Friday, February 3, 2012

How Hot is Too Hot for Healthy Hair?

I like to use a blow-dryer with a diffuser attachment on my hair after I have put hair gel in it for about 5 minutes. The heat and quicker-than-air-drying treatment “sets” the waves and curls in my fine hair. Then I let it air-dry until dry. If I let it air-dry exclusively, the weight of the water tends to lengthen the curls and waves somewhat. I worried that the heat would be damaging to my hair – so a little testing and research were in order.
©Science-y Hair Blog 2013
I put a thermometer’s sensor in the bowl of the diffuser right where I put my hair. 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, or in the same direction as the hair hangs (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:
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. So 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 or lick it 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. A 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.

List of proven heat-protecting ingredients:

  • Quaternium 70 (Stearamidopropyl Dimethyl Ammonium Chloride) 
  • Stearamidopropyl Dimethylamine (Same ingredient as the above)
  • P/DMAPA Acrylates Copolymer 
  • 2% glycerin (that is a lot for any formulation) 
  • Polyquaternium-55 
  • Hydrolyzed wheat protein 
  • Hydrolyzed vegetable protein PG-propyl silanetriol 
  • Hydrolyzed wheat protein PG-propyl silanetriol 
  • Sodium polystyrene sulfonate 
  • Sodium laureth-40 maleate/styrene sulfonate copolymer 
Products containing these heat protectants available in the 'States
  • AG Recoil Curl Activator 
  • Aphogee ApHogee Keratin and Green Tea Restructurizer 
  • Biolage RAW Heat Styling Primer
  • Biosilk Silk Therapy Thermal Shield
  • Bumble and Bumble Hairdresser's Invisible Oil Heat and UV Primer
  • Carol's Daughter Cupucau Anti-Frizz Smoothing Blow Dry Cream
  • CHI 44 Iron Guard Protection Spray 
  • CHI Infra Therma Protective Treatment
  • Drybar Hot Toddy Heat & UV Protectant
  • Hask Keratin Sleek Blow-Out Balm
  • John Frieda Frizz Ease Heat Defeat Protective Spray
  • Kenra Professional Platinum Blow Dry Spray
  • Kenra Thermal Styling Spray 19
  • Kenra Perfect Blowout Light Hold Styling Creme
  • Not Your Mother's Smooth Moves Frizz Control Hair Cream
  • Paul Mitchell Round Trip 
  • Ouidad Climate Control Heat and Humidity Gel
  • Suave Professionals Keratin Infusion Heat Defense Leave-In Conditioner 
  • Thermafuse Fixxe Volume Mousse
  • Thermafuse One Pass Hot Iron Activated Smoothing Creme
  • Tresemme Keratin Smooth Flat Iron Spray 

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