Environmental Challenges 4 (2021) 100204 Contents lists available at ScienceDirect Environmental Challenges journal homepage: www.elsevier.com/locate/envc Studying temporal variations of indoor radon as a vital step towards rational and harmonized international regulation Andrey Tsapalov, Konstantin Kovler ∗ National Building Research Institute - Faculty of Civil and Environmental Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel a r t i c l e i n f o Keywords: Indoor radon Annual monitoring Temporal uncertainty Reference level Action level Measurement protocol a b s t r a c t Regulations and measurement protocols for indoor radon testing differ between Europe and the US, with Europe implementing a reference level as opposed to the American two-step approach based on an action level. Moreover, none of the afore-mentioned regulatory approaches considers the temporal uncertainty of radon, a factor that usually significantly exceeds instrumental uncertainty. Discussed hereafter is the innovative principle of indoor radon regulation that considers both temporal and instrumental uncertainties. A quantitative relation between Action and Reference levels is being established for the first time. A statistical method for assessing the coefficient of temporal radon variation K(t) depending on the mode and duration of measurements is discussed. New data on the values of K(t) in hot climates and unstable geology typical for Israel are obtained. It is also shown that the influence of meteorological factors, tidal forces and seismic activity on the behavior of indoor radon does not improve the measurement protocol. It is concluded that building a statistically representative array of calcu- lated coefficients of temporal radon variation K(t) with a large number (200–300) of continuous annual indoor radon monitoring in different countries is a vital step towards establishing rational and harmonized international regulation. 1. Introduction Radon is a dangerous carcinogen causing lung cancer and accounting for 42% (1.26 mSv/y) of the total dose of ionizing radiation (3.0 mSv/y) from all known natural and man-made sources (UNSCEAR, 2008). At the same time, the contribution from pollution caused by technogenic sources, for example, the Chernobyl accident or nuclear power genera- tion (including uranium mining) does not exceed 0.1% and 0.01%, re- spectively (UNSCEAR, 2008). Thus, contrary to popular fears as to per- ceived dangers of nuclear power facilities, the main factor of human exposure is in fact in our own homes due to radon, since about 90% of the time people spend inside buildings (Cincinelli and Martellini, 2017). According to the World Health Organization (WHO), between 3% and 14% of lung cancers are caused by radon (WHO, 2009). For example, the premature mortality from radon in the USA is estimated at 21,000 peo- ple per year, exceeding the annual death rate from all other household risks (US EPA, 2016). In contrast to other sources, such as cosmic or terrestrial radiation, radon exposure can be regulated and mitigated. According to the rec- ommendations of WHO (2018) and EU-BSS (2014), the annual average indoor radon (AAIR) concentration should be limited to 300 Bq m -3 . The national reference levels (RL) vary in different countries due to differences in regional levels of indoor radon and usually range from ∗ Corresponding author. E-mail address: cvrkost@technion.ac.il (K. Kovler). 100 to 300 Bq m -3 . However, a reliable decision as to the compli- ance of estimated AAIR by measurement result with the RL remains an unsolved problem, due to the fact that radon concentration in build- ings has significant temporal variations (daily, weekly and seasonal). This factor is the main source of the AAIR uncertainty, but it still does not have a quantitative assessment in place, that is necessary in or- der to establish rational and harmonized international regulation. This unresolved issue renders assessing the building compliance with radon safety criteria a serious challenge, significantly complicates making an informed decision as to whether radon mitigation measures are needed, and greatly reduces the effectiveness of National Radon Action Plans established in EU Member States according to the requirements of the EU-BSS (2014). In addition, due to the lognormal distribution of radon in buildings (Bossew, 2010), high indoor concentrations can occur not only in Radon Priority Areas: “Any home on any parcel of land can have a radon problem. Testing is the only way to know… Radon concentration cannot be predicted based on state, local or neighborhood radon measure- ments” (ANSI/AARST MAH, 2019, p.1). This means that any existing building has a potential for radon risk, and therefore must be surveyed (US EPA, 2016; ANSI/AARST MAH, 2014; 2019). Today, there are two main approaches towards indoor radon test- ing. Radon regulation in European countries is based on traditionally established national measurement protocols (IAEA, 2017). To this day, https://doi.org/10.1016/j.envc.2021.100204 Received 22 May 2021; Received in revised form 1 July 2021; Accepted 3 July 2021 2667-0100/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)