Wisconsin · County air quality · 2025
Marathon County, Wisconsin
Median AQI 36 (Good). 76.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 36
- Median AQI
- 76.6%
- Good-air days
- 4
- Unhealthy days
- 48%
- Cleaner than US counties
Marathon County's 2025 annual median AQI is 36 (Good), with 76.6% Good days
According to the U.S. EPA Air Quality System, Marathon County, Wisconsin posted a 2025 median AQI of 36 (Good) across 342 monitored days, with 76.6% rated Good and 4 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Marathon County's 76.6% Good-day share places it cleaner than 48% of US counties in 2025.
- Closest median-AQI peer
- Ashland County (81.6% Good, median AQI 36) is the closest nationwide median-AQI peer to Marathon County.
- 2020–2025 trend
- Marathon County's floor-qualifying Good-day share is worsening from 2020 to 2025 (97.0% → 76.6%).
- Median AQI
- Median AQI: 36
According to the 2025 EPA AQS county release on PlainAirData, Marathon County, Wisconsin ranks #233 of 951 by median AQI (36) and #496 by good-day share (76.6%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- Marathon County's Good-day share fell from 85.0% to 76.6% between the 2024 and 2025 EPA annual releases, a change of -8.4 percentage points.
- Marathon County's Unhealthy-category days rose from 1 to 4 between the 2024 and 2025 EPA annual releases, a change of +3 days.
- Marathon County's peak single-day AQI rose from 151 to 168 between the 2024 and 2025 EPA annual releases, a change of +17 points.
Data updated August 2026
- Good · 262 d · 76.6%
- Moderate · 71 d · 20.8%
- USG · 5 d · 1.5%
- Unhealthy · 4 d · 1.2%
How did Marathon County's air break down in 2025?
Across 342 monitored days, 262 fell in the Good band (AQI 0–50) and 4 reached Unhealthy or worse. The worst single day peaked at AQI 168 (Unhealthy).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Marathon County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Marathon County's Good-day share is 76.6% - cleaner than 48% of that comparison set.
Marathon County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
76.6% 48th percentile higher than 48% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Marathon County.
- PM2.5 · 229 days · 67.0%
- Ozone · 113 days · 33.0%
2024 daily-monitor rhythm in Marathon County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Jul posted the highest typical readings (median AQI 45, peak 80); Mar was cleanest at median 21. Summer months (Jun–Aug) averaged median AQI 40 vs 31 in Dec–Feb. 1 day crossed Unhealthy (AQI >150) at least once that year.
Five-year seasonal record
EPA day-level county files · 2020–20241,186 daily readings show a recurring local pattern: Jun has the highest matched-month median AQI (42), while Nov is lowest (23). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Marathon County's air quality is rated Good based on 76.6% good air days. There were 4 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 5 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 168. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 229 of 342 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Marathon County. Highest Good-day share: 2020 (97.0%, 193 of 199 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 193 | 6 | 0 | 0 | 31 | 74 |
| 2021 | 191 | 9 | 0 | 0 | 36 | 87 |
| 2022 | 197 | 8 | 0 | 0 | 35 | 64 |
| 2023 | 168 | 39 | 8 | 1 | 41 | 166 |
| 2024 | 311 | 54 | 0 | 1 | 37 | 151 |
| 2025 | 262 | 71 | 5 | 4 | 36 | 168 |
How does Marathon County compare?
How Marathon County compares
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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Marathon County, Wisconsin · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
Every figure on PlainAirData 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.