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.