Air quality has taken a serious hit across the United States as a twofold blow of wildfires in the West and in Canada have sent smoke pouring across large sections of the country — a problem that’s set to linger for days.

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.

Wildfires in Canada are sending heavy smoke across the northern U.S., and New Jersey is experiencing some unhealthy air pollution. The air quality has some Northeasterners wondering, what is the AQI, or Air Quality Index? AQI, or Air Quality Index, is the measure of how healthy or unhealthy the air is.