Breathing gas

Breathing gas

Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.

Air is the most common and only natural breathing gas. Other artificial gases, either 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, and anaesthetic machines.[1][2][3]

Most breathing gases 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 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 has three essential features:

  • it must contain sufficient oxygen to support the life, consciousness and work rate of the breather.[1][2][3]
  • it must not contain harmful gases. Carbon monoxide and carbon dioxide are common poisons in breathing gases. There are many others.[1][2][3]
  • it must not become toxic when being breathed at high pressure such as when underwater. Oxygen and nitrogen are examples of gases that become toxic under pressure.[1][2][3]

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


Common diving breathing gases

Common diving breathing gases are:

  • Air is a mixture of 21% oxygen, 78% nitrogen, and approximately 1% other trace gases; 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.[1][2][3] 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).[1][3][6]
  • Pure oxygen is mainly used to speed the shallow decompression stops at the end of a military, commercial or technical dive and is only safe down to a depth of 6 meters (maximum operating depth) before oxygen toxicity steps in.[1][2][3][6] It was much used in frogmen's rebreathers.[2][6][7][8]
  • 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).[1][2][3][9]
  • 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.[1][2][3]
  • 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.[1][2][3][10]
  • 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.[3][11]
  • Hydreliox is a mixture of oxygen, helium, and hydrogen and is used for dives below 130 metres in commercial diving.[1][3][10][12][13]
  • Hydrox, a gas mixture of hydrogen and oxygen is used as a breathing gas in very deep diving.[1][3][10][12][14]
  • Neox (also called neonox) is a mixture of oxygen and neon sometimes employed for 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.[1][3][10][15]

Individual component gases

NEDU gas analysis lab


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:

  • hypoxic mixes, strictly, contain less than 21% oxygen, although often a boundary of 16% is used, and are designed only to be breathed at depth as a "bottom gas" where the higher pressure increases the partial pressure of oxygen to a safe level.[1][2][3] Trimix, Heliox and Heliair create typical hypoxic mixes and are used in technical diving as deep breathing gases.[1][3]
  • normoxic mixes have the same proportion of oxygen as air, 21%.[1][3] The maximum operating depth of a normoxic mix could be as shallow as 47 metres (155 feet). Trimix with between 17% and 21% oxygen is often described as normoxic because it contains a high enough proportion of oxygen to be safe to breathe at the surface.
  • hyperoxic mixes have more than 21% oxygen. Enriched Air Nitrox (EANx) is a typical hyperoxic breathing gas.[1][3][9] Hyperoxic mixtures, when compared to air, cause oxygen toxicity at shallower depths but can be used to shorten decompression stops by drawing dissolved inert gases out of the body more quickly.[6][9]

The fraction of the oxygen determines the deepest the mixture gas 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 both 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 x volume fraction of gas component

For the oxygen component:

ppO2 = P x FO2
ppO2 = partial pressure of oxygen
P = total pressure
FO2 = volume fraction of oxygen

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) but for dives of less than three hours 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 including a seizure.[1][2] Each breathing gas has a maximum operating depth that is determined by its oxygen content.[1][2][3][6][9]

Oxygen analysers 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 "created" Divox and registered it as a trademark 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, in order to more easily identify the exact manufacturing trail of a "lot" of oxygen, in case problems are later found with its purity.


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 due 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.


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 oxygen is cleared from the lungs, the blood stream and the breathing equipment before breathing hydrogen starts. Like helium, it increases the timbre of the diver's voice. The hydrogen-oxygen mix when used as a diving gas is sometimes referred to as Hydrox.

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]

Carbon monoxide

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

  • Internal combustion engine exhaust gas containing CO in the air being drawn into a diving air compressor. CO in the intake air cannot be stopped by any filter. The exhausts of all internal combustion engines running on petroleum fuels contain some CO, and this is a particular problem on boats, where the intake of the compressor cannot be arbitrarily moved as far as desired from the engine and compressor exhausts.
  • Heating of lubricants inside the compressor may vaporize them sufficiently to be available to a compressor intake or intake system line.
  • In some cases hydrocarbon lubricating oil may be drawn into the compressor's cylinder directly through damaged or worn seals, and the oil may (and usually will) then undergo combustion, being ignited by the immense compression ratio and subsequent temperature rise. Since heavy oils don't burn well - especially when not atomized properly - incomplete combustion will result in carbon monoxide production.
  • A similar process is thought to potentially happen to any particulate material, which contains "organic" (carbon-containing) matter, especially in cylinders which are used for hyperoxic gas mixtures. If the compressor air filter(s) fail, ordinary dust will be introduced to the cylinder, which contains organic matter (since it usually contains humus). A more severe danger is that air particulates on boats and industrial areas, where cylinders are filled, often contain carbon-particulate combustion products (these are what makes a dirt rag black), and these represent a more severe CO danger when introduced into a cylinder.


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]

  • They can act as a fuel in combustion increasing the risk of explosion, especially in high-oxygen gas mixtures.
  • Inhaling oil mist can damage the lungs and ultimately cause the lungs to degenerate with severe lipid pneumonia or emphysema.

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 to remove carbon dioxide 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.


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