## Materials and Methods  
### Numerical Model of EMF Source and Its Validation  
The considered WSNs (using sample devices developed at CIOP-PIB) consist of a specific type of wearable device that is able to perform Wi-Fi and/or Bluetooth connections [15; 16; 17]. Wearable devices are worn by workers and used to alert them to any hazards, or to inform them of the necessary safety measures in the monitored area, for example to alert workers using hearing protectors against approaching vehicles. The electronic circuits of both wearable devices consist of one double-sided printed circuit board (PCB). A radiofrequency module is mounted on the PCB's top side, as shown in Figure 3. The PCB has a cut out in the antenna area of the radio module. A small Li-Po cell, a vibration motor and a miniature loudspeaker are located below the PCB. The case for both devices was made of polylactic acid (PLA) polymer on a 3D printer and has walls 1.8 mm thick. Figure 3: _Cont._  
The heart of both wearable devices in the considered applications is the commercial radiofrequency (RF) module ESP32-WROOM-32. This module was chosen because it enables wireless communication in both Wi-Fi and Bluetooth standards (necessary in considered WSN to transmit data to the main network unit or receive data to alert workers to hazards and to localise workers) and is easily programmable and available. Moreover, it is based on system-on-chip (SoC), with relatively high computing power equipped with flash memory. This allows a wearable device to be built based on the radio module itself, without the need for an additional microcontroller. The ESP32-WROOM-32 is equipped with an omnidirectional meandered inverted-F antenna (MIFA). It should be mentioned that other RF modules from the same manufacturer, such as the ESP32-S2, are also equipped with the same MIFA. Therefore, the investigation results presented in this article also apply to such modules or modules of a similar structure. The MIFA was designed to operate in the ISM frequency band (2400-2483.5 MHz) and supports Wi-Fi 2G and Bluetooth communication technologies compliant with the requirements of international standards IEEE 802.11 and IEEE 802.15.1. Maximum EIRP declared by the manufacturer for the ESP32-WROOM-32 module is 100 mW (20 dBm) for Wi-Fi communication (i.e., the maximum value for which use does not require special administrative permission) while for Bluetooth it is 8 mW (9 dBm) (i.e., the value lower than the maximum one not requiring special administrative permission) [18]. The numerical model of the MIFA was an antenna with outer dimensions of \(15.3\times 5.9\) mm and ground of \(18.0\times 19.0\) mm placed on a substrate of \(18.0\times 25.5\) mm (Figure 4). Copper with a thickness of 0.035 mm, an electric conductivity of \(5.813\times 10^{7}\) S/m and a relative permittivity of 1 was used for the antenna and ground; FR-4 (glass-reinforced epoxy laminate material) with a thickness of 0.7 mm, an electric conductivity of \(3.8\times 10^{-3}\) S/m and a relative permittivity of 4 was used as the substrate. Pilot tests were carried out for exposure scenarios with the numerical model of the wearable device with the MIFA plane at a distance of 7 mm from the model of the head. This consisted of the RF module and a battery placed below it, as shown in Figure 3c (i.e., the battery located between the numerical model of head and the RF module in such a way that it did not cover the MIFA) and with wearable devices consisting only of an RF module. The results showed insignificant differences between the obtained maximum SAR10g values (0.2%) and the WHSAR (SAR evaluated as averaged over the entire exposed head) values (1%). The results obtained from the pilot simulations supported the use of a simplified structure of the model of the wearable device (only the model of the MIFA without a battery), as in other published studies [31; 32].