The purpose is to extend the breathing endurance of a limited gas supply, while also eliminating the bubbles otherwise produced by an open circuit system.

On land they are used in industrial applications where poisonous gases may be present or oxygen may be absent, firefighting, where firefighters may be required to operate in an atmosphere immediately dangerous to life and health for extended periods, in hospital anaesthesia breathing systems to supply controlled concentrations of anaesthetic gases to patients without contaminating the air that the staff breathe, and at high altitude, where the partial pressure of oxygen is low, for high altitude mountaineering.

In aerospace there are applications in unpressurised aircraft and for high altitude parachute drops, and above the Earth's atmosphere, in space suits for extra-vehicular activity.

The recycling of breathing gas comes at the cost of technological complexity and specific hazards, some of which depend on the application and type of rebreather used.

The breathing reflex is triggered by CO2 concentration in the blood, not by the oxygen concentration, so even a small buildup of CO2 in the inhaled gas quickly becomes intolerable; if a person tries to directly rebreathe their exhaled breathing gas, they will soon feel an acute sense of suffocation, so rebreathers must remove the CO2 in a component known as a carbon dioxide scrubber.

[4] By adding sufficient oxygen to compensate for the metabolic usage, removing the carbon dioxide, and rebreathing the gas, most of the volume is conserved.

[6][8] This process also heats and humidifies the air, which is desirable for diving in cold water, or climbing at high altitudes, but not for working in hot environments.

They have a wide enough bore to minimise flow resistance at the ambient pressure in the operational range for the equipment, are usually circular in cross section, and may be corrugated to let the user's head move about without the tube collapsing at kinks.

[6] Each end has an airtight connection to the adjacent component, and they may contain a one-way valve to keep the gas circulating the right way in a loop system.

This is the earliest type of rebreather and was commonly used by navies for submarine escape and shallow water diving work, for mine rescue, high altitude mountaineering and flight, and in industrial applications from the early twentieth century.

Carbon dioxide is considered a waste product, and in a correctly functioning rebreather, is effectively removed when the gas passes through the scrubber.

Potassium superoxide reacts vigorously with liquid water, releasing considerable heat and oxygen, and causing a fire hazard, so the more successful applications have been for space-suits, fire-fighting and mine rescue.

A diving rebreather is safety-critical life-support equipment – some modes of failure can kill the diver without warning, others can require immediate appropriate response for survival.

Submarines, underwater habitats, bomb shelters, space stations, and other living spaces occupied by several people over medium to long periods on a limited gas supply, are equivalent to closed circuit rebreathers in principle, but generally rely on mechanical circulation of breathing gas through the scrubbers.

Purging should be done while breathing off the unit so that the inert gas in the user's lungs and body tissues that finds its way into the loop is also removed from the system.

Continued use of a rebreather with an ineffective scrubber is not possible for very long, as the levels will become toxic and the user will experience extreme respiratory distress, ultimately leading to loss of consciousness and death.

[31][32][33] The only recourse is to vent the expelled breath outside the closed system, therefore not reusing the oxygen, and thereby increasing use of the gas mixture, but this is not an option in every field of application.

The term "break-through" means the failure of the scrubber to continue removing sufficient carbon dioxide from the gas circulating in the loop.

There are several ways that the scrubber may fail or become less efficient: Around 1620, Cornelius Drebbel discovered that heating saltpetre (potassium nitrate) would generate oxygen.

[34] The first basic rebreather based on carbon dioxide absorption was patented in France in 1808 by Pierre-Marie Touboulic from Brest, a mechanic in Napoleon's Imperial Navy.

[39][36] It had a large back mounted oxygen tank with working pressure of about 13.3 bar, and two scrubbers containing sponges soaked in a caustic soda solution.

[36] The first commercially practical closed-circuit scuba was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London.

His apparatus was first used under operational conditions in 1880 by Alexander Lambert, the lead diver on the Severn Tunnel construction project, who was able to travel 1000 feet in the darkness to close several submerged sluice doors in the tunnel; this had defeated his best efforts with standard diving dress due to the danger of the air supply hose becoming fouled on submerged debris, and the strong water currents in the workings.

[36] Fleuss continually improved his apparatus, adding a demand regulator and tanks capable of holding greater amounts of oxygen at higher pressure.

[41] In 1912 the German firm Dräger began mass production of their own version of standard diving dress with the air supply from a rebreather.

[44] In the 1930s, after some tragic accidents in the 1920s, the United States Navy began to equip Porpoise- and Salmon-class submarines with primitive rebreathers called Momsen lungs, which were in use until the 1960s.

This practice soon came to the attention of the Italian Navy, which developed an extensively upgraded model designed by Teseo Tesei and Angelo Belloni [it] that was used by its frogman unit Decima Flottiglia MAS with good results during World War II.

The earliest of these breathing sets may have been modified Davis Submerged Escape Apparatus; their fullface masks were the type intended for the Siebe Gorman Salvus, but in later operations different designs were used, leading to a fullface mask with one big face window, at first circular or oval and later rectangular (mostly flat, but the sides curved back to allow better vision sideways).

Due to the military importance of the rebreather, amply demonstrated during the naval campaigns of the Second World War, most governments were reluctant to issue the technology into the public domain.

Eventually the Cold War ended, and in 1989 the Communist Bloc collapsed, and as a result the perceived risk of sabotage attacks by combat divers dwindled, and Western armed forces had less reason to requisition civilian rebreather patents, and automatic and semi-automatic recreational diving rebreathers with oxygen partial pressure sensors started to appear.