You might want to check the label on your at-home keratin treatment.
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Most people are exposed to formaldehyde — a colorless, strong-smelling gas that can be hazardous to your health — at some point in their lives, some more than others. Formaldehyde is found in cigarettes, some e-cigarettes, certain building materials, industrial cleaning products, and some beauty products, according to the National Cancer Institute. Yes, you read that right: beauty products.

Wait, there's formaldehyde in beauty products?!

Yup. "Formaldehyde is a great preservative," explains Papri Sarkar, M.D., a dermatologist. "That's why formalin (the liquid form of formaldehyde) is used to preserve cadavers that med students use in their anatomy courses," she says.

"Similarly, you can make an amazing cleanser or moisturizer or beauty product, but without a preservative, it will likely only last a few weeks or months," says Dr. Sarkar. Formaldehyde-releasers were first put into cosmetics to keep them from spoiling and causing bacterial or fungal infections and to prolong their shelf life." Formaldehyde-releasers are, essentially, substances that release formaldehyde over time, keeping the product fresh. (BTW, here's the difference between clean and natural beauty products.)

And while many brands that once used formaldehyde as a preservative have stopped doing so thanks to the wealth of evidence that it's not-so-great for you (Johnson & Johnson, for example), there are plenty of manufacturers that still use the stuff to cheaply preserve their products.

To be fair, inhalation of formaldehyde in gas form is the biggest concern, notes David Pollock, an independent beauty chemist. "However, up to 60 percent of chemicals applied to your skin can be absorbed by your body," he says. While the U.S. Food and Drug Administration (FDA) doesn't require formal approval of cosmetics with formaldehyde-releasing ingredients, the European Union has straight-up banned formaldehyde in beauty products because it's a known carcinogen. (Related: How to Make the Switch to a Clean, Nontoxic Beauty Regimen)

The top culprits in the beauty space? "The worst offenders are nail polishes and nail polish removers," says Dr. Sarkar. Hair products in general, as well as baby shampoo and soap, also can contain formaldehyde or formaldehyde-releasers, says Ava Shamban, M.D.

Old-school hair straightening products, including the old formulation of the Brazilian blowout and certain keratin treatments, also used to have a significant amount of formaldehyde, but have reportedly been improved. Again, though, since these products don't require FDA approval, some keratin treatments do still contain formaldehyde-releasers. Interestingly, the FDA reportedly once considered taking certain keratin treatments off the market after the agency's scientists deemed their formaldehyde-releasing ingredients "unsafe," according to The New York Times. Clearly, though, the FDA never actually wound up banning the products, despite those reported recommendations from its internal experts.

So...what should you do?

"My opinion is that everyone should be concerned," says Dr. Shamban. "You are exposed to these products on a daily basis, and over time, these products can build up in fatty tissue and potentially create serious health problems."

That being said, it's worth noting that most of these products contain only small amounts of formaldehyde, meaning that they're not as dangerous as other sources of the chemical, like embalming fluid used on cadavers and building materials that contain it.

But if you'd rather be safe than sorry, finding clean beauty products, which are formaldehyde-free, is easier than ever. "The Environmental Working Group has a list of not only formaldehyde-containing products but also products that contain formaldehyde releasers," says Dr. Shamban.

You can check your favorite products for these ingredients, which contain and/or release formaldehyde: methylene glycol, DMDM hydantoin, imidazolidinyl urea, diazolidinyl urea, quaternium 15, bronopol, 5-bromo-5-nitro-1,3 dioxane, and hydroxymethylglycinate. (Related: The Best Clean Beauty Products You Can Buy at Sephora)

Lastly, you can always rely on retailers who specialize in clean products. "Sephora has a clean beauty label that only includes products that don't include formaldehyde, and there are now many large retailers that only stock or make products that are formaldehyde-free such as Credo, The Detox Market, Follain, and Beauty Counter," says Dr. Sarkar. "They take the guesswork out of it."