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.

Breathing in polluted air for extended periods of time can pose a serious health risk for people, especially for vulnerable populations.

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.