Bad Air Days in San Francisco County, California: 3 a Year

3 bad air days a year

About the middle of monitored US counties. Ranked 325 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (97% particle days).

Which months to watch

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

What actually helps 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.

Better or worse than it was

In 2000 to 2009 this county averaged 7 bad days a year; in 2016 to 2025, 3. The air has improved. The worst recent year was 2018, with 14 bad days on 365 monitored days.

Date	AQI	Main pollutant
2018-11-16	252	PM2.5
2020-09-11	223	PM2.5
2018-11-15	221	PM2.5
2020-09-10	198	PM2.5
2020-09-13	190	PM2.5

Most days, not the worst ones

Across 3,255 monitored days from 2016 to 2024, the air in San Francisco County rated Good on 68% of them, less often than the 72% typical of monitored counties, and Moderate on 31%. The median day sat at AQI 39; one day in ten went above 61.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)63%2,053 days
ozone32%1,048 days
nitrogen dioxide5%147 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2024	334	254	80	0	0	33	94
2023	365	299	65	1	0	32	117
2022	364	260	104	0	0	36	88
2021	365	255	110	0	0	39	86
2020	366	227	131	8	5	42	223
2019	365	251	113	1	0	39	108
2018	365	222	129	14	12	44	252
2017	365	211	147	7	0	44	136
2016	366	236	130	0	0	41	71
2015	365	246	119	0	0	39	100
2014	365	249	116	0	0	39	97
2013	365	193	170	2	0	48	133
2012	364	238	124	2	0	42	105
2011	365	190	173	2	0	48	131
2010	365	171	191	3	0	51	125

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.