Missouri · County air quality · 2025
Saint Louis County, Missouri
Median AQI 43 (Good). 73.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 73.4%
- Good-air days
- 1
- Unhealthy days
- 40%
- Cleaner than US counties
Saint Louis County's 2025 annual median AQI is 43 (Good), with 73.4% Good days
According to the U.S. EPA Air Quality System, Saint Louis County, Missouri posted a 2025 median AQI of 43 (Good) across 365 monitored days, with 73.4% 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 Louis County's 73.4% Good-day share places it cleaner than 40% of US counties in 2025.
- Closest median-AQI peer
- Ashtabula County (81.2% Good, median AQI 43) is the closest nationwide median-AQI peer to Saint Louis County.
- 2020–2025 trend
- Saint Louis County's floor-qualifying Good-day share is improving from 2020 to 2025 (67.2% → 73.4%).
- Median AQI
- Median AQI: 43
According to the 2025 EPA AQS county release on PlainAirData, Saint Louis County, Missouri ranks #631 of 951 by median AQI (43) and #566 by good-day share (73.4%). 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 Louis County's peak single-day AQI rose from 122 to 151 between the 2024 and 2025 EPA annual releases, a change of +29 points.
- Saint Louis 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 · 268 d · 73.4%
- Moderate · 91 d · 24.9%
- USG · 5 d · 1.4%
- Unhealthy · 1 d · 0.3%
How did Saint Louis County's air break down in 2025?
Across 365 monitored days, 268 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 151 (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 Louis County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Saint Louis County's Good-day share is 73.4% - cleaner than 40% of that comparison set.
Saint Louis County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
73.4% 40th percentile higher than 40% 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 Louis County.
- Ozone · 191 days · 52.3%
- PM2.5 · 158 days · 43.3%
- NO₂ · 16 days · 4.4%
2024 daily-monitor rhythm in Saint Louis County
Month-by-month median AQI from EPA day-level county monitors (362 reported days in 2024). Jul posted the highest typical readings (median AQI 48, peak 70); Nov was cleanest at median 32. Summer months (Jun–Aug) averaged median AQI 46 vs 36 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,823 daily readings show a recurring local pattern: Jun has the highest matched-month median AQI (59), while Nov is lowest (35). 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 Louis County's air quality is rated Moderate based on 73.4% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 5 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 151. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 191 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Saint Louis County. Highest Good-day share: 2024 (73.5%, 266 of 362 monitored 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 | 268 | 91 | 5 | 1 | 43 | 151 |
How does Saint Louis County compare?
How Saint Louis 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 Louis 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.