Which county in Washington has the best air quality?

Based on 2024 EPA AQS data, Pacific County had the best air quality in Washington with a median AQI of 16 and 329 good-air days.

2024 monitoring year Public dataset EPA AQS

Washington Air Quality Rankings

Compiled by PlainEnviro Editorial, Environmental Data Editorial Team

County-level AQI rollup using EPA Air Quality System monitor records. Refreshed as new EPA AQS data lands.

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County-by-county air quality data for Washington based on EPA AQS annual AQI readings. Data covers 32 monitored counties for 2024.

Washington (2024): 32 counties monitored. State average median AQI: 28 (national average: 38.8). 86% of monitored days rated "Good." Most unhealthy air: Asotin County (4 days above AQI 100).

Washington air quality on this page is drawn from the EPA Air Quality System (AQS), which aggregates daily readings from federal, state, tribal, and local monitors into annual summaries per county. The current dataset covers 32 Washington counties with at least ten days of valid AQI readings in 2024, out of the state's total monitored counties.

The state-average median AQI across those counties is 28, compared with a national average of 38.8 in the same reporting period. On average, 86% of monitored days fell into the "Good" AQI bucket (0–50). The heaviest burden of unhealthy days lands on Asotin County (4 days above AQI 100), while Pacific County records the cleanest air with a median AQI of 16.

AQI values translate ambient concentrations of pollutants (ozone, PM2.5, PM10, NO₂, SO₂, CO) onto a single 0–500 scale with six health-based bands. A low median AQI reflects typical daily conditions; peak values matter separately because short-term exposure drives most acute health effects. Counties without monitors are not "clean" - they are simply unmeasured by EPA AQS. For site-specific exposure guidance, check AirNow.gov alongside the county detail links below.

Counties with Data

State Avg Median AQI

National Avg Median AQI

38.8

Avg Good Days %

86%

Which Washington counties have the worst air quality?

Ranked by median AQI (higher = worse). Click a county for full environmental data.

#	County	Median AQI	Good Days	Good %	Unhealthy Days	Peak AQI
1	Pierce	44	265	72%	1	166
2	Asotin	42	224	62%	4	196
3	King	41	255	70%	0	119
4	Snohomish	38	262	72%	0	99
5	Okanogan	38	225	61%	0	102
6	Spokane	38	280	77%	0	106
7	Yakima	37	248	68%	1	153
8	Clallam	36	321	88%	0	69
9	Columbia	36	331	90%	0	90
10	Stevens	34	257	70%	0	98
11	Benton	34	299	82%	0	126
12	Whatcom	33	335	93%	0	82
13	Clark	31	321	88%	0	105
14	Chelan	28	293	80%	1	158
15	Skagit	26	357	98%	0	59
16	Thurston	26	338	93%	0	85
17	Grant	26	318	87%	0	84
18	Pend Oreille	26	207	88%	0	81
19	Walla Walla	24	314	86%	0	87
20	Kittitas	23	320	87%	0	94
21	Lewis	22	336	93%	0	129
22	Cowlitz	22	349	95%	0	67
23	Jefferson	21	300	96%	0	57
24	Mason	21	341	93%	0	73
25	Grays Harbor	20	353	96%	0	64
26	Whitman	20	310	85%	0	97
27	Franklin	20	313	90%	0	75
28	Kitsap	19	341	97%	0	69
29	Adams	18	328	93%	0	71
30	Klickitat	17	101	95%	0	64
31	Garfield	17	314	90%	0	85
32	Pacific	16	329	100%	0	54

Frequently Asked Questions

What is the air quality like in Washington?

In 2024, Washington had an average median AQI of 28 across 32 monitored counties, with 86% of monitored days rated "Good" (AQI 0–50). The national average median AQI is 38.8.

Which county in Washington has the worst air quality?

Based on 2024 EPA AQS data, Asotin County had the most unhealthy air days in Washington - 4 days above AQI 100, with a median AQI of 42.

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National AQI Rankings → Washington Full Profile → Washington Water Quality →

About This Data

Air quality data is from the EPA Air Quality System (AQS), Annual AQI by County dataset. Counties with fewer than 10 monitored days are excluded. "Unhealthy days" counts days with AQI above 100 (Unhealthy for Sensitive Groups or worse). All data is for 2024.

Source: U.S. Environmental Protection Agency, Air Quality System (AQS) U.S. Environmental Protection Agency, Air Quality System (AQS)

Read our methodology - how this data is sourced, computed, and verified.

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Data sourced from the U.S. Environmental Protection Agency (EPA). See our methodology for details. Retrieved and formatted by PlainEnviro Editorial