## 1 Introduction
### Examples of Wearable WSN and IoT Devices to Monitor and Reduce Hazards in the Work Environment  
WSNs and the IoT can be used for various purposes in the work environment. WSNs used for monitoring the work environment and alerting workers to hazards consist of measuring devices, wearable devices and Bluetooth beacons (Figure 1) [15; 16]. Measuring (sensor) devices are deployed in the workplace area and used to monitor harmful agents in specific areas of the workplace. Sensor devices using Wi-Fi connections send the data to the network main unit (MU) to processes them in order to assess hazards in the workplace area. The collected data can also be analysed using an external computer (EC) connected to the Internet. Wearable devices are used to alert workers to hazards and inform them of the necessary safety measures. They communicate with the network and MU using Wi-Fi, and are also used to determine whether the worker should be alerted when approaching the area under any hazard (i.e., danger zone). The location of the worker is evaluated based on the level of electromagnetic signal received by wearable devices from Bluetooth beacons deployed in the workplace. Another example is a system warning against approaching vehicles, dedicated for workers using hearing protectors for reducing direct acoustic hazards in the work environment [17]. It consists of three types of component: transmitters mounted on every vehicle moving around the workplace, and wearable--basic (primary) and additional (secondary) receivers--used by each worker having access there, as shown in Figure 2. The operation of the WSN is based on an analysis of the strength of the received signal, which is a metric of the distance between the receiver (worker) and the transmitter (vehicle). It is implemented with the use of a received signal strength indicator (RSSI) that can be read by a microprocessor. Transmitters mounted on every vehicle are Bluetooth beacons broadcasting specific radio signals with a constant transmission power. The danger zone around the vehicle is defined as a minimum distance from a vehicle, within which the worker should be warned about a moving vehicle. This distance, at the constant power of the transmitter, also sets out the minimum strength of the signal received by wearable devices, above which the worker should be warned. In this way, the system reduces the risk that the worker is hit by approaching vehicle. The wearable basic (primary) signal receiver was designed to be worn on the wrist by a worker and is integrated with alarm systems producing vibration (haptic), light and sound warning signals, of which the vibration signal is best received in a noisy e.g., industrial work environment. The Bluetooth radio signal (at 2.4 GHz frequency) can be highly attenuated in the human body (in the field tests, RSSI can differ by up to 20 dBm depending on the position of the worker in relation to the transmitter). Because of that, an additional (secondary) receiver, attached to the worker's helmet (on the opposite side of the body to the basic receiver) was used to improve the directional sensitivity of the system. Based on an analysis of the level of the signals received by the basic and additional receivers, the warnings to the worker are either generated or not. For this purpose an additional transmitter/receiver (located at the helmet) communicates with the basic receiver (located at the wrist) using Bluetooth communication technology. Figure 1: Basic structure of a wireless sensor network for monitoring the work environment and alerting workers to hazards (M—measuring device; W—wearable device; B—Bluetooth beacon; AP—Wi-Fi access point; MU—network main unit; H/G—network hub and gate; EC—external computer; grey arrows—Wi-Fi connections; black dashed arrows—Bluetooth connections; black arrows—Ethernet connections).