Under Pressure

Cryogenic materials are used in a wide variety of laboratories. They are common in chemistry, healthcare, and physics labs, and are becoming more prevalent as stem cell, nanotechnology, computer chip design, and cryotube research advance.

Cryogenic fluids are liquefied gases that must be kept at extremely low temperatures. Boiling points for these gases are usually below -150°C (-238°F). For this article, we will consider materials with boiling points below -75°C as cryogenic (so as to include dry ice).¹ Due to its physical properties, a very small amount of the liquid can expand into a very large volume of gas.

The most common cryogens include helium, hydrogen, nitrogen, and oxygen. You will encounter them on a daily basis if you receive shipments packed with dry ice, preserve samples with liquid nitrogen, remove impurities with cold traps or baths, or keep equipment cooled. All these tasks can be done safely if we recognize the hazards and endeavor to control them. Should we become cavalier, things can go very wrong. Here are a few examples:

A laboratory assistant died and four other people were injured from a chemical leak at a hospital in Edinburgh, Scotland. The assistant died after liquid nitrogen spilled in a basement storage room.²
A university investigator was blinded in one eye when a cryotube exploded while being thawed. The probable cause of the explosion was the rapid expansion of liquid nitrogen that had entered the tube through a small crack during storage.³
A researcher at a university reported that a vial of potentially infectious materials “exploded” when she removed it from liquid nitrogen. The “explosion” occurred when the liquid nitrogen leaked into a vial and expanded when removed from the cold.³

Generally, cryogens present similar hazards as compressed gases. The primary hazards fall into two categories: physiological and physical.

Physiological Hazards

Physiological hazards produce bodily harm and are divided into two main categories—those that damage tissue from direct contact and those that can cause asphyxiation.

Direct contact

Cryogenic materials flow freely just as other liquids do and, as a result, may splash and spill. The effect produced from accidental splashes or contact with extremely cold cryogens is similar to liquid nitrogen used by your dermatologist. Direct contact with liquid nitrogen or other cryogens freezes and kills tissue rapidly.

Asphyxiation

Cryogenic liquids contain a tremendous amount of potential gas volume. One-unit volume of liquid nitrogen, for example, will expand to produce almost 700 times the volume of gas when vaporized. This rapid and extreme expansion can lead to oxygen displacement. A leak or vessel breakage can result in an oxygen-deficient atmosphere rather quickly, especially in small areas with poor ventilation.

Physical Hazards

Physical hazards include explosion risks from pressure buildup and those from chemical reactions or combustion.

Explosion—pressure

As mentioned above, the gas volume generated from the vaporization of the liquid phase is very large. If this phase change occurs in a vessel unable to contain the pressure exerted, it can fail catastrophically from over-pressurization.

Related Article: Safe Handling of Liquid Nitrogen

Explosion—chemical

Cryogenic fluids with a boiling point below that of liquid oxygen, such as nitrogen, are able to condense and accumulate oxygen from the atmosphere. Violent reactions—for example, rapid combustion or explosion— may occur if incompatible materials, such as most common organic compounds, come in contact with the oxygen.

Accident Prevention

Anyone who handles or uses cryogenic liquids must have adequate knowledge of the particular material’s properties and safe handling practices.⁴ Specific understanding acquired through proper training should include:

Properties of the cryogen as a liquid, solid, or gas
Materials compatible for use with that cryogen (e.g., compatible with the temperatures and pressures of the material)
Protective equipment required and its proper use
Understanding of the equipment being used, including its safety devices
Emergency procedures, including first aid and treatment

Ensure all employees handling and using cryogens read and understand the safety data sheet. In addition, develop and follow standard operating procedures (SOPs) whenever handling or using cryogenic materials. To jumpstart your training and SOPs, here are some quick tips:

Remove metal jewelry/watches on your hands and wrists before working with cryogens. If exposed to cryogenic liquids or boil-off gases, the jewelry can freeze to your skin.
Protect your eyes by wearing safety goggles whenever working with cryogenic fluids or samples recently removed from cryogenic temperatures. Full face shields should be used in the following situations: a) when a cryogen is poured; b) for open transfers; c) if fluid in an open container is likely to bubble.
Wear a cryogen apron when working with cryogenic liquids.
Try to cover all exposed skin by wearing long-sleeved shirts, long pants/skirts, a long-sleeved lab coat, well-fitted leather shoes (no sneakers), and gloves. Gloves should be loosefitting, lightweight, flexible, and insulated so that they can be quickly removed if cryogenic fluids are spilled on them.
Do not transfer liquefied gases from one tank to another the first time without supervision.
Use care when filling portable dewars and do not overfill them.
Transfer or pour cryogens slowly in order to minimize boiling and splashing. To reduce the amount of splatter when transferring cryogenic liquids from one container to another, always start slowly, allowing the vaporization to chill the receiving container before filling it. Use a phase separator or special filling funnel (the top of the funnel should be partly covered to reduce splashing). If the liquid cannot be poured, use a cryogenic liquid withdrawal device for the transfer (be sure to follow all instructions provided with the device).
Use tubes specifically designed for cryogenic storage and place them in a heavy-walled container or behind a safety shield while thawing.
When hand-carrying a cryogen-containing dewar, ensure that it is your only load (no books, coffee, or other items). Watch carefully for people who may run into you, and ensure that the dewar is carried with both hands and as far away from your face as comfortably possible.
Ensure dewars are properly labeled with the identity of the cryogen. Do not mix different cryogens.
To avoid asphyxiation, an oxygen monitor in good working order is recommended if you are working with a cryogen in a confined space.
Do not permit smoking or open flame in any area where oxygen or hydrogen is stored, handled, or used.
Do not lubricate oxygen equipment with oil, grease, or unapproved lubricants.

References:

1. Prudent Practice in the Laboratory: Handling and Disposal of Chemicals, National Academies Press, National Resource Council, Washington, DC. 1995. http://www.nap.edu/catalog/4911/prudent-practices-in-the-laboratory-handling-and-disposal-of-chemicals

2. “Inquiry after man dies in chemical leak,” BBC Online News, Monday, October 25, 1999. http://news.bbc.co.uk/1/hi/scotland/484813.stm

3. “Lab Safety Cryogens Incidents,” American Industrial Hygiene Association, Laboratory Health and Safety Committee. 2015. https://www.aiha.org/get-involved/VolunteerGroups/LabHSCommittee/Incident%20Pages/Lab-Safety-Cryogens-Incidents.aspx

4. “How Do I Work Safely with Cryogenic Liquids?,” Canadian Center for Occupational Health and Safety, Hamilton, ON, Canada. 2015. http://www.ccohs.ca/oshanswers/prevention/cryogens.html

Download the Infographic: Cryogen and Cold Storage Safety

Under Pressure

Cryogenic safety hazards and what it takes to control them