By far the most intense source of UV radiation in our region of space is the sun. The solar UV flux reaching our atmosphere is intense, and fortunately all but a small fraction is absorbed by earth's ozone layer or reflected back to space. The harmful effects of excessive UV exposure are well known; they include sunburn and cataracts, and increased risk of skin cancers. On the other hand, some UV is required for health. UV is necessary for the synthesis of vitamin D by the body, which is important for healthy bones and may also help in the treatment of some bone diseases. Many experts believe that managed doses of UV benefit skin conditions such as acne, psoriasis and eczema. Interestingly, recent scientific research suggests that UV may have helped induce the formation of important prebiotic molecules on the early earth.
UV reaches earth from deep space, although ordinarily the amount is insignificant compared to solar UV. This cosmic UV flux has been mapped by scientists in recent years. UV radiation is produced by many astrophysical objects and phenomena including stars, pulsars, quasars, starbursts, black holes and supernovae (exploding stars). In absolute terms, the quantities of UV produced in the universe at large are almost unimaginable. For example, quasars (primeval galaxies) radiate so strongly in UV that they are easily studied by astrophysicists at distances of several billion light years. Although exceedingly unlikely, if a supernova were to occur within a few light years of earth, the UV flux would probably destroy most of the life on the planet.
Lightning is a significant natural source of UV radiation. Lightning flashes are not only extremely bright in the visible portion of the electromagnetic spectrum, they also produce intense fluxes of infrared and ultraviolet radiation. Lightning flashes are enormous bursts of energy generated by the movement of air masses. At close range this energy can cause temporarily blindness and serious eye damage. Scientists estimate that, globally, lightning strikes the ground about 100 times each second, or around 8 million times each day. Incredibly, the number of cloud-to-cloud flashes is up to 10 times higher than that.