Keratin smoothing: what to know beforehand
A keratin smoothing treatment lasts between three and five months, and its critical point is the composition of the product, since European cosmetics rules limit free formaldehyde to 0,2 per cent.
gelisted editorial team · Updated 15 August 2026
The short version
- The legal limit for free formaldehyde in cosmetics is 0,2 per cent.
- The treatment costs 120 € to 300 € depending on length.
- The salon appointment takes 2 to 4 hours.
- Sulphate free shampoo noticeably extends the result.
It works, and precisely because it works it is worth knowing what it works with. The difference between a compliant product and an irregular one is a percentage that has to be on the label.
What the treatment does
A product is applied that fills the hair fibre and is then sealed with heat from high temperature irons. The result is smoother hair with less frizz and more shine, retaining some movement. It is not a permanent straightening and it does not break the bonds irreversibly: the effect fades with washing and new growth comes through with its original texture.
| Aspect | Figure | Note |
|---|---|---|
| Duration of the effect | 3 to 5 months | by washing and hair type |
| Appointment length | 2 to 4 hours | by length |
| Price on short hair | 120 to 180 € | |
| Price on long hair | 180 to 300 € | |
| Legal free formaldehyde limit | 0,2 % | European rules |
| Wait before first wash | 48 to 72 hours | by product |
| Recommended shampoo | sulphate free | extends the result |
The question to ask
European cosmetic product rules limit free formaldehyde and impose labelling obligations above a threshold. Products sold legally respect that limit, but formulations imported outside proper channels circulate with far higher concentrations, producing eye and respiratory irritation during application. Asking about the product and seeing the label before starting is reasonable, and no serious salon objects.
Signs a salon works properly
Adequate ventilation during application, with extraction or open windows. A labelled, visible product. A mask worn by the stylist during the heat sealing stage. A strand test beforehand on lightened or badly damaged hair. And a clear explanation of the aftercare. If any of that is missing, ask before sitting down rather than halfway through with the product already applied.
It is not advised during pregnancy or breastfeeding, and on recently lightened hair it is better to wait 2 to 4 weeks.
Aftercare
No washing for 48 to 72 hours and no elastics or clips that mark the hair. After that, sulphate free and salt free shampoo, which extends the result by several weeks against a conventional one. Heat protection before a dryer or irons. And in summer, a fresh water rinse after swimming in the sea or a pool, since chlorine and salt speed the loss of the effect noticeably.
Frequently asked
+How long does a keratin treatment last?
Between three and five months, depending on hair type, washing frequency and shampoo used.
+What does it cost?
One hundred and twenty to one hundred and eighty euros on short hair and 180 € to 300 € on long hair.
+What is the legal formaldehyde limit?
European cosmetics rules limit free formaldehyde to 0,2 per cent.
+When can I wash my hair?
After 48 to 72 hours depending on the product, avoiding elastics and clips before that.
+Can it be done during pregnancy?
It is not advised during pregnancy or breastfeeding.
Sources
- 1Regulation (EC) 1223/2009 on cosmetic products, European Union (August 2026)
- 2Ministerio de Sanidad, Gobierno de España (August 2026)