High Thoron Research Articles

Pilot Study of Thoron Concentration in an Underground Thorium Mine.

The Steenkampskraal mine in the Western Cape Province in South Africa provides some interesting challenges for radiation protection practitioners in view of the high thoron values encountered in this mine. The mine contains high natural thorium concentrations that lead to high thoron activity concentrations, as will be shown in this paper. The influence of ventilation has been studied, and the source term has been investigated by considering the thorium content of the rocks and the thoron exhalation. The thoron activity concentrations are around 10 kBq m -3 at a monazite seam, and the thorium exhalation is consistent with these levels. The thoron concentrations can be reduced by ventilation but not eliminated. However, the thoron progeny can probably be reduced dramatically. Issues that affect the thoron levels are also discussed. Further studies are needed, but the thoron may well not be a radiation protection problem despite the high thoron concentrations.

Thoron Interference on Performance of Continuous Radon Monitors: An Experimental Study on Four Devices and a Proposal of an Indirect Method to Estimate Thoron Concentration.

The performance of continuous radon monitors (CRMs) is usually evaluated under controlled conditions in a radon chamber during calibrations or intercomparison exercises. The impact of thoron on CRMs response is rarely evaluated; in case the evaluation is performed, it is carried out in a controlled atmosphere with relatively constant, homogeneous, and generally high thoron concentrations and very low radon levels. In a real indoor environment, both radon and thoron concentrations are extremely variable, so the thoron interference evaluations reported in the literature are generally not applicable to CRMs used to measure radon concentration indoors. For this reason, an experimental study was carried out with four different CRMs in an indoor environment (an office room) where medium-to-high concentrations of both radon and thoron were expected. Thoron concentration has been separately evaluated throughout two different active monitors. Three CRMs resulted in overestimations of radon concentration by about 10% due to thoron interference, whereas such interference results were negligible for the fourth CRM. However, the thoron interference can also be used to assess thoron concentration by using CRM not specifically designed to do so. Based on the results of this study, an indirect method to assess thoron concentration is indeed proposed, relying on the combination of two identical monitors (one placed right close to the wall and the other one far enough from there).

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.

REVIEW OF RADON AND THORON RESEARCH IN KENYA: 1997-2017.

Six research studies involving radon and thoron in Kenya were carried out between 1997 and 2007. The studies were mainly small scale and involved a few areas scattered across Nairobi, Rift Valley, Coast and Western regions. The results were captured in seven online journal articles. This paper relooks at the journal articles with the view of underscoring the high radon and thoron areas in Kenya, raising awareness on the sources and risks of radon isotopes in indoor environments and highlighting the need for more research on the isotopes in the country.

Dosimetric evaluation of thoron exposure in the three typical rural indoor environments of China

In order to evaluate thoron exposure in indoor environments, field measurements of both concentration and its size distribution were carried out in three typical rural residential houses in China, and exposure dose was evaluated using dosimetric method. Results show that the thoron progeny size distributions of rural indoor environments (AMAD: 76.5 nm; GSD: 2.7) are much smaller than those of urban (AMAD: 115 nm; GSD: 2.0), which makes the dose conversion factors of thoron in rural environments [307.4 nSv/(Bq m−3 h−1)] are much higher than those in urban [113.4 nSv/(Bq m−3 h−1)]. The highest thoron exposure (10.12 mSv a−1) was found in mud house of Yangjiang, the high radiation background area.

Prospecting for groundwater discharge in the canals of Bangkok via natural radon and thoron

We conducted surveys of several canals in Bangkok, Thailand using continuous measurements of naturally occurring 222Rn (“radon”) and 220Rn (“thoron”). Shallow groundwater seeping into these canals is an important pathway for contamination of surface waters. Radon, with a half-life (3.82 days) shorter than the suspected flushing time of the canals, is widely distributed throughout the waterway. It can thus be used to estimate discharge via a mass balance approach but cannot specify precisely where the discharge is occurring. Thoron, on the other hand, with its rapid decay (56 s half-life) will only occur very close to points of entry. Thus, if one detects thoron in the environment, there must be a source nearby – a good ‘prospecting’ tool. We found thoron spikes in Klong Bangkok Noi during a survey in August 2009. We repeated the same survey route in June 2013 and found essentially the same pattern of high thoron peaks (indicating points of discharge) adjacent to several temples along the canal. The connection to temples is thought to be a consequence of these structures being built on relatively higher ground and having sandy substrates.

Levels of thoron and progeny in high background radiation area of southeastern coast of Odisha, India

Exposure to radon, (222)Rn, is assumed to be the most significant source of natural radiation to human beings in most cases. It is thought that radon and its progeny are major factors that cause cancer. The presence of thoron, (220)Rn, was often neglected because it was considered that the quantity of thoron in the environment is less than that of radon. However, recent studies have shown that a high thoron concentration was found in some regions and the exposure to (220)Rn and its progeny can equal or several time exceed that of (220)Rn and its progeny. The results of thoron and its progeny measurements in the houses of high background radiation area (HBRA) of the southeastern coast of Odisha, India presented here. This area is one of the high background radiation areas in India with a large deposit of monazite sand which is the probable source of thoron. Both active and passive methods were employed for the measurement of thoron and its progeny in cement, brick and mud houses in the study area. Thoron concentration was measured using RAD-7 and Raduet. A CR-39 track detector was employed for the measurement of environmental thoron progeny, both in active and passive modes. Thoron and its progeny concentrations were found to be comparatively high in the area. A comparison between the results obtained with various techniques is presented in this paper.

A simple technique for determining the equilibrium equivalent Thoron concentration using a CR-39 detector: Application in mineral treatment industry

The paper presents a simple technique for determination of the Equilibrium Equivalent Thoron Concentration (EETC). A solid-state nuclear alpha track detector (SSNATD) named CR-39 manufactured by BARYOTRAK is used as the detector. This technique can be utilized anywhere, provided that air sampling is carried out with a DC driving pump. Although the measurement of Thoron concentrations is useful enough for screening high Thoron exposure, it is still necessary to determine Thoron progeny concentrations in order to complete dose assessment. After applying this technique for determining the EETC in some selected areas in southern Thailand, it has been found that the obtained EETC values are consistent with the values presented in the report compiled by UNSCEAR in 2000.

Radon survey in dwellings of Gansu, China: the influence of thoron and an attempt for correction

Alpha track detectors used in a previous investigation of the US National Cancer Institute and the China Ministry of Health on indoor radon ((222)Rn) in Gansu, China, proved to be influenced by (220)Rn (thoron), thus overestimating the (222)Rn level. Therefore, the detector was improved used in the previous survey. The new detectors allow discrimination between the two isotopes without any disturbance of the (222)Rn measurement. With this detector, a semi-annual study was conducted in 49 traditional dwellings of a village in Gansu. The arithmetic (AM) and geometric (GM) mean (222)Rn concentrations were 120 +/- 61 and 105 Bq m(-3) (with geometric standard deviation GSD = 1.8), respectively, while the mean (220)Rn concentrations at 2.5 cm wall distance were 430 +/- 210 Bq m(-3) (AM) and 350 Bq m(-3) (GM) with GSD = 2.3. The high thoron concentrations demonstrate the importance of the (220)Rn contribution to radiation exposure, in the investigated area. The actual level of indoor (222)Rn was about three times lower than that in the previous investigation which was affected by (220)Rn. A correction method for the radon results of the previous study is proposed, which provides (222)Rn and (220)Rn values comparable with those obtained in the study presented here.

Radon–Thoron Discriminative Measurements in Gansu Province, China, and Their Implication for Dose Estimates

Indoor radon measurements were carried out in cave dwellings of the Chinese loess plateau in Gansu province, where previously the Laboratory of Industrial Hygiene (LIH), China, and the U.S. National Cancer Institute (NCI) had conducted an international collaborative epidemiological study. The LIH–NCI study showed an increased lung cancer risk due to high residential radon levels, and estimated the excess odds ratio at 100 Bq/m3 to be 0.19 (Wang et al., 2002). The present study used two types of newly developed passive monitors: One is a discriminative monitor for radon and thoron; the other is a selective monitor for thoron decay products. The arithmetic mean concentrations of indoor radon and thoron were 91 and 351 Bq/m3, respectively. As reported by our previous study in Shanxi and Shaanxi provinces (Tokonami et al., 2004), the presence of high thoron concentration was confirmed and thoron was predominant over radon in the cave dwellings. However, the mean equilibrium equivalent thoron concentration (EETC) was found to be much lower than expected when assuming the equilibrium factor of 0.1 provided by the UNSCEAR (2000) report. The effective dose by radon and thoron decay products was estimated to be 3.08 mSv/yr. It was significantly lower than the dose of 8.22 mSv/yr estimated from the measurements that did not take into consideration any discrimination between radon and thoron. Excess relative risk of lung cancer per sievert may be much higher than the risk estimated from the LIH–NCI study, considering that discriminative measurements were not used in their study.

Rn–Tn discriminative measurements and their dose estimates in Chinese loess plateau

Numerous epidemiological studies have been carried out to clarify the health effects caused by indoor radon exposure. Some of passive monitors for radon are sensitive to thoron, and thus the results of measurements might be possibly overestimated as radon concentration. Recently, we developed a simple radon–thoron discriminative monitor by using two passive detectors and a special method for concentration measurements of thoron decay products, and applied them to field measurements in conducted in Chinese loess plateau where cave dwellings are widely distributed. Our new monitors revealed the presence of high thoron concentration there. The highest concentration of 1471 Bq/m 3 was observed and the mean was 240 Bq/m 3. The mean concentration of thoron decay products was 2.2 Bq/m 3. It was very low, compared with the expected concentration from the equilibrium factor of 0.1 adopted by UNSCEAR 2000. On the other hand, the mean radon concentration was 73 Bq/m 3. The effective dose estimated from our discriminative measurements was 2.4 mSv/year, which is below a half of 5.9 mSv/year estimated from the results of the nondiscriminative measurements. This means the significance of radon and thoron discriminative measurement and the direct measurement of thoron decay products.

Thoron in the living environments of Japan

High thoron concentrations have been investigated in various living environments with radon–thoron discriminative measurements; however, behavior of the thoron and its decay products and their effects on human health have not been clearly elucidated yet. In Japan, some kinds of building materials rich in natural radionuclides of thorium series, such as clay and soil plaster, are commonly used for construction and/or surface finishing of wall in traditional Japanese style houses. Measurements with different types of monitor for thoron and its decay products were carried out to investigate general characteristics of thoron in Japanese living environments. From the result of the measurements, thoron concentrations ranged from almost 0 to more than thousands Bq m −3. Extremely high concentrations can be found in the places near wall surface, and it decreases drastically as distance from the surface of the wall increases. Thoron concentration was found to be changed drastically with time. The equilibrium equivalent thoron concentrations (EETC) ranged from nearly 0 to about 6 Bq m −3. Correlation between the thoron concentration and EETC was not good. This report summarized the results of an investigation of thoron in living environments of Japan.

The Levels of Indoor Thoron and Its Progeny in Four Areas in China

Small surveys on thoron, radon and their progeny were carried out in four areas in China. Both high thoron concentrations near to walls and high thoron progeny concentrations were found in some dwellings through the surveys. It is suggested from the limited data that attention shall be paid to the exposure from the inhalation of thoron and its progeny in some kinds of dwellings or areas, and much works on thoron study in detail is necessary.

The Levels of Indoor Thoron and Its Progeny in Four Areas in China.

Small surveys on thoron, radon and their progeny were carried out in four areas in China. Both high thoron concentrations near to walls and high thoron progeny concentrations were found in some dwellings through the surveys. It is suggested from the limited data that attention shall be paid to the exposure from the inhalation of thoron and its progeny in some kinds of dwellings or areas, and much works on thoron study in detail is necessary.

Potential of High Thoron Exposure in China

Compared with radon (222Rn) and its progeny, thoron (220Rn) and its progeny have not been well studied in the natural radiation fields. In this paper, some important factors (232Th content in soil and building material as well as building structure) relating to the level of indoor thoron and its progeny in China are reviewed. Limited results of small-scale surveys are also cited. The facts indicate that the public exposure from the inhalation of thoron progeny may be higher than the representative value in the UNSCEAR 1993 report, and thoron and its progeny may be a significant indoor pollutant in some regions or specific dwellings in China.

Potential of High Thoron Exposure in China.

Compared with radon (222Rn) and its progeny, thoron (220Rn) and its progeny have not been well studied in the natural radiation fields. In this paper, some important factors (232Th content in soil and building material as well as building structure) relating to the level of indoor thoron and its progeny in China are reviewed. Limited results of small-scale surveys are also cited. The facts indicate that the public exposure from the inhalation of thoron progeny may be higher than the representative value in the UNSCEAR 1993 report, and thoron and its progeny may be a significant indoor pollutant in some regions or specific dwellings in China.

Preliminary Survey on Radon and Thoron Concentrations in Korea

A preliminary survey was conducted to determine atmospheric radon and thoron concentrations, and soil gas radon concentrations in South Korea. The atmospheric radon and thoron measurements were made with a radon-thoron discriminative monitor and soil gas radon measurements with a hand-made radon monitor over a year. Six cities were chosen as the measurement site in South Korea. A polycarbonate film and a CR-39 detector were used as detectors for the radon-thoron discriminative monitor and the soil gas radon monitor, respectively. The detectors were replaced every 2-3 months during the survey period so as to observe seasonal variations of the concentration. From the limited data, the outdoor radon concentration has appeared to be somewhat higher than the typical value in the UNSCEAR report. The mean outdoor to indoor concentration ratio was estimated to be 0.6. Since high thoron concentrations were found in some areas, attention must be paid to thoron for accurate dose assessment. The soil gas radon concentration was observed in the range of 3.9-23.1 kBq.m-3.

Thoron Exposure of Man: A Negligible Issue?

Abstract The existing database on thoron (Tn) in the environment, the experimental validation of dosimetric models and potential health effects are scarce. Available information on Tn and its decay products (Tn-d) indoors and outdoors is associated with partly significant uncertainties due to methodological shortcomings. Dosimetric modelling reveals that ThB (212Pb) data are to be considered as the most important input parameter. Under typical indoor exposure conditions the dose due to Tn-d is about one order of magnitude less than from radon decay products (Rn-d). However, special circumstances have been identified where the Tn-dose contribution can exceed the corresponding Rn-d values. Several health effects have been associated with elevated exposure among workers in thoron-related industries and residents in high thoron background areas.

Measurements of Thoron and Thoron Progeny Indoors in Sweden

In order to assess the risk for high thoron levels indoors, measurements of thoron progeny have been performed in a number of buildings in Sweden. Some of the measurements have been made in detached houses in areas with elevated natural thorium concentrations. The radon progeny concentration was measured at the same time. Up to the summer of 1991 measurements of thoron progeny concentrations have been made in nine dwellings, eight cellars and four workplaces in 12 different buildings. The mean thoron progeny concentration in the measured dwellings was 0.3 Bq.m-3 (EET) and in the cellars 1.5 Bq.m-3 (EET). The highest thoron progeny value measured so far in this investigation in the living space of a dwelling was 0.6 Bq.m-3. In a cellar of a detached house we have measured 3.09 Bq.m-3. The highest values have been found in houses built on an esker in central Sweden with a high natural thorium concentration (120 Bq.kg-1). Elevated radon progeny levels were also found in these houses.