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That makes sense, thanks, since our threshold for co2 is less than 0.5%.
I may have worded my question poorly; I’m more asking why low oxygen is a problem vs low nitrogen. In retrospect, my climate focus may be distracting. It was what made me wonder about this in the first place, but the medical and scuba points are much more relevant. That has little to do with co2 (I think?) and more to do with the relative compounds in our air.
I’m still confused why we hear about oxygen but never nitrogen. Another example: when we look for habitable planets, the focus is ‘oxygen rich atmosphere’, but not ‘nitrogen rich’.
As someone else pointed out, nitrogen is non-reactive. Almost any gas would work, as long as it was plentiful enough to maintain the necessary air pressure, and non-reactive. You don't need nitrogen to live; you just need oxygen. Just, not so much that you get acute oxygen toxicity, which mainly happens with pure oxygen at regular atmospheric pressure for extended periods of time. There are even applications where pure oxygen is administered to people, usually at lower than atmospheric pressure.
Nitrogen is a filler gas. It's there to take up space and keep the air molecules bouncing around at the appropriate pressure. (Or perhaps it would be more accurate to say our lungs require a certain pressure because this is where we evolved; that pressure happens to be maintained mostly by nitrogen.)
We aren't exploring other planets in person yet, but if we were, we'd need to filter out all the bad shit in the air, keep the oxygen, and maintain the normal pressure. If we were lucky enough to encounter an atmosphere with oxygen, a non-reactive filler gas, and no toxins, we might be able to just breathe it; or to breathe it after compressing it to the appropriate pressure. Nitrogen wouldn't need to be there at all.
The confusing thing about the scuba application is that nitrogen isn't in the mix because you need the nitrogen. It's there because it reduces the pressure of toxic gases to a threshhold you can survive.
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?
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.
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.
Maybe nitrogen could be replaced with other gases, but we need oxygen in our lungs and bloodstream to survive. So maybe it's more important for our survival?
Here's an interesting post about the different formulations and pressures required to have breathable oxygen mixtures.
https://space.stackexchange.com/questions/28280/could-we-breathe-an-atmosphere-that-is-not-nitrogen-based#28283
Based on OP's question, we don't care about nitrogen, because it's not strictly necessary for our survival.
That explains it very well, thank you!
So from what I understand, we need a rather precise amount of oxygen plus a large amount of an inert gas – pretty much any inert gas, barring a few that have narcotic effects. So nitrogen isn’t special, except that it’s inert and doesn’t get us high.
But I’m also curious whether the reactive gas in low quantities (oxygen) can also be replaced. I’m not a chemist, and this is fascinating. I’ll keep reading.
Thanks again!
We don't need a precise amount of oxygen - we can survive in a fairly wide range. Think about living in the mountains vs by the ocean.
Nitrogen gets us absolutely high. Balls to the wall high. It's why gas narcosis used to be called nitrogen narcosis. Also known as the "rapture of the deep".
Also, oxygen gets you high. Also, oxygen kills you, but that's another matter.
It’s pretty amazing we’re alive at all, when you put it that way.
I'm not a biologist, doctor, or chemist, but my guess is "no." We have evolved to use oxygen to create energy within our cells, not some other gas.
I would hazard an additional guess that it's not a simple matter to just swap out the oxygen molecules for something else. Carbon monoxide binds better and more readily to our cells, yet that mixture would asphyxiate you.
https://www.nhlbi.nih.gov/health/lungs/breathing-benefits
See my reply to someone below.