Internal exposure to radon gas progeny can lead to serious biologic damages to the lung tissue. The aim of this study was to evaluate the absorbed dose by lung tissue due to the exposure from short-lived radioactive products of radon (222Rn) decay using Monte Carlo simulation. Material and Methods: A lung equivalent phantom including 64 air sacs was simulated by MCNPX code. Then, the absorbed dose from short-lived radioactive products of radon decay chain including 218Po, 214Po, 214Pb and 214Bi

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... was calculated for both suspended and deposited states of daughter nuclides inside the lung. Results: The results showed that alpha decay has more contribution to the lung absorbed dose in comparison with the beta and gamma decay. Furthermore, the received dose by the lung was higher when the radon progenies were deposited inside the lung so that the maximum received dose to lung was 100 times higher than that of calculated in suspended state. Conclusion: Short-lived daughter radionuclides of radon decay chain, especially alpha emitter products, can be considered as dangerous internal radiation sources. The biological effects of these daughter radionuclides is more severe when are suspended inside the respiratory system.