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

12 bad air days a year

Among the worst tenth of monitored US counties. Ranked 65 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (2% particle days).

When the bad days come in Arapahoe

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 Arapahoe

In 2000 to 2009 this county averaged 16 bad days a year; in 2016 to 2025, 12. The air has improved. The worst recent year was 2021, with 37 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-07-22	161	Ozone
2024-07-24	161	Ozone
2020-08-23	159	Ozone
2021-07-12	159	Ozone
2018-07-06	156	Ozone

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Arapahoe County rated Good on 62% of them, less often than the 72% typical of monitored counties, and Moderate on 34%. The median day sat at AQI 47; one day in ten went above 82.

The pollutant setting the AQI on an ordinary day here:

ozone94%3,447 days
fine particles (PM2.5)6%206 days

Arapahoe County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	212	151	2	0	48	101
2024	366	196	153	17	2	49	161
2023	365	239	118	8	0	47	136
2022	365	237	116	12	0	46	129
2021	365	220	108	37	3	46	161
2020	366	243	112	11	2	46	159
2019	365	222	137	6	0	47	140
2018	365	227	123	15	1	46	156
2017	365	221	137	7	0	47	119
2016	366	261	99	6	0	44	119
2015	361	257	102	2	0	45	136
2014	364	261	102	1	0	45	122
2013	365	206	141	18	0	47	150
2012	365	199	142	24	2	48	172
2011	364	202	139	23	3	49	159
2010	365	216	142	7	0	48	122

*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.