Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI)

Yakima County had the highest share of days unhealthy for sensitive groups or worse (US EPA AQI) of 31 counties in 2020, at 4.1%. Skagit County had the lowest, at 0%. The median was 2.5%. Source: EPA.

Highest4.1Yakima County

Lowest0Skagit County

Median2.531 counties

Map of counties, 2020

Under 1.9
1.9 to 2.4
2.4 to 2.5
2.5 to 2.7
2.7 to 3
3 and over
No data

%, 2020. 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, 2020

%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

Rank	Counties	2020 (%)	Change on 2019
1	Yakima County	4.1	+1.4 pts
2	Okanogan County	3.6	+3.6 pts
3	Benton County	3.3	+3.3 pts
3	Chelan County	3.3	+3 pts
3	King County	3.3	+3.3 pts
6	Kittitas County	3	+3 pts
6	Pierce County	3	+2.5 pts
8	Klickitat County	2.8	+2.8 pts
9	Garfield County	2.7	+2.7 pts
9	Grant County	2.7	+2.7 pts
9	Snohomish County	2.7	+2.4 pts
12	Asotin County	2.6	+2.3 pts
13	Clark County	2.5	+2.2 pts
13	Cowlitz County	2.5	+2.5 pts
13	Kitsap County	2.5	+2.5 pts
13	Mason County	2.5	+2.5 pts
13	Stevens County	2.5	+2.2 pts
13	Walla Walla County	2.5	+2.5 pts
13	Whatcom County	2.5	+2.5 pts
20	Franklin County	2.4	+2.4 pts
20	Jefferson County	2.4	+2.4 pts
22	Adams County	2.2	+2.2 pts
22	Columbia County	2.2	+2.2 pts
22	Grays Harbor County	2.2	+2.2 pts
22	Whitman County	2.2	+2.2 pts
26	Clallam County	1.9	+1.9 pts
26	Spokane County	1.9	+1.6 pts
26	Thurston County	1.9	+1.9 pts
29	Lewis County	0.3	+0.3 pts
30	Pend Oreille County	0	n/a
30	Skagit County	0	0 pts

Other periods

About this ranking

Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) 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; the counts are stored with each value.

Use the share rather than the number of days to compare counties, since the days with an AQI differ. 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 2020 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: days with an AQI above 100 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2020) [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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2020. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/washington-counties?period=2020. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2020. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/washington-counties?period=2020.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2020 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/unhealthy-air-quality-days-share/washington-counties?period=2020 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_washington_by_2020,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2020}},
  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/unhealthy-air-quality-days-share/washington-counties?period=2020. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Washington by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2020
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/unhealthy-air-quality-days-share/washington-counties?period=2020
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/washington-counties?period=2020.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 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 unhealthy for sensitive groups or worse (US EPA AQI)?

Yakima County, with 4.1% in 2020, according to EPA.

Which county has the lowest share of days unhealthy for sensitive groups or worse (US EPA AQI)?

Skagit County, with 0% in 2020.

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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 3 counties have no value for 2020 and are not ranked.