Daily Air Quality Index summaries from EPA monitors in Cass, North Dakota, 1985–2025.
76%
Good days in 2025
37
Median daily AQI, 2025
11
Days above 100 in 2025
361
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Cass, North Dakota on 361 days. 275 of those days (76%) were in the Good range, and the median daily AQI was 37. That median is about the same as the typical monitored county in North Dakota (median 38 across 9 counties), and lower than the typical monitored U.S. county (40). The main pollutant was ozone on 206 days. Across the years with fuller monitoring, the share of Good days fell: an average of 99% in 1986–1992 versus 68% in 2023–2025.
Years with fewer than 120 monitored days (1985, 1987, 1988, 1989, 1990) 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
206
57%
Fine particles (PM2.5)
155
43%
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
361
275
75
4
7
0
0
37
71
192
Ozone
2024
366
288
76
1
1
0
0
38
58
158
PM2.5
2023
358
180
166
10
2
0
0
50
84
175
PM2.5
2022
365
285
79
1
0
0
0
40
59
111
PM2.5
2021
363
227
119
8
8
1
0
42
74
211
PM2.5
2020
363
303
60
0
0
0
0
37
53
80
Ozone
2019
363
301
60
1
1
0
0
36
54
157
Ozone
2018
360
271
88
1
0
0
0
39
58
121
Ozone
2017
362
232
130
0
0
0
0
44
57
79
PM2.5
2016
366
263
103
0
0
0
0
37
57
87
Ozone
2015
365
235
127
3
0
0
0
46
58
140
PM2.5
2014
365
285
79
1
0
0
0
37
57
113
Ozone
2013
355
272
83
0
0
0
0
42
59
89
PM2.5
2012
366
241
125
0
0
0
0
43
62
92
PM2.5
2011
350
253
97
0
0
0
0
41
60
78
PM2.5
2010
365
225
139
1
0
0
0
44
65
124
PM2.5
2009
365
315
48
2
0
0
0
34
53
102
Ozone
2008
366
312
53
0
1
0
0
36
53
161
Ozone
2007
364
310
51
1
2
0
0
34
55
171
Ozone
2006
364
296
67
1
0
0
0
38
58
118
Ozone
2005
362
305
56
0
1
0
0
36
55
155
Ozone
2004
366
321
45
0
0
0
0
35
54
88
Ozone
2003
365
291
73
1
0
0
0
38
59
131
Ozone
2002
358
308
50
0
0
0
0
34
54
98
Ozone
2001
365
301
63
1
0
0
0
37
58
104
Ozone
2000
366
321
44
1
0
0
0
34
53
101
Ozone
1999
361
298
63
0
0
0
0
36
58
100
Ozone
1998
344
311
33
0
0
0
0
32
50
84
Ozone
1997
365
338
26
1
0
0
0
34
49
105
Ozone
1996
366
337
28
1
0
0
0
36
49
108
Ozone
1995
161
160
1
0
0
0
0
23
33
51
Ozone
1994
172
169
3
0
0
0
0
15
40
53
CO
1993
350
346
4
0
0
0
0
22
40
71
Ozone
1992
190
189
1
0
0
0
0
14
27
52
CO
1991
187
186
1
0
0
0
0
13
24
53
CO
1990
71
68
2
1
0
0
0
15
31
125
PM10
1989
61
61
0
0
0
0
0
16
34
46
PM10
1988
60
59
1
0
0
0
0
16
32
51
PM10
1987
61
60
1
0
0
0
0
17
29
56
PM10
1986
178
175
3
0
0
0
0
24
41
88
PM10
1985
62
62
0
0
0
0
0
24
38
50
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 North Dakota with a similar median AQI in 2025
