SUMMARY: The American Cancer Society estimates that approximately 66,200 new cases of uterine cancer will be diagnosed in 2023 and about 13,030 will die of the disease. Uterine cancer is the second most prevalent gynecologic cancer in women worldwide, and its incidence has been increasing. Risk factors include age, factors that influence hormone levels such as obesity and estrogen replacement therapy, Type 2 diabetes, family history, diet and exercise, drugs such as Tamoxifen, and delayed menopause. Exposure to excess estrogen and a hormonal imbalance of estrogen and progesterone have been identified as key risk factors for uterine cancer. Further, synthetic estrogenic compounds such as endocrine-disrupting chemicals have been implicated, because of their ability to alter hormonal actions.
It is estimated that up to 95% of self-identified adult African American women in the U.S. report ever use of hair relaxers. Chemical straighteners or relaxers are commonly used by Black women to straighten curly or tightly coiled hair mostly due to societal pressure to straighten hair. As a result, chemical hair relaxers are heavily marketed to Black women. These straighteners have been found to have potentially harmful toxicants such as phthalates, parabens, cyclosiloxanes and metals that may act as carcinogens or endocrine disruptors, and may release formaldehyde when heated. These products when left on the hair longer than advised can injure the scalp, making it easier to absorb the chemicals. High concentrations of metabolites of phthalates and parabens have been detected in urine samples from women who use chemical hair relaxers.
Endocrine disrupting chemicals may cause hormonal imbalance through either estrogenic or anti-estrogenic activity and has been associated with earlier puberty, infertility, and uterine fibroids. A study published in the International Journal of Cancer in 2020 found that, women who used chemical hair straighteners more than 6 times a year, had about a 30% higher risk of breast cancer. In a study published in the journal Carcinogenesis in 2021 and in the Journal of the National Cancer Institute in 2022 (The Sister Study), women who used chemical hair straighteners more than 4 times a year, were twice as likely to develop ovarian cancer and more than twice as likely to develop uterine cancer as compared to women who did not use chemical hair straighteners. However in the Sister Study there were only 17 exposed cases among Black women. The researchers in this study sought to evaluate the possible association of hair relaxer use with uterine cancer risk in a large prospective cohort of Black women.
The US-based Black Women’s Health Study (BWHS) included 44,798 Black women with an intact uterus and no prior history of cancer, between the ages of 21 and 69, who were followed from 1997 until 2019. All participants completed a self-administered questionnaire about their personal and family medical history as well as behavioral and other factors such as medication use, cigarette smoking, and diet. The researchers used adjusted multivariable Cox proportional hazards regression models, to estimate Hazard Ratios (HRs) and 95% Confidence Intervals (CIs) for associations of hair relaxer use with risk of uterine cancer. The participants were followed for up to 22 years, and the rates of uterine cancer among women who reported frequent or long-term use of hair relaxers were compared to rates among women who never or rarely used them.
It was noted that compared to women who never used hair relaxers or used them infrequently (less than 4 years and 1-2 times or less/year), the Hazard Ratio for uterine cancer associated with heavy use (15 years or more and at least 5 times/year) was 1.18. However, among postmenopausal women, the rates of uterine cancer were statistically significantly higher for those who commonly used hair relaxers even after adjustment for other potential risk factors. In postmenopausal women compared to never/light use, the Hazard Ratio for moderate use was 1.60, the Hazard Ratio for heavy use was 1.64, and the Hazard Ratio for 20 or more years of use regardless of frequency was 1.71. Postmenopausal women who reported using hair relaxers more than twice a year, or for more than five years had a greater than 50% increased risk of uterine cancer, compared to women who never or rarely used hair relaxers.
It was concluded from this large study that long-term use of chemical hair relaxers was associated with increased risk of uterine cancer among postmenopausal women, even after adjustment for other potential risk factors, but not among premenopausal women. These findings suggest that hair relaxer use may be a potentially modifiable risk factor for uterine cancer and the researchers hope these results will raise awareness of the potential toxic effects of these products and promote efforts to reduce exposure.
Hair relaxer use and risk of uterine cancer in the Black Women’s Health Study. Bertrand KA, Delp L, Coogan PF, et al. Environmental Research 2023. Volume 239, Part 1, 15 December 2023, 117228. https://doi.org/10.1016/j.envres.2023.117228
