The air was rated very unhealthy, or code purple, the second-worst rating for air quality levels, as of 9 a.m. Wednesday, according to IQAir.

The smoke from the Canadian wildfires is causing air quality issues in the U.S. See where.

Toronto had the worst air quality of any major city in the world on Wednesday, the Swiss firm IQAir said, as Canadian authorities urged people to stay indoors.

The wildfires are burning in Ontario, Canada, Minnesota, and nearby regions, with smoke already affecting cities in the Midwest.