September 23, 2010 

MEMORANDUM

SUBJECT: Transmittal of Final Assessment Documents: Quantitative Health Risk 

Assessment for Particulate Matter and Particulate Matter Urban-Focused 

Visibility Assessment

FROM: Lydia N. Wegman, Director 

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 the final assessment documents: Quantitative Health Risk Assessment for 

Particulate Matter (Risk Assessment or RA, June 2010) and Particulate Matter Urban-Focused 

Visibility Assessment (Visibility Assessment or UFVA, July 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) and secondary 

(welfare-based) national ambient air quality standards (NAAQS) for particulate matter (PM). 

These final documents draw upon information presented in the final Integrated Science 

Assessment for Particulate Matter (ISA, December 2009) prepared by EPA’s National Center for 

Environmental Assessment, Research Triangle Park, NC (NCEA-RTP). They also reflect 

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

second draft assessment documents, which were reviewed by the Panel at a meeting on March 

10-11, 2010. 

Risk Assessment 

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

provided to us in an April 15, 2010 letter (Samet, 2010a)1

, are briefly summarized below 

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

Risk Assessment. 

 

1

 Samet, J. (2010a). Letter from Clean Air Scientific Advisory Committee to the Honorable Lisa P. Jackson, 

Administrator, US EPA. CASAC Review of Quantitative Health Risk Assessment for Particulate Matter – Second 

External Review Draft (February 2010). April 15, 2010. Available: 

http://yosemite.epa.gov/sab/sabproduct.nsf/BC4F6E77B6385155852577070002F09F/$File/EPA-CASAC-10-008-

unsigned.pdf
2

 Regarding methods used to simulate just meeting current and alternative standards, the Panel 

recommended that EPA: (a) include a graphical/mathematical description of each rollback 

method, (b) give more emphasis to the hybrid rollback method, using the other two rollback 

methods to represent bounding estimates, and (c) rename the “peak shaving” method to more 

clearly describe the actual methods used. 

RESPONSE: In the final Risk Assessment, EPA included an example calculation of the 

three rollback methods illustrating mathematically the application of the three rollback 

methods as applied to the Detroit urban study area in simulating just meeting current standard 

levels (Appendix B, section B.3). In presenting estimated risk results generated using the 

three rollback methods for simulating alternative standard levels, we noted the potential for 

the hybrid method to reflect a more representative approach bounded by the other two 

rollback methods considered in Chapters 4 and 5. In addition, we renamed the “peak 

shaving” approach the “locally focused rollback method" (section 3.2.3). 

 Regarding the national-scale assessment, the Panel concluded that the key findings of this 

work presented in Chapter 5 of the second draft RA related to the representativeness of the 

15 urban study areas. The Panel recommended moving the overall discussion of this analysis 

to an appendix while retaining the key findings. 

RESPONSE: For the final Risk Assessment, we moved discussion of the methods used in 

the national-scale risk assessment into Appendix G. In the main body of the document, 

(Chapters 4 and 5), we focused the discussion of that analysis to the representativeness of the 

15 urban study areas. 

 Regarding the confidence in the risk estimates, the Panel recommended that EPA develop 

and apply specific criteria for determining the lower-bound exposure concentrations to be 

considered in the risk assessment. Specifically, the Panel suggested consideration should be 

given to the range of concentrations evaluated in the epidemiology studies providing the 

concentration-response functions for the quantitative risk assessment as well as the degree of 

protection afforded to susceptible populations under various scenarios. 

RESPONSE: For the final Risk Assessment, we expanded our discussion of the confidence 

associated with the core risk estimates by more fully considering how the simulated 

composite monitor values underlying the risk estimates compared to ambient measurements 

used to support concentration-response functions in the epidemiological studies (section 

5.1.6. The consideration of the protection afforded by the current and alternative suites of 

standards, including protection afforded to susceptible populations, is discussed in the Policy 

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

 The Panel recommended that EPA provide additional clarification on the selection of 

exposure scenarios used in the risk assessment. 
3

RESPONSE: For the final Risk Assessment, we expanded the discussion related to selection 

of alternative suites of standards evaluated to more fully discuss the iterative nature of the 

process we used to identify alternative standard levels to model (section 2.4) . 

 The Panel recommended additional rewriting and clarification of content presented in the 

integrative discussion (Chapter 6 of the second draft RA). 

RESPONSE: For the final Risk Assessment, we made extensive edits in the integrative 

discussion to streamline the text and focus on critical observations (Chapter 5). 

 The Panel recommended including a very brief description of how the design values were 

calculated in the analyses and expressed concern about the method used in the second draft 

Risk Assessment to impute missing measurement data. The Panel further suggested EPA 

consider alternative methods for interpolating missing data. 

RESPONSE: In the final Risk Assessment, we included a more rigorous description of how 

design values are calculated (section 3.2.3.1). We also considered, as part of a sensitivity 

analysis, an alternative method for generating design values that used a different approach for 

interpolating missing data (Appendix B, section B1). 

 Regarding characterization of uncertainties, the Panel recommended that EPA clarify the 

approach used to assign high, medium or low classifiers to potential sources of uncertainty. 

The Panel also suggested EPA more directly incorporate the results from the qualitative 

analysis of uncertainty presented in Chapter 4 into the integrative discussion chapter. 

Furthermore, the Panel suggested that interpretation of the sensitivity analysis results include 

consideration of the absolute magnitude of the change in risk results in addition to focusing 

on the direction and percent change. 

RESPONSE: For the final Risk Assessment, we clarified the approach used to assign 

descriptors to potential sources of uncertainty (section 3.5.3). We also more fully integrated 

the results of the qualitative discussion of uncertainty into the integrative discussion (Chapter 

5). In addition, we expanded the presentation of the results of the sensitivity analysis to 

include information on the magnitude of the core risk estimates as presented in Table 4-4. 

 The Panel recommended that the scatter diagrams depicting 24-hour and annual design 

values be enhanced by including information on geographical regions. 

RESPONSE: For the final Risk Assessment, in considering the relationships between annual 

and 24-hour design values for specific areas, we color coded these areas by geographic 

region to facilitate our ability to observe regional differences (Figure 4-21). 

 The Panel recommended that the term "peaky" as used to describe PM patterns be more 

clearly defined and applied consistently. 
4

RESPONSE: In the final Risk Assessment, we clarified the definition of the term “peaky” 

and used it consistently in the discussion and interpretation of the risk estimates (Chapters 4 

and 5). 

Visibility Assessment 

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

provided to us in an April 20, 2010 letter (Samet, 2010b)2

, are briefly summarized below 

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

Visibility Assessment. 

 The Panel recommended that it would be important to see a more detailed comparison of the 

affected PM2.5 concentrations, species compositions and levels of visibility protection that 

would result from the use of alternative PM10 light extinction, PM2.5 light extinction or PM2.5 

mass indicators. 

RESPONSE: In the final Visibility Assessment, the effectiveness for reducing visibility 

effects of PM by the use of various 1-hour PM10 light extinction and 24-hour and annual 

PM2.5 mass concentration indicators are compared based on the current design values and the 

results of a rollback exercise to just meet each NAAQS scenario for 15 urban areas. These 

comparisons are presented principally by using tables and box plots to show the magnitude of 

current and projected rollback levels of PM10 light extinction. To improve the comparisons 

of the species compositions for highest 1-hour PM10 light extinction values as selected by the 

two forms (i.e. maximum daylight daily 1-hour and all daylight1-hour forms), stacked bar 

plots in Figure 3-13 were changed in the final version of the Visibility Assessment so that 

both forms included hourly contributions to PM10 light extinction. These figures showed 

similar composition for hours selected by the two forms as described in section 3.4.5. 

Additional analysis utilizing similar plots to characterize the composition of hours selected 

by PM2.5 mass concentration indicators were included in Appendix 4-C of the Policy 

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

Second External Review Draft. 

 The Panel recommended the addition of an integrative summary chapter. 

RESPONSE: An integrative summary chapter was added to the final document (Chapter 5). 

 The Panel recommended the addition of a section on further research needs, including future 

urban visibility preference studies. 

RESPONSE: The final Visibility Assessment included brief descriptions of the potential 

benefits from future research as part of the Technical Issues subsections in Chapter 5, 

 

2

 Samet, J. (2010b). Letter from Clean Air Scientific Advisory Committee to the Honorable Lisa P. Jackson, 

Administrator, US EPA. CASAC Review of Particulate Matter Urban-Focused Visibility Assessment – Second 

External Review Draft (January 2010). April 20, 2010. Available: 

http://yosemite.epa.gov/sab/sabproduct.nsf/0D5CB76AFE7FA77C8525770D004EED55/$File/EPA-CASAC-10-

009-unsigned.pdf 
5

including a discussion of the benefits of addition urban visibility preference studies (section 

5.1). A more comprehensive discussion of future research needs is included in section 4.5 of 

the second draft Policy Assessment. 

 The Panel recommended reduction of the level of detail in the description of the logit 

analysis in Chapter 2. 

RESPONSE: As recommended, we streamlined the description of the logit analysis in 

chapter 2 and included a complete technical description of the analysis in a newly added 

Appendix J. 

 The Panel requested a more direct “apples to apples” comparison of the composition between 

different forms for the plots of hourly PM composition for the various urban areas. 

RESPONSE: Stacked bar plots in Figure 3-13 were modified and a summary plot of urban 

area averaged composition in Figure 3-14 was added. These plots compare the PM 

composition selected by the two alternative forms more directly than was done in the second 

review draft of the UFVA. 

 The Panel expressed concern for the high PM10-2.5 mass contributions for St. Louis and Los 

Angeles and recommended closer scrutiny of the data used. 

RESPONSE: The final Visibility Assessment acknowledged the concerns raised by CASAC 

with respect to the credibility of unrealistically high PM10-2.5 mass measurement data for St. 

Louis (section 3.2.1 including footnote 10 on page 3-13). Since this concern was raised late 

in the development of the Visibility Assessment (i.e. beyond the point when alternative data 

could be use as input to a reassessment), the final document was modified to caution the 

reader with respect to the St. Louis results wherever they are discussed. The second draft 

Policy Assessment excluded results from St. Louis. With regard to Los Angeles PM10-2.5 

mass contributions, we acknowledge that the data may be of greater uncertainty than other 

urban areas with collocated PM2.5 and PM10 monitors, however unlike for St. Louis, PM10-2.5

mass rarely contributes significantly in the periods of time featured in the assessments 

conducted for Los Angeles, so no change or additional cautionary warnings to the reader 

were included in the final UFVA. 

Availability of Final Documents 

The final assessment documents are available to the CASAC PM Panel in the form of the 

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

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. Should you have any 

questions regarding the final PM Risk Assessment, please contact Dr. Zach Pekar (919 541-

3704; email pekar.zachary@epa.gov). Should you have any questions regarding the final 

Visibility Assessment, please contact Ms. Vicki Sandiford (919 541-2629; email 

sandiford.vicki@epa.gov). 

Attachments 
6

cc: Vanessa Vu, SAB, OA 

Karen Martin, OAQPS/HEID 

Beth Hassett-Sipple, OAQPS/HEID 

Zach Pekar, OAQPS/HEID 

Marc Pitchford, NOAA 

Pradeep Rajan, OAQPS/HEID 

Mary Ross, ORD/NCEA-RTP 

Vicki Sandiford, OAQPS/HEID 

Lindsay Stanek, ORD/NCEA-RTP 

Jason Sacks, ORD/NCEA-RTP 

 John Vandenberg, ORD/NCEA-RTP