Radon is a natural radioactive gas that can easily enter to the respiratory tract and cause lung cancer. The aim of this study was to determine the absorbed dose from radioactive products of radon’s decay chain in respiratory system using Monte Carlo simulation. A cubic lung equivalent phantom including 64 air sacs was simulated by MCNPX Monte Carlo code. Then, the absorbed dose from alpha and gamma-emitter products of radon decay chain was separately calculated. The daughter radionuclides understudy were considered in two different geometric states. At the first state, the daughter radionuclides were considered as suspended particles inside the air sacs volume, while in the second one, it was supposed that the radionuclides are deposited on the inner wall of air sacs. The results showed that alpha decay has more contribution to the absorbed dose by lung in comparison with gamma decay. The administered dose by alpha decay in suspended and deposited states were 6×106 MeV/gr-decay and 4.6×106 MeV/gr-decay, respectively. Polonium 214 was the most dangerous daughter nucleus which had the highest delivered dose in both states understudy. The absorbed dose by gamma decay in each above mentioned states was also equal to the 6.2×104 MeV/gr-decay and 5.3×104 MeV/gr-decay, respectively. In both geometric states understudy, Pb-214 and Bi-214 had almost the same contribution in the absorbed dose by gamma radiation. From the results it can be concluded that the daughter radionuclides from decay chain of Radon, especially alpha emitter products, can be considered as a dangerous internal radiation sources. In addition, the biological effects of these daughter radionuclides is more severe when are suspended inside the respiratory system. Reducing the biologic effects of these internal radiation sources requires especial schemes to avoid entering the radon and its radioactive daughters to human respiratory system, as much as possible.

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