Washington Air Quality: The Seasonal Shape

Q: Which months have the worst air quality in Washington?

In EPA's monitoring record for 2020 to 2024, the worst stretch in Washington is August through October. September leads: 7.9% of monitored days reached Unhealthy for Sensitive Groups or worse and 28% were at least Moderate, with the index most often set by fine particles (PM2.5). August runs second. The month-by-month chart on this page shows the full shape.

Q: When is smoke season in Washington?

The data puts it in late summer: the months ranked worst in Washington are August through October, driven by fine particles, which in that part of the calendar means smoke. The season tracks the fire map upwind rather than a fixed date, so treat those months as the watch window rather than a guarantee either way.

Q: What is the cleanest month for air quality in Washington?

May, in the 2020 to 2024 record: only 4% of monitored days even reached Moderate, and 0.0% went beyond it. If you are choosing a month for a visit or an outdoor project on air quality grounds, that is the statistical answer.

Q: Where do I check the live AQI for Washington today?

AirNow.gov, EPA's official public AQI service, publishes the current reading and forecast at its Washington state page. This page adds the calendar (which months are like this every year, and why) plus the live NWS alert products above, reproduced as issued.

Q: How is this monthly calendar computed?

From EPA's Air Quality System pre-generated daily AQI files, the official regulatory record: every daily AQI value for every monitored Washington county, 2020 to 2024, grouped by calendar month into EPA's six categories. The snapshot is rebuilt annually when EPA finalizes a new year. It is climatology, not a forecast: it says what the months are usually like, not what next week holds.

Smoke season. Which months are worst in Washington, why, and what the EPA record actually shows.

Worst stretch in the EPA record: August through October

Worst month

September

Cleanest month

May

Season type

Smoke season

Washington air quality at a glance

EPA record, 2020 to 2024

September

worst month

7.9% of days USG or worse

fine particles

sets the index then

statewide, in September

AQI over 500

worst single day

September 12, 2020

May

cleanest month

4% of days even reach Moderate

Computed from EPA's official daily AQI record for every monitored county in Washington, 2020 to 2024. Shares are of monitored county days, so they describe how often air somewhere in the state crossed each line.

The calendar

Which months are worst for air quality in Washington

The chart below is five years of EPA's regulatory monitoring record, 2020 to 2024, folded into a single year: for each month, the share of monitored days in each official AQI category, in EPA's exact colors. Under each bar is the pollutant that most often set the index that month, which is the mechanism in one word: ozone is a sunshine chemistry problem, PM2.5 in winter is a trapped-air problem, PM2.5 in late summer is usually smoke.

JanPM2.5

FebPM2.5

MarPM2.5

AprPM2.5

MayPM2.5

JunPM2.5

JulPM2.5

AugPM2.5

SepPM2.5

OctPM2.5

NovPM2.5

DecPM2.5

Share of monitored days in each EPA AQI category by month, all Washington counties with monitors, 2020 to 2024. Source: EPA Air Quality System daily AQI files.

The three worst months, ranked

September

7.9% of monitored days reached Unhealthy for Sensitive Groups or worse, and 28% at least Moderate. The index was most often set by fine particles (PM2.5). Worst single day: AQI over 500 on September 12, 2020.

August

4.2% of monitored days reached Unhealthy for Sensitive Groups or worse, and 27% at least Moderate. The index was most often set by fine particles (PM2.5). Worst single day: AQI 447 on August 19, 2023.

October

2.0% of monitored days reached Unhealthy for Sensitive Groups or worse, and 26% at least Moderate. The index was most often set by fine particles (PM2.5). Worst single day: AQI 362 on October 19, 2022.

Ranked by share of days reaching Unhealthy for Sensitive Groups or worse, the first category EPA ties to health effects for anyone. A single bad fire year inside the window can move a month; that is not noise, that is what living with a smoke era means.

The mechanism

Why Washington air goes bad when it does

Washington's air is excellent until it abruptly is not: September leads the data because that is when east winds and peak fire season align, and a normally clean Seattle or Spokane can spend a week in the worst air in the world. Winter stagnation is the second, milder season.

Wildfire smoke from the Cascades, eastern Washington, Oregon and British Columbia, worst in August and September.
Easterly wind events that pour smoke over the Cascades onto the Puget Sound metros.
Winter wood smoke under stagnation, the state's quieter cool-season pattern.

“Smoke from wildfires is the largest source of particle pollution in Washington. Breathing in smoke causes wheezing and coughing, heart and lung disease, and death.”
Washington State Department of Ecology, Wildfire smoke information, quoted verbatim.

Satellite view of wildfire smoke filling every mountain valley across a wide stretch of the northern Rockies, with active fire detections marked in red — Wildfire smoke filling the valleys of the Sawtooth Range and Salmon River Mountains, photographed by NASA's MODIS on September 12, 2022. This is what a smoke episode looks like from orbit: not a plume but a flood, following the terrain. The same valley-filling behavior drives Washington's smoke season. “Smoke Shrouds the Sawtooth Range and Salmon River Mountains (MODIS 2022-09-12)” — MODIS Land Rapid Response Team, NASA GSFC — Public domain

Live, as issued

Air quality alerts in Washington right now

These are the active National Weather Service air quality products whose zones touch Washington, reproduced as issued with the issuing office attached. An Air Quality Alert is written by or for your state's air agency; NWS also issues its own smoke, dust and stagnation advisories. Each is reproduced in full.

Checking api.weather.gov for active air quality alert products…

Today's AQI is at AirNow

EPA's AirNow page for Washington publishes the current reading and the official forecast, fed by the same agencies whose alerts appear above. This page carries the other half of the picture: which months are like this every year, and why.

EPA air sampling instruments on tripods inside a fenced monitoring site, with a technician in an EPA contractor shirt walking past — EPA air sampling stations at a monitoring site. Every number on this page traces back to instruments like these: the monthly chart is computed from EPA's official daily AQI record for every monitored Washington county, 2020 to 2024. “EPA Air Sampling Stations” — US EPA — Public domain

Reading the index

The AQI scale, in EPA's words

Every category on this page uses EPA's official names, ranges and colors, quoted rather than paraphrased. The line that matters most for planning is 101: Unhealthy for Sensitive Groups, the first category EPA ties to health effects, and the threshold the worst-month ranking above counts.

AQI	Category	What it means
0 to 50	Good	Air quality is satisfactory, and air pollution poses little or no risk.
51 to 100	Moderate	Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
101 to 150	Unhealthy for Sensitive Groups	Members of sensitive groups may experience health effects. The general public is less likely to be affected.
151 to 200	Unhealthy	Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
201 to 300	Very Unhealthy	Health alert: The risk of health effects is increased for everyone.
301 and higher	Hazardous	Health warning of emergency conditions: everyone is more likely to be affected.

Category names, index ranges and colors are reproduced exactly from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Tables 1 and 2. Meanings quoted from AirNow.gov, AQI Basics.

Who EPA means by sensitive groups

When this pollutant has an AQI above 100	EPA lists these sensitive groups
Ozone	People with lung disease, such as asthma; children, including teenagers; people who are active outdoors; and older adults.
Fine particle pollution (PM2.5) and particle pollution (PM10)	People with heart or lung disease; older adults; and children, including teenagers. In addition, some minority groups, people with lower incomes, and outdoor workers may experience higher exposure that can worsen underlying health conditions.
Carbon monoxide (CO)	People with heart disease.
Nitrogen dioxide (NO2)	People with lung disease, such as asthma; children, including teenagers; and older adults.
Sulfur dioxide (SO2)	People with asthma; children, including teenagers; and older adults.

What EPA says sensitive groups should do on an orange day

Sensitive groups: Make outdoor activities shorter and less intense. Take more breaks. Watch for symptoms such as coughing or shortness of breath. Plan outdoor activities in the morning when ozone is lower. People with asthma: Follow your asthma action plan and keep quick-relief medicine handy.
EPA cautionary statement for Ozone (8-hour), Unhealthy for Sensitive Groups, quoted from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Table 5.

Sensitive groups: Make outdoor activities shorter and less intense. It's OK to be active outdoors but take more breaks. Watch for symptoms such as coughing or shortness of breath. People with asthma: Follow your asthma action plan and keep quick relief medicine handy. People with heart disease: Symptoms such as palpitations, shortness of breath, or unusual fatigue may indicate a serious problem. If you have any of these, contact your health care provider.
EPA cautionary statement for PM2.5 (24-hour), Unhealthy for Sensitive Groups, quoted from the US EPA, Technical Assistance Document for the Reporting of Daily Air Quality (EPA-454/B-24-002, May 2024), Table 5.

Everything above is EPA’s language, quoted. On any day that matters, the decision belongs with the official product for your location and your own clinician.

San Francisco Bay Bridge and downtown skyline at midday under a dark orange sky during the September 2020 wildfire smoke event — September 9, 2020: the Bay Bridge at midday under smoke from the North Complex fire. The orange sky day became the West's reference point for how completely a smoke season can rewrite daylight. “North Complex smoke in San Francisco - Bay Bridge and Financial District” by Christopher Michel, CC BY 2.0

Closer to home

Washington metro pages

Statewide numbers blur the places where the question is actually asked. These metros have enough of an air quality calendar to deserve their own page, with the same five year EPA record cut to the metro area.

Common questions

Washington air quality FAQ

Which months have the worst air quality in Washington?

When is smoke season in Washington?

Why is air quality bad in Washington in September?

What is the cleanest month for air quality in Washington?

Where do I check the live AQI for Washington today?

What does Unhealthy for Sensitive Groups actually mean?

EPA's definition, verbatim: "Members of sensitive groups may experience health effects. The general public is less likely to be affected." It covers AQI 101 to 150, shown in orange. Who counts as a sensitive group depends on the pollutant; for ozone EPA lists "People with lung disease, such as asthma; children, including teenagers; people who are active outdoors; and older adults." The full pollutant-by-pollutant table is on this page, quoted from EPA's Technical Assistance Document.

Should I run or exercise outside today in Washington?

Check the current category on AirNow, then apply EPA's cautionary statement for that category. For orange days and ozone, EPA's guidance reads: "Sensitive groups: Make outdoor activities shorter and less intense. Take more breaks. Watch for symptoms such as coughing or shortness of breath. Plan outdoor activities in the morning when ozone is lower." The equivalent statements for other pollutants are quoted in the scale section above.

How is this monthly calendar computed?

An airport control tower barely visible through dense orange wildfire smoke during the June 2023 Canadian smoke event over the New York area — The New York City area at midday on June 7, 2023, under Canadian wildfire smoke. The episode ended the idea that distance from the fire belt is protection: every calendar on this site now carries a smoke wildcard. “Quebec Canada Wildfire Smoke Consumes New Jersey and New York City June 7 2023” by Anthony Quintano, CC BY 2.0

Everywhere else

Air quality seasons in other states

Washington tick forecastThe other thing to check before a long outdoor day, especially in brushy or grassy country.Washington mosquito forecastWarm stagnant evenings that hold haze are the same evenings mosquitoes love.Air quality seasons nationwideSmoke, ozone, inversion and dust seasons across every state, on one page.

Monthly figures are computed from EPA Air Quality System daily AQI files (public domain regulatory data), 2020 to 2024, refreshed annually. Alerts are read live in your browser from api.weather.gov and reproduced as issued. AQI category definitions are EPA's, quoted verbatim. Today's measured reading is on EPA's AirNow page for Washington.
Hero image: “Smokey skyline view of Downtown Seattle from Rizal Bridge - Sept 11, 2020” by SounderBruce, CC BY-SA 4.0