Median AQI
41
Good
Wisconsin · County air quality · 2025
Jefferson County, Wisconsin
Median AQI 41 (Good). 82.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 82.6%
- Good-air days
- 0
- Unhealthy days
- 59%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Jefferson County's air is good on balance, cleaner than the median US county, a median AQI of 41 with 82.6% of 92 monitored days in the Good band, driven mainly by Ozone.
- 41
- median AQI · Good
- 82.6%
- days rated Good
- Ozone
- main pollutant
- 59%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 105.
Data updated July 2026
- Good · 76 d · 82.6%
- Moderate · 15 d · 16.3%
- USG · 1 d · 1.1%
How did Jefferson County's air break down in 2025?
Across 92 monitored days, 76 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 105 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 92 days monitored. Median AQI: 41 (Good).
Insights: Jefferson County, Wisconsin. EPA monitors recorded a median Air Quality Index of 41 across 92 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 76 (83%) fell in the Good band (AQI 0–50), 15 (16%) registered Moderate readings, and 1 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 54, while the worst single-day peak hit 105 (USG). Residents averaged about 24.8 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (92 days). Across the 2020–2025 trend window, good days shifted by -113 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 105, usg but never Unhealthy. Nearby counties show comparable readings: Ashland (85.6% good, AQI 39); Brown (79.0% good, AQI 37); Columbia (84.6% good, AQI 42); Dane (71.8% good, AQI 40); Dodge (76.4% good, AQI 39); Door (85.7% good, AQI 40).
Air Quality Summary
Good Days
82.6%
76 of 92 monitored days
7.6pp vs 75% target
Max AQI Recorded
105
USG
Health Grade
Good
90th-pct AQI 54
Jefferson County, Good days share 82.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Jefferson County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 92 monitored days across EPA AQI categories.
Good
76
83%
Moderate
15
16%
USG
1
1%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Jefferson County.
- Ozone · 92 days · 100.0%
Health Impact Assessment
Good
Jefferson County's air quality is rated Good based on 82.6% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 82.6% of monitored days in 2025, with a median AQI of 41. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 92 of 92 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Jefferson County. Best year: 2020 (189 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 189 | 17 | 4 | 0 | 38 | 136 |
| 2021 | 178 | 29 | 1 | 0 | 40 | 101 |
| 2022 | 185 | 15 | 0 | 0 | 38 | 87 |
| 2023 | 145 | 43 | 15 | 0 | 44 | 140 |
| 2024 | 183 | 22 | 0 | 0 | 39 | 93 |
| 2025 | 76 | 15 | 1 | 0 | 41 | 105 |
How does Jefferson County compare?
How Jefferson County compares
Good-day share better than US avg
Jefferson County 82.6%
Ashland County, Wisconsin 85.6%
US average 76.5%
Median AQI worse than US avg
Jefferson County 41
Ashland County, Wisconsin 39
US average 38
Unhealthy days / yr about average
Jefferson County 0
Ashland County, Wisconsin 1
US average 0
Other Counties in Wisconsin
Air quality comparison for nearby counties in Wisconsin.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Jefferson County.
Albemarle County, Virginia
AQI 41 · 77.9% good
Anderson County, South Carolina
AQI 41 · 90.0% good
Berkshire County, Massachusetts
AQI 41 · 71.6% good
Berrien County, Michigan
AQI 41 · 80.9% good
Blair County, Pennsylvania
AQI 41 · 71.9% good
Bonner County, Idaho
AQI 41 · 71.3% good
What Jefferson County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 41 means for exposure. Health guidance
- See how Jefferson County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Jefferson County with its Wisconsin neighbours before drawing a conclusion about the region. All of Wisconsin
Based on 92 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Jefferson County, Wisconsin? ▼
Jefferson County has a median Air Quality Index (AQI) of 41, rated "Good." 82.6% of monitored days (76 out of 92) had good air quality in 2025.
What is the main pollutant in Jefferson County? ▼
The primary pollutant in Jefferson County is Ozone (O3), which was the main AQI driver on 92 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Jefferson County have? ▼
In 2025, Jefferson County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 105.
Is the air quality in Jefferson County getting better or worse? ▼
Air quality in Jefferson County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 189 good days. Worst year: 2025 with 76 good days.
What does AQI 41 mean for health? ▼
Jefferson County's median AQI of 41 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Jefferson County compare to other counties in Wisconsin? ▼
Jefferson County has 82.6% good air days. Nearby counties in Wisconsin: Ashland (85.6% good), Brown (79.0% good), Columbia (84.6% good), Dane (71.8% good), Dodge (76.4% good), Door (85.7% good).
What is the 90th percentile AQI in Jefferson County? ▼
The 90th percentile AQI for Jefferson County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 105 (USG).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.