Bad Air Days in San Joaquin County, California: 18 a Year

18 bad air days a year

Among the worst tenth of monitored US counties. Ranked 38 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (76% particle days).

When the bad days come in San Joaquin

Most bad days here fall in December and August 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.
Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.

Better or worse than it was

In 2000 to 2009 this county averaged 27 bad days a year; in 2016 to 2025, 18. The air has improved. The worst recent year was 2017, with 33 bad days on 365 monitored days.

Date	AQI	Main pollutant
2018-11-16	365	PM2.5
2018-11-17	253	PM2.5
2018-11-11	226	PM2.5
2018-11-15	217	PM2.5
2020-09-11	215	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in San Joaquin County rated Good on 44% of them, less often than the 72% typical of monitored counties, and Moderate on 51%. The median day sat at AQI 52; one day in ten went above 87.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)58%2,125 days
ozone39%1,414 days
coarse dust (PM10)3%110 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	219	146	0	0	46	93
2024	366	178	177	11	0	51	140
2023	365	208	148	9	0	47	119
2022	365	192	164	9	1	48	208
2021	365	140	209	16	1	55	153
2020	366	162	173	31	15	54	215
2019	365	202	151	12	1	47	190
2018	365	91	241	33	13	59	365
2017	365	141	191	33	0	55	146
2016	366	62	278	26	1	60	166
2015	365	83	246	36	4	60	164
2014	365	112	219	34	1	58	152
2013	365	43	289	33	9	62	159
2012	366	88	236	42	4	60	182
2011	365	128	194	43	7	57	158
2010	365	168	184	13	1	52	174

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

California compared

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.