Radon (Rn-222), a naturally occurring radioactive gas, is the second leading cause of lung cancer after tobacco smoking. Its primary source in indoor environments is:

• A Combustion of coal in indoor stoves
• B Outgassing from polyvinyl chloride building materials
• C Leakage from uranium processing facilities
• D Decay of radium-226 in soil and rock beneath buildings ✓

Explanation

Radon-222 is a decay product of uranium-238 → radium-226 → radon-222 in the decay chain. It diffuses from the soil and rock beneath and around buildings, accumulating in ground-floor and basement spaces with poor ventilation. It is a noble gas (chemically inert) but its short-lived decay products (Po-218, Pb-214, Bi-214, Po-214) are charged and adhere to respiratory epithelium, emitting alpha particles that cause DNA strand breaks and lung cancer. Indoor radon is measured in Becquerels/m³; WHO action level is 300 Bq/m³ (or 100 Bq/m³ reference level). Mitigation involves sub-slab depressurization and improved ventilation.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.