Daily Air Quality Index summaries from EPA monitors in Chester, Pennsylvania, 1990–2025.
71%
Good days in 2025
43
Median daily AQI, 2025
2
Days above 100 in 2025
363
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Chester, Pennsylvania on 363 days. 259 of those days (71%) were in the Good range, and the median daily AQI was 43. That median is higher than the typical monitored county in Pennsylvania (median 40 across 38 counties), and higher than the typical monitored U.S. county (40). The main pollutant was ozone on 227 days. Across the years with fuller monitoring, the share of Good days rose: an average of 52% in 2000–2002 versus 68% in 2023–2025.
Years with fewer than 120 monitored days (1992, 1993, 1994, 1995, 1996, 1997…) 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
Ozone
227
63%
Fine particles (PM2.5)
136
37%
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
363
259
102
1
1
0
0
43
64
157
Ozone
2024
363
264
96
3
0
0
0
41
61
143
Ozone
2023
361
220
134
3
2
2
0
45
66
248
PM2.5
2022
364
249
115
0
0
0
0
45
58
80
PM2.5
2021
347
215
130
2
0
0
0
45
63
138
PM2.5
2020
355
233
121
1
0
0
0
44
59
143
PM2.5
2019
364
223
139
2
0
0
0
46
67
115
Ozone
2018
359
220
138
1
0
0
0
44
66
112
PM2.5
2017
364
137
222
5
0
0
0
54
69
136
PM2.5
2016
365
158
198
9
0
0
0
52
71
143
PM2.5
2015
365
133
230
1
1
0
0
55
75
166
PM2.5
2014
363
154
205
4
0
0
0
52
74
129
PM2.5
2013
365
154
208
3
0
0
0
53
73
113
PM2.5
2012
355
170
166
16
3
0
0
51
84
151
Ozone
2011
345
127
207
9
2
0
0
56
81
161
PM2.5
2010
351
117
211
22
1
0
0
57
93
151
PM2.5
2009
348
81
263
4
0
0
0
58
80
126
PM2.5
2008
259
128
111
17
3
0
0
51
93
172
Ozone
2007
254
105
123
24
1
1
0
57
101
212
Ozone
2006
249
120
105
20
4
0
0
51
100
187
Ozone
2005
251
102
109
32
8
0
0
58
115
195
Ozone
2004
253
121
106
23
3
0
0
51
101
174
Ozone
2003
263
119
118
23
1
2
0
53
99
207
Ozone
2002
261
95
105
35
21
5
0
61
150
227
Ozone
2001
214
100
65
30
16
3
0
58
147
204
Ozone
2000
120
88
27
4
1
0
0
38
84
154
Ozone
1997
59
50
9
0
0
0
0
29
54
76
PM10
1996
54
50
4
0
0
0
0
26
45
63
PM10
1995
55
44
11
0
0
0
0
21
55
87
PM10
1994
47
40
7
0
0
0
0
29
56
62
PM10
1993
54
47
7
0
0
0
0
24
56
62
PM10
1992
52
51
1
0
0
0
0
25
39
56
PM10
1991
52
46
6
0
0
0
0
28
52
87
PM10
1990
54
45
9
0
0
0
0
25
62
70
PM10
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 Pennsylvania with a similar median AQI in 2025
