Bad Air Days in Stanislaus County, California: 35 a Year

35 bad air days a year

Among the worst tenth of monitored US counties. Ranked 13 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (41% particle days).

Which months to watch

Most bad days here fall in July and August and December and September. 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.
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.
Sign up for EnviroFlash ozone alerts for this area during the worst months.

Better or worse than it was

In 2000 to 2009 this county averaged 58 bad days a year; in 2016 to 2025, 35. The air has improved. The worst recent year was 2017, with 63 bad days on 365 monitored days.

Date	AQI	Main pollutant
2018-11-16	265	PM2.5
2018-11-17	221	PM2.5
2018-11-18	218	PM2.5
2018-11-15	207	PM2.5
2020-09-13	195	PM2.5

Most days, not the worst ones

Across 3,653 monitored days from 2016 to 2025, the air in Stanislaus County rated Good on 37% of them, less often than the 72% typical of monitored counties, and Moderate on 54%. The median day sat at AQI 57; one day in ten went above 99.

The pollutant setting the AQI on an ordinary day here:

ozone49%1,806 days
fine particles (PM2.5)48%1,759 days
coarse dust (PM10)2%85 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	182	180	3	0	51	115
2024	366	166	162	38	0	53	150
2023	365	166	176	23	0	54	150
2022	365	108	234	23	1	57	166
2021	365	84	234	47	5	61	172
2020	366	131	198	37	16	60	195
2019	365	168	174	23	1	52	177
2018	365	109	209	47	15	59	265
2017	365	127	175	63	8	61	185
2016	366	102	221	43	3	59	164
2015	365	92	225	48	5	62	187
2014	365	112	194	59	6	60	164
2013	365	117	183	65	11	64	170
2012	366	95	192	79	12	64	201
2011	365	82	212	71	14	67	169
2010	365	125	202	38	6	58	177

*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, and on why a purifier does not touch ozone in smoke vs ozone.