Bad Air Days in Marion County, Indiana: 8 a Year

8 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 109 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (59% particle days).

The bad-air season here

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

What to buy, and what not to

Particles (wildfire smoke, dust) cause enough bad days here to matter. A HEPA purifier sized to your bedroom removes them; run it with windows shut on bad days.
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.

Better or worse than it was

In 2000 to 2009 this county averaged 28 bad days a year; in 2016 to 2025, 8. The air has improved. The worst recent year was 2023, with 21 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-28	259	PM2.5
2023-06-27	191	PM2.5
2023-06-29	181	PM2.5
2025-07-05	170	PM2.5
2018-07-05	158	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Marion County rated Good on 30% of them, less often than the 72% typical of monitored counties, and Moderate on 68%. The median day sat at AQI 56; one day in ten went above 76.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)82%2,981 days
ozone18%663 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	146	214	5	2	54	170
2024	366	134	228	4	0	54	138
2023	365	84	260	21	4	60	259
2022	365	101	251	13	1	56	154
2021	365	81	278	6	0	58	114
2020	366	110	250	6	0	55	144
2019	365	78	283	4	0	58	112
2018	365	105	249	11	1	55	158
2017	365	134	227	4	0	54	122
2016	366	111	244	11	0	55	143
2015	365	129	233	3	1	56	159
2014	365	85	273	7	1	58	209
2013	365	59	300	6	0	61	124
2012	366	37	308	21	2	64	169
2011	365	20	323	22	0	64	141
2010	365	53	299	13	0	64	143

*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.