Keratin—the protein that helps strengthen hair to prevent breakage, heat damage, and frizz—is vitally important for maintaining strong and healthy hair. But keratin straightening, the salon treatments that promise silky-smooth hair, have long come with major health concerns. To get the breakdown of everything you need to know about keratin treatments, we spoke with celebrity hairstylist and NatureLab Tokyo brand ambassador Andrew FitzsimonsEva NYC hairstylist T. Cooper, and celebrity hairstylist Annagjid “Kee” Taylor.

What Is Keratin?

Keratin is a structural protein found in our hair, skin, and nails. It’s also commonly found in styling products to help strengthen hair—but the term keratin treatment is actually a misnomer. “Keratin treatments are a semi-permanent hair straightening treatment that smooth and add shine to frizzy hair,” says Fitzsimons. How the treatments work is not through the use of keratin, though.

To make hair straighter, a solution containing a formaldehyde derivative or (the much safer) glyoxylic acid is worked through the hair to break the bonds and reseal them in a straighter position. The solution is then blow-dried and sealed with a flat iron, and the results can last anywhere from three to six months.

He says the treatments work well on most hair types and recommends it for anyone who wants to cut out blow-drying or straightening their hair on a regular basis, reduce frizz, or boost shine.

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How Does a Keratin Treatment Differ for Those With Natural Curls?

Taylor agrees with Fitzsimmons that keratin treatments work well on most hair types. For curly hair in particular, it will completely smooth frizz and add more shine. “Curly hair reflects light less easily than straight hair, so you’ll notice more shine after your keratin treatment,” she says.

Another added benefit is that it will keep freshly-colored hair vibrant. She recommends talking to your stylist and colorist about about getting your color refreshed right before your keratin treatment. The treatment seals in the color, making it last longer and appear brighter.

Is Keratin the Same as a Japanese Treatment or Relaxer?

While similar to other hair straightening treatments, keratin treatments are still distinctively different. Taylor explains that keratin treatments are a less harsh option to other straightening treatments like relaxers. “Keratin is great for those who have frizzy hair and want long-term straight and smooth hair,” she says. “The chemicals with keratin are a little less harmful than those found with other straightening treatments, like a relaxing treatment, which is very harsh.”

“The main difference between a keratin treatment and a relaxer is how long they straighten the hair,” explains Cooper. “A keratin treatment temporarily loosens your curl pattern, and fades out over time. Your hair texture will eventually return to its natural state. With a relaxer, that is not the case at all. Relaxers break down the molecular bonds in hair, and permanently straightens it.” Because Japanese hair treatments and traditional relaxers permanently break your hair’s bonds using ammonium thioglycolate and sodium hydroxide, they are far more effective on coily hair—but also more damaging. The growing-out phase will also be more intense than with a keratin treatments, since there will be a line of demarkation when your natural texture grows back in.

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Why Won’t Some Hairstylists Perform Keratin Treatments?

Part of the concern with keratin treatments revolve around one of the ingredients found in most traditional salon formulas: formaldehyde. The Centers for Disease Control (CDC) defines it as a colorless, strong-smelling gas that is usually used to make building materials, household products like glue and fiberboard, and used as a preservative when dissolved in water. In keratin treatments, it’s responsible for locking the hair into that new straight and smooth position for months. But these treatments don’t actually contain formaldehyde, because, well, it’s a known carcinogen. What they contain instead is ingredients like methylene glycol, formalin, methanal, and methanediol, that release the carcinogenic compound when mixed with water during the treatment. So while the formula might technically be formaldehyde-free, it’s not once mixed with water.

This poses potential risk not only to you—but also the salon technicians who preform the treatments over and over again in enclosed spaces. “Some stylists refuse to do keratin treatments, because there is research that shows that continued use of formaldehyde overtime can have some serious consequences,” says Cooper. “The risks can be as minor as watery eyes and skin irritation, to as serious as cancer and respiratory illness.”

Indeed, the CDC lists common side effects to formaldehyde exposure include sore throat, nose bleeds, and scratchy eyes. The U.S. Food and Drug Administration (FDA) also warns about potential health hazards for those exposed and notes that formulas and products that claim they’re natural, organic, or formaldehyde-free may not be entirely true. The FDA encourages people to always read the label, ask hair professionals for an ingredient list, and to report any and all bad reactions.

These so-called keratin treatments can also do a lot of harm to your hair over time. Fitzsimons says because these treatment require high-heat to lock your hair into place, he wouldn’t recommend it for those with super-fine or damaged hair. If you’re looking for a less-damaging, truly formaldehyde-free treatment, ask your salon if they use a treatment containing glyoxylic acid instead. They don’t straighten hair as dramatically (they’re more for boosting shine and fighting frizz), but they’re also a lot safer.

How Long Do Keratin Treatments Last?

The cardinal rule of keratin treatments is to stay away from water and ponytails for at least three days. You don’t want to risk creating a bend in your newly-straightened hair. Fitzsimons says the key to maintaining your treatment for as long as possible is using a sulfate-free shampoo on the days you wash your hair.

For those with natural curls, Taylor says not to wet your hair for three days. She also recommends using sodium chloride-free hair products and to wrap your hair in a silk or satin scarf (or pillowcase) to keep the moisture locked in your hair as the treatment can cause your hair to dry out faster. She says that treatments can last up to six months, but does caution that it might impact your curl pattern permanently.

But most importantly, Fitzsimons says to consider a few key things before booking that appointment. “There are so many effective ways to smooth the hair while keeping it protected these by using the right products and techniques—so a keratin treatment might not even be the right option,” he adds. “Depending on your hair’s particular texture, a different treatment, such as a chemical relaxer, might make more sense for you.”

This article originally appeared on Harper’s BAZAAR US.

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