The activity size distribution Z is important regarding the distribution of radon progeny in respect to the three modes that Porstendoefer first introduced: Nucleation mode, Accumulation, Coarse mode. Depending on Z, the,  what was traditionally called free and attached progeny, is altered. It is very difficult to measure Z because it necessitates advanced techniques, for example aerosol spectroscopy (~100 kEuro purchase) . Otherwise, other methods should be employed, such as discrimination through grids of various size values. Differentiation in Z, imposes alteration in distribution of all progeny nuclei. Monte-Carlo is a good method in that.

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