Using chemical relaxers that straighten curly or tightly coiled hair has been linked to an increased risk in uterine cancer in Black women after menopause, according to a large study from Boston University.

Researchers used responses from the Black Women’s Health Study, which interviewed 59,000 self-identified Black women in 1995, then between the ages of 21 and 69. The women in the study did follow-up questionnaires every two years after that.

They found that postmenopausal women who used chemical straighteners a moderate to heavy amount (“heavy” was defined as more than 5 times a year over more than 15 years), or regularly at any amount over 20 years, were more likely to get uterine cancer than women who never or very rarely used chemical straighteners.

“Our study suggests that moderate and heavy use of chemical hair relaxers may be associated with higher risk of uterine cancer among postmenopausal Black women,” author Kimberly Bertrand, an associate professor of medicine at Boston University, said in a release. She also noted that Black women “have higher rates of aggressive subtypes of uterine cancer and are nearly twice as likely to die from their disease.”

Evidence Doesn’t Prove a Connection, But Analysis Shows Higher Rates Than Expected

It’s important to note that the study does not prove that chemical relaxers definitely lead to uterine cancer. Of the nearly 45,000 women who qualified for this particular study, only 347 had uterine cancer. But the statistical analysis done by study authors found that rate of cancer was higher than expected among women using hair relaxers.

Researchers said this finding was important, citing that around 90% of Black adult women in the United States had used hair relaxers. The exact ingredients used in hair straighteners are proprietary, but study authors wrote that “potentially harmful toxicants and carcinogens such as formaldehyde and heavy metals may be present,” and that the products are “known to contain phthalates and parabens and other endocrine disrupting chemicals.”

And those “endocrine disrupting chemicals” have been linked in prior studies to changes in women’s reproductive systems, possibly because the chemicals are encouraging more estrogen production or inhibiting it.