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Weinbaum, Jiji, and Lemos model

Furthermore, Weinbaum, Jiji, and Lemos [160] and Jiji, Weinbaum, and Lemos [161] concluded that the main contribution of local blood perfusion to metabolic heat transfer in tissue is associated with incomplete countercurrent metabolic heat exchange between pairs of arteries and veins (large vessels), not with metabolic heat exchange at the capillary level (small vessels). Eventually, they proposed a model to describe metabolic heat transfer involving arterial and venous blood and skin tissue. Their model is stated as

where qmet = metabolic heat (J); Ta = arterial temperature (°C); T = tissue temperature (° C); (p)b = blood density (kg/m3); (c)b = blood-specific heat (Jkg °C); k=conductivity (W m/°C); g is the volumetric rate of the bleed-off blood flow (the flow out of or into the blood vessel via the connecting capillaries) (m3 m-2 s-1); n = the vessel number; rb = the vessel radius (cm); and V = the blood velocity within the vessel (m/s).

The earlier discussion confirms that metabolic heat transfer occurs from the core of the human body toward the skin tissue; eventually, this metabolic heat dissipates from the skin’s epidermis to the ambient environment [162,163]. At the same time, the body’s sweat glands activate and start to produce sweat, which accumulates on the skin and gradually dissipates into the ambient environment. This dissipation process converts the sweat into vapor by absorbing the metabolic heat from the body and, in turn, cools down the body. In this context, it is notable that clothing acts as a barrier between firefighters’ bodies and their ambient environment (Fig. 1.1). If the clothing does not properly transfer the metabolic heat and sweat-vapor, it results in a rapid increase of core body temperature (Tcore) from a normal 37°C. Because their work places them in ambient environments where this situation is likely to transpire, a great amount of heat stress (cardiovascular and thermoregulatory strains) occurs to firefighters [28-30,164-166]. This heat stress, which decreases work capacity and increases exhaustion, is the primary cause of firefighter fatalities [14].

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