EnviroNearby

Lewis and Clark County, Montana air quality history

Daily Air Quality Index summaries from EPA monitors in Lewis and Clark, Montana, 1985–2025.

72%
Good days in 2025
44
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 Lewis and Clark, Montana on 365 days. 261 of those days (72%) were in the Good range, and the median daily AQI was 44. That median is higher than the typical monitored county in Montana (median 25.5 across 18 counties), and higher than the typical monitored U.S. county (40). The main pollutant was ozone on 194 days. Across the years with fuller monitoring, the share of Good days fell: an average of 92% in 1988–1990 versus 67% in 2023–2025.

Years with fewer than 120 monitored days (1985, 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%1985: Good 35 of 41 days1985: Moderate 6 of 41 days19851986: Good 46 of 60 days1986: Moderate 14 of 60 days1987: Good 87 of 92 days1987: Moderate 5 of 92 days1988: Good 207 of 234 days1988: Moderate 27 of 234 days1989: Good 253 of 273 days1989: Moderate 20 of 273 days1990: Good 170 of 179 days1990: Moderate 9 of 179 days19901991: Good 138 of 159 days1991: Moderate 21 of 159 days1992: Good 157 of 180 days1992: Moderate 23 of 180 days1993: Good 153 of 183 days1993: Moderate 30 of 183 days1994: Good 329 of 365 days1994: Moderate 36 of 365 days1995: Good 336 of 357 days1995: Moderate 21 of 357 days19951996: Good 339 of 351 days1996: Moderate 12 of 351 days1997: Good 351 of 361 days1997: Moderate 10 of 361 days1998: Good 322 of 338 days1998: Moderate 15 of 338 days1998: Unhealthy for Sensitive Groups 1 of 338 days1999: Good 340 of 365 days1999: Moderate 25 of 365 days2000: Good 308 of 364 days2000: Moderate 50 of 364 days2000: Unhealthy for Sensitive Groups 6 of 364 days20002001: Good 314 of 360 days2001: Moderate 43 of 360 days2001: Unhealthy for Sensitive Groups 3 of 360 days2002: Good 311 of 348 days2002: Moderate 34 of 348 days2002: Unhealthy for Sensitive Groups 3 of 348 days2003: Good 305 of 346 days2003: Moderate 37 of 346 days2003: Unhealthy for Sensitive Groups 2 of 346 days2003: Unhealthy 2 of 346 days2004: Good 281 of 311 days2004: Moderate 28 of 311 days2004: Unhealthy for Sensitive Groups 2 of 311 days2005: Good 326 of 358 days2005: Moderate 29 of 358 days2005: Unhealthy for Sensitive Groups 3 of 358 days20052006: Good 302 of 357 days2006: Moderate 54 of 357 days2006: Unhealthy for Sensitive Groups 1 of 357 days2007: Good 225 of 353 days2007: Moderate 117 of 353 days2007: Unhealthy for Sensitive Groups 7 of 353 days2007: Unhealthy 4 of 353 days2008: Good 268 of 364 days2008: Moderate 94 of 364 days2008: Unhealthy for Sensitive Groups 2 of 364 days2009: Good 265 of 358 days2009: Moderate 90 of 358 days2009: Unhealthy for Sensitive Groups 3 of 358 days2010: Good 248 of 360 days2010: Moderate 98 of 360 days2010: Unhealthy for Sensitive Groups 11 of 360 days2010: Unhealthy 3 of 360 days20102011: Good 247 of 365 days2011: Moderate 116 of 365 days2011: Unhealthy for Sensitive Groups 2 of 365 days2012: Good 232 of 366 days2012: Moderate 127 of 366 days2012: Unhealthy for Sensitive Groups 7 of 366 days2013: Good 278 of 365 days2013: Moderate 87 of 365 days2014: Good 268 of 365 days2014: Moderate 92 of 365 days2014: Unhealthy for Sensitive Groups 5 of 365 days2015: Good 254 of 365 days2015: Moderate 98 of 365 days2015: Unhealthy for Sensitive Groups 8 of 365 days2015: Unhealthy 5 of 365 days20152016: Good 281 of 366 days2016: Moderate 78 of 366 days2016: Unhealthy for Sensitive Groups 7 of 366 days2017: Good 209 of 365 days2017: Moderate 127 of 365 days2017: Unhealthy for Sensitive Groups 17 of 365 days2017: Unhealthy 12 of 365 days2018: Good 238 of 365 days2018: Moderate 117 of 365 days2018: Unhealthy for Sensitive Groups 7 of 365 days2018: Unhealthy 3 of 365 days2019: Good 251 of 365 days2019: Moderate 112 of 365 days2019: Unhealthy for Sensitive Groups 2 of 365 days2020: Good 277 of 366 days2020: Moderate 84 of 366 days2020: Unhealthy for Sensitive Groups 1 of 366 days2020: Unhealthy 4 of 366 days20202021: Good 249 of 365 days2021: Moderate 99 of 365 days2021: Unhealthy for Sensitive Groups 14 of 365 days2021: Unhealthy 3 of 365 days2022: Good 239 of 365 days2022: Moderate 119 of 365 days2022: Unhealthy for Sensitive Groups 4 of 365 days2022: Unhealthy 3 of 365 days2023: Good 240 of 365 days2023: Moderate 120 of 365 days2023: Unhealthy for Sensitive Groups 3 of 365 days2023: Unhealthy 2 of 365 days2024: Good 233 of 366 days2024: Moderate 127 of 366 days2024: Unhealthy for Sensitive Groups 5 of 366 days2024: Unhealthy 1 of 366 days2025: Good 261 of 365 days2025: Moderate 104 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 531986: 90th percentile AQI 711987: 90th percentile AQI 481988: 90th percentile AQI 511989: 90th percentile AQI 471990: 90th percentile AQI 441991: 90th percentile AQI 541992: 90th percentile AQI 531993: 90th percentile AQI 561994: 90th percentile AQI 501995: 90th percentile AQI 411996: 90th percentile AQI 361997: 90th percentile AQI 391998: 90th percentile AQI 391999: 90th percentile AQI 432000: 90th percentile AQI 612001: 90th percentile AQI 562002: 90th percentile AQI 522003: 90th percentile AQI 532004: 90th percentile AQI 492005: 90th percentile AQI 492006: 90th percentile AQI 542007: 90th percentile AQI 752008: 90th percentile AQI 582009: 90th percentile AQI 602010: 90th percentile AQI 722011: 90th percentile AQI 602012: 90th percentile AQI 702013: 90th percentile AQI 622014: 90th percentile AQI 652015: 90th percentile AQI 692016: 90th percentile AQI 582017: 90th percentile AQI 932018: 90th percentile AQI 672019: 90th percentile AQI 682020: 90th percentile AQI 602021: 90th percentile AQI 772022: 90th percentile AQI 632023: 90th percentile AQI 632024: 90th percentile AQI 732025: 90th percentile AQI 651985: Median AQI 311986: Median AQI 301987: Median AQI 241988: Median AQI 231989: Median AQI 191990: Median AQI 251991: Median AQI 231992: Median AQI 301993: Median AQI 281994: Median AQI 241995: Median AQI 181996: Median AQI 181997: Median AQI 181998: Median AQI 191999: Median AQI 192000: Median AQI 192001: Median AQI 222002: Median AQI 212003: Median AQI 192004: Median AQI 182005: Median AQI 182006: Median AQI 242007: Median AQI 392008: Median AQI 342009: Median AQI 352010: Median AQI 332011: Median AQI 442012: Median AQI 432013: Median AQI 402014: Median AQI 422015: Median AQI 442016: Median AQI 422017: Median AQI 472018: Median AQI 462019: Median AQI 432020: Median AQI 422021: Median AQI 442022: Median AQI 452023: Median AQI 452024: Median AQI 462025: Median AQI 44198519901995200020052010201520202025
  • 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
Ozone19453%
Fine particles (PM2.5)17147%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20253652611040000446593Ozone
202436623312751004673160Ozone
202336524012032004563165Ozone
202236523911943004563167PM2.5
202136524999143004477156Ozone
20203662778414004260168Ozone
201936525111220004368144Ozone
201836523811773004667169Ozone
20173652091271712004793177Ozone
20163662817870004258146Ozone
20153652549885004469178Ozone
20143652689250004265150Ozone
20133652788700004062100PM2.5
201236623212770004370148PM2.5
201136524711620004460115Ozone
201036024898113003372160PM2.5
20093582659030003560143PM2.5
20083642689420003458138PM2.5
200735322511774003975169PM2.5
20063573025410002454102PM10
20053583262930001849113PM10
20043112812820001849132PM10
20033463053722001953161PM10
20023483113430002152125PM10
20013603144330002256124PM10
20003643085060001961124PM10
1999365340250000194390PM10
19983383221510001939120PM10
1997361351100000183987PM10
1996351339120000183669PM10
1995357336210000184177PM10
1994365329360000245090PM10
1993183153300000285683PM10
1992180157230000305385PM10
1991159138210000235494PM10
199017917090000254479PM10
1989273253200000194783PM10
1988234207270000235192PM10
1987928750000244859PM10
19866046140000307193PM10
1985413560000315392PM10

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