Radon is a decay product of uranium in bedrock. The Hudson Valley has a great deal of granite and a great deal of fractured shale, which is why the EPA maps most of Ulster and Dutchess as Zone 1 — the highest of the three risk tiers.
In practice, about a third of the houses I test come back at or above 4.0 pCi/L, the level at which the EPA recommends action. That proportion does not vary much by the age of the house, which surprises people. A tight new build with a sealed basement can read higher than a draughty 1920s house, because the draughts were doing something useful.
The test
A continuous monitor, placed in the lowest livable level, left for 48 hours with the windows shut. It logs hourly, which matters — a single-figure charcoal canister cannot tell you whether the number is a stable average or one bad night.
Mitigation
A sub-slab depressurisation system is $1,200 to $1,800 installed around here, and it is close to a solved problem: a fan, a pipe, and a reading that drops to under 1.0 within a day. It is worth knowing before closing because it is a reasonable thing to ask a seller to cover, and an awkward thing to raise afterwards.