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the presence in or introduction into the environment of a substance which has harmful or poisonous effects
It is of various types like
Air pollution
Water pollution
Noise pollution
Soil pollution

Air pollution
Air pollution is a mixture of solid particles and gases in the air. Car emissions, chemicals from factories, dust, pollen and mold spores may be suspended as particles. Ozone, a gas, is a major part of air pollution in cities. When ozone forms air pollution, it's also called smog. Some air pollutants are poisonous.