Breathing gas

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural, breathing gas - but a range of pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, submarines, space suits, spacecraft, medical life support and first aid equipment, high-altitude mountaineering and anaesthetic machines.[1][2][3]

Oxygen is the essential component for any breathing gas, at a partial pressure of between roughly 0.16 and 1.60 bar at the ambient pressure. The oxygen is usually the only metabolically active component unless the gas is an anaesthetic mixture. Some of the oxygen in the breathing gas is consumed by the metabolic processes, and the inert components are unchanged, and serve mainly to dilute the oxygen to an appropriate concentration, and are therefore also known as diluent gases. Most breathing gases therefore are a mixture of oxygen and one or more inert gases.[1][3] Other breathing gases have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving.[1][3]

A safe breathing gas for hyperbaric use has three essential features:

The techniques used to fill diving cylinders with gases other than air are called gas blending.[4][5]

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.

Common diving breathing gases

These common diving breathing gases are used:

Individual component gases

NEDU gas analysis lab


For the main article on the medical use of oxygen, see oxygen therapy.

Oxygen (O2) must be present in every breathing gas.[1][2][3] This is because it is essential to the human body's metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness and death result. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.

Filling a diving cylinder with pure oxygen costs around five times more than filling it with compressed air. As oxygen supports combustion and causes rust in diving cylinders, it should be handled with caution when gas blending.[4][5]

Oxygen has historically been obtained by fractional distillation of liquid air, but is increasingly obtained by non-cryogenic technologies such as pressure swing adsorption (PSA) and vacuum-pressure swing adsorption (VPSA) technologies.[16]

Fraction of oxygen

The fraction of the oxygen component of a breathing gas mixture is sometimes used when naming the mix:

The fraction of the oxygen determines the greatest depth at which the mixture can safely be used to avoid oxygen toxicity. This depth is called the maximum operating depth.[1][3][6][9]

Partial pressure of oxygen

The concentration of oxygen in a gas mix depends on the fraction and the pressure of the mixture. It is expressed by the partial pressure of oxygen (ppO2).[1][3][6][9]

The partial pressure of any component gas in a mixture is calculated as:

partial pressure = total absolute pressure × volume fraction of gas component

For the oxygen component,

ppO2 = P × FO2
ppO2 = partial pressure of oxygen
P = total pressure
FO2 = volume fraction of oxygen content

The minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16 kPa (0.16 bar). Below this partial pressure the diver may be at risk of unconsciousness and death due to hypoxia, depending on factors including individual physiology and level of exertion. When a hypoxic mix is breathed in shallow water it may not have a high enough ppO2 to keep the diver conscious. For this reason normoxic or hyperoxic "travel gases" are used at medium depth between the "bottom" and "decompression" phases of the dive.

The maximum safe ppO2 in a breathing gas depends on exposure time, the level of exercise and the security of the breathing equipment being used. It is typically between 100 kPa (1 bar) and 160 kPa (1.6 bar); for dives of less than three hours it is commonly considered to be 140 kPa (1.4 bar), although the U.S. Navy has been known to authorize dives with a ppO2 of as much as 180 kPa (1.8 bar).[1][2][3][6][9] At high ppO2 or longer exposures, the diver risks oxygen toxicity which may result in a seizure.[1][2] Each breathing gas has a maximum operating depth that is determined by its oxygen content.[1][2][3][6][9] For therapeutic recompression and hyperbaric oxygen therapy partial pressures of 2.8 bar are commonly used in the chamber, but there is no risk of drowning if the occupant loses consciousness.[2]

Oxygen analysers are used to measure the ppO2 in the gas mix.[4]


"Divox" is oxygen. In the Netherlands, pure oxygen for breathing purposes is regarded as medicinal as opposed to industrial oxygen, such as that used in welding, and is only available on medical prescription. The diving industry registered Divox as a trademark for breathing grade oxygen to circumvent the strict rules concerning medicinal oxygen thus making it easier for (recreational) scuba divers to obtain oxygen for blending their breathing gas. In most countries, there is no difference in purity in medical oxygen and industrial oxygen, as they are produced by exactly the same methods and manufacturers, but labeled and tanked differently. The chief difference between them is that the paper record-keeping trail is much more extensive for medical oxygen, to more easily identify the exact manufacturing trail of a "lot" or batch of oxygen, in case problems with its purity are discovered. Aviation grade oxygen is similar to medical oxygen, but may have a lower moisture content.[4]


Nitrogen (N2) is a diatomic gas and the main component of air, the cheapest and most common breathing gas used for diving. It causes nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause decompression sickness.[1][2][3][17]

Equivalent air depth is used to estimate the decompression requirements of a nitrox (oxygen/nitrogen) mixture. Equivalent narcotic depth is used to estimate the narcotic potency of trimix (oxygen/helium/nitrogen mixture). Many divers find that the level of narcosis caused by a 30 m (100 ft) dive, whilst breathing air, is a comfortable maximum.[1][2][3][18][19]

Nitrogen in a gas mix is almost always obtained by adding air to the mix.


Helium (He) is an inert gas that is less narcotic than nitrogen at equivalent pressure (in fact there is no evidence for any narcosis from helium at all), so it is more suitable for deeper dives than nitrogen.[1][3] Helium is equally able to cause decompression sickness. At high pressures, helium also causes high-pressure nervous syndrome, which is a CNS irritation syndrome which is in some ways opposite to narcosis.[1][2][3][20]

Helium fills typically cost ten times more than an equivalent air fill.

Helium is not very suitable for dry suit inflation owing to its poor thermal insulation properties – helium is a very good conductor of heat (compared to air which is a rather poor, making it more of an insulator).[1][3] Helium's low molecular weight (monatomic MW=4, compared with diatomic nitrogen MW=28) increases the timbre of the breather's voice, which may impede communication.[1][3][21] This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonance frequency of the vocal cords.[1][21] Helium leaks from damaged or faulty valves more readily than other gases because atoms of helium are smaller allowing them to pass through smaller gaps in seals.

Helium is found in significant amounts only in natural gas, from which it is extracted at low temperatures by fractional distillation.


"Neox" redirects here. For Spanish TV channel, see Neox (TV channel).

Neon (Ne) is an inert gas sometimes used in deep commercial diving but is very expensive.[1][3][10][15] Like helium, it is less narcotic than nitrogen, but unlike helium, it does not distort the diver's voice.


Hydrogen (H2) has been used in deep diving gas mixes but is very explosive when mixed with more than about 4 to 5% oxygen (such as the oxygen found in breathing gas).[1][3][10][12] This limits use of hydrogen to deep dives and imposes complicated protocols to ensure that excess oxygen is cleared from the breathing equipment before breathing hydrogen starts. Like helium, it raises the timbre of the diver's voice. The hydrogen-oxygen mix when used as a diving gas is sometimes referred to as Hydrox. Mixtures containing both hydrogen and helium as diluents are termed Hydreliox.

Unwelcome components of breathing gases

Many gases are not suitable for use in diving breathing gases.[5][22] Here is an incomplete list of gases commonly present in a diving environment:


Argon (Ar) is an inert gas that is more narcotic than nitrogen, so is not generally suitable as a diving breathing gas.[23] Argox is used for decompression research.[1][3][24][25] It is sometimes used for dry suit inflation by divers whose primary breathing gas is helium-based, because of argon's good thermal insulation properties. Argon is more expensive than air or oxygen, but considerably less expensive than helium.

Carbon dioxide

Carbon dioxide (CO2) is produced by the metabolism in the human body and can cause carbon dioxide poisoning.[22][26][27] When breathing gas is recycled in a rebreather or life support system, the carbon dioxide is removed by scrubbers before the gas is re-used.

Carbon monoxide

Carbon monoxide (CO) is produced by incomplete combustion.[1][2][5][22] See carbon monoxide poisoning. Four common sources are:


Hydrocarbons (CxHy) are present in compressor lubricants and fuels. They can enter diving cylinders as a result of contamination, leaks, or due to incomplete combustion near the air intake.[2][4][5][22][28]

Moisture content

The process of compressing gas into a diving cylinder removes moisture from the gas.[5][22] This is good for corrosion prevention in the cylinder but means that the diver inhales very dry gas. The dry gas extracts moisture from the diver's lungs while underwater contributing to dehydration, which is also thought to be a predisposing risk factor of decompression sickness. It is also uncomfortable, causing a dry mouth and throat and making the diver thirsty. This problem is reduced in rebreathers because the soda lime reaction, which removes carbon dioxide, also puts moisture back into the breathing gas.[8] In hot climates, open circuit diving can accelerate heat exhaustion because of dehydration. Another concern with regard to moisture content is the tendency of moisture to condense as the gas is decompressed while passing through the regulator; this coupled with the extreme reduction in temperature, also due to the decompression can cause the moisture to solidify as ice. This icing up in a regulator can cause moving parts to seize and the regulator to fail or free flow. It is for this reason that SCUBA regulators are generally constructed from brass, and chrome plated (for protection). Brass, with its good thermal conductive properties, quickly conducts heat from the surrounding water to the cold, newly decompressed air, helping to prevent icing up.

Gas detection and measurement

Divers find it difficult to detect most gases that are likely to be present in diving cylinders because they are colourless, odourless and tasteless. Electronic sensors exist for some gases, such as oxygen analysers, helium analyser, carbon monoxide detectors and carbon dioxide detectors.[2][4][5] Oxygen analysers are commonly found underwater in rebreathers.[8] Oxygen and helium analysers are often used on the surface during gas blending to determine the percentage of oxygen or helium in a breathing gas mix.[4] Chemical and other types of gas detection methods are not often used in recreational diving, but are used for periodical quality testing of compressed breathing air from diving air compressors.[4]

See also


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