Plymouth County, Massachusetts air quality history
Daily Air Quality Index summaries from EPA monitors in Plymouth, Massachusetts, 1987–2025.
84%
Good days in 2025
37
Median daily AQI, 2025
1
Days above 100 in 2025
365
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Plymouth, Massachusetts on 365 days. 308 of those days (84%) were in the Good range, and the median daily AQI was 37. That median is about the same as the typical monitored county in Massachusetts (median 38 across 13 counties), and lower than the typical monitored U.S. county (40). The main pollutant was ozone on 251 days. Across the years with fuller monitoring, the share of Good days rose: an average of 75% in 1987–1989 versus 83% in 2023–2025.
Years with fewer than 120 monitored days (2001, 2002, 2003, 2005, 2007, 2009…) 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
251
69%
Fine particles (PM2.5)
114
31%
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
365
308
56
1
0
0
0
37
54
108
Ozone
2024
366
319
47
0
0
0
0
37
53
97
Ozone
2023
365
278
83
4
0
0
0
40
58
114
Ozone
2022
360
235
125
0
0
0
0
45
57
93
PM2.5
2021
365
257
106
2
0
0
0
39
58
112
PM2.5
2020
366
267
99
0
0
0
0
42
56
80
PM2.5
2019
363
289
74
0
0
0
0
40
54
90
Ozone
2018
364
279
82
3
0
0
0
39
56
126
Ozone
2017
365
281
80
4
0
0
0
39
55
147
Ozone
2016
366
298
67
1
0
0
0
39
56
105
Ozone
2015
363
246
116
1
0
0
0
40
61
126
PM2.5
2014
363
229
134
0
0
0
0
44
60
93
PM2.5
2013
192
139
53
0
0
0
0
37
59
75
PM2.5
2012
121
82
39
0
0
0
0
39
59
78
PM2.5
2011
121
73
48
0
0
0
0
40
62
74
PM2.5
2010
122
85
37
0
0
0
0
39
60
87
PM2.5
2009
118
77
41
0
0
0
0
41
62
79
PM2.5
2008
121
68
53
0
0
0
0
47
63
90
PM2.5
2007
119
67
51
1
0
0
0
42
68
103
PM2.5
2006
120
72
48
0
0
0
0
43
69
99
PM2.5
2005
118
55
62
1
0
0
0
52
70
105
PM2.5
2004
124
66
58
0
0
0
0
47
69
99
PM2.5
2003
115
65
48
2
0
0
0
47
71
115
PM2.5
2002
111
54
53
2
2
0
0
51
73
158
PM2.5
2001
106
37
69
0
0
0
0
54
83
99
PM2.5
2000
96
44
52
0
0
0
0
53
72
86
PM2.5
1999
104
49
52
3
0
0
0
52
77
121
PM2.5
1997
179
170
5
3
1
0
0
23
42
156
Ozone
1996
211
201
9
1
0
0
0
28
45
108
Ozone
1995
191
167
17
6
1
0
0
31
61
159
Ozone
1994
211
180
22
7
2
0
0
33
64
156
Ozone
1993
211
174
26
8
3
0
0
32
74
172
Ozone
1992
206
163
34
5
4
0
0
38
77
187
Ozone
1991
202
159
27
8
8
0
0
38
93
200
Ozone
1990
209
194
13
2
0
0
0
31
49
143
Ozone
1989
203
137
45
12
7
2
0
42
101
210
Ozone
1988
198
150
22
10
10
6
0
36
136
232
Ozone
1987
163
132
16
12
3
0
0
36
97
161
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 Massachusetts with a similar median AQI in 2025
