≡ Menu

How do keratin hair straighteners work? Episode 43

How do hair straightening products work? Are they safe? How long do they last?

Show notes

Silicones and ice cream cones

In a clever summer time, non-beauty science twist I quiz Randy about ice cream cones. Listed to the show and test your conical trivia knowledge.

Question of the week: How do keratin hair smoothers work?

Jennifer asks… Will they help my daughter’s hair? Are they bad for hair, I don’t understand how they work. Will they prevent frizz in high humidity Florida weather?

What makes hair curly?

Hair is made of proteins which are long chain molecules. The shape of hair is determined by the chemical bonds that connect these chains together.  There are hydrogen bonds and salt bridges which are weak and easily formed and broken. There are also disulfide bonds which are difficult to form and to break.

To visualize how these bonds shape hair, think of a ladder. The sides of a ladder are two different protein chains and the rungs that connect them are like disulfide bonds.  In straight hair the rungs hold the sides rigid and perpendicular. The first bond on the left side is connect to the first bond on the right side and so on. But what if you sawed through those ladder rungs? Then you could freely move the two sides of the ladder. You could twist the sides move them up or down, shift them however you like.  Then if you glued those rungs back together, you would have changed the shape of the ladder. Now instead of the first bond on the left linked to first bond on the right maybe you’ve moved the sides such that the first bond on left is linked to the second or third bond on the right. That gives the ladder a twisted shape. It’s kind of like turning a straight ladder into a spiral staircase. Straight hair is like the ladder, curly hair is like the spiral staircase.  Now let’s talk about three kinds of hair straightening products and how they work.

Surface treatment products (No disulfide bond changes)

First of all – they DON’T work because of keratin. Keratin is the type of protein that hair is made of but just adding keratin to hair doesn’t cause it to change from straight to curly. It’s more accurate to think of these as treatments that smooth keratin rather than as products that smooth with keratin.

Remember these disulfide bonds are NOT easy to change so there’s an inherent tradeoff when you straighten your hair. The more complete and long lasting the straightness, the more damage to your hair. It’s a trade off.

Products that only coat the hair do not  break or reform bonds at all.  These are least effective but also least damaging. They are temporary and may last through a few shampoos depending on the formula. With current technology, the best you can do is a hair spray resin coating that is unneutralized which means its very hard to wash off.

Bond breaking products

These are the most effective but also the most damaging because they break and reforms the most bonds.  They are permanent until hair grows out. (Examples: Relaxing hair with sodium hydroxide  or perming hair with Ammonium thioglycolate or Bisulfite

Cross-linking free bonds

This is the class that most so called keratin smoothing treatments fall into. They are effective and only cause minimal to moderate damage. They work by reforming a percentage of free bonds (and reducing a few bonds as well.). They are semi-Permanent because not all bonds are affected and the reformed bonds can break again.  The performance depends on degree of curliness of hair. Examples include the following:

  • Formaldehyde (methylene glycol)
  • Oxoacetamide/Oxoacetic acid/Glyoxylic acid (behaves like formaldehyde)
  • Cysteine and Ethanolamine or cysteamine hydrochloride

Why does humidity make hair frizzy?

The inner region of hair, the cortex, consists of two different types of protein.  One type is more hydrophilic than the other which means it absorbs water more easily. So when hair is exposed to high humidity, one region absorbs more water than the other which causes the hair fiber to swell unevenly. This uneven swelling twists the hair fiber and causes it to frizz. The degree of frizz depends on several factors: The porosity of your hair, the mixture of the different protein regions, and the strength and completeness of the bonds in the hair.

How can you fight humidity? Water proof hair with a silicone, reconfigure bonds with reactive hair products, and/or lock hair in place with a ton of styling products.

The Beauty Brains bottom line

Keratin smoothers don’t really work by adding keratin. Instead they work by either coating the hair, cross-linking some of the internal bonds, or breaking and reforming a bunch of bonds. By looking at the claims and ingredients you can figure out which is which and you can also pick the best product to keep your hair frizz free.

LIL buy it now button

Buy your copy of It’s OK to Have Lead in Your Lipstick to learn more about:

  • Clever lies that the beauty companies tell you.
  • The straight scoop of which beauty myths are true and which are just urban legends.
  • Which ingredients are really scary and which ones are just scaremongering by the media to incite an irrational fear of chemicals.
  • How to tell the difference between the products that are really green and the ones that are just trying to get more of your hard earned money by labeling them “natural” or “organic.

Click here for all the The Beauty Brains podcasts.

Comments on this entry are closed.

  • jake August 12, 2014, 7:30 pm

    You forgot to mention “Brazilians” use high heat to infuse NOT Keratin, but silicone in the hair. First, they destroy some bonds with formaldehyde (they reduce more than a few), then they use high heat to infuse the silicone for cross-linking of broken bonds. Some newer ones use urea to break bonds or loosen them. But, the point is, they are more silicone than keratin, and the silicone is not just coating the hair.

    • Randy Schueller August 13, 2014, 6:54 am

      Jake: Very interesting points. My understanding is that keratin cross links but doesn’t destroy bonds and silicones (unless they’re some specially modified version) can’t cross link with keratin. Do you have any references to further explain how what you say is possible? I’d love to learn more.

  • Sara August 18, 2014, 9:00 am

    I have naturally curly hair and I tried a “Brazilian Keratin” straightening once…..the moment I washed my hair my curls came back. Decided my curl was too strong to break without resorting to more damaging means so I abandoned the idea!


    • Randy Schueller August 18, 2014, 2:22 pm

      Yes, Sara. That’s the tradeoff – if you have really tight curls you’ll have to damage your hair more to make it straight.

  • dianejwright September 6, 2014, 8:21 am

    Hi. Please explain how shampoos and conditioners affect keratin smoothing. Some info says to avoid salt as an ingredient but is all sodium salt? Some ingredients lists say “salt-free” yet list sodium-related ingredients such as Sodium Cocoyl Isethionate, Sodium Lauryl Benzene Sulfonate, Disodium Laureth Sulfosuccinate, and Sodium Lauroyl Sarcosinate. How do these affect the “hydrogen bonds and salt bridges”? Thanks.

    • Randy Schueller September 6, 2014, 2:48 pm

      Hi Diane. I’ve heard the same warnings about avoiding “salt” after smoothing hair but I’ve never seen a shred of scientific evidence that would support such a claim. The truth is almost ALL shampoo formulas contain some residual salt (typically sodium chloride). As we mentioned in the podcast, water (as well as the surfactants you mentioned) will relax hydrogen bonds and salt bridges. However, these are not the bonds that are altered during hair straightening so breaking these bonds does not affect hair straightness.

      • Cora West September 18, 2014, 12:13 am


  • Kim November 7, 2014, 1:57 pm

    I recently got a keratin treatment at a salon (not a Brazilian), and I liked the results, but I’m confused as to whether it’s a good thing to keep getting, and what I should use or avoid when I’ve had it. I wasn’t looking to straighten my hair, but just smooth it since it’s dry, frizzy and brittle and has a lot of breakage. My stylist was trying to sell me on the product line to maintain it with keratin shampoo, but she also warned me that too much keratin would cause more breakage, and that she couldn’t verify other shampoos or conditioners because the keratin might be different. The last thing I want is more breakage, since that’s what I was trying to fix in the first place, but what’s the answer? Should I keep getting these treatments, or will they start to damage my hair after a while? And if I should keep getting them, how do I keep from damaging my hair myself in the meantime?

    • Randy Schueller November 7, 2014, 2:32 pm

      Hi Kim: “Too much keratin” does not cause hair breakage. Excessively damaging hair with products that chemically interact with hair structure can. As we noted in the article, the more straightening that a product achieves, the more damage that it causes.

  • Fran Mc November 17, 2014, 7:17 pm

    got a keratin treatment 2 weeks ago. After just 2 shampoos,it’s wavy & frizzing? I purchased ION brand shampoo,conditioner & leave in cond(all recommended by the hairdresser who did my straightening treatment. She said they are sodium & sulfate free. The ingredients list:sodium cocoyl isethionate,sodium C14-16 Olefin sulfonate, sodium cocoamphoacetate & sodium lauroyl sarcosinate What went wrong?

    • Randy Schueller November 18, 2014, 8:10 am

      Hi Fran. Sorry to hear about the problems you’re having with your keratin treatment. I don’t know what product your stylist used or how she applied it so I think you’d better check back with her. (Especially if you spent a lot of money on the treatment!) Based on the ingredients, I doubt the shampoo had anything to do with it. Write back and tell us what you find out. Good luck!

  • Becci February 25, 2015, 9:47 am

    I’m still confused and concerned about the use of straightener products with formaldehyde or other toxic fumes…I have seen beautiful results with theses hair products but at what cost health wise to the stylist (me) as well as the client?
    BTW – Cool book and website guys 🙂 Thanks!

    • Randy Schueller February 25, 2015, 10:31 am

      Hi Becci. Straighteners that release high levels of formaldehyde gas ARE dangerous – to the client who uses them occasionally and especially the stylist who may use them daily. (I’m glad you like our book and website! Have you listened to our podcasts yet? If you have, we’d love for you to write a short review on iTunes. Thank you! Here’s the link: https://itunes.apple.com/us/podcast/the-beauty-brains/id733300242?mt=2

  • Meesa April 2, 2016, 6:07 pm

    Hi Randy,
    I wonder how does straitening with brazilian keratin lasts for months? Is it true that they form a coating on hair and prevent water from reaching hair?

    • Randy Schueller April 5, 2016, 10:58 am

      True “Brazilian” straighteners change the chemical bonds in hair. That’s why it’s so long lasting.

  • Meetu April 25, 2016, 3:23 pm

    What is the impact of Sun/heat along with dust and dryness on the hairs. And which treatment will be most suitable for such an environment.