Liquid nitrogen has been part of my scientific life since I first entered a lab. See, there's two types of cells you can grow (in petri dishes, flasks, 6-96 well plates, or, in the case of one microbiology undergrad, on the flesh of your back). Primary cells have been taken directly from the source, be it a human or an animal,via biopsy or just by cutting it open and taking out an organ or two. You 'split' (ie. take a smaller amount of cells from a container that's full of 'em and transport to a new, fresh container) primary cells about 20 times at most. After that, you can't be sure they're the same cells you started with - primary cells have a bad habit of changing.
The second type are the immortalised cells. The first of these was the HeLa cells, and they were originally the cervical cancer cells of a woman called Henrietta Lacks (hence, the HeLa), who died from the disease. She was also royally screwed over by the doctor who propogated these cells from her biopsy without her permission or knowledge and, eventually commercialised those 'progeny'. You're not allowed to do that today (says the woman who had to write a 25 page ethics application to ask permission to use people's blood cells which we'd then dispose of, not alter and make shitloads of money off). Immortialised cell-lines don't change. This makes them awesome. What little comfort Henrietta's family can take from the entire shitty mess is that HeLa cells have been instrumental in everything from development of the Sacks vaccine for Polio to understanding of telemeres and their role in cancer. Next time you're having a drink, raise your glass to Henrietta Lacks.
Regardless of what type of cell you're using, you need a stock of them. Here's where liquid nitrogen comes in. Take one large container, fill it with racks containing boxes of your cells samples. Add liquid nitrogen and there you have it, long term cell storage. The temperature of liquid nitrogen is about -200C, AKA, Really Fucking Cold. This stops the cells from doing, well, anything. It's like those cold mornings when you don't want to get out of bed, multiplied by a million. Here's a demonstration picture of a random scientist and their cells:
The Occupation Health and Safety demon would choke if they saw this picture. Sure, she's got the glasses and the gloves, but look at her feet! Sandals! While fucking around with a vat full of LN2! She either hates her toes, or her job, and she wants rid of one/both.
I'm ranting because one of my jobs is to keep the level of LN2 high enough to keep those cells nice and frozen. This means I have been trained in using it, because LN2 will kill you if you don't respect it.
Obviously, being really fucking cold, spilling is an issue. Whenever I take the (smaller, 10L) dewer down to the basement to fill from the (enormous, 120L) tank, I have to wear:
a) gloves that give me all the dexterity of a Disney employee in a mickey mouse costume AND
b) a big fuck-off face mask. Because nobody likes their face frozen away.
c) Lab coat and (take note, Pink T-shirt) covered shoes. Actually, I have to do this no matter what I'm doing in the lab. Some countries have different rules, of course, but personally, I'd like as much between me and the -200C splashy stuff as possible.
But there's another feature of liquid nitrogen that isn't so well known. At room temperature, it evaporates. When it evaporates, it expands to about 7 times what it is in liquid form, shoving the oxygen and carbon dioxide away. This has a bad effect on humans, because our lungs are set up according to the usual ratio of oxygen:carbon dioxide: nitrogen, and if it's not right, you're not conscious. What makes it worse is that nitrogen is heavier than oxygen and carbon dioxide and when people pass out, they tend to fall downwards. Where it's mostly nitrogen.
This can be fatal. That's not me being drunkenly snarky, that's a fact.
So, here's your take-home message. Remember that scene from Terminator 2? This one?
(well, okay, the scene about two minute before when the tanker crashes, but I couldn't find a picture of that)
Sarah and John would have asphyxiated long before they got to blast the hell out of the T1000. Which, given how hard they'd been fighting the bastard, would have been very anti-climactic.
I have a lot of respect for liquid nitrogen.
Bonus helpful tip - if you ever find yourself filling a dewar with liquid nitrogen, they're not transparent, so it's hard to know how full it is. Fortunately, the super-cold LN2 creates a vapour in the air, and how that vapour is behaving lets you know how full the container is.
When you first turn it on, there'll be a great jet of vapour, which will probably try to envelop your face. This is because the bottom of the dewar is empty, and the hose completely exposed. When the exit point of the hose is covered, the vapour will settle down, 'bubbling' over the edge of the dewar. When the level of LN2 gets closer to the top of the dewar, the smooth bubbling will become much more irregular, with puffs of vapour shooting off in every direction. This is the point where you should prepare to turn the hose off, because very soon, you'll find more than just vapour bubbling over the edge.
There you go, just one of the things I work with. Which is why MY income protection insurance was 3 times that of the ITGeek and the same amount as Life, Death, Disability and 'big health problems' insurance for the both of us (guess what we went for in the end?)