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Photosensitivity is the medical term used to describe an abnormal reaction of the skin to sunlight, especially to the sun’s ultraviolet (UV) rays. Photosensitive individuals often develop a skin rash triggered by exposure to ultraviolet radiation (UVR).

All skin is photosensitive to some degree. That is, it undergoes changes when exposed to UV light to protect against damage. When skin is exposed to sunlight or other sources of UVR (e.g., sun lamps, fluorescent lamps, tanning beds), the outer protective layer of skin (epidermis) darkens to block UV light from penetrating deeper into the tissue. This occurs because the pigment-producing skin cells (melanocytes) make increased amounts of melanin, the pigment that gives skin its color. The new melanin eventually travels to the surface of the skin, and the skin becomes darker in appearance (tans).