Washington · County air quality · 2025
King County, Washington
Median AQI 43 (Good). 67.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 67.1%
- Good-air days
- 0
- Unhealthy days
- 26%
- Cleaner than US counties
King County's 2025 annual median AQI is 43 (Good), with 67.1% Good days
According to the U.S. EPA Air Quality System, King County, Washington posted a 2025 median AQI of 43 (Good) across 365 monitored days, with 67.1% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- King County's 67.1% Good-day share places it cleaner than 26% of US counties in 2025.
- Closest median-AQI peer
- Ashtabula County (81.2% Good, median AQI 43) is the closest nationwide median-AQI peer to King County.
- 2020–2025 trend
- King County's floor-qualifying Good-day share is stable from 2020 to 2025 (68.7% → 67.1%).
- Median AQI
- Median AQI: 43
According to the 2025 EPA AQS county release on PlainAirData, King County, Washington ranks #631 of 951 by median AQI (43) and #701 by good-day share (67.1%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- King County's Good-day share fell from 69.7% to 67.1% between the 2024 and 2025 EPA annual releases, a change of -2.5 percentage points.
- King County's peak single-day AQI fell from 119 to 97 between the 2024 and 2025 EPA annual releases, a change of -22 points.
Data updated August 2026
- Good · 245 d · 67.1%
- Moderate · 120 d · 32.9%
How did King County's air break down in 2025?
Across 365 monitored days, 245 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 97 (Moderate).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where King County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. King County's Good-day share is 67.1% - cleaner than 26% of that comparison set.
King County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
67.1% 26th percentile higher than 26% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in King County.
- PM2.5 · 217 days · 59.5%
- Ozone · 138 days · 37.8%
- NO₂ · 10 days · 2.7%
2024 daily-monitor rhythm in King County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Aug posted the highest typical readings (median AQI 48, peak 90); May was cleanest at median 37. Summer months (Jun–Aug) averaged median AQI 44 vs 46 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,827 daily readings show a recurring local pattern: Oct has the highest matched-month median AQI (52), while Jun is lowest (38). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
King County's air quality is rated Moderate based on 67.1% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 67.1% 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 217 of 365 monitored days.
5-Year Air Quality Trend
Air quality is stable in King County. Highest Good-day share: 2024 (69.7%, 255 of 366 monitored 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 | 245 | 120 | 0 | 0 | 43 | 97 |
How does King County compare?
How King County compares
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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for King County, Washington · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
Every figure on PlainAirData 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.