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

Some smoke from fires in the western United States and Canada is not uncommon in the summer months in the D.C. area. Occasionally, smoke enters the region from fires nearby, particularly during hot, dry spells when blazes ignite locally.

Smoke from hundreds of wildfires in Canada is lingering over the northern U.S., spreading across to roughly a dozen states from Wisconsin to Maine.