SmogRank / Episodes / United States

Colorado, United States, September 2025

For 1 day, on 4 September 2025, 8 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Thornton, Colorado, United States: 30.9 µg/m³ on 4 September, about 5.5× its usual 5.6.

No cause namedWe have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached8in 1 country
Days14 September 2025
Worst day4 September8 cities counted
Furthest above usual5.9×Cheyenne

Day by day

4 September 8 cities at twice their usual or more
Fort CollinsGreeleyDenverLakewoodLittletonKen CarylLaramieColorado SpringsCañon CityPuebloCasperGrand JunctionRock SpringsNorthglennBoulderBrightonLongmontArvadaThorntonWindsorCheyenne
1 Sep234567
Every measured city in the area, coloured by its PM2.5 that day; a dark ring marks a city at twice its usual or more. Bars: cities counted each day, coloured by their median level. Choose a day or press play; without JavaScript, the days link to that day's world ranking.

How it spread

ThorntonThornton, 1 September: 8.2 µg/m³, 1.5× usualThornton, 2 September: 11.3 µg/m³, 2.0× usualThornton, 3 September: 15.4 µg/m³, 2.8× usualThornton, 4 September: 30.9 µg/m³, 5.5× usualThornton, 5 September: 23.0 µg/m³, 4.1× usualThornton, 6 September: 23.5 µg/m³, 4.2× usualThornton, 7 September: 14.4 µg/m³, 2.6× usualCheyenneCheyenne, 1 September: 5.2 µg/m³, 1.0× usualCheyenne, 2 September: 4.8 µg/m³, 1.0× usualCheyenne, 3 September: 8.7 µg/m³, 1.7× usualCheyenne, 4 September: 29.5 µg/m³, 5.9× usualCheyenne, 5 September: 13.8 µg/m³, 2.8× usualCheyenne, 6 September: 14.9 µg/m³, 3.0× usualCheyenne, 7 September: 8.1 µg/m³, 1.6× usualWindsorWindsor, 1 September: 6.9 µg/m³, 1.4× usualWindsor, 2 September: 8.2 µg/m³, 1.6× usualWindsor, 3 September: 12.6 µg/m³, 2.5× usualWindsor, 4 September: 28.6 µg/m³, 5.7× usualWindsor, 5 September: 20.0 µg/m³, 4.0× usualWindsor, 6 September: 20.3 µg/m³, 4.1× usualWindsor, 7 September: 10.3 µg/m³, 2.1× usualNorthglennNorthglenn, 1 September: 8.8 µg/m³, 1.6× usualNorthglenn, 2 September: 10.6 µg/m³, 1.9× usualNorthglenn, 3 September: 14.9 µg/m³, 2.7× usualNorthglenn, 4 September: 26.7 µg/m³, 4.9× usualNorthglenn, 5 September: no readingNorthglenn, 6 September: 22.8 µg/m³, 4.1× usualNorthglenn, 7 September: 14.6 µg/m³, 2.7× usualArvadaArvada, 1 September: 8.9 µg/m³, 1.8× usualArvada, 2 September: 11.3 µg/m³, 2.3× usualArvada, 3 September: 14.7 µg/m³, 2.9× usualArvada, 4 September: 26.4 µg/m³, 5.3× usualArvada, 5 September: 24.1 µg/m³, 4.8× usualArvada, 6 September: 24.6 µg/m³, 4.9× usualArvada, 7 September: 15.8 µg/m³, 3.2× usualBrightonBrighton, 1 September: 8.6 µg/m³, 1.7× usualBrighton, 2 September: 10.5 µg/m³, 2.0× usualBrighton, 3 September: 14.8 µg/m³, 2.8× usualBrighton, 4 September: 26.3 µg/m³, 5.1× usualBrighton, 5 September: 21.0 µg/m³, 4.0× usualBrighton, 6 September: 21.6 µg/m³, 4.2× usualBrighton, 7 September: 14.6 µg/m³, 2.8× usualLongmontLongmont, 1 September: 7.7 µg/m³, 1.5× usualLongmont, 2 September: 8.9 µg/m³, 1.8× usualLongmont, 3 September: 12.8 µg/m³, 2.6× usualLongmont, 4 September: 26.0 µg/m³, 5.2× usualLongmont, 5 September: 23.1 µg/m³, 4.6× usualLongmont, 6 September: 22.5 µg/m³, 4.5× usualLongmont, 7 September: 12.5 µg/m³, 2.5× usualBoulderBoulder, 1 September: 6.7 µg/m³, 1.3× usualBoulder, 2 September: 8.4 µg/m³, 1.7× usualBoulder, 3 September: 11.9 µg/m³, 2.4× usualBoulder, 4 September: 25.0 µg/m³, 5.0× usualBoulder, 5 September: 22.4 µg/m³, 4.5× usualBoulder, 6 September: 22.0 µg/m³, 4.4× usualBoulder, 7 September: 10.1 µg/m³, 2.0× usual1 Sep234567
Every counted city, in the order the episode reached them, 3 days before to 3 after. Each square is a day, coloured by that day's PM2.5; outlined: at twice the city's usual or more.
Good0–50
Moderate51–100
Sensitive groups101–150
Unhealthy151–200
Very unhealthy201–300
Hazardous301+

Every city it reached

CityDaysHighestUsualAbove usual
CheyenneWyoming, United States1 29.54 September3.25.9×
WindsorColorado, United States1 28.64 September4.45.7×
ThorntonColorado, United States1 30.94 September5.65.5×
ArvadaColorado, United States1 26.44 September4.55.3×
LongmontColorado, United States1 26.04 September5.05.2×
BrightonColorado, United States1 26.34 September5.25.1×
BoulderColorado, United States1 25.04 September3.85.0×
NorthglennColorado, United States1 26.74 September5.54.9×

Daily means in µg/m³. Usual: the city's median for this time of year (its days within 21 days of the date, every year, leaving out the week around it). Above usual: its highest day against that.

Against other episodes in the United States

Number 92 of 98 episodes in the United States since our records began, by cities reached.

Bars: cities reached; colour: the level of the episode's highest daily reading. All 98.

Use this page

SmogRank, Colorado, United States, September 2025: 8 cities with PM2.5 at twice their usual or more, 4 September 2025, smogrank.com/episodes/2025-09-colorado-united-states (accessed 2 October 2026).

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You're welcome to reuse a single chart, image or figure with this citation and a link. Terms.

An episode is a run of days when many cities close to each other (four or more within 350 km on a day, 8 or more in all) each had a daily PM2.5 mean of at least 25 µg/m³ and at least 2 times their usual for that time of year. Found in our own measurements, not picked by hand. How we find episodes. Values are each city's daily mean of its own monitors; causes are the usual kinds for a pattern like this, not a confirmed source.