SmogRank / World / North America / United States / Colorado

Most polluted cities in Colorado, United States

19 measured cities, last 30 days; 0 above the WHO 24-hour guideline. Worst: Northglenn at 5.2 µg/m³, #2,148 in the world.

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.

Arizona: 16 measured citiesNew Mexico: 8 measured citiesTexas: 33 measured citiesUtah: 20 measured citiesWyoming: 10 measured citiesColorado: 19 measured citiesNebraska: 6 measured citiesKansas: 7 measured citiesOklahoma: 10 measured cities#19 Aspen: 2.6 µg/m³#18 Steamboat Springs: 2.8 µg/m³#17 Littleton: 2.9 µg/m³#16 Cañon City: 3.1 µg/m³#15 Ken Caryl: 3.2 µg/m³#13 Delta: 3.3 µg/m³#14 Boulder: 3.3 µg/m³#12 Alamosa: 3.4 µg/m³#11 Arvada: 3.6 µg/m³#10 Pueblo: 4.0 µg/m³#9 Windsor: 4.1 µg/m³#8 Colorado Springs: 4.2 µg/m³#7 Denver: 4.4 µg/m³#5 Lamar: 4.5 µg/m³#6 Longmont: 4.5 µg/m³#4 Fort Collins: 4.7 µg/m³#2 Thornton: 5.1 µg/m³#3 Greeley: 5.1 µg/m³#1 Northglenn: 5.2 µg/m³
Each dot is a measured city, coloured by its PM2.5; tap a dot to open the city.
← Northglenn 0% 0.75%0
Every measured city in Colorado, United States, 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 sensorslast full day 3 Oct 0%
  2. 2Thorntonlast full day 3 Oct 0%
  3. 2Greeleylast full day 3 Oct 0%
  4. 4Fort Collinslast full day 3 Oct 0%
  5. 5Lamarlast full day 3 Oct 0%
  6. 5Longmontlast full day 3 Oct 0%
  7. 7Denverlast full day 3 Oct 0%
  8. 8Colorado Springslast full day 3 Oct 0%
  9. 9Windsorlast full day 3 Oct 0%
  10. 10Pueblolast full day 3 Oct 0%
  11. 11Arvadalow-cost sensors 0%
  12. 12Alamosalast full day 3 Oct 0%
  13. 13Deltalast full day 3 Oct 0%
  14. 13Boulderlast full day 3 Oct 0%
  15. 15Ken Caryllast full day 3 Oct 0%
  16. 16Cañon Citylast full day 3 Oct 0%
  17. 17Littletonlast full day 3 Oct 0%
  18. 18Steamboat Springslast full day 3 Oct 0%
  19. 19Aspenlast full day 3 Oct 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

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.

City7-day estimateBand
Aurora≈ 5 µg/m³Good
Lakewood≈ 4 µg/m³Good
Westminster≈ 4 µg/m³Good
Centennial≈ 5 µg/m³Good
Highlands Ranch≈ 5 µg/m³Good
Loveland≈ 3 µg/m³Good
Broomfield≈ 4 µg/m³Good
Grand Junction≈ 3 µg/m³Good
Castle Rock≈ 3 µg/m³Good
Commerce City≈ 5 µg/m³Good
Parker≈ 5 µg/m³Good
Dakota Ridge≈ 4 µg/m³Good
Englewood≈ 5 µg/m³Good
Security-Widefield≈ 4 µg/m³Good
Wheat Ridge≈ 4 µg/m³Good
Pueblo West≈ 3 µg/m³Good
Fountain≈ 4 µg/m³Good
Lafayette≈ 4 µg/m³Good
Columbine≈ 4 µg/m³Good
Erie≈ 4 µg/m³Good
Evans≈ 4 µg/m³Good
Louisville≈ 4 µg/m³Good
Golden≈ 4 µg/m³Good
Clifton≈ 2 µg/m³Good
Montrose≈ 1 µg/m³Good
Sherrelwood≈ 4 µg/m³Good
Durango≈ 2 µg/m³Good
Greenwood Village≈ 5 µg/m³Good

Bad-air episodes in Colorado

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, last 30 days to 6 Oct 2026, 22:00 UTC, smogrank.com/us/region/colorado?w=30d.

Download top 10 (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.

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.