Bad Air Days in Winnebago County, Illinois: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 297 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (38% particle days).

Which months to watch

Most bad days here fall in June and May and July. 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 6 bad days a year; in 2016 to 2025, 4. The air has improved. The worst recent year was 2023, with 20 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-28	242	PM2.5
2023-06-27	215	PM2.5
2023-06-29	191	PM2.5
2025-07-31	169	PM2.5
2018-05-29	151	Ozone

An ordinary day in Winnebago

Across 3,581 monitored days from 2016 to 2025, the air in Winnebago County rated Good on 66% of them, less often than the 72% typical of monitored counties, and Moderate on 33%. The median day sat at AQI 43; one day in ten went above 63.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)51%1,844 days
ozone49%1,737 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	355	244	105	6	1	40	169
2024	361	249	109	3	0	43	119
2023	365	167	178	20	3	52	242
2022	363	237	126	0	0	43	97
2021	360	190	170	0	0	49	100
2020	366	205	160	1	0	47	101
2019	341	182	158	1	0	49	102
2018	362	310	49	3	1	36	151
2017	354	296	58	0	0	36	87
2016	354	300	51	3	0	36	105
2015	362	300	61	1	0	36	108
2014	342	304	36	2	0	34	105
2013	363	340	23	0	0	31	84
2012	366	313	47	6	0	33	115
2011	365	329	35	1	0	32	115
2010	365	299	64	2	0	37	117

*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.