Bad Air Days in Yolo County, California: 6 a Year

6 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 164 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (80% particle days).

The bad-air season here

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

What actually helps 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 record for Yolo

In 2000 to 2009 this county averaged 18 bad days a year; in 2016 to 2025, 6. The air has improved. The worst recent year was 2020, with 28 bad days on 366 monitored days.

Date	AQI	Main pollutant
2018-11-15	260	PM2.5
2018-11-16	241	PM2.5
2020-09-12	209	PM2.5
2020-09-11	200	PM2.5
2020-09-13	186	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Yolo County rated Good on 65% of them, less often than the 72% typical of monitored counties, and Moderate on 33%. The median day sat at AQI 44; one day in ten went above 68.

The pollutant setting the AQI on an ordinary day here:

ozone58%2,135 days
fine particles (PM2.5)41%1,511 days

Yolo County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	213	152	0	0	47	100
2024	366	238	127	1	0	45	118
2023	365	246	117	2	0	43	108
2022	365	283	80	2	0	42	101
2021	365	242	117	6	2	42	159
2020	366	215	123	28	14	45	209
2019	365	252	112	1	0	43	136
2018	365	216	135	14	9	45	260
2017	365	228	130	7	1	45	154
2016	366	246	116	4	0	43	115
2015	365	185	176	4	0	50	103
2014	365	215	149	1	0	46	101
2013	365	249	116	0	0	42	92
2012	366	201	157	8	0	49	133
2011	365	135	221	9	0	56	140
2010	362	273	86	3	0	41	109

*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.