Pollution, has its origins in the Latin word polluere, means 'contamination of any feature of the environment'. Pollution has the following characteristics:

1. It is the addition of substance at a faster rate than the environment can accomodate. Certain pollutants like arsenic or mercury have natural levels in nature, but only if these levels exceed certain critical values can they consider to be pollutants.
2. Pollutants are not only chemicals, but also forms of energy like heat, sound (so noise pollution) and radioactive rays.
   
   

   These are pollutants.

   
   
3. To be a pollutant, a material has to be potentially harmful to life.
4. Pollution is the only result of human activities.

The layer of air which supports life extends about 8 km above the Earth's surface and is known as troposphere. The composition of air remains remarkably constant except there may be small localised variations. Air pollution has existed since humans first used fire. The problem becomes significant since the industrial revolution in the 19th century. Almost all air pollutants is the result of burning fossil fuels, either in the home, by industry or in internal combustion engine.

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