A Preliminary Thoron Survey in the Upper Midwest

Although thoron (2201Zn) was recognized as a potential source of radiation exposure in homes twenty yearsago, only a few homes in the United States have been measured for thoron. Early indoor surveys suggested that thoron concentrations would generally be low, highly variable, and make a poor surrogate for thoron pro eny dose. Howeyer, 522 recent reports of significant thoron interference with some radon ( Rn) detectors have renewed interest in thoron. In the upper Midwest, a region where three of the North American radon-lung cancer studies took place, basement living spaces near thorongenerating soil are common raising the possibility of thoron interference in those studies. Passive, integrating, radon-thoron discriminating detectors (ATDs) and nondiscriminating detectors were developed and exposed for 90 days in 76 Minnesota homes and 19 eastern Iowa homesat interior locations where ATD radon measurements are routinely made. The overall thoron and radon distributions are lognormal with geometric mean (geometric standard deviation); and arithmetic mean of 20 (1.8), 35 Bq m-3 respectively and 140 (2.6), 210 Bq m"3. Thoron and radon were correlated, especially for basement measurements where both isotopes had higher concentrations than on first floors. In the nondiscriminatory ATD, the median percentage of tracks attributable to thoron was 17%. This contamination was enough to cause some ATD results to be false positives for the current radon action level in some of the homes measured. The thoron interference would create a contamination of the radon-related dose estimate that averages about lo%, assuming current equilibrium ratios and dose model estimates are correct. INTRODUCTION During the 1 9 8 0 ' ~ ~ the discovery of unexpectedly high concentration of radon gas (%n) in some US homes led to a rapid expansion of residential radiation measurements. In addition to a few measurements were made for the presence of ^Rn, the radon isotope commonly called thoron (Schery 1985, 1990). Thoron, like radon, can pose a health risk because it can generate progeny that can deliver dose to lung and other tissues. However, the short half-life of thoron limits its transport from sources like soil and building materials to indoor living spaces. Thus, it was generally believed that thoron concentrations indoors would be low and have large spatial variation making thoron exposures diffi cult to assess (Nero 1988). However, one decay product of thoron, ^ ~ b , can accumulate significant potential alpha energy concentrations (PAEC) even at low thoron concentrations because it has a long half life. These characteristics of the thoron decay chain led early researchers to focus their surveys on 'I2pb rather than thoron to better quantify the potential radiation dose related to thoron. They found that thoron can generate a significant fraction of the total PAEC in homes and can be an important fraction of the dose delivered to the lungs (Schery 1985, 1990, Dudney et al. 1990, Martz Copyright