**1. Introduction**

Radon (222Rn), thoron (220Rn) and their progeny species are large contributors to the annual exposure of an effective dose to the general population. 222Rn and its progeny species contribute about half of the annual effective dose due to natural radiation based on the world mean dose. According to the United Nations Scientific Committee on the Effects of Atomic Radiation [1], the annual effective dose from natural radiation sources has been calculated to be 2.4 mSv as the worldwide average, whereas 222Rn and 220Rn contribute 1.2 and 0.1 mSv, respectively. 222Rn and 220Rn are products of the decay chains of natural radionuclides, such as the 238U and 232Th series, and have half-lives of 3.825 days and 55 s, respectively. The 220Rn half-life is very short compared with 222Rn. Thus, only a very small amount of 220Rn can enter a room from the outside. It is considered that a 220Rn concentration gradient exists near the mud-based walls and floors in low ventilated houses [2]. Therefore, if a mud mortar wall is present in housing materials which have high concentrations of thorium, 220Rn and its decay products may enter houses and cause potential health problems. In particular, traditional wooden houses with mud mortar walls are a common house type in Japan.

The International Commission on Radiological Protection (ICRP) [3] have issued new dose conversion factors for 222Rn and 220Rn progeny species based on a dosimetric approach in Publication 137. The values specified are 16.8 and 107 nSv (Bq m−<sup>3</sup> h)−1, respectively. This means that even small amounts of 220Rn progeny species will cause higher radiation exposure compared to 222Rn [4]. Therefore, interest in 220Rn exposure is growing among the health sciences communities. Recently, a number of 220Rn surveys have been carried out in local regions and nationwide, and the results have been published enabling an evaluation of exposures from 220Rn [5–21]. Also, the need to adopt reliable 220Rn measurement techniques has been argued in several papers [22].

**Citation:** Sanada, T. Measurement of Indoor Thoron Gas Concentrations Using a Radon-Thoron Discriminative Passive Type Monitor: Nationwide Survey in Japan. *IJERPH* **2021**, *18*, 1299.

https://doi.org/10.3390/ijerph18031299

Academic Editor: Paul B. Tchounwou Received: 31 December 2020 Accepted: 29 January 2021 Published: 1 February 2021

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An indoor 222Rn survey was conducted on 940 houses nationwide in Japan from 1993 to 1996 using 222Rn–220Rn discriminative passive type monitors [23]. The passive monitor, developed by Doi and Kobayashi [2], was placed in either a bedroom or a living room where residents spent most of their time. Indoor 222Rn concentrations were determined at 20 dwellings in each prefecture for four successive three-month periods to cover an entire year. In the survey, to eliminate the influence of 220Rn on 222Rn measurement, the 220Rn concentration was performed at the same time for referencing purposes. The 222Rn and 220Rn calibration experiments were performed in a standard radon chamber at the National Radiological Protection Board (Didcot, UK) and using the 222Rn–220Rn mixed chamber of Waseda University (Tokyo, Japan), respectively. This study is concerned with the results for the indoor 220Rn concentrations using the reference data from the nationwide survey which was conducted to determine the 222Rn concentrations in Japan [23]. Furthermore, the seasonal and regional variations were investigated, and the influence of the type of house structure was examined as mentioned previously. However, this study does not include a dose assessment of 220Rn because the 220Rn concentration varies widely in rooms and it is not easy to measure the activity concentration given the short half-life of the radioisotope [22].

#### **2. Materials and Methods**
