Boulder, CO Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Boulder, CO recorded 3 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 1 days. Since 2000, unhealthy days are 50% lower on a consistent definition.
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 | 214 | 59% |
| Moderate | 148 | 41% |
| Unhealthy for sensitive groups | 3 | 1% |
| Unhealthy or worse | 0 | 0% |
Median AQI 48 · 90th-percentile AQI 71 · worst day 119. Scored under current rules.
What drove the unhealthy days
Of the 3 days above 100 in 2025, 3 were driven by ozone and 0 by fine particles. 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. 2 of these 3 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| May 10 | 101 | Ozone | 87 (not unhealthy) |
| Jul 3 | 115 | Ozone | 100 (not unhealthy) |
| Aug 4 | 119 | Ozone | 101 |
Statewide context
Boulder, CO sits in Colorado.
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 5 | 2 | 366 |
| 2001 | 5 | 2 | 365 |
| 2002 | 12 | 5 | 365 |
| 2003 | 23 | 18 | 365 |
| 2004 | 0 | 0 | 366 |
| 2005 | 8 | 4 | 365 |
| 2006 | 19 | 10 | 365 |
| 2007 | 22 | 8 | 365 |
| 2008 | 16 | 8 | 366 |
| 2009 | 10 | 3 | 365 |
| 2010 | 12 | 7 | 365 |
| 2011 | 16 | 5 | 365 |
| 2012 | 34 | 17 | 362 |
| 2013 | 16 | 7 | 364 |
| 2014 | 5 | 2 | 362 |
| 2015 | 7 | 3 | 365 |
| 2016 | 0 | 0 | 204 |
| 2017 | 11 | 2 | 359 |
| 2018 | 29 | 6 | 363 |
| 2019 | 5 | 2 | 365 |
| 2020 | 21 | 14 | 366 |
| 2021 | 35 | 21 | 365 |
| 2022 | 7 | 2 | 365 |
| 2023 | 6 | 3 | 365 |
| 2024 | 10 | 7 | 366 |
| 2025 | 3 | 1 | 365 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.