Bad Air Days in Union County, South Dakota: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 547 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (60% particle days).

When the bad days come in Union

Most bad days here fall in June and July and April and May. 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.

The worst days on file

In 2000 to 2009 this county averaged 1 bad days a year; in 2016 to 2025, 2. Not much has changed. The worst recent year was 2021, with 4 bad days on 273 monitored days.

Date	AQI	Main pollutant
2021-07-30	167	PM2.5
2021-07-31	153	PM2.5
2020-04-11	141	PM2.5
2021-07-29	126	PM2.5
2021-06-16	115	Ozone

Most days, not the worst ones

Across 2,099 monitored days from 2016 to 2021, the air in Union County rated Good on 77% of them, more often than the 72% typical of monitored counties, and Moderate on 22%. The median day sat at AQI 40; one day in ten went above 59.

The pollutant setting the AQI on an ordinary day here:

ozone58%1,218 days
fine particles (PM2.5)38%804 days
coarse dust (PM10)4%74 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2021	273	179	90	4	2	44	167
2020	366	304	61	1	0	39	141
2019	364	306	58	0	0	37	97
2018	365	256	106	3	0	43	112
2017	365	278	87	0	0	41	100
2016	366	295	69	2	0	38	112
2015	365	243	120	2	0	43	108
2014	365	244	121	0	0	43	100
2013	365	170	194	1	0	51	114
2012	366	152	211	3	0	52	111
2011	365	188	177	0	0	49	100
2010	365	207	156	2	0	47	127

South Dakota compared

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.