Portland, ME Air Quality in Winter
Across 2019–2025, Portland's winter (December–February) air quality had a median AQI of 40 (Good) over 629 monitored days in the Portland-South Portland, ME area. 0.2% of winter days (1 of 629) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 49.6% of days. The cleanest winter on record here was 2021 (median AQI 36, 0 days above 100) and the worst was 2019 (median 50, 0 days above 100, peak AQI 81).
- Good: 71.5%
- Moderate: 28.3%
- Unhealthy for Sensitive Groups: 0.2%
- 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.
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 Portland in winter?
Good on a typical day: median AQI 40 across 2019–2025, with 0.2% of winter days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓71.5% of winter days were Good (AQI 0–50), 28.3% Moderate (51–100).
- ✓1 day of 629 reached AQI 101+; the winter peak was 109.
- ✓Cleanest month: December; worst month: February.
Frequently asked questions
What is the air quality like in Portland in winter?▾
Across 2019–2025, the median daily AQI in Portland in winter was 40 (Good). 71.5% of days were Good, 28.3% Moderate, and 0.2% reached Unhealthy for Sensitive Groups or worse, based on 629 monitored days in EPA's Air Quality System.
Is winter a good time for outdoor exercise in Portland?▾
A typical winter day in Portland sits in the Good range (median AQI 40), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of winter days reached AQI 101+ in 2019–2025.
Which pollutant causes Portland's winter air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 49.6% of winter days in Portland. The highest winter AQI recorded in 2019–2025 was 109 (Unhealthy for Sensitive Groups), in 2019.
Portland winter air quality, year by year
Portland winter 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 | 90 | 50 | 0 | 81 | PM2.5 |
| 2020 | 91 | 47 | 0 | 73 | PM2.5 |
| 2021cleanest | 90 | 36 | 0 | 65 | O3 |
| 2022 | 87 | 37 | 1 | 109 | O3 |
| 2023 | 90 | 37 | 0 | 78 | O3 |
| 2024 | 91 | 39 | 0 | 79 | O3 |
| 2025 | 90 | 42 | 0 | 79 | PM2.5 |
What winter air means in Portland
A typical winter day in Portland sits in the Good range (median AQI 40), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of winter days reached AQI 101+ in 2019–2025.
Winter's median AQI of 40 is in line with Portland's whole-year median of 40, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median Winter AQI across every EPA metro area (278,620 metro-days, 2019–2025) is 40, so Portland runs in line with the national Winter norm; 0.2% of its Winter days topped AQI 100 versus 0.6% nationally (about the same).
Within the season, December is the cleanest month (median AQI 36, 0.0% of days above 100) and February the worst (median 42, 0.5% above 100).
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 season 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.