Excess oxygen is actually harmful to humans, but all the climate warnings are about losing oxygen, not nitrogen edit: but when we look for habitable planets, our focus is ‘oxygen rich atmosphere’, not ‘nitrogen rich’, and in medical settings, we’re always concerned about low oxygen, not nitrogen.
Deep sea divers also use a nitrogen mix (nitrox) to stay alive and help prevent the bends, so nitrogen seems pretty important.
It seems weird that our main focus is oxygen when our main air intake is nitrogen. What am I missing?
edit: my climate example was poor and I think misleading. Added a better example instead.
Thank you for your detailed response. That explains things very well. I don’t know a lot about chemistry, but is oxygen specifically required for cell metabolism or could that be replaced with a similarly reactive gas, too?
We’re pretty hyper-specialized to use it, but there are organisms on earth that don’t need it and in fact find oxygen deadly; they are called anaerobic. They still need chemical energy, it’s just not provided by oxygen. (As I was looking this up I discovered there’s even a creature in the animal kingdom that doesn’t breathe oxygen.) Some gases, like carbon monoxide, will actually participate in gas exchange in your lungs and react with your body chemistry, but in a way that rapidly breaks down cell functioning.
So, yes, there are definitely other forms of biochemistry that can process non-oxygenated environments and extract energy from them, just not us, not by a long shot.
This is why:
A) in spaceships, you can have 100% oxygen environments, at low pressures
B) scuba divers replace nitrogen with helium for deep dives (trimix) - and reduce oxygen.
As for replace oxygen: yes, but that would kill us very quickly.