Chicago faces an air quality alert due to Canadian wildfire smoke and high ozone levels, with risks for sensitive groups.

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.

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.