Background: The potential hazards of exposure to radiation from radon have been of great concern worldwide, as it is associated with an increased risk of lung cancer. Radon (222Rn) and its progeny are the main sources of radioactivity in the environment. The half-life of 222Rn (3.82 days) is long enough for it to diffuse into and build up in homes. 220Rn or thoron from the 232Th series, and 219Rn—a decay product of 235U—have very short half-lives (55.6 and 3.96 sec, respectively) and are of minor significance compared to 222Rn in the 238U series. However, these sources, like thoron, are important in uranium mines. Radon and thoron progeny decay by emitting β particles accompanied by γ radiation. Although γ rays are comparatively less damaging to the respiratory system than α and β particles, it is the principal deposited energy in other organs. Methods: Comparison of radon and thoron γ radiation dosimetry was performed using a modified Oak Ridge national laboratory (ORNL) adult mathematical phantom and the Monte Carlo N-particle transport code (MCNP). Results: The results showed that the highest amount of the absorbed dose was in the lung followed by the thymus and heart, according to the ICRP103 publication. Conclusions: The absorbed dose from thoron was quite large in comparison to radon because thoron has more high-energy particles than radon.

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