# Dangers of using liquid nitrogen

So, I remember in college physics the prof using liquid nitrogen in a demonstration. When he was done, he threw the container of LN at the front row, and of course it evaporated (or whatever) before it got to the students.

I am watching a cooking show now and they are using LN -- if they touched that, what would happen? Would it be possible for the chefs to pour the LN onto their skin accidentally, or would it evaporate before it reached their skin?

-
Voted to close: this should belong in a medical forum –  Sklivvz Jan 8 '11 at 15:11
This is a bit off-topic, but I'll allow it for now. –  Noldorin Jan 8 '11 at 15:12
@Sklivvz: the questions is mainly about the physical properties of cool liquids. –  Marek Jan 8 '11 at 17:54
The question Would it be (...) skin? is 100% on-topic to me. The other question about touching it is 70% off-topic. –  Malabarba Jan 8 '11 at 17:58
@Marek: look, @mbq's answer is a bit about physics even if half of if is hearsay; @dmckee is about the medical consequences; @Omega_centauri's is about his personal experiences with the substance. So there's barely any physics in both the OP and the answers... In any case that's why there is a voting system. I can't close answers, but I can vote my opinion :-) –  Sklivvz Jan 8 '11 at 20:28

Liquid nitrogen boils when it comes in contact with skin, so small amounts of spatter are no danger at all-- the droplets just bounce off. I regularly pour a liter or so (a bit at a time) out on a lab table when I do liquid nitrogen demos, with no problems or safety gear.

The biggest risk from the low temperature is getting it into fabric of some sort, which will hold it in closer proximity to skin for longer than the drops by themselves will. I have, on occasion spilled some on my pants, which is annoyingly cold, but not too bad unless you're wearing really tight clothing.

Really, the biggest hazard from any of the nitrogen demos I do is not the temperature but the expansion. When it boils, it expands to something like 700 times the volume of the liquid, so if you put some in a sealed container, it can make a big bang. I know of a case where a grad student at MIT destroyed a bathroom with a 2-liter bottle of liquid nitrogen.

You can use this expansion for a kind of cool demonstration if you take one of those little dropper bottles with the angled spouts that are common in chem labs, and seal a little nitrogen inside. Put it down on the floor, and it spins like a firework. The problem with that is, you almost always end up getting a little water vapor condensing in the spout, which plugs it up, and then the bottle will go bang. I had a student a few years ago who had one go off in his hand, and he said it stung pretty badly.

I have seen videos of people (Jearl Walker in particular) "drinking" liquid nitrogen by taking a small amount into their mouth, and holding it there for a second or so-- the instant boiling will keep it from giving you oral frostbite for a little while, and you can spit the liquid out after breathing out over it, which makes an enormous plume of steam. It's really cool to see, but kind of risky to do. I've never had the guts to try it myself.

-
Good for picking up the expansion hazard. In addition to explossion risks, large quantities of cryogenics in confined spaces have the potential to displace enough air to create regions of dangerously low $O_2$ partial pressures---called Oxygen Deficiency (or Displacement) Hazard (ODH). Facilities where the preconditions pertain are usually instrumented for $O_2$ levels and equipped with alarms and flashing beacons. –  dmckee Jan 8 '11 at 21:56
You can also get nasty cold burns by touching metal pipes that have had LN2 in them. –  Martin Beckett Aug 13 '11 at 4:01

Liquid nitrogen will not moisten human skin, so short contact with small amount of it should not be too harmful -- it would just float on a evaporated portion of itself like water over very hot pan; yet of course putting a hand into a container with it is not a good idea.

From what I have heard, the biggest problem is when you pour it on your shoes, because it moistures them creating a direct contact with skin and extending its length -- also it takes a significant amount of time to take shoes off.

-
Actually it is possible to immerse the hand very briefly in a container of LN. The boil off creates a layer of thermally isolating gas which keeps the full effects of the cooling at bay for very short amounts of time. –  Brendan Jan 12 '11 at 17:46

You can get localized soft tissue damage rather like a burn from sustained contact with moderate amounts of cryogenic liquids, and large amounts can freeze flesh solid---which is really bad.

Small amounts will dance on your skin because of the vapor barrier that develops as they vaporize.

Treat cryogenic materials with respect. Think about what you're doing around them.

-

Not recommended, however I recall doing just that as a student, it boils on contact with your skin, you barely feel it. If it hits a less conductive part, such as a callous, it might not boil but instead freeze it. I seem to remember things like zippers also getting quite cold. btw, it is threw not through.

-