Portland, OR Air Quality in September
Across 2019–2025, Portland's September air quality had a median AQI of 40 (Good) over 210 monitored days in the Portland-Vancouver-Hillsboro, OR-WA area. 5.7% of September days (12 of 210) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 55.2% of days. The cleanest September on record here was 2019 (median AQI 36, 0 days above 100) and the worst was 2020 (median 55, 11 days above 100, peak AQI 874).
- Good: 69.5%
- Moderate: 24.8%
- Unhealthy for Sensitive Groups: 1.9%
- Unhealthy or worse: 3.8%
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How is the air in Portland in September?
Good on a typical day: median AQI 40 across 2019–2025, with 5.7% of September days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓69.5% of September days were Good (AQI 0–50), 24.8% Moderate (51–100).
- ✓12 days of 210 reached AQI 101+; the September peak was 874.
- ✓Best September: 2019; worst September: 2020.
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 40 (Good). 69.5% of days were Good, 24.8% Moderate, and 5.7% 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 40), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 18 of September days (5.7%) reached AQI 101+ in 2019–2025.
Which pollutant causes Portland's September air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 55.2% of September days in Portland. The highest September AQI recorded in 2019–2025 was 874 (Hazardous), in 2020.
Portland September air quality, year by year
Portland September air quality by year (Portland-Vancouver-Hillsboro, OR-WA, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019cleanest | 30 | 36 | 0 | 55 | PM2.5 |
| 2020worst | 30 | 55 | 11 | 874 | PM2.5 |
| 2021 | 30 | 39 | 0 | 73 | O3 |
| 2022 | 30 | 43 | 0 | 97 | PM2.5 |
| 2023 | 30 | 38 | 0 | 87 | O3 |
| 2024 | 30 | 34 | 1 | 120 | O3 |
| 2025 | 30 | 46 | 0 | 68 | PM2.5 |
What September air means in Portland
A typical September 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; about 1 day in 18 of September days (5.7%) reached AQI 101+ in 2019–2025.
September's median AQI of 40 is in line with Portland's whole-year median of 41, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median September AQI across every EPA metro area (100,699 metro-days, 2019–2025) is 43, so Portland runs in line with the national September norm; 5.7% of its September days topped AQI 100 versus 2.7% nationally (above 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 (PM2.5). 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-Vancouver-Hillsboro, OR-WA), 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.