Average Radon Levels in Pennsylvania: EPA Zone Data and What It Means
Pennsylvania Is Entirely EPA Zone 1
Every county in Pennsylvania carries an EPA Zone 1 designation, the highest possible radon risk classification. Zone 1 indicates a predicted average indoor radon level above 4 pCi/L, the EPA’s recommended action level. Pennsylvania consistently ranks among the top three to five states nationally for radon, alongside Iowa, North Dakota, and Minnesota. The difference is that Pennsylvania’s high radon is not confined to one geological formation or one region. It spans the entire state, driven by different geological mechanisms in each part of the commonwealth.
Pennsylvania DEP estimates that 40 percent or more of Pennsylvania homes test above 4 pCi/L. This is roughly two to three times the national average percentage. For context, nationally about one in fifteen homes tests above the EPA action level. In Pennsylvania, the ratio is closer to one in two to three, and in the Reading Prong counties it is higher still.
The Reading Prong: The Most Notorious Radon Zone in the United States
The Reading Prong geological formation runs southwest to northeast through southeastern Pennsylvania, covering most of Bucks, Montgomery, Berks, Lehigh, and Northampton counties before continuing into New Jersey and New York. It is composed of Precambrian metamorphic and igneous rocks including hornblende gneiss, biotite schist, and metabasalt, all containing elevated concentrations of uranium-bearing minerals. As uranium decays, it produces radium, which in turn produces radon gas. In the Reading Prong, this decay process occurs in rocks that are fractured and permeable, allowing radon to migrate upward through soil and into building foundations at unusually high rates.
The Reading Prong became nationally known through a single, startling event in December 1984. Stanley Watras, a construction engineer at the Limerick Nuclear Power Plant in Limerick Township, Montgomery County, began triggering radiation alarms at the plant entrance every morning. Plant officials investigated and found that Watras was not picking up radiation at work. He was bringing it from home. Investigators tested his house in Boyertown, Berks County, and found radon levels exceeding 2,700 pCi/L, roughly 675 times the EPA action level and equivalent in lung cancer risk to smoking more than 200 cigarettes per day. The discovery made national headlines and prompted the EPA to launch its radon testing and mitigation programs, fundamentally changing how the United States approaches indoor air quality.
The Watras case was extreme but not isolated. Pre-mitigation radon levels of 20 to 100 pCi/L are documented regularly across Berks, Bucks, and Montgomery counties. Lehigh and Northampton counties show similar patterns. Homes built on slab foundations or with basement slabs sitting directly on fractured gneiss and schist can accumulate radon very rapidly, particularly during winter months when windows are closed and negative pressure inside the home draws soil gas inward.
County-by-County Radon Overview
Reading Prong Counties: Very High to Extreme
Berks County: Ground zero for radon awareness in the US. Pre-mitigation levels above 20 pCi/L are common; levels above 50 pCi/L occur. Boyertown, Reading, and surrounding municipalities have among the highest documented residential radon levels in North America. Testing and mitigation are effectively universal in real estate transactions.
Bucks County: Doylestown, Lansdale, Quakertown, and communities across Bucks County sit on Reading Prong geology. Pre-mitigation levels of 10 to 40 pCi/L are typical. The county has one of the highest per-capita concentrations of DEP-certified radon mitigators in Pennsylvania, reflecting decades of active remediation work.
Montgomery County: Including Norristown, Lansdale, Pottstown, and the Philadelphia western suburbs. High radon throughout, with elevated rates in areas underlain by Precambrian metamorphics. Radon testing is routine in all real estate transactions.
Lehigh County: Allentown, Emmaus, Macungie, and the Lehigh Valley. The county sits on the Reading Prong and produces consistently high results. Pre-mitigation levels above 10 pCi/L are common in the western and northern parts of the county.
Northampton County: Bethlehem, Easton, and the eastern Lehigh Valley. Similar geology to Lehigh County, with high radon throughout. The Slate Belt municipalities in the northern part of the county also show elevated readings due to metamorphic rock.
Lancaster and York Counties: High
Lancaster County sits on a combination of the Lancaster Plain limestone karst and Piedmont crystalline rock. Limestone karst geology concentrates radon in unusual ways: the porous structure of karst allows radon-bearing water and soil gas to move efficiently, and dissolution features can channel gas directly under foundations. Lancaster city and many surrounding townships show high radon rates. York County has similar geology and comparable radon levels. Testing rates in Lancaster and York are high by statewide standards, partly because both counties have active real estate markets with experienced inspection professionals who routinely include radon testing.
Adams County: High
Adams County, including Gettysburg and the South Mountain communities, lies on Blue Ridge crystalline rock and is classified as high-radon. The South Mountain Outlier, an extension of the Blue Ridge formation, contains metamorphic rocks with elevated uranium. Results above 10 pCi/L are not unusual in this county.
Centre County: High
Centre County, home to State College and Penn State University, sits primarily in the Nittany Valley, a limestone karst basin in the Valley and Ridge province. Limestone karst in this region produces consistently high radon through the same soil-gas channeling mechanism seen in Lancaster County. Radon testing in State College and surrounding communities routinely finds levels above 4 pCi/L, and results in the 8 to 20 pCi/L range are common in older homes with block foundations.
Allegheny County (Pittsburgh Metro): High to Very High
Allegheny County and the Pittsburgh metro area sit on Devonian shale and coal-bearing rock formations. These formations contain significant uranium and produce radon through a different mechanism than the Reading Prong: shale weathers into fine particles that concentrate uranium near the surface, and the fractured nature of shale allows soil gas to move readily. Pittsburgh metro radon levels are consistently high, with a significant percentage of homes above 4 pCi/L. Pre-mitigation results of 6 to 20 pCi/L are common. While less extreme than the Reading Prong counties, Allegheny County has sufficient radon prevalence that testing is standard in real estate transactions and recommended for all existing homeowners.
Lackawanna and Luzerne Counties (Scranton/Wilkes-Barre): Complex Radon
The anthracite coal region covering Lackawanna and Luzerne counties presents a complex radon picture. Anthracite coal measures are themselves relatively low in uranium, but the surrounding sedimentary rocks and underclay formations produce elevated radon. Mining history adds complexity: old mine workings, mine drainage tunnels, and disturbed soils can create unexpected pathways for soil gas movement. Radon levels in Scranton, Wilkes-Barre, and surrounding communities vary considerably. Testing is essential; the mine-influenced geology makes regional generalizations unreliable, and individual homes can have dramatically different results from neighboring properties.
Lycoming County and North-Central Pennsylvania: Moderate to High
Lycoming County and the north-central Pennsylvania counties sit on mixed shale, sandstone, and glacial deposits in the Allegheny Plateau province. Radon levels are generally moderate compared to the Reading Prong or Pittsburgh metro, but elevated results above 4 pCi/L occur throughout the region. Testing is advisable for all homeowners.
Erie County and Northwestern Pennsylvania: Moderate
Erie County occupies glaciated lake plain geology, deposited during the retreat of glaciers from Lake Erie. This geology is generally lower in uranium than the rest of the state, and average radon levels in Erie County are moderate by Pennsylvania standards. However, even in Erie County individual homes can test above the action level. The key variable is foundation construction and soil permeability in the specific location of the home, not just the regional geology.
Why Statewide Testing Matters
Pennsylvania’s Zone 1 designation applies to every county. The geological variation across the state means radon occurs through different pathways in different regions, but the result statewide is an elevated baseline risk. Several factors cause individual homes to differ significantly from the regional average:
- Foundation type: Full basements, especially with poured concrete or block walls, tend to accumulate more radon than slab-on-grade homes. Crawl spaces with unvented dirt floors are also high-risk entry points.
- Soil permeability: Coarse, fractured, or disturbed soils allow radon to move more freely than dense clay soils.
- Heating season: Pennsylvania’s cold winters mean homes are sealed for extended periods, and the stack effect (warm indoor air rising and escaping) creates negative pressure at the foundation level that draws radon inward. Tests conducted in winter typically return higher results than summer tests.
- House construction era: Older homes with unfinished basements, block foundations, and floor drains often test higher than modern construction with sealed slabs.
These variables mean that your neighbor’s radon result tells you very little about your own. Two houses on the same block can produce results that differ by a factor of ten. The only reliable way to know your home’s radon level is to test.
The EPA Action Level and Health Risk
The EPA recommends mitigation for any home testing at or above 4 pCi/L. Radon is the second leading cause of lung cancer in the United States after smoking, responsible for approximately 21,000 deaths per year nationally. In Pennsylvania, where radon levels are above the national average in most counties, the cumulative exposure risk over decades of living in an untreated home is significant. The World Health Organization recommends a lower action threshold of 2.7 pCi/L. Many Pennsylvania homeowners and buyers in high-radon areas choose to mitigate at any result above 2 pCi/L, particularly in homes with children.
Find Certified Radon Testers in Pennsylvania
Testing is the first step. Our directory lists certified radon testers in Pennsylvania who hold current DEP certification under Act 73, as required for real estate testing and recommended for all residential tests. Search by county to find a measurement professional serving your area.
Frequently Asked Questions
What is the average radon level in Pennsylvania homes?
Pennsylvania DEP estimates that roughly 40 percent or more of Pennsylvania homes test above the EPA action level of 4 pCi/L. The state average is significantly above the national indoor average of 1.3 pCi/L. In Reading Prong counties such as Berks, Bucks, Montgomery, and Lehigh, the percentage of homes above 4 pCi/L is substantially higher, and pre-mitigation levels of 10 to 50 pCi/L are common.
Why is Pennsylvania radon so much higher than other states?
Pennsylvania sits on geology that concentrates uranium and radium across much of the state. The Reading Prong in the southeast contains some of the most uranium-rich Precambrian metamorphic rock in the eastern US. Central Pennsylvania has limestone karst formations that channel radon-bearing soil gas into homes. Western Pennsylvania has Devonian shale and coal measures. Each geological province produces elevated radon through a different mechanism, but the result across the state is the same: elevated risk compared to national averages.
Is Erie County or northwestern Pennsylvania safer for radon?
Northwestern Pennsylvania, including Erie County, sits on glaciated lake plain geology that is generally lower in uranium than the rest of the state. Radon levels in Erie County are typically moderate, below the state average. But lower risk is not no risk. Any individual home in any Pennsylvania county can have elevated radon regardless of the regional average, because soil permeability, foundation construction, and ventilation patterns matter as much as bedrock geology.
What radon levels were found in the Stanley Watras house?
Investigators testing Stanley Watras’s home in Boyertown, Berks County found radon levels above 2,700 pCi/L, roughly 675 times the EPA action level and comparable to working in a uranium mine. This 1984 discovery was the event that launched national radon awareness in the United States and led the EPA to develop its radon testing and mitigation programs.
Should I test even if I live outside the Reading Prong?
Yes. Every Pennsylvania county is EPA Zone 1, meaning the entire state has high radon potential. Central Pennsylvania limestone counties like Centre and Lancaster have high radon rates. Pittsburgh metro (Allegheny County) tests high regularly. Even counties with generally moderate geology can have individual homes with elevated radon due to local soil conditions, foundation type, and house construction. Testing is the only way to know your specific home’s level.