Sunlight is subdivided into visible light, ranging from 400 nm (violet) to 700 nm (red); longer infrared, “above red” or >700 nm, also called heat; and shorter ultraviolet radiation (UVR), “below violet” or <400 nm. UVR is further subdivided into UV-A (320–400 nm), also called black (invisible) light; UV-B (290–320 nm), which is more skin-penetrating; and UV-C (<290 nm). UV-B constitutes <0.5% of sunlight reaching the earth's surface, but is responsible for most of the acute and chronic sun-related damage to normal skin.1 Most UVR is absorbed by stratospheric ozone.
UV-B has greater intensity in summer than in winter, at midday than in morning or late afternoon, in places closer to the equator, and at high altitudes. Sand, snow, concrete, and water can reflect up to 85% of sunlight, thus intensifying exposure.1
Exposure to solar radiation causes vasodilatation and increases the volume of blood in the dermis, resulting...
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