SmogRank / World / United States / Colorado / 2023
Most polluted cities in Colorado, United States, 2023
8 cities in Colorado, United States ranked by their average PM2.5 over the 365 days; 7 above the WHO annual guideline (5.0 µg/m³). Worst: Thornton at 8.3 µg/m³, #789 of 1,302 cities in the world. Cleanest: Grand Junction at 4.3. A city needs measurements on 70% of the days to be ranked.
OpenAQ's archive, where our readings come from, has almost none for 56 of these 365 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.
- 1Thornton 1%
- 2Longmont 1%
- 3Colorado Springs 1%
- 4Ken Caryl 1%
- 5Greeley 1%
- 6Denver 1%
- 7Fort Collins 0%
- 8Grand Junction 0%
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
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SmogRank, most polluted cities in Colorado, United States, PM2.5, 2023, smogrank.com/us/region/colorado/history/2023 (accessed 6 Oct 2026).
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.