There are two kinds of radon test and they answer two different questions. Choosing the wrong one is how people end up mitigating a house that did not need it, or not mitigating one that did.
The short-term test
A short-term test runs for at least 48 hours under closed-house conditions, using either a continuous monitor or charcoal detectors. It answers the question what is this house doing right now, under the conditions most likely to produce a high reading. It is fast, it is cheap, typically somewhere in the region of $150 to $325 in this area, and it is what every real-estate transaction runs on because it is the only kind that fits inside an inspection period.
Its weakness is that it is a snapshot. Radon levels move with the weather, the season, and how the house is being operated. A February reading during a cold snap and a mild-April reading in the same house can differ considerably, and both are true.
The long-term test
A long-term detector sits in place for 90 days to a year and returns an annual average. It answers the better question — what is this household actually exposed to over a year — and it is the measurement you would want if you were deciding about a house you intend to live in for a decade. It typically runs around $175 to $400.
Its weakness is obvious: it cannot be done in three weeks, so it is useless in a transaction.
When each one is right
A sale, either side of it: short-term, no argument. The only decision is whether it is done early enough to be useful.
A result well above 4.0 pCi/L: you do not need another measurement to decide. Move to design and installation.
A result between 2 and 4 pCi/L: this is exactly what long-term testing is for. New Jersey suggests considering mitigation in this band, and a snapshot is a thin basis for a decision that costs a couple of thousand dollars either way. Ninety days of data is a much better one.
Two short-term tests that disagreed: long-term, and stop re-running short-term tests hoping for a tie-breaker.
A system that has just been installed: short-term, run between 24 hours and 30 days after commissioning. That is the clearance test, and it is not optional if you want to know whether the money worked.
What invalidates a test
Open windows or exterior doors during the closed-house period. Whole-house fans or window air conditioners running. A detector placed in a draft, against an exterior wall, near a heat source, or in a bathroom, kitchen or laundry. Placement on the wrong floor — the test belongs at the lowest level you live in or reasonably might, which for a lot of Hunterdon houses means the finished basement, not the ground floor. And in New Jersey, a test placed or read by someone without NJDEP measurement certification is not a test that counts in a transaction.
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Get free quotes →General information for Hunterdon County, NJ property owners — not a diagnosis, not a quote, and not a reading for any particular building. Every property is different; a certified professional confirms specifics on site.