Hair Damage 374 the atmosphere and it may be only experienced in space, or by those engaged in certain occupations (welding, for example). The World Health Organization (WHO) has introduced, through the international program Intersun, a UV Index for describing the level of UV radiation in any given place.24,25 The value relates to the amount of energy coming from UV radiation, and it is expressed on a scale from 0 (minimum exposure, no risk) to 10 (dangerous exposure). Its intention is to aid in reducing people’s exposure to harmful radiation.26 The reduction in ozone protection in areas of the Southern Hemisphere has becoming acute and values of 11 or more have become common in certain areas. Both UVA and UVB radiation produce photodegradation of hair, and induce damage in keratin and melanin which compete for the absorption of photons between 254 nm and 345 nm.27 The mechanism proposed for photodegradation of keratin is based on the fact that only cystine and the aromatic residues from tyrosine, phenylalanine, and tryptophan absorb light within the wavelength range of UVA and UVB. This is also supported by studies of wool degrading by sunlight.28 The photochemical oxidation of cystine follows the C-S scission pathway, yielding S-sulfonic acid that is finally degraded by light to cysteic acid:29 R-S-S-R à R-S-S-OH à R-S-SO 2 H à R-S-SO 3 H à R-SO 3 H + H 2 SO 4 + R-OH There is also evidence for an alternative route of C-C scission, with the production of alanine and glycine, and with lysinolalanine, or lanthionine as end products that cross-link the irradiated keratins.30 The aromatic amino acids are also reduced by irradiation and there is research suggesting that tryptophan is damaged before cystine, explaining why weathered hair has lower levels of tryptophan and elevated amounts of cysteic acid compared to