Median AQI
36
Good
Illinois · County air quality · 2025
Clark County, Illinois
Median AQI 36 (Good). 92.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 36
- Median AQI
- 92.1%
- Good-air days
- 0
- Unhealthy days
- 80%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Clark County's air is good on balance, cleaner than most US counties, a median AQI of 36 with 92.1% of 267 monitored days in the Good band, driven mainly by Ozone.
- 36
- median AQI · Good
- 92.1%
- days rated Good
- Ozone
- main pollutant
- 80%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 87.
Data updated July 2026
- Good · 246 d · 92.1%
- Moderate · 21 d · 7.9%
How did Clark County's air break down in 2025?
Across 267 monitored days, 246 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 87 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 267 days monitored. Median AQI: 36 (Good).
Insights: Clark County, Illinois. EPA monitors recorded a median Air Quality Index of 36 across 267 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 246 (92%) fell in the Good band (AQI 0–50), 21 (8%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 49, while the worst single-day peak hit 87 (Moderate). Residents averaged about 27.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (267 days). Across the 2020–2025 trend window, good days shifted by -101 and the median AQI moved +5 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 87, moderate but never Unhealthy. Nearby counties show comparable readings: Adams (94.4% good, AQI 38); Champaign (63.7% good, AQI 44); Cook (40.5% good, AQI 53); DuPage (61.2% good, AQI 44); Effingham (95.7% good, AQI 35); Hamilton (66.5% good, AQI 43).
Air Quality Summary
Good Days
92.1%
246 of 267 monitored days
17.1pp vs 75% target
Max AQI Recorded
87
Moderate
Health Grade
Excellent
90th-pct AQI 49
Clark County, Good days share 92.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Clark County, Unhealthy-or-worse share 0.0%
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 267 monitored days across EPA AQI categories.
Good
246
92%
Moderate
21
8%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Clark County.
- Ozone · 267 days · 100.0%
Health Impact Assessment
Excellent
Clark County's air quality is rated Excellent based on 92.1% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 92.1% of monitored days in 2025, with a median AQI of 36. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 267 of 267 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Clark County. Best year: 2020 (347 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 347 | 16 | 0 | 0 | 31 | 97 |
| 2021 | 344 | 19 | 0 | 0 | 34 | 84 |
| 2022 | 296 | 15 | 1 | 0 | 34 | 101 |
| 2023 | 320 | 33 | 5 | 0 | 36 | 136 |
| 2024 | 332 | 31 | 0 | 0 | 36 | 90 |
| 2025 | 246 | 21 | 0 | 0 | 36 | 87 |
How does Clark County compare?
How Clark County compares
Good-day share better than US avg
Clark County 92.1%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI better than US avg
Clark County 36
Adams County, Illinois 38
US average 38
Unhealthy days / yr about average
Clark County 0
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 Clark County.
Aroostook County, Maine
AQI 36 · 84.1% good
Berkeley County, South Carolina
AQI 36 · 92.2% good
Carter County, Kentucky
AQI 36 · 89.8% good
Claiborne County, Tennessee
AQI 36 · 96.3% good
Cobb County, Georgia
AQI 36 · 92.5% good
Fauquier County, Virginia
AQI 36 · 97.2% good
What Clark County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 36 means for exposure. Health guidance
- See how Clark County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Clark County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 267 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 Clark County, Illinois? ▼
Clark County has a median Air Quality Index (AQI) of 36, rated "Good." 92.1% of monitored days (246 out of 267) had good air quality in 2025.
What is the main pollutant in Clark County? ▼
The primary pollutant in Clark County is Ozone (O3), which was the main AQI driver on 267 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Clark County have? ▼
In 2025, Clark County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 87.
Is the air quality in Clark County getting better or worse? ▼
Air quality in Clark County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 347 good days. Worst year: 2025 with 246 good days.
What does AQI 36 mean for health? ▼
Clark County's median AQI of 36 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 Clark County compare to other counties in Illinois? ▼
Clark County has 92.1% good air days. Nearby counties in Illinois: Adams (94.4% good), Champaign (63.7% good), Cook (40.5% good), DuPage (61.2% good), Effingham (95.7% good), Hamilton (66.5% good).
What is the 90th percentile AQI in Clark County? ▼
The 90th percentile AQI for Clark County is 49, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 87 (Moderate).
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.