BLEACHING OF HAIR 183 RESULTS AND DISCUSSION An interesting aspect of the photobleaching phenomenon seen in keratin fibers such as wool and hair, is that solar radiation lightens both the non-pigmented and pigmented fibers. In the former case, however, the bleaching is an ephemeral first step of a photo- chemical process that results in severe fiber discoloration (photo-yellowing). There is an imposing body of literature devoted to the mechanism of photo-yellowing of keratin [see an excellent summary in (5)], but virtually nothing is known of the chemical changes associated with the bleaching stage. The situation is somewhat different for pigmented fibers where, in addition to the keratin matrix, the fibers contain multiple melanosomes. It appears that in this case the observed color changes are dominated by the photochemical reactivity of the melanin pigment. It has been amply documented (6,7) that the latter undergoes progressive lightening with an increase in solar exposure, the photo-bleaching process being aided by high values of moisture regain. In this general sense, the photo-response of red and brown hair conforms to the expected be- havior (Tables I and II). Both types of hair lighten and they do so much more under moist conditions. A closer examination of the data suggests, however, that there is a significant difference in the photochemical response between the eu- and pheomelanin. Not only does brown hair lighten more than red hair (increases in L values are 35% and 14%, respectively), but it also becomes more reddish in the process. The reflectance 25 2O 15 10 I I I i i 400 500 600 Wavelength (nm) i ! 700 Figure 3. Reflectance spectra of red hair before (C)) and after (O) photo-bleaching.

184 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 20 15 L J ! I I ! I 400 500 600 700 Wavelength (D•) Figure4. Reflectancespectraofbrownhairbe•re(¸)anda•er(O)photo-bleaching. spectra, which provide more appearance-related color changes, demonstrate the effect of photo-bleaching for these two types of hair (reflectance of 192 hour--photobleached hair, Figures 3 and 4). Clearly, the changes in reflectance of brown hair are distinctly different from those observed with red hair. The magnitude of this differential effect and its shift with wavelength is summarized in Figure 5. We have alluded earlier to the tendency of keratin fibers for photo-yellowing. An im- portant point arises as to whether the observed differences in photo-response between the red and brown hair are not optical artifacts caused by the discoloration process of the keratin matrix. This is unlikely since photo-yellowing of keratin would be similar in both cases and thus parallel changes in reflectance should be observed. The results of recent studies of the photoreactivity of eu- and pheomelanin provide, however, strong corroborative support for the view that the observed changes are primarily related to the pigment component of the fiber. It has been found that on irradiation of aqueous solu- tions of pheo- and eumelanins, both undergo photo-bleaching but at different rates (8). The pheomelanin photo-bleaches more slowly indeed, in the initial stage of irradiation, the solution shows an increase in optical density. The photo-bleaching effect observed with a solution of eumelanin is reproduced in Figure 6. An interesting characteristic of this photo-bleaching process is the monotonic decrease in absorbance over the entire UV-VIS spectrum. Upon photolysis, the solution becomes uniformly lighter, with no observable spectral shifts. This is in contrast to the observed trend in brown hair which, upon exposure to solar radiation, develops a reddish tone. One might be tempted to explain this difference in photoresponse by assuming that brown hair is not purely eumelanic but contains a mixture of pigments with both eu- and pheomelanins present.