Dry Ice Shipments in Air Transport
Adding dry ice in perishable shipments has been used as a cost effective solution to maintain cold temperatures for a limited time. Dry ice is solidified carbon dioxide (CO2) at very low temperature. Phase change diagrams establish the relationship between temperature, pressure and states of matter for carbon dioxide. Dry ice undergoes a phase change from solid to gas at -78oC under atmospheric pressure, without passing through the liquid state in a process called sublimation. Dry ice can be found in various particle sizes and shapes such as blocks, pellets or snow. Phase change occurs at higher rates for dry ice assemblies with larger surface areas. Due to very low temperatures, dry ice can cause severe skin damage by frostbites.
Dry ice is classified as a hazardous material by the International Air Transport Association (IATA). Since high concentrations of odorless gaseous CO2 in confined spaces such as airplanes can lead to breathing problems or even suffocation, the Federal Aviation Administration (FAA) in the United States has published a guideline for determining dry ice capacities. The maximum CO2 concentration per volume allowed on board an aircraft is typically taken as 0.5% for freighter models and 0.25% for passenger aircraft, implying that cargo airlines can carry twice the amount of dry ice compared to passenger airlines for the same cargo compartment. Phase change rates are variable since sublimation rates depend on environmental conditions, the form of dry ice, packaging materials, insulating properties, and wrapping procedures. While aircraft volume is an invariant for each specific airplane model, the maximum amount of dry ice allowed during flight increases with the number of scheduled fresh air exchanges. However this number depends on the operating aircraft and the flight procedures associated with emergency measures. Airplanes used on international routes are typically allowed to carry around 3 tonnes of dry ice during flight. The presence of animals on board reduces this dry ice amount tremendously by roughly a factor 6 or even more.
As aircraft are becoming more fuel efficient and non-smoking rules have been adopted by the industry, the number of air exchanges has been reduced by aircraft manufacturers. Consequently, dry ice capacities are decreasing as airlines are modernizing their fleet. This trend is becoming a challenge for air transportation providers who may be obliged to leave some dry ice shipments on tarmacs. FedEx Priority Alert service allows healthcare customers to secure priority boarding and thereby reduces the risk of their time and temperature sensitive shipments being left behind. In addition, re-icing services are offered at various FedEx locations as part of contingency planning for various unplanned delays such as shipments that cannot be placed in airplanes that have reached their dry ice limits.
Unfortunately these specialized services do not solve the overall dry ice challenge facing both the airline and healthcare industries today. While the lack of data on the amount of dry ice transported by air does not allow us to absolutely determine if air cargo capacity growth is being outpaced by dry ice shipping growth rate, it is clear that the move to more fuel efficient aircraft by air transporters will further limit the amount of dry ice which can be transported by air. Environmental concerns may not be sufficient to encourage shippers to consider alternative solutions to dry ice, as those alternatives are generally more costly and often multi-use by design (which then requires an additional return leg of transportation to get the packaging or container back to the shipper for reuse). If no viable alternatives become available airlines may be forced to implement new surcharges for dry ice shipments or increase existing ones as pricing measures. FedEx as an industry leader in transport of time and temperature sensitive goods is committed to face and address this challenge by exploring viable, economical options that provide the same benefits as dry ice in the shipping environment.
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