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.
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.
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?
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.
If you have a dry or sensitive scalp, the lower temps may still be too high for more than a short burst. Air that dries hair also dries skin.
Gamez-Garcia M. 1998. The Cracking of Human Hair Cuticles by Cyclical Thermal Stresses. Journal of Cosmetic Science, 49, 141-153.
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