SmogRank / World / United States / Colorado / January 2023

Most unhealthy days in Colorado, United States, January 2023

8 cities in Colorado, United States ranked by their share of unhealthy days over the 31 days. The most unhealthy days: Thornton, United States, 0 of 28 measured days. A city needs measurements on 70% of the days to be ranked.

OpenAQ's archive, where our readings come from, has almost none for 7 of these 31 days. Cities in the United States and Europe have them from the EPA's and the European Environment Agency's own records of the same monitors; elsewhere a city's average is over its other days. Why.

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.

← Thornton 0% 0.75%0
Every city in Colorado, United States ranked for January 2023, 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. 1Thornton
    0 of 28
  2. 2Greeley
    0 of 31
  3. 3Fort Collins
    0 of 31
  4. 4Denver
    0 of 31
  5. 5Boulder
    0 of 22
  6. 6Ken Caryl
    0 of 30
  7. 7Grand Junction
    0 of 31
  8. 8Colorado Springs
    0 of 31
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, January 2023 · All cities in United States, January 2023 · Colorado now

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SmogRank, most polluted cities in Colorado, United States, by unhealthy days, PM2.5, January 2023, smogrank.com/us/region/colorado/history/2023-01?by=days (accessed 7 Oct 2026).

Download top 10 by average (PNG)

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.