there is no single, scientifically definable distinction between “safe” and “unsafe” exposures, risk management necessarily incorporates components of sound, practical decision making that are not addressed by the risk assessment process (NRC, 1983, 1994).

Although a risk assessment requires that information be organized in rather specific ways, its conduct does not require any specific scientific methodologies for evaluating that information. Rather, it asks risk assessors to evaluate scientific information using what are, in their judgments, appropriate methodologies and to make explicit the bases for their judgments. Risk assessment also requires explicit recognition of uncertainties in risk estimates and the acknowledgment, when appropriate, that alternative interpretations of the available data may be scientifically plausible (NRC, 1994; OTA, 1993).

Risk assessment is subject to two types of scientific uncertainties: (1) those related to data and (2) those associated with any inferences that are required when directly applicable data are not available (NRC, 1994). Data uncertainties arise in the evaluation of information obtained from the epidemiology and toxicology studies and investigations of nutrient intake levels that are the basis for risk assessments. The use of data from experimental animals to estimate responses in humans, and the selection of so-called uncertainty factors to estimate inter- and intraspecies variabilities in response to toxic substances are examples of the use of inferences in risk assessment. Uncertainties regarding the appropriate inferences to be made arise whenever attempts are made to estimate or predict adverse health effects in humans (in which there are often inadequate or nonexistent direct empirical data) based on extrapolations of data obtained under dissimilar conditions (for example, experimental animal studies). Data on nutrient toxicity are generally available from studies in human populations and, therefore, may not be subject to the same uncertainties (related to interspecies extrapolations) associated with the available data on nonessential chemicals. Options for dealing with uncertainties are discussed below and in detail in Appendix C.

Steps in the Risk Assessment Process

Although various terms are used to describe the specific organizing steps of the risk assessment process (for example, FAO/WHO, 1995), there appears to be widespread agreement among risk assessors on the content of those steps. The organization of risk assessment in this report is based on a model proposed by the NRC (1983,



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