Bad Air Days in Custer County, South Dakota: 3 a Year

3 bad air days a year

About the middle of monitored US counties. Ranked 397 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (57% particle days).

The bad-air season here

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

What to buy, and what not to

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

Better or worse than it was

In 2000 to 2009 this county averaged 2 bad days a year; in 2016 to 2025, 3. Not much has changed. The worst recent year was 2023, with 9 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-07-30	209	PM2.5
2023-05-18	167	PM2.5
2023-07-30	166	Ozone
2017-09-03	162	PM2.5
2023-09-06	157	PM2.5

The air on a normal day

Across 3,639 monitored days from 2016 to 2025, the air in Custer County rated Good on 84% of them, more often than the 72% typical of monitored counties, and Moderate on 15%. The median day sat at AQI 41; one day in ten went above 56.

The pollutant setting the AQI on an ordinary day here:

ozone86%3,121 days
fine particles (PM2.5)14%503 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	360	265	95	0	0	45	87
2024	366	276	81	9	0	44	126
2023	365	308	48	9	5	43	167
2022	362	298	62	2	0	41	130
2021	364	302	59	3	1	39	209
2020	366	323	41	2	0	41	119
2019	362	352	10	0	0	40	84
2018	365	311	54	0	0	41	89
2017	363	299	61	3	1	42	162
2016	366	340	26	0	0	39	97
2015	365	326	37	2	0	40	133
2014	365	359	6	0	0	39	100
2013	365	325	40	0	0	41	74
2012	364	264	97	3	0	43	112
2011	365	334	31	0	0	40	67
2010	364	328	34	2	1	40	193

*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.