Saint Charles, Missouri Air Quality Today
AirHistory tracks long-run EPA monitoring rather than live readings, so for the live number check AirNow.gov below. As a baseline, Saint Charles, Missouri's most recent EPA year (2023) posted a median AQI of 45 (Good) against a 5-year median of 42 and an overall Grade of C. The dominant pollutant is Ground-Level Ozone, which tells you which days are most likely to spike.
Check Today's Live AQI in Saint Charles, Missouri
AirHistory is built on 10 years of EPA Air Quality System records, so it shows you what air quality in Saint Charles, Missouri typically looks like — not the live reading for this exact hour. For today's real-time AQI, check AirNow.gov (the EPA's official live index) or the AirNow Fire and Smoke Map during wildfire season.
That said, the history is the best predictor of a normal day. In 2023, Saint Charles, Missouri posted a median AQI of 45 (Good), with 156 "Good" days and 15 days that crossed into "Unhealthy for Sensitive Groups" or worse. The dominant pollutant, Ground-Level Ozone, is the one most likely to push today's number up — Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Saint Charles, Missouri Air Quality Snapshot
| Air Quality Grade | C63/100 |
| 5-Year Median AQI | 42 (Good) |
| Most Recent Median AQI (2023) | 45 (Good) |
| Dominant Pollutant | Ground-Level Ozone |
| 10-Year Trend | Stable (-0.07 AQI/yr) |
| Unhealthy Days (last 5 yr) | 32 |
| National Rank (cleanest = #1) | #600 of 1,020 (59th most polluted percentile) |
| Missouri Rank | #14 of 21 |
What Does the C Grade Mean?
Saint Charles, Missouri earns a C — air quality is fair, but not great. With a 5-year median AQI of 42, the city sees a meaningful number of "Moderate" days each year, when the EPA flags air as a concern for unusually sensitive people.
Saint Charles, Missouri's 5-year median AQI of 42 is right around the national average of 41 across the 1,020 monitored U.S. cities tracked here. Within Missouri, Saint Charles, Missouri's air quality is roughly typical for the state, where the average city posts a 5-year median AQI of 41.
For context within Missouri: Taney, Missouri currently holds the state's cleanest grade (A, AQI 26), while St. Louis City, Missouri sits at the bottom (C, AQI 55).
What's in Saint Charles, Missouri's Air?
The dominant pollutant in Saint Charles, Missouri is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Ground-Level Ozone | 245 | 100% |
Is the Air Getting Better or Worse?
Air quality in Saint Charles, Missouri has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Saint Charles, Missouri posted a median AQI of 43. By 2023 that figure was 45 — a rise of 2 AQI points dirtier across 10 years of EPA records.
Year-by-Year AQI in Saint Charles, Missouri
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 43 | 166 | 8 | Ozone |
| 2015 | 42 | 174 | 3 | Ozone |
| 2016 | 44 | 154 | 12 | Ozone |
| 2017 | 44 | 185 | 4 | Ozone |
| 2018 | 42 | 190 | 8 | Ozone |
| 2019 | 42 | 202 | 4 | Ozone |
| 2020 | 39 | 216 | 4 | Ozone |
| 2021 | 41 | 209 | 2 | Ozone |
| 2022 | 42 | 206 | 7 | Ozone |
| 2023 | 45 | 156 | 15 | Ozone |
Health Context for Saint Charles, Missouri
Across the past five years, this area has logged 32 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 6 days per year. That is roughly typical for a U.S. metro, with most caution days clustered in summer (ozone) or wildfire season.
Healthy adults can continue normal outdoor activity in most weather, but should pay attention to AQI alerts during the worst pollution windows. People with asthma, heart disease, or pregnancy should reduce prolonged or intense outdoor exertion on flagged days, and consider running an indoor HEPA air cleaner during peak season. Because ozone peaks in the afternoon on hot sunny days, plan outdoor exercise for early morning or after sunset on bad-air days.
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
More about Saint Charles, Missouri
Saint Charles, Missouri has an Air Quality Grade of C (fair) with a 5-year median AQI of 42. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
The data source behind this answer is the EPA Air Quality System (AQS). Every figure on the page traces back to that source; the methodology page describes the inputs and the refresh cadence in full detail.
A practical caveat: the headline answer above reflects the most recent the EPA Air Quality System (AQS) vintage; underlying data is often revised for months after first publication, and the right reference for any specific decision is whichever vintage is current at the time of the decision. The as-of date is stamped on every page.
Source: EPA Outdoor Air Quality Data, 2026.