EnviroNearby

Sheridan County, Wyoming air quality history

Daily Air Quality Index summaries from EPA monitors in Sheridan, Wyoming, 1985–2025.

89%
Good days in 2025
27
Median daily AQI, 2025
0
Days above 100 in 2025
359
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Sheridan, Wyoming on 359 days. 320 of those days (89%) were in the Good range, and the median daily AQI was 27. That median is lower than the typical monitored county in Wyoming (median 41 across 17 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 279 days. Across the years with fuller monitoring, the share of Good days rose: an average of 71% in 1987–1989 versus 81% in 2023–2025.

Years with fewer than 120 monitored days (1985, 1986) 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%1985: Good 22 of 54 days1985: Moderate 30 of 54 days1985: Unhealthy for Sensitive Groups 2 of 54 days19851986: Good 35 of 61 days1986: Moderate 24 of 61 days1986: Unhealthy for Sensitive Groups 2 of 61 days1987: Good 120 of 203 days1987: Moderate 83 of 203 days1988: Good 206 of 278 days1988: Moderate 70 of 278 days1988: Unhealthy for Sensitive Groups 2 of 278 days1989: Good 136 of 171 days1989: Moderate 35 of 171 days1990: Good 128 of 163 days1990: Moderate 35 of 163 days19901991: Good 129 of 190 days1991: Moderate 60 of 190 days1991: Unhealthy for Sensitive Groups 1 of 190 days1992: Good 211 of 265 days1992: Moderate 54 of 265 days1993: Good 211 of 269 days1993: Moderate 58 of 269 days1994: Good 214 of 264 days1994: Moderate 50 of 264 days1995: Good 213 of 264 days1995: Moderate 51 of 264 days19951996: Good 235 of 257 days1996: Moderate 22 of 257 days1997: Good 227 of 248 days1997: Moderate 20 of 248 days1997: Unhealthy for Sensitive Groups 1 of 248 days1998: Good 216 of 256 days1998: Moderate 39 of 256 days1998: Unhealthy for Sensitive Groups 1 of 256 days1999: Good 203 of 283 days1999: Moderate 80 of 283 days2000: Good 164 of 226 days2000: Moderate 58 of 226 days2000: Unhealthy for Sensitive Groups 4 of 226 days20002001: Good 177 of 241 days2001: Moderate 60 of 241 days2001: Unhealthy for Sensitive Groups 4 of 241 days2002: Good 179 of 242 days2002: Moderate 62 of 242 days2002: Unhealthy for Sensitive Groups 1 of 242 days2003: Good 171 of 238 days2003: Moderate 67 of 238 days2004: Good 173 of 237 days2004: Moderate 59 of 237 days2004: Unhealthy for Sensitive Groups 5 of 237 days2005: Good 173 of 246 days2005: Moderate 72 of 246 days2005: Unhealthy for Sensitive Groups 1 of 246 days20052006: Good 167 of 237 days2006: Moderate 70 of 237 days2007: Good 204 of 259 days2007: Moderate 53 of 259 days2007: Unhealthy for Sensitive Groups 2 of 259 days2008: Good 318 of 363 days2008: Moderate 45 of 363 days2009: Good 315 of 363 days2009: Moderate 47 of 363 days2009: Unhealthy for Sensitive Groups 1 of 363 days2010: Good 320 of 365 days2010: Moderate 45 of 365 days20102011: Good 327 of 362 days2011: Moderate 35 of 362 days2012: Good 307 of 358 days2012: Moderate 51 of 358 days2013: Good 325 of 365 days2013: Moderate 40 of 365 days2014: Good 336 of 365 days2014: Moderate 29 of 365 days2015: Good 318 of 365 days2015: Moderate 45 of 365 days2015: Unhealthy for Sensitive Groups 2 of 365 days20152016: Good 326 of 366 days2016: Moderate 39 of 366 days2016: Unhealthy for Sensitive Groups 1 of 366 days2017: Good 250 of 365 days2017: Moderate 111 of 365 days2017: Unhealthy for Sensitive Groups 3 of 365 days2017: Unhealthy 1 of 365 days2018: Good 241 of 365 days2018: Moderate 121 of 365 days2018: Unhealthy for Sensitive Groups 3 of 365 days2019: Good 315 of 365 days2019: Moderate 50 of 365 days2020: Good 334 of 366 days2020: Moderate 32 of 366 days20202021: Good 302 of 364 days2021: Moderate 55 of 364 days2021: Unhealthy for Sensitive Groups 7 of 364 days2022: Good 270 of 365 days2022: Moderate 95 of 365 days2023: Good 283 of 365 days2023: Moderate 79 of 365 days2023: Unhealthy for Sensitive Groups 2 of 365 days2023: Unhealthy 1 of 365 days2024: Good 281 of 366 days2024: Moderate 69 of 366 days2024: Unhealthy for Sensitive Groups 9 of 366 days2024: Unhealthy 6 of 366 days2024: Very Unhealthy 1 of 366 days2025: Good 320 of 359 days2025: Moderate 39 of 359 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 year0501001985: 90th percentile AQI 851986: 90th percentile AQI 791987: 90th percentile AQI 681988: 90th percentile AQI 621989: 90th percentile AQI 561990: 90th percentile AQI 581991: 90th percentile AQI 631992: 90th percentile AQI 581993: 90th percentile AQI 581994: 90th percentile AQI 571995: 90th percentile AQI 581996: 90th percentile AQI 491997: 90th percentile AQI 481998: 90th percentile AQI 551999: 90th percentile AQI 612000: 90th percentile AQI 722001: 90th percentile AQI 682002: 90th percentile AQI 612003: 90th percentile AQI 632004: 90th percentile AQI 622005: 90th percentile AQI 622006: 90th percentile AQI 612007: 90th percentile AQI 592008: 90th percentile AQI 522009: 90th percentile AQI 532010: 90th percentile AQI 542011: 90th percentile AQI 502012: 90th percentile AQI 532013: 90th percentile AQI 512014: 90th percentile AQI 482015: 90th percentile AQI 542016: 90th percentile AQI 542017: 90th percentile AQI 672018: 90th percentile AQI 682019: 90th percentile AQI 542020: 90th percentile AQI 482021: 90th percentile AQI 672022: 90th percentile AQI 582023: 90th percentile AQI 592024: 90th percentile AQI 672025: 90th percentile AQI 511985: Median AQI 541986: Median AQI 421987: Median AQI 461988: Median AQI 381989: Median AQI 311990: Median AQI 321991: Median AQI 361992: Median AQI 311993: Median AQI 301994: Median AQI 291995: Median AQI 311996: Median AQI 241997: Median AQI 271998: Median AQI 281999: Median AQI 362000: Median AQI 362001: Median AQI 342002: Median AQI 362003: Median AQI 352004: Median AQI 362005: Median AQI 352006: Median AQI 362007: Median AQI 312008: Median AQI 242009: Median AQI 242010: Median AQI 222011: Median AQI 202012: Median AQI 232013: Median AQI 402014: Median AQI 382015: Median AQI 402016: Median AQI 392017: Median AQI 432018: Median AQI 462019: Median AQI 392020: Median AQI 402021: Median AQI 402022: Median AQI 442023: Median AQI 432024: Median AQI 442025: Median AQI 27198519901995200020052010201520202025
  • 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)27978%
Ozone4713%
Coarse particles (PM10)308%
Nitrogen dioxide (NO₂)31%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025359320390000275175PM2.5
20243662816996104467205Ozone
20233652837921004359175Ozone
2022365270950000445890Ozone
20213643025570004067115Ozone
20203663343200004048100Ozone
2019365315500000395490Ozone
201836524112130004668123Ozone
201736525011131004367157Ozone
20163663263910003954110Ozone
20153653184520004054146Ozone
2014365336290000384884Ozone
2013365325400000405178Ozone
2012358307510000235380PM10
2011362327350000205091PM10
2010365320450000225495PM10
20093633154710002453109PM10
2008363318450000245285PM10
20072592045320003159107PM10
2006237167700000366193PM10
20052461737210003562106PM10
20042371735950003662116PM10
2003238171670000356393PM10
20022421796210003661106PM10
20012411776040003468131PM10
20002261645840003672124PM2.5
1999283203800000366198PM10
19982562163910002855104PM10
19972482272010002748114PM10
1996257235220000244985PM10
1995264213510000315890PM10
1994264214500000295785PM10
1993269211580000305879PM10
1992265211540000315886PM10
19911901296010003663122PM10
19901631283500003258100PM10
1989171136350000315678PM10
19882782067020003862115PM10
1987203120830000466893PM10
198661352420004279109PM10
198554223020005485126PM10

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 Wyoming with a similar median AQI in 2025

All monitored counties in Wyoming

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