dermatologic disorders
Overview of Effects of Sunlight
Ultraviolet (UV) radiation
The sun emits a wide range of electromagnetic radiation. Most of the dermatologic effects of sunlight are caused by UV radiation, which is divided into 3 bands: UVA (320 to 400 nm), UVB (280 to 320 nm), and UVC (100 to 280 nm). Because the atmosphere filters the radiation, only UVA and UVB reach the earth’s surface. The character and amount of sunburn-producing rays (primarily wavelengths < 320 nm) reaching the earth's surface vary greatly with the following factors: Atmospheric and surface conditions Latitude Season Time of day Altitude Ozone layer Exposure of skin to sunlight also depends on multiple lifestyle factors (eg, clothing, occupation, recreational activities). Sunburn-producing rays are filtered out by glass and to a great extent by heavy clouds, smoke, and smog; however, they may still pass through light clouds, fog, or 30 cm of clear water, potentially causing severe burns. Snow, sand, and water enhance exposure by reflecting the rays. Exposure is increased at low latitudes (nearer the equator), in the summer, and during midday (10 AM to 4 PM) because sunlight passes through the atmosphere more directly (ie, at less of an angle) in these settings. Exposure is also increased at high altitudes primarily because of a thinner atmosphere. Stratospheric ozone, which filters out UV radiation, especially shorter wavelengths, is depleted by man-made chlorofluorocarbons (eg, in refrigerants and aerosols). A decreased ozone layer increases the amount of UVA and UVB reaching the earth's surface. "Sun"-tanning lamps use artificial light that is more UVA than UVB. This UVA use is often advertised as a “safer” way to tan; however, many of the same long-term deleterious effects occur as with UVB exposure, including Photoaging and 