Breathing apparatus


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 apparatus 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
  • 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

, 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 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. 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.
The UK Health and Safety Executive distinguishes between respirators and breathing apparatus. Respirators are described as 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.
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.
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.

Breathing gas source

The US Occupational Safety and Health Administration 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:
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 respirators and self-contained breathing apparatus.
A self-contained breathing apparatus is a type of atmosphere-supplying breathing apparatus in which the breathing gas source is carried by the user.
A supplied-air respirator, 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.
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. 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, oxygen or blended gas mixtures, liquid oxygen, chemically generated oxygen, or a combination of ambient atmosphere and another of these sources.

Breathing gas regulator

When using a pressurised gas supply, the breathing gas must be supplied to the respiratory interface at a suitable pressure for inhalation, which is close to ambient pressure. This is generally done by a breathing gas regulator, a pressure reduction regulator, which reduces the gas supply pressure from the supply line. Exhalation is usually to the surroundings at ambient pressure, but in special cases such as built-in breathing systems and gas reclaim systems, it may be exhausted to a significantly lower pressure, sometimes at a remote location, and may require a back-pressure regulator to do this safely.

Breathing circuit type

Supplied gas breathing apparatus can be categorised by how the gas is supplied to the user. There are several combinations of options

Constant flow

The gas can be supplied continuously, in what is known as a constant flow, continuous flow, or free-flow system. The user inhales from the stream of fresh gas passing the face, and exhales back into the same stream. Supply rate must be sufficient that at reasonably foreseeable work rates, the inhaled gas does not include too much of the previously exhaled gas. This is simple, but wasteful of supplied gas.

Demand

The gas is supplied on demand when the user inhales, using the pressure drop at the start of inhalation to control the opening of a demand valve and automatic stops when there is no demand. This is more conservative in gas usage but has a higher work of breathing. It requires a facepiece that seals moderately well to the user with a small internal volume to limit dead space. Some demand supplied breathing apparatus can be switched to continuous flow mode.

Open circuit

is any breathing apparatus that does not recycle any of the breathing gas, and discharges it all to the surroundings.
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.

Enriched gas

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.

Closed and semi-closed circuit

Closed and semi-closed circuit breathing sets, also known as rebreathers and gas extenders, are breathing apparatus that absorb the carbon dioxide from, and add oxygen to, a user's exhaled breath, allowing unused oxygen and diluent 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 or loop flow path configuration, and the gas can be circulated by the breathing of the user through non-return valves,, by the energy of the injected fresh gas,, or by an external power input.
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.

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.
Remotely supplied applications include:
  • Surface-supplied diving equipment 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.
  • Supplied-air respirators. or airline respirators, use a hose to supply breathing gas from a source which is not carried by the user.
  • Built-in breathing systems in submarines or hyperbaric chambers.
  • Some space suits and pressure suits,
  • Some diving rebreathers. When remotely supplied they are likely to be semi-closed circuit, and called gas extenders, and their main function is likely to be to save expensive helium diluent gas. In helium reclaim systems, exhaled gas is returned to the surface to be recycled.
  • Flight crew breathing apparatus
  • Aircraft emergency oxygen systems for passengers in commercial airliners.
Self-contained applications include: