Ultraviolet a protective sunscreens

Abstract

A wide range of topical preparations for protecting the skin against sun exposure is now available. In general, these preparations are cosmetically elegant, and they have been shown to be effective and to conform to standards laid down by various regulatory bodies. Development of these compounds has grown along with acceptance by the general public of potential long-term adverse effects of unprotected sun exposure. It must be stressed, however, that the assessment procedures now used for topical sunscreens essentially measure their protection against the ultraviolet B (UVB, 290–320 nm) content of natural sunlight. Topical sunscreens are usually assessed by means of the sun protection factor (SPF), which compares the minimal erythema dose (MED) of protected normal skin with the MED of unprotected normal skin. 1 The dose of UVB is much lower than the dose of UVA (320–400 nm) for the production of erythema in normal skin. When natural or artificial broad band sources are used to produce erythema, the UVB content will produce an effect before a sufficient dose of UVA can be delivered. If exposure times are prolonged enough to deliver sufficient UVA with these sources, erythema from the UVB content will mask any UVA effect; thus, for practical purposes, the current testing procedures that utilize erythema to develop an SPF provide information concerning protection against UVB. There are two factors that generally limit an individual's exposure to natural sunlight. The first is discomfort from heat, the infrared effect, which induces a person to seek shade. The second is the knowledge that long periods of unprotected exposure will lead a few hours later to a painful sunburn, due mainly to UVB. If the UVB effect of natural sunlight can be obviated and current high-potency topical sunscreens are very successful in this regard, then long exposures become more likely. With these long exposures, greater amounts of UVA are received than would otherwise be possible. UVA is certainly not innocuous. It has a crucial role in some photosensitive skin diseases, especially those produced by drugs and chemicals. 2,3 Studies have also shown the adverse effects of UVA in other situations; eg, this part of the spectrum can produce both epidermal 4 and dermal 5 effects in experimental animals.