Skin Disorders Guide

Sunburn and photoinduced carcinogenesis occur as the result of irradiation in the ultraviolet B (UVB) (280- to 320-nm) range of light. Blockage of these wavelengths can be obtained through the use of sunscreens containing benzophenones, salicylates, p-aminobenzoic acid (PABA), esters of PABA, and cinnamates. Efficacy of these products is determined by measuring their sun protective factor (SPF). This factor is calculated as the ratio of minutes of sunlight necessary to produce minimal redness in sunscreen-protected skin versus that for unprotected skin. Thus if redness develops in 15 minutes when no sunscreen is used but appears only after 150 minutes following sunscreen use, that sunscreen is said to have an SPF of 10. Sunscreens with an SPF less than 15 offer only partial protection, those with an SPF above 15 are almost totally protective. Some tanning occurs with all of these products as a result of their relative inability to block ultraviolet A (UVA)-type light (320- to 400-nm range). An additional measure of sunscreen effectiveness is the degree to which it protects after heavy sweating or swimming. This property is called substantivity. Currently, there is no standard means of measuring substantivity, but it is known that for all sunscreens substantivity is improved when they are applied 1 hour or more prior to the onset of sweating or swimming. However, it is probably desirable to apply additional sunscreen about every hour when swimming or sweating heavily.The sunscreens discussed above are designed for the UVB spectrum and offer little protection for the longer wavelengths of the UVA spectrum. These longer wavelengths are responsible for some skin diseases such as porphyria cutanea tarda, solar urticaria, polymorphous light eruption, and some medication reactions. UVA irradiation, through the process known as photoaugmentation, may even play a role in skin aging and carcinogenesis. Only partial protection in the UVA spectrum can be obtained with the products described above. Even opaque materials such as zinc oxide or titanium dioxide (A-fil, RVPaque) are surprisingly ineffective at blocking the longer wavelengths of ultraviolet light (UVL).

Topically applied sunscreens are occasionally associated with the development of allergic contact dermatitis. When faced with a “sunburn” that was unexpected, given the patient’s use of an appropriate sunscreen, the clinician needs to consider the development of sunscreen allergy as a possible explanation. Recently, a question has been raised regarding sunscreen safety. Specifically, because the use of sunscreens so effectively prevents sunburn, people stay out much longer than before and receive very large doses of UVL outside the UVB spectrum. Speculation has arisen regarding the potentially adverse effects of this additional UVL irradiation, Nevertheless, at the practical level, only beneficial outcomes have been observed during the past 20 years of regular sunscreen use.