Ambulance staff were called to treat six passengers at Heathrow airport after they fainted while on a British Airways flight from the US.
None of those assessed needed hospital treatment and all were able to continue their journeys, the airline said.
London Ambulance Service was asked to meet the flight from Newark at Terminal Five at 0650 GMT on Saturday.
Emergency services teams initially wore protective suits as it was unclear what had caused the passengers to faint.
A British Airways spokeswoman said: “A handful of passengers on the aircraft fainted during the flight.
“As a precaution, medical services met the aircraft.”
An investigation is under way into what caused the passengers to faint.