Median AQI
41
Good
Washington · County air quality · 2025
Snohomish County, Washington
Median AQI 41 (Good). 69.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 69.1%
- Good-air days
- 0
- Unhealthy days
- 30%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Snohomish County's air is good on balance, more polluted than the median US county, a median AQI of 41 with 69.1% of 243 monitored days in the Good band, driven mainly by PM2.5.
- 41
- median AQI · Good
- 69.1%
- days rated Good
- PM2.5
- main pollutant
- 30%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 87.
Data updated July 2026
- Good · 168 d · 69.1%
- Moderate · 75 d · 30.9%
How did Snohomish County's air break down in 2025?
Across 243 monitored days, 168 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 87 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 243 days monitored. Median AQI: 41 (Good).
Insights: Snohomish County, Washington. EPA monitors recorded a median Air Quality Index of 41 across 243 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 168 (69%) fell in the Good band (AQI 0–50), 75 (31%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 62, while the worst single-day peak hit 87 (Moderate). Residents averaged about 20.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (243 days). Across the 2020–2025 trend window, good days shifted by -66 and the median AQI moved +1 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 87, 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
69.1%
168 of 243 monitored days
-5.9pp vs 75% target
Max AQI Recorded
87
Moderate
Health Grade
Moderate
90th-pct AQI 62
Snohomish County, Good days share 69.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Snohomish 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
168
69%
Moderate
75
31%
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 Snohomish County.
- PM2.5 · 243 days · 100.0%
Health Impact Assessment
Moderate
Snohomish County's air quality is rated Moderate based on 69.1% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 69.1% of monitored days in 2025, with a median AQI of 41. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 243 of 243 monitored days.
5-Year Air Quality Trend
Air quality is improving in Snohomish County. Best year: 2024 (262 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 234 | 122 | 3 | 3 | 40 | 250 |
| 2021 | 248 | 114 | 3 | 0 | 35 | 137 |
| 2022 | 210 | 129 | 13 | 6 | 39 | 362 |
| 2023 | 246 | 116 | 2 | 1 | 38 | 175 |
| 2024 | 262 | 104 | 0 | 0 | 38 | 99 |
| 2025 | 168 | 75 | 0 | 0 | 41 | 87 |
How does Snohomish County compare?
How Snohomish County compares
Good-day share worse than US avg
Snohomish County 69.1%
Adams County, Washington 94.7%
US average 76.5%
Median AQI worse than US avg
Snohomish County 41
Adams County, Washington 19
US average 38
Unhealthy days / yr about average
Snohomish 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 Snohomish County.
Albemarle County, Virginia
AQI 41 · 77.9% good
Anderson County, South Carolina
AQI 41 · 90.0% good
Berkshire County, Massachusetts
AQI 41 · 71.6% good
Berrien County, Michigan
AQI 41 · 80.9% good
Blair County, Pennsylvania
AQI 41 · 71.9% good
Bonner County, Idaho
AQI 41 · 71.3% good
What Snohomish 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 41 means for exposure. Health guidance
- See how Snohomish County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Snohomish 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 Snohomish County, Washington? ▼
Snohomish County has a median Air Quality Index (AQI) of 41, rated "Good." 69.1% of monitored days (168 out of 243) had good air quality in 2025.
What is the main pollutant in Snohomish County? ▼
The primary pollutant in Snohomish County is Fine Particles (PM2.5), which was the main AQI driver on 243 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Snohomish County have? ▼
In 2025, Snohomish County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 87.
Is the air quality in Snohomish County getting better or worse? ▼
Air quality in Snohomish County is improving based on 5-year trends (2020-2025). Best year: 2024 with 262 good days. Worst year: 2025 with 168 good days.
What does AQI 41 mean for health? ▼
Snohomish County's median AQI of 41 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 Snohomish County compare to other counties in Washington? ▼
Snohomish County has 69.1% 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 Snohomish County? ▼
The 90th percentile AQI for Snohomish County is 62, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 87 (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.