Smoke from more than 100 active wildfires in northern Ontario have made Toronto’s air quality the current worst in the world and caused yellow, smoky air in cities across the north-east US.

Environment Canada issued health warnings on Wednesday after the sky over country’s largest city turned a sickly yellow and was ranked the worst in the world according to IQAir, the Swiss technology company that racks global air quality.

The city is also suffering a heatwave that shattered a three-decade record after reaching 37.3C in the downtown core, with runways at its main international airport hitting 55C.

The wildfires burning across northwestern Ontario have prompted mandatory evacuations from a number of First Nations communities.

Striking video footage of a train near the community of Armstrong, Ontario, highlighted the speed and ferocity of the fires. “This could potentially overtake us here … This has gotten a little scary,” says a crew member as a wall of flames whips across the windows.

The hazy Toronto skyline on Wednesday. Photograph: Anadolu/Getty Images

“We’re encased in flames now.”

The railway company Canadian National confirmed the crew had been “safely evacuated” from the area.

Other images showed families fleeing their homes by boat against the backdrop of massive plumes of smoke. “My family hometown, Collins Ontario, is GONE,” Nadya Kwandibens, a photographer, posted on social media.

Residents of Namaygoosisagagun First Nation said they had only minutes warning before fleeing across Collins Lake in the northwest part of the province.

“What we are witnessing right now is devastating,” said Sol Mamakwa, a member of the province’s New Democratic party. “An entire First Nation community has been erased because of this disaster. With wildfires closing highways and threatening communities across the north, we urge everyone to follow the guidance of emergency officials and remain prepared in case evacuations are necessary.

“Collins has burned to the ground. This is a tragedy and we are grateful that everyone got out safely,” said Lise Vaugeois, the provincial representative for the region.

“Fires are part of a natural cycle, but the extreme temperatures we are experiencing across the county and the growing severity of weather events are indicators of climate change.”

CNN reported that air quality alerts due to spoke have been issued across large parts of Michigan, Minnesota and Wisconsin, and that thicker smoke is forecast to move over New York, Washington and other cities across the eastern seaboard later in the week.