Astronauts breathe air that is manufactured, scrubbed, and pushed through their spacecraft by an elaborate set of machines collectively known as the Environmental Control and Life Support System, or ECLSS. On the International Space Station, electricity splits water molecules to produce oxygen, chemical beds strip exhaled carbon dioxide from the cabin, and fans force the air through every module because, without gravity, there is no natural air circulation at all. The whole arrangement is more intricate than most people realize, and keeping it running is one of the highest priorities aboard any crewed vehicle.
Where the Oxygen Comes From
The primary oxygen source on the ISS is a device called the Oxygen Generation System, or OGS, which sits in the U.S. Destiny laboratory module. It works by electrolysis: an electric current passes through water, splitting each molecule into hydrogen and oxygen. The oxygen is vented into the cabin for crew breathing, and the hydrogen is routed to a separate system (more on that shortly). The water itself comes from a combination of supply shipments and the station’s own water-recycling loop, which recovers moisture from humidity, sweat, and even urine.
On the Russian side of the station, a similar electrolysis unit called Elektron does the same job. Between the two systems, the station can keep its cabin atmosphere at roughly the same oxygen percentage found at sea level on Earth, around 21 percent. The station operates at about one atmosphere of total pressure, so from a breathing standpoint, it feels a lot like being in a building on the ground.
Getting Rid of Carbon Dioxide
Generating oxygen is only half the problem. Every exhaled breath adds carbon dioxide to the cabin, and in a sealed environment that CO2 accumulates fast. The ISS relies on the Carbon Dioxide Removal Assembly, or CDRA, to handle this. The CDRA uses beds of a material called zeolite, a type of molecular sieve, that selectively adsorbs CO2 from the air. Once a bed is saturated, the system heats it and exposes it to the vacuum of space, driving off the captured CO2 and regenerating the sieve for another cycle.1SAE International. International Space Station Carbon Dioxide Removal Assembly (ISS CDRA) Troubleshooting and Evaluation This approach has been the backbone of CO2 scrubbing since the station’s early years, though it has required ongoing troubleshooting and redesign to deal with dust contamination and mechanical wear.
The captured CO2 does not simply get dumped overboard and forgotten. Since 2010, a reactor called the Sabatier system has been converting some of that CO2 back into water by combining it with the hydrogen left over from electrolysis. The reaction produces methane as a byproduct, which is currently vented into space, but the water it creates feeds back into the electrolysis loop to make more oxygen.2iScience. Electrochemical methane production from CO2 for orbital and interplanetary refueling This closed-loop recycling is a critical step toward reducing how much water needs to be launched from Earth. Even so, the loop is not perfectly closed: some hydrogen is lost with the vented methane, so the station still needs periodic water resupply.
Integrated testing of the four-bed molecular sieve, a mechanical compressor, and the Sabatier unit together has been part of ongoing development to tighten this loop further for future missions.3SAE Technical Paper Series. Integrated Test and Evaluation of a 4-Bed Molecular Sieve (4BMS) Carbon Dioxide Removal System (CDRA), Mechanical Compressor Engineering Development Unit (EDU), and Sabatier Engineering Development Unit (EDU)
Why Air Has to Be Pushed Around Mechanically
On Earth you rarely think about air movement. Warm air rises, cool air sinks, and the gentle convection currents that result keep the air in a room reasonably well mixed. In microgravity, none of that happens. Without buoyancy-driven convection, exhaled CO2 simply hangs in a cloud around your face unless something physically moves it away.455th International Conference on Environmental Systems. Airflow Optimization in Microgravity: Computational Insights from Starlab Space Station Ventilation Design
This is why every module on the ISS has fans and ventilation ducts running constantly. The forced airflow pushes fresh, scrubbed air toward crew areas and pulls stale air back to the CDRA and other processors. The system also helps regulate temperature and humidity, since microgravity removes the natural mixing that keeps those in check on Earth.
Even with this ventilation, some spots remain poorly served. The crew quarters where astronauts sleep are small, enclosed compartments, and studies have found that exhaled CO2 can pool around a sleeping astronaut’s face because the ventilation does not always penetrate that far into the breathing zone. There have been reports of astronauts waking with headaches or symptoms consistent with mild CO2 exposure after spending hours in their crew quarters, leading researchers to recommend more targeted personal ventilation for those spaces.5ScienceDirect (Elsevier / Building and Environment). Accumulation and spatial distribution of CO2 in the astronaut’s crew quarters on the International Space Station
The CO2 Problem Is Harder Than It Looks
Even when the CDRA is working well, CO2 levels on the ISS tend to run higher than what you would encounter in a typical building on Earth. Average cabin concentrations often hover around 3 to 6 mmHg of CO2 partial pressure, which is several times higher than outdoor air and somewhat above the levels most office-building standards allow. This chronic low-level exposure has drawn increasing attention from researchers.
A review of the evidence on long-term mild CO2 elevation found that while people can work safely in these mildly elevated conditions for extended periods, chronic exposure carries risks including bone demineralization, kidney calcification, systemic inflammation, and impairments in cognitive function and visuomotor skills.6PubMed Central. Confined spaces in space: Cerebral implications of chronic elevations of inspired carbon dioxide and implications for long-duration space travel The concern is not acute CO2 poisoning but the subtle, cumulative toll that slightly elevated levels might take over a months-long mission.
Ground-based studies simulating spaceflight conditions have tried to tease apart whether the cognitive effects astronauts report come from CO2 specifically or from other aspects of the spaceflight environment. One study using head-down tilt bed rest (which mimics some of the fluid shifts of microgravity) found a modest slowing in cognitive performance across multiple domains, but the effect was similar whether subjects breathed normal air or air with elevated CO2, suggesting that the body-position changes themselves play a substantial role.7PubMed Central. Effects of head-down tilt bed rest plus elevated CO2 on cognitive performance A separate study looking at cerebrovascular responses under similar conditions found that 30 days of mild CO2 elevation did not detectably change blood CO2 levels or brain blood-flow reactivity, suggesting the body compensates up to a point.8PubMed. Unchanged cerebrovascular CO(2) reactivity and hypercapnic ventilatory response during strict head-down tilt bed rest in a mild hypercapnic environment
The upshot is that researchers are still sorting out exactly how much of a problem chronic CO2 exposure is in space, but lowering cabin CO2 remains a design priority for future spacecraft, especially for missions measured in years rather than months.
Trace Contaminants and the Air You Cannot See
Oxygen and CO2 are the headline gases, but the station’s atmosphere also accumulates low-level contaminants from outgassing materials, cleaning solvents, personal care products, and the crew themselves. A dedicated Trace Contaminant Control Subassembly uses a combination of activated-charcoal adsorption beds and a catalytic oxidizer to break down volatile organic compounds and other pollutants before they can reach harmful concentrations.9SAE International. Study of Trace Contaminant Control System for Space Station A Major Constituent Analyzer continuously monitors the relative proportions of oxygen, nitrogen, CO2, and other gases so that controllers on the ground and crew aboard can spot any drift in cabin air composition.10SAE International. International Space Station Environmental Control and Life Support System Technology Evolution
This monitoring matters because changes in cabin composition that would be harmless on Earth can become dangerous in a sealed spacecraft. A small leak in the ammonia coolant loop, for example, would fill the cabin with a toxic gas that cannot escape through a window. The trace contaminant system is one piece of the safety net, but it also has limits, which is where emergency gear comes in.
Emergency Breathing Equipment
If a fire breaks out or ammonia leaks into the cabin, the crew cannot simply step outside. The ISS carries emergency masks that provide respiratory protection during these events. The current Emergency Mask design is a one-size-fits-all unit that uses interchangeable filter cartridges: fire cartridges for post-fire smoke and fumes, or ammonia-specific cartridges for coolant leaks. With appropriate cartridge swaps, a single mask can provide at least eight hours of respiratory protection.11NASA Technical Reports Server. International Space Station (ISS) Emergency Mask (EM) Development
For a more fundamental backup, the station is required to carry enough contingency oxygen to sustain the crew for 45 days even if the primary electrolysis systems fail. Part of this requirement is met by the Russian Solid Fuel Oxygen Generator, which burns lithium perchlorate “candles” to release oxygen chemically. NASA also developed a Backup Oxygen Candle System consisting of 33 chlorate candles and a thermal-containment apparatus to ensure the oxygen supply remains available during early assembly stages or system failures.12SAE International. Development of a Solid Chlorate Backup Oxygen Delivery System for the International Space Station These chemical generators are simple and reliable: they need no electricity and no water, just a controlled chemical reaction.
Breathing During a Spacewalk
Inside the station, astronauts share the cabin atmosphere. Outside during an extravehicular activity, or EVA, they breathe pure oxygen at reduced pressure inside their spacesuit. The suit operates at roughly a third of sea-level atmospheric pressure, which is enough to keep the fabric flexible enough to move in while still delivering adequate oxygen to the lungs.
The catch is decompression sickness. Going from the station’s higher-pressure, nitrogen-rich cabin air to a low-pressure pure-oxygen suit is analogous to a scuba diver surfacing too fast: dissolved nitrogen can form bubbles in the blood and tissues. To prevent this, astronauts follow a “prebreathe” protocol, spending time on pure oxygen to flush nitrogen out of their bodies before suiting up. NASA has modeled a 15-minute prebreathe protocol for future exploration-class suits operating in a cabin atmosphere with higher oxygen content (around 34 percent O2 at reduced total pressure), and estimated that decompression sickness risk under various EVA scenarios using that protocol would fall between roughly 6 and 12 percent.13Acta Astronautica. Modeling a 15-min extravehicular activity prebreathe protocol using NASA׳s exploration atmosphere (56.5 kPa/34% O2)
This is one reason future exploration vehicles may not use the same sea-level atmosphere the ISS does. A cabin atmosphere with modestly higher oxygen and lower total pressure could dramatically shorten prebreathe times and simplify EVA operations, but it comes with trade-offs, especially around fire safety.
Fire Behaves Differently Up There
Choosing a cabin atmosphere involves a direct tension between making it easy to breathe during spacewalks and keeping fire risk manageable inside the vehicle. Higher oxygen concentrations make materials more flammable, and microgravity changes fire behavior in unexpected ways. Research aboard the ISS has shown that flames can spread at lower oxygen concentrations in microgravity than on Earth, and under certain airflow conditions, the flame-spread rate in weightlessness actually exceeds what you see in normal gravity.14PubMed Central. The Effect of Gravity on Flame Spread over PMMA Cylinders
NASA’s Solid Fuel Ignition and Extinction (SoFIE) experiments have pushed this further by testing material flammability at elevated oxygen levels in quiescent (still-air) microgravity conditions. At 30 percent oxygen, thin acrylic sheets burned readily, and near the point of extinction, flames exhibited pulsating and oscillatory behavior that sometimes persisted for several minutes before finally dying out.15Proceedings of the Combustion Institute. Quiescent flame spread and extinction over acrylic sheets in microgravity at increased oxygen concentration These findings are not academic curiosities. Every material that goes on a spacecraft has to be tested for flammability in the actual atmosphere the vehicle will use.
The fire risk extends to what the crew wears. Research on fabric flammability at higher oxygen concentrations found that synthetic fabrics that pose little burn risk under standard conditions become dangerous in oxygen-enriched cabin atmospheres, producing hazardous burn patterns even when there is an air gap between the fabric and skin.16Fire Safety Journal. Predicting fabric flammability and skin burn injury risk in high oxygen concentration normoxic atmospheres This is one reason the ISS sticks with a roughly Earth-normal atmosphere: the risk calculus for enriched-oxygen cabins has not yet been fully resolved for long-duration habitats.
Dust and Airlocks on the Moon
When astronauts eventually return to the lunar surface, keeping the air clean will involve a hazard the ISS largely avoids: dust. Lunar dust is extremely fine, abrasive, and has been a concern since the Apollo era, when it clogged equipment and irritated the astronauts’ airways within hours. A key strategy for future lunar habitats will be preventing that dust from entering the pressurized cabin in the first place. Proposed designs include multi-stage HEPA filtration inside the habitat, supplemented with magnetic filters that exploit the nanoscale metallic iron embedded in lunar dust particles to pull them from the air.17npj Microgravity. Overview of lunar dust toxicity risk
Airlock design becomes a critical part of air management in this context. Each time an astronaut passes through an airlock after a moonwalk, some amount of dust is carried in on suits and tools. Minimizing the dust that gets past that barrier is not just a housekeeping problem: it affects the life of the ECLSS filters, the health of the crew, and the integrity of sensitive equipment.
Making Oxygen on Mars
For missions beyond Earth orbit, shipping all the oxygen from home becomes impractical. A Mars crew would need to breathe for years, and launch costs make it prohibitively expensive to pack that much gas. The alternative is to produce oxygen on site using resources already available at the destination, a concept called in-situ resource utilization (ISRU).
The first real test of this idea came with MOXIE, the Mars Oxygen ISRU Experiment, which rode aboard NASA’s Perseverance rover. MOXIE demonstrated that oxygen could be produced directly from the carbon dioxide that makes up about 96 percent of the Martian atmosphere, using a process called solid oxide electrolysis. It was a small-scale proof of concept, but it worked across multiple Martian seasons, validating the basic chemistry for eventual human missions.18PubMed Central. Mars Oxygen ISRU Experiment (MOXIE)-Preparing for human Mars exploration A full-scale version would need to produce oxygen at rates hundreds of times higher, but the demonstration showed the approach is viable.
Plants and Algae as Living Air Processors
An even more ambitious approach to air revitalization replaces some of the machinery with biology. Photobioreactors, essentially contained tanks of algae or cyanobacteria, can absorb CO2 and release oxygen through photosynthesis, mimicking what forests do on Earth. Research into these bioregenerative systems has shown that microbial photosynthesis could serve a dual purpose aboard a spacecraft: removing CO2 and producing edible biomass at the same time.19PubMed Central. Use of Photobioreactors in Regenerative Life Support Systems for Human Space Exploration
No crewed mission has relied on biological air processing as a primary system. The challenges are significant: living organisms are sensitive to temperature, light, and contamination, and they do not respond instantly to changing crew demands the way a chemical scrubber does. But for a multi-year Mars transit or a permanent lunar base, the appeal of a system that recycles carbon, produces food, and processes air without consuming expendable chemicals is hard to ignore. Current designs envision photobioreactors as supplements to mechanical systems rather than replacements, gradually closing the loop further as the technology matures.
What Future Systems Need to Do Better
The ISS life support system is a remarkable piece of engineering, but it was designed for a station that receives regular resupply from Earth and has ground-based mission control monitoring its systems around the clock. A spacecraft heading to Mars would be on its own for months at a time, with no resupply possible. Future ECLSS designs need to be more autonomous, more efficient at closing the resource loop, and more resilient to failures than anything currently flying.20Space Habitation. Toward sustainable living in space: A review of environmental control and life support system technologies
One concrete goal is closing the oxygen loop more completely. The current Sabatier system recovers some hydrogen from CO2, but the methane byproduct carries hydrogen away when it is vented. Future systems may use different catalysts or reactor designs to extract more water from CO2, reducing the amount of make-up water that needs to be launched. Combining ISRU-generated oxygen with tighter onboard recycling could eventually make a crewed Mars habitat largely self-sufficient for breathing gas, a requirement that is non-negotiable for any permanent settlement beyond Earth.