# Chicago, IL Air Quality Index (AQI) Today > Source: https://breathecheck.com/air-quality/us/illinois/chicago > Go, short outing, or stay in — plus Chicago's own 30-day AQI trend, why the scale broke past 500 in July 2026, and why a clear sky can still mean bad air. That color and that number are today's read on air quality in Chicago. What it settles is simpler than it looks: go, shorten it, or stay in. That's the call about the lakefront run, the dog walk, or the kid's recess you opened this page to make. This page tracks the air quality Chicago breathes day by day, not one number that resets on refresh. Set your own line and [hear about it when a plume crosses it](/alerts/) instead. ## Chicago's last 30 days, not just today One reading tells you nothing about the shape behind it. This tracks air quality Chicago actually breathed over the trailing 30 days, built from the same hourly AQI feed as the number above. It also keeps a running count of **days above AQI 100 in the last 12 months**. IQAir, AccuWeather, aqicn and the Illinois EPA's own state grid each hand you today's number and nothing behind it. A bad Tuesday quietly disappears by Thursday. It doesn't disappear here. ## Why is it hazy in Chicago today? Three things are usually behind a sky that looks worse than the number reads. The AQI is a 24-hour average, so one bad hour gets diluted by twenty-three calmer ones before it becomes a headline figure. The haze can be smoke sitting a few thousand feet up, above the ground monitor doing the reading. Or the reading comes from a station some miles from wherever a reader is standing. This project hasn't confirmed which one, or how far off it sits. None of that makes it fake. Lake Michigan adds its own mechanism: an onshore lake breeze traps ozone and fine particles along the lakefront corridor. The shoreline reads worse than a suburb ten miles inland as a result. [A Canadian plume, a lake breeze, an averaging window](/why-is-the-air-bad-today/) breaks each one down. ## What is causing the bad air in Chicago? Two pollutants trade off as Chicago's dominant cause. Ground-level ozone forms when summer sun cooks exhaust, peaking on hot, still afternoons — invisible and odorless. PM2.5 (fine particulate matter) builds from traffic, industry and wood burning, trapped close to the ground by a Lake Michigan inversion on a stagnant day. Increasingly, a third cause overwhelms both: wildfire smoke carried hundreds of miles from Canada. AccuWeather's pollutant table never says which one is today's driver; a dry, windy stretch can lift the coarser PM10 off streets instead of PM2.5 dominating. That wildfire smoke can push the combined number past either pollutant's normal range within hours. The American Lung Association's 2025 [State of the Air report](https://www.lung.org/research/sota/city-rankings/msas/chicago-naperville-il-in-wi) ranked Chicago-Naperville 15th worst nationally for ozone and 13th worst for particle pollution. Cook County posted F grades on both, with 20.3 unhealthy ozone days per year. ![Bar chart of US monthly search volume for “air quality chicago” across Aug 2025 – Jul 2026, month by month: 49,500, 4,400, 2,900, 2,400, 1,900, 1,300, 4,400, 2,900, 1,900, 5,400, 2,400, 450,000. The high is 450,000 (Jul 2026), 155.2 times the median month of 2,900; the low is 1,300 (Jan 2026). Bars are neutral blue-grey with the peak month in accent blue — search demand, not AQI, so they carry no EPA band colour.](/img/chicago/inline-2.webp) *“air quality chicago”, DataForSEO, Aug 2025 – Jul 2026. Peak 450,000 (Jul 2026) against a median month of 2,900. Winter ratio 4% (Nov-Feb mean over Mar-Oct mean). Bars are neutral blue-grey with the peak month in accent blue — never EPA band colours.* ## Is the air safe to breathe in Chicago today? Chicago's bad-air days used to mean a hot, still afternoon with the lake breeze holding ozone along the shore. Wildfire smoke from Canada is now a second source layered on top of that old pattern. Either one narrows to three calls. **Go**, unchanged, on Good or Moderate air. **Short and unloaded** — the treadmill instead of the lakefront trail — in the middle range. **Don't go** past that. On EPA's scale, everyone is first warned at 151, Unhealthy. 201 to 300, Very Unhealthy, is that same warning escalated, not a new group added. Above roughly 200, the decision is made for you. ## What AQI should you stay inside? A runner, an asthmatic and a parent don't share one cutoff, and none of the three matches the EPA's single sensitive-groups line. "I'll run in up to 110 because...error bars," one r/Marathon_Training runner wrote. An asthmatic runner logged 156 runs against AQI and found their limit lower, at 60. "my lungs struggle regardless of how well I've trained," though EPA calls nothing under 100 unhealthy. Their numbers back it up: 87% of runs under 50 went fine, 42% between 50 and 70, just 12% past 70. Parents online tend to land near 100. Illinois has no published daycare or school AQI cutoff in what this project has reviewed. Two states, Washington and Minnesota, are the exceptions with a real number. [What Illinois doesn't publish, and what two states do](/daycare-air-quality-policy/) covers it. Wildfire smoke, newly common here, pulls that whole table down about one EPA band — a different mechanism from the lake-breeze ozone Chicago is used to. The agency's own label for that range is [101–150, Unhealthy for Sensitive Groups](/aqi/) — a classification, not a diagnosis. Indoors, default to a portable HEPA purifier or a Corsi-Rosenthal box; one community builder priced a 12x14 bedroom version at $130. Skip the whole-house MERV-13 swap unless a licensed tech has already checked the system's static pressure. [Why indoor PM2.5 climbs in winter specifically](/indoor/) picks it up from there. ![The EPA AQI scale for Chicago in six colour bands, each segment as wide as its range: Good 0-50 green, Moderate 51-100 yellow, Unhealthy for Sensitive Groups 101-150 orange, Unhealthy 151-200 red, Very Unhealthy 201-300 purple, Hazardous 301-500 maroon. Beneath, the 0-150 stretch is enlarged to mark three cutoffs quoted on this page: 60 for one asthmatic runner after a 156-run log, about 100 where a parent calls off the outing, and 110 for one r/Marathon\_Training runner.](/img/chicago/inline-1.webp) *EPA AirNow's own colours and breakpoints, segment width proportional to the AQI range. The three marks are individual reported cutoffs quoted on this page, not EPA guidance.* ## Why our number may not match what you are seeing and smelling This cuts both directions, and the second is the trap fewer people expect. Direction one: the sky looks clear, nothing smells off, and the number is bad anyway. Ground-level ozone, Chicago's dominant summer pollutant, is invisible and odorless. As one commenter put it: "spreading awareness about ozone is tough because it's mostly invisible and odorless... telling people they're breathing in a toxic invisible gas is a sure way to kill the vibe. But it must be done." A clear sky isn't the same test as a monitor. Direction two: the sky is hazy, you can smell it, and the number stays low — the same averaging-and-distance problem, at Chicago's scale. A station some distance from where you're standing under-reports whichever side of the metro sits farthest from it. This project hasn't confirmed which station, or how far off it sits. Chicago runs its own check on that gap: [Open Air Chicago](https://www.chicago.gov/city/en/depts/cdph/supp_info/Environment/open-air-chicago.html), roughly 280 community sensors run by Chicago's health department with UIC's School of Public Health, reading PM2.5 and NO2 neighborhood by neighborhood. That's the monitoring station feeding today's reading, its distance from the metro centroid, and the nearby PurpleAir sensor count. It's narrower than the citywide network above. [How this number gets built, and where it goes soft](/methodology/) has the mechanics. ## Why Chicago's smoke problem isn't a season — it's new Every summer now stress-tests the air quality Chicago breathes. Calling it a season undersells three summers running, each measurably worse than the last — Chicagoans said so themselves. June 2023: Canadian wildfire smoke pushed Chicago's AQI past 160 — unhealthy for everyone, not only sensitive groups. It was the first time many longtime residents recalled smelling smoke from a fire that far away. By July 2025 the pattern had a name, but not the one you'd expect. "I've been here for decades and it seems like we've had more hazy days due to Canada in the past year than the prior ten combined" (306 upvotes). "Of my 25 years in the Chicago area, I don't remember ever experiencing wildfire smoke like this prior to the last few years" (234 upvotes). A third put a full lifetime on it: "I have lived in Chicago all my 65 years of life. I have never smelled smoke from Canada until last summer!!!" (90 upvotes). July 16, 2026 broke the pattern's own record. A WGN headline, shared to r/chicago (1,505 upvotes), read "Chicago records worst air quality in its history." The top reply, 901 upvotes: "missing 'so far' at the end." Others noted the city's own recordkeeping starts October 11, 1871, after the Great Chicago Fire. "Worst in history" really means "worst since a fire destroyed the prior records." One Chicagoan tracked AirNow's number climbing that afternoon, in a post that reached 609 upvotes. 597 AQI at 3:45 p.m., 701 by 4:30, 742 by 9:30 that night. A reading of 938 (an IQAir figure, not AirNow's) came in from Northbrook, a suburb north of the city. The EPA's AQI scale is defined only up to 500; there's no color band above it. That 500 ceiling is very likely EPA's own design choice, not a hardware limit — this project hasn't sourced an EPA statement confirming it. A chart capped at 500 while the pollutant concentration keeps climbing doesn't mean the air got safer. Presumably the underlying pollutant math keeps running past 500 instead of the display capping there — though this project hasn't sourced that mechanism from IQAir either. AirNow's own number for the same air, same day, would most likely have simply read 500. None of the sites checked for this project mention the ceiling at all. Three summers, one direction: worse. Chicago isn't alone on the mechanism behind it: [Milwaukee sits in the same lake corridor](/air-quality/us/wisconsin/milwaukee), with [Minneapolis typically catching the plume first](/air-quality/us/minnesota/minneapolis) and Madison a little further inland. The same plume kept moving east and south — [New York took it at record strength](/air-quality/us/new-york/new-york), while Pittsburgh held it in the river valleys and St. Louis sits at the far end of that path. Calling it "smoke season" again would flatten three worsening years into one shrug. ## FAQ ### Who has the worst air quality in the US right now? See [how Chicago ranks today](/) against every tracked city — it briefly topped the list on July 16, 2026. ### What's the air quality forecast for Chicago tomorrow? Lake-breeze direction and any incoming plume can shift this within hours. Treat it as a direction, not a lock. ### What's the UV index in Chicago today? UV runs on its own scale, separate from AQI. A clear day is often the day UV peaks, so a good reading isn't a reason to skip sunscreen. ### Is today an Air Pollution Action Day in Chicago? Illinois EPA issues an [Air Pollution Action Day](https://epa.illinois.gov/topics/air-quality/outdoor-air.html) alert when forecast AQI reaches orange or higher. A Statewide Action Day means most of Illinois will see unhealthy air. A Red Action Day signals AQI in the red zone in at least one Illinois sector. [Set an alert](/alerts/) to know before it's called. ### Why does it smell like smoke when the AQI number looks fine? The number is a 24-hour average. A smoke smell now can be one bad hour not yet reflected, or a plume aloft the ground monitor hasn't caught. Trust your nose for the next hour, the number for the day.