In the first step called initiation (2,3), a free radical is created by homolysis.
Homolysis can be brought about by heat or ultraviolet light, but also by radical initiators such as organic peroxides or azo compounds.
UV Light is used to create two free radicals from one diatomic species.
Many oxidation and reduction reactions in organic chemistry have free radical intermediates, for example the oxidation of aldehydes to carboxylic acids with chromic acid.
Auto-oxidation is a process responsible for deterioration of paints and food, as well as production of certain lab hazards such as diethyl ether peroxide.