Daily Air Quality Index summaries from EPA monitors in Fergus, Montana, 1988–2025.
91%
Good days in 2025
41
Median daily AQI, 2025
1
Days above 100 in 2025
364
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Fergus, Montana on 364 days. 330 of those days (91%) were in the Good range, and the median daily AQI was 41. That median is higher than the typical monitored county in Montana (median 25.5 across 18 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 291 days. Across the years with fuller monitoring, the share of Good days fell: an average of 98% in 2005–2010 versus 87% in 2023–2025.
Years with fewer than 120 monitored days (2003, 2004, 2007, 2008, 2009, 2011…) 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
291
80%
Fine particles (PM2.5)
69
19%
Coarse particles (PM10)
4
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
364
330
33
1
0
0
0
41
49
131
Ozone
2024
366
312
50
3
1
0
0
40
56
155
Ozone
2023
364
315
43
4
2
0
0
41
54
185
Ozone
2022
365
341
22
2
0
0
0
40
49
122
Ozone
2021
365
288
62
14
1
0
0
41
77
156
Ozone
2020
366
330
33
1
2
0
0
39
50
159
Ozone
2019
365
343
22
0
0
0
0
38
48
86
Ozone
2018
365
313
46
5
1
0
0
40
54
161
Ozone
2017
365
296
62
5
2
0
0
42
60
155
Ozone
2016
366
349
17
0
0
0
0
36
44
88
Ozone
2015
365
328
28
5
4
0
0
37
52
167
Ozone
2014
365
337
27
1
0
0
0
37
46
135
Ozone
2013
365
348
17
0
0
0
0
35
45
62
Ozone
2012
217
206
11
0
0
0
0
27
34
71
Ozone
2011
114
114
0
0
0
0
0
12
29
47
PM2.5
2010
121
118
3
0
0
0
0
14
29
58
PM2.5
2009
110
109
1
0
0
0
0
13
29
55
PM2.5
2008
115
113
2
0
0
0
0
13
36
72
PM2.5
2007
109
102
6
1
0
0
0
12
38
109
PM2.5
2006
121
118
3
0
0
0
0
14
38
71
PM2.5
2005
122
121
1
0
0
0
0
13
27
61
PM2.5
2004
119
117
2
0
0
0
0
13
27
78
PM2.5
2003
106
105
1
0
0
0
0
14
34
54
PM2.5
2002
107
107
0
0
0
0
0
15
29
39
PM2.5
2001
104
101
3
0
0
0
0
14
31
54
PM2.5
2000
89
85
4
0
0
0
0
16
33
87
PM2.5
1997
73
73
0
0
0
0
0
9
19
22
PM10
1996
86
86
0
0
0
0
0
7
19
44
PM10
1995
88
88
0
0
0
0
0
7
16
29
PM10
1994
88
78
9
1
0
0
0
19
53
104
PM10
1993
88
86
2
0
0
0
0
15
33
97
PM10
1992
87
82
5
0
0
0
0
13
34
68
PM10
1991
50
49
1
0
0
0
0
12
37
51
PM10
1990
56
55
1
0
0
0
0
14
39
60
PM10
1989
39
36
3
0
0
0
0
8
47
89
PM10
1988
14
11
3
0
0
0
0
22
81
84
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
