Bad Air Days in Cook County, Illinois: 18 a Year

18 bad air days a year

Among the worst tenth of monitored US counties. Ranked 36 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (14% particle days).

Which months to watch

Most bad days here fall in June and July and August. Averaged over 2016 to 2025.

What actually helps here

Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
Sign up for EnviroFlash ozone alerts for this area during the worst months.

Better or worse than it was

In 2000 to 2009 this county averaged 39 bad days a year; in 2016 to 2025, 18. The air has improved. The worst recent year was 2023, with 34 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-27	223	PM2.5
2023-06-28	213	PM2.5
2023-07-25	185	Ozone
2017-07-18	177	Ozone
2018-05-27	177	Ozone

Most days, not the worst ones

Across 3,653 monitored days from 2016 to 2025, the air in Cook County rated Good on 29% of them, less often than the 72% typical of monitored counties, and Moderate on 66%. The median day sat at AQI 56; one day in ten went above 84.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)68%2,493 days
ozone24%880 days
nitrogen dioxide4%160 days
coarse dust (PM10)3%120 days

Cook County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	149	191	25	5	53	168
2024	366	131	224	11	1	54	166
2023	365	104	227	34	7	57	223
2022	365	117	240	8	1	54	151
2021	365	87	257	21	1	59	156
2020	366	86	260	20	4	58	164
2019	365	80	276	9	0	58	133
2018	365	86	260	19	4	59	177
2017	365	114	232	19	1	55	177
2016	366	115	233	18	2	56	169
2015	365	73	285	7	2	57	195
2014	365	76	281	8	0	59	143
2013	365	76	278	11	0	58	136
2012	366	53	271	42	10	61	195
2011	365	52	297	16	4	62	159
2010	365	43	298	24	0	62	140

*2025 may be a partial year in EPA's file.

Illinois compared

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.