J. Cosmet. Sci.J 55, 95-113 Qanuary/February 2004) Review of the current understanding of the effect of ultraviolet and visible radiation on hair structure and options for photoprotection VITTORIA SIGNORI, BASF Corporation, 1705 Route 46 West, Ledgewood, NJ 07852. Accepted for publication June 18, 2003. Synopsis This article describes the current understanding of the effect of ultraviolet (UV) radiation and visible light on the structure and integrity of human hair fibers furthermore, it discusses current and past approaches to the protection of hair from UV rays. Relevant literature is reviewed. INTRODUCTION Personal care products are formulated to deliver many benefits to the consumer. As the industry has become increasingly competitive, and the consumer has become more educated and at times rightfully skeptical of miracle-like claims printed on cosmetic products, the personal care scientific community has risen to the task of investigating the true cause of the effectiveness of some cosmetic treatments, and a great deal of under­ standing has been gained through these efforts. This knowledge is kept as a trade secret, patented, and delivered to the consumer in the form of innovative products, or shared openly in a scientific forum for the participating community to take part in the dis­ cussion. Either way, the whole personal care industry, some more enthusiastically than others, has embraced a more scrupulous way of looking at products and the scientific data in support of claims. In the hair care discussion, hair damage has long been recognized to be an issue of great concern to most consumers. Hair damage is a generally recognized term that encompasses a variety of attributes, among which are dryness, ease to breakage, split ends, a coarse feel, lack of manageability, and lack of luster. The following will give an overview of light and its interaction with organic compounds, solar simulators and their role in reproducing solar radiation, and the current under­ standing of the effect of ultraviolet (UV) radiation on the structure of human hair, as well as approaches aiming to limit these negative effects. 95
96 JOURNAL OF COSMETIC SCIENCE LIGHT, RADIATION, AND ENERGY Exposure to radiation is a cause of concern to the personal care chemist, and in particular the radiation that has a great amount of energy, like UV, is known to cause many undesirable effects. For example, dyes, fragrances and carbomer-type thickeners exhibit varying degrees of instability upon exposure to light. This article explores the potential for structural damage caused by UV radiation to human hair. Appendix I shows a number of basic equations that define the relationship of quantities and units of measures commonly used to describe light, radiation, and wavelength Appendix II is a summary table of more complex functions like irradiance, radiant power, and radiant energy. The following discussion will consider, light and its energy, chemical bonds and the energy required to break them, and the radiant energy coming from the sun that reaches the earth's surface and the spectral distribution of such energy. Table I shows the wavelength, frequency, and energy of different radiation. Table II shows the energy required to break chemical bonds (bond dissociation energy) it is evident that the energy associated with UV is sufficient to break chemical bonds within the organic substrates (proteins, keratin, hair) in addition, the radiation of such energy as UV is capable of inducing free radical reactions that would have a lower activation energy and be even more likely than bond cleavage. The generally accepted mechanism for photo-oxidation is well presented by Robbins (2). Figure 1 shows the actual values of the radiation power reaching the earth's surface at a specific point in time and geographical area. A large variation in the spectral power distribution is to be expected between times of the year, hours of the day, and the geographic location at which the measurements are taken (3 ). The unit of measure of radiant power received per unit surface area is irradiance (W m- 2 ). Ir radiance is often associated with a wavelength or wavelength range (W m - 2 nm - 1 ) for instance, an average 1.5 W m- 2 nm- 1 over a wavelength range of 200 nm would give a total 300 W m- 2 total irradiance. The summation of irradiance over a period of time is called irradiation energy (or radiant exposure) e.g., irradiance of 1. 5 W m- 2 over a period of five hours(= 18,000 s) is equal to 27,000 J m- 2 . A radiant exposure of 3-6 KWh m- 2 day- 1 (= 10.8 x 106 - 21.6 x 106 J m- 2 day- 1 ) is the average for the US during the months of July and August (6). In comparison, in Australia, the global solar exposure in August is between 13 and 22 x 106 J m- 2 day- 1 (6). The values reported above refer to the total solar radiation intensity, visible (VIS), and UV and infrared (IR) rays. Table III gives a breakdown of the ratio between VIS and UV radiation. As seen in Table III, the irradiance ratio between UV and VIS radiation is approximately 1 to 10. Generally speaking, when the measurement of the solar radiation is referred to as total} that means that it includes visible, ultraviolet, and infrared (VIS, UV, IR) rays when it is referred to as global} it means that the measurement was carried out in such a way as to capture the radiation from the sky, avoiding the radiation resulting from reflection on the ground. When the total radiation is considered, the relative proportions of UV, VIS, and IR, respectively, are 4-6%, up to 52%, and up to 42%.
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