Indoor thoron and radon

Significance of thoron in radiation protection

The radioactive noble gas radon (the nuclide 222Rn from the radium series) and its decay products have been know as a health risk in indoor air for a long time. In contrast to that, the radon isotope thoron (220Rn from the thorium series) and its decay products gained less notice due to its relatively short half-life. Two reasons, however, resulted in increasing attention of radiation protection to thoron lately: Firstly, even smaller concentrations of radioactive noble gases in the air are considered as noxious; secondly, thoron concentrations in the order of magnitude of common radon concentrations were found in houses built of clay. Questions such as those on which conditions increased thoron concentrations occur, which properties (e.g. which size distribution) the decay products possess, and how the exposure to thoron can be mitigated are current topics in radiation protection and are treated in the Research Group Preventative Radiation Protection at the Institute of Radiation Medicine.

Thoron occurs in the decay series of the heavy metal thorium, which has always been present in the ground around the world. It features as half-life of only 56 seconds (that of radon is 3.8 days) and can enter indoor air from the ground or the building material within a few minutes. There, it decays via several different metallic decay products (among them 212Pb and 212Bi in particular), in which also alpha radiation is emitted. As it is the case with radon, the gas itself contributes to the inhalation dose only little. But if the decay products are inhaled as water clusters or after having attached to indoor aerosol, it is exactly this alpha radiation which can cause a significant radiation dose in the respiratory tract.

Zerfallsschema von Thoron und Radon
Fig. 1: Decay scheme of thoron and radon

In modern buildings, which exhibit a tight foundation, thoron cannot reach the indoor air from the ground below the building due to its short half-life. Most building materials are not know as thoron sources either as they contain little thorium or are to tight for the exhalation of thoron. Particularly high thoron concentrations in a large number of houses, however, were found in the traditional cave dwellings and mudbrick buildings on the Central-Chinese Loess Plateau. The Research Group Preventative Radiation Protection could accomplish their own measurement campaign in these houses in cooperation with radiation protection scientists of the Chinese Health Agency CDC (Ms Prof. B. Shang).

Wohnhöhlen und Häuser aus Lehm
Fig. 2: On the Central-Chinese Loess Plateau, many people live in cave dwellings…
Fig. 3: … or in mudbrick buildings. Particularly high thoron concentrations were found there.

Assessment of concentrations with the thoron model

During the last years, a model for the calculation of the concentrations of thoron and its decay products in buildings was developed in the Research Group Preventative Radiation Protection. For this purpose, the influences on the genesis of the gas and its decay products, i.e. their sources and sinks, were identified and related to each other. Subsequently, the parameters which occurred in the resulting equations were determined in measurements. For these measurements, a unique laboratory was erected at Helmholtz Zentrum München: the replica of a traditional mudbrick dwelling from the Central-Chinese Loess Plateau. Many different environmental parameters can be set up in that room (such as the air exchange rate by opening and closing the windows or the humidity with humidifiers and air dryers). The equipment comprises the following measurement devices:

RAD7thoron concentration
passive SSNT detectors (CR-39)thoron concentration
working level monitors with filters and wire-meshesconcentration of the decay products (in total and unattached)
PIPS detectors (alpha spectrometers)deposition of the decay products
condensation particle counteraerosol concentration
SMPS, APS, DMAsize distribution of the aerosol particles
Berner cascade impactorsize distribution of the decay products
HPGe detectors (gamma spectrometers)activity of sampled aerosol
Pt100 sensorstemperature (along with its spatial distribution)
capacitive sensorshumidity
microwave sensormoisture of the building material
CO2 sensorsair exchange rate
Der Modell-Lehmraum des HMGU
Fig. 4: A unique laboratory exists at Helmholtz Zentrum München: the replica of a traditional Chinese mudbrick dwelling at a scale of 1:2.25.

The results of the measurements in the experimental mudbrick dwelling could already be confirmed in measurement campaigns in China (in cooperation with CDC Beijing, Ms Prof. B. Shang) and in northern India (in cooperation with the GND University Amritsar, Mr Prof B.S. Bajwa). The gained information can be applied to different rooms made of clay or other building materials with known thoron exhalation rate. In this way, an annual inhalation dose of about 2 mSv for the inhabitants of such Chinese and Indian clay dwellings is calculated assuming the usual edificial conditions of those areas.

Is there thoron in German houses as well?

Houses with clay as building material are also common in Germany. In most old half-timbered houses, clay is used to fill the wall panels and to plaster interior walls (Fig. 5). In modern ecological civil engineering, clay is in vogue as a healthy building material and popular for inside plastering. According to the known thorium concentrations in Central-European clay, significant thoron concentrations can be expected in German clay buildings as well. A pilot study was therefore performed to obtain a first survey of thoron indoor exposure. Indoor concentration of thoron and its decay products was measured in 17 Bavarian houses in which clay was used as building material. The houses were selected randomly; half-timbered houses but also modern houses with either clay plaster or clay panels were investigated. The measurement period during late winter/spring lasted eight weeks.

Ein Fachwerkhaus
Fig. 5: Clay was used as building material in most half-timbered houses.

In all but two houses significant thoron concentrations were measured, however rather inhomogeneously distributed in space because of the short half-life of thoron: high close to the walls and decreasing towards the center of the room. For dose assessment, the measurement of the decay products proved to be more meaningful, because it is homogeneously distributed due to its longer half-life. In the nine dwellings in which the decay products were measured, the annual dose was in the range of 0.6-4 mSv (with the assumption of 10 h exposure time). Interestingly, two of the modern dwellings showed the highest measurement values (Fig. 6). The low ventilation rate in the well isolated new homes caused a high level of thoron decay products.

Fig. 6: Decay product concentration and resultant inhalation dose in the investigated dwellings.

A detailed comparative study was performed with different measurement techniques in two dwellings, a half-timbered house and a modern low-energy house. The concentration profile of thoron was measured at different walls of a room to determine the total inventory. Besides the known decrease of concentration towards the room center, different courses of the profiles were found due to turbulent mixing. Even different concentrations close to the wall were measured, reflecting locally different exhalation from the wall (Fig. 7). These are additional arguments for the direct measurement of the decay products of thoron by which even potential advection from adjacent rooms would be incorporated. However, a separate measurement of the unattached progeny is not necessary because of its generally low concentration.

Fig. 7: Thoron concentration in indoor air of several rooms of a half-timbered house in dependence of the wall distance. In room 1, the profile was measured at two walls; the rooms 5 and 6 are without any clay surfaces.

The ventilation rate is generally very low in well isolated modern dwellings, which favours high indoor concentrations. Measurements could be performed in a low-energy house with active ventilation at different air exchange. A clear decrease of resulting inhalation dose was observed at high ventilation rate (Fig. 8). The importance of active ventilation for preventative radiation protection gets even clearer if these doses are compared to the annual dose of 7.8 ± 3.2 mSv which was determined in the same house in a not actively ventilated room.

Fig. 8: Effect of active ventilation on the annual dose caused by inhalation of thoron decay products in a modern low-energy house with indoor clay plaster.

References

Tschiersch, J. ; Li, W.B. ; Meisenberg, O.
Increased indoor thoron concentrations and implication to inhalation dosimetry.

Radiat. Prot. Dosim. 127, 73-78 (2007)

Shang, B. ; Tschiersch, J. ; Cui, H. ; Xia, Y.
Radon survey in dwellings of Gansu, China: The influence of thoron and an attempt for correction.

Radiat. Environ. Biophys. 47, 367-373 (2008)

Tschiersch, J. ; Meisenberg, O.
The HMGU thoron experimental house: A new tool for exposure assessment.

Radiat. Prot. Dosim. 141, 395-399 (2010)

Meisenberg, O. ; Tschiersch, J.
Thoron in indoor air: Modeling for a better exposure estimate.

Indoor Air 21, 240-252 (2011)

Gierl, S. ; Meisenberg, O. ; Feistenauer, P. ; Tschiersch, J.
Thoron and thoron progeny measurements in German clay houses.

Radiat. Prot. Dosim. 160, 160-163 (2014)

Meisenberg, O. ; Mishra, R. ; Joshi, M. ; Gierl, S. ; Rout, R. ; Guo, L. ; Agarwal, T. ; Kanse, S.M. ; Irlinger, J. ; Sapra, B.K. ; Tschiersch, J.
Radon and thoron inhalation doses in dwellings with earthen architecture: Comparison of measurement methods.

Sci. Total Environ. 579, 1855-1862 (2017)