NEW YORK, Aug 04 (Reuters) -- Drinking chlorinated water may increase the risk of colon cancer, a study suggests.
A large-scale, prospective women's health study found a greater than expected likelihood of colon cancer, depending on the level of exposure to chloroform and other byproducts associated with chlorine.
As a disinfectant, chlorine is added to about 75% of the nation's drinking water.
"Our findings are consistent with some previous studies, including animal and human studies, that showed the same association," says Dr. Wei Zheng, associate professor of epidemiology at the University of Minnesota School of Public Health in Minneapolis.
In a report in the American Journal of Public Health, Zheng and his colleagues note that earlier epidemiologic findings had linked chlorination of drinking water to increased risks of bladder and colorectal cancers. However, all but one of these studies were retrospective estimates and did not follow large groups of people over time.
"We did a prospective, follow-up study of more than 40,000 women in Iowa because the state has all the historical data on chloroform levels in their drinking water sources," he explains, adding that chloroform is one of the major byproducts of chlorine.
Participants were drawn from the Iowa Women's Health Study, which originally enrolled about 42,000 postmenopausal women. Of the original study sample, 28,000 reported municipal water as their main source of home drinking water. An additional 6,600 reported drinking private well water, and over 2,000 more drank bottled water (or water from other sources).
The women were then followed for eight years. Of the group 3,567 developed at least one new cancer during the study period. The investigators chose as the reference category women served by municipal groundwater sources. This was because relatively lower levels of chlorine byproducts are generally detected in groundwater sources.
Compared to women using municipal groundwater sources, women using municipal surface water had a not-statistically significant increased risk of all cancers combined, Zheng and his colleagues explain.
When the researchers focused specifically on colon cancer, they found "a clear dose-response relationship" -- that is, higher exposures to chloroform were associated with increased cancer risks. Women who reported municipal surface water sources were at increased risk of colon cancer.
The chloroform levels in the drinking water were measured statewide in 1979 and 1986. Four categories of increasing chloroform levels yielded four colon cancer relative risk levels (0%, 6%, 39%, and 68%). Only the 68% increase was statistically significant, Zheng says.
Zheng says the increase in risk for all cancers combined "was generated by colon cancer, and if you exclude colon cancer, then all the other increases in risk are eliminated."
Zheng says he does not want people to be alarmed by the findings. Other "lifestyle" factors not studied -- such as diet and exercise -- may play important roles. "There's a possibility that confounding factors such as these may have caused the association we found," he says.
The researcher also notes that chlorination of drinking water "is the most effective way to clean the water and reduces a lot of disease."
SOURCE: American Journal of Public Health (1997;87:1168-1176)
