Abstract— Human red hairs, equilibrated at 79.8%, 58.0%, 32.3% humidity and at dry conditions, were irradiated with UV‐visible (Λ > 290 nm) light. ESR intensity of the photo‐induced transient free radicals was measured, at room temperature, as a function of time during and after exposure to light. Photoenhancement was found to be most pronounced in 79.8%‐humidity conditioned hair and least in dry hair. The lifetimes of the transient free radicals in these samples were measured as 360s (79.8%), 180s (58.0%), 140s (32.3%), and 50s (dry). The gradual increase in lifetime as a function of humidity conditions was attributed to an increasing number of decay rates of the centers varying from fast to slow, the slow ones being unable to decay unless the fast ones do. This process is best described by a stretched exponential time function, I(t) =Ioexp[‐(t/to)α].