A closed-circuit apparatus works on a different principle: exhaled air is not vented but passes through a carbon-dioxide absorbent and is re-oxygenated. That lets a compact unit run for 30, 60 or even 90 minutes.
Oxygen source
- Chemical — potassium superoxide (KO₂), which reacts with exhaled moisture and CO₂ to release oxygen.
- Compressed — a small oxygen cylinder with a separate CO₂ absorbent.
Where it is used
These are classically used in mining, where escape routes are long and methane or carbon monoxide levels high. They are also found in tunnels, chemical plants and on ships.
One important characteristic: the chemical reaction produces heat, so the inhaled air is noticeably warm. This is normal and must be explained in training, so that a wearer does not remove the unit in alarm.