The acute effects of long-wave ultraviolet radiation on human skin.

The erythematogenic and melanogenic properties of polychromatic long-wave ultraviolet light (UV-A) has been re-examined. Redness appeared immediately after exposure and persisted for 24 hr with doses of about 50 Joules/cm2. With threshold erythemal doses, about 13 J/cm2, the redness faded after a few minutes. The response was not biphasic. Pigmentation also appeared immediately after exposure and faded rapidly with threshold doses of 4 J/cm2. With larger doses (18 J/cm2) immediate pigmentation gave way without fading to delayed pigmentation (true melanogenesis). Thus, the acute effects of UV-A, unlike other wavelengths within the UV-spectrum, are immediate and appear without latency. The responses are also most intense immediately after exposure.