Chemical hair relaxers and keratin straightening treatments — safety profile

What is this product?

Chemical hair relaxers and keratin smoothing treatments are two distinct product categories that chemically alter the structure of hair to reduce or eliminate natural curl patterns. Alkali hair relaxers — by far the more widely used category, predominantly among Black women — use either sodium hydroxide (lye, pH 12–14) or calcium hydroxide plus guanidine carbonate (no-lye systems, pH 11–13) to break disulfide bonds in the hair keratin protein, permanently restructuring the hair shaft. Keratin smoothing treatments (Brazilian blowout, keratin complex, various salon formulations) coat the hair shaft with a keratin-formaldehyde resin system and use flat-iron heat (200–230°C) to seal the coating — a process that volatilizes formaldehyde from methylene glycol in solution. The safety concerns for these two product types are distinct but overlap in one important area: endocrine-disrupting chemical contamination of commercial relaxer formulas. The 2022 NIH SISTER Study (Sister Study cohort, n=33,497 women) published in Environmental Health Perspectives found that women who reported ≥4 chemical relaxer uses per year had a 2.5-fold increased risk of uterine cancer compared to non-users. This finding is mechanistically plausible: chemical analysis of commercial hair relaxer products has detected parabens, phthalates, BPA, and benzophenone UV filters — known or suspected estrogen-disrupting chemicals — in the formulations, and uterine cancer is a hormone-sensitive cancer. The relaxer application method — direct scalp contact with an alkaline chemical system at pH 12–14 for 15–30 minutes, often causing intentional or inadvertent scalp irritation — may enhance dermal absorption of these endocrine-disrupting additives relative to rinse-off products with intact skin barriers. For keratin/Brazilian blowout treatments, OSHA issued an explicit hazard alert in 2011 documenting formaldehyde air concentrations exceeding OSHA's permissible exposure limit (0.75 ppm) during treatment application in multiple salon measurements — some exceeding 5 ppm. At 230°C flat-iron heat, methylene glycol (the hydrated form of formaldehyde in solution) releases free formaldehyde gas. Formaldehyde is a Group 1 human carcinogen (IARC) causing nasopharyngeal cancer and leukemia. Stylist occupational exposure is the primary concern, though high-end salon clients may also receive clinically significant exposures during prolonged treatment sessions.

What's in it

Click any compound name for its full safety profile, regulatory consensus, and exposure data.

Who's most at risk

• Pregnant Women — Dermal absorption of endocrine disruptors; fetal exposure
• Children — Thinner skin, higher surface-area-to-body-weight ratio

How to use it more safely

• Perform patch test 24-48 hours before application on small skin area
• Apply only to hair, never to scalp or skin; use protective barrier cream
• Use in well-ventilated area to minimize inhalation of fumes
• Wear protective gloves and follow manufacturer instructions exactly

Red flags — when to walk away

• Chemical hair relaxer use ≥4 times per year, especially if use began in childhood/adolescence and has continued for years to decades — This is the use pattern associated with a 2.5× increase in uterine cancer risk in the NIH SISTER Study. The risk appears cumulative — longer duration and higher frequency of use correlates with greater cancer risk. Uterine cancer (endometrial cancer) is the most common gynecologic cancer in the US; an elevated risk factor that has been present for years merits awareness even though absolute risk remains modest for any individual.
• Brazilian blowout or keratin straightening treatment in a poorly ventilated salon space — burning eyes, throat irritation, headache during or after treatment — Burning eyes and throat irritation are symptoms of formaldehyde exposure exceeding irritation thresholds (~0.1 ppm). Headache at higher exposures. These symptoms indicate formaldehyde levels above the OSHA action level during your treatment. Salons performing keratin treatments should have NIOSH-approved exhaust ventilation and personal protective equipment for stylists; clients should be in well-ventilated spaces.

Green flags — what to look for

• Hair relaxer formulated without parabens, phthalates, synthetic fragrance, BPA — ideally EWG-rated or with disclosed ingredient list showing these compounds absent — The endocrine disruptor concern in relaxers stems from formulation additives, not the active alkaline chemistry. Products that have removed these additives address the primary mechanism linking relaxer use to uterine cancer in the SISTER Study hypothesis.

Safer alternatives

• Silk press (heat styling) — Temporary straightening without harsh chemicals; reversible with lower toxicity
• Low-lye or no-lye relaxers — Gentler formulations with reduced caustic alkalinity and lower irritation risk
• Natural hair care and protective styling — Braids, twists, and conditioning avoid chemical exposure entirely

Frequently asked questions