What Is the Air Quality in McHenry, Illinois?
McHenry, Illinois has an Air Quality Grade of C (fair) with a 5-year median AQI of 49. The dominant pollutant is Fine Particulate Matter (PM2.5), and air quality has been worsening over the past decade.
McHenry, Illinois Air Quality Snapshot
| Air Quality Grade | C54/100 |
| 5-Year Median AQI | 49 (Good) |
| Most Recent Median AQI (2023) | 49 (Good) |
| Dominant Pollutant | Fine Particulate Matter (PM2.5) |
| 10-Year Trend | Worsening (+0.45 AQI/yr) |
| Unhealthy Days (last 5 yr) | 37 |
| National Rank (cleanest = #1) | #905 of 1,020 (89th most polluted percentile) |
| Illinois Rank | #17 of 23 |
What Does the C Grade Mean?
McHenry, Illinois earns a C — air quality is fair, but not great. With a 5-year median AQI of 49, the city sees a meaningful number of "Moderate" days each year, when the EPA flags air as a concern for unusually sensitive people.
McHenry, Illinois's 5-year median AQI of 49 is 8 points above the national average of 41 — meaningfully more polluted than the typical U.S. metro tracked here. Within Illinois, McHenry, Illinois runs more polluted than the state average of 45 — local sources or geography are concentrating pollution above the state's typical reading.
For context within Illinois: Clark, Illinois currently holds the state's cleanest grade (B, AQI 34), while Rock Island, Illinois sits at the bottom (D, AQI 47).
What's in McHenry, Illinois's Air?
The dominant pollutant in McHenry, Illinois is Fine Particulate Matter (PM2.5). Fine particulate matter — particles less than 2.5 micrometers across — comes mostly from combustion: vehicle exhaust, wildfire smoke, residential wood burning, and industrial emissions. Because these particles are small enough to enter the bloodstream, PM2.5 is the pollutant most strongly linked to cardiovascular disease, respiratory illness, and premature death.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Fine Particulate Matter (PM2.5) | 247 | 68% |
| Ground-Level Ozone | 118 | 32% |
Is the Air Getting Better or Worse?
Air quality in McHenry, Illinois has been getting worse over the past decade, with median AQI climbing by roughly 0.5 points per year. That bucks the national trend of broad improvement, and most often reflects either growing wildfire smoke exposure (particularly across the West) or rising local emissions from population and freight growth.
In 2014, McHenry, Illinois posted a median AQI of 42. By 2023 that figure was 49 — a rise of 7 AQI points dirtier across 10 years of EPA records.
Year-by-Year AQI in McHenry, Illinois
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 42 | 248 | 3 | PM2.5 |
| 2015 | 51 | 173 | 2 | PM2.5 |
| 2016 | 44 | 241 | 6 | PM2.5 |
| 2017 | 44 | 226 | 3 | PM2.5 |
| 2018 | 46 | 213 | 8 | Ozone |
| 2019 | 52 | 161 | 2 | PM2.5 |
| 2020 | 47 | 192 | 9 | PM2.5 |
| 2021 | 49 | 187 | 2 | PM2.5 |
| 2022 | 47 | 199 | 3 | PM2.5 |
| 2023 | 49 | 191 | 21 | PM2.5 |
Health Context for McHenry, Illinois
Across the past five years, this area has logged 37 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 7 days per year. That is roughly typical for a U.S. metro, with most caution days clustered in summer (ozone) or wildfire season.
Healthy adults can continue normal outdoor activity in most weather, but should pay attention to AQI alerts during the worst pollution windows. People with asthma, heart disease, or pregnancy should reduce prolonged or intense outdoor exertion on flagged days, and consider running an indoor HEPA air cleaner during peak season. Because PM2.5 penetrates deep into the lungs and bloodstream, an N95 or KN95 mask provides meaningful protection on smoky or high-particulate days — surgical masks do not.
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
McHenry, Illinois has an Air Quality Grade of C (fair) with a 5-year median AQI of 49. The dominant pollutant is Fine Particulate Matter (PM2.5), and air quality has been worsening over the past decade.
This answer pulls from the EPA Air Quality System (AQS), the authoritative federal source for U.S. air quality and pollution monitoring. The headline number above is the direct answer; what follows is the additional context most readers need to use the answer for a real decision rather than just a fact lookup.
A practical caveat: the headline answer above reflects the most recent the EPA Air Quality System (AQS) vintage; underlying data is often revised for months after first publication, and the right reference for any specific decision is whichever vintage is current at the time of the decision. The as-of date is stamped on every page.
Source: EPA Outdoor Air Quality Data, 2026.