Seattle Air Quality in Winter
Across 2019–2025, Seattle's winter (December–February) air quality had a median AQI of 55 (Moderate) over 632 monitored days in the Seattle-Tacoma-Bellevue, WA area. 1.3% of winter days (8 of 632) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 81.5% of days. The cleanest winter on record here was 2020 (median AQI 51, 0 days above 100) and the worst was 2022 (median 59, 4 days above 100, peak AQI 108).
- Good: 33.2%
- Moderate: 65.5%
- Unhealthy for Sensitive Groups: 1.3%
- Unhealthy or worse: 0.0%
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How is the air in Seattle in winter?
Moderate on a typical day: median AQI 55 across 2019–2025, with 1.3% of winter days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓33.2% of winter days were Good (AQI 0–50), 65.5% Moderate (51–100).
- ✓8 days of 632 reached AQI 101+; the winter peak was 142.
- ✓Cleanest month: February; worst month: January.
Frequently asked questions
What is the air quality like in Seattle in winter?▾
Across 2019–2025, the median daily AQI in Seattle in winter was 55 (Moderate). 33.2% of days were Good, 65.5% Moderate, and 1.3% reached Unhealthy for Sensitive Groups or worse, based on 632 monitored days in EPA's Air Quality System.
Is winter a good time for outdoor exercise in Seattle?▾
A typical winter day in Seattle is Moderate (median AQI 55): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 77 of winter days (1.3%) 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 winter air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 81.5% of winter days in Seattle. The highest winter AQI recorded in 2019–2025 was 142 (Unhealthy for Sensitive Groups), in 2022.
Seattle winter air quality, year by year
Seattle winter 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 | 90 | 62 | 2 | 142 | PM2.5 |
| 2020cleanest | 91 | 51 | 0 | 99 | PM2.5 |
| 2021 | 90 | 55 | 2 | 108 | PM2.5 |
| 2022worst | 90 | 59 | 4 | 108 | PM2.5 |
| 2023 | 90 | 54 | 0 | 85 | PM2.5 |
| 2024 | 91 | 54 | 0 | 96 | PM2.5 |
| 2025 | 90 | 57 | 0 | 89 | PM2.5 |
What winter air means in Seattle
A typical winter day in Seattle is Moderate (median AQI 55): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 77 of winter days (1.3%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Winter's median AQI of 55 is above Seattle's whole-year median of 49, making it one of the area's dirtier periods. Nationally, the median Winter AQI across every EPA metro area (278,620 metro-days, 2019–2025) is 40, so Seattle runs above the national Winter norm; 1.3% of its Winter days topped AQI 100 versus 0.6% nationally (about the same).
Within the season, February is the cleanest month (median AQI 54, 0.0% of days above 100) and January the worst (median 58, 1.8% above 100).
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Seattle
The latest EPA AirNow observation puts Seattle's AQI at 37 (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.