Daily Air Quality Index summaries from EPA monitors in Hartford, Connecticut, 1980–2025.
68%
Good days in 2025
42
Median daily AQI, 2025
8
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 Hartford, Connecticut on 365 days. 248 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 Connecticut (median 41 across 8 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 213 days. Across the years with fuller monitoring, the share of Good days rose: an average of 41% in 1980–1982 versus 69% 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
Ozone
213
58%
Fine particles (PM2.5)
143
39%
Nitrogen dioxide (NO₂)
9
2%
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
248
109
6
2
0
0
42
68
164
Ozone
2024
366
280
79
6
1
0
0
39
59
185
Ozone
2023
365
226
132
4
3
0
0
43
64
166
PM2.5
2022
365
260
101
4
0
0
0
42
61
136
Ozone
2021
365
247
115
3
0
0
0
42
63
136
PM2.5
2020
366
255
111
0
0
0
0
42
60
98
PM2.5
2019
365
252
108
5
0
0
0
43
59
122
PM2.5
2018
365
240
123
2
0
0
0
42
64
143
PM2.5
2017
365
219
143
3
0
0
0
46
64
133
PM2.5
2016
366
207
152
6
1
0
0
47
68
169
PM2.5
2015
365
167
190
8
0
0
0
52
75
147
PM2.5
2014
365
159
196
10
0
0
0
52
71
143
PM2.5
2013
365
121
233
10
1
0
0
54
74
151
PM2.5
2012
366
173
182
10
1
0
0
52
74
164
PM2.5
2011
365
222
136
6
1
0
0
44
67
166
PM2.5
2010
365
248
111
5
1
0
0
40
69
159
PM2.5
2009
365
255
107
3
0
0
0
39
67
119
PM2.5
2008
366
206
151
7
2
0
0
46
71
159
PM2.5
2007
365
213
136
11
4
1
0
44
80
202
PM2.5
2006
365
209
147
5
3
1
0
45
77
206
PM2.5
2005
365
190
159
11
5
0
0
50
80
195
PM2.5
2004
366
198
156
11
1
0
0
47
75
154
PM2.5
2003
365
167
186
12
0
0
0
52
77
147
PM2.5
2002
365
182
156
15
11
1
0
51
90
222
PM2.5
2001
365
157
182
18
7
1
0
53
87
209
PM2.5
2000
366
187
165
12
2
0
0
49
74
179
PM2.5
1999
365
199
145
10
11
0
0
47
80
200
PM2.5
1998
365
289
63
11
2
0
0
41
67
195
NO2
1997
365
309
42
8
4
2
0
38
60
234
Ozone
1996
361
319
35
7
0
0
0
28
54
136
CO
1995
365
292
50
15
6
2
0
39
77
208
Ozone
1994
365
256
82
15
11
1
0
43
89
232
Ozone
1993
365
288
59
12
5
1
0
40
69
210
NO2
1992
366
303
54
6
3
0
0
37
61
187
CO
1991
365
267
73
12
9
4
0
41
83
225
CO
1990
365
279
72
8
4
2
0
39
74
210
Ozone
1989
365
233
116
9
4
3
0
45
76
223
CO
1988
366
206
128
11
12
9
0
48
91
243
CO
1987
365
182
168
13
1
1
0
51
81
203
CO
1986
365
182
174
8
1
0
0
51
79
182
CO
1985
365
152
188
23
1
1
0
55
90
205
CO
1984
366
148
188
16
14
0
0
56
97
200
CO
1983
365
137
185
18
17
8
0
56
112
237
CO
1982
365
156
169
21
12
7
0
54
105
220
CO
1981
365
147
185
16
9
8
0
55
100
228
CO
1980
366
143
170
21
17
15
0
55
119
260
CO
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 Connecticut with a similar median AQI in 2025
