*3.2. Definition of Workers' Unsafe Actions*

Fall-from-height and struck-by-object accidents are prone to happen when workers are working at heights; therefore, in this research, we analyzed workers' unsafe actions through investigating the regulations and reports on these two accident types [1,59,60]. Simultaneously, it should be considered whether the unsafe actions have features that could be identified through computer vision. The five main unsafe actions are summarized in Table 1, namely, throwing objects downwards, relying on railings, lying on scaffold boards and operating platforms, jumping up and down levels, and not wearing a helmet.


All of the unsafe actions are momentary actions, except for not wearing a helmet, and therefore, it is difficult for security managers to observe these actions instantly. Whether or not to wear a helmet is a relatively stable state of a person's action, which security managers can clearly observe and prevent in time. However, wearing a helmet is very important to the safety of workers, especially for working at heights; therefore, it is worthwhile including it in the research content.
