Bad Air Days in Jefferson County, Colorado: 28 a Year

28 bad air days a year

Among the worst tenth of monitored US counties. Ranked 19 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 July and August and June. Averaged over 2016 to 2025.

What actually helps here

Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
Sign up for EnviroFlash ozone alerts for this area during the worst months.

Better or worse than it was

In 2000 to 2009 this county averaged 32 bad days a year; in 2016 to 2025, 28. The air has improved. The worst recent year was 2021, with 55 bad days on 365 monitored days.

Date	AQI	Main pollutant
2024-07-24	174	Ozone
2021-06-15	172	Ozone
2021-07-12	172	Ozone
2024-07-23	169	Ozone
2022-06-16	166	Ozone

An ordinary day in Jefferson

Across 3,653 monitored days from 2016 to 2025, the air in Jefferson County rated Good on 61% of them, less often than the 72% typical of monitored counties, and Moderate on 31%. The median day sat at AQI 47; one day in ten went above 94.

The pollutant setting the AQI on an ordinary day here:

ozone100%3,647 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	215	137	13	0	48	119
2024	366	192	137	37	7	50	174
2023	365	206	141	18	0	49	143
2022	365	234	100	31	1	47	166
2021	365	222	88	55	11	47	172
2020	366	231	108	27	4	46	161
2019	365	238	111	16	0	46	129
2018	365	229	99	37	2	46	151
2017	365	214	123	28	0	47	136
2016	366	245	101	20	3	46	159
2015	365	244	104	17	2	45	164
2014	365	228	121	16	0	46	140
2013	365	209	121	35	4	47	169
2012	366	201	113	52	4	48	190
2011	365	198	129	38	2	49	197
2010	365	233	106	26	0	46	143

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

Elsewhere in Colorado

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.