Bad Air Days in Hamilton County, Illinois: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 510 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (21% particle days).

Which months to watch

Most bad days here fall in June and May. 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.

The record for Hamilton

In 2000 to 2009 this county averaged 15 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2023, with 11 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-28	157	PM2.5
2023-06-06	136	Ozone
2023-06-03	122	Ozone
2020-06-27	118	PM2.5
2021-06-14	115	Ozone

Most days, not the worst ones

Across 3,530 monitored days from 2016 to 2025, the air in Hamilton County rated Good on 62% of them, less often than the 72% typical of monitored counties, and Moderate on 38%. The median day sat at AQI 45; one day in ten went above 62.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)66%2,316 days
ozone34%1,214 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	361	245	116	0	0	42	98
2024	366	236	129	1	0	44	112
2023	365	213	141	11	1	46	157
2022	365	227	138	0	0	45	88
2021	364	207	155	2	0	48	115
2020	353	223	129	1	0	46	118
2019	344	204	139	1	0	47	105
2018	362	212	147	3	0	47	105
2017	365	220	145	0	0	46	87
2016	285	197	88	0	0	42	100
2015	261	203	57	1	0	41	105
2014	231	177	54	0	0	41	93
2013	210	175	35	0	0	41	87
2012	214	118	71	25	3	49	164
2011	212	143	64	5	0	45	126
2010	249	152	86	11	0	47	119

*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.