## 3 Results and Discussion
### Analysis Results of PPG Measurement  
Figure 9 shows the variation of the pulse wave of the driver according to the cabin and vent discharge air temperatures in the cooling mode under solar radiation. Figure 9a shows the variations of the stress index, TP, and LF/HF. At room temperature (25 \({}^{\circ}\)C), which was the preparation condition before the experiment, the stress index, TP, and LF/HF showed general values, because it was the common indoor temperature. Unlike the results of questionnaires, the stress index, TP, and LF/HF did not change significantly when the cabin and vent discharge air temperatures were 35 and 12.5 \({}^{\circ}\)C, respectively. This may be because it was too short a time after the subject started to drive, and the bodies' response to feeling was not immediately linked to the pulse wave. Thus, the pulse wave of subjects did not change immediately according to the operating condition. After the body adapted for a certain period at a high temperature, the stress index increased rapidly at the cabin and vent discharge air temperatures of 32.5 and 14.5 \({}^{\circ}\)C, respectively. Accordingly, TP tended to decrease slightly. This indicates that the ability of the driver to regulate the external environment change and to cope with stress deteriorate during a long time under high temperatures, which also results in an increase in the stress index of the driver. At cabin and vent discharge air temperatures of 30 and 16.5 \({}^{\circ}\)C, the TP increased and the stress index decreased; therefore, it can be judged that the subjects were relatively stable and comfortable when driving under this condition. However, the stress index rapidly increased again at cabin and vent discharge air temperatures of 25 and 20.5 \({}^{\circ}\)C.  
The discomfort of the driver was caused by the direct contact with a low air temperature and the fatigue from a long driving time. The result of the PPG analysis confirmed that there was a time gap between the emotional feeling and the PPG reaction during driving. Figure 9b shows the variations in the SDNN and RMSSD in the pulse wave of the subjects during driving. The SDNN and RMSSD at a cabin temperature of 35 \({}^{\circ}\)C without vent cooling were higher than those at the indoor temperature of 25 \({}^{\circ}\)C, which was the  
Figure 9: Variations in driver’s PPG in cooling condition. (**a**) stress index, total power (TP), and LF/HF; (**b**) the standard deviation of N-N interval (SDNN) and root mean square of successive differences (RMSSD). preparation condition. This is because the subject suddenly entered a hot environment and the body temperature of the subject increased rapidly. This phenomenon occurred at the stage of human body adaptation in a hot environment. Afterwards, the cabin temperature tended to decrease continuously until the cabin and vent discharge air temperatures of 32.5 and 14.5 \({}^{\circ}\)C, respectively, which means that the driver felt uncomfortable under these temperature conditions. SDNN and RMSSD at cabin and vent discharge air temperatures of 30 and 16.5 \({}^{\circ}\)C increased slightly, which is similar to the variation in the stress index. This was because the body of the driver adapted to the thermal environment, which provided a relatively comfortable environment to the driver. In the results of the PPG analysis under a cooling condition with solar radiation in the summer, the stress index of the driver decreased at cabin and vent discharge air temperatures of 30 and 16.5 \({}^{\circ}\)C, as well as 27.5 and 18.5 \({}^{\circ}\)C, respectively, while the TP, SDNN, and RMSSD increased simultaneously. Therefore, these two temperature conditions can provide a relatively comfortable driving environment for the driver. [PERSON] et al.