SUNLIGHT AND THE SKIN

SUNLIGHT AND THE SKIN

SUNLIGHT AND THE SKIN

Of the energy reaching the earth's surface, 99% is ultraviolet through to middle infrared. The shorter wavelengths of ultraviolet are filtered out by ozone in the stratosphereand by the atmosphere. The response of skin to solar radiationdepends on the wavelength and on natural defences,the most important being melanin deposition. UltravioletB (UVB) is responsible for acute sunburn and the chronicchanges of sun exposure: atrophy, dryness, blotchy pigmentationand wrinkling, solar keratoses and many skincancers. The consequences of failure to repair damage inflicted on cellular DNA by UVB are seen in the rare autosomal recessively inherited condition xerodermapigmentosum, in which sun-exposed skin becomes prematurely aged and skin cancers can appear in childhood.UVA has a much less obvious effect on normal skin, but isoften responsible for photosensitive drug reactions andpolymorphic light eruption. A clinical clue that UVA isplaying a part is the provocation of a sun-related skin disorderbehind window glass, as this filters out UVB. Theaction spectrum, i.e. the determination of exactly whichwavelengths are responsible for a photosensitivity disorder,is known for only a few diseases. Visible light does notproduce reactions on normal skin, but is responsible fo rphotosensitivity in some diseases, notably the porphyrias,where the maximal reactivity is around 400 nm.Although most of the consequences of solar radiation onthe skin appear harmful, one benefit is the synthesis ofvitamin D3. UV radiation is essential to break the steroidB ring and convert 7-dihydrocholesterol to previtamin D3,which then isomerizes spontaneously to vitamin D3.The immune system in the skin is modified by UV exposure,which may be important in the development of skincancer.

Polymorphic light eruption Polymorphic light eruption is a common disorder thatoccurs mainly in females. Usually in spring, a day or soafter sun exposure, itchy red papules, plaques and sometimesvesicles appear on sun-exposed sites. The conditiongradually settles with continued sun exposure.

Drug photosensitivity It is usually not known whether allergic or toxic mechanismsare involved in drug photosensitivity.

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