Portland, ME Air Quality in September
Across 2019–2025, Portland's September air quality had a median AQI of 34 (Good) over 210 monitored days in the Portland-South Portland, ME area. 1.0% of September days (2 of 210) reached Unhealthy for Sensitive Groups or worse, and ground-level ozone set the daily AQI on 60.0% of days. The cleanest September on record here was 2022 (median AQI 30, 0 days above 100) and the worst was 2023 (median 46, 2 days above 100, peak AQI 110).
- Good: 87.1%
- Moderate: 11.9%
- Unhealthy for Sensitive Groups: 1.0%
- 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.
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 September?
Good on a typical day: median AQI 34 across 2019–2025, with 1.0% of September days above AQI 100. Ground-level ozone is the main driver.
- ✓87.1% of September days were Good (AQI 0–50), 11.9% Moderate (51–100).
- ✓2 days of 210 reached AQI 101+; the September peak was 110.
- ✓Best September: 2022; worst September: 2023.
Frequently asked questions
What is the air quality like in Portland in September?▾
Across 2019–2025, the median daily AQI in Portland in September was 34 (Good). 87.1% of days were Good, 11.9% Moderate, and 1.0% reached Unhealthy for Sensitive Groups or worse, based on 210 monitored days in EPA's Air Quality System.
Is September a good time for outdoor exercise in Portland?▾
A typical September day in Portland sits in the Good range (median AQI 34), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 100 of September days (1.0%) reached AQI 101+ in 2019–2025.
Which pollutant causes Portland's September air quality problems?▾
Ground-level ozone set the daily AQI on 60.0% of September days in Portland. The highest September AQI recorded in 2019–2025 was 110 (Unhealthy for Sensitive Groups), in 2023.
Portland September air quality, year by year
Portland September 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 | 30 | 45 | 0 | 84 | PM2.5 |
| 2020 | 30 | 31 | 0 | 54 | O3 |
| 2021 | 30 | 31 | 0 | 49 | O3 |
| 2022cleanest | 30 | 30 | 0 | 48 | O3 |
| 2023worst | 30 | 46 | 2 | 110 | PM2.5 |
| 2024 | 30 | 34 | 0 | 52 | O3 |
| 2025 | 30 | 32 | 0 | 56 | O3 |
What September air means in Portland
A typical September day in Portland sits in the Good range (median AQI 34), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 100 of September days (1.0%) reached AQI 101+ in 2019–2025.
September's median AQI of 34 is below Portland's whole-year median of 40, making it one of the area's cleaner periods. Nationally, the median September AQI across every EPA metro area (100,699 metro-days, 2019–2025) is 43, so Portland runs below the national September norm; 1.0% of its September days topped AQI 100 versus 2.7% nationally (below average).
September is part of Portland's fall season (September, October, November); the fall 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.