A pollutant may be defined as ‘anything living or non-living or any physical agent (e.g., heat, sound, etc.), that in its excess makes any part of the environment undesirable for survival.
Polluted air is undesirable for breathing, for photosynthesis by plants; polluted water is undesirable for drinking and for all uses in day-to-day life; polluted soil and land are undesirable for growing food for humans and fodder for animals.
Excessive heat due to deliberate burning of forests as well as high pitched sound (noise), are also pollutants as they affect living organisms.
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Some substances which do not seem to be pollutants can also pollute, e.g., the fertilizers used in agriculture if used in large concentration can pollute lakes, rivers, ponds, etc.
Pollutants can be the wastes of solid, liquid, gas as well as energy. Some of the examples are –
Solid wastes:
Garbage, rubbish, ashes, animal wastes, agricultural wastes, mercury, etc.
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Liquid wastes:
Organic liquids, inorganic acids, alkalis, etc. Gaseous wastes: Carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, etc.
Energy wastes:
Heat, noise and radioactive wastes.
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Some pollutants are bio-degradable whereas others are not. Some of the biodegradable pollutants are wastes of plants or animals and are generally organic substances.
The non-biodegradable substances can be pesticides, heavy metals like lead, mercury and their salts, plastics, glasses, etc.
Depending upon the nature of pollutants (solid, liquid, gas, and energy) and the type of environment (air, water and land), pollution can be classified into six major categories: (1) Air pollution (2) Water pollution (3) Land pollution (4) Noise pollution (5) Thermal pollution (6) Radiation pollution.