Median AQI
40
Good
Illinois · County air quality · 2025
Lake County, Illinois
Median AQI 40 (Good). 83.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 83.5%
- Good-air days
- 0
- Unhealthy days
- 62%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Lake County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 83.5% of 200 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 83.5%
- days rated Good
- Ozone
- main pollutant
- 62%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 108.
Data updated July 2026
- Good · 167 d · 83.5%
- Moderate · 30 d · 15.0%
- USG · 3 d · 1.5%
How did Lake County's air break down in 2025?
Across 200 monitored days, 167 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 108 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 200 days monitored. Median AQI: 40 (Good).
Insights: Lake County, Illinois. EPA monitors recorded a median Air Quality Index of 40 across 200 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 167 (84%) fell in the Good band (AQI 0–50), 30 (15%) registered Moderate readings, and 3 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 67, while the worst single-day peak hit 108 (USG). Residents averaged about 25.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (200 days). Across the 2020–2025 trend window, good days shifted by -32 and the median AQI moved +1 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 108, usg but never Unhealthy. 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
83.5%
167 of 200 monitored days
8.5pp vs 75% target
Max AQI Recorded
108
USG
Health Grade
Good
90th-pct AQI 67
Lake County, Good days share 83.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Lake 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 200 monitored days across EPA AQI categories.
Good
167
84%
Moderate
30
15%
USG
3
2%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Lake County.
- Ozone · 200 days · 100.0%
Health Impact Assessment
Good
Lake County's air quality is rated Good based on 83.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 83.5% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 200 of 200 monitored days.
5-Year Air Quality Trend
Air quality is stable in Lake County. Best year: 2024 (201 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 199 | 31 | 10 | 1 | 39 | 185 |
| 2021 | 180 | 55 | 9 | 0 | 42 | 136 |
| 2022 | 199 | 41 | 2 | 1 | 39 | 159 |
| 2023 | 182 | 47 | 13 | 1 | 42 | 179 |
| 2024 | 201 | 39 | 5 | 0 | 40 | 140 |
| 2025 | 167 | 30 | 3 | 0 | 40 | 108 |
How does Lake County compare?
How Lake County compares
Good-day share better than US avg
Lake County 83.5%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
Lake County 40
Adams County, Illinois 38
US average 38
Unhealthy days / yr about average
Lake 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 Lake County.
Amador County, California
AQI 40 · 84.8% good
Anne Arundel County, Maryland
AQI 40 · 87.2% good
Arlington County, Virginia
AQI 40 · 84.8% good
Bergen County, New Jersey
AQI 40 · 71.1% good
Berkeley County, West Virginia
AQI 40 · 81.1% good
Billings County, North Dakota
AQI 40 · 80.3% good
What Lake 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 40 means for exposure. Health guidance
- See how Lake County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Lake County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 200 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 Lake County, Illinois? ▼
Lake County has a median Air Quality Index (AQI) of 40, rated "Good." 83.5% of monitored days (167 out of 200) had good air quality in 2025.
What is the main pollutant in Lake County? ▼
The primary pollutant in Lake County is Ozone (O3), which was the main AQI driver on 200 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Lake County have? ▼
In 2025, Lake County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 108.
Is the air quality in Lake County getting better or worse? ▼
Air quality in Lake County is stable based on 5-year trends (2020-2025). Best year: 2024 with 201 good days. Worst year: 2025 with 167 good days.
What does AQI 40 mean for health? ▼
Lake County's median AQI of 40 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 Lake County compare to other counties in Illinois? ▼
Lake County has 83.5% 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 Lake County? ▼
The 90th percentile AQI for Lake County is 67, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 108 (USG).
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.