Saturday, 20 August 2011

Blow Drying Reduces Moisture in Hair (even with low heat!)

Does blow drying on cool damage your hair?

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
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
110°C /230°F -1.85 -17.9

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


  1. Thanks for posting this Jc. For the past 4yrs. I've been air drying my hair and only blow dried my hair one time out of those four years. My hair got dry after that incident.That is why I just air dry my hair because of the lack of moisture I got from blow drying. If I ever do blow dry my hair again, I will blow dry on a low temperature.

  2. Thanks for the post!

  3. Nice post! This explains why blow-drying (on cool using the tension method) made my hair feel dry in the past. If I ever revist blow-drying in the future, I'll do so with the tips in this post in mind.

  4. How interesting. Thank you so much for this info, as always. I haven't blow dried my hair in over a year, and it is thriving. Thriving so much that I've got about 14 inches of hair to deal with now...which has led to me giving my blow drier contemplative, longing (and sexy, might I add) glances, wondering if using it on cool might be a way to reincorporate it and make life a bit easier. I just may try it, now that I'm armed with this info. Thanks again, Jc!

  5. Now all I need is a thermometer! Great info. Just wondered about your statement about using a comb / brush. I blow dry with an afro pic attachment attached to the hair dryer. I use a wide tooth comb one for a couple strokes, then attach a finer tooth comb on the dryer. If I use it on the cooler setting, would this method be ok?

  6. I already realised this, I stopped blowdrying over a year ago. My hair feels softer when airdried and the frizzies are easier to contain.

    I noticed that in your table, blowdrying at 70 degrees C shows less moisture loss than either 40 or 50 degrees C. Is this explained in the paper?


  7. Lela - if your hair is not complaining then you should be fine. Science is only a guide, you should observe how your hair responds (in terms of breakage or possible dryness). Using a lower temperature is always beneficial to hair.

    Anon - The paper did not offer a direct explanation of this but they did offer a theory as to why hair loses its moisture. In short, water is bound to hair via hydrogen bonds and when you dry the hair, some of these bonds are broken which make hair able to form new crosslinks with itself instead of with water. These new links prevent full hydration.

    As you go up to higher temperatures some of these bonds remain permanently and that is why at very high temperatures, hair cannot ever go back to its normal state of moisture. This happens at the 110°C point

    I would theorise from this explanation that at 70 degrees it may be the case that hair is transitioning and a cocooning effect may happen where hydrogen bonds break and the hair starts to form some of the permanent crosslinks. But as the temperature is not yet quite high enough, these crosslinks act to prevent heat access further inside the cortex and prevent water loss.
    This may in part explain why at 70 degrees there appears to be a shielding effect where the hair seems to not loose as much water but in actual fact it may just be that the heat is starting to deform hair and the energy is focused on this process and not on dehydration.

    I don't know if that made sense......I hope it did.

    Some scientists though would attribute it to experimental error or the fact that the average is considered instead of the most common repeating values.......

  8. Thanks for this post Jc. Do you think the results are the same for hooded dryer use (not steamers)?

  9. Does the same information hold true to hair dryers (bonnet)?

  10. Mo and Medina - Yes I would say the same applies to a bonnet dryer, the air is still heated up and therefore it constitutes 'heat drying'

  11. Thank you soooo much JC! A really timely post considering I was reconsidering blow drying my hair monthly. I'll be sure to do it less frequently to hold on to as much moisture as I can, whilst still getting my fab styles.

  12. So I was wondering how do heat protectants help?

  13. I was just thinking about this over the weekend, thanks for the post. I rarely if ever blowdry my hair and I notice that the times that I have that my hair always felt crispy.

  14. Very interesting. Though the publishing year of the study you mentioned causes me to assume that the information pre-dates the cosmetic use of infrared heat to dry hair. I would think with the way it works, this reduction of moisture is lessened if the heat is infrared versus not. Perhaps a follow-up on this information can be considered?

  15. I was curious about this, but I'm not sure if your definition of "cool" is the same that I was thinking. The lowest temperature you have there is 40˚C, but I'm actually wondering about air cooler than that. I'm assuming that if you blow 40˚C air over the back of your hand it will feel rather warm to the touch. I actually would like to know what would happen if you blow-dry hair with air that actually felt cool to the touch, as in "sticking your head out of the window, while going 80 mph on the highway" cool. As in "room-temperature-cool" air. Thanks! -L.u


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