Daily Air Quality Index summaries from EPA monitors in St. Louis City, Missouri, 1980–2025.
34%
Good days in 2025
55
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 St. Louis City, Missouri on 365 days. 125 of those days (34%) were in the Good range, and the median daily AQI was 55. That median is higher than the typical monitored county in Missouri (median 41 across 19 counties), and higher than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 261 days. Across the years with fuller monitoring, the share of Good days fell: an average of 38% in 1980–1984 versus 32% in 2023–2025.
Years with fewer than 120 monitored days (1982) 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)
261
72%
Ozone
75
21%
Coarse particles (PM10)
24
7%
Nitrogen dioxide (NO₂)
5
1%
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
125
232
6
0
0
2
55
77
738
PM2.5
2024
366
137
219
9
1
0
0
54
74
156
PM2.5
2023
365
89
260
14
2
0
0
58
84
172
PM2.5
2022
365
127
233
5
0
0
0
55
75
117
PM2.5
2021
365
114
244
6
1
0
0
55
71
187
PM2.5
2020
366
137
224
4
1
0
0
54
70
156
PM2.5
2019
365
135
225
5
0
0
0
54
72
129
PM2.5
2018
365
129
226
9
1
0
0
55
73
182
PM2.5
2017
365
165
198
2
0
0
0
52
67
115
PM2.5
2016
366
140
223
2
1
0
0
53
66
162
PM2.5
2015
365
122
237
4
2
0
0
55
72
155
PM2.5
2014
365
111
247
7
0
0
0
56
75
148
PM2.5
2013
365
108
255
2
0
0
0
57
72
133
PM2.5
2012
366
75
259
27
5
0
0
61
97
174
PM2.5
2011
365
73
271
19
2
0
0
61
88
179
PM2.5
2010
365
66
293
5
1
0
0
61
78
162
PM2.5
2009
365
80
282
2
1
0
0
58
74
166
PM2.5
2008
366
59
297
10
0
0
0
59
83
133
PM2.5
2007
365
45
291
24
4
1
0
62
93
208
PM2.5
2006
365
20
308
33
4
0
0
72
101
183
PM2.5
2005
365
35
282
41
7
0
0
70
115
185
PM2.5
2004
366
62
286
18
0
0
0
62
87
141
PM2.5
2003
365
50
285
26
2
2
0
64
98
212
PM2.5
2002
365
48
262
40
13
2
0
67
122
206
PM2.5
2001
365
60
264
41
0
0
0
65
105
147
PM2.5
2000
366
36
309
17
4
0
0
66
92
166
PM2.5
1999
365
71
263
24
6
1
0
63
97
204
PM2.5
1998
365
249
92
22
2
0
0
44
84
197
NO2
1997
365
273
75
14
3
0
0
42
67
182
NO2
1996
366
251
88
20
7
0
0
42
90
179
NO2
1995
365
244
95
22
3
1
0
43
85
208
NO2
1994
365
232
118
11
4
0
0
44
74
177
NO2
1993
365
264
96
3
2
0
0
42
67
159
NO2
1992
366
268
88
10
0
0
0
42
66
126
NO2
1991
365
277
72
14
2
0
0
41
74
156
NO2
1990
365
269
68
22
6
0
0
42
80
185
NO2
1989
365
242
103
15
5
0
0
45
80
169
NO2
1988
366
203
123
24
14
2
0
47
105
222
NO2
1987
364
202
134
17
11
0
0
48
95
195
NO2
1986
365
141
160
58
5
1
0
57
108
202
NO2
1985
363
183
166
12
2
0
0
50
76
192
NO2
1984
365
178
153
27
4
3
0
52
98
219
NO2
1983
365
178
149
20
12
6
0
51
101
238
CO
1982
92
49
42
1
0
0
0
49
82
101
CO
1980
359
54
171
117
14
3
0
85
131
220
NO2
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 Missouri with a similar median AQI in 2025
