Two common beauty products, which are permanent hair dyes and chemical straighteners (or hair relaxing), may be associated with an elevated risk for breast cancer, according to a new study published in the International Journal of Cancer.
Hair dyes have been linked to other cancers before, though the research is inconclusive. Bladder and blood cancers have been examined most closely, according to the American Cancer Society, with the most consistent results pointing to a small increase in bladder cancer risk for salon employees. Meanwhile, most studies to date looking specifically at dye and breast cancer have not found a connection.
The picture is similarly unclear for hair straighteners. While a major study using data from the mid-1990s did not find a link between straighteners and breast cancer, other, more recent studies have-and the researchers behind the new paper note that some straightening formulas popularized since the 1990s, namely keratin treatments, have been found to either contain the carcinogen formaldehyde, or release it during the application process.
The new study, which was funded by the National Institutes of Health (NIH) and the National Institute of Environmental Health Sciences, tracked 46,700 U.S. women enrolled in the Sister Study, which recruited breast-cancer-free women whose sisters had been diagnosed with the disease. At enrollment, the women ranged in age from 35 to 74. They answered questions about their health, lifestyle (including hair product use) and demographics at the beginning of the study, and provided researchers with updates over a follow-up period of, on average, eight years.
More than half of the women reported use of permanent hair dyes in the year before they joined the study, and about 10% said they had used chemical straighteners. These women, the researchers found, had a greater chance of being among the nearly 2,800 study participants who ended up developing breast cancer-especially if they identified as black.
Overall, using permanent dye was associated with a 9% higher risk of developing breast cancer, compared to non-use. But black women who used permanent dye had a 45% higher risk of breast cancer, compared to non-users, and those who used these products every eight weeks or more often had a 60% higher risk.
Black women were also far more likely to report using chemical straighteners-74% had, compared to 3% of white women-which were associated with an 18% higher risk of breast cancer in the study population as a whole.
Alexandra White, an National Institute of Environmental Health Sciences epidemiologist and one of the study authors, notes that this may be due to differences in the formulations of the dyes and straighteners used by women identifying as black compared to those identifying as white. Her co-author Dale Sandler adds that coarser, thicker hair may also absorb more dye. More research is required to confirm those hypotheses, but Sandler says it's important for doctors to know about the differences, especially since black women are more likely to die from breast cancer than white women, and since most previous research on the health effects of hair dye have been on white women.