Dr. Cheema says that what you see is what you breathe. "Especially with these particles and smoke, you know, they can sometimes embed in your lungs, and then it irritates the lungs, and then you start coughing. You start coughing up all that stuff," Dr. Cheema said.

Smoke from wildfires burning in three Canadian provinces is spreading into the U.S. Upper Midwest, stirring memories of the severe pollution that drifted south from Canada two years ago during that country's worst fire season on record.