A substance, gaseous material or
aerosol which has been introduced into the air (either by human activity or by natural processes)
in sufficient concentration to produce a measurable effect on humans, animals, vegetation
or materials (monuments, etc.):
SO2,
NO2,
H2S,
CO,
hydrocarbons, etc.
Source:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2172