What happened
United Airlines Flight 1 was cruising at flight level 310 approximately 1.8 hours after takeoff when a flight attendant reported experiencing headaches and lightheadedness. The aircraft subsequently climbed to flight level 350, at which point another flight attendant and several passengers reported similar symptoms. At the time of these reports, the tropopause was located at 40,000 feet. As the flight continued, the altitude of the tropopause lowered slightly before increasing again. Subsequently, several other occupants on board experienced similar symptoms. The captain activated the 'no smoking' sign in an attempt to mitigate the issue, but this action did not appear to help. Those affected by the possible irritating or gaseous substance were located in zones B, C, D, E, or on the upper deck of the aircraft. No occupants of zone A or the flight deck were affected, with the exception of the second officer who reported a slight pain between his eyebrows after spending 10 to 20 minutes in the passenger cabin.
The investigation
An examination was conducted to determine if hazardous cargo had leaked or if passengers admitted to carrying butane or similar substances; neither was found. An investigation for a possible ozone encounter was also performed, but no specific evidence was discovered. The catalytic converters in the air conditioning system were tested and found to be 51.2% efficient.