Boston Air Quality in December
Across 2019–2025, Boston's December air quality had a median AQI of 48 (Good) over 217 monitored days in the Boston-Cambridge-Newton, MA-NH area. 0.0% of December days (0 of 217) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 80.2% of days. The cleanest December on record here was 2024 (median AQI 39, 0 days above 100) and the worst was 2020 (median 52, 0 days above 100, peak AQI 81).
- Good: 57.1%
- Moderate: 42.9%
- 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.
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How is the air in Boston in December?
Good on a typical day: median AQI 48 across 2019–2025, with 0.0% of December days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓57.1% of December days were Good (AQI 0–50), 42.9% Moderate (51–100).
- ✓0 days of 217 reached AQI 101+; the December peak was 84.
- ✓Best December: 2024; worst December: 2020.
Frequently asked questions
What is the air quality like in Boston in December?▾
Across 2019–2025, the median daily AQI in Boston in December was 48 (Good). 57.1% of days were Good, 42.9% Moderate, and 0.0% reached Unhealthy for Sensitive Groups or worse, based on 217 monitored days in EPA's Air Quality System.
Is December a good time for outdoor exercise in Boston?▾
A typical December day in Boston sits in the Good range (median AQI 48), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of December days reached AQI 101+ in 2019–2025.
Which pollutant causes Boston's December air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 80.2% of December days in Boston. The highest December AQI recorded in 2019–2025 was 84 (Moderate), in 2020.
Boston December air quality, year by year
Boston December 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 | 31 | 48 | 0 | 72 | PM2.5 |
| 2020worst | 31 | 52 | 0 | 81 | PM2.5 |
| 2021 | 31 | 50 | 0 | 73 | PM2.5 |
| 2022 | 31 | 51 | 0 | 84 | PM2.5 |
| 2023 | 31 | 42 | 0 | 71 | PM2.5 |
| 2024cleanest | 31 | 39 | 0 | 73 | PM2.5 |
| 2025 | 31 | 41 | 0 | 69 | PM2.5 |
What December air means in Boston
A typical December day in Boston sits in the Good range (median AQI 48), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of December days reached AQI 101+ in 2019–2025.
December's median AQI of 48 is in line with Boston's whole-year median of 48, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median December AQI across every EPA metro area (95,590 metro-days, 2019–2025) is 39, so Boston runs above the national December norm; 0.0% of its December days topped AQI 100 versus 0.7% nationally (about the same).
December is part of Boston's winter season (December, January, February); the winter 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.