Bad Air Days in Mariposa County, California: 25 a Year

25 bad air days a year

Among the worst tenth of monitored US counties. Ranked 24 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (62% particle days).

When the bad days come in Mariposa

Most bad days here fall in August and September and July and October. 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 68 bad days a year; in 2016 to 2025, 25. The air has improved. The worst recent year was 2020, with 48 bad days on 366 monitored days.

Date	AQI	Main pollutant
2020-09-17	1073	PM2.5
2020-09-15	670	PM2.5
2020-09-16	568	PM2.5
2020-09-14	442	PM2.5
2020-09-10	376	PM2.5

The air on a normal day

Across 3,628 monitored days from 2016 to 2025, the air in Mariposa County rated Good on 56% of them, less often than the 72% typical of monitored counties, and Moderate on 37%. The median day sat at AQI 49; one day in ten went above 90.

The pollutant setting the AQI on an ordinary day here:

ozone74%2,697 days
fine particles (PM2.5)25%913 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	340	274	66	0	0	44	90
2024	366	255	105	6	0	45	133
2023	365	217	131	17	7	47	264
2022	365	211	125	29	21	48	198
2021	365	193	140	32	7	49	162
2020	366	195	123	48	21	49	1073
2019	365	192	159	14	5	50	185
2018	365	156	169	40	30	53	352
2017	365	146	179	40	9	54	189
2016	366	197	149	20	1	48	159
2015	365	220	134	11	5	47	240
2014	365	160	178	27	1	53	153
2013	365	128	215	22	1	56	255
2012	366	132	188	46	2	58	166
2011	365	173	164	28	0	51	150
2010	365	152	187	26	3	52	187

*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.