Why does natural radon account for a large share of human exposure to ionizing radiation?

Natural radon gas accounts for a significant portion of human exposure to ionizing radiation primarily because it is a naturally occurring byproduct of the radioactive decay of uranium and radium found in soil and rocks.12

Because radon is a dense gas, it can diffuse through the ground and seep into buildings, where it often becomes trapped and concentrated in poorly ventilated spaces like basements. When inhaled, radon and its radioactive decay products—known as "daughters"—can be deposited in the lungs. These decay products emit [alpha particles], which, while having limited penetration power, are highly effective at damaging cellular DNA when the source is inside the body. This internal exposure is a major health concern, as it is associated with an increased risk of lung cancer.123