Median AQI
39
Good
Illinois · County air quality · 2025
Lake County, Illinois
Median AQI 39 (Good). 84.8% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 84.8%
- Good-air days
- 0
- Unhealthy days
- 68%
- Cleaner than US counties
Lake County's 2025 annual median AQI is 39 (Good), with 84.8% Good days
According to the U.S. EPA Air Quality System, Lake County, Illinois posted a 2025 median AQI of 39 (Good) across 231 monitored days, with 84.8% 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
- Lake County's 84.8% Good-day share places it cleaner than 68% of US counties in 2025.
- Closest median-AQI peer
- Aiken County (90.6% Good, median AQI 39) is the closest nationwide median-AQI peer to Lake County.
- 2020–2025 trend
- Lake County's floor-qualifying Good-day share is stable from 2020 to 2025 (82.6% → 84.8%).
- Median AQI
- Median AQI: 39
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 39 with 84.8% of 231 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 84.8%
- days rated Good
- Ozone
- main pollutant
- 68%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 108.
Annual release change
What changed in the 2025 EPA annual release
- Lake County's peak single-day AQI fell from 140 to 108 between the 2024 and 2025 EPA annual releases, a change of -32 points.
- Lake County's Good-day share rose from 82.0% to 84.8% between the 2024 and 2025 EPA annual releases, a change of +2.8 percentage points.
Data updated August 2026
- Good · 196 d · 84.8%
- Moderate · 32 d · 13.9%
- USG · 3 d · 1.3%
How did Lake County's air break down in 2025?
Across 231 monitored days, 196 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). 231 days monitored. Median AQI: 39 (Good).
Insights: Lake County, Illinois. EPA monitors recorded a median Air Quality Index of 39 across 231 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 196 (85%) fell in the Good band (AQI 0–50), 32 (14%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 108 (USG). Residents averaged about 25.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (231 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +2.3 points and the median AQI moved +0 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 (94.7% good, AQI 38); Champaign (65.8% good, AQI 43); Clark (93.8% good, AQI 33); Cook (40.8% good, AQI 53); DuPage (63.8% good, AQI 42); Effingham (95.5% good, AQI 35).
Air Quality Summary
Good Days
84.8%
196 of 231 monitored days
9.8pp vs 75% target
Max AQI Recorded
108
USG
Health Grade
Good
90th-pct AQI 61
Lake County, Good days share 84.8%
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 231 monitored days across EPA AQI categories.
Good
196
85%
Moderate
32
14%
USG
3
1%
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 · 231 days · 100.0%
Health Impact Assessment
Good
Lake County's air quality is rated Good based on 84.8% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 84.8% of monitored days in 2025, with a median AQI of 39. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 231 of 231 monitored days.
5-Year Air Quality Trend
Air quality is stable in Lake County. Highest Good-day share: 2025 (84.8%, 196 of 231 monitored 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 | 196 | 32 | 3 | 0 | 39 | 108 |
How does Lake County compare?
How Lake County compares
Good-day share better than typical
Lake County 84.8%
Aiken County, South Carolina 90.6%
Typical US county 76.0%
Median AQI about typical
Lake County 39
Aiken County, South Carolina 39
Typical US county 39
Unhealthy days / yr better than typical
Lake County 0
Aiken County, South Carolina 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Lake County (39), not geographic neighbors.
Aiken County, South Carolina
Median AQI 39 · 90.6% Good-day share
Albany County, New York
Median AQI 39 · 68.5% Good-day share
Anderson County, Tennessee
Median AQI 39 · 94.3% Good-day share
Bay County, Michigan
Median AQI 39 · 70.4% Good-day share
Berkeley County, West Virginia
Median AQI 39 · 83.6% Good-day share
Boyd County, Kentucky
Median AQI 39 · 80.0% 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).
Columbia County, Wisconsin
84.8% Good-day share · median AQI 40
Jefferson County, Wisconsin
84.8% Good-day share · median AQI 40
Kewaunee County, Wisconsin
84.9% Good-day share · median AQI 39
Hillsborough County, New Hampshire
84.9% Good-day share · median AQI 38
Teton County, Wyoming
84.9% Good-day share · median AQI 44
Perry County, Missouri
84.7% Good-day share · median AQI 41
Illinois counties on the map
Geographic containment only, other monitored counties in Illinois, independent of AQI or pollutant match.
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 39 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 231 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 Lake County, Illinois? ▼
Lake County has a median Air Quality Index (AQI) of 39, rated "Good." 84.8% of monitored days (196 out of 231) 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 231 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 across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2025 at 84.8% (196 of 231 days). Lowest qualifying Good-day share: 2021 at 73.8% (180 of 244 days).
What does AQI 39 mean for health? ▼
Lake County's median AQI of 39 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Lake County compare to other counties in Illinois? ▼
Lake County has 84.8% good air days. Nearby counties in Illinois: Adams (94.7% good), Champaign (65.8% good), Clark (93.8% good), Cook (40.8% good), DuPage (63.8% good), Effingham (95.5% good).
What is the 90th percentile AQI in Lake County? ▼
The 90th percentile AQI for Lake County is 61, 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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Lake 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.