Effects of breathing high pressure gas

Decompression sickness

The diver must avoid the formation of gas bubbles in the body, called decompression sickness or 'the bends', by releasing the water pressure on the body slowly at the end of the dive. This is done by making decompression stops and ascending slowly using dive computers or decompression tables for guidance. Decompression sickness must be treated promptly, typically in a recompression chamber. Administering a higher concentration of oxygen to a decompression sickness stricken diver on the surface is a good form of first aid for decompression sickness, although fatality or permanent disability may still occur.

Nitrogen narcosis

Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth. Being "narced" can impair judgement and make diving very dangerous. It occurs at any depth, but in most cases doesn't become noticeable until deeper depths; typically when breathing air at around 30m/100 ft. Jacques Cousteau famously described it as the "rapture of the deep".

information: Wikipedia