ABSTRACT
The assessment steps and the risk characterization, the risk assessor should also distinguish between variability and uncertainty. Variability arises from true heterogeneity in characteristics such as dose-response differences within a population or differences in the contaminant levels in the environment. Assessments should address the resulting variability in doses received by members of the target population. The central tendency and high-end individual risk descriptors are intended to capture the variability in exposure, lifestyles, and other factors that lead to a distribution of risk across a population. Some of the working groups EPA asked to consider relative risk had difficulty ranking 31 environmental problem issues they were presented with because there are no accepted guidelines for quantitatively assessing relative risks. As described earlier, additional uncertainties are incorporated in the risk assessment when exposures to a several substances across multiple pathways are summed.
