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

3 bad air days a year

About the middle of monitored US counties. Ranked 337 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (97% particle days).

When the bad days come in Marin

Most bad days here fall in November 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 0 bad days a year; in 2016 to 2025, 3. It has got worse. The worst recent year was 2018, with 13 bad days on 365 monitored days.

Date	AQI	Main pollutant
2018-11-16	243	PM2.5
2020-09-11	231	PM2.5
2018-11-17	196	PM2.5
2018-11-15	191	PM2.5
2020-09-12	178	PM2.5

Most days, not the worst ones

Across 3,370 monitored days from 2016 to 2025, the air in Marin County rated Good on 76% of them, more often than the 72% typical of monitored counties, and Moderate on 23%. The median day sat at AQI 37; one day in ten went above 57.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)64%2,154 days
ozone35%1,166 days
nitrogen dioxide1%49 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	95	87	8	0	0	21	66
2024	357	308	49	0	0	28	86
2023	363	315	48	0	0	34	99
2022	365	275	90	0	0	37	91
2021	365	267	98	0	0	38	88
2020	366	272	85	9	5	36	231
2019	363	291	71	1	0	37	133
2018	365	235	117	13	10	43	243
2017	365	224	133	8	3	44	164
2016	366	296	70	0	0	36	90
2015	365	226	137	2	0	43	103
2014	365	130	234	1	0	53	107
2013	365	157	206	2	0	52	124
2012	366	254	112	0	0	42	83
2011	365	183	181	1	0	50	117
2010	365	191	170	4	0	49	128

*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.