Bad Air Days in Mendocino County, California: 7 a Year

7 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 155 of 996 monitored counties, where 1 is worst.

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

The bad-air season here

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

Is a purifier worth it here?

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 under 1 bad days a year; in 2016 to 2025, 7. It has got worse. The worst recent year was 2020, with 23 bad days on 366 monitored days.

Date	AQI	Main pollutant
2020-09-09	716	PM2.5
2020-09-10	428	PM2.5
2018-11-09	376	PM2.5
2020-09-08	333	PM2.5
2018-11-10	273	PM2.5

The air on a normal day

Across 3,646 monitored days from 2016 to 2025, the air in Mendocino County rated Good on 62% of them, less often than the 72% typical of monitored counties, and Moderate on 36%. The median day sat at AQI 44; one day in ten went above 65.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)74%2,692 days
ozone26%937 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	269	96	0	0	38	81
2024	366	244	122	0	0	43	83
2023	365	144	216	5	0	53	140
2022	365	129	234	2	0	54	103
2021	365	210	147	8	2	46	185
2020	366	240	103	23	13	40	716
2019	361	290	71	0	0	36	80
2018	364	217	126	21	8	46	376
2017	365	226	131	8	1	45	203
2016	364	287	77	0	0	39	70
2015	365	213	148	4	2	46	164
2014	365	224	141	0	0	44	96
2013	331	245	86	0	0	29	82
2012	357	277	80	0	0	38	80
2011	361	157	204	0	0	52	82
2010	361	250	111	0	0	42	75

*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.