Bad Air Days in Jo Daviess County, Illinois: 1 a Year

1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 611 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 Jo Daviess

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

What actually helps here

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

Better or worse than it was

In 2000 to 2009 this county averaged 8 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2023, with 9 bad days on 353 monitored days.

Date	AQI	Main pollutant
2023-06-28	154	Ozone
2023-06-19	129	Ozone
2023-06-03	122	Ozone
2018-05-29	119	Ozone
2023-05-31	119	Ozone

The air on a normal day

Across 3,482 monitored days from 2016 to 2025, the air in Jo Daviess County rated Good on 94% of them, more often than the 72% typical of monitored counties, and Moderate on 6%. The median day sat at AQI 34; one day in ten went above 48.

The pollutant setting the AQI on an ordinary day here:

ozone100%3,482 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	341	325	16	0	0	33	87
2024	349	327	21	1	0	36	101
2023	353	302	42	9	1	36	154
2022	343	328	15	0	0	34	84
2021	362	336	26	0	0	36	97
2020	353	337	16	0	0	32	87
2019	363	351	12	0	0	34	74
2018	344	323	19	2	0	33	119
2017	324	303	21	0	0	34	87
2016	350	331	18	1	0	33	108
2015	327	309	18	0	0	34	90
2014	353	324	28	1	0	35	108
2013	317	284	32	1	0	37	105
2012	341	275	56	10	0	36	129
2011	315	287	27	1	0	35	105
2010	357	327	28	2	0	37	105

*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.