Air Quality

How has air quality changed across the United States?

On average, the nation’s air quality is improving. However, improvement or decline depends on location. The Air Quality Index (AQI) measures the concentration of five key air pollutants regulated under the Clean Air Act. The scale values are divided into six categories which indicate varying degrees of health concern: good, moderate, unhealthy for sensitive groups, unhealthy, very unhealthy, and hazardous. The Environmental Protection Agency defines “Good” air quality as an index level between 0 and 50. The higher the AQI value, the more polluted the air.

The blue line indicates the national average AQI, grey lines indicate the average AQI by state. Selectable metro areas are displayed in green.

Air quality is generally expected to be worse in more populated areas due to several factors including industrial facilities, vehicle emissions, and construction. By default, each line on the chart represents an average of the air quality across the counties: this provides an indication to the air quality in the average place. Weighting counties by their population makes it possible to see the air quality for the average person. The average person is worse off because that person is more likely to live in densely populated areas.

Use the tool to explore trends in states and large cities, to see where air quality is improving and where it isn’t.

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