Women who have used hair straightening chemicals, or relaxers, may be at higher risk of developing uterine cancer.          

Women who have used hair straightening chemicals, or relaxers, may be at higher risk of developing uterine cancer, according to a new study by researchers at the National Institutes of Health.

The study, released earlier this week, joins prior research in showing an increased risk of hormone-related cancer in women who have used the chemicals in their hair — a finding that researchers note may be especially concerning for Black women, who are far more likely to report using such products.

“We estimated that 1.64% of women who never used hair straighteners would go on to develop uterine cancer by the age of 70; but for frequent users, that risk goes up to 4.05%,” said lead author Alexandra White, the head of the NIEHS Environment and Cancer Epidemiology group.

The data comes from the Sister Study, a large research project led by the National Institute of Environmental Health Sciences, which is part of NIH. The project collects medical records and lifestyle surveys from tens of thousands of women ages 35 to 74, all of whom are sisters to women with breast cancer but did not have cancer themselves, in an effort to identify risk factors for breast cancer and other diseases.

Over the course of 11 years, 378 cases of uterine cancer were diagnosed among the study’s 33,497 participants. Women who had ever reported using straightening products in the last year were slightly likelier to develop cancer than those who’d never used them. Women who’d used the products more than 4 times in the past year were even more at risk — though researchers caution that uterine cancer is still relatively rare.

The study did not ask participants to report particular brands or chemicals. Other hair products, like hair dyes, highlights and perms, did not have the same link to uterine cancer, researchers said. (Hair dye, along with straighteners, had previously been linked to breast and ovarian cancer by the same research group.)

Thought uterine cancer is rare, Black women develop it at higher rates than women of other races, according to the National Cancer Institute.

And Black women, too, face intense societal pressure about hair. Natural hair, braids and other hairstyles traditionally worn by Black women (and men) have been subject to bans in settings from K-12 schools to the U.S. military. A 2017 study suggested that many people may still have implicit bias against textured hair. In many states, hair discrimination is still legal; earlier this year, the House passed a bill designed to ban it.

“Because Black women use hair straightening or relaxer products more frequently and tend to initiate use at earlier ages than other races and ethnicities, these findings may be even more relevant for them,” said Che-Jung Chang, another author on the study.