Air to breathe : Air consist of gases and not some chemical compound. It is absolutely essential for the maintenance of the life. The two main functions of the air are: interchanging of gases in the process of respiration and regulation of body temperature. The pure air to breathe has approximately 20.95% oxygen; 0.03% carbon dioxide; 79.0% nitrogen; water vapors (varies with temperature); ammonia, ozone, argon and salts of mineral substances (variable in traces).
In the open air this composition remains remarkably constant owing to the diffusion of air currents due to the fact that the plants by virtue of their chlorophyll content, take up carbon dioxide from air and give off oxygen, thus compensating for the consumption of oxygen and the formation of carbon dioxide, which is always going on as a result of the existence of the animal life, combustion, etc. the process is however reversed at the night. The wind also helps producing a uniformity of composition of air and renders assistance in the removal of impurities by distributing them.
At certain temperature, air can hold only a definite amount of water vapor. The air is said to be saturated when it can hold no more amount of water vapors at a particular temperature and at that point, its humidity is said to be 100%. About 65% to 75% humidity is considered as conductive for health. The chief impurities in the air are due to respiration, combustion, decomposition of organic matter, dust and bacteria.