Get the number first, then decide what it means

A calibrated monitor goes in the lowest livable level of the house and runs for at least 48 hours with the building kept closed. Because it logs every hour rather than producing one averaged figure, the record shows how the level moves overnight, as weather shifts, and as heating or cooling cycles. A charcoal kit from a hardware store returns a single number with none of that behind it, which is fine as a rough screen and thin as a basis for spending money.

Where the decision is not urgent, a long-term test averaging 90 days or more gives the most representative picture of what a household is genuinely exposed to. That is the better instrument when you are weighing up whether to finish a basement, and the short test is the right one when a deadline is driving the question.

The report gives you an average in picocuries per liter, the hourly readings underneath it, the conditions the test ran in, and a plain explanation of what the figure means. At or above 4.0 pCi/L the EPA recommends fixing the house. Between 2.0 and 4.0 there is a genuine judgment to make, and it turns largely on how the lower level is used: a storage basement and a bedroom are different exposures at the same reading.

Below that, the sensible answer is usually to do nothing and test again in a couple of years. We will say so when that is the case. A referral from someone we talked out of a system is worth more than the system would have been.

Inspection periods do not leave much room, which is why we keep slots open each week for homes under contract. The device runs its 48 hours, gets collected, and the result goes out that same day, leaving time to negotiate instead of time to request an extension.

For a buyer that means an independent figure, and someone explaining it, before anything gets waived. For sellers and agents, testing ahead of listing takes a late shock out of the process entirely, and if a system does turn out to be warranted, a documented installation is a routine disclosure rather than a reopened negotiation.

Every system we install gets a confirming test afterward. That record matters at a closing, and it matters again a couple of years later when somebody wants to know whether the fan on the side of the house is achieving anything at all.

We also test houses with systems somebody else installed, which is worth doing more often than people realize. Fans have a service life and eventually stop, sump lids get lifted during plumbing work and not resealed, and an addition can change what is happening under the floor without anyone connecting the two.