Radon has been considered the second leading cause of lung cancer and leading environmental cause of cancer mortality by the EPA, with the first one being smoking. Others have reached similar conclusions for the United Kingdom and France. Radon exposure in homes and offices may arise from certain subsurface rock formations, and also from certain building materials (e.g., some granites). The greatest risk of radon exposure arises in buildings that are airtight, insufficiently ventilated, and have foundation leaks that allow air from the soil into basements and dwelling rooms.