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The ''Red Book" paradigm should be supplemented by applying a cross-cutting approach that uses those themes. Such an approach could ameliorate the following problems in risk assessment as it is currently practiced within the agency:

The differing opinions in the scientific community on the merits of particular scientific evidence and the resulting lack of credibility caused by periodic revisions of particular "risk numbers" (e.g., those for dioxin).

The reluctance to incorporate new scientific information into risk assessments when it might (erroneously) appear to increase uncertainty.

The incompatibility of various inputs to risk characterization, e.g., dose estimates in units that cannot be combined with more sophisticated dose-response evaluations, or hazard-identification evidence that cannot readily be integrated into potency assessment.

The emphasis on theoretical modeling over measurement.

The production of risk assessments that are either insufficiently informative or too detailed for the needs of risk managers, and the related problem of lack of clear signals to guide risk-assessment research.

Considering the six cross-cutting themes in the planning and analysis of risk assessment will not solve the problems of risk assessment by itself. Indeed, too much emphasis on a cross-cutting vision of risk assessment might create unanticipated problems. On balance, however, the view of risk assessment proposed in Chapters 6-11 will serve two important purposes: it will give the individual cross-cutting themes a more prominent place in the risk-assessment process, and it will encourage the gradual evolution of attempts to improve risk assessment from its current, somewhat piecemeal orientation to a more holistic one, with the goal of improving the precision, comprehensibility, and usefulness for regulatory decision-making of the entire risk-assessment process. Whatever conceptual framework is used, the committee believes that EPA must develop principles for choosing default options and for judging when and how to depart from them. This controversial issue is described in the next section.

The Need For Risk-Assessment Principles

Our scientific knowledge of hazardous air pollutants has numerous gaps. Hence, there are many uncertainties in the health risk assessments of those pollutants. Some of these can be referred to as model uncertainties—for example, uncertainties regarding dose-response model choices due to a lack of knowledge about the mechanisms by which hazardous air pollutants elicit toxicity. As discussed more fully in Chapter 6, EPA has developed "default options" to use when such uncertainties arise. These options are used in the absence of convincing scientific information on which of several competing models and theories is correct. The options are not rules that bind the agency; rather, they constitute

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