SmogRank / World / United States / Colorado / October 2026
Most polluted cities in Colorado, United States, October 2026
2 cities in Colorado, United States ranked by their average PM2.5 so far (5 of 31 days). Worst: Brighton at 5.1 µg/m³, #1,773 of 2,246 cities in the world. Cleanest: Arvada at 4.1. A city needs measurements on 70% of the days to be ranked.
Colorado has 19 cities ranked over the last 30 days, but only 2 here: most of its monitors have no readings for October 2026 in OpenAQ's archive, where ours come from, or too few to rank. Cities missing from this board weren't clean, just not measured.
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.
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.
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, October 2026 · All cities in United States, October 2026 · Colorado now
Use this board
SmogRank, most polluted cities in Colorado, United States, PM2.5, October 2026 (so far), smogrank.com/us/region/colorado/history/2026-10 (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.