The study also looked at occupational exposure ? barbers and salon workers who use dyes. Though this study is the first to look at how often people are exposed to hair dye and their risk of developing bladder cancer, Manuela Gago-Dominguez, MD, PhD, of the University of Southern California School of Medicine in Los Angeles, says, "Unfortunately, at this moment we do not have the necessary information to make a recommendation to the public."

"In our study we have found an association between hair dye use and bladder cancer, which does not necessarily mean that hair dye use is a major cause of bladder cancer," says Gago-Dominguez

Gago-Dominguez and her colleagues compared 897 patients with bladder cancer who used hair dye, with a similar number of adults without bladder cancer. The researchers found that the women who used permanent hair dye at least once a month were twice as likely as women who did not use permanent hair dye to develop bladder cancer. These results held true whether or not the women smoked. Smoking is the greatest risk factor for bladder cancer.

Women who reported using permanent hair dye at least once a month for 15 years were three times more likely to have bladder cancer, and subjects who worked for 10 or more years as hairdressers or barbers were five times more likely to have bladder cancer compared to individuals not exposed, according to the study.

"We hope that our study can serve as an impetus to resume the lines of research that are needed for the resolution of this important public health issue," Gago-Dominguez says.

But John Corbett, PhD, spokesperson for the Cosmetic, Toiletry, and Fragrance Association, a trade association, points out that "several larger continuing studies have not shown increases in bladder cancer risk, including studies by the American Cancer Society of more than 570,000 women and a study at Harvard University of about 120,000 women."

Corbett also criticizes the study for not asking the appropriate questions about hair dye use. "The only things they asked about were frequency of use and duration of use," says Corbett, who notes that the most important factor in exposure is the shade that is used.

"All permanent hair colors use the same basic ingredients, but darker shades have about 100 times more chemicals than the lightest blonde shade," he explains. "The shade used is the most important measure of exposure to chemicals, not the number of times the hair dye was used," he says.

"No chemicals in hair dyes currently on the market have been found to cause cancers in laboratory animals," says Corbett.

"I think that this study is out of line with the findings of other studies," says Michael Thun, MD, vice president of epidemiology and surveillance for the American Cancer Society (ACS) and a researcher on the large ACS study.

"If the use of permanent hair dye is a risk factor, it is an extremely weak risk factor," Thun says, "so whether someone continues to dye their hair might depend on the way they deal with risk."

He suggests that people could either avoid using hair dyes, use the dyes for a shorter duration, use lighter shades, or reduce their skin contact with the dye. He also suggests that people could focus on eliminating behaviors that are more strongly suspected to cause bladder cancer, such as smoking or physical inactivity, rather than changing their hair dye use.

"People should base their decisions on their own judgment and value systems," he says.