Data / Rankings / Median Air Quality Index (US EPA) / Counties in Washington

Counties in Washington by median Air Quality Index (US EPA)

King County had the highest median Air Quality Index (US EPA) of 29 counties in 2015, at 53. Columbia County had the lowest, at 19. The median was 33. Source: the U.S. Environmental Protection Agency.

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Highest53King County
Lowest19Columbia County
Median3329 counties

Map of counties, 2015

Adams County: 27 indexAsotin County: 36 indexBenton County: 37 indexChelan County: 33 indexClallam County: 41 indexClark County: 40 indexColumbia County: 19 indexCowlitz County: 27 indexFranklin County: 27 indexGrant County: 31 indexGrays Harbor County: 29 indexJefferson County: 26 indexKing County: 53 indexKitsap County: 23 indexKittitas County: 28 indexKlickitat County: 21 indexLewis County: 30 indexMason County: 29 indexOkanogan County: 52 indexPierce County: 44 indexSkagit County: 39 indexSnohomish County: 38 indexSpokane County: 46 indexStevens County: 36 indexThurston County: 34 indexWalla Walla County: 26 indexWhatcom County: 34 indexWhitman County: 26 indexYakima County: 48 index
  • Under 27
  • 27 to 29
  • 29 to 36
  • 36 to 40
  • 40 and over
  • No data
index, 2015. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2015

King County: 53 indexKing County53Okanogan County: 52 indexOkanogan County52Yakima County: 48 indexYakima County48Spokane County: 46 indexSpokane County46Pierce County: 44 indexPierce County44Clallam County: 41 indexClallam County41Clark County: 40 indexClark County40Skagit County: 39 indexSkagit County39Snohomish County: 38 indexSnohomish County38Benton County: 37 indexBenton County37Asotin County: 36 indexAsotin County36Stevens County: 36 indexStevens County36Thurston County: 34 indexThurston County34Whatcom County: 34 indexWhatcom County34Chelan County: 33 indexChelan County33Grant County: 31 indexGrant County31Lewis County: 30 indexLewis County30Grays Harbor County: 29 indexGrays Harbor County29Mason County: 29 indexMason County29Kittitas County: 28 indexKittitas County28Adams County: 27 indexAdams County27Cowlitz County: 27 indexCowlitz County27Franklin County: 27 indexFranklin County27Jefferson County: 26 indexJefferson County26Walla Walla County: 26 indexWalla Walla County26Whitman County: 26 indexWhitman County26Kitsap County: 23 indexKitsap County23Klickitat County: 21 indexKlickitat County21Columbia County: 19 indexColumbia County19King County: 53 indexKing County53Okanogan County: 52 indexOkanogan County52Yakima County: 48 indexYakima County48Spokane County: 46 indexSpokane County46Pierce County: 44 indexPierce County44Clallam County: 41 indexClallam County41Clark County: 40 indexClark County40Skagit County: 39 indexSkagit County39Snohomish County: 38 indexSnohomish County38Benton County: 37 indexBenton County37Asotin County: 36 indexAsotin County36Stevens County: 36 indexStevens County36Thurston County: 34 indexThurston County34Whatcom County: 34 indexWhatcom County34Chelan County: 33 indexChelan County33Grant County: 31 indexGrant County31Lewis County: 30 indexLewis County30Grays Harbor County: 29 indexGrays Harbor County29Mason County: 29 indexMason County29Kittitas County: 28 indexKittitas County28Adams County: 27 indexAdams County27Cowlitz County: 27 indexCowlitz County27Franklin County: 27 indexFranklin County27Jefferson County: 26 indexJefferson County26Walla Walla County: 26 indexWalla Walla County26Whitman County: 26 indexWhitman County26Kitsap County: 23 indexKitsap County23Klickitat County: 21 indexKlickitat County21Columbia County: 19 indexColumbia County19
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties2015
index		Change on 2014
1King County53+4
2Okanogan County52+8
3Yakima County48+9
4Spokane County46+4
5Pierce County44+2
6Clallam County41+1
7Clark County40+3
8Skagit County39+3
9Snohomish County38-1
10Benton County37+11
11Asotin County36+3
11Stevens County36-3
13Thurston County34+2
13Whatcom County34-4
15Chelan County330
16Grant County31+4
17Lewis County30-1
18Grays Harbor County29+2
18Mason County29+2
20Kittitas County28+7
21Adams County27+6
21Cowlitz County27-1
21Franklin County27+2
24Jefferson County26-1
24Walla Walla County26+4
24Whitman County260
27Kitsap County23+1
28Klickitat County210
29Columbia County19-3

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. 5 counties with data in other periods have no value for 2015 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

The publisher is the author. Invokely is named only as the place the figures were read, where a style asks for it.

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), 2015) [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), 2015. 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=2015. 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), 2015. 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=2015.

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), 2015 [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=2015 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_washington_by_2015,
  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), 2015}},
  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=2015. 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), 2015
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=2015
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/washington-counties?period=2015.
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  -
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Rank Counties in Washington by median Air Quality Index (US EPA) using official data, and cite the source.
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Questions

Which county has the highest median Air Quality Index (US EPA)?

King County, with 53 in 2015, according to the U.S. Environmental Protection Agency.

Which county has the lowest median Air Quality Index (US EPA)?

Columbia County, with 19 in 2015.

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. 5 counties have no value for 2015 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Washington: more rankings