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 other mixtures of gases, or pure gases, are also 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:

  • It must contain sufficient oxygen to support life, consciousness and work rate of the breather.[1][2][3]
  • It must not contain harmful contaminants. Carbon monoxide and carbon dioxide are common poisons which may contaminate breathing gases. There are many other possibilities.[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]

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.

Flickr - Official U.S. Navy Imagery - Sailors check breathing devices at sea.
Sailors check breathing devices at sea.
Trimix label
Trimix scuba cylinder label

For diving and other hyperbaric use

Closed diving bell 20151203 132327
A closed bell used for saturation diving showing emergency gas supply cylinders

These common diving breathing gases are used:

  • 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.[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. Risk of acute oxygen toxicity increases rapidly at pressures greater than 6 metres sea water.[1][2][3][6] It was much used in frogmen's rebreathers, and is still used by attack swimmers.[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 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]
Commonly accepted breathing gas container colour coding in the offshore diving industry.[16]
Gas Symbol Typical shoulder colours Cylinder shoulder Quad upper frame/
frame valve end
Medical oxygen O2
IMCA Oxygen shoulder
White White
Oxygen and helium mixtures
O2/He IMCA Heliox shoulder quarteredIMCA Heliox shoulder Brown and white
quarters or bands
Brown and white
short (8 inches (20 cm))
alternating bands
Oxygen, helium and nitrogen
mixtures (Trimix)
O2/He/N2 IMCA Trimix shoulder quarteredIMCA Trimix shoulder Black, white and brown
quarters or bands
Black, white and brown
short (8 inches (20 cm))
alternating bands
Oxygen and nitrogen mixtures
(Nitrox) including air
N2/O2 IMCA Nitrox shoulder quarteredIMCA Nitrox shoulder Black and white
quarters or bands
Black and white
short (8 inches (20 cm))
alternating bands

Classification by oxygen fraction

Breathing gases for diving are classified by oxygen fraction. The boundaries set by authorities may differ slightly, as the effects vary gradually with concentration and between people, and are not accurately predictable.

where the oxygen content does not differ greatly from that of air and allows continuous safe use at atmospheric pressure.
Hyperoxic, or oxygen enriched
where the oxygen content exceeds atmospheric levels, generally to a level where there is some measurable physiological effect over long term use, and sometimes requiring special procedures for handling due to increased fire hazard. The associated risks are oxygen toxicity at depth and fire, particularly in the breathing apparatus.
where the oxygen content is less than that of air, generally to the extent that there is a significant risk of measurable physiological effect over the short term. The immediate risk is usually hypoxic incapacitation at or near the surface.

Individual component gases

Breathing gases for diving are mixed from a small number of component gases which provide special characteristics to the mixture which are not available from atmospheric air.


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 swing adsorption (VSA) technologies.[17]

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 are gas blends commonly used for hypoxic mixes and are used in professional and 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 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]

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 (PO2).[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,

PO2 = P × FO2


PO2 = 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 PO2 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 PO2 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 PO2 of as much as 180 kPa (1.8 bar).[1][2][3][6][9] At high PO2 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 oxygen partial pressure in the gas mix.[4]

Divox is breathing grade oxygen labelled for diving use. 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 filled differently. The chief difference between them is that the 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][18]

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][19][20]

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


Helium Quad 20151203 133932
2% Heliox storage quad. 2% oxygen by volume is sufficient at pressures exceeding 90 msw.

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 central nervous system irritation syndrome which is in some ways opposite to narcosis.[1][2][3][21]

Helium mixture fills are considerably more expensive than air fills due to the cost of helium and the cost of mixing and compressing the mix.

Helium is not suitable for dry suit inflation owing to its poor thermal insulation properties – compared to air, which is regarded as a reasonable insulator, helium has six times the thermal conductivity.[22] 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][23] 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][23] 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. Compared to helium, neon has superior thermal insulating properties.[24]


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 for diving

Many gases are not suitable for use in diving breathing gases.[5][25] 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.[26] Argox is used for decompression research.[1][3][27][28] 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. Argon is a component of natural air, and constitutes 0.934% by volume of the Earth's atmosphere.[29]

Carbon dioxide

Carbon dioxide (CO2) is produced by the metabolism in the human body and can cause carbon dioxide poisoning.[25][30][31] 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][25] 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.

Carbon monoxide is generally avoided as far as is reasonably practicable by positioning of the air intake in uncontaminated air, filtration of particulates from the intake air, use of suitable compressor design and appropriate lubricants, and ensuring that running temperatures are not excessive. Where the residual risk is excessive, a hopcalite catalyst can be used in the high pressure filter to convert carbon monoxide into carbon dioxide, which is far less toxic.


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][25][32]

  • 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[33] or emphysema.

Moisture content

The process of compressing gas into a diving cylinder removes moisture from the gas.[5][25] 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. This is one of the reasons 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

Electro-galvanic fuel cell
Electro-galvanic fuel cell as used in a diving rebreather

It is 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]

Breathing gas standards

Standards for breathing gas quality are published by national and international organisations, and may be enforced in terms of legislation. In the UK, the Health and Safety Executive indicate that the requirements for breathing gases for divers are based on the BS EN 12021:2014. The specifications are listed for oxygen compatible air, nitrox mixtures produced by adding oxygen, removing nitrogen, or mixing nitrogen and oxygen, mixtures of helium and oxygen (heliox), mixtures of helium, nitrogen and oxygen (trimix), and pure oxygen, for both open circuit and reclaim systems, and for high pressure and low pressure supply (above and below 40 bar supply).[34]

Oxygen content is variable depending on the operating depth, but the tolerance depends on the gas fraction range, being ±0.25% for an oxygen fraction below 10% by volume, ±0.5% for a fraction between 10% and 20%, and ±1% for a fraction over 20%.[34]

Water content is limited by risks of icing of control valves, and corrosion of containment surfaces – higher humidity is not a physiological problem – and is generally a factor of dew point.[34]

Other specified contaminants are carbon dioxide, carbon monoxide, oil, and volatile hydrocarbons, which are limited by toxic effects. Other possible contaminants should be analysed based on risk assessment, and the required frequency of testing for contaminants is also based on risk assessment.[34]

In Australia breathing air quality is specified by Australian Standard 2299.1, Section 3.13 Breathing Gas Quality.[35]

Diving gas blending

Gas blending equipment
Air, oxygen and helium partial pressure gas blending system
Nitrox continuous blending compressor installation P8160005
Nitrox continuous blending compressor installation

Gas blending (or Gas mixing) of breathing gases for diving is the filling of gas cylinders with non-air breathing gases.

Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion due to the use of high-pressure gases. The composition of the mix must be safe for the depth and duration of the planned dive. If the concentration of oxygen is too lean the diver may lose consciousness due to hypoxia and if it is too rich the diver may suffer oxygen toxicity. The concentration of inert gases, such as nitrogen and helium, are planned and checked to avoid nitrogen narcosis and decompression sickness.

Methods used include batch mixing by partial pressure or by mass fraction, and continuous blending processes. Completed blends are analysed for composition for the safety of the user. Gas blenders may be required by legislation to prove competence if filling for other persons.

Hypobaric breathing gases

Astronaut in an Orlan space suit, outside the International Space Station

Breathing gases for use at reduced ambient pressure are used for high altitude flight in unpressurised aircraft, in space flight, particularly in space suits, and for high altitude mountaineering. In all these cases, the primary consideration is providing an adequate partial pressure of oxygen. In some cases the breathing gas has oxygen added to make up a sufficient concentration, and in other cases the breathing gas may be pure or nearly pure oxygen. Closed circuit systems may be used to conserve the breathing gas, which may be in limited supply - in the case of mountaineering the user must carry the supplemental oxygen, and in space flight the cost of lifting mass into orbit is very high.

Medical breathing gases

Medical use of breathing gases other than air include oxygen therapy and anesthesia applications.

Oxygen therapy

Simple face mask
A person wearing a simple face mask for oxygen therapy

Oxygen is required by people for normal cell metabolism.[36] Air is typically 21% oxygen by volume.[37] This is normally sufficient, but in some circumstances the oxygen supply to tissues is compromised.

Oxygen therapy, also known as supplemental oxygen, is the use of oxygen as a medical treatment.[38] This can include for low blood oxygen, carbon monoxide toxicity, cluster headaches, and to maintain enough oxygen while inhaled anesthetics are given.[39] Long term oxygen is often useful in people with chronically low oxygen such as from severe COPD or cystic fibrosis.[40][38] Oxygen can be given in a number of ways including nasal cannula, face mask, and inside a hyperbaric chamber.[41][42]

High concentrations of oxygen can cause oxygen toxicity such as lung damage or result in respiratory failure in those who are predisposed.[39][37] It can also dry out the nose and increase the risk of fires in those who smoke. The target oxygen saturation recommended depends on the condition being treated. In most conditions a saturation of 94-98% is recommended, while in those at risk of carbon dioxide retention saturations of 88-92% are preferred, and in those with carbon monoxide toxicity or cardiac arrest the saturation should be as high as possible.[38]

The use of oxygen in medicine become common around 1917.[43][44] It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.[45] The cost of home oxygen is about US$150 a month in Brazil and US$400 a month in the United States.[40] Home oxygen can be provided either by oxygen tanks or an oxygen concentrator.[38] Oxygen is believed to be the most common treatment given in hospitals in the developed world.[46][38]

Anaesthetic gases

Vaporizer Sevoflurane 001 JPN
A vaporizer holds a liquid anesthetic and converts it to gas for inhalation (in this case sevoflurane)
Maquet Flow-I anesthesia machine
An anaesthetic machine.
Bottles of sevoflurane, isoflurane, enflurane, and desflurane, the common fluorinated ether anaesthetics used in clinical practice. These agents are colour-coded for safety purposes. Note the special fitting for desflurane, which boils at room temperature.

The most common approach to general anaesthesia is through the use of inhaled general anesthetics. Each has its own potency which is correlated to its solubility in oil. This relationship exists because the drugs bind directly to cavities in proteins of the central nervous system, although several theories of general anaesthetic action have been described. Inhalational anesthetics are thought to exact their effects on different parts of the central nervous system. For instance, the immobilizing effect of inhaled anesthetics results from an effect on the spinal cord whereas sedation, hypnosis and amnesia involve sites in the brain.[47]:515

An inhalational anaesthetic is a chemical compound possessing general anaesthetic properties that can be delivered via inhalation. Agents of significant contemporary clinical interest include volatile anaesthetic agents such as isoflurane, sevoflurane and desflurane, and anaesthetic gases such as nitrous oxide and xenon.


Anaesthetic gases are administered by anaesthetists (a term which includes anaesthesiologists, nurse anaesthetists, and anaesthesiologist assistants) through an anaesthesia mask, laryngeal mask airway or tracheal tube connected to an anaesthetic vaporiser and an anaesthetic delivery system. The anaesthetic machine (UK English) or anesthesia machine (US English) or Boyle's machine is used to support the administration of anaesthesia. The most common type of anaesthetic machine in use in the developed world is the continuous-flow anaesthetic machine, which is designed to provide an accurate and continuous supply of medical gases (such as oxygen and nitrous oxide), mixed with an accurate concentration of anaesthetic vapour (such as isoflurane), and deliver this to the patient at a safe pressure and flow. Modern machines incorporate a ventilator, suction unit, and patient monitoring devices.

See also


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External links

Alternative air source

In underwater diving, an alternative air source, or more generally alternative breathing gas source, is a secondary supply of air or other breathing gas for use by the diver in an emergency. Examples include an auxiliary demand valve, a pony bottle and bailout bottle.An alternative air source may be fully redundant (completely independent of any part of the main air supply system) or non-redundant, if it can be compromised by any failure of the main air supply. From the diver's point of view, air supplied by a buddy or rescue diver is fully redundant, as it is unaffected by the diver's own air supply in any way, but a second regulator on a double cylinder valve or a secondary demand valve (octopus) is not redundant to the diver carrying it, as it is attached to his or her main air supply. Decompression gas can be considered an alternative gas supply only when the risk of breathing it at the current depth is acceptable.

Effective use of any alternate air source requires competence in the associated skill set. The procedures for receiving air from another diver or from one's own equipment are most effective and least likely to result in a life-threatening incident if well trained to the extent that they do not distract the diver from other essential matters. A major difference from buddy breathing is that the diver using a redundant alternative air source need not alternate breathing with the donor, which can be a substantial advantage in many circumstances. There is a further significant advantage when the alternate air source is carried by the diver using it, in that it is not necessary to locate the buddy before it is available, but this comes at the cost of extra equipment.

Bailout bottle

A bailout bottle (BoB) or bailout cylinder is a scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure. A bailout cylinder may be carried by a scuba diver in addition to the primary scuba set, or by a surface supplied diver using either free-flow or demand systems. The bailout gas is not intended for use during the dive except in an emergency. The term may be used to refer to just the cylinder, or the bailout set or emergency gas supply (EGS), which is the cylinder with the gas delivery system attached.

In solo diving, a buddy bottle is a bailout cylinder carried as a substitute for an emergency gas supply from a diving buddy.

Rebreathers also have bailout systems, often including an open-circuit bailout bottle.

Diving chamber

A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of breathing gas for the occupants.

There are two main functions for diving chambers:

as a simple form of submersible vessel to transport divers underwater and to provide a temporary base and retrieval system in the depths;

as a land, ship or offshore platform-based hyperbaric chamber or system, to artificially reproduce the hyperbaric conditions under the sea. Internal pressures above normal atmospheric pressure are provided for diving-related applications such as saturation diving and diver decompression, and non-diving medical applications such as hyperbaric medicine.

Diving helmet

A Diving helmet is a rigid head enclosure with a breathing gas supply used in underwater diving. They are worn mainly by professional divers engaged in surface-supplied diving, though some models can be used with scuba equipment. The upper part of the helmet, known colloquially as the hat or bonnet, may be secured to the diver or diving suit by a lower part, known as a neck dam or corselet, depending on the construction.

The helmet seals the whole of the diver's head from the water, allows the diver to see clearly underwater, provides the diver with breathing gas, protects the diver's head when doing heavy or dangerous work, and usually provides voice communications with the surface (and possibly other divers). If a helmeted diver becomes unconscious but is still breathing, the helmet will remain in place and continue to deliver breathing gas until the diver can be rescued. In contrast, the scuba regulator typically used by recreational divers must be held in the mouth, otherwise it can fall out of an unconscious diver's mouth and result in drowning (this does not apply to a full face mask which also continues to serve air if the diver is unconscious).

Before the invention of the demand regulator, all diving helmets used a free-flow design. Gas was delivered at a constant rate, regardless of the diver's breathing, and flowed out through an exhaust valve. Most modern helmets incorporate a demand valve so the helmet only delivers breathing gas when the diver inhales. Free-flow helmets use much larger quantities of gas than demand helmets, which can cause logistical difficulties and is very expensive when special breathing gases (such as heliox) are used. They also produce a constant noise inside the helmet, which can cause communication difficulties. Free-flow helmets are still preferred for hazardous materials diving, because their positive-pressure nature can prevent the ingress of hazardous material in case the integrity of the suit or helmet is compromised. They also remain relatively common in shallow-water air diving, where gas consumption is of little concern, and in nuclear diving because they must be disposed of after some period of use due to irradiation; free-flow helmets are significantly less expensive to purchase and maintain than demand types.

Most modern helmet designs are sealed at the neck using a neoprene "neck dam" which is independent of the suit, allowing the diver his choice of suits depending on the dive conditions. When a neck dam is installed into a drysuit, however, the entire body is isolated from the surrounding liquid, giving an additional degree of warmth and protection. When divers must work in hazardous environments such as sewage or dangerous chemicals, a helmet (usually of the free-flow type or using a series exhaust valve system) is sealed to a special drysuit (commonly made of a fabric with a smooth vulcanised rubber outer surface) to completely isolate and protect the diver. This equipment is the modern equivalent of the historic Mark V "Standard Diving Dress".

Diving suit

A diving suit is a garment or device designed to protect a diver from the underwater environment. A diving suit may also incorporate a breathing gas supply (i.e. Standard diving dress or atmospheric diving suit). but in most cases applies only to the environmental protective covering worn by the diver. The breathing gas supply is usually referred to separately. There is no generic term for the combination of suit and breathing apparatus alone. It is generally referred to as diving equipment or dive gear along with any other equipment necessary for the dive.

Diving suits can be divided into two classes: "soft" or ambient pressure diving suits – examples are wetsuits, dry suits, semi-dry suits and dive skins –, and "hard" or atmospheric pressure diving suits, armored suits that keep the diver at atmospheric pressure at any depth within the operating range of the suit.

Emergency ascent

An emergency ascent is an ascent to the surface by a diver in an emergency. More specifically it refers to any of several procedures for reaching the surface in the event of an out-of-air emergency, generally while scuba diving.

Emergency ascents may be broadly categorised as independent ascents, where the diver is alone and manages the ascent by him/herself, and dependent ascents, where the diver is assisted by another diver, who generally provides breathing gas, but may also provide transportation or other assistance. The extreme case of a dependent ascent is underwater rescue or recovery of an unconscious or unresponsive diver, but this is more usually referred to as diver rescue, and emergency ascent is usually used for cases where the distressed diver is at least partially able to contribute to the management of the ascent.

An emergency ascent usually implies that the diver initiated the ascent voluntarily, and made the choice of the procedure. Ascents that are involuntary or get out of control unintentionally are more accurately classed as accidents.

An emergency ascent may be made for any one of several reasons, including failure or imminent failure of the breathing gas supply.

Equivalent narcotic depth

Equivalent narcotic depth (END) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as heliox and trimix. The method is, for a given mix and depth, to calculate the depth which would produce the same narcotic effect when breathing air.

The equivalent narcotic depth of a breathing gas mix at a particular depth is calculated by finding the depth of a dive when breathing air that would have the same total partial pressure of nitrogen and oxygen as the breathing gas in question.

For example, a trimix containing 20% oxygen, 40% helium, 40% nitrogen (trimix 20/40) being used at 60 metres (200 ft) has an END of 32 metres (105 ft).

Since air is composed of approximately 21% oxygen and 79% nitrogen, the narcotic gases make up 100% of the mix, or equivalently the fraction of the total gases which are narcotic is 1.0. Oxygen is assumed equivalent in narcotic effect to nitrogen for this purpose. In contrast, the oxygen and nitrogen component in a trimix containing, for example, 40% helium accounts for only 60% of the mix, i.e. a fraction of 0.6. In a trimix, the fraction of narcotic gases (oxygen and nitrogen) is equal to 1.0 minus the fraction of non-narcotic gas (helium).

Full face diving mask

A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece, demand valve or constant flow gas supply that provides the diver with breathing gas. The full face mask has several functions: it lets the diver see clearly underwater, it provides the diver's face with some protection from cold and polluted water and from stings, such as from jellyfish or coral. It increases breathing security and provides a space for equipment that lets the diver communicate with the surface support team.

Full face masks can be more secure than breathing from an independent mouthpiece; if the diver becomes unconscious or suffers an oxygen toxicity convulsion, the diver can continue to breathe from the mask. unlike a scuba mouthpiece which is normally gripped between the teeth.Full-face diving masks are often used in professional diving. They are relatively rarely used in recreational diving.


Heliox is a breathing gas composed of a mixture of helium (He) and oxygen (O2).

Heliox is a medical treatment for patients with difficulty breathing. The mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs.

Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of the low density of the gas. Heliox is also used in saturation diving and sometimes during the deep phase of technical dives.

Hydrox (breathing gas)

Hydrox, a gas mixture of hydrogen and oxygen, was used as a breathing gas in very deep diving. It allows divers to descend several hundred metres.Precautions are necessary when using hydrox, since mixtures containing more than a few percent of both oxygen and hydrogen are explosive if ignited. Hydrogen is the lightest gas (half the weight of helium) but still has a narcotic potential and may cause hydrogen narcosis.

List of diving hazards and precautions

Divers face specific physical and health risks when they go underwater with scuba or other diving equipment, or use high pressure breathing gas. Some of these factors also affect people who work in raised pressure environments out of water, for example in caissons. This article lists hazards that a diver may be exposed to during a dive, and possible consequences of these hazards, with some details of the proximate causes of the listed consequences. A listing is also given of precautions that may be taken to reduce vulnerability, either by reducing the risk or mitigating the consequences. A hazard that is understood and acknowledged may present a lower risk if appropriate precautions are taken, and the consequences may be less severe if mitigation procedures are planned and in place.

A hazard is any agent or situation that poses a level of threat to life, health, property, or environment. Most hazards remain dormant or potential, with only a theoretical risk of harm, and when a hazard becomes active, and produces undesirable consequences, it is called an incident and may culminate in an emergency or accident. Hazard and vulnerability interact with likelihood of occurrence to create risk, which can be the probability of a specific undesirable consequence of a specific hazard, or the combined probability of undesirable consequences of all the hazards of a specific activity. The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident. The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations tend to be less tolerant of risk than recreational, particularly technical divers, who are less constrained by occupational health and safety legislation.

Decompression sickness and arterial gas embolism in recreational diving are associated with certain demographic, environmental, and dive style factors. A statistical study published in 2005 tested potential risk factors: age, gender, body mass index, smoking, asthma, diabetes, cardiovascular disease, previous decompression illness, years since certification, dives in last year, number of diving days, number of dives in a repetitive series, last dive depth, nitrox use, and drysuit use. No significant associations with decompression sickness or arterial gas embolism were found for asthma, diabetes, cardiovascular disease, smoking, or body mass index. Increased depth, previous DCI, days diving, and being male were associated with higher risk for decompression sickness and arterial gas embolism. Nitrox and drysuit use, greater frequency of diving in the past year, increasing age, and years since certification were associated with lower risk, possibly as indicators of more extensive training and experience.According to a North American 1972 analysis of calendar year 1970 data, diving was, based on man hours, 96 times riskier than driving an automobile, and according to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving.

List of signs and symptoms of diving disorders

Diving disorders are medical conditions specifically arising from underwater diving. The signs and symptoms of these may present during a dive, on surfacing, or up to several hours after a dive. Divers have to breathe a gas which is at the same pressure as their surroundings (ambient pressure), which can be much greater than on the surface. The ambient pressure underwater increases by 1 standard atmosphere (100 kPa) for every 10 metres (33 ft) of depth.The principal conditions are: decompression illness (which covers decompression sickness and arterial gas embolism); nitrogen narcosis; high pressure nervous syndrome; oxygen toxicity; and pulmonary barotrauma (burst lung). Although some of these may occur in other settings, they are of particular concern during diving activities.The disorders are caused by breathing gas at the high pressures encountered at depth, and divers will often breathe a gas mixture different from air to mitigate these effects. Nitrox, which contains more oxygen and less nitrogen, is commonly used as a breathing gas to reduce the risk of decompression sickness at recreational depths (up to about 40 metres (130 ft)). Helium may be added to reduce the amount of nitrogen and oxygen in the gas mixture when diving deeper, to reduce the effects of narcosis and to avoid the risk of oxygen toxicity. This is complicated at depths beyond about 150 metres (500 ft), because a helium–oxygen mixture (heliox) then causes high pressure nervous syndrome. More exotic mixtures such as hydreliox, a hydrogen–helium–oxygen mixture, are used at extreme depths to counteract this.

Outline of underwater diving

The following outline is provided as an overview of and topical guide to underwater diving:

Underwater diving – as a human activity, is the practice of descending below the water's surface to interact with the environment.

Pony bottle

A pony bottle is a small diving cylinder which is fitted with an independent regulator, and carried by a scuba diver as an extension to the scuba set. In an emergency, such as depletion of the diver's main air supply, it can be used as an alternative air source or bailout bottle to allow a normal ascent in place of a controlled emergency swimming ascent. The key attribute of a pony bottle is that it provides a totally independent and redundant source of breathing gas for the diver. The name pony is due to the smaller size, often of only a few litres capacity.

Pony bottles are used by divers who understand that no matter their preparation and planning, accidents may happen, and cannot, or do not choose to depend on another diver for emergency breathing gas. They are carried by the diver in one of several alternative configurations, and the capacity and contents should be sufficient to allow a safe ascent from any point in the planned dive profile.


A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from an open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment.

Rebreather technology may be used where breathing gas supply is limited, such as underwater or in space, where the environment is toxic or hypoxic, as in firefighting, mine rescue and high-altitude operations, or where the breathing gas is specially enriched or contains expensive components, such as helium diluent or anaesthetic gases.

Rebreather technology is used in many environments:

Underwater – as a self-contained breathing apparatus, where it is sometimes known as "closed circuit scuba" as opposed to "open circuit scuba" where the diver exhales breathing gas into the surrounding water. Surface-supplied diving equipment may incorporate rebreather technology either as a gas reclaim system, where the surface-supplied breathing gas is returned and scrubbed at the surface, or as a self-contained diver bailout system.

Mine rescue and other industrial applications – where poisonous gases may be present or oxygen may be absent.

Crewed spacecraft and space suits – outer space is, effectively, a vacuum without oxygen to support life.

Hospital anaesthesia breathing systems – to supply controlled concentrations of anaesthetic gases to patients without contaminating the air that the staff breathe.

Himalayan mountaineering. High altitude reduces the partial pressure of oxygen in the ambient air, which reduces the ability of the climber to function effectively. Mountaineering rebreathers provide a higher partial pressure of oxygen to the climber.

Submarines, underwater habitats, and saturation diving systems use a scrubber system working on the same principles as a rebreather.

Fire fighting, where firefighters may be required to operate in an IDLH atmosphere for extended periods of time, longer than an open-circuit SCBA can provide air forThis may be compared with some applications of open-circuit breathing apparatus:

The oxygen enrichment systems primarily used by medical patients, high altitude mountaineers and commercial aircraft emergency systems, in which the user breathes ambient air which is enriched by the addition of pure oxygen,

Open circuit breathing apparatus used by firefighters and underwater divers, which supplies fresh gas for each breath, which is then discharged into the environment.

Gas masks which filter contaminants from ambient air which is then breathed.The recycling of breathing gas comes at the cost of mass, bulk, technological complexity and specific hazards, which depend on the specific application and type of rebreather used.

Scuba diving

Scuba diving is a mode of underwater diving where the diver uses a self-contained underwater breathing apparatus (scuba), which is completely independent of surface supply, to breathe underwater. Scuba divers carry their own source of breathing gas, usually compressed air, allowing them greater independence and freedom of movement than surface-supplied divers, and longer underwater endurance than breath-hold divers. Although the use of compressed air is common, a new mixture called enriched air (Nitrox) has been gaining popularity due to its benefit of reduced nitrogen intake during repetitive dives. Open circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver through a regulator. They may include additional cylinders for range extension, decompression gas or emergency breathing gas. Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. The volume of gas used is reduced compared to that of open circuit, so a smaller cylinder or cylinders may be used for an equivalent dive duration. Rebreathers extend the time spent underwater compared to open circuit for the same gas consumption; they produce fewer bubbles and less noise than open circuit scuba which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media divers to avoid bubble interference.Scuba diving may be done recreationally or professionally in a number of applications, including scientific, military and public safety roles, but most commercial diving uses surface-supplied diving equipment when this is practicable. Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.A scuba diver primarily moves underwater by using fins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface. Other equipment includes a mask to improve underwater vision, exposure protection, equipment to control buoyancy, and equipment related to the specific circumstances and purpose of the dive. Some scuba divers use a snorkel when swimming on the surface. Scuba divers are trained in the procedures and skills appropriate to their level of certification by instructors affiliated to the diver certification organisations which issue these certifications. These include standard operating procedures for using the equipment and dealing with the general hazards of the underwater environment, and emergency procedures for self-help and assistance of a similarly equipped diver experiencing problems. A minimum level of fitness and health is required by most training organisations, but a higher level of fitness may be appropriate for some applications.

Scuba gas planning

Scuba gas planning is the aspect of dive planning which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive profile. It usually assumes that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.Gas planning includes the following aspects:

Choice of breathing gases

Choice of Scuba configuration

Estimation of gas required for the planned dive, including bottom gas, travel gas, and decompression gases, as appropriate to the profile.

Estimation of gas quantities for reasonably foreseeable contingencies. Under stress it is likely that a diver will increase breathing rate and decrease swimming speed. Both of these lead to a higher gas consumption during an emergency exit or ascent.

Choice of cylinders to carry the required gases. Each cylinder volume and working pressure must be sufficient to contain the required quantity of gas.

Calculation of the pressures for each of the gases in each of the cylinders to provide the required quantities.

Specifying the critical pressures of relevant gas mixtures for appropriate stages (waypoints) of the planned dive profile.Gas planning is one of the stages of scuba gas management. The other stages include:

Knowledge of personal and team members' gas consumption rates under varying conditions

basic consumption at the surface for variations in workload

variation in consumption due to depth variation

variation in consumption due to dive conditions and personal physical and mental condition

Monitoring the contents of the cylinders during a dive

Awareness of the critical pressures and using them to manage the dive

Efficient use of the available gas during the planned dive and during an emergency

Limiting the risk of equipment malfunctions that could cause a loss of breathing gas

Trimix (breathing gas)

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is often used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.The helium is included as a substitute for some of the nitrogen, to reduce the narcotic effect of the breathing gas at depth. With a mixture of three gases it is possible to create mixes suitable for different depths or purposes by adjusting the proportions of each gas. Oxygen content can be optimised for the depth to limit the risk of toxicity, and the inert component balanced between nitrogen (which is cheap but narcotic) and helium (which is not narcotic and reduces work of breathing, but is more expensive and increases heat loss).

The mixture of helium and oxygen with a 0% nitrogen content is generally known as Heliox. This is frequently used as a breathing gas in deep commercial diving operations, where it is often recycled to save the expensive helium component. Analysis of two-component gases is much simpler than three-component gases.

Work of breathing

Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing.

In a normal resting state the work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.

Recreational diving
Diving equipment
Diving safety:
risks and
History of
underwater diving
Breathing gas
Diving cylinders
Diving regulators
Helmets and masks
Open-circuit scuba
Other related

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