All passengers on BA Flight 286 disembarked from the aircraft safely, but 25 crew members were treated in various hospitals in Vancouver "as a precaution," the airline said.
A spokesman for Vancouver Coastal Health, Gavin Wilson, told CNN that all patients have since been discharged.
When asked about media reports that crew members were treated for smoke inhalation, British Airways' London office told CNN that a number of people had been treated after becoming unwell.
Wilson said that "the earlier reports of smoke inhalation weren't substantiated."
BA spokeswoman Caroline Niven said that the airline is investigating the incident.
"Our team in Vancouver has ensured that all customers have been accommodated on different flights. We apologize for the disruption to their journeys," she wrote in a statement. She added that the crew would be returning to London.
Passenger Steve Lowy said he didn't notice anything odd or different while he was on the flight before it landed in Vancouver. He said people seemed "calm and a little nervous when the announcement happened, but that's it." He said he was on the top deck of the plane and didn't notice anyone feeling sick on the top deck.
Although he would be delayed 24 hours in returning to London, he told CNN: "We were all safe and well and that ultimately is the most important thing."
He later tweeted his dissatisfaction over the instructions provided by British Airways.
Another apparent passenger, Laura Dashwood, also tweeted about the situation, saying the airline had provided "zero info."
Kent Matthiesen, who says on Twitter that he works for an air systems service company in Vancouver, posted pictures of emergency vehicles around the plane with red lights shining in the darkness.