Massive plumes of Canadian wildfire smoke have started to pour over the border into the United States, bringing dangerous air quality to more than 100 million people in the Midwest and Northeast.

Over 800 wildfires are burning through forests in Canada, prompting heavy smoke warnings for millions of Americans.

Smoke from northern Minnesota and western Ontario wildfires is pouring over the Great Lakes, mid-Atlantic and Northeast, bringing record levels of hazardous air pollution to major…