Formaldehyde shows up in two main categories of hair products: professional straightening and smoothing treatments, which can contain alarmingly high concentrations, and everyday retail products like shampoos and conditioners, which use preservatives that slowly release small amounts of formaldehyde over time. Knowing which ingredients to look for on a label is the key to avoiding it, because the word “formaldehyde” almost never appears.
Keratin Treatments and Hair Straighteners
Professional hair smoothing treatments are the biggest source of formaldehyde in the hair care world. These products work by using formaldehyde (or chemicals that produce it) to cross-link proteins in your hair, locking strands into a straighter shape. The process typically involves applying a liquid solution and then sealing it with a flat iron at high heat, which releases formaldehyde gas into the air of the salon.
The Environmental Working Group investigated 16 companies making hair-straightening products and found that 15 of them claimed their products contained little to no formaldehyde, even though lab testing told a different story. Brazilian Blowout, one of the most well-known brands, previously labeled its product “CONTAINS NO FORMALDEHYDE!” while independent testing found formaldehyde levels up to 11.8%. Cadiveu products were found to contain 7% formaldehyde. Marcia Teixeira treatments tested at up to 5.87%. Coppola’s Keratin Complex, which claimed its “bonded aldehydes” were not harmful, tested at up to 2.3%. Other brands like IBS Beauty (marketed as i-Straight or Love-Straight), Keratin Express, R&L, and Tahe all tested positive for formaldehyde despite making “formaldehyde-free” claims.
The trick many of these companies use is listing the ingredient as “methylene glycol” instead of formaldehyde. Methylene glycol is simply formaldehyde dissolved in water. When heat is applied during the straightening process, it releases formaldehyde gas, sometimes at levels that far exceed workplace safety limits.
Products Labeled “Formaldehyde-Free”
Even products that technically avoid formaldehyde as an ingredient can still produce it. A newer generation of hair straighteners uses glyoxylic acid (sometimes listed as glyoxyloyl carbocysteine) as the active ingredient. These products are marketed as safer alternatives, but glyoxylic acid contains an aldehyde group that releases formaldehyde gas when heated during the straightening process. In some cases, the levels exceed safe exposure limits.
Glyoxylic acid also carries a separate risk: once absorbed through the scalp, the body can convert it into oxalic acid, which can damage the kidneys. Case reports have documented acute kidney injury following exposure to these so-called formaldehyde-free products. The “formaldehyde-free” label on a hair straightener does not guarantee the product is safe or that you won’t be exposed to formaldehyde during use.
Everyday Shampoos, Conditioners, and Styling Products
Formaldehyde isn’t limited to salon treatments. Roughly 58% of hair products that contain formaldehyde-releasing preservatives use one called DMDM hydantoin, making it the most common source in everyday items like shampoos and conditioners. These preservatives work by slowly releasing tiny amounts of formaldehyde to kill bacteria and extend shelf life. The concentrations are far lower than what you’d find in a straightening treatment, but they add up with daily use across multiple products.
The most common formaldehyde-releasing preservatives found in retail hair products include:
- DMDM hydantoin (also listed as dimethyloldimethylhydantoin or Glydant)
- Quaternium-15 (also listed as Dowicil 75, Dowicil 200, or chloroallyl hexaminium chloride)
- Diazolidinyl urea (also listed as Germall II or Germaben II)
- Imidazolidinyl urea
- Sodium hydroxymethyl glycinate
- Bronopol (2-bromo-2-nitropropane-1,3-diol)
- Methenamine
- Benzylhemiformal
These ingredients appear in shampoos, conditioners, leave-in treatments, styling gels, and hair sprays. Research from Columbia University’s Mailman School of Public Health confirmed that formaldehyde-releasing preservatives are present across a wide range of personal care products, not just professional salon treatments.
How to Read Labels
You won’t find the word “formaldehyde” on most product labels. Manufacturers use the chemical synonyms listed above, and unless you know what to look for, they’re easy to miss. The fastest approach is to scan ingredient lists for “DMDM,” “quaternium,” “urea” (specifically diazolidinyl or imidazolidinyl), “methylene glycol,” or “glyoxylic acid.” Any of these means the product either contains formaldehyde or will release it during use.
For salon treatments specifically, be skeptical of any “formaldehyde-free” claim on a keratin or smoothing product. Testing has repeatedly shown these claims to be misleading. If the product straightens your hair using heat activation and the results last for weeks or months, there is a good chance formaldehyde or a formaldehyde-releasing chemical is involved in the process.
Why Formaldehyde in Hair Products Matters
The International Agency for Research on Cancer classifies formaldehyde as a known human carcinogen. For salon workers who perform straightening treatments repeatedly, the risks are highest: chronic exposure is associated with increased rates of headaches, asthma, and contact dermatitis (an itchy, red rash on the skin). Long-term exposure has been linked to certain cancers.
Clients getting a single treatment face lower risk than stylists, but the formaldehyde gas released during the flat-iron step exposes everyone in the room. The FDA has acknowledged that most hair smoothing and straightening products release formaldehyde gas during use and has called regulation of these products a priority. A proposed rule to ban formaldehyde in hair straightening products had a target date at the end of 2025, but as of early 2026, no official action had been finalized.
Lower-Risk Alternatives
If you want smoother or straighter hair without formaldehyde exposure, your options are more limited than marketing suggests. Treatments based on amino acids, tannins, or polyethylene glycol exist, but they generally produce less dramatic or shorter-lasting results than formaldehyde-based keratin treatments. Heat-based styling with a flat iron or blow dryer, combined with silicone-based smoothing serums, offers temporary straightening without chemical exposure. For preservative-free daily products, look for shampoos and conditioners that use alternatives like phenoxyethanol, potassium sorbate, or sodium benzoate instead of the formaldehyde releasers listed above.