## 1 Introduction  
Firefighters are often exposed to dangerous environments with high temperatures and levels of radiation. Researchers have indicated that burns and thermal stress caused by the accumulation of heat in the microclimate within clothing, rather than thermal damage, are the main causes of injury [1]. The accumulation of heat in the thick and heavy uniforms with low air permeability worn by firefighters can increase the temperature in the microclimate between the clothing and the body [2]. When the temperature of the microclimate within clothing is 5 \({}^{\circ}\)C higher than the ambient temperature, increased perspiration can cause the body to rapidly lose water, create a salt-water imbalance [3], and exacerbate heat stress by increasing body temperature and heartbeat. This process may cause heat illness and can even threaten a firefighter's life [4]. When core body temperature decreases by 2 \({}^{\circ}\)C, the low temperature may cause firefighters to lose consciousness, and can even result in death [5]. Therefore, research on the heat and humidity transfer mechanism of heat-protective clothing is crucial. Mitigating heat stress, preventing burns and physical injury, and increasing firefighters' work efficiency can maximize the likelihood of successful rescues. Several studies on the four-layer fabric used in firefighter uniforms have discovered that thermal protection can be enhanced by increasing fabric count and thickness; however, the increase in material would compromise the quality of the uniform and result in a loss of body heat [6]. Phase-change materials, air cooling, and liquid cooling are often used to reduce microclimate temperature [7]. Phase-change materials exhibit a fast thermal response, but the heat released in the solidification process and the heat accumulated in the multilayer firefighter uniform increase the risk of burns [8], which limits their use in firefighter uniforms. In terms of air cooling methods, externally connected pumps for air transmission can easily explode under high temperatures, and the forced convection accelerates water evaporation on the skin, thus endangering firefighters. Liquid cooling methods are widely used because of their safety and effectiveness. Liquid cooling is considered the most effective method of regulating heat within clothing [9]. [PERSON] [10] designed liquid cooling clothing for pilots in the British Royal Air Force. [PERSON] [11] evaluated the effect of liquid cooling by placing various numbers of cooling strips in vests. The results revealed that the liquid cooling vests slowed the increase in skin temperature, maintained the clothing microclimate, and were comfortable. Blocking external heat and reducing the temperature in the clothing microclimate are the primary solutions to addressing the thermal stress caused by firefighter uniforms. The commonly used methods of optimizing fabric performance and increasing layers of clothing to block heat disrupt heat dissipation and exacerbate heat stress. When the ambient temperature is higher than the skin temperature, body heat mainly dissipates through evaporation [12; 13]. Evaporation is caused by differences in water vapor pressure, temperature, and relative humidity between the air and skin. During long periods of high-stress work, firefighters' skin temperature increases, which reduces the difference between the clothing microclimate and ambient temperatures, thereby preventing the body from regulating temperature by removing excess heat through the evaporation of perspiration. A reduction in skin temperature could affect core temperature [14]. Liquid cooling clothing can protect those working in high-temperature environments and prevent injury and discomfort caused by heat. This study applied a human-factors-engineering-based methodology to design a cooling vest by quantitatively measuring factors related to the body and the surface conduction, convection, radiation, and heat evaporation of the vest [15] on the basis of physiology and thermodynamics [16].