September 8, 2009 

MEMORANDUM

SUBJECT: CASAC Review of Risk Assessment to Support the Review of the PM Primary 

National Ambient Air Quality Standards - External Review Draft and Particulate 

Matter Urban-Focused Visibility Assessment – 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 are two draft assessment documents: (1) Risk Assessment to Support the Review 

of the PM Primary National Ambient Air Quality Standards - External Review Draft (September 

2009) and (2) Particulate Matter Urban-Focused Visibility Assessment – External Review Draft 

(September 2009), 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) and secondary (welfare-based) national ambient air quality standards (NAAQS) 

for particulate matter (PM). These documents will be the focus of a review 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 October 5–6, 2009. In addition, on or about 

September 15, 2009, we plan to release a very preliminary draft of another document, Policy 

Assessment for the Review of the Particulate Matter National Ambient Air Quality Standards: 

Preliminary Draft (Policy Assessment), to facilitate discussion with the CASAC PM Panel on 

our ongoing efforts to prepare an external review draft Policy Assessment. I am requesting that 

you forward these draft documents to the CASAC PM Panel to prepare for the October meeting. 

As part of EPA’s review of the PM NAAQS, the Agency is conducting quantitative 

assessments characterizing (1) the health risks associated with exposure to ambient PM and (2) 

urban visibility impairment associated with ambient PM. The two attached draft assessment 

documents convey the approaches taken to assess and characterize PM-associated health risks 

and urban visibility impairment. These documents include descriptions of the scope of the 

assessments and the methodologies used as well as the initial key results, observations, and 

related uncertainties associated with the quantitative analyses conducted. 
2

The EPA’s plans for conducting the health risk and urban-focused visibility assessments, 

including the proposed scope and methods of the analyses, were presented in two planning 

documents, Particulate Matter National Ambient Air Quality Standards: Scope and Methods 

Plan for Health Risk and Exposure Assessment (February 2009) and Particulate Matter National 

Ambient Air Quality Standards: Scope and Methods Plan for Urban Visibility Impact 

Assessment (February 2009), henceforth, Scope and Methods Plans, see 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html. These planning documents 

were the subject of a consultation with CASAC on April 2, 2009. Comments from the CASAC 

PM Panel as well as public comments were considered in developing the draft assessment 

documents being released at this time. 

These draft documents draw upon information presented in the Integrated Science 

Assessment for Particulate Matter: Second External Review Draft (ISA, July 2009) prepared by 

EPA’s National Center for Environmental Assessment, Research Triangle Park, NC (NCEARTP), which the CASAC PM Panel will review at the same meeting. The CASAC and public 

comments on the draft assessment documents will be taken into consideration in making 

revisions to the draft assessment documents and in the preparation of an external review draft 

Policy Assessment. 

Draft documents are being made available to the CASAC PM Panel in the form of 

attached electronic files. The documents are also available from the EPA website at 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html. Printed copies of these 

documents will be sent to CASAC PM Panel members via Federal Express. With regard to the 

Risk Assessment document, 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 two draft assessment documents, but we would appreciate comments on any other 

topics as well. 

 Risk Assessment: Risk Assessment to Support the Review of the PM Primary National 

Ambient Air Quality Standards - External Review Draft (September 2009) 

Following an introductory chapter, this document discusses the scope of the risk assessment, 

including modifications based on CASAC consultation and public comment on the Scope 

and Methods Plan; the methods used to estimate risks associated with recent air quality and 

with air quality concentrations simulated to just meet current and alternative standards; the 

results including core analyses and alternative estimates based on a number of sensitivity 

analyses, as well as an evaluation of the representativeness of the urban study areas in a 

national context; and a national-scale assessment of premature mortality associated with 

long-term PM2.5 exposures based on recent air quality. Throughout these chapters, key issues 

and uncertainties are discussed.

 
3

Chapters 2 and 3 – Scope of the assessment and Methods used for the urban case studies

1) After careful consideration of evidence provided in the second draft ISA and of the views 

expressed by the Panel in consulting on the Scope and Methods Plan, we have decided to 

quantitatively assess risk associated with both short- and long-term exposure to PM2.5 only. 

Based on our consideration of the evidence for health effects potentially associated with 

short-term exposure to PM10-2.5, as well as to ultrafine particles and specific components, and 

in recognition of the limited available air quality data, we decided not to assess risk 

quantitatively for PM10-2.5, ultrafine particles, or specific PM components as part of the 

current assessment. Is the Panel generally supportive of this scope? To what extent is the 

rationale for this decision clear and appropriate? 

2) The final set of health effect categories included in the risk assessment for PM2.5 are 

consistent with those outlined in the Scope and Methods Plan (i.e., those classified as having 

a causal or likely causal association with PM2.5 exposure, as presented in the second draft 

ISA). We decided not to include any of the health effect categories classified as suggestive 

of a casual association in the second draft ISA, based on a number of considerations as 

described in section 3.3.1. Please comment on the approach taken and on the clarity of the 

rationale for selecting health effect categories for inclusion in the quantitative risk 

assessment. 

3) Based on consideration of evidence presented in the second draft ISA, we have identified 

four combinations of 24-hour and annual alternative standard levels for analysis in the risk 

assessment. Please comment on the extent to which the rationale provided in section 2.5 

appropriately supports these combinations of alternative standard levels for this assessment. 

4) General approach 

a) For this assessment, we have developed a primary set of risk results based on the 

application of modeling element choices (e.g., concentration-response (C-R) functions, 

lag periods) that we believe have the greatest overall support in the literature (referred to 

as the “core” results). As discussed in sections 2.4.1, 3.1 and 4.0, while it is not possible 

at this time to assign quantitative levels of confidence to these core risk estimates, staff 

believes that these estimates are generally based on inputs having higher overall levels of 

confidence, relative to risk estimates that could have been generated using other inputs 

identified in the literature. Consequently, the core risk estimates receive greater focus 

when we present, summarize and discuss risk estimates. What is the Panel’s view on the 

approach used and does the Panel consider it to be described appropriately and clearly? 

b) Based on consideration of uncertainties associated with specifying C-R functions below 

the lowest measured level (LML) from a particular epidemiological study, we have 

decided to model risk for long-term PM2.5 exposures down to the LML, but not to 

extrapolate down to policy-relevant background (PRB). In contrast, when estimating risk 

associated with short-term PM2.5 exposures, because the LML is generally below the 

range of PRB values on some days during the study periods evaluated, we decided to 

model short-term risk down to PRB (see section 3.1). Is the Panel generally supportive 

of these approaches? 
4

5) Air quality inputs 

For this assessment, we have included an alternative approach for simulating air quality 

levels that just meet either current or alternative suites of standards in addition to the 

proportional analysis that has been used in previous analyses. Specifically, we have 

employed a hybrid (non-proportional) air quality adjustment procedure which simulates a 

combination of regional and local controls. The non-proportional rollback approach was 

used as part of a sensitivity analysis to examine uncertainty associated with this aspect of the 

risk assessment, while the historical proportional approach was used for the core analysis. 

To what extent does the Panel support the use of the alternative non-proportional rollback 

approach in the context of the sensitivity analyses? Please provide comments on the 

alternative approach as presented in section 3.2.3 and Appendix B. 

6) Selection of urban study areas 

We have included 15 urban study areas in the risk assessment, with the selection of these 

areas being based on a number of criteria as presented in section 3.3.2. To what extent does 

the Panel support the rationale provided for selection of the urban study areas and the 

specific locations considered? 

7) Selection of epidemiological studies and C-R functions within those studies: 

In estimating risks associated with PM2.5 exposures, we focused on selecting C-R functions 

from large multi-city studies based on staff’s conclusion that these studies provided more 

defensible effect estimates (see section 3.3.1). Concentration-response functions from 

several single-city studies evaluating short-term PM2.5 exposures were also included to 

provide coverage for additional health effect endpoints (e.g., emergency department visits). 

To what extent is the Panel supportive of this approach for selecting C-R functions for 

modeling risk related to short-term and long-term PM2.5 exposures? 

a) Specifically with regard to short-term exposure-related mortality, focusing on a study of 

112 US cities by Zanobetti and Schwartz (2009), we obtained Empirical Bayes 

“shrunken” city-specific estimates from the study authors that provided a distinct C-R 

function for each urban study area location. For short-term exposure-related morbidity, 

focusing on a study of 202 U.S. counties by Bell et al. (2008), we used regionallydifferentiated effect estimates provided by the study authors. Please comment on the 

selection of C-R functions for evaluating short-term morbidity and mortality effects. To 

what extent do the Panel members consider the rationales supporting the selection of C-R 

functions to be clearly and appropriately presented? 

b) Specifically with regard to long-term exposure-related mortality, we identified a number 

of effect estimates using the extended follow-up of the American Cancer Society (ACS) 

study to use in the core analysis (Krewski et al., 2009). These effect estimates include 

standard Cox proportional hazards models, with 44 individual and 7 ecologic covariates, 

derived using two separate PM2.5 monitoring data sets (i.e., 1979-1983 and 1999-2000) 

(see section 3.3.3 of the RA). To what extent is the rationale for these choices clear and 

sufficiently justified as the basis for the core analysis involving long-term PM2.5-related 

mortality? 
5

8) Addressing uncertainty and variability 

a) The treatment of uncertainty and variability in the analysis is based on the multi-tiered 

approach presented in a recent WHO document (WHO, 2008). Specifically, as outlined 

in section 3.5, we have included qualitative analysis of both variability and uncertainty 

(WHO Tier 1), as well as single-factor and multi-factor sensitivity analyses aimed at 

identifying which potential sources of uncertainty have the greatest impact on the core 

risk estimates (WHO Tier 2). In addition, the sensitivity analyses have been designed to 

provide a reasonable set of alternate risk estimates to supplement the core risk estimates 

and inform consideration of uncertainty associated with the core analysis. To what extent 

does the Panel support the overall approach for addressing uncertainty and variability? 

To what extent does the Panel agree that the overall approach is appropriate and 

consistent with the goals of the risk assessment as outlined in chapter 1? Does the Panel 

have any recommendations for improving the characterization of variability and/or 

uncertainty? 

b) The qualitative discussion of key sources of variability, and the degree to which the 

analysis design captures those sources of variability, are presented in section 3.5.2. 

Please provide comments on the approach used. Specifically, do the analyses sufficiently 

address the issue of variability? Are there key sources of variability that have not been 

addressed within the qualitative analysis but which could have an important impact on 

modeling population-level risk associated with PM2.5 exposure? 

c) Table 3-13 provides a qualitative characterization of uncertainties including the potential 

direction, magnitude, and degree of confidence associated with our understanding of the 

sources of uncertainty. To what extent does the Panel support the characterizations of the 

key sources of uncertainties identified and the relative rankings of the importance of 

those sources of uncertainty? Are there additional uncertainties that should be 

considered? 

d) The results of the sensitivity analyses have been used to gain insights into which sources 

of uncertainty significantly impact the core risk estimates and to provide a reasonable set 

of alternate risk estimates to supplement the core analysis. We are mindful that these 

estimates do not represent a true uncertainty distribution. With regard to the single- and 

multi-factor sensitivity analyses, to what extent is the Panel supportive of the approach 

used to conduct and characterize the results of the sensitivity analyses? Please provide 

comments on the extent to which the presentation of the results of the sensitivity analyses 

are clearly and reasonably described? Does the Panel have any recommendations for 

how the results of the sensitivity analyses could be used more effectively or appropriately 

in characterizing uncertainty associated with the core risk estimates? 

Chapter 4 – Results 

9) A number of risk metrics as well as different approaches for presenting these metrics are 

included in tabular and graphical format for both the core analysis and sensitivity analyses. 

Please comment on the extent to which the risk estimates are clearly and appropriately 

characterized and presented. 
6

10) Evaluation of the representativeness of the urban study areas in the national context 

We completed a comparison of the 15 urban study areas against national distributions for key 

PM risk-related attributes. The goal of this analysis was to determine whether the urban 

study areas are more nationally-representative of these attributes, or are more focused on a 

particular portion of the distribution for a given parameter. In particular, given that one of 

the goals of the risk analysis is to provide estimates of risks for those areas likely to 

experience high levels of PM exposure and risk, this assessment provides insights as to the 

extent to which the assessment represents high PM2.5 risk locations. The results of this 

analysis were then used to evaluate, in part, whether the set of 15 urban study areas is likely 

to reflect the broader U.S. population with regard to PM2.5–related risk, including coverage 

for those locations that represent specific at-risk populations. To what extent does the Panel 

support the approaches used? Please provide comments on the clarity with which the 

methods, results, and insights gained from this analysis are described. 

Chapter 5 – National-Scale Assessment of Long-Term Mortality Related to PM2.5 Exposures 

11) We completed a national scale assessment focused on long-term mortality associated with 

recent air quality conditions. To what extent does the Panel support the approach used? 

Please comment on the clarity and appropriateness with which the methods, results, and 

insights gained from this analysis are described. 

12) The national-scale long-term mortality risk assessment provides perspective for where the 31 

counties associated with our 15 urban study area analysis fall along the national distribution 

of mortality risk. We note that this analysis is distinct from the representativeness analysis 

referenced above (and described in section 4.4) in that this analysis focuses on coverage of 

the 15 urban study areas for long-term mortality risk, while the earlier representativeness 

analysis focuses on coverage for PM risk-related factors. To what extent is the Panel 

supportive of this specific analysis and its intended use to provide insights into the extent to 

which the urban study area analysis broadly represents urban PM2.5-related risk in the U.S? 

 Visibility Assessment Particulate Matter Urban-Focused Visibility Assessment – External 

Review Draft (Visibility Assessment, September 2009) 

Following an introductory chapter, this document discusses the current scope of the urban 

focused visibility assessment, including modifications based on CASAC consultation and 

information provided through public comment; the results of a reanalysis of urban visibility 

preference studies; the methods used to evaluate recent PM2.5 air quality and light extinction 

levels with PM2.5 air quality concentrations simulated to just meet current and alternative 

PM2.5 and PM light extinction standards; and results of the evaluations. Throughout these 

chapters, key issues and uncertainties are discussed. 

Chapter 1 – Scope of Visibility Assessment 

1) After careful consideration of the evidence provided in the second draft ISA, and in 

particular the significant body of work that has been conducted by the Regional Planning 

Organizations under the Regional Haze Rule, (i.e. information on urban and rural PM 

concentrations and compositions), we have decided to continue to focus this assessment on 
7

the PM induced visibility impairment that is occurring in urban areas. What is the Panel’s 

view on this approach? Is the rationale supporting the selected approach clear and 

appropriate? 

2) After further considering the nature of urban versus more remote area PM, and in light of 

discussions with CASAC at the April 2, 2009 meeting, we have decided not to develop an 

urban optimized algorithm at this time, but instead to rely on the original IMPROVE 

algorithm to relate urban PM to local haze (PM light extinction). Is the Panel generally 

supportive of this approach? Is the rationale supporting this decision appropriate and clearly 

presented? 

3) In a change from the planned approach presented in the Scope and Methods Plan, we have 

decided to conduct a reanalysis of the urban visibility preference studies available at the time 

of the 2006 PM NAAQS review, rather than conducting new public preference studies since 

it is highly unlikely that the results of new studies could be completed in time to inform this 

review. This reanalysis was designed to explore the similarities and differences 

(comparability) between the current studies and to assess what information could be drawn 

from these results to inform the selection of visual air quality (VAQ) candidate protection 

levels (CPLs) to be used in subsequent impact assessments. This reanalysis also includes a 

recent study by Smith and Howell (2009) for Washington, D.C. which was presented to the 

CASAC during the public comment phase of the April 2, 2009 meeting and later provided to 

EPA staff. Does the Panel agree that the information provided by this reanalysis is useful to 

inform the selection of CPLs? Does the Panel agree that inclusion of the Smith and Howell 

(2009) is appropriate in both the ISA and Visibility Assessment? To what extent does the 

Panel consider that the reanalysis of the urban visibility preference studies is clearly and 

appropriately characterized? 

4) We have chosen to use the range that represents the 50th acceptability criteria across the four 

cites studied (i.e., the VAQ level that best divides the photographs shown into two groups: 

those with a VAQ rated as acceptable by the majority of the participants, and those rated not 

acceptable by the majority of participants) as CPLs to characterize the nature of the impact 

on urban VAQ associated with current PM levels. Please comment on the clarity and 

appropriateness of the rationale supporting this decision. Does the Panel have suggestions 

for alternative ranges to consider? 

5) A new indicator relating ambient PM to urban VAQ (i.e., PM light extinction) has been 

evaluated to improve our characterization of the relationship between ambient PM and 

visibility impairment. Is the Panel generally supportive of this approach? To what extent 

have we provided an adequate justification for the new indicator used? 

6) An averaging time of one hour as a practical minimum time period was used in this 

assessment in recognition that, while the visibility impacts are nearly instantaneous, the 

urban VAQ does not generally change significantly from minute to minute, but does vary 

from hour to hour. To what extent does the Panel support this approach? Does the Panel 

consider the rationale supporting this approach to be clearly and appropriately presented? 

7) We have chosen to use the 90th and 95th percentile forms in our assessment of alternative 

secondary (welfare-based) standards. Please comment on the use of these alternative forms. 
8

Chapter 2 – Urban Visibility Preference Studies

8) To what extent does the Panel support the graphical displays presented in this chapter? As 

currently presented, do these figures clearly summarize the assessment results? We have 

combined data from multiple studies for two locations - British Columbia and Washington, 

D.C. - (Abt Associates, 2001; Smith and Howell, 2009 test 1) as presented in Figure 2-14. 

Does the Panel agree with developing a composite dataset for each of these two urban areas? 

9) Despite significant differences in study characteristics (e.g., size, location), to what extent 

does the Panel support combining and comparing the results from the four cities, as shown in 

Figure 2-14? What is the Panel’s view on the clarity and adequacy of the descriptions of the 

uncertainties and limitations associated with such a combined assessment and the 

conclusions that can be drawn from the assessments? Please provide comments on additional 

insights, uncertainties, or caveats that should be considered. 

10) We have used the combined results presented in this chapter to develop a range of CPLs that 

are used in subsequent steps of the assessment. To what extent does the Panel support the 

range of CPLs used and the justification provided for selecting this range? Does the Panel 

recommend consideration of any alternative approaches or criteria for selecting CPLs? 

11) Overall, we consider this assessment useful for providing information for the design of future 

urban visibility preference studies. Does the Panel support this conclusion and does the 

Panel have any recommendations for changes that could be made in the discussions of this 

information to enhance its usefulness for this purpose? 

Chapter 3 – Estimation of Current PM Concentrations and Light Extinction

12) Are the goals articulated in the first paragraph of this chapter achieved in the remainder of 

the assessment? If not, does the Panel have suggestions for additional assessments that 

should be done? 

13) Are the methods and approaches taken in these assessments, including those for monitor site 

selection, incomplete data adjustments, and the use of the CMAQ model to augment 

speciation data, appropriate and is the rationale for their selection clearly articulated? 

14) Is the approach used to estimate PRB as described in chapter 3 and Appendix C appropriate? 

15) We consider the results generated by these analyses to be reasonable based on PM 

composition and relative humidity data. Does the Panel agree? Are there other tests of 

reasonableness that could be applied? 

16) In addition to a qualitative discussion of possible sources of uncertainty and variability, are 

there quantitative methods for addressing uncertainty and variability associated with these 

assessments that the Panel would recommend? 

17) A number of appendices are provided at the end of this document. Does the Panel agree that 

this information is useful to retain? Does the Panel agree with the level of detail provided in 

the body of the report and its organization and distribution throughout the document? 
9

Chapter 4 – Total Light Extinction Under “What If” Conditions of Just Meeting Specific 

Alternative Secondary NAAQS 

18) Does the Panel agree with the approaches used to simulate just meeting air quality conditions 

for the current and alternative PM standards? In particular, is use of the proportional rollback 

approach appropriate in the context of the urban PM visibility assessment? 

19) To what extent does the Panel consider the presentation of "what if" scenarios for retention of 

the current secondary PM2.5 NAAQS and consideration of alternative, more protective 

secondary NAAQS in sections 4.2 and 4.3 to be clearly written with an appropriate level of 

detail? Do the correlation analyses presented in Appendix D provide sufficient insight into 

the suitability of alternative indicators based on sub-24 hour averaging periods for PM2.5? 

Are there additional alternative standard scenarios that should be evaluated? 

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

meeting. Should you have any questions regarding these draft documents, please contact Dr. 

Zach Pekar regarding the draft risk assessment (919-541-3704; email pekar.zachary@epa.gov) or 

Ms. Vicki Sandiford regarding the draft visibility assessment (919-541-2629; email 

sandiford.vicki@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 

Meredith Lassiter, OAQPS/HEID 

Phil Lorang, OAQPS/AQAD 

Zach Pekar, OAQPS/HEID 

Marc Pitchford, OAQPS/HEID 

Pradeep Rajan, OAQPS/HEID 

Harvey Richmond, OAQPS/HEID 

Mary Ross, ORD/NCEA-RTP 

Vicki Sandiford, OAQPS/HEID 

Lindsay Stanek, ORD/NCEA-RTP 

Susan Stone, OAQPS/HEID 

 John Vandenberg, ORD/NCEA-RTP