For 2 days, from 24 to 25 December 2025, 11 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Wheaton, Illinois, United States: 32.9 µg/m³ on 25 December, about 3.0× its usual 11.1. Ended 25 December.
Few fires were detected near these cities on those days. Spreads like this usually come from still weather trapping smoke from heating, traffic and industry, or from dust.
Cities reached11in 1 country
Days224 to 25 December 2025
Worst day25 December7 cities counted
Furthest above usual4.0×Muscatine
Day by day
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
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.
Daily means in µg/m³, over the days each city counted in this episode. Highest: its highest of those days. Usual: the city's median for the time of year of that day (its days within 21 days of the date, every year, leaving out the week around it). Above usual: its highest day against that, with a usual under 5 µg/m³ taken as 5.
Against other episodes in the United States
Number 111 of 151 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 151.
Use this page
SmogRank, Northern United States, December 2025: 11 cities with PM2.5 at twice their usual or more, 24 to 25 December 2025, smogrank.com/episodes/2025-12-northern-united-states (accessed 5 October 2026).
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. Fire detections from NASA FIRMS; smoke maps from NOAA HMS.
