EnviroNearby

Saint Charles County, Missouri air quality history

Daily Air Quality Index summaries from EPA monitors in Saint Charles, Missouri, 1980–2025.

80%
Good days in 2025
40
Median daily AQI, 2025
4
Days above 100 in 2025
309
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Saint Charles, Missouri on 309 days. 248 of those days (80%) were in the Good range, and the median daily AQI was 40. That median is about the same as the typical monitored county in Missouri (median 41 across 19 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 248 days. Across the years with fuller monitoring, the share of Good days fell: an average of 78% in 1980–1982 versus 72% in 2023–2025.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1980: Good 293 of 366 days1980: Moderate 36 of 366 days1980: Unhealthy for Sensitive Groups 23 of 366 days1980: Unhealthy 11 of 366 days1980: Very Unhealthy 3 of 366 days19801981: Good 254 of 325 days1981: Moderate 43 of 325 days1981: Unhealthy for Sensitive Groups 17 of 325 days1981: Unhealthy 9 of 325 days1981: Very Unhealthy 2 of 325 days1982: Good 260 of 340 days1982: Moderate 49 of 340 days1982: Unhealthy for Sensitive Groups 29 of 340 days1982: Unhealthy 2 of 340 days1983: Good 336 of 359 days1983: Moderate 21 of 359 days1983: Unhealthy for Sensitive Groups 2 of 359 days1984: Good 272 of 320 days1984: Moderate 24 of 320 days1984: Unhealthy for Sensitive Groups 14 of 320 days1984: Unhealthy 10 of 320 days1985: Good 336 of 361 days1985: Moderate 23 of 361 days1985: Unhealthy 2 of 361 days19851986: Good 278 of 342 days1986: Moderate 36 of 342 days1986: Unhealthy for Sensitive Groups 18 of 342 days1986: Unhealthy 7 of 342 days1986: Very Unhealthy 3 of 342 days1987: Good 275 of 360 days1987: Moderate 45 of 360 days1987: Unhealthy for Sensitive Groups 31 of 360 days1987: Unhealthy 7 of 360 days1987: Very Unhealthy 2 of 360 days1988: Good 250 of 353 days1988: Moderate 46 of 353 days1988: Unhealthy for Sensitive Groups 33 of 353 days1988: Unhealthy 17 of 353 days1988: Very Unhealthy 7 of 353 days1989: Good 283 of 356 days1989: Moderate 52 of 356 days1989: Unhealthy for Sensitive Groups 16 of 356 days1989: Unhealthy 4 of 356 days1989: Very Unhealthy 1 of 356 days1990: Good 297 of 350 days1990: Moderate 33 of 350 days1990: Unhealthy for Sensitive Groups 12 of 350 days1990: Unhealthy 7 of 350 days1990: Very Unhealthy 1 of 350 days19901991: Good 283 of 362 days1991: Moderate 45 of 362 days1991: Unhealthy for Sensitive Groups 25 of 362 days1991: Unhealthy 9 of 362 days1992: Good 288 of 358 days1992: Moderate 48 of 358 days1992: Unhealthy for Sensitive Groups 17 of 358 days1992: Unhealthy 5 of 358 days1993: Good 154 of 186 days1993: Moderate 23 of 186 days1993: Unhealthy for Sensitive Groups 6 of 186 days1993: Unhealthy 3 of 186 days1994: Good 167 of 273 days1994: Moderate 49 of 273 days1994: Unhealthy for Sensitive Groups 37 of 273 days1994: Unhealthy 18 of 273 days1994: Very Unhealthy 2 of 273 days1995: Good 253 of 365 days1995: Moderate 52 of 365 days1995: Unhealthy for Sensitive Groups 30 of 365 days1995: Unhealthy 23 of 365 days1995: Very Unhealthy 7 of 365 days19951996: Good 259 of 366 days1996: Moderate 69 of 366 days1996: Unhealthy for Sensitive Groups 20 of 366 days1996: Unhealthy 16 of 366 days1996: Very Unhealthy 2 of 366 days1997: Good 268 of 365 days1997: Moderate 58 of 365 days1997: Unhealthy for Sensitive Groups 34 of 365 days1997: Unhealthy 4 of 365 days1997: Very Unhealthy 1 of 365 days1998: Good 251 of 365 days1998: Moderate 64 of 365 days1998: Unhealthy for Sensitive Groups 32 of 365 days1998: Unhealthy 17 of 365 days1998: Very Unhealthy 1 of 365 days1999: Good 215 of 364 days1999: Moderate 104 of 364 days1999: Unhealthy for Sensitive Groups 26 of 364 days1999: Unhealthy 17 of 364 days1999: Very Unhealthy 2 of 364 days2000: Good 202 of 357 days2000: Moderate 130 of 357 days2000: Unhealthy for Sensitive Groups 15 of 357 days2000: Unhealthy 10 of 357 days20002001: Good 214 of 365 days2001: Moderate 116 of 365 days2001: Unhealthy for Sensitive Groups 30 of 365 days2001: Unhealthy 5 of 365 days2002: Good 200 of 354 days2002: Moderate 106 of 354 days2002: Unhealthy for Sensitive Groups 28 of 354 days2002: Unhealthy 18 of 354 days2002: Very Unhealthy 2 of 354 days2003: Good 217 of 364 days2003: Moderate 114 of 364 days2003: Unhealthy for Sensitive Groups 24 of 364 days2003: Unhealthy 9 of 364 days2004: Good 213 of 338 days2004: Moderate 110 of 338 days2004: Unhealthy for Sensitive Groups 15 of 338 days2005: Good 209 of 364 days2005: Moderate 106 of 364 days2005: Unhealthy for Sensitive Groups 35 of 364 days2005: Unhealthy 14 of 364 days20052006: Good 225 of 365 days2006: Moderate 109 of 365 days2006: Unhealthy for Sensitive Groups 20 of 365 days2006: Unhealthy 11 of 365 days2007: Good 125 of 365 days2007: Moderate 206 of 365 days2007: Unhealthy for Sensitive Groups 23 of 365 days2007: Unhealthy 11 of 365 days2008: Good 245 of 348 days2008: Moderate 96 of 348 days2008: Unhealthy for Sensitive Groups 6 of 348 days2008: Unhealthy 1 of 348 days2009: Good 306 of 365 days2009: Moderate 52 of 365 days2009: Unhealthy for Sensitive Groups 7 of 365 days2010: Good 222 of 304 days2010: Moderate 54 of 304 days2010: Unhealthy for Sensitive Groups 25 of 304 days2010: Unhealthy 3 of 304 days20102011: Good 161 of 245 days2011: Moderate 59 of 245 days2011: Unhealthy for Sensitive Groups 22 of 245 days2011: Unhealthy 3 of 245 days2012: Good 121 of 214 days2012: Moderate 61 of 214 days2012: Unhealthy for Sensitive Groups 26 of 214 days2012: Unhealthy 6 of 214 days2013: Good 152 of 214 days2013: Moderate 57 of 214 days2013: Unhealthy for Sensitive Groups 4 of 214 days2013: Unhealthy 1 of 214 days2014: Good 166 of 214 days2014: Moderate 40 of 214 days2014: Unhealthy for Sensitive Groups 8 of 214 days2015: Good 174 of 214 days2015: Moderate 37 of 214 days2015: Unhealthy for Sensitive Groups 3 of 214 days20152016: Good 154 of 214 days2016: Moderate 48 of 214 days2016: Unhealthy for Sensitive Groups 11 of 214 days2016: Unhealthy 1 of 214 days2017: Good 185 of 245 days2017: Moderate 56 of 245 days2017: Unhealthy for Sensitive Groups 3 of 245 days2017: Unhealthy 1 of 245 days2018: Good 190 of 245 days2018: Moderate 47 of 245 days2018: Unhealthy for Sensitive Groups 8 of 245 days2019: Good 202 of 244 days2019: Moderate 38 of 244 days2019: Unhealthy for Sensitive Groups 3 of 244 days2019: Unhealthy 1 of 244 days2020: Good 216 of 245 days2020: Moderate 25 of 245 days2020: Unhealthy for Sensitive Groups 4 of 245 days20202021: Good 209 of 245 days2021: Moderate 34 of 245 days2021: Unhealthy for Sensitive Groups 2 of 245 days2022: Good 206 of 245 days2022: Moderate 32 of 245 days2022: Unhealthy for Sensitive Groups 7 of 245 days2023: Good 156 of 245 days2023: Moderate 74 of 245 days2023: Unhealthy for Sensitive Groups 15 of 245 days2024: Good 194 of 265 days2024: Moderate 68 of 265 days2024: Unhealthy for Sensitive Groups 3 of 265 days2025: Good 248 of 309 days2025: Moderate 57 of 309 days2025: Unhealthy for Sensitive Groups 3 of 309 days2025: Unhealthy 1 of 309 days2025
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001501980: 90th percentile AQI 1011981: 90th percentile AQI 931982: 90th percentile AQI 1001983: 90th percentile AQI 441984: 90th percentile AQI 801985: 90th percentile AQI 461986: 90th percentile AQI 931987: 90th percentile AQI 1081988: 90th percentile AQI 1291989: 90th percentile AQI 771990: 90th percentile AQI 771991: 90th percentile AQI 931992: 90th percentile AQI 771993: 90th percentile AQI 741994: 90th percentile AQI 1331995: 90th percentile AQI 1361996: 90th percentile AQI 1011997: 90th percentile AQI 1011998: 90th percentile AQI 1081999: 90th percentile AQI 1152000: 90th percentile AQI 872001: 90th percentile AQI 972002: 90th percentile AQI 1262003: 90th percentile AQI 1002004: 90th percentile AQI 802005: 90th percentile AQI 1122006: 90th percentile AQI 972007: 90th percentile AQI 972008: 90th percentile AQI 742009: 90th percentile AQI 612010: 90th percentile AQI 1002011: 90th percentile AQI 1012012: 90th percentile AQI 1192013: 90th percentile AQI 802014: 90th percentile AQI 842015: 90th percentile AQI 742016: 90th percentile AQI 772017: 90th percentile AQI 712018: 90th percentile AQI 712019: 90th percentile AQI 582020: 90th percentile AQI 542021: 90th percentile AQI 542022: 90th percentile AQI 642023: 90th percentile AQI 872024: 90th percentile AQI 742025: 90th percentile AQI 711980: Median AQI 341981: Median AQI 361982: Median AQI 371983: Median AQI 261984: Median AQI 301985: Median AQI 291986: Median AQI 311987: Median AQI 371988: Median AQI 371989: Median AQI 351990: Median AQI 341991: Median AQI 351992: Median AQI 351993: Median AQI 371994: Median AQI 411995: Median AQI 391996: Median AQI 381997: Median AQI 381998: Median AQI 381999: Median AQI 452000: Median AQI 462001: Median AQI 442002: Median AQI 442003: Median AQI 442004: Median AQI 442005: Median AQI 462006: Median AQI 442007: Median AQI 582008: Median AQI 412009: Median AQI 312010: Median AQI 402011: Median AQI 442012: Median AQI 492013: Median AQI 442014: Median AQI 432015: Median AQI 422016: Median AQI 442017: Median AQI 442018: Median AQI 422019: Median AQI 422020: Median AQI 392021: Median AQI 412022: Median AQI 422023: Median AQI 452024: Median AQI 442025: Median AQI 401980198519901995200020052010201520202025
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2025

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Ozone24880%
Nitrogen dioxide (NO₂)6120%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253092485731004071164Ozone
20242651946830004474129Ozone
202324515674150004587147Ozone
20222452063270004264129Ozone
20212452093420004154119Ozone
20202452162540003954112Ozone
20192442023831004258151Ozone
20182451904780004271147Ozone
20172451855631004471159Ozone
201621415448111004477151Ozone
20152141743730004274126Ozone
20142141664080004384126Ozone
20132141525741004480156Ozone
2012214121612660049119185Ozone
2011245161592230044101169Ozone
2010304222542530040100164Ozone
20093653065270003161136Ozone
20083482459661004174159Ozone
20073651252062311005897185PM2.5
20063652251092011004497179Ozone
200536420910635140046112190Ozone
2004338213110150004480136Ozone
20033642171142490044100179Ozone
200235420010628182044126209Ozone
2001365214116305004497185Ozone
20003572021301510004687185Ozone
199936421510426172045115225Ozone
19983652516432171038108212Ozone
1997365268583441038101207Ozone
19963662596920162038101232Ozone
19953652535230237039136221Ozone
19942731674937182041133220Ozone
19931861542363003774185Ozone
199235828848175003577195Ozone
199136228345259003593195Ozone
199035029733127103477205Ozone
198935628352164103577202Ozone
19883532504633177037129238Ozone
1987360275453172037108222Ozone
198634227836187303193209Ozone
19853613362302002946177Ozone
1984320272241410003080197Ozone
19833593362120002644122NO2
1982340260492920037100159Ozone
198132525443179203693223Ozone
19803662933623113034101215Ozone

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

Counties in Missouri with a similar median AQI in 2025

All monitored counties in Missouri

Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem