Monitor ranks Toronto as having the worst air quality on earth, surpassing Kinshasa, DR Congo, and New Delhi, India.

Toronto’s air quality has ranked the worst among all major cities in the world as smoke from wildfires in northwestern Ontario blankets the skies and spreads into the northeastern United States, triggering multiple health warnings and evacuations.

Wildfires continued burning through sparsely populated areas hundreds of miles from Toronto, Canada’s largest city, on Wednesday, sending smoke over a wide area, although cities in the area are not being threatened.

Environment Canada reported an Air Quality Health Index reading of 10+, classified as “very high risk”, for Toronto. Forecasts suggested that hazardous conditions could persist through Thursday night.

IQAir, a Swiss air quality technology company, ranked Toronto as having the worst air quality across the globe, surpassing the Democratic Republic of the Congo’s Kinshasa and India’s New Delhi.

“The biggest contributor to Toronto’s spike in air pollution right now is wildfires, though the higher-than-average temperatures are also playing a role,” Armen Araradian of IQAir told the AFP news agency.

While this year’s wildfire season in Canada has been fairly muted compared with recent years, there are more than 800 active fires nationwide.

A video that went viral on social media showed a Canadian National train surrounded by fire near Armstrong, Ontario. Canadian National employees in the area and residents of Armstrong were evacuated on Monday night, the railroad operator said in a statement. It suspended rail operations near Armstrong as a precaution.

Smoke from the wildfires also worsened air quality across the border in the US, with the states of Pennsylvania, New York, Connecticut, Massachusetts, Maine and New Hampshire particularly affected.

Authorities in New York City have issued an alert over unhealthy air quality, urging residents to reduce strenuous outdoor activity and take extra breaks if they are outside on Wednesday and Thursday.

The National Weather Service said smoke could linger until the end of the week.

“We probably haven’t seen the worst of it yet for New York City. We probably haven’t seen the worst of it yet for the Great Lakes and upstate, and New England yet either,” Dan Westervelt, Lamont associate research professor at Columbia University, told the Reuters news agency.

More than 80,000 people are expected to attend the FIFA World Cup final at an open-air stadium in New Jersey on Sunday, with another 50,000 planning to watch the game from New York City’s Central Park, where skies appeared hazy.

New York Governor Kathy Hochul urged people, especially those with health conditions, to exercise caution.

The Canadian government has said that wildfire season began more slowly this year than in 2023 or 2025 – the two worst seasons for wildfires – but warned that fires were likely, due to warmer-than-usual temperatures across the country.

It said some 835 active fires were burning across the country on Wednesday, with 112 considered out of control, and most in the central provinces of Manitoba, Saskatchewan and Ontario.

They have burned 1.9 million hectares (4.7 million acres) so far.

Greg Evans, a professor of chemical engineering and applied chemistry at the University of Toronto, said the city had been simultaneously hit with severe heat and wildfire smoke.

“I expect that this will occur more frequently over the coming decades, so cities and residents need to prepare for this in the future,” he said.