Bad Air Days in Larimer County, Colorado: 18 a Year
18 bad air days a year
Among the worst tenth of monitored US counties. Ranked 39 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (10% particle days).
Which months to watch
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in July and August and June. Averaged over 2016 to 2025.
What actually helps here
- Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
- Sign up for EnviroFlash ozone alerts for this area during the worst months.
Better or worse than it was
In 2000 to 2009 this county averaged 23 bad days a year; in 2016 to 2025, 18. The air has improved. The worst recent year was 2021, with 50 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2018-06-11 | 169 | Ozone |
| 2024-07-23 | 161 | Ozone |
| 2020-10-21 | 159 | PM2.5 |
| 2021-07-27 | 159 | Ozone |
| 2024-07-24 | 159 | Ozone |
An ordinary day in Larimer
Across 3,653 monitored days from 2016 to 2025, the air in Larimer County rated Good on 47% of them, less often than the 72% typical of monitored counties, and Moderate on 48%. The median day sat at AQI 51; one day in ten went above 88.
The pollutant setting the AQI on an ordinary day here:
- ozone81%2,961 days
- fine particles (PM2.5)19%691 days
Larimer County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 185 | 169 | 11 | 0 | 50 | 119 |
| 2024 | 366 | 156 | 187 | 23 | 2 | 54 | 161 |
| 2023 | 365 | 175 | 185 | 5 | 1 | 51 | 156 |
| 2022 | 365 | 176 | 179 | 10 | 0 | 51 | 126 |
| 2021 | 365 | 164 | 151 | 50 | 4 | 54 | 159 |
| 2020 | 366 | 197 | 147 | 22 | 2 | 49 | 159 |
| 2019 | 365 | 183 | 176 | 6 | 0 | 50 | 129 |
| 2018 | 365 | 160 | 177 | 28 | 3 | 53 | 169 |
| 2017 | 365 | 165 | 187 | 13 | 1 | 52 | 154 |
| 2016 | 366 | 162 | 196 | 8 | 1 | 51 | 151 |
| 2015 | 365 | 177 | 171 | 17 | 1 | 51 | 165 |
| 2014 | 365 | 196 | 152 | 17 | 0 | 50 | 140 |
| 2013 | 365 | 184 | 153 | 28 | 2 | 50 | 164 |
| 2012 | 366 | 160 | 156 | 50 | 5 | 54 | 172 |
| 2011 | 365 | 143 | 189 | 33 | 1 | 58 | 151 |
| 2010 | 365 | 166 | 178 | 21 | 0 | 53 | 140 |
*2025 may be a partial year in EPA's file.
Colorado compared
- Jefferson County28/yr
- Teller Countythin data
- Douglas County20/yr
- Boulder County13/yr
- Weld County12/yr
- Arapahoe County12/yr
- Denver County12/yr
- Clear Creek County11/yr
How this is counted
A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.