Median AQI
39
Good
Missouri · County air quality · 2025
Saint Louis County, Missouri
Median AQI 39 (Good). 88.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 88.5%
- Good-air days
- 0
- Unhealthy days
- 72%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Saint Louis County's air is good on balance, cleaner than the median US county, a median AQI of 39 with 88.5% of 148 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 88.5%
- days rated Good
- Ozone
- main pollutant
- 72%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 93.
Data updated July 2026
- Good · 131 d · 88.5%
- Moderate · 17 d · 11.5%
How did Saint Louis County's air break down in 2025?
Across 148 monitored days, 131 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 93 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 148 days monitored. Median AQI: 39 (Good).
Insights: Saint Louis County, Missouri. EPA monitors recorded a median Air Quality Index of 39 across 148 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 131 (89%) fell in the Good band (AQI 0–50), 17 (11%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 51, while the worst single-day peak hit 93 (Moderate). Residents averaged about 26.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (96 days), NO2 (40 days), PM2.5 (12 days). Across the 2020–2025 trend window, good days shifted by -115 and the median AQI moved -5 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 93, moderate but never Unhealthy. Nearby counties show comparable readings: Andrew (84.3% good, AQI 42); Boone (89.1% good, AQI 41); Buchanan (64.2% good, AQI 39); Callaway (85.6% good, AQI 42); Cass (82.2% good, AQI 41); Cedar (81.8% good, AQI 40).
Air Quality Summary
Good Days
88.5%
131 of 148 monitored days
13.5pp vs 75% target
Max AQI Recorded
93
Moderate
Health Grade
Good
90th-pct AQI 51
Saint Louis County, Good days share 88.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Saint Louis 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 148 monitored days across EPA AQI categories.
Good
131
89%
Moderate
17
11%
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 Saint Louis County.
- Ozone · 96 days · 64.9%
- NO₂ · 40 days · 27.0%
- PM2.5 · 12 days · 8.1%
Health Impact Assessment
Good
Saint Louis County's air quality is rated Good based on 88.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 88.5% of monitored days in 2025, with a median AQI of 39. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 96 of 148 monitored days.
5-Year Air Quality Trend
Air quality is improving in Saint Louis County. Best year: 2024 (266 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 246 | 114 | 6 | 0 | 44 | 144 |
| 2021 | 251 | 111 | 3 | 0 | 42 | 126 |
| 2022 | 263 | 100 | 2 | 0 | 44 | 140 |
| 2023 | 218 | 130 | 14 | 3 | 46 | 166 |
| 2024 | 266 | 94 | 2 | 0 | 43 | 122 |
| 2025 | 131 | 17 | 0 | 0 | 39 | 93 |
How does Saint Louis County compare?
How Saint Louis County compares
Good-day share better than US avg
Saint Louis County 88.5%
Andrew County, Missouri 84.3%
US average 76.5%
Median AQI worse than US avg
Saint Louis County 39
Andrew County, Missouri 42
US average 38
Unhealthy days / yr about average
Saint Louis County 0
Andrew County, Missouri 0
US average 0
Other Counties in Missouri
Air quality comparison for nearby counties in Missouri.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Saint Louis County.
Ada County, Idaho
AQI 39 · 80.6% good
Aiken County, South Carolina
AQI 39 · 89.1% good
Albany County, New York
AQI 39 · 67.4% good
Alexander County, North Carolina
AQI 39 · 92.4% good
Ashland County, Wisconsin
AQI 39 · 85.6% good
Bennington County, Vermont
AQI 39 · 82.9% good
What Saint Louis 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 39 means for exposure. Health guidance
- See how Saint Louis County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Saint Louis County with its Missouri neighbours before drawing a conclusion about the region. All of Missouri
Based on 148 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 Saint Louis County, Missouri? ▼
Saint Louis County has a median Air Quality Index (AQI) of 39, rated "Good." 88.5% of monitored days (131 out of 148) had good air quality in 2025.
What is the main pollutant in Saint Louis County? ▼
The primary pollutant in Saint Louis County is Ozone (O3), which was the main AQI driver on 96 days (65% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Saint Louis County have? ▼
In 2025, Saint Louis County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 93.
Is the air quality in Saint Louis County getting better or worse? ▼
Air quality in Saint Louis County is improving based on 5-year trends (2020-2025). Best year: 2024 with 266 good days. Worst year: 2025 with 131 good days.
What does AQI 39 mean for health? ▼
Saint Louis County's median AQI of 39 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 Saint Louis County compare to other counties in Missouri? ▼
Saint Louis County has 88.5% good air days. Nearby counties in Missouri: Andrew (84.3% good), Boone (89.1% good), Buchanan (64.2% good), Callaway (85.6% good), Cass (82.2% good), Cedar (81.8% good).
What is the 90th percentile AQI in Saint Louis County? ▼
The 90th percentile AQI for Saint Louis County is 51, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 93 (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.