Bad Air Days in Saint Clair County, Illinois: 3 a Year

3 bad air days a year

About the middle of monitored US counties. Ranked 320 of 996 monitored counties, where 1 is worst.

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

When the bad days come in Saint Clair

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.

Better or worse than it was

In 2000 to 2009 this county averaged 13 bad days a year; in 2016 to 2025, 3. The air has improved. The worst recent year was 2023, with 14 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-07-28	185	Ozone
2018-06-06	174	Ozone
2023-06-06	169	Ozone
2025-03-15	161	PM2.5
2023-06-28	159	PM2.5

Most days, not the worst ones

Across 3,651 monitored days from 2016 to 2025, the air in Saint Clair 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 65.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)64%2,352 days
ozone29%1,075 days
nitrogen dioxide6%221 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	213	149	3	1	47	161
2024	366	218	145	3	0	46	112
2023	365	128	223	14	2	54	169
2022	365	165	199	1	0	52	105
2021	365	141	221	3	1	53	185
2020	365	260	105	0	0	42	100
2019	365	217	147	1	0	47	108
2018	364	152	207	5	1	52	174
2017	365	237	127	1	0	45	101
2016	366	226	136	4	0	44	126
2015	365	205	158	2	0	48	117
2014	365	194	169	2	0	49	125
2013	365	177	186	2	0	51	101
2012	366	182	153	31	4	51	281
2011	365	141	214	10	1	54	151
2010	365	177	181	7	0	52	150

*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.