Chapter 057. Photosensitivity and Other Reactions to Light (Part 1)

Harrisons Internal Medicine Chapter 57. Photosensitivity and Other Reactions to LightSolar RadiationSunlight is the most visible and obvious source of comfort in the environment. The sun provides the beneficial effects of warmth and vitamin D synthesis; however, acute and chronic sun exposure also have pathologic consequences.Few effects of sun exposure beyond those affecting the skin have been identified, but cutaneous exposure to sunlight is the major cause of human skin cancer and can exert immunosuppressive effects as well.The suns energy reaching the earths surface is limited to components of the ultraviolet (UV), the visible, and portions of the...
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Chapter 057. Photosensitivity and Other Reactions to Light (Part 1) Chapter 057. Photosensitivity and Other Reactions to Light (Part 1) Harrisons Internal Medicine > Chapter 57. Photosensitivity and OtherReactions to Light Solar Radiation Sunlight is the most visible and obvious source of comfort in theenvironment. The sun provides the beneficial effects of warmth and vitamin Dsynthesis; however, acute and chronic sun exposure also have pathologicconsequences. Few effects of sun exposure beyond those affecting the skin have beenidentified, but cutaneous exposure to sunlight is the major cause of human skincancer and can exert immunosuppressive effects as well. The suns energy reaching the earths surface is limited to components ofthe ultraviolet (UV), the visible, and portions of the infrared spectra. The cutoff atthe short end of the UV is at ~290 nm; this is due primarily to stratospheric ozoneformed by highly energetic ionizing radiation, thereby preventing penetration tothe earths surface of the shorter, more energetic, potentially more harmfulwavelengths of solar radiation. Indeed, concern about destruction of the ozone layer bychlorofluorocarbons released into the atmosphere has led to internationalagreements to reduce production of these chemicals. Measurements of solar flux indicate that there is a twentyfold regionalvariation in the amount of energy at 300 nm that reaches the earths surface. Thisvariability relates to seasonal effects; the path of sunlight transmission throughozone and air; the altitude (4% increase for each 300 m of elevation); the latitude(increasing intensity with decreasing latitude); and the amount of cloud cover, fog,and pollution. The major components of the photobiologic action spectrum capable ofaffecting human skin include the UV and visible wavelengths between 290 and700 nm. In addition, the wavelengths beyond 700 nm in the infrared spectrumprimarily emit heat and under certain circumstances may exacerbate the pathologiceffects of energy in the UV and visible spectra. The UV spectrum reaching the earth represents Photosensitivity occurs when a photon-absorbing chemical (chromophore)present in the skin absorbs incident energy, becomes excited, and transfers theabsorbed energy to various structures or to oxygen. UV Radiation (UVR) and Skin Structure and Function Skin consists of two major compartments: the outer epidermis, a stratifiedsquamous epithelium, and the underlying dermis rich in matrix proteins such ascollagen and elastin. Both of these compartments are susceptible to damage fromsun exposure. The epidermis and the dermis contain several chromophores capable ofabsorbing incident solar energy including nucleic acids, proteins, and lipids. Theoutermost epidermal layer, the stratum corneum, is a major absorber of UV-B, and In contrast, UV-A readily penetrates to the dermis and is capable of alteringstructural and matrix proteins that contribute to photoaging of chronically sun-exposed skin, particularly in individuals of light complexion.