High-Performance Apparel and Wearable Devices for Hot Environments

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Introduction

Textiles and apparel have always been an interface between the environment and the human body, protecting people from uncomfortable temperatures and solar radiation. Clothing has truly wearable features, and nowadays even traditional fashion items can be provided with high-performance properties like heating panels, sensors or lighting (Angelova, 2018).

Textiles and apparel support the thermophysiological comfort of people in both cold and hot environment. Living and working in a cold climate requires high insulation clothing layers and shelter to be regularly used so as the human body to be protected from the low temperatures (Wang, Ji & Su, 2018). In the case of a hot environment, where the textile barriers are not needed for thermal protection, the presence of apparel is required by aesthetic, social or other protective (e.g. biological or chemical) reasons. The clothes, however, serve as obstacles, which impeded the transmission of heat and water vapor from the body to the environment (Raccuglia, Sales, Heyde, Havenith & Hodder, 2018).

Nowadays, prolonged periods of hot weather are becoming frequent, due to climate change and global warming (Li, Gu, Bi, Yang & Liu, 2015). The increased metabolism can provoke the same thermophysiological reactions of the human body, as the hot environment (Chmura, Konefał, Andrzejewski, Kosowski, Rokita & Chmura, 2017). Due to physical activity, stress, medications and other reasons the metabolic rate can increase the internal heat of the human body, triggering thermophysiological reactions that aim to decrease the warming of the body and transmitting the excessive heat to the environment (Pryor, Bennett, O’Connor, Young & Asplund, 2015).

Being homeothermic creatures, humans maintain their core temperature constantly around 37 °C (36.5 °C ÷ 37.5 °C), independently from the outside (environmental) temperature (Carvalho, Carvalho, Fontes, Martins & Abelha, 2017). The human body is in a state of thermal neutrality when the heat gained or produced in the body is equal to the heat losses to the surrounding environment. The heat balance equation gives all the factors, which precondition the comfort of the human body (Fanger, 1970):

(1) where M is the body’s metabolic rate (in W or W/m²), W is the mechanical work (in W or W/m²), E is the heat transfer by evaporation (in W or W/m²), R is the heat transfer by radiation (in W or W/m²), C is the heat transfer by convection (in W or W/m²), K is the heat transfer by conduction (in W or W/m²), and S is the body’s heat storage (in W or W/m²).

When the body is in the state of thermal neutrality conditions, the heat storage remains zero (Wang & Hu, 2018). Heat stress appears when which means that the body produces or gains from the environment more heat that it could transfer through the skin to the environment. The inability of the thermoregulatory system to cool the body through the textiles layer leads to thermophysiological discomfort and hot-related injuries, the heatstroke being the most severe of them.

The heat balance equation (1) clarifies all the situations when the heat storage can be :

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  Because of the high temperature of the environment;

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  Because of active mechanical work (mainly muscles work);

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  Because of increased metabolic activity of the body due to reasons, different from mechanical work (e.g. alcohol, drugs).

The influence of the clothing on the human thermophysiological comfort in a hot environment is undoubted, due to the presence of one or more textile layers between the human skin and the surrounding air. Two types of protective interactions are possible: passive and active (Figure 1):