Augmenting Our Use of the Risk Assessment Paradigm To Conduct Cumulative Risk (CR) Assessments: Application to Drinking Water Chemical Contaminants. L. K. Teuschler, G. E. Rice, and R. C. Hertzberg, U.S. EPA
U.S. EPA generally follows the NAS Risk Assessment Paradigm to evaluate human health risks. This paradigm requires augmentation beyond its traditional use to conduct a CR assessment, defined as multiple chemical, multiple route exposures over time. Augmentations include: hazard characterization must evaluate interaction effects from multiple chemicals and their combined doses; dose-response assessment must consider potential effects below individual chemical thresholds when exposure is to the mixture and incorporate judgments of similarity of mode-of-action among chemicals; exposure assessments must address changes in the mixture’s analytic chemistry over time, aggregate multiple route exposures, and chemical characterization of unidentified material in the mixture; risk characterization must evaluate how well the assumptions made about interaction effects, cross-chemical similarity of toxicologic mode-of-action, and exposure variability are supported by the data. Drinking water exposures provide a classic example of how the paradigm’s use must be broadened for CR applications. Exposures include multiple chemicals by multiple routes, with variation due to source waters, weather, disinfection processes and human water uses. Potential health outcomes include acute (e.g., developmental effects), requiring peak or average daily exposure estimates, and chronic health effects (e.g., cancer), requiring annual or longer-term exposure estimates. Depending on the types of data that are available for analysis (component, complex mixture, similar mixture) different questions about health risk can be addressed. Cogent discussions and quantitative treatment of uncertainty and variability increase in importance with the complexity of the risk assessment.
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