Bad Air Days in Marathon County, Wisconsin: 3 a Year
3 bad air days a year
About the middle of monitored US counties. Ranked 392 of 996 monitored counties, where 1 is worst.
What makes them bad: a mix of particles and ozone (53% particle days).
Which months to watch
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and May and August and July. 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 record for Marathon
In 2000 to 2009 this county averaged 7 bad days a year; in 2016 to 2025, 3. The air has improved. The worst recent year was 2023, with 9 bad days on 216 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2025-07-30 | 168 | PM2.5 |
| 2023-06-28 | 166 | Ozone |
| 2025-07-31 | 165 | PM2.5 |
| 2025-07-13 | 153 | PM2.5 |
| 2025-08-02 | 152 | PM2.5 |
An ordinary day in Marathon
Across 2,394 monitored days from 2016 to 2025, the air in Marathon County rated Good on 89% of them, more often than the 72% typical of monitored counties, and Moderate on 11%. The median day sat at AQI 36; one day in ten went above 54.
The pollutant setting the AQI on an ordinary day here:
- ozone82%1,954 days
- fine particles (PM2.5)18%440 days
Every recent year on file
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 342 | 262 | 71 | 9 | 4 | 36 | 168 |
| 2024 | 366 | 311 | 54 | 1 | 1 | 37 | 151 |
| 2023 | 216 | 168 | 39 | 9 | 1 | 41 | 166 |
| 2022 | 205 | 197 | 8 | 0 | 0 | 35 | 64 |
| 2021 | 200 | 191 | 9 | 0 | 0 | 36 | 87 |
| 2020 | 199 | 193 | 6 | 0 | 0 | 31 | 74 |
| 2019 | 205 | 196 | 9 | 0 | 0 | 34 | 84 |
| 2018 | 232 | 205 | 27 | 0 | 0 | 39 | 97 |
| 2017 | 237 | 223 | 14 | 0 | 0 | 36 | 90 |
| 2016 | 192 | 176 | 16 | 0 | 0 | 35 | 100 |
| 2015 | 204 | 192 | 12 | 0 | 0 | 36 | 90 |
| 2014 | 194 | 179 | 14 | 1 | 0 | 38 | 101 |
| 2013 | 203 | 173 | 30 | 0 | 0 | 39 | 84 |
| 2012 | 221 | 188 | 31 | 2 | 0 | 41 | 101 |
| 2011 | 198 | 174 | 24 | 0 | 0 | 39 | 100 |
| 2010 | 208 | 195 | 13 | 0 | 0 | 38 | 80 |
*2025 may be a partial year in EPA's file.
Nearby counties on file
- Sheboygan County16/yr
- Racine County15/yr
- Kenosha County12/yr
- Ozaukee County9/yr
- Milwaukee County8/yr
- Manitowoc County7/yr
- Door County7/yr
- Walworth County6/yr
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.