Boston Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for Boston (Boston-Cambridge-Newton, MA-NH area) was November 10, 2024, when the daily AQI reached 1513 (Hazardous), set by fine particulate matter (PM2.5). Across 2,557 monitored days in 2019–2025, 5 of the 10 worst days were Unhealthy (AQI 151+) or worse; 5 of the 10 were driven by ground-level ozone and 5 by fine particulate matter (PM2.5), and 5 of them fell in 2024. The cleanest day was October 23, 2023 at AQI 17 (Good); the 10 best days span 2 different years.
Reduce Indoor Ozone in Boston
Today's main pollutant in Boston is ground-level ozone (AQI 33). Important: standard HEPA filters do NOT remove ozone — only a purifier with a substantial activated-carbon bed adsorbs it. The biggest wins are cutting indoor sources (gas appliances, certain "ionizer" purifiers) and ventilating when outdoor ozone is low (early morning). A high-carbon unit helps with the gases and VOCs that linger indoors.
Recommended: Levoit Core 300 — True HEPA + activated carbon · CADR ~141 CFM · rooms up to 219 sq ft
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When were Boston's worst and best air quality days?
Worst: November 10, 2024, AQI 1513 (Hazardous, PM2.5). Best: October 23, 2023, AQI 17 (Good). Based on 2,557 EPA-monitored days in the Boston-Cambridge-Newton, MA-NH area, 2019–2025.
- ✓5 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 122 (Unhealthy for Sensitive Groups).
- ✓Ground-level ozone drove 5 of the 10 worst days.
- ✓2024 accounts for 5 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in Boston?▾
November 10, 2024: the Boston-Cambridge-Newton, MA-NH area's daily AQI hit 1513 (Hazardous), driven by fine particulate matter (PM2.5). That is the highest of 2,557 monitored days in 2019–2025.
What was the best air quality day in Boston?▾
October 23, 2023, with a daily AQI of 17 (Good). The daily AQI is the highest reading from any monitor in the area, so even the cleanest day reflects the area's worst monitor.
What was Boston's worst air quality day in 2025?▾
August 12, 2025, AQI 150 (Unhealthy for Sensitive Groups), set by ground-level ozone, in peak ozone season.
Which pollutant causes Boston's worst air days?▾
Ground-level ozone set the AQI on 5 of the 10 worst days; fine particulate matter (PM2.5) on 5. The tables above list the defining pollutant for every day.
Boston's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the Boston-Cambridge-Newton, MA-NH area (EPA AQS). The daily AQI is the highest reading from any monitor in the area that day.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2024 page) | 1513 | Hazardous | PM2.5 — fine particulate matter (PM2.5) |
| 2 | (2024 page) | 493 | Hazardous | PM2.5 — fine particulate matter (PM2.5) |
| 3 | (2024 page) | 162 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 4 | (2024 page) | 157 | Unhealthy | PM2.5 — fine particulate matter (PM2.5) |
| 5 | (2021 page) | 153 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 6 | (2025 page) | 150 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 7 | (2023 page) | 136 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 8 | (2025 page) | 133 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 9 | (2024 page) | 122 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 10 | (2022 page) | 122 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
Worst day of each year in Boston
The single highest-AQI day in each year on record. Click a year for its full month-by-month statistics.
| Year | Worst day | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 2019 | 122 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2020 | 97 | Moderate | O3 — ground-level ozone, in peak ozone season | |
| 2021 | 153 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2022 | 122 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2023 | 136 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2024 | 1513 | Hazardous | PM2.5 — fine particulate matter (PM2.5) | |
| 2025 | 150 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
Boston's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the Boston-Cambridge-Newton, MA-NH area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2023 page) | 17 | Good | O3 — ground-level ozone |
| 2 | (2023 page) | 19 | Good | O3 — ground-level ozone |
| 3 | (2023 page) | 20 | Good | O3 — ground-level ozone |
| 4 | (2023 page) | 20 | Good | O3 — ground-level ozone |
| 5 | (2025 page) | 21 | Good | O3 — ground-level ozone |
| 6 | (2025 page) | 21 | Good | O3 — ground-level ozone |
| 7 | (2025 page) | 23 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 8 | (2025 page) | 23 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 9 | (2023 page) | 23 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 10 | (2025 page) | 24 | Good | O3 — ground-level ozone |
Right now in Boston
The latest EPA AirNow observation puts Boston's AQI at 33 (O3). The days above are historical records, not today's reading.
View current Boston air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Boston-Cambridge-Newton, MA-NH), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.