Friday, February 3, 2012

How Hot is Too Hot for Healthy Hair?

Updated: March 2024

I put a thermometer’s sensor in the bowl of the diffuser I use on my hair dryer - if you dry your hair up close like this, that's the right place to test the temperature. The hair dryer was set on "warm" and "low." The temperature rose to 125°F (52°C). Now, looking at an article published in the Journal of Cosmetic Science titled, "The Cracking of Human Hair Cuticles by Cyclical Thermal Stresses," I have some answers. The author tested hair alternately immersed in water, then blow dried at various temperatures from 86° to 212° F (30°C to 100° C), re-wetted, and blow dried again up to 100 times. Then they looked for cracks in the cuticle and protein loss. These cracks are very small – limited to individual cuticle scales and always longitudinal, (up and down, not across). For example, when hair samples were treated with 10 seconds of wetting in water, followed by 10 seconds of blow drying at 203° F (75° C), the hair progressed from having 0 cracks to having 600 cracks per millimeter of hair after 30 wetting and drying cycles. Hair which was not subjected to these treatments was also examined, showing no increase in this type of microscopic cuticle cracking.©Science-y Hair Blog 2013

An interesting twist: When you begin to heat-dry your hair with a diffuser, the water in your hair actually lowers the temperature overall. Water absorbs extra heat during the process of changing from a liquid to a gas (water vapor) and this brings down the temperature of the air around your hair as long as it is damp by a few degrees or more. As your hair dries, there is less and less water to moderate the heat from the diffuser.

How Hot is Too hot?©Science-y Hair Blog 2013

They found that blow-dried hair, held at temps between 167° and 203° F (75 to 95°C) for at least 10 seconds produced the most cracks/hair damage. These cracks occur because of the fast change from wet to dry. The sudden vaporization of water causes the cuticle scales to contract and become rigid - and crack. That leads to more porosity and greater potential protein loss when wetted and dehydration in general. It's worse if you heat hair very fast - a slow heating produced fewer cracks than applying high heat suddenly.

Below 122°F (50°C) cracks did not appear during blow-drying and the author suspected that temps up to 150°F (65°C) did not produce that super-fast vaporization that causes cuticle cracks.

If you want to test your hair dryer, place an oven thermometer or candy thermometer wherever your hair would be in proximity to the warm air coming out of the blow dryer or diffuser attachment. Then, refer to this:
©Science-y Hair Blog 2013
"Safe" zone: 122° F or 50° C or lower
Probably safe: 122-150°F or 65°C
Danger zone: 167-203°F or 75-95°C and above

Curling irons, straightening or “flatirons” fall into the “danger zone” as well. The “safe” temperatures probably are the “low” temperature settings on blow dryers, but it doesn’t hurt to test yours.

Heat Protectants:©Science-y Hair Blog 2013

It was notable that when hair was wetted in a solution containing 2% glycerin or propylene glycol instead of plain water, the cracks did not develop. But “conditioning agents” such as Polyquaternium-11, Cetrimonium chloride,  and Stearylkonium chloride did not prevent cuticle cracking. Triglycerides, silicones, mineral oils and petrolatum also did not prevent cracking. The protein polymer, hydrolyzed wheat protein polysiloxane copolymer was found to prevent cuticle cracking. Other proteins and conditioners were not tested. Other testing has demonstrated that P/DMAPA Acrylates Copolymer and Polyquaternium-55 have good heat-protecting qualities. I am taking these data from the literature distributed by the manufacturers of these two ingredients, heat damage in this case was determined by protein loss – and when the cuticle has many cracks in it, it will lose protein when wetted. Protein-loss is a good indicator of hair damage with high heat.

Oils and Silicones
Oils and silicones disperse heat - like when you dip your finger in water before touching the heated surface to test the temperature of an iron or frying pan. Or like putting on a light-weight oven mitt. You couldn't hold a hot pan for 2 or 3 minutes, even with an oven mitt on - but you can hold it for a short while without getting burned. Another very important benefit of oils and silicones in heat protectants is that they keep your hair flexible and lubricated, which helps both in styling and prevents a dry feeling.
Silicones in particular, like Dimethicone can provide an effective layer of heat dispersal during blow-drying, flat-ironing or curling iron styling. Amodimethicone and other amino-modified silicones interact with hair in a unique way, providing a protective (but micro-thin!) coating to further protect against moisture loss and heat-damage.

Best Application Method?©Science-y Hair Blog 2013

A product that spreads over your hair evenly can protect it the best. That gives an advantage to rinse-out conditioners and styling creams. If you prefer sprays, it's recommended to spray individual sections before curling or straightening.

List of proven heat-protecting ingredients:

  • Amodimethicone
  • Amodimethicone/Morpholinomethyl Silsesquioxane Copolymer
  • Dimethicone
  • Glycerin (2% - and that is a lot for any formulation)
  • Hydrolyzed wheat protein (any hydrolyzed protein will likely be helpful)
  • Hydrolyzed vegetable protein PG-propyl silanetriol 
  • Hydrolyzed wheat protein PG-propyl silanetriol 
  • Sodium Laneth-40 Maleate/Styrene Sulfonate Copolymer
  • P/DMAPA Acrylates Copolymer 
  • Polyquaternium-55 
  • Polyquaternium-68
  • Polyquaternium-37 (may offer some heat protection)
  • Propoxytetramethyl Piperidinyl Dimethicone
  • Propylene glycol
  • VP/DMAPA Acrylates Copolymer
  • VA/Crotonates/Vinyl Neodecanoate Copolymer
  • Quaternium 70 (Stearamidopropyl Dimethyl Ammonium Chloride) 
  • Stearamidopropyl Dimethylamine (Same ingredient as the above)
    • These 2 (above) are very common in rinse-out conditioners, check yours!
  • Sodium polystyrene sulfonate 
  • Sodium Laneth-40 Maleate/Styrene Sulfonate Copolymer
Products containing these heat protectants available in the 'States

May be more suitable for Fine/Thin hair: 

Richer products for blowouts or straightening

Rinse-Out or Leave-In Conditioner + Heat Protectant

Heat Protectants for Naturally Curly Styles

©Science-y Hair Blog 2013

Gamez-Garcia M. 1998. The Cracking of Human Hair Cuticles by Cyclical Thermal Stresses. Journal of Cosmetic Science, 49, 141-153.


  1. Hello!

    Thank you for your article! I would like know: Is it better to do several passes at low temperature while flat ironing or one pass at high temperature (more than 150°C)?
    Thanks in advance

  2. Those temperatures are very high for hair. Anything which would harm the skin will harm the hair, so try to have as little exposure to the heat as necessary. That said, you should definitely invest in a heat-protectant product - as a spray or mousse or leave-in conditioner. They usually say "heat protectant" or something like that on the packaging. Ingredients to look for are:
    glycerin or propylene glycol
    P/DMAPA Acrylates Copolymer
    hydrolyzed wheat protein polysiloxane copolymer

  3. Hi WS. I hope you see this! I'm almost 10 years late to the thread. Anyway, I use a diffuser and a "hovering" technique, where I slowly move it around holding it a few inches away from my hair. At that distance, the air temperature is 160 F. When I took the temperature of my hair while drying it, using an IR thermometer, it only gets up to 120. Which measurement should I use?