Bad Air Days in Loudoun County, Virginia: 1 a Year

1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 653 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (36% particle days).

Which months to watch

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

What actually helps 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.

The record for Loudoun

In 2000 to 2009 this county averaged 23 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2023, with 4 bad days on 364 monitored days.

Date	AQI	Main pollutant
2023-06-29	167	PM2.5
2023-06-08	164	PM2.5
2021-05-20	122	Ozone
2025-07-04	120	PM2.5
2016-06-01	112	Ozone

An ordinary day in Loudoun

Across 3,622 monitored days from 2016 to 2025, the air in Loudoun County rated Good on 83% of them, more often than the 72% typical of monitored counties, and Moderate on 16%. The median day sat at AQI 38; one day in ten went above 55.

The pollutant setting the AQI on an ordinary day here:

ozone67%2,440 days
fine particles (PM2.5)21%748 days
nitrogen dioxide12%434 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	294	70	1	0	40	120
2024	366	245	121	0	0	43	87
2023	364	306	54	4	2	39	167
2022	364	328	34	2	0	37	112
2021	364	308	55	1	0	37	122
2020	366	339	26	1	0	34	108
2019	337	286	50	1	0	38	112
2018	365	313	52	0	0	35	93
2017	365	297	68	0	0	38	100
2016	366	302	63	1	0	36	112
2015	361	294	63	4	0	38	115
2014	355	285	69	1	0	40	105
2013	365	300	63	2	0	35	115
2012	363	274	85	4	0	38	129
2011	365	265	93	7	1	38	151
2010	363	245	107	11	1	42	166

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

Virginia 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.