Explain the differences between pure air and polluted air.

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.

https://www.tiwariacademy.com/ncert-solutions/class-8/science/chapter-18/