Rare and aggressive uterine cancers are rapidly rising in the United States, especially among people of color, and a new study suggests chemical straighteners might be partly to blame.

For almost 11 years, researchers at the US National Institute of Environmental Health Sciences (NIEHS) followed 33,947 adults who had a uterus upon enrollment in the study. During this time, 378 uterine cancer cases were identified.

Those who had used a chemical 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.

To put that in perspective, those who never used hair straightening products would have a 1.64 percent chance of being diagnosed with uterine cancer by their 70th birthday. That number creeps up to 4.05 percent among those who frequently straighten their hair chemically – a still small, but appreciably higher risk.

Hair dyes, meanwhile, were not linked to uterine cancer.

"These findings are the first epidemiologic evidence of association between use of straightening products and uterine cancer," researchers at NIEHS write.

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 products can explain the associated carcinogenic effects, but parabens, pthalates, and formaldehydes are some of the leading suspects.

The human scalp also readily absorbs chemicals that might not make it through thicker areas of skin, or those with fewer hair follicles, such as the palms or abdomen. Flat ironing or blow drying could also thermally decompose chemicals on the hair, leading to potentially more hazardous effects.

"We observed stronger associations with straightener use among women with low physical activity," the authors write.

"Because physical activity has been associated with decreased sex steroid hormones and less chronic inflammation, women with higher physical activity might be less susceptible to other risk factors for uterine cancer. However, more studies are warranted to understand the interrelationship between physical activity, hair product use, and uterine cancer."

Roughly half of all women in the US have used permanent hair dye at some point, and smoothing keratin treatments and Brazilian blowouts are growing more popular.

Hairdressers and their clients deserve to know what these products are doing to their health.

The study was published in the Journal of the National Cancer Institute.