SmogRank / World / North America / United States / Colorado
Most polluted cities in Colorado, United States
17 measured cities, this year; 13 above the WHO annual guideline. Worst: Delta at 7.5 µg/m³, #2,036 in the world.
About 836 people in these 2 cities breathed unhealthy air on an average day so far this year.
Ranked by people: each city's population times its share of unhealthy days, those with a daily mean over 35.4 µg/m³ (US AQI 101+). That is how many people living there breathed unhealthy air on an average day. Only cities measured at 2 or more places at least 3 km apart are ranked: one monitor shouldn't speak for a whole city. Population is the city itself, not its wider metro area.
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
Estimated: cities without a monitor
Colorado cities we can't measure, with model estimates for the last 7 days. Listed by population, not ranked: estimates aren't precise enough to rank. Model estimates from the Copernicus Atmosphere Monitoring Service (CAMS), adjusted to measured cities nearby where there are any. Adjusted, a single day lands within ×1.5 of what a monitor would read about two times in three; unadjusted, it can be off by half or double. Treat them as a guide, not a measurement.
| City | 7-day estimate |
|---|---|
| Aurora | ≈ 5 µg/m³ |
| Lakewood | ≈ 4 µg/m³ |
| Westminster | ≈ 4 µg/m³ |
| Centennial | ≈ 5 µg/m³ |
| Highlands Ranch | ≈ 5 µg/m³ |
| Loveland | ≈ 3 µg/m³ |
| Broomfield | ≈ 4 µg/m³ |
| Grand Junction | ≈ 3 µg/m³ |
| Castle Rock | ≈ 3 µg/m³ |
| Commerce City | ≈ 5 µg/m³ |
| Parker | ≈ 5 µg/m³ |
| Dakota Ridge | ≈ 4 µg/m³ |
| Englewood | ≈ 5 µg/m³ |
| Security-Widefield | ≈ 4 µg/m³ |
| Wheat Ridge | ≈ 4 µg/m³ |
| Pueblo West | ≈ 3 µg/m³ |
| Fountain | ≈ 4 µg/m³ |
| Lafayette | ≈ 4 µg/m³ |
| Columbine | ≈ 4 µg/m³ |
| Erie | ≈ 4 µg/m³ |
| Evans | ≈ 4 µg/m³ |
| Louisville | ≈ 4 µg/m³ |
| Golden | ≈ 4 µg/m³ |
| Clifton | ≈ 2 µg/m³ |
| Montrose | ≈ 1 µg/m³ |
| Sherrelwood | ≈ 4 µg/m³ |
| Durango | ≈ 2 µg/m³ |
| Greenwood Village | ≈ 5 µg/m³ |
Bad-air episodes in Colorado
- North-western United States and Canada, July and August 202616 July to 4 August 2026, 20 days118 cities168 µg/m³peak, Prineville
- Utah and Colorado, United States, June 202623 to 24 June 2026, 2 days16 cities66 µg/m³peak, Delta
- Colorado and Wyoming, United States, September 20254 September 2025, 1 day9 cities31 µg/m³peak, Thornton
- North-western United States and Canada, July 202419 to 31 July 2024, 13 days92 cities217 µg/m³peak, Grand Centre
- North-western United States and Canada, May 202319 to 23 May 2023, 5 days63 cities357 µg/m³peak, Grand Centre
Times when many cities around here had PM2.5 at twice their usual or more on the same days.
Use this board
SmogRank, most polluted cities in Colorado, United States, PM2.5, this year to 6 Oct 2026, 22:00 UTC, smogrank.com/us/region/colorado?w=year.
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.
Updated 2026-10-06 22:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index.