Portland, ME Air Quality in December
Across 2019–2025, Portland's December air quality had a median AQI of 36 (Good) over 214 monitored days in the Portland-South Portland, ME area. 0.0% of December days (0 of 214) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 50.5% of days. The cleanest December on record here was 2022 (median AQI 31, 0 days above 100) and the worst was 2019 (median 48, 0 days above 100, peak AQI 70).
- Good: 73.4%
- Moderate: 26.6%
- Unhealthy for Sensitive Groups: 0.0%
- Unhealthy or worse: 0.0%
Reduce Indoor Ozone in Portland
Today's main pollutant in Portland is ground-level ozone (AQI 29). 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 Portland in December?
Good on a typical day: median AQI 36 across 2019–2025, with 0.0% of December days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓73.4% of December days were Good (AQI 0–50), 26.6% Moderate (51–100).
- ✓0 days of 214 reached AQI 101+; the December peak was 78.
- ✓Best December: 2022; worst December: 2019.
Frequently asked questions
What is the air quality like in Portland in December?▾
Across 2019–2025, the median daily AQI in Portland in December was 36 (Good). 73.4% of days were Good, 26.6% Moderate, and 0.0% reached Unhealthy for Sensitive Groups or worse, based on 214 monitored days in EPA's Air Quality System.
Is December a good time for outdoor exercise in Portland?▾
A typical December day in Portland sits in the Good range (median AQI 36), 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 Portland's December air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 50.5% of December days in Portland. The highest December AQI recorded in 2019–2025 was 78 (Moderate), in 2019.
Portland December air quality, year by year
Portland December air quality by year (Portland-South Portland, ME, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019worst | 31 | 48 | 0 | 70 | PM2.5 |
| 2020 | 31 | 36 | 0 | 67 | PM2.5 |
| 2021 | 31 | 33 | 0 | 54 | O3 |
| 2022cleanest | 28 | 31 | 0 | 73 | O3 |
| 2023 | 31 | 40 | 0 | 78 | PM2.5 |
| 2024 | 31 | 35 | 0 | 73 | O3 |
| 2025 | 31 | 42 | 0 | 63 | PM2.5 |
What December air means in Portland
A typical December day in Portland sits in the Good range (median AQI 36), 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 36 is below Portland's whole-year median of 40, making it one of the area's cleaner periods. Nationally, the median December AQI across every EPA metro area (95,590 metro-days, 2019–2025) is 39, so Portland runs in line with the national December norm; 0.0% of its December days topped AQI 100 versus 0.7% nationally (about the same).
December is part of Portland's winter season (December, January, February); the winter page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Portland
The latest EPA AirNow observation puts Portland's AQI at 29 (O3). The figures above are multi-year month statistics, not today's reading.
View current Portland air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Portland-South Portland, ME), 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.