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.
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- Hunger Jr, WL; Bennett, Peter B (1974). "The causes, mechanisms and prevention of the high pressure nervous syndrome". Undersea Biomedical Research 1 (1): 1–28. ISSN 0093-5387. OCLC 2068005. PMID 4619860. Retrieved 2008-06-24.
- Abraini, JH; Gardette-Chauffour, MC; Martinez,, E; Rostain, JC; Lemaire, C (1994). "Psychophysiological reactions in humans during an open sea dive to 500 m with a hydrogen-helium-oxygen mixture". Journal of Applied Physiology (American Physiological Society) 76 (3): 1113–8. OCLC 8750-7587/94. PMID 8005852. Retrieved 1 March 2009.