Straightening Facts :How hot is too hot?

Well, the first thing to note is that every person's hair is different (but similar). Therefore there is a range of temperatures to consider and what might be low heat for you may be high heat for someone else. I would suggest that if you are straightening, you should attempt to use the lowest possible heat for the shortest possible time.

Human hair has a similar temperature profile to wool as they are both composed of the protein keratin (Thermochimica Acta, pg5-9, 1999). Wool was more comprehensively reported and therefore I'll quote those figures (Adv Exp Med Biol. p 329-344, 1977)

Stage 1 : At and below 150 degrees C (302F)
Loosely bound water and tightly bound water is lost from the hair

Stage 2: At 160-175 degrees C (320 -347F)
Hair undergoes a glass transition phase (meaning the hair begins to flow as hot glass would). Plastic deformation is possible (meaning in a 'normal' hydrated state, hair is elastic and can stretch and return back to its original length. Hair does also have temporary plasticity which is why styles like roller sets, twist outs/knot outs can occur. However at this heating stage, the plasticity is NOT temporary, the hair may return to a 'normal' looking state but it will not be exactly the same)

Stage 3: Between 215 and 235 degrees C ( 419-455F)
The keratin in hair has a natural twist to it knows as an alpha helix (α-helix). This twist is present in all keratin, whether straight hair or curly hair. At this stage the helices of the hair protein melt (not reversible).

Therefore this research suggests that under 150 degrees C (302F) is probably the best for straightening. Yes you will have to trade moisture for straightening. If your hair doesn't get straight, then it probably isn't meant to be, don't turn up the heat! My hair for example has never been straight, even when I relaxed, the kinks like me and I love them :)
Does your flat iron have a temperature regulator? Do you know how hot it actually is?


  1. I THINK mine does..My aunt usually straightens it for me, but I think it's probably in the stage 1 bordering stage 2 area.

    The IDEA of a 235C hot tool touching my hair is SO scary! You BAKE CAKES with temperatures lower than that!! @_@

  2. I just had to post!! I am a BIG fan of your blog--I discovered it a couple days ago, and have been reading your blog currently from start (March'09) to finish. I wanted to post to let you know I was a fan, because my husband and I (both of us scientists) can discuss these issues (we are also both naturals--8 years!). Especially for me as a Materials Scientist, you took me waaaay back when you explained elastic and plastic deformation in this post, a true materials expert you are! We are truly appreciate of your work, tres tres supa cool JC,
    ~~Dr. Charlee Bennett

  3. Hi

    If I understand this correctly, if you flat iron your hair at above 215 to 235 it melts the natural curl pattern and that is not reversible.

    So if you melt or change the helixes of the hair protein do you lose your natural curl pattern or do you hair become weak and break off.

    If so probably why girls say stay away for styling tools..

    Warm Regards,

  4. Hi. I just discovered your blog but i am curious to know. I watch youtube videos all the time of women straightening their hair and their curl pattern reverts. Not all of them use high heat but the ones that do never go over 350.

    How can you explain their situations? Is it that everyone's hair is different because like i said, their hair reverts.

  5. Ouch. I think I'll be turning my flatiron down from now on, even if it does take more to straighten it. Is there any way to prevent crispy split ends when flatironing?

  6. So glad I no longer use my flat-iron! Well.. except for craft projects - just not on my hair. That's freakin scary, especially when you look at your own hair and see exactly what it's doing

  7. Wow interesting article,my starighteners don't have a clear thermometer it's one of those you turn so i have no choice but to buy one with a clear one.I only straighten quaterly for length checks.I look forward to testing my shed hairs i feel like im back in the chemistry room when i read this blog! :)

  8. I read this at first and was stunned! Then I realized that the first temps are in Celsius! I'm very ok with using 300 degrees Fahrenheit for straightening.

  9. The straighter I use goes up to 450 and I normally use it between 350 and 450. My hair is burnt so bad and now it doesn't even go naturally curly so I continue to straighten it.. any ideas that don't involve heat? Also, once my hair grows out will it be better?

  10. So at 165c-175c, does it mean your hair is damaged?

  11. So at 165c-175c, when the curl pattern is changed, doesn't that mean heat damage? I flat ironed my hair at 450f, and it killed my ends, but it also changed the rest of my hair as well. I have an S pattern in my hair now. 3.5 years down the drain. BIG SIGH...

    1. Yes as I wrote in the article there is a high likelihood of damage at that temperature.

  12. Hi JC! I love your blog, one the the most informative sites I've found. I am wondering if u can discuss direct heat vs. indirect heat. from this post I can't really tell if roller-setting is better than blowdrying of if blowdrying is better than airdrying+flat-ironing. Are temp-rules the same no matter how applied. Can you also discuss the structural damage caused by cool air while blowdrying.

  13. I have a friend who is a hair stylist and she told me that hair burns above 400F, so I have been keeping my flat iron between 375-400F. My hair is not fried and I have not lost any curl pattern. But I do notice my hair getting blonder and blonder and I suspect that is because of the flat iron. I'm still on the lookout for a good heat protectant. I can't seem to grasp the concept of a heat protectant that is sticky like hairspray. Every one that I've ever used seems sticky like you're spraying hairspray on your hair before you straighten it, which just doesn't seem right. Any suggestions?

  14. @Amy I use One and Only Argan Oil Spray Treatment $11 for 6 oz for heat protection when blowdrying or flat ironing. A little bit goes a long way. Leaves my hair shiny and silky. I wear my shoulder length very salt and pepper hair flat ironed but not relaxed. I am new to flat ironing and started at 250F on washed and blowdried hair and found that 325F works best on my hair.

  15. If only someone had told me about stage 3, 1 month ago. I straightened my hair at 220 C for the first time ever- having only flat ironed twice in my whole life. I am now transitioning from major heat damage and every time my hair gets wet, I smell burning. My Mom thinks the wash out straightening serum I blow dried with is what has kept my hair straight. She says that heat damage doesn't exist because she hot combed her hair and it never got damaged. She doesn't believe that flat irons can get too hot. Is there anyway to prove to her that heat damage exists?

  16. No wonder why my hair never got straight! Everytime I used my flat iron I dont go above 270F because I always get so scared to burn or dry out my hair, but I always complain that my hair doesnt stay straight for more than 5mins haha. And that's the reason why I only use my flat iron for my bangs. This explains everything!


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