A dense band of wildfire smoke stretching from the Upper Midwest and Canada across the Great Lakes and into New England darkened skies across large parts of North America again Thursday, pushing air quality readings to dangerous levels in cities including Chicago, Cleveland, Detroit, Minneapolis and Toronto.

Satellite imagery showed the smoke plume extending from active wildfires burning in Minnesota, Wisconsin and Ontario, sweeping southeast through southern Ontario and New England before reaching New York City, with portions of the plume even drifting out over the Atlantic Ocean and curling back toward Canada's far eastern coastline. Forecasters said Thursday was expected to unfold much like the day before, with the densest smoke moving south throughout the day and potentially reaching as far as Maryland.

The worst air quality Thursday morning was concentrated in Minnesota, Wisconsin and Ontario, where the wildfires were actively burning. Among U.S. cities, Minneapolis, St. Paul, Chicago, Milwaukee, Detroit and Cleveland recorded the most severe readings. But forecasters warned that unhealthy air quality was likely to extend as far east as Toronto and New York throughout the day, with the worst conditions around New York City expected in the afternoon and evening hours.

Toronto has borne the brunt of the smoke's impact for a second consecutive day. On Thursday morning, the city's Air Quality Index reading approached 400, placing conditions........

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