Smoke from wildfires in the western U.S. and Canada is blanketing much of the continent and reaching unhealthy levels this week in communities from Washington state to Washington D.C.

An air quality alert prompted by Canadian wildfires ended this morning, but it’s probably far from the last one of the summer. Fires across the border in Ontario and Manitoba will continue burning through the rest of the summer, perhaps until the fall when temperatures drop and maybe the state gets more rainfall. Until then, parts of Minnesota will likely continue getting doses of smoky air when winds shift to the north.

Growing research points to potential long-term health damage from breathing in microscopic particles of smoke, with millions of people potentially at risk far from where huge fires burn.