Median AQI
43
Good
Washington · County air quality · 2025
King County, Washington
Median AQI 43 (Good). 67.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 67.5%
- Good-air days
- 0
- Unhealthy days
- 27%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, King County's air is good on balance, more polluted than the median US county, a median AQI of 43 with 67.5% of 243 monitored days in the Good band, driven mainly by PM2.5.
- 43
- median AQI · Good
- 67.5%
- days rated Good
- PM2.5
- main pollutant
- 27%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 90.
Data updated July 2026
- Good · 164 d · 67.5%
- Moderate · 79 d · 32.5%
How did King County's air break down in 2025?
Across 243 monitored days, 164 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 90 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 243 days monitored. Median AQI: 43 (Good).
Insights: King County, Washington. EPA monitors recorded a median Air Quality Index of 43 across 243 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 164 (67%) fell in the Good band (AQI 0–50), 79 (33%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 90 (Moderate). Residents averaged about 20.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (138 days), Ozone (98 days), NO2 (7 days). Across the 2020–2025 trend window, good days shifted by -84 and the median AQI moved +0 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 90, moderate but never Unhealthy. Nearby counties show comparable readings: Adams (94.7% good, AQI 19); Asotin (73.8% good, AQI 35); Benton (88.9% good, AQI 35); Chelan (87.2% good, AQI 27); Clallam (88.5% good, AQI 26); Clark (89.3% good, AQI 31).
Air Quality Summary
Good Days
67.5%
164 of 243 monitored days
-7.5pp vs 75% target
Max AQI Recorded
90
Moderate
Health Grade
Moderate
90th-pct AQI 61
King County, Good days share 67.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
King County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 243 monitored days across EPA AQI categories.
Good
164
67%
Moderate
79
33%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in King County.
- PM2.5 · 138 days · 56.8%
- Ozone · 98 days · 40.3%
- NO₂ · 7 days · 2.9%
Health Impact Assessment
Moderate
King County's air quality is rated Moderate based on 67.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 67.5% of monitored days in 2025, with a median AQI of 43. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 138 of 243 monitored days.
5-Year Air Quality Trend
Air quality is stable in King County. Best year: 2024 (255 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 248 | 106 | 3 | 4 | 43 | 299 |
| 2021 | 248 | 112 | 3 | 2 | 43 | 177 |
| 2022 | 216 | 130 | 14 | 3 | 46 | 237 |
| 2023 | 214 | 145 | 5 | 1 | 46 | 165 |
| 2024 | 255 | 108 | 3 | 0 | 41 | 119 |
| 2025 | 164 | 79 | 0 | 0 | 43 | 90 |
How does King County compare?
How King County compares
Good-day share worse than US avg
King County 67.5%
Adams County, Washington 94.7%
US average 76.5%
Median AQI worse than US avg
King County 43
Adams County, Washington 19
US average 38
Unhealthy days / yr about average
King County 0
Adams County, Washington 0
US average 0
Other Counties in Washington
Air quality comparison for nearby counties in Washington.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to King County.
Allegan County, Michigan
AQI 43 · 71.2% good
Allen County, Ohio
AQI 43 · 65.9% good
Ashtabula County, Ohio
AQI 43 · 81.2% good
Bell County, Kentucky
AQI 43 · 63.5% good
Boise County, Idaho
AQI 43 · 72.9% good
Brazos County, Texas
AQI 43 · 63.4% good
What King County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 43 means for exposure. Health guidance
- See how King County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare King County with its Washington neighbours before drawing a conclusion about the region. All of Washington
Based on 243 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in King County, Washington? ▼
King County has a median Air Quality Index (AQI) of 43, rated "Good." 67.5% of monitored days (164 out of 243) had good air quality in 2025.
What is the main pollutant in King County? ▼
The primary pollutant in King County is Fine Particles (PM2.5), which was the main AQI driver on 138 days (57% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does King County have? ▼
In 2025, King County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 90.
Is the air quality in King County getting better or worse? ▼
Air quality in King County is stable based on 5-year trends (2020-2025). Best year: 2024 with 255 good days. Worst year: 2025 with 164 good days.
What does AQI 43 mean for health? ▼
King County's median AQI of 43 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does King County compare to other counties in Washington? ▼
King County has 67.5% good air days. Nearby counties in Washington: Adams (94.7% good), Asotin (73.8% good), Benton (88.9% good), Chelan (87.2% good), Clallam (88.5% good), Clark (89.3% good).
What is the 90th percentile AQI in King County? ▼
The 90th percentile AQI for King County is 61, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 90 (Moderate).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.