Radon basics
Thoron (Rn-220)
A radioactive isotope of radon produced in the thorium-232 decay chain. Its half-life is only about 56 seconds, so it rarely travels far from its source; indoor exposure usually matters only where building materials contain significant thorium.
Related terms
Radon (Rn-222)A naturally occurring radioactive noble gas produced by the decay of radium-226 in the uranium-238 chain. It is colorless, odorless and tasteless, seeps out of soils and rocks, and can accumulate indoors. The WHO classifies it as the second leading cause of lung cancer after smoking.Uranium decay chainThe sequence of radioactive decays that starts with uranium-238, present in most soils and rocks, and produces radium-226 and then radon-222. Because uranium is ubiquitous in the Earth’s crust, radon is generated everywhere — what varies is how much reaches indoor air.Soil gasThe air contained in the pores of the ground beneath and around a building. Soil gas typically carries much higher radon concentrations than outdoor air and is the main source of indoor radon when it is drawn into the building through the foundation.Stack effectThe pressure difference created when warm indoor air rises and escapes through upper parts of a building, drawing replacement air — including radon-laden soil gas — in through the lowest levels. It explains why indoor radon is usually highest in winter and at night.Radon entry routesThe openings through which soil gas enters a building: cracks in slabs and walls, construction joints, gaps around service pipes and cables, sumps, floor drains and crawl spaces. Identifying entry routes is a key step in designing an effective mitigation system.