Liquid nitrogen N2 (l) is a clear fluid with a very low viscosity, it is extremely cold (-196° C) and therefore it belongs to the Cryogenic liquids. Liquid nitrogen is odourless, it is not flammable and it is inactive (does not react chemically).
The most common dangers while handling N2 (l) are:
Frost damages, especially eye injuries
Serious frost damages can occur if the N2 (l) get in contact with the skin. If bare skin freezes onto surfaces that are cooled down with N2 (l), it will cause severe damage. Sustained cooling of the skin can lead to frostbites. Inhalation of the cold gas can be very harmful to the lungs. Especially the eyes are sensitive, so even small splashes or aerosols of N2 (l) can cause immediate freezing of eye tissue with permanent damages as a result.
Suffocation
N2 (l) is not dangerous in itself, but its presence can cause suffocation, because the liquid converts into gas and the volume increases drastically.
1 Liter of N2 (l) converts into 700 L of gas, which will reduce the amount of oxygen in the room.
Normal air contains 20.9 % oxygen and it is prohibited to stay in a room with less than 19.5 % oxygen. If the oxygen content decreases further, this will cause dizziness, nausea, vomit, fainting and death. Be aware! No warning is given in advance, i.e.´feelings of suffocation´.
If the content of a 10 L Dewar (container) evaporates in a room sized 4 m x 3.6 x 2.5 m = 36 m3 , the concentration of oxygen will drop to 16.9%!
Eksplosion
Since liquid nitrogen expands dramatically when it evaporates, incorrect storage can cause vigorous explosions. Large containers for N2 (l) are therefore equipped with overpressure valves regulating the pressure in the container. Smaller containers are provided with loose lids.
Ingestion of N2 (l), intentionally or by mistake, will almost certainly make the stomach explode.
Due to the low viscosity of N2 (l) it will penetrate small holes and cracks. If you pour N2 (l) into a normal kitchen thermos, the N2 (l) will most likely diffuse into the cavity between the glass container and the rubber sealing. When the N2 (l) converts to gas, the pressure will rise and the thermos can explode.
If N2 (l) is stored in open containers, the vapour condenses the moisture in the air so the nitrogen will obtain a content of ice crystals. Even more serious is, that oxygen from the air will condense onto surfaces with low temperature because of the N2 (l), so a reservoir of liquid oxygen builds up. This oxygen can react violently with certain organic chemicals leading to a fire or explosion.
The abovementioned is a translation of the official (Department of Physics and Astronomy, Aarhus) safety instruction: phys.medarbejdere.au.dk/fileadmin/site_files/arbejdsmiljoe/Sikkerhed_LN2.pdf