Pure air consists of a mixture of gases in proper amount which is good for living being. If by volume, about 78% of this mixture is nitrogen, about 21% is oxygen, carbon dioxide, argon, methane, ozone and water vapour are also present in very small quantities, then it is called pure air.
When air is contaminated by unwanted substances which have a harmful effect on both the living and the non-living, it is referred to as polluted air.

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