Among the following components of natural background ionising radiation, the LARGEST contribution to the average annual effective dose received by the general population comes from:
- A Cosmic rays
- B Radon and thoron inhaled indoors ✓
- C Terrestrial gamma rays from building materials
- D Internally deposited potassium-40
Correct answer: B. Radon and thoron inhaled indoors
Explanation
Per UNSCEAR estimates, inhalation of radon and thoron accounts for roughly half of the natural background dose, about 1.25 mSv of the approximate 2.4 mSv yearly total. Radon, an alpha-emitting daughter of radium-226, accumulates in poorly ventilated dwellings. Cosmic rays, terrestrial gamma and potassium-40 each contribute substantially less, which is why radon dominates indoor radiation risk.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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