Median AQI
21
Good
Colorado · County air quality · 2025
San Miguel County, Colorado
Median AQI 21 (Good). 93.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 21
- Median AQI
- 93.0%
- Good-air days
- 0
- Unhealthy days
- 83%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, San Miguel County's air is good on balance, cleaner than most US counties, a median AQI of 21 with 93.0% of 273 monitored days in the Good band, driven mainly by PM2.5.
- 21
- median AQI · Good
- 93.0%
- days rated Good
- PM2.5
- main pollutant
- 83%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 94.
Data updated July 2026
- Good · 254 d · 93.0%
- Moderate · 19 d · 7.0%
How did San Miguel County's air break down in 2025?
Across 273 monitored days, 254 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 94 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 273 days monitored. Median AQI: 21 (Good).
Insights: San Miguel County, Colorado. EPA monitors recorded a median Air Quality Index of 21 across 273 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 254 (93%) fell in the Good band (AQI 0–50), 19 (7%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 44, while the worst single-day peak hit 94 (Moderate). Residents averaged about 27.9 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (216 days), PM10 (57 days). Across the 2020–2025 trend window, good days shifted by +99 and the median AQI moved -27 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 94, moderate 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
93.0%
254 of 273 monitored days
18.0pp vs 75% target
Max AQI Recorded
94
Moderate
Health Grade
Excellent
90th-pct AQI 44
San Miguel County, Good days share 93.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
San Miguel 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
254
93%
Moderate
19
7%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in San Miguel County.
- PM2.5 · 216 days · 79.1%
- PM10 · 57 days · 20.9%
Health Impact Assessment
Excellent
San Miguel County's air quality is rated Excellent based on 93.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 93.0% of monitored days in 2025, with a median AQI of 21. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 216 of 273 monitored days.
5-Year Air Quality Trend
Air quality is improving in San Miguel County. Best year: 2024 (311 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 155 | 90 | 2 | 0 | 48 | 136 |
| 2021 | 165 | 87 | 0 | 0 | 48 | 97 |
| 2022 | 194 | 60 | 1 | 0 | 45 | 108 |
| 2023 | 185 | 68 | 0 | 0 | 47 | 84 |
| 2024 | 311 | 50 | 0 | 0 | 19 | 97 |
| 2025 | 254 | 19 | 0 | 0 | 21 | 94 |
How does San Miguel County compare?
How San Miguel County compares
Good-day share better than US avg
San Miguel County 93.0%
Adams County, Colorado 50.2%
US average 76.5%
Median AQI better than US avg
San Miguel County 21
Adams County, Colorado 50
US average 38
Unhealthy days / yr about average
San Miguel 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 San Miguel County.
Cowlitz County, Washington
AQI 21 · 91.6% good
Juneau County, Alaska
AQI 21 · 85.1% good
Kitsap County, Washington
AQI 21 · 96.2% good
Marin County, California
AQI 21 · 100.0% good
Mason County, Washington
AQI 21 · 92.6% good
Pitkin County, Colorado
AQI 21 · 91.6% good
What San Miguel 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 21 means for exposure. Health guidance
- See how San Miguel County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare San Miguel 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 San Miguel County, Colorado? ▼
San Miguel County has a median Air Quality Index (AQI) of 21, rated "Good." 93.0% of monitored days (254 out of 273) had good air quality in 2025.
What is the main pollutant in San Miguel County? ▼
The primary pollutant in San Miguel County is Fine Particles (PM2.5), which was the main AQI driver on 216 days (79% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does San Miguel County have? ▼
In 2025, San Miguel County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 94.
Is the air quality in San Miguel County getting better or worse? ▼
Air quality in San Miguel County is improving based on 5-year trends (2020-2025). Best year: 2024 with 311 good days. Worst year: 2020 with 155 good days.
What does AQI 21 mean for health? ▼
San Miguel County's median AQI of 21 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 San Miguel County compare to other counties in Colorado? ▼
San Miguel County has 93.0% 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 San Miguel County? ▼
The 90th percentile AQI for San Miguel County is 44, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 94 (Moderate).
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.