The development and evaluation of UVA (320–400 nm) sunscreens is important because UVA sunlight can penetrate deep into human skin and cause severe internal damage, as well as erythema and photoaging. Human phototesting is usually used for determining the sunscreen protection factor (SPF), defined as the ability of a sunscreen to prevent a threshold erythema from solar radiation. The SPF definition is now considered as the tentative final FDA standard for measuring SPF. The SPF measured with this method, however, results primarily from UVB (280–320 nm) and not from UVA. Phototesting is unsuitable for the evaluation of UVA protection factors (PFA) because of the extremely long exposure time necessary to generate erythema from UVA radiation and the concerns of reciprocity. Recently, several in vivo and in vitro methods using animals have been proposed. However, none of the methods is capable of measuring the efficacy of UVA sunscreens in vivo on human skin.
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