Smoke from raging Canadian wildfires hung over the U.S. Midwest and parts of the East Coast on Wednesday, creating hazy skies and worsening air quality, making for dangerous, unhealthy conditions for millions of Americans.

A wide swath of the Midwest, reaching from western Iowa through Illinois and Wisconsin and into Michigan, was under an air quality alert that was expected to last through the day and into Thursday or even longer, the National Weather Service said.

Air quality alerts were also in effect for Western New York and Pennsylvania, the Washington, D.C., area and parts of North Carolina.

Forecasters urged people living in those areas, especially children, the elderly and those affected by respiratory illness to limit prolonged or heavy exertion and, if they can, to stay indoors or wear a mask.

In Chicago on Wednesday, hundreds of thousands of residents woke up to a smoke-induced fog that washed out the summer sun and the air smelled of burning lumber. The city’s air quality in was categorized as “very unhealthy” by AirNow.gov, a government website that tracks pollution.

The smoke was caused by prolonged wildfires in Canada’s two biggest provinces, Ontario and Quebec.

In Toronto, the Air Quality Health Index was forecast to reach 9 on a 10-point scale on Wednesday, indicating a high level of risk. Authorities were encouraging residents to limit outdoor activities.

Canada is wrestling with its worst-ever start to wildfire season, which has already burned 6.5 million hectares (16 million acres), an area a little bigger than West Virginia.

In the U.S. South, Florida and California, high temperatures combined with high humidity were the big worry, with some 56 million people expected to experience stifling heat throughout the day and into the weekend, the weather service said in its forecast.

Heat indexes – which use humidity and temperature to calculate how hot it feels – were expected to climb to the equivalent of 100 degrees Fahrenheit (38 Celsius). In some spots, the heat index was forecast to reach 115 degrees, the service said, urging people to stay indoors and drink plenty of water.
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