Does calcium in drinking water modify the association between trihalomethanes and the risk of death from colon cancer?

The objectives of this study were (1) to examine the relationship between total trihalomethanes (TTHM) levels in public water supplies and mortality attributed to colon cancer and (2) to determine whether calcium levels (Ca) in drinking water modify the effects of TTHM on risk to develop colon cancer. A matched cancer case-control study was used to investigate the relationship between the risk of death attributed to colon cancer and exposure to TTHM in drinking water in 53 municipalities in Taiwan. All colon cancer deaths in the 53 municipalities from 1998 through 2007 were obtained from the Bureau of Vital Statistics of the Taiwan Provincial Department of Health. Controls were deaths from other causes and were pair-matched to the cancer cases by gender, year of birth, and year of death. Each matched control was selected randomly from the set of possible controls for each cancer case. Data on TTHM levels in drinking water were collected from the Taiwan Environmental Protection Administration. Information on the levels of Ca in drinking water was obtained from the Taiwan Water Supply Corporation. The municipality of residence for cancer cases and controls was presumed to be the source of the subject's TTHM and Ca exposure via drinking water. Relative to individuals whose TTHM exposure level was <4.9 ppb, the adjusted OR (95% CI) for colon cancer was 1.14 (1.01-1.28) for individuals who resided in municipalities served by drinking water with a TTHM exposure > or =4.9 ppb. Data demonstrated evidence of an interaction between drinking-water TTHM concentrations and Ca intake via drinking water. Our findings showed that the correlation between TTHM exposure and risk of colon cancer development is influenced by Ca in drinking water. Increased knowledge of the interaction between Ca and TTHM in reducing colon cancer risk will aid in public policymaking and standard setting.