Science and Judgment in Risk Assessment -- Copyright -- Other Recent Reports Of The Board On Environmental Studies And Toxicology -- Preface -- Contents -- Executive Summary -- REGULATION OF HAZARDOUS AIR POLLUTANTS -- CHARGE TO THE STUDY COMMITTEE -- CURRENT RISK-ASSESSMENT PRACTICES -- STRATEGIES FOR RISK ASSESSMENT -- Flexibility and the Use of Default Options -- Recommendations -- Validation: Methods and Models -- Recommendations -- Priority-Setting and Data Needs -- Recommendations -- Variability -- Recommendations -- Uncertainty -- Recommendations -- Aggregation -- Recommendations -- Communicating Risk -- Recommendation -- An Iterative Approach -- Recommendations -- OVERALL CONCLUSIONS AND RECOMMENDATIONS -- 1 Introduction -- Charge To The Committee -- Conceptual Framework Of The Report -- Part I Current Approaches to Risk Assessment -- 2 Risk Assessment and Its Social and Regulatory Contexts -- General Concepts -- What is Risk Assessment? -- How Is Risk Assessment Conducted? -- What is the Relationship Between Risk Assessment and Research? -- What is the Relationship Between Risk Assessment and Regulatory Decision-Making? -- What Is a Default Option? -- Historical Roots -- Early Efforts to Establish Safe Limits of Exposure to Toxic Substances -- The Problem of Carcinogens -- NRC Study Of Risk Assessment In The Federal Government -- Events After Release Of The 1983 NRC Report -- Uses Of Risk Assessment In The Regulation Of Hazardous Air Pollutants -- Noncancer Risk Associated With Hazardous Air Pollutants -- Public Criticism Of Conduct And Uses Of Risk Assessment -- Criticisms Pertaining to Conduct of Risk Assessment -- Criticisms Pertaining to the Relationship Between Risk Assessment and Risk Management -- Note -- 3 Exposure Assessment -- Introduction -- 1992 Exposure-Assessment Guidelines.

Exposure Calculation and the Maximally Exposed Individual -- Emission Characterization -- Sources of Emissions -- Emission Estimation Methods -- Measurement Methods -- Modeling Used In Exposure Assessment -- Modeling Airborne Concentrations -- Modeling Multimedia Exposure to Air Pollutants -- Alternative Transport and Fate Models -- Time-Activity Patterns -- Exposure-Assessment Models -- Long-Term Exposure Modeling -- Short-Term Exposure Modeling -- 4 Assessment of Toxicity -- Introduction -- Principles Of Toxicity Assessment -- Hazard Identification -- Epidemiologic Studies -- Animal Studies -- Dose-Response Assessment -- Toxic Effects Other Than Cancer -- Cancer -- New Trends In Toxicity Assessment -- 5 Risk Characterization -- Introduction -- Elements Of Risk Characterization -- Quantitative Estimates of Risk -- Individual Risk -- Population Risk -- Description of Uncertainty -- Superfund Risk-Assessment Guidance -- Uncertainty Analysis for Radionuclide Risk -- Presentation of Risk Estimates -- 1992 Exposure-Assessment Guidelines -- Risk-Characterization Memorandum -- Communication of Risk -- Part II Strategies for Improving Risk Assessment -- The Need For Risk-Assessment Principles -- Reporting Risk Assessments -- The Iterative Approach -- 6 Default Options -- Adoption Of Guidelines -- Departures From Default Options -- Current Epa Practice In Departing From Default options -- Example 1: Use of Animal-Cancer Bioassay Data -- Example 2: Linkages Between Exposure, Dose, and Response -- Methylene Chloride -- Formaldehyde -- Trichloroethylene -- Cadmium -- Nickel -- Dioxins -- Example 3: Modeling Exposure-Response Relationship -- Findings And Recommendations -- Use of Default Options -- Articulation of Defaults -- Justification for Defaults -- Alternatives to Default Options -- Process For Departures -- Missing Defaults -- 7 Models, Methods, and Data.

Introduction -- Emission Characterization -- Exposure Assessment -- Population -- Air-Quality Model Evaluation -- Assessment Of Toxicity -- Cancer -- Qualitative Considerations -- Quantitative Considerations -- Carcinogen Classification -- Other End Points of Toxicity -- Impact of Pharmacokinetic Information in Risk Assessment -- Conclusions -- Findings And Recommendations -- Predictive Accuracy and Uncertainty of Models -- Emission Characterization -- Guidelines -- Uncertainty -- External Collaboration -- Exposure Assessment -- Gaussian-Plume Models -- Exposure Models -- Population Data -- Human-Exposure Model -- Assessment of Toxicity -- Extrapolation from Animal Data for Carcinogens -- Extrapolation of Animal Data on Noncarcinogens -- Classification of Evidence of Carcinogenicity -- Potency Estimates -- 8 Data Needs -- Context Of Data Needs -- Implications For Priority-Setting -- Data Needed For Risk Assessment -- Emissions -- Priorities for Collecting Data -- Data Availability -- Environmental Fate and Transport -- Priorities for Collecting Data -- Data Availability -- Exposure -- Priorities for Collecting Data -- Data Availability -- Toxicity -- Priorities for Collecting Data -- Data Availability -- Overall Priority Setting -- Screening risk assessment -- Full risk assessment -- Data Management -- Findings And Recommendations -- Insufficient Data for Risk Assessment -- Need for Data-Gathering Guidelines -- Inadequacy of Emission and Exposure Data -- Inadequacy of TRI Database as a Source of Emission Data for Risk-Assessment Purposes -- Lack of Adequate Natural Background-Exposure Database -- Inadequate Explanation of Analytical Techniques -- Need for System of Data Management for Risk Assessment -- 9 Uncertainty -- Context Of Uncertainty Analysis -- Nature Of Uncertainty -- Taxonomies -- Problems With EPA's Current Approach To Uncertainty.

Some Alternatives To EPA's Approach -- Probability Distributions -- Subjective Probability Distributions -- Model Uncertainty: "Unconditional" Versus "Conditional" PDFs -- Specific Guidance On Uncertainty Analysis -- Generating Probability Distributions -- Statistical Analysis of Generated Probabilities -- Barriers to Quantitative Uncertainty Analysis -- Uncertainty Guidelines -- Risk Management And Uncertainty Analysis -- Single Estimates of Risk -- Risk Communication -- Comparison, Ranking, And Harmonization Of Risk Assessments -- Findings And Recommendations -- Single Point Estimates and Uncertainty -- Uncertainty Guidelines -- Comparison of Risk Estimates -- Harmonization of Risk Assessment Methods -- Ranking of Risk -- Notes -- 10 Variability -- Introduction And Background -- Exposure Variability -- Emissions Variability -- Atmospheric Process Variability -- Microenvironmental and Personal-Activity Variability -- Variability In Human Susceptibility -- Overall Susceptibility -- Conclusions -- Exposure Variability and the Maximally Exposed Individual -- Susceptibility -- Other Changes in Risk-Assessment Methods -- Risk Communication -- Identifiability and Risk Assessment -- Findings And Recommendations -- Exposure -- Potency -- Susceptibility -- Risk Communication -- Notes -- 11 Aggregation -- Introduction -- Exposure Routes -- Risk-Inducing Agents -- Types of Nonthreshold Risk -- Cancer -- Other Nonthreshold End Points -- Genetic Effects -- Reproductive/Developmental Risks -- Measures And Characteristics Of Risk -- Overall Characterization Goals -- Consistency in Characterization: Example of Aggregation of Uncertainty -- Uncertainty and Variability -- Aggregation of Uncertainty and Variability -- Findings And Recommendations -- Multiple Routes of Exposure -- Multiple Compounds and End Points -- Genetic Effects.

Reproductive and Developmental Toxicants -- "Upper-Bound Estimates" versus "Best Estimates" -- Uncertainty versus Variability -- Note -- Part III Implementation of Findings -- 12 Implementation -- Priority-Setting And Section 112 -- Iterative Risk Assessment -- Exposure Assessment -- Assessment of Toxicity -- Summary -- Epa Practices: Points To Consider -- Implications for Priority-Setting for Title III Activities -- Model Evaluation and Data Quality -- Default Options -- Uncertainty Analysis -- Institutional Issues In Risk Assessment And Management -- Stability and Change -- Management As Guide To Assessment -- Comparisons of Risk -- Risk Management and Research -- Risk Assessment as a Policy Guide -- Social and Cultural Factors -- Summary -- Findings And Recommendations -- Tiered vs. Iterative Risk Assessment -- Verification of Amount of Risk-Assessment Conservatism -- Full Set of Exposure Models -- IRIS Data Quality -- Toxicity Data Development -- Full Data Set for Priority-Setting -- Iterative Priority-Setting -- Full and Complete Documentation of Priority-Setting -- Guidelines vs. Requirements -- Process for Public Review and Comment -- Petitions for Departure from Default Options -- Iterative Uncertainty Analysis -- Risk Assessment vs. Risk Management -- Comparisons of Risk -- Policy Focus on Stationary Sources -- Risk Management and Research -- References -- Appendixes -- Appendix A Risk Assessment Methodologies: EPA's Responses to Questions from the National Academy of Sciences -- Disclaimer -- Table of Contents -- Question 1: What Does EPA Consider To Be the Risk Assessment Requirements Needed To Implement the Clean Air Act of 1990? -- I.A. Introduction -- I.B. Regulatory Flow and Chronology of Title III Implementation -- I.C Levels of Risk Assessment -- I.D Risk Assessment Review Requirements -- I.E Title III Risk-Related Provisions.

II. Question 2: What has EPA done in the past toward those or similar risk assessment requirements, and why did EPA...