What was the worst air-quality day in Salt Lake City?

February 16, 2004, when daily PM2.5 reached 94.2 µg/m³ (AQI 178), about 4.3 cigarette-equivalents. It ranks first of 9,961 monitored days since 1999.

What is the highest AQI recorded in Salt Lake City?

On the worst day, the daily PM2.5 AQI reached 178 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 178 on February 16, 2004.

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 Salt Lake City get air this bad?

Rarely. The record sits at the far end of 9,961 measured days; Salt Lake City averages 4 "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

Salt Lake City's worst air quality days

#61 of 100 US metros by worst day · Salt Lake City, UT metro area

Based on the available regulatory-monitor record since 1999, February 16, 2004 ranks as Salt Lake City's most severe daily PM2.5 day: 94.2 µg/m³ (AQI 178, unhealthy), about 4.3 cigarettes in a day.

Daily rank: #1
Period of record: Since 1999
Peak daily PM2.5: 94.2 µg/m³
About: 4.3 cigarettes
Status: Finalized

Current conditions on the smoke map → Data through May 31, 2026 · page built July 26, 2026

Photo: Unknown creator (Public domain)

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 20 EPA monitors in the Salt Lake City, UT 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 178 on February 16, 2004 (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 Salt Lake City's worst days?

Rank	Date	Daily PM2.5
#1	February 16, 20042-day stretch	94.2 µg/m³
#2	February 7, 20022-day stretch	91.6 µg/m³
#3	January 27, 20075-day stretch	86.6 µg/m³
#4	February 15, 2004	85.9 µg/m³
#5	February 6, 2002	85.4 µg/m³
#6	January 28, 2007	81 µg/m³
#7	December 29, 20013-day stretch	80.8 µg/m³
#8	January 29, 2007	80.6 µg/m³
#9	January 30, 2007	80.6 µg/m³
#10	January 24, 2004	79.6 µg/m³
#11	January 26, 2007	79.2 µg/m³
#12	July 4, 2007	78.2 µg/m³
#13	December 27, 2001	77.2 µg/m³
#14	December 28, 2001	76.8 µg/m³
#15	January 12, 2010	76.7 µg/m³

What caused Salt Lake City's worst days?

The event catalog has named 4 episodes affecting Salt Lake City, 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 2020-08-21 2-area wildfire-smoke episode Aug 21, 2020 · 71.5 µg/m³ here , #23 all-time · from Ogden-Clearfield, UT
Winter inversion January 19-22, 2009 Wasatch Front winter-inversion episode Jan 21, 2009 · 74.3 µg/m³ here , #16 all-time · from Wasatch Front, Utah
Fireworks Salt Lake City Rose Park July 4 fireworks event Jul 4, 2007 · 78.2 µg/m³ here , #12 all-time · from Salt Lake City, Utah
Winter inversion 2002-02-06 winter inversion episode affecting Salt Lake City, UT, Ogden-Clearfield, UT Feb 7, 2002 · 91.6 µg/m³ here , #2 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: 94.2 µg/m³ (February 16, 2004) against 91.6 µg/m³ (February 7, 2002).

How unusual is air like this?

11× a typical day here, at the record

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

1 µg/m³94 µg/m³

Every measured day near Salt Lake City, by daily PM2.5; the record stands far out on the right. 417 days at "unhealthy for sensitive groups" or worse, 100 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 9,961 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 Salt Lake City?: February 16, 2004, when daily PM2.5 reached 94.2 µg/m³ (AQI 178), about 4.3 cigarette-equivalents. It ranks first of 9,961 monitored days since 1999.
What is the highest AQI recorded in Salt Lake City?: On the worst day, the daily PM2.5 AQI reached 178 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 178 on February 16, 2004.
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 Salt Lake City get air this bad?: Rarely. The record sits at the far end of 9,961 measured days; Salt Lake City averages 4 "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: salt-lake-city.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: Salt Lake City, UT. iaqng.com/records/salt-lake-city. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.