Air quality has taken a serious hit across the United States as a two-fold 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.

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.