UV adaptation: Pigmentation and protection against overexposure.

The skin is known to adapt to UV exposures, that is become less sensitive to sunburn. Reported decreases in sensitivity vary widely from well over 10-fold down to a negligible 10%. This appears to depend importantly on the UV irradiation spectrum to which the skin adapts and on the UV irradiation spectrum that is used to test the sensitivity. The sensitivity is conventionally and generally assessed by the reciprocal of the minimal erythema dose (MED): the UV dose causing a just perceptible reddening of the skin after 8-24 hours. However, MED is much too subtle for everyday life: people will not notice a minimal reddening and commonly consider themselves sunburnt at considerably higher UV doses causing an intense reddening. Levels of adaptation of a well-tanned skin may be substantially higher at these more intense levels of reddening than MED levels. This expectation is based on the fact that people with a constitutively coloured skin may show moderate differences in MED compared with fair-skinned people but far less steep increases in reddening with overexposures to solar-simulated radiation (SSR). UVA exposure is known to enhance pigmentation and may thus be important in protection against overexposure to SSR.