Hazy, smoke-filled skies from raging Canadian wildfires hovered Wednesday over the U.S. Midwest and East, where millions of Americans were urged to limit outdoor activities as the poor air quality made for dangerous, unhealthy conditions.

Much of the Midwest - from western Iowa through Illinois and Wisconsin and into Michigan and Ohio - was under an air quality alert 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 New York state, New Jersey, Pennsylvania, Washington, D.C., and parts of North Carolina. In all, more than 100 million Americans were dealing with poor air quality on Wednesday afternoon.

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 possible, to stay indoors or wear a mask.

On Wednesday afternoon, an eerie smoke-induced fog hung over Chicago, washing out the summer sun, and the air smelled of burning lumber. The city's air quality was categorized as "very unhealthy" by AirNow.gov

By midday, the worst air quality in the United States was detected in Decatur, Illinois, about 180 miles (290 km) south of Chicago, according to AirNow, which categorized the air in the city of 70,000 as "Hazardous."

"Everyone should stay indoors and reduce activity levels," said the government website, which tracks pollution. "Stay tuned to local news media for advisories."

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

In Toronto, Ontario's most populous city, 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 experiencing stifling heat that was to persist through 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 F (46 C), the service said, urging people to stay indoors and drink plenty of water.