## 2 Search Procedure of the Paper Survey  
Figure 1 depicts the overall survey procedure of this paper. The first step is to search research papers using keywords. A combination of two keywords, keyword1 and keyword2, was used in hopes that relevant research papers would be collected and the keywords are listed in Table 1. Keyword1 specifies the technologies that should be utilized and exploited in research papers for our survey. As shown in Table 1, this paper considers two technologies, DT and cyber-physical system (CPS), for keyword1. CPS is also an important Industry 4.0 technology, where a physical asset and the associated DT constitute a closed-loop control system. In other words, DT is used as a controller for the associated  
physical asset in CPS [11; 14]. In this context, DT and CPS are closely related to each other, and both of them are considered for keyword1. On the other hand, five terms related to OSH issues are used as keyword2.  
To ensure the quality of the collected research papers, only research papers published in academic journals indexed on the Web of Science (WoS) and written in English were considered in this step. Research papers published in conference proceedings or journals not indexed in WoS were out of the scope of our survey. Note that about 800 research papers were collected in this step.  
This paper focuses on research papers satisfying the following three conditions, and other research papers are out of the scope of this paper. Hence, the authors manually checked the searched papers, and a research paper was excluded from our survey if it did not meet one or more conditions in the filtering phase, as shown in Figure 1.  
1. Specific applications for OSH should be presented.
2. Data about human workers should be collected and utilized.
3. Visual representation of human workers should be provided.  
Condition (1) indicates that only research papers that propose specific applications--DT systems designed and developed to solve an OSH-related problem--were considered in our survey. In contrast, research papers on conceptual frameworks, general guidelines, and literature reviews of DT for OSH were excluded. Moreover, research papers about single components of DT systems, such as sensor units, or DT applications for non-OSH issues such as production scheduling, were also not considered in this paper.  
The other two conditions imply that the DT application should be human-focused, which means that a DT for human workers must be provided and visualized. Some research papers proposed that DT applications designed to collect and utilize data about facilities or buildings can be used to prevent injuries and accidents in workplaces [20; 21]. However, those papers do not satisfy condition (2). Condition (3) clarifies that this paper emphasizes the visual representation of human workers. Some researchers have pointed out that visual representation is not an integral part of the DT paradigm, where any \"digital\" representation can be adopted to develop DT applications [12]. Nevertheless,  
Figure 1: Overall survey procedure.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Keyword1** & **Keyword2** \\ \hline \hline \multicolumn{3}{c}{Occupational safety and health} \\ Digital twin & Injury \\ Cyber physical system & Fatigue \\ \multicolumn{3}{c}{Workplace hazard} \\ \multicolumn{3}{c}{Ergonomics} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Keywords for the initial search.  
visual representation is emerging as an important element of DT application, since it can be used for monitoring, user interaction, and analysis [22; 23]. Thus, only DT applications with a visual representation of human workers were considered in this paper, and simple wearable devices or sensor applications do not satisfy condition (3).