You should not let your hair air-dry because water causes the fiber to swell and damages its internal structure, known as the cell membrane complex. Instead, it is recommended to blow-dry your hair at a low temperature, keeping a distance of 15 cm and using constant motion, while also avoiding leaving the scalp damp to prevent the development of fungi.

noli's read

This recommendation is based on a solid experimental study published in 2011 in the journal Annals of Dermatology by researchers at Yonsei University in Korea. Their laboratory work revealed that natural drying keeps the hair fiber swollen with water for too long, which fatigues and cracks the cell membrane complex, the internal 'cement' that ensures the cohesion of the hair. Conversely, using a blow-dryer at 15 cm with continuous movement preserves this internal structure, even if direct heat can slightly sensitize the outer cuticle if not controlled. Furthermore, the claim regarding moisture promoting fungi is consistent with the fact that yeasts naturally present on the head multiply in a humid environment, which can cause dandruff or discomfort. Nevertheless, the absolute injunction to 'never' let hair air-dry is exaggerated: for very short or very fine hair that dries in a few minutes, the risk of water-related fatigue is minimal. The real benefits of this method will therefore always depend on your hair type, its natural porosity, and the thickness of your hair.

the sources

No named source held this one up — that absence is part of the verdict.