Drinking water and cancer incidence in Iowa. I. Trends and incidence by source of drinking water and size of municipality.

Age-adjusted, sex-specific cancer incidence rates for the years 1969-1978 were determined for municipalities in Iowa having a population of 1000 or over and public water supply that had remained stable for a minimum of 14 years. The incidence rates for the municipalities were examined according to major source of water supply (surface or ground) and depth of well. As municipality size increased, incidence rates increased for cancer of the lung among males and females. A slight gradient of increasing cancer incidence was seen for cancer of the bladder among males and females. When stratified for population size, incidence rates for cancers of the lung and rectum among males and females were higher for municipalities on surface water compared with those on ground sources. Incidence rates for cancer of the prostate rose as well depth increased, while a trend was seen toward decreasing incidence rates for cancer of the colon among females as well depth increased. When time trends were examined, an increase in cancer rates over time was seen for several cancer sites, with the increase most noticeable in municipalities supplied by surface water or shallow wells. These results are not always consistent with the hypothesis of an association between cancer and chlorinated water.