Something in the water? A health impact assessment of disinfection by-products in New Zealand.

Disinfection by-products (DBP) are a large group of halogenated chemicals formed by the reaction of disinfectant agents with naturally-occurring organic substances in water. Numerous studies have found associations between DBP and some cancers and adverse reproductive outcomes. For both cancer and birth defects the relative risk associated with exposure to DBP is about 1.5. About 66% of New Zealanders, or 2.4 million people, use chlorinated water supplies and are exposed to DBP. New Zealand's unique combination of flora, climate and geology will create unique mixtures of DBP but little detailed information is available on the level or composition of DBP in New Zealand. The population attributable risk per cent, for cancers and birth defects in New Zealand, is about 25%. In other words, a quarter of all bladder, colon and rectal cancers and birth defects may be preventable by reducing DBP exposure. This is equal to 329 preventable cancer deaths in 1995 and 94 preventable birth defects in 1996. DBP exposure can be reduced without compromising microbiological safety of water supplies. The health effects of DBPs must be weighed against the cost of DBP reduction and not against the potential water borne disease prevented by disinfection. Some aspects of New Zealand's water supplies and population provide a unique opportunity to undertake research. Further research is needed on the occurrence of DBPs and their health consequences in order to undertake a properly informed risk assessment.