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.

Air quality alerts are issued when pollution levels rise to unhealthy levels, warning the public, especially vulnerable groups, to limit outdoor exposure for health and safety.