Washington Air Quality in September
Across 2019–2025, Washington's September air quality had a median AQI of 48 (Good) over 210 monitored days in the Washington-Arlington-Alexandria, DC-VA-MD-WV area. 0.5% of September days (1 of 210) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 50.0% of days. The cleanest September on record here was 2024 (median AQI 44, 0 days above 100) and the worst was 2019 (median 55, 1 day above 100, peak AQI 112).
- Good: 57.1%
- Moderate: 42.4%
- Unhealthy for Sensitive Groups: 0.5%
- Unhealthy or worse: 0.0%
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How is the air in Washington in September?
Good on a typical day: median AQI 48 across 2019–2025, with 0.5% of September days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓57.1% of September days were Good (AQI 0–50), 42.4% Moderate (51–100).
- ✓1 day of 210 reached AQI 101+; the September peak was 112.
- ✓Best September: 2024; worst September: 2019.
Frequently asked questions
What is the air quality like in Washington in September?▾
Across 2019–2025, the median daily AQI in Washington in September was 48 (Good). 57.1% of days were Good, 42.4% Moderate, and 0.5% 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 Washington?▾
A typical September day in Washington sits in the Good range (median AQI 48), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of September days reached AQI 101+ in 2019–2025.
Which pollutant causes Washington's September air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 50.0% of September days in Washington. The highest September AQI recorded in 2019–2025 was 112 (Unhealthy for Sensitive Groups), in 2019.
Washington September air quality, year by year
Washington September air quality by year (Washington-Arlington-Alexandria, DC-VA-MD-WV, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019worst | 30 | 55 | 1 | 112 | O3 |
| 2020 | 30 | 46 | 0 | 58 | PM2.5 |
| 2021 | 30 | 49 | 0 | 73 | PM2.5 |
| 2022 | 30 | 46 | 0 | 80 | PM2.5 |
| 2023 | 30 | 48 | 0 | 84 | O3 |
| 2024cleanest | 30 | 44 | 0 | 87 | O3 |
| 2025 | 30 | 50 | 0 | 90 | O3 |
What September air means in Washington
A typical September day in Washington sits in the Good range (median AQI 48), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of September days reached AQI 101+ in 2019–2025.
September's median AQI of 48 is below Washington's whole-year median of 52, 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 Washington runs above the national September norm; 0.5% of its September days topped AQI 100 versus 2.7% nationally (below average).
September is part of Washington's fall season (September, October, November); the fall page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Washington
The latest EPA AirNow observation puts Washington's AQI at 41 (PM2.5). The figures above are multi-year month statistics, not today's reading.
View current Washington air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Washington-Arlington-Alexandria, DC-VA-MD-WV), 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.