124 JOURNAL OF COSMETIC SCIENCE 20 • • • 20 t6 16 • 12 Figure 17. Loss •n hair color due to progressive light exposure in the QUV and "AW," shown as increase in transmission intensity (%) of hair fibers due to light exposure. underlying cuticle layer. The characteristic differentiation of the surface cuticle cell of untreated hair gradually disappears with increasing exposure. Besides thinning and fusion of the surface cuticle cell, there also occurs fusion of the complete cuticular sheath into a rigid, brittle unit. While intercellular cohesion within the cuticular sheath is high, possibly due to free radical-initiated crosslinking of the CMC, the cuticle cells themselves are brittle. A newly observed fracture pattern of long-term UV-exposed fibers suggests fusion of the regions attacked by UV light into one rigid and brittle mass, incapable of extension due to loss of all original elastic properties. While chemical oxidation results in partial (1 h H202) and then complete solubilization (4 h H102) of the melanin granules, photochemical oxidation produces entirely different results. Even after long-term exposure to severe conditions of UV irradiation/ humidification cycling, the melanin granules appear physically intact with little change in their physical nature. Loss of color does not occur as long as the melanin granules are intact. Long-term UV irradiation/humidification cycling has severely damaged the hair proteins and preconditioned them for accelerated disintegration during subsequent treatments with alkaline hydrogen peroxide. Such contact restfits in rapid disintegration and dis- solution of elements of the cuticle cells, restilting in formation of Allw/Srden sacs via osmosis. UV irradiation has also severely damaged the melanoproteins and precondi- tioned them for accelerated disintegration and solubilization by the alkaline hydrogen peroxide, as indicated by SEM micrographs of the empty cavities where once the melanin granules were housed. While photochemical oxidation of hair occurs at all humidity conditions, it appears to be more pronounced and rapid at high relative humidity and/or humidification cycling.

PHOTODEGRADATION OF HUMAN HAIR 125 Continuous UV irradiation at constant 10% RH (QUV), without alternating cycles of humidification, does not show the thinning and fusion of the surface cuticle cells. However, photochemical degradation has occurred, as subsequent immersion in water causes cuticle thinning and fusion of a magnitude similar to that experienced by fibers exposed to UV irradiation/humidification cycling (QUV). Low and moderate RH conditions during exposure to unfiltered solar light in the "AW" also fail to produce cuticle thinning and fusion, and hair fibers still display the char- acteristic cuticle cell differentiation. Subsequent immersion in water shows only mod- erate cuticle thinning and fusion. This suggests that the solar spectrum of the "AW" causes less severe levels of photo-oxidative degradation of the proteins than the UV irradiation in the QUV. High relative humidity is the primary contributing factor to accelerated loss in hair color. The combination of both the high RH conditions and the unfiltered solar light have the most damaging effects on hair color. REFERENCES (1) E. Hoting, M. Zimmermann, and S. Hilterhaus-Bong, Photochemical alterations in human hair. I. Artificial irradiation and investigations of hair proteins,J. Soc. Cosmet. Chem., 46, 85-99 (1995). (2) L.J. Wolfram and L. Albrecht, Chemical- and photobleaching of brown and red hair, J. Soc. Cosmet. Chem., 82, 179-191 (1987). (3) C. Dubief, Experiments with hair photodegradation, Cosmet. Toilerr., 107, 95 (1992). (4) J. B. Speakman and P. R. McMahon, The action of light on wool and related fibers, N. Z.J. Sci. Tech., 20 (1939). (5) C. Robbins and M. Bahl, J. Soc. Cosmet. Chem., 35, 379-390 (1984). (6) S. B. Ruetsch, Y. Kamath, and H.-D. Weigmann, unpublished observations. (7) C. R. Robbins,J. Soc. Cosmet. Chem., 22, 339 (1971). (8) W. E. Savige and J. A. Maclaren, in The Chemistry of Organic Salfar Compoands, N. Kharash, C.Y. Meyers, Eds. (Pergamon Press, New York, 1966), Vol. 2, pp. 367-402. (9) C. R. Robbins, Chemical and Physical Behavior of Haman Hair, 2nd ed. (Springer-Verlag, New York, 1994), pp. 103-104, 108. (10) L.J. Wolfram and K. Hall,J. Soc. Costa. Chem., 26, 247 (1975). (11) S. Ratnapandian, S. B. Warner, and K. Kamath, Photodegradation of human hair,J. Soc. Cosmet. Chem., 49, 309-320 (1998). (12) F. Leroy, A. Deftandre, and J. C. Garson, Photoaging of human hair, 7th International Hair Science Symposiam (Bad-Neuenahr, 1990). (13) C. R. Robbins, Chemical and Physical Behavior of Haman Hair, 3rd ed. (Springer-Verlag, New York, 1994), p. 78.