Bad Air Days in Delta County, Colorado: under 1 a Year

under 1 bad air day a year

Rare: among the cleanest-air counties EPA monitors. Ranked 797 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (100% particle days).

Which months to watch

Most bad days here fall in August and July. 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.

Better or worse than it was

The worst recent year was 2021, with 2 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-08-07	141	PM2.5
2021-08-08	126	PM2.5
2025-07-16	103	PM2.5
2020-09-07	97	PM2.5
2025-07-15	96	PM2.5

An ordinary day in Delta

Across 1,978 monitored days from 2016 to 2025, the air in Delta County rated Good on 88% of them, more often than the 72% typical of monitored counties, and Moderate on 12%. The median day sat at AQI 37; one day in ten went above 51.

The pollutant setting the AQI on an ordinary day here:

ozone66%1,299 days
fine particles (PM2.5)19%367 days
coarse dust (PM10)16%311 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Median AQI	Highest AQI
2025*	209	179	29	1	29	103
2022	143	125	18	0	44	80
2021	365	294	69	2	43	141
2020	365	312	53	0	41	97
2019	365	338	27	0	41	80
2018	297	264	33	0	36	85
2017	115	114	1	0	19	52
2016	119	117	2	0	16	64
2015	114	113	1	0	16	51
2014	118	117	1	0	21	77
2013	121	120	1	0	19	55
2012	121	119	2	0	21	56
2011	121	121	0	0	19	47
2010	115	111	4	0	19	86

*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.