Formaldehyde
for-MAL-duh-hyde
What Is Formaldehyde?
Formaldehyde is a colorless, strong-smelling gas used as a preservative and in keratin hair straightening treatments. It is classified as a known human carcinogen by multiple health agencies. While pure formaldehyde is rarely listed directly on ingredient labels, several preservatives (formaldehyde donors) release it slowly over time. Formaldehyde-based keratin treatments pose significant health risks.
Source: Chemically synthesized from methanol
Origin: Synthetic
Also known as: Formalin, Methanal, Methylene Oxide, Formol
What Does Formaldehyde Do to Your Hair?
In hair care, formaldehyde cross-links keratin proteins in the hair shaft, creating a temporary straightening effect that can last several months. As a preservative, it kills bacteria and other microorganisms to prevent product spoilage. Formaldehyde-releasing preservatives slowly release small amounts of formaldehyde to maintain antimicrobial protection over time.
Best For
None recommended due to safety concerns
Not Recommended For
All hair types, all individuals
Concentration Matters
0.01-0.2% (preservative), up to 12% (illegal straightening treatments)
Hair Type Compatibility
Method Compatibility
Safety & Regulation
Known human carcinogen (IARC Group 1). Causes eye, nose, and throat irritation. Salon workers applying keratin treatments face significant occupational exposure risks. Can trigger asthma, allergic reactions, and chemical burns at higher concentrations. Long-term exposure linked to nasopharyngeal cancer and leukemia.
FDA Status: Restricted
FDA has issued multiple warnings about formaldehyde in hair straightening products. Products containing more than 0.2% formaldehyde must carry a warning label in the EU. Some states have introduced additional restrictions. Brazil has banned concentrations above 0.2% in hair products.
EWG Score: 10/10
HairAide Verdict
Formaldehyde is an ingredient HairAide strongly advises avoiding entirely. The cancer risk is real and well-documented, particularly from salon keratin straightening treatments. If you want a straightening treatment, seek out formaldehyde-free options and verify the ingredient list yourself. No amount of smoothing is worth the health risk.
Products That Contain Formaldehyde
Keratin straightening treatments, some nail polishes, some older preservative systems
Alternatives to Formaldehyde
If you want to avoid Formaldehyde, look for products with these ingredients instead:
Glyoxylic acid-based straightening treatments, formaldehyde-free keratin treatments
Frequently Asked Questions
Formaldehyde has a safety rating of 1/10 and may be problematic for many hair types. Known human carcinogen (IARC Group 1). Causes eye, nose, and throat irritation. Salon workers applying keratin treatments face significant occupational exposure risks. Can trigger asthma, allergic reactions, and chemical burns at higher concentrations. Long-term exposure linked to nasopharyngeal cancer and leukemia.
Formaldehyde is not CG-method approved. Curly hair types may want to avoid this ingredient or use products containing it sparingly.
No, Formaldehyde is not approved for the Curly Girl method.
Some alternatives to Formaldehyde include: Glyoxylic acid-based straightening treatments, formaldehyde-free keratin treatments
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