United States EPA Science Advisory EPA-SAB-EC-ADV-02-001

Environmental Board (1400A) December 2001

Protection Agency Washington DC www.epa.gov/sab

NATA - EVALUATING THE

NATIONAL-SCALE AIR

TOXICS ASSESSMENT

1996 DATA - AN SAB

ADVISORY

AN ADVISORY BY THE EPA

SCIENCE ADVISORY BOARD

(SAB)
December 20, 2001

EPA-SAB-EC-ADV-02-001

Honorable Christine Todd Whitman 

Administrator

U.S. Environmental Protection Agency

1200 Pennsylvania Avenue, NW

Washington, DC 20460

RE: NATA - Evaluating the National-Scale Air Toxics Assessment 1996 Data - An

SAB Advisory 

Dear Governor Whitman:

On March 20-21, 2001 the EPA Science Advisory Board's (SAB's) National-Scale

Air Toxics Assessment (NATA) Subcommittee (also referred to as the NATA Review

Panel) conducted a review of the Agency's NATA program. The NATA Review Panel

produced this advisory on the initial NATA of the potential health risks associated with

inhalation exposures to 32 air toxics identified as priority pollutants by the Agency’s

Integrated Urban Air Toxics Strategy, plus diesel emissions.

While a number of the elements of this assessment plan have already undergone

scientific peer review, the entire assembly of these elements and application of the full

NATA approach have not. The Agency asked the SAB’s NATA Review Panel to comment

on the appropriateness of the overall approach, including the data, models, and methods

used, and the ways these elements have been integrated, as well as to suggest ways to

improve these approaches for subsequent national-scale assessments. The advice and

insights contained herein are focused on changes that can be made to the current (1996)

NATA, as well as to the future (1999 and beyond) NATA exercises (the years 1996 and

1999 refer to time periods for which the estimates in the study are made). 

The NATA Review Panel met on February 21, 2001 in a public conference call to

provide Panel members and consultants (M/C) with the opportunity to clarify the Charge

questions, request any supplemental materials from the Agency, ask questions on materials

already received from the Agency, and discuss preparations for a public review meeting of

the NATA Review Panel on March 20 & 21, 2001 held in Research Triangle Park, NC. The

Panel M/C met in numerous public conference call follow-up technical editing work

sessions and there were several opportunities where public comments were formally

solicited through the process of developing this advisory. A detailed description of the

SAB process is found in Appendix A of this advisory.

The Agency posed nine charge questions to the NATA review Panel. These

questions addressed: 1) the adequacy of air toxic emissions estimates in the National

Toxics Inventory; 2) the appropriateness of the models and methods used to assess the

transport, fate and exposure to air toxics; 3) whether available dose-response information is

used appropriately; 4) whether predicted cancer and non-cancer risks are appropriately
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characterized and aggregated; 5) whether the discussion in the NATA on diesel particulate

matter is appropriate; 6) whether uncertainty and variability in NATA estimates are properly

characterized; 7) whether results are appropriately and clearly communicated; 8) whether

the NATA methodology and results can be used for national scale benefits analysis under

Section 812 of the Clean Air Act; and 9) suggestions for research priorities to improve the

scientific basis for future NATAs and related air toxics activities.

The Panel found that the Agency has done a very good job in assembling and using

available data and models for the 1996 NATA, and that the integration of this information

represents a new and significant advancement in the national capability for air toxics

assessment. We commend the Agency for its efforts and progress in addressing such a

broad and difficult, but important task. However, this effort continues to be a work in

progress, and limitations in the available data and the lack of scientific understanding of key

processes affecting emissions, transport, fate, exposure and health effects processes for air

toxics is such that the NATA results cannot yet be used for regulatory purposes. More

refined and source-specific data and assessments will be necessary to develop risk-based

regulations. These limitations are explicitly recognized by the Agency in the current NATA

document. Still, the Agency’s effort and the NATA results serve a critical purpose of

prescribing the current state of knowledge for a number of air toxics in the United States;

characterizing the general level and uncertainty in estimates of emissions, ambient

concentrations, exposures and health risks; and identifying where further data collection

and research efforts are needed. NATA’s potential to identify the types of further data

needed for its estimates is particularly important in motivating industry, states, concerned

citizens and the Agency to continue to expand their data collection and reporting effort. 

Improving input data is the most critical way to improve future NATA estimates.

We provide a number of specific findings and recommendations to you in this

advisory. Most of the recommendations address the specific charge questions posed by the

Agency, though some are more general in nature. A total of 56 recommendations are

provided; 30 of these involve short-term steps needed to improve the 1996 NATA and the

NATA process in general; 13 involve recommendations appropriate for the 1999 NATA;

and 11 apply to long-term research and methods improvement needed for future NATAs

beyond 1999. These recommendations are summarized in tabular form at the end of the

executive summary, and this table can be used by the Agency to track progress in

responding to this advisory. We note that our evaluation focused on the general

methodology presented in the NATA document, and not the specific values of inputs and

parameters used to implement it (though specific examples are identified to be illustrative

of apparent problems and areas of concern). Separate peer review is required for the

specific parameter values and factors used to implement the NATA.

Key recommendations provided for each charge question are as follows:

1. Improvements in the National Toxics Inventory (NTI) are critical to the

NATA and should be facilitated through the provision of uniform national

reporting protocols and rules; the provision of incentives for industry to

measure, validate and report their emissions; and the use of visualization

tools (e.g., GIS database and mapping programs) for the NTI. Methods for

cross-validation of emission estimates and for development of industryspecific emission factors for use in other applications are also needed.

2. Once the specific recommendations for the 1996 NATA are implemented,
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the model predictions of ambient concentrations and human exposure should

be acceptable for presentation to the public. However, NATA’s estimates for

secondary air pollutants – those that form as a result of chemical reactions in

the atmosphere – are likely to be incorrect (biased low) because the ASPEN

model used by NATA to predict ambient concentrations does not directly

consider nonlinear chemical formation processes. High priority should be

given to the local-scale adaptation and application of a model platform able

to simulate nonlinear chemistry for secondary air toxics and address the

larger-scale transport processes important for pollutants with significant

background concentrations for future NATAs. In addition, the Panel found

that EPA’s application of the HAPEM4 model, used to estimate indoor

exposures to pollutants, lacked appropriate consideration of inter-individual

exposure variability and (as acknowledged in the NATA report) indoor

sources of air pollution. Recognizing these HAPEM4 limitations, we

recommend that the current NATA results be accompanied by presentation of

exposure and risk estimates based on simpler transformations (or direct use)

of modeled and measured ambient pollutant concentrations and, information

on time spent indoors, in parallel with results based on the current HAPEM4

exposure module. In addition, a demonstration and validation of the full

modeling procedure now proposed for future NATAs should be made for a

well-characterized air toxic, such as benzene. These results would reflect

total exposure to the chemical from both outdoor and indoor sources. 

3. The NATA study makes generally appropriate use of available dose-response

information, consistent with currently accepted protocols. Dose-response

tables used for cancer and non-cancer health effects estimation should be

checked for accuracy and expanded to identify the date of the assessment, the

source of the data, the level of peer review provided, and whether or not the

chemical is currently undergoing re-review. When new changes are being

considered to replace those currently in EPA’s toxicity database (IRIS), the

NATA evaluation should conduct a scenario-based assessment to identify the

implications of the possible changes. Ongoing improvements to IRIS are

critically important for a number of Agency programs, including NATA.

4. NATA’s overall conceptual approach to risk characterization is reasonable

and generally follows EPA guidelines and procedures. However, NATA’s

approach to summing carcinogens is not conventional, nor is it appropriate. 

It would be appropriate and certainly more precautionary for the Agency to

combine and report the Class A and Class B carcinogens separate from the

Class C carcinogens Changes in the 1996 NATA are also needed to ensure

that the addition of non-cancer effects follows current mixtures guidance

limiting such aggregation to effects with a common mode of action. Finally,

future NATAs should address additional (non-inhalation) pathways for

exposure and sub-chronic (less than lifetime) effects. 

5. The lack of an accepted unit risk estimate for diesel cancer risk prevents the

treatment of these important emissions in parallel with the other toxics

evaluated by NATA. Diesel should be treated in a separate, succinct section

of the report in which the calculations for assessing exposures and the

present knowledge of risks are described clearly, including the concerns for

health effects associated with fine particulate matter.
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6. Methods and supporting information are not yet sufficient to adequately

represent uncertainty in each of the NATA model components. It would be

valuable for EPA to supplement its current “top down” approach for assessing

uncertainty with a scenario-based approach to identify the key model and data

uncertainties.

7. As EPA recognizes, it is a challenge to clearly communicate the NATA

results to the public. To this end, our panel recommends that NATA results

should be presented in a hierarchical manner (e.g., on different, color-coded

web pages) to differentiate between data and model predictions based on

scientific results at different stages of development and with different

degrees of confidence.

8. The current exposure methodology and results in NATA are not ready for use

in the national scale benefits analysis required in Section 812 of the Clean

Air Act. Such estimates should consider the full distribution of exposure

and risk to affected populations (not just the county median values computed

in the current NATA) and should also address less than lifetime health

effects. The Agency’s NATA and Section 812 study teams should work

together to ensure that the important goals of these related assessments are

attained in a timely manner.

9. Because the Agency’s air toxics research program has been historically

under-funded, significant, well-focused new research is needed to provide an

improved basis for future NATAs. The Agency’s research strategy for this

purpose should be reviewed by this or a similar Panel.

In summary, we believe that very effective and innovative work and progress have

been accomplished to date in developing the framework and methodology for the Agency’s

NATA. The Panel emphasizes the need for continued, improved monitoring and data

collection to allow validation with measured data in support of the assessment. An

expanded set of measurements is needed to evaluate and develop confidence in the models,

and to provide independent information about spatial distributions and trends of pollutants

over time. In this, we reiterate a critical comment that was made during the SAB’s review

of the Cumulative Exposure Project (Phase 1) in 1996, which was the genesis of the 1996

NATA. The current NATA Review Panel still believes this comment to be very relevant

today. “We also encourage the Agency to begin examining ways in which environmental

data collected for regulatory purposes might be collected in ways that would make these

data simultaneously useful for scientific purposes. With some thought, . . . it should be

possible to develop improved guidelines for the collection of some environmental data so

that it could be used for the dual purpose of assessing regulatory compliance and advancing

environmental science in order to improve the future protection of public health.”
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We appreciate the opportunity to provide advice on this effort. The Agency staff

was open, collegial, cognizant of shortcomings in the document, and accepting of the

NATA Panel’s suggestions. We look forward to your response, particularly to the points

highlighted in this letter. We look forward to being of further assistance to the Agency

with follow-up advice on the 1999 and future NATAs.

Sincerely,

/signed/ /signed/

 Dr. William Glaze, Chair Dr. Mitchell J. Small, Chair

EPA Science Advisory Board NATA Review Panel

EPA Science Advisory Board
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NOTICE

This report has been written as part of the activities of the EPA Science Advisory

Board, a public advisory group providing extramural scientific information and advice to the

Administrator and other officials of the Environmental Protection Agency. The Board is

structured to provide balanced, expert assessment of scientific matters related to 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

Environmental Protection Agency, nor of other agencies in the Executive Branch of the

Federal government, nor does mention of trade names or commercial products constitute a

recommendation for use.

Distribution and Availability: This EPA Science Advisory Board report is provided to the

EPA Administrator, senior Agency management, appropriate program staff, interested

members of the public, and is posted on the SAB website (www.epa.gov/sab). Information

on its availability is also provided in the SAB’s monthly newsletter (Happenings at the

Science Advisory Board). Additional copies and further information are available from the

SAB Staff [US EPA Science Advisory Board (1400A), 1200 Pennsylvania Avenue, NW,
ii

Washington, DC 20460-0001; 202-564-4533].
iii

ABSTRACT

This advisory provides a response to a request by the Agency to the EPA Science

Advisory Board’s (SAB) Executive Committee, to review the initial (for the year 1996)

National-Scale Air Toxics Assessment (NATA) developed by the EPA/Office of Air

Quality Planning and Standards (OAQPS). The major review meeting took place on March

20 & 21, 2001, with public teleconferences held prior to and following this meeting.

The Panel found that the Agency has done a very good job in assembling and using

available data and models for the 1996 NATA, and that the integration of this information

represents a significant advancement in the national capability for air toxics assessment,

and provides focus and motivation for ongoing improvements. However, the limitations in

the available data and scientific understanding are such that the NATA results cannot yet be

used for regulatory purposes. Topics reviewed in the advisory deal with the National

Toxics Inventory (NTI), model issues (specifically for ASPEN and HAPEM4), doseresponse information, risk characterization, diesel emissions, uncertainty analysis,

communication of results, use in future benefits assessments, and future research

priorities. The Panel provided advice and recommendations for the 1996 NATA, as well as

for the 1999 and subsequent NATAs, including 56 specific recommendations that can be

used by the Agency to track its response to this advisory. The Panel emphasized that an

expanded set of measurements and research is needed to further advance, evaluate and

develop confidence in the models and the associated exposure and risk estimates.

Keywords: hazardous air pollutants, air toxics, monitoring, emissions, transport, fate,

exposure, risk, models, ASPEN, HAPEM, NATA
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U.S. ENVIRONMENTAL PROTECTION AGENCY

EPA SCIENCE ADVISORY BOARD (SAB)

NATIONAL-SCALE AIR TOXICS ASSESSMENT (NATA) REVIEW

PANEL

CHAIR

Dr. Mitchell J. Small, Professor, Departments of Civil & Environmental Engineering and Engineering &

Public Policy, Carnegie Mellon University, Pittsburgh, PA

SAB MEMBERS*

Dr. Henry A. Anderson, M.D., Chief Medical Officer, Wisconsin Bureau of Public Health, Madison,

WI

Dr. Steven M. Bartell, Principal, Cadmus Group, Inc. Oak Ridge, TN

Dr. Calvin Chien, Senior Environmental Fellow, E.I. DuPont Company, Wilmington, DE

Dr. Linda E. Greer, Senior Scientist, Natural Resources Defense Council (NRDC), Washington, DC 

Dr. Kai-Shen Liu, Epidemiologist, California Department of Health Services, Berkeley, CA 

Dr. Joe L. Mauderly, Director of National Environmental Respiratory Center, Lovelace Respiratory

Research Institute, Albuquerque, NM 

Dr. Paulette Middleton, Director, RAND Environmental, Boulder, CO

SAB CONSULTANTS*

Dr. David R. Brown, Public Health Toxicologist, Northeastern States for Coordinated Air Use

Management (NESCAUM), Boston, MA

Mr. Thomas J. Gentile, Chief, Toxics Assessment Section, Division of Air Resources, New York State

Department of Environmental Conservation, Albany, NY

Dr. Panos G. Georgopoulos, Associate Professor, Environmental and Community Medicine, UMDNJRobert Wood Johnson Medical School, Piscataway, NJ

Dr. Carol J. Henry, Vice President, Science and Research, American Chemistry Council, Arlington, VA

Dr. Jana Milford, Associate Professor, Department of Mechanical Engineering, University of Colorado,

Boulder, CO

EPA SCIENCE ADVISORY BOARD STAFF

 Dr. K. Jack Kooyoomjian, Designated Federal Officer, US Environmental Protection Agency, EPA

Science Advisory Board (1400A), Washington, DC

Ms. Betty B. Fortune, Office Assistant, US Environmental Protection Agency, EPA Science Advisory

Board (1400A), Washington, DC

* Members of this SAB Panel consist of the following:

a. SAB Members: Experts appointed by the Administrator to two-year terms to serve on one of the 10 SAB Standing

Committees.

b. SAB Consultants: Experts appointed by the SAB Staff Director to a one-year term to serve on ad hoc Panels formed to

address a particular issue; in this case, the review of the Agency's National-Scale Air Toxics Assessment (NATA) for
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1996 and to provide recommendations for the 1999 and subsequent NATAs.
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TABLE OF CONTENTS

1. EXECUTIVE SUMMARY .................................................... 1

TABLE 1-1 - SUMMARY TABLE OF NATA REVIEW PANEL

RECOMMENDATIONS ........................................... 12

2.0 INTRODUCTION ......................................................... 18

2.1 Background ........................................................ 18

2.2 Charge ............................................................ 19

2.3 SAB Review Process ................................................. 20

3. EVALUATION OF THE DRAFT 1996 NATA .................................... 21

3.1 General Findings ..................................................... 21

3.2 Responses to Specific Charge Questions ................................... 22

3.2.1 Charge Question 1 ............................................ 22

3.2.1.1 National Toxics Inventory (NTI) ........................... 23

3.2.1.2 Reactivity Class Decay Rates ............................ 25

3.2.1.3 Temporal Allocations ................................. 25

3.2.1.4 Quality Analysis and Quality Control (QA/QC) and the

Reduction of Uncertainties ............................... 26

3.2.2 Charge Question 2 ............................................ 29

3.2.2.1 General Comments .................................... 29

3.2.2.2 Specific Concerns and Recommendations .................... 30

3.2.2.3 Summary Recommendations for Charge Question 2 ............ 34

3.2.3 Charge Question 3 ............................................ 35

3.2.3.1 Degree of Conservatism in Health ......................... 36

3.2.3.2 Validating Dose-Response Predictions ...................... 37

3.2.3.3 Use of Oral vs. Inhalation Data ........................... 37

3.2.3.4 Deviations from Linearity ............................... 37

3.2.3.5 Other Issues With Respect to Dose Response ................ 38

3.2.3.6 Indirect exposures ..................................... 38

3.2.3.7 Uncertainties in the Dose Response ........................ 38

3.2.3.8 Micro Environments and Dose Response .................... 39

3.2.4 Charge Question 4 ............................................ 39

3.2.4.1 Strengths of the Overall Conceptual Approach ............... 40

3.2.4.2 Weaknesses of the Overall Conceptual Approach ............. 40

3.2.4.3 Aggregate and Cumulative Risk Issues ...................... 41

3.2.4.4 Alternative Risk Evaluations .............................. 45

3.2.4.5 On the Issue of Children ................................ 46

3.2.4.6 Additional Clarification Issues ............................ 47

3.2.5 Charge Question 5 ............................................ 48

3.2.6 Charge Question 6 ............................................ 50

3.2.7 Charge Question 7 ............................................ 53

3.2.8 Charge Question 8 ............................................ 56

3.2.9 Charge Question 9 ............................................ 57

REFERENCES .............................................................. R-1

APPENDIX A - A MORE DETAILED DESCRIPTION OF THE SAB PROCESS ........ A-1

APPENDIX B - AREAS OF FOCUS IDENTIFIED BY PANEL MEMBERS 

FOR RESEARCH TO IMPROVE FUTURE NATA STUDIES ....... B-1

A) General Methods Research ................................ B-1

B) Chemical-Specific Information Needs ......................... B-1

APPENDIX C – GLOSSARY ................................................. C-1
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1. EXECUTIVE SUMMARY

On March 20-21, 2001 the EPA Science Advisory Board's (SAB's) National-Scale

Air Toxics Assessment (NATA) Subcommittee (also referred to as the NATA Review

Panel, or the “Panel”) of the SAB Executive Committee conducted a peer review of the

Agency's NATA program. The NATA study represents the most current effort by the EPA

to provide a nationwide quantitative assessment of health risks associated with the

inhalation of 32 priority pollutants and diesel emissions identified as contributing

significantly to human exposures and risks in urban areas. The EPA draft document which

is the subject of this review is entitled “National-Scale Air Toxics Assessment for 1996,”

EPA-453/R-01-003, January 2001 (See U.S. EPA/OAQPS. 2001).

The NATA Review Panel wishes to compliment the Agency for undertaking this

most difficult and important task. The development of the NATA document (U.S.

EPA/OAQPS, 2001) has clearly involved a major effort by a small, but dedicated staff of

Agency scientists and engineers working across disciplinary boundaries, and with little

previous precedence upon which to base model development and integration. In this regard,

the NATA report has done much to define the state-of-the-art in broadscale, national

assessment of air toxics, identifying what is possible with current tools and data, and where

these tools and data must be improved. We are especially appreciative to the authors for

their thorough documentation of methods and assumptions, facilitating our ability to review

their work and to contribute to this effort. While we focus on answering the charge

questions that seek advice on where improvements are needed in the current and future

NATAs, we wish to note that we offer these suggestions with full respect for the difficulty

involved, and with an understanding of the limited, evolving state of the science and

available information upon which such methods development can be based.

The Panel found that the draft NATA 1996 document represents an extensive and

comprehensive effort to systematically evaluate and link the various components of the risk

paradigm relevant to HAP impacts, including emissions, atmospheric transport, human

exposure and risk. In the absence of widespread measurements, the 1996 NATA relies on

modeling to estimate some elements of the emissions inventory, as well as ambient

concentrations and exposures. While some aspects of the current data collection and

modeling are advanced enough for confident prediction, others are still highly uncertain. 

An expanded set of measurements is needed to fully evaluate and develop confidence in the

models, and to provide independent information about spatial distributions and trends over

time. 

As part of our review, we have identified specific areas where the current NATA is

especially problematic. Some of these difficulties can and should be addressed for the

current 1996 assessment. Others suggested improvements will require a more long-term

effort, and should be targeted for the 1999 and future NATA’s. In the recommendations

that follow in our advisory, short- vs. long-term targets for implementation are identified. 

It is also recognized that, in order to meet the objective of NATA of establishing a baseline

for tracking trends and progress in reducing air toxics emissions, concentrations,

exposures and risks, it will be necessary in the future to revisit earlier NATAs, so as to

update them with the improved methods that become available. It will thus be important for

the Agency to carefully document the changes in methodology used for successive NATAs. 

The NATA framework and results may then be used by industry, the states, citizen groups
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and other stakeholders as a basis for improving and validating their inputs to the process and

better focusing their efforts for data collection, risk management and risk communication.

In structuring the NATA, the Agency has had to make a number of choices cognizant

of the limitations in scientific understanding, available data, and the time and resources

available for the assessment. A key choice has involved the selection of the spatial scale of

aggregation for conducting the NATA, and for reporting the results. The census tract is

utilized as a basis for estimating emissions (at times inferred from information at higher

levels of aggregation, such as the county level), predicting atmospheric transport, defining

receptor populations, and computing their exposures and risks. The results are then

aggregated back up to the county level for reporting purposes. While we agree with this

basic strategy for assessment and reporting, there are a number of difficulties that arise in

its implementation. The census tract is a good unit for defining the demographic

characteristics of receptor populations, but it is not a good geographic unit for air pollution

modeling and assessment. In particular, densely populated census tracts are small, while

those in sparsely populated areas tend to be large. This tends to misrepresent the allocation

of emissions and bias the calculation of representative ambient and exposure

concentrations for densely vs. sparsely populated areas. This problem needs to be

identified in the current NATA, and addressed in future NATAs through conversion to a

regular spatial grid for emissions tracking and the calculation of ambient concentrations,

with subsequent conversion back to underlying census tracts for population exposure and

risk calculations.

A major finding of the Panel is that parts of the NATA are based on relatively

reliable data and/or well-established scientific estimation and modeling methods, while

other aspects are based on more limited data and methods that are in an earlier,

developmental stage. This applies to all aspects of the NATA, including emissions

estimates, estimates of ambient concentrations based on the ASPEN model, estimates of

exposure based on the HAPEM modeling system (or, as suggested in our report, other,

simpler methods that should be considered in parallel with the HAPEM predictions), and

risk estimates requiring the use of toxicity values based on differing amounts of scientific

information and consensus. To help citizens and other users of NATA better understand the

varying bases for different NATA results, we recommend use of a hierarchical presentation

of results that distinguishes between quantities measured or modeled at different levels of

scientific development, and with differing levels of available data and confidence.

While we have attempted to provide specific information and recommendations to

improve the 1996 and future NATA studies, we recognize that much of the need for

improved information applies generally to the field of air toxics health risk assessment, and

is not specific to the NATA. When uncertainties and concerns are apparent in the NATA

methodology, we have attempted in a number of cases to distinguish between those specific

to NATA and those more broadly applicable across the field of environmental health risk

assessment. We also note that we have focused on the general methodology presented in

the NATA document, and not the specific values of inputs and parameters used to

implement it (though specific examples are identified to be illustrative of apparent

problems and areas of concern). The absence of comment on specific emission,

atmospheric transport, exposure and toxicity factors should not be construed to indicate

Panel review and approval of these values. Separate peer review is required for the specific

parameter values and factors used to implement the NATA.
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The Panel addressed the following set of nine charge questions, modified through

negotiation from those originally proposed by the Agency. The principal findings and

recommendations of the Subcommittee applicable to each question follow. A summary of

all 56 recommendations of the Panel is provided in Table 1-1 at the end of this Executive

Summary.

1. Given the nature of the NTI and the methods by which it was developed and reviewed, have

available emissions data been appropriately adapted for use in this assessment? Can you

suggest improvements to EPA’s application of the NTI for use in future initial national-scale

assessments?

a) Can you suggest improvements to the treatment of compound classes (e.g., chromium

and compounds), given the nature of the information available in the inventory? 

b) Can you suggest improvements to the methods used to spatially distribute area and

mobile source emissions? 

c) Can you suggest improvements to the methods used to specify default point source

emission characteristics in lieu of missing emissions data?

The Panel finds that the continued collection and compilation of air toxics

emissions data is of vital importance to the national capacity for environmental health

assessment and management. Continued presentation of inventory results to the states,

industry and other stakeholders is encouraged, in order to identify errors and to encourage

more complete reporting and data quality assurance. Improvements in the National Toxics

Inventory (NTI) would be facilitated through the provision of uniform national reporting

protocols and rules; the provision of incentives for industry to measure, validate and report

their emissions; and the use of visualization tools (e.g., GIS database and mapping

programs) for the NTI. While disaggregating emissions estimates to census tracts is

necessary for subsequent fate-and-transport modeling, continuing to limit the reporting of

emissions to the county level is supported. It should be noted however, that emission

estimates averaged over a county or a census track will spatially distribute emissions from

hot spot locations, such as those occurring near highways, leading to a subsequent

underestimation of the variability in ambient concentrations and interindividual exposure

and risk.

The NATA document (U.S. EPA/OAQPS, 2001) should provide a clearer

presentation of the methods used for data collection, analysis and interpretation within the

NTI, in comparison to those used for the National Emission Trends [NET] database for

criteria pollutants. Methods for direct cross-validation of emission estimates are needed. 

Additional approaches that do not depend entirely on ambient concentration measurements

and models should be pursued. Comparisons of emission inventories for similar point and

area source categories across the States should be made using the 1996 NTI. Comparison

of emission estimates from state reporting, National Emission Standards for Hazardous Air

Pollutants (NESHAP) information collection requests, and TRI information, should be

made when these are available. Diagnostic study of relationships between economic

activity (e.g., production, employment) for industrial sectors in an area and the emissions

estimated for those sectors, can also be to used to identify possible mismatches or

outliers. These relationships may also help in the development of industry-specific

emission factors for use in other applications. 
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For a number of metals, such as chromium and nickel, emissions estimates and

calculations in the subsequent NATA modules should differentiate between important

species (e.g., Cr6+ vs Cr3+) wherever feasible.

There is a need to better validate and document methods used to estimate mobile

source emissions, especially for non-road mobile sources. In particular, more information

should be provided on the methods used to allocate mobile-source emissions to census

tracts. Non-road emission estimates should be further checked and validated where

possible, since these are predicted to have a significant impact on ambient concentrations,

exposures and risks. For on-road mobile sources, state data based on vehicle miles traveled

(VMT) and other state generated input data (e.g., average vehicle speed and vehicle fleet

mix) should be used to estimate on-road emissions when available on a county basis.

2. Is the approach taken for the geographic aggregation of ambient and exposure

concentrations generated by the ASPEN and HAPEM4 models appropriate in light of the

limitations of the models, the available emissions data, and the results of the comparisons of

ambient predictions with ambient monitoring data? 

The Panel is concerned about a number of aspects of the current implementation of

ASPEN (the atmospheric transport model used to compute ambient concentrations from

HAP emissions) and HAPEM4 (the time-activity model used to compute human exposure

from predicted ambient concentrations) within NATA. Many of these concerns are already

recognized and acknowledged in the Agency report and documentation. For the current

(1996) assessment, HAPs should be classified to identify (a) those where ASPEN is

expected to provide an appropriate basis for analysis; (b) those for which ASPEN is

potentially applicable, but still uncertain, and improvements/refinements are needed; and

(c) those for which the model is highly uncertain, and use for these compounds is close to,

or even beyond, the range of scientifically defensible applicability for ASPEN. This latter

group includes chemicals that occur to an important extent as secondary pollutants (e.g.,

formaldehyde, acetaldehyde, acrolein), and those for which background or regional areal

sources dominate (e.g., lead in most communities). Furthermore, geographic regions

where ASPEN predictions are likely to provide accurate vs. inaccurate predictions should

be identified, based on terrain and climatology. For future assessments, ASPEN

capabilities for NATA should include the ability to address seasonal variations in

climatology and emissions. For secondary pollutants, ASPEN cannot be utilized in a

reliable manner, and high priority should be given to the local-scale adaptation and

application of MODELS-3, or a similar model platform, able to simulate nonlinear

chemistry for secondary air toxics and address the larger-scale transport processes

important for pollutants with significant background concentrations. Because of these

limitations of ASPEN, the NATA report likely underestimates concentrations of these

secondary contaminants.

The current implementation of HAPEM4 is incomplete limited in its representation

of exposure variability. The selection of different individuals within a cohort in the

Consolidated Human Activity Database (CHAD) for each day of a simulation over a year

greatly suppresses the individual-to-individual variability between simulations. While this

might be an appropriate method for estimating the mean or median exposure in a census

tract or county, the subsequent presentation with probability intervals is misleading, since it

implies that the presented quantiles represent the population exposure distribution across

the targeted area. There are three approaches that can be used to address this problem in

the short term (ideally, all three options should be evaluated and their results compared). 

First, model risk estimates based solely on ambient concentrations can be calculated and
1 The term, “threshold and non-threshold,” is more correct than use of the term, “cancer and non-cancer,”

since some carcinogens have been observed to have effective thresholds, and many agents controlled for

their non-cancer effects (PM, O3, Pb, CO) do not. The NATA study refers to mechanisms (“threshold or

non-threshold”). A few cancer assessments will be based on threshold mechanisms, but they still will be

referred to as cancer assessments.

5

reported [as done in the current Cumulative Exposure Project (CEP)]. Second, a simple

outdoor-indoor correction factor can be introduced to simulate the effects of interindividual variability in the fraction of time spent indoors and the overall effective

penetration factor for each individual’s indoor environments. Third, the HAPEM model can

be implemented as currently formulated, but only to compute (and report) the median

exposure predictions and risk measures for each census tract (and county). As noted

elsewhere, hierarchical presentation of results from all three approaches is recommended,

indicating information and estimates based on quantities measured or modeled at different

levels of scientific development, and with differing levels of available data and confidence. 

Further discussion and methods development is needed to address concerns about whether

certain demographic groups, especially poor and transient populations, are underrepresented in the time-activity databases used in the HAPEM model.

To demonstrate application of ASPEN and HAPEM4 for a case where the models

and available data are adequate to provide for reasonable prediction, we recommend that a

full-scale analysis of exposure to benzene, or another well-studied, -monitored and

–characterized compound, be conducted across the US. This would include the

development of improved activity pattern selection methods to allow a reasonable

simulation of interindividual variability in long-term exposure. This will help to build

confidence in the overall NATA approach, and the improvements in methodology that are

developed would then be available for application to other compounds in future NATA

studies. Methods development should also begin for the consideration of indoor sources

of hazardous air pollutants (based, for example, on EPA’s recent study of indoor air

pollution, U.S. EPA/IED. 2000) and the incorporation of other important pathways of

exposure for multi-media pollutants, such as the fish ingestion route for methyl mercury

and soil ingestion for lead.

3. Has available dose-response information (e.g., different sources of information, a different

prioritization scheme) been appropriately used in this assessment? Can you suggest methods

that could improve upon the use of available dose-response information?

The NATA study (U.S. EPA/OAQPS, 2001) makes generally appropriate use of

available dose-response information, consistent with currently accepted protocols by

federal and state agencies. The dose-response tables for threshold and non-threshold (also

referred to as, cancer and non-cancer)1 effects should be checked for accuracy and should

be expanded to allow the reader to identify the sources for the values used (e.g., IRIS,

CalEPA), the date of the assessment, whether or not the value has been subjected to

external peer review, whether or not the chemical is currently undergoing re-review, and a

qualitative evaluation of whether significant new studies have become available since the

assessment date. The “citation” (e.g., IRIS, CalEPA) should enable the reader to easily find

a complete source document for the value used. If this is not possible (e.g., if the authors

have performed additional calculations), this should be clearly identified and a reference

provided to that additional information. Full justification is needed for the use of

alternative methods in cases where it is decided to take a different approach from the

standard protocol for determining dose-response factors. Differences in NATA
6

predictions should be illustrated when current potencies or benchmark dose factors are

used vs. different values that may be under consideration or proposed for change. 

Since significant uncertainty is present in chemical dose-response factors, no

matter which exposure and risk assessment method is used, care should be taken to isolate

and separately report these uncertainties from those introduced through the assessment

procedures specific to NATA. Significant uncertainties in IRIS and other chemical toxicity

databases suggest that high priority be given to ongoing research to update and improve the

knowledge base for dose-response assessment of air toxics. 

4. What are the strengths and the weaknesses of the overall conceptual approach to risk

characterization used in this assessment? Given the underlying science and the intended

purposes of the assessment, can you suggest ways in which the risk characterization could be

improved?

a) Is the method used to aggregate cancer risks appropriate? The aggregation of

carcinogenic risk within two categories, based on weight-of-evidence classifications, is of

particular interest. 

b) Is the method used to aggregate non-cancer hazards appropriate? The summation of

hazard quotients within target organs, the categorization of sums by ranges of

uncertainty factors, and the inclusion of all target organs (as opposed to only the organs

associated with the critical effect) are of particular interest.

The overall conceptual approach to the risk characterization is reasonable. It

generally follows the guidelines and procedures of risk assessment (with exceptions noted

later for mixtures). However, as detailed below, some of the key specific elements in

implementation of the conceptual approach are not consistent with current assessment

guidelines or best practices. 

The current NATA (U.S. EPA/OAQPS, 2001) includes only chronic inhalation

health effects from exposure to outdoor sources of air toxics. The document is quite clear

on this, but the resulting limitations of the assessment need to be more explicitly

discussed. Effects from less-than-lifetime exposures and total exposure to air toxics are

key issues requiring further evaluation. Changes in the 1996 NATA are also needed to

ensure that the addition of non-cancer effects follows current mixtures guidance limiting

such aggregation to effects with a common mode of action. The 1999 NATA needs to

incorporate these issues, especially assessments based on the multiple pathways of

exposure to outdoor sources of air toxics. Future NATAs should address additional (noninhalation) pathways for exposure and sub-chronic (less than lifetime) effects.

In the current EPA cancer guidelines, chemicals are classed according to the weight

of evidence in support of the inference that they are carcinogenic. The classes for known

or suspected carcinogens include: 

A: “Known” Carcinogens based on sufficient evidence of carcinogenicity

from epidemiologic studies to support a causal association between exposure

to the agents and cancer;

B1: “Probable” HumanCarcinogens based on limited evidence of

carcinogenicity from epidemiologic studies, but sufficient evidence from

animal studies;
7

B2: “Probable” HumanCarcinogens based on sufficient evidence of

carcinogenicity from animal studies, but inadequate evidence or no data

from epidemiologic studies. 

C: “Possible” HumanCarcinogens used for agents with limited evidence of

carcinogenicity in animals, and the absence of human (epidemiological) data.

Known human carcinogens are summed separately from probable human

carcinogens in the NATA document. Probable human carcinogens are lumped with possible

carcinogens. This is not conventional. The only difference between the known and

probable classes of carcinogens is the extent of available data from human studies, and

human studies of these compounds are relatively rare. Thus, it seems more correct and

certainly more precautionary for the Agency to combine and report the Class A and Class B

separate from the Class C carcinogens. Because many of the IRIS values are based on

assessments performed more than 10 years ago, it is essential that EPA re-evaluate the

scientific appropriateness of those values for future NATAs. Ongoing improvements to

IRIS are important for a number of Agency programs; they are particularly important for

providing improved scientific capabilities for assessing air toxics. Also, the Agency

should provide an estimate for all types of cancers summed together and then break them

out by group. These revised calculations should be feasible for the 1996 NATA. 

The Hazard Quotient, HQ, equal to the exposure to a given chemical divided by its

reference concentration (RfC), and a Hazard Index, HI, equal to the sum of HQs for

multiple compounds, are common means for assessing and characterizing noncancer risks. 

As everyone agrees, there is a high degree of uncertainty in this approach. Nevertheless,

there are standard, generally-accepted approaches for implementing these calculations, and

the methods in the draft NATA document deviate from these. In particular, the NATA HI

calculations do not follow current EPA guidelines and are scientifically questionable, and

therefore need to be improved.

The HI methodology is commonly accepted for aggregating noncancer effects for

chemicals having a common mode or mechanism of action. In the absence of data, some

assessors default to using a common organ (in accordance with EPA mixtures assessment

guidelines). However, in some cases, chemicals having known different

modes/mechanisms were added together in computing an HI (e.g., formaldehyde which

produces nasal effects was added to cadmium which produces lung effects through

different mechanisms). This needs to be corrected. It is also important that problems in

computing HI’s (due to uncertainties in both the methodology and the supporting data) be

clearly identified in the text as a significant limitation.

The calculation of greatest concern is the target-organ-specific-hazard index

(TOSHI). This HI was calculated by taking the RfC for a chemical based upon the critical

effect and dose to one organ and transferring this RfC to all other organs affected by that

chemical. The RfC is based on the most sensitive indicator of effects, to which

conservative uncertainty factors are applied. To take this value and apply it directly to other

organs (deemed inappropriate by EPA for the original RfC calculation) is scientifically

questionable. If EPA wishes to use a TOSHI approach, it is essential for the Agency to go

back to the database for each chemical and actually develop TOSHIs with a high level of

scientific rigor.
8

As discussed later in response to Charge Questions 6 and 7, the very large

uncertainty in exposure estimates and toxicity values creates a considerable challenge to

the Agency, as to how they should characterize and present the uncertainty and confidence

that can be placed in the resulting risk estimates. To help characterize the level of

confidence that is warranted, the Agency should implement some selective

“groundtruthing” exercises for the predicted exposures and risks for some of the selected

air toxics. EPA should identify a data-rich air toxic that would be evaluated to compare

various risk characterization approaches in the 1996 NATA. Benzene could serve as such a

test compound, but others should also be considered. The 1999 NATA should include

more such comparisons, as well as consideration of different scenarios that would

facilitate a better understanding of the relative importance of exposure and toxicity value

uncertainties.

5. Although EPA has concluded that available data are not sufficient to develop a reliable

quantitative estimate of cancer unit risk for diesel emissions, it is clear that this pollutant class

may be of significant concern in a number of urban settings. The risk characterization in this

report includes a discussion of diesel particulate matter to help states and local areas frame the

importance of this pollutant compared to the other air toxics. In the context of this assessment,

is the discussion in this report regarding making risk comparisons among other air toxics

appropriate? Can you provide any suggestions that would improve upon this approach to

comparing the toxic health effects of diesel particulate matter with other pollutants?

The inclusion of an assessment of diesel emissions in the current NATA (U.S. EPA/

OAQPS, 2001) is appropriate. Furthermore, the caveats used in the report to describe the

current state of knowledge about diesel particle health risks are reasonable and generally

consistent with the latest CASAC findings and recommendations. The exposure assessment

is especially valuable. However, the attempt to treat diesel emissions in a fully integrated

and step-wise manner, in parallel to the other air toxics addressed in the report, is awkward,

and the required frequent repetition of the Agencies “belief statement”, that diesel particles

are (or may be) among the most significant health risks among air toxics, is not adequately

supported in the report. The current status of our knowledge of the risks from diesel

emissions should be summarized more clearly in a separate and succinct section of the

report, and the calculations used for computing diesel exposures and risks expounded upon

in that section. The set of diesel health risks addressed in this section of the report should

be expanded to include the concerns for respiratory disease mortality and morbidity

generally associated with fine particulate matter (PM).

6. Given the limitations inherent in this preliminary assessment, have uncertainty and variability

been appropriately characterized?

a) Can you suggest ways that the characterization of uncertainty and variability could be

improved, made more transparent, or integrated more effectively into the risk

characterization?

b) Can you suggest methods for quantifying individual as well as composite uncertainties

associated with the emissions inventory, dispersion modeling, exposure modeling, doseresponse assessment, quantitative risk estimates, and accumulation of risk across air

toxics?

Given the high degree of conceptual uncertainty in the modeling of air toxic

emissions, exposures and risks, and the significant gaps in available data for supporting

these, the more aggregate, ‘top-down’ approach for assessing uncertainty proposed in the
2 Wherever the term, “conceptual uncertainty” is used, it refers to the model constructs, the supporting data,

as well as the methods and supporting information to assign probability distribution functions for

representing uncertainty in each of the NATA components, and the combination of these to estimate a

probability distribution for the resulting prediction of risk. The Panel recommends a scenario-based (that is,

a systematic parametric analysis) approach to capture key conceptual and data uncertainties. 

9

NATA document (U.S. EPA/OAQPS, 2001) is appropriate.2 However, the current

implementation requires significant further work before meaningful results and insights can

be obtained. In particular, the methods and supporting information are not yet sufficient to

allow the assignment of probability distribution functions for representing uncertainty in

each of the NATA components (emissions, fate-and-transport, exposure, and doseresponse) and the combination of these to estimate a probability distribution for the

resulting prediction of risk. Instead, a scenario-based approach should be used to capture

and discuss key conceptual and data uncertainties in the NATA. This would allow the focus

to be upon the assumptions and data-gaps that might contribute to inaccuracies in the

assessment, rather than a focus on imprecision implied by the current probabilistic method

and results (with the implication that the central tendency of the estimate has a degree of

reliability that in many cases may not be justified). 

For each of the components of NATA, summary tables should first be developed

summarizing the amount of available vs. missing data for the assessment. A sequential

outcome (or ‘event’) tree, with different branches to represent the adoption of each of the

major conceptual or data-source assumptions could then be constructed. For the emissions

component, the alternative scenarios could consider use of information from the different

available sources and databases. For the fate-and-transport model predictions of the ratio

of ambient and exposure-unit concentrations to emissions, the scenarios can address

compounds and conditions where ASPEN is applicable, vs. those where it is not. As noted

above, the current implementation of HAPEM is inappropriate for representing interindividual variability in the target population exposures, and alternative approaches (when

developed) could also form the basis for different scenario evaluations in the assessment. 

For the dose-response component of the model, reliance on different databases or the use

of currently accepted vs. proposed (or ‘under review’) toxicity values would allow insight

into the impact of these assumptions. 

When combined, this scenario tree would provide insight into which combinations

of assumptions lead to the most important differences in predicted exposure and risk (and

air toxic prioritization), and which assumptions in turn require further discussion with

stakeholders and improved resolution through further data collection and model

development. This would also help to provide insight as to which sources of uncertainty are

specific to the NATA and which are common to all health risk characterization efforts,

suggesting specific needs for NATA improvements as well as more general priorities for

air toxics research in ORD. 

The use of a detailed (‘bottom-up’) Monte Carlo simulation for characterizing

uncertainty in NATA predictions is not recommended at this time, though such an approach

should be used as part of the ongoing studies to explore the sensitivity of the component

models to different parameter inputs.

7. Have the results of the assessment been appropriately and clearly presented? Can you

suggest alternative methods or formats that could improve the presentation and communication

of these results?
10

The NATA document (U.S. EPA/OAQPS, 2001) reflects a proper concern with the

importance of effective communication of results, to encourage a holistic understanding of

air toxic risks and the options available for addressing them; and to address the various

information needs of decision makers and stakeholders in the EPA, other federal and state

agencies, industry, environmental and other interest groups, and the general citizenry. A

problem facing EPA staff in this task is finding a means to clearly communicate which

pieces of the assessment are understood and characterized with a relatively high degree of

confidence, and which require further data gathering and model improvement before

reliable estimates can be assured. Given the importance of environmental pollution

information such as this (e.g., the widespread use of the TRI and the current NTI data by

business, environmental groups and citizens), we recommend that the Agency clearly

distinguish between those parts of NATA that are well established, vs. those which are in an

earlier, developmental stage, based upon less certain science and models, and more limited

data. In developing the web page for communicating results, the EPA should consider use

of a hierarchical set of pages to differentiate between:

a) Information that is based solely on data or data reports, e.g., emissions data

sets and ambient concentration and personal monitoring datasets for different

compounds in different locations; 

b) Information that is based on relatively simple or highly confident model

calculations, such as ambient air concentration values computed by ASPEN

for well-characterized air toxics that are not affected by secondary pollutant

formation processes, in areas (terrain and meteorology) where ASPEN can

provide reliable prediction, or total exposures to ambient pollutants

computed assuming a simple indoor-outdoor penetration factor; and

c) Information based on new model developments, where research is ongoing to

improve the basis for prediction.

These pages could be color coded and titled to indicate: a) existing NATA data

(using, for example, a blue background); b) existing NATA models (pale green background);

and c) models undergoing research and development (yellow for caution). Graphic

representations, such as a thermometer type graph, could be used to display the levels at

which different health effects are seen, or to present different cancer risk levels.

The current NATA document was written to some extent for this Panel, with a

number of the discussions directed towards an SAB advisory. A more general report for a

broader audience should be written. This revised report should include an executive

summary which highlights key findings and important compounds and issues from the

beginning. Many of the graphics used for summarizing risks across the multiple

compounds and in different locations are very clear and effective (though this does make

the responsibility even greater for ensuring that these results are accurate and reliable).

Members of the Panel held differing opinions as to whether model exposure and

risk estimates or rankings should be presented for specific counties in the U.S. Such

information might include an alphabetical list of the 100 counties with the highest

exposures and risks (or the top Y% of counties). Such a listing should include information

to help readers discern the particular reasons why (and the set of assumptions under which)

the county is included in the list. Some members of the Panel felt strongly that states,

citizens and other stakeholders would greatly benefit from this information and that, in any
3 Sub-chronic health effects is referred to here in the context of generally accepted animal toxicology studies. 

11

case, other organizations will be able to access and manipulate the NATA results to produce

it. Others felt just as strongly that the uncertainty in NATA estimates is too great to justify

identification of specific “hot-spot”, high-risk counties, and that even if others could

generate such a list, this was preferable to the EPA itself producing it (with the implied

“official support” that this would entail). We note this disagreement within the Panel and

hope that we have clarified (here and in the main report) the advantages and disadvantages to

the Agency of producing a list of counties with high estimated NATA exposures and risks.

8. The exposure methodology in NATA is being considered as one candidate for providing the

basis for a national scale benefits analysis (as required in Section 812 of the CAA). Please

comment on the strengths and weaknesses of this approach, recognizing the limitations outlined

in the NATA report. 

The current exposure methodology and results in NATA are not yet ready for use in

the national scale benefits analysis required in Section 812 of the Clean Air Act. Once the

needed improvements noted above are implemented with a few more iterations of the

approach, application to benefits assessment can be considered. In particular, a meaningful

benefits assessment must consider the full distribution of exposure and risk (not just

median values) and should also address sub-chronic health effects.3 Once exposure

predictions are improved and validated, the cost-effectiveness of alternative toxics

management strategies (for emissions and exposure reductions) could be compared,

stopping short of a full benefits assessment (that would be based on health risks, mortality

and morbidity avoided). If a full distribution of exposure and risk is estimated for an

information-rich HAP, such as benzene, as part of the current NATA, then the 812 study

could attempt an initial benefits assessment for that HAP, to illustrate the type of analysis

that is envisioned for the future. Another precaution that is needed for such a calculation is

that best-estimate values of dose-response metrics should be used to obtain best-estimate

values of health benefits. In contrast, upper-bound estimates of toxicity values, such as

those typically found in IRIS, yield conservatively high estimates of health benefits

(assuming that these upper-bound toxicity values are combined with best-estimate values of

exposure). Since EPA’s NATA and Section 812 studies must address many of the same

issues related to exposure and health effects, the study teams should work together to

assure that the important goals of these related assessments are attained in a timely manner.

9. Do you have suggestions for research priorities that would improve such air toxics

assessments in the future?

An extensive research effort should be mounted to address the wide array of the data

and model development areas needed to significantly improve the scientific foundation for

future NATAs, as well as regulations based on the health risks of air toxics. The needs

(addressed in detail in the NATA document) include both fundamental and chemicalspecific research and span the whole of the risk paradigm (i.e., emissions, ambient

concentrations, exposures, effects, and risks). Because air toxics research has been underfunded by the Agency for so long, considerable new resources are needed. Fortunately, the

NATA allows identification of the uncertainties that are inhibiting the development of

reliable quantitative assessments so that the new resources could be well-focused. We

understand that the EPA ORD is completing a strategic plan for air toxics research, so there

is no need for the SAB to duplicate this effort. We recommend that the Agency’s research
12

strategy be developed with full knowledge of, and in concert with, the efforts of other EPA

offices, external organizations and experts (for example, the Health Effects Institute is now

preparing a Mobile Source Air Toxics research strategy), and that the subsequent draft be

reviewed by this or a similar Panel. Research needs for diesel particles can be obtained

from EPA’s recent diesel health assessment. 

While significant data limitations and the high degree of uncertainty present in the

scientific understanding of processes affecting air toxic emissions, fate, transport,

exposure and risk are likely to continue to limit our ability to develop accurate and precise

risk estimates, we believe that specific, well-focused research can be conducted to insure

that improved methods and data are available for future NATAs. Because developing a

research strategy and implementing it takes considerable time, the Panel recommends that

EPA develop a plan that describes what work (information collection, research, and

assessments) it will perform with existing resources over the next few years that will

directly improve the 1999 NATA.

Using the information developed in research programs is just as important as

generating the information. Thus, no air toxics research program can be useful until it is

incorporated in Agency models for assessments and until, for example, the new doseresponse assessment information is entered into IRIS. Given the reliance on IRIS, keeping

it scientifically robust is a crucial need. Thus, re-evaluating the need to update all the air

toxics and then proceeding to do updates, as appropriate, is essential for the next NATA

(the 1999 NATA). These activities also need appropriate resources. 
13

TABLE 1-1 - SUMMARY TABLE OF NATA REVIEW PANEL RECOMMENDATIONS

No. CHARG

E No. SUBJECT ABBREVIATED RECOMMENDATION

SECTION WHERE

DISCUSSION CAN BE

FOUND

RECOMMENDED SCHEDULE

FOR IMPLEMENTATION

1 none General Findings Separate peer review should be conducted for the specific input parameters and

values assumed for the different modules of the NATA model. 3.1 1996, 1999 and FUTURENATAs

2 1

National Toxics

Inventory

Implement additional QA/QC measures to ensure that a satisfactory level of

nationwide completeness and accuracy is achieved for the point and area source

emission inventories.

3.2.1.1 1999 NATA 

3 1

National Toxics

Inventory

Continue the development of the on-road model to accept input parameters

developed from State & Local Air Pollution Control Agencies for the development

of the 1999 on-road emission inventory. Provide more detail on how the on-road

HAP emission factors for the MobTox 5b model were developed.

3.2.1.1 1999 NATA 

4 1

National Toxics

Invewntory

Critically re-evaluate surrogates used to estimate the non-road emissions inventory

and make adjustments where necessary. Continue the development and

verification of the non-road emission inventory & non-road model for future

iterations of NATA by expanding the research agenda to fill known important data

gaps. These data gaps should be prioritized to reduce the most significant

uncertainties associated with the non-road emission inventory and model

predictions.

3.2.1.1 1999 NATA

5 1

Reactivitiy Class

Decay Rates

Reactivity categories and decay rates should be identified for each HAP modeled in

ASPEN. Critical assumptions and uncertainties associated with the assignment of

reactivity classifications for HAPS should be discussed 

3.2.1.2 1996 NATA

6 1

Reactivity Class

Decay Rates

Update reactivity categories assignments and decay rates by incorporating HAP

specific information when available. For HAPs identified as important risk drivers or

regional contributors evaluate the impact of the assumption that each pollutant

witihn a specific reactivity class is assumed to decay at the same rate.

3.2.1.2 1999 NATA 

7 1

QA/QC and

Reduction of

Uncertainties 

Implement additional QA/QC measures to ensure that a satisfactory level of

completeness and accuracy is reached for all emission inventories. 3.2.1.4 1999 NATA

8 1

QA/QC and

Reduction of

Uncertainties 

The Agency should apportion Cr6+ for each source category in the EMS-HAP stage

and have two separate inputs into the model as chromium and Cr6+using the

available literature on this subject. In addition, a reactivity decay rate will have to

be developed and incorporated into EMS-HAP for Cr6 

3.2.1.4 1999 NATA

9 1

QA/QC and

Reduction of

Uncertainties

Consider an alternative modeling approach for counties with major metropolitan

areas and small census tracts which would involve the mapping of all averages

using a uniform grid approach. This type of analysis would provide results which

are directly comparable from one metropolitan area of the country to another. 

3.2.1.4 FUTURE NATAs

10 1

QA/QC and

Reduction of

Uncertainties

To avoid the use of default stack parameters, request that State and Local Air

Pollution Agencies or industry summarize any stack parameter information

contained in stack test reports if available for facilities that have been assigned

default stack parameters.

3.2.1.4 1999 NATA
No. CHARG

E No. SUBJECT ABBREVIATED RECOMMENDATION

SECTION WHERE

DISCUSSION CAN BE

FOUND

RECOMMENDED SCHEDULE

FOR IMPLEMENTATION

14

11 2

Model Issues -

ASPEN

Explicitly identify the level of confidence/uncertainty associated with ASPEN

predictions for the specific contaminants considered (using the three group

classification recommended in this review), for particular geographical regions and

locales

3.2.2.3 1996NATA 

12 2

Model Issues - 

ASPEN

Explain and discuss the fact that only a single component (county to county

differences in the median) of exposure variability is characterized in the current

application

3.2.2.3 1996 NATA 

13 2

Model Issues -

ASPEN

Discuss explicitly the limitations of the 1996 NATA approach (i.e., those associated

with the treatment of long range transport and characterization of background,

nonlinear chemistry of secondary air toxic formation, seasonal variability in

emission climatology, etc.) 

3.2.2.3 1996 NATA 

14 2

Model Issues -

HAPEM

While continued development of HAPEM is encouraged, until this occurs, exposure

and risk estimates based on simpler transformations (or direct use) of ambient

concentrations should be presented in parallel with those based upon HAPEM

results. A discussion of possible biases in HAPEM results associated with underrepresentation of certain demographic groups in available time-activity databases

should be included in the NATA report.

3.2.2.3 FUTURE NATAs

15 2

Model Issues -

HAPEM

A “full-fledged HAPEM” calculation for benzene should be performed and included

in the 1996 NATA report as a prototype example for future applications to other

toxics: this application should account for exposure to indoor as well as outdoor

sources and correctly treat day-to-day correlations in activity patterns for individuals

in order to properly address exposure variability.

3.2.2.3 1996, 1999 and FUTURE

NATAs

16 2

Model Issues -

Future Applications

Future NATA applications should address the limitations identified in this review

and, for example, consider the effects of factors such as seasonal variability in

emission, climatology and resulting ambient concentrations, improve the treatment

of outdoor air quality concentration gradients within a census tract, consider the

contribution of indoor sources of air toxics to total exposure, and account properly

for inter- and intra-individual variability of exposure. Further efforts should be

made to ensure that all demographic groups in the United States are represented in

the exposure estimates, either by extending current time-activity databases, or by

applying appropriate statistical corrections that have been tested and validated.

3.2.2.3 FUTURE NATAs

17 2

Model Issues -

Future Applications

Future NATA applications should test, adapt, and employ (a) more comprehensive,

multi-scale, air quality models, such as Models-3, that can account for both local

and long range transport and for nonlinear chemical transformation, as well as (b)

evolving modeling tools for exposure analysis that are currently under development

by USEPA and other organizations.

3.2.2.3 FUTURE NATAs

18 2

Model Issues -

Future Applications

Future applications should also focus on the development and application of a

consistent, integrated, framework that incorporates multiple routes and pathways of

exposure for multi-media pollutants.

3.2.2.3 FUTURE NATAs

19 3

Dose-Response

Information

For the 1996 NATA, recheck the accuracy of the Tables of dose-response values and

add columns to identify whether the value has been externally peer-reviewed, the

date of the assessment, and a qualitative indication of whether significant new

studies have become available since that date. The “citation” (e.g., IRIS, CalEPA)

should enable the reader to easily find a complete source document for the value

used. If this is not possible (e.g., if the authors have performed additional

calculations), this should be clearly identified and a reference provided to that

additional information. For chemicals that do not use the NATA protocol, show the

rationale for the assessment in detail. For the 1999 NATA, EPA is encouraged to

update all IRIS cancer and non-cancer dose response values for those chemicals

having new health effects data since the existing IRIS assessment.

3.2.3 1996 & 1999 and FUTURE

NATAs
No. CHARG

E No. SUBJECT ABBREVIATED RECOMMENDATION

SECTION WHERE

DISCUSSION CAN BE

FOUND

RECOMMENDED SCHEDULE

FOR IMPLEMENTATION

15

20 3

Dose-Response

Information 

For the 1999 NATA include dioxins. Also, consider establishing a specific schedule

for periodic update of the NATA risk estimates, by setting a calendar date that will

be used for selection of reference information from secondary sources (i.e., only

data available “as of” the given date will be used for the update).

3.2.3 1999 and FUTURE NATAs

21 3

Degree of

Conservatism in

Health

Indicate in the document the differences in relative risk expected if MLEs were to be

used instead of upper bound estimates of cancer potency, in cases where both are

available. Provide comment on the effect of different uncertainty factors on the

selection of specific HAPs as risk drivers.

3.2.3.1 1996, 1999 and FUTURE

NATAs

22 3

Validating DoseResponse

Predictions

For 1999, request that States provide reference concentrations as part of inventory

or state review of NATA. The State estimates could be provided in an appendix

table for compilation purposes.

3.2.3.2 1999 and FUTURE NATAs

23 3

Use of Ora l vs.

Inhalation Data 

For 1996, provide an estimate of the potential variability of the oral to inhalation

extrapolation, and the implications of this for the derived toxicity values. 3.2.3.3 1996 NATA

24 3

Deviations from

Linearity

Consideration should be given in future NATAs to possible deviations from linearity

in the dose-response functions for non-cancer risk. 3.2.3.4 1999 and FUTURE NATA 

25 3 Indirect Exposures The 1999 NATA should include the effects of indirect (non-inhalation) exposures

for PBTs. 3.2.3.6 1999 and FUTURE NATAs

26 3

Uncertainties in the

Dose Response

For the 1996 NATA more clearly indicate which of the uncertainties are due to the

ASPEN/HAPEM process and which are due to the more general risk assessment

process.

3.2.3.7 1996 NATA

27 3

Micro Environments

and Dose Response

As acute health effects are considered for evaluation in future NATAs, a careful

matching of toxicity value estimates and exposure estimates will be needed.

Similar concern is needed when considering the effects of background and indoor

sources of HAPs on health impact estimates that are subject to threshold effects.

3.2.3.8 FUTURE NATAs 

28 4

Risk

Characterization:

Weaknesses of the

Overall Approach

For the 1996 NATA, include more discussion of the implications of considering only

chronic health effects. For the 1999 NATA, include less-than-lifetime exposure

health assessments, exposure assessments, and risk assessments, if possible. Some

of thses actions will require the development of standard assessment guidelines and

new evaluations and entries into IRIS, as well as modification in estimation

procedures and data in all phases of the NATA to begin to address short-term, acute

effects. 

3.2.4.2 1996, 1999 and FUTURE

NATAs 

29 4

Risk

Characterization:

Weaknesses of the

Overall Approach

For the 1996 NATA, increase discussion of potential impacts of total exposure,

including the indoor source issue. For the 1999 NATA, include other sources of

exposure in the risk analysis.

3.2.4.2 1996,1999 and FUTURE

NATAs 

30 4

Risk

Characterization:

Weaknesses of the

Overall Approach

For 1996 NATA, provide a more balanced discussion of the possible sources of

under- versus- over-estimations of HAP exposures and risks. 3.2.4.2 1996 NATA 

31 4

Aggregate and

Cumulative Risk

Issues

For the 1996 NATA expand the discussion of the rationale for the approaches used

to aggregate cancer and non-cancer risks and the impacts of these approaches on

uncertainty. Also, expand the discussion on the possible extent of the influence of

background concentrations and other model assumptions on the risk outcomes,

3.2.4.3 1996 NATA 
No. CHARG

E No. SUBJECT ABBREVIATED RECOMMENDATION

SECTION WHERE

DISCUSSION CAN BE

FOUND

RECOMMENDED SCHEDULE

FOR IMPLEMENTATION

16

32 4

Aggregation and

Characterization of

Cancer Risks.

For the 1996 NATA, evaluate the impacts of combining the A and B1 carcinogens,

leaving the B2 and C carcinogens as separate entities, and see whether this

changes the conclusions about risk drivers or the risk drivers characterization. If this

evaluation has significant impact, decide on the optimal approach for the main

presentations and provide an appendix with an alternate approach(es), along with

an evaluation that integrates Class A, B1, B2, and C carcinogens. When deciding

on one approach over another, document the rationale for the selection and any

history of use of a particular approach. 

3.2.4.3.1 1996, 1999 and FUTURE

NATAs 

33 4

Aggregation and

Characterization of

Cancer Risks.

For the 1996 NATA, the section that discusses which HAPs are important risk drivers

should take note of the possibility that other compounds underestimated by the

model could be risk drivers.

3.2.4.3.1 1996 NATA 

34 4

Aggregation and

Characterization of

Cancer Risks.

For the 1996 NATA, please clarify this issue of the difference between seeking a

relative ranking vs. an absolute risk and the differential influence that conservative

assumptions employed when aggregating risk may have on these.

3.2.3.4.1 1996 NATA 

35 4

Aggregation and

Characterization of

Non-Cancer Risks.

For the 1996 NATA, either create the HI based on mode/mechanism of action or

remove the HI, applying it properly in the 1999 NATA. 3.2.4.3.2 1996, 1999 and FUTURENATAs 

36 4

Aggregation and

Characterization of

Non-Cancer Risks.

For the 1996 NATA, either reexamine the IRIS database and calculate target-organ

specific “RfC’s” based on NOAELs (or Benchmark dose equivalents) for each organ

considered, or delete the TOSHI. If the TOSHI are deleted here, they should be

developed (with up-to-date, target-organ specific data) for the 1999 NATA.

3.2.4.3.2 1996. 1999 and FUTURE

NATAs 

37 4

Alternative Risk

Evaluations.

For the 1999 NATA, consider running the risk analysis using alternative toxicity

values for a few key chemicals to provide a scenario-based approach for identifying

the importance of these values in the overall assessment. This action should be

taken in the near future to help inform priorities on research areas. 

3.2.4.4 1999 NATA 

38 4

Alternative Risk

Evaluations.

For the 1996 NATA, select 1 or 2 air toxics having substantial databases and

develop a risk assessment based on their data and compare it to the model results of

the current draft. For the 1999 NATA, explicitly incorporate all the credible data in

the assessments and incorporate the results of validation/evaluation research in the

selection and parameterization of models.

3.2.4.4 1996, 1999 and FUTURE

NATAs

39 4

On the Issue of

Children.

For the 1996 NATA, the discussion of children should be clarified to indicate that

they are an important life stage to be considered and therefore are already

incorporated in the chronic assessments. However, the exact degree to which these

assessments either under- or over-estimate risks to children is unknown. 

3.2.4.5 1996 NATA

40 4

On the Issue of

Children.

When future NATA’s consider less-than-lifetime exposure effects, special attention

must be paid to children, because they are likely to have different short-term

exposures and sensitivities compared to adults, and thus the risks may be different.

3.2.4.5 1999 and FUTURE NATAs

41 4

Additional

Clarification Issues

For the most part, the document is internally consistent, except for a few instances

(a through i). For the 1996 NATA, consider clarifications of the above points. 3.2.4.6 1996 NATA 

42 5 Diesel Emissions 

Diesel emissions should be included in the NATA. A specific section should be

devoted to a clear, succinct explanation of the basis for the Agency’s conclusions

regarding health risks from DEP. The section should address both cancer and noncancer risks, and links to risks attributed to ambient particulate matter. The wording

should be moderated to more accurately reflect the uncertainty of the health risks

and CASAC’s position regarding the cancer risk range in the Diesel HAD. 

3.2.5 1996 NATA 

43 6

Uncertainty and

Variability

For the 1996 NATA, use the scenario-based approach described above to represent

the uncertainty in the analysis, placing the emphasis on inaccuracies, rather than

imprecision.

3.2.6 1996 NATA 
No. CHARG

E No. SUBJECT ABBREVIATED RECOMMENDATION

SECTION WHERE

DISCUSSION CAN BE

FOUND

RECOMMENDED SCHEDULE

FOR IMPLEMENTATION

17

44 6

Uncertainty and

Variability: Specific

Comments

For the 1996 NATA, differentiate between NATA-specific and universal sources of

uncertainty, and between major and minor sources of uncertainty. 3.2.6.1 1996, 1999 and FUTURENATAs 

45 6

Uncertainty and

Variability: Specific

Comments

Use the scenario analysis to help bound the NATA risk estimates and avoid

oversimplified characterization of the “nominal” results as conservative. 3.2.6.1 1996, 1999 and FUTURENATAs

46 6

Uncertainty and

Variability: Specific

Comments

Provide more detail in the main NATA documentation on uncertainties associated

with emissions from area, on-road mobile and non-road mobile sources. 3.2.6.1 1996, 1999 and FUTURENATAs

47 7

Uncertainty and

Variability: Specific

Comments

Distinguish between reducible uncertainty (due to lack of information) and

irreducible variability. 3.2.6.1 1996, 1999 and FUTURENATAs

48 7

Uncertainty and

Variability: Specific

Comments

If uncertainty estimates are to be extended to aggregate risks, careful consideration

needs to be given to which sources of uncertainty act independently across

pollutants versus those uncertainties that simultaneously affect multiple pollutants.

3.2.6.1 1996, 1999 and FUTURE

NATAs

49 7

Uncertainty and

Variability: Specific

Comments

Should lists of high-exposure/high-risk counties be developed as part of the NATA

results, information should be provided on the key factors that determine whether or

not a county is included on the list, and the sensitivity of the list to alternative

scenarios considered in the scenario-tree evaluations. 

3.2.6.1 1996, 1999 and FUTURE

NATAs

50 7 Communications For the 1996 NATA, it would be most useful if there were an Executive Summary

that would summarize the key findings and conclusions. 3.2.7 1996 NATA 

51 7 Communications

For the 1996 NATA, at the start of each section, it would be helpful to have the

authors describe the top 5 or 6 limitations that they believe have the greatest

impact on the results/conclusions.

3.2.7 1996 NATA 

52 7 Communications 

For the 1996 NATA, the Agency especially in materials intended for non-technical

individuals, should clearly distinguish between those parts of NATA that are well

established, vs. those which are in an earlier, developmental stage.

3.2.7 1996 NATA 

53 7 Communications

For the 1996 NATA, for the lay public, it will be important to place the

consequences of exposure into the public health context. A graphic representation

such as a “thermometer” type graph could be used to display the levels at which

different health effects are seen, or to present different cancer risk levels. Whatever

approach the Agency chooses, all communication materials intended for the

general public should be pre-tested to assure comprehension.

3.2.7 1996 NATA

54 7 Communications

For the 1996 and 1999 NATA, we recommend that the Agency consider developing

a qualitative ranking with perhaps an alphabetic listing in a table of the counties

that score in the top grouping in terms of exposure and risk, but that this table be

accompanied by an indication of the factors that contribute to each county being

among the high exposure/high risk grouping, and the degree of confidence that can

be placed in these factors.

3.2.7 1996 NATA

55 8

Benefits Analysis 

 

For the 1996 NATA, results from the proposed assessment, for an information-rich

HAP such as benzene, would be appropriate for the CAAA Section 812 study and

should be considered. Descriptions of the limitations of the NATA for the CAAA

Section 812 national benefits assessment need to be clearly articulated in both the

NATA and the CAAA Section 812 studies. NATA and CAAA Section 812 study

teams should work together to assure that the important goals of these related

assessments are attained in a timely manner.

3.2.8 1996 NATA 
No. CHARG

E No. SUBJECT ABBREVIATED RECOMMENDATION

SECTION WHERE

DISCUSSION CAN BE

FOUND

RECOMMENDED SCHEDULE

FOR IMPLEMENTATION

18

56 9

Future Research

Priorities

EPA should rapidly develop a research plan to identify the work (information

collection, research, and assessments) it will perform with existing resources over the

next few years that will directly improve the 1999 NATA. This plan should be

closely linked to, and consistent with, the overall Air Toxics Research Strategy and

should be reviewed by this or similar Panel. 

3.2.9 1996, 1999 and FUTURE

NATAs
4 The Agency’s approach to reducing air toxics also includes control of criteria air pollutants, including

particulate matter (PM), ozone (O3), nitrogen dioxide(NO2), sulfur dioxide(SO2), carbon monoxide(CO) and

lead (Pb), with special focus in recent years on the precursors of PM and O3. However, the term air toxics is

usually associated with non-criteria hazardous air pollutants (HAPs) and their precursors, and efforts aimed

at criteria pollutants are not a focus of the NATA exercise. An exception is lead, which is both a criteria

pollutant and a HAP addressed in the NATA study.

5 The Integrated Urban Air Toxics Strategy is documented in 64 FR 38705. See U.S. EPA .1999. Also

available on-line at http://www.epa.gov/ttn/uatw/urban/urbanpg.html. 

6 Exposure to air toxics occurs directly through inhalation, but also indirectly due to the partitioning of HAPs

to other media, such as soil, water and food, and subsequent ingestion or dermal exposure. The 1996

NATA study considers only the direct inhalation pathway.

19

2.0 INTRODUCTION

2.1 Background

The air toxics program was authorized under the 1970 Clean Air Act and

reauthorized through the 1990 Amendments to the Clean Air Act (CAA). Since 1990, EPA

and its regulatory partners, including State, local, and tribal governments, have made

considerable progress in reducing emissions of air toxics through regulatory, voluntary, and

other programs. To date, the overall air toxics program has focused on reducing emissions

of air toxics from major stationary sources through the implementation of technologybased emissions standards. These actions, as well as actions to address mobile and

stationary sources under other CAA programs, have achieved substantial reductions in air

toxics emissions. The EPA expects, however, that the emission reductions that result from

these actions may only be part of what is necessary to protect public health and the

environment from air toxics. The Agency’s approach to reducing air toxics risks consists

of four key components4: a) source-specific and sector-based standards (e.g., risk-based

standards, under the Residual Risk Program5; area source standards, through the Integrated

Urban Air Toxics Strategy)5 (See U.S. EPA. 1999); b) national, regional, and communitybased initiatives; c) National Air Toxics Assessment (NATA) activities; and d) education

and outreach.

As a primary component of the EPA’s national air toxics program, NATA activities

include all data gathering, analyses, assessments, characterizations, and related research

needed to support the other components of the EPA air toxics program. More specifically,

NATA activities include: expanding air toxics monitoring; improving and periodically

updating emissions inventories; periodically conducting national- and local-scale air

quality, multi-media and exposure modeling; characterizing risks associated with air toxics

exposures; and continuing research on health and environmental effects of, and exposures

to, both ambient and indoor sources of air toxics. The EPA plans to use these technical

support activities to help set program priorities, characterize risks, and track progress

toward meeting overall national air toxics program goals, as well as specific risk-based

goals such as those of the Integrated Urban Air Toxics Strategy. 

As part of the NATA activities, the EPA Office of Air Quality Planning and

Standards (OAQPS) has completed an initial national-scale assessment that demonstrates

an approach to characterizing air toxics risks nationwide. This initial assessment provides

preliminary information for characterizing, on a national scale, potential health risks

associated with inhalation exposures to 32 air toxics identified as priority pollutants in the

EPA Integrated Urban Air Toxics Strategy6. In addition, the assessment examines the
20

inhalation exposure resulting from emissions of diesel particulate matter. The primary

stated goals of the initial national-scale assessment are to assist in:

a) Identifying air toxics of greatest potential concern, in terms of contribution

to population risk;

b) Characterizing the relative contributions to air toxics concentrations and

population exposures from different types of air toxics emission sources;

c) Setting priorities for the collection of additional air toxics data (e.g.,

emission data, ambient monitoring data, data from personal exposure

monitoring) for use in local-scale and multipathway modeling and

assessments, and for future research to improve estimates of air toxics

concentrations and their potential public health impacts;

d) Establishing a baseline for tracking trends over time in modeled ambient

concentrations of air toxics; and

e) Establishing a baseline for measuring progress toward meeting goals for

inhalation risk reduction from ambient air toxics.

2.2 Charge

In the months leading up to the SAB NATA Review Panel meeting, the Agency and

the Board negotiated a Charge consisting of the nine questions below as follows: 

1. Given the nature of the NTI and the methods by which it was developed and reviewed, have

available emissions data been appropriately adapted for use in this assessment? Can you

suggest improvements to EPA’s application of the NTI for use in future initial national-scale

assessments? 

a) Can you suggest improvements to the treatment of compound classes (e.g., chromium

and compounds), given the nature of the information available in the inventory?

b) Can you suggest improvements to the methods used to spatially distribute area and

mobile source emissions?

c) Can you suggest improvements to the methods used to specify default point source

emission characteristics in lieu of missing emissions data?

2. Is the approach taken for the geographic aggregation of ambient and exposure

concentrations generated by the ASPEN and HAPEM4 models appropriate in light of the

limitations of the models, the available emissions data, and the results of the comparisons of

ambient predictions with ambient monitoring data? 

3. Has available dose-response information (e.g., different sources of information, a different

prioritization scheme) been appropriately used in this assessment? Can you suggest methods

that could improve upon the use of available dose-response information?

4. What are the strengths and the weaknesses of the overall conceptual approach to risk

characterization used in this assessment? Given the underlying science and the intended

purposes of the assessment, can you suggest ways in which the risk characterization could be

improved?
21

a) Is the method used to aggregate cancer risks appropriate? The aggregation of

carcinogenic risk within two categories, based on weight-of-evidence classifications, is of

particular interest. 

b) Is the method used to aggregate non-cancer hazards appropriate? The summation of

hazard quotients within target organs, the categorization of sums by ranges of uncertainty

factors, andthe inclusion of all target organs(as opposed to only the organs associated with

the critical effect) are of particular interest.

5. Although EPA has concluded that available data are not sufficient to develop a reliable

quantitative estimate of cancer unit risk for diesel emissions, it is clear that this pollutant class

may be of significant concern in a number of urban settings. The risk characterization in this

report includes a discussion of diesel particulate matter to help states and local areas frame the

importance of this pollutant compared to the other air toxics. In the context of this assessment,

is the discussion in this report regarding making risk comparisons among other air toxics

appropriate? Can you provide any suggestions that would improve upon this approach to

comparing the toxic health effects of diesel particulate matter with other pollutants?

6. Given the limitations inherent in this preliminary assessment, have uncertainty and variability

been appropriately characterized?

a) Can you suggest ways that the characterization of uncertainty and variability could be

improved, made more transparent, or integrated more effectively into the risk

characterization?

b) Can you suggest methods for quantifying individual as well as composite uncertainties

associated with the emissions inventory, dispersion modeling, exposure modeling, doseresponse assessment, quantitative risk estimates, and accumulation of risk across air

toxics?

7. Have the results of the assessment been appropriately and clearly presented? Can you

suggest alternative methods or formats that could improve the presentation and communication

of these results?

8. The exposure methodology in NATA is being considered as one candidate for providing the

basis for a national scale benefits analysis (as required in Section 812 of the CAA). Please

comment on the strengths and weaknesses of this approach, recognizing the limitations outlined

in the NATA report. 

9. Do you have suggestions for research priorities that would improve such air toxics

assessments in the future?

2.3 SAB Review Process 

The SAB Panel was recruited following nominations received from SAB Members

and Consultants, the Agency, and outside organizations. The group met in public session on

March 20 -21, 2001 at the Radisson Governor’s Inn in Research Triangle Park, NC. 

Written comments were prepared before, during and after the meeting by Panel members

and consultants, and made available at the meeting, which formed the basis for this report. 

A more detailed description of the SAB process for this review can be found in

Appendix A.
22
23

3. EVALUATION OF THE DRAFT 1996 NATA

3.1 General Findings

The Panel found that the draft NATA 1996 document (U.S. EPA/OAQPS, 2001)

represents an extensive and comprehensive effort to systematically evaluate and link the

various components of the risk paradigm relevant to HAP impacts, including emissions,

atmospheric transport, human exposure and risk. In the absence of widespread

measurements, the 1996 NATA relies on modeling to estimate some elements of the

emissions inventory, as well as ambient concentrations and exposures. While some aspects

of the current data collection and modeling are advanced enough for confident prediction,

others are still highly uncertain. An expanded set of measurements is needed to evaluate

and develop confidence in the models, and to provide independent information about spatial

distributions and trends over time. 

As part of our review, we have identified specific areas where the current NATA is

especially problematic. Some of these difficulties can and should be addressed for the

current 1996 assessment. Others suggested improvements will require a more long-term

effort, and should be targeted for the 1999 and future NATA’s. In the recommendations

that follow in this advisory, short- vs. long-term targets for implementation are identified.

The development of a nationwide assessment of air toxic emissions, atmospheric

transport, human exposure and risk is a daunting task, and the Agency has had to make a

number of choices cognizant of the limitations in scientific understanding, available data,

and the time and resources available for the assessment. A key choice has involved the

selection of the spatial scale of aggregation for conducting the NATA, and for reporting the

results. The census tract is utilized as a basis for estimating emissions (at times inferred

from information at higher levels of aggregation, such as the county level), predicting

atmospheric transport, defining receptor populations, and computing their exposures and

risks. The results are then aggregated back up to the county level for reporting purposes. 

While we agree with this basic strategy for assessment and reporting, there are a number of

difficulties that arise in its implementation. 

The census tract is a good unit for defining the demographic characteristics of

receptor populations, but it is not a good geographic unit for air pollution modeling and

assessment. In particular, densely populated census tracts are small, while those in sparsely

populated areas tend to be large. This tends to misrepresent the allocation of emissions and

bias the calculation of representative ambient and exposure calculations for densely vs.

sparsely populated areas. This problem needs to be identified in the current NATA, and

addressed in future NATAs through conversion to a regular spatial grid for emissions

tracking and the calculation of ambient concentrations, with subsequent conversion back to

underlying census tracts for population exposure and risk calculations. 

A major finding of the Panel is that parts of the NATA are based on relatively

reliable data and/or well-established scientific estimation and modeling methods, while

other aspects are based on more limited data and methods that are in an earlier,

developmental stage. This applies to all aspects of the NATA, including emissions

estimates, estimates of ambient concentrations based on the ASPEN model, estimates of

exposure based on the HAPEM modeling system (or, as suggested in our report, other,

simpler methods that should be considered in parallel with the HAPEM predictions), and
24

risk estimates requiring the use of toxicity values based on different amounts of scientific

information and consensus. To help citizens and other users of NATA better understand the

differing bases for NATA results, we recommend use of a hierarchical presentation of

results that distinguishes between quantities measured or modeled at different levels of

scientific development, and with differing levels of available data and confidence.

The scientific basis for EPA’s NATA will continue to evolve as new data and

improved methods are developed for estimating emissions, concentration, exposures and

health effects. It is thus important for the Agency to carefully document the changes in

methodology used for successive NATA’s. The current NATA document is largely

successful in meeting this objective (though further changes are expected in response to

the specific recommendation provided in this report). It is also important for the Agency

to maintain the capability of updating past NATAs as new ones are performed. This is

essential for the Agency in meeting the fourth and fifth goals (see end of Section 2.1 of

this report) of establishing a baseline for tracking trends and progress in reducing air toxics

emissions, concentrations, exposures and risks. In this manner, the NATA may be used by

industry, the states, citizen groups and other stakeholders as a basis for improving and

validating their data inputs and better focusing their efforts for data collection, risk

management and risk communication.

While we have attempted to provide specific information and recommendations to

improve the 1996 and future NATA studies, we recognize that much of the need for

improved information applies generally to the field of air toxics and risk assessment and is

not specific to the NATA. When uncertainties and concerns are apparent in the NATA

methodology, we have attempted to distinguish between those specific to NATA and those

more broadly applicable across the field of environmental health risk assessment. We also

note that we have focused on the general methodology presented in the NATA document,

and not the specific values of inputs and parameters used to implement it (though specific

examples are identified to be illustrative of apparent problems and areas of concern). The

absence of comment on specific emission, atmospheric transport, exposure and toxicity

factors should not be construed to indicate Panel review and approval of these values. 

Separate peer review is required for the specific parameter values and factors used to

implement the NATA.

Recommendation #1: Separate peer review should be conducted for the specific input

parameters and values assumed for the different modules of the NATA model.

3.2 Responses to Specific Charge Questions

3.2.1 Charge Question 1

Given the nature of the NTI and the methods by which it was developed and reviewed,

have available emissions data been appropriately adapted for use in this assessment? Can you

suggest improvements to EPA’s application of the NTI for use in future initial national-scale

assessments? 

Given the enormity of this task, the Agency has made a valiant effort to compile a

model-ready national air toxics inventory for the point, area, on-road and non-road source

sectors for 1996. The NATA document (U.S. EPA/OAQPS, 2001) appropriately

acknowledges the limitations in the information and implications of this for the

development of the 1996 NTI. The Emissions Modeling System for Hazardous Air
25

Pollutants (EMS-HAP) which was developed to process the emissions inventory data for

subsequent air quality modeling (see Appendix C of the NATA report) is impressive. 

However, there are a number of steps that should be taken to further improve the accuracy

of the results of the assessment and reduce the uncertainties. Our comments address

improvements that could be considered in future applications and iterations of the NTI and

the National-Scale Air Toxics Assessment (NATA). They specifically address

improvements for the collection of raw HAP emission inventories and the application of

EMS-HAP for the various source sectors (i.e., point, non-point, on-road and non-road

sources).

3.2.1.1 National Toxics Inventory (NTI)

Improvements in the development of the 1996 National Toxics Inventory (NTI) are

evident when compared to the inventory that was prepared for the 1990 Cumulative

Exposure Project (CEP). There are significant differences in the national emissions totals

between the two studies presented in Table 4-4 of the NATA report. We believe that much

of this difference is a result of improved data, progress made by the Agency in resolving

the emissions inventory discrepancies, and the development of more advanced emission

inventory methodologies. The emission inventory developed for the CEP relied heavily on

VOC and PM emission estimates from an interim 1990 National Emissions Trends (NET)

Inventory. The criteria pollutant emissions were converted to individual HAP emissions via

speciation profiles which are now considered dated and are no longer used by the Agency to

estimate HAP emissions. We are supportive of the iterative approach taken by the Agency

to improve the emissions inventory and continue to view the development of future national

air toxics inventories as a work in progress. The inclusion of emission and facility

specific information collected by State and Local Air Pollution Control Programs for point

sources represents a significant advancement in this effort. 

The Table 4-5 Facility Count Summary by state provides the reader with some

insight about the extent of the state point and area source inventories that were available to

the Agency in developing the 1996 NTI. We understand that there could be some overlap

between the NTI and the NET, so the word “unique” should be removed from the Table

since it may suggest to the reader that the two inventories are mutually exclusive of one

another. We agree that the NET provides a good resource for checking NTI’s

completeness. A quick examination of the NTI/NET facility count ratio indicates a range of

0.07 to 4.60. We are concerned that facilities may be missing from the 1996 NTI in states

where this ratio is well below one. This would result in an underestimation of emissions

for these states, directly impacting predicted ambient ASPEN concentrations and

subsequent risk predictions. 

In the next round of data collection for the 1999 NTI, the Agency should consider

implementing some quality assurance/quality control measures to ensure that a satisfactory

level of completeness and accuracy is achieved. This would include a careful review of the

NET facility files for the states with extremely low ratios to determine how many HAP

point and area sources are missing. Once these facilities are identified, an effort could be

undertaken with the affected state or industry to review the necessary raw HAP emissions

information. The current emission inventory format developed by the Agency in the AIRS

database, which lists the HAP emissions associated with each facility, provides an excellent

way to efficiently review and verify the large amounts of emissions information. The

identification of all missing point sources in the NTI will be a difficult task. The best

future solution will be the development of a consistent national HAP emissions inventory
26

data collection and reporting rule, with proper incentives for industry to participate and

comply. This would help to eliminate the potential bias of missing facility emissions and

the associated underestimation of exposure and risk that currently exist for these point

sources in the 1996 NATA.

Recommendation #2: For 1999 NATA, implement additional QA/QC measures to

ensure that a satisfactory level of nationwide completeness and accuracy is achieved for the

point and area source emission inventories.

In future NTI assessments of on-road emissions, the Agency should make an effort

to incorporate State and Local Air Pollution Control Program data for on-road emissions. 

Some States have county specific (vehicle miles traveled) VMT and VOC data sets that are

prepared as part of their State Implementation Plans (SIPs). The NTI uses HAP vehicular

emission factors generated by MobTox5b and then multiplies them by county VMT

estimates that are based on a population surrogate. An analysis comparing the VMT

estimates for the New York Metropolitan Area prepared by the EPA and New York State

indicated large differences in emission estimates (NESCAUM, 1999). The state VMT

estimate in the NESCAUM report is based on actual vehicle count data from the

Department of Transportation. The EPA VMT estimate is based on a population surrogate. 

In the above data sets, the patterns resulting in county differences in VMT indicate that the

EPA method will result in underestimation of on-road emissions in more suburban

counties, while largely overestimating on-road emissions in urban counties. Estimating

VMT on state populations will also not reflect on-road emission increases in those states

which have a significant seasonal increase in transient populations (e.g., tourists).

In addition, Colorado’s Department of Public Health and Environment sent EPA an

analysis that suggested that HAP inventory estimates developed by the Agency in the draft

NATA for seven Colorado counties were higher than what would have been estimated using

more refined input parameters from the State of Colorado (Silva and Wells, 2001). Using

default values for input variables, such as average vehicle speed and the percentage of cold

starts, can result in the underestimation or overestimation of local scale inventories. In

future NATA assessments, on-road models that incorporate state- or urban-specific input

variables (e.g., vehicle speeds, vehicle fleet type and age, etc.) should be developed to

estimate on-road HAP emissions. 

The NY State Department of Environmental Conservation (NYSDEC) attempted to

verify the HAP emission factors generated by the MobTox 5b model (NESCAUM, 1999). 

To address this problem the MobTox input files were placed into the Mobile Model which

generates emission factors for total organic gases (TOG), but not air HAPs. These TOG

factors were then compared to the VOC emission factors generated in the SIP

demonstration for the New York Metropolitan Area (9 counties). The results of this

analysis indicated that EPA’s MobTox inputs tended to underestimate TOG emissions, at

least for New York City, which suggest that HAP emissions are similarly underestimated. 

The development and application of the hydrocarbon mass metrics used to generate HAP

emission factors by MobTox 5b needs to be discussed in more detail to create transparency

for this critical portion of the emissions inventory.

Recommendation #3: For 1999 NATA, continue the development of the on-road model to

accept input parameters developed by State and Local Air Pollution Control Agencies for the

development of the 1999 on-road emission inventory. Provide more detail on how the on-road

HAP emission factors for the MobTox 5b model were developed. 
27

The determination of the non-road emission inventory appears to be one of the

weakest links in the NATA document (U.S. EPA/OAQPS, 2001). The NATA document

does note the limitations associated with the development of the nonroad emissions

inventory and acknowledges the recent 202(l)(2) rulemaking which outlines a research

strategy to improve the non-road emissions inventory for future NATA studies. We

reviewed Appendix C and the paper on the Geographic Allocation of State Level Non-Road

Engine Population Data to the County Level (9/16/98) to take a more in-depth look at the

factors used in NATA 1996 for determining and allocating non-road emissions. The

document indicates that non-road construction equipment emissions were estimated by

assuming there was a proportional relationship between the dollar value of construction and

the amount of construction in a given area. This is not a good surrogate to use when

estimating these emissions for urban counties in the northeast and perhaps in some other

areas of the country where housing and commercial building prices are extremely high. For

example, the relative contributions of non-road diesel PM contributions are unrealistically

high for the NYC Metropolitan counties. While the dollar value of construction is high in

these counties, less of this construction is at new sites where non-road diesel is used

extensively for earth moving. Rather, construction occurs more at existing sites, where

the ground is already level (and, for example, much of the work is done by in-place cranes). 

A similar over-estimation of non-road diesel emissions is likely to occur in other urban

areas that are already highly developed, given that these emissions are based primarily on

the dollar value of construction. 

The relationship between the cost of construction expenditures and non-road diesel

emissions varies across the country and the potential impact of the use of this emissions

surrogate needs to be evaluated in future NATA assessments. This factor may also be

impacting emission estimates for other HAPs (besides diesel) associated with nonroad

construction (e.g., formaldehyde, benzene, acrolein, and acetaldehyde) in these urban areas. 

Recommendation #4: For 1999 NATA, critically re-evaluate surrogates used to

estimate the non-road emissions inventory and make adjustments where necessary. Continue

the development and verification of the non-road emission inventory and non-road model for

future iterations of NATA by expanding the research agenda to fill known important data

gaps. These data gaps should be prioritized to reduce the most significant uncertainties

associated with the non-road emission inventory and model predictions.

3.2.1.2 Reactivity Class Decay Rates 

The reactivity categories and decay rates should be identified for each HAP modeled

in the NATA. We are specifically concerned about how EMS-HAP handles emissions of

1,3-butadiene, a chemical that undergoes rapid decay in the daylight (estimated half-life =

1.6 hours), but slower decay at night (estimated half-life = 9 hours) ( CARB, 1992; Harley

and Cass, 1994). We believe that EMS-HAP processing should account for seasonal

variations in decay rates. Critical assumptions and uncertainties associated with the

assignments of reactivity classifications for HAPs, and decay rates for various stability

categories for modeling should be discussed in more detail. It is important for this

emissions characterization and processing aspect of NATA to be scientifically defendable.

Recommendation #5: For 1996 NATA, reactivity categories and decay rates should be

identified for each HAP modeled in ASPEN. Critical assumptions and uncertainties

associated with the assignment of reactivity classifications for HAPs should be discussed.
28

Recommendation # 6: For 1999 NATA, update reactivity categories assignments and

decay rates by incorporating HAP specific information when available. For HAPs identified

as important risk drivers or regional contributors evaluate the impact of the assumption that

each pollutant within a specific reactivity class is assumed to decay at the same rate.

3.2.1.3 Temporal Allocations 

The use of the eight 3-hour blocks to calculate annual ambient concentrations for

each time block in each census tract is a strong feature for anticipated downstream uses. It

allows HAPEM to account for daily variations in HAP exposure by using the activity

patterns for the point, area, onroad and off-road source sectors as presented in Appendix D

of the EMS-HAP Users Guide. The emissions Equation 5-1 in Appendix C provides an

excellent example of how emissions are divided to provide a grams/second emission rate

for each three-hour period during the day. For example, emissions rates for mobile source

HAPs are higher during the 3-hour blocks which contain rush hours. Therefore, the

potential HAP exposure while driving or walking during these time periods would be higher

and can be accounted for by activity patterns contained in HAPEM. Figure 3-3 provides an

excellent example of the daily fluctuations of a HAP concentration overlying the daily

activity scenario of a cohort. This appears to be a very good approach for capturing daily

variability in ambient exposure concentrations in relation to activity patterns. 

It would be interesting to see the range of predicted daily values for some of the

HAPs identified as risk drivers in future assessments. While the approach for diurnal

dissaggregation of emissions is appropriate, we do note in the following section that, in its

coupling with HAPEM, ignoring seasonal variation and using a sequence of independently

sampled person-days to represent annual exposure does lead to a misrepresentation of

long-term individual to individual variations in exposure, and that the result may only be

appropriate for estimating the median (rather than the full distribution of) exposures in a

census block or county. 

3.2.1.4 Quality Analysis and Quality Control (QA/QC) and the Reduction of Uncertainties 

Under Section 3.5.2.6 of the Agency’s Guidelines for Exposure Assessment, it is

stated: “Any data developed through previous studies should be validated with respect to

both quality and extrapolation to current use. One should consider how long ago the data

were collected and whether they are still representative.” Although the Agency stated in the

report that it went through three rounds of review with state and local agencies, this review

process was apparently not stringent enough to be considered as a QA/QC evaluation. This

is pointed out in the NATA document, when it states that, “EPA has not undertaken a full

QA/QC evaluation of the NTI,” (page 56) and “EPA did not attempt to verify the methods by

which emissions were estimated or undertake a full quality control evaluation of the NTI”

(page 104). The results of any assessment conducted by using models can only be as good

as the quality of the input data used for the analysis. The importance of QA/QC processes

is obvious and the needs for further reduction of the uncertainties stated in subsequent

discussion in this review report should also be clear. 

Recommendation # 7: For 1999 NATA, implement additional QA/QC measures to

ensure that a satisfactory level of completeness and accuracy is reached for all emission

inventories. 
29

a) Can you suggest improvements to the treatment of compound classes (e.g., chromium and

compounds), given the nature of the information available in the inventory?

While in some instances ignoring speciation effects for an element or grouping

compounds with similar behavior can lead to beneficial simplifications for analysis, this

must be done with great care. The grouping of chromium compounds to improve modeling

efficiency creates downstream problems for the proper risk characterization of these

compounds and introduces more uncertainty than necessary. The issue of how much

hexavalent chromium (Cr6+) is present in total chromium stack and ambient measurements

has been investigated by numerous researchers over the past decade (Bell and Hipfner,

1997; Grohse, et al, 1998; Scott et al., 1997). The use of the assumption that 34% of the

total ambient chromium is present in the carcinogenic hexavalent form clearly results in

regional over and underestimations of risk. Chromium compounds should not be grouped

and should be segregated based on valence state using the SIC codes when the inventory is

developed. For example, census tracts which contain chromium electroplaters or chromate

production facilities will have a much higher proportion of ambient Cr6+ than census tracts

impacted by municipal waste combustion facilities. The Agency should apportion Cr6+ for

each source category in the EMS-HAP stage and have two separate inputs into the model as

chromium and Cr6+ using the available literature on this subject. Different fate-andtransport factors for chromium and Cr6+, such as reactivity decay rates, should also be

utilized for ASPEN and other transport model calculations.

The use of the assumption that 65% of the predicted total ambient nickel is

insoluble and in the crystalline form is a conservative assumption for assessing cancer risk. 

It is more conservative than the 50% assumption used in the Utility Study (EPA, 1998a). 

The Agency should investigate if the available literature on this issue would support a

source-specific speciation approach as suggested above for Cr6+. 

Given the available emissions information for polycyclic organic matter (POM), the

grouping of POM species into two groups is appropriate. The inclusion of the toxicity

equivalency factors (TEF) approach for dioxin compounds in EMS-HAP is also appropriate.

Recommendation # 8: For 1999 NATA, the Agency should apportion Cr6+ for each

source category in the EMS-HAP stage and have two separate inputs into the model as

chromium and Cr6+ using the available literature on this subject. In addition, a reactivity

decay rate will have to be developed and incorporated into EMS-HAP for Cr6+.

b) Can you suggest improvements to the methods used to spatially distribute area and mobile

source emissions?

The Agency recognizes the uncertainty associated with estimates for area and

mobile emissions sources that are compiled on a county-wide basis, and then allocated

using spatial allocation factors (SAFs) to census tracts within the county. While it is

difficult with current information to estimate emissions from these sources and to allocate

the emissions in a more refined manner than is currently done in the NATA, suggestions are

provided for future NATAs.

EMS-HAP handles point source location defaulting within census tracts by

eliminating census tracts with a radius less than or equal to 0.5 km, because the ASPEN

model would calculate excessively high concentrations for these small areas. A default

consolidation mechanism should also be developed for area, on-road and non-road
30

emission census tract spatial allocations in these small census tracts. A possible spatial

allocation method for future iterations of NATA is discussed below.

The initial screening assessment may result in the generation of biased results, since

the annual average concentrations computed by county and state are greatly influenced by

area (e.g. square miles) and population densities. Therefore, in future iterations of NATA

the Agency should consider an alternative approach before there is any attempt to

characterize potential public health risk due to the inhalation of air toxics. This step would

involve the isolation of counties with major metropolitan areas and the mapping of all

averages in these locations using a uniform regular spatial grid approach for emissions

tracking and calculation of ambient concentrations. Once ambient concentrations are

computed for each point on the grid, concentrations in each census tract and county would

be computed as the average of the appropriately assigned grid points. This would remove

the dilution effect of using large areas and would limit the influence of small census tracts,

since the size of a census tract is based on population density, not source activity. Source

activity should determine the magnitude of predicted concentrations. This type of analysis

in future NATAs would provide results that are directly comparable from one metropolitan

area of the country to another. For the current NATA, the Agency should consider

developing a quantitative measure of the extent to which the variable size of the census

tracts can distort the concentration and exposure estimates.

Our concern is illustrated by the following brief discussion. Those counties in

highly populated areas are predicted to have higher average concentrations while those in

the lower population areas have lower predicted concentrations. While this is in part due to

the presence of some air toxics sources (particularly area and mobile sources) that do

properly correlate (to some extent) with population, it also occurs because census tracts

are not uniform in size: some may be as small as 0.03 km2 while others are as big as 3084.2

km2. Thus for the smaller census tracts, concentrations are calculated much closer to the

source and therefore tend to be much higher on average. In larger tracts, however, the

average concentration may not be representative of average exposure concentrations,

especially where the population is more concentrated near urban (or industrial) sources. 

The results indicate that the distributions in the larger tracts represent the averages of the

averages. Therefore, when you look at predominantly rural States you observe very narrow

bands of concentrations. In contrast, there is a wider distribution of concentrations in more

highly populated States. Many of these smaller distribution bands may be valid, while

others may not. As a result, small urban areas which may be of public health concern could

be missed or overlooked. The approach taken to properly identify and characterize

locations with high air toxics exposure will be critical in developing future risk

management strategies. 

Recommendation #9: For future iterations of NATA, consider an alternative

modeling approach for counties with major metropolitan areas and small census tracts which

would involve the mapping of all averages using a uniform grid approach. This type of

analysis would provide results which are directly comparable from one metropolitan area of

the country to another.

 c) Can you suggest improvements to the methods used to specify default point source emission

characteristics in lieu of missing emissions data?

The point source defaults used in the NATA for location and stack parameters are

conservative approaches and appropriate. While it is reasonable to enter some default stack
31

data for modeling purposes, it is not reasonable to use these values to create default

emission data for facilities where all aspects of the needed data are missing. EPA must

work with the facilities themselves and State and Local government agencies to gather

realistic information. In most cases, it is better to enter no information at all than to create

surrogate emissions data for specific plants and facilities (though efforts to estimate the

overall magnitude of omitted emissions in a county or a census track may be appropriate

when known emitters have been left off of the inventory. 

Some suggestions for removing stack parameter defaults for facilities that have not

provided actual stack information would be to request information from the states for stack

testing information which should be available for NET sources in many states, and ask the

states or industry if they could summarize any stack parameter information contained in the

test reports. This would entail a large effort, but it would help to avoid the use of default

parameters and refine the results and contributions to exposure and risk from the point

source inventory.

Recommendation # 10: For 1999 NATA, to avoid the use of default stack parameters,

request that State and Local Air Pollution Agencies or industry summarize any stack

parameter information contained in stack test reports if available for facilities that have been

assigned default stack parameters.
32

3.2.2 Charge Question 2 

Is the approach taken for the geographic aggregation of ambient and exposure

concentrations generated by the ASPEN and HAPEM4 models appropriate in light of the

limitations of the models, the available emissions data, and the results of the comparisons of

ambient predictions with ambient monitoring data? 

3.2.2.1 General Comments 

The NATA efforts at modeling HAP airborne fate, transport and exposure represent

a serious, diligent effort; and the USEPA NATA team should be commended for this work. 

A substantial effort has been made explaining and explicitly documenting caveats and

limitations of the individual components and steps of the NATA approach. The choice of

the census tract as a statistical receptor/exposure unit is a good starting compromise that

allows for future coupling with multimedia/multipathway assessments. The choice of

county-level aggregation for the presentation of results is generally appropriate (for most

of the air toxics considered) as long as limitations and caveats are clearly identified. 

The local (rather than national-scale or even long-range) character of ASPEN

calculations offers the practical advantage that it allows for independent local evaluation

and refinement of estimates by State and local agencies. Since ASPEN incorporates wellestablished practices and techniques that local agency personnel should be quite familiar

with, it should be expected that such local evaluations would be straightforward and

productive. Clearly, the NATA effort represents work in progress; it should be expected

that refinements and changes in the NATA approach will take place in both the present and

future phases. In particular, HAPEM4 is an essentially new (for the field of air toxics) and

potentially valuable element that has been added in this phase. This is the new component,

that, from a methodological point of view, takes us from the ambient concentration-based

approach of the CEP, to an actual population exposure assessment process. It is important,

however, in order for a local application, evaluation, and refinement process to be

successful – in fact, in order for such a process to start in the first place – that sufficient

guidance and support be provided by USEPA to the State and local agencies regarding the

use of new tools, such as HAPEM4. The Agency should provide the necessary resources

so that, at a minimum, detailed and thoroughly tested user guides, that fully explain the

methods and rationale behind the HAPEM4 approach, combined with demonstration case

studies, are developed and provided to the State and local agencies. 

As with every new effort, there are problems with data gaps, etc., nevertheless, the

incorporation of HAPEM4 into the NATA process is a step in the right direction. It is

important that the NATA team distinguish between successes and failures, and identify

causes for both. In fact, it is important to ask not only why a model fails in a modelobservations comparison, but, also, if a model performs well, if it does so for the right

reasons. 

For ASPEN, HAREM4, and all other models that might be used in future NATA

studies, the Panel emphasizes the need for continued, improved monitoring and data

collection to allow validation with measured data in support of the assessment. An

expanded set of measurements is needed to evaluate and develop confidence in the models,

and to provide independent information about spatial distributions and trends over time. In

this, we would also like to reiterate a critical comment that was made during the SAB’s

review of the Cumulative Exposure Project (Phase 1) in 1996, which was the genesis of the
33

1996 NATA. The current NATA Review Panel still believes this comment to be very

relevant today. “ We also encourage the Agency to begin examining ways in which

environmental data collected for regulatory purposes might be collected in ways that would

make these data simultaneously useful for scientific purposes. With some thought, . . . it

should be possible to develop improved guidelines for the collection of some

environmental data so that it could be used for the dual purpose of assessing regulatory

compliance and advancing environmental science in order to improve the future protection

of public health.”

3.2.2.2 Specific Concerns and Recommendations 

The following is a list of major concerns and areas for possible improvement

regarding the specific application of ASPEN in NATA. It should be noted that ASPEN

relies on a standard Gaussian plume model formulation (specifically the Industrial Source

Complex [ISC] model) and therefore has the well-known inherent limitations of Gaussian

models, such as the inability to handle nonlinear chemical transformations or dispersion of

contaminants in complex atmospheric flow fields (e.g. sea and lake breezes, etc.). In fact,

some of the concerns discussed below arise precisely from the attempt to apply ASPEN, in

the NATA approach, to situations that are beyond the range of applicability of its underlying

classical Gaussian plume model formulation. 

The study attempts to use a single model, ASPEN, to model the fate and transport of

each HAP. ASPEN is principally designed for use in predicting ambient concentrations of

primary pollutants under relatively simple transport conditions. While modifications to

ASPEN are made to attempt to account for secondary pollutant formulation, these

modifications are generally ad-hoc and do not account for the fundamental nonlinear and

time-variable (diurnal and seasonal) reaction kinetics that control secondary pollutant

formation. These processes, as well as a number of complex terrain and meteorological

effects (e.g., regular patterns of on- and off-shore, sea-breeze winds), have important

regional and seasonal components that are not captured by ASPEN. The Agency should

identify where the model is applicable and works well, and where it does not, and correct

and refine the modeling approach for these applications.

There is limited quality assurance of available input data (especially emission

inventories). The Agency should adopt the use of visual GIS-based tools for inventory

development/testing and for emissions preprocessing. 

There is no consideration of regional/seasonal variability of background (in fact, no

clear definition of what is meant by background is given). The NATA report should define

what background is; perform refined statistical analysis to identify trends and clustering in

background concentrations; and consider in the future simplified seasonal grid-based

modeling for the prediction of background. 

The ASPEN model assessment provides no consideration of long-range transport

(LRT). The study should identify specific toxics with LRT concerns and perform grid

based modeling (as e.g. in the CMAQ [Community Multiscale Air Quality] Hg modeling

project). 

There is no consideration of seasonal patterns in the local ASPEN calculations (in

addition to diurnal variation). In reality, both meteorology and emissions (as well as

chemical transformations) can exhibit strong seasonal patterns and dependencies. For
34

example, there is often a significant temperature dependence for fugitive emissions that

occur via volatilization, so that emission rates will differ as a function of season. 

Similarly, certain activities that generate HAP emissions (such as lawn mower use in

northern states) have a strong seasonal component; distributing these emissions uniformly

over the year is inappropriate. Seasonal emissions preprocessing and seasonal evaluations

of NATA should be used in the next iteration of NATA (i.e., for the 1999 assessment). 

The ASPEN model is restricted to an overly simplified, inappropriate treatment of

secondary air toxics (such as formaldehyde, acetaldehyde, and acrolein) that exhibit

nonlinear chemistry. This problem is emphasized by the inconsistency of the ASPEN

estimates with the OZIP (OZone Isopleth Plotting program) predictions for the percentage

formed versus emitted, and the known dependencies of photochemical transformations on

the variability of ambient conditions. The NATA 1996 study should specifically state the

uncertainties and limitations associated with the treatment of secondary species involved in

complex (nonlinear) photochemistry (as discussed further in the following), and the

Agency should plan for development of a more appropriate approach for the next phase. As

useful background, the Agency needs to more clearly state what it did to predict secondary

species formation in the NATA document and not rely heavily on referenced reports for

this description.

The NATA study provides for no consideration of regional limitations in the ASPEN

model applicability, and the corresponding increase in model structure-related uncertainty

in areas with complex terrain, sea/lake breeze effects, or other conditions not addressed by

the ASPEN model. The NATA report should incorporate regional limitations in uncertainty

characterizations by defining topographical/climatological regimes associated with ASPEN

applicability (i.e., regimes with different structural uncertainty ranges). 

There is no consideration of how representative (in addition to complete) the

meteorological data are, in particular, with regard to where stations are located relative to

emissions and exposed populations. Maps should be provided indicating the locations of

the meteorological stations versus the above topographical/climatological regimes and the

distribution of census tract centroids. 

The Agency has conducted very little diagnostic evaluation of ASPEN. The limited

available HAP monitoring data from across the US should be used in an informal, case-bycase, diagnostic analysis, to answer questions such as: Does the model perform better in

cases where parametric/input uncertainties are lower? Does the model perform better

where model structural uncertainty is lower (i.e., where confidence and applicability are

expected to be higher)?

The report utilizes inconsistent or ad hoc terminology for terms such as ‘nationalscale’ (rather than “national level” or “nationwide”), ‘background’, ‘cumulative/aggregate’,

‘grid model’ (a term used the for OZIP – which is based on a single box formulation), and

‘exposure-related’ (rather than demographics-related). There should be an attempt to

streamline the terminology and semantics conventions used in the report. 

To address these and other uncertainties, we recommend that for the 1996 NATA,

the air toxics considered be classified in terms of where ASPEN is expected to provide

reasonable results. We recommend three categories: confident; in need of

improvement/refinement; and uncertain. Secondary compounds, such as formaldehyde, that

are formed in the atmosphere through nonlinear chemical reactions, should be placed in the

uncertain category, as should compounds for which background concentrations were found
35

to dominate. The secondary formation of formaldehyde, acetaldehyde, and acrolein in the

ASPEN model is calculated by estimating the amount of known precursors which would

react in the atmosphere (based on annual average decay rates) and then estimating the

amount of product which that amount would form (based on average stoichiometric

coefficients or a weighted emissions scheme). More specifically, ASPEN tries to account

for secondary species by adding a surrogate "precursor" species that can then be transported

like any other species in the dispersion model. Emissions of the precursor species are

calculated as a weighted sum of the emissions of some of the species whose reactions lead

to formation of the compound. For formaldehyde, for example, emissions of 23

compounds are included in the precursor sum. The approach used to modify ASPEN for

application to secondary pollutants does consider relative humidity and nitrogen dioxide in

the atmosphere in a parametric way, as well as sunlight and temperature to some extent (see

pages 5-9 through 5-11 of the CEP report “Modeling Cumulative Outdoor Concentrations

of Hazardous Air Pollutants,” SYSAPP-99-96, February 1999, available at

www.epa.gov/oppecumm/air/air.htm. (U.S. EPA, 1999a), however, this approach is not

based on the fundamental reaction kinetics represented in photochemical air pollution

models.

The first specific problem with the ASPEN approach for predicting the formation of

compounds, such as formaldehyde, is that these compounds are generally formed as a result

of the reaction of many precursor species. For example, formaldehyde is formed as a

product of many more than the 23 primary organic compounds considered in the current

ASPEN formulation. It is also a product of many secondary compounds (e.g., higher

aldehydes and ketones) that a weighted emissions scheme cannot capture. Another problem

is that the extent of reaction of the primary species (and hence the amount of secondary

species production) depends on spatial, diurnal and seasonal variations in relative humidity,

sunlight intensity, temperature and the amount of other organic compounds and nitrogen

oxides present in the atmosphere that cannot be captured in detail with the current,

aggregate ASPEN approach. The reaction systems are very nonlinear, in that formaldehyde

and acetaldehyde themselves react to produce radicals that speed the production of

secondary species from other organic compounds. These new secondary species include

more formaldehyde and acetaldehyde. Lacking a detailed treatment of the coupled

chemical reactions of many compounds, the ASPEN model cannot properly account for

these nonlinear interactions. 

All the (known or suspected) reasons for assigning an air toxic to one of the three

categories (confident, applicable but in need of improvement, and uncertain) should be

listed and clearly explained in the report. For example, it has been pointed out that

potential causes for ASPEN underpredicting monitored values for metals involve both (a)

inadequacies of emission inventories; and (b) the fact that the metal monitors are generally

located next to sources (i.e., in a "hotspot"), and the ASPEN modeling approach is not

finely resolved enough to capture these hotspots.

The document (U.S. EPA/OAQPS, 2001) should also classify geographic regions in

terms of ASPEN’s expected performance. Areas with complex terrain or meteorology

should be distinguished from areas where Gaussian-type models are most applicable. 

Furthermore, in future assessments, the air quality modeling should be improved by

capturing seasonal variations in emissions and fate and transport for all of the toxics. 

Priority should also be given to the adaptation and application of developing models such as

CMAQ (Community Multiscale Air Quality model, a component of USEPA’s Models-3

system) that are capable of treating secondary compounds and long-range transport of toxic
36

air pollutants. Since these models often predict ambient concentrations on a coarser grid

than that obtained with local air quality models, methods for interpolating predicted

concentrations on to the finer grid (necessary for development of a consistent set of

predicted concentrations and exposures across the modeled HAPs) will be needed. 

In contrast to the well known methods (and of their limitations) incorporated in

ASPEN, HAPEM4 represents application of relatively new, and therefore not as welldeveloped or –tested, methods for assessing personal exposure to air toxics. Most

applications of exposure assessment of this type have been limited to the criteria pollutants

(CO, O3, PM). The benefits of including the HAPEM4 calculations in the overall NATA

process are significant. In particular, the incorporation of HAPEM4 sets a framework in

place for the future – allowing iterative improvements in exposure assessments. 

Specifically, the current application allows correction for the fact that census tract

populations are not concentrated at the tract centroid (even if this is the only concentration

calculated for the tract). Also, the commuting feature in the current HAPEM4 applications

allows cohorts to move from tract to tract: this can be very important in urban areas with

large concentration gradients from tract to tract.

The limitations of this first use of HAPEM4 for NATA have been presented in

considerable detail in the NATA document and indeed, they are not trivial. Of particular

concern are:

a) the use of single, best value estimates rather than statistical distributions for

microenvironmental parameters;

b) that there is no consideration of geographic or seasonal variability in

microenvironmental parameters; and

c) indoor sources are not considered in this phase. While not a scientific/technical

limitation per se, this could present some problems when comparing predicted

exposures to monitored personal exposures, and in communicating the relevant

results in an effective manner. 

Another serious issue is the artificially low variability in exposure calculated by

HAPEM4 within each census tract. It is understood that this occurs since, (a) the current

variability predicted by the model reflects only demographic variability, since ASPEN does

not consider air quality gradients within a tract; and (b) the demographic variability is not

adequately represented because the current treatment fails to incorporate day-to-day

correlations in activity patterns for individuals. Due to these limitations, the 1996 NATA

should be restricted to reporting median estimates from HAPEM, not distributions, though

even for the prediction of medians, the effects of failing to include individual persistence

in the day-to-day behavior of individuals are uncertain. Either way, Figures such as 4-15

and 4-16 should be correctly labeled and clearly explained to indicate that they are not

population distributions, rather distributions of county medians, and that the percentiles of

the distributions shown only represent a small component of the overall variability in

individual exposure. Differences in exposure concentration distributions (e.g., between

states) in presentations such as Figure 4-15 of the current NATA report are due primarily

to differences in predicted ambient concentrations, not proper accounting of either

individual-to-individual variation in time-activity patterns nor regional differences in these,

and the communication of current results should not leave readers with the impression that

these factors play a role (although hopefully in future NATAs, proper handling of
37

individual-to-individual variation within HAPEM will allow these factors to properly affect

the estimates). 

Questions have been raised about how representative HAPEM exposure predictions

are, considering the demographics of the available time-activity databases used in the

model, and whether the population therefore simulated by HAPEM is skewed towards

middle class workers, failing to take into account less fortunate populations and their

lifestyle and workplaces (see, for example, NESCAUM,. 1999). This could occur because

poor and more transient individuals may be less likely to be participate in the time-activity

diary studies upon which the databases are based. While EPA has apparently attempted to

adjust results to account for such groups that have been under-represented in the past, it is

not clear whether these adjustments have been adequate.

In addition, it should be noted that the enhanced exposures due to hot spot

emissions, such as those near roadways, are not taken into account. Emissions are averaged

over the census tract or county and exposures are estimated based on these spatial averages.

Improving the basis for individual exposure modeling is necessary both to compute

the full range of individual exposure in targeted census tracts and counties, and for ensuring

that the median estimates for these locations are accurate. While continued development

of HAPEM is encouraged, until this occurs, exposure and risk estimates based on simpler

transformations (or direct use) of ambient concentrations should be presented in parallel

with those based upon HAPEM results. There are three approaches that can be used for this

(ideally, all three options should be evaluated and their results compared). First, model risk

estimates based solely on ambient concentrations can be calculated and reported [as done

in the current Cumulative Exposure Project (CEP)]. Second, a simple outdoor-indoor

correction factor can be introduced to simulate the effects of inter-individual variability in

the fraction of time spent indoors and the overall effective penetration factor for each

individual’s indoor environments. Third, the HAPEM model can be implemented as

currently formulated, but only to compute (and report) the median exposure predictions and

risk measures for each census tract (and county). As noted elsewhere, hierarchical

presentation of results from all three approaches is recommended, indicating information

and estimates based on quantities measured or modeled at different levels of scientific

development, and with differing levels of available data and confidence.

To illustrate these benefits of exposure estimates properly computed using

HAPEM, and to demonstrate the significance of indoor sources, we recommend that the

Agency consider including a full-fledged HAPEM calculation for benzene. This example

should account for exposure to indoor as well as outdoor sources and correctly treat dayto-day correlations in activity patterns for individuals. The output from this particular

example could be useful for the toxics portion of the 812 benefit/cost analysis. This

example also should be helpful in guiding future efforts to characterize exposure for the

full set of air toxics. Furthermore, there should be a coordinated effort for future

iterations of NATA to utilize and test the new tools and methods currently under

development at USEPA (such as the neighborhood scale version of Models-3, the various

outcomes of the Human Exposure and Dose Simulation program, etc.) in addition to any

refinements that are expected to be incorporated in the approaches currently used (ASPEN

and HAPEM). Future efforts should also focus on the incorporation of other important

pathways of exposure for multi-media pollutants, such as the fish ingestion route for

methyl mercury, drinking water ingestion for arsenic, and soil ingestion for lead.
38

3.2.2.3 Summary Recommendations for Charge Question 2 

For the 1996 NATA:

1. The NATA document should be modified as per the specific recommendations of the

previous section, i.e. to:

a) Explicitly identify the level of confidence/uncertainty associated with ASPEN

predictions for the specific contaminants considered (using the three group

classification recommended in this review), for particular geographical sregions and

locales (Recommendation # 11); 

b) Explain and discuss the fact that only a single component (county to county

differences in the median) of exposure variability is characterized in the current

application (Recommendation # 12); and 

c) Discuss explicitly the limitations of the 1996 NATA approach (i.e. those associated

with the treatment of long range transport and characterization of background,

nonlinear chemistry of secondary air toxic formation, seasonal variability in

emissions and climatology, etc.) (Recommendation # 13)

2. While continued development of HAPEM is encouraged, until this occurs, exposure and

risk estimates based on simpler transformations (or direct use) of ambient concentrations

should be presented in parallel with those based upon HAPEM results. A discussion of

possible biases in HAPEM results associated with under-representation of certain

demographic groups in available time-activity databases should be included in the NATA

report. (Recommendation # 14)

3. A “full-fledged HAPEM” calculation for benzene should be performed and included in the

1996 NATA report as a prototype example for future applications to other toxics: this

application should account for exposure to indoor as well as outdoor sources and correctly

treat day-to-day correlations in activity patterns for individuals in order to properly address

exposure variability. (Recommendation # 15) 

For future NATA applications:

1. Future NATA applications should address the limitations identified in this review and,

for example, consider the effects of factors such as seasonal variability in emission,

climatology and resulting ambient concentrations, improve the treatment of outdoor air

quality concentration gradients within a census tract, consider the contribution of indoor

sources of air toxics to total exposure, and account properly for inter- and intra-individual

variability of exposure. Further efforts should be made to ensure that all demographic

groups in the United States are represented in the exposure estimates, either by extending

current time-activity databases, or by applying appropriate statistical corrections that have

been tested and validated. (Recommendation # 16)

2. Future NATA applications should test, adapt, and employ (a) more comprehensive,

multiscale, air quality models, such as Models-3, that can account for both local and long

range transport and for nonlinear chemical transformations, as well as (b) evolving modeling

tools for exposure analysis that are currently under development by USEPA and other

organizations, and (Recommendation # 17)
39

3. Future applications should also focus on the development and application of a consistent,

integrated, framework that incorporates multiple routes and pathways of exposure for multimedia pollutants. (Recommendation # 18)

3.2.3 Charge Question 3 

Has available dose-response information (e.g., different sources of information, a

different prioritization scheme) been appropriately used in this assessment? Can you suggest

methods that could improve upon the use of available dose-response information?

The NATA document (U.S. EPA/OAQPS, 2001) does a generally good job of

evaluating and using available dose response information for the assessment. The approach

used to determine the dose-response based on the level of confidence in the quantitative

information from secondary data sources parallels that used by state and federal health

agencies when setting guidelines and standards for air toxics. The preferences

implemented in the current assessment proceed from using IRIS values of RfCs and UREs

to the use of ATSDR MRLs (noncancer), and finally to the use of California EPA RELs and

UREs. This order of preferences is reasonable and recognizes that the RfCs, MRLs, and

RELs are measures of similar, but not exactly the same human health endpoints. Of the 30

UREs reported in Appendix G, 21 derive from IRIS, four are from Cal EPA data, and one

derives from EPA NCEA.

The toxicity values reported in Appendix G and used in the NATA study were not

examined in detail by the Panel to ascertain whether they are the most recently reported

values. It is the practice of state health assessors to review the most current data even when

using federal or other secondary databases such as IRIS to assess the impact of new

information. New studies are ongoing or have been completed for a number of chemicals,

some of whose potency values are a decade or more old (for example, formaldehyde,

butadiene and ethylene oxide), and the process of incorporating this new information into

established databases can be slow and uncertain. The EPA is re-examining the carcinogenic

potency of 19 of the assessed HAPs. Presuming that these re-evaluations are ongoing, how

will the NATA assessment process incorporate new or revised estimates of cancer and

noncancer dose-response information in its periodic reappraisal of risks posed by toxic

HAPs? Will any revisions to UREs as a result of this activity be incorporated into a revised

1996 air quality assessment or future assessments? The dose-response information

summarized in Tables 3-5 and 3-6 should include some characterization of how recent are

the IRIS (and other sources of) estimates of cancer and non-cancer data. In addition, if

UREs or RfCs are undergoing re-evaluation, this should be indicated in the same tables. 

Dioxins are not included and the 1996 NATA study, and should be included in future

assessments.

Recommendation # 19: For the 1996 NATA, recheck the accuracy of the Tables of

dose-response values and add columns to identify whether the value has been externally peerreviewed, the date of the assessment, and a qualitative indication of whether significant new

studies have become available since that date. The “citation” (e.g., IRIS, CalEPA) should

enable the reader to easily find a complete source document for the value used. If this is not

possible (e.g., if the authors have performed additional calculations), this should be clearly

identified and a reference provided to that additional information. For chemicals that do not

use the NATA protocol, show the rationale for the assessment in detail. For the 1999 NATA,

EPA is encouraged to update all IRIS cancer and non-cancer dose-response values for those

chemicals having new health effects data since the existing IRIS assessment. 
40

Recommendation # 20: For the 1999 NATA include dioxins. Also, consider

establishing a specific schedule for periodic update of the NATA risk estimates, by setting a

calendar date that will be used for selection of reference information from secondary sources

(i.e., only data available “as of” the given date will be used for the update). 

3.2.3.1 Degree of Conservatism in Health 

 

The NATA document (U.S. EPA/OAQPS, 2001) uses UREs (unit risk estimates)

developed by the USEPA and the California EPA to determine plausible upper bound

estimates according to the priority system present in Appendix G of the NATA report. In

so doing, it is clear that these estimates are designed to provide a degree of conservatism in

health estimates. In places in the report it is noted that actual HAP risks “are likely to be

lower, but may be greater (than those reported in the document).” While true, the

conservative nature of health factor estimates are widely recognized, so that repeated use

of such statements is not necessary and often misleading.

For some chemicals in the NATA, toxicity values based on MLEs are available and

utilized, while for others, upper bound estimates based on upper confidence limits (UCLs)

are used. Since UCLs, generally used when fewer data are available, are more conservative

than MLEs, it is likely that these choices affect the relative likelihood of different

compounds being included among the list of risk driving HAPs. Furthermore, as noted in

response to Charge Question 4, summing cancer risks based on UCL’s can lead to an even

greater (though unspecified) level of conservatism in the estimate of the aggregate risk

from multiple compounds. 

Similar effects may occur when considering noncancer impacts for cases with high

uncertainty factors. How might the prioritization of different compounds change if

different (higher or lower) uncertainty factors were used for each? For both cancer and

noncancer effects, the use of conservatively high dose-response metrics causes estimates

of risk to be conservatively high. 

Recommendation #21: Indicate in the document the differences in relative risk

expected if MLEs were to be used instead of upper bound estimates of cancer potency, in cases

where both are available. Provide comment on the effect of different uncertainty factors on

the selection of specific HAPs as risk drivers. 

3.2.3.2 Validating Dose-Response Predictions 

For the CEP analysis, the uncertainties in the dose-response data were considered

by many users of these results to be small compared to the differences between compounds

in their relative exposure estimates, based both on the ASPEN estimates and the state

monitoring data that were used to corroborate these estimates. For NATA, it will also be

important to “ground truth” the risk estimates through comparison with Health Based

Guidelines and standards determined by Public Health scientists in the states to support

state air toxics regulations.

Recommendation #22: For 1999, request that States provide reference concentrations as part

of inventory or state review of NATA. The State estimates could be provided in an appendix

table for comparison purposes.

3.2.3.3 Use of Oral vs. Inhalation Data 
41

Two unit risk estimates were extrapolated from oral exposure data. The process

used is scientifically consistent with the process used by states when faced with similar

needs. In most cases the extrapolation is best based on estimates of blood levels, either

measured or calculated through use of a pharmacokinetic methodology, rather than based

solely on an overall body weight comparison. For example, there is concern in the 1996

NATA report that one of the highest UREs , that for quinoline (3.4 x 10-3), is based on an

inhalation potency derived from oral exposure values.

Recommendation #23: For 1996, provide an estimate of the potential variability of the oral

to inhalation extrapolation, and the implications of this for the derived toxicity values.

3.2.3.4 Deviations from Linearity 

For some cancer and non-cancer risks, the risk is not linear throughout all possible

range of exposures. However, the risk is likely to be very-nearly linear over the relatively

narrow range of ambient air exposures that occur in the vast majority of locations. The

probability that an exposure exceeds a reference value needs to be first established and

followed by assessment of the dose-response relationships. This process must show the

severity of the outcome. If the dose-response data are based on different outcomes with

some very severe compared to others, the short-term reversible effects could be ranked

incorrectly. The selection of endpoint could also alter the dose-response reference values.

While none of the 33 compounds in the 1996 NATA (U.S. EPA/OAQPS, 2001) are

likely to exhibit linearity throughout the entire range of dose-response, it is important to

keep in mind that these compounds were selected from 188 HAP chemicals based on their

higher toxic risk and potential exposure in urban areas. When NATA is extended to less

potent compounds, deviations from linearity in the dose-response relationship could be of

greater importance.

Recommendation #24: Consideration should be given in future NATAs to possible

deviations from linearity in the dose-response functions for noncancer risk.

3.2.3.5 Other Issues With Respect to Dose Response 

Some members of the Panel cautioned against using the available dose-response

RFCs in combining risk estimates. The aggregation of risks and grouping by target organ is

an undefined approximation and for some members of the Panel that is a concern.

The grouping of hazards by endpoint or by target organ is helpful for planning of

interventions to reduce risk. Interventions usually consider route of exposures. It is

important to determine whether the reference risk value is valid across target organs when a

compound has toxicity in different organ systems. Since NATA is a screening rather than a

regulatory process, the errors in including compounds with a common target organ and

different mode of action are less important. Combining different modes of action should

be less of a problem in assigning risk drivers. 

3.2.3.6 Indirect exposures 

The omission of indirect routes of exposure is a serious public health limitation in

the NATA risk estimates that must be addressed in future assessments. The persistent
42

bioaccumulating toxics (PBT’s ) should at least be assessed for food and water impact as

they represent a major potential health concern at the state regulatory level. 

Recommendation #25: The 1999 NATA should include the effects of indirect (noninhalation) exposures for PBTs. 

3.2.3.7 Uncertainties in the Dose Response 

Uncertainties listed in Section 3.4.4.1 (of the NATA document , pp. 49-51) are

included in the URE but these are the standard uncertainties and are not unique to the NATA

process. The report fully emphasizes these risks but in doing so tends to overstate the

uncertainty in the NATA process. In fact, every risk assessment has these uncertainties. 

The problem is a more general one owing to the lack of scientific study and data. That

uncertainty should be clearly conveyed to the public as what it is, an inability of the current

toxicological research agenda (with current levels of funding and resources) to meet the

regulatory demands for dose-response information. The reference concentration

uncertainties in the NATA document (Table 3-7), in which UF and MF are combined, are

inappropriate, confusing the NATA and dose-response uncertainties. The discussion in the

current NATA draft seems to indicate that the NATA process increases the dose-response

uncertainty found in population risk calculations. It does not do so, rather it conveys the

high level of uncertainty present in current dose-response factors. 

Recommendation #26: For the 1996 NATA more clearly indicate which of the

uncertainties are due to the ASPEN/HAPEM process and which are due to the more general

risk assessment process. 

3.2.3.8 Micro Environments and Dose Response 

It is difficult to evaluate the use of dose-response information in the NATA

independently from the approach used to compute exposures, since their levels of

specificity (e.g., the exposure modes – inhalation, ingestion, dermal, etc., and the time

scales of exposure – long- vs. short-term) must be compatible to allow for their effective

integration in a risk assessment. With the current emphasis on chronic risks, less temporal

detail is required in the exposure estimates. However, in future NATAs, as subchronic and

acute effects are increasingly considered, improvements will be needed in both the

methods used to estimate exposure and the available dose-response information. In

particular, new acute (noncancer) dose-response data will be needed. 

Recent changes in HAPEM have improved the exposure modeling and the potential

ability to obtain short-term risk estimates. The use of 3-hour time blocks of exposures and

stochastic match-up of the exposures is very important for the acute risk estimates. Once

such an approach is properly implemented (and the accuracy of the local inventory verified

through comparisons with the local, county and state exposures), acute risks can be

included as part of the NATA. Stronger dose response rationale will be needed at that time

to avoid errors in estimating the actual short-term risks. 

There is an ongoing issue with background levels (that is most important for health

effects thought to occur only above a threshold exposure concentration). EPA needs to

provide a discussion of the possible magnitude of the background effect. Because the acute

dose-response data are based on cumulative exposures, all exposure sources need to be

considered including the added risk over background. The backgrounds from long-range

transport and natural sources, as well as the contributions from indoor sources, could raise
43

the exposures toward the threshold, thereby increasing the risk contributions from other

sources. Proper characterization of non-cancer risks that are subject to thresholds requires

appropriate incorporation of background and indoor-source exposures. Knowledge gained

as a result of the most recent EPA indoor air quality assessments (EPA/SAB 1998b,

EPA/SAB 1999g, EPA/SAB 1999h, EPA/SAB 2000b and EPA 1998b) should be very

helpful in this effort. 

Recommendation # 27: As acute health effects are considered for evaluation in future

NATAs, a careful matching of toxicity value estimates and exposure estimates will be needed. 

 Similar concern is needed when considering the effects of background and indoor sources of

HAPs on health impact estimates that are subject to threshold effects. 

3.2.4 Charge Question 4 

What are the strengths and the weaknesses of the overall conceptual approach to risk

characterization used in this assessment? Given the underlying science and the intended

purposes of the assessment, can the Panel suggest ways in which the risk characterization could

be improved?

a) Is the method used to aggregate cancer risks appropriate? The aggregation of

carcinogenic risk within two categories, based on weight-of-evidence classifications, is of

particular interest. 

b) Is the method used to aggregate non-cancer hazards appropriate? The summation of

hazard quotients within target organs, the categorization of sums by ranges of

uncertainty factors, and the inclusion of all target organs (as opposed to only the organs

associated with the critical effect) are of particular interest.

3.2.4.1 Strengths of the Overall Conceptual Approach 

The overall conceptual approach to the risk characterization is reasonable. It

generally follows the guidelines and procedures of risk assessment (with exceptions noted

later for mixtures). Pollutant-specific risks to populations are generated and pollutants are

grouped into national and regional risk drivers as well as important national and regional

contributors. Risks of multiple pollutants are aggregated to generate national cancer and

non-cancer hazards by sources (major, area, on-road mobile, non-road mobile, and

background). However, as detailed subsequently, some of the key specific elements in

implementation of the conceptual approach are not consistent with assessment guidelines

or current best practices. 

The Agency faces two challenges in characterizing risks from this analysis. First, it

must find a technically valid way to aggregate predictions and summarize findings for a very

large set of individual estimates for individual chemicals at numerous locations. It is a very

difficult task to summarize information in a way that does not bury some of the important

fine points. Second, it must develop a lucid presentation for consumption by both a

sophisticated technical or policy analysis audience as well as the general public. In many

areas the Agency has done a good job and met these challenges. However, there are also a

number of key areas where decisions to summarize and generalize findings are

questionable. 
44

This charge deals with the integration of the dose-response assessment and the

exposure assessment. Thus, it encompasses the strengths and weaknesses of these risk

components. 

3.2.4.2 Weaknesses of the Overall Conceptual Approach 

Some fundamental issues are raised, but not fully discussed about the scope of the

NATA, namely issues about effects from less-than-lifetime exposures and total exposure to

air toxics. The assessment includes only chronic health effects and not acute or subchronic

health effects. In actual environmental health assessments, acute health effects are very

important for the evaluation of mortality and morbidity from outdoor air pollutants. By not

including acute or subchronic health effects in this assessment, it is not possible to

evaluate critical short-term health effects of outdoor air pollutants. The current models

available for use in NATA do not have the necessary level of spatial and temporal detail nor

accuracy to allow for acute, short-term predictions. Modification of estimation

procedures and the inclusion of new data will be necessary in all phases of the NATA

(emissions, fate-and-transport, exposure and dose-response) to allow for consideration of

such acute effects .

Recommendation # 28: For the 1996 NATA, include more discussion of the

implications of considering only chronic health effects. For the 1999 NATA, include lessthan-lifetime exposure health assessments, exposure assessments, and risk assessments, if

possible. Some of these actions will require the development of standard assessment

guidelines and new evaluations and entries into IRIS, as well modification in estimation

procedures and data in all phases of the NATA to begin to address short-term, acute effects. 

The NATA focuses on inhalation risks from outdoor sources of air toxics, including

exposures that occur outdoors and indoors as related to penetration of outdoor air. If

exposures from indoor sources of air toxics are not included, the potential risk to the

public from total exposure to these chemicals cannot be understood, given that some air

toxics have substantial and others insignificant indoor sources. Additional pathways (e.g.,

some air toxics deposited on the ground or bodies of water can enter the food chain) are

not considered. Basically, even if the NATA findings on inhalation risks from outdoor

sources of air toxics were perfect, important elements of risk from these chemicals are

being ignored, rendering the entire assessment more limited than portrayed. Such

“missing” information will, in some cases, have a significant impact on total risk. Air

toxics regulatory authority covers outdoor sources, including all pathways, making this

important for NATA. However, including risks from indoor sources (see U.S. EPA, 1994;

U.S. EPA, 1999g; U.S. EPA/SAB, 1998; U.S. EPA/SAB, 1998a;) is important to the “total”

risk issue and provides guidance to risk managers and the public on all of the potentially

most effective approaches to reducing risks from these chemicals. It is also essential when

computing health effects when the dose-response function is nonlinear or has a non-zero

threshold, since outdoor sources may not be sufficient to cause thresholds to be exceeded

(or steeper portions of the nonlinear dose-response function to be reached), however, such

thresholds may be exceeded when other sources of exposure are included. 

Recommendation # 29: For the 1996 NATA, increase discussion of potential impacts

of total exposure, including the indoor source issue. For the 1999 NATA, include other

sources of exposure in the risk analysis. 
45

The Agency states that this assessment was undertaken to: help identify pollutants of

greatest potential concern, prioritize efforts to reduce emissions, provide a baseline for

measuring future trends, and help set research priorities. The document appears to

discourage applications on a local or regional level, yet it provides information at the

county level. Clarification of the appropriate scale for application of the information

would be useful. 

There is much discussion of how the NATA results could be overestimating risk, but

not much in terms of how the results might be underestimating risk. For example, as noted

in response to Charge Question 2, questions have been raised concerning the demographics

of available time-activity databases and whether the population therefore simulated by

HAPEM is skewed towards middle class workers, failing to take into account less fortunate

populations and their lifestyle and workplaces. The factors in HAPEM are generalized

factors, which do not account for variability in exposure to outdoor air across the country,

e.g., areas of the country where windows are left open for more days of the years than

others. As discussed in response to Charge Question 2, day-to-day correlations in

activities are not preserved in the activity pattern sequences, which means, for example, that

a day 1 activity pattern may specify a house with an attached garage, and in day 2 a house

with no attached garage. Furthermore, the exposure estimates represent midrange

estimates, and results from the high end of exposure are not provided. On the hazard

number side, OAQPS relies in many instances on MLEs, which are based on “best

estimates” rather than high-end estimates for some chemicals (Table 3-5). The total risk

estimates also could not include the estimated risks from diesel, though elsewhere in the

NATA document diesel is indicated to be a significant source of hazardous air pollutants

(refer to the Panels’ related response to Charge Question 5, in Section 3.2.5). Finally, as is

discussed elsewhere, a check between ASPEN model predictions and monitoring data

shows that the model often (indeed, in most cases examined) underestimates observed

ambient concentrations. 

Recommendation # 30: For 1996 NATA, provide a more balanced discussion of the

possible sources of under- versus over-estimations of HAP exposures and risks. 

3.2.4.3 Aggregate and Cumulative Risk Issues 

The NATA evaluates the relative importance of various source sectors (major, area,

mobile on-road, mobile non-road, and background) by aggregating health risks (cancer and

noncancer) across pollutants to estimate populations affected by different source sectors. 

The procedure used for aggregating cancer risks is based on three underlying assumptions. 

They are linearity, additive effects, and comparable units. To derive UREs, a linear doseresponse model is used to extrapolate risks from high to low doses. To estimate

population risks, linear extrapolation is again applied to the range of population exposures

based on UREs. The assumption of linearity will not be violated if dose-response curves

used in the procedures are linear. Even if some of the dose–response curves are not linear;

it is assumed that they are approximately linear around UREs. It is also assumed that they

are approximately linear from the UREs down to population exposure levels. 

The assumption of additive effects is used for estimating cumulative risks resulting

from multiple pollutants. Since there is very little good information available on the

interactive or synergistic effects among multiple pollutants, it is logical to assume that all

pollutants act independently and additively. This assumption allows the risks of multiple
46

pollutants to be computed by simply adding up all individual risks. However, due to the lack

of related studies, the validity of this assumption is difficult to test. 

The third assumption follows the second assumption, in that summation of risks is

only meaningful if the risk units to be added up are equal or at least comparable. 

Population risks are determined by population exposures and UREs. To aggregate

population risk across pollutants appropriately, population exposures should be unbiased

and UREs should be comparable. An ideal URE should have the property of reflecting the

severity of cancer risk with minimum uncertainties. A URE is actually an estimate of

cancer potency with uncertainties. There are two kinds of uncertainties associated with

UREs. One is the weight-of-evidence (i.e., the classification of known, probable, and

possible human carcinogens). Another uncertainty involves the actual value of the UREs

(i.e., upper bound estimates). The aggregation of cancer risks based on weight-of-evidence

has the advantage of increasing comparability. Determining UREs using the same method,

such as MLE, for all pollutants, is another way to increase the comparability of risk units. 

The same underlying assumptions can also be used to judge if the method used to

aggregate non-cancer hazards is appropriate. Risk characterization is based on exposure

and dose-response curves, regardless of whether it is a cancer or non-cancer risk. 

However, the nature of the RfC is more complicated than the URE. To generate a risk unit

for non-cancer hazard, the NOAEL or LOAEL is divided by an uncertainty factor (UF) and a

modification factor (MF) to determine the RfC (RfC = N(or L)OAEL / [UF X MF]). For

the air toxics in NATA, the values of UF X MF range from 1 to 1,000. This uncertainty

factor moves the RfC away from its original dose-response curve. Therefore, unlike the

URE of cancer risk, it is not possible to apply linear extrapolation to population exposure

levels from RfC’s. To evaluate risks at population exposure levels, the HQ is generated as a

function of exposure by dividing it (the exposure) by the RfC. The HQ cannot be

interpreted as a probability of non-cancer risk. The HQ is a measure of potential health

risk, but lacks a clearly defined meaning of risk. 

To add up hazard quotients across pollutants within target organs, the assumption of

additive effects is needed. This assumptions is often invoked even though, within the same

target organ, different pollutants have different modes of action. For many such effects,

additivity is a simple and logical assumption, but it lacks the support of empirical data. 

Regarding comparability, RfCs are far less useful in terms of their statistical comparability

across multiple compounds than are UREs. The UCL used for an URE is a conservative

measure with statistical reference, while the UF is a measure of uncertainty with limited

theoretical (statistical or biological) justification. Because of the large size of the

uncertainty factor in certain cases, the UF’s used could be a key factor driving the

estimated population risk. Take the example of acrolein; the UF of 1,000 is assigned to its

RfC due to interspecies extrapolation (a UF of 10), lack of chronic studies (another UF of

10), and accounting for sensitive human populations (an additional UF of 10). Because of

the above uncertainties, the RfC (2.0E-05) of acrolein becomes 1,000 (10 X 10 X 10)

times lower than the LOAEL (2.0-E-02) estimated from animal studies. The resulting high

computed values of HQ for acrolein contribute to estimated risk across a large affected

population, however, this results in significant part due to its large uncertainty factor, and

not necessarily due to its high potency (or low threshold) of non-cancer health effects. As

a leading national hazard driver, the estimated population risk of acrolein can certainly be

attributable partially, maybe even largely, to the UF of 1,000. 
47

For noncancer hazards, efforts were also made to increase comparability for

aggregated risks. To increase comparability of HQs across different pollutants, TOSHIs

were developed grouping noncancer risks by target organs. The categorization of sums by

ranges of uncertainty factors (UF>100 and UF 1-100) is another way to increase the

comparability of risk aggregation. 

The issue of background exposure to some of the air toxics was raised in the text. 

However, it is difficult to discern its chemical-specific impact. For example, Figures 5-3

and 5-4 include background as a source, suggesting a cancer risk in excess of 1 in a million. 

This is a significant statement, making more discussion useful. For example, it would be

useful for the reader to know which compounds in Figure 5-6 had a significant background

component to their risks (note- this is a figure of exceedances of HQ levels based on all

source sectors). A simple indicator (e.g., use of an asterisk for those chemicals having

significant background contributions) would be helpful. As noted below, a general

recommendation is made for greater explication of the reasons why different compounds

are predicted to be risk drivers.

Recommendation # 31: For the 1996 NATA expand the discussion of the rationale

for the approaches used to aggregate cancer and noncancer risks and the impacts of these

approaches on uncertainty. Also, expand the discussion on the possible extent of the

influence of background concentrations and other model assumptions on the risk outcomes. 

3.2.4.3.1 Aggregation and Characterization of Cancer Risks 

NATA’s overall conceptual approach to risk characterization is reasonable and

generally follows EPA guidelines and procedures. Known human carcinogens are summed

separately from probable human carcinogens in the NATA document. Probable human

carcinogens are lumped with possible carcinogens. This is not conventional, nor is it

appropriate. The only difference between known and probable classes of carcinogens is

data from human studies, and human studies of these compounds are relatively rare. Thus,

it seems more appropriate and certainly more precautionary for the Agency to combine and

report the Class A and Class B separate from the Class C carcinogens. Also, the Agency

should provide an estimate for all types of cancers summed together and then break the

results out by group. Changes in the 1996 NATA are also needed to ensure that the addition

of non-cancer effects follows current mixtures guidelines limiting such aggregation to

effects with a common mode of action. Finally, future NATAs should address additional

(non-inhalation) pathways for exposure and sub-chronic (less than lifetime) effects. 

Recommendation # 32: For the 1996 NATA, evaluate the impacts of combining the A

and the B1 carcinogens, leaving the B2 and C carcinogens as a separate entities, and see

whether this changes the conclusions about risk drivers or the risk characterization. If this

evaluation has significant impact, decide on the optimal approach for the main presentations

and provide an appendix with an alternate approach(es), along with an evaluation that

integrates Class A, B1, B2 and C carcinogens. When deciding on one approach over another,

document the rationale for the selection and any history of use of a particular approach. 

Uneven and unsystematic biases may amplify or cancel each other following the

many steps of the risk modeling process, and thus, the end results might change the actual

rank order of risks in an undesirable manner. For example, all Unit Risk Estimates (UREs)

used in this assessment are based on linear extrapolation. For some pollutants, which are

less than linear, this process may overestimate the risk. In contrast, most UREs used in this

assessment are based on upper confidence limit (UCL), but a few are based on maximum
48

likelihood estimates (MLEs). Estimates based on the MLE are less conservative than

those based on the UCL.

It is very helpful that the Agency identifies those chemicals disproportionately

responsible for the risks in the study; again, using the analysis to identify a priority list of

HAPs is a useful and practical application for the study. However, this section does not

discuss or take into account some contaminants previously identified in the report as

particularly underestimated in the model. In particular, when ambient chromium, cadmium,

and lead concentrations predicted and reported in the 1996 NATA study (from estimated

emissions for these compounds and ASPEN model predictions) are compared to observed

data at available monitoring stations, the predicted concentrations are indicated to be

significantly lower than the measured concentrations. Since the ambient concentrations of

these compounds are underestimated in the assessment, their risks may be as well (see

1996 NATA document, Sections 4.3 and 5.2, US EPA/OAQPS, 2001.). 

Recommendation # 33: For the 1996 NATA, the section that discusses which HAPs

are important risk drivers should take note of the possibility that other compounds

underestimated by the model could also be risk drivers. 

There is a concern with the “addition” of upper bound cancer estimates to estimate

the overall aggregate risk. The sum of multiple 95th percentiles yields a value that is

generally much further out on the tail (i.e., much more conservative) than the 95th

percentile value for the sum. That concern is especially valid when the slope functions

differ significantly from chemical to chemical or if an exact risk for a specific population

is desired. In the case of the former, comparison with the MLE estimates should be used to

reveal any discrepancies in estimates that might occur due to adding multiple upper 95th

percentile values that differ significantly from their respective MLE estimates. That should

be noted in a footnote of the document. In the case of the latter, it can be noted that NATA

is not attempting to determine the exact aggregate cancer risk for any area, but to determine

relationships between regional risks and risk drivers. Thus while the use of MLE estimates

would be more accurate, when summing cancer risks, the summation of upper bound

estimates may in many cases be employed without altering the risk ranking of the

compounds. 

Recommendation # 34: For the 1996 NATA, please clarify this issue of the difference

between seeking a relative ranking vs. an absolute risk and the differential influence that

conservative assumptions employed when aggregating risk may have on these. 

3.2.4.3.2 Aggregation and Characterization of Non-Cancer Risks 

A HQ and HI approach are common means of assessing non-cancer risks. As

everyone agrees, there is a high degree of uncertainty in this approach. However, the means

of doing this calculation presented in the draft NATA document do not follow EPA

guidelines and are scientifically questionable and therefore need to be revisited. 

The HI methodology is commonly accepted for chemicals having a common

mode/mechanism of action. In the absence of data, some assessors default to using a

common organ (in accordance with EPA mixtures assessment guidelines). The key phrase

is, in the absence of data. In some cases, chemicals having known different

modes/mechanisms were added (e.g., formaldehyde which produces nasal effects was added

to cadmium which produces lung effects through different mechanisms). 
49

Recommendation # 35: For the 1996 NATA, either create the HI based on

mode/mechanism of action or remove the HI, applying it properly in the 1999 NATA. 

The calculation of greatest concern is the target-organ-specific-hazard index

(TOSHI). As described on pages 46 and 92 of the draft NATA document, TOSHIs were

developed by summing the HQs (the exposure divided by the RfC) for individual air toxics

that affect the same organ or organ system. It was calculated by taking the RfC for a

chemical based upon the critical effect and dose to one organ and transferring this RfC to

all other organs affected by that chemical. The RfC methodology begins with the

identification of the “critical effect” commonly defined as that endpoint having the lowest

NOAEL (or LOAEL) (or the benchmark equivalent); it is a human equivalent concentration,

including an estimate of dose to the target organ. Uncertainty factors and modifying

factors are then used, according to the guidelines. An RfC results from these calculations. 

Often other organs are affected, but at higher NOAELs, so they are not the “critical effect”. 

An RfC based on such a higher NOAEL would be higher. Dose calculations would also be

different. Even more uncertainty can result. If EPA wishes to use a TOSHI approach, it is

essential that EPA goes back to the database for each chemical and actually develops

TOSHI’s with a high level of scientific rigor. Without that effort, they should be eliminated

from the document. 

It is recognized that the IRIS database for many of these substances is out-of-date,

but timing considerations for revision of this version of NATA may restrict the TOSHI

reevaluation to this IRIS database. Although this would compound any errors due to the

date of evaluation, it is preferable to the incorrect approach now used in the draft

document. 

With respect to Table 3-7 and the discussion on TOSHI in Section 3.4.3 of the draft

NATA document (U.S. EPA/OAQPS, 2001), some chemicals appear in more than one

group (e.g., Cr is listed for the respiratory, liver/kidney, and immune systems). Please

clarify whether they are counted more than once. Are they counted in all categories, or in

only one? If the former, is this double counting?

Recommendation #36: For the 1996 NATA, either reexamine the IRIS database and

calculate target-organ specific “RfC’s” based on NOAELs (or Benchmark dose equivalents)

for each organ considered, or delete the TOSHI. If the TOSHI are deleted here, they should

be developed (with up-to-date, target-organ specific data) for the 1999 NATA. 

3.2.4.4 Alternative Risk Evaluations 

The integration of an exposure assessment with a health assessment is extremely

difficult, even under data-rich circumstances. Because this luxury does not exist for air

toxics, there will be considerable errors in unknown directions as data collected for one

purpose are used for another purpose in unvalidated models. It therefore would be of value

to know the relative influence of errors in exposure vs. errors in health factors. One issue

of particular concern is the magnitude of the net uncertainty factors in the RfC’s. It would

be of interest to know the degree to which the uncertainty was driving the risk. For

example, acrolein is identified as having a higher noncancer risk than other compounds. Is

this due more to the uncertainties in the dose-response assessment or the exposure

assessment? 
50

Recommendation # 37: For the 1999 NATA, consider running the risk analysis using

alternative toxicity values for a few key chemicals to provide a scenario-based approach for

identifying the importance of these values in the overall assessment. This action should be

taken in the near future to help inform priorities on research areas. 

Many places in the text discuss the uncertainties and variabilities inherent in NATA

and the current inability to quantify the impacts of these unknowns. However, many

choices were made in the assessment, e.g., using modeled exposure estimates without

estimates based on the measurements of exposure from various sources like NHEXAS,

TEAM, or other literature sources; using one health value rather than another (e.g., for

butadiene), and it would be interesting to consider some selective groundtruthing for some

selected air toxics. The Agency should select the air toxics for such an analysis based on

available databases. Benzene is one example where a groundtruthing exercise would be

informative. 

The NATA risk classification for air toxics is based upon a reasonable logic that the

broader the risk distribution, the more likely the source was local. For some of the air

toxics, the database should be rich enough to perform a source apportionment. For

example, source apportionments of benzene have been published years ago and more recent

ones may be available for use. For example, a review article by Wallace (Wallace, 1995)

illustrates a source apportionment based on the TEAM studies. This analysis estimated that

82% of benzene emissions are due to cars, 14% are due to industry, and the remaining 4%

are due to cigarettes, personal, and home sources. However, this same analysis found that

40% of monitored personal benzene exposure is due to smoking cigarettes, 5% is due to

environmental tobacco smoke, 18% is due to automobile exhaust, 18% is due to personal

activities, 6% to home sources, and 3% to industry sources. When such information is

available, it should be used for conducting further evalautions, and these should be

compared to the results obtained using the basic NATA methodology. 

Recommendation # 38: For the 1996 NATA, select 1 or 2 air toxics having

substantial databases and develop a risk assessment based on their data and compare it to the

model results of the current draft. For the 1999 NATA, explicitly incorporate all the credible

data in the assessments and incorporate the results of validation/evaluation research in the

selection and parameterization of models. 

3.2.4.5 On the Issue of Children 

On page 99, under 5.5.3, paragraph 1, the NATA document (U.S. EPA/OAQPS, 2001)

states, “it is necessary to consider adults and children separately.” On page 100, in the top

paragraph discussion on children; line 4, the text states, “dose-response assessments for

non-cancer effects developed by EPA... do not currently include separate reference

concentrations...for adults and children.” These comments are misleading. Indeed, there

are not separate RfC’s. As stated in several places in the document, the definition of the

RfC includes the coverage of “sensitive sub-groups.” This part of the definition is derived

from the use of an uncertainty factor of up to 10 for intraspecies extrapolation (i.e., from

average to sensitive sub-groups). There has been much debate engendered by the Food

Quality Protection Act (FQPA) and its requirement for an additional factor of 10 to ensure

protection of children from pesticides. Is EPA implying that additional protection (beyond

the standard uncertainty factor) is required for children exposed to air toxics? If so, EPA 

should provide the scientific basis for this. As mentioned above, the RfC, being based on

lifetime exposure, is not an appropriate index for children who have not lived for 70 years. 
51

Where children are a special concern, the data need to be evaluated and assessed

appropriately. The paragraph ends with a comment about higher TOSHI’s for adults than for

children. This compounds errors, and the entire discussion in this section needs to be

revisited.

Concern over the need for additional, special consideration and assessment designed

to protect children is especially great when health effects from less-than-lifetime exposures

(such as asthma) are considered. Since we do recommend that health effects from lessthan-lifetime exposures be considered in future NATA’s, the data collection, research and

assessment activities necessary to develop exposure and susceptibility estimates for

children relevant to these sub-chronic effects should begin now. Until such time that results

from these more-targeted efforts are realized, greater uncertainty is likely to be present in

both acute and sub-chronic exposure and health assessments for children. The current

NATA document has addressed some (but not all) of the uncertainties and issues related to

children in describing the key data collection, modeling and characterization issues for

exposure calculation. 

Recommendation # 39: For the 1996 NATA, the discussion of children should be

clarified to indicate that they are an important life stage to be considered and therefore are

already incorporated in the chronic assessments. However, the exact degree to which these

assessments either under- or over-estimate risks to children is unknown. 

Recommendation # 40: When future NATA’s consider less-than-lifetime exposure

effects, special attention must be paid to children, because they are likely to have different

short-term exposures and sensitivities compared to adults, and thus the risks may be different. 

3.2.4.6 Additional Clarification Issues 

For the most part, the document (U.S. EPA/OAQPS, 2001) is internally consistent,

except for a few instances.

a) Page 18, L 4 says that “current Agency risk assessment...guidelines” were used. As

described elsewhere, in some cases the assessment practices of others (e.g.,

CALEPA) were used and procedures can be different;

b) Page 35, Microenvironmental data, para 1, last line. This says that an ADD factor was

used “that accounts for ...i.e., indoor emission sources.” However, in many other

places the document said that indoor sources were not considered. Page 37 says that

the ADD factor was set to zero;

c) Page 84 discusses the interpretation of census tract and higher order aggregations. 

As mentioned elsewhere, the census-level is too uncertain to be used. Then the next

paragraph says that ‘The results of the exposure assessment are only meaningful when

examined at the individual county level or above.” Is this “meaningful” comment

really true, given the caveats?

d) Page 91 line 2. This sentence says that the “risk characterization focused on results

at the national level, which is the level at which EPA believes the results are most

meaningful.” If this is correct, why provide county-level data?
52

e) Page 41, Section 3.4 The risk characterization section is a mixture of dose-response

assessments and risk characterizations. They should be separated for more clarity; 

f) Page 42 line 11 from bottom. Clarify terminology: why is cancer a risk and noncancer a hazard?;

g) These analyses were “based on the median exposure within each of the approximately

61,000 census tracts nationwide.”(Page 93 and elsewhere in this area.). In many

earlier sections, the document states that the variability of the data at the census tract

level causes the authors to only show the information at the county level. Other

places say that the exposure assessment is “only meaningful when examined at the

individual county level or above.” (Page 84). It would be useful to further justify the

quality of using such aggregations of information;

h) The document should be slightly reorganized. Chapter 4 is the exposure assessment,

but Chapter 5 jumps right into the risk characterization. A new Chapter 5 should be

constructed to contain the hazard identification and dose-response information for

the health assessment. The next chapter would be the integration—the risk

characterization; and

i) Page 99, under 5.3.3, paragraph 1: This section on aggregate TOSHI implies that

non-cancer aggregate risk is more complex than cancer risk because for non-cancer,

“it is necessary to consider different toxic effects and mechanisms...” However,

cancer mechanisms also differ, so this should be reworded. 

Recommendation # 41: For the most part, the document is internally consistent, except

for a few instances (a through i as identified above). For the 1996 NATA, consider

clarifications of the above points. 

3.2.5 Charge Question 5 

Although EPA has concluded that available data are not sufficient to develop a reliable

quantitative estimate of cancer unit risk for diesel emissions, it is clear that this pollutant class

may be of significant concern in a number of urban settings. The risk characterization in this

report includes a discussion of diesel particulate matter to help states and local areas frame the

importance of this pollutant compared to the other air toxics. In the context of this assessment, is

the discussion in this report regarding making risk comparisons among other air toxics

appropriate? Can you provide any suggestions that would improve upon this approach to

comparing the toxic health effects of diesel particulate matter with other pollutants? 

The inclusion of diesel exhaust particles (DEP) as an air toxic in the context of this

Assessment is arguable. It can be argued on the basis of: a) the lack of a unit risk estimate

(URE); and b) the complex nature of DEP; that the material should not be included at all. 

Moveover, diesel exhaust particles consist of multiple particle types and similar particles

are emitted from other sources which are not discussed specifically in this document. It is

the view of the Panel, however, that it is appropriate for DEP to be included in some manner

in this assessment. There is a widespread and longstanding concern for the health impacts of

DEP, and the public and other users of the NATA would expect it to be included. The

exposure to DEP is ubiquitous, and the exposure assessment included in this document

provides useful perspectives. Although the level of risk is not known and continues to be

debated strongly, some level of risk is plausible. 
7 Note that the usage of the term, PMfine in this context is essentially equivalent to discussion of PM 2.5,

however, by its use, we recognize that health effects from particulate matter, including that associated with

diesel emissions, could in the future be identified with an even smaller size fraction of PM (e.g., PM1.0 or PM

0.5). 

53

The Agency was interested in whether or not the caveats they included in the NATA

document (U.S. EPA/OAQPS, 2001) are consistent with the recommendations of the Clean

Air Scientific Advisory Committee concerning the diesel Hazard Assessment Document

(HAD) (not yet published). In general, the caveats concerning the uncertainty of the level of

risk and the decision not to use a specific URE for lung cancer were appropriately stated,

with the exception of perhaps two issues. First, the wording suggests that CASAC endorsed

the range of probable cancer risk portrayed in the document. Although CASAC agreed to

close on the diesel HAD with the range included, there was not consensus regarding the

appropriateness of its inclusion or the validity of the values bounding the range. Opinion

was divided, thus, although CASAC agreed that inclusion of the range would not prevent

closure, there was not a consensus to endorse the range and there were members who were

opposed to its inclusion. Second, the explanation provided in the NATA document was not

sufficient to give an uniformed reader a good sense of why the Agency did not adopt a URE

for DEP cancer risk, or why it did not adopt the California URE as a backup (as it did for

some of the other air toxics). 

The attempt to treat the risk from DEP in parallel with the risks from other species

results in an obviously awkward construction. Given that there is no acceptable URE for

DEP cancer risk for this exercise, the insertion of repeated statements that the Agency

believes that DEP is one of the most important of the air toxics appears incongruous, and a

circumvention of the process used for the other species considered. In fact, the Agency

may be correct in its belief, but it may also be incorrect. If we knew with acceptable

certainty, we would have an acceptable URE. Without better explanation, the reader

perceives that if the Agency decides an air toxic is important as a carcinogen, it can state

this as a belief without the rigor of establishing a URE. The present explanation does not

give the reader a very solid understanding of why this conclusion was reached for DEP. It is

understandable how exposures, or at least regional concentrations, of DEP are estimated,

but it is not very understandable from the present treatment what the situation is with respect

to risk. 

The Panel suggests that the Agency develop a more thorough explanation of the

current status of knowledge concerning DEP health risks, and place it in one section devoted

to that purpose. The section need not be a separate chapter, nor need it be very long. 

Perhaps a few pages would suffice. The Panel also recommends that the section include a

summary of non-cancer as well as cancer risks. It is plausible that the non-cancer health

burden from environmental diesel emissions may exceed the health burden from cancer. It

would also be useful for this section to mention links between health issues associated with

DEP and those associated more generally with ambient fine particulate matter (PMfine)

7

. 

Because DEP comprises a minor, but significant portion of PMfine in urban inventories, and a

major portion in certain microenvironments, the health effects of DEP must be integral to

those attributed to PMfine, including possible mortality and morbidity effects associated with

cardiopulmonary disease, influenza and asthma. Mentions of DEP at other steps of the

Assessment can be referenced to this section. As a result: (a) the reader will have a better

understanding of the Agency’s views and the reasons for them; and (b) the construction will

appear less awkward and will give less impression of a circumvention of the process

established and used consistently for the other air toxics. 
54

Recommendation # 42: Diesel emissions should be included in the NATA. A specific

section should be devoted to a clear, succinct explanation of the basis for the Agency’s

conclusions regarding health risks from DEP. The section should address both cancer and

non-cancer risks, and links to risks attributed to ambient particulate matter. The wording

should be moderated to more accurately reflect the uncertainty of the health risks and

CASAC’s position regarding the cancer risk range in the Diesel HAD. 

3.2.6 Charge Question 6 

Given the limitations inherent in this preliminary assessment, have uncertainty and

variability been appropriately characterized? 

a) Can you suggest ways that the characterization of uncertainty and variability could be

improved, made more transparent, or integrated more effectively into the risk

characterization? 

b) Can you suggest methods for quantifying individual as well as composite uncertainties

associated with the emissions inventory, dispersion modeling, exposure modeling, doseresponse assessment, quantitative risk estimates, and accumulation of risk across air

toxics? 

The NATA 1996 document (U.S. EPA/OAQPS, 2001) provided to the SAB presents a

variety of qualitative discussions of sources of uncertainty in the risk assessment and a topdown effort to characterize the overall uncertainty in the analysis. We support the overall

approach of estimating the top-down uncertainty factors based on the multiplicative

elements of the assessment. A top-down approach is well suited to the preliminary nature of

the overall assessment. In contrast, a more detailed effort to propagate uncertainties from

the bottom up would not be viable in the current assessment, given the limitations of the

baseline analysis. 

Although the NATA review panel generally supports the use of a top-down approach,

the current implementation requires significant additional work. In particular, the methods

and supporting information used in the assessment are not yet adequate to allow the

assignment and propagation of probability distribution functions for representing

uncertainty in each of the NATA components (emissions, fate-and-transport, exposure and

dose-response). 

 The top-down uncertainty estimates presented in Section 5.5 of the NATA document

(U.S. EPA/OAQPS, 2001) consider three factors: modeled ambient concentrations from

ASPEN, the ratio of personal exposures to ambient concentrations, and dose-response

factors. The monitor-to-model comparison used is a reasonable approach for estimating

uncertainty in the ASPEN modeling results, and makes effective use of the limited

monitoring data that are available. However, the use of measured correlations between

personal exposure and ambient concentrations is not an appropriate means of estimating

uncertainty in the exposure/ concentration ratios used in NATA. Although the NATA

deliberatively excluded exposures due to indoor sources and personal activities, these

sources strongly influence and may even dominate measured exposures for certain

chemicals. Moreover, the use of observed exposure/concentration ratios for fine

particulate matter (PM) and ozone to gain insight into the exposure/concentration ratios

expected for the air toxics addressed in NATA is inappropriate, since fine PM and ozone are

not good surrogates for most of these compounds. In particular, the daily and seasonal time

scales, and spatial distributions of fine PM and ozone are likely to differ significantly from

those for air toxic compounds which are present predominantly as primary pollutants, and
8

“Conceptual uncertainty” is used here to refer to uncertainty in the choice of model structures (rather than

uncertainty in the choice of input values to models with a fixed structure), including alternate ways of

formulating and combining the models used in the risk assessment. 

55

these differences in spatial and temporal patterns can have a significant impact on personal

exposure. Furthermore, the uniform distributions used in the illustrative calculations for

PM and ozone exposure variability are completely arbitrary, and the uniform distribution

used to represent uncertainty in the dose-response factors also appears to be arbitrary. 

Since current data are not available to support development of probability distribution

functions, a scenario-based approach for representing uncertainty should be used instead.8 

Scenario analysis also has the advantage that it would emphasize data gaps and assumptions

that might contribute to inaccuracies in the assessment. At this stage, highlighting possible

inaccuracies is more important than the focus on imprecision implied by the use of

continuous probability distribution functions in Section 5.5. The approach proposed in

Section 5.5 may suggest that the estimated central tendency of a predicted quantity has a

misleadingly high degree of reliability. 

For each of the components of the NATA, summary tables should first be developed

identifying alternative assumptions or data sources along with the amount of available versus

missing data for the assessment. The "scenario" analysis would then combine high and low

estimates of each factor, or estimates based on the major alternative sources of data or

methods for calculation, rather than requiring distributions. For example, results straight

out of ASPEN could provide the "low" value of metals concentrations, while the factor of

five that reflects the model's underestimation compared to measurements could be

incorporated to provide the "high" estimates. Similarly, in cases in which UREs are being or

have been re-evaluated, risks calculated using previous versus current or proposed values

could be compared to demonstrate the range of uncertainty in the estimates. An event, or

“scenario tree” could be used to represent the adoption of each of the major conceptual or

data-source assumptions in the combined assessment, and indicate the implications of each. 

The scenario tree would provide insight into which combinations of assumptions lead to the

most important differences in predicted exposure and risk, and consequently in

prioritization of air toxics, and which assessment components warrant highest priority for

further research or data collection. 

An important use of the recommended scenario analysis is to guide the collection of

new information to refine the study. For example, if the uncertainty associated with an

estimated risk for a given compound is dominated by the uncertainty factors used in the

derivation of the dose-response relations, investments in refined exposure modeling will not

payoff proportionally in improving the risk estimate. Under such circumstances, there

should be some mechanism for the NATA to communicate to the appropriate group (within,

or outside of the Agency) the need for more accurate and precise dose-response

information. At a minimum, the NATA process should clearly indicate which risk estimates

are dominated by uncertainties in exposure estimates and which are determined by uncertain

dose-response information as part of the risk characterization. 

Recommendation # 43: For the 1996 NATA, use the scenario-based approach

described above to represent the uncertainty in the analysis, placing the emphasis on

inaccuracies, rather than imprecision. 

3.2.6.1 Specific Comments 
56

The qualitative discussions of uncertainty sources given throughout the current

document (U.S. EPA/OAQPS, 2001) are valuable. However, the document should more

carefully distinguish between sources of uncertainty that are specific to the NATA and

sources of uncertainty that are common to all health risk characterization efforts. Where

possible, greater delineation of major versus relatively minor sources of uncertainty would

also be valuable. 

Recommendation #44: For the 1996 NATA, differentiate between NATA-specific and

universal sources of uncertainty, and between major and minor sources of uncertainty. 

In Section 3.4.4 (U.S. EPA/OAQPS, 2001), more consideration needs to be given to

interpretation of the NATA results in view of the fact that the UREs and RfCs are thought to

be "conservative" but the exposures are likely to be underestimated. The report generally

implies that the assessment results are more likely to err on the side of overestimating risks

than underestimating them. However, it is not clear that this is the case, since emissions

and ambient concentrations appear to be underestimated, indoor sources are neglected, only

median populations are considered, and dose-response estimates do not differentiate

between healthy adults, children and other sensitive populations. 

Recommendation # 45: Use the scenario analysis to help bound the NATA risk

estimates and avoid oversimplified characterization of the "nominal" results as conservative.

Section 4.2.2 of the NATA document (U.S. EPA/OAQPS, 2001) should clarify the

uncertainties associated with the various aspects of the emissions inventory to create more

transparency about potential over and under estimations for each source sector. Tables 4-3

and 4-5 provide a good overview of the uncertainty associated with the major point source

inventory. However, it is difficult to draw clear inferences from comparisons of some of

the emission estimates, since these comparisons mix differences due to methodology, time

period and the set of sources that are addressed. Moreover, the uncertainty associated with

area source, on-road mobile source and non-road mobile sources needs to be presented in

greater detail in the current version of NATA. 

A table should be included which provides the reader with an estimate of the

confidence (high, medium or low) for each EPA-generated emission factor and the activity

data used to generate the NTI for all non-point stationary sources (area sources). This is

extremely important since these factors account for 70% of all of the non-point emissions. 

The Agency should make an effort to make the non-point emissions inventory more

transparent in the main document. Readers should not have to probe through layer upon

layer of references in order to understand how this part of the NTI was developed. These

same transparency concerns exist for the on-road and off-road mobile source emissions

inventory. In order to improve future NATA assessments and spur future research, some

degree of confidence needs to be included in the current NATA assessment for each

individual component of the NTI. We recommend that the limitations of the NTI at least be

ranked in order of importance for each general source sector (e.g. major, area/other, onroad mobile, and non-road mobile).

Recommendation # 46: Provide more detail in the main NATA documentation on

uncertainties associated with emissions from area, on-road mobile and non-road mobile

sources.
57

In Section 4.3.4.2 (U.S. EPA/OAQPS, 2001), characterizing the difference in results

obtained using 1990 versus 1996 meteorological data as uncertainty is misleading. The

differences reflect both uncertainty and variability.

Recommendation # 47: Distinguish between reducible uncertainty (due to lack of

information) and irreducible variability.

In Section 5.5.7 (U.S. EPA/OAQPS, 2001), the discussion of uncertainties in risks

aggregated across pollutants rests on the unlikely assumption that the uncertainties

associated with each pollutant are independent. Some discussion should be added of how

uncertainties in aggregate risks might behave if the assessment uncertainties are correlated

across pollutants, as is likely in some cases. For example, uncertainties in motor vehicle

activity factors simultaneously affect benzene, 1,3-butadiene and other air toxics associated

with this source.

Recommendation # 48: If uncertainty estimates are to be extended to aggregate risks,

careful consideration needs to be given to which sources of uncertainty act independently

across pollutants versus those uncertainties that simultaneously affect multiple pollutants.

A major output of the NATA may involve lists of counties estimated to be among the

top X (or top Y%) of counties in terms of computed exposure and risk for all compounds,

or selected HAPs. Should such lists be developed as part of NATA, it will be very important

to identify the sensitivity of the results to differences in assumptions, using the scenario

tree approach described above. Readers should be able to identify the specific reasons why

a county is included in any list, for example, due to high estimated emissions of a particular

type (facility, area, mobile on-road or off-road) for particular sets of compounds; low

ambient dilution and dispersion (due either to local meteorology or the presence of small

census tracts with high emissions); or specific demographic or time-activity factors. The

presentation should also indicate the plausible scenarios under which the county is not

included in the list.

Recommendation # 49: Should lists of high-exposure/high-risk counties be developed

as part of the NATA results, information should be provided on the key factors that determine

whether or not a county is included on the list, and the sensitivity of the list to alternative

scenarios considered in the scenario-tree evaluations.

3.2.7 Charge Question 7 

Have the results of the assessment been appropriately and clearly presented? Can you

suggest alternative methods or formats that could improve the presentation and communication

of these results?

The NATA assessment is complex and presents a challenge for compilation into a

single document that flows well and leads the reader through the processes that are used. 

The current document is intended for use by technical experts. It will be critical to develop

the summary documents to accurately communicate with non-technical audiences. The

WEB page is apt to be the primary tool for communicating with such non-technical readers. 

The draft is organized logically along the risk assessment paradigm and transparently

takes the reader through the steps of the assessment. The steps are clearly described as well

as the results. However, the detail necessary to make the assessment fully transparent also
58

makes the document very long. It would be most useful if there were an executive summary

that would summarize the key findings and conclusions. The introduction clearly describes

the goals of the assessment and could form the outline for an executive summary. These

distilled conclusions could then become the answers for a "Frequently asked questions"

section on the public Web page. The assessment document and appendices do address each

of the stated goals of the NATA study, but often it is difficult to find them. Thus an

executive summary could for example, include statements such as in 6.3.1 which succinctly

addresses Goal 1 - Identifying air toxics of greatest potential concern. If the readers can

start with the core of the results, they will then have the context to critically follow the

supporting materials to see that the results are appropriate. 

The limitations at each step are clearly described and, if anything, are too

comprehensive, giving the reader the impression that there is little confidence in the results. 

In some instances there is considerable confidence and others the model results are more

speculative. While all the caveats are important for transparency, it would also be helpful in

the beginning to have the authors describe the top 5 or 6 limitations that they believe have

the greatest impact on the results and conclusions. In some of the chapters this is done very

nicely and a qualitative as well as quantitative description is provided. If the limitations are

agent specific, then that also needs to be described as is done with diesel particulate. The

maps and graphical displays of results are very helpful and compactly present the complexity

of the project components and results. 

The Web page will likely be the prime method for communicating with the general

public. All materials developed for the general public and for use on the Web page should

be pre-tested prior to distribution to assure public understanding. Frequently a focus group

approach is an efficient approach to pre-test materials and obtain suggestions for

improvement. The current page is a good start for distilling the assessment down to

manageable materials without losing critical information. This will be a critical

communication tool to reach the majority of the public. Again the key will be to choose and

display those aspects and results that the Agency finds most important and in which it has the

greatest degree of confidence. 

A challenge presented by the complexity of the document is to find a means to

clearly communicate to the lay public which pieces of the assessment are understood and

characterized with a relatively high degree of confidence, and which require further data

gathering and model improvement before reliable estimates can be assured. Given the

importance of environmental pollution information such as this (e.g., the widespread use of

the TRI and the current NTI data by business, environmental groups and citizens), we

recommend that the Agency, especially in materials intended for non-technical individuals,

clearly distinguish between those parts of the NATA that are well established, vs. those

which are in an earlier, developmental stage. In developing the web page for communicating

results, the EPA should consider use of a hierarchical set of pages to differentiate between:

a) Information that is based solely on data or data reports, e.g., emissions datasets and

ambient concentration and personal monitoring datasets for different compounds in

different locations; 

b) Information that is based on relatively simple or highly confident model calculations,

such as ambient air concentration values computed by ASPEN for well-characterized

air toxics that are not affected by secondary pollutant formation processes, in areas

(terrain and meteorology) where ASPEN can provide reliable prediction, or total
59

exposures to ambient pollutants computed assuming a simple indoor-outdoor

penetration factor; and

c) Information based on new model developments, where research is ongoing to

improved the basis for prediction.

These pages could be color coded and titled to indicate: a) existing NATA data

(using, for example, a blue background); b) existing NATA models (pale green background);

and c) models undergoing research and development (yellow for caution).

For the lay public it will be important to place the consequences of exposure into a

public heath context. A “thermometer” type graph could be used to display the levels at

which different effects are seen, or to present different cancer risk levels. Examples of the

types of displays that might be used can be seen in the Agency for Toxic Substances and

Disease Registry (ATSDR) toxicological profiles as well as in materials developed by the

State of New York. See, for example, 

http://www.health.state.ny.us/nysdoh/environ/btsa.htm and

http://www.health.state.ny.us/nysdoh/environ/btsa/figure1.pdf.

However, it is critical that adequate explanations are provided about the information which is

portrayed in these type of graphs. These graphs should have consistent units, explanations of

the different units used, and should not be overly cluttered with multiple health endpoints

and text.

The public will be very interested to learn which counties in the United States rank

highest for exposures and cumulative risks. In earlier sections of this report we have

identified the significant uncertainty that we believe to be present in the quantitative scores

derived for each county and that such rankings pose significant concern, given the

limitations in the data used. However, despite any recommendations and cautions to avoid

comparative ranking, the data in the report will allow others to do such comparisons if EPA

does not provide such descriptive summary information. 

The Panel is divided concerning the wisdom of presenting results of any type that

identify specific counties as “hot-spot”, high-exposure/ high-risk locations. Some members

of the Panel believe strongly that states, citizens and other stakeholders will greatly benefit

from this information and that, since other organizations will be able to access and

manipulate the NATA results to produce it, it is better to have the Agency perform this

service. Others feel just as strongly that the uncertainty in NATA estimates is too great to

justify identification of specific “hot-spot”, high-risk counties, and that even if others could

generate such a list, this was preferable to the EPA itself producing it (with the implied

“official support” that this would entail). We note this disagreement within the Panel and

hope that we have clarified the advantages and disadvantages to the Agency of producing a

list of counties with high estimated NATA exposures and risks.

Should the Agency elect to produce a list of high exposure/high risk counties as part

of the NATA, we recommend that the Agency do this by developing a qualitative ranking with

perhaps an alphabetic listing in a table of the counties that score in the top Y (e.g., 1 to 5)%

of exposure and risk, along with an indication of each variable that contributes to this high

ranking (emissions by source type, local meteorological conditions, demographic or timeactivity factors, or particular compound classes or toxicity assumptions associated with

those compounds). Across the table could be listed the factors that contribute to the ranking
60

and an “X” could be placed in the table when a listed county is in the top percentage group

for that variable. This would allow the reader to identify which counties were in the top

group as a result of the key contributing factor(s), rather than just their presence on the list

as a result of the final, aggregated estimate of risk. While comparative ranking between

individual counties within the top grouping (i.e. which is #1) would be highly problematic, it

is likely that there is sufficient stability in the predictions to indicate that those in the top

grouping as a result of factors known with a relatively high degree of confidence do deserve

closer scrutiny. 

Recommendation # 50: For the 1996 NATA, it would be most useful if there were an

executive summary that would summarize the key findings and conclusions.

Recommendation # 51: For the 1996 NATA, at the start of each section, it would be

helpful to have the authors describe the top 5 or 6 limitations that they believe have the

greatest impact on the results/conclusions.

Recommendation #52: For the 1996 NATA, the Agency, especially in materials

intended for non-technical individuals, should clearly distinguish between those parts of

NATA that are well established, vs. those which are in an earlier, developmental stage.

Recommendation # 53: For the 1996 NATA , for the lay public it will be important to

place the consequences of exposure into public heath context. A graphic representation such

as a “thermometer” type graph could be used to display the levels at which different health

effects are seen, or to present different cancer risk levels. Whatever approach the Agency

chooses, all communication materials intended for the general public should be pretested to

assure comprehension.

Recommendation # 54: For the 1996 and 1999 NATA, we recommend that the Agency

consider developing a qualitative ranking with perhaps an alphabetic listing in a table of the

counties that score in the top grouping in terms of exposure and risk, but that this table be

accompanied by an indication of the factors that contribute to each county being among the

high exposure/ high risk grouping, and the degree of confidence that can be placed in these

factors.

3.2.8 Charge Question 8 

The exposure methodology in NATA is being considered as one candidate for providing

the basis for a national scale benefits analysis (as required in section 812 CAA). Please comment

on the strengths and weaknesses of this approach, recognizing the limitations outlined in the

NATA report.

Section 812 of the Clean Air Act Amendments of 1990 requires the EPA to

periodically assess the effects of the Act on the public heath, environment and the economy. 

These assessments seek to compare benefits (e.g., health expressed in various monetary

terms) and costs (e.g., costs of emission management options). Air toxics represent one

aspect of the assessment that has not yet been quantified. The NATA exposure methodology

is being considered as one viable approach to quantifying the relationships between

emissions, concentrations, exposures and risks. In the 812 studies, the risks are then

translated into monetary values to be compared to emission management option costs.
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Given the needs of the 812 study for an approach that can provide a sound basis for

estimating benefits, the Panel must conclude at this point that the current exposure

methodology and results in NATA are not yet ready for use in a national scale benefits

analysis. This review has already noted the limitations of the models and data bases being

used in NATA. Use of the current approach in the 812 studies would be subject to the same

critiques.

Once the needed improvements noted above are implemented, application to benefits

assessment can be considered. The particular improvements that have been listed as

essential deal with the shortcomings of the models and the fact that a meaningful benefits

assessment must consider the full distribution of exposure and risk (not just median values). 

It should also address sub-chronic health effects. Once exposure predictions are improved

as noted and then validated, the cost-effectiveness of alternative toxics management

strategies (for emissions and exposure reductions) could be compared, stopping short of a

full benefits assessment. A full benefits assessment would need to consider health risks,

mortality and morbidity avoided. 

Another precaution that is needed for such a calculation is that best-estimate values of

toxicity dose-response metrics should be used to obtain best-estimate values of health

benefits. In contrast, upper-bound estimates of toxicity values, such as those typically found

in IRIS, yield conservatively high estimates of health benefits (assuming that these upperbound toxicity values are combined with best-estimate values of exposure).

In our response to questions 2 and 4, we recommended that a full distribution

analysis of exposures and risks be conducted for a HAP for which there are adequate data

available across the US. One candidate HAP is benzene since adequate information is

available for benzene to be able to do the analysis. If this recommended analysis is

conducted, then it would be possible to conduct an initial benefits assessment for that HAP,

to illustrate the type of analysis that is envisioned for a broader benefits assessment

involving multiple toxics in the future.

Recommendation # 55: For the 1996 NATA, results from the proposed assessment, for

an information-rich HAP such as benzene, would be appropriate for the 812 study and should

be considered. Descriptions of the limitations of the NATA for the 812 national benefits

assessment need to be clearly articulated in both the NATA and the 812 studies. NATA and

Section 812 study teams should work together to assure that the important goals of these

related assessments are attained in a timely manner.

3.2.9 Charge Question 9 

Do you have suggestions for research priorities that would improve such air toxics

assessments in the future?

An extensive research effort should be mounted to address the wide array of the data

and model development needs to significantly improve the scientific foundation for future

NATA studies as well as regulations based on the health risks of air toxics. The needs

include both fundamental and chemical-specific research and span the whole of the risk

paradigm (i.e., emissions, ambient concentrations, exposures, effects, and risks). The NATA

document (U.S. EPA/OAQPS, 2001, pp. 126-127) does a good job of outlining the variety

of research needs. Because air toxics research has been under-funded by the Agency for so

long, considerable new resources are needed to address these needs. Fortunately, the NATA

allows identification of the uncertainties that are inhibiting the development of reliable
62

quantitative assessments, so that new resources could be well-focused. Prioritization is

always difficult when there are so many needs, but perhaps this effort could be assisted by

some sensitivity analyses based on the NATA. 

Using the information developed in research programs is just as important as

generating the information. Thus, no air research program can be useful until it is

incorporated in Agency models for assessments. In the case of new research on health

effects and dose-response factors, such information must be entered into IRIS. In numerous

sections of this document, the importance of having an up-to-date, current IRIS database has

been discussed. Support of IRIS also needs appropriate resources.

We understand that the EPA ORD is completing a research strategy for air toxics, so

there in no need for SAB to duplicate this effort. We recommend that this plan be

developed in concert with external experts on the related topics and that the subsequent draft

be reviewed by this or a similar Panel. The Health Effects Institute is also preparing a

Mobile Source Air Toxics research strategy, so ORD might also derive benefit from this

activity. In addition, research needs on diesel particulate matter can be gleaned from the

recent diesel assessment (U.S. EPA. 2000). All of this must happen rapidly if new research

is to be completed in time to impact the next NATA (and imminent air toxics regulatory

assessments). The issue of near-term and long-term research needs to be explicitly

addressed. It will likely take EPA some time to complete the Air Toxics Research Strategy,

and then implementation will require lead times consistent with future budget development. 

In the meantime, the knowledge base and dose-response assessment base for the 1999

NATA must be improved. In Appendix B we describe specific areas of focus that the Panel

has identified as important for such a research effort. A more rigorous delineation of the

Agency’s research plan, for air toxics in general and NATA in particular, should be made

considering this and other inputs and information, and subject to SAB review.

Recommendation # 56: EPA should rapidly develop a research plan to identify the

work (information collection, research, and assessments) it will perform with existing

resources over the next few years that will directly improve the 1999 NATA. This plan should

be closely linked to, and consistent with, the overall Air Toxics Research Strategy and should

be reviewed by this or a similar Panel.
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EPA, Research Triangle Park, North Carolina, June 1, 1994 
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Study of Costs and Benefits (1999): Advisory by the Health and Ecological Effects

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Study of Costs and Benefits (1999): Advisory by the Council on Clean Air

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Wallace. 1995. Clinical and Experimental Allergy, 1995, 25:4-9

Website Address for Charge 7 New York State Department of Health (NYSDOH) Toxicity

Trees is as follows:

http://www.health.state.ny.us/nysdoh/environ/btsa.htm

http://www.health.state.ny.us/nysdoh/environ/btsa/figure1.pdf
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APPENDIX A - A MORE DETAILED DESCRIPTION OF THE SAB

PROCESS

The SAB Staff recruited Dr. Mitchell Small, Chair of the Executive Committee's

Environmental Models Subcommittee (EMS) and the H. John Heinz, III Professor of

Environmental Engineering in the Departments of Civil & Environmental Engineering and

Engineering & Public Policy at Carnegie Mellon University, to serve as Chair of the

Subcommittee. Working with the Chair, other SAB Members and Consultants, Agency

Staff, and suggestions from the public, the SAB Staff compiled a list of over 50 scientists

and engineers ("Wide Cast") whose expertise appeared to be relevant to answering the

questions in the Charge. Subsequently, the Chair, the Staff Director and the DFO reviewed

the list in some detail and identified 22 individuals (“Narrow Cast") to contact regarding

their interest and availability to participate on the Panel. Based on this information and the

importance of having a balanced range of views on the technical issues represented on the

Panel, the Chair and the DFO made recommendations for membership to the Staff Director,

who made the final decision on the composition of the Panel. This process included

assigning different members Lead and Associate responsibilities for each of the Charge

questions.

The Agency transmitted review materials to the Subcommittee members in late

January, 2001. On February 21 the SAB Staff convened a publicly-accessible, Federal

Register-noticed conference call meeting between Panel members and Agency staff. The

goal of this information-gathering meeting was to clarify any questions that Panel Members

might have, to identify any gaps in the information sent to the Panel, and to identify areas

that the Agency should be prepared to clarify at the face-to-face meeting. Minutes of the

meeting were posted on the SAB Website: www.epa.gov/sab. In addition, public comments

were received and distributed to the Panel Members at the February 21, 2001 informational

conference call meeting from many of the groups that attended and spoke at the March 20 &

21, 2001 meeting. 

On March 20-21, 2001 the Panel convened in the ballroom of the Raddison

Governor’s Inn Hotel, Research Triangle Park, NC. Those groups providing formal written

public comments are listed below. All parties spoke during the public comments session on

March 20th, except for the latter two groups, which transmitted written public comments

without attending the meeting. The groups and presenters are listed as follows:

a. The Acrylonitrile Group, Mr. Chuck Elkins,

b. The Residual Risk Coalition, Mr. Chuck Elkins,

c. The Colorado Air Pollution Control Division, Ms. Lisa J. Silva,

d. The Ethylene Oxide Council, Dr Jane Teta, 

e. The Engine Manufacturers Association, Mr. Timothy French

f. The Halogenated Solvents Industry Alliance, Mr. Stephen P. Risotto,

g. The Hydrazene Panel of the American Chemistry Council, Ms. Claudia O’Brien of

Latham and Watkins,

h. The International Truck and Engine Corporation, Ms. Claudia O’Brien of Latham &

Watkins,

i. Dr. Robert J. Carton, Chief of Environmental Protection, U.S. Army Medical

Research & Materiel Command, Fort Dietrick, MD (written comments submitted,

but not in attendance at meeting), and
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j. Dr. Amy D. Kyle, Univ of Calif, Berkeley, CA (written comments submitted, but not

in attendance at meeting).

During the March 20 & 21, 2001 public meeting, the NATA Review Panel heard

presentations from the Agency staff on the first day, as well as public comments. This was

followed by detailed discussion by the NATA Panelists on the nine charge questions. The

second day saw the discussion being completed by the NATA Review Panel on the Charge

questions in the morning, followed by preparation for a poster session by the NATA Review

Panel members and consultants (M/C) on key points within each charge question, as well as

re-writing of the pre-meeting written comments by the NATA Panelists to their assigned

charge questions, and teaming in groups by the NATA Panelists to develop merged language

edits.

By the end of the first day, the individual comments and merged edits were

incorporated into a template for a first draft, which was given to the Chair to synthesize into

a second draft. Dr. Small emailed the second draft to the NATA Panel on April 6th. There

was a contingency provision announced in the Federal Register Vol. 66, No. 29, February

12, 2001, pages 9846-9847, to hold a public conference call on April 24th, should it be

needed. The NATA Review Panel decided to exercise this option, and planned to conduct a

technical editing public conference call in which the public can follow the NATA Review

Panel’s discussions on their working draft, which is not yet a public consensus report. The

NATA Review Panel anticipated that a public consensus draft would be completed around

May 1st, and planned to hold a public conference call to reach closure on edits to that draft

report on May 14th in order to give the NATA Panelists and the public adequate reading on

the draft report. The draft took longer to develop, and consequently the Panel M/C met in public

conference call follow-up technical editing work sessions on April 24th, May 14th and May 25th where the

public listened in, but no public comments were solicited. The first “working” public draft was

developed on June 6th and posted onto the SAB website on June 7th (www.epa.gov/sab under

“draft reports”) for discussions on June 13th.

The NATA Review Panel held a public conference call on June 13th in which the first

public draft report, dated June 6th was shared with all parties and on which public comments

were solicited. Following receipt of Panel and public comments, a revised working draft

dated July 20th was prepared and the Panel convened a technical editing (non-FACA) work

session on July 31st to complete the edits. Following this work session, the edits were

incorporated into a second public draft report dated August 10th. This draft was posted onto

the SAB web site (www.epa.gov/sab under “draft reports”) for access by the public

(including the Agency). A public closure meeting was held on Wednesday, August 29, 2001

in which the NATA Review Panel conducted final edits and the public was given an

opportunity for closure comments. Following this August 29th meeting, a September 5th

public draft was prepared for a vetting review by the SAB’s Executive Committee on

September 17th, at which public meeting the public was invited to comment by the Chair of

the SAB Executive Committee. The Chair of the NATA Review Panel conferred with the

SAB Executive Committee discussants and completed the edits to this advisory, resulting in

this final version being submitted to the Administrator.

NOTE: Throughout the process, the SAB has provided announcements in the Federal

Register, as well as posting notices, agendas, and the publically-available draft reports onto

the SAB website (www.epa.gov/sab), along with related efforts to reach out to all potentially

affected and interested parties. This also included development of a wide-cast list and

narrow-cast list of candidates for the NATA Review Panel, as well as a conference call
A-3

meeting one month prior to the March face-to-face public meeting to discuss and negotiate

the charge, determine if the review materials are adequate, and begin the pre-meeting review

and writing process. The Agency also provided a URL site for all Agency review materials,

appendices, background briefings and related materials. 
B-2

APPENDIX B - AREAS OF FOCUS IDENTIFIED BY PANEL MEMBERS

FOR RESEARCH TO IMPROVE FUTURE NATA STUDIES 

The NATA Review Panel recognizes that evaluation of the NATA national-scale

results is an iterative process and supports the research needs already recognized by the

Agency, as discussed in the 1996 NATA document (U.S. EPA/OAQPS, 2001), including

(pages 126-127):

a) Improve the quality of emission data;

b) Improve the support for urban-scale modeling;

c) Improve the characterization of background concentrations of air toxics;

d) Provide support for future model-to-monitor comparisons for ambient air toxics

concentrations;

e) Provide support for future model-to-monitor comparisons for exposure;

f) Improve dose-response information;

g) Extend EPA risk assessment guidelines to be more inclusive of children and other

vulnerable subpopulations; and

h) Improve modeling to include multipathway exposures.

As mentioned in the main text, we also encourage the Agency to complete its Air

Toxics Research Strategy and take advantage of the related activities of other organizations. 

The following text offers additional thoughts on research needs, which are similar to some

of those already identified by EPA (see pages 126-127 of the NATA document).

A) General Methods Research: Research is needed on fundamental, general tools and

methodology. These will provide the methods for estimating uncertainty and variability for

population distributions of exposure and risk to the general populace and susceptible

populations. 

1) Improved multimedia, multipathway, multipollutant transport, fate, and

transformation (including secondary pollutant formation) models that have been

scientifically evaluated (e.g., validated) and that estimate the relationship between

sources (outdoors and indoors) and environmental levels;

2) Improved multimedia, multipathway, multipollutant exposure and dose models

(that have been scientifically evaluated/validated) to relate environmental

concentrations to the population distribution of actual human exposure and dose; 

3) Improved and harmonized cancer and noncancer assessment methods that can be

applied to air toxics as multimedia, multipathway chemicals;

4) Improved methods to estimate distributions of risks for individual air toxics as

well as mixtures of air toxics; and 

5) Improved treatment of exposure to hot spot emissions.

B) Chemical-Specific Information Needs: Research, testing and data collection are needed to

estimate specific emission, fate-and-transport, exposure and toxicity values for air toxics.
B-3

1) Improved emissions inventories to obtain better environmental, exposure, and

dose measurements to enable development, evaluation, and verification of models; 

2) Use of Geographic Information System (GIS) tools for displaying and

communicating emissions estimates. The Agency should focus on developing

improved methods for direct cross-validation of emission estimates. This might

include use of Geographic Information System (GIS) tools for displaying and

communicating emissions estimates to state and local agencies and stakeholder

groups that are well-positioned to ground-truth the data;

3) Improve Estimates for Non-Road Mobile Source Emissions. Non-road mobile

source emissions appear to be major contributors to risks associated with toxic air

pollutants. However emissions models and inventory development methods for nonroad mobile sources are not as well developed as those for on-road vehicles. The

efforts to improve methods for estimating emissions from non- road mobile sources

that are underway at the Agency deserve priority, and should be followed closely by

staff working on NATA;

4) Improve background concentration estimates for air toxics. The NATA Review

Panel agrees with the Agency that improving the characterization of background

concentrations for air toxics so that they can be treated as region and season-specific

is an important priority; 

5) Improvements in knowledge of emissions from indoor sources for the air toxics

of interest to NATA. The main text recommends that future NATAs consider total

human exposure to air toxics. This requires exposure models that can make such

estimates (as addressed under fundamental scientific needs) and total (outdoor and

indoor) emissions information on specific chemicals;

6) Improvements in longitudinal activity patterns for different cohorts are necessary. 

At present, only daily-time activity information has been used in the NATA. In future

assessments, the implementation of the HAPEM model needs to be improved to

adequately reflect the full range of interindividual variability in air toxics exposures. 

To support this, the collection of multi-day time activity pattern data is needed to

allow characterization of long-term persistence in individual behavior and exposure. 

One research need for doing this correctly is to investigate and incorporate

longitudinal activity pattern data for different cohorts. 

7) Improve the current “zero” value used for the ADD factor (indoor and background

sources of exposure) in HAPEM. This would be facilitated by a review TEAM and

NEXHAS data to determine their relevance for incorporation to improve HAPEM;

8) Fundamental studies are needed on the behavior of gases and particles, and their

interactions, in the respiratory system; and

9) Dose-response and mechanistic studies are needed targeted to the specific

uncertainties that drive the risk for the chemicals of higher concern.
C-1

APPENDIX C – GLOSSARY

ADD Additive Factor (Used in the exposure model HAPEM4 to account for

the contribution from indoor sources to personal exposures)

AIRS-A Aerometric Information Retrieval System (Data base)

ASPEN Assessment System for Population Exposure Nationwide (dispersion 

model)

ATSDR Agency for Toxic Substances and Disease Registry

CAA Clean Air Act

CAAA Clean Air Act Amendments

CASAC Clean Air Scientific Advisory Committee (of the U.S. EPA/SAB)

CEP Cumulative Exposure Project

CHAD Consolidated Human Activity Database (an EPA database for 40 cohort

groups)

CMAQ Community Multi-scale Air Quality (model)

CO Carbon Monoxide

Cr Chromium and Isotopes (e.g., Cr+3 - Trivalent and Cr+6 - Hexavalent

Chromium)

DEP Diesel Exhaust Particles

EMS Emissions Modeling System

EPA U.S. Environmental Protection Agency (U.S. EPA)

FQPA Food Quality Protection Act

GIS Geographic Information System 

HAD Hazard Assessment Document

HAP Hazardous Air Pollutant

HAPEM Hazardous Air Pollutant Exposure Model

HEI Health Effects Institute

Hg Mercury

HQs Hazard Quotients

IRIS Integrated Risk Information System (data base)

ISC Industrial Source Complex (model)

IUATA Integrated Urban Air Toxics Assessment (Strategy)

LOAEL Lowest Observed Adverse Effects Level

LRT Long Range Transport

MACT Maximum Achievable Control Technology

MF Modification Factor

MLEs Maximum Likelihood Estimates

MobTox Mobile Toxic Emission Model (for mobile sources, e.g., MobTox5b)

MODELS3 A Comprehensive Modeling Framework Currently Under Development

by U.S. EPA/ORD 

MRL Minimum Risk Level 

MS Mobile Sources

MSAT Mobile Source Air Toxics

NAAQS National Ambient Air Quality Standards

NATA National-Scale Air Toxics Assessment (also National Air Toxics Assessment)

NCEA National Center for Environmental Assessment (U.S.

EPA/ORD/NCEA)

NET National Emission Trends

NHEXHAS National Human Exposure Health Assessment Survey 

NLEV National Low Emission Vehicle
C-2

NTI National Toxics Inventory

NYC New York City

NYS New York State

NYSDEC New York State Department of Environmental Conservation

O3 Ozone

OAEL Observed Adverse Effects Level

OAQPS Office of Air Quality Planning and Standards (U.S. EPA/OAR/OAQPS)

OAR Office of Air and Radiation (U.S. EPA/OAR)

ORD Office of Research and Development (U.S. EPA/ORD)

OTAQ Office of Transportation and Air Quality (U.S. EPA/ORD)

OZIP OZone Isopleth Plotting Model (for predicting ozone in urban areas)

PAH Polynuclear Aromatic Hydrocarbons (one type of POM)

PBTs Persistent Bioaccumlative Toxics

PM Particulate Matter

POM Polycyclic Organic Matter 

QA/QC Quality Analysis and Quality Control

RELs Reference Exposure Levels

RfCs Reference Concentrations

RFG Reformulated Gasoline

SAF Spatial Allocation Factors

SIC Standard Industrial Classification

TEAM Total Exposure Assessment Methodology

TEF Toxicity Equivalency Factor ?

TMDL Total Maximum Daily Load

TOG Total Organic Gasses

TOSHI Target Organ-Specific Hazard Index

TRI Toxics Release Inventory

UCL Upper Confidence Limit

UF Uncertainty Factor

UREs Unit Risk Estimates

URF Unit Risk Factor

U.S. United States

VMT Vehicle Miles Traveled

VOC Volatile Organic Compounds