Millions of women are using hair-straightening treatments and products that might be harmful to their health.

More research is linking chemicals in these products to an increased risk of uterineovarian and breast cancer. Studies also suggest that frequent use of such products can negatively affect puberty and pregnancy.

Researchers don’t know exactly what chemicals may be leading to these increased health risks. The U.S. Food and Drug Administration suspects one culprit may be formaldehyde, which is often used in salon-based keratin straightening treatments and has been linked to certain cancers. The agency is working on a proposal that would ban the chemical from hair smoothing and straightening products.

Drugstore-bought hair straighteners and relaxers don’t typically contain formaldehyde. However, some contain chemicals that can disrupt the endocrine system, such as phthalates, phenols and parabens. Endocrine disrupters may behave like estrogen or other hormones that can lead to an increased risk of cancer, says Dale Sandler, chief of the National Institute of Environmental Health Sciences epidemiology branch.

Hair straighteners and relaxers, including cream-based products as well as oils and lotions used for maintenance, are often used by and marketed to Black women and other women of color. Many keratin-based products are used by a wide range of women to smooth curly or wavy hair.

What the science says

Womenwho told researchers they used hair-straightening products four or more times a year were more than twice as likely to develop uterine cancer compared with those who didn’t, according to a 2022 study in the Journal of the National Cancer Institute.

A separate October studyin the journal Environmental Research found that postmenopausal women who regularly used hair straighteners had a 50% to 70% increased risk of developing uterine cancer.

Other research has suggested links with ovarian and breast cancers, though the breast cancer data is more mixed. Women who frequently used straighteners or had perms during adolescence had an increased risk of breast cancer, while women who used hair-straightening products or relaxers four or more times a year were twice as likely to develop ovarian cancer compared with those who didn’t use the products, according to two separate 2021 studies.

“What our research has shown is that there are multiple harmful chemicals in these products,” says Tamarra James-Todd, an associate professor of environmental reproductive epidemiology at Harvard T.H. Chan School of Public Health.

None of the research showed that the products caused the cancers, but rather that use of the products was associated with higher risks.

It’s unclear exactly what chemicals or combinations of chemicals may be leading to increased risk of those cancers, says Lauren Wise, an epidemiology professor at Boston University School of Public Health.But high doses and frequent exposures of formaldehyde have been linked to leukemia and nasopharyngeal cancer.

The Personal Care Products Council, a trade association, noted that hair-straightening products are regulated by the FDA and that companies must ensure their products are safe. It pointed to what it perceived as flaws in recent research and said that finding a linkisn’t the same thing as proving a cause. It said it would support the FDA’s proposed ban, which would bar formaldehyde as well as formaldehyde-releasing ingredients in hair straighteners, and that “ensuring product safety, consumer trust and confidence through science-backed measures is our utmost priority.”

What you can do

Many products such as keratin treatments are marketed as formaldehyde-free but contain methylene glycol, which when heated releases formaldehyde, researchers say. The FDA says to look out for formaldehyde-related ingredients, which include formalin and methylene glycol.

The Environmental Working Group, a Washington, D.C., nonprofit, has a Skin Deep cosmetics database, which rates products for personal safety, and includes a list of other commonly used names for formaldehyde. EWG also has a Healthy Living App where consumers can scan personal care products and see information about them.

Other resources include apps like the Detox Me app from the Silent Spring Institute. The Breast Cancer Prevention Partners’ Campaign for Safe Cosmetics also developed a database of products it deems safer that aremade by Black-owned beauty brands.

With salon treatments, it’s harder to know what’s in the products than in store-bought relaxers, says David Andrews, a senior scientist for EWG. He recommends asking your stylist what’s in the products.

It’s difficult to tell if a straightener contains endocrine-disruptors, says Kimberly Bertrand, an associate professor of medicine at Boston University who was first author of the October study linking hair straighteners with an increased risk of uterine cancer.

Some will list long names under ingredients that end in “paraben” or “phthalate.” But manufacturers aren’t required to disclose specific ingredients so sometimes they will be displayed as “preservative” or “fragrance,” says Bertrand.

Dr. Ryland Gore, a surgical oncologist in Atlanta, recently went to get her hair chemically straightened and asked her hair stylist about the products, scrutinizing the labels. Gore says she has been using straighteners regularly since she was a child. But as more research emerges, she thinks about her own risk.

“We cannot ignore the associated cancer risk,” she says. “I’m so glad that this is a conversation that is happening.”

Regulating hair products

The FDA regulates hair straighteners as cosmetics, not drugs; they don’t go through the same level of safety testing and approvals process that medications do.

Recent studies have fueled a flurry of lawsuits across the country. More than 7,000 lawsuits have been consolidated in federal court in Illinois, filedagainst more than a dozen manufacturers of hair straighteners and relaxers. The suit claims companies including L’Oréal and Revlon knowingly marketed and sold products with harmful chemicals in them, failing to warn customers of their potential cancer risks.

Revlon declined to comment on the suit and on the health concerns more broadly. A spokesperson for L’Oréal said the company is confident in the safety of its products, believes the suit has no merit and that “our products are subject to a rigorous scientific evaluation of their safety by experts who also ensure that we strictly follow all regulations in every market in which we operate.”

Write to Sumathi Reddy at