Portland, ME Air Quality in Fall
Across 2019–2025, Portland's fall (September–November) air quality had a median AQI of 33 (Good) over 637 monitored days in the Portland-South Portland, ME area. 0.5% of fall days (3 of 637) reached Unhealthy for Sensitive Groups or worse, and ground-level ozone set the daily AQI on 58.9% of days. The cleanest fall on record here was 2021 (median AQI 30, 0 days above 100) and the worst was 2023 (median 38, 3 days above 100, peak AQI 120).
- Good: 87.3%
- Moderate: 12.2%
- Unhealthy for Sensitive Groups: 0.5%
- 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
As an Amazon Associate, we earn from qualifying purchases.
How is the air in Portland in fall?
Good on a typical day: median AQI 33 across 2019–2025, with 0.5% of fall days above AQI 100. Ground-level ozone is the main driver.
- ✓87.3% of fall days were Good (AQI 0–50), 12.2% Moderate (51–100).
- ✓3 days of 637 reached AQI 101+; the fall peak was 120.
- ✓Cleanest month: November; worst month: September.
Frequently asked questions
What is the air quality like in Portland in fall?▾
Across 2019–2025, the median daily AQI in Portland in fall was 33 (Good). 87.3% of days were Good, 12.2% Moderate, and 0.5% 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 Portland?▾
A typical fall day in Portland sits in the Good range (median AQI 33), 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 Portland's fall air quality problems?▾
Ground-level ozone set the daily AQI on 58.9% of fall days in Portland. The highest fall AQI recorded in 2019–2025 was 120 (Unhealthy for Sensitive Groups), in 2023.
Portland fall air quality, year by year
Portland fall air quality by year (Portland-South Portland, ME, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 91 | 44 | 0 | 84 | PM2.5 |
| 2020 | 91 | 32 | 0 | 97 | O3 |
| 2021cleanest | 91 | 30 | 0 | 56 | O3 |
| 2022 | 91 | 31 | 0 | 67 | O3 |
| 2023worst | 91 | 38 | 3 | 120 | PM2.5 |
| 2024 | 91 | 32 | 0 | 60 | O3 |
| 2025 | 91 | 31 | 0 | 74 | O3 |
What fall air means in Portland
A typical fall day in Portland sits in the Good range (median AQI 33), 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 33 is below Portland's whole-year median of 40, 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 Portland runs below the national Fall norm; 0.5% of its Fall days topped AQI 100 versus 1.5% nationally (about the same).
Within the season, November is the cleanest month (median AQI 33, 0.0% of days above 100) and September the worst (median 34, 1.0% 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.