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What is the actual composition of Liquid Nitrogen fumes? Are the harmful anyways if directly came in contact with?

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fumes from liquid nitrogen contain nitrogen $N2$ in gaseous state with some amounts of $N2O5$ which are not that harmful as the liquid nitrogen itself, anyway if you have inhaled those vapours and you are suffocating now then better visit a doctor, but its contact with keratinised epidermis would'nt be that harmful –  agha rehan abbas Aug 8 at 16:12
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@agharehanabbas N2O5 in the fumes? Seriously? By what mechanism can this possibly be formed in any notable quantity at this temperature? –  matt_black Aug 8 at 22:57
    
Ignoring the question how it's formed, how does it end up in the fume? The stuff boils at 320K, it's frozen solid at 77K. –  MSalters Aug 11 at 16:53

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The visible fog that forms when liquid nitrogen is poured into an open container is almost entirely water fog: the boiling nitrogen chills the air above it, causing the humidity in the air to condense into fog.

There may be some microdroplets of liquid nitrogen in the fog too, but the air, even after it's been chilled, is still much warmer than the boiling point of nitrogen, so there won't be much. (You can condense air by running a continuous stream of liquid nitrogen through a heat exchanger and blowing air over it, but mostly you get liquid oxygen when you do that.)

Liquid nitrogen has a relatively small heat of vaporization, as these things go: 199.2 kJ/kg according to Air Liquide. (Compare 572.2 kJ/kg for dry ice.) That plus the Leidenfrost effect means it's actually rather difficult to freeze yourself with the stuff. There's a standard demo where you pour liquid nitrogen into an audience member's cupped hands. The Leidenfrost effect prevents it ever coming in contact with their skin, and it only leaves a mild chill behind. Try this with dry ice and your "volunteer" will need emergency treatment for frostbite.

Gaseous nitrogen comprises something like 77% of ordinary air by volume; it's harmless in and of itself. However, as was mentioned in an earlier answer (which seems to have been deleted), if you let liquid nitrogen evaporate in an enclosed space, it can "displace" the oxygen that makes up the other 22% of the air, at which point anyone in the area may asphyxiate. The sensation of suffocation is triggered by having too much CO2 in your bloodstream, not by inadequate oxygen supply, so you might not even notice that anything was wrong until you collapsed. That's the second most dangerous thing about evaporating liquid nitrogen.

The most dangerous thing is, if you let liquid nitrogen evaporate in a sealed container, it's liable to explode on you.

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The fog / vapor / fumes that come off a container of any liquefied gas will be a combination of condensed water vapor and whatever it is that is boiling. It is no more or less dangerous than the gas itself. In the case of nitrogen, oxygen, helium, neon, argon, sulfur hexaflouride, methane etc. you can put your nose right up to the liquid's surface and take a really deep breath, no harm will come to you unless you are stupid enough to breathe it for an extended period. Oxygen, of course, is the exception but too much O2 is bad for you as well. Don't try it with chlorine.

Regarding the Leidenfrost effect and pouring liquid N2 into someone's cupped hand, lets be sure the palm is facing down, not up. Leidenfrost only works for a couple of seconds on room-temperature objects like people.

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In my experience, pouring liquid N2 onto the back of someone's hand is much more likely to give them (mild) frostbite than the reverse, because people tend to have hair on the backs of their hands, which will disrupt the Leidenfrost layer and/or draw the liquid to the skin by capillary action. As for the effect only working a couple seconds, I'm not sure that's true in this case; even a substantial quantity of lN2 will have serious trouble chilling your hands below the Leidenfrost point for lN2 (I don't have that number to hand but I'm guessing it's order of 100 Kelvin). –  Zack Aug 9 at 12:43
    
It's your hand. Would you prefer I pour liquid N2 on the back, where it can spill off, or into your cupped palm, where it will sit and boil? And it doesn't have to chill your hand enough to stop the LE, just enough to kill some tissue. –  david Aug 10 at 1:34
    
I've done this, both ways, on multiple occasions. I predictably get frost rash on the back of my hand, and have never had any problems with the palms. I think you're overestimating the amount of cold in lN2 -- let's assume all of the heat required to vaporize 10 cc of it is drawn directly from someone's hands (which is unlikely), we have 10 cm^3 * 0.8 g/cm^3 (density of lN2) * 199.2 kJ/kg = 1.5 kJ. It takes maybe ten seconds to vaporize under these conditions, so that's 0.15 kW heat draw for ten seconds. (cont'd) –  Zack Aug 10 at 14:09
    
Your average human at rest generates 100 W of heat output over their entire body surface, so yes, if all the heat required to vaporize that much lN2 were drawn directly from a small patch of skin you would be injured. But this is not what happens; most of the heat required is drawn from the air -- room temperature is 300K, lN2 boils at 77K; even with the low heat capacity and density of dry air (~1kJ/kg.K, 1.225 kg/m^3 -> 1.225 mJ/cm^3.K) that is enough to vaporize all of the lN2 while only cooling the air a few degrees. –  Zack Aug 10 at 14:28
    
(The actual boiling process is convective and explosive. If you modeled it in detail you might find that the Leidenfrost effect doesn't even come into play, the glob of lN2 is instead suspended on a convective updraft for the second or so that it takes to fragment into hundreds of smaller droplets which are flung all over the surrounding area.) –  Zack Aug 10 at 14:31

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