Bad Air Days in Washtenaw County, Michigan: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 258 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (30% particle days).

Which months to watch

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

What to buy, and what not to

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 worst days on file

In 2000 to 2009 this county averaged 16 bad days a year; in 2016 to 2025, 4. The air has improved. The worst recent year was 2023, with 13 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-28	218	PM2.5
2023-06-27	214	PM2.5
2023-06-29	190	PM2.5
2020-07-05	160	PM2.5
2023-06-07	152	PM2.5

An ordinary day in Washtenaw

Across 3,653 monitored days from 2016 to 2025, the air in Washtenaw County rated Good on 62% of them, less often than the 72% typical of monitored counties, and Moderate on 36%. The median day sat at AQI 45; one day in ten went above 65.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)54%1,986 days
ozone46%1,667 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	253	104	8	1	41	152
2024	366	253	113	0	0	43	87
2023	365	168	184	13	4	52	218
2022	365	222	142	1	0	46	101
2021	365	212	153	0	0	46	100
2020	366	236	124	6	1	43	160
2019	365	248	117	0	0	44	89
2018	365	238	121	6	0	46	150
2017	365	216	146	3	0	46	119
2016	366	233	127	6	0	44	122
2015	365	190	175	0	0	49	99
2014	365	174	187	4	0	51	126
2013	364	282	81	1	0	38	101
2012	364	249	93	22	2	42	164
2011	357	261	89	7	1	39	154
2010	362	273	88	1	0	39	119

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

Elsewhere in Michigan

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.