Bad Air Days in Hamilton County, Indiana: 3 a Year

3 bad air days a year

About the middle of monitored US counties. Ranked 388 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (31% particle days).

The bad-air season here

Most bad days here fall in June and July and September. 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

The worst recent year was 2023, with 10 bad days on 364 monitored days.

Date	AQI	Main pollutant
2023-06-28	190	PM2.5
2023-06-27	176	PM2.5
2023-06-29	136	PM2.5
2018-06-15	129	Ozone
2018-05-25	119	Ozone

An ordinary day in Hamilton

Across 3,644 monitored days from 2016 to 2025, the air in Hamilton County rated Good on 63% of them, less often than the 72% typical of monitored counties, and Moderate on 37%. The median day sat at AQI 45; one day in ten went above 63.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)59%2,143 days
ozone41%1,501 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	364	233	126	5	0	44	118
2024	366	244	121	1	0	44	108
2023	364	220	134	10	2	46	190
2022	365	241	124	0	0	43	100
2021	365	232	133	0	0	44	100
2020	366	247	119	0	0	44	97
2019	365	214	149	2	0	47	119
2018	365	229	130	6	0	45	129
2017	365	229	134	2	0	46	105
2016	359	189	167	3	0	49	115
2015	360	213	147	0	0	44	93
2014	358	191	165	2	0	48	129
2013	182	166	16	0	0	39	84
2012	183	114	56	13	0	48	126
2011	173	127	41	5	0	44	150
2010	180	119	53	8	0	44	122

*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.