Bad Air Days in San Juan County, Utah: 2 a Year

2 bad air days a year

About the middle of monitored US counties. Ranked 469 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (0% particle days).

Which months to watch

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

What actually helps 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 San Juan

In 2000 to 2009 this county averaged 5 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2018, with 7 bad days on 356 monitored days.

Date	AQI	Main pollutant
2019-07-31	268	Ozone
2019-07-30	247	Ozone
2019-08-02	207	Ozone
2019-08-03	187	Ozone
2019-08-01	150	Ozone

An ordinary day in San Juan

Across 3,601 monitored days from 2016 to 2025, the air in San Juan County rated Good on 78% of them, more often than the 72% typical of monitored counties, and Moderate on 22%. The median day sat at AQI 44; one day in ten went above 62.

The pollutant setting the AQI on an ordinary day here:

ozone99%3,567 days
fine particles (PM2.5)1%34 days

San Juan County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	361	274	86	1	0	44	108
2024	356	276	77	3	0	44	126
2023	357	267	90	0	0	45	87
2022	365	306	59	0	0	44	87
2021	364	276	86	2	0	44	115
2020	363	294	67	2	0	43	119
2019	363	286	70	7	4	44	268
2018	356	245	104	7	0	44	119
2017	351	263	88	0	0	44	100
2016	365	309	56	0	0	44	93
2015	362	285	76	1	0	43	105
2014	355	278	77	0	0	44	100
2013	343	269	74	0	0	44	97
2012	365	238	122	5	0	46	129
2011	354	236	115	3	0	47	108
2010	354	244	108	2	0	47	119

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

Elsewhere in Utah

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.