Drinking water disinfection byproducts: Review and approach to toxicity evaluation

There is widespread potential for human exposure to disinfection byproducts (DBPs) in drinking water because everyone drinks, bathes, cooks, and clean s with water. The need for clean and safe water led the U.S. Congress to pa ss the Safe Drinking Water Act more than 20 years ago in 1974. In 1976, chl oroform, a trihalomethane (THM) and a principal DBP, was shown to be carcin ogenic in rodents. This prompted the U.S. Environmental Protection Agency ( U.S. EPA) in 1979 to develop a drinking water rule that would provide guida nce on the levels of THMs allowed in drinking water. Further concern was ra ised by epidemiology studies suggesting a weak association between the cons umption of chlorinated drinking water and the occurrence of bladder, colon, and rectal cancer. In 1992 the U.S. EPA initiated a negotiated rulemaking to evaluate the need for additional controls for microbial pathogens and DB Ps. The goal was to develop an approach that would reduce the level of expo sure from disinfectants and DBPs without undermining the control of microbi al pathogens. The product of these deliberations was a proposed stage 1 DBP rule. It was agreed that additional information was necessary on how to op timize the use of disinfectants while maintaining control of pathogens befo re further controls to reduce exposure beyond stage 1 were warranted. In re sponse to this need, the U.S. EPA developed a 5-year research plan to suppo rt the development of the longer term rules to control microbial pathogens and DBPs. A considerable body of toxicologic data has been developed on DBP s that occur in the drinking water, but the main emphasis has been on THMs. Given the complexity of the problem and the need for additional data to su pport the drinking water DBP rules, the U.S. EPA, the National institute of Environmental Health Sciences, and the U.S. Army are working together to d evelop a comprehensive biologic and mechanistic DBP database. Selected DBPs will be tested using 2-year toxicity and carcinogenicity studies in standa rd rodent models, transgenic mouse models and small fish models; in vitro m echanistic and toxicokinetic studies; and reproductive, immunotoxicity, and developmental studies, The goal is to create a toxicity database that refl ects a wide range of DBPs resulting from different disinfection practices. This paper describes the approach developed by these agencies to provide th e information needed to make scientifically based regulatory decisions.