## 3 Materials and Methods  
This research was conducted to carry out a state-of-the-art survey on IoT technologies for radon monitoring and active mitigation by using the following research engines: Google Scholar, IEEE Xplore, DOAJ, and PubMed with the search query using the following combination of keywords:  
* In Google Scholar (by title): _allintitle: radon \"Air Quality Control\" OR IoT OR \"Internet of Things\"_
* In other databases (by all fields): _(\"Air Quality Control\" OR IoT OR \"Internet of Things\") AND Radon_ The search will focus on articles and publications related to methods, techniques, and technologies used for radon monitoring and mitigation in indoor or outdoor environments, excluding the remaining. Table 2 includes the criteria defined to assist with the selection of the most relevant publications from the existing databases. The association of radon sensors with IoT technologies is a recent trend, so there are a few reliable studies concerning the integration of monitoring radon concentration campaigns with IoT-based integrated systems. Figure 1 summarizes the flow diagram of the adopted procedure and quantifies the number of processed documents in each specific phase:  
* First, from the 46 indexed publications found, all duplicates were removed along with those written in languages other than English;
* Second, from the remaining records, some titles and abstracts were excluded for lack of relevance or because they are not focused on the survey subject;
* Third, the publications that did not refer to integrated systems with active sensing on radon monitoring, or denote the absence of web architectures, and the ones with unclear or incomplete details about used technologies were removed;
* Finally, following this approach, 9 publications that comply with all these requirements were included in the survey. These baseline studies undertaken by the seminal publications taken as a reference will help to respond to the following questions: (i) What technologies have been recently in use, both in academia and/or available on the market, that integrates radon assessment with IoT technologies? (ii) how do they operate? (iii) what type of radon detection mechanisms \begin{table}
\begin{tabular}{l l l} \hline \hline \multicolumn{1}{c}{**Eligibility Criteria**} & \multicolumn{1}{c}{**Exclusion Criteria**} \\ \hline \multicolumn{3}{l}{Articles or publications that:} \\ \(\bullet\) & Are based on IoT technologies for radon monitoring & Articles or publications that: \\  & and management. & \(\bullet\) & Are duplicated. \\ \(\bullet\) & Include active sensing IoT devices for radon monitoring and & \(\bullet\) & Are written in languages other than English. \\  & an overview of the system architecture. & \(\bullet\) & Are not focused on the survey subject. \\ \(\bullet\) & Provide insights into relevant methods, techniques, and technologies. & \(\bullet\) & Contain unclear details about the used technologies. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Eligibility and exclusion criteria for the survey. do these technologies use? and (iv) what type of system architectures, components, and communication technologies have been used to assist the referred technologies? An in-depth analysis was implemented to respond to each of the four previous questions, highlighting the innovative functions as well as the best functionalities of the IoT-based architectures and their components (hardware and software). As a result, the limitations, opportunities, and future challenges were discussed and insights into the state-of-the-art of these radon-based and mitigation systems were provided. This work aims to contribute to the design of better, smarter, and more sustainable systems for indoor radon assessment and control, taking advantage of IoT and ICT technologies. Researchers can use it as a guide for achieving a more transparent and harmonious integration between technology and the built environment.