Believed to be scientifically exhausted 25 years ago, radon (Rn) research is livelier than ever. Steps towards regulation since the late 2000s (IAEA 1 , EU 2) and integration into the general radioprotection regime (ICRP 3 ) motivated various research projects, and their number seems to be still increasing. It turned out that compliance with action according to regulation, aimed to prevent, mitigate and remediate Rn exposure in a quality assured manner raised questions that warranted thorough investigation. In this contribution, we address a few of them.

Hazard and risk: Until not long ago, the terms have mostly been used interchangeably. However, this blurs an important difference which has consequences for Rn abatement policy. Hazard denotes the physical cause of a detriment, but only under certain conditions (summarized as the vulnerability factor) and the presence of persons affected, it becomes risk. While it is important to know the geographical distribution of hazard (leading to Rn maps), policy that aims to reduce the detriment to society (represented by collective risk) also requires to investigate the distribution of risk. Due to different factors contributing, Rn hazard and risk maps look differently.

Decision QA: Decisions to comply with regulation can have far-reaching economical and political consequences. They must therefore be legally proof, which means that their elements must be QAed. This implies QA of all steps whose result is a decision; therefore one speaks of the QA chain. Part of QA is estimation of uncertainty, which for decisions means assessment of the chance of ill-decision.

Temporal dynamic of radon concentration: Maps that visualize the spatial dynamic of Rn concentration, usually have long-term means as their target variable. However, indoor Rn in particular is known to be subject to strong temporal variability. This is practically relevant if decisions about action must be based on measurements shorter than one year. The question is: Under which circumstances and how can one derive reliable decisions about whether certain action (such as remediating a building) is necessary.

Mapping techniques: Maps are important tools to visualize the presence of ambient hazards and resulting risks, and thus are important as decision support. This has motivated, among other, the European Atlas of Natural Radiation 4 which covers Rn among other agents that may represent environmental hazards. Global Rn mapping is in discussion since recently. Progress in mapping techniques is still ongoing, in particular regarding inclusion of multiple predictors and classification problems, typically delineation of Rn priority areas.

1. Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards; GSR Part 3; www-pub.iaea.org/MTCD/Publications/PDF/Pub1578_web-57265295.pdf
2. European Council Directive 2013/59/Euratom laying down basic safety standards, etc. Official Journal of the European Union 2014; 57(L13), 1 – 73. http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=OJ:L:2014:013:FULL&from=EN
3. e.g., ICRP 137 (2017). Occupational Intakes of Radionuclides: Part 3. Ann. ICRP 46(3/4); https://www.icrp.org/publication.asp?id=ICRP%20Publication%20137
4. Cinelli G. et al. / European Commission, 2019: European Atlas of Natural Radiation, doi:10.2760/520053