# An Accident Model with Considering Physical Processes for Indoor Environment Safety
## 1 Introduction  
Accident models provide a conceptual representation of accident causation [1]. Their state-of-the-art development is on the phase of systemic models [1; 2; 3], i.e., accident models based on system theory rather than reliability. They were specifically applied to understand accidents in industrial areas, e.g., deepwater well control [4], railway [5], and aviation [6]. Some others relate to places that have been exposed to poisonous or dangerous substances, e.g., oil transportation [7] and nuclear power plants [8]. These poisonous or dangerous substances are hazards in nature, which can directly cause harm when leaked or released in workplaces. The workplace is a strictly managed environment for work. Safety-critical systems are taken as preventative measures for leakage and release. The home environment refers to the indoor space with respect to physical processes of indoor climate, e.g., temperature change, which is different from safety-critical environments in workplaces. Hereafter, we use physical process to represent the physical process of indoor climate. The home environment is not a hazardous place in general. However, it can become hazardous when the physical process transfers from a normal state (e.g., temperature for thermal comfort), then through some intermediate states (e.g., temperature for thermal discomfort), finally reaching a hazardous state (e.g., temperature for heat stroke). The home environment is a place for everyday living, and it is not as strictly managed as that in workplaces. Smart home systems are developed to maintain the home environment in desired states, not only as preventative measures. In systemic accident models [1; 2; 3], accidents are the result of the violation of a set of constraints on the behaviors of the system components, i.e., management, humans, and technology. If directly applying a systemic accident model to the smart home system, the information of physical processes is missing. For example, a smart home system violated its constraint on adjusting the indoor temperature for thermal comfort, and resulted in high temperature for heat stroke. The physical process of temperature change from one of thermal comfort, to some intermediate states for discomfort, then to a high-temperature state that can cause, e.g., heat stroke, is missing. This process is important. First, it assists in understanding how system behaviors could result in accident through intermediate state(s). Second, we need the anomalies' information of the physical process and their causes to deploy reactions and precautionary measures. We can take advantage of hazard analysis techniques to identify the causes in the system to the anomalies. When an intermediate abnormal state or hazardous state of the physical process is detected, with considering the corresponding causes in the system, an effective precautionary or reaction measures can be selected. Generally, if something undesired is not considered in the very beginning of risk analysis, the causes in the following analysis cannot be identified [2; 9]. Therefore, it is necessary to extend the systemic accident model by including the physical process. A newly developing systemic accident model, i.e., Systems-Theoretic Accident Model and Process (STAMP) [2], is considered in this paper, as its underlying rationale has been widely acknowledged by comparing with other systemic accident models [10]. It is based on general system theory for understanding accident causality of sociotechnical systems. A brief introduction of it is presented in Section 3.1. We extend it by considering the following facets. First, the home environment is not inherently hazardous. It can ensure a comfortable life in some physical process states and cause harms in others. Therefore, we take the physical process into account in understanding accident formation. Second, the indoor environment is greatly affected by the behaviors of smart home systems. This is because physical processes are the result of smart home systems and the outdoor climate.