Hondo, lying again
Photochemical smog was first described in the 1950s. Photochemical smog is a mixture of air pollutants which have been chemically altered into further noxious compounds by exposure to sunlight.
The main components of photochemical smog are nitrogen oxides, Volatile Organic Compounds (VOCs), tropospheric ozone, and PAN (peroxyacytyl nitrate). All of these substances are very chemically reactive and are irritating to humans and other living things. They tend to smell bad, degrade materials, cause respiratory problems and illnesses, and irritate the eyes and other mucous membranes.