Cryogens and Dry Ice
Overview of Hazards
Cryogens are liquefied gases such as nitrogen, argon, or helium that pose significant hazards due to their low temperatures. They are colorless, odorless and extremely cold. Solid carbon dioxide, referred to as dry ice, has handling hazards due extreme cold (-78 ºC). The boiling points or sublimation points for cryogens are listed in the table below:
Note that this document does NOT cover hazards associated with using liquid oxygen or liquid hydrogen.
Due to the low temperatures all cryogens and dry ice pose the following hazards:
The extremely low temperatures cause tissues to freeze immediately upon contact, leading to severe burns and frostbite to skin and eyes. Contact with cryogenic vapor or a non-insulated vessel containing cryogen is sufficient to sustain such injuries.
The high rate of evaporation of cryogens displaces oxygen in the surrounding air and can reduce oxygen levels to the point where rapid suffocation can occur without warning. One volume of a cryogenic liquid will expand to about 700 equivalents of gas when evaporated. Oxygen deficiency sets in when the oxygen content drops below 19.5%. The decrease in oxygen accompanied by gradual asphyxia is usually NOT noticeable by the victim. One pound of dry ice will produce 250 L of carbon dioxide gas. Within 24 hours, 5-10 pounds of dry ice can sublimate. Concentrations of more than 0.5% (5000 ppm) carbon dioxide in the air can lead to unconsciousness.
Evaporation also leads to a rapid increase of pressure and possible vessel rupture if cryogens are stored in a container that is not adequately vented.
Oxygen Enrichment (Liquid Nitrogen and Helium Only)
Because the boiling points of liquid nitrogen and helium are lower than that of oxygen, oxygen can condense and liquefy on outer surfaces of non-insulated lines. Open liquid cryogen containers can condense oxygen from the air into the liquid and reach levels as high as 80% O2, potentially increasing the flammability of combustible materials.
Liquid nitrogen can also cause argon to condense and form liquid argon. This is most common in laboratories that use argon gas in equipment that can potentially be introduced to a vacuum cooled with liquid nitrogen (e.g., manifolds).
Protect yourself from skin contact by always wearing Personal Protective Equipment (PPE) when handling cryogens.
Wear cryogenic gloves that fit loosely so they can be removed quickly should a splash occur. Do NOT touch dry ice or non-insulated vessels containing cryogenic liquid without wearing cryogenic gloves as your skin will stick to the extremely cold material.
Do not wear rubber gloves, as they will harden instantly and you may not be able to remove them.
- Eye / Face Protection:
Splash goggles are a minimum when handling cryogenic liquids. A face shield is required when a splash is possible, e.g., when filling from a pressurized container into an open dewar.
Wear a lab coat, closed-toe shoes, and long pants without cuffs to prevent cryogenic liquid from being trapped next to the skin. Leather shoes are preferred, as leather will shed spilled liquid. Textile or canvas shoes absorb the liquid, bringing it closer to the skin. Such shoes will also be impossible to remove when frozen.
Pressurized cryogenic liquids are stored in cylinders equipped with pressure safety devices such as pressure relief valves and a rupture disc. Never tamper with or plug the safety devices. Be familiar with the functioning of the cylinder before using it. Low pressure cylinders are equipped with a liquid valve for extracting liquid and a vent valve. They operate at up to 22 psig. High pressure cylinders are equipped with a vent valve, liquid valve, gas valve, and pressure building valve. They can operate at up to 350 psig. Read the pressure gauge of a cylinder to make sure it is at the required pressure. The vent valve can be used to vent excess pressure into a well-ventilated area. Cylinders for helium may have additional valves for transportation. Refer to the manufacturer’s instructions on correct valve positions and how to extract liquid.
Make sure that ice buildup is not blocking the safety devices of the cylinder.
An occasional hissing sound from a cylinder indicates normal operation of the pressure relief valve as the liquid slowly evaporates.
Contact the manufacturer or vendor and have the cylinder checked in the case of:
- Ice build-up on the outside of the cylinder,
- Continuous venting through a pressure-relief valve,
- Any visible damage or malfunctioning of the cylinder.
Transferring cryogenic liquids
Attach only transfer lines with the correct fitting. Never attempt to change or remove any fittings of the cylinder, and do NOT use adaptors.
Any pipe or transfer line in which cryogenic liquid can become trapped must be equipped with a pressure relief valve.
When transferring cryogenic liquid into experimental equipment, the recommended procedure is to purge air out of the transfer line and the equipment by using dry gas of the cryogenic liquid. This is especially important for helium, as it is cold enough to solidify air.
Use an appropriate transfer line with phase separator to avoid splashing when filling into an open container. Do NOT use plastic or Teflon tubing as they will become brittle from the cold temperature.
Non-pressurized containers for holding cryogenic liquids are referred to as dewars. They are equipped with a loose-fitting cap or lid to prevent air from entering while allowing evolved gas to escape.
Fill cryogenic liquids only into manufacturer-approved containers that can withstand the extremely low temperatures. A regular thermos bottle is NOT an adequate vessel.
Small, open dewars used as transfer vessels usually come with loosely fitting stoppers. Use the stoppers to prevent oxygen condensation and ice buildup.
Fill dewars to no more than 80% of capacity; evaporation during warming can cause pressure buildup.
NEVER seal a vessel containing a cryogen, including dry ice.
When checking the liquid level in a container, do not use hollow rods as dipsticks. Use solid wood or metal.
Use tongs with insulated handles to withdraw objects immersed in cryogenic liquids.
When filling cryogenic liquids into equipment from an open dewar, avoid pouring over your head level, as a spill could cause serious injury. Use a step ladder and work from above with nobody below.
If cryogenic liquids have to be used or stored in a poorly ventilated area, or if large amounts could potentially be released and cause oxygen deficiency, an oxygen monitor should be installed.
Thawing out samples in cryotubes
Cryotubes stored under liquid nitrogen can explode while thawing. This is thought to be caused by liquid nitrogen entering the tubes through minute cracks and then expanding rapidly as the tube warms up. Place cryotubes in a thick-wall container, e.g., a desiccator, or behind a safety shield while thawing.
Wear cryogenic gloves and eye protection when transporting containers. Never tip, slide, or roll them on their side. Keep containers vertical at all times. Avoid mechanical or thermal shock. Always push containers on wheels. Pulling can cause injury if they tip and fall on you.
Use appropriate carts when moving heavy containers with no wheels, and always secure the container during transport.
Do not transport containers holding cryogens inside elevators at the same time as people. Put the container into the elevator and place a sign on it the cylinder facing the door “Do not enter elevator, asphyxiation hazard”. Send the elevator to the desired floor and have somebody there to pick it up. It is best to use service elevators with good ventilation (e.g., with screen walls or doors).
Respond immediately if there is an accidental condensation of oxygen or argon. Open the system to the atmosphere. Shut off the source of vacuum if it is present. Place a blast shield in front of the apparatus and allow the system to slowly warm to room temperature. It is important that this is an open system to minimize the risk of an explosion upon warming. The sudden boiling of the cryogenic liquid is a significant hazard. Liquid oxygen mixed with organics increases the explosion hazard significantly. Alert researchers in the area and inform DRS.
If skin or the eyes are exposed to cryogens or the cold vapor, use warm water (up to 108 ºF/42 ºC, NOT above 112 ºF/44 ºC) to restore normal body temperature. Do NOT rub the frozen skin. Seek medical attention. Remove or loosen clothing that may restrict blood flow to the frozen area.
In the case of a large spill or rupture of a container, evacuate the area while alerting others. Oxygen deficiency might make the area unsafe to enter. Call 911.
Store all containers with cryogens, pressurized or non-pressurized, in well ventilated areas. Dry ice can be placed in Styrofoam boxes or non-sealed, insulated coolers. Store all cryogenic liquids away from combustible material and do not leave containers uncovered for long periods of times. Use a loose fitting stopper or lid. Note that a cold room does NOT slow down the evaporation rate. Cold rooms are not well ventilated and can quickly become an asphyxiation hazard when cryogens are stored in them.
Allow excess cryogens to evaporate naturally in a well-ventilated place. NEVER pour cryogenic liquids down the drain, as the pipes are not made to withstand the low temperature.
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Stanford University: SLAC Environment, Safety, and Health Manual, Chapter 36: Cryogenic and Oxygen Deficiency Hazard Safety http://www-group.slac.stanford.edu/esh/eshmanual/pdfs/ESHch36.pdf (accessed Dec 2013).
Environment, Health and Safety Information for the Berkeley Campus #77: Cryogenic Liquids Precautions and Safe Handling Procedures. http://www.advancedlab.org/mediawiki/images/b/b4/77cryogenic.pdf (accessed Dec 2013).
Lawrence Berkeley National Laboratory: ES&H Manual, Chapter 29: Safe Handling of cryogenic liquids http://www.lbl.gov/ehs/pub3000/CH29/CH29.html (accessed Dec 2013).
Last Update: 3/24/2014