Heliox

Heliox is a breathing gas mixture of helium and oxygen used as a medical treatment for patients with difficulty breathing, because the mixture creates less resistance than atmospheric air passing through the lungs' airways and so requires less effort to breathe. It has been used medically since the 1930s, initially to relieve symptoms of upper airway obstruction, with its range of medical uses since expanding, largely because of the gas's low density; it is also used as a breathing gas for deep ambient pressure diving because it is not narcotic at high pressure and has a low work of breathing.