Toronto's air quality has been ranked the worst among major global cities as smoke from wildfires in northwestern Ontario blankets the skies and spreads into the northeastern United States. The severe haze has triggered multiple health warnings and prompted evacuations in some affected areas.

Wildfires continue to burn through sparsely populated regions hundreds of miles from Canada's largest city, with the smoke creating hazardous conditions. While the fires themselves do not directly threaten urban centers, the widespread smoke has significantly impacted air quality across a vast geographical area.

Environment Canada reported an Air Quality Health Index reading of 10+, classified as "very high risk," for Toronto, with forecasts indicating these hazardous conditions could persist through Thursday night. IQAir, a Swiss air quality technology company, placed Toronto at the top of its global ranking for the worst air quality, surpassing Kinshasa, Democratic Republic of the Congo, and New Delhi, India.

"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 AFP. He noted that despite a relatively muted wildfire season compared to recent years, over 800 active fires are currently burning nationwide.

The unfolding situation has had tangible consequences, with a video surfacing on social media showing a Canadian National train surrounded by fire near Armstrong, Ontario. Canadian National confirmed that employees in the area and residents of Armstrong were evacuated on Monday night, and the company suspended rail operations near Armstrong as a precautionary measure.

Smoke from the ongoing wildfires has also exacerbated air quality concerns across the border in the United States. States such as Pennsylvania, New York, Connecticut, Massachusetts, Maine, and New Hampshire are particularly affected. New York City authorities have issued an alert advising residents to limit strenuous outdoor activities and take frequent breaks if outdoors.

Experts warn that the worst may not yet have passed for some regions. Dan Westervelt, a Lamont associate research professor at Columbia University, told Reuters that "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." The National Weather Service anticipates the smoke could linger until the end of the week.

The potential impact on major upcoming events is also a concern. More than 80,000 people are expected to attend the FIFA World Cup final in an open-air stadium in New Jersey on Sunday, with an additional 50,000 planning to watch the game from New York City, raising questions about public health advisories for these large gatherings.