Any undesirable change in the physical, chemical or biological characteristics of our environment i.e. air, water and soil that may adversely affect humans or other organisms of our biosphere is called pollution.

The substances in solids, liquids or gaseous states are present in such concentrations which may be injurious to environment cause pollution. Such substances are called as pollutants.

The pollutants can be grouped into two broad categories.i.e. Biodegradable and Non- biodegradable.

Biodegradable Pollutants. Substances which break down to an acceptable form are called as biodegradable substances. There are two types of biodegradable pollutants.

(a)  Rapidly biodegradable substances e.g. Animal and crop wastes.

(b) Slowly biodegradable substances include fertilizers, agrochemicals etc. e.g. DDT, PCBS.

Non- biodegradable substances do not break down or change and so accumulate in the biosphere to dangerous levels. e.g. Heavy metals, pesticides, lead compounds, radioactive substances etc. Some of them combine with other compounds and thus produce toxic substances.