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I know we need nitrogen to live, but what happen if we reduce the proportion of nitrogen in air (replacing it with another substance)?

What percentage can I replace without risking health of humans or animals in my fantasy world?

EDIT: To be more clear in my current case: I have invented an new element in my fantasy world that does not exist in reality. I want integrate it into the atmosphere (mix into the air) of my world. If I just add this element without replacing anything else, the air will be more dense. Instead of that, I am asking how much I can reduce the percentage of nitrogen and replace it with my new substance without preventing humans and animals from breathing.

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  • $\begingroup$ Do you need this atmosphere to be scientifically explicable (high neon/argon atmosphere would be difficult to explain)? $\endgroup$ – Alexander Dec 26 '19 at 19:28
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    $\begingroup$ I've tweaked your question slightly to improve the English... feel free to revert it if you don't like the changes. $\endgroup$ – Starfish Prime Dec 26 '19 at 20:08
  • $\begingroup$ We don't need nitrogen to live; in fact, about 1% of Earth's atmosphere is argon, a noble gas. Nitrogen is however crucially important in the make-up of Earth life; the symbiotic fungi which take nitrogen from the air and supply it to the plants which then proceed to make the proteins on which we all live may balk if the partial pressure of nitrogen was decreased too much. $\endgroup$ – AlexP Dec 26 '19 at 20:10
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    $\begingroup$ We don't need to breathe atmospheric nitrogen to live, but we do need nitrogen incorporated in chemicals such as proteins. Plants need fixed nitrogen in order to grow, which is taken up through their roots. It's produced naturally mainly by the action of nitrogen-fixing bacteria, found in the roots of legumes, and by lightning. (And these days, from artificial fertilizers.) So your world needs enough atmospheric nitrogen for this to happen. $\endgroup$ – jamesqf Dec 26 '19 at 20:32
  • $\begingroup$ From linked prior question: "This planet's atmosphere has oxygen at a partial pressure that is breathable to humans, and the rest of the atmosphere is, at least, not made up primarily of Nitrogen (that is, it's not just Earth's atmosphere with marginally different partial pressures)". I think that is the same as this. $\endgroup$ – Willk Dec 26 '19 at 23:48
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From a mammalian respiration point of view, you can replace Nitrogen with any inert gas from the periodic table. For example, Helium-Oxygen mixtures are used by saturation divers to avoid nitrogen dissolving into their blood at the high pressures of deep-sea diving. While there are medical complications arising from the high atmospheric pressure, the gases they breath don't appear to pose health risks.

From a planetary lifecycle, everything living thing needs nitrogen since DNA uses nitrogen bases. For terrestrial species, bacteria in the ground and lightening generate nitrogen compounds that plants absorb. We get our nitrogen from eating plants and animals that eat plants.

To maintain the lifecycle of our planet without altering how nitrogen fixation works, you'll need to keep $N_2$ concentrations relatively high to ensure that $N_2$ diffusion into the soil (where the bacteria live) still works effectively. The concentration of $N_2$ in the soil approximates the atmospheric levels.

If the concentration of atmospheric nitrogen was $35\%$ instead of ~$79\%$ the concentration of $N_2$ in the soil couldn't rise above that level since it is determined by the principles of diffusion. How this would impact plant life is difficult to estimate. At some threshold, you'd constrain the amount of plant life, but where that threshold is was not something I could determine in my casual research of the topic.

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  • $\begingroup$ thx, I have edit my question with some details.. $\endgroup$ – Matrix Dec 28 '19 at 0:35
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There are various gasses which are more or less inert, but they're a lot less common in the universe than nitrogen. Argon is a good alternative if your atmospheric pressure is fairly Earth-like, but it can become unhealthy at higher pressures (causing an effect similar to nitrogen narcosis, but occuring at much lower partial pressures). Neon has no such problem, but is rarer still on Earth, though more common than nitrogen in the rest of the solar system and sider Universe. Divers also use helium instead of nitrogen to avoid nitrogen narcosis] at depth, but having a lot of helium in a planetary atmosphere is problematic for other reasons (because of the additional gravity required to keep it from flying away into space).

You could replace nitrogen in its entirety with argon or neon with no immediate health issues, though there would be other interesting knock-on effects. Firstly, neon is somewhat easier to ionise than nitrogen (hence the common occurence of neon lights) so lightning would be perhaps more frequent and much more red than blue.

More importantly though, your planetary nitrogen cycle would have to be radically different... no nitrogen fixing, as there's little atmospheric nitrogen, and nitrogen gas produced by denitrifying organisms would be effectively lost to the system which would be extremely problematic. You could handwave some other reservoir of nitrogen, but preventing it from being released as gas over geological timescales seems like it would be quite difficult. Without a nitrogen cycle, life on your planet will have to be radically different to Earth.

More broadly, if you wanted to remove some or all nitrogen from all of your universe, not just a planetary atmosphere, you risk messing up the C-N-O fusion cycle which occurs in larger stars, or having to perform implausible handwaves to reduce its comsic occurrence. Probably best not to do that.

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  • $\begingroup$ Helium is also used in spacecraft, probably to save mass. $\endgroup$ – Jeff Zeitlin Dec 26 '19 at 19:26
  • $\begingroup$ @JeffZeitlin apparently the ISS, Soyuz and Space Shuttle all use(d) a conventional Terrestrial gas mix and pressure. I know all sorts of other things have been used elsewhere, including low pressure pure oxygen. Who used a helium mix? $\endgroup$ – Starfish Prime Dec 26 '19 at 19:46
  • $\begingroup$ I seem to recall that the Skylab astronauts ended up speaking 'donaldduck' because the N2 was at least in great part replaced by He2. $\endgroup$ – Jeff Zeitlin Dec 26 '19 at 19:48
  • $\begingroup$ Heinlein put heliox in space suits in Have Space Suit, Will Travel -- his were open circuit air supply, and the heliox gave much more cooling than pure oxygen (however, it would also have required up to five times higher total pressure, making the suit much stiffer). $\endgroup$ – Zeiss Ikon Dec 26 '19 at 19:51
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    $\begingroup$ @JeffZeitlin this NASA page suggests that skylab used 74% O2 26% N2 at a mere 5psi. $\endgroup$ – Starfish Prime Dec 26 '19 at 19:52
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There are many possible atmosphere mixtures which could work for human life. If we keep the partial pressure of O2 constant, not much about humans would need to change.

But in the long term, changing the density of the atmosphere will effect many other aspects of life, such as the ability of birds and insects to fly, and transpiration through vegetation.

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  • $\begingroup$ Can you be more specific about impact of change dansity of air ? (increase density => harder to fly?) $\endgroup$ – Matrix Dec 28 '19 at 0:36

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