# Experimental Investigation into the Thermal Performance of Personal Cooling Mechanisms
## 1 Introduction  
The heat balance equation [1] indicates that the body achieves thermal equilibrium by balancing internal heat production with heat exchange during the respiratory process and through the skin. In the respiratory system, heat exchange mainly occurs through convection and evaporation. Heat exchange occurs via convection, evaporation, conduction, and radiation for the skin. The body's heat loss in hot environments is primarily through evaporative heat exchange [2]. This process involves the transfer of heat from the body to water on the skin, leading to the conversion of water into water vapour, which then disperses into the surrounding environment. Heat exposure affects the thermoregulation of the person [3] because heat transfer can be compromised in this environment. A person's core temperature is between 37.0 and 37.5 \({}^{\circ}\)C [4], with a mean skin temperature of around 32.0-34.0 \({}^{\circ}\)C [5]. In hot conditions, with an air temperature above 33.0 \({}^{\circ}\)C, the core temperature can be increased because sweat evaporation can be compromised, leading to heatstress [6]. This can significantly impact a person's cognitive response and ability, especially during complex work or activities [7]. It is alarming that one-third of heat-related deaths over the past 30 years can be attributed to climate change [8]. The Copernicus Climate Change Service (C3S) [9] has confirmed that the last decade has been the warmest period, with 2023 and 2024 marking the highest temperatures. Projections indicate an average surface warming increase of +2.7 \({}^{\circ}\)C by the end of the 21 st century, which could lead to a staggering 370% rise in heat-related fatalities due to intensified heatwaves. The 2024 report from the Lancet Countdown on Health and Climate Change [10] reveals that 2024 was the hottest year since 1850, with a global average air temperature of 15.10 \({}^{\circ}\)C, an increase of +0.12 \({}^{\circ}\)C compared to the previous record holder, 2023. Moreover, 2024 is the first year recorded with a worldwide air temperature exceeding 1.5 \({}^{\circ}\)C above pre-industrial levels, surpassing the internationally agreed target set at the UN Climate Change Conference (COP21) in Paris [11], with a notable increase of 1.60 \({}^{\circ}\)C. This highlights the urgent necessity for effective cooling mechanisms to mitigate the impact of climate change on human health, and our study represents a crucial effort toward addressing this pressing need. Employees who work outside are at a higher risk of experiencing thermal stress in severe weather conditions, as these conditions cannot be controlled. How outdoor workers perceive temperature affects the heat exchange between their bodies and the environment as well as their physiological and psychological well-being. To safeguard outdoor workers from such environmental conditions, the primary strategy is to prevent exposure to extreme conditions or to provide them with Personal Protective Equipment. The certification standards for protective clothing in cold environments include the EN 342-2017 [12] standard for ensembles and garments designed for protection against cold and the EN 14058-2017 [13] standard for garments intended for use in cool environments. Currently, there are no standards for protective garments in hot environments, so workers exposed to hot outdoor conditions may not have appropriate personal protective equipment to mitigate this risk, and they are unsafe. Understanding the role of clothing in maintaining the thermal comfort of outdoor workers is crucial. In heat-stress environments, it is essential for clothing to facilitate the evaporation of moisture from the skin, thereby maintaining thermal comfort and preventing heat stress. The materials and fabrics of PPE need to be breathable, allowing for the efficient passage of sweat from the user's skin to the environment.