Ozone, the triatomic form of oxygen (O3), is a gaseous atmospheric constituent. In the troposphere - at ground level - it is created both naturally and by photochemical reactions involving gases resulting from human activities (photochemical smog). In high concentrations, tropospheric ozone can be harmful to a wide range of living organisms. In the stratosphere, ozone is created by the interaction between solar ultraviolet (UV) radiation and molecular oxygen (O2). Stratospheric ozone plays a decisive role in protecting life on earth from ultraviolet radiation from the sun.