Smoke: keep one room clean
Shut the windows and run an air purifier sized for the room.
Wildfire smoke guide →Air quality in Illinois, county by county
23 Illinois counties have enough EPA monitor data to count bad-air days. Weighted by population, they averaged about 13 days a year from 2021 to 2025 when the air was unhealthy for sensitive groups or worse.
CDC's model, which estimates every county from monitors and satellites, gives the average Illinois county, weighted by population, almost no smoky days (fine particles over 35.4 µg/m³, mostly wildfire smoke) and about 8 high-ozone days a year at its worst spot (2018 to 2022). See every county on the map.
Every Illinois county with monitors
Most bad-air days first. 18 have a full report. Compare with the worst and the cleanest counties in the US.
| County | Bad-air days a year | |
|---|---|---|
| Cook CountyChicagoOzone | 19.8 | |
| Madison CountyGranite CityOzone | 8.2 | |
| McHenry CountyCrystal LakeOzone | 7.8 | |
| Kane CountyAuroraOzone | 7.0 | |
| Lake CountyWaukeganOzoneparticles not measured | 6.8 | |
| Jersey CountyOzone | 6.0 | |
| Winnebago CountyRockfordOzone | 5.8 | |
| Sangamon CountySpringfieldOzone | 5.6 | |
| Will CountyJolietOzone | 5.6 | |
| Peoria CountyPeoriaOzone | 5.4 | |
| DuPage CountyNapervilleOzone | 5.2 | |
| Macon CountyDecaturOzone | 5.0 | |
| Rock Island CountyMolineOzone | 5.0 | |
| St. Clair CountyBellevilleOzone | 4.8 | |
| McLean CountyBloomingtonOzone | 4.2 | |
| Champaign CountyChampaignOzone | 3.6 | |
| Randolph CountyOzone | 3.4 | |
| Hamilton CountyOzone | 2.8 | |
| Macoupin CountyOzoneparticles not measured | 2.8 | |
| Adams CountyQuincyOzoneparticles not measured | 2.4 | |
| Effingham CountyEffinghamOzoneparticles not measured | 2.2 | |
| Jo Daviess CountyOzoneparticles not measured | 2.0 | |
| Clark CountyOzoneparticles not measured | 1.2 | |
| No Illinois county with a monitor matches “”. Counties without a monitor aren't listed; a neighbor's numbers are a fair guide, or find it on the map. | ||
Counties without a monitor aren't listed. Air doesn't stop at the county line, so a neighbor's numbers are a fair guide.
What helps on a bad-air day
Check AirNow for today's reading and tomorrow's forecast, then match the step to the pollutant.
Upgrade the furnace filter
MERV 13 catches far more smoke than thin fiberglass filters.
Furnace filters →Questions people ask about air quality in Illinois
Which county in Illinois has the worst air?
Among Illinois counties with EPA monitors, Cook County had the most bad-air days from 2021 to 2025: about 20 bad-air days a year with an AQI over 100. That's about 1 day in 18.
How often is the air bad in Illinois?
Weighted by population, Illinois counties with monitors averaged about 13 bad-air days a year from 2021 to 2025 with an AQI over 100, when children, older adults and people with asthma should cut back outdoors. That's about 1 day in 28.
Which county in Illinois has the cleanest air?
Of the 23 counties with enough monitor data, Clark County had the fewest: 1.2 a year from 2021 to 2025. Counties without monitors aren't ranked.
Where can I check today's air quality in Illinois?
AirNow, run by the EPA and its partners, shows current readings and forecasts by ZIP code, and its fire and smoke map adds smoke plumes and low-cost sensors.
Sources: EPA AirData county AQI summaries, 2021 to 2025; CDC Environmental Public Health Tracking Network modeled PM2.5 and ozone, 2018 to 2022. Updated November 8, 2026.