Seattle Air Quality (AQI)
Washington · Pop. 4M metro
One of the cleanest major US cities — except wildfire smoke season
Good
PM2.5: 6.1 μg/m³ · Ozone AQI: 44
Seattle Monthly AQI Pattern
April–June: cleanest months. August–September: wildfire smoke risk.
Seattle's Two Air Quality Seasons
The Clean Season (Oct–Jun)
Pacific storms bring consistent rainfall that washes PM2.5 from the air. Wind direction is predominantly onshore (west/southwest), delivering marine air that has no pollution sources for thousands of miles. Seattle's April–June is among the cleanest air of any major US city.
- PM2.5 averages 3–5 μg/m³ in spring — well below WHO guidelines
- April is statistically Seattle's cleanest month
- Exceptions: wood smoke inversions in winter (Nov–Feb) when winds calm
The Smoke Season (Jul–Sep)
As the Pacific high pressure builds, winds shift offshore (east/northeast). This pattern draws smoke from the dry eastside fires and Pacific Coast megafires into the Puget Sound basin. Once smoke is trapped under high pressure, it can persist for days with no rain to clear it.
- 2020: 7+ consecutive hazardous AQI days — all-time worst
- 2023: Canadian fires pushed AQI above 100 for 10+ days
- Climate trend: smoke season starting earlier, lasting longer
Seattle Smoke Season Preparedness
- •Puget Sound Clean Air Agency (pscleanair.gov) — local alerts
- •AirNow.gov Fire and Smoke Map — regional overview
- •Purple Air community sensors — neighborhood-level PM2.5
- •NWSSeattle smoke forecasts during active events
- •HEPA air purifier: size appropriately (CADR ≥ 2/3 of room sq ft)
- •Seal gaps under doors with towels or draft stoppers
- •Set HVAC to recirculate (not fresh air intake) during smoke events
- •DIY Corsi-Rosenthal box fan + MERV-13 filters: ~$50, very effective
- •N95 respirator (NIOSH-approved): surgical masks do not filter PM2.5
- •Limit strenuous outdoor activity when AQI > 100
- •No outdoor exercise when AQI > 150
- •Cyclists: check air quality before morning commutes Jul–Sep
- •Asthma: prepare written action plan, refill inhalers before smoke season
- •Elderly: most sensitive — stay indoors when AQI > 100
- •Children: cancel outdoor recess and sports when AQI > 100
- •Pets: dogs need shorter walks; don't let them exercise hard in smoke
Seattle Air Quality FAQ
Why does Seattle have smoke every summer?
Seattle's summer smoke comes primarily from wildfires in Eastern Washington, British Columbia, Oregon, and increasingly California. The Cascade Mountains separate the wet westside from the dry eastside fire country. Upper-level winds, particularly ridge-of-high-pressure patterns, channel smoke from hundreds of miles away into the Puget Sound lowlands. Climate change is making Washington's fire season longer — fire starts that used to be confined to August now extend from July into October.
When was Seattle's worst air quality on record?
September 2020 was Seattle's worst air quality on record. The combined smoke from the Oregon and California Labor Day fires, plus the Cascade fires, created sustained hazardous air quality (AQI 200+) for over a week. The sky turned an eerie orange-red. Prior worst events: August 2018 (Hicks/Rattlesnake fires), August 2015 (Okanogan Complex — largest in WA history). 2023 was also notable when Canadian wildfire smoke affected the entire Pacific Northwest.
Is Seattle good or bad for air quality overall?
Seattle has excellent baseline air quality — one of the best among major US cities. Frequent Pacific Ocean rainfall (38 inches per year) washes pollution from the air. There are no heavy industrial facilities in the city core. The main air quality problems are episodic: wildfire smoke in August–September and residential wood burning in winter. If you pick the right months to visit (April–June, October–November), Seattle's air is among the cleanest in the US.
How do I prepare for Seattle smoke season?
Essential preparation: (1) Get a HEPA air purifier sized for your living space — run it on high when AQI exceeds 100. (2) Stock N95 respirators — check the NIOSH approval number. (3) Download Puget Sound Clean Air Agency's app for local air quality alerts. (4) Seal door and window gaps with weatherstripping. (5) Learn to make a DIY box fan air purifier (Corsi-Rosenthal box) with Merv-13 filters — effective and low-cost backup.