EFFECT OF UV RADIATION ON HAIR STRUCTURE 101 systematically compares the results from solar simulator to sunlight, and for this reason it is a valuable point of reference. The findings refer to the effect of irradiation on hair structure: 1. Hair exposed to both sunlight and artificial light showed an increase in IR absorption bands characteristic of C = 0 (1720 cm- 1 ) and RSO 3 H (1041 cm- 1 ) bonds. This indicates a change in the chemical "composition" of the hair the hair is photo­ oxidized. 2. Hair exposed to natural sunlight showed a decrease in tensile strength and was more prone to alkaline attack (alkaline solubility). Visible sunlight (no UV) increased the alkaline solubility and more than doubled the cysteic acid content of the hair (com­ pared to unirradiated hair). The complete VIS + UV (sun) radiation further promoted an increase in alkaline solubility (almost quadrupling it from the unirradiated hair) and in cysteic acid content (more than quadrupling it with respect to the unirradiated hair). 3. The UV portion of the solar radiation promoted the changes in the fibers that lead to less protein being extracted (three months, 127,000 J cm - 2 ( = 1.27 x 109 J m - 2 )). This was interpreted as possible evidence of protein cross-linking. 4. The main factor in photobleaching brown hair appeared to be the amount of VIS light. Only after approximately 100,000 J cm - 2 ( = 109 J m - 2 ), the samples exposed to VIS + UV radiation showed a greater photobleaching effect than with just visible light. 5. When brown hair was exposed to up to 150,000 J cm- 2 (= 1.5 x 109 J m- 2 ) in the Xenon test at 88% RH, the fibers showed substantially more photobleaching than when they were exposed to the same radiation intensity in the Sun test at only 5 % RH. The photobleaching obtained when the hair was exposed to natural sunlight, for the same radiant exposure, was distinctively greater than that obtained by artificial irradiation. This study shows that there is some, but not perfect, correlation between the solar simulators used and the natural sun radiation. Some of the changes monitored, such as photobleaching, are more pronounced when the hair is exposed to natural light than to the solar simulators used. Relative humidity is shown to play a very important role in accelerating or increasing photobleaching. These findings are consistent with the work of other authors (as reported below). The magnitude of discrepancy between simulated light and natural light should be taken into account when interpreting data that was generated solely by sunlight simulators such studies will also be reviewed in the following paragraphs. A systematic study of the effect of UVB, UV A, VIS, and IR energies on human hair was carried out by Hoting et al. in a two-part paper (7, 10). The authors investigated the effect of radiation on both chemically untreated and chemically treated human hair of different pigmentation levels (light to dark brown). The equipment used was especially designed to reproduce the different portions of the solar radiation, and the hair was exposed for a prolonged period of time, 1008 hr, at 25°-48°C, 70-94% R.H. 1. VIS radiation produces more photobleaching that UV A or UVB radiation, UVB being the feast effective. In all cases, to different extents, permed, dyed, and par­ ticularly bleached hair is more prone to photobleaching than hair that was not cosmetically treated. 2. Dark hair is more protected against photobleaching than light brown hair. 3. UVB radiation produces the most apparent changes in the chemical composition of

102 JOURNAL OF COSMETIC SCIENCE the cuticle of both dark or light brown hair (photo-oxidation). A high concentration of melanin pigments (dark brown hair) protects the whole fiber, but not the cuticle, against photo-oxidation (melanin granules are present in the cortex of the fiber, not in the cuticle). 4. Evidence of cross-linking of keratin was found when the hair was exposed by either UV A or UVB radiation. Although the energy to which the samples were exposed was perhaps a little excessive (see Table IV), the results are consistent with the work of other authors, and this paper is particularly informative for differentiating the effect of UV A and UVB radiation on hair structure, which is a topic often discussed while choosing the appropriate UV absorber for photoprotection. A study by Ruetsch, Karnath, and Weigmann (11) also studied hair photodamage under conditions of prolonged exposure to UV radiation, and hair has been found to undergo substantial changes, both chemical and morphological. These changes were more pro­ nounced, as more humidity/moisture was available during exposure. The number of hours of exposure used in this study was up to 700 hr. Two artificial light sources were used in this study: the Accelerated Weathering Testerc (QUV), simulating UV radiation and the Atlas Weather-Ometerd (AW), simulating UV and VIS radiation. The humid­ ity, the temperature, and humidification cycles were controlled throughout the experi­ ments. This comprehensive study confirmed findings from other authors (no UV absorbers were tested in these experiments): 1. Both UVB and UV A radiation caused photodegradation of hair involving both proteins (keratin) and pigments (melanin). Photo-oxidation occurs at the cystine C-S bond to yield 1 mole of cysteic acid among the products of reaction, and the mechanism is thought to be free radical in nature ( 12). This mechanism is different from chemical oxidation that follows the S-S scission pathway yielding 2 moles of cysteic acid. 2. Photodegradation of hair proteins was more pronounced in unpigmented/blonde hair, the highest level of photodegradation occurring in the cuticular region, where cystine is present at its highest concentration. 3. Prolonged (700 hr) UV irradiation (290-400 nm, with QUV, 95% RH at 42°C) produced the thinning and the fusion of cuticular cells the proposed theory is that the outer proteic layers would photodegrade to smaller, lower-molecular-weight peptides, which would then diffuse into the hair structure when enough humidity would allow for the fiber's swelling. 4. In prolonged exposure to UV radiation (700 hr) and also to humidification cycles (but no water immersion or shampoo cycles) using the QUV simulator, the authors found that melanin was not degraded, the pigmented hair retained its color, and the c Simulates sunlight in the range of 290-400 nm. The irradiance at 340 nm is equal to 0.97 W m - 2 the total irradiance between 300 and 400 nm was equal to 5.06 mW cm- 2 . cl This instrument emitted UV as well as visible light the irradiance at 340 nm was kept constant at 0.3 Wm ~ 2 the irradiance between 300 and 400 nm was equal to 4.46 mW cm ~ 2 . The total irradiance of this source was 41.272 mW cm- 2 .