A rebreather is the type of breathing apparatus that recycles the user’s exhaled air. Unlike standard open-circuit systems that vent every breath into the environment, a rebreather captures exhaled gas, removes the carbon dioxide, replenishes oxygen, and sends it back to the user in a continuous loop. Rebreathers are used in scuba diving, firefighting, mining rescue, and military operations, and they come in several designs with different levels of automation.
How a Rebreather Works
Every time you exhale, your breath still contains a significant amount of usable oxygen. An open-circuit system wastes all of that by dumping it into the water or surrounding air. A rebreather instead directs your exhaled breath through a closed loop: first through a canister that chemically removes carbon dioxide, then back to you with fresh oxygen added to replace what your body consumed.
The CO2 removal happens inside a “scrubber” canister filled with a chemical absorbent, most commonly soda lime. When your exhaled carbon dioxide passes through this material, it reacts with sodium hydroxide and calcium hydroxide in a series of chemical reactions that convert the CO2 into calcium carbonate, water, and heat. The absorbent is consumed over time and must be replaced between uses. Other absorbent formulations use lithium-based compounds, which work through a similar process.
Once the CO2 is stripped out, the system adds oxygen back into the loop to replace what your body metabolized. How that oxygen gets added is what distinguishes the different types of rebreathers from one another.
Closed-Circuit Rebreathers
A fully closed-circuit rebreather (CCR) recycles all exhaled gas with no venting to the outside. This makes it the most gas-efficient design and produces no bubbles underwater, which is why military and scientific divers favor it. CCRs come in three main variants.
Manual closed-circuit rebreathers (mCCR) rely on the diver to monitor oxygen levels in the breathing loop and manually inject oxygen by pressing a button. This gives experienced divers direct control but demands constant attention.
Electronic closed-circuit rebreathers (eCCR) automate the process. Oxygen sensors in the loop feed data to an onboard computer, which activates a solenoid valve to inject a small burst of oxygen whenever levels drop below a target. That target, called a setpoint, is typically kept at a partial pressure of 1.2 or 1.3. A heads-up display mounted near the mouthpiece shows the diver real-time oxygen readings from multiple sensors, so they always know what’s happening inside the loop.
Hybrid closed-circuit rebreathers combine elements of both, using electronic monitoring with manual override capability.
Semi-Closed Circuit Rebreathers
A semi-closed circuit rebreather recycles most of the exhaled air but vents a small portion to the outside with each breathing cycle. Fresh breathing gas flows continuously into the loop through a fixed-rate orifice, and roughly one-fifth of each breath is released. This keeps the oxygen fraction in the loop within about 2% to 3% of a preset level without requiring electronic sensors or manual intervention.
The trade-off is reduced gas efficiency compared to a fully closed system, and the periodic venting produces some bubbles. Because the counterlung volume changes as gas is vented, buoyancy can fluctuate slightly. Semi-closed systems are mechanically simpler, though, which makes them less expensive and easier to maintain.
Gas Efficiency and Duration
The practical advantage of recycling exhaled air is dramatically longer operating time from smaller gas supplies. A standard open-circuit self-contained breathing apparatus tops out at about 75 minutes. A closed-circuit system can run from 30 minutes to 4 hours depending on the application, the scrubber capacity, and the user’s oxygen consumption rate.
The gas savings are substantial. For deep diving, you can fill seven rebreather cylinders with the same volume of mixed gas it would take to fill a single set of large back-mounted doubles on an open-circuit rig. That efficiency matters most on deep or long dives where carrying enough open-circuit gas becomes physically impractical.
Uses Beyond Diving
Rebreather technology is not limited to underwater work. In underground mining, regulations require that self-contained self-rescuers be available to every worker. These are compact, wearable rebreathers designed for emergency escape from toxic atmospheres. U.S. federal mining safety standards mandate devices providing at least one hour of breathable air, with some shorter-duration units (10 minutes or more) paired with a one-hour backup canister stored along escape routes.
Firefighters and hazmat responders also use closed-circuit breathing apparatus for extended operations in environments where open-circuit tanks would run out too quickly. The same core principle applies: scrub the CO2, replenish the oxygen, and recirculate the air.
Risks of Recycled Breathing Gas
Rebreathers are more complex than open-circuit systems, and the consequences of a malfunction are severe. The three primary dangers are sometimes called “the three Hs”: hypercapnia (too much CO2), hypoxia (too little oxygen), and hyperoxia (too much oxygen). Any of these can cause incapacitation and unconsciousness.
Hypoxia is particularly dangerous because it gives little warning. In controlled studies of rebreather divers exposed to declining oxygen, some participants became unresponsive at oxygen saturation levels as high as 85%, while others remained functional down to 43%. On average, divers who recognized the problem and switched to a backup gas source did so when their blood oxygen saturation had already fallen to somewhere between 73% and 81%, values that sit right at the edge of rapid decline. Below that point, oxygen delivery to the brain drops precipitously.
A failing CO2 scrubber is equally insidious. If the absorbent is exhausted or improperly packed, carbon dioxide builds up in the loop. At depth, dense gas is harder to move through the airways, which already impairs the body’s ability to expel CO2 naturally. The combination can progress from breathlessness and anxiety to unconsciousness.
Hyperoxia, caused by oxygen partial pressure climbing too high, can trigger seizures. This is why electronic rebreathers constantly monitor oxygen levels and why manual rebreather divers are trained to check their displays obsessively.
How Rebreathers Compare to Open-Circuit Systems
- Gas recycling: Rebreathers recirculate exhaled air after scrubbing CO2. Open-circuit systems exhaust every breath.
- Duration: Rebreathers can operate up to 4 hours; open-circuit systems max out around 75 minutes.
- Noise and signature: Closed-circuit rebreathers produce no bubbles, making them nearly silent. Open-circuit systems release a burst of bubbles with every exhalation.
- Complexity: Rebreathers require more training, more pre-dive preparation, and more vigilant monitoring during use.
- Failure mode: An open-circuit regulator failure is usually obvious and survivable. A rebreather failure can be subtle, with the user losing consciousness before recognizing the problem.