## 4 Results
### Examined Works  
There are two types of radon detection devices: passive and active. Passive detectors are used for measuring radon gas for long periods (3-12 months) without the need for electric energy to function since in many of this devices sampling is carried out by diffusion. They differ from active detector measurements as these can provide continuous measurements or averages by day, week, short-term or long-term and report the results instantly in a digital display [2]. These probes require active components such as pumps or electric energy to carry out sampling and are based on different detection principles: electret ion chambers, scintillation cells, current or pulse ionization chambers, or solid-state silicon detectors [2]. This current investigation only focuses on active detectors-related works. In [68], [PERSON] et al. presented a cost-effective IoT radon gas remote monitoring system. The IoT device is based on the adaptation of a commercial radon detector (Safety Siren Pro Series 3) interfaced to an ESP8266 MCU. The collected data are then transmitted via Wi-Fi to a cloud database and the results are made available to remote users through a web application that presents a real-time dashboard with reporting capabilities, and a notification service configured to generate alert messages when specific thresholds are exceeded, e.g., 300 Bq/m\({}^{3}\)[65], which may be delivered using distinct mechanisms, e.g., email, SMS. If the threshold is exceeded, an alarm notification is sent to warn users, or automatically activate mitigation mechanisms, such as forced ventilation. After testing the device, the authors concluded that the external hardware (Wi-Fi transceivers and embedded System-on-Chip) do not interfere with radon measurements. Sensor data are collected once an hour for the remote monitoring service, and to guarantee time synchronization and measurements traceability, in the case of network failure, all transmitted messages have a timestamp generated locally at the device, whose synchronization is updated daily via Network Time Protocol (NTP) or forced after a reboot. In [69], [PERSON] et al. presented the design and implementation of the RnProbe, an IoT Edge Device, that collects, aggregates, and transmits up to the cloud several IAQ parameters. The RnProbe prototype is based on the RD200M Radon Gas Sensor from FTLab, and is capable of sensing indoor radon level, atmospheric pressure, air temperature, relative humidity, and CO\({}_{2}\) level. These devices transmit IAQ measurements to the cloud for re-al-time processing and analysis. In an alarm situation, the system automatically notifies the building administrator to perform manual or mechanical ventilation to reduce radon gas concentration levels. The common use-case of the detection system is made in three steps: (a) Detects a higher radon level in a specific room of a building; (b) sends an alarm message to the building janitor; and (c) a manual ventilation action is carried out by the building administrator. The conceptual system architecture is an online network with three core elements that depict a common use-case: (i) IoT Edge Devices with LoRaWAN modulation, gateway, and server, (ii) cloud/analytics engine, and (iii) client app/dashboard with notifications. The remaining components are software-based and include an end-to-end security mechanism (AES128 + SSL) and MQTT Secure and HTTPS protocols. The IoT Edge device is equipped with two communication technologies (LoRa and Wi-Fi) for ensuring long-range, low-power, and that the data are always transmitted. More details regarding the overall RnMonitor platform, of which the RnProbe is part, can be found in [70; 71; 72], respectively. In [73], [PERSON] et al. developed a secure and low-cost radon monitoring and alert system, based on the radon gas sensor RD200M with other additional sensors, a processing unit, a communication module, and a backend with open-source technologies to predict radon levels, issue alerts, and to show the data and alert status in a web page.