On a journey to find the answer above I came across a lovely little scientific study (J Soc Cosmet Chem, pp 27-36, 1981) which showed that even with low temperature blow drying hair never gets back to its original moisture level until it is soaked in water then air dried or placed at a much higher humidity.
In other words, heat drying your hair actually does dry your hair out. To explain this, I will start off with a reminder of the basic structure of hair.
The next step is to wet the hair as you normally would do with washing and this causes water to travel into the cortex.
Finally there are two options to dry hair. Air drying allows the hair to get back to its original pre-wash moisture level which is in balance with the water in air (humidity). However, heat drying hair actually drives water out of the hair and interestingly it does not balance back with humidity. There is always a slight loss of water from the cortex which cannot be regained unless the hair is re-soaked in water or placed into much more humid air.
The paper helpfully showed the loss levels according to the amount of heat used. The table shows that blow drying results in a moisture loss from anywhere between 6% and 18%.
Moisture loss due to blow drying
Weight loss due to heat drying (%)
Total Moisture Loss (%)
|40°C / 104°F||-0.63||-6.1|
|50°C / 122°F||-0.66||-6.4|
|70°C / 158°F||-0.48||-4.6|
|90°C / 194°F||-1.00||-9.7|
If you are having difficulty maintaining moisture in your hair and you regularly blow dry your hair, you may have an explanation now for why this is the case.
The positive news is that drying hair at low temperature (up to around 50°C) does not greatly affect the strength of hair meaning that it can generally resist the same amount of force as it would do if air dried. However getting to the higher temperature (around 100°C) hair does begin to lose some strength - about 4% for untreated hair.
So going back to the question, does blow drying on cool damage your hair?
The simple answer to that is no and the complex answer is no if you do not
1. blow dry dripping wet hair (bubble hair!)
2. tug too hard on the hair or use a comb/brush that breaks your hair
3. your blow dryer does not over heat