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220Rn (Thoron) Geohazard in Room Air of Earthen Dwellings in Vietnam
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.
Radon is a naturally occurring colorless, odorless, and tasteless gas, which, at high exposure levels, is recognized to cause lung cancer [1, 2]. Radon has more than thirty characterized radioactive isotopes [3, 4], but 222Rn (called radon) of the uranium (238U) decay series and 220Rn (called thoron, commonly abbreviated Tn) of the thorium (232Th) decay series are considered the major sources of natural radiation to the global human population . The dangers of radon have been well established and constitute about 50% of natural radiation exposure to humans. Additional significant sources of natural radiation include cosmic radiation and 40K exposure. However, unlike indoor radon exposure, the latter sources cannot be managed . The dangers of indoor radon exposure have led to guidelines by the World Health Organization (WHO) and public health legislation in many countries to require radon monitoring and mitigation . The α-decay of radon isotopes generates multiple radioactive metallic progeny that tend to become adsorbed to aerosol and dust particles. These particles, once inhaled, collect in lung fluids and adsorb onto lung tissues, thereby concentrating nuclear radiation near living cells. All types of radiation from radioactive decay can induce harmful random biochemical reactions, including damage to DNA . The cell damage from exposure to high radon concentrations is known to increase the incidence of lung cancer.
Radon has been shown to pose a health risk to occupants if concentrations exceed 200 Bq m-3 in domestic residences and 300 Bq m-3 in workplaces [1, 8–12]. Unlike the focus that has been granted to 222Rn studies for decades, acquiring data sets for indoor thoron concentrations and its decay products has been overlooked due to the general perception that its exposure levels are negligible. The disproportionate focus on 222Rn ignored the contribution of 220Rn to the total inhalation dose when evaluating significant sources of natural radiation. Thoron and its progeny have not even been officially considered into the contexts of official safety thresholds of indoor radiation exposure . This misconception was based on knowledge of its relatively short half-life of 55.6 seconds and its short diffusion distance from emission sources ([13–16]).
Only in recent years has indoor thoron exposure gained attention as a public health concern when increased concentrations were found in dwellings dug into clay-rich soil and in certain types of traditional houses with unfired earthen architecture in China [17–19], India , Germany , Hungary , and Japan . These studies demonstrated that in nearly all the housing types examined, thoron concentrations exceeded those of radon, especially in locations proximal to interior walls. Nearly all dry natural mud surfaces contain sufficient amounts of radioisotopes that provide a significant source for thoron exhalation . The surface layer of soil or other earthen building materials containing 232Th exhales thoron into indoor air [5, 25]. The results of these studies identified thoron as a significant contributor to the indoor radiation exposure of inhabitants.
High concentrations of thoron in Vietnam were initially discovered in karst caves and earthen dwellings of the Đồng Văn Karst Plateau Geopark (http://www.globalgeopark.org/aboutggn/list/vietnam/6509.htm), with values in excess of 1000 Bq m-3 [26–28]. The aforementioned recommendations, by UNSCEAR  and ICRP , for the control of radon radiation exposure in domestic and workplace dwellings are based on average environmental background concentrations of 10 Bq m-3 for thoron and 100 Bq m-3 for radon. Observed thoron levels inside evaluated earthen dwellings of northern Vietnam were orders of magnitude higher than the average environmental background thoron concentration in outdoor air.
The conventional earthen dwelling on the Đồng Văn karst plateau is constructed with fresh local clay-rich soil that is compacted and dried to form the walls and floor of the mud house. Traditional inexpensive mud houses have been used for centuries by several ethnic groups in the mountainous region of northern Vietnam where quality timber and bricks have been too costly. The inhabitants of mud houses often position their beds next to the bare mud walls inside their homes. This arrangement of furniture next to the uncovered exhaling source of thoron has not received adequate scrutiny in terms of the potential radiation health risk for occupants.
In this study, thoron and radon concentrations were systematically surveyed in the main room of a traditional mud house (termed ED 4) on the Đồng Văn karst plateau to establish the spatial distribution of 220Rn and 222Rn where inhabitants spend time during their daily activities. The primary emphasis of this study is the evaluation of thoron as a contributor to the total indoor radiation exposure affecting occupants of mud houses.
2. Materials and Methods
2.1. The Traditional Earthen Dwelling
The model mud house ED 4, a traditional ethnic house on the Đồng Văn karst plateau, was constructed using local soil with an earthen-wall thickness of 60 cm. The dimensions of the main room measured , which is typical of a mud house in the Đồng Văn karst region. The same local soil used to construct the walls also comprised the bare mud floor of the dwelling. As illustrated in Figure 1, the main entrance with a door (1) is located in the center of the front wall while two windows (2) are symmetrically arranged in the front wall. This layout divides the main room of the mud house into two symmetric halves with their corners being bed positions (3) of inhabitants. A opening in the eastern mud wall (4) connects the mud-constructed kitchen room to the main room. The structure of the mud house provides ventilation through openings between the top of the walls and the loose fitting roof (Figure 1). Each earthen dwelling is usually occupied by a family of 6 to 8 people, typically with three generations in one household.
2.2. Measurements of Thoron and Radon Concentrations in Indoor Air
This study used two separate portable radon detectors. Most α-radiation of indoor air derives from 222Rn and 220Rn and their radiogenic progenies 218Po, 214Po, and 214Bi from radon and 216Po, 212Po, and 212Bi from thoron. The two instruments used α-spectroscopy to distinguish the nuclide-specific source of α-radiation in terms of radiation energy. (i) Air was sampled through a plastic hose, prefiltered, and measured using a SARAD® RTM 2200 instrument in “slow mode” to determine nuclide-specific α-radiation intensity in 10-minute intervals. The SARAD® RTM 2200 instrument additionally recorded temperature, barometric pressure, GPS location, relative humidity, and the carbon dioxide concentration. (ii) A RAD7 instrument (serial number 1572) sampled air through a plastic hose, filtered the air through a 1.0 μm membrane filter to retain airborne particulate matter, and passed the air sample through a Drierite desiccant trap before gas entered the detector chamber. The instrument quantified the α-decay of both radon isotopes in “sniff mode” .
Initial measurements indicating elevated thoron concentrations in indoor air, on the Đồng Văn karst plateau in March 2016, were performed with a SARAD® RTM 2200 instrument in different dwellings constructed with compacted soil, unfired-soil bricks, fired-clay bricks, and/or concrete. Measurements were performed at interior and exterior sites of the dwellings. In December 2016, both instruments were simultaneously operated side by side to record thoron levels in room air at identical locations (i.e., 40 cm above the compacted earthen floor—comparable to the location of a bed or daily family life activities). Radon and thoron data from both instruments were compatible.
With the support of the builder and home owner of the traditional mud house ED 4 featured in Figure 1, we first visited the house in December 2016 (i.e., cold season) and surveyed the air with the SARAD® RTM 2200 in the western portion of the main room, whereas the RAD7 surveyed the eastern half of the main room (Figure 1(c)). We returned in July 2017 during the contrasting warm season  for repeat measurements using the RAD7. With door and windows closed, detailed measurements were taken in a grid-like pattern at ~1 m increments (Figure 1) to obtain the spatial distribution of radon and thoron concentrations at night. Duplicate measurements were recorded at each point. Reported results are averages of duplicate measurements.
3. Results and Discussion
3.1. Recognition of High Thoron Concentrations in Indoor Air of Earthen Dwellings on the Đồng Văn Karst Plateau
Radon and thoron concentrations of indoor and outdoor air at the dwellings constructed from (i) compacted soil (4 mud houses), (ii) unfired-soil bricks (1 house), and (iii) fired-clay bricks and concrete (1 house) are summarized in Table 1. The average 222Rn level of indoor air was <100 Bq m-3 in all housing types. The fired-clay brick house showed no significant 222Rn difference when measurements were taken at the center of rooms versus areas close to walls. Conversely, a trend of increasing thoron concentrations was observed from the center of rooms to locations close to the interior walls of dwellings (Figure 2). In addition, thoron concentrations were far higher than radon in the majority of the surveyed dwellings. Maximum thoron concentrations of up to 725 Bq m-3 were measured in air close to mud walls and did not decrease below 100 Bq m-3 in the center of rooms. The indoor air thoron concentration of 480 Bq m-3 next to interior unfired-soil brick walls was similar to that of measurements next to compacted earthen walls, but swiftly declined to 86 Bq m-3 towards the center of the room. Thoron concentrations were generally below the detection limit in rooms constructed with fired-clay bricks and concrete, except 44 Bq m-3 in the immediate vicinity of some interior walls.
ED = earthen mud house from compacted soil; EBD = unfired-earthen brick dwelling; FBH = dwelling built by fired-clay brick and/or concrete.
Additional experiments in other mud houses evaluated the effects on thoron concentrations in the interstitial air between dry mud surfaces and inexpensive surface coverings like polyethylene foil or multiple layers of newspaper. Data from the SARAD® RTM 2200 measured at different locations with or without surface treatments along the interior and exterior of mud houses indicated that thoron concentrations in interstitial air behind surface covers increased drastically. In one instance, thoron levels were observed to exceed 3,000 Bq m-3 behind polyethylene foil. In contrast, measurements of air in the center of the room showed concentrations of at least an order of magnitude less than the thoron levels recorded behind the polyethylene foil covering (Figure 3). These results further demonstrate that thoron is being exhaled from earthen-sourced building materials and diffuses further into the room air. Any surface covering impedes the advective dispersion of thoron into the room air  and results in enhanced thoron concentrations in the limited trapped air volume behind the coverings.
3.2. Detailed Thoron Distribution in Surveyed Traditional Earthen Dwelling
Detailed surveys of the distribution of radon and thoron in a traditional earthen dwelling were conducted in the compacted soil (mud) house ED 4 by both portable SARAD® RTM 2200 and RAD7 instruments (Figure 4(a)). Thoron concentrations in the air of the western half of the main room, furthest from the kitchen entrance, measured by SARAD® RTM 2200 in December 2016, varied from 258 to 2,383 Bq m-3 with an average of 744 Bq m-3 (Figure 4(b)). The eastern half of the main room, adjacent to the kitchen entrance, measured by RAD7, featured thoron concentrations from 128 Bq m-3 to 798 Bq m-3 (average 456 Bq m-3) in December 2016 (Figure 4(c)) and from 88 Bq m-3 to 2,030 Bq m-3 (average 643 Bq m-3) in July 2017 (Figure 4(d)). The highest concentrations of thoron in both cold and warm seasons exceeded 2,000 Bq m-3 at locations proximal to mud walls (Figures 4(b) and 4(d)).
In the eastern half of the main room, the average value of thoron in the warm season was higher than during the cold season. The discrepancy between seasons is from warmer air with higher humidity enhancing the diffusive exhalation of thoron from porous, more humid source materials [32, 33]. In the cold season, air in the western half of the room had higher thoron levels than the other half of the room. The difference is likely due to ventilation and uneven air flow in the room as well as through the kitchen entrance.
The results demonstrate that thoron concentrations tend to increase from the center of the room towards the surface of earthen walls (Figures 4(b)–4(d)) with maxima occurring near corners between two mud walls where air flow is limited and the ratio of thoron-exhaling mud surface versus adjacent air volume is highest (Figures 4(b) and 4(d)).
Despite emissions from multiple sources of dry mud in the dwelling, thoron in indoor air rapidly declines in concentration outward from mud walls toward the center of the room. Thoron concentrations diminished by almost 70% at a distance of 120 cm from the walls (Figures 5 and 6). The center of the room measures only 20 to 25% of the thoron concentration measured proximal to mud walls. The observed spatial distribution of thoron in the mud house does not demonstrate an exponential decrease from the wall surface to the center of the structure, which is expected in the absence of convection (). Instead, the observed pattern is consistent with a combination of factors, including an uneven distribution of thoron-exhaling mud surfaces, slow convection of room air (enhanced by the movement and thermal disturbance from the presence of people), and external wind forcing the mild ventilation through open spaces between the top of the mud walls and the roof, as well as through the imperfectly fitted door and windows.
From a human health perspective, it is important to note that the average thoron concentration in room air is far higher in the sleeping area near earthen walls (up to ~2,500 Bq m-3) than in the center of the room (Figure 7). The inhabitants of earthen dwellings often place their beds at the corner of two mud walls and/or next to mud walls. Sleeping close to thoron-exhaling mud walls greatly enhances the radiation exposure for the inhabitants. The closure of doors and windows at night diminishes air convection and further exacerbates the indoor exposure from radionuclides during sleeping hours.
3.3. Estimated Annual Inhalation Dose for Inhabitants of Mud House ED 4
The effective dose of radon and thoron inhalation, along with its radioactive progeny, was calculated for the main room of mud house ED 4 using the following UNSCEAR  algorithm: with
: total annual inhalation dose exposure to radon (mSv a-1);
Rn and Tn: abbreviations of radon and thoron, respectively;
: solubility coefficient of radon in blood (; );
: inhalation dose conversion factor (nSv/(Bq h m-3)) (; );
: indoor equilibrium factor (; );
: average duration of exposure per year (h);
: concentration of radon (Bq m-3);
0.8: occupancy factor for the study region with exposure duration of one year;
10-6: factor to convert nSv into mSv.
Due to ethnic custom and economic constraints, a family living in a traditional northern Vietnamese mud house commonly encompasses three generations living together (i.e., infants to adolescents, working age adults ~16 to 50 in age, and elderly with ages above 50). All members of a family are usually at home together from the hours of 5 : 00 pm in the evening to 6 : 00 am the next morning. They stay at home approximately 13 hours per day with about 8 hours spent sleeping and 5 hours participating in family life activities (i.e., cooking, eating, children playing, etc.). Therefore, we assume that an inhabitant’s average exposure to indoor radon and thoron lasts for a duration of at least 13 hours per day in mud houses. This estimate is most appropriately applied to adults of working age, but exposure time for children and the elderly generation may be higher because of their likely extended time at the earthen structures. The elderly generation often stays at home, especially during colder months and during the rainy season, and children still attend school in some earthen buildings.
The average annual effective dose from radon and thoron and their progenies to mud house inhabitants was estimated to be 37.4 mSv a-1. Thoron and its progenies account for 97% of the combined average radiation dose from radon and thoron, amounting to 36.2 mSv a-1 (Table 2). Furthermore, the average annual inhalation dose due to the exposure of indoor thoron and its progenies amounts to 9.0 mSv a-1 during 5 hours of daytime daily spent in mud houses, in addition to 27.1 mSv a-1 while sleeping in mud houses for 8 hours daily. The annual inhalation dose during sleeping hours is 3 times higher than the dose of thoron during daytime activities in mud houses.
For each respective parameter, numbers in regular font represent average values, whereas numbers in italics and in brackets indicate observed ranges.
The spatial variation of the annual inhalation dose from thoron and its progenies in indoor air reflecting 5 hours per day of daytime presence in the mud house is represented in Figure 8. The annual inhalation doses from thoron and its progenies in proximity either to a single mud wall or at the corner of two adjoining mud walls have been estimated to be ~13 mSv a-1 and ~19 mSv a-1, respectively. These values decrease to one-third at a distance of ~1 m from mud surfaces and remain rather constant towards the center of the room.
Based on the UNSCEAR  guidelines for annual doses of ionizing radiation by source, the recommended upper threshold inhalation dose of total radon and thoron and their progenies is 1.26 mSv a-1, with a typical range of observed doses up to 10 mSv a-1. The estimated average inhalation dose on an annual basis from only thoron and its progenies of 36.1 mSv a-1 experienced by inhabitants in earthen dwelling ED 4 in northern Vietnam is substantially higher than the total annual average dose from natural sources of 2.4 mSv a-1 .
Thoron and its airborne radioactive progeny were found to pose a significant health risk to inhabitants living in traditional earthen dwellings (i.e., mud houses) in northern Vietnam’s karst region. High levels of thoron in indoor air with typical values of 450–650 Bq m-3 cannot be mitigated by enhanced ventilation because thoron’s short half-life of 55.6 s causes a maximum concentration in the vicinity of exhaling mud surfaces. The distribution of thoron in the indoor air of mud houses is influenced by convection and also depends strongly on the available area and porosity of sources (i.e., mud walls and floors) as well as the distance from mud surfaces . The common practice of positioning beds next to mud walls, especially in the corner of a room, exacerbates the radiation hazard during sleeping hours. The estimated average inhalation dose for daily occupancy, exclusively from thoron exposure during sleeping hours, on an annual basis reaches up to 37.4 mSv a-1 and is 15 times larger than the recommended annual safety threshold for the public of 2.4 mSv a-1 . Most ethnic groups in the northern karst region of Vietnam live in earthen dwellings constructed with compacted local soil and thus are disproportionately at a higher risk of exposure to thoron compared to inhabitants of more modern homes made from nonearthen materials. Practical mitigation strategies that are needed must be socially acceptable and economically feasible.
The data used to support the findings of this study are included within the article. No extra data were used to support this study.
Highlights. (i) Thoron exhales from bare dried mud walls and floors into indoor air of traditional northern Vietnamese mud houses. (ii) The thoron concentration in indoor air is elevated near mud surfaces and decreases towards the center of the room. (iii) The thoron inhalation dose of inhabitants increases in proportion to the amount of time spent near mud surfaces, especially when sleeping close to mud walls. (iv) High thoron concentrations in indoor air of traditional northern Vietnamese mud houses pose a human radiation health hazard.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The content of this manuscript is based upon work supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) grant number 105.99-2016.16. Measurements with the SARAD® RTM 2200 were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Award Number DE-SC0006978. We are indebted to Mr. Nùng Văn Minh and his family for permission to use their home for measurements. We are grateful for the cultural liaison and logistics facilitated by Minh Ngọc Schimmelmann. We thank Dr. Thomas Streil from SARAD® GmbH and Assoc. Prof. Dr. Trần Tuấn Anh from the Institute of Geological Science, Vietnam Academy of Science and Technology, for the expert guidance on radon measurements.
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