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.

States across the U.S. are feeling the impact of the Canadian wildfires as air quality takes a hit from the smoke, according to a government website that tracks air quality in the United States.

Millions are under air quality alerts as wildfire smoke blankets large swaths of U.S. Millions of people across the Upper Midwest and the Northeast are under air quality alerts Monday, as smoke from wildfires in Canada drift over the region.