What happened
During taxi operations prior to takeoff, all four occupants of a DC-8 freighter began experiencing shortness of breath. In response to the physiological symptoms, the crew donned oxygen masks and transitioned the aircraft back to the ramp.
The cargo on board consisted of 960 pounds of dry ice (solid carbon dioxide) distributed in 198 separate blocks, each weighing 4.85 pounds. This dry ice was being utilized as a refrigerant for the transport of frozen shrimp.
The investigation
The investigation focused on the sublimation of the dry ice into gaseous carbon dioxide (CO2). At the time of the incident, the aircraft was at an altitude of 23 feet MSL with an ambient temperature of 66 degrees F.
Calculations determined that at sea level and 70 degrees F, just 0.4 pounds of CO2 gas in a DC-8 cockpit is sufficient to reach a 0.5% concentration, which can trigger shortness of breath. A concentration of 3.3 pounds of CO2 gas would result in a 4% concentration, causing rapid and deep breathing.
While a sublimation rate of 1% per hour is a standard figure used for calculating safe dry ice transport, the investigation noted that this rate may not be applicable to bulk shipments. At a 1% hourly sublimation rate, the 960 pounds of cargo would produce 1.6 pounds of CO2 gas in 10 minutes. However, at a higher sublimation rate of 14% per hour—a rate observed using a single 5-pound block—the same amount of cargo would produce 22.4 pounds of CO2 gas in 10 minutes.