No Open Windows in Orbit: How the International Space Station Maintains Air Quality

by | Aug 24, 2026

On Earth, a stuffy room has a simple solution: open a window.

Astronauts aboard the International Space Station do not have that option. The station is a sealed environment surrounded by vacuum, so the air inside must be continuously circulated, cleaned, monitored, and chemically balanced.

Every breath adds carbon dioxide and water vapor. People, electronics, plastics, experiments, cleaning products, and equipment release trace chemicals. Skin cells, fibers, food particles, microorganisms, and ordinary dust enter the cabin air as well.

Keeping astronauts alive therefore requires something more sophisticated than an air conditioner. A collection of systems known as the Environmental Control and Life Support System, or ECLSS, continuously manages the station’s atmosphere.

Carbon Dioxide Must Be Continuously Removed

Astronauts inhale oxygen and exhale carbon dioxide just as people do on Earth.

Inside a house, carbon dioxide can be diluted by outdoor ventilation. On the space station, it must be actively removed.

NASA’s Carbon Dioxide Removal Assembly, or CDRA, uses materials called molecular sieves. Cabin air passes through beds containing materials that selectively capture carbon dioxide molecules.

The system operates through alternating beds. While one portion removes carbon dioxide from the cabin air, another can be heated and exposed to the vacuum of space so the captured CO₂ is released and the material can be reused.

This regenerative design is important because carrying disposable carbon dioxide filters from Earth for every mission would require enormous amounts of cargo.

Carbon dioxide control is also more than a comfort issue. Excessive concentrations can contribute to headaches, fatigue and other physiological effects, making continuous removal essential for long-duration missions.

Oxygen Can Be Made From Water

Removing carbon dioxide solves only half of the breathing equation. Astronauts also continuously consume oxygen.

The station’s Oxygen Generation System produces new oxygen through electrolysis.

Electricity is passed through water, separating water molecules into oxygen and hydrogen. The oxygen is released into the cabin atmosphere for the crew to breathe.

The hydrogen does not simply have to become waste. It can be combined with captured carbon dioxide in a Sabatier reaction. This process produces water and methane.

The recovered water can return to the station’s life-support cycle, while methane is discharged.

This connection between air and water is one of the most interesting aspects of spacecraft environmental control. Moisture from astronauts can eventually become drinking water, and some of that water can later become breathable oxygen.

Humidity Has Nowhere Else to Go

People continuously release water vapor by breathing and sweating.

On Earth, excess moisture may leave a building through ventilation. On the station, it remains inside unless machinery removes it.

Common Cabin Air Assemblies circulate cabin air through cooling equipment containing condensing heat exchangers. As humid air contacts the cold exchanger, water condenses from it.

Microgravity makes this process more complicated than ordinary air conditioning. Water does not simply drip downward into a drain.

Special air-and-water separators collect the condensed moisture and direct it into the station’s water-recovery system.

Humidity control helps keep astronauts comfortable, but it also protects the spacecraft. Excess moisture could encourage microbial growth, create condensation on sensitive surfaces, and interfere with electrical and mechanical equipment.

Tiny Particles Do Not Fall to the Floor

Dust behaves differently in microgravity.

On Earth, gravity continuously pulls many larger airborne particles toward floors and furniture. On the space station, particles can remain suspended much longer.

Astronauts themselves generate skin flakes, hair, clothing fibers, food crumbs, and other debris. Equipment and experiments can produce additional particulate matter.

The U.S. portion of the station uses distributed high-efficiency particulate filters known as Bacterial Filter Elements. These use HEPA filtration to capture suspended particles while the ventilation system continuously moves cabin air through the station.

Particle filtration protects both astronauts and machinery. Floating debris that enters a fan, heat exchanger, electronics package, or scientific instrument can create operational problems even if it poses little direct health risk.

Regular housekeeping is therefore surprisingly important in orbit.

Trace Chemicals Need Their Own System

Carbon dioxide is not the only gas produced inside a sealed spacecraft.

Humans release small amounts of chemicals through breathing, perspiration and normal metabolism. Plastics, electronics, adhesives, experiments and equipment can also release volatile compounds.

On Earth, many of these substances would simply become diluted outdoors. In a spacecraft, even small emissions can gradually accumulate.

The Trace Contaminant Control System handles these pollutants.

Cabin air passes through activated charcoal capable of trapping many chemical vapors. Other contaminants are processed through a catalytic oxidizer, which chemically converts them into less troublesome substances. Additional sorbent material captures products produced during those reactions.

This allows the station to continuously control low concentrations of chemicals that would otherwise slowly build up during months of habitation.

Air Must Keep Moving

Ventilation serves an especially important function in microgravity.

Warm exhaled air does not naturally rise away from an astronaut in the same way it does on Earth. Without forced circulation, pockets of carbon dioxide could form around a sleeping or stationary crew member.

Fans continuously mix the atmosphere and move air between occupied areas and life-support equipment.

Ventilation also distributes oxygen, transports humidity toward condensing systems, carries carbon dioxide toward scrubbers and moves particles toward filters.

The air does not merely need to be clean. It needs to keep moving.

Sensors Watch the Atmosphere

A spacecraft cannot rely on smell to determine whether the air is safe.

The station uses instruments to monitor major atmospheric components such as oxygen, nitrogen, carbon dioxide, hydrogen, methane and water vapor.

Additional instruments can look for trace contaminants. The European Space Agency’s ANITA-2 system, for example, has been used aboard the station to monitor dozens of trace gases in cabin air.

Monitoring provides an early warning when equipment malfunctions, an experiment releases an unexpected chemical, or a life-support system begins performing differently.

The Station Is a Model for Closed Environments

The International Space Station effectively contains its own tiny artificial atmosphere.

Instead of solving pollution through unlimited outdoor ventilation, it must identify contaminants, remove them selectively, recover useful resources and return cleaned air to the crew.

Carbon dioxide becomes material for water recovery. Humidity becomes usable water. Water becomes oxygen. Particles are filtered, trace gases are chemically removed, and sensors continuously verify that the system is working.

Future missions to the Moon and Mars will make these technologies even more important because replacement filters, water and oxygen will be much farther away.

For astronauts, air quality is not simply about comfort.

In a spacecraft where opening a window is impossible, maintaining clean air is one of the machines keeping the entire habitat alive.

References