EnviroNearby

Flathead County, Montana air quality history

Daily Air Quality Index summaries from EPA monitors in Flathead, Montana, 1985–2025.

74%
Good days in 2025
41
Median daily AQI, 2025
1
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 Flathead, Montana on 365 days. 269 of those days (74%) were in the Good range, and the median daily AQI was 41. 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 fine particles (PM2.5) on 177 days. Across the years with fuller monitoring, the share of Good days held roughly steady: an average of 70% in 1987–1989 versus 69% 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 27 of 38 days1985: Moderate 11 of 38 days19851986: Good 32 of 63 days1986: Moderate 28 of 63 days1986: Unhealthy for Sensitive Groups 2 of 63 days1986: Unhealthy 1 of 63 days1987: Good 79 of 134 days1987: Moderate 53 of 134 days1987: Unhealthy for Sensitive Groups 2 of 134 days1988: Good 258 of 360 days1988: Moderate 99 of 360 days1988: Unhealthy for Sensitive Groups 1 of 360 days1988: Unhealthy 1 of 360 days1988: Very Unhealthy 1 of 360 days1989: Good 293 of 363 days1989: Moderate 67 of 363 days1989: Unhealthy for Sensitive Groups 3 of 363 days1990: Good 298 of 365 days1990: Moderate 67 of 365 days19901991: Good 281 of 365 days1991: Moderate 81 of 365 days1991: Unhealthy for Sensitive Groups 3 of 365 days1992: Good 281 of 364 days1992: Moderate 75 of 364 days1992: Unhealthy for Sensitive Groups 6 of 364 days1992: Unhealthy 2 of 364 days1993: Good 300 of 365 days1993: Moderate 65 of 365 days1994: Good 273 of 365 days1994: Moderate 89 of 365 days1994: Unhealthy for Sensitive Groups 3 of 365 days1995: Good 307 of 365 days1995: Moderate 58 of 365 days19951996: Good 294 of 366 days1996: Moderate 68 of 366 days1996: Unhealthy for Sensitive Groups 4 of 366 days1997: Good 311 of 365 days1997: Moderate 53 of 365 days1997: Unhealthy for Sensitive Groups 1 of 365 days1998: Good 313 of 365 days1998: Moderate 50 of 365 days1998: Unhealthy for Sensitive Groups 2 of 365 days1999: Good 267 of 365 days1999: Moderate 96 of 365 days1999: Unhealthy for Sensitive Groups 2 of 365 days2000: Good 276 of 366 days2000: Moderate 89 of 366 days2000: Unhealthy for Sensitive Groups 1 of 366 days20002001: Good 312 of 365 days2001: Moderate 51 of 365 days2001: Unhealthy for Sensitive Groups 2 of 365 days2002: Good 303 of 365 days2002: Moderate 62 of 365 days2003: Good 271 of 365 days2003: Moderate 84 of 365 days2003: Unhealthy for Sensitive Groups 6 of 365 days2003: Unhealthy 4 of 365 days2004: Good 287 of 366 days2004: Moderate 78 of 366 days2004: Unhealthy 1 of 366 days2005: Good 282 of 365 days2005: Moderate 82 of 365 days2005: Unhealthy for Sensitive Groups 1 of 365 days20052006: Good 281 of 365 days2006: Moderate 83 of 365 days2006: Unhealthy for Sensitive Groups 1 of 365 days2007: Good 270 of 365 days2007: Moderate 91 of 365 days2007: Unhealthy for Sensitive Groups 2 of 365 days2007: Unhealthy 2 of 365 days2008: Good 209 of 366 days2008: Moderate 157 of 366 days2009: Good 122 of 365 days2009: Moderate 243 of 365 days2010: Good 188 of 365 days2010: Moderate 177 of 365 days20102011: Good 221 of 365 days2011: Moderate 144 of 365 days2012: Good 258 of 366 days2012: Moderate 108 of 366 days2013: Good 235 of 365 days2013: Moderate 126 of 365 days2013: Unhealthy for Sensitive Groups 4 of 365 days2014: Good 254 of 365 days2014: Moderate 109 of 365 days2014: Unhealthy for Sensitive Groups 2 of 365 days2015: Good 216 of 365 days2015: Moderate 137 of 365 days2015: Unhealthy for Sensitive Groups 3 of 365 days2015: Unhealthy 9 of 365 days20152016: Good 298 of 366 days2016: Moderate 68 of 366 days2017: Good 250 of 365 days2017: Moderate 104 of 365 days2017: Unhealthy for Sensitive Groups 5 of 365 days2017: Unhealthy 3 of 365 days2017: Very Unhealthy 3 of 365 days2018: Good 222 of 365 days2018: Moderate 127 of 365 days2018: Unhealthy for Sensitive Groups 11 of 365 days2018: Unhealthy 5 of 365 days2019: Good 267 of 365 days2019: Moderate 97 of 365 days2019: Unhealthy for Sensitive Groups 1 of 365 days2020: Good 286 of 366 days2020: Moderate 72 of 366 days2020: Unhealthy for Sensitive Groups 3 of 366 days2020: Unhealthy 4 of 366 days2020: Very Unhealthy 1 of 366 days20202021: Good 268 of 365 days2021: Moderate 88 of 365 days2021: Unhealthy for Sensitive Groups 8 of 365 days2021: Unhealthy 1 of 365 days2022: Good 249 of 365 days2022: Moderate 113 of 365 days2022: Unhealthy for Sensitive Groups 2 of 365 days2022: Unhealthy 1 of 365 days2023: Good 237 of 365 days2023: Moderate 120 of 365 days2023: Unhealthy for Sensitive Groups 6 of 365 days2023: Unhealthy 2 of 365 days2024: Good 253 of 366 days2024: Moderate 113 of 366 days2025: Good 269 of 365 days2025: Moderate 95 of 365 days2025: Unhealthy for Sensitive Groups 1 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 year0501001985: 90th percentile AQI 581986: 90th percentile AQI 721987: 90th percentile AQI 641988: 90th percentile AQI 651989: 90th percentile AQI 561990: 90th percentile AQI 571991: 90th percentile AQI 611992: 90th percentile AQI 611993: 90th percentile AQI 551994: 90th percentile AQI 691995: 90th percentile AQI 561996: 90th percentile AQI 571997: 90th percentile AQI 541998: 90th percentile AQI 531999: 90th percentile AQI 622000: 90th percentile AQI 632001: 90th percentile AQI 552002: 90th percentile AQI 562003: 90th percentile AQI 652004: 90th percentile AQI 582005: 90th percentile AQI 602006: 90th percentile AQI 592007: 90th percentile AQI 612008: 90th percentile AQI 662009: 90th percentile AQI 662010: 90th percentile AQI 662011: 90th percentile AQI 652012: 90th percentile AQI 652013: 90th percentile AQI 642014: 90th percentile AQI 642015: 90th percentile AQI 742016: 90th percentile AQI 562017: 90th percentile AQI 712018: 90th percentile AQI 702019: 90th percentile AQI 612020: 90th percentile AQI 592021: 90th percentile AQI 662022: 90th percentile AQI 642023: 90th percentile AQI 682024: 90th percentile AQI 632025: 90th percentile AQI 621985: Median AQI 411986: Median AQI 501987: Median AQI 441988: Median AQI 401989: Median AQI 381990: Median AQI 381991: Median AQI 401992: Median AQI 381993: Median AQI 351994: Median AQI 401995: Median AQI 351996: Median AQI 411997: Median AQI 361998: Median AQI 361999: Median AQI 402000: Median AQI 382001: Median AQI 352002: Median AQI 372003: Median AQI 392004: Median AQI 382005: Median AQI 392006: Median AQI 392007: Median AQI 382008: Median AQI 472009: Median AQI 542010: Median AQI 492011: Median AQI 452012: Median AQI 412013: Median AQI 442014: Median AQI 412015: Median AQI 442016: Median AQI 372017: Median AQI 422018: Median AQI 452019: Median AQI 392020: Median AQI 372021: Median AQI 392022: Median AQI 412023: Median AQI 432024: Median AQI 422025: Median AQI 41198519901995200020052010201520202025
  • 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)17748%
Ozone17548%
Coarse particles (PM10)134%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253652699510004162103PM2.5
20243662531130000426395PM2.5
202336523712062004368160PM2.5
202236524911321004164152PM2.5
20213652688881003966155Ozone
20203662867234103759206Ozone
20193652679710003961103Ozone
2018365222127115004570193PM2.5
201736525010453304271275PM2.5
2016366298680000375680Ozone
201536521613739004474190PM2.5
201436525410920004164141PM2.5
201336523512640004464125PM2.5
20123662581080000416595PM2.5
20113652211440000456589PM2.5
20103651881770000496696PM2.5
20093651222430000546699PM2.5
20083662091570000476683PM2.5
20073652709122003861159Ozone
20063652818310003959105Ozone
20053652828210003960105Ozone
20043662877801003858152Ozone
20033652718464003965170Ozone
2002365303620000375691Ozone
20013653125120003555139Ozone
20003662768910003863132Ozone
19993652679620004062145Ozone
19983653135020003653128Ozone
19973653115310003654112PM10
19963662946840004157133Ozone
1995365307580000355693Ozone
19943652738930004069110Ozone
1993365300650000355592Ozone
19923642817562003861190Ozone
19913652818130004061121Ozone
1990365298670000385779Ozone
19893632936730003856132Ozone
19883602589911104065256PM10
1987134795320004464116PM10
198663322821005072153PM10
19853827110000415862PM10

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 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