Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. Although long-wavelength ultraviolet radiation is not energetic enough to cause ionization, it can cause chemical reactions and causes many substances to glow or fluoresce. Fluorescent minerals and some synthetic dyes absorb ultraviolet light and re-emit it at visible wavelengths; this property is exploited in mineral prospecting and forensic science.
Most man-made sources of UV rays are artificial lights used in industry, medicine, recreation (such as tanning beds or blacklights), or consumer products (such as bug zappers). These devices emit large amounts of low-frequency UV rays below 200 nm that can damage DNA if exposed for long periods of time; such exposure increases the risk of skin cancer. High-frequency artificial UV rays between 200–400 nm are used in germicidal lamps to sterilize surfaces and equipment by destroying the nucleic acids in microorganisms on them; this type of lamp emits little visible light so as not to alert people that they are being exposed to potentially harmful levels of UV radiation.
The Sun emits all types of ultraviolet rays: UVA (~315–400 nm), near ultraviolet or NUV (~200–315 nm), far ultraviolet or FUV (~100–200 nm), extreme ultraviolet or EUV (~10–100 nm). The amount reaching Earth’s surface—and thus the amount humans are exposed to—varies widely depending on factors such as latitude (the lower the latitude, the higher the sun intensity during summer months), time of day (direct sunlight vs. indirect sunlight), season (summer vs. winter), cloud cover, smog, elevation above sea level (high altitudes have less atmospheric filtering of UV rays), sunscreen use habits/amounts applied properly/incorrectly etc..
Exposure to solar UV rays can cause immediate skin damage such as sunburn; it also contributes to long-term risks such as premature skin aging and an increased risk for developing skin cancer including melanoma later in life. However, moderate exposure to UV rays can also result in health benefits such as vitamin D production which helps maintain bones strong by aiding calcium absorption; vitamin D has also been linked with reducing risks for certain cancers(e.g., colon) along with a host other potential benefits like lowering blood pressure & improving cognitive function . Some researchers believe there may be an “ideal” range of sun exposure that would provide optimal benefit without excessive harm though more study is needed on this topic before any definitive recommendations could be made – too much sun exposure obviously increase risks for skin cancer while too little may limit potential benefits like vitamin D production .