## 2 Experimental Methods
### Index and Calculation Methods  
The wet bulb globe temperature (_WBGT_) index is the most commonly used index to evaluate environmental thermal stress. It is mainly used to determine safe exposure limits in hot and humid environments. This index is calculated using three parameters: black globe temperature (\(t_{g}\)) which reflects the solar radiation, wet bulb temperature (\(t_{w}\)) and dry bulb temperature (\(t_{a}\)). The calculation formula of _WBGT_ in an outdoor environment with solar radiation is as follows [27]:  
\[WBGT=0.7t_{w}+0.2t_{g}+0.1t_{a} \tag{1}\]  
An 8-point weighted method with high reliability and sensitivity was used to estimate the mean skin temperature (\(T_{sk}\)) of the test human bodies [28]:  
\[ \begin{split} T_{sk}=0.07t_{forehead}+0.07t_{upper\ arm}+0.07t_{ lower\ arm}\\ +0.05t_{hand}+0.175t_{back}+0.175t_{chest}+0.19t_{thigh}+0.2t_{ calf}\end{split}  \tag{2}\]  
\(T_{sk}\) and heart rate (\(HR\)) were recorded every one second, and averages and standard deviations were calculated every five minutes to obtain values to monitor variation over time. A weighing method was used to measure sweat production (\(SW_{p}\)), determined by the change in clothing weight before and after the experiment. Due to the barrier properties of the protective clothing, evaporation of sweat discharged by the human body is difficult, and hence the amount of evaporative loss is very small and can be ignored. Hence, sweat production was estimated by the weight gain of the absorbing surgical gown and protective suit before and after the experiment and the weight gain of a towel that was used to wipe residual sweat which remained on the body after the experiment. In order to reduce errors, an average value was obtained using multiple measurements. The formula for the estimation of sweat production (\(SW_{p}\)) is as follows [29]:  
\[SW_{p}=W_{PC2}+W_{SG2}+W_{T2}+W_{G2}-W_{PC1}-W_{SG1}-W_{T1}-W_{G1} \tag{3}\]  
where \(W_{PC1}\) and \(W_{PC2}\) are the weight of the protective suit before and after the experiment (g), \(W_{SG1}\) and \(W_{SG2}\) are the weight of the surgical gown before and after the experiment (g), \(W_{T1}\) and \(W_{T2}\) are the weight of the towel before and after the experiment (g) and \(W_{G1}\) and \(W_{G2}\) are the weight of the protective disposable gloves before and after the experiment (g). The metabolic rate is generally obtained by direct measurement of oxygen consumption [30]. However, it was difficult to measure the oxygen consumed by the participants in this experiment. Considering these constraints, a simpler method was adopted to directly calculate the weight loss due to the metabolism of the human body. The metabolic weight-loss (\(M\)) was estimated using the weight difference of a test participant, i.e., the difference between the total weight of a participant before the beginning of the experiment and the total weight of the participant after the end of the experiment, as follows [31]:  
\[M=M_{1}-M_{2} \tag{4}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Instrument Type** & **Parameter Range** & **Accuracy** & **Resolution** & **Instrument** \\ \hline Human body scale & ES-100 KT-5 & 0.005–100 kg & \(\pm\)10 g & 5 g & \\ \hline Electronic balance & Meilen-MTB & 0–3 kg & \(\pm\)0.01 g & 0.01 g & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._where \(M_{1}\) and \(M_{2}\) are the total weight of participants before and after the experiment (g). The physiological strain index (\(PSI\)), which reflects the body temperature regulation system and the cardiovascular system, is one of the most widely used indicators to describe heat strain in the human body [16]. \(PSI\) ranges from 0 (no heat strain) to 10 (maximum heat strain), with 1 indicating no/little heat strain, 2 and 3 indicating low heat strain, 4 and 5 indicating moderate heat strain, 6 and 7 indicating high heat strain and 8 and 9 indicating very high heat strain.