Median AQI
43
Good
Missouri · County air quality · 2025
Saint Charles County, Missouri
Median AQI 43 (Good). 81.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 81.5%
- Good-air days
- 0
- Unhealthy days
- 56%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Saint Charles County's air is good on balance, cleaner than the median US county, a median AQI of 43 with 81.5% of 92 monitored days in the Good band, driven mainly by Ozone.
- 43
- median AQI · Good
- 81.5%
- days rated Good
- Ozone
- main pollutant
- 56%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 84.
Data updated July 2026
- Good · 75 d · 81.5%
- Moderate · 17 d · 18.5%
How did Saint Charles County's air break down in 2025?
Across 92 monitored days, 75 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 84 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 92 days monitored. Median AQI: 43 (Good).
Insights: Saint Charles County, Missouri. EPA monitors recorded a median Air Quality Index of 43 across 92 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 75 (82%) fell in the Good band (AQI 0–50), 17 (18%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 58, while the worst single-day peak hit 84 (Moderate). Residents averaged about 24.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (92 days). Across the 2020–2025 trend window, good days shifted by -141 and the median AQI moved +4 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 84, 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
81.5%
75 of 92 monitored days
6.5pp vs 75% target
Max AQI Recorded
84
Moderate
Health Grade
Good
90th-pct AQI 58
Saint Charles County, Good days share 81.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Saint Charles 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 92 monitored days across EPA AQI categories.
Good
75
82%
Moderate
17
18%
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 Charles County.
- Ozone · 92 days · 100.0%
Health Impact Assessment
Good
Saint Charles County's air quality is rated Good based on 81.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 81.5% of monitored days in 2025, with a median AQI of 43. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 92 of 92 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Saint Charles County. Best year: 2020 (216 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 216 | 25 | 4 | 0 | 39 | 112 |
| 2021 | 209 | 34 | 2 | 0 | 41 | 119 |
| 2022 | 206 | 32 | 7 | 0 | 42 | 129 |
| 2023 | 156 | 74 | 15 | 0 | 45 | 147 |
| 2024 | 191 | 67 | 3 | 0 | 44 | 129 |
| 2025 | 75 | 17 | 0 | 0 | 43 | 84 |
How does Saint Charles County compare?
How Saint Charles County compares
Good-day share better than US avg
Saint Charles County 81.5%
Andrew County, Missouri 84.3%
US average 76.5%
Median AQI worse than US avg
Saint Charles County 43
Andrew County, Missouri 42
US average 38
Unhealthy days / yr about average
Saint Charles 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 Charles County.
Allegan County, Michigan
AQI 43 · 71.2% good
Allen County, Ohio
AQI 43 · 65.9% good
Ashtabula County, Ohio
AQI 43 · 81.2% good
Bell County, Kentucky
AQI 43 · 63.5% good
Boise County, Idaho
AQI 43 · 72.9% good
Brazos County, Texas
AQI 43 · 63.4% good
What Saint Charles 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 43 means for exposure. Health guidance
- See how Saint Charles County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Saint Charles County with its Missouri neighbours before drawing a conclusion about the region. All of Missouri
Based on 92 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 Charles County, Missouri? ▼
Saint Charles County has a median Air Quality Index (AQI) of 43, rated "Good." 81.5% of monitored days (75 out of 92) had good air quality in 2025.
What is the main pollutant in Saint Charles County? ▼
The primary pollutant in Saint Charles County is Ozone (O3), which was the main AQI driver on 92 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Saint Charles County have? ▼
In 2025, Saint Charles County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 84.
Is the air quality in Saint Charles County getting better or worse? ▼
Air quality in Saint Charles County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 216 good days. Worst year: 2025 with 75 good days.
What does AQI 43 mean for health? ▼
Saint Charles County's median AQI of 43 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 Charles County compare to other counties in Missouri? ▼
Saint Charles County has 81.5% 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 Charles County? ▼
The 90th percentile AQI for Saint Charles County is 58, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 84 (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.