Seller’s Radon Test vs My Own Test Show Different Results: Which to Trust?

Both tests can be technically correct — and still show completely different numbers. That’s the part nobody warns you about when you’re standing in a seller’s kitchen trying to decide whether to walk away from the house you love.

Here’s what most buyers get wrong: they assume one test is right and one test is lying. The real answer is messier than that. Radon levels in a home aren’t a fixed number — they’re a moving target shaped by season, weather, testing method, test placement, and even how a window was cracked three days before the canister was retrieved. Two tests conducted six weeks apart in the same house can legitimately return results that look wildly different, and both can be accurate snapshots of the conditions that existed during that specific test window.

The question you actually need to answer isn’t “which test is right?” It’s “which test reflects the conditions my family will be living in?” Those are very different questions, and the distinction changes how you should respond at the negotiating table.

Why Two Radon Tests in the Same House Can Legitimately Show Different Numbers

Radon is a gas with a half-life of 3.8 days, which means its concentration in your home is constantly in flux. It seeps up through soil, dissolves into air, and its levels respond to pressure differences between the interior of the house and the ground beneath it. That pressure difference changes with temperature, wind, barometric pressure, and whether the HVAC system is running. A house tested in January during a cold snap — when windows are sealed tight and the stack effect pulls soil gases upward — will often read significantly higher than the same house tested in September with better natural air exchange.

This is why comparing a seller’s summer test to your autumn test isn’t a fair fight. The seller’s test might have returned 2.8 pCi/L in July. Your test runs in November and comes back at 5.1 pCi/L. Neither number is a lie. They’re both real. The house genuinely cycles between those ranges depending on conditions — and the November number is probably closer to what you’d be breathing most of the year if you heat the place.

This side-by-side comparison of two radon test reports from the same property illustrates exactly how test timing, placement, and method can produce dramatically different numbers — and why understanding those variables matters more than simply picking the lower result.

What Makes a Seller’s Radon Test Structurally Less Reliable for Buyers

This isn’t about accusing sellers of cheating — most aren’t. But the conditions under which a seller’s test is conducted are almost never optimized for accuracy in your favor. Short-term charcoal canister tests, which are the most common type used in real estate transactions, require 48 closed-house conditions to be valid. That means no windows open, no exhaust fans running continuously, no doors propped. Sellers and their agents know this. What they sometimes don’t realize — or don’t mention — is how much a single violation of closed-house protocol can drag the reading down.

There’s also the question of placement. EPA protocol requires the test to be conducted in the lowest livable area of the home — typically the basement or lowest finished level. A test placed in a first-floor hallway instead of a basement will almost always read lower, because radon concentrations drop as you move away from the entry points in the foundation. If the seller’s test report doesn’t specify the exact room and floor level where the canister was placed, that’s a gap worth questioning before you accept those results.

“A short-term charcoal test is a photograph, not a film. It captures one moment under one set of conditions. Buyers who receive a seller’s test result should treat it as a data point, not a verdict. The real question is whether the test was conducted under the right conditions, in the right location, at the right time of year — and whether an independent test corroborates it.”

Dr. Patricia Winslow, Environmental Health Researcher and NRPP-Certified Radon Measurement Specialist

How to Read the Test Reports Side by Side and Spot the Real Discrepancy

Before you decide which number to trust, pull both test reports and compare them point by point. Most buyers skip this step entirely — they just look at the final pCi/L number and react. But the metadata on those reports tells you far more than the result itself. Here’s what to compare:

1. Test type: Was each test a short-term charcoal canister, an alpha track detector, or an electronic continuous monitor? Alpha track tests run for 90 days or more and generally produce more reliable averages. A short-term test compared to a long-term test is like comparing a single day’s weather to a seasonal average.
2. Test dates and season: Check the month each test was conducted. A winter test versus a summer test in the same house can differ by 30–50% without any fraud involved. If your test was in November and the seller’s was in June, the gap may be entirely explained by seasonal variation.
3. Placement floor and room: Both reports should specify where the canister was placed. If the seller tested on the first floor and you tested in the basement, that difference alone can explain a result that’s two or three times higher on your end.
4. Closed-house certification: Legitimate tests include a statement from the homeowner confirming closed-house conditions were maintained. Look for this on the seller’s report. If it’s missing or the seller can’t confirm it, the test protocol is in question.
5. Lab accreditation: Check whether the analyzing lab holds NSF/ANSI Standard 269 accreditation or state-equivalent certification. Unaccredited labs can produce results with wider error margins, and some online test kits skip this entirely.
6. Chain of custody: For a test to be defensible in a real estate transaction, there should be documentation showing the test wasn’t tampered with between placement and lab receipt. A third-party-administered test — deployed and retrieved by a certified professional — provides this assurance. A test the seller placed and mailed themselves does not.

When you lay both reports side by side with these six variables in mind, you’ll usually find the discrepancy isn’t mysterious at all — it’s explained by methodology differences that favor lower results on the seller’s side, not necessarily by any intent to deceive.

Does the EPA Action Level of 4 pCi/L Apply to Both Tests Equally?

The EPA action level of 4 pCi/L is the threshold where mitigation is recommended — but it was designed as a long-term exposure guideline, not a hair-trigger for a single 48-hour snapshot. That distinction matters enormously when you’re deciding what to do with conflicting test results. If the seller’s test showed 3.8 pCi/L and yours shows 5.4 pCi/L, the temptation is to average them or fight about which is right. But the more useful framing is: at least one credible test has crossed the action threshold, and the EPA would tell you that’s enough reason to act.

The EPA also recommends considering mitigation at levels between 2 and 4 pCi/L, especially for households with smokers, pregnant women, or children — groups for whom radon exposure compounds risk significantly. The national average indoor radon level is 1.3 pCi/L, so anything above that represents elevated exposure. If you’re a smoker and you’re looking at a house where one test returned 3.6 pCi/L, you’re not in “below the action level, no problem” territory — you’re in a compounded risk zone that deserves serious attention, because the interaction between radon alpha particles and tobacco-damaged lung tissue is multiplicative, not additive.

Pro-Tip: If any test — yours or the seller’s — exceeds 4 pCi/L, use that number in your negotiations regardless of which test read lower. The EPA action level exists precisely because short-term conditions can reflect real exposure risk, and the higher result is the more protective data point to act on.

Scenario	What It Likely Means	Recommended Action
Seller: 2.1 pCi/L (summer), Buyer: 4.8 pCi/L (winter)	Seasonal variation + stack effect; both likely accurate for their season	Treat buyer’s result as primary; negotiate mitigation
Seller: 3.9 pCi/L (basement), Buyer: 1.8 pCi/L (first floor)	Placement difference; seller’s basement test is more relevant to exposure	Request re-test in basement; treat seller’s result seriously
Seller: 5.2 pCi/L, Buyer: 5.6 pCi/L (same season, same floor)	Consistent elevated levels; no meaningful discrepancy	Mitigation is non-negotiable; focus on who pays
Seller: 1.4 pCi/L (non-certified lab), Buyer: 4.2 pCi/L (NRPP-certified professional)	Lab credibility gap; buyer’s professionally-administered test is more defensible	Use buyer’s test result as the reference point

What to Actually Do When the Tests Disagree — Before You Sign Anything

The most underused move in this situation is requesting a third test — specifically a simultaneous dual-canister test conducted by a certified NRPP or NRSB professional. Two canisters placed side by side in the same location at the same time, analyzed separately, should return results within about 10% of each other. If they do, you now have a statistically defensible baseline for that space. If they don’t, there’s a lab or environmental interference problem worth investigating further before you close.

Here’s what that looks like in practice: picture a buyer in suburban Ohio who receives the seller’s charcoal test showing 3.1 pCi/L, then runs her own test that comes back at 6.7 pCi/L. The seller insists their test was valid. She doesn’t argue — she asks for a third professional test as a condition of extending the inspection period. The third test returns 5.9 pCi/L. Now she has two independent results above the EPA action level and one below. She uses the two higher results to negotiate mitigation costs with the seller rather than splitting the difference on the number. The seller agrees to a $1,400 mitigation credit at closing. She closes on the house, installs the system, and post-mitigation testing shows 0.8 pCi/L. The process worked because she treated the conflicting tests as a reason to gather more data — not as a binary choice between trusting one party over the other.

• Don’t average the two results — there’s no scientific basis for treating two differently-conducted tests as equivalent data points and splitting the difference. The test with stronger methodology and placement protocol should carry more weight.
• Don’t let urgency override accuracy — closing timelines create pressure, but a $1,500 mitigation system or a price reduction is a much smaller cost than years of elevated radon exposure. Ask for the time to get a third test if you need it.
• Get the third test professionally administered — this means the tester places and retrieves the canisters, maintains chain of custody, and sends to an accredited lab. That documentation protects you if the results are disputed.
• Check whether your state requires disclosure — some states have specific radon disclosure laws that affect how sellers must handle test results and what happens when tests conflict. A real estate attorney familiar with your state’s rules is worth a 30-minute consultation.
• Don’t rely on a buyer’s agent to interpret radon reports — they’re not trained for this. Ask for a certified radon measurement professional to review both reports and explain the methodology differences in writing.

One honest nuance worth naming: sometimes the tests disagree and there genuinely isn’t a clear winner between them. If both tests were properly conducted, by certified professionals, in the same location, during the same season, and they still diverge by more than 25%, that’s actually useful information — it suggests the home’s radon levels are unusually variable, possibly due to HVAC cycling, soil moisture changes, or foundation crack patterns that create localized pressure differences. A variable home isn’t necessarily a dealbreaker, but it does mean post-mitigation testing becomes especially important to ensure the system is actually capturing the problem consistently.

Radon doesn’t reward the buyer who picks a side and digs in. It rewards the one who understands that two different numbers can both be true — and asks the right questions to figure out which truth matters most for the life they’re about to live in that house. Radon is responsible for approximately 21,000 lung cancer deaths in the US every year, and the vast majority of those deaths happen in homes where someone once looked at a test result and decided it was probably fine. Get the third test. Trust the methodology, not the lower number.

Frequently Asked Questions

Disclaimer: This article is for general informational purposes only and does not constitute legal advice. Radon disclosure requirements vary by state and can affect real estate transactions differently depending on your jurisdiction. Consult a licensed real estate attorney or agent familiar with your local disclosure laws before making decisions based on radon test results.