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Hair straightening products linked to higher uterine cancer risk

A new study from the National Institutes of Health has found that women who use chemical hair straightening products are at a higher risk of developing uterine cancer compared to those who have never used such products. The findings, published in the Journal of the National Cancer Institute, could explain the increasing incidence of the disease in the US and point to new avenues for public health approaches.

Sister scrutiny

Uterine cancer only makes up about 3% of all new cancer cases, yet it is the most common gynaecologic cancer. Studies have pointed to increasing rates of uterine cancer in the US – particularly among Black women – and more than 65,950 cases (12,550 deaths) are anticipated in 2022.

Hormonal imbalances have been identified as key risk factors for uterine cancer, so it seems plausible that any chemical capable of disrupting normal hormonal functions could result in an increased risk of developing the disease. Hair products are known to contain chemicals that could be hazardous to our health, including endocrine-disrupting and carcinogenic compounds. Previous studies have linked their use to hormone-sensitive cancers (such as breast and ovarian cancer), but no research has been done on their association with uterine cancer. That is, until now.

To test whether the use of hair products is linked to uterine cancer, researchers at the National Institute of Environmental Health Sciences (NIEHS) turned to the Sister Study. Led by the NIEHS, the Sister Study aims to identify risk factors for breast cancer and other health conditions by profiling women with at least one sister who has developed breast cancer. Eligible participants were breast cancer-free and aged 35-74 at time of enrolment. All in all, 50,884 women were enrolled between 2003-2009 and have been followed for almost 11 years.

The team narrowed this down to 33,497 US women who met criteria for their hair product study and found that 378 cases of uterine cancer (including endometrial cancer, uterine sarcoma and other cancers of the uterus) were diagnosed in this time. The study population consisted of 7.4% Black/African American, 4.4% Hispanic/Latina non-Black, 85.6% non-Hispanic White, and 2.5% all other race and ethnicity.

Telling it straight

The authors discovered that those women who used hair straightening products frequently – 4 times or more in the previous year – were more than twice as likely to develop uterine cancer compared to those who did not use the products. No higher risk was found for other products such as hair dyes, bleach, highlights, or perms.

“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 Alexandra White, head of the NIEHS Environment and Cancer Epidemiology group and lead author on the new study. “This doubling rate is concerning. However, it is important to put this information into context – uterine cancer is a relatively rare type of cancer.”

Though differences were found in the hazard ratios between racial and ethnic groups, approximately 60% of the participants who reported using straighteners in the previous year were self-identified Black women.

“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, co-author of the study and a research fellow in the NIEHS Epidemiology Branch.

What now?

The latest study follows on from the team’s previous work showing that permanent hair dye and straighteners increase breast and ovarian cancer risk. Though the paper does not investigate specific brands or products, the team suggests certain ingredients like parabens, bisphenol A, metals, and formaldehyde could contribute to increased risk. They also suggest that hair products are more dangerous due to increased absorption through the scalp.

The findings are concerning, but more research is needed to replicate them in other scenarios and identify the specific chemicals driving the observation. “To our knowledge this is the first epidemiologic study to examine the relationship between straightener use and uterine cancer,” said White. “More research is needed to confirm these findings in different populations, to determine if hair products contribute to health disparities in uterine cancer, and to identify the specific chemicals that may be increasing the risk of cancers in women.”