25 of this county's 26 towns are Tier 1. The only question is your number.

Nothing. We are paid by the certified radon businesses that take the job referrals, never by homeowners. There is no fee, no membership, and no obligation to hire anyone we introduce you to.

No. We are a referral service and we hold no radon certification. We collect what you know, pass it to businesses certified by the New Jersey Department of Environmental Protection, and step out of the way. Any agreement you reach is between you and that business.

The EPA and NJDEP action level is 4.0 pCi/L: at or above that, fixing it is recommended. Between 2 and 4 pCi/L, New Jersey suggests you consider mitigating, because there is no level of radon that is known to be risk-free. Below 2 pCi/L, current guidance is that reduction is generally not warranted. Only a certified measurement professional can tell you whether your particular test was valid and what it means for your home.

Two reasons. The north and west of the county sit on the Reading Prong — a belt of uranium-bearing Precambrian granite and gneiss that runs from Pennsylvania into New York and produces some of the highest indoor radon readings in the country. The rest of the county sits on Newark Basin rock, including the dark Lockatong argillite that also carries uranium. In NJDEP's 2015 Radon Tier Assignment Report, 25 of the county's 26 municipalities are classified Tier 1 — the highest category, meaning a quarter or more of the test results the state holds for that town were at or above 4 pCi/L. In Califon it is 49 percent. The single exception is Union Township at 14 percent, which NJDEP places in Tier 2.

Yes. Under N.J.A.C. 7:28-27, testing for radon or installing mitigation systems for a fee in New Jersey requires certification by NJDEP. Measurement and mitigation are certified separately, so a business certified to install a system is not automatically certified to place and read the detectors. We check both against the NJDEP register before a business receives anything from us, and we record the date we checked.

New Jersey does not require a radon test on every sale, but it does require a seller who has had one to give the buyer the results, and radon testing is a routine part of most home inspections here. Separately, if the property is on a private well, the Private Well Testing Act requires water testing at the point of sale, and gross alpha — a screen that can indicate radon and other radioactivity in the water — is a required parameter for Hunterdon County wells.

Yes, and it is common in this county because so many homes draw from fractured bedrock. Radon dissolved in well water is released into the air whenever water is agitated — showers especially — so a house can have two separate sources at once. A soil-gas system does nothing about the water side; that needs its own treatment, usually aeration or a carbon bed at the point of entry.

One. We pass your request to a single certified business that covers your town and holds the certification for the specific work you need, so you are not fielding calls from three companies about the same reading. If that business cannot help, tell us and we will pass it to another.

Yes. Where there is no slab to work through, the usual approach is a sealed membrane laid over the crawl-space floor with suction drawn from beneath it. It is a bigger job than a standard sub-slab system and the quality of the sealing is most of the result, which is why it is worth asking a contractor how they terminate the membrane at the walls and around the piers.

The common causes are a small number: too few suction points for the way the slabs are laid out, a fan undersized for the pressure the system actually runs at, discharge in the wrong place, or sealing that was never done. Diagnosis comes first — a bigger fan on a badly placed system mostly just makes it louder. And if nobody has re-tested since the system went in, nothing is known about it at all.