Washington · County air quality · 2025
Mason County, Washington
Median AQI 23 (Good). 90.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 23
- Median AQI
- 90.4%
- Good-air days
- 1
- Unhealthy days
- 81%
- Cleaner than US counties
Mason County's 2025 annual median AQI is 23 (Good), with 90.4% Good days
According to the U.S. EPA Air Quality System, Mason County, Washington posted a 2025 median AQI of 23 (Good) across 365 monitored days, with 90.4% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Mason County's 90.4% Good-day share places it cleaner than 81% of US counties in 2025.
- Closest median-AQI peer
- Deschutes County (89.3% Good, median AQI 23) is the closest nationwide median-AQI peer to Mason County.
- 2020–2025 trend
- Mason County's floor-qualifying Good-day share is stable from 2020 to 2025 (88.6% → 90.4%).
- Median AQI
- Median AQI: 23
According to the 2025 EPA AQS county release on PlainAirData, Mason County, Washington ranks #51 of 951 by median AQI (23) and #182 by good-day share (90.4%). 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
- Mason County's peak single-day AQI rose from 73 to 159 between the 2024 and 2025 EPA annual releases, a change of +86 points.
- Mason County's Good-day share fell from 93.2% to 90.4% between the 2024 and 2025 EPA annual releases, a change of -2.8 percentage points.
- Mason County's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.
Data updated August 2026
- Good · 330 d · 90.4%
- Moderate · 34 d · 9.3%
- Unhealthy · 1 d · 0.3%
How did Mason County's air break down in 2025?
Across 365 monitored days, 330 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 159 (Unhealthy).
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 Mason County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Mason County's Good-day share is 90.4% - cleaner than 81% of that comparison set.
Mason County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
90.4% Cleaner than 81% higher than 81% 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 Mason County.
- PM2.5 · 365 days · 100.0%
2024 daily-monitor rhythm in Mason County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Dec posted the highest typical readings (median AQI 31, peak 69); May was cleanest at median 10. Summer months (Jun–Aug) averaged median AQI 18 vs 26 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,812 daily readings show a recurring local pattern: Nov has the highest matched-month median AQI (39), while Jun is lowest (14). 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
Mason County's air quality is rated Excellent based on 90.4% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 0 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 159. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 365 of 365 monitored days.
5-Year Air Quality Trend
Air quality is stable in Mason County. Highest Good-day share: 2021 (93.9%, 339 of 361 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 312 | 36 | 2 | 2 | 23 | 397 |
| 2021 | 339 | 22 | 0 | 0 | 22 | 70 |
| 2022 | 302 | 60 | 3 | 0 | 23 | 125 |
| 2023 | 313 | 49 | 1 | 0 | 24 | 106 |
| 2024 | 341 | 25 | 0 | 0 | 21 | 73 |
| 2025 | 330 | 34 | 0 | 1 | 23 | 159 |
How does Mason County compare?
How Mason 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 Mason 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.