The aircraft had taken off from London Heathrow Airport. During taxi, takeoff and initial climb, the crew felt unusually warm and noticed very low airflow from the flight deck vents. As the flight reached initial cruise level, all three crew members on the flight deck started to feel unwell and opened the cockpit door to improve ventilation. The decision was made to return to Heathrow with all three crew members on oxygen and the cockpit door closed.
An uneventful landing was carried out and it was found that debris in the conditioned air duct below the cockpit floor was almost completely blocking airflow to the flight deck. The source of the debris and how long it had been present could not be determined.