Smoke from Canadian wildfires has swept across the United States, causing severe air pollution that now affects approximately 109 million people.
The smoke has blanketed the midwest, mid-Atlantic, and north-east regions, leading to widespread health warnings.
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Government data from AirNow showed the air quality index reached a hazardous level of 361 in some midwestern locations.
Major cities including Chicago and Detroit experienced dangerous conditions, prompting officials to advise residents to stay indoors and limit physical activity.
The thick haze moved into Baltimore and Washington DC overnight, driving air quality index values to very unhealthy levels of 281 and 247, respectively, by 6am eastern time.
In New York City, where smoky conditions have persisted since Tuesday, the air quality index registered an unhealthy level of 184.
Philadelphia and Cleveland reported very unhealthy readings around 260, while parts of Michigan, Minnesota, and Wisconsin recorded numbers in the hazardous range.
Global monitoring showed that pollution in five major North American cities surpassed levels seen in Kinshasa or Nairobi.
Satellite imagery from NASA's Fire Information for Resource Management System revealed intense wildfire pockets extending deep into Canada's Northwest Territories.
The Michigan Department of Environment, Great Lakes, and Energy advised residents to close windows, minimize opening doors, and use HVAC systems rated MERV-13 or higher.
Some areas in the north-east and New England anticipate relief as stronger winds from Quebec are expected to displace the smoke from western Ontario.
