radon-222
... that radiation from radon-222 and its decay products is the second-leading cause of lung cancer in the United States after cigarette smoking?
... that radiation from <b id="mwBSg"><a href="./Radon-222" id="mwBSk" rel="mw:WikiLink" title="Radon-222">radon-222</a></b> and its <a href="./Decay_chain#Uranium_series" id="mwBSo" rel="mw:WikiLink" title="Decay chain">decay products</a> is the second-leading cause of lung cancer in the United States after cigarette smoking?