The Tool Desk
Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →The International Space Station replenishes oxygen primarily by splitting recovered water through electrolysis. Separate air-revitalization equipment removes carbon dioxide and trace contaminants from cabin air. A chemical reaction returns some carbon dioxide’s oxygen to the water supply, but the process vents methane and cannot close the loop completely, so the station still depends on resupply.
How oxygen and air are replenished
The ISS life-support system combines water recovery, oxygen generation, and air cleaning. They work together, but they are not the same process: cleaning cabin air removes contaminants, while electrolysis produces oxygen.
- Recover and purify water. The station collects wastewater, including urine, humidity condensate, and water associated with spacesuit hydration. Filtration and catalytic oxidation treat it, and conductivity sensors help assess purity. Water that does not meet requirements is reprocessed. NASA’s ECLSS reference says the Water Recovery System recovers and recycles about 90 percent of the station’s water. NASA’s ECLSS overview was last updated April 4, 2025. This is a water-recovery figure, not the percentage of oxygen recovered from exhaled carbon dioxide.
- Make oxygen from water. The Oxygen Generation Assembly uses electrolysis to split purified water into oxygen and hydrogen. The oxygen is supplied to the cabin atmosphere. The hydrogen can be vented or sent to the carbon dioxide reduction process.
- Clean the cabin air. Air-revitalization equipment circulates cabin air through molecular sieves that capture carbon dioxide. Dedicated treatment units also remove trace contaminants associated with materials such as electronics and plastics, as well as substances released by people. This keeps the air cleaner; it does not itself generate oxygen.
- Recover some oxygen through a chemical loop. The Sabatier reactor combines captured carbon dioxide with hydrogen to produce water and methane. The water can be purified and returned to oxygen generation. The methane is vented, carrying hydrogen out of the system.
- Replace what the system loses. Oxygen is needed to make up for losses associated with experiments, airlock depressurization, module leakage, and carbon dioxide venting. NASA’s reviewed sources do not provide a current complete mass balance of oxygen produced onboard versus delivered, so a precise present-day split cannot be stated.
How much oxygen does the ISS recover?
NASA’s Spacecraft Oxygen Recovery (SCOR) overview describes the state-of-the-art ISS system as recovering about 50 percent of the oxygen from exhaled carbon dioxide. That is a different measure from the roughly 90 percent water-recovery figure: one concerns oxygen recovered from metabolic carbon dioxide, the other concerns water recycled by the station’s Water Recovery System. NASA’s SCOR overview explains the oxygen-recovery work.
A NASA-authored 2024 technical paper gives estimates for a described system architecture, rather than a current station-wide supply tally. It estimates water needed for breathable oxygen at about 0.459 kilograms per crew member per day with Sabatier processing, compared with about 0.891 kilograms without it. For a crew of four, the paper reports about 670 kilograms of water per year for breathable oxygen under that architecture. These estimates illustrate the benefit of recovering some oxygen, but they should not be read as a live ISS inventory or delivery rate. NASA’s 2024 paper provides the technical context.
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Why the loop is only partly closed
Electrolysis consumes water to produce oxygen and hydrogen. Sabatier processing can use some of that hydrogen with captured carbon dioxide to make water again, but the reaction also produces methane. Because the methane is vented, hydrogen leaves the system rather than being recovered for repeated use. The process therefore returns some water and enables partial oxygen recovery, not complete recycling of the oxygen people breathe out.
The station also has losses and demands that the chemical loop does not eliminate, including oxygen used in experiments and air lost during operations or through leakage. Resupply remains part of the life-support plan.
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What is operating now, and what is being developed?
The routine ISS architecture described by NASA uses water electrolysis and Sabatier carbon dioxide reduction. NASA’s SCOR project is developing higher-recovery approaches, including a Continuous Bosch Reactor intended to make water and elemental carbon from hydrogen and carbon dioxide, and hydrogen recovery through carbon vapor deposition to reclaim hydrogen that would otherwise leave in methane. These are development pathways, not equipment to treat as the station’s current routine oxygen-replenishment system.
A NASA-authored 2024 paper discusses a 75–90 percent oxygen-recovery target for long-duration missions beyond low Earth orbit. That is a future technology target, not the ISS’s current recovery performance. Higher recovery matters for missions farther from Earth because reducing life-support consumables can reduce the mass that must be launched or resupplied. The paper’s estimates and targets describe a mission architecture, not an operational ISS guarantee.
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What “water balance” means for station operations
In NASA technical literature, water balance is the planning task of matching water inputs and storage with crew needs, visiting-vehicle traffic, and the availability of systems such as carbon dioxide removal, Sabatier processing, and oxygen generation. It is an operational planning concept, not a separate device that produces oxygen. NASA’s technical paper on water balance describes how changing mission conditions affect that planning.
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