Hydreliox is an exotic breathing gas mixture of helium, oxygen and hydrogen. For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.
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 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. To avoid the risk of explosion, as a rule of thumb hydrogen is only considered for use in breathing mixtures if the proportion of oxygen in the mixture is less than 5%. However, the pressure during the dive must be such that the partial pressure of 5% oxygen is sufficient to sustain the diver. (The flammability of the mixture also depends to some degree on the pressure)
Hydreliox has been tested to simulated depths in excess of 700 metres (2,300 ft) 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 500 metres (1,600 ft).