In addition to oxygen (O₂) and nitrogen (N₂), which account for about 99% of its composition, air may also contain air pollutants: particulate matter (PM), ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen oxides (NOx), lead (Pb), benzene... This is a complex mixture of compounds, of natural or anthropogenic origin, emitted directly by various sources of pollution or formed secondarily during chemical reactions occurring in the atmosphere. (R.A.)