Pennsylvania is one of the highest-radon states in the country — and Philadelphia's geology puts many neighborhoods at real risk. Yet radon remains one of the most overlooked issues in Philadelphia real estate transactions. This guide covers how radon works, how to test for it, what mitigation costs, and exactly what buyers and sellers need to do under Pennsylvania law.
Radon is a colorless, odorless radioactive gas produced by the natural decay of uranium in soil and rock. It seeps up through foundation cracks, floor drains, sump pits, and construction joints — accumulating in enclosed spaces like basements, crawl spaces, and lower-level rooms.
The EPA divides the country into three radon zones based on predicted average indoor levels. Pennsylvania is predominantly Zone 1 — the highest-risk category, with average indoor radon concentrations predicted above 4 pCi/L (picocuries per liter). The Pennsylvania Department of Environmental Protection (DEP) estimates that approximately 40% of Pennsylvania homes have radon levels above the EPA action level.
Philadelphia sits on a mix of geologic formations — including the Piedmont region's crystalline bedrock and soils that are particularly prone to radon generation and transport. The city's dense rowhouse and attached housing stock, with shared party walls and limited sub-slab ventilation, can concentrate radon more than detached suburban homes. Lower levels, basements, and partially below-grade units face the most exposure risk.
Second-leading cause of lung cancer: The EPA and Surgeon General rank radon as the second-leading cause of lung cancer in the United States — responsible for approximately 21,000 deaths per year. Non-smokers are not immune: radon is the leading cause of lung cancer among non-smokers. This is not a disclosure technicality — it's a genuine health issue.
The EPA recommends fixing any home with an indoor radon level at or above 4 pCi/L. The agency also suggests considering mitigation for levels between 2–4 pCi/L, since there is no truly "safe" level of radon exposure — only managed risk.
To put the numbers in perspective:
| Radon Level | Risk Context | Recommended Action |
|---|---|---|
| Below 2 pCi/L | Roughly the average U.S. indoor level; background risk | No action required; retest periodically |
| 2–4 pCi/L | Elevated; EPA considers mitigation | Mitigate if cost-effective; retest after any renovation |
| 4–8 pCi/L | Above EPA action level | Fix within months |
| 8–20 pCi/L | Significantly elevated | Fix within weeks |
| Above 20 pCi/L | Very high — comparable to occupational exposure standards | Fix as soon as possible |
The outdoor average in the U.S. is about 0.4 pCi/L. Any indoor level above 4 pCi/L represents meaningful cumulative exposure risk over time, particularly for long-term occupants.
Understanding entry points helps you evaluate both test results and mitigation strategies:
Philadelphia's rowhouse construction — with shared party walls, below-grade front rooms, and original stone or brick rubble foundations — creates particular vulnerabilities. Many pre-war rowhouses have rubble-stone foundations with poor separation between the sub-slab soil and interior air. Basement finish work without proper vapor barriers or ventilation can worsen accumulation.
There are two primary categories of radon tests, and which one you use depends on your situation:
Charcoal canisters are the most widely used short-term test method. You place the canister in the lowest livable area of the home — typically the basement or lowest floor — for 48–96 hours with windows and doors closed, then mail it to a certified lab. Results come back within a week. Cost: $15–$35 for a DIY kit from a hardware store, or $100–$200 for a professional placement and analysis.
Short-term tests are standard practice in real estate transactions because of the time pressure. However, they capture only a snapshot — radon levels vary significantly with weather, season, barometric pressure, and occupant behavior. A single short-term test has meaningful uncertainty.
Electret ion chamber (E-PERM) devices and alpha track detectors are left in place for 90 days to a year, then sent to a lab. Long-term tests produce a far more accurate picture of average annual exposure. They're recommended for post-mitigation verification and for homeowners who want to truly understand their radon environment rather than just satisfy a transaction requirement.
The Pennsylvania DEP recommends long-term testing as the most reliable method outside of time-constrained transactions.
Certified testers and labs: Pennsylvania requires that radon testing in real estate transactions be performed by a PA DEP-certified radon tester or by the homeowner using a PA DEP-listed device. For real estate transactions, using a certified professional is strongly recommended — results are more defensible in negotiation and less likely to be challenged.
The optimal time to test is during the home inspection period — after an Agreement of Sale is signed but before the inspection contingency deadline. Most home inspection contracts in the Philadelphia area include radon testing as an add-on service ($100–$200 above the base inspection fee).
Practically, the test is placed at the start of the inspection period and picked up 48–72 hours later. Results typically come back within 24–48 hours of the lab receiving the canister, giving buyers time to negotiate before the contingency deadline expires.
Key timing points:
If your test comes back at or above 4 pCi/L, mitigation is warranted. The good news: radon mitigation is well-understood, reliably effective, and not catastrophically expensive.
Active Sub-Slab Depressurization (ASD) is the most common and effective mitigation method. A licensed mitigator drills one or more suction points through the concrete slab, installs PVC piping routed to the exterior of the home, and connects a continuously running fan that draws radon-laden soil gas out before it can enter the living space. The system exhausts above the roofline or away from windows and doors.
ASD typically reduces radon levels by 80–99%. A well-designed system can bring a home from 20 pCi/L down to below 2 pCi/L. The fan runs continuously and uses about as much electricity as a light bulb (~$30–$60/year).
| Mitigation Scenario | Typical Cost Range | Notes |
|---|---|---|
| ASD — standard slab foundation (rowhouse/twin) | $800–$1,400 | 1–2 suction points; one day installation |
| ASD — larger footprint / multiple zones | $1,200–$1,800 | Detached home or complex sub-slab conditions |
| ASD — crawl space with vapor barrier | $1,500–$2,500 | Requires sealing crawl space floor before ASD |
| Block wall depressurization | $1,000–$2,000 | For hollow concrete block foundations common in older Philly stock |
| Post-installation retest (recommended) | $75–$150 | Verify results 24h after system installation |
All radon mitigators working in Pennsylvania must be PA DEP-certified. Always ask for the mitigator's certification number before signing a contract. The PA DEP maintains a searchable list of certified contractors on its website.
In some situations, additional or alternative methods are used:
Investor note: If you're buying a property with below-grade units — basement apartments, garden units, or any unit partially below grade — factor radon mitigation into your pre-purchase due diligence and your renovation budget. In Philadelphia's dense rental market, habitability and tenant health liability are real concerns. A mitigation system installed during renovation is far cheaper than retrofitting after tenants are in place.
Pennsylvania's Real Estate Seller Disclosure Law (RESDL) requires sellers of residential property to complete a standardized disclosure form before signing an Agreement of Sale. The PA standard disclosure form includes a radon section with the following required disclosures:
Key points about PA radon disclosure:
As-is sales and radon: Even in as-is transactions, sellers must complete and deliver the RESDL disclosure form. "As-is" means the seller won't make repairs — it does not eliminate the disclosure obligation. Buyers in as-is transactions should always test during any contingency period they negotiate.
If a radon test comes back at or above 4 pCi/L, buyers have several options under the standard Greater Philadelphia Association of Realtors (GPAR) Agreement of Sale:
| Approach | How It Works | Best Used When |
|---|---|---|
| Seller installs mitigation system | Seller hires PA DEP-certified contractor; buyer approves contractor and receives post-mitigation test showing <4 pCi/L | Seller is cooperative and has time; protects buyer from inheriting the problem |
| Seller provides credit at closing | Seller credits buyer estimated mitigation cost ($900–$1,500 typical); buyer installs system after closing | Tight timelines; buyer wants to choose contractor; faster to close |
| Price reduction | Parties renegotiate sale price to reflect mitigation cost | Alternative to seller credit; useful when seller resists lender-prohibited credits |
| Buyer accepts condition | Buyer proceeds with full knowledge and plans to mitigate after closing | Investor purchases; buyer is informed and comfortable with cost |
| Buyer terminates | Buyer exercises inspection contingency if radon is deal-breaker | Rare; radon is almost always fixable — termination is usually an overreaction |
Radon is almost never a deal-killer — the fix is straightforward and the costs are modest relative to Philadelphia home prices. The more common mistake is buyers who skip radon testing entirely and discover the issue after moving in.
Radon in rental housing raises distinct considerations:
See L&I violations, open permits, 311 complaints, tax liens, and flood zone status before you buy, rent, or invest.
Run a Free ReportWhile any Philadelphia property can have elevated radon — it depends on local soil conditions, construction type, and building-specific conditions — certain geologic factors create elevated baseline risk in parts of the region:
The bottom line: geology provides useful context, but neighborhood averages don't predict individual property levels. A Manayunk rowhouse might test at 2 pCi/L while its neighbor tests at 9 pCi/L depending on foundation type, sub-slab conditions, and airflow. Always test the specific property — don't rely on neighborhood averages.
PA DEP radon resources: The Pennsylvania Department of Environmental Protection maintains a free database of certified radon testers and mitigators, a radon test kit order program, and zip-code-level radon data at dep.pa.gov/radon. Buyers can also order certified short-term test kits by mail at low or no cost through the PA Radon Division.
Radon is invisible, odorless, and well-documented as a serious health risk — and Pennsylvania's geology makes it a real concern across the Philadelphia area. The good news is that it's also one of the most solvable problems in residential real estate. A $150 test and an $800–$1,500 mitigation system can eliminate the risk for the life of the building.
For buyers: test every purchase, regardless of neighborhood. For sellers: disclose honestly — a mitigated property with documentation is not a problem, it's a selling point. For investors and landlords: proactively test basement units and include mitigation in any below-grade renovation budget. Don't skip this one.