Most individuals are exposed to large amounts of ultraviolet (UV) radiation throughout their lifetimes due primarily to sunlight exposure. Sunlight includes two types of UV rays: long wave ultraviolet A (UVA) and short wave ultraviolet B (UVB), both of which can damage skin. UVA rays account for up to 95 percent of the UV radiation reaching the Earth's surface. Although they are less intense than UVB rays, UVA rays are 30 to 50 times more prevalent. They are present with relatively equal intensity during all daylight hours throughout the year, and can penetrate clouds and glass.
UVA rays penetrate the skin more deeply than UVB rays and have long been known to play a major part in skin aging and wrinkling (photo-aging), but until recently scientists believed that UVA rays did not cause significant damage to the epidermis (outermost skin layer) where most skin cancers occur. Studies over the past two decades, however, show that UVA radiation damages skin cells called keratinocytes in the basal layer of the epidermis.
Both UVA and UVB radiation contribute to skin damage that accelerates the appearance of aging, for example, loss of skin elasticity and the appearance of wrinkles. This process is commonly referred to as photo-aging. As skin ages, the outer skin layer (epidermis) thins, even though the number of cells remain largely unchanged. The number of pigment-containing cells (melanocytes), however, decreases. Therefore, the skin appears pale and translucent. Large pigmented spots (age spots, liver spots, or lentigos) may appear in sun-exposed areas. Changes in the connective tissue reduce the skin's strength and elasticity, which becomes more noticeable in sun-exposed areas (solar elastosis). Elastosis produces the leathery, weather-beaten appearance common to farmers, sailors, and others who spend a large amount of time outdoors.