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Safety

Cryogens

Cryogens are liquefied gases maintained at extremely low temperatures, typically below -150°C (-238°F). In the clinical laboratory, the two most ubiquitous cryogens are Liquid Nitrogen (LN2), used for long-term storage of cells and tissues in Blood Bank or Microbiology, and Dry Ice (Solid Carbon Dioxide), used for specimen transport. While indispensable, cryogens pose three distinct life-threatening hazards: extreme cold (tissue damage), rapid expansion (explosion), and asphyxiation (oxygen displacement)

The Physics of the Hazard

To respect the danger of cryogens, laboratory personnel must understand their physical properties:

  1. Extreme Temperature: Liquid Nitrogen boils at -196°C (-320°F). Dry Ice sublimates at -78.5°C (-109°F). Contact with skin causes instantaneous freezing of water in the cells, leading to severe frostbite or “cryogenic burns” indistinguishable from third-degree thermal burns
  2. Expansion Ratio: The liquid-to-gas expansion ratio of Nitrogen is approximately 694:1. This means 1 liter of liquid nitrogen expands to create nearly 700 liters of nitrogen gas
    • Explosion Risk: If LN2 is trapped in a sealed container (like a screw-top cryovial) and warms up, the pressure will shatter the vessel
    • Asphyxiation Risk: This massive volume of gas rapidly displaces breathable oxygen in the room

Personal Protective Equipment (PPE)

Standard laboratory PPE (latex gloves and a lab coat) is insufficient for handling cryogens. Specific gear is required to prevent splash injuries

  • Cryogenic Gloves: Must be loose-fitting, insulated gloves designed for extreme cold
    • Why Loose-Fitting? If liquid nitrogen splashes inside the glove, the user must be able to shake the glove off instantly. Tight gloves trap the liquid against the skin, guaranteeing severe necrosis
  • Face Protection: A full Face Shield is mandatory when pouring or transferring liquid nitrogen. Safety glasses alone do not protect the face or neck from splashes
  • Body Protection
    • Lab Coat: Must be fully buttoned
    • Apron: A non-porous rubber or leather apron is recommended for large volume transfers
    • Footwear: Closed-toe shoes are mandatory. Ideally, trousers should cover the shoe opening (cuffless pants) so spilled liquid runs off onto the floor rather than pooling inside the shoe. Sandals or mesh-top sneakers are strictly prohibited

Handling & Storage Protocols

Liquid Nitrogen (LN2)

  • Dewars: LN2 must be stored in specialized vacuum-insulated containers called Dewars
    • Venting: Dewars have loose-fitting lids or pressure relief valves. Never seal a Dewar with a tight plug. The boil-off gas must be allowed to escape, or the vessel will explode
    • Glass Dewars: If using older glass Dewars, they must be wrapped in tape to contain shards in case of implosion
  • Transfer
    • Pour slowly to minimize boiling and splashing
    • Use a phase separator or funnel to direct the flow
    • Never dip a hollow tube into LN2; liquid can spray out the top (the “geyser effect”)

Dry Ice (Solid CO2)

  • Handling: Use tongs or insulated gloves. Never handle dry ice with bare hands
  • Disposal: Do not dump dry ice into a sink or toilet. The extreme cold can crack ceramic and pipes. Allow it to sublimate in a well-ventilated area (like a fume hood) or a designated cooler
  • Sealed Containers: Never store dry ice in a tightly sealed jar or cooler. As it sublimates into CO2 gas, pressure builds up until the container bursts

The Asphyxiation Hazard (Oxygen Deficiency)

This is the most insidious danger of cryogens because Nitrogen and CO2 are odorless, colorless, and tasteless. A leak or spill gives no sensory warning

  • The Mechanism: As the cryogen boils or sublimates, it expands and pushes the lighter Oxygen out of the room
    • Normal air is ~21% Oxygen
    • At <19.5%, cognitive impairment begins
    • At <12%, unconsciousness is rapid
    • At <6%, death occurs within minutes
  • High-Risk Areas: Small, unventilated rooms like walk-in freezers, elevators, or small closet-sized storage rooms
  • Oxygen Monitors: Rooms storing large volumes of LN2 should be equipped with wall-mounted Oxygen Deficiency Monitors. If the alarm sounds, evacuate immediately. Do not enter to rescue a colleague without self-contained breathing apparatus (SCBA)

Specimen Transport Safety

  • Cryovials: When removing cryovials from liquid nitrogen, they may explode if liquid nitrogen has seeped inside the cap
    • Safety Rule: Place the vial in a heavy-walled container or behind a shield immediately upon removal to thaw. Do not hold it near your face
  • Vehicle Transport: Never transport Dewars or large coolers of dry ice in the passenger cabin of a car. If the container vents, the driver can lose consciousness. Transport them in the separate trunk of a vehicle or a truck bed

First Aid for Cryogenic Burns

  1. Remove: Move the victim away from the hazard
  2. Warm: Immerse the affected area in tepid (warm) water (100-105°F / 37-40°C)
    • Crucial: Do NOT: use hot water. Do NOT rub or massage the area (this destroys frozen tissue). Do NOT use dry heat (hair dryer)
  3. Protect: Cover with a sterile dressing
  4. Evaluate: Seek immediate medical attention. Deep tissue damage may not be immediately visible