Bad Air Days in Lake County, Illinois: 9 a Year

9 bad air days a year

Among the worst tenth of monitored US counties. Ranked 91 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (0% particle days).

When the bad days come in Lake

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

What to buy, and what not to

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.

Better or worse than it was

In 2000 to 2009 this county averaged 11 bad days a year; in 2016 to 2025, 9. Not much has changed. The worst recent year was 2023, with 14 bad days on 243 monitored days.

Date	AQI	Main pollutant
2020-06-19	185	Ozone
2023-07-25	179	Ozone
2018-05-25	164	Ozone
2022-06-20	159	Ozone
2018-05-27	156	Ozone

Most days, not the worst ones

Across 2,748 monitored days from 2016 to 2025, the air in Lake County rated Good on 83% of them, more often than the 72% typical of monitored counties, and Moderate on 14%. The median day sat at AQI 38; one day in ten went above 64.

The pollutant setting the AQI on an ordinary day here:

ozone100%2,748 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	231	196	32	3	0	39	108
2024	245	201	39	5	0	40	140
2023	243	182	47	14	1	42	179
2022	243	199	41	3	1	39	159
2021	244	180	55	9	0	42	136
2020	241	199	31	11	1	39	185
2019	245	210	33	2	0	40	115
2018	352	304	40	8	2	35	164
2017	358	316	35	7	0	35	129
2016	346	306	32	8	0	34	140
2015	362	332	27	3	0	35	122
2014	355	317	33	5	0	35	129
2013	351	305	39	7	0	34	136
2012	361	296	37	28	8	34	195
2011	359	316	36	7	1	34	174
2010	365	287	67	11	1	38	156

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

Elsewhere in Illinois

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.