Radon in Adams County, Pennsylvania
Radon in Adams County: 2,978 DEP-recorded basement tests average 4.98 pCi/L — above EPA's 4.0 pCi/L action level, but roughly half the level measured in the three counties Adams borders. First-floor tests average 2.01 pCi/L. Adams is EPA Zone 1, and Gettysburg-area homes should be tested regardless of the county figure.
Above the Action Level, Below Every Neighbor
Adams County records a test-weighted basement average of 4.98 pCi/L across 2,978 PA DEP-recorded tests. That is above the 4.0 pCi/L level at which EPA recommends mitigation, and below the 6.44 pCi/L test-weighted average across the 67 counties in the PA Radon Hub dataset. Adams ranks 51st of 67 by basement average — a county that clears the action level without approaching the extremes.
The comparison that matters is local. Adams borders three counties, and it reads roughly half of each: Cumberland averages 10.83 pCi/L, York 10.33, and Franklin 10.06. A homeowner who has read about South Central Pennsylvania radon has almost certainly encountered numbers from those three counties. Adams is not that county, and treating Cumberland-level figures as applicable here overstates the local picture by a factor of two.
The first-floor number reinforces it. Adams first-floor tests average 2.01 pCi/L across 899 measurements — comfortably below the action level, and a basement-to-first-floor ratio of 2.48, the steepest gradient among the counties compared on this page. In Adams the elevated concentrations are concentrated below grade, which is a more tractable mitigation problem than a county where the exposure has already worked its way into the living space.
The 899 first-floor tests are worth noting on their own. Against 2,978 basement tests, that is an unusually high proportion of above-grade measurement for a county this size, and it is why the gradient figure above can be stated with reasonable confidence rather than inferred.
Diabase Contact Zones Under the Gettysburg Basin
Adams County sits on the Gettysburg Basin — a Triassic complex of shale and siltstone cut by diabase intrusions. The Gettysburg diabase is a Jurassic-age igneous body that pushed up through those sedimentary rocks, and uranium concentrates at the contact zones where the diabase meets the surrounding shale and siltstone. Those contact zones weather into uranium-enriched soils, and uranium-enriched soil is what produces radon.
This is geology you can see. The diabase surfaces as the dark, rocky ridges across the battlefield landscape around Gettysburg. Homes built on or near those ridges and outcrops sit closest to the contact zones and carry the highest local risk.
The practical consequence is variability rather than uniform elevation. A county built on a single uniform formation produces a tight distribution of results; a county built on intrusions cutting through sedimentary rock produces a wide one, because the distance from a given house to the nearest contact zone can change over a few hundred yards. The 4.98 pCi/L county average is the midpoint of a genuinely wide spread, and the highest single reading on record here is 269.0 pCi/L.
Two Housing Stocks, Two Mitigation Problems
Adams County has the newest housing stock of the counties profiled on this page: a Census-reported median year built of 1980 for owner-occupied housing, against 1960 in Armstrong and 1957 in Allegheny. Gettysburg borough, at a median year built of 1942, is 38 years older than the county around it. Adams is a Civil War-era core surrounded by four decades of newer construction.
Those two stocks fail differently. Post-1970 construction across the county was built to tighter thermal specifications — better insulation and reduced air infiltration raise the stack effect during the heating season, which pulls more soil gas across the slab. A modern Adams County home is not protected by its age; its envelope is doing the opposite.
The borough core is the other problem. Historic stone and brick homes from the Civil War era frequently have fieldstone foundations, exposed dirt basements, and minimal vapor barriers, all of which increase radon entry. These structures need mitigation designs that achieve depressurization without damaging original masonry or requiring visible exterior piping — a specialist scope, and one worth naming when requesting a proposal.
The county carries 107,914 residents, a median owner-occupied home value of $256,100, and a Gettysburg-area median sale price of $320,000. Radon testing is standard in Adams County real estate transactions, and tourist-area rental properties around the battlefield warrant testing on the same basis as owner-occupied homes.
Measured Radon Data for Adams County
This unusually high reading may reflect a source data anomaly or a single-home outlier; individual home results vary widely.
Source: PA DEP RadonZip Tool, January 1990 – December 2025 · Updated 2026-05-20
Adams is a mid-sample county: 36th of 67 by basement test volume, with 2,978 basement and 899 first-floor measurements drawn from ZIP 17325. That is enough measurement to state a county average with confidence, and not so much that the average smooths away the underlying spread.
The 269.0 pCi/L peak is the reminder of that spread. In a county whose radon is driven by proximity to diabase contact zones rather than by a uniform formation, a result two orders of magnitude above the county average is what the geology predicts — and it is why the 4.98 pCi/L figure should be read as a starting expectation rather than a ceiling.
1 DEP-certified mitigation firm is physically based in Adams County. Firms from adjacent counties routinely serve here as well.
Source: PA DEP Certified Mitigation Firms Directory, July 2026. This reflects business address only — firms based elsewhere may also serve Adams County.
Cities and Boroughs in Adams County
Gettysburg carries the Adams County dataset. Its page holds the ZIP-level DEP averages behind the numbers above, guidance for historic fieldstone-foundation properties, and connections to DEP-certified professionals serving the county.
Adjacent Counties
Adams reads roughly half the level of all three counties it borders — Cumberland at 10.83 pCi/L, York at 10.33, and Franklin at 10.06. That contrast is the clearest argument against applying regional South Central Pennsylvania radon figures to an Adams County address.
Comparing radon risk across the wider Commonwealth? Browse All 67 Pennsylvania county radon pages for EPA zone classification and measured basement averages statewide.
Connect with a DEP-Certified Radon Professional
Submit your request and we will connect you with a DEP-certified professional serving Adams County. All providers verified against the PA Department of Environmental Protection Bureau of Radiation Protection directory.
(717) 461-2427Connect with a DEP-certified radon proPA Radon Hub is an independent informational resource. We do not perform radon testing or mitigation. We connect Pennsylvania property owners with independent, DEP-certified professionals — verify any contractor's certification at dep.pa.gov/radon before hiring.
Testing and Mitigation Options in Adams County
Testing a County With a Steep Basement Gradient
Adams has the widest basement-to-first-floor gap of the counties compared here — 4.98 pCi/L below grade against 2.01 above it. Test the basement, and treat a first-floor result as a separate question rather than a proxy. Short-term charcoal canisters and continuous monitors run 48–96 hours at $15–$50; long-term alpha-track detectors run 90 days or more and average out the heating-season stack effect, which matters more than usual in the county's post-1970 housing because tighter envelopes amplify that seasonal swing.
If the property sits on or near a diabase ridge, a long-term test is the better first instrument. Proximity to a contact zone drives a wide range of possible results, and a 48-hour snapshot in that setting is a weaker basis for a decision than a 90-day average.
Historic Foundations Change the Cost Picture
Active sub-slab depressurization systems in the Gettysburg area run $900–$2,400, and where a property falls in that band is mostly a function of when it was built. A post-1970 home on a poured-concrete slab is a routine installation at the lower end. A Civil War-era property with a fieldstone foundation and an exposed dirt basement needs sub-membrane depressurization, sealing work that respects original masonry, and often concealed routing in place of exterior piping — all of which push toward the upper end.
One Firm in County, Several Across the Line
The PA DEP Certified Mitigation Firms Directory lists one mitigation firm with a business address in Adams County. Cumberland County lists seven and York two, and firms from both routinely work in Adams. For a historic Gettysburg property it is worth asking specifically about prior work on fieldstone foundations before comparing proposals. Certification under 25 Pa. Code § 240 is required statewide — confirm the certification number is current before signing a contract or accepting payment.
Frequently Asked Questions
What is the average radon level in Adams County?
PA DEP-recorded basement tests in Adams County average 4.98 pCi/L across 2,978 tests, above the EPA action level of 4.0 pCi/L. First-floor tests average 2.01 pCi/L across 899 tests. Adams ranks 51st of the 67 counties in the PA Radon Hub dataset by basement average, below the statewide test-weighted average of 6.44 pCi/L. The highest single result on record is 269.0 pCi/L.
Why does Adams County read lower than York and Cumberland?
Adams averages 4.98 pCi/L against 10.83 pCi/L in Cumberland, 10.33 in York, and 10.06 in Franklin — roughly half its neighbors. Adams sits on the Gettysburg Basin, a Triassic shale and siltstone complex cut by diabase intrusions, where radon production concentrates at the contact zones between the igneous rock and the surrounding sediments rather than being generated uniformly across the county. That produces a lower county average with a wider spread between individual properties.
How does Gettysburg diabase affect radon levels?
The Gettysburg diabase is a Jurassic-age igneous intrusion that penetrated the Triassic sedimentary rocks of the Gettysburg Basin. Uranium concentrates at the contact zones where the diabase meets the surrounding shale and siltstone, and those zones weather into uranium-enriched soils that generate elevated radon. The diabase is visible as the dark, rocky terrain across the battlefield area. Homes near those ridges and outcrops carry the highest local risk, which is why Adams County results vary widely between properties a short distance apart.
Do historic Gettysburg homes need different radon mitigation?
Often, yes. Civil War-era stone and brick homes frequently have fieldstone foundations, exposed dirt basements, and minimal vapor barriers, all of which increase radon entry. Mitigating them typically means sub-membrane depressurization, sealing that respects original masonry, and concealed routing rather than visible exterior piping. DEP-certified contractors experienced with historic preservation can design systems that reach the required depressurization without altering the structure. Expect these properties toward the upper end of the $900–$2,400 Adams County range.
Does newer construction in Adams County have lower radon risk?
No. Adams County has a Census-reported median year built of 1980 — newer than most Pennsylvania counties — and post-1970 construction is not protective. Tighter insulation and reduced air infiltration raise the stack effect during the heating season, drawing more soil gas across the slab. Sump pits, plumbing chases, and slab penetrations remain entry paths regardless of construction era. Test the house, not the build year.
Is radon testing expected in Adams County real estate transactions?
Radon testing is standard in Adams County real estate transactions, against a county median owner-occupied home value of $256,100 and a Gettysburg-area median sale price of $320,000. Pennsylvania's Real Estate Seller Disclosure Law (68 Pa.C.S. §§ 7301–7315) requires sellers to disclose known material defects (§ 7303), with radon recorded on the state disclosure form under 'presence of hazardous substances' at § 7304(b)(14); § 7308 imposes no duty to test. Tourist-area rental properties around the battlefield warrant testing on the same basis as owner-occupied homes.
Which ZIP codes does the Adams County radon data cover?
One: 17325. That single ZCTA spans Gettysburg borough and the townships around it, which is a wider footprint than a ZIP code usually implies and takes in a substantial share of the diabase terrain driving local variability. Every measurement summarized above — 2,978 below grade, 899 above it — originates inside that boundary, drawn from the PA DEP RadonZip Tool. Municipalities elsewhere in Adams County are not represented in the figures on this page.
Does Adams County's Zone 1 classification match its measured data?
Yes, though narrowly. EPA Zone 1 predicts a county-wide average indoor radon level above 4.0 pCi/L, and the measured basement average for Adams County is 4.98 pCi/L — above the threshold, and closer to it than the 6.44 pCi/L test-weighted average across the 67 counties in this dataset. The classification and the measurement agree that Adams County homes should be tested; neither predicts what any individual property will read, which in a county whose radon is driven by proximity to diabase contact zones can vary widely between neighboring addresses.
Related Reading
Independent resource — we do not perform testing. PA Radon Hub is an independent informational resource connecting Pennsylvania property owners with independent, DEP-certified radon professionals. We do not perform radon testing or mitigation. Verify contractor certification against the PA DEP Radon Services Directory before hiring.