St. Louis Air Quality in Fall
Across 2019–2025, St. Louis's fall (September–November) air quality had a median AQI of 57 (Moderate) over 637 monitored days in the St. Louis, MO-IL area. 3.4% of fall days (22 of 637) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 74.3% of days. The cleanest fall on record here was 2020 (median AQI 57, 1 day above 100) and the worst was 2025 (median 60, 9 days above 100, peak AQI 164).
- Good: 22.6%
- Moderate: 73.9%
- Unhealthy for Sensitive Groups: 3.1%
- Unhealthy or worse: 0.3%
Extra Protection for St. Louis Homes
Air quality in St. Louis is moderate (AQI 60), with fine particles (PM2.5) the main concern. A True-HEPA purifier gives sensitive groups — kids, seniors and anyone with asthma or allergies — cleaner air indoors on days like today.
Recommended: Coway AP-1512HH Mighty — True HEPA + carbon · CADR ~246 CFM · rooms up to 361 sq ft
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How is the air in St. Louis in fall?
Moderate on a typical day: median AQI 57 across 2019–2025, with 3.4% of fall days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓22.6% of fall days were Good (AQI 0–50), 73.9% Moderate (51–100).
- ✓22 days of 637 reached AQI 101+; the fall peak was 164.
- ✓Cleanest month: November; worst month: September.
Frequently asked questions
What is the air quality like in St. Louis in fall?▾
Across 2019–2025, the median daily AQI in St. Louis in fall was 57 (Moderate). 22.6% of days were Good, 73.9% Moderate, and 3.4% 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 St. Louis?▾
A typical fall day in St. Louis is Moderate (median AQI 57): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 29 of fall days (3.4%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes St. Louis's fall air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 74.3% of fall days in St. Louis. The highest fall AQI recorded in 2019–2025 was 164 (Unhealthy), in 2025.
St. Louis fall air quality, year by year
St. Louis fall air quality by year (St. Louis, MO-IL, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 91 | 57 | 2 | 129 | PM2.5 |
| 2020cleanest | 91 | 57 | 1 | 111 | PM2.5 |
| 2021 | 91 | 57 | 1 | 119 | PM2.5 |
| 2022 | 91 | 57 | 2 | 111 | PM2.5 |
| 2023 | 91 | 59 | 1 | 108 | PM2.5 |
| 2024 | 91 | 55 | 6 | 131 | PM2.5 |
| 2025worst | 91 | 60 | 9 | 164 | PM2.5 |
What fall air means in St. Louis
A typical fall day in St. Louis is Moderate (median AQI 57): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 29 of fall days (3.4%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Fall's median AQI of 57 is in line with St. Louis's whole-year median of 58, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median Fall AQI across every EPA metro area (296,275 metro-days, 2019–2025) is 40, so St. Louis runs above the national Fall norm; 3.4% of its Fall days topped AQI 100 versus 1.5% nationally (above average).
Within the season, November is the cleanest month (median AQI 56, 1.0% of days above 100) and September the worst (median 59, 6.7% above 100).
Activity-specific thresholds: running, cycling, kids' sports.
Right now in St. Louis
The latest EPA AirNow observation puts St. Louis's AQI at 60 (PM2.5). The figures above are multi-year season statistics, not today's reading.
View current St. Louis air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (St. Louis, MO-IL), 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.