Bad Air Days in Alameda County, California: 11 a Year

11 bad air days a year

Among the worst tenth of monitored US counties. Ranked 73 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (45% particle days).

Which months to watch

Most bad days here fall in August and September and July. 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.

Better or worse than it was

In 2000 to 2009 this county averaged 16 bad days a year; in 2016 to 2025, 11. The air has improved. The worst recent year was 2020, with 20 bad days on 366 monitored days.

Date	AQI	Main pollutant
2018-11-16	248	PM2.5
2020-09-11	243	PM2.5
2018-11-15	228	PM2.5
2018-11-17	209	PM2.5
2017-09-01	205	Ozone

Most days, not the worst ones

Across 3,288 monitored days from 2016 to 2024, the air in Alameda County rated Good on 43% of them, less often than the 72% typical of monitored counties, and Moderate on 54%. The median day sat at AQI 52; one day in ten went above 73.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)76%2,513 days
ozone22%727 days
nitrogen dioxide1%47 days

Alameda County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2024	366	194	171	1	0	49	105
2023	365	197	155	13	0	48	130
2022	365	185	175	5	0	49	122
2021	365	155	200	10	1	53	151
2020	366	169	177	20	12	52	243
2019	365	183	174	8	0	50	150
2018	365	77	271	17	12	57	248
2017	365	106	245	14	5	56	205
2016	366	155	196	15	2	52	154
2015	365	146	207	12	0	54	147
2014	365	169	186	10	0	52	133
2013	365	105	252	8	0	56	122
2012	366	186	176	4	1	50	161
2011	365	145	209	11	0	53	147
2010	365	161	199	5	1	52	179

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.