acute cutaneous inflammation caused by overexposure to ultraviolet (ultraviolet radiation) (UV) radiation of the so-called UVB wavelength band (290–320 nanometre; a nanometre is 10^-9 metre), which originates from sunlight or artificial sources. Reactions to overexposure range in severity from mild redness and tenderness to intense pain, edema (swelling), and blistering; systemic symptoms include shock, chills, fever, and nausea. The visible manifestations of sunburn usually begin within 6–12 hours after the first ultraviolet exposure and peak within 24–28 hours, followed by a gradual easing of symptoms and light tanning or “peeling” (the sloughing off of the skin), depending on the severity of the burn.

Sunburn begins within 15 minutes after exposure to UV rays, triggering inflammation (the erythema, or redness). To limit epidermal damage, the pigment melanin (which is produced by epidermal cells called melanocytes) darkens through oxidation. Melanocytes increase in both size and number within two to three days, producing more melanin. Within days, a protective tan (in mild cases of sunburn) is formed.

Cold compresses applied to the affected skin and analgesic medicines can relieve some of the pain of sunburn. Severe sunburn can be prevented by limiting exposure to ultraviolet rays until a sufficient protective tan has developed or by applying a sunscreen that contains either para-aminobenzoic acid (PABA) or benzophenones. A long-term effect of prolonged and repeated exposure to sunlight can cause a number of skin disorders, including basal-cell carcinomas (small, smooth nodules) that usually appear on the face.