Incident Description
While taxiing for takeoff, all four occupants of a DC-8 freighter experienced shortness of breath. They donned oxygen masks and the flight returned to the ramp. The cargo included 960 pounds of dry ice (solid carbon dioxide) in 198 separate 4.85-pound blocks, used as a refrigerant for frozen shrimp.
Carbon Dioxide Concentration and Effects
Sublimation of dry ice to the gaseous state occurs constantly. When carbon dioxide (CO2) gas concentration reaches 0.5%, shortness of breath may occur. At 2%, depth of respiration increases. Above 4%, breathing becomes rapid and very deep.
At sea level and 70 degrees Fahrenheit (incident conditions were 23 feet MSL and 66 degrees Fahrenheit), 0.4 pounds of CO2 gas in the cockpit of a DC-8 produces a 0.5% concentration, and 3.3 pounds produces a 4% concentration. In 10 minutes, at sublimation rates of 1% and 14% per hour, 960 pounds of dry ice produces 1.6 and 22.4 pounds of CO2 gas, respectively.
Sublimation Rate Considerations
The 1% sublimation rate is commonly used when calculating amounts of dry ice that can safely be carried on aircraft; however, it was determined for bulk shipments of dry ice and may not be applicable to dry ice used as a refrigerant. The 14% rate was determined using a single 5-pound block of ice.