Bad Air Days in Box Elder County, Utah: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 278 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (49% particle days).

Which months to watch

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

Is a purifier worth it here?

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 record for Box Elder

In 2000 to 2009 this county averaged 25 bad days a year; in 2016 to 2025, 4. The air has improved. The worst recent year was 2017, with 11 bad days on 365 monitored days.

Date	AQI	Main pollutant
2017-02-02	154	PM2.5
2018-08-11	151	Ozone
2020-08-21	147	Ozone
2017-01-19	145	PM2.5
2020-08-22	143	Ozone

An ordinary day in Box Elder

Across 3,550 monitored days from 2016 to 2025, the air in Box Elder County rated Good on 73% of them, close to the 72% typical of monitored counties, and Moderate on 26%. The median day sat at AQI 43; one day in ten went above 66.

The pollutant setting the AQI on an ordinary day here:

ozone79%2,788 days
fine particles (PM2.5)17%611 days
nitrogen dioxide4%146 days

Box Elder County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	236	128	1	0	45	105
2024	366	214	147	5	0	47	133
2023	360	302	58	0	0	38	96
2022	281	266	15	0	0	35	90
2021	363	281	78	4	0	42	119
2020	355	316	36	3	0	40	147
2019	364	300	62	2	0	41	121
2018	365	201	155	9	1	48	151
2017	365	207	147	11	1	48	154
2016	366	269	93	4	0	41	120
2015	365	249	111	5	0	45	120
2014	365	256	103	6	1	45	156
2013	365	200	143	22	2	49	153
2012	366	212	140	14	0	48	126
2011	365	219	138	8	4	47	160
2010	364	221	132	11	0	47	142

*2025 may be a partial year in EPA's file.

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