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

4 bad air days a year

About the middle of monitored US counties. Ranked 300 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (35% particle days).

Which months to watch

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

What to buy, and what not to

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.

The record for Will

In 2000 to 2009 this county averaged 10 bad days a year; in 2016 to 2025, 4. The air has improved. The worst recent year was 2023, with 19 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-28	246	PM2.5
2023-06-27	222	PM2.5
2020-07-05	156	PM2.5
2025-07-31	156	PM2.5
2023-06-29	154	PM2.5

Most days, not the worst ones

Across 3,648 monitored days from 2016 to 2025, the air in Will County rated Good on 54% of them, less often than the 72% typical of monitored counties, and Moderate on 45%. The median day sat at AQI 48; one day in ten went above 67.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)70%2,557 days
ozone30%1,091 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	225	135	5	1	44	156
2024	365	245	120	0	0	44	84
2023	365	155	191	19	5	53	246
2022	365	193	170	2	0	49	101
2021	365	157	206	2	0	52	105
2020	363	183	176	4	1	50	156
2019	365	179	186	0	0	51	84
2018	365	194	167	4	0	49	126
2017	365	231	134	0	0	45	93
2016	365	201	163	1	0	48	105
2015	364	153	211	0	0	53	93
2014	362	171	191	0	0	51	97
2013	365	271	94	0	0	41	91
2012	359	303	52	4	0	35	119
2011	359	334	23	2	1	33	164
2010	351	292	57	2	0	38	115

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

Nearby counties on file

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.