Diving gases which are used (in order of frequency in everyday use):

• Most scuba dives use conventional air as a gas mixture.

• However, some divers use nitrox, which is oxygen enriched air. The increased oxygen content (well, actually it is the reduced nitrogen content) allows divers to stay underwater for longer without decompression stops.

• Some advanced technical divers who go deeper use a mix of oxygen, nitrogen and helium called trimix. The helium in the mix reduces the effects of nitrogen narcosis, and the oxygen content is usually reduced to lower the risk of oxygen toxicity at great depths.

• If the dive is so deep that nitrogen cannot be present at all, the diver may simply use a helium-oxygen mix, usually called heliox.

• For very-very-very deep diving (usually very deep commercial or military operations), the diver can use a blend of hydrogen, helium and oxygen called hydreliox. The hydrogen is used to reduce the effect of high pressure nervous syndrome (HPNS) caused by breathing helium at great pressures.

• Also extremely rare, and used on occasionally in commercial diving is a mix of neon and oxygen called neox. Neox has the same benefits as heliox, but helium can affect the voice and so if divers need to communicate vocally underwater they may elect to use neox.

• There are two other "varieties" of trimix which have been given their own names over the years. Heliair is used to refer to any mix of helium and ordinary air. It is quicker and cheaper to blend than customised trimix, and so is often called "poor man's trimix". More common in the 70s and 80s, it is rarely used today. Conversely, some divers used trimix with hyperoxic contents (ie. more than 21% oxygen). This can be used in the narrow corridor of water between about 100 and 160 feet to offset nitrogen narcosis and also to marginally reduce decompression limits. Such mixes are referred to either as TriOx or as Helitrox, but are not in common usage.