Missouri · County air quality · 2025
Saint Charles County, Missouri
Median AQI 40 (Good). 80.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 80.3%
- Good-air days
- 1
- Unhealthy days
- 57%
- Cleaner than US counties
Saint Charles County's 2025 annual median AQI is 40 (Good), with 80.3% Good days
According to the U.S. EPA Air Quality System, Saint Charles County, Missouri posted a 2025 median AQI of 40 (Good) across 309 monitored days, with 80.3% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Saint Charles County's 80.3% Good-day share places it cleaner than 57% of US counties in 2025.
- Closest median-AQI peer
- Anderson County (91.3% Good, median AQI 40) is the closest nationwide median-AQI peer to Saint Charles County.
- 2020–2025 trend
- Saint Charles County's floor-qualifying Good-day share is worsening from 2020 to 2025 (88.2% → 80.3%).
- Median AQI
- Median AQI: 40
According to the 2025 EPA AQS county release on PlainAirData, Saint Charles County, Missouri ranks #462 of 951 by median AQI (40) and #409 by good-day share (80.3%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- Saint Charles County's Good-day share rose from 73.2% to 80.3% between the 2024 and 2025 EPA annual releases, a change of +7.1 percentage points.
- Saint Charles County's peak single-day AQI rose from 129 to 164 between the 2024 and 2025 EPA annual releases, a change of +35 points.
- Saint Charles County's median AQI fell from 44 to 40 between the 2024 and 2025 EPA annual releases, a change of -4 points.
- Saint Charles County's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.
Data updated August 2026
- Good · 248 d · 80.3%
- Moderate · 57 d · 18.4%
- USG · 3 d · 1.0%
- Unhealthy · 1 d · 0.3%
How did Saint Charles County's air break down in 2025?
Across 309 monitored days, 248 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 164 (Unhealthy).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Saint Charles County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Saint Charles County's Good-day share is 80.3% - cleaner than 57% of that comparison set.
Saint Charles County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
80.3% Cleaner than 57% higher than 57% of 951 US counties
Each bar is a band; taller bars hold more US counties. 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
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Saint Charles County.
- Ozone · 248 days · 80.3%
- NO₂ · 61 days · 19.7%
2024 daily-monitor rhythm in Saint Charles County
Month-by-month median AQI from EPA day-level county monitors (261 reported days in 2024). Jun posted the highest typical readings (median AQI 50, peak 129); Nov was cleanest at median 31. Summer months (Jun–Aug) averaged median AQI 46 vs 39 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,241 daily readings show a recurring local pattern: Jun has the highest matched-month median AQI (58), while Nov is lowest (31). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Saint Charles County's air quality is rated Good based on 80.3% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 3 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 164. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 248 of 309 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Saint Charles County. Highest Good-day share: 2020 (88.2%, 216 of 245 monitored 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 | 248 | 57 | 3 | 1 | 40 | 164 |
How does Saint Charles County compare?
How Saint Charles County compares
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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Saint Charles County, Missouri · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
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.