Counties in Washington by median Air Quality Index (US EPA)
Okanogan County had the highest median Air Quality Index (US EPA) of 31 counties in 2021, at 54. Thurston County had the lowest, at 19. The median was 28. Source: the U.S. Environmental Protection Agency.
Map of counties, 2021
- Under 21
- 21 to 24
- 24 to 28
- 28 to 35
- 35 to 43
- 43 and over
- No data
All counties, 2021
Full ranking
| Rank | Counties | 2021 | Change on 2020 |
|---|---|---|---|
| 1 | Okanogan County | 54 | +7 |
| 2 | Yakima County | 52 | +6 |
| 3 | Asotin County | 48 | +10 |
| 4 | Stevens County | 44 | +2 |
| 5 | King County | 43 | 0 |
| 5 | Spokane County | 43 | +7 |
| 7 | Pierce County | 42 | 0 |
| 8 | Columbia County | 39 | +16 |
| 9 | Clallam County | 37 | 0 |
| 10 | Benton County | 36 | +1 |
| 11 | Pend Oreille County | 35 | -12 |
| 11 | Snohomish County | 35 | -5 |
| 13 | Clark County | 34 | +2 |
| 14 | Chelan County | 31 | +3 |
| 15 | Kittitas County | 28 | +4 |
| 15 | Skagit County | 28 | +7 |
| 15 | Whatcom County | 28 | +4 |
| 18 | Grays Harbor County | 26 | -1 |
| 18 | Kitsap County | 26 | +3 |
| 20 | Grant County | 25 | +4 |
| 21 | Walla Walla County | 24 | -2 |
| 22 | Lewis County | 23 | -2 |
| 23 | Mason County | 22 | -1 |
| 23 | Whitman County | 22 | +3 |
| 25 | Cowlitz County | 21 | 0 |
| 25 | Franklin County | 21 | +3 |
| 25 | Klickitat County | 21 | -1 |
| 28 | Jefferson County | 20 | -3 |
| 29 | Adams County | 19 | +2 |
| 29 | Garfield County | 19 | 0 |
| 29 | Thurston County | 19 | -9 |
Other periods
About this ranking
Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries.
More on how it is measured
- The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide).
- Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.
United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.
Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 3 counties with data in other periods have no value for 2021 and are left out rather than filled from another source. Equal values share a rank.
How to cite
U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.Citation styles
APA 7
U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: median daily AQI (Counties in Washington by median Air Quality Index (US EPA), 2021) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
MLA 9
EPA AirData annual AQI by county: median daily AQI, Counties in Washington by median Air Quality Index (US EPA), 2021. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2021. Accessed 12 Oct. 2026.
Chicago (author-date)
U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Counties in Washington by median Air Quality Index (US EPA), 2021. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2021.
Harvard
U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Counties in Washington by median Air Quality Index (US EPA), 2021 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2021 (Accessed: 12 October 2026).
BibTeX
@misc{environmental_counties_in_washington_by_2021,
author = {{U.S. Environmental Protection Agency}},
title = {{EPA AirData annual AQI by county: median daily AQI: Counties in Washington by median Air Quality Index (US EPA), 2021}},
year = {2026},
howpublished = {Data set},
url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
urldate = {2026-10-12},
note = {Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2021. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}
RIS
TY - DATA
AU - U.S. Environmental Protection Agency
TI - EPA AirData annual AQI by county: median daily AQI: Counties in Washington by median Air Quality Index (US EPA), 2021
PY - 2026
DA - 2026/07/13
PB - U.S. Environmental Protection Agency
DB - Invokely
UR - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2 - https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2021
Y2 - 2026/10/12
N1 - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2021.
N1 - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER -
Use this data in your AI agent
Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:
Rank Counties in Washington by median Air Quality Index (US EPA) using official data, and cite the source.
rank_places {"indicator":"aqi_median","level":"province","within":"Washington","limit":10}
GET https://invokely.ai/api/v1/rank?indicator=aqi_median&level=province&within=Washington&limit=10Questions
Which county has the highest median Air Quality Index (US EPA)?
Okanogan County, with 54 in 2021, according to the U.S. Environmental Protection Agency.
Which county has the lowest median Air Quality Index (US EPA)?
Thurston County, with 19 in 2021.
Where does this ranking come from?
Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12. 3 counties have no value for 2021 and are not ranked.