## 1 Introduction  
Occupational illness and injuries can result in short-term and long-term lowered quality of life or disability in workers, which can lead to significant costs and economic burdens to households, companies, and governments. The objective of occupational safety and health (OSH) is to minimize hazards and risks within workplaces to prevent occupational illness and injuries, and employers and companies are the ones responsible for OSH in many countries [1]. During the past few decades, OSH has been one of the most challenging issues in practical workplaces. Additionally, in the Industry 4.0 era, some parts of the manufacturing process are expected to become automated, while other parts will still be handled by human workers. Thus, OSH is anticipated to remain challenging in the Industry 4.0 era [2]. Recently, modern technologies, including the Internet of Things (IoT), sensor networks, wireless communication, big data, artificial intelligence (AI), cloud computing, robotics, etc., have emerged as promising tools for improving productivity and cost reduction [3, 4]. This megatrend is called Industry 4.0, or the fourth industrial revolution. Such Industry4.0 technologies enhance competitiveness in a wide range of industries, including manufacturing, construction, and warehouses; however, they can cause new types of hazards and risks [2; 5]. For instance, automated facilities and collaborative robots interacting with human workers can cause unintentional contacts and collisions between human workers and physical objects such as machines and robots, which may lead to accidents and injuries [6; 7; 8]. Thus, OSH should be carefully considered when applying Industry 4.0 technologies accompanying potential hazards and risks. On the other hand, some existing OSH problems can be dealt with more efficiently by applying Industry 4.0 technologies to collect and process various data more quickly and to automate OSH management procedures [9; 10]. Consequently, the impacts of Industry 4.0 technologies on OSH are expected to be significant. This paper focuses on the digital twin (DT), one of the most important Industry 4.0 technologies [8]. DT is a computerized representation in the digital world of a physical asset available in the real world. To reflect a physical asset's status and characteristics, DT must be integrated and synchronized with the associated physical asset [11; 12; 13]. Recently, more and more researchers and organizations have attempted to apply DT technology to workplaces for cost reduction, the efficient utilization of resources, improved productivity, and so on [14; 15]. Additionally, some researchers have pointed out that DTs can be a novel approach to dealing with various issues related to OSH since they enable objective decision-making based on data analyses and real-time feedback [15; 16; 17]. Nevertheless, the number of research papers on DT applications for OSH has been fairly limited so far. The main obstacle to developing such applications is probably that the DT system is implemented by integrating several modern technologies [16]. Specifically, DT for OSH should be equipped with software and hardware suitable for collecting and processing data about human workers and for representing and visualizing the status of human workers in the digital world. In other words, DT for OSH is a complex system from the technology perspective since it should focus on human workers. Another issue surrounding DT for OSH is that classical OSH methods, such as checklists and expert assessments, are still widely used in practical workplaces [18; 19]. DT technology is expected to be applicable to a wider range of OSH problems, and the benefits of DT for OSH need to be further studied. In these contexts, designing and developing DT applications for OSH are still challenging. This paper aims to provide practical insights into DT applications for OSH. First, a brief survey of relevant research papers is presented, focusing on the visual representation of human workers.