Experts Downplay Cancer Risk of Chlorinated Water : Health: Report uses 10 earlier studies to make estimates. But officials say benefits of disinfection outweigh the danger.

Disinfection of drinking water with chlorine is associated with a slight increase in the risk of rectal and bladder cancers, but the benefits of chlorination far outweigh the possible risks, experts said Wednesday.

Chlorination of water may lead to an additional 6,500 cases of rectal cancer and 4,200 cases of bladder cancer each year, according to a report in the July issue of the American Journal of Public Health by researchers at the Medical College of Wisconsin and the Harvard School of Public Health.

Scientists have long known that chlorine interacts with organic chemicals in water to produce possible carcinogens, but the new report--which combines results from 10 previous studies--is the first to yield good projections of the potential risk.

Experts cautioned, however, that people should not overreact to the potential risk from chlorination. “The known risk of waterborne disease in humans that occurs if water is not disinfected is much greater than the theoretical risk of developing cancer from the consumption of chlorinated drinking water,” said toxicologist Jennifer Orme of the U.S. Environmental Protection Agency.

“Nonetheless, these findings should provide an impetus to . . . develop disinfection strategies that are not associated with adverse risks,” the authors wrote.

For most Southern Californians, the question is moot. The Los Angeles Department of Water and Power, which supplies the city of Los Angeles, and the Metropolitan Water District, which sells water throughout the region, normally use a different disinfectant technique that carries less risk.

But the San Joaquin Reservoir in Orange County, for example, uses chlorine routinely because it is an uncovered reservoir subject to potential contamination with disease-causing microorganisms.

Chlorination of water has been used routinely in this country since it was first adopted in the Chicago stockyards in 1908. Adoption of the technique produced sharp drops in the incidence of infectious disease, said epidemiologist Robert D. Morris of the Medical College of Wisconsin.

But in 1974, researchers found that chlorine can interact with certain organic materials in river water to produce chlorinated byproducts that have been shown to produce cancer in animals. The EPA established new standards to limit the concentration of such chemicals in drinking water.

One way to meet those standards was to shift to an alternate disinfection process using a combination of chlorine and ammonia called chloramine, which kills bacteria but is less likely to produce toxic chemicals.

Several studies have attempted to determine the risk of chlorinated water, but most have been too small to produce definitive results. Morris, with epidemiologist Thomas C. Chalmers and his colleagues at Harvard, used a new technique called meta-analysis to combine the results from the 10 best studies, yielding the new findings.

They report that people drinking chlorinated water over long periods have a 21% increase in the risk of contracting bladder cancer and a 38% increase in the risk of rectal cancer. Morris estimates that consumption of chlorinated drinking water accounts for 9% of all bladder cancers and 15% of all rectal cancers in the United States.

But the EPA and the International Agency for Research on Cancer say the studies that Morris and Chalmers drew on are “inadequate” for determining cancer risk, said EPA’s Orme. She said the EPA has been very careful in establishing standards for allowable concentrations of the chlorinated byproducts in water.

“We don’t want to develop standards that are so restrictive that people will stop disinfecting water,” she said.