Air.

Air
Air is a kind of a gas. It is rather a mixture of gases.
Air consists of around 78% nitrogen, 21% oxygen and other gases.
Air is colorless odorless tasteless gaseous mixture that surrounds the atmosphere of the earth,
on account of which existence of life on earth is possible. Air has weight. The air reaches up so
high in the sky that its weight puts a pressure of around 7 Kg. on every square inch of the earth.
Secondly the oxygen in the air makes it possible for things to burn.
Air is also an inexhaustible resource in the sense that the uses made of it have little effect on its total quantity.
The quality of air, however, as measured by its chemical composition or physical state, is subject to human interference.
For life to exist on Earth there must be a proper balance among the nitrogen, oxygen, carbon dioxide, water vapor, and other components of the atmosphere.
A layer of the gas ozone, for example, must be maintained in the upper atmosphere to screen out damaging ultraviolet light from the Sun.
The accumulation of toxic materials in the air must be kept to a minimum, and the concentration of solid and liquid particles in the atmosphere must not be allowed to reach a level that interferes with the flux of solar radiation.
All of these factors are affected by human activity and by the effects of this activity on other forms of life.