Historical PM2.5 records
Boston's worst air quality days
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 August 26, 2026
Photo: Robert Linsdell from St. Andrews, Canada (CC BY 2.0)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 | Air event | PM2.5 / cigarettes |
|---|---|---|---|
| #1 | August 25, 1999 | Cause not established after standard review Review complete | 70.1 µg/m³ · 3.2 cigs |
| #2 | July 7, 20022-day stretch | Wildfire smoke EPA monitor flag | 68.9 µg/m³ · 3.1 cigs |
| #3 | July 1, 2002 | Regional secondary PM Possible | 66.9 µg/m³ · 3.0 cigs |
| #4 | May 31, 2010 | Wildfire smoke EPA monitor flag | 60.5 µg/m³ · 2.8 cigs |
| #5 | July 26, 2021 | Wildfire smoke EPA monitor flag | 59 µg/m³ · 2.7 cigs |
| #6 | August 19, 2001 | Cause not established after standard review Review complete | 57 µg/m³ · 2.6 cigs |
| #7 | July 17, 1999 | Regional secondary PM Likely | 54.3 µg/m³ · 2.5 cigs |
| #8 | July 9, 2002 | Wildfire smoke EPA monitor flag | 54.1 µg/m³ · 2.5 cigs |
| #9 | October 9, 2003 | Cause not established after standard review Review complete | 53.7 µg/m³ · 2.4 cigs |
| #10 | August 13, 20022-day stretch | Regional secondary PM Likely | 51.2 µg/m³ · 2.3 cigs |
| #11 | October 10, 1999 | Cause not established after review; leads retained Review complete | 50.8 µg/m³ · 2.3 cigs |
| #12 | August 14, 2002 | Regional secondary PM Likely | 50.8 µg/m³ · 2.3 cigs |
| #13 | June 27, 2003 | Stagnant pollution mixture Possible | 50.7 µg/m³ · 2.3 cigs |
| #14 | June 27, 2007 | Stagnant pollution mixture Possible | 50.2 µg/m³ · 2.3 cigs |
| #15 | October 13, 2003 | Cause not established after standard review Review complete | 48 µg/m³ · 2.2 cigs |
What caused Boston's worst days?
The air-event catalog has named 14 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
- Wildfire smoke 2021-07-26 5-area wildfire-smoke episode
- Wildfire smoke 2010-05-31 wildfire-smoke episode affecting Boston-Cambridge-Newton, MA-NH
- Stagnant pollution mixture 2007-06-27 Boston-Cambridge-Newton, MA-NH, Hartford-West Hartford-East Hartford, CT, and 1 other metros stagnant mixed-pollution episode
- Stagnant pollution mixture 2005-08-13 Boston-Cambridge-Newton, MA-NH stagnant mixed-pollution episode
- Regional secondary PM 2005-06-26 Boston-Cambridge-Newton, MA-NH regional secondary-particle pollution episode
8 more, plus every named episode and its evidence is in the air events.
Boston air season · summer so far, through July 27
Boston's summer is the 21st-worst of 28 on record.
The summer season in every year since 1999, measured over the same window, June 1 to July 27, so each is compared at the same point. All fine particulate the metro's monitors recorded, from every source.
Where 2026 stands
The race, summer by summer
Every summer on record
| Rank | Season | Cigs | Average µg/m³ | Worst day | Unhealthy days | Coverage |
|---|---|---|---|---|---|---|
| 1 | 2005 | 49.1 cig · 18.9 µg/m³ avg | 18.9 | 46.9 | 0 | 100% |
| 2 | 20021 documented smoke event | 44.8 cig · 19.3 µg/m³ avg | 19.3 | 68.9 | 2 | 90% |
| 3 | 2008 | 44.3 cig · 17.1 µg/m³ avg | 17.1 | 34.1 | 0 | 100% |
| 4 | 2006 | 42.5 cig · 16.4 µg/m³ avg | 16.4 | 41.6 | 0 | 100% |
| 5 | 2003 | 42.1 cig · 16.8 µg/m³ avg | 16.8 | 50.7 | 0 | 97% |
| 6 | 2007 | 40.4 cig · 15.6 µg/m³ avg | 15.6 | 50.2 | 0 | 100% |
| 7 | 20041 documented smoke event | 39.0 cig · 15 µg/m³ avg | 15 | 46.4 | 0 | 100% |
| 8 | 1999 | 36.3 cig · 19 µg/m³ avg | 19 | 54.3 | 0 | 74% |
| 9 | 2023 | 36.1 cig · 13.9 µg/m³ avg | 13.9 | 40.9 | 0 | 100% |
| 10 | 2013 | 35.4 cig · 13.7 µg/m³ avg | 13.7 | 29.4 | 0 | 100% |
| 11 | 20101 documented smoke event | 33.9 cig · 13.1 µg/m³ avg | 13.1 | 31.5 | 0 | 100% |
| 12 | 2001 | 33.7 cig · 14.8 µg/m³ avg | 14.8 | 34.6 | 0 | 88% |
| 13 | 20211 documented smoke event | 33.1 cig · 12.8 µg/m³ avg | 12.8 | 59 | 1 | 100% |
| 14 | 2011 | 32.5 cig · 12.6 µg/m³ avg | 12.6 | 29.5 | 0 | 100% |
| 15 | 2018 | 32.1 cig · 12.4 µg/m³ avg | 12.4 | 29.1 | 0 | 100% |
| 16 | 2000 | 31.3 cig · 14.9 µg/m³ avg | 14.9 | 31.9 | 0 | 81% |
| 17 | 2012 | 29.6 cig · 11.4 µg/m³ avg | 11.4 | 23.7 | 0 | 100% |
| 18 | 2025 | 29.4 cig · 11.4 µg/m³ avg | 11.4 | 37.5 | 0 | 100% |
| 19 | 2015 | 28.9 cig · 11.1 µg/m³ avg | 11.1 | 29.8 | 0 | 100% |
| 20 | 2017 | 27.5 cig · 10.6 µg/m³ avg | 10.6 | 30 | 0 | 100% |
| 21 | 20261 documented smoke event | 26.9 cig · 10.4 µg/m³ avg | 10.4 | 43.8 | 0 | 100% |
| 22 | 2014 | 26.5 cig · 10.2 µg/m³ avg | 10.2 | 17.3 | 0 | 100% |
| 23 | 2022 | 26.2 cig · 10.1 µg/m³ avg | 10.1 | 18.6 | 0 | 100% |
| 24 | 2020 | 25.8 cig · 10 µg/m³ avg | 10 | 25.6 | 0 | 100% |
| 25 | 2019 | 25.8 cig · 9.9 µg/m³ avg | 9.9 | 28.3 | 0 | 100% |
| 26 | 2016 | 25.4 cig · 9.8 µg/m³ avg | 9.8 | 19.3 | 0 | 100% |
| 27 | 2009 | 25.0 cig · 10 µg/m³ avg | 10 | 21.5 | 0 | 97% |
| 28 | 2024 | 22.0 cig · 8.5 µg/m³ avg | 8.5 | 23.4 | 0 | 100% |
Documented smoke events this summer
What this measures, and what it does not
Each bar is the sum of Boston's robust metro daily PM2.5 readings across the summer season through July 27, in µg/m³-days, divided by 22 µg/m³ to give cigarette-equivalents. Earlier seasons are cut at the same point in the season, so this is like-for-like rather than full-season totals. The cigarette figure is an illustration of exposure, not a medical equivalence.
It counts all fine particulate the monitors recorded, whatever produced it: wildfire smoke, traffic, industry, fireworks. It is not a smoke-only measure. The documented smoke events shown alongside explain particular spikes; they do not define the ranking, and that catalog only begins in 2002.
Context for reading the ranking: regional fine-particulate levels have fallen substantially since the early 2000s, which is why seasons from that era rank high on season totals while recent seasons rank high on single-day peaks.
A season is ranked only where monitors reported on at least 60% of the window's days, since below that a low total would reflect missing instruments rather than clean air. No season currently falls below that threshold. Readings for 2026 after the certified EPA archive are provisional AirNow values and may be revised.
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?
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 August 26, 2026.