Funded by Nafosted (2016-2019)
A chance discovery during field work for the first project about methane in the Đồng Văn district, Hà Giang province in 2015, yielded intriguingly high radon-222 concentrations in Rồng Cave near the Sảng Tủng commune.
Even higher levels of environmental radioactivity were measured in March 2016 when our group was relaxing in an ancient mud-built house in Đồng Văn town that serves as a restaurant. The latter geohazard for human health was discovered in the form of high thoron (radon-220) concentrations in room air near mud walls and the mud floor. Our discoveries led us to our second project entitled “radon isotopes in Northeastern Vietnam’s karst environment” (recently funded by Vietnam’s NAFOSTED science grant agency).
Our study uses modern, portable radiation-spectroscopic analytical instrumentation to isotope-specifically assess the radon-induced health hazards in houses, in the open air in villages and towns, on fields, and in caves and karst sinkholes in northeastern Vietnam during climatically different seasons.
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.
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 China, Germany, Hungary, India and Japan.
Thoron concentrations in excess of 1000 Bq m-3 were frequently encountered in room air close to mud walls of northern Vietnamese mud houses. Inhabitants often place their beds next to walls where thoron concentrations are much higher than in the center of the room. 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. An inhaled atom of 222Rn 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 aware of the wide-spread thoron geohazard. Enhanced ventilation of rooms is unable to significantly decrease the concentration of thoron near mud walls. The literature provides no example of a feasible remediation strategy that fits the needs of developing countries. Barring expensive filtering approaches, the only promising strategy is to apply a diffusion barrier on inside walls and 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 assessments 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 using RAD7 and SARAD® RTM2200 instruments.
We were able to develop methodologies and test the results from specific methods of surface sealants to prevent thoron from entering room air. We met our goal to develop methods that are effective, affordable, non-toxic, environmentally sustainable, and socially acceptable. The required materials are readily available in developing countries.
In our upcoming communication with the IAEA, we would contribute towards our outreach and training efforts in developing countries. There may be a possibility that the IAEA will be willing to financially support training activities in Vietnam (and other developing countries), and in such a case we would want to apply for travel money to enable experts to participate in person.
- Nguyễn-Thùy D, Nguyễn-Văn H, Schimmelmann A, Nguyễn-Ánh N, Đặng PT, 2016. Radon concentrations in karst caves in Dong Van karst plateau. VNU Journal of Science – Earth and Environmental Sciences Vol. 32, No. 2S, 2016 [Full PDF].
- Đặng PT, Nguyễn-Thùy D, Nguyễn-Ánh N, Nguyễn-Văn H, Schimmelmann A, 2016. Preliminary investigation into radiological environment in Dong Van district, Ha Giang province. VNU Journal of Science – Earth and Environmental Sciences Vol. 32, No. 2S, 2016 [Full PDF].
- Nguyễn-Ánh N, Nguyễn-Thùy D, Schimmelmann A, Nguyễn-Văn H, Tạ HP, Đặng PT, Ma NG, 2016. Radon concentration in Rong cave in Dong Van Karst Plateau Geopark. Full paper in Proceeding of International Symposium Hanoi Geoengineering 2016 [Full PDF] [Poster PDF].
- Nguyen-Thuy, Duong, A. Schimmelmann, Phuong Ta Hoa, Agnieszka Drobniak, and Minh Schimmelmann (2016) Radon and carbon dioxide in northern Vietnamese limestone caves. 5th International Conference on Earth Science & Climate Change, July 25-27, 2016, Bangkok, Thailand. http://earthscience.conferenceseries.com/abstract/2016/radon-and-carbon-dioxide-in-northern-vietnamese-limestone-caves