HEALTH RISK ASSESSMENT OF DRINKING-WATER CONTAMINANTS IN CANADA - THEAPPLICABILITY OF MIXTURE RISK ASSESSMENT METHODS

The objectives of this article are: (i) to review the current approach es of Health Canada to the risk assessment of drinking water contamina nts, and (ii) to examine the applicability of mixture risk assessment methods to drinking water contaminants. Health Canada's current approa ches to drinking water risk assessment, like those of many regulatory agencies, focus almost solely on the effects of individual chemicals. As such, no formal method is currently used for developing mixtures gu idelines or for modifying guidelines of individual chemicals to accoun t for the possibility of the occurrence of interactions (supraadditive or infraadditive), Recent interest in the risk assessment of mixtures , at least in part, stems from concerns over the potential health risk s of mixtures of very commonly occurring compounds in Canadian drinkin g water supplies, namely the disinfection byproducts. Before any mixtu res methods can be considered for incorporation into Health Canada's c urrent approaches to the risk assessment of drinking water contaminant s, it is essential to consider the limitations and data requirements o f the various mixture risk assessment methods (i.e., whole mixture app roach, similar mixture approach, components-based approaches, interact ions-based assessment). Among the existing mixture risk assessment met hods, the components-based and interactions-based approaches could be applicable to drinking water contaminants. Specifically, among the com ponents-based approaches, dose-addition response-addition, and the tox ic equivalency factor approaches are the most applicable ones for drin king water contaminants. Until an interactions-based, mechanistic risk assessment approach (e.g., physiological model-based approach) become s available for routine use, the components-based approaches remain th e default methods for consideration. Progress in the development and v alidation of an interactions-based risk assessment methodology should facilitate a more realistic assessment of risk due to drinking water c ontaminants without increasing the levels of uncertainty in risk estim ates above those associated with existing single-chemical methods. (C) 1997 Academic Press.