How radon testing works
Every decision about radon rests on a measurement, and measurements are easy to get wrong in ways that are invisible in the report. A number produced under broken conditions looks exactly like a number produced correctly.
This is what a valid measurement requires, and what quietly ruins one.
The devices, and what each is for
Short-term detectors — charcoal canisters, alpha-track devices deployed briefly, and electret devices — measure over roughly two to seven days. They are inexpensive, fast, and the standard instrument for a real-estate transaction where a number is needed within a contingency period.
Long-term alpha-track detectors stay in place ninety days or more and report an annual-style average. They cost little, they tolerate normal living, and they are the better instrument for a house you already own — particularly if a short-term result landed in the 2 to 4 pCi/L band.
Continuous monitors are professional instruments that log hourly readings. Their value is the data trail: they show whether conditions actually held during the test, which is why they are common in transactions.
Where the detector goes
On the lowest level you live in or could reasonably finish. In a house with a finished basement family room, that is the basement. In a house with an unheated cellar nobody enters, measure the lowest floor you actually occupy — and measure the cellar too if you might ever finish it.
Within the room: at least twenty inches off the floor, away from exterior walls, away from drafts, away from supply registers, away from sump pits, and not in a kitchen, bathroom, or laundry where humidity interferes. Record the exact spot, because every later test belongs in the same place.
Closed-building conditions
This is the requirement most often broken and the one that most reliably invalidates a result. Exterior doors and windows stay closed except for normal entry and exit, for twelve hours before a short-term test begins and throughout the test itself.
Whole-house fans stay off. Normal heating and air conditioning operate as usual. A propped door for a pet, an open window on a warm afternoon, or a house being aired out for showings all break the condition — and the resulting low number is exactly the kind that sends someone into a closing confident about a house nobody actually measured.
What makes a result trustworthy
Four things: the right level, the right placement, genuine closed-building conditions, and a laboratory or instrument that can be identified. A report that documents all four is usable by both sides of a transaction.
A result that cannot describe its own conditions is a number without a context, and it is the reason so many radon negotiations end up paying for a second test.
How testing works: common questions
Do hardware-store kits actually work?
Yes, when used correctly. A kit from a recognized laboratory, placed properly on the lowest lived-in level under closed-building conditions and mailed promptly so the lab reads it inside its valid window, is a genuine measurement — EPA treats do-it-yourself testing as a legitimate first step. Where they fail is protocol: wrong level, wrong placement, or a house that was never actually closed.
Why do two tests in the same house give different numbers?
Because they measured different conditions. Weather, wind, barometric pressure, heating system operation, occupancy, and season all move indoor concentrations, sometimes substantially. That variability is why EPA recommends confirming a short-term result, and why a ninety-day detector is more informative than two weekend tests.
How long before a real-estate closing should testing happen?
As early in the inspection period as possible, because everything downstream depends on it. A short-term test needs twelve hours of closed conditions beforehand plus two to seven days of measurement, and if the result is elevated you will want time for a confirming test and written quotes. Starting on day one of the contingency rather than day five routinely saves a week.
Keep reading
- Monitors vs short-term tests — Which instrument to use, and why transactions favour monitors.
- Post-mitigation testing — The measurement that proves the system worked — and what if it did not.
- EPA recommendations — The action level, the consideration band, and what guidance is not.
- Failed test during inspection — The inspection-window playbook, in the order the clock demands.
- All homeowner guides — every decision page in one place.
Lowest lived-in level, twenty inches up, away from walls and drafts, house genuinely closed, and the exact location written down. Get those five right and the number means something.
Sources
- U.S. EPA — Radon
- U.S. EPA — A Citizen’s Guide to Radon
- U.S. EPA — Consumer’s Guide to Radon Reduction
Elevated result? Get quotes while you confirm
Requesting quotes runs in parallel with a confirming test, which is how the whole sequence stays inside an inspection window. Free, no obligation.