Insights: Saint Charles County, Missouri. EPA monitors recorded a median Air Quality Index of 40 across 309 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 248 (80%) fell in the Good band (AQI 0–50), 57 (18%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 71, while the worst single-day peak hit 164 (Unhealthy). Residents averaged about 24.1 clean-air days per 30-day window during the monitoring year.

The full pollutant mix ranks as Ozone (248 days), NO2 (61 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -7.9 points (measured against 245 monitored days in 2020 and 309 in 2025, so raw day counts are not comparable) and the median AQI moved +1 points, placing the multi-year trajectory at worsening. The annual record contains 1 day at Unhealthy or worse. On that day, 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).