Hints of a chlorine-cancer connection - meta-analysis suggests chlorine may form cancer causing by-products - Brief Article

Science News, July 11, 1992 by Kathy A. Fackelmann

The chlorine added to drinking water in the United States has produced dramatic reductions in water-borne diseases such as typhoid fever. Yet this chemical's role as a public health hero may be tarnished if hints of a link with cancer prove true.

A new statistical analysis indicates that people who drink chlorinated water run a 21 percent greater risk of bladder cancer and a 38 percent greater risk of rectal cancer than people who drink water with little or no chlorine. There is no proof that chlorine itself actually causes cancer; however, investigators think something in chlorinated water may act as a carcinogen.

"I am quite convinced, based on this study, that there is an association between cancer and chlorinated water," says Robert D. Morris of the Medical College of Wisconsin in Milwaukee, who directed the new study. Previous investigations showed no clear link between chlorinated drinking water and bladder or rectal cancer, but those efforts may have lacked the statistical power to find a connection, Morris says.

He and his colleagues turned to a formidable statistical method called meta-analysis. They began their inquiry by using a computer to comb the medical literature for studies on chlorinated water and cancer. Their search turned up 22 studies, but only 10 met certain quality standards. In these 10 studies, for example, investigators had determined whether subjects consumed chlorinated water and then used that information to define control groups. Some of the 10 studies indicated that people drinking chlorinated water had an elevated risk of cancer; others showed no excess risk.

Meta-analysis enabled Morris' group to pool the data from the 10 studies, far surpassing the statistical power of any single study. The analysis turned up a significantly increased risk of developing rectal and bladder cancer for people who drank chlorinated water -- a risk that went up as the dose increased. In the July AMERICAN JOURNAL OF PUBLIC HEALTH, the researchers suggest that at least 4,200 cases of bladder cancer and 6,500 cases of rectal cancer in the United States each year may trace to consumption of chlorinated water.

Animal studies have suggested that chlorine forms cancer-causing by-products when it interacts with organic compounds in water. Morris speculates that the rectum and bladder, which act as holding tanks for concentrated human waste, may be espcially vulnerable to such chemicals.

"We recognize that there could be an association between exposure to chlorination by-products and cancer," says Fred S. Hauchman at the Environmental Protection Agency's health effects lab in Research Triangle Park, N.C.

However, he and Morris emphasize that the public health benefits of clean water far outweigh the potential health risks of chlorination. At the same time, researchers continue to study other methods of disinfecting water, including a process called chloramination, which adds both chlorine and ammonia to water. Chloramination may prove safer than simple chlorination because it yields fewer dangerous by-products, Morris says.