Daily Air Quality Index summaries from EPA monitors in Washington, Oregon, 1999–2025.
79%
Good days in 2025
36
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 Washington, Oregon on 365 days. 289 of those days (79%) were in the Good range, and the median daily AQI was 36. That median is higher than the typical monitored county in Oregon (median 29 across 21 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 204 days. Across the years with fuller monitoring, the share of Good days rose: an average of 67% in 1999–2001 versus 79% in 2023–2025.
Years with fewer than 120 monitored days (2003) 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)
204
56%
Ozone
154
42%
Nitrogen dioxide (NO₂)
7
2%
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
289
76
0
0
0
0
36
57
88
PM2.5
2024
366
309
55
2
0
0
0
33
53
105
Ozone
2023
365
269
94
2
0
0
0
36
59
115
PM2.5
2022
365
244
118
1
2
0
0
37
67
164
PM2.5
2021
365
276
89
0
0
0
0
36
58
100
PM2.5
2020
366
265
91
2
0
2
6
34
61
595
PM2.5
2019
365
268
96
1
0
0
0
37
62
102
PM2.5
2018
365
260
98
7
0
0
0
36
64
150
PM2.5
2017
365
257
96
10
2
0
0
37
71
169
PM2.5
2016
366
292
74
0
0
0
0
35
58
84
PM2.5
2015
365
254
109
1
1
0
0
40
61
157
PM2.5
2014
365
281
82
2
0
0
0
35
59
119
PM2.5
2013
365
247
109
8
1
0
0
36
70
152
PM2.5
2012
366
269
96
1
0
0
0
37
62
109
PM2.5
2011
365
262
98
5
0
0
0
35
71
147
PM2.5
2010
365
278
85
2
0
0
0
33
60
113
PM2.5
2009
365
224
135
6
0
0
0
40
77
143
PM2.5
2008
366
247
114
5
0
0
0
33
71
137
PM2.5
2007
365
252
101
11
1
0
0
30
76
158
PM2.5
2006
364
273
86
5
0
0
0
28
68
148
PM2.5
2005
365
245
110
7
3
0
0
33
74
155
PM2.5
2004
303
185
110
7
1
0
0
42
80
163
PM2.5
2003
116
78
37
1
0
0
0
34
67
107
PM2.5
2002
122
78
40
3
1
0
0
38
74
158
PM2.5
2001
123
80
42
1
0
0
0
38
70
128
PM2.5
2000
123
74
47
2
0
0
0
40
76
117
PM2.5
1999
269
207
60
2
0
0
0
29
59
138
PM2.5
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 Oregon with a similar median AQI in 2025
