Median AQI
54
Moderate
Colorado · County air quality · 2025
Larimer County, Colorado
Median AQI 54 (Moderate). 40.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 54
- Median AQI
- 40.1%
- Good-air days
- 0
- Unhealthy days
- 3%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Larimer County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 54 with 40.1% of 274 monitored days in the Good band, driven mainly by Ozone.
- 54
- median AQI · Moderate
- 40.1%
- days rated Good
- Ozone
- main pollutant
- 3%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 119.
Data updated July 2026
- Good · 110 d · 40.1%
- Moderate · 153 d · 55.8%
- USG · 11 d · 4.0%
How did Larimer County's air break down in 2025?
Across 274 monitored days, 110 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 119 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 274 days monitored. Median AQI: 54 (Moderate).
Insights: Larimer County, Colorado. EPA monitors recorded a median Air Quality Index of 54 across 274 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 110 (40%) fell in the Good band (AQI 0–50), 153 (56%) registered Moderate readings, and 11 (4%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 87, while the worst single-day peak hit 119 (USG). Residents averaged about 12.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (236 days), PM2.5 (37 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -87 and the median AQI moved +5 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 119, usg but never Unhealthy. Nearby counties show comparable readings: Adams (50.2% good, AQI 50); Alamosa (90.8% good, AQI 24); Arapahoe (47.3% good, AQI 51); Archuleta (79.5% good, AQI 32); Boulder (50.9% good, AQI 50); Clear Creek (64.4% good, AQI 48).
Air Quality Summary
Good Days
40.1%
110 of 274 monitored days
-34.9pp vs 75% target
Max AQI Recorded
119
USG
Health Grade
Poor
90th-pct AQI 87
Larimer County, Good days share 40.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Larimer County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 274 monitored days across EPA AQI categories.
Good
110
40%
Moderate
153
56%
USG
11
4%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Larimer County.
- Ozone · 236 days · 86.1%
- PM2.5 · 37 days · 13.5%
- CO · 1 days · 0.4%
Health Impact Assessment
Poor
Larimer County's air quality is rated Poor based on 40.1% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 40.1% of monitored days in 2025, with a median AQI of 54. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 236 of 274 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Larimer County. Best year: 2020 (197 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 197 | 147 | 20 | 2 | 49 | 159 |
| 2021 | 164 | 151 | 46 | 4 | 54 | 159 |
| 2022 | 176 | 179 | 10 | 0 | 51 | 126 |
| 2023 | 175 | 185 | 4 | 1 | 51 | 156 |
| 2024 | 156 | 187 | 21 | 2 | 54 | 161 |
| 2025 | 110 | 153 | 11 | 0 | 54 | 119 |
How does Larimer County compare?
How Larimer County compares
Good-day share worse than US avg
Larimer County 40.1%
Adams County, Colorado 50.2%
US average 76.5%
Median AQI worse than US avg
Larimer County 54
Adams County, Colorado 50
US average 38
Unhealthy days / yr about average
Larimer County 0
Adams County, Colorado 0
US average 0
Other Counties in Colorado
Air quality comparison for nearby counties in Colorado.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Larimer County.
Denver County, Colorado
AQI 54 · 44.2% good
Franklin County, Ohio
AQI 54 · 39.8% good
Fresno County, California
AQI 54 · 40.1% good
Hamilton County, Ohio
AQI 54 · 32.1% good
Hidalgo County, Texas
AQI 54 · 36.2% good
Inyo County, California
AQI 54 · 40.7% good
What Larimer County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 54 means for exposure. Health guidance
- See how Larimer County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Larimer County with its Colorado neighbours before drawing a conclusion about the region. All of Colorado
Based on 274 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Larimer County, Colorado? ▼
Larimer County has a median Air Quality Index (AQI) of 54, rated "Moderate." 40.1% of monitored days (110 out of 274) had good air quality in 2025.
What is the main pollutant in Larimer County? ▼
The primary pollutant in Larimer County is Ozone (O3), which was the main AQI driver on 236 days (86% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Larimer County have? ▼
In 2025, Larimer County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 119.
Is the air quality in Larimer County getting better or worse? ▼
Air quality in Larimer County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 197 good days. Worst year: 2025 with 110 good days.
What does AQI 54 mean for health? ▼
Larimer County's median AQI of 54 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Larimer County compare to other counties in Colorado? ▼
Larimer County has 40.1% good air days. Nearby counties in Colorado: Adams (50.2% good), Alamosa (90.8% good), Arapahoe (47.3% good), Archuleta (79.5% good), Boulder (50.9% good), Clear Creek (64.4% good).
What is the 90th percentile AQI in Larimer County? ▼
The 90th percentile AQI for Larimer County is 87, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 119 (USG).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.