Bad Air Days in Orange County, California: 19 a Year

19 bad air days a year

Among the worst tenth of monitored US counties. Ranked 35 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (28% particle days).

The bad-air season here

Most bad days here fall in September 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.
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 45 bad days a year; in 2016 to 2025, 19. The air has improved. The worst recent year was 2020, with 41 bad days on 366 monitored days.

Date	AQI	Main pollutant
2020-09-05	218	Ozone
2020-09-06	208	Ozone
2020-08-18	195	Ozone
2019-10-06	172	Ozone
2024-09-07	172	Ozone

An ordinary day in Orange

Across 3,653 monitored days from 2016 to 2025, the air in Orange County rated Good on 34% of them, less often than the 72% typical of monitored counties, and Moderate on 61%. The median day sat at AQI 55; one day in ten went above 86.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)59%2,170 days
ozone37%1,343 days
nitrogen dioxide3%106 days
coarse dust (PM10)1%34 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	138	214	13	2	53	159
2024	366	119	231	16	3	54	172
2023	365	160	195	10	1	52	166
2022	365	143	215	7	2	53	156
2021	365	109	238	18	0	55	148
2020	366	118	207	41	13	56	218
2019	365	157	192	16	1	53	172
2018	365	114	233	18	3	54	160
2017	365	86	247	32	2	59	151
2016	366	87	264	15	2	58	169
2015	365	57	287	21	2	61	156
2014	365	35	309	21	2	63	156
2013	365	79	274	12	0	59	143
2012	366	75	279	12	0	60	147
2011	365	43	308	14	0	62	143
2010	365	65	292	8	1	61	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.