What gas do humans need to breathe to live?

Oxygen, about 21 per cent of the air, and the way your body monitors it is the counterintuitive part. The urge to breathe is not triggered by oxygen running low. It is triggered by carbon dioxide building up, detected by receptors that respond to the resulting change in blood acidity.

That has a lethal consequence. In an atmosphere with no oxygen but no carbon dioxide either - nitrogen, helium, argon - you feel nothing at all. No panic, no gasping, just a few lucid seconds and then unconsciousness. It is why inert gas leaks in industrial spaces kill people who had time to leave and no reason to think they should, and why would-be rescuers so often become the second and third casualties.

The gas is also fundamentally corrosive, and using it is a managed risk. A fraction of the oxygen you process becomes reactive fragments that damage DNA and proteins, and repairing that is a continuous internal job. Breathing pure oxygen under pressure is toxic and causes convulsions, which is why deep divers breathe mixtures containing less of it, not more.