Seasonal variation of skin pigmentation.

We measured skin pigmentation by skin reflectance monthly from May 1992 to April 1993 in 36 healthy Caucasians. Pigmentation was measured at four UV-exposed sites at the forehead, the upper chest, the inside of the upper arm, and at the upper back. The pigmentation and UV sensitivity were simultaneously measured at UV-protected buttock skin. The results showed a considerable seasonal variation for skin pigmentation at the UV-exposed sites. Buttock skin had a pigmentation and UV sensitivity that varied only marginally. We recommend that measurements of genetically controlled skin pigmentation and constitutive UV sensitivity should be performed at UV-unexposed skin on the buttocks, except in persons that expose this site to artificial or natural sunlight.