The decay chain of radon only ends with the stable lead isotope Lead-206. The polonium isotope Polonium-210 occurs within the chain with a half-life of more than twenty years. After a very long time, each decayed radon atom generates another decay in the measuring chamber, which is not related to the current radon concentration. The more radon has been measured in the past, the greater the number of Polonium-210 decays. This is known as long-term contamination. Devices with an ionization or scintillation chamber cannot differentiate between radon and the various nuclides of the radon decay chain. They therefore interpret the Polonium-210 decays as a radon signal, which indicates that the concentration is too high, depending on the history of the device. Devices with α-spectroscopy, on the other hand, physically separate Polonium-210 decays, so that long-term contamination is irrelevant.

In all cases where measurements are made at a relatively constant concentration level over the whole lifetime of the instrument, long-term contamination plays a subordinate role. Only after twenty years would the device display a value that had increased by 50%. However, if high radon concentrations are measured with a device (e.g. in soil air), then the higher contamination has a very strong effect when measuring lower concentrations.

Some non-spectroscopic devices allow the subtraction of the contamination based background previously determined, e.g. by a zero measurement. If contamination is present, however, this leads to a sharp increase in the measurement uncertainty and the lower detection limit.