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A keratin treatment is a process that smooths and often straightens hair. You might also hear it called a Brazilian keratin treatment or a Brazilian blowout. It involves applying products to the hair, blow-drying it, and sealing it with a flat iron.

Keratin is a natural type of protein in your skin, hair, and nails. A keratin treatment adds additional keratin to your hair, which can help to reduce frizz, increase shine, and strengthen hair. If you have curly or wavy hair, keratin treatments can also make your hair straighter.

But keratin treatments can also come with some unwanted side effects, and could present some safety issues.

Keratin used in beauty treatments is usually from these the skin, hair, or nails or animals. While keratin is a natural protein, these products are made with several other added ingredients, including a chemical called formaldehyde.

The American Cancer Society warns that formaldehyde is a known carcinogen. This means that it may cause cancer or help cancer to grow. Formaldehyde may also trigger other health side effects.

The Food and Drug Administration warns that formaldehyde and related compounds in keratin treatments may contribute to:

  • eye irritation
  • headaches
  • dizziness
  • sore throat
  • coughing
  • wheezing
  • nausea
  • chest pain
  • vomiting
  • rash

These side effects have been reported by both consumers who received keratin treatments and stylists who provided them.

The effects tend to happen during or shortly after a keratin treatment using products containing formaldehyde. It’s still unclear whether keratin treatments containing formaldehyde have long-term health effects or what those effects might be.

The specific benefits you’ll get from a keratin treatment depend on your hair type and the products used.

Generally, you can expect the following benefits from a keratin treatment:

  • Silky hair. Exposure to sun, hair dye, and other chemical treatments can strip your hair of naturally occurring keratin, leaving behind porous areas that are more vulnerable to breakage and other damage. Keratin treatments work to “refill” these areas that have lost keratin. The result is a smoother hair shaft, which translates to silky, shinier hair.
  • Straighter hair. Depending on the products used, a keratin treatment can straighten curly or wavy hair.
  • Smoother curls. That’s right — a keratin treatment doesn’t have to result in stick-straight hair. Certain formulations will simply reduce frizziness, providing softer, smoother curls or waves.
  • Stronger hair. By replenishing lost keratin in your hair shaft, keratin treatments can make hair stronger and less prone to breakage. For some people, this might result in being able to grow hair longer, as the strands are less likely to break.
  • Thicker-looking hair. In addition to strengthening hair, replenishing keratin through a keratin treatment can make hair appear thicker.

If you decide to get a keratin treatment, be sure to tell your stylist about what you’re looking to get out of the treatment. They can adjust the products and process to help you achieve what you’re looking for.

Part of the issue with formaldehyde in keratin treatments is that products may contain much more formaldehyde than advertised.

For example, a 2014 study of keratin brands marketed in South Africa found that 6 out of 7 products contained 0.96 percent to 1.4 percent formaldehyde levels — five times higher than the recommended safe level of 0.2 percent.

An earlier study from 2011 similarly found higher-than-advertised levels of formaldehyde across 4 different brands of keratin treatments.

It’s worth noting that keratin treatments are typically done in a salon, where others are likely receiving keratin treatments throughout the day.

The Agency for Toxic Substances and Disease Registry notes that exposure to a formaldehyde level 0.1 to 0.5 parts per million (ppm) can cause nose and eye irritation, neurological effects, and increased risk of asthma and allergies. At levels of 0.6 to 1.9 ppm, people might experience eczema or changes in their lung function.

The 2011 study mentioned above found that formaldehyde concentrations in the salon during the blow-drying portion of a keratin treatment ranged from 0.08 to 3.47 ppm. During the flat-ironing portion of the treatment, it ranged from 0.08 to 1.05 ppm. The upper limit of both ranges is well beyond what most people can tolerate.

If you’re just going in for a single treatment, you might not feel many effects. But if you’re a stylist regularly working in a salon, this kind of exposure could have a larger health impact.

A variety of formaldehyde-free keratin treatments have popped up in recent years, but these may also have labelling issues. Five of the brands that tested positive for formaldehyde in the 2012 study mentioned above, for example, were labeled as formaldehyde-free.

In some cases, the manufacturer might be listing formaldehyde by another name, such as:

  • aldehyde
  • bonded aldehyde
  • formalin
  • formic aldehyde
  • methanediol
  • methanal
  • methyl aldehyde
  • methylene glycol
  • methylene oxide
  • morbicid acid

In others, the product might contain preservatives that give off formaldehyde, like:

  • benzylhemiformal
  • diazolidinyl urea
  • imidazolidinyl urea
  • quaternium-15

Formaldehyde is part of what makes keratin treatments so effective. As a result, it’s hard to find a product that’s 100 percent formaldehyde-free and works. If you’re considering a keratin treatment, research various brands first and take a look at the ingredients in each. You can also ask a stylist who specializes in keratin treatments. They’ll often provide a consultation to help you find the safest option for you.

If you’re concerned about the risk of formaldehyde exposure, there are other ways to achieve similar effects to those you’d get from a keratin treatment.

Heat styling with a blow-dryer or flat iron can temporarily smooth your hair.

You can also try to keep your health strong and healthy by keeping it moisturized (especially if you use heat-styling tools).

Try to wash your hair as little as possible, as shampoo can remove natural hair oils. You can also try switching to the “no poo” method.

You can also apply products containing natural moisturizers, like:

Looking for more tips? Our guide to getting shiny hair can help.

Keratin hair treatments can be an effective way to get smoother, shinier hair, but they usually involve the use of formaldehyde, which can lead to a range of side effects.

While some manufacturers advertise products that are said to be formaldehyde-free, many of these still contain some form of formaldehyde.

It’s unclear whether getting the occasional keratin treatment has long-term health impacts, but if you’re particularly sensitive or have asthma or allergies, you may want to skip it.

It’s also best to avoid keratin treatments if you’re pregnant or breastfeeding.