Health significance of chlorination byproducts in drinking water: the Houston experience.

In 1954, following the construction of Lake Houston, a change from lightly chlorinated ground sources to a heavily chlorinated surface source of drinking water took place for a sizable part of the population in the city of Houston, Texas. This has provided the opportunity to compare the incidence of urinary tract cancer mortality in populations exposed to heavily chlorinated and lightly chlorinated drinking water. The spatial, diurnal, and seasonal concentrations of chlorination byproducts (trihalomethanes) in Houston water were assessed. The range of concentrations varied from below the limits of detection in treated ground water, to more than 200 mg/l (twice the level allowed by US drinking water standards) in treated lake water. The mortality experience by gender, by race, and by age cohorts for the period 1940 to 1970 from urinary tract cancers and three comparison causes was determined for 56 of Houston's census tracts classified by the duration of exposure to the surface water. By the 1970's 20 years following the switch to surface water, an increase was detected in urinary cancer mortality rates for white females without a corresponding increase observed for white males. No clear-cut trends were found for the non-white population. On balance, a detrimental urinary cancer effect associated with a switch to chlorinated surface water has not been demonstrated yet.