The fires have forced more than 27,000 Canadians in three provinces to flee their homes, and the smoke has even reached Europe

The smell of smoke hung over the Minneapolis-St Paul area on Tuesday morning despite rain that obscured the full measure of the dirty air

The Minnesota Pollution Control Agency issued an alert for almost the entire state into Wednesday, but the Twin Cities area got the worst of it in the Midwest on Tuesday

"As the smoke continues to move across the state Tuesday, air quality will slowly improve from northwest to southeast for the remainder of the alert area," the agency said. "The smoke is expected to leave the state by Wednesday at noon"

The Iowa Department of Natural Resources warned that air quality in a band from the state`s southwest corner to the northeast could fall into the unhealthy category through Thursday morning

The agency recommended that people, especially those with heart and lung disease, avoid long or intense activities and to take extra breaks while doing strenuous actions outdoors

Smoky conditions that have reached the US periodically in recent weeks extended as far east Tuesday as Michigan, west into the Dakotas and Nebraska, and as far to the southeast as Georgia