Radon presents the greatest health risk when it is

• A. In open air
• B. In well-ventilated areas
• C. Trapped and concentrates in inadequately ventilated areas
• D. Heated and circulated

Answer: (C)

Radon is a radioactive gas that seeps from the ground into buildings. The health risk comes from inhaling its decay products, which irradiate the lungs. This danger is greatest when radon builds up in enclosed, poorly ventilated spaces because there is little air exchange to dilute the gas, leading to higher concentration and greater exposure. In open air it dissipates quickly, and in well-ventilated areas the concentration is kept low. Heating or circulating air by itself doesn’t create risk unless it happens in a space where ventilation is poor and radon can accumulate. So, the scenario where radon is trapped and concentrates in inadequately ventilated areas poses the highest health risk.