EnviroNearby

Silver Bow County, Montana air quality history

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

85%
Good days in 2025
28
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 Silver Bow, Montana on 365 days. 310 of those days (85%) were in the Good range, and the median daily AQI was 28. That median is higher than the typical monitored county in Montana (median 25.5 across 18 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 350 days. Across the years with fuller monitoring, the share of Good days rose: an average of 74% in 1980–1988 versus 76% in 2023–2025.

Years with fewer than 120 monitored days (1981, 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%1980: Good 144 of 176 days1980: Moderate 26 of 176 days1980: Unhealthy for Sensitive Groups 6 of 176 days19801981: Good 60 of 99 days1981: Moderate 32 of 99 days1981: Unhealthy for Sensitive Groups 7 of 99 days1985: Good 17 of 28 days1985: Moderate 4 of 28 days1985: Unhealthy for Sensitive Groups 6 of 28 days1985: Unhealthy 1 of 28 days1986: Good 25 of 40 days1986: Moderate 9 of 40 days1986: Unhealthy for Sensitive Groups 6 of 40 days1987: Good 81 of 140 days1987: Moderate 54 of 140 days1987: Unhealthy for Sensitive Groups 5 of 140 days1988: Good 288 of 354 days1988: Moderate 55 of 354 days1988: Unhealthy for Sensitive Groups 9 of 354 days1988: Unhealthy 2 of 354 days19881989: Good 300 of 341 days1989: Moderate 40 of 341 days1989: Unhealthy for Sensitive Groups 1 of 341 days1990: Good 319 of 335 days1990: Moderate 16 of 335 days1991: Good 301 of 348 days1991: Moderate 46 of 348 days1991: Unhealthy for Sensitive Groups 1 of 348 days1992: Good 279 of 327 days1992: Moderate 47 of 327 days1992: Unhealthy for Sensitive Groups 1 of 327 days1993: Good 270 of 303 days1993: Moderate 33 of 303 days19931994: Good 326 of 360 days1994: Moderate 34 of 360 days1995: Good 344 of 358 days1995: Moderate 14 of 358 days1996: Good 340 of 352 days1996: Moderate 12 of 352 days1997: Good 325 of 339 days1997: Moderate 14 of 339 days1998: Good 349 of 365 days1998: Moderate 15 of 365 days1998: Unhealthy for Sensitive Groups 1 of 365 days19981999: Good 337 of 365 days1999: Moderate 27 of 365 days1999: Unhealthy for Sensitive Groups 1 of 365 days2000: Good 319 of 366 days2000: Moderate 41 of 366 days2000: Unhealthy for Sensitive Groups 3 of 366 days2000: Unhealthy 2 of 366 days2000: Very Unhealthy 1 of 366 days2001: Good 333 of 365 days2001: Moderate 31 of 365 days2001: Unhealthy for Sensitive Groups 1 of 365 days2002: Good 344 of 365 days2002: Moderate 21 of 365 days2003: Good 334 of 365 days2003: Moderate 28 of 365 days2003: Unhealthy for Sensitive Groups 3 of 365 days20032004: Good 332 of 366 days2004: Moderate 32 of 366 days2004: Unhealthy for Sensitive Groups 2 of 366 days2005: Good 314 of 365 days2005: Moderate 48 of 365 days2005: Unhealthy for Sensitive Groups 3 of 365 days2006: Good 302 of 365 days2006: Moderate 57 of 365 days2006: Unhealthy for Sensitive Groups 6 of 365 days2007: Good 261 of 360 days2007: Moderate 92 of 360 days2007: Unhealthy for Sensitive Groups 6 of 360 days2007: Unhealthy 1 of 360 days2008: Good 254 of 365 days2008: Moderate 109 of 365 days2008: Unhealthy for Sensitive Groups 2 of 365 days20082009: Good 264 of 364 days2009: Moderate 94 of 364 days2009: Unhealthy for Sensitive Groups 6 of 364 days2010: Good 224 of 362 days2010: Moderate 124 of 362 days2010: Unhealthy for Sensitive Groups 14 of 362 days2011: Good 233 of 364 days2011: Moderate 126 of 364 days2011: Unhealthy for Sensitive Groups 5 of 364 days2012: Good 225 of 363 days2012: Moderate 125 of 363 days2012: Unhealthy for Sensitive Groups 8 of 363 days2012: Unhealthy 5 of 363 days2013: Good 238 of 365 days2013: Moderate 121 of 365 days2013: Unhealthy for Sensitive Groups 5 of 365 days2013: Unhealthy 1 of 365 days20132014: Good 232 of 362 days2014: Moderate 129 of 362 days2014: Unhealthy for Sensitive Groups 1 of 362 days2015: Good 239 of 365 days2015: Moderate 116 of 365 days2015: Unhealthy for Sensitive Groups 4 of 365 days2015: Unhealthy 6 of 365 days2016: Good 283 of 366 days2016: Moderate 83 of 366 days2017: Good 188 of 354 days2017: Moderate 142 of 354 days2017: Unhealthy for Sensitive Groups 20 of 354 days2017: Unhealthy 4 of 354 days2018: Good 284 of 363 days2018: Moderate 78 of 363 days2018: Unhealthy for Sensitive Groups 1 of 363 days20182019: Good 293 of 363 days2019: Moderate 70 of 363 days2020: Good 289 of 366 days2020: Moderate 69 of 366 days2020: Unhealthy for Sensitive Groups 5 of 366 days2020: Unhealthy 3 of 366 days2021: Good 256 of 365 days2021: Moderate 89 of 365 days2021: Unhealthy for Sensitive Groups 14 of 365 days2021: Unhealthy 6 of 365 days2022: Good 268 of 365 days2022: Moderate 87 of 365 days2022: Unhealthy for Sensitive Groups 8 of 365 days2022: Unhealthy 2 of 365 days2023: Good 268 of 365 days2023: Moderate 94 of 365 days2023: Unhealthy for Sensitive Groups 3 of 365 days20232024: Good 258 of 366 days2024: Moderate 103 of 366 days2024: Unhealthy for Sensitive Groups 3 of 366 days2024: Unhealthy 2 of 366 days2025: Good 310 of 365 days2025: Moderate 55 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 year0501001501980: 90th percentile AQI 711981: 90th percentile AQI 881985: 90th percentile AQI 1411986: 90th percentile AQI 1181987: 90th percentile AQI 751988: 90th percentile AQI 631989: 90th percentile AQI 521990: 90th percentile AQI 431991: 90th percentile AQI 561992: 90th percentile AQI 541993: 90th percentile AQI 511994: 90th percentile AQI 481995: 90th percentile AQI 371996: 90th percentile AQI 391997: 90th percentile AQI 411998: 90th percentile AQI 421999: 90th percentile AQI 442000: 90th percentile AQI 562001: 90th percentile AQI 462002: 90th percentile AQI 362003: 90th percentile AQI 452004: 90th percentile AQI 492005: 90th percentile AQI 572006: 90th percentile AQI 582007: 90th percentile AQI 672008: 90th percentile AQI 642009: 90th percentile AQI 632010: 90th percentile AQI 752011: 90th percentile AQI 732012: 90th percentile AQI 802013: 90th percentile AQI 682014: 90th percentile AQI 662015: 90th percentile AQI 692016: 90th percentile AQI 632017: 90th percentile AQI 932018: 90th percentile AQI 612019: 90th percentile AQI 602020: 90th percentile AQI 612021: 90th percentile AQI 802022: 90th percentile AQI 672023: 90th percentile AQI 662024: 90th percentile AQI 662025: 90th percentile AQI 541980: Median AQI 361981: Median AQI 431985: Median AQI 351986: Median AQI 451987: Median AQI 451988: Median AQI 261989: Median AQI 191990: Median AQI 211991: Median AQI 211992: Median AQI 221993: Median AQI 201994: Median AQI 231995: Median AQI 171996: Median AQI 191997: Median AQI 191998: Median AQI 241999: Median AQI 262000: Median AQI 242001: Median AQI 222002: Median AQI 192003: Median AQI 212004: Median AQI 202005: Median AQI 232006: Median AQI 242007: Median AQI 332008: Median AQI 382009: Median AQI 352010: Median AQI 392011: Median AQI 412012: Median AQI 392013: Median AQI 372014: Median AQI 402015: Median AQI 392016: Median AQI 292017: Median AQI 462018: Median AQI 252019: Median AQI 242020: Median AQI 222021: Median AQI 332022: Median AQI 272023: Median AQI 312024: Median AQI 312025: Median AQI 281980198819931998200320082013201820232025
  • 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)35096%
Coarse particles (PM10)154%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025365310550000285496PM2.5
202436625810332003166168PM2.5
20233652689430003166124PM2.5
20223652688782002767162PM2.5
202136525689146003380163PM2.5
20203662896953002261181PM2.5
2019363293700000246092PM2.5
20183632847810002561120PM2.5
2017354188142204004693191PM2.5
2016366283830000296394PM2.5
201536523911646003969175PM2.5
201436223212910004066108PM2.5
201336523812151003768153PM2.5
201236322512585003980183PM2.5
201136423312650004173135PM2.5
2010362224124140003975139PM2.5
20093642649460003563138PM2.5
200836525410920003864102PM2.5
20073602619261003367173PM10
20063653025760002458145PM10
20053653144830002357137PM10
20043663323220002049120PM10
20033653342830002145123CO
2002365344210000193691CO
20013653333110002246105CO
20003663194132102456209CO
19993653372710002644107CO
19983653491510002442109CO
1997339325140000194185PM10
1996352340120000193969PM10
1995358344140000173772PM10
1994360326340000234877PM10
1993303270330000205169PM10
19923272794710002254106PM10
19913483014610002156103PM10
1990335319160000214369PM10
19893413004010001952102PM10
19883542885592002663165PM10
1987140815450004575136PM10
198640259600045118130PM10
198528174610035141174PM10
198199603270004388110NO2
19801761442660003671112Ozone

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