EnviroNearby

Jefferson County, Colorado air quality history

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

59%
Good days in 2025
48
Median daily AQI, 2025
13
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 Jefferson, Colorado on 365 days. 215 of those days (59%) were in the Good range, and the median daily AQI was 48. That median is about the same as 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 365 days. Across the years with fuller monitoring, the share of Good days held roughly steady: an average of 55% in 1980–1982 versus 56% 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 194 of 362 days1980: Moderate 140 of 362 days1980: Unhealthy for Sensitive Groups 26 of 362 days1980: Unhealthy 2 of 362 days19801981: Good 205 of 364 days1981: Moderate 131 of 364 days1981: Unhealthy for Sensitive Groups 23 of 364 days1981: Unhealthy 4 of 364 days1981: Very Unhealthy 1 of 364 days1982: Good 199 of 363 days1982: Moderate 134 of 363 days1982: Unhealthy for Sensitive Groups 23 of 363 days1982: Unhealthy 7 of 363 days1983: Good 197 of 365 days1983: Moderate 120 of 365 days1983: Unhealthy for Sensitive Groups 38 of 365 days1983: Unhealthy 10 of 365 days1984: Good 206 of 365 days1984: Moderate 134 of 365 days1984: Unhealthy for Sensitive Groups 22 of 365 days1984: Unhealthy 3 of 365 days1985: Good 220 of 364 days1985: Moderate 129 of 364 days1985: Unhealthy for Sensitive Groups 15 of 364 days19851986: Good 223 of 364 days1986: Moderate 121 of 364 days1986: Unhealthy for Sensitive Groups 20 of 364 days1987: Good 207 of 365 days1987: Moderate 134 of 365 days1987: Unhealthy for Sensitive Groups 21 of 365 days1987: Unhealthy 2 of 365 days1987: Very Unhealthy 1 of 365 days1988: Good 240 of 366 days1988: Moderate 108 of 366 days1988: Unhealthy for Sensitive Groups 15 of 366 days1988: Unhealthy 3 of 366 days1989: Good 234 of 365 days1989: Moderate 94 of 365 days1989: Unhealthy for Sensitive Groups 33 of 365 days1989: Unhealthy 4 of 365 days1990: Good 285 of 365 days1990: Moderate 72 of 365 days1990: Unhealthy for Sensitive Groups 7 of 365 days1990: Unhealthy 1 of 365 days19901991: Good 267 of 360 days1991: Moderate 80 of 360 days1991: Unhealthy for Sensitive Groups 13 of 360 days1992: Good 263 of 365 days1992: Moderate 89 of 365 days1992: Unhealthy for Sensitive Groups 10 of 365 days1992: Unhealthy 3 of 365 days1993: Good 232 of 365 days1993: Moderate 105 of 365 days1993: Unhealthy for Sensitive Groups 23 of 365 days1993: Unhealthy 5 of 365 days1994: Good 225 of 365 days1994: Moderate 111 of 365 days1994: Unhealthy for Sensitive Groups 27 of 365 days1994: Unhealthy 2 of 365 days1995: Good 261 of 365 days1995: Moderate 82 of 365 days1995: Unhealthy for Sensitive Groups 19 of 365 days1995: Unhealthy 3 of 365 days19951996: Good 233 of 366 days1996: Moderate 98 of 366 days1996: Unhealthy for Sensitive Groups 35 of 366 days1997: Good 240 of 365 days1997: Moderate 103 of 365 days1997: Unhealthy for Sensitive Groups 22 of 365 days1998: Good 219 of 365 days1998: Moderate 108 of 365 days1998: Unhealthy for Sensitive Groups 31 of 365 days1998: Unhealthy 7 of 365 days1999: Good 237 of 365 days1999: Moderate 105 of 365 days1999: Unhealthy for Sensitive Groups 21 of 365 days1999: Unhealthy 2 of 365 days2000: Good 249 of 366 days2000: Moderate 78 of 366 days2000: Unhealthy for Sensitive Groups 38 of 366 days2000: Unhealthy 1 of 366 days20002001: Good 265 of 365 days2001: Moderate 75 of 365 days2001: Unhealthy for Sensitive Groups 23 of 365 days2001: Unhealthy 2 of 365 days2002: Good 247 of 365 days2002: Moderate 79 of 365 days2002: Unhealthy for Sensitive Groups 32 of 365 days2002: Unhealthy 7 of 365 days2003: Good 250 of 365 days2003: Moderate 69 of 365 days2003: Unhealthy for Sensitive Groups 33 of 365 days2003: Unhealthy 13 of 365 days2004: Good 256 of 366 days2004: Moderate 93 of 366 days2004: Unhealthy for Sensitive Groups 17 of 366 days2005: Good 258 of 365 days2005: Moderate 83 of 365 days2005: Unhealthy for Sensitive Groups 24 of 365 days20052006: Good 222 of 365 days2006: Moderate 95 of 365 days2006: Unhealthy for Sensitive Groups 41 of 365 days2006: Unhealthy 7 of 365 days2007: Good 225 of 365 days2007: Moderate 97 of 365 days2007: Unhealthy for Sensitive Groups 37 of 365 days2007: Unhealthy 6 of 365 days2008: Good 212 of 366 days2008: Moderate 125 of 366 days2008: Unhealthy for Sensitive Groups 28 of 366 days2008: Unhealthy 1 of 366 days2009: Good 263 of 365 days2009: Moderate 92 of 365 days2009: Unhealthy for Sensitive Groups 9 of 365 days2009: Unhealthy 1 of 365 days2010: Good 233 of 365 days2010: Moderate 106 of 365 days2010: Unhealthy for Sensitive Groups 26 of 365 days20102011: Good 198 of 365 days2011: Moderate 129 of 365 days2011: Unhealthy for Sensitive Groups 36 of 365 days2011: Unhealthy 2 of 365 days2012: Good 201 of 366 days2012: Moderate 113 of 366 days2012: Unhealthy for Sensitive Groups 48 of 366 days2012: Unhealthy 4 of 366 days2013: Good 209 of 365 days2013: Moderate 121 of 365 days2013: Unhealthy for Sensitive Groups 31 of 365 days2013: Unhealthy 4 of 365 days2014: Good 228 of 365 days2014: Moderate 121 of 365 days2014: Unhealthy for Sensitive Groups 16 of 365 days2015: Good 244 of 365 days2015: Moderate 104 of 365 days2015: Unhealthy for Sensitive Groups 15 of 365 days2015: Unhealthy 2 of 365 days20152016: Good 245 of 366 days2016: Moderate 101 of 366 days2016: Unhealthy for Sensitive Groups 17 of 366 days2016: Unhealthy 3 of 366 days2017: Good 214 of 365 days2017: Moderate 123 of 365 days2017: Unhealthy for Sensitive Groups 28 of 365 days2018: Good 229 of 365 days2018: Moderate 99 of 365 days2018: Unhealthy for Sensitive Groups 35 of 365 days2018: Unhealthy 2 of 365 days2019: Good 238 of 365 days2019: Moderate 111 of 365 days2019: Unhealthy for Sensitive Groups 16 of 365 days2020: Good 231 of 366 days2020: Moderate 108 of 366 days2020: Unhealthy for Sensitive Groups 23 of 366 days2020: Unhealthy 4 of 366 days20202021: Good 222 of 365 days2021: Moderate 88 of 365 days2021: Unhealthy for Sensitive Groups 44 of 365 days2021: Unhealthy 11 of 365 days2022: Good 234 of 365 days2022: Moderate 100 of 365 days2022: Unhealthy for Sensitive Groups 30 of 365 days2022: Unhealthy 1 of 365 days2023: Good 206 of 365 days2023: Moderate 141 of 365 days2023: Unhealthy for Sensitive Groups 18 of 365 days2024: Good 192 of 366 days2024: Moderate 137 of 366 days2024: Unhealthy for Sensitive Groups 30 of 366 days2024: Unhealthy 7 of 366 days2025: Good 215 of 365 days2025: Moderate 137 of 365 days2025: Unhealthy for Sensitive Groups 13 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 901981: 90th percentile AQI 971982: 90th percentile AQI 931983: 90th percentile AQI 1151984: 90th percentile AQI 931985: 90th percentile AQI 871986: 90th percentile AQI 841987: 90th percentile AQI 931988: 90th percentile AQI 841989: 90th percentile AQI 1011990: 90th percentile AQI 711991: 90th percentile AQI 771992: 90th percentile AQI 811993: 90th percentile AQI 931994: 90th percentile AQI 971995: 90th percentile AQI 871996: 90th percentile AQI 971997: 90th percentile AQI 841998: 90th percentile AQI 1011999: 90th percentile AQI 902000: 90th percentile AQI 1012001: 90th percentile AQI 872002: 90th percentile AQI 1012003: 90th percentile AQI 1052004: 90th percentile AQI 842005: 90th percentile AQI 872006: 90th percentile AQI 1082007: 90th percentile AQI 1052008: 90th percentile AQI 1002009: 90th percentile AQI 772010: 90th percentile AQI 932011: 90th percentile AQI 1012012: 90th percentile AQI 1082013: 90th percentile AQI 1002014: 90th percentile AQI 842015: 90th percentile AQI 842016: 90th percentile AQI 872017: 90th percentile AQI 932018: 90th percentile AQI 1012019: 90th percentile AQI 872020: 90th percentile AQI 902021: 90th percentile AQI 1152022: 90th percentile AQI 972023: 90th percentile AQI 902024: 90th percentile AQI 1012025: 90th percentile AQI 841980: Median AQI 491981: Median AQI 481982: Median AQI 481983: Median AQI 481984: Median AQI 481985: Median AQI 451986: Median AQI 441987: Median AQI 451988: Median AQI 441989: Median AQI 451990: Median AQI 401991: Median AQI 411992: Median AQI 411993: Median AQI 441994: Median AQI 461995: Median AQI 441996: Median AQI 441997: Median AQI 451998: Median AQI 461999: Median AQI 442000: Median AQI 442001: Median AQI 422002: Median AQI 432003: Median AQI 442004: Median AQI 432005: Median AQI 442006: Median AQI 452007: Median AQI 452008: Median AQI 482009: Median AQI 442010: Median AQI 462011: Median AQI 492012: Median AQI 482013: Median AQI 472014: Median AQI 462015: Median AQI 452016: Median AQI 462017: Median AQI 472018: Median AQI 462019: Median AQI 462020: Median AQI 462021: Median AQI 472022: Median AQI 472023: Median AQI 492024: Median AQI 502025: Median AQI 481980198519901995200020052010201520202025
  • 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
Ozone365100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025365215137130004884119Ozone
20243661921373070050101174Ozone
2023365206141180004990143Ozone
2022365234100301004797166Ozone
20213652228844110047115172Ozone
2020366231108234004690161Ozone
2019365238111160004687129Ozone
2018365229993520046101151Ozone
2017365214123280004793136Ozone
2016366245101173004687159Ozone
2015365244104152004584164Ozone
2014365228121160004684140Ozone
20133652091213140047100169Ozone
20123662011134840048108190Ozone
20113651981293620049101197Ozone
2010365233106260004693143Ozone
20093652639291004477151Ozone
20083662121252810048100174Ozone
2007365225973760045105182Ozone
2006365222954170045108177Ozone
200536525883240004487150Ozone
200436625693170004384136Ozone
20033652506933130044105192Ozone
2002365247793270043101166Ozone
200136526575232004287161Ozone
2000366249783810044101159Ozone
1999365237105212004490166Ozone
19983652191083170046101195Ozone
1997365240103220004584143Ozone
199636623398350004497150Ozone
199536526182193004487156Ozone
1994365225111272004697156Ozone
1993365232105235004493192Ozone
199236526389103004181172Ozone
199136026780130004177147Ozone
19903652857271004071177Ozone
1989365234943340045101161Ozone
1988366240108153004484159Ozone
1987365207134212104593202Ozone
1986364223121200004484150Ozone
1985364220129150004587140Ozone
1984365206134223004893200Ozone
198336519712038100048115190Ozone
1982363199134237004893200Ozone
1981364205131234104897202Ozone
1980362194140262004990166Ozone

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