Fort Collins Air Quality in Fall
Across 2019–2025, Fort Collins's fall (September–November) air quality had a median AQI of 48 (Good) over 637 monitored days in the Fort Collins, CO area. 4.1% of fall days (26 of 637) reached Unhealthy for Sensitive Groups or worse, and ground-level ozone set the daily AQI on 74.7% of days. The cleanest fall on record here was 2025 (median AQI 47, 0 days above 100) and the worst was 2020 (median 55, 11 days above 100, peak AQI 159).
- Good: 59.7%
- Moderate: 36.3%
- Unhealthy for Sensitive Groups: 3.8%
- Unhealthy or worse: 0.3%
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 fall?
Good on a typical day: median AQI 48 across 2019–2025, with 4.1% of fall days above AQI 100. Ground-level ozone is the main driver.
- ✓59.7% of fall days were Good (AQI 0–50), 36.3% Moderate (51–100).
- ✓26 days of 637 reached AQI 101+; the fall peak was 159.
- ✓Cleanest month: November; worst month: September.
Frequently asked questions
What is the air quality like in Fort Collins in fall?▾
Across 2019–2025, the median daily AQI in Fort Collins in fall was 48 (Good). 59.7% of days were Good, 36.3% Moderate, and 4.1% 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 Fort Collins?▾
A typical fall day in Fort Collins sits in the Good range (median AQI 48), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 24 of fall days (4.1%) reached AQI 101+ in 2019–2025.
Which pollutant causes Fort Collins's fall air quality problems?▾
Ground-level ozone set the daily AQI on 74.7% of fall days in Fort Collins. The highest fall AQI recorded in 2019–2025 was 159 (Unhealthy), in 2020.
Fort Collins fall air quality, year by year
Fort Collins fall air quality by year (Fort Collins, CO, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019 | 91 | 46 | 1 | 101 | O3 |
| 2020worst | 91 | 55 | 11 | 159 | PM2.5 |
| 2021 | 91 | 48 | 11 | 143 | O3 |
| 2022 | 91 | 47 | 3 | 112 | O3 |
| 2023 | 91 | 48 | 0 | 90 | O3 |
| 2024 | 91 | 54 | 0 | 90 | O3 |
| 2025cleanest | 91 | 47 | 0 | 88 | O3 |
What fall air means in Fort Collins
A typical fall day in Fort Collins sits in the Good range (median AQI 48), so running, cycling and children's outdoor play are fine for everyone on most days; about 1 day in 24 of fall days (4.1%) reached AQI 101+ in 2019–2025.
Fall's median AQI of 48 is in line with Fort Collins's whole-year median of 51, 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 Fort Collins runs above the national Fall norm; 4.1% of its Fall days topped AQI 100 versus 1.5% nationally (above average).
Within the season, November is the cleanest month (median AQI 45, 0.0% of days above 100) and September the worst (median 58, 8.6% above 100).
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 season 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.