Uncertainties in health-risk assessment: an integrated case study based on tetrachloroethylene in California groundwater.

Population risk to an environmental contaminant is represented as the product of a source term; the exposure function, which converts the source into a lifetime-equivalent contact rate in the population; the fraction metabolized; the toxic potency associated with the delivered dose; and the size of the exposed population. Using case studies for contamination of groundwater in California with tetrachloroethylene (PCE), the combined uncertainty in exposure and dose-response models is characterized. Three key issues are addressed: (a) uncertainty in quantifying pathway exposure factors that relate measured environmental concentrations to levels of contact in the exposed population; (b) the uncertainty in the human dose-response models derived from animal data; and (c) an overview of important contributors to the overall uncertainty in population risk estimates.