Smoke: keep one room clean
Shut the windows and run an air purifier sized for the room.
Wildfire smoke guide →Air quality in Pennsylvania, county by county
38 Pennsylvania counties have enough EPA monitor data to count bad-air days. Weighted by population, they averaged about 5 days a year from 2021 to 2025 when the air was unhealthy for sensitive groups or worse.
CDC's model, which estimates every county from monitors and satellites, gives the average Pennsylvania county, weighted by population, about 1 smoky day (fine particles over 35.4 µg/m³, mostly wildfire smoke) and about 2 high-ozone days a year at its worst spot (2018 to 2022). See every county on the map.
Every Pennsylvania county with monitors
Most bad-air days first. 33 have a full report. Compare with the worst and the cleanest counties in the US.
| County | Bad-air days a year | |
|---|---|---|
| Allegheny CountyPittsburghWinter particles | 9.6 | |
| Philadelphia CountyOzone | 9.4 | |
| Lancaster CountyLancasterWinter particles | 6.8 | |
| Bucks CountyOzoneparticles not measured | 6.2 | |
| Berks CountyReadingOzone | 4.8 | |
| Dauphin CountyHarrisburgWinter particles | 4.0 | |
| Lebanon CountyLebanonOzone and particles | 3.6 | |
| Adams CountyGettysburgSummer smoke | 3.4 | |
| Delaware CountyChesterOzone | 3.4 | |
| Beaver CountyOzone and particles | 3.2 | |
| Northampton CountyBethlehemOzone and particles | 3.2 | |
| Montgomery CountyNorristownOzone | 3.0 | |
| York CountyYorkOzone and particles | 3.0 | |
| Chester CountyWest ChesterOzone | 2.8 | |
| Indiana CountyIndianaOzone and particles | 2.4 | |
| Lackawanna CountyScrantonSummer smoke | 2.4 | |
| Mercer CountyHermitageOzone | 2.4 | |
| Washington CountyWashingtonOzone and particles | 2.4 | |
| Armstrong CountyOzone | 2.2 | |
| Lehigh CountyAllentownOzone and particles | 2.2 | |
| Centre CountyState CollegeSummer smoke | 2.0 | |
| Erie CountyErieOzone and particles | 2.0 | |
| Blair CountyAltoonaSummer smoke | 1.8 | |
| Cumberland CountyCarlisleWinter particlesozone not measured | 1.8 | |
| Cambria CountyJohnstownSummer smoke | 1.2 | |
| Lycoming CountyWilliamsportOzone | 1.2 | |
| Monroe CountyEast StroudsburgOzone | 1.2 | |
| Fayette CountyOzone | 1.0 | |
| Lawrence CountyNew CastleOzoneparticles not measured | 1.0 | |
| Susquehanna CountySmokeozone not measured | 1.0 | |
| Bradford CountySayreSummer smoke | 0.8 | |
| Greene CountyOzone | 0.8 | |
| Wyoming CountySmokeozone not measured | 0.8 | |
| Tioga CountySummer smoke | 0.6 | |
| Westmoreland CountyMurrysvilleOzone | 0.6 | |
| Franklin CountyChambersburgRarely badparticles not measured | 0.4 | |
| Elk CountySt. MarysRarely badparticles not measured | 0.2 | |
| Somerset CountySomersetRarely badparticles not measured | 0.0 | |
| No Pennsylvania county with a monitor matches “”. Counties without a monitor aren't listed; a neighbor's numbers are a fair guide, or find it on the map. | ||
Counties without a monitor aren't listed. Air doesn't stop at the county line, so a neighbor's numbers are a fair guide.
What helps on a bad-air day
Check AirNow for today's reading and tomorrow's forecast, then match the step to the pollutant.
Upgrade the furnace filter
MERV 13 catches far more smoke than thin fiberglass filters.
Furnace filters →Questions people ask about air quality in Pennsylvania
Which county in Pennsylvania has the worst air?
Among Pennsylvania counties with EPA monitors, Allegheny County had the most bad-air days from 2021 to 2025: about 10 bad-air days a year with an AQI over 100. That's about 1 day in 38.
How often is the air bad in Pennsylvania?
Weighted by population, Pennsylvania counties with monitors averaged about 5 bad-air days a year from 2021 to 2025 with an AQI over 100, when children, older adults and people with asthma should cut back outdoors. That's about 1 day in 77.
Which county in Pennsylvania has the cleanest air?
Of the 38 counties with enough monitor data, Somerset County had the fewest, with no bad-air days at all from 2021 to 2025. Counties without monitors aren't ranked.
Where can I check today's air quality in Pennsylvania?
AirNow, run by the EPA and its partners, shows current readings and forecasts by ZIP code, and its fire and smoke map adds smoke plumes and low-cost sensors.
Sources: EPA AirData county AQI summaries, 2021 to 2025; CDC Environmental Public Health Tracking Network modeled PM2.5 and ozone, 2018 to 2022. Updated November 8, 2026.