Seattle air quality this fall: no smoke, a Moderate peak of 68, and burn ban season next

Seattle air quality stayed Good or Moderate on every day from 15 September to 23 October 2026. In Good Air's archive of EPA AirNow readings, Seattle's daily US AQI averaged 48 over those 39 days, peaked at 68 (Moderate) on 8 October and never reached 101, the level where air becomes Unhealthy for Sensitive Groups.

That is a quiet autumn for a city whose summers are now often smoky. Wildfire smoke had cleared from western Washington by late September. The next risk is different: wood smoke from fireplaces and stoves, trapped near the ground on cold, still nights. That is when the Puget Sound Clean Air Agency calls burn bans.

For the reading right now, see the live Seattle AQI page. When we checked it at 8:58 pm Pacific time on 23 October, it showed a US AQI of 55 (Moderate), with PM2.5 as the main pollutant.

Seattle's air from 15 September to 23 October 2026

• Average daily US AQI: 48, just inside the Good band (0 to 50). The median day was also 48.
• 16 of 39 days were Moderate (a daily average above 50). None was worse than Moderate.
• The worst days were 8 October (68), 5 October (64) and 22 October (64). The cleanest was 29 September (28).
• October was a little worse than September. Seattle averaged 45 from 15 to 30 September and 50 from 1 to 23 October.
• No Puget Sound monitor came close to a smoke day. The highest daily PM2.5 at any of the 17 monitors we checked was 16.9 µg/m³, at Seattle's South Park monitor on 8 October. That is less than half of 35 µg/m³, the level the Puget Sound Clean Air Agency uses to call a burn ban in King County.

Here is how Seattle compares with three other Pacific Northwest cities that have their own AirNow readings. All figures are daily averages of the hourly US AQI.

Seattle and Portland moved in step for much of the period, and both had their second-worst stretch on 21 and 22 October. In our earlier ranking of 266 US cities from 15 September to 19 October, Seattle placed 39th, with dirtier air on average than Chicago (77th), New York City (97th) or Boston (202nd) (most polluted US cities this autumn).

Why the worst days were in October

Smoke was not the cause. Washington's interagency smoke blog closed its 2026 season on 30 September, saying wet, cool weather had helped wind down the wildfire season, with some smoke lingering near fires in the Cascades (Washington Smoke Blog, 30 September 2026). Its mid-September posts described smoke pooling in Cascade valleys such as Lake Chelan and Cle Elum, not in Seattle (Washington Smoke Blog, 18 September 2026).

The bad days instead line up with calm, foggy or dry weather, when local pollution builds up instead of blowing away:

• 8 October (68). Seattle woke under fog and mist that morning, before a warm afternoon close to 70°F. A weak front with light rain arrived that night (Hoodline, 8 October 2026). A National Weather Service meteorologist put the fog down to cool temperatures, moisture and limited air movement (MyNorthwest, 8 October 2026). The next day, after the front, Seattle's AQI fell to 37.
• 21 and 22 October (58 and 64). By 21 October Seattle had recorded only 0.08 inches of rain all month, the 5th driest October on record to that point (FOX 13 Seattle, 21 October 2026). Hourly readings on 22 October peaked at 78 around 1 am and 77 around 10 am Pacific time, then fell to the mid-30s by mid-afternoon. Overnight into 22 October, monitors in Tukwila, Tacoma and Auburn reached hourly PM2.5 levels of about 23 µg/m³.

We can't say exactly which sources pushed each day up. AirNow's city readings report the leading pollutant, which was PM2.5 in all but 7 of the 192 hours we hold from 16 to 23 October, but not where it came from.

One reading we don't rely on: on 10 October, Seattle's city feed logged a single hour at 215 (Very Unhealthy), while the day averaged 53. We could not match it to any reported event, and only the South Park monitor was raised that day (16.4 µg/m³), so it was either very local or a faulty reading.

Which parts of Puget Sound had the dirtiest air

Averaged over the whole period, the monitors with the most fine particles (PM2.5) were in Seattle's industrial south end and the Green River valley. The cleanest in the core urban area were in Bellevue and Everett.

Nine µg/m³ matters because it is where the US AQI for PM2.5 moves from Good to Moderate. South Park ranked 127th of the 1,110 US monitors with enough data, so even the dirtiest Puget Sound site was nowhere near the top of the national list.

Seattle's bigger long-term problem is short smoke episodes. The American Lung Association's State of the Air 2026 report ranked Seattle-Tacoma 8th worst in the US for short-term particle pollution, driven by wildfire smoke, but outside the worst 25 for year-round particles and ozone (KOMO News, 18 May 2026).

What comes next: wood smoke and burn bans

Until early spring, Seattle's air depends mostly on the weather. Rainy, windy spells, including atmospheric rivers, flush pollution out. Cold, clear, still spells do the opposite: warm air traps cold air near the ground (an inversion), and smoke from fireplaces and wood stoves builds up overnight.

The Puget Sound Clean Air Agency calls burn bans for King, Kitsap, Pierce and Snohomish counties. Bans "typically occur during fall and winter months" and usually last a few days (Puget Sound Clean Air Agency, About Air Quality Burn Bans, viewed 24 October 2026):

• Stage 1 is called when stagnant weather is forecast to push the 24-hour PM2.5 average above 35 µg/m³ within 48 hours in King and Kitsap counties, or above 30 µg/m³ within 72 hours in Pierce and Snohomish. Wood fireplaces and uncertified wood stoves and inserts are banned, and so is all outdoor burning. EPA-certified stoves may still be used, but must not make visible smoke.
• Stage 2 is called when levels reach a higher trigger set by state law, and can come without a Stage 1 first. All wood fireplaces, wood stoves and inserts, certified or not, and pellet stoves are banned.
• The only exemption is for homes where the stove is the only adequate source of heat and the owner has an approved exemption.

The agency's FAQ says Pierce and Snohomish counties get burn bans more often than King or Kitsap. Inspectors who see a violation issue a Notice of Violation to the property owner (Puget Sound Clean Air Agency, Burn Ban FAQs, viewed 24 October 2026). You can check the current status on the agency's burn ban status page or the burn ban line, 800-595-4341.

We found no 2026–27 burn ban announced as of 24 October. FOX 13 forecast a weak system that could bring rain late Saturday into Sunday morning, then high pressure rebuilding early next week (FOX 13 Seattle, 21 October 2026). When the first ban comes depends on the first long cold, calm spell; nobody can say yet.

Read how the Bay Area's wood burning ban works, or how Salt Lake City's inversion season builds up.

How to check Seattle's AQI today, and what the numbers mean

Our Seattle page shows the live US AQI from EPA AirNow, the main pollutant, every nearby monitor and a 90-day history. For towns north of the city, use Marysville or Bellingham. For Oregon, see Portland.

The US AQI runs from 0 to 500 (AirNow, AQI basics, viewed 24 October 2026):

For runners and cyclists, our guide on whether it's safe to exercise outside at each AQI gives level-by-level advice. If smoke does come back, our wildfire smoke safety guide covers masks and air purifiers.

How we measured

• Data. Hourly readings from EPA's AirNow system, collected by Good Air: the US AQI AirNow publishes for each city reporting area (Seattle, Portland, Marysville, Bellingham), and PM2.5 from individual government monitors. No low-cost sensors, satellites or models.
• City values. The mean of each day's hourly US AQI, counting days with at least 12 hourly readings (all 39 days qualified for all four cities). This is not EPA's official daily AQI, which uses a 24-hour average for PM2.5, so it can differ a little from AirNow's daily figures. Tacoma, Everett and Bellevue have no city reading of their own in our archive, so we use their monitors.
• Monitor values. The mean of daily PM2.5 averages, counting days with at least 18 hourly readings. Most monitors had all 39 days; Marysville had 33, Beacon Hill 34 and Duwamish 36. The US median uses 1,110 monitors with at least 33 days.
• Period. 15 September to 23 October 2026, days in UTC. Hourly detail is only kept for the last eight days, so the hourly figures cover 16 to 23 October.