Dry ice, the solid form of carbon dioxide (\(\text{CO}_2\)), is widely used for its extremely cold temperature and ability to keep items chilled without leaving a liquid residue. Dry ice is classified as a hazardous material, particularly for transportation purposes. While it is not toxic or flammable, its physical properties—including its very low temperature and its tendency to change directly into a gas—create distinct hazards. These dangers stem from the sublimation process and the thermal effects of extreme cold, requiring specific safety measures for handling and storage.
Regulatory Classification for Transport
Dry ice is formally classified as a hazardous material for shipment by major transportation bodies worldwide. Transportation regulations identify dry ice as a Class 9 Miscellaneous Hazardous Material, or Dangerous Good, due to the potential for asphyxiation and pressure buildup in transport vehicles and aircraft. International and domestic agencies, such as the Department of Transportation (DOT) and the International Air Transport Association (IATA), assign dry ice the United Nations (UN) identification number \(\text{UN 1845}\). This classification dictates strict requirements for packaging, labeling, and documentation for air and vessel transport. Packaging must be designed to permit the continuous release of the \(\text{CO}_2\) gas to prevent pressure buildup that could cause the container to rupture. The outside of the package must be clearly marked with the Class 9 label, the \(\text{UN 1845}\) number, and the net weight of the dry ice in kilograms.
Specific Physical Hazards
The hazards posed by dry ice arise directly from its physical state and the process of sublimation, where it transitions from a solid directly to a gas. One of the most significant risks is asphyxiation, which occurs because the solid \(\text{CO}_2\) produces a large volume of carbon dioxide gas that is heavier than air. This dense \(\text{CO}_2\) gas can accumulate in low-lying areas and confined spaces, displacing the oxygen necessary for breathing. Since carbon dioxide is colorless and odorless, a person may not realize the oxygen level has dropped until they experience symptoms like shortness of breath or dizziness.
The extreme cold of dry ice, which maintains a temperature of approximately \(-109.3^\circ\text{F}\) (or \(-78.5^\circ\text{C}\)), presents the second major threat. Direct contact with the solid can cause immediate and severe thermal injury, similar to a burn from intense heat. This rapid freezing of tissues is known as a cryogenic burn or severe frostbite, and it can occur within seconds of bare skin contact.
A third major hazard is the risk of explosion when dry ice is stored improperly in sealed containers. As dry ice sublimates, one pound of the solid can produce about 250 liters of carbon dioxide gas. If this large volume of gas is trapped in an airtight container, the pressure buildup can be sufficient to cause a violent rupture of the container.
Essential Safety and Handling Procedures
Managing the hazards of dry ice requires strict adherence to safety procedures focused on ventilation, storage, and personal protection. Because of the risk of oxygen displacement, dry ice must always be used and stored in a well-ventilated area. This prevents the sublimating \(\text{CO}_2\) gas from accumulating to dangerous concentrations, especially in confined spaces like basements or walk-in freezers.
To prevent the pressure hazard, dry ice must never be placed in any sealed or airtight container. Storage containers should allow the gas to vent freely; insulated coolers or Styrofoam chests that are not airtight are the most appropriate options. Insulated gloves or tongs should always be worn when handling dry ice to prevent direct skin contact and cryogenic burns.
Safe disposal involves allowing the dry ice to fully sublimate to gas in a secure, well-ventilated location away from people and pets. Dry ice should never be disposed of in a sink, toilet, or drain, as the extreme cold can damage plumbing. It should also not be placed directly into a trash receptacle, where accumulating gas could pose a risk to sanitation workers.