WA air quality ranked among best in the U.S. Which counties have the best and worst air?

On Earth Day, April 22, many Washingtonians took to the outdoors, soaking in the Earth and all it has to offer. In general, the Evergreen State is a great place to do be outdoors, with clean air, an abundance of natural beauty and plenty of sunshine as the weather warms up.

While the climate differs from the east to the west side of the state, the natural wonders of Washington are present in both.

In fact, a recent study out of BetArizona analyzed the air quality index of each state between March 2023 and 2024, in order to determine the states with the best air quality. Washington ranked in the top three.

Air quality by state

Washington state’s average AQI is 24.1, the third-lowest in the U.S. The lower the AQI, the better the air quality.

The only states with a lower air quality index were Alaska, with just 0.1 less than Washington, and Hawai’i with an AQI of 22.

Western states dominated the top of the charts, with Oregon, Idaho and Montana following Washington in that order.

The five states with the worst air quality, according to the study, are:

  • Oklahoma - 45.9

  • Illinois - 46

  • Delaware - 47

  • Utah - 47.6

  • Missouri - 48.1

WA counties ranked by air quality

There is also county-specific data used for the study. In the one-year period used for the study, most Washington counties had AQI readings available on 181 days. The only counties that saw additional readings were King County, with 182 readings, and Columbia County, with 212 readings.

The worst maximum AQI in the study time period was in Walla Walla County, at 119. Benton County wasn’t far behind, with a maximum of 116, and King County followed after with 112, according to BetArizona.

Pierce County had one some of the worst average air quality readings in the state over the study period, at 41.

For average AQI, the worst five counties were:

  • Benton - 34

  • Clallam - 37

  • King - 38

  • Columbia - 39

  • Pierce - 41

Garfield County had the best average AQI at 7.