It's about CO2 concentration, not about oxygen production.
this post was submitted on 04 Dec 2023
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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?
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.
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.
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
The cells need this oxygen to make the energy your body needs to work. When cells make that energy, they create the waste product carbon dioxide.
Is there some threat to the worlds supply of atmospheric nitrogen?
My climate example was poor, I’m sorry. A better example of what I mean is that when we look for habitable planets, our focus is ‘oxygen rich atmosphere’, not ‘nitrogen rich’, though most of our breathable air is nitrogen and too much oxygen will actually kill us.
I suppose because we don’t really use the nitrogen - it’s inert, unlike oxygen which is part of vital respiration. I’m no expert but it’s conceivable some other mix of gases could work as the inert portion besides nitrogen, but oxygen is required. Seems like it would take a lot of luck to find the right concentration though.
Calling it "inert" is misleading. It's involved in all kinds of chemical reactions that are essential for life (and lots of non-biological reactions, too). It's only inert in the sense that most living things can't use it directly from the air and rely on nitrogen-fixing plants and bacteria to make it into molecules we can use.
Sure, I understand it’s one of the 3 basic plant fertilizers and plays a role in human biology. I’m referring nitrogen as a gas and its role in human respiration. It’s commonly referred to as an inert gas. I think that mainly refers to respiration and combustion.
Now I'm imagining a planet with a helium atmosphere that's breathable for humans. Best. Episode. Of. Star Trek. Ever. I'm envisioning TOS, super serious scenes where Scotty has fallen near dead, Kirk looks to Bones for some reassurance, and in Mickey Mouses voice Bones mournfully tells him "He's dead, Jim"
This needs to be an episode of Lower Decks.
e: and Boimler’s voice doesn’t change.
I think it could work? Not really sure. Probably sound enough scientifically for a 1st gen Star Trek episode.
It would work in the sense that you could breathe it. It would not work in the sense that the gravity of a planet that actually holds a helium atmosphere (as opposed to it flying off into space) would be uncomfortable.
Would argon work?
Nitrogen is usually in the form of N2 and is very stable. We don't really do much with this form of nitrogen because chemistry is hard so with each breath it just hangs around. The oxygen on the other hand js readily absorbed and used, converting it into CO2. We have to remove the CO2 to prevent toxicity and add O2 to prevent suffocation.
We (humans) don't need atmospheric nitrogen to survive, we just need the "other" in the atmosphere to not be toxic. My guess is any element in the same periodic table column as nitrogen that could somehow exist as a gas at STP would be fine (hypothetically).
Don't they replace nitrogen with like helium for some SCUBA applications?
Other life on Earth obviously does metabolize nitrogen (I think we do a little bit too). So to preserve our biosphere we should not throw off the nitrogen balance. But as others have observed it's not clear there's any imbalance coming.
Overall you're not on a bad track with thinking that other elements in the same column could substitute for nitrogen, often those elements will have similar properties.
However it doesn't really apply in this particular case, the other elements in its group are all pretty reactive and solid, and include elements like phosphorus and arsenic. Even nitrogen compounds can be pretty damn reactive, it's just kind of a quirk of chemistry that diatomic hydrogen (N2) which makes up most of our atmosphere is pretty stable and nonreactive under most circumstances.
We need certain nitrogen compounds as part of our biological processes but we don't get that nitrogen from the air, we get it from food we eat, which ultimately get it from the air mostly from microbes in the soil that are actually able to take nitrogen from the air and turn it into other chemicals.
However since atmospheric nitrogen is, for the purposes of this conversation, inert, you can pretty much replace it with any other nonpoisonous gas, like we often do for deep sea divers, because under high pressure nitrogen will dissolve into our blood and then when we resurface it creates nitrogen bubbles in our blood which is very bad.
Nitrogen is, as far as our biology is concerned, effectively inert, we don't really do anything with it, it more or less just goes in and out of our lungs. For most practical purposes under normal atmospheric conditions it could pretty much be replaced with just about any nonpoisonous gas. As far as your body is concerned that part of the atmosphere might as well be helium, and in fact for certain deep sea diving applications and such we do replace some or all of the gas mix with things like helium because nitrogen will sort of dissolve into your bleed at high pressure, which causes issues when you start to resurface and it creates nitrogen bubbles in your blood (known as "the bends" or more technically as "decompression sickness") and those other gasses don't dissolve into our blood as readily.
Pretty much as long as oxygen is at the right percentage, your body doesn't care what the rest of the gas mix is as long as it's not outright poisonous.
Now there could be issues for nitroget-fixing bacteria that do use atmospheric nitrogen and convert it into other nitrogen compounds that are absolutely necessary for plants and such to grow (and by extension for us and everything else that eats those plants, or eats things that eat plants) to live, and I'll be honest, I have no idea at what level of atmospheric nitrogen that would start to be a problem, and unless we want to start growing crops in underwater domes it's probably not something we ever really need to worry about on earth, nitrogen is very plentiful in our atmosphere. It could possibly be something worth investing for long-term space exploration and such, but we're not quite there yet.
New comment based on your edits.
When we breath, we use the oxygen, but we do not use the nitrogen. The nitrogen can actually be replaced with another inert gas and the "air" is breathable. Thinking about diving specifically, nitrox is actually an oxygen rich (nitrogen poor) mixture. More extreme mixtures use helium in place of some nitrogen (and sometime oxygen depending on the depth).
In your body, the amount of oxygen in our blood is critical for survival. Having a lot of nitrogen is actually not good. Too much is what causes the "bends", again related to diving.
When looking at exoplanet atmospheres, we look for oxygen rich because it likely indicates water. We believe that planets with a high amount of water are more likely to support life similar to ours. It is possible that another form of life exists that doesn't need oxygen or water, but we know for certain that oxygen and water can support life.
Atmospheric nitrogen is useless to most life, as it's extremely hard to break down into other nitrogen compounds. Certain bacteria are the exception, and they're very important both to ecology and human agriculture (although less so since the Haber process was invented and artificial fertilisers became available). The other natural source of nitrogen compounds is lightning strikes.
Oxygen is completely the opposite. It's unstable in an Earth-like environment (which is why fires happen), and if you find it in such an environment there must be something special producing it continuously. It's not the only biomarker astronomers look for, either. There was a planet with insane amounts of a chemical called DMS found recently, and that's just as eye-catching, if weirder.
Deep sea divers also use a nitrogen mix (nitrox) to stay alive and help prevent the bends
You've actually got that somewhat backwards. To go really deep you switch to heliox or similar. Nitrox is for intermediate depths where you need less oxygen than in the normal nitrogen-oxygen atmosphere, but nitrogen narcosis isn't an issue yet.
When scuba diving recreationaly, Nitrox tanks actually have higher concentrations of oxygen, leaving less "space" for nitrogen and other gases.
Normal air tanks are filled at 21% oxygen. A nitrox tank will have more than 21%. The ones I use have 32% oxygen, wich helps when being at depths between ~20m and ~40m
I know that technical divers do use other gas concentrations, and they even change the nitrogen in the tanks for other inert gasses. But I've no idea of which gasses they use.
The most famous is Heliox, 98% helium and 2% oxygen iirc.
N2 is (mostly) inert when it comes to respiration. What your body needs is oxygen and low concentrations of anything that might also be metabolically active. For scuba diving, N2 is used to dilute the oxygen and is used specifically because of how non-reactive it is. At high concentrations though, it can result in nitrogen narcosis - helium is sometimes used as the diluent gas instead to mitigate this.
As far as habitability is concerned, atmospheric nitrogen is essential for life on Earth at least, as it's a major part of the nitrogen cycle (specifically, nitrogen fixation). Without it, we wouldn't have nitrogen-containing organic compounds like amino acids (and, therefore, proteins), at least not nearly in the same quantities that we currently do. This doesn't mean it's essential for life outside earth, but it is for life as we know it, so its presence should increase our credence (if only a little) for whether a given planet is habitable or not. However, when looking for signs of life, it's better to look for atmospheric signatures that are heavily influenced by life, rather than just those that facilitate it. The oxygen in Earth's atmosphere was largely produced by life, and so its presence in the atmospheres of other planets would be a good (though not definitive) indication of habitability.
I'm not fully aware of all the issues you reference, but my first guess is that oxygen is reactive and can be used up (apparently based on your statements, although I'm not familiar with that line of reasoning). Whereas nitrogen is not very reactive, so doesn't get used up nearly as much.