Bad Air Days in San Bernardino County, California: 138 a Year

138 bad air days a year

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

What makes them bad: mostly ozone: summer smog (8% particle days).

When the bad days come in San Bernardino

Most bad days here fall in July and August and June and September. 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 worst days on file

In 2000 to 2009 this county averaged 158 bad days a year; in 2016 to 2025, 138. The air has improved. The worst recent year was 2020, with 163 bad days on 366 monitored days.

Date	AQI	Main pollutant
2024-09-11	593	PM2.5
2021-10-11	486	PM10
2020-08-20	236	Ozone
2017-07-07	233	Ozone
2020-08-14	233	Ozone

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in San Bernardino County rated Good on 7% of them, less often than the 72% typical of monitored counties, and Moderate on 55%. The median day sat at AQI 83; one day in ten went above 172.

The pollutant setting the AQI on an ordinary day here:

ozone56%2,051 days
fine particles (PM2.5)38%1,406 days
coarse dust (PM10)3%114 days
nitrogen dioxide2%82 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	46	209	110	43	77	207
2024	366	40	182	144	88	87	593
2023	365	50	197	118	51	71	214
2022	365	23	212	130	63	77	225
2021	365	17	202	146	73	87	486
2020	366	33	170	163	87	93	236
2019	365	29	203	133	62	81	213
2018	365	14	197	154	58	89	221
2017	365	9	209	147	74	84	233
2016	366	9	219	138	60	84	217
2015	365	14	221	130	42	84	223
2014	365	21	217	127	44	82	201
2013	365	35	195	135	52	84	218
2012	366	20	203	143	59	90	207
2011	365	21	197	147	64	84	233
2010	365	12	218	135	55	84	219

*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, and on why a purifier does not touch ozone in smoke vs ozone.