Skin pigmentation kinetics after UVB exposure.

There have been few previous studies of the kinetics of pigmentation following ultraviolet B (UVB) exposure, and these have included only fair-skinned persons. The current study investigated pigmentation increase to steady state and fading in 12 Scandinavians and 12 Indians/Pakistanis. Over a period of 3 weeks the subjects were UV-irradiated 6 times on the right side of the back and 12 times on the left side using a Solar Simulator and narrowband UVB with equal sub-Minimal Melanogenesis Doses (individually predetermined). Pigmentation was measured from skin remittance at 555 nm and 660 nm (allowing correction for erythema). The absolute pigmentation increase was independent of pre-exposure pigmentation, therefore the percentage pigmentation increase was higher in fair-skinned volunteers. The UV dose to minimal pigmentation was higher in darker-skinned persons for single and multiple UV exposures for both UV sources. Going from a single exposure to 6 and 12 exposures, the required dose to minimal pigmentation was reduced by factors of 2 and 3, respectively, for both UV sources, thus reducing the risk of sunburn, but the cumulative dose increased 3- and 4-fold, respectively. This result was independent of skin type and pre-exposure pigmentation. Fading took 4-5 months and was not related to frequency of UV exposure or to ethnic origin.