Daily Air Quality Index summaries from EPA monitors in Hamilton, Illinois, 1997–2025.
68%
Good days in 2025
42
Median daily AQI, 2025
0
Days above 100 in 2025
361
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Hamilton, Illinois on 361 days. 245 of those days (68%) were in the Good range, and the median daily AQI was 42. That median is about the same as the typical monitored county in Illinois (median 42 across 23 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 208 days. Across the years with fuller monitoring, the share of Good days fell: an average of 69% in 1997–1999 versus 64% in 2023–2025.
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)
208
58%
Ozone
153
42%
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
361
245
116
0
0
0
0
42
61
98
PM2.5
2024
366
236
129
1
0
0
0
44
60
112
PM2.5
2023
365
213
141
10
1
0
0
46
67
157
PM2.5
2022
365
227
138
0
0
0
0
45
61
88
PM2.5
2021
364
207
155
2
0
0
0
48
63
115
PM2.5
2020
353
223
129
1
0
0
0
46
61
118
PM2.5
2019
344
204
139
1
0
0
0
47
61
105
PM2.5
2018
362
212
147
3
0
0
0
47
63
105
PM2.5
2017
365
220
145
0
0
0
0
46
60
87
PM2.5
2016
285
197
88
0
0
0
0
42
63
100
Ozone
2015
261
203
57
1
0
0
0
41
61
105
Ozone
2014
231
177
54
0
0
0
0
41
61
93
Ozone
2013
210
175
35
0
0
0
0
41
61
87
Ozone
2012
214
118
71
22
3
0
0
49
105
164
Ozone
2011
212
143
64
5
0
0
0
45
74
126
Ozone
2010
249
152
86
11
0
0
0
47
87
119
Ozone
2009
247
178
67
2
0
0
0
44
61
105
Ozone
2008
245
178
65
2
0
0
0
43
67
109
Ozone
2007
316
74
231
11
0
0
0
59
90
147
PM2.5
2006
251
170
81
0
0
0
0
44
73
100
Ozone
2005
216
118
87
11
0
0
0
49
93
136
Ozone
2004
213
152
57
4
0
0
0
44
80
105
Ozone
2003
209
119
75
15
0
0
0
48
93
133
Ozone
2002
210
120
58
25
7
0
0
48
122
172
Ozone
2001
208
149
53
6
0
0
0
44
77
122
Ozone
2000
228
163
53
11
1
0
0
43
84
156
Ozone
1999
225
158
53
11
3
0
0
44
90
166
Ozone
1998
225
146
69
10
0
0
0
46
84
150
Ozone
1997
220
159
49
12
0
0
0
44
87
129
Ozone
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
