Three natural occuring isotopes of radon are known:
- 222Rn (radon) in the decay chain of 238U (uranium)
- 220Rn (thoron) in the decay chain of 232Th (thorium)
- 219Rn (actinon) in the decay chain of 235U (uranium).
In general, 220Rn and 219Rn are not relevant for the human radioactive exposure due to their very short half-life (less than 1 minute in each case). Therefore the main focus concerning measuring devices is set on 222Rn.
The short-lived decay products of 222Rn are electrically charged and therefore attached to aerosol particles in the air. This causes a radioactive disequilibrium between radon and its decay products in the natural environment. Radon decay products always exist in smaller concentrations than radon itself. Hence, measuring devices for radon can be grouped in instruments for analyzing radon gas (exclusively 222Rn) and/or instruments for radon decay products.
Because most of the relevant guidelines and reference levels are related to the 222Rn activity concentration, distributes measuring devices of this kind.