Radon is the second leading cause of lung cancer. Now a new study has found exposure to this invisible, odorless gas is also linked to an increased risk of stroke. The study, which examined exposures in middle age to older female participants, found an increased risk of stroke among those exposed to high and even moderate concentrations of the gas compared to those exposed to the lowest concentrations. The study does not prove that exposure to radon causes stroke; it only shows an association.