The strip of dense wildfire smoke can be seen on satellites stretching from the Upper Midwest and Canada, across the Great Lakes, through southern Ontario and New England and down to New York City. Some is even traveling out over the Atlantic Ocean and back up to the far eastern coastline of Canada.

Weather forecasters expect the smoke to darken skies again on Thursday across North America and send air quality readings surging to dangerous levels in many places.

The E.P.A. estimated that many places that were affected by smoke on Wednesday would experience similar conditions on Thursday — or even a little worse.

Air quality readings of 100 or higher are a warning to people who have respiratory conditions to take precautions. Above 150, the air is considered “unhealthy” for people who are not in sensitive groups. Above 200, the air is “very unhealthy.” Above 300, the air is ”hazardous.”

The scale, which runs from 0 to 500, was designed by the E.P.A. and measures the density of five different pollutants in the air: ground-level ozone, particulates, carbon monoxide, nitrogen dioxide and sulfur dioxide.

Several spots in northeastern Minnesota, closest to the fires recorded had A.Q.I. readings well into the “hazardous” range on Wednesday.

Toronto’s A.Q.I. was briefly among the worst in the world on Wednesday, and by the evening every U.S. state from Minnesota to Connecticut had some location where the air quality index had reached unhealthy levels. At 10 p.m. on Wednesday night, as some of the thickest plumes of smoke moved well south of the international border, Minneapolis was at 287, Detroit was at 196, New York City was at 192 and Scranton, Pa., was at 157, according to AirNow, run by the E.P.A.