Cambridge Air Quality in 2021
In 2021, Cambridge's air quality (Boston-Cambridge-Newton, MA-NH area) had a median daily AQI of 48 (Good) across 365 monitored days. 5 days (1.4%) reached Unhealthy for Sensitive Groups or worse, 206 were Good, and the year's peak AQI was 153 (Unhealthy). Fine particulate matter (PM2.5) set the daily AQI on 265 of those days. The cleanest month of 2021 was October (median AQI 42, 0 days above 100) and the worst was June (median 57, 2 days above 100, peak 122).
- Good: 56.4%
- Moderate: 42.2%
- Unhealthy for Sensitive Groups: 1.1%
- Unhealthy or worse: 0.3%
Reduce Indoor Ozone in Cambridge
Today's main pollutant in Cambridge is ground-level ozone (AQI 28). 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 was Cambridge's air quality in 2021?
Good on a typical day: median AQI 48, with 5 days above AQI 100 out of 365 monitored — the 5th cleanest of 7 years on record for the area.
- ✓Peak AQI in 2021: 153 (Unhealthy).
- ✓Fine particulate matter (PM2.5) was the main pollutant on 265 days.
- ✓Cleanest month: October; worst: June.
Frequently asked questions
What was the average AQI in Cambridge in 2021?▾
EPA's Air Quality System puts Cambridge's (Boston-Cambridge-Newton, MA-NH) median daily AQI for 2021 at 48, which is Good. 5 days were Unhealthy for Sensitive Groups or worse and the year's highest AQI was 153.
Which month had the worst air quality in Cambridge in 2021?▾
The cleanest month of 2021 was October (median AQI 42, 0 days above 100) and the worst was June (median 57, 2 days above 100, peak 122).
Was 2021 a good or bad year for Cambridge's air?▾
Among the 7 years EPA has published for the area (2019–2025), 2021 ranks 5th cleanest by days above AQI 100. 2021's median of 48 is in line with the area's typical annual median of 48.
Cambridge month by month in 2021
Cambridge air quality month by month, 2021 (Boston-Cambridge-Newton, MA-NH, EPA AQS daily AQI)
| Month | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| January | 31 | 43 | 0 | 89 | PM2.5 |
| February | 28 | 49 | 0 | 66 | PM2.5 |
| March | 31 | 47 | 0 | 77 | PM2.5 |
| April | 30 | 45 | 0 | 74 | O3 |
| May | 31 | 44 | 0 | 100 | O3 |
| Juneworst | 30 | 57 | 2 | 122 | PM2.5 |
| July | 31 | 54 | 2 | 153 | PM2.5 |
| August | 31 | 53 | 1 | 122 | PM2.5 |
| September | 30 | 45 | 0 | 57 | PM2.5 |
| Octobercleanest | 31 | 42 | 0 | 55 | PM2.5 |
| November | 30 | 44 | 0 | 70 | PM2.5 |
| December | 31 | 50 | 0 | 73 | PM2.5 |
How 2021 compares
Among the 7 years EPA has published for the area (2019–2025), 2021 ranks 5th cleanest by days above AQI 100.
2021's median of 48 is in line with the area's typical annual median of 48.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Cambridge
The latest EPA AirNow observation puts Cambridge's AQI at 28 (O3). The figures above are multi-year annual 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.