EnviroNearby

Jefferson County, Wisconsin air quality history

Daily Air Quality Index summaries from EPA monitors in Jefferson, Wisconsin, 1985–2025.

85%
Good days in 2025
40
Median daily AQI, 2025
1
Days above 100 in 2025
198
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Jefferson, Wisconsin on 198 days. 168 of those days (85%) were in the Good range, and the median daily AQI was 40. That median is about the same as the typical monitored county in Wisconsin (median 38 across 29 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 198 days. Across the years with fuller monitoring, the share of Good days rose: an average of 72% in 1985–1987 versus 82% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (198 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

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%1985: Good 124 of 175 days1985: Moderate 37 of 175 days1985: Unhealthy for Sensitive Groups 12 of 175 days1985: Unhealthy 1 of 175 days1985: Very Unhealthy 1 of 175 days19851986: Good 132 of 172 days1986: Moderate 29 of 172 days1986: Unhealthy for Sensitive Groups 10 of 172 days1986: Unhealthy 1 of 172 days1987: Good 114 of 168 days1987: Moderate 41 of 168 days1987: Unhealthy for Sensitive Groups 9 of 168 days1987: Unhealthy 4 of 168 days1988: Good 86 of 180 days1988: Moderate 46 of 180 days1988: Unhealthy for Sensitive Groups 33 of 180 days1988: Unhealthy 14 of 180 days1988: Very Unhealthy 1 of 180 days1989: Good 105 of 184 days1989: Moderate 47 of 184 days1989: Unhealthy for Sensitive Groups 28 of 184 days1989: Unhealthy 3 of 184 days1989: Very Unhealthy 1 of 184 days1990: Good 122 of 181 days1990: Moderate 49 of 181 days1990: Unhealthy for Sensitive Groups 8 of 181 days1990: Unhealthy 2 of 181 days19901991: Good 120 of 181 days1991: Moderate 44 of 181 days1991: Unhealthy for Sensitive Groups 15 of 181 days1991: Unhealthy 2 of 181 days1992: Good 148 of 184 days1992: Moderate 22 of 184 days1992: Unhealthy for Sensitive Groups 13 of 184 days1992: Unhealthy 1 of 184 days1993: Good 158 of 180 days1993: Moderate 17 of 180 days1993: Unhealthy for Sensitive Groups 5 of 180 days1994: Good 153 of 182 days1994: Moderate 23 of 182 days1994: Unhealthy for Sensitive Groups 6 of 182 days1995: Good 126 of 182 days1995: Moderate 42 of 182 days1995: Unhealthy for Sensitive Groups 10 of 182 days1995: Unhealthy 4 of 182 days19951996: Good 146 of 178 days1996: Moderate 23 of 178 days1996: Unhealthy for Sensitive Groups 8 of 178 days1996: Unhealthy 1 of 178 days1997: Good 136 of 182 days1997: Moderate 38 of 182 days1997: Unhealthy for Sensitive Groups 8 of 182 days1998: Good 114 of 184 days1998: Moderate 56 of 184 days1998: Unhealthy for Sensitive Groups 13 of 184 days1998: Unhealthy 1 of 184 days1999: Good 108 of 237 days1999: Moderate 101 of 237 days1999: Unhealthy for Sensitive Groups 21 of 237 days1999: Unhealthy 7 of 237 days2000: Good 102 of 207 days2000: Moderate 96 of 207 days2000: Unhealthy for Sensitive Groups 7 of 207 days2000: Unhealthy 2 of 207 days20002001: Good 139 of 243 days2001: Moderate 88 of 243 days2001: Unhealthy for Sensitive Groups 12 of 243 days2001: Unhealthy 4 of 243 days2002: Good 132 of 244 days2002: Moderate 95 of 244 days2002: Unhealthy for Sensitive Groups 14 of 244 days2002: Unhealthy 3 of 244 days2003: Good 126 of 204 days2003: Moderate 60 of 204 days2003: Unhealthy for Sensitive Groups 18 of 204 days2004: Good 162 of 192 days2004: Moderate 29 of 192 days2004: Unhealthy for Sensitive Groups 1 of 192 days2005: Good 116 of 185 days2005: Moderate 52 of 185 days2005: Unhealthy for Sensitive Groups 16 of 185 days2005: Unhealthy 1 of 185 days20052006: Good 148 of 185 days2006: Moderate 34 of 185 days2006: Unhealthy for Sensitive Groups 3 of 185 days2007: Good 131 of 184 days2007: Moderate 42 of 184 days2007: Unhealthy for Sensitive Groups 10 of 184 days2007: Unhealthy 1 of 184 days2008: Good 166 of 200 days2008: Moderate 34 of 200 days2009: Good 149 of 180 days2009: Moderate 30 of 180 days2009: Unhealthy for Sensitive Groups 1 of 180 days2010: Good 168 of 194 days2010: Moderate 25 of 194 days2010: Unhealthy for Sensitive Groups 1 of 194 days20102011: Good 157 of 193 days2011: Moderate 34 of 193 days2011: Unhealthy for Sensitive Groups 2 of 193 days2012: Good 130 of 193 days2012: Moderate 51 of 193 days2012: Unhealthy for Sensitive Groups 11 of 193 days2012: Unhealthy 1 of 193 days2013: Good 155 of 193 days2013: Moderate 35 of 193 days2013: Unhealthy for Sensitive Groups 3 of 193 days2014: Good 161 of 195 days2014: Moderate 30 of 195 days2014: Unhealthy for Sensitive Groups 4 of 195 days2015: Good 171 of 198 days2015: Moderate 27 of 198 days20152016: Good 176 of 201 days2016: Moderate 21 of 201 days2016: Unhealthy for Sensitive Groups 4 of 201 days2017: Good 214 of 245 days2017: Moderate 31 of 245 days2018: Good 209 of 243 days2018: Moderate 32 of 243 days2018: Unhealthy for Sensitive Groups 2 of 243 days2019: Good 191 of 203 days2019: Moderate 12 of 203 days2020: Good 189 of 210 days2020: Moderate 17 of 210 days2020: Unhealthy for Sensitive Groups 4 of 210 days20202021: Good 178 of 208 days2021: Moderate 29 of 208 days2021: Unhealthy for Sensitive Groups 1 of 208 days2022: Good 185 of 200 days2022: Moderate 15 of 200 days2023: Good 145 of 203 days2023: Moderate 43 of 203 days2023: Unhealthy for Sensitive Groups 15 of 203 days2024: Good 183 of 205 days2024: Moderate 22 of 205 days2025: Good 168 of 198 days2025: Moderate 29 of 198 days2025: Unhealthy for Sensitive Groups 1 of 198 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 year0501001501985: 90th percentile AQI 931986: 90th percentile AQI 841987: 90th percentile AQI 931988: 90th percentile AQI 1471989: 90th percentile AQI 1191990: 90th percentile AQI 931991: 90th percentile AQI 971992: 90th percentile AQI 741993: 90th percentile AQI 511994: 90th percentile AQI 581995: 90th percentile AQI 1001996: 90th percentile AQI 741997: 90th percentile AQI 801998: 90th percentile AQI 931999: 90th percentile AQI 1122000: 90th percentile AQI 832001: 90th percentile AQI 872002: 90th percentile AQI 932003: 90th percentile AQI 982004: 90th percentile AQI 672005: 90th percentile AQI 1002006: 90th percentile AQI 642007: 90th percentile AQI 872008: 90th percentile AQI 612009: 90th percentile AQI 642010: 90th percentile AQI 582011: 90th percentile AQI 642012: 90th percentile AQI 842013: 90th percentile AQI 642014: 90th percentile AQI 642015: 90th percentile AQI 582016: 90th percentile AQI 612017: 90th percentile AQI 542018: 90th percentile AQI 582019: 90th percentile AQI 482020: 90th percentile AQI 512021: 90th percentile AQI 612022: 90th percentile AQI 492023: 90th percentile AQI 972024: 90th percentile AQI 512025: 90th percentile AQI 541985: Median AQI 431986: Median AQI 401987: Median AQI 441988: Median AQI 511989: Median AQI 471990: Median AQI 451991: Median AQI 441992: Median AQI 361993: Median AQI 371994: Median AQI 391995: Median AQI 421996: Median AQI 391997: Median AQI 421998: Median AQI 471999: Median AQI 532000: Median AQI 512001: Median AQI 472002: Median AQI 482003: Median AQI 462004: Median AQI 392005: Median AQI 452006: Median AQI 412007: Median AQI 442008: Median AQI 412009: Median AQI 382010: Median AQI 412011: Median AQI 402012: Median AQI 442013: Median AQI 412014: Median AQI 422015: Median AQI 402016: Median AQI 402017: Median AQI 392018: Median AQI 392019: Median AQI 372020: Median AQI 382021: Median AQI 402022: Median AQI 382023: Median AQI 442024: Median AQI 392025: Median AQI 40198519901995200020052010201520202025
  • 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
Ozone198100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20251981682910004054105Ozone
2024205183220000395193Ozone
202320314543150004497140Ozone
2022200185150000384987Ozone
20212081782910004061101Ozone
20202101891740003851136Ozone
20192031911200003748100Ozone
20182432093220003958126Ozone
2017245214310000395490Ozone
20162011762140004061108Ozone
2015198171270000405893Ozone
20141951613040004264126Ozone
20131931553530004164112Ozone
201219313051111004484166Ozone
20111931573420004064112Ozone
20101941682510004158108Ozone
20091801493010003864112Ozone
2008200166340000416193Ozone
200718413142101004487154Ozone
20061851483430004164112Ozone
2005185116521610045100159Ozone
20041921622910003967136Ozone
200320412660180004698147Ozone
200224413295143004893161Ozone
200124313988124004787174Ozone
20002071029672005183164Ozone
19992371081012170053112195Ozone
199818411456131004793151Ozone
19971821363880004280143Ozone
19961781462381003974166Ozone
1995182126421040042100172Ozone
19941821532360003958136Ozone
19931801581750003751150Ozone
199218414822131003674169Ozone
199118112044152004497182Ozone
19901811224982004593169Ozone
1989184105472831047119203Ozone
1988180864633141051147207Ozone
19871681144194004493169Ozone
198617213229101004084154Ozone
198517512437121104393204Ozone

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 1985–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem