Clay as building material

Common mineral house building materials such as concrete, brick, gypsum and aerated concrete contain natural radionuclides. This does usually not involve radiation exposure relevant to the house residents' health.

Clay as a building material is gaining importance again in the context of sustainable building practices: its ecological and physical properties ensure a good indoor climate. Time and again it is a matter of debate whether unfired clay building materials may result in radiation exposures critical to health. The reason for this is because unfired clay can emit the radioactive gas thoron into indoor air. It cannot be ruled out that elevated thoron levels occur in indoor air in particular cases.

Radon-222 and Radon-220 (also termed thoron) both are isotopes of the natural gaseous element radon. The term "radon" usually signifies the isotope Radon-222 produced during the decay of uranium. The term "thoron" points to the fact that Radon-220 originates from the decay of thorium.

Unlike radon of which both the occurrence in buildings and health effects have been studied well, thoron requires further research to facilitate a reliable evaluation of its health implications.

Radon and thoron in buildings

A radon problem mainly occurs when a lot of radon from the subsoil enters a house. It has been established that elevated radon concentrations in buildings increase the lung cancer risk.

Thoron is formed in the ground, too. However, with a half-live of only 55 seconds, it decays almost completely on its way from the ground into a house. Therefore, the subsoil is not a noteworthy source as to thoron – in contrast to radon – indoors. Elevated thoron levels can only occur when greater amounts of thorium from building materials are directly released into a house.

The assumption that unfired clay could give rise to health-relevant radiation exposure indoors is traced back to studies on traditional Chinese clay housings. Clay basically does not contain more uranium or thorium than other building materials, but it has a greater surface because it is very finely granulated. It is via that greater surface that more radon and thoron are released into indoor air than for instance with fired clay bricks. During stoving of bricks the grains fuse together, thereby reducing the surface. This is why fired clay bricks do not release relevant levels of radon and thoron.

The amount of radon and thoron actually occurring in clay correlates with its uranium and thorium level which varies considerably depending on its region of origin.

Due to the natural tightness and adsorption of humidity as well as its ability to store heat, clay creates a pleasant room climate, which often does not require increased ventilation of the interior. To reduce the radon and thoron concentration, however, when using unfired clay as a building material, care should be taken as a precaution to ventilate affected rooms regularly.

Further research required

Based on expansive scientific research it has been well established that radon in buildings can cause lung cancer. The risk of disease depends on the radon concentration.

As a general rule, thoron also has the potential to induce lung cancer. However, the concentration required to cause a marked increase in risk has been far less well established for thoron in indoor air than for radon. Only few studies have been carried out so far on the occurrence of thoron in buildings in Germany as compared to radon.

Further research is therefore necessary to facilitate reliable assessment of the health impacts of thoron in building materials in Germany.

Detection of thoron is difficult

For this reason, the Federal Office for Radiation Protection (BfS) has made important proposals to facilitate quality assured thoron measurements: Within the scope of departmental research the BfS has initiated and supervised the construction of calibration devices for thoron measurements at the Physikalisch-Technische Bundesanstalt (PTB, the National Metrology Institute of Germany) as well as a study of the Helmholtz-Zentrum Munich concerning the fitness of thoron measurement devices for national surveys.

BfS itself offers factory calibrations of thoron measurement instruments made in its accredited radon laboratory. Here, measuring instruments are exposed to exactly known thoron concentrations in order to ensure the accuracy of measurement results. This is a prerequisite for quality assured performance of thoron measurements which are technically very sophisticated.