Both 238U and 232Th are naturally occurring radioactive isotopes found in the Earth's crust, and they undergo radioactive decay by emitting alpha particles. However, the decay chain of 238U includes the isotope 222Rn, which is a gas and can easily be inhaled into the lungs. This presents a greater threat of lung cancer because the alpha particles emitted by 222Rn and its decay products can damage the cells in the lining of the lungs.
Furthermore, 222Rn has a relatively short half-life of 3.8 days, which means that it decays relatively quickly into other radioactive isotopes, including polonium-218 and lead-214, which are also alpha emitters. This continuous decay chain can create a buildup of radioactive particles in the lungs, increasing the risk of lung cancer.
On the other hand, the decay chain of 232Th does not include any gas isotopes, and its decay products tend to be heavy metals rather than alpha emitters. This means that the decay products are less likely to be inhaled into the lungs, and even if they are, they are less likely to cause as much damage as alpha particles.
In summary, the natural underground decay of 238U poses a greater threat of lung cancer than the decay of 232Th because it includes a gas isotope (222Rn) that can easily be inhaled into the lungs, and its decay products are alpha emitters that can cause significant damage to the cells in the lining of the lungs.