UNATTACHED 212PB FRACTION IN THE WORKER'S BREATHING ZONE AT HIGH THORON CONCENTRATIONS.

Classical aerosol physics predicts that it is impossible to observe noticeable values of the unattached fraction of decay products of thoron at typical concentrations of aerosol particles. The experimental data have shown that the unattached fraction of the thoron decay products in a monazite storage facility is in the range 0.3-0.6. This effect is caused by local increase in air exchange near the inlet of the sampling device at high 220Rn concentrations. It was found that a dynamic shift between the unattached and attached thoron decay products is also observed in the worker's breathing zone at a high concentration of thoron. The 212Pb unattached fraction in worker's breathing zone was ~0.4. It demonstrates that when evaluating inhalation exposure to the products of the decay of thoron, it is necessary to take into account the increase of the 212Pb unattached fraction in the worker's breathing zone.