This book evaluates the risks to human health posed by disinfectants and disinfectant by-products found in treated drinking-water. Noting that chlorine and other widely-used disinfectants were approved for use almost 100 years ago, when toxicological data were limited, the report responds to the need for reassurance that consumption of treated drinking-water will not have adverse effects on health. Particular concern centres on the potential of chlorine to react with natural organic matter and form a large number of by-products, some of which have been intensively studied as potential human carcinogens. With these concerns in mind, the report evaluates over 800 recent studies in an effort to clarify understanding of the chemistry and toxicology of disinfectants and disinfectant by-products, and provide a balanced assessment of the associated risks to human health. The report is issued at a time when public health authorities and utilities providers in several countries are considering alternative methods of disinfection aimed at reducing the formation of specific by-products. In this context, the report stresses the overriding importance of microbiological safety, and warns that adequate disinfection must not be compromised by efforts to control chemical by-products.