Cambridge Air Quality in Fall
Across 2019–2025, Cambridge's fall (September–November) air quality had a median AQI of 42 (Good) over 637 monitored days in the Boston-Cambridge-Newton, MA-NH area. 0.8% of fall days (5 of 637) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 68.6% of days. The cleanest fall on record here was 2025 (median AQI 36, 0 days above 100) and the worst was 2024 (median 40, 4 days above 100, peak AQI 1513).
- Good: 69.4%
- Moderate: 29.8%
- Unhealthy for Sensitive Groups: 0.2%
- Unhealthy or worse: 0.6%
Reduce Indoor Ozone in Cambridge
Today's main pollutant in Cambridge 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.
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How is the air in Cambridge in fall?
Good on a typical day: median AQI 42 across 2019–2025, with 0.8% of fall days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓69.4% of fall days were Good (AQI 0–50), 29.8% Moderate (51–100).
- ✓5 days of 637 reached AQI 101+; the fall peak was 1513.
- ✓Cleanest month: September; worst month: November.
Frequently asked questions
What is the air quality like in Cambridge in fall?▾
Across 2019–2025, the median daily AQI in Cambridge in fall was 42 (Good). 69.4% of days were Good, 29.8% Moderate, and 0.8% reached Unhealthy for Sensitive Groups or worse, based on 637 monitored days in EPA's Air Quality System.
Is fall a good time for outdoor exercise in Cambridge?▾
A typical fall day in Cambridge 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 fall days reached AQI 101+ in 2019–2025.
Which pollutant causes Cambridge's fall air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 68.6% of fall days in Cambridge. The highest fall AQI recorded in 2019–2025 was 1513 (Hazardous), in 2024.
Cambridge fall air quality, year by year
Cambridge fall 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 | 91 | 47 | 0 | 80 | PM2.5 |
| 2020 | 91 | 47 | 0 | 82 | PM2.5 |
| 2021 | 91 | 44 | 0 | 70 | PM2.5 |
| 2022 | 91 | 42 | 0 | 74 | PM2.5 |
| 2023 | 91 | 38 | 1 | 102 | PM2.5 |
| 2024worst | 91 | 40 | 4 | 1513 | O3 |
| 2025cleanest | 91 | 36 | 0 | 74 | O3 |
What fall air means in Cambridge
A typical fall day in Cambridge 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 fall days reached AQI 101+ in 2019–2025.
Fall's median AQI of 42 is below Cambridge's whole-year median of 48, making it one of the area's cleaner periods. Nationally, the median Fall AQI across every EPA metro area (296,275 metro-days, 2019–2025) is 40, so Cambridge runs in line with the national Fall norm; 0.8% of its Fall days topped AQI 100 versus 1.5% nationally (about the same).
Within the season, September is the cleanest month (median AQI 42, 0.0% of days above 100) and November the worst (median 42, 1.4% above 100).
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Cambridge
The latest EPA AirNow observation puts Cambridge's AQI at 33 (O3). The figures above are multi-year season statistics, not today's reading.
View current Cambridge 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.