## 3 Experimental Results and Analysis  
### Physiological Parameters  
The variations of the average values of \(T_{sk}\) and \(HR\) over time under the four working conditions are shown in Figure 8. It can be seen from the figure that \(T_{sk}\) rose rapidly for the first 15 min. Subsequently, there was a significant correlation between \(T_{sk}\) and WBGT (\(p\leq 0.01\)). When working conditions changed from category 1 to category 2, category 2 to category 3 and category 3 to category 4, the mean skin temperature increased by 0.57, 1.04 and 1.94 \({}^{\circ}\)C, respectively. This indicates that the higher the thermal stress level was, the more obvious the skin temperature rise was. Figure 7: Schematic diagram of the experimental procedures. In this study, when the WBGT was 25-27 \({}^{\circ}\)C, the average ambient temperature was 29.85 \({}^{\circ}\)C and the average humidity was 57.83%. The mean skin temperature of the test participants rose from 33.87 to 35.47 \({}^{\circ}\)C, and the participants began to feel uncomfortable. When the WBGT was 27-29 \({}^{\circ}\)C, the average ambient temperature was 35.12 \({}^{\circ}\)C, the average humidity was 39.30% and the mean skin temperature of the participants fluctuated from 33.89 to 36.40 \({}^{\circ}\)C. When this occurred, the temperature of human skin was high and the thermal sensation was obvious. After the first 15 min, \(HR\) and WBGT also were significantly correlated (\(p\leq 0.01\)). When working conditions were in category 1, category 2, category 3 and category 4, the average \(HR\) of the test participants were 91.27, 93.66, 98.95 and 106.12 ppm, respectively. With the increase in thermal stress level, the rising trend of \(HR\) was obvious. In the high-temperature environment of 27-29 \({}^{\circ}\)C, the heart rate of the participants who stood for rest was more than 100 bpm. Figure 9 shows the variation of skin temperature at the eight body locations (forehead, chest, back, upper arm, lower arm, hand, thigh and calf) of the test participants under the four working conditions. As can be seen from the figure, with the increase in thermal stress level, the skin temperature of each part increases to varying degrees. The forehead skin temperature was the highest, and the average forehead temperature reached 36.99 \({}^{\circ}\)C at 27-29 \({}^{\circ}\)C. The second was the back and chest, with average temperatures of 36.79 and 36.43 \({}^{\circ}\)C, respectively. The skin temperature of the thigh was the lowest. The skin temperature of the hands was significantly greater than that of the lower and upper arm. The main reason is that the medical rubber gloves blocked heat dissipation from the hand. Figure 10 shows the average amount of sweat production and metabolism of the test participants under the four working conditions. It can be seen from the figure that there were significant differences in sweat production and metabolic rate between the different thermal stress levels. Between working condition level 1 (21-23 \({}^{\circ}\)C) and level 4 (27-29 \({}^{\circ}\)C), sweat production increased 113.67 g and metabolic rate increased 118.08 g. Sweat could not be evaporated from the protective clothing and hot air could not be discharged quickly, resulting in an increase in temperature and humidity in a microclimate inside the clothing. Hence, the thermal environment under the clothing was more extreme than the external thermal environment, which increased the physiological pressure on the test participants. Figure 11 shows typical sweat patterns after the experiment, confirming that sweat production was obvious. Figure 8: Variation of \(T_{sk}\) and \(HR\) with time under four different working conditions (i.e., 21–23 \({}^{\circ}\)C, 23–25 \({}^{\circ}\)C, 25–27 \({}^{\circ}\)C and 27–29 \({}^{\circ}\)C). **(a)** Mean skin temperature, **(b)** Heart Rate. Note: \({}^{\star}\) and \({}^{\star\star}\) represent the correlation between WBGT and various parameters; \({}^{\star}\)\(p<0.05\), \({}^{\star\star}\)\(p<0.01\).