Chlorination of drinking water and cancer incidence

Epidemiological studies report associations between cancer mortality and type of drinking water source. Different studies appeared to lend some support to the hypothesis that surface waters contain higher levels of human carcinogens than groundwater sources. The possible contaminants that might play such a role are the halogenated organic compounds produced by the chemical reaction between organic materials in water and chlorine used for disinfection. Chlorine reacts with naturally occurring organic compounds in the water to form haloforms. Valencia Province of Spain relies almost exclusively on groundwater for its public drinking water, but the city and surrounding urban areas rely on superficial water. The chlorination used in the superficial water includes several phases: prechlorination, coagulation, filtration, and, finally, postchlorination, which shows the possible formation of organohalogenated compounds. To find an association between stomach and bladder cancer mortality with the type of water source, we studied 261 municipalities of Valencia Province. We calculated sex-specific standardized mortality ratios for deaths occurring between the 1985 to 1989 (census statistics), and we carried out a relative-risk calculation associated with the differences between superficial and groundwater supplies.