<P> Breathing gases for use at ambient pressures below normal atmospheric pressure are usually air enriched with oxygen to provide sufficient oxygen to maintain life and consciousness, or to allow higher levels of exertion than would be possible using air . It is common to provide the additional oxygen as a pure gas added to the breathing air at inhalation, or though a life - support system . </P> <P> These common diving breathing gases are used: </P> <Ul> <Li> Air is a mixture of 21% oxygen, 78% nitrogen, and approximately 1% other trace gases, primarily argon; to simplify calculations this last 1% is usually treated as if it were nitrogen . Being cheap and simple to use, it is the most common diving gas . As its nitrogen component causes nitrogen narcosis, it is considered to have a safe depth limit of about 40 metres (130 feet) for most divers, although the maximum operating depth of air is 66.2 metres (218 feet). </Li> <Li> Pure oxygen is mainly used to speed the shallow decompression stops at the end of a military, commercial, or technical dive . Risk of acute oxygen toxicity increases rapidly at pressures greater than 6 metres sea water . It was much used in frogmen's rebreathers, and is still used by attack swimmers . </Li> <Li> Nitrox is a mixture of oxygen and air, and generally refers to mixtures which are more than 21% oxygen . It can be used as a tool to accelerate in - water decompression stops or to decrease the risk of decompression sickness and thus prolong a dive (a common misconception is that the diver can go deeper, this is not true owing to a shallower maximum operating depth than on conventional air). </Li> <Li> Trimix is a mixture of oxygen, nitrogen and helium and is often used at depth in technical diving and commercial diving instead of air to reduce nitrogen narcosis and to avoid the dangers of oxygen toxicity . </Li> <Li> Heliox is a mixture of oxygen and helium and is often used in the deep phase of a commercial deep dive to eliminate nitrogen narcosis . </Li> <Li> Heliair is a form of trimix that is easily blended from helium and air without using pure oxygen . It always has a 21: 79 ratio of oxygen to nitrogen; the balance of the mix is helium . </Li> <Li> Hydreliox is a mixture of oxygen, helium, and hydrogen and is used for dives below 130 metres in commercial diving . </Li> <Li> Hydrox, a gas mixture of hydrogen and oxygen, is used as a breathing gas in very deep diving . </Li> <Li> Neox (also called neonox) is a mixture of oxygen and neon sometimes employed in deep commercial diving . It is rarely used due to its cost . Also, DCS symptoms produced by neon ("neox bends") have a poor reputation, being widely reported to be more severe than those produced by an exactly equivalent dive - table and mix with helium . </Li> </Ul> <Li> Air is a mixture of 21% oxygen, 78% nitrogen, and approximately 1% other trace gases, primarily argon; to simplify calculations this last 1% is usually treated as if it were nitrogen . Being cheap and simple to use, it is the most common diving gas . As its nitrogen component causes nitrogen narcosis, it is considered to have a safe depth limit of about 40 metres (130 feet) for most divers, although the maximum operating depth of air is 66.2 metres (218 feet). </Li>

The inert gas which is substituted for nitrogen in the air used by deep sea divers for breathing is
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