Some hair dyes contain chemicals that are known to cause cancer. But that doesn't mean that coloring your hair will make you sick. On the other hand, an awful lot of people use hair dye—so if using these hair dyes increases your risk of cancer, as some epidemiologists fear, it's something public health officials should be concerned about. Researchers in Spain and Canada collected a raft of studies done on hair dye and cancer risk to see if a pattern emerged when they pooled the results.

What the researchers wanted to know: What is the scientific evidence on a potential link between hair dye and cancer?

What they did: The researchers searched for studies published from 1966 to January 2005 on hair dye and cancer. They were interested in only two types of studies: case-control studies, in which people with cancer were matched with people without cancer, to see if they'd had differences in their exposures to hair dye; and cohort studies, in which people who do and do not use hair dye are followed over time to see who develops cancer. They found 14 studies on breast cancer that met their standards, 10 on bladder cancer, and 40 on hematopoietic cancers, those related to blood cell formation, such as leukemia and Hodgkin's disease.

What they found: Hair dye was not linked to breast cancer or bladder cancer; people who used dye and those who didn't had roughly equal chances of getting those two types of cancer. For hematopoietic cancers, studies found a slightly increased risk of cancer in people who had used hair dye. But when the researchers looked more closely, they found that using hair dye intensively or using permanent dye, which may contain more carcinogens than temporary dyes, exclusively did not increase the risk of hematopoietic cancers, suggesting that hair dye wasn't to blame for the cancers.

What the study means to you: While this meta-analysis did find a small increase in the risk of hematopoietic cancers in people who use hair dye, the researchers say this could be a coincidence. Overall, they say, hair dye is not a major public health concern. More studies might look at the risk of cancer from exposure to hair dye at work, which the researchers excluded from this analysis.

Caveats: More than half of the studies didn't adjust for smoking, which is an important risk factor for cancer. If people who dye their hair are also more likely to smoke—or vice versa—that might color the results.

Find out more: Check out information on hair dye from the Food and Drug Administration.

Read a study on hair dye and leukemia published in July of last year at USNews.com.

Read the article: Takkouche, B., et al. "Personal Use of Hair Dyes and Risk of Cancer." Journal of the American Medical Association. May 25, 2005, Vol. 293, No. 20, pp. 2516-2525.

Abstract online: http://jama.ama-assn.org