 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.WHO presented in 2009 a recommended reference level (the national reference level), 100 Bq/m3, for radon in dwellings.