Formaldehyde is such an effective preservative—it's used to embalm bodies, after all—that it’s in all kinds of products, including nail polish, shampoo, and body wash. In fact, research in the journal Contact Dermatitis suggests it's in about 20% of personal care products in the US. Most often, you’ll find it masquerading in what’s called "formaldehyde releasers," meaning chemicals that break down into formaldehyde.
While the FDA says small quantities in products don’t pose a threat, the preservative is a known carcinogen, and has been associated with skin allergies. You can avoid it by checking your ingredient label for these red flags: quaternium-15, diazolidnyl urea, DMDM hydantoin, and imidazolidinyl urea. Skip hair-smoothing products—especially those sold in salons, as salon-based products are exempt from labeling laws.