Posted: 26 September 2001

An unstable and chemically-reactive gas containing three oxygen atoms, formed at high altitudes by the action of sunlight on molecular oxygen. Present at low concentration in the stratosphere, ozone absorbs ultraviolet radiation from the sun and reduces the amount of this damaging radiation that reaches the Earth's surface. Ozone is also formed at ground level - by the interaction of sunlight with exhaust gases from automobiles and industry, and by the action of sunlight on nitrogen oxides and hydrocarbons-where it is a primary component of smog that aggravates breathing problems and damages plants.