Also, its solid decay products, and their respective products, tend to form a fine dust which can easily enter the airways and become permanently stuck in lung tissue, producing heavy localized exposure. Rooms where radium, actinium, or thorium are stored should be well-ventilated in order to prevent build-up in the air. The build-up of radon is a potential health hazard in uranium and some lead mines. Build-up of radon in homes has also been a more recent health concern and many lung cancer cases are attributed to radon exposure each year.
Radon is of greatly increased danger to smokers. The solid decay products of radon staying in the atmosphere in dust form can fix themselves on the micro particles in tobacco smoke, which then enter the lungs.
Radon can cause lung cancer. Many deaths from lung cancer are caused by radon exposure. In the U.S. it is recommended that remedial action be taken if the air in homes exceeds 4 pCi/l.