Bad Air Days in Jefferson County, Wisconsin: 5 a Year
5 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 239 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 Jefferson
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and May. Averaged over 2016 to 2025.
Is a purifier worth it here?
- 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.
The worst days on file
In 2000 to 2009 this county averaged 17 bad days a year; in 2016 to 2025, 5. The air has improved. The worst recent year was 2023, with 15 bad days on 203 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-19 | 140 | Ozone |
| 2020-06-08 | 136 | Ozone |
| 2023-06-28 | 136 | Ozone |
| 2020-06-18 | 129 | Ozone |
| 2023-05-30 | 129 | Ozone |
The air on a normal day
Across 2,116 monitored days from 2016 to 2025, the air in Jefferson County rated Good on 87% of them, more often than the 72% typical of monitored counties, and Moderate on 12%. The median day sat at AQI 39; one day in ten went above 58.
The pollutant setting the AQI on an ordinary day here:
- ozone100%2,116 days
Year by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 198 | 168 | 29 | 1 | 0 | 40 | 105 |
| 2024 | 205 | 183 | 22 | 0 | 0 | 39 | 93 |
| 2023 | 203 | 145 | 43 | 15 | 0 | 44 | 140 |
| 2022 | 200 | 185 | 15 | 0 | 0 | 38 | 87 |
| 2021 | 208 | 178 | 29 | 1 | 0 | 40 | 101 |
| 2020 | 210 | 189 | 17 | 4 | 0 | 38 | 136 |
| 2019 | 203 | 191 | 12 | 0 | 0 | 37 | 100 |
| 2018 | 243 | 209 | 32 | 2 | 0 | 39 | 126 |
| 2017 | 245 | 214 | 31 | 0 | 0 | 39 | 90 |
| 2016 | 201 | 176 | 21 | 4 | 0 | 40 | 108 |
| 2015 | 198 | 171 | 27 | 0 | 0 | 40 | 93 |
| 2014 | 195 | 161 | 30 | 4 | 0 | 42 | 126 |
| 2013 | 193 | 155 | 35 | 3 | 0 | 41 | 112 |
| 2012 | 193 | 130 | 51 | 12 | 1 | 44 | 166 |
| 2011 | 193 | 157 | 34 | 2 | 0 | 40 | 112 |
| 2010 | 194 | 168 | 25 | 1 | 0 | 41 | 108 |
*2025 may be a partial year in EPA's file.
Elsewhere in Wisconsin
- 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.