# Hydrox (breathing gas)

Hydrox, a gas mixture of hydrogen and oxygen, is occasionally used as an experimental breathing gas in very deep diving. It allows divers to descend several hundred metres. Hydrox has been used experimentally in surface supplied, saturation, and scuba diving, both on open circuit and with closed circuit rebreathers.
Precautions are necessary when using hydrox, since mixtures containing more than four percent of oxygen in hydrogen are explosive if ignited. Hydrogen is the lightest gas (one quarter the atomic mass of helium or one half the molecular mass of helium) but still has a slight narcotic potential and may cause hydrogen narcosis. Also like nitrogen, it appears to mitigate the symptoms of high pressure nervous syndrome (HPNS) on deep bounce dives, but reduces the density of the gas, unlike nitrogen.

## History

Although the first reported use of hydrogen seems to be Antoine Lavoisier (1743–1794) experimenting on guinea pigs, the actual first uses of this gas in diving are usually attributed to trials by the Swedish engineer, Arne Zetterström in 1945.
Zetterström showed that hydrogen was perfectly usable to great depths. Following a fault in using the surface equipment, he died during a demonstration dive. The study of hydrogen was not resumed until several years later by the United States Navy and by the Compagnie maritime d'expertises (Comex), initially during their Hydra I and Hydra II experiments, in 1968 and 1969. Comex subsequently developed procedures allowing dives between 500 and 700 m (1,640 and 2,297 ft) in depth, while breathing gas mixtures based on hydrogen, called hydrox (hydrogen-oxygen) or hydreliox (hydrogen-helium-oxygen).

### Memorial dives

In July 2012, after about a year of preparation and planning, members of the Swedish Historical Diving Society and the Royal Institute of Technology Diving Club, performed a series of hydrox dives in memory of Arne Zetterström, who was accidentally killed during the ascent from his record dive using hydrox in August 1945. The memorial dives were performed using the same breathing mixture of 96% hydrogen and 4% oxygen as was developed and tested by Zetterström in the 1940s. The dives were made to a depth of 40 metres (131 ft), just deep enough to be able to use the oxygen-lean gas mixture. Project Leader Ola Lindh commented that in order to repeat Zetterström's record the team would need to make a dive to 160 metres (525 ft), and even today a dive to that depth requires planning and equipment beyond the capabilities of most divers.

### Experimental rebreather dive

A 230 m hydrox dive in the Pearse Resurgence in New Zealand was made on 14 February 2023 by Richard Harris, using a Megalodon rebreather. This dive is estimated to be the 54th reported experimental hydrogen dive conducted in the last 80 years by military, commercial and technical divers, and the first reported hydrogen dive using a rebreather. Two Megalodon rebreathers connected at the bailout valve were used for the dive. One with trimix diluent (O$_2$, N$_2$, He), the other with hydreliox (O$_2$, H$_2$, He). It was also the first hydrogen diluent dive in a cave.

## Use

Hydrox may be used for combating high pressure nervous syndrome (HPNS), commonly occurring during very deep bounce dives. and as a low density breathing gas to minimise work of breathing at extreme depths.
The COMEX experimental series culminated in a simulated dive to 701 metres (2,300 ft), by Théo Mavrostomos on 20 November 1990 at Toulon, during the COMEX Hydra X decompression chamber experiments. This dive made him "the deepest diver in the world".

## Biochemical decompression

The United States Navy has evaluated the use of intestinal bacteria to speed decompression from hydrox diving.