A highly reactive molecule, ozone (O₃) is composed of three linked oxygen atoms. Ozone is created by electric discharges such as lightning and as well as by high intensity light. It forms a colorless gas with a biting, acrid odor that is a major component of urban smog. A strong oxidizer, ozone in ambient air (that normally around us at ground level) burns eyes, degrades paint and rubber, kills vegetation, and has a number of other undesirable effects. Consequently, air pollution controls restrict actions that increase ozone levels in urban air. In the upper atmosphere, however, the situation is very different. Because it absorbs ultraviolet (UV) radiation very effectively, ozone in the stratosphere [that part of the atmosphere between about 9-30 mi (15-50 km) above Earth's surface] provides an irreplaceable protective shield. Without this layer, living organisms on Earth's surface would be subjected to...