Formaldehyde and formaldehyde releasers (methylene glycol)

What it is

In smoothing treatments formaldehyde is dissolved in water as methylene glycol (formalin), and related ingredients can release it. In the US, cosmetics are not FDA-approved, and the FDA names formalin and methylene glycol on its smoothing-product page. FDA

How it works

The aldehyde cross-links hair proteins. Heat from flat-ironing sets the straightness and releases the gas.

What the studies show

• A simulation of four branded professional products measured airborne formaldehyde up to 3.47 ppm during blow-drying. Bulk content ranged from 11.5% to 0%, and even products labelled formaldehyde-free could meet or exceed occupational limits. Pierce 2011
• The FDA states that heating releases formaldehyde gas, that air levels above 0.1 ppm can cause eye, nose and throat symptoms and that IARC classes formaldehyde as a human carcinogen. It has issued two warning letters. FDA
• A 2026 scoping review of 21 studies found keratin treatments more often contain formaldehyde or releasers, and that salon workers may have higher exposure. Weaver 2026

How it's used in studies

Salon smoothing treatments are applied and then heat-sealed with a flat iron at high temperature.

Side effects and safety

Serious. Formaldehyde is a carcinogen, a skin sensitiser and can trigger asthma. Avoid it in pregnancy as a precaution. We could not confirm the current US regulatory status of a proposed ban, so check the latest FDA position.

Bottom line

Avoid it. Heat-activated smoothing carries an inhalation risk to you and the stylist. Safer routes are heat styling, wash-and-go routines or accepting your texture. The evidence of harm is strong. See relaxers, perms and smoothing.