Bad Air Days in Merced County, California: 24 a Year

24 bad air days a year

Among the worst tenth of monitored US counties. Ranked 25 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (39% particle days).

The bad-air season here

Most bad days here fall in August and July and September. 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.
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 77 bad days a year; in 2016 to 2025, 24. The air has improved. The worst recent year was 2018, with 41 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-08-20	194	PM2.5
2020-09-13	194	PM2.5
2020-08-22	183	PM2.5
2020-08-23	181	PM2.5
2018-11-16	178	PM2.5

The air on a normal day

Across 3,652 monitored days from 2016 to 2025, the air in Merced County rated Good on 40% of them, less often than the 72% typical of monitored counties, and Moderate on 53%. The median day sat at AQI 54; one day in ten went above 93.

The pollutant setting the AQI on an ordinary day here:

ozone53%1,946 days
fine particles (PM2.5)44%1,625 days
coarse dust (PM10)2%80 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	186	179	0	0	50	100
2024	366	168	163	35	1	52	151
2023	365	171	179	15	0	51	129
2022	365	140	212	13	0	56	143
2021	365	155	177	33	2	54	166
2020	366	131	203	32	15	57	194
2019	365	176	182	7	0	51	119
2018	365	110	214	41	9	56	178
2017	364	127	203	34	3	58	161
2016	366	115	216	35	2	58	151
2015	365	107	213	45	4	61	159
2014	364	120	186	58	2	61	157
2013	364	105	211	48	5	62	165
2012	366	136	196	34	1	57	151
2011	362	70	232	60	2	67	156
2010	365	106	220	39	6	59	177

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