Bad Air Days in King County, Washington: 8 a Year

8 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 126 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (61% particle days).

The bad-air season here

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

What to buy, and what not to

Particles (wildfire smoke, dust) cause enough bad days here to matter. A HEPA purifier sized to your bedroom removes them; run it with windows shut on bad days.
Upgrade the furnace or AC filter to MERV 13 so the whole house is filtered, not just one room.

The worst days on file

In 2000 to 2009 this county averaged 14 bad days a year; in 2016 to 2025, 8. The air has improved. The worst recent year was 2022, with 19 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-09-14	299	PM2.5
2020-09-15	268	PM2.5
2020-09-13	263	PM2.5
2020-09-12	256	PM2.5
2020-09-11	248	PM2.5

An ordinary day in King

Across 3,653 monitored days from 2016 to 2025, the air in King County rated Good on 62% of them, less often than the 72% typical of monitored counties, and Moderate on 35%. The median day sat at AQI 45; one day in ten went above 66.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)71%2,599 days
ozone24%866 days
nitrogen dioxide5%188 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	245	120	0	0	43	97
2024	366	255	108	3	0	41	119
2023	365	214	145	6	1	46	165
2022	365	216	130	19	5	46	237
2021	365	248	112	5	2	43	177
2020	366	248	106	12	9	43	299
2019	365	208	157	0	0	48	90
2018	365	212	139	14	6	46	210
2017	365	194	153	18	8	48	227
2016	366	240	126	0	0	44	100
2015	364	131	224	9	0	53	129
2014	365	188	174	3	0	49	150
2013	365	113	246	6	0	55	149
2012	366	181	178	7	0	51	140
2011	365	205	160	0	0	48	99
2010	365	221	141	3	0	45	122

*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.