The Science: How Hair Relaxers Cause Cancer

Last updated: March 2026

The NIH Sister Study (October 2022)

The most significant research linking hair relaxers to cancer was published in the Journal of the National Cancer Institute on October 17, 2022. Conducted by researchers at the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health (NIH), the Sister Study is one of the most comprehensive environmental health studies ever conducted on American women.

What Is a 2.55× Risk Increase?

To put the 2.55× figure in context: the baseline rate of uterine cancer in the United States is approximately 1.5% for women over a lifetime. Women who used hair relaxers more than four times per year had a risk of approximately 4.05% — more than double the general population.

A hazard ratio of 2.55 is considered a strong effect in epidemiological research. For comparison:

• The relative risk of lung cancer from smoking is approximately 10–30×
• The relative risk of cervical cancer from HPV infection is approximately 10×
• A hazard ratio of 2.5 for a product used by tens of millions of women over decades is epidemiologically significant

The Mechanism: How Endocrine Disruptors Cause Cancer

Chemical hair relaxers do not cause cancer through one single pathway. Instead, they contain multiple classes of endocrine-disrupting chemicals (EDCs) that interfere with the body's hormonal system in ways that promote cancer growth.

What Are Endocrine Disruptors?

Endocrine disruptors are synthetic chemicals that interfere with the body's hormonal signaling. They can:

• Mimic hormones — binding to hormone receptors and triggering inappropriate cellular responses
• Block hormones — preventing natural hormones from binding to their receptors
• Alter hormone production — changing the levels of hormones produced by the body
• Affect hormone metabolism — changing how hormones are broken down and eliminated

The uterus and ovaries are highly sensitive to estrogen signaling. Uterine cancer (endometrial cancer) is overwhelmingly driven by estrogen exposure. Chemicals that mimic estrogen or increase estrogen activity promote the growth of uterine tissue and can trigger cancerous changes.

Parabens and Estrogen Mimicry

Parabens are preservatives found in many hair care products, including relaxers. They are known to bind to estrogen receptors in the body — acting like estrogen even though they are synthetic chemicals. Parabens have been detected in breast tumor biopsies, and their estrogenic activity is linked to hormonally-driven cancers including uterine and ovarian cancer.

Phthalates and Hormone Disruption

Phthalates are plasticizers used in fragrances and other cosmetic ingredients. They are classified as endocrine disruptors linked to:

• Uterine fibroids (studies show significantly higher phthalate levels in women with fibroids)
• Endometriosis
• Hormone disruption affecting reproductive function

Formaldehyde and Direct DNA Damage

Formaldehyde is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC) — meaning there is sufficient evidence that it causes cancer in humans. Formaldehyde-releasing agents in hair relaxers can cause direct DNA damage, leading to mutations that trigger cancer development. Hair straightening treatments (especially Brazilian Blowout-type treatments and thermal straighteners) can release formaldehyde vapor during application.

The Scalp Absorption Problem

Hair relaxers are applied directly to the scalp — not just the hair shaft. The scalp is one of the most permeable areas of skin on the body. Chemical absorption through the scalp is significantly higher than absorption through skin on other parts of the body.

This means the toxic chemicals in hair relaxers — parabens, phthalates, BPA, formaldehyde-releasing agents — are absorbed into the bloodstream at higher rates than if the same chemicals were applied elsewhere. They then circulate throughout the body, reaching the uterus, ovaries, and other reproductive organs.

Additional Research Supporting the Link

The NIH Sister Study was not the first research to examine this connection:

• 2019 study (International Journal of Cancer): Found that straightener use was associated with a significantly higher risk of uterine fibroids in Black women
• 2020 study (Carcinogenesis): Found associations between hair product use and breast cancer risk
• Multiple studies on uterine fibroids have found phthalate metabolites in the urine of women with fibroids at higher levels than women without fibroids
• Environmental Working Group analyses have documented dozens of potentially harmful chemicals in popular hair relaxer brands

Why Black Women Bear the Highest Risk

Black women are disproportionately affected by this litigation for two compounding reasons:

1. Higher use rates: Approximately 60% of Black women in the US have used chemical hair relaxers, compared to lower rates in other demographic groups. Many began using relaxers as children.
2. Higher baseline uterine cancer rates: Black women already have higher rates of uterine cancer diagnosis and significantly higher mortality rates from the disease than white women. The NIH Sister Study found that the association between hair relaxer use and uterine cancer was particularly strong among Black participants.

This is an environmental justice issue. The burden of decades of aggressive marketing, cultural pressure, and inadequate product safety testing has fallen disproportionately on Black women and girls.