Ozone is an oxygen molecule which consists of three oxygen atoms. It is sometimes referred to as trivalent oxygen. Breathable oxygen consists of two oxygen atoms; ozone is breathable by humans but doesn't support life in the same way as O2 oxygen. Ozone is an irritant to the eyes, throat and respiratory system, and prolonged exposure to high levels can result in rapidly accelerated aging and damage to many types of living tissue.

Most ground-level ozone occurs as a result of sunlight causing reactions between nitrogen oxides emitted from fossil fuel combustion and naturally-occurring atmospheric components. Ozone occurs naturally in the presence of lightning or fire. Ozone is an invisible gas, but concentrations of ground-level ozone decrease the transparency of the atmosphere. The smog that reduces visibility in urban areas is largely composed of ozone. Ozone serves a different purpose in the upper atmosphere. Concentrations of ozone in the stratosphere actually serve as a protective shield against harmful types of solar radiation. Reductions in stratospheric ozone may be even more harmful than concentrations of ground-level ozone.

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