Median AQI
28
Good
Colorado · County air quality · 2025
Delta County, Colorado
Median AQI 28 (Good). 88.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 28
- Median AQI
- 88.0%
- Good-air days
- 0
- Unhealthy days
- 71%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Delta County's air is good on balance, cleaner than the median US county, a median AQI of 28 with 88.0% of 117 monitored days in the Good band, driven mainly by PM2.5.
- 28
- median AQI · Good
- 88.0%
- days rated Good
- PM2.5
- main pollutant
- 71%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 103.
Data updated July 2026
- Good · 103 d · 88.0%
- Moderate · 13 d · 11.1%
- USG · 1 d · 0.9%
How did Delta County's air break down in 2025?
Across 117 monitored days, 103 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 103 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 117 days monitored. Median AQI: 28 (Good).
Insights: Delta County, Colorado. EPA monitors recorded a median Air Quality Index of 28 across 117 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 103 (88%) fell in the Good band (AQI 0–50), 13 (11%) registered Moderate readings, and 1 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 55, while the worst single-day peak hit 103 (USG). Residents averaged about 26.4 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (97 days), PM10 (20 days). Across the 2020–2025 trend window, good days shifted by -209 and the median AQI moved -13 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 103, 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
88.0%
103 of 117 monitored days
13.0pp vs 75% target
Max AQI Recorded
103
USG
Health Grade
Good
90th-pct AQI 55
Delta County, Good days share 88.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Delta 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 117 monitored days across EPA AQI categories.
Good
103
88%
Moderate
13
11%
USG
1
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 Delta County.
- PM2.5 · 97 days · 82.9%
- PM10 · 20 days · 17.1%
Health Impact Assessment
Good
Delta County's air quality is rated Good based on 88.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 88.0% of monitored days in 2025, with a median AQI of 28. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 97 of 117 monitored days.
5-Year Air Quality Trend
Air quality is improving in Delta County. Best year: 2020 (312 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 312 | 53 | 0 | 0 | 41 | 97 |
| 2021 | 294 | 69 | 2 | 0 | 43 | 141 |
| 2022 | 125 | 18 | 0 | 0 | 44 | 80 |
| 2025 | 103 | 13 | 1 | 0 | 28 | 103 |
How does Delta County compare?
How Delta County compares
Good-day share better than US avg
Delta County 88.0%
Adams County, Colorado 50.2%
US average 76.5%
Median AQI better than US avg
Delta County 28
Adams County, Colorado 50
US average 38
Unhealthy days / yr about average
Delta 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 Delta County.
Martin County, North Carolina
AQI 28 · 100.0% good
Oneida County, New York
AQI 28 · 80.8% good
Orange County, New York
AQI 28 · 85.4% good
Bayamon County, Puerto Rico
AQI 29 · 88.5% good
Chelan County, Washington
AQI 27 · 87.2% good
Clackamas County, Oregon
AQI 27 · 93.9% good
What Delta County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 28 means for exposure. Health guidance
- See how Delta County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Delta County with its Colorado neighbours before drawing a conclusion about the region. All of Colorado
Based on 117 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 Delta County, Colorado? ▼
Delta County has a median Air Quality Index (AQI) of 28, rated "Good." 88.0% of monitored days (103 out of 117) had good air quality in 2025.
What is the main pollutant in Delta County? ▼
The primary pollutant in Delta County is Fine Particles (PM2.5), which was the main AQI driver on 97 days (83% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Delta County have? ▼
In 2025, Delta County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 103.
Is the air quality in Delta County getting better or worse? ▼
Air quality in Delta County is improving based on 5-year trends (2020-2025). Best year: 2020 with 312 good days. Worst year: 2025 with 103 good days.
What does AQI 28 mean for health? ▼
Delta County's median AQI of 28 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 Delta County compare to other counties in Colorado? ▼
Delta County has 88.0% 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 Delta County? ▼
The 90th percentile AQI for Delta County is 55, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 103 (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.