Carcinogenicity of Disinfection Byproducts in Humans: Epidemiological Studies

The cancer risk associated with consumption of chlorinated drinking water or exposure to specific disinfection by-products (DBPs) has been evaluated in several epidemiological studies. The first studies were ecological in design or were based on death certificates, which identified bladder cancer as one of the cancers at increased risk. These studies had limited information on exposure to DBPs and did not consider other risk factors for the cancers examined. Following these first studies, case-control studies with markedly improved exposure assessment at the individual level were conducted. The studies on bladder cancer reported the most consistent positive associations with chlorination by-products exposure and indicate an increased risk at trihalomethane (THM) levels commonly observed in industrialized countries. One study evaluated the association with ozonation and found a protective effect for subjects supplied with ozonated water compared to chlorinated water. Only one study evaluated different routes of exposure and showed that water activities that would lead to exposure to THMs via inhalation and dermal absorption were associated with a higher bladder cancer risk than exposure via water ingestion. Finally, only one study has evaluated the potential interaction between specific genes and exposure to DBPs and has shown that a part of the population appears more susceptible to the effect of THMs. The evidence for other cancers is less conclusive, with studies on colon cancer providing some support for an association with consumption of chlorinated drinking water.