Fort Collins Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for Fort Collins (Fort Collins, CO area) was July 23, 2024, when the daily AQI reached 161 (Unhealthy), set by ground-level ozone, in peak ozone season. Across 2,557 monitored days in 2019–2025, 9 of the 10 worst days were Unhealthy (AQI 151+) or worse; 8 of the 10 were driven by ground-level ozone and 2 by fine particulate matter (PM2.5), and 4 of them fell in 2021. The cleanest day was October 1, 2019 at AQI 19 (Good); the 10 best days span 4 different years.
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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When were Fort Collins's worst and best air quality days?
Worst: July 23, 2024, AQI 161 (Unhealthy, O3). Best: October 1, 2019, AQI 19 (Good). Based on 2,557 EPA-monitored days in the Fort Collins, CO area, 2019–2025.
- ✓9 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 150 (Unhealthy for Sensitive Groups).
- ✓Ground-level ozone drove 8 of the 10 worst days.
- ✓2021 accounts for 4 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in Fort Collins?▾
July 23, 2024: the Fort Collins, CO area's daily AQI hit 161 (Unhealthy), driven by ground-level ozone, in peak ozone season. That is the highest of 2,557 monitored days in 2019–2025.
What was the best air quality day in Fort Collins?▾
October 1, 2019, with a daily AQI of 19 (Good). The daily AQI is the highest reading from any monitor in the area, so even the cleanest day reflects the area's worst monitor.
What was Fort Collins's worst air quality day in 2025?▾
June 19, 2025, AQI 119 (Unhealthy for Sensitive Groups), set by ground-level ozone, in peak ozone season.
Which pollutant causes Fort Collins's worst air days?▾
Ground-level ozone set the AQI on 8 of the 10 worst days; fine particulate matter (PM2.5) on 2. The tables above list the defining pollutant for every day.
Fort Collins's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the Fort Collins, CO area (EPA AQS). The daily AQI is the highest reading from any monitor in the area that day.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2024 page) | 161 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 2 | (2024 page) | 159 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 3 | (2021 page) | 159 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 4 | (2020 page) | 159 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 5 | (2023 page) | 156 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 6 | (2021 page) | 156 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 7 | (2020 page) | 155 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 8 | (2021 page) | 154 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 9 | (2021 page) | 151 | Unhealthy | O3 — ground-level ozone |
| 10 | (2024 page) | 150 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
Worst day of each year in Fort Collins
The single highest-AQI day in each year on record. Click a year for its full month-by-month statistics.
| Year | Worst day | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 2019 | 129 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2020 | 159 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2021 | 159 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2022 | 126 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2023 | 156 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2024 | 161 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2025 | 119 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
Fort Collins's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the Fort Collins, CO area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2019 page) | 19 | Good | O3 — ground-level ozone |
| 2 | (2025 page) | 20 | Good | O3 — ground-level ozone |
| 3 | (2020 page) | 21 | Good | O3 — ground-level ozone |
| 4 | (2019 page) | 24 | Good | O3 — ground-level ozone |
| 5 | (2025 page) | 27 | Good | O3 — ground-level ozone |
| 6 | (2019 page) | 27 | Good | O3 — ground-level ozone |
| 7 | (2020 page) | 29 | Good | O3 — ground-level ozone |
| 8 | (2023 page) | 30 | Good | O3 — ground-level ozone |
| 9 | (2020 page) | 30 | Good | O3 — ground-level ozone |
| 10 | (2019 page) | 30 | Good | O3 — ground-level ozone |
Right now in Fort Collins
The latest EPA AirNow observation puts Fort Collins's AQI at 36 (O3). The days above are historical records, 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.