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

2 bad air days a year

About the middle of monitored US counties. Ranked 474 of 996 monitored counties, where 1 is worst.

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

Which months to watch

Most bad days here fall in September and August and October. 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 worst days on file

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 357 monitored days.

Date	AQI	Main pollutant
2020-09-14	397	PM2.5
2020-09-13	329	PM2.5
2020-09-12	305	PM2.5
2020-09-11	303	PM2.5
2020-09-15	293	PM2.5

An ordinary day in Mason

Across 3,632 monitored days from 2016 to 2025, the air in Mason County rated Good on 87% of them, more often than the 72% typical of monitored counties, and Moderate on 12%. The median day sat at AQI 24; one day in ten went above 51.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)100%3,632 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	330	34	1	1	23	159
2024	366	341	25	0	0	21	73
2023	363	313	49	1	0	24	106
2022	365	302	60	3	0	23	125
2021	361	339	22	0	0	22	70
2020	357	312	36	9	7	23	397
2019	361	305	56	0	0	28	75
2018	365	304	55	6	2	28	168
2017	363	301	60	2	1	25	156
2016	366	327	39	0	0	27	72
2015	362	316	45	1	0	29	102
2014	365	319	46	0	0	27	80
2013	336	233	103	0	0	33	78
2012	366	278	87	1	0	33	116
2011	360	272	88	0	0	33	83
2010	365	316	49	0	0	28	76

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

Elsewhere in Washington

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.