Chlorination Disinfection By-Products in Drinking Water
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The challenge of judging and managing public health risks “caused by” chlorination disinfection by-products (DBPs) in drinking water is likely the most complex issue faced by the drinking water industry in the developed world over the past three decades. Public health professionals must be sure that precautionary efforts in managing DBP risks are never allowed to compromise necessary measures to prevent the ever-present threat of waterborne disease.
This primer provides a review of relevant strengths and limitations of epidemiology, toxicology, and risk assessment for judging evidence of possible health effects of chlorination DBPs. The
epidemiological and relevant toxicological evidence regarding risks of cancer and adverse reproductive outcomes has been summarized and briefly analyzed. The resulting challenges for risk management decision-making have been reviewed.
The recent regulatory focus on trihalomethanes (THMs) has been mainly rationalized as providing a means to reduce the occurrence of bladder cancer. However, evidence suggests there is no causal connection between THMs and bladder cancer. In addition, evidence for adverse reproductive outcomes has been inconsistent at best, with evidence for birth defects caused by chlorination DBPs being primarily negative.
Given the inevitable uncertainties presented in this report, drinking water professionals need to view the subject of DBPs and public health as a major issue that must continue to be managed in a precautionary manner. This should be accepted even though over 30 years of health related research into DBPs in drinking water appears to warrant an over-all rating of evidence as indicating there is no “certain” health effect that has been proven between any DBP within currently regulated levels and any specific health outcome. However, the possibility of some causal association remains a viable hypothesis. For example, more focused attention on causes of bladder cancer is necessary because a large proportion of the comparisons of high chlorination DBP exposures with lower chlorination DBP exposures involve comparing exposure to disinfected surface water vs. lightly or non-disinfected groundwater. It is necessary to maintain a sensible, precautionary approach to managing DBPs that recognizes it is possible there may be no adverse health effects from current disinfection practices but future research may yet be able to establish a more convincing causal relationship for one or more DBPs and specified outcomes.
The bottom line for public health practitioners, who recognize the importance of maintaining their credibility, is to justify the case for control of chlorination DBPs in drinking water on a position of reasonable precaution. In most circumstances, there is no need or justification provided by current evidence to advocate taking urgent or extreme action on chlorination DBPs, based on any realistic expectation of observing adverse health outcomes.