A full-size model of a mud-built home was constructed in a Hanoi suburb in 2017. The structure is being used for development and testing of remediation strategies to mitigate radiation geohazards in mud-built homes in Vietnam as well as developing countries.

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 (UNSCEAR, 1993; WHO, 2009).

Our project entitled “radon isotopes in Northeastern Vietnam’s karst environment” aims to survey the concentrations and distributions of radon and thoron in mud house air in Hà Giang province, northern Vietnam. Utilizing RAD7 and SARAD® RTM2200 instruments, thoron concentrations in excess of 1000 Bq m^-3 were frequently encountered in room air close to mud surfaces of mud houses.

Inhabitants often place beds near walls of mud houses where thoron concentrations are much higher than elsewhere in rooms. In contrast to thoron (²²⁰Rn) 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. An inhaled atom of ²²²Rn will likely be exhaled over the next day before it decays in a human body. In contrast, 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 are sufficiently aware of the wide-spread thoron geohazard. Enhanced ventilation of rooms is unable to significantly decrease the concentration of thoron near mud surfaces.

A detailed study by Meisenberg et al. (2017) laid the foundation for accurate measurements of the ²²⁰Rn inhalation hazard in mud houses, yet it seems that no feasible remediation strategy has been available to fit the needs of developing countries. A promising strategy is a diffusion barrier on inside walls to delay the escape of ²²⁰Rn until the short half-life of ~55 seconds has caused safe decay within the porous mud wall.

Our group performed numerous practical tests of surface sealing techniques both in an authentic mud house in northern Vietnam and using standard-sized artificial mud bricks in the laboratory in Hanoi. Abundant deep cracks in most mud walls prevent layers of paint to provide an adequate seal. Hung tapestry, sheets of paper, and foil pinned to walls also proved to be inadequate. Isolated renovation efforts in historic mud houses sealed interior walls and unwittingly remediated the ²²⁰Rn problem, but the high costs of drywall, plastering, wall paper and paint make such approaches unaffordable for most inhabitants of mud houses.

In December 2016, our remediation strategy in an authentic mud house reduced the thoron concentration in room air close to walls from above 1000 Bq m^-3 to below detection limit. The applied diffusion barrier is inexpensive, non-toxic, non-flammable, resistant to biodegradation, and easily applicable using regionally available materials. The final application can be pigmented or include a dye. The material cost to remediate an average mud house in northern Vietnam is estimated to US $25. A crew of two low-skilled workers can remediate an average mud house in two days using simple tools. Mud houses are abundant in many parts of the world, and mud-based ‘green’ building technologies are gaining acceptance in developed countries. Effective outreach and remediation need to be customized for each country.

Vietnam’s Hà Giang province is located in the northernmost mountainous region. The time-consuming travel between Hà Giang and Hanoi prevents our group from performing frequent measurements in inhabited mud houses. In October 2017, Mr. Minh’s family and friends from a village in Hà Giang province helped the EOS group transporting local mud to Hanoi, and then used traditional mud-house construction techniques to build a small mud house in a Hanoi suburb. The full-size  mud house is being used for assessment of various remediation strategies.

Our study 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.

Related publications

1. Nguyễn, N.T.A., Nguyễn-Thuỳ, D., Nguyễn-Văn, H., Nguyễn-Hải, N., Schimmelmann, A., (2021). Radioactive thoron ²²⁰Rn exhalation from unfired mud building material into room air of earthen dwellings.  Front. Earth Sci. – Geochemistry, 9, 629241. https://doi.org/10.3389/feart.2021.629241.
2. Nguyễn-Văn H., Nguyễn-Thuỳ, D., Nguyễn, N.T.A., Streil, T., Schimmelmann, J., , Doiron, K., Nguyễn-Đình, T., Nguyễn-Thị H, Schimmelmann, A., 2020. Excessive radon-based radiation in indoor air caused by soil building materials in traditional homes on Đồng Văn karst plateau, northern Vietnam. Chemosphere. Vol. 257, Oct. 2020, 127119. https://doi.org/10.1016/j.chemosphere.2020.127119. [VSI: Air pollution in developing countries].
3. Dương Nguyễn-Thuỳ, Hướng Nguyễn-Văn, Jan P. Schimmelmann, Nguyệt Thị Ánh Nguyễn, Kelsey Doiron and Arndt Schimmelmann (2019) ²²⁰Rn (thoron) geohazard in room air of earthen dwellings in Vietnam.  Geofluids, Vol. 2019, Article ID 7202616, 11 pages; DOI: 10.1155/2019/7202616
4. Schimmelmann, A., Dương Nguyễn-Thuỳ, Hướng Nguyễn-Văn, Nguyệt Thị Ánh Nguyễn, Minh Ngọc Schimmelmann (2017) Thoron (²²⁰Rn) exhalation into room air of earthen dwellings in northern Vietnam: Recognition of health geohazard and strategy for remediation. 14^th International Conference on Gas Geochemistry 2017, ICGG-14, 24-28 Sep., Wrocław and Świeradów, Poland, Book of Abstracts 139-141 [PDF].
5. 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.