ASSESSMENT OF THE CARCINOGENIC POTENTIAL OF CHLORINATED WATER - EXPERIMENTAL STUDIES OF CHLORINE, CHLORAMINE, AND TRIHALOMETHANES

Background: Water chlorination has been one of the major disease preve ntion treatments of this century. While epidemiologic studies suggest an association between cancer in humans and consumption of chlorinatio n byproducts in drinking water, these studies have not been adequate t o draw definite conclusions about the carcinogenic potential of the in dividual byproducts. Purpose: The purpose of this study was to investi gate the carcinogenic potential of chlorinated or chloraminated drinki ng water and of four organic trihalomethane byproducts of chlorination (chloroform, bromodichloromethane, chlorodibromomethane, and bromofor m) in rats and mice. Methods: Bromodichloromethane, chlorodibromometha ne, bromoform, chlorine, or chloramine was administered to both sexes of F344/N rats and (C57BL/6 X C3H)F1 mice (hereafter called B6C3F1 mic e). Chloroform was given to both sexes of Osborne-Mendel rats and B6C3 F1 mice. Chlorine or chloramine was administered daily in the drinking water for 2 years at doses ranging from 0.05 to 0.3 mmol/kg per day. The trihalomethanes were administered by gavage in corn oil at doses r anging from 0.15 to 4.0 mmol/kg per day for 2 years, with the exceptio n of chloroform, which was given for 78 weeks. Results: The trihalomet hanes were carcinogenic in the liver, kidney, and/or intestine of rode nts. There was equivocal evidence for carcinogenicity in female rats t hat received chlorinated or chloraminated drinking water; this evidenc e was based on a marginal increase in the incidence of mononuclear cel l leukemia. Rodents were generally exposed to lower doses of chlorine and chloramine than to the trihalomethanes, but the doses in these stu dies were the maximum that the animals would consume in the drinking w ater. The highest doses used in the chlorine and chloramine studies we re equivalent to a daily gavage dose of bromodichloromethane that indu ced neoplasms of the large intestine in rats. In contrast to the resul ts with the trihalomethanes, administration of chlorine or chloramine did not cause a clear carcinogenic response in rats or mice after long -term exposure. Conclusion: These results suggest that organic byprodu cts of chlorination are the chemicals of greatest concern in assessmen t of the carcinogenic potential of chlorinated drinking water.