There was a sense of mounting anxiety as firefighters clad in respiratory gear passed through a British Airways flight diverted to Vancouver amid fears the cabin crew was sick, witnesses said.
The Airbus 380 and its 400 passengers were travelling to Heathrow from San Francisco when they were forced to land in the Canadian city so crew members could undergo precautionary checks at local hospitals.
In a statement, the airline apologised for the delay but said the well-being of its crew and customers was its "top priority".
A Vancouver Coastal Health spokesman confirmed 25 people from the flight were taken to area hospitals for assessment and "treated if required" before all being discharged, with the airline denying they were treated for smoke inhalation.
Passenger Liz Keller, a London-based advertising strategy director, said patrons were initially told the aircraft was diverting to Calgary before heading towards Vancouver due to a "crew illness".
Once they got there, she said they were told to stay in their seats while some "fully kitted up" firefighters walked down the aisles wearing respiratory equipment.
She said: "That's when things got a little more frightening in terms of the lack of information and knowledge. I only saw two (firefighters) on the plane but then when we got off there were loads more and tonnes of police as well."
The 38-year-old, who was returning home after a holiday visiting friends, described the incident as anxiety-inducing but said passengers remained quiet and didn't panic as their route altered.
"It was very serious. You never want to hear something's wrong with your plane when you're in the air. There was no sense of real panic, just mounting anxiety," she said, adding she didn't see any crew members who looked obviously ill.
In its statement, the airline said the ground team at Vancouver had arranged hotel accommodation for the customers and would book them on alternative flights as soon as possible
But Ms Keller said passengers had since been told they would have to re-book their own flights home.
British Airways check-in desks at Vancouver Airport are expected to open at 10am local time to help re-book passengers delayed by the diversion.