Pollutant: When the concentration of a substance already present in nature or of a new substance increases to undesirable proportions causing danger to human beings, other animals or vegetation and other materials, the substance is treated as a pollutant.
The common pollutants are:
(i) Gases like carbon monoxide, sulphur dioxide, oxides of nitrogen etc.
(ii) Compounds of metals like lead, mercury zinc, cadmium, arsenic etc.
Contaminant: A substance which does not occur is nature but is introduced by activity into the atmosphere, affecting its composition is called contaminant. For example, pyrosulphuric acid (H2S2O7) leaked from a defective tank killed many persons and caused skin and breathing problems to many persons in Delhi.
