Move workouts to the morning
Ozone peaks on hot, sunny afternoons. Mornings are usually cleaner.
Ozone guide →Air quality in St. Charles County, MO
Includes O'Fallon, St. Charles and St. Peters · EPA monitor readings, 2021 to 2025 (ozone only) · 405,000 people
For today's air and tomorrow's forecast, check AirNow by ZIP code.
Bad-air days a year
6
days with an AQI over 100
About 1 day in 59. That's down from about 31 a year in the early 2000s. St. Charles County has the 3rd most bad-air days of the 19 Missouri counties with monitors.
<1day unhealthy for everyoneAQI over 150, a year
42AQI on a typical dayGood: fine for everyone
May, Jun and Sepworst monthsOzone, the only pollutant measured
164worst day since 2016Sep 18, 2025, ozone
How to read the AQIThe EPA's air score runs from 0 to 500. Higher is worse. Over 100 counts as a bad-air day.
Typical day 42 (Good)Worst day 164 (Unhealthy)
Good0 to 50Moderate51 to 100Sensitive groups101 to 150Unhealthy151 to 200Very unhealthy201 to 300Hazardous301 to 500
When bad air happens in St. Charles County
Average bad-air days in each month, from 10 years of daily readings since 2016. Most fall in May, June and September. Only ozone is measured here.
Jan0
Feb0
Mar0
Apr0
May0.8
Jun3.0
Jul0.5
Aug0.8
Sep1.1
Oct0.1
Nov0
Dec0
Fine particles (smoke, soot)Ozone (smog)Bars stack in that order, from the bottom.
See the numbers by month
| Month | Ozone | Fine particles | All |
|---|---|---|---|
| January | 0 | 0 | 0 |
| February | 0 | 0 | 0 |
| March | 0 | 0 | 0 |
| April | 0 | 0 | 0 |
| May | 0.8 | 0 | 0.8 |
| June | 3.0 | 0 | 3.0 |
| July | 0.5 | 0 | 0.5 |
| August | 0.8 | 0 | 0.8 |
| September | 1.1 | 0 | 1.1 |
| October | 0.1 | 0 | 0.1 |
| November | 0 | 0 | 0 |
| December | 0 | 0 | 0 |
The worst day since 2016 was September 18, 2025: AQI 164, from ozone.
What causes it
Ozone is the only pollutant measured here. Every bad day counted here was an ozone day: smog that builds on hot, sunny afternoons. Monitors here don't measure fine particles, so smoky days aren't in these counts.
CDC's model estimates about 0.6 days a year with high fine particles at the county's worst spot, 2018 to 2022. See the estimates on the map
CDC estimates by month, 2001 to 2022
The CDC's model counts bad-air days at the county's worst spot, from monitors and satellites. Particles (brown) fill the inside of the ring and ozone (blue) the outside.
See the numbers by month
| Month | Fine particles | Ozone |
|---|---|---|
| January | 0 | 0 |
| February | 0.2 | 0 |
| March | 0.2 | 0 |
| April | 0 | 0.4 |
| May | 0 | 0.8 |
| June | 0.3 | 4.1 |
| July | 0.4 | 4.3 |
| August | 0.4 | 4.4 |
| September | 0.6 | 2.7 |
| October | 0 | 0 |
| November | 0 | 0 |
| December | 0 | 0 |
What helps in St. Charles County
Monitors here track ozone, which builds on hot afternoons, so timing helps. They don't track smoke, so it's worth having a clean room ready too.
Check tomorrow's forecast
AirNow posts an air quality forecast by ZIP code, usually the day before.
AirNow →Keep one room clean on smoky days
Shut the windows and run an air purifier sized for the room.
Choosing a purifier →For today's reading and tomorrow's forecast, check AirNow by ZIP code. Its fire and smoke map adds smoke plumes.
Purifiers catch smoke but don't remove ozone, and smoky days are rare here. If you want one for wildfire season anyway, see the 3 models we'd buy, each with a verified smoke rating.
Every year since 2000
Bad-air days each year. The striped part is days unhealthy for everyone, over AQI 150. The worst recent year was 2023, with 15 bad-air days.
Unhealthy for sensitive groups (101 to 150)Unhealthy or worse (over 150)
See the numbers by year
| Year | Bad-air days | Over 150 |
|---|---|---|
| 2000 | 25 | 10 |
| 2001 | 35 | 5 |
| 2002 | 48 | 20 |
| 2003 | 33 | 9 |
| 2004 | 15 | 0 |
| 2005 | 49 | 14 |
| 2006 | 31 | 11 |
| 2007 | 34 | 11 |
| 2008 | 7 | 1 |
| 2009 | 7 | 0 |
| 2010 | 28 | 3 |
| 2011 | 25 | 3 |
| 2012 | 32 | 6 |
| 2013 | 5 | 1 |
| 2014 | 8 | 0 |
| 2015 | 3 | 0 |
| 2016 | 12 | 1 |
| 2017 | 4 | 1 |
| 2018 | 8 | 0 |
| 2019 | 4 | 1 |
| 2020 | 4 | 0 |
| 2021 | 2 | 0 |
| 2022 | 7 | 0 |
| 2023 | 15 | 0 |
| 2024 | 3 | 0 |
| 2025 | 4 | 1 |
St. Charles County and its neighbors
Bad-air days a year, 2021 to 2025. Missouri counties with monitors averaged about 5 bad-air days a year, weighted by population.
Questions people ask about air in St. Charles County
How often is the air bad in St. Charles County?
From 2021 to 2025, monitors in St. Charles County recorded about 6 bad-air days a year with an AQI above 100, which is unhealthy for sensitive groups or worse. The worst year in that stretch was 2023, with 15 bad-air days.
What causes bad air in St. Charles County?
Every bad day counted here was an ozone day: smog that builds on hot, sunny afternoons. Bad days cluster in May, June and September. Monitors here don't measure fine particles, so smoky days aren't in these counts.
Where can I see today's air quality in St. Charles County?
AirNow, run by the EPA and its partners, shows the current AQI and tomorrow's forecast for your ZIP code. Its fire and smoke map adds smoke plumes. On a typical day the AQI in St. Charles County is about 42, which is good, fine for everyone. Check it most in May, June and September, when bad days here cluster.
About these numbers
Day counts come from the EPA's county air quality summaries, which take the worst reading across a county's monitors each day. Monitors here reported on about 262 days a year from 2021 to 2025; some run only in ozone season, so a few bad days can go uncounted. A county is large, and your street can be cleaner or worse. CDC's model, which estimates every county from monitors and satellites, gives the worst spot in St. Charles County about 1 high-particle day and about 4 high-ozone days a year from 2018 to 2022.
Sources: EPA AirData county AQI summaries (2000 to 2025) and daily county AQI (2016 to 2025); CDC Environmental Public Health Tracking Network modeled PM2.5 and ozone (2001 to 2022). Updated November 8, 2026.