(VIDEO) Canadian Wildfire Smoke Darkens Skies Again Across US and Canada as Air Quality Hits Hazardous Levels

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 well within the “hazardous” category, with forecasters projecting the air would not return to healthier levels until around 7 p.m. That followed an even more severe stretch Wednesday, when Toronto’s air quality index briefly ranked among the worst of any major city in the world. By Wednesday evening, every U.S. state stretching from Minnesota to Connecticut had at least one location where the index had climbed into unhealthy territory. At 10 p.m. Wednesday, as some of the thickest smoke plumes pushed south of the international border, Minneapolis recorded a reading of 287, Detroit registered 196, New York City reached 192, and Scranton, Pennsylvania, hit 157, according to data from AirNow, the monitoring network run by the Environmental Protection Agency.

The EPA’s Air Quality Index scale runs from 0 to 500 and measures the concentration of five pollutants: ground-level ozone, particulates, carbon monoxide, nitrogen dioxide and sulfur dioxide. Readings of 100 or higher serve as a warning for people with respiratory conditions to take precautions. Once the index climbs above 150, air is considered unhealthy even for people outside sensitive groups; above 200, conditions are classified as “very unhealthy”; and above 300, the air is deemed “hazardous.” Several locations in northeastern Minnesota, closest to the active fires, recorded readings well into the hazardous range on Wednesday.

Meteorologists said the unusually widespread reach of the smoke this week stems from the same atmospheric conditions responsible for the brutal heat gripping the Midwest and Northeast. A heat dome parked over the region has trapped the smoke close to the ground rather than allowing it to disperse at higher altitudes, compounding both the heat and the pollution simultaneously. Forecasters expect conditions to begin easing in the Northeast by the weekend, as another weather system moves in and pushes the hottest air out of the region. However, areas closer to the fires themselves, including much of the Upper Midwest and Ontario, are likely to see the smoke linger longer even as conditions improve further east.

The Environmental Protection Agency estimated that many locations affected by smoke on Wednesday would experience similar or even slightly worse conditions on Thursday, underscoring the persistence of the smoke event across the region.

Health officials and researchers have pointed to a broader pattern behind weeks like this one. As climate change continues to push global temperatures to record levels, the frequency of days that combine extreme heat with heavy air pollution has been increasing, a dynamic that compounds health risks for vulnerable populations during already dangerous heat events. Wildfire smoke in particular can travel enormous distances from its source, meaning cities far removed from any active fire can still experience unhealthy or even hazardous air quality, as this week’s smoke plume reaching from Minnesota and Ontario down to New York and Maryland has demonstrated.

Public health guidance for smoky conditions generally centers on limiting outdoor exposure, particularly for children, older adults and people with existing heart or lung conditions. Officials recommend keeping windows closed, running air conditioning or air purifiers on recirculating settings where possible, and monitoring for symptoms such as coughing, difficulty breathing or eye and throat irritation. Athletes and others who exercise outdoors are advised to pay especially close attention to real-time air quality readings before heading out, since exertion during smoky conditions increases the amount of polluted air the body takes in and can heighten health risks even for otherwise healthy individuals.

The current smoke event follows a familiar pattern seen in recent years, as Canadian wildfires have increasingly sent smoke drifting deep into the United States during summer months, disrupting outdoor activities and prompting air quality alerts across a wide swath of the country far from where the fires themselves are burning. With active fires continuing to burn in Minnesota, Wisconsin and Ontario, forecasters said they would continue monitoring the smoke’s movement and updating air quality projections as conditions evolve through the rest of the week, particularly as the heat dome responsible for trapping the smoke near the ground begins to break down heading into the weekend.