Can people breath totally nitrogen-free atmosphere for a long time? I know, nitrogen is essential for life, and in big quantities, but maybe people can take it entirely from food, from proteins, etc?
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2$\begingroup$ Some short comments - Nitrogen gas is pretty much inert and nutritional nitrogen is mainly obtained from the diet. Inhaling pure oxygen will kill you in the longer run. So what exactly is your question? If N2 gas has dietary value, or whether nitrogen-less air where ~79% of some other non-toxic gas substitutes for N2 can sustain life, or whether 100% O2 can possibly sustain life? $\endgroup$– AliceD ♦Commented Apr 13, 2021 at 7:23
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Do people need nitrogen from air for health?
No. This is well established!
Can people breath totally nitrogen-free atmosphere for a long time?
Yes. Also well established!
I know, nitrogen is essential for life, and in big quantities, but maybe people can take it entirely from food, from proteins, etc?
As expounded on in the comments by AliceD, nitrogen gas (N2) is entirely taken in by humans from their diet, since atmospheric N2 is pretty inert. It is also not necessary for gas exchange to take place in the lungs.
Some extra questions as an addendum:
Can one substitute N2 in atmospheric air with another gas, and sustain human life?
Yes. All side effects would be attributable to the effects of the substituting gas (e.g. it may be heavier or lighter or equally or more reactive than N2).
Can 100% O2 sustain human life?
No, and such a case has never been recorded. Oxygen toxicity following hyperoxia is very dangerous. Ironically, too much oxygen damages lung tissue and epithelial membranes via oxidation and render the lungs unable to exchange gases correctly; and so death by suffocation follows in such extreme cases such as with 100% O2.
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5$\begingroup$ A pedantic point: dietary nitrogen is 'fixed' nitrogen, not N(2) which (to quote the Wikipedia article) is 'metabolically useless" to all but a few (very important) microorganisms. N(2) is, however, primarily responsible for 'the bends' (decompression sickness). (Nitrogen 'fixation' is why the Harber process is so important) $\endgroup$ Commented Apr 13, 2021 at 13:01
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1$\begingroup$ Is it known that none of the microbes in the microbiome require nitrogen in the air from inside the intestines for protein synthesis? $\endgroup$ Commented Apr 15, 2021 at 12:54
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1$\begingroup$ Do they have on ISS nitrogen-based air? $\endgroup$– AnixxCommented Apr 16, 2021 at 11:28
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1$\begingroup$ I think negative effects of oxygen come frm its partial pressure, not from its percentage. Thus, 100% oxygen at 20% atmospheric pressure should be harmless, no? $\endgroup$– AnixxCommented Nov 18, 2022 at 3:30
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$\begingroup$ Oxygen toxicity depends on partial pressure, sure. That relates to how many O2 molecules there are. Fewer make it less toxic, no doubt. But the act of breathing in low atmospheric pressure is more difficult too, it's another problem entirely. During inhalation, lung volume increases, lung pressure decreases below that of atmospheric pressure. This draws air into the lungs. At lower atmospheric pressure, less total air is drawn in, so breathing is more difficult at lower atmospheric pressures (or at higher altitudes due to the same phenomenon). So you're describing a trade-off. $\endgroup$– S PrCommented Nov 18, 2022 at 10:11