Colorado · County air quality · 2025

Delta County, Colorado

Median AQI 29 (Good). 85.7% of monitored days rated Good, direct from EPA Air Quality System stations.

29
Median AQI
85.7%
Good-air days
0
Unhealthy days
71%
Cleaner than US counties

Delta County's 2025 annual median AQI is 29 (Good), with 85.7% Good days

According to the U.S. EPA Air Quality System, Delta County, Colorado posted a 2025 median AQI of 29 (Good) across 209 monitored days, with 85.7% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.

Data source EPA Air Quality System (AQS)

Cleaner than US
Delta County's 85.7% Good-day share places it cleaner than 71% of US counties in 2025.
Closest median-AQI peer
Baker County (83.3% Good, median AQI 29) is the closest nationwide median-AQI peer to Delta County.
2020–2025 trend
Delta County's floor-qualifying Good-day share is improving from 2020 to 2025 (85.5% → 85.6%).
Median AQI
Median AQI: 29

According to the 2025 EPA AQS county release on PlainAirData, Delta County, Colorado ranks #89 of 951 by median AQI (29) and #273 by good-day share (85.7%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.

Annual release change

What changed in the 2025 EPA annual release

2022 → 2025
  • Delta County's median AQI fell from 44 to 29 between the 2022 and 2025 EPA annual releases, a change of -15 points.
  • Delta County's peak single-day AQI rose from 80 to 103 between the 2022 and 2025 EPA annual releases, a change of +23 points.
  • Delta County's Good-day share fell from 87.4% to 85.6% between the 2022 and 2025 EPA annual releases, a change of -1.8 percentage points.

Compared against 143 monitored days in 2022 and 209 in 2025. These are arithmetic differences between two stored EPA annual summaries, not a cause claim or a live advisory.

National release changes Two-year CSV

Data updated August 2026

Delta County AQI category distribution Stacked-segment bar showing the share of monitored days that fell in each EPA AQI category, Good, Moderate, USG, Unhealthy, Very Unhealthy, Hazardous, with median AQI marker. AQI category distribution 209 monitored days G M Median AQI 29 0 50 100 150 200 300 500 Delta County
  • Good · 179 d · 85.6%
  • Moderate · 29 d · 13.9%
  • USG · 1 d · 0.5%

How did Delta County's air break down in 2025?

Across 209 monitored days, 179 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 103 (USG).

EPA AQS →
Delta County, Good days share 85.7%
National avg

Share of monitored days at AQI 0-50 (Good band).

Delta County, Unhealthy-or-worse share 0.0%
National avg

Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).

Where Delta County sits nationally

COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Delta County's Good-day share is 85.7% - cleaner than 71% of that comparison set.

Delta County Good-day share vs. every US county

Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release

85.7% Cleaner than 71% higher than 71% of 951 US counties

0.0%–5.0%: 0 US counties (0%). Below this entry. 5.0%–10.0%: 1 US counties (0%). Below this entry. 10.0%–15.0%: 4 US counties (0%). Below this entry. 15.0%–20.0%: 4 US counties (0%). Below this entry. 20.0%–25.0%: 1 US counties (0%). Below this entry. 25.0%–30.0%: 5 US counties (1%). Below this entry. 30.0%–35.0%: 9 US counties (1%). Below this entry. 35.0%–40.0%: 4 US counties (0%). Below this entry. 40.0%–45.0%: 16 US counties (2%). Below this entry. 45.0%–50.0%: 16 US counties (2%). Below this entry. 50.0%–55.0%: 32 US counties (3%). Below this entry. 55.0%–60.0%: 50 US counties (5%). Below this entry. 60.0%–65.0%: 77 US counties (8%). Below this entry. 65.0%–70.0%: 93 US counties (10%). Below this entry. 70.0%–75.0%: 109 US counties (11%). Below this entry. 75.0%–80.0%: 114 US counties (12%). Below this entry. 80.0%–85.0%: 123 US counties (13%). Below this entry. 85.0%–90.0%: 99 US counties (10%). This entry sits in this band. 90.0%–95.0%: 117 US counties (12%). Above this entry. 95.0%–100.0%: 70 US counties (7%). Above this entry. 100.0%–105.0%: 7 US counties (1%). Above this entry. This county 0.0% 100% every floor-qualified US county, bucketed by value

Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.

Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025

Pollutant Breakdown

Days where each pollutant was the primary driver of AQI readings in Delta County.

Delta County, Colorado criteria pollutant distribution Stratified-band visualization showing the share of monitored days each criteria pollutant (PM2.5, PM10, O3, NO2, CO) was the primary AQI driver. PM2.5 · 87.1% PM10 · 12.9% 0% 25% 50% 75% 100% Delta County, Colorado · 209 monitored days Criteria pollutant column
  • PM2.5 · 182 days · 87.1%
  • PM10 · 27 days · 12.9%

Five-year seasonal record

EPA day-level county files · 2020–2022

873 daily readings show a recurring local pattern: Aug has the highest matched-month median AQI (55), while Nov is lowest (34). Each cell is that month’s median daily AQI, not a current-air forecast.

JanFebMarAprMayJunJulAugSepOctNovDec
2020 374142444545425143383533
2021 364044465045525944373336
2022 3943444748-------

Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.

Health Impact Assessment

Good

Delta County's air quality is rated Good based on 85.7% good air days. No unhealthy air days were recorded in 2025.

Air quality was acceptable on 85.7% of monitored days in 2025, with a median AQI of 29. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.

The dominant pollutant is PM2.5, driving AQI on 182 of 209 monitored days.

5-Year Air Quality Trend

Air quality is improving in Delta County. Highest Good-day share: 2022 (87.4%, 125 of 143 monitored days).

Year Good Moderate USG Unhealthy Median AQI
2020 312 53 0 0 41
2021 294 69 2 0 43
2022 125 18 0 0 44
2025 179 29 1 0 29

How does Delta County compare?

How Delta County compares

Good-day share better than typical
Delta County 85.7%
Baker County, Oregon 83.3%
Typical US county 76.0%
Median AQI better than typical
Delta County 29
Baker County, Oregon 29
Typical US county 39
Unhealthy days / yr better than typical
Delta County 0
Baker County, Oregon 0
Typical US county 1

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer. Spotted a figure that looks wrong? Report a correction.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Delta County, Colorado · 2020–2025

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Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.

Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.