Smoke from hundreds of wildfires burning in Canada and the northern United States has drifted south, enveloping major population centres from the Great Lakes to the Eastern Seaboard in a thick, orange-tinged haze. Air quality alerts were in effect on Thursday for at least 20 states, from Minnesota to Virginia, as authorities urged millions of residents to stay indoors and limit physical activity.

In Detroit, the Air Quality Index surpassed 600 early Thursday, a level classified as hazardous by the US Environmental Protection Agency, according to monitoring firm IQAir. Minneapolis and Toronto also ranked among the world’s most polluted major cities, with readings in the “very unhealthy” to “hazardous” range. Canadian officials reported that the air quality health index in Toronto hit 10+, the top of the scale, prompting the city to close outdoor pools, cancel summer camps and suspend a public viewing event for the World Cup semi-final. In New York, Governor Kathy Hochul announced the distribution of more than 100,000 N95 masks, while Mayor Zohran Mamdani warned that the combination of extreme heat and poor air posed a serious health threat.

North of the border, the Ontario Provincial Police confirmed mandatory evacuations in several First Nations communities, including Armstrong and Whitesand First Nation, as fast-moving flames damaged homes and infrastructure. A freight train crew was safely rescued after their locomotive became surrounded by fire near Armstrong, the railway operator Canadian National Rail said. The union representing rail workers accused the company of knowingly sending the crew into an active fire zone, a claim the company did not directly address. The Canadian Interagency Forest Fire Centre reported 857 active fires nationwide, with 189 burning out of control, though figures varied slightly across official updates.

Meteorologists attributed the smoke’s persistence to a stationary high-pressure system that is trapping hot air and particulates near the surface. While rain and a shift in wind direction are expected to bring some relief by the weekend, US National Weather Service officials cautioned that smoke could linger over the Northeast into Friday, potentially affecting visibility for Sunday’s World Cup final in New Jersey. Authorities on both sides of the border said they were monitoring conditions closely, but no definitive forecast had been issued for the match.