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Does Liquid Air mitigate the asphyxiation risks associated with Liquid Nitrogen? As Oxygen and Nitrogen have marginally different boiling points, do you run the risk of all the nitrogen boiling off first, leaving behind a very flammable, very dangerous pure liquid Oxygen? Assuming you were to store the liquid air in a dewar tank, approximately how long would it take for this to become a concern, if at all?

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    $\begingroup$ Effective ventilation reduces the risk. Why would liquid air be better than that? $\endgroup$
    – matt_black
    Feb 23 at 11:15
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    $\begingroup$ Since they will boil off at different temperatures, no, it just causes other problems. $\endgroup$
    – Jon Custer
    Feb 23 at 14:28
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    $\begingroup$ Oxygen is flammable only in reducing atmosphere, like H2, CH4 or NH3. $\endgroup$
    – Poutnik
    Feb 23 at 14:32
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    $\begingroup$ Well, if you are running pure oxygen in non-rated plumbing it will (un)happily make some exciting flames just fine… $\endgroup$
    – Jon Custer
    Feb 23 at 15:26
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    $\begingroup$ @JonCuster, what are these other problems? Really I'm trying to understand whether you could safely have a dewar of liquid air in a confined space without suffocating anyone. $\endgroup$
    – 19172281
    Feb 23 at 18:10

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If we agree that asphyxiation is lack of providing cells of the the human organism with oxygen, liquid air is not safe because of its low temperature. Once the isolating buffer of air (Leidenfrost effect) between the liquid air and the gaseous air is off of the alveoli, the tissue will be frozen (and become brittle)* and blood behind these membranes will freeze, too. Don't forget blood contains much of water, which expands above and below about $\pu{4 ^\circ{}C}$, too.

At atmospheric pressure, the boiling temperature of nitrogen is about $ \pu{-196 ^\circ{}C}$ (ref), the of oxygen about $\pu{-183 ^\circ{}C}$ (ref). While you argue about liquid air, it actually is a known issue as potential danger when handling liquid nitrogen as a cryogenic. The very first example which comes to my mind are the Dewar traps installed to protect oil pumps filled with LN instead of dry ice and iPrOH. The ones filled with LN are cool enough to condense oxygen available in the ambient air. Because condensed oxygen supports combustion so much better than gaseous oxygen diluted in air (which is about 78% nitrogen, and 21% oxygen), you just don't mindlessly refill these traps with LN again and again. It isn't safe to await for the blue color of liquid oxygen as an indicator of a risk (and compared to liquid ozone one may trap behind an ozonolysis, its blue is much fainter), nor to use liquid air as a cryogenic.

Running a Linde machine to harvest nitrogen actually requires a number of steps to filter, wash, dry air to remove e.g., dust, pollen, humidity, $\ce{CO2}$, $\ce{N2O}$ before air is liquefied; and uses a distillation column to remove oxygen trapped by the process. Hence you see these machines more likely in locales with both reliable and affordable supply of electricity (for the compressors) and water (for the cooling) to provide you with purified (colorless) LN from the pressurized truck / local tank for your lab can.

* Some lectures display shatter e.g., a flexible rubber tube just cooled in liquid nitrogen with a hammer. More seriously though, cryogenic grinding is technique to eventually amend (heat sensitive) samples of polymers and organic materials to an analysis to lower the rates of chemical reactions e.g., if cells are broken up, or because deep frozen samples tend to be less sticky to the grinder.

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