106 JOURNAL OF COSMETIC SCIENCE for photo-oxidative damage in a Hitachi S-4500 digital field emission scanning electron microscope (FESEM). RESULTS AND DISCUSSION BACKGROUND Photochemical degradation of hair affects both hair proteins and hair pigments and has been shown to occur primarily in the wavelength region of 254 to 400. In recent infrared spectroscopic studies of hair exposed to weathering, Dubief (3) shows a decrease in lysine and histidine, and a drastic increase in cysteic acid content in tip ends of hair compared to root ends. This suggests scission of the disulfide crosslinks during the photochemical oxidation process and formation of cysteic acid. This assumption is supported by Rob- bins and Bahl (5), who have shown by electron spectroscopy for chemical analysis (ESCA) that both UV-B (280-320 nm) and UV-A (320-400 nm) oxidize sulfur in hair. Oxi- dation was shown to occur primarily in the fiber periphery, namely, the cuticular sheath, producing a steep gradient to less oxidized hair in the fiber core. These results are not surprising, since disulfide is at its highest concentration in the A-layer and the exocu- ticle, where the highest level of photochemical oxidation occurs. The ESCA spectra suggest that high levels of cystine S-sulfonate and cysteic acid are formed in hair exposed to photochemical oxidation in the wavelength region of 254-500 nm. Our research (6) strongly supports the results of Dubief (3) and Robbins and Bahl (5). Using UV microspectrophotometry, we have shown that high levels of photodegradation products are formed throughout the hair fiber cross section during long-term exposure in the 290-400 nm range. UV irradiation-induced photodegradation products of the hair proteins are revealed by an extension of the absorbance plateau and a shift in peaks from 290 nm in untreated hair to 315 nm in UV-exposed hair. There is also the development of an absorbance shoulder in the 330-340 nm range, well isolated from the absorbance of the bulk of the hair fiber (Figure 1). Formation of the photo-oxidized hair proteins can be traced and mapped, even quantified, by scanning across hair fiber cross sections at the wavelengths of the absorbance shoulder, (}t m -- 330 nm). These photo- degradation products are especially pronounced in blond (unpigmented) Piedmont hair, with the highest level ofphotodegradation occurring in the cuticular region (A-layer and exocuticle), where cystine is at its highest concentration (Figure 2). It is a generally accepted concept that the mechanism for photochemical oxidation of cystine follows the C-S scission pathway, whereby oxidative scission yields S-sulfonic acid that is finally degraded by light to cysteic acid (7,8). In contrast to the chemical oxidation that follows the S-S scission pathway and yields two moles of cysteic acid per mole of reacted disulfide, only one mole of cysteic acid is produced from each mole of reacted disulfide in the C-S scission pathway. The progressive oxidation pathways for the two types of scission are: S-S Scission (chemical oxidation): R-S-S-R --- R-SO-S-R • R-SO2-S-R -• R-SO2-SO-R -• R-SO2-SO2R -• 2R-SO3H C-S Scission (photochemical oxidation): R-S-S-R • R-S-S-OH • R-S-SO2H • R-S-SO3H • R-SO3H + H2SO 4 + R-OH

PHOTODEGRADATION OF HUMAN HAIR 107 2.0 J• f. o 1.2 0.8 0.4 Untreated Hair .o 230. o 264. o 298. o 332. o 366. o 400. COLLAPSE AND FUSION OF THE SURFACE CUTICLE CELL The conditions we have chosen in this study may seem somewhat extreme however, we felt they were necessary to be able to learn about the behavior of the hair fiber and the changes of its physical nature brought about by severe UV irradiation. Observations in the FESEM had shown that nearly all hair fibers exposed to long-term alternating cycles of UV irradiation (290-400 nm kma x = 340 nm) and humidification (95% RH 42øC) displayed an unusual topography. The hair fibers have a smooth topography similar to that of a man-made fiber, with little of the characteristic differentiation of the cuticle cell of undamaged hair. Figure 3a shows the typical appearance and thickness of a normal cuticle cell of unaltered, untreated hair fibers. However, after only 100 hours of UV irradiation and humidification in the QUV, a slight thinning of the surface cuticle cell and fusion at the scale edges are apparent (Figure 3b). After 300 hours of UV exposure, a more pronounced collapse of the surface cuticles and fusion of the scale edges to the underlying cuticle cells is seen (Figure 3c). 700 hours of UV exposure has produced hair fibers with a smooth topography, lacking clear differentiation of the cuticle cells (Figure 3d) because of extreme cuticular thinning and fusion to the underlying cuticle cells. The overall decrease in thickness of the surface cuticle cell as a function of exposure time to UV irradiation was obtained in the FESEM using built-in software for measuring distances on a nanometer scale in the axial, radial, and diagonal direction of fibers. Decreases in scale thickness were measured at the same high magnification and con- verted to the appropriate scale. The results are shown in Figure 4. Our hypothesis explaining this photochemical damage phenomenon is as follows: this progressive thinning and fusion of the surface cuticle cell, under the conditions we used, is most likely due to photochemical degradation of the proteins in the surface cuticle cell