Yes. Combined tobacco use and high radon dramatically increases your lung cancer risks than either alone. Health Canada has some information on this here. If you are exposed to or use tobacco products, testing your home for radon (and mitigating if high) is one way of lowering your chance of getting lung cancer. It is important to note that there are other verified causes of lung cancer that when combined radon exposure, will dramatically increase your lifetime relative risk of getting lung cancer. These are: Exposure to air pollution such as forest fire smoke and combustion particulates (called particulate matter less than 2.5 microns aka PM 2.5) Exposure to heavy metals such as Arsenic Exposure to Asbestos Previous history of severe lung inflammation (such as pneumonia and tuberculosis)