Presented in Chapter 10 were qualitative descriptions of the mechanisms through which contaminants can affect normal structures and functions in the body as well as the ultimate effects on human health. The risk assessment process as presented in Chapter 1 requires a quantitative characterization of the human health risk posed by contaminant exposures. This is accomplished through dose-response assessment, which is the process of quantifying the relationship between contaminant dose and the resulting toxicological response.
In the contemporary practice of risk assessment, human health risk is characterized by one of two generic metrics. Selection of the appropriate metric is dictated by the dose-response relationship. Some contaminant effects are only expressed once a toxic threshold is exceeded, and the typical risk management objective is prevention of the effect. This can be accomplished simply by keeping exposures under the threshold. The generic metric used for these types of effects is the margin of safety, which is a comparison, usually a ratio, between the estimated dose and the dose at which adverse effects are expected to occur. In contrast, some contaminant effects can be expressed at any dose (i.e., there is a nonzero response at any nonzero dose). The only way to prevent the risk is to prevent or eliminate exposure, which is usually not possible. Thus, the typical risk management objective is to minimize the risk by keeping it below an allowable level. The generic metric used to characterize nonthreshold effects is the fractional response, which is the probability of the effect occurring in the exposed population. With respect to the deterministic-stochastic categorization of effects introduced in Chapter 1 and described in more detail below, deterministic effects are frequently characterized by a margin of safety, whereas most stochastic effects are characterized by some type of fractional response metric.
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