Parameter uncertainty and variability in evaluative fate and exposure models

The human toxicity potential, a weighting scheme used to evaluate toxic emi ssions for life cycle assessment and toxics release inventories, is based o n potential dose calculations and toxicity factors. This paper evaluates th e variance in potential dose calculations that can be attributed to the unc ertainty in chemical-specific input parameters as well as the variability i n exposure factors and landscape parameters. A knowledge of the uncertainty allows us to assess the robustness of a decision based on the toxicity pot ential; a knowledge of the sources of uncertainty allows us to focus our re sources if we want to reduce the uncertainty. The potential dose of 236 che micals was assessed. The chemicals were grouped by dominant exposure route, and a Monte Carlo analysis was conducted for one representative chemical i n each group. The variance is typically one to two orders of magnitude. For comparison, the point estimates in potential dose for 236 chemicals span t en orders of magnitude. Most of the variance in the potential dose is due t o chemical-specific input parameters, especially half-lives, although expos ure factors such as fish intake and the source of drinking water can be imp ortant for chemicals whose dominant exposure is through indirect routes, La ndscape characteristics are generally of minor importance.