The objectives of this study were (1) to examine the relationship between total
trihalomethanes (TTHM) levels in public water supplies and risk of
rectal cancer development and (2) to determine whether
calcium (Ca) and
magnesium (Mg) levels in
drinking water might modify the effects of TTHM on risk of developing
rectal cancer. A matched
cancer case-control study was used to investigate the relationship between the risk of death attributed to
rectal cancer and exposure to TTHM in
drinking water in 53 municipalities in Taiwan. All
rectal 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
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 and Mg 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, Ca, and Mg exposure via
drinking water. Relative to individuals whose TTHM exposure level was <4.9 ppb, the adjusted OR (95% CI) for
rectal cancer occurrence was 1.04 (0.88-1.22) for individuals who resided in municipalities served by
drinking water with a TTHM exposure >or=4.9 ppb. There was no evidence of an interaction of
drinking-water TTHM levels with low Ca intake via
drinking water. However, evidence of an interaction was noted between
drinking-water TTHM concentrations and Mg intake via
drinking water. Our findings showed that the correlation between TTHM exposure and risk of
rectal cancer is influenced by Mg in
drinking water. Increased knowledge of the interaction between Mg and TTHM in reducing
rectal cancer risk will aid in public policymaking and standard setting.