SmogRank / World / United States / Colorado / September 2026

Most polluted cities in Colorado, United States, September 2026

19 cities in Colorado, United States ranked by their average PM2.5 over the 30 days. Worst: Northglenn at 5.1 µg/m³, #2,241 of 2,949 cities in the world. Cleanest: Aspen at 2.5. A city needs measurements on 70% of the days to be ranked.

Ranked by unhealthy days: the share of each city's measured days with a daily mean over 35.4 µg/m³ (US AQI 101+), Unhealthy for Sensitive Groups or worse on the US EPA's scale. Ties go to the higher average.

← Northglenn 0% 0.75%0
Every city in Colorado, United States ranked for September 2026, one column each, ranked by unhealthy days: height is the share of its measured days that were unhealthy, colour its average. Point at it to read a city; tap to open it.
  1. 1Northglennlow-cost sensors
    0%
  2. 2Thornton
    0%
  3. 3Lamar
    0%
  4. 3Greeley
    0%
  5. 5Fort Collins
    0%
  6. 6Longmont
    0%
  7. 7Denver
    0%
  8. 8Colorado Springs
    0%
  9. 9Windsor
    0%
  10. 10Pueblo
    0%
  11. 11Alamosa
    0%
  12. 11Arvadalow-cost sensors
    0%
  13. 13Boulder
    0%
  14. 14Delta
    0%
  15. 14Ken Caryl
    0%
  16. 16Cañon City
    0%
  17. 17Littleton
    0%
  18. 18Steamboat Springs
    0%
  19. 19Aspen
    0%

Low-cost sensors: consumer units, counted only in cities with no government-grade monitor; they read close to reference monitors on the whole, but 15 to 25% high in humid air. How we use them.

Good0–50
Moderate51–100
Sensitive groups101–150
Unhealthy151–200
Very unhealthy201–300
Hazardous301+

The US EPA's Air Quality Index for PM2.5 (2024 breakpoints). Show values as: µg/m³ · US AQI · India AQI · China AQI · European AQI · UK DAQI · Canada AQHI+. About the scales

All cities in the world, September 2026 · All cities in United States, September 2026 · Colorado now

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SmogRank, most polluted cities in Colorado, United States, PM2.5, September 2026, smogrank.com/us/region/colorado/history/2026-09 (accessed 6 Oct 2026).

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You're welcome to reuse a single chart, image or figure with this citation and a link. Terms. Raw monitor readings in bulk are published by OpenAQ, under each provider's licence.