Bad Air Days in San Diego County, California: 38 a Year

38 bad air days a year

Among the worst tenth of monitored US counties. Ranked 11 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (14% particle days).

Which months to watch

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

What to buy, and what not to

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.

The record for San Diego

In 2000 to 2009 this county averaged 71 bad days a year; in 2016 to 2025, 38. The air has improved. The worst recent year was 2017, with 62 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-09-05	192	Ozone
2020-09-14	180	PM2.5
2020-10-05	177	PM2.5
2017-07-07	174	Ozone
2020-10-06	173	PM2.5

An ordinary day in San Diego

Across 3,653 monitored days from 2016 to 2025, the air in San Diego County rated Good on 12% of them, less often than the 72% typical of monitored counties, and Moderate on 78%. The median day sat at AQI 68; one day in ten went above 101.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)52%1,915 days
ozone45%1,650 days
coarse dust (PM10)2%83 days

San Diego County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	64	268	33	7	67	161
2024	366	47	267	52	4	71	161
2023	365	61	260	44	0	67	150
2022	365	42	297	26	1	63	156
2021	365	52	297	16	0	67	133
2020	366	33	284	49	14	74	192
2019	365	38	302	25	2	66	169
2018	365	28	302	35	0	67	143
2017	365	41	262	62	6	68	174
2016	366	30	294	42	2	68	164
2015	365	46	278	41	1	65	156
2014	365	48	278	39	2	65	170
2013	365	23	305	37	2	67	630
2012	366	26	304	36	1	67	170
2011	365	21	310	34	4	66	169
2010	365	35	305	25	4	64	192

*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.