Dry ice is the solid form of carbon dioxide, existing at an extremely cold temperature of approximately -109.3°F (-78.5°C). It is used as a cooling agent because it changes directly into a gas (sublimation) rather than melting into a liquid. Choosing the correct container is paramount for safety and maintaining effectiveness during short-term storage or transport. Safe containment requires managing the gas released during sublimation and protecting handlers from the extreme cold.
Understanding the Sublimation Process and Pressure Risks
The need for a specific type of container stems from dry ice’s tendency to sublime into carbon dioxide (CO2) gas. The resulting gas occupies a far greater volume than the solid form; one pound of dry ice produces approximately 250 liters of CO2 gas. If dry ice is placed in a completely sealed or airtight container, the continuous gas expansion creates immense pressure. This rapid pressure buildup can lead to a violent rupture or explosion of the container. Consequently, any container used for dry ice must be designed or modified to allow the CO2 gas to vent safely into the surrounding atmosphere.
Recommended Containers for Storage and Transport
The most suitable containers for dry ice are those that are well-insulated yet not airtight, as this combination slows the sublimation rate while permitting gas release. Heavy-duty insulated coolers, such as those made from Styrofoam or high-quality plastic with thick foam insulation, are ideal choices. These containers minimize heat transfer from the outside, which helps preserve the dry ice longer than an uninsulated container.
When using a standard insulated cooler, ensure the lid is not clamped down or sealed completely. Leaving the lid slightly ajar, or using a cooler with a built-in vent hole, is a simple but effective way to prevent pressure accumulation. Specialized dry ice storage chests are commercially available, engineered with durable double-wall polyethylene construction and urethane foam insulation to minimize sublimation rates while allowing ventilation.
For storage, the dry ice itself should be wrapped in a material like paper, a towel, or a heavy cloth before being placed in the cooler. This wrapping provides an additional layer of insulation, which further slows the rate of sublimation and protects the interior walls of the container from potential damage caused by the extreme cold. The insulation of the container determines the longevity, with dry ice typically sublimating at a rate of five to ten pounds every 24 hours in a standard cooler.
Containers and Materials to Strictly Avoid
A strict rule for dry ice containment is to never use any vessel that is designed to be completely airtight, as this introduces the explosion risk. Specific examples of containers that must be avoided include glass jars, sealed plastic bottles, and metal drums with clamped or gasketed lids. The gas pressure generated can easily exceed the structural limits of these materials, leading to catastrophic failure.
Glass containers are particularly unsuitable because the extreme cold temperature of dry ice can cause the material to become brittle and shatter even before significant pressure has built up. Similarly, placing dry ice in a standard freezer or refrigerator is inadvisable. These appliances are not designed to vent the gas created by dry ice, which can lead to a buildup of CO2 gas within the appliance, risking asphyxiation and pressure damage. Thin, uninsulated metal containers should also be avoided because they rapidly conduct heat, causing the dry ice to sublime much faster.
Safe Handling and Ventilation Practices
Beyond the container itself, safe handling and proper ventilation are necessary to mitigate the remaining risks associated with dry ice. The extremely cold temperature of -109.3°F can cause severe frostbite or cold burns upon direct contact with skin. Personal protection is therefore necessary, requiring the use of insulated gloves or tongs when manipulating the dry ice.
The colorless, odorless carbon dioxide gas released during sublimation is heavier than air and can accumulate in low-lying, confined areas. Storing or using dry ice in a small, unventilated room, a basement, or a closed vehicle cabin poses a risk of asphyxiation. The CO2 gas displaces the oxygen in the air, leading to dizziness, rapid breathing, or loss of consciousness.
Dry ice should always be stored and used in an area with good air circulation to disperse the accumulating gas. When transporting dry ice in a vehicle, the container should be placed in the trunk or a cargo area separate from the passengers, and windows should be opened to ensure continuous fresh air. For disposal, any remaining dry ice should be allowed to fully sublime in a well-ventilated location, such as outdoors, away from traffic and unauthorized access.