Median AQI
33
Good
Illinois · County air quality · 2025
Clark County, Illinois
Median AQI 33 (Good). 93.8% of monitored days rated Good, direct from EPA Air Quality System stations.
- 33
- Median AQI
- 93.8%
- Good-air days
- 0
- Unhealthy days
- 89%
- Cleaner than US counties
Clark County's 2025 annual median AQI is 33 (Good), with 93.8% Good days
According to the U.S. EPA Air Quality System, Clark County, Illinois posted a 2025 median AQI of 33 (Good) across 355 monitored days, with 93.8% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Clark County's 93.8% Good-day share places it cleaner than 89% of US counties in 2025.
- Closest median-AQI peer
- Apache County (86.2% Good, median AQI 33) is the closest nationwide median-AQI peer to Clark County.
- 2020–2025 trend
- Clark County's floor-qualifying Good-day share is stable from 2020 to 2025 (95.6% → 93.8%).
- Median AQI
- Median AQI: 33
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 33 with 93.8% of 355 monitored days in the Good band, driven mainly by Ozone.
- 33
- median AQI · Good
- 93.8%
- days rated Good
- Ozone
- main pollutant
- 89%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 87.
Annual release change
What changed in the 2025 EPA annual release
- Clark County's Good-day share rose from 91.5% to 93.8% between the 2024 and 2025 EPA annual releases, a change of +2.3 percentage points.
- Clark County's median AQI fell from 36 to 33 between the 2024 and 2025 EPA annual releases, a change of -3 points.
Data updated August 2026
- Good · 333 d · 93.8%
- Moderate · 22 d · 6.2%
How did Clark County's air break down in 2025?
Across 355 monitored days, 333 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). 355 days monitored. Median AQI: 33 (Good).
Insights: Clark County, Illinois. EPA monitors recorded a median Air Quality Index of 33 across 355 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 333 (94%) fell in the Good band (AQI 0–50), 22 (6%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 48, while the worst single-day peak hit 87 (Moderate). Residents averaged about 28.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (355 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -1.8 points and the median AQI moved +2 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 87, moderate but never Unhealthy. Nearby counties show comparable readings: Adams (94.7% good, AQI 38); Champaign (65.8% good, AQI 43); Cook (40.8% good, AQI 53); DuPage (63.8% good, AQI 42); Effingham (95.5% good, AQI 35); Hamilton (67.9% good, AQI 42).
Air Quality Summary
Good Days
93.8%
333 of 355 monitored days
18.8pp vs 75% target
Max AQI Recorded
87
Moderate
Health Grade
Excellent
90th-pct AQI 48
Clark County, Good days share 93.8%
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 355 monitored days across EPA AQI categories.
Good
333
94%
Moderate
22
6%
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 · 355 days · 100.0%
Health Impact Assessment
Excellent
Clark County's air quality is rated Excellent based on 93.8% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 93.8% of monitored days in 2025, with a median AQI of 33. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 355 of 355 monitored days.
5-Year Air Quality Trend
Air quality is stable in Clark County. Highest Good-day share: 2020 (95.6%, 347 of 363 monitored 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 | 333 | 22 | 0 | 0 | 33 | 87 |
How does Clark County compare?
How Clark County compares
Good-day share better than typical
Clark County 93.8%
Apache County, Arizona 86.2%
Typical US county 76.0%
Median AQI better than typical
Clark County 33
Apache County, Arizona 33
Typical US county 39
Unhealthy days / yr better than typical
Clark County 0
Apache County, Arizona 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Clark County (33), not geographic neighbors.
Apache County, Arizona
Median AQI 33 · 86.2% Good-day share
Ascension County, Louisiana
Median AQI 33 · 95.6% Good-day share
Carlton County, Minnesota
Median AQI 33 · 82.8% Good-day share
Clark County, Washington
Median AQI 33 · 82.7% Good-day share
Clay County, Alabama
Median AQI 33 · 81.1% Good-day share
Denali County, Alaska
Median AQI 33 · 99.7% Good-day share
Same primary pollutant: Ozone
Ozone led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
St. James County, Louisiana
93.9% Good-day share · median AQI 33
Okaloosa County, Florida
93.9% Good-day share · median AQI 36
Pointe Coupee County, Louisiana
93.7% Good-day share · median AQI 35
Barnstable County, Massachusetts
93.7% Good-day share · median AQI 36
Rockbridge County, Virginia
93.9% Good-day share · median AQI 36
Windham County, Connecticut
93.7% Good-day share · median AQI 35
Illinois counties on the map
Geographic containment only, other monitored counties in Illinois, independent of AQI or pollutant match.
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 33 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 355 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Clark County, Illinois? ▼
Clark County has a median Air Quality Index (AQI) of 33, rated "Good." 93.8% of monitored days (333 out of 355) 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 355 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 stable across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 95.6% (347 of 363 days). Lowest qualifying Good-day share: 2023 at 89.4% (320 of 358 days).
What does AQI 33 mean for health? ▼
Clark County's median AQI of 33 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Clark County compare to other counties in Illinois? ▼
Clark County has 93.8% good air days. Nearby counties in Illinois: Adams (94.7% good), Champaign (65.8% good), Cook (40.8% good), DuPage (63.8% good), Effingham (95.5% good), Hamilton (67.9% good).
What is the 90th percentile AQI in Clark County? ▼
The 90th percentile AQI for Clark County is 48, 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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Clark County, Illinois · 2020–2025
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Every figure on PlainAirData 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.