What is "hydreliox"

Hydreliox is an exotic breathing gas mixture of helium, oxygen and hydrogen.

It is used primarily for research and scientific deep diving, usually below . Below this depth, extended breathing of heliox gas mixtures may cause high pressure nervous syndrome (HPNS). Two gas mixtures exist that attempt to combat this problem: trimix and hydreliox. Like trimix, hydreliox contains helium and oxygen and a third gas to counteract HPNS. The third gas in trimix is nitrogen and the third gas in hydreliox is hydrogen. Because hydrogen is the lightest element, it is easier to breathe than nitrogen under high pressure.

Hydreliox has been tested to simulated depths in excess of in a chamber by COMEX S.A., a French diving company. Although breathing hydreliox improves the symptoms seen in HPNS, tests have shown that hydrogen narcosis becomes a factor at depths of .