SmogRank / Episodes / United States
Western United States and Mexico, December 2025
For 6 days, from 13 to 18 December 2025, 25 cities in the United States and Mexico had PM2.5 at least twice their usual for the time of year. The worst reading was Mexicali, Baja California, Mexico: 76.3 µg/m³ on 16 December, about 2.7× its usual 28.4.
No cause named We have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached 25 in 2 countries
Days 6 13 to 18 December 2025
Worst day 13 December 18 cities counted
Furthest above usual 9.8× Pasadena
Day by day
Play 13 December 18 cities at twice their usual or more
Banning Victorville Lake Elsinore Indio Mission Viejo Lancaster Poway La Crescenta-Montrose Burbank Ridgecrest Santa Clarita Brawley Agoura Simi Valley El Centro Thousand Oaks Fillmore Enterprise Summerlin South Spring Valley Oxnard Bakersfield Henderson Boulder City Yuma Porterville Delano Santa Barbara Goleta Tulare Visalia Corcoran Hanford Lompoc Orcutt Nipomo Fresno Arroyo Grande San Luis Obispo Atascadero Prescott Glendale Phoenix Tempe Salinas Prunedale Tijuana Las Vegas Hawthorne Chula Vista Mexicali Carson El Cajon Compton Torrance San Diego Inglewood Fontana Los Angeles Anaheim Lomita Long Beach Oceanside Huntington Beach Santa Monica Calexico Whittier Riverside Jurupa Valley Glendora Pasadena 10 Dec 11 12 13 14 15 16 17 18 19 20 21 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
Mexicali Mexicali, 10 December: 54.1 µg/m³, 1.9× usual Mexicali, 11 December: 64.3 µg/m³, 2.3× usual Mexicali, 12 December: 43.6 µg/m³, 1.5× usual Mexicali, 13 December: 48.8 µg/m³, 1.7× usual Mexicali, 14 December: 43.8 µg/m³, 1.5× usual Mexicali, 15 December: 43.8 µg/m³, 1.5× usual Mexicali, 16 December: 76.3 µg/m³, 2.7× usual Mexicali, 17 December: 67.3 µg/m³, 2.4× usual Mexicali, 18 December: 47.7 µg/m³, 1.7× usual Mexicali, 19 December: 50.3 µg/m³, 1.8× usual Mexicali, 20 December: 30.7 µg/m³, 1.1× usual Mexicali, 21 December: 43.3 µg/m³, 1.5× usual Huntington Beach Huntington Beach, 10 December: 19.5 µg/m³, 1.4× usual Huntington Beach, 11 December: 27.5 µg/m³, 2.0× usual Huntington Beach, 12 December: 37.6 µg/m³, 2.7× usual Huntington Beach, 13 December: 51.1 µg/m³, 3.7× usual Huntington Beach, 14 December: 43.2 µg/m³, 3.1× usual Huntington Beach, 15 December: 38.5 µg/m³, 2.8× usual Huntington Beach, 16 December: 45.3 µg/m³, 3.2× usual Huntington Beach, 17 December: 48.6 µg/m³, 3.5× usual Huntington Beach, 18 December: 39.0 µg/m³, 2.8× usual Huntington Beach, 19 December: 31.1 µg/m³, 2.2× usual Huntington Beach, 20 December: 21.4 µg/m³, 1.5× usual Huntington Beach, 21 December: 40.6 µg/m³, 2.9× usual Inglewood Inglewood, 10 December: 9.4 µg/m³, 0.9× usual Inglewood, 11 December: 14.7 µg/m³, 1.4× usual Inglewood, 12 December: 27.2 µg/m³, 2.5× usual Inglewood, 13 December: 33.3 µg/m³, 3.1× usual Inglewood, 14 December: 18.2 µg/m³, 1.7× usual Inglewood, 15 December: no reading Inglewood, 16 December: 29.2 µg/m³, 2.7× usual Inglewood, 17 December: 29.9 µg/m³, 2.8× usual Inglewood, 18 December: 34.0 µg/m³, 3.1× usual Inglewood, 19 December: 23.9 µg/m³, 2.2× usual Inglewood, 20 December: 29.0 µg/m³, 2.7× usual Inglewood, 21 December: 29.0 µg/m³, 2.7× usual Jurupa Valley Jurupa Valley, 10 December: 13.4 µg/m³, 0.9× usual Jurupa Valley, 11 December: 20.8 µg/m³, 1.4× usual Jurupa Valley, 12 December: 26.7 µg/m³, 1.8× usual Jurupa Valley, 13 December: 50.9 µg/m³, 3.4× usual Jurupa Valley, 14 December: 66.4 µg/m³, 4.4× usual Jurupa Valley, 15 December: 45.2 µg/m³, 3.0× usual Jurupa Valley, 16 December: 25.9 µg/m³, 1.7× usual Jurupa Valley, 17 December: 38.4 µg/m³, 2.5× usual Jurupa Valley, 18 December: 35.2 µg/m³, 2.3× usual Jurupa Valley, 19 December: 24.2 µg/m³, 1.6× usual Jurupa Valley, 20 December: 26.4 µg/m³, 1.7× usual Jurupa Valley, 21 December: 35.6 µg/m³, 2.4× usual Pasadena Pasadena, 10 December: 0.4 µg/m³, 0.1× usual Pasadena, 11 December: 7.5 µg/m³, 1.5× usual Pasadena, 12 December: 20.9 µg/m³, 4.2× usual Pasadena, 13 December: 49.2 µg/m³, 9.8× usual Pasadena, 14 December: 40.2 µg/m³, 8.0× usual Pasadena, 15 December: 28.3 µg/m³, 5.7× usual Pasadena, 16 December: 17.5 µg/m³, 3.5× usual Pasadena, 17 December: 43.6 µg/m³, 8.7× usual Pasadena, 18 December: 10.5 µg/m³, 2.1× usual Pasadena, 19 December: 20.5 µg/m³, 4.1× usual Pasadena, 20 December: 42.5 µg/m³, 8.5× usual Pasadena, 21 December: 34.5 µg/m³, 6.9× usual Compton Compton, 10 December: 14.7 µg/m³, 0.9× usual Compton, 11 December: 20.7 µg/m³, 1.2× usual Compton, 12 December: 25.6 µg/m³, 1.5× usual Compton, 13 December: 47.1 µg/m³, 2.8× usual Compton, 14 December: 21.5 µg/m³, 1.3× usual Compton, 15 December: 18.7 µg/m³, 1.1× usual Compton, 16 December: 26.4 µg/m³, 1.6× usual Compton, 17 December: 45.4 µg/m³, 2.7× usual Compton, 18 December: 42.7 µg/m³, 2.5× usual Compton, 19 December: 26.0 µg/m³, 1.5× usual Compton, 20 December: 22.0 µg/m³, 1.3× usual Compton, 21 December: 33.9 µg/m³, 2.0× usual Whittier Whittier, 10 December: 13.2 µg/m³, 1.0× usual Whittier, 11 December: no reading Whittier, 12 December: 34.3 µg/m³, 2.6× usual Whittier, 13 December: 44.6 µg/m³, 3.4× usual Whittier, 14 December: 37.7 µg/m³, 2.8× usual Whittier, 15 December: no reading Whittier, 16 December: 39.4 µg/m³, 3.0× usual Whittier, 17 December: 52.8 µg/m³, 4.0× usual Whittier, 18 December: 46.5 µg/m³, 3.5× usual Whittier, 19 December: 32.5 µg/m³, 2.4× usual Whittier, 20 December: 29.3 µg/m³, 2.2× usual Whittier, 21 December: 29.0 µg/m³, 2.2× usual Riverside Riverside, 10 December: 11.2 µg/m³, 0.9× usual Riverside, 11 December: 13.2 µg/m³, 1.1× usual Riverside, 12 December: 21.5 µg/m³, 1.7× usual Riverside, 13 December: 40.2 µg/m³, 3.2× usual Riverside, 14 December: 53.1 µg/m³, 4.3× usual Riverside, 15 December: 35.1 µg/m³, 2.8× usual Riverside, 16 December: 17.0 µg/m³, 1.4× usual Riverside, 17 December: 29.0 µg/m³, 2.3× usual Riverside, 18 December: 26.9 µg/m³, 2.2× usual Riverside, 19 December: 19.4 µg/m³, 1.6× usual Riverside, 20 December: 25.4 µg/m³, 2.0× usual Riverside, 21 December: 29.9 µg/m³, 2.4× usual Long Beach Long Beach, 10 December: 15.2 µg/m³, 1.2× usual Long Beach, 11 December: 19.0 µg/m³, 1.6× usual Long Beach, 12 December: 16.8 µg/m³, 1.4× usual Long Beach, 13 December: 39.6 µg/m³, 3.2× usual Long Beach, 14 December: 18.8 µg/m³, 1.5× usual Long Beach, 15 December: 20.5 µg/m³, 1.7× usual Long Beach, 16 December: 23.6 µg/m³, 1.9× usual Long Beach, 17 December: 43.6 µg/m³, 3.6× usual Long Beach, 18 December: 29.6 µg/m³, 2.4× usual Long Beach, 19 December: 13.4 µg/m³, 1.1× usual Long Beach, 20 December: 13.4 µg/m³, 1.1× usual Long Beach, 21 December: 23.4 µg/m³, 1.9× usual Anaheim Anaheim, 10 December: 13.2 µg/m³, 1.0× usual Anaheim, 11 December: 17.6 µg/m³, 1.4× usual Anaheim, 12 December: 24.1 µg/m³, 1.9× usual Anaheim, 13 December: 38.5 µg/m³, 3.0× usual Anaheim, 14 December: 33.1 µg/m³, 2.5× usual Anaheim, 15 December: 23.4 µg/m³, 1.8× usual Anaheim, 16 December: 27.9 µg/m³, 2.1× usual Anaheim, 17 December: 43.2 µg/m³, 3.3× usual Anaheim, 18 December: 36.3 µg/m³, 2.8× usual Anaheim, 19 December: 27.1 µg/m³, 2.1× usual Anaheim, 20 December: 14.1 µg/m³, 1.1× usual Anaheim, 21 December: 23.9 µg/m³, 1.8× usual Carson Carson, 10 December: 11.9 µg/m³, 0.9× usual Carson, 11 December: 17.9 µg/m³, 1.3× usual Carson, 12 December: 25.4 µg/m³, 1.9× usual Carson, 13 December: 35.1 µg/m³, 2.6× usual Carson, 14 December: 19.2 µg/m³, 1.4× usual Carson, 15 December: no reading Carson, 16 December: 30.8 µg/m³, 2.3× usual Carson, 17 December: 36.5 µg/m³, 2.7× usual Carson, 18 December: 30.1 µg/m³, 2.2× usual Carson, 19 December: 19.5 µg/m³, 1.4× usual Carson, 20 December: 19.5 µg/m³, 1.4× usual Carson, 21 December: 26.4 µg/m³, 2.0× usual Hawthorne Hawthorne, 10 December: 14.8 µg/m³, 0.9× usual Hawthorne, 11 December: 18.7 µg/m³, 1.2× usual Hawthorne, 12 December: 27.0 µg/m³, 1.7× usual Hawthorne, 13 December: 32.2 µg/m³, 2.1× usual Hawthorne, 14 December: 17.9 µg/m³, 1.1× usual Hawthorne, 15 December: no reading Hawthorne, 16 December: 28.9 µg/m³, 1.9× usual Hawthorne, 17 December: 37.6 µg/m³, 2.4× usual Hawthorne, 18 December: 32.4 µg/m³, 2.1× usual Hawthorne, 19 December: 23.8 µg/m³, 1.5× usual Hawthorne, 20 December: 25.3 µg/m³, 1.6× usual Hawthorne, 21 December: 30.6 µg/m³, 2.0× usual San Diego San Diego, 10 December: 9.5 µg/m³, 0.9× usual San Diego, 11 December: 16.2 µg/m³, 1.5× usual San Diego, 12 December: 22.7 µg/m³, 2.0× usual San Diego, 13 December: 31.2 µg/m³, 2.8× usual San Diego, 14 December: 32.0 µg/m³, 2.9× usual San Diego, 15 December: 20.4 µg/m³, 1.8× usual San Diego, 16 December: 34.4 µg/m³, 3.1× usual San Diego, 17 December: 31.9 µg/m³, 2.9× usual San Diego, 18 December: 20.5 µg/m³, 1.8× usual San Diego, 19 December: 16.4 µg/m³, 1.5× usual San Diego, 20 December: 16.0 µg/m³, 1.4× usual San Diego, 21 December: 22.7 µg/m³, 2.0× usual Fontana Fontana, 10 December: 6.0 µg/m³, 0.6× usual Fontana, 11 December: 10.0 µg/m³, 0.9× usual Fontana, 12 December: 20.6 µg/m³, 1.9× usual Fontana, 13 December: 30.9 µg/m³, 2.8× usual Fontana, 14 December: 33.8 µg/m³, 3.1× usual Fontana, 15 December: 24.6 µg/m³, 2.3× usual Fontana, 16 December: 13.8 µg/m³, 1.3× usual Fontana, 17 December: 19.8 µg/m³, 1.8× usual Fontana, 18 December: 17.4 µg/m³, 1.6× usual Fontana, 19 December: 17.3 µg/m³, 1.6× usual Fontana, 20 December: 26.0 µg/m³, 2.4× usual Fontana, 21 December: 18.3 µg/m³, 1.7× usual Torrance Torrance, 10 December: 8.2 µg/m³, 0.7× usual Torrance, 11 December: 15.4 µg/m³, 1.4× usual Torrance, 12 December: 23.3 µg/m³, 2.0× usual Torrance, 13 December: 30.1 µg/m³, 2.6× usual Torrance, 14 December: 16.6 µg/m³, 1.5× usual Torrance, 15 December: 12.9 µg/m³, 1.1× usual Torrance, 16 December: 28.8 µg/m³, 2.5× usual Torrance, 17 December: 34.0 µg/m³, 3.0× usual Torrance, 18 December: 25.8 µg/m³, 2.3× usual Torrance, 19 December: 17.3 µg/m³, 1.5× usual Torrance, 20 December: 20.7 µg/m³, 1.8× usual Torrance, 21 December: 24.9 µg/m³, 2.2× usual Los Angeles Los Angeles, 10 December: 9.0 µg/m³, 0.7× usual Los Angeles, 11 December: 12.4 µg/m³, 1.0× usual Los Angeles, 12 December: 22.6 µg/m³, 1.9× usual Los Angeles, 13 December: 29.7 µg/m³, 2.4× usual Los Angeles, 14 December: 20.6 µg/m³, 1.7× usual Los Angeles, 15 December: 19.4 µg/m³, 1.6× usual Los Angeles, 16 December: 29.3 µg/m³, 2.4× usual Los Angeles, 17 December: 38.9 µg/m³, 3.2× usual Los Angeles, 18 December: 32.9 µg/m³, 2.7× usual Los Angeles, 19 December: 21.1 µg/m³, 1.7× usual Los Angeles, 20 December: no reading Los Angeles, 21 December: no reading Lomita Lomita, 10 December: 6.3 µg/m³, 0.7× usual Lomita, 11 December: 12.0 µg/m³, 1.4× usual Lomita, 12 December: 20.3 µg/m³, 2.4× usual Lomita, 13 December: 28.9 µg/m³, 3.4× usual Lomita, 14 December: 15.4 µg/m³, 1.8× usual Lomita, 15 December: no reading Lomita, 16 December: 29.0 µg/m³, 3.4× usual Lomita, 17 December: 25.6 µg/m³, 3.0× usual Lomita, 18 December: 17.1 µg/m³, 2.0× usual Lomita, 19 December: 15.3 µg/m³, 1.8× usual Lomita, 20 December: 16.5 µg/m³, 1.9× usual Lomita, 21 December: 21.9 µg/m³, 2.6× usual Glendora Glendora, 10 December: 1.7 µg/m³, 0.3× usual Glendora, 11 December: 3.6 µg/m³, 0.6× usual Glendora, 12 December: 11.9 µg/m³, 2.0× usual Glendora, 13 December: 26.9 µg/m³, 4.5× usual Glendora, 14 December: 22.7 µg/m³, 3.8× usual Glendora, 15 December: 13.7 µg/m³, 2.3× usual Glendora, 16 December: 13.1 µg/m³, 2.2× usual Glendora, 17 December: 18.9 µg/m³, 3.2× usual Glendora, 18 December: 6.0 µg/m³, 1.0× usual Glendora, 19 December: 9.1 µg/m³, 1.5× usual Glendora, 20 December: 19.4 µg/m³, 3.2× usual Glendora, 21 December: 16.4 µg/m³, 2.7× usual Santa Monica Santa Monica, 10 December: 5.5 µg/m³, 0.7× usual Santa Monica, 11 December: 11.1 µg/m³, 1.3× usual Santa Monica, 12 December: 21.1 µg/m³, 2.5× usual Santa Monica, 13 December: 26.7 µg/m³, 3.2× usual Santa Monica, 14 December: 15.7 µg/m³, 1.9× usual Santa Monica, 15 December: 31.6 µg/m³, 3.8× usual Santa Monica, 16 December: 23.4 µg/m³, 2.8× usual Santa Monica, 17 December: 21.7 µg/m³, 2.6× usual Santa Monica, 18 December: 24.2 µg/m³, 2.9× usual Santa Monica, 19 December: 16.9 µg/m³, 2.0× usual Santa Monica, 20 December: 28.2 µg/m³, 3.4× usual Santa Monica, 21 December: 25.7 µg/m³, 3.1× usual Oceanside Oceanside, 10 December: 6.4 µg/m³, 0.8× usual Oceanside, 11 December: 6.2 µg/m³, 0.8× usual Oceanside, 12 December: 7.6 µg/m³, 1.0× usual Oceanside, 13 December: 20.1 µg/m³, 2.5× usual Oceanside, 14 December: 29.5 µg/m³, 3.7× usual Oceanside, 15 December: 18.4 µg/m³, 2.3× usual Oceanside, 16 December: 14.0 µg/m³, 1.8× usual Oceanside, 17 December: 18.2 µg/m³, 2.3× usual Oceanside, 18 December: 15.6 µg/m³, 2.0× usual Oceanside, 19 December: 8.5 µg/m³, 1.1× usual Oceanside, 20 December: 11.5 µg/m³, 1.4× usual Oceanside, 21 December: 19.6 µg/m³, 2.5× usual Las Vegas Las Vegas, 10 December: 19.4 µg/m³, 1.5× usual Las Vegas, 11 December: 20.6 µg/m³, 1.6× usual Las Vegas, 12 December: 19.8 µg/m³, 1.6× usual Las Vegas, 13 December: 23.0 µg/m³, 1.8× usual Las Vegas, 14 December: 27.7 µg/m³, 2.2× usual Las Vegas, 15 December: 21.5 µg/m³, 1.7× usual Las Vegas, 16 December: 20.2 µg/m³, 1.6× usual Las Vegas, 17 December: no reading Las Vegas, 18 December: 22.0 µg/m³, 1.7× usual Las Vegas, 19 December: 25.6 µg/m³, 2.0× usual Las Vegas, 20 December: 24.0 µg/m³, 1.9× usual Las Vegas, 21 December: 22.7 µg/m³, 1.8× usual Tijuana Tijuana, 10 December: 20.0 µg/m³, 1.0× usual Tijuana, 11 December: 32.9 µg/m³, 1.6× usual Tijuana, 12 December: 37.9 µg/m³, 1.8× usual Tijuana, 13 December: 38.6 µg/m³, 1.9× usual Tijuana, 14 December: 31.7 µg/m³, 1.5× usual Tijuana, 15 December: 24.4 µg/m³, 1.2× usual Tijuana, 16 December: 42.3 µg/m³, 2.1× usual Tijuana, 17 December: 36.6 µg/m³, 1.8× usual Tijuana, 18 December: 29.4 µg/m³, 1.4× usual Tijuana, 19 December: 39.0 µg/m³, 1.9× usual Tijuana, 20 December: 21.8 µg/m³, 1.1× usual Tijuana, 21 December: 23.6 µg/m³, 1.2× usual Chula Vista Chula Vista, 10 December: 5.1 µg/m³, 0.5× usual Chula Vista, 11 December: 11.2 µg/m³, 1.1× usual Chula Vista, 12 December: 18.1 µg/m³, 1.8× usual Chula Vista, 13 December: 22.6 µg/m³, 2.2× usual Chula Vista, 14 December: 17.0 µg/m³, 1.7× usual Chula Vista, 15 December: 13.4 µg/m³, 1.3× usual Chula Vista, 16 December: 26.2 µg/m³, 2.5× usual Chula Vista, 17 December: 18.7 µg/m³, 1.8× usual Chula Vista, 18 December: 16.5 µg/m³, 1.6× usual Chula Vista, 19 December: 13.8 µg/m³, 1.3× usual Chula Vista, 20 December: 15.6 µg/m³, 1.5× usual Chula Vista, 21 December: 16.6 µg/m³, 1.6× usual El Cajon El Cajon, 10 December: 8.5 µg/m³, 0.8× usual El Cajon, 11 December: 10.9 µg/m³, 1.1× usual El Cajon, 12 December: 16.6 µg/m³, 1.6× usual El Cajon, 13 December: 22.4 µg/m³, 2.2× usual El Cajon, 14 December: 18.3 µg/m³, 1.8× usual El Cajon, 15 December: 13.2 µg/m³, 1.3× usual El Cajon, 16 December: 25.7 µg/m³, 2.5× usual El Cajon, 17 December: 28.2 µg/m³, 2.8× usual El Cajon, 18 December: 14.9 µg/m³, 1.5× usual El Cajon, 19 December: 12.8 µg/m³, 1.3× usual El Cajon, 20 December: 14.4 µg/m³, 1.4× usual El Cajon, 21 December: 14.9 µg/m³, 1.5× usual Calexico Calexico, 10 December: 12.3 µg/m³, 0.9× usual Calexico, 11 December: 19.6 µg/m³, 1.4× usual Calexico, 12 December: 20.3 µg/m³, 1.5× usual Calexico, 13 December: 20.3 µg/m³, 1.5× usual Calexico, 14 December: 9.3 µg/m³, 0.7× usual Calexico, 15 December: 8.6 µg/m³, 0.6× usual Calexico, 16 December: 12.9 µg/m³, 0.9× usual Calexico, 17 December: 53.8 µg/m³, 4.0× usual Calexico, 18 December: 27.9 µg/m³, 2.1× usual Calexico, 19 December: no reading Calexico, 20 December: no reading Calexico, 21 December: no reading 10 Dec 12 14 16 18 20 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. Good 0–50
Moderate 51–100
Sensitive groups 101–150
Unhealthy 151–200
Very unhealthy 201–300
Hazardous 301+
Every city it reached
City Days Highest Usual Above usual
Pasadena California, United States 4 13 December 4.4 9.8× Glendora California, United States 1 13 December 6.0 4.5× Jurupa Valley California, United States 5 14 December 15.1 4.4× Riverside California, United States 5 14 December 12.4 4.3× Whittier California, United States 5 17 December 13.3 4.0× Calexico California, United States 2 17 December 13.6 4.0× Santa Monica California, United States 2 15 December 8.4 3.8× Oceanside California, United States 1 14 December 8.0 3.7× Huntington Beach California, United States 6 13 December 14.0 3.7× Long Beach California, United States 3 17 December 12.2 3.6× Lomita California, United States 3 16 December 8.5 3.4× Anaheim California, United States 5 17 December 13.0 3.3× Los Angeles California, United States 4 17 December 12.2 3.2× Inglewood California, United States 4 18 December 10.8 3.1× Fontana California, United States 2 14 December 10.9 3.1× San Diego California, United States 4 16 December 11.1 3.1× Torrance California, United States 4 17 December 11.4 3.0× El Cajon California, United States 2 17 December 10.1 2.8× Compton California, United States 3 13 December 17.0 2.8× Carson California, United States 4 17 December 13.5 2.7× Mexicali Baja California, Mexico 2 16 December 28.4 2.7× Chula Vista California, United States 1 16 December 10.3 2.5× Hawthorne California, United States 3 17 December 15.6 2.4× Las Vegas Nevada, United States 1 14 December 12.6 2.2× Tijuana Baja California, Mexico 1 16 December 20.5 2.1×
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 and Mexico
Number 33 of 106 episodes in the United States and Mexico since our records began, by cities reached.
North-eastern United States and Canada, June and July 2023 279 North-eastern United States and Canada, July 2026 264 North-eastern United States and Canada, May and June 2023 260 North-eastern United States and Canada, July and August 2025 241 North-eastern United States and Canada, July 2023 241 North-eastern United States and Canada, June 2025 196 North-eastern United States and Canada, June 2023 161 North-eastern United States and Canada, July 2025 119 North-western United States and Canada, August 2023 110 North-eastern United States, June 2025 104 North-western United States and Canada, August 2026 99 North-eastern United States and Canada, February 2026 96 Western United States and Mexico, December 2025 25 Bars: cities reached; colour: the level of the episode's highest daily reading. All 106 .
Use this page SmogRank, Western United States and Mexico, December 2025: 25 cities with PM2.5 at twice their usual or more, 13 to 18 December 2025, smogrank.com/episodes/2025-12-western-united-states-and-mexico (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.
