Sunlight is the most visible and obvious source of comfort in the environment. This natural proclivity for the sun has the beneficial results of warmth and vitamin D synthesis but also can produce pathologic consequences. Few effects of sun exposure beyond those affecting the skin have been identified, but cutaneous exposure to sunlight can evoke immunosuppressive responses and genetic changes that may be relevant to the pathogenesis of nonmelanoma skin cancer and perhaps infections such as herpes simplex.
The UV spectrum is arbitrarily divided into three major segments: C, B, and A. This includes the wavelengths between 10 and 400 nm. Ultraviolet C (UV-C) consists of wavelengths between 10 and 290 nm and does not reach the earth because of its absorption by stratospheric ozone. These wavelengths are not a cause of photosensitivity except in occupational settings where artificial sources of this energy are employed¾e.g., for germicidal effects.
The energy possessed by photons in the visible spectrum is not capable of damaging human skin in the absence of a photosensitizing chemical. The absorption of energy is critical to the development of photosensitivity. Thus the absorption spectrum of a molecule is defined as the range of wavelengths absorbed by it, whereas the action spectrum for an effect of incident radiation is defined as the range of wavelengths that evoke the response.
Photosensitivity occurs when a photon-absorbing chemical (chromophore) present in the skin absorbs incident energy, becomes excited, and transfers the absorbed energy to various structures or to oxygen. The absorbed energy must be dissipated by processes including heat, fluorescence, and phosphorescence. It is important to emphasize that absorption spectra and action spectra need not be superimposable, but there must be overlap at some point to produce photosensitization.
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