Chlorination products linked to cancer.

A report published last summer offered the first strong link between chlorinated drinking water and an increased risk of human bladder and rectal cancers. But this epidemiologic study could not tease out the agent responsible -- chlorine, by-products of the disinfectant's reaction with water contaminants, or both (SN: 7/11/92, p.23). A new animal study now fingers the by-products as the most likely culprits.

For two years, scientists at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., fed hundreds of mice and rats water containing a drinking-water disinfectant -- either chlorine ([Cl.sub.2]) or chloramine ([NH.sub.2]Cl). In a concurrent study, they laced the diets of rodents with chloroform, bromodichloromethane, chlorodibromomethane, or bromoform -- trihalomethanes (THMs) that typically form in chlorinated water. Each experiment included 50 mice and 50 rats of each sex.

In animals administered chlorine or chloramine, "the only evidence of carcinogenicity...was an equivocal response for leukemia in female rats," June K. Dunnick and Ronald L. Melnick report in the May 19 JOURNAL OF THE NATIONAL CANCER INSTITUTE. However, compared to rodents eating an ordinary diet, those dining on food containing a THM showed not only liver and kidney toxicity, but also unusually high rates of cancer.

For instance, tumors of the kidney and colorectum normally occur in fewer than 1 percent of rats, the investigators note. The new research shows that one-quarter to one-third of rats treated with chloroform, bromoform, or bromodichloromethane developed kidney cancers. Between 25 and 90 percent of the rats fed one of those three THMs also developed both precancerous and malignant colorectal tumors.

"This study provides some of the clearest evidence yet of [chlorination's] carcinogenicity, particularly its colorectal carcinogencity," says Robert D. Morris of the Medical College of Wisconsin in Milwaukee. However, THMs may not be the only carcinogens formed, he says, noting that only about half of chlorination's by-products have so far been identified.

The study also provides some of the first information on the relative potency of individual THMs, notes Stephen W. Clark of EPA's Office of Drinking Water Standards in Washington, D.C. Indeed, he says, these data will probably influence the new rules for THMs in drinking water now being negotiated by EPA and industry and environmental leaders.