Daily Air Quality Index summaries from EPA monitors in Trinity, California, 1987–2025.
70%
Good days in 2025
31
Median daily AQI, 2025
2
Days above 100 in 2025
332
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Trinity, California on 332 days. 231 of those days (70%) were in the Good range, and the median daily AQI was 31. That median is lower than the typical monitored county in California (median 45.5 across 46 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 332 days. Across the years with fuller monitoring, the share of Good days fell: an average of 98% in 1987–2004 versus 70% in 2023–2025.
Years with fewer than 120 monitored days (2002, 2005, 2007, 2009, 2014, 2015…) 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)
332
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
332
231
99
1
1
0
0
31
65
164
PM2.5
2024
357
258
98
1
0
0
0
31
62
113
PM2.5
2023
311
209
93
6
3
0
0
31
65
174
PM2.5
2022
353
227
111
5
5
3
2
34
72
342
PM2.5
2021
328
201
81
4
19
9
14
38
168
1217
PM2.5
2020
343
215
100
12
13
3
0
36
81
211
PM2.5
2019
269
225
42
2
0
0
0
23
56
113
PM2.5
2018
291
117
155
12
5
2
0
54
82
209
PM2.5
2017
203
76
116
6
1
1
3
57
91
844
PM2.5
2016
167
83
84
0
0
0
0
51
73
97
PM2.5
2015
68
67
1
0
0
0
0
12
29
56
PM2.5
2014
112
102
10
0
0
0
0
12
44
90
PM2.5
2013
218
140
62
16
0
0
0
34
97
143
PM2.5
2012
343
156
152
28
6
1
0
54
101
243
PM2.5
2011
210
116
61
31
2
0
0
34
113
160
PM2.5
2010
122
122
0
0
0
0
0
11
26
46
PM2.5
2009
119
118
1
0
0
0
0
17
33
53
PM2.5
2008
122
101
10
2
8
1
0
19
94
203
PM2.5
2007
118
116
2
0
0
0
0
14
34
64
PM2.5
2006
126
112
9
4
1
0
0
17
52
178
PM2.5
2005
113
113
0
0
0
0
0
11
28
43
PM2.5
2004
140
136
4
0
0
0
0
14
32
66
PM2.5
2003
125
122
3
0
0
0
0
15
37
55
PM2.5
2002
112
109
3
0
0
0
0
13
36
62
PM2.5
2001
118
106
12
0
0
0
0
17
51
67
PM2.5
2000
81
81
0
0
0
0
0
14
28
46
PM10
1999
61
57
4
0
0
0
0
18
43
73
PM10
1998
58
58
0
0
0
0
0
15
31
43
PM10
1997
60
60
0
0
0
0
0
14
30
50
PM10
1996
58
56
2
0
0
0
0
15
31
59
PM10
1995
55
55
0
0
0
0
0
15
26
38
PM10
1988
39
37
2
0
0
0
0
16
35
81
PM10
1987
246
245
1
0
0
0
0
11
20
62
CO
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 California with a similar median AQI in 2025
