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April 20, 2011 

MEMORANDUM

SUBJECT: Transmittal of Policy Assessment for the Review of the Particulate Matter National 

Ambient Air Quality Standards – Final Document

FROM: Lydia N. Wegman, Director /s/ 

Health and Environmental Impacts Division 

Office of Air Quality Planning and Standards 

TO: Holly Stallworth 

 Designated Federal Officer 

 Clean Air Scientific Advisory Committee 

 EPA Science Advisory Board Staff Office 

Attached is the final document, Policy Assessment for the Review of the Particulate Matter 

National Ambient Air Quality Standards (Policy Assessment, April 2011), prepared by the 

Environmental Protection Agency’s (EPA) Office of Air Quality Planning and Standards 

(OAQPS) staff as part of EPA’s ongoing review of the national ambient air quality standards 

(NAAQS) for particulate matter (PM). The Policy Assessment presents considerations and 

conclusions relevant for EPA’s review of the current primary (health-based) standards and 

secondary (welfare-based) standards for both fine and coarse particles. This final document 

draws upon the evidence and information assessed and presented in the Integrated Science 

Assessment for Particulate Matter (ISA) prepared by EPA’s National Center for Environmental 

Assessment and two risk and exposure assessment documents (REAs) prepared by OAQPS, the 

Quantitative Health Risk Assessment for Particulate Matter (Risk Assessment) and the UrbanFocused Visibility Assessment for Particulate Matter (Visibility Assessment).1

 These three final 

documents reflect consideration of comments from the Clean Air Scientific Advisory Committee 

(CASAC) Particular Matter Review Panel (the Panel) and the public on earlier drafts. 

The Policy Assessment reflects consideration of comments from the Panel, as well as 

public comments, on the second draft Policy Assessment, which was reviewed by the Panel at a 

meeting on July 26-27, 2010. Comments from the Panel on the second draft Policy Assessment 

were provided to us in a September 10, 2010 letter (Samet, 2010a).2

 These comments, and the 

 

1

 All documents related to the current PM NAAQS review are available at: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html. 2

Samet, J (2010a). Letter from Dr, Jonathan M. Samet, Chair, Clean Air Scientific Advisory Committee to the 

Honorable Lisa P. Jackson, Administrator, US EPA. CASAC Review of Policy Assessment for the Review of the PM 

NAAQS – Second External Review Draft (June 2010). September 10, 2010. Available at: 

http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a4/CCF9F4C0500C500F8525779D00

73C593/$File/EPA-CASAC-10-015-unsigned.pdf 
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changes made in the final Policy Assessment in response to them, are summarized in the 

Attachment. 

Availability of Final Document

The Policy Assessment is being made available to the Panel in the form of the attached 

electronic file, which we request that you forward to members of the Panel. This document is 

also available on the EPA website: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 

 Please accept my gratitude for the advice the Panel has provided throughout our review 

of the Particulate Matter NAAQS. I look forward to continuing our productive relationship. 

Should you have any questions regarding the final Policy Assessment, please contact Ms. Beth 

Hassett-Sipple (919-541-4605; email - hassett-sipple.beth@epa.gov). 

cc: Vanessa Vu, SAB, OA 

Rosalina Rodriguez, OAQPS/HEID 

Karen Martin, OAQPS/HEID 

John Vandenberg, ORD/NCEA-RTP 

Debra Walsh, ORD/NCEA-RTP 

Mary Ross, ORD/NCEA-RTP 

Doug Johns, ORD/NCEA-RTP 

Lindsay Stanek, ORD/NCEA-RTP 

Beth Hassett-Sipple, OAQPS/HEID 

Zachary Pekar, OAQPS/HEID 

Pradeep Rajan, OAQPS/HEID 

Scott Jenkins, OAQPS/HEID 

Vicki Sandiford, OAQPS/HEID 

Meredith Lassiter, OAQPS/HEID 

Marc Pitchford, NOAA 

Chet Wayland, OAQPS/AQAD 

Phil Lorang, OAQPS/AQPD 

Lewis Weinstock, OAQPS/AQAD 

Patrick Dolwick, OAQPS/AQAD 

James Hemby, OAQPS/AQAD
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Attachment 

CASAC Comments on Second Draft PM Policy Assessment and Responses to those 

Comments 

The CASAC PM Panel’s comments and recommendations on the second draft of the PM 

Policy Assessment, as well as changes made in the final document in response to those 

comments for chapters 2 through 4, are summarized below. 

Chapter 2 (Primary Standards for Fine Particles)

The Panel expressed agreement with a number of staff conclusions in the second draft Policy 

Assessment, and supported the following: 

 The general approach for translating the available epidemiological evidence, risk 

information, and air quality information into the basis for reaching conclusions on the 

adequacy of the current standards and on alternative standards that are appropriate for 

consideration; 

 Considering revisions to the current PM2.5 primary standard to provide increased public 

health protection; 

 Returning to the strategy used in 1997 that considers a suite of standard levels in which 

the annual standard would be the “generally controlling” standard to provide primary 

protection for health effects associated with both long- and short-term PM2.5 exposures, 

in conjunction with a 24-hour standard set to provide supplemental protection; 

 Revising the form of the annual standard to eliminate spatial averaging; and 

 Revising the annual standard level within the range of 13 to 11 μg/m3

 in conjunction 

with retaining or revising the level of the 24-hour standard within a range of 35 to 30 

μg/m3. The Panel stated that these ranges were “supported by the epidemiological and 

toxicological evidence, as well as by the risk and air quality information...” However, 

the Panel requested further clarification on the rationale for the combinations of annual 

and 24-hour standards most strongly supported by the currently available information, 

as well the rationale for excluding other combinations within the ranges considered. 

 In addition, the Panel 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 annual standard levels (Samet, 2010a, p. 2). 

 The Panel also commented that the approach presented in the second draft Policy 

Assessment to identify alternative 24-hour standard levels which focused on peak-tomean ratios was not relevant for informing the actual level (Samet 2010a, p. 4) 

Changes made in chapter 2 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 potential alternative standard levels and more clearly articulating the basis for 

staff’s conclusions related to alternative standard levels that are appropriate for consideration. 

Specifically, we have updated and streamlined the discussion of the general approach used in the 

current review (section 2.1.3). We have modified or added a number of figures that summarize the 

available epidemiological evidence, air quality information, and risk-based considerations (Figures 

2-4 to 2-12). 
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In our discussion of evidence-based considerations to inform alternative annual standard levels 

(section 2.3.4.1), we expanded and clarified the different approaches for characterizing the range of 

PM2.5 concentrations over which we have the most confidence in the associations observed in the 

epidemiological studies. Consistent with the Panel’s comments to consider more information from 

epidemiological studies related to the concentrations that were most influential in generating the 

health effect estimates in individual studies to inform staff conclusions on alternative annual 

standard levels that would provide appropriate protection for both long- and short-term exposures, 

we contacted several study investigators to obtain additional information on population-level data 

(i.e., health events, number of study participants). In new analyses using distributional statistics, 

we considered these data in conjunction with air quality data to identify the broader range of PM2.5

concentrations that were most influential in generating health effect estimates in epidemiological 

studies, and, specifically, the range of PM2.5 concentrations below the long-term means over which 

we continue to have confidence in the associations observed in these epidemiological studies 

(Figures 2-7 and 2-8 and associated text). 

In our discussion of evidence-based considerations to inform potential alternative 24-hour 

standards, consistent with the Panel’s comments we focused on the epidemiological evidence from 

multi- and single-city short-term exposure studies to inform staff conclusions on alternative 24-

hour standard levels appropriate to consider (section 2.3.4.1, Figures 2-6 and 2-9). In addition to 

the epidemiological evidence, we also considered air quality information to understand the 

implications of different combinations of alternative annual and 24-hour standards that would 

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

24-hour standard that would provide supplementation protection (Figure 2-10). 

In the second draft Policy Assessment, staff reached the preliminary conclusion that consideration 

should be given to alternative annual PM2.5 standard levels in the range of 13 to 11 μg/m3, in 

conjunction with retaining the current 24-hour PM2.5 standard level of 35 μg/m3, and that 

consideration could also be given to an alternative 24-hour PM2.5 standard level of 30 μg/m3

particularly in conjunction with an annual standard of 11 μg/m3. As noted in section 2.3.4.3, staff 

conclusions in the final Policy Assessment regarding alternative pairings of annual and 24-hour 

standards that are appropriate to consider differ somewhat from the pairings of annual and 24-hour 

standards discussed in the second draft Policy Assessment upon which the Panel based its advice. 

In the final Policy Assessment, staff again concludes that consideration should be given to revising 

the current annual PM2.5 standard level to a level within the range of 13 to 11 μg/m3

. Staff further 

concludes that the evidence most strongly supports consideration of an alternative annual standard 

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

range of 12 to 11 μg/m3

, staff concludes it is appropriate to consider retaining the current 24-hour 

PM2.5 standard level at 35 μg/m3. In conjunction with consideration of an annual standard level of 

13 μg/m3

, staff concludes there is limited support to consider revising the 24-hour PM2.5 standard 

level to somewhat below 35 μg/m3, such as down to 30 μg/m3. 

These final staff conclusions reflect consideration of the Panel’s comments as well as public 

comments on the second draft Policy Assessment and additional analyses, as described above. 
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Chapter 3 (Primary Standard for Coarse Particles)

In their review of the second draft Policy Assessment, the Panel recommended the following: 

 Revising the current PM10 primary standard to provide increased public health 

protection against effects associated with exposures to PM10-2.5; 

 Retaining PM10 as the indicator for thoracic coarse particles, while also encouraging 

research to provide information for future NAAQS reviews to inform consideration of 

alternative sizes (e.g., PM10-2.5) as well as components and sources; 

 Retaining the 24-hour averaging time; 

 Revising the form to the 98th percentile, while also noting that additional 

characterization is needed regarding the potential implications of such a change in 

form; and 

 Setting the level of the revised standard within the range of 75 to 65 g/m3. 

Changes made to chapter 3 since the second draft Policy Assessment have been focused largely on 

improving and expanding the discussion of potential alternative standards in section 3.3. In 

sections 3.3.3 and 3.3.4, we have included additional analysis and discussion of the potential public 

health impacts of revising the form and level of the current PM10 standard. Specifically, section 

3.3.3 discusses a new analysis of PM10 and PM10-2.5 air quality concentrations in U.S. locations and 

section 3.3.4 now includes an expanded discussion of PM10 air quality concentrations in U.S. study 

locations. Section 3.3.4 also includes an expanded discussion of the relationship between the 

current standard level and 98th percentile PM10 concentrations. Figure 3-8 presents a regional 

breakdown of this relationship, and the integration discussion in section 3.3.4 now includes more 

extensive consideration of different approaches to characterizing this relationship. In addition, 

section 3.3.4 includes an expanded discussion of potential alternative standard levels, more clearly 

articulating the basis for staff’s conclusions related to alternative levels that are appropriate for 

consideration. 

Chapter 4 (Secondary Standard for PM-related Visibility)

The Panel expressed agreement with a number of staff conclusions included in the second draft 

Policy Assessment, including: 

 Considering revisions to the current PM2.5 standards to provide increased protection 

against PM-related visibility impairment; 

 Considering a PM2.5 light extinction indicator in preference to a PM2.5 mass indicator; 

and 

 Considering secondary standards for a PM2.5 light extinction indicator in a range of 20 

to 30 deciviews. 

The Panel expressed its strong preference for directly measuring light extinction over using 

estimates based on mass measurements and recommended a fuller discussion of the reasons EPA 

staff believes this is not a feasible approach for this review. We have expanded the discussion of 

this issue and added a citation to the recommendations of the advice we received from the Panel’s 

Ambient Air Monitoring and Methods Subcommittee (Russell and Samet, 2010).3

 

 

3

 Russell A, Samet JM (2010). Letter from the Clean Air Science Advisory Committee (CASAC) Ambient Air 

Monitoring and Methods Subcommittee (AAMMS) to the Honorable Lisa P, Jackson, Administrator, US EPA. Review 

of the White Paper on Particulate Matter (PM) Light Extinction Measurements. April 29, 2010, Available at: 
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In its review of the first draft Policy Assessment, the Panel agreed with the staff conclusion that 

PM effects on visibility can vary widely and rapidly over the course of a day and such changes are 

almost instantaneously perceptible to human observers.4

 Based in part on this consideration, the 

Panel agreed that a 1-hour averaging time would be appropriate to consider in conjunction with a 

directly measured PM light extinction indicator, noting that a 1-hour averaging time is well within 

the instrument response times of the various currently available and developing optical monitoring 

methods that directly measure light extinction. At that time, the Panel also advised that if a PM2.5

mass indicator were to be used, it would be appropriate to consider “somewhat longer averaging 

times – 2 to 4 hours – to assure a more stable instrumental response” (Samet, 2010b, p. 19). Thus, 

the Panel’s views on averaging times that would be appropriate for consideration were predicated 

in part on the capabilities of monitoring methods that were available for the alternative indicators 

discussed in the draft PA. We have interpreted the Panel’s views on a multi-hour averaging time 

as extending to a sub-daily calculated PM2.5 light extinction indicator since PM2.5 mass 

measurements are also required for this indicator. 

In preparing the final Policy Assessment, EPA staff conducted analyses to better understand data 

quality issues associated with the use of continuous Federal Equivalent Methods (FEMs) that were 

considered as the basis for hourly PM2.5 mass measurements (section 4.3.1.1). Based in part on 

these analyses, EPA staff concludes that it is appropriate to consider a 24-hour averaging time, in 

conjunction with a calculated PM2.5 light extinction indicator and appropriately specified standard 

level (sections 4.3.2.2 and 4.3.2.4). This conclusion reflects the judgment that PM2.5 light 

extinction calculated on a 24-hour basis is a reasonable and appropriate surrogate for sub-daily 

PM2.5 light extinction calculated on a 4-hour average basis. This conclusion is also predicated on 

consideration of a 24-hour average standard level which we have interpreted as providing 

information on the acceptability of daytime visual air quality over an hourly or multi-hour 

exposure period (section 4.3.4). 

Other changes included in the final Policy Assessment include an analysis of the performance of 

two somewhat differently defined calculated PM2.5 light extinction indicators using 2007-2009 data 

(revised Appendix F), removal of detailed material from Chapter 4 regarding a PM2.5 mass 

indicator (this material is still provided in Appendix E and several technical memos), and the 

addition of two new appendices, Appendix G and Appendix H. Appendix G discusses the use of 

calculated 24-hour average PM2.5 light extinction as the indicator and averaging period for a 

secondary PM2.5 NAAQS that would be aimed at controlling daily maximum daylight 4-hour 

average light extinction conditions. Appendix H provides information on the number and 

percentage of counties that likely would not meet the current and alternative secondary standards, 

 

http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a4/92C9F5AA09A76A938525771500

04A782/$File/EPA-CASAC-10-010-unsigned.pdf. 4

 Samet J (2010b). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific Advisory Committee to the 

Honorable Lisa P. Jackson, Administrator, US EPA. CASAC Review of Policy Assessment for the Review of the 

PM NAAQS – First External Review Draft (March 2010). May 17, 2010. Available at: 

http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a4/E504EE3276D87A9E85257727

00647AFB/$File/EPA-CASAC-10-011-unsigned.pdf/. 
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similar to Appendices C and D which provides information related to the current and alternative 

primary standards. 

We note that since earlier drafts of this Policy Assessment did not include discussion of a 

calculated PM2.5 indicator based on a 24-hour averaging time, CASAC did not have a basis to 

express any views regarding a 24-hour averaging time.