What was the worst air-quality day in Spokane?

September 13, 2020, when daily PM2.5 reached 396.8 µg/m³ (AQI 500), about 18.0 cigarette-equivalents. It ranks first of 6,741 monitored days since 1999.

What is the highest AQI recorded in Spokane?

On the worst day, the daily PM2.5 AQI reached 500 (Hazardous). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 957 on September 13, 2020.

Were these days caused by wildfire smoke?

This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.

How often does Spokane get air this bad?

Rarely. The record sits at the far end of 6,741 measured days; Spokane averages 2 "unhealthy" days a year.

Why did different apps show different numbers that day?

Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.

← All 100 US metros ranked

Historical PM2.5 records

Spokane's worst air quality days

#2 of 100 US metros by worst day · Spokane-Spokane Valley, WA metro area

Based on the available regulatory-monitor record since 1999, September 13, 2020 ranks as Spokane's most severe daily PM2.5 day: 396.8 µg/m³ (AQI 500, hazardous), about 18.0 cigarettes in a day.

Daily rank: #1
Period of record: Since 1999
Peak daily PM2.5: 396.8 µg/m³
About: 18.0 cigarettes
Latest event: 2 days, September 2025
Status: Finalized

Current conditions on the smoke map → Data through Feb 28, 2026 · page built July 26, 2026

Show

The worst-hit part of the metro: its highest credible monitor each day.

How this is measured sources, the daily measure, and what this ranking is not

Source. Daily PM2.5 from the U.S. EPA Air Quality System (AQS) , back to 1999, across 9 EPA monitors in the Spokane-Spokane Valley, WA metro area (CBSA). Days after February 28, 2026 are preliminary AirNow readings, marked provisional, and re-rank once EPA certifies them (usually the following spring).

The daily measure. "Worst area" is the metro's highest credible monitor each day, with lone bad-monitor spikes dropped (the raw single-monitor max before that filter is in the data but not charted). "Metro average" averages every monitor. The unit choice re-reads the same value as µg/m³, cigarette-equivalents (PM2.5 ÷ 22, Berkeley Earth, an illustration rather than a medical reading), or the EPA 2024 PM2.5 AQI.

Official AQI. EPA's reported metro AQI uses the single worst monitor of any pollutant, so it can run higher than the values here. The highest official daily AQI among the ranked days is 957 on September 13, 2020 (defining pollutant PM2.5, above the scale's usual 500 cap).

What this ranking is not. It does not attribute days to wildfire smoke: a high-PM2.5 day can be dust, fireworks, an inversion, or industry. The wildfire-smoke event ranking below arrives with a published attribution method.

Latest major event September 2025 2 of the worst days on record in one stretch, Sep 5, 2025 to Sep 6, 2025, peaking at 168.1 µg/m³, #10 all-time. See where it ranks ↓

What were Spokane's worst days?

Rank	Date	Daily PM2.5
#1	September 13, 20205-day stretch	396.8 µg/m³
#2	September 14, 2020	305 µg/m³
#3	September 12, 2020	296.3 µg/m³
#4	August 19, 20232-day stretch	239.7 µg/m³
#5	September 15, 2020	217.6 µg/m³
#6	September 7, 20174-day stretch	194.9 µg/m³
#7	September 5, 2017	193 µg/m³
#8	September 6, 2017	186 µg/m³
#9	August 19, 20182-day stretch	185 µg/m³
#10	September 6, 20252-day stretch	168.1 µg/m³
#11	August 20, 2023	163.8 µg/m³
#12	August 20, 2018	150.3 µg/m³
#13	September 4, 2017	145.1 µg/m³
#14	September 16, 2020	145 µg/m³
#15	September 5, 2025	143.2 µg/m³

What caused Spokane's worst days?

The event catalog has named 10 episodes affecting Spokane, each resolved to a cause only where retained evidence supports it: EPA monitor qualifiers, NWS alerts, agency statements, and reporting. The ranking above stays cause-neutral; this is where the causes live.

Wildfire smoke 2026-07-17 wildfire smoke episode affecting Spokane-Spokane Valley, WA Jul 17, 2026 · 130.6 µg/m³ here , #16 all-time
Wildfire smoke 2025-09-04 wildfire smoke episode affecting Spokane-Spokane Valley, WA Sep 6, 2025 · 168.1 µg/m³ here , #10 all-time
Wildfire smoke 2023 mid-August Canadian wildfire-smoke episode Aug 19, 2023 · 239.7 µg/m³ here , #4 all-time · Gray Fire
Wildfire smoke 2022-09-12 Spokane-Spokane Valley, WA wildfire smoke episode Sep 12, 2022 · 103.5 µg/m³ here , #21 all-time
Wildfire smoke 2021-08-13 Spokane-Spokane Valley wildfire-smoke episode Aug 13, 2021 · 123.7 µg/m³ here , #17 all-time · from Spokane-Spokane Valley, WA
Wildfire smoke 2021-08-02 Spokane-Spokane Valley wildfire-smoke episode Aug 2, 2021 · 105 µg/m³ here , #20 all-time · from Spokane-Spokane Valley, WA

4 more, plus every named episode and its evidence is in the event catalog.

How does it compare?

The record is 30% higher than the next-worst day: 396.8 µg/m³ (September 13, 2020) against 305 µg/m³ (September 14, 2020).

How unusual is air like this?

54× a typical day here, at the record

2 days a year, on average, at "unhealthy" or worse

0 µg/m³397 µg/m³

Every measured day near Spokane, by daily PM2.5; the record stands far out on the right. 111 days at "unhealthy for sensitive groups" or worse, 42 at "unhealthy", 16 at "very unhealthy" (2024 EPA thresholds, across 6,741 measured days).

Year by year (1999–2026)

Worst single day each year, by the selected measure.

Why did reported numbers differ?

During a major event, AirNow, weather apps, news reports, and neighborhood monitors can all show different figures at the same moment. All of them can be right:

Concentration vs AQI. µg/m³ is the measurement; AQI is a 0 to 500 index derived from it. "330 µg/m³" and "AQI 500" can describe the same air.
Hourly vs NowCast vs daily. An hourly peak, the NowCast rolling estimate apps display, and the full-day average answer different questions; this page ranks full days.
One monitor vs a composite. A single sensor can read far above or below a citywide figure. The measure toggle above shows both framings.
Live vs certified. Real-time feeds get corrected when EPA certifies data months later, which is why recent days here carry a provisional label.

What about indoors?

Citywide monitors can tell you what happened outside. They cannot tell you what happened inside a particular home, school, or facility: filtration, ventilation, and building operation decide that. During and after days like the ones above, outdoor particles become an indoor load question.

Check current conditions and the next clean-air window → Estimate recent filter load where you are →

Common questions

What was the worst air-quality day in Spokane?: September 13, 2020, when daily PM2.5 reached 396.8 µg/m³ (AQI 500), about 18.0 cigarette-equivalents. It ranks first of 6,741 monitored days since 1999.
What is the highest AQI recorded in Spokane?: On the worst day, the daily PM2.5 AQI reached 500 (Hazardous). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 957 on September 13, 2020.
Were these days caused by wildfire smoke?: This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.
How often does Spokane get air this bad?: Rarely. The record sits at the far end of 6,741 measured days; Spokane averages 2 "unhealthy" days a year.
Why did different apps show different numbers that day?: Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.

Data and citation

Ranked days for both measures are downloadable, with units and status flags: spokane.csv. Source: U.S. EPA Air Quality System (AQS) daily PM2.5, with preliminary AirNow readings for the uncertified tail. Cite as:

IAQng Air Records: Spokane, WA. iaqng.com/records/spokane. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.