Bad Air Days in Tuolumne County, California: 12 a Year

12 bad air days a year

Among the worst tenth of monitored US counties. Ranked 68 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (0% particle days).

When the bad days come in Tuolumne

Most bad days here fall in August and July and June. Averaged over 2016 to 2025.

Is a purifier worth it here?

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.

The worst days on file

In 2000 to 2009 this county averaged 43 bad days a year; in 2016 to 2025, 12. The air has improved. The worst recent year was 2016, with 45 bad days on 354 monitored days.

Date	AQI	Main pollutant
2016-07-25	164	Ozone
2016-07-01	159	Ozone
2016-07-27	159	Ozone
2016-07-28	156	Ozone
2016-07-15	154	Ozone

The air on a normal day

Across 3,367 monitored days from 2016 to 2025, the air in Tuolumne County rated Good on 77% of them, more often than the 72% typical of monitored counties, and Moderate on 19%. The median day sat at AQI 42; one day in ten went above 71.

The pollutant setting the AQI on an ordinary day here:

ozone100%3,367 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	338	294	43	1	0	39	105
2024	354	286	64	4	0	40	115
2023	290	237	51	2	0	43	112
2022	250	237	12	1	0	39	108
2021	360	295	63	2	0	41	136
2020	353	277	71	5	0	41	143
2019	358	273	83	2	0	43	108
2018	348	254	73	21	1	43	154
2017	362	231	106	25	0	44	143
2016	354	223	86	45	7	44	164
2015	364	264	89	11	0	43	126
2014	334	214	104	16	0	46	133
2013	364	284	78	2	0	43	119
2012	364	246	105	13	0	42	136
2011	355	259	84	12	0	41	126
2010	365	274	83	8	0	41	147

*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.