Daily Air Quality Index summaries from EPA monitors in Lake, Montana, 1985–2025.
98%
Good days in 2016
8
Median daily AQI, 2016
1
Days above 100 in 2016
120
Days with an AQI in 2016 (of 366)
In 2016, EPA reported a daily Air Quality Index (AQI) for Lake, Montana on 120 days. 117 of those days (98%) were in the Good range, and the median daily AQI was 8. That median is lower than the typical monitored county in Montana (median 35 across 16 counties), and lower than the typical monitored U.S. county (41). The main pollutant was fine particles (PM2.5) on 120 days. Across the years with fuller monitoring, the share of Good days rose: an average of 87% in 1988–1990 versus 95% in 2013–2016.
EPA did not report an AQI for every day of 2016 here (120 of 366 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 (2020, 2021, 2022, 2023, 2024, 2025…) 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, 2016
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)
120
100%
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
95
92
3
0
0
0
0
11
33
79
PM2.5
2024
115
108
7
0
0
0
0
9
34
77
PM2.5
2023
109
101
6
1
1
0
0
13
43
157
PM2.5
2022
97
93
3
1
0
0
0
11
37
120
PM2.5
2021
117
105
9
2
1
0
0
9
57
152
PM2.5
2020
117
111
3
1
2
0
0
7
27
183
PM2.5
2019
93
92
1
0
0
0
0
10
26
53
PM2.5
2018
118
107
8
3
0
0
0
11
48
117
PM2.5
2017
110
99
9
1
1
0
0
9
55
195
PM2.5
2016
120
117
2
1
0
0
0
8
24
130
PM2.5
2015
120
110
8
1
1
0
0
12
48
172
PM2.5
2014
117
111
6
0
0
0
0
11
26
90
PM2.5
2013
120
116
4
0
0
0
0
11
32
56
PM2.5
2012
120
112
8
0
0
0
0
9
39
87
PM2.5
2011
226
221
4
1
0
0
0
11
25
102
PM10
2010
359
352
7
0
0
0
0
13
29
62
PM10
2009
119
119
0
0
0
0
0
12
31
49
PM2.5
2008
341
336
5
0
0
0
0
13
30
84
PM10
2007
351
335
15
0
1
0
0
15
36
162
PM10
2006
349
331
18
0
0
0
0
20
44
82
PM10
2005
353
304
49
0
0
0
0
21
55
98
PM10
2004
364
304
59
1
0
0
0
22
57
120
PM10
2003
358
295
57
3
3
0
0
22
58
188
PM10
2002
357
309
48
0
0
0
0
21
56
99
PM10
2001
365
312
50
2
1
0
0
21
57
156
PM10
2000
366
286
78
1
1
0
0
24
62
161
PM10
1999
364
347
16
1
0
0
0
17
40
107
PM10
1998
365
339
25
1
0
0
0
18
44
104
PM10
1997
365
358
7
0
0
0
0
18
32
59
PM10
1996
363
346
17
0
0
0
0
18
37
85
PM10
1995
365
341
24
0
0
0
0
20
46
87
PM10
1994
315
281
34
0
0
0
0
25
53
91
PM10
1993
249
232
17
0
0
0
0
24
48
69
PM10
1992
351
300
50
1
0
0
0
28
55
133
PM10
1991
364
322
41
1
0
0
0
25
52
131
PM10
1990
360
292
65
3
0
0
0
25
57
135
PM10
1989
364
336
27
1
0
0
0
21
44
119
PM10
1988
151
134
17
0
0
0
0
26
53
67
PM10
1987
48
30
18
0
0
0
0
37
74
95
PM10
1986
48
34
12
2
0
0
0
40
61
119
PM10
1985
23
19
4
0
0
0
0
37
55
66
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.
