From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov

Cc: martin.karen@epa.gov, hannon.john@epa.gov, Joseph-J Dougherty/DC/USEPA/US@EPA, Scott 

Jenkins/RTP/USEPA/US@EPA, Beth Hassett-Sipple/RTP/USEPA/US@EPA, Steven 

Silverman/DC/USEPA/US@EPA

Date: 06/08/2012 04:39 PM

Subject: Revisions to the PM2.5 level section of the PM NAAQS preamble

Nathan, 

Please find attached a redline file showing revisions to the Administrator's proposed conclusions on the primary 

PM2.5 standard levels (subsection III.E.4.e). This file includes the entire section on the PM2.5 levels. The 

Administrator's proposed conclusions subsection is the last third of this section.

We are continuing to edit the earlier subsections of the level section to address the interagency comments we 

received on June 6 related to the health effects evidence and specific comments regarding the primary standard. 

The attached file does not include any of our responses to those comments. We anticipate sending those 

additional edits to you on Tuesday morning. Please let me know if you have any questions. Thanks. 

In a moment, I will send you our edits to the AQI section of the preamble. 

Lydia
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1. 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 longterm exposure studies as the principal basis for selecting the level of the annual PM2.5 standard that 

would protect against effects associated with long-term exposures. The EPA relied upon evidence from 

the short-term exposures studies as the principal basis for selecting the level of the 24-hour PM2.5

standard that would protect against effects associated with short-term exposures. As summarized in 

section III.A.2 above, the 2006 decision to retain the level of the annual PM2.5 standard at 15 μg/m3 1

was challenged and on judicial review, the D.C. Circuit remanded the primary annual PM2.5 standard to 

the EPA, finding that EPA’s explanation for its approach to setting the level of the annual standard was 

inadequate. 

a. Approach Used in the Policy Assessment

Building upon the lessons learned in the previous PM NAAQS reviews, in considering 

alternative standard levels supported by the currently available scientific information, the Policy 

Assessment uses an approach that integrates evidence-based and risk-based considerations, takes into 

account CASAC advice, and considers the issues raised by the court in remanding the primary annual 

PM2.5 standard. Following the general approach outlined in section III.A.3, for the reasons discussed 

below, the Policy Assessment concludes it is appropriate to consider the protection afforded by the 

annual and 24-hour standards taken together against mortality and morbidity effects associated with both 

long- and short-term PM2.5 exposures. This is consistent with the approach taken in the review 

completed in 1997 rather than considering each standard separately, as was done in the review 

 

1

Throughout this section, the annual standard level is denoted as an integer value for simplicity, 

although, as noted above in section II.B.1, Table 1, footnote 7, the standard level is defined to one 

decimal place, such that the current standard level is 15.0 μg/m3. Alternative standard levels discussed in 

this section are similarly defined to one decimal place.
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completed in 2006. 

Beyond looking directly at the relevant epidemiologic evidence, the Policy Assessment considers 

the extent to which specific alternative PM2.5 standard levels are likely to reduce the nature and 

magnitude of both long-term exposure-related mortality risk and short-term exposure-related mortality 

and morbidity risk (US EPA, 2011a, section 2.3.4.2; U.S.EPA, 2010a, section 4.2.2). As noted in section 

III.C.3 above, patterns of increasing estimated risk reductions are generally observed as either the annual 

or 24-hour standard, or both, are reduced below the level of the current standards (US 2011a, Figures 

2-11 and 2-12; US EPA, 2010a, sections 4.2.2, 5.2.2, and 5.2.3). 

Based on the quantitative risk assessment, the Policy Assessment observes, as discussed in 

section III.A.3, that analyses conducted for this and previous reviews demonstrate that much, if not 

most, of the aggregate risk associated with short-term exposures results from the large number of days 

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

concentrations (US EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C.3, the Risk 

Assessment observes that alternative annual standard levels, when controlling, resulted in more 

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

degree of public health protection (US EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk Assessment 

notes that while the results of simulating alternative suites of PM2.5 standards including different 

combinations of alternative annual and 24-hour standard levels suggest that an alternative 24-hour 

standard level can produce additional estimated risk reductions beyond that provided by an alternative 

annual standard alone. However, the degree of estimated risk reduction provided by alternative 24-hour 

standard levels is highly variable, in part due to the choice of rollback approached used (US EPA, 

2010a, p. 5-17). 

Therefore, the Policy Assessment concludes, consistent with CASAC advice (Samet 2010c, p. 

1), that it is appropriate to set a “generally controlling” annual standard that will lower a wide range of 
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ambient 24-hour concentrations. The Policy Assessment concludes this approach would likely reduce 

aggregate risks associated with both long- and short-term exposures with more consistency than a 

generally controlling 24-hour standard and would be the most effective and efficient way to reduce total 

PM2-5-related population risk and so provide appropriate protection. The staff believes this approach, in 

contrast to one focusing on a generally controlling 24-hour standard, would likely reduce aggregate risks 

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

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

setting standards that are either more or less stringent than necessary in different geographical areas. 

The Policy Assessment recognizes that an annual standard intended to serve as the primary 

means for providing protection against effects associated with both long- and short-term PM2.5

exposures cannot be expected to offer an adequate margin of safety against the effects of all short-term 

PM2.5 exposures. As a result, in conjunction with a generally controlling annual standard, the Policy 

Assessment concludes it is appropriate to consider setting a 24-hour standard to provide supplemental 

protection, particularly for areas with high peak-to-mean ratios possibly associated with strong local or 

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

periods.

Based on the above considerations, the approach used in the Policy Assessment to identify 

alternative standard levels that are appropriate for consideration focuses on translating information from 

epidemiological studies into the basis for staff conclusions on levels. This approach is broader and more 

integrative than the general approach used by the EPA in previous reviews (see summary in section 

III.A.3 above; US EPA, 2011a, sections 2.1.3 and 2.3.4.1) and reflects the more extensive and stronger 

body of scientific evidence now available on health effects related to long- and short-term PM2.5

exposures, a more comprehensive quantitative risk assessment, and more extensive PM2.5 air quality 

data. In considering the currently available information, the Policy Assessment focuses on identifying 
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levels for an annual standard and a 24-hour standard that, in combination, provide protection against 

health effects associated with both long- and short-term PM2.5 exposures. The Policy Assessment also 

considers the extent to which various combinations of annual and 24-hour standards reflect setting a 

generally controlling annual standard with a 24-hour standard providing supplemental protection (US 

EPA, 2011a, sections 2.1.3, 2.3.4.1). 

As discussed in the Policy Assessment, EPA staff recognizes that there is no single factor or 

criterion that comprises the “correct” approach for reaching conclusions on alternative standard levels 

for consideration, but rather there are various approaches that are reasonable to consider (US EPA, 

2011a, section 2.3.4.1). In reaching conclusions in the Policy Assessment on the ranges of standard 

levels that are appropriate to consider, staff considered the relative weight to place on different evidence. 

The Policy Assessment initially focuses on long- and short-term PM2.5 exposure studies conducted in the 

U.S. and Canada and places the greatest weight on health outcomes judged in the Integrated Science 

Assessment as having evidence to support a causal or likely causal relationship. The Policy Assessment 

also considers the evidence for a broader range of health outcomes judged in the Integrated Science 

Assessment to have evidence suggestive of a causal relationship, specifically studies that focus on 

effects in susceptible populations, to evaluate whether this evidence provides support for considering 

lower alternative standard levels. 

Several factors were taken into account in placing relative weight on the body of available 

epidemiological studies, for example, study characteristics, including study design (e.g., time period of 

air quality monitoring, control for potential confounders); strength of the study (in terms of statistical 

significance and precision of results); and availability of population-level and air quality distribution 

data. As noted above in section III.A.3, the Policy Assessment places greatest weight on information 

from multi-city epidemiological studies to inform staff conclusions regarding alternative annual standard 
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levels. These studies have a number of advantages compared to single-city studies2that include 

providing representation of ambient PM2.5 concentrations and potential health impacts across a range of 

diverse locations providing spatial coverage for different regions across the country, reflecting 

differences in PM2.5 sources, composition, and potentially other exposure-related factors which might 

impact PM2.5-related risks; lack of ‘publication bias’ (US EPA, 2004, p. 8-30); and consideration of 

larger study populations that afford the possibility of generalizing to the broader national population and 

provide higher statistical power than single-city studies to detect potentially statistically significant 

associations with relatively more precise effect estimates. 

In reaching conclusions in the Policy Assessment regarding alternative 24-hour standard levels 

that are appropriate to consider, staff also considers relevant information from single-city short-term 

PM2.5 exposure studies. Although, as discussed above, multi-city studies have greater power to detect 

associations and provide broader geographic coverage in comparison to single-city studies, the extent to 

which effects reported in multi-city short-term PM2.5 exposure studies are associated with the specific 

short-term air quality in any particular location is unclear, especially when considering short-term 

concentrations at the upper end of the air quality distribution (i.e., at the 98th percentile value) for a 

given study area. In contrast, single-city studies are more limited in terms of power and geographic 

coverage but the link between reported health effects and the air quality in a given study area is more 

straightforward. Therefore, the Policy Assessment considers the results of both multi-city and single-city 

short-term exposure studies to inform staff conclusions regarding alternative levels that are appropriate 

to consider for a 24-hour standard that is intended to provide supplemental protection in areas where the 

 

2 As discussed in section III.B.1 above, the Policy Assessment recognizes that single-city studies 

provide ancillary evidence to multi-city studies in support of calling into question the adequacy of the 

current suite of standards. However, in light of the mixed findings reported in single-city short-term 

PM2.5 exposure studies, and the likelihood that these results are influenced by localized events and not 

representative of air quality across the country, the Policy Assessment places comparatively greater 

weight on the results from multi-city studies in considering alternative annual and 24-hour standard 

levels (US EPA, 2011a, p. 2-64). 
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annual standard may not offer appropriate protection against the effects of all short-term exposures (US 

EPA, 2011a, pp. 2-62 to 2-65). 

b. Consideration of the Annual Standard in the Policy Assessment

In recognizing the absence of a discernible population threshold below which effects would not 

occur, the Policy Assessment’s general approach for identifying alternative annual standard levels that 

are appropriate to consider focuses on characterizing the range of PM2.5 concentrations over which we 

have the most confidence in the associations reported in the epidemiological studies, and conversely 

where our confidence in the association becomes appreciably lower. The most direct approach to 

address this issue, consistent with CASAC advise (Samet, 2010c, p.10), is to consider epidemiological 

studies reporting confidence intervals around concentration-response relationships (US EPA, 2011a, p. 

2-63). Based on a thorough search of the available evidence, the Policy Assessment identified three 

long-term PM2.5 exposure studies reporting confidence intervals around concentration-response 

functions (i.e., Schwartz et al., 2008; Pope et al., 2002; Miller et al., 2007; US EPA, 2011a, pp. 2-65 to 

2-70 and Figure 2-3).3In its assessment of these studies, the Policy Assessment places greater weight on 

analyses that averaged across multiple concentration-response models since this approach represents a 

more robust examination of the underlying concentration-response relationship than analyses 

considering a single concentration-response model. Although these analyses of long-term exposure to 

PM2.5 provide information on the lack of any discernible population threshold, only Schwartz et al. 

(2008) conducted a multi-model analysis to characterize confidence intervals around the estimated 

concentration-response relationship that can help inform at what PM2.5 concentrations we have 

appreciably less confidence in the nature of the underlying concentration-response relationship. 

Although analyses of confidence intervals associated with concentration-response relationships can help 

 

3

The EPA carefully analyzed the published evidence, but was unable to identify any short-term PM2.5

exposure studies that characterized confidence intervals around concentration-response relationships. 

Nor did CASAC or public comments on this issue, as addressed in their comments on the second draft 

Policy Assessment, identify any additional analyses.
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inform consideration of alternative standard levels, the Policy Assessment concludes that the single 

relevant analysis now available is too limited to serve as the principal basis for identifying alternative 

standard levels in this review (US EPA, 2011a, p. 2-70).

The Policy Assessment explores other approaches that considered different statistical metrics to 

identify ranges of long-term mean PM2.5 concentrations that were most influential in generating health 

effect estimates in long- and short-term epidemiological studies, placing greatest weight on those studies 

that reported positive and statistically significant associations (US EPA, 2011a, p. 2-63). First, as 

discussed in section III.A.3 above, the Policy Assessment considered the statistical metric used in 

previous reviews. This approach recognizes that the strongest evidence of associations occurs at 

concentrations around the long-term mean concentration. Thus, in earlier reviews, the EPA focused on 

identifying standard levels that were somewhat below the long-term mean concentrations reported in 

PM2.5 exposure studies. The long-term mean concentrations represent air quality data typically used in 

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

health effects. Further, these data are available for all long- and short-term exposure studies analyzed 

and, therefore, represent the data set available for the broadest set of epidemiological studies. 

However, consistent with CASAC’s comments on the second draft Policy Assessment4(Samet, 

2010d, p. 2), in preparing the final Policy Assessment, EPA staff explored ways to take into account 

additional information from epidemiological studies, when available (Rajan et al., 2011). These analyses 

focused on evaluating different statistical metrics, beyond the long-term mean concentration, to 

characterize the range of PM2.5 concentrations down through which staff continued to have confidence 

in the associations observed in epidemiological studies and below which there is a comparative lack of 

data such that the staff’s confidence in the relationship was appreciably less. This would also be the 

 

4 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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range of PM2.5 concentrations which has the most influence on generating the health effect estimates 

reported in epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment 

recognizes there is no one percentile value within a given distribution that is the most appropriate or 

“correct” way to characterize where our confidence in the associations becomes appreciably lower. The 

Policy Assessment concludes that focusing on concentrations within the lower quartile of a distribution, 

such as the range from the 25th to the 10th percentile, is reasonable to consider as a region within which 

we begin to have appreciably less confidence in the associations observed in epidemiological studies.5In 

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.

As outlined in section III.A.3 above, the Policy Assessment recognizes that there are two types 

of population-level information to consider in identifying the range of PM2.5 concentrations which have 

the most influence on generating the health effect estimates reported in epidemiological studies. The 

most relevant information to consider is the number of health events (e.g., deaths, hospitalizations) 

occurring within a study population in relation to the distribution of PM2.5 concentrations likely 

experienced by study participants. Although not as directly relevant, the number of participants within 

each study area can be used as a surrogate for health event data in relation to the distribution of PM2.5

concentrations. 

In applying this approach, the Policy Assessment focuses on identifying the broader range of 

 

5

In the last review, staff believed it was appropriate to consider a level for an annual PM2.5 standard that 

was somewhat below the averages of the long-term concentrations across the cities in each of the key 

long-term exposures studies, recognizing that the evidence of an association in any such study was 

strongest at and around the long-term average where the data in the study are most concentrated. For 

example, the interquartile range of long-term average concentrations within a study and a range within 

one standard deviation around the study mean were considered reasonable approaches for characterizing 

the range over which the evidence of association is strongest (US EPA, 2005, pp. 5-22 to 5-23). In this 

review, the Policy Assessment noted the interrelatedness of the distributional statistics and a range of 

one standard deviation around the mean which contains approximately 68 percent of normally 

distributed data, in that one standard deviation below the mean falls between the 25th and 10th percentiles 

(US EPA, 2011a, p. 2-71).
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PM2.5 concentrations which had the most influence on generating health effect estimates in 

epidemiological studies. As discussed below, in working with study investigators, EPA staff was able to 

obtain health event data for three large multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 

2009; Bell et al., 2008) and population data for the same three studies and one additional long-term 

exposure study (Miller et al., 2007); as documented in a staff memorandum (Rajan et al., 2011).

Figures 1 through 3 summarize multi-city, long- and short-term exposure studies that evaluated 

endpoints classified in the Integrated Science Assessment as having evidence of a causal or likely causal 

relationship or suggestive of a causal relationship, showing the studies with long-term mean PM2.5

concentrations below 17 μg/m3.

6

Figures 1 and 3 provide summaries of long- and short-term exposure 

studies that evaluated mortality (evidence of a causal relationship); cardiovascular effects (evidence of a 

causal relationship); and respiratory effects (evidence of a likely causal relationship) in the general 

population, as well as in older adults (a susceptible population). Figure 2 provides a summary of studies 

that evaluated respiratory effects (evidence of a likely causal relationship) in children, a susceptible 

population, as well as long-term exposure studies of children that evaluated developmental effects 

(evidence suggestive of a causal relationship). 

As outlined above in section III.A.3, epidemiological studies typically report concentrations 

based upon a composite monitor distribution; that is, concentrations averaged across ambient monitors 

within each area included in a given study are averaged across study areas to calculate an overall study 

mean concentration. As noted above in section III.A.3 and discussed in the Policy Assessment, a policy 

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

and then compares those levels to concentrations at appropriate maximum monitors to determine if an 

area meets a given standard, inherently has the potential to build in some margin of safety (US EPA, 

2011a, p. 2-14).

 

6 Additional studies presented and assessed in the Integrated Science Assessment report effects at higher 

long-term mean PM2.5 concentrations (e.g., US EPA, 2009a, Figures 2-1, 2-2, 7-6, and 7-7).
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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 Adults

Source: US 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: US EPA, 2011a, Figure 2-5
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Figure 3. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Short-term PM2.5 Exposure 

Studies of the General Population and Older Adults

Source: US EPA, 2011a, Figure 2-6
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In analyses conducted by EPA staff based on selected long- and short-term exposure studies, the 

Policy Assessment notes that the differences between the maximum and composite distributions were 

greater for studies with fewer years of air quality data (i.e., 1 to 3 years) and smaller numbers of study 

areas (i.e., 36 to 51 study areas). The differences in the maximum and composite monitor distribution 

were much smaller for studies with more years of air quality data (i.e., up to 6 years) and larger numbers 

of study areas (i.e., 112 to 204 study areas) (Hassett-Sipple et al., 2010; US EPA, 2010f, section 2.3.4.1). 

Therefore, any margin of safety that may be provided by a policy approach that uses data based on 

composite monitor distributions to identify alternative standard levels, and then compares those levels to 

concentrations at appropriate maximum monitors to determine if an area meets a given standard, will 

vary depending upon the number of monitors and air quality distributions within a given area. See also, 

footnote 27 in section III.A.3 above and associated text.

The Policy Assessment initially considers distributional statistics for both long- and short-term 

exposure studies that evaluated mortality (causal relationship), cardiovascular effects (causal 

relationship) or respiratory effects (likely causal relationship). In looking first at the long-term mean 

concentrations in the long-term exposure studies, the Policy Assessment observes positive and often 

statistically significant associations at long-term mean PM2.5 concentrations ranging from 16.4 to 12.9 

μg/m3 7(Laden et al., 2006; Lipfert et al., 2006; Krewski et al., 2009; Goss et al., 2004; Miller et al.; 

2007; Zeger et al., 2008; Eftim et al., 2008; Dockery et al., 1996; McConnell et al., 2003; see Figures 1 

and 2). In considering the one long-term PM2.5 exposure study for which health event data are available 

(Krewski et al., 2009), the Policy Assessment observes that the long-term mean PM2.5 concentrations 

corresponding with study areas contributing to the 25th and 10th percentiles of the distribution of 

 

7 As discussed in footnote 58 above, the lowest long-term mean PM2.5 concentration reported in the 

long-term exposure studies was based on updated air quality data for Miller et al. (2007). As noted in the 

Policy Assessment, these air quality data were based on only one year of ambient measurements (2000) 

and in comparison to other long-term exposure studies that considered multiple years of air quality data, 

were much more limited (US EPA, 2011a, pp. 2-81 to 2-82).
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mortality data are 12.0 μg/m3and 10.2 μg/m3, respectively (US EPA, 2011a, Figure 2-7; Rajan et al., 

2011, Table 1). As identified above, although less directly relevant than event data, the number of 

participants within each study area can be used as a surrogate for health event data in relation to the 

distribution of PM2.5 concentrations. The long-term mean PM2.5 concentrations corresponding with study 

areas contributing to the 25th and 10th percentiles of the distribution of study participants for Miller et al. 

(2007) were 11.2 μg/m3and 9.7 μg/m3, respectively (US EPA, 2011a, Figure 2-7; Rajan et al., 2011, 

Table 1). 

In then considering information from multi-city, short-term exposure studies reporting positive 

and statistically significant associations with these same broad health effect categories, the Policy 

Assessment observes positive and statistically significant associations at long-term mean PM2.5

concentrations in a similar range of 15.6 to 12.8 μg/m3(Franklin et al., 2007, 2008; Klemm and Mason, 

2003; Burnett and Goldberg, 2003; Zanobetti and Schwartz, 2009; Burnett et al., 2004; Bell et al., 2008; 

Dominici et al., 2006a; see Figure 3). In considering the two multi-city, short-term PM2.5 exposure 

studies for which health event data are available, the Policy Assessment observes that the long-term 

mean PM2.5 concentrations corresponding with study areas contributing to the 25th and 10th percentiles 

of the distribution of deaths and cardiovascular-related hospitalizations are 12.5 μg/m3and 10.3 μg/m3, 

respectively, for Zanobetti and Schwartz (2009), and 11.5 μg/m3and 9.8 μg/m3, respectively, for Bell et 

al. (2008) (US EPA, 2011a, Figure 2-7; Rajan et al., 2011, Table 1). 

Taking into consideration additional studies of specific susceptible lifestages (i.e., childhood), 

the Policy Assessment expands its evaluation of the long-term exposure studies to include a broader 

range of health outcomes judged in the Integrated Science Assessment to have evidence suggestive of a 

causal relationship. This evidence was taken into account to evaluate whether it provides support for 

considering lower alternative levels than if weight were only placed on studies for which health effects 

have been judged in the Integrated Science Assessment to have evidence supporting a causal or likely 
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causal relationship. The Policy Assessment makes note of a limited number of studies that provide 

emerging evidence for PM2.5-related low birth weight and infant mortality, especially related to 

respiratory causes during the post-neonatal period. This more limited body of evidence indicates positive 

and often statistically significant effects associated with long-term PM2.5 mean concentrations in the 

range of 14.9 to 11.9 μg/m

3

(Woodruff et al., 2008; Liu et al., 2007; Bell et al., 2007; see Figure 2). As 

illustrated in Figure 2, although Parker and Woodruff (2008) did not observe an association between 

quarterly estimates of exposure to PM2.5 and low birth weight in a multi-city U.S. study, other U.S. and 

Canadian studies did report positive and statistically significant associations between PM2.5 and low 

birth weight at lower ambient concentrations (Bell et al., 2007; Liu et al., 2007).8There remain 

significant limitations (e.g., identifying the etiologically relevant time period) in the evaluation of 

evidence on the relationship between PM2.5 exposures and birth outcomes (US EPA, 2009a, pp. 7-48 and 

7-56) which should be taken into consideration in reaching judgments about how to weigh these studies 

of potential impacts on specific susceptible populations in considering alternative standard levels that 

provide protection with an appropriate margin of safety.

With respect to carcinogenicity, mutagenicity, and genotoxicity (evidence suggestive of a causal 

relationship), the strongest evidence currently available is from long-term prospective cohort studies that 

report positive associations between PM2.5 and lung cancer mortality. At this time, the PM2.5

concentrations reported in studies evaluating these effects generally included ambient concentrations 

that are equal to or greater than ambient concentrations observed in studies that reported mortality and 

cardiovascular and respiratory effects (US EPA, 2009a, section 7.5). Therefore, in selecting alternative 

standard levels appropriate to consider, the Policy Assessment noted that, in providing protection against 

mortality and cardiovascular and respiratory effects it is reasonable to anticipate that protection will also 

 

8 As noted in section 7.4 of the Integrated Science Assessment, Parker et al. (2005) reported that over a 

9-month exposure period (mean PM2.5 concentration of 15.4 μg/m3) a significant decrease in birth 

weight was associated with infants in the highest quartile of PM2.5 exposure as compared to infants 

exposed in the lowest quartile.
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be provided for carcinogenicity, mutagenicity, and genotoxicity effects (US EPA, 2011a, p. 2-78). 

In summarizing the currently available evidence and air quality information within the context of 

identifying potential alternative annual standard levels for consideration, the Policy Assessment first 

notes that the Integrated Science Assessment concludes there is no evidence of a discernible population 

threshold below which effects would not occur. Thus, health effects may occur over the full range of 

concentrations observed in the epidemiological studies. In the absence of any discernible thresholds, the 

general approach used in the Policy Assessment for identifying alternative standard levels that would 

provide appropriate protection against effects observed in epidemiological studies has focused on the 

central question of identifying the range of PM2.5 concentrations below the long-term mean 

concentrations where we continue to have confidence in the associations observed in epidemiological 

studies. 

In considering the evidence, the Policy Assessment recognizes that NAAQS are standards set so 

as to provide requisite protection, neither more nor less stringent than necessary to protect public health 

with an adequate margin of safety. This judgment, ultimately made by the Administrator, involves 

weighing the strength of the evidence and the inherent uncertainties and limitations of that evidence. 

Therefore, depending on the weight placed on different aspects of the evidence and inherent 

uncertainties, considerations of different alternative standard levels could be supported. 

As discussed in section III.A.3 above, the Policy Assessment recognizes that by applying the 

general policy approach used in previous reviews, one could focus on identifying alternative standard 

levels for the annual standard that are somewhat below the long-term mean PM2.5 concentrations 

reported in the long- and short-term exposure studies. By expanding this approach to consider additional 

population-level information from epidemiological studies, one could also focus on the range of PM2.5

concentrations below the long-term mean concentrations over which we continue to have confidence in 

the associations observed in epidemiological studies, as well as the extent to which our confidence in the 
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associations is appreciably less at lower concentrations, to identify alternative annual standard levels. 

Figure 4 provides a summary of the long-term mean PM2.5 concentrations for the long- and short-term 

exposure studies presented in Figures 1 through 3 as well as the range of PM2.5 concentrations 

corresponding with the 25th to 10th percentiles of health event or number of study participants data from 

the four multi-city studies, for which distributional statistics are available9(US EPA, 2011a, Figure 2-7; 

Rajan et al., 2011, Table 1). 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 effect estimates in those studies. The EPA 

anticipates that, in characterizing where the bulk of the data exist, patterns similar to those presented in 

Figure 4 would occur for the additional long- and short-term PM2.5 exposure studies.

Given the currently available evidence and considering the various approaches discussed above, 

the Policy Assessment concludes it is appropriate to focus on an annual standard level within a range of 

about 12 to 11 μg/m3(US EPA, 2011a, pp. 2-82, 2-101, and 2-106). As illustrated in Figure 4, a 

standard level of 12 μg/m3, at the upper end of this range, is somewhat below the long-term mean PM2.5

concentrations reported in all the multi-city long- and short-term exposure studies that provide evidence 

of positive and statistically significant associations with health effects classified as having evidence of a 

causal or likely causal relationship, including premature mortality and hospitalizations and emergency 

department visits for cardiovascular and respiratory effects as well as respiratory effects in children. 

Further, a level of 12 μg/m3 would reflect consideration of additional population-level information from 

such epidemiological studies in that it generally corresponds with approximately the 25th percentile of 

the available distributions of health events data in the studies for which population-level information 

 

9 Health event data (e.g., number of deaths, hospitalizations) occurring in a study population were 

obtained for three multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et al., 

2008) and study population data were obtained for the same three studies and one additional study 

(Miller et al., 2007) (US EPA, 2011a, p.2-71).
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was available.10 In addition, a level of 12 μg/m3 would reflect some consideration of studies that provide 

more limited evidence of reproductive and developmental effects, which are suggestive of a causal 

relationship, in that it is about at the same level as the lowest long-term mean PM2.5 concentrations 

reported in such studies (see Figure 4). 

Alternatively, an annual standard level of 11 μg/m3, at the lower end of this range, is well below 

the lowest long-term mean PM2.5 concentrations reported in all multi-city long- and short-term exposure 

studies that provide evidence of positive and statistically significant associations with health effects 

classified as having evidence of a causal or likely causal relationship. A level of 11 μg/m3 would reflect 

placing more weight on the distributions of health event and population data, in that this level is within 

the range of PM2.5 concentrations corresponding to the 25th and 10th percentiles of all the available 

distributions of such data.11 In addition, a level of 11 μg/m3is somewhat below the lowest long-term 

mean PM2.5 concentrations reported in reproductive and developmental effects studies that are 

suggestive of a causal relationship. Thus, a level of 11 μg/m3 would reflect an approach to translating 

the available evidence that places relatively more emphasis on margin of safety considerations than 

would a standard set at a higher level. Such a policy approach would tend to weigh uncertainties in the 

evidence in such a way as to avoid potentially underestimating PM2.5-related risks to public health. 

Further, recognizing the uncertainties inherent in identifying any particular point at which our 

confidence in reported associations becomes appreciably less, the Policy Assessment concludes that the 

available evidence does not provide a sufficient basis to consider alternative annual standard levels 

below 11 μg/m3(US EPA, 2011a, p. 2-81).

 

10 As outlined in section III.A.3, the Policy Assessment considers the 25th percentile to be the start of the 

range of PM2.5 concentrations below the mean within which the data become appreciably more sparse 

and, thus, where our confidence in the associations observed in epidemiological studies begins to 

become appreciably less. 

11 As discussed in section III.A.3, the Policy Assessment identifies the range from the 25th to the 10th

percentiles as a reasonable range to consider, in that it is a range where we have appreciably less 

confidence in the associations observed in epidemiological studies (US EPA, 2011a, p. 2-12). 
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1 Figure 4. Translating Epidemiological Evidence from Multi-City Exposure Studies into an Annual PM2.5 Standard

2

3 Source: US EPA, 2011a, Figure 2-8
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The Policy Assessment also considers the extent to which the available evidence provides a basis 

for considering alternative annual standard levels above 12 μg/m3. As discussed below, the Policy 

Assessment concludes that it could be reasonable to consider a standard level up to 13 μg/m3based on a 

policy approach that tends to weigh uncertainties in the evidence in such a way as to avoid potentially 

overestimating PM2.5-related risks to public health, especially to the extent that primary emphasis is 

placed on long-term exposure studies as a basis for an annual standard level. A level of 13 μg/m3is 

somewhat below the long-term mean PM2.5 concentrations reported in all but one of the long-term 

exposure studies providing evidence of positive and statistically significant associations with 

PM2.5-related health effects classified as having a causal or likely causal relationship. As shown in 

Figure 4, the one long-term exposure study with a long-term mean PM2.5 concentration just below 13 

μg/m3is the WHI study (Miller et al., 2007). As noted in section III.D.1.a above, including footnote 58, 

the Policy Assessment observes that in comparison to other long-term exposure studies, the WHI study 

was more limited in that it was based on only one year of air quality data (US EPA, 2011a, pp. 2-81 to 

2-82). Thus, to the extent that less weight is placed on the WHI study than on other long-term exposure 

studies with more robust air quality data, a level of 13 μg/m3could be considered as being protective of 

long-term exposure related effects classified as having a causal or likely causal relationship. In also 

considering short-term exposure studies, the Policy Assessment notes that a level of 13 μg/m3is below 

the long-term mean PM2.5 concentrations reported in most such studies, but is above the long-term 

means of 12.8 and 12.9 μg/m3reported in Burnett et al. (2004) and Bell et al. (2008), respectively. In 

considering these studies, the Policy Assessment finds no basis to conclude that these two studies are 

any more limited or uncertain than the other short-term exposure studies shown in Figures 3 and 4 (US 

EPA, 2011a, p. 2-82). On this basis, as discussed below, the Policy Assessment concludes that 

consideration of an annual standard level of 13 μg/m3 would have implications for the degree of 

protection that would need to be provided by the 24-hour standard, such that taken together the suite of 
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PM2.5 standards would provide appropriate protection from effects on public health related to short-term 

exposure to PM2.5 (US EPA, 2011a, p. 2-82).

The Policy Assessment also notes that a standard level of 13 μg/m3 would reflect a judgment that 

the uncertainties in the epidemiological evidence as summarized in section III.B.2 above, including 

uncertainties related to the heterogeneity observed in the epidemiological studies in the eastern versus 

western parts of the U.S., the relative toxicity of PM2.5 components, and the potential role of 

co-pollutants, are too great to warrant placing any weight on the distributions of health event and 

population data that extend down below the mean into the lower quartile of the data. This level would 

also reflect a judgment that the evidence from reproductive and developmental effects studies that is 

suggestive of a causal relationship is too uncertain to support consideration of any lower level.

Taken together, the Policy Assessment concludes that consideration of alternative annual 

standard levels in the range of 13 to 11 μg/m3 may be appropriate. Furthermore, the Policy Assessment 

concludes that the currently available evidence most strongly supports consideration of an alternative 

annual standard level in the range of 12 to 11 μg/m3(US EPA, 2011a, p. 2-82). The Policy Assessment 

concludes that an alternative level within the range of 12 to 11 μg/m3 would more fully take into 

consideration the available information from all long- and short-term PM2.5 exposure studies, including 

studies of susceptible populations, than would a higher level. This range would also reflect placing 

weight on information from studies that help to characterize the range of PM2.5 concentrations over 

which we continue to have confidence in the associations observed in epidemiological studies, as well as 

the extent to which our confidence in the associations is appreciably less at lower concentrations. 

c. Consideration of the 24-Hour Standard in the Policy Assessment

As recognized in section III.A.3 above, an annual standard intended to serve as the primary 

means for providing protection from effects associated with both long- and short-term PM2.5 exposures 

is not expected to provide appropriate protection against the effects of all short-term PM2.5 exposures 
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(unless established at a level so low as to undoubtedly provide more protection than necessary for longterm exposures). Of particular concern are areas with high peak-to-mean ratios possibly associated with 

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

exposure periods. As a result, the Administrator believesPolicy Assessment concludes that it is 

appropriate to consider alternative 24-hour PM2.5 standard levels that would supplement the protection 

provided by an annual standard. 

As outlined in section III.A.3 above, the Policy Assessment considers the available evidence 

from short-term PM2.5 exposure studies, as well as the uncertainties and limitations in that evidence, to 

assess the degree to which alternative annual and 24-hour PM2.5 standards can be expected to reduce the 

estimated risks attributed to short-term fine particle exposures. In considering the available 

epidemiological evidence, the Policy Assessment takes into account information from multi-city studies 

as well as single-city studies. The Policy Assessment considers the distributions of 24-hour PM2.5

concentrations reported in short-term exposure studies, focusing on the 98th percentile concentrations to 

match the form of the 24-hour standard as discussed in section III.E.3.b above. In recognizing that the 

annual and 24-hour standards work together to provide protection from effects associated with 

short-term PM2.5 exposures, the Policy Assessment also considers information on the long-term mean 

PM2.5 concentrations from these studies. 

In addition to considering the epidemiological evidence, the Policy Assessment also considers air 

quality information, specifically peak-to-mean ratios using county-level 24-hour and annual design 

values, to characterize air quality patterns in areas possibly associated with strong local or seasonal 

sources. These patterns help in understanding the extent to which different combinations of annual and 

24-hour standards would be consistent with the policy goal of setting a generally controlling annual 

standard with a 24-hour standard that provides supplemental protection especially for areas with high 

peak-to-mean ratios (US EPA, 2011a, p. 2-14). 
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In considering the information provided by the short-term exposure studies, the Policy 

Assessment recognizes that to the extent these studies were conducted in areas that likely did not meet 

one or both of the current standards, such studies do not help inform the characterization of the potential 

public health improvements of alternative standards set at lower levels. Therefore, in considering the 

short-term exposure studies to inform staff conclusions regarding levels of the 24-hour standard that are 

appropriate to consider, the Policy Assessment places greatest weight on studies conducted in areas that 

likely met both the current annual and 24-hour standards.

With regard to multi-city studies that evaluated effects associated with short-term PM2.5

exposures, as summarized in Figure 3, the Policy Assessment observes an overall pattern of positive and 

statistically significant associations in studies with 98th percentile values averaged across study areas in 

the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; 

Dominici et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The Policy Assessment notes 

that, to the extent air quality distributions were reduced to reflect just meeting the current 24-hour 

standard, additional protection would be anticipated for the effects observed in the three multi-city 

studies with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett and Goldberg, 

2003; Franklin et al., 2008). In the three additional studies with 98th percentile values below 35 μg/m3, 

specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 μg/m3, the Policy Assessment notes 

that these studies reported long-term mean PM2.5 concentrations of 12.9, 13.2, and 13.4 μg/m3, 

respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). To the extent that 

consideration is given to revising the level of the annual standard, as discussed above in section 

III.E.4.b, the Policy Assessment recognizes that potential changes associated with meeting such an 

alternative annual standard would result in lowering risks associated with both long- and short-term 

PM2.5 exposures. Consequently, in considering a 24-hour standard that would work in conjunction with 

an annual standard to provide appropriate public health protection, the Policy Assessment notes that to 
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the extent that the level of the annual standard is revised to within a range of 13 to 11 μg/m3, in 

particular in the range of 12 to 11 μg/m3, additional protection would be provided for the effects 

observed in these multi-city studies (US EPA, 2011a, p. 2-84). 

In summary, the Policy Assessment concludes that the multi-city, short-term exposure studies 

generally provide support for retaining the 24-hour standard level at 35 μg/m3in conjunction with an 

annual standard level revised to within a range of 12 to 11 μg/m3(US EPA, 2011a, p. 2-84). 

Alternatively, in conjunction with an annual standard level of 13 μg/m3, the Policy Assessment 

concludes that the multi-city studies provide limited support for revising the 24-hour standard level 

somewhat below 35 μg/m3, such as down to 30 μg/m3, based on one study (Bell et al., 2008) that 

reported positive and statistically significant effects with an overall 98th percentile value below the level 

of the current 24-hour standard in conjunction with an overall long-term mean concentration slightly less 

than 13 μg/m3(Figure 3; US EPA, 2011a, p. 2-84). 

In reaching staff conclusions regarding alternative 24-hour standard levels that are appropriate to 

consider, the Policy Assessment also takes into account relevant information from single-city studies 

that evaluated effects associated with short-term PM2.5 exposures. The Policy Assessment recognizes 

that these studies may provide additional insights regarding impacts on susceptible populations and/or 

on areas with isolated peak concentrations. Although, as discussed in section III.E.4.a above, multi-city 

studies have advantages over single-city studies in terms of statistical power to detect associations and 

broader geographic coverage as well as other factors such as less likelihood of publication bias, 

reflecting differences in PM2.5 sources, composition, and potentially other factors that could impact 

PM2.5-related effects, multi-city studies often present overall effect estimates rather than single-city 

effect estimates. Since short-term air quality can vary considerably across cities, the extent to which 

effects reported in multi-city studies are associated with short-term air quality in any particular location 

is uncertain, especially when considering short-term concentrations at the upper end of the distribution 
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of daily PM2.5 concentrations (i.e., at the 98th percentile value). In contrast, single-city studies are more 

limited in terms of power and geographic coverage but the link between reported health effects and the 

air quality in a given study area is more straightforward to establish. Therefore, the Policy Assessment 

also considers evidence from single-city, short-term exposure studies to inform staff conclusions 

regarding alternative levels that are appropriate to consider for a 24-hour standard that is intended to 

provide supplemental protection in areas where the annual standard may not provide an adequate margin 

of safety against the effects of all short-term PM2.5 exposures.

As discussed above for the multi-city studies, the Policy Assessment takes into account both the 

24-hour PM2.5 concentrations in the single-city studies, focusing on the 98th percentile air quality values, 

as well as the long-term mean PM2.5 concentrations. The Policy Assessment considers single-city studies 

conducted in areas that would likely have met the current suite of PM2.5 standards as most useful for 

informing staff conclusions related to the level of the 24-hour standard (US EPA, 2011a, Figure 2-9). 

The Policy Assessment notes that additional single-city studies summarized in that Figure 2-9 were 

conducted in areas that would likely have met one but not both of the current PM2.5 standards. To the 

extent changes in air quality designed to just meet the current suite of PM2.5 standards are undertaken, 

one could reasonably anticipate additional public health protection will occur in these study areas. 

Therefore, the Policy Assessment concludes that these studies are not helpful to inform staff conclusions 

regarding alternative standard levels that are appropriate to consider (US EPA, 2011a, p. 2-87).

With regard to single-city studies that were conducted in areas that would likely have met both 

the current 24-hour and annual standards, the Policy Assessment first considers studies that reported 

positive and statistically significant associations. In considering this group of studies, the Policy 

Assessment notes Mar et al. (2003) reported a positive and statistically significant association for 

premature mortality in Phoenix with a long-term mean concentration of 13.5 μg/m3in conjunction with 

a 98th percentile value of 32.2 μg/m3(US EPA, 2011a, Figure 2-9). To the extent that consideration is 
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given to revising the level of the annual standard, within a range of 13 to 11 μg/m3, as discussed above, 

additional protection would be provided for the effects observed in this study (US EPA, 2011a, p. 2-87). 

Four additional studies reported positive and statistically significant associations with 98th

percentile values within a range of 31.2 to 25.8 μg/m3and long-term mean concentrations within a range 

of 12.1 to 8.5 μg/m3(Delfino et al., 1997; Peters et al., 2001; Stieb et al., 2000; and Mar et al., 2004; US 

EPA, 2011a, Figure 2-9). Delfino et al. (1997) reported statistically significant associations between 

PM2.5 and respiratory emergency department visits for older adults (greater than 64 years old) but not 

young children (less than 2 years old), in one part of the study period (summer 1993) but not the other 

(summer 1992). Peters et al. (2001) reported a positive and statistically significant association between 

short-term exposure to PM2.5 (2-hour and 24-hour averaging times) and onset of acute myocardial 

infarction in Boston. Stieb et al. (2000) reported positive and statistically significant associations with 

cardiovascular- and respiratory-related emergency department visits in Saint John, Canada, in single 

pollutant models but not in multi-pollutant models (US EPA, 2004, pp. 8-154 and 8-252 to 8-253). Mar 

et al. (2004) reported a positive and statistically significant association for short-term PM2.5 exposures in 

relation to respiratory symptoms among children but not adults in Spokane, however, this study had very 

limited statistical power because of the small number of children and adults evaluated. 

The Policy Assessment also considers short-term single-city PM2.5 exposure studies that reported 

positive but nonstatistically significant associations for cardiovascular and respiratory endpoints in areas 

that would likely have met both the current 24-hour and annual standards. The 98th percentile values 

reported in these studies ranged from 31.6 to 17.2 μg/m3and the long-term mean concentrations ranged 

from 13.0 to 7.0 μg/m3(US EPA, 2011a, Figure 2-9). These studies included consideration of 

cardiovascular-related mortality effects in Phoenix (Wilson et al., 2007), asthma medication use in 

children in Denver (Rabinovitch et al., 2006), hospital admissions for hemorrhagic and ischemic stroke 

in Edmonton, Canada (Villeneuve et al., 2006), and hospital admissions for ischemic stroke/transient 
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ischemic attack in Nueces County, TX (Lisabeth et al., 2008). 

Lastly, the Policy Assessment considers single-city studies conducted in areas that would likely 

have met both the current 24-hour and annual standards that reported null findings. The 98th percentile 

values reported in these studies ranged from 29.6 to 24.0 μg/m3and the long-term mean concentrations 

ranged from 10.8 to 8.5 μg/m3(US EPA, 2011a, Figure 2-9). These studies reported no associations with 

short-term PM2.5 exposures and cardiovascular-related hospital admissions and respiratory-related 

emergency department visits (Slaughter et al., 2005) and cardiovascular-related emergency department 

visits (Schreuder et al., 2006) in Spokane; asthma exacerbation in children in Denver (Rabinovitch et al., 

2004); and hospital admissions for transient ischemic attack in Edmonton, Canada (Villeneuve et al., 

2006). 

Viewing the evidence as a whole, the Policy Assessment observes a limited number of singlecity studies that reported positive and statistically significant associations for a range of health endpoints 

related to short-term PM2.5 concentrations in areas that would likely have met the current suite of PM2.5

standards. Many of these studies had significant limitations (e.g., limited statistical power, limited 

exposure data) or equivocal results (i.e., mixed results within the same study area) as briefly identified 

above and discussed in more detail in the Policy Assessment (US EPA, 2011a, p. 2-88). Other studies 

reported positive but not statistically significant results or null associations also in areas that would 

likely have met the current suite of PM2.5 standards. Overall, the entire body of results from these singlecity studies is mixed, particularly as 24-hour 98th percentile concentrations go below 35 μg/m3

. 

Although a number of single-city studies report effects at appreciably lower PM2.5 concentrations 

than multi-city short-term exposure studies, the uncertainties and limitations associated with the singlecity studies were greater and, thus, the Policy Assessment concludes there is less confidence in using 

these studies as a basis for setting the level of a standard. Therefore, the Policy Assessment concludes 

that the multi-city short-term exposure studies provide the strongest evidence to inform decisions on the 
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level of the 24-hour standard, and the single-city studies do not warrant consideration of 24-hour 

standard levels different from those supported by the multi-city studies (US EPA, 2011a, p. 2-88). 

In addition to considering the epidemiological evidence, the Policy Assessment takes into 

account air quality information based on county-level 24-hour and annual design values to understand 

the implications of the alternative standard levels supported by the currently available scientific 

evidence, as discussed in section III.E.4.b above. As discussed in section III.A.3 above, the Policy 

Assessment concludes that a policy goal which includes setting the annual standard to be the “generally 

controlling” standard in conjunction with setting the 24-hour standard to provide supplemental 

protection, to the extent that additional protection is warranted, is the most effective and efficient way to 

reduce total population risk associated with both long- and short-term PM2.5 exposures, resulting in more 

uniform protection across the U.S than the alternative of setting the 24-hour standard to be the 

controlling standard. Therefore, the Policy Assessment considers the extent to which different 

combinations of alternative annual and 24-hour standard levels based on the evidence would support this 

policy goal (US EPA, 2011a, pp 2-88 to 2-91, Figure 2-10). 

Using information on the relationship of the 24-hour and annual design values, the Policy 

Assessment examines the implications of three alternative suites of PM2.5 standards identified as 

appropriate to consider based on the currently available scientific evidence, as discussed above. The 

Policy Assessment concludes that an alternative suite of PM2.5 standards that would include an annual 

standard level of 11 or 12 μg/m3and a 24-hour standard with a level of 35 μg/m3(i.e., 11/35 or 12/35) 

would result in the annual standard being the generally controlling standard in most areas although the 

24-hour standard would continue to be the generally controlling standard in the Northwest (US EPA, 

2011a, pp. 2-89 to 2-91 and Figure 2-10). These Northwest counties generally represent areas where the 

annual mean PM2.5 concentrations have historically been low but where relatively high 24-hour 

concentrations occur, often related to seasonal wood smoke emissions. Alternatively, combining an 
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alternative annual standard of 13 μg/m3 with a 24-hour standard of 30 μg/m3 would result in many more 

areas across the country in which the 24-hour standard would likely become the controlling standard 

than if an alternative annual standard of 12 or 11 μg/m3 were paired with the current level of the 24-hour 

standard (i.e., 35 μg/m3). 

The Policy Assessment concludes that consideration of retaining the 24-hour standard level at 35 

μg/m3 would reflect placing greatest weight on evidence from multi-city studies that reported positive 

and statistically significant associations with health effects classified as having a causal or likely causal 

relationship. In conjunction with lowering the annual standard level, especially within a range of 12 to 

11 μg/m3, this alternative would recognize additional public health protection against effects associated 

with short-term PM2.5 exposures which would be provided by lowering the annual standard such that 

revision to the 24-hour standard would not be warranted (US EPA, 2011a, p. 2-91). 

The Policy Assessment also recognizes an alternative approach to considering the evidence that 

provides some support for revising the level below 35 μg/m3, perhaps as low as 30 μg/m3(US EPA, 

2011a, p. 2-92). This alternative 24-hour standard level would be more compatible with an alternative 

annual standard of 13 μg/m3based on placing greater weight on one multi-city short-term exposure 

study (Bell et al., 2008) that reported positive and statistically significant effects at a 98th percentile 

value less than 35 μg/m3(i.e., 34.2 μg/m3) in conjunction with a long-term mean concentration less than 

13 μg/m3(i.e., 12.9 μg/m3). 

Taken together, the Policy Assessment concludes that while it is appropriate to consider an 

alternative 24-hour standard level within a range of 35 to 30 μg/m3, the currently available evidence 

most strongly supports consideration for retaining the current 24-hour standard level at 35 μg/m3in 

conjunction with lowering the level of the annual standard within a range of 12 to 11 μg/m3(US EPA, 

2011a, p. 2-92).

d. CASAC Advice
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Based on its review of the second draft Policy Assessment, CASAC agreed with the general 

approach for translating the available epidemiological evidence, risk information, and air quality 

information into the basis for reaching conclusions on alternative standards for consideration. 

Furthermore, CASAC agreed “that it is appropriate to return to the strategy used in 1997 that considers 

the annual and the short-term standards together, with the annual standard as the controlling standard, 

and the short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have evidence 

supportive of a causal or likely causal relationship as presented in the Integrated Science Assessment 

(Samet, 2010d, p. 1). 

CASAC concluded that the range of levels presented in the second draft Policy Assessment (i.e., 

alternative annual standard levels within a range of 13 to 11 μg/m3and alternative 24-hour standard 

levels within a range of 35 to 30 μg/m3) “are supported by the epidemiological and toxicological 

evidence, as well as by the risk and air quality information compiled” in the Integrated Science 

Assessment, Risk Assessment, and second draft Policy Assessment. CASAC further noted that 

“[a]lthough there is increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a 

level below which there is no risk for adverse health effects)” (Samet, 2010d, p. ii). 

Although CASAC supported the alternative standard level ranges presented in the second draft 

Policy Assessment, it did not express support for any specific levels or combinations of standards. 

Rather, CASAC encouraged the EPA to develop a clearer rationale in the final Policy Assessment for 

staff conclusions regarding annual and 24-hour standards that are appropriate to consider, including 

consideration of the combination of these standards supported by the available information (Samet, 

2010d, p. ii). Specifically, CASAC encouraged staff to focus on information related to the 

concentrations that were most influential in generating the health effect estimates in individual studies to 

inform alternative standard levels (Samet, 2010d, p. 2). CASAC also commented that the approach 
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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, EPA staff conducted additional analyses and modified their conclusions regarding 

alternative standard levels that are appropriate to consider. The staff conclusions in the final Policy 

Assessment (US EPA, 2011a, section 2.3.4.4) differ somewhat from the alternative standard levels 

discussed in the second draft Policy Assessment (US EPA, 2010f, section 2.3.4.3), upon which CASAC 

based its advice. Changes made in the final Policy Assessment were primarily focused on improving and 

clarifying the approach for translating the epidemiological evidence into a basis for staff conclusions on 

the broadest range of alternative standard levels supported by the available scientific information and 

more clearly articulating the rationale for the staff’s conclusions (Wegman, 2011, pp. 1 to 2). In 

commenting on draft staff conclusions in the second draft Policy Assessment, CASAC did not have an 

opportunity to review the analyses of distributional statistics conducted by staff to identify the broader 

range of PM2.5 concentrations that were most influential in generating health effect estimates in 

epidemiological studies (Rajan et al., 2011). In addition, CASAC was not aware of the revised long-term 

mean PM2.5 concentration in the WHI study as discussed in section III.D.1.a above, specifically footnote 
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58, or the staff’s inclusion of that value in its evaluation of the evidence (i.e., in Figure 4 above and 

related discussion). The WHI study is the only long-term cohort study that provides information 

regarding effects classified as having evidence of a causal or likely causal relationship associated with a 

long-term PM2.5 concentration below 13 μg/m3. However, as noted in section III.D.1.a above, this study 

was based on only one year of air quality data and linked to effects that occurred in earlier years. 

Furthermore, CASAC did not have an opportunity to review the staff’s revised rationale for the 

combinations of alternative standards suggested in the final Policy Assessment. 

e. Administrator’s Proposed Conclusions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate alternative standard levels to consider, the 

Administrator has considered the epidemiological and other scientific evidence, estimates of risk 

reductions associated with just meeting alternative annual and/or 24-hour standards, air quality analyses, 

related limitations and uncertainties and the advice of CASAC. As an initial matter, the Administrator 

agrees with the approach discussed in the Policy Assessment as summarized in sections III.A.3 and 

III.E.4.a above, and supported by CASAC, of considering the protection afforded by the annual and 

24-hour standards taken together for mortality and morbidity effects associated with both long- and 

short-term exposures to PM2.5. This is consistent with the approach taken in the review completed in 

1997, in contrast to considering each standard separately, as was done in the review completed in 2006. 

Furthermore, based on the evidence and quantitative risk assessment, the Administrator provisionally 

concludes it is appropriate to set a “generally controlling” annual standard that will lower a wide range 

of ambient 24-hour concentrations, with a 24-hour standard focused on providing supplemental 

protection, particularly for areas with high peak-to-mean ratios possibly associated with strong local or 

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

periods. The Administrator provisionally concludes this approach would likely reduce aggregate risks 

associated with both long- and short-term exposures more consistently than a generally controlling 
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24-hour standard and would be the most effective and efficient way to reduce total PM2-5-related 

population risk and so provide appropriate protection. In reaching decisions on alternative standard 

levels to propose, the Administrator judges that it is appropriate to examine where the evidence of 

associations observed in the epidemiological studies is strongest and, conversely, where she has 

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

Based on the characterization and assessment of the epidemiological and other studies presented 

and assessed in the Integrated Science Assessment and the Policy Assessment, the Administrator 

recognizes the substantial increase in the number and diversity of studies available in this review 

including extended analyses of the seminal studies of long-term PM2.5 exposures (i.e., ACS and Harvard 

Six Cities studies) as well as important new long-term exposure studies (as summarized in Figures 1 and 

2). Collectively, the Administrator takes note that these studies, along with evidence available in the last 

review, provide consistent and stronger evidence of an association with premature mortality, with the 

strongest evidence related to cardiovascular-related mortality, at lower ambient concentrations than 

previously observed. The Administrator also recognizes the availability of stronger evidence of 

morbidity effects associated with long-term PM2.5 exposures, including evidence of cardiovascular 

effects from the WHI study and respiratory effects, including decreased lung function growth, from the 

extended analyses for the Southern California Children’s Health Study. Furthermore, the Administrator 

recognizes new U.S. multi-city studies that greatly expand and reinforce our understanding of mortality 

and morbidity effects associated with short-term PM2.5 exposures, providing stronger evidence of 

associations at ambient concentrations similar to those previously observed (as summarized in Figure 3). 

The newly available scientific evidence builds upon the previous scientific data base to provide 

evidence of generally robust associations and to provide a basis for greater confidence in the reported 

associations than in the last review. The Administrator recognizes that the strengthweight of the 

scientific evidence for evaluating potential health effects associated with fine particles and notes, as 
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evaluated in the Integrated Science Assessment concludes that this body of , is strongest for health 

endpoints classified as having evidence supportsof a causal relationship. These relationships include 

those between long- and short-term PM2.5 exposures and mortality and cardiovascular effects and . She 

recognizes that the weight of evidence is also strong for health endpoints classified as having evidence 

of a likely causal relationship, which include those between long- and short-term PM2.5 exposures and 

respiratory effects. In addition, the Administrator makes note of additional, butthe much more limited 

evidence for a broader range of health endpoints includingclassified as having evidence suggestive of a 

causal relationship for, including developmental and, reproductive effects as well as forand 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 long- and short-term epidemiological studies and that no discernible threshold for any effects can be 

identified based on the currently available evidence (US EPA, 2009a, section 2.4.3). She also

recognizes, in taking note of CASAC advice and the distributional statistics analysis discussed in the 

Policy Assessment, that there is significantly greater confidence in observed associations over certain 

parts of the air quality distributiondistributions in the studies, and conversely, that there is significantly 

diminished confidence in ascribing effects to concentrations fromtoward the lowestlower part of the 

rangedistributions.

Consistent with the general approach summarized in section III.A.3 above, and supported by 

CASAC as discussed in section III.E.4.d above, the Administrator generally agrees that it is appropriate 

to consider a level for an annual standard that is somewhat below the long-term mean PM2.5

concentrations reported in long- and short-term exposure studies, recognizing that the evidence of an 

association in any such study is strongest at and around the long-term average where the data in the 

study are most concentrated. The Administrator notes that long-term mean PM2.5 concentrations are 
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available for each study considered and, therefore, represent the most robust data set to inform her 

decisions on appropriate annual standard levels. In considering such long-term mean concentrations, the 

Administrator understands that it is appropriate to consider the weight of evidence for the health 

endpoints evaluated in such studies in giving weight to this information.

Based on the information summarized in Figure 4 and presented in more detail in the Policy 

Assessment (US EPA, 2011a, chapter 2),) for effects classified in the Integrated Science Assessment as 

having a causal or likely causal relationship with PM2.5 exposures, the Administrator observes an overall 

pattern of statistically significant associations reported in studies of long-term PM2.5 exposures with 

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) and). She 

observes a similar pattern of statistically significant associations in studies of short-term PM2.5

exposures with long-term mean concentrations ranging from around 15 μg/m3down to 12.8 μg/m3(in 

Burnett et al., 2004) for ). With regard to effects classified in the Integrated Science Assessment as 

having a causal or likely causal relationship with PM2.5 exposures. 

In looking at information for a broader range of health endpoints, the Administrator also notes 

more limited evidence of serious effects in susceptible populationsas providing evidence suggestive of a 

causal relationship (e.g., developmental and reproductive effects, cancer). With regard to developmental 

and reproductive effects, the Administrator observes a small number of studies related to developmental 

and reproductive effects that reported statistically significant associations for long-term PM2.5

concentrations with long-term means down to 11.9 μg/m3(in Bell et al., 2007). 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 (US EPA, 2009a, section 7.5). Therefore, the Administrator provisionally concludes 
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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.12

The Administrator also considers additional information from epidemiological studies, consistent 

with CASAC advice, to take into account the broader distribution of PM2.5 concentrations and the degree 

of confidence in the observed associations over the broader air quality distribution. In considering this 

additional information, she considers the range from the 25th to the 10th percentiles, consistent with the 

Policy Assessment analyses. More specifically, the Administrator recognizes the 25th percentile as the 

start of the range of PM2.5 concentrations below the mean within which the data become appreciably 

more sparse and, thus, where her confidence in the associations observed in epidemiological studies 

begins to become appreciably less. As discussed in section III.E.4.b above and summarized in Figure 4,

the Administrator takes note of additional population-level data that are available for four studies 

(Krewski et al., 2009; Miller et al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009).), each of 

which report statistically significant associations with health endpoints classified as having evidence of a 

causal relationship. In considering the long-term mean PM2.5 concentrations associated with the 25th

percentile values for these four studies, she observes that these values range from somewhat above to 

somewhat below 12 μg/m3(Figure 4). The Administrator recognizes that these four studies represent 

some of the strongest evidence available within the overall body of scientific evidence and notes that 

three of these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) were used 

as the basis for concentration-response functions used in the quantitative risk assessment (US EPA, 

2010a, section 3.3.3). However, the Administrator also recognizes that additional population-level data 

 

12 With respect to suggestive evidence related to cancer, mutagenic, and genotoxic effects, the PM2.5

concentrations reported in studies generally included ambient concentrations that are equal to or greater 

than ambient concentrations observed in studies that reported mortality and cardiovascular and 

respiratory effects (US EPA, 2009a, section 7.5), such that in selecting alternative standard levels that 

provide protection from mortality and cardiovascular and respiratory effects, it is reasonable to 

anticipate that protection will also be provided for carcinogenic effects.
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are available for only these four studies and, therefore, she believes that these studies comprise a more 

limited data set than one based on long-term mean PM2.5 concentrations for which data are available for 

all studies considered, as discussed above. Nonetheless, consistent with CASAC adviceIn considering 

this information, the Administrator considersnotes that CASAC advised that information about the longterm PM2.5 concentrations that were most influential in generating the health effect estimates in 

epidemiological studies as helpingcan help to inform selection of an appropriate annual standard level. 

The Administrator recognizes, as summarized in section III.B.2 above, that important 

uncertainties remain in the evidence and information considered in this review of the primary fine 

particle standards. These uncertainties are generally related to understanding the relative toxicity of the 

different components in the fine particle mixture, the role of PM2.5 in the complex ambient mixture, 

exposure measurement errors inherent in epidemiological studies based on concentrations measured at 

fixed monitor sites, and the nature and, magnitude of, and confidence in estimated risks related to 

increasingly lower ambient PM2.5 concentrations. Furthermore, the Administrator notes that 

epidemiological studies have reported heterogeneity in responses both within and between cities and 

geographic regions across the U.S. She recognizes that this heterogeneity may be attributed, in part, to 

differences in fine particle composition in different regions and cities. 

With regard to The Administrator also recognizes that there are additional limitations associated 

with evidence for reproductive and developmental effects, identified as being suggestive of a causal 

relationship with long-term PM2.5 exposures, 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 
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for considering that serious effects may be occurring in a susceptible population at concentrations lower 

than those associated with effects classified as having a causal or likely causal relationship with longterm PM2.5 exposures (i.e., mortality, cardiovascular, and respiratory effects).

The Administrator is mindful that considering what standards are requisite to protect public 

health with an adequate margin of safety requires public health policy judgments that neither overstate 

nor understate the strength and limitations of the evidence or the appropriate inferences to be drawn 

from the evidence. In considering how to translate the available information into appropriate standard 

levels, the Administrator weighs the available scientific information and associated uncertainties and 

limitations. For the purpose of determining what standard levels are appropriate to propose, the 

Administrator recognizes, as did EPA staff in the Policy Assessment, that there is no single factor or 

criterion that comprises the “correct” approach to weighing the various types of available evidence and 

information, but rather there are various approaches that are appropriate to consider. The Administrator 

further recognizes that different evaluations of the evidence and other information before the 

Administrator could reflect placing different weight on the relative strengths and limitations of the 

scientific information, and different judgments could be made as to how such information should 

appropriately be used in making public health policy decisions on standard levels. This recognition leads 

the Administrator to consider various approaches to weighing the evidence so as to identify an 

appropriate range of alternative standard levels, rather than a single level, to propose. In so doing, the 

Administrator encourages extensive public comment on alternative approaches to weighing the evidence 

so as to inform her public health policy judgments before reaching final decisions on appropriate 

standard levels.

In considering thisthe available information, the Administrator notes the advice of CASAC that 

the currently available scientific information provides 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 
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addition, the Administrator recognizes that the Policy Assessment concludes that the evidence most 

strongly supports consideration of an annual standard level in the range of 12 to 11 μg/m3. In 

considering how the annual and 24-hour standards work together to provide appropriate public health 

protection, the Administrator observes that the Policy Assessment concludes it is appropriate to consider 

retaining the current 24-hour standard level at 35 μg/m3in conjunction with an annual standard in the 

range of 12 to 11 μg/m3, as well as to consider an alternative 24-hour standard level of 30 μg/m3, 

particularly in conjunction with an annual standard level of 13 μg/m3. As noted in section III.E.4.d 

above, the Administrator observes that CASAC did not express support for any specific levels or 

combinations of standards within in these ranges, although she recognizes that CASAC did not have an 

opportunity to review additional information and analyses presented in the final Policy Assessment

prepared in response to CASAC’s recommendations on the second draft Policy Assessment. Nor did 

CASAC have an opportunity to review the EPA staff’s revised rationale for the combinations of 

alternative standards presented in the final document.

In considering the extent to which the currently available evidence, as summarized in Figure 4 

and discussed more broadly above, the and information on risk,provide support for specific standard 

levels within the ranges identified by CASAC advice, and the conclusions of the Policy Assessment, the 

Administrator provisionally concludes that an annual standard level of 12 μg/m3 would provide as 

appropriate protection against effects associated with long- and short-term PM2.5 exposures. In the 

Administrator’s judgment, an annual standard of 12 μg/m3reflects placing greatest weight on evidence 

of effects for which the Integrated Science Assessment has determined there is a causal or likely causal 

relationship with long- and short-term PM2.5 exposures. An annual standard level of 12 μg/m3 would be 

below the for consideration, the Administrator initially considers standard levels within the range of 13 

to 11 μg/m3for the annual standard. In so doing, the Administrator first considers the long-term mean 

PM2.5 concentrations reported in these studies. Furthermore, an annual standard level of 12 μg/m3
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reflects placing some weight on additional population-level information studies of effects classified as 

having evidence of a causal or likely causal relationship, as summarized in Figure 4 and discussed more

broadly above. She notes that a level of 13 would be below most but not all the mean concentrations 

from a limited number of epidemiological studies providingthe multi-city studies of long- and short-term 

exposures, whereas a somewhat lower level, such as 12 μg/m3or lower, would be below all such mean 

concentrations. In considering the appropriate weight to place on this information, the Administrator 

again notes that the evidence of an association in any such study is strongest at and around the long-term 

average where the data in the study are most concentrated, and that long-term mean PM2.5

concentrations are available for each study considered and, therefore, represent the most robust data set 

to inform her decisions on appropriate annual standard levels.

In considering the long-term mean PM2.5 concentrations reported in studies of effects classified 

as having evidence suggestive of a causal relationship, as summarized in Figure 4 for reproductive and 

developmental effects, the Administrator notes that a level of 13 would be below the mean concentration 

in one of the three studies, while lower levels such as 12 or 11 μg/m3 would be below the mean 

concentration in two or three of these studies, respectively. In considering the appropriate weight to 

place on this information, the Administrator notes the very limited nature of this evidence of such effects 

and the additional uncertainties in these epidemiological studies relative to the studies that provide 

evidence of causal or likely causal relationships.

In considering additional distributional statistics of population-level information that were 

available from four of the epidemiological studies that provide evidence of effects identified as having a 

causal relationship with long- or short-term PM2.5 concentrations. More specifically,, the Administrator 

notes that a level of 12 μg/m3is the PM2.5 concentration that generally corresponds with approximately 

the 25th percentile of the available distributions of health events data available in thethree of these

studies for which population-level information was available. The Administrator also recognizes a level 
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of 12notes that higher standard levels up to 13 μg/m3 would reflect placing significantly less weight on 

this statistical information, whereas lower standard levels down to 11 μg/m3 would reflect some 

consideration for studies that provide more limited evidence of reproductive and developmental effects 

(e.g., infant mortality, low birth weight) identified as having evidence suggestive of a causal relationship 

with long-termplacing significantly more weight on this information. In considering this information, the 

Administrator notes that there is no bright line that delineates the start of the range of PM2.5

concentrations. A level of 12 μg/m3is approximately at the same level as the lowest long-term mean 

concentration reported in such studies (Figures 2 and 4; 11.9 μg/m3for Bell et al., 2007). below the 

mean within which the data become appreciably more sparse and, thus, where her confidence in the 

associations observed in epidemiological studies begins to become appreciably less.

The Administrator also recognizes that anIn considering mean PM2.5 concentrations and 

distributional statistics from the various sets of epidemiological studies noted above, the Administrator 

recognizes that such studies typically report concentrations based on composite monitor distributions, in 

which concentrations may be averaged across multiple ambient monitors that may be present within 

each area included in a given study. Thus, as noted above, a policy approach that uses data based on 

composite monitor distributionsmonitors to identify potential alternative standard levels, and then 

compares those levels to would inherently build in a margin of safety of some degree relative to an 

alternative standard level based on information from maximum monitors in an area, since once a 

standard is set, concentrations at appropriate maximum monitors within an area are generally used to 

determine if an area meets a given standard, inherently builds in some margin of safety. Using such an 

approach, an annual standard set at a level of 12 μg/m3 would thus provide some.

The Administrator also recognizes that judgments about the appropriate weight to place on any 

of the factors considered above should reflect consideration not only of the relative strength of the 

evidence but also on the important uncertainties that remain in the evidence and information being 
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considered in this review. The Administrator notes that the extent to which these uncertainties influence 

judgments about appropriate annual and daily standard levels within the range of 11 to 13 μg/m3 would 

likely be greater for standard levels in the lower part of the range which would necessarily be based on 

fewer available studies than would higher levels within the range.

Based on the above considerations, the Administrator concludes that it is appropriate to propose 

to set a level for the primary annual PM2.5 standard within the range of 12 to 13 μg/m3. The 

Administrator provisionally concludes that a standard set within this range would reflect alternative 

approaches to appropriately placing the most weight on the strongest available evidence, while placing 

less weight on much more limited evidence and on more uncertain analyses of information available 

from a relatively small number of studies. Further, she provisionally concludes that a standard level 

within this range would reflect alternative approaches to appropriately providing an adequate margin of 

safety for this reason. Anthe 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 recognizes that setting an annual standard level at the lower 

end of this range would reflect an approach that places more emphasis on the strength of the evidence, 

including the analysis of the distribution of air quality concentrations in the studies, and more emphasis 

on margin of safety considerations, than would setting a level at the upper end. Conversely, an approach

that would support a level at the upper end of this range would place greater weight on the remaining 

uncertainties in the evidence to avoid potentially overestimating public health improvements, and would 

generally support a view that the uncertainties remaining in the evidence are too great to warrant setting 

a lower annual standard level of 12.

While the Administrator recognizes that CASAC advised, and the Policy Assessment concluded,

that the available scientific information provides support for considering a range that extended down to 

11 μg/m3, she concludes that proposing such an extended range would reflect a public health policy 
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approach that places more weight on relatively limited evidence and more uncertain information and 

analyses than she considers appropriate at this time. Nonetheless, the Administrator solicits comment on 

a level down to 11 μg/m3 would also provide some margin of safety since it is somewhat lower than the 

long-term mean concentrations of the studies noted immediately aboveas well as on approaches for 

translating scientific evidence and rationales that would support such a level. Such an approach might 

reflect a view that the uncertainties associated with the available scientific information warrant a highly 

precautionary public health policy response that would incorporate a large margin of safety.

The Administrator recognizes that potential air quality changes associated with meeting an 

annual standard set at a level within the range of 12 to 13 μg/m3 will result in lowering risks associated 

with both long- and short-term PM2.5 exposures. However, the Administrator recognizes that such an 

annual standard intended to serve as the primary means for providing protection from effects associated

with both long- and short-term PM2.5 exposures would not by itself be expected to offer requisite 

protection with an adequate margin of safety against the effects of all short-term PM2.5 exposures. As a 

result, in conjunction with consideringproposing an annual standard level in the range of 12 to 13 

μg/m3, the Administrator provisionally concludes that it is appropriate to continue to provide 

supplemental protection by means of a 24-hour standard set at the appropriate level, particularly for 

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

PM2.5-related effects that may be associated with shorter-than-daily exposure periods. 

Based on the approach discussed in section III.A.3 above, the Administrator has relied upon 

evidence from the short-term exposure studies as the principal basis for selecting the level of the 24-hour 

standard. In considering these studies as a basis for the level of a 24-hour standard, and having selected a 

98th percentile form for the standard, the Administrator agrees with the focus in the Policy Assessment 

of looking at the 98th percentile values, as well as at the long-term mean PM2.5 concentrations in these 

studies.
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In considering the information provided by the short-term exposure studies, the Administrator 

recognizes that to the extent these studies were conducted in areas that likely did not meet one or both of 

the current standards, such studies do not help inform the characterization of the potential public health 

improvements of alternative standards set at lower levels. By reducing the PM2.5 concentrations in such 

areas to just meet the current standards, the Administrator anticipates that additional public health 

protection will occur. Therefore, the Administrator has focused on studies that reported positive and 

statistically significant associations in areas that would likely have met both the current 24-hour and 

annual standards. She has also considered whether or not these studies were conducted in areas that 

would likely have met an annual standard level of 12 to 13 μg/m3to inform her decision regarding an 

appropriate 24-hour standard level. As discussed in section III.E.4.a, the Administrator concludes that 

multi-city, short-term exposure studies provide the strongest data set for informing her decisions on 

appropriate 24-hour standard levels. The Administrator views the single-city, short-term exposure 

studies as a much more limited data set providing mixed results and, therefore, she has less confidence 

in using these studies as a basis for setting the level of a 24-hour standard. With regard to the limited 

number of single-city studies that reported positive and statistically significant associations for a range 

of health endpoints related to short-term PM2.5 concentrations in areas that would likely have met the 

current suite of PM2.5 standards, the Administrator recognizes that many of these studies had significant 

limitations (e.g., limited statistical power, limited exposure data) or equivocal results (mixed results 

within the same study area) that make them unsuitable to form the basis for setting the level of a 24-hour 

standard. 

With regard to multi-city studies that evaluated effects associated with short-term PM2.5

exposures, the Administrator observes an overall pattern of positive and statistically significant 

associations in studies with 98th percentile values averaged across study areas in the range of 45.8 to 

34.2 μg/m3(Burnett et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, 
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Burnett and Goldberg, 2003; Franklin et al., 2008). The Administrator notes that, to the extent air quality 

distributions are reduced to reflect just meeting the current 24-hour standard, additional protection 

would be anticipated for the effects observed in the three multi-city studies with 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 notes that these studies reported longterm mean PM2.5 concentrations of 12.9, 13.2, and 13.4 μg/m3

, respectively (Bell et al., 2008; Zanobetti 

and Schwartz, 2009; Dominici et al., 2006a).

In considering revisingproposing to revise the level of the annual standard to 12within the range 

of 12 to 13 μg/m3, as discussed above, the Administrator recognizes that additional protection would be 

provided for the short-term effects observed in these multi-city studies such that revision to the 24-hour 

standard would not be warranted. Therefore, based onin conjunction with an annual standard level of 12 

μg/m3, and in two of these three studies in conjunction with an annual standard level of 13 μg/m3. Based 

on this, it would be reasonable to conclude that revision to the 24-hour standard would not be warranted

in either case. However, the Administrator has less confidence in judging that a level of 35 μg/m3 would 

be appropriate in conjunction with an annual level at the upper end of the range. Based on the above 

considerations related to the epidemiological evidence, the Administrator provisionally concludes that it 

is appropriate to retain the level of the 24-hour standard at 35 μg/m3, in conjunction with a revised 

annual standard level in the proposed range of 12 to 13 μg/m3.

In addition to considering the epidemiological evidence, the Administrator also has taken into 

account air quality information based on county-level 24-hour and annual design values to understand 

the implications of revising the annual standard level from 15 to 12 μg/m3in conjunction with retaining 

the 24-hour standard level at 35 μg/m3in conjunction with an annual standard level within the proposed 

range of 12 to 13 μg/m3. She has considered whether this suite of standards would meet a public health 
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policy goal which includes setting the annual standard to be the “generally controlling” standard in 

conjunction with setting the 24-hour standard to provide supplemental protection to the extent that 

additional protection is warranted. As discussed above, the Administrator provisionally concludes that 

this approach is the most effective and efficient way to reduce total population risk associated with both 

long- and short-term PM2.5 exposures, resulting in more uniform protection across the U.S. than the 

alternative of setting the 24-hour standard to be the controlling standard. 

In considering the air quality information, the Administrator observes that a suite of PM2.5

standards that includes an annual standard level of 12 μg/m3and a 24-hour standard level of 35 μg/m3

(i.e., 12/35) would result in the annual standard as the generally controlling standard in most regions 

across the country, except for certain areas in the Northwest, where the annual mean PM2.5

concentrations have historically been low but where relatively high 24-hour concentrations occur, often 

related to seasonal wood smoke emissions (US EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). Although 

not explicitly delineated on Figure 2-10 in the Policy Assessment, an annual standard of 13 μg/m3 would 

be somewhat less likely to be the generally controlling standard in some regions of the U.S. outside the 

Northwest in conjunction with a 24-hour standard level of 35 μg/m3.

Taking the above considerations into account, the Administrator proposes to revise the level of 

the primary annual PM2.5 standard from 15.0 μg/m3to 12within 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, thissuch a suite of 

primary PM2.5 standards and the rationale supporting thesesuch levels appropriately reflectscould 

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. She judges that this suite of standards addresses

The Administrator recognizes that the final suite of standards selected from within the proposed 

range of annual standard levels, or the broader range of annual standard levels on which public comment
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is solicited, must be clearly responsive to the issues raised by the D.C. Circuit’s remand of the 2006 

primary annual PM2.5 standard. Furthermore, in the the final suite of standards will reflect the 

Administrator’s ultimate judgment, this 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. It would 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 short-term PM2.5 exposures. These standards would also 

provide an appropriately more limited degree of protection against other health effects for which there is 

more limited evidence of effects and causality, such as reproductive and developmental effects. ThisThe 

final judgment to be made by the Administrator will appropriately considersconsider the requirement for 

a standard that is neither more nor less stringent than necessary and recognizeswill recognize that the 

CAA does not require that primary standards be set at a zero-risk level, but rather at a level that reduces 

risk sufficiently so as to protect public health with an adequate margin of safety. 

Having reached her decisionprovisional judgment to propose revising the annual standard level 

from 15.0 to 12within a range of 12.0 to 13.0 μg/m3and to propose retaining the 24-hour standard level 

at 35 μg/m3, the Administrator recognizes that different evaluations ofsolicits public comment on this 

range of levels and on approaches to considering the available 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 could be used 

in making public health policy decisions on proposed standards. In particular, the Administrator 

observes that the Policy Assessment concludes that the currently available information could provide 

that would support for an annual standard level as high as 13 μg/m3and as low as 11 μg/m3, but that the 

evidence most strongly supports considerationthe choice of an alternative annual standard level within a 

range of 12 to 11 μg/m3(US EPA, 2011a, p. 2-101). With regard to the 24-hour standard, the 
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Administrator observes that the Policy Assessment concludes that there is limited support to consider 

revising the level to somewhat below 35 μg/m3, such as down to 30 μg/m3, in conjunction with revising 

the annual standard level to 13 μg/m3and to consider retaining the level at 35 μg/m3in conjunction with

revising the annual standard level to 11 μg/m3. 

The Administrator also recognizes that CASAC endorsed annual standard levels within a range 

of 13 to 11 μg/m3and alternative 24-hour standard levels within athis range of 35 to 30 μg/m3, 

consistent with the general ranges identified in the Policy Assessment (Samet, 2010d, p. ii). As noted in 

section III.E.4.d above, the Administrator observes that CASAC, in offering its advice on alternative 

standard levels, did not express support for any specific levels or combinations of standards within in 

these ranges. However, she also observes, as noted in section III.E.4.d above, that CASAC did not have 

an opportunity to review additional information and analyses presented in the final Policy Assessment 

nor did it have an opportunity to review the EPA staff’s revised rationale for the combinations of 

alternative standards presented in the final document.

The Administrator recognizes that consideration for setting a higher annual standard level, e.g., 

up to 13 μg/m3, would reflect an approach that places significantly less emphasis on margin of safety 

considerations than a standard set at 12 μg/m3. It would also reflect placing essentially no weight on 

more limited evidence of serious effects in susceptible populations (e.g., infant mortality, low birth 

weight) and would likewise place essentially no weight on additional population-level data from a 

limited number of epidemiological studies. In addition, this approach would require the Administrator to 

explain why a standard established above the level of the long-term mean PM2.5 concentrations reported 

in several robust well-conducted studies, notably the Burnett et al. (2004) and Bell et al. (2008) studies, 

would provide requisite protection with an adequate margin of safety.13 The Administrator is not 

 

13 In this regard, the Administrator is less influenced by the reported long-term mean PM2.5

concentration from the WHI study (12.9 μg/m3) because that value is extrapolated from a single year of 

air quality data to the whole study, and the data year (2000) comes from a later year than the years of 
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presently aware of any such explanations. This approach would place greater weight on the remaining 

uncertainties in the evidence to avoid potentially overestimating public health improvements. Such an 

approach could generally support a view that the uncertainties remaining in the evidence 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 components within the fine particle mixtures14 and the 

potential role of co-pollutants (i.e., distinguishing the effects of PM2.5 from the effects of other pollutants 

within the ambient mixture) are too great to warrant setting a lower annual standard level.

The Administrator recognizes that consideration for setting a lower annual standard level, e.g., 

down to 11 μg/m3, would reflect an approach that places relatively more emphasis on margin of safety 

considerations than a standard set at 12 μg/m3. It would also reflect placing greater weight on more 

limited evidence of serious effects in susceptible populations (e.g., infant mortality, low birth weight), as 

well as assuming associations exist for effects for which at present there is only suggestive evidence of 

causality. This approach would place less weight on the uncertainties remaining in the evidence 

including understanding the heterogeneity in responses observed in the epidemiological studies, to avoid 

potentially underestimating public health improvements. Such an approach could generally support 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, particularly 

in light of the serious nature of some of the health effects at issue.

The Administrator solicits public comment on these approaches and on related standard levels 

such as alternative annual standard levels up to 13 μg/m3and down to 11 μg/m3or 24-hour standard 

 

health events considered (1994 to 1998) so that air quality concentrations could have been higher in the 

study’s earlier years.

14 As explained in section III.E.1, however, 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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levels down to 30 μg/m3.. The Administrator also solicits public commentscomment on alternative 

annual standard levels down to 11 μg/m3and on the combination of annual and 24-hour standards that 

commenters may believe areis appropriate, along with the approaches and rationales used to support 

such levels. In addition, given the importance the evidence from epidemiologic studies plays in 

considering the appropriate annual and 24-hour levels, the Administrator solicits commentspublic 

comment on issues related to translating epidemiological evidence into standards, including approaches 

for addressing the uncertainties and limitations associated with this evidence and quantitative risk 

assessment.

C. Administrator’s Proposed Decisions on Primary PM2.5 Standards

For the reasons discussed above, and taking into account the information and assessments 

presented in the Integrated Science Assessment, Risk Assessment, and Policy Assessment, the advice 

and recommendations of CASAC, and public comments to date, the Administrator proposes to revise the 

current primary PM2.5 standards. Specifically, the Administrator proposes to revise: (1) the level of the 

primary annual PM2.5 standard to 12a level within the range of 12.0 to 13.0 μg/m3and (2) the form of 

the primary annual PM2.5 standard to one based on the highest appropriate area-wide monitor in an area, 

with no allowance for spatial averaging. In conjunction with revising the primary annual PM2.5 standard 

to provide protection from effects associated with long- and short-term PM2.5 exposures, the 

Administrator proposes to retain the level and form of the primary 24-hour PM2.5 standard to provide 

supplemental protection for areas with high peak PM2.5 concentrations. The Administrator provisionally 

concludes that such a revised suite of standards, including a revised annual standard together with the 

current 24-hour standard, wouldcould provide requisite protection against health effects potentially 

associated with long- and short-term PM2.5 exposures. The Administrator is not proposing any revisions 

to the current PM2.5 indicator and the annual and 24-hour averaging times for the primary PM2.5

standards. Data handling conventions are specified in proposed revisions to appendix N, as discussed in 
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section VII below. The Administrator solicits comment on all aspects of this proposed decision. In 

particular, the Administrator solicits comments on alternative combinations of standards including: (1) 

an annual standard level up to 13.0 μg/m3in conjunction with an alternative 24-hour standard level 

down to 30 μg/m3, (2) an annual standard level down to 11.0 μg/m3in conjunction with retaining the 24-

hour standard level at 35 μg/m3, or (3) other combinations of annual and 24-hour standards.