Cryogenic liquids, often referred to as cryogens or cryogenics, are liquefied gases that must be maintained at very low temperatures to remain a liquid. Cryogenic liquids, by definition, are very cold, with a boiling point below -150°C (- 238°F). At room temperature, these liquids boil off and become gases. They are odorless and colorless, therefore able to escape their containers unnoticed by those in their presence. Although liquid nitrogen, argon, and helium are the more common cryogens it is important to be aware of other liquid cryogens such as oxygen, methane, and hydrogen.
All use of cryogens must be part of your lab’s chemical hygiene plan, and a risk assessment must be performed to determine any and all necessary controls to perform the work safely.
By campus policy, the PI/Facility Manager or designee must contact EH&S prior to initial purchase, rental, or use of inert cryogenic liquids in excess of 25 liters. EH&S is responsible for assessing the facilities housing cryogenic liquids. To initiate a cryogen hazard analysis with EH&S, please fill out this form. EH&S will review the information and either approve the use or follow up to discuss further.
Request Cryogen Assessment
Contact
Office of Chemical Safety
chemsafety@wisc.edu
Phone: 608-265-5700
Hazards
The primary hazards associated with inert cryogenic liquids are their extreme cold temperatures, which can lead to cold burns or frostbite, and oxygen deficient environments. There is the potential for pressurization of containers and the embrittlement of materials. Other more reactive cryogens are also flammable or can greatly increase the flammability of other materials.
Oxygen Deficiency
The high expansion ratio of inert cryogenic liquids significantly increases the risk of creating oxygen-deficient atmospheres, which can lead to asphyxiation of individuals working in or entering affected areas. The gases released by these liquids are colorless, odorless, and tasteless, making leaks or exposures difficult to detect without appropriate monitoring systems. Without a thorough hazard analysis and the implementation of effective control measures, university students and staff may be at serious risk of an adverse event.
Hazard Evaluation Process
When evaluating cryogenic safety risks, consider the following factors:
- The amount of cryogen present
- How the cryogen is being used
- The size and layout of the laboratory
- The adequacy of room ventilation
- Potential failure modes, including worst-case scenarios that may result in hazardous conditions
Using these criteria, the likelihood of a failure leading to an Oxygen Deficiency Hazard will be assessed. If the laboratory/facility operations change or the quantity of cryogenic liquid in the space is increased, the space must be reassessed. You may also contact EH&S to request a reevaluation if cryogen use is decreased or discontinued.
To initiate a cryogen hazard analysis with EH&S, please fill out this form. EH&S will review the information and either approve the use or follow up to discuss further.
Low Temperatures
The extremely low temperatures of cryogenic liquids can cause severe frostbite and cryogenic burns on contact with skin. When filling tanks or dewars with cryogens you’ll want to cover your legs, feet, arms, hands and face to protect against splashes. In case of skin exposure to cryogens rinse the area in warm (not hot) water. Don’t rub the affected area because the skin may be fragile and susceptible to tearing. If your clothing is exposed, it may be necessary to remove it before it has a chance to react with your skin.
The cold temperatures are not only for the liquids themselves but also for anything that the cryogens have been in contact with – especially metals. Be careful when touching any surface that has been in contact with cryogenic liquids.
Embrittlement of materials
Another important hazard to consider is the effect of extremely low temperatures on the structural integrity of materials. Many have witnessed demonstrations in which everyday objects—such as bananas or rubber items—are submerged in liquid nitrogen and subsequently shattered due to brittleness. This illustrates how cryogenic temperatures can significantly alter material properties. Therefore, it is essential to carefully design any system that involves cryogenic liquids to ensure all components are compatible with such conditions. For instance, latex hoses are preferable to Tygon® tubing, as they are better suited to withstand the extreme cold of liquid nitrogen without becoming brittle or failing.
Pressurization of Containers
When treating or storing samples in liquid nitrogen, containers should never be completely sealed unless they are specifically engineered for cryogenic use. As liquid nitrogen warms and vaporizes, it generates a significant increase in internal pressure. If the container is not vented or pressure-rated, this buildup can cause it to crack, rupture, or even explode, posing a serious safety risk.
Oxidizing and Flammable Cryogenic Liquids
Oxygen
Liquid oxygen poses significant fire hazards. It can cause materials that are normally non-combustible to ignite and burn rapidly. Organic substances, in particular, may react violently when exposed to liquid oxygen. If a fire breaks out in an area where liquid oxygen is present, it is critical to immediately stop the flow by shutting off the supply valve. Additionally, any clothing that comes into contact with liquid oxygen should be removed right away and thoroughly aired out. Clothing saturated with oxygen becomes highly flammable and can burn with intense heat.
Liquid oxygen is not frequently used on campus, but there have been instances where oxygen has been accidentally condensed in liquid nitrogen cooled traps used to capture solvents and other contaminants in vacuum systems – a common example being the traps used with Schlenk lines in chemical syntheses. Nitrogen has a lower boiling point than oxygen so any leaks in a vacuum system can lead to oxygen in the atmosphere being collected along with other organic materials. As mentioned above, liquid oxygen can form an explosive mixture with organic substances. If you see a pale blue color in a trap, indicative of liquid oxygen, remove the liquid nitrogen dewar, vent the trap, close the sash of the hood, warn others of the danger, and leave the lab. Allow the trap to slowly warm and cautiously check every 30 minutes to see if the liquid oxygen has evaporated.
Flammable Cryogens
Besides the more common inert cryogens, there are some cryogenic liquids, such as hydrogen and methane that are flammable. Liquid hydrogen is a rocket fuel and is also used to cool neutrons for neutron scattering. It is easily ignited and is a serious fire hazard. It has a wide flammability range and can burn at as little as 4% of the air volume. The flame produced by its combustion is colorless and difficult to see. Additionally, since liquid hydrogen is colder than the boiling point of liquid oxygen, there is a potential for liquefying oxygen leading to a significant explosion hazard. Extreme precautions need to be taken when working with this chemical. It is prudent to limit the amount of liquid hydrogen used indoors. If a fire starts you must first turn off the gas flow, if it can be done without putting yourself at risk. If the hydrogen supply cannot be turned off it may be best to allow it to burn while keeping the fire contained. Attempting to smother the fire while the gas is still flowing could cause re-ignition or an explosion.
Methane is another gas that is sometimes used in cryogenic form. Like hydrogen, it poses a flammability risk; however, it does not condense oxygen.
Engineering Controls
Ventilation
The space where any cryogenic liquids are stored or used must be properly ventilated according to National and State standards while also considering any additional ventilation needs due to the amount of material in that space. Under no circumstances shall cryogenic liquids be stored in an unventilated room.
Emergency ventilation may also be required where oxygen depletion can occur rapidly. This type of ventilation system will immediately increase the air exchange within the facility when a release is detected.
Oxygen Monitors
Using the hazard evaluation process described above, EH&S will determine if a permanently installed oxygen monitor is necessary in a space using or storing inert cryogenic liquids. If installed, monitors will be set to alarm when the concentration of oxygen drops below 19.5%. The number of monitors and their placement will be determined by EH&S. Oxygen monitors are typically mounted in the breathing zone (4-6 ft off the ground).
The oxygen monitors must give both audible and visual alarm when oxygen levels drop below the alarm point. The alarm must be noticeable both from within the room, and from outside the room before entering. All occupants should exit the room when an alarm sounds.
When EH&S determines an oxygen monitor is necessary, EH&S will work with the Physical Plant and/or contractor to install equipment consistent with campus standards and to enter the monitors into the campus preventative maintenance program. The monitors in this program will be maintained in collaboration with EH&S and the Physical Plant. The lab (meaning the PI/Facility Manager or designee) is responsible for costs associated with the installation and maintenance of all oxygen monitors. The lab must also ensuring that the oxygen monitors operate properly by completing a monthly status form and by reporting any issues to EH&S and/or the Physical Plant. All monitors must be properly maintained according to the manufacturer’s requirements. If monitors are not part of the campus preventative maintenance program, the lab/facility is responsible for ensuring monitors are properly maintained according to the equipment manufacturer’s specification by a contracted third-party providing auditable records of the maintenance. Contact EH&S if you feel a monitor is no longer needed—EH&S will determine if decommissioning is appropriate.
Administrative Controls
Administrative controls and workplace-specific rules should be in place to address any hazards in the lab. Common administrative controls that may be necessary include, but aren’t limited to:
- Maximum quantity limits for room space.
- Written safe working procedures.
- Prohibition on working alone in a space with cryogens.
- Limited access to hazardous areas.
- Emergency response procedures.
The appropriate administrative controls will be determined during the hazard assessment.
Personal Protective Equipment (PPE)
Appropriate PPE must be worn when handling or dispensing cryogenic liquids. When handling cryogenic liquids it is typically necessary to wear eye protection, closed-toed shoes, long sleeved shirts, and long pants always. Face shields and thermal gloves should be worn whenever filling a dewar or transferring large amounts of cryogenic liquid. These items must be provided by the employer and available to anyone working with cryogens.
Whenever handling a flammable cryogen or liquid oxygen PPE should include a flammable resistant lab coat along with less flammable clothing under the lab coat, such as clothing made of cotton or wool.
Storage
Cryogens should be stored in containers specifically designed to house them. The containers should be insulated and double walled. Store all cryogenic liquid containers upright in well- ventilated areas. Handle them carefully, and avoid dropping, rolling or tipping them on their sides. Cryogen tanks and containers should not be stored near elevators, walkways and unprotected platform edges or in locations where heavy moving objects may strike or fall on them.
Cryogens should never be stored in cold rooms or in any room without ventilation.
Transporting Cryogenic Liquids
Cryogenic liquid containers should always be moved using a hand truck, cart, or another suitable transportation device. It is essential to secure containers during transport and keep them upright at all times to prevent spills and maintain safety. When transporting inert cryogenic liquids by elevator, it is important to carefully evaluate routes and procedures to ensure safe movement. In the event of a power failure, a passenger could become trapped in the confined elevator space with the cryogenic containers. As the liquids evaporate, oxygen displacement may occur, creating a hazardous environment. For this reason, best practice is to transport cryogens only in unoccupied elevators. Ideally, two people should be involved in the process: one to load the container into the elevator and another to receive it on the destination floor.
Evaluation of the routes should consider the amount of material being transported, the vessel used, typical evaluation rates, and ventilation in all locations, including elevators. EH&S is available to assist in performing this risk assessment.
Training
Students and staff working with or around cryogenic liquids must be trained on the procedures for its use and be made aware of the hazards involved. Training must be documented. EH&S Chemical Safety provides general Cryogen Safety Training. Facility specific training must also be provided by the PI/Facility Manager or designee. The training received shall provide information on the following topics:
- Properties and hazards of the cryogen being used;
- Personal Protective Equipment (PPE) requirements.
- Facility-specific procedures, including appropriate handling and filling methods;
- Proper use and function of engineering controls, including oxygen monitors, instrument interlocks, fume hoods, and other room ventilation.
- Review of all administrative controls.
- Incident/Exposure response and emergency contact.
- Transporting cryogenic liquids.
Signage/Postings
The presence of cryogenic liquids shall be indicated on the Laboratory Emergency Information Card (aka Laboratory door card). EH&S will also determine the need for signage to be posted outside of spaces utilizing inert cryogenic liquids. Spaces with oxygen monitors will have signage indicating the presence of the monitors and containing descriptions of what to do when the alarm sounds. EH&S may also recommend signage for other spaces containing cryogenic liquids.
REFERENCES
- Compressed Gas Association, Pamphlet P-12, Safe Handling of Cryogenic Liquids
- NFPA 45 Standard on Fire Protection for Laboratories Using Chemicals, Chapter 10, Compressed and Liquefied Gases
- NFPA 55, Standard for the Storage, Use, and Handling of Compressed Gases and Cryogenic Liquids in Portable and Stationary Containers, Cylinders, and Tanks
- Fluids Practical Cryogenics, by N.H. Balshaw, Oxford Instruments
- https://www.sciencedirect.com/topics/chemical-engineering/cryogenic-liquid
- Safe Handling of Cryogenic products: Air Products.
- Warzyniec; Ed. Safe handling of compressed gases and cryogenic liquids, Chem. Health & Saf. May/June 2000, 34-36.