## 2 Literature Review
The third tier generates the miner's health report and is also responsible for alerting the monitoring and rescue team in case of the worst situation. A built-in GPS tracker aids in tracking the worker's current location. [PERSON] and [PERSON] [20] proposed a real-time helmet-mounted system to monitor hazardous gases (methane, hydrogen sulfide, ammonia, and carbon monoxide), oxygen, temperature, and humidity. Workers outside and inside confined spaces receive alerts every second to immediately initiate the rescue operation in the event of a hazard. To develop the system, they used Arduino Mega and the HC-05 Bluetooth module which sent the recorded data and alerts to the workers outside of the confined place. A printed circuit board (PCB) was designed to combine gas sensors into a single unit. The designed helmet-mounted safety system was tested under normal, toxic, and confined atmospheric conditions. Regarding privacy aspects, according to [PERSON] [24], wearable devices have the potential to create multiple opportunities in measuring and tracking health data and occupational risks, helping companies to significantly improve organizational performance. However, attention is needed to extend the use of wearables from the professional to the private domain. The study conducted by [PERSON] and [PERSON] [25] provided a quantitative analysis of wearable safety devices in the construction industry and analyzed the reasons why workers resist using this type of equipment. The authors highlighted that employees hesitate to adopt this type of technology because they fear that the devices might capture data that they consider to be personal and private. In addition, employees mentioned that they do not want to be continuously monitored while they work. Another concern raised was about the use of private health data in decisions regarding health insurance or employment. Privacy aspects in the context of the use of IoT/Industry 4.0 technologies in OSH are also discussed in the study by [PERSON] et al. [26]. Other issues mentioned by the authors are that not all OSH criteria can be easily digitalized and that for some OSH risks, there is no clear prevention criterion, aside from the need for monitoring devices to be comfortable to use and not cause any inconvenience during working. [PERSON] et al. [27] discussed the challenges of adopting IoT in small construction projects. Among the main points raised are the complexity of data architecture, the need for the implementation of a heterogeneous distributed IoT system, the occurrence of false alarms, data privacy concerns, energy and device management, a lack of standards, a lack of legal and regulatory requirements, inexistent or inadequate government policies and incentives, and the need for technical education for workers. [PERSON] et al. [28] presented a research work conducted in the U.S.A. with 330 workers with experience in the use of wearable sensing devices. The authors investigated the types of devices preferred by field workers and managers and the types of functions that they consider to be essential. Smartphones are the most accepted option by all respondents. According to the authors, the preference for smartphones could be due to the reported resistance workers have against putting on additional PPE or carrying additional equipment. Other options that are among the most accepted by workers are smart safety vests, smart hats, and safety glasses. Managers also mentioned smart hats, smart glasses, and attachable devices among the preferred options. The authors comment that the use of smartphones as wearable sensing devices comes with some safety risks because they could cause distractions that lead to accidents. Functions that were considered more useful were the detection of smoke and fire, proximity to energized cables, toxic gases or chemicals, and proximity to potential falling objects.