Pennsylvania County Radon Risk Index
49 of Pennsylvania's 67 counties are EPA Zone 1, the highest radon-potential tier. Fulton, Juniata, Perry, Lebanon, and Mifflin counties have the state's highest shares of basement tests at or above EPA's 4.0 pCi/L action level: 60% to 70% of tests reported to PA DEP. 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. Jefferson County, labelled EPA Zone 2, ranks 9th highest of 67 counties by share of basement tests at or above 4.0 pCi/L.
Counties With the Highest Share of Tests at or Above 4.0 pCi/L
Predicted zones and measured results are two different things. The table below ranks Pennsylvania counties by the share of basement tests at or above 4.0 pCi/L in the PA DEP test record, which holds 2,629,492 basement tests statewide. Across the state, 37% of those tests measured 4.0 pCi/L or higher and the median result is 2.8 pCi/L.
| County | Basement tests at or above 4.0 pCi/L | Median | Basement tests | EPA zone |
|---|---|---|---|---|
| Fulton County | 70% | 7.6 pCi/L | 706 | Zone 1 |
| Juniata County | 62% | 5.6 pCi/L | 1,798 | Zone 1 |
| Perry County | 62% | 6.0 pCi/L | 5,945 | Zone 1 |
| Lebanon County | 60% | 6.4 pCi/L | 26,666 | Zone 1 |
| Mifflin County | 60% | 5.6 pCi/L | 3,746 | Zone 1 |
| Franklin County | 59% | 5.2 pCi/L | 20,716 | Zone 1 |
| York County | 57% | 4.9 pCi/L | 81,620 | Zone 1 |
| Centre County | 56% | 5.2 pCi/L | 39,672 | Zone 1 |
Sample size changes how much weight each figure deserves. Fulton County's figure rests on 706 tests. Restrict the ranking to counties with at least 3,000 basement tests and Perry County is highest, at 62% of 5,945 tests.
Source: PA Department of Environmental Protection, Radon Test Results, data.pa.gov. Tests through July 2026. Retrieved September 29, 2026.
Why a County Figure Does Not Describe One Home
Radon results are heavily skewed. Most tests in any county cluster near the low end while a long tail runs far above it. Statewide, the median basement result is 2.8 pCi/L, yet 6% of basement tests measured 20 pCi/L or higher, five times the action level. In Fulton County the median is 7.6 pCi/L and 24% of tests reached 20 pCi/L. No county-level statistic says which houses those are.
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 figure.
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, basement and first-floor test results, 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
The 67 counties below are listed alphabetically. Each card shows the county's EPA radon zone, the number of PA Radon Hub city pages inside it, and the share of its basement tests at or above 4.0 pCi/L with the number of tests behind that share, drawn from test results reported to the Pennsylvania Department of Environmental Protection. In 22 of the 67 counties the median basement result is at or above the EPA action level of 4.0 pCi/L. Read the test count alongside the share: Forest County's figure rests on 259 tests, Allegheny County's on 310,884.
Pennsylvania County Radon: Frequently Asked Questions
Which Pennsylvania county has the highest radon levels?
Fulton County has the highest share of basement tests at or above 4.0 pCi/L in the PA DEP test record: 70% of 706 tests, with a median of 7.6 pCi/L. Among counties with at least 3,000 basement tests, Perry County is highest at 62% of 5,945 tests. The figures count tests, not homes, and they describe a county: individual homes in lower-ranked counties can 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 come from the radon test results PA DEP publishes on data.pa.gov, one row per test, grouped by the county recorded for the test. Each figure covers basement tests in homes; tests in schools and non-residential buildings are excluded. The index shows the share of tests at or above 4.0 pCi/L and the median result. Figures count tests, not homes. A home tested more than once is counted each time, and tests run after a mitigation system was installed are included because the dataset does not separate them. The record holds 2,629,492 basement tests 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.