Median AQI
51
Moderate
Colorado · County air quality · 2025
Jefferson County, Colorado
Median AQI 51 (Moderate). 46.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 51
- Median AQI
- 46.5%
- Good-air days
- 0
- Unhealthy days
- 5%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Jefferson County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 51 with 46.5% of 273 monitored days in the Good band, driven mainly by Ozone.
- 51
- median AQI · Moderate
- 46.5%
- days rated Good
- Ozone
- main pollutant
- 5%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 119.
Data updated July 2026
- Good · 127 d · 46.5%
- Moderate · 133 d · 48.7%
- USG · 13 d · 4.8%
How did Jefferson County's air break down in 2025?
Across 273 monitored days, 127 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: 51 (Moderate).
Insights: Jefferson County, Colorado. EPA monitors recorded a median Air Quality Index of 51 across 273 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 127 (47%) fell in the Good band (AQI 0–50), 133 (49%) registered Moderate readings, and 13 (5%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 90, while the worst single-day peak hit 119 (USG). Residents averaged about 14.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (273 days). Across the 2020–2025 trend window, good days shifted by -104 and the median AQI moved +5 points, placing the multi-year trajectory at worsening. 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); Boulder (50.9% good, AQI 50); Clear Creek (64.4% good, AQI 48).
Air Quality Summary
Good Days
46.5%
127 of 273 monitored days
-28.5pp vs 75% target
Max AQI Recorded
119
USG
Health Grade
Poor
90th-pct AQI 90
Jefferson County, Good days share 46.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Jefferson 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
127
47%
Moderate
133
49%
USG
13
5%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Jefferson County.
- Ozone · 273 days · 100.0%
Health Impact Assessment
Poor
Jefferson County's air quality is rated Poor based on 46.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 46.5% of monitored days in 2025, with a median AQI of 51. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 273 of 273 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Jefferson County. Best year: 2022 (234 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 231 | 108 | 23 | 4 | 46 | 161 |
| 2021 | 222 | 88 | 44 | 11 | 47 | 172 |
| 2022 | 234 | 100 | 30 | 1 | 47 | 166 |
| 2023 | 206 | 141 | 18 | 0 | 49 | 143 |
| 2024 | 192 | 137 | 30 | 7 | 50 | 174 |
| 2025 | 127 | 133 | 13 | 0 | 51 | 119 |
How does Jefferson County compare?
How Jefferson County compares
Good-day share worse than US avg
Jefferson County 46.5%
Adams County, Colorado 50.2%
US average 76.5%
Median AQI worse than US avg
Jefferson County 51
Adams County, Colorado 50
US average 38
Unhealthy days / yr about average
Jefferson 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 Jefferson County.
Arapahoe County, Colorado
AQI 51 · 47.3% good
Bibb County, Georgia
AQI 51 · 49.5% good
East Baton Rouge County, Louisiana
AQI 51 · 49.3% good
Eddy County, New Mexico
AQI 51 · 46.9% good
Hillsborough County, Florida
AQI 51 · 49.3% good
Mecklenburg County, North Carolina
AQI 51 · 49.2% good
What Jefferson 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 51 means for exposure. Health guidance
- See how Jefferson County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Jefferson 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 Jefferson County, Colorado? ▼
Jefferson County has a median Air Quality Index (AQI) of 51, rated "Moderate." 46.5% of monitored days (127 out of 273) had good air quality in 2025.
What is the main pollutant in Jefferson County? ▼
The primary pollutant in Jefferson County is Ozone (O3), which was the main AQI driver on 273 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Jefferson County have? ▼
In 2025, Jefferson County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 119.
Is the air quality in Jefferson County getting better or worse? ▼
Air quality in Jefferson County is worsening based on 5-year trends (2020-2025). Best year: 2022 with 234 good days. Worst year: 2025 with 127 good days.
What does AQI 51 mean for health? ▼
Jefferson County's median AQI of 51 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Jefferson County compare to other counties in Colorado? ▼
Jefferson County has 46.5% good air days. Nearby counties in Colorado: Adams (50.2% good), Alamosa (90.8% good), Arapahoe (47.3% good), Archuleta (79.5% good), Boulder (50.9% good), Clear Creek (64.4% good).
What is the 90th percentile AQI in Jefferson County? ▼
The 90th percentile AQI for Jefferson County is 90, 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.