Why It Matters
You need a radon test on any property with a basement or ground-level unit before closing. The EPA action threshold is 4 pCi/L — above that, mitigate. Systems run $800–$2,500 and reliably drive levels below 2 pCi/L. Get the test during due diligence, use the result to negotiate, and disclose any known radon history to future buyers and tenants. This is a fixable problem with a known price tag.
At a Glance
- Measured in picocuries per liter (pCi/L); EPA action threshold is 4 pCi/L
- Estimated 21,000 lung cancer deaths annually in the U.S. — second only to cigarette smoke
- Short-term tests: 2–7 days; long-term tests: 90+ days (more accurate)
- National average indoor level: 1.3 pCi/L; EPA recommends considering mitigation above 2 pCi/L
- EPA Zone 1 counties (highest risk) cover large portions of the Midwest and Mountain West
- Mitigation cost: $800–$2,500 for sub-slab depressurization; post-mitigation typically below 2 pCi/L
- Most states require disclosure of known results; some mandate testing before sale
- Radon-resistant new construction (RRNC) practices can reduce entry at the build stage
How It Works
Where radon comes from. Uranium decays naturally in soil and rock, releasing radon gas that migrates upward and enters buildings through foundation cracks, floor-wall joints, sump pits, and porous block walls. Buildings sit at slightly lower pressure than surrounding soil, acting like a vacuum — pulling radon in from below.
Testing. Short-term tests use charcoal canisters in the lowest livable area for 2–7 days, then mailed to a lab. During environmental-compliance due diligence, a short-term closed-door test is standard. Cost: $15–$30 for a DIY kit; $100–$200 professional. The EPA action threshold is 4 pCi/L; between 2 and 4, mitigation is recommended; below 2, no action required.
Mitigation. The standard fix is sub-slab depressurization (SSD): a contractor cores a PVC vent through the slab, adds a fan, and exhausts above the roofline. The fan reverses the vacuum effect and typically brings levels below 2 pCi/L. Crawl-space properties use a sealed ground barrier plus vent fan. Operating cost: roughly $30/year in electricity.
Disclosure. Federal and state rules require sellers to disclose known radon results — some states mandate testing before sale. Landlords who find elevated radon must disclose to prospective tenants in most jurisdictions. Once you have results, they follow the property. This is the same habitability disclosure logic that applies to asbestos and lead paint.
Real-World Example
David was buying a 1978 three-unit in Columbus, Ohio — EPA Zone 2 — listed at $298,000. The seller had no radon test on file.
David ordered a short-term test during his inspection period. The certified tester placed canisters in the basement unit for five days under closed-door conditions. Result: 7.8 pCi/L — nearly twice the EPA action threshold.
Two mitigation quotes came in at $1,100 and $1,350. David asked the seller for a $1,500 credit. The seller countered at $1,000. David held firm: the result was documented, remediation was required under Ohio disclosure rules, and the $1,100 quote was in writing. Seller accepted $1,350.
Post-mitigation test result: 0.9 pCi/L. David filed the certificate and both reports in the property folder, added a radon disclosure to the basement lease, and moved on. Total delay: 12 days.
Pros & Cons
- Mitigation systems are reliable, proven, and relatively inexpensive — $800–$2,500 for most residential properties
- Post-mitigation levels consistently drop below 2 pCi/L, eliminating habitability liability
- A documented test and mitigation certificate satisfies future disclosure obligations and protects against tenant health claims
- Properties with disclosed-but-mitigated radon rarely trade at a significant discount — buyers accept certified systems
- Elevated results during due diligence add a negotiation variable and can complicate contract timelines
- Mitigation systems require annual fan and seal checks — maintenance the owner must track
- In multi-unit buildings, testing every unit adds cost; basement and first-floor units present the highest exposure risk
- States with mandatory pre-sale testing requirements add a line item to closing timelines regardless of expected results
Watch Out
Testing the wrong floor. Radon enters from below. Testing must happen in the lowest livable space — the basement unit or ground-floor apartment, not a second-floor bedroom. A test on the wrong floor understates risk and gives false confidence.
Relying on old results. Any test more than two years old should be treated as unreliable. Renovation, new HVAC, or foundation changes all affect radon entry rates.
Skipping the post-mitigation test. System installation doesn't guarantee success. Verify with a follow-up test 24–48 hours after the fan is running. Budget $100–$150 if the contractor doesn't include it. File results with the original report for future environmental-compliance disclosures.
Treating zone maps as a substitute for testing. EPA Zone 1 flags the highest-risk counties — not every property in the zone tests above threshold. Zone 3 buildings can still come back elevated. Test the actual building.
Ask an Investor
The Takeaway
Radon is a manageable due diligence item with a known solution. Test in the lowest livable space, mitigate above 4 pCi/L, document everything. The system costs less than a month's rent. The alternative — undisclosed elevated radon surfacing in a tenant health claim — is not something you can budget for retroactively.