Formaldehyde is added to water-based hair care products to prevent microbial growth. According to the CSC, it is a known human carcinogen recognized by the US Toxicity Program, banned in toiletries and cosmetics in Japan and Sweden, and severely restricted in the EU and Canada. This chemical is found primarily in shampoos and conditioners, but it can also be found in body soaps, hair gels, and hair smoothing products.
The FDA notes that exposure to formaldehyde can cause both short-term and long-term health effects. If released into the air as a gas, it can irritate the eyes, nose and lungs. It can also cause allergies and dermatitis. Prolonged exposure to high concentrations of formaldehyde can cause dizziness, headache, chest pain, nausea, and vomiting. At very high concentrations, it can also cause chemical burns, which is more of a concern for stylists and those who work in salons.
To avoid this, look for ingredient names such as formaldehyde, methylene glycol, formalin, quaternium 15, dimethyldimethyl (DMDM) hydantoin, and bromopol when purchasing hair care products for home use.