How the ISS Makes Oxygen and Keeps the Air Breathable

The International Space Station keeps its cabin close to normal sea-level air: about 21 percent oxygen, mostly nitrogen, at a comfortable pressure. Doing that in the vacuum of space, with a crew constantly breathing, is a nonstop engineering job handled by the station's life support system, known as ECLSS. Our live dashboard tracks the resulting cabin pressure and temperature in real time.

Making oxygen from water

The main trick is electrolysis: running an electric current through water to split it into hydrogen and oxygen. The station has two systems that do this, powered by its large solar arrays:

• The Oxygen Generation Assembly (OGA) on the US segment splits water into breathable oxygen, released into the cabin, and hydrogen, which is vented or reused.
• The Elektron unit on the Russian segment does the same job, giving the station redundancy in case one system needs maintenance.

Because oxygen comes from water, water is one of the most precious resources on board, which is why the station recycles almost all of it. We cover that loop in how the ISS recycles water.

Removing carbon dioxide

Making oxygen is only half the problem. The crew constantly exhales carbon dioxide, which is toxic as it builds up. The Carbon Dioxide Removal Assembly (CDRA) scrubs CO2 out of the air using materials that absorb it, then dump it overboard or feed it to a Sabatier reactor. That reactor combines the CO2 with leftover hydrogen from oxygen generation to make water and methane, recovering even more water in the process.

Backups for a closed world

In a place where you cannot open a window, redundancy is everything. Alongside the generators, the station keeps other oxygen sources ready:

1. Pressurized oxygen and nitrogen tanks, refilled by visiting cargo ships.
2. Solid fuel oxygen generators, sometimes called oxygen candles, for emergencies.
3. Oxygen delivered directly in the tanks of Progress and other resupply vehicles.

Why it matters

Together these systems turn the station into a small, mostly self-sustaining world. The same closed-loop technology being refined on the ISS is exactly what future crews will need for long trips to the Moon and Mars, where resupply from Earth is slow or impossible. For the full picture of the station's scale and systems, see ISS by the numbers.