Boston Air Quality in September
Across 2019–2025, Boston's September air quality had a median AQI of 42 (Good) over 210 monitored days in the Boston-Cambridge-Newton, MA-NH area. 0.0% of September days (0 of 210) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 62.9% of days. The cleanest September on record here was 2024 (median AQI 39, 0 days above 100) and the worst was 2023 (median 50, 0 days above 100, peak AQI 81).
- Good: 73.8%
- Moderate: 26.2%
- Unhealthy for Sensitive Groups: 0.0%
- Unhealthy or worse: 0.0%
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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How is the air in Boston in September?
Good on a typical day: median AQI 42 across 2019–2025, with 0.0% of September days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓73.8% of September days were Good (AQI 0–50), 26.2% Moderate (51–100).
- ✓0 days of 210 reached AQI 101+; the September peak was 87.
- ✓Best September: 2024; worst September: 2023.
Frequently asked questions
What is the air quality like in Boston in September?▾
Across 2019–2025, the median daily AQI in Boston in September was 42 (Good). 73.8% of days were Good, 26.2% Moderate, and 0.0% reached Unhealthy for Sensitive Groups or worse, based on 210 monitored days in EPA's Air Quality System.
Is September a good time for outdoor exercise in Boston?▾
A typical September day in Boston sits in the Good range (median AQI 42), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of September days reached AQI 101+ in 2019–2025.
Which pollutant causes Boston's September air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 62.9% of September days in Boston. The highest September AQI recorded in 2019–2025 was 87 (Moderate), in 2023.
Boston September air quality, year by year
Boston September air quality by year (Boston-Cambridge-Newton, MA-NH, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 30 | 43 | 0 | 80 | PM2.5 |
| 2020 | 30 | 45 | 0 | 71 | PM2.5 |
| 2021 | 30 | 45 | 0 | 57 | PM2.5 |
| 2022 | 30 | 40 | 0 | 61 | PM2.5 |
| 2023worst | 30 | 50 | 0 | 81 | PM2.5 |
| 2024cleanest | 30 | 39 | 0 | 87 | O3 |
| 2025 | 30 | 39 | 0 | 55 | O3 |
What September air means in Boston
A typical September day in Boston sits in the Good range (median AQI 42), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of September days reached AQI 101+ in 2019–2025.
September's median AQI of 42 is below Boston's whole-year median of 48, making it one of the area's cleaner periods. Nationally, the median September AQI across every EPA metro area (100,699 metro-days, 2019–2025) is 43, so Boston runs in line with the national September norm; 0.0% of its September days topped AQI 100 versus 2.7% nationally (below average).
September is part of Boston's fall season (September, October, November); the fall page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Boston
The latest EPA AirNow observation puts Boston's AQI at 33 (O3). The figures above are multi-year month statistics, 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.