Smoothing treatments are among the most requested salon services in the country. Understanding what is in them takes about five minutes and changes the conversation you have with your stylist entirely.
The chemistry behind a smoothing treatment is straightforward. A solution is applied to the hair, heat is added, and the combination bonds the formula to the keratin in each strand, relaxing its structure and creating a smooth, frizz-resistant finish that can last several months.
What varies between products is the ingredient doing that bonding. In many of the most widely used treatments, that ingredient is formaldehyde, or a compound that becomes formaldehyde once heat is applied. It is worth knowing which one you are getting before you sit in the chair.
How Formaldehyde Works in Smoothing Treatments
Formaldehyde bonds readily to proteins under heat, which is what makes it effective in these treatments. When a flat iron passes over the formula, it activates the chemical reaction that locks the hair into its smoothed state. The same heat also converts liquid formaldehyde into a gas, which becomes airborne in the salon during the service.
Formaldehyde is classified as a known human carcinogen. Regulatory guidelines recommend that hair treatments contain no more than 0.2 percent. FDA testing has found products significantly above that threshold, with at least one widely used Brazilian blowout formula testing between 8.7 and 10.4 percent. That product was sold under a formaldehyde-free label.
How Formaldehyde Gets a Different Name
Formaldehyde does not always appear by name. The most common substitution is methylene glycol, the liquid form of formaldehyde dissolved in water. Before heat is applied, they are technically distinct compounds. After heat is applied, they are not. The FDA issued a warning letter to one manufacturer specifically for using this distinction to support a formaldehyde free claim on packaging.
There is also a separate category of formaldehyde releasing preservatives, found not just in smoothing treatments but in everyday shampoos and conditioners. These release formaldehyde gradually over time rather than in a concentrated burst under heat. Both categories are worth understanding.
| Name on label | What it is |
|---|---|
| Methylene glycol | Liquid formaldehyde in water. Converts to gas under flat iron heat. Most common in treatments marketed as formaldehyde free. |
| Formalin | Formaldehyde dissolved in water. Same substance, different name. |
| Methanal | The official chemical name for formaldehyde. |
| DMDM hydantoin | A preservative that releases formaldehyde gradually. Found in shampoos, conditioners, and some smoothing formulas. |
| Quaternium-15 | The heaviest formaldehyde releaser in common cosmetic use. |
What the Research Shows
Short term exposure to formaldehyde gas during a salon service can cause eye and throat irritation, headaches, and dizziness. These effects are more pronounced for stylists, who are in contact with the product and its fumes repeatedly over the course of a career.
A National Institutes of Health study following more than 33,000 women over eleven years found that using chemical hair straightening products more than four times a year was associated with more than double the risk of uterine cancer compared to non-users. The researchers did not isolate formaldehyde as a single cause, noting that several chemicals present in these products may contribute. A subsequent analysis from the same cohort found associations with additional cancer types. Black women made up roughly sixty percent of frequent straightener users in the study, a detail researchers noted given how early and consistently these products tend to be introduced within that population.
Frequency matters more than any single appointment. That is the consistent finding across the research.
Where Regulation Stands
Formaldehyde in hair smoothing products is not federally banned in the United States. The FDA proposed a rule to restrict it, with an original target date of October 2023. That deadline has been missed five times, most recently December 31, 2025. Canada has restricted the chemical. No federal ban is currently in effect in the United States.
In the absence of a ban, ingredient disclosure remains voluntary in practice. Salons are not required to share product information with clients unprompted. The ingredient list on the product itself is the most reliable source.
"Keratin treatment" or "smoothing treatment" describes a category, not a formula. Ask your hair care professional which specific product they use, then look up the ingredient list before your appointment.
Check the ingredient list for methylene glycol, formalin, methanal, DMDM hydantoin, and quaternium-15. A formaldehyde-free label does not guarantee their absence.
The NIH research points to frequency as the key variable. One treatment reads very differently in the research than four or more treatments per year.
Before reserving an appointment, ask which product will be used. Research it. Arrive having already decided.
If you have had a smoothing treatment, do you know which product was used? The Strands Journal exists because that gap matters.