What was the worst air-quality day in Chicago?

July 16, 2026 (provisional), when daily PM2.5 reached 330.9 µg/m³ (AQI 500), about 15.0 cigarette-equivalents. It ranks first of 9,929 monitored days since 1999.

What is the highest AQI recorded in Chicago?

On the worst day, the daily PM2.5 AQI reached 500 (Hazardous). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 246 on June 28, 2023.

Is the July 2026 ranking final?

No. Days after April 30, 2026 are preliminary AirNow readings. They rank now so a current event is visible, and they may shift once EPA certifies the data, usually the following spring.

Were these days caused by wildfire smoke?

This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.

How often does Chicago get air this bad?

Rarely. The record sits at the far end of 9,929 measured days; Chicago averages 1 "unhealthy" day a year.

Why did different apps show different numbers that day?

Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.

← All 100 US metros ranked

Historical PM2.5 records

Chicago's worst air quality days

#4 of 100 US metros by worst day · Chicago-Naperville-Elgin, IL-IN-WI metro area

Based on the available regulatory-monitor record since 1999, July 16, 2026 provisionally ranks as Chicago's most severe daily PM2.5 day: 330.9 µg/m³ (AQI 500, hazardous), about 15.0 cigarettes in a day. The ranking updates when EPA finalizes the data.

Daily rank: #1 provisional
Period of record: Since 1999
Peak daily PM2.5: 330.9 µg/m³
About: 15.0 cigarettes
Latest event: 4 days, July 2026
Status: Preliminary

Current conditions on the smoke map → Data through Jul 19, 2026 · page built July 26, 2026

Photo: James Conkis (CC BY-SA 4.0)

Show

The worst-hit part of the metro: its highest credible monitor each day.

How this is measured sources, the daily measure, and what this ranking is not

Source. Daily PM2.5 from the U.S. EPA Air Quality System (AQS) , back to 1999, across 40 EPA monitors in the Chicago-Naperville-Elgin, IL-IN-WI metro area (CBSA). Days after April 30, 2026 are preliminary AirNow readings, marked provisional, and re-rank once EPA certifies them (usually the following spring).

The daily measure. "Worst area" is the metro's highest credible monitor each day, with lone bad-monitor spikes dropped (the raw single-monitor max before that filter is in the data but not charted). "Metro average" averages every monitor. The unit choice re-reads the same value as µg/m³, cigarette-equivalents (PM2.5 ÷ 22, Berkeley Earth, an illustration rather than a medical reading), or the EPA 2024 PM2.5 AQI.

Official AQI. EPA's reported metro AQI uses the single worst monitor of any pollutant, so it can run higher than the values here. The highest official daily AQI among the ranked days is 246 on June 28, 2023 (defining pollutant PM2.5).

What this ranking is not. It does not attribute days to wildfire smoke: a high-PM2.5 day can be dust, fireworks, an inversion, or industry. The wildfire-smoke event ranking below arrives with a published attribution method.

Latest major event July 2026 3 of the worst days on record in one stretch, Jul 16, 2026 to Jul 19, 2026, peaking at 330.9 µg/m³, a new city record. Provisional until EPA certifies the data. See where it ranks ↓

What were Chicago's worst days?

Rank	Date	Daily PM2.5
#1	July 16, 2026provisional3-day stretch	330.9 µg/m³
#2	June 28, 20233-day stretch	170.6 µg/m³
#3	June 27, 2023	158.8 µg/m³
#4	July 17, 2026provisional	144.7 µg/m³
#5	July 5, 2015	117.8 µg/m³
#6	November 4, 2015	80 µg/m³
#7	June 29, 2023	79.3 µg/m³
#8	July 31, 2025	79.1 µg/m³
#9	March 27, 2012	77.2 µg/m³
#10	July 19, 2026provisional	72.6 µg/m³
#11	May 30, 1999	67.5 µg/m³
#12	July 5, 20112-day stretch	65.2 µg/m³
#13	July 4, 2011	65.1 µg/m³
#14	February 16, 2001	64.3 µg/m³
#15	October 23, 2000	63.7 µg/m³

Days marked provisional are preliminary AirNow readings after April 30, 2026, the latest date EPA has certified for Chicago; they may shift once finalized.

What caused Chicago's worst days?

The event catalog has named 9 episodes affecting Chicago, each resolved to a cause only where retained evidence supports it: EPA monitor qualifiers, NWS alerts, agency statements, and reporting. The ranking above stays cause-neutral; this is where the causes live.

Wildfire smoke July 15–20, 2026 Great Lakes and eastern U.S. wildfire-smoke episode Jul 16, 2026 · 330.9 µg/m³ here , #1 all-time · from Canada
Wildfire smoke July 30–August 5, 2025 Midwest Canadian wildfire-smoke episode Jul 31, 2025 · 79.1 µg/m³ here , #8 all-time · from Canada
Wildfire smoke June 3–6, 2025 Upper Midwest Canadian wildfire-smoke episode Jun 5, 2025 · 59.4 µg/m³ here , #21 all-time · from Canada
Wildfire smoke 2023 mid-July Canadian wildfire-smoke episode Jul 16, 2023 · 58.8 µg/m³ here , #23 all-time · from Canada
Wildfire smoke 2023 late-June Canadian wildfire-smoke episode Jun 28, 2023 · 170.6 µg/m³ here , #2 all-time · from Canada
Fireworks Fireworks episode, 2015-07-04 through 2015-07-05 Jul 5, 2015 · 117.8 µg/m³ here , #5 all-time

3 more, plus every named episode and its evidence is in the event catalog.

How does it compare?

The current provisional record is 94% higher than the previous certified record: 330.9 µg/m³ (July 16, 2026) against 170.6 µg/m³ (June 28, 2023).

How unusual is air like this?

25× a typical day here, at the record

1 day a year, on average, at "unhealthy" or worse

0 µg/m³331 µg/m³

Every measured day near Chicago, by daily PM2.5; the record stands far out on the right. 291 days at "unhealthy for sensitive groups" or worse, 32 at "unhealthy", 4 at "very unhealthy" (2024 EPA thresholds, across 9,929 measured days).

Year by year (1999–2026)

Worst single day each year, by the selected measure.

Why did reported numbers differ?

During a major event, AirNow, weather apps, news reports, and neighborhood monitors can all show different figures at the same moment. All of them can be right:

Concentration vs AQI. µg/m³ is the measurement; AQI is a 0 to 500 index derived from it. "330 µg/m³" and "AQI 500" can describe the same air.
Hourly vs NowCast vs daily. An hourly peak, the NowCast rolling estimate apps display, and the full-day average answer different questions; this page ranks full days.
One monitor vs a composite. A single sensor can read far above or below a citywide figure. The measure toggle above shows both framings.
Live vs certified. Real-time feeds get corrected when EPA certifies data months later, which is why recent days here carry a provisional label.

What about indoors?

Citywide monitors can tell you what happened outside. They cannot tell you what happened inside a particular home, school, or facility: filtration, ventilation, and building operation decide that. During and after days like the ones above, outdoor particles become an indoor load question.

Check current conditions and the next clean-air window → Estimate recent filter load where you are →

Common questions

What was the worst air-quality day in Chicago?: July 16, 2026 (provisional), when daily PM2.5 reached 330.9 µg/m³ (AQI 500), about 15.0 cigarette-equivalents. It ranks first of 9,929 monitored days since 1999.
What is the highest AQI recorded in Chicago?: On the worst day, the daily PM2.5 AQI reached 500 (Hazardous). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 246 on June 28, 2023.
Is the July 2026 ranking final?: No. Days after April 30, 2026 are preliminary AirNow readings. They rank now so a current event is visible, and they may shift once EPA certifies the data, usually the following spring.
Were these days caused by wildfire smoke?: This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.
How often does Chicago get air this bad?: Rarely. The record sits at the far end of 9,929 measured days; Chicago averages 1 "unhealthy" day a year.
Why did different apps show different numbers that day?: Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.

Data and citation

Ranked days for both measures are downloadable, with units and status flags: chicago.csv. Source: U.S. EPA Air Quality System (AQS) daily PM2.5, with preliminary AirNow readings for the uncertified tail. Cite as:

IAQng Air Records: Chicago, IL-IN-WI. iaqng.com/records/chicago. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.