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 oxygen, are also used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, high-altitude mountaineering, high-flying aircraft, submarines, space suits, spacecraft, medical life support and first aid equipment, and anaesthetic machines.
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. 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.
A safe breathing gas for use has three essential features:
The techniques used to fill diving cylinders with gases other than air are called gas blending.
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.

For diving and other hyperbaric use

These common diving breathing gases are used:
GasSymbolTypical shoulder coloursCylinder shoulderQuad upper frame/
frame valve end
Medical oxygenO2WhiteWhite
Oxygen and helium mixtures
O2/HeBrown and white
quarters or bands
Brown and white
short
alternating bands
Oxygen, helium and nitrogen
mixtures
O2/He/N2Black, white and brown
quarters or bands
Black, white and brown
short
alternating bands
Oxygen and nitrogen mixtures
including air
N2/O2Black and white
quarters or bands
Black and white
short
alternating bands

Breathing air

Breathing air is atmospheric air with a standard of purity suitable for human breathing in the specified application. For hyperbaric use the partial pressure of contaminants is increased in proportion to the absolute pressure, and must be limited to a safe composition for the depth or pressure range in which it is to be used.

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.
;Normoxic: 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.
;Hypoxic: 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

Oxygen must be present in every breathing gas. 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 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.
Oxygen has historically been obtained by fractional distillation of liquid air, but is increasingly obtained by non-cryogenic technologies such as pressure swing adsorption and vacuum swing adsorption technologies.
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.
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.
The partial pressure of any component gas in a mixture is calculated as:
For the oxygen component,

where:
The minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16 kPa. 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 and 160 kPa ; for dives of less than three hours it is commonly considered to be 140 kPa, although the U.S. Navy has been known to authorize dives with a PO2 of as much as 180 kPa. At high PO2 or longer exposures, the diver risks oxygen toxicity which may result in a seizure. Each breathing gas has a maximum operating depth that is determined by its oxygen content. 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.
Oxygen analysers are used to measure the oxygen partial pressure in the gas mix.
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 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.

Nitrogen

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.
Equivalent air depth is used to estimate the decompression requirements of a nitrox mixture. Equivalent narcotic depth is used to estimate the narcotic potency of trimix. Many divers find that the level of narcosis caused by a dive, whilst breathing air, is a comfortable maximum.
Nitrogen in a gas mix is almost always obtained by adding air to the mix.

Helium

is an inert gas that is less narcotic than nitrogen at equivalent pressure, so it is more suitable for deeper dives than nitrogen. 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.
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. Helium's low molecular weight increases the timbre of the breather's voice, which may impede communication. This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonance frequency of the vocal cords. 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

is an inert gas sometimes used in deep commercial diving but is very expensive. 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.

Hydrogen

has been used in deep diving gas mixes but is very explosive when mixed with more than about 4 to 5% oxygen. 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. Here is an incomplete list of gases commonly present in a diving environment:

Argon

is an inert gas that is more narcotic than nitrogen, so is not generally suitable as a diving breathing gas. Argox is used for decompression research. 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.

Carbon dioxide

is produced by the metabolism in the human body and can cause carbon dioxide poisoning. 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

is produced by incomplete combustion. See carbon monoxide poisoning. Four common sources are:
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

s 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.
The process of compressing gas into a diving cylinder removes moisture from the gas. 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. 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. 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 analysis

Gas mixtures must generally be analysed either in process or after blending for quality control. This is particularly important for breathing gas mixtures where errors can affect the health and safety of the end user. 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. Oxygen analysers are commonly found underwater in rebreathers. 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. 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.

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, mixtures of helium, nitrogen and oxygen, and pure oxygen, for both open circuit and reclaim systems, and for high pressure and low pressure supply.
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%.
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.
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.
In Australia breathing air quality is specified by Australian Standard 2299.1, Section 3.13 Breathing Gas Quality.

Diving gas blending

Gas blending 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

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

Oxygen is required by people for normal cell metabolism. Air is typically 21% oxygen by volume. 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. This can include for low blood oxygen, carbon monoxide toxicity, cluster headaches, and to maintain enough oxygen while inhaled anesthetics are given. Long term oxygen is often useful in people with chronically low oxygen such as from severe COPD or cystic fibrosis. Oxygen can be given in a number of ways including nasal cannula, face mask, and inside a hyperbaric chamber.
High concentrations of oxygen can cause oxygen toxicity such as lung damage or result in respiratory failure in those who are predisposed. 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.
The use of oxygen in medicine become common around 1917. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system. The cost of home oxygen is about US$150 a month in Brazil and US$400 a month in the United States. Home oxygen can be provided either by oxygen tanks or an oxygen concentrator. Oxygen is believed to be the most common treatment given in hospitals in the developed world.

Anaesthetic gases

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.
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.

Administration

Anaesthetic gases are administered by anaesthetists through an anaesthesia mask, laryngeal mask airway or tracheal tube connected to an anaesthetic vaporiser and an anaesthetic delivery system.
The anaesthetic machine or anesthesia machine 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, mixed with an accurate concentration of anaesthetic vapour, and deliver this to the patient at a safe pressure and flow. Modern machines incorporate a ventilator, suction unit, and patient monitoring devices.