Women who use permanent dye or chemically straighten their hair may be at a higher risk of developing breast cancer, according to a recent study published in the International Journal of Cancer.

However, cancer researchers warned that the findings do not show causation and should be viewed in the appropriate context.

Study details

For the study, researchers at the National Institute of Environmental Health Sciences, part of NIH, looked at data from 46,709 women who are part of the ongoing Sister Study. All of the women in the Sister Study are at a high risk for breast cancer because they had a sister who had breast cancer.

The women in the study were between ages 35 and 74, and 9% were black.

Study shows link between hair dye use and breast cancer—and large variation by race

Overall, the researchers found the hazard ratio for breast cancer was 1.09 for permanent dye use, compared with women who did not use the dye. However, the researchers found the increased breast cancer risk varied greatly by race.

For instance, the researchers found that among black women, the risk of developing breast cancer was 45% higher among those who used permanent hair dye than among women who did not use hair dye. That risk increased 60% among black women who reported using hair dye every five to eight weeks, the researchers found.

Among non-Hispanic white women, use of permanent hair dye was associated with a 7% increase in the risk of breast cancer.

The researchers said they were unsure what ingredients in hair dye products could be concerning, as the study didn’t look at the specific ingredients in the hair dyes women used.

The researchers also looked at the relationship between chemical straighteners and cancer risks.

Overall, the researchers found that women who used chemical hair straighteners were about 30% more likely to develop breast cancer than those who didn’t. The researchers did not observe a difference between black and white women when it came to breast cancer risk and chemical straightener use. However, black women were far more likely to use chemical hair straighteners, with about 75% of black women in the study reporting using them, compared with 3% of non-Hispanic white women.

Alexandra White, an author on the study and an investigator with the National Institute of Environmental Health Sciences, said that “one of the big concerns [with chemical straighteners] is formaldehyde, which is a known carcinogen.”

Why experts are urging caution

Otis Brawley, a medical oncologist at Johns Hopkins University, who was not involved in the study, said the actual cancer risk found in this study was fairly low, especially compared to other carcinogens, such as tobacco or radiation. “This is a very weak signal that these things might be causing cancer in the population,” he said.

Brawley added that more research would be needed. However, he said that the type of study necessary, namely a long-term clinical trial with a control group and a placebo, “would be difficult if not impossible to do.”

Similarly, Larry Norton, medical director of the Evelyn H. Lauder Breast Center at Memorial Sloan Kettering Cancer Center, who was not involved in the study, said, “You cannot, based on [this] data, make the statement that hair dyes and straighteners cause breast cancer. These effects were small.”

Doris Browne, a medical oncologist and former president of the National Medical Association, who was not involved in the study, said women should have a conversation with their doctor about their breast cancer risk.

Browne suggests patients discuss with their doctors their use of hair dyes and straighteners alongside other aspects of their “social history,” such as alcohol consumption, smoking, and obesity.

Robin Dodson, a research scientist at the Silent Spring Institute, who was not involved in the study, recommended a “cautionary approach” to using hair dyes and straighteners in response to the study.

“Most products put out there on the market today are not adequately tested for safety, and they aren’t tested for endocrine-disrupting chemicals,” Dodson said. “Most people are very surprised to learn that there’s nobody really minding the store” (Neighmond, “Shots,” NPR, 12/4; Rabin, New York Times, 12/4; Ducharme, TIME, 12/4; Andrew, CNN, 12/6).