Keratin Hair Straightening — The Chemistry, Formaldehyde Risks and Safe Alternatives

What is keratin straightening?

Keratin hair straightening — also known as the Brazilian blowout — is a treatment that saturates the hair shaft with liquid keratin and then locks in the result using high heat. The goal is to relax the hair's curl pattern, seal the cuticle scales and add long-lasting shine. The effect typically lasts between 8 and 16 weeks, depending on your hair type and the products used.

It sounds simple, but the chemistry behind the treatment is far more complex — and it carries real health risks if the formula contains hazardous substances.

The role of formaldehyde and its derivatives

A key ingredient in many keratin straightening formulas is formaldehyde (methanal) or its derivatives — most often methylene glycol and glyoxylic acid. Formaldehyde acts as a cross-linking agent: it forms cross-bonds between the keratin chains in the hair cortex, which is what holds the hair in its straightened shape.

The problem is that formaldehyde is a Group 1 carcinogen according to the IARC. During the treatment, the heat of the flat iron (often 200–230 °C) causes it to be released as a gas and inhaled by both the client and the stylist.

Many products market themselves as "formaldehyde-free", but in reality they contain derivatives that release formaldehyde when heated. On the label, these may appear under names such as:

• Methylene glycol
• Formalin
• Methanediol
• Glyoxyloyl carbocysteine
• Glyoxylic acid

If you want to check whether your product contains these substances, use our ingredient search — simply enter the INCI name.

Safer alternatives

Several formulas have appeared on the market that rely on different straightening mechanisms and don't require formaldehyde:

Glycolic acid

Some modern formulas use glycolic acid in combination with hydrolyzed keratin. The result is gentler but far safer. The hair is smoothed, although the curl doesn't disappear completely.

Cysteine-based formulas

Cysteine — an amino acid naturally present in keratin — can break and re-form the disulfide bonds within the hair shaft. Cysteine-based treatments work more gently than classic keratin straightening, but they are safe for your health.

Tannin-based treatments

Tannin (a plant-derived compound) forms a coating around the hair, smoothing the cuticle mechanically. It doesn't penetrate the hair cortex as deeply as formaldehyde, so the effect lasts a shorter time (4–8 weeks), but the health risk is minimal.

Aftercare — which ingredients should you look for?

After keratin straightening, your hair needs a specific care routine. The key rules:

• Sulphate-free shampoos — classic surfactants (SLS, SLES) wash keratin out of the hair quickly. Look for products with cocamidopropyl betaine or sodium cocoyl isethionate as the main cleansing agent.
• Hydrolyzed keratin in conditioners — replenishes the keratin lost from the hair cortex. Check our ingredient database to see how this ingredient works.
• Sealing oils — argan, jojoba or squalane help keep the cuticle smooth.
• Avoid salt and chlorine — seawater and pool water speed up the loss of the straightening effect.

When choosing a shampoo and conditioner for aftercare, check their full ingredient list — it's the key to a long-lasting result.

Who should avoid keratin straightening?

The treatment is not recommended for people with:

• Asthma or respiratory conditions
• A contact allergy to formaldehyde
• Very damaged, severely dried-out hair (the treatment may make the problem worse)
• Pregnancy or breastfeeding

If you have any doubts, it's worth consulting a trichologist and asking for the full ingredient list of the formula before the treatment.

Summary

Keratin straightening can deliver spectacular visual results, but choosing the right formula is essential for your safety. Avoid products containing formaldehyde and its derivatives, always ask for the full INCI list, and consider gentler alternatives. After the treatment, invest in sulphate-free shampoos and conditioners with hydrolyzed keratin.