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.
The city's air quality was categorized as "very unhealthy" by AirNow.govBy 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.
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.
Persons:
Brendan O'Brien, Jonathan Oatis
Organizations:
U.S . Midwest, National Weather Service, D.C, AirNow.gov, Air, Thomson
Locations:
Iowa, Illinois, Wisconsin, Michigan, Ohio, New York, New Jersey , Pennsylvania, Washington, North Carolina, Chicago, United States, Decatur , Illinois, Ontario, Quebec, Toronto, Ontario's, Canada, West Virginia, U.S, , Florida, California