Bad Air Days in Williamson County, Tennessee: under 1 a Year

under 1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 704 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (0% particle days).

The bad-air season here

Most bad days here fall in June and March. 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 Williamson

In 2000 to 2009 this county averaged 28 bad days a year; in 2016 to 2025, under 1. The air has improved. The worst recent year was 2022, with 2 bad days on 244 monitored days.

Date	AQI	Main pollutant
2022-06-29	122	Ozone
2023-06-09	122	Ozone
2016-06-09	108	Ozone
2018-06-07	108	Ozone
2022-03-04	101	Ozone

An ordinary day in Williamson

Across 2,402 monitored days from 2016 to 2025, the air in Williamson County rated Good on 88% of them, more often than the 72% typical of monitored counties, and Moderate on 11%. The median day sat at AQI 40; one day in ten went above 53.

The pollutant setting the AQI on an ordinary day here:

ozone100%2,402 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Median AQI	Highest AQI
2025*	243	216	27	0	40	100
2024	238	200	38	0	43	87
2023	244	197	45	2	43	122
2022	244	209	33	2	41	122
2021	243	225	18	0	39	67
2020	241	231	10	0	37	71
2019	245	215	30	0	42	84
2018	245	212	32	1	39	108
2017	215	197	18	0	40	80
2016	244	219	24	1	40	108
2015	241	223	18	0	36	84
2014	234	210	23	1	38	105
2013	244	227	17	0	37	87
2012	244	185	52	7	44	136
2011	243	154	83	6	47	129
2010	240	162	69	9	45	126

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

Elsewhere in Tennessee

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.