EnviroNearby

McHenry County, Illinois air quality history

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

68%
Good days in 2025
42
Median daily AQI, 2025
12
Days above 100 in 2025
364
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for McHenry, Illinois on 364 days. 247 of those days (68%) were in the Good range, and the median daily AQI was 42. 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 185 days. Across the years with fuller monitoring, the share of Good days fell: an average of 84% in 1980–1982 versus 62% 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 99 of 123 days1980: Moderate 16 of 123 days1980: Unhealthy for Sensitive Groups 6 of 123 days1980: Unhealthy 2 of 123 days19801981: Good 281 of 324 days1981: Moderate 31 of 324 days1981: Unhealthy for Sensitive Groups 9 of 324 days1981: Unhealthy 2 of 324 days1981: Very Unhealthy 1 of 324 days1982: Good 296 of 346 days1982: Moderate 36 of 346 days1982: Unhealthy for Sensitive Groups 11 of 346 days1982: Unhealthy 2 of 346 days1982: Very Unhealthy 1 of 346 days1983: Good 307 of 358 days1983: Moderate 31 of 358 days1983: Unhealthy for Sensitive Groups 16 of 358 days1983: Unhealthy 3 of 358 days1983: Very Unhealthy 1 of 358 days1984: Good 328 of 353 days1984: Moderate 20 of 353 days1984: Unhealthy for Sensitive Groups 4 of 353 days1984: Unhealthy 1 of 353 days1985: Good 324 of 351 days1985: Moderate 23 of 351 days1985: Unhealthy for Sensitive Groups 4 of 351 days19851986: Good 288 of 316 days1986: Moderate 22 of 316 days1986: Unhealthy for Sensitive Groups 5 of 316 days1986: Unhealthy 1 of 316 days1987: Good 309 of 362 days1987: Moderate 39 of 362 days1987: Unhealthy for Sensitive Groups 11 of 362 days1987: Unhealthy 2 of 362 days1987: Very Unhealthy 1 of 362 days1988: Good 250 of 322 days1988: Moderate 44 of 322 days1988: Unhealthy for Sensitive Groups 24 of 322 days1988: Unhealthy 3 of 322 days1988: Very Unhealthy 1 of 322 days1989: Good 304 of 363 days1989: Moderate 44 of 363 days1989: Unhealthy for Sensitive Groups 11 of 363 days1989: Unhealthy 4 of 363 days1990: Good 311 of 345 days1990: Moderate 32 of 345 days1990: Unhealthy for Sensitive Groups 2 of 345 days19901991: Good 295 of 354 days1991: Moderate 44 of 354 days1991: Unhealthy for Sensitive Groups 12 of 354 days1991: Unhealthy 2 of 354 days1991: Very Unhealthy 1 of 354 days1992: Good 266 of 291 days1992: Moderate 20 of 291 days1992: Unhealthy for Sensitive Groups 4 of 291 days1992: Unhealthy 1 of 291 days1993: Good 197 of 239 days1993: Moderate 33 of 239 days1993: Unhealthy for Sensitive Groups 8 of 239 days1993: Unhealthy 1 of 239 days1994: Good 303 of 343 days1994: Moderate 35 of 343 days1994: Unhealthy for Sensitive Groups 2 of 343 days1994: Unhealthy 3 of 343 days1995: Good 291 of 359 days1995: Moderate 46 of 359 days1995: Unhealthy for Sensitive Groups 14 of 359 days1995: Unhealthy 8 of 359 days19951996: Good 323 of 366 days1996: Moderate 35 of 366 days1996: Unhealthy for Sensitive Groups 5 of 366 days1996: Unhealthy 3 of 366 days1997: Good 319 of 364 days1997: Moderate 37 of 364 days1997: Unhealthy for Sensitive Groups 5 of 364 days1997: Unhealthy 3 of 364 days1998: Good 312 of 364 days1998: Moderate 37 of 364 days1998: Unhealthy for Sensitive Groups 15 of 364 days1999: Good 305 of 358 days1999: Moderate 34 of 358 days1999: Unhealthy for Sensitive Groups 13 of 358 days1999: Unhealthy 6 of 358 days2000: Good 269 of 363 days2000: Moderate 85 of 363 days2000: Unhealthy for Sensitive Groups 9 of 363 days20002001: Good 245 of 362 days2001: Moderate 101 of 362 days2001: Unhealthy for Sensitive Groups 13 of 362 days2001: Unhealthy 3 of 362 days2002: Good 254 of 364 days2002: Moderate 86 of 364 days2002: Unhealthy for Sensitive Groups 19 of 364 days2002: Unhealthy 5 of 364 days2003: Good 271 of 363 days2003: Moderate 86 of 363 days2003: Unhealthy for Sensitive Groups 6 of 363 days2004: Good 271 of 358 days2004: Moderate 84 of 358 days2004: Unhealthy for Sensitive Groups 3 of 358 days2005: Good 240 of 363 days2005: Moderate 99 of 363 days2005: Unhealthy for Sensitive Groups 20 of 363 days2005: Unhealthy 4 of 363 days20052006: Good 285 of 363 days2006: Moderate 78 of 363 days2007: Good 149 of 365 days2007: Moderate 207 of 365 days2007: Unhealthy for Sensitive Groups 9 of 365 days2008: Good 194 of 366 days2008: Moderate 172 of 366 days2009: Good 178 of 365 days2009: Moderate 187 of 365 days2010: Good 225 of 362 days2010: Moderate 136 of 362 days2010: Unhealthy for Sensitive Groups 1 of 362 days20102011: Good 135 of 363 days2011: Moderate 223 of 363 days2011: Unhealthy for Sensitive Groups 5 of 363 days2012: Good 187 of 363 days2012: Moderate 167 of 363 days2012: Unhealthy for Sensitive Groups 8 of 363 days2012: Unhealthy 1 of 363 days2013: Good 238 of 361 days2013: Moderate 122 of 361 days2013: Unhealthy for Sensitive Groups 1 of 361 days2014: Good 248 of 364 days2014: Moderate 113 of 364 days2014: Unhealthy for Sensitive Groups 3 of 364 days2015: Good 173 of 364 days2015: Moderate 189 of 364 days2015: Unhealthy for Sensitive Groups 2 of 364 days20152016: Good 241 of 362 days2016: Moderate 115 of 362 days2016: Unhealthy for Sensitive Groups 6 of 362 days2017: Good 226 of 357 days2017: Moderate 128 of 357 days2017: Unhealthy for Sensitive Groups 3 of 357 days2018: Good 213 of 362 days2018: Moderate 141 of 362 days2018: Unhealthy for Sensitive Groups 8 of 362 days2019: Good 161 of 340 days2019: Moderate 177 of 340 days2019: Unhealthy for Sensitive Groups 2 of 340 days2020: Good 192 of 334 days2020: Moderate 133 of 334 days2020: Unhealthy for Sensitive Groups 9 of 334 days20202021: Good 187 of 363 days2021: Moderate 174 of 363 days2021: Unhealthy for Sensitive Groups 2 of 363 days2022: Good 199 of 365 days2022: Moderate 163 of 365 days2022: Unhealthy for Sensitive Groups 3 of 365 days2023: Good 191 of 365 days2023: Moderate 153 of 365 days2023: Unhealthy for Sensitive Groups 16 of 365 days2023: Unhealthy 4 of 365 days2023: Very Unhealthy 1 of 365 days2024: Good 245 of 366 days2024: Moderate 120 of 366 days2024: Unhealthy for Sensitive Groups 1 of 366 days2025: Good 247 of 364 days2025: Moderate 105 of 364 days2025: Unhealthy for Sensitive Groups 11 of 364 days2025: Unhealthy 1 of 364 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 841981: 90th percentile AQI 671982: 90th percentile AQI 641983: 90th percentile AQI 711984: 90th percentile AQI 471985: 90th percentile AQI 471986: 90th percentile AQI 481987: 90th percentile AQI 641988: 90th percentile AQI 871989: 90th percentile AQI 671990: 90th percentile AQI 501991: 90th percentile AQI 711992: 90th percentile AQI 491993: 90th percentile AQI 671994: 90th percentile AQI 541995: 90th percentile AQI 771996: 90th percentile AQI 511997: 90th percentile AQI 581998: 90th percentile AQI 641999: 90th percentile AQI 742000: 90th percentile AQI 702001: 90th percentile AQI 842002: 90th percentile AQI 842003: 90th percentile AQI 672004: 90th percentile AQI 642005: 90th percentile AQI 882006: 90th percentile AQI 612007: 90th percentile AQI 772008: 90th percentile AQI 682009: 90th percentile AQI 702010: 90th percentile AQI 682011: 90th percentile AQI 722012: 90th percentile AQI 772013: 90th percentile AQI 642014: 90th percentile AQI 612015: 90th percentile AQI 672016: 90th percentile AQI 592017: 90th percentile AQI 642018: 90th percentile AQI 712019: 90th percentile AQI 652020: 90th percentile AQI 622021: 90th percentile AQI 682022: 90th percentile AQI 662023: 90th percentile AQI 822024: 90th percentile AQI 612025: 90th percentile AQI 711980: Median AQI 321981: Median AQI 301982: Median AQI 311983: Median AQI 301984: Median AQI 281985: Median AQI 301986: Median AQI 301987: Median AQI 291988: Median AQI 311989: Median AQI 321990: Median AQI 301991: Median AQI 311992: Median AQI 311993: Median AQI 331994: Median AQI 321995: Median AQI 311996: Median AQI 311997: Median AQI 311998: Median AQI 311999: Median AQI 332000: Median AQI 352001: Median AQI 382002: Median AQI 372003: Median AQI 342004: Median AQI 342005: Median AQI 392006: Median AQI 342007: Median AQI 542008: Median AQI 482009: Median AQI 522010: Median AQI 442011: Median AQI 542012: Median AQI 492013: Median AQI 432014: Median AQI 422015: Median AQI 512016: Median AQI 442017: Median AQI 442018: Median AQI 462019: Median AQI 522020: Median AQI 472021: Median AQI 492022: Median AQI 472023: Median AQI 492024: Median AQI 432025: Median AQI 421980198519901995200020052010201520202025
  • 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)18551%
Ozone17949%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025364247105111004271165PM2.5
202436624512010004361108PM2.5
2023365191153164104982230PM2.5
202236519916330004766126PM2.5
202136318717420004968105PM2.5
202033419213390004762147PM2.5
201934016117720005265126PM2.5
201836221314180004671122Ozone
201735722612830004464133PM2.5
201636224111560004459129PM2.5
201536417318920005167103PM2.5
201436424811330004261136PM2.5
201336123812210004364119PM2.5
201236318716781004977177PM2.5
201136313522350005472129PM2.5
201036222513610004468115PM2.5
200936517818700005270100PM2.5
200836619417200004868100PM2.5
200736514920790005477133PM2.5
2006363285780000346198Ozone
200536324099204003988172Ozone
20043582718430003464105Ozone
20033632718660003467147Ozone
200236425486195003784169Ozone
2001362245101133003884159Ozone
20003632698590003570136Ozone
199935830534136003374187Ozone
199836431237150003164150Ozone
19973643193753003158164Ozone
19963663233553003151161Ozone
199535929146148003177197Ozone
19943433033523003254164Ozone
19932391973381003367151Ozone
19922912662041003149166Ozone
199135429544122103171209Ozone
19903453113220003050126Ozone
198936330444114003267166Ozone
198832225044243103187203Ozone
198736230939112102964205Ozone
19863162882251003048151Ozone
19853513242340003047112Ozone
19843533282041002847151Ozone
198335830731163103071201Ozone
198234629636112103164212Ozone
19813242813192103067213Ozone
1980123991662003284192Ozone

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