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 of it 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. As of Thursday morning, air quality readings surged to dangerous levels in many places, including Chicago, Cleveland, Detroit, Minneapolis and Toronto.
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.
On Thursday morning, the A.Q.I. in Toronto was close to 400, with hazardous conditions forecast for much of the morning. The air quality was expected to go back to healthier levels in Toronto around 7 p.m. on Thursday.
It was the second day of extremely unhealthy air in Toronto. On Wednesday, the city’s A.Q.I. was briefly among the worst in the world 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.
What else to know about the smoke:
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As climate change drives up global temperatures past record levels, the frequency of days when it is both hot and polluted has also been increasing. Claire Brown and Christina Kelso of The Times’s Climate team wrote about several factors that are converging to amplify health risks during sweltering days. Read more here ›
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Wildfires can generate unhealthy levels of smoke. Dani Blum of Well wrote about what you can do to protect yourself, even far from the fires. Read more here ›
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When the air is smoky, it’s important to consider whether it’s safe to exercise outdoors — and how to protect yourself if you choose to head out. Melinda Wenner Moyer of Well wrote about what athletes should know. Read more here ›
Claire Moses contributed reporting.
