hair damaged by sunlight, in most cases, the amino acids of the
cuticle are altered to a greater extent than those of the cortex because the outer
layers of the fiber receive higher intensities of radiation. Hair protein degradation
by light radiation has been shown to occur primarily in the wavelength region of
254–400 nm. More recent work by Hoting and Zimmerman [38] shows that the
proteins of the cuticle are degraded by UV-B and UV-A, but less by visible light
and that cystine, proline and valine are degraded more in light brown hair than in
black hair. In other words the photo-protective effect of melanin is much better in
dark hair than in light hair.