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. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or reducing work of breathing and allowing safer deep diving.

Description

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. 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, occasionally lower for high altitude mountaineering, or higher for hyperbaric oxygen treatment. 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 metabolically inert gases. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving. The techniques used to fill diving cylinders with gases other than air or pure oxygen are called gas blending.
Breathing gases for use at ambient pressures below normal atmospheric pressure are usually pure oxygen or 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

When used by an underwater diver, a breathing gas may also be referred to as a diving gas. A safe breathing gas for hyperbaric use has four essential features:

It must contain sufficient oxygen to support life, consciousness and work rate of the breather.
It must not contain harmful contaminants. Carbon monoxide and carbon dioxide are common poisons which may contaminate breathing gases. There are many other possibilities.
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.
It must not be too dense to breathe. Work of breathing increases with density and viscosity. Maximum ventilation drops by about 50% when density is equivalent to air at 30 msw, and carbon dioxide levels rise unacceptably for moderate exercise with a gas density exceeding 6 g/litre. Breathing gas density of 10 g/litre or more may cause runaway hypercapnia even at very low work levels, with potentially fatal effects.

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 freely available and simple to use, it is the most common diving gas. As its nitrogen component causes nitrogen narcosis, it is considered to have a safe depth limit of about for most divers, although the maximum operating depth of air taking an allowable oxygen partial pressure of 1,6 bar is. Breathing air is air meeting specified standards for contaminants.
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. It was much used in frogmen's rebreathers, and is still used by attack swimmers.
are mixtures of two or more gases specifically blended for use as breathing gas for divers. They may be mixed to the composition required before the dive, for open circuit use, or produced in the breathing circuit of a rebreather during the dive. Diving using mixed gases is referred to as mixed gas diving.
* Nitrox is a mixture of oxygen and nitrogen, either by mixing oxygen with air, or by removing nitrogen from 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.
* 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 decrease density, reduce nitrogen narcosis and to avoid the dangers of oxygen toxicity.
* 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 and reduce density, to limit work of breathing. Heliox is the standard mixture type for deep offshore saturation diving.
* 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.
* Hydreliox is a mixture of oxygen, helium, and hydrogen and is used for dives below 130 metres in commercial diving. Experimental work using hydreliox is also done in deep technical diving, where the hydrogen is used to reduce HPNS.
* Hydrox, a gas mixture of hydrogen and oxygen, is used as a breathing gas in very deep diving.
* Neox 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 have a poor reputation, being widely reported to be more severe than those produced by an exactly equivalent dive-table and mix with helium.

Gas	Symbol	Typical shoulder colours	Cylinder shoulder	Quad upper frame/ frame valve end
Medical oxygen	O₂		White	White
Oxygen and helium mixtures	O₂/He		Brown and white quarters or bands	Brown and white short alternating bands
Oxygen, helium and nitrogen mixtures	O₂/He/N₂		Black, white and brown quarters or bands	Black, white and brown short alternating bands
Oxygen and nitrogen mixtures including air	N₂/O₂		Black 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.
;: where the oxygen content does not differ greatly from that of air and allows continuous safe use at atmospheric pressure.
;, or : 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

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 considerably 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:

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. Trimix, Heliox and Heliair are gas blends commonly used for hypoxic mixes and are used in professional and technical diving as deep breathing gases. A may be assigned to a hypoxic gas mixture, based on the depth at which the partial pressure is equal to the minimum oxygen partial pressure acceptable to the person or organisation using the gas.
normoxic mixes have the same proportion of oxygen as air, 21%. The maximum operating depth of a normoxic mix could be as shallow as. 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 is a typical hyperoxic breathing gas. 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.

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 P_O₂ 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 P_O₂ 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 P_O₂ of as much as 180 kPa. At high P_O₂ 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. For longer periods such as in saturation diving, 0.4 bar can be tolerated over several weeks.
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.