Bad Air Days in Sonoma County, California: 3 a Year

3 bad air days a year

About the middle of monitored US counties. Ranked 411 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (96% particle days).

Which months to watch

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

What actually helps 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.

Better or worse than it was

In 2000 to 2009 this county averaged 2 bad days a year; in 2016 to 2025, 3. Not much has changed. The worst recent year was 2018, with 13 bad days on 365 monitored days.

Date	AQI	Main pollutant
2018-11-09	250	PM2.5
2018-11-10	233	PM2.5
2020-09-10	199	PM2.5
2020-09-11	199	PM2.5
2018-11-13	179	PM2.5

Most days, not the worst ones

Across 3,653 monitored days from 2016 to 2025, the air in Sonoma County rated Good on 85% of them, more often than the 72% typical of monitored counties, and Moderate on 14%. The median day sat at AQI 32; one day in ten went above 50.

The pollutant setting the AQI on an ordinary day here:

ozone46%1,681 days
fine particles (PM2.5)40%1,449 days
coarse dust (PM10)14%517 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	365	0	0	0	9	39
2024	366	348	18	0	0	19	72
2023	365	323	40	2	0	33	117
2022	365	286	79	0	0	36	82
2021	365	283	82	0	0	37	89
2020	366	290	69	7	6	36	199
2019	365	318	47	0	0	36	87
2018	365	304	48	13	10	39	250
2017	365	267	93	5	3	41	169
2016	366	323	43	0	0	34	87
2015	365	293	72	0	0	37	90
2014	365	257	108	0	0	42	83
2013	365	240	125	0	0	41	86
2012	366	240	126	0	0	41	82
2011	365	225	140	0	0	40	96
2010	365	268	97	0	0	36	93

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

Elsewhere in California

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.