For 1 day, on 15 September 2026, 8 cities in the United States and Mexico had PM2.5 at least twice their usual for the time of year. The worst reading was Port Isabel, Texas, United States: 36.8 µg/m³ on 15 September, about 4.3× its usual 8.6. Ended 15 September.
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 reached8in 2 countries
Days115 September 2026
Worst day15 September8 cities counted
Furthest above usual5.3×Heroica Matamoros
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 and Mexico
Number 148 of 163 episodes in the United States and Mexico since our records began, by cities reached.
Bars: cities reached; colour: the level of the episode's highest daily reading. All 163.
Use this page
SmogRank, Southern United States and Mexico, September 2026: 8 cities with PM2.5 at twice their usual or more, 15 September 2026, smogrank.com/episodes/2026-09-southern-united-states-and-mexico (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.
