EnviroNearby

Weld County, Colorado air quality history

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

45%
Good days in 2025
52
Median daily AQI, 2025
6
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 Weld, Colorado on 365 days. 163 of those days (45%) were in the Good range, and the median daily AQI was 52. 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 229 days. Across the years with fuller monitoring, the share of Good days fell: an average of 70% in 1980–1982 versus 42% 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 263 of 366 days1980: Moderate 88 of 366 days1980: Unhealthy for Sensitive Groups 12 of 366 days1980: Unhealthy 3 of 366 days19801981: Good 203 of 297 days1981: Moderate 73 of 297 days1981: Unhealthy for Sensitive Groups 19 of 297 days1981: Unhealthy 2 of 297 days1982: Good 258 of 364 days1982: Moderate 96 of 364 days1982: Unhealthy for Sensitive Groups 10 of 364 days1983: Good 232 of 334 days1983: Moderate 85 of 334 days1983: Unhealthy for Sensitive Groups 17 of 334 days1984: Good 258 of 366 days1984: Moderate 91 of 366 days1984: Unhealthy for Sensitive Groups 14 of 366 days1984: Unhealthy 1 of 366 days1984: Very Unhealthy 2 of 366 days1985: Good 272 of 365 days1985: Moderate 89 of 365 days1985: Unhealthy for Sensitive Groups 3 of 365 days1985: Unhealthy 1 of 365 days19851986: Good 256 of 365 days1986: Moderate 100 of 365 days1986: Unhealthy for Sensitive Groups 7 of 365 days1986: Unhealthy 2 of 365 days1987: Good 246 of 365 days1987: Moderate 112 of 365 days1987: Unhealthy for Sensitive Groups 7 of 365 days1988: Good 250 of 366 days1988: Moderate 107 of 366 days1988: Unhealthy for Sensitive Groups 9 of 366 days1989: Good 275 of 365 days1989: Moderate 86 of 365 days1989: Unhealthy for Sensitive Groups 4 of 365 days1990: Good 290 of 365 days1990: Moderate 69 of 365 days1990: Unhealthy for Sensitive Groups 5 of 365 days1990: Unhealthy 1 of 365 days19901991: Good 282 of 365 days1991: Moderate 78 of 365 days1991: Unhealthy for Sensitive Groups 5 of 365 days1992: Good 305 of 364 days1992: Moderate 58 of 364 days1992: Unhealthy for Sensitive Groups 1 of 364 days1993: Good 316 of 365 days1993: Moderate 48 of 365 days1993: Unhealthy for Sensitive Groups 1 of 365 days1994: Good 294 of 365 days1994: Moderate 67 of 365 days1994: Unhealthy for Sensitive Groups 4 of 365 days1995: Good 300 of 365 days1995: Moderate 61 of 365 days1995: Unhealthy for Sensitive Groups 4 of 365 days19951996: Good 287 of 366 days1996: Moderate 76 of 366 days1996: Unhealthy for Sensitive Groups 3 of 366 days1997: Good 303 of 365 days1997: Moderate 59 of 365 days1997: Unhealthy for Sensitive Groups 3 of 365 days1998: Good 276 of 365 days1998: Moderate 77 of 365 days1998: Unhealthy for Sensitive Groups 12 of 365 days1999: Good 274 of 365 days1999: Moderate 88 of 365 days1999: Unhealthy for Sensitive Groups 3 of 365 days2000: Good 266 of 366 days2000: Moderate 96 of 366 days2000: Unhealthy for Sensitive Groups 4 of 366 days20002001: Good 251 of 365 days2001: Moderate 104 of 365 days2001: Unhealthy for Sensitive Groups 10 of 365 days2002: Good 210 of 356 days2002: Moderate 129 of 356 days2002: Unhealthy for Sensitive Groups 17 of 356 days2003: Good 203 of 360 days2003: Moderate 130 of 360 days2003: Unhealthy for Sensitive Groups 26 of 360 days2003: Unhealthy 1 of 360 days2004: Good 225 of 366 days2004: Moderate 139 of 366 days2004: Unhealthy for Sensitive Groups 2 of 366 days2005: Good 227 of 365 days2005: Moderate 119 of 365 days2005: Unhealthy for Sensitive Groups 19 of 365 days20052006: Good 185 of 365 days2006: Moderate 153 of 365 days2006: Unhealthy for Sensitive Groups 25 of 365 days2006: Unhealthy 2 of 365 days2007: Good 207 of 365 days2007: Moderate 142 of 365 days2007: Unhealthy for Sensitive Groups 16 of 365 days2008: Good 235 of 366 days2008: Moderate 123 of 366 days2008: Unhealthy for Sensitive Groups 8 of 366 days2009: Good 265 of 365 days2009: Moderate 98 of 365 days2009: Unhealthy for Sensitive Groups 2 of 365 days2010: Good 212 of 365 days2010: Moderate 146 of 365 days2010: Unhealthy for Sensitive Groups 7 of 365 days20102011: Good 206 of 365 days2011: Moderate 147 of 365 days2011: Unhealthy for Sensitive Groups 12 of 365 days2012: Good 196 of 366 days2012: Moderate 150 of 366 days2012: Unhealthy for Sensitive Groups 18 of 366 days2012: Unhealthy 2 of 366 days2013: Good 216 of 365 days2013: Moderate 138 of 365 days2013: Unhealthy for Sensitive Groups 11 of 365 days2014: Good 239 of 365 days2014: Moderate 122 of 365 days2014: Unhealthy for Sensitive Groups 4 of 365 days2015: Good 224 of 365 days2015: Moderate 132 of 365 days2015: Unhealthy for Sensitive Groups 9 of 365 days20152016: Good 201 of 366 days2016: Moderate 162 of 366 days2016: Unhealthy for Sensitive Groups 3 of 366 days2017: Good 173 of 365 days2017: Moderate 184 of 365 days2017: Unhealthy for Sensitive Groups 7 of 365 days2017: Unhealthy 1 of 365 days2018: Good 138 of 365 days2018: Moderate 217 of 365 days2018: Unhealthy for Sensitive Groups 10 of 365 days2019: Good 152 of 365 days2019: Moderate 205 of 365 days2019: Unhealthy for Sensitive Groups 8 of 365 days2020: Good 166 of 366 days2020: Moderate 187 of 366 days2020: Unhealthy for Sensitive Groups 13 of 366 days20202021: Good 156 of 365 days2021: Moderate 169 of 365 days2021: Unhealthy for Sensitive Groups 39 of 365 days2021: Unhealthy 1 of 365 days2022: Good 170 of 365 days2022: Moderate 185 of 365 days2022: Unhealthy for Sensitive Groups 10 of 365 days2023: Good 146 of 365 days2023: Moderate 214 of 365 days2023: Unhealthy for Sensitive Groups 4 of 365 days2023: Unhealthy 1 of 365 days2024: Good 149 of 366 days2024: Moderate 196 of 366 days2024: Unhealthy for Sensitive Groups 19 of 366 days2024: Unhealthy 2 of 366 days2025: Good 163 of 365 days2025: Moderate 196 of 365 days2025: Unhealthy for Sensitive Groups 6 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 771981: 90th percentile AQI 931982: 90th percentile AQI 761983: 90th percentile AQI 801984: 90th percentile AQI 781985: 90th percentile AQI 691986: 90th percentile AQI 841987: 90th percentile AQI 771988: 90th percentile AQI 741989: 90th percentile AQI 701990: 90th percentile AQI 641991: 90th percentile AQI 671992: 90th percentile AQI 601993: 90th percentile AQI 571994: 90th percentile AQI 641995: 90th percentile AQI 581996: 90th percentile AQI 711997: 90th percentile AQI 641998: 90th percentile AQI 741999: 90th percentile AQI 672000: 90th percentile AQI 642001: 90th percentile AQI 742002: 90th percentile AQI 902003: 90th percentile AQI 932004: 90th percentile AQI 742005: 90th percentile AQI 842006: 90th percentile AQI 972007: 90th percentile AQI 932008: 90th percentile AQI 802009: 90th percentile AQI 712010: 90th percentile AQI 772011: 90th percentile AQI 842012: 90th percentile AQI 902013: 90th percentile AQI 872014: 90th percentile AQI 742015: 90th percentile AQI 802016: 90th percentile AQI 712017: 90th percentile AQI 792018: 90th percentile AQI 872019: 90th percentile AQI 772020: 90th percentile AQI 842021: 90th percentile AQI 1012022: 90th percentile AQI 872023: 90th percentile AQI 802024: 90th percentile AQI 872025: 90th percentile AQI 801980: Median AQI 441981: Median AQI 441982: Median AQI 431983: Median AQI 431984: Median AQI 421985: Median AQI 441986: Median AQI 401987: Median AQI 441988: Median AQI 441989: Median AQI 421990: Median AQI 401991: Median AQI 391992: Median AQI 361993: Median AQI 381994: Median AQI 381995: Median AQI 381996: Median AQI 371997: Median AQI 391998: Median AQI 401999: Median AQI 412000: Median AQI 412001: Median AQI 432002: Median AQI 442003: Median AQI 472004: Median AQI 452005: Median AQI 452006: Median AQI 502007: Median AQI 472008: Median AQI 442009: Median AQI 442010: Median AQI 472011: Median AQI 472012: Median AQI 492013: Median AQI 482014: Median AQI 462015: Median AQI 472016: Median AQI 492017: Median AQI 512018: Median AQI 542019: Median AQI 532020: Median AQI 522021: Median AQI 542022: Median AQI 512023: Median AQI 552024: Median AQI 542025: Median AQI 521980198519901995200020052010201520202025
  • 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
Ozone22963%
Fine particles (PM2.5)13637%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
202536516319660005280119Ozone
2024366149196192005487164Ozone
202336514621441005580162Ozone
2022365170185100005187133Ozone
20213651561693910054101154Ozone
2020366166187130005284140Ozone
201936515220580005377129Ozone
2018365138217100005487133Ozone
201736517318471005179155Ozone
201636620116230004971129Ozone
201536522413290004780133Ozone
201436523912240004674126Ozone
2013365216138110004887133Ozone
2012366196150182004990161Ozone
2011365206147120004784136Ozone
201036521214670004777126Ozone
20093652659820004471108Ozone
200836623512380004480122Ozone
2007365207142160004793139Ozone
2006365185153252005097161Ozone
2005365227119190004584147Ozone
200436622513920004574101Ozone
2003360203130261004793169Ozone
2002356210129170004490140Ozone
2001365251104100004374147Ozone
20003662669640004164119Ozone
19993652748830004167108Ozone
199836527677120004074140Ozone
19973653035930003964108Ozone
19963662877630003771112Ozone
19953653006140003858115Ozone
19943652946740003864119Ozone
19933653164810003857115Ozone
19923643055810003660105Ozone
19913652827850003967147Ozone
19903652906951004064156Ozone
19893652758640004270136Ozone
198836625010790004474143Ozone
198736524611270004477126Ozone
198636525610072004084190Ozone
19853652728931004469154Ozone
198436625891141204278210Ozone
198333423285170004380136Ozone
198236425896100004376138Ozone
198129720373192004493173Ozone
198036626388123004477192Ozone

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