Right now: Boulder is at 44, good.
Is the air safe to breathe in Boulder today?
In Boulder, BreatheCheck reads 44 as Go. The cutoffs: go below 101, keep it short and easy from 101 to 150, move it indoors above 150. These are our own thresholds, drawn on the EPA bands — EPA itself classifies 101–150 as Unhealthy for Sensitive Groups and does not tell you how long to stay out.
Go AQI 0–100
Normal outdoor plans. No change needed.
Short & unloaded AQI 101–150
A 20-minute dog walk, not a 2.5-hour run. Keep it brief and easy.
Don’t go AQI 151+
Move it indoors. Above 200 that becomes everyone, not just sensitive groups.
Above 200 this stops being a personal call: everyone indoors.
Duration and intensity are two separate dials
| Under 20 min | 20–60 min | Over an hour | |
|---|---|---|---|
| Easy | Go | Go | Go |
| Moderate | Go | Go | Go |
| Hard | Go | Go | Go |
Go Every combination lands the same way at this reading — a hard hour is no different from an easy twenty minutes. The two dials separate above 80.
Where people actually draw the line
All the thresholds people reported, and who reported them
- 60 Asthmatic runnerone runner’s own 156-run log
- 75–100 Daycare / schoolparents and staff in one r/toddlers thread, Aug 2025 — not a published rule
- 100 Parentcommunity-reported, escalating at 150 and 200
- 110 Runnercommunity-reported
If you’re staying in, filter the room you’re actually in
Ozone that arrives on the afternoon upslope
Boulder County is inside the Denver Metro/North Front Range ozone nonattainment area, along with Denver, Adams, Arapahoe, Broomfield, Douglas, Jefferson and parts of Larimer and Weld. The mechanism behind that designation is partly geographic. On weak-wind summer days, thermally driven mountain-valley circulation pulls air upslope toward the foothills through the afternoon and can recirculate it rather than flush it out, so the terrain at the western edge of the metro helps hold the air it receives.
What rides in on that flow has an unusual source split. Urban nitrogen oxides from the US 36 and I-25 corridors combine with volatile organic compounds from the Denver-Julesburg oil and gas field spread across Weld County to the northeast. The FRAPPÉ and DISCOVER-AQ field campaigns found that on individual days the oil and gas precursors alone can add more than 30 parts per billion of ozone, which the work linked directly to exceedances of the federal standard.
The season runs roughly May through September, peaks in the middle and late afternoon, and foothills monitors often read higher than downtown Denver. Winter works against a buildup rather than for one here. The downslope windstorms that hit this stretch of foothills are severe ventilators: in the December 2021 event, NCAR’s Mesa Lab clocked a 112 mph gust.
Where this number sits
Of the 121 US metros reporting to us right now, Boulder is number 49 and number 2 of the 4 we track in Colorado. The pollutant driving it today is O3.
Nearby metros we also measure, and what they read at the same hour:
- Denver, CO — 24 miles away, AQI 44
- Fort Collins, CO — 41 miles away, AQI 38
- Cheyenne, WY — 82 miles away, AQI 35
- Colorado Springs, CO — 85 miles away, AQI 42
Where this reading comes from
AirNow’s Denver-Boulder reporting area, about 14.3 miles from the metro centre.
What has air quality in Boulder actually been?
13 of the 30 days we have recorded so far went above AQI 100.
| Date | Daily max AQI | EPA band | Hours read |
|---|---|---|---|
| 106 | Unhealthy for Sensitive Groups | 22 | |
| 80 | Moderate | 23 | |
| 84 | Moderate | 23 | |
| 115 | Unhealthy for Sensitive Groups | 23 | |
| 117 | Unhealthy for Sensitive Groups | 23 | |
| 77 | Moderate | 22 | |
| 74 | Moderate | 23 | |
| 108 | Unhealthy for Sensitive Groups | 23 | |
| 108 | Unhealthy for Sensitive Groups | 23 | |
| 64 | Moderate | 23 | |
| 89 | Moderate | 23 | |
| 74 | Moderate | 24 | |
| 87 | Moderate | 23 | |
| 58 | Moderate | 23 | |
| 73 | Moderate | 23 | |
| 61 | Moderate | 23 | |
| 80 | Moderate | 23 | |
| 80 | Moderate | 22 | |
| 154 | Unhealthy | 23 | |
| 156 | Unhealthy | 23 | |
| 93 | Moderate | 23 | |
| 122 | Unhealthy for Sensitive Groups | 24 | |
| 80 | Moderate | 23 | |
| 93 | Moderate | 23 | |
| 108 | Unhealthy for Sensitive Groups | 23 | |
| 108 | Unhealthy for Sensitive Groups | 23 | |
| 119 | Unhealthy for Sensitive Groups | 23 | |
| 119 | Unhealthy for Sensitive Groups | 23 | |
| 97 | Moderate | 26 | |
| 108 | Unhealthy for Sensitive Groups | 6 — partial day |
Questions about Boulder
What is the air quality in Boulder right now?
Which monitor does this number come from?
How does Boulder compare with metros nearby?
Does this include pollen?
Put this number in context
Updated