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Your shampoo may seem harmless, but it could be contributing to the formation of a mysterious, cancer-causing substance, a new study says.

New research reveals that common household products such as shampoo can interact with disinfectants at U.S. wastewater treatment plants to form a little-studied class of cancer-causing substances. These substances, called nitrosamines, can end up in drinking water, experts say.

Several nitrosamines, including the chemical NDMA, a focus of the new Yale study, are classified by the U.S. Environmental Protection Agency (EPA) as probable human carcinogens.

Nitrosamines form in small amounts when exposed to chloramine, the disinfectant of choice at the nation's wastewater treatment plants. The chemical—a combination of chlorine and ammonia—has been used increasingly in drinking water disinfection since the EPA set limits for better-known toxic substances that can arise from the use of chlorine, the traditional disinfectant.

Though inconclusive, the study suggests "it's entirely possible that we're producing more problems—and maybe even worse problems—with chloramines," said David Reckhow, an environmental engineer at the University of Massachusetts in Amherst who was not involved in the new study.

Cost of Beauty?

Nitrosamines are found in a wide variety of sources, including processed meats and tobacco smoke, but what sparks their formation in drinking water has long baffled scientists.

Past studies with cosmetics have hinted that substances called quaternary amines, which are also ingredients in household cleaning agents, may play a role in creating nitrosamines.

Other work has shown the drug ranitidine, a stomach acid reducer also known as Zantac, can be transformed into NDMA. But those chemicals occur in trace concentrations in wastewater, and "ranitidine itself has not been detected as far as I know," said study co-author William Mitch, a chemical engineer at Yale University. "We started to think about chemicals that are used in high concentrations in consumer products."

In search of more sources of nitrosamines, Mitch and his colleagues tested water at three treatment plants in Connecticut. The team then analyzed the relative amounts of quaternary amines and nitrosamines before and after treatment with chloramine.

They also tested four common household products, chosen at random, for chemicals that readily form nitrosamines: Suave and Pantene shampoos, Dawn dishwashing soap, and Cheer laundry detergent.

Mixed Results

The researchers found that although sewage treatment plants remove some of the quaternary amines that form NDMA, quaternary amines are used in such large quantities that some may make it through the treatment process.

For example, if 80 percent of people in an area served by a treatment plant used a typical amount of Suave shampoo daily, that amount of shampoo would account for up to 3 percent of nitrosamines in the treated wastewater, the authors wrote. Dawn detergent created 26 times more NDMA than Suave, according to the study in the January 19 edition of the journal Environmental Science & Technology.

Some products, such as Cheer laundry detergent and Pantene shampoo, did not form nitrosamines. And the highly preliminary results do not indicate whether other brands of household products would.

Mitch said that while some quaternary amines might be removed during wastewater treatment, others might be biologically transformed into tertiary amines, which yield even higher amounts of nitrosamines.

"The problem is that we don't know because many of these materials are polymeric, and current analytical techniques are not geared to measure polymer concentrations," he said. Polymers are molecules made up of chains of simple, repeated units.

"So at this point, it's a suggestion, because we cannot measure polymeric quaternary amines to demonstrate a link."

Don't Pee in the Pool?

Mitch said the combination of disinfectants and quaternary amines can occur in places other than treatment plants—when different household cleaners are combined, for instance, or in swimming pools.

"In fact, we measured nitrosamine concentrations in swimming pools, and the precursors are either consumer products or human waste," he said, referring to the fact that people sometimes urinate in pools. Those results were published in 2008 in the journal Environmental Science & Technology.

Yet no regulations exist so far in the United States—the University of Massachusetts' Reckhow said it's not unusual for regulations to lag behind scientific discovery.

"There’s no way of getting around that, end even the science isn’t really sure" about the consequences of combining chloramination at treatment plants with household products, he said.

Still, the work represents an important step, said Susan Richardson, a research chemist with the National Exposure Research Laboratory in Athens, Georgia, who was not involved in the research.