In contrast, the use 

of WinHaze in the Phoenix (and Washington, DC) study reduced variations that affect 

scene appearance preference rating and avoided the uncertainty inherent in using ambient 

measurements to represent sight path-averaged light extinction values. Reducing these 

sources of noisiness and uncertainty in the results of future studies of sensitive urban 

scenes could provide more confidence in the selection of a low CPL value.

Based on the above considerations, and recognizing the limitations in the 

currently available information, the Policy Assessment concluded that it is reasonable to 

 

123 In order to examine this issue, an effort would have to be made to see if scenes in such 

areas could be found that would be generally comparable to the western scenes (e.g., 

scenes that contain valued scenic elements at more sensitive distances than that used in 

the eastern study). This is only one of a family of issues concerning how exposure to 

urban scenes of varying sensitivity affects public perception for which no preference 

study information is currently available.
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consider a range of CPL values including a high value of 30 dv, a mid-range value of 25 

dv, and a low value of 20 dv (U.S. EPA, 2011a, p. 4-27). Based on its review of the 

second draft Policy Assessment, CASAC also supported this set of CPLs for 

consideration by the EPA in this review. CASAC noted that these CPL values were based 

on all available visibility preference data and that they bound the study results as 

represented by the 50 percent acceptability criteria. While recommending that further 

visibility preference studies be conducted to reduce remaining uncertainties,124 CASAC 

concluded that this range of levels was “adequately supported by the evidence presented”

(Samet, 2010d, p. iii).

c. Summary of Proposed Conclusions

i. Adequacy of the Current Standards for PM-Related Visibility Impairment

At the time of proposal, the Administrator provisionally concluded that the 

current suite of secondary PM standards is not sufficiently protective of visual air quality, 

and that consideration should be given to an alternative secondary standard that would 

provide additional protection against PM-related visibility impairment, with a focus 

primarily in urban areas. This proposed conclusion was based on the information 

presented in the proposal with regard to the nature of PM-related visibility impairment, 

the results of public perception surveys on the acceptability of varying degrees of 

visibility impairment in urban areas, analyses of the number of days that are estimated to 

exceed a range of candidate protection levels under conditions simulated to just meet the 

current standards, and the advice of CASAC. This section summarizes key points from 

 

124 “CASAC has also identified needs for the next review cycle in terms of further 

research on a number of topics related to urban visibility; .... In particular, there is a need 

for the Agency to conduct additional urban visibility preference studies over a broad 

range of urban areas and viewing conditions, to further evaluate and refine the range of 

visibility levels considered to be acceptable in the current assessment.” (Samet, 2010a)
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section VI.C of the proposal regarding visibility under current conditions, the degree of 

protection afforded by the current standards, and CASAC’s advice regarding the 

adequacy of the current standards.

As discussed in section VI.C.1 of the proposal, to evaluate visibility under current 

conditions the Visibility Assessment and Policy Assessment estimated PM-related light 

extinction125 levels for 15 urban areas126 in the United States. Consistent with the 

emphasis in this review on the hourly or multi-hour time periods that might reasonably 

characterize the visibility effects experienced by various segments of the population, 

these analyses focused on using maximum 1-hour and 4-hour values of PM light 

extinction during daylight hours for purposes of evaluating the degree of visibility 

impairment. Hourly average PM-related light extinction was analyzed in terms of both 

PM10 and PM2.5 light extinction. For reasons discussed above, hours with relative 

humidity greater than 90 percent were excluded from consideration. Recent visibility 

conditions in these urban areas were then compared to the CPLs identified above. The 

Visibility Assessment, which focused on PM10 light extinction in 14 of the 15 urban areas 

during the 2005 to 2007 time period,127 found that all 14 areas had daily maximum hourly 

 

125 PM-related light extinction is used here to refer to the light extinction caused by PM 

regardless of particle size; PM10 light extinction refers to the contribution by particles 

sampled through an inlet with a particle size 50 percent cutpoint of 10 μm diameter; and 

PM2.5 light extinction refers to the contribution by particles sampled through an inlet with 

a particle size 50 percent cutpoint of 2.5 μm diameter.

126 The 15 urban areas are Tacoma, Fresno, Los Angeles, Phoenix, Salt Lake City, Dallas, 

Houston, St. Louis, Birmingham, Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, 

and New York. 

127 Comments on the second draft Visibility Assessment from those familiar with the 

monitoring sites in St. Louis indicated that the site selected to provide continuous PM10

monitoring, although less than a mile from the site of the PM2.5 data, was not 

representative of the urban area and resulted in unrealistically large PM10-2.5 values. The 

EPA staff considered these comments credible and set aside the St. Louis assessment 

results for PM10 light extinction. Thus, results and statements in the Policy Assessment 
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PM10 light extinction values estimated to exceed even the highest CPL some of the days. 

Except for the two Texas areas and the non-California western urban areas, all of the 

other urban areas were estimated to have maximum hourly PM10 concentrations that 

exceeded the high CPL on about 20 percent to over 60 percent of the days. All 14 of the 

urban areas were estimated to have maximum hourly PM10 concentrations that exceeded 

the low CPL on about 40 percent to over 90 percent of the days. In general, areas in the 

East and in California tend to have a higher frequency of hourly visibility conditions 

estimated to be above the high CPL compared with those in the western U.S. 

The Policy Assessment repeated the Visibility Assessment-type modeling based 

on PM2.5 light extinction and data from the more recent 2007 to 2009 time period for the 

same 15 study areas (including St. Louis). While the estimates of the percentage of daily 

maximum hourly PM2.5 light extinction values exceeding the CPLs were somewhat lower 

than for PM10 light extinction, the patterns of these estimates across the study areas was 

found to be similar. More specifically, except for the two Texas and the non-California 

western urban areas, all of the other urban areas were estimated to have maximum hourly 

PM2.5 concentrations that exceeded the high CPL on about 10 percent up to about 50 

percent of the days based on PM2.5 light extinction, while all 15 areas were estimated to 

have maximum hourly PM2.5 concentrations that exceeded the low CPL on over 10 

percent to over 90 percent of the days. 

To evaluate how PM-related visibility would be affected by just meeting the 

current suite of PM2.5 secondary standards, the Policy Assessment applied the 

proportional rollback approach described in section VI.C.2 of the proposal to all the 

 

regarding PM10 light extinction applied to only the other 14 areas. However, results 

regarding PM2.5 light extinction in most cases applied to all 15 study areas because the St. 

Louis estimates for PM2.5 light extinction were not affected by the PM10 monitoring issue.
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PM2.5 monitoring sites in each study area.128 After adjusting for composition, the Policy 

Assessment applied the original IMPROVE algorithm to calculate the PM10 light 

extinction, using “rolled back” PM2.5 component concentrations, the current conditions 

PM10-2.5 concentration for the day and hour, and relative humidity for the day and hour.

In these analyses, the Policy Assessment estimated both PM2.5 and PM10 light 

extinction in terms of both daily maximum 1-hour average values and multi-hour (i.e., 4-

hour) average values for daylight hours. Figure 4-7 and Table 4-6 of the Policy 

Assessment displayed the results of the rollback procedures as a box and whisker plot of 

daily maximum daylight 1-hour PM2.5 light extinction and the percentage of daily 

maximum hourly PM2.5 light extinction values estimated to exceed the CPLs when just 

meeting the current suite of PM2.5 secondary standards for all 15 areas considered in the 

Visibility Assessment (including St. Louis) (excluding hours with relative humidity 

greater than 90 percent). These displays showed that the daily maximum 1-hour average 

PM2.5 light extinction values in all of the study areas other than the three western nonCalifornia areas were estimated to exceed the high CPL on about 8 percent up to over 30 

percent of the days and to exceed the middle CPL on about 30 percent up to about 70 

percent of the days, while all areas except Phoenix were estimated to have daily 

maximum 1-hour average PM2.5 light extinction values that exceeded the low CPL on 

over 15 percent to about 90 percent of the days. Figure 4-8 and Table 4-7 of the Policy 

Assessment present results based on daily maximum 4-hour average values. These 

displays show that the daily maximum 4-hour average PM2.5 light extinction values in all 

of the study areas other than the three western non-California areas and the two areas in 

 

128 Phoenix and Salt Lake City met the current PM2.5 NAAQS under current conditions 

and required no reduction.
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Texas were estimated to exceed the high CPL on about 4 percent up to over 15 percent of 

the days and to exceed the middle CPL on about 15 percent up to about 45 percent of the 

days, while all areas except Phoenix were estimated to have daily maximum 4-hour 

average PM2.5 light extinction values that exceeded the low CPL on over 10 percent to 

about 75 percent of the days. A similar set of figures and tables were developed in terms 

of PM10 light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and 4-5). 

Taking the results of these analyses focusing on 1-hour and 4-hour maximum 

light extinction values into account, the Policy Assessment concluded that the available 

information in this review clearly called into question the adequacy of the current suite of 

PM2.5 standards in the context of public welfare protection from visibility impairment, 

primarily in urban areas, and supported consideration of alternative standards to provide 

appropriate protection (U.S. EPA, 2011a, p. 4-39). This conclusion was based in part on 

the large percentage of days, in many urban areas, that were estimated to have maximum 

1-hour or 4-hour light extinction values that exceed the range of CPLs identified for 

consideration under simulations of conditions that would just meet the current suite of 

PM2.5 secondary standards. In particular, for air quality that was simulated to just meet 

the current PM2.5 standards, greater than 10 percent of the days were estimated to have 

peak light extinction values that exceed the highest, least protective CPL of 30 dv in 

terms of PM2.5 light extinction for 9 of the 15 urban areas, based on 1-hour average 

values, and would thus likely fail to meet a 90th percentile-based standard at that level. 

For these areas, the percent of days estimated to have maximum 1-hour values that 

exceed the highest CPL ranged from over 10 percent to over 30 percent. Similarly, when 

the middle CPL of 25 dv was considered, greater than 30 percent up to approximately 70 
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percent of the days were estimated to have peak light extinction that exceeded that CPL 

in terms of PM2.5 light extinction, for 11 of the 15 urban areas, based on 1-hour average 

values. Based on a 4-hour averaging time, 5 of the areas were estimated to have at least 

10 percent of the days with peak light extinction exceeding the highest CPL in terms of 

PM2.5 light extinction, and 8 of the areas were estimated to have at least 30 percent of the 

days with peak light extinction exceeding the middle CPL in terms of PM2.5 light 

extinction. For the lowest CPL of 20 dv, the percentages of days with 4-hour maximum 

light extinction estimated to exceed that CPL are even higher for all cases considered. 

Based on all of the above, the Policy Assessment concluded that PM light extinction 

estimated to be associated with just meeting the current suite of PM2.5 secondary 

standards in many areas across the country exceeded levels and percentages of days that 

could reasonably be considered to be important from a public welfare perspective (U.S. 

EPA, 2011a, p. 4-40). 

Further, the Policy Assessment concluded that use of the current indicator of 

PM2.5 mass, in conjunction with the current 24-hour and annual averaging times, is 

clearly called into question for a national standard intended to protect public welfare from 

PM-related visibility impairment (U.S. EPA, 2011a, p. 4-40). This is because such a 

standard is inherently variable in the degree of protection provided because of regional 

differences in relative humidity and species composition of PM2.5, which are critical 

factors in the relationship between the mix of fine particles in the ambient air and the 

associated impairment of visibility. The Policy Assessment noted that this concern was 

one of the important elements in the court’s decision to remand the PM2.5 secondary 

standards set in 2006 to the Agency. 
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Thus, in addition to concluding that the available information clearly calls into 

question the adequacy of the protection against PM-related visibility impairment afforded 

by the current suite of PM2.5 standards, the Policy Assessment also concluded that it 

clearly calls into question the appropriateness of each of the current standard elements: 

indicator, averaging time, form, and level (U.S. EPA, 2011a, p. 4-40).

After reviewing the information and analysis in the second draft Policy 

Assessment, CASAC concluded that the “currently available information clearly calls 

into question the adequacy of the current standards and that consideration should be 

given to revising the suite of standards to provide increased public welfare protection” 

(Samet, 2010d, p. iii). CASAC noted that the detailed estimates of hourly PM light 

extinction associated with just meeting the current standards “clearly demonstrate that 

current standards do not protect against levels of visual air quality which have been 

judged to be unacceptable in all of the available urban visibility preference studies.” 

Further, CASAC stated, with respect to the current suite of secondary PM2.5 standards, 

that “[T]he levels are too high, the averaging times are too long, and the PM2.5 mass 

indicator could be improved to correspond more closely to the light scattering and 

absorption properties of suspended particles in the ambient air” (Samet, 2010d, p. 9).

After considering the available evidence and the advice of CASAC, the 

Administrator concluded at the time of proposal that such information did provide an 

appropriate basis to inform a conclusion as to whether the current standards afford 

adequate protection against PM-related visibility impairment in urban areas. The 

Administrator took into account the information discussed above with regard to the 

nature of PM-related visibility impairment, the results of public perception surveys on the 
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acceptability of varying degrees of visibility impairment in urban areas, analyses of the 

number of days on which peak 1-hour or 4-hour light extinction values are estimated to 

exceed a range of candidate protection levels under conditions simulated to just meet the 

current standards, and the advice of CASAC. She noted the clear causal relationship 

between PM in the ambient air and impairment of visibility, the evidence from the 

visibility preference studies, and the rationale for determining a range of candidate 

protection levels based on those studies. She also noted the relatively large number of 

days when maximum 1-hour or 4-hour light extinction values were estimated to exceed 

the three candidate protection levels, including the highest level of 30 dv, under the 

current standards. While recognizing the limitations in the available information on 

public perceptions of the acceptability of varying degree of visibility impairment and the 

information on the number of days estimated to exceed the CPLs, she concluded that 

such information provided an appropriate basis to inform a conclusion as to whether the 

current standards provide adequate protection against PM-related visibility impairment in 

urban areas. Based on these considerations, and placing great importance on the advice of 

CASAC, the Administrator provisionally concluded that the current standards are not 

sufficiently protective of visual air quality, and that consideration should be given to an 

alternative secondary standard that would provide additional protection against PMrelated visibility impairment, with a focus primarily in urban areas.

Having reached this conclusion, the Administrator also stated at the time of 

proposal that the current indicator of PM2.5 mass, in conjunction with the current 24-hour 

and annual averaging times, is not well suited for a national standard intended to protect 

public welfare from PM-related visibility impairment. As noted in the proposal, the 
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current standards do not incorporate information on the concentrations of various species 

within the mix of ambient particles, nor do they incorporate information on relative 

humidity, both of which play a central role in determining the relationship between the 

mix of PM in the ambient air and impairment of visibility. Such considerations were 

reflected both in CASAC’s advice to set a distinct secondary standard that would more 

directly reflect the relationship between ambient PM and visibility impairment and in the 

court’s remand of the current secondary PM2.5 standards. Based on the above 

considerations, at the time of proposal the Administrator provisionally concluded that the 

current secondary PM2.5 standards, taken together, are neither sufficiently protective nor 

suitably structured to provide an appropriate degree of public welfare protection from 

PM-related visibility impairment, primarily in urban areas. This led the EPA to consider 

alternative standards by looking at each of the elements of the standards—indicator, 

averaging time, form, and level—as discussed below. 

ii. Indicator

At the time of proposal, the EPA considered three alternative indicators for a 

PM2.5 standard designed to protect against visibility impairment: the current PM2.5 mass 

indicator; directly measured PM2.5 light extinction; and calculated PM2.5 light extinction. 

Directly measured PM2.5 light extinction is a measurement (or combination of

measurements) of the light absorption and scattering caused by PM2.5 under ambient 

conditions. Calculated PM2.5 light extinction uses the IMPROVE algorithm to calculate 

PM2.5 light extinction using measured PM2.5 mass, speciated PM2.5 mass, and measured 

relative humidity. The Policy Assessment evaluated each of these alternatives, finally 

concluding that consideration should be given to establishing a new calculated PM2.5 light 
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extinction indicator (U.S. EPA, 2011a, p. 4-51).

As discussed in section VI.D.1 of the proposal, the Policy Assessment concluded 

that consideration of the use of either directly measured PM2.5 light extinction or 

calculated PM2.5 light extinction as an indicator is justified because light extinction is a 

physically meaningful measure of the characteristic of ambient PM2.5 that is most 

relevant and directly related to PM-related visibility effects (U.S. EPA, 2011a, p. 4-41). 

Further, as noted above, PM2.5 is the component of PM responsible for most of the 

visibility impairment in most urban areas. In these areas, the contribution of PM10–2.5 is a 

minor contributor to visibility impairment most of the time. The Policy Assessment also 

indicated that the available evidence demonstrated a strong correspondence between 

calculated PM2.5 light extinction and PM-related visibility impairment, as well as the 

significant degree of variability in visibility protection across the U.S. allowed by a PM2.5

mass indicator. The Policy Assessment recognized that while in the future it would be 

appropriate to consider a direct measurement of PM2.5 light extinction it was not an 

appropriate option in this review because a suitable specification of the equipment and 

associated performance verification procedures cannot be developed in the time frame for 

this review.

(a) PM2.5 Mass 

In terms of utilizing a PM2.5 mass indicator, the proposal noted that PM2.5 mass 

monitoring methods are in widespread use, including the FRM involving the collection of 

periodic (usually 1-day-in-6 or 1-day-in-3) 24-hour filter samples. However, these routine 

monitoring activities do not include measurement of the full water content of the ambient 

PM2.5 that contributes, often significantly, to visibility impacts. Further, the PM2.5 mass 
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concentration monitors do not provide information on the composition of the ambient 

PM2.5, which plays a central role in the relationship between PM-related visibility 

impairment and ambient PM2.5 mass concentrations. Additional analyses discussed in the 

proposal that looked at the contribution of PM2.5 to total PM-related light extinction 

(defined in terms of hourly PM10 calculated light extinction) indicate that there is a poor 

correlation between hourly PM10 light extinction and hourly PM2.5 mass principally due 

to the impact of the water content of the particles on light extinction, which depends on 

both the composition of the PM2.5 and the ambient relative humidity. Both composition 

and especially relative humidity vary during a single day, as well as from day-to-day, at 

any site and time of year. Also, there are systematic regional and seasonal differences in 

the distribution of ambient humidity and PM2.5 composition conditions that make it 

impossible to select a PM2.5 concentration that generally would correspond to the same 

PM-related light extinction levels across all areas of the nation. Analyses discussed in the 

proposal quantify the projected uneven protection that would result from the use of 1-

hour average PM2.5 mass as the indicator. 

(b) Directly Measured PM2.5 Light Extinction

PM light extinction has a nearly one-to-one relationship to light extinction, unlike 

PM2.5 mass concentration. As explained above, PM2.5 is the component responsible for 

the large majority of PM light extinction in most places and times. PM2.5 light extinction 

can be directly measured using several instrumental methods, some of which have been 

used for decades to routinely monitor the two components of PM2.5 light extinction (light 

scattering and absorption) or to jointly measure both as total light extinction (from which 

Rayleigh scattering is subtracted to get PM2.5 light extinction). As noted at the time of 
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proposal, there are a number of advantages to direct measurements of light extinction for 

use in a secondary standard relative to estimates of PM2.5 light extinction calculated using 

PM2.5 mass and speciation data. These include greater accuracy of direct measurements 

with shorter averaging times and overall greater simplicity when compared to the need 

for measurements of multiple parameters to calculate PM light extinction.

In evaluating whether direct measurement of PM2.5 or PM10 light extinction is 

appropriate to consider in the context of this PM NAAQS review, the EPA solicited 

comment from the Ambient Air Monitoring and Methods Subcommittee (AAMMS) of 

CASAC. The CASAC AAMMS recommended that consideration of direct measurement 

should be limited to PM2.5 light extinction, and that although instruments suitable for this 

purpose are commercially available at present, research is expected to produce even 

better instruments in the near term. The CASAC AAMMS advised against choosing any 

currently available commercial instrument, or even a general measurement approach, as 

an FRM because to do so could discourage development of other potentially superior 

approaches. Instead, the CASAC AAMMS recommended that the EPA develop 

performance-based approval criteria for direct measurement methods in order to put all 

approaches on a level playing field.

At the present time, the EPA has not undertaken to develop and test such 

performance-base approval criteria. The EPA anticipates that if an effort were begun it 

would take at least several years before such criteria would be ready for regulatory use. 

Thus, the Policy Assessment concluded that while in the future it would be appropriate to 

consider a direct measurement of PM2.5 light extinction, or the sum of separate 

measurements of light scattering and light absorption, as the indicator for the secondary 
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PM2.5 standard, this is not an appropriate option in this review because a suitable 

specification of the equipment or appropriate performance-based verification procedures 

cannot be developed in the time frame for this review (U.S. EPA, 2011a, p. 4-51, -52).

(c) Calculated PM2.5 Light Extinction 

For the reasons discussed above, the Policy Assessment concluded that a 

calculated PM2.5 light extinction indicator would be the preferred approach. PM2.5 light 

extinction can be calculated from PM2.5 mass, combined with speciated PM2.5 mass 

concentration data plus relative humidity data, as is presently routinely done on a 24-hour 

average basis under the Regional Haze Program using data from the rural IMPROVE 

monitoring network. This same calculation procedure, using a 24-hour average basis, 

could be used for a NAAQS focused on protecting against PM-related visibility 

impairment primarily in urban areas. This approach would use the type of data that is 

routinely collected from the urban CSN129 in combination with monthly average relative 

humidity data based on long-term climatological means as used in the Regional Haze 

Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal discussed the 

complex approach utilized in the Visibility Assessment for calculating hourly PM2.5 light 

extinction130 and discussed various simplified approaches for calculating these hourly 

values that were analyzed in the Policy Assessment. The Policy Assessment concluded 

that each of these simplified approaches provided reasonably good estimates of PM2.5

 

129 About 200 sites in the CSN routinely measure 24-hour average PM2.5 chemical 

components using filter-based samplers and chemical analysis in a laboratory, on either a 

1-day-in-3 or 1-day-in-6 schedule (U.S. EPA, 2011a, Appendix B, section B.1.3).

130 As noted at the time of proposal, the sheer size of the ambient air quality, 

meteorological, and chemical transport modeling data files involved with the Visibility 

Assessment approach would make it very difficult for state agencies or any interested 

party to consistently apply such an approach on a routine basis for the purpose of 

implementing a national standard defined in terms of the Visibility Assessment approach.
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light extinction and each would be appropriate to consider as the indicator for a distinct 

hourly or multi-hour secondary standard (U.S. EPA, 2011a, p. 4-48). The proposal also 

recognized that the Policy Assessment identified a number of variations on these 

simplified approaches that it would be appropriate to consider, including:

(1) the use of the split-component mass extinction efficiency approach from the 

revised IMPROVE algorithm131

(2) the use of more refined value(s) for the organic carbon multiplier132

(3) the use of the reconstructed 24-hour PM2.5 mass (i.e., the sum of the five PM2.5

components from speciated monitoring) as a normalization value for the 

hourly measurements from the PM2.5 instrument as a way of better reflecting 

ambient nitrate concentrations

(4) the use of historical monthly or seasonal, or regional, speciation averages

Overall, the analyses conducted for the Visibility Assessment and Policy 

Assessment indicated that the use of a calculated PM2.5 light extinction indicator would 

provide a much higher degree of uniformity in terms of the degree of protection from 

visibility impairment across the country than a PM2.5 mass indicator, because a calculated 

PM2.5 light extinction indicator would directly incorporate the effects of humidity and 

PM2.5 composition differences between various regions. Further, the proposal noted that 

the Policy Assessment concluded that consideration could be given to defining a 

calculated PM2.5 light extinction indicator on either a 24-hour or a sub-daily basis (U.S. 

 

131 If the revised IMPROVE algorithm were used to define the calculated PM2.5 massbased indicator, it would not be possible to algebraically reduce the revised algorithm to a 

two-factor version as described above and in Appendix F of the Policy Assessment for 

the simplified approaches. Instead, five component fractions would be determined from

each day of speciated sampling, and then either applied to hourly measurements of PM2.5

mass on the same day or averaged across a month and then applied to measurements of 

PM2.5 mass on each day of the month. 

132 An organic carbon (OC)-to-organic mass (OM) multiplier of 1.6 was used for the 

assessment, which was found to produce a value of OM comparable to the one derived 

with the original, albeit more complex Visibility Assessment method.
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EPA, 2011a, p. 4-52). However, the Policy Assessment noted that approval of continuous 

FEM monitors has been based only on 24-hour average, not hourly, PM2.5 mass. In 

addition, there are mixed results of data quality assessments on a 24-hour basis for these 

monitors, as well as the near absence of performance data for sub-daily averaging 

periods. Thus, while it is possible to utilize data from PM2.5 continuous FEMs on a 1-

hour or multi-hour (e.g., 4-hour) basis, these factors increase the uncertainty of utilizing 

continuous methods to support 1-hour or 4-hour PM2.5 mass measurements as an input to 

the light extinction calculation. Therefore, as noted at the time of proposal, until issues 

regarding the comparability of 24-hour PM2.5 mass values derived from continuous FEMs 

and filter-based FRMs133 are resolved, there is reason to be cautious about relying on a 

calculation procedure that uses hourly PM2.5 mass values reported by continuous FEMs in 

combination with speciated PM2.5 mass values from 24-hour filter-based samplers.

(d) CASAC Advice

In reviewing the second draft Policy Assessment, CASAC stated that it 

“overwhelmingly ... would prefer the direct measurement of light extinction,” 

recognizing it as the property of the atmosphere that most directly relates to visibility 

effects (Samet, 2010d, p. iii). CASAC noted that “[I]t has the advantage of relating 

directly to the demonstrated harmful welfare effect of ambient PM on human visual 

perception.” However, CASAC also concluded that the calculated PM2.5 light extinction 

indicator “appears to be a reasonable approach for estimating hourly light extinction” 

(Samet, 2010d, p. 11). Further, based on CASAC’s understanding of the time that would 

be required to develop an FRM for this indicator, CASAC agreed with the staff 

 

133 Filter-based FRMs are designed to adequately quantify the amount of PM2.5 collected 

over 24-hours. They cannot be presumed to be appropriate for quantifying average 

concentrations over 1-hour or 4-hour periods.
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preference presented in the second draft Policy Assessment for a calculated PM2.5 light 

extinction indicator. CASAC noted that “[I]ts reliance on procedures that have already 

been implemented in the CSN and routinely collected continuous PM2.5 data suggest that 

it could be implemented much sooner than a directly measured indicator” (Samet, 2010d, 

p. iii).134

(e) Administrator's Proposed Conclusions on Indicator

At the time of proposal, while agreeing with CASAC that a directly measured PM 

light extinction indicator would provide the most direct link between PM in the ambient 

air and PM-related light extinction, the Administrator provisionally concluded that this 

was not an appropriate option in this review because a suitable specification of currently 

available equipment or performance-based verification procedures cannot be developed 

in the time frame of this review. Taking all of the above considerations and CASAC 

advice into account, the Administrator provisionally concluded that a new calculated 

PM2.5 light extinction indicator, similar to that used in the Regional Haze Program (i.e., 

using an IMPROVE algorithm as translated into the deciview scale), was the appropriate 

indicator to replace the current PM2.5 mass indicator. Such an indicator, referred to as a 

PM2.5 visibility index, would appropriately reflect the relationship between ambient PM 

and PM-related light extinction, based on the analyses discussed in the proposal and 

incorporation of factors based on measured PM2.5 speciation concentrations and relative 

humidity data. In addition, selection of this type of indicator would address, in part, the 

issues raised in the court’s remand of the 2006 PM2.5 standards. The Administrator also 

 

134 In commenting on the second draft Policy Assessment, CASAC did not have an 

opportunity to review the assessment of continuous PM2.5 FEMs compared to collocated 

FRMs (Hanley and Reff, 2011) as presented and discussed in the final Policy Assessment 

(U.S. EPA, 2011a, p. 4-50).
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noted that such a PM2.5 visibility index would afford a relatively high degree of 

uniformity of visual air quality protection in areas across the country by virtue of directly 

incorporating the effects of differences in PM2.5 composition and relative humidity across 

the country. 

Based on these above considerations, the Administrator proposed to set a distinct 

secondary standard for PM2.5 defined in terms of a PM2.5 visibility index (i.e., a 

calculated PM2.5 light extinction indicator, translated into the deciview scale) to protect 

against PM-related visibility impairment primarily in urban areas. The Administrator 

proposed that such an index be based on the original IMPROVE algorithm in conjunction 

with monthly average relative humidity data based on long-term climatological means as 

used in the Regional Haze Program. The EPA solicited comment on all aspects of the 

proposed indicator, especially:

(1) the proposed use of a PM2.5 visibility index rather than a PM10 visibility index 

which would include an additional term for coarse particles;

(2) using the revised IMPROVE algorithm rather than the original IMPROVE 

algorithm;

(3) the use of alternative values for the organic carbon multiplier in conjunction with 

either the original or revised IMPROVE algorithm;

(4) the use of historical monthly, seasonal, or regional speciation averages;

(5) alternative approaches to determining relative humidity; and

(6) simplified approaches to generating hourly PM2.5 light extinction values for 

purposes of calculating an hourly or multi-hour indicator.

iii. Averaging Times

In this review, as discussed in section VI.D.2 of the proposal, consideration of 

appropriate averaging times for use in conjunction with a PM2.5 visibility index was 

informed by information related to the nature of PM visibility effects and the nature of 
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inputs to the calculation of PM2.5 light extinction, as discussed above. The EPA 

considered both sub-daily (1- and 4-hour averaging times) and 24-hour averaging times. 

In considering sub-daily averaging times, the EPA has also considered what diurnal 

periods and ambient relative humidity conditions would be appropriate to consider in 

conjunction with such an averaging time.

As an initial matter, the Policy Assessment considered sub-daily averaging times. 

Taking into account what is known from available studies concerning how quickly people

experience and judge visibility conditions, the possibility that some fraction of the public 

experiences infrequent or short periods of exposure to ambient visibility conditions, and 

the typical rate of change of the path-averaged PM light extinction over urban areas, the 

initial analyses conducted as part of the Visibility Assessment focused on a 1-hour 

averaging time. In its review of the first draft Policy Assessment, CASAC agreed that a 

1-hour averaging time would be appropriate to consider, noting that PM effects on 

visibility can vary widely and rapidly over the course of a day and such changes are 

almost instantaneously perceptible to human observers (Samet, 2010c, p. 19). The Policy 

Assessment noted that this view related specifically to a standard defined in terms of a 

directly measured PM light extinction indicator, in that CASAC also noted that a 1-hour 

averaging time is well within the instrument response times of the various currently 

available and developing optical monitoring methods. 

However, CASAC also advised that if a PM2.5 mass indicator were to be used, it 

would be appropriate to consider “somewhat longer averaging times – 2 to 4 hours – to 

assure a more stable instrumental response” (Samet, 2010c, p. 19). In considering this 

advice, the Policy Assessment concluded that since a calculated PM2.5 light extinction 
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indicator relies in part on measured PM2.5 mass, it would be appropriate to consider a 

multi-hour averaging time on the order of a few hours (e.g. 4-hours). A multi-hour 

averaging time might reasonably characterize the visibility effects experienced by the 

segment of the population who have access to visibility conditions often or continuously 

throughout the day. For this segment of the population, it may be that their perception of

visual air quality reflects some degree of offsetting an hour with poor visual air quality 

with one or more hours of clearer visual conditions. Further, the Policy Assessment 

recognized that a multi-hour averaging time would have the effect of averaging away 

peak hourly visibility impairment, which can change significantly from one hour to the 

next (U.S. EPA, 2011a, p. 4-53; U.S. EPA, 2010b, Figure 3-12). 

In considering either 1-hour or multi-hour averaging times, the Policy Assessment 

recognized that no data are available with regard to how the duration and variation of 

time a person spends outdoors during the daytime impacts his or her judgment of the 

acceptability of different degrees of visibility impairment. As a consequence, it is not 

clear to what degree, if at all, the protection levels found to be acceptable in the public 

preference studies would change for a multi-hour averaging time as compared to a 1-hour 

averaging time. Thus, the Policy Assessment concluded that it is appropriate to consider a 

1-hour or multi-hour (e.g., 4-hour) averaging time as the basis for a sub-daily standard 

defined in terms of a calculated PM2.5 light extinction indicator (U.S. EPA, 2011a, p. 4-

53).

In addition, as discussed above, some data quality uncertainties have been 

observed with regard to hourly data collected by FEMs. Specifically, as part of the review 

of data from all continuous FEM PM2.5 instruments operating at state/local monitoring 
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sites, the Policy Assessment noted that the occurrence of questionable outliers in 1-hour 

data submitted to AQS from continuous FEM PM2.5 instruments had been observed at 

some of these sites (Evangelista, 2011). Some of these outliers were questionable simply 

by virtue of their extreme magnitude, as high as 985 μg/m3, whereas other values were 

questionable because they were isolated to single hours with much lower values before 

and after, a pattern that is much less plausible than if the high concentrations were more 

sustained.135 The Policy Assessment noted that any current data quality problems might 

be resolved in the normal course of monitoring program evolution as operators become 

more adept at instrument operation and maintenance and data validation or by improving 

the approval criteria and testing requirements for continuous instruments. Regardless, the 

Policy Assessment noted that multi-hour averaging of FEM data could serve to reduce 

the effects of such outliers relative to the use of a 1-hour averaging time. 

The Policy Assessment noted that there are significant reasons to consider using 

PM2.5 light extinction calculated on a 24-hour basis to reduce the various data quality 

concerns described above with respect to relying on continuous PM2.5 monitoring data. 

However, the Policy Assessment recognized that 24 hours is far longer than the hourly or 

multi-hour time periods that might reasonably characterize the visibility effects 

experienced by various segments of the population, including both those who do and do 

not have access to visibility conditions often or continuously throughout the day. Thus, 

the Policy Assessment concluded that the appropriateness of considering a 24-hour 

 

135 Similarly questionable hourly data were not observed in the 2005 to 2007 continuous 

PM2.5 data used in the Visibility Assessment, all of which came from early-generation 

continuous instruments that had not been approved as FEMs. However, only 15 sites and 

instruments were involved in the Visibility Assessment analyses, versus about 180 

currently operating FEM instruments submitting data to AQS. Therefore, there were 

more opportunities for very infrequent measurement errors to be observed in the larger 

FEM data set. 
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averaging time would depend upon the extent to which PM-related light extinction 

calculated on a 24-hour average basis would be a reasonable and appropriate surrogate 

for PM-related light extinction calculated on a sub-daily basis. 

To examine this relationship, the EPA conducted comparative analyses of 24-hour 

and 4-hour averaging times in conjunction with a calculated PM2.5 indicator. For these 

analyses, 4-hour average PM2.5 light extinction was calculated based on using the 

Visibility Assessment approach. The 24-hour average PM2.5 light extinction was 

calculated using the original IMPROVE algorithm and long-term relative humidity 

conditions to calculate PM2.5 light extinction. Based on these analyses,136 which are 

presented and discussed in Appendix G of the Policy Assessment, scatter plots comparing 

24-hour and 4-hour calculated PM2.5 light extinction were constructed for each of the 15 

cities included in the Visibility Assessment and for all 15 cities pooled together (U.S. 

EPA, 2011a, Figures G-4 and G-5). Though there was some scatter around the regression 

line for each city because the calculated 4-hour light extinction values included dayspecific and hour-specific influences that are not captured by the simpler 24-hour 

approach, these analyses generally showed good correlation between 24-hour and 4-hour 

average PM2.5 light extinction, as evidenced by reasonably high city-specific and pooled 

R

2

values, generally in the range of over 0.6 to over 0.8.137 This suggested that PM2.5 light 

extinction calculated on a 24-hour basis is a reasonable and appropriate surrogate to 

PM2.5 light extinction calculated on a sub-daily basis. 

Taking the above considerations and CASAC’s advice into account, the Policy 

 

136 These analyses are also based on the use of a 90th percentile form, averaged over 3 

years, as discussed below in section VI.D.3 and in section 4.3.3 of the Policy Assessment 

(U.S. EPA, 2011a).

137 The EPA staff noted that the R2

value (0.44) for Houston was notably lower than for 

the other cities.
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Assessment concluded that it would be appropriate to consider a 24-hour averaging time,

in conjunction with a calculated PM2.5 light extinction indicator and an appropriately 

specified standard level, as discussed below. By using site-specific daily data on PM2.5

composition and site-specific long-term relative humidity conditions, this 24-hour 

average indicator would provide more consistent protection from PM2.5-related visibility 

impairment than would a secondary PM2.5 NAAQS based only on 24-hour or annual 

average PM2.5 mass. In particular, this approach would account for the systematic 

difference in humidity conditions between most eastern states and most western states. 

The Policy Assessment also concluded that it would also be appropriate to consider a 

multi-hour, sub-daily averaging time, for example a period of 4 hours, in conjunction 

with a calculated PM2.5 light extinction indicator and with further consideration of the 

data quality issues discussed above. Such an averaging time, to the extent that data 

quality issues can be appropriately addressed, would be more directly related to the shortterm nature of the perception of visibility impairment, short-term variability in PMrelated visual air quality, and the short-term nature (hourly to multiple hours) of relevant 

exposure periods for segments of the viewing public. Such an averaging time would still 

result in an indicator that is less sensitive than a 1-hour averaging time to short-term 

instrument variability with respect to PM2.5 mass measurement. In conjunction with 

consideration of a multi-hour, sub-daily averaging time, the Policy Assessment concluded 

that consideration should be given to including daylight hours only and to applying a 

relative humidity screen of approximately 90 percent to remove hours in which fog or 

precipitation is much more likely to contribute to the observed visibility impairment (U.S. 

EPA, 2011a, p. 4-58). Recognizing that a 1-hour averaging time would be even more 
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sensitive to data quality issues, including short-term variability in hourly data from 

currently available continuous monitoring methods, the Policy Assessment concluded 

that it would not be appropriate to consider a 1-hour averaging time in conjunction with a 

calculated PM2.5 light extinction indicator in this review (U.S. EPA, 2011a, p. 4-58). 

As noted above, in its review of the first draft Policy Assessment, CASAC 

concluded that PM effects on visibility can vary widely and rapidly over the course of a 

day and such changes are almost instantaneously perceptible to human observers (Samet, 

2010c, p. 19). Based in part on this consideration, CASAC agreed that a 1-hour averaging 

time would be appropriate to consider in conjunction with a directly measured PM light 

extinction indicator, noting that a 1-hour averaging time is well within the instrument 

response times of the various currently available and developing optical monitoring 

methods. At that time, CASAC also advised that if a PM2.5 mass indicator were to be 

used, it would be appropriate to consider “somewhat longer averaging times – 2- to 4-

hours – to assure a more stable instrumental response” (Samet, 2010c, p. 19). Thus, 

CASAC’s advice on averaging times that would be appropriate for consideration was 

predicated in part on the capabilities of monitoring methods that were available for the 

alternative indicators discussed in the draft Policy Assessment. CASAC’s views on a 

multi-hour averaging time would also apply to the calculated PM2.5 light extinction 

indicator since hourly PM2.5 mass measurements are also required for this indicator when 

calculated on a sub-daily basis.

It is important to note that at the time it provided advice on suitable averaging 

times, CASAC did not have the benefit of EPA’s subsequent assessment of the data 

quality issues associated with the use of continuous FEMs as the basis for hourly PM2.5
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mass measurements. Furthermore, since CASAC only commented on the first and second 

drafts of the Policy Assessment, neither of which included discussion of a calculated 

PM2.5 indicator based on a 24-hour averaging time, CASAC did not have a basis to offer 

advice regarding a 24-hour averaging time. In addition, the 24-hour averaging time is not 

based on consideration of 24-hours as a relevant exposure period, but rather as a 

surrogate for a sub-daily period of 4 hours, which is consistent with CASAC’s advice 

concerning an averaging time associated with the use of a PM2.5 mass indicator.

Taking into account the information discussed above with regard to analyses and 

conclusions presented in the final Policy Assessment the Administrator recognized that 

hourly or sub-daily, multi-hour averaging times, within daylight hours and excluding 

hours with relative humidity above approximately 90 percent, are more directly related 

than a 24-hour averaging time to the short-term nature of the perception of PM-related 

visibility impairment and the relevant exposure periods for segments of the viewing 

public. On the other hand, she recognized that data quality uncertainties have recently 

been associated with currently available instruments that would be used to provide the 

hourly PM2.5 mass measurements that would be needed in conjunction with an averaging 

time shorter than 24-hours. As a result, while the Administrator recognized the 

desirability of a sub-daily averaging time, she had strong reservations about proposing to 

set a standard at this time in terms of a sub-daily averaging time.

In considering the information and analyses related to consideration of a 24-hour 

averaging time, the Administrator recognized that the Policy Assessment concluded that 

PM2.5 light extinction calculated on a 24-hour averaging basis is a reasonable and 

appropriate surrogate for sub-daily PM2.5 light extinction calculated on a 4-hour average 
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basis. In light of this finding and the views of CASAC based on its reviews of the first 

and second drafts of the Policy Assessment, the Administrator proposed to set a distinct 

secondary standard with a 24-hour averaging time in conjunction with a PM2.5 visibility 

index. 

iv. Form

As discussed in section VI.D.3 of the proposal, the “form” of a standard defines 

the air quality statistic that is to be compared to the level of the standard in determining 

whether the standard is achieved. The form of the current 24-hour PM2.5 NAAQS is such 

that the level of the standard is compared to the 3-year average of the annual 98th

percentile value of the measured indicator. The purpose in averaging for three years is to 

provide stability from the occasional effects of inter-annual meteorological variability 

that can result in unusually high pollution levels for a particular year. The use of a multiyear percentile form, among other things, makes the standard less subject to the 

possibility of transient violations caused by statistically unusual indicator values, thereby 

providing more stability to the air quality management process that may enhance the 

practical effectiveness of efforts to implement the NAAQS. Also, a percentile form can 

be used to take into account the number of times an exposure might occur as part of the 

judgment on protectiveness in setting a NAAQS. For all of these reasons, the Policy 

Assessment concluded it would be appropriate to consider defining the form of a distinct 

secondary standard in terms of a 3-year average of a specified percentile air quality 

statistic (U.S. EPA, 2011a, p. 4-58).

The urban visibility preference studies that provided results leading to the range 

of CPLs being considered in this review offer no information that addresses the 
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frequency of time that visibility levels should be below those values. Given this lack of 

information, and recognizing that the nature of the public welfare effect is one of 

aesthetics and/or feelings of well-being, the Policy Assessment concluded that it would 

not be appropriate to consider eliminating all exposures above the level of the standard 

and that allowing some number of hours/days with reduced visibility can reasonably be 

considered (U.S. EPA, 2011a, p. 4-59). In the Visibility Assessment, 90th, 95th, and 98th

percentile forms were assessed for alternative PM light extinction standards (U.S. EPA, 

2010b, section 4.3.3). In considering these alternative percentiles, the Policy Assessment 

noted that the Regional Haze Program targets the 20 percent most impaired days for 

improvements in visual air quality in Federal Class I areas. If improvement in the 20 

percent most impaired days were similarly judged to be appropriate for protecting visual 

air quality in urban areas, a percentile well above the 80th percentile would be appropriate 

to increase the likelihood that all days in this range would be improved by control 

strategies intended to attain the standard. A focus on improving the 20 percent most 

impaired days suggests that the 90th percentile, which represents the median of the 

distribution of the 20 percent worst days, would be an appropriate form to consider. 

Strategies that are implemented so that 90 percent of days have visual air quality that is at 

or below the level of the standard would reasonably be expected to lead to improvements 

in visual air quality for the 20 percent most impaired days. Higher percentile values 

within the range assessed could have the effect of limiting the occurrence of days with 

peak PM-related light extinction in urban areas to a greater degree. In considering the 

limited information available from the public preference studies, the Policy Assessment 

found no basis to conclude that it would be appropriate to consider limiting the 
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occurrence of days with peak PM-related light extinction in urban areas to a greater 

degree.

Another aspect of the form discussed in the proposal for a sub-daily averaging 

time was whether to include all daylight hours or only the maximum daily daylight 

hour(s). The maximum daily daylight 1-hour or multi-hour form would be most directly 

protective of the welfare of people who have limited, infrequent or intermittent exposure 

to visibility during the day (e.g., during commutes), but spend most of their time without 

an outdoor view. For such people a view of poor visibility during their morning commute 

may represent their perception of the day’s visibility conditions until the next time they 

venture outside during daylight, which may be hours later or perhaps the next day. Other 

people have exposure to visibility conditions throughout the day. For those people, it 

might be more appropriate to include every daylight hour in assessing compliance with a 

standard, since it is more likely that each daylight hour could affect their welfare. 

The Policy Assessment did not have information regarding the fraction of the 

public that has only one or a few opportunities to experience visibility during the day, nor 

did it have information on the role the duration of the observed visibility conditions has 

on wellbeing effects associated with those visibility conditions. However, it is logical to 

conclude that people with limited opportunities to experience visibility conditions on a 

daily basis would experience the entire impact associated with visibility based on their 

short-term exposure. The impact of visibility for those who have access to visibility 

conditions often or continuously during the day may be based on varying conditions 

throughout the day. 

In light of these considerations, the analyses conducted as part of the Visibility 
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Assessment analyses included both the maximum daily hour and the all daylight hours 

forms. The Policy Assessment noted that there is a close correspondence between the 

level of protection afforded for all 15 urban areas by a maximum daily daylight 1-hour 

approach using the 90th percentile form and an all daylight hours approach combined with 

the 98th percentile form (U.S. EPA, 2010b, section 4.1.4). This suggested that reductions 

in visibility impairment required to meet either form of the standard would provide 

protection to both fractions of the public (i.e., those with limited opportunities and those 

with greater opportunities to view PM-related visibility conditions). CASAC generally 

supported consideration of both types of forms without expressing a preference based on 

its review of information presented in the second draft Policy Assessment (Samet, 2010d, 

p. 11).

In conjunction with a calculated PM2.5 light extinction indicator and alternative 

24-hour or sub-daily (e.g., 4-hour) averaging times, based on the above considerations, 

and given the lack of information on and the high degree of uncertainty over the impact 

on public welfare of the number of days with visibility impairment over a year, the Policy 

Assessment concluded that it would be appropriate to give primary consideration to a 90th

percentile form, averaged over three years (U.S. EPA, 2011a, p. 4-60). Further, in the 

case of a multi-hour, sub-daily alternative standard, the Policy Assessment concluded that 

it would be appropriate to give primary consideration to a form based on the maximum 

daily multi-hour period in conjunction with the 90th percentile form (U.S. EPA, 2011a, p. 

4-60). This sub-daily form would be expected to provide appropriate protection for 

various segments of the population, including those with limited opportunities during a 

day and those with more extended opportunities over the daylight hours to experience 
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PM-related visual air quality.

Though CASAC did not provide advice as to a specific form that would be 

appropriate, it took note of the alternative forms considered in that document and 

encouraged further analyses in the final Policy Assessment that might help to clarify a 

basis for selecting from within the range of forms identified. In considering the available 

information and the conclusions in the final Policy Assessment in light of CASAC’s 

comments, at the time of proposal the Administrator concluded that a 90th percentile 

form, averaged over 3 years, is appropriate, and proposed such a form in conjunction 

with a PM2.5 visibility index and a 24-hour averaging time.

v. Level

As discussed in section VI.D.4 of the proposal, in considering appropriate levels 

for a 24-hour standard defined in terms of a PM2.5 visibility index and an 90th percentile 

form, averaged over 3 years, the Policy Assessment took into account the evidence- and 

impact-based considerations discussed above, with a focus on the results of public 

perception and attitude surveys related to the acceptability of various levels of visual air 

quality and on the important limitations in the design and scope of such available studies. 

The Policy Assessment considered a variety of approaches for identifying appropriate 

levels for such a standard, including utilizing both adjusted and unadjusted CPLs derived

from the visibility preference studies. 

The Policy Assessment interpreted the results from the visibility preferences 

studies conducted in four urban areas to define a range of low, middle, and high CPLs for 

a sub-daily standard (e.g., 1- to 4-hour averaging time) of 20, 25, and 30 dv, which are 

approximately equivalent to PM2.5 light extinction of values of 65, 110, and 190 Mm-1.
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The CASAC generally supported this approach, noting that the “EPA staff’s approach for 

translating and presenting the technical evidence and assessment results is logically 

conceived and clearly presented. The 20-30 deciview range of levels chosen by EPA staff as 

“Candidate Protection Levels” is adequately supported by the evidence presented” (Samet, 

2010d, p. 11).138 The Policy Assessment also recognized that to define a range of 

alternative levels that would be appropriate to consider for a 24-hour calculated PM2.5

light extinction standard, it would be appropriate to consider whether some adjustment to 

these CPLs is warranted since these preference studies cannot be directly interpreted as 

applying to a 24-hour exposure period (as noted above and in Policy Assessment section 

4.3.1). Considerations related to such adjustments are more specifically discussed below.

In considering alternative levels for a sub-daily standard based directly on the four 

preference study results, the Policy Assessment noted that the individual low and high 

CPLs are in fact generally reflective of the results from the Denver and Washington, DC 

studies respectively, and the middle CPL is very near to the 50th percentile criteria result 

from the Phoenix study, which was by far the best of the studies, providing somewhat 

more support for the middle CPL.

In considering the results from the four visibility preference studies, the Policy 

Assessment recognized that currently available studies are limited in that they were 

conducted in only four areas, three in the U.S. and one in Canada. Further, the Policy 

Assessment recognized that available studies provide no information on how the duration 

 

138 In 2009, the D.C. Circuit remanded the secondary PM2.5 standards to the EPA in part 

because the Agency failed to identify a target level of protection, even though EPA staff 

and CASAC had identified a range of target levels of protection that were appropriate for 

consideration. The court determined that the Agency’s failure to identify a target level of 

protection as part of its final decision was contrary to the statute and therefore unlawful, 

and that it deprived EPA’s decision-making of a reasoned basis. See 559F. 3d at 528-31; 

see also section VI.A.2 above and the Policy Assessment, section 4.1.2.
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and variation of time a person spends outdoors during the daytime may impact their 

judgment of the acceptability of different degrees of visibility impairment. As such, there 

is a relatively high degree of uncertainty associated with using the results of these studies 

to inform consideration of a national standard for any specific averaging time. 

Nonetheless, the Policy Assessment concluded, as did CASAC, that these studies are 

appropriate to use for this purpose (U.S. EPA, 2011a, p. 4-61).

Using approaches described in section VI.C.4 of the proposal, the Policy 

Assessment explored various approaches to adjusting the CPLs derived from the 

preference studies to inform alternative levels for a 24-hour standard. These various 

approaches, based on analyses of 2007 – 2009 data from the 15 urban areas assessed in 

the Visibility Assessment, focused on estimating CPLs for a 24-hour standard that would 

provide generally equivalent protection as that provided by a 4-hour standard with CPLs

of 20, 25, and 30 dv. In conducting these analyses, staff initially expected that the values 

of 24-hour average PM2.5 light extinction and daily maximum daylight 4-hour average 

PM2.5 light extinction would differ on any given day, with the shorter term peak value 

generally being larger. This would mean that, in concept, the level of a 24-hour standard 

should include a downward adjustment compared to the level of a 4-hour standard to 

provide generally equivalent protection. As discussed more fully in section G.5 of 

Appendix G and summarized below, this initial expectation was not found to be the case 

across the range of CPLs considered. In fact, as shown in Tables G-7 and G-8 of 

Appendix G and in the corrected version of Table G-6 found in Frank et al. (2012b),

139 in 

 

139Note that the city-specific ranges shown in Table G-6, Appendix G of the Policy 

Assessment are incorrectly stated for Approaches C and E. Drawing from the more 

detailed and correct results for Approaches C and E presented in Tables G-7 and G-8, 

respectively, the city-specific ranges in Table G-6 for Approach C should be 17 – 21 dv 
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considering estimates aggregated or averaged over all 15 cities as well as the range of 

city-specific estimates for the various approaches considered, these analyses indicated

that the generally equivalent 24-hour levels ranged from somewhat below the 4-hour 

level to just above the 4-hour level for each of the CPLs.140 In all cases, the range of cityspecific estimates of generally equivalent 24-hour levels included the 4-hour level for 

each of the CPLs of 20, 25, and 30 dv. As noted in the proposal, looking more broadly at 

these results could support consideration of using the same CPL for a 24-hour standard as 

for a 4-hour standard, recognizing that there is no one approach that can most closely 

identify a generally equivalent 24-hour standard level in each urban area for each CPL. 

The use of such an unadjusted CPL for a 24-hour standard would place more emphasis on 

the relatively high degree of spatial and temporal variability in relative humidity and fine 

particle composition observed in urban areas across the country, so as to reduce the 

potential of setting a 24-hour standard level that would require more than the intended 

degree of protection in some areas.

In considering the appropriate level of a secondary standard focused on protection 

from PM-related urban visibility impairment based on either a 24-hour or a multi-hour, 

 

for the CPL of 20 dv; 21 – 25 dv for the CPL of 25 dv; and 24 – 30 dv for the CPL of 30 

dv; the city-specific ranges in Table G-6 for Approach E should be 17 – 21 dv for the 

CPL of 20 dv; 21 – 26 dv for the CPL of 25 dv; and 25 – 31 dv for the CPL of 30 dv. In 

the EPA’s reanalysis comparing 4- vs. 24-hour values, Frank et al. (2012b) recreated 

Table G-6 using the correct values from Tables G-7 and G-8.

140As discussed in more detail in Appendix G of the Policy Assessment, some days have 

higher values for 24-hour average light extinction than for daily maximum 4-hour 

daylight light extinction, and consequently an adjusted "equivalent" 24-hour CPL can be 

greater than the original 4-hour CPL. This can happen for two reasons. First, the use of 

monthly average historical RH data will lead to cases in which the f(RH) values used for 

the calculation of 24-hour average light extinction are higher than all or some of the four 

hourly values of f(RH) used to determine daily maximum 4-hour daylight light extinction 

on the same day. Second, PM2.5 concentrations may be greater during non-daylight 

periods than during daylight hours.
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sub-daily (e.g., 4-hour) averaging time, the EPA has been mindful of the important 

limitations in the available evidence from public preference studies. These uncertainties 

and limitations are due in part to the small number of stated preference studies available 

for this review; the relatively small number of study participants and the extent to which 

the study participants may not be representative of the broader study area population in 

some of the studies; and the variations in the specific materials and methods used in each 

study such as scene characteristics, the range of VAQ levels presented to study 

participants, image presentation methods and specific wording used to frame the 

questions used in the group interviews. In addition the EPA has noted that the scenic 

vistas available on a daily basis in many urban areas across the country generally may not 

have the inherent visual interest or the distance between viewer and object of greatest 

intrinsic value as in the Denver and Phoenix preference studies, and that there is the 

possibility that there could be regional differences in individual preferences for VAQ.

It is also important to note that as in past reviews, the EPA is considering a 

national visibility standard in conjunction with the Regional Haze Program as a means of 

achieving appropriate levels of protection against PM-related visibility impairment in 

urban, non-urban, and Federal Class I areas across the country. The EPA recognizes that 

programs implemented to meet a national standard focused primarily on the visibility 

problems in urban areas can be expected to improve visual air quality in surrounding nonurban areas as well, as would programs now being developed to address the requirements 

of the Regional Haze Program established for protection of visual air quality in Federal 

Class I areas. The EPA also believes that the development of local programs, such as 

those in Denver and Phoenix, can continue to be an effective and appropriate approach to 
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provide additional protection, beyond that afforded by a national standard, for unique 

scenic resources in and around certain urban areas that are particularly highly valued by 

people living in those areas.

The Policy Assessment concluded that it is appropriate to give primary 

consideration to alternative standard levels toward the upper end of the ranges identified 

above for 24-hour and sub-daily standards, respectively (U.S. EPA, 2011a, p. 4-63). 

Thus, the Policy Assessment concluded it is appropriate to consider the following 

alternative levels: a level of 28 dv or somewhat below, down to 25 dv, for a standard 

defined in terms of a calculated PM2.5 light extinction indicator, a 90th percentile form, 

and a 24-hour averaging time; and a standard level of 30 dv or somewhat below, down to 

25 dv, for a similar standard but with a 4-hour averaging time (U.S. EPA, 2011a, p. 4-

63). The Policy Assessment judged that such standards would provide appropriate 

protection against PM-related visibility impairment primarily in urban areas. The Policy 

Assessment noted that CASAC generally supported consideration of the 20 – 30 dv range 

as CPLs and, more specifically, that support for consideration of the upper part of the 

range of the CPLs derived from the public preference studies was expressed by some 

CASAC Panel members during the public meeting on the second draft Policy 

Assessment. The Policy Assessment concluded that such a standard would be appropriate 

in conjunction with the Regional Haze Program to achieve appropriate levels of 

protection against PM-related visibility impairment in areas across the country (U.S. 

EPA, 2011a, p. 4-63).

Based on the considerations discussed above and in section VI.D.4 of the 

proposal, and taking into account the advice of CASAC, at the time of proposal the 
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Administrator concluded that it would be appropriate to establish a target level of 

protection – for a standard defined in terms of a PM2.5 visibility index; a 90th percentile 

form averaged over 3 years; and a 24-hour averaging time – equivalent to the protection 

afforded by such a sub-daily (i.e., 4-hour) standard at a level of 30 dv, which is the upper 

end of the range of CPLs identified in the Policy Assessment and generally supported by 

CASAC. More specifically, the Administrator provisionally concluded that a 24-hour 

level of either 30 dv or 28 dv could be construed as providing such a degree of protection, 

and that either level was supported by the available information and was generally 

consistent with the advice of CASAC. Thus, the EPA proposed two options for the level 

of a new 24-hour standard (defined in terms of a PM2.5 visibility index and a 90th

percentile form, averaged over 3 years) to provide appropriate protection from PMrelated visibility impairment: either 30 dv or 28 dv. As noted in the proposal, the option 

of setting such a 24-hour standard at a level of 30 dv would reflect recognition that there 

is considerable spatial and temporal variability in the key factors that determine the value 

of the PM2.5 visibility index in any given urban area, such that there is a relatively high 

degree of uncertainty as to the most appropriate approach to use in selecting a 24-hour 

standard level that would be generally equivalent to a specific 4-hour standard level. 

Selecting a 24-hour standard level of 30 dv would reflect a judgment that such substantial 

degrees of variability and uncertainty should be reflected in a higher standard level than 

would be appropriate if the underlying information were more consistent and certain. 

Alternatively, the option of setting such a 24-hour standard at a level of 28 dv would 

reflect placing more weight on statistical analyses of aggregated data from across the 

study cities and not placing as much emphasis on the city-to-city variability as a basis for 
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determining an appropriate degree of protection on a national scale.

The information available for the Administrator to consider when setting the 

secondary PM standard raises a number of uncertainties. While CASAC supported 

moving forward with a new standard on the basis of the available information, CASAC 

also recognized these uncertainties, referencing the discussion of key uncertainties and 

areas for future research in the second draft of the Policy Assessment. In discussing areas 

of future research, CASAC stated that: “The range of 50% acceptability values discussed 

as possible standards are based on just four studies (Figure 4-2), which, given the large 

spread in values, provide only limited confidence that the benchmark candidate 

protection levels cover the appropriate range of preference values. Studies using a range 

of urban scenes (including, but not limited to, iconic scenes – “valued scenic elements” 

such as those in the Washington, DC study), should also be considered” (Samet, 2010d, 

p. 12). The EPA solicited comment on how the Administrator should weigh those 

uncertainties as well as any additional comments and information to inform her 

consideration of these uncertainties. 

In addition, the EPA solicited comment on a number of other issues related to the 

level of the standard, including: 

(1) Both of the proposed levels and the various approaches to identifying generally 

equivalent levels upon which the alternative proposed levels are based.

(2) A broader range of levels down to 25 dv in conjunction with a 24-hour averaging 

time. 

(3) A range of alternative levels from 30 to 25 dv in conjunction with a sub-daily 

(e.g., 4-hour) averaging time.

(4) The strengths and limitations associated with the public preference studies and the 

use of these studies to inform the selection of a range of levels that could be used 

to provide an appropriate degree of public welfare protection when combined 

with the other elements of the standard (i.e. indicator, form and averaging time).
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(5) Specific aspects of the public preference studies, including the extent to which the 

50 percent acceptability criterion is an appropriate basis for establishing target 

protection levels in the context of establishing a distinct secondary NAAQS to 

address PM-related visibility impairment in urban areas; how the variability 

among preference studies in the extent to which study participants may be 

representative of the broader study area population should be weighed in the 

context of considering these studies in reaching proposed conclusions on a 

distinct secondary NAAQS; and the extent to which the ranges of VAQ levels 

presented to participants in each of the studies may have influenced study results 

and on how this aspect of the study designs should appropriately be weighed in 

the context of considering these studies in the context of this review.

vi. Administrator’s Proposed Conclusions Regarding PM Standards to Protect 

Visibility 

At the time of proposal, based on the considerations described above, the 

Administrator proposed to revise the suite of secondary PM standards by adding a 

distinct standard for PM2.5 to address PM-related visibility impairment, focused primarily 

on visibility in urban areas. This proposed visibility standard was to be defined in terms 

of a PM2.5 visibility index, which would use measured PM2.5 mass, combined with PM2.5

speciation data and relative humidity data, to calculate PM2.5 light extinction, translated 

into the deciview (dv) scale; a 24-hour averaging time; a 90th percentile form, averaged 

over 3 years; and a level of 28-30 dv. 

vii. Related Technical Analysis

At the time of proposal, the EPA conducted a two-pronged technical analysis of 

the relationships between the proposed PM2.5 visibility index standard and the current 24-

hour PM2.5 mass-based standard (Kelly, et al., 2012a). This analysis was designed to 

provide technical information to inform key issues related to implementing a distinct 

secondary standard for visibility as proposed. Specifically, the EPA recognized that 

significant technical issues were likely to arise for new or modified emissions sources 
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conducting air quality analyses for purposes of demonstrating that they would not cause 

or contribute to a violation of the visibility standard under the Prevention of Significant 

Deterioration (PSD) program. Such a demonstration for the proposed secondary PM2.5

visibility index standard could require each PSD applicant to predict, via air quality 

modeling, the increase in visibility impairment, in terms of the proposed PM2.5 visibility 

index, that would result from the proposed source's emissions in conjunction with an 

assessment of existing air quality (visibility impairment) conditions in terms of the 

proposed PM2.5 visibility index. The EPA noted that if this demonstration were to be 

attempted using the six-step procedure that the EPA proposed to use for calculating PM2.5

visibility index design values from monitored air concentrations of PM2.5 components, 

significant technical issues with the modeling procedures could arise.

To address these technical issues, the EPA sought to explore whether sources that 

met the requirements pertaining to the 24-hour mass-based standard of 35 μg/m3 would 

also meet the requirements pertaining to the proposed visibility index standard. As 

described in Kelly et al. (2012a), the first prong of the analysis addressed aspects of a 

PSD significant impact analysis by evaluating whether an individual source’s impact 

resulting in a small increase in the ambient PM2.5 concentration would produce a 

comparably small increase in visibility impairment. This analysis included estimates of 

PM2.5 speciation profiles based on direct PM2.5 emission profiles for a broad range of 

source categories and for theoretical upper and lower bound scenarios.

The second prong of the analysis addressed aspects of a PSD cumulative impact 

analysis by exploring the relationship between the three-year design values for the 

existing 24-hour PM2.5 standard and coincident design values for the proposed PM2.5
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visibility index standard based on recent air quality data. This aspect of the analysis 

indicated that increases in 24-hour PM2.5 design values generally correspond to increases 

in visibility index design values, and vice-versa. The analysis further explored the 

appropriateness of using a demonstration that a source does not cause or contribute to a 

violation of the 24-hour PM2.5 standard as a surrogate for a demonstration that a source 

does not cause or contribute to a violation of the proposed secondary PM2.5 visibility 

index standard. This analysis was based on 2008 to 2010 air quality data, and compared

the proposed level of 35 μg/m3for the 24-hour PM2.5 standard and for illustrative 

purposes an alternative standard level of 12 μg/m3for the annual PM2.5 standard with the 

proposed levels of 28 or 30 dv for the secondary PM2.5 visibility index standard with a 

24-hour averaging time and a 90th percentile form. The results indicated that all (for the

30 dv level) or nearly all (for the 28 dv level) areas in attainment of the 24-hour PM2.5

standard would also have been in attainment of the proposed secondary PM2.5 visibility 

index standard.

Based on this technical analysis, the EPA proposed that there is sufficient 

evidence that a demonstration that a source does not cause or contribute to a violation of 

the mass-based 24-hour PM2.5 standard serves as a suitable surrogate for demonstrating 

that a source does not cause or contribute to a violation of the proposed secondary 24-

hour PM2.5 visibility index standard under the PSD program. As such, the EPA proposed 

to conclude that many or all sources undergoing PSD review for PM2.5 could rely upon 

their analysis for demonstrating that they do not cause or contribute to a violation of the 

mass-based 24-hour PM2.5 standard to also show that they do not cause or contribute to a 

violation of the proposed secondary PM2.5 visibility index standard, if a distinct visibility 
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standard were finalized. 

Although this proposed “surrogacy policy” was designed to address an 

implementation-related issue, the second prong of the technical analysis addresses the 

broader technical question of the relationship between the existing 24-hour PM2.5

standard and the proposed PM2.5 visibility index standard in terms of the degree of 

protection likely to be afforded by each standard. Specifically, the analysis indicated that 

depending on the level of the proposed PM2.5 visibility index standard, the existing 24-

hour PM2.5 mass-based standard would be as protective or in some areas more protective 

of visibility than a distinct secondary standard set within the range of levels proposed. 

Commenters on the proposed PM2.5 visibility index explored the implications of this 

analysis at length, as discussed further below in section VI.C.1.f. For this reason, the 

analysis is described in some detail here.

Kelly et al. (2012a) noted that the relationship between design values for the 24-

hour PM2.5 standard and the proposed secondary visibility index standard is not obvious a 

priori because of differences in design value calculations for the standards. However, 

closer examination of this relationship indicated that increases or decreases in 24-hour 

PM2.5 design values correspond, respectively, to increases or decreases in visibility index 

values. Specifically, based on measurements from 102 sites with complete data from 

2008-2010, Kelly et al. (2012a) found linear correlations between the 24-hour PM2.5

design values and the visibility index design values with r2values ranging from 0.65 to 

0.98 across these sites, with an average r2value of 0.75 across all U.S. sites. Moreover, 

the data indicated that no design value existed where the visibility index design value 

exceeded 30 dv, but the 24-hour PM2.5 standard level of 35 μg/m3 was attained. Visibility 
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index design values for certain sites in the Industrial Midwest were shown to exceed 28 

dv despite the fact that the 24-hour PM2.5 design values for these sites were below 35 

μg/m3. This was attributed to the combination of high nitrate and sulfate fractions, 

substantial RH adjustment factors, and PM2.5 distribution characteristics that led to 

relatively high visibility index design values for a given 24-hour PM2.5 design value for 

counties in the Industrial Midwest.141 Kelly et al. (2012a) concluded that the “overall, 

design values based on 2008-2010 data suggest that counties that attain 24-hour PM2.5

NAAQS level of 35 μg/m3 would attain the proposed secondary PM2.5 visibility index 

NAAQS level of 30 dv and generally attain the level of 28 dv” (pp. 17-18). In addition, 

the Kelly et al. analysis indicated that at sites that violated both the 24-hour PM2.5 level 

and the proposed visibility index 30 dv level, the proposed level of 30 dv would likely be 

attained if PM2.5 concentrations were reduced such that the 24-hour PM2.5 level of 35 

μg/m3 was attained (Kelly et al.,2012a, p.15).

142 A key implication of this analysis, 

therefore, was that within the range of levels proposed by the EPA for a visibility index 

standard (28-30 dv), the 24-hour PM2.5 standard of 35 μg/m3 would be controlling in 

almost all (at 28 dv) or all (at 30 dv) instances.

2. Other (Non-Visibility) PM-related Welfare Effects

In the 2006 review, the EPA concluded that there was insufficient information to 

consider a distinct secondary standard based on PM-related impacts to ecosystems, 

 

141 Kelly et al. (2012a) also noted that “Regional reductions in sulfate PM2.5 due to 

emission controls planned as part of national rules as well as emission reductions 

associated with potential annual standard violations are expected to improve visibility in 

this region” (p. 17).

142 The analysis also showed that attaining the 24-hour PM2.5 standard level of 35 μg/m3

would result in achieving a lower PM2.5 visibility index level in certain areas of the 

country, largely western areas, than would be achieved in other areas of the country. This 

is due to differences in the composition of ambient PM2.5 and the lower relative humidity 

in those areas.
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materials damage and soiling, and climatic and radiative processes (71 FR 61144, 

October 17, 2006). Specifically, there was a lack of evidence linking various nonvisibility welfare effects to specific levels of ambient PM. In that review, to provide a 

level of protection for these welfare-related effects, the secondary standards were set 

equal to the revised primary standards to directionally improve the level of protection 

afforded vegetation, ecosystems, and materials (71 FR 61210, October 17, 2006).

This section briefly outlines key conclusions discussed more fully in section VI.E 

of the proposal regarding the non-visibility welfare effects of PM. These conclusions 

relate to the climate, ecological (including effects on plants, soil and nutrient cycling, 

wildlife and water) and materials damage effects of PM. For all of these effects, the 

Policy Assessment concluded that there is insufficient information at this time to revise 

the current suite of secondary standards. It is important to note that the Policy 

Assessment explicitly excluded discussion of the effects associated with deposited 

particulate matter components of NOx and SOx and their transformation products which 

are addressed fully in the joint review of the secondary NO2 and SO2 NAAQS.

a. Evidence of Other Welfare Effects Related to PM

With regard to the role of PM in climate, the proposal noted that there is 

considerable ongoing research focused on understanding aerosol contributions to changes 

in global mean temperature and precipitation patterns. The Integrated Science 

Assessment concluded “that a causal relationship exists between PM and effects on 

climate, including both direct effects on radiative forcing and indirect effects that involve 

cloud feedbacks that influence precipitation formation and cloud lifetimes” (U.S. EPA, 

2009a, section 9.3.10). These effects are discussed in more detail in section VI.E.1 of the 
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proposal, which provides information on the major aerosol components of interest for 

climate processes, including black carbon (BC), organic carbon (OC), sulfates, nitrates, 

and mineral dusts, and the nature, magnitude, and direction (e.g., cooling vs. warming) of 

various aerosol impacts on climate.143 The Policy Assessment concluded that aerosols 

alter climate processes directly through radiative forcing and by indirect effects on cloud 

brightness, changes in precipitation, and possible changes in cloud lifetimes (U.S. EPA, 

2011a, p. 5-10). Further, the Policy Assessment noted that the major aerosol components 

that contribute to climate processes (i.e. BC, OC, sulfate, nitrate and mineral dusts) vary 

in their reflectivity, forcing efficiencies and even in the direction of climate forcing, 

though there is an overall net climate cooling associated with aerosols in the global 

atmosphere (U.S. EPA, 2009a, section 9.2.10). The Policy Assessment concluded that the 

current mass-based PM2.5 and PM10 secondary standards were not an appropriate or 

effective means of focusing protection against PM-associated climate effects due to these 

differences in components (U.S. EPA, 2011a, p. 5-11). In addition, in light of the 

significant uncertainties in current scientific information and the lack of sufficient data, 

the Policy Assessment concluded it is not currently feasible to conduct a quantitative 

analysis for the purpose of informing revisions of the current secondary PM standards 

based on climate (U.S. EPA, 2011a, p. 5-11). Overall the Policy Assessment concluded 

that there is insufficient information at this time to base a national ambient standard on 

climate impacts associated with current ambient concentrations of PM or its constituents 

(U.S. EPA, 2011a, p. 5-11, -12).144

 

143 Atmospheric PM is referred to as aerosols in the remainder of this section to be 

consistent with the Integrated Science Assessment.

144 This conclusion would apply for both the secondary (welfare-based) and the primary 

(health-based) standards. 
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With regard to ecological effects, the proposal noted that several ecosystem 

components (e.g., plants, soils and nutrient cycling, wildlife and water) are impacted by 

PM air pollution, which may alter the services provided by affected ecosystems. 

Ecological effects include both direct effects due to deposition (e.g., wet, dry or occult) to 

vegetation surfaces and indirect effects occurring via deposition to ecosystem soils or 

surface waters where the deposited constituents of PM then interact with biological 

organisms. Some of the ecological effects considered in this review include direct effects 

to metabolic processes of plant foliage; contribution to total metal loading resulting in 

alteration of soil biogeochemistry and microbiology, and plant and animal growth and 

reproduction; and contribution to total organics loading resulting in bioaccumulation and 

biomagnification across trophic levels. Section VI.E.2 of the proposal summarizes key 

findings related to:

(1) Impacts on plants and the ecosystem services they provide due to deposition of 

PM to vegetative surfaces, which alters the radiation received by the plant, and 

uptake of deposited PM components by plants from soil or foliage, which can lead 

to stress and decreased photosynthesis;

(2) Impacts on ecosystem support services such as nutrient cycling, products such as 

crops and the regulation of flooding and water quality;

(3) Impacts on wildlife, especially due to biomagnification of heavy metals 

(especially Hg) up the food chain and bioconcentration of POPs and PBDEs; and

(4) Impacts of deposited PM, especially metals and organics, on the ecosystem 

services provided by water bodies, including primary production, provision of 

fresh water, regulation of climate and floods, recreational fishing and water 

purification.

The proposal noted that the Integrated Science Assessment had concluded that 

ecological evidence is sufficient to conclude that a causal relationship is likely to exist 

between deposition of PM and a variety of effects on individual organisms and 

ecosystems (U.S. EPA, 2009a, sections 2.5.3 and 9.4.7), and also noted that vegetation 
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and other ecosystem components are affected more by particulate chemistry than size 

fraction. However, the proposal also pointed to the Integrated Science Assessment 

conclusion that it is generally difficult to characterize the nature and magnitude of effects 

and to quantify relationships between ambient concentrations of PM and ecosystem 

response due to significant data gaps and uncertainties as well as considerable variability 

that exists in the components of PM and their various ecological effects. There are few 

studies that link ambient PM concentrations to observed effect. Most direct ecosystem 

effects associated with particulate pollution occur in severely polluted areas near 

industrial point sources (quarries, cement kilns, metal smelting) (U.S. EPA, 2009a, 

sections 9.4.3 and 9.4.5.7).

Based on the evidence available at this time, the proposal noted the following key 

conclusions in the Policy Assessment:

(1) A number of significant environmental effects that either have already occurred or 

are currently occurring are linked to deposition of chemical constituents found in 

ambient PM.

(2) Ecosystem services can be adversely impacted by PM in the environment, 

including supporting, provisioning, regulating and cultural services.

(3) The lack of sufficient information to relate specific ambient concentrations of 

particulate metals and organics to a degree of impairment of a specific ecological 

endpoint hinders the identification of a range of appropriate indicators, levels, 

forms and averaging times of a distinct secondary standard to protect against 

associated effects.

(4) Data from regionally-based ecological studies can be used to establish probable 

local, regional and/or global sources of deposited PM components and their 

concurrent effects on ecological receptors.

The proposal noted that the Policy Assessment had concluded that the currently 

available information is insufficient for purposes of assessing the adequacy of the 

protection for ecosystems afforded by the current suite of PM secondary standards or 
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establishing a distinct national standard for ambient PM based on ecosystem effects of 

particulates not addressed in the NOx/SOx secondary review (e.g., metals, organics) (U.S. 

EPA, 2011a, p. 5-24). Furthermore, the Policy Assessment had concluded that in the 

absence of information providing a basis for specific standards in terms of particle 

composition, the observations continue to support retaining an appropriate degree of 

control on both fine and coarse particles to help address effects to ecosystems and 

ecosystem components associated with PM (U.S. EPA, 2011a, p. 5-24).

With regard to materials damage, the proposal discussed effects associated with 

deposition of PM, including both physical damage (materials damage effects) and 

impaired aesthetic qualities (soiling effects). As with the other categories of welfare 

effects discussed above, the Integrated Science Assessment concluded that evidence is 

sufficient to support a causal relationship between PM and effects on materials (U.S. 

EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition of PM can physically affect 

materials, adding to the effects of natural weathering processes, by potentially promoting 

or accelerating the corrosion of metals, by degrading paints and by deteriorating building 

materials such as stone, concrete and marble (U.S. EPA, 2009a, section 9.5). In addition, 

the deposition of ambient PM can reduce the aesthetic appeal of buildings and objects 

through soiling. The Policy Assessment made the following observations:

(1) Materials damage and soiling that occur through natural weathering processes are 

enhanced by exposure to atmospheric pollutants, most notably sulfur dioxide and 

particulate sulfates. 

(2) While ambient particles play a role in the corrosion of metals and in the 

weathering of materials, no quantitative relationships between ambient particle 

concentrations and rates of damage have been established.

(3) While soiling associated with fine and course particles can result in increased 

cleaning frequency and repainting of surfaces, no quantitative relationships 

between particle characteristics and the frequency of cleaning or repainting have 
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been established.

(4) Limited new data on the role of microbial colonizers in biodeterioration processes 

and contributions of black crust to soiling are not sufficient for quantitative 

analysis.

(5) While several studies in the PM Integrated Science Assessment and NOx/SOx

Integrated Science Assessment suggest that particles can promote corrosion of 

metals there remains insufficient evidence to relate corrosive effects to specific 

particulate levels or to establish a quantitative relationship between ambient PM 

and metal degradation. With respect to damage to calcareous stone, numerous 

studies suggest that wet or dry deposition of particles and dry deposition of 

gypsum particles can enhance natural weathering processes.

The Policy Assessment concluded that none of the new evidence in this review 

called into question the adequacy of the current standards for protecting against material 

damage effects, that such effects could play no quantitative role in determining whether 

revisions to the secondary PM NAAQS are appropriate at this time, and that observations 

continue to support retaining an appropriate degree of control on both fine and coarse 

particles to help address materials damage and soiling associated with PM (U.S. EPA, 

2011a, p. 5-29).

b. CASAC Advice

In advising the EPA regarding the non-visibility welfare effects, CASAC stated 

that it “concurs with the Policy Assessment’s conclusions that while these effects are 

important, and should be the focus of future research efforts, there is not currently a 

strong technical basis to support revisions of the current standards to protect against these 

other welfare effects” (Samet, 2010c). More specifically, with regard to climate impacts, 

CASAC concluded that while there is insufficient information on which to base a 

national standard, the causal relationship is established and the risk of impacts is high, so 

further research on a regional basis is urgently needed (Samet, 2010c, p. 5). CASAC also 

noted that reducing certain aerosol components could lead to increased radiative forcing 
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and regional climate warming while having a beneficial effect on PM-related visibility. 

As a consequence, CASAC noted that a secondary standard directed toward reducing 

PM-related visibility impairment has the potential to be accompanied by regional

warming if light scattering aerosols are preferentially targeted.

With regard to ecological effects, CASAC concluded that the published literature 

is insufficient to support a national standard for PM effects on ecosystem services 

(Samet, 2010c, p.23). CASAC noted that the best-established effects are related to 

particles containing nitrogen and sulfur, which are being considered in the EPA’s 

ongoing review of the secondary NAAQS for NOx/SOx. With regard to PM-related 

effects on materials, CASAC concluded that the published literature, including literature 

published since the last review, is insufficient either to call into question the current level 

of the standard or to support any specific national standard for PM effects on materials 

(Samet, 2010c, p.23). Nonetheless, with regard to both types of effects, CASAC noted 

the importance of maintaining an appropriate degree of control of both fine and coarse 

particles to address such effects, even in the current absence of sufficient information to 

develop a standard.

c. Summary of Proposed Decisions Regarding Other Welfare Effects

Based on the above considerations and the advice of CASAC, at the time of 

proposal the Administrator provisionally concluded that it would not be appropriate to 

establish any distinct secondary PM standards to address other non-visibility PM-related 

welfare effects, including ecological effects, effects on materials, and climate impacts. 

Nonetheless, the Administrator concurred with the conclusions of the Policy Assessment 

and CASAC advice that it is important to maintain an appropriate degree of control of 
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both fine and coarse particles to address such effects. Noting that there is an absence of 

information that would support any different standards, the Administrator proposed 

generally to retain the current suite of secondary PM standards145 to address non-visibility 

welfare effects. Specifically, the Administrator proposed to retain all aspects of the 

current secondary 24-hour PM2.5 and PM10 standards. With regard to the secondary 

annual PM2.5 standard, the Administrator proposed to retain the level of the current 

standard and to revise the form of the standard by removing the option for spatial 

averaging consistent with this change to the primary annual PM2.5 standard.

C. Public Comments on Proposed Decisions regarding Secondary PM Standards

The EPA received a large number of comments on its proposed decisions with 

regard to secondary PM standards, with the large majority of those comments focusing on 

the proposal to set a distinct standard to protect against visibility impairment, discussed 

below in section VI.C.1. Very few commenters addressed the proposal to retain the 

existing secondary standards for non-visibility welfare effects, discussed below in section 

VI.C.2. As discussed in section VI.D. below, the Administrator has decided to retain the 

current suite of secondary PM standards generally, while revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. The Administrator has also 

decided, contrary to what was proposed, not to establish a distinct secondary standard to 

address PM-related visibility impairment. This section discusses EPA’s responses to the 

comments EPA received on its proposal, and the rationale behind the Administrator’s 

final decisions is discussed in section VI.D below. 

 

145 As summarized in section VI.A and Table 1 above, the current suite of secondary PM 

standards includes annual and 24-hour PM2.5 standards and a 24-hour PM10 standard. 
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1. Comments on Proposed Secondary Standard for Visibility Protection 

a. Overview of Comments

Among those commenting on the proposal to set a distinct secondary PM2.5

visibility index standard, a large majority of commenters, including more than 25 state 

and local agencies; regional organizations such as NACAA, NESCAUM, and WESTAR; 

and industry commenters, such as ACC, API, BP, EPRI, NCBA, NEDA-CAP, NMA, 

NSSGA, and UARG, opposed setting a distinct secondary standard for visibility at this 

time. Many commenters in this group expressed the view that such a standard was not 

needed, primarily on the basis that adequate protection was provided by the existing 24-

hour secondary PM2.5 standard. Some of these commenters also expressed legal concerns 

with the nature of the proposed standard. Other commenters in this group supported a 

distinct secondary standard for visibility in concept, but expressed the view that it was 

premature to set such a standard pending collection of additional visibility preference 

study data and the resolution of a number of key technical issues. Support for setting such 

a distinct secondary standard for visibility at this time came from a second group of 

commenters, including the Department of the Interior (National Park Service), several 

states, the Mid-Atlantic/Northeast Visibility Union (MANE-VU), the National Tribal Air 

Association (NTAA), environmental organizations such as the Appalachian Mountain 

Club, National Parks Conservation Association, Earthjustice (AMC, et al.) and the 

League of Women Voters of Texas. These commenters argued that the existing 

secondary standards are not sufficiently protective of visual air quality, and that a distinct 

secondary standard similar to the proposed visibility index standard is both necessary and 

appropriate to ensure adequate protection of visibility.
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Commenters in both groups expressed concerns about various aspects of the 

proposed distinct secondary standard, including the indicator, averaging time, level, and 

form. In addition, a large number of commenters, including commenters from both 

groups, expressed concern and/or confusion over the relationship between the Regional 

Haze Program and the proposed distinct secondary standard for visibility, raising issues 

such as analytical differences in methods between the programs, monitoring issues, and 

other implementation challenges.

A discussion of the significant comments outlined above, including EPA’s 

responses to the comments, is presented here, with more detailed discussion in the 

Response to Comments document. Comments relating to the specific elements of the 

proposed standard – indicator, averaging time, form and level– are discussed in sections 

VI.C.1.b-e, respectively. Comments related to the need for a distinct secondary standard 

at this time are discussed in section VI.C.f. Legal issues raised by commenters opposed to 

setting a secondary standard based on the proposed visibility index are discussed in 

section VI.C.g. Finally, comments related to the relationship between a distinct secondary 

standard and the Regional Haze Program are discussed in section VI.C.h.146 While the 

EPA concludes in section VI.D below to retain the current suite of secondary PM2.5

standards, the appropriateness of the protection that would be provided by the proposed 

PM2.5 visibility index standard, and the relationship between this degree of protection and 

that provided by the current secondary 24-hour secondary PM2.5 standard, are key 

elements in the Administrator's decision, and are discussed below. 

 

146 Comments pertaining to implementation issues, which the Administrator may not 

consider in making decisions about setting national ambient air quality standards, are 

discussed in the Response to Comments document, as are comments regarding 

monitoring issues related to the proposed distinct visibility index standard.
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b. Indicator

Numerous commenters, both those supporting a distinct secondary standard and 

those opposed to setting such a standard, expressed views on the suitability of utilizing a 

PM2.5 calculated light extinction indicator for the standard as proposed. While these 

groups of commenters differed in terms of their views on the appropriateness of using 

calculated PM2.5 light extinction as the basis for the indicator rather than relying on direct 

measurements of PM2.5 light extinction, commenters from both groups expressed concern 

over specific elements of the proposed method of calculating PM2.5 light extinction. In 

particular, commenters expressed differing views on which IMPROVE algorithm should 

be utilized; whether it is appropriate to exclude coarse particles from the indicator; and 

whether the proposed protocols for incorporating data on relative humidity and PM2.5

species are appropriate.147

i. Comments on Calculated vs. Directly Measured Light Extinction

The majority of commenters in both groups noted the uncertainties associated 

with relying on a calculated light extinction indicator and stated a preference for utilizing 

direct light extinction measurements. However, recognizing the limitations on applying 

direct measurements at present, commenters supporting the proposal to set a distinct 

standard argued that relying on “calculated light extinction is a reasonable first approach” 

(DOI, p. 2). These commenters pointed to the advice of CASAC, which had 

acknowledged that it was not possible for the EPA to develop an FRM for direct 

measurement of light extinction within the time frame of this review and had concluded 

that relying on a calculated PM2.5 light extinction indicator represented a reasonable 

 

147 Some commenters expressed concern about the omission of other contributors to 

visibility impairment from the visibility index, as discussed in the Response to Comments 

document.
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approach that could be implemented sooner than a directly measured indicator. These 

commenters generally supported the proposal to adopt a calculated PM2.5 light extinction 

indicator, at least as an interim approach.

Commenters opposed to setting a distinct standard generally argued that it was 

inappropriate to rely on a calculated light extinction indicator rather than direct 

measurements. Some of these commenters argued that the proposed calculated light 

extinction indictor is ill suited for a bright line standard because the method uses average 

humidity and a reconstructed visibility measurement calculated from PM2.5 speciation 

filter analysis, rather than measuring what is actually observed by individuals. A number 

of commenters advocated postponing setting a distinct standard until an approach based 

on direct light extinction measurements can be adopted. Many of these commenters 

stated that relying on direct light extinction measurements would enable a standard to be 

based on a shorter averaging time, either 1-hour or sub-daily (4 to 6 hours), consistent 

with the more instantaneous nature of perceptions of visual air quality and the advice of 

CASAC in this review.

The EPA generally agrees with commenters that an indicator based on directly 

measured light extinction would provide the most direct link between PM in the ambient 

air and PM-related light extinction. However, as noted at the time of proposal and in 

accordance with the advice of CASAC, the EPA has concluded that this is not an 

appropriate option in this review because a suitable specification of currently available 

equipment or performance-based verification procedures could not be developed in the 

time frame of this review. Moreover, CASAC concluded that relying on a calculated 

PM2.5 light extinction indicator based on PM2.5 chemical speciation and relative humidity
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data represented a reasonable approach. The inputs that are necessary include 

measurements that are available through existing monitoring networks and approved 

protocols. Thus, the EPA remains confident that the available evidence demonstrates that 

a strong correspondence exists between calculated PM2.5 light extinction and PM-related 

visibility impairment. Furthermore, CASAC agreed, noting that the proposed calculated 

PM2.5 light extinction indicator based on the original IMPROVE algorithm “appears to be 

a reasonable approach for estimating hourly light extinction” (Samet, 2010d, p. 11) and 

“its reliance on procedures that have already been implemented in the CSN and routinely 

collected continuous PM2.5 data suggest that it could be implemented much sooner than a 

directly measured indicator” (Samet, 2010d, p. iii). Thus it would not be appropriate to 

postpone setting a distinct secondary standard until an approach based on direct light 

extinction measurements could be adopted.

ii. Comments on Specific Aspects of Calculated Light Extinction Indicator 

Some commenters, even those supporting the adoption of a calculated light 

extinction indicator, also expressed concern over specific aspects of the proposed 

indicator. First, a number of commenters expressed concern over the proposal to use the 

original IMPROVE algorithm as the basis for the calculated light extinction indicator. 

These commenters noted that the original IMPROVE algorithm has been shown to have 

consistent biases at both low and high levels of light extinction. In particular, these 

commenters expressed concern with the algorithm’s bias at higher levels of light 

extinction, which they pointed out were the conditions that might be encountered on 

hazier days in urban areas. 

Some commenters supported use of the revised IMPROVE algorithm. These 
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commenters noted that the revised equation has been through a peer review which 

confirmed that it is based on the best science and corrects the biases inherent in the 

original algorithm. Commenters also noted that this revised algorithm has been widely 

incorporated into Regional Haze plans, and urged the EPA to use this same equation in 

the visibility index for the sake of consistency: “EPA approved this approach for regional 

haze and does not dispute its greater accuracy. Therefore, a national secondary ambient 

air quality standard based on criteria that accurately reflect the latest scientific knowledge 

logically should not revert to the original IMPROVE algorithm” (Oklahoma DEQ, p. 2). 

Other commenters noted that both the original and the revised IMPROVE algorithms 

were designed in support of the Regional Haze Program which is focused on largely rural 

Class I areas, and that neither algorithm is necessarily suitable for urban areas. Noting 

that the EPA has not thoroughly evaluated the applicability of either IMPROVE 

algorithm in urban areas, these commenters urged additional research to evaluate the 

suitability of either algorithm (or an alternative approach) in urban areas.

Second, a number of commenters argued that exclusion of coarse PM from the 

calculated light extinction indicator was inappropriate. These commenters noted that 

coarse particulate matter is an important contributor to visibility impairment in many 

areas, particularly in the western U.S., and that the levels of “acceptable” visual air 

quality derived from the visibility preference studies reflected total light extinction due to 

the full mix of particles (including coarse PM) in ambient air. A few commenters noted 

that due to the exclusion of coarse particles, a “deciview” calculated for purposes of the 

proposed PM2.5 visibility index is inconsistent with the unit as conventionally defined 

under the Regional Haze Program. Other commenters, however, supported the proposal 
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to exclude coarse PM from the calculated light extinction indicator, noting the important 

role that PM2.5 plays in urban visibility and arguing it would be more difficult to control 

the contribution of coarse particle sources such as wind-blown dust to urban visibility 

impairment. 

Third, some commenters questioned why the EPA was proposing to rely on 

monthly average relative humidity (f(RH)) values when hourly humidity data are widely 

available, particularly in urban areas. One commenter argued that the EPA’s proposed 

approach involves “guessing relative humidity” rather than relying on accurate, readily 

available measurements (Oklahoma DEQ, p. 1). The commenter stated that since relative 

humidity is highly variable and weather dependent, the proposed approach “effectively 

undermines the capacity of the prescribed monitoring regime to identify periods when 

PM2.5 adversely affects visibility.” Other commenters supported this view, noting that 

relative humidity can vary substantially even within a 24-hour period, and that light 

extinction can be very sensitive to these changes. These commenters recommended that 

hourly or daily humidity measurements should be utilized in place of the proposed 

monthly average f(RH) values. 

Some commenters also recommended that the EPA should utilize a 90 percent 

relative humidity screen rather than 95 percent cap for purposes of eliminating periods in 

which visibility impairment is due to rain or fog. These commenters claimed that under a 

95 percent cap, both the average f(RH) values and the PM2.5 visibility index values could 

be inflated in locations frequently affected by fog and/or precipitation. These commenters 

preferred the approach of excluding hours with relative humidity above 90 percent on the 

grounds that this approach would eliminate foggy/rainy hours irrespective of the 
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frequency of occurrence.

The EPA does not agree with commenters who advocated using the revised 

IMPROVE algorithm. Both the original and the revised IMPROVE algorithms have been 

evaluated by comparing the calculated estimates of light extinction with coincident 

optical measurements. As discussed above in section VI.B.1.a.i, the revised algorithm 

was developed to address observed biases in the predictions using the original algorithm 

under very low and very high light extinction conditions, with further modifications and 

additions to better account for differences in particle composition and aging in remote 

areas.

148 However, the EPA does not believe that these same modifications and additions 

would necessarily be appropriate for calculating light extinction in urban areas. Instead, 

the EPA considers the original algorithm to be suitable for purposes of calculating urban 

light-extinction, although some adjustments may be appropriate for urban environments 

as well. The reasons why the original algorithm is suited to urban environments are 

discussed further below, along with adjustments that the EPA believes are likely 

appropriate based on the current (limited) state of knowledge.

First, the EPA considers that the multiplier of 1.8 used to convert OC to OM in 

the revised IMPROVE algorithm is too high for urban environments. The EPA is aware 

that there has been considerable debate within the research community about the 

appropriate multiplier to use to best represent urban environments. As discussed in 

Appendix F of the Policy Assessment (U.S. EPA, 2011a), the EPA used the SANDWICH 

 

148 Specifically, the revised IMPROVE algorithm incorporates additional terms to 

account for particles representing the different dry extinction and water uptake (f(RH)) 

from two size modes of sulfate, nitrate and organic mass, as well as adding a term for 

hygroscopic sea salt. There are also adjustments for the calculation of OM as 1.8*OC 

compared to 1.4*OC in the original algorithm to better account for the more aged PM 

organic components found in remote areas.
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mass closure approach in the Urban Focused Visibility Assessment (U.S. EPA, 2010b)

for purposes of calculating maximum daylight hourly PM2.5 light extinction and 

evaluated which multiplier would produce 24-hour results most similar to the 

SANDWICH approach using 24-hour PM2.5 organic carbon derived from the new 

Chemical Speciation Network (CSN) carbon monitoring protocol established in 2007.

149

Analyses presented in Appendix F of the Policy Assessment indicate that a multiplier of 

1.6 is most appropriate for purposes of comparing the hourly PM2.5 light extinction with 

calculated 24-hour extinction (see Appendix F, section F.6 for a full explanation). The 

EPA also considers this higher multiplier to be a better approach for urban CSN

monitoring sites where the new measurements of organic carbon tend to be lower than 

those produced by the older NIOSH-type monitoring protocol (Malm, 2011). A multiplier 

of 1.6 is now used to calculate OM from OC measurements at CSN sites.

At the time of proposal, the EPA proposed to use the original IMPROVE 

algorithm with its 1.4 multiplier for converting OC to OM, but requested comment on 

whether this value was appropriate. Comments received by the Agency generally indicate 

that the OC-to-OM multiplier of 1.4 used in the original IMPROVE algorithm is too low

for urban areas. Based on the analyses presented in Appendix F of the Policy Assessment, 

the EPA agrees with these commenters. However, the EPA also believes that it would be 

inappropriate to use a multiplier as high as 1.8 to convert OC to OM in urban areas. As 

noted by commenters, the organic mass contribution to visibility impairment can be large, 

and generally OM is significantly larger in urban areas compared to surrounding rural 

 

149 Starting in 2007, the CSN adopted the IMPROVE monitoring protocol for the 

measurement of organic and elemental carbon using the IMPROVE analytical method 

and an IMPROVE-like sampler. The transition was completed in 2009. ( “Modification 

of Carbon Procedures in the Speciation Network,”

http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbon.pdf)
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areas.

150 Because a large portion of the organic component of urban PM results from 

nearby emissions sources, the total OM mass is generally closer to the measured OC from 

which it is derived. This means it is appropriate to use a smaller multiplier to convert OC 

to OM in urban areas as compared to the value of 1.8 used in the revised algorithm,

which is tailored to remote areas. The CASAC noted that urban OM includes fresh 

emissions and the EPA concluded in the Visibility Assessment that “the original version 

is considered more representative of urban situations when emissions are still fresh rather 

than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3-19). Although the 

revised algorithm represents the best science of estimating extinction in remote areas with 

its aged aerosol, the commenters did not address how the EPA should modify the revised

algorithm to best represent the more complex and different urban aerosol, particularly for 

OM. In light of all of these considerations, in particular the analyses the EPA conducted 

for Appendix F of the Policy Assessment and the fact that the monitoring method for 

organic carbon has recently changed in the CSN network, the EPA judges that a 

multiplier of 1.6 for urban areas would be most appropriate for purposes of calculating 

PM2.5 light extinction in urban areas.151 In formulating this judgment, the EPA recognizes 

that neither the original nor the revised IMPROVE algorithm has been tested for 

suitability in urban areas and that additional research is necessary to reduce the 

uncertainties about the most appropriate value for the OC to OM multiplier in urban 

environments. With regard to other changes between the original and revised IMPROVE 

 

150 The difference between higher PM2.5 mass in urban areas compared to surrounding 

regions, known as the urban excess, is largely attributed to organic mass (U.S. EPA 

Particle Pollution Report, 2004).

151 The implications of this shift to a 1.6 multiplier for OC in urban areas for decisions 

about averaging time, level, and need for a distinct secondary standard are discussed 

further below in sections VI.C.1.c, VI.C.1.e, and VI.C.1.f, respectively.
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algorithms, the EPA also does not believe that it would be appropriate to include a term 

for hygroscopic sea salt for urban light extinction, or to differentiate between different 

size modes of sulfate, nitrate, and organic mass as empirically defined by the revised 

IMPROVE algorithm. Unlike in some remote coastal locations, sea salt is not major 

contributor to light extinction in urban areas. Moreover, urban sources of salt include 

sanding of roads during the winter and those re-entrained particles are mostly in the 

coarse size range. 

Like in remote areas, small and large size modes of sulfate, nitrate and organic 

mass would exist in the urban environment. However, the apportionment of the total fine 

particle concentration of each of the three PM2.5 components into the concentrations of 

the small and large size fractions would likely need a different approach than that used 

for remote areas. This is because of the closer proximity of urban sources to their 

emissions. This is a particular concern not only for organic mass, which as explained 

previously has a large contribution from nearby urban emission sources, but also for 

PM2.5 nitrate whose concentrations are also higher in urban areas compared to the 

surrounding regions. Thus, a higher portion of the total urban concentration may be in the 

small mode compared to remote areas and thus a different apportionment algorithm 

would be needed.

Finally, the EPA does not consider it necessary to employ site-specific Rayleigh 

light scattering terms in place of a universal Rayleigh light scattering value for purposes 

of calculating light extinction in urban areas for purposes of calculating the 90th

percentile values. The site-specific Rayleigh value is most important to accurately 
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estimate extinction on the best visibility days which is an essential metric for the regional 

haze program.

For all of these reasons, the EPA considers the original IMPROVE algorithm 

better suited to the task of calculating urban light extinction than the revised IMPROVE 

algorithm. However, the EPA does consider it appropriate to make certain adjustments to 

the original algorithm for purposes of calculating urban light extinction. As discussed 

above, the EPA believes it is appropriate to use a 1.6 multiplier to convert OC to OM in 

urban areas. In addition, the EPA believes it is appropriate to exclude the term for coarse 

particles from the equation. The EPA does not agree with commenters who suggested 

that coarse particles should be included in the calculated light extinction indicator. As 

noted in the proposal, PM2.5 is the component of PM responsible for most of the visibility 

impairment in most urban areas. Currently available data suggest that PM10–2.5 is a minor 

contributor to visibility impairment most of the time, although at some locations (U.S. 

EPA, 2010b, Figure 3-13 for Phoenix) PM10–2.5 can be a major contributor to urban 

visibility effects. While it is reasonable to assume that other urban areas in the desert 

southwestern region of the country may have conditions similar to the conditions shown 

for Phoenix, in fact few urban areas conduct continuous PM10-2.5 monitoring. This 

significantly increases the difficulty of assessing the role of coarse particles in urban 

visibility impairment. For example, among the 15 urban areas assessed in this review, 

only four areas had collocated continuous PM10 data allowing calculation of hourly PM10-

2.5 data for 2005 to 2007. In addition, PM10-2.5 is generally less homogenous in urban 

areas than PM2.5 in that coarse particle concentrations exhibit greater temporal variability 

and a steeper gradient across urban areas than fine particles (U.S. EPA, 2009a, p. 3-72). 
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This makes it more challenging to select sites that would adequately represent urban 

visibility conditions. Thus, while it would be possible to include a PM10–2.5 light 

extinction term in a calculated light extinction indicator, as was done in the Visibility 

Assessment, there is insufficient information available at this time to assess the impact 

and effectiveness of such a refinement in providing public welfare protection in areas 

across the country (U.S. EPA, 2011a, pp. 4-41 to 4-42). Therefore, the EPA concludes 

that it is not appropriate to set a standard based on a calculated light extinction indicator 

that includes coarse particles at this time, and the calculated indicator should be based on 

PM2.5 light extinction.

With regard to the suggestion by some commenters that the calculated light 

extinction indicator should be calculated using hourly humidity data, the EPA disagrees 

that concurrent humidity measurements should be used. The use of longer-term averages 

for each monitoring site adequately captures the seasonal variability of relative humidity 

and its effects of visibility impairment, and this approach focuses more on the underlying 

aerosol contributions to visibility impairment and less on the day-to-day variations in 

humidity. This provides a more stable indicator for comparison to the NAAQS and one 

that is more directly related to the underlying emissions that contribute to visibility 

impairment. 

With regard to the comments advocating the use of a 90 percent humidity screen 

as opposed to a 95 percent humidity cap, the EPA believes that relying on monthly 

average relative humidity values based on 10 years of climatological data appropriately 

reduces the effect of fog and precipitation. Although the approach of using a 95 percent 

humidity cap, as in the Regional Haze Program, includes some hours with relative 
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humidity between 90-95 percent, the general approach of using a longer-term average for 

each monitoring site effectively eliminates the effect of very high humidity conditions on 

visibility at those locations.

Therefore, taking all of the above considerations and CASAC advice into account, 

the EPA continues to conclude that a calculated PM2.5 light extinction indicator, similar 

to that used in the Regional Haze Program (i.e., using an IMPROVE algorithm as 

translated into the deciview scale), would be the most appropriate indicator to replace the 

current PM2.5 mass indicator for a distinct secondary standard. Moreover, the EPA 

continues to conclude that this calculated indicator should based on the original 

IMPROVE algorithm, adjusted to use a 1.6 OC multiplier and exclude the term for coarse 

particles, in conjunction with monthly average relative humidity data (i.e., f(RH) values) 

based on long-term climatological means as used in the Regional Haze Program. A PM2.5

visibility index defined in this way would appropriately reflect the relationship between 

ambient PM and PM-related light extinction, based on the analyses discussed in the 

proposal and reflecting the aerosol and relative humidity contributions to visibility 

impairment by incorporation of factors based on measured PM2.5 speciation 

concentrations and climatological average relative humidity data. In addition, this type of 

indicator would address, in part, the issues raised in the court’s remand of the 2006 PM2.5

standards. Such a PM2.5 visibility index would afford a relatively high degree of 

uniformity of visual air quality protection in areas across the country by virtue of directly 

incorporating the effects of differences in PM2.5 composition and relative humidity across 

the country. 

c. Averaging time
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Few commenters specifically addressed the issue of averaging time. Those who 

did generally expressed the view that an hourly or sub-daily averaging time would be the 

most appropriate approach, as supported by CASAC and the EPA’s own analyses in this 

review. These comments were generally consistent with the emphasis among all 

commenters on the desirability of adopting a directly measured light extinction indicator 

that could be measured on an hourly or sub-daily time scale. Some commenters noted 

that a standard based on a 4-6 hour averaging time would better capture peak daily light 

extinction while allowing stable signal quality; others urged EPA to adopt a 1-hour 

averaging time in conjunction with direct measurements. Commenters pointed to 

significant limitations associated with using a 24-hour averaging time, including the 

uncertainties in translating hourly or sub-daily visibility index values into 24-hour 

equivalent values. Some commenters criticized the analysis presented in the Policy 

Assessment comparing the 24-hour calculated light extinction values to the maximum 

daylight 4-hour calculated light extinction values. These commenters stated that the 

scatter plots and regressions presented in the Policy Assessment indicate there is 

considerable variation in the 24-hour vs. 4-hour relationship, and interpreted this to mean 

that 24-hour light extinction values are a poor surrogate for 4-hour values. For example, 

several industry commenters cited an analysis which noted that the correlation coefficient 

between the 24-hour and 4-hour values was as low as r2= 0.42 in Houston, and stated 

that the EPA was being overly “optimistic” in concluding that city-specific and pooled r2

values in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2, p. 27).

In addition, some commenters expressed concern over potential bias and greater 

uncertainty introduced by the inclusion of nighttime hours, noting that because relative 
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humidity tends to be higher at night, inclusion of these hours could cause areas to “record 

NAAQS exceedances that have no corresponding visibility impairment value” (UARG, p. 

36). Commenters also emphasized the poor fit of a 24-hour averaging time with the near 

instantaneous judgments about visibility impairment reflected in the visibility preference 

studies. Commenters also noted that there is greater hourly variation in PM 

concentrations and resulting visibility conditions in urban areas than in Class I areas; 

thus, while the Regional Haze Program uses 24-hour IMPROVE data, the commenters 

stated that a shorter averaging time is needed for an urban-focused PM2.5 visibility 

standard. Some commenters objected to a 24-hour averaging time as unsupported by the 

record in this review: “Because the science the Administrator relies on for the other 

elements of the proposed visibility standard is tied to short-term exposures to visibility 

impairment, the EPA has no basis for promulgating a standard that uses a 24-hour 

averaging time” (API, p. 43). These commenters claimed that while the EPA may not 

have the information or infrastructure in place to allow the Agency to set a standard 

based on a 1-hour or other sub-daily averaging time, this does not justify moving to a 24-

hour averaging time. 

Among commenters supporting the proposed distinct secondary standard for 

visibility, many commenters recognized the limitations on monitoring methods and 

currently available data that led to the EPA’s proposal to adopt a standard based on a 24-

hour averaging time. Most of these commenters acknowledged that the lack of reliable 

hourly speciation data means that a 24-hour averaging time is the only workable 

approach for a standard based on calculated light extinction. Commenters advocating a 

distinct secondary standard for visibility therefore generally supported the proposal to 
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adopt a 24-hour averaging time, at least as an interim approach until a directly measured 

light extinction indicator could be adopted in the future. This approach was also 

supported by a few industry commenters who noted that since a visibility index standard 

would be based on data from the IMPROVE and CSN monitors, which operate on a 24-

hour basis with 1-in-3 (or 1-in-6) day sampling, “it is imperative that EPA retain a 24-

hour averaging time if a secondary visibility standard is promulgated” (API, Attachment 

2, p. 9).

In response to comments supporting a 1-hour or sub-daily (4- to 6- hour) 

averaging time in conjunction with a direct light extinction measurements, the EPA notes 

that, as discussed above in the response to comments on indicator, the Agency has 

concluded that a directly measured light extinction indicator is not an appropriate option 

in this review, independent of the decision on averaging time. Having reached the 

conclusion that a calculated PM2.5 light extinction indicator would be most appropriate, 

the EPA has next considered what averaging time would be most desirable for such an 

indicator. As noted in the proposal, the EPA has recognized that hourly or sub-daily (4-

to 6-hour) averaging times, within daylight hours and excluding hours with high relative 

humidity, are more directly related than a 24-hour averaging time to the short-term nature 

of the perception of PM-related visibility impairment and the relevant exposure periods 

for segments of the viewing public. Thus, the Agency agrees with commenters’ general 

point that, as a starting premise, a sub-daily averaging time would generally be 

preferable. 

However, as noted at the time of proposal and discussed above in section 

VI.B.1.c, important data quality uncertainties have recently been identified in association 
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with currently available instruments that would be used to provide the hourly PM2.5 mass 

measurements that would be needed in conjunction with an averaging time shorter than 

24 hours. As a result, at this time the Agency has strong technical reservations about a 

secondary standard that would be defined in terms of a sub-daily averaging time. The 

data quality issues which have been identified, including short-term variability in hourly 

data from currently available continuous monitoring methods, effectively preclude 

adoption of a 1-hour averaging time in this review, given the sensitivity of a 1-hour 

averaging time to these data quality limitations. Even with regard to multi-hour averaging 

times, the EPA continues to conclude that the data quality concerns preclude adoption of 

a sub-daily averaging time.

Moreover, analyses conducted for the Policy Assessment indicate that PM2.5 light 

extinction calculated on a 24-hour average basis would be a reasonable and appropriate 

surrogate for PM2.5 light extinction calculated on a 4-hour basis. The scatter plots 

comparing 24-hour and 4-hour calculated PM2.5 light extinction in the Policy Assessment 

(U.S. EPA, 2011a, Figures G-4 and G-5) do show some scatter around the regression line 

for each city. This was to be expected, since the calculated 4-hour light extinction 

includes day-specific and hour-specific influences that are not captured by the simpler 

24-hour approach. Overall, however, in the EPA’s view, both the city-specific and pooled 

15-city 24-hour vs. 4-hour comparisons show strong correlation between the two 

averaging times. Moreover, the 90th percentile design values calculated for 4-hour vs. 

24-hour light extinction are much more closely correlated than are the values for 

individual days in particular urban areas calculated using these two approaches. Thus, 

while the EPA agrees with commenters who pointed out the relatively low correlation 
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between 4- and 24-hour values in cities such as Houston, the Agency points out that the 

correlations of 90th percentile values are much higher, particularly when one considers 

the average values across urban areas. In general, the 90th percentile values line up better 

and demonstrate closer to a one-to-one relationship. 

The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships 

between estimated 24-hour and 4-hour visibility impairment based on the variety of 

metrics discussed in Appendix G of the Policy Assessment that further supports this 

finding. The reanalysis more appropriately considered the uncertainty of the calculated 4-

hour values. It also considered the effect of changing the OC to OM multiplier used in 

urban areas with the new CSN monitoring protocol from 1.4 to 1.6. The revised analysis 

shows that the 24-hour values are generally closer to the 4-hour values than originally 

estimated. 

Since conclusions in the proposal about the relationship between 4-hour and 24-

hour values were drawn not just on the basis of the city-specific results but also on the 

more robust 90th percentile values, the EPA disagrees with commenters who state that the 

Agency was overly optimistic in considering 24-hour values an appropriate surrogate for 

4-hour values. Also, it is appropriate to focus on the 90th percentile design value 

comparison since the design values would determine attainment status and the degree of 

improvement in air quality that could be expected in areas instituting controls to meet the 

NAAQS. Therefore the EPA does not agree with commenters who state that a 24-hour 

averaging time cannot serve as an appropriate surrogate for sub-daily periods of visibility 

impairment. On the contrary, the EPA continues to conclude, on the basis of this analysis, 

that PM2.5 light extinction calculated on a 24-hour basis is a reasonable and appropriate 
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surrogate for sub-daily PM2.5 light extinction calculated on a 4-hour basis.

The EPA recognizes that the effect of adopting a 24-hour averaging time may be 

to smooth out some of the hour-by-hour variability in visibility index values. (Indeed, this 

is true if we compare a 4-hour averaging time to a 1-hour averaging time as well.) Hourspecific influences which would be evident if an hourly or sub-daily averaging time were 

to be used will be masked to some extent when those hours are averaged together with 

other hours. This means, in part, that a 24-hour averaging time may effectively reduce 

peak values by means of averaging them together with other hours, which may have 

lower values. However, given the well documented variability in hourly visibility 

conditions, especially in urban areas, as noted by commenters, it is reasonable to assume 

that in some cases peak hours may be significantly influenced by atypical conditions, 

making it appropriate to adopt an averaging time that is sufficiently long to ensure that 

hour-specific influences are balanced against more typical conditions. Perhaps even more 

important is the concern that many peak hourly measurements may be significantly 

influenced by atypical instrument performance; this reinforces the conclusion that it is 

appropriate to adopt a longer averaging time, to ensure that hour-specific uncertainties 

are balanced against more robust measurements.

Thus, in agreement with commenters who supported a daily averaging time, the 

EPA concludes that a 24-hour averaging time would be appropriate for a distinct 

secondary standard based on a calculated PM2.5 light extinction indicator. 

d. Form

The EPA received very few comments with regard to the proposal to adopt a 90th

percentile form, averaged over 3-years, in conjunction with a PM2.5 visibility index and a 
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24-hour averaging time. One commenter stated that it was inappropriate to use a 90th

percentile form, noting that this would result in the exclusion of a minimum of 36 days of 

data annually. The commenter expressed particular concern that this proposed approach, 

in combination with a 24-hour standard based on an unadjusted CPL, would not capture 

the worst visibility impairment and that this would undermine “the intent of setting a 

meaningful secondary visibility standard” (AMC, et al., p. 2). Another commenter argued 

that the EPA had provided no scientific basis for why the 90th percentile form was 

suitable, and claimed that the Agency was making “a somewhat arbitrary judgment that 

people’s welfare would be affected only if adverse urban visibility were to occur more 

than 10 percent of the time” (API, Attachment 2, p. 4). 

On other hand, a few commenters who appeared to generally support the proposal 

to use a 90th percentile form advocated averaging the 90th percentile values over longer 

time periods, arguing that averaging over only 3 years would not provide a stable 

assessment of visual air quality in the West because this time period is insufficient to 

properly account for western drought and fire cycles. These commenters pointed to the 

approach in the Regional Haze Program of averaging visibility impairment over 5 years, 

and noted that even within this longer time period data can be significantly influenced by 

high emissions during significant fire years.

The EPA disagrees with all of these comments. With regard to the comment 

opposing the 90th percentile form as inappropriately excluding the worst visibility days, 

the EPA notes that there is a significant lack of information on, and a high degree of 

uncertainty regarding, the impact on public welfare of the number of days with visibility 

impairment over the course of a year. For example, the visibility preference studies used 
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to derive the range of CPLs considered in this review offered no information regarding 

the frequency of time that visibility levels should be below those values. Based on this 

limitation, the EPA concluded in the Policy Assessment that it would not be appropriate 

to consider eliminating all exposures above the level of the standard and that it was 

reasonable to consider allowing some number of days with reduced visibility. 

Recognizing that the Regional Haze Program focuses attention on the 20 percent worst 

visibility days (i.e., those at or above the 80th percentile of visibility impairment), the 

EPA continues to believe, as noted in the proposal, that a percentile well above the 80th

percentile would be appropriate to increase the likelihood that all days in this range 

would be improved by control strategies intended to help areas attain the standard. 

Focusing on the 90th percentile, which represents the median of the distribution of the 20 

percent worst visibility days, could be reasonably expected to lead to improvements in 

visual air quality on the 20 percent most impaired days. Thus, the EPA has made a 

reasoned judgment based on a full consideration of the upper end of the distribution of 

visibility impairment conditions and continues to conclude that it is appropriate to focus 

on the 90th percentile of visibility impairment values.

With regard to comments requesting the EPA adopt a longer multi-year averaging 

period for the 90th percentile values, the EPA disagrees that it would be appropriate to 

average the 90th percentile values over periods longer than 3 years. The EPA recognizes 

that a multi-year percentile form offers greater stability to the air quality management 

process by reducing the possibility that statistically unusual indicator values will lead to 

transient violations of the standard. Utilizing a 3-year average form provides stability 

from the occasional effects of inter-annual meteorological variability that can result in 
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unusually high pollution levels for a particular year. The Agency has adopted this

approach in other NAAQS, including the current secondary 24-hour PM2.5 NAAQS, 

which has a 98th percentile form averaged over 3 years. However, adopting a multi-year 

averaging period longer than 3 years would increase the number of days with visibility 

impairment above the target level of protection and would therefore reduce the 

protectiveness of the standard. Based on this the EPA does not believe it would be 

appropriate to average 90th percentile values over a period as long as five years. 

Therefore, the EPA continues to conclude that a 90th percentile form, averaged over 3

years, would be appropriate, in conjunction with a calculated PM2.5 light extinction 

indicator and a 24-hour averaging time. 

e. Level

With regard to level, commenters focused on two main themes. First, a large 

number of commenters addressed the information available from the public preference 

studies with regard to the acceptability of various levels of visual air quality. These 

comments, which are discussed in subsection VI.C.1.e.i below, address the EPA’s use of 

visibility preference studies as the basis for the selection of a range of appropriate levels 

for the Administrator to consider. Many commenters challenged the use of these studies 

as the basis for setting a distinct secondary standard, arguing that limitations in these 

studies rendered them an unsuitable and insufficient basis on which to establish such a 

standard. Second, commenters expressed different views as to what level(s) of a distinct 

secondary standard would be appropriate, if the EPA were to set such a standard. These 

comments reflected consideration of the results of the public preference studies as well as 

analyses conducted in the Visibility Assessment and the Policy Assessment, as discussed 
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in the proposal. Comments addressing the appropriateness of specific levels are discussed 

in subsection VI.C.1.e.ii below. 

i. Comments on Visibility Preference Studies 

A majority of commenters expressed the view that the existing preference studies 

provide an insufficient basis for selection by the Administrator of an appropriate level of 

public welfare visibility protection for a national standard. These commenters highlighted 

a number of limitations and uncertainties (enumerated below) associated with these 

studies as support for this view. In contrast, other commenters felt that despite certain 

limitations, these studies do provide a sufficient basis on which the Administrator can 

select an appropriate level of a standard to provide national public welfare visibility 

protection. The remainder of this section organizes and discusses these comments under 

four broad topic areas, including: (a) limitations and uncertainties associated with the 

visibility preference studies; (b) preference study methods and design; (c) use of 

preference study results for determining adversity; (d) the appropriateness of using 

regionally varying preference study results to select a single level for a national standard. 

(a) Preference Study Limitations and Uncertainties

A large and diverse number of limitations and uncertainties associated with the 

visibility preference studies have been identified and discussed in the public comments. 

Many of these same limitations and uncertainties were also identified and discussed by 

the EPA in the various documents developed throughout this review. The most important 

and fundamental limitations and uncertainties will be discussed here in the preamble, 

while more specific, unique or detailed comments will be addressed in the Response to 

Comments document. 

The primary or most frequent limitation cited by many commenters relates to the 
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small number of preference studies that are available in this review. In particular, some 

commenters note that these preference studies cover just four locations, only three of 

which occur in the U.S., that the two studies conducted in Washington, DC were pilot

studies, not full preference studies, and/or that three of the preference studies were 

conducted in the West, while only one was conducted in the East, providing only limited 

geographic coverage. Typically, these same commenters also pointed out that taken 

together, these limited studies only included a total of 852 participants, which they 

claimed was too small a sample size and unrepresentative nationally. These commenters 

thus concluded that there is insufficient information, both geographically and 

demographically, upon which to select a national level of a visibility index for purposes 

of visibility protection. 

In contrast, several commenters stated support for using the preference studies, 

concluding they provide an adequate basis, in spite of their limited nature. In particular, 

AMC et al. state:

We believe that these studies provide sufficient results to inform setting a national 

visibility standard. While the number of studies is small, they do incorporate 

spatial variation and, in the case of Denver and Phoenix, varied populations.... 

EPA should have confidence, rather than uncertainty, in the fact that these studies 

used different methods and respondents and yield a range of 20 – 24 dv, with one 

outlier of 29. (AMC, et al., pp. 6-7)

Regarding the first group of commenters, the EPA notes that it is well aware of 

the limited nature of the information, which it has described in great detail in the 

Integrated Science Assessment, Visibility Assessment, and Policy Assessment, as well as 

in section VI.B.2 of the proposed rule (77 FR 38973). The EPA further notes, however, 

that limited information does not preclude the Administrator from making judgments 

based on the best available science, taking into account the existing uncertainties and 
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limitations associated with that available science. Thus, in reaching judgments based on 

the science, the Administrator appropriately weighs the associated uncertainties. The 

CASAC supported this view and concluded that the available information provided a 

sufficient basis on which the Administrator could form a judgment about requisite PMrelated public welfare visibility protection. Specifically, CASAC stated “[t]he 20-30 

deciview range of levels chosen by EPA staff as ‘Candidate Protection Levels’ is 

adequately supported by the evidence presented” (Samet, 2010b, p. iii). As discussed in 

the proposed rule (77 FR 38990), the Administrator recognized and explicitly took into 

account the uncertainties and limitations in the science in determining an appropriate 

degree of protection when she proposed a level at the upper end of the recommended 

range. As discussed below, the Administrator continues to be mindful of these 

uncertainties and limitations in reaching her final determination regarding what 

constitutes an appropriate degree of protection with respect to visibility impairment. 

With respect to the comments of AMC et al., the EPA agrees that these studies 

provide a sufficient basis to inform the Administrator’s judgments regarding an 

appropriate level of protection from PM-related visibility impairment, but she recognizes 

that these studies, which are the only studies before her, are a limited source of 

information. However, the EPA does not agree that the Washington, DC, results represent 

an outlier and thus the EPA believes that these results are appropriately included in the 

range identified for the Administrator to consider. 

Some commenters made the point that the EPA relied on much of this same 

evidence to reach the conclusion in 2006 that the information was too limited to allow 

selection of a national standard. For example, API stated: 
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[T]he bulk of the VAQ preference studies were available during the previous PM 

NAAQS review and were considered by the Agency in its establishment of the 

2006 PM secondary NAAQS.... The Proposed Rule does not mention this fact 

and does not explain why many of these same studies now compel EPA to 

propose this new secondary NAAQS....The Proposed Rule notes in passing that, 

since the last review of the PM NAAQS, ‘limited information that has become 

available regarding the characterization of public preferences in urban areas has 

provided some new perspectives on the usefulness of this information in 

informing the selection of target levels of urban visibility protection.’ 77 Fed. 

Reg. at 38969/2. It is a serious oversight that the Proposed Rule makes no attempt 

to explain what that information is or how it affects the interpretation of the VAQ 

preference studies. This ‘limited information’ is an apparent reference to 

information provided by Dr. Anne Smith. (API, p. 37)

The EPA disagrees with these commenters. First, the EPA disagrees that it failed 

to distinguish between studies that were available in the previous review and the current 

review. The discussion in section VI.A.1 of the proposal specifically identifies the studies 

from Denver, Phoenix and British Columbia (77 FR 38967/2) as being considered in the 

last review. The EPA further disagrees with the implication that it is being circumspect 

about identifying the “limited information that has become available regarding the 

characterization of public preferences in urban areas.” Beginning in section VI.A.3 of 

the proposed rule (77 FR 38969), the EPA was clear about what information, both 

preexisting and new, it relied upon in this review to inform its views and provide the 

basis for its proposal. In section VI.B.2, the EPA elaborates on the specific information, 

tools, methods and data which are considered in relation to the public preference studies, 

including the new information available since the last review. 

As noted above and in the proposal, in addition to the substantial PM urban air 

quality information and analyses new to this review, there are three other sources of 

information that have specifically “provided some new perspectives on the usefulness of” 

the preference studies “in informing the selection of target levels of urban visibility 
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protection” (77 FR 38969). They include: (1) results from additional urban visibility 

preference study experiments conducted for Washington, DC by Smith and Howell 

(2009) which added to the preference data for that location and shed light on the role of

location in preference responses152; (2) a review and reanalysis of the urban visibility 

public preference studies from the four urban areas, including the newly available Smith 

and Howell (2009) experiments which examined the similarities and differences between 

the studies and evaluated the potential significance of those differences on the study 

results153; and (3) additional analyses, including most importantly a logit analysis 

(discussed in Chapter 2 and Appendix J of the Visibility Assessment), which was

requested and reviewed by CASAC, which showed that each city’s responses represented 

unique and statistically different curves. 154 Taken together, these sources contributed to 

the EPA’s current knowledge and understanding of each survey study’s results, the 

appropriateness of comparing each study’s results to the others, and the key uncertainties 

relevant to data interpretation. In addition, in the last review the decision to not adopt a 

distinct secondary standard was remanded as contrary to law and failing to provide a 

reasoned explanation for the decision. As such it is not appropriate for purposes of 

comparison with the Administrator's judgment and reasoning in this review. 

(b) Preference Study Methods and Design 

In addition to the limitations and uncertainties noted above, many comments also 

 

152 Smith, AE; Howell S (2009). An assessment of the robustness of visual air quality 

preference study results. CRA International. Washington, DC. 

http://yosemite.epa.gov/sab/sabproduct.nsf/B55911DF9796E5E385257592006FB737/$F

ile/CRA+VAQ+Pref+Robustness+Study+3+30+09+final.pdf.

153 Stratus Consulting, 2009. Review of Urban Visibility Public Preference Studies: Final 

Report. EPA-HQ-OAR-2007-0492-0042.

154 Deck L., Lawson M. 2010. Statistical Analysis of Existing Urban Visibility Preference 

Studies. EPA-HQ-OAR-2007-0492-0089.
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asserted the methodologies used in the preference studies are fundamentally flawed. 

Many commenters cited some of the same issues that have already been identified by the 

EPA as sources of uncertainty and potential factors in producing the statistically different 

study results (see section VI.B.1.b above). As noted above, the EPA is well aware of the 

issues raised regarding the adequacy of the preference studies to serve as a basis for a 

secondary NAAQS (see 77 FR 38975) and solicited comment on how these uncertainties 

should be considered (see 77 FR 38990). Most of these same commenters also pointed to 

an assessment of the preference studies methodology provided by Smith and Howell 

(2009) as the basis for their views, as indicated by the following comments:

Smith and Howell (2009) show that VAQ preference study outcomes are 

malleable and depend entirely on the design of the study. Accordingly, such 

studies do not identify any meaningful threshold of acceptable visibility 

conditions. Despite Smith and Howell’s conclusions, EPA continues to assert that 

the VAQ preference studies can be used to identify minimally acceptable 

visibility conditions even though the Agency has never provided any valid 

scientific basis for discounting the Smith and Howell (2009) results. (API, p. 38)

Well-controlled preference studies discussed by Anne Smith of Charles River 

Associates at the March 2010 CASAC meeting demonstrated that the judgment of 

panel members was affected by the order in which photographs were presented 

and tendency to identify the middle of the range of visibility degredation as a 

threshold of acceptability. This points to a potential flaw in these studies and that 

artifacts caused by these tendencies may have influenced study results. 

Dismissing these inherent flaws in the existing preference studies and then using 

these studies to set a secondary NAAQS is arbitrary and capricious. (API, 

Attachment 2, p. 12) 

EPA also fails to acknowledge that the only study conducted since the last review 

rebuts the validity of the VAQ preference studies previously conducted. (UARG, 

Attachment 2, p. 28)

As is explained in a more detailed discussion in the Response to Comments 

document, the EPA disagrees that the study conducted by Smith and Howell (2009) 

supports the conclusion that the preference study methodologies were fundamentally 
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flawed; however, the EPA notes that their experiments do identify areas where additional 

research would be useful to further inform our limited understanding of public 

preferences in urban areas. The EPA views the Smith and Howell experiments as 

increasing the EPA’s knowledge and understanding of the findings of the 2001 

Washington, DC focus group pilot study (Abt, 2001) in several important ways, although 

this information still remains limited overall. Specifically, the Smith and Howell results 

suggest: 1) the 2001 s results, while based on a small sample size of 9, were consistent 

with results from a larger sample of the general Washington, DC population; 2) an 

individual’s preferences for visibility in one location may not depend on whether they 

live in that location; and 3) presentation method (i.e., changing from slide projection to 

computer monitor) did not appear to affect the reported preferences. 

(c) Preference Study Results and Adversity

A number of comments were received regarding the EPA’s use of preference 

study results to make the determination that adverse PM2.5-related visibility effects on the 

public welfare are occurring. In this context, several commenters questioned whether the 

EPA had made the case that unacceptable levels of visual air quality based on preference 

study results alone can be equated with an adverse public welfare effect. These 

commenters suggested that unless preference study information is linked to personal 

comfort and well-being or other associated welfare effects, it cannot form the basis of a 

determination of adversity. For example, Kennecott Utah Copper LLC stated that: 

Thus, EPA seemingly was building the foundation for a determination of what 

constitutes an adverse effect on visibility in the context of public welfare. 

However ... EPA subsequently veered toward an oversimplified focus on public 

acceptance of visibility conditions.... EPA’s discussion of visibility in the Policy 

Assessment and its proposed rule in the Federal Register focuses entirely on 

“acceptable” and “unacceptable” visual air quality and make no mention of an 

“adverse effect” in the context of visibility. EPA’s reliance on only 3 urban 
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preference studies represents a paucity of data and a wholesale abandonment of 

any effort to seek a scientifically measurable adverse effect. (Kennecott Utah 

Copper LLC, p. 26)

In response, the EPA first notes that the definition of effects on welfare included 

in section 302(h) of the CAA identifies both visibility and the broader category of effects 

on personal comfort and well-being as effects on welfare. In setting a secondary standard 

to address visibility impairment, the EPA considers the effect on the public from 

impairment of visibility as a separate and distinct welfare effect in its own right. The EPA 

is not required to translate this into terms of personal comfort and well-being, as visibility 

impairment is designated explicitly by Congress as an effect on welfare. While there may 

be a large degree of overlap among these different welfare effects, the EPA properly 

focuses on evaluating all of the information before the Agency on the effect visibility 

impairment has on the public, whether or not this impairment would also be categorized 

as having an adverse effect on personal comfort and well-being. It is in the context of all 

of this information that the EPA makes the judgment as to the appropriate degree of 

protection from known and anticipated adverse effects on the public from visibility 

impairment. The EPA recognizes that there is uncertainty about the degree of adversity to 

the public welfare associated with PM-related visibility impairment. However a 

secondary standard is designed to provide protection from "known or anticipated" 

adverse effects, and a bright line determination of adversity is not required in judging the 

requisite degree of protection under section 109(b)(2).Furthermore, the EPA disagrees 

that it has abandoned its consideration of visibility-related impacts on the welfare effect 

of personal comfort and well-being, as is made clear in the following quote:

Research has demonstrated that people are emotionally affected by low visual air 

quality, that perception of pollution is correlated with stress, annoyance, and 

symptoms of depression, and that visual air quality is deeply intertwined with a 
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‘‘sense of place,’’ affecting people’s sense of the desirability of a neighborhood 

(U.S. EPA, 2009a, section 9.2.4). Though it is not known to what extent these 

emotional effects are linked to different periods of exposure to poor visual air 

quality, providing additional protection against short-term exposures to levels of 

visual air quality considered unacceptable by subjects in the context of the 

preference studies would be expected to provide some degree of protection 

against the risk of loss in the public’s ‘‘sense of well-being.’’ (77 FR 38973/1, 

emphasis added)

The approach taken to address such qualitative, but policy-relevant, information 

in this review is the same as in other NAAQS reviews. The review is initiated with a 

comprehensive assessment of all possible public health and welfare effects associated 

with PM in the Integrated Science Assessment. Then policy-relevant effects for which 

there is sufficient quantitative information to allow a determination of the change in risks 

associated with incremental changes in air quality are assessed (in this review, in the 

Visibility Assessment) and used to provide a quantitative basis to inform the selection of 

an appropriate range of levels for further consideration in the Policy Assessment. In the 

Policy Assessment, the EPA considers all important policy-relevant evidence and 

information, both quantitative and qualitative, in making recommendations regarding the 

range of policy options appropriate for the Administrator to consider. It is in the context 

of all of this information that the Administrator makes her final judgment as to the 

appropriate degree of protection from known and anticipated adverse effects on the 

public from visibility impairment.

Another issue raised in the comments regarding adversity is the EPA’s decision to 

use the 50 percent acceptability criterion from the public preference studies in 

determining candidate protection levels of visibility impairment for the selection of a 

national level of visibility protection. For example, AMC et al. recommended “a 75% 

acceptability criterion as a target that is in line with protecting the broader public from 
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the negative effects of visibility impairment” (AMC, et al., p. 9).

In the Visibility Assessment, the EPA noted that the use of the 50 percent 

acceptance level for urban visibility was first presented in Ely et al. (1991) (U.S. EPA, 

2010b, p. 2-5). Ely discussed the use of the 50 percent acceptability criterion as a 

reasonable basis for setting an urban visibility standard.

The standard was determined based on a 50% acceptability criterion, that is, the 

standard was set at the level of extinction that would divide the slides into two 

groups: those judged acceptable and those judged unacceptable by a majority of 

the people in the study. The criterion is politically reasonable because it defines 

the point where a majority of the study participants begin to judge slides as 

representing unacceptable visibility. It is also consistent with psychological 

scaling theory which indicates that a “true score” exceeds a standard when more 

than 50% of the “observed scores” exceed that standard. (Ely et al., 1991, p. 11)

As Ely described, the 50 percent acceptability criterion and the preference study 

conducted by Ely were used as the basis for setting the level of the Denver Visibility 

Standard in 1990. That same criterion was judged appropriate and selected for use in the 

Phoenix preference study (BBC research, 2003) and as the basis for setting the level of 

the Phoenix Visibility Standard in 2003. Most recently, the 50 percent acceptability 

criterion has been recommended by the British Columbia Visibility Coordinating 

Committee as the basis for the visibility standard currently under consideration by British 

Columbia, Canada. Furthermore, CASAC supported this approach, while recognizing the 

uncertainty associated with this issue. Specifically, CASAC agreed that “the 50th percentile 

for the acceptability criteria is logical, given the noted similarities in methodologies 

employed in the 4 study areas. ... In terms of choosing a specific percentile from the 

preference studies, we note that there may not be a “preferred” one, but in assessing 

preference studies to propose a PM secondary NAAQS, the 50th percentile is sufficient, as it 

is the basis for existing visibility indexes used in the Denver/Colorado Front Range and 
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Phoenix metropolitan areas” (Samet, 2009c, pp. 8-9). Therefore, after considering the 

information that served as the original basis for its selection as described in Ely et al., 

1991, and given its acceptance and use in existing visibility programs, the EPA continues 

to conclude, consistent with the advice of CASAC, that it is reasonable to use the 50 

percent acceptability criterion in determining target levels of protection from visibility 

impairment. 

(d) Appropriateness of using regionally varying preference study results to select 

a single level for a national standard. 

A number of commenters raised concerns regarding the bases for and implications 

of the differences observed in the preference study results, concluding that these results 

were due to regionally varying factors and thus could not be used to set a national 

standard. For example, some commenters asserted that because the confidence intervals 

around the four 50 percent acceptability levels do not overlap at all, and because there are 

variations in preference study designs and inherent differences in the visual setting 

among cities and panels, the four preference curves and their associated 50 percent dv 

values are city-specific and statistically different. The commenters concluded, therefore, 

that it was inappropriate to aggregate the 50th percentile dv values from multiple studies 

and that they should instead be evaluated individually. 

Other commenters expressed the related view that the preference study results 

cannot be used to set a national standard for visibility impairment because the results 

show that visibility preferences vary regionally. For example, API stated that: 

The ‘one-size-fits-all’ approach...is not viable because it does not account for 

regional and city-specific factors that have been made evident in the disparity of 

preference study data.... It is well known, for example, that the level of light 

extinction to which people in different areas of the country are accustomed, as 

well as the urban setting, are the primary factors that affect a person’s visual 

perception of an urban vista. Thus, the degree to which extinction threshold can 
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be related to human welfare is inevitably regionally-dependent. (API, Attachment 

2, p. 4)

Some of these commenters argued that because acceptable visual air quality is regionally 

dependent, it would be more appropriate to develop distinct visibility standards at the 

state or local level. Others pointed out that areas which lack “important visibility vistas” 

might not need standards at all, since flat areas without significant terrain have a limited 

maximum visual range (NEDA/CAP, p. 3).

Other commenters stated that due to regionally varying factors, such as relative 

humidity, it is not possible to select a single level for a national standard to protect 

visibility across the United States. In particular, these commenters pointed to differences 

between Eastern and Western areas, arguing that a single national standard could not 

offer the appropriate degree of protection in locations with distinct characteristics. For 

example:

[T]he proposed method falls short because it is not temporally or geographically 

representative enough to have any meaning.... The uncertainty evidenced in these 

studies and the non-uniformity between the western and eastern vistas makes it 

impossible at this time to set an acceptable light extinction value that would 

appropriately address visibility concerns in non-Class I areas. (New York 

DOH/DEC, pp. 5-6)

The EPA agrees that the preference curves and the 50 percent dv levels are 

separate and distinct data points representing four different VAQ preference curves for 

four unique urban scenes. However, the EPA does not consider the fact that the four 

curves are distinct as a weakness of the approach or a reason that the results cannot be 

compared. In addition, the EPA does not agree that the study results necessarily support a 

conclusion that preferences are regionally dependent. In particular, the EPA notes that the 

results of Smith and Howell (2009) which show that participants in Houston and 

Washington, DC did not have significantly different views on acceptable air quality in 
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Washington, DC, provide limited support for the conclusion that people’s preferences 

differ less because of where they live and more because of the scene they are viewing. 

On the other hand, the existing literature indicates that people’s preferences for 

VAQ depend in large part on the characteristics and sensitivity of the scene being 

viewed. The EPA understands there is a wide variety or range of urban scenes within the 

United States. These sensitive urban scenes include those with natural vistas such as the 

Colorado Rocky Mountains as well as those with iconic man-made urban structures like 

the Washington Monument. The EPA believes that the scenes presented in the four urban 

areas include important types of sensitive valued urban scenes and therefore, when 

considered together, can inform the selection of a level of acceptable urban VAQ at the 

national scale, taking into account the variation across the country evidenced in the 

studies. This is discussed further in the next section, below.

The EPA does agree with commenters that there are regionally varying factors 

that are important to take into account when setting a national standard for visibility 

protection. Section VI.A above regarding the history of the secondary PM NAAQS 

review discusses the evolution of the EPA’s understanding regarding the regional 

differences in PM concentrations, relative humidity and other factors. As a result, the 

current review has gone to great lengths to address these factors, leading to the EPA’s 

proposal to use the IMPROVE algorithm to calculate light extinction in order to take into 

account the varying effects of relative humidity and speciated PM. While this approach 

does not result in a uniform level of ambient PM2.5, it does ensure a nationally uniform 

level of visibility protection. The EPA refers the reader to other sections of the final rule, 

including sections VI.B.1.a, VI.B.1.c, VI.C.1.b and VI.C.1.f, and the Response to 
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Comments document for a more detailed response as to how it is taking these variables 

into account. 

ii. Specific Comments on Level

The EPA received relatively few comments endorsing a specific level for a 

distinct secondary standard for visibility. In general, commenters who opposed setting a 

distinct secondary standard at this time did not address the question of what level would 

be appropriate if the EPA were to set a distinct secondary standard for visibility; 

similarly, commenters who supported adopting a distinct secondary standard at this time 

generally did not recommend a specific level. However, a few commenters did provide 

comments in support of a specific level or range of levels, with some commenters 

advocating standards at the upper end of the range of proposed levels (i.e., 30 dv), while 

others supported levels below the lower end of the proposed range (i.e., below 28 dv). 

As discussed above, a large number of commenters argued that the currently 

available data are insufficient to determine what constitutes a standard that would be 

neither more nor less protective than necessary and that no standard should be set at this 

time. These commenters pointed to the limitations and uncertainties in the preference 

studies discussed above as the basis for this claim. These commenters pointed to 

significant variation in the results of the preference studies in support of their arguments 

that the studies should not be used to derive a level for a distinct secondary standard for 

visibility. For example, one consultant cited by several industry commenters argued that 

the proposed level of 28 or 30 dv did not reflect the substantial difference in visibility 

preferences between the East and the West reflected in the preference studies (UARG, 

Attachment 2, p. 11), and that it did not reflect the full range of preferences (i.e., potential 
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50 percent acceptability levels) likely to exist nationwide (UARG, Attachment 2, p. 19). 

This commenter further objected to the EPA’s proposal for a level of 28 or 30 dv on the 

grounds that the EPA had inaccurately adjusted 4-hour values into 24-hour values. Based 

on his analysis, the consultant concluded that “a range of adjusted values from 28 to 32 

dv is needed” to account for the majority of the spread between the 4-hour vs. 24-hour 

equivalent values at the upper end of the distribution of values. 

A number of commenters questioned whether the proposed range of levels was 

appropriate. One industry commenter claimed that the EPA had not explicitly justified 

why a standard within the proposed range was requisite, stating that “EPA makes no 

attempt to explain how the proposed level of the standard is neither lower nor higher than 

necessary to protect public welfare” (NSSGA, p. 15). Arizona DEQ noted that since the 

proposed calculated light extinction indicator excluded coarse particles and Rayleigh 

scattering, the proposed levels of 28 or 30 dv were inconsistent with the visibility 

preference studies, which considered total light extinction. Noting these perceived 

problems with the proposed range of levels, a few commenters noted that if the EPA were 

to set a distinct secondary standard, the level should be set no lower than 30 dv, “to 

account for inconsistent value judgments, a great deal of spatial and temporal variability, 

and a very high level of uncertainty” (Texas CEQ, p. 7). 

In contrast, some commenters supporting the EPA’s proposal for a distinct 

secondary standard for visibility stated that the proposed range of levels from 28-30 dv 

was insufficiently protective based on a 24-hour averaging time, and recommended a 

lower level for the visibility index standard. These commenters expressed the view that 

the proposed levels of 28 or 30 dv represented neither adequate surrogates for equivalent 
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4-hour values, as the EPA claimed, nor sufficiently protective levels based on recent air 

quality data. Several commenters stated that the EPA’s own analyses suggested that a 

standard set at a level of 28 or 30 dv was insufficiently protective based on a 24-hour 

averaging time. One commenter emphasized that the Policy Assessment had indicated a 

level between 25-28 dv was appropriate for a standard calculated on a 24-hour average, 

and encouraged the EPA to adopt a standard level of 25 dv. Several environmental 

groups provided comments stating that a 24-hour average would underestimate a 4-hour 

value by 13-42 percent and certain areas of the country—particularly the Northeast—

would be affected disproportionately. These commenters suggested that a 24-hour PM2.5

visibility index standard should be set at a level of 18.6-20 dv. The Department of the 

Interior pointed to recent air quality data indicating that visibility on the 20% worst days 

in several large metropolitan areas, including Birmingham, Fresno, New York City, 

Phoenix, and Washington, DC was below 29 dv. While noting that these calculations 

were based on IMPROVE calculations which include contributions from coarse PM 

mass, DOI expressed the view that the proposed level of 28 to 30 dv would not provide 

adequate visibility protection compared to the current 24-hour PM2.5 standard of 35 

μg/m

3

and recommended that the standard be set at a level of 25 dv consistent with the 

results of the Phoenix visibility preference study. 

In contrast, the states of Arizona and Colorado submitted comments arguing that 

the visibility preference studies conducted in Phoenix and Denver, respectively, were 

designed to address a specific local problem and that the results of these studies were not 

an appropriate basis for selecting the level of a national standard. For example, Arizona 

DEQ noted: 
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The cited studies were conducted considering total light extinction; including 

extinction resulting from particulate matter and Rayleigh scattering. Visibility 

impairment due to coarse particulate matter can be an important contributor in 

Arizona, specifically in the Phoenix area where ongoing measurements have been 

made. Therefore, ADEQ believes that the proposed levels of the secondary 

visibility standard are inconsistent with applicable urban studies. (Arizona DEQ, 

p. 2)

Similarly, the Colorado Department of Public Health and the Environment noted that the 

Denver visibility standard was designed to address “brown clouds”, i.e., strong inversions 

that occur in the Denver metropolitan area, and that this standard “is based on a specific 

view of Denver” associated with particular sight paths and direct measurement methods. 

The commenter stated that this standard “is applicable only to this location,” and that 

these limitations make it potentially unsuitable for application as “a national secondary 

standard, particularly a proposed standard that does not use a direct measurement 

method” (Colorado DPHE, p. 2). 

While acknowledging the uncertainties and limitations associated with the 

visibility preference studies as discussed above, the EPA continues to conclude, as did 

CASAC, that the preference studies are appropriate to use as the basis for selecting a 

target level of protection from visibility impairment. However, the EPA agrees with 

commenters who emphasize the high degree of variability in visibility conditions and the 

potential variability in visibility preferences across different parts of the country. In light 

of the associated uncertainty, as noted in the proposal, the Administrator judged it 

appropriate to establish a target level of protection equivalent to the upper end of the 

range of Candidate Protection Levels (CPLs) identified in the Policy Assessment and 

generally supported by CASAC. Thus, the EPA proposed to set a 24-hour visibility index 

standard that would provide protection equivalent to the protection afforded by a 4-hour 

standard set at a level of 30 dv. In light of the comments received on the proposal, in 
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particular comments emphasizing the uncertainty and variability in the results of the 

public preference studies, the EPA continues to conclude that this approach is warranted, 

and that it is appropriate to set a target level of protection equivalent to the protection that 

would be afforded by a 4-hour, 30 dv visibility index standard.

Moreover, the EPA disagrees with commenters who argued that the EPA’s 

approach for translating 4-hour CPLs into equivalent 24-hour values was inappropriate. 

In adjusting 4-hour values for purposes of defining an appropriate level for a 24-hour 

standard, the EPA noted at the time of proposal that there were multiple approaches for 

estimating generally equivalent levels on a city-specific or national basis. While 

expressing the view that it was appropriate to consider the two approaches with the 

highest r2values (Approaches A and B in Appendix G of the Policy Assessment),155

which used regressions of 90th percentile light extinction values, the EPA determined it 

would also be appropriate to consider the city-specific estimates resulting from 

Approaches C and E which showed greater variability than the aggregated estimates. 

Approaches C and E generated a range of city-specific estimates of generally equivalent 

24-hour levels that encompassed the range of levels considered appropriate for 4-hour 

CPLs, including the CPL of 30 dv at the upper end of that range. This information 

provided support for using the same CPL for a 24-hour standard as for a 4-hour standard, 

since no single approach could generate an equivalent 24-hour standard level in each 

urban area for each CPL. The EPA continues to conclude, as it did at the time of 

 

155In particular, EPA staff expressed a preference for Approach B in the Policy 

Assessment. However, in light of the additional information provided by the other 

approaches explored in Appendix G of the Policy Assessment and the reanalysis in 

Frank, et al. (2012b), the EPA judges it more appropriate to consider the range of values 

resulting from all five analytical approaches for purposes of informing decisions about 

the equivalent level of a 24-hour standard.
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proposal, that using an unadjusted 4-hour CPL for purposes of establishing a target level 

of protection for a 24-hour standard is appropriate because this approach places more 

emphasis on the relatively high degree of spatial and temporal variability in relative 

humidity and fine particle composition observed in urban areas across the country, 

consistent with EPA’s reanalysis discussed below. 

The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships 

between estimated 24-hour and 4-hour visibility impairment based on the variety of 

metrics discussed in Appendix G of the Policy Assessment. The reanalysis has more 

appropriately considered the uncertainty of the calculated 4-hour values. The revised 

analysis shows that the 24-hour equivalent level is generally closer to the 4-hour value at 

the upper end of the range of CPLs than originally estimated, as can be seen in the results 

for Approaches B, C, and D.156 For example, the reanalysis indicates that Approach B 

yields an adjusted 24-hour CPL of 29 dv157 as generally being equivalent to a 4-hour CPL

of 30 dv, while Approach C yields a 24-hour equivalent CPL of 29 averaged across cities 

and a range of city-specific values from 25-36 dv.

158,159 Not only are the 90th percentile 

and pooled average values closer to the 4-hour CPL of 30 dv, the range of city-specific 

results shows a wider spread that clearly encompasses the unadjusted 4-hour value of 30 

 

156 Approach E as presented in the Policy Assessment is based on the median values for 

each city; these results are not affected by the regression analyses. Therefore, Approach E 

was not included in the reanalysis, and the results remain unchanged from those reported 

in the corrected Table G-6 as reported in Frank, et al., 2012b.

157 In Appendix G of the Policy Assessment, a 24-hour adjusted CPL of 28 was estimated 

to be equivalent to a 4-hour value of 30 under Approach B.

158 In Appendix G of the Policy Assessment, a 24-hour adjusted CPL of 27 dv was 

estimated to be equivalent to a 4-hour CPL of 30 dv when averaged across cities, while 

city-specific values were estimated to range from 24-30 dv.

159 In the reanalysis, Approach D (all days pooled regression) generated results of 28 dv 

for the 24-hour CPL equivalent to a 4-hour value of 30 dv as compared to a value of 27 

dv in the original analysis described in Appendix G.
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dv near the midpoint of the city-specific range. This provides support for concluding that 

the EPA’s approach to translating of 4-hour CPLs into equivalent 24-hour values was 

appropriate, and that it is appropriate to use unadjusted 4-hour values for purposes of 

selecting a level for a standard based on a 24-hour averaging time.

160

Moreover, the EPA disagrees with commenters who argue that the currently

available evidence is sufficient to justify establishing a target level of protection at 25 dv 

or below. The EPA recognizes that 25 dv represents the middle of the range of 50 percent 

acceptability levels from the 4 cities studied, and represents the 50 percent acceptability 

level from the Phoenix study, which the Agency has acknowledged as the best of the four 

studies in terms of having the least noise in the preference study results and the most 

representative selection of participants. The EPA also notes the caveats discussed in the 

proposal regarding whether it would be appropriate to interpret results from the western 

studies as generally representative of a broader range of scenic vistas in urban areas 

across the country. The Policy Assessment noted significant differences in the 

characteristics of the urban scenes used in each study, with western urban visibility 

preference study scenes including mountains in the background and objects at greater 

distances, while scenes in the eastern study did not. Since objects at a greater distance 

 

160 The analysis in Appendix G of the Policy Assessment used the 4-hour light extinction 

value treated as the independent (x-axis) variable in an ordinary least squares regression. 

The EPA now concludes that this regression approach was inappropriate because that 

variable has error and in fact may be more uncertain than the calculated 24-hour 

extinction values. The Frank et al. (2012b) reanalysis uses an orthogonal regression 

instead of ordinary least squares regression and results in slopes closer to the 1:1 line for 

all the results, particularly for Dallas, TX. Furthermore, consistent with the EPA’s 

conclusion that a higher multiplier for converting OC to OM would be appropriate (see 

section VI.C.1.b.ii above), the reanalysis substitutes a 1.6 multiplier for converting OC to 

OM in the calculation of 24-hour values instead of the value of 1.4 that was used in 

calculating 24-hour values for Appendix G. The higher multiplier is more consistent with 

the SANDWICH approach used to calculate the 4-hour values found in Appendix G. See 

Frank et al. (2012b) for a more detailed explanation.
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have a greater sensitivity to perceived visibility changes as light extinction changes 

compared to otherwise similar scenes with objects at a shorter range, this likely explains 

part of the difference between the results of the eastern study and results of the western 

studies. In the proposal, the EPA noted that the scenic vistas available on a daily basis in 

many urban areas across the country generally do not have the inherent visual interest or 

the distance between viewer and object of greatest intrinsic value as in the Denver and 

Phoenix preference studies. Also, the Agency takes note of the caution expressed by 

Colorado and Arizona about using the results of the Denver and Phoenix preference 

studies, which were aimed at addressing specific local visibility problems, to inform the 

choice of level for a national standard. Therefore, the Agency considers it reasonable to 

conclude, especially in light of the significant uncertainties, that it is appropriate to place 

less weight on the western preference results and that the high CPL value (30 dv) that is 

based on the eastern preference results is likely to be more representative of urban areas 

that do not have associated mountains or other valued objects visible in the distant 

background. These areas would include the middle of the country and many areas in the 

eastern U.S., as well as some western areas. As a result, the EPA concludes that it is more 

appropriate to establish a target level of protection at the upper end of the range of 24-

hour CPLs considered, recognizing that no one level will be "correct" for every urban 

area in the country.

In considering the upper end of this range, the EPA must identify a target level of 

protection that is considered requisite to protect public welfare from a national 

perspective, recognizing that the same target level would apply in all locations. Making 

this judgment requires a balancing of the risks to the public welfare and the substantial 
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uncertainties surrounding appropriate levels of visibility protection. As acknowledged in 

the proposal, the EPA recognizes that setting a target level of protection for a 24-hour 

standard at 30 dv would reflect a judgment that the current substantial degrees of 

variability and uncertainty inherent in the public preference studies should be reflected in 

a higher target protection level than would be appropriate if the underlying information 

were more consistent and certain. Also, a 24-hour visibility index at a level of 30 dv 

would reflect recognition that there is considerable spatial and temporal variability in the 

key factors that determine the value of the PM2.5 visibility index in any given urban area, 

such that there is a relatively high degree of uncertainty as to the most appropriate 

approach to use in selecting a 24-hour standard level that would be generally equivalent 

to a specific 4-hour standard level. In light of these uncertainties, the EPA continues to 

believe that it is appropriate to establish a target level of protection for visual air quality 

of 30 dv, averaged over 24-hours, with a form as discussed above.

In reaching this conclusion, the EPA notes that any national ambient air quality 

standard for visibility would be designed to work in conjunction with the Regional Haze 

Program as a means of achieving appropriate levels of protection against PM-related 

visibility impairment in all areas of the country, including urban, non-urban, and Federal 

Class I areas. While the Regional Haze Program is focused on improving visibility in 

Federal Class I areas and a secondary visibility index NAAQS would focus on protecting 

visual air quality principally in urban areas, both programs could be expected to provide 

benefits in surrounding areas. In addition, the development of local programs, such as 

those in Denver and Phoenix, can continue to be an effective and appropriate approach to 

provide additional protection, beyond that afforded by a national standard, for unique 
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scenic resources in and around certain urban areas that are particularly highly valued by 

people living in those areas. With regard to comments from the Department of Interior 

noting that many large metropolitan areas have 24-hour IMPROVE values below 30 dv 

on the worst 20 percent of days already, the EPA notes that the purpose of establishing 

NAAQS is to ensure adequate protection of public welfare everywhere, not to mandate 

continuous improvements in areas that may already be relatively clean. In fact, the 

evidence from the IMPROVE program that many urban areas have total 24-hour PMrelated light extinction below 29 dv on the 20 percent worst visibility days suggests that 

many areas have relatively good visual air quality already.

f. Need for a Distinct Secondary Standard to Protect Visibility

Numerous commenters questioned whether a distinct secondary standard for 

visibility is necessary in light of the analysis described in section VI.B.1.c.vii above 

(Kelly et al., 2012a) which indicated that a 24-hour mass-based PM2.5 standard of 35 

μg/m3 would protect against visibility impacts exceeding the range of levels considered 

in the proposal (28-30 dv). While this analysis was conducted in support of proposed 

implementation requirements for a distinct secondary standard (specifically, the modeling 

demonstrations that would be required under the PSD program), the second prong of the 

analysis showed that within the range of levels proposed by the EPA for the visibility 

index NAAQS (28-30 dv), the 24-hour PM2.5 standard of 35 μg/m3 would generally be 

controlling. Kelly et al. (2012a) concluded that “overall, design values based on 2008-

2010 data suggest that counties that attain 24-hour PM2.5 NAAQS level of 35 μg/m3

would attain the proposed secondary PM2.5 visibility index NAAQS level of 30 dv and 

generally attain the level of 28 dv” (pp. 17-18). 
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Citing this conclusion, many state and local agencies and industry commenters 

argued that a visibility index standard in the range proposed (28-30 dv) would provide no 

additional protection beyond that afforded by the existing secondary 24-hour PM2.5

NAAQS, and therefore no distinct visibility standard was necessary. These commenters 

advocated retaining the current 24-hour PM2.5 mass-based standard to protect against 

visibility effects. “Since the 24-hour PM2.5 standard already protects the welfare the 24-

hour PM2.5 visibility standard is designed to protect, the new standard is duplicative and 

unnecessary” (South Dakota DENR, p. 2). Furthermore, a number of state commenters 

objected to the additional resource burden associated with implementing a standard 

which had, in their view, no practical effect: “If the 24-hour PM2.5 mass standard has the 

same effect as the visibility standard, crafting complex regulations to implement another 

standard seems redundant” (South Carolina DHEC, p. 3). Other states agreed: “A PM2.5-

related Visibility Index appears redundant since the benefits achieved from the current 

primary and secondary annual and 24-hour PM2.5 standards already provide reductions 

that would improve visibility. Establishing a new PM2.5 secondary standard for visibility 

would be an additional complication and burden to the states that is not warranted” 

(Indiana DEM, p. 5). 

In addition, several commenters submitted additional analyses supporting their 

position that a 35 μg/m324-hour PM2.5 standard would provide at least equivalent 

protection to a distinct 24-hour visibility standard within the range of levels proposed 

(API, Attachment 2, p. 8 and Attachment 3, p. 1). 

In responding to these comments stating that a distinct visibility standard is not 

needed, the EPA notes as an initial matter that the Administrator provisionally concluded 
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at the time of proposal that the current PM standards were not sufficiently protective of 

visual air quality, and that consideration should be given to an alternative secondary 

standard that would provide additional protection against PM-related visibility 

impairment, especially in urban areas. This provisional conclusion was based on the 

results of public preference surveys on the acceptability of varying degrees of visibility 

impairment in urban areas, analyses of the number of days on which peak 1-hour or 4-

hour light extinction values were estimated to exceed a range of CPLs under conditions 

simulated to just meet the current standards, and the advice of CASAC. The 

Administrator also noted that the current indicator of PM2.5 mass, in conjunction with the 

current 24-hour and annual averaging times, was not well suited for purposes of 

protecting visibility, since it does not incorporate species composition or relative 

humidity, both of which play a central role in determining the impact of ambient PM on 

visibility. Taking into account the advice of CASAC and the court’s remand of the 

current secondary PM2.5 standards, the Administrator provisionally concluded that the 

current secondary standards were neither sufficiently protective nor suitably structured to 

provide an appropriate degree of public welfare protection from PM-related visibility 

impairment. As a result, the EPA proposed a new, distinct secondary standard that was 

designed to address these deficiencies.

The EPA notes that in critiquing the proposed secondary standard, commenters 

generally did not advocate that the form of the existing mass-based PM2.5 standards was

better suited scientifically to the task of protecting against visibility impairment. Rather, 

the commenters’ position that a distinct secondary standard was not needed for purposes 

of protecting visibility was based almost entirely on the relative degree of protection
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likely to be afforded by the existing standards (in particular, the existing 24-hour PM2.5

standard) as compared to the proposed visibility index, along with the relatively large 

uncertainties associated with the latter. Thus, for all the reasons discussed in the proposal 

with regard to the scientific appropriateness of an indicator that takes into account both 

species composition and relative humidity, the EPA continues to conclude that the 

proposed standard based on a visibility index would be appropriate scientifically to 

provide targeted protection of visibility, since it would provide a measure of PM-related 

light extinction that directly takes into account the factors (i.e., species composition and 

relative humidity) that influence the relationship between PM2.5 in the ambient air and 

PM-related visibility impairment. 

Furthermore, the EPA disagrees with commenters who stated that implementation 

concerns, in particular the additional resource burden associated with implementing a 

distinct secondary standard, should alter the Agency’s decision making with regard to a 

standard to protect visibility. The EPA may not take the costs of implementation into 

account in setting or revising the NAAQS. 

However, in light of the results of the Kelly et al. (2012a) analysis and the views 

expressed by commenters on the implications of this analysis for conclusions regarding 

the adequacy of the current secondary 24-hour PM2.5 standard, the EPA has reconsidered 

some of the conclusions drawn at the time of proposal, in particular with regard to the 

degree of protection that would be provided by the current secondary standard. Based on 

a review of comments related to indicator, averaging time, form and level, the Agency 

has concluded that (as described in sections VI.C.1b-e above) a standard defined in terms 

of a PM2.5 visibility index (based on speciated PM2.5 mass concentrations and relative 
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humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and a 90th

percentile form, averaged over 3 years, and a level of 30 dv, would provide sufficient but 

not more than necessary protection of the public welfare with regard to visual air quality. 

Having identified this target level of protection, the EPA is now in a position to compare 

it specifically to the existing secondary 24-hour PM2.5 standard of 35 μg/m3for purposes 

of determining whether it would provide more, the same, or less protection from visibility 

impairment. The EPA must consider both whether the existing secondary 24-hour PM2.5

standard of 35 μg/m3is sufficient (i.e. not under-protective) and whether it is more 

stringent than necessary (i.e. over-protective).

With regard to the degree to which the existing secondary 24-hour PM2.5 standard 

provides sufficient but not more than necessary protection for visibility, the EPA first 

notes that the kind of area-specific analysis conducted in Kelly et al. (2012a) is essential 

for addressing the court remand of the 2006 secondary standards. In the case of the 2006 

secondary standards, the EPA had argued that the 35 μg/m324-hour PM2.5 standard was 

requisite because one part of the proposed range for a distinct secondary standard the 

Agency had considered would affect the attainment status of a somewhat fewer counties 

than the 35 μg/m324-hour PM2.5 standard. The court rejected this kind of rough 

balancing, finding that the EPA’s equivalency analysis based on percentages of counties 

demonstrated nothing about the relative protection offered by the different standards. 

Based on this, an area-by-area evaluation of the relative degree of protection offered by 

different standards should be conducted to the extent air quality data is available. 

Kelly et al. (2012a) performed such an evaluation. Based on 2008-2010 data, 

there are no areas that would have exceeded a 30 dv, 24-hour visibility index standard 
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that would not also have exceeded a 24-hour PM2.5 standard of 35 μg/m3. Stated another 

way, all areas that met the 24-hour PM2.5 standard of 35 μg/m3 would have had visual air 

quality at least as good as 30 dv (24-hour average, based on 90th percentile form averaged 

over 3 years). The Kelly (2012a) analysis also showed that for some areas, particularly in 

the West, areas that would have met a 24-hour PM2.5 standard of 35 μg/m3 would have 

had visual air quality better than 30 dv for the PM2.5 visibility index standard, and that at 

sites that violated both the 24-hour level and the visibility index 30 dv level, the visibility 

index level of 30 dv would likely be attained if PM2.5 concentrations were reduced such 

that the 24-hour PM2.5 level of 35 μg/m3 was attained. 

The EPA has conducted a reanalysis (Kelly et al., 2012b) to update the area-byarea analysis in the original Kelly et al. (2012a) analysis in three respects. First, noting 

that the original Kelly at al. (2012a) analysis used a 1.4 multiplier to convert OC to OM 

at those monitors not using the new CSN monitoring protocol, the EPA recalculated the 

visibility index design values for 2008-2010 using a higher multiplier for converting OC 

to OM at monitors not already using the new CSN monitoring protocol SANDWICH 

approach, consistent with the Agency’s view that it is more appropriate to use a 

multiplier of 1.6 at such monitors as compared to 1.4, as described in section VI.C.1.a.ii, 

above.161 The recomputed visibility design index values for 2008-2010 show the same 

overall relationship to 24-hour PM2.5 design values as presented in Kelly et al., 2012a. 

Second, the EPA repeated the calculations comparing visibility index design

values with 24-hour PM2.5 design values using 2009-2011 data, the most recent three 

 

161 Some of the OC measurements were produced with CSN’s newer monitoring protocol 

and did not require a change in the computed OM.
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years of air quality information currently available.162 Third, the EPA modified the areaby-area evaluation to ensure consistency with the data completeness criteria of 40 CFR 

part 50, Appendix N, including the removal of data approved by EPA as exceptional 

events, for the current 24–hour PM2.5 standard and the proposed visibility index standard.

The results of this reanalysis, as presented in Kelly et al. (2012b), show a similar 

pattern to that described in the original Kelly memo. Specifically, the analysis indicates 

that there were no areas with visibility impairment above 30 dv that did not also exceed 

the 24-hour PM2.5 standard of 35 μg/m3. The updated memo concludes that the results for 

2009-2011 corroborate the findings for 2008-2010.

Based on these analyses (Kelly et al., 2012a; 2012b), the EPA concludes with a 

high degree of confidence that having air quality that meets the 24-hour PM2.5 standard of 

35 μg/m3 would be sufficient to ensure areas would not exceed 30 dv. The EPA notes that 

this conclusion from Kelly et al. (2012a) is supported by two analyses submitted by 

industry commenters (API, Attachments 2 and 3).

At the time of proposal, the EPA had reached a different conclusion, specifically 

that the 35 μg/m324-hour PM2.5 standard was not sufficiently protective. This conclusion 

was based, in part, on the analyses conducted for the Visibility Assessment and Policy 

Assessment regarding 1- to 4-hour peak light extinction values based on 2007-2009 data. 

For the reasons outlined above in sections VI.B.1.c and VI.C.1.c, the EPA originally 

focused on hourly or sub-daily timeframes for evaluating visibility conditions. However, 

data quality concerns effectively precluded adoption of a 1-hour or sub-daily averaging 

time in this review, and ultimately the EPA has concluded that a 24-hour averaging time 

can serve as an appropriate surrogate. In reaching this conclusion, the EPA has 

 

162 The 2011 air quality data were not yet available at the time of proposal.
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recognized that adopting a 24-hour averaging time will likely smooth out some of the 

hour-by-hour variability in visibility index values, and will effectively reduce peak values

by averaging them together with other hours. In concluding it is appropriate to adopt a 

24-hour averaging time, which limits the impact of hour-specific influences, the Agency 

is now placing less weight on the results of the 1-hour and 4-hour analyses presented in 

the Visibility Assessment and the Policy Assessment which focused on identifying the 

percent of days with peak hourly light extinction above various CPLs. In light of the 

Agency’s conclusion that a 24-hour averaging time would be appropriate, the Agency has 

determined to place more weight on analyses of visibility conditions over a 24-hour time 

period, especially the results in Kelly et al. (2012a and 2012b). In addition, the EPA notes 

that the Kelly et al. analyses reflects updated air quality information from more recent 

years of data (2008-2010 for Kelly et al., 2012a; 2009-2011 for Kelly et al. 2012b) as 

compared to the air quality information used in the Visibility Assessment and Policy 

Assessment. 

In light of all of these considerations, including the results of the Kelly et al. 

(2012a; 2012b) analyses, and the supporting comments provided by a broad range of 

public commenters, the EPA now concludes that the 24-hour PM2.5 standard of 35 μg/m3

provides sufficient protection in all areas against the effects of visibility impairment. The 

EPA concludes that the existing 24-hour PM2.5 standard would provide at least the target 

level of protection for visual air quality defined by a visibility index set at 30 dv, as 

described above, which the EPA judges appropriate.

However, the EPA also recognizes that it is important to evaluate whether such a 

standard would be over-protective (i.e. more stringent than necessary to protect public 
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welfare). The analyses presented in Kelly et al. (2012a; 2012b) indicates that the 24-hour 

PM2.5 standard of 35 μg/m3 would achieve more than the target level of protection of 

visual air quality (30 dv) in some areas. That is, when meeting a mass-based standard of 

35 μg/m3, some areas would have levels of PM-related visibility impairment far below 30 

dv. Thus, when considered by itself and without consideration of the secondary standards 

adopted for purposes of non-visibility welfare effects, the 24-hour PM2.5 standard of 35 

μg/m3 would be over-protective of visibility in some areas. However, it is important to 

note that as long as the current secondary 24-hour PM2.5 standard of 35 μg/m3remains in 

effect, this overprotection for visibility would occur, regardless of whether a distinct 

secondary standard based on a visibility index set at 30 dv were adopted. These issues are 

discussed more fully in section VI.D, which outlines the Administrator’s final 

conclusions on the secondary PM standards, below.

g. Legal Issues

Some commenters opposed the proposal to establish a distinct secondary 

standard that would be defined in terms of a PM2.5 visibility index. The proposed 

standard would use measured PM2.5 mass concentration, in combination with speciated 

PM2.5 mass concentration and relative humidity data, to calculate PM2.5 light extinction, 

translated to the deciview (dv) scale. The standard would also be defined in terms of a 

specified averaging time and form, and a level for the PM2.5 visibility index set at one of 

two options—either 30 dv or 28 dv. The commenters argued that the entire approach 

proposed by the EPA is inconsistent with the requirements of CAA section 109(b). They 

pointed to a number of different aspects of the proposal which in their view made it 

incompatible with the CAA. For example, the Utility Air Resources Group (UARG) 
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stated:

In the past, EPA has always used a measure of PM mass as the indicator for both 

primary and secondary PM NAAQS. Such a standard is, as a general matter, 

consistent with the directive in the CAA that the NAAQS “specify a level of air 

quality” and targets for control the listed criteria air pollutant. CAA § 109(b)(2). 

The standard contained in EPA’s proposed rule does neither of these things. 

Instead, it would (1) regulate relative humidity, which is not a criteria pollutant; 

(2) fail to “specify a level of air quality” as required by section 109(b)(2) of the 

CAA; and (3) result in a standard necessitating nationally variable PM 

concentrations instead of a standard establishing a nationally uniform, minimally 

acceptable PM concentration. (UARG, p. 22-23)

Other commenters raised similar or related issues, arguing that the EPA 

improperly set a visibility standard, and not a PM2.5 standard, and that NAAQS can only 

be set in terms of a level or concentration of the air pollutant. Commenters also argued 

that an endangerment finding and air quality criteria would be needed before the EPA 

could set a standard based on PM components. Each of these comments is discussed 

below.

As an initial matter, the commenters argued that the proposed standard is 

unlawful because it is “not a PM2.5 standard at all, but rather a visibility standard, and 

visibility is neither an air pollutant nor a criteria pollutant for which a NAAQS may be 

promulgated” (NMA/NCBA, p. 21). According to these commenters, the CAA requires 

that NAAQS be established as limits on the concentration of an air pollutant in ambient 

air, not limits on the “identifiable effects” caused by that air pollutant. These commenters 

claimed that reduced visibility due to light extinction is not an air pollutant but instead is 

an effect, noting that “the Act’s definition of ‘air pollutant’ speaks in terms of specific 

substances or matter in the ambient air” (NSSGA, p. 8). The commenters pointed to the 

use of the term “air pollutant” in sections 109(a)(1)(A) and (b)(2) as support for their 

argument, as these provisions refer to setting standards for the “air pollutant” to address 
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the effects associated with the presence of the air pollutant in the ambient air. They 

likewise pointed to section 108(a)(2)’s reference to the presence of the air pollutant in the 

ambient air. Since reduced visibility is not an air pollutant, they argue the EPA cannot set 

a NAAQS that is a standard for visibility. They argue that the proposed secondary 

standard it is not a PM2.5 standard as it does not limit the concentration of PM2.5 or any 

other fraction of particulate matter in the ambient air and therefore is not an “ambient air 

quality standard” for any pollutant.

One commenter argued that the EPA is required to “specify a level of air quality” 

under section 109(b)(2), which Congress intended as an acceptable concentration level of 

the air pollutant in the ambient air, noting that specification of acceptable visibility 

conditions is not the same as an acceptable air pollution concentration level. Citing 

American Farm Bureau v. EPA, 559 F.3d at 516, one commenter claimed that the court 

had affirmed that “the NAAQS—whether primary or secondary—is a mass-based 

standard” (Nevada DEP, p. 5). Commenters also refer to the legislative history of the 

1970 amendments, referring to NAAQS as setting the “maximum permissible ambient air 

level” for an air pollutant. The commenters argue that the proposed standard is improper 

because it does not limit the concentration of PM2.5 or any fraction of PM in ambient air, 

but improperly sets a limit on visibility effects.

With regard to humidity, these commenters argued that the proposed standard 

improperly regulates relative humidity because it is included in the calculation to 

determine the value of the visibility index. According to these commenters, the CAA 

allows the EPA to control criteria air pollutants through the NAAQS program, but not 

other various substances. The commenters stated that the EPA recognized this in the last 
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review, treating humidity as a confounding factor and considering addressing it by 

measuring PM2.5 mass-based concentration over the midday hours, when humidity would 

have the least effect. This would target the effects caused by PM, and not by humidity. 

Referring to American Farm Bureau v. EPA, 559 F.3d 512, 528 (D.C. Cir. 2009) and 77 

Fed. Reg. at 38979 n.153. UARG contested the proposed calculated visibility index as it 

does not approach relative humidity as a confounding factor but instead “embraces it and 

treats it as if it were a PM effect” (UARG p. 24).

The commenters also stated that the use of a calculated visibility index, and the 

failure to exclude the effects of humidity, would result in acceptable PM concentrations 

that vary across the nation. These commenters claimed that such a standard is 

inconsistent with the requirements of the CAA because the proposed approach fails to 

establish a nationally uniform PM concentration standard. For example, API argued that 

the proposed visibility index approach is “essentially specifying levels—not a level—of 

air quality” (API, p. 29). UARG agreed, and stated that the Act “requires that criteria 

pollutant concentrations throughout the nation reach, at the least, a single, specified 

ambient concentration level” (UARG, p. 25, emphasis in original). The commenters 

argue that a PM2.5 visibility index standard cannot provide equal protection nationwide 

due to geographic variation in key factors such as relative humidity that affect level of 

particles allowed in different areas. The commenters noted that establishing a single 

national level for the PM2.5 visibility index would necessarily result in unequal acceptable 

PM2.5 levels in different areas of the country, with lowest allowable PM2.5 levels in urban 

areas in the Southeast and highest allowable levels in the arid West. UARG recognized 

that under section 108 the air quality criteria are to “address those variable factors 
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(including atmospheric conditions) which of themselves or in combination with other 

factors may alter the effects on public health or welfare of such air pollutant,” but stated 

that while section 108 “allows” this, it has no bearing on this issue. Instead, the 

commenter stated that the EPA may take such information into account in setting a 

permissible concentration of the pollutant that is uniform and national (UARG, p. 25). 

In addition, some commenters opposed to the proposed distinct secondary 

standard argued that in order to base a standard on measured levels of several speciated 

substances, the EPA must first make an endangerment finding and issue air quality 

criteria for each of the speciated substances included in the calculation of PM2.5 light 

extinction. According to these commenters, “EPA cannot use NAAQS to indirectly 

regulate multiple substances which are not criteria pollutants under the guise of 

establishing a visibility standard” (NMA/NCBA, p. 21). Noting that air quality criteria 

for particulate matter were issued in 1969, NMA/NCBA argued that the 1969 Criteria 

Document “did not establish air quality criteria for individual constituents that occur in 

particle form, instead it established criteria for particulate matter as a whole” (p. 27). In 

light of the fact that criteria have never been issued for “individual speciated components 

of particulate matter,” these commenters argued, “if EPA wishes to promulgate a rule 

such as its secondary visibility NAAQS, it first must make a finding that the speciated 

components listed in Appendix N endanger public health or welfare and then issue an air 

quality criteria document for those components” (NMA/NCBA, p. 29). According to 

these commenters, the approach the EPA adopted in promulgating a NAAQS for lead 

supports this view: 

When EPA promulgated a NAAQS for lead, an individual substance in particle 

form, it did not assert that an endangerment finding or criteria document for lead 
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was unnecessary because lead was already covered by the PM Criteria Document. 

Instead, EPA complied with the Section 108 and 109 NAAQS prerequisites for 

lead, just as it must do for Appendix N substances if it intends to promulgate a 

NAAQS for those substances. ... [In 1976], EPA listed lead as an air pollutant 

that adversely affected public health or welfare, issued an air quality criteria 

document for lead, and promulgated a NAAQS for lead. 43 Fed. Reg. 46246 (Oct. 

5, 1976). (NMA/NCBA, p. 29)

Finally, UARG argued that the EPA has in the past recognized that the secondary 

NAAQS is an inappropriate vehicle for regulating PM-related visibility, referring to 62 

Fed. Reg. at 38680, including fn 49. UARG claimed the same situation continues, and the 

EPA has not provided a valid basis for changing this conclusion. 

The EPA disagrees with the points raised by these commenters. While the EPA is 

not adopting the proposed secondary standard, as explained below, this decision is not 

based on concern over the EPA’s authority to adopt a secondary standard such as the one 

proposed. 

The proposed distinct secondary standard is a standard for PM2.5, and is not a 

“visibility standard.” The proposed secondary standard is based on the mass 

concentration of PM2.5 in the ambient air. The standard is defined in terms of calculated 

PM2.5 light extinction which is based on the measurement of the mass concentration of 

ambient PM2.5 over a 24-hour period. The measured mass concentration is adjusted based 

on information on the speciated mass components of the PM2.5 and the relative humidity, 

resulting in a calculated visibility index. The level of the visibility index, combined with 

the form of the standard and averaging time, identifies whether a level of ambient mass 

concentration of PM2.5 achieves the standard or not. Given any specific mass 

concentration of ambient PM2.5, combined with information on speciation and relative 

humidity, it can be determined whether the specific mass concentration of ambient PM2.5

achieved the NAAQS. Hence, the proposed secondary NAAQS specifies acceptable 
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levels of ambient mass concentration of PM2.5.

The combination of indicator, averaging time, form, and level of the proposed 

NAAQS is designed to provide the appropriate degree of protection from visibility 

impairment caused by ambient levels of PM2.5. It does this by calculating the light 

extinction associated with ambient concentrations of PM2.5 and specifying the level of 

acceptable PM2.5 mass concentration in terms of this calculation. However this does not 

change the fact that the standard is for the air pollutant PM2.5, and defines acceptable 

ambient levels of this air pollutant. It does not transform the standard into a “visibility 

standard” and not a standard for PM2.5. While the commenters had additional concerns 

over the use of relative humidity in the calculation, and the variation around the country 

of acceptable mass concentrations, those issues are separate and do not change the fact 

that the proposed standard defined in terms of calculated PM2.5 light extinction is based 

on measurement of PM2.5 concentration in the ambient air, and is a NAAQS for PM2.5. 

With regard to the contention that section 109(b) limits the EPA to setting a 

standard that is based on the concentration of the pollutant in the ambient air, we note 

that the term “concentration” typically means some measure of relative content. For 

example, this would include relative measures such as mass per unit of volume or parts 

per million. The EPA has often used such metrics to define the NAAQS, largely because 

the scientific evidence of health or welfare effects supporting the NAAQS typically use 

such metrics in air pollution studies. For example, the current secondary standards for 

PM are defined in terms of the concentration of PM2.5 and PM10 in the ambient air, 

measured as the dried mass of the particulate matter per unit of air. However section 

109(b) does not require that a NAAQS be defined this way. 
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Sections 109(a) and (b) both use the general term “air quality” when discussing 

the EPA’s obligation to set NAAQS. The NAAQS are clearly national ambient “air 

quality” standards under section 109(b), which specifies that the primary NAAQS “shall 

be ambient air quality standards” and the secondary NAAQS “shall specify a level of air 

quality.” Both the primary and secondary NAAQS are to be based on the “air quality 

criteria,” which are to accurately reflect the latest scientific knowledge on the effects on 

public health and welfare associated with “the presence of such air pollutant in the 

ambient air, in varying quantities.” Section 109(b), 108(a)(2). Congress spoke in broad 

terms, tasking the EPA with assessing the latest scientific knowledge about the public 

health and welfare associated with the presence of the pollutant in the air, without 

limiting this to consideration of only those effects associated with one or more measures 

of concentration of the air pollutant. Congress referred to any and all effects associated 

with the presence of the pollutant in the ambient air, not just the effects associated with 

the concentration of the pollutant in the ambient air. Based on this knowledge, the EPA is 

required to set standards for the quality of the air that will provide the appropriate degree 

of protection from these health and welfare effects, without limitation on how to measure 

or define air quality. While concentration in the air has typically been an appropriate way 

to set a standard to achieve these requirements, the more general terms used in section 

108(a) and 109(b) do not limit the EPA to using concentration as the only way to measure 

air quality for purposes of setting a NAAQS. The EPA is charged with setting air quality 

standards, and has the discretion under section 109(b) to choose the metric for defining 

air quality that is appropriate to address the health or welfare effect at issue.

Congress did refer to “concentration” in certain situations. In section 109(c) 
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Congress required the EPA to set a primary NAAQS for NO2 concentration over 3 hours. 

This addressed Congress’ concern over whether the then current NO2 standard, which 

used concentration as a metric, provided adequate protection. Congress also called on 

CASAC to advise the Administrator on the relative contribution to “air pollution 

concentrations” of natural and anthropogenic sources, under section 109(d)(2)(C)(iii). 

This information is in addition to the advice CASAC is required to provide concerning 

appropriate revisions to the “air quality criteria” and to the NAAQS under section 

109(d)(2)(B).163 While these provisions refer to ambient concentrations of pollutants, this 

reflects the EPA’s standard practice to date in setting NAAQS, and none of them change 

or limit the range of discretion provided under section 109(b) in setting NAAQS. They do 

not change the fact that the EPA is to set “air quality” standards, and is not limited to “air 

concentration” standards. The reference in the legislative history to a maximum 

permissible ambient air level for the pollutant also does not limit the EPA to a level of air 

pollutant concentration, as compared to a different metric for specifying the level of air

quality, if that is judged to be appropriate. 

The text of sections 108 and 109 does not support the limited interpretation 

commenters suggest. Instead these provisions provide the EPA with significant discretion 

in determining the metric for air quality that is appropriate to achieve the required degree 

of protection of public welfare. The commenters’ interpretation would improperly limit 

this discretion, interfering with achieving the goals of section 109(b).

 

163 In a provision that is not part of the CAA, in 1990 Congress required EPA to request a 

report from the National Academy of Sciences on the role of secondary national ambient 

air quality standards, including information on the “effects on welfare and the 

environment which are caused by ambient concentrations of pollutants” listed under 

section 108, and the “ambient concentrations of each such pollutant which would be 

adequate to protect welfare and the environment from such effects.” Section 817(a) of the 

CAA Amendments of 1990, P.L. 101-549.
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For example, in this review the EPA considered whether it would be appropriate 

to base a secondary NAAQS on direct measurement of the light extinction caused by 

PM2.5. See 77 FR 38890, 38980-1 (June 29, 2012). There are several instrumental 

methods that directly measure PM2.5 light extinction – the amount of light extinction 

caused by the presence of PM2.5 in the ambient air. This is not a measure of the 

concentration of PM2.5 in the air, but a measure of the light extinction caused by PM2.5. 

This is clearly an effect associated with the presence of PM2.5 in the ambient air, and this 

atmospheric property is directly related to visibility effects. Unlike PM2.5 mass 

concentration, there is a close scientific relationship between directly measured PM2.5

light extinction and visibility effects.

It would appear straightforward to say that PM2.5 light extinction is a quality of 

the ambient air, and a secondary NAAQS that specified an acceptable level of PM2.5

based on directly measured PM2.5 light extinction would be an “ambient air quality 

standard” for the air pollutant that specifies a “level of air quality” designed to provide 

protection against visibility impairment. Unlike directly measured PM2.5 light extinction, 

the mass concentration of PM2.5 does not have the same direct relationship to light 

extinction, and specifying an acceptable level of mass concentration of PM2.5 would be a 

more indirect and less effective way to provide protection from visibility impairment 

caused by the presence of PM2.5 in the ambient air. Under the commenters’ interpretation, 

the EPA would be precluded from specifying a level of air quality in terms of directly 

measured PM2.5 light extinction, the more scientifically appropriate and direct measure of 

the effect PM2.5 has on visibility. Instead the EPA would be limited to the more indirect 

and less effective specification of a level of concentration of PM2.5.
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The commenters also objected to the inclusion of relative humidity as an 

adjustment factor in the calculation of PM2.5 light extinction. Contrary to the claims of 

these commenters, the use of calculated PM2.5 light extinction does not regulate relative 

humidity. The proposed secondary standard would define acceptable levels of ambient 

PM2.5, not acceptable levels of relative humidity. In addition, section 108 explicitly 

requires that the air quality criteria include information on the atmospheric conditions 

that can alter the effects of the air pollutant on public health or welfare, and relative 

humidity certainly has this kind of impact. Section 109(b) requires that the standard be 

based on the air quality criteria, indicating that this information can and should be taken 

into account in setting the standard. Including relative humidity as an adjustment factor in 

the calculation of PM2.5 light extinction is a reasonable and straightforward way to use 

the scientific information in the air quality criteria in establishing a standard to provide 

protection from visibility impairment.164

Some commenters pointed to the EPA’s position in the last review, stating that the 

EPA properly treated relative humidity as a confounding factor, and in this review 

improperly moves away from that position. See 77 FR at 38979, 71 FR 61144, 61205 

(October 17, 2006). In the last review the EPA considered a distinct PM2.5 mass-based 

secondary standard. In that context, limiting the measurement of PM2.5 mass 

concentration to the mid-day hours when relative humidity had the least impact would 

promote the correlation between measured PM2.5 mass concentration and light extinction, 

which would promote achievement of a relatively consistent degree of visibility 

protection across the country. However in this rulemaking the proposed calculated PM2.5

 

164 UARG recognizes these provisions, but argues, as above, that this is limited by the 

requirement that EPA set a NAAQS based solely on ambient concentration. 
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light extinction standard achieves a consistent degree of visibility protection by directly 

accounting for humidity, in a scientifically defensible manner. The goal has not changed 

– achieving the desired degree of protection across the country. What has changed is that 

calculated PM2.5 light extinction is a more direct and scientifically appropriate way to 

achieve that result.

Finally, it should be made clear that water is not a separate compound from PM2.5

that confounds the impact PM2.5 has on light extinction. As described in the Integrated 

Science Assessment, “PM is the generic term for a broad class of chemically and 

physically diverse substances that exist as discrete particles (liquid droplets or solids) 

over a wide range of sizes” (U.S. EPA, 2009a, p. 1-4). “Particles composed of water 

soluble inorganic salts (i.e., ammoniated sulfate, ammonium nitrate, sodium chloride, 

etc.) are hygroscopic in that they absorb water as a function of relative humidity to form a 

liquid solution droplet. Aside from the chemical consequences of this water growth, the 

droplets become larger when relative humidity increases, resulting in increased light 

scattering. Hence, the same PM dry concentration produces more haze” (U.S. EPA, 

2009a, p. 9-6). Thus water is not a compound that is separate and apart from the particle 

that acts as an extraneous confounding factor.165 The effect of relative humidity occurs 

after the water becomes part of the particle. Certain water soluble salts absorb water and 

the resulting particle is larger in size and scatters more light, increasing the visibility 

impact of the particle. But the particle is still a PM2.5 particle. The fact that the PM 

NAAQS traditionally uses a measurement of the dried mass of the particles as the metric 

 

165 According to the Integrated Science Assessment, “Confounding is ‘...a confusion of 

effects. Specifically, the apparent effect of the exposure of interest is distorted because 

the effect of an extraneous factor is mistaken for or mixed with the actual exposure effect 

(which may be null)’ (Rothman and Greenland, 1998, 086599)” (U.S. EPA, 2009a, p. 1-

16).
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for the standard does not change the fact that the particles in the air include liquid 

droplets and particles that have increased in size because of absorption of water. These 

ambient PM2.5 particles are what is in the air and impacting visibility, not just the dried 

mass of PM2.5 that is measured in the laboratory and is currently used as the indicator for 

the PM NAAQS. Thus the commenters improperly claimed that the proposed secondary 

standard regulates water or relative humidity, and not PM2.5, when in fact the proposed 

secondary standard accounts in a scientific manner for the fact that some PM2.5 particles 

are larger in size and have a greater impact on light extinction when the relative humidity 

increases. 

The commenters also raised concerns that a standard based on calculated PM2.5

light extinction, compared to a standard based on just PM2.5 mass concentration, 

improperly results in variable levels of acceptable PM2.5 mass concentrations across the 

country. This stems from the adjustments in the calculation for speciated components of 

PM2.5 and relative humidity. According to commenters, this is improper as section 109(b) 

requires that the NAAQS set a single, specified ambient concentration that is nationally 

uniform across the country.

As discussed above, the text of section 109(b) does not specify this limitation of a 

single national acceptable concentration. Instead the secondary NAAQS is to specify a 

level of air quality that achieves the appropriate degree of protection. The proposed 

secondary standard would do just that – specify a level of air quality, defined in terms of 

calculated PM2.5 light extinction, that would achieve the desired degree of protection. The 

fact that this results in varying allowable levels of PM2.5 mass concentrations is not 

inconsistent with the Act. The D.C. Circuit recently approved such a result. In the last 
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review of the PM10 primary NAAQS, the court approved the EPA’s choice of an 

indicator that was designed to allow varying levels of acceptable coarse PM. The court 

stated that:

The industry petitioners next argue that the 150 μg/m3 standard for PM10 will 

result in arbitrarily varying levels of coarse PM, and that the agency should 

instead have used a PM10-2.5 indicator. The EPA does not dispute that using the 

PM10 indicator will result in coarse PM levels that vary within the limit of 150 

μg/m3. As the EPA explains: "Because the PM10 indicator includes both coarse 

PM (PM10-2.5) and fine PM (PM2.5), the concentration of PM10-2.5 allowed by a 

PM10 standard set at a single level declines as the concentration of PM2.5

increases. Thus, the level of coarse particles allowed varies depending on the level 

of fine particles present." Id. at 61,195.

Although the EPA acknowledges that a PM10 indicator will result in varying 

coarse PM levels, it does not agree that the variance will be arbitrary. The EPA 

agrees with the industry petitioners that protection from coarse particles should be 

targeted at urban areas, where coarse particles have been shown to pose the 

greatest danger. Id. at 61,194. But the agency argues that targeting of urban areas 

is effectively accomplished by using an indicator that permits the varying levels 

that the industry petitioners challenge. ... Id. at 61,195-96 (citations omitted). In 

other words: "The varying levels of coarse particles allowed by a PM10 indicator 

will therefore target protection in urban and industrial areas where the evidence of 

adverse health effects associated with exposure to coarse particles is strongest." 

Id.

The EPA also offers a further rationale for tying the stringency of coarse PM 

regulation to increases in the level of PM2.5. ... EPA argues that it is "logical to 

allow lower levels of coarse particles when fine particle concentrations are high. . 

. . [I]nclusion of PM2.5 in the PM10 indicator for purposes of coarse particle 

protection would appropriately reflect the contribution that contaminants emitted 

in fine particle form can make to the overall health risk posed by coarse particles." 

Id. 

In sum, we find that the EPA has provided a reasonable explanation for its 

decision[ ] ... to utilize a standard that allows targeted variance in coarse PM 

levels in an inverse relationship to the amount of fine PM in the air. American 

Farm Bureau v. EPA, 559 F.3d 512, 534-5 (D.C. Cir. 2009). 

A similar result applies here. Under the proposed secondary standard there would 

be a single level of air quality specified for the NAAQS. The standard would apply across 

the nation; it would not be a regional standard. The proposed standard would be the same 
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standard everywhere—the acceptable level of mass concentration of PM2.5 would be 

defined the same way across the nation, using the same method of calculating the 

allowable concentration of PM2.5. The same degree of protection from visibility 

impairment would apply across the country. While the allowable amount of PM2.5 could 

vary, this would be a reasoned way to achieve the desired degree of protection from 

visibility impairment. The requirements of section 109(b) would be satisfied.

Commenters also objected that the EPA could not set a NAAQS for the separate 

components of PM2.5 without listing the components of PM2.5 under section 108, based 

on an endangerment finding, and issuing air quality criteria for these components. They 

argued that the issuance of air quality criteria for particulate matter starting in 1969 did 

not provide a lawful basis for a proposed secondary standard that is based on components 

of PM, as the 1969 air quality was for particulate matter “as a whole,” defining PM as 

particles smaller than 500 micrometers (NMA/NCBA, p. 27). However, as discussed 

above, the proposed standard sets the allowable limit on ambient concentrations of PM2.5. 

Information on both the speciated components of PM2.5 and the relative humidity affect 

how much light extinction is associated with any specific level of PM2.5, but the standard 

is for PM2.5. The D.C. Circuit has made it clear that PM2.5, just like PM10 and TSP before 

that, is an appropriate subset of PM for the EPA to focus on in setting the NAAQS based 

on the scientific evidence before the EPA. This focus of the NAAQS does not make the 

subset a new pollutant that requires listing and new air quality criteria under section 108 

before setting a NAAQS. American Trucking Association et al. v. EPA, 175 F.3d 1027, 

1055 (D.C. Cir. 1999). Commenters’ interpretation would apply to PM2.5 as well as to 

components of PM2.5, and is inconsistent with the ATA decision. In addition, it is clear 
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that the current air quality criteria do address the scientific basis for calculating PM2.5

light extinction as the EPA proposed (U.S. EPA, 2009a, pp. 9-5 to 9-8). 

Finally, at least one commenter argued that the EPA has concluded in prior 

reviews that the secondary NAAQS program is an inappropriate vehicle for regulating 

PM related visibility impairment (UARG, p. 26). UARG mischaracterized the EPA’s past 

decision-making. In past reviews the EPA has been clear that the EPA should take into 

account the existence of the visibility program under section 169A, the regional haze 

program, when considering a secondary NAAQS and should not treat the secondary 

NAAQS as the sole mechanism to address visibility impairment across the country. That 

is the approach the EPA has taken in this and prior reviews. See 77 FR at 38990. 

h. Relationship with Regional Haze Program

A large number of commenters expressed confusion and concern over differences 

between the proposed visibility index standard and the Regional Haze Program. This 

included commenters who supported setting a distinct secondary standard to protect 

visibility as well as those opposed to setting such a standard. A number of these 

commenters noted that visibility impairment would be assessed differently under the two 

approaches due to differences in the way light extinction is calculated, including different 

IMPROVE equations and differences in the inclusion and weighting of specific species 

and components. The commenters argued it would be inappropriate to have two different 

regimes for managing visibility impairment in the exact same location. These 

commenters claimed that since data from the IMPROVE monitoring network would 

inform nonattainment designations, as well as an area’s obligations under the Regional 

Haze Program, there could be considerable confusion over how to draw nonattainment 
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boundaries and what requirements would affect large sources in rural areas. These 

commenters also noted the resource burden associated with maintaining two different 

programs aimed at protecting visibility in the same geographic area. Some commenters 

argued that a visibility NAAQS should not apply to rural areas. The Department of the 

Interior requested that the EPA clearly define the geographic area to which the visibility 

index standard would be applicable, and suggested that Class I and Class II areas should 

generally be excluded from the standard. As discussed above, commenters questioned the 

need for a distinct visibility standard, arguing that the existing primary PM standards 

combined with the Regional Haze Program ensured adequate protection of visibility, 

even in urban areas.

In response to these comments relating to the overlap between the Regional Haze 

program and a distinct secondary standard designed to protect visibility principally in 

urban areas, the EPA notes that the objectives of each program are distinct. While the 

Regional Haze program is designed to eliminate man-made impairment of visibility in 

Federal Class I areas over the course of several decades, a distinct secondary standard for 

PM-related visibility impairment would be focused on providing a nationally applicable 

level of protection for all areas, particularly urban areas which do not receive targeted 

protection under the Regional Haze Program. Moreover, the metric used to assess 

visibility impairment differs between the two programs precisely because each program 

is aimed at a different aspect of the problem. Recognizing the importance of fresh 

emissions for urban visibility, the Visibility Assessment focused on visibility impairment 

as measured by the original IMPROVE equation because “the original version is 

considered more representative of urban situations when emissions are still fresh rather 
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than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3-19). The Regional Haze 

Program, on the other hand, has shifted to a revised IMPROVE algorithm more suited to 

remote locations. While this difference is discussed in more detail in section VI.C.1.b 

above, the result is that each program would appropriately measure those aspects of 

visibility impairment most closely related to the problem the program is trying to prevent. 

Since the same data can be used to calculate both visibility impairment under the 

Regional Haze approach and the proposed visibility index, the additional calculation 

burden for state and local agencies would be light. Also, to the extent that there is any 

difference in terms of the emissions control obligations the two different programs would 

impose upon state and local areas, this is likely appropriate given the extent and nature of 

visibility impairment in those areas. The EPA notes that in general, there is likely to be 

substantial overlap in the control strategies a state or local area would pursue under either 

program. Thus, the EPA disagrees with commenters who stated that a distinct visibility 

standard as proposed would inherently conflict with the Regional Haze Program or that it 

would be appropriate to draw geographical distinctions that would explicitly exclude 

some areas (e.g., Class I areas) from the NAAQS. The EPA notes that the CAA requires 

that NAAQS be national in scope, and that the specific requirements laid out in the 

proposal for the distinct secondary standard would ensure that the protection it afforded 

would be appropriately targeted toward urban areas so that it could work in conjunction 

with—not be in conflict with—the Regional Haze Program under sections 169A and 

169B of the CAA.

2. Comments on the Proposed Decision Regarding Non-Visibility Welfare Effects

Relatively few commenters addressed the proposal to retain the existing suite of 
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secondary PM standards to address non-visibility welfare effects. A couple of states, 

including Mississippi and South Dakota, offered brief endorsements of the proposal. A 

few other commenters offered more extensive comments on the proposal to retain the 

existing secondary standards, and these commenters opposed this aspect of the proposal 

for one of two reasons. First, some commenters opposed the proposal to retain the current 

secondary annual PM2.5 standard of 15 μg/m3in light of the proposal to revise the level of 

the primary annual PM2.5 standard to a level between 12-13 μg/m3. Expressing concern 

over the implications of this decision for the air quality planning obligations of states, 

these commenters argued that the EPA should revise the secondary PM2.5 standards to be 

equivalent in all respects to the primary PM2.5 standards. For example, the American 

Association of State Highway and Transportation Officials (AASHTO) supported 

“retaining secondary standards that are consistent with the primary standards in order to 

reduce the complexity of the transportation and air quality planning processes, as well as 

the transportation conformity process” (AASHTO, p. 3). Thus, if the EPA were to adopt a 

lower level for the primary annual PM2.5 standard, the commenters recommended that the 

EPA adopt this same lower level for the primary secondary PM2.5 standard as well. 

In response to these comments, the EPA notes that the Agency lacks an 

appropriate scientific basis for revising the level of the secondary annual PM2.5 standard. 

As noted above in section VI.B.2, there is an absence of information that would support 

any different secondary standards for PM. Comments related to the implementation 

challenges associated with distinct primary and secondary standards are not relevant to 

the Administrator’s final decisions regarding what standards are requisite to protect the 

public welfare. Therefore, the EPA continues to conclude that it would be appropriate to 
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retain the current suite of secondary PM standards166 to address non-visibility welfare 

effects, while revising only the form of the secondary annual PM2.5 standard to remove 

the option for spatial averaging consistent with this change to the primary annual PM2.5

standard, as proposed.

Other commenters focused on the impacts of particulate matter on climate. One 

commenter cited a number of recent studies that considered mobile source black carbon 

emissions and associated climate impacts, and urged the EPA to protect the public 

welfare by setting “higher standards for gasoline quality” (Urban Air Initiative, p. 4). 

This commenter did not, however, advocate specific secondary NAAQS to address 

climate impacts of PM. More extensive comments on this same subject were provided by 

the Center for Biological Diversity (CBD), which urged the EPA to “set a separate limit 

for black carbon within the overall PM2.5 standard” to ensure that public welfare is fully 

protected “from the serious climate impacts of black carbon” (CBD, p. 2). This 

commenter argued that “[p]recaution is required for secondary NAAQS,” citing 

American Trucking Associations, Inc. v. EPA, 283 F.3d 355, 369 (D.C. Cir. 2002):

[N]othing in the Clean Air Act requires EPA to wait until it has perfect 

information before adopting a protective secondary NAAQS. Rather, the Act 

mandates promulgation of secondary standards requisite to protect public welfare 

from any “anticipated adverse effects associated with” regulated pollutants, 42 

U.S.C. § 7409(b)(2) (emphasis added), suggesting that EPA must act as soon as it 

has enough information (even if crude) to “anticipate[]” such effects[.]

The commenter stressed the growing scientific evidence regarding the impacts of black 

carbon on climate, and argued that the EPA’s proposal ignores important research studies 

published within the last five years which provide improved estimates of the radiative 

forcing associated with black carbon, and the effects of black carbon on snow and ice, the 

 

166 As summarized in section VI.A and Table 1 above, the current suite of secondary PM 

standards includes annual and 24-hour PM2.5 standards and a 24-hour PM10 standard. 
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Arctic climate, water availability and climate “tipping points.” The commenter also noted 

that reductions in cooling aerosol species, particularly sulfate, due to pollution control 

programs are leading to an “unmasking” of the true extent of warming due to the 

accumulation of greenhouse gases in the atmosphere. The commenter argued that this 

unmasking effect can be offset by ensuring “that sufficient black carbon reductions 

accompany reductions in overall aerosol pollution” (CBD, p. 10). The commenter also 

argued that the EPA did not consider the negative impacts of climate change on public 

health adequately in the proposal.

The commenter stated that the EPA had an obligation to address the impacts of 

black carbon in the PM NAAQS, despite the remaining uncertainties. The commenter 

pointed to the U.S. EPA’s report to Congress on Black Carbon (U.S. EPA, 2012c), stating 

that the “report shows that EPA is aware of the climate science and public health 

information that point to the importance of addressing black carbon pollution. EPA must 

use this information in its relevant decisionmaking” (CBD, p. 13). The commenter also 

noted that the U.S. participates in a number of international forums that have recognized 

the need to take action on black carbon, and argued that the U.S. has “an obligation under 

the Gothenburg Protocol to address black carbon pollution.” The commenter challenged 

the uncertainties cited by EPA with regard to the climate impacts of aerosols generally, 

arguing that they “do not apply to the regulation of black carbon” (CBD, p. 14). 

Specifically, the commenter stated that “there are significant anthropogenic sources of 

black carbon that contribute a large proportion of total black carbon emissions”; that 

“there is enough information related to black carbon’s impact to know that global 

temperatures will rise due to black carbon emissions”; that spatial and temporal 
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heterogeneity in black carbon emissions do not matter for estimating likely climate 

effects; that “[b]lack carbon’s negative climate impacts do not depend upon details of 

cloud interactions with aerosols”; and that the EPA does not need to be able to quantify 

the health or climate benefits precisely to know that it is appropriate to control black 

carbon as a specific component of PM under the CAA (CBD, pp. 14-15). 

As a result, the commenter concluded that the current size-based PM mass 

standard “is insufficient to fully protect health and welfare,” and that the EPA was 

obligated to establish a specific limit on black carbon as a component of PM. The 

commenter argued that “Black carbon must be regulated separately and in addition to 

PM2.5 because absent separate standards sulfates and nitrates may be more likely to be 

mitigated than the black carbon component of PM” (CBD, p. 17). To support this point, 

the commenter cited the conclusion in the Policy Assessment that:

The current standards that are defined in terms of aggregate size mass cannot be 

expected to appropriately target controls on components of fine and coarse 

particles that are related to climate forcing effects. Thus, the current mass-based 

PM2.5 and PM10 secondary standards are not an appropriate or effective means of 

focusing protection against PM-associated climate effects due to these differences 

in components. (U.S. EPA, 2011a, p. 5-11)

The commenter also noted that existing regulations on diesel engines, which are 

the largest source of black carbon in the United States, do not affect existing engines and 

vehicles, and stated that “The NAAQS program is one of the few opportunities to reduce 

black carbon from existing engines, industrial and biofuel sources within the United 

States and rapidly reduce emissions from this pollutant” (CBD, p. 18).

The EPA agrees with the commenters’ assertion that the scientific information 

about the impacts of aerosol species on climate is developing rapidly, and that 

understanding of the magnitude of aerosol effects on climate and the contribution of 
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individual aerosol components to those effects has improved substantially over the past 

decade. The EPA also agrees that certain species, in particular black carbon, play a 

significant role in multiple aspects of climate. The Policy Assessment recognized that 

“Aerosols can impact glaciers, snowpack, regional water supplies, precipitation and 

climate patterns,” and may contribute to the melting of ice and snow, a decrease in 

surface albedo, and climate impacts in the Arctic and other locations (U.S. EPA, 2011a, 

p. 5-9). The contribution of black carbon to these effects is discussed in detail in the 

EPA’s recent Report to Congress on Black Carbon (U.S. EPA, 2012c). In particular, 

black carbon plays an important role in heating the lower atmosphere by absorbing 

incoming solar radiation and outgoing terrestrial radiation, i.e. via “direct” radiative 

forcing.

However, the EPA disagrees that there is sufficient information available at this 

time to establish a NAAQS to protect against the climate impacts associated with current 

ambient concentrations of black carbon or other PM constituents. While the Integrated 

Science Assessment concluded that “a causal relationship exists between PM and effects 

on climate, including both direct effects on radiative forcing and indirect effects that 

involve cloud feedbacks that influence precipitation formation and cloud lifetime” (U.S. 

EPA, 2009a, section 9.3.10), it also identified substantial remaining uncertainties with 

regard to the contribution of individual aerosol species to these climate effects. The 

contribution of individual aerosol components to total aerosol direct radiative forcing is 

more uncertain than the global average (U.S. EPA, 2009a, section 9.3.6.6), and the 

indirect effects of aerosols and aerosol components remain highly uncertain, in particular 

with regard to their complex interactions with clouds.
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With regard to black carbon, for example, the EPA disagrees with CBD’s claims 

that “black carbon’s negative climate impacts do not depend upon details of cloud 

interactions with aerosols” and that the uncertainties associated with climate impacts of 

aerosols generally do not apply to black carbon. In fact, the EPA has pointed to cloud 

interactions as the area of greatest uncertainty with regard to black carbon: recognizing 

that black carbon affects cloud reflectivity (albedo), lifetime, and stability as well as 

precipitation, the Report to Congress on Black Carbon noted that “few quantitative 

estimates of these effects are available, and significant uncertainty remains. Due to all of 

the remaining gaps in scientific knowledge, it is difficult to place quantitative bounds on 

the forcing attributable to [black carbon] impacts on clouds at present” (U.S. EPA, 2012c, 

p. 4). The Report acknowledged that “most estimates of the forcing from aerosol indirect 

effects are based on all aerosol species (e.g. total PM) and are not estimated for 

individual species (e.g, BC alone)” (U.S. EPA, 2012c, p. 40). The Report concluded that 

it remains unclear the extent to which black carbon contributes to the overall aerosol 

indirect effect, and did not assign any central estimate or even a range of possible values 

to the role of black carbon in the overall aerosol indirect effect. With regard to black 

carbon’s net contribution to climate, therefore, the Report concluded:

The direct and snow/ice albedo effects of BC are widely understood to lead to 

climate warming. However, the globally averaged net climate effect of BC also 

includes the effects associated with cloud interactions, which are not well 

quantified and may cause either warming or cooling. Therefore, though most 

estimates indicate that BC has a net warming influence, a net cooling effect 

cannot be ruled out. It is also important to note that the net radiative effect of all 

aerosols combined (including sulfates, nitrates, BC and OC) is widely understood 

to be negative (cooling) on a global average basis. (U.S. EPA, 2012c, p. 3)

Given the remaining uncertainties about the impact of aerosols on climate, there is even 

greater uncertainty with regard to how aerosol-induced climate change will affect public 
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health. At this time, it is not possible to estimate the extent to which aerosols in general, 

let alone particular aerosol components, contribute to the occurrence or exacerbation of 

adverse health outcomes due to climate change. The EPA therefore disagrees with CBD’s 

claim that the EPA should pursue black carbon reductions for purposes of reducing the

impacts of climate change on public health.

The Report to Congress on Black Carbon also stressed the importance of 

considering co-emitted PM species, such as SO2 and NOx, in evaluating the benefits of 

black carbon mitigation options. Noting that many of these co-emitted particles and gases 

have a cooling influence on climate, the Report noted the difficulty of estimating the net 

effect of various mitigation measures on net radiative forcing or other climate variables. 

The EPA concluded that the location and timing of emissions reductions would be 

critically important for achieving climate benefits, and that “more research is needed on 

the benefits of individual control measures in specific locations to support policy 

decisions made at the national level” (U.S. EPA, 2012c, p. 140). Thus, the EPA disagrees 

with CBD’s claim that spatial and temporal heterogeneity in black carbon emissions do 

not matter for estimating likely climate effects, and continues to believe that being able to 

quantify the climate impacts of various aerosol species, alone and in combination, is 

essential for informing any possible revisions to the current secondary PM standards 

based on climate.

Furthermore, while the EPA agrees with the commenter that a large percentage of 

black carbon emissions come from anthropogenic sources, including diesel engines and 

vehicles, the EPA notes that existing regulations on mobile diesel engines are already 

reducing these emissions substantially. Between 1990 and 2005, new engine 
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requirements resulted in a 32 percent reduction in black carbon emissions from mobile 

sources, and a further 86 percent reduction from 2005 levels is projected to occur by 2030 

as vehicles and engines meeting existing regulations are phased into the fleet (U.S. EPA, 

2012c, p. 175). Long-term historic data indicate that there has been a dramatic overall 

decline in black carbon emissions over the past century, due to changes in fuel use, more 

efficient combustion practices, and implementation of PM controls. Therefore, the EPA 

disagrees with CBD’s claim that a distinct black carbon NAAQS is necessary to achieve 

reductions in black carbon emissions. Clearly, U.S. emissions of black carbon are already 

declining substantially, suggesting that the existing mass-based PM standards, though not 

targeting black carbon specifically, have been effective in achieving black carbon 

emissions reductions in practice. As acknowledged in the Report to Congress on Black 

Carbon, “While [black carbon] is not the direct target of existing programs, it has been 

reduced through controls aimed at reducing ambient PM2.5 concentrations and/or direct 

particle emissions” (U.S. EPA, 2012c, p. 161). The EPA has acknowledged the need to 

encourage PM mitigation strategies that focus on reducing directly emitted PM2.5 for 

purposes of reducing black carbon, and this is reflected in U.S. commitments under the 

Gothenburg Protocol: the new provisions in the Protocol pertaining to PM encourage 

parties to develop national inventories and projections for black carbon, and to “give 

priority” to black carbon when implementing measures to control PM. However, the EPA 

notes that the U.S. has not yet ratified the PM amendments to the Gothenburg Protocol, 

and furthermore, these amendments do not require action specifically to reduce black 

carbon, but rather encourage countries to take such actions voluntarily within the context 

of their broader PM reduction strategies. Thus the EPA disagrees with the commenter 
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that the U.S. has an “obligation” to reduce black carbon under the Gothenburg Protocol, 

or that it has “agree[d] to choose mitigation options for particulate matter that focus on 

black carbon reductions” under the Protocol (CBD, p. 13).

In sum, the EPA notes the substantial remaining the uncertainties and gaps with 

regard to the climate impacts of PM components, including black carbon. These include 

the uncertainties associated with the spatial and temporal heterogeneity of PM 

components that contribute to climate forcing; the uncertainties associated with 

measurement of aerosol components; the inadequate consideration of aerosol impacts in 

climate modeling; and the currently insufficient data on local and regional microclimate 

variations and the heterogeneity of cloud formations. As a result, the EPA continues to 

conclude that it is not currently feasible to conduct a quantitative analysis for the purpose 

of informing revisions of the current secondary PM standards based on climate, and that 

there is insufficient information at this time to base a national ambient standard on 

climate impacts associated with current ambient concentrations of PM or any of its 

constituents.167

D. Conclusions on Secondary PM Standards

This section describes the Administrator’s conclusions regarding the secondary 

PM standards and the rationale leading to the Administrator’s final decision to retain the 

current suite of secondary PM standards, including an annual PM2.5 standard of 15 μg/m3

a 24-hour PM2.5 standard of 35 μg/m3, and a 24-hour PM10 standard of 150 μg/m3, to 

address PM-related visibility impairment as well as other PM-related welfare effects, 

including ecological effects, effects on materials, and climate impacts. Specifically, this 

 

167 This conclusion applies for both the secondary (welfare-based) and the primary 

(health-based) standards. 
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section explains the Administrator’s decision, consistent with the proposal, to retain the 

current suite of secondary PM standards generally, while revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. It also explains the 

Administrator’s decision, contrary to what was proposed, not to establish a distinct 

standard to address PM-related visibility impairment. 

In reaching conclusions regarding the need to revise the secondary PM standards 

for both visibility and non-visibility welfare effects, the Administrator has taken into 

account several key factors, including: (1) The latest scientific information on both 

visibility and non-visibility welfare effects associated with PM, as previously described; 

2) the advice of CASAC; and 3) the comments received during the public comment 

period, as discussed above. Based on this information, the Administrator has reached 

final conclusions about the secondary PM standards and made final decisions about those 

standards, as outlined below. Because the Administrator’s final conclusions with regard 

to the need to establish a distinct secondary standard to protect against visibility 

impairment reflect, in part, her conclusions on secondary PM standards for non-visibility 

welfare effects, section VI.D.1 first outlines her conclusions regarding secondary PM 

standards to address non-visibility welfare effects. This is followed by section VI.D.2 

which outlines her conclusions regarding a secondary PM standard to address PM-related 

visibility impairment. Finally, section VI.D.3 summarizes the Administrator’s final 

decisions with regard to the secondary PM standards for both visibility and non-visibility 

welfare effects.

1. Conclusions Regarding Secondary PM Standards to Address Non-Visibility Welfare 
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Effects

With regard to the secondary PM standards to address non-visibility welfare 

effects, the Administrator concludes that it is generally appropriate to retain the existing 

secondary standards and that it is not appropriate to establish any distinct secondary PM 

standards to address non-visibility PM-related welfare effects. This conclusion is based 

on the considerations discussed above in section VI.B.2, including the latest scientific 

information and the advice of CASAC, and the public comments received on the 

proposal, as discussed above in section VI.C.2. The Administrator concurs with the 

advice of CASAC and the conclusions expressed at the time of proposal that it is 

important to maintain an appropriate degree of control of both fine and coarse particles to 

address non-visibility welfare effects, including ecological effects, effects on materials, 

and climate impacts. In the absence of information that would support any different 

standards the Administrator concludes that it is appropriate to retain the existing suite of 

secondary standards to address non-visibility welfare effects, as proposed. More 

specifically, the Administrator concludes it is appropriate to retain all aspects of the 

current 24-hour PM2.5 and PM10 standards. With regard to the secondary annual PM2.5

standard, the Administrator concludes that it is appropriate to retain a level of 15.0 μg/m3

for this standard while revising only the form of the secondary annual PM2.5 standard to 

remove the option for spatial averaging consistent with this change to the primary annual 

PM2.5 standard. In reaching this conclusion, the Administrator notes that no areas in the 

country are currently using the option for spatial averaging to demonstrate attainment 

with the secondary annual PM2.5 standard.

2. Conclusions Regarding Secondary PM Standards for Visibility Protection
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Having reached the conclusion that it is generally appropriate to retain the 

existing secondary standards to protect against non-visibility welfare effects, the 

Administrator next considered the target level of protection that would be requisite to 

protect public welfare with regard to visual air quality. The Administrator then 

determined whether to adopt a distinct secondary standard to achieve this target level of 

protection. In making this decision, the Administrator compared the degree of protection 

for visibility that would be provided by such a distinct secondary standard to the degree 

of protection provided by the existing secondary standards, focusing specifically on the 

secondary 24-hour PM2.5 standard of 35 μg/m3.

168

Based on the considerations discussed above in section VI.B and VI.C, the 

Administrator first concludes that a target level of protection for a secondary standard is 

most appropriately defined in terms of a PM2.5 visibility index as proposed, since it would 

provide a measure of PM-related light extinction that directly takes into account the 

factors (i.e., species composition and relative humidity) that influence the relationship 

between PM2.5 in the ambient air and PM-related visibility impairment. Such a PM2.5

visibility index standard would afford a relatively high degree of uniformity of visual air 

quality protection in areas across the country by virtue of directly incorporating the 

effects of differences in PM2.5 composition and relative humidity across the country. 

In defining a target level of protection based on a PM2.5 visibility index, the 

Administrator has considered specific aspects of such an index, including the appropriate 

indicator, averaging time, level, and form. First, with regard to indicator, the 

 

168 This focus on the 24-hour PM2.5 standard reflects the Administrator’s judgments that 

PM-related visibility impairment is principally related to fine particle concentrations and 

that perception of visibility impairment is most directly related to short-term levels of 

visual air quality. 
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Administrator notes the conclusion of CASAC that relying on a calculated PM2.5 light 

extinction indicator based on PM2.5 chemical speciation and relative humidity data 

represented a reasonable approach. Based on the analyses conducted in support of this 

rulemaking, as described above, as well as the advice of CASAC, the Administrator 

concludes that a calculated PM2.5 light extinction indicator that utilizes the original 

IMPROVE algorithm, adjusted to use a 1.6 OC multiplier and exclude the term for coarse 

particles, in conjunction with monthly average relative humidity data (i.e., f(RH) values) 

based on long-term climatological means would be the most appropriate indicator for a 

PM2.5 visibility index standard. 

With regard to averaging time, the Administrator notes that both CASAC and 

EPA staff have concluded that hourly or sub-daily (4- to 6-hour) averaging times, within 

daylight hours and excluding hours with high relative humidity, are more directly related 

than a 24-hour averaging time to the short-term nature of the perception of PM-related 

visibility impairment and the relevant exposure periods for segments of the viewing 

public. However, in light of the important data quality uncertainties that have recently 

been identified in association with currently available instruments that would be used to 

provide the hourly PM2.5 mass measurements that would be needed in conjunction with 

an averaging time shorter than 24 hours, the Administrator concludes it would not be 

appropriate at this time to set a standard based on a sub-daily averaging time. Moreover, 

the Administrator notes that analyses conducted by the EPA during this review clearly 

indicate that PM2.5 light extinction calculated on a 24-hour average basis would be a 

reasonable and appropriate surrogate for PM2.5 light extinction calculated on a 4-hour 

basis. Thus, the Administrator concludes that a 24-hour averaging time would be 
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appropriate for a PM2.5 visibility index. The Administrator recognizes that a 24-hour 

averaging time would effectively reduce the influence of peak hours of visibility 

impairment on visibility index values, but concludes that in light of the concern that peak 

hourly measurements may be significantly influenced by atypical conditions and/or 

atypical instrument performance, it is appropriate to adopt a longer averaging time to 

ensure that hour-specific influences and uncertainties are balanced against more robust 

measurements. 

With regard to form, the Administrator notes that consistent with the approach 

taken in other NAAQS, including the current 24-hour PM2.5 NAAQS, a multi-year 

percentile form offers greater stability to the air quality management process by reducing 

the possibility that statistically unusual indicator values will lead to transient violations of 

the standard. Utilizing a three-year average form provides stability from the occasional 

effects of inter-annual meteorological variability that can result in unusually high 

pollution levels for a particular year. Moreover, considering the lack of information on 

and the high degree of uncertainty regarding the impact on public welfare of the number 

of days with visibility impairment over the course of a year, the Administrator considers 

it reasonable to focus on the 90

th percentile, which represents the median of the 

distribution of the 20 percent worst visibility days, a key focus of the Regional Haze 

program. The Administrator concludes that ensuring that 90 percent of days have visual 

air quality that is at or below the target level of protection could be reasonably expected 

to lead to improvements in visual air quality on the 20 percent most impaired days, and 

that the limited information available in this review provides no basis for adopting a 

different form which would limit the occurrence of days with peak PM-related light 
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extinction in urban areas to a greater degree. Therefore, the Administrator concludes that 

a 90th percentile form, averaged over 3 years, is appropriate, for purposes of establishing 

a target level of protection in terms of a 24-hour PM2.5 visibility index.

With regard to level, the Administrator concludes that in light of the uncertainty 

associated with the high degree of variability in visibility conditions and the potential 

variability in visibility preferences across different parts of the country, it is appropriate 

to establish a target level of protection based on the upper end of the range of Candidate 

Protection Levels (CPLs) identified in the Policy Assessment (i.e., 20-30 dv) and 

generally supported by CASAC. Thus, the Administrator concludes that it would be 

appropriate to set a target level of protection in terms of a PM2.5 visibility index with a 

24-hour averaging time that would provide protection equivalent to the protection 

afforded by a 4-hour PM2.5 visibility index with a level of 30 dv. Furthermore, the 

Administrator notes that the approaches used to estimate generally equivalent levels for a 

24-hour PM2.5 visibility index generated 90th percentile 24-hour values similar to the 4-

hour values and a range of city-specific estimates of generally equivalent 24-hour levels 

that encompassed the range of levels considered appropriate for 4-hour CPLs, including 

the CPL of 30 dv at the upper end of that range. The Administrator thus concludes that it 

would be appropriate to use an unadjusted 4-hour CPL for purposes of establishing a 

target level of protection in terms of a 24-hour PM2.5 visibility index. 

In considering the alternative levels proposed for a 24-hour standard, either 28 dv 

or 30 dv, the Administrator concludes that the current substantial degrees of variability 

and uncertainty inherent in the public preference studies should be reflected in a higher 

target protection level than would be appropriate if the underlying information were more 
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consistent and certain. In addition, she concludes that, in light of the significant 

uncertainties, it is appropriate to place less weight on the results of western visibility 

preference studies and that the CPL value (30 dv) that is based on the eastern preference 

study results is likely to be more representative of urban areas that do not have associated 

mountains or other valued objects visible in the distant background For all of these 

reasons, the Administrator concludes that it is appropriate to set a target level of 

protection in terms of a 24-hour PM2.5 visibility index at 30 dv. 

In summary, in light of all the information available in this review, the 

Administrator concludes that the protection provided by a standard defined in terms of a 

PM2.5 visibility index (based on speciated PM2.5 mass concentrations and relative 

humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and a 90th

percentile form, averaged over 3 years, set at a level of 30 dv, would be requisite to 

protect public welfare with regard to visual air quality. 

In reaching this conclusion, the Administrator notes that any national ambient air 

quality standard to address PM-related visibility impairment would be designed to work 

in conjunction with the Regional Haze Program as a means of achieving appropriate 

levels of protection against PM-related visibility impairment in all areas of the country, 

including urban, non-urban, and Federal Class I areas. While the Regional Haze Program 

is focused on improving visibility in Federal Class I areas and a secondary NAAQS to 

address PM-related visibility impairment would focus on protecting visual air quality 

principally in urban areas, both programs could be expected to provide benefits in 

surrounding areas. In addition, the development of local programs, such as those in 

Denver and Phoenix, could continue to be an effective and appropriate approach to 
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provide additional protection, beyond that afforded by a national standard, for unique 

scenic resources in and around certain urban areas that are particularly highly valued by 

people living in those areas.

Having concluded that the protection provided by a standard defined in terms of a 

PM2.5 visibility index, with a 24-hour averaging time, and a 90th percentile form, 

averaged over 3 years, set at a level of 30 dv, would be requisite to protect public welfare 

with regard to visual air quality, the Administrator next has to determine whether to adopt 

such a visibility index as a distinct secondary standard. This determination requires 

considering such a secondary standard not in isolation but in the context of the full suite 

of secondary standards. As discussed above, the Administrator has determined to retain 

the current suite of secondary PM standards to address non-visibility welfare effects 

(except for the form of the annual standard). A distinct secondary standard to address 

visibility impairment is properly considered in a context where there is also a 24-hour 

PM2.5 standard of 35 μg/m3. 

In this context, the Administrator has considered the degree of protection from 

visibility impairment afforded by the existing secondary PM2.5 standards. The 

Administrator has considered both whether the existing 24-hour PM2.5 standard of 35 

μg/m3is sufficient (i.e. not under-protective) and whether it is not more stringent than 

necessary (i.e. not over-protective). 

As discussed above in section VI.C.1.f, the results of the Kelly et al. (2012a; 

2012b) analyses indicate that based on 2008-2010 and 2009-2011 data, all areas meeting 

the 24-hour PM2.5 standard of 35 μg/m3had visual air quality at least as good as 30 dv 

(24-hour average, based on 90th percentile form averaged over 3 years). This means that 
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it is highly likely that the secondary 24-hour PM2.5 standard of 35 μg/m3 would be 

controlling relative to a 24-hour standard based on a PM2.5 visibility index set at a level of 

30 dv, and highly unlikely that areas would exceed the target level of protection for 

visibility of 30 dv without also exceeding the existing secondary 24-hour standard. On 

the basis of this evidence, and the supporting public comments, the Administrator judges 

that the 24-hour PM2.5 standard of 35 μg/m3provides sufficient protection in all areas 

against the effects of visibility impairment—i.e., that the existing 24-hour PM2.5 standard 

would provide at least the target level of protection for visual air quality of 30 dv which 

the Administrator judges appropriate.

The Administrator also recognizes that the analyses presented in Kelly et al. 

(2012a; 2012b) indicate that the 24-hour PM2.5 standard of 35 μg/m3also would likely 

achieve more than the target level of protection of visual air quality (30 dv) in some 

areas. That is, when meeting a mass-based standard of 35 μg/m3, some areas would have 

levels of PM-related visibility impairment below 30 dv. Thus, the 24-hour PM2.5 standard 

of 35 μg/m3 would be over-protective in some areas (i.e. more stringent than necessary) 

relative to the target level of protection for visibility. This is not surprising, as the current 

mass-based standard does not account for variation in particle species and relative 

humidity. The 24-hour PM2.5 standard of 35 μg/m3 would provide more than the 

necessary protection in the areas where this would be expected, for example western 

areas with lower relative humidity. 

In light of the Administrator’s conclusion that it is appropriate to retain the 

current secondary 24-hour PM2.5 standard of 35 μg/m3for non-visibility welfare effects, 

the Administrator notes that this standard will remain in place regardless of whether she 
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elects to set a distinct secondary standard in terms of a PM2.5 visibility index. The issue is 

not whether to adopt a PM2.5 visibility index standard when viewed in isolation, but 

whether such a distinct secondary standard should be adopted in addition to the current 

secondary 24-hour PM2.5 standard of 35 μg/m3. The EPA notes that adoption of such a 

distinct secondary standard is not needed to provide sufficient protection from visibility 

impairment with respect to the target level of protection determined above. In addition, 

adoption of such a distinct secondary standard would not change the fact that the current 

secondary 24-hour PM2.5 standard of 35 μg/m3 would result in over-protection from 

visibility impairment in certain areas of the country. Such over-protection will occur 

whether or not such a distinct secondary standard is adopted. In effect, adopting such a 

distinct secondary standard would have no impact on the degree of protection provided 

from visibility impairment. Since sufficient protection from visibility impairment would 

be provided for all areas of the country without adoption of a distinct secondary standard, 

and adoption of a distinct secondary standard will not change the degree of overprotection provided for some areas of the country, the Administrator judges that adoption 

of such a distinct secondary standard is not needed to provide requisite protection for 

both visibility and non-visibility related welfare effects. 

It is important to note that this conclusion is based on the specific target level of 

protection determined above, and the specific set of current secondary standards. The 

Administrator’s conclusion with regard to the sufficiency of the protection provided by 

the current suite of secondary standards is based on comparing the a 30 dv target level of 

protection for a PM2.5 visibility index standard against the degree of protection provided 

by the current secondary 24-hour PM2.5 standard of 35 μg/m3. It is the combination of the 
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specific target level of protection and the current suite of secondary standards that is the 

basis for the decision not to adopt a distinct secondary standard in terms of a PM2.5

visibility index at this time. 

The EPA recognizes that, as in the last review, the final decision is to not adopt a 

distinct secondary standard to address visibility impairment. While the D.C. Circuit 

remanded the decision on a secondary standard in the last review, the EPA’s decision in 

this review has addressed the issues raised in the court’s remand. Here the EPA has 

clearly identified the target degree of protection (defined in terms of a PM2.5 visibility 

index at a level of 30 dv based on a 24-hour averaging time, and a 90th percentile form, 

averaged over 3 years) that would be requisite to protect public welfare with regard to 

visual air quality. The EPA has carefully compared this degree of protection with that 

provided by the current secondary 24-hour PM2.5 standard of 35 μg/m3, based on an areaspecific analysis of recent air quality data and concluded that the degree of protection 

from visibility impairment provided by the current secondary standard is sufficient to 

protect public welfare consistent with section 109(b)(2). This provides a clear basis for 

judging that the current secondary 24-hour PM2.5 standard of 35 μg/m3 would provide 

sufficient protection. The analysis also shows that the current secondary 24-hour PM2.5

standard would provide more protection than is needed in some areas, largely because it 

does not take into account variable factors such as relative humidity. However, the EPA 

has recognized that adoption of a distinct secondary standard to address visibility, in 

addition to retaining the current secondary standard, would not change this result. The 

EPA has therefore concluded that adoption of such a distinct secondary standard, in 

addition to the current suite of secondary PM standards, is not needed to provide requisite 
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protection for both visibility and non-visibility related welfare effects. Thus the EPA’s 

decision has carefully considered and accounted for the views of the court in the remand 

of the 2006 NAAQS. 

E. Administrator’s Final Decisions on Secondary PM Standards

To address PM-related welfare effects, including ecological effects, effects on 

materials, climate impacts, and visibility impairment, the Administrator is retaining the 

current suite of secondary PM standards, except for a change to the form of the annual

standard. Specifically, to address PM-related non-visibility welfare effects including 

ecological effects, effects on materials, and climate impacts, the EPA is retaining the 

current secondary 24-hour PM2.5 and PM10 standard and is revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. With respect to PM-related 

visibility impairment, the Administrator has identified a target degree of protection, 

defined in terms of a PM2.5 visibility index (based on speciated PM2.5 mass 

concentrations and relative humidity data to calculate PM2.5 light extinction), a 24-hour 

averaging time, and a 90th percentile form, averaged over 3 years, and a level of 30

deciviews (dv), which she judges to be requisite to protect public welfare with regard to 

visual air quality. The EPA’s analysis of monitoring data provides the basis for 

concluding that the current secondary 24-hour PM2.5 standard would provide sufficient 

protection, and in some areas greater protection, relative to this target protection level. 

Adding a distinct secondary standard to address PM-related visibility impairment would 

not affect this protection. Since sufficient protection from visibility impairment will be 

provided for all areas of the country without adoption of a distinct secondary standard, 
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and adoption of a distinct secondary standard will not change the degree of overprotection of visual air quality provided for some areas of the country by the secondary 

24-hour PM2.5 standard, the Administrator judges that adoption of a distinct secondary 

standard, in addition to the current suite of secondary standards, is not needed to provide 

requisite protection for both visibility and non-visibility related welfare effects.