EnviroNearby

Delta County, Colorado air quality history

Daily Air Quality Index summaries from EPA monitors in Delta, Colorado, 1988–2025.

86%
Good days in 2025
29
Median daily AQI, 2025
1
Days above 100 in 2025
209
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Delta, Colorado on 209 days. 179 of those days (86%) were in the Good range, and the median daily AQI was 29. That median is lower than the typical monitored county in Colorado (median 46 across 27 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 182 days. Across the years with fuller monitoring, the share of Good days held roughly steady: an average of 82% in 1989–1991 versus 85% in 2021–2025.

EPA did not report an AQI for every day of 2025 here (209 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

Years with fewer than 120 monitored days (2008, 2010, 2014, 2015, 2016, 2017…) 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%1988: Good 4 of 5 days1988: Moderate 1 of 5 days19881989: Good 143 of 169 days1989: Moderate 26 of 169 days1990: Good 155 of 175 days1990: Moderate 20 of 175 days1991: Good 130 of 177 days1991: Moderate 47 of 177 days1992: Good 152 of 173 days1992: Moderate 21 of 173 days1993: Good 221 of 236 days1993: Moderate 15 of 236 days19931994: Good 291 of 329 days1994: Moderate 38 of 329 days1995: Good 333 of 342 days1995: Moderate 9 of 342 days1996: Good 333 of 340 days1996: Moderate 7 of 340 days1997: Good 198 of 203 days1997: Moderate 5 of 203 days1998: Good 57 of 60 days1998: Moderate 3 of 60 days19981999: Good 57 of 61 days1999: Moderate 3 of 61 days1999: Hazardous 1 of 61 days2000: Good 54 of 66 days2000: Moderate 12 of 66 days2001: Good 48 of 61 days2001: Moderate 13 of 61 days2002: Good 46 of 64 days2002: Moderate 17 of 64 days2002: Unhealthy for Sensitive Groups 1 of 64 days2003: Good 50 of 63 days2003: Moderate 12 of 63 days2003: Unhealthy for Sensitive Groups 1 of 63 days20032004: Good 50 of 64 days2004: Moderate 14 of 64 days2005: Good 71 of 87 days2005: Moderate 16 of 87 days2006: Good 100 of 114 days2006: Moderate 14 of 114 days2007: Good 102 of 106 days2007: Moderate 4 of 106 days2008: Good 111 of 115 days2008: Moderate 4 of 115 days20082009: Good 116 of 121 days2009: Moderate 4 of 121 days2009: Unhealthy for Sensitive Groups 1 of 121 days2010: Good 111 of 115 days2010: Moderate 4 of 115 days2011: Good 121 of 121 days2012: Good 119 of 121 days2012: Moderate 2 of 121 days2013: Good 120 of 121 days2013: Moderate 1 of 121 days20132014: Good 117 of 118 days2014: Moderate 1 of 118 days2015: Good 113 of 114 days2015: Moderate 1 of 114 days2016: Good 117 of 119 days2016: Moderate 2 of 119 days2017: Good 114 of 115 days2017: Moderate 1 of 115 days2018: Good 264 of 297 days2018: Moderate 33 of 297 days20182019: Good 338 of 365 days2019: Moderate 27 of 365 days2020: Good 312 of 365 days2020: Moderate 53 of 365 days2021: Good 294 of 365 days2021: Moderate 69 of 365 days2021: Unhealthy for Sensitive Groups 2 of 365 days2022: Good 125 of 143 days2022: Moderate 18 of 143 days2025: Good 179 of 209 days2025: Moderate 29 of 209 days2025: Unhealthy for Sensitive Groups 1 of 209 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 year0501001988: 90th percentile AQI 541989: 90th percentile AQI 551990: 90th percentile AQI 531991: 90th percentile AQI 671992: 90th percentile AQI 531993: 90th percentile AQI 441994: 90th percentile AQI 531995: 90th percentile AQI 381996: 90th percentile AQI 371997: 90th percentile AQI 391998: 90th percentile AQI 391999: 90th percentile AQI 442000: 90th percentile AQI 632001: 90th percentile AQI 542002: 90th percentile AQI 592003: 90th percentile AQI 582004: 90th percentile AQI 612005: 90th percentile AQI 542006: 90th percentile AQI 522007: 90th percentile AQI 372008: 90th percentile AQI 332009: 90th percentile AQI 372010: 90th percentile AQI 322011: 90th percentile AQI 312012: 90th percentile AQI 362013: 90th percentile AQI 312014: 90th percentile AQI 322015: 90th percentile AQI 272016: 90th percentile AQI 312017: 90th percentile AQI 332018: 90th percentile AQI 522019: 90th percentile AQI 502020: 90th percentile AQI 542021: 90th percentile AQI 592022: 90th percentile AQI 562025: 90th percentile AQI 531988: Median AQI 111989: Median AQI 311990: Median AQI 271991: Median AQI 291992: Median AQI 271993: Median AQI 261994: Median AQI 261995: Median AQI 201996: Median AQI 231997: Median AQI 221998: Median AQI 221999: Median AQI 232000: Median AQI 292001: Median AQI 342002: Median AQI 372003: Median AQI 332004: Median AQI 282005: Median AQI 312006: Median AQI 282007: Median AQI 232008: Median AQI 222009: Median AQI 222010: Median AQI 192011: Median AQI 192012: Median AQI 212013: Median AQI 192014: Median AQI 212015: Median AQI 162016: Median AQI 162017: Median AQI 192018: Median AQI 362019: Median AQI 412020: Median AQI 412021: Median AQI 432022: Median AQI 442025: Median AQI 2919881993199820032008201320182025
  • 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)18287%
Coarse particles (PM10)2713%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20252091792910002953103PM2.5
2022143125180000445680Ozone
20213652946920004359141Ozone
2020365312530000415497Ozone
2019365338270000415080Ozone
2018297264330000365285Ozone
201711511410000193352PM10
201611911720000163164PM10
201511411310000162751PM10
201411811710000213277PM10
201312112010000193155PM10
201212111920000213656PM10
201112112100000193147PM10
201011511140000193286PM10
2009121116410002237116PM10
200811511140000223369PM10
200710610240000233766PM10
2006114100140000285268PM10
20058771160000315465PM2.5
20046450140000286192PM2.5
200363501210003358131PM2.5
200264461710003759107PM2.5
20016148130000345471PM2.5
20006654120000296373PM2.5
19996157300012344351PM10
1998605730000223962PM10
199720319850000223975PM10
199634033370000233759PM10
199534233390000203858PM10
1994329291380000265397PM10
1993236221150000264473PM10
1992173152210000275368PM10
1991177130470000296792PM10
1990175155200000275386PM10
1989169143260000315578PM10
19885410000115454PM10

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 1988–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem