What Is the Air Quality in Larimer, Colorado?
Larimer, Colorado has an Air Quality Grade of C (fair) with a 5-year median AQI of 51. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
Larimer, Colorado Air Quality Snapshot
| Air Quality Grade | C51/100 |
| 5-Year Median AQI | 51 (Moderate) |
| Most Recent Median AQI (2023) | 51 (Moderate) |
| Dominant Pollutant | Ground-Level Ozone |
| 10-Year Trend | Stable (+0.07 AQI/yr) |
| Unhealthy Days (last 5 yr) | 93 |
| National Rank (cleanest = #1) | #950 of 1,020 (93th most polluted percentile) |
| Colorado Rank | #29 of 32 |
What Does the C Grade Mean?
Larimer, Colorado earns a C — air quality is fair, but not great. With a 5-year median AQI of 51, the city sees a meaningful number of "Moderate" days each year, when the EPA flags air as a concern for unusually sensitive people.
Larimer, Colorado's 5-year median AQI of 51 is 10 points above the national average of 41 — meaningfully more polluted than the typical U.S. metro tracked here. Within Colorado, Larimer, Colorado runs more polluted than the state average of 39 — local sources or geography are concentrating pollution above the state's typical reading.
For context within Colorado: Alamosa, Colorado currently holds the state's cleanest grade (A, AQI 14), while Jefferson, Colorado sits at the bottom (D, AQI 47).
What's in Larimer, Colorado's Air?
The dominant pollutant in Larimer, Colorado is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Ground-Level Ozone | 304 | 83% |
| Fine Particulate Matter (PM2.5) | 61 | 17% |
Is the Air Getting Better or Worse?
Air quality in Larimer, Colorado has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Larimer, Colorado posted a median AQI of 50. By 2023 that figure was 51 — a rise of 1 AQI points dirtier across 10 years of EPA records.
Year-by-Year AQI in Larimer, Colorado
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 50 | 196 | 17 | Ozone |
| 2015 | 51 | 177 | 17 | Ozone |
| 2016 | 51 | 162 | 8 | Ozone |
| 2017 | 52 | 165 | 13 | Ozone |
| 2018 | 53 | 160 | 28 | Ozone |
| 2019 | 50 | 183 | 6 | Ozone |
| 2020 | 49 | 197 | 22 | Ozone |
| 2021 | 54 | 164 | 50 | Ozone |
| 2022 | 51 | 176 | 10 | Ozone |
| 2023 | 51 | 175 | 5 | Ozone |
Health Context for Larimer, Colorado
Across the past five years, this area has logged 93 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 19 days per year. That is roughly typical for a U.S. metro, with most caution days clustered in summer (ozone) or wildfire season.
Healthy adults can continue normal outdoor activity in most weather, but should pay attention to AQI alerts during the worst pollution windows. People with asthma, heart disease, or pregnancy should reduce prolonged or intense outdoor exertion on flagged days, and consider running an indoor HEPA air cleaner during peak season. Because ozone peaks in the afternoon on hot sunny days, plan outdoor exercise for early morning or after sunset on bad-air days.
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
Larimer, Colorado has an Air Quality Grade of C (fair) with a 5-year median AQI of 51. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
The data source behind this answer is the EPA Air Quality System (AQS). Every figure on the page traces back to that source; the methodology page describes the inputs and the refresh cadence in full detail.
A practical caveat: the headline answer above reflects the most recent the EPA Air Quality System (AQS) vintage; underlying data is often revised for months after first publication, and the right reference for any specific decision is whichever vintage is current at the time of the decision. The as-of date is stamped on every page.
Source: EPA Outdoor Air Quality Data, 2026.