Smoke from Canadian wildfires is causing air quality concerns for millions of Americans in the Midwest and Northeast.

Since then, about 2,000 federal firefighters have been sent to Canada in rotations. As of July 17, there were 401 federal firefighters in Canada, many of them in Quebec, the agency said.

Maps show the extent of the wildfires spreading across the Northwest Territories in Canada, with the capital of the province asking all its residents to evacuate. Yellowknife, a city on the Great ...