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 national AQI is the average AQI across all counties. For each state, the AQI shown is the average air quality across the counties in the state. This provides the air quality for the average place. Since Americans do not live equally distributed across the country, this is not an effective measure of the air quality that the average person is living with. Look at the population-weighted version of this measure to better understand that aspect of the issue.
Interact more with the air quality data at The State of the Earth.