Hundreds of wildfires in Canada are creating unhealthy air quality conditions across the Midwest, including Michigan, where some areas ranked among the worst air quality in the world Monday.

It’s not fog. Wildfires burning hundreds of miles away in Canada are sending plumes of smoke all over the American Midwest, and that smoke makes breathing hard and sometimes affects our health in untold ways. We know breathing wildfire smoke near the source is harmful, but there’s still uncertainty about what happens as the smoke travels.