Pennsylvania County Radon Risk Index
49 of Pennsylvania's 67 counties are EPA Zone 1, the highest radon-potential tier. Potter, Forest, Lebanon, Snyder, and Carbon counties record the state's highest measured basement averages — 13.6 to 16.3 pCi/L against the EPA action level of 4.0 pCi/L. Only Philadelphia County is Zone 3. Every zone rating is a county-wide prediction, not a measurement of any individual home.
How EPA Radon Zone Classification Works
The EPA Map of Radon Zones assigns each U.S. county to one of three tiers. The assignment is the output of a predictive model, not a survey of homes: it combines indoor radon measurements available at the time of publication with underlying geology, aerial radioactivity readings, soil permeability, and the mix of foundation types common to the area. The result is a planning tool for building codes and public health targeting.
- Zone 1 — predicted average indoor screening level above 4.0 pCi/L. The EPA expects the typical home here to test at or above the action level.
- Zone 2 — predicted average between 2.0 and 4.0 pCi/L. Below the action level on paper, comfortably above it in a large share of individual homes.
- Zone 3 — predicted average below 2.0 pCi/L. The lowest tier, and still not a clearance: elevated individual results occur in every Zone 3 county in the country.
The critical limitation is that the model was calibrated at county resolution. It was never designed to answer whether one address is safe, and the EPA has been explicit that it does not replace testing.
How Pennsylvania's 67 Counties Distribute Across the Three Zones
Pennsylvania is among the most radon-affected states in the country, and the zone distribution shows it. Of the Commonwealth's 67 counties, 49 are classified EPA Zone 1, 17 are Zone 2, and a single county — Philadelphia — is Zone 3. Roughly three quarters of the state sits in the highest tier, which is why radon is treated as a default consideration in Pennsylvania real-estate transactions rather than a regional curiosity.
The Zone 2 group is geographically concentrated in the northwest and the far northeast, among them Cambria, Crawford, Elk, Erie, Fayette, and Forest. A Zone 2 label reflects a lower modeled county average, not a lower ceiling on what any given home can measure, and several Zone 2 counties turn out to carry the highest measured averages in the state once actual test results are counted.
Counties With the Highest Measured Basement Averages
Predicted zones and measured results are two different things. The table below ranks Pennsylvania counties by average basement radon in the DEP measurement record, which currently covers 528,409 basement tests statewide (PA DEP RadonZip Tool, January 1990 – December 2025).
| County | Basement average | Homes tested | EPA zone |
|---|---|---|---|
| Potter County | 16.3 pCi/L | 957 | Zone 2 |
| Forest County | 15.8 pCi/L | 98 | Zone 2 |
| Lebanon County | 14.0 pCi/L | 9,501 | Zone 1 |
| Snyder County | 14.0 pCi/L | 580 | Zone 1 |
| Carbon County | 13.6 pCi/L | 3,677 | Zone 1 |
| Juniata County | 13.2 pCi/L | 483 | Zone 1 |
| Venango County | 11.7 pCi/L | 1,276 | Zone 1 |
| Berks County | 11.7 pCi/L | 7,339 | Zone 1 |
Sample size changes how much weight each figure deserves. Potter County tops the measured ranking at 16.3 pCi/L despite carrying an EPA Zone 2 label, on a pool of 957 tests. Restrict the ranking to counties with at least 3,000 recorded tests and Lebanon County leads at 14.0 pCi/L across 9,501 homes — a figure built on enough measurements to be difficult to dismiss.
Why County Averages Understate Individual-Home Variation
An average compresses a distribution into one number, and the radon distribution is heavily skewed. Most homes in any county cluster near the low end while a long tail runs far above it, so the county figure sits well below what the worst-affected properties actually measure. The highest individual reading in the Pennsylvania record is 2,902 pCi/L, logged in Union County — a county whose overall basement average is 7.4 pCi/L. No county-level statistic would have flagged that house.
The spread has physical causes that operate at property scale rather than county scale. Bedrock fracturing varies over tens of meters. Soil permeability changes with drainage and fill. A sealed slab and a block-wall basement on the same street pull soil gas at different rates, and a home with a tight envelope and strong winter stack effect will draw more of it than its neighbor. None of that is visible in a county-wide mean.
Treat the county figure as a prior probability, not a diagnosis. It tells you how seriously to take the possibility before you test. It cannot tell you the number for a specific address, and no aggregate ever will.
How to Use This Index
Start at the county. Each of the 67 entries below opens a county page carrying its EPA zone, measured basement and first-floor averages, housing-stock age profile, adjacent-county links, and connections to DEP-certified professionals working that territory.
From the county page, follow the constituent city links down to the municipality that matches your address. PA Radon Hub maintains 96 dedicated city and borough pages, each carrying local test data and neighborhood-level context that a county aggregate flattens out. County first, then city, is the fastest path to the most specific data available for your property.
PA Radon Hub is an independent resource — we do not perform testing. We connect Pennsylvania property owners with independent, DEP-certified radon professionals, and we encourage you to verify any contractor's certification against the PA DEP Radon Services Directory before hiring.
Looking for a specific municipality rather than a county? All Pennsylvania city radon pages lists every city and borough covered by PA Radon Hub, each with local testing data and DEP-certified contractor connections.
Browse Every Pennsylvania County
Pennsylvania County Radon: Frequently Asked Questions
Which Pennsylvania county has the highest radon levels?
Potter County records the highest average basement radon of any Pennsylvania county in the DEP measurement record, at 16.3 pCi/L across 957 tested homes. Among counties with large test pools, Lebanon County leads at 14.0 pCi/L across 9,501 homes. Both figures are multiples of the EPA 4.0 pCi/L action level, and both are averages — individual homes in lower-ranked counties routinely test higher.
How many Pennsylvania counties are EPA Zone 1?
49 of Pennsylvania's 67 counties carry the EPA Zone 1 designation, the highest of the three radon-potential tiers. 17 are Zone 2 and just one — Philadelphia County — is Zone 3. No Pennsylvania county is exempt from radon risk; the zone tiers describe predicted county averages, not the presence or absence of hazard.
Does radon risk stop at county lines?
No. County borders are administrative lines drawn for governance, not geological ones. The uranium-bearing rock formations that generate radon run in bands that cut across borders, so two homes a mile apart on opposite sides of a county line sit on the same geology and carry the same risk despite holding different EPA zone labels. Use the zone as orientation, not as a boundary.
How is Pennsylvania county radon data collected?
County figures on this index aggregate individual test results reported to the Pennsylvania Department of Environmental Protection (PA DEP RadonZip Tool, January 1990 – December 2025). Results are submitted by certified testers and laboratories, recorded against the property ZIP code, then rolled up to county level. Counts are weighted by the number of homes tested, so counties with larger test pools carry proportionally more influence. The record covers 528,409 basement measurements statewide.
Do EPA radon zones tell you whether a specific home is safe?
No. EPA zone maps were built from predictive modeling of indoor measurements, geology, soil permeability, and foundation types — not from testing individual properties. A Zone 3 designation does not mean a home is clear, and a Zone 1 designation does not guarantee an elevated result. Only a test of the specific building produces an actionable number.