Chapter 11 375 untreated fibers.31 One of the roles of melanin in hair is considered to involve photoprotection from incoming radiation.32 In this process, the melanins, both eu- and pheomelanin, were shown to be also damaged by UV radiation,33,34 following the extensive destruction of their quinone system, a process called photobleaching. In this respect, eumelanin seems to be more stable during irradiation and therefore does a better job protecting the keratin than pheomelanin.34 The amount of melanin pigment in hair fibers is also important for its photoprotective role, with gray hair being more severely damaged (tryptophan depletion, mechanic strength) by UV radiation than dark brown fibers from the same head.35 Since melanin grains are present only in the cortex, then this photoprotection does not extend to the cuticle. As a result, the cuticle has been noticed to thin and the scales to fuse after exposure to radiation.29 This effect is greatly enhanced by humidity. Visible Radiation: Most of the effects mentioned for UV radiation also occur under visible light (VIS) irradiation however, UV radiation is more effective in damaging cystine and tryptophan than VIS light.27 Yet there are exceptions to this. The absorbance of melanin grains is more sensitive to the wavelengths of visible light than UV, which means that the photobleaching is related mainly to the intensity of visible light.27, 32 It was also shown that degradation of the internal lipids occurs to a considerably higher extent under visible light than when exposed to UVB and UVA, and that the photochemical destruction of lipids is largely retarded by eumelanin pigment.36 Lipids provide hydrophobicity to the hair fiber surface and contribute to its smoothness and flexibility therefore long periods of exposure to visible radiation are detrimental.27,37 Gamma Radiation: The effect of gamma radiation on hair has seen less investigation than the aforementioned forms. Some data has been acquired on wool, which indicates that gamma radiation also significantly decreases the amounts of cystine, tyrosine and tryptophan. The process induced by small doses of radiation (below 10 Mrad) is thought to involve a cross-linking of the matrix, as the