Bad Air Days in Weld County, Colorado: 12 a Year

12 bad air days a year

Among the worst tenth of monitored US counties. Ranked 64 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (19% particle days).

When the bad days come in Weld

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

What to buy, and what not to

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 worst days on file

In 2000 to 2009 this county averaged 13 bad days a year; in 2016 to 2025, 12. Not much has changed. The worst recent year was 2021, with 40 bad days on 365 monitored days.

Date	AQI	Main pollutant
2024-07-24	164	Ozone
2023-05-19	162	PM2.5
2017-09-04	155	PM2.5
2021-03-19	154	Ozone
2024-07-23	154	Ozone

Most days, not the worst ones

Across 3,653 monitored days from 2016 to 2025, the air in Weld County rated Good on 44% of them, less often than the 72% typical of monitored counties, and Moderate on 52%. The median day sat at AQI 52; one day in ten went above 83.

The pollutant setting the AQI on an ordinary day here:

ozone66%2,406 days
fine particles (PM2.5)34%1,243 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	163	196	6	0	52	119
2024	366	149	196	21	2	54	164
2023	365	146	214	5	1	55	162
2022	365	170	185	10	0	51	133
2021	365	156	169	40	1	54	154
2020	366	166	187	13	0	52	140
2019	365	152	205	8	0	53	129
2018	365	138	217	10	0	54	133
2017	365	173	184	8	1	51	155
2016	366	201	162	3	0	49	129
2015	365	224	132	9	0	47	133
2014	365	239	122	4	0	46	126
2013	365	216	138	11	0	48	133
2012	366	196	150	20	2	49	161
2011	365	206	147	12	0	47	136
2010	365	212	146	7	0	47	126

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

Colorado 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.