Bad Air Days in Cowlitz County, Washington: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 544 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (100% particle days).

When the bad days come in Cowlitz

Most bad days here fall in September and August. 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 record for Cowlitz

In 2000 to 2009 this county averaged under 1 bad days a year; in 2016 to 2025, 2. Not much has changed. The worst recent year was 2020, with 9 bad days on 366 monitored days.

Date	AQI	Main pollutant
2020-09-13	282	PM2.5
2020-09-14	242	PM2.5
2020-09-15	231	PM2.5
2020-09-12	216	PM2.5
2020-09-16	210	PM2.5

Most days, not the worst ones

Across 3,528 monitored days from 2016 to 2025, the air in Cowlitz County rated Good on 89% of them, more often than the 72% typical of monitored counties, and Moderate on 10%. The median day sat at AQI 23; one day in ten went above 50.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)100%3,528 days

Cowlitz County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	250	226	24	0	0	22	72
2024	366	349	17	0	0	22	67
2023	365	323	42	0	0	22	89
2022	363	316	45	2	0	21	144
2021	365	355	10	0	0	21	64
2020	366	331	26	9	8	21	282
2019	360	301	59	0	0	24	79
2018	365	310	51	4	2	27	153
2017	365	301	62	2	1	26	156
2016	363	331	32	0	0	23	77
2015	362	308	53	1	0	27	109
2014	323	274	49	0	0	28	86
2013	365	283	82	0	0	29	84
2012	351	304	47	0	0	26	74
2011	363	311	52	0	0	18	80
2010	364	338	26	0	0	21	68

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

Washington 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.