318 JOURNAL OF COSMETIC SCIENCE Table III Characteristic Bands in IR Spectra of Hair Fiber Peak Number Molecular group Band (cm -•) 1 Amide-II 1513 2 CH 1449 3 Amide-III 1232 4 Cystine monoxide 1073 5 Cysteic acid 1042 Source: reference (2). 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 220 240 260 280 •00 •20 •40 •60 •80 400 420 Wavelength ( nm ) Figure 8. Absorption spectra of Piedmont hair. increase as the ambient moisture level increases during irradiation. However, the changes in mechanical properties observed in this study, as functions of relative humid- ity, are related to the diffusibility of the radicals. At low humidity levels, the radicals cannot diffuse and terminate. Therefore, the number of radicals increases, causing ex- tensive damage. Low-molecular-weight residues will diffuse out of these fibers when immersed in water. At intermediate RH levels the radicals can diffuse and terminate, thus limiting the number of radicals and the damage caused by them, as shown in Figures 4 and 5. The details associated with the chemical interactions of moisture with hair keratin may be the keys to understanding the role of moisture in hair photolysis. According to Feughelman and Haly (18) the mobility of bound water in keratin increases with increasing moisture content. Water molecules associate more with each other than with keratin as the moisture content increases. Thus, at high RH levels the amount of loosely bound water in hair is large. The loosely bound water may act as a transfer agent for the free radicals as well as residues, thereby actively participating in the photolysis reaction. The removal of degraded protein residues exposes new protein segments to photolysis that accentuates the damage to the fiber.

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).