Does hair dryer damage your hair? If yes, how much damage does it cause?
Here’s a comparison between natural hair drying and drying your hair with a hair dryer.

This article contains my own opinions and also excerpts/proof from the paper – “Hair shaft damage from heat and drying time of hair dryer” published in May 30, 2011 in a reputed dermatology magazine called Annals of Dermatology. It is available under Creative Commons Attribution Non-Commercial License.

To identify the effects on the hair, the following characteristics of hair were analysed.
1. Hair Surface damage
2. Hair cuticle and cortex
3. Cell Membrane complex

To see the results without all the details, click Results.

Hair sample preparation

For their expirement, they prepared five hair samples in the following ways
(a) no treatment
(b) shampooing and drying without using a hair dryer (room temperature, 20oC)
(c) shampooing and drying with a hair dryer for 60 seconds at a distance of 15 cm (47oC)
(d) shampooing and drying with a hair dryer for 30 seconds at a distance of 10 cm (61oC)
(e) shampooing and drying with a hair dryer for 15 seconds at a distance of 5 cm (95oC)

Each treatment was done once a day (24 hours) . Shampooing and drying process for the expirement was done 30 times for 30 days.

It is a widely accepted fact that hair gets damaged due to excessive shampooing, bleaching, perming and by the use of other chemical agents. Hair also gets damaged due to friction caused when drying it with a towel. Hair dryers also damage hair due to their heat. Split ends and hair fall are some of the side effects of damaged hair.

With all these factors that damage hair, which mechanism produces the least amount of damage? The researchers try to provide an answer to this question by analyzing the various hair samples under a microscope and other instruments and by looking at magnified views of the hair surface to quantify the damages caused.

Hair type and drying mechanismHair Surface DamageTransmission Electron MicroscopyLipid Transmission Electron Microscopy 
No Treatment
A - hair surface damage

A - hair surface damage

A - Hair Cuticle

A - Hair Cuticle

A - Cell Membrance Complex

A - Cell Membrance Complex

Natural Drying
B - hair surface damage

B - hair surface damage

B - Hair Cuticle

B - Hair Cuticle

B - Cell Membrance Complex

B - Cell Membrance Complex

Hair Dryer
15cm, 60s
47C
C - hair surface damage

C - hair surface damage

C - Hair Cuticle

C - Hair Cuticle

C - Cell Membrance Complex

C - Cell Membrance Complex

Hair Dryer
10cm, 30s
61C
D - hair surface damage

D - hair surface damage

D - Hair Cuticle

D - Hair Cuticle

D - Cell Membrance Complex

D - Cell Membrance Complex

Hair Dryer
5cm, 15s
95C
E - hair surface damage

E - hair surface damage

E - Hair Cuticle

E - Hair Cuticle

E - Cell Membrance Complex

E - Cell Membrance Complex

Hair Surface Damage

  • Untreated and naturally dried showed no cracks on surface
  • 47 and 67 ° C showed some longitudinal cracks and the 95 ° showed the most severe damage

Conclusion
Cuticle gets damaged by heat. It develops cracks, holes and jagged borders. So, closer the temperature to room temperature, the better it is for hair.

Hair Cuticle and Cortex

  • No significant damage seen to the cuticle at lower temperatures
  • At 95 °C holes were seen on the cuticle.
  • The cortex was not damaged in any of the hair samples

Conclusion
Cuticle damage was seen at 95°C and not at lower temperatures.

P.S. Hair surface damage was measured using scanning electron microscope(SEM) and the cuticle damage using transmission electron microscope(TEM). SEM is used to analyze the surface layer where as TEM can provide crystallographic information. For more info, read this post on researchgate.

Cell Membrane Complex

  • Only naturally dried hair showed bulging of the cell membrane. This is shown with arrows on the image in the table above

Conclusion
Bulging is a sign of damaged Cell membrance complex. It is quite possible that hair gets damaged when it stays wet for a long time. Further study is required to identify the correlation of hair wet time and the damage to the cell membrane.

Results

Using a hair dryer causes surface damage to hair. But, this can be reduced by using a conditioner or any other surface protectors available in the market. So, according to this study, using a hair dryer at low temperatures causes slight damage to the surface of the hair and not to the cortex. The inner cell membrane is only damaged when hair stays wet for longer periods of time. The temperature or heat affecting your hair can be significantly reduced by constantly moving the dryer along the length of the hair.

Hence, we can say that drying hair with a hair dryer held atleast 15 cm from the hair at lower temperatures is better than natural drying.

References
Lee, Y., Kim, Y. D., Hyun, H. J., Pi, L. Q., Jin, X., & Lee, W. S. (2011). Hair Shaft Damage from Heat and Drying Time of Hair Dryer. Annals of dermatology, 23(4), 455-462.

But, what about towel drying and brushing ? How much damage does it cause?