## 5 Discussion

Table 6: Comparative study for radon monitoring technologies. signing complex systems for building management applications such as real-time radon monitoring and management. To achieve this goal, each edge device must provide accurate ra-don sensing (among other collected IAQ parameters/indicators), computing power, and communication capabilities. Typically, all remaining tasks (data storage included) are performed in a cloud computing approach. The system architectures, communication technologies, and hardware requirements were analyzed in-depth to provide insights into the state-of-the-art of IoT-based systems for radon monitoring and management. The general architecture of these IoT-based systems follows the ITU-T Y.4113 Recommendation, which is composed of 3 blocks: IoT area network, access network, and core network. The IoT area network includes all the various devices capable of capturing data (sensors), actuators, and MCUs with storage and bi-directional communication capabilities. The access network is responsible for the communication systems, enabling IoT de-vices to connect to the core networks using different technologies, either wired or wireless (Wi-Fi, BLE, ZigBee, 4 G/LTE, etc.). The core network provides the connectivity infrastructure from the access network to the service provider. It provides the connection to web servers, which will process the data and store it into a data storage system (online or physical storage), permits the visualization of received data, analyzes the impact of pollutants in the target premises, and helps end-users to get instant updates about radon and IAQ levels. It is possible to identify six different types of radon sensors (three commercial sensors and three academic prototypes), using different radon detection techniques, with professional solutions also including IAQ sensing parameters (temperature, relative humidity, and barometric pressure). The preferred architectures for connecting these sensors include different MCUs and interfaces being the most used, the Espressif, Arduino, and Raspberry Pi solutions. The adopted communication technologies for real-time data collection typically include SigFox, LoRaWAN, and Wi-Fi, being the latter widely preferred. The use of electronic systems, in general, involves costs and requires electrical power, however, the authors do not comply with this precept given the absence of estimations for overall power consumption and prices. The sensor's calibration is also an issue, and although referred to by some authors, there's equally a lack of calibration procedures, mostly on prototype solutions, so for these aspects, the comparisons are meager. On the one hand, the results show that 77.8% of the studies focused on radon monitoring with defined threshold-based alerts and corrective actions or measures taken by human operators (manual ventilation, population alerts, etc.,). On the other hand, 22.2% focused not only on radon monitoring but also on radon management, which included autonomous airflow control systems (ventilated systems, motorized windows, etc.,). Additionally, there are 22.2%, 33.3%, and 22.2% of systems based on ESP8266, Arduino, and Raspberry Pi controllers, respectively. At least 44.4% of the sensors were calibrated individually before the system implementation.