State air quality · 2025

Illinois Air Quality Ranking

Across 23 monitored counties, the typical county in Illinois posts a median AQI of 42 (Good) and 70.0% of monitored days were rated Good, from EPA AQS.

42
Avg median AQI
70.0%
Good-air days
23
Counties monitored
19%
Cleaner than US states

Illinois's typical county median AQI is 42 (Good) across 23 monitored counties

According to the U.S. EPA Air Quality System, Illinois monitors 23 counties with a day-weighted Good-day share of 70.0%. Cleanest: Effingham County (95.5% Good). Most polluted: Cook County (40.8%). These are annual summaries, not live AirNow readings.

Data source EPA Air Quality System (AQS)

Cleaner than US states
Illinois's day-weighted Good-day share of 70.0% places it cleaner than 19% of US states.
In-state spread
Within Illinois, Effingham County leads at 95.5% Good days while Cook County sits at 40.8% across 23 monitored counties.
2024→2025 release
Illinois's Good-day share moved -1.6 points vs the 2024 EPA annual release.
Typical median AQI
Typical median AQI: 42

Verify with AirNow → · EPA AQS →

EPA state monitoring brief

AQS state key ILLINOIS

Illinois · 2025 EPA AQS · 23 counties · med AQI 42 · 70.0% Good · Effingham→Cook spread

  • GOOD 42 Median AQI · Good
  • SHARE 70.0% Good-day share
  • COUNT 23 Counties monitored
  • UNH 19 Unhealthy+ days
  • RANK 19% Cleaner than states
  • O3 50.9% Main pollutant share

Good-day share peers

South Carolina71.3%Arkansas71.2%Illinois (this)70%New Mexico69.1%
Good-day share neighbourhood · nearest ABS peers vs Illinois

Illinois's 70.0% Good-day share is cleaner than 19% of monitored US states; Effingham and Cook County bracket the in-state range. Open Effingham County →

According to EPA AQS annual summaries on PlainAirData, Illinois ranks #42 of 52 by day-weighted good-day share (70.0%) and #40 by mean county median AQI (41.7). Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%).

Data updated August 2026

What changed, what drives it, and how thick is the sample

Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.

Release change

70.0%

Good-day share in 2025, down 1.6 percentage points vs 2024. County-mean median AQI 42, 1.0 points lower (cleaner) than 2024.

Pollutant mix

Ozone

Main driver on 50.9% of pollutant-assigned days.

  • Ozone 50.9%
  • PM2.5 46.8%
  • NO2 2.1%
  • PM10 0.2%

Monitoring coverage

346

Average monitored days per county in the latest release. Every monitored county clears the 100-day ranking floor.

Counties Monitored

23

Good Days %

70.0%

Avg Median AQI

42

Total Unhealthy Days

19

Best Air Quality

Effingham County

Worst Air Quality

Cook County

Where Illinois sits nationally

COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Illinois's day-weighted Good-day share is 70.0% - cleaner than 19% of that comparison set.

Illinois Good-day share vs. every US state

Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release

70.0% 19th percentile higher than 19% of 52 US states

0.0%–5.0%: 0 US states (0%). Below this entry. 5.0%–10.0%: 0 US states (0%). Below this entry. 10.0%–15.0%: 0 US states (0%). Below this entry. 15.0%–20.0%: 0 US states (0%). Below this entry. 20.0%–25.0%: 0 US states (0%). Below this entry. 25.0%–30.0%: 0 US states (0%). Below this entry. 30.0%–35.0%: 0 US states (0%). Below this entry. 35.0%–40.0%: 0 US states (0%). Below this entry. 40.0%–45.0%: 0 US states (0%). Below this entry. 45.0%–50.0%: 0 US states (0%). Below this entry. 50.0%–55.0%: 0 US states (0%). Below this entry. 55.0%–60.0%: 0 US states (0%). Below this entry. 60.0%–65.0%: 2 US states (4%). Below this entry. 65.0%–70.0%: 9 US states (17%). This entry sits in this band. 70.0%–75.0%: 11 US states (21%). Above this entry. 75.0%–80.0%: 11 US states (21%). Above this entry. 80.0%–85.0%: 13 US states (25%). Above this entry. 85.0%–90.0%: 5 US states (10%). Above this entry. 90.0%–95.0%: 1 US states (2%). Above this entry. 95.0%–100.0%: 0 US states (0%). Above this entry. 100.0%–105.0%: 0 US states (0%). Above this entry. This state 0.0% 100% every floor-qualified US state, bucketed by value

Each bar is a band; taller bars hold more US states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.

Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025

Which counties in Illinois have the cleanest and most polluted air?

Cleanest counties in Illinois

Share of monitored days rated Good (AQI 0–50)

  1. 1 Effingham County 95.5%
  2. 2 Jo Daviess County 95.3%
  3. 3 Adams County 94.7%
  4. 4 Clark County 93.8%
  5. 5 Macoupin County 91.8%
  6. 6 Lake County 84.9%
  7. 7 Jersey County 72.4%
  8. 8 Sangamon County 70.1%
  9. 9 Winnebago County 68.7%
  10. 10 Randolph County 68.1%
  11. 11 Hamilton County 67.9%
  12. 12 McHenry County 67.9%

Good-day share · EPA Air Quality System

County Good % Median AQI
Effingham 95.5% 35
Jo Daviess 95.3% 33
Adams 94.7% 38
Clark 93.8% 33
Macoupin 91.8% 34
Lake 84.8% 39
Jersey 72.3% 42
Sangamon 70.1% 41
Winnebago 68.7% 40
Randolph 68.1% 43
Hamilton 67.9% 42
McHenry 67.9% 42
Rock Island 66.8% 41
McLean 66.0% 44
Champaign 65.8% 43
Macon 64.9% 44
DuPage 63.8% 42
Kane 61.9% 45
Will 61.6% 44
Peoria 59.9% 45
Saint Clair 58.4% 47
Madison 52.3% 49
Cook 40.8% 53

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.

Coverage: 23 counties in Illinois with active monitoring stations. Unmonitored counties are not represented.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025

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.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer.