Heliox

A two-gas blend of helium and oxygen with no nitrogen, used mainly in commercial and military deep diving to eliminate nitrogen narcosis entirely.

Heliox is a breathing gas made of only helium and oxygen, with no nitrogen at all. With nitrogen removed, nitrogen narcosis is eliminated completely, which is why heliox has long been the standard gas of commercial and military deep diving, including saturation diving operations where divers live under pressure for weeks. Its diving history goes back to the 1930s, when the US Navy used it for deep salvage work.

In technical diving, heliox is less common than trimix, mostly on grounds of cost and decompression character: helium is expensive, and an all helium diluent changes tissue loading compared with a mix that retains some nitrogen. Some rebreather divers and deep projects still choose heliox for its clean narcotic profile and low gas density, which keeps work of breathing manageable at depth. Like any mix, a heliox blend is defined by its oxygen fraction, which sets its maximum operating depth, and hypoxic blends need a travel gas near the surface.

For most recreational and technical divers, heliox is a term met in theory classes rather than in their own cylinders. It still earns its place in the curriculum because it isolates the variables cleanly: comparing heliox with air and trimix is the easiest way to understand what nitrogen contributes to narcosis, what helium does to decompression and voice, and why oxygen fraction alone sets the depth ceiling of any mix.