February 9, 2010 

MEMORANDUM

SUBJECT: CASAC Review of Quantitative Health Risk Assessment for Particulate Matter -- 

Second External Review Draft

FROM: Lydia N. Wegman, Director /s/ 

Health and Environmental Impacts Division (C504-02) 

Office of Air Quality Planning and Standards 

United States Environmental Protection Agency 

TO: Holly Stallworth 

Designated Federal Officer 

Clean Air Scientific Advisory Committee 

EPA Science Advisory Board Staff Office 

Attached is the second draft assessment document: Quantitative Health Risk Assessment 

for Particulate Matter (February 2010), 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 primary (health-based) national ambient air quality standards (NAAQS) for 

particulate matter (PM). This document will be one of the documents to be reviewed by the 

Clean Air Scientific Advisory Committee (CASAC) PM NAAQS Review Panel (the CASAC 

PM Panel) at a public meeting to be held in Chapel Hill, NC on March 10-11, 2010. I am 

requesting that you forward this draft document to the CASAC PM Panel to prepare for the 

March meeting. 

The other document to be reviewed at the March meeting, Particulate Matter UrbanFocused Visibility Assessment – Second External Review Draft, was sent to you on January 20, 

2010. As I noted at that time, we are also preparing another draft document, Policy Assessment 

for the Review of the Particulate Matter National Ambient Air Quality Standards – First 

External Review Draft, to be reviewed by the CASAC PM Panel by teleconference on April 8-9, 

2010. I expect to send the draft Policy Assessment to you around the beginning of March, in 

advance of the March meeting. 

This draft document draws upon information presented in the final Integrated Science 

Assessment for Particulate Matter (ISA, December 2010) prepared by EPA’s National Center for 

Environmental Assessment, Research Triangle Park, NC (NCEA-RTP). It also reflects 

consideration of comments from the CASAC PM Panel, as well as public comments, on the first 

draft PM Risk Assessment, which was reviewed by the Panel at a meeting on October 5-6, 2009. 
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The CASAC and public comments on this second draft Risk Assessment will be taken into 

consideration in preparing the final PM Risk Assessment. 

The main comments from the CASAC PM Panel on the first draft Risk Assessment, 

provided to us in a November 24, 2009 letter (Samet, 2009), are briefly summarized below 

together with a brief description of how we have considered the comments in preparing this 

second draft PM Risk Assessment. 

 The Panel recommended that we include an integrated discussion that draws on the 

uncertainty and variability analyses as well as the various national-scale analyses completed 

as part of this assessment, to interpret the core risk estimates. 

RESPONSE: We have added a new chapter 6 to provide an integrative discussion of the 

risk-related analyses presented throughout this second draft Risk Assessment, including the 

PM2.5-related risk estimates generated for the set of urban study areas and the related 

uncertainty and sensitivity analyses, the representativeness analyses, and the national-scale 

long-term exposure PM2.5 mortality assessment. The goal of this integrative discussion is to 

inform our understanding of important policy-relevant risk-based questions. 

 Regarding EPA staff’s decision to focus the first draft risk assessment on PM2.5 and not to 

estimate risk for coarse-fraction PM, the Panel generally agreed that the focus on PM2.5 was 

appropriate, although the Panel suggested that consideration be given to ways to carry out 

some form of exploratory analyses related specifically to PM10-2.5. While acknowledging the 

uncertainty that would be involved, the Panel also recommended that we generate semiquantitative risk estimates for both reproductive effects and lung cancer. 

RESPONSE: We have not included a quantitative assessment of risk associated with PM10-

2.5 due to continued concerns that the uncertainty in any such estimates that we could 

generate would be so great that they would not serve the purpose of informing our review of 

the PM NAAQS. We discuss the basis for this decision briefly in section 2.3 of the 

document with additional discussion provided in Appendix H. We also note that the 

evidence-based considerations to be presented in the upcoming draft Policy Assessment 

document will address PM10-2.5 exposure and risk. Regarding reproductive and 

developmental effects and lung cancer, we note that the risk assessment does include 

estimates of lung-cancer mortality as part of modeling long-term exposure-related mortality. 

However, in the case of reproductive and developmental effects, we continue to believe that 

there are insufficient data at this time (particularly with regard to identifying appropriate 

exposure windows) to support quantitative risk assessment. Nonetheless, these endpoints 

will be covered as part of the evidence-based considerations to be presented in the draft 

Policy Assessment. 

 The Panel expressed the view that stronger justification was needed for designating some of 

the risk estimates as core estimates. The Panel also recommended that we provide further 

explanation for how specific long-term and short-term multi-city epidemiological studies 

were selected for inclusion in the risk assessment and why other studies of potential interest 

were not included. For long-term exposure-related risk, the Panel suggested that we provide 

additional rationale for the specific set of C-R functions and effect estimates selected from 

the Krewski et al. (2009) study for modeling long-term exposure-related mortality for the 

core and sensitivity analyses. 
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RESPONSE: We have expanded the discussion (in section 3.3.3) of our rationale for 

identifying modeling choices comprising the core risk model, including both the selection of 

specific epidemiological studies for use in deriving C-R functions as well as specific choices 

made in specifying those functions. In addition, we have added clarification for why 

alternative candidate epidemiological studies and alternative C-R function specifications 

were not included in the core risk assessment. 

 Regarding the characterization of variability, the Panel recommended that additional detail be 

provided on how different sources of variability were compared and prioritized. In addition, 

the Panel suggested that the sources of variability considered in the analysis be augmented to 

include differences in PM copollutant concentrations and discussion of land use, source 

locations, housing stock, and SES. 

RESPONSE: We have added clarification for the process used to evaluate sources of 

variability as part of section 3.5.2 of the document. In addition, we added coverage for the 

specific sources of variability referenced in the Panel’s comments. 

 With regard to the qualitative assessment of uncertainty, the Panel recommended that 

additional detail be provided on the method used to rank and categorize sources of 

uncertainty, and that we discuss implications for interpreting the urban study area risk 

estimates. The Panel encouraged us to comment on the degree to which dependencies exist 

between pair-wise combinations of sources of uncertainty. The Panel also recommended that 

we address more directly the potential for exposure misclassification (in the epidemiological 

studies providing the C-R functions) to introduce uncertainty into the risk assessment. 

RESPONSE: We have expanded our discussion of the qualitative analysis of uncertainty (in 

section 3.5.3) to more explicitly describe the process used. The integrative discussion in the 

new chapter 6 includes consideration for how the qualitative analysis of uncertainty impacts 

the interpretation of core risk estimates. We have also included comparisons of selected pairwise interactions of sources of uncertainty that have the potential to impact risk estimates. 

 Regarding the sensitivity analysis, the Panel made two specific recommendations for 

additional analyses: consider alternative lags for short-term morbidity endpoints based on 

Bell et al. (2008) and consider risk for short-term mortality using regional effect estimates 

from Zanobetti and Schwartz (2009). More generally, the Panel recommended that we 

compare the impact of uncertainty in the statistical fit of the effect estimates used in the core 

analysis with the impact of other sources of uncertainty considered in the sensitivity analysis. 

The Panel also suggested that we do the following: 1) provide additional rationale for 

focusing on long-term exposure-related mortality in using the results of the sensitivity 

analysis to inform consideration of uncertainty in the core risk estimates; 2) discuss whether 

the lower and upper bounds of the sensitivity analysis represent plausible upper and lower 

bounds on the core risk estimates; and 3) consider the results of the sensitivity analysis 

together with the results of the qualitative uncertainty analysis to inform interpretation of the 

core risk estimates. 

RESPONSE: We have expanded the sensitivity analysis to include consideration for 

alternative lag periods in modeling risk for short-term morbidity using Bell et al., 2008 (see 

section 3.5.4). However, we concluded that expanding the sensitivity analysis to include 

coverage for regional effect estimates (in addition to the city-specific effect estimates used in 

the core risk assessment) was not warranted in light of the resources and time that would 
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have been required. We have expanded our discussion of uncertainty (in section 3.5.3) to 

include explicit consideration for uncertainty in the statistical fit of the C-R functions and 

specifically how this compares to other factors assessed as part of the sensitivity analysis. 

The discussion of the sensitivity analysis has been expanded to address the use of the results 

as an alternative set of reasonable risk estimates for informing consideration of uncertainty in 

the core risk estimates (including the degree to which we believe the sensitivity analysis 

captured plausible high- and low-bound risk estimates – see section 4.3.2). The integrated 

discussion in the new chapter 6 integrates consideration of the sensitivity analysis results 

with other analyses completed for the risk assessment, including the qualitative uncertainty 

analysis as noted above, to inform consideration of overall confidence in the core risk 

estimates. 

 With regard to the national-scale long-term exposure-related mortality analysis, the Panel 

recommended that risk estimates be generated for the full set of air quality scenarios included 

in the risk assessment (i.e., current and alternative suites of standards, in addition to a recent 

conditions analysis) and that additional health effect endpoints matching those modeled for 

the 15 urban study areas be included in the national-scale analysis. 

RESPONSE: Following deliberative review of available data and methods, combined with 

consideration for the policy goals of the national-scale mortality analysis (as differentiated 

from the health benefits assessment that the Agency conducts as part of a Regulatory Impact 

Analysis), and taking into consideration the resources and time needed for such an expanded 

assessment, we have decided not to expand the national-scale mortality analysis to include 

consideration of either alternative suites of standards or additional health endpoints. We have 

included our rationale for this decision in section 5.1. 

Document Availability 

The draft document is being made available to the CASAC PM Panel in the form of the 

attached electronic file. The document is also available from the EPA website at 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. Printed copies of this 

document will be sent to CASAC PM Panel members via Federal Express. We will mail a 

printed copy of the main body of the document only. The appendices can be accessed 

electronically as noted above and printed copies of the appendices will be made available to 

Panel members upon request.

Charge to the CASAC PM Review Panel 

We ask the CASAC PM Panel to focus on the charge questions listed below in their 

review of the Quantitative Health Risk Assessment for Particulate Matter, but we would 

appreciate comments on any other topics as well. 

Chapter 3 – Urban Case Study Analysis Methods

1) Air quality inputs (section 3.2): We have expanded the consideration of alternative 

approaches to simulating just meeting the current and alternative suites of PM2.5 standards 
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(i.e., rollback approaches) to include a peak shaving approach, in addition to the hybrid and 

proportional approaches considered in the first draft assessment. This peak shaving approach 

is intended to represent more localized, rather than regional, patterns of PM2.5 reductions 

(discussed in section 3.2.3.3). 

a) To what extent does the Panel believe that the use of the peak shaving approach provides 

useful additional exploration of variability associated with how ambient PM2.5

concentrations are simulated to change upon just meeting the current and alternative 

suites of standards? 

b) We have used comparisons of composite monitor annual averages generated using the 

different rollback approaches as a surrogate for differences in long-term exposure-related 

mortality in looking across all three rollback approaches. To what extent does the Panel 

believe that this is a reasonable approach for assessing the impact of variability 

associated with simulating changes in air quality patterns on estimates of long-term 

exposure-related mortality? 

2) Selection of model inputs (section 3.3): We have expanded and clarified the discussion of 

our rationale for identifying modeling choices comprising the core risk model, focusing in 

particular on selection of C-R functions (section 3.3.3). To what extent does the Panel 

consider this discussion to be clear and the model selections appropriate? 

3) Addressing uncertainty and variability (section 3.5): We have clarified the process used to 

evaluate sources of variability and added coverage for specific sources of variability (section 

3.5.2); expanded our discussion of the qualitative analysis of uncertainty (section 3.5.3); and 

included analyses of pair-wise interactions of sources of uncertainty (section 3.5.4). To what 

extent does the Panel consider these discussions to be clear and appropriate? 

Chapter 4 – Urban Case Study Results

4) Sensitivity analysis results (section 4.3): We have included a discussion of how the results of 

the sensitivity analysis can be used as an additional set of reasonable risk estimates to inform 

consideration of uncertainty in the core risk estimates (see section 4.3.2). What are the 

Panel’s views on how we have used the sensitivity analysis results to support consideration 

of uncertainty in the core risk estimates? 

5) Consideration of design values and patterns of PM2.5 monitoring data in interpreting core risk 

estimates (section 4.5): To enhance our interpretation of the patterns of core risk estimates 

generated for both the current and alternative suites of standards, we have included analyses 

of 24-hour and annual design values together with patterns of PM2.5 monitoring data for the 

15 urban study areas. This reflects the fact that these two factors play a key role in 

determining the degree of risk reduction estimated upon just meeting the current and 

alternative suites of standards under alternative rollback approaches. As part of the 

consideration of design values, we have also contrasted the 15 urban study areas with 

patterns of design values seen for the broader set of urban areas in the U.S. in order to help 

place the urban study area in a broader national context 

a) To what extent is the Panel supportive of these additional assessments? 
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b) Does the Panel have any recommendations for additional insights based on consideration 

of patterns in design values and PM2.5 monitoring data across the 15 urban study areas 

and at the national level? 

Chapter 6 – Integrative Discussion of PM2.5-related Risks

6) We have developed an integrated discussion of the PM2.5-related risk estimates which 

considers the results of the qualitative and quantitative treatment of uncertainty and 

variability together with the various national-scale assessments completed for the analysis to 

support interpretation of the core risk estimates. As part of the integrative discussion, we 

also provide key observations that bear on policy-relevant risk-based questions. 

a) To what extent does the Panel believe that we have captured the key policy-relevant 

questions that can be addressed by this risk assessment? 

b) We provide a set of key observations related to estimates of risk associated with 

simulations of just meeting the current and alternative suites of standards. These 

observations are based not only on consideration of trends in risk reduction across 

alternative suites of standards and residual risk remaining after simulation of just meeting 

specific suites of standards, but also on additional factors that can impact risk (e.g., the 

role of annual and 24-hour design values, the peakiness of PM2.5 distributions within a 

study area, and application of different rollback approaches). To what extent do the 

Panel members believe that the observations presented in section 6.2 are well supported 

by the results of the analyses? Are there other observations that might be made that 

would help to address the policy-relevant questions identified at the beginning of the 

chapter? 

c) Part of our interpretation of the core risk estimates presented in section 6.2 is our 

characterization of confidence in the core risk estimates and in observations made based 

on those estimates. These assessments of confidence are based on consideration of the 

results of the sensitivity analysis as well as on the qualitative assessment of uncertainty 

and variability. To what extent does the Panel believe that the characterizations of 

confidence in the core risk estimates and associated policy-related observations are 

reasonable given available information? 

d) As part of the integrative discussion, we use the results of several national-scale analyses 

(i.e., the national scale PM2.5 mortality analysis, the representativeness analysis, and the 

new exploration of design values and patterns of PM2.5 monitoring data presented in 

section 4.5) to place the results of the risk assessment in a broader national-context. 

What are the Panel members’ views on appropriateness of this effort to place results of 

the analysis in a national context? 

e) We conclude chapter 6 with a list of key observations. Does the Panel believe that we 

have appropriately highlighted key findings of the risk assessment in these observations? 

Of particular note is the observation that, while alternative 24-hour standard levels can be 

used to reduce annual-average PM2.5 concentrations and thus to reduce estimated risk, the 

results are likely to be highly variable across urban areas. More consistent lowering of 

annual-average PM2.5 concentrations across study areas, and thus more consistent 

reductions in estimated risk, may result from application of alternative annual standard 

levels. We also note that simulation of the alternative 24-hour standard level of 25 μg/m3
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resulted in reductions in annual-average PM2.5 levels for some study areas that were well 

below the lowest annual standard level assessed (i.e., below 12 μg/m3

). As a 

consequence, we observed risk reductions reflecting these changes in annual-average 

PM2.5 levels below 12 μg/m3

. Given these results, does the Panel believe that there is 

utility in estimating risks for alternative annual standard levels below 12 μg/m3

? 

We look forward to discussing these issues with the CASAC PM Panel at our upcoming 

meeting. Should you have any questions regarding this draft document, please contact Dr. Zach 

Pekar (919-541-3704; email pekar.zachary@epa.gov). 

Attachments 

cc: Vanessa Vu, SAB, OA 

Karen Martin, OAQPS/HEID 

Neal Fann, OAQPS/HEID 

Beth Hassett-Sipple, OAQPS/HEID 

Bryan Hubbell, OAQPS/HEID 

Phil Lorang, OAQPS/AQAD 

Zach Pekar, OAQPS/HEID 

Pradeep Rajan, OAQPS/HEID 

Mary Ross, ORD/NCEA-RTP 

Lindsay Stanek, ORD/NCEA-RTP 

Susan Stone, OAQPS/HEID 

 John Vandenberg, ORD/NCEA-RTP 

Jason Sacks, ORD/NCEA-RTP