# An Accident Model with Considering Physical Processes for Indoor Environment Safety
## 2 Preliminaries
### Home Environment Safety  
If we refer to a dictionary, the definition of safety could be the condition of being protected from or unlikely to cause harm, injury or loss. In system safety [2], safety is taken as an emergent property of systems. It is defined as freedom from conditions that can cause death, injury, occupational illness, damage to or loss of equipment or property, or damage to the environment [19]. However, it cannot be freedom from the conditions in practice. Safety is, thus, the condition of risk that has been reduced to an acceptable level, e.g., as low as reasonably practicable [20]. Home environment safety could be understood as that the risk of indoor climate has been reduced to a level of no harm to the health of occupants.  
As is known to all, indoor climate is affected by outdoor climate. Bad weather such as heatwaves has been occurring frequently in recent years due to global warming and weather anomalies around the world. For example, many places globally experienced intense heat in the summer of 2018. It was observed that the highest temperature record in Japan was broken and reached a new level of 41.1 \({}^{\circ}\)C (Japan Meteorological Agency [visited 2018.10.03] [[http://www.jma.go.jp/jma/index.html](http://www.jma.go.jp/jma/index.html)]([http://www.jma.go.jp/jma/index.html](http://www.jma.go.jp/jma/index.html))). Indoor climate can thus be endangered by outdoor climate anomalies.  
The indoor climate is adjusted by dedicated home appliances, e.g., use of air-conditioner to adjust indoor temperature. Different home appliances are integrated via home networks that yield value-added integrated services [21; 22]. In order to ensure thermal comfort, an indoor temperature adjustment service is an example of the integrated service, which can potentially adopt a window, curtain, and air-conditioning unit. Each involved home appliance may have safety instructions. However, due to the complexity of the smart home, they still could be used in scenarios that cause safety problems. For example, the heating mode of an air-conditioner was used when it should not be. Since the smart home is an application area, appliances inside it may be replaced from time to time. Once an appliance is introduced into the smart home, it also brings about risk. For example, the predefined integrated service may not be aware of the new item and cause safety problems when using it. Therefore, home environment safety depends on the proper use of the indoor climate adjustment service with respect to related home appliances if they were properly designed and manufactured.  
To understand home environment safety, we also need to discuss how people relate to the smart home. This is mostly because home appliances are produced and operated by people. One group of people are professionals. These relate to the activities of design, manufacture, transport, installation, and disposal of appliances, home networks, and so on. One distinguishing characteristic is that they have expertise in a certain field. The other group is occupants who are non-professionals. They are the customers who use the various home appliances. Both groups of people can affect home environment safety in different ways. Professionals are responsible for designing, manufacturing, etc. safe systems and home appliances. Occupants care more about operational safety, since they are more error-prone in operations. We talk about occupants in this paper.