Dry ice is the solid form of carbon dioxide (\(\text{CO}_2\)) and is widely used as a refrigerant for perishable goods, medical specimens, and other temperature-sensitive items. Its unique physical properties necessitate strict safety protocols, classifying it as a regulated material for transport. Dry ice is classified as a Class 9 hazard under international and domestic regulations due to the risks it poses during handling and transportation.
The Science Behind Dry Ice
Dry ice is solid carbon dioxide, existing at an extremely low temperature of \(-78.5^\circ \text{C}\) (or \(-109.3^\circ \text{F}\)) at standard atmospheric pressure. This intense cold makes it an effective cooling agent, offering a temperature significantly lower than that of regular water ice. The unique characteristic of dry ice is sublimation, meaning it transitions directly from a solid state into a gas without passing through a liquid phase.
As the surrounding temperature rises, the solid \(\text{CO}_2\) absorbs heat and rapidly converts into colorless, odorless, non-flammable carbon dioxide gas. This phase change is accompanied by a massive increase in volume; one pound of dry ice can generate about 250 liters of gas. This rapid expansion is the underlying scientific reason for the material’s safety hazards.
Defining Dangerous Goods Class 9
Regulatory bodies, including the International Air Transport Association (IATA) and the U.S. Department of Transportation (DOT), categorize dry ice as a Class 9 Miscellaneous Dangerous Good. This category is reserved for substances that present a hazard during transport but do not fit into the other eight primary hazard classes. Dry ice is formally identified by the United Nations (UN) number UN 1845, with the proper shipping name “Carbon Dioxide, Solid.”
The classification acknowledges that the material’s danger stems from the consequences of its sublimation. Since dry ice is non-toxic and non-flammable, it is not classified as a poison or a fire risk. Instead, its physical change creates significant pressure and atmospheric risks. The UN 1845 designation flags the material for its unique physical hazards, ensuring carriers and handlers are prepared for the risks associated with the large volume of gas produced during transport.
Essential Safety Hazards
The hazards associated with dry ice stem from its extreme cold and its sublimation into gas. Direct contact with the solid material can cause severe tissue damage, similar to a burn, due to the rapid freezing of skin cells. This cryogenic burn, or frostbite, can occur almost immediately upon touching the \(-78.5^\circ \text{C}\) surface without insulated protection.
The continuous release of carbon dioxide gas poses a serious asphyxiation risk, particularly in enclosed or poorly ventilated spaces. Since \(\text{CO}_2\) gas is heavier than air, it settles in low areas, displacing breathable oxygen. If the oxygen concentration drops below 19.5%, a person can suffer unconsciousness or death because the gas is odorless.
The most immediate physical danger during transport is the risk of rupture or explosion from pressure buildup. When dry ice sublimates inside a sealed container, the gas expansion ratio is enormous, reaching approximately 760 times the volume of the solid. If the packaging cannot vent this rapidly expanding gas, the internal pressure will quickly burst the container, causing injury or property damage.
Shipping and Handling Compliance
The Class 9 classification dictates specific requirements for compliant shipping and handling. Packaging for dry ice must be designed to permit the continuous release of carbon dioxide gas, preventing dangerous pressure accumulation. Containers must be robust enough to maintain integrity but should never be completely sealed with an airtight closure.
All packages containing dry ice must be clearly marked and labeled with a Class 9 miscellaneous hazard diamond label. The exterior must also display the UN identification number UN 1845, the proper shipping name “Dry Ice” or “Carbon Dioxide, Solid,” and the net weight in kilograms. For air transport, a Shipper’s Declaration of Dangerous Goods may be required.
The air waybill must contain the specific dry ice information, even when it is used only as a refrigerant for non-hazardous goods. Transportation in vehicles or aircraft cargo holds requires adequate ventilation to prevent the buildup of gaseous carbon dioxide and protect personnel from asphyxiation.