The greatest source of natural background radiation to humans is which substance?

• A. X-rays
• B. Delta rays
• C. Radon
• D. Radium

Answer: (C)

Radon is the largest natural source of background radiation because it is a radioactive gas produced by the natural decay of uranium in soil and rocks. It seeps from the ground into buildings and the air people breathe, so indoor exposure to radon is common and often substantial. When radon decays, it forms short‑lived progeny that emit alpha particles inside the lungs, delivering a significant dose over time. Because people spend most of their time indoors, this accumulation makes radon the dominant contributor to natural background radiation in many populations. Other sources listed—X-rays are largely from medical imaging, delta rays are a type of secondary radiation, and radium, while radioactive, does not contribute as much to typical natural background exposure as radon.