Radon "Build Up" In Vacant House?

What missing here is the fact that when we test for radon, we are measuring radiation. Radiation from radon, like all other radiation decays at a specific rate. With radon, it’s half life is approximately 3.8 days. All radon that is captive in a structure will be cut in half in about 4 days. If radon numbers are high in a house, vacant or occupied, it is because the decaying radon is replaced with new radon at a rate greater than the decay.