Wildfires have swept across Ontario, prompting dozens of communities to evacuate and sending acrid smoke to New York City and across the U.S. northeast.

Parts of Canada, including its most populous city, Toronto, woke Wednesday to a hazy, orange sky, the air smelling strongly of wood smoke. Air quality readings for the city triggered a warning from Environment Canada, a federal department, about the health risks of spending time outdoors.

“We’re really being fumigated,” said Brian Proctor, a meteorologist at Environment Canada, adding that people across New York state can also anticipate worse air quality measures through the day tomorrow as the smoke makes its way southeast.

“In general terms, expect it to get worse before it’s going to get better in the next 36 hours,” said Mr. Proctor.

At least six rural and First Nations communities have been evacuated around Thunder Bay, Ontario, a city on Lake Superior about an hour’s drive from the Minnesota border. Members of Namaygoosisagagun First Nation, a community of about 40 residents, evacuated their homes by boat.

“An entire first nations community has been erased because of this disaster,” Sol Mamakwa, an Indigenous member of provincial parliament, said in a statement, offering his support to residents of the community, also known as Collins First Nation.

Three trains transporting flammable cargo were threatened by flames along the tracks and held at a safer location, the Ontario Provincial Police said.

A dramatic video, taken from inside the locomotive cab of a train passing through a fire zone in Armstrong, Ontario, shows at least two engineers radioing into their dispatch crew about the rush of flames through the trees in front of them.

The Canadian National Railway did not immediately respond to a request for comment.

Municipalities have banned open flames, including camp fires, as a protective measure, particularly after an oppressively hot day on Tuesday, reaching 99 degrees Fahrenheit, or 37 Celsius in Toronto.

Moister conditions across much of Canada — rainfall in the west, cooler temperatures in the east — had tempered the start of wildfire season, which typically lasts from April to October.

There are 838 active wildfires across the country, according to the Canadian Interagency Forest Fire Center, a national coordinating body. So far this year, 4.7 million forest acres have burned, less than a quarter of the 22.2 million acres scorched in 2025, the second-worst year on record for Canada, after 2023.

The haze amid a heat wave across Ontario and the northeastern United States is expected to continue and could even worsen in some places on Wednesday. The heat dome that has pushed temperatures close to 100 degrees Fahrenheit in Ontario and the U.S. northeast this week traps the smoke from faraway wildfires close to the ground.

While the air quality in Toronto on Wednesday was unusually bad for Canada, it was comparable to cities like Delhi and Dhaka at other times of year.