Are chemical hair straighteners and relaxers linked to cancer risk?

What's actually in it

Chemical hair straighteners and relaxers work by breaking and reshaping the protein bonds in your hair. To do this, they contain harsh chemicals like formaldehyde, lye (sodium hydroxide), phthalates, and various endocrine-disrupting compounds. Many "formaldehyde-free" products actually contain chemicals that release formaldehyde when heated, like methylene glycol.

During a straightening treatment, you're exposed through your scalp (which absorbs chemicals quickly) and through your lungs (because the chemicals become airborne when heated with a flat iron). Salon workers face even higher exposure from treating multiple clients a day.

What the research says

A 2026 study in the J Natl Cancer Inst examined the link between hair straightener and chemical relaxer use and the incidence of non-reproductive cancers. The study followed a large group of women and found that those who used these products regularly had a higher rate of certain cancers compared to non-users.

This study is significant because it's published in one of the top cancer research journals and focuses specifically on non-reproductive cancers, expanding beyond earlier findings that linked these products to uterine and breast cancer.

The chemicals in hair straighteners don't just sit on the surface of your hair. Formaldehyde is classified as a known human carcinogen. Your scalp is highly permeable, meaning chemicals applied to it can enter the bloodstream quickly. Heat from flat irons makes the exposure worse by turning liquid chemicals into gas you breathe in.

If you want straight hair without these chemicals, look for heat-only straightening methods, or products that use plant-based proteins and natural oils for smoothing. Always check ingredient lists and avoid anything containing formaldehyde, methylene glycol, or formalin.