Bad Air Days in Multnomah County, Oregon: 3 a Year

3 bad air days a year

About the middle of monitored US counties. Ranked 354 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (81% particle days).

The bad-air season here

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

What to buy, and what not to

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 record for Multnomah

In 2000 to 2009 this county averaged 5 bad days a year; in 2016 to 2025, 3. Not much has changed. The worst recent year was 2018, with 9 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-09-13	779	PM2.5
2020-09-14	670	PM2.5
2020-09-12	526	PM2.5
2020-09-15	517	PM2.5
2020-09-16	413	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Multnomah County rated Good on 78% of them, more often than the 72% typical of monitored counties, and Moderate on 21%. The median day sat at AQI 36; one day in ten went above 59.

The pollutant setting the AQI on an ordinary day here:

ozone51%1,858 days
fine particles (PM2.5)48%1,750 days
nitrogen dioxide1%45 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	283	82	0	0	37	87
2024	366	311	55	0	0	35	93
2023	365	267	97	1	0	38	117
2022	365	266	94	5	2	36	165
2021	365	294	70	1	0	36	105
2020	366	275	82	9	8	36	779
2019	365	283	82	0	0	36	91
2018	365	272	84	9	1	37	152
2017	365	278	81	6	5	38	166
2016	366	324	41	1	0	33	108
2015	365	277	86	2	2	39	167
2014	365	290	74	1	0	36	115
2013	365	245	115	5	0	40	116
2012	366	263	102	1	0	38	106
2011	365	260	99	6	0	37	127
2010	365	283	82	0	0	36	99

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

Nearby counties on file

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.