NIEHS followed over 33,000 adults for 11 years, and 378 uterine cancer cases were identified during this time. Those who had used a chemical hair straightening product more than four times in the 12 months prior to being surveyed were 155 percent more likely to later be diagnosed with uterine cancer, compared with those who had never received a straightening treatment. A 1.64% chance of being diagnosed with uterine cancer by their 70th birthday increased to 4.05% among those who frequently straighten their hair chemically. It is small; however, as the article from Science Alert notes:

“The worrisome results are the latest from a recent string of studies on widely-used chemicals that disrupt the endocrine system; messenger chemicals in our bodies that connect hormones to their target organs. Excess hormones like estrogen and progesterone have been tied to uterine cancer in the past, and many hair products can mimic these natural hormones and bind to their receptors.

In 2018, researchers found endocrine-disrupting chemicals in 18 tested hair products. What’s more, 84 percent of the chemicals identified were not listed on product labels, and 11 products contained chemicals prohibited under the European Union’s Cosmetics Directive or regulated under California law.

Currently, federal regulations in the US require testing for estrogen receptor activity only when it comes to pesticides and drinking water contaminants. Hair products are left out, and that could be seriously harming public health. In 2019, a study funded by the NIH found permanent hair dye and straightening chemicals are associated with a higher risk of breast cancer, especially among black women who tend to receive these treatments more often. In 2021, a follow-up study found permanent hair dye and straightening chemicals are also associated with a higher risk of ovarian cancer.

Now, uterine cancer can be added to the list as well. The study on this latest type of cancer did not identify differences between racial and ethnic groups, but the authors note that because Black women use straightening products at younger ages, higher rates, and higher concentrations, the hazards may be greater. One study found that 89 percent of African American women reported using chemical relaxers or straighteners.

Researchers at the NIH are still figuring out which specific chemicals in hair straighteners can explain the associated carcinogenic effects, but parabens, pthalates, and formaldehydes are some of the leading suspects.