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{{Short description|Equipment allowing or assisting the user to breath in a hostile environment}}
[[File:RAF Firefighter MOD 45154261.jpg|thumb|In an atmosphere that may be oxygen-deficient, or toxic, an air supply can be carried on the back.]]
A '''breathing apparatus''' or '''breathing set''' is equipment which allows a person to breathe in a hostile environment where breathing would otherwise be impossible, difficult, harmful, or hazardous, or assists a person to breathe.  A [[respirator]], [[medical ventilator]], or [[resuscitator]] may also be considered to be breathing apparatus. Equipment that supplies or recycles breathing gas other than ambient air in a space used by several people is usually referred to as being part of a [[life-support system]], and a life-support system for one person may include breathing apparatus, when the breathing gas is specifically supplied to the user rather than to the enclosure in which the user is the occupant.

Breathing sets may be classified by type in several ways:
* by breathing gas source: self-contained gas supply,  remotely supplied gas, or purified ambient air,
* by environment: underwater/hyperbaric, terrestrial/normobaric, or high altitude/hypobaric,
* by breathing circuit type: open, semi-closed, or closed circuit, 
* by gas supply type: constant flow, supply on demand, or supplemental,
* by ventilatory driving force: the breathing effort of the user, or mechanical work from an external source,
* by operational pressure regime: at ambient pressure or in isolation from ambient pressure,<!--is there a standard term for this?-->
* by gas mixture: air, oxygen enriched air, pure oxygen or mixed gases,
* by purpose: underwater diving, mountaineering, aeronautical, industrial, emergency and escape, and medical.

The user respiratory interface is the delivery system by which the breathing apparatus guides the breathing gas flow to and from the user. Some form of facepiece, hood or helmet is usual, but for some medical interventions an invasive method may be necessary.

Any given unit is a member of several types. The well-known recreational scuba set is a self-contained, open circuit, demand supplied, high pressure stored air, ambient pressure, underwater diving type, delivered through a bite-grip secured mouthpiece.

==Definition and scope==
[[Semantically]], the term ''breathing apparatus'' implies any set of equipment and materials specifically intended to enable or facilitate breathing, which could include equipment as basic as a [[Snorkel (swimming)|snorkel]] or [[artificial airway]], or as complex as an [[anaesthetic machine]] or a [[space suit]]. Actual usage varies, and breathing apparatus, breathing set, ventilator and respirator have similar and overlapping meanings which vary depending on the sources chosen.<ref name="Merriam Webster usage notes" /> ''Breathing set'' appears to be a secondary synonym for breathing apparatus, as internet searches appear to all be redirected to breathing apparatus. According to [[Merriam-Webster]], a ventilator can be a medical device to provide artificially assisted respiration, or equipment to circulate fresh air through a space, while a [[respirator]] is usually a mask worn to protect the user from particulate contaminants in the air, but can also mean a device for providing artificial respiration. The usage in the sense of a filtering mask dates to the early 19th century and the artificial respiration sense dates to the second half of the 19th century, so both are well established.<ref name="Merriam Webster usage notes" />

The UK [[Health and Safety Executive]] (HSE) distinguishes between respirators and breathing apparatus. Respirators are described as [[Air filter|filtering devices]], which may be powered, using a motor to pass ambient air through the filter, or unpowered, relying on the wearer's breathing to draw ambient air through the filter. The distinguishing features of a respirator in this context appear to be that the air is not significantly compressed at any stage, is filtered, and is at approximately ambient pressure. The HSE definition for breathing apparatus is that they use a supply of breathing quality gas from an independent source, such as air compressors or compressed gas cylinders. In this case compression of the supply gas at some stage is implied. Both respirators and breathing apparatus are classed as [[respiratory protective equipment]] by the HSE.<ref name=HSE />

Vocabulary.com describes a breathing apparatus as "a device that facilitates breathing in cases of respiratory failure", which is a functional description of a medical ventilator, or a resuscitator.<ref name="vocabulary.com" />

McGraw-Hill Dictionary of Scientific & Technical Terms defines breathing apparatus as "An appliance that enables a person to function in irrespirable or poisonous gases or fluids; contains a supply of oxygen and a regenerator which removes the carbon dioxide exhaled", which is the description of any type or application of rebreather.<ref name="McGraw-Hill"/>

===Breathing gas source===
The US [[Occupational Safety and Health Administration]] (OSHA) uses the source of the breathing gas to distinguish between types of breathing apparatus, and considers respirators to be a type or class of breathing apparatus:<ref name="OSHA definitions" />

An atmosphere-supplying respirator is a breathing apparatus that supplies the user with breathing gas from a source independent of the ambient atmosphere, such as [[supplied-air respirator]]s (SARs) and [[self-contained breathing apparatus]] (SCBA).<ref name="OSHA definitions" />

A self-contained breathing apparatus (SCBA) is a type of atmosphere-supplying breathing apparatus in which the breathing gas source is carried by the user.<ref name="OSHA definitions" />

A supplied-air respirator (SAR), or airline respirator, is a type of atmosphere-supplying breathing apparatus which uses a hose to supply breathing gas from a source which is not carried by the user.<ref name="OSHA definitions" /> 

An [[air-purifying respirator]] is a breathing apparatus which uses a filter, cartridge, or canister, to remove specific air contaminants by passing ambient air through the air-purifying component.<ref name="OSHA definitions" /> No distinction is made based on the mechanism of passing the air through the purifying component – it may be the lungs of the user or a mechanical device.

The breathing gas source may be the ambient atmosphere, [[compressed air]] supplied from a low pressure compressor in real time, [[oxygen enriched air]] supplied from an oxygen concentrator, high-pressure stored compressed air, [[supercritical compressed air]],<ref name="Tech briefs"/> oxygen or blended gas mixtures, [[liquid oxygen]], chemically generated oxygen, or a combination of ambient atmosphere and another of these sources.

==Breathing circuit type==
Breathing apparatus can be categorised by whether the gas is used once only, then discharged to waste in open circuit, or largely recycled in a closed or semi-closed circuit rebreather system.

===Open circuit===
{{Further|Underwater breathing apparatus#Classification}}
[[Open circuit breathing apparatus]] is any breathing apparatus that does not recycle any of the breathing gas, and discharges it all to the surroundings.<ref name="National Environmental Trainers" /> Open circuit breathing apparatus may be further classified as continuous flow (or free-flow), where the gas flows to the user at a constant flow rate, and some is inhaled in passing, or demand supplied, where flow is triggered by a pressure difference caused by inhalation, and automatically stops when there is no demand.<ref name="Harlow" /> Demand supply can be further classified as positive and negative pressure systems, based on the pressure maintained when flow has stopped, and whether the breathing gas pressure in the apparatus ever drops below ambient pressure. Open circuit systems without mixing during delivery are simple and the gas supplied is consistent and reliable.

Both constant flow and demand supply can also provide gas from two sources, one of them being the ambient atmosphere, in what is generally referred to as supplemental oxygen provision, frequently used for medical purposes where the user is at risk for medical hypoxia, and at high altitudes where the oxygen partial pressure is naturally low.<ref name="Drake 1974" />

===Closed and semi-closed circuit===
{{main|Rebreather|Diving rebreather}}
Closed and semi-closed circuit breathing sets, also known as [[rebreather]]s and [[gas extender]]s, are breathing apparatus that absorb the carbon dioxide from, and add oxygen to, a user's exhaled breath, allowing unused oxygen and diluent (if present) to be recycled. A rebreather system may be used for any application of a supplied gas breathing set. It may be more complex than open circuit if the mixture must be controlled, and for short endurance applications may be heavier. There may be a greater fire hazard due to high oxygen concentration. In other applications, when long endurance and reasonably light weight is required, it may allow a large saving of gas and be much simpler or lighter than the equivalent open circuit option. Rebreather systems can be closed or semi-closed circuit, have a [[pendulum rebreather|pendulum]] or [[breathing loop|loop]] flow path configuration, and the gas can be circulated by the breathing of the user through non-return valves, (almost all self-contained units), by the energy of the injected fresh gas, (Dräger Modell 1915 "Bubikopf", DM20 and DM40,<ref name="Dekker Modell 1912" /> and US Navy Mk V helium helmet gas extenders,<ref name="USNDM R1" />), or by an external power input (the oxygen in a space suit is circulated by an electric fan).<ref name="Paul et al 2010" />
When powered by breathing effort, rebreather units will have an elevated work of breathing, particularly with high gas densities at great depth, which is a limiting factor for diving rebreathers, even when the diluent is helium.<ref name="Anthony and Mitchell 2016" />

==Self-contained or remotely supplied==
Breathing apparatus can also be categorised as self-contained, where everything is carried by the user, or remotely supplied, with a hose to supply gas from the supply panel and in some cases a return hose for the exhaled gas.<ref name="US Navy Diving Manual R6" />

Remotely supplied applications include:
*[[Surface-supplied diving|Surface-supplied diving equipment]] (SSDE) is diving equipment supplied with breathing gas using a diver's umbilical or airline from the surface, such as from a boat or offshore platform.<ref name="US Navy Diving Manual R6" />
*[[supplied-air respirator]]s (SAR). or airline respirators, use a hose to supply breathing gas from a source which is not carried by the user.<ref name="OSHA definitions" /> 
*[[Built-in breathing systems]] in submarines or hyperbaric chambers.<ref name="US Navy Diving Manual R6" />
* Some space suits and pressure suits,<ref name="thomas" />
* Some [[diving rebreather]]s. When remotely supplied they are likely to be semi-closed circuit, and called [[gas extender]]s, and their main function is likely to be to save expensive helium diluent gas. In [[helium reclaim system]]s, exhaled gas is returned to the surface to be recycled.<ref name="MGD" />
*Flight crew breathing apparatus<ref name="cfinotebook" />
*Aircraft emergency oxygen systems for passengers in commercial airliners.
[[File:Recovery of Casualties After Primero Explosion.jpg|thumb|Recovery of casualties after the explosion at the Primero coal mine in Colorado, 1910]]
Self-contained applications include:
*[[Self-contained breathing apparatus]] (SCBA), used out of water, worn by rescue workers, firefighters and others in contaminated, toxic or hypoxic atmospheres
*[[Self-contained underwater breathing apparatus]] (scuba), used underwater for recreational and occupational diving<ref name="US Navy Diving Manual R6" />
*[[Escape set]]s, including underwater diving [[bailout set]]s
*Most [[rebreather]] sets,
*[[Mountaineering breathing apparatus]], which provides supplementary oxygen from a supply carried by the user,
*[[Air-purifying respirator]]s, which filter contaminants from ambient air.

==User respiratory interface==
The user respiratory interface, also commonly referred to as the facepiece, is the delivery system by which the breathing apparatus controls breathing gas flow to and from the user. The choice of interface type and the fit can significantly influence convenience, effectiveness, comfort, and sometimes safety. Several types are in use:<ref name="Bahamann et al 2018" />

===Nasal cannula===
A [[nasal cannula]] is relatively unobtrusive and is widely used for supplemental oxygen. The basic version is used to deliver continuous flow supplemental oxygen at rates from 1 to 6 litres per minute. It has two short prongs that fit into the nostrils for delivery, that are connected to a common tube, which is usually hooked over the ears for support.<ref name="Medical Dictionary" /> The more complex [[reservoir cannula]] is an oxygen conserving supplemental oxygen administration device which accumulates constant flow oxygen in a small reservoir below the nose during exhalation and delivers it in a bolus at the beginning of the next inhalation, which ensures that most of it reaches the parts of the lung in which gas exchange occurs, and little is wasted in dead space.<ref name="Dumont and Tiep 2002" />

===Nasal mask===
A [[nasal mask]] covers the nose and seals against the upper lip, the sides of the nose, and the bridge of the nose.<ref name="Bahamann et al 2018" />

===Nasal pillow mask===
A [[nasal pillow mask]] seals on the rim of the nostrils. It is used in stable patients with sleep-disordered breathing.<ref name="Bahamann et al 2018" />

===Artificial airway===
An [[artificial airway]] uses a medical device to provide a patent airway. This requires intervention by a competent person, and may be supraglottic, infraglottic, or surgically placed. These applications are mostly used in emergency medicine and surgery.

===Mouthpiece===
A [[Mouthpiece (scuba)|mouthpiece]], usually held in place by a bite-grip, and sealed by the lips, is common in scuba equipment, [[snorkel (swimming)|snorkels]], and some types of escape breathing apparatus.<ref name="NOAA Diving Manual 2001" /> A mouthpiece is simple and effective, with minimal dead space, and reliably seals without need for adjustment, but must be actively held in place by the user, and can cause jaw fatigue over long periods. A mouthpiece only allows mouth breathing of the delivered gas, and it may be necessary to block the nose to prevent bypass. A mouthpiece makes intelligible speech difficult or impossible, and eating or drinking require temporary removal.

===Oral mask===
An [[oral mask]] fits inside the mouth between the teeth and lips, with a guide to prevent the tongue from obstructing the airway. They are not often used.<ref name="Bahamann et al 2018" />

===Breathing mask===
A breathing mask, also called a facepiece, is a component which covers the mouth and nose, sometimes also the eyes and other parts of the face, and may seal against the face. A breathing mask is usually effective, allows mouth and nose breathing, and can usually be sealed adequately without effort by the user. A wide range of designs are available depending on the application. The disadvantages are that the user cannot eat or drink while the mask is in place, and some models may interfere with speech, while others may have relatively large dead space. Three basic configurations are distinguished by the area they cover.

The orinasal mask, also called oro-nasal, oral-nasal, or quarter mask, covers the mouth and nostrils and seals to the front of the face on the bridge and sides of the nose and mouth and the chin with little dead space.<ref name="Bahamann et al 2018" />

The half-mask extends below the chin, and the full-face mask covers the eyes as well as the nose and mouth, and can have a dead space so large that an inner orinasal mask is provided to reduce the dead space. 

A full-face mask is generally only used when including the eyes in the protected space is necessary, and often includes an inner orinasal mask to reduce dead space.

===Breathing hood===
A [[breathing hood]] is a type of respiratory interface that completely covers the head and neck, and optionally the shoulders or upper torso,<ref name="Reg 1910.134" /> with a loose-fitting bag, which may have a neck seal or be relatively close fitting at the neck or shoulders. They are used in escape breathing apparatus of several kinds ([[escape hood]]s),<ref name="safetygas" /> and as a route for supplementary oxygen ([[oxygen hood]]s). Breathing hoods with full length visors are commonly used with free-flow supplied air respirators for industrial work like in spray painting, boatbuilding, and woodworking workshops.<ref name="SATA" />

===Breathing helmet===
A [[breathing helmet]] is usually defined as a rigid respiratory user interface covering the head that also provides head protection against impact and penetration.<ref name="Reg 1910.134" /> In medical terminology, a breathing helmet is synonymous with a breathing hood, and need not have any rigid protective structure.<ref name="Easton and Wood 2020" /><ref name="Bahamann et al 2018" />

==Ambient pressure regimes==
Breathing apparatus may be used in various pressure regimes: [[wikt:hyperbaric|hyperbaric]] for diving, tunneling, and [[Caisson (engineering)|caisson]] work, [[wikt:normobaric|normobaric]] where the ambient atmosphere is unbreathable, or supplemental oxygen is needed for medical reasons, and [[wikt:hypobaric|hypobaric]] at high altitudes and in space.

===Hypobaric systems===
[[High altitude breathing apparatus]] is used for unpressurised (ambient pressure) aeronautical and mountaineering activities where the oxygen content of the natural atmosphere is insufficient for maintaining physical activity, consciousness, or life, but the atmospheric pressure is sufficient that a pressure suit is not needed.

Both rebreather and open circuit equipment have been used in this application, where either pure oxygen or supplemental oxygen is provided by the equipment. Minor leakage in either direction usually only affects efficiency and gas endurance, as the ambient air is usually only hypoxic due to low ambient pressure.<ref name="Drake 1974" />

===Normobaric systems===

Breathing apparatus are used for mine escape and rescue, firefighting, or working in hypoxic or toxic atmospheres at pressures near to normal [[atmospheric pressure]]. These may supply breathing gas at a slight overpressure, also known as [[positive pressure breathing|positive pressure]], to prevent contamination by ambient gas, as leakage from the breathing set is generally less harmful than breathing the ambient gas.

This subclass includes both self-contained and airline supplied units, and self-contained units may use rebreather technology to extend gas endurance.

===Hyperbaric systems===
A breathing set intended for use in hyperbaric environments must not supply gas with a [[oxygen toxicity|toxic concentration of oxygen]].<ref name="NOAA Diving Manual 2001" /> Most breathing sets for hyperbaric use are ambient pressure underwater breathing apparatus, but breathing apparatus may be necessary in a pressurised tunnel or caisson due to contamination by hazardous materials. Minor leakage to the environment is usually of little importance.

Open and closed circuit, self-contained, and remotely supplied systems are all in common use, but gas composition choice is complicated by the possibility of oxygen toxicity and decompression requirements. The possibilities of [[nitrogen narcosis]] and excessive gas density causing unacceptably high [[work of breathing]] make the use of [[helium]] as a diluent necessary for use at greater depths. The large range of pressures possible complicate decompression necessary to avoid [[decompression sickness]], and the use of special gas mixtures to [[accelerated decompression|accelerate decompression]] is fairly common. This either requires the diver to use [[gas switching|several mixtures]] at different depths which could be toxic if used at the wrong depth, or for closed circuit apparatus which provides reliable control and monitoring of the gas mixture. As a malfunction which cuts off breathing gas supply to a diver at depth could be rapidly fatal, [[bailout set|bailout breathing apparatus]] may be carried in addition to the primary gas supply.<ref name="US Navy Diving Manual R6" />

==Isolation from ambient pressure==
At extremes of ambient pressure, the user must be isolated from the environment to survive, as in [[single atmosphere diving suit]]s, where the occupant is kept at surface atmospheric pressure, isolated from the high ambient pressure of the deep underwater environment, and pressure suits and [[space suit]]s where the interior of the suit is pressurised above the external environmental pressure. In these applications it is usual to use oxygen rebreather systems, as they are relatively safe, simple and efficient compared to open circuit, and do not inherently affect suit internal pressure. [[Liquid air]] has also been used for space suits,<ref name="NASA" /><!--Liquid Air Backpack on the Mk III suit--> which implies an internal suit pressure close to normal atmospheric pressure, and open circuit. Leakage to or from the outside environment generally indicates a system failure and an emergency.<ref name="Thornton 2000" />

==Positive and negative pressure systems==
{{Further|Work of breathing|Breathing performance of regulators}}
Positive and negative pressure systems can have slightly different meanings in the context of breathing apparatus depending on whether the context is medical or non-medical applications. 

===Occupational health and safety===
In this context these terms refer to the protection of the breathing circuit against leakage of contaminants.

Positive pressure means that the area around the mouth or nose inside the facepiece remains slightly higher than the ambient pressure outside the breathing apparatus facepiece at all times while in use, so that ambient gas or liquid cannot leak into the breathing space.<ref name="OSHA definitions" /> This also has the physiological effect of assisting inhalation and resisting exhalation, but should not affect the total work of breathing.

Negative pressure means that the pressure inside the facepiece is lower than the ambient pressure outside the facepiece at some point during inhalation, and a good seal on the facepiece is required to prevent leakage of the ambient gas or liquid into the breathing space.<ref name="OSHA definitions" /> This pressure offset is usually constant over all or several breaths, depending on the cause, and has the opposite effect of assisting exhalation and resisting inhalation, also with no net effect on the total work of breathing.

===Underwater diving===
{{see|Human physiology of underwater diving#Positive and negative pressure breathing}}
The ambient pressure underwater varies with the depth, and diver attitude in the water can affect the variation in pressure between the lungs and the delivered gas in the mouthpiece by up to about 250&nbsp;mm water (25&nbsp;mb), but usually less, which can be positive or negative depending on the relative position of the lungs to the demand valve, exhaust valve of a free-flow helmet, or counterlung of a rebreather. Also called {{diving term|positive static lung load}} and {{diving term|negative static lung load}}<ref name="Anthony and Mitchell 2016" />

===Medical ventilation===
{{Further|Mechanical ventilation}}
In this context the terms refer to the mechanism of inducing inhalation in a person who is not spontaneously breathing. 

Positive pressure ventilation occurs when the breathing gas is delivered at a pressure higher than ambient, and gas is blown into the respiratory passages, inflating the lungs. This system is used by most mechanical ventilators and resuscitators.

Negative pressure ventilation occurs when the torso of the patient is subjected to an external pressure below ambient pressure, and ambient pressure air is drawn unto the lungs by the pressure difference induced by expansion of the chest. The equipment is traditionally known as an [[iron lung]].

==Applications==
Breathing sets may be used for breathing gas supply in a range of applications where the environment does not provide  suitable breathing gas:

===Underwater breathing apparatus===
{{main|Underwater breathing apparatus}}
[[File:Водолазные костюмы.jpg|thumb|A museum display of diving dry suits with different breathing apparatus configurations]]
Underwater breathing apparatus is any breathing apparatus intended to allow the user to breathe underwater, and includes open circuit scuba, diving rebreathers and surface supplied diving equipment, and both ambient pressure and single atmosphere systems.<ref name="US Navy Diving Manual R6" />

The major categories of [[ambient pressure]] underwater breathing apparatus are:
*[[Scuba set]], any breathing set that is carried entirely by an underwater diver and provides the diver with breathing gas at the ambient pressure:<ref name="NOAA Diving Manual 2001" />
**[[Open circuit scuba]], where the diver carries the gas supply, and exhaled gas is exhausted to the environment<ref name="US Navy Diving Manual R6" />
**[[Diving rebreather]]s, where the diver carries the gas supply, and exhaled gas is partly or entirely recycled for further use,<ref name="US Navy Diving Manual R6" /> and
*[[Surface supplied diving|Surface supplied diving equipment]], where the gas supply is provided from the surface through a hose in a [[diver's umbilical]]<ref name="US Navy Diving Manual R6" /> This may also be open circuit, either demand open circuit, or free-flow open circuit, semi-closed circuit [[gas extender]]s, or closed circuit [[helium reclaim system|helium reclaim]].<ref name="MGD" />

Two other types may also be identified:
*[[Escape set]]s provide a limited amount of breathing gas to allow the user to reach the surface from a disabled vessel or vehicle, such as a disabled submarine, a sunken armoured vehicle, or a ditched helicopter. These may also be open or closed circuit.
*Atmospheric pressure underwater breathing apparatus is also used, in the form of armoured [[atmospheric diving suit]]s, which maintain an internal pressure approximating surface pressure. Their breathing apparatus tend to be closed circuit [[oxygen rebreather]]s.<ref name="Thornton 2000" />

===Industrial breathing apparatus===
{{See also|Self-contained breathing apparatus|Rebreather#Fields of application}}
Breathing gas must be supplied for work in unbreathable normobaric atmospheres, which may be toxic, irritant, narcotic or hypoxic, and may include firefighting, damage control, exploration, and rescue work, and in [[Glossary of breathing apparatus terminology#normobaric environment|normobaric environments]] where contamination of the person ([[hazmat environment]]s) or the environment ([[clean room]]) must be avoided. Open circuit and rebreather systems can be used, and self-contained (SCBA) and remotely supplied systems are used depending on the requirement for mobility. Positive or negative pressure equipment may be appropriate, depending on what is to be protected from contamination.

A supplied-air respirator (SAR), also called an airline respirator, is a type of respiratory protection equipment used where the ambient atmosphere is unsuitable to breathe directly or after filtering at the user.<ref name="Homeland security" /><ref name="pksafety" /> The equipment may provide air on demand, at positive pressure, or may supply a constant flow at a rate greater than the user's peak demand rate.

Depending on the nature of the hazardous atmosphere, the user may need to wear personal protective equipment to isolate the entire body from the environment ([[hazmat suit]]).

===Emergency and escape breathing sets===
{{main|Escape breathing apparatus|Smoke hood|Built-in breathing system|Emergency oxygen system}}
[[File:US Navy 030312-N-0068T-036 Fireman Charles Phillips uses an Emergency Escape Breathing Device (EEBD) as he climbs up the emergence escape trunk while conducting a main machinery space fire drill.jpg|thumb|US Navy [[Emergency Escape Breathing Device]] (EEBD)]]
Escape breathing apparatus are a class of self contained atmosphere supplying or air purifying breathing apparatus for use in emergencies, intended to allow the user to pass through areas without a breathable atmosphere to a place of relative safety where the ambient air is safe to breathe. These are ambient pressure systems, and include:
*[[Helicopter escape set]]
*[[Mine escape set]]
*[[Submarine escape set]]
*[[Amphibious Tank Escape Apparatus]]
*[[Smoke hood]]

Early escape sets were often [[rebreather]]s and were typically used to escape from [[submarine]]s that were unable to surface. Escape sets are also used ashore, in the [[mining]] industry, and by the military for escape from tanks.

The small open-circuit scuba [[Helicopter Aircrew Breathing Device]] has the similar purpose of providing breathing gas to escape from a ditched helicopter.

Another type of emergency breathing set, which is remotely supplied, is [[built-in breathing systems]] in submarines and hyperbaric chambers.

[[File:US Navy 070208-N-8268B-001 Navy Diver 1st class Mike Barnett and Navy Diver 1st Class Chad Christensen test built-in breathing masks inside a recompression chamber.jpg|thumb|Navy divers testing the built-in breathing masks inside a recompression chamber]]
A ''built-in breathing system'' is a source of [[breathing gas]] installed in a confined space where an alternative to the ambient gas may be required for medical treatment, emergency use, or to minimise a hazard. They are found in [[diving chamber]]s, [[Hyperbaric medicine|hyperbaric treatment]] chambers, and [[submarines]].

The use in hyperbaric treatment chambers is usually to supply an oxygen rich treatment gas which if used as the chamber atmosphere, would constitute an unacceptable [[fire hazard]].<ref name="Divex" /><ref name="USNDM Ch21" /> In this application the exhaust gas is vented outside of the chamber.<ref name="Divex" /> In [[saturation diving]] chambers and [[surface decompression]] chamber the application is similar, but a further function is a supply of breathable gas in case of toxic contamination of the chamber atmosphere.<ref name="Divex" /> This function does not require external venting, but the same equipment is typically used for supply of oxygen enriched gases, so they are generally vented to the exterior.

In submarines the function is to supply a breathable gas in an emergency, which may be contamination of the ambient internal atmosphere, or flooding. In this application venting to the interior is both acceptable and generally the only feasible option, as the exterior is typically at a higher pressure than the interior, and external venting is not possible by passive means.

The [[Emergency oxygen system|emergency oxygen]] supplied to passengers in commercial airliners that have lost cabin pressure is also a basic form of built-in breathing system, where the oxygen is generated and supplied as a constant flow for a limited period, which should be sufficient to allow the aircraft to safely descend to an altitude where the ambient air oxygen content is sufficient to support consciousness.

[[Smoke hood]]s and other [[escape respirator]]s are used in many industrial environments where they may be needed to evacuate a building in a fire or other incident which may compromise the ambient air quality but there is likely to be sufficient oxygen remaining to sustain the necessary activity.

Emergency and escape breathing apparatus may provide purified ambient air where it has sufficient oxygen and it is reasonably practicable to purify it, or may supply stored breathing gas that is known to be respirable.

===Supplemental oxygen provision===
{{Main|Oxygen mask|Rebreather#Mountaineering rebreathers|Rebreather#Rebreathers for unpressurised aircraft and high altitude parachuting}}
Supplemental oxygen is oxygen additional to that available from atmospheric air at the ambient pressure. This may be necessary or desirable in hypobaric environments, or for medical purposes in any pressure regime. With supplemental oxygen the flow rate is often stipulated, but it is the partial pressure in the alveoli that is important to achieve the desired result, and that is strongly dependent on the delivery system of the breathing apparatus and the ambient pressure. Systems providing a constant flow rate of open circuit oxygen at the nose or mouth will waste a lot of the gas to dead space and during exhalation.

====Oxygen conserving devices====
{{See|Oxygen therapy#Oxygen conserving devices}}
A closed circuit rebreather is highly effective at conserving stored oxygen, but it makes no use of ambient oxygen, so its effectiveness at minimising use of stored oxygen depends on where it is used. It is most applicable where it is not possible to use enriched ambient gas, either because there is none (underwater and in space), because its pressure is too low (extreme altitude), because it does not contain a useful partial pressure of oxygen, or because the contaminants make the risk unacceptable.

The delivery of open circuit supplemental oxygen is most effective if it is made at a point in the breathing cycle when it will be inhaled to the alveoli, where gas transfer occurs. This is during the first part of inhalation. Oxygen delivered later in the cycle will be inhaled into [[physiological dead space]], where it serves no useful purpose as it cannot diffuse into the blood. Oxygen delivered during stages of the breathing cycle in which it is not inhaled is also wasted, unless it is stored temporarily.<ref name="Tiep and Carter 2008" />

A continuous constant flow rate delivered to the mouth and nose uses a simple regulator, but is inefficient as a high percentage of the delivered gas does not reach the alveoli, and over half is not inhaled at all. A system which accumulates free-flow oxygen during resting and exhalation stages, ([[reservoir cannula]]s, [[partial rebreather mask]]s and [[non-rebreather mask]]s) makes a larger part of the oxygen available for inhalation, and it will be selectively inhaled during the initial part of inhalation, which reaches furthest into the lungs, and may also recover the volume inhaled into dead space for re-use on the next breath if it can be accommodated by the reservoir bag. The flow rate must be matched to the breathing interface storage volume  and the user's breathing tidal volume and breathing rate for best efficiency, and the tidal volume and breathing rate can vary considerably over a short period with changes in exertion, so these methods are not very effective for an active user.

Delivery by demand valve avoids wastage of oxygen when the user is not actively inhaling, and when combined with a suitably calibrated dilution orifice can conserve a large proportion of the stored oxygen, but it still wastes oxygen to fill the anatomical and mechanical dead spaces, and it requires some physical effort by the user. 

Since the 1980s, devices have been available which conserve stored oxygen by delivering it during the stage of the breathing cycle when it is more effectively used. This has the effect of stored oxygen lasting longer, or a smaller, and therefore lighter, portable oxygen delivery system being practicable. This class of device can also be used with portable oxygen concentrators, making them more efficient.<ref name="Tiep and Carter 2008" />

A [[pulse dose oxygen conserving device]], (or demand pulse device) senses the start of inhalation and provides a metered bolus, which, if correctly matched to requirements, will be sufficient and effectively inhaled into the alveoli. Such systems can be pneumatically or electrically controlled.<ref name="Tiep and Carter 2008" />

Adaptive demand systems are a development in pulse demand delivery. They are devices that automatically adjust the volume of the pulsed bolus to suit the activity level of the user. This adaptive response is intended to reduce desaturation responses caused by exercise rate variation.<ref name="Tiep and Carter 2008" /> The exhaled gas from these devices is discharged to the environment, and the oxygen is lost, so they are less gas-efficient than closed circuit rebreathers, but do not have a carbon dioxide scrubber or counterlungs, which is a saving on weight and bulk, and make use of the oxygen available in the ambient air, so their efficiency is better at lower altitudes.

====High altitude supplemental oxygen====
[[Mountaineering breathing apparatus]] provides oxygen at a higher concentration than available from atmospheric air in a naturally hypoxic environment. It needs to be lightweight and to be reliable in severe cold, including not getting choked with deposited frost from the exhaled gas, which is saturated with water vapour at body temperature.<ref name=Hendricks1999 /> 

Both chemically generated and compressed gas oxygen have been used in experimental closed-circuit oxygen mountaineering systems, but open circuit has also been used, although relatively wasteful, as the equipment is reliable.<ref name="Hunt 1953" /><ref name="Drake 1974" />
For mountaineering at high altitudes where the user has to carry the stored oxygen, open circuit demand or closed circuit may be used to maximise endurance of the set.<ref name="Drake 1974" />

Breathing pure oxygen results in an elevated partial pressure of oxygen in the blood: a climber breathing pure oxygen at the summit of Mt. Everest has a greater arterial oxygen partial pressure than breathing air at sea level.  This results in being able to exert greater physical effort at altitude. The exothermic carbon dioxide absorption reaction of a rebreather helps keep the scrubber contents from freezing while it is in use, and helps reduce heat loss from the user.

Unpressurised aircraft and [[high altitude parachuting]] have similar requirements and working environment to mountaineering, but weight is less of a problem. <ref name="Drake 1974" />

====Oxygen therapy====
{{Main|Oxygen therapy}}
Oxygen therapy is the use of supplemental oxygen as [[medical therapy]].<ref name=BNF69 /> Acute indications for therapy include [[hypoxemia]] (low blood oxygen levels), [[carbon monoxide toxicity]], [[cluster headache]] and [[decompression illness]]. It may also be prophylactically given to maintain blood oxygen levels during the induction of [[anesthesia]].<ref name="WHO2008" /> Oxygen therapy is often useful in chronic hypoxemia caused by conditions such as severe [[COPD]] or [[cystic fibrosis]].<ref name="Jami2006" /><ref name=BNF69/> Partial pressures administered range from low flow rates giving slight increases over ambient air up to 2.8 bar absolute used in [[hyperbaric oxygen treatment]] of decompression illness and some other indications. Oxygen can be delivered to spontaneously breathing patients via [[nasal cannula]], [[Oxygen mask|face mask]], [[artificial airway]], or by [[built-in breathing system]] demand mask or [[oxygen hood]] in a [[hyperbaric chamber]].<ref name="Macintosh and Moore 1999" /><ref name="Dart 2004" /> Delivery may be by continuous flow, by [[bag reservoir mask]], on demand, or on [[pulse demand]].<ref name="Tiep and Carter 2008" />

Patients who are not able to breathe sufficiently for themselves are provided with breathing gas by ventilator or resuscitator.

===Medical breathing apparatus===
{{Main|Anaesthetic machine|Mechanical ventilator|Resuscitator}}
[[file:Maquet Flow-I anesthesia machine.jpg|thumb|An anaesthetic machine]]
An ''anaesthetic machine'' ([[British English]]) or ''anesthesia machine'' ([[American English]]) is a [[medical device]] used to generate and mix a fresh gas flow of medical gases and [[inhalational anaesthetic]] agents for the purpose of inducing and maintaining [[anaesthesia]].

====Anaesthetic machines====
The anaesthetic machine is commonly used together with a [[mechanical ventilator]], [[breathing circuit|breathing system]], [[suction (medicine)|suction equipment]], and [[Medical monitor|patient monitoring]] devices; strictly speaking, the term "anaesthetic machine" refers only to the component which generates the gas flow, but modern machines usually integrate all these devices into one combined freestanding unit, which is colloquially referred to as the anaesthetic machine for the sake of simplicity. In the developed world, the most frequent type in use is the ''continuous-flow anaesthetic machine'', which is designed to provide a supply of medical gases mixed with an accurate concentration of anaesthetic vapour, and to deliver this continuously to the patient at a safe [[pressure]] and flow. This is distinct from ''intermittent-flow anaesthetic machines'', which provide gas flow only on demand when triggered by the patient's own inspiration.

====Mechanical ventilators and resuscitators====
[[Mechanical ventilation]] is the provision of breathing gas to the user by the ventilator or resuscitator, when the user is unable to provide the driving forces to induce gas flow. Such [[artificial ventilation]] is a characteristic of resuscitation and may be provided by medical ventilators when needed. Two basic types of mechanical ventilation may be distinguished by the limiting mechanism. Some are pressure controlled, in which the delivery stops when a limiting pressure is reached, and others are volume controlled, in which a set volume is delivered for each breath. Both of these methods have limitations and may work sub-optimally in some circumstances.<ref name="Bishop" />

A ''ventilator'' is a type of equipment that provides [[mechanical ventilation]] by moving breathable air into and out of the [[lungs]], to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. Ventilators are [[computer]]ized [[microprocessor control|microprocessor-controlled]] machines, but patients can also be ventilated with a simple, hand-operated [[bag valve mask]].  Ventilators are chiefly used in [[intensive-care medicine]], [[home care]], and [[emergency medicine]] (as standalone units) and in [[anesthesiology]] (as a component of an [[anesthesia machine]]).

A ''resuscitator'' is a device using positive pressure to inflate the lungs of an [[Syncope (medicine)|unconscious]] person who is [[apnea|not breathing]], in order to keep them [[oxygen]]ated and alive.<ref name="Merriam Webster" /> There are three basic types: a ''manual'' version (also known as a ''[[bag valve mask]]'') consisting of a mask and a large hand-squeezed plastic bulb using ambient air, or with supplemental oxygen from a high-pressure tank. The second type is the ''expired air'' or breath powered resuscitator, and the third type is an ''oxygen powered'' resuscitator. These are driven by pressurized gas delivered by a regulator, and can either be automatic or manually controlled. The most popular type of gas powered resuscitator are time cycled, volume controlled ventilators, (or constant volume ventilators). In the early days of pre-hospital emergency services, pressure cycled devices like the [[Pulmotor]] were popular but gave less satisfactory results. Most modern resuscitators are designed to allow the patient to breathe on their own should they recover the ability to do so. All resuscitation devices should be able to deliver more than 85% oxygen when a gas source is available.{{cn|date=July 2023}}

There is considerable overlap between ''ventilator'' and ''resuscitator''. The difference may mainly be in the way the equipment is used.

There are three modes of mechanical ventilation, which are the ways in which a breath is delivered by a medical ventilator: In control mode, each breath is mechanically delivered, but may be  triggered by a timing mechanism or by patient effort. These breaths may be volume or pressure controlled. In supported or spontaneous mode, each breath is triggered by the patient, and supported by ventilator. In combination mode, there is a combination of controlled and supported breaths, and there may be a combination of volume controlled and pressure supported or controlled breaths.<ref name="ventilator terminology" />

===High altitude breathing apparatus===
[[File:Edmund Hillary and Tenzing Norgay.jpg|thumb|Edmund Hillary and Tenzing Norgay, 29 May 1953 after successfully completing the first ascent of Mount Everest using open circuit supplemental oxygen]]
{{main|High altitude breathing apparatus}}
{{See also|Aircraft breathing apparatus|Mountaineering breathing apparatus}}
[[High altitude breathing apparatus]] is used in aviation as standard equipment in unpressurised aircraft capable of high altitude flight, as emergency equipment in unpressurised aircraft, and in high altitude mountaineering.

====Environmental influence====
At ''high altitude'', from {{convert|1500 to 3500|m|feet}} there are physiological effects of the reduced oxygen partial pressure which include reduced exercise performance and increased respiratory rate. [[Arterial oxygen saturation]] is generally still over 90% in healthy people, but arterial P<sub>O<sub>2</sub></sub> is reduced.<ref name="Paralikar and Paralikar" /> 

At ''very high altitude'', from {{convert|3500 to 5500|m|feet}} arterial oxygen saturation falls below 90% and arterial P<sub>O<sub>2</sub></sub> is reduced to the extent that extreme [[hypoxemia]] may occur during exercise and sleep, and if [[high altitude pulmonary edema]] occurs. In this range severe altitude illness is common.<ref name="Paralikar and Paralikar" />

At ''extreme altitude'', above {{convert|5500|m|feet}}, one can expect significant hypoxemia, [[hypocapnia]] and [[alkalosis]], with progressive deterioration of physiological function, which exceeds acclimatisation. Consequently, there is no human habitation in this altitude range.<ref name="Paralikar and Paralikar" />

====Physiological effects====
{{See also|Altitude sickness|Effects of high altitude on humans}}
In the region from sea level to around {{convert|3000|m|abbr=on|-3}}, known as the ''physiological-efficient zone'', oxygen levels are usually high enough for humans to function without [[supplemental oxygen]] and [[altitude decompression sickness]] is rare.

The ''physiological-deficient'' zone extends from {{convert|3600|m|abbr=on|-3}} to about {{convert|15000|m|abbr=on|-4}}. In this zone there is an increased risk of [[Hypoxia (medical)|hypoxia]], trapped-gas [[dysbarism]] (where gas trapped in the body expands), and evolved-gas dysbarism (where dissolved gases such as nitrogen may form in the tissues, i.e. [[decompression sickness]]).<ref name="altitude" /> Above approximately {{convert|4300|m|abbr=on|-3}} oxygen-rich [[Breathing gas|breathing mixture]] is required to approximate the oxygen available in the lower atmosphere,<ref name="cfinotebook" /> while above {{convert|12000|m|abbr=on|-4}} oxygen must be provided under positive pressure.  Above {{convert|15000|m|abbr=on}}, respiration is not possible because the pressure at which the lungs excrete carbon dioxide (approximately 87 mmHg) exceeds outside air pressure.{{citation needed|date=July 2023}}  Above {{convert|19000|m|abbr=on}}, known as the [[Armstrong limit]], exposed fluids in the throat and lungs will boil away at normal body temperature, and pressure suits are needed. Generally, 100% oxygen is used to maintain an equivalent altitude of {{convert|3000|m|abbr=on|-3}}.

====Physiogical acclimatisation====
{{See|Altitude acclimatisation}}
People can become acclimatised to an altitude of {{convert|17,000 to 18,000|feet|m|order=flip}} if they remain at high altitude for long enough, but for high altitude rescue work, rescue teams must be rapidly deployed, and the time necessary to acclimatise is not available, making oxygen breathing equipment necessary above approximately {{convert|12,000|feet|m|order=flip}}.<ref name="Drake 1974" />

====Theoretical solutions====
An oxygen partial pressure equivalent to sea level can be maintained at an altitude of {{convert|34,000 |feet|m|order=flip}} with 100% oxygen. Above {{convert|40,000|feet|m|order=flip}}, positive pressure breathing with 100% oxygen is essential, as without positive pressure even very short exposures to altitudes above {{convert|43,000 |feet|m|order=flip}} lead to loss of consciousness.<ref name="Pilmanis and Sears 2003" /> Oxygen conservation devices may be used with open circuit breathing apparatus to improve efficiency of gas use at lower altitudes where ambient pressure breathing is viable.

====Management====
At high enough altitudes the partial pressure of oxygen in the air is insufficient to support useful work and consciousness, even after acclimatisation, and at even higher altitudes it cannot support human life. At altitudes where the problem is hypoxia, breathing gas with a higher oxygen content at ambient pressure is a viable solution. Supplemental oxygen sufficient to provide an equivalent altitude of a pressurised aircraft cabin (about 8000ft) is sufficient for many purposes, but higher concentrations, such as sea level equivalent (P<sub>O<sub>2</sub></sub> of about 0.21 bar), can allow a greater capacity for aerobic work. Balanced against this is the need to conserve oxygen and to minimise the weight carried by the user of breathing apparatus. 

====Practical aspects====
Where the user must carry the supplementary oxygen supply, and also perform significant work over a fairly long period, as in mountaineering and rescue work, the efficiency of oxygen use and the reliability of the breathing apparatus are more important, and there is a trade-off of these characteristics with the weight that must be carried.

The amount of supplementary oxygen needed to bring the inhaled partial pressure to sea level equivalent, or any other fixed value greater than that of the ambient atmosphere is a function of the altitude, and increases with an increase in altitude in direct proportion to pressure drop. The amount of supplementary oxygen actually used is also proportional to [[respiratory minute volume]], which depends on the level of exertion. 

====Oxygen concentrators====
{{main|Oxygen concentrator}}
When there is no limitation on power use and the work is to be done at a fixed location, oxygen concentrators may be an effective solution.<ref name="oxygen concentrator" /> An oxygen concentrator is a device that concentrates the [[oxygen]] from a gas supply (typically ambient air) by selectively removing nitrogen to supply an oxygen-enriched product gas stream. They are also used industrially and as [[medical device]]s for [[oxygen therapy]].<ref name="oxygentimes" /> Two methods in common use are [[pressure swing adsorption]] and [[membrane gas separation]]. They are most efficient when the supplemental oxygen does not need to be at a high percentage.

Pressure swing adsorption oxygen concentrators use a [[molecular sieve]] to adsorb gases and operate on the principle of [[Pressure swing adsorption#Rapid PSA|rapid pressure swing adsorption]] of atmospheric [[nitrogen]] onto [[zeolite]] minerals at high pressure. This type of adsorption system is therefore functionally a nitrogen scrubber, leaving the other atmospheric gases to pass through, with oxygen as the primary gas remaining.<ref name="Ruthven 1993" /> Gas separation across a membrane is also a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is a generally non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The membrane gas separation equipment typically pumps gas into the membrane module and the targeted gases are separated based on difference in diffusivity and solubility.<ref name="Chong et al 2016" /> Product gas can be delivered directly to the user through a suitable breathing apparatus.

Pulse dose (also called intermittent-flow or on-demand) [[portable oxygen concentrator]]s are the smallest units, which may weigh as little as {{convert|5|lb|kg|order=flip}} Their small size enables the user to waste less of the energy gained from the treatment on carrying them. The unit administers a set volume (bolus) of oxygen enriched air at the start of each breath, which is the part of the breath most likely to reach the gas exchange regions of the lung beyond the physiological dead space. Their ability to make efficient use of oxygen is key to keeping the units compact.<ref name="Pulse" />

====Closed circuit oxygen rebreathers====
{{See|Oxygen rebreather}}
In a closed circuit system, any unused oxygen is retained and rebreathed, so the utilisation is close to 100%, with some losses possible due to expansion on increased altitude and incidental leakage from the breathing loop. 

There is a risk of [[pulmonary oxygen toxicity]] if the pressure of the oxygen exceeds about 0.5 bar for extended periods, which could happen at altitudes below 5500 m, where atmospheric pressure is about half of the value at sea level.<ref name="NOAA Diving Manual 2001" />

A closed circuit oxygen rebreather is the most efficient in terms of oxygen use, but is relatively bulky and requires the use of a carbon dioxide absorbent, which must either be sufficient for the oxygen supply, or must be periodically replaced. If the oxygen supply fails, the loop gas can become more hypoxic than ambient atmosphere if the loop was not adequately purged or if it gets contaminated by ambient air. In the absence of oxygen monitoring the user may not notice the reduction in oxygen concentration.<ref name="Drake 1974" />

A closed-circuit oxygen system was tested by [[Tom Bourdillon]] and [[Charles Evans (mountaineer)|Charles Evans]] during the 1953 British expedition to Mount Everest.<ref name="Drake 1974" />

====Open circuit dilutor demand regulator====
The [[dilutor demand regulator]] was developed and extensively used for high altitude flying during WWII. A dilutor demand regulator draws ambient air into the mask through an orifice in the regulator, while concurrently being fed with pure oxygen through a demand valve in the regulator. For aeronautical use the size of the ambient air orifice is controlled by an aneroid valve operator and is directly proportional to atmospheric pressure. As the altitude increases, the pressure decreases and the orifice gets smaller, so the user is provided with a higher proportion of oxygen, and when correctly calibrated, the partial pressure of oxygen in the mixture remains fairly consistent at a value similar to the 0.21 bar at sea level. This system makes efficient use of a combination of ambient and stored oxygen.<ref name="Drake 1974" /> 
The function of the aneroid valve operator can be substituted for terrestrial use by a simpler, lighter, and more rugged manually operated orifice selector knob, giving a stepwise range of concentrations which is lighter, more reliable, a bit less efficient, and requires appropriate selection by the user. It also allows the user to manually adjust the mixture to match personal needs. As it is manually selected, It is less suitable for flying, and more suitable for pedestrians who will not change altitude rapidly.<ref name="Drake 1974" />
The flow rates through the orifice and regulator are sensitive to flow rate of inhalation, and can be designed to provide a somewhat higher oxygen partial pressure at higher inhalation flow rates, which helps compensate for higher exertion.<ref name="Drake 1974" />

====Obligatory pressurisation zone====
{{main|Pressure suit|Space suit}}
<!-- if there is a formal name or definition for the zone, I am unaware of it-->
This is the zone where 100% oxygen at ambient pressure is insufficient, and some form of pressurisation is required to provide a viable inhalation oxygen pressure. The options are partial pressurisation  and full pressurisation.

A pressure suit is a [[protective suit]] worn by high-altitude pilots who may fly at altitudes where the [[air pressure]] is too low for an unprotected person to survive, even breathing pure oxygen at [[positive pressure]]. Such suits may be either full-pressure (e.g., a [[space suit]]) or partial-pressure (as used by [[aircrew]]). Partial-pressure suits work by providing mechanical counter-pressure to assist breathing at altitude.

[[File:Aldrin Apollo 11 cropped.jpg|thumb|upright|[[Apollo/Skylab spacesuit|Apollo spacesuit]] worn by astronaut [[Buzz Aldrin]] on [[Apollo 11]], with completely self-contained life support for lunar excursions.]]
[[File:Iss009e29620.jpg|thumb|upright|[[Orlan space suit]] worn by astronaut [[Michael Fincke]] outside the [[International Space Station]], which has a remote supply via the umbilical.]]
A [[space suit]] is a garment worn to keep a human alive in the harsh environment of [[outer space]], primarily as protection from [[Vacuum (outer space)|vacuum]] and temperature extremes. The breathing gas is pure oxygen, which allows the lowest suit pressure. Space suits are often worn inside [[spacecraft]] as a safety precaution in case of loss of cabin [[pressure]], and are essential for [[extravehicular activity]] (EVA). Modern space suits augment the basic pressure garment with a complex system of equipment and environmental systems designed to keep the wearer comfortable, and to minimize the effort required to bend the limbs, resisting a soft pressure garment's natural tendency to stiffen against the vacuum. A self-contained [[oxygen]] supply and environmental control system may be used to allow greater freedom of movement, independent of the spacecraft.<ref name="thomas" />

Three types of space suits exist for different purposes: IVA (intravehicular activity), EVA (extravehicular activity), and IEVA (intra/extravehicular activity). IVA suits are meant to be worn inside a pressurized spacecraft, and are therefore lighter and more comfortable. IEVA suits are meant for use inside and outside the spacecraft, such as the [[Gemini space suit|Gemini G4C]] suit. They include more protection from the harsh conditions of space, such as protection from [[micrometeoroid]]s and extreme temperature change. EVA suits, such as the [[Extravehicular Mobility Unit|EMU]], are used outside spacecraft, for either planetary exploration or spacewalks. They must protect the wearer against all conditions of space, as well as provide mobility and functionality.<ref name=thomas />

==Safety==
Breathing apparatus is usually used as personal protective equipment, and the user should be safer using it than without it in the same environment if it is needed, but there are hazards associated with its use. Some are specific to the apparatus and others are more general. The more obvious generic hazards are loss of gas supply, contamination of gas supply, and inappropriate gas supply. The consequences may include hypoxia, hyperoxia, hypercapnia, and poisoning or infection by contamination of the breathing gas due to leaks. Where high oxygen concentrations are provided, there may be a fire hazard, where high pressure gas storage is used, there are hazards associated with the high pressure equipment, and where liquid oxygen is used there are hazards of extreme cold. 

The usual methods of risk management include  design standards, quality control during manufacture, testing and certification of equipment, appropriate training of operators, regulation of use, as appropriate to the specific equipment and situations in which it is used, and correct selection of equipment for the situation. For some equipment, proper maintenance and pre-use inspection and testing are required.

==Human factors in breathing apparatus design==
{{See also|Human factors in diving equipment design#Breathing apparatus}}
Human factors in breathing apparatus design are the influence of the interaction between the user and the equipment on the design of the equipment. The user of breathing apparatus relies on the equipment to stay alive or healthy, in reasonable comfort and to perform the tasks required during use of the equipment. The design of the equipment can strongly influence its effectiveness in performing the desired functions. It should be comfortable to wear, and not cause [[stress injury]] or [[allergic reactions]] to component materials. It must be reliable and should not require constant attention or adjustment during use, and if possible performance should degrade gradually in the event of malfunctions, allowing time for corrective action to be taken with minimum risk.<ref name="Lundgren and Warkander 2000" />

Users vary considerably in [[Anthropometry|anthropometric dimensions]], [[physical strength]], stamina, joint flexibility, etc. Breathing apparatus should allow as full a range of physical function as reasonably practicable and should be matched to the user, the environment and the task. The interface between equipment and user can strongly influence functionality.<ref name="Bitterman" /> Breathing apparatus may be used by a wide range of users and must work for them all. Where correct operation and use of equipment is critical to user safety, it is desirable that different makes and models for the same application should work similarly, to facilitate rapid familiarisation with new equipment. Where this is not possible, additional training for the required skills may be necessary, and for medical interventions it may be necessary for a skilled operator to set up the apparatus and monitor its function while in use.

The user of breathing apparatus may be supported by a team who are available to assist to the extent necessary to reduce the risk associated with the use of the apparatus to a level acceptable in terms of the governing regulations and codes of practice. <ref name="CoP Inshore" /><ref name="Diving at Work Regulations 1997" /><ref name="IMCA D014" />

Breathing apparatus is used to facilitate breathing in hazardous conditions or where the user needs assistance to respire adequately. The primary requirements are to keep the user alive and healthy during and after use. Secondary requirements include providing user comfort, and sufficient capacity to perform the intended activities. The user is an integral part of the system, which may rely on user competence as well as correct equipment function for safe operation.<ref name="AAUSascents Lang et al" />

[[Fault tolerance]] is the property that enables a system to continue operating properly in the event of the failure of some of its components. If its operating quality decreases at all, the decrease is proportional to the severity of the failure, as compared to a naively designed system, in which even a small failure can cause total breakdown. Fault tolerance is particularly important in [[high availability]] or [[safety-critical system]]s. The ability to maintain functionality when portions of a system break down is referred to as '[[graceful degradation]]'.<ref name="Gonzalez et al 1997" /> Some items of breathing apparatus, and the user, may be regarded as safety critical components of the system, and should therefore be tolerant of faults. In the case of the user, this is achieved by sufficient fitness to perform the intended task, [[Competence (human resources)|competence]], and [[situational awareness]]. Equipment must be selected which is appropriate for the specific use, and can be designed, manufactured, and maintained to provide appropriate fault tolerance. Good [[ergonomic design]] minimises the opportunity for user error.

===Work of breathing===
{{Main|Work of breathing}}
[[File:Breathing Resistance.svg|thumb|Graph of the breathing resistance of an open-circuit demand regulator. The area of the graph (green) is proportional to the net mechanical work of breathing for a single breathing cycle]]
Breathing apparatus must allow the user to breathe with minimum added [[work of breathing]], and minimise additional [[Dead space (physiology)|dead space]].

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.<ref name="Medical dictionary HPN" />

The total work of breathing when using a breathing apparatus is the sum of the physiological work of breathing and the mechanical work of breathing of the apparatus.
In a normal resting state the physiological work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness<ref name="Mosby's" /> or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.<ref name="Mitchell" />

===Breathing mask construction===
{{See also|Human factors in diving equipment design#Masks and helmets}}
[[File:Craig Mainprize.png|thumb|Ocean Reef Full Face Mask (IDM)]]
[[File:Inside a KM37 diving helmet PA268016.jpg|thumb|Inside view of a Kirby Morgan 37 showing the oral-nasal mask used to minimise dead space, the microphone and a loudspeaker of the communications system]] 
Breathing masks and diving helmets both provide a supply of breathing gas to the user. Other functions may differ or partly overlap.<ref name="adolfson" />

==== Seal ====
If the mask is intended to be used in a hostile environment and contamination of the gas supply must be prevented, the mask must form an airtight or watertight seal around the edges, regardless of the position of the user. This seal is between the elastomer skirt of the mask and the skin of the face. Fit of mask affects the seal and comfort and must account for variability of face shapes and sizes. This is less of a problem with full-face masks and less again with diving helmets, but other problems affect these, like overall head size, and neck length and circumference, so there is still a need for adjustment and a few size options.<ref name="NOAA Diving Manual 2001" />

Seals may be compromised by hair passing under the seal, and the amount of leakage will depend on the amount of hair and in some cases, the position of the compromised part of the seal.<ref name="NOAA Diving Manual 2001" />

==== Ear clearing ====
{{see also|Ear clearing}}
The gas space in a breathing mask is inherently self-equalising for reasonably gradual pressure changes. If the mask is to be used where the ambient pressure may change significantly, the user must be able to equalise the pressure in the middle ears, which for many people requires a method to block the nostrils.<ref name="NOAA Diving Manual 2001" />

Diving helmets and most full-face masks do not allow the user finger access to the nose, and various mechanical aids have been tried with varying levels of comfort and convenience.<ref name="PDC manual" /><ref name="NOAA Diving Manual 2001" /> Masks for supplemental oxygen may be soft enough to close the nose with the mask in place, or may be temporarily removed. Masks for use in contaminated atmospheres are usually used at a constant ambient pressure, so this problem may not arise.

==== Vision ====
{{see also|Underwater vision}}
[[File:Lieutenant Eddie, executive officer of Underwater Construction Team 1, tests Mark 12 diving gear during a training exercise - DPLA - 1b3ae13550c309fdf7e5018af2ff811a.jpeg|thumb|Diver using US Navy Mark 12 freeflow helmet which has unusually large viewports.]]
The [[field of vision]] of the user of a full-face breathing mask or helmet is reduced by opaque parts of the helmet or mask. Peripheral vision can be particularly reduced in the lower areas due to the bulk of the demand valve. Helmet design is a compromise between low mass and inertia, with relatively small interior volume and viewports affording restricted field of vision, and large viewports with large interior volume. Siting the viewport close to the eyes helps provide a better view but is complicated by the need for sufficient clearance in front of the nose for a wide range of divers. Cylindrically curved viewports can introduce [[Underwater vision#Humans|visual distortions]] underwater that can reduce the effectiveness of the diver at judging distance, but are common in masks used in air. Spherical viewport surfaces are generally used in recent atmospheric suits for structural reasons, and work well when the interior volume is large enough. They can be made wide enough for adequate peripheral vision. Field of vision in helmets is affected by mobility of the helmet. A helmet directly supported by the head can rotate with the head, allowing the user to aim the viewport at the target, but peripheral vision is constrained by the dimensions of the viewport. The weight in air and unbalanced buoyancy forces when immersed must be carried by the neck, and inertial and hydrodynamic loads must be carried by the neck. A helmet fixed to a breastplate or space suit is supported by the torso, which can safely support much greater loads, but it does not rotate with the head. The entire upper body must rotate to direct the field of vision. This makes it necessary to use larger viewports so the user has an acceptable field of vision at times when rotating the body is impractical. The need to rotate the head inside the non-rotatable helmet requires internal clearance, therefore a large volume.

<!-- Vision: Fogging -->
[[File:IDA-71 mask, DSV and breathing hoses P5167748.JPG|thumb|IDA-71 mask, showing the central wiper blade which is operated using the handle projecting from the top of the mask]]
The inside surface of the viewport of a mask or helmet tends to be prone to fogging when the external environment is colder than the dew pint of the gas inside, where tiny droplets of condensed water disperse light passing through the transparent material, blurring the view. Treating the inside surface with a defogging surfactant can reduce fogging, but it may occur anyway, and it must be possible to actively defog, either by rinsing with water or by blowing dry air over it until it is clear. A [[spitcock]] may be provided on [[standard helmet]]s for rinsing. Demand helmets may have a free-flow supply valve which directs dry air over the inside of the facepiece. Full-face diving masks may use either rinsing or free-flow, depending on whether they are intended primarily for scuba or surface-supply. Full-face masks and helmets may also direct the flow of fresh dry gas over the inside surface of the viewport before it is inhaled, and prevent warm, moist exhaled gas from reaching the faceplate by using an oro-nasal insert with a non-return valve in the exhaust flow path.<!--Kirby Morgan EXA and possibly others --> In situations where rinsing is not an option, an [[anti-fog|anti-fogging surfactant]] can be applied to the viewport surface to prevent the formation of droplets. A manually operated wiper blade has also occasionally been used for removing condensation from the viewport. <!--IDA71 rebreather, maybe others-->

<!-- Vision: Optical correction -->
Users who need optical correction have choices. Contact lenses can be worn under all types of masks and helmets. Regular spectacles can be worn in most helmets, but can not be adjusted. Corrective lenses can be fitted to the inside of some full-face masks, but the distance from the eyes to the lenses may not be optimal. Bifocal arrangements are available. Defogging of bonded lenses is the same as for plain glass.

<!-- Vision: Bubbles -->
Open circuit breathing apparatus for diving produces exhalation gas bubbles at the exhaust ports. Free-flow systems produce the largest volumes, but the outlet can be behind the viewports so it does not obscure the diver's vision. Demand systems must have the second stage diaphragm and exhaust ports at approximately the same depth as the mouth or lungs to minimise work of breathing. To get consistent breathing effort for the range of postures the diver may need to assume, this is most practicable when the exhaust ports and valves are close to the mouth, so some form of ducting is required to direct the bubbles away from the viewports of helmet or mask. This generally diverts exhaust gases round the sides of the head, where they tend to be rather noisy as the bubbles rise past the ears. Closed circuit systems vent far less gas, which can be released behind the diver, and are significantly quieter. Diffuser systems have been tried, but have not been successful for open circuit equipment, though they have been used on rebreathers, where they improve stealth characteristics.

==== Security ====
Masks held in place by adjustable straps can be knocked off or moved from the correct position, allowing ambient atmosphere or water to flood in, and the loss of breathing gas. Full-face breathing masks are more easily dislodged due to their size, and need to be more securely supported, usually by 4 or 5 adjustable straps, connected at the back of the head, but it is possible for them to be dislodged, so it must be possible for the user to refit them and purge the mask sufficiently to continue breathing. Helmets are much more securely attached, and it is considered an emergency if they come off the head.

==== Internal volume ====
The volume of [[Dead space (physiology)|dead space]] is important for all breathing apparatus. Internal [[oro-nasal mask]]s are often used to minimise internal dead space in helmets and full-face masks, and may also reduce the tendency to fog the inner surface of the viewport.

==== Helmet buoyancy ====
{{See|Human factors in diving equipment design#Helmet buoyancy}}
A [[lightweight demand diving helmet]] is ballasted to be nearly neutrally buoyant underwater so it is not an excessive static load on the neck.

[[Free-flow diving helmet]]s compensate for a potentially large dead space by a high gas flow rate, so that exhaled gas is flushed away before it can be rebreathed. They tend to have a large internal volume, and be heavier than demand helmets, and usually rest on the shoulders to prevent over-stressing the neck, so do not move with the head. Underwater, excess buoyancy is counteracted by connection to the diver's [[Diving weighting system|weighting system]] or by {{diving term|jocking strap}}s.

====SCBA harness====
When the user must carry the gas supply, weight, balance and inertia of the apparatus and the load distribution of the harness can make a large difference to comfort and safety, particularly when the user may be required to do heavy work in difficult conditions, as in firefighting, rescue and mountain climbing. The user must have as much freedom of movement as is reasonably practicable, and at least enough to safely carry out the expected tasks' while the set must remain securely in place during the necessary maneuvers. Access to the valves and pressure gauge is important for gas management, and it is helpful when equipment is shared by a team that the fit can be easily and quickly adjusted to suit the individual.
For diving the buoyancy and buoyancy distribution are important to safety.

===Gas management===
{{See also|Scuba gas management}}
[[File:Комбинированный СИЗОД ШР и АДА.jpg|thumb|Airline supplied respirator with emergency gas supply cylinder.]]
For supplied gas breathing apparatus, it is usually highly undesirable, and may well be an emergency, to run out of gas unexpectedly. Monitoring remaining gas, identifying low gas levels in time to take appropriate action, and where necessary, bailing out to an available backup system are necessary items of gas management.<ref name="Jablonski 2006" />

The most fundamental aspect of gas management is to have a realistic idea of the expected [[gas endurance|endurance]] of the currently available gas, and how this will be affected by exertion in foreseeable circumstances.
Periodical checks on the remaining gas pressure is the usual monitoring method, for which the usual equipment is a [[cylinder pressure gauge]] attached to the regulator first stage. When the user is likely to be [[Task loading|task loaded]] to the extent that failing to check gas pressure is reasonably likely, a low gas alarm. a manual reserve switchover, or both are prudent. All three of these may be found on industrial breathing sets used for rescue and firefighting. For underwater diving, pressure gauges are standard, with an [[alternative gas supply]] system selected from an acceptable option of scuba bailout set, gas supplied by a buddy diver, or emergency ascent to the surface. The choice depends on the risk assessment, and in some cases standard operating procedures or code of practice. [[Reserve valve]]s are also occasionally still used in low visibility conditions. The [[buddy system]] and [[emergency ascent]]s are frequently used by recreational divers in shallow open water with no planned decompression obligation. [[Occupational diver]]s may be obliged to carry an independent [[bailout set]],<ref name="IMCA D014" /> and [[technical diver]]s may have several contingency plans for the reasonably foreseeable situations that could occur that would compromise their breathing gas supply.<ref name="Jablonski 2006" />

When more than one breathing gas mixture is available, the risk of selecting a gas unsuitable for the current situation must be minimised.<ref name="Jablonski 2006" />

In medical equipment, the user interface of the control and monitoring system can influence the probability of operator error.<ref name="Jiang et al 2018" />

==See also==
*[[Glossary of breathing apparatus terminology]]
* {{annotated link|Breathing mask (disambiguation)}} 
* {{annotated link|Emergency Air Breather}}
* {{annotated link|Gas mask}}
* {{annotated link|Hazmat suit}}
* {{annotated link|Iron lung}}
* {{annotated link|Life-support system}}
* {{annotated link|Respirator}} 
* {{annotated link|Resuscitator}}
* {{annotated link|Ventilator}}

==References==
{{Reflist|refs=

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<ref name="Divex">{{cite web |url=http://www.beta.jfdefence.com/wp-content/uploads/2015/09/DE-MDS-540-R0.pdf |title=Ultralite 2 BIBS Mask (DE-MDS-540-R0) |publisher=Divex |access-date=25 September 2018 |archive-date=25 September 2018 |archive-url=https://web.archive.org/web/20180925180701/http://www.beta.jfdefence.com/wp-content/uploads/2015/09/DE-MDS-540-R0.pdf |url-status=live }}</ref>

<ref name="Diving at Work Regulations 1997" >{{cite web |url=http://www.legislation.gov.uk/uksi/1997/2776/made |title=The Diving at Work Regulations 1997 |author=<!--Staff writer(s); no by-line.--> |year=1977 |work=Statutory Instruments 1997 No. 2776 Health and Safety |publisher=Her Majesty's Stationery Office (HMSO) |location=Kew, Richmond, Surrey |access-date=6 November 2016 |archive-date=31 October 2019 |archive-url=https://web.archive.org/web/20191031191337/http://www.legislation.gov.uk/uksi/1997/2776/made |url-status=live }}</ref>

<ref name="Drake 1974" >{{cite report |url=https://apps.dtic.mil/sti/tr/pdf/AD0776384.pdf |title=Oxygen Breathing Equipment For High Altitude Operations |first=Frederick M. |last=Drake |date=January 1974 |publisher=US Army Land Warfare Laboratory |location=Aberdeen Proving Ground, MD |work=Report No. 74-06 |access-date=2023-07-15 |archive-date=2023-07-15 |archive-url=https://web.archive.org/web/20230715182335/https://apps.dtic.mil/sti/tr/pdf/AD0776384.pdf |url-status=live }}</ref>

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<ref name="Harlow" >{{cite book |last=Harlow |first=Vance |title=Scuba regulator maintenance and repair |year=1999 |publisher=Airspeed Press |location=Warner, New Hampshire |isbn=0-9678873-0-5 }}</ref>

<ref name=Hendricks1999>{{cite journal|author1=Hendricks, David M|author2=Pollock, Neal W|author3=Natoli, Michael J|author4=[[Gene Hobbs|Hobbs, Gene W]]|author5=Gabrielova, Ivana|author6=Vann, Richard D|title=Mountaineering oxygen mask efficiency at 4572 m.|journal=In: Roach RC, Wagner PD, Hackett PH. Hypoxia: Into the Next Millennium (Advances in Experimental Medicine and Biology Series)|publisher=Kluwer Academic: New York|year=1999|pages=387–388}}</ref>

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<ref name=HSE >{{cite web |url=https://www.hse.gov.uk/respiratory-protective-equipment/what-is-rpe.htm |title=What is RPE? |website=www.hse.gov.uk |access-date=12 July 2023 |archive-date=12 July 2023 |archive-url=https://web.archive.org/web/20230712153722/https://www.hse.gov.uk/respiratory-protective-equipment/what-is-rpe.htm |url-status=live }}</ref>

<ref name="Hunt 1953">{{cite book |last=Hunt |first=John |title=The Ascent of Everest |url=https://archive.org/details/ascentofeverest0000hunt |url-access=registration |year=1953 |publisher=Hodder & Stoughton |location=London |pages=[https://archive.org/details/ascentofeverest0000hunt/page/257 257]–262 }}</ref>

<ref name="IMCA D014" >{{cite book |title=IMCA International Code of Practice for Offshore Diving: IMCA D 014|author=<!--Staff writer(s); no by-line.--> |date=February 2014 |edition=Rev. 2 |publisher=International Marine Contractor's Association |location=London }}</ref>

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<ref name="Macintosh and Moore 1999" >{{cite book |last1=Macintosh |first1=M. |last2=Moore |first2=T. |title=Caring for the Seriously Ill Patient 2E |date=1999 |publisher=CRC Press |isbn=9780340705827 |page=57 |edition=2 |url=https://books.google.com/books?id=T8-owObnIgUC&pg=PA57 |language=en |url-status=live |archive-url=https://web.archive.org/web/20170118051021/https://books.google.ca/books?id=T8-owObnIgUC&pg=PA57 |archive-date=2017-01-18 }}</ref>

<ref name="McGraw-Hill">{{cite web |title=breathing apparatus. |work=McGraw-Hill Dictionary of Scientific & Technical Terms |edition=6 |date=2003 |publisher=The McGraw-Hill Companies, Inc. |access-date=13 July 2023 |url=https://encyclopedia2.thefreedictionary.com/breathing+apparatus |archive-date=13 July 2023 |archive-url=https://web.archive.org/web/20230713111614/https://encyclopedia2.thefreedictionary.com/breathing+apparatus |url-status=live }}</ref>

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<ref name="Medical dictionary HPN" >Medical Dictionary for the Health Professions and Nursing. S.v. "work of breathing." Retrieved September 8, 2015, from http://medical-dictionary.thefreedictionary.com/work+of+breathing {{Webarchive|url=https://web.archive.org/web/20230729133852/https://medical-dictionary.thefreedictionary.com/work+of+breathing |date=2023-07-29 }}</ref>

<ref name="Merriam Webster" >{{cite web |title=Resuscitator |work=Merriam-Webster.com Dictionary |publisher=Merriam-Webster |url=https://www.merriam-webster.com/dictionary/resuscitator |access-date=17 September 2023 }}</ref>

<ref name="Merriam Webster usage notes" >{{cite web |url=https://www.merriam-webster.com/words-at-play/ventilator-or-respirator-difference |work=Merriam-Webster usage notes |title='Ventilator' or 'Respirator'? How they differ and overlap |access-date=12 July 2023 |archive-date=12 July 2023 |archive-url=https://web.archive.org/web/20230712150338/https://www.merriam-webster.com/words-at-play/ventilator-or-respirator-difference |url-status=live }}</ref>

<ref name="MGD" >{{cite book |first1=Heinz |last1=Lettnin |year=1999 |title= International textbook of Mixed Gas Diving |publisher= Best Publishing Company |location= Flagstaff, AZ |isbn= 0-941332--50-0 }}</ref>

<ref name="Mitchell" >{{cite web |title=Respiratory failure in technical diving |url=https://www.youtube.com/watch?v=QBajM3xmOtc |website=www.youtube.com |publisher=DAN Southern Africa |last=Mitchell |first=Simon |date=2015 |access-date=6 October 2021 |archive-date=9 October 2021 |archive-url=https://web.archive.org/web/20211009210228/https://www.youtube.com/watch?v=QBajM3xmOtc&feature=youtu.be |url-status=live }}</ref>

<ref name="Mosby's" >Mosby's Medical Dictionary, 8th edition. S.v. "work of breathing." Retrieved September 8, 2015, from http://medical-dictionary.thefreedictionary.com/work+of+breathing {{Webarchive|url=https://web.archive.org/web/20230719141546/https://medical-dictionary.thefreedictionary.com/work+of+breathing |date=2023-07-19 }}</ref>

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<ref name="National Environmental Trainers" >{{cite web |url=https://www.natlenvtrainers.com/blog/article/selfcontained-breathing-apparatus-scba |title=Self-Contained Breathing Apparatus (SCBA) |website=www.natlenvtrainers.com |publisher=National Environmental Trainers |access-date=16 July 2023 |archive-date=16 July 2023 |archive-url=https://web.archive.org/web/20230716065115/https://www.natlenvtrainers.com/blog/article/selfcontained-breathing-apparatus-scba |url-status=live }}</ref>

<ref name="NOAA Diving Manual 2001" >{{cite book |title=NOAA Diving Manual, Diving for Science and Technology |author=NOAA Diving Program (U.S.) |edition=4th |editor-first=James T. |editor-last=Joiner |date=2001 |publisher=National Oceanic and Atmospheric Administration, Office of Oceanic and Atmospheric Research, National Undersea Research Program |location=Silver Spring, Maryland |isbn=978-0-941332-70-5 }}
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<ref name="OSHA definitions" >{{cite web |url=https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.134 |title=1910.134 - Respiratory protection - Definitions |publisher=US Department of Labor, Occupational Safety and Health Administration |access-date=13 July 2023 |archive-date=13 July 2023 |archive-url=https://web.archive.org/web/20230713053702/https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.134 |url-status=live }}</ref>

<ref name="oxygen concentrator" >{{cite web |url=https://www.oxygenconcentratorsupplies.com/articles/tips-for-using-oxygen-at-high-altitudes/ |title=Tips for Using Oxygen Concentrators at High Altitudes |website=www.oxygenconcentratorsupplies.com |access-date=16 July 2023 |archive-date=16 July 2023 |archive-url=https://web.archive.org/web/20230716090533/https://www.oxygenconcentratorsupplies.com/articles/tips-for-using-oxygen-at-high-altitudes/ |url-status=live }}</ref>

<ref name="oxygentimes" >{{cite web |url=https://www.oxygentimes.com/guides/how-oxygen-concentrators-work |title=How does an Oxygen Concentrator Work? |website=oxygentimes.com |access-date=10 August 2021 |archive-date=24 July 2021 |archive-url=https://web.archive.org/web/20210724021344/https://www.oxygentimes.com/guides/how-oxygen-concentrators-work |url-status=live }}</ref>

<ref name="Paralikar and Paralikar" >{{cite journal |last1=Paralikar |first1=S.J. |last2=Paralikar |first2=J.H. |title=High-altitude medicine |journal=Indian J Occup Environ Med |date=January 2010 |volume=14 |issue=1 |pages=6–12 |doi=10.4103/0019-5278.64608 |pmid=20808661 |pmc=2923424 |doi-access=free }}</ref>

<ref name="Paul et al 2010" >{{cite conference |url=https://ntrs.nasa.gov/citations/20100019156 |title=Development of a Fan for Future Space Suit Applications |last1=Paul |first1=Heather L. |last2=Converse |first2=David |last3=Dionne |first3=Steven |last4=Moser |first4=Jeff |date=1 January 2010
|conference=40th International Conference on Environmental Systems |location=Barcelona }}</ref>

<ref name="PDC manual" >{{cite book |title=New Commercial Air Diving Manual |first=Grant |last=Jameson |publisher=Professional Diving Centre |location=Durban, South Africa }}</ref>

<ref name="Pilmanis and Sears 2003" >{{cite journal |journal=Lancet |volume=362 Issue=Special issue |pages=s16–s17 |title=Physiological hazards of flight at high altitude |first1=Andrew A. |last1=Pilmanis |first2=William J. |last2=Sears |date=December 2003 |doi=10.1016/S0140-6736(03)15059-3 |pmid=14698113 |s2cid=8210206 |doi-access=free }}</ref>

<ref name="pksafety" >{{cite web |url=https://pksafety.com/airline-respirators |title=Airline Respirators |website=pksafety.com |access-date=12 July 2023 |archive-date=12 July 2023 |archive-url=https://web.archive.org/web/20230712135615/https://pksafety.com/airline-respirators |url-status=live }}</ref>

<ref name="Pulse">{{cite web |url=http://hme-business.com/Articles/2009/09/01/Continuous-Flow-vs-Pulse-Dose.aspx |title=Continuous Flow vs. Pulse Dose |website=business.com |publisher=Home Medical Equipment Business |access-date=27 January 2015 |archive-date=17 April 2015 |archive-url=https://web.archive.org/web/20150417235443/http://hme-business.com/Articles/2009/09/01/Continuous-Flow-vs-Pulse-Dose.aspx |url-status=live }}</ref>

<ref name="Reg 1910.134" >{{cite web |url=https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.134 |title=1910.134 - Respiratory protection. |publisher=US Department of Labor: Occupational Safety and Health Administration |website=www.osha.gov |access-date=2023-07-13 |archive-date=2023-07-13 |archive-url=https://web.archive.org/web/20230713053702/https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.134 |url-status=live }}</ref>

<ref name="Ruthven 1993" >{{cite book |isbn=978-0-471-18818-6 |title=Pressure Swing Adsorption |last1=Ruthven |first1=Douglas M. |first2=Shamsuzzman |last2=Farooq |first3=Kent S. |last3=Knaebel |year=1993 |publisher=Wiley-VCH |page=6,304 }}</ref>

<ref name="safetygas" >{{cite web |url=https://en.safetygas.com/respiratory-protection/eebd/escape-respirator-hoods |title=Emergency escape respirators & hoods: The emergency escape hood |website=en.safetygas.com |access-date=27 July 2023 |archive-date=27 July 2023 |archive-url=https://web.archive.org/web/20230727055406/https://en.safetygas.com/respiratory-protection/eebd/escape-respirator-hoods |url-status=live }}</ref>

<ref name="SATA" >{{cite web |url=https://www.sata.com/en-eco/products/breathing-protection/full-hoods-respiratory-protection/sata-air-vision-5000 |title=SATA air vision 5000: Product details |website=www.sata.com |access-date=27 July 2023 |archive-date=27 June 2023 |archive-url=https://web.archive.org/web/20230627050315/https://www.sata.com/en-eco/products/breathing-protection/full-hoods-respiratory-protection/sata-air-vision-5000 |url-status=live }}</ref>

<ref name="Tech briefs">{{cite web |url=https://www.techbriefs.com/component/content/article/tb/pub/briefs/physical-sciences/30036 |title=Breathing Apparatus Stores Cold Supercritical Air |first=Harold L. |last=Gier |date=1 November 1999 |website=www.techbriefs.com |publisher=John F. Kennedy Space Center, Florida |access-date=10 October 2023 }}</ref>

<ref name=thomas>{{cite book |last1=Thomas |first1=Kenneth S. |last2=McMann |first2=Harold J. |title=U.S. Spacesuits |date=23 November 2011 |publisher=Springer Science & Business Media}}</ref>

<ref name="Thornton 2000" >{{cite report |url=https://apps.dtic.mil/sti/pdfs/ADA387646.pdf |title=A Survey and Engineering Design of Atmospheric Diving Suits |first=Michael Albert |last=Thornton |date=December 2000 |publisher=Texas A&M University }}</ref>

<ref name="Tiep and Carter 2008" >{{cite journal |last1=Tiep |first1=B. |last2=Carter |first2=R. |title=Oxygen conserving devices and methodologies |journal=Chronic Respiratory Disease |volume=5 |issue=2 |pages=109–114 |date=2008 |pmid=18539725 |doi=10.1177/1479972308090691 |publisher=crd.sagepub.com |s2cid=6141420 |doi-access=free }}</ref>

<ref name="US Navy Diving Manual R6" >{{cite book |title=US Navy Diving Manual, 6th revision |year=2006 |chapter=21 |publisher=US Naval Sea Systems Command |location=Washington, DC |url=http://www.navsea.navy.mil/Home/SUPSALV/00C3-Diving/Diving-Publications/ |access-date=6 August 2016 |author=US Navy |archive-date=3 December 2020 |archive-url=https://web.archive.org/web/20201203031428/https://www.navsea.navy.mil/Home/SUPSALV/00C3-Diving/Diving-Publications/ |url-status=live }}</ref>

<ref name="USNDM Ch21">{{cite book |author=U.S. Navy Supervisor of Diving |title=U.S. Navy Diving Manual. Volume 5: Diving Medicine and Recompression Chamber Operations |version=SS521-AG-PRO-010, Revision 6 |chapter=Chapter 21: Recompression Chamber Operation |date=April 2008 |publisher=U.S. Naval Sea Systems Command |url=http://www.usu.edu/scuba/navy_manual6.pdf |access-date=2009-06-29 |archive-url=https://web.archive.org/web/20140331063312/http://www.usu.edu/scuba/navy_manual6.pdf |archive-date=31 March 2014 |url-status=live}}</ref>

<ref name="USNDM R1" >{{cite book|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a112710.pdf |archive-url=https://web.archive.org/web/20190702185411/https://apps.dtic.mil/dtic/tr/fulltext/u2/a112710.pdf |url-status=live |archive-date=2 July 2019 |title=US Navy Diving Manual Revision 1 Navsea-0994-LP001-9020 |date=July 1981 |volume=2 |chapter=12 |publisher=Navy Department |location=Washington DC}}</ref>

<ref name="ventilator terminology" >{{cite web |url=https://www.passy-muir.com/sites/default/files/pdf/ventilator_terminology.pdf |title=Ventilator Terminology |website=www.passy-muir.com |access-date=16 July 2023 |archive-date=16 July 2023 |archive-url=https://web.archive.org/web/20230716101616/https://www.passy-muir.com/sites/default/files/pdf/ventilator_terminology.pdf |url-status=live }}</ref>

<ref name="vocabulary.com" >{{cite web |title=Breathing apparatus |work=Vocabulary.com Dictionary |publisher=Vocabulary.com |url=https://www.vocabulary.com/dictionary/breathing |access-date=12 July 2023 |archive-date=12 July 2023 |archive-url=https://web.archive.org/web/20230712161904/https://www.vocabulary.com/dictionary/breathing |url-status=live }}</ref>

<ref name="WHO2008">{{cite book |title=WHO Model Formulary 2008 |year=2009 |isbn=9789241547659 |vauthors=((World Health Organization)) |editor1-last=Stuart |editor1-first=M.C. |editor2-last=Kouimtzi |editor2-first=M. |editor3-last=Hill |editor3-first=S.R. |hdl=10665/44053 |author-link=World Health Organization |publisher=World Health Organization |hdl-access=free |page=20 }}</ref>

}}

{{Breathing apparatus}}

[[Category:Breathing apparatus]]