while the other is likely to fight for air.

The body doesn’t actually sense the amount of oxygen in your blood. Instead, there’s assorted ways it detects elevated levels of CO2.

That works great when you’re e.g. swimming under water or are holding your breath. Low oxygen naturally coincides with excess CO2.

But it’s dangerous in low oxygen environments. CO2 doesn’t build up, so you don’t sense that you’re running low on oxygen.

People die accidentally from inert gas asphyxiation. It’s sneaky like that.