WEDNESDAY, March 26 (HealthDay News) -- Hairdressers and barbers who used now-banned hair dyes that had high concentrations of coloring ingredients appear to be at a slightly elevated risk of bladder cancer, a new report contends.

The authors said the findings confirm what was first suspected back in the 1970s -- that hair dyes appear to increase the risk of cancer. These coloring agents were discontinued in the 1970s when they tested positive for cancer in rodents.

"This report updates an earlier review done in the 1990s, which called the evidence inadequate to determine the risk of cancer," said Dr. Michael J. Thun, head of epidemiological research at the American Cancer Society.

There is a question, however, whether the same cancer risk exists with the chemicals currently used in hair dyes, Thun said.

"These studies were done over years, and cancer takes years to develop. So the relevant exposures would have been in the past, and the products have changed," Thun said. "So this report doesn't provide any evidence about the risk of current exposure."

The new report is published in the April issue of The Lancet Oncology.

Dr. Robert Baan, of the International Agency for Research on Cancer, in Lyon, France, and his colleagues reviewed studies on cancers in hairdressers, beauticians and barbers that were done after 1993.

Hair dyes are classified as permanent, semi-permanent or temporary. Permanent dyes make up about 80 percent of the market. These dyes contain chemicals that, when mixed with peroxide, cause a chemical reaction that produces the dye. Dark hair dyes have the highest concentration of coloring agents, Baan's group noted.

The researchers found that the use of these dyes was linked to a small but discernible increased risk of bladder cancer. "Bladder cancer is not rare, but it's not common," Thun said.

"A small, but consistent, risk of bladder cancer was reported in male hairdressers and barbers. Because of few supporting findings by duration or period of exposure, the Working Group considered these data as limited evidence of carcinogenicity and reaffirmed occupational exposures of hairdressers and barbers as 'probably carcinogenic' to humans," the researchers wrote.

Baan's team also looked at whether people using hair dyes at home had similar risks for cancer. They found there wasn't enough evidence to make a definitive conclusion about the personal use of hair dyes and the potential for cancer.

The researchers also looked at other chemicals that belong to the same chemical group as hair dyes. They noted that one chemical -- ortho-toluidine, which is used in making dyes, pigments and rubber chemicals -- is classified as carcinogenic to humans.

Thun noted that people are exposed to naturally occurring carcinogens all the time -- in food, in air and water. "The goal is not to increase that load. So the goal here is to have these products be free of substances that are potential carcinogens and to minimize the exposure of people who work with them," he said.

More information

For more information on hair dyes, visit the U.S. Food and Drug Administration.

SOURCES: Michael J. Thun, M.D., head, epidemiological research, American Cancer Society, Atlanta; April 2008 Lancet Oncology

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