SmogRank / Episodes / United States
North-western United States, September 2024
For 4 days, from 7 to 10 September 2024, 20 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Butte, Montana, United States: 79.7 µg/m³ on 10 September, about 16× its usual 4.9.
No cause named We have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached 20 in 1 country
Days 4 7 to 10 September 2024
Worst day 8 September 15 cities counted
Furthest above usual 16× Butte
Day by day
Play 8 September 15 cities at twice their usual or more
Rexburg Idaho Falls Pocatello Conda Twin Falls Great Falls Boise Meridian Nampa Coeur d'Alene Spokane Valley Walla Walla Tri-Cities Moses Lake Casper Gillette Wenatchee Ellensburg Altamont Gresham Auburn Renton Bellevue Columbia City Milwaukie Salmon Creek Vancouver Portland Parkland Marysville Canby Seattle Tacoma Picnic Point-North Lynnwood Shoreline Tigard Beaverton Woodburn Bremerton Hillsboro Longview Salem Lacey Albany Centralia Eugene Forest Grove McMinnville Corvallis Dallas Roseburg Grants Pass Aberdeen Coos Bay Sunnyside Spokane Rapid City Yakima Medford Lewiston Hermiston Ashland Redmond Billings Pullman Moscow Sheridan Pendleton The Dalles Bend Bozeman Helena Missoula Butte 4 Sep 5 6 7 8 9 10 11 12 13 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
Redmond Redmond, 4 September: 31.5 µg/m³, 4.8× usual Redmond, 5 September: 39.2 µg/m³, 6.0× usual Redmond, 6 September: 50.1 µg/m³, 7.7× usual Redmond, 7 September: 42.0 µg/m³, 6.5× usual Redmond, 8 September: 34.3 µg/m³, 5.3× usual Redmond, 9 September: 38.7 µg/m³, 6.0× usual Redmond, 10 September: 35.9 µg/m³, 5.5× usual Redmond, 11 September: 14.5 µg/m³, 2.2× usual Redmond, 12 September: 4.4 µg/m³, 0.7× usual Redmond, 13 September: 10.4 µg/m³, 1.6× usual Bend Bend, 4 September: 18.1 µg/m³, 3.2× usual Bend, 5 September: 31.4 µg/m³, 5.6× usual Bend, 6 September: 39.9 µg/m³, 7.1× usual Bend, 7 September: 34.8 µg/m³, 6.2× usual Bend, 8 September: 50.2 µg/m³, 9.0× usual Bend, 9 September: 65.7 µg/m³, 12× usual Bend, 10 September: 46.7 µg/m³, 8.3× usual Bend, 11 September: 36.9 µg/m³, 6.6× usual Bend, 12 September: 9.2 µg/m³, 1.6× usual Bend, 13 September: 40.3 µg/m³, 7.2× usual Moscow Moscow, 4 September: 11.4 µg/m³, 1.6× usual Moscow, 5 September: 13.5 µg/m³, 1.8× usual Moscow, 6 September: 25.5 µg/m³, 3.5× usual Moscow, 7 September: 31.6 µg/m³, 4.3× usual Moscow, 8 September: 41.1 µg/m³, 5.6× usual Moscow, 9 September: 22.5 µg/m³, 3.1× usual Moscow, 10 September: 8.2 µg/m³, 1.1× usual Moscow, 11 September: 16.6 µg/m³, 2.3× usual Moscow, 12 September: 5.5 µg/m³, 0.8× usual Moscow, 13 September: 6.5 µg/m³, 0.9× usual Lewiston Lewiston, 4 September: 12.6 µg/m³, 1.4× usual Lewiston, 5 September: 14.8 µg/m³, 1.7× usual Lewiston, 6 September: 22.7 µg/m³, 2.6× usual Lewiston, 7 September: 37.8 µg/m³, 4.3× usual Lewiston, 8 September: 44.3 µg/m³, 5.1× usual Lewiston, 9 September: 37.6 µg/m³, 4.3× usual Lewiston, 10 September: 10.6 µg/m³, 1.2× usual Lewiston, 11 September: 21.5 µg/m³, 2.5× usual Lewiston, 12 September: 4.6 µg/m³, 0.5× usual Lewiston, 13 September: 4.9 µg/m³, 0.6× usual Pullman Pullman, 4 September: 10.3 µg/m³, 1.7× usual Pullman, 5 September: 11.1 µg/m³, 1.8× usual Pullman, 6 September: 24.8 µg/m³, 4.1× usual Pullman, 7 September: 33.3 µg/m³, 5.5× usual Pullman, 8 September: 33.6 µg/m³, 5.5× usual Pullman, 9 September: 16.7 µg/m³, 2.7× usual Pullman, 10 September: 4.5 µg/m³, 0.7× usual Pullman, 11 September: 15.2 µg/m³, 2.5× usual Pullman, 12 September: 3.0 µg/m³, 0.5× usual Pullman, 13 September: 3.9 µg/m³, 0.6× usual Helena Helena, 4 September: 13.4 µg/m³, 1.9× usual Helena, 5 September: 12.4 µg/m³, 1.8× usual Helena, 6 September: 18.4 µg/m³, 2.6× usual Helena, 7 September: 28.3 µg/m³, 4.0× usual Helena, 8 September: 39.3 µg/m³, 5.6× usual Helena, 9 September: 69.2 µg/m³, 9.9× usual Helena, 10 September: 25.1 µg/m³, 3.6× usual Helena, 11 September: 27.7 µg/m³, 4.0× usual Helena, 12 September: 8.6 µg/m³, 1.2× usual Helena, 13 September: 8.1 µg/m³, 1.2× usual Butte Butte, 4 September: 10.3 µg/m³, 2.1× usual Butte, 5 September: 10.1 µg/m³, 2.0× usual Butte, 6 September: 16.4 µg/m³, 3.3× usual Butte, 7 September: 28.0 µg/m³, 5.6× usual Butte, 8 September: 27.6 µg/m³, 5.5× usual Butte, 9 September: 70.5 µg/m³, 14× usual Butte, 10 September: 79.7 µg/m³, 16× usual Butte, 11 September: 34.7 µg/m³, 6.9× usual Butte, 12 September: 5.0 µg/m³, 1.0× usual Butte, 13 September: 3.5 µg/m³, 0.7× usual Missoula Missoula, 4 September: 13.5 µg/m³, 2.7× usual Missoula, 5 September: 11.7 µg/m³, 2.3× usual Missoula, 6 September: 16.7 µg/m³, 3.3× usual Missoula, 7 September: 26.3 µg/m³, 5.3× usual Missoula, 8 September: 51.6 µg/m³, 10× usual Missoula, 9 September: 57.5 µg/m³, 12× usual Missoula, 10 September: 29.4 µg/m³, 5.9× usual Missoula, 11 September: 28.1 µg/m³, 5.6× usual Missoula, 12 September: 2.1 µg/m³, 0.4× usual Missoula, 13 September: 2.3 µg/m³, 0.5× usual Spokane Spokane, 4 September: 11.6 µg/m³, 1.7× usual Spokane, 5 September: 9.3 µg/m³, 1.4× usual Spokane, 6 September: 12.9 µg/m³, 1.9× usual Spokane, 7 September: 26.0 µg/m³, 3.9× usual Spokane, 8 September: 22.8 µg/m³, 3.4× usual Spokane, 9 September: 17.7 µg/m³, 2.6× usual Spokane, 10 September: 5.3 µg/m³, 0.8× usual Spokane, 11 September: 5.2 µg/m³, 0.8× usual Spokane, 12 September: 3.7 µg/m³, 0.6× usual Spokane, 13 September: 4.1 µg/m³, 0.6× usual The Dalles The Dalles, 4 September: 6.4 µg/m³, 1.3× usual The Dalles, 5 September: 13.3 µg/m³, 2.7× usual The Dalles, 6 September: 19.3 µg/m³, 3.9× usual The Dalles, 7 September: 20.3 µg/m³, 4.1× usual The Dalles, 8 September: 31.9 µg/m³, 6.4× usual The Dalles, 9 September: 8.2 µg/m³, 1.6× usual The Dalles, 10 September: 5.4 µg/m³, 1.1× usual The Dalles, 11 September: 5.0 µg/m³, 1.0× usual The Dalles, 12 September: 3.5 µg/m³, 0.7× usual The Dalles, 13 September: 4.5 µg/m³, 0.9× usual Hermiston Hermiston, 4 September: 15.1 µg/m³, 2.5× usual Hermiston, 5 September: 11.3 µg/m³, 1.9× usual Hermiston, 6 September: 15.4 µg/m³, 2.6× usual Hermiston, 7 September: 23.7 µg/m³, 3.9× usual Hermiston, 8 September: 31.1 µg/m³, 5.2× usual Hermiston, 9 September: 12.9 µg/m³, 2.2× usual Hermiston, 10 September: 6.5 µg/m³, 1.1× usual Hermiston, 11 September: 5.1 µg/m³, 0.9× usual Hermiston, 12 September: 2.9 µg/m³, 0.5× usual Hermiston, 13 September: 4.1 µg/m³, 0.7× usual Pendleton Pendleton, 4 September: 15.7 µg/m³, 3.1× usual Pendleton, 5 September: 11.9 µg/m³, 2.4× usual Pendleton, 6 September: 16.3 µg/m³, 3.3× usual Pendleton, 7 September: 23.8 µg/m³, 4.8× usual Pendleton, 8 September: 30.9 µg/m³, 6.2× usual Pendleton, 9 September: 19.2 µg/m³, 3.8× usual Pendleton, 10 September: 5.3 µg/m³, 1.1× usual Pendleton, 11 September: 4.7 µg/m³, 0.9× usual Pendleton, 12 September: 2.6 µg/m³, 0.5× usual Pendleton, 13 September: 3.7 µg/m³, 0.7× usual Yakima Yakima, 4 September: 4.6 µg/m³, 0.7× usual Yakima, 5 September: 7.2 µg/m³, 1.1× usual Yakima, 6 September: 13.8 µg/m³, 2.2× usual Yakima, 7 September: 18.6 µg/m³, 2.9× usual Yakima, 8 September: 27.8 µg/m³, 4.3× usual Yakima, 9 September: 10.2 µg/m³, 1.6× usual Yakima, 10 September: 2.0 µg/m³, 0.3× usual Yakima, 11 September: 2.6 µg/m³, 0.4× usual Yakima, 12 September: 3.0 µg/m³, 0.5× usual Yakima, 13 September: 4.1 µg/m³, 0.6× usual Medford Medford, 4 September: 15.5 µg/m³, 2.9× usual Medford, 5 September: 19.8 µg/m³, 3.7× usual Medford, 6 September: 15.7 µg/m³, 2.9× usual Medford, 7 September: 15.9 µg/m³, 2.9× usual Medford, 8 September: 26.4 µg/m³, 4.9× usual Medford, 9 September: 18.1 µg/m³, 3.4× usual Medford, 10 September: 9.1 µg/m³, 1.7× usual Medford, 11 September: 2.8 µg/m³, 0.5× usual Medford, 12 September: 2.9 µg/m³, 0.5× usual Medford, 13 September: 5.3 µg/m³, 1.0× usual Ashland Ashland, 4 September: 13.0 µg/m³, 2.6× usual Ashland, 5 September: 17.6 µg/m³, 3.5× usual Ashland, 6 September: 12.4 µg/m³, 2.5× usual Ashland, 7 September: 16.2 µg/m³, 3.2× usual Ashland, 8 September: 25.9 µg/m³, 5.2× usual Ashland, 9 September: 14.0 µg/m³, 2.8× usual Ashland, 10 September: 9.1 µg/m³, 1.8× usual Ashland, 11 September: 2.4 µg/m³, 0.5× usual Ashland, 12 September: 2.6 µg/m³, 0.5× usual Ashland, 13 September: 4.5 µg/m³, 0.9× usual Sunnyside Sunnyside, 4 September: 9.5 µg/m³, 1.3× usual Sunnyside, 5 September: 10.4 µg/m³, 1.5× usual Sunnyside, 6 September: 15.5 µg/m³, 2.2× usual Sunnyside, 7 September: 21.6 µg/m³, 3.0× usual Sunnyside, 8 September: 25.6 µg/m³, 3.6× usual Sunnyside, 9 September: 16.2 µg/m³, 2.3× usual Sunnyside, 10 September: 4.7 µg/m³, 0.7× usual Sunnyside, 11 September: 3.5 µg/m³, 0.5× usual Sunnyside, 12 September: 4.0 µg/m³, 0.6× usual Sunnyside, 13 September: 3.8 µg/m³, 0.5× usual Bozeman Bozeman, 4 September: 9.0 µg/m³, 1.5× usual Bozeman, 5 September: 9.3 µg/m³, 1.6× usual Bozeman, 6 September: 13.1 µg/m³, 2.2× usual Bozeman, 7 September: 18.8 µg/m³, 3.2× usual Bozeman, 8 September: 24.7 µg/m³, 4.2× usual Bozeman, 9 September: 38.2 µg/m³, 6.5× usual Bozeman, 10 September: 56.5 µg/m³, 9.6× usual Bozeman, 11 September: 13.2 µg/m³, 2.2× usual Bozeman, 12 September: 9.8 µg/m³, 1.7× usual Bozeman, 13 September: 5.9 µg/m³, 1.0× usual Billings Billings, 4 September: no reading Billings, 5 September: 8.2 µg/m³, 1.2× usual Billings, 6 September: 9.4 µg/m³, 1.4× usual Billings, 7 September: 13.1 µg/m³, 2.0× usual Billings, 8 September: 19.0 µg/m³, 2.8× usual Billings, 9 September: 32.6 µg/m³, 4.9× usual Billings, 10 September: 36.9 µg/m³, 5.5× usual Billings, 11 September: 15.9 µg/m³, 2.4× usual Billings, 12 September: 3.9 µg/m³, 0.6× usual Billings, 13 September: 3.1 µg/m³, 0.5× usual Sheridan Sheridan, 4 September: 7.2 µg/m³, 1.4× usual Sheridan, 5 September: 8.8 µg/m³, 1.8× usual Sheridan, 6 September: 8.1 µg/m³, 1.6× usual Sheridan, 7 September: 10.1 µg/m³, 2.0× usual Sheridan, 8 September: 14.4 µg/m³, 2.9× usual Sheridan, 9 September: 14.2 µg/m³, 2.8× usual Sheridan, 10 September: 30.3 µg/m³, 6.1× usual Sheridan, 11 September: 19.3 µg/m³, 3.9× usual Sheridan, 12 September: 5.2 µg/m³, 1.0× usual Sheridan, 13 September: 3.5 µg/m³, 0.7× usual Rapid City Rapid City, 4 September: 12.3 µg/m³, 1.8× usual Rapid City, 5 September: 19.6 µg/m³, 2.9× usual Rapid City, 6 September: 12.1 µg/m³, 1.8× usual Rapid City, 7 September: no reading Rapid City, 8 September: no reading Rapid City, 9 September: no reading Rapid City, 10 September: 27.6 µg/m³, 4.1× usual Rapid City, 11 September: 12.9 µg/m³, 1.9× usual Rapid City, 12 September: 6.9 µg/m³, 1.0× usual Rapid City, 13 September: 10.6 µg/m³, 1.6× usual 4 Sep 5 6 7 8 9 10 11 12 13 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
Butte Montana, United States 4 10 September 4.9 16× Bend Oregon, United States 1 9 September 5.6 12× Missoula Montana, United States 4 9 September 5.0 12× Helena Montana, United States 4 9 September 7.0 9.9× Bozeman Montana, United States 2 10 September 5.9 9.6× Redmond Oregon, United States 1 6 September 6.5 7.7× The Dalles Oregon, United States 1 8 September 4.8 6.4× Pendleton Oregon, United States 1 8 September 4.9 6.2× Sheridan Wyoming, United States 1 10 September 4.6 6.1× Moscow Idaho, United States 2 8 September 7.3 5.6× Pullman Washington, United States 2 8 September 6.1 5.5× Billings Montana, United States 2 10 September 6.7 5.5× Hermiston Oregon, United States 1 8 September 6.0 5.2× Ashland Oregon, United States 1 8 September 4.8 5.2× Lewiston Idaho, United States 3 8 September 8.7 5.1× Medford Oregon, United States 1 8 September 5.4 4.9× Yakima Washington, United States 1 8 September 6.4 4.3× Rapid City South Dakota, United States 1 10 September 6.7 4.1× Spokane Washington, United States 1 7 September 6.7 3.9× Sunnyside Washington, United States 1 8 September 7.1 3.6×
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 52 of 131 episodes in the United States since our records began, by cities reached.
North-eastern United States and Canada, June and July 2023 280 North-eastern United States and Canada, July 2026 264 North-eastern United States and Canada, May and June 2023 261 North-eastern United States and Canada, July and August 2025 241 North-eastern United States and Canada, July 2023 240 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 North-western United States, September 2024 20 Bars: cities reached; colour: the level of the episode's highest daily reading. All 131 .
Use this page SmogRank, North-western United States, September 2024: 20 cities with PM2.5 at twice their usual or more, 7 to 10 September 2024, smogrank.com/episodes/2024-09-north-western-united-states (accessed 3 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.
