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.

Red flag warnings remain in place for parts of Nevada, Utah, Colorado and Wyoming due to very dry conditions, low humidity and gusty winds.

Another air quality alert is in effect Tuesday for NYC, western Long Island, the Hudson Valley, northwest New Jersey and southwest Connecticut.