EnviroNearby

Marathon County, Wisconsin air quality history

Daily Air Quality Index summaries from EPA monitors in Marathon, Wisconsin, 1980–2025.

77%
Good days in 2025
36
Median daily AQI, 2025
9
Days above 100 in 2025
342
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Marathon, Wisconsin on 342 days. 262 of those days (77%) were in the Good range, and the median daily AQI was 36. That median is about the same as the typical monitored county in Wisconsin (median 38 across 29 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 229 days. Across the years with fuller monitoring, the share of Good days held roughly steady: an average of 81% in 1980–1984 versus 80% in 2023–2025.

Years with fewer than 120 monitored days (1981, 1991) are shown in the table but left out of comparisons and trend statements.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1980: Good 202 of 219 days1980: Moderate 14 of 219 days1980: Unhealthy for Sensitive Groups 3 of 219 days19801981: Good 87 of 87 days1983: Good 98 of 137 days1983: Moderate 28 of 137 days1983: Unhealthy for Sensitive Groups 9 of 137 days1983: Unhealthy 2 of 137 days1984: Good 149 of 189 days1984: Moderate 33 of 189 days1984: Unhealthy for Sensitive Groups 6 of 189 days1984: Unhealthy 1 of 189 days1985: Good 132 of 174 days1985: Moderate 35 of 174 days1985: Unhealthy for Sensitive Groups 7 of 174 days1986: Good 132 of 171 days1986: Moderate 32 of 171 days1986: Unhealthy for Sensitive Groups 7 of 171 days19861987: Good 124 of 176 days1987: Moderate 39 of 176 days1987: Unhealthy for Sensitive Groups 11 of 176 days1987: Unhealthy 2 of 176 days1991: Good 49 of 52 days1991: Moderate 3 of 52 days1992: Good 233 of 270 days1992: Moderate 29 of 270 days1992: Unhealthy for Sensitive Groups 7 of 270 days1992: Unhealthy 1 of 270 days1993: Good 217 of 234 days1993: Moderate 15 of 234 days1993: Unhealthy for Sensitive Groups 2 of 234 days1994: Good 187 of 207 days1994: Moderate 19 of 207 days1994: Unhealthy for Sensitive Groups 1 of 207 days19941995: Good 165 of 206 days1995: Moderate 36 of 206 days1995: Unhealthy for Sensitive Groups 5 of 206 days1996: Good 174 of 211 days1996: Moderate 34 of 211 days1996: Unhealthy for Sensitive Groups 3 of 211 days1997: Good 173 of 212 days1997: Moderate 36 of 212 days1997: Unhealthy for Sensitive Groups 3 of 212 days1998: Good 155 of 213 days1998: Moderate 50 of 213 days1998: Unhealthy for Sensitive Groups 7 of 213 days1998: Unhealthy 1 of 213 days1999: Good 154 of 206 days1999: Moderate 41 of 206 days1999: Unhealthy for Sensitive Groups 8 of 206 days1999: Unhealthy 3 of 206 days19992000: Good 155 of 186 days2000: Moderate 26 of 186 days2000: Unhealthy for Sensitive Groups 5 of 186 days2001: Good 141 of 176 days2001: Moderate 30 of 176 days2001: Unhealthy for Sensitive Groups 5 of 176 days2002: Good 143 of 182 days2002: Moderate 35 of 182 days2002: Unhealthy for Sensitive Groups 4 of 182 days2003: Good 129 of 178 days2003: Moderate 39 of 178 days2003: Unhealthy for Sensitive Groups 10 of 178 days2004: Good 163 of 183 days2004: Moderate 20 of 183 days20042005: Good 143 of 186 days2005: Moderate 37 of 186 days2005: Unhealthy for Sensitive Groups 5 of 186 days2005: Unhealthy 1 of 186 days2006: Good 146 of 185 days2006: Moderate 38 of 185 days2006: Unhealthy for Sensitive Groups 1 of 185 days2007: Good 167 of 203 days2007: Moderate 30 of 203 days2007: Unhealthy for Sensitive Groups 5 of 203 days2007: Unhealthy 1 of 203 days2008: Good 192 of 215 days2008: Moderate 22 of 215 days2008: Unhealthy for Sensitive Groups 1 of 215 days2009: Good 200 of 217 days2009: Moderate 17 of 217 days20092010: Good 195 of 208 days2010: Moderate 13 of 208 days2011: Good 174 of 198 days2011: Moderate 24 of 198 days2012: Good 188 of 221 days2012: Moderate 31 of 221 days2012: Unhealthy for Sensitive Groups 2 of 221 days2013: Good 173 of 203 days2013: Moderate 30 of 203 days2014: Good 179 of 194 days2014: Moderate 14 of 194 days2014: Unhealthy for Sensitive Groups 1 of 194 days20142015: Good 192 of 204 days2015: Moderate 12 of 204 days2016: Good 176 of 192 days2016: Moderate 16 of 192 days2017: Good 223 of 237 days2017: Moderate 14 of 237 days2018: Good 205 of 232 days2018: Moderate 27 of 232 days2019: Good 196 of 205 days2019: Moderate 9 of 205 days20192020: Good 193 of 199 days2020: Moderate 6 of 199 days2021: Good 191 of 200 days2021: Moderate 9 of 200 days2022: Good 197 of 205 days2022: Moderate 8 of 205 days2023: Good 168 of 216 days2023: Moderate 39 of 216 days2023: Unhealthy for Sensitive Groups 8 of 216 days2023: Unhealthy 1 of 216 days2024: Good 311 of 366 days2024: Moderate 54 of 366 days2024: Unhealthy 1 of 366 days20242025: Good 262 of 342 days2025: Moderate 71 of 342 days2025: Unhealthy for Sensitive Groups 5 of 342 days2025: Unhealthy 4 of 342 days2025
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001980: 90th percentile AQI 481981: 90th percentile AQI 321983: 90th percentile AQI 901984: 90th percentile AQI 741985: 90th percentile AQI 771986: 90th percentile AQI 771987: 90th percentile AQI 901991: 90th percentile AQI 411992: 90th percentile AQI 671993: 90th percentile AQI 491994: 90th percentile AQI 501995: 90th percentile AQI 741996: 90th percentile AQI 641997: 90th percentile AQI 671998: 90th percentile AQI 741999: 90th percentile AQI 842000: 90th percentile AQI 612001: 90th percentile AQI 772002: 90th percentile AQI 712003: 90th percentile AQI 872004: 90th percentile AQI 542005: 90th percentile AQI 802006: 90th percentile AQI 612007: 90th percentile AQI 642008: 90th percentile AQI 512009: 90th percentile AQI 492010: 90th percentile AQI 492011: 90th percentile AQI 512012: 90th percentile AQI 612013: 90th percentile AQI 542014: 90th percentile AQI 492015: 90th percentile AQI 482016: 90th percentile AQI 492017: 90th percentile AQI 472018: 90th percentile AQI 542019: 90th percentile AQI 472020: 90th percentile AQI 452021: 90th percentile AQI 492022: 90th percentile AQI 462023: 90th percentile AQI 842024: 90th percentile AQI 532025: 90th percentile AQI 611980: Median AQI 291981: Median AQI 221983: Median AQI 411984: Median AQI 431985: Median AQI 421986: Median AQI 411987: Median AQI 431991: Median AQI 301992: Median AQI 351993: Median AQI 351994: Median AQI 381995: Median AQI 381996: Median AQI 401997: Median AQI 371998: Median AQI 441999: Median AQI 402000: Median AQI 392001: Median AQI 392002: Median AQI 422003: Median AQI 442004: Median AQI 402005: Median AQI 432006: Median AQI 412007: Median AQI 412008: Median AQI 392009: Median AQI 372010: Median AQI 382011: Median AQI 392012: Median AQI 412013: Median AQI 392014: Median AQI 382015: Median AQI 362016: Median AQI 352017: Median AQI 362018: Median AQI 392019: Median AQI 342020: Median AQI 312021: Median AQI 362022: Median AQI 352023: Median AQI 412024: Median AQI 372025: Median AQI 361980198619941999200420092014201920242025
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2025

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Fine particles (PM2.5)22967%
Ozone11333%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253422627154003661168PM2.5
20243663115401003753151PM2.5
20232161683981004184166Ozone
202220519780000354664Ozone
202120019190000364987Ozone
202019919360000314574Ozone
201920519690000344784Ozone
2018232205270000395497Ozone
2017237223140000364790Ozone
20161921761600003549100Ozone
2015204192120000364890Ozone
20141941791410003849101Ozone
2013203173300000395484Ozone
20122211883120004161101Ozone
20111981742400003951100Ozone
2010208195130000384980Ozone
2009217200170000374987Ozone
20082151922210003951108Ozone
20072031673051004164156Ozone
20061851463810004161105Ozone
20051861433751004380156Ozone
20041831632000004054100Ozone
200317812939100004487126Ozone
20021821433540004271122Ozone
20011761413050003977126Ozone
20001861552650003961150Ozone
19992061544183004084166Ozone
19982131555071004474174Ozone
19972121733630003767115Ozone
19962111743430004064119Ozone
19952061653650003874136Ozone
19942071871910003850112Ozone
19932342171520003549126Ozone
19922702332971003567166Ozone
1991524930000304167Ozone
198717612439112004390174Ozone
19861711323270004177115Ozone
19851741323570004277143Ozone
19841891493361004374164Ozone
1983137982892004190156Ozone
1981878700000223244Ozone
19802192021430002948150Ozone

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

Counties in Wisconsin with a similar median AQI in 2025

All monitored counties in Wisconsin

Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem