occurring radioactive materials (NORM), such as uranium, thorium
and potassium as well as any of their decay products, such as radium
and radon. The indoor radon concentrations mainly depend on radon
that has penetrated from the surrounding soil through gaps, cracks,
etc., but also on radon exhalation from building materials and radon
in domestic water supplies.
Recent long-term surveys of indoor thoron and its progeny
showed that doses from the thoron series should no longer be
considered as negligible. In the report a dosimetric approach was
used to calculate the effective dose per unit of equilibrium
equivalent concentration (EEC) for thoron progeny and it was
almost four times greater than that of radon progeny.
In this context the dose contributions of thoron (
220
Rn) and its
decay products can exceed the corresponding radon (
222
Rn)
values, as in the indoor environment thoron is considered more
likely to originate from exhalation of building materials rather
than from soils due to its shorter half-life. Consequently indoor
radon and thoron concentrations should be monitored and building materials should be classified on the basis of their radon and
thoron exhalation rates which can then be correlated with their
activity concentration index (I) value, based on
232
Th,
226
Ra and
40
K concentrations.