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Phototherapy
An interaction between natural sunlight and the skin is inescapable. The potential for harm depends on the type and length of exposure. Sunlight can help in certain skin diseases (p. 61). Both ultraviolet (UV)B and UVA are extensively used therapeutically. UVA is usually used combined with photosensitizing psoralens given systemically or topically, although by itself it has some therapeutic effects. Photoageing is a growing problem (p. 130), because of an increasingly aged population and a rise in the average individual exposure to UV radiation.
The sun’s radiation spectrum
The sun’s emission of electromagnetic radiation ranges from low-wavelength ionizing cosmic, gamma and X-rays to the non-ionizing UV, visible and infrared higher wavelengths ( Fig. 62.1 ). The ozone layer absorbs UVC, but UVA and smaller amounts of UVB reach ground level. UV radiation is maximal in the middle of the day (11.00–15.00 hours) and is increased by reflection from snow, water and sand. UVA penetrates the epidermis to reach the dermis. UVB is mostly absorbed by the stratum corneum—only 10% reaches the dermis. Most window glass absorbs UV <320 nm in wavelength. Artificial UV sources emit in the UVB or UVA spectrum. Sunbeds largely emit UVA.
Effects of light on normal skin
Physiological
UVB promotes the synthesis of vitamin D3 from its precursors in the skin, and UVA and UVB stimulate immediate pigmentation (due to photooxidation of melanin precursors), melanogenesis and epidermal thickening as a protective measure against UV damage (p. 7).
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