UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C. 20460 

 OFFICE OF THE ADMINISTRATOR 

 SCIENCE ADVISORY BOARD 

May 17, 2010 

EPA-CASAC-10-011 

The Honorable Lisa P. Jackson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Subject: CASAC Review of Policy Assessment for the Review of the PM NAAQS - First 

External Review Draft (March 2010) 

Dear Administrator Jackson, 

The Clean Air Scientific Advisory Committee (CASAC) Particulate Matter (PM) Review 

Panel teleconferenced on April 8, 2010 and April 9, 2010 to review the Policy Assessment for the 

Review of the Particulate Matter National Ambient Air Quality Standards (NAAQS) - First 

External Review Draft (March 2010). The Policy Assessment is intended to help “bridge the gap” 

between the relevant scientific information and assessments and the judgment required of the EPA 

Administrator in determining whether and how to revise the National Ambient Air Quality 

Standards (NAAQS) for Particulate Matter (PM). This letter highlights CASAC’s main comments 

on the Policy Assessment, followed by our consensus responses to the charge questions and 

comments from individual Panel members. Before addressing specific issues associated with the 

Policy Assessment, we offer some general thoughts below for your consideration. 

General Comments 

In 2006, EPA revised the primary and secondary NAAQS for PM to provide increased 

protection of public health and welfare, setting the 24-h PM2.5 standard at 35 μg/m3

 and retaining 

the annual standard at 15 μg/m3

.

 In the Policy Assessment released earlier this year, EPA is 

considering revisions to these primary standards, using PM2.5 as the indicator for fine particles, and 

either PM10-2.5 (as measured), or the combination (difference) of PM2.5 and PM10 as the indicator 

for thoracic coarse particles. EPA has also, for the first time, proposed consideration of a 

secondary PM standard that would be based on a different indicator – PM light extinction – than 

the mass-based indicators which have long been used for the primary PM standards. The CASAC 

applauds this initiative and encourages the Agency to move aggressively forward toward 

developing the next generation of indicators for primary PM standards as well. 

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EPA has a history of continuously improving indicators for primary PM NAAQS as new 

evidence becomes available. Since the first standard for airborne PM, Total Suspended Particles 

(TSP), was promulgated in 1971, the EPA has implemented PM NAAQS with progressively more 

specific indicators from both the pollution source and biological perspectives. In 1987, EPA 

changed the indicator from TSP to PM10 (thoracic fraction penetrating into the thorax) and in 1997, 

fine PM (PM2.5), which differs substantially in chemical composition from the coarse particle 

fraction within PM10 and which penetrates to the gas exchange region of the lung, was adopted to 

supplement the protection provided by the PM10 standard. 

PM2.5 has been a useful surrogate index since it was adopted in the 1997 PM NAAQS 

promulgation, but may become an increasingly inadequate index of health risk as the mass 

concentration limits are reduced to the levels being contemplated in the current Policy Assessment. 

PM10 is a non-specific indicator of the respiratory system risks associated with PM10-2.5, as widely 

acknowledged in the Panel's discussions of the Integrated Science Assessment (ISA), Quantitative 

Health Risk Assessment, and Policy Assessment documents. Neither index is directly related to 

health risks associated with ultra-fine particles (UFP). The Panel also notes the challenge posed by 

the promulgation of a PM NAAQS directed at coarse particles, which vary widely in mass and 

composition across the country and between urban and rural areas. 

While research evidence on PM and health has evolved, we urge the Agency to undertake 

additional efforts to leverage the gains made thus far. Now is the time to look ahead to future 

review cycles and reinvigorate support for the development of evidence that might lead to newer 

indicators that may correlate better with the health effects associated with ambient air 

concentrations of particulate matter (PM) and for more comprehensive PM monitoring data, 

including expanding the range of sizes, and adding measurements of numbers, chemistry, species, 

and related emissions characteristics of particles. There is an inherent feedback in the cycle from 

research to policy formulation whereby researchers use the monitoring data that are gathered 

primarily for regulatory purposes with available indicators, and, in turn, expand the scientific basis 

for regulation. If EPA initiates efforts with air pollution research and monitoring communities 

now to create a more robust monitoring platform for research, the Agency will be better positioned 

to make an evidence-based transition to the “next generation” of indicators of PM-related health 

risks. 

Draft Policy Assessment (March 2010) 

Overall, the first draft Policy Assessment provides a solid start as EPA completes further 

analyses and deliberations related to thoracic coarse particles and visibility-related effects. As the 

first instance of moving through the sequence from an Integrated Science Assessment (ISA) 

through a Policy Assessment, the documents and the underlying flow of evidence and analytic 

findings provide an ordered and transparent basis for decision-making. CASAC was unanimous 

in recommending that the document be condensed and carefully edited to remove redundancy. 

While the document needs to be self-contained, the text should quote only the most critical 

evidence and rely more extensively on citations to the ISA and Risk Assessment. CASAC also 

suggests that the underlying decision-making algorithm that flows through the major questions 

posed in the text be made more explicit. (Text, and perhaps a figure, could be provided in Chapter 

1, or at the start of Chapters 2-5.) 

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Primary Standards 

The Policy Assessment recommends consideration of NAAQS that cover both fine 

particles, using PM2.5 as the indicator, and thoracic coarse particles, using either PM10-2.5 or the 

combination of PM2.5 and PM10. At the present time, CASAC agrees that these indicators are the 

best available given scientific knowledge to date. The evidence presented in the Integrated 

Science Assessment, Quantitative Health Risk Assessment and Policy Assessment shows that health 

effects have been observed in areas that meet current standards for PM2.5. Thus, CASAC supports 

EPA’s conclusion that consideration should be given to revising the current PM2.5 primary 

standards to provide increased public health protection. Since the last NAAQS review, findings 

from additional epidemiological studies support the association between PM10-2.5 and 

cardiovascular and respiratory morbidity and mortality. While the certainty of the associations 

between health effects for the association with PM10-2.5 exposure does not match that for PM2.5, 

CASAC also agrees with EPA’s conclusion that there is sufficient evidence to consider an 

adjustment of the PM10 standard. 

The choice of indicator is driven by findings of epidemiological studies that have drawn on 

monitoring data for PM10 and PM2.5 as exposure measures, as well as biological and source-related 

considerations. For the fine particle standard, the Policy Assessment draws heavily on the analyses 

in the Quantitative Health Risk Assessment for Particulate Matter, which are based on the 

assumption of a linear no-threshold model for risk. The Risk Assessment covers the evidence 

supporting this model, drawing on the findings of the ISA, particularly the finding of associations 

of PM indicators with morbidity and mortality at concentrations measured in the United States. In 

commenting on the Second External Review Draft of the Quantitative Health Risk Assessment, 

CASAC recommended that EPA develop criteria for setting the lower bound for the scenarios 

considered in the risk analysis. That same issue emerges with the Policy Assessment as lower 

bounds are considered for the range for the annual and 24-hr PM2.5 standards. The approach could 

be statistically based, using, for example, interquartile range, standard deviation, 10th percentile 

level, or lower-bound confidence interval, but also based on broader consideration of uncertainty 

and of the level of health protection to be achieved. In fact, selection of the lower bound of the 

inter-quartile range may not be sufficiently health protective. Specifying the criterion by which a 

lower bound concentration is chosen becomes central in decision-making under a no-threshold 

model, since any suite of NAAQS above policy-relevant background leaves residual morbidity and 

premature mortality. 

Secondary Standard 

 

With respect to EPA’s consideration of a secondary PM standard, CASAC supports the 

recommendations for secondary PM standards to protect public welfare as presented in the first 

draft Policy Assessment. The focus on protecting visibility is appropriate given the advanced 

scientific understanding of the direct effects of PM on visual air quality, as set out in the Integrated 

Science Assessment. The Policy Assessment establishes that the current secondary PM standards 

are inadequate to protect visibility, as their levels are too high, their averaging times are too long, 

and their (dried) mass-based indicators do not fully represent PM effects on visual air quality in the 

ambient atmosphere. 

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 CASAC supports the proposed “PM light extinction” indicator, as it represents an actual 

and measurable consequence of particles in the ambient air and it relates directly to the 

demonstrated harmful welfare effect of ambient PM on human visual perception. PM light 

extinction also captures the physical and chemical properties of ambient aerosols and associated 

meteorological conditions. The Policy Assessment also proposes an optional PM2.5 mass indicator, 

which the Panel finds to be a less direct measure of light extinction, but an adequate indicator for a 

secondary PM standard, especially if the mass scattering/absorption efficiencies are adjusted on 

seasonal, site-specific and/or regional basis to reflect differences in aerosol composition and 

relative humidity. Moreover, a mass-based standard could be considered on an interim basis while 

methods to measure the preferred PM light extinction indicator are being developed, tested and 

deployed. 

 

 At most locations, a majority of PM light extinction is caused by fine particles. Coarse 

particles are less of a contributing factor for light extinction, while posing measurement problems 

for instruments measuring light scattering or adsorption. For these reasons, an initial focus on 

PM2.5 light extinction may be appropriate. However, coarse particle visibility effects can be very 

important at some locations, and a variety of approaches could be employed to measure or estimate 

coarse particle effects, regardless of whether light extinction or mass-based indicators are 

employed (see responses to questions 14, 17 and individual panelist comments for more detail). 

 

 CASAC supports the use of a one-hour averaging time for a secondary visibility standard, 

as this time span approximates the nearly instantaneous nature of human perception of visual air 

quality, which itself can change rapidly over the course of a day. An hourly averaging time would 

also be well within the instrumental response capabilities of currently available light extinction 

monitoring instruments, although if a PM2.5 mass indicator were used, a somewhat longer 

averaging time may be necessary to accommodate the larger response variability of some 

continuous PM2.5 monitors. The current focus on daytime visibility is also appropriate, given the 

fact that nighttime visibility, while important, has a complex relationship with PM and other 

factors such as light pollution, for which scientific understanding is less advanced than for daylight 

conditions. 

 

 The Policy Assessment presents a large number of optional combinations of levels and 

forms, including levels for PM light extinction in the range of 20 to 30 deciviews, with forms 

ranging between the 90th and 98th percentiles, which would be calculated based on either the single 

worst hour in a day (with 90th or 95th percentile) or based on all daylight hours (with 95th or 98th

percentile). These many options are further complicated by consideration of PM2.5 mass as a 

possible alternative indicator. While the range of options proposed appears reasonable, CASAC 

recommends that the next draft of the Policy Assessment provide more detailed illustrations of the 

similarities and differences in the many optional combinations, to provide a more informed view of 

the policy implications of these options. Specifically, it would be useful to describe the levels of 

visibility protection and/or PM2.5 mass concentration, the PM species, the sources and event types 

that would be most affected, and the temporal and spatial variations in these factors that would 

result from different combinations of optional indicators, levels and forms. 

 In addition to its principal focus on visibility protection, the Policy Assessment also 

includes very brief discussions of effects of PM on climate, ecosystems and materials. CASAC 

 4
 

concurs with the Policy Assessment’s conclusions that while these effects are important, and should 

be the focus of future research efforts, there is not currently a strong technical basis to support 

revisions of the current standards to protect against these other welfare effects. Because different 

PM components that affect visibility can have differential (cooling or warming) effects on climate, 

CASAC recommends revising the discussion of climate effects to better inform policy makers that 

reducing certain aerosol components could lead to increased radiative forcing and regional climate 

warming while having a beneficial effect on visibility. However, the causal relationship between 

PM and climate is established, and the risk of aerosol impact on climate is high; therefore further 

research on a regional basis is urgently required. 

 

Finally, CASAC reiterates its expectation that the revised Policy Assessment will be 

accompanied by a delineation of key changes from the first draft. This will enhance the efficiency 

and targeting of our review, and will provide a transparent record of the basis for these changes. 

As always, we appreciate the opportunity to provide advice on this important subject and look 

forward to continuing our dialogue with the Agency on PM. 

 

Sincerely, 

 /Signed/ 

 

Dr. Jonathan M. Samet 

Chair 

Clean Air Scientific Advisory Committee 

Enclosures A: CASAC Particulate Matter Review Panel Roster 

 B: CASAC Responses to Charge Questions 

 C: Individual Panelists’ Responses to Charge Questions 

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NOTICE 

This report has been written as part of the activities of the EPA's Clean Air Scientific Advisory 

Committee (CASAC), a federal advisory committee independently chartered to provide extramural 

scientific information and advice to the Administrator and other officials of the EPA. CASAC 

provides balanced, expert assessment of scientific matters related to issues and problems facing the 

Agency. This report has not been reviewed for approval by the Agency and, hence, the contents of 

this report do not necessarily represent the views and policies of the EPA, nor of other agencies 

within the Executive Branch of the federal government. In addition, any mention of trade names or 

commercial products does not constitute a recommendation for use. CASAC reports are posted on 

the EPA Web site at: http://www.epa.gov/casac. 

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Enclosure A

Clean Air Scientific Advisory Committee 

Particulate Matter Review Panel 

CHAIR 

Dr. Jonathan M. Samet, Professor and Chair, Department of Preventive Medicine, University of 

Southern California, Los Angeles, CA 

CASAC MEMBERS 

Dr. Joseph Brain, Philip Drinker Professor of Environmental Physiology, Department of 

Environmental Health, Harvard School of Public Health, Harvard University, Boston, MA 

Dr. H. Christopher Frey, Professor, Department of Civil, Construction and Environmental 

Engineering, College of Engineering, North Carolina State University, Raleigh, NC 

Dr. Donna Kenski, Data Analysis Director, Lake Michigan Air Directors Consortium, Rosemont, 

IL 

Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering, 

Georgia Institute of Technology, Atlanta, GA 

Dr. Helen Suh, Associate Professor, Harvard School of Public Health, Harvard University, Boston, 

MA 

Dr. Kathy Weathers, Senior Scientist, Cary Institute of Ecosystem Studies, Millbrook, NY 

CONSULTANTS 

Dr. Lowell Ashbaugh, Associate Research Ecologist, Crocker Nuclear Lab, University of 

California, Davis, Davis, CA 

Prof. Ed Avol, Professor, Preventive Medicine, Keck School of Medicine, University of Southern 

California, Los Angeles, CA 

Dr. Wayne Cascio, Professor, Medicine, Cardiology, Brody School of Medicine at East Carolina 

University, Greenville, NC 

Dr. David Grantz, Director, Botany and Plant Sciences and Air Pollution Research Center, 

Riverside Campus and Kearney Agricultural Center, University of California, Parlier, CA 

Dr. Joseph Helble, Dean and Professor, Thayer School of Engineering, Dartmouth College, 

Hanover, NH 

 7
 

Dr. Rogene Henderson, Senior Scientist Emeritus, Lovelace Respiratory Research Institute, 

Albuquerque, NM 

Dr. Philip Hopke, Bayard D. Clarkson Distinguished Professor, Department of Chemical 

Engineering, Clarkson University, Potsdam, NY 

Dr. Morton Lippmann, Professor, Nelson Institute of Environmental Medicine, New York 

University School of Medicine, Tuxedo, NY 

Dr. William Malm, Research Physicist, National Park Service Air Resources Division, 

Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 

Mr. Charles Thomas (Tom) Moore, Jr., Air Quality Program Manager, Western Governors' 

Association, Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort 

Collins, CO 

Dr. Robert F. Phalen, Professor, Department of Community & Environmental Medicine; Director, 

Air Pollution Health Effects Laboratory; Professor of Occupational & Environmental Health, 

Center for Occupation & Environment Health, College of Medicine, University of California 

Irvine, Irvine, CA 

Dr. Kent Pinkerton, Professor, Regents of the University of California, Center for Health and the 

Environment, University of California, Davis, CA 

Mr. Richard L. Poirot, Environmental Analyst, Air Pollution Control Division, Department of 

Environmental Conservation, Vermont Agency of Natural Resources, Waterbury, VT 

Dr. Frank Speizer, Edward Kass Professor of Medicine, Channing Laboratory, Harvard Medical 

School, Boston, MA 

Dr. Sverre Vedal, Professor, Department of Environmental and Occupational Health Sciences, 

School of Public Health and Community Medicine, University of Washington, Seattle, WA 

SCIENCE ADVISORY BOARD STAFF 

Dr. Holly Stallworth, Designated Federal Officer, EPA Science Advisory Board Staff Office 

 8
 

Enclosure B

CASAC Responses to EPA Charge Questions on the First Draft Policy Assessment for the 

Review of the Particulate Matter NAAQS 

1. Organizational Structure 

a. Does the Panel find the introductory and background material, including that 

pertaining to previous reviews of the PM standards and the current review, to be 

clearly communicated and appropriately characterized? 

The introductory and background material in the Policy Assessment for the Review of the 

Particulate Matter NAAQS (PA), including the summary of previous reviews, is 

appropriate and clearly communicated; however, the discussion could be condensed or 

summarized, thereby improving clarity. 

b. To what extent does the Panel find that the questions posed appropriately reflect the 

important policy-relevant issues for this review? Are there additional policy-relevant 

questions that should be addressed? 

The questions posed in the Policy Assessment reflect important policy-relevant issues 

relevant to this consideration of the PM NAAQS. As discussed in our letter to the 

Administrator, we recommend the Agency begin to consider the evidence needed to 

continue the refinement of the PM indicator for such variables as chemical composition 

and optimal size ranges. 

c. What are the Panel’s views regarding the level of detail used to address these policyrelevant questions? Please comment on the extent to which the discussions are 

appropriately focused in addressing the questions. 

The level of detail is excessive. It should be cut back to provide succinct summaries of the 

information extracted from the Integrated Science Assessment (ISA) and Quantitative 

Health Risk Assessment (RA) documents. An Executive Summary would be very helpful, 

and could be based on the summaries offered throughout the Policy Assessment. 

2. Technical Content 

a. Do the discussions accurately reflect and clearly communicate the currently available 

health and welfare effects evidence, including important uncertainties, as 

characterized in the final ISA? 

Yes, the Policy Assessment captures the currently available health and welfare evidence, as 

well as important uncertainties as characterized in the final ISA; however, much of the 

evidence could be summarized and referenced to the ISA, making the Policy Assessment a 

more "reader friendly" document. The Policy Assessment satisfactorily captures the range 

of proven and possible health effects literature that has expanded since previous PM 

reviews. 

The discussion of the welfare effects on visibility accurately reflects the currently available 

scientific evidence. The rationale for selecting the 1-hour averaging time is particularly 

clear and convincing. The summary of the differences between nighttime visibility and 

 9
 

daytime visibility, and the basis for considering the human impact of short-term visibility 

vs. all daylight hours, is clearly stated. 

b. Do the discussions accurately reflect and clearly communicate policy-relevant 

information from the human health risk and visibility assessments, including 

important uncertainties, as characterized in the second draft REAs? 

Yes, the Policy Assessment captures the policy-relevant information and important 

uncertainties as characterized in the second draft RA. However, more of the evidence could 

be summarized and referenced to the RA. The policy-relevant information for the visibility 

assessment is clearly communicated, particularly the discussion of the nighttime visibility 

effects and the rationale for selecting the 1-hour averaging time over all daylight hours. 

3. Approach (section 2.1.3): Various approaches are described for translating different air 

quality metrics from epidemiological studies into the basis for reaching preliminary staff 

conclusions on the adequacy of the current PM2.5 standards and on alternative 

standards for consideration. What are the Panel’s views regarding the appropriate 

weight to place on these various approaches; should specific approaches be afforded 

more or less weight? Are there additional approaches that should be considered?

The selection of both evidence- and risk-based approaches is sound, as is the consideration of 

the utility of both annual and 24-hour concentration limits, and of various pairings of the two 

in exploring the degree of public health protection offered under possible NAAQS. The 

balance between evidence- and risk-based approaches is appropriate. 

Charge question #3 also pertains to approaches used to translate different air quality metrics 

from epidemiological studies into the basis for identifying concentration ranges (levels) 

relevant to assessing the adequacy of the current PM2.5 standards and to suggesting alternative 

standards for consideration. These approaches included using concentrations somewhat below 

the aggregate mean, at one standard deviation below the mean, and at the lowest quartile of the 

interquartile range. A more direct approach would use confidence bounds on the 

concentration-response relationships, such as those found in time-series studies. Attention 

could then be focused on the region where these bounds and other sources of uncertainty 

indicate an unacceptable degree of uncertainty about the concentration-response relationship. 

The less direct approaches used in the Policy Assessment instead make use of the 

concentrations observed in the epidemiological studies without consideration of the estimates 

of effect. The confidence bounds along a concentration-response relationship and 

distributional measures of concentrations are intrinsically related, since the estimated 

exposure-response relationship is most certain in a concentration range where most of the data 

reside. We recommend that EPA consider exploring the use of confidence bounds on the 

concentration-response relationships, where they are available, rather than the distributional 

measures of concentrations.

4. Adequacy of current suite of PM2.5 standards (section 2.2): 

 What are the Panel’s views on the following: 

a. The appropriateness and characterization of the sets of long- and short-term PM2.5

exposure studies highlighted? (section 2.2.1) 

 10
 

• Are there specific studies that should be given more or less emphasis? 

The Policy Assessment features appropriate studies and they are given appropriate 

emphasis. The discussion would be enhanced if there were an added indicator in Figure 2.1 

to specify which of the studies were multicity vs. single-city studies. This would visually 

emphasize that more confidence should be given to the multicity studies (since the 

confidence intervals would be tighter) and provide justification for preferentially relying on 

multi-city studies. The Krewski et al. (2009) study receives greater emphasis in comparison 

to the other highlighted studies of annual mortality, however, because the study population 

was a relatively high SES group of volunteers, the estimates from the study may 

underestimate the risk for the US population in general. The Policy Assessment should 

acknowledge this potential underestimation, and present the findings of this study in the 

context of the other US cohort studies. 

• Are there additional studies that should be focused on? 

Yes, the section concludes that older adults and children along with those with pre-existing 

cardiopulmonary diseases should be considered as part of the susceptible population. In 

addition, CASAC suggests that additional groups (as identified in the ISA) should be 

mentioned, including diabetics, and people with specific genetic polymorphisms that were 

reported in the ISA. 

• Does the Panel generally support placing greater weight on multi-city vs. singlecity short-term exposure studies? 

Yes, see above in response to the first bullet. 

• Does the Panel agree with the characterizations of PM2.5 air quality at which 

associations have been observed? 

Yes, the presentation of results indicating PM2.5 effects below the current 24 hour and 

annual standard is appropriately summarized from multiple studies, adds to the 

observations previously made, and supports the conclusion about the inadequacy of the 

current two primary PM standards, However, the selection of the lower bound of the interquartile range may not be sufficiently health protective. Further consideration should be 

given to using the 10th percentile as a level for assessing various scenarios of levels for the 

PM NAAQS. 

b. The focus on persons with lower socioeconomic status as a susceptible population in 

addition to consideration of newly available evidence for susceptible populations 

identified in previous reviews (e.g., children, older adults, persons with pre-existing 

cardiac and respiratory disease)? (section 2.2.1) 

The data presented fully justifies consideration of lower socioeconomic status (SES) people 

as a susceptible group. This consideration would build from the integration of recent 

findings in the ISA that expand the identification of susceptible groups. 

c. The preliminary staff conclusion that the estimated risks remaining upon simulation 

of just meeting the current suite of standards can reasonably be judged to be 

important from a public health perspective? (section 2.2.2) 

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A substantial body of empirical evidence is summarized indicating that significant health 

effects are reported at or just below the current 24 hour and annual standards, justifying the 

conclusion in the Policy Assessment that the observed effects are important from a public 

health perspective. Thus, the discussion of estimated public health risks remaining if 

ambient air quality levels were to just meet the current standard is valuable and should 

contribute to the deliberations made by the Administrator in setting a PM NAAQS to 

protect the public with an adequate margin of safety. 

d. The relative roles of the annual and 24-hour standards in providing public health 

protection as informed by the quantitative risk assessment, specifically in focusing on 

simulation of estimated risks remaining upon just meeting the current suite of 

standards? (section 2.2.2) 

The approach used to characterize the relative contributions of the annual and 24-hour 

concentrations is clearly presented. The Policy Assessment clearly explains why greater 

confidence can be placed on the roll-back estimates when the annual standard estimates are 

controlling, as compared to when the 24-hour values are controlling. The link to the 

characteristics of the data available is made clear. As CASAC wrote in discussing the RA, 

we would have preferred that EPA use Total Cardiovascular Mortality for both 

comparisons, although the conclusions would not likely change substantially. The RA and 

Policy Assessment do explain their use of ischemic heart disease (IHD) mortality. 

e. The integration of evidence-based and risk-based considerations to inform the 

preliminary staff conclusion that the available information clearly calls into question 

the adequacy of the current suite of PM2.5 standards and provides strong support for 

giving consideration to revising the current standards to provide increased public 

health protection? (section 2.2.3) 

With regard to the integration of evidence-based and risk-based considerations, CASAC 

concurs with EPA’s conclusion that the new data strengthens the evidence available on 

associations previously considered in the last round of the assessment of the PM2.5 standard. 

CASAC also agrees that there are significant public health consequences at the current 

levels of the standard that justify consideration of lowering the PM2.5 NAAQS further. 

More text might be provided to describe specifically the uncertainties that have been 

addressed by the new evidence since the last review. 

5. Indicator (section 2.3.1): 

 What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is reasonable to retain PM2.5 as an indicator 

for fine particles? 

This conclusion is reasonable for this round of the PM NAAQS review. There was 

insufficient peer-reviewed literature to support any other indicator at this time. However, it 

is essential that EPA continue to develop the ambient air quality database so that the suite 

of PM indicators considered in epidemiological studies can be expanded. Population-based 

studies will be needed to begin to assess more refined PM indicators, including chemical 

speciation of fine particles and measurements of ultrafine particles (UFP) that may more 

directly reflect the risk of PM2.5 to human health. 

 12
 

b. The preliminary staff conclusion that the currently available information is too 

limited to support a distinct PM standard for ultrafine particles? 

This conclusion was unavoidable in that there was insufficient peer-reviewed literature on 

ultrafine particle (UFP) exposures and exposure-response relationships to support a UFP 

indicator at this time. As for fine PM (above), EPA needs to implement a network of UFP 

monitors to support broader research on associations between UFP exposures and health. 

c. The preliminary staff conclusion that the currently available information is not 

sufficient to support consideration of a separate indicator for a specific PM2.5 

component or for the mix of fine particles from any specific source category, and that 

data are too limited to consider eliminating any component or source category from 

the mix of fine particles included in the PM2.5 mass-based indicator? 

See answer to 5.a. above. 

6. Averaging Times (section 2.3.2): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is reasonable to retain annual and 24-hour 

averaging times? 

This conclusion is reasonable in view of the current limited evidence pointing towards 

other averaging times. 

b. The preliminary staff conclusions that the currently available evidence is too limited 

to support additional averaging times to address sub-daily exposures or seasonal 

exposures? 

This conclusion is reasonable. To the extent that EPA supplements or replaces 24-hour fine 

PM samplers with continuous monitors, it may become possible to conduct studies that 

demonstrate that other averaging times for acute responses may be more appropriate than 

24-hour averages, for at least some effects. 

7. Forms (section 2.3.3): 

What are the Panel’s views on the utility of additional analyses to inform consideration 

of eliminating provisions included in the current form of the annual standard that allow 

for spatial averaging across monitors, specifically with regard to the potential for 

disproportionate impacts on susceptible populations with lower socioeconomic status? 

Given mounting evidence showing that persons with lower SES levels are a susceptible group 

for PM-related health risks, CASAC recommends that the provisions that allow for spatial 

averaging across monitors be eliminated for the reasons cited in the Policy Assessment. 

Moreover, it should be recognized that monitors are located specifically to reflect exposures 

to surrounding populations, which may vary spatially. 

 13
 

Given this recommendation and its basis, additional analyses are not needed to inform a 

decision about spatial averaging. Should EPA decide that additional analyses are warranted, 

however, a description of the aims, methods, and the decision criteria to apply these analyses 

are needed to assess their utility. CASAC notes the need for additional analyses and models 

to assess local heterogeneity in PM concentrations, both in terms of assessing population 

exposure, especially for low SES populations, and as a guide in future monitor siting 

decisions. 

8. Levels (sections 2.3.4, 2.3.5, and 2.3.6): 

 What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding alternative standard levels that are 

appropriate for consideration, and the rationale upon which those conclusions are 

based? 

CASAC believes that the document provides a clear and appropriate rationale for using the 

annual PM2.5 standard as the controlling standard and the 24-h standard to provide 

additional protection. As discussed in more detail in our response to Charge Question 3, 

the text could be clarified substantially, specifically with regard to the rationale used to 

select the two sets of alternate standards considered in the Policy Assessment. This 

explanation should include the rationale behind the use of a particular lower bound to 

define annual PM2.5 standards and the relation of the alternative standards to policy-relevant 

background levels. 

b. The insights that can be gained from the quantitative risk assessment to inform our 

understanding of the roles that the annual and 24-hour standards play in providing 

public health protection, specifically in focusing on simulations of estimated risks 

remaining upon just meeting alternative suites of standards? (sections 2.3.4.2 and 

2.3.5.2) 

CASAC supports the use of the quantitative risk assessment to illustrate the impact of 

alternative standards on health risk. The value of the risk assessment to this process would 

be increased substantially if the discussion was clarified and condensed. 

c. The policy implications of the uncertainties associated with estimating risks, including 

potential underestimation of risk, in reaching conclusions regarding standards that 

would provide public health protection with an adequate margin of safety? 

CASAC recommends that the impacts of model choices on long- and short-term risk 

estimates should be expanded to assess whether and how estimated risks would differ if: 

(1) comparisons were made to the LML (as compared to the PRB); and (2) alternative 

concentration-response (C-R) functions were selected. The selection of C-R functions is 

relevant to margin of safety considerations. For example, use of C-R functions from 

Krewski et al. (2009) may underestimate risks for lower SES groups. 

d. A policy approach for identifying a suite of standard levels in which the annual 

standard would be the “generally controlling” standard to provide protection for both 

long- and short-term PM2.5 exposures, in conjunction with a 24-hour standard set to 

provide supplemental protection against days with high peak concentrations 

 14
 

associated with localized “hotspots” and risk arising from seasonal emission that 

might not be controlled by a national annual standard? (section 2.3.6) 

The policy approach for identifying the suite of standard levels was clearly described. 

e. Additional considerations that could inform our conclusions related to alternative 

suites of fine particle standards?

CASAC identified a few areas for further consideration, including potential alternative 

lower bounds for estimating risk, margin of safety issues (as described above) with regard 

to the impacts on estimated risks of model choices, particle speciation and size (e.g., coarse 

and ultra-fine). 

9. Adequacy of the current 24-hour PM10 standard (Sect. 3.2): 

 What are the Panel’s views on the following: 

a. The preliminary staff conclusion that the available scientific evidence supports 

maintaining a standard that provides protection against health effects associated with 

exposure to thoracic coarse particles? 

CASAC concurs that the evidence since the last review is stronger in linking adverse health 

effects to the thoracic course fraction. One complexity is the finding that after taking into 

account the component of the PM10 that is made up of PM2.5, urban thoracic coarse 

particles appear to be more toxic than rural PM10. Thus, no single measure of thoracic 

coarse fraction can be applied uniformly across all sites. 

b. The various approaches described for translating the evidence and related 

uncertainties into the basis for conclusions on the adequacy of the current PM10

standard? 

The Policy Assessment describes the uncertainty coming from having insufficient PM10-2.5 

monitors and limited geographic coverage, particularly for the purpose of comparing the 

thoracic coarse fraction in rural and urban areas. See the answer to question 13. Further 

insight could be gained by using the data from all locations with co-located PM samplers 

so that PM10-2.5 could be calculated as PM10 concentration minus PM2.5 concentration. 

c. The adequacy of the public health protection afforded by the current PM10 standard 

against exposures to thoracic coarse particles? 

The relatively toxicity of the total PM10 fraction depends on several factors. The 

proportion of the PM10 that is in fact PM2.5 is different between urban and rural sites. In 

addition, although the limited evidence is only suggestive, there are independent 

differential potential health effects of the PM10-2.5 portion of PM10 in both urban and rural 

areas that can be separated from the health impact of PM2.5. Since such effects have been 

reported in both urban and rural areas at concentrations below the current standard, it is 

clear that the current PM10 standard is not adequate to protect the public health. It is also 

the case that there is no evidence to guide the choice between PM10 or PM10-2.5 that would 

be appropriate as a standard for both urban and rural regions as a single national standard. 

Since there are insufficient spatial PM10-2.5 measurements at this time, CASAC suggests 

that we continue with the PM10 measure as the indicator, but encourage EPA to work on a 

 15
 

national monitoring scheme for a range of particles including both smaller and larger 

fractions. 

10. Indicator (sections 3.3.1, 3.3.2, and 3.3.3): 

 What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is appropriate to maintain a standard that 

provides protection against all thoracic coarse particles, regardless of their source of 

origin or composition? 

This conclusion is justifiable at this time, in view of the evidence from the PM ISA 

showing that coarse thoracic PM exposures are associated with acute respiratory system 

responses that are not accounted for by fine PM. However, it is important to recognize that 

there is a lack of data on the influence of coarse thoracic PM composition. While it is 

reasonable to not consider chemical composition in the current round of review of a coarse 

thoracic NAAQS, it is essential that a suitable database for PM10-2.5 concentrations and 

composition be developed over the next several years so that a more satisfactory judgment 

on the need for composition-specific indicators can be made in the next review cycle. 

b. The appropriateness of maintaining an indicator that allows lower PM10-2.5 

concentrations in urban areas than non-urban areas? 

Because PM2.5 concentrations are generally higher in urban areas than rural areas, particles 

in this size range constitute a greater proportion of PM10 in such areas.. In view of the 

evidence that there is a greater proportion of road dust, incinerator effluents, and other 

coarse PM that contain known toxicants in urban areas than in windblown soil dusts in 

rural areas, it is appropriate to maintain a coarse thoracic PM indicator that requires lower 

concentrations in urban areas than non-urban areas. In this review cycle, PM10 can serve as 

a prudent index of coarse thoracic PM. However, as noted (in 10.a. above) chemical 

speciation of PM10-2.5 may provide a more robust basis for future coarse thoracic PM 

NAAQS. 

c. The appropriateness of either a PM10 or PM10-2.5 indicator for a standard meant to 

protect against exposures to thoracic coarse particles? 

While a PM10-2.5 indicator for a standard meant to protect against exposures to thoracic 

coarse particles would be preferable, a PM10 indicator is an acceptable substitute in this 

round of review. However, in light of the implications of different source profiles leading 

to differences in the current PM10 nonattainment areas in the U.S., CASAC notes potential 

problems with use of either the PM10 or PM10-2.5 fractions as the indicator of coarse PM. In 

the short term, information from the ISA on PM10-2.5 health effects and regional differences 

(in particular, section 6.5.2.3) could be brought forward into the Policy Assessment to 

motivate consideration of alternative coarse PM indicators that could allow for a more 

focused and health relevant coarse PM standard. Further analyses based on this 

information might also be considered. In the long term, information on chemically 

speciated coarse PM health effects would allow consideration of a coarse PM standard that 

reflects differential toxicity related to composition. 

11. Averaging Time (section 3.3.4): 

 16
 

What are the Panel’s views on the preliminary staff conclusions that the currently 

available evidence supports maintaining a 24-hour standard to protect against health 

effects associated with short-term exposures to thoracic coarse particles, and does not 

support consideration of a long-term thoracic coarse particle standard? 

This conclusion is reasonable. 

 

12. Form (section 3.3.6): 

What are the Panel’s views on the preliminary staff conclusion that it is appropriate to 

consider a 98th percentile form for a revised 24-hour standard meant to protect against 

short-term exposures to thoracic coarse particles? 

CASAC finds the 98th percentile approach to be appropriate, but some members questioned 

the approach used to calculate the 98th percentile, noting that in the absence of additional 

information, a 98th-percentile calculation from the multi-year distribution of values was more 

defensible than a year-by-year calculation over three successive years. 

13. Level (section 3.3.7): 

What are the Panel’s views on the appropriateness of considering 98th percentile, 24-

hour PM10 or PM10-2.5 concentrations in epidemiological study locations for identifying a 

range of potential alternative standard levels for consideration, recognizing the more 

limited air quality information available for PM10-2.5? 

This portion of the Policy Assessment provided only a promissory note that additional 

analyses would be done, dependent upon the conclusions as to whether CASAC would 

recommend staying with PM10 or moving to PM10-2.5 as the indicator for coarse thoracic 

particles. The proposal to use PM10 for alternative levels parallels what was done with PM2.5 

and seems appropriate. Because of the potential lack of sufficient spatial data available for 

PM10-2.5, it is not clear that similar analyses can be accomplished. However, there may be 

sufficient data coming from locations with co-located PM2.5 and PM10 monitors, so that by 

subtraction an estimate of PM10-2.5 could be made. It would be valuable to conduct some 

additional analyses if there is a smaller subset with sufficient data to carry out such estimates 

of PM10-2.5. In addition there may be data to make direct comparisons of the estimated levels 

with directly measured PM10-2.5 in selected sites for purposes of testing the sensitivity of the 

subtraction estimates. This comparison would serve the purpose of demonstrating relative 

differences (or lack thereof) of the different indicators. 

14. Adequacy (section 4.2): What are the Panel’s views on the preliminary staff conclusion 

that the currently available information clearly calls into question the adequacy of the 

current suite of PM2.5 standards to provide public welfare protection from unacceptable 

levels of visibility impairment, primarily in urban areas, and supports consideration of 

alternative standards to provide appropriate protection? 

CASAC agrees that the Policy Assessment clearly describes how the form and level of the 

current PM standard are inadequate to protect the public from unacceptable levels of 

visibility impairment. Specifically, the levels of the current PM2.5 and PM10 standards are too 

 17
 

high, and their averaging times are too long, to guard against levels of visual air quality 

considered adverse over the short (hour or less) time periods during which changes in visual 

air quality are perceptible. The current mass-based indicators are also not ideal, and could be 

made much more directly responsive to visibility effects by using a PM light extinction 

indicator. Although an alternative PM2.5 mass indicator applied over shorter daylight hourly 

averaging times and at lower levels is not ideal to protect visibility, it would represent a 

substantial improvement over the current 24-hour and annual means. 

The relationship between the maximum daily 1-hour 90th percentile and the all daylight hours 

98th percentile is clearly and convincingly presented. The reasoning for excluding hours with 

relative humidity above 90% is also readily understood. The box-and-whisker plots of PM 

light extinction clearly show that rolling back the PM2.5 concentrations to meet the current 

standards would not protect visibility in most locations, regardless of which candidate 

protection level (CPL) or percentile form is considered. It appears from Figure 4-3 that 

Phoenix (along with Tacoma) would also not exceed the least restrictive CPL more than 2% 

of the time. If so, Phoenix should also be mentioned on line 17 of page 4-26. 

The summary in section 4.2.2 is very clear in describing why the current PM standards are 

not protective of visibility. An addition that could strengthen the chapter in future drafts 

would be to show – with data examples – how high a 1-hour PM2.5 mass concentration or a 1-

hour PM light extinction level could be on (a) day(s) when the 24-hour PM2.5 mass 

concentration does not exceed the current 35 μg/m3 standard. In addition, some photographic 

illustrations using actual photos or Winhaze images would be helpful to show what visual air 

quality looks like at levels of the current standards (and at the higher 1-hour levels that are 

possible without exceeding the 24-hour average standards). 

15. Indicator (section 4.3.1): What are the Panel’s views on the following: 

a. The appropriateness of considering PM light extinction and PM2.5 mass as indicators 

for a distinct secondary PM standard for visibility protection, and the relative 

advantages and disadvantages associated with each indicator? 

b. The appropriateness of considering the contribution of coarse particles with respect 

to a potential PM light extinction indicator? 

CASAC agrees it would be preferable to use PM light extinction as the indicator since it is 

directly related to visibility and can be directly measured. Light extinction is a 

fundamental physical property of the ambient aerosol and thus, it represents a very 

appropriate indicator for a secondary visibility standard. While PM light extinction is 

much preferred, PM2.5 mass (with appropriate averaging time, level and form) could also 

be an adequate indicator for PM visibility effects, and may be more reflective of other 

possible welfare outcomes. 

Since a continuous PM2.5 mass monitoring network is already in operation and robust 

methods for widespread PM light extinction measurements will take several years to 

develop, evaluate and deploy, the Agency might consider a phased approach in which a 

standard is set based on PM light extinction, with compliance measurements initially made 

using Federal Equivalent Method PM2.5 mass as a surrogate for light extinction, until such 

 18
 

time as a more direct PM light extinction network is in place. To facilitate a better 

agreement between mass and extinction some consideration should be given to adjusting 

the mass scattering efficiencies that relate mass to extinction in such a way as to reflect 

spatial and seasonal variability in the relative composition of ambient aerosols and regional 

differences in relative humidity. Setting a standard to any specific level of PM2.5 mass 

concentration would still allow a wide range of visibility effects at different times and 

locations and would provide much less efficient protection than could be attained with a 

light extinction indicator. CASAC still encourages the Agency to develop suitable methods 

for measuring PM light extinction, and to evaluate these methods in a small pilot field 

network. 

The bulk of the light extinction at a majority of locations is driven by fine particles (PM2.5). 

There are significant problems with the measurement of both light scattering and light 

absorption from larger particle sizes. Consequently, it is appropriate to focus initially on 

measurement of PM2.5 light extinction or its components. Another approach that would 

improve the quality of measurements of extinction from both fine and coarse particles 

would be to deploy a system that periodically switches between PM2.5 and PM10 inlets for 

the same measurement device (nephelometer or aethalometer). A third approach would be 

to limit light extinction measurements to instruments with PM2.5 inlets (which would also 

reduce instrumental maintenance requirements), and estimate coarse particle extinction 

from continuous PM10-2.5 mass measurements, based on the difference method. 

16. Averaging times (section 4.3.2): What are the Panel’s views on the following: 

a. The preliminary staff conclusion that consideration should be given to a 1-hour 

averaging time? 

CASAC concurs that a one-hour averaging time is most appropriate for a secondary 

standard to protect visibility, since 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. An hourly averaging time is also well within the instrumental response times of 

the various currently available and developing optical monitoring methods. If a PM2.5 mass 

indicator is used, the Agency may wish to consider somewhat longer averaging times - 2 to 

4 hours - to assure a more stable instrumental response, although obtaining reliable hourly 

data from currently available continuous PM2.5 monitors should be both feasible and a 

highly desirable objective for a variety of other applications. 

b. The advantages and disadvantages of focusing on a 1-hour daily maximum or each 1-

hour average concentration during all daylight hours? 

As noted in the second draft Urban-Focused Visibility Assessment (UFVA), there is 

relatively little difference in the levels of visibility protection that would be provided by a 

PM light extinction standard based on considering the single worst daylight hour in a day 

combined with a relatively low 90th percentile form and one based on considering all 

daylight hours combined with a higher 98th percentile form. It is likely that these two 

approaches may tend to emphasize different kinds of visibility events and source 

influences, and in its recent review of the UFVA, CASAC requested that the Agency 

prepare additional analyses to better illustrate and evaluate what these differences may be. 

 19
 

We expect that the results of these analyses will be helpful to Staff in recommending the 

best approach and to CASAC in commenting on it more objectively. 

While it may be that the single worst daylight hour is judged to be a better approach, 

CASAC did not think that the stated rationale of protecting the most “sensitive” or 

“susceptible” individuals who may only have a short period of outdoor time each day to 

view scenery was an especially strong basis for this selection. We suggest that a different 

justification needs to be provided if this approach is considered superior to one based on all 

daylight hours. In addition, CASAC recognizes that there are other complexities associated 

with evaluating and protecting nighttime visibility – such as the combined influence of PM 

and light pollution. The Policy Assessment should make clear that the current focus on 

daytime visibility is not because nighttime visibility is unimportant, but rather because the 

scientific understanding of PM visibility effects under daylight conditions is currently 

much more advanced than for nighttime effects. 

17. Levels and Forms (section 4.3.3): What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding ranges of options with varying levels and 

forms in combination with different indicators (light extinction and PM2.5 mass-based 

indicators) and averaging times (1-hour daily maximum or each 1-hour average 

concentration during all daylight hours)? 

There is general agreement that the standard should be based on 1-hour averaging time. 

Furthermore, CASAC found that a standard based on extinction is preferable to a massbased standard. However, because of potential issues in implementing monitoring for light 

extinction, a mass-based indicator may be acceptable, at least on an interim basis, given the 

right averaging time and percentile level. However, the consensus of the committee was 

that not enough information is provided in the first draft Policy Assessment to clearly 

evaluate the differences in protection associated with the various combinations of levels, 

percentiles, and 1 hr worst visibility vs. all daylight hours. The Policy Assessment more 

specifically does not address how the protection afforded by these various combinations 

would be further modified by the choice of an extinction vs. a mass indicator. For the 

limited comparisons illustrated in the Policy Assessment, CASAC agreed that 20 to 30 

deciviews was an appropriate range of levels for a PM light extinction indicator, but 

concludes that the upper bound of 40 – 60 μg/m3

 was unreasonably high for a PM2.5 mass 

indicator and that a range more along the lines of 15-35 μg/m3

 should be evaluated in 

future drafts. 

In considering the implications of using a mass vs. an extinction indicator, it is critical to 

understand what level of average or median mass concentration corresponds to a given 

level of extinction as a function of atmospheric characteristics in representative eastern and 

western cities. A given level of visibility protection will correspond to differing mass 

concentrations around the country, while a given level of PM2.5 mass will correspond to 

differences in associated visibility protection. More detailed comparisons should be 

included in the Policy Assessment to better illustrate the differences in visibility protection 

as the degree of protection varies with mass concentration and species composition. 

Regional and seasonal variation in protection should also be explored. 

 20
 

While it has been demonstrated that a 90th percentile form combined with the worst 

daylight hour each day would generally result in similar average levels of visibility 

protection as the 98th percentile based on all daylight hours, these two options may tend to 

focus on different kinds of visibility impairing events, PM species compositions and 

emissions sources. The Policy Assessment should provide clearer illustrations of the 

differences (or similarities) between these approaches, and how they might be further 

modified if a PM2.5 mass indicator is selected rather than the preferred light extinction 

indicator. 

b. Additional approaches that could help inform our consideration of alternative levels 

and forms? 

The 90% RH screen may not be needed if a PM2.5 indicator is used. Some consideration 

should be given to using a nephelometer that cycles between a PM2.5 and a PM10 inlet. 

This approach would allow for the very accurate measurement of PM2.5 scattering with a 

reasonable estimate of coarse particle scattering. The same approach could be used for a 

continuous mass or light absorption monitoring system. 

As indicated in previous comments on the UFVA, a “progress-based” attainment test rather 

than the traditional “threshold-based” attainment test would be preferable from a scientific 

perspective. While the Agency has indicated this is not an option in the standard-setting 

process, this approach could and should be considered in the implementation phase (where, 

for example, the Agency could provide guidance on what would be considered reasonable 

rates of progress). Possibly an example of how this approach might work could be 

provided in the next draft Policy Assessment. 

It would be helpful if the next draft of the Policy Assessment included a section identifying 

and prioritizing key near-term monitoring and research needs to support refinements and 

revisions to secondary PM standards in the future. This could include establishing a small 

pilot urban visibility network to field-test and evaluate alternative PM light extinction 

measurement methods, additional visibility preference studies to better define levels 

(and/or frequencies) of visual air quality considered acceptable over a wider range of 

locations and viewing conditions, and chemically speciated measurements of fine particles 

to support the linkage between PM mass and visibility impairment. 

18. Climate (section 5.2): What are the Panel’s views on the preliminary staff conclusion 

that there is insufficient information to base a standard on climate impacts associated 

with current ambient PM concentrations? 

CASAC agrees that the preliminary staff conclusions on the climate impact of aerosols are 

correct – there is insufficient evidence on which to base a national standard. However, the 

causal relationship is established, and the risk of aerosol impact on climate is high so further 

research on a regional basis is urgently required. This need should be strongly voiced in the

Policy Assessment, and research should be undertaken sooner rather than later. If possible, 

research should be designed and begun now to be included in future assessments of the 

NAAQS. 

 21
 

While there may be insufficient information to base a secondary standard on potential climate 

change, CASAC emphasizes that policy makers should be better informed that reducing certain 

aerosol components could lead to increased radiative forcing and regional climate warming 

while having a beneficial effect on visibility. There is abundant information demonstrating that 

aerosols that scatter visible light (e.g., nitrates and sulfates) are cooling, and can lead to 

increased/longer lived/more reflective cloud formation. The Integrated Science Assessment

(based largely on other assessments) demonstrates that light scattering aerosols lead to a net 

decrease in radiative forcing, and while the uncertainty is significant, it suggests that the effect 

may be quite large. Further, there is sufficient evidence that light absorbing aerosols (e.g., 

black carbon) can lead to warming. To go forward with a secondary standard based on 

reducing light extinction, the Policy Assessment needs to more directly weigh the potential that 

regional warming could accompany improved visibility if light scattering aerosols are 

preferentially targeted. There also may be significant regional heterogeneity to controls that 

would address a visibility-based secondary standard which should be addressed. For example, 

the areas that currently have lower PM concentrations may benefit most from maintaining or 

improving visibility, but the areas with regionally high sulfate levels (and reduced visibility) 

would likely benefit most from the reduced radiative forcing. At a minimum, EPA needs to 

address how a secondary standard might impact regional radiative forcing and climate, assess 

the uncertainties, and provide a reasoned analysis weighing the two issues. Also, on Page 5-11, 

lines 10-11 and lines 31-32, it is noted that “aerosols that are warming are co-emitted with 

aerosols that are cooling.” While this statement is correct in some instances, it does not 

accurately represent the aerosols with these competing effects. Most of the warming aerosols 

in the US are emitted by biomass burning and internal engine combustion. Much of the 

cooling aerosol is formed in the atmosphere by oxidation of SO2 or VOCs. Thus, a set of 

controls to control warming PM would not necessarily have that much impact on cooling PM 

and vice versa. These statements should be modified accordingly. 

Finally putative difficulties in relating PM to climate effects are laid out in sections 5.2.3 and 

5.2.4 (pages 5-11 to 5-13) and again in section 5.3.4 (page 5-24). However, these apply to all 

other potential effects of PM, and it is not appropriate to emphasize them here, as if they were 

uniquely related to climate. For example, other PM-effects relationships would be improved by 

using chemically-speciated mass measurements; other relationships are expected to vary 

locally and regionally; others have offsetting positive and negative effects (warming vs. 

cooling, radiation attenuation vs. radiation scattering); others are confounded by effects at a 

distance from sources, by the difficulty of distinguishing anthropogenic from natural 

constituents (PRB issues should be explicitly mentioned here - section 5.2.4); by spatial and 

temporal heterogeneity; etc. In addition, many of the caveats in section 5.3.4 are equally true of 

nitrogen oxides, sulfur oxides, and ozone, but have not presented insurmountable obstacles to 

revising NAAQS for those contaminants. 

19. Ecological effects (section 5.3): What are the Panel’s views on the preliminary staff 

conclusion that data are insufficient to support establishing a distinct standard for 

ambient PM based on ecosystem effects not addressed in the ongoing NOx/SOx 

secondary NAAQS review? 

 22
 

 23

CASAC agrees with the conclusions of the Policy Assessment that the published literature is 

insufficient to support a national standard for PM effects on ecosystem services. The best 

established effects are related to particles containing nitrogen and sulfur, which are no longer 

considered in this assessment of particulate matter. CASAC considers that some causal 

relationships are known, but that sensitivities are not well characterized. Further research 

should be advocated, including progress toward a monitoring network that will yield time 

resolved information on chemically speciated particulate mass that is likely to be most closely 

related to ecosystem endpoints. 

The references in the Policy Assessment to welfare valuation associated with end use, taken 

from the ozone assessment documents, and to ecosystem services as specific and individual 

endpoints, taken from the Millennium Ecological Assessment, may prove useful in structuring 

this research. 

CASAC recommends incorporation of text into Section 5.3 that is similar to that in the current 

draft of Section 5.4, regarding the importance of maintaining an appropriate degree of control 

of both fine and coarse particles to address ecosystem effects attributed to them, even in the 

current absence of sufficient information to develop a standard. 

20. Material effects (section 5.4): What are the Panel’s views on the preliminary staff 

conclusion that no new evidence calls into question the adequacy of the protection 

afforded by the current suite of PM standards, and that there continues to be support 

for retaining an appropriate degree of control for both fine and coarse particles to 

provide protection against materials damage and soiling? 

CASAC agrees with the conclusions of the Policy Assessment that the published literature, 

including literature published since the last review, is insufficient either to call into question the 

current level of the standard or to support any specific national standard for PM effects on 

materials. However, causal relationships are well established, particularly with regard to soiling 

of buildings and monuments. CASAC believes this end point is particularly amenable to 

demonstration of relationships between PM mass (particularly chemically speciated mass) and 

levels of soiling, because the non- linear effects that characterize ecosystem endpoints may be 

less significant. Soiling effects of PM on materials may be linked to atmospheric burdens 

simply through an appropriate deposition velocity for specific components. 

We believe it is important to retain the text in the current draft of Section 5.4 regarding the 

importance of maintaining an appropriate degree of control of both fine and coarse particles to 

address materials damage and soiling attributed to them, even in the absence of sufficient 

information to develop a new standard.
 

Enclosure C 

Compendium of Individual Comments 

on Particulate Matter Policy Assessment (March 2010) 

Comments from Dr. Lowell Ashbaugh....................................................................................... 2 

Comments from Mr. Ed Avol ...................................................................................................... 6 

Comments from Dr. Joe Brain .................................................................................................. 12 

Comments from Dr. Wayne Cascio........................................................................................... 14 

Comments from Dr. Chris Frey ................................................................................................ 17 

Comments from Dr. David Grantz............................................................................................ 18 

Comments from Dr. Joseph Helble ........................................................................................... 21 

Comments from Dr. Rogene Henderson................................................................................... 22 

Comments from Dr. Mort Lippmann ....................................................................................... 23 

Comments from Dr. Phil Hopke................................................................................................ 31 

Comments from Dr. Robert Phalen .......................................................................................... 37 

Comments from Mr. Rich Poirot .............................................................................................. 38 

Comments from Dr. Armistead Russell.................................................................................... 46 

Comments from Dr. Frank Speizer........................................................................................... 48 

Comments from Dr. Helen Suh ................................................................................................. 53 

Comments from Dr. Sverre Vedal............................................................................................. 55 

 1
 

Comments from Dr. Lowell Ashbaugh 

Question 2a: Do the discussions accurately reflect and clearly communicate the currently 

available health and welfare effects evidence, including important uncertainties, as 

characterized in the final ISA? 

I believe the discussions of the health effects evidence accurately reflect the currently available 

evidence, but I’ll focus more on the welfare effects and leave it to the other reviewers to provide 

more detail on the health effects evidence. 

The discussion of the welfare effects on visibility is quite complete and it accurately reflects the 

currently available science. The rationale for selecting the 1-hour averaging time is particularly 

clear and convincing. The summary of the differences between nighttime visibility and daytime 

visibility, and the reasoning for human impact of short-term visibility vs. all daylight hours is 

clearly stated. 

The term ‘deciview’ is introduced on page 4-18, but it is not defined. A simple description of its 

relationship to extinction, along with guidance on how to understand it, would be useful. For 

example, it would help to explain that a one deciview change is just noticeable, and that higher 

values correspond to reduced visibility. 

The discussion of the CPL values and how they were arrived at is very good. It is particularly 

useful to point out that the greater uncertainty, represented by the more scattered data points, for 

the Denver and British Columbia results may arise from the use of actual photos taken under 

varying conditions, while the better fit of the Phoenix results could be due to the use of WinHaze 

to alter the same photograph to display different visibility conditions. 

The distinction between the Washington, DC results and the results from western cities is 

accurately and clearly described. The results presented by Smith (2009) on a new study at 

Washington, DC are not discussed here, though, and probably should receive some attention. 

Although I believe there are good reasons to discard the results for the Policy Assessment, they 

should be mentioned. 

Question 2b: Do the discussions accurately reflect and clearly communicate policy-relevant 

information from the human health risk and visibility assessments, including important 

uncertainties, as characterized in the second draft REAs? 

The policy-relevant information for the visibility assessment is clearly communicated. I 

particularly like the discussion of the nighttime visibility effects and the rationale for selecting the 

1-hour standard over all daylight hours. The selection of CPLs is described well and the 

justification for selecting them is consistent with the evidence presented. The only caveat I would 

make is that the results of the Smith (2009) study presented at the April 2009 CASAC meeting 

should be discussed. The results of that study provide an opportunity to explain uncertainties in 

visibility studies and why these results should not be included in the final assessment. 

 2
 

Question 14: Adequacy (section 4.2): What are the Panel’s views on the preliminary staff 

conclusion that the currently available information clearly calls into question the adequacy of 

the current suite of PM2.5 standards to provide public welfare protection from unacceptable 

levels of visibility impairment, primarily in urban areas, and supports consideration of 

alternative standards to provide appropriate protection?

The Policy Assessment clearly describes how the form and level of the current PM standard is not 

adequate to provide public welfare protection from unacceptable levels of visibility impairment. 

The relationship between the maximum daily 1-hour 90th percentile and the all daylight hours 98th

percentile is clear and convincing. The reasoning for excluding hours with relative humidity above 

90% is also clear. The box-and-whisker plots of PM light extinction are clear evidence that the 

current levels of PM standards are not protective of visibility, particularly combined with the 

estimated light extinction if PM levels are rolled back to just meet the current standard. It appears 

from Figure 4-3 that Phoenix (along with Tacoma) would also not exceed the least restrictive CPL 

more than 2% of the time. If so, it should be included in line 17 of page 4-26. 

The summary in section 4.2.2 is very clear in describing why the current PM standards are not 

protective of visibility. 

Question 15: Indicator (section 4.3.1): What are the Panel’s views on the following: 

a. The appropriateness of considering PM light extinction and PM2.5 mass as 

indicators for a distinct secondary PM standard for visibility protection, and the 

relative advantages and disadvantages associated with each indicator? 

b. The appropriateness of considering the contribution of coarse particles with respect 

to a potential PM light extinction indicator? 

The issues associated with selecting PM light extinction as a standard are described well. The 

limitations of using 24-hour PM2.5 mass to predict 1-hour visibility impacts could be better 

described. In particular, the discussion of visibility impacts clearly points out that short-term 

visibility is most clearly representative of the welfare effects of visibility. PM mass, though, is not 

currently measured on a 1-hour time scale, so it is not able to adequately represent visibility 

impacts. Figure 4.3-1 shows the relationship between 24-hour visibility and 1-hour light extinction. 

The uncertainty associated with this relationship could be discussed in more detail to illustrate the 

shortcomings of PM2.5 mass as an indicator of short-term visibility. 

It’s appropriate to consider the contribution of coarse particles with respect to a potential light 

extinction indicator. Although coarse particles are not a significant contributor in some parts of the 

country, they are significant in other parts – notably the southwestern U.S. It’s important that EPA 

acknowledge these differences by including coarse particles in the indicator. 

Question 16: Averaging times (section 4.3.2): What are the Panel’s views on the following: 

a. The preliminary staff conclusion that consideration should be given to a 1-hour 

averaging time? 

b. The advantages and disadvantages of focusing on a 1-hour daily maximum or each 

1-hour average concentration during all daylight hours? 

The discussion of visibility effects on welfare presents clear and convincing evidence that a 1-hour 

averaging time is appropriate for a visibility indicator. The assessment points out that a significant 

fraction of the population is exposed to visibility conditions for only brief periods during the day, 

and their welfare is affected by these brief exposures. The relationship between the 1-hour daily 

 3
 

maximum and 1-hour average daylight visibility indicators is clearly explained and the protective 

benefits of using the 1-hour maximum indicator is justified in the Policy Assessment. 

Question 17: Levels and Forms (section 4.3.3): What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding ranges of options with varying levels 

and forms in combination with different indicators (light extinction and PM2.5 

mass-based indicators) and averaging times (1-hour daily maximum or each 1-hour 

average concentration during all daylight hours)? 

b. Additional approaches that could help inform our consideration of alternative 

levels and forms? 

The staff conclusions regarding the levels and forms of the light extinction indicator are related 

well to the evidence presented earlier in the Policy Assessment. The difficulty in relating PM levels 

to light extinction is also described well, and the selection of alternative PM2.5 mass concentration 

levels is well justified. In the discussion on page 4-34 it is not clear why the 98th percentile form is 

not considered. A brief comment on why it is not included would be helpful. I infer from the 

discussion that EPA staff considers it too restrictive, but this could be explained more explicitly. 

The discussion of the maximum daily 1-hour indicator versus the average daylight 1-hour visibility 

is well done. I like the characterization of people who have infrequent access to daytime visibility 

as a susceptible population, and the explanation that the 1-hour daily maximum indicator would be 

also protective of people who have access to visibility throughout the day is effective. 

The box-and-whisker plots and the tables showing the effect of “just meeting” the alternative 

standards are very effective in conveying the anticipated results. The use of color-coding in Figure 

4.3-2 is very helpful in comparing the alternative standards. The blue shading in the table of 

Appendix A is too light; it should be darker to more effectively highlight those conditions. 

Question 18: Climate (section 5.2): What are the Panel’s views on the preliminary staff 

conclusion that there is insufficient information to base a standard on climate impacts 

associated with current ambient PM concentrations? 

The preliminary staff conclusions on the climate impact of aerosols are accurate – there is 

insufficient evidence on which to base a national standard. The risk of aerosol impact on climate is 

high, though, so this issue needs further development. It’s important to state that research into the 

effects of aerosols on climate is needed and should be undertaken sooner rather than later. If 

possible, research should be designed and begun now to be included in future assessments of the 

NAAQS. 

Question 19: Ecological effects (section 5.3): What are the Panel’s views on the preliminary 

staff conclusion that data are insufficient to support establishing a distinct standard for 

ambient PM based on ecosystem effects not addressed in the ongoing NOx/SOx secondary 

NAAQS review? 

The preliminary staff conclusion is appropriate for ecological effects. While acknowledging that 

ecological effects occur, it is not possible at this time to establish a PM level that would be 

protective of ecosystems. 

Question 20: Material effects (section 5.4): What are the Panel’s views on the preliminary 

staff conclusion that no new evidence calls into question the adequacy of the protection 

afforded by the current suite of PM standards, and that there continues to be support for 

 4
 

retaining an appropriate degree of control for both fine and coarse particles to provide 

protection against materials damage and soiling? 

This preliminary staff conclusion is also appropriate. The wording of this conclusion could be 

applied to the climate effects and the ecological effects sections. In particular, a version of the 

following sentence from the materials effects section could be applied to the other two sections: 

“However, in the absence of information that provides a basis for establishing a different level of 

control, observations continue to support retaining an appropriate degree of control on both fine 

and coarse particles to help address materials damage and soiling associated with PM.”

 5
 

Comments from Mr. Ed Avol 

General Comments: The objectives of the Policy Assessment (PA) document are laudatory – to 

provide the Administrator with a review of the policy implications arising from the conclusions 

drawn from the ISA and RA. The Policy Assessment has all of the elements to make a persuasive 

and compelling presentation of the assembled data. However, in my opinion, the current Policy 

Assessment is too repetitive, too wordy, and devotes too many pages to re-visiting previous 

discussions presented in the ISA and RA documents, rather than summarizing the findings and 

moving on to the policy implications. 

An inordinate amount of space is afforded to the 2004 AQCD, with regular discussion of what was 

presented and/or concluded in the 2004 review document. The ISA and RA presented the relevant 

science and evaluated it in the context of risk estimates and levels of uncertainty; what are the 

policy implications of these findings? The current document should focus on what the cumulative 

information, through the current review, tells us now about PM exposure, effects, levels, and 

susceptibility, so that an informed judgment can be made at this point in time. It may be 

interesting to compare what was known several years ago with what is known now, but that is not 

the central issue at hand. As a result of this approach, this Policy Assessment document is much 

too long, making it frustrating to read. This style of presentation results in weakening the impact of 

the document, and masking its key messages. I personally found it very difficult to wade through, 

even though I realize it contains important information that will be critical to the Administrator’s 

determinations. This needs to dramatically improve in the next draft, in order to make this 

document accessible and worthwhile. 

Specific Comments: 

P2-2, line 19 to P2-9, line 8 (sections 2.1.1 and 2.1.2) – This is historically interesting, helps to lay 

the groundwork for how EPA got to where it is with the fine particle standard...and is arguably 

completely irrelevant to the current document and discussion. The issue for the current policy 

document is interpretation of the currently available information for re-examining the current

standard. Details about previous cycles of review and their outcomes could be placed in an 

appendix, but do not centrally address the focused theme of this document – the basis for review of 

the current standard. 

P2-9, lines24 & 25 – “...the annual and 24-hour standards taken together, rather than to consider 

each standard separately...” But didn’t the RA clearly demonstrate that the annual standard was 

“controlling” (i.e., set at a given annual level, it effectively achieved a number of 24-hour standards 

being considered in conjunction with it? 

P2-10 through2-13 – all this preparatory introduction about what is going to be discussed is 

unnecessary meandering (in my opinion); the document should be a focused application of the data 

presented in the ISA and RA, presented clearly and then clearly summarized; these pages could 

(and should) be distilled down to a paragraph or two. The Policy document should be brief, 

focused, and clear; this document has the potential to become that, but isn’t there yet. 

 6
 

P2-10, line 9 – Declaring a focus on US and Canadian studies side-steps the issue that careful and 

well-performed science (or poorly-performed science, for that matter) can be performed in other 

countries, just as they can be performed in the US and Canada. 

P2-10, lines 26 & 27 – This conclusion line is not needed here; first present the information and 

then summarize the document’s position in a succinct integrative final chapter or section. 

P2-13, lines 30 to 35 – not necessary, can be deleted without impacting presentation. 

P2-14, lines 11 to 18 – not necessary, can be deleted without impacting presentation. 

P2-17, lines 25 to 34 – The text is slipping into a listing and re-discussion of studies and findings, 

which was done quite well in the ISA and re-visited in the RA. This document should be a crafted, 

directed summary, rather than a re-listing of the studies...so I think this section could be edited to a 

line or two. The policy-relevant comment is in lines 25 through 27 (“...locations with the largest 

reductions in PM2.5 saw the largest improvements in reduced mortality rates, while those with the 

smallest decreases in PM2.5 concentrations saw the smallest improvements in reduced mortality 

rates...”); the rest is repetitive carryover from the ISA. Figure 2.1 (which is on an un-numbered 

page that might be P2-23) could be refered to and used as the basis for comments about what was 

reported in the ISA...without going through and talking about several of the studies in the text. 

P2-19, lines 9-17 – Here, again, I think the point has been made and re-listing ISA references does 

not add to the presentation; I think these can be deleted. The current presentation seems to have 

slipped back into a review and listing of the body of evidence that supports a given conclusion 

(which was already done in the ISA), rather than presenting the conclusions brought forward from 

the ISA and RA and presenting a discussion that begins with those ISA and RA conclusions being 

carried forward. 

P2-24, lines 33 to 35 – The notion of a summary is great and much-appreciated, but this is too 

wordy and meandering. Delete lines 33 to 35 and get to the point! 

P2-25, lines 1 and 2 – same as previous comment, delete this and get to the point. 

P2-25, line 26 to p2-26, line 2 – this is yet another example of unnecessary meandering and reopening an issue that does not need to be re-visited in this document. Earlier versions of ISAs, and 

the PM ISA, discussed and evolved towards a functional and useful definition of susceptibility, and 

this document should begin from that point, not re-visit what the evolution was. 

P2-26, line 29 – change the word “factors” to “observations”. 

P2-27, lines 8 to 11 – Is this consistent with the previous comment (P2-10, line9) that the focus 

would be on US and Canadian studies (a focus with which I respectfully disagree)? 

P2-43, line 4 – replace the phrase “peak shaving” with a more accurate one that describes the 

approach. 

 

 7
 

P2-44, lines 3 to 7 – This statement about the importance of annual average PM2.5 concentrations 

on interpreting risk results for both short and long-term exposures is a key observation that should 

be highlighted, bolded, underlines, or otherwise clearly identified. 

P2-45, line 9 and line 21 – The comments about lung cancer here (when the argument was 

previously made about basing the presentation on “causal” levels of determination) raises the issue 

of staff confidence and belief in the strength of association between PM2.5 long-term exposure and 

lung cancer. What is the determination on this matter that Staff wants to transmit to the 

Administrator? 

P2-45, line 11 to 14 – Does this comment about 10 of the 15 study areas having “...design values 

which result in the current 24-hour standard controlling...” contradict the previous one (p2-44, 

lines 3-7), which asserted that annual averages were controlling for both short and long-term 

exposures? Together with a later comment (p2-49, lines 27 to 29, “...This pattern is generally 

characteristic of the larger set of urban areas across the US that do not meet the current suite of 

standards...”), this leaves the reader somewhat confused –which standard is controlling? What is 

the policy implication? 

P2-51, line 27 on to P2-52, line 33 – This discussion of the representativeness of the study areas 

seems out of place here, 50+ pages into the discussion...Shouldn’t this be established early in the 

presentation so that the credibility of all that follows is increased? 

P2-52, line 34 to p2-55, line 3 – This seems like an obvious and unnecessary comment – Does 

Staff really think that someone might conclude that these 15 areas in the presentation are the 

ONLY 15 areas in the country for which this discussion is relevant? 

P2-52, line 4 to p2-55, line10 – This question and response seems unnecessary and out of place 

here. The purpose of the NAAQS review is to periodically assess the current information relevant 

to the current standard and make a judgment about the adequacy of the standard to protect the 

public health. The determination as to whether there are “risks remaining upon simulation of the 

current suite of standards...” will be addressed when the body of information is presented and the 

Administrator makes that determination. 

P2-54, lines 24 to 28 – This is yet another example of meandering monologue – rephrase this to 

present the conclusion clearly and succinctly – the data suggest that level of the current PM2.5 

standard needs to be re-considered. 

P2-57, lines 6 & 7 – Change this line to read, “These techniques, while widely used in aerosol 

research, have not yet been widely used in health effects studies, or systematically deployed for 

extensive time periods in local or regional air monitoring networks.” 

P2-57, lines 23 to 28 – Unnecessary and can be deleted... 

P2-58, lines 25 to 29 – This is certainly true and potentially a self-fulfilling prophecy (in that there 

will never be national-level ultra-fine data until and unless there is directed interest to support the 

 8
 

collection of such data). Could an appendix of “Research Needs To Advance Future Reviews” be 

added? 

P2-62 – The presentation has slid back to a review of data, which has already been done in the ISA 

(where it rightfully belongs, not here). 

P2-64, lines 24 to 34 – This is a useful summary comment. Based on the history of PM research 

(from mass-based assessments to particle size, constituent, and biological toxicity) and the PM 

NAAQS (gradually moving to smaller size fractions), it seems reasonable to anticipate that a 

component of PM might be the biological actor of interest. With the STN network and large-scale 

chemical characterization studies, we are on the cusp of perhaps collecting the information needed 

to identify the important chemical constituents of biological significance. This could and should 

be added to the “Research Needs” list. 

(The same generic comments apply for most of the next 50+ pages– too wordy, unnecessary detail, 

substantial editing needed!) 

P2-107, section 2.4 – This is a well-constructed, focused presentation and should serve as the 

template for the previous 100 pages. These comments could be supported by references to the ISA 

or RA, but the specific supporting materials do not need to be here to make a convincing 

presentation (and to the authors’ credit, the supporting materials do not appear here). 

P2-109, Section 2.5 – Key Uncertainties – This section can and should be a focused, terse, and 

brief presentation...a few pages at most, possibly only a page or less. It is somewhat surprising 

that in this first draft, given the excellent discussion in the RA, there is only a title and placeholder 

here... 

Chapter 3, Thoracic Particles – The presentation scheme is analogous to the previous chapter, and 

in doing so, suffers from the same generic shortfalls (too much detail, too much repetition, only a 

placeholder for the uncertainty discussion, etc). 

Chapters 4 and 5 – defer to my visibility colleagues on these sections... 

Specific Charge Question Assignments: 

Question 4: Adequacy of current suite of PM2.5 standards (section 2.2):

What are the Panel’s views on the following: 

a. The appropriateness and characterization of the sets of long- and short-term PM2.5

exposure studies highlighted? (Section 2.2.1). Too-long-winded a discussion but studies 

are the appropriate ones. 

• Are there specific studies that should be given more or less emphasis? No, and the 

studies’ discussion should be referenced to ISA and RA, not re-visited in the manner 

and extent here.

• Are there additional studies that should be focused on? No.

 9
 

• Does the Panel generally support placing greater weight on multi-city vs. singlecity short-term exposure studies? Favor multi-city studies to better substantiate 

findings and make findings that much more representative.

• Does the Panel agree with the characterizations of PM2.5 air quality at which 

associations have been observed? Yes.

b. The focus on persons with lower socioeconomic status as a susceptible population in 

addition to consideration of newly available evidence for susceptible populations 

identified in previous reviews (e.g., children, older adults, persons with pre-existing 

cardiac and respiratory disease)? (section 2.2.1). This is appropriate and could arguably 

be included in those with compromised health, because lower SES carries with it a host of 

related increased risks (access to health care, poor diet, increased smoking rates, increased 

exposure (roadway proximity of residences and occupational exposures), increased 

violence and stress, etc). 

c. The preliminary staff conclusion that the estimated risks remaining upon simulation 

of just meeting the current suite of standards can reasonably be judged to be 

important from a public health perspective? (section 2.2.2). Based on the referenced 

health risk estimates, this is a logical conclusion.

d. The relative roles of the annual and 24-hour standards in providing public health 

protection as informed by the quantitative risk assessment, specifically in focusing on 

simulation of estimated risks remaining upon just meeting the current suite of 

standards? (section 2.2.2). Agreed and acceptable.

e. The integration of evidence-based and risk-based considerations to inform the 

preliminary staff conclusion that the available information clearly calls into question 

the adequacy of the current suite of PM2.5 standards and provides strong support for 

giving consideration to revising the current standards to provide increased public 

health protection? (section 2.2.3). Agreed and acceptable, but raises a policy issue as to 

whether PM speciation is potentially more informative rather than total mass.

Question 8: Levels (sections 2.3.4, 2.3.5, and 2.3.6):

What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding alternative standard levels that are 

appropriate for consideration, and the rationale upon which those conclusions are 

based? The conclusions are fine – appropriate for the foundational arguments made. The 

presentation, however, is too wordy, too long, too repetitious, and in need of substantial 

editing.

b. The insights that can be gained from the quantitative risk assessment to inform our 

understanding of the roles that the annual and 24-hour standards play in providing 

public health protection, specifically in focusing on simulations of estimated risks 

remaining upon just meeting alternative suites of standards? (sections 2.3.4.2 and 

2.3.5.2) Useful and warranted. Presentation-wise, the figures and the summaries, with 

reference to the RA, would have been sufficient to make the point. 

c. The policy implications of the uncertainties associated with estimating risks, including 

potential underestimation of risk, in reaching conclusions regarding standards that 

 10
 

would provide public health protection with an adequate margin of safety? My 

preference would have been for a clearer and concise section directly discussing 

uncertainty here, rather than comments about “increased variability” scattered throughout. 

d. A policy approach for identifying a suite of standard levels in which the annual 

standard would be the “generally controlling” standard to provide protection for 

both long- and short-term PM2.5 exposures, in conjunction with a 24-hour standard 

set to provide supplemental protection against days with high peak concentrations 

associated with localized “hotspots” and risk arising from seasonal emission that 

might not be controlled by a national annual standard? (section 2.3.6) The question 

itself is indicative of the systematic problem permeating the document – lack of clear, 

succinct and focused presentation.

e. Additional considerations that could inform our conclusions related to alternative 

suites of fine particle standards? Clarity, focus, and brevity....

Question 12: Form (section 3.3.6): What are the Panel’s views on the preliminary staff 

conclusion that it is appropriate to consider a 98th percentile form for a revised 24-hour 

standard meant to protect against short-term exposures to thoracic coarse particles? The 

argument for utilizing a concentration-based approach rather than an exceedance-based approach is 

compelling, and the presentation is terse but clear.

Typos: 

P2-1, line 16 – missing a period. 

P2-43, line32 – change “references” to “reference’. 

P2-51, line 23 – insert the word “to” after “...24-hour standard is controlling due...” 

 11
 

Comments from Dr. Joe Brain 

1. Organizational Structure 

a. Does the Panel find the introductory and background material, including that 

pertaining to previous reviews of the PM standards and the current review, to be 

clearly communicated and appropriately characterized? 

 

The panel views the Policy Assessment (PA) document as critically important to the 

overall process of standard setting. It is the culmination of a deliberate process leading 

from the ISA to the REA, and ultimately to an informed decision by the Administrator. In 

toto, the panel approves of the current versions of the ISA and the REA, and feels that key 

ideas from them now appear in the Policy Assessment. In particular, the panel feels that 

the discussion of previous reviews of the PM standards is adequate, appropriate, and 

clearly communicated. 

 

b. To what extent does the Panel find that the questions posed appropriately reflect the 

important policy-relevant issues for this review? Are there additional policy-relevant 

questions that should be addressed? 

 

The panel feels that the questions posed (questions 2-20) are pertinent to an overall 

assessment of the evidence for the regulation of PM. The questions – and the answers – 

focus on policy-relevant issues. Other than minor aspects addressed by answers to other 

charge questions, the panel feels that there are no major additional questions that should be 

addressed. 

 

c. What are the Panel’s views regarding the level of detail used to address these policyrelevant questions? Please comment on the extent to which the discussions are 

appropriately focused in addressing the questions. 

 

The panel was divided regarding the level of detail present in the current Policy 

Assessment. Some thought the scope of the present Policy Assessment was appropriate. It 

may be necessary to include the considerable detail which characterizes the current 

document. However, a substantial minority felt that an ideal policy assessment should be 

far shorter and more focused. Key staff and the administrator can turn to the REA and the 

ISA as necessary. The Policy Assessment could be one third its current length and be a 

brief summary of the evidence and how that relates to alternative PM levels. Moreover, a 

concise description of how the form of the standard is important would also be useful. 

 

2. Technical Content 

a. Do the discussions accurately reflect and clearly communicate the currently available 

health and welfare effects evidence, including important uncertainties, as 

characterized in the final ISA? 

 

The panel welcomes the expansion of material relating to health and welfare effects. The range 

of proven and possible health effects has been expanded since previous PM reviews. We 

welcome a discussion of how PM affects visibility and is involved in climate change. We 

 12
 

believe these topics are important, and we applaud the authors’ attempt to concisely inform the 

Administrator regarding the importance of these relevant outcomes. 

 

b. Do the discussions accurately reflect and clearly communicate policy-relevant 

information from the human health risk and visibility assessments, including 

important uncertainties, as characterized in the second draft REAs? 

 

The panel believes that the current Policy Assessment discussion does clearly communicate 

information relating to human health risks and visibility. There is a good discussion of 

uncertainties and how these uncertainties impact quantitative risk assessments. 

As noted earlier, some panel members thought that the Policy Assessment would be more 

effective if it were shorter. More of the evidence contained in it could be summarized and 

references to the REA or ISA – as appropriate – could be included to compensate for some of 

the missing or truncated detail. 

 

 13
 

Comments from Dr. Wayne Cascio 

Question 4: Adequacy of current suite of PM2.5 standards (section 2.2): What are the Panel’s 

views on the following? 

a. The appropriateness and characterization of the sets of long- and short-term PM2.5 

exposure studies highlighted? (Section 2.2.1). Are there specific studies that should be 

given more or less emphasis? Are there additional studies that should be focused on? 

Does the Panel generally support placing greater weight on multi-city vs. single-city 

short-term exposure studies? Does the Panel agree with the characterizations of 

PM2.5 air quality at which associations have been observed? 

Comment: EPA staff in authoring the Policy Assessment has achieved an appropriate and balanced 

emphasis between the strengths and weaknesses of the studies, and weighing their relative 

contribution to the assessment. There are no additional studies that need to be introduced from the 

ISA in support of the Policy Assessment. Multi-city studies do merit greater emphasis (or weight) 

for the purpose of establishing the association between PM2.5 exposure and health effects. Table 2-

1 provides an informative overview summary of the causality determinations for PM2.5. While the 

present ISA does not specifically address long-term neurocognitive effects of PM2.5, it is likely to 

be a factor to be evaluated in the next ISA. As such it might be worthwhile placing such an 

outcome in the Table. One could use “Central Nervous System Effects” as is already present in the 

Short-term category and indicating that the causal determination is “inadequate”. Figure 2-1 

provides an informative summary of the effect estimates and air quality distributions for long-term 

exposure studies. An added designation that would be useful would be to identify each study as 

single- or multi-city study and note the number of cities included in the multi-city studies. The 

value of multi-city studies is emphasized in the ISA, RA and Policy Assessment therefore it would 

be useful to see the specific number. Have the region of the country present in the table as has 

already been done is useful. 

b. The focus on persons with lower socioeconomic status as a susceptible population in 

addition to consideration of newly available evidence for susceptible populations 

identified in previous reviews (e.g., children, older adults, persons with pre-existing 

cardiac and respiratory disease)? (Section 2.2.1) 

Comment: The identification of individuals having a lower socioeconomic status as a susceptible 

population is appropriate. Two noticeable omissions are the absence of a description of the 

association with certain gene polymorphisms associated with enzymes modulating oxidant stress, 

and diabetes. While more data needs to be obtained to better establish the magnitude of these risks 

and define the mechanism accounting for the increased risks, there appears to sufficient evidence 

(Chapters 4, 6 and 8) in the ISA to comment in the Policy Assessment. 

c. The preliminary staff conclusion that the estimated risks remaining upon simulation 

of just meeting the current suite of standards can reasonably be judged to be 

important from a public health perspective? (Section 2.2.2) 

Comment: The conclusion that simulations of just meeting the current suite of standards 

demonstrates persist risk to the health of the public is justified and important. Yet, it must be noted 

 14
 

that these estimates of excess risk are extrapolated to PM2.5 levels for which there are only limited 

real world data and higher degrees of uncertainly. Consequently, the rationale for further adjusting 

the PM standard rests with assuring a margin of safety. 

d. The relative roles of the annual and 24-hour standards in providing public health 

protection as informed by the quantitative risk assessment, specifically in focusing on 

simulation of estimated risks remaining upon just meeting the current suite of 

standards? (Section 2.2.2) 

Comment: Comparisons of short-term and long-term cardiovascular risk based are challenging 

based on Tables 2-2 and 2-3 that describe different health effects end-points, incidence of ischemic 

heart disease mortality in Table 2-2 and total cardiovascular mortality in Table 2-3. Providing total 

cardiovascular mortality in both tables or in the text would allow a more direct comparison of the 

relative roles of the annual and 24-hour standard. 

e. Does the current suite of PM2.5 standards provide adequate protection of public 

health? (Section 2.2.3) EPA staff in the Policy Assessment has provided a logical 

approach and justification of the conclusion that the current PM2.5 standard is 

insufficient to protect health and should be revised. 

Comment: Taken in total the data presented in the ISA and summarized in the Policy Assessment

provide strong justification for reconsidering the current PM2.5 standard. Convincing evidence 

continues to be present indicating that serious cardiovascular and respiratory health effects and 

adverse heath outcomes occur at the level of the current standard, and that a lower standard will 

result in greater protection of health. 

Question 9: Adequacy of the current 24-hour PM standard (section 3.2): What are the 

Panel’s views on the following? 

a. The preliminary staff conclusion that the available scientific evidence supports 

maintaining a standard that provides protection against health effects associated with 

exposure to thoracic coarse particles? 

Comment: While data from epidemiological studies are suggestive of short-term cardiovascular, 

respiratory and mortality effects, at the present time significant uncertainties exist with respect to 

the health impact of coarse PM (PM10-2.5). Limitations exist with respect to exposure assessment, 

and non-human studies addressing biological plausibility. Since the last review new information 

has generally supported an association but an important multi-city Canadian study did not find 

association between PM10-2.5 statistically significant, thus emphasizing the persistent uncertainty of 

the impact of this PM fraction. On page 3-12 (L. 26-28) it is stated that “Considered as a whole, the 

ISA notes that epidemiologic studies that have evaluated thoracic coarse particles have reported 

consistent, positive associations between PM10-2.5 and mortality (US EPA, 2009a, section 6.5.2.3)”. 

It should further be stated, “..., yet, in many cases the associations did not achieve statistical 

significance.” 

b. The various approaches described for translating the evidence and related 

uncertainties into the basis for conclusions on the adequacy of the current PM10

 15
 

standard? 

Comment: Evidence appearing since the last review shows that mortality is increased even in some 

cities where the current PM10 standard is met. This suggests that the PM10 standard should be 

reconsidered. Issues related to the uncertainty of air quality estimates in the epidemiological 

studies reporting the health effects of PM10-2.5 remain, as does the limitation of few studies 

addressing confounding by co-pollutants. For these reasons in contrast to PM2.5 significant 

uncertainties persist regarding association between PM10-2.5 and overall mortality as well as cause 

specific mortality. 

c. The adequacy of the public health protection afforded by the current PM10 standard 

against exposures to thoracic coarse particles? 

Comment: Since the last review additional epidemiological studies support the association between 

PM10-2.5 and cardiovascular and respiratory morbidity and mortality. While the precision of the 

associations between health effects for the association with PM10-2.5 exposure does not match that 

for PM2.5 there is sufficient evidence to consider an adjustment of the PM10 standard. Because 

evidence is presented in the ISA that shows that health effects have been observed in areas that 

meet current standards there is reason to expect that a downward adjustment of the standard will 

benefit the health of the public. Importantly, these associations at lower than the current standard 

levels are preserved even in two-pollutant models. And there is limited but supportive evidence 

from human exposure studies and non-human toxicological studies. While the available evidence 

continues to be less robust than the PM2.5 by the limitations noted, one can argue that the standard 

should be lowered to provide a greater margin of protection. 

 16
 

Comments from Dr. Chris Frey 

Initial comments on the draft Policy Assessment 

‐ It is unusual that C-R estimates based on the low end of a plausible range of values would 

be used. In public health applications, it is more typical to user upper bound estimates of 

dose-response relationships in order to be protective of public health. Thus, one could 

argue that EPA has biased the risk characterization to be very low and, therefore, is on a 

path that will not be adequately protective of public health. 

‐ P 2-73: The draft Policy Assessment seems to say that the form of the standard will not 

be assessed until the 2nd draft of the Policy Assessment. This would seem to effectively 

disengage CASAC from being able to comment on a fully specified alternative standard 

that takes into account indicator, level, averaging time, and form. All of these must be 

considered jointly in order to assess a regulatory alternative. 

‐ The Policy Assessment is very very long. While it may be useful to provide background 

for people who have not read the ISA or the REA, the text is simply too verbose. Avoid 

being so tentative with statements. Avoid passive phrases such as “we note that...”. The 

text will have more impact over all if things are said just once and not repeated. While it 

is useful to summarize material from the ISA and REA, there does not need to be an 

extensive verbose discussion of these. The interested reader can access these documents 

for more detail. 

‐ the summary paragraphs at the end of each major section are very useful. These 

summary paragraphs should be the basis of creating an executive summary or synthesis 

chapter, that would enable a reader to get the main points without having to labor through 

hundreds of pages of material. 

‐ The objective in evaluating alternative standards should not be risk reduction but rather 

seeking a standard that is protective of public health with an adequate margin of safety. 

(e.g., page 2-87) 

‐ P 2-97 – should also take into account that sensitivity analysis of the C-R functions 

generally lead to higher effects estimates that the core estimates. Furthermore, the 

assessment does not adequately take into account that the C-R functions are likely to be 

underestimated for reasons not quantified, such as exposure error. 

‐ Section 2 does not seem to deal with the number of adverse effects cases associated with 

each alternative standard. This is necessary to consider in order to satisfy the legal 

objective/statutory mandate for the NAAQS. 

 17
 

Comments from Dr. David Grantz 

Overall, this Policy Assessment is well laid out and presented. It moves smoothly and clearly 

through the relevant issues and provides abundant information on each. There is some repetitive 

introductory text which might be condensed. The amount of information presented is large, and 

may be excessive since it has been already carried through the ISA and one or both of the Risk 

Assessments. What needs to be highlighted in the Policy Assessment are staff conclusions, their 

implications, and the type of information on which they are based. 

Question 14. Adequacy. The Policy Assessment does a nice job of demonstrating that, for a 

visibility endpoint, the current standard is not protective of public welfare. In the text describing 

Fig. 4-3 it should be noted that Pheonix as well as Tacoma would meet the candidate level 98% of 

the time. 

Question 15. Indicator. The Policy Assessment adequately presents the advantages of a visibilitybased indicator. It also, however, demonstrates that a mass-based PM 2.5 indicator would work 

almost as well (subject to modest additional spatial variability in achieved visibility). 

There are advantages to retaining a mass-based indicator for the Secondary Standard: 

1. Choosing a visibility-based indicator would be analogous to choosing one of the several 

health endpoints as the indicator for the Primary Standard. In either case, this 

inappropriately diminishes the importance of other endpoints, whether or not there is 

sufficient evidence to use them in the current review cycle. 

2. Mass is more conducive to future development of concentration-effects relationships for 

other endpoints. Research should be advocated in the Policy Assessment to develop 

appropriate deposition velocities for PM to link atmospheric concentrations to loads on 

ecosystems and materials, and to develop relationships between loading and diverse 

effects. A similar advantage of the mass approach holds for effects on cloud nucleation 

(climate) and radiation scattering (penetration into plant canopies). 

3. Nothing is sacrificed with respect to visibility protection if the level of the mass-based 

standard is derived from known physical relationships between visibility and PM2.5 mass 

(accepting some variability) and the human preference data (also with considerable 

regional variability). 

A goal to be advocated in the Policy Assessment is movement towards a chemically speciated, 

mass-based standard. This appears to be equally true for the Primary and Secondary standards. The

Policy Assessment should endorse further research on relationships between chemically speciated 

mass and various endpoints, and further proliferation of monitoring networks to support such a 

standard in the future. There is insufficient information to consider such an indicator during this 

review cycle. 

Because the coarse PM fraction contributes significantly to visibility impairment in some locations, 

it is appropriate to consider a visibility-based standard for coarse PM mass. 

 18
 

Question 16. Averaging time. A visibility-based criterion is reasonably based on a worst one 

hour averaging time, since the Policy Assessment demonstrates that visibility is essentially 

instantaneously perceived. 

It is not appropriate to apply the concept of a “sensitive sub-population” to individuals who only go 

outside occasionally (page 4-15; line 22). It is equally reasonable to postulate that people who 

spend their whole day outside are exposed to repeated instantaneous updates regarding poor 

visibility, and that they are the true “sensitive sub-population”. 

Any standard based on a one hour averaging time will require a new monitoring protocol. This 

could be for visibility, or it could be used to push towards a real-time (continuous) mass 

measurement network. The latter would be useful for more purposes, has been advocated 

previously by CASAC, and therefore should be given serious consideration at this time. 

While visibility and corresponding mass levels could be assessed during daylight hours, the 

standard could apply at all hours if a mass indicator were selected. This could have potential 

benefits, for example in control of “dumping” of PM emissions at night when visibility is not 

monitored. 

Question 17. Levels and Forms. The options are well laid out, in the Policy Assessment and 

Appendix A. These are presented relative to the Candidate Protection Levels, based on the 

visibility preference studies. The underlying data regarding visibility preferences are somewhat 

weak, as appropriately noted in the Policy Assessment, and further research will be required to 

determine whether there is real regional variability or these relationships can be collapsed into a 

population mean. 

It is appropriate to consider a <100% form, allowing some hours of exceedance. This is more to 

stabilize the statistic than, as stated in the current draft Policy Assessment, because this is 

appropriate for an aesthetic standard. A relatively high percentile form, with a correspondingly 

lower level, would support the significance of the chosen level by seeking to minimize its 

exceedance. 

It is appropriate to consider a screen at 90% RH, to remove non-PM associated visibility 

impairments. 

Question 18. Climate. The Policy Assessment appropriately concludes that there is insufficient 

information to proceed toward a national standard for PM effects on climate. However, the causal 

relationship is established, and further research on a regional basis is urgently required. This should 

be advocated in the Policy Assessment. 

A number of putative difficulties in relating PM to climate effects are laid out in sections 5.2.3 and 

5.2.4 (pages 5-11 to 5-13) and again in section 5.3.4 (page 5-24). However, these apply to all other 

potential effects of PM, and it is not appropriate to emphasize them here as if they were uniquely 

related to climate. For example, other PM-effects relationships would be improved by using 

chemically-speciated mass measurements; other relationships are expected to vary locally and 

regionally; others have offsetting positive and negative effects (warming vs.cooling, radiation 

 19
 

attenuation vs. radiation scattering); others are confounded by effects at a distance from sources, by 

the difficulty of distinguishing anthropogenic from natural constituents (PRB issues should be 

explicitly mentioned here (section 5.2.4); by spatial and temporal heterogeneity; etc. In addition, 

many of the caveats in section 5.3.4 are equally true of N, S and ozone, but have not prevented 

those effects from being used in evaluation of the NAAQS for those contaminants. 

Question 19. Ecological Effects. The Policy Assessment appropriately concludes that there is 

insufficient information to proceed toward a national standard for PM effects on ecosystem 

services. However, causal relationships are known, and further research should be an Agency 

priority. This should be advocated in the Policy Assessment, incorporating the desirability of 

chemically speciated mass determinations. 

The framing of the discussion (section 5.3.1) in terms of the end use of the product, taken from the 

ozone documents, and in terms of ecosystem services, following the Millennium Assessment, is 

very good. Structuring future research in terms of effects on individual ecosystem services may 

lead to these effects becoming useful in development of a NAAQS for PM. 

Question 20. Materials. The Policy Assessment appropriately concludes that there is insufficient 

information to proceed toward a national standard for PM effects on materials. However, causal 

relationships are well established. Further research should be advocated in the Policy Assessment, 

though other potential welfare effects may be more significant. Soiling effects on materials may, 

however, be the most straightforward to relate to atmospheric PM, since the linkage is only 

through deposition velocity for specific components. 

Two editorial comments 

1. On page 5-16, line 36, it should be added that plant genotype also determines uptake of 

metals and organics. 

2. On page 5-19, line 32, I assume this is “peroxisomal” rather than “peroxidomal”.? 

 20
 

Comments from Dr. Joseph Helble 

Charge Question 3: Approach (section 2.1.3) 

The discussion in section 2.1.3 is very general, and not particularly well-addressed by charge 

question 3. The text in this section, while clearly written, is longer than necessary for making the 

general points that 24 hour and annual standards should be considered together, since controls 

directed towards one will affect the other, but that the Agency feels that “more consistent risk 

reductions are likely to be accomplished through strategies aimed at meeting an annual standard” 

(p 2-11). This is clear. The balance of the section (p. 2-12 and 2-13) is an overview of the 

approach taken in the rest of the document to developing potential standards. My recollection is 

that this was addressed adequately in the RA; a one paragraph summary with a reference back to 

the RA would be better here. 

Small point, but one that arose earlier and persists: the scientific rationale for focusing on studies 

conducted primarily in the U.S. and Canada (p 2-10, line 9) is not clear, and not addressed in the

Policy Assessment. 

Charge Question 7: Forms (section 2.3.3) 

The question addresses the appropriateness of allowing spatial averaging in determining 

compliance with the annual standard. The discussion both in this section and elsewhere in the

Policy Assessment indicates that EPA is moving away from the use of spatial averaging in this 

context. 

The utility of spatial averaging depends upon the heterogeneity or “spikiness” of the PM 

concentration distribution relative to the local population distribution. It will therefore strongly 

depend upon monitor placement, with respect to population density, source location, and local 

airflow patterns. EPA notes in the Policy Assessment that it will address these questions through 

“additional analysis” (section 2.3.4.1) that will be addressed in the 2nd draft Policy Assessment. 

Some description and discussion of the planned “additional analysis” would be helpful, as it was 

not addressed here, nor in the RA. 

Further, this is one area where detailed modeling of PM concentration distributions could be 

helpful in assessing local heterogeneity of PM concentrations, both in terms of assessing 

population exposure and as a guide in future monitor siting decisions. 

Additional Comment 

In the footnote on p 1-5, is this backwards? The wording is not clear, but particles with dp > d50

are those captured with higher efficiency than those at d50. Smaller particles (particles with dp < 

d50) are collected with lower efficiency. 

 21
 

Comments from Dr. Rogene Henderson 

Charge Questions 1 and 2, Organizational structure and technical content: I found the 

organizational structure to be helpful in following the logic of the conclusions of the staff. The 

technical content was excellent. I especially liked the summary paragraphs. It would be helpful to 

reduce the amount of verbiage by referring back to the ISA or the RA more often, rather than 

repeating the content of those documents. 

Charge Question 8. Levels: Based on the lack of ability to detect a threshold level for adverse 

health effects in epidemiology studies, and the need to protect sensitive sub populations with a 

margin of safety, we are beginning to arrive at risk estimates that drive our estimates of healthprotective levels down almost to background levels. I think this must be discussed in the Policy 

Assessment document. As a science advisory body, it is not our place to make policy 

recommendations as to where to draw the line. However, the Agency should discuss the criteria 

they will use to determine where to draw the line. This is their policy choice, but it would be wise 

to have their choice informed by discussions from CASAC.. 

Charge Question 9. Coarse Particles 

a. I agree with the staff conclusion that the available scientific evidence supports maintaining 

standard for coarse particles. 

b. I agree with the staff approaches to considering the adequacy of the current PM 10 standard. 

Charge Question 8: Levels for Coarse Particle Standards 

I agree with the described approach. 

 22
 

Comments from Dr. Mort Lippmann 

Major Comment and Concern: 

On the basis of the scientific evidence and risk assessment, the OAQPS Staff has recommended 

that a revised PM2.5 annual NAAQS in the range of 13 – 12 ug/m3

 is needed to provide better 

public health protection, and furthermore that consideration should also be given to a PM2.5 annual 

NAAQS in the range of 11 – 10 ug/m3

. I find that these options were well considered, and 

scientifically justifiable on the basis of the analyses cited in the PM Policy Assessment draft. Their 

implementation would be consistent with the Administrator’s obligations under the Clean Air Act 

with respect to setting NAAQS that would protect the public health. 

However, I also believe that 2nd draft of the PM Policy Assessment needs to more explicitly address 

the policy implications of the selection of a concentration limit that, even at 13 ug/m3

 is 

considerably more stringent than the current limit of 15 ug/m3. Promulgation of this concentration 

limit would require: a broad array of new controls on emissions of PM2.5 and its precursors; would 

result in much more non-attainment over long periods of time; and would only be achievable in a 

time frame measured in years in most urban areas in the eastern and Midwestern States and 

California. For the even lower alternate concentration limits (12 -10 ug/m3

), the requisite controls 

on emissions of fossil fuel combustion effluents would require huge capital investments in new 

power plants and/or in alternate energy sources. These considerations lead me to conclude it is now 

time to begin the transition to alternate indicators that better target the health-risks that have been 

associated with PM2.5 mass concentration. I come to this conclusion on the basis that: 

1. There is a rapidly growing body of scientific evidence, much of it cited in the PM ISA, that 

indicates that PM2.5-associated cardiovascular mortality and morbidity risk coefficients 

vary widely across the US, as does the PM2.5 chemical composition. 

2. The establishment and maintenance of a chemical speciation network (CSN) over the past 

decade has made it possible for well-targeted epidemiological studies to demonstrate that 

the urban areas with the highest health-based risks are those most heavily exposed to PM2.5 

components originating from the combustion of coal and residual oil, as well as the 

pollutants associated with motor vehicle traffic. Thus, emission controls focused on 

reductions of effluents from these source categories are highly likely to reduce PM2.5-

associated health risks more than emission reductions from other source categories or for 

PM2.5 as a whole. 

3. Unfortunately, the current limitations of the CSN, in terms of temporal and spatial 

coverage, have greatly limited the opportunities for epidemiologists to establish 

associations between PM2.5-components and source categories and health effects. Having 

data for only every third or sixth day severely limits the ability to study acute responses and 

their lag structure, while having only one or a few sampling sites in an urban area severely 

limits studies of both the acute and chronic effects of components and source-related 

mixtures that are not uniformly distributed an urban area. 

4. Moving toward the designation of better-targeted indicators for primary PM NAAQS is not 

a new strategy. In 1987, PM penetrating into the thorax (PM10) replaced total suspended 

particulate matter (TSP), and in 1997, fine PM (PM2.5), which differs substantially from the 

coarse particle fraction within PM10, and which penetrates in the gas exchange region of the 

lung, was adopted to supplement the protection provided by PM10. 

5. While the scientific evidence to support a designation of one or more new indicators of 

PM2.5–related health risks is inadequate for this PM NAAQS review round, EPA needs to 

 23
 

initiate actions to ensure that an adequate scientific basis will exist within the next 4 to 5 

years for the designation of better and more efficient indices of PM2.5–related health risks 

for the next round of PM NAAQS review. 

6. The most urgent need for data to support new indicators of health risks is to enhance the 

temporal and spatial coverage of the CSN network for PM2.5, and if possible, for PM10 or 

PM10-2.5, and ultrafine PM as well. An expanded CSN network can be supplemented by 

arranging for the creation and maintenance of an archive for daily FRN Teflon filters that 

can be made available for epidemiological studies focused on the health effects of inhaled 

trace metals and black carbon. The archived filters could support retrospective as well as 

prospective studies. 

General Comments: 

1. The OAQPS Staff have done an excellent job in summarizing the most relevant findings 

reported in the PM ISA and REA, and translating them into a suitable policy framework to 

facilitate and enable rational and defensible decisions by the Administrator on the 

promulgation of the next suite of primary PM NAAQS. 

2. The OAQPS Staff warrants commendation for its straightforward and well-reasoned 

interpretations of the health effects evidence and risk estimates for PM2.5, and their 

implications to the choice of the most suitable indicators, forms, levels for further 

consideration, and monitoring site data, as well as the optimal combinations of 24h and 

annual limits for public health protection. They point to the need for selection of a much 

more stringent annual limit, which will result in exceedances in most US urban areas. 

3. Chapter 2 of the Policy Assessment document, while comprehensive in terms of the issues 

that needed to be addressed, has not yet been sufficiently edited to remove the 

redundancies. In too many places, there are repetitions of the same words, phrases, and 

whole sentences (e.g., in the introductory paragraphs, discussion of the long-term exposurerelated evidence and risk estimates, the discussion of the short-term exposure-related 

evidence and risk estimates, and the options for suites of long- and short-term exposure 

limits). These common issues and themes should be presented only once in full, and cited 

or summarized briefly, where they apply, in the later sections. 

4. The content of this first draft of Chapter 3 on a NAAQS for PM10 or PM10-2.5, represents a 

place holder, and contains only boiler-plate wording adapted from the more complete 

Chapter 2 that deals with PM2.5 NAAQS. Critical comments on Chapter 3 will have to wait 

until we have a chance to see the second draft. 

Specific Comments: 

Page Line Comment 

2-33 Fig. 2-1 Separate listings of mortality effects from those of other effects 

2-25 17 change “fine particles at least as” to “PM2.5 NAAQS as, or more” 

2-25 23 insert “the extensive” before “interindividual” 

2-25 24 insert “likely to be” before “at” 

2-26 25 change “to” to “for” 

2-26 29 insert “of outdoor origin” after “PM” 

2-27 18 change “which” to “that” 

2-31 14 insert “chemical composition, which varies” after “along” 

2-36 17 change “This reduction was” to “ Reductions were” 

 24
 

2-37 19 The ACS and 6-cities based risk estimates were not similar to and 6-cities or those of the 

other cited studies 

2-41 16 The entry for the brackets is missing 

2-43 32 change “references” to “reference” 

2-48 23 change “effected” to “affected” 

2-59 7 change “larger particle sizes” to “PM2.5 particle mass” 

2-60 33 insert “metals,” before “sulfate” for consistency with other text entries 

2-74 32 “MCAPS” has not been defined in the text 

2-76 30 change “which” to “that” 

2-85 35 What city, other than New York, should be cited within the bracket? 

2-86 3 correct the 3.2% entry 

Question 1: Organizational Structure 

a. Does the Panel find the introductory and background material, including that 

pertaining to previous reviews of the PM standards and the current review, to be 

clearly communicated and appropriately characterized? 

 Yes 

b. To what extent does the Panel find that the questions posed appropriately reflect the 

important policy-relevant issues for this review? 

 Yes, but not all of the policy-relevant issues. 

Are there additional policy-relevant questions that should be addressed? 

 Yes. Specifically, the mismatch between associations between PM2.5 mass 

concentrations and adverse health effects, and the public health need to control exposures 

to PM2.5 components that account for the effects. As the PM2.5 level approaches background 

concentrations, the PM2.5 indicator becomes much less defensible. 

c. What are the Panel’s views regarding the level of detail used to address these policyrelevant questions? Please comment on the extent to which the discussions are 

appropriately focused in addressing the questions. 

 The level of detail is excessive. It should be cut back to provide succinct summaries 

of the information extracted from the ISA and REA documents. 

Question 2: Technical Content 

a. Do the discussions accurately reflect and clearly communicate the currently 

available health and welfare effects evidence, including important 

uncertainties, as characterized in the final ISA? 

 Yes. 

b. Do the discussions accurately reflect and clearly communicate policy-relevant 

information from the human health risk and visibility assessments, including 

important uncertainties, as characterized in the second draft REAs? 

 Yes. 

 25
 

Question 3: Approach (section 2.1.3): Various approaches are described for translating 

different air quality metrics from epidemiological studies into the basis for reaching 

preliminary staff conclusions on the adequacy of the current PM2.5 standards and on 

alternative standards for consideration. What are the Panel’s views regarding the 

appropriate weight to place on these various approaches; should specific approaches be 

afforded more or less weight? Are there additional approaches that should be considered?

The selection of both evidence- and risk-based approaches is sound, as is the consideration of 

the utility of both annual and 24 hour concentration limits, and of their combinations in 

providing public health protection. 

Question 4: Adequacy of current suite of PM2.5 standards (section 2.2): What are the 

Panel’s views on the following: 

a. The appropriateness and characterization of the sets of long- and short-term 

PM2.5 exposure studies highlighted? (section 2.2.1) 

 The highlighted data sets are appropriate selections. 

• Are there specific studies that should be given more or less emphasis? 

 The Krewski et al. (2009) study is overemphasized in relation to the other 

highlighted studies of annual mortality in that it relies on a higher SES population than 

the others and thereby underestimates responses for the US population as a whole. 

• Are there additional studies that should be focused on? 

 The 6-cities study, having a more representative population, should be given greater 

emphasis, at least for Eastern and Midwestern populations, where the coefficients of 

response are greater than for Western populations. If the question refers to additional 

studies to be highlighted, the answer is no. 

• Does the Panel generally support placing greater weight on multi-city vs. singlecity short-term exposure studies? 

 Yes. 

• Does the Panel agree with the characterizations of PM2.5 air quality at which 

associations have been observed? 

 Yes, but the selection of the lower bound of the inter-quartile range may not be 

sufficiently health protective. Further consideration should be given to using the 10th

percentile as a level at which associations have been observed. 

b. The focus on persons with lower socioeconomic status as a susceptible 

population in addition to consideration of newly available evidence for 

susceptible populations identified in previous reviews (e.g., children, older 

adults, persons with pre-existing cardiac and respiratory disease)? (section 

2.2.1) 

 This was appropriate. 

 26
 

c. The preliminary staff conclusion that the estimated risks remaining upon 

simulation of just meeting the current suite of standards can reasonably be 

judged to be important from a public health perspective? (section 2.2.2) 

 This was appropriate. 

d. The relative roles of the annual and 24-hour standards in providing public 

health protection as informed by the quantitative risk assessment, specifically 

in focusing on simulation of estimated risks remaining upon just meeting the 

current suite of standards? (section 2.2.2) 

 This was appropriate. 

e. The integration of evidence-based and risk-based considerations to inform the 

preliminary staff conclusion that the available information clearly calls into 

question the adequacy of the current suite of PM2.5 standards and provides 

strong support for giving consideration to revising the current standards to 

provide increased public health protection? (section 2.2.3) 

 This was appropriate. 

Question 5: Indicator (section 2.3.1): What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is reasonable to retain PM2.5 as an 

indicator for fine particles? 

It was reasonable only because there was insufficient peer-reviewed literature to 

support any other indicator at this time. However, it is essential that EPA develop the 

database, over the next four years, to support the identification of one or more 

indicators that are better suited to represent the risks of exposure to PM2.5, especially 

of its components that, based on emerging evidence, represent the greatest risks to 

public health. 

b. The preliminary staff conclusion that the currently available information is too 

limited to support a distinct PM standard for ultrafine particles? 

This conclusion was unavoidable in that there was insufficient peer-reviewed 

literature to support a UFP indicator at this time. 

c. The preliminary staff conclusion that the currently available information is not 

sufficient to support consideration of a separate indicator for a specific PM2.5 

component or for the mix of fine particles from any specific source category, and 

that data are too limited to consider eliminating any component or source 

category from the mix of fine particles included in the PM2.5 mass-based 

indicator? 

 See answer to 5.a. above. 

Question 6: Averaging Times (section 2.3.2): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is reasonable to retain annual and 24- hour 

averaging times? 

 27
 

 It was appropriate. 

b. The preliminary staff conclusions that the currently available evidence is too limited 

to support additional averaging times to address sub-daily exposures or seasonal 

exposures? 

 It was appropriate. 

Question 7: Forms (section 2.3.3): 

What are the Panel’s views on the utility of additional analyses to inform consideration of 

eliminating provisions included in the current form of the annual standard that allow for 

spatial averaging across monitors, specifically with regard to the potential for 

disproportionate impacts on susceptible populations with lower socioeconomic status? 

 Such analyses are warranted. 

Question 8: Levels (sections 2.3.4, 2.3.5, and 2.3.6): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding alternative standard levels that are 

appropriate for consideration, and the rationale upon which those conclusions are 

based? 

 They were appropriate. 

b. The insights that can be gained from the quantitative risk assessment to inform our 

understanding of the roles that the annual and 24-hour standards play in providing 

public health protection, specifically in focusing on simulations of estimated risks 

remaining upon just meeting alternative suites of standards? (sections 2.3.4.2 and 

2.3.5.2) 

 The insights gained were useful and informative. 

c. The policy implications of the uncertainties associated with estimating risks, including 

potential underestimation of risk, in reaching conclusions regarding standards that 

would provide public health protection with an adequate margin of safety? 

 Uncertainties need to be defined as well as current information allows. On the other 

hand, using uncertainty as an excuse for choosing a minimally stringent level is not 

consistent with having an adequate margin of safety. 

d. A policy approach for identifying a suite of standard levels in which the annual 

standard would be the “generally controlling” standard to provide protection for 

both long- and short-term PM2.5 exposures, in conjunction with a 24-hour standard 

set to provide supplemental protection against days with high peak concentrations 

associated with localized “hotspots” and risk arising from seasonal emission that 

might not be controlled by a national annual standard? (section 2.3.6) 

 This approach is most appropriate. 

e. Additional considerations that could inform our conclusions related to 

alternative suites of fine particle standards? 

 The alternatives that were selected were very reasonable choices. 

 28
 

Question 9: Adequacy of the current 24-hour PM10 standard (Sect. 3.2): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that the available scientific evidence supports 

maintaining a standard that provides protection against health effects associated with 

exposure to thoracic coarse particles? 

 This preliminary conclusion was warranted. 

b. The various approaches described for translating the evidence and related 

uncertainties into the basis for conclusions on the adequacy of the current PM10

standard? 

 The various approaches that were described were appropriate. 

c. The adequacy of the public health protection afforded by the current PM10 standard 

against exposures to thoracic coarse particles? 

 The adequacy is uncertain in view of the very limited database. While it is 

reasonable to maintain the current PM10 NAAQS in this round, it is essential that a suitable 

database for PM10-2.5 concentrations be developed over the next four years so that a more 

satisfactory judgment can be made in the next review cycle, 

 

Question 10: Indicator (sections 3.3.1, 3.3.2, and 3.3.3): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is appropriate to maintain a standard that 

provides protection against all thoracic coarse particles, regardless of their source 

of origin or composition? 

 This conclusion is justifiable at this time in view of the absence of data on the 

influence of coarse thoracic PM composition. While it is reasonable to not consider 

chemical composition in the current round of review of a coarse thoracic NAAQS, it is 

essential that a suitable database for PM10-2.5 concentrations and composition be developed 

over the next four years so that a more satisfactory judgment can be made in the next 

review cycle. 

b. The appropriateness of maintaining an indicator that allows lower PM10-2.5 

concentrations in urban areas than non-urban areas? 

 On the basis that PM2.5 is higher in urban areas than rural areas, it will constitute a 

greater proportion of PM10 in urban areas. Therefore, one can expect that for the same PM10 

concentration, that the concentration of PM10-2.5 will be lower in urban areas. Thus, it is 

appropriate to maintain an indicator that allows lower PM10-2.5 concentrations in urban areas 

than non-urban areas. 

c. The appropriateness of either a PM10 or PM10-2.5 indicator for a standard meant to 

protect against exposures to thoracic coarse particles? 

 While a PM10-2.5 indicator for a standard meant to protect against exposures to 

thoracic coarse particles would be preferable, a PM10 indicator would be an acceptable 

substitute in this round of review. 

 29
 

Question 11: Averaging Time (section 3.3.4): 

What are the Panel’s views on the preliminary staff conclusions that the currently 

available evidence supports maintaining a 24-hour standard to protect against health 

effects associated with short-term exposures to thoracic coarse particles, and does not 

support consideration of a long-term thoracic coarse particle standard? 

 We agree. 

 

Question 12: Form (section 3.3.6): 

What are the Panel’s views on the preliminary staff conclusion that it is appropriate to 

consider a 98th percentile form for a revised 24-hour standard meant to protect against 

short-term exposures to thoracic coarse particles? 

 We agree. 

Question 13: Level (section 3.3.7): What are the Panel’s views on the appropriateness of 

considering 98th percentile, 24-hour PM10 or PM10-2.5 concentrations in epidemiological 

study locations for identifying a range of potential alternative standard levels for 

consideration, recognizing the more limited air quality information available for PM10-2.5? 

 We consider it to be appropriate. 

 

 30
 

Comments from Dr. Phil Hopke 

Question 5: Indicator (section 2.3.1): What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is reasonable to retain PM2.5 as an indicator 

for fine particles? 

b. The preliminary staff conclusion that the currently available information is too 

limited to support a distinct PM standard for ultrafine particles? 

c. The preliminary staff conclusion that the currently available information is not 

sufficient to support consideration of a separate indicator for a specific PM2.5 

component or for the mix of fine particles from any specific source category, and that 

data are too limited to consider eliminating any component or source category from 

the mix of fine particles included in the PM2.5 mass-based indicator? 

These all represent self-fulfilling prophecies. Given the very limited number of measurements of 

PM1 or UFP, there is clearly no basis for extensive epidemiology and thus, no basis for an 

indicator. If we drop PM10 and move to a coarse particle indicator as the companion standard to 

PM2.5, it would really make more sense based on the nature of the atmospheric aerosol and the 

sources that would need to be controlled to provide attainment of one or both sized standards to 

switch from 2.5 μm to 1 μm (or possibly 1.5 μm) as a better separation point and eliminate most of 

the tail from coarse particle sources penetrating into the fine particle indicator. 

In spite of significant toxicological evidence of the role of UFP through the work of two PM 

centers, the Agency has been unwilling to deploy adequate UFP monitors to provide the data 

needed for epidemiology. Thus, although it seems reasonable to assume some level of toxicity 

associated with these particles beyond that which is directly related to their mass, there is no basis 

for setting an effective number based standard. At the moment, there seems to be no indication that 

monitoring will be initiated in time for the data and epidemiological results to be available in the 

next round of review. There is clearly a major disconnect between the laboratory results and 

activities in the field that would permit an informed basis for deciding if an UFP standard were 

needed. 

We measure limited composition data, but with varying relationships to observed health effects. 

There may be some “toxic” metals (Ni) in the PM, but we have no significant data on organic 

components and particularly on reactive organic components. There will need to be an expanded, 

but targeted monitoring based on clearly defined hypotheses to develop initial composition – health 

effects links to begin to serious consider component specific 

A bigger issue with respect to the indicator is the measurement method since PM mass is 

operationally defined. PM mass is defined by the sampling system, filter, and post-sample 

treatment process. This results in a precise result with totally unknown accuracy. We now have 

continuous monitors that can provide a better measure of the actual airborne particle mass. These 

measurements are almost uniformly higher than the corresponding FRM mass. It would be useful 

to consider changing the FRM to a more direct, time resolved system that better reflects the actual 

airborne mass concentrations. 

 31
 

Question 6: Averaging Times (section 2.3.2): What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is reasonable to retain annual and 24- hour 

averaging times? 

b. The preliminary staff conclusions that the currently available evidence is too limited to 

support additional averaging times to address sub-daily exposures or seasonal exposures? 

Again, we have only started to collect a reasonable amount of more highly time resolved particle 

mass data. However, there may not be sufficiently well time marked health data to begin to look at 

whether shorter, higher exposures (peaks) are more important than the 24 hour average value that 

are now available for health effects models. There are a few hints of short, high exposures 

inducing acute cardiac effects, but clearly we need far more data and analyses to permit the 

exploration of time intervals less than 24 hours. Thus, for the moment, we are stuck, but there 

should be more emphasis on using continuous monitors and not forcing them to reproduce the 

known negative bias in mass measurements inherent in the FRM values. 

Question 7: Forms (section 2.3.3): What are the Panel’s views on the utility of additional 

analyses to inform consideration of eliminating provisions included in the current form of the 

annual standard that allow for spatial averaging across monitors, specifically with regard to 

the potential for disproportionate impacts on susceptible populations with lower 

socioeconomic status? 

Monitors are to be placed to provide population exposure rather than exploring “hot spots.” Thus, 

it does not make sense to average values across an area since that provides an uneven level of 

protection to the populations that live in high versus low areas. 

There should be consideration of looking at three years data together rather than on a year by year 

basis. The idea of averaging the 98th percentile values determined for each of the three year period 

does not make a lot of statistical sense. We should be looking at the complete distribution of 

values measured in that three year period (assuming an adequate number of samples in each 

period) to determine the 98th percentile value. 

Question 8: Levels (sections 2.3.4, 2.3.5, and 2.3.6): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding alternative standard levels that are 

appropriate for consideration, and the rationale upon which those conclusions are based? 

b. The insights that can be gained from the quantitative risk assessment to inform our 

understanding of the roles that the annual and 24-hour standards play in providing 

public health protection, specifically in focusing on simulations of estimated risks 

remaining upon just meeting alternative suites of standards? (sections 2.3.4.2 and 2.3.5.2) 

c. The policy implications of the uncertainties associated with estimating risks, including 

potential underestimation of risk, in reaching conclusions regarding standards that 

would provide public health protection with an adequate margin of safety? 

d. A policy approach for identifying a suite of standard levels in which the annual standard 

would be the “generally controlling” standard to provide protection for both long- and 

 32
 

e. Additional considerations that could inform our conclusions related to alternative suites 

of fine particle standards? 

As noted above, the level cannot be separated from the measurement scheme since PM mass is 

NOT a fundamental property of the aerosol as defined by the standard. The PM mass would be a 

fundamental property IF it included the particle-bound water. Thus, we have an operational 

definition of “mass” that is known to have a significant negative bias resulting from the loss of 

semivolatile species (nitrate and SVOC). It would be better to make it clear that there is such an 

interdependency and move toward more accurate measurement technologies. 

I find the evidence adequate to lower the levels somewhat (13 to 12 μg/m3 and 35 to 30 μg/m3) but 

I do not believe there is a strong basis from going below these values. Again an additional 

measure of protection would be afforded by moving to monitoring methods that do not have the 

known negative bias inherent in the current FRM approach. 

Question 9: Adequacy of the current 24-hour PM10 standard (Sect. 3.2): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that the available scientific evidence supports 

maintaining a standard that provides protection against health effects associated with 

exposure to thoracic coarse particles? 

b. The various approaches described for translating the evidence and related uncertainties 

into the basis for conclusions on the adequacy of the current PM10 standard? 

c. The adequacy of the public health protection afforded by the current PM10 standard 

against exposures to thoracic coarse particles? 

There is certainly adequate evidence to suggest the need for a standard to protect health against 

coarse particles. However, it is clear that PM10 is not adequate given the wide variation in the 

PM2.5 to PM10 ratio. 

 

Question 10: Indicator (sections 3.3.1, 3.3.2, and 3.3.3): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is appropriate to maintain a standard that 

provides protection against all thoracic coarse particles, regardless of their source of 

origin or composition? 

b. The appropriateness of maintaining an indicator that allows lower PM10-2.5 

concentrations in urban areas than non-urban areas? 

c. The appropriateness of either a PM10 or PM10-2.5 indicator for a standard meant to 

protect against exposures to thoracic coarse particles? 

 33
 

The arguments in favor of retaining PM10 as the indicator are unpersuasive. It makes much more 

sense to measure coarse particles directly. There is currently a disparity in the level of protection 

afforded by the present standard depending on the nature of the fine to coarse ratio in any given 

location. It does not appear to me that there is significantly less information on PM coarse now as 

there was on PM2.5 in 1996 when we moved ahead with the new indicator. Thus, it is time to 

move to a direct indicator of coarse PM. We have as much basis to set a PM coarse standard as we 

did to set the initial PM2.5 standard and it is time to move away from a corrupted measure like 

PM10. 

Question 11: Averaging Time (section 3.3.4): What are the Panel’s views on the preliminary 

staff conclusions that the currently available evidence supports maintaining a 24-hour 

standard to protect against health effects associated with short-term exposures to thoracic 

coarse particles, and does not support consideration of a long-term thoracic coarse particle 

standard? 

Same arguments as made above regarding short term averaging periods can be made for the coarse 

particle indicator. We have no basis for looking at anything else and we need to initiate adequate 

measurements of PM coarse on a continuous basis so that we can actually have the data to 

ascertain the adequacy of the various possible averaging times. 

 

Question 12: Form (section 3.3.6): What are the Panel’s views on the preliminary staff 

conclusion that it is appropriate to consider a 98th percentile form for a revised 24-hour 

standard meant to protect against short-term exposures to thoracic coarse particles? 

I suggest the same approach as noted above with respect to the form of the PM2.5 standard. 

Question 13: Level (section 3.3.7): What are the Panel’s views on the appropriateness of 

considering 98th percentile, 24-hour PM10 or PM10-2.5 concentrations in epidemiological study 

locations for identifying a range of potential alternative standard levels for consideration, 

recognizing the more limited air quality information available for PM10-2.5? 

There are no specific ranges discussed. At this point, it would make sense to do what was done 

with PM2.5 and set a standard based on the smaller end of the range of PM10-PM2.5 differences to 

provide a first estimate on the standard. Such a value will be similar to the protection offered by 

the PM10 standard, but will apply uniformly. 

Question 14: Adequacy (section 4.2): What are the Panel’s views on the preliminary staff 

conclusion that the currently available information clearly calls into question the adequacy of 

the current suite of PM2.5 standards to provide public welfare protection from unacceptable 

levels of visibility impairment, primarily in urban areas, and supports consideration of 

alternative standards to provide appropriate protection?

I thought there was adequate justification for a visibility secondary standard in the last round and 

there remains a strong justification for setting such a standard particularly one based directly on 

measured light extinction. There is strong public perception that environmental quality has 

degraded rather than improved as the data clearly demonstrate. Thus, visibility provides a graphic 

illustration of improved air quality and is a useful part of the improvement of the public welfare. 

 34
 

Question 15: Indicator (section 4.3.1): 

What are the Panel’s views on the following: 

a. The appropriateness of considering PM light extinction and PM2.5 mass as indicators 

for a distinct secondary PM standard for visibility protection, and the relative 

advantages and disadvantages associated with each indicator? 

b. The appropriateness of considering the contribution of coarse particles with respect 

to a potential PM light extinction indicator? 

As indicated in the discussion of PM coarse, it is best to have an indicator that can be directly 

measured. Light extinction is a fundamental physical property of the ambient aerosol and thus, it 

represents a very appropriate indicator for a secondary visibility standard. The bulk of the 

extinction in the vast majority of locations is driven by fine particles (PM2.5). There are 

significant problems with the measurement of the light extinction from all particle sizes. Thus, the 

initial secondary standard should use light extinction by PM2.5 particles as the indicator. It could 

be possible to extend the size up to 4 to 5 μm without extensive increasing the measurement 

difficulty and that could provide some additional measure of protection to those locations that are 

more prone to high coarse particle mass concentrations. 

Question 16: Averaging times (section 4.3.2): What are the Panel’s views on the following: 

a. The preliminary staff conclusion that consideration should be given to a 1-hour averaging 

time? 

b. The advantages and disadvantages of focusing on a 1-hour daily maximum or each 1-

hour average concentration during all daylight hours? 

At this point, there is no really sound scientific basis for picking an averaging time. An hour is a 

reasonable starting point. 

Question 17: Levels and Forms (section 4.3.3): What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding ranges of options with varying levels and 

forms in combination with different indicators (light extinction and PM2.5 mass-based 

indicators) and averaging times (1-hour daily maximum or each 1-hour average 

concentration during all daylight hours)? 

b. Additional approaches that could help inform our consideration of alternative levels and 

forms? 

The range of proposed levels is overly large. There is no point in an alternative standard unless it 

is going to be significantly different from the primary standard. Thus, the level should be set 

within the lower third of the proposed range.

Question 18: Climate (section 5.2): What are the Panel’s views on the preliminary staff 

conclusion that there is insufficient information to base a standard on climate impacts 

associated with current ambient PM concentrations? 

 35
 

There are both cooling and warming effects of particles as well as the indirect effects related to 

cloud formation and brightening. At this point there are far too great uncertainties in the climate 

system and the role of particles to propose a standard with a strong scientific foundation. 

Question 19: Ecological effects (section 5.3): What are the Panel’s views on the preliminary 

staff conclusion that data are insufficient to support establishing a distinct standard for 

ambient PM based on ecosystem effects not addressed in the ongoing NOx/SOx secondary 

NAAQS review? 

In reviewing the Policy Assessment and the ISA, there does not seem to be adequate information 

available to conclude that there is need for a separate standard to provide ecosystem protection. 

Thus, the staff conclusion appears justified. 

Question 20: Material effects (section 5.4): What are the Panel’s views on the preliminary 

staff conclusion that no new evidence calls into question the adequacy of the protection 

afforded by the current suite of PM standards, and that there continues to be support for 

retaining an appropriate degree of control for both fine and coarse particles to provide 

protection against materials damage and soiling? 

Neither the ISA nor the Policy Assessment provides evidence to support the consideration of an 

alternative or new secondary standard to protect against materials degradation. 

Other Comments 

On page 4-11, it says “The amount of light extinction contributed by PM depends on the particle 

size distribution and composition, as well as its concentration.” However, particle shape also 

matters. Not all particles are spherical. 

Please do not use “particulate” when you mean “particle” 

 36
 

Comments from Dr. Robert Phalen 

Regarding sections 2.3.1 (Indicator) and 2.3.2 (Averaging Times), I have a few comments. 

1. Both sections are well-written and adequately thorough in describing the many 

uncertainties in relation to the indicator and averaging times for PM2.5. 

2. It is clear that additional research is required before specific components, and/or specific 

co-pollutant interactions, can be targeted for health-related policy assessments. For the 

present time, PM2.5 is the only feasible default indicator for health-effect considerations. 

Sadly, this will place some U.S. regions at a disadvantage in protecting public health using 

a mass-based indicator (PM2.5). 

3. Similarly, as the available health-associated data are robust for 24 hour and annual 

averaging times, the conclusion to retain these averaging times is appropriate. 

4. The lucid discussions of the limitations and uncertainties associated with the indicator and 

averaging times, will be very helpful for stimulating the needed research. The EPA staff 

has provided useful guidance in this respect. 

5. As a minor editing suggestion, I would consider eliminating the word “strong” on line 6, 

pg. 2-70, as it conflicts with the lucid discussion of the limitations and uncertainties 

previously presented. This is not a strong suggestion, but it should be considered. 

 37
 

Comments from Mr. Rich Poirot 

Overall, Chapter 4 of the Policy Assessment represents an excellent first draft presentation of 

justifications and policy options for setting secondary PM standards to protect visual air quality. 

While there are advantages to using an optical indicator (PM light extinction) and to using the 

single worst daylight hour for a regulatory metric, I think there are also certain advantages to using 

a PM2.5 mass indicator, and/or to considering effects in all daylight hours, and that plusses and 

minuses associated with these various alternatives could be presented in a somewhat more 

balanced way. 

14. Adequacy (section 4.2): What are the Panel’s views on the preliminary staff conclusion 

that the currently available information clearly calls into question the adequacy of the 

current suite of PM2.5 standards to provide public welfare protection from unacceptable 

levels of visibility impairment, primarily in urban areas, and supports consideration of 

alternative standards to provide appropriate protection?

The Policy Assessment does a good job of demonstrating that current PM standards (for which 24-

hour PM2.5 is most relevant) are not adequate for protecting visibility in most, if not all sections of 

the country. Specifically, the levels of current PM2.5 and PM10 standards are too high and their 

averaging times are too long to guard against levels of visual air quality considered adverse, over 

the short (hour or less) time periods over which changes in visual air quality are perceptible. The 

current mass-based indicators are also not ideal and could be made much more directly responsive 

to visibility effects by using a PM light extinction indicator. An optional PM2.5 mass indicator, 

applied over shorter daylight hourly averaging times and at lower levels would be adequate, and 

would represent a substantial improvement over the current 24-hour and annual means. 

A possible addition which might strengthen the chapter in future drafts would be to show – with 

data examples – how extreme a 1-hour PM2.5 mass concentration or 1-hour PM light extinction 

level could be on (a) day(s) when the 24-hour PM2.5 mass concentration does not exceed the 

current 35 μg/m3

 standard. In addition, you might include some photographic illustrations – using 

actual haze cam photos or Winhaze images showing what visual air quality looks like at levels of 

the current standards (and at the higher 1-hour levels that are possible without exceeding the 24-

hour average standards). 

15. Indicator (section 4.3.1): What are the Panel’s views on the following: 

a. The appropriateness of considering PM light extinction and PM2.5 mass as indicators for 

a distinct secondary PM standard for visibility protection, and the relative advantages 

and disadvantages associated with each indicator? 

The Policy Assessment does a good job of demonstrating the appropriateness and benefits of a PM 

light extinction indicator, while showing that a (daylight only, hourly) PM2.5 mass-based indicator 

would also be adequate, and a substantial improvement over the status quo. To a certain extent, 

arguments for use of a light extinction indicator can be strengthened by emphasizing the 

shortcomings of a PM2.5 mass-based indicator. However, I think the Policy Assessment could 

provide a somewhat more balanced view of some of the advantages and disadvantages of both 

alternative indicators. 

 38
 

A PM light extinction indicator has the major obvious advantage that the indicator would 

essentially be the effect. PM light extinction is an actual and measurable characteristic of particles 

in the atmosphere, which can be directly related on the one hand to the size and composition 

(including aerosol water) of airborne particles which cause it, and on the other hand to the levels of 

visual air quality considered unacceptable by human observers. There are however also 

disadvantages to a PM light extinction indicator (and advantages to a PM2.5 mass indicator) that 

could be presented more clearly in the Policy Assessment. 

A disadvantage of a PM light extinction indicator is that there is currently no monitoring network 

in place to assess compliance, nor have standard methods been developed which have been 

deployed in anything other than research-grade applications. At a minimum, at least 2 separate 

instruments (nephelometer and aethalometer) would be required using currently available 

technology, and these will function best if coarse particles are excluded. Thus the measured 

indicator would actually be PM2.5 light extinction, although this could be enhanced where need be 

with measurements of and estimates of extinction from continuous PM10-2.5 mass (requiring two 

additional instruments). Accurate RH measurements will also be more important for this indicator. 

Conversely, there is already a large network of continuous PM2.5 instruments in place that could be 

used immediately to assess compliance with an hourly PM2.5 mass indicator, and the quality of that 

data – and its utility for assessing human health or other public welfare effects – could be 

substantially enhanced by forcing closer scrutiny of the accuracy and precision on an hourly basis. 

A PM2.5 mass indicator would allow somewhat higher extinction in the more humid (and generally 

less scenic) East, and provide relatively better protection in the less humid (and generally more 

scenic) West – which is not necessarily a disadvantage. It would also tend to place relatively more 

emphasis on organic carbon and elemental carbon aerosols, which are more negatively associated 

with other public welfare (soiling and climate) effects, and which are also generally more 

influenced by local emissions than are the sulfate and nitrate compounds which are emphasized by 

the PM light extinction indicator. 

b. The appropriateness of considering the contribution of coarse particles with respect to a 

potential PM light extinction indicator? 

Since coarse particles make relatively minor contributions to light extinction at most locations, and 

also present both measurement uncertainties as well as increased measurement maintenance needs, 

it may be reasonable to either: 

• Exclude coarse particles from the regulatory metric (either re-name it “PM2.5 light extinction” 

or keep the same name but exclude coarse particles from monitoring requirements) or, 

• Add (either on an all-site basis, or at sites where PM10:PM2.5 > X) continuous mass 

measurements of PM10-2.5 by the difference method, estimating the associated PM light 

extinction increment by multiplying by 0.6 m2/g as in the IMPROVE formula. 

• Conceptually a similar treatment of PM10-2.5 effects could be added to a PM2.5 mass indicator, 

where the revised mass indicator might be something like PM2.5 + 0.1 x PM10-2.5. 

16. Averaging times (section 4.3.2): What are the Panel’s views on the following: 

 39
 

a. The preliminary staff conclusion that consideration should be given to a 1-hour averaging 

time? 

The one-hour averaging time (during daylight hours) is appropriate, given the nearly instantaneous 

nature of perceived impairment of visual air quality. At the same time, I think somewhat longer 

averaging times (of say 4 daylight hours) could also be justified for several reasons, including: 

• To minimize the influence of measurement errors – which could be important if multiple 

monitors are used for an optical indicator, and also because some of the continuous PM2.5

monitors produce “noisy” data over short 1-hour averaging times. 

• To minimize the influence of short-term, localized meteorological extremes – such as the 

concurrence of high RH, low wind speed and low mixing heights on cold, clear winter 

mornings. 

• To intentionally focus on visibility-impairing events which last for more than a few hours, 

and are arguably more objectionable, and effect more people, than events of short duration. 

• To follow the reasoning used and precedents set in locations like Denver and Phoenix where 

visibility standards have been established, and based on 4-hour averages in both cases. 

b. The advantages and disadvantages of focusing on a 1-hour daily maximum or each 1-

hour average concentration during all daylight hours? 

I much prefer a metric based on using all available daylight hours (combined with a higher 

percentile like 98th) over one based on only the single worst daylight hour (combined with a low 

percentile like 90th) for several reasons: 

• I think “sensitive” or “susceptible” are poor choices of terms for those who care so much 

about viewing scenery that they chose to work in windowless indoor environments, stay 

inside during lunch, and (returning home from their 8-hour shifts) further choose not to go 

outside or look out their windows during the other 2 to 6 hours of daylight that would be 

available, during 10 to 14-hour days in different seasons at a mid-latitude location in the US. 

Do they also stay inside and away from windows on weekends, holidays and vacations? I 

doubt these are the folks who indicate that only the very clearest levels are acceptable in the 

urban visibility preference studies. 

• There’s an implied, but illogical assumption that the single worst hour will coincide with the 

one hour when the poor “shut-ins” are let outside, but that often won’t occur. At best, you’re 

disregarding their views on either the to-work or from-work halves of their commutes. What 

if they work “9 to 5” (as many do), but the worst hour comes at 6 to 7 AM (or PM) or 7 to 8 

AM or PM (as it often will), or in mid morning or mid afternoon, or what if they work the 

second or third shift? By considering only the single worst hour, you are specifically not 

affording protection to all the non-shut-ins for most of the day, and providing no protection at 

all for that fraction of the shut-ins whose hour outside doesn’t coincide with the worst hour of 

the day. 

• It puts the maximum possible emphasis on uncontrollable natural meteorological influences, 

including the convergence of maximum RH, minimum wind speed and lowest mixing height 

(along with whatever weather influences a 90% RH screen might miss) in the hour just after 

sunrise. 

• This early morning met effect will be most extreme in mountain valley locations in winter, 

but these coldest, calmest winter days often offer the best visibility later in the day when the 

 40
 

inversion burns off (or at slightly higher elevations just above the inversion), so benefits of 

the extreme pollution controls needed to reduce pollutant concentrations at winter dawn, are 

likely to be spatially and temporally limited. 

• There are not really any viable 1-hour control strategies – especially for the hygroscopic SO4

and NO3 species which are emphasized by the PM extinction indicator. Conversely, when 

only the single worst hour Bext is considered, there will be the most minimal knowledge of 

the pollutants and sources responsible. 

• By emphasizing early morning RH and hygroscopic species, it tends to emphasize East/West 

differences and deemphasize the importance of organic carbon and elemental carbon 

aerosols, which have other welfare effects such as on soiling and climate. 

• For a PM light extinction indicator, measurements might include nephelometer and 

aethalometer (preferably with PM2.5 heads), plus an RH sensor, and could conceivably also 

include continuous PM10 and PM2.5 mass at some locations to estimate coarse mass 

extinction. That’s a lot of things that can potentially go wrong, for which a single worst 

hour/day measurement will be much more sensitive to measurement errors or failures in any 

of the above than if a more robust metric based on all daylight hours were considered. 

• It seems especially illogical when combined with a low percentile: “even a single bad hour 

will ruin my day, but I don’t mind a bit until it happens more than 37 days a year.” 

17. Levels and Forms (section 4.3.3): What are the Panel’s views on the following: 

a. The preliminary staff conclusions regarding ranges of options with varying levels and 

forms in combination with different indicators (light extinction and PM2.5 mass-based 

indicators) and averaging times (1-hour daily maximum or each 1-hour average 

concentration during all daylight hours)? 

I think the ranges of options combining various levels, forms, indicators and averaging times are 

reasonable, with focus on mid-range combinations while considering modest upper and lower 

bounds. For the PM2.5 mass indicator, I think the upper bound concentration levels considered (40 

μg/m3 and 60 μg/m3) are rather unreasonably high and could be replaced in future drafts with 

smaller 5 μg/m3 increment calculations within the range of 15 to 35 μg/m3

. As indicated 

previously, I much prefer consideration of all daylight hours, rather than focusing on the single 

worst hour, and would like to see a full set of calculations based on that metric for both the optical 

and fine mass indicators included in the next draft. Along similar lines, the Policy Assessment

indicates (p. 4-44, lines 18, 19) an intended focus on the more lenient 90th and 95th percentiles – 

which I think should specifically pertain only to the single worst hour/day indicator. Higher 

percentiles (95 to 98th) would be appropriate if all daylight hours are considered. I suggest 

rewording this to “... 90th or 95th percentile forms is appropriate if based on the single worst 

daylight hour, while consideration of the 95th or 98th percentiles is more appropriate if based on all 

daylight hours.” 

Would it be possible to show estimates of what attaining the various hourly optical or PM2.5-based 

secondary standards would do to (or mean in terms of) changes in annual and 24-hour average 

PM2.5 levels in the test cities? Along similar lines, would it be possible to show the “incremental 

% reductions” in PM2.5 that would be needed to, or associated with efforts to meet the various 

optical or PM2.5-based secondary standards after the current and proposed new primary PM2.5 

standards were first attained? 

 41
 

I’ve become confused about the differences that would result if PM light extinction levels or PM2.5 

concentrations were rolled back to just meet standards based on single worst hour compared to 

using all hours (and a higher percentile). At any given level of PM light extinction or PM2.5 mass, 

it would seem like rollback to attainment would produce similar results, since once the worst hour 

is “fixed”, the second worst hour takes precedence, and then the third, etc. until all hours above 

threshold are reduced – in which case, the single worst hour approach becomes very similar to the 

all hours approach. Can this be explored/explained in more detail? 

b. Additional approaches that could help inform our consideration of alternative levels and 

forms? 

As indicated above under Q 16a, I think a 4-hour averaging could be considered and justified, 

especially if the occasionally noisy PM2.5 indicator is used. I think the need for a 90 % RH screen 

becomes less urgent, and might possibly be dropped if a PM2.5 indicator is used (see other 

comments on p. 4-44 below). It is definitely needed if an optical indicator is used, and 

consideration needs to be given to whether an hour would be eliminated if the average RH exceeds 

90%, or if any or many (1-minute) RH levels within the hour exceed 90%. 

Because of measurement issues with coarse particle extinction, consideration might be given to 

using (and measuring) PM2.5 light extinction as an indicator. Alternatively, measurements of 

continuous PM10-2.5 mass (by the difference method), combined with an assumed extinction 

efficiency of say 0.6 m2

/g could be used to supplement PM2.5 light extinction measurements – 

either at all sites or at sites with relatively high coarse particle concentrations. Conceptually, 

coarse mass by difference could also be used to enhance a PM2.5 mass indicator, using a 

relationship of something like PM2.5 + 0.1 x PM10-2.5 (assumes a generic PM2.5 extinction efficiency 

of about 6 m2/gram – which would represent ammonium sulfate or nitrate at 66% RH – or a mix of 

these and organic matter at somewhat higher RH). 

PM light extinction is an actual and measurable characteristic of particles in the atmosphere, which 

can be directly related to the levels of visual air quality considered unacceptable by human 

observers and to the size and composition (including aerosol water) of airborne particles which 

cause it. Given this, a level of a secondary standard could be selected, even in the absence of – or 

in advance of a monitoring network to determine compliance. Along similar lines, a PM (or PM2.5) 

light extinction standard could be set, and PM2.5 mass measurements specified as an FEM to 

determine compliance – until such time as an appropriate PM light extinction measurement 

network can be established. 

As indicated in previous comments on the UFVA, a “progress-based” attainment test rather than 

the traditional “threshold-based” attainment test would be preferable from a scientific perspective. 

While the Agency has indicated this is not an option in the standard-setting process, it is something 

that could and should be considered in the implementation phase (where, for example, the Agency 

could provide guidance on what would be considered reasonable rates of progress). Possibly an 

illustrative example of how this might work could be provided in the next draft Policy Assessment. 

I think it would be appropriate if the next draft of the Policy Assessment included a section 

identifying and prioritizing key near-term monitoring and research needs. This could include 

things like establishing a small pilot urban visibility network to field-test and evaluate alternative 

PM light extinction measurement methods, and should also include recommendations for 

 42
 

additional visibility preference studies to better define levels (and/or frequencies) of visual air 

quality considered acceptable over a wider range of locations and viewing conditions, to support 

refinements and revisions to secondary PM standards in the future. 

Other Comments on Chapter 4

P 4-5 line 23: You could add “most” at the end of the line. 

P 4-5 lines 34-35: You could add “(all but one)” after “most CASAC Panel members”. 

P 4-15 lines 15-31: As indicated above, I’m not a fan of the single worst hour approach, and think 

it’s a stretch to claim its needed to protect the most “sensitive” individuals...since they are neither 

sensitive, nor does it protect them most of the time. 

P 4-16, lines 15-23: This is generally true, but especially during the winter. In warmer months 

(when people spend more time outdoors), RH still peaks at dawn, but morning rush hour occurs 

several hours later, nocturnal inversions are weaker, SO2 plumes aloft don’t mix down until mid 

morning, and secondary formation rates for sulfates and organics typically peak in mid-afternoon. 

P 4-20 line 17: The “24.3 dv” level cited for the 50th percentile acceptability response in Phoenix is 

different from the “24.15 dv” 50% level for Phoenix in Table 4-1. 

P 4-21 lines 16-31: You might also add something like “It should also be noted that the current 

version of the Winhaze model does not include clouds in the base photos. The visibility of clouds 

and their contrast with the background sky would be sensitive to changes in light extinction, even 

in urban city scenes which otherwise lack distant objects in the background.” 

P 4-22 line 13: You could add “and Eastern urban scenes with more distant vistas or with 

background clouds” after “...Western urban scenes.” 

P 4-29 lines 9-13: As previously indicated, I think the argument to protect the “sensitive” shut-ins 

is not the strongest. Better arguments are the instantaneous nature of the perceived visibility effect, 

and you might also add an illustration, based on data, showing how high a one-hour PM light 

extinction (or PM2.5 mass concentration) could be on a day when the current 35 μg/m3

 secondary 

PM2.5 standard is not exceeded. 

P 4-30 lines 21-32: Maybe you could set this point up a bit by zooming in from the concept of light 

extinction to PM light extinction (excluding Rayleigh and NO2) – which I assume would be the 

preferred indicator. Then you might consider but exclude the very small fraction of PM extinction 

from ultra-coarse particles (> 10 μm) – not because they contribute nothing, but because their 

contribution is so minor. Then make the point that PM10-2.5 particles also usually contribute very 

little (and their extinction effect is more difficult to measure), but can occasionally be important. 

This might lead to an optional approach in which “PM light extinction” could be the indicator, 

measured as “PM2.5 light extinction” in most areas, but with supplemental methods to estimate 

PM10-2.5 extinction in areas where coarse particles are important. 

P 4-30 line 23: Change “many” to “may”. 

 43
 

P 4-30 line 34: I think “one to two years” may be optimistic, and would constitute “rushing” with 

all of its various associated pitfalls. 

P 4-31 line 2: Use of the term “promulgation/implementation” implies that the two things need to 

be concurrent, but I don’t think they need to be so. Conceivably, a PM light extinction standard 

could be promulgated, with an implementation period delayed until a future time period (perhaps 

to allow a reasonable time frame to develop, test and deploy appropriate monitoring methods). 

P 4-31 line 14: No space needed in “widespread”. 

P 4-31 line 17: Add something like: “24-hour chemical composition data and” between “produce” 

and “hourly”, or otherwise correct this sentence. 

P 4-31 line 19: Delete “produces”. 

P 4-33 lines 1-4: Presumably the frequency of visibility-impairing “weather” is very small, but not 

zero, when RH is less than 90%.. It might be informative to know that there may still be some 

weather influence and that 90% RH is a reasonable compromise, which eliminates most of the 

weather events. 

P 4-33, line 28: I think 60 μg/m3

 is a very extreme upper bound, and that its nearly impossible to 

have PM extinction < 191 Mm-1 with PM2.5 as high as 60 μg/m3

 (and extinction of 191 Mm-1 with 

PM2.5 of 40 μg/m3 would be relatively rare as well) . In the entire IMPROVE network for the 5-

year period from 2000 through 2004, there were 73,410 samples with reconstructed PM extinction 

of less than 191 Mm-1. None of these had PM2.5 mass concentrations as high as 60 μg/m3, and only 

2 samples and 9 samples (of 73,410) with reconstructed extinction < 191 Mm-1 had fine mass 

concentrations above 50 and 40 μg/m3

 respectively. 

P 4-35, lines 8-9: As indicated before, I don’t buy the logic that a focus on the worst one hour is 

needed to protect the “susceptible” shut-ins. For this to be true, the single hour of poorest visibility 

would need to correspond to all the shut-ins’ hour outside. 

P 4-36 line 4: Remove “could” or the “s” from “affords”. Also, I’m not sure that providing a 

uniform degree of visibility protection nationwide is such a great advantage. You could add to the 

end of this sentence “, although efforts to attain it would require substantially greater pollution 

control efforts in the humid East, where scenic vistas are typically more limited. By comparison, an 

“equally stringent” nationally uniform PM2.5 mass indicator would allow somewhat higher levels 

of visibility impairment in the East, but would provide somewhat better visibility protection in 

dryer, and typically more scenic, Western urban areas.” 

P 4-38: I don’t understand why, for Tacoma and Phoenix, there are changes from column a to b 

(90th to 95th percentiles) when both are below the 191 Mm-1 threshold? 

P 4-41: This table - with the color-coding - is very helpful! Possibly future versions might focus 

on a more realistic range of say 35 to 15 μg/m3

, with smaller (5 μg/m3) increments between bins. 

 44
 

Would there be a way to estimate the changes in and remaining concentrations of in 24-hr average 

PM2.5 mass that would result if these hourly levels (or the hourly Bext levels in Table 4.3-1) were 

obtained? It will be a lot more work, but you should include all the same calculations based on the 

98th percentile using all daylight hours. Without these you will be stuck with illustrations that only 

apply to your (questionably justified) worst 1 hour metric. 

 

P 4-42, lines 11-13: Although presumably there would be substantial overlap between the worst 

10% fine mass days (hours) and the worst 10% Bext days (hours). Could this be explored? 

P 4-43, line 21: Is there an intent to specifically emphasize “urban areas”, or could this be 

rephrased to “primarily in those rural, suburban and urban areas which are not protected by the 

1999 Regional Haze Rule”. 

P 4-44 lines 18, 19: By focusing on the 90th or 95th percentile, it sounds like you’ve already made 

the (questionable) decision to use the single worst daylight hour. Otherwise, you could change this 

to “... 90th or 95th percentile forms is appropriate if based on the single worst daylight hour, while 

consideration of the 95th or 98th percentiles is more appropriate if based on all daylight hours.” 

P 4-44, line 19: Conceptually, a 90% RH screen could be used for the same weather-screening 

reasons if a PM2.5 mass, rather than a PM light extinction indicator was used. As a practical matter, 

it may be less critical for a mass indicator for several reasons. Higher RH will tend to “push” 

hours into the high percentile light extinction bins, but not so for high PM mass hours. In fact any 

weather (rain, fog) concurrent with high RH is likely to push high PM hours toward lower PM 

percentiles. Also, with a PM2.5 mass indicator, there’s no chance of requiring reductions in very 

low pollution concentrations that happen to concur with extremely high RH conditions, or of 

having any public perception, however misguided, that EPA is trying to regulate the weather. 

There is also no benefit from or need to specifically exclude higher RH conditions from (dried) 

PM2.5 mass measurements, nor would errors in the RH sensor measurements have large influences 

on whether the “right” or “wrong” hours were excluded or included from the regulatory metrics. 

P 4-44 lines 29-30: I assume this is a typo and that you meant to say “focus on a level of 20 

μg/m3

as well as levels down to 10 μg/m3 and up to 30 μg/m3”. 

 45
 

Comments from Dr. Armistead Russell 

Question 7: Forms (section 2.3.3): What are the Panel’s views on the utility of additional 

analyses to inform consideration of eliminating provisions included in the current form of the 

annual standard that allow for spatial averaging across monitors, specifically with regard to 

the potential for disproportionate impacts on susceptible populations with lower 

socioeconomic status? 

I am not sure that there was sufficient justification for spatial averaging in the past, and do not see 

that there is much reason to continue the practice. It is likely that a monitor with elevated levels is 

being impacted with fresh emissions, and unless that monitor is directly adjacent to the source(s), 

there are areas with even higher levels. Further, those elevated levels may be due to a source 

whose emissions are more harmful than the regional background PM more evenly distributed 

across a city. Allowing spatial averaging could lead to a specific area being significantly and 

consistently adversely impacted beyond the region as a whole. 

Question 10: Indicator (sections 3.3.1, 3.3.2, and 3.3.3): 

What are the Panel’s views on the following: 

a. The preliminary staff conclusion that it is appropriate to maintain a standard that 

provides protection against all thoracic coarse particles, regardless of their source of 

origin or composition? Agree. 

b. The appropriateness of maintaining an indicator that allows lower PM10-2.5 

concentrations in urban areas than non-urban areas? See below. 

c. The appropriateness of either a PM10 or PM10-2.5 indicator for a standard meant to 

protect against exposures to thoracic coarse particles? 

While the association of the observed health effects with larger particles is viewed as being 

only suggestive of a causal relationship, sufficient information is available to continue to 

have a standard that includes particles above 2.5 um. While certain sources may primarily 

emit particles that are found in the coarse size range (2.5-10 um), the tail of the size 

distribution of those emissions contains PM in the sub-2.5 um range, and those particles 

presumably contribute to the observed health effects in studies using either PM10 or PM10-

2.5. Using a PM10-2.5 indicator would allow dismissing how this tail contributes. Further, as 

discussed in the Policy Assessment, using a PM10 standard would address the (rather small 

amount of) evidence that urban coarse particles appear to have greater health impacts than 

wind blown dust. The epidemiology can support either range, and controls aimed at 

controlling PM10 (beyond those used to reduce PM2.5) would reduce levels of PM10-2.5 and 

vice versa. Given the above, a PM10 standard would appear to be preferred. Question (b) 

should be rephrased as to allowing higher PM concentrations in non-urban areas. 

Question 11: Averaging Time (section 3.3.4): What are the Panel’s views on the 

preliminary staff conclusions that the currently available evidence supports maintaining a 

24-hour standard to protect against health effects associated with short-term exposures to 

thoracic coarse particles, and does not support consideration of a long-term thoracic coarse 

particle standard? Agree. 

 46
 

Question 18: Climate (section 5.2): What are the Panel’s views on the preliminary staff 

conclusion that there is insufficient information to base a standard on climate impacts 

associated with current ambient PM concentrations? 

While there may be insufficient information to base a secondary standard on potential climate 

change, the current discussion does not adequately inform policy makers as to the potential that a 

standard limiting light extinction would do more harm than good. There is sufficient information 

demonstrating that some aerosols lead to cooling, in particular aerosols that scatter visible light 

and can lead to increased/longer lived/more reflective cloud formation. The REA (based largely 

on other assessments) demonstrates that light scattering aerosols lead to a net decrease in radiative 

forcing, and while the uncertainty is significant, it suggests that the effect may be quite large. 

Further, there is sufficient evidence that light absorbing aerosols can lead to warming. To go 

forward with a secondary standard based on reducing light extinction, the Policy Assessment

needs to more directly weigh the potential dis-benefits of regional warming with improved 

visibility unless light absorbing aerosols are preferentially targeted. There may be significant 

regional heterogeneity to controls that would address a visibility-based secondary standard which 

should be addressed. For example, it is the areas that have the lower PM concentrations that may 

benefit most from maintaining/improving visibility, but the areas with regionally high sulfate 

levels likely benefit most from the reduced radiative forcing. At a minimum, EPA staff needs to 

address how a secondary standard might impact regional radiative forcing and climate, assess the 

uncertainties, and provide a reasoned analysis weighing the two issues. Also, on Page 5-11, lines 

10-11 and lines 31-31, it is noted that “aerosols that are warming are co-emitted with aerosols that 

are cooling.” While this is true to some degree in some cases, it skews one’s perception of the 

actual case. Most of the warming aerosols in the US are emitted by biomass burning and internal 

engine combustion. Much of the cooling aerosol is formed in the atmosphere by oxidation of 

SO2 or VOCs. Thus, a set of controls to control warming PM would not necessarily have that 

much impact on cooling PM and vice versa. These statements should be modified. 

Question 20: Material effects (section 5.4): What are the Panel’s views on the preliminary 

staff conclusion that no new evidence calls into question the adequacy of the protection 

afforded by the current suite of PM standards, and that there continues to be support for 

retaining an appropriate degree of control for both fine and coarse particles to provide 

protection against materials damage and soiling? Agree. 

Minor: 

P 5-22, l 13. Replace “impossible” with “difficult” 

 47
 

Comments from Dr. Frank Speizer 

Charge Question 3: Approach (Section 2.1.3) 

Page 2.10-2.12: With regard to the weight given to the various approaches Staff appears to 

indicate that the lower level of the interquartile range for both the long term and short term 

empirical evidence should be considered, in spite of indicating no threshold. This may be 

absolutely the correct approach, but some further discussion as to why this decision was made 

might be warranted. Indicate why not the 90th or 95th %tile level. What does “notably wider” 

mean in discussing confidence intervals? How much wider? At what point do they cross zero? 

Page 2.12-13: In discussing the use of the risk estimates Staff indicates that the uncertainties are 

taken into account by considering the sensitivity of the “core” risk. May need to define “core” risk 

more explicitly. In addition it is not clear how the two separate approaches are to be combined and 

whether the weighting of one over the other will dominate or be the determining factor. If I read it 

one way I might conclude that the evidence based values will be used and the risk assessment is 

simply a test of the sensitivity of reaching a conclusion. Alternatively (and most likely) somehow 

the use of the total information will be used to reach a judgment. 

Charge Question 4: Adequacy of current suite of PM2.5 standards (Section 2.2) 

a. Appropriateness and characterization of sets of studies. It would appear that the specific 

studies chosen are appropriate and there are none other that need to be added. However, 

Figure 3.1 seems not to be adequately discussed in terms of the presentation of differences 

in weights to be put on any given set of studies. (There are also some specific questions 

about the way the figure is presented). Clearly the nature of the confidence intervals 

indicated in the figure provide as estimate of both the nature and size of each study. A 

more explicit indication on the figure as to which were single city vs multicity studies 

would be helpful. This would provide further evidence of why relying on the multicity 

studies seems justified. Specifically with regard to the figure and characterizing the PM 

exposure there are some questions raised by what is in the figure. For example for a 

number of the studies the Lower IQR value matches the lower bound of the Range. How 

does this occur? 

b. Page 2.34 The question of susceptible groups seems to conclude with the standard group of 

older adults and children along with pre existing cardiopulmonary diseases. The paragraph 

justifying lower SES is appropriate. However, no mention is made of diabetics (perhaps 

much of the data is post ISA, but not all). This is an important group, the data are at least 

as firm as some other groups, and it is a growing issue in the face of the obesity epidemic. 

I would therefore suggest a brief discussion of the risk data and it be added to the list. 

c. Use of risk assessment, judge correct. Page 2.34, line 14: Minor point—top of page 2.37, 

line 1: Not sure where the conclusion that the WHI represented a “healthier cohort 

population” comes from. If I had noted it in the ISA I would have objected. These women 

were selected as post menopausal and thus were aged ~45+. In the ACS cohort I believe 

the lower bound age was 35 and the cohort was probably of a generally higher social class, 

and in Six Cities it was a random sample of households and generally younger (and 

probably healthier). Before getting to the question of risk based consideration the several 

pages leading up to page 2.42 provide substantial evidence that the various studies 

considered demonstrate empirically that significant and substantial health effects have been 

reported by multiple investigators at or just below the current standards. It would seem that 

 48
 

the conclusion that the current standards are inadequate to protect the public and should be 

so stated before making the obvious concluding sentence on page 2.42, starting on line 5. 

In fact, it is an interesting contrast that one could take the position that since the current 

standards fail to protect the public and since there are not a lot of studies that show effects 

below the current standard levels, the very reason for doing the risk assessment is to help 

the administrator find an appropriate level that includes a margin of safety (rather than 

simply trying to add the risk assessment information to the empirical data to come to some 

“combined judgment” I admit this may just be semantics but because risk assessment 

carries the implication of uncertainty it would seem better to put it into this context. 

d. The argument used for justifying the relative contributions of the annual and 24 hour 

concentrations is logically presented and clearly stated by summarizing the results of table 

2.2. and text on page 2.45. As we indicated in discussing the RA, although the conclusions 

would not have changed, we understood why IHD mortality was used but would have 

preferred Staff using Total Cardiovascular mortality. The problem is further complicated 

by trying to compare directly the results as presented in table 2.3 with table 2.2 where the 

contrast is between the annual and the 24 hour. Since table 2.2 is IHD results and table 2.3 

is cardiovascular and the magnitudes of effect are ~3-10 times larger for IHD the tables do 

not appear to be directly comparable. The argument as to why there is greater confidence 

in the roll back estimates from where the annual standard is controlling than where the 24 

hour is controlling is straight forward and clearly related to the differences in the volume of 

data for each scenario from which to make the estimates. (The argument presented on page 

2.52 for the use of the 15 urban study areas for estimating the contrast between annual and 

24 hour values is persuasive and provides the relative limitations of the sites as a fair 

assessment.) 

e. With regard to the integration of the evidence presented in section 2.2.3, Staff is correct in 

asserting that “newly available information strengthens the associations between ...” We 

also agree that there are significant public health consequences at the current levels of the 

standard that justify consideration of lower standards in the further. However, what is not 

is this section is the fact that this is not different from the conclusions of the previous 

review, but only strengthened by additional data obtained in the last five years. Remember 

it was the Administrator that when outside the recommendation of both CASAC and Staff 

the during the last review. Further without specifying the nature of the uncertainties that 

were reduced since the last review (page 2.54, line 30) the text does a disservice to the 

previous conlusions that the standards should have been lowered. What are the specific 

uncertainties that have been lowered? What are the mechanisms that were not known 5 

years ago that are better articulated now? It is true that there are a greater suite of health 

effects, however, they do not meet a higher degree of certainty and thus add little to the 

quantitative argument available at the last round. (One cannot avoid the obvious 

conclusion that the Administrator in the last round simply ignored the available scientific 

data.) 

Charge Question 5—Indicator 

a. With regard to maintaining PM2.5 Staff’s argument that the data have expanded over the 

last several years is correct. They are also correct in indicating that in spite of there 

being significant advance in being able to better characterize both in terms of size, 

number (and perhaps chemistry) these latter measures have generally not be applied 

 49
 

sufficiently in health based research to be able to change the indicator (at least for the 

fine fraction from the current one. Of PM2.5. 

b. With regard to Ultrafines (UFP) the data are certainly suggestive of health effects. 

Staff rightfully points out (Page 2.58, Paragraph beginning line 25) there is currently no 

network of UFP samplers and thus only limited site-specific data sets. These data, 

however, are compelling and point to the need for the establishment of a network of 

samplers under some federal guidelines to position EPA to be more effective in making 

judgments on future particle standards. 

c. With regard to speciation and sources perhaps the most important summary statement 

is that on Page 2.64, sentence beginning line 17, “...evidence not sufficient to support 

eliminating any component or source from the mix...” However, the suggestion that 

could find a specific component that might be eliminated is probably a wrong way to 

conceive of the problem. PM is obviously a summary measure of a mixture of both 

complex air chemistry and source components. The likelihood of being able to 

identify or eliminate one of those components is small. More important is the use of 

this complex chemistry to more accurately identify and label sources that need to be 

better controlled. Thus far the data point most specifically to oil burning and traffic 

pollution sources, and less conclusively to wood burning. This obviously is important 

information for considerations beyond the Standard to the implementation phase. 

Rather than simply indicating that the work to date does not justify expanding the 

Indicator to be more specific in terms of speciation, a more positive statement of the 

sources could be made. 

Charge Question 6--Averaging times 

a. Agree with Staff’s position that the annual and 24 hour average need to both be 

retained as the standards. 

b. Also agree that that there is insufficient information to consider a shorter than 24 hour 

average or a seasonal average at this time. However, there is mounting clinical 

evidence that short term exposures (substantially less than 24 hours) are associated with 

reversible cardiovascular changes. The degree that these changes either from acute or 

repeated exposures trigger significant adverse events or result in more sustained 

adverse effects is as present highly suggestive, They certainly point to the need to 

consider from a policy perspective related to susceptible groups how the 24 hours 

standard should be modeled to protect the population with a margin of safety. Are 

there additional analyses that could be done to inform the Administrator on the 

distributions of the frequencies at which any specific level of exposure for , say up to 2 

hours, would occur at specific levels of the 24 hour standard? 

Charge Question 7—Form 

 The argument that there may be difference in the protective effects of a given standard 

based upon spatial averaging vs ‘community-wide air quality” recording the highest value only is 

sufficiently well presented to justify further analyses. 

Charge Question 8—Level 

 a-c. The rationale presented regarding alternative standard levels is well presented, 

although it is not clear how Staff intends will eventually focus of the which of the two very logical 

 50
 

alternatives (range 13-10ug/m3, 30-35ug/m3 vs. 11-10ug/m3, 30-25ug/m3) will become the focus 

of discussion in the second draft. Perhaps it would be useful for CASAC to weigh in more heavily 

on these alternatives. (Obviously, the latter provides a greater degree of safety and is more 

consistent with an “adequate margin of safety” 

d. Clearly the idea that the alternative annual standard will shift the distribution of 24 hour 

measures is well presented. However, it is also well presented that “hot spots” would remain and it 

may be these very hot spots that put the most vulnerable parts of the population at greatest risk. 

Thus, it becomes appropriate to continue to also regulate 24 hour levels, and thus logical to 

consider and maintain an alternative 24 hour standard. 

e. Other considerations. Because one national standard may not deal effectively with the 

seasonal or spatial variations that might occur (nor the potential localization of susceptible subpopulations at greater risk) it would seem more logical to consider more conservative levels for the 

standards. Are there additional analyses that could be undertaken to provide more quantitative 

estimates of the magnitude of the populations that would be left at risk for any given suite of 

standards? This might be one of the topics to be covered in the second draft of the Policy 

Assessment. 

 

Charge Question 9 –Adequacy of current 24 hour PM10 standard. 

a. Maintaining a Course Fraction particle standard. If anything the evidence since the last 

review is stronger, although still only suggestive that there are significant adverse 

health effects associated with the thoracic course fraction. It would appear that 

uncertainty issues remaining relate mostly to either lack or sufficient numbers of 

monitors from which to make appropriate statistical associations and adequacy of 

spatial estimates of exposure, potential for co-pollutant effects (mostly separating the 

effects of fine particles from course particles) or difficulties in technically being able to 

carry out toxicological studies of enough variety to provide a foundation for the 

potential mechanisms (different from those that have already been confirmed for 

PM2.5). Clearly the public health concern remains high (potentially related more to 

morbidity effects rather than mortality as evidenced by the relation to hospitalization 

and emergency room associations). Thus, in spite of the uncertainties maintaining a 

thoracic course particle standard is warranted. Further to the degree that the 

associations reported are at least as significant as in the last round of review, and I 

believe they are even more suggestive, strengthening the standard can also be defended. 

Charge Question 10 Indicator. 

 a-b. Appropriateness of maintaining a standard that protects against particles regardless of 

source of origin and composition. The answer to this question seems obvious. The issue is that the 

potential fraction of total course fraction that is combustion generated vs. dust generated may or 

may not be important. The wind blown dust studies that show impacts at far distances are 

suggestive that irrespective of source and thus potential toxicity (from what we currently know) 

may not make as much difference. Therefore accepting that some fraction of the PM10 may be 

related to the amount of combustion source (read PM2.5) and thus a portion of the toxicity noted 

does not negate the importance of reducing total particles. Staff seems to be arguing that being 

able to reduce PM10 from any source will work to the benefit of both urban and rural sources. I 

think I agree with this, but it still would be worth while a full discussion at CASAC. 

 51
 

 c. With regard to which measure (PM10 or PM10-2.5) is appropriate, I believe we will 

need to discuss this fully. We cannot leave the question open to the Administrator to decide. 

Therefore what we need is some estimates of exposure fractions with which to make a tentative 

judgment of what the alternatives would predict. 

Charge Question 11—Averaging time. 

 Staff concludes that the 24 hour average provides the best estimate of exposure to course 

fraction and is the exposure value with the most data suggesting an association. I would agree. It 

probably make the most biological sense and the suggestion that control of accumulated repeated 

24 hour exposures would reduce the potential for longer term averaging time effects is reasonable. 

Charge Question 12. Form. 

 Concur with the use of the 98th percentile form averaged over 3 years. 

Charge Question 13—Level 

 We currently have a promissory note that Staff will conduct some analyses of alternative 

levels of the 24 hour standard for PM10 and PM10-2.5. More specific criteria of a “stopping 

point” are needed. Will the lower bound be at 140,120, 100ug/m3 for PM10, and if so on what 

basis? For PM10-2.5 will it be at 75%, 60%, 50%, 40%, lower(?) of PM10 and on what basis? (If 

it were at all possible to see some of these analyses before the Draft 2 it would be useful to include 

in our discussion.) by indicating that the focus will be on the upper end of the distributions of daily 

PM will Staff will need to take into consideration the differences between total PM urban and rural 

and may have to run two sets of analyses to compare differences. 

 52
 

Comments from Dr. Helen Suh 

The first draft Policy Assessment is a good start at presenting and discussing the policyrelevant issues in the review of the PM NAAQS. It is comprehensive in its inclusion of relevant 

evidence and risk based information. The summaries (e.g., sections 2.3.6, 2.4, 3.2.2, 3.4) were 

particularly notable, as they were generally succinct and thoughtful, clearly and cogently 

presenting the staff conclusions. Other sections of the Assessment, however, suffered from too 

much detail, seemingly recounting much of information presented in the ISA and REA. This 

rehashing of information obscured the key policy related issues and brought a lack of focus and 

clarity to the document. Further distillation and tightening of the document is needed. While not 

yet written, the sections on “Key Uncertainties and Needs for Further Research” (for both PM2.5

and PM10-2.5) are welcomed and important. These sections should certainly include discussions of 

PM composition, with respect to whether particulate mass is the appropriate indicator. Finally, at 

its current length, the document would benefit from a chapter either at the beginning or end of the 

Assessment that summarizes staff conclusions for both the primary and secondary standards. 

Question 7: Forms (section 2.3.3): What are the Panel’s views on the utility of additional 

analyses to inform consideration of eliminating provisions included in the current form of the 

annual standard that allow for spatial averaging across monitors, specifically with regard to 

the potential for disproportionate impacts on susceptible populations with lower 

socioeconomic status? 

The rationale to conduct additional analyses to assess whether spatial averaging provisions should 

be eliminated is sensible and appropriate. Further, the consideration of alternate forms for the 

annual PM2.5 standard, specifically the highest community monitor level, is appropriate. However, 

the process by which such considerations (and from this the appropriate standard form) will be 

made is unclear and should be clearly set forth in the second Policy Assessment. In particular, the 

second Policy Assessment should describe how likely regional and temporal variation in the 

comparisons and the relationship between highest monitor levels and low SES populations was 

considered. 

c. Levels (sections 2.3.4, 2.3.5, and 2.3.6): What are the Panel’s views on the following: 

d. The preliminary staff conclusions regarding alternative standard levels that are appropriate 

for consideration, and the rationale upon which those conclusions are based? 

The alternate standard levels to be considered and their rationale are appropriate. 

e. The insights that can be gained from the quantitative risk assessment to inform our 

understanding of the roles that the annual and 24-hour standards play in providing 

public health protection, specifically in focusing on simulations of estimated risks 

remaining upon just meeting alternative suites of standards? (sections 2.3.4.2 and 

2.3.5.2) 

The quantitative risk assessment is a useful and illustrative tool to illustrate the impact of 

alternative standards on health risk. The usefulness of the risk assessment to this process 

 53
 

would be increased if the discussion of the risk assessment results were condensed even 

further to focus on the major points. 

f. The policy implications of the uncertainties associated with estimating risks, including 

potential underestimation of risk, in reaching conclusions regarding standards that 

would provide public health protection with an adequate margin of safety? 

As noted in discussions and comments from the March 2010 meeting, the impacts of model 

choices on long- and short-term risk estimates should be expanded (perhaps on page 2-43, 

paragraph beginning on line 9), as this information is central to determinations of margin of 

safety. As an example, the potential under- (and/or over-) estimation of long-term 

mortality risks from reliance on C-R functions from Krewski et al. (2009) should be 

discussed. 

g. A policy approach for identifying a suite of standard levels in which the annual 

standard would be the “generally controlling” standard to provide protection for both 

long- and short-term PM2.5 exposures, in conjunction with a 24-hour standard set to 

provide supplemental protection against days with high peak concentrations 

associated with localized “hotspots” and risk arising from seasonal emission that 

might not be controlled by a national annual standard? (section 2.3.6) 

The policy approach for identifying the suite of standard levels was well written and clear. 

h. Additional considerations that could inform our conclusions related to alternative 

suites of fine particle standards? 

With regard to a margin of safety, it may be important to include a statement in the risk 

assessment consideration of results (page 2-104) regarding the potential implications of 

model choices regarding under- or over-estimation of risks. 

Review of the Primary Standard for Thoracic Coarse Particles (Chapter 3)

[As a general note, the chapter on thoracic coarse particle seems to be less formed and more 

preliminary than corresponding sections for PM2.5.] 

6. Form (section 3.3.6): What are the Panel’s views on the preliminary staff conclusion that it 

is appropriate to consider a 98th percentile form for a revised 24-hour standard meant to 

protect against short-term exposures to thoracic coarse particles? 

This conclusion seems appropriate for practical reasons. However, it is not clear that the 

logic used to support a 98th percentile form for NO2 and PM2.5 pertains to thoracic coarse 

particles given that the high end of the PM10 concentration distribution may reflect higher 

contributions of PM2.5 and not PM10-2.5 in many parts of the US. Although perhaps this 

issue can not be addressed, this uncertainty should probably be mentioned. 

 54
 

Comments from Dr. Sverre Vedal 

Charge Question 1: Organizational structure.

a. Intro and Background. 

These are well-organized and informative for both naïve and informed readers. 

b. Questions posed. 

All questions are appropriate - I can’t think of additional ones that would be useful. 

c. Level of detail and focus. 

The balance between too little and too much detail is good, in general. There is 

repetitiveness as parallel arguments are followed through in dealing with long-term and short-term 

exposure risks, but I think this is largely unavoidable. The historical review in the coarse PM 

section (3-26 through 3-30) is perhaps too extensive. 

Charge Question 2: Technical content. 

a. ISA evidence and uncertainties. 

The Policy Assessment adequately and faithfully summarizes the relevant points from the 

ISA. I still maintain a minority opinion that the link between long-term exposure and 

cardiovascular disease is appropriately classified as causal but that with total mortality should be 

classified as likely causal (p. 2-16, table 2-1). Regardless, the Policy Assessment is faithful to the 

last version of the ISA, which has the links with both cardiovascular and total mortality classified 

as causal. 

 The section on susceptibility is somewhat poorly focused, including a discussion of lung 

function deficits in adulthood (p. 2-27) of limited relevance. Also, studies of a single stratum of 

the population (e.g., post-menopausal women, p. 2-28) that are unable to compare risk across 

different strata provide little evidence of enhanced subgroup susceptibility. The bottom line, 

however, is appropriate (p.2-29). 

b. Risk assessment policy relevant information and uncertainties. 

As in by review of the RA, I take issue with the statement that the magnitude of both longterm and short-term risk depends primarily on annual-average PM2.5 concentrations (p. 2-44, line 4 

and p. 2-48). I know this is intended to refer to short-term effects being driven by concentrations 

in the middle part of the distribution, but the statement as stands is not strictly correct. 

Charge Question 3: Fine PM primary standard approaches (Section 2.1.3). Approach. 

EPA staff is using evidence- and risk-based approaches, as before, to guide consideration 

of the adequacy of the current standards and the choice of alternative standards. Both have their 

roles and are appropriate to use for these purposes. 

There is asymmetry in the way EPA staff uses evidence from long-term and short-term 

exposure studies in assessing protection afforded by the annual and 25-hour standards: evidence 

from long-term exposure studies is used only in assessing protection afforded by the annual 

standard (p. 2-10, line 18), whereas evidence from short-term exposure studies is used in assessing 

protection afforded by both the annual and 24-hour standards (p. 2-11, line 10). Is there a rationale 

for also considering the evidence from long-term studies in assessing the 24-hour standard? It has 

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been argued that it is the cumulative effect of repeated daily PM concentration increases that is 

responsible for the chronic effect of PM exposure observed in long-term exposure studies (Brook 

RD, Rajagopalan S. Air pollution and cardiovascular events. N Engl J Med 2007;356:2104-5). It is 

therefore possible that chronic effects could be better controlled through the 24-hour standard than 

the annual standard. 

The relative weighting. 

I agree with using both the annual and 24-hr standard together in providing protection 

against PM effects. The balance between evidence- and risk-based approaches is fine. 

Additional approaches? 

None to suggest at this time. 

Charge Question 8: PM2.5 level. 

a. Conclusions on alternative standard levels and rationale. 

I find the use of the 1SD below the mean or at the lower IQR to define annual mean 

concentrations of concern to be less appealing that the use of “somewhat below the long-term 

mean” concentrations, although either is somewhat arbitrary. I know that we are to limit ourselves 

here to scientific considerations and ignore issues of feasibility and cost-benefit, but as ever lower 

standards are being considered, this becomes more difficult. 

b. Insights from the quantitative RA on annual and 24-hour standards, especially simulations of 

remaining risks with alternative standards (sections 2.3.4.2 and 2.3.5.2). 

 The quantitative RA results are very important. The information presented is very dense, 

for example, in detailing and contrasting effects in individual cities – but, I think this is preferable 

to being vague. The two scenarios used for alternative 24-hour standards (13/30 and 12/25) may 

provide most of the information needed, but additional options may be helpful unless it can be 

justified that they aren’t. 

c. Policy implications of the uncertainties in estimating risks. 

Use of the LML rather than PRB as the lower level for estimating risk is appropriate, in my 

opinion. 

d. Approach with annual standard “generally controlling” standard for both long- and short-term 

PM2.5exposures (section 2.3.6). 

 I believe the arguments made in favor of the annual standard as the controlling standard are 

well laid out and ultimately persuasive. 

e. Additional considerations related to alternative standards? 

It isn’t clear how the selected studies were chosen to be included in Fig. 2-4 (p. 2-91). 

Charge Question 13: PM10-2.5 level. Appropriateness of 98th %ile, 24-hour PM10 or PM10-2.5 

in epi study locations for identifying range of standards, in light of limited monitoring data 

for PM10-2.5 (Section 3.37)? 

I am disposed to emphasizing the specific PM10-2.5 indicator/measure in assessing level 

rather than PM10. I find that the extent to which PM10 reflects PM2.5 (ie, more so in urban areas) 

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may make it insurmountably problematic in using it for reviewing evidence and deliberating on the 

level of a standard to protect against effects of PM10-2.5. I remain open to attempts to accomplish 

that, however.