Published: October 25, 2022 · Updated: March 2026

NIH Study: Hair Relaxers Double Uterine Cancer Risk

On October 17, 2022, researchers at the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health (NIH), published a study in the Journal of the National Cancer Institute that changed everything in the conversation about chemical hair relaxers and cancer.

The study — part of the ongoing NIH Sister Study — followed 33,497 women for approximately 11 years. Its central finding was stark and alarming: women who used chemical hair straightening products more than four times per year had 2.55 times the risk of developing uterine cancer compared to women who did not use these products.

What the Study Found

The Dose-Response Relationship

One of the most significant aspects of the study was its demonstration of a dose-response relationship — meaning the more frequently women used hair straightening products, the higher their cancer risk. This pattern is a hallmark of causal relationships in epidemiology and strengthens the argument that chemical hair relaxers actually cause uterine cancer, rather than merely being associated with it.

• Women who used relaxers 1–3 times per year: moderately elevated risk
• Women who used relaxers more than 4 times per year: 2.55× the risk
• Women who never used hair straightening products: baseline risk

The Racial Justice Dimension

The NIH researchers explicitly noted that Black women were disproportionately represented among frequent hair straightener users in the study. While the study found associations across all racial groups, Black women's higher rates of use — driven by decades of aggressive marketing and social pressure to conform to Eurocentric beauty standards — means they bear a disproportionate burden of the harm.

Dr. Alexandra White, the study's lead researcher, stated: "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%."

What Causes the Cancer?

The study did not identify the specific chemicals responsible — that would require additional research. However, the researchers pointed to endocrine-disrupting chemicals (EDCs) commonly found in hair relaxers as the likely culprits. These include:

• Parabens — preservatives that mimic estrogen
• Phthalates — plasticizers linked to reproductive harm
• Bisphenol A (BPA) — a known xenoestrogen
• Formaldehyde-releasing agents — classified carcinogens

Because chemical hair relaxers are applied directly to the highly permeable scalp skin, these chemicals can be absorbed into the bloodstream at higher rates than if applied elsewhere on the body.

What This Means for Lawsuits

The NIH Sister Study is the foundational scientific evidence supporting the hair relaxer cancer litigation. Since its publication in October 2022, thousands of women have filed lawsuits against hair relaxer manufacturers including L'Oréal, Revlon, Namaste Laboratories, and others.

The study is also significant for the statute of limitations: for many women, the October 2022 publication date is when they first learned (or should have learned) that their cancer diagnosis could be linked to hair relaxer use. Under the discovery rule in most states, this means many women's filing deadlines may not expire until 2024 or 2025 — and some may still have time to file.