Cryogens and Dry Ice
Cryogens, substances used to produce very low temperatures [below -153°C (-243°F)], such as liquid nitrogen (LN2) which has a boiling point of -196°C (-321°F), are commonly used in laboratories.
Although not a cryogen, solid carbon dioxide or dry ice which converts directly to carbon dioxide gas at -78°C (-109°F) is also often used in laboratories. Shipments packed with dry ice, samples preserved with liquid nitrogen, and, in some cases, techniques that use cryogenic liquids, such as cryogenic grinding of samples, present potential hazards in the laboratory.
Overview of Cryogenic Safety Hazards
The safety hazards associated with the use of cryogenic liquids are categorized as follows:
- Cold contact burns. Liquid or low-temperature gas from any cryogenic substance will produce effects on the skin similar to a burn.
- Asphyxiation. Degrees of asphyxia will occur when the oxygen content of the working environment is less than 20.9% by volume. This decrease in oxygen content can be caused by a failure/leak of a cryogenic vessel or transfer line and subsequent vaporization of the cryogen. Effects from oxygen deficiency become noticeable at levels below approximately 18% and sudden death may occur at approximately 6% oxygen content by volume.
- Explosion - Pressure. Heat flux into the cryogen from the environment will vaporize the liquid and potentially cause pressure buildup in cryogenic containment vessels and transfer lines. Adequate pressure relief should be provided to all parts of a system to permit this routine outgassing and prevent explosion.
- Explosion - Chemical. Cryogenic fluids with a boiling point below that of liquid oxygen are able to condense oxygen from the atmosphere. Repeated replenishment of the system can thereby cause oxygen to accumulate as an unwanted contaminant. Similar oxygen enrichment may occur where condensed air accumulates on the exterior of cryogenic piping. Violent reactions (e.g., rapid combustion or explosion) may occur if the materials that make contact with the oxygen are combustible.
Whenever working with cryogenic fluids, be sure to use the following personal protective equipment:
- Face shield or safety goggles. Eye protection is required at all times when working with cryogenic fluids. When pouring a cryogen, working with a wide-mouth Dewar flask or around the exhaust of cold boil-off gas, use of a full face shield is recommended.
- Safety gloves. Hand protection is required to guard against the hazard of touching cold surfaces. It is recommended that Cryogen Safety Gloves be used by the worker.
- Long-sleeved shirts, lab coats, and aprons. Chemical-resistant sleeves should be used to provide additional protection. To protect against the risk of a splash hazard, consider using a chemical-resistant apron, such as those made from rubber, neoprene or PVC (depending on compatibility). Lab coat materials may be made of materials for limited reuse, or disposable one time use. If chemicals penetrate the coat or sleeve material, take it off and be sure to wash any affected area of your skin for 15 minutes.
For more information on laboratory PPE, see Lawrence Berkeley National Laboratory (Berkeley Lab) PPE requirements, Chapter 19 .
Knowledge Check Choose the best answer for the question.
9-4. Which of the following PPE is recommended when pouring a cryogen?
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