What was the worst air-quality day in Boston?

August 25, 1999, when daily PM2.5 reached 70.1 µg/m³ (AQI 161), about 3.2 cigarette-equivalents. It ranks first of 9,693 monitored days since 1999.

What is the highest AQI recorded in Boston?

On the worst day, the daily PM2.5 AQI reached 161 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 230 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 Boston get air this bad?

Rarely. The record sits at the far end of 9,693 measured days; Boston averages about one "unhealthy" day every 5 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.

← All 100 US metros ranked

Historical PM2.5 records

Boston's worst air quality days

#86 of 100 US metros by worst day · Boston-Cambridge-Newton, MA-NH metro area

Based on the available regulatory-monitor record since 1999, August 25, 1999 ranks as Boston's most severe daily PM2.5 day: 70.1 µg/m³ (AQI 161, unhealthy), about 3.2 cigarettes in a day.

Daily rank: #1
Period of record: Since 1999
Peak daily PM2.5: 70.1 µg/m³
About: 3.2 cigarettes
Status: Finalized

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

Photo: Robert Linsdell from St. Andrews, Canada (CC BY 2.0)

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 27 EPA monitors in the Boston-Cambridge-Newton, MA-NH metro area (CBSA). Days after March 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 230 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.

What were Boston's worst days?

Rank	Date	Daily PM2.5
#1	August 25, 1999	70.1 µg/m³
#2	July 7, 20022-day stretch	68.9 µg/m³
#3	July 1, 2002	66.9 µg/m³
#4	May 31, 2010	60.5 µg/m³
#5	July 26, 2021	59 µg/m³
#6	August 19, 2001	57 µg/m³
#7	July 17, 1999	54.3 µg/m³
#8	July 9, 2002	54.1 µg/m³
#9	October 9, 2003	53.7 µg/m³
#10	August 13, 20022-day stretch	51.2 µg/m³
#11	October 10, 1999	50.8 µg/m³
#12	August 14, 2002	50.8 µg/m³
#13	June 27, 2003	50.7 µg/m³
#14	June 27, 2007	50.2 µg/m³
#15	October 13, 2003	48 µg/m³

What caused Boston's worst days?

The event catalog has named 5 episodes affecting Boston, 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 · 43.8 µg/m³ here , #20 all-time · from Canada
Wildfire smoke 2021-07-26 5-area wildfire-smoke episode Jul 26, 2021 · 59 µg/m³ here , #5 all-time · from Boston-Cambridge-Newton, MA-NH
Wildfire smoke 2010-05-31 wildfire-smoke episode affecting Boston-Cambridge-Newton, MA-NH May 31, 2010 · 60.5 µg/m³ here , #4 all-time · from Canada
Wildfire smoke July 2004 eastern U.S. Alaskan wildfire-smoke episode Jul 23, 2004 · 46.4 µg/m³ here , #18 all-time · from Alaska
Wildfire smoke July 2002 northeastern U.S. wildfire-smoke episode Jul 7, 2002 · 68.9 µg/m³ here , #2 all-time · from Quebec

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

How does it compare?

The record is 2% higher than the next-worst day: 70.1 µg/m³ (August 25, 1999) against 68.9 µg/m³ (July 7, 2002).

How unusual is air like this?

7× a typical day here, at the record

1 in 5 years, roughly, sees an "unhealthy" day here

1 µg/m³70 µg/m³

Every measured day near Boston, by daily PM2.5; the record stands far out on the right. 65 days at "unhealthy for sensitive groups" or worse, 6 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 9,693 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 Boston?: August 25, 1999, when daily PM2.5 reached 70.1 µg/m³ (AQI 161), about 3.2 cigarette-equivalents. It ranks first of 9,693 monitored days since 1999.
What is the highest AQI recorded in Boston?: On the worst day, the daily PM2.5 AQI reached 161 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 230 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 Boston get air this bad?: Rarely. The record sits at the far end of 9,693 measured days; Boston averages about one "unhealthy" day every 5 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: boston.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: Boston, MA-NH. iaqng.com/records/boston. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.