Bad Air Days in Boulder County, Colorado: 13 a Year

13 bad air days a year

Among the worst tenth of monitored US counties. Ranked 58 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (21% particle days).

Which months to watch

Most bad days here fall in July and August. 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.

The record for Boulder

In 2000 to 2009 this county averaged 12 bad days a year; in 2016 to 2025, 13. Not much has changed. The worst recent year was 2021, with 35 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-10-16	181	PM2.5
2023-05-19	181	PM2.5
2020-09-06	164	PM2.5
2021-03-19	160	PM2.5
2018-07-10	159	Ozone

Most days, not the worst ones

Across 3,480 monitored days from 2016 to 2025, the air in Boulder County rated Good on 57% of them, less often than the 72% typical of monitored counties, and Moderate on 40%. The median day sat at AQI 49; one day in ten went above 80.

The pollutant setting the AQI on an ordinary day here:

ozone68%2,382 days
fine particles (PM2.5)31%1,076 days
coarse dust (PM10)1%22 days

Boulder County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	213	149	3	0	48	119
2024	366	212	144	10	1	49	154
2023	365	192	167	6	2	50	181
2022	365	224	134	7	0	47	129
2021	365	154	176	35	3	52	160
2020	366	173	172	21	2	51	181
2019	365	192	168	5	0	50	119
2018	362	198	135	29	1	49	159
2017	357	233	113	11	0	46	149
2016	204	178	26	0	0	39	97
2015	365	253	105	7	0	44	129
2014	360	254	101	5	0	45	126
2013	360	228	116	16	1	46	151
2012	359	185	139	35	4	50	172
2011	365	212	137	16	0	48	140
2010	365	226	127	12	0	46	143

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