²²⁰Rn (Thoron) Geohazard in Room Air of Earthen Dwellings in Vietnam

Abstract

Thoron’s (220Rn) contribution to α-radiation exposure is usually considered negligible compared to that of 222Rn (radon). Despite its short half-life of 55.6 seconds, thoron can be exhaled from porous surface layers of building materials into indoor air where people subsequently inhale radioisotopes, including metallic radioactive progeny. Bare surfaces of dry porous soil with relatively high 232Th content can pose a thoron radiation hazard in indoor air. On northern Vietnam’s Đồng Văn karst plateau, the spatial distribution of thoron was determined in indoor air of traditional earthen and other types of dwellings using portable RAD7 and SARAD® RTM 2200 detectors. “Mud houses” are constructed with local compacted soil and typically do not have any floor or wall coverings (i.e., no plaster, wallpaper, or paint). Detailed measurements in a mud house revealed levels of thoron in room air averaging >500 Bq m-3. The spatial distribution of α-radiation from thoron in indoor air at a distance of about 1 m from interior walls was fairly homogeneous and averaged ~200 Bq m-3. Most concerning, from a human health perspective, were the high thoron concentrations of up to 884 Bq m-3 in sleeping areas near mud walls. The average annual thoron radiation dose to inhabitants of mud houses was estimated based on 13 hours of daily occupancy, including daily activities and sleeping. The estimated average thoron inhalation dose of 27.1 mSv a-1 during sleeping hours near mud surfaces accounts for nearly 75% of the total estimated radon and thoron inhalation dose of 37.4 mSv a-1 from indoor mud house air. Our conservative annual radiation dose estimates do not include subsequent radiation from inhaled metallic progeny of thoron. Our data demonstrate a significant human health risk from radiation exposure and a critical need for remediation in traditional northern Vietnamese mud house dwellings.

Radioactive Thoron 220Rn Exhalation From Unfired Mud Building Material Into Room Air of Earthen Dwellings

“…The concentration of thoron in indoor air declines with increasing distance from mud walls and floors toward the center of the room. High concentrations of α-emitting thoron and progenies in indoor air of mud houses in the center of rooms (>100 Bq m −3 ) and especially near bare mud walls (>350 Bq m −3 ) are responsible for > 80 % of the average effective dosage affecting inhabitants (Nguyễn-Thùy et al, 2019;Nguyễn-Vȃn et al, 2020). Severely elevated concentrations of thoron up to 1500 Bq m −3 close to bare mud walls of traditional dwellings were also found in China (Shang et al, 2008) and in an experimental room constructed with unfired mud bricks in a laboratory at the Helmholtz Zentrum in Munich, Germany (Meisenberg and Tschiersch, 2011), although in both cases 232 Th in the mud building material only produced γ-radiation with an average activity concentration of 40-50 Bq kg −1 .…”

“…A typical family of six to eight people, encompassing three generations, lives in a mud house with bare earthen walls and floor. Surveys of indoor air proved that the radiation geohazard of 220 Rn substantially exceeds the average environmental background thoron concentration in outdoor air, especially at close distances to earthen walls and floor (Nguyễn-Thùy et al, 2019), while thoron and its progeny account for > 80 % of the combined average effective dose and represent the predominant radioactive inhalation hazard for residents of mud houses (Nguyễn-Vȃn et al, 2020).…”