Daily Air Quality Index summaries from EPA monitors in McLean, Illinois, 2000–2025.
66%
Good days in 2025
44
Median daily AQI, 2025
4
Days above 100 in 2025
365
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for McLean, Illinois on 365 days. 241 of those days (66%) were in the Good range, and the median daily AQI was 44. That median is about the same as the typical monitored county in Illinois (median 42 across 23 counties), and higher than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 207 days. Across the years with fuller monitoring, the share of Good days fell: an average of 72% in 2001–2003 versus 62% in 2023–2025.
Years with fewer than 120 monitored days (2000) are shown in the table but left out of comparisons and trend statements.
How the days broke down each year
Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.
Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous
Typical and high-end AQI over time
The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.
Median daily AQI
90th-percentile daily AQI
Main pollutant by day, 2025
On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.
Pollutant
Days as main pollutant
Share of monitored days
Fine particles (PM2.5)
207
57%
Ozone
158
43%
Year-by-year data
Annual AQI summary by year
Year
Days with AQI
Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous
Median AQI
90th pct.
Max AQI
Main pollutant
2025
365
241
120
3
1
0
0
44
67
154
PM2.5
2024
366
254
111
1
0
0
0
44
61
112
PM2.5
2023
365
184
168
10
2
1
0
50
74
289
PM2.5
2022
365
207
156
2
0
0
0
47
63
129
PM2.5
2021
365
191
173
1
0
0
0
48
63
107
PM2.5
2020
364
207
154
3
0
0
0
48
61
136
PM2.5
2019
364
187
177
0
0
0
0
49
65
100
PM2.5
2018
365
169
195
1
0
0
0
52
67
108
PM2.5
2017
364
201
163
0
0
0
0
48
61
100
PM2.5
2016
366
283
82
1
0
0
0
39
61
108
Ozone
2015
363
304
59
0
0
0
0
37
55
90
Ozone
2014
363
312
51
0
0
0
0
37
58
100
Ozone
2013
355
303
51
1
0
0
0
37
58
105
Ozone
2012
359
271
71
17
0
0
0
38
87
133
Ozone
2011
359
311
46
2
0
0
0
34
58
147
Ozone
2010
365
271
92
2
0
0
0
41
64
112
Ozone
2009
364
268
92
4
0
0
0
38
64
105
Ozone
2008
347
253
93
1
0
0
0
40
64
112
Ozone
2007
365
252
103
9
1
0
0
40
84
166
Ozone
2006
365
273
85
7
0
0
0
36
71
119
Ozone
2005
364
254
95
15
0
0
0
40
86
140
Ozone
2004
361
296
65
0
0
0
0
36
61
97
Ozone
2003
362
275
78
9
0
0
0
38
74
126
Ozone
2002
362
271
76
13
2
0
0
37
76
156
Ozone
2001
287
189
88
10
0
0
0
44
80
150
Ozone
2000
55
15
39
1
0
0
0
58
85
112
PM2.5
What this data can and can’t tell you
It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.
It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.
Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.
Counties in Illinois with a similar median AQI in 2025
