Hydreliox is an exotic breathing gas mixture of helium, oxygen and hydrogen.
It is used primarily for research and scientific deep diving, usually below 130 metres (430 ft). 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 prevent 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 depths in excess of 500 metres (1,600 ft) by COMEX S.A., a French diving company. Unfortunately, although it was initially believed that breathing hydreliox would prevent the sort of symptoms seen in nitrogen narcosis, the COMEX tests showed that hydrogen narcosis became a factor at depths deeper than 300 metres.