Chlorinated water and overall risk of cancer: a systematic review

Abstract

Introduction: Chlorine was introduced into large-scale public water supplies early in the 20th century as an inexpensive and expedient solution to render sewage contaminated and infectious water supplies non-infectious. Redraw and recycling of contaminated and infectious water downstream continues to this day around the world e.g. Mississippi River, United States (US) and the Murray-Darling River, Australia. However, today disinfection of redrawn water with chlorine allows consumption of recycled water without causing gastrointestinal epidemics including cholera and typhoid. Trihalomethanes, sourced from chlorinated water drawn from the bottom of the Mississippi River in New Orleans, were first identified in 1974 as the potential carcinogenic agents responsible for the very high rates of cancer occurring there. Since then chlorine disinfection by-products (CDBs) have been a serious public health concern. Exposure to them is widespread and associated with increased cancer risk and adverse reproductive and developmental outcomes. To date increased cancer risk has only been confirmed for bladder and colorectal cancers. The objective of this review is to determine the evidence for an association between chlorinated water and cancer, other than colorectal and bladder. Method: The Joanna Briggs Institute methodology for systematic reviews was used, with multiple databases date limited to 1974. Data was included according to an a priori protocol, and was synthesized through meta-analysis or presented narratively. Results: Twelve statistically significant point estimates were identified: all reported cancers combined risk index 1.13 (1.07, 1.2) p = 0.000, geographical regions of North America and Europe, for males and females as well the following cancer sites breast, female reproductive, melanoma, non-Hodgkin’s and lung. Non-Hodgkin’s and lung had significant predictive intervals. Overall, for all reported cancers a relative increase in risk of 13% was observed. This risk is discussed in terms of the absolute incident and mortality associated with CDBs. Conclusion: The results from the present work support the association between CDBs and cancer originally made by Rook in 1974 and strengthen the argument for a causal basis. Recommendations for best practice: The US Environmental Protection Agency (US EPA) since 1974 has been proactive and set the benchmark for regulating CDBs to reduce population risk and exposure. Despite the encouraging action of the US EPA the overwhelming attitude towards disinfection of water supplies with chlorine in public health and within the water industry generally remains one of, “infection control is paramount and in no way should be compromised to lower exposure risk to CDBs”. Large urban populations prospered without the need to disinfect water supplies with chlorine for centuries including throughout the 19th century. However, CDBs are now considered a serious public health concern and are recognised in the US as contributing to the burden of chronic non-communicable diseases; including cancer now at epidemic rates. The results of our systematic review help put scale and magnitude to the potential size of the problem. Improved knowledge of biologically safe water, new technology and the hygienic management of sewage to prevent contamination of water supplies will help mitigate the need for disinfection. Ongoing research to determine the most desirable water qualities for health and how to deliver this quality of water to end users will set the gold standard to strive for in the entire water supply chain.