Daily Air Quality Index summaries from EPA monitors in Whatcom, Washington, 1986–2025.
94%
Good days in 2025
31
Median daily AQI, 2025
1
Days above 100 in 2025
364
Days with an AQI in 2025 (of 365)
In 2025, EPA reported a daily Air Quality Index (AQI) for Whatcom, Washington on 364 days. 342 of those days (94%) were in the Good range, and the median daily AQI was 31. That median is higher than the typical monitored county in Washington (median 28 across 30 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 278 days. Across the years with fuller monitoring, the share of Good days held roughly steady: an average of 92% in 1988–1990 versus 93% in 2023–2025.
Years with fewer than 120 monitored days (1986, 1987) 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)
278
76%
Ozone
86
24%
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
364
342
21
0
1
0
0
31
46
152
PM2.5
2024
361
335
26
0
0
0
0
33
47
82
PM2.5
2023
360
327
33
0
0
0
0
31
49
94
PM2.5
2022
343
294
40
3
5
1
0
32
56
243
Ozone
2021
332
323
7
2
0
0
0
28
42
129
PM2.5
2020
366
341
16
2
4
3
0
24
44
237
PM2.5
2019
363
334
29
0
0
0
0
29
47
66
PM2.5
2018
361
319
33
6
3
0
0
29
52
193
PM2.5
2017
348
300
41
4
3
0
0
31
54
187
PM2.5
2016
349
322
27
0
0
0
0
28
46
64
PM2.5
2015
331
279
52
0
0
0
0
34
53
91
PM2.5
2014
339
236
103
0
0
0
0
38
57
81
PM2.5
2013
348
274
74
0
0
0
0
33
56
78
PM2.5
2012
351
330
21
0
0
0
0
26
42
97
PM2.5
2011
354
326
28
0
0
0
0
30
47
66
PM2.5
2010
355
311
44
0
0
0
0
32
52
75
PM2.5
2009
365
288
77
0
0
0
0
33
57
78
PM2.5
2008
366
315
51
0
0
0
0
31
53
69
PM2.5
2007
347
270
77
0
0
0
0
32
57
83
PM2.5
2006
362
305
57
0
0
0
0
34
54
76
PM2.5
2005
365
293
72
0
0
0
0
33
56
80
PM2.5
2004
363
323
40
0
0
0
0
31
51
97
PM2.5
2003
364
280
84
0
0
0
0
36
57
80
PM2.5
2002
282
205
77
0
0
0
0
36
62
90
PM2.5
2001
229
195
34
0
0
0
0
33
53
78
Ozone
2000
235
201
34
0
0
0
0
31
56
82
Ozone
1999
217
187
30
0
0
0
0
33
53
81
Ozone
1998
237
227
10
0
0
0
0
30
42
90
Ozone
1997
252
231
21
0
0
0
0
31
46
85
Ozone
1996
248
238
10
0
0
0
0
31
42
100
Ozone
1995
235
231
4
0
0
0
0
31
42
100
Ozone
1994
232
226
5
1
0
0
0
31
44
112
Ozone
1993
237
229
8
0
0
0
0
31
43
87
Ozone
1992
212
207
5
0
0
0
0
34
43
74
Ozone
1991
240
228
12
0
0
0
0
31
44
84
Ozone
1990
320
299
21
0
0
0
0
30
48
100
Ozone
1989
320
286
34
0
0
0
0
27
51
99
CO
1988
358
330
28
0
0
0
0
24
49
89
CO
1987
84
52
32
0
0
0
0
43
64
81
PM10
1986
44
28
16
0
0
0
0
44
63
74
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 Washington with a similar median AQI in 2025
