For 3 days, from 6 to 8 October 2024, 11 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Sheridan, Wyoming, United States: 84.4 µg/m³ on 6 October, about 17× its usual 3.4. Ended 8 October.
Likely smoke from firesFires were detected within 300 km of the city on 71% of the counted city-days and smoke was mapped over the city on 43%. Satellites see crop, field and forest fires alike, so this doesn't say which burned.
Cities reached11in 1 country
Days36 to 8 October 2024
Worst day7 October7 cities counted
Furthest above usual17×Sheridan
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 114 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, North-western United States, October 2024: 11 cities with PM2.5 at twice their usual or more, 6 to 8 October 2024, smogrank.com/episodes/2024-10-north-western-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.
