The Air Quality Index, or AQI, is a daily public report that the Department of Environmental Quality is required (by EPA) to make in each metropolitan area with a population of more than 350,000. The AQI is based on the previous day’s monitored concentrations of the criteria pollutants (i.e. particulate matter, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide). The AQI report can also be used to predict the levels of the criteria pollutants. This is important when the pollutants are expected to reach unhealthy levels because it gives sensitive people time to take precautions.