SmogRank / Episodes / United States

North-western United States, August and September 2024

For 4 days, from 29 August to 1 September 2024, 10 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Meridian, Idaho, United States: 96.3 µg/m³ on 31 August, about 14× its usual 6.7. Ended 1 September.

No cause namedWe have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached10in 1 country
Days429 August to 1 September 2024
Worst day30 August9 cities counted
Furthest above usual17×Garden City

Day by day

30 August 9 cities at twice their usual or more
BurleyPocatelloIdaho FallsRexburgCondaNorth OgdenBountifulSalt Lake CityCodyLanderLa GrandeWorlandBaker CitySmithfieldPrestonOntarioRock SpringsWeiserTwin FallsNampaMeridianGarden City
26 Aug27282930311 Sep234
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

Garden CityGarden City, 26 August: 4.1 µg/m³Garden City, 27 August: no readingGarden City, 28 August: 4.4 µg/m³Garden City, 29 August: 48.6 µg/m³, 8.2× its usual 5.9Garden City, 30 August: 82.5 µg/m³, 14× its usual 5.7Garden City, 31 August: 92.6 µg/m³, 17× its usual 5.5Garden City, 1 September: 48.6 µg/m³, 9.2× its usual 5.3Garden City, 2 September: 14.8 µg/m³Garden City, 3 September: 26.2 µg/m³Garden City, 4 September: 36.0 µg/m³MeridianMeridian, 26 August: 5.2 µg/m³Meridian, 27 August: no readingMeridian, 28 August: 4.9 µg/m³Meridian, 29 August: 47.0 µg/m³, 6.4× its usual 7.3Meridian, 30 August: 92.1 µg/m³, 13× its usual 7.0Meridian, 31 August: 96.3 µg/m³, 14× its usual 6.7Meridian, 1 September: 47.0 µg/m³, 7.2× its usual 6.5Meridian, 2 September: 15.6 µg/m³Meridian, 3 September: 26.2 µg/m³Meridian, 4 September: 38.4 µg/m³Twin FallsTwin Falls, 26 August: 7.5 µg/m³Twin Falls, 27 August: no readingTwin Falls, 28 August: 30.4 µg/m³Twin Falls, 29 August: 38.6 µg/m³, 7.1× its usual 5.4Twin Falls, 30 August: 59.0 µg/m³, 11× its usual 5.3Twin Falls, 31 August: 28.9 µg/m³, 5.5× its usual 5.3Twin Falls, 1 September: 15.5 µg/m³Twin Falls, 2 September: 11.2 µg/m³Twin Falls, 3 September: 34.7 µg/m³Twin Falls, 4 September: 49.3 µg/m³NampaNampa, 26 August: 4.3 µg/m³Nampa, 27 August: no readingNampa, 28 August: 6.0 µg/m³Nampa, 29 August: 33.1 µg/m³, 4.0× its usual 8.2Nampa, 30 August: 73.9 µg/m³, 9.5× its usual 7.8Nampa, 31 August: 92.4 µg/m³, 12× its usual 7.7Nampa, 1 September: 48.8 µg/m³, 6.3× its usual 7.7Nampa, 2 September: 13.0 µg/m³Nampa, 3 September: 23.6 µg/m³Nampa, 4 September: 31.2 µg/m³WeiserWeiser, 26 August: 8.0 µg/m³Weiser, 27 August: no readingWeiser, 28 August: 8.4 µg/m³Weiser, 29 August: 8.6 µg/m³Weiser, 30 August: 50.4 µg/m³, 5.3× its usual 9.5Weiser, 31 August: 75.1 µg/m³, 8.0× its usual 9.4Weiser, 1 September: 60.5 µg/m³, 6.5× its usual 9.3Weiser, 2 September: 21.4 µg/m³Weiser, 3 September: 17.4 µg/m³Weiser, 4 September: 13.2 µg/m³Rock SpringsRock Springs, 26 August: 4.3 µg/m³Rock Springs, 27 August: 4.4 µg/m³Rock Springs, 28 August: 4.5 µg/m³Rock Springs, 29 August: 15.2 µg/m³Rock Springs, 30 August: 38.0 µg/m³, 7.6× its usual 4.0Rock Springs, 31 August: 10.0 µg/m³Rock Springs, 1 September: 7.2 µg/m³Rock Springs, 2 September: 4.5 µg/m³Rock Springs, 3 September: 3.5 µg/m³Rock Springs, 4 September: 5.2 µg/m³OntarioOntario, 26 August: 3.1 µg/m³Ontario, 27 August: 3.6 µg/m³Ontario, 28 August: 2.8 µg/m³Ontario, 29 August: 11.6 µg/m³Ontario, 30 August: 31.0 µg/m³, 5.7× its usual 5.4Ontario, 31 August: 36.5 µg/m³, 6.9× its usual 5.3Ontario, 1 September: 26.1 µg/m³, 4.9× its usual 5.3Ontario, 2 September: 12.6 µg/m³Ontario, 3 September: 11.2 µg/m³Ontario, 4 September: 10.8 µg/m³SmithfieldSmithfield, 26 August: 7.7 µg/m³Smithfield, 27 August: 7.7 µg/m³Smithfield, 28 August: 8.2 µg/m³Smithfield, 29 August: 15.8 µg/m³Smithfield, 30 August: 31.0 µg/m³, 4.8× its usual 6.4Smithfield, 31 August: 16.3 µg/m³Smithfield, 1 September: 10.0 µg/m³Smithfield, 2 September: 7.2 µg/m³Smithfield, 3 September: 7.8 µg/m³Smithfield, 4 September: 31.4 µg/m³PrestonPreston, 26 August: 4.7 µg/m³Preston, 27 August: no readingPreston, 28 August: 3.2 µg/m³Preston, 29 August: 22.1 µg/m³Preston, 30 August: 26.8 µg/m³, 5.4× its usual 3.5Preston, 31 August: 11.1 µg/m³Preston, 1 September: 8.3 µg/m³Preston, 2 September: 5.2 µg/m³Preston, 3 September: 5.3 µg/m³Preston, 4 September: 25.0 µg/m³Baker CityBaker City, 26 August: 9.3 µg/m³Baker City, 27 August: 9.3 µg/m³Baker City, 28 August: 7.4 µg/m³Baker City, 29 August: 5.3 µg/m³Baker City, 30 August: 16.6 µg/m³Baker City, 31 August: 19.8 µg/m³Baker City, 1 September: 26.1 µg/m³, 3.8× its usual 6.9Baker City, 2 September: 16.5 µg/m³Baker City, 3 September: 18.8 µg/m³Baker City, 4 September: 13.3 µg/m³26 Aug27282930311 Sep234
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.
Good0–50
Moderate51–100
Sensitive groups101–150
Unhealthy151–200
Very unhealthy201–300
Hazardous301+

Every city it reached

CityDaysHighestUsualAbove usual
Garden CityIdaho, United States4 92.631 August5.517×
MeridianIdaho, United States4 96.331 August6.714×
NampaIdaho, United States4 92.431 August7.712×
Twin FallsIdaho, United States3 59.030 August5.311×
WeiserIdaho, United States3 75.131 August9.48.0×
Rock SpringsWyoming, United States1 38.030 August4.07.6×
OntarioOregon, United States3 36.531 August5.36.9×
PrestonIdaho, United States1 26.830 August3.55.4×
SmithfieldUtah, United States1 31.030 August6.44.8×
Baker CityOregon, United States1 26.11 September6.93.8×

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 123 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, August and September 2024: 10 cities with PM2.5 at twice their usual or more, 29 August to 1 September 2024, smogrank.com/episodes/2024-08-north-western-united-states (accessed 5 October 2026).

Download top 10 (PNG)

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.