EnviroNearby

Winnebago County, Illinois air quality history

Daily Air Quality Index summaries from EPA monitors in Winnebago, Illinois, 1980–2025.

69%
Good days in 2025
40
Median daily AQI, 2025
6
Days above 100 in 2025
355
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Winnebago, Illinois on 355 days. 244 of those days (69%) were in the Good range, and the median daily AQI was 40. That median is about the same as the typical monitored county in Illinois (median 42 across 23 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 215 days. Across the years with fuller monitoring, the share of Good days fell: an average of 74% in 1980–1982 versus 61% in 2023–2025.

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 245 of 278 days1980: Moderate 25 of 278 days1980: Unhealthy for Sensitive Groups 7 of 278 days1980: Unhealthy 1 of 278 days19801981: Good 244 of 365 days1981: Moderate 111 of 365 days1981: Unhealthy for Sensitive Groups 10 of 365 days1982: Good 244 of 360 days1982: Moderate 105 of 360 days1982: Unhealthy for Sensitive Groups 9 of 360 days1982: Unhealthy 2 of 360 days1983: Good 251 of 364 days1983: Moderate 90 of 364 days1983: Unhealthy for Sensitive Groups 19 of 364 days1983: Unhealthy 4 of 364 days1984: Good 234 of 366 days1984: Moderate 121 of 366 days1984: Unhealthy for Sensitive Groups 10 of 366 days1984: Unhealthy 1 of 366 days1985: Good 256 of 363 days1985: Moderate 90 of 363 days1985: Unhealthy for Sensitive Groups 15 of 363 days1985: Unhealthy 2 of 363 days19851986: Good 244 of 362 days1986: Moderate 109 of 362 days1986: Unhealthy for Sensitive Groups 8 of 362 days1986: Unhealthy 1 of 362 days1987: Good 253 of 363 days1987: Moderate 88 of 363 days1987: Unhealthy for Sensitive Groups 20 of 363 days1987: Unhealthy 2 of 363 days1988: Good 220 of 355 days1988: Moderate 92 of 355 days1988: Unhealthy for Sensitive Groups 34 of 355 days1988: Unhealthy 8 of 355 days1988: Very Unhealthy 1 of 355 days1989: Good 255 of 365 days1989: Moderate 94 of 365 days1989: Unhealthy for Sensitive Groups 15 of 365 days1989: Unhealthy 1 of 365 days1990: Good 316 of 363 days1990: Moderate 43 of 363 days1990: Unhealthy for Sensitive Groups 4 of 363 days19901991: Good 307 of 365 days1991: Moderate 43 of 365 days1991: Unhealthy for Sensitive Groups 14 of 365 days1991: Unhealthy 1 of 365 days1992: Good 328 of 366 days1992: Moderate 23 of 366 days1992: Unhealthy for Sensitive Groups 13 of 366 days1992: Unhealthy 2 of 366 days1993: Good 347 of 365 days1993: Moderate 16 of 365 days1993: Unhealthy for Sensitive Groups 2 of 365 days1994: Good 323 of 365 days1994: Moderate 34 of 365 days1994: Unhealthy for Sensitive Groups 6 of 365 days1994: Unhealthy 2 of 365 days1995: Good 305 of 365 days1995: Moderate 44 of 365 days1995: Unhealthy for Sensitive Groups 12 of 365 days1995: Unhealthy 4 of 365 days19951996: Good 332 of 366 days1996: Moderate 25 of 366 days1996: Unhealthy for Sensitive Groups 8 of 366 days1996: Unhealthy 1 of 366 days1997: Good 324 of 365 days1997: Moderate 37 of 365 days1997: Unhealthy for Sensitive Groups 4 of 365 days1998: Good 307 of 365 days1998: Moderate 51 of 365 days1998: Unhealthy for Sensitive Groups 7 of 365 days1999: Good 287 of 365 days1999: Moderate 68 of 365 days1999: Unhealthy for Sensitive Groups 10 of 365 days2000: Good 260 of 366 days2000: Moderate 101 of 366 days2000: Unhealthy for Sensitive Groups 5 of 366 days20002001: Good 260 of 365 days2001: Moderate 96 of 365 days2001: Unhealthy for Sensitive Groups 8 of 365 days2001: Unhealthy 1 of 365 days2002: Good 238 of 365 days2002: Moderate 113 of 365 days2002: Unhealthy for Sensitive Groups 12 of 365 days2002: Unhealthy 2 of 365 days2003: Good 286 of 365 days2003: Moderate 72 of 365 days2003: Unhealthy for Sensitive Groups 7 of 365 days2004: Good 314 of 366 days2004: Moderate 48 of 366 days2004: Unhealthy for Sensitive Groups 4 of 366 days2005: Good 273 of 365 days2005: Moderate 82 of 365 days2005: Unhealthy for Sensitive Groups 10 of 365 days20052006: Good 318 of 365 days2006: Moderate 47 of 365 days2007: Good 269 of 365 days2007: Moderate 89 of 365 days2007: Unhealthy for Sensitive Groups 7 of 365 days2008: Good 296 of 366 days2008: Moderate 69 of 366 days2008: Unhealthy for Sensitive Groups 1 of 366 days2009: Good 301 of 365 days2009: Moderate 63 of 365 days2009: Unhealthy for Sensitive Groups 1 of 365 days2010: Good 299 of 365 days2010: Moderate 64 of 365 days2010: Unhealthy for Sensitive Groups 2 of 365 days20102011: Good 329 of 365 days2011: Moderate 35 of 365 days2011: Unhealthy for Sensitive Groups 1 of 365 days2012: Good 313 of 366 days2012: Moderate 47 of 366 days2012: Unhealthy for Sensitive Groups 6 of 366 days2013: Good 340 of 363 days2013: Moderate 23 of 363 days2014: Good 304 of 342 days2014: Moderate 36 of 342 days2014: Unhealthy for Sensitive Groups 2 of 342 days2015: Good 300 of 362 days2015: Moderate 61 of 362 days2015: Unhealthy for Sensitive Groups 1 of 362 days20152016: Good 300 of 354 days2016: Moderate 51 of 354 days2016: Unhealthy for Sensitive Groups 3 of 354 days2017: Good 296 of 354 days2017: Moderate 58 of 354 days2018: Good 310 of 362 days2018: Moderate 49 of 362 days2018: Unhealthy for Sensitive Groups 2 of 362 days2018: Unhealthy 1 of 362 days2019: Good 182 of 341 days2019: Moderate 158 of 341 days2019: Unhealthy for Sensitive Groups 1 of 341 days2020: Good 205 of 366 days2020: Moderate 160 of 366 days2020: Unhealthy for Sensitive Groups 1 of 366 days20202021: Good 190 of 360 days2021: Moderate 170 of 360 days2022: Good 237 of 363 days2022: Moderate 126 of 363 days2023: Good 167 of 365 days2023: Moderate 178 of 365 days2023: Unhealthy for Sensitive Groups 17 of 365 days2023: Unhealthy 1 of 365 days2023: Very Unhealthy 2 of 365 days2024: Good 249 of 361 days2024: Moderate 109 of 361 days2024: Unhealthy for Sensitive Groups 3 of 361 days2025: Good 244 of 355 days2025: Moderate 105 of 355 days2025: Unhealthy for Sensitive Groups 5 of 355 days2025: Unhealthy 1 of 355 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 year0501001501980: 90th percentile AQI 581981: 90th percentile AQI 741982: 90th percentile AQI 801983: 90th percentile AQI 901984: 90th percentile AQI 791985: 90th percentile AQI 781986: 90th percentile AQI 751987: 90th percentile AQI 821988: 90th percentile AQI 1051989: 90th percentile AQI 731990: 90th percentile AQI 551991: 90th percentile AQI 641992: 90th percentile AQI 541993: 90th percentile AQI 471994: 90th percentile AQI 541995: 90th percentile AQI 741996: 90th percentile AQI 491997: 90th percentile AQI 541998: 90th percentile AQI 641999: 90th percentile AQI 712000: 90th percentile AQI 742001: 90th percentile AQI 722002: 90th percentile AQI 802003: 90th percentile AQI 672004: 90th percentile AQI 572005: 90th percentile AQI 772006: 90th percentile AQI 582007: 90th percentile AQI 682008: 90th percentile AQI 602009: 90th percentile AQI 572010: 90th percentile AQI 582011: 90th percentile AQI 502012: 90th percentile AQI 612013: 90th percentile AQI 482014: 90th percentile AQI 532015: 90th percentile AQI 582016: 90th percentile AQI 562017: 90th percentile AQI 572018: 90th percentile AQI 542019: 90th percentile AQI 662020: 90th percentile AQI 642021: 90th percentile AQI 682022: 90th percentile AQI 612023: 90th percentile AQI 772024: 90th percentile AQI 592025: 90th percentile AQI 671980: Median AQI 261981: Median AQI 441982: Median AQI 411983: Median AQI 391984: Median AQI 431985: Median AQI 411986: Median AQI 421987: Median AQI 431988: Median AQI 441989: Median AQI 421990: Median AQI 351991: Median AQI 341992: Median AQI 311993: Median AQI 291994: Median AQI 331995: Median AQI 351996: Median AQI 311997: Median AQI 331998: Median AQI 341999: Median AQI 362000: Median AQI 382001: Median AQI 372002: Median AQI 392003: Median AQI 342004: Median AQI 332005: Median AQI 352006: Median AQI 332007: Median AQI 372008: Median AQI 362009: Median AQI 342010: Median AQI 372011: Median AQI 322012: Median AQI 332013: Median AQI 312014: Median AQI 342015: Median AQI 362016: Median AQI 362017: Median AQI 362018: Median AQI 362019: Median AQI 492020: Median AQI 472021: Median AQI 492022: Median AQI 432023: Median AQI 522024: Median AQI 432025: Median AQI 401980198519901995200020052010201520202025
  • 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)21561%
Ozone14039%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
202535524410551004067169PM2.5
202436124910930004359119PM2.5
2023365167178171205277242PM2.5
20223632371260000436197PM2.5
202136019017000004968100PM2.5
202036620516010004764101PM2.5
201934118215810004966102PM2.5
20183623104921003654151Ozone
2017354296580000365787Ozone
20163543005130003656105Ozone
20153623006110003658108Ozone
20143423043620003453105Ozone
2013363340230000314884Ozone
20123663134760003361115Ozone
20113653293510003250115Ozone
20103652996420003758117Ozone
20093653016310003457102Ozone
20083662966910003660128Ozone
20073652698970003768122Ozone
2006365318470000335896Ozone
200536527382100003577135Ozone
20043663144840003357132Ozone
20033652867270003467136Ozone
2002365238113122003980166Ozone
20013652609681003772153Ozone
200036626010150003874126Ozone
199936528768100003671150Ozone
19983653075170003464122Ozone
19973653243740003354140Ozone
19963663322581003149161Ozone
199536530544124003574182Ozone
19943653233462003354179Ozone
19933653471620002947101Ozone
199236632823132003154159Ozone
199136530743141003464159Ozone
19903633164340003555143Ozone
198936525594151004273177Ozone
1988355220923481044105203Ozone
198736325388202004382174Ozone
198636224410981004275156CO
198536325690152004178174Ozone
1984366234121101004379154Ozone
198336425190194003990179Ozone
198236024410592004180154Ozone
1981365244111100004474147CO
19802782452571002658169Ozone

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 Illinois with a similar median AQI in 2025

All monitored counties in Illinois

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