Air Quality

How has air quality changed across the United States since 1980?

On average, air quality is improving in the United States. However, improvement or decline depends on location. The Air Quality Index (AQI) is a value that 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. 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 blue line indicates the national average AQI, grey lines indicate the average AQI by state and selectable metro areas are displayed in green.

In general, air quality is expected to be worse in more populated areas due to several factors including emissions from vehicles. 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. If we weight the counties by their population we 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 of high and low-density states, and large cities to see where air quality is improving and where it isn’t.

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