AQI measures the concentration of five key air pollutants regulated under the Clean Air Act. Higher AQI values are worse and lower ones are better — and “good” air quality is defined as having an air quality index level between 0 and 50. The AQI here is the average AQI across all counties weighted by the population. Weighting the counties by their population shows the air quality experienced by the average person in the nation or a state. Average air quality experienced per person tends to be worse than the average air quality across the country or state, because more densely-populated places like cities often have more pollution than rural, less-populated areas.
Interact more with the air quality data at The State of the Earth.