Bad Air Days in Knox County, Indiana: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 511 of 996 monitored counties, where 1 is worst.

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

When the bad days come in Knox

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

Is a purifier worth it 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.

Better or worse than it was

In 2000 to 2009 this county averaged 15 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2023, with 8 bad days on 364 monitored days.

Date	AQI	Main pollutant
2023-06-06	126	Ozone
2018-06-08	122	Ozone
2023-05-24	122	Ozone
2016-06-10	119	Ozone
2016-06-25	119	Ozone

Most days, not the worst ones

Across 3,553 monitored days from 2016 to 2025, the air in Knox County rated Good on 90% of them, more often than the 72% typical of monitored counties, and Moderate on 10%. The median day sat at AQI 36; one day in ten went above 52.

The pollutant setting the AQI on an ordinary day here:

ozone100%3,553 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	344	314	30	0	0	36	93
2024	360	314	46	0	0	38	90
2023	364	313	43	8	0	39	126
2022	344	321	23	0	0	36	87
2021	360	320	38	2	0	36	108
2020	351	328	22	1	0	33	105
2019	359	328	31	0	0	36	87
2018	357	311	42	4	0	35	122
2017	355	323	32	0	0	36	100
2016	359	313	42	4	0	35	119
2015	349	314	35	0	0	34	93
2014	348	313	35	0	0	34	90
2013	350	318	32	0	0	36	90
2012	351	263	63	25	0	39	136
2011	353	236	111	6	2	44	154
2010	362	252	106	4	0	42	112

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

Elsewhere in Indiana

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.