Median AQI
44
Good
Illinois · County air quality · 2025
Saint Clair County, Illinois
Median AQI 44 (Good). 64.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- Median AQI
- 64.4%
- Good-air days
- 1
- Unhealthy days
- 22%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Saint Clair County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 44 with 64.4% of 298 monitored days in the Good band, driven mainly by PM2.5.
- 44
- median AQI · Good
- 64.4%
- days rated Good
- PM2.5
- main pollutant
- 22%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 161.
Data updated July 2026
- Good · 192 d · 64.4%
- Moderate · 103 d · 34.6%
- USG · 2 d · 0.7%
- Unhealthy · 1 d · 0.3%
How did Saint Clair County's air break down in 2025?
Across 298 monitored days, 192 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 161 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 298 days monitored. Median AQI: 44 (Good).
Insights: Saint Clair County, Illinois. EPA monitors recorded a median Air Quality Index of 44 across 298 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 192 (64%) fell in the Good band (AQI 0–50), 103 (35%) registered Moderate readings, and 2 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 63, while the worst single-day peak hit 161 (Unhealthy). Residents averaged about 19.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (194 days), Ozone (98 days), NO2 (6 days). Across the 2020–2025 trend window, good days shifted by -68 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. Residents and visitors experienced 1 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Adams (94.4% good, AQI 38); Champaign (63.7% good, AQI 44); Clark (92.1% good, AQI 36); Cook (40.5% good, AQI 53); DuPage (61.2% good, AQI 44); Effingham (95.7% good, AQI 35).
Air Quality Summary
Good Days
64.4%
192 of 298 monitored days
-10.6pp vs 75% target
Max AQI Recorded
161
Unhealthy
Health Grade
Moderate
90th-pct AQI 63
Saint Clair County, Good days share 64.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Saint Clair County, Unhealthy-or-worse share 0.3%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 298 monitored days across EPA AQI categories.
Good
192
64%
Moderate
103
35%
USG
2
1%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Saint Clair County.
- PM2.5 · 194 days · 65.1%
- Ozone · 98 days · 32.9%
- NO₂ · 6 days · 2.0%
Health Impact Assessment
Moderate
Saint Clair County's air quality is rated Moderate based on 64.4% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 2 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 161. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 194 of 298 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Saint Clair County. Best year: 2020 (260 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 260 | 105 | 0 | 0 | 42 | 100 |
| 2021 | 141 | 221 | 2 | 1 | 53 | 185 |
| 2022 | 165 | 199 | 1 | 0 | 52 | 105 |
| 2023 | 128 | 223 | 12 | 2 | 54 | 169 |
| 2024 | 218 | 145 | 3 | 0 | 46 | 112 |
| 2025 | 192 | 103 | 2 | 1 | 44 | 161 |
How does Saint Clair County compare?
How Saint Clair County compares
Good-day share worse than US avg
Saint Clair County 64.4%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
Saint Clair County 44
Adams County, Illinois 38
US average 38
Unhealthy days / yr worse than US avg
Saint Clair County 1
Adams County, Illinois 0
US average 0
Other Counties in Illinois
Air quality comparison for nearby counties in Illinois.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Saint Clair County.
Baltimore County, Maryland
AQI 44 · 70.4% good
Baltimore (City) County, Maryland
AQI 44 · 68.7% good
Bartholomew County, Indiana
AQI 44 · 69.2% good
Berks County, Pennsylvania
AQI 44 · 65.6% good
Bolivar County, Mississippi
AQI 44 · 70.3% good
Brewster County, Texas
AQI 44 · 69.0% good
What Saint Clair County's air means for you
On most days the air here is fine, but PM2.5 drives 1 unhealthy-or-worse day a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 44 means for exposure. Health guidance
- See how Saint Clair County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Saint Clair County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 298 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Saint Clair County, Illinois? ▼
Saint Clair County has a median Air Quality Index (AQI) of 44, rated "Good." 64.4% of monitored days (192 out of 298) had good air quality in 2025.
What is the main pollutant in Saint Clair County? ▼
The primary pollutant in Saint Clair County is Fine Particles (PM2.5), which was the main AQI driver on 194 days (65% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Saint Clair County have? ▼
In 2025, Saint Clair County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 161.
Is the air quality in Saint Clair County getting better or worse? ▼
Air quality in Saint Clair County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 260 good days. Worst year: 2023 with 128 good days.
What does AQI 44 mean for health? ▼
Saint Clair County's median AQI of 44 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Saint Clair County compare to other counties in Illinois? ▼
Saint Clair County has 64.4% good air days. Nearby counties in Illinois: Adams (94.4% good), Champaign (63.7% good), Clark (92.1% good), Cook (40.5% good), DuPage (61.2% good), Effingham (95.7% good).
What is the 90th percentile AQI in Saint Clair County? ▼
The 90th percentile AQI for Saint Clair County is 63, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 161 (Unhealthy).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.