## 3 System Overview
### Environment and Methods  
In this chapter, we analyze the diffusion of CO\({}_{2}\) generated by human breath by simultaneously measuring CO\({}_{2}\) concentrations at multiple locations in a room. All experiments in this chapter were performed in the same studio with a volume of 197.0 m\({}^{3}\). During the experiment, all door gaps, vents and openings were sealed up to prevent the CO\({}_{2}\) concentration from being biased by the outside air and the studio's ventilation system is turned off (only natural ventilation). In addition, the studio is located in the basement, and there are no windows or other openings in contact with the outside air. The reason for the use of the basement is to minimize the effect of the air entering through the gaps in the windows on the amount of ventilation, depending on the weather conditions of that day. After the experiment, the door to the outside was opened, the studio's ventilation system was activated and the subjects left and were ventilated for 60 min. The number of subjects in the studio was decided to be three to four because we could place the subjects evenly in the four corners of the studio due to the nature of the experiment. All the subjects were males over 18 years of age and were instructed to breathe steadily in a seated position during the experiment and were allowed to speak to some extent. The duration of the experiment was 60 min for all conditions, and the mean value of the experiment was used as the experimental result. After the experiment, we ventilated the room sufficiently and confirmed that the CO\({}_{2}\) concentration in the room had decreased to the same level as the outside air in the building (about 400 to 500 ppm according to the measurement device) before starting the next experiment. To show that there was no sudden temperature change in the room, the room temperature was measured every minute during the experiment. The experiments in this chapter are referred to as Case 1, Case 2 and Case 3. In Case 1, we measured and analyzed how the distance between the subject and the measurement device. The height difference between the subject and the measurement device in Case 2 and the position of several subjects in Case 3, respectively, affect the diffusion of CO\({}_{2}\) in the room.  
In Case 1, the measurement devices were placed in a straight line from three subjects at 1 m intervals, and the distance between subjects was 30 cm. The arrangement of the subjects and devices in Case 1 is shown in Figure 3, where the measurement device was placed on the floor and the height of the device was set at 0 m.  
In Case 2, three subjects were placed at the edge of the room, and the measurement devices were placed at six locations at a height of 0 m (floor), 1 m and 2 m, and 1 m and 8 m away from the subjects, respectively. The arrangement of the subjects and devices in Case 2 is shown in Figure 4.  
Figure 3: Position of subjects and measurement devices in Case 1.  
In Case 3, four subjects were placed in the four corners or the center of the room. Case 3a is the experiment in which subjects are placed in the four corners of the room, and Case 3b is the experiment in which subjects are placed in the center. The measurement devices were placed at 2 m intervals on 4 \(\times\) 3 blocks to cover the room, and the distance between subjects was set to 8 m or 6.5 m when subjects were placed at the four corners. The measurement device was placed on the floor and its height was set at 0 m. The distance between subjects was set at 1 m when the subjects were placed in the center of the study. The arrangement of the subjects and devices in the experiment is shown in Figure 5.