Ann Arbor Air Quality in September
Across 2019–2025, Ann Arbor's September air quality had a median AQI of 43 (Good) over 210 monitored days in the Ann Arbor, MI area. 1.0% of September days (2 of 210) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 54.3% of days. The cleanest September on record here was 2019 (median AQI 41, 0 days above 100) and the worst was 2025 (median 44, 2 days above 100, peak AQI 108).
- Good: 70.0%
- Moderate: 29.0%
- Unhealthy for Sensitive Groups: 1.0%
- Unhealthy or worse: 0.0%
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How is the air in Ann Arbor in September?
Good on a typical day: median AQI 43 across 2019–2025, with 1.0% of September days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓70.0% of September days were Good (AQI 0–50), 29.0% Moderate (51–100).
- ✓2 days of 210 reached AQI 101+; the September peak was 108.
- ✓Best September: 2019; worst September: 2025.
Frequently asked questions
What is the air quality like in Ann Arbor in September?▾
Across 2019–2025, the median daily AQI in Ann Arbor in September was 43 (Good). 70.0% of days were Good, 29.0% 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 Ann Arbor?▾
A typical September day in Ann Arbor sits in the Good range (median AQI 43), 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 Ann Arbor's September air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 54.3% of September days in Ann Arbor. The highest September AQI recorded in 2019–2025 was 108 (Unhealthy for Sensitive Groups), in 2025.
Ann Arbor September air quality, year by year
Ann Arbor September air quality by year (Ann Arbor, MI, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019cleanest | 30 | 41 | 0 | 64 | PM2.5 |
| 2020 | 30 | 43 | 0 | 61 | PM2.5 |
| 2021 | 30 | 42 | 0 | 58 | O3 |
| 2022 | 30 | 44 | 0 | 59 | PM2.5 |
| 2023 | 30 | 49 | 0 | 59 | PM2.5 |
| 2024 | 30 | 43 | 0 | 80 | O3 |
| 2025worst | 30 | 44 | 2 | 108 | O3 |
What September air means in Ann Arbor
A typical September day in Ann Arbor sits in the Good range (median AQI 43), 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 43 is in line with Ann Arbor's whole-year median of 44, 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 Ann Arbor runs in line with the national September norm; 1.0% of its September days topped AQI 100 versus 2.7% nationally (below average).
September is part of Ann Arbor's fall season (September, October, November); the fall page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Ann Arbor
The latest EPA AirNow observation puts Ann Arbor's AQI at 29 (PM2.5). The figures above are multi-year month statistics, not today's reading.
View current Ann Arbor air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Ann Arbor, MI), 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.