Daily Air Quality Index summaries from EPA monitors in Bay, Michigan, 1980–2025.
70%
Good days in 2025
39
Median daily AQI, 2025
3
Days above 100 in 2025
358
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Bay, Michigan on 358 days. 252 of those days (70%) were in the Good range, and the median daily AQI was 39. That median is about the same as the typical monitored county in Michigan (median 39 across 26 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 358 days. Across the years with fuller monitoring, the share of Good days fell: an average of 82% in 1980–1982 versus 70% in 2023–2025.
Years with fewer than 120 monitored days (2007, 2008, 2009, 2011, 2012, 2013…) 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)
358
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
358
252
103
3
0
0
0
39
60
139
PM2.5
2024
363
277
86
0
0
0
0
37
56
73
PM2.5
2023
363
229
128
3
3
0
0
42
63
192
PM2.5
2022
354
287
67
0
0
0
0
32
55
85
PM2.5
2021
363
290
73
0
0
0
0
28
56
86
PM2.5
2020
365
325
40
0
0
0
0
22
52
77
PM2.5
2019
316
252
64
0
0
0
0
34
55
85
PM2.5
2018
356
266
90
0
0
0
0
37
56
85
PM2.5
2017
362
279
83
0
0
0
0
36
54
82
PM2.5
2016
355
264
91
0
0
0
0
37
56
83
PM2.5
2015
361
220
141
0
0
0
0
44
61
82
PM2.5
2014
348
205
143
0
0
0
0
45
61
96
PM2.5
2013
114
74
40
0
0
0
0
38
59
71
PM2.5
2012
119
83
36
0
0
0
0
36
61
87
PM2.5
2011
118
79
39
0
0
0
0
32
65
79
PM2.5
2010
121
86
34
1
0
0
0
33
65
103
PM2.5
2009
116
73
43
0
0
0
0
36
62
95
PM2.5
2008
114
67
47
0
0
0
0
38
67
87
PM2.5
2007
112
55
57
0
0
0
0
51
70
90
PM2.5
2006
74
39
34
1
0
0
0
43
72
123
PM2.5
2005
117
56
56
5
0
0
0
52
80
124
PM2.5
2004
120
74
46
0
0
0
0
41
76
97
PM2.5
2003
113
56
57
0
0
0
0
51
71
97
PM2.5
2002
105
50
55
0
0
0
0
53
73
98
PM2.5
2001
119
60
57
2
0
0
0
50
81
119
PM2.5
2000
39
21
18
0
0
0
0
46
77
86
PM2.5
1993
89
86
3
0
0
0
0
22
37
93
PM10
1992
250
212
36
0
1
0
1
24
57
378
PM10
1991
15
13
2
0
0
0
0
20
51
62
PM10
1990
59
54
5
0
0
0
0
23
49
62
PM10
1989
15
11
3
1
0
0
0
23
73
133
PM10
1982
204
154
37
13
0
0
0
40
84
140
Ozone
1981
278
233
21
18
6
0
0
35
93
182
Ozone
1980
362
318
22
17
4
1
0
30
61
213
Ozone
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 Michigan with a similar median AQI in 2025
