## 4 Conclusions  
In this study, the bio-signals (EEG, PPG, and skin temperature) of subjects were measured, and subjective surveys (TSV, CSV, and CLV) were conducted for the indoor environment of cars in summer conditions to investigate the thermal comfort of drivers under solar radiation, according to the cabin and vent discharge air temperatures. The following conclusions were obtained. 1. In the measurement result for the mean surface temperature, when a vent discharge air temperature of 12.5 \({}^{\circ}\)C was used at a cabin temperature of 35 \({}^{\circ}\)C, the average surface temperatures on the forehead, left hand, right hand, and abdomen of the subject decreased by 2, 0.97, 2.18, and 5.86, respectively. 2. In the results of the subjective survey, the TSV decreased significantly from 2.89 to 1.33 when the vent with a discharge air temperature of 12.5 \({}^{\circ}\)C was operated at a cabin temperature of 35 \({}^{\circ}\)C. In addition, the CSV increased from \(-\)233 to \(-\)0.44 and the CLV increased from \(-\)1.78 to \(-\)0.33 under the same condition. The vent cooling provided a thermally pleasant environment for the driver and greatly improved the concentration on the driving. The CSV and CLV at cabin and vent discharge air temperatures of 25 and 20.5 \({}^{\circ}\)C were 1.33 and 1.33, respectively, which were the highest; thus, drivers felt the most comfortable in this condition. 3. In the PPG experimental results, TP, SDNN, and RMSSD increased and the stress index decreased at cabin and vent discharge air temperatures of 30 and 16.5 \({}^{\circ}\)C and 27.5 and 18.5 \({}^{\circ}\)C, respectively. This means that the subject was relatively stable and comfortable driving under these conditions. In the results of the EEG experiment, the relative SMR wave in the frontal lobe was highest at the cabin and vent discharge air temperatures of 30 and 16.5 \({}^{\circ}\)C, respectively. Under this condition, the subject was in a state of slight tension, which is considered to result in the highest concentration during driving. When the cabin and vent discharge air temperatures were 25 and 20.5 \({}^{\circ}\)C, respectively, the concentration decreased and the relative \(\beta/\alpha\) was the highest; therefore, the concentration on driving of the subject was slightly reduced owing to a long driving time and some fatigue during driving. Figure 11: Variations of relative \(\beta/\alpha\)-, \(\alpha\)-, and \(\beta\)-waves at the occipital lobe. (**a**) Relative \(\alpha\) and \(\beta\); (**b**) Relative \(\beta/\alpha\). 4. In the results of the bio-signal analysis (MST, PPG, and EEG) of the driver, the driving concentration was high at cabin and vent discharge air temperatures of 30-27.5 \({}^{\circ}\)C and 16.5-18.5 \({}^{\circ}\)C, respectively. However, the results of the subjective surveys (TSV, CSV, and CLV) showed that the driver was comfortable and had high concentration during driving at cabin and vent discharge air temperatures of 30-25 \({}^{\circ}\)C and 16.5-20.5 \({}^{\circ}\)C, respectively. Therefore, the results of the bio-signals of the human body may be slightly different from feelings of subjects because of the differences between the reaction of the body and feelings for temperature changes of the environment. 5. As comprehensively analysis of the subjective survey, PPG, and EEG results, TP, SDNN, and RMSSD of subjects increased and stress index decreased at the cabin and vent discharge air temperatures of 30-27.5 \({}^{\circ}\)C and 16.5-18.5 \({}^{\circ}\)C, respectively. Besides, the relative SMR wave and CI index of the frontal lobe tended to increase under same temperature conditions. Accordingly, it was confirmed that these temperature conditions provided a pleasant driving environment to the driver and increased the concentration on driving. Conceptualization, Y.S. and J.H. ; methodology Y.S. and H.C.; validation, H.C., Y.S. ; formal analysis, J.H. ; investigation, J.H. and Y.S. ; resources, H.C.; writing--original draft preparation, J.H. ; writing--review and editing, Y.S.