Seattle Air Quality in Fall
Across 2019–2025, Seattle's fall (September–November) air quality had a median AQI of 52 (Moderate) over 637 monitored days in the Seattle-Tacoma-Bellevue, WA area. 5.4% of fall days (34 of 637) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 83.5% of days. The cleanest fall on record here was 2021 (median AQI 47, 0 days above 100) and the worst was 2022 (median 63, 22 days above 100, peak AQI 362).
- Good: 43.5%
- Moderate: 51.2%
- Unhealthy for Sensitive Groups: 2.4%
- Unhealthy or worse: 3.0%
Improve Indoor Air Quality
Even with good outdoor air in Seattle, indoor PM2.5, dust and allergens build up. A True-HEPA purifier keeps a bedroom or office consistently clean.
Recommended: Levoit Core 400S — True HEPA · CADR ~141 CFM · rooms up to 219 sq ft
As an Amazon Associate, we earn from qualifying purchases.
How is the air in Seattle in fall?
Moderate on a typical day: median AQI 52 across 2019–2025, with 5.4% of fall days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓43.5% of fall days were Good (AQI 0–50), 51.2% Moderate (51–100).
- ✓34 days of 637 reached AQI 101+; the fall peak was 362.
- ✓Cleanest month: November; worst month: September.
Frequently asked questions
What is the air quality like in Seattle in fall?▾
Across 2019–2025, the median daily AQI in Seattle in fall was 52 (Moderate). 43.5% of days were Good, 51.2% Moderate, and 5.4% reached Unhealthy for Sensitive Groups or worse, based on 637 monitored days in EPA's Air Quality System.
Is fall a good time for outdoor exercise in Seattle?▾
A typical fall day in Seattle is Moderate (median AQI 52): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 19 of fall days (5.4%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes Seattle's fall air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 83.5% of fall days in Seattle. The highest fall AQI recorded in 2019–2025 was 362 (Hazardous), in 2022.
Seattle fall air quality, year by year
Seattle fall air quality by year (Seattle-Tacoma-Bellevue, WA, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 91 | 54 | 0 | 94 | PM2.5 |
| 2020 | 91 | 53 | 11 | 299 | PM2.5 |
| 2021cleanest | 91 | 47 | 0 | 88 | PM2.5 |
| 2022worst | 91 | 63 | 22 | 362 | PM2.5 |
| 2023 | 91 | 52 | 0 | 92 | PM2.5 |
| 2024 | 91 | 48 | 0 | 96 | PM2.5 |
| 2025 | 91 | 52 | 1 | 125 | PM2.5 |
What fall air means in Seattle
A typical fall day in Seattle is Moderate (median AQI 52): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 19 of fall days (5.4%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Fall's median AQI of 52 is in line with Seattle's whole-year median of 49, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median Fall AQI across every EPA metro area (296,275 metro-days, 2019–2025) is 40, so Seattle runs above the national Fall norm; 5.4% of its Fall days topped AQI 100 versus 1.5% nationally (above average).
Within the season, November is the cleanest month (median AQI 54, 1.9% of days above 100) and September the worst (median 48, 8.1% above 100).
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Seattle
The latest EPA AirNow observation puts Seattle's AQI at 42 (PM2.5). The figures above are multi-year season statistics, not today's reading.
View current Seattle air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Seattle-Tacoma-Bellevue, WA), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.