Median AQI
55
Moderate
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
The verdict
According to EPA Air Quality System monitoring data, St. Louis City County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 55 with 34.3% of 365 monitored days in the Good band, driven mainly by PM2.5.
- 55
- median AQI · Moderate
- 34.3%
- days rated Good
- PM2.5
- main pollutant
- 2%
- cleaner than US counties
2 days reached Unhealthy or worse; the worst single reading hit AQI 738 - a reading beyond the EPA’s standard 0–500 AQI scale, reflecting an extreme wildfire-smoke event or a transient monitor anomaly.
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 →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 55 (Moderate).
Insights: St. Louis City County, Missouri. EPA monitors recorded a median Air Quality Index of 55 across 365 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 125 (34%) fell in the Good band (AQI 0–50), 232 (64%) registered Moderate readings, and 6 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 77, while the worst single-day peak hit 738 (Hazardous). Residents averaged about 10.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (261 days), Ozone (75 days), PM10 (24 days), NO2 (5 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -3.2 points and the median AQI moved +1 points, placing the multi-year trajectory at stable. The annual record contains 2 days at Unhealthy or worse. On those days, the EPA recommends that sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Andrew (86.9% good, AQI 40); Boone (93.2% good, AQI 37); Buchanan (56.4% good, AQI 46); Callaway (89.6% good, AQI 41); Cass (77.6% good, AQI 41); Cedar (81.6% good, AQI 39).
Air Quality Summary
Good Days
34.3%
125 of 365 monitored days
-40.8pp vs 75% target
Max AQI Recorded
738
Hazardous
Health Grade
Poor
90th-pct AQI 77
St. Louis City County, Good days share 34.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
St. Louis City County, Unhealthy-or-worse share 5.5%
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 365 monitored days across EPA AQI categories.
Good
125
34%
Moderate
232
64%
USG
6
2%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
2
1%
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%
Health Impact Assessment
Poor
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
Good-day share worse than typical
St. Louis City County 34.3%
Butler County, Ohio 33.1%
Typical US county 76.0%
Median AQI worse than typical
St. Louis City County 55
Butler County, Ohio 55
Typical US county 39
Unhealthy days / yr worse than typical
St. Louis City County 2
Butler County, Ohio 1
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as St. Louis City County (55), not geographic neighbors.
Butler County, Ohio
Median AQI 55 · 33.1% Good-day share
Inyo County, California
Median AQI 55 · 37.3% Good-day share
Pulaski County, Arkansas
Median AQI 55 · 31.2% Good-day share
Hamilton County, Ohio
Median AQI 54 · 34.3% Good-day share
Marion County, Indiana
Median AQI 54 · 40.0% Good-day share
Pima County, Arizona
Median AQI 54 · 33.7% Good-day share
Same primary pollutant: PM2.5
PM2.5 led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Kings County, California
34.5% Good-day share · median AQI 57
Wayne County, Michigan
36.2% Good-day share · median AQI 54
Tarrant County, Texas
31.8% Good-day share · median AQI 56
Orange County, California
37.8% Good-day share · median AQI 53
Cameron County, Texas
29.9% Good-day share · median AQI 57
Montgomery County, Texas
39.1% Good-day share · median AQI 53
Missouri counties on the map
Geographic containment only, other monitored counties in Missouri, independent of AQI or pollutant match.
What St. Louis City County's air means for you
On most days the air here is fine, but PM2.5 drives 2 unhealthy-or-worse days a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 55 means for exposure. Health guidance
- See how St. Louis City County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare St. Louis City County with its Missouri neighbours before drawing a conclusion about the region. All of Missouri
Based on 365 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in St. Louis City County, Missouri? ▼
St. Louis City County has a median Air Quality Index (AQI) of 55, rated "Moderate." 34.3% of monitored days (125 out of 365) had good air quality in 2025.
What is the main pollutant in St. Louis City County? ▼
The primary pollutant in St. Louis City County is Fine Particles (PM2.5), which was the main AQI driver on 261 days (72% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does St. Louis City County have? ▼
In 2025, St. Louis City County recorded 2 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 2 Hazardous). The maximum AQI reached 738 - a reading beyond the EPA’s standard 0–500 AQI scale, reflecting an extreme wildfire-smoke event or a transient monitor anomaly.
Is the air quality in St. Louis City County getting better or worse? ▼
Air quality in St. Louis City County is stable across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2024 at 38.0% (139 of 366 days). Lowest qualifying Good-day share: 2023 at 24.4% (89 of 365 days).
What does AQI 55 mean for health? ▼
St. Louis City County's median AQI of 55 falls in the "Moderate" category. See the AQI category guide for what each range means for health.
How does St. Louis City County compare to other counties in Missouri? ▼
St. Louis City County has 34.3% good air days. Nearby counties in Missouri: Andrew (86.9% good), Boone (93.2% good), Buchanan (56.4% good), Callaway (89.6% good), Cass (77.6% good), Cedar (81.6% good).
What is the 90th percentile AQI in St. Louis City County? ▼
The 90th percentile AQI for St. Louis City County is 77, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 738 - a reading beyond the EPA’s standard 0–500 AQI scale, reflecting an extreme wildfire-smoke event or a transient monitor anomaly (Hazardous).
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
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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.