United States EPA Science Advisory EPA-SAB-EC-ADV-02-001
Environmental Board (1400A) December 2001
&EPA NATA - EVALUATING THE
NATIONAL-SCALE AIR
TOXICS ASSESSMENT
1996 DATA -AN SAB
ADVISORY
AN ADVISORY BY THE EPA
SCIENCE ADVISORY BOARD
(SAB)
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December 20, 2001
EP A-S AB-EC-AD V-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 paniculate
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 industry-
specific 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 NAT As. 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, collegia!, cognizant of shortcomings in the document, and accepting of the
NAT A 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 NAT As.
Sincerely,
/signed/ /signed/
Dr. William daze, 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,
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Washington, DC 20460-0001; 202-564-4533].
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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), dose-
response 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 NAT As, 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, UMDNJ-
Robert 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-l
APPENDIX A - A MORE DETAILED DESCRIPTION OF THE SAB PROCESS A-l
APPENDIX B - AREAS OF FOCUS IDENTIFIED BY PANEL MEMBERS
FOR RESEARCH TO IMPROVE FUTURE NATA STUDIES B-l
A) General Methods Research B-l
B) Chemical-Specific Information Needs B-l
APPENDIX C - GLOSSARY C-l
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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 NAT A, 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 Cr+) 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
FLAP emissions) and FLAPEM4 (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, FLAPs 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 FLAPEM4 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
-------
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.
Further discussion and methods development is needed to address concerns about whether
certain demographic groups, especially poor and transient populations, are under-
represented 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/EED. 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
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.
-------
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 NAT A. 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 NAT A (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 NAT As should address additional (non-
inhalation) 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:
Aj "Known" Carcinogens based on sufficient evidence of carcinogenicity
from epidemiologic studies to support a causal association between exposure
to the agents and cancer;
Bl: "Probable" Human Carcinogens based on limited evidence of
carcinogenicity from epidemiologic studies, but sufficient evidence from
animal studies;
-------
B2: "Probable" Human Carcinogens based on sufficient evidence of
carcinogenicity from animal studies, but inadequate evidence or no data
from epidemiologic studies.
Cj "Possible" Human Carcinogens 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 NAT As. 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.
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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 NAT A. 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 par ticulate 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 par ticulate 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, dose-
response 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
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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 dose-
response) 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 inter-
individual 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? Canyon
suggest alternative methods or formats that could improve the presentation and communication
of these results?
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.
-------
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
10
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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 chemical-
specific research and span the whole of the risk paradigm (i.e., emissions, ambient
concentrations, exposures, effects, and risks). Because air toxics research has been under-
funded 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
3 Sub-chronic health effects is referred to here in the context of generally accepted animal toxicology studies.
11
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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 NAT A.
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 dose-
response 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.
12
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TABLE 1-1 - SUMMARY TABLE OF NATA REVIEW PANEL RECOMMENDATIONS
No.
CHARG
ENo.
SUBJECT
ABBREVIATED RECOMMENDATION
SECTION WHERE
DISCUSSION CAN BE
FOUND
RECOMMENDED SCHEDULE
FOR IMPLEMENTATION
1
2
3
4
5
6
7
8
9
10
none
1
1
1
1
1
1
1
1
1
General Findings
National Toxics
Inventory
National Toxics
Inventory
National Toxics
Invewntory
Reactivitiy Class
Decay Rates
Reactivity Class
Decay Rates
QA/QC and
Reduction of
Uncertainties
QA/QC and
Reduction of
Uncertainties
QA/QC and
Reduction of
Uncertainties
QA/QC and
Reduction of
Uncertainties
Separate peer review should be conducted for the specific input parameters and
values assumed for the different modules of the NATA model.
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.
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.
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.
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
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.
Implement additional QA/QC measures to ensure that a satisfactory level of
completeness and accuracy is reached for all emission inventories.
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 Causing the
available literature on this subject. In addition, a reactivity decay rate will have to
be developed and incorporated into EMS-HAP for Cr6
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.
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.
3.1
3.2.1.1
3.2.1.1
3.2.1.1
3.2.1.2
3.2.1.2
3.2.1.4
3.2.1.4
3.2.1.4
3.2.1.4
1996, 1999 and FUTURE
NAT As
1999 NATA
1999 NATA
1999 NATA
1996 NATA
1999 NATA
1999 NATA
1999 NATA
FUTURE NATAs
1999 NATA
13
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No.
11
12
13
14
15
16
17
18
19
CHARG
ENo.
2
2
2
2
2
2
2
2
3
SUBJECT
Model Issues -
ASPEN
Model Issues -
ASPEN
Model Issues -
ASPEN
Model Issues -
HAPEM
Model Issues -
HAPEM
Model Issues -
Future Applications
Model Issues -
Future Applications
Model Issues -
Future Applications
Dose-Response
Information
ABBREVIATED RECOMMENDATION
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
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
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.)
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.
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.
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.
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.
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.
For the 1 996 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.
SECTION WHERE
DISCUSSION CAN BE
FOUND
3.2.2.3
3.2.2.3
3.2.2.3
3.2.2.3
3.2.2.3
3.2.2.3
3.2.2.3
3.2.2.3
3.2.3
RECOMMENDED SCHEDULE
FOR IMPLEMENTATION
1996NATA
1996 NATA
1996 NATA
FUTURE NATAs
1996, 1999 and FUTURE
NATAs
FUTURE NATAs
FUTURE NATAs
FUTURE NATAs
1996 & 1999 and FUTURE
NATAs
14
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No.
20
21
22
23
24
25
26
27
28
29
30
31
CHARG
ENo.
3
3
3
3
3
3
3
3
4
4
4
4
SUBJECT
Dose-Response
Information
Degree of
Conservatism in
Health
Validating Dose-
Response
Predictions
Use of Ora I vs.
Inhalation Data
Deviations from
Linearity
Indirect Exposures
Uncertainties in the
Dose Response
Micro Environments
and Dose Response
Risk
Characterization:
Weaknesses of the
Overall Approach
Risk
Characterization:
Weaknesses of the
Overall Approach
Risk
Characterization:
Weaknesses of the
Overall Approach
Aggregate and
Cumulative Risk
Issues
ABBREVIATED RECOMMENDATION
For the 1 999 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).
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.
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.
For 1 996, provide an estimate of the potential variability of the oral to inhalation
extrapolation, and the implications of this for the derived toxicity values.
Consideration should be given in future NAT As to possible deviations from linearity
in the dose-response functions for non-cancer risk.
The 1999 NATA should include the effects of indirect (non-inhalation) exposures
for PBTs.
For the 1 996 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.
As acute health effects are considered for evaluation in future NAT As, 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.
For the 1 996 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.
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.
For 1996 NATA, provide a more balanced discussion of the possible sources of
under- versus- over-estimations of HAP exposures and risks.
For the 1 996 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,
SECTION WHERE
DISCUSSION CAN BE
FOUND
3.2.3
3.2.3.1
3.2.3.2
3.2.3.3
3.2.3.4
3.2.3.6
3.2.3.7
3.2.3.8
3.2.4.2
3.2.4.2
3.2.4.2
3.2.4.3
RECOMMENDED SCHEDULE
FOR IMPLEMENTATION
1999 and FUTURE NAT As
1996, 1999 and FUTURE
NAT As
1999 and FUTURE NAT As
1996 NATA
1999 and FUTURE NATA
1999 and FUTURE NAT As
1996 NATA
FUTURE NATAs
1996, 1999 and FUTURE
NATAs
1996,1999 and FUTURE
NATAs
1996 NATA
1996 NATA
15
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No.
32
33
34
35
36
37
38
39
40
41
42
43
CHARG
ENo.
4
4
4
4
4
4
4
4
4
4
5
6
SUBJECT
Aggregation and
Characterization of
Cancer Risks.
Aggregation and
Characterization of
Cancer Risks.
Aggregation and
Characterization of
Cancer Risks.
Aggregation and
Characterization of
Non-Cancer Risks.
Aggregation and
Characterization of
Non-Cancer Risks.
Alternative Risk
Evaluations.
Alternative Risk
Evaluations.
On the Issue of
Children.
On the Issue of
Children.
Additional
Clarification Issues
Diesel Emissions
Uncertainty and
Variability
ABBREVIATED RECOMMENDATION
For the 1 996 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.
For the 1 996 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.
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.
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.
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.
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.
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 1 999 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.
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.
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.
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.
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.
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.
SECTION WHERE
DISCUSSION CAN BE
FOUND
3.2.4.3.1
3.2.4.3.1
3.2.3.4.1
3.2.4.3.2
3.2.4.3.2
3.2.4.4
3.2.4.4
3.2.4.5
3.2.4.5
3.2.4.6
3.2.5
3.2.6
RECOMMENDED SCHEDULE
FOR IMPLEMENTATION
1996, 1999 and FUTURE
NATAs
1996 NATA
1996 NATA
1996, 1999 and FUTURE
NATAs
1996. 1999 and FUTURE
NATAs
1999 NATA
1996, 1999 and FUTURE
NATAs
1996 NATA
1999 and FUTURE NATAs
1996 NATA
1996 NATA
1996 NATA
16
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No.
44
45
46
47
48
49
50
51
52
53
54
55
CHARG
ENo.
6
6
6
7
7
7
7
7
7
7
7
8
SUBJECT
Uncertainty and
Variability: Specific
Comments
Uncertainty and
Variability: Specific
Comments
Uncertainty and
Variability: Specific
Comments
Uncertainty and
Variability: Specific
Comments
Uncertainty and
Variability: Specific
Comments
Uncertainty and
Variability: Specific
Comments
Communications
Communications
Communications
Communications
Communications
Benefits Analysis
ABBREVIATED RECOMMENDATION
For the 1996 NATA, differentiate between NATA-specific and universal sources of
uncertainty, and between major and minor sources of uncertainty.
Use the scenario analysis to help bound the NATA risk estimates and avoid
oversimplified characterization of the "nominal" results as conservative.
Provide more detail in the main NATA documentation on uncertainties associated
with emissions from area, on-road mobile and non-road mobile sources.
Distinguish between reducible uncertainty (due to lack of information) and
irreducible variability.
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.
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.
For the 1996 NATA, it would be most useful if there were an Executive Summary
that would summarize the key findings and conclusions.
For the 1 996 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.
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.
For the 1 996 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.
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.
For the 1 996 NATA, results from the proposed assessment, for an information-rich
HAP such as benzene, would be appropriate for the CAAA Section 81 2 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 81 2 studies. NATA and CAAA Section 81 2 study
teams should work together to assure that the important goals of these related
assessments are attained in a timely manner.
SECTION WHERE
DISCUSSION CAN BE
FOUND
3.2.6.1
3.2.6.1
3.2.6.1
3.2.6.1
3.2.6.1
3.2.6.1
3.2.7
3.2.7
3.2.7
3.2.7
3.2.7
3.2.8
RECOMMENDED SCHEDULE
FOR IMPLEMENTATION
1996, 1999 and FUTURE
NATAs
1996, 1999 and FUTURE
NATAs
1996, 1999 and FUTURE
NATAs
1996, 1999 and FUTURE
NATAs
1996, 1999 and FUTURE
NATAs
1996, 1999 and FUTURE
NATAs
1996 NATA
1996 NATA
1996 NATA
1996 NATA
1996 NATA
1996 NATA
17
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No.
56
CHARG
ENo.
9
SUBJECT
Future Research
Priorities
ABBREVIATED RECOMMENDATION
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.
SECTION WHERE
DISCUSSION CAN BE
FOUND
3.2.9
RECOMMENDED SCHEDULE
FOR IMPLEMENTATION
1996, 1999 and FUTURE
NAT As
18
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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 technology-
based 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 community-
based 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
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.
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.
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
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inhalation exposure resulting from emissions of diesel paniculate 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?
20
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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.
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, dose-
response 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
21
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22
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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
23
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risk estimates requiring the use of toxicity values based on different amounts of scientific
information and consensus. To help citizens and other users of NAT A better understand the
differing bases for NAT A results, we recommend use of a hierarchical presentation of
results mat 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 NAT As 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
24
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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 mat 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 NIT/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
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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 MobToxSb 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.
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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(1)(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.
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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.
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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 (Ci6+) 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 Ci6+ man census tracts
impacted by municipal waste combustion facilities. The Agency should apportion Ci6+ 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-and-
transport 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 C/+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 C/+.
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
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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 NAT As 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) Canyon 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
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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 NAT A, 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.
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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 well-
established 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 model-
observations 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
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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
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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-by-
case, 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 'national-
scale' (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
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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
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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 well-
developed 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
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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 day-
to-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 NAT A 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.
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3.2.2.3 Summary Recommendations for Charge Question 2
For the 1996 NAT A:
1. The NA TA document should be modified as per the specific recommendations of the
previous section, l.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 sreglons 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 ofHAPEM 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-representatlon 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-lndlvldual
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,
multlscale, 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)
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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 multi-
media 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 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.
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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
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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
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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 (non-
inhalation) 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
lexicological 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/HAPEMprocess 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
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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 oftoxicity 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.
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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 less-
than-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.
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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 NA TA, 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 dose-
response 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
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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 = Nfor L)OAEL I [UF X MF]). For
the air toxics in NAT A, 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.
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For noncancer hazards, efforts were also made to increase comparability for
aggregated risks. To increase comparability of HQs across different pollutants, TOSFfls
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 NAT As 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 Bl 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, Bl, 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
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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).
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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?
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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 1996NATA, 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.
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Where children are a special concern, the data need to be evaluated and assessed
appropriately. The paragraph ends with a comment about higher TO SHI'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 less-
than-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?
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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 non-
cancer 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 199 6 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 par ticulate 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 paniculate 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.
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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 CAS AC endorsed
the range of probable cancer risk portrayed in the document. Although CAS AC 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 CAS AC 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 (PMfme)7.
Because DEP comprises a minor, but significant portion of PMfme in urban inventories, and a
major portion in certain microenvironments, the health effects of DEP must be integral to
those attributed to PMfme, 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.
Note that the usage of the term, PMfme 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., PM[ 0 or PM
0.5).
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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 ambientparticulate 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, dose-
response 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 top-
down 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
uncertainly 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
paniculate 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
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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 NAT A, 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 1996NATA, 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
"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.
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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 1996NATA, 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, on-
road 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.
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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? Canyon
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
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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
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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) lexicological profiles as well as in materials developed by the
State of New York. See, for example,
http ://www.health. state.ny .us/ny sdoh/environ/btsa.htm and
http://www.health.state.ny.us/nysdoh/environ/btsa/figurel.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 time-
activity 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
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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 upper-
bound 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
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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 NA TA. 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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Website Address for Charge 7 New York State Department of Health (NYSDOH) Toxicity
Trees is as follows:
http ://www.health. state.ny .us/ny sdoh/environ/btsa.htm
http://www.health.state.nv.us/nvsdoh/environ/btsa/figurel.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, UJ 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
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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.
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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.
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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 non-
road 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 NAT A;
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 NAT As 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.
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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
CAS AC 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., MobToxSb)
MODELS3 A Comprehensive Modeling Framework Currently Under Development
byU.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
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NTI
NYC
NYS
NYSDEC
03
GAEL
OAQPS
OAR
ORD
OTAQ
OZIP
PAH
PBTs
PM
POM
QA/QC
RELs
RfCs
RFG
SAP
SIC
TEAM
TEF
TMDL
TOG
TOSHI
TRI
UCL
UF
UREs
URF
U.S.
VMT
VOC
National Toxics Inventory
New York City
New York State
New York State Department of Environmental Conservation
Ozone
Observed Adverse Effects Level
Office of Air Quality Planning and Standards (U.S. EPA/OAR/OAQPS)
Office of Air and Radiation (U.S. EPA/OAR)
Office of Research and Development (U.S. EPA/ORD)
Office of Transportation and Air Quality (U.S. EPA/ORD)
OZone Isopleth Plotting Model (for predicting ozone in urban areas)
Polynuclear Aromatic Hydrocarbons (one type of POM)
Persistent Bioaccumlative Toxics
Particulate Matter
Polycyclic Organic Matter
Qualify Analysis and Quality Control
Reference Exposure Levels
Reference Concentrations
Reformulated Gasoline
Spatial Allocation Factors
Standard Industrial Classification
Total Exposure Assessment Methodology
Toxicity Equivalency Factor ?
Total Maximum Daily Load
Total Organic Gasses
Target Organ-Specific Hazard Index
Toxics Release Inventory
Upper Confidence Limit
Uncertainty Factor
Unit Risk Estimates
Unit Risk Factor
United States
Vehicle Miles Traveled
Volatile Organic Compounds
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