Any undesirable solid, liquid or gaseous matter in a solid, liquid or gaseous environmental medium.

Note 1: ‘Undesirability’ is often concentration-dependent, low concentrations of most substances being tolerable or even essential in many cases.

Note 2: A primary pollutant is one emitted into the atmosphere, water, sediments or soil from an identifiable source.

Note 3: A secondary pollutant is a pollutant formed by chemical reaction in the atmosphere, water, sediments, or soil.