Median AQI
51
Moderate
Missouri · County air quality · 2025
St. Louis City County, Missouri
Median AQI 51 (Moderate). 47.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 51
- Median AQI
- 47.7%
- Good-air days
- 2
- Unhealthy days
- 5%
- Cleaner than US counties
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 51 with 47.7% of 151 monitored days in the Good band, driven mainly by PM2.5.
- 51
- median AQI · Moderate
- 47.7%
- days rated Good
- PM2.5
- main pollutant
- 5%
- 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.
Data updated July 2026
- Good · 72 d · 47.7%
- Moderate · 77 d · 51.0%
- Hazardous · 2 d · 1.3%
How did St. Louis City County's air break down in 2025?
Across 151 monitored days, 72 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). 151 days monitored. Median AQI: 51 (Moderate).
Insights: St. Louis City County, Missouri. EPA monitors recorded a median Air Quality Index of 51 across 151 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 72 (48%) fell in the Good band (AQI 0–50), 77 (51%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 68, while the worst single-day peak hit 738 (Hazardous). Residents averaged about 14.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (99 days), Ozone (33 days), PM10 (15 days), NO2 (3 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -65 and the median AQI moved -3 points, placing the multi-year trajectory at improving. Residents and visitors experienced 2 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. 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
47.7%
72 of 151 monitored days
-27.3pp vs 75% target
Max AQI Recorded
738
Hazardous
Health Grade
Poor
90th-pct AQI 68
St. Louis City County, Good days share 47.7%
National avg
Share of monitored days at AQI 0-50 (Good band).
St. Louis City County, Unhealthy-or-worse share 1.3%
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 151 monitored days across EPA AQI categories.
Good
72
48%
Moderate
77
51%
USG
0
0%
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 · 99 days · 65.6%
- Ozone · 33 days · 21.9%
- PM10 · 15 days · 9.9%
- NO₂ · 3 days · 2.0%
- CO · 1 days · 0.7%
Health Impact Assessment
Poor
St. Louis City County's air quality is rated Poor based on 47.7% 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 99 of 151 monitored days.
5-Year Air Quality Trend
Air quality is improving in St. Louis City County. Best year: 2024 (139 good 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 | 72 | 77 | 0 | 0 | 51 | 738 |
How does St. Louis City County compare?
How St. Louis City County compares
Good-day share worse than US avg
St. Louis City County 47.7%
Andrew County, Missouri 84.3%
US average 76.5%
Median AQI worse than US avg
St. Louis City County 51
Andrew County, Missouri 42
US average 38
Unhealthy days / yr worse than US avg
St. Louis City County 2
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 St. Louis City County.
Arapahoe County, Colorado
AQI 51 · 47.3% good
Bibb County, Georgia
AQI 51 · 49.5% good
East Baton Rouge County, Louisiana
AQI 51 · 49.3% good
Eddy County, New Mexico
AQI 51 · 46.9% good
Hillsborough County, Florida
AQI 51 · 49.3% good
Jefferson County, Colorado
AQI 51 · 46.5% good
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 51 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 151 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 St. Louis City County, Missouri? ▼
St. Louis City County has a median Air Quality Index (AQI) of 51, rated "Moderate." 47.7% of monitored days (72 out of 151) 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 99 days (66% 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 improving based on 5-year trends (2020-2025). Best year: 2024 with 139 good days. Worst year: 2025 with 72 good days.
What does AQI 51 mean for health? ▼
St. Louis City County's median AQI of 51 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does St. Louis City County compare to other counties in Missouri? ▼
St. Louis City County has 47.7% 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 St. Louis City County? ▼
The 90th percentile AQI for St. Louis City County is 68, 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.
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.