Median AQI
52
Moderate
Colorado · County air quality · 2025
Weld County, Colorado
Median AQI 52 (Moderate). 42.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 52
- Median AQI
- 42.7%
- Good-air days
- 0
- Unhealthy days
- 4%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Weld County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 52 with 42.7% of 274 monitored days in the Good band, driven mainly by Ozone.
- 52
- median AQI · Moderate
- 42.7%
- days rated Good
- Ozone
- main pollutant
- 4%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 119.
Data updated July 2026
- Good · 117 d · 42.7%
- Moderate · 151 d · 55.1%
- USG · 6 d · 2.2%
How did Weld County's air break down in 2025?
Across 274 monitored days, 117 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). 274 days monitored. Median AQI: 52 (Moderate).
Insights: Weld County, Colorado. EPA monitors recorded a median Air Quality Index of 52 across 274 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 117 (43%) fell in the Good band (AQI 0–50), 151 (55%) registered Moderate readings, and 6 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 84, while the worst single-day peak hit 119 (USG). Residents averaged about 12.8 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (194 days), PM2.5 (78 days), NO2 (1 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -49 and the median AQI moved +0 points, placing the multi-year trajectory at stable. 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
42.7%
117 of 274 monitored days
-32.3pp vs 75% target
Max AQI Recorded
119
USG
Health Grade
Poor
90th-pct AQI 84
Weld County, Good days share 42.7%
National avg
Share of monitored days at AQI 0-50 (Good band).
Weld 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 274 monitored days across EPA AQI categories.
Good
117
43%
Moderate
151
55%
USG
6
2%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Weld County.
- Ozone · 194 days · 70.8%
- PM2.5 · 78 days · 28.5%
- NO₂ · 1 days · 0.4%
- CO · 1 days · 0.4%
Health Impact Assessment
Poor
Weld County's air quality is rated Poor based on 42.7% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 42.7% of monitored days in 2025, with a median AQI of 52. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 194 of 274 monitored days.
5-Year Air Quality Trend
Air quality is stable in Weld County. Best year: 2022 (170 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 166 | 187 | 13 | 0 | 52 | 140 |
| 2021 | 156 | 169 | 39 | 1 | 54 | 154 |
| 2022 | 170 | 185 | 10 | 0 | 51 | 133 |
| 2023 | 146 | 214 | 4 | 1 | 55 | 162 |
| 2024 | 149 | 196 | 19 | 2 | 54 | 164 |
| 2025 | 117 | 151 | 6 | 0 | 52 | 119 |
How does Weld County compare?
How Weld County compares
Good-day share worse than US avg
Weld County 42.7%
Adams County, Colorado 50.2%
US average 76.5%
Median AQI worse than US avg
Weld County 52
Adams County, Colorado 50
US average 38
Unhealthy days / yr about average
Weld 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 Weld County.
Duval County, Florida
AQI 52 · 40.7% good
Hinds County, Mississippi
AQI 52 · 43.4% good
Jefferson County, Texas
AQI 52 · 42.4% good
McLennan County, Texas
AQI 52 · 43.4% good
Orange County, California
AQI 52 · 44.5% good
Union County, Arkansas
AQI 52 · 44.0% good
What Weld 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 52 means for exposure. Health guidance
- See how Weld County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Weld County with its Colorado neighbours before drawing a conclusion about the region. All of Colorado
Based on 274 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 Weld County, Colorado? ▼
Weld County has a median Air Quality Index (AQI) of 52, rated "Moderate." 42.7% of monitored days (117 out of 274) had good air quality in 2025.
What is the main pollutant in Weld County? ▼
The primary pollutant in Weld County is Ozone (O3), which was the main AQI driver on 194 days (71% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Weld County have? ▼
In 2025, Weld County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 119.
Is the air quality in Weld County getting better or worse? ▼
Air quality in Weld County is stable based on 5-year trends (2020-2025). Best year: 2022 with 170 good days. Worst year: 2025 with 117 good days.
What does AQI 52 mean for health? ▼
Weld County's median AQI of 52 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 Weld County compare to other counties in Colorado? ▼
Weld County has 42.7% 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 Weld County? ▼
The 90th percentile AQI for Weld County is 84, 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.