range of breathing rates (0.5 to 1.2 m3/h). Currently no such data are available quantifying the influence of the different air conditioning systems used in Hong Kong on the f-value.
Although of a lesser influence on the dose as f, the data base available in Hong Kong on the equilibrium value (F) between radon, respectively thoron, and their decay products is based on a few grab samples only. The overall uncertainties in the dose.risk assessment will be significantly reduced, if the specific fp- and F-values are established for homes, offices and public buildings.
6.4. AEROSOL SIZE DISTRIBUTION
In dependence of lung geometry and retention characteristics the effective dose (normalized per WLM) ranges from about 15 mSv to 5 mSv over the range of activity median diameters encountered in non-mining environments, i.e. 0.1 μm to 0.25 μm. No data are available on the size distribution of radon decay products attached to indoor aerosols typical for Hong Kong's mixture of urban pollution and industrial pollution, modified by air conditioning systems.
6.5. CONTIBUTION BY THORON AND DECAY PRODUCTS
The high thorium content in the subsoil indicates that thoron (220Rn) and its short-lived decay products may represent a non-negligible contributor to the total dose from inhalation in Hong Kong. This is confirmed by the few grab sample measurements of thoron-working level indoors (see discussion above). Also the single excessively high reading by some electrets exposed indoors could be indicative of an elevated thoron comparent rather than "pure-radon" readings, since the conventional radon detection systems cannot differentiate between the two radon isotopes. Therefore a special thoron/thoron-decay product indoor survey in areas with increased 232Th in the subsoil and/or construction material will provide important classification on the radon situation in Hong Kong. No data are available on the thoron-exhalation rate from building materials used in Hong Kong.
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