Hazards of liquid nitrogen

Explosive release of trapped gas

iquid nitrogen is not just a frostbite hazard: sample vials stored in the liquid phase of liquid nitrogen can cause serious injury if improperly sealed. Although these vials are made of a tough plastic that prevents cracking at the liquid nitrogen temperatures of −196°C (−321°F), the vials can easily explode if liquid nitrogen gets into the vial during storage. Although this is not an explosion in the usual sense of a rapid chemical reaction, the rapidly expanding gas can be just as dangerous.

A simple calculation shows that 0.5 g of liquid nitrogen in a 1.5 ml vial will generate a pressure of 4,053 psi when it evaporates. Failure of the screw threads can turn the cap into a projectile with an initial velocity of up to 296 miles per hour (132 meters/sec), and up to 8.4% as much kinetic energy as a 22-caliber bullet!

Liquid nitrogen can get into a vial in several ways:

• If the vial is defective.
• If the vial is used after its expiration date. Nunc tubes must be used within three years of their date of sterilization because the silicone gasket deteriorates over time.
• If the vial is under- or overtightened, or if there is water on the threads.
• If the wrong type of vial is used. Samples for liquid nitrogen should only be stored in Nunc CryoTubes.

Precautions:

• Wear safety goggles at all times while handling liquid nitrogen.
• Never place liquid nitrogen or dry ice in a sealed container or any object that could cause entrapment of the gas.
• Never mix dry ice or liquid nitrogen with water or water ice or pour it down the sink. Ice can solidify around it, trapping the gas at a high pressure.

Vials to be stored in contact with liquid phase nitrogen must always be sealed with Nunc CryoFlex, which is a type of heat-shrinkable tubing.

Frostbite Hazard

If a sufficient quantity of liquid nitrogen comes in contact with the body, a "cold burn" results. Small amounts will rapidly evaporate and will only provide a small sensation similar to a pin prick. The danger comes from larger quantities which do not evaporate quickly. Should a larger quantity come in contact with a person, the person should immediately take action to get away.

• Wear protective clothing to keep smaller amounts from contacting the body.
• If a large spill occurs, discontinue filling and leave the room until the liquid evaporates.
• If clothing becomes soaked, hold it away from the body until it warms, or if a larger area is soaked, remove the clothing.
• If a cold burn occurs, once warmed up, it will appear very similar to, and should be treated the same as, a sunburn of comparable magnitude. Warm affected skin slowly using cold (NOT HOT) water.
• Any serious cold burn should be treated by a doctor.

Care should be taken when using liquid nitrogen not to spill the liquid on clothing, since this can easily become saturated with the liquid and then hold it next to the skin for a significant period of time, leading to serious burns. For this reason, cloth gloves may be worse than nothing at all.

Asphyxiation

Liquid nitrogen rapidly evaporates giving nitrogen gas. Just one liter of liquid produces around 700 liters of gas at atmospheric pressure, displacing significant quantities of breathable air if the gas is released in a confined space such as a laboratory, cold room, or storage area. The problem is compounded by nitrogen's tendency to accumulate at low levels where it is less easily dispersed than the ambient atmosphere. Even an apparently small spillage could lead to dangerously low oxygen levels, presenting a serious hazard to personnel working in the area.

The human body does not detect oxygen deficiencies very well. The feeling of being suffocated comes from excess carbon dioxide, not from a lack of oxygen, so symptoms may not be evident. The normal oxygen level in the atmosphere is 20.9%. Physical and intellectual performance may be inhibited if levels fall below 17%, and at levels just a few per cent lower symptoms of asphyxia, such as gasping, vomiting or collapse, will set in. Victims may well not be aware of their condition, especially if the oxygen level falls rapidly, and in the event of a major leak or spillage could fall unconscious without ever being aware of the danger. When the oxygen content of air is reduced to around 10%, unconsciousness can be immediate with virtually no warning.

Under normal circumstances, lab ventilation is usually sufficient to remove this hazard. However, dry ice and liquid nitrogen have caused fatalities after being spilled in closed areas such as a cold room. A microbiologist died of suffocation at CSIRO in Australia in December 2001 because of a liquid nitrogen leak in a cold room. If there should be a large spill anywhere in the building, don't wait around making "Hasta la vista" jokes - notify everyone that they need to evacuate the area. As Arnold Schwarzenegger would say, "Get Aooooout!"

Note: The cloudy vapor that appears when liquid nitrogen is exposed to the air is condensed water vapor, not the gas itself. The issuing gas is invisible.

Liquid nitrogen should never be carried in an elevator, because of the remote possibility that the Dewar containing the liquid could get broken or go "soft" (i.e., the vacuum in the Dewar flask will be lost). You would have unpleasant scenario of an elevator packed with people, unable to breathe. The liquid nitrogen freezer - and all nitrogen cylinders - are meant to be kept in a well-ventilated area. Don't move them into a lab unless an oxygen alarm system is installed in the lab. If your boss tells you to do it anyway---quit!

Also, never put liquid nitrogen, dry ice, or any other hazardous liquid in styrofoam coffee cups. Although the styrofoam will keep the liquid from evaporating, the risk is too great that someone will walk by and mistake it for Ginger Ale and try to take a sip. Acetone-dry ice mixtures look exactly like Seven-Up and ice. However, the taste is quite different indeed.

This brings me to my next pet peeve: eating in the lab. People might call you "anal" if you stop them from eating or drinking in the lab. But that's nothing compared to what their next of kin will call you if you don't.

Swallowing liquid nitrogen

Yes, there are people who do tricks with liquid nitrogen. One low-temperature physicist makes a toast with liquid nitrogen. It is dangerous - don't do it.

The danger of toasting with liquid nitrogen is the possibility that you might swallow a bit by accident. If this were to happen, then the valve at the top of your stomach might close, trapping some liquid nitrogen inside you. Then some very bad things would happen:

1. The liquid nitrogen would be held in contact with your tissues for long enough to cause damage, possibly freezing your epiglottis or destroying the valve at the top of your stomach or some other part of your mouth, throat or stomach.
2. Second, the rapidly boiling liquid would generate a huge amount of vapor inside you. You might not explode, but it would be extremely unpleasant.

First aid for liquid nitrogen exposure

If a person seems to become dizzy or loses consciousness while working with liquid nitrogen, move to a well-ventilated area immediately. If breathing has stopped, apply artificial respiration. If breathing is difficult, give oxygen. Call a physician. Keep warm and at rest. Keep the patient warm too.

If exposed to liquid or cold gas, restore tissue to normal body temperature 98.6° F (37° C) as rapidly as possible, followed by protection of the injured tissue from further damage and infection. Remove or loosen clothing that may constrict blood circulation to the frozen area. Call a physician. Rapid warming of the affected part is best achieved by using water at 108° F (42° C). Under no circumstances should the water be over 112° F (44° C), nor should the frozen part be rubbed either before or after rewarming. The patient should neither smoke nor drink alcohol.

Hasta la vista baby! And please, no more Ted Williams jokes.