radon
noun [ U ] uk/ˈreɪ.dɒn/ us/ˈreɪ.dɑːn/ (symbol Rn)
氡（镭衰减时形成的一种放射性元素，发现自然存在于岩石和土壤中）
a chemical element that is a radioactive gas formed when radium decays and found naturally in rock and soil
Tests showed high radon levels. 试验表明氡含量较高。