Missouri · County air quality · 2025
St. Louis City County, Missouri
Median AQI 55 (Moderate). 34.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 55
- Median AQI
- 34.3%
- Good-air days
- 2
- Unhealthy days
- 2%
- Cleaner than US counties
St. Louis City County's 2025 annual median AQI is 55 (Moderate), with 34.3% Good days
According to the U.S. EPA Air Quality System, St. Louis City County, Missouri posted a 2025 median AQI of 55 (Moderate) across 365 monitored days, with 34.3% rated Good and 2 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- St. Louis City County's 34.3% Good-day share places it cleaner than 2% of US counties in 2025.
- Closest median-AQI peer
- Butler County (33.1% Good, median AQI 55) is the closest nationwide median-AQI peer to St. Louis City County.
- 2020–2025 trend
- St. Louis City County's floor-qualifying Good-day share is stable from 2020 to 2025 (37.4% → 34.4%).
- Median AQI
- Median AQI: 55
According to the 2025 EPA AQS county release on PlainAirData, St. Louis City County, Missouri ranks #929 of 951 by median AQI (55) and #929 by good-day share (34.3%). 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
- St. Louis City County's peak single-day AQI rose from 156 to 738 between the 2024 and 2025 EPA annual releases, a change of +582 points.
- St. Louis City County's Good-day share fell from 38.0% to 34.2% between the 2024 and 2025 EPA annual releases, a change of -3.7 percentage points.
- St. Louis City County's Unhealthy-category days fell from 1 to 0 between the 2024 and 2025 EPA annual releases, a change of -1 days.
Data updated August 2026
- Good · 125 d · 34.2%
- Moderate · 232 d · 63.6%
- USG · 6 d · 1.6%
- Hazardous · 2 d · 0.5%
How did St. Louis City County's air break down in 2025?
Across 365 monitored days, 125 fell in the Good band (AQI 0–50) and 2 reached Unhealthy or worse. The worst single day peaked at AQI 738 (Hazardous) - a reading beyond the EPA’s standard 0–500 AQI scale, reflecting an extreme wildfire-smoke event or a transient monitor anomaly.
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 St. Louis City County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. St. Louis City County's Good-day share is 34.3% - cleaner than 2% of that comparison set.
St. Louis City County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
34.3% 2nd percentile higher than 2% 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 St. Louis City County.
- PM2.5 · 261 days · 71.5%
- Ozone · 75 days · 20.5%
- PM10 · 24 days · 6.6%
- NO₂ · 5 days · 1.4%
2024 daily-monitor rhythm in St. Louis City County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Oct posted the highest typical readings (median AQI 66, peak 131); Nov was cleanest at median 43. Summer months (Jun–Aug) averaged median AQI 60 vs 50 in Dec–Feb. 1 day crossed Unhealthy (AQI >150) at least once that year.
Five-year seasonal record
EPA day-level county files · 2020–20241,827 daily readings show a recurring local pattern: Jun has the highest matched-month median AQI (67), while Apr is lowest (53). 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
St. Louis City County's air quality is rated Poor based on 34.3% good air days. There were 2 unhealthy days in 2025 when outdoor activity should be limited.
Air quality posed significant health risk in 2025. Sensitive groups, including people with asthma, COPD, cardiovascular conditions, children, pregnant women, and adults over 65, should monitor AirNow.gov alerts and limit outdoor activity on Unhealthy (AQI 151+) and Very Unhealthy (201+) days. Peak readings reached AQI 738, beyond EPA's standard 0-500 scale, indicating an extreme event.
Sensitive group advisory
People with asthma, COPD, heart disease, children, pregnant women, and adults 65+ should follow EPA AQI alerts daily and limit outdoor exertion when AQI exceeds 100. Real-time conditions: airnow.gov.
Note: EPA's standard AQI scale caps at 500 (Hazardous category 301-500). The 738 peak reading represents an extreme event, typically wildfire smoke, extrapolated beyond the index's design range. See EPA wildfire smoke guidance.
The dominant pollutant is PM2.5, driving AQI on 261 of 365 monitored days.
5-Year Air Quality Trend
Air quality is stable in St. Louis City County. Highest Good-day share: 2024 (38.0%, 139 of 366 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 137 | 224 | 4 | 1 | 54 | 156 |
| 2021 | 114 | 244 | 6 | 1 | 55 | 187 |
| 2022 | 127 | 233 | 5 | 0 | 55 | 117 |
| 2023 | 89 | 260 | 14 | 2 | 58 | 172 |
| 2024 | 139 | 217 | 9 | 1 | 54 | 156 |
| 2025 | 125 | 232 | 6 | 0 | 55 | 738 |
How does St. Louis City County compare?
How St. Louis City 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 St. Louis City 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.