You should never let your hair air dry, because residual moisture on the scalp promotes the proliferation of fungi and can trigger irritation.

noli's read

The fact that prolonged scalp moisture promotes the proliferation of naturally occurring microorganisms, such as the yeast Malassezia, is scientifically validated by hair specialists, as explained by dermatologist Dr. Aamna Adel (expert opinion). This skin microbiome imbalance can indeed irritate the skin and lead to discomfort or dandruff. Nevertheless, strictly forbidding air drying is an exaggeration. For short, fine, or thin hair that dries quickly, this natural method poses no risk to scalp health. The issue primarily arises for thick hair that retains water for hours, or when going to sleep with wet hair. A 2011 biomechanical study (laboratory study) notably revealed that gentle blow-drying (at 15 cm and a lukewarm temperature) stresses the hair fiber less than prolonged natural drying, as it limits the hair's extended swelling from water. This advice is therefore highly relevant for optimizing your routine and protecting your scalp, but it should be nuanced according to the thickness of your hair.

the sources

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