Objectives

This study assessed the association of drinking water source and chlorination by-product exposure with cancer incidence.

METHODS

A cohort of 28,237 Iowa women reported their drinking water source. Exposure to chlorination by-products was determined from statewide water quality data.

RESULTS

In comparison with women who used municipal ground-water sources, women with municipal surface water sources were at an increased risk of colon cancer and all cancers combined. A clear dose-response relation was observed between four categories of increasing chloroform levels in finished drinking water and the risk of colon cancer and all cancers combined. The relative risks were 1.00, 1.06, 1.39, and 1.68 for colon cancer and 1.00, 1.04, 1.24, and 1.25 for total cancers. No consistent association with either water source or chloroform concentration was observed for other cancer sites.

CONCLUSIONS

These results suggest that exposure to chlorination by-products in drinking water is associated with increased risk of colon cancer.