Fort Collins Air Quality in September
Across 2019–2025, Fort Collins's September air quality had a median AQI of 58 (Moderate) over 210 monitored days in the Fort Collins, CO area. 8.6% of September days (18 of 210) reached Unhealthy for Sensitive Groups or worse, and ground-level ozone set the daily AQI on 87.6% of days. The cleanest September on record here was 2025 (median AQI 49, 0 days above 100) and the worst was 2021 (median 93, 11 days above 100, peak AQI 143).
- Good: 35.2%
- Moderate: 56.2%
- Unhealthy for Sensitive Groups: 8.6%
- Unhealthy or worse: 0.0%
Reduce Indoor Ozone in Fort Collins
Today's main pollutant in Fort Collins is ground-level ozone (AQI 36). 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
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How is the air in Fort Collins in September?
Moderate on a typical day: median AQI 58 across 2019–2025, with 8.6% of September days above AQI 100. Ground-level ozone is the main driver.
- ✓35.2% of September days were Good (AQI 0–50), 56.2% Moderate (51–100).
- ✓18 days of 210 reached AQI 101+; the September peak was 143.
- ✓Best September: 2025; worst September: 2021.
Frequently asked questions
What is the air quality like in Fort Collins in September?▾
Across 2019–2025, the median daily AQI in Fort Collins in September was 58 (Moderate). 35.2% of days were Good, 56.2% Moderate, and 8.6% 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 Fort Collins?▾
A typical September day in Fort Collins is Moderate (median AQI 58): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 12 of September days (8.6%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes Fort Collins's September air quality problems?▾
Ground-level ozone set the daily AQI on 87.6% of September days in Fort Collins. The highest September AQI recorded in 2019–2025 was 143 (Unhealthy for Sensitive Groups), in 2021.
Fort Collins September air quality, year by year
Fort Collins September air quality by year (Fort Collins, CO, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 30 | 53 | 1 | 101 | O3 |
| 2020 | 30 | 73 | 3 | 133 | PM2.5 |
| 2021worst | 30 | 93 | 11 | 143 | O3 |
| 2022 | 30 | 53 | 3 | 112 | O3 |
| 2023 | 30 | 54 | 0 | 90 | O3 |
| 2024 | 30 | 64 | 0 | 90 | O3 |
| 2025cleanest | 30 | 49 | 0 | 88 | O3 |
What September air means in Fort Collins
A typical September day in Fort Collins is Moderate (median AQI 58): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 12 of September days (8.6%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
September's median AQI of 58 is above Fort Collins's whole-year median of 51, making it one of the area's dirtier periods. Nationally, the median September AQI across every EPA metro area (100,699 metro-days, 2019–2025) is 43, so Fort Collins runs above the national September norm; 8.6% of its September days topped AQI 100 versus 2.7% nationally (above average).
September is part of Fort Collins's fall season (September, October, November); the fall page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Fort Collins
The latest EPA AirNow observation puts Fort Collins's AQI at 36 (O3). The figures above are multi-year month statistics, not today's reading.
View current Fort Collins air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Fort Collins, CO), 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.