The size distribution of nanometer 218 Po is a key-parameter in radon risk evaluation. Indeed, assessment of the doses delivered to the target tissues of the pulmonary tract requires a precise knowledge of the inhaled-particle size distribution (Marsh et al., 2002). The diffusional properties of this component affect its attachment to the present aerosols along with its deposition onto surfaces; these two fundamental processes are currently used in the room model to calculate the activities of radon progeny in indoor environment. Unfortunately, only few literature data about the nanometric size characterisation of 218 Po are presently available; in addition, it is likely that the available experimental data are not definitive mainly because of the missing optimisation of the tools used for such a measurement. For example, within the European RARAD project (Monchaux, 1999) indoor measurements evidenced 3 modes at 0.55, 0.85 and 1.35 nm depending on the environmental conditions, i.e. relative humidity, gas composition (organic compounds at trace levels), radon activity concentration… Indoors, Huet (1999) measured median diameters within 0.5 – 1.25 nm. But, as the smallest cut-off diameter of the instruments used to determine the unattached 218 Po size distribution is 0.85 nm, precise measurement at this size or below is forbidden.
This is why two new diffusion batteries have been respectively developed by INTE-UPC group at the Technical University of Catalonia and the IRSN-UBO group; their cut–off diameters fall in the range 0.5 -3.7 nm. In other respects, this type of measurement is challenging because of missing size-standard for particles below 5 nm. The only way to check such systems is, therefore, through intercomparisons like the one reported here.