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.

“Smoke plumes can travel quite a bit. They can travel hundreds and thousands of miles, so our air quality right now is being impacted by neighboring fires along with other fires across the western United States,” said Sally Born, the Environmental Health Manager at Mesa County Public Health.

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.