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

A thin veil of smoke from wildfires burning in Canada is worsening air quality and putting a lid on temperatures in the D.C. area. The smoke, which has resulted in some of the world’s worst air quality in the Upper Midwest and Northeast, is only wafting through the Mid-Atlantic in limited quantities, but it’s enough to have a small impact.

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.