the public health costs incurred because of a delay in the implementation of controls. Complete certainty is an unattainable ideal.

Health benefits analyses compare alternative scenarios that would result with and without regulatory action. As a consequence, these analyses are inherently speculative and their results unverifiable. Because only one regulatory option can be chosen by decision-makers, the outcomes of the remaining regulatory options, including the baseline with no action (if not chosen), can never be directly observed.

Analyses of health benefits should represent the uncertainties in the choices facing decision-makers and society at large (Hattis and Anderson 1999). Analyses should attempt to provide insight into the variability of impacts (among persons, places, and other dimensions of interest) and the extent and sources of uncertainties in the results. The representation of uncertainty requires a good faith appraisal of the imperfection in the state of information about these impacts (Hattis and Burmaster 1994). Uncertainty assessment should not overrepresent or underrepresent the quality and completeness of available information.

This chapter discusses EPA’s current approach to assessing uncertainty in health benefits analyses for air pollution control regulations. The agency’s analysis of the health benefits for the final Tier 2 vehicle emissions standards and gasoline sulfur control rule-making (EPA 1999a) is used for illustration. The chapter outlines a revised approach that would reflect overall uncertainty more realistically, in part by using probabilistic expressions of expert judgment. The chapter also briefly reviews the history of probabilistic uncertainty assessment in EPA health benefits analyses under the Clean Air Act.

This chapter is confined to uncertainty in the analysis of health benefits expressed solely in terms of health. Although uncertainties in the monetary valuation of health benefits and in the analysis of regulatory costs are not considered, the committee notes that there are great uncertainties in those analyses as well.

EPA’S APPROACH TO UNCERTAINTY ANALYSIS

EPA uses a two-part approach to assessing uncertainty in health benefits analyses that rely on epidemiological studies as the source of estimated concentration-response functions, although different approaches are some



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