Smoke from hundreds of wildfires in Canada is lingering over the northern U.S., spreading across to roughly a dozen states from Wisconsin to Maine.

Smoke from Canadian wildfires continues to affect air quality across the Midwest, though Monday’s conditions were an improvement for most of the Chicago area.

The hazy skies in the Chicago area are the visible sign of the poor air quality on Thursday as smoke from Canadian wildfires sent Chicago to the top of the list of most polluted cities in the world.