The only other radon isotope that can occur indoors in significant amounts is
radon-220 (half-life 55.6 seconds). Radon-220 is referred to as thoron and is a
member of the thorium-232 (half-life 1.4 × 1010 years) decay series. Its immediate
parent is radium-224 (half-life 4.6 days). It should be noted that there has been an increasing interest in indoor thoron in recent years. Owing to its short
half-life, thoron in soil gas beneath a building, in most situations, cannot survive
long enough to enter a house and thereby contribute to the level of thoron in indoor
air. Indoor thoron is due to the exhalation of thoron from thorium that may
be present in the materials forming the internal surfaces of the building. Some
building materials, such as volcanic tuff in Italy, have been found to have a high
thoron exhalation rate. While in general indoor thoron levels are low, research in
recent years has identified uncommon situations, such as cave dwellings, where
the doses from airborne thoron decay products can be significant and can even
exceed those from the radon decay products in the same location (18). In this
context, it should be noted that for the same exposure (i.e. concentration by time)
the dose from thoron decay products is estimated to be about four times that
of radon decay products (1,19). From the perspective of radiation dose to lung
tissue due to inhalation, the most important airborne thoron decay product is
lead-210 (half-life 10.64 hours). While lead-210 itself is a beta particle emitter
when it decays in the lung, it gives rise to the alpha-emitting decay products
bismuth-212 (half-life 60.5 minutes, 36% alpha particle energy Eα = 5.5–6.1
MeV) and polonium-212 (half-life 3 ×10–7 seconds, alpha particle energy Eα =
8.68 MeV).