The main causes of pollution are :
1. Sewage :
The household sewage usually is taken to and fall in rivers, large tanks and lakes. Urine has urea which produces ammonia by hydrolysis. It is also produced by decay of other nitrogenous substances present in sewage. Thus water gets polluted giving rise to foul smell and becomes useless for drinking and bathing purposes because it may cause various skin diseases.
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2. Household detergents:
There are such chemicals which are used for washing clothes, clearing bathrooms, hospitals, etc. These include soaps, surf, nirma, fab etc. Because they are not completely oxidised, they produce CO2, alcohol and organic acids which pollute water and harm living organisms.
3. Pesticides :
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These chemicals are sprayed on fields, garden, sewage outlets to kill rats and insects. These chemicals are gases, liquids or solids. DOT is a white coloured substance used to kill insects, flies and worms. SO2 is a colourless gas used to kill insects. Formaldehyde, chlorine, carbolic acid, phenyl, potassium permagnate are other such substances. Lime is used for whitewash of buildings and sewage outlets to kill insects and worms, Bleaching powder (a mixture of chlorine and lime) is used to purify water reservoirs and wells.
These chemicals may become more harmful than useful because they kill many animals that feed upon plants and small animals upon which these chemicals were sprayed. Chemicals which are sprayed by helicopter to kill mosquitos etc. may cause death of many fishes and herbs. Also small insects, earthworms and fungi may be killed due to which fertility of soil is reduced. Pesticides may also cause genetic defects.
4. Weedicides :
2,4,-D; 2,4,5-T and other weedicides get mixed with the soil and cause soil pollution.
5. Smoke :
Smoke released from industrial plants, fuel and railway engines cause air pollution. Smoke mostly consists of CO2 and water vapour. Along with these carbon monoxide and other carbon containing compounds, nitrogen are also present which cause air pollution if their amount increases. These cause respiratory trouble and ill effects on eye.
6. Automobile exhausts :
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Jet planes, tractors, motor, buses and scooters etc in which petrol, diesel and kerosine is burnt produce different gases which are cause of air pollution.
7. Chemical discharge from industries :
Industrial effluents decreases oxygen in water. The amount of chlorides, nitrates and sulphates increases. By discharge of effluents into rivers most rivers of country become polluted and water becomes toxic to fishes and aquatic plants. Lead, Zinc and Iron compounds specially contribute to the pollution.
8. Decay and putrefaction of household wastes and dead bodies :
With death of animals, certain anaerobic bacteria putrefy the dead body as a result of which ammonia and H2S are produced giving foul smell. This causes air pollution.
9. Radioactive substance :
This topic has already been discussed in section of radioactive pollution.
10. Biopollutants :
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Some of the allergic diseases like asthma, common cold, eczema and other skin ailments are caused by biological organisms like fungal spores, pollen, bacteria. Some higher plants that are allergic are, kikar, mulberry, castor (Ricinus), carrot weed (Parthenium) and Chilbil (Holoptelea).