Use of liquid nitrogen (LN2) has grown rapidly the last several years, due in part to the growth in stem cell research and other medical uses. Unfortunately, so have the number of serious accidents and injuries. These incidents have occurred not only during use, but also during transport and storage.
A wide variety of laboratories use LN2. Most common are chemistry and physics labs, nanotechnology, computer chip design, and production and cryotube research. There is also growing use in the healthcare arena in the rising need for dermatology and skin cancer treatment and as research and development with stem cells advances.
LN2 is classed as a cryogenic fluid, being a liquefied gas that must be kept at extremely low temperature. Boiling points for cryogens are defined as below -150°C (-238°F). LN2 boils at -196°C (-320°F). This fact presents the primary hazard: contact will produce severe burns and more serious injuries due to instant freezing.
As most of you probably know, nitrogen is also an inert gas. Due to LN2’s physical properties, a very small amount of the liquid can expand into very large volumes of gas. This presents the second most important hazard: displacement of oxygen and possibility of asphyxiation.
In addition to the physiological hazards above, physical hazards abound with LN2. Handling and using LN2 can be done safely, if we recognize the hazards and strive to control them. If we become cavalier and things go wrong, serious injury and/or death is usually the end result. Here is one recent example:
- A 50-year-old female employee at a Canadian sperm bank died of asphyxiation after attempting to correct an LN2 leak.
Hazard recognition: Physiological
As mentioned above, the hazards fall into two categories: physiological and physical. Physiological hazards produce bodily harm and fall into two main categories, those that damage tissue from direct contact and those that can cause asphyxiation.
LN2 flows freely as a liquid and, as a result, may splash and spill. Accidental splashes or contact with an extremely cold cryogen freezes and kills tissue instantaneously. Therefore, direct contact must be prevented at all costs.
Cryogenic liquids contain a tremendous amount of potential gas volume. For example, one unit volume of LN2 will expand to produce 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 very quickly, especially in small enclosed areas with poor ventilation. Due to LN2’s odorless and colorless properties, this situation is very hard to recognize.
Hazard recognition: Physical
Physical hazards from LN2 include explosion risks from pressure buildup. As mentioned above, the gas volume generated from the vaporization of the liquid phase is very large and happens rapidly. If this phase change occurs in a vessel unable to contain the pressures exerted, it can fail catastrophically from over-pressurization.
Control and Prevention
Anyone who handles or uses cryogenic liquids must have adequate knowledge of the particular material’s properties and the safe handling practices. Specific understanding acquired through proper training must include:
- Physical properties of LN2 as a liquid, solid, or a gas
- Materials compatible for use with LN2 (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 LN2 read and understand the safety data sheet. In addition, develop and follow standard operating procedures (SOPs) whenever handling or using LN2. To jump-start your training and SOPs, here are some quick tips:
- Remove metal jewelry and watches on your hands and wrists before working with LN2. If exposed to cryogenic liquids or boil-off gases, the jewelry can freeze to the skin.
- Protect your eyes by wearing safety goggles whenever working with LN2 or working with 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 LN2.
- Try to cover all exposed skin by wearing long-sleeve shirts, long pants (skirts), a long-sleeve lab coat, well-fitted leather shoes (no sneakers), and gloves. Gloves should be loose-fitting, lightweight, flexible, and insulated so that they can be quickly removed if LN2 is spilled on them.
- Use care when filling portable Dewars and do not overfill them.
- Transfer or pour slowly 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 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 and body as comfortably possible.
- Ensure Dewars are properly labeled. 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.