If the biological effect of radon is due mostly to radon decay products and not the radon gas itself, why are most measurements taken of radon gas, rather than RDPs? There are several reasons for this:
Fewer variables in radon gas measurement facilitate greater certainty in representative results. For instance, unlike RDPs, the gas concentration is not affected by circulation or filtration devices. It is generally easier to make time-averaged measurements of radon gas than of RDPs. Measuring radon gas also can be a good indicator of RDPs.
However, sometimes it is still preferable to measure RDPs since they are the source of health effects. Typically, health-related research is conducted with WL monitors, which are also necessary when making side-by-side measurements to determine the actual equilibrium ratio, or when evaluating an air treatment device designed to reduce RDP concentrations.