St. Louis Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for St. Louis (St. Louis, MO-IL area) was March 14, 2025, when the daily AQI reached 738 (Hazardous), set by coarse dust (PM10). Across 2,557 monitored days in 2019–2025, 10 of the 10 worst days were Unhealthy (AQI 151+) or worse; 4 of the 10 were driven by ground-level ozone and 3 by coarse dust (PM10), and 3 of them fell in 2023. The cleanest day was September 24, 2024 at AQI 24 (Good); the 10 best days span 4 different years.
Extra Protection for St. Louis Homes
Air quality in St. Louis is moderate (AQI 60), with fine particles (PM2.5) the main concern. A True-HEPA purifier gives sensitive groups — kids, seniors and anyone with asthma or allergies — cleaner air indoors on days like today.
Recommended: Coway AP-1512HH Mighty — True HEPA + carbon · CADR ~246 CFM · rooms up to 361 sq ft
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When were St. Louis's worst and best air quality days?
Worst: March 14, 2025, AQI 738 (Hazardous, PM10). Best: September 24, 2024, AQI 24 (Good). Based on 2,557 EPA-monitored days in the St. Louis, MO-IL area, 2019–2025.
- ✓10 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 156 (Unhealthy).
- ✓Ground-level ozone drove 4 of the 10 worst days.
- ✓2023 accounts for 3 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in St. Louis?▾
March 14, 2025: the St. Louis, MO-IL area's daily AQI hit 738 (Hazardous), driven by coarse dust (PM10). That is the highest of 2,557 monitored days in 2019–2025.
What was the best air quality day in St. Louis?▾
September 24, 2024, with a daily AQI of 24 (Good). The daily AQI is the highest reading from any monitor in the area, so even the cleanest day reflects the area's worst monitor.
What was St. Louis's worst air quality day in 2025?▾
March 14, 2025, AQI 738 (Hazardous), set by coarse dust (PM10).
Which pollutant causes St. Louis's worst air days?▾
Ground-level ozone set the AQI on 4 of the 10 worst days; coarse dust (PM10) on 3. The tables above list the defining pollutant for every day.
St. Louis's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the St. Louis, MO-IL area (EPA AQS). The daily AQI is the highest reading from any monitor in the area that day.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2025 page) | 738 | Hazardous | PM10 — coarse dust (PM10) |
| 2 | (2025 page) | 343 | Hazardous | PM10 — coarse dust (PM10) |
| 3 | (2021 page) | 187 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 4 | (2023 page) | 185 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 5 | (2023 page) | 167 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 6 | (2024 page) | 165 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 7 | (2025 page) | 164 | Unhealthy | O3 — ground-level ozone |
| 8 | (2021 page) | 164 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 9 | (2024 page) | 156 | Unhealthy | PM10 — coarse dust (PM10) |
| 10 | (2023 page) | 156 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
Worst day of each year in St. Louis
The single highest-AQI day in each year on record. Click a year for its full month-by-month statistics.
| Year | Worst day | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 2019 | 156 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2020 | 156 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2021 | 187 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2022 | 147 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2023 | 185 | Unhealthy | O3 — ground-level ozone, in peak ozone season | |
| 2024 | 165 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2025 | 738 | Hazardous | PM10 — coarse dust (PM10) |
St. Louis's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the St. Louis, MO-IL area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2024 page) | 24 | Good | O3 — ground-level ozone |
| 2 | (2024 page) | 24 | Good | NO2 — nitrogen dioxide |
| 3 | (2023 page) | 24 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 4 | (2024 page) | 25 | Good | O3 — ground-level ozone |
| 5 | (2021 page) | 26 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 6 | (2025 page) | 27 | Good | O3 — ground-level ozone |
| 7 | (2025 page) | 27 | Good | O3 — ground-level ozone |
| 8 | (2024 page) | 27 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 9 | (2025 page) | 28 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 10 | (2024 page) | 28 | Good | PM2.5 — fine particulate matter (PM2.5) |
Right now in St. Louis
The latest EPA AirNow observation puts St. Louis's AQI at 60 (PM2.5). The days above are historical records, not today's reading.
View current St. Louis air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (St. Louis, MO-IL), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.