Wildfire smoke degrades air quality in swaths of Ontario, with Toronto ranking among the worst in the world on Monday.

The EPA's Air Quality Index can tell you whether breathing outside air can be harmful. Here's how to read the scale to plan accordingly in Michigan.

EDT on Tuesday, the National Weather Service reported an air quality alert in effect until Thursday at midnight EDT for Muskegon, Ottawa, Kent, Allegan and Van Buren counties.