Daily Air Quality Index summaries from EPA monitors in Humboldt, California, 1986–2025.
78%
Good days in 2025
38
Median daily AQI, 2025
0
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 Humboldt, California on 365 days. 285 of those days (78%) were in the Good range, and the median daily AQI was 38. That median is lower than the typical monitored county in California (median 45.5 across 46 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 199 days. Across the years with fuller monitoring, the share of Good days fell: an average of 98% in 1989–1991 versus 76% in 2023–2025.
Years with fewer than 120 monitored days (1999, 2000, 2003, 2004, 2005, 2006…) 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)
199
55%
Ozone
148
41%
Coarse particles (PM10)
18
5%
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
285
80
0
0
0
0
38
55
87
PM2.5
2024
366
266
100
0
0
0
0
38
58
86
PM2.5
2023
365
284
70
6
1
4
0
32
60
299
Ozone
2022
365
340
25
0
0
0
0
24
42
74
Ozone
2021
365
333
32
0
0
0
0
29
48
64
Ozone
2020
366
318
44
3
1
0
0
29
53
180
Ozone
2019
363
340
23
0
0
0
0
28
44
69
Ozone
2018
365
301
58
5
1
0
0
35
58
167
Ozone
2017
365
298
59
5
3
0
0
37
61
180
Ozone
2016
366
288
78
0
0
0
0
38
57
77
Ozone
2015
365
241
124
0
0
0
0
40
59
91
PM2.5
2014
365
152
207
5
1
0
0
53
69
169
PM2.5
2013
365
226
138
1
0
0
0
44
62
108
PM2.5
2012
366
252
114
0
0
0
0
39
58
76
PM2.5
2011
365
277
88
0
0
0
0
38
60
80
Ozone
2010
365
348
17
0
0
0
0
30
41
75
Ozone
2009
362
333
29
0
0
0
0
28
46
90
Ozone
2008
364
332
32
0
0
0
0
29
47
88
Ozone
2007
339
311
28
0
0
0
0
29
47
97
Ozone
2006
101
71
30
0
0
0
0
32
63
99
PM2.5
2005
37
26
11
0
0
0
0
32
69
93
PM2.5
2004
64
44
20
0
0
0
0
32
62
82
PM2.5
2003
64
50
13
1
0
0
0
34
59
102
PM2.5
2002
211
183
26
2
0
0
0
9
52
123
PM10
2001
323
293
30
0
0
0
0
6
49
95
PM10
2000
77
55
22
0
0
0
0
36
64
79
PM2.5
1999
68
45
22
1
0
0
0
36
63
104
PM2.5
1998
58
58
0
0
0
0
0
12
23
40
PM10
1997
60
59
1
0
0
0
0
18
33
51
PM10
1996
68
66
2
0
0
0
0
15
31
67
PM10
1995
57
56
1
0
0
0
0
18
30
57
PM10
1994
61
59
2
0
0
0
0
20
35
62
PM10
1992
117
117
0
0
0
0
0
19
28
37
Ozone
1991
353
353
0
0
0
0
0
15
28
39
Ozone
1990
184
179
5
0
0
0
0
14
33
65
CO
1989
344
337
7
0
0
0
0
11
32
69
CO
1988
64
52
12
0
0
0
0
27
59
72
PM10
1987
60
51
9
0
0
0
0
30
56
72
PM10
1986
10
7
3
0
0
0
0
42
72
72
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 California with a similar median AQI in 2025
