The air capacity of the human lungs varies between one to six litres. As humans do not use up all the oxygen present in the outside air in one breath, one can rebreathe the same air several times. With a capacity larger than the lungs, a rebreather can be held closed for a longer time until the oxygen levels drop too low and more importantly, the CO2 content rises to lethal levels.
Physical exercise and excitement makes the body use more oxygen and thus considerably shorten the allowed rebreathing time.
Never use them alone.
And don't try it unless you are absolutely certain that you have considered everything that could go wrong, from tipping over to a heart attack (of either person present).
Uses outside the fetish scene
Some military tanks have gas masks with connected rebreathers for the crew to allow them to escape in an event of a smoke-emitting fire. That equates to maybe 20 seconds' use during physical tasks and the air capacities cited are in the range of 10 to 20 litres.
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