Smoke from Canadian wildfires is causing air quality concerns for millions of Americans in the Midwest and Northeast.

Wildfires burning in Canada were causing unhealthy air quality across parts of North America on Monday. Canadian wildfires burning across northern Manitoba and Saskatchewan are sending smoke through central Canada, the Great Lakes region and the northeastern United States, reducing air quality and visibility in major cities.

Hundreds of wildfires still burning in Canada are expected to undermine air quality across the Upper Midwest and Northeast into Tuesday.