The change (disturbance) in any component of the environment which leads to its deterioration is termed as pollution of the environment. It is the undesirable change in the physical or biological components which adversely alters the environment.

Pollutants: The substances which are present in harmful concentration and is the agent who causes pollution is termed as the pollutant.

CLASSIFICATION OF POLLUTANTS:

On the basis of existence in nature 

a)       Quantitative Pollutants- The substances which are already present in the environment, but are termed as pollutants when their concentration (quantity) increases in the environment. eg. CO₂  is present in the environment in greater quantity than normal and is hence termed as a quantitative pollutant.

b)       Qualitative pollutant- The substances which are not normally present in the environment and are added by human beings and are pollutants by nature. Eg. insecticides, pesticides

On the basis of the form in which they persist

a)       Primary Pollutants- The substances which are directly emitted from the source and remain in that form are termed as primary pollutants eg, smoke, fumes, ash, dust, nitric oxide and sulphur dioxide

b)       Secondary pollutants- The substances which are formed by chemical reaction between the primary pollutants and constituents of the environment (i.e. those which are already present in the environment) .eg smog, ozone, sulphur trioxide, nitrogen dioxide.

On the basis of disposal  

a)       Bio-degradable pollutants- the pollutants which are decomposed by natural processes eg domestic (municipal) sewage.

b)       Non bio-degradable pollutants- The pollutants which don’t decompose naturally or decompose slowly eg DDT, aluminium cans.