Drinking Water
For 100 years, public water supplies have been treated with disinfectants, such as chlorine, to reduce the risks of infectious disease from waterborne pathogens. Although water disinfection has been highly effective in reducing the incidence of certain diseases, such as cholera and typhoid, the continued occurrence of waterborne disease outbreaks demonstrates that contamination of drinking water with pathogenic bacteria, viruses, and parasites still poses a health risk when treatment is inadequate. The use of disinfectants, while reducing microbial risks, creates new potential problems as chemical by-products are formed during the treatment process. Some of these disinfection by-products (DBPs) have been shown to cause cancer and other toxic effects in experimental animals. In humans, however, the scientific evidence is inconclusive. In addition, surface water and groundwater supplies that are used as a source of drinking water may be contaminated by a variety of chemicals of potential public health concern. For example, arsenic, a naturally occurring contaminant of drinking water in some areas, has been shown to cause cancer, cardiovascular disease and other effects in exposed individuals. Research is required to obtain sufficient understanding of the health risks posed by these chemical and microbiological contaminants, and to develop a strong scientific basis for setting new drinking water standards that may be needed to protect public health. NOTE: Information on drinking water laws and regulations, publications and other resources can be found at the EPA Office of Water's website located at: www.epa.gov/ow/.
Problem (DBPs): A large number of organic and inorganic chemicals, such as trihalomethanes, haloacids, bromate and chlorate, are found in disinfected drinking water as a result of reactions between disinfectants and chemical precursors in the source water. Many of these disinfection by-products (DBPs) have been shown to cause adverse health effects at high doses in experimental animals, and epidemiology studies have suggested potential risks of cancer and adverse reproductive outcomes in human populations. EPA is currently working with the water industry and other stakeholders to determine the need for more stringent standards for DBPs found in drinking water. NOTE: A more detailed description of the DBP research program, including recent NHEERL accomplishments, is available at www.epa.gov/ord/publications under Research and Development 1997-98 Research Accomplishments. Scientific Questions:
- What are the health effects of DBPs in communities served by disinfected drinking water?
- What is the toxicity of individual chemical contaminants and mixtures of DBPs at low doses?
Approach: NHEERL is supporting the establishment of scientifically sound regulatory decisions on DBPs by conducting health effects research to address key uncertainties in the risk assessment for these contaminants. This research includes epidemiology and toxicology studies across a variety of scientific disciplines for characterizing the risks associated with exposure to DBPs in drinking water. An emphasis is being placed on studies of adverse reproductive outcomes, but research will also be conducted on cancer and, to the extent necessary, neurotoxic and immunotoxic effects. Epidemiology research will emphasize an evaluation of the potential reproductive risks that may be associated with DBP exposures. Toxicology research on DBPs will include dose-response studies and research on mode of action and pharmacokinetics to provide information needed to support more biologically based risk assessments for the most important by-products. Research on DBP mixtures will address hypotheses concerning the relative risks of different complex mixtures as a function of treatment characteristics and source water quality, and the nature of possible by-product interactions at low doses. Recent Highlights: A summary of recent NHEERL accomplishments in DBP research may be found in the publication Research and Development 1997-98 Research Accomplishments at: www.epa.gov/ord/publications.
- Our studies were the first to demonstrate that dichloroacetic acid (DCA) is a hepatocarcinogen in the rat. We also showed that DCA induces a unique rasoncogene mutation in hepatocellular carcinomas in rodents. This information may help elucidate the mechanisms by which this chemical causes liver cancer in animals, and may provide information to assist in extrapolation of the experimental data to humans.
- Our investigators found that renal tube injury caused by chloroform was present in rodents only at doses that resulted in increased tumor incidence, thereby lending support for a risk assessment approach based on a non-linear mode of action.
- We discovered a new pathway by which certain trihalomethanes (THMs) cause cancer in laboratory animals. Our findings reveal that the metabolism of THMs containing bromine produces highly reactive metabolites that damage DNA and cause gene mutations. Interestingly, this new genotoxic pathway does not appear to occur with chloroform, a THM that lacks bromine. The metabolic pathway involves the enzyme glutathione-s-transferase in rodents. Because an analogous enzyme is found in humans, a similar pathway is likely to be active in people.
- In a chronic exposure study in which rats and mice were exposed to bromate in drinking water, we found that bromate causes cancer in the male rat at 3 sites: the kidney, thyroid, and mesothelium. EPA used these results in its decision to maintain the MCLG for bromate at zero for the final Stage 1 DBP rule.
|
