# An Accident Model with Considering Physical Processes for Indoor Environment Safety
## 5 Discussion
In the very beginning, the warning signals will be sent to occupants who could leave the scene or do something else to ensure safety. If the reconfiguration in the reaction stage cannot restore the system to a safe situation, the warning mechanism will inform an emergency department, e.g., a hospital, through networks. Accident models can be roughly classified into three categories [1; 2], i.e., sequential models, epidemiological models, and systemic models. Sequential models describe accidents as the result of time-ordered sequences of discrete events. Epidemiological models view accidents as a combination of latent and active failures within a system, which is analogous to the spreading of a disease. Latent conditions, e.g., management practices or organizational culture, can lie dormant within a system for a long time, which can finally create conditions at a local level to result in active failures. The STAMP model is based on system theory and so is the STAMP-PP model. It describes physical process anomalies that cause uncomfortableness and health problems as a result of abnormal Behaviors of the Performers System. Furthermore, to better understand the causal relation between the Performers System and physical processes and better represent the time order of physical process evolvement, the terms in the STAMP-PP model can be taken as events. Hazard analysis techniques are devised based on accident models. The STPA was invented based on the STAMP model to identify causes that conventional approaches overlooked [2; 9]. We found it is an efficient tool for assisting in the identification of Defects, i.e., ICAs and UCAs (e.g., the results shown in Figures 8 and 9). However, hazard analysis heavily depends on expertise in a specific area. Accident models and hazard analysis techniques are to assist in understanding accidents and guiding for causal analysis. The STAMP-PP model offers a way to understand physical process anomalies with respect to the behaviors of smart home systems. To apply the tailored STPA to that in our case (some results of which are shown in Figures 8 and 9), one may need to possess knowledge in at least the fields of computer science, software engineering, computer networks, and even physics. This makes the authors feel that safety research in different areas is like different research fields. In the first step of STPA, the accidents and system hazards provided mostly depend on the interest of an organization or the government [2]. For example, in this paper, Accident is defined as physical harm of occupants due to heat stroke, as illustrated in Table 3. It can also, however, encompass other symptoms aside from heat stroke, e.g., severe cold. Concrete accidents are different due to the variety of environments, e.g., workplaces and the home environment. It is also determined by budget, severity of occurrence, and frequency of occurrence [2]. In step two of STPA, originally, the taxonomies were provided to identify UCAs with respect to control actions. Physical process anomalies (not Hazards) are the result of abnormal Behaviors of the Performers System. The behaviors are the representations of the functions which are achieved by the control actions issued by the Performers System. For example, stopping the Performers System from working is achieved by the control action \"set OFF\" as shown in Figure 8. Thus, the taxonomies are applicable for identifying ICAs. This ensures that all possible ICAs can be effectively identified. The states of physical process change from a state of comfort to one of discomfort, then to one of hazard. When a state of discomfort is detected, precautionary measures are adopted to prevent the physical process from transferring into a state of hazard. Precutionary measures refer to control actions achieved by reconfigurations. Thus, the taxonomies are also applicable to the precautionary measures. For example, the control action \"set OFF\" in Figure 8 can both be the original control action and the precautionary measure.