Sunscreens delay the induction of sunburn by absorbing or reflecting a portion of the UVR reaching the epidermis. Organic sunscreens are aromatic compounds that absorb light energy in the UV region, and therefore reduce the amount of UVR reaching the stratum cor-neum. A benzene ring has the ability to transform high energy UVR into harmless long wave radiation above the 380-nm range, which is emitted from the skin as heat (294). The conversion of UVR to longer wavelength occurs through resonance delocalization. Most chemical sunscreens absorb up to 95% of the UVB spectrum, but do not absorb in the UVA range. Avobenzone (22) is the only organic
sunscreen that provides a high degree of UVA protection. Benzophenones and anthranilates provide partial protection in the UVA range. Sunscreens are almost always used in combinations to broaden the absorbance spectrum and increase the SPF value of the product.
Little is known regarding the quantitative ability of sunscreens to prevent UVR-induced adverse effects other than sunburn. For example, there is no information on the threshold or dose-response for UVR-induced immunosuppression and DNA damage (237), but there is strong evidence that regular use of Sunscreens reduces the incidence of precancerous lesions. A large population study in Australia proved the effectiveness of sunscreens in reducing the incidence of solar keratosis (291). Solar keratosis is a known precursor for squa-mous cell carcinoma and an established risk factor for basal cell carcinoma and melanoma.
It has been suggested that sunscreens augment the defense mechanism against oxida-tive damage by UV-generated free radicals. This defense mechanism is mediated by thio-redoxin reductase in the human keratocytes, which reduces superoxide anion radicals through hydrogen peroxide to water (295). However, the oxygen radicals concentrations generated by UVA and UVB radiations, even below the minimal erythemal dose, are high enough to cause considerable deactivation of thioredoxin reductase. Sunscreens have been shown to protect the thioredoxin reductase against both UVA and UVB in human skin of types I and II. The same sunscreen, however,
failed to protect the enzyme in more pigmented skin (types III and IV).
8.2.4 Sun Protection Factor. Sunscreens were originally developed to prevent sunburn and minimize erythema (224). The efficacy of sunscreen products is rated by their sun protection factor (SPF), which is defined as the ratio of UV energy required to produce a minimal erythemal dose (MED) on protected skin to the UV energy required to produce MED on unprotected skin. It is calculated as the ratio cf time of W^ exposure necessary to produce minimal erythema in sunscreen protected skin to that time in unprotected skin. This SPF measures the effect of UVB only and does not account for the W A effect. Few methods have been proposed for a W A protection factor, including an in vivo pigment darkening method, and an in vitro critical wavelength test proposed to the FDA by the Cosmetic, Toiletry and Fragrance Association (CTFA). However, there is still no generally acceptable method for UVA testing, and the FDA has not yet reached a conclusion. It is also noteworthy that the SPF is not a measure of other photodamage protective properties of sunscreens protection against skin cancer and im-munologic protection). However, it is generally agreed that a higher SPF sunscreen will provide better protection against UVB-in-duced damage. There have been several attempts at determining the immune protection factor of sunscreens (293,296). All these studies seemed to indicate some immune protection of sunscreens, but with no correlation with the SPF. It seems that the extent of immune protection is much less than erythema protection, which suggests that the wavelengths that cause immune suppression are different from those that cause erythema (293).
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Complete Guide to Preventing Skin Cancer. We all know enough to fear the name, just as we do the words tumor and malignant. But apart from that, most of us know very little at all about cancer, especially skin cancer in itself. If I were to ask you to tell me about skin cancer right now, what would you say? Apart from the fact that its a cancer on the skin, that is.