Air is important for the survival of all kind of life forms on earth. We cannot see it, touch it but feel it. Air is invisible. It takes up space and puts pressure or pushes on everything. If it applies pressure on us, then why don’t we feel it? It is because the fluid pressure from inside the body balances the outside atmospheric pressure. Air has got weight. Earth’s gravity pulls the air towards the ground. That is why the air is densest near the ground and gets thinner and thinner as we go high up.
Dry air is a mixture of nitrogen, oxygen, carbon dioxide and many more gases. All the gases are composed of large number of molecules which are in constant, random motion. Humans and animals breathe in oxygen and breathe out carbon dioxide. Contamination of air by impurities is called air pollution and the impurities are called pollutants. Air pollution is a major cause for respiratory diseases. Major such diseases are asthma, bronchitis and lung cancer.
In order to breathe underwater, one must overcome outside pressure. When diving under water, we immediately feel pressure on our ears. This is because water above us puts lots of pressure on us that cannot be balanced by air pressure inside our ears. When a person scuba dives with compressed air, they take in extra oxygen and nitrogen so that they breathe air at the same pressure as the surrounding air. The body uses the oxygen, but the nitrogen is dissolved into the blood. During the dive it remains in blood.
As the diver swims back toward the surface after a deep dive the surrounding water pressure decreases. If this pressure change happens too quickly, nitrogen will not get sufficient time to clear from the blood. It gets separated from the blood and causes bubbles within tissues or blood. The nitrogen bubbles thus formed sometimes lead to decompression sickness.