SmogRank / Episodes / United States

Pennsylvania, United States, January 2024

For 2 days, from 23 to 24 January 2024, 8 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Lancaster, Pennsylvania, United States: 38.6 µg/m³ on 23 January, about 4.4× its usual 8.8.

No cause namedWe have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached8in 1 country
Days223 to 24 January 2024
Worst day24 January8 cities counted
Furthest above usual4.4×Hagerstown

Day by day

24 January 8 cities at twice their usual or more
State CollegeHunt ValleyRockvilleAshburnScaggsvilleBaltimoreSouth LaurelAltoonaWashingtonSpringfieldRose HillEdgewoodReadingJohnstownElktonNewarkBearWilmingtonAllentownChesterDoverNorristownBethlehemHarrisonburgScrantonCharlottesvillePhiladelphiaSewellPennsaukenSouth VinelandRichmondMcKeesportWilkinsburgEwingPittsburghTrentonPleasantvilleRandolphNew BrunswickAtlantic CityWeirton HeightsWeirtonToms RiverLindenSteubenvilleElizabethHermitageWheelingBayonnePatersonJersey CityErieYoungstownNew York CityFort LeeNew CityWarrenNewburghWhite PlainsCantonDanburyPainesvilleAkronBridgeportShaker HeightsCleveland HeightsCharlestonClevelandMedinaWaterburyNew HavenPittsfieldHartfordSpringfieldHuntingtonNorth ChicopeeChathamAshlandNew LondonLebanonWest MifflinYorkBensalemHarrisburgCarlisleLancasterHagerstown
20 Jan21222324252627
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

LancasterLancaster, 20 January: 3.6 µg/m³, 0.4× usualLancaster, 21 January: 3.7 µg/m³, 0.4× usualLancaster, 22 January: 13.1 µg/m³, 1.5× usualLancaster, 23 January: 38.6 µg/m³, 4.4× usualLancaster, 24 January: 25.7 µg/m³, 2.9× usualLancaster, 25 January: 28.4 µg/m³, 3.2× usualLancaster, 26 January: 6.9 µg/m³, 0.8× usualLancaster, 27 January: 8.2 µg/m³, 0.9× usualCarlisleCarlisle, 20 January: 2.7 µg/m³, 0.3× usualCarlisle, 21 January: 3.5 µg/m³, 0.4× usualCarlisle, 22 January: 17.6 µg/m³, 2.2× usualCarlisle, 23 January: 31.5 µg/m³, 3.9× usualCarlisle, 24 January: 29.4 µg/m³, 3.7× usualCarlisle, 25 January: 21.7 µg/m³, 2.7× usualCarlisle, 26 January: 6.4 µg/m³, 0.8× usualCarlisle, 27 January: 3.6 µg/m³, 0.5× usualBensalemBensalem, 20 January: 9.0 µg/m³, 1.0× usualBensalem, 21 January: 7.8 µg/m³, 0.8× usualBensalem, 22 January: 18.5 µg/m³, 2.0× usualBensalem, 23 January: 30.7 µg/m³, 3.3× usualBensalem, 24 January: 27.8 µg/m³, 3.0× usualBensalem, 25 January: 22.2 µg/m³, 2.4× usualBensalem, 26 January: 9.9 µg/m³, 1.1× usualBensalem, 27 January: 7.9 µg/m³, 0.9× usualHarrisburgHarrisburg, 20 January: 3.1 µg/m³, 0.4× usualHarrisburg, 21 January: 3.6 µg/m³, 0.4× usualHarrisburg, 22 January: 14.8 µg/m³, 1.7× usualHarrisburg, 23 January: 28.7 µg/m³, 3.3× usualHarrisburg, 24 January: 30.7 µg/m³, 3.6× usualHarrisburg, 25 January: 22.2 µg/m³, 2.6× usualHarrisburg, 26 January: 7.9 µg/m³, 0.9× usualHarrisburg, 27 January: 4.7 µg/m³, 0.5× usualHagerstownHagerstown, 20 January: 4.4 µg/m³, 0.7× usualHagerstown, 21 January: 5.7 µg/m³, 0.9× usualHagerstown, 22 January: 11.8 µg/m³, 1.9× usualHagerstown, 23 January: 27.3 µg/m³, 4.4× usualHagerstown, 24 January: 26.1 µg/m³, 4.2× usualHagerstown, 25 January: 26.5 µg/m³, 4.3× usualHagerstown, 26 January: 5.6 µg/m³, 0.9× usualHagerstown, 27 January: 2.7 µg/m³, 0.4× usualYorkYork, 20 January: 3.5 µg/m³, 0.4× usualYork, 21 January: 3.8 µg/m³, 0.4× usualYork, 22 January: 13.4 µg/m³, 1.5× usualYork, 23 January: 26.2 µg/m³, 2.9× usualYork, 24 January: 28.6 µg/m³, 3.2× usualYork, 25 January: 33.1 µg/m³, 3.7× usualYork, 26 January: 8.5 µg/m³, 0.9× usualYork, 27 January: 5.3 µg/m³, 0.6× usualWest MifflinWest Mifflin, 20 January: 5.2 µg/m³, 0.6× usualWest Mifflin, 21 January: 10.4 µg/m³, 1.3× usualWest Mifflin, 22 January: 18.2 µg/m³, 2.2× usualWest Mifflin, 23 January: 11.7 µg/m³, 1.4× usualWest Mifflin, 24 January: 26.0 µg/m³, 3.2× usualWest Mifflin, 25 January: 14.5 µg/m³, 1.8× usualWest Mifflin, 26 January: 4.5 µg/m³, 0.5× usualWest Mifflin, 27 January: 4.6 µg/m³, 0.6× usualLebanonLebanon, 20 January: 2.7 µg/m³, 0.3× usualLebanon, 21 January: 3.1 µg/m³, 0.4× usualLebanon, 22 January: 13.4 µg/m³, 1.6× usualLebanon, 23 January: 23.9 µg/m³, 2.8× usualLebanon, 24 January: 25.0 µg/m³, 2.9× usualLebanon, 25 January: 16.6 µg/m³, 1.9× usualLebanon, 26 January: 7.2 µg/m³, 0.8× usualLebanon, 27 January: 7.7 µg/m³, 0.9× usual20 Jan21222324252627
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
HagerstownMaryland, United States2 27.323 January6.24.4×
LancasterPennsylvania, United States2 38.623 January8.84.4×
CarlislePennsylvania, United States2 31.523 January8.03.9×
YorkPennsylvania, United States2 33.125 January9.03.7×
HarrisburgPennsylvania, United States2 30.724 January8.63.6×
BensalemPennsylvania, United States2 30.723 January9.23.3×
West MifflinPennsylvania, United States1 26.024 January8.23.2×
LebanonPennsylvania, United States1 25.024 January8.62.9×

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 127 of 131 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 131.

Use this page

SmogRank, Pennsylvania, United States, January 2024: 8 cities with PM2.5 at twice their usual or more, 23 to 24 January 2024, smogrank.com/episodes/2024-01-pennsylvania-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.