EnviroNearby

Larimer County, Colorado air quality history

Daily Air Quality Index summaries from EPA monitors in Larimer, Colorado, 1980–2025.

51%
Good days in 2025
50
Median daily AQI, 2025
11
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 Larimer, Colorado on 365 days. 185 of those days (51%) were in the Good range, and the median daily AQI was 50. That median is higher than the typical monitored county in Colorado (median 46 across 27 counties), and higher than the typical monitored U.S. county (40). The main pollutant was ozone on 307 days. Across the years with fuller monitoring, the share of Good days fell: an average of 72% in 1980–1982 versus 47% in 2023–2025.

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 220 of 327 days1980: Moderate 91 of 327 days1980: Unhealthy for Sensitive Groups 11 of 327 days1980: Unhealthy 5 of 327 days19801981: Good 269 of 363 days1981: Moderate 82 of 363 days1981: Unhealthy for Sensitive Groups 9 of 363 days1981: Unhealthy 3 of 363 days1982: Good 253 of 346 days1982: Moderate 87 of 346 days1982: Unhealthy for Sensitive Groups 5 of 346 days1982: Unhealthy 1 of 346 days1983: Good 234 of 332 days1983: Moderate 89 of 332 days1983: Unhealthy for Sensitive Groups 7 of 332 days1983: Very Unhealthy 2 of 332 days1984: Good 259 of 366 days1984: Moderate 97 of 366 days1984: Unhealthy for Sensitive Groups 6 of 366 days1984: Unhealthy 1 of 366 days1984: Very Unhealthy 3 of 366 days1985: Good 273 of 362 days1985: Moderate 84 of 362 days1985: Unhealthy for Sensitive Groups 3 of 362 days1985: Unhealthy 2 of 362 days19851986: Good 275 of 363 days1986: Moderate 75 of 363 days1986: Unhealthy for Sensitive Groups 10 of 363 days1986: Unhealthy 2 of 363 days1986: Very Unhealthy 1 of 363 days1987: Good 237 of 365 days1987: Moderate 119 of 365 days1987: Unhealthy for Sensitive Groups 6 of 365 days1987: Unhealthy 3 of 365 days1988: Good 203 of 366 days1988: Moderate 139 of 366 days1988: Unhealthy for Sensitive Groups 21 of 366 days1988: Unhealthy 3 of 366 days1989: Good 253 of 365 days1989: Moderate 102 of 365 days1989: Unhealthy for Sensitive Groups 10 of 365 days1990: Good 284 of 365 days1990: Moderate 76 of 365 days1990: Unhealthy for Sensitive Groups 4 of 365 days1990: Unhealthy 1 of 365 days19901991: Good 257 of 365 days1991: Moderate 95 of 365 days1991: Unhealthy for Sensitive Groups 13 of 365 days1992: Good 274 of 366 days1992: Moderate 86 of 366 days1992: Unhealthy for Sensitive Groups 6 of 366 days1993: Good 266 of 365 days1993: Moderate 92 of 365 days1993: Unhealthy for Sensitive Groups 6 of 365 days1993: Unhealthy 1 of 365 days1994: Good 237 of 365 days1994: Moderate 118 of 365 days1994: Unhealthy for Sensitive Groups 10 of 365 days1995: Good 268 of 365 days1995: Moderate 90 of 365 days1995: Unhealthy for Sensitive Groups 7 of 365 days19951996: Good 245 of 366 days1996: Moderate 113 of 366 days1996: Unhealthy for Sensitive Groups 8 of 366 days1997: Good 270 of 365 days1997: Moderate 92 of 365 days1997: Unhealthy for Sensitive Groups 3 of 365 days1998: Good 222 of 365 days1998: Moderate 124 of 365 days1998: Unhealthy for Sensitive Groups 19 of 365 days1999: Good 250 of 365 days1999: Moderate 108 of 365 days1999: Unhealthy for Sensitive Groups 7 of 365 days2000: Good 199 of 366 days2000: Moderate 145 of 366 days2000: Unhealthy for Sensitive Groups 20 of 366 days2000: Unhealthy 2 of 366 days20002001: Good 243 of 365 days2001: Moderate 111 of 365 days2001: Unhealthy for Sensitive Groups 11 of 365 days2002: Good 167 of 365 days2002: Moderate 156 of 365 days2002: Unhealthy for Sensitive Groups 38 of 365 days2002: Unhealthy 4 of 365 days2003: Good 182 of 365 days2003: Moderate 149 of 365 days2003: Unhealthy for Sensitive Groups 30 of 365 days2003: Unhealthy 4 of 365 days2004: Good 206 of 366 days2004: Moderate 151 of 366 days2004: Unhealthy for Sensitive Groups 9 of 366 days2005: Good 229 of 365 days2005: Moderate 124 of 365 days2005: Unhealthy for Sensitive Groups 11 of 365 days2005: Unhealthy 1 of 365 days20052006: Good 175 of 365 days2006: Moderate 151 of 365 days2006: Unhealthy for Sensitive Groups 33 of 365 days2006: Unhealthy 6 of 365 days2007: Good 176 of 365 days2007: Moderate 154 of 365 days2007: Unhealthy for Sensitive Groups 33 of 365 days2007: Unhealthy 2 of 365 days2008: Good 196 of 366 days2008: Moderate 149 of 366 days2008: Unhealthy for Sensitive Groups 21 of 366 days2009: Good 219 of 365 days2009: Moderate 138 of 365 days2009: Unhealthy for Sensitive Groups 8 of 365 days2010: Good 166 of 365 days2010: Moderate 178 of 365 days2010: Unhealthy for Sensitive Groups 21 of 365 days20102011: Good 143 of 365 days2011: Moderate 189 of 365 days2011: Unhealthy for Sensitive Groups 32 of 365 days2011: Unhealthy 1 of 365 days2012: Good 160 of 366 days2012: Moderate 156 of 366 days2012: Unhealthy for Sensitive Groups 45 of 366 days2012: Unhealthy 5 of 366 days2013: Good 184 of 365 days2013: Moderate 153 of 365 days2013: Unhealthy for Sensitive Groups 26 of 365 days2013: Unhealthy 2 of 365 days2014: Good 196 of 365 days2014: Moderate 152 of 365 days2014: Unhealthy for Sensitive Groups 17 of 365 days2015: Good 177 of 365 days2015: Moderate 171 of 365 days2015: Unhealthy for Sensitive Groups 16 of 365 days2015: Unhealthy 1 of 365 days20152016: Good 162 of 366 days2016: Moderate 196 of 366 days2016: Unhealthy for Sensitive Groups 7 of 366 days2016: Unhealthy 1 of 366 days2017: Good 165 of 365 days2017: Moderate 187 of 365 days2017: Unhealthy for Sensitive Groups 12 of 365 days2017: Unhealthy 1 of 365 days2018: Good 160 of 365 days2018: Moderate 177 of 365 days2018: Unhealthy for Sensitive Groups 25 of 365 days2018: Unhealthy 3 of 365 days2019: Good 183 of 365 days2019: Moderate 176 of 365 days2019: Unhealthy for Sensitive Groups 6 of 365 days2020: Good 197 of 366 days2020: Moderate 147 of 366 days2020: Unhealthy for Sensitive Groups 20 of 366 days2020: Unhealthy 2 of 366 days20202021: Good 164 of 365 days2021: Moderate 151 of 365 days2021: Unhealthy for Sensitive Groups 46 of 365 days2021: Unhealthy 4 of 365 days2022: Good 176 of 365 days2022: Moderate 179 of 365 days2022: Unhealthy for Sensitive Groups 10 of 365 days2023: Good 175 of 365 days2023: Moderate 185 of 365 days2023: Unhealthy for Sensitive Groups 4 of 365 days2023: Unhealthy 1 of 365 days2024: Good 156 of 366 days2024: Moderate 187 of 366 days2024: Unhealthy for Sensitive Groups 21 of 366 days2024: Unhealthy 2 of 366 days2025: Good 185 of 365 days2025: Moderate 169 of 365 days2025: Unhealthy for Sensitive Groups 11 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 841981: 90th percentile AQI 771982: 90th percentile AQI 691983: 90th percentile AQI 771984: 90th percentile AQI 771985: 90th percentile AQI 671986: 90th percentile AQI 731987: 90th percentile AQI 831988: 90th percentile AQI 901989: 90th percentile AQI 771990: 90th percentile AQI 641991: 90th percentile AQI 771992: 90th percentile AQI 691993: 90th percentile AQI 691994: 90th percentile AQI 771995: 90th percentile AQI 671996: 90th percentile AQI 741997: 90th percentile AQI 671998: 90th percentile AQI 901999: 90th percentile AQI 772000: 90th percentile AQI 902001: 90th percentile AQI 772002: 90th percentile AQI 1082003: 90th percentile AQI 1002004: 90th percentile AQI 802005: 90th percentile AQI 802006: 90th percentile AQI 1012007: 90th percentile AQI 1002008: 90th percentile AQI 902009: 90th percentile AQI 802010: 90th percentile AQI 902011: 90th percentile AQI 1002012: 90th percentile AQI 1052013: 90th percentile AQI 932014: 90th percentile AQI 802015: 90th percentile AQI 802016: 90th percentile AQI 872017: 90th percentile AQI 872018: 90th percentile AQI 972019: 90th percentile AQI 772020: 90th percentile AQI 892021: 90th percentile AQI 1122022: 90th percentile AQI 842023: 90th percentile AQI 772024: 90th percentile AQI 872025: 90th percentile AQI 841980: Median AQI 341981: Median AQI 411982: Median AQI 411983: Median AQI 401984: Median AQI 411985: Median AQI 411986: Median AQI 401987: Median AQI 441988: Median AQI 491989: Median AQI 441990: Median AQI 401991: Median AQI 451992: Median AQI 441993: Median AQI 451994: Median AQI 471995: Median AQI 451996: Median AQI 451997: Median AQI 441998: Median AQI 481999: Median AQI 452000: Median AQI 492001: Median AQI 462002: Median AQI 522003: Median AQI 512004: Median AQI 492005: Median AQI 482006: Median AQI 512007: Median AQI 512008: Median AQI 502009: Median AQI 492010: Median AQI 532011: Median AQI 582012: Median AQI 542013: Median AQI 502014: Median AQI 502015: Median AQI 512016: Median AQI 512017: Median AQI 522018: Median AQI 532019: Median AQI 502020: Median AQI 492021: Median AQI 542022: Median AQI 512023: Median AQI 512024: Median AQI 542025: Median AQI 501980198519901995200020052010201520202025
  • 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
Ozone30784%
Fine particles (PM2.5)5816%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025365185169110005084119Ozone
2024366156187212005487161Ozone
202336517518541005177156Ozone
2022365176179100005184126Ozone
20213651641514640054112159Ozone
2020366197147202004989159Ozone
201936518317660005077129Ozone
2018365160177253005397169Ozone
2017365165187121005287154Ozone
201636616219671005187151Ozone
2015365177171161005180165Ozone
2014365196152170005080140Ozone
2013365184153262005093164Ozone
20123661601564550054105172Ozone
20113651431893210058100151Ozone
2010365166178210005390140Ozone
200936521913880004980140Ozone
2008366196149210005090143Ozone
20073651761543320051100156Ozone
20063651751513360051101179Ozone
2005365229124111004880151Ozone
200436620615190004980119Ozone
20033651821493040051100166Ozone
20023651671563840052108169Ozone
2001365243111110004677140Ozone
2000366199145202004990159Ozone
199936525010870004577150Ozone
1998365222124190004890150Ozone
19973652709230004467129Ozone
199636624511380004574129Ozone
19953652689070004567136Ozone
1994365237118100004777143Ozone
19933652669261004569169Ozone
19923662748660004469143Ozone
199136525795130004577133Ozone
19903652847641004064154Ozone
1989365253102100004477126Ozone
1988366203139213004990195Ozone
198736523711963004483175Ozone
198636327575102104073206Ozone
19853622738432004167163Ozone
19843662599761304177227Ozone
19833322348970204077277Ozone
19823462538751004169151Ozone
19813632698293004177186CO
198032722091115003484190CO

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 Colorado with a similar median AQI in 2025

All monitored counties in Colorado

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