Denver-Aurora-Lakewood, CO Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Denver-Aurora-Lakewood, CO recorded 17 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 3 days. Since 2000, unhealthy days are 89% 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
Ozone since 1990
The index counts above only start in 2000, because nationwide fine-particle monitoring did not exist before 1999 and an index without it would measure the monitoring network rather than the air. Ozone does have a 35-year record. This is Denver-Aurora-Lakewood, CO's, from EPA's long-running trend monitors: 76 ppb in 1990 to 78 ppb in 2024, 3% higher.
Peak ozone, 1990 to 2024
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 106 | 29% |
| Moderate | 242 | 66% |
| Unhealthy for sensitive groups | 17 | 5% |
| Unhealthy or worse | 0 | 0% |
Median AQI 58 · 90th-percentile AQI 89 · worst day 122. Scored under current rules.
What drove the unhealthy days
Of the 17 days above 100 in 2025, 17 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. 14 of these 17 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| Apr 12 | 105 | Ozone | 90 (not unhealthy) |
| May 10 | 115 | Ozone | 100 (not unhealthy) |
| May 12 | 101 | Ozone | 87 (not unhealthy) |
| May 13 | 101 | Ozone | 87 (not unhealthy) |
| May 23 | 115 | Ozone | 100 (not unhealthy) |
| Jun 16 | 108 | Ozone | 93 (not unhealthy) |
| Jun 26 | 105 | Ozone | 90 (not unhealthy) |
| Jul 1 | 105 | Ozone | 90 (not unhealthy) |
| Jul 2 | 101 | Ozone | 87 (not unhealthy) |
| Jul 3 | 115 | Ozone | 100 (not unhealthy) |
| Jul 11 | 105 | Ozone | 90 (not unhealthy) |
| Jul 12 | 122 | Ozone | 104 |
| Jul 13 | 119 | Ozone | 101 |
| Jul 19 | 108 | Ozone | 93 (not unhealthy) |
| Aug 20 | 101 | Ozone | 87 (not unhealthy) |
| Aug 21 | 119 | Ozone | 101 |
| Sep 4 | 108 | Ozone | 93 (not unhealthy) |
Statewide context
Denver-Aurora-Lakewood, CO sits in Colorado.
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 51 | 27 | 366 |
| 2001 | 40 | 26 | 365 |
| 2002 | 45 | 30 | 365 |
| 2003 | 62 | 44 | 365 |
| 2004 | 26 | 11 | 366 |
| 2005 | 41 | 18 | 365 |
| 2006 | 60 | 33 | 365 |
| 2007 | 90 | 59 | 365 |
| 2008 | 53 | 32 | 366 |
| 2009 | 15 | 8 | 365 |
| 2010 | 38 | 18 | 365 |
| 2011 | 54 | 18 | 365 |
| 2012 | 68 | 32 | 366 |
| 2013 | 44 | 24 | 365 |
| 2014 | 34 | 12 | 365 |
| 2015 | 26 | 15 | 365 |
| 2016 | 28 | 16 | 366 |
| 2017 | 39 | 10 | 365 |
| 2018 | 56 | 34 | 365 |
| 2019 | 24 | 7 | 365 |
| 2020 | 35 | 24 | 366 |
| 2021 | 69 | 45 | 365 |
| 2022 | 36 | 17 | 365 |
| 2023 | 29 | 14 | 365 |
| 2024 | 41 | 22 | 366 |
| 2025 | 17 | 3 | 365 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.