From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov, dmancini@omb.eop.gov

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

Dougherty/DC/USEPA/US@EPA, Janet McCabe/DC/USEPA/US@EPA, Gina 

McCarthy/DC/USEPA/US@EPA

Date: 06/14/2012 04:08 PM

Subject: Edits to the conclusion section on level of the PM2.5 primary standards

Nathan and Dom, 

Attached are revisions to the section on the level of the primary PM2.5 standards. These revisions address the 

comments we received from the interagency reviewers on this morning's call. Please let me know if you have any 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

associated with both long- and short-term exposures more consistently than a generally controlling 

24-hour standard and would be the most effective and efficient way to reduce total PM2-5-related 

population risk.

In reaching decisions on alternative standard levels to propose, the Administrator judges that it is 

most appropriate to examine where the evidence of associations observed in the epidemiological studies 

is strongest and, conversely, where she has appreciably less confidence in the associations observed in 

the epidemiological studies. Based on the characterization and assessment of the epidemiological and 
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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 weight of evidence, as 

evaluated in the Integrated Science Assessment, is strongest for health endpoints classified as having 

evidence of a causal relationship. These relationships include those between long- and short-term PM2.5

exposures and mortality and cardiovascular effects. She recognizes that the weight of evidence is also 

strong for health endpoints classified as having evidence of a likely causal relationship, which include 

those between long- and short-term PM2.5 exposures and respiratory effects. In addition, the 

Administrator makes note of the much more limited evidence for health endpoints classified as having 
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evidence suggestive of a causal relationship, including developmental, reproductive and 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 section 

III.E.4.b above and in the Policy Assessment, that there is significantly greater confidence in observed 

associations over certain parts of the air quality distributions in the studies, and conversely, that there is 

significantly diminished confidence in ascribing effects to concentrations toward the lower part of the 

distributions.

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

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

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

concentrations reported in long- and short-term exposure studies. In 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, she understands that this approach does not provide a bright line for 

reaching decisions about appropriate standard levels. The Administrator notes that long-term mean 

PM2.5 concentrations are available for each study considered and, therefore, represent the most robust 

data set to inform her decisions on appropriate annual standard levels. She also notes that the overall 

study mean PM2.5 concentrations are generally calculated based on monitored concentrations averaged 

across monitors in each study area with multiple monitors, referred to as a composite monitor 

concentration, in contrast to the highest concentration monitored in study area, referred to as a maximum 

monitor concentration, which are used to determine whether an area meets a given standard. In 
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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). She 

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

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

Burnett et al., 2004). With regard to effects classified as providing evidence suggestive of a causal 

relationship, the Administrator observes a small number of long-term exposure studies related to 

developmental and reproductive effects that reported statistically significant associations with overall 

study mean PM2.5 concentrations down to 11.9 μg/m3(in Bell et al., 2007).1

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 understands that the Policy Assessment presented information on the 25th

and 10th percentiles of the distributions of PM2.5 concentrations available from four multi-city studies to 

provide a general frame of reference as to the part of the distribution within which the data become 

 

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

studies would 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 PM2.5 concentrations associated with the 25th

percentile values of the population-level data 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 are available for only these four studies and, therefore, she believes that these 

studies comprise a more limited data set than one based on long-term mean PM2.5 concentrations for 

which data are available for all studies considered, as discussed above. In considering this information, 

the Administrator notes that CASAC advised that information about the long-term PM2.5 concentrations 

that were most influential in generating the health effect estimates in epidemiological studies can help to 

inform selection of an appropriate annual standard level. 

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

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

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

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

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

fixed monitor sites, and the nature, magnitude, and confidence in estimated risks related to increasingly 
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lower ambient PM2.5 concentrations. Furthermore, the Administrator notes that epidemiological studies 

have reported heterogeneity in responses both within and between cities and geographic regions across 

the U.S. She recognizes that this heterogeneity may be attributed, in part, to differences in fine particle 

composition in different regions and cities. The Administrator also recognizes that there are additional 

limitations associated with evidence for reproductive and developmental effects, identified as being 

suggestive of a causal relationship with long-term PM2.5 exposures, including: the limited number of 

studies evaluating such effects; uncertainties related to identifying the relevant exposure time periods of 

concern; and limited toxicological evidence providing little information on the mode of action(s) or 

biological plausibility for an association between long-term PM2.5 exposures and adverse birth 

outcomes.

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

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

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

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

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

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

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

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

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

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

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

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

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

the Administrator to consider various approaches to weighing the evidence so as to identify appropriate 
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standard levels to propose. In so doing, the Administrator encourages extensive public comment on 

alternative approaches to weighing the evidence and other information so as to inform her public health 

policy judgments before reaching final decisions on appropriate standard levels.

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

currently available scientific information, including epidemiological and toxicological evidence as well 

as risk and air quality 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 addition, the 

Administrator recognizes that the Policy Assessment concludes that the available evidence and riskbased information support consideration of annual standard levels in the range of 13 to 11 μg/m3

, and 

that the Policy Assessment also 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 

CASAC did not express support for any specific levels or combinations of standards within in these 

ranges, although she recognizes that CASAC did not have an opportunity to review additional 

information and analyses presented in the final Policy Assessment prepared in response to CASAC’s 

recommendations on the second draft Policy Assessment. Nor did CASAC have an opportunity to 

review the EPA staff’s revised rationale for the combinations of alternative standards presented in the 

final document.

In considering the extent to which the currently available evidence and information provide 

support for specific standard levels within the ranges identified by CASAC and the Policy Assessment 

as appropriate for consideration, the Administrator initially considers standard levels within the range of 

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

PM2.5 concentrations reported in studies of effects classified as having evidence of a causal or likely 

causal relationship, as summarized in Figure 4 and discussed more broadly above. She notes that a level
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at the upper end of this range of 13 μg/m3 would be below most but not all the overall study mean 

concentrations from the multi-city studies of long- and short-term exposures, whereas a somewhat lower 

levels, such as 12 μg/m3or lower, within this range would be below all such overall study mean 

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

again 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. Further, she is mindful that this approach 

does not provide a bright line for reaching decisions about appropriate 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 13at the upper end of this range would be 

below the overall study mean concentration in one of the three studies, while lower levels in the mid- to 

lower part of this rangesuch as 12 or 11 μg/m3 would be below the overall study mean concentrations 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.

The Administrator also considers In considering additional distributional statistics of populationlevel information that were available from four of the epidemiological studies that provide evidence of 

effects identified as having a causal relationship with long- or short-term PM2.5 concentrations for 

annual standard levels within the same range of 13 to 11 μg/m3, . In so doing, the Administrator first 

notes that a level in the mid-part of this range of 12 μg/m3is the PM2.5 concentration that generally 

corresponds with approximately the 25th percentile of the distributions of health events data available in 
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three of these studies. The Administrator also notes that higher sstandard levels toward the upper part of 

this range up to 13 μg/m3 would reflect placing significantlysubstantially less weight on this statistical 

information, whereas lower standard levels down to 11 μg/m3toward the lower part of this range would 

reflect placing significantlysubstantially more weight on this information. In considering this 

information, the Administrator notes that there is no bright line that delineates the part of the distribution 

of PM2.5 concentrations within which the data become appreciably more sparse and, thus, where her 

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

In considering mean PM2.5 concentrations and distributional statistics from the various sets of 

epidemiological studies noted above, the Administrator is mindful, as noted above, that such studies 

typically report concentrations based on composite monitor distributions, in which concentrations may 

be averaged across multiple ambient monitors that may be present within each area included in a given 

study. Thus, a policy approach that uses data based on composite monitors to identify potential 

alternative standard levels would inherently build in a margin of safety of some degree relative to an 

alternative standard level based on measurements at the monitor within an area that records the highest 

concentration, or the maximum monitor, since once a standard is set, concentrations at appropriate 

maximum monitors within an area are generally used to determine if an area meets a given standard.

The Administrator also recognizes that judgments about the appropriate weight to place on any 

of the factors discussed above should reflect consideration not only of the relative strength of the 

evidence but also on the important uncertainties that remain in the evidence and information being 

considered in this review. The Administrator notes that the extent to which these uncertainties influence 

judgments about appropriate annual standard levels within the range of 113 to 131 μg/m3 would likely 

be greater for standard levels in the lower part of theis range which would necessarily be based on fewer 

available studies than would higher levels within thise range.

Based on the above considerations, the Administrator concludes that it is appropriate to propose 
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to set a level for the primary annual PM2.5 standard within the range of 12 to 13 μg/m3. The 

Administrator provisionally concludes that a standard set within this range would reflect alternative 

approaches to appropriately placing the most weight on the strongest available evidence, while placing 

less weight on much more limited evidence and on more uncertain analyses of information available 

from a relatively small number of studies. Further, she provisionally concludes that a standard level 

within this range would reflect alternative approaches to appropriately providing an adequate margin of 

safety for the populations at risk for the serious health effects classified as having evidence of a causal or 

likely causal relationship, depending in part on the emphasis placed on margin of safety considerations. 

The Administrator recognizes that setting an annual standard level at the lower end of this range would 

reflect an approach that places more emphasis on the strength of the evidence, including the analysis of 

the distribution of air quality concentrations most influential in generating health effect estimates in the 

studies, and more emphasis on margin of safety considerations, than would setting a level at the upper 

end of the range. Conversely, an approach that would support a level at the upper end of this range

would place greater weight on the remaining uncertainties in the evidence to avoid potentially 

overestimating public health improvements, and would generally support a view that the uncertainties 

remaining in the evidence are too great to warrant setting a lower annual standard level.

While the Administrator recognizes that CASAC advised, and the Policy Assessment concluded,

that the available scientific information provides support for considering a range that extended down to 

11 μg/m3, she concludes that proposing such an extended range would reflect a public health policy 

approach that places more weight on relatively limited evidence and more uncertain information and 

analyses than she considers appropriate at this time. Nonetheless, the Administrator solicits comment on 

a level down to 11 μg/m3as well as on approaches for translating scientific evidence and rationales that 

would support such a level. Such an approach might reflect a view that the uncertainties associated with 

the available scientific information warrant a highly precautionary public health policy response that 
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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 proposing an annual standard level in the range of 12 to 13 μg/m3, the 

Administrator provisionally concludes that it is appropriate to continue to provide supplemental 

protection by means of a 24-hour standard set at the appropriate level, particularly for areas with high 

peak-to-mean ratios possibly associated with strong local or seasonal sources, or for PM2.5-related 

effects that may be associated with shorter-than-daily exposure periods. 

Based on the approach discussed in section III.A.3 above, the Administrator has relied upon 

evidence from the short-term exposure studies as the principal basis for selecting the level of the 24-hour 

standard. In considering these studies as a basis for the level of a 24-hour standard, and having selected a 

98th percentile form for the standard, the Administrator agrees with the focus in the Policy Assessment 

of looking at the 98th percentile values, as well as at the long-term mean PM2.5 concentrations in these 

studies.

In considering the information provided by the short-term exposure studies, the Administrator 

recognizes that to the extent these studies were conducted in areas that likely did not meet one or both of 

the current standards, such studies do not help inform the characterization of the potential public health 

improvements of alternative standards set at lower levels. By reducing the PM2.5 concentrations in such 

areas to just meet the current standards, the Administrator anticipates that additional public health 

protection will occur. Therefore, the Administrator has focused on studies that reported positive and 
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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, 

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 long-
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term mean PM2.5 concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti 

and Schwartz, 2009; Dominici et al., 2006a).

In proposing to revise the level of the annual standard to within the range of 12 to 13 μg/m3, as 

discussed above, the Administrator recognizes that additional protection would be provided for the 

short-term effects observed in these multi-city studies in conjunction with an annual standard level of 12 

μg/m3, and in two of these three studies in conjunction with an annual standard level of 13 μg/m3. She 

notes that the study-wide mean concentrations are based on averaging across monitors within study 

areas and that compliance with the standard would be based on concentrations measured at the monitor 

reporting the highest concentration within each area. The Administrator believes it would be reasonable 

to conclude that revision to the 24-hour standard would not be warranted in either casein conjunction 

with an annual standard within this range. However, she has less confidence in judging that a level of 35

μg/m3 would be appropriate in conjunction with an annual level at the upper end of the range. Based on 

the above considerations related to the epidemiological evidence, the Administrator provisionally 

concludes that it is appropriate to retain the level of the 24-hour standard at 35 μg/m3, in conjunction 

with a revised annual standard level in the proposed range of 12 to 13 μg/m3.

In addition to considering the epidemiological evidence, the Administrator also has taken into 

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

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

standard level within the proposed range of 12 to 13 μg/m3. She has considered whether this suite of 

standards would meet a public health policy goal which includes setting the annual standard to be the 

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

supplemental protection to the extent that additional protection is warranted. As discussed above, the 

Administrator provisionally concludes that this approach is the most effective and efficient way to 

reduce total population risk associated with both long- and short-term PM2.5 exposures, resulting in more 
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uniform protection across the U.S. than the alternative of setting the 24-hour standard to be the 

controlling standard. 

In considering the air quality information, the Administrator first recognizes that there is no 

annual standard within the proposed range of levels, when combined with a 24-hour standard at the 

proposed level of 35 μg/m3, for which the annual standard would be the generally controlling standard in 

all areas of the country. She further observes that such a suite of PM2.5 standards with 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 within the range of 12.0 to 13.0 μg/m3and to 

retain the 24-hour standard level at 35 μg/m3. In the Administrator’s judgment, such a suite of primary 

PM2.5 standards and the rationale supporting such levels could reasonably be judged to reflect alternative

approaches to the appropriate consideration of the strength of the available evidence and other 

information and their associated uncertainties and the advice of CASAC.

The Administrator recognizes that the final suite of standards selected from within the proposed 

range of annual standard levels, or the broader range of annual standard levels on which public comment

is solicited, must be clearly responsive to the issues raised by the D.C. Circuit’s remand of the 2006 

primary annual PM2.5 standard. Furthermore, the final suite of standards will reflect the Administrator’s 
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ultimate judgment in the final rulemaking as to the suite of primary PM2.5 standards that would be 

requisite to protect the public health with an adequate margin of safety from effects associated with fine 

particle exposures. The final judgment to be made by the Administrator will appropriately consider the 

requirement for a standard that is neither more nor less stringent than necessary and will recognize that 

the CAA does not require that primary standards be set at a zero-risk level, but rather at a level that 

reduces risk sufficiently so as to protect public health with an adequate margin of safety. 

Having reached her provisional judgment to propose revising the annual standard level from 15.0 

to within a range of 12.0 to 13.0 μg/m3and to propose retaining the 24-hour standard level at 35 μg/m3, 

the Administrator solicits public comment on this range of levels and on approaches to considering the 

available evidence and information that would support the choice of levels within this range. The 

Administrator also solicits public comment on alternative annual standard levels down to 11 μg/m3and

on the combination of annual and 24-hour standards that commenters may believe is appropriate, along 

with the approaches and rationales used to support such levels. In addition, given the importance the 

evidence from epidemiologic studies plays in considering the appropriate annual and 24-hour levels, the 

Administrator solicits public comment on issues related to translating epidemiological evidence into 

standards, including approaches for addressing the uncertainties and limitations associated with this 

evidence.