What was the worst air-quality day in Worcester?

July 7, 2002, when daily PM2.5 reached 76 µg/m³ (AQI 165), about 3.5 cigarette-equivalents. It ranks first of 6,973 monitored days since 1999.

What is the highest AQI recorded in Worcester?

On the worst day, the daily PM2.5 AQI reached 165 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 214 on August 13, 2002.

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 Worcester get air this bad?

Rarely. The record sits at the far end of 6,973 measured days; Worcester averages about one "unhealthy" day every 14 years.

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

Worcester's worst air quality days

#78 of 100 US metros by worst day · Worcester, MA-CT metro area

Based on the available regulatory-monitor record since 1999, July 7, 2002 ranks as Worcester's most severe daily PM2.5 day: 76 µg/m³ (AQI 165, unhealthy), about 3.5 cigarettes in a day.

Daily rank: #1
Period of record: Since 1999
Peak daily PM2.5: 76 µg/m³
About: 3.5 cigarettes
Latest event: 1 day, July 2026
Status: Finalized

Current conditions on the smoke map → Data through Jul 18, 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 5 EPA monitors in the Worcester, MA-CT 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 214 on August 13, 2002 (defining pollutant Ozone).

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 July 2026 One ranked day, Jul 18, 2026, peaking at 41.6 µg/m³, #12 all-time. Provisional until EPA certifies the data. See where it ranks ↓

What were Worcester's worst days?

Rank	Date	Daily PM2.5
#1	July 7, 20022-day stretch	76 µg/m³
#2	August 13, 20022-day stretch	56.2 µg/m³
#3	June 6, 2023	54.4 µg/m³
#4	July 1, 2023	53.3 µg/m³
#5	January 28, 2003	49.5 µg/m³
#6	July 17, 1999	49.1 µg/m³
#7	July 9, 2002	46.8 µg/m³
#8	August 14, 2002	45.5 µg/m³
#9	July 26, 2021	45.3 µg/m³
#10	July 5, 2003	43.3 µg/m³
#11	August 10, 2001	41.8 µg/m³
#12	July 18, 2026provisional	41.6 µg/m³
#13	June 19, 2006	41.2 µg/m³
#14	June 27, 2003	40.8 µg/m³
#15	October 1, 2023	40.5 µg/m³

Days marked provisional are preliminary AirNow readings after February 28, 2026, the latest date EPA has certified for Worcester; they may shift once finalized.

What caused Worcester's worst days?

The event catalog has named 8 episodes affecting Worcester, 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 July 15–20, 2026 Great Lakes and eastern U.S. wildfire-smoke episode Jul 18, 2026 · 41.6 µg/m³ here , #12 all-time · from Canada
Wildfire smoke 2023 early-October Canadian wildfire-smoke episode Oct 1, 2023 · 40.5 µg/m³ here , #15 all-time · from Canada
Wildfire smoke 2023 late-June Canadian wildfire-smoke episode Jul 1, 2023 · 53.3 µg/m³ here , #4 all-time · from Canada
Wildfire smoke 2023 early-June Canadian wildfire-smoke episode Jun 6, 2023 · 54.4 µg/m³ here , #3 all-time · from Canada
Wildfire smoke 2021-07-26 5-area wildfire-smoke episode Jul 26, 2021 · 45.3 µg/m³ here , #9 all-time · from Boston-Cambridge-Newton, MA-NH
Wildfire smoke 2021-07-20 11-area wildfire-smoke episode Jul 20, 2021 · 36.5 µg/m³ here , #22 all-time · from Albany-Schenectady-Troy, NY

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

How does it compare?

The record is 35% higher than the next-worst day: 76 µg/m³ (July 7, 2002) against 56.2 µg/m³ (August 13, 2002).

How unusual is air like this?

10× a typical day here, at the record

1 in 14 years, roughly, sees an "unhealthy" day here

0 µg/m³76 µg/m³

Every measured day near Worcester, by daily PM2.5; the record stands far out on the right. 31 days at "unhealthy for sensitive groups" or worse, 2 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 6,973 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 Worcester?: July 7, 2002, when daily PM2.5 reached 76 µg/m³ (AQI 165), about 3.5 cigarette-equivalents. It ranks first of 6,973 monitored days since 1999.
What is the highest AQI recorded in Worcester?: On the worst day, the daily PM2.5 AQI reached 165 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 214 on August 13, 2002.
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 Worcester get air this bad?: Rarely. The record sits at the far end of 6,973 measured days; Worcester averages about one "unhealthy" day every 14 years.
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: worcester.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: Worcester, MA-CT. iaqng.com/records/worcester. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.