PHOTODEGRADATION OF HUMAN HAIR 319 The moisture itself may act as an additional source of free radicals. According to Walling (19), dissociation of water may be schematically represented as: 3H20 •3 H' + 'OH + H202 + H 2 (+ perhaps other products). The peroxide can further dissociate into two radicals that can damage the fiber. The net result is a fiber damaged to an extent similar to that of fiber irradiated under low humidity conditions. Some naturally occurring trace elements in hair may generate free radicals on irradiation (1). At low RH levels, there is minimal or no fiber swelling. Under such conditions, the free radicals have limited mobility. According to Walling (19), reduced free-radical mobility during condensation polymerization leads to polymerization in multiple di- rections instead of lengthwise chain propagation. This process is termed "popcorn polymerization," after the shape of the resulting polymeric structure. A similar depo- lymerizing reaction may be occurring in hair photolysis, leading to extensive local damage and loss of fiber mechanical properties. The dual role of moisture explains the results obtained for work-to-20%-strain and work-to-break for fibers exposed at 30% RH. It also explains the changes in post-yield modulus and strain-to-break data with weathering at various humidity levels. At 70% RH, for example, low-level photolysis of the primary peptide bond may occur, leading to free movement of the molecular chains. New crosslinks may form at a later time. The arguments presented above explain the current results satisfactorily, but in an indirect manner. They fall short of providing a definite proof of the exact mechanisms by which moisture affects photolysis of hair. CONCLUSIONS © Weathering damages hair at any given RH, the damage increasing with length of exposure. The wet mechanical properties of irradiated fibers decrease with extent of damage. © Damage is more severe when hair is weathered at high or low RH. This is consistent with the results reported on wool weathered under similar conditions (15). The wet tensile properties of irradiated fibers are least affected when they are exposed at a RH of 30%. © The free-radical mechanism for keratin photolysis proposed by Wei (7) has been used to explain the swelling and mechanical properties of irradiated fibers. © Damage to the cuticle is not significantly affected by humidity during irradiation. This concurs with the work reported by Hoting et al. (2). ACKNOWLEDGMENTS The authors thank the sponsor for this research, the Lawrence W. Gelb Foundation of Clairol Inc. They also thank Dr. Gall Eaton, Executive Director, TRI/Princeton, for allowing the use of the facilities of the institute. REFERENCES (1) E. Tolgyesi, Weathering of hair, Cosmet. Toiletr. 98, 29-33 (1983). (2) E. Hoting et al. Photochemical alterations in human hair. I. Artificial irradiation and investigation of hair proteins, J. Soc. Cosmet. Chem., 46, 85-99 (1995).

320 JOURNAL OF COSMETIC SCIENCE (3) S. Ramapandian, Photodegradation of Human Hair, M.S. thesis, University of Massachusetts, Dart- mouth, 1997. (4) C. R. Robbins and M. Bahl, Analysis of hair by electron spectroscopy for chemical analysis, J. Soc. Cosmet. Chem., 35,379-390 (1984). (5) J. B. Speakman and P. R. McMahon, The action of light on wool and related fibers, N. Z.J. Sci. Tech., 20 (1939). (6) L.J. Wolfram, in Hair Research, Status and Future Aspects, Organos, Montagna, and Sturtgen, Eds., 1981, pp. 479-500. (7) S. Wei, Efj•cts of Graying Process on Hair Properties, M. S. thesis, Philadelphia College of Textiles and Science, 1995. (8) F. Leroy, A. Deftandre, and J. C. Garson, Photoaging of Human Hair, 7th International Hair Science Symposium, Bad-Nevenahr, 1990. (9) C. Dubief, Experiments with hair photodegradation, Cosmet. Toiletr., 95, 107 (1992). (10) H. Zahn, J. Soc. Cosmet. Chem., 17, 687 (1966). (11) Instrument literature from Atlas Materials Testing Solutions, Chicago, Illinois. (12) Instrument literature from Mitutoyo Corporation, Japan, 1996. (13) Instrument literature from Diastron, UK, 1993. (14) P.S. Bhandare, Private communications, July 1997. (15) L. A. Holt and P.J. Waters, Factors affecting the degradation of wool by light--Wavelength, tem- perature, moisture content, Proc. 7th Int. Wool Text. Res. Conf, Tokyo, IV, 1 (1985). (16) C. R. Robbins, Chemical and Physical Behavior of Human Hair (Springer-Verlag, New York, 1988). (17) J. Arnaud, Int. J. Cosmet. Sci., 6, 71 (1984). (18) M. Feughelman and A. R. Haly, The physical properties of wool fibers at various regains, Textile Res. J., 32, 966 (1962). (19) C. Walling, Free Radicals in Solution (John Wiley & Sons, New York, 1957).