A big swath of the U.S. is under red and purple air quality alerts from Canada's smoke

Pittsburgh's Duquesne Incline ascends Mount Washington as smoke, from Canadian wildfires, hanging over the U.S. Midwest and parts of the East Coast create hazy skies on Wednesday. (Quinn Glabicki/Reuters)

As the sun rose on Madison, Wis., Thursday morning, the air quality was measured at 242 — a "very unhealthy" purple alert rating — due to smoke from Canada's wildfires.

Madison wasn't alone: from Cedar Rapids, Iowa, to Columbus, Ohio, a swath of the Midwest was marked by a dense mass of purple dots on the U.S. AirNow fire and smoke map, signaling the need for people to limit their time outside.