What was the worst air-quality day in Fresno?

September 16, 2020, when daily PM2.5 reached 199.3 µg/m³ (AQI 274), about 9.1 cigarette-equivalents. It ranks first of 9,683 monitored days since 1999.

What is the highest AQI recorded in Fresno?

On the worst day, the daily PM2.5 AQI reached 274 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 346 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 Fresno get air this bad?

Rarely. The record sits at the far end of 9,683 measured days; Fresno averages 12 "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.

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Historical PM2.5 records

Fresno's worst air quality days

#18 of 100 US metros by worst day · Fresno, CA metro area

Based on the available regulatory-monitor record since 1999, September 16, 2020 ranks as Fresno's most severe daily PM2.5 day: 199.3 µg/m³ (AQI 274, very unhealthy), about 9.1 cigarettes in a day.

Daily rank: #1
Period of record: Since 1999
Peak daily PM2.5: 199.3 µg/m³
About: 9.1 cigarettes
Status: Finalized

Current conditions on the smoke map → Data through May 31, 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 7 EPA monitors in the Fresno, CA metro area (CBSA). Days after May 31, 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 346 on September 13, 2020 (defining pollutant PM2.5).

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.

What were Fresno's worst days?

Rank	Date	Daily PM2.5
#1	September 16, 20205-day stretch	199.3 µg/m³
#2	September 14, 2020	193.7 µg/m³
#3	September 17, 2020	184.4 µg/m³
#4	January 9, 2000	160 µg/m³
#5	September 13, 2020	157.1 µg/m³
#6	August 22, 20202-day stretch	153.5 µg/m³
#7	September 15, 2020	149.2 µg/m³
#8	October 3, 2021	147.7 µg/m³
#9	August 21, 2020	146.2 µg/m³
#10	December 31, 1999	136 µg/m³
#11	December 24, 19993-day stretch	134 µg/m³
#12	December 22, 1999	132 µg/m³
#13	October 5, 2020	131.6 µg/m³
#14	December 25, 1999	131 µg/m³
#15	January 15, 1999	128 µg/m³

What caused Fresno's worst days?

The event catalog has named 6 episodes affecting Fresno, 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 2021-10-03 Fresno wildfire-smoke episode Oct 3, 2021 · 147.7 µg/m³ here , #8 all-time · from Fresno, CA
Wildfire smoke 2020-10-04 Fresno wildfire-smoke episode Oct 5, 2020 · 131.6 µg/m³ here , #13 all-time · from Fresno, CA
Wildfire smoke 2020-09-10 8-area wildfire-smoke episode Sep 16, 2020 · 199.3 µg/m³ here , #1 all-time · from Fresno, CA
Wildfire smoke 2020-08-19 6-area wildfire-smoke episode Aug 22, 2020 · 153.5 µg/m³ here , #6 all-time · from Bakersfield, CA
Other January 2000 San Joaquin Valley limited-dispersion pollution episode Jan 9, 2000 · 160 µg/m³ here , #4 all-time · from San Joaquin Valley, California
Other 1999-11-12 other documented PM source or accumulation mechanism episode affecting Bakersfield, CA, Fresno, CA Nov 14, 1999 · 115 µg/m³ here , #24 all-time

every named episode and its evidence is in the event catalog.

How does it compare?

The record is 3% higher than the next-worst day: 199.3 µg/m³ (September 16, 2020) against 193.7 µg/m³ (September 14, 2020).

How unusual is air like this?

16× a typical day here, at the record

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

1 µg/m³199 µg/m³

Every measured day near Fresno, by daily PM2.5; the record stands far out on the right. 1,038 days at "unhealthy for sensitive groups" or worse, 333 at "unhealthy", 16 at "very unhealthy" (2024 EPA thresholds, across 9,683 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 Fresno?: September 16, 2020, when daily PM2.5 reached 199.3 µg/m³ (AQI 274), about 9.1 cigarette-equivalents. It ranks first of 9,683 monitored days since 1999.
What is the highest AQI recorded in Fresno?: On the worst day, the daily PM2.5 AQI reached 274 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 346 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 Fresno get air this bad?: Rarely. The record sits at the far end of 9,683 measured days; Fresno averages 12 "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: fresno.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: Fresno, CA. iaqng.com/records/fresno. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.