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

2 bad air days a year

About the middle of monitored US counties. Ranked 460 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (18% 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 Randolph

In 2000 to 2009 this county averaged 8 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2023, with 13 bad days on 365 monitored days.

Date	AQI	Main pollutant
2025-03-15	161	PM2.5
2023-06-06	156	Ozone
2023-06-03	154	Ozone
2023-06-10	140	Ozone
2023-06-15	133	Ozone

Most days, not the worst ones

Across 3,506 monitored days from 2016 to 2025, the air in Randolph County rated Good on 65% of them, less often than the 72% typical of monitored counties, and Moderate on 34%. The median day sat at AQI 45; one day in ten went above 61.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)60%2,102 days
ozone40%1,404 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	364	248	115	1	1	43	161
2024	366	289	77	0	0	40	100
2023	365	215	137	13	2	46	156
2022	365	244	119	2	0	45	108
2021	355	223	131	1	0	45	122
2020	366	243	121	2	0	44	103
2019	324	234	90	0	0	43	97
2018	363	225	137	1	0	46	129
2017	343	153	189	1	0	52	115
2016	295	209	85	1	0	42	119
2015	257	198	59	0	0	42	90
2014	230	159	67	4	0	44	122
2013	301	270	31	0	0	38	93
2012	355	271	60	24	2	38	161
2011	353	314	39	0	0	36	97
2010	353	305	48	0	0	36	97

*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.