EFFECT OF UV RADIATION ON HAIR STRUCTURE 109 SUMMARY AND CONCLUSIONS From the discussion presented it is clear that there is no real doubt as to the undesirable effect of visible and/or UV radiation on hair structure and physical integrity. Surpris­ ingly, there is very little data to show the correlation between natural radiation and artificial radiation. In any case, a number of findings are consistent among all reports. The following is a summary of the commonly shared results: 1. For the most part, photobleaching, the attack to the melanin granules, is caused by visible light the UV portion of the radiation promotes/accelerates it. UV A radia­ tion is more effective than UVB radiation in inducing photobleaching. Humidity and moisture contribute significantly, enhancing this phenomenon. 2. Photo-oxidation of the hair fiber follows a different mechanistic pathway than chemical oxidation water is a crucial medium in which free radicals diffuse, thereby enhancing the chemical reactions taking place during photodegradation. 3. Damage to protein and lipids in the cuticle of the hair fiber are caused by UVA and UVB irradiation, but only marginally by VIS rays. 4. It was shown that irradiation of hair with UVB light induces damage to the Sl003A disulphide bonds that link this protein to the rest of the hair matrix, resulting in the ability to elute the protein from the hair fiber and indicating structural damage produced by irradiation. 5. Both VIS and UV radiation can, independently, promote the conversion of cystine to cysteic acid (photo-oxidation) UV radiation is more effective at promoting this degradation. 6. The amino acid tryptophan can be used as an early indicator of photodegradation of human hair. 7. Unpigmented (gray or Piedmont) and lightly pigmented (light brown) hair fibers are substantially more prone to photo-induced damage than pigmented fibers (dark brown and black). Melanin does not protect the cuticle layer (which is free from melanin granules) against the damage induced by UV radiation, but it does protect the whole fiber (cortex, where the melanin is present). 8. Eumelanins are less prone to be degraded by irradiation than pheomelanins on the contrary, eumelanins are more prone to chemical oxidation (bleaching) than phe­ omelanin. 9. UV radiation has clear effects on the physical measurements correlated to the strength and integrity of the hair fiber: it decreases the stress-to-break, the Young's modulus (fiber strength), and dynamic contact angle (hydrophobicity), and it in­ creases the wet-combing force (coarser, difficult to manage hair), copper up­ take (negative sites in the fiber), and the transverse swelling of the hair fiber (index of the level of unaffected cross-linking of the proteic matrix constituent, the hair cortex). 10. Prolonged exposure to UV radiation can cause dramatic changes to the physical properties of human hair in extreme cases irradiation can cause the whole cuticular layer to disintegrate, exposing the cortex. The studies that report on the effectiveness of UV absorbers in reducing the damage present a rather more complex picture the underlying problem that every author was faced with was the one of choosing an appropriate formulation in which to incorporate the UV absorber. Even more challenging was the choice of one or more formulations that

110 JOURNAL OF COSMETIC SCIENCE would be suitable to compare different UV absorbers (some are hydrophobic, some are water-soluble, some are oily substances, some are powders!). While a number of authors made a considerable effort in the comparison of two or more UV filters, there was no straightforward solution to the problem: a system that would work with some UV absorbers would not work for others, making the comparison difficult. It is clear, though, that a number of UV absorbers were shown to "protect" the hair fiber from physical degradation as well as from color degradation. UV absorbers that were specifi­ cally developed for hair care applications (cationically modified), as well as vehicles that would impart affinity to the hair to oil-soluble sunscreens, showed effectiveness and ease of application. A different approach to photoprotection is offered by the evidence that shows that artificial hair color (permanent or not) protects the hair's mechanical integrity from the damaging effect of light (VIS or UV). Another note of considerable importance is the possible, and in many cases probable, lack of correlation between the radiation emitted by the sun and that emitted by the solar simulator. More importantly, it is known that even small differences, at very high energy frequencies (UV), can result in big differences in results when comparing the effect of natural light exposure to that of artificial light. A comparison between the irradiance or irradiation energy used by the different authors whose articles are reviewed is shown in Table IV. The values were converted into SI units and, where it was not reported by the authors, the irradiance was transformed into irradiation energy and vice versa, thus simplifying the task of comparing different exposure conditions. It is important to note that although it may appear that some of the exposure conditions for solar simulators are similar to the irradiation energy coming from the sun for a period of a few days or a few months, this table does not mean to imply that equal radiant energy of exposure is a guarantee of realistic results (i.e., in vitro = in vivo). Since it is useful to present some hypothetical cases of exposure to sunlight, the follow­ ing cases are rough approximations of possible exposures and are all the result of calculation and not actual measurements: 1. Spending all day outdoors in a sunny week of summer: between 70 x 106 and 140 x 106 J m- 2 week- 1 (UV-VIS-IR) and 6 x 106 J m- 2 week- 1 (UV only). 2. Spending half a day outdoors: approximately 60 x 106 J m- 2 week- 1 (UV-VIS-IR) and 3 x 106 J m- 2 week- 1 (UV only). 3. Spending only a few hours per day outdoors, probably in the later afternoon and evening: approximately 20 x 106 J m- 2 week- 1 (UV-VIS-IR) and 1.15 x 106 J m- 2 week- 1 (UV only). Most of the authors used reasonable exposure conditions, although the only real way to know would have been to carry out a parallel test using natural sunlight. Most of the irradiation energies used in the studies reviewed were in the same order of magnitude as the exposure for a whole year (case 1 calculated above). For future experiments involving hair photoprotection, it would be advisable to carefully balance the steps in the testing protocol to reflect a reasonable amount of UV-VIS radiation and moisture and to allow for the hair to be washed with shampoo and perhaps treated with a rinse-off conditioner, in order to achieve a higher degree of confidence for the correlation of laboratory versus real-life conditions. Despite the many differences in approach that the different authors chose to adopt, there is an overall agreement on what the most important factors are in hair photodamage.