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Why can radon buildup indoors increase the risk of lung cancer?

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Neon — noble gas A road in Tokyo lined with neon signs.

Radon buildup indoors increases the risk of lung cancer primarily due to its radioactive decay products rather than the gas itself.1

As radon decays, it produces a series of radioactive "daughters"—such as isotopes of polonium, bismuth, and lead—that exist in particulate form. When these particles are inhaled, they become deposited on the lining of the respiratory tract. Once settled, these daughter nuclides release alpha particles, which provide intense, localized irradiation to lung tissue. This chronic exposure to ionizing radiation can damage cells and lead to the development of lung cancer.21

Radon gas enters buildings by diffusing through soil and rocks containing uranium mineral deposits, often accumulating in poorly ventilated spaces like basements. Because it is a noble gas, radon is chemically relatively inactive, but its radioactive nature and the subsequent accumulation of its decay products make it a significant health hazard.21