There may be good reasons to stop dyeing your hair. But despite persistent rumors to the contrary, concern for your health probably isn't one of them.

While hair dyes contain several substances that have been shown to cause cancer in animals, no strong link has ever been found between hair dye and cancer in humans. Some studies have associated hair dye with a higher risk of disease, but these studies have generally been small or considered less reliable.

If hair dyes nonetheless produce some unease, it may be partly because they exist in an area lacking strong federal regulatory authority. In 1938 dye makers successfully lobbied for a special exemption from the Food, Drug and Cosmetic Act, which generally requires pre-market testing of cosmetic products by the Food and Drug Administration (FDA). By the FDA's own admission, this leaves consumers pretty much on their own in assessing product safety.

In a small 2001 study involving 203 women with bladder cancer, Manuela Gago-Dominguez, a researcher at the University of Southern California's Keck School of Medicine, found that those who reported using permanent hair dye at least once a month were twice as likely to develop bladder cancer as those who did not use permanent hair dye. Gago-Dominguez's research suggests that genetics may put certain women more at risk, along with frequency of application and duration of use.

Over the last 20 years, manufacturers have removed some carcinogens from hair dyes, but not all of the aromatic amines, says Gago-Dominguez, even though some of those compounds have been banned in Europe. ". . . I think that to play down the risks associated with hair dyeing based on the data available to us is not a prudent assessment," Gago-Dominguez said.

Most experts, however, say any cancer risk from hair color is infinitesimal, if it exists at all. Among them is epidemiologist Michael Thun, vice president of epidemiology and surveillance for the American Cancer Society (ACS). His conclusions are based on a landmark survey of nearly 600,000 women, nearly one-third of whom had used hair dye, that ACS conducted jointly with the FDA in the early 1990s.

Thun calls that data more reliable not just because of the study size but because the study was prospective (subjects, randomly selected, were followed forward in time). In retrospective studies like Gago-Dominguez's, says Thun, subjects' reporting of possible risk factors may be influenced by the knowledge that they have the disease.

"For us to be unable to detect an association between hair dye and cancer deaths shows that this association must be very small or very elusive," said Thun, who noted that his wife colors her hair. He recommends that people minimize their risk by using products less frequently, choosing lighter colors, which have a lower concentration of potentially harmful chemicals, and wearing gloves when applying dye.

In 1998, Elizabeth Holly, head of the division of cancer epidemiology at the University of California at San Francisco School of Medicine, ruled out hair dye as a factor contributing to non-Hodgkin's lymphoma after an eight-year survey of more than 4,000 people. Last year, after a study of nearly 1,350 women, she eliminated hair color used immediately before and during pregnancy as a factor leading to brain tumors in their children.

Other studies have failed to link hair dye with other types of cancer, including breast cancer.

"It's difficult to come up with a causality between cancer and hair coloring," said Linda Katz, director of the FDA's Office of Cosmetics and Colors.

-- Rita Zeidner