PHOTODEGRADATION OF HUMAN HAIR 1 I9 J. B0•011 5.0 kV Figure 13. Longitudinal (a,b) and cross-sectional (c,d) views of long-term UV-irradiated hair fibers and subsequent 2-h treatment with 6% alkaline hydrogen peroxide. structures (Figure 14b,c). Eventually, these sac-like structures (swollen cuticle cells) burst and their contents drain (Figure 14d), leaving behind membranes or shells of cuticle cells (Figure 14e), which may detach from the hair fiber and drift away (Figure 14f), or may remain attached and form a thin film-like layer enveloping the outer cortical cells, as has been shown in the SEM study. EFFECTS OF RELATIVE HUMIDITY AND SPECTRAL ENERGY DISTRIBUTION ON PHOTOCHEMICAL OXIDATION Moisture plays an important role in the photobleaching of human hair (3,11,12). Pho- tochemical oxidation (also termed "weathering" or "photo-bleaching") damages hair at any given RH and increases with duration of exposure. The wet mechanical properties of photo-bleached hair fibers decrease with increasing extent of damage (11). Reduction in wet mechanical properties was found to be the most severe when hair is weathered at high or low RH. These fiber properties are least affected when hair is exposed at a RH of 30%. We have also investigated the effects of both the relative humidity and the spectral energy distribution on photochemical oxidation of hair fibers. We compared the results obtained at various relative humidities in two different fading units, namely, the QUV accelerated weathering tester and the Atlas Weather-Ometer ©.

120 JOURNAL OF COSMETIC SCIENCE Figure 14. Longitudinal views of long-term UV-irradiated hair fibers and subsequent exposure for various times to 6% alkaline hydrogen peroxide: (a) no peroxide post-treatment, (b) 5 s, (c) 20 s, (d) 15 rain, (e) 15 rain (focused on dome of fiber), and (f) 60 rain of peroxide post-treatment. UV exposure in the QUV. Fibers exposed to continuous UV light at constant 10% hu- midity in the QUV do not show thinning and fusion of the cuticle cells (Figure 15a), as was observed in our earlier studies in which hair had been exposed to UV irradiation/ humidification (at 95% RH) cycling in the QUV. However, upon subsequent immer- sion in water, collapse and fusion of the surface cuticle cell occurs (Figure 15b), with a severity similar to that observed in hair fibers exposed to irradiation/humidification cycling in the QUV. This indicates that photodegradation had occurred at a low con- centration of water in the fiber structure, but that the degraded protein fractions (unlike in the swollen fiber), were not able to diffuse from the cuticle cell due to lack of adequate amounts of water to promote swelling. This thinning and fusion phenomenon can be easily explained. At very low relative