Short wavelength UV light exhibits more quantum properties than its visible and infrared counterparts.  Ultraviolet light is arbitrarily broken down into three bands, according to its anecdotal effects. UV-A is the least harmful and most commonly found type of UV light, because it has the least energy.  UV-A light is often called black light, and is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light - thus appearing to glow in the dark.  Most phototherapy and tanning booths use UV-A lamps. UV-B is typically the most destructive form of UV light, because it has enough energy to damage biological tissues, yet not quite enough to be completely absorbed by the atmosphere.  UV-B is known to cause skin cancer.  Since most of the extraterrestrial UV-B light is blocked by the atmosphere, a small change in the ozone layer could dramatically increase the danger of skin cancer. Short wavelength UV-C is almost completely absorbed in air within a few hundred meters.  When UV-C photons collide with oxygen atoms, the energy exchange causes the formation of ozone.  UV-C is almost never observed in nature, since it is absorbed so quickly.  Germicidal UV-C lamps are often used to purify air and water, because of their ability to kill bacteria.

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