Bad Air Days in San Luis Obispo County, California: 9 a Year

9 bad air days a year

Among the worst tenth of monitored US counties. Ranked 90 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (17% particle days).

The bad-air season here

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

What to buy, and what not to

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.

The record for San Luis Obispo

In 2000 to 2009 this county averaged 25 bad days a year; in 2016 to 2025, 9. The air has improved. The worst recent year was 2020, with 31 bad days on 366 monitored days.

Date	AQI	Main pollutant
2020-08-20	334	PM2.5
2020-08-21	322	PM2.5
2020-08-19	211	PM2.5
2020-09-14	192	PM2.5
2020-08-22	191	PM2.5

An ordinary day in San Luis Obispo

Across 3,653 monitored days from 2016 to 2025, the air in San Luis Obispo County rated Good on 44% of them, less often than the 72% typical of monitored counties, and Moderate on 53%. The median day sat at AQI 52; one day in ten went above 78.

The pollutant setting the AQI on an ordinary day here:

ozone55%2,004 days
fine particles (PM2.5)42%1,543 days
coarse dust (PM10)3%103 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	191	167	7	0	50	119
2024	366	193	163	10	0	49	122
2023	365	207	152	6	0	47	122
2022	365	117	245	3	0	55	115
2021	365	154	203	8	0	53	133
2020	366	161	174	31	10	53	334
2019	365	196	166	3	0	49	108
2018	365	147	209	9	0	53	133
2017	365	107	250	8	0	58	133
2016	366	140	217	9	1	55	156
2015	365	95	265	5	0	58	126
2014	365	81	270	14	0	60	126
2013	365	65	286	14	0	62	136
2012	366	113	216	37	2	57	154
2011	365	78	268	19	0	61	133
2010	365	124	207	34	1	55	161

*2025 may be a partial year in EPA's file.

Nearby counties on file

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.