What Is the Air Quality in Lake, Illinois?
Lake, Illinois has an Air Quality Grade of C (fair) with a 5-year median AQI of 40. The dominant pollutant is Ground-Level Ozone, and air quality has been worsening over the past decade.
Lake, Illinois Air Quality Snapshot
| Air Quality Grade | C55/100 |
| 5-Year Median AQI | 40 (Good) |
| Most Recent Median AQI (2023) | 42 (Good) |
| Dominant Pollutant | Ground-Level Ozone |
| 10-Year Trend | Worsening (+0.90 AQI/yr) |
| Unhealthy Days (last 5 yr) | 39 |
| National Rank (cleanest = #1) | #511 of 1,020 (50th cleanest percentile) |
| Illinois Rank | #6 of 23 |
What Does the C Grade Mean?
Lake, Illinois earns a C — air quality is fair, but not great. With a 5-year median AQI of 40, the city sees a meaningful number of "Moderate" days each year, when the EPA flags air as a concern for unusually sensitive people.
Lake, Illinois's 5-year median AQI of 40 is right around the national average of 41 across the 1,020 monitored U.S. cities tracked here. Within Illinois, Lake, Illinois runs cleaner than the state average of 45 — a positive signal that local conditions (terrain, wind patterns, emission sources) are working in residents' favor.
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 Lake, Illinois's Air?
The dominant pollutant in Lake, Illinois is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Ground-Level Ozone | 243 | 100% |
Is the Air Getting Better or Worse?
Air quality in Lake, Illinois has been getting worse over the past decade, with median AQI climbing by roughly 0.9 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, Lake, Illinois posted a median AQI of 35. By 2023 that figure was 42 — a rise of 7 AQI points dirtier across 10 years of EPA records.
Year-by-Year AQI in Lake, Illinois
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 35 | 317 | 5 | Ozone |
| 2015 | 35 | 332 | 3 | Ozone |
| 2016 | 34 | 306 | 8 | Ozone |
| 2017 | 35 | 316 | 7 | Ozone |
| 2018 | 35 | 304 | 8 | Ozone |
| 2019 | 40 | 210 | 2 | Ozone |
| 2020 | 39 | 199 | 11 | Ozone |
| 2021 | 42 | 180 | 9 | Ozone |
| 2022 | 39 | 199 | 3 | Ozone |
| 2023 | 42 | 182 | 14 | Ozone |
Health Context for Lake, Illinois
Across the past five years, this area has logged 39 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 8 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 ozone peaks in the afternoon on hot sunny days, plan outdoor exercise for early morning or after sunset on bad-air days.
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.
Lake, Illinois has an Air Quality Grade of C (fair) with a 5-year median AQI of 40. The dominant pollutant is Ground-Level Ozone, 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.