Thermal damage and UV-related damage alter the hair fiber through distinct mechanisms: heat primarily degrades the outer protective layer, while UV rays cause internal chemical degradation similar to bleaching, affecting both color and protein structure.

noli's read

The distinction between these two types of stress is scientifically relevant. Research confirms that excessive heat (styling tools) weakens the cuticle and alters the moisture balance, which reduces hair suppleness (Meta-analysis, Journal of Cosmetic Science). Regarding UV rays, studies indeed indicate photochemical degradation of amino acids and sulfur bonds (disulfide bridges), confirming the 'chemical' aspect of the damage that leads to discoloration and loss of strength (In vitro studies, International Journal of Cosmetic Science). However, it is important to note that these processes are not completely isolated: a cuticle already damaged by heat allows UV rays to penetrate more deeply into the cortex. While the mechanism is well-documented, the extent of the damage varies significantly depending on individual hair genetics and actual exposure. The scientific community strongly agrees on this differentiation, supporting its findings with solid technical references.

the sources

1. 1r/HaircareScience