Colorado Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Colorado recorded 32 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by ozone. Under the AQI rules that applied before 2013, the same air would have scored 11 days. Since 2000, unhealthy days are 79% lower on a consistent definition.
How to read a statewide figure. The Air Quality Index describes one place at one time. A statewide number is the worst monitor anywhere in the state on each day, so it answers “was the air unhealthy somewhere in Colorado?” — not “what did a typical resident breathe?” In a large state with many monitors it will read high even if most of the state was clean. EPA publishes the AQI for metropolitan areas rather than states for exactly this reason. For what people actually breathed, use the metro pages below.
Unhealthy days by year, scored two ways
EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.
Scored by today's rules
Scored by the pre-2013 rules
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 53 | 15% |
| Moderate | 280 | 77% |
| Unhealthy for sensitive groups | 29 | 8% |
| Unhealthy or worse | 3 | 1% |
Median AQI 64 · 90th-percentile AQI 97 · worst day 652. Scored under current rules.
What drove the unhealthy days
Of the 32 days above 100 in 2025, 26 were driven by ozone and 4 by fine particles, with 2 from another pollutant. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke. In the western United States, ozone is a different problem: the floor is rising, with even the cleanest summer days at high elevation now near 40 ppb, and bad years track wildfire seasons. Domestic emission cuts have less room to work against that.
The unhealthy days in 2025
Every day the index passed 100, what it reached, and which pollutant drove it. 21 of these 32 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| Mar 14 | 652 | Coarse particles | 637 |
| Mar 18 | 261 | Coarse particles | 261 |
| Apr 12 | 105 | Ozone | 90 (not unhealthy) |
| Apr 28 | 101 | Ozone | 87 (not unhealthy) |
| May 9 | 101 | Ozone | 87 (not unhealthy) |
| May 10 | 119 | Ozone | 101 |
| May 12 | 108 | Ozone | 93 (not unhealthy) |
| May 13 | 108 | Ozone | 93 (not unhealthy) |
| May 14 | 105 | Ozone | 90 (not unhealthy) |
| May 23 | 115 | Ozone | 100 (not unhealthy) |
| Jun 15 | 115 | Ozone | 100 (not unhealthy) |
| Jun 16 | 108 | Ozone | 93 (not unhealthy) |
| Jun 18 | 115 | Ozone | 100 (not unhealthy) |
| Jun 19 | 119 | Ozone | 101 |
| Jun 21 | 108 | Ozone | 93 (not unhealthy) |
| Jun 26 | 105 | Ozone | 90 (not unhealthy) |
| Jun 27 | 101 | Ozone | 87 (not unhealthy) |
| Jul 1 | 105 | Ozone | 90 (not unhealthy) |
| Jul 2 | 108 | Ozone | 93 (not unhealthy) |
| Jul 3 | 115 | Ozone | 100 (not unhealthy) |
| Jul 11 | 112 | Ozone | 97 (not unhealthy) |
| Jul 12 | 122 | Ozone | 104 |
| Jul 13 | 119 | Ozone | 101 |
| Jul 16 | 103 | Fine particles | 92 (not unhealthy) |
| Jul 19 | 108 | Ozone | 93 (not unhealthy) |
| Jul 21 | 101 | Ozone | 87 (not unhealthy) |
| Aug 4 | 119 | Ozone | 101 |
| Aug 9 | 131 | Fine particles | 115 |
| Aug 10 | 154 | Fine particles | 138 |
| Aug 20 | 101 | Ozone | 87 (not unhealthy) |
| Aug 21 | 119 | Ozone | 101 |
| Sep 4 | 119 | Fine particles | 106 |
Metro areas in Colorado
Ranked by unhealthy days in their latest year. These are the geographies EPA actually reports the AQI for.
| Metro area | Unhealthy days | Pre-2013 rules | Main driver |
|---|---|---|---|
| Denver-Aurora-Lakewood, CO | 17 | 3 | ozone |
| Fort Collins, CO | 11 | 2 | ozone |
| Greeley, CO | 6 | 1 | ozone |
| Boulder, CO | 3 | 1 | ozone |
| Durango, CO | 2 | 0 | ozone |
| Glenwood Springs, CO | 2 | 2 | fine particles |
| Colorado Springs, CO | 0 | 0 | ozone and fine particles equally |
| Grand Junction, CO | 0 | 0 | ozone and fine particles equally |
| Steamboat Springs, CO | 0 | 0 | ozone and fine particles equally |
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 82 | 53 | 366 |
| 2001 | 48 | 30 | 365 |
| 2002 | 72 | 50 | 365 |
| 2003 | 77 | 53 | 365 |
| 2004 | 35 | 13 | 366 |
| 2005 | 57 | 31 | 365 |
| 2006 | 82 | 49 | 365 |
| 2007 | 102 | 67 | 365 |
| 2008 | 71 | 43 | 366 |
| 2009 | 46 | 26 | 365 |
| 2010 | 62 | 34 | 365 |
| 2011 | 77 | 40 | 365 |
| 2012 | 104 | 58 | 366 |
| 2013 | 79 | 45 | 365 |
| 2014 | 51 | 25 | 365 |
| 2015 | 40 | 24 | 365 |
| 2016 | 42 | 20 | 366 |
| 2017 | 48 | 16 | 365 |
| 2018 | 76 | 46 | 365 |
| 2019 | 33 | 11 | 365 |
| 2020 | 52 | 41 | 366 |
| 2021 | 83 | 55 | 365 |
| 2022 | 46 | 24 | 365 |
| 2023 | 48 | 20 | 365 |
| 2024 | 48 | 24 | 366 |
| 2025 | 32 | 11 | 365 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.