Erythema induction by ultraviolet radiation points to a possible acquired defense mechanism in chronically sun-exposed human skin.

Abstract

It is generally accepted that a UVA-induced erythema is difficult to detect except in the most sensitive individuals. As UVA effects on human skin and skin cells have been shown to depend strongly on anatomical body sites, UVA I, UVA I + II and solar simulator radiations were compared in their ability to induce erythema and melanin pigmentation responses in individuals with skin types I-IV on both previously sun-exposed (arms, forearms, thighs) and nonexposed body sites (buttocks). Erythema induction by UVA I on previously nonexposed skin sites followed a dose response in all skin types which was contrary to the absence of erythema induction seen on previously sun-exposed sites. Melanin expression followed a dose and skin type response and was shown to be more enhanced in previously exposed skin and in skin types III and IV. In contrast, UVA I + II induced erythema on nonexposed skin areas and to a lesser extent on frequently sun-exposed skin. Melanin production by UVA I + II was similar to that seen with UVA I alone in individuals of skin types II and III. Solar simulator radiation was very efficient in erythema induction regardless of previous sun exposure of skin. We have found that contrary to the widespread opinion that UVA and in particular UVA I could not induce a significant erythema, this waveband is capable of measurable erythema induction on skin nonexposed to sunlight. The diminished erythema induction by UVA I on chronically sun-exposed skin suggests the possibility of a defense mechanism against UVA-induced damage in this tissue.