why is helium used in diving apparatus?

Helium is an inert gas and is not toxic. However breathing pure helium is dangerous because it excludes needed oxygen. Deep sea divers can remain at depth only for controlled periods and must come up slowly and even remain at specified depths to avoid the bends. When ordinary air is used for diving the oxygen in the air is used up by the body but the nitrogen builds up to a new equilibrium state at the higher pressure required at great depths. One danger that deep sea divers faced was that if they adjusted their air control valves improperly their suit might inflate preventing their hands from reaching the controls to correct the problem. They would then race all the way to the surface like a giant bubble. The excess nitrogen would then come out of tissues and form bubbles (like the fizz in a opened can of soda) in the blood causing the bends and great pain with possible death. The diver could be placed in a re-compression chamber and pressurized to squeeze the nitrogen bubbles back into the tissues for controlled release during gradual decompression. The expanding nitrogen bubbles in tissues during rapid decompression may also drive away necessary oxygen leading to bone necrosis (bone death). One solution would be to use pure oxygen rather than pure air. However pure oxygen can be toxic at great concentrations at depth. Therefore oxygen can be mixed with helium to reduce oxygen concentration while eliminating nitrogen. During controlled decompression the helium would also diffuse out of tissues and the lungs more easily than nitrogen avoiding the bends. Use of helium for relatively shallow scuba diving would likely permit longer diving times with less threat of the bends.