## 1 Introduction
### Survey on Wearable Systems for Monitoring Biophysical and Environmental Parameters  
Wearable devices are playing an important role in different application fields, including personalized healthcare monitoring, protection, and security, given their capability to acquire, process, and store data in several domains, such as biophysical, psychophysical, behavioral, and environmental parameters [20; 21; 22; 23]. Several examples are reported in the literature in which single wearable devices [24; 25] or body area networks (BANs) [26] are proposed to acquire vital signs and environmental parameters, such as the concentration of dangerous gas species. Fundamental requirements for wearable devices are flexibility, lightness, and reduced weight. In particular, wrist-worn solutions represent discreet and unobtrusive solutions for acquiring data directly from the human body. [PERSON] et al. developed a low-power multisensor platform for acquiring both users' vital signs and the chemical composition of the surrounding ambient air (NO\({}_{2}\), CO\({}_{2}\), and NH\({}_{3}\) compounds) [24]. The system relies on a flexible printed circuit board (PCB) integrated into a silicone wristband. Gas, humidity and temperature, and activity sensors are embedded in the platform, whereas other sensors can be connected, such as the ISFET sweat/pH biosensor. Similarly, in [25], the authors proposed a wrist-worn wearable device to continuously monitor environmental parameters, personal activity, and vital signs of workers in their workplaces and patients with chronic obstructive pulmonary disease (COPD). The device, based on a multi-physical layer approach (3D-MPL), contains gas sensors to detect pollutants and toxic gases, along with biomedical sensors in direct contact with the user's skin to monitor skin temperature and PPG. A UV radiation sensor and a microphone are also included to measure the sound level of the surrounding environment. Similarly, in [27], the authors introduced a wrist-worn device, called Ubiqsense, to measure environmental parameters, such as pollutant levels, toxic gases, UV index, noise, air pressure, humidity, and temperature to assess indoor and outdoor air quality. Moreover, a flexible IoT gateway was implemented on a smartphone as an intermediate hub between the physical layer and the server, allowing data collection and synchronization to permit end-to-end communication between the user and the medic in real time. Among the different application fields, the mining industry is certainly one that could benefit from using wearable monitoring devices. In [16], the authors proposed a smart helmet to monitor miners' conditions [28]; it is equipped with a temperature and humidity sensor, CH\({}_{4}\) sensor, and collision detection sensor. The acquired data are wirelessly forwarded to a cloud platform that processes them and sends warning notifications if anomalous parameters are detected. Additionally, a smart jacket was proposed in [29] for monitoring a miner's vital signs and environmental conditions. Notably, the wearable device comprises several sensors for detecting the heart rate of the miner, the presence of dangerous gas species, and air temperature and humidity; in addition, the device acquires the current user's position by using an integrated GPS module. In relation to the BAN approach, [PERSON] et al. presented a WSN-based environmental and health monitoring approach for soldiers on the battlefield [30]. It comprises a soldier's unit, integrated into a belt, based on a Raspberry Pi 3 board, which collects data from different sensors and sends the encrypted information to the base station. The system tracks the heartbeat, body temperature, and SpO\({}_{2}\). The device also integrates gas sensors for detecting combustible gas, smoke, and harmful gases. The data are processed and compared to different threshold values to determine the soldier's health condition, which is then displayed through the graphical user interface (GUI). In addition, [PERSON] et al.