State air quality · 2025
Missouri Air Quality Ranking
Across 21 monitored counties, the typical county in Missouri posts a median AQI of 40 (Good) and 75.9% of monitored days were rated Good, from EPA AQS.
- 40
- Avg median AQI
- 75.9%
- Good-air days
- 21
- Counties monitored
- 44%
- Cleaner than US states
Missouri's typical county median AQI is 40 (Good) across 21 monitored counties
According to the U.S. EPA Air Quality System, Missouri monitors 21 counties with a day-weighted Good-day share of 75.9%. Cleanest: Monroe County (94.0% Good). Most polluted: St. Louis City County (34.3%). These are annual summaries, not live AirNow readings.
Data source EPA Air Quality System (AQS)
- Cleaner than US states
- Missouri's day-weighted Good-day share of 75.9% places it cleaner than 44% of US states.
- In-state spread
- Within Missouri, Stoddard County leads at 100.0% Good days while St. Louis City County sits at 34.3% across 21 monitored counties.
- 2024→2025 release
- Missouri's Good-day share moved -1.6 points vs the 2024 EPA annual release.
- Typical median AQI
- Typical median AQI: 40
Verify with AirNow → · EPA AQS →
EPA state monitoring brief
AQS state key MISSOURI
Missouri · 2025 EPA AQS · 21 counties · med AQI 40 · 75.9% Good · Monroe→St. Louis City spread
- GOOD 40 Median AQI · Good
- SHARE 75.9% Good-day share
- COUNT 21 Counties monitored
- UNH 10 Unhealthy+ days
- RANK 44% Cleaner than states
- O3 59.3% Main pollutant share
Good-day share peers
Missouri's 75.9% Good-day share is cleaner than 44% of monitored US states; Monroe and St. Louis City County bracket the in-state range. Open Stoddard County →
According to EPA AQS annual summaries on PlainAirData, Missouri ranks #29 of 52 by day-weighted good-day share (75.9%) and #32 by mean county median AQI (40.2). Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%).
Data updated August 2026
What changed, what drives it, and how thick is the sample
Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.
Release change
75.9%
Good-day share in 2025, down 1.6 percentage points vs 2024. County-mean median AQI 40, 0.8 points lower (cleaner) than 2024.
Pollutant mix
Ozone
Main driver on 59.3% of pollutant-assigned days.
- Ozone 59.3%
- PM2.5 34.5%
- PM10 4.4%
- NO2 1.9%
Monitoring coverage
295
Average monitored days per county in the latest release. 2 of 21 counties sit below the 100-day floor used for cleanest/most-polluted claims.
Counties Monitored
21
Good Days %
75.9%
Avg Median AQI
40
Total Unhealthy Days
10
Best Air Quality
Monroe County
Worst Air Quality
St. Louis City County
Where Missouri sits nationally
COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Missouri's day-weighted Good-day share is 75.9% - cleaner than 44% of that comparison set.
Missouri Good-day share vs. every US state
Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
75.9% 44th percentile higher than 44% of 52 US states
Each bar is a band; taller bars hold more US states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Which counties in Missouri have the cleanest and most polluted air?
Cleanest counties in Missouri
| County | Good % | Median AQI | Max AQI | Unhealthy Days |
|---|---|---|---|---|
| Stoddard | 100.0% | 21 | 43 | 0 |
| Monroe | 94.0% | 35 | 97 | 0 |
| Boone | 93.2% | 37 | 87 | 0 |
| Sainte Genevieve | 91.4% | 41 | 108 | 0 |
| Taney | 89.8% | 28 | 79 | 0 |
| Callaway | 89.6% | 41 | 77 | 0 |
| Andrew | 86.9% | 40 | 93 | 0 |
| Lincoln | 85.0% | 41 | 122 | 0 |
| Perry | 84.7% | 41 | 105 | 0 |
| Clinton | 84.6% | 41 | 122 | 0 |
| Jasper | 83.6% | 38 | 162 | 1 |
| Cedar | 81.6% | 39 | 106 | 0 |
| Saint Charles | 80.3% | 40 | 164 | 1 |
| Cass | 77.6% | 41 | 123 | 0 |
| Clay | 74.4% | 42 | 156 | 1 |
| Jefferson | 73.4% | 42 | 150 | 0 |
| Saint Louis | 73.4% | 43 | 151 | 1 |
| Greene | 62.6% | 45 | 160 | 1 |
| Jackson | 57.3% | 47 | 293 | 2 |
| Buchanan | 56.4% | 46 | 256 | 1 |
| St. Louis City | 34.3% | 55 | 738 | 2 |
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.
Coverage: 21 counties in Missouri with active monitoring stations. Unmonitored counties are not represented.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025
Every figure on PlainAirData 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.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.