A photoinitiator is any chemical compound that decomposes into free radicals when exposed to light. Photoinitiators are found both in nature (in photochemical smog) and in industry (for example, in plastics production).
In nature, photoinitiators are present throughout the atmosphere. For instance, nitrogen dioxide is produced in large quantities by gasoline-burning internal combustion engines. NO2 in the troposphere gives smog its brown coloration and catalyzes production of toxic ground-level ozone. Molecular oxygen (O2) also serves as a photoinitiator in the stratosphere, breaking down into atomic oxygen and in order to form the ozone in the ozone layer.
In industry, photoinitiators are primarily used to promote polymerization reactions, notably in the production of polyethylene plastic. There are a handful of medical applications as well; for instance, benzoyl peroxide creams are commonly prescribed as acne medication.