We breathe out a mix of gases, not just one thing.

Main things we breathe out

  • A lot of nitrogen (about the same as in normal air, roughly 78%).
  • Less oxygen than we inhaled (air in has about 21% oxygen, air out has about 16%).
  • More carbon dioxide (CO₂), which is a waste product from our cells making energy.
  • Small amounts of water vapor and trace gases.

So: when people ask “what do we breathe out?”, the simple answer is “mostly nitrogen, some leftover oxygen, and extra carbon dioxide plus water vapor.”

Inside the lungs, oxygen moves from the air into the blood, and carbon dioxide moves from the blood into the air; when you exhale, that CO₂-rich air leaves your body.

A neat extra fact: the carbon in the CO₂ we breathe out originally came from the food we ate (plants or animals that ate plants), so it’s part of a natural carbon cycle.