Assessment and remediation of radon and thoron radiation health hazard in mud-built homes in developing countries
Re-submitting to IGCP
Radioactive radon gas is being generated from uranium and thorium trace metals in soil. Dry, porous mud walls and floors of mud-built homes in developing countries emit radon into room air. Radiation damage in lung tissue results from two radon isotopes and their radioactive decay products. The longer-lived radon-222 with a half-life of ca. 3.8 days can be effectively diluted by venting with outside air. In contrast, radon-220 (called thoron) with a half-life of only ca. 55 seconds can only be mitigated by sealing of porous surfaces.
Typical tight sleeping quarters are often close to walls and floors where thoron originates. A high radiation dose may cause lung cancer. The thoron health hazard is unique to mud houses because porous mud surfaces are typically not sealed by gas-impermeable wall coverings or tiles.
Our project mainly focuses on: (1) developing recommendations for affordable and reasonable detection methods for radon and thoron in mud-built homes; (2) assessing the concentrations and distributions of thoron and radon in mud house air in Vietnam (as an example of developing countries); (3) affordable, non-toxic and socially acceptable sealing of porous mud surfaces in homes to prevent escape of thoron and radon, and to improve public health.
- Duong Nguyen-Thuy, Huong Nguyen-Van, Nguyet Thi-Anh-Nguyen, A.Schimmelmann, M.N. Schimmelmann, 2017. Recognition of health geohazard of thoron (Rn-220) exhalation into room air of earthen dwellings in northern Vietnam. 4th International Conference on Radioecology & Environmental Radioactivity, 3-8 September 2017, Berlin, Abstracts Book, ISBN 978-2-9545237-7-4 [PDF] [Poster JPEG]
Low-cost housing construction in developing countries often relies on the use of local soil or clay that is compacted and dried to form the walls of dwellings. Depending on the local geological context, the soil and clay building materials may contain enough thorium and uranium to produce significant exhalation of radon isotopes. Unlike their metallic precursor elements, monoatomic noble gas radon can diffuse into the room air where it can be inhaled by humans, dissolve in lymph fluid, and pose a radiation health hazard not only due to radon’s own radioactive decay, but also due to the subsequent radioactive decay chains of their unstable metallic daughter nuclides in the human body (WHO, 2009).
Among all radon isotopes, traditionally only 222Rn and its progeny have been considered a health risk in indoor environments due to (i) their contribution to the overall radiation dose and their potential for inducing lung cancer, (ii) widespread occurrence in buildings in developed countries, and (iii) because 222Rn is relatively easy to quantify in room air. In contrast, the contribution of 220Rn (called thoron) to the radiation dose in room air has not received sufficient international attention, in spite of pioneering research in Germany, China, Hungary, India and Japan (see Meisenberg et al., 2017, and references therein).
Materials and Methods
Our group assessed the thoron exhalation from (1) walls and floors of authentic mud houses in northern Vietnam, and from (2) standard-size mud bricks with and without various surface treatments in the laboratory in Hanoi using RAD7 and SARAD® RTM2200 instruments.
Thoron distribution in room air of a mud-house
Thoron concentrations in excess of 1000 Bq m-3 were frequently encountered in room air close to mud walls of northern Vietnamese mud houses. Mud walls exhale less thoron during ‘cold and dry’ winter (mean temperature 15oC; relative humidity 60 to 65 %) than during ‘warm and wet’ summer (mean temperature 24oC; relative humidity 90 to 95 %. Inhabitants of mud houses often place their beds next to walls where thoron concentrations are much higher than in the center of rooms (Fig. 4), thus exposing inhabitants to high dose rates at night.
In contrast to thoron with its short half-life of ~55 seconds, the longer-lived 222Rn with a half-life of ~3.8 days is rarely of concern in mud houses because their typically drafty construction allows for fast ventilation of room air and removal of 222Rn (< 100 Bq m-3). An inhaled atom of 222Rn from room air will likely be exhaled over the next day before it decays in a human body, whereas an inhaled atom of thoron that readily dissolved in the lung’s fluid will almost certainly decay in the human body and contribute to radiation damage in tissue.
Neither the population nor governmental and public health authorities in Vietnam have been sufficiently aware of the wide-spread thoron geohazard in northern Vietnamese mud houses. Enhanced porosity, the presence of holes and cracks, and elevated humidity in mud walls seem to increase thoron concentrations in indoor air of mud houses.
Indoor thoron levels in the air of mud-built houses were found to be much higher than the average 220Rn concentration in the environment (10 Bq m-3; UNSCEAR, 1993), whereas 222Rn abundances were typically lower than the recommended WHO (2009) action level. Rare renovation efforts in historic mud houses sealed interior walls and unwittingly remediated the 220Rn problem, but the high cost of drywall, plastering, wall paper and paint makes such approaches unaffordable for most inhabitants of traditional mud houses.