How the Space Station Turns Exhaled Breath Back into Oxygen

Long-duration space missions require closed-loop life support systems to recycle waste into essentials. The International Space Station's Advanced Closed Loop System (ACLS) demonstrates this by converting exhaled carbon dioxide back into breathable oxygen through a multi-stage process.

Daron Krikorian

October 11, 2026 · 4 min read

For humans to travel to the Moon, Mars, and beyond, they must solve a fundamental problem: how to survive for months or years far from Earth’s life-sustaining resources. The sheer quantity of air, water, and food required for a long-duration mission makes it impractical to simply pack everything at the start. The solution lies in developing closed-loop life support systems that recycle waste products into the essentials for survival, creating a self-sustaining environment inside a spacecraft. These regenerative systems are critical for making a long-term human presence beyond Earth both possible and sustainable. A potential mission to Mars, for instance, would need to support a crew for nearly two years.

Instead of carrying an enormous stockpile of supplies that must be launched from Earth, a circular recycling system can produce the oxygen, water, and food the crew needs by continuously processing their waste.

The Limits of Open-Loop Systems

Early and shorter-duration space missions relied on what engineers call "open-loop" life support. In this model, all necessary resources—such as oxygen, water, and food—are brought from Earth in storage tanks and containers. As the crew consumes these supplies, their waste products, like exhaled carbon dioxide and wastewater, are collected and stored for eventual disposal. This approach is straightforward but has a significant drawback: the amount of required supplies increases in direct proportion to the mission's length and the size of the crew. For a short trip to low Earth orbit, this is manageable. But for a multi-year journey to another planet, the mass and volume of stored resources would become prohibitively large and expensive to launch. Similarly, systems like carbon dioxide scrubbers become ineffective over time and would require frequent replacement, a logistical impossibility on a deep-space voyage.

The Three Pillars of a Closed-Loop System

To overcome the limitations of open-loop designs, space agencies are focused on creating "closed-loop" or regenerative systems. These systems require an initial supply of resources but are designed to process waste products to recover and reuse valuable materials, dramatically reducing dependence on resupply from Earth. A fully self-sufficient system is built around three primary functions, often called "loops," that manage the flow of essential resources. First is the air revitalization loop, which focuses on maintaining a breathable atmosphere. This involves actively removing the carbon dioxide (CO₂) exhaled by the crew and generating fresh oxygen to replace what is consumed. Second is the water recovery loop.

This system is designed to collect and purify all sources of wastewater, including urine, hygiene water, and condensation captured from the cabin air. The goal is to turn this water back into clean, potable water for drinking and other uses. The third and most complex pillar is the food production and waste management loop. This aims to grow food onboard and recycle organic waste. The European Space Agency’s MELiSSA program, for example, is a long-term effort to develop a bioregenerative system that uses microorganisms and higher plants to recycle waste, produce food, and contribute to air and water regeneration.

ESA's Advanced Closed Loop System

A key example of a modern air revitalization system is the European Space Agency's Advanced Closed Loop System (ACLS), which operates aboard the International Space Station. This hardware represents a major step toward creating a fully self-sufficient environment for astronauts. The ACLS is designed to take the carbon dioxide exhaled by the crew and, through a multi-stage process, convert it back into breathable oxygen. This technology is considered a crucial advancement for human spaceflight as agencies prepare for exploration farther from Earth. For future outposts like the lunar Gateway, which is envisioned as a staging post for missions to the Moon and Mars, regenerative life support is a necessity. The great distances involved will make resupply missions far more difficult and expensive than they are for the International Space Station today.

The ACLS Air Revitalization Process: CO2 to O2

The Advanced Closed Loop System produces water from the carbon dioxide astronauts exhale, and that water is then used to generate oxygen for the crew. According to the European Space Agency, its operation can be broken down into three distinct stages, each using a specific technology to transform molecules from one form to another.

A step-by-step breakdown of how the Advanced Closed Loop System recycles carbon dioxide into oxygen.
Process Stage	Key Technology/Component	Inputs	Outputs
1. CO₂ Capture	Carbon dioxide Concentration Assembly (CCA)	Cabin air containing exhaled CO₂	Concentrated carbon dioxide
2. Water Production	Sabatier Reactor	Concentrated CO₂ and hydrogen (H₂)	Water (H₂O) and methane (CH₄)
3. Oxygen Generation	Oxygen Generation Assembly (OGA) / Electrolyser	Water (H₂O)	Breathable oxygen (O₂) and hydrogen (H₂)

A step-by-step breakdown of how the Advanced Closed Loop System recycles carbon dioxide into oxygen.

Process Stage

Key Technology/Component

Inputs

Outputs

1. CO₂ Capture

Carbon dioxide Concentration Assembly (CCA)

Cabin air containing exhaled CO₂

Concentrated carbon dioxide

2. Water Production

Sabatier Reactor

Concentrated CO₂ and hydrogen (H₂)

Water (H₂O) and methane (CH₄)

3. Oxygen Generation

Oxygen Generation Assembly (OGA) / Electrolyser

Water (H₂O)

Breathable oxygen (O₂) and hydrogen (H₂)

The process begins with the first major function, the Carbon dioxide Concentration Assembly (CCA). This component draws in cabin air and concentrates the CO₂ to keep it within acceptable, breathable levels for the crew. Once captured, the concentrated carbon dioxide is extracted using steam and transferred to the next stage. In the second stage, the CO₂ is fed into a Sabatier reactor. Here, it is combined with hydrogen in a chemical reaction that produces two new substances: water (H₂O) and methane (CH₄). The production of water is a critical step, as it converts a waste gas into one of the most valuable resources in space.

The newly created water is then sent to the third component, the Oxygen Generation Assembly (OGA). This assembly functions as an electrolyser, using electricity to split the water molecules into their constituent parts: breathable oxygen and hydrogen. The oxygen is released into the cabin for the crew, while the hydrogen is recycled back to the Sabatier reactor to be used again in the second stage. This recycling of hydrogen is what makes the system a "closed loop." The methane produced as a byproduct in the Sabatier reactor is vented into space.