The formation of ozone (O₃) in the earth’s atmosphere. In the stratosphere, about 20–50 km above the surface of the earth, oxygen molecules (O₂) dissociate into their constituent atoms under the influence of ultraviolet radiation of short wavelength (below about 240 nm). These atoms combine with oxygen molecules to form ozone (see ozone layer). Ozone is also formed in the lower atmosphere (troposphere) from nitrogen oxides and other pollutants by photochemical reactions (see photochemical smog). Through its oxidative damage to leaves and their photosynthetic apparatus, tropospheric ozone is increasingly a cause of impaired plant growth and reduced crop yields. It also poses a threat to human health by irritating the respiratory tract and triggering or exacerbating diseases such as asthma and lung diseases.