What do astronauts breathe in and which gas is it?

Astronauts breathe in a mixture of gases, primarily oxygen and nitrogen, similar to Earth's atmosphere. However, in spacecraft and space stations, the atmosphere is typically enriched with oxygen, around 21% to 30%, to ensure safety and comfort. The primary gas they breathe is oxygen (O₂), which is essential for respiration. Carbon dioxide (CO₂) is also present, but it must be carefully monitored and removed from the environment to maintain a safe atmosphere.