Daily Air Quality Index summaries from EPA monitors in Lincoln, Montana, 1985–2025.
54%
Good days in 2025
45
Median daily AQI, 2025
2
Days above 100 in 2025
182
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Lincoln, Montana on 182 days. 99 of those days (54%) were in the Good range, and the median daily AQI was 45. That median is higher than the typical monitored county in Montana (median 25.5 across 18 counties), and higher than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 181 days. Across the years with fuller monitoring, the share of Good days rose: an average of 41% in 1987–1989 versus 49% in 2023–2025.
EPA did not report an AQI for every day of 2025 here (182 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.
Years with fewer than 120 monitored days (1985, 1986) 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)
181
99%
Coarse particles (PM10)
1
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
182
99
81
2
0
0
0
45
67
118
PM2.5
2024
365
185
178
2
0
0
0
49
71
116
PM2.5
2023
364
151
209
2
2
0
0
53
73
189
PM2.5
2022
365
184
175
4
2
0
0
49
77
174
PM2.5
2021
365
125
223
11
6
0
0
56
79
177
PM2.5
2020
366
175
182
2
4
1
2
52
75
403
PM2.5
2019
364
189
174
1
0
0
0
48
75
102
PM2.5
2018
365
139
206
8
12
0
0
55
80
186
PM2.5
2017
363
151
199
6
5
2
0
53
81
241
PM2.5
2016
366
212
154
0
0
0
0
44
71
99
PM2.5
2015
365
168
176
8
12
1
0
52
85
240
PM2.5
2014
365
209
154
2
0
0
0
41
71
133
PM2.5
2013
365
198
167
0
0
0
0
47
75
94
PM2.5
2012
364
180
184
0
0
0
0
51
75
96
PM2.5
2011
362
253
108
1
0
0
0
36
69
102
PM2.5
2010
365
171
190
4
0
0
0
52
81
107
PM2.5
2009
364
164
198
2
0
0
0
52
73
112
PM2.5
2008
366
164
199
3
0
0
0
53
79
118
PM2.5
2007
365
150
211
4
0
0
0
54
80
116
PM2.5
2006
363
139
210
13
1
0
0
56
87
160
PM2.5
2005
363
241
108
11
3
0
0
31
87
165
PM10
2004
362
282
76
4
0
0
0
30
69
114
PM10
2003
354
275
71
8
0
0
0
23
75
129
PM10
2002
362
273
78
10
1
0
0
27
74
157
PM10
2001
331
250
71
10
0
0
0
27
74
134
PM10
2000
360
283
70
7
0
0
0
26
72
128
PM10
1999
353
292
56
4
1
0
0
26
61
159
PM10
1998
361
333
28
0
0
0
0
24
45
84
PM10
1997
361
345
16
0
0
0
0
25
42
66
PM10
1996
354
316
38
0
0
0
0
24
51
80
PM10
1995
359
328
31
0
0
0
0
22
48
70
PM10
1994
353
297
56
0
0
0
0
30
56
83
PM10
1993
164
112
52
0
0
0
0
44
58
75
PM10
1992
154
100
54
0
0
0
0
42
66
93
PM10
1991
356
232
117
7
0
0
0
41
68
136
PM10
1990
352
202
138
12
0
0
0
45
72
150
PM10
1989
330
180
141
9
0
0
0
47
80
150
PM10
1988
356
172
170
13
1
0
0
51
87
155
PM10
1987
137
28
102
7
0
0
0
67
93
140
PM10
1986
56
16
34
5
1
0
0
63
102
184
PM10
1985
34
16
17
1
0
0
0
53
72
102
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
