A British Airways flight made an emergency landing in March at London Heathrow airport after pilots reported oxygen starvation, according to UPI.

The captain and his two co-pilots began experiencing symptoms such as nausea, headache, light-headedness, and difficulty breathing on the March 6 flight. The plane was at an altitude of 34,000 feet when the symptoms began, according to ABC News.

The flight, with 235 passengers and crewmembers onboard, was heading for Seattle when the crew inside the cockpit was forced to put on emergency oxygen masks. The crew had to open the cockpit door, violating a policy that requires the door to be closed. The door was guarded by two crewmembers.

Upon inspection, the air vents into the cockpit were found to have been blocked by debris such as wire, bubble wrap, and insulation, the Daily Mail reported. This stopped the flow of oxygen into the cockpit.

The blocked vents were noticed before the flight, but the flight crew prepared for takeoff anyway. As there symptoms worsened, however, the crew decided to head back to Heathrow airport whilst getting oxygen from their emergency masks.

In the month before the incident, two reports were filed by pilots noting inadequate airflow and abnormally high temperatures in the cockpit, according to ABC News.