EML Thoron Gas Measurements

The Environmental Measurements Laboratory's experience with two methods of measuring thoron gas, and its findings on the feasibility of using these measurements to diagnose indoor air flow paths, are presented. One method is an updated version of the two-filter tube, and the other is a modified Falk-More-Nyblom delayed coincidence method. Measurements made with these instruments in six houses indicated that thoron concentrations are very low (median about 11 Bq.m-3); this is consistent with values previously reported for US housing. Both methods had difficulty measuring these low levels, particularly in houses with high radon gas levels. At one house, thoron levels measured outdoors over bare earth were higher than indoor levels. At the low levels encountered and with the current measuring technology, it seems unlikely that thoron gas measurements can be used to trace indoor air motion.