Drinking Water and Health, Volume 9 Selected Issues in Risk Assessment (1989) / Chapter Skim
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1 Introduction
1 Introduction
Pages 101-107
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From page 101... ...
of regulating exposure to toxic chemicals, its Office of Drinking Water asked the Safe Drinking Water Committee of the National Research Council's Board on Environmental Studies and Toxicology to hold a small workshop, with each participant addressing some aspect of the methodology for assessing the risk associated with exposure to mixtures of chemicals found in drinking water. This report is the product of that workshop, held in October 1987 in Washington, D.C., and of the deliberations of the Subcommittee on Mixtures in a followup meeting.
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From page 102... ...
A recent study showed a statistically significant association between the ingestion of chlorinated surface water and human bladder cancer (Cantor et al., 1987~. Although the specific components responsible for that association remain unidentified, the by-products of chlorine disinfection are currently the prime suspects.
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From page 103... ...
Synergistic interactions between chemicals have been suspected of causing health effects in humans that could not be predicted by simply adding the expected effects of the component chemicals. One such case was the incident of mass organophosphorus insecticide poisoning among field workers in Pakistan in 1976 (Baker et al., 19781; two of the pesticide formulations contained contaminants, which could well have increased the toxicity attributed to the designated active pesticidal ingredient, malathion, by inhibiting its detoxification (see Chapter 41.
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From page 104... ...
, the risk of one end point associated with exposure to a mixture of carcinogens at low concentrations can be theoretically approximated as the sum of risks associated with the individual carcinogens; i.e., additivity of response or risk is usually assumed for carcinogens associated with relative risks of less than 1.01. However, the subcommittee recognizes that this assumption of low-dose additivity of response does not have much empirical foundation.
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From page 105... ...
In grouping chemical mixtures by whatever method, a "toxic-equivalence" approach can be considered assigning numerical potency values to individual mixture components that are representatives of specific classes, estimating potencies of other class members that are present relative to those of the appropriate representative chemicals, and then summing the products of the relative potencies and concentrations of all the chemicals present across all end points. Risks associated with exposure to polycyclic aromatic hydrocarbons (Clement Associates, 1988)
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From page 106... ...
Chapter 6 shows how the assumptions inherent in EPA's risk assessment methods for carcinogens can be used to combine the estimated risks associated with individual carcinogenic components in a mixture. Although the workshop led to an affirmation of current methods for the risk assessment of mixtures in drinking water, attempts at developing a firmer empirical base and reevaluation should continue.
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From page 107... ...
EPA (U.S. Environmental Protection Agency)
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