Bad Air Days in Washington County, Oregon: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 303 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (78% particle days).

The bad-air season here

Most bad days here fall in August and September. Averaged over 2016 to 2025.

Is a purifier worth it here?

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The worst days on file

In 2000 to 2009 this county averaged 8 bad days a year; in 2016 to 2025, 4. The air has improved. The worst recent year was 2017, with 12 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-09-15	595	PM2.5
2020-09-14	586	PM2.5
2020-09-13	584	PM2.5
2020-09-12	411	PM2.5
2020-09-16	409	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Washington County rated Good on 75% of them, close to the 72% typical of monitored counties, and Moderate on 24%. The median day sat at AQI 36; one day in ten went above 61.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)59%2,150 days
ozone38%1,401 days
nitrogen dioxide3%101 days

Washington County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	289	76	0	0	36	88
2024	366	309	55	2	0	33	105
2023	365	269	94	2	0	36	115
2022	365	244	118	3	2	37	164
2021	365	276	89	0	0	36	100
2020	366	265	91	10	8	34	595
2019	365	268	96	1	0	37	102
2018	365	260	98	7	0	36	150
2017	365	257	96	12	2	37	169
2016	366	292	74	0	0	35	84
2015	365	254	109	2	1	40	157
2014	365	281	82	2	0	35	119
2013	365	247	109	9	1	36	152
2012	366	269	96	1	0	37	109
2011	365	262	98	5	0	35	147
2010	365	278	85	2	0	33	113

*2025 may be a partial year in EPA's file.

Oregon compared

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day.