What's Really Inside Hair Relaxers

Last updated: March 2026

For decades, the manufacturers of chemical hair relaxers assured customers that their products were safe. Independent scientific research tells a different story. Studies have found that many common hair relaxers contain chemicals that are classified as carcinogens, endocrine disruptors, and reproductive toxins.

1. Formaldehyde and Formaldehyde-Releasing Agents

Formaldehyde is a colorless, reactive gas used as a preservative and chemical bonding agent in many hair straightening treatments. The International Agency for Research on Cancer (IARC) classifies formaldehyde as a Group 1 carcinogen — meaning there is conclusive evidence it causes cancer in humans.

Even when formaldehyde is not listed as a direct ingredient, many hair relaxer products contain formaldehyde-releasing preservatives that generate formaldehyde over time. These include DMDM hydantoin, quaternium-15, imidazolidinyl urea, and diazolidinyl urea.

When applied to the scalp, formaldehyde and formaldehyde-releasing chemicals can be absorbed into the bloodstream, where they cause direct DNA damage and promote cancer development.

2. Parabens

Parabens are synthetic preservatives found in a wide range of cosmetic and personal care products, including hair relaxers. Multiple studies have found that parabens act as xenoestrogens — foreign chemicals that bind to the body's estrogen receptors and trigger estrogenic responses.

This is particularly dangerous for hormone-sensitive cancers like uterine (endometrial) cancer, which is predominantly driven by estrogen exposure. The body's cells cannot distinguish between natural estrogen and the estrogen-like effects of parabens — the result is inappropriate hormonal stimulation of uterine and ovarian tissue.

Parabens have been detected in breast tumor tissue biopsies, establishing that these chemicals penetrate skin, enter the bloodstream, and accumulate in reproductive organs.

3. Phthalates

Phthalates are a family of chemicals used as plasticizers and fragrance stabilizers. They are among the most pervasive environmental contaminants and are found in many cosmetic and personal care products including hair relaxers.

Studies have found significantly higher levels of phthalate metabolites in the urine of women with uterine fibroids compared to women without fibroids. Research has also linked phthalate exposure to endometriosis and ovarian dysfunction.

Because phthalates are often embedded in fragrance formulas (listed simply as "fragrance" or "parfum" on ingredient labels), consumers have no way of knowing how much phthalate they are being exposed to from a given product.

4. Bisphenol A (BPA)

BPA is one of the most extensively studied endocrine disruptors. It binds to estrogen receptors with high affinity and has been found to promote the growth of hormone-sensitive cancer cells in laboratory studies.

BPA exposure has been linked to increased risk of uterine cancer, breast cancer, and a range of reproductive health conditions. BPA can leach from product packaging into the cosmetic itself, meaning even products that do not intentionally include BPA as an ingredient may contain it.

5. Cyclosiloxanes (D4, D5, D6)

Cyclosiloxanes are silicone-based compounds commonly found in hair care products for their conditioning properties. D4 (cyclotetasiloxane) is classified as an endocrine disruptor by the European Chemicals Agency and has demonstrated reproductive and uterine effects in animal studies. D5 and D6 are persistent, bioaccumulative, and toxic (PBT) substances.

The Cocktail Effect

Hair relaxers do not contain just one of these chemicals — they often contain several endocrine-disrupting chemicals simultaneously. Research increasingly shows that the combination of multiple EDCs can have effects greater than any single chemical alone. This "cocktail effect" means that the total harm from regular hair relaxer use may exceed what any single-chemical study could capture.

Women who used hair relaxers multiple times per year for decades were potentially absorbing a complex mixture of hormone-disrupting chemicals through their highly permeable scalp skin — year after year, starting in some cases from early childhood.