Daily Air Quality Index summaries from EPA monitors in Delaware, Ohio, 1997–2025.
84%
Good days in 2025
40
Median daily AQI, 2025
3
Days above 100 in 2025
242
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Delaware, Ohio on 242 days. 204 of those days (84%) were in the Good range, and the median daily AQI was 40. That median is lower than the typical monitored county in Ohio (median 43 across 36 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 242 days. Across the years with fuller monitoring, the share of Good days rose: an average of 54% in 1997–1999 versus 84% in 2023–2025.
EPA did not report an AQI for every day of 2025 here (242 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.
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
Ozone
242
100%
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
242
204
35
3
0
0
0
40
61
119
Ozone
2024
242
200
41
1
0
0
0
42
61
105
Ozone
2023
245
211
33
1
0
0
0
41
54
108
Ozone
2022
244
230
14
0
0
0
0
37
47
100
Ozone
2021
240
222
18
0
0
0
0
38
49
77
Ozone
2020
244
222
21
1
0
0
0
38
50
101
Ozone
2019
241
214
27
0
0
0
0
41
51
84
Ozone
2018
244
212
32
0
0
0
0
39
54
100
Ozone
2017
245
228
17
0
0
0
0
38
49
90
Ozone
2016
212
179
32
1
0
0
0
41
67
112
Ozone
2015
214
187
26
1
0
0
0
39
54
108
Ozone
2014
214
188
24
2
0
0
0
41
54
105
Ozone
2013
211
175
33
3
0
0
0
41
58
112
Ozone
2012
213
128
70
14
1
0
0
46
93
185
Ozone
2011
214
156
51
7
0
0
0
43
71
126
Ozone
2010
212
138
66
8
0
0
0
44
84
133
Ozone
2009
214
170
42
2
0
0
0
41
67
115
Ozone
2008
213
147
55
11
0
0
0
46
80
133
Ozone
2007
213
130
56
26
1
0
0
48
105
154
Ozone
2006
214
142
61
10
1
0
0
45
84
151
Ozone
2005
214
119
71
23
1
0
0
47
108
172
Ozone
2004
214
158
43
13
0
0
0
44
84
129
Ozone
2003
213
139
56
13
4
1
0
45
97
201
Ozone
2002
213
116
53
27
17
0
0
48
136
200
Ozone
2001
213
117
65
25
6
0
0
48
112
177
Ozone
2000
214
148
55
10
1
0
0
45
84
177
Ozone
1999
207
117
50
24
13
3
0
47
133
226
Ozone
1998
211
91
76
31
12
1
0
54
133
203
Ozone
1997
214
133
57
19
5
0
0
44
105
185
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 Ohio with a similar median AQI in 2025
