100 JOURNAL OF COSMETIC SCIENCE 5. There is no rule to predict what the acceleration factor is for one instrument (solar simulator or UV source) and one substrate in a specific application and testing protocol. There is no value in taking studies that report acceleration factors obtained on paints and applying them to plastics or textiles or other substrates (e.g., hair). In accelerated stability tests it is often tempting to do unrealistic extrapolations the researcher needs to keep in mind that if the test involves personal care formulations that are intended for hair care, for instance, the protocol should include reasonable cycles of application of the formulation and cleaning of the hair by shampoo. A good reference paper, although not developed for personal care products, is Crump's work on correlation study between different solar simulators and Florida sunlight as it applies to gel coatings (8). This type of paper will give the reader an example of how to set up a study to correlate an artificial radiation source to natural sun radiation. 6 The four points of developing a testing protocol can be summarized as follows: • Choice of the hair substrate (pigmented, unpigmented, untreated, chemically treated, etc.), • Choice of the type of formulations to be tested (shampoo, conditioner, hairspray, hair gel, etc.), • Choice of the irradiation procedure, time, and light intensity (light sources, total energy delivered over the time set for the experiment, frequency of re-application of the formulations to the hair, relative humidity, temperature, etc.) • Choice of analytical methods for the determination of the physical/chemical damage or changes occurring during exposure (tensile strength, amino acid analysis, IR spec­ troscopy, etc.). LITERATURE REVIEW EFFECT OF IRRADIATION ON THE STRUCTURE OF HUMAN HAIR This section is dedicated to an in-depth understanding of the effect of visible and UV radiation on human hair as reported by a variety of authors. It is clear that there is no standard protocol on how to expose human hair to light and/or UV radiation, and there is no standard test for assessing the damage that UV radiation causes. The findings reported in the following mentioned papers confirm that both UV and VIS radiation can have a very significant effect on hair. Among the literature available on this and related topics, the following papers were chosen because of the relevance of the work presented to the way photoprotection for human hair is often considered from the formulation point of view. The paper by Dubief (9) reported data obtained by exposing human hair to natural sunlight (with and without the UV portion of the radiation) as well as to artificial light no UV absorbers were tested in these experiments. This is one of the few papers that 6 For background information, the reader may find useful consulting two ASTM publications: G 15 5 and G151.

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