Median AQI
43
Good
Illinois · County air quality · 2025
McHenry County, Illinois
Median AQI 43 (Good). 66.2% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 66.2%
- Good-air days
- 1
- Unhealthy days
- 25%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, McHenry County's air is good on balance, more polluted than the median US county, a median AQI of 43 with 66.2% of 272 monitored days in the Good band, driven mainly by Ozone.
- 43
- median AQI · Good
- 66.2%
- days rated Good
- Ozone
- main pollutant
- 25%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 165.
Data updated July 2026
- Good · 180 d · 66.2%
- Moderate · 80 d · 29.4%
- USG · 11 d · 4.0%
- Unhealthy · 1 d · 0.4%
How did McHenry County's air break down in 2025?
Across 272 monitored days, 180 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 165 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 272 days monitored. Median AQI: 43 (Good).
Insights: McHenry County, Illinois. EPA monitors recorded a median Air Quality Index of 43 across 272 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 180 (66%) fell in the Good band (AQI 0–50), 80 (29%) registered Moderate readings, and 11 (4%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 79, while the worst single-day peak hit 165 (Unhealthy). Residents averaged about 19.9 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (154 days), PM2.5 (118 days). Across the 2020–2025 trend window, good days shifted by -12 and the median AQI moved -4 points, placing the multi-year trajectory at improving. Residents and visitors experienced 1 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. 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
66.2%
180 of 272 monitored days
-8.8pp vs 75% target
Max AQI Recorded
165
Unhealthy
Health Grade
Moderate
90th-pct AQI 79
McHenry County, Good days share 66.2%
National avg
Share of monitored days at AQI 0-50 (Good band).
McHenry County, Unhealthy-or-worse share 0.4%
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 272 monitored days across EPA AQI categories.
Good
180
66%
Moderate
80
29%
USG
11
4%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in McHenry County.
- Ozone · 154 days · 56.6%
- PM2.5 · 118 days · 43.4%
Health Impact Assessment
Moderate
McHenry County's air quality is rated Moderate based on 66.2% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 11 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 165. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 154 of 272 monitored days.
5-Year Air Quality Trend
Air quality is improving in McHenry County. Best year: 2024 (245 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 192 | 133 | 9 | 0 | 47 | 147 |
| 2021 | 187 | 174 | 2 | 0 | 49 | 105 |
| 2022 | 199 | 163 | 3 | 0 | 47 | 126 |
| 2023 | 191 | 153 | 16 | 4 | 49 | 230 |
| 2024 | 245 | 120 | 1 | 0 | 43 | 108 |
| 2025 | 180 | 80 | 11 | 1 | 43 | 165 |
How does McHenry County compare?
How McHenry County compares
Good-day share worse than US avg
McHenry County 66.2%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
McHenry County 43
Adams County, Illinois 38
US average 38
Unhealthy days / yr worse than US avg
McHenry County 1
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 McHenry County.
Allegan County, Michigan
AQI 43 · 71.2% good
Allen County, Ohio
AQI 43 · 65.9% good
Ashtabula County, Ohio
AQI 43 · 81.2% good
Bell County, Kentucky
AQI 43 · 63.5% good
Boise County, Idaho
AQI 43 · 72.9% good
Brazos County, Texas
AQI 43 · 63.4% good
What McHenry County's air means for you
On most days the air here is fine, but Ozone drives 1 unhealthy-or-worse day a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 43 means for exposure. Health guidance
- See how McHenry County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare McHenry County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 272 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 McHenry County, Illinois? ▼
McHenry County has a median Air Quality Index (AQI) of 43, rated "Good." 66.2% of monitored days (180 out of 272) had good air quality in 2025.
What is the main pollutant in McHenry County? ▼
The primary pollutant in McHenry County is Ozone (O3), which was the main AQI driver on 154 days (57% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does McHenry County have? ▼
In 2025, McHenry County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 165.
Is the air quality in McHenry County getting better or worse? ▼
Air quality in McHenry County is improving based on 5-year trends (2020-2025). Best year: 2024 with 245 good days. Worst year: 2025 with 180 good days.
What does AQI 43 mean for health? ▼
McHenry County's median AQI of 43 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 McHenry County compare to other counties in Illinois? ▼
McHenry County has 66.2% 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 McHenry County? ▼
The 90th percentile AQI for McHenry County is 79, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 165 (Unhealthy).
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.