Since the discovery of chlorination byproducts in drinking water in 1974, numerous toxicological studies (studies on the health effects from exposure to high dosages contaminants usually involving animals in a lab) have been conducted. These studies have shown several disinfection byproducts to be carcinogenic in laboratory animals (e.g., including bromate, certain trihalomethanes and haloacetic acids). Some disinfection byproducts have also been shown to cause adverse reproductive or developmental effects in laboratory animals (e.g., chlorite and certain trihalomethanes and haloacetic acids). However, there is considerable uncertainty involved in using the results of high-dose, toxicological studies of some byproducts occurring in disinfected drinking water to estimate the risk to humans from chronic exposure to low doses of these and other byproducts.
In the area of epidemiology (studies of the factors that influence disease in human populations), a number of studies have been completed investigating the relationship between exposure to chlorinated surface water and cancer. Some have suggested an increased cancer risk to those exposed to chlorinated waters while others have demonstrated none. In issuing the Stage 1 Disinfectants/Disinfection Byproducts Rule based on evidence then available, EPA stated that while the agency cannot conclude there is a causal link between exposure to chlorinated surface water and cancer, these studies have suggested an association, albeit small, between bladder, rectal, and colon cancer and exposure to chlorinated surface water. There are fewer epidemiology studies evaluating the association between exposure to disinfection byproducts and reproductive and developmental effects. Again, some have suggested an increased risk from exposure to disinfection byproducts while others have shown none. There remains considerable debate in the scientific community on the significance of these contradictory findings concerning chlorinated water and disinfection byproducts. As with cancer, EPA stated in its Stage 1 Disinfectants/Disinfection Byproducts Rule that, based on data then available, the agency can not conclude there is a causal link between exposure to disinfection byproducts and reproductive and developmental effects. In sum, EPA believes the weight-of-evidence presented by the available epidemiological studies on chlorinated drinking water and toxicological studies on individual disinfection byproducts support a potential hazard concern and warrant regulatory action at this time such as that taken in the Stage 1 Disinfectants/Disinfection Byproducts Rule.
An extensive research effort (1997 Update to ORD's Strategic Plan, National Toxicology Program) is currently underway to better understand the potential risks attending exposure to disinfection byproducts. While this research is being completed, an agreement among water suppliers, environmental groups, consumer groups, and regulatory agencies has been reached, resulting in the publication of a Stage 1 Disinfectants/Disinfection Byproducts Rule by EPA in the Federal Register in December 1998. The participants recommended that while additional information, especially on health effects, is needed, the Stage 1 Disinfectants/Disinfection Byproducts Rule was considered the best course of action to reduce potential risks from disinfection byproducts in the near term.