Median AQI
50
Good
Colorado · County air quality · 2025
Boulder County, Colorado
Median AQI 50 (Good). 50.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 50
- Median AQI
- 50.9%
- Good-air days
- 0
- Unhealthy days
- 7%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Boulder County's air is good on balance, among the most polluted counties nationally, a median AQI of 50 with 50.9% of 273 monitored days in the Good band, driven mainly by Ozone.
- 50
- median AQI · Good
- 50.9%
- days rated Good
- Ozone
- main pollutant
- 7%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 119.
Data updated July 2026
- Good · 139 d · 50.9%
- Moderate · 131 d · 48.0%
- USG · 3 d · 1.1%
How did Boulder County's air break down in 2025?
Across 273 monitored days, 139 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 119 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 273 days monitored. Median AQI: 50 (Good).
Insights: Boulder County, Colorado. EPA monitors recorded a median Air Quality Index of 50 across 273 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 139 (51%) fell in the Good band (AQI 0–50), 131 (48%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 76, while the worst single-day peak hit 119 (USG). Residents averaged about 15.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (202 days), PM2.5 (70 days), PM10 (1 days). Across the 2020–2025 trend window, good days shifted by -34 and the median AQI moved -1 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 119, usg but never Unhealthy. Nearby counties show comparable readings: Adams (50.2% good, AQI 50); Alamosa (90.8% good, AQI 24); Arapahoe (47.3% good, AQI 51); Archuleta (79.5% good, AQI 32); Clear Creek (64.4% good, AQI 48); Delta (88.0% good, AQI 28).
Air Quality Summary
Good Days
50.9%
139 of 273 monitored days
-24.1pp vs 75% target
Max AQI Recorded
119
USG
Health Grade
Moderate
90th-pct AQI 76
Boulder County, Good days share 50.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Boulder County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 273 monitored days across EPA AQI categories.
Good
139
51%
Moderate
131
48%
USG
3
1%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Boulder County.
- Ozone · 202 days · 74.0%
- PM2.5 · 70 days · 25.6%
- PM10 · 1 days · 0.4%
Health Impact Assessment
Moderate
Boulder County's air quality is rated Moderate based on 50.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 50.9% of monitored days in 2025, with a median AQI of 50. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 202 of 273 monitored days.
5-Year Air Quality Trend
Air quality is improving in Boulder County. Best year: 2022 (224 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 173 | 172 | 19 | 2 | 51 | 181 |
| 2021 | 154 | 176 | 32 | 3 | 52 | 160 |
| 2022 | 224 | 134 | 7 | 0 | 47 | 129 |
| 2023 | 192 | 167 | 4 | 2 | 50 | 181 |
| 2024 | 212 | 144 | 9 | 1 | 49 | 154 |
| 2025 | 139 | 131 | 3 | 0 | 50 | 119 |
How does Boulder County compare?
How Boulder County compares
Good-day share worse than US avg
Boulder County 50.9%
Adams County, Colorado 50.2%
US average 76.5%
Median AQI worse than US avg
Boulder County 50
Adams County, Colorado 50
US average 38
Unhealthy days / yr about average
Boulder County 0
Adams County, Colorado 0
US average 0
Other Counties in Colorado
Air quality comparison for nearby counties in Colorado.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Boulder County.
Adams County, Colorado
AQI 50 · 50.2% good
Albany County, Wyoming
AQI 50 · 51.9% good
Davidson County, North Carolina
AQI 50 · 50.2% good
Davidson County, Tennessee
AQI 50 · 51.8% good
Gila County, Arizona
AQI 50 · 52.0% good
Jefferson County, Ohio
AQI 50 · 50.5% good
What Boulder County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 50 means for exposure. Health guidance
- See how Boulder County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Boulder County with its Colorado neighbours before drawing a conclusion about the region. All of Colorado
Based on 273 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Boulder County, Colorado? ▼
Boulder County has a median Air Quality Index (AQI) of 50, rated "Good." 50.9% of monitored days (139 out of 273) had good air quality in 2025.
What is the main pollutant in Boulder County? ▼
The primary pollutant in Boulder County is Ozone (O3), which was the main AQI driver on 202 days (74% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Boulder County have? ▼
In 2025, Boulder County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 119.
Is the air quality in Boulder County getting better or worse? ▼
Air quality in Boulder County is improving based on 5-year trends (2020-2025). Best year: 2022 with 224 good days. Worst year: 2025 with 139 good days.
What does AQI 50 mean for health? ▼
Boulder County's median AQI of 50 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Boulder County compare to other counties in Colorado? ▼
Boulder County has 50.9% good air days. Nearby counties in Colorado: Adams (50.2% good), Alamosa (90.8% good), Arapahoe (47.3% good), Archuleta (79.5% good), Clear Creek (64.4% good), Delta (88.0% good).
What is the 90th percentile AQI in Boulder County? ▼
The 90th percentile AQI for Boulder County is 76, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 119 (USG).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.