RADON AND THORON PROGENY IN DUTCH DWELLINGS

Development of a High-Resolution Indoor Radon Map Using a New Machine Learning-Based Probabilistic Model and German Radon Survey Data

BACKGROUND

Radon is a carcinogenic, radioactive gas that can accumulate indoors and is undetected by human senses. Therefore, accurate knowledge of indoor radon concentration is crucial for assessing radon-related health effects or identifying radon-prone areas.

OBJECTIVES

Indoor radon concentration at the national scale is usually estimated on the basis of extensive measurement campaigns. However, characteristics of the sampled households often differ from the characteristics of the target population owing to the large number of relevant factors that control the indoor radon concentration, such as the availability of geogenic radon or floor level. Furthermore, the sample size usually does not allow estimation with high spatial resolution. We propose a model-based approach that allows a more realistic estimation of indoor radon distribution with a higher spatial resolution than a purely data-based approach.

METHODS

A multistage modeling approach was used by applying a quantile regression forest that uses environmental and building data as predictors to estimate the probability distribution function of indoor radon for each floor level of each residential building in Germany. Based on the estimated probability distribution function, a probabilistic Monte Carlo sampling technique was applied, enabling the combination and population weighting of floor-level predictions. In this way, the uncertainty of the individual predictions is effectively propagated into the estimate of variability at the aggregated level.

RESULTS

The results show an approximate lognormal distribution of indoor radon in dwellings in Germany with an arithmetic mean of 63  Bq / m 3 , a geometric mean of 41  Bq / m 3 , and a 95th percentile of 180  Bq / m 3 . The exceedance probabilities for 100 and 300  Bq / m 3 are 12.5% (10.5 million people affected) and 2.2% (1.9 million people affected), respectively. In large cities, individual indoor radon concentration is generally estimated to be lower than in rural areas, which is due to the different distribution of the population on floor levels.

DISCUSSION

The advantages of our approach are that is yields a) an accurate estimation of indoor radon concentration even if the survey is not fully representative with respect to floor level and radon concentration in soil, and b) an estimate of the indoor radon distribution with a much higher spatial resolution than basic descriptive statistics. https://doi.org/10.1289/EHP14171.

The First Attempt to Reevaluate Radon and Thoron Exposure in Gansu Province Study Using Radon-Thoron Discriminating Measurement Technique

Although the epidemiological studies provide evidence for an increased risk of lung cancer risk associated with residential radon, an issue of radon-thoron discrimination remains to be solved. In this study, an updated evaluation of lung cancer risk among the residents in Gansu, China was performed where one of the major epidemiological studies on indoor radon demonstrated an increased risk of lung cancer. We analyzed data from a hospital-based case-control study that included 30 lung cancer cases and 39 controls with special attention to internal exposure assessment based on the discriminative measurement technique of radon isotopes. Results from the analyses showed non-significant increased lung cancer risks; odds ratios (ORs) adjusted for age, smoking, and total income were 0.35 (95% CI: 0.07–1.74) and 0.27 (95% CI: 0.04–1.74) for groups living in residences with indoor radon concentrations of 50–100 Bq m⁻³ and over 100 Bq m⁻³, respectively, compared with those with < 50 Bq m⁻³ indoor radon concentrations. Although the small sample size hampers the usefulness of present analyses, our study suggests that reevaluation of lung cancer risk associated with residential radon in the epidemiological studies will be required on the basis of precise exposure assessment.

Application of TLD devices for radon and thoron PAEC measurements in air is the concept of "total PAEC" useful?

The calibration of monitors of radon decay products is a difficult task, and even more so in the case of monitors detecting the products of thoron decay. An important issue here is the lack of primary standards, both in relation to the products of radon as well as thoron decay. In Poland, for many years, measurements of potential alpha energy concentration (PAEC) were carried out with the application of special devices, called ALFA probes. These in turn featured thermo-luminescent detectors (TLD), the readout of which provided information about the potential alpha energy, with no dependence on equilibrium or other factors alike. In this paper, we propose modifying this technique, which had been used only for measurements of products of radon decay. We specifically examine how it can be used to allow simultaneous measurements of radon and thoron PAECs. In employing the method, the idea is to use two sets of TL detectors in the same device during and after sampling. The first set of detectors is used to store the energy of alpha particles during the course and for a short while after the sampling, up to 5 h after its end. Using this method, the first set of detectors holds respectively the full and partial PAECs of products of radon and thoron decay. After that, the first set of TLDs is replaced with a second one. These are then left in the device for another 55-60 h. This set contains only the PAEC from thoron decay products. Afterwards, it is necessary to subtract thoron PAEC from the first set result and add it to the result provided by the second set in order to estimate the PAEC for the products of thoron decay. On a practical level, using such a procedure can potentially difficult during underground or field monitoring nevertheless. This is why another approach can be adopted as well. This is specifically to leave the device for 60 h after sampling without any changes, the original set of TLDs providing a "total" PAEC readout - this being a sum of radon and thoron PAECs. In this paper, we will explore the value of using such a readout for the measurement that is carried out.

A novel method based on 220Rn (thoron) exhalation rate of indoor surfaces for robust estimates of 220Rn concentration and equilibrium factor to compute inhalation dose

The research into ²²⁰Rn (thoron) has generated an increasing interest in recent times due to the realisation of its radiological importance in many indoor environments. Though it is assumed that the contribution of ²²⁰Rn, per se, to the inhalation dose is negligible in comparison with that of its decay products, this may not be always true. Correct estimation of inhalation dose due to thoron requires a reliable method to measure the concentration of both ²²⁰Rn and its decay products in indoor air. However, due to its very short half-life (55.6 s) ²²⁰Rn shows large variation in its indoor activity concentration. This makes it difficult to have a robust value of ²²⁰Rn concentration which can be considered representative of a house, thus making the dose estimation unreliable. This issue has been addressed in the present study by developing a novel method that utilises the ²²⁰Rn exhalation rate from indoor surfaces as the basis for estimation of average ²²⁰Rn concentration in indoor air. The ²²⁰Rn concentration estimated in this manner can be converted to decay products concentration using a suitable equilibrium factor and finally the inhalation dose using appropriate dose conversion factors. A wall mounting accumulator setup has been developed for easy in-situ measurement of ²²⁰Rn exhalation from room surfaces. The method has been validated through comprehensive measurements in 25 dwellings in two different regions of India. The developed method is very good for large scale field surveys because of fast and easy applicability.

Measurement of Indoor Thoron Gas Concentrations Using a Radon-Thoron Discriminative Passive Type Monitor: Nationwide Survey in Japan

As part of a nationwide survey of thoron (220Rn) in Japan, the indoor 220Rn gas concentrations in 940 dwellings were measured throughout one year, from 1993 to 1996, using a passive type 222Rn-220Rn discriminative monitor. The monitor was placed in a bedroom or a living room in each house for four successive three-month periods. The mean annual indoor 220Rn concentration was estimated from the four measurements in each house. The arithmetic mean, the median and the geometric mean for indoor 220Rn concentrations in 899 dwellings were 20.1, 9.6 and 10.0 Bq m-3, respectively. The 220Rn concentrations exhibited a log-normal distribution. It was found that the 220Rn concentrations were dependent on the nature of the materials used for wall construction and also on the distance of measurement from the wall. Significant seasonal variations in the 220Rn concentration were not observed. It would seem that the nature of the wall material contributed to the increased indoor 220Rn concentrations.

Characteristics of Thoron (220Rn) and Its Progeny in the Indoor Environment

The present paper outlines characteristics of thoron and its progeny in the indoor environment. Since the half-life of thoron (220Rn) is very short (55.6 s), its behavior is quite different from the isotope radon (222Rn, half-life 3.8 days) in the environment. Analyses of radon and lung cancer risk have revealed a clearly positive relationship in epidemiological studies among miners and residents. However, there is no epidemiological evidence for thoron exposure causing lung cancer risk. In contrast to this, a dosimetric approach has been approved in the International Commission on Radiological Protection (ICRP) Publication 137, from which new dose conversion factors for radon and thoron progenies can be obtained. They are given as 16.8 and 107 nSv (Bq m-3 h)-1, respectively. It implies that even a small quantity of thoron progeny will induce higher radiation exposure compared to radon. Thus, an interest in thoron exposure is increasing among the relevant scientific communities. As measurement technologies for thoron and its progeny have been developed, they are now readily available. This paper reviews measurement technologies, activity levels, dosimetry and resulting doses. Although thoron has been underestimated in the past, recent findings have revealed that reassessment of risks due to radon exposure may need to take the presence of thoron and its progeny into account.

The analysis of results of radon/thoron measurements performed with the use of nuclear track detectors

Radon has been identified as one of the most important hazards, causing lung cancer. The most important isotope of radon is²²²Rn (3.83 d), while thoron²²⁰Rn (55 s) is treated as the less important isotope due to its short half-life. The radon/thoron hazard for people is related to inhalation of their decay products, but usually, only measurements of radon gas are done in dwellings. For such a purpose nuclear track detectors are used in most of the cases. Since several years simultaneous measurements are done to estimate thoron contribution to indoor radon and thoron exposure with the use of track detectors, too. Typically, a set of two detectors are applied and thoron concentrations are calculated on the basis of discriminative calculations. Unfortunately, very often results of these surveys are not accurate due to underestimation of the lower limit of detection (LLD) for thoron in the presence of elevated radon concentrations. Therefore an analysis of thoron LLDs in relationship to radon concentrations is presented.

A method for the simultaneous measurement of radon and thoron PAEC concentrations in air using a TLD monitor

The idea of using a device with thermo-luminescent detectors (TLD) for the simultaneous measurement of radon (Rn-222) and thoron (Rn-220) decay products' concentrations was invented and developed in the Silesian Centre for Environmental Radioactivity at the Central Mining Institute, Katowice, Poland. The results of a preliminary analysis of the technical applicability, the required minimum period of air sampling and the optimised time schedule proved that such measurements can provide information about the potential alpha energy concentrations (PAECs) of radon and thoron decay products (TnDP).Following the analysis, preliminary measurements were performed at several locations-in a thoron chamber, in dwellings and even outdoors. Surprisingly, the maximum PAEC of the TnDP in the basement of a twin house in the Upper Silesia region was as high as 0.68 ± 0.15µJ m^-3. This paper presents the results of those measurements.

A practical approach to limit the radiation dose from building materials applied in dwellings, in compliance with the Euratom Basic Safety Standards

Individuals receive a significant part of their radiation exposure indoors. We anticipate that this exposure is likely to increase in the near future, due to a growing use in the building industry of recycled materials and materials previously regarded as waste. Such materials often contain elevated levels of natural radionuclides. Directive 2013/59/Euratom ('Basic Safety Standards', BSS) pays comprehensive attention to indoor exposure from natural radionuclides, but proper implementation of all corresponding BSS regulations is not straightforward, especially when regarding the regulation of building materials containing so-called Annex XIII materials. In this paper, we discuss the most relevant deficiencies in the BSS and present a practical approach to cope with these. Our most important observation is that adequate methods for assessing the annual dose due to gamma radiation from building materials are not provided by the BSS. This is in particular difficult because compliance of single building materials has to be tested, but the corresponding BSS reference level refers to gamma radiation emitted by all building materials present in a room. Based on a simple model of three layers of building materials, we present a set of operational conditions for building materials, either used for construction purposes ('bulk layers') or for the finishing of walls, floors and ceilings ('superficial layers'). Any customary combination of building materials meeting these conditions will stay below the BSS reference level for gamma radiation. This statement holds for the middle of a reference room, but is not always the case close to the walls, especially when low density materials with a relatively high content of natural radionuclides are present at the inner side of the room. This can be avoided by applying more strict conditions for those kind of materials than presented in this paper. We further focus on the indoor exposure to thoron progeny. Building materials that pass the test for gamma radiation can still be a significant source for indoor air concentrations of thoron progeny. When the average annual thoron inhalation dose were to be restricted to 1 mSv a^-1 - a level comparable to the BSS reference level for gamma radiation - the activity concentration of Ra-224 in (especially porous) building materials used for wall finishing purposes should be limited to a value of typically 50 Bq kg^-1. Even if our suggested approach of the BSS regulations is fully implemented, it still allows for a significant increase in the average radiation exposure in dwellings due to external radiation and thoron progeny. However, the situation will be worse if a less strict interpretation of the BSS regulations will be applied.

Thoron exposure in Dutch dwellings - An overview

In the Netherlands considerable attention has been given to the exposure from thoron progeny in dwellings. For this purpose a nationwide survey on the thoron exhalation and thoron progeny concentration has been completed in 2015. Furthermore, extensive laboratory studies have been performed to measure activity concentrations and thoron exhalation rates from regular Dutch building materials. The purpose of this study is to demonstrate if the findings from both field experiments and laboratory results are consistent. For this reason measured properties of building materials and surface barriers, in-situ measurements on air ventilation and thoron(progeny) in dwellings as well as advanced computational modelling on indoor air and aerosol behaviour have been used. The results demonstrate that median and mean thoron progeny concentrations of 0.53 and 0.64 Bq·m^-3 found in the survey are comparable with the mean concentration of 0.57 Bq·m^-3 obtained from laboratory testing and calculation. Furthermore, upper thoron progeny concentrations from the survey and the calculations are with respectively 13 and 14 Bq·m^-3 also in good agreement. Such elevated concentrations lead to an effective doses of around 4 mSv per year. The study also includes worst-case scenarios on the application of surface materials high on ²³²Th, and the expected reduction in thoron progeny when using mainstream mitigation measures.