Bad Air Days in Butte County, California: 16 a Year

16 bad air days a year

Among the worst tenth of monitored US counties. Ranked 44 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (69% particle days).

Which months to watch

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

In 2000 to 2009 this county averaged 49 bad days a year; in 2016 to 2025, 16. The air has improved. The worst recent year was 2018, with 40 bad days on 365 monitored days.

Date	AQI	Main pollutant
2018-11-16	682	PM2.5
2020-09-13	508	PM2.5
2018-11-15	462	PM2.5
2018-11-09	409	PM2.5
2018-11-14	371	PM2.5

Most days, not the worst ones

Across 3,653 monitored days from 2016 to 2025, the air in Butte County rated Good on 45% of them, less often than the 72% typical of monitored counties, and Moderate on 51%. The median day sat at AQI 52; one day in ten went above 84.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)51%1,875 days
ozone48%1,755 days
coarse dust (PM10)1%23 days

Butte County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	238	127	0	0	46	96
2024	366	231	133	2	2	46	213
2023	365	191	174	0	0	50	100
2022	365	191	171	3	0	50	119
2021	365	160	174	31	13	53	257
2020	366	149	178	39	21	54	508
2019	365	171	189	5	2	51	180
2018	365	97	228	40	18	59	682
2017	365	100	241	24	1	59	155
2016	366	115	235	16	0	57	127
2015	365	138	214	13	1	55	153
2014	365	127	224	14	2	57	156
2013	365	104	252	9	1	59	169
2012	366	116	219	31	8	57	199
2011	365	86	255	24	0	60	149
2010	365	167	185	13	0	53	126

*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.