The International Space Station gets most of its oxygen by running an electric current through water to split the molecules into breathable oxygen gas and hydrogen byproduct. Onboard hardware continuously recycles humidity, sweat, and urine into clean water to sustain this reaction. Backup supplies include pressurized storage tanks, solid-fuel chemical candles, and cargo deliveries from Earth.
Oxygen From Splitting Water
The space station produces most of its breathable air by splitting water into oxygen and hydrogen through electrolysis. An electrical charge breaks the bond between hydrogen and oxygen atoms in liquid water. The resulting oxygen gas vents directly into the cabin atmosphere for the crew to breathe. The remaining hydrogen gas is routed away so it does not build up inside the pressurized modules.
Two separate hardware suites handle this job on orbit. The United States segment operates the Oxygen Generation System (OGS), while the Russian segment runs the Elektron unit. Each system operates on water electrolysis principles, giving the station independent mechanical redundancy. If one system requires maintenance or spare parts, the other unit continues delivering air to both sides of the complex.
This continuous production avoids the severe weight penalties of launching all cabin air as compressed gas. Launching liquid water or relying on recycled onboard fluids is far safer and more volume-efficient than launching thousands of kilograms of high-pressure gas cylinders. To see how launchers carry heavy logistics into orbit, read our guide on how rocket launches work.
Where the Water Comes From
The water that feeds the oxygen generators comes from a closed-loop recycling setup called the Water Recovery System (WRS). Astronauts exhale moisture into the air, perspire during daily exercise, and produce urine. Cabin air-conditioning systems pull moisture out of the air as humidity condensate, while specialized plumbing collects crew urine for chemical processing.
The recovery hardware purifies this wastewater through chemical treatment, distillation, and fine filtration. NASA reported in 2023 that the system reached a 98 percent water recovery milestone. That efficiency level allows the station to run for extended stretches without massive freshwater deliveries from Earth.
The crew drinks this recycled water, uses it to rehydrate freeze-dried food pouches, and feeds the surplus into the oxygen generators. Future private platforms profiled in our guide to commercial space stations rely on identical closed-loop fluid physics to control launch mass.
Managing Carbon Dioxide and Hydrogen
Crew respiration continuously consumes oxygen and releases carbon dioxide (CO2). Without active scrubbing, carbon dioxide concentrations would quickly rise, causing headaches, mental confusion, and asphyxiation. Specialized sorbent beds absorb carbon dioxide from the circulating cabin air stream to keep the environment safe.
Once the carbon dioxide is captured, station systems either vent the gas into space or pump it into a Sabatier reactor. The Sabatier system combines carbon dioxide with the hydrogen gas generated as a byproduct of water electrolysis. Under high temperatures and with a catalyst, this chemical reaction produces water and methane gas.
The newly formed water loops directly back into the Water Recovery System to be purified and split into oxygen again. The methane gas is vented overboard into space as a waste product. This chemical loop recovers usable oxygen atoms that would otherwise be lost when astronauts exhale carbon dioxide.
Redundant Backups and Resupply Flights
Mechanical hardware in space eventually suffers valve jams, sensor failures, or pump wear. To ensure the crew never faces an immediate breathing emergency, the station maintains several layers of oxygen backup.
| Source | Method | Primary purpose | Operational state |
|---|---|---|---|
| Oxygen Generation System and Elektron | Water electrolysis | Primary daily breathing air | Active continuous run |
| Stored high-pressure gas tanks | Compressed gaseous oxygen | Cabin re-pressurization and system reserve | Standby reserve |
| Solid-Fuel Oxygen Generation candles | Chemical burn | Emergency oxygen production | Standby reserve |
| Uncrewed cargo ships | Delivered water and gas tanks | System replenishment | Periodic resupply |
Pressurized tanks mounted to the station exterior store gaseous nitrogen and oxygen. Crew systems use these tanks to adjust the cabin pressure or re-pressurize airlocks following spacewalks. When mechanical generators drop offline for repairs, flight controllers can bleed oxygen directly from these reserve tanks into the cabin.
For severe contingencies where electrical power or plumbing loops fail, the station stores solid-fuel oxygen generators, commonly called oxygen candles. An oxygen candle releases breathable oxygen as it burns chemically, giving engineers time to troubleshoot primary systems.
Periodic uncrewed resupply capsules launched on platforms like the Falcon 9 deliver fresh tanks of nitrogen, water bags, and spare filtration beds. Agencies like the European Space Agency collaborate with NASA to plan cargo manifests so that consumable inventories never drop below safety thresholds.
Cabin Air Composition and Pressure
Human lungs evolved to operate within specific atmospheric pressures and gas mixtures. Spacecraft designers match those biological parameters rather than forcing astronauts to breathe pure oxygen.
The International Space Station maintains an internal atmospheric pressure near Earth sea-level pressure, about 14.7 pounds per square inch (psi). The air inside consists of roughly 78 percent nitrogen and 21 percent oxygen, closely mirroring the ambient air on Earth. Keeping sea-level pressure lets the crew work in ordinary clothing without pressure suits.
Nitrogen does not react chemically or break down inside the body, but it slowly leaks into vacuum through airlocks, module hatch seals, and structural penetrations. Life support sensors monitor the partial pressure of nitrogen and bleed fresh gas from high-pressure tanks to maintain the 14.7 psi target. Balancing the oxygen-to-nitrogen ratio prevents fire risks, as pure oxygen atmospheres turn ordinary fabrics and plastics into intense fire hazards.
Closing the Loop for Deep Space
Operating the station in low Earth orbit provides a continuous test bed for the life-support systems required for long-duration spaceflight. Low Earth orbit allows resupply vehicles to deliver spare parts and extra water within days of an anomaly. Missions outbound to the Moon or Mars will not have access to emergency cargo flights.
At OrbitalIntel, we track how life-support system endurance shapes mission architectures for deep space exploration. A crew traveling to Mars must carry hardware capable of running for years without resupply. If life support equipment cannot recover at least 98 percent of crew water and convert exhalations back into breathable gas, the propellant mass required to launch extra consumables makes interplanetary flight impractical. For details on how deep-space propulsion systems compare when moving large payloads between planets, read our overview of nuclear propulsion in space.
The lessons learned from running the Oxygen Generation System, the Elektron unit, and the Sabatier reactor show where mechanical parts wear down and how microgravity disrupts fluid separation. Refining these recycling loops on the space station ensures that long-range exploration platforms maintain a stable, self-replenishing breathing atmosphere throughout their journeys.
To prepare for future exploration requirements, monitor NASA life-support hardware upgrades scheduled for delivery on upcoming cargo missions.