Air Quality Index Seattle Today: What the Numbers Mean and What to Do

By Audrey Hayes

Updated November 2026

The air quality index for Seattle is a 0-to-500 scale that tells you how clean or polluted outdoor air is right now, with higher numbers meaning more health risk. Most days the Seattle air quality index sits in the “Good” range (0 to 50), but wildfire smoke from eastern Washington, Oregon and British Columbia can push it into “Unhealthy” or worse for days at a time. Knowing the number helps you decide whether to exercise outside, open windows or run your air purifier.

Why Seattle’s Air Quality Index Changes So Much

Seattle usually has some of the cleanest air of any large US city, thanks to steady marine winds off Puget Sound and frequent rain that washes particles out of the atmosphere. The city’s air quality index is rarely driven by local factors alone. Instead, the big swings come from fire season, when smoke can travel hundreds of miles on prevailing winds and settle into the valleys and along the Sound. Temperature inversions, which trap cool air near the ground, can also hold smoke and vehicle exhaust in place for several days.

Shopping for one? See our guide: 5 Best Air Purifiers for Air Quality Index Delhi (2026): Expert Picks.

Local sources still matter. Traffic on I-5 and SR-99, residential wood smoke in colder months, and construction dust all add fine particles. These local contributions tend to show up as short spikes rather than long, sustained highs.

The Factors Behind the Number

The AQI is calculated from several pollutants, and the reported value is the highest sub-index among them. In Seattle, the two that most often set the number are:

• PM2.5: fine particles 2.5 microns or smaller. These penetrate deep into the lungs and drive most smoke-related AQI readings. Anything above roughly 35.5 micrograms per cubic meter (24-hour average) lands in the “Unhealthy for Sensitive Groups” band or higher.
• Ozone: a summer pollutant that forms when sunlight reacts with vehicle exhaust and other emissions. It is less common in Seattle than in inland cities but can appear on hot, still afternoons.

The EPA’s AQI color scale runs from green (Good, 0 to 50) through yellow (Moderate, 51 to 100), orange (Unhealthy for Sensitive Groups, 101 to 150), red (Unhealthy, 151 to 200), purple (Very Unhealthy, 201 to 300) and maroon (Hazardous, 301 and up). Children, older adults, pregnant people and anyone with asthma or heart disease should start limiting outdoor activity at the orange level.

How to Check the Number Accurately

For the most reliable reading, use the official AirNow map from the EPA or the Puget Sound Clean Air Agency’s data, which pulls from regulatory monitors. Weather apps often show a modeled estimate that can differ from what is measured at ground level, especially during smoke events where conditions change by the hour. Look at the hourly trend, not just the single current value. A reading that is climbing from 80 to 140 over three hours calls for different action than one that is steadily falling.

What to Do When the Index Rises

Your response should match the category, not just the color. Here is a practical approach:

1. Under 100 (Good or Moderate): Normal activity is fine for most people. Sensitive individuals can keep an eye on symptoms.
2. 101 to 150 (Unhealthy for Sensitive Groups): Shorten outdoor workouts, move strenuous yard work to another day, and keep rescue inhalers handy if you have asthma.
3. 151 and above (Unhealthy or worse): Stay indoors with windows and doors closed, run a HEPA air purifier on a higher fan speed, and wear a well-fitted N95 or KN95 respirator if you must go outside.

Protecting Your Home During Smoke Season

Keeping indoor air cleaner is the most effective step you can take during a smoke event. Close windows, set your HVAC system to recirculate, and replace filters with a rating of at least MERV 13 if your system can handle it. Portable air purifiers rated with a clean air delivery rate suited to your room size will reduce fine particles noticeably. Check the manufacturer’s CADR and room-size specifications before buying, since a unit sized for a small bedroom will struggle in a open-plan living room.

Avoid adding to indoor pollution during bad air days. Skip candles, incense, gas-stove use with the hood off, and vacuuming without a HEPA filter, since each of these can raise fine particle levels inside.

Practical Tips

• Bookmark the AirNow site and set a daily check before outdoor plans.
• Use a smartphone alert if your local air agency offers one.
• Pre-stock HEPA filters and N95 masks before fire season begins in late summer.
• Seal gaps around doors and window frames to reduce infiltration.
• Run your purifier continuously during smoke events rather than only when you notice odor.
• If you have a medical condition, talk with your doctor about a written action plan for high-AQI days.

What We’d Use

For a bedroom or a single living area, a compact HEPA purifier with a sealed carbon layer is a practical starting point, and View on Amazon is a widely recognized model in that category. For larger open spaces, a higher-capacity unit such as the View on Amazon is often suggested by owners in homes with more square footage. To handle the outdoor air you must breathe on the way in and out, keep a stock of respirators on hand, such as View on Amazon.

For background on how smoke and particles behave in the region, see Reno Nevada Air Quality: How to Check, Read and Protect Your Home for a comparable western-state guide, and Smoke and Air Purifier: 7 Best Picks for Wildfire Smoke (2026) for purifier selection during fire season. If you want to measure your own rooms, Indoor Air Quality Monitoring: How to Set Up and Read Your Results (2026) explains how to interpret home readings.

Related Questions

Is an AQI of 100 dangerous?

At 100 the air is in the Moderate range. Most healthy adults are fine, but people with asthma or heart conditions may notice symptoms, so pay attention to how you feel.

Should I open windows when the Seattle air quality index is high?

Not when the index is in the orange range or above. Keep windows closed and run a purifier, then ventilate briefly when readings drop.

Do HEPA purifiers remove wildfire smoke?

They capture the fine particles that make up most smoke, which lowers indoor exposure. They do not remove gases, so pair them with activated carbon if odor is a concern.

How often should I change the filter during smoke season?

Check the manufacturer’s guidance and inspect the filter regularly. Heavy use during a long smoke event can clog a filter faster than the standard schedule suggests.

When should I see a doctor about smoke exposure?

Seek care for chest pain, severe shortness of breath, persistent wheezing that does not respond to your inhaler, or confusion. Consult a healthcare professional for personalized advice.

About Audrey Hayes

Audrey Hayes is part of our editorial team and reviews and edits our indoor air guides, checking product details against manufacturer specifications and owner feedback.