Chapter 11 373 Water also tends to break the hydrogen bonds of hair, and in so doing affects the mechanics of hair fibers. A molecular model involving the contribution of hydrogen bonds to the elasticity of fibers found that the Young modulus, E, is proportional to the amount of effective hydrogen bonds of a unit volume, N:22,23 E = k N1/3 This model fits well with experimental data on hair and suggests that at low relative humidity most of the contribution is from hydrogen bonds, while disulphide bridges contribute more at higher stages.23 Effect of Radiation UV Radiation: In recent years, the effects of ultraviolet (UV) radiation have become more notable due to the deterioration of the stratospheric ozone layer, and its effect on hair has been increasingly studied. The Earth’s atmosphere filters out most short-wavelength radiation below 295 nm. However, depletion of the ozone layer and the corresponding reduction of this screening effect allow the most energetic component of the UV spectrum to reach the Earth’s surface. The UV range of sunlight is divided into three or energetic regions, namely: a. UVA (wavelength range 320‑400 nm) which plays a helpful role in the formation of vitamin D when absorbed by the skin, although prolonged exposure may cause sunburn on skin and cataracts in eyes. b. UVB (280‑320 nm) which causes sunburn at even short times of exposure, can cause damage at the molecular level (i.e. damage to deoxyribonucleic acid [DNA]) and is normally filtered out by the ozone layer. c. UVC (below 280 nm) which is the most dangerous form of UV radiation. UVC radiation is totally filtered out by