EnviroNearby

Yellowstone County, Montana air quality history

Daily Air Quality Index summaries from EPA monitors in Yellowstone, Montana, 1980–2025.

86%
Good days in 2025
38
Median daily AQI, 2025
0
Days above 100 in 2025
365
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Yellowstone, Montana on 365 days. 315 of those days (86%) were in the Good range, and the median daily AQI was 38. That median is higher than the typical monitored county in Montana (median 25.5 across 18 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 220 days. Across the years with fuller monitoring, the share of Good days fell: an average of 93% in 1980–1982 versus 83% in 2023–2025.

Years with fewer than 120 monitored days (1986, 1987) 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 201 of 213 days1980: Moderate 12 of 213 days19801981: Good 247 of 270 days1981: Moderate 23 of 270 days1982: Good 263 of 282 days1982: Moderate 19 of 282 days1983: Good 190 of 219 days1983: Moderate 27 of 219 days1983: Unhealthy for Sensitive Groups 2 of 219 days1984: Good 131 of 139 days1984: Moderate 8 of 139 days1985: Good 304 of 305 days1985: Moderate 1 of 305 days19851986: Good 107 of 116 days1986: Moderate 9 of 116 days1987: Good 24 of 24 days1988: Good 309 of 336 days1988: Moderate 27 of 336 days1989: Good 302 of 315 days1989: Moderate 13 of 315 days1990: Good 350 of 361 days1990: Moderate 11 of 361 days19901991: Good 350 of 365 days1991: Moderate 15 of 365 days1992: Good 336 of 354 days1992: Moderate 18 of 354 days1993: Good 330 of 365 days1993: Moderate 35 of 365 days1994: Good 340 of 365 days1994: Moderate 25 of 365 days1995: Good 349 of 365 days1995: Moderate 16 of 365 days19951996: Good 345 of 366 days1996: Moderate 21 of 366 days1997: Good 349 of 365 days1997: Moderate 16 of 365 days1998: Good 341 of 365 days1998: Moderate 24 of 365 days1999: Good 328 of 365 days1999: Moderate 36 of 365 days1999: Unhealthy for Sensitive Groups 1 of 365 days2000: Good 311 of 348 days2000: Moderate 36 of 348 days2000: Unhealthy for Sensitive Groups 1 of 348 days20002001: Good 331 of 365 days2001: Moderate 34 of 365 days2002: Good 343 of 365 days2002: Moderate 22 of 365 days2003: Good 333 of 365 days2003: Moderate 32 of 365 days2004: Good 306 of 327 days2004: Moderate 20 of 327 days2004: Very Unhealthy 1 of 327 days2005: Good 331 of 365 days2005: Moderate 34 of 365 days20052006: Good 283 of 365 days2006: Moderate 82 of 365 days2007: Good 311 of 365 days2007: Moderate 53 of 365 days2007: Unhealthy for Sensitive Groups 1 of 365 days2008: Good 319 of 366 days2008: Moderate 47 of 366 days2009: Good 336 of 365 days2009: Moderate 29 of 365 days2010: Good 319 of 365 days2010: Moderate 46 of 365 days20102011: Good 319 of 362 days2011: Moderate 43 of 362 days2012: Good 303 of 362 days2012: Moderate 59 of 362 days2013: Good 325 of 355 days2013: Moderate 30 of 355 days2014: Good 247 of 324 days2014: Moderate 77 of 324 days2015: Good 260 of 325 days2015: Moderate 64 of 325 days2015: Unhealthy for Sensitive Groups 1 of 325 days20152016: Good 273 of 323 days2016: Moderate 49 of 323 days2016: Unhealthy for Sensitive Groups 1 of 323 days2017: Good 272 of 356 days2017: Moderate 79 of 356 days2017: Unhealthy for Sensitive Groups 5 of 356 days2018: Good 251 of 345 days2018: Moderate 91 of 345 days2018: Unhealthy for Sensitive Groups 2 of 345 days2018: Unhealthy 1 of 345 days2019: Good 271 of 292 days2019: Moderate 20 of 292 days2019: Unhealthy 1 of 292 days2020: Good 304 of 364 days2020: Moderate 57 of 364 days2020: Unhealthy for Sensitive Groups 2 of 364 days2020: Unhealthy 1 of 364 days20202021: Good 212 of 340 days2021: Moderate 118 of 340 days2021: Unhealthy for Sensitive Groups 9 of 340 days2021: Unhealthy 1 of 340 days2022: Good 307 of 365 days2022: Moderate 56 of 365 days2022: Unhealthy for Sensitive Groups 1 of 365 days2022: Unhealthy 1 of 365 days2023: Good 306 of 357 days2023: Moderate 47 of 357 days2023: Unhealthy for Sensitive Groups 3 of 357 days2023: Unhealthy 1 of 357 days2024: Good 272 of 357 days2024: Moderate 84 of 357 days2024: Unhealthy for Sensitive Groups 1 of 357 days2025: Good 315 of 365 days2025: Moderate 50 of 365 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 421981: 90th percentile AQI 491982: 90th percentile AQI 441983: 90th percentile AQI 551984: 90th percentile AQI 481985: 90th percentile AQI 301986: 90th percentile AQI 451987: 90th percentile AQI 371988: 90th percentile AQI 491989: 90th percentile AQI 431990: 90th percentile AQI 401991: 90th percentile AQI 401992: 90th percentile AQI 411993: 90th percentile AQI 501994: 90th percentile AQI 441995: 90th percentile AQI 401996: 90th percentile AQI 411997: 90th percentile AQI 391998: 90th percentile AQI 441999: 90th percentile AQI 512000: 90th percentile AQI 522001: 90th percentile AQI 492002: 90th percentile AQI 432003: 90th percentile AQI 472004: 90th percentile AQI 452005: 90th percentile AQI 502006: 90th percentile AQI 582007: 90th percentile AQI 532008: 90th percentile AQI 522009: 90th percentile AQI 472010: 90th percentile AQI 532011: 90th percentile AQI 512012: 90th percentile AQI 542013: 90th percentile AQI 482014: 90th percentile AQI 572015: 90th percentile AQI 542016: 90th percentile AQI 532017: 90th percentile AQI 632018: 90th percentile AQI 602019: 90th percentile AQI 472020: 90th percentile AQI 552021: 90th percentile AQI 702022: 90th percentile AQI 542023: 90th percentile AQI 532024: 90th percentile AQI 612025: 90th percentile AQI 541980: Median AQI 281981: Median AQI 191982: Median AQI 241983: Median AQI 221984: Median AQI 221985: Median AQI 171986: Median AQI 261987: Median AQI 291988: Median AQI 361989: Median AQI 321990: Median AQI 211991: Median AQI 221992: Median AQI 231993: Median AQI 271994: Median AQI 231995: Median AQI 231996: Median AQI 201997: Median AQI 201998: Median AQI 201999: Median AQI 202000: Median AQI 202001: Median AQI 192002: Median AQI 162003: Median AQI 172004: Median AQI 172005: Median AQI 312006: Median AQI 412007: Median AQI 362008: Median AQI 212009: Median AQI 222010: Median AQI 262011: Median AQI 262012: Median AQI 232013: Median AQI 192014: Median AQI 332015: Median AQI 372016: Median AQI 312017: Median AQI 332018: Median AQI 372019: Median AQI 312020: Median AQI 312021: Median AQI 402022: Median AQI 322023: Median AQI 292024: Median AQI 392025: Median AQI 381980198519901995200020052010201520202025
  • 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
Ozone22060%
Fine particles (PM2.5)13537%
Nitrogen dioxide (NO₂)103%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025365315500000385491Ozone
20243572728410003961105Ozone
20233573064731002953191PM2.5
20223653075611003254155PM2.5
202134021211891004070167PM2.5
20203643045721003155163PM2.5
20192922712001003147159PM2.5
20183452519121003760155PM2.5
20173562727950003363141PM2.5
20163232734910003153124PM2.5
20153252606410003754122PM2.5
2014324247770000335778PM2.5
2013355325300000194879PM2.5
2012362303590000235495PM2.5
2011362319430000265180PM2.5
2010365319460000265384PM2.5
2009365336290000224772PM2.5
2008366319470000215297PM2.5
20073653115310003653102Ozone
2006365283820000415890Ozone
2005365331340000315092Ozone
20043273062000101745227CO
20033653333200001747100CO
2002365343220000164367CO
2001365331340000194986CO
20003483113610002052102CO
19993653283610002051107CO
1998365341240000204475CO
1997365349160000203993CO
1996366345210000204188CO
1995365349160000234077CO
1994365340250000234487CO
1993365330350000275077CO
1992354336180000234171CO
1991365350150000224064CO
1990361350110000214064CO
1989315302130000324377Ozone
1988336309270000364977Ozone
1987242400000293749PM10
198611610790000264565CO
198530530410000173051CO
198413913180000224877CO
19832191902720002255106CO
1982282263190000244479CO
1981270247230000194992CO
1980213201120000284286CO

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 Montana with a similar median AQI in 2025

All monitored counties in Montana

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