Data / Rankings / Share of days with good air quality (US EPA AQI) / Counties in Washington

Counties in Washington by share of days with good air quality (US EPA AQI)

Columbia County had the highest share of days with good air quality (US EPA AQI) of 29 counties in 2015, at 94.5%. King County had the lowest, at 36%. The median was 79.8%. Source: EPA.

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Highest94.5Columbia County
Lowest36King County
Median79.829 counties

Map of counties, 2015

Adams County: 82.6 %Asotin County: 66.3 %Benton County: 74.3 %Chelan County: 67.1 %Clallam County: 72.3 %Clark County: 70.6 %Columbia County: 94.5 %Cowlitz County: 85.1 %Franklin County: 83.6 %Grant County: 80.3 %Grays Harbor County: 88.3 %Jefferson County: 89.6 %King County: 36 %Kitsap County: 93 %Kittitas County: 77.4 %Klickitat County: 89.2 %Lewis County: 86.1 %Mason County: 87.3 %Okanogan County: 47.7 %Pierce County: 64.4 %Skagit County: 77.8 %Snohomish County: 65.7 %Spokane County: 59.1 %Stevens County: 61.9 %Thurston County: 82.6 %Walla Walla County: 79.8 %Whatcom County: 84.3 %Whitman County: 82 %Yakima County: 52.3 %
  • Under 65
  • 65 to 75
  • 75 to 82
  • 82 to 87
  • 87 and over
  • No data
%, 2015. Hatched areas have no value for this period. Source: Invokely calculation with data from EPA. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2015

Columbia County: 94.5 %Columbia County94.5Kitsap County: 93 %Kitsap County93Jefferson County: 89.6 %Jefferson County89.6Klickitat County: 89.2 %Klickitat County89.2Grays Harbor County: 88.3 %Grays Harbor County88.3Mason County: 87.3 %Mason County87.3Lewis County: 86.1 %Lewis County86.1Cowlitz County: 85.1 %Cowlitz County85.1Whatcom County: 84.3 %Whatcom County84.3Franklin County: 83.6 %Franklin County83.6Adams County: 82.6 %Adams County82.6Thurston County: 82.6 %Thurston County82.6Whitman County: 82 %Whitman County82Grant County: 80.3 %Grant County80.3Walla Walla County: 79.8 %Walla Walla County79.8Skagit County: 77.8 %Skagit County77.8Kittitas County: 77.4 %Kittitas County77.4Benton County: 74.3 %Benton County74.3Clallam County: 72.3 %Clallam County72.3Clark County: 70.6 %Clark County70.6Chelan County: 67.1 %Chelan County67.1Asotin County: 66.3 %Asotin County66.3Snohomish County: 65.7 %Snohomish County65.7Pierce County: 64.4 %Pierce County64.4Stevens County: 61.9 %Stevens County61.9Spokane County: 59.1 %Spokane County59.1Yakima County: 52.3 %Yakima County52.3Okanogan County: 47.7 %Okanogan County47.7King County: 36 %King County36Columbia County: 94.5 %Columbia County94.5Kitsap County: 93 %Kitsap County93Jefferson County: 89.6 %Jefferson County89.6Klickitat County: 89.2 %Klickitat County89.2Grays Harbor County: 88.3 %Grays Harbor County88.3Mason County: 87.3 %Mason County87.3Lewis County: 86.1 %Lewis County86.1Cowlitz County: 85.1 %Cowlitz County85.1Whatcom County: 84.3 %Whatcom County84.3Franklin County: 83.6 %Franklin County83.6Adams County: 82.6 %Adams County82.6Thurston County: 82.6 %Thurston County82.6Whitman County: 82 %Whitman County82Grant County: 80.3 %Grant County80.3Walla Walla County: 79.8 %Walla Walla County79.8Skagit County: 77.8 %Skagit County77.8Kittitas County: 77.4 %Kittitas County77.4Benton County: 74.3 %Benton County74.3Clallam County: 72.3 %Clallam County72.3Clark County: 70.6 %Clark County70.6Chelan County: 67.1 %Chelan County67.1Asotin County: 66.3 %Asotin County66.3Snohomish County: 65.7 %Snohomish County65.7Pierce County: 64.4 %Pierce County64.4Stevens County: 61.9 %Stevens County61.9Spokane County: 59.1 %Spokane County59.1Yakima County: 52.3 %Yakima County52.3Okanogan County: 47.7 %Okanogan County47.7King County: 36 %King County36
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2015 (%)Change on 2014
1Columbia County94.5+3.7 pts
2Kitsap County93+0.2 pts
3Jefferson County89.6+1.3 pts
4Klickitat County89.2-6.6 pts
5Grays Harbor County88.3+0.9 pts
6Mason County87.3-0.1 pts
7Lewis County86.1+0.6 pts
8Cowlitz County85.1+0.3 pts
9Whatcom County84.3+14.7 pts
10Franklin County83.6-3.9 pts
11Adams County82.6-9 pts
11Thurston County82.6+1.6 pts
13Whitman County82-6.2 pts
14Grant County80.3-2.7 pts
15Walla Walla County79.8-10.5 pts
16Skagit County77.8-9.6 pts
17Kittitas County77.4-6.8 pts
18Benton County74.3-10.5 pts
19Clallam County72.3-4.1 pts
20Clark County70.6-2.3 pts
21Chelan County67.1+2.5 pts
22Asotin County66.3-9.9 pts
23Snohomish County65.7-0.3 pts
24Pierce County64.4-4.4 pts
25Stevens County61.9+0.8 pts
26Spokane County59.1-3.9 pts
27Yakima County52.3-9.9 pts
28Okanogan County47.7-7.6 pts
29King County36-15.5 pts

Other periods

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; both counts are stored with each value.

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: good days as a share of days with an AQI, calculated with stored counts. 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: good days as a share of days with an AQI, calculated with stored counts (Counties in Washington by share of days with good air quality (US EPA AQI), 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: good days as a share of days with an AQI, calculated with stored counts, Counties in Washington by share of days with good air quality (US EPA AQI), 2015. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/washington-counties?period=2015. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in Washington by share of days with good air quality (US EPA AQI), 2015. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/washington-counties?period=2015.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Washington by share of days with good air quality (US EPA AQI), 2015 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/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: good days as a share of days with an AQI, calculated with stored counts: Counties in Washington by share of days with good air quality (US EPA AQI), 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/good-air-quality-days-share/washington-counties?period=2015. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. 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: good days as a share of days with an AQI, calculated with stored counts: Counties in Washington by share of days with good air quality (US EPA AQI), 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/good-air-quality-days-share/washington-counties?period=2015
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/washington-counties?period=2015.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -
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Questions

Which county has the highest share of days with good air quality (US EPA AQI)?

Columbia County, with 94.5% in 2015, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

King County, with 36% 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: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 5 counties have no value for 2015 and are not ranked.

Share of days with good air quality (US EPA AQI): other rankings

Counties in Washington: more rankings