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In estimating the relative hazards created by the inhalation of thoron and thoron daughters, certain assumptions were required because of the incomplete knowledge of the biological behavior of these elements or were employed to simplify the calculations. These assumptions were that all daughters produced by the decay of thoron in the lungs were retained in the lungs until they decay to stable lead; the potential alpha energy of the daughters was calculated only from the energy released by the decay of ThB to stable lead; thorium A decayed 100% of the time to thorium B; thorium C was considered to decay 33.2% of the time by emission of 6.07 Mev alpha particles; and that only the tissue dose delivered by the alpha particles was considered. These calculations resulted in several interesting conclusions. They showed that veutilation is much more effective in reducing atmospheric concentrations of thoron daughters than it is for radon daughters. Conversely, practical rates of ventilation will not appreciably lower atmospheric concentrations of thoron. In comparison with radon daughters, much longer sampling times are feasible. This is important, as thoron daughter samples collected in the field will usually have quite small amounts of alpha activity and will requiremore » a more sensitive instrument than those which are satisfactory for radon daughter measurements. The calculations indicate that a concentration of only 8 mu mu c of thoron per liter at equilibrium with its daughters is equivalent in potential alpha energy to a concentration of l00 mu mu c radon per liter at equilibrium (1.3 x 10/sup 5/ Mev/1). At equilibrium the calculated lung radiation dose from thoron is insignificant compared to the calculated lung radiation dose from thoron daughters. For example, a concentration of 5360 mu mu c of thoron per liter delivers a lung radiation dose equivalent to that from a working level concentration of thoron daughters. The calculations also indicate that, on an activity basis, the permissible atmospheric concentration of thoron daughters should be much less than the level for radon daughters. Therefore, all situations involving the prceessing and use of appreciable amounts of thorium ores, salts, or metal should be surveyed to determine if significant amounts of these elements are present in the workroom atmosphere. (Public Health Engr. Abstr. 40 (1960) Dec.)« less