Ultraviolet irradiation from the sun has deleterious effects on human skin, including sunburn, immune suppression, cancer and premature aging (phtoaging). Sunburn and immune suppression occur acutely in response to excessive exposure to the sun, whereas skin cancer and photoaging result form accumulated damage caused by repeated exposures. Skin cancer, the most prevalent form of cancer in humans, typically occurs in skin that is photodamaged. Photoaged skin is charaterized by wrinkles, uneven pigmentation, brown spots and a leathery appearance. In contrast, chronologically aged skin that has been protected from the sun is thin andhas reduced elasticity but is otherwise smooth and unblemished.