Cancer mortality and the method of chlorination of public drinking water: St. Louis City and St. Louis County, Missouri.

St. Louis City and St. Louis County, Missouri share the same public drinking water source, namely the Missouri River. The 'all cancer' and most organ specific cancer mortality rates have been consistently and considerably higher for St. Louis City than for St. Louis County for the period 1960 through 1972. A change in the St. Louis County water treatment process, which included increasing the chlorine dosage and delaying the addition of ammonia to form chloramines until just prior to distribution, was instituted in 1955. St. Louis City has, by contrast, continued the lower chlorine level and early ammoniation. Trend analysis using the period 1960-67 and 1972-76 showed higher percentage as well as net cancer mortality rate per million increases for large bowel, liver and bladder cancers for St. Louis County. An apparent association between a probable increase in trihalomethane production in the St. Louis County water since 1955 and an increase in these specific cancer rates which exceed the increases in the St. Louis City rates appears to have been shown. This does not imply causality but is in general agreement with other studies which have examined water chlorination and cancer mortality.