What was the worst air-quality day in Los Angeles?

July 5, 2020, when daily PM2.5 reached 175 µg/m³ (AQI 250), about 8.0 cigarette-equivalents. It ranks first of 9,647 monitored days since 1999.

What is the highest AQI recorded in Los Angeles?

On the worst day, the daily PM2.5 AQI reached 250 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 250 on July 5, 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 Los Angeles get air this bad?

Rarely. The record sits at the far end of 9,647 measured days; Los Angeles averages 5 "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

Los Angeles's worst air quality days

#28 of 100 US metros by worst day · Los Angeles-Long Beach-Anaheim, CA metro area

Based on the available regulatory-monitor record since 1999, July 5, 2020 ranks as Los Angeles's most severe daily PM2.5 day: 175 µg/m³ (AQI 250, very unhealthy), about 8.0 cigarettes in a day.

Daily rank: #1
Period of record: Since 1999
Peak daily PM2.5: 175 µg/m³
About: 8.0 cigarettes
Status: Finalized

Current conditions on the smoke map → Data through Apr 30, 2026 · page built July 26, 2026

Photo: Alek Leckszas (CC BY-SA 4.0)

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 19 EPA monitors in the Los Angeles-Long Beach-Anaheim, CA metro area (CBSA). Days after April 30, 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 250 on July 5, 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 Los Angeles's worst days?

Rank	Date	Daily PM2.5
#1	July 5, 20202-day stretch	175 µg/m³
#2	July 5, 2003	121.2 µg/m³
#3	October 26, 20033-day stretch	115.5 µg/m³
#4	December 2, 20003-day stretch	113.9 µg/m³
#5	January 1, 2018	111 µg/m³
#6	October 27, 2003	107.2 µg/m³
#7	September 12, 2020	102.7 µg/m³
#8	December 3, 2000	99 µg/m³
#9	October 28, 2003	96 µg/m³
#10	November 6, 2001	94.7 µg/m³
#11	October 20, 2000	92.5 µg/m³
#12	July 4, 2020	90.2 µg/m³
#13	December 25, 2017	89.9 µg/m³
#14	December 1, 2000	87.8 µg/m³
#15	November 25, 2000	87.5 µg/m³

What caused Los Angeles's worst days?

The event catalog has named 7 episodes affecting Los Angeles, 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 2020-09-10 8-area wildfire-smoke episode Sep 12, 2020 · 102.7 µg/m³ here , #7 all-time · from Fresno, CA
Fireworks 2020-07-04 Los Angeles-Long Beach-Anaheim fireworks episode Jul 5, 2020 · 175 µg/m³ here , #1 all-time · from Los Angeles-Long Beach-Anaheim, CA
Fireworks Fireworks episode, 2017-12-31 through 2018-01-02 Jan 1, 2018 · 111 µg/m³ here , #5 all-time
Mixed causes Mixed-source episode, 2017-12-25 Dec 25, 2017 · 89.9 µg/m³ here , #13 all-time
Wildfire smoke 2008-11-16 wildfire smoke episode affecting Los Angeles-Long Beach-Anaheim, CA Nov 16, 2008 · 78.3 µg/m³ here , #23 all-time
Fireworks 2004-07-05 fireworks episode affecting Los Angeles-Long Beach-Anaheim, CA, Riverside-San Bernardino-Ontario, CA Jul 5, 2004 · 75.6 µg/m³ here , #25 all-time

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

How does it compare?

The record is 44% higher than the next-worst day: 175 µg/m³ (July 5, 2020) against 121.2 µg/m³ (July 5, 2003).

How unusual is air like this?

11× a typical day here, at the record

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

3 µg/m³175 µg/m³

Every measured day near Los Angeles, by daily PM2.5; the record stands far out on the right. 702 days at "unhealthy for sensitive groups" or worse, 144 at "unhealthy", 1 at "very unhealthy" (2024 EPA thresholds, across 9,647 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 Los Angeles?: July 5, 2020, when daily PM2.5 reached 175 µg/m³ (AQI 250), about 8.0 cigarette-equivalents. It ranks first of 9,647 monitored days since 1999.
What is the highest AQI recorded in Los Angeles?: On the worst day, the daily PM2.5 AQI reached 250 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 250 on July 5, 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 Los Angeles get air this bad?: Rarely. The record sits at the far end of 9,647 measured days; Los Angeles averages 5 "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: los-angeles.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: Los Angeles, CA. iaqng.com/records/los-angeles. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.