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Appendixes NOTE: The following appendixes have been typeset from a variety of original sources. The typographical appearance of the originals, as well as the editorial content and style, has been preserved wherever possible.

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Page 289 Appendix A Risk Assessment Methodologies: EPA's Responses to Questions from the National Academy of Sciences Disclaimer This document was prepared primarily by the staff of the Pollutant Assessment Branch within the Office of Air Quality Planning and Standards. Some of the responses that describe future risk assessment procedures and policies represent the opinions of the authors within the Office of Air Quality Planning and Standards and do not necessarily represent the U.S. Environmental Protection Agency policy.

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Page 290 Table of Contents   Page List of Figures 292 List of Tables 293 I. Risk Assessment Requirements (Question 1) 294   A. Introduction 294   B. Regulatory Flow and Chronology of Title III Implementation 294   C. Levels of Risk Assessment 294   D. Risk Assessment Review Requirements 296   E. Title III Risk Assessment Provisions 297 II. Past and Present Examples (Question 2) 306   A. Introduction 306   B. Generic Discussion 306     1. Risk Assessment Guideline Development 306     2. Hazard Assessment Document Development 307     3. Exposure Methodology 308     4. Risk Characterization and Treatment of Uncertainty 310     5. Some Differences Between Past and Present Risk Assessments 312   C. Examples of Past Assessments 313     1. Problem Definition 313     2. Hazard Assessment 314     3. Hazard Ranking 315     4. Risk Ranking 315     5. Quantitative Risk Assessment 317   D. Examples of Present Assessments 322     1. Problem Definition 322     2. Hazard Assessment 323     3. Hazard Ranking 324     4. Risk Ranking 325     5. Quantitative Risk Assessment 326     5. Evolution of Exposure and Risk Assessment 329 III. Data Availability (Question 3) 333   A. Introduction 333   B. Summaries of Available Data 333

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Page 291 IV. Prioritization of Data Gathering (Question 4) 344   A. Introduction 344   B. Criteria for Effects Data-Gathering Plan 345   C. Options for Scope of Effects Data-Gathering 346   D. Mechanisms for Obtaining Effects Data 346   E. Improving Data Bases for Estimating Exposures to HAPs 347 V. Risk Management Issues (Question 5) 349

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Page 292 List Of Figures Figure Page 1 Title III Regulatory Flow 295 2 Chronology of Title III Implementation 296 3 Risk Assessment Guidelines Development Process 307 4 Hazard Assessment Document Development 309 5 Integration of Exposure Assessment into Risk Assessment Process 310 6 Identification, Assessment, and Regulation of Hazardous Air Pollutants 315 7 Chronology of Section 112 Regulatory Policy Development 318

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Page 293 List Of Tables Table Page 1 Overview of CAA Title III Risk-Related Requirements 298 2 1984-87 Hazardous Air Pollutant Decisions 316 3 Data Sources for Exposure Assessment 328 4 Exposure Modeling Parameters 332 5 Summary of Health Effects Data 333 6 Data on Hazardous Air Pollutants 334

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Page 294 Question 1: What Does EPA Consider To Be the Risk Assessment Requirements Needed To Implement the Clean Air Act of 1990? I.A. Introduction Implementation of Title III of the Clean Air Act (CAA) requires the development and consideration of risk and hazard assessment in several provisions. The extent of assessment appropriate for each implementation activity is dependent on various factors. These include, but are not limited to, the purpose of the specific provision, the statutory timing and relationship to other provisions, and the availability of data and analytical methods. The next sections describe the regulatory flow and timing of Title III implementation, identify the levels of assessment and review, and describe the provisions with risk-related requirements. I.B. Regulatory Flow and Chronology of Title III Implementation Regulation under Title III is comprised of two major steps: the application of technology-based emission standards to categories of major stationary industrial sources, followed by the evaluation of residual risks and the development of further standards, as necessary, to insure that public health is being protected with an ample margin of safety. Affected source categories are identified based on emissions of listed pollutants. The list of source categories and agenda for regulation are required to be published. Extensions from compliance with the technology-based standards are available with demonstration of voluntary emissions reductions, documented problems with the installation of controls, or recently installed controls. Following compliance with the technology-based standards (maximum achievable control technology or MACT), EPA is required to evaluate residual risks and promulgate further standards, if necessary. Compliance and enforcement of the regulations is implemented through an operating permit program at the State level. The flow of the regulatory program under Title III is summarized in Figure 1. In addition to the regulatory requirements, there are a number of studies in Title III that require reports to Congress on various schedules. The timing of these studies and the principal regulatory milestones are illustrated in Figure 2. I.C Levels of Risk Assessment Table 1 presents a brief overview of those Title III provisions which contain elements of risk assessment. Included is a categorization of the level of analysis associated with each activity and the level of review. These are briefly described below. Their use, as exemplified in the past and present or future efforts is presented in the response to Question 2.

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Page 295 FIGURE 1 Title III Regulatory Flow. a. Problem Definition: Problem definition activities generally include scoping studies to broadly assess the potential magnitude of the air toxics problem. b. Hazard Assessment: A hazard assessment is the evaluation of the potential of a substance to cause human health or environmental effects. It would include an assessment of the available effects data and additional information such as environmental fate, potential for bioaccumulation, and identification of sensitive subpopulations. c. Hazard Ranking: A hazard ranking is the relative comparison of information identified in individual pollutant hazard assessments. The purpose of this type of analysis is to rank or group pollutants that pose similar hazards to public health or the environment.

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Page 296 FIGURE 2 Chronology of Title III Risk-Related Activities. d. Risk Ranking: A risk ranking is the comparative ranking that considers both emissions or exposure information and health effects data. The data may vary in quality depending upon the needs of the specific project. e. Ouantitative Risk Assessment: Quantitative risk assessment is the quantitative characterization of individual and population risk. It is typically conducted for individual sources, but the results may also be aggregated across an industrial source category. This level of analysis requires the most extensive collection of data and analytical resources. I.D Risk Assessment Review Requirements The assessments and methods used to implement various aspects of the air toxics program undergo a series of internal and external review procedures. The level of the review varies but will generally fall into one or more of the categories. The levels of review intended for each implementation activity under Title III are indicated in Table 1 and are broadly described below. It should be noted that individual components of a risk assessment may have a formal peer review.

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Page 297 For example, hazard assessment documents always undergo external peer review. a. Internal Review: This generally consists of review by EPA technical and scientific staff, supervisors, and senior management. It may also include review by Agency-wide committees such as the Risk Assessment Forum (RAF) or the Risk Assessment Council (RAC). Internal review is included in all phases of regulatory and methods development. b. External Review by Individuals: This review is conducted by individuals outside the Agency who are selected for their expertise in a specific area. c. External Review by Panels: Such review is the result of a workshop or meeting of experts and representatives of interested groups or affected organizations. d. Public Review: This consists of review by the public of all supporting documentation as part of the formal rulemaking process, and follows publication of a proposed rule in the Federal Register. e. Formal External Review: This is review by established advisory committees (e.g., EPA's Science Advisory Board (SAB), National Academy of Sciences (NAS), National Air Pollution Control Techniques Advisory Committee (NAPCTAC)).1 I.E Title III Risk-Related Provisions Several provisions of Title III contain requirements for risk or hazard assessment. Beginning on the following page, Table 1 summarizes these provisions. The levels of analysis and review identified on the Table correspond to the levels discussed above. The codes used in the Table are explained in notes on the last page of the Table. 1 The NAPCTAC is a committee composed of representatives of industry, environmental groups, and State and local agencies. It was established pursuant to Section 117 of the CAA. The primary focus of NAPCTAC is the review of control technology alternatives considered in the development of emission standards. The role has expanded to include other areas relevant to Title III implementation.

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Page 340 CAS Chemical Name                   #     M A C T 1991 1990 1989 1988       Y E A R Emis Emis Emis Emis       2 4 7 10 (T/yr) (T/yr) (T/yr) (T/yr)   Column Number       1   2 2 2 2     #SC                 108394 m-Cresol 1 X       38.9 3.8 6.3 9.2 108383 m-Xylene 6 X   X   718.1 601.1 583.7 1012.1 121697 N,N-Dimethyl aniline 1 X       25.6 25.4 45.9 49.5 91203 Naphthalene 2 X X     1335.9 1853.1 1656.9 1932.1 0 Nickel Compounds (subsulfide) 16   X X X 121.6 132.8 466.5 284.1 98953 Nitrobenzene 1 X       26.3 33.1 19.4 19.6 62759 N-Nitrosodimethylamine 1     X           59892 N-Nitrosomorpholine                   684935 N-Nitroso-N-methylurea                   90040 o-Anisidine 1 X       0.5 0.9 1.1 1.1 95487 o-Cresol 1 X       30.6 19.6 29.8 44.5 95534 o-Toluidine 1 X       5.4 3.7 12.8 23.5 95476 o-Xylene 23 X X X X 864.9 952.3 899.6 979.6 56382 Parathion           0.3 0.3 0.8 1.6 82688 Pentachloronitrobenzene (Quintobenzene)           0.1 0.1 1.1 0.5 87865 Pentachlorophenol 4 X   X X 6.2 11.6 5.6 7.1 108952 Phenol 12 X X X X 3165.6 3827.4 5264.1 5083.3 75445 Phosgene 2 X   X   2.2 2.4 4.1 10.8 7803512 Phosphine                   7723140 Phosphorus 5   X X X 11.7 12.1 30.1 9.6 85449 Phthalic anhydride 5 X   X X 315.9 343.7 325.1 273.6 1336363 Polychlorinated biphenyls (PCB's)             0.0   0.1 0 Polycylic Organic Matter 12   X X X         123386 Propionaldehyde 4 X     X 694.1 494.5 453.8 523.1 114261 Propoxur (Baygon)             0.1 0.3 0.1 78875 Propylene dichloride (1,2-Dichloropropane) 1 X       386.7 315.3 616.7 682.1 75569 Propylene oxide 16 X X X   533.3 680.0 897.1 1482.8 106445 p-Cresol 1 X       67.8 119.5 127.4 320.4 106503 p-Phenylenediamine 1 X       1.8 0.4 2.1 56.9 106423 p-Xylene 3 X   X   2639.2 2969.3 2360.1 3153.1 91225 Quinoline           22.5 13.8 31.8 24.7 106514 Quinone (1,4-benzoquinone) 1 X       2.1 0.8 0.9 5.7 0 Radionuclides (including radon)                   0 Selenium Compounds 15   X X X 18.5 15.3 16.7 14.8 100425 Styrene 15 X X X X 14238.2 15838.3 16650.9 17386.3 96093 Styrene oxide 1 X       0.8 1.2 0.4 1.2 127184 Tetrachloroethylene (Perchloroethylene) 11 X X X X 8343.7 10822.5 12752.4 15794.3 7550450 Titanium tetrachloride           16.8 27.2 28.6 39.3 108883 Toluene 39 X X X X 99260.1 116912.8 127718.9 13581 8001352 Toxaphene (chlorinated camphene)                   79016 Trichloroethylene 8 X X X X 17529.2 18949.0 22162.8 24092.3 121448 Triethylamine 1 X               1582098 Trifluralin           5.6 7.8 2.1 1.6 108054 Vinyl acetate 5 X   X   2743.2 2778.4 2699.1 2869.6 593602 Vinyl bromide (bromoethene)           1.8 5.1 0.4 2.5 75014 Vinyl chloride 4 X   X X 523.7 567.9 634.4 687.2 75354 Vinylidene chloride (1,1-Dichloroethylene) 3 X     X 142.6 151.8 110.3 149.7 1330207 Xylenes (isomers and mixture) 23 X X X X 57776.5 69988.4 73743.4 71332.8     Total Emis         708443.8  

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Page 341   IUR OUR EPA RfC LARC Exp. Genetic Toxicity Date Repr/   per per WOE mg/m3 WOE Asses. MVV MVT NM Dev   ug/m3 ug/L   Stat     So G M C S E Data   5 6 7 8 9 10 11   12   13   14 108394     C NV                   108383       NV     -       -   X 121697                           91203 4.2E-6   C       -       -     0 4.8E-4   A UR 1 B + - -+ + + -   98953     D UR     -       -   X 62759 1.4E-2 1.4E-3 B2   2A     - + + + +   59892         2B                 684935     B2                     90040       NV 2B                 95487     C NV                   95534         2B   -   +- +- - -   95476       NV     -       -   X 56382     C   3                 82688     UR   3                 87865   3.0E-6 B2 UR 2B       - + -     108952     D NV   B -   +   v     75445       NV                   7803512     D                     7723140     D                     85449                           1336363   2.2E-4 B2   2A   - -   - -   X 0     UR     B               123386       NV                   114261     UR                     78875     UR 6.0E-3 3               X 75569 3.7E-6 6.8E-6 B2 3.0E-2 2A   + - + + + +   106445     C NV                   106503         3                 106423       NV     -       -   X 91225       NV                   106514       NV                   0     UR     A               0     D   3                 100425     UR 1.0E+0 2B   +-     + +   X* 96093         2A   - - + + + + X 127184         2B B   - +   - - X 7550450                           108883     D 4.0E-1   B +   -     - X* 8001352 3.2E-4 3.2E-5 B2   2B     -     +     79016     UR UR 3 B + - +- +- + + X 121448       7.0E-3                   1582098   2.2E-7 C                     108054     UR 2.0E-1 3         + -   X 593602       3.0E-3 2A           +     75014 8.4E-5 5.4E-5 A   1 A + - +   + + X* 75354 5.0E-5 1.7E-5 C UR 3 B - - - - + + X 1330207     D NV     -     - - - X   800960.9   863337.1   906614.9   39.0 44.0          

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Page 342 Column Numbers and Footnotes to Table 1 Chemicals emitted from sources that will be regulated within the 2, 4, 7, and 10 year deadlines for maximum achievable control technology standards (# preceding X indicates the # of source categories (#SC) HAP in). 2 Toxic Release Inventory Data (TRI) in tons/year (1988,1989,1990,1991) 5 IUR= Inhalation unit risk estimate per ug/m3; Source is EPA's Integrated Risk Information System (IRIS) 6 OUR= Oral unit risk estimate per ug/1; Source is EPA's IRIS data base 7 WOE= Weight of Evidence classification; Source is EPA's IRIS data base 8 RfC workgroup; verified, on IRIS= conc. given in mg/m3 verified, not on IRIS= 'v' 9 IARC (International Agency for Research on Cancer) WOE 10 Exposure Assessments:     A) HAPS with risk assessments done for development of Section 112 standards     B) HAPS with screening assessments done for listing purposes 11, 12, 13 Information on Genetic Toxicology; Source= Genetic Activity Profiles data base provided by Dr. Michael Waters, EPA's Health Effects Research Lab. (Data as of 1992) The + or - represents the overall call for that group which may contain more than one assay. When discrepancies exist within a group, this is indicated by a +- (or-+). The first symbol represents the majority call for that group.     MVV= Mammalian, In Vivo       So= Somatic cell; G= Germ Cell     MVT= Mammalian, In Vitro       M= Mutation; C= Chromosome aberration     NM= Non-mammalian       S= Salmonella typhi; E= Eschericia coli 14 Data from Non-cancer Health Effects Database, prepared and provided by Dr. John Vandenberg, EPA's Health Effects Research Lab. 'X' indicates data available; X* indicates some human data available Note: Data includes effects on maternal toxicity: All data from Inhalation exposure Symbols used: UR= under review       V= verified     NA= not available   NV= not verifiable IARC vs. EPA: Classification Differences EPA Modifications to IARC Approach: 1. Considers statistically significant association between an agent and life-threatening benign tumors when evaluation human risk 2. Added "no data available" category 3. Added "no evidence of carcinogenicity" category

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Page 343 By Category EPA Group A - Known human carcinogen Group B - Probable human carcinogen     B1 - Limited human data     B2 - Inadequate human data, sufficient animal data Group C - Possible human carcinogen LARC Group 1 - Known human carcinogen Group 2A - Probable human carcinogen Group 2B - Possible human carcinogen Group 3 - Not classifiable as to human carcinogenicity Group 4 - Probably not carcinogenic to humans

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Page 344 IV. Ouestion 4: What does EPA consider to be the prioritization of the information gathering needs? What criteria would EPA use for determining this prioritization? IV.A Introduction Existing data on effects and exposure to the hazardous air pollutants (HAPs) listed under Section 112 have supported a variety of decisions under Title III of the Clean Air Act (CAA). Rules that use these data and additional data collected in a timely fashion will continue to be issued on CAA schedules that extend to the year 2010. Future information gathering on the HAPs will support residual risk decisions, biennial Great Waters reports, urban air toxics reports, and other continuous activities required to administer Section 112 provisions.     Interest in the HAPs exists beyond the CAA. Other EPA-administered programs and programs of other agencies address many of the same chemicals and mixtures. Therefore, whatever data are gathered will be gathered with an eye to serving needs beyond Section 112. The process of prioritizing data collection activities must consider many factors. Decisions for gathering information will have both science and management components. Important considerations include: the types of information needed to make a statutory finding, the current state-of-the-science, priorities given other EPA work, budget constraints, and statutory deadlines. The EPA has not, as yet, made decisions about the extent, mechanism, or timing of data gathering activities. The information presented below generally describes EPA's initial thoughts regarding the gathering of information needed to effectively implement Title III of the CAA. Under Title IX of the CAA, EPA and other agencies will be looking generally at the research needs for all of the HAPs. This Title provides a forum for planning research to advance the state of the art beyond standard testing. The plans for carrying out this Title are currently being formulated as the Title was added after the FY91 appropriation process was completed. Overall, the goals by which the priorities and needs can be balanced may be stated as: • ensure that the data collected meet the requirements of the statutory finding(s) that must be made • ensure that the data are collected in a timely fashion • ensure efficient use of resources, given the parallel data gathering efforts of others

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Page 345 • ensure that adequate resources are invested in HAPs that are emitted in significant volumes • avoid enriching an already rich data base of one HAP at the expense of another HAP of importance IV.B Criteria for Effects Data-Gathering Plan The major focus in planning for health and environmental effects data collection activities is to ensure that adequate data are available to conduct the residual risk determinations that will be made under Section 112(f). In order to obtain the data necessary to support these decisions required later in the decade, EPA must begin collection efforts immediately. The Agency anticipates that activities will begin with a ranking of HAPs that takes several factors into account. These factors include: • promulgation dates of control technology standards • estimation of the extent to which a particular HAP will contribute to risks resulting from combined HAP emissions from sources in a source category (using effects and exposure data available now) • importance of a HAP to the Great Waters or Urban Area Source programs • overlapping priority/interest of other EPA programs or governmental agencies (e.g. timing of ongoing Agency for Toxic Substances and Disease Registry or National Toxicology Program activities) Decisions on the extent and type of data to be gathered on potential adverse effects associated with exposure to a HAP will also require a balancing of several factors including incorporation of professional judgment on the likelihood that additional data may significantly alter current opinions on the toxicity of a specific HAP. Critical elements will include: • the richness of the current data base • the need for data to enable route-to-route extrapolation of existing toxicity data • the need to expand a data set on an already identified endpoint in order to improve dose-response characterization • the need to extend the scope of data to cover endpoints other than those previously identified • the need for research beyond standard test protocols to understand biological fate and transformation or mechanism of action

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Page 346 IV.C Options for Scope of Effects Data-gathering While the alternatives have not been exhaustively explored, and substantial work remains to be done, there are three general options that are being considered. These options are: 1. Board Scope. This approach would use staged testing for a large number of HAPs, screening a range of endpoints and proceeding to full endpoint tests as the screening assays indicate. 2. Medium Scope. This option would focus screening tests on those HAPs with the most significant emissions. Testing strategies would be more robust and address critical endpoints (carcinogenicity and developmental toxicity, at a minimum). Other HAPs with significant emissions would be considered under the narrow scope testing identified below. 3. Narrow Scope. Under this alternative, testing would focus on complimenting and making more useable existing data bases. For example, HAPs with significant emissions may be studied to "convert" oral to inhalation data or to elucidate dose-response relationships. This narrow scope testing could include: pharmacokinetics studies, a 90 day subchronic inhalation study, or a repeat of a previous study on an endpoint to better define the dose-response relationship. IV.D Mechanisms for Obtaining Effects Data There are a variety of mechanisms that may be accessed for collecting effects data, all of which will likely be employed. Major data gathering efforts are underway that will complement data collected specifically for Section 112 use. For example, the Superfund Amendments and Reauthorization Act of 1986 (SARA) requires the Agency for Toxic Substances and Disease Registry (ATSDR) to prepare toxicity profiles for over 200 pollutants. These profiles identify data gaps and efforts will be put forth to fill these gaps. Of the pollutants studied by ATSDR, 76 are HAPs. A second example is efforts being undertaken by the European communities. They are interested in generating data for a list of chemicals that overlaps the HAPs list. In addition, the National Toxicology Program (NTP) is working with EPA to identify testing and research the NTP can undertake for several HAPs. The EPA's Health and Environmental Research Laboratory (HERL) has ongoing research that addresses several HAPs, as well as urban toxics issues. This laboratory also conducts fundamental research on pharmacokinetics applicable to the HAPs. Additional EPA laboratories are conducting research on environmental fate, ecological effects, etc. Another alternative for collecting data is to access the regulatory test program under the Toxic Substan-

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Page 347 ces Control Act (TSCA) to require that industry conduct testing. Finally, the CAA Title IX research program will be pursued for research on HAPs. Making these overlapping efforts work together will be part of any data gathering EPA does on the HAPs. IV.E Improving Data Bases for Estimating Exposure to HAPs In addition to developing quantitative relationships between HAP concentrations and health or environmental effects, it is critical that the EPA pursue parallel efforts to support accurate characterization of the levels of exposure associated with sources of the HAPs. In the past, efforts to obtain sufficient information to accurately characterize HAP exposure levels in the vicinity of an industrial source have focused on one pollutant at a time. These efforts have been severely limited by lack of information on the source(s) being evaluated. In lieu of site-specific data for exposure characterization, EPA has settled for "model plant" types of analyses, which rely on only a sampling of data from one type of source and extrapolate exposure estimates to the rest of the source population. These analyses by nature must be very conservative, and therefore tend to overestimate ambient exposure levels due to any one type of industrial source. As a result, these analyses are often criticized by industry as being "overly conservative". It is clear that the CAA mandate for residual risk analyses (after the implementation of MACT) would require that such analyses be based on site-specific data rather than "model plant" scenarios. These analyses must therefore require more site-specific data than are currently available. In addition, the analyses will differ from past analyses in that they will be directed at assessing the exposure to multiple pollutants being emitted from a source in a particular source category. The EPA must begin now to develop the tools and process for obtaining the necessary data to perform residual risk analyses. While such efforts may build on past efforts, there are several new and challenging aspects that must be addressed, including: 1) Emission levels of each of the HAPs from each source within a source category must be obtained. Since EPA-approved measurement methods are not available for all HAPs, this will entail research and development efforts for both measurement methods and site-specific emission estimation techniques. It is hoped that cooperative efforts can be undertaken with industry to expand the publicly-available expertise in this area. 2) Data are to be obtained on a source category-by-source category basis. Since most currently available data bases are on a pollutant-by-pollutant basis, most of the current data will be inadequate for this purpose.

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Page 348 3) Exact stack, vent, and fugitive emission locations as well as fenceline locations for each facility are crucial for reducing the uncertainty of exposure assessments. Very little data are available in this regard, and it is unclear whether most industries will be willing to provide such data. 4) Development of guidelines is needed to explore the use of monitoring data or other more direct measures of exposure in assessing exposures resulting from emissions of HAPS. Specifically, the use of these data to complement modeling analyses needs to be examined. 5) Development of a user-friendly, easy-access, centralized data base and retrieval system (such as an electronic bulletin board system) may be desirable to provide a convenient vehicle for obtaining the necessary data. Industry input and cooperation in such development would be crucial to its success. Making sure that industry realizes that, without the necessary data, EPA efforts to assess exposure will be "conservative", may provide the needed cooperation of industry. Development of a data base system that is easy to use will substantially reduce the burden on industry as well as reduce the paperwork that would otherwise be necessary for such an information request. 6) Efforts to check and assure the quality of the data obtained for exposure assessments may prove to be a large part of the data gathering process. 7) Efforts to appropriately include population mobility and microenvironment exposures into the overall exposure assessment process have already begun. Sensitivity studies are needed, however, to determine the extent to which such factors can affect the overall exposure and risk assessment results. 8) Inclusion of short-term exposure quantification is important for many HAPs. Some modeling techniques are already available to address this quantification, but data on short-term emissions variability are generally lacking. The extent to which such information becomes available will dictate the extent to which EPA can incorporate such variability in exposure assessments. 9) Concentration measurements to assist in the validation of human exposure modeling results are generally lacking for most HAPs. While validation of air dispersion models in the field has been done, indoor/outdoor partitioning and multiple route exposures have not received the same level of validation efforts. This is an area where more data would be helpful. EPA welcomes comments and suggestions from the Committee on the plans for improving the accuracy of exposure and risk assessments required to implement the CAA. Of specific interest are the recommendations of the panel for prioritizing the vast amount of work that is required to fill the existing data gaps.

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Page 349 V. Question 5: What does EPA consider to be some of the critical management aspects of risk assessment decision-making that may not be apparent to an outside observer? The current regulatory process places a number of challenging demands on the risk manager. Depending on the nature of the regulation and the legislative authority, he or she must try to assimilate a variety of analyses—legal, economic, social and scientific—of which risk characterization is only one part. Because of this diversity, the risk manager must relay on the products of experts in a range of disciplines. In making risk management decisions, there are a number of considerations and factors to be weighed that may not be apparent to outside observers. Some of the factors influencing these decisions are described below. 1. In dealing with scientific issues, the risk manager is typically a generalist with no particular expertise in the area of risk assessment. This places particular requirements on the risk assessment process. Thus, the products of the risk assessment process must be designed to aid these individuals in decision-making. Risk managers are often frustrated by complex discussions of scientific uncertainties (mechanism of action, uncertainty in extrapolation, etc.). Rather they tend to desire bottom-line characterizations of the likelihood and magnitude of potential problems. In many respects, the popularity of the current cancer classification system lies in its ability to characterize the overall weight of evidence by readily-comprehended categories (e.g., known, probable, possible carcinogen) and the presentation of a measure of carcinogenic potency.   The Agency has increased the emphasis placed on the risk characterization component of risk assessment, and is moving toward a more comprehensive examination of the assumptions and uncertainties in risk assessment. The fact remains, however, that communication of the critical elements of a risk assessment to risk managers remains a challenge. 2. Consistency is important. This does not mean that all risk assessments should look the same. But it is important that a consistent terminology be adopted, even if the terminology draws controversy, and that the risk managers understand and can communicate that understanding. Decision-makers build on previous decisions and examples to put current issues in context. If formats or meanings differ from case to case, this process becomes difficult, if not impossible. 3. Risk managers do not expect perfect information. Critics of risk assessment's imperfections must recognize that public policy is often a blunt instrument rather than a surgeon's scalpel. Decisions are often based on broad bands

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Page 350   of uncertainty within which even differences of several fold may not affect the decision.   It is important for both risk managers and critics of risk assessment to avoid pursuing the ideal risk assessment. These individuals must bear in mind the limits of the real world. These limits include time, money, and the state of scientific knowledge. 4. Statutory mandates may place constraints on the development and use of risk characterization data that are not consistent with our understanding of the underlying science. The establishment of risk targets (or bright lines) such as 10-6, for example, have been criticized as not allowing the consideration of weight-of-evidence in decision making. Another example is the requirement that the Agency consider the risk to the ''person most exposed" to emissions from an air toxics source. Thus, the statutory framework constrains full consideration of the distribution of risk across the exposed population. 5. Statutes or court action often mandate regulation at a specific time, effectively mandating decision-making based upon available data. This is exacerbated by the fact that the development of robust health and safety data (e.g., well-conducted animal bioassays, epidemiological, or exposure studies) are both resource- and time-intensive. 6. The risk management process is often the focus of considerable outside attention and controversy. This is particularly true where the impacts of decisions are costly, or where they adversely affect well-organized groups. On these circumstances, there is a natural tendency to continue the process of data development and analysis, rather than to make decisions in an atmosphere of uncertainty. While such an environment can cause delay, it can also have the effect of encouraging more rigorous examination of data and careful consideration of options. 7. Persistent requests for information and more studies lead to paralysis by analysis and the waste of limited resources. The risk of inaction is often forgotten. Additional information needs must be balanced against the need to take timely action where it is warranted. This is particularly true in the risk assessment process, where the limitations of the current state of the science often prevent definitive answers, and can encourage continual additional data development. Reviewers of Agency risk assessments must consider the reasonable resource constraints under which the Agency operates.