Smoke from wildfires burning in northern Ontario and the Prairies of Canada caused poor air quality and reduced visibility in major cities including Toronto, Ottawa and Montreal on Friday.

Another large fire exacerbated by climate change is threatening the health of Chicagoans. Government officials issued an air pollution "action day" on Thursday.

Wildfires are still raging in Canada, where there have been dozens of active fires, and over 33,000 people have been forced to evacuate their homes.