Allowing hair to air dry is not inherently more damaging than using a hair dryer, but prolonged exposure to moisture can weaken the hair fiber and promote certain scalp imbalances.

noli's read

Hair science, as popularized by experts like Lab Muffin, indicates that a wet state makes hair more vulnerable to mechanical stress because its internal structure (keratin) is temporarily swollen and weakened. A frequently cited study (Journal of Cosmetic Dermatology, observational) suggests that the controlled use of a hair dryer at a moderate temperature, while maintaining an adequate distance, can sometimes be less deleterious than very long air drying, which keeps the hair in a state of prolonged fragility. The major risk of air drying lies in associated habits, such as sleeping with wet hair, which generates excessive friction that promotes breakage, or creating an environment conducive to the proliferation of microorganisms on the scalp. Conversely, excessive heat applied abruptly can cause permanent structural damage (the formation of bubbles inside the hair). The scientific community largely converges on the idea that there is no universal rule, but rather a trade-off between thermal risk and the mechanical risk linked to the duration of moisture.

the sources

1. 1r/HaircareScience