Daily Air Quality Index summaries from EPA monitors in Gallatin, Montana, 1985–2025.
98%
Good days in 2025
11
Median daily AQI, 2025
0
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 Gallatin, Montana on 365 days. 358 of those days (98%) were in the Good range, and the median daily AQI was 11. That median is lower than the typical monitored county in Montana (median 25.5 across 18 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 232 days. Across the years with fuller monitoring, the share of Good days rose: an average of 80% in 1992–1995 versus 93% in 2023–2025.
Years with fewer than 120 monitored days (1987, 1988, 1989, 1990, 1991, 1994…) 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)
232
64%
Nitrogen dioxide (NO₂)
91
25%
Carbon monoxide (CO)
42
12%
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
358
7
0
0
0
0
11
26
70
PM2.5
2024
365
329
36
0
0
0
0
18
50
84
PM2.5
2023
365
331
34
0
0
0
0
26
48
98
PM2.5
2022
365
290
71
4
0
0
0
32
58
148
PM2.5
2021
365
243
108
13
1
0
0
41
78
158
PM2.5
2020
366
290
65
9
2
0
0
29
57
162
PM2.5
2019
365
301
64
0
0
0
0
33
55
92
PM2.5
2018
365
289
70
5
1
0
0
32
57
154
PM2.5
2017
365
263
99
2
1
0
0
31
70
152
PM2.5
2016
366
298
58
6
3
0
1
32
60
479
PM2.5
2015
365
284
77
3
1
0
0
29
60
183
PM2.5
2014
365
287
78
0
0
0
0
33
57
87
PM2.5
2013
365
294
71
0
0
0
0
32
55
79
PM2.5
2012
366
272
86
8
0
0
0
33
66
138
PM2.5
2011
365
204
160
1
0
0
0
47
62
103
PM2.5
2010
365
239
124
2
0
0
0
38
69
124
PM2.5
2009
365
216
148
1
0
0
0
44
68
108
PM2.5
2008
366
269
97
0
0
0
0
34
59
85
PM2.5
2007
365
258
100
5
2
0
0
36
67
182
PM2.5
2006
365
294
68
2
0
1
0
23
60
202
PM2.5
2005
365
325
37
3
0
0
0
19
52
124
PM2.5
2004
366
326
39
0
1
0
0
29
52
156
PM2.5
2003
358
334
24
0
0
0
0
9
39
94
CO
2002
365
341
24
0
0
0
0
6
43
93
CO
2001
365
333
31
1
0
0
0
6
46
101
CO
2000
348
311
33
2
2
0
0
11
52
173
CO
1999
320
309
11
0
0
0
0
8
32
95
CO
1998
156
145
11
0
0
0
0
19
45
59
PM10
1997
123
111
12
0
0
0
0
25
49
75
PM10
1996
123
115
8
0
0
0
0
29
47
60
PM10
1995
122
105
17
0
0
0
0
24
53
97
PM10
1994
117
102
15
0
0
0
0
28
51
70
PM10
1993
122
92
30
0
0
0
0
37
61
92
PM10
1992
133
105
28
0
0
0
0
32
60
90
PM10
1991
105
89
16
0
0
0
0
28
59
87
PM10
1990
50
47
3
0
0
0
0
23
45
73
PM10
1989
55
54
1
0
0
0
0
19
30
91
PM10
1988
55
51
4
0
0
0
0
22
42
69
PM10
1987
50
49
1
0
0
0
0
22
38
54
PM10
1986
32
26
6
0
0
0
0
35
54
67
PM10
1985
6
5
1
0
0
0
0
38
55
55
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 Montana with a similar median AQI in 2025
