I'm certainly no expert on this. But I've been wondering about it for a long time. I think it's primarily a question of temperature - though I think we would all agree that air drying, on average, is the least damaging. The last time I went to the salon, they put me under (what they called) their "lightning fast dryer". Just before I went under, they placed these soft fabric ear protectors over my ears. At first, I thought it was a nice, professional jesture. As I sat there, I started thinking, if they expect my ears to get hot, what is happening to my scalp?
When I was a child, I lived under a hair dryer. One day my father taped a note on the hood that read, "Oh heat, dry up my brain. - Shakespeare."
Just speculating,
Cheveux